Ryan Bethencourt

David Coelho

Jessica Gobbo

Magali Frugier

Rolf Postina

Sophie Rome

Stefano Marzi

Timothé, Laeya Liza

Fanny

Florent Jean-Malo Paul

Pichot F, Marchand V Galvanin A Ayadi

Roignant Jy

Motorin Leidel Sa

Helm M Angew Chem

-RNA ribose methylation (2'-O-methylation): Occurrence, biosynthesis and biological function

RNA are molecules that play roles far more important than a simple transfer of information from the head: the sacred stored genetic material to the working-class proteins. Indeed, RNA are not only the molecules which make the link between DNA and proteins, RNA are even in the centre, we can consider them at the wise of the cell. At the beginning of the life on Earth, they were there, pioneers, having catalytic activities before bequeathing them to enzyme proteins. At this time in the evolution, it was what we call "the RNA world" where these nucleic acids were ruling the whole cell, they were at the base for everything. Now, a lot of other molecular components took over but RNA are still strongly present in the cell and are involved in a multitude of essential functions. They have so much properties and attributions that it is impossible to list them in an exhaustive manner. If we add a layer of complexity and add RNA modification, splicing event in the equation, it becomes completely crazy. This is why molecular biology and especially the study of RNA are a huge field in fundamental research where we never stop discovering new things. In a world where technology flows in our veins, where high throughput techniques allowed to make progress science, I am glad to contribute to this vast domain by writing my phD thesis.

"Our world is built on biology and once we begin to understand it, it then becomes a technology."

Figure list:

Table list:

Table 1 Comparison between sequencing by ligation, sequencing by nucleotide addition and

General introduction I. Ribonucleic acids: RNA

Together with proteins, lipids and complex polysaccharides, nucleic acids are the most important macromolecules and are essential for the life and cell functions. The major functions of these biomolecules are the storage of genetic information for deoxyribonucleic acid (DNA) and the conversion of the genetic information to active biomolecules for ribonucleic acid (RNA). Nucleic acids are composed of nucleosides consisting of a sugar deoxyribose (DNA) or ribose (RNA) connected to the nucleobase via a glycosidic bond. Adenosine (A) and Guanosine (G) belong to the purine family while Uridine (U)/ Thymidine (T) and Cytosine (C) are pyrimidines. Nucleosides are attached to a ribose-phosphate chain. In contrast to DNA which is structured in a rigid double stranded helix (1,2) in order to keep stability for storage of genetic information, RNA are single stranded, but can form complex 2D and 3D structures via hydrogen bonding between complementary bases: A-U (2 hydrogen bonds) or C-G (3 hydrogens bonds) (1).

The genetic code, stored as DNA, is transcribed in coding RNA: messenger RNA (mRNA) and further is translated to proteins with the help of non-coding RNA species, namely the transfer RNA (tRNA) and ribosomal RNA (rRNA). Besides their role in translation, ncRNA are involved in a multitude of other important biological processes. Some of them, like the small nuclear RNA (snRNA) are involved in splicing, while small nucleolar RNA (snoRNA) and small cajal-bodies RNA (scaRNA) play a role in nucleotide modifications (methylation and pseudouridylation) of rRNA and snRNA, respectively. Among small ncRNA, there are also interference RNA that regulate gene expression, such as microRNA (miRNA), piwi-interacting RNA (piRNA) and small-interference RNA (siRNA). There are also various long ncRNA (lncRNA,. For instance, there are RNA playing a role in epigenetic regulation by chromatin modification (e.g. RNA Xist responsible for X chromosome inactivation, Air and HOTAIR (HOX transcript antisense) RNA...), or in regulation of gene expression by acting on transcription (7SK RNA) or on translation (7SL RNA) (3)(4)(5).

II. Nucleic acids sequencing from 60s to now and later

The strong diversity of RNA makes their study a real challenge. Since decades, scientists strive to characterize their sequences and optimize DNA/RNA sequencing.

a. 1st Generation

In 1953 Watson and Crick determined the 3D structure of DNA but reading their sequenced remained impossible for some years. Techniques used for protein sequence determination were not transposable since DNA had only 4 different nucleobases with a longer sequence than proteins (6). This is why, researchers had to find other strategies. First, they combined the techniques used for nucleic acid composition determination with treatment by selective ribonucleases to get information of their order via specific produced fragments. In 1965, Robert

Holley and colleagues were able to sequence a whole alanine-specific tRNA from S. cerevisiae (7). In parallel Frederick Sanger developed a method based on the detection of radiolabelled fragments that were partially digested and fractionated by two-dimensional gel coupled with thin layer chromatography (TLC) (8). Using this method Sanger's and then Fier's laboratories sequenced a first pool of ribosomal and tRNA sequences as well as, few years later, the whole bacteriophage MS2 RNA in 1972 (9)(10)(11)(12)(13)(14)(15). Then, Ray Wu and Dale Kaiser used DNA polymerase with radioactively-labelled nucleotide triphosphates. They managed to determine the sequence by measuring the radioactivity of the labelled nucleotides added one by one (16,17). However, this required a considerable amount of analytical chemistry and the necessity to use sophisticated 2-D fractionation techniques (electrophoresis coupled to chromatography).

Better resolution in a single separation step was achieved by the use of polyacrylamide gels.

Two protocols were simultaneously developed: Alan Coulson and Frederick Sanger created the "plus and minus" system in 1975 while Allan Maxam and Walter Gilbert used chemical cleavage techniques (18,19). "Plus and minus" technique is an optimization of the previously described method where two DNA polymerisation experiments are done in parallel. The "plus" reaction uses only one labelled nucleotide triphosphate and thus all sequences seen on the gel end by this nucleotide. The "minus" reaction uses the three other nucleotide triphosphates. In contrast, Maxam and Gilbert used radiolabelled DNA treated with chemicals able to cleave DNA at specific bases. This last technique was considered as the first generation for nucleic acids sequencing.

The major event in this field occurred in 1977 with the development of the protocol which we call now Sanger sequencing by chain termination (20). This protocol uses dideoxynucleotides triphosphate (ddNTP) which are analogues of deoxyribonucleotides triphosphate (dNTP) but lacking the 3' hydroxyl group and thus preventing further extension of the DNA chain.

Radiolabelled ddNTP are added to DNA extension reaction containing standard dNTP at higher concentration and provoke termination of DNA chains at all possible lengths, since ddNTP are randomly incorporated. On the polyacrylamide gel, one can read the sequence by following the DNA bands for all four nucleotides in parallel like it is shown in the figure 1. For years, Sanger sequencing became the most used technology to sequence DNA. Since 70s, several improvements were proposed such as the use of fluorophores instead of radioactivity (allowing to use all four fluorophores together in one experiment) or the detection of DNA chains by capillary based electrophoresis (21)(22)(23)(24)(25)(26). These improvements considerably change the way analyses were performed and sequencing could be automatized with machines (27).

b. 2 nd Generation

For years and even decades, DNA sequencing was tedious and expensive. For instance, in 2003 the human genome sequencing costed around 2.2 billion euros (28). Now with the upcoming of 2 nd generation of sequencing in the 2010s, cost decreases considerably to around 1,000 euros in 2016. The second generation of sequencing methods gathers all high-throughput methods including a phase of amplification of a given DNA in a restricted area in order to be able to detect its corresponding signal over background. Moreover, these methods are able to analyse only short reads: generally below 300 nts. One can distinguish two types of sequencing, the sequencing by ligation (SBL) and the sequencing by synthesis (SBS) similar to Sanger Sequencing.

Amplification procedures

Before detecting signal from the sequencing, DNA amplification is required. Several strategies exist for the generation of clonal DNA template. Globally DNA is first fragmented in pieces and adapters are ligated to the extremities allowing their amplification and sequencing. For beads-based amplification, one of the adaptors is complementary to an oligonucleotide that is covalently attached to the beads. By doing an emulsion Polymerase Chain Reaction (emPCR):

micelle droplets containing all reagents for PCR, single original DNA molecule is amplified

and forms what we call polonies (29). Beads containing statistically a unique DNA population can be distributed on a glass surface (30) or arrayed on a PicoTiterPlate (31).

Alternative on-surface PCR amplification (32) avoids the use of emPCR since DNA is directly amplified on a surface (33). Both adaptors (one at the time) can hybridize to oligonucleotides covalently bound to the surface. This is called a "bridge amplification" since the new synthetized DNA strand has to arch over to rich the neighbouring primer for the next cycle of polymerisation (32,34,35).

The last type of amplification used is the one which can be done in solution and employs to the rolling circle amplification process (36) leading to DNA nanoballs. In that case, DNA undergoes a ligation, circularization and cleavage process to create a circular template.

Nanoballs are then distributed on a slide such as the polonies.

2. Sequencing by ligation (SBL): SOLiD SBL approaches consist in the hybridization and ligation (37) of labelled probes to a DNA strand. In SOLiD system (figure 2), once the probe is bound to DNA template coming from emPCR amplification, imaging of the fluorescence is performed and one can identify two nucleotides. A new cycle begins after cleavage of the probe in order to remove the fluorophore.

SOLiD is composed of a series of probe binding, ligation, imaging, cleavage cycles to elongate the complementary strand. Since determined dinucleotides are not next to each other, it is important to use other probes with first part offset of one nucleotide to ensure sequencing of every base.

Sequencing by synthesis

Sequencing by synthesis groups all the methods using DNA polymerase. Some techniques use single nucleotide addition (pyrosequencing and Ion torrent sequencing), while the others employ cyclic reversible termination (Illumina sequencing).

• Pyrosequencing and Ion torrent sequencing

In contrast to most described strategies, sequencing by single nucleotide addition doesn't rely on the detection of radioactivity or fluorescence. Here, a specific signal produced after the addition of a natural dNTP during polymerisation is measured. Thus, each of the four dNTP has to be added one by one to make sure the signal is specific to each nucleobase.

454 pyrosequencing (39), was the first system using this principle. After amplification by emPCR on beads, sequence is determined via direct nucleotide incorporation and an enzymatic cascade. Each time a dNTP is incorporated, a pyrophosphate (PPi) is produced as a by-product: (40). The enzyme ATP sulfurylase converts PPi to ATP, which is then used as a substrate for luciferase leading to light emission which is proportional to the amount of pyrophosphate released (figure 3a).

Ion torrent sequencing (41) doesn't use an enzymatic system but directly detects the change of pH due to release of H+ ions when a dNTP is incorporated. Detection is done by the integrated complementary metal-oxide-semiconductor (CMOS) circuit (Figure 3b).

Major issue for these two techniques is the difficulty to sequence homopolymer regions since the signal detected is not completely proportional to the number of nucleotides incorporated. b) Ion Torrent. The principle is the same than 454 pyrosequencing except the fact that the by-product released during nucleotide incorporation is H+ leading to a change of pH that can be detected via an integrated complementary metal-oxide semiconductor (CMOS) and an ion-sensitive field-effect transistor (IFSET) device.

• Illumina sequencing (derived from Sanger sequencing)

Illumina sequencing is derived from the original Sanger protocol with the use of fluorescentlylabelled dNTP blocked at 3'OH thus preventing the further cDNA elongation (42,43) (Figure 4). DNA templates are transformed into double stranded DNA with adaptors at both extremities.

DNA clusters are performed via the bridge amplification directly on a flow cell. At every cycle, the four individually labelled and 3'-blocked dNTP are added. One is incorporated by the DNA polymerase where the three unbound are washed away. The surface is then imaged and the nature of incorporated dNTP into each DNA cluster is identified by specific fluorescent signal.

The fluorophore and the blocking group are removed and another cycle begins. Illumina is also able to produce paired-end (PE) data where each DNA cluster can be sequenced from both extremities. Once a first sequencing by synthesis from a single stranded DNA bound to the flow cell is finished, sequencer changes its orientation by bridge amplification and starts another round from the opposite end. This provides a greater amount of data and improves accuracy, especially for repeated sequences.

Comparison of the different methods

Even if all these methods are rather accurate (> 99%), Sequencing by ligation (SOLiD) and pyrosequencing (454) are no longer commercialised due to their lower performance and especially a higher cost. In the field, Ion Torrent and especially Illumina sequencing are the pillars in the market and have the monopoly. In the table 1 specificities of each of the three methods are represented. Pacific Bioscience technology is also based on the detection of fluorescent dNTP incorporated by a polymerase but without bridge amplification (33,(50)(51)(52). Here, DNA polymerase and template are attached to the bottom of a nanostructure called zero-mode waveguide (ZMW).

These nanostructures exploit the properties of light passing through a window with a diameter -High level of compatibilities with other platforms -technology mastery over decades since it derives from Sanger sequencing -some under-representation for AT and GC rich regions as well as some substitutions (44,(47)(48)(49) lower than the light wavelength. Thus, only the very bottom of the well is illuminated (where the DNA polymerase is attached). Then DNA polymerisation can be followed in real time since only the fluorescent dNTP that is incorporated can be detected. Pacific Bioscience process can sequence single molecules in a very short time. However, the level of sequencing errors is high and it becomes difficult for DNA templates longer than 3 kb. Nanopore Oxford Technology ( 53) is based on the direct detection of DNA composition using a protein nanopore. DNA strand attached to a motor protein is translocated through a nanopore, modulating the electric current passing through. This change is called the squiggle space and shifts in voltage are characteristic of the particular DNA sequence in the pore. This very promising system is able to sequence up to 500 bp per second and read sequence of length up to 200 kb (54).

III. RNA-Sequencing

RNA sequencing relies on the same principles as DNA sequencing. With the exception of nanopore technology where extremely recent protocols propose to sequence directly RNA (55,56). All other methods convert RNA into cDNA via a retro-transcription (RT) step .

Depending on the aim, target RNA species can be first enriched or non-target RNA can be depleted. In the case of transcriptome wide analysis, mRNA can be enriched via by a polyA selection with an oligo-dT oligonucleotide or rRNA (that represent more than 90% of a total RNA population) can be depleted using complementary oligonucleotides. Another possibility is to use the natural chemical properties at the extremity of some RNA for the ligation of adapters. For instance, miRNA have 5'phosphate and 3'OH extremities which allows direct adaptors ligation during library preparation.

High-throughput sequencing is widely applied in biomedical research and considerably contributed to advancement in transcriptomes knowledge over the last few years. A large set of applications can be performed using this technique. Two application areas can be distinguished: the exploratory domain and the comparison and quantification domain. On one hand, one can mention several RNA sequencing applications such as the discovery of new transcripts that can be involved in cell regulation (57)(58)(59), better knowledge of transcriptional start sites (60)(61)(62)(63)(64) or nucleotide polymorphisms (SNP) (65)(66)(67)(68). On the other hand, RNA sequencing is also commonly used for the comparison of expression level of diverse target RNA under two (or more) different conditions. RNA sequencing is not only used for RNA characterization, this technology is available for a lot of different purposes such as determination of RNA/protein interaction by all the Cross-Linking and Immuno-Precipitation (CLIP) procedures (69). Another example would be the detection and quantification of modified nucleotides by RNA sequencing that will be described in more details in the chapter II of this manuscript.

During my PhD thesis, I was working in tight collaboration with the Next Generation Sequencing core facility of the "Unité Mixte de Service" UMS2008 IBSLor directed and managed respectively by my Director of Thesis Pr. Iouri Motorine and Dr. Virginie Marchand.

I could get better insights into Illumina sequencing system. In particular, I have used highthroughput sequencing in two projects that will be presented in the following chapters. The first one is about the characterization of extracellular RNA from human plasma and the second one is the dynamics of 2'O-methylation in E. coli under stress conditions. On one hand, some RNA and especially miRNA are found in "soluble state" and are included in ribonucleoprotein (RNP) complexes, containing both ribonucleic acids and associated proteins (figure 5) (70), or, more rarely, as a part of particles such as low-and high-density lipoproteins (LDL and HDL) (71) or exomeres (72). However, it is still not clear if these RNA can really enter recipient cells and join the RNA-induced silencing complex (RISC) and how these RNA are recruited into lipoprotein vesicles.

CHAPTER I: Characterization of extracellular RNA by high-throughput sequencing

On the other hand, RNAs are also found in another kind of particles released from the cell that are called extracellular vesicles (EV) (73)(74)(75)(76). Extracellular vesicles are delimited by a doubleleaflet membrane. There are evolving recommendations to characterize their subtypes particularly in biological fluids. To date, there is no consensus on specific markers of EV subtypes (77,78). Nevertheless, three major types of EV are mostly described according to their size and biogenesis (Figure 5): apoptotic bodies (79)(80)(81), microvesicles (or budding vesicles) (82,83) and exosomes (84)(85)(86). Apoptotic bodies are 1 -5 µm vesicles and arise from apoptotic cells by invagination. They are composed predominantly of fragmented DNA and histones.

Microvesicles, from 100 nm to ~1 µm in size, are formed by budding of the plasma membrane and their content depends on where the budding occurs. Their membrane composition is similar to the parental cells (87). Microvesicles are enriched in phosphatidylserin, lipid rafts, cell lineage markers and cell surface receptors (87)(88)(89). In contrast to apoptotic bodies which result from indiscriminate surface blebbing, nascent microvesicles come from a distinct blebbing mechanism leading to localized changes in plasma membrane protein and lipid composition.

This modulates changes in membrane curvature and rigidity (90,91). Moreover, they carry proteins and nucleic acids components selectively enriched while others are excluded (89). The third type of EV called exosomes are particles of 50 -150 nm in size (92) originating from classical endocytic pathway with a specific biogenesis and cargo components recruitments (85,93). Extracellular RNA can be a part of extracellular vesicles. Apoptotic bodies 1-5 um arise from apoptotic cells by invagination, microvesicles 100 nm to 1µm are shed from the plasma membrane and exosomes 50-150 nm are secreted from the cell due to a specific biosynthesis. After endocytic pathway mediated by CCV, multivesicular bodies containing intraluminal vesicles are formed via the ESCRT and then fuse the plasma membrane in order to release exosomes. exRNA can also be part of RNP complexes or more rarely of lipoproteins.

Taking into account these multiple secretion pathways, exRNA composition is extremely diverse and heterogeneous (79,94). In addition, exRNA are attractive biomarkers of pathologies and diseases, with growing application in diagnostics and prognostics (95). Indeed, highlighting new biomarkers from theses populations that are present in the circulating blood could help for diagnostic by using liquid biopsy instead of classical invasive tissue biopsy. Among all these subpopulations, microvesicles and exosomes present a strong interest for the determination of disease biomarkers since they have a specific biogenesis, specific proteins and nucleic acids cargo recruitment and are known to play a role in different diseases such as cancers (92,96,97).

We choose to focus our interest on exosomes because they are the most studied in the field.

Moreover, their small size allows them to cross the blood-brain barrier and tissue matrix, and thus to be harvested in biofluids (98,99).

b. Exosomes: biogenesis and composition

Biogenesis

Exosomes were first discovered nearly 45 years ago when vesicles were observed within the cartilage matrix (100,101) . Exosomes are now known to be secreted by almost any cell types (immune cells, cancer cells, stem cells, hepatocytes, neurons...) and are found in all biological fluids (plasma, urine, milk, amniotic fluids...) (see http://exocarta.com).

The general mechanism for secretion of exosomes outside the cell begins with a classical endocytic pathway commonly mediated via clathrin-coated vesicles (CCV). In the early endosomes, there is a formation of intraluminal vesicles (ILV) by invagination of the endosome membrane. These vesicles form multivesicular bodies (MVB). MVB often fuse with the lysosome where cellular waste products, proteins, lipids, carbohydrates and other macromolecules are cleaved down into simple compounds and recycled as new cell building material. Nevertheless, MVB can also fuse with the plasma membrane thereby releasing ILV, among which are found exosomes, to the extracellular space (Figure 5) (92,93,102,103).

The MVB fate, between lysosomal pathway and exosome release, depends on its biochemical composition. MVB rich in cholesterol and poor in lysobisphophatidic acid fuse with the plasma membrane and release exosomes while if their composition is inverted they are destined to the lysosome (104-106). Moreover, in the secretion pathway itself, there are two different options:

the pathway involving the endosomal sorting complexes required for transport (ESCRT), and the lipid pathway (92,(106)(107)(108). ESCRT complexes are implicated in the membrane remodelling that allows the formation of ILV. In the minor lipid pathway, exosome formation is independent from ESCRT complexes, but depends on the sphingomyelinase enzyme, which produces ceramide.

Composition

The molecular composition of exosomes does not reflect exactly the composition of the secreting cell (109,110). In fact, exosomes are enriched in specific proteins, lipids and RNA, while some other components are totally absent. Thus, sorting and recruitment of molecules into exosomes are controlled by specialized mechanisms.

Membranes of exosomes are enriched in specific lipids, like cholesterol, sphingolipids, glycerophospholipids and ceramides, but also in some proteins such as integrins, growth factor receptors or immunoglobulins (85) (Figure 6). The most abundant exosomal transmembrane proteins belong to the tetraspanin family (e.g. CD9, CD63 and CD81). They are organized in tetraspanins enriched microdomains (TEM) and have four transmembrane domains (111). Even if this protein family is also found on cell membrane, exosomes are highly enriched in tetraspanins (between seven to hundred times compared to cells and other vesicles). This makes tetraspanins good candidates to be specific biological markers for exosomes. Inside exosomes, other proteins are also found: heat shock proteins (hsp70, hsp90), cytoskeleton proteins (tubulin, actin), vesicle trafficking proteins (Rab GTPases, annexin, flotilin) required for MVB transport to the plasma membrane and ESCRT-related proteins (Alix, Tsg101) (112).

Concerning RNA composition, this part will be described in more details in section (A. III. a.

exRNA composition: state of the art). The exact mechanisms allowing protein recruitment into exosomes are not yet fully understood, even if the protein composition seems to be determined by the exosome secretion pathway.

Since ESCRT complexes play a role in ILV budding, proteins that are able to interact with ESCRT complexes are preferentially sorted into ILV during their formation. In contrast, for production of ceramide-rich exosomes, ESCRT complexes are not involved. In this case, ceramide produced by the sphingomyelinase 2 is able to cause spontaneous curvature of the endosomal membrane to form ILV (113).

The existence of ESCRT-dependent and independent mechanisms for the loading of proteins into exosomes is not necessarily contradictory. Indeed, this heterogeneity allows the possibility to enhance or inhibit the secretion of only one sub-population of exosomes (114,115).

c. Functions

After their secretion, exosomes are released in the extracellular medium. While initial studies considered these EV as "garbage bags" of the cells (116,117), exosomes are now recognized to serve as "vehicles" for intercellular communication (118,119). This communication can be limited to cell-to-cell in close proximity or affect the whole organism by secreting signals such as hormones released from the cell and acting in autocrine and paracrine manner. How this regulation is insured? Exosomes can act through receptor-receptor interaction at the plasma membrane, but most probably their content is incorporated into the recipient cell. Exosomes can be either internalized entirely by endocytose into the cell and then join the lysosomes or they can fuse with the plasma membrane and deliver their molecules inside the cell (92,120) (Figure 7).

Figure 7 Exosome uptake in recipient cell

Exosomes can be internalized to the recipient cell by three potential different ways. They can fuse directly with the plasma membrane, interact with specific receptor or be internalized by the endocytic pathway, brought to the lysosome and release their content into the cell.

By this way, while the first reported function of exosomes was the elimination of outdated molecules in a catabolic pathway, exosomes have a lot of different functions, depending on their origin, their target cells, as well as on their composition. Exosomes can transfer specific proteins and nucleic acids (miRNA) to target cells for the delivery of signaling pathways and gene regulation (121)(122)(123). Exosomes affect the adaptive and innate immune system (124), they can bear peptide-major histocompatibility complexes (MHC) or antigen and activate T cell response or carry bacterial molecules that directly stimulate innate immune system (125-127).

Moreover, depending on their producing cells, exosomes can protect recipient cells from tumor

d. Use for biotechnologies: drug delivery system

Because liposomes look like exosomes, they were naturally exploited for drug delivery system.

They have particular benefits such as specificity, safety, and stability. Due to their nanometric size, exosomes can be transferred between cells and are able to avoid phagocytosis. They fuse with the plasma membrane and bypass the lysosome pathway (134-136). Their lipid bilayer membrane protects their cargo containing bioactive molecules from degradation in the extracellular milieu (136). Moreover, exosomes show a lower immunogenicity and toxicity than other drug delivery system (136,136). By their intrinsic characteristics of intercellular communication, exosomes can deliver their cargo over a long distance with a strong stability in the blood under physiological and pathological conditions. In addition, exosomes have a hydrophilic core making them suitable to carry water-soluble drug (137).

Several methods are used to load cargos containing anticancer agents, siRNA or proteins. After their isolation from cell culture conditioned medium, exosomes are subjected to electroporation, sonication or they are simply incubated with the cargo to be mixed with it (138,139).

To target the recipient cells, either natural characteristics of exosomes can be used. For instance, exosomes bearing specific integrins can directly go to targeted organs (140). Otherwise, peptides have to be fused on the membrane surface (141). For instance, fused αv integrinspecific RGD (Arg-Gly-Asp) on exosome carrying anti-cancer drug, allow their uptake in αv integrin-positive cancer cells (142).

Most of application for exosome drug delivery is in the treatment of cancer diseases. As an example, this technique is used as a cell-free vaccine where cancer antigens are loaded in exosomes derived from autologous dendritic cells. By inducing natural killer activation, it facilitates the anticancer immune response in patients with non-small-cell lung cancer (143).

II.

Studying extracellular RNA: from fractionation of subpopulations to

RNA sequencing a. Exosome isolation procedures

The gold standard for exosome isolation is differential centrifugation (144,145) (figure 8). This method consists in three steps: first, a low-speed centrifugation to remove cells and apoptotic debris, then a medium speed spin to eliminate larger vesicles and finally high-speed centrifugation (ultracentrifugation) to sediment exosomes. The second popular method is the polymer-driven precipitation proposed by several companies (figure 8). These precipitation kits such as Exoquick (System Biosciences), Total Exosome Isolation Reagent (Invitrogen), miRCURY™ Exosome Isolation Kit (Exiqon) are based on the formation of a mesh-like net that embeds EV with a size ranging from 60 to 180 nm (146). However, both of these procedures are barely suitable for complex and viscous biological fluids such as plasma or serum (147)(148)(149). Indeed, exosomes isolated from cell culture medium have a "simple" composition leading to a good yield and purity level. But due to their intrinsic characteristic, ultracentrifugation and precipitation methods lead to aggregation of proteins, RNP or lipids that are contaminating the exosome pellet and could prevent exosome detection due to high background (150). Thus, depending on the exosome source and their designed downstream applications, it may be important to use alternative methods to purify exosomes from other vesicles instead of only enriching them.

Among other proposed purification methods, size exclusion and ultrafiltration techniques bring much better purity (figure 8). For ultrafiltration protocol, after their enrichment by precipitation or ultracentrifugation, exosomes can be separated from other soluble proteins and aggregates using matrices with size exclusion limits (151,152). Concerning size exclusion chromatography (SEC), it has been shown that this technique avoids protein aggregation and subsequently allows EV purification without huge contamination. Due to pores of a diameter < 70 nm, proteins and RNP contaminants enter the beads and thus elute later than exosomes that are present in a void volume. Several comparative studies concluded SEC is the best method for EV isolation from plasma (153-155).

Immunoaffinity based protocol (150,156,157) target transmembrane proteins such as CD9, CD63 or CD81 and is the only one based on the biological properties of exosomes and not simply their size (figure 8). Here, there is no contamination by non-exosomal components, however, this kind of exosome enrichment is risky because some potentially important EV populations without target marker protein may be excluded. In addition, this methods heavily relays on specificity and selectivity of antibodies used (158).

Other procedures that allow EV purification exist, but frequently require particular expertise and costly material/equipment. An often-used method is the density gradient/cushion centrifugation, where particles are separated by their floating density allowing separating soluble protein aggregates from EV. However, a swinging bucket ultracentrifuge is required (159-161). Another example among others is the technique proposed by Ibsen et al, 2017 (162) where EV are purified by an alternative current electrokinetic (ACE) microarray chip. This original method is certainly very promising but requires specific equipment.

Almost all theses procedures use the nanoscale size property of exosomes in order to enrich/isolate them. De facto, it is not possible to exclude other EV such as some microvesicles that may also have a nanoscale size. Moreover, depending on the target used in immunoaffinitybased methods, other EV can be isolated together with exosomes. For instance, tetraspanins are naturally found in the membrane of microvesicles since they come from a budding cell and therefore their membrane composition is similar to the parental cell containing tetraspanins (however, they are not enriched like it is the case for exosomes) (87). Nevertheless, this "contamination" remains relatively limited.

b. Exosome characterization

After EV purification, it is crucial to validate that isolated particles are indeed exosomes. There are physical and biological validations based on different methods. However, while successfully applied to exosomes obtained from conditioned medium, all these methods are not entirely suitable for complex biological fluids such as plasma. The most popular is Nanosight system with a dedicated software called Nanoparticle Tracking Analysis (NTA) (163) that analyses videos captured giving a particle size distribution and particle count based upon tracking of each particle's Brownian motion. This tracking is due to the diffraction of a laser beam in contact with the particles. By this way, it is possible to analyse vesicles in solution and to determine their size distribution and their concentration. This method is very helpful but does not make the difference between EV and protein aggregates. Thus, exosome concentrations are highly overestimated (154) when an inappropriate isolation method is used leading to protein and RNP contaminations. One can also mention dynamic light scattering (DLS) that gives size distribution, but does not provide information about particle concentration (164,165).

Transmission electron microscopy (TEM) (166-168) gives information about size and morphological characteristics such as lipid bilayer and shape. TEM can characterize EV but there is no information about their concentration. Moreover, it is not possible to make the difference between exosomes and microvesicles of the same size.

Biological validation includes immunoblotting targeting specific markers such as internal markers: Tsg101 (46kDa), Alix (96 kDa) Another option could be flow cytometry. However, because of their nanoscale, exosomes cannot be directly loaded on a classical flow cytometer in order to analyse the presence of specific markers on their surface. The sensibility of the cytometer is not appropriate for the analysis of these vesicles. To make EV detection possible, exosomes have to be bound to beads (4 µm in diameter) first (174). Now nanoscale flow cytometer exists (175,176) but require specific and expensive equipment.

c. RNA extraction

On whole plasma or with exosomes, it is well-established that exRNA are mostly below 200 nt in length (177,178). It has been shown that classical phenol/chloroform or TRIzol extraction is not efficient enough for this kind of RNA (179) because their yield is too low and small RNA (< 200 nt) are lost. Thus, the most important is to use a procedure that does not excludes some species: small and/or large RNA. Several kits are available on the market with more or less the same principle.

d. RNA sequencing

The first methods used for RNA characterization were mostly real-time quantitative polymerase chain reaction (RT-PCR) (70,180,181) or hybridization-based assays (70,182). However, these methods require the design of oligonucleotides recognizing known targets. Thus, it is not possible to analyse non-expected transcripts. Nowadays, high-throughput sequencing encounters this issue by sequencing all RNA in the sample.

III. exRNA: state of the art

In the field of liquid biopsies and diagnostic biomarkers determination, the most used biological fluid is blood: either plasma or serum. In this part, we will first describe how RNA are recruited into exosomes and then make an overview of exRNA content either from whole plasma/serum or from EV subpopulations.

a. Mechanisms of RNA recruitment into exosomes

Exosomal RNA composition is different to cellular total RNA of the donor cell (109). This means that in exosomes, there is a specific recruitment at least for some RNA species. At the moment, most studies were focused on miRNA, so only these specific recruitment mechanisms can be described in details, but it is clear that other RNA categories have dedicated recruitment mechanisms as well. For instance, ESCRT II and III were found to be RNA binding proteins that can contribute to the recruitment of various RNA into intraluminal vesicles ILV (183,184).

Four potential modes for miRNA recruitment into ILV, were described, although the underlying mechanisms still remain largely elusive (122). The first is the neural sphingomyelinase 2 (nSMase2)-dependent pathway. It was shown that overexpression of nSMase2 increases the amount of exosomal miRNA and, in contrast, its inhibition reduced exosomal miRNA number (185). The second mechanism is the miRNA motif and sumoylated heterogeneous nuclear ribonucleoproteins (hnRNP)-dependent pathway. hnRNPA1 is able to recognize a specific sequence in the 3' portion of miRNA: GGAG (186). The third is the 3'-end of the miRNA sequence-dependent pathway. In fact Koppers-Lalic et al, 2014 (187) discovered that 3'-end adenylated miRNA are relatively enriched in cells (B cells), whereas 3'-end uridylated isoforms appear over-represented in exosomes derived from B cells or found in urine. According to the two last mechanisms, it seems that 3'-end of miRNA sequence is crucial for the recruitment into exosomes. Finally, the last mechanism is the miRNA induced silencing complex (miRISC)-related pathway. When Ago2 protein (main component of RISC) together with the 182 kDa glycine-triptophane protein: GW182 is inactivated, abundance of miRNA in exosomes drastically decreases (188).

b. exRNA composition in human blood

The initial studies on the composition of exRNA from extracellular vesicles showed the presence of miRNA and more than a thousand of different mRNAs (189). These mRNAs seem to be even functional because after transfer of mouse extravesicular RNAs into human mast cells, new mouse proteins were found to be produced in the recipient cell, indicating that transferred exosomal mRNA can be translated after entering another cell. Nevertheless, another study demonstrated that mRNA found in exosomes are mostly fragments from 3' untranslated region that could have functions other than translation, such as regulation stability or localization in recipient cells (190). However, in these studies authors used miRNA and mRNA specific microarrays for exRNA characterization. As mentioned before, these methods prevent the determination of unexpected RNA transcripts while high-throughput sequencing encounters this issue. In the last decade, more than a dozen of papers described exRNA content from plasma/serum and/or respective extracellular vesicles (94,170,(191)(192)(193)(194)(195)(196)(197)(198)(199). Using RNA sequencing, almost all known RNA species were found in blood and extracellular vesicles: mRNA, rRNA, tRNA, miRNA, piRNA, snoRNA, vault RNA, Y RNA, lncRNA... However, the reported proportion of different RNA varies dramatically from one study to another (Table 2). For instance, protein coding RNA and miRNA are described at 24.5% and 8.8% respectively (170), while another study with similar design reported a proportion of 20% and 42% (192).

Moreover, the proportion of rRNA was reported to be low (<10%) in (197), while the same rRNA was found to be a major species at ~50% in another study (193). This most probably reflects the extreme heterogeneity of sample preparation and the techniques used to isolate and characterize extracellular vesicle fractions (200). Discrepancies may also arise from the methods used in library preparation for sequencing, as well as from the bioinformatic pipeline used for data analysis and interpretation (201).

Analysis of human miRNAs, which are widely employed for biomarker discovery, may be highly biased, depending on the used purification procedures. The miRNA content of the exosomes isolated by centrifugation was shown to be very similar to RNA composition of the human plasma, demonstrating that inappropriate EV purification method yields preparations highly contaminated by non-exosomal RNA (202). In addition, several comparative studies highlighted strong differences in exosomal RNA content when extracellular vesicles were isolated from the same biological sample by different methods (153,(203)(204)(205). Not only the global miRNA content was found to be different (204), but also the level of individual miRNA such as let -7-d, miR-25, miR-127-3p, miR-16 and miR-451 (204,205). Such differences in exosomal RNA content were described for plasma-derived exosomes, and, surprisingly also for cell-derived exosomes obtained from rather well characterized cell-culture media (203). This demonstrates that biases related to exosomes purification methods may be important and may lead to lower quality results, even when the biological source is not a complex biofluid like plasma. In the study comparing ultracentrifugation, density gradient protocol and a precipitation based kit (153) only 40% of miRNAs were in common in the three protocols, moreover, mass spectrometry analyses pointed out that precipitation methods lead to contamination by polyethylene glycol (PEG) and serum proteins (206).

Globally, it is a real challenge to characterize exRNA content when plasma is used as source of exRNA (207). It is therefore critical to carefully design the study and to be aware of the possible contamination issues, which may affect sample composition.

IV. Objectives

As it was mentioned before, exRNA can be found in almost every biological fluid and show a strong interest in the field of liquid biopsy. During the first period of my PhD thesis we decided to focus on the deep sequencing characterization of exRNA from blood due to its potential clinical interest. By having better knowledge on exRNA, we could use it to define diagnostic biomarkers in cancer, cardiovascular diseases or even infection by some viruses.

In this work, we approached the exhaustive characterization of exRNA from healthy human plasma using combination of extraction/characterization methods. We determined the exRNA composition from either whole plasma or from enriched or purified exosomes.

B. Results

I. Study design a. Source for blood biological sample

Most common ways for blood pre-treatment consist in preparation of plasma or serum fractions.

Plasma is obtained by centrifugation in the presence of anticoagulants, while serum is prepared by centrifugation of coagulated samples. Technically, working with serum is easier, but it is known that platelets release a lot of EV containing miRNA in response to coagulation (209) and this considerably affects the composition of exRNA originally present in circulating blood. This is the reason why plasma is the preferred source when studying exRNA as potential biomarkers (210). However, some popular anticoagulants, like heparin and EDTA, are not compatible with downstream applications (e.g. polymerase chain reaction, PCR) so we used citrate anticoagulant (211)(212)(213). Special care should be taken during these initial centrifugation steps to prevent release of platelet-derived EV (212).

b. Plasma treatment

Because plasma is a viscous and complex biological fluid, we had to first find the best method for exosome isolation since a lot of contaminants are commonly found in ultracentrifugation or precipitation-based kits procedures (148,149). The first part of this project was to compare several isolation methods including: kits based on precipitation, ultracentrifugation, density gradients and size exclusion chromatography. We also developed another method which consists in degrading soluble proteins in plasma with proteinase K treatment. Therefore, RNA from RNP are vulnerable while the one from EV are still protected by the lipid bilayer. Indeed, proteinase K is not able to make EV permeable. Then, non-protected RNA are degraded by RNase A treatment. This method should allow a good exosome purity by removing proteins and RNP contaminants (214).

c. Inclusion of degradation products

In the past decades, exosomes were thought to be only garbage bags of the cells but now this idea disappeared in the profit of intercellular communication function for EV. However, most of studies performed on RNA composition by deep sequencing include only maturation products. Indeed, when working with exRNA that mostly are below 200 nts in length, library preparation kits require 5'-P and 3'-OH extremities. However, degradation products probably present in whole plasma or exosomes have 5'-OH and 3'-P (or 3'-cyclophosphate) ends. Thus, we decided to include theses RNA species in the analysis by adding a preliminary dephosphorylation/phosphorylation step that lead to the right extremities for all RNA present in the sample. Without any treatment, only maturation products are converted to library and sequenced, while degradation products are included only after this optional step. The goal is to perform the analysis with or without a dephosphorylation/phosphorylation step in order to compare maturation products to the pool of exRNA.

Nevertheless, we have to keep in mind that eukaryotic RNA are not only 5'P/3'OH or vice et versa (figure 9). While rRNA, tRNA, small RNA involved in regulation (miRNA, piRNA), most of snoRNA and scaRNA carry these extremities, this is not the case for mRNA, and most of lncRNA that carry a 5'm 7 G-ppp cap and for snoRNA U3 that has a 5'm 227 G-ppp cap. Even if exRNA is composed in majority of small transcripts (< 200nts), this does not exclude potential presence of RNA of a longer size. Thus, we have to be aware that potential extracellular mRNA, snoRNA U3 and most of lncRNA are excluded from the analysis because the alkaline phosphatase is not able to remove these caps.

d. exRNA sequencing

Small RNA sequencing kits are the most appropriate to convert RNA fragments into sequencing libraries. Here is described the library preparation protocol proposed by New England Biolabs (NEB) that is compatible with widespread sequencing system: Illumina and is called NEBNext® Multiplex Small RNA Library Prep Set for Illumina (figure 10). To perform such library preparation, 5'-phosphate (5'-P) and 3'-hydroxyl (3'-OH) extremities of RNA are required. A 3' adaptor with a 5' pre-adenylated (App) and 3' blocked DNA sequence is ligated specifically to 3'OH extremities from RNA sample via a thermostable 5´ App DNA/RNA ligase enzyme which is ATP independent. The mutant ligase is unable to adenylate the 5´-phosphate of RNA or single stranded DNA (ssDNA), which reduces the formation of undesired ligation products (concatemers and circles). Then, the reverse transcription primer is hybridized on 3'adaptor. This step is important to prevent adaptor-dimer formation. The RT Primer hybridizes to the excess of 3´ single read (SR) Adaptor (that remains free after the 3´-ligation reaction) and transforms the single stranded DNA adaptor into a double-stranded DNA molecule that cannot be substrate for the T4 RNA ligase. Thus, during the ligation of 5'adaptor, none of them will hybridize to 3'-adaptor/RT primer dimer. Finally, the first complementary DNA (cDNA) strand is synthesized by a reverse transcriptase and an amplification by PCR is performed. Primers contain either P5 sequence either both a barcode and P7 sequence. P5 and P7 sequence are required to amplify libraries and fix them on the flow cell. The barcode is required to identify the library during demultiplexing after sequencing.

II. Major Results

All data concerning this first project are reported in the article Galvanin et al "Diversity and heterogeneity of extracellular RNA in human plasma" published in Biochimie in May 2019 in the annexe 1 of this manuscript (208).

Briefly, exRNA preparations from whole healthy human plasma samples were isolated directly or plasma was first fractionated by ultracentrifugation, or by precipitation-based commercial kits. Two other treatments were performed on samples previously treated with a precipitation kit in order to enrich exosomes. These two methods are the size exclusion chromatography and the combination of proteinase K and RNase A treatments. For whole plasma and samples prepared by ultracentrifugation and precipitation-based kits, both mature RNA and degradation products were analysed, using prior deP/P treatment of exRNA species.

In this study, we demonstrated that exRNA are found in low amounts (between 1 to 8 ng of RNA/ mL of starting plasma) and have a similar profile on Bioanalyzer whatever were their treatment. In fact, exRNA are in majority below 200 nt and even below 70 nt in length.

Analysis of fragment size in whole plasma without further treatment highlighted the presence of two major peaks at 21 and 33 nt corresponding to miRNA and hY4 fragments respectively.

When degradation products are included the profile changes with a broad peak of small fragment sizes that correspond mostly to rRNA fragments.

Ultracentrifugation or precipitation methods show a similar profile in fragment size and RNA composition than with whole plasma. The only difference was a moderate increase of nonhuman RNA in some preparations obtained by precipitation-based kits. These data demonstrate the inefficiency of both methods for the purification of exosomes and are in correlation with the literature describing how they contribute to contamination of other non exosomal RNA and proteins.

However, the profile of exRNA obtained from EVs purified by SEC or PK/RNase A treatment is completely different clearly indicating that another population of exRNA is present inside of exosomes. Here, size of RNA fragments is mostly over 50 nt without any characteristic miRNA or hY4 peaks. In addition, the proportion of nonhuman RNA considerably increases compared to the whole plasma without treatment.

III. Discussion and conclusion

In this article, we showed the real challenge for the analysis of exRNA from the complex and viscous biological fluid plasma, especially when one wants to enrich particular sub-populations like exosomes or microvesicles. Such issues are not encountered when working with cell secretome from cell culture, since its composition is much better controlled and exosome enrichment does not lead to contamination due to inappropriate treatment. However, in the field of liquid biopsies, biological fluids and more especially blood samples stay the only source available for diagnostics. Due to complexity of human plasma, it remains difficult to obtain pure extracellular vesicles such as exosomes. In the literature, a number of studies already suggested exosomes from blood as good candidates for definition of diagnostic biomarkers in term of proteins, but also RNA, like miRNA. One should pay attention on how these studies were designed, because discovered biomarkers may not belong to extracellular vesicles (215-220). When plasma is submitted to either SEC or PK/RNase treatment, a subpopulation of exRNA is highlighted with a completely different profile compared to the whole plasma and corresponds certainly to exosomal RNA. Here, miRNA are rare, but these data are not in correlation with the literature where lots of exosomal miRNA were pointed out as diagnostic biomarkers. Nevertheless, in term of clinical research, it does not really matter what is the exact origin of defined biomarkers, as far as they are able to discriminate healthy from unhealthy.

Really promising studies even showed how extracellular RNA can be a marker for early stages of cancer (215,221,222). However, if the purpose is to study certain subpopulations of extracellular RNA, one should be really careful on the study design and be aware of potential contamination in samples.

In SEC or PK/RNase A treated samples, exRNA contains a huge proportion of nonhuman RNA species. This was also previously reporteded in Wang et al, 2012 (223). In addition, it has been demonstrated in the literature that viruses are able to use exosome biogenesis, release and uptake pathway in order to spread themselves into their host (133). Viruses enter the cell via clathrin-mediated endocytosis and hijack the ESCRT vesicular trafficking complex and then assimilate into exosomes (224)(225)(226)(227). Inside intraluminal vesicles, viruses hide their genome and thus can persist in the host without activating the immune system. Hepatitis C virus (HCV), Zika virus (ZV), West Nile virus (WNV), and dengue viruses are able to antagonize the late endosome leading to the release of their genome to the cytoplasm. For HCV, viral genome can also stay in the multivesicular body and be secreted into exosomes. They can then perform as infectious particles (224,228,229) and infect specific targets through cellular contacts and therefore creating a productive infection. Thus, exosomes or at least plasma treated with SEC or PK/RNase A could be used for the detection of several viruses.

The original point of our project is a pre-treatment of isolated RNA before library preparation for sequencing. This deP/P treatment allows to include RNA that don't have naturally 5'P and 3'OH, such as degradation products. In fact, most of studies don't perform this additional step and that's why miRNA (with the extremities compatible to direct ligation) are the most frequently found species in plasma. Finally, this represents a strong technical bias because many reported library preparation protocols don't take into account degradation products as well as all nucleic acid without 5'P such as capped RNA (mRNA, snRNA …).

Concerning soluble exRNA in plasma, the major species are miRNA included into RNP via interaction with Ago2 (230). These RNA represent a huge proportion of whole exRNA and our data are in perfect agreement with this observation. It could be of interest to isolate this population via Ago2 immunoprecipitation and compare their profile to data obtained with whole plasma and samples treated with ultracentrifugation and precipitation kits.

Regarding Y4 RNA, its role outside the cell is not well understood. This RNA exists in 4 isoforms between 83 to 112 nt in length (figure 11) and is composed by a loop domain, an upper and lower stem and a polyuridine tail. In the cell, Y RNA has two major functions. The upper stem is involved in DNA replication. The lower stem and the polyuridine tail bind to Ro60 and La proteins, respectively and are involved in RNA stability and quality control (231). However, in plasma, Y RNA is not found as a full-length RNA but mostly as fragments of 31 nt in length derived from the 5′-end (figure 12). These fragments are generated by cleavage within a predicted internal loop (232). Up to now, function(s) of the 5′-Y RNA fragment(s) is(are) unknown. However, the processing and secretion of specific 5'-Y RNA fragments in plasma are consistent with a signalling function of this RNA. Only one study provided an evidence for the potential role of hY4 RNA fragment that can form a 5'-half-tRNA-like structure and bind a target RNA to form a pre-tRNA-like structure in order to guide its cleavage by tRNase Z L (233).

Moreover, it has been shown that hY1 and hY3 isoforms are over-expressed in tumours such as carcinomas, while hY4 is the most abundant isoform in healthy cells (234). So, Y RNA fragments may be potential candidates for cancer biomarkers. To conclude, studying exRNA from plasma is really complicated and one must remember the study design is essential because little changes can strongly affect the resulting exRNA composition. Obtaining an exhaustive characterization of exRNA from either whole plasma or subpopulation (exosomes) is a real challenge. However, its use as diagnostic biomarkers in the field of liquid biopsies is very exciting and promising. Depending on the question asked, one can use whole plasma or subpopulation obtained by specific treatment (SEC or PK/RNase A).

For instance, whole plasma (and not exosomes) is well appropriate for cancer diagnostics. On the contrary, samples enriched by SEC or PK/RNase A treatment can be used for detailed analysis of exRNA originating from the human microbiota.

CHAPTER II: Dynamics of Escherichia coli tRNA 2'O-methylations

A. Literature review I.

E. coli tRNA

tRNA are noncoding RNA playing a crucial role in translation. Individual tRNA are specific adaptors that results in the productive decoding of messenger RNA into proteins. They transfer the appropriate amino acid to an elongating polypeptide chain associated with the ribosome.

After their transcription and processing, tRNA are strongly modified and then aminoacylated.

The global structure of a tRNA consists in 5' monophosphate extremity followed by an acceptor stem, the D-loop, the anticodon loop, a variable loop and the TΨ loop. tRNA ends with the second strand of the acceptor stem and a 3'CCA-end (figure 14).

Historically, first studies on tRNA were done already in the 50s when Hoagland et al (235) discovered an enzyme able to activate amino acids and produce aminoacyl-adenylates in rat liver extract. Zamecnik' team demonstrated that this amino acceptor molecule was tRNA (235-238). The first tRNA sequence was described in yeast model (7,239) and with the increasing development of RNA sequencing (see general introduction on sequencing ) most of tRNA sequences were determined in E. coli, S. cerevisiae and in human.

a. tRNA biosynthesis

In bacteria including E. coli, the genetic material encoding tRNA is mostly found in clusters where multiple gene copies for a single species are common (240). There are three different types of tRNA-encoding regions: units including only tRNA genes, those that contain tRNA genes between ribosomal RNA regions and tRNA operons including specific protein-encoding gene (241).

tRNA sequence varies significantly between each individual tRNA, but always respects a consensus of four (or five when there is a variable loop) inverted repeats that are responsible for stem-loop elements formation (242).

In contrast to eukaryotic system where 3'CCA sequence is added post-transcriptionally, tRNA genes of bacteria (including E. coli) encode directly this extremity feature (240). However, bacterial CCA-adding enzyme exist, they are not required for maturation, but contributes to efficient cell growth by restoring damaged CCA-ends (243,244).

Processing

Because tRNA genes are transcribed mainly in polycistronic regions, newly transcribed tRNA need first to be cleaved by endoribonucleases such as RNase E and RNase III in order to obtain smaller precursor molecules that will be then processed at 5' and 3'ends (245-247) (figure 13).

Both RNase E and RNase III play a role in the initial processing of tRNA precursors and give the accessibility for RNase P activity (required for 5'-processing), which is inhibited by long trailing regions, and for 3'-processing by exonucleases. Concerning the homodimer RNase III, compared to RNase E, its role in polycistronic tRNA transcript processing is less significant (248,249).

After endonucleolytic cleavage in the 3'-trailer region of the tRNA precursor, some extra nucleotides after the 3'CCA-end need to be removed. Final maturation at the 3'-end then requires an exonuclease that can trim right up to the critical CCA-end without disrupting it.

RNase D plays this role and is not able to hydrolyse the CCA 3'-end (250,251) (figure 13).

5'-end tRNA processing is carried out by the ribonuclease RNase P (252) (figure 13). Bacterial

RNase P is composed of one basic protein of 18 kDa (257,258) and one RNA. The catalytic activity carried by a 375-377 nt RNA and cleavage can be achieved without the protein component (257,258). However, the presence of the protein accelerates tRNA processing due to stabilization of RNase P RNA component (259-261). Three specific features in the tRNA substrate are required for an accurate recognition by the RNA of RNase P: the TΨ loop, the acceptor system and the CCA-end (262).

Modifications

Post-and co-transcriptional modifications on tRNA confer chemical diversity and increase Prior studies demonstrated the importance of tRNA modification for translation or tRNA stability, but it is clear now that they play a role much more complex in cell regulation by providing connections to protein synthesis, metabolism and stress response. In the section (II.

Stress and effect on tRNA) will be detailed how environmental stress can change tRNA modifications.

Aminoacylation

After tRNA transcription, processing and addition of their modifications, tRNA need to be aminoacylated at the 3'-end to be completely functional for translation on the ribosome.

Aminoacylation is performed in two enzymatic steps by specific amino acyl tRNA synthetases TrmH (or SpoU) is responsible for Gm18 and was found in the gmk-rpoZ-spoTspoU-recG operon (279,280). This enzyme is conserved in Gram negative species and in species living in harsh conditions such as T. thermophilus (281).

II. Stress and effect on tRNA

tRNA are strongly regulated and can be influenced by environmental stresses such as change in temperature, nutrient deprivation or even oxidative conditions (292,293). There are documented cases of stress-induced regulation of tRNA from transcripts to translation. In addition, modifications may play discrete but elegant and essential role in fine tune of translation (292). Since few studies were done on E. coli or bacteria in general, both bacteria and eukaryotes will be considered in this section.

a. Transcriptional regulation of tRNA induced by stress

Eukaryotic tRNA transcription is performed by RNA polymerase III and, in yeast, is enhanced by a protein Maf1 that can be regulated by stress factors (294). In favourable conditions, Maf1 is phosphorylated and doesn't inhibit RNA polymerase III activity. However, in stress caused by DNA damage or nutrient deprivation for instance, Maf1 is not phosphorylated leading to the inhibition of the polymerase and thus reduced level of tRNA transcription (295-299). In human, the protein La is regulated by the same way: when it is not phosphorylated due to accumulating DNA damage, RNase P is inhibited and 5'processing is inhibited (300,301). Concerning 3-end processing, it has been demonstrated in S. cerevisiae that there is an accumulation of aberrant pre-tRNA under elevated temperatures or in glycerol-containing medium (302).

Translation fidelity certainly depends on the correct codon recognition by tRNA anticodon but also on the appropriate amino acid incorporation during aminoacylation process. In oxidative stress induced by reactive oxygen species (ROS), there is an increase of non-methionyl-tRNA charged with methionine (303,304). These misincorporations of methionine into nascent proteins allow a better detoxification capacity against ROS and belong to cellular mechanisms responsible for the protection against oxidative stress-induced damage.

In eukaryotes, tRNAs can also be cleaved into tRNA-derived ncRNAs (tRF) and this event generally occurs during stress conditions (305-311). When vertebrate tRNA are exposed to oxidative stress, heat shock and UV irradiation, the protein angiogenin is activated and cleaves tRNA in tRNA halves: sequences of 30-35 nucleotides in length derived from either 5'-or 3'end of mature tRNA (312). In S. cerevisiae, Rny1 is responsible for this cleavage and is sensitive to oxidative stress, methionine starvation, extended growth and heat but not to UV irradiation and glucose starvation (307). Roles of tRNA halves are still poorly understood, but several of them are involved in the inhibition for translation initiation (313,314). Others inhibit stress-induced apoptosis by binding to cytochrome-c (315). tRNA can be also cleaved at other positions in the mature tRNA sequences leading to diverse set of tRNA-derived fragments (tRF) with a shorter length (between 13 to 32 nt). Some tRF, in higher eukaryotes, are associated with Argonaute proteins and play a role in gene expression regulation such as regulatory RNA (316,317).

In stress conditions, tRNA Val from archaea Haloferax volcanii is cleaved in tRF and then binds to small ribosome subunits in order to fine tune the rate of protein synthesis and regulate gene expression (318). Although E. coli is not producing tRNA halves or fragments, it has been highlighted that bacterial tRNA instability could be a mechanism for stress response in nutrient/ amino acid deprivation condition (319). In addition to the regulation at the level of synthesis, tRNA could be subjected to demand-based regulation. However, further experiments require to be performed to validate this hypothesis.

b. Stress impact on tRNA modifications

While tRNA modification influence role in tRNA stability, maintaining the efficiency and fidelity of translation or play a role in the RNA recognition by innate immune system, they also have a more general role in cell regulation by being involved in cellular metabolism and mostly in the response to cellular and environmental stress (292,293). This is reinforced by the fact that several tRNA modifications were found to be modulated by environmental factors such as growth rate, oxygen level or temperature indicating that these modifications may play a role in stress response pathway (240).

In bacterial domain, relation between stress and tRNA modification is described. Several thermophilic bacteria showed an increase in the level of tRNA modifications when growth temperature changes (320). Concerning E. coli, TruB-effected pseudouridines 55 modification of tRNA is not essential, but contributes to thermal stress (321) . In addition to high temperature correlation, enzymes responsible for thiolated ms 2 i 6 A, τm 5 s 2 U tRNA modification are also necessary for oxidative stress survival (322). Content of thiolated tRNA correlates with growth rate (323). TruA-effected pseudouridines 38/39 are also important for high temperature growth (322) . However very few studies show the dynamic of these modifications for the regulation of stress conditions. For instance, E. coli ms 2 i 6 A mediated by MiaB has a role in response to stress by affecting the steady-state levels of RpoS. Indeed, the rpoS coding sequence is significantly enriched for Leu codons that use MiaA-modified tRNA ms 2 i 6 A (324). Moreover, a study on Mycobacterium bovis demonstrated that early hypoxia increases wobble cmo 5 U in tRNA Thr (cmo 5 UGU) inducing codon-biased translation. This allows the production of DosR protein required for hypoxia survival.

Concerning eukaryotes, similar findings were described. Some studies demonstrated that modification level changes upon elevated growth temperatures or under growth arrest conditions (325-328). In oxidative stress, Dedon's lab highlighted with an elegant method for tRNA modification detection and quantification by LC-MS/MS that S. cerevisiae Cm, m 5 C and m 2 2G are increasing after exposure to H2O2,but not in response to NaOCl: another oxidative stress-inducing agent (329). Moreover, when the strain lacks enzymes for the modification responsible to oxidative stress, it shows a cytotoxic hypersensitivity to this stress. Thus, tRNA modification are involved in cell survival after ROS stress by playing a role in specific pathways.

Apart from oxidative stress, this very interesting high-throughput method reported change in S. cerevisiae modification level for 23 of the 25 analysed modification in response to the toxicants used such as NaAsO2 (arsenite toxicity) or methyl methanesulfonate (MMS) (DNA damage) (329). This clearly demonstrate the importance of tRNA modification in response to stress event.

When modification dynamics is observed in the tRNA anticodon loop and more specifically at the wobble 34 th position, such as m 5 C and mcm 5 U, after certain stresses in S. cerevisiae, it has been demonstrated that this behaviour is crucial for codon-biased translation. The idea is that specific modifications are regulating gene expression. Indeed, these modification tunable transcripts called MoTTs may influence codon usage patterns. Peter Dedon's lab, demonstrated that modification on the wobble base is coordinated with stress-specific reprogramming leading to the translation of proteins involved in stress response (figure 16). Therefore, tRNA modifications serves as a regulatory system for translational control in stress response (292,329,330).

III. Detection of modified nucleosides a. Methods for the detection of modification by high-throughput sequencing

Detection of RNA modification is performed for decades by various techniques of chromatography mostly by thin layer chromatography (TLC), but also high-performance liquid chromatography (HPLC) and combinations of liquid chromatography with mass spectrometry (LC-MS). However, these methods are not high-throughput and they don't give information about the position of the modification. This is why, detection of RNA modifications by deep sequencing becomes a very promising field and started less than 10 years ago (331-335). Three major principles are used for the detection of RNA modifications: targeting modifications with specific antibodies, using the RT signature induced naturally of after a particular chemical treatment or exploiting particular chemical properties of the RNA modifications.

Antibody based methods

This approach is widely used for the detection of m 6 A and related modifications (331). The method called MeRIP-Seq and then improved leading to miCLIP technique (m 6 A individualnucleotide-resolution cross-linking and immunoprecipitation) (336) consists in the enrichment of RNA fragments containing the modification by immunoprecipitation with antibodies recognizing m 6 A. Then, UV crosslinking treatment leads to formation of a covalent bound between antibody and targeted RNA fragment allowing a better enrichment of modified fragments, and also mapping of modification using the mismatch signature.

RT signature methods

Some modifications naturally cause a specific signature during the RT allowing to detect them by subsequent sequencing. Depending on the nature of the modification, it can either provoke a nucleotide misincorporation into cDNA or RT can be even aborted. m 3 C, m 1 A, m 1 I, m 2 2G and m 1 G were detected using such approach called HAMR (High-throughput Annotation of Modified Ribonucleotides) (337).

Unfortunately, only limited number of post-transcriptional modifications naturally generate visible RT signature. Most of them are RT-silent but their RT signature can be enhanced by engineering of either the reverse transcriptase (338), dNTP chemical properties or the RNA template (339-341).

Methods based on modification of chemical properties

A multitude of chemical treatments can be performed on the RNA template before the synthesis of cDNA. These reagents can either act specifically on a given modification or selectively affect non-modified nucleotides. For example, m 5 C can be detected using bisulfite treatment (342), while 1-cyclohexyl-(2-morpholinoethyl) carbodiimide metho-p-toluene sulfonate (CMCT) treatment allows the detection of pseudouridines (333,(343)(344)(345)(346)(347).

Recently, Virginie Marchand and Yuri Motorin at the NGS core facility UMS IBSLor 2008 developed a method for the detection for m 7 G, m 3 C, but also D and ho 5 C residues. The socalled AlkAnilineSeq method (348) relies on the enrichment of RNA fragments by selective ligation. First step is to make an alkaline hydrolysis that results in abasic sites for the mentioned modifications. This abasic site is then cleaved by aniline leaving 5'P at the N+1 nucleotide in the sequence. Such RNA extremity is compatible with ligation of the 5'adaptor during library preparation. Thus, fragments containing the nucleotide N+1 to the modified residue in RNA are positively selected.

b. Specific detection of 2'O-methylation

Several methods were proposed for detection of 2'O-methylation. In particular conditions such as at low dNTP concentration , these modifications generate an RT signature by RT arrest (349).

This method was extensively used for mapping of 2'O-methylation in rRNA and was now coupled to deep sequencing in a protocol called 2OMe-seq (350,351).

At normal dNTP concentration, 2'O-methylation don't create visible RT signature except if an engineered RT enzyme is used. In fact, engineered mutant of KlenTaq DNA polymerase is sensitive to ribose methylated residues, and generates RT stop (352). However, this method has not yet been applied in a high-throughput manner. (356,357). The principle (represented in figure 19) relies on the protection of 3'-adjacent phosphodiester bond against alkaline hydrolysis of the nucleotide after the 2'O-methylation position. All other phosphodiester bonds remain sensitive to random alkaline cleavage, creating a more or less regular cleavage profile. So, the number of fragments that begin or finish at each position (5' and 3' end coverage) should be drastically low at N+1 to a ribose methylation. This method was further adapted for tRNA 2'O-methylation detection by optimizing the fragmentation step, non-redundant tRNA dataset for alignment with reduced number of ambiguously mapped reads and the bioinformatic pipeline for score calculation (357).

Adapted from Schwartz and Motorin, 2017. RNA Biology (358) Figure 17 RiboMethSeq method After alkaline random fragmentation, only methylated ribonucleosides are protected leading to a drop in the 5' and 3'end coverage after library preparation and sequencing. These drops can be transformed into MethScore thanks to appropriate score calculation.

c. Sequencing data validation

When working with deep sequencing for RNA modification detection and quantification, it is absolutely necessary to validate the results by alternative analytical approach since such highthroughput techniques always lead to multiple biases such as errors in the RT, library preparation, detection of the fluorescent signal or computational errors (359-361) .

For 2'O-methylation, several methods are commonly used for the validation of sequencing results. The first one is to perform sequencing gels after RT-PCR in low dNTP concentration leading to RT arrest visible on the polyacrylamide gel. However, this approach is not appropriate for tRNA, since their strong 2D structure gives false positive RT stop. Moreover, this technique is not appropriate for relative quantification of modifications.

Combination of liquid chromatography (LC) and tandem mass spectrometry (MS/MS) has become the method of choice for identification and quantification of RNA modifications, providing high accuracy and sensitivity (362,363) and can also be used for validation of deep sequencing results. RNA samples are digested in nucleosides and analysed by LC-MS/MS where modifications are then determined according to their specific mass. Briefly, by feeding 13 C-glucose as sole carbon source, a stable isotope-labelled internal standard (SIL-IS) is generated and facilitates relative comparison of all modifications.

However, as mentioned above, the major issue of this technique is that RNA must be digested to nucleosides for quantitative LC-MS analysis and thus no information about the position of the modification can be obtained from this type of analysis. To overcome this problem, we first need to isolate RNA fragment containing only one modification. For tRNA species, it is necessary to purify individual tRNA priory to RNA digestion. Then, the obtained result will correspond to a specific position within the RNA sequence. Of note, it is possible to work with RNA sequences containing up to four 2'O-methylation modified residues as long as the methylation is carried by different nucleosides (Am, Gm, Cm and Um/Tm) because they have a specific mass. In the case of tRNA in E. coli, there are three potential methylated residues: Gm18, Cm/Um32 and Cm/Um34 but each individual tRNA carries always a unique 2'Omethylation for each of the three nucleosides pool in the sample. So, after appropriate and specific tRNA isolation, one can have information about the position and validate sequencing data.

IV. Objectives

In my PhD thesis project, in addition to characterization of the extracellular RNA content from human plasma, our aim was to explore the dynamics of bacterial tRNA modification by analysing levels of 2'O-methylation under several stress growth conditions. Most of studies addressing tRNA modification dynamics were performed on eukaryotes, so we focused our interest on the model bacteria E. coli and have chosen environmental stresses that could mimic the stress during host invasion. I first tested several common stress conditions such as change in temperature, oxygen level, nutrient limitation and non-lethal concentrations of antibiotics.

As antibiotics, we have chosen several aminoglycosides affecting ribosomal translation either by modulating translation fidelity or by reduction of its speed. To detect and quantify these modifications, I used Illumina high-throughput sequencing coupled with the RiboMethSeq protocol adapted for tRNA (357). After a first screening, we highlighted modified tRNA positions of particular interest and stress conditions and performed experiments in biological replicates. RiboMethSeq data were then validated by LC-MS/MS measurements of the isolated and digested targeted tRNA. The LC-MS/MS part was performed in Mainz in the Institute of Pharmacy in the Johannes Gutenberg University where I was as part of my "cotutelle" PhD contract.

B. Results

I.

High-throughput sequencing a. Choice of stress conditions Before applying any stress to growing E. coli culture, it was imperative to control growth conditions as strictly as possible. Indeed, cell physiology of the culture grown to the stationary phase is totally different from the one in the exponential phase of growth. To compare between control and stress conditions, it was necessary to establish first the exact growth conditions for the culture where only one parameter will change during the stress studied. We decided to use the E. coli laboratory strain DH5α at the exponential phase for all the cultures; in order to produce sufficient amount of cells without excessive stress linked to the lack of nutrients characteristic to stationary phase. Thus, the growth curve was systematically taken in order to determine the Optical Density (OD) at 600 nm corresponding to middle exponential phase. In the figure 18 (blue lane for both graphs) one notice that around 1OD600nm, E. coli cultures are still in the middle of the exponential phase under normal conditions. For all cultures, harvest of cells was done when OD600nm reached 0.8 to 1.0. In order to achieve the best reproducibility, other growth parameters were also strictly controlled, the same incubator set to shaking speed of 190 rpm, as well as the same type of Erlenmeyer flasks (volume, shape: control of oxygenation level) were used. Seeding was performed at OD600nm 0.01 -0.05 from a preculture also grown to middle exponential phase. The same preculture was used for control and stress conditions for the same batch of experiments.

At beginning of the project, we performed a screening of several possible stress conditions in order to check if E. coli tRNA 2'O-methyation is affected. Thus, we tested varying temperatures (20°C, 37°C and 42°C), level of nutrients (standard LB medium, minimal medium M9 or starvation in Phosphate-Buffered Saline (PBS)). We also attempted to reduce the level of oxygen available in the medium (hypoxia-like conditions) or to induce antibiotic-driven stress using aminoglycosides affecting the ribosome (streptomycin, spectinomycin conditions). Some stress conditions were combined together such as low or high temperature with reduced level of oxygen or in M9 minimal medium. The table 3 describes all the conditions tested for the initial screening as well as growth parameters used.

In order to explore as much as possible potential impact of stress conditions, variants of previously reported protocols were also applied. For reduced oxygen level (hypoxia-like) conditions, we used bacteria grown in tightly closed glass bottles under normal agitation speed (190 rpm). Under these conditions (protocol A) the level of oxygen is moderate since no oxygen can enter during the culture, but residual oxygen dissolved in a media and present in the air of the bottle can be used by bacteria. The protocol B relies on a culture without any agitation to prevent oxygenation. In addition, culture is grown in 50 mL plastic Falcon tubes completely filled by medium (no air in the Falcon). Only the oxygen initially dissolved in the medium was not removed. For these samples, OD600nm was not checked regularly, in order to avoid disruption of the hypoxia-like conditions. This explains why for some samples the OD600nm was inferior to expected 0.8-1.2 OD600nm.

For non-lethal antibiotic stress conditions, protocol A consisted in growing the bacteria directly in the medium supplemented by non-lethal concentration of antibiotic. We selected a concentration leading to a growth delay around 30% (figure 18): 1.5 µg/mL for streptomycin and 2.5 µg/mL for spectinomycin. For the protocol B, the stress is more stringent. First cells are grown under normal conditions in LB at 37°C until middle exponential phase and are then submitted to stress with higher final antibiotic concentration, leading to a growth delay of around 90% (figure 18): 5 µg/mL for streptomycin and 10 µg/mL for spectinomycin.

Figure 18 Growth curve in Streptomycin and Spectinomycin condition (protocol A)

Culture was seeded at 0.05 OD600nm in a given antibiotic concentration. The OD600nm was followed during 1250 min. displayed on a heatmap (figure 19). Each square corresponds the MethScore (relative methylation level) normalised to the average value for a given modification in different samples. Thus, when it is blue, the 2'O-methylation level in the given condition is lower compared to the average, while if the square is in pink, the level of 2'O-methylation is higher than the average for the given stress condition. Data are shown for one, two or three biological replicates tested depending on the condition.

Heatmap shows that MethScore values, obtained for the majority of analysed tRNA 2'-Omethylated positions, remain stable even upon stress or their variation is not related to the applied stress conditions. This is true for variable growth temperature, use of poor growth medium (M9), hypoxia-like conditions and protocol A for antibiotics. However, much stronger effect was observed for some of tRNA 2'-O-methylations under starvation in PBS, as well as for streptomycin and spectinomycin protocol B conditions. Here, over six methylated residues show increased methylation level under stress. For antibiotic stress, protocol A shows the same tendency, but clearly, the protocol B enhances strongly this effect of stress on tRNA 2'-Omethylation. Culture was seeded at 0.05 OD600nm in a given chloramphenicol concentration. The OD600nm was followed during 1600 min.

Finally, we performed "return-back" experiment in order to check if the level of tRNA 2'-Omethylation in bacteria under stress conditions can come back to the level observed for control phenotype when source of stress is not there anymore.

The whole experimental design is summarized in the figure 21.

Figure 21 Study design for starvation and antibiotics conditions and restoration experiments

A large volume of control culture was grown in LB at 37°C until 1 OD600nm, 200 mL were harvest and used for control condition while the rest of the culture was split for the four stress conditions. 300 mL was centrifuged and the pellet resuspended and incubated for 24h in PBS 1x. For antibiotics conditions, the appropriate concentration of antibiotics was added to the 200 mL of culture and the culture was incubated for 24h. After, one day 20 mL of each culture is seeded in 180 mL of LB medium and cultured until 1OD600nm for "return-back" experiments. The rest of the culture is pelleted for the stress conditions. In the table 4, is listed the different cultures parameters.

Major results

Results for biological quadruplates or triplicates for starvation and antibiotics conditions are presented in the figure 22 as a heatmap. MethScore levels were normalized to the average as described previously. Three different behaviour profiles can be described. The first one groups together almost the half of studied positions where there are no visible differences between control and stress conditions (bottom part of the heatmap). As already noticed in the initial screening, several modifications (Nm32 and Gm18) are not impacted when a growth stress is induced. However, in the upper part of the heatmap, two dynamics profiles are clearly visible.

First, six tRNA modifications (tRNA Ser (cmo 5 UGA)-Cm32, tRNA Ser (CGA)-Gm18, tRNA Leu (cmnm 5 UmAA)-Gm18, tRNA Ser (cmo 5 UGA)-Gm18, tRNA Gln (CUG)-Gm18 and tRNA Leu (CmAA)-Gm18) show an increase in their 2'O-methylation level in all stress conditions compared to the control one. Modifications concerned are mostly Gm18 (5/6) and predominantly on tRNA Ser or tRNA Leu , which are tRNA with a long variable loop. Out of the 9

Gm18 positions studied, more than the half (5) show such behaviour. On the contrary, out of the 6 studied modifications at the position 32, only one Cm32 was found to be increased under stress conditions. The second profile for response to stress is observed for positions 34: tRNA Leu (CmAA)-Cm34 and tRNA Leu (cmnm 5 UmAA)-Um34. Here the profile is different. Modification at those two positions is not affected in starvation and spectinomycin stress. Only a slight decrease for Cm34 in tRNA Leu (CmAA) was observed in upon spectinomycin stress.

However, under streptomycin stress and particularly under chloramphenicol-induced stress, the level of these two 2'O-methylations was substantially decreased.

Thus, this experiment highlights two possible behaviours: a global increase of certain Gm18

and Cm32 modifications and a specific decrease for position 34 under streptomycin and chloramphenicol stress conditions.

Figure 22 Global heatmap: heat map representing MethScores in starvation and antibiotics conditions performed in biological replicates

The figure 23 represents a boxplot for the 8 most affected positions. Here, MethScore levels are not normalised to the average deviation, but raw values are used. This allows a better individual analysis (position per position) and better illustrates variability between biological replicates.

Clearly, for all studied methylations, the figure shows a significant difference between MethScore levels observed in controls and under stress conditions.

"Return-back" experiment

Finally, we tested if the observed stress-related changes in profile of tRNA 2'-O-methylation are reversible. Thus, after growth under stress conditions the cells were placed back to standard LB medium and grown at 37°C as for the control conditions. The figure 24 shows a heatmap of MethScore values obtained for "return-back" experiments. As seen before, Gm18 and one Cm32 methylation levels are increased under stress conditions, while methylations at position 34 are decreased in the presence of streptomycin and chloramphenicol. However, after "returnback" growth in LB at 37°C, the MethScore values are rather similar to control values in LB 37°C cultures. Thus, we can consider that the response to stress involving modulation of tRNA 2'O-methylation is reversible. These data highlight the fact that modifications in E. coli tRNA are dynamically regulated in response of stress and this may be a regulation mechanism allowing adaptation to growth environment. The second protocol relies on the same hybridization principle but here oligonucleotides have 3'NH2 extremity that forms a covalent link with N-Hydroxysuccinimide (NHS) group of preactivated agarose, placed into Hi Trap column (figure 26).

Figure 26 Two methods for individual tRNA isolation

Both methods rely on the hybridization of the target tRNA to DNA oligonucleotide. The method on the left consists in the interaction of biotin coupled to the oligo with streptavidin-coated magnetic beads. Using a magnet, beads with specific tRNA are retained and the non-targeted species are washed away. The second method (on the right) uses DNA oligonucleotide with a primary amine at the 3'-end that reacts with NHS-activated agarose placed in the HiTrap column. Then by passing a total tRNA sample through the column, only the targeted one will stay in the column and can then be eluted by decreasing salt concentrations.

To perform isolation of individual tRNA species for LC-MS/MS validation of Nm content, we ordered four oligonucleotides targeting altogether six 2'O-methylations as described in the table 11 in the section (Chapter II, C, I, c: Oligonucleotides). These modifications showed most prominent increase under stress condition for Gm18 and Cm32 position in RiboMethSeq data, while substantial decrease was observed for Cm34 under chloramphenicol and streptomycin stress. For application of biotin/streptavidin protocol, DNA oligonucleotides were fully complementary to the target tRNA with a biotin coupled at the 5'-end and a fluorescein at the 3'-end. For NH2/NHS-agarose protocol, DNA oligonucleotides were of 40 nt in length with complementarity to the 3'-end of the target tRNA and were carrying a primary amine at the 3'end.

1. Optimization of the biotin/streptavidin method Since our goal was to isolate several tRNA species for analysis, we decided to optimize the isolation protocol for parallel isolation of different individual tRNA from one total tRNA sample. DNA oligonucleotide is first bound to magnetic beads via the biotin/streptavidin interaction and then, total tRNA sample is added to the beads for hybridization. Beads containing individual tRNA are sedimented in the magnetic field, washed to remove unspecific contaminants and bound material eluted in buffer with low salt concentration.

• Determination of the best DNA/RNA ratio for hybridization

The first step was to determine the best ratio between DNA oligonucleotide and amount of total tRNA. Analysis of RNA/DNA duplex was performed by separation on native polyacrylamide gel followed by detection of the fluorescence signal emitted by fluorescein attached to the 3'ends of DNA oligonucleotides. In the native gel the RNA/DNA duplex and free DNA have different migration behaviour, and thus can be distinguished.

To determine the optimal tRNA amount, constant amount of DNA oligonucleotide (2 pmol) was titrated by increasing amount of total tRNA preparation. Optimal amount was defined as the point insuring 100% of hybridization for DNA oligonucleotide. Figure 27 illustrates this principle for tRNA Leu (cmnm 5 UmAA) where 3,5 µg of total tRNA fraction was sufficient for complete binding with 2 pmol of DNA oligonucleotide. The table 5 shows the different amounts of total tRNA required for complete binding of all four DNA oligonucleotides used. Amount of total tRNA thus depends on individual tRNA content in the pool. For tRNA Ser CGA, due to its low concentration in total tRNA pool, the point of 100% of hybridization was not reached even with 12 µg of total tRNA. For this tRNA, isolation was done with 12 µg of tot tRNA leading to a lower hybridization level (and lower yield). • Optimization to limit DNA leakage in flow-through fractions

After the determination of DNA/RNA ratio, we performed the isolation procedure consecutively with four DNA oligonucleotides with a unique total tRNA preparation, to limit amount of sample required. We first isolated tRNA Leu (cmnm 5 UmAA) and used the flowthrough containing all other tRNA for tRNA Ser (cmo 5 UGA) isolation. To continue in this direction, isolation of tRNA Leu (CmAA) was performed from total tRNA depleted for tRNA Leu (cmnm 5 UmAA) and tRNA Ser (cmo 5 UGA) and the last tRNA Ser (CGA) on a pool of tRNA depleted for all three previously isolated tRNA. In this procedure the risk is the leakage of DNA oligonucleotides into the flow-through and thus contamination of consecutive fractions by other tRNA species.

The figure 28 a) shows that the flow-through from tRNA Leu (cmnm 5 UmAA) isolation still contains DNA oligonucleotide after one, two or even three consecutive washes. This result shows that biotin/streptavidin binding is not sufficiently stable and there is a substantial leakage of bound DNA oligonucleotide into the non-target tRNA fraction. As a consequence, the second isolated tRNA is contaminated by the tRNA isolated in the first round (figure 28 b)). We can see a DNA leakage in all washes b) Serial isolation of first tRNA Leu (cmnm 5 UmAA), tRNA Ser (cmo 5 UGA), tRNA Leu (CmAA) and finally tRNA Ser (CGA) using at each time the flow through containing tot tRNA minus previously isolated tRNA. Samples were loaded on a denaturing gel and RNA was detected via gel red staining. Red star corresponds to non-targeted tRNA isolation while green star represents the degradation of the specific isolated tRNA c) After DNase treatment and loading on a denaturing gel, there is no more full-length oligonucleotides but only digested nucleotides.

It is essential to have strictly purified tRNA. Indeed, LC-MS/MS measurement require the digestion of RNA sample and if individual tRNA are not purified, we wouldn't be able to attribute the signal to a particular modification. Thus, we couldn't use this protocol with a DNA leakage. This is why we decided to treat the flow-through with DNase in order to digest oligonucleotides so that they can't be involved in further contaminations. Because the buffer composition in the flow-through (SSC 5X : 0,75 M NaCl; 75 mM trisodiumcitrate pH7) inhibits

DNase enzyme, we first had to precipitate the flow-through and then proceed to DNase treatment. The figure 28 c) shows that there are no more full-length oligonucleotides in DNase treatment sample but only digested nucleotides.

The last step before the next cycle of individual tRNA isolation is the removal of deoxyribonucleotides monophosphates (dNMP), DNase and buffer. We performed this step using MEGAclear™ kit which is normally designed for rapid purification of RNA longer than 100 nt. Since tRNA of interest are shorter, the ethanol concentration was adjusted accordingly, to retain tRNA on the hydrophobic filter (675 µL instead of 250 µL recommended for longer RNA). In this optimized protocol the estimated loss of total tRNA preparation is 15% -25% between two cycles.

The global yield observed for these four isolations is described in the table 6. Observed differences between four tRNA is due to their content on total tRNA pool. Since a minimum of 100 ng is required for LC-MS/MS analysis, no extra purification steps, such as gel excision procedure, were applied to these samples. Since we had contamination/loss issues with the biotin/streptavidin protocol (DNA leakage, non-reusable magnetic beads, loss of material in repetitive precipitation steps), we decided evaluate the performance of DNA-NH2/NHS isolation protocol. In brief, the principle is very similar to the biotin/streptavidin protocol (figure 26), but complementary DNA oligonucleotides carry a primary amine at the 5'-end and covalently bind to NHS-activated agarose gel. In an appropriate carbonate buffer at basic pH (9-10), DNA-NH2 and NHS groups readily react and create a covalent amide bond. This covalent link should prevent DNA oligonucleotide leakage. Moreover, HiTrap agarose columns coupled to specific DNA oligonucleotides can be used repetitively, since the NH2/NHS covalent link is quite stable. With this technique, 1 mL HiTrap columns are used instead of classical 1.5 mL Eppendorf tubes. It is not an issue when HiTrap columns are used for the purification of proteins at room temperature but for individual tRNA isolation, hybridization with DNA oligonucleotide is done at 65°C. Moreover, for a better hybridization and reduced aspecific binding, total tRNA pools were recirculating through the column by a peristaltic pump. This closed circuit allows to increase the contact tile and thus the binding efficiency and the yield of the individual tRNA.

In addition, with an appropriate binding conditions, other non-target tRNA are not retained on the column by aspecific interactions. Technically, we used an oven capable to accommodate recipient for total tRNA sample, HiTrap 1mL column and a peristaltic pump. Washing step is done with a buffer at lower salt concentration and removes the residual unspecific tRNA. For elution, the column is placed into a water bath at 75°C with a buffer without salt leading to the elution of the individual tRNA from the column. Since the elution volume is substantial (several mL), eluted RNA is precipitated to concentrate it. In order to obtain a pure individual tRNA, sample is then loaded on a denaturing gel and band corresponding to the correct size is excised.

This additional step (compared to the biotin/streptavidin method) allows a better purity level but considerably decreases the yield.

The figure 29 observe a shift for all three fragments (b1, b2, b3) meaning that they correspond to degradation products from the specific tRNA of interest. Hybridization is only partial, since molar ratio between RNA and DNA oligonucleotide was not determined for this verification. The band b3 also hybridizes to the oligo indicating that it is not leaked DNA oligonucleotide from the column. Thus, the flow-through can be used for further tRNA isolations without contamination. Since shorter fragments are degradation products of the same RNA this leads to a substantial loss of material. This degradation should be reduced or avoided as much as possible. Buffer used for tRNA hybridization and washing contains MgCl2. Although many buffers used for RNA manipulations contain MgCl2, it was demonstrated the Mg2+ ion enhances RNA degradation (366) especially at high temperature (like it is in hybridization, washing and elution steps). Thus, we decided to remove MgCl2 from buffers to avoid tRNA degradation and increase the final yield.

Finally, after several other smaller optimizations such as the determination of the incubation time for tRNA hybridization, temperature and volumes during washing and elution, we obtained almost pure isolated tRNA (figure 30). Here all four tRNA isolated simultaneously from one total tRNA pool (1 mg) are presented. Indeed, it is possible to connect two HiTrap columns together. Moreover, the peristaltic pomp can hold several tubings. Here by pumping in parallel the same total tRNA pool through four HiTrap columns connected in parallel (2x2, figure 31), we isolated four specific tRNA.

Figure Profile of individual tRNA after final optimizations

From mg of tot tRNA, all four tRNA were isolated in parallel and the whole sample was loaded on a denaturing gel and RNA was detected after gel red staining. The four profile show a specific band corresponding to each isolated tRNA and without RNA degradation.

Figure Set up for the parallel isolation of four individual tRNA

Two tubes 1 and 2 (red and blue respectively) are recirculating the same total tRNA sample via a peristaltic pump. For each tubing, two columns (a and b) can be connected.

For total tRNA under control conditions, the yield for the four tRNA is indicated in the table 7.

Since there is an additional step of purification by gel and isolation is done in parallel, it is hard to compare these yields with the ones obtained with the biotin/streptavidin method. To conclude on this technique, an important advantage is the possibility to re-use columns coupled with a specific DNA oligonucleotide for many repetitive isolation cycles and thus decreases considerably the cost compared to the streptavidin/biotin method, where new beads and biotinylated DNA oligo are used at every time. In addition, there is no DNA oligonucleotide leakage, no cross-contamination and we were able to isolate four (possible even more) tRNA at the same time. These are the reasons why we decided to use this individual tRNA isolation method for further LC-MS/MS experiment for validation of the RiboMethSeq data.

c. LC-MS/MS validation

With successful isolation of individual tRNA, we proceeded to the digestion of RNA into nucleosides and determined the level of specific 2'O-methylations. Six candidates already showing substantial differences in RiboMethSeq data were measured: tRNA Ser (CGA)-Gm18, tRNA Leu (cmnm 5 UmAA)-Gm18, tRNA Ser (cmo 5 UGA)-Gm18 and -Cm32 and tRNA Leu (CmAA)-Gm18 and -Cm34. The figure 32 shows the comparison between MethScores from RiboMethSeq and MS quantification data. Comparison of modification level between control and stress conditions is expressed as % of increase or decrease. For all Gm18 positions MS data validate the increased modification level observed by RiboMethSeq. A significant increase is visible for tRNA Ser (CGA)-Gm18, tRNA Leu (cmnm 5 UmAA)-Gm18, tRNA Ser (cmo 5 UGA)-Gm18. Concerning tRNA Leu (CmAA)-Gm18, for both RiboMethSeq and MS results, the effect was weaker but still quite consistent. Since the precision of RiboMethSeq (~5%) is better than for MS quantification (~10%), difference is not statistically significant, but the tendency stays the same. For the position 34 with tRNA Leu (CmAA)-Cm34, we were expecting a strong decrease of the modification level under chloramphenicol stress and a modest decrease for streptomycin. Similar profile is also observed for the MS data. Finally, for tRNA Ser (cmo 5 UGA)-Cm32, the increase of the modification level was expected from RiboMethSeq data, but MS quantification does not show statistically significant changes. This may be due to the presence of another strong modification at position 34 in the same tRNA and variation of modification level at position 34 may affect RiboMethSeq quantification for position 32.

Indeed, MethScore calculations take into account two neighbouring nucleotides, thus including the position 34. Moreover, this tRNA contains i 6 A37 and this proximal nucleotide may affect 5'-and 3'-end coverage, thus altering MethScore for anticodon loop positions. In it noteworthy that tRNA Ser (cmo 5 UGA)-Cm32 was the only Nm32 position to be involved in stress response where all the others remained unaffected.

Thus, by LC-MS/MS quantification, we were able to validate the two behaviours observed in RiboMethSeq data, namely the global increase of Gm18 in several tRNA and the decrease of the modification level for position 34 under chloramphenicol and streptomycin stress.

Figure 32 Comparison between RiboMethSeq and LC-MS measurements

The six studied individual tRNA are represented in relative quantification. Data for LC-MS and RiboMethSeq are presented in percentage of increasing or decreasing compared to the control at 100%. Each black dot corresponds to a biological replicate.

III. tRNA distribution

We studied the dynamics of tRNA 2'O-methylation under stress conditions and confirmed that tRNA modifications are involved in stress response. However, we were wondering if the observed increase or decrease correlates to the change in the tRNA proportion in the global tRNA pool. To answer this question, we also measured alterations of tRNA distribution upon stress.

a. Global tRNA sequencing data tRNA distribution can be deduced directly from RiboMethSeq sequencing data, since all tRNA species are randomly fragmented, fragments are converted to libraries and sequenced. Thus, variation of the tRNA reads count correlates with changes in tRNA pool. However, due to highly biased amplification, the relative molar proportions of different tRNA cannot be defined essentially from these data.

We compared the number of reads for each tRNA in control and under stress conditions. In the figure 33, tRNA counts are first normalized to the total size of the library (tRNA aligned reads)

and then to the average value for individual tRNA species. Globally, composition of the tRNA pool remains rather constant, except for tRNA Ile (k 2 CAU), Val (GAC) and Arg (CCG) which seems to be increased under streptomycin stress. This may be related to their overexpression in response to stress, or by changes in tRNA modification profile, allowing better amplification during library preparation. However, none of these tRNA are known to carry a 2'O-methylation.

In conclusion, stress conditions have a stronger impact on tRNA 2'O-methylations, while only minor changes in tRNA distribution were observed. Moreover, modulation of global tRNA 2'-O-methylation observed for total E. coli tRNA by LC-MS/MS measurements, is explained by a change in the modification level in tRNA population and not to the change of the tRNA distribution. Additionally, some tRNA such as tRNA Leu (CmAA) and tRNA Leu (cmnm 5 UmAA), carry modified nucleotides with an opposite behaviour under stress conditions. Our results show that there is little (if any) correlation between the changes in tRNA 2'-O-modification profile and alteration of tRNA distribution under stress conditions in bacteria.

Figure 33 Changes of tRNA proportion under stress conditions

Raw tRNA count were first normalized on the total tRNA counts for each condition. A second normalization was done to obtain average deviation from the median. A value of one corresponds to 100% of increase compared to the average.

b. Analysis of tRNA distribution by microscale thermophoresis (MST)

Since analysis of tRNA composition by deep sequencing is known to be highly biased due to unequal amplification and ligation of different fragments (367), this method provides only some broad indications and relative quantification. Thus, the observed changes in tRNA pool compositions have to be validated by independent approach. For this validation we employed an alternative method that allows precise quantification for molar concentrations of tRNA (339,369,370). In other words, some tRNA species will be more amplified than other and thus lead to a bias in tRNA distribution. Secondly, modifications present in tRNA are susceptible to block the reverse transcription, limiting representation of these sequences in the sequencing data. Therefore, MST can be used for validation of these data even if tRNA distribution is not strongly affected under stress conditions used in our project.

MST measurements were thus performed with the same four fluorescently labelled full-length complementary DNA probes used previously for tRNA isolation in the biotin/streptavidin protocol. Since a substantial amount of total tRNA fraction are required for MST measurements, comparison was done only for conditions of spectinomycin stress.

First of all, tRNAs showing modulation of their 2'O-methylation level in response to growth stress are not well represented in the total E. coli tRNA. While some tRNA can represent until 15% of the pool, these tRNA are only present at 0.22% -2% under control conditions (table 8).

In stress conditions we observe an increase of at least two-fold for tRNA Leu (cmnm 5 UmAA) and a decrease of ~50% for tRNA Ser (cmo 5 UGA) and tRNA Ser (CGA). No change was observed for tRNA Leu (CmAA). Fold change and even absolute level of expression between deep sequencing data and MST experiment are relatively similar. This validates the relative quantification of tRNA in deep sequencing experiment and changes observed under stress conditions.

IV. Discussion conclusion

In this project we discovered dynamic E. coli tRNA 2'O-methylation as a response to stress growth conditions. Two major behaviour were observed. Half of tRNA Gm18 sites increase in starvation, streptomycin, spectinomycin and chloramphenicol stress conditions, while all 2'Omethylated in the position 34 are decreasing in chloramphenicol and streptomycin stress.

The observed increase of tRNA 2'-O-methylation may be explained either by increased expression of the corresponding RNA: modification enzyme or by the increased "contact" time between the substrate tRNA and enzyme under slow growth conditions. Indeed, in all cases increased tRNA Gm18 methylation was observed in slowly growing cells (starvation, antibiotic stress). However, low growth temperature (20°C) and/or poor medium (M9) do not lead to increased tRNA modification. Thus, the effect is probably related to the increased expression of the corresponding RNA: modification enzyme. Unfortunately, antibodies against bacterial is involved in the regulation of gene expression. The so-called modification tunable transcripts (MoTTs) may change the codon usage and modulate the efficiency of mRNA decoding. This would lead to a stress reprogramming system leading to the production of proteins required for the stress response. In fact, the mRNA codon usage is not random and depending on the expressed gene, only a subset of synonymous codons may be enriched compared to others. So MoTTs are able to stimulate translation only of certain transcripts which are enriched for specific codons. This will increase translational fidelity, but also allow a faster translation for some codons leading to the production of particular proteins in response to stress. Modifications known to be involved in such regulation are mostly m 5 C and mcm 5 U residues and the majority of studies was done for eukaryotic species (S. cerevisiae) (292,329,330). Nevertheless, no evidences were provided for the same function of ribose. One can imagine that E. coli Nm34 have the same function that are already described for MoTTs. It would be of interest to perform ribosome profiling combined to proteomic analysis for our samples in order to identify codon bias in transcripts when they are translated as well as upregulated proteins.

In most of Gram-negative bacteria RNA modification found at the position 18 in D-loop, is exclusively a Gm. It is likely that the primary function for this modification is not stress-related response, but it can escape from the innate immune system during a host invasion. Indeed, it is now well established that RNA 2'-O-methylations act as immunosuppressors.

In the field of innate immunity, Toll-like receptors (TLR) play a crucial role. They recognize microbial associated molecular patterns (MAMP) and thus activate immune cells. TLR3, 7, 8, 9 and 13 (mouse specific) are known to be activated by different nucleic acids DNA and RNA.

Indeed, TLR3 is specific for double stranded RNA, mouse TLR13 for bacterial 23S rRNA and TLR 9 for unmethylated CpG-DNA. Moreover, TLR7 and TLR8 are activated by single stranded RNA of bacterial origin. In humans, TLR7 is expressed only in plasmacytoid dendritic and B cells and induces the production of type I interferon such as IFN-α, whereas TLR8 is found in monocytes, myeloid dendritic cells and regulatory T cells and its activation leads the production of cytokines (371). The innate immune system is able to make a difference between "self" and "non-self" RNA molecules due to specific RNA modifications, spatial restrictions and sequence positions in nucleic acids. Concerning 2'O-methylation, in vitro studies showed that their incorporation in in vitro RNA transcripts, siRNA or 18S rRNA prevents TLR activation (371,372). In fact, in native RNA, it was demonstrated that Gm18 in bacterial E. coli tRNA is responsible for escape from the immune response and that bacterial tRNA Tyr seems to play a major role (267,268). The molecular mechanism of immunosupression is still poorly understood. 2'-O-methylated tRNA is a TLR7 antagonist (373) with a recognition pattern of two nucleotides including Gm18 and its downstream neighbouring nucleotide (374). In addition, it was shown that 2'-O-methylation acts as an inhibitor for TLR7, but not for TLR8 (371). However, other bacterial species that lack Gm18, such as Gram-positive strains (e.g. S.

aureus) also inhibit TLR7 response. This shows that 2'-O-methylations at other positions in tRNA or rRNA may also have immunosuppression properties (268).

Thus, in perspective of this work, we started a collaboration with Pr. Alexander Dalpke now located at the Institute of Medical Microbiology and Hygiene in Dresden. We were wondering if the increase of Gm18 content during starvation and antibiotic stress response is able to modulate the innate immune system. We hypothesized that stress conditions used in our study may somehow mimic the environment during host invasion (starvation or antibiotics conditions). Under these conditions, the increased Gm18 level could help bacteria to reduce the host immune response during invasion and thus increase the infectivity/pathogenicity.

In order to evaluate immunostimulation properties of total tRNA samples extracted from control and stress condition (chloramphenicol stress was used here) were sent to Pr. A. Dalpke lab for interferon alpha (IFN-α) production assay with peripheral blood mononuclear cells (PBMC).

Unfortunately, the amount of tRNA required for these tests is quite substantial (several µg) and does not allow measurements of immunostimulation properties of individual isolated tRNA. If first data on total tRNA reveal interesting results, additional experiments with scaled-up cultures can be performed in order to get enough tRNA species for measurements.

In our project, we were using E. coli DH5α strain commonly used in laboratories for fundamental research. Thanks to the collaboration with Pr. A. Dalpke, we will have the opportunity to analyse also clinical E. coli isolates coming from patients. It would be important to perform the same stress experiment pipeline (control, starvation, antibiotics conditions) on these strains in order to know if the same behaviour is also observed for bacteria that already were involved in host invasion. Collection of these clinical isolates disposes strains without any resistance, but also gentamycin resistant strains. Gentamycin is another antibiotic from aminoglycoside group, it binds to 16S rRNA and large sub-unit of the ribosome and interferes with the initiation complex leading to misreading of mRNA (decreases the translation fidelity).

Since gentamycin was not initially included in our study design, we first checked if such stress gives the same alterations of the tRNA modification profile. The figure 36 represents the heatmap for MethScores in duplicate of control conditions and duplicate under gentamycin stress. We observed a profile which is similar to previous data obtained for streptomycin stress, namely the increased level of four Gm18 and one Cm32 (position excluded due to bias in RiboMethSeq quantification): tRNA Ser (cmo 5 UGA)-Cm32, tRNA Ser (CGA)-Gm18, tRNA Leu (cmnm 5 UmAA)-Gm18, tRNA Ser (cmo 5 UGA)-Gm18, tRNA Gln (CUG)-Gm18. Only for tRNA Leu (CmAA)-Gm18, we don't see a difference, but it was the one with the lowest change in our previous experiments too. For, tRNA Leu (cmnm 5 UmAA)-Um34 and tRNA Leu (CmAA)-Cm34, there is a slight decrease such as it was observed under streptomycin condition. So, the use of gentamycin stress can be further pursued in collaboration with Pr. A. Dalpke.

Figure 36 Heatmap for the level of 2'O-methylation in control and gentamycin stress condition

Gentamycin stress was done with the same pipeline as other stress conditions. Concentration was determined by a growth curve in different antibiotics conditions. A concentration of 5µg/ mL was leading to 90% reduction of the growth late, so we used it for gentamycin stress.

In Summary for this chapter, E. coli 2'O-methylations are involved in stress response and could play a role either as MoTTs or to reduce innate immune system response to bacterial tRNA.

This creates a lot of new possibilities for 2'O-methylations and contributes to the better understanding of complex post-transcriptionally regulation systems.

General discussion

During the three years of my PhD thesis, I had the opportunity to work on diverse projects related to applications of high-throughput sequencing. In particular, the two chapters described in this manuscript show how this technology can be used for different RNA studies. First, I investigated the composition of exRNA population present in human plasma. Here, the goal was to use non-biased technology without the need to guess RNA target, like microarrays or RT-PCR. Using deep sequencing, I was able to determine the global composition of RNA in the whole plasma and in its sub-populations. Secondly, I used deep sequencing in order to measure the level of post-transcriptional modifications in E. coli tRNA species. In this case, the goal was not to sequence RNA species, but rather use deep sequencing as a tool, in order to detect and quantify specific RNA modifications, namely 2'O-methylations. I was able to demonstrate how Gm18 in E. coli tRNA is important for bacterial stress response.

These two projects show that high-throughput sequencing is a powerful and efficient technology that can be applied for a large subset of purposes. Nevertheless, it is crucial to always pay attention on the experimental biases that go hand in hand with this technology. This is why, in both projects I used a rigorous methodology. In exRNA project, I carefully designed the study in order to avoid as much as possible technical and bioinformatical errors. I spend a lot of time to determine what were the best methods for exRNA isolation and study. However, we had to be aware of the non-visible biases that could interfere in our data. Concerning the second project, I used other complementary techniques (LC-MS/MS, MST) in order to validate our data obtained by deep sequencing. Altogether, I was able to measure the importance of high-throughput sequencing in molecular biology research and especially in the complex and diverse RNA field.

When we talk about extracellular RNA of non-human origin or bacterial tRNA modifications that play a role in host immunomodulation properties, we instinctively think of infectious diseases and pathogenicity. There are three major strategies employed by bacteria, viruses and parasites to fight against either the innate or the acquired immune system (well reviewed in Finlay et McFadden, Cell. 2006 (375)).

The first strategy is to hide from recognition by the immune system. Viruses and bacteria can use a camouflage by dysplaing host-derived proteins on their surface/envelop (pretend they are elements of the host) or by producing carbohydrate capsules (to hide their bacterial antigens) respectively. Pathogens also use antigenic variation to fool the acquired immune system: by modifying their surface antigens, pathogens can re-infect the host without activating the acquired immune cells that don't recognize them. Another way to escape from the innate immune system is to attenuate/inhibit the immune response by preventing the production of cytokines. From this standpoint, microorganisms can attenuate/inhibit TLR activation. As On the other side, EV and exosomes in particular, are clearly involved in infection diseases.

Indeed, we and others showed that EV contain microbiota's molecular elements (nucleic acids, proteins) ( 223) but some pathogens like bacteria and viruses (as already described in the discussion section of the chapter I) use EV to better invade their host. In fact, they play a role in each of the three major strategies described below: hide, attack and/or manipulate. Pathogens enter the cell via the endosomal pathway and affect composition of nacent exosomes by modifying their composition with foreign nucleic acids or proteins. Some are also able to introduce themselves entirely in exosomes. As described above, pathogens need to hide from the immune system and can do it by three different ways. They can be protected inside exosomes and thus they don't trigger the immune system, exosomes can modify pathogenrelated substances or exosomes can suppress immune molecules. As examples, hepatitis C and genome of hepatitis A viruses stay in multivesicular body, they are secreted into exosomes and can then infect specific targets (224,228,229). Here, Alix and VPS4b exosomal proteins are involved (380). Moreover, RNA from hepatitis E are found in exosomal fractions into infectious particles without triggering antibodies. To escape their surveillance, particles have to be modified but the specific mechanism is still unknown (381). Interestingly, in Epstein-Barr virus infection, pathogens escape the immune system by sequestering immune effectors like caspase 1 and interleukines into exosomes (382). Finnally, pathogens can then manipulate the EV system to regulate the functions of recipient cells in order induce pathological consequences.

For instance, exosomes infected by S. aureus contain bacterial pore forming the α-toxin, exosomes will deliver these virulence factors to distant cells in order to kill them (383).

Moreover, some viruses, spread viral protein RNA and even miRNA by exosome uptake system in recipient cells in order to regulate their functions such as promoting the apoptosis of cells in interaction with T-and monocytes cells (384,385).

Thus, both projects I've carried out during my PhD thesis enrol completely in the field of infection disease and immunity. In the world where bacteria are more and more resistant to classical antibiotics treatments, the need to develop alternative antimicrobial therapies is critical. This is why, continue to study how pathogens are able to infect their host without triggering the innate or acquired system is very important. We could better understand how they work and determine their weaknesses. This knowledge is essential for potential implementation of new preventive and therapeutic strategies.

Material and Methods

All material and methods concerning exRNA project is described in Galvanin et al, 2019, Biochimie (Annexe 1) (208). For the second project, total tRNA isolation procedure, library preparation, sequencing and bioinformatics procedure are described in the article Galvanin et al, 2019, Methods in Molecular Biology (Annexe 2) (364).

I. Material : a. Chemicals, reagents, enzymes and ready-to-use buffers

The principal chemical, enzymes and ready-to-use buffers are described in the following table 9.

b. Buffers and mediums

The made buffers and medium used are described in the following table 10 For all E. coli culture, DH5α was used. A first preculture from a glycerol stock was performed in 10 mL LB overnight at 37°C and at 190 rpm. Then, a second precultured was seeded at 0.05

OD600nm and cultured until 1-2 OD600nm. This is this second preculture that was used for the seeding of control and stress conditions in each batch of experiment.

-Control condition: from the second preculture, E. coli is seeded between 0.01-0.05

OD600nm and grown for few hours in LB medium, 37°C, 190 rpm until 0.8-1.2 OD600nm. For growth curves, cells were cultures until the stationary phase and OD600nm was monitored several times.

-Temperature conditions: the same culture parameters than the condition one are used except bacteria were cultured at either 42°C or 20°C.

-Minimal medium condition: the same culture parameters than the condition one are used except bacteria were cultured in minimal medium M9.

-Hypoxia like condition o protocol A: bacteria were grown in a closed glass bottle instead of an Erlenmeyer in LB medium (the bottle was not full of medium) at 37°C or 20°C, at 190 rpm.

o protocol B: bacteria were grown in 50 mL Falcon tubes full of LB medium and incubated without shaking at either 37°C or 20°C.

For both conditions, cells were harvest when the medium is trouble and corresponds to the expected OD600nm. It is not possible to put back again the culture once we opened it to determine its OD600nm.

-Starvation: Bacteria were grown as the control condition. When the culture reached the expected OD600nm. Cells are harvest in 50 mL Falcon tubes for 5 min at 3,500xg and washed with 5 mL of 1X PBS. After another centrifuge step of 5 min at 3500xg. Pellet is resuspended in the same amount of initial LB medium and incubated for 24h at 37°C, 190 rpm.

-Antibiotics conditions Antibiotics were prepared from dried powder and resuspend in a final concentration of 1mg/mL in milliQ water for all of them except chloramphenicol that was resuspended in Ethanol.

Antibiotics solutions were filtered at 0.22 µm for further sterile use. In sterile condition, solution were aliquoted in 1.5 mL Eppendorf tubes and stocked at -20°C. For the determination of antibiotic concentrations, bacteria were grown with several antibiotic concentrations, the OD600nm was monitored during one day.

o Protocol A: bacteria were seeded between 0.01-0.05 µg/mL in LB medium containing a determined concentration of antibiotics leading to a growth delay of around 30%

(1.5 µg/mL for streptomycin and 2.5µg/mL for spectinomycin). Cultures were harvest between 0.8-1.2 OD600nm.

o Protocol B: bacteria were grown as the control condition. When the culture reached the expected OD600nm, antibiotics were added to the medium in the determined final concentration (5 µg/mL for streptomycin, 10 µg/mL for spectinomycin, 5 µg/mL for chloramphenicol and 5 µg/mL for gentamycin) and cells were incubated at 37°C, 190 rpm for 24h.

-Concerning experiments performed in biological quadruplates and triplicates, in this case a huge control culture was reached to the expected OD600nm and then was split for the stress conditions (see figure 21 in the results section chapter II, B, I, d, 1: Study design).

For all conditions, cells were pelleted in 50 mL Falcon tubes at 3,500xg for 5min. Pellet was washed with 5 mL of PBS 1X. After another centrifuge step of 5 min at 3,500g, pellet was directly used for further tot tRNA isolation (without storage at -20°C).

You can find the whole material and methods in Galvanin et al, 2019 (364) for tot tRNA isolation procedure and further experiments for the detection and quantification of 2'Omethylations by high-throughput sequencing.

b. Specific tRNA isolation 1. Biotin/streptavidin method Before individual tRNA isolation, we determined the amount of total tRNA required for 100% of hybridization with full-length complementary probes (described in table 11). A constant amount of the fluorescently labelled DNA probe (in this case 2 pmol), were hybridized in 1X

PBS to increasing amounts of total tRNA (0-12 µg). Samples (total volume of 100 µL) were denaturated 3 min at 90°C and hybridized for 10 min à 65°C. After cooling down to room temperature, the samples were mixed with non-denaturing loading dye (50% (v/v) glycerol, 10% (v/v) 10X TBE buffer) and were analysed by 15% native PAGE. The fluorescence of the DNA probe was detected with a Typhoon 9400 device (excitation: 532 nm, emission filter: 526 SP).

Four individual tRNA were isolated from total tRNA by using the hybridization of a complementary DNA probe. The sequences of all used DNA probes are listed in the table 11.

The DNA probe carries a fluorophore (fluorescein) on the 3'-end and a biotin label for separation by immobilization on streptavidin-coated magnetic beads (Dynabeads® MyOneTM) on the 5'-end. 200 pmol of the respective DNA probe were hybridized to previously determined amount of total tRNA in 5X SSC buffer in a total volume of 100 μL and incubated for 3 minutes at 90°C followed by an incubation for 10 minutes at 65°C. After this, samples were cooled down to room temperature. The magnetic beads were prepared: 50 µL of beads were placed on a 1.5 mL Eppendorf tube and placed on the magnetic rack, after removing supernatant and removing the beads from the magnetic rack, beads were washed with the same volume of 1X B&W buffer (5 mM Tris-HCl (pH 7,5), 0.5 mM EDTA, 1 M NaCl) three times. Then, beads were resuspended in 5X SCC buffer. For RNA/DNA binding on the beads, the samples were incubated with the magnetic beads 30 minutes at 25 °C under slight shaking at 650 rpm. After placing the tubes on the magnetic rack and removing of the supernatant (flow through containing non-targeted tRNA), the beads were washed once in 1X SSC buffer and three times in 0.1X SSC buffer. Finally, the pellet of beads was resuspended in 50 μL MilliQ-H2O and the RNA was eluted by heating for 3 min at 75 °C and the RNA containing supernatant was removed. Samples were stored in -20°C for further LC-MS/MS experiments.

In

NHS/NH2 method

For the isolation of individual tRNA with the NHS/NH2 method, we used the GE Healthcare HiTrap™ NHS-Activated HP Columns (1mL).

We bound four oligonucleotides with a primary amine in 5' (table 11) on the columns according to manufacturer's instruction for the four targeted individual tRNA isolation (Leu cmnm 5 UmAA, Ser cmo 5 UGA, Leu CmAA and Ser CGA). Column previously put at RT for 15 min was flushed with 6 mL of cold 1 mM HCl. 100 nmol of lyophilized oligonucleotide was resuspended in 1.2 mL of coupling buffer: 0.2 M NaHCO3, 0.5 M NaCl pH 8.3) and was added to the column. Binding was done during 1-2 hours. The column was washed with 6 mL of buffer A (ethanolamine 0.5 M, NaCl 0.5 M, pH 8.3) and buffer B (0.1 M sodium acetate, 0.5 M NaCl, pH 4). A second wash of buffer A is performed with an incubation time of at least 30 min in order to quench the residual free NHS groups. Then, the column was washed with 6 mL of buffer B, buffer A and buffer B. The column is ready for individual tRNA isolation or can be stored in a neutral phosphate buffer (50 mM Na2HPO4 pH 7.1, NaN3 0.1%).

For individual tRNA isolation, the column is first equilibrated for few minutes at 65°C (in an oven capable of containing a peristaltic pomp and the tubes for total tRNA recirculation). In parallel, total tRNA sample was prepared. 1 mg of RNA was heated 2 min at 70°C in 1 mL of tRNA binding buffer (1.2 M NaCl, 30 mM Hepes KOH ph7,4) and directly put on ice. Then sample was transferred in a 50 mL Falcon tube containing 14 mL of tRNA binding buffer and was connected to the Hi-trap column. Via a peristaltic pomp, total tRNA sample was recirculated at 65°C for at least 1 hour. Column was washed with 10-15 mL of wash buffer (10 mM Hepes, 100 mM NaCl) at 55°C. Individual tRNA was eluted with 7 mL of elution buffer (1 mM Edta, 1 mM Hepes) after putting both buffer and column in preheated water bath at 75°C for 5 min. Elution fractions were precipitated with isopropanol (see the Method section Chapter II, C, II, b, 3: section RNA/DNA precipitation). Samples resuspended in 20 µL milliQ water were loaded on a 10% denaturing gel (see section). Band of interests were cut, frozen for 30 min in -20°C and eluted with 0.5 M NH4oAc at RT 750 rpm (200 µL) overnight. After shaking, the gel suspension was filtered through centrifugal filters (NanoSeps) and the resulting filtrate was ethanol precipitated as described in the section (Method section Chapter II, C, II, b, 3:

section RNA/DNA precipitation).

Before our optimization experiment tRNA binding buffers and wash buffer contained 10 mM MgCl2.

For the determination of individual tRNA contamination or degradation, we did a hybridization assay on native gels described in the section (Chapter II, C, II, b, 4: Gel electrophoresis).

RNA/DNA precipitation

Samples were supplemented with 1/10 volume of a 5 M ammonium acetate solution (pH 5.2), 1µL of GlycoBlue T , 2.5 volumes ice cold pure ethanol (-80 °C) (in the case of biotin/streptavidin method) or 0.4 volume of cold isopropanol (in the case of NHS/NH2 method) were mixed by vortexing and stored overnight at -20 °C. Samples were centrifuged at least 30 min, full speed at -4°C. Pellet was washed with 500 µL of Ethanol 80% and centrifuged again 10 min, at full speed at -4°C. Supernatant was removed and pellet was dried at room temperature for 5 min and then resuspended in the required amount of milliQ water.

Gel electrophoresis

-Denaturing:

10% denaturing gels, containing 8 M urea, 50 mL of the respective 12% PAGE pre-mixes ( 40(v/v) denaturing gel concentrate, 50% (v/v) gel diluent, 10% (v/v) gel buffer concentrate) were mixed with 50 μL tetramethylethylenediamine solution (TEMED) and 400 μL 10% (m/v) ammonium persulfate (APS). After polymerization, gels were pre-run in 1× TBE (Tris-borate-EDTA) buffer for 30 minutes before RNA samples (mixed 1:1 with denaturing loading dye containing 90% (v/v) formamide and 10% (v/v) 10× TBE) were loaded. Gels were run at 10-15 W for around an hour.

-Native:

15% native polyacrylamide gels were prepared by mixing 50 mL native PAGE pre-mix (37.5%

(v/v) native gel concentrate (40%), 10% (v/v) 10X TBE in H2O) with 50 μL TEMED solution and 1 mL 10% (m/v) APS. After polymerization and a pre-run in 1X TBE buffer for 30 minutes, samples (mixed 1:1 with native loading dye containing 50% (v/v) glycerol and 10% (v/v) 10X

TBE buffer in H2O) were loaded. Gels were run at 15 W for an hour. was applied with a flow rate at 0.35 mL/min as described in the figure 37. For quantification experiments the dMRM mode of the LC-MS/MS was used. Following are the QQQ settings required for adjustment in the table 13.

99

Subsequent analysis of the peaks with predefined m/z values was performed with the Agilent MassHunter Software, Qualitative Analysis (V. 5.0.519.0). Extraction of ion chromatograms allowed integration of respective peaks, providing values for the area under the curve. In order to normalize these values, the following calculation was performed: ) × 100 where x corresponds to previously calculated values.

𝑥 = 𝑑𝑀𝑅𝑀 (

d. Thermophoresis experiment

To determine the tRNA distribution, MicroScale Thermophproesis (MST) experiments were performed. Increasing total tRNA samples were hybridized with a constant amount of 5'Fluorescein labelled probes (as described in the table 14) in 1X PBS and 0.1% PEG.

Sequences of probes are reported in tables 11 and12

Abstract:

For less than a decade, high-throughput sequencing became a very powerful, sensitive and precise technique for the study of ribonucleic acids. During my PhD thesis, I have used this technology for in-depth characterization of the extracellular RNA (exRNA) content of human plasma. exRNA in plasma exists either in a "soluble state" as a component of ribonucleoprotein (RNP) complexes or encapsulated into extracellular vesicles (EV) of diverse origins (exosomes, microvesicles, …). In this project, I have demonstrated that whole human plasma contains mostly micro RNAs and the fragment of RNA hY4, as well as degraded ribosomal RNA. Moreover, using a rigorous strategy via size exclusion chromatography or consecutive proteinase K/RNase A treatments, highly purified EVs can be obtained. miRNAs and RNA hY4 fragments were nor present in majority of samples, demonstrating a huge difference between soluble exRNA and exRNA from purified EVs. The RNA content of these EVs mainly reflects RNA composition of human microbiota. In addition, I have also performed a comparative analysis of commercially available "exosome-enrichment" kits which supposed to purify human exosomes by precipitation. RNA composition of such fractions was found to be almost identical to human plasma, showing strong uncontrolled contamination by soluble RNPs. Based on this study, we were able to propose a protocol for studies in exRNA in the field of liquid biopsies with clinical sample in order to discover new diagnostic biomarkers.

Apart from the characterization of RNA, high-throughput sequencing can be used for detection and quantification of RNA posttranscriptional modifications. During my PhD thesis I have applied deep sequencing for analysis of transfer RNA (tRNA) 2'-Omethylations in model bacteria (E. coli) using RiboMethSeq. Under several stress conditions, such as starvation and non-lethal antibiotics concentrations, some 2'-O-methylated nucleotides show an adaptive response. While over than half of Gm18 show a global increase under all investigated stress conditions, ribomethylated residues at position 34 show an opposite effect for some antibiotic treatments: mostly for chloramphenicol and streptomycin. Each of these dynamic profiles can be linked to cell regulation in response to stress. Change at the tRNA wobble base (position 34) could be a way to regulate translation by modifying the codon usage. Concerning Gm18, its role in the escape from the human innate immune system during host invasion is currently elucidated.

e. Les souspopulation d'ARN extracellulaires

Certains ARN et notamment les micro ARN (miARN) sont retrouvés à l'état « soluble » et sont inclus au sein de complexes ribonucléoprotéiques (RNP), contenant à la fois des ARN and des protéines associées (3) (figure 1) ou moins fréquemment dans des particules lipidiques telles que les lipoprotéines à faible et haute densité (LDL et HDL) (4).

Le second type d'ARN extracellulaires est retrouvé à l'intérieur de particules relâchées par la cellule appelées les vésicules extracellulaires (VE) (5)(6)(7)(8). Les VE sont délimitées par une bicouche lipidique et ne peuvent pas se répliquer. Trois types majeurs de VE sont repertoriés selon leur taille et leur biogénèse (figure 1) : les corps apoptotiques (9)(10)(11), les microvésicules (12,13) et les exosomes (14)(15)(16). Les corps apoptotiques sont des vésicules de 1 à 5 µm et proviennent de l'invagination des cellules en apoptose. Ils sont principalement composés de fragments d'ADN et d'histones. Les microvésicules, de 100 nm à 1 µm, sont formées par bourgeonnement de la membrane plasmique et leur contenu dépend le là où a lieu ce bourgeonnement. La composition de leur membrane est similaire à celle de la cellule dont elles proviennent (17). Les microvésicules sont enrichies en radeaux lipidiques de phosphatidylsérine, marqueurs de lignées cellulaires et récepteurs de surface cellulaire (17)(18)(19).

Contrairement aux corps apoptotiques qui résultent d'un bourgeonnement non discriminé de la membrane plasmique, les microvésicules naissantes proviennent d'un mécanisme de bourgeonnement distinct menant à des changements locaux des composants protéiques et lipidiques de la membrane. Ceci module des changements dans la courbure et dans la rigidité de la membrane (20,21). De plus, ils portent des protéines et des acide nucléiques sélectivement enrichis alors que d'autres en sont exclus (19). Le troisième type de VE appelé exosome est une particule entre 50 et 150 nm (22) et est originaire d'un mécanisme classique d'endocytose avec une biogénèse et un recrutement de cargos spécifiques (15,23). En prenant en compte tous ces mécanismes de secrétions d'ARNex, leur composition se révèle extrêmement diverse et hétérogène (9,24). De plus, les ARNex sont des biomarqueurs potentiels très attractifs pour certaines pathologies, avec une application croissante en diagnostic et pronostic (25). En effet, la découverte de nouveaux biomarqueurs parmi ces sous-populations d'ARNex présentes dans la circulation sanguine pourrait aider au diagnostic par l'utilisation de biopsies liquides au lieu des classiques biopsies tissulaires invasives et douloureuses. Parmi toutes ces sous-populations, les microvésicules et surtout les exosomes présentent un fort intérêt dans ce domaine car ils ont une biogénèse spécifique et un contenu contrôlé en protéines et acides nucléiques qui sont connus pour jouer un rôle dans plusieurs maladies tels que le cancer (22,26,27). C'est pourquoi, nous nous sommes intéressés de plus près aux exosomes.

f. Exosomes : biogénèse et composition

Biogénèse

Les exosomes sont connus pour être sécrétés par quasiment tous les types cellulaires et sont retrouvés dans tous les liquides biologiques tels que le plasma. Le mécanisme général pour la sécrétion des exosomes en dehors de la cellule commence avec une endocytose classique médiée par des vésicules recouvertes de clathrine. Dans l'endosome précoce, il y a formation de vésicules intraluminales (VIL) par invagination de la membrane de l'endosome. Ces vésicules forment les corps multivésiculaires (CMV). Souvent ils fusionnent avec le lysosome où les déchets cellulaires (protéines, lipides, carbohydrates, …) sont clivés en simple composés puis recyclés en nouveaux matériaux pour la construction de la cellule. Néanmoins, les CMV peuvent aussi fusionner avec la membrane plasmique et de ce fait sécréter les VIL, qui dorénavant sont appelés les exosomes, dans le milieu extracellulaire (figure 1) (22,23,28,29).

La plupart du temps, le destin pour la sécrétion des VIL en exosomes dépend du complexe « endosomal sorting complexe required for transport » (ESCRT) (22,(30)(31)(32). Il est notamment impliqué dans le remodelage de la membrane endosomale permettant la formation des VIL.

Composition

La composition des exosomes ne reflète pas exactement celle de la cellule sécrétrice (33,34).

V. Composition des ARNex : état de l'art

Dans le domaine des biopsies liquides et de la détermination de biomarqueurs de diagnostic, la source biologique la plus utilisée est le sang : plasma ou sérum. Ce paragraphe décrit la composition en ARNex du plasma/sérum total ou de la sous-population d'exosomes.

Les premières études ont mis en évidence la présence d'ARNm et de miARN (36) 44), alors qu'une autre étude avec des paramètres d'étude similaires rapportent une proportion de 20% et 42% (38). Ceci reflète probablement l'extrême hétérogénéité lors de la préparation des échantillons dues aux techniques utilisées pour l'isolement et la caractérisation des VE (47).

Des différences peuvent aussi provenir des méthodes utilisées pour la préparation des libraires pour le séquençage ainsi que la stratégie bio-informatique pour l'analyse des données et leur interprétation (48).

Globalement, c'est un réel challenge pour caractériser le contenu en ARNex lorsqu'ils proviennent du plasma/sérum (49). Il est donc essentiel de bien concevoir l'étude et de prendre conscience des éventuels problèmes de contamination, pouvant affecter la composition de l'échantillon.

VIII. Principaux résultats

XI. Modification des ARNt j. Vue d'ensemble des modifications des ARNt

Les modifications post-et co-transcriptionnelles sur l'ARNt confèrent une diversité chimique et augmentent la fonctionnalité de l'ARN. Jusqu'à présent, plus de 40 modifications chimiques différentes ont été rapportées chez E. coli (figure 6), générant divers dérivés des quatre nucléotides classiques A, C, G et U. Il n'est donc pas surprenant que 1 à 10% du génome code des protéines impliquées dans le processus de modification de l'ARNt (66,67).

Les nucléosides modifiés peuvent être trouvés à plusieurs endroits de la séquence d'ARNt conduisant à des fonctions différentes, mais la fréquence et la diversité de modifications les plus élevées sont observées aux positions 34 et 37 de la tige boucle de l'anticodon (68). La modification de la position 37 est associée au maintien de l'efficacité et de la fidélité de la traduction (69). En effet, les modifications à ce niveau garantissent que le cadre de lecture correct est pris en charge. Ainsi, elles permettent une meilleure stabilisation de l'interaction codon-anticodon, en particulier à la position de la première paire de bases (67,69). La modification de la position 34 est impliquée dans la fidélité de la traduction en influençant le choix et la discrimination des codons. Ceci est vraiment important lorsque deux acides aminés correspondent à deux codons dont un seul des trois nucléotides est différent.

Outre la région anticodon, plusieurs autres positions sont connues pour être modifiées. Leur fonction principale est la stabilisation de la structure de l'ARNt telle que les pseudouridines (66,69). Les autres fonctions pour la modification des ARNt sont des fonctions de régulation telles que leur rôle dans la réponse immunitaire de l'hôte (70,71).

Des études antérieures ont démontré l'importance des modifications de l'ARNt pour la traduction ou la stabilité de l'ARNt, mais il est clair maintenant qu'elles jouent un rôle beaucoup plus complexe dans la régulation cellulaire en fournissant des liens avec la synthèse des protéines, le métabolisme et la réponse au stress. En effet, il a été démontré que Gm18 échappait au système immunitaire inné en inactivant les récepteurs de type Toll (TLR) impliqués dans la réponse immunitaire innée (71).

XII. Impact du stress sur les modifications des ARNt

Les ARNt sont fortement régulés et peuvent être influencés par des contraintes environnementales telles que les changements de température, la privation de nutriments ou même des conditions d'oxydation (90,91). Il existe des cas documentés de de régulation induites par le stress sur les ARNt au niveau des ARNt eux même et de leur rôle dans la traduction. De plus, les modifications peuvent jouer un rôle discret mais élégant et essentiel dans le réglage précis de la traduction (90).

Certaines études ont démontré que le niveau de modification change pendant la croissance à des températures élevées ou dans des conditions d'arrêt de la croissance (92)(93)(94)(95). Plusieurs bactéries thermophiles ont montré une augmentation des modifications de l'ARN lorsque la température de croissance était modifiée (96), chez E. coli une augmentation des résidus thiolés est observée (97). Des recherches ont déterminé l'importance de l'absence de mcm 5 S 2 U ou de la présence de la modification m 5 C38 chez Kluyveromyces lactis et la drosophile, respectivement, pour la protection de certains ARNt contre le clivage induit par le stress (98,99). 8). Par conséquent, les modifications des ARNt servent de système de régulation pour le contrôle de la traduction dans la réponse au stress (90,103,104). Les résultats pour les quadruplats ou triplicats biologiques en condition contrôle ou de stress suite au manque de nutriments ou de la présence d'antibiotiques sont présentés dans la figure 9 sous forme de heatmap. Nous pouvons décrire trois profils différents. Le premier groupe concerne la moitié des positions étudiées où il n'y a pas de différence entre le contrôle et les conditions de stress (partie basse du heatmap). Plusieurs modifications (Nm32 et Gm18) ne sont pas impactées lorsqu'un stress de croissance est induit. Cependant, dans la partie supérieure du heatmap, deux profils de dynamique sont visibles. Premièrement, six modifications de l'ARNt (ARNt Ser (cmo 5 UGA) -Cm32, ARNt Ser (CGA) -Gm18, ARNt Leu (cmnm 5 UmAA) -Gm18, ARNt Ser (cmo 5 UGA) -Gm18, ARNt Gln (CUG) -Gm18 et ARNt Leu (CmAA ) -Gm18) montrent une augmentation de leur niveau de 2'O-méthylation dans toutes les conditions de stress par rapport à celle du témoin. Les modifications impliquées concernent principalement Gm18 (5/6) et surtout au niveau des ARNt Ser ou Leu, qui sont des ARNt avec une grande boucle variable. Parmi les 9 Gm18 étudiées, plus de la moitié (5) sont impliquées dans ce comportement. Au contraire, sur les 6 modifications étudiées en position 32, un seul Cm32 a été augmenté en condition de stress. Le deuxième type de réponse au stress est observé pour toutes les positions 34 : ARNt Leu (CmAA) -Cm34 et ARNt Leu (cmnm 5 UmAA) -Um34. Ici le profil est différent. Il n'y a pas de réelle différence avec le contrôle pour la condition sans nutriments et celle avec l'antibiotique spectinomycine. Nous avons seulement pu constater une légère diminution en condition spectinomycine uniquement pour l'ARNt Leu (CmAA) -Cm34.

Cependant, dans les conditions avec la streptomycine et particulièrement dans les conditions au chloramphénicol, on peut voir une diminution du niveau de 2'O-méthylations. Ainsi, nous avons pu valider les deux comportements observés dans les données de séquençage via l'expérience LC-MS/MS, c'est-à-dire l'augmentation globale de plusieurs Gm18 et la diminution de la position 34 des conditions de chloramphénicol et de streptomycine.

XVI. Discussion et conclusion

Dans ce projet, nous avons mis en évidence les propriétés de dynamisme de la 2'O-méthylation à partir d'ARNt d'E. coli en conditions de stress. Deux comportements majeurs ont été décrits.

La moitié des Gm18 augmente dans les conditions sans nutriments, de streptomycine, de spectinomycine et de chloramphénicol, tandis que toutes les 2'O-méthylations en position 34 diminuent dans les conditions de chloramphénicol et de streptomycine.

Nous nous demandions si l'effet de stress sur les 2'O-méthhylations était dû à une augmentation de la production de la modification ou à une diminution de la dégradation de l'ARNt.

Abstract:

For less than a decade, high-throughput sequencing became a very powerful, sensitive and precise technique for the study of ribonucleic acids. During my PhD thesis, I have used this technology for in-depth characterization of the extracellular RNA (exRNA) content of human plasma. exRNA in plasma exists either in a "soluble state" as a component of ribonucleoprotein (RNP) complexes or encapsulated into extracellular vesicles (EV) of diverse origins (exosomes, microvesicles, …). In this project, I have demonstrated that whole human plasma contains mostly micro RNAs and the fragment of RNA hY4, as well as degraded ribosomal RNA. Moreover, using a rigorous strategy via size exclusion chromatography or consecutive proteinase K/RNase A treatments, highly purified EVs can be obtained. miRNAs and RNA hY4 fragments were nor present in majority of samples, demonstrating a huge difference between soluble exRNA and exRNA from purified EVs. The RNA content of these EVs mainly reflects RNA composition of human microbiota. In addition, I have also performed a comparative analysis of commercially available "exosome-enrichment" kits which supposed to purify human exosomes by precipitation. RNA composition of such fractions was found to be almost identical to human plasma, showing strong uncontrolled contamination by soluble RNPs. Based on this study, we were able to propose a protocol for studies in exRNA in the field of liquid biopsies with clinical sample in order to discover new diagnostic biomarkers.

Apart from the characterization of RNA, high-throughput sequencing can be used for detection and quantification of RNA posttranscriptional modifications. During my PhD thesis I have applied deep sequencing for analysis of transfer RNA (tRNA) 2'-Omethylations in model bacteria (E. coli) using RiboMethSeq. Under several stress conditions, such as starvation and non-lethal antibiotics concentrations, some 2'-O-methylated nucleotides show an adaptive response. While over than half of Gm18 show a global increase under all investigated stress conditions, ribomethylated residues at position 34 show an opposite effect for some antibiotic treatments: mostly for chloramphenicol and streptomycin. Each of these dynamic profiles can be linked to cell regulation in response to stress. Change at the tRNA wobble base (position 34) could be a way to regulate translation by modifying the codon usage. Concerning Gm18, its role in the escape from the human innate immune system during host invasion is currently elucidated.

Keywords: high-throughput sequencing, extracellular RNA, extracellular vesicles, tRNA modifications; 2'O-methylation Résumé :

Depuis une petite dizaine d'années, le séquençage à haut débit est devenu une technique très puissante, sensible et précise pour l'étude des acides ribonucléiques (ARN). Pendant ma thèse de doctorat, nous avons utilisé cette technologie pour la caractérisation en profondeur des ARN extracellulaire (ARNex) provenant du plasma humain. Les ARNex du plasma peuvent être retrouvés soit à l'état « soluble » sous forme de complexes ribonucléoprotéiques (RNP) ou encapsulés au sein de vésicules extracellulaires (VE) de diverses origines (exosomes, microvesicles, …). Dans ce projet, j'ai démontré que le plasma entier contenait principalement des micro ARN et le fragment hY4 ainsi que des ARN ribosomiques dégradés. Par ailleurs, suite à une stratégie rigoureuse via chromatographie à exclusion de tailles ou par traitement consécutif à la protéinase K et à la RNase A, des EV hautement purifiés peuvent être obtenus. Les Micro ARN et le fragment d'ARN hY4 ne sont pas présents en majorité dans ces échantillons, démontrant une grande différence de composition entre les ARNex solubles et ceux des vésicules purifiées. Le contenu ARN de ces VE reflète principalement une composition en ARN du microbiote humain. Par ailleurs, j'ai également effectué une étude comparative de kits commerciaux pour l'enrichissement des exosomes qui sont supposés purifier les exosomes par précipitation. La composition en ARN de ces fractions est très similaire au plasma humain total, montrant une forte contamination non contrôlée par les RNP solubles. Sur la base de cette étude, nous sommes en mesure de proposer un protocole pour l'études des ARNex dans le cadre de biopsies liquides avec des échantillons cliniques afin de découvrir de potentiels biomarqueurs de diagnostic.

Au-delà de la caractérisation d'ARN, le séquençage à haut-débit peut être utilisé pour la détection et quantification des modifications post-transcriptionnelles. Pendant ma thèse, j'ai utilisé le séquençage en profondeur pour l'analyse des 2'Ométhylation des ARN de transfert (ARNt) chez le model bactérien E. coli par RiboMethSeq. Sous plusieurs conditions de stress telles que le manque de nutriments ou des concentrations non létales d'antibiotiques, certaines 2'O-méthylations montrent une réponse adaptative. Alors que plus de la moitié des 2'O-méthylation en position 18 sont augmentées dans toutes les conditions de stress étudiées, les résidus 2'O-methylés à la positon 34 eux montrent un effet opposé avec une diminution dans certains stress comme le chloramphénicol et la streptomycine. Chacun de ces deux comportements dynamiques peut être relié à un phénomène de régulation cellulaire en réponse au stress. Un changement au niveau de la « wobble base » (position 34) pourrait être un moyen de réguler la traduction en modifiant l'usage des codons. En ce qui concerne la modification Gm18, son rôle dans l'évasion du système immunitaire inné lors de l'invasion d'un hôte est en cours d'élucidation.

Mots clés : Séquençage à haut débit, ARN extracellulaires, vésicules extracellulaires, modification des ARNt, 2'O-méthylations

Foreword

  Figure list: ......................................................................................................................................

Figure 1

 1 Figure 1 Sanger sequencing .......................................................................................................

Figure 2

 2 Figure 2 Sequencing by ligation: SOLiD ...................................................................................

Figure 3

 3 Figure 3 Sequencing by synthesis: single nucleotide addition ...................................................

Figure 4

 4 Figure 4 Sequencing by synthesis: Illumina ..............................................................................

Figure 5

 5 Figure 5 Extracellular RNA overview in plasma .....................................................................

Figure 6

 6 Figure 6 Exosome composition ................................................................................................

Figure 7

 7 Figure 7 Exosome uptake in recipient cell ...............................................................................

Figure 8

 8 Figure 8 Methods for exosome isolation ..................................................................................

Figure 9

 9 Figure 9 Types of RNA included or excluded in the analysis according to their extremities..

Figure 10

 10 Figure 10 Library preparation adapted for small RNA ............................................................

Figure 11 :

 11 Figure 11: Structure of human Y RNA isoforms .....................................................................

Figure 12 :

 12 Figure 12: hY4 fragments found in plasma ..............................................................................

Figure 13 E

 13 Figure 13 E. coli tRNA processing ..........................................................................................

Figure 14 E

 14 Figure 14 E. coli tRNA modification: positions and involved enzymes ..................................

Figure 15 E

 15 Figure 15 E. coli tRNA 2'O-methylations: positions and enzymes involved ...........................

Figure 16

 16 Figure 16 Model for the regulation of MoTT ..........................................................................

Figure 17

 17 Figure 17 RiboMethSeq method ..............................................................................................

Figure 18

 18 Figure 18 Growth curve in Streptomycin and Spectinomycin condition (protocol A) ............

Figure 19

 19 Figure 19 First screening: heat map representing MethScore levels under several stress conditions for a subset of analysed 2'O-methylations ..............................................................

Figure 20

 20 Figure 20 Growth curve in the presence of varying concentrations of chloramphenicol ........

Figure 21 Figure 22

 2122 Figure 21 Study design for starvation and antibiotics conditions and restoration experiments59 Figure 22 Global heatmap: heat map representing MethScores in starvation and antibiotics conditions performed in biological replicates ..........................................................................

Figure 23

 23 Figure 23 Box plot on 8 interesting positions leading to a modification dynamism ...............

Figure 24

 24 Figure 24 Heat map representing "Return-back" experiment ..................................................

Figure 25

 25 Figure 25 Relative quantification on MS signal from tot tRNA samples ................................

Figure 26

 26 Figure 26 Two methods for individual tRNA isolation ...........................................................

Figure 27

 27 Figure 27 Determination of DNA/RNA optimal ratio for individual tRNA hybridization .....

Figure 28

 28 Figure 28 Leakage of oligonucleotides leads to cross-contamination .....................................

Figure 29 First

 29 Figure 29 First individual isolated tRNA are degraded ...........................................................

Figure 30

 30 Figure 30 Profile of individual tRNA after final optimizations ...............................................

Figure 31

 31 Figure 31 Set up for the parallel isolation of four individual tRNA ........................................

Figure 32

 32 Figure 32 Comparison between RiboMethSeq and LC-MS measurements ............................

Figure 33

 33 Figure 33 Changes of tRNA proportion under stress conditions .............................................

Figure 34

 34 Figure 34 Microscale thermophoresis (MST) principle ...........................................................

Figure 35

 35 Figure 35 tRNA distribution in E. coli measured by deep sequencing and MST experiments Figure 36 Heatmap for the level of 2'O-methylation in control and gentamycin stress condition ..................................................................................................................................................

Figure 37

 37 Figure 37 LC gradient ..............................................................................................................

  Illumina sequencing ................................................................................................................... Table 2 Published studies of human blood exRNA characterization. ...................................... Table 3 Stress conditions for the first screening ...................................................................... Table 4 Stress conditions for biological replicates ................................................................... Table 5 Determined DNA/RNA ratios for the four studied tRNA...........................................

  Abbreviation list:

Figure 1

 1 Figure 1 Sanger sequencing Sanger sequencing: generated fragments are visualized on a polyacrylamide gel. By following the increasing bands in size, one can determine the sequence because each fragment ends by labelled ddNTP. Here the sequence is TATCGCGTCGA.

Figure 2

 2 Figure 2 Sequencing by ligation: SOLiDAfter library amplification on beads, fragments are sequenced by ligation. A probe encoded with two known bases (blue) and several degenerated and universal nucleotides (pink) are labelled with a fluorophore and is added to the libraries. The two bases probe is ligated onto the anchor (light purple) that is complementary to a 5' adaptor (red). Fluorescence is detected and the two first bases are identified. The probe is cleaved and the fluorophore and a part of the degenerate and universal nucleotides are released. The process is repeated ten times. Then, the entire strand is reset by removing all the ligated probes and the anchor. A novel anchor with a N+1 offset is ligated and another cycle of ligation, imaging and cleavage is performed. This continues with until N+4 offset anchor in order to read the whole sequence.

Figure 3

 3 Figure 3 Sequencing by synthesis: single nucleotide addition a) 454 pyrosequencing. After library amplification on beads arrayed onto a microtitre plate along primers and beads containing an enzyme cocktail. A single nucleotide is added and each complementary base is incorporated in new synthetized strand. During the dNTP incorporation, pyrophosphate molecules are released. ATP sulfurylase transforms APS (adenosine 5'phosphosulfate) in ATP via pyrophosphates. Then, ATP, which is a cofactor, allows the conversion of luciferin to oxyluciferin by luciferase activity. Light is a by-product of the reaction and can be detected by a charge-coupled device (CCD) camera. Finally, apyrase degrades unincorporated dNTP and another base is added to the wells.

Figure 4

 4 Figure 4 Sequencing by synthesis: Illumina After library amplification on a flow cell, a mixture of primers, DNA polymerase and labelled 3'-blocked ddNTP are added. At each cycle, only one ddNTP is incorporated due to the 3' blockage. Unincorporated bases are washed, fluorescence signal is measured and incorporated nucleotide is determined. The dye is then cleaved and the 3' group is regenerated by reduction. A new cycle of nucleotide addition, wash, imaging, cleavage, regeneration is possible.

c. 3

 3 rd Generation: Pacific bioscience and Nanopore Oxford Technology The third generation of sequencing groups mostly techniques of single molecule sequencing and real time sequencing allowing the sequencing of long reads (several kb). Two major approaches were developed: Pacific Bioscience (PacBio) and Oxford Nanopore technologies (ONT).

Figure 5

 5 Figure 5 Extracellular RNA overview in plasma Legend: CCV: clathrin-coated vesicle; ESCRT: endosomal sorting complexes required for transport; MVB: multivesicular body; RNP: ribonucleoprotein particle.

Figure 8

 8 Figure 8 Methods for exosome isolationSeveral methods are available for the isolation or at least the enrichment of exosomes. Differential centrifugation finishing by ultracentrifugation and methods based on exosomes precipitation are the most known. Filtration and size exclusion chromatography lead to a better purification like immunoaffinity beads. However, this last method can avoid certain subtypes of exosomes. Lastly, other methods are interesting but require specific material such as density gradient or alternative current electrokinetic (ACE).

Figure 9

 9 Figure 9 Types of RNA included or excluded in the analysis according to their extremities Included rRNA, tRNA, miRNA, snoRNA, scaRNA have correct 5'P and 3'OH extremities, piwiRNA have 5'P and are 2'Omethylated in 3'extremities and degradation products have 5'OH and 3'P or cyclophosphate ends. In contrast excluded RNA are capped: mRNA and most lncRNA have a m 7 G-ppp cap while snRNA and snoRNA U3 have a m2,2,7 G-ppp cap.

Figure 10

 10 Figure 10 Library preparation adapted for small RNA 5'App: 5´ pre-adenylated; X: 3´-blocked A 3' adaptor with a 5' pre-adenylated (App) and 3' blocked DNA sequence is ligated specifically to 3'OH extremities from RNA sample. Then, the reverse transcription primer is hybridized on 3'adaptor. This step is important to prevent adaptordimer formation. After ligation of 5'adaptor, the first cDNA strand is synthesized by a reverse transcriptase and an amplification by PCR is performed. Primers contain either P5 sequence or both a barcode and P7 sequence. P5 and P7 sequence are required to amplify libraries and fix them on the flow cell. The barcode is required to identify the library during demultiplexing after sequencing.

Figure 11 :

 11 Figure 11: Structure of human Y RNA isoforms In human cells, Y RNA has four isoforms hY1, hY3, hY4 and hY5 composed by a least conservative loop domain, a upper stem domain implicated in DNA replication, a lower stem domain and a polyuridine tail implicated in the interaction with Ro60 and La proteins playing a role in RNA stability and quality control.

Figure 12 :

 12 Figure 12: hY4 fragments found in plasma Y RNA and mostly its hY4 isoform is found in plasma as fragments of 31 nt at the 5'-end and 22 nt at 3'-end.

Figure 13 E

 13 Figure 13 E. coli tRNA processing Newly transcribed tRNA by RNA polymerase III are cleaved by endonucleases RNase E and in a minor way RNase III if they are polycistronic. 5' processing removes the 5' leader sequence via the endonuclease RNase P while 3'-end processing is performed by the exonuclease RNase D.

  Figure 14 E. coli tRNA modification: positions and involved enzymes

(

  Figure 15 E. coli tRNA 2'O-methylations: positions and enzymes involved E. coli tRNA carry three modifications with methylated ribonucleotides at positions 18, 32 and 34. They are catalysed by the methyltransferases TrmH, TrmJ and TrmL respectively. The addition of the methyl group to the ribose to form 2'O-methylation is performed by protein stand-alone enzymes: methyltransferases (MTases). They are specific to RNA and belong to a vast group of MTases that catalyze the transfer of a CH3-group (Me-group) from a methyl donor to a biomolecule (273-275). The almost universal methyl donor is S-adenosyl-Lmethionine (SAM or AdoMet). All MTases share the same core structure of a mixed seven stranded β-sheets. Most 2'-OMTases belong to the superfamily of SPOUT MTases. In the past RNA 2'-O-MTases were separated into two groups: Rossman-like fold MTases (RMT) and SPOUT, but recently it has been demonstrated that they share a common evolutionary origin and they all form a single superfamily (274,276-278). Bacterial tRNA 2'-O-methyltransferases enzymes involved in tRNA 2'-O-methylation are rather well studied in E. coli. Each known position is formed by a specific tRNA MTase.

Figure 16

 16 Figure 16 Model for the regulation of MoTTWhen general transcripts are modified, a pool of proteins are translated. If tRNA are modified tunable transcripts (moTT), another pool of protein is translated. In stress condition, these modifications can be increased and lead to the more important production of the corresponding proteins. Without tRNA modification on moTT, fewer corresponding proteins are produced. Thus, depending on the level of modification on moTT, translation is regulated.

  2'O-methylated ribose is more stable to NaIO4 oxidation compared to unmodified ribose moiety. So, after NaIO4 treatment, non-methylated riboses are unsuitable for 3' ligation during library preparation and only the modified one can be transformed in cDNA for sequencing. Two methods use this principle: the Nm-Seq (353) and RibOxi-Seq (354). The last, and the most used, technique for 2'O-methylation detection relies on their chemical protective properties against alkaline cleavage. The protocol was first published by Nielsen lab in 2015 (355) and was further improved and optimized by the UMS IBSLor 2008 NGS core facility

For

  sequencing experiments, I prepared all the samples, performed library preparations and analysed data. Virginie Marchand, Lilia Ayadi and Valérie Igel-Bourguignon at the NGS core facility (UMS 2008 IBSLor) performed the sequencing and Pr Yuri Motorin conducted the bioinformatics analysis of the RiboMethSeq raw data. c. High throughput sequencing: first screening Our purpose was to determine what kind of growth stress can induce a change in level of tRNA 2'O-methylations in E. coli. The results of this initial screening of possible stress conditions are

Figure 19 Figure 20

 1920 Figure 19 First screening: heat map representing MethScore levels under several stress conditions for a subset of analysed 2'O-methylations Each square corresponds to the MethScore level normalised on the average value for a given modification.d. Focus on starvation and antibiotics conditions

Figure 23

 23 Figure 23 Box plot on 8 interesting positions leading to a modification dynamism Rectangle in the box plot represents data from 1 st to 3 rd quartile and is cut by the median. Segments (or dots) in the extremities show the extreme values.

Figure 24

 24

Figure 25

 25 Figure 25 Relative quantification on MS signal from tot tRNA samples MS values were reported between 0 and 1. 1 represents the highest MS value. Black circles, triangles and rectangles represents first, second and third biological replicates respectively. Each sample was analysed in technical duplicates. b. Individual tRNA isolation procedure

Figure 27

 27 Figure 27 Determination of DNA/RNA optimal ratio for individual tRNA hybridization 2pmol of oligonucleotide complementary to tRNA Leu (cmnm 5 UmAA) was hybridized with an increasing amount of total tRNA preparation and loaded on a native gel. Upper shift observed after measuring fluorescein signal corresponds to oligonucleotide hybridized to the target tRNA (RNA/DNA duplex) while the lower band represents the free DNA oligonucleotide. 3.5 µg of tot tRNA is the minimal amount to obtain 100% of hybridization.

Figure 28

 28 Figure 28 Leakage of oligonucleotides leads to cross-contamination a) 100µg of tot tRNA were used for tRNA Leu (cmnm 5 UmAA) isolation. First, second and third washes are loaded on a denaturing gel and the potential presence of oligonucleotides is detected by measuring the fluorescein fluorescence. We can see a DNA leakage in all washes

  (left part) represents the first optimization experiments. Apart from the band corresponding to the target tRNA, we can notice several bands (b1, b2 and b3) corresponding to fragments of a lower size. At this point it was not clear if these fragments correspond to tRNA contaminants (b1 and b2) or oligonucleotide leakage (b3) (in these experiments DNA oligonucleotides are 40 nt in length). Since, NH2-DNA oligonucleotides are not fluorescein modified, DNA leakage cannot be detected by measuring the fluorescence signal. However, the nature of band was verified by hybridization of each eluted fragment with full-length DNA oligonucleotides with 5'biotin and 3'fluorescein. Duplex formation was assessed by loading the potential RNA/DNA-FL duplex on a native gel. On the right part of the figure 29, we

Figure 29 First

 29 Figure 29 First individual isolated tRNA are degraded Left part: Isolated tRNA Leu (cmnm 5 UmAA) from a protocol where buffers contain MgCl2 was loaded on a denaturing gel and signal was measured after gel red staining. Apart from the full-length RNA corresponding to the upper band, there are three other bands with a lower size Right part: after gel excision, RNA elution and precipitation from the three bands, RNA are hybridized to a full-length oligonucleotide with fluorescein molecule in 3'-end. Potential duplexes are loaded on a native gel. b1, b2 and b3 hybridize to the oligonucleotide and leads to an upper shift (compared to control with oligonucleotides but without any RNA). These fragments are thus only degradation products from the targeted tRNA but don't come from contamination or DNA leakage. Legends: FL Fluorescein/ Ctol Control

Figure 34

 34 figure 34) where parameters such as the inflection point (EC50) can be determined. This parameter corresponds to the concentration of total RNA required for 50% of hybridization. If the concentration of the DNA probe is known, the absolute amount of individual tRNA can be determined (see Material and Methods section for more details Chapter II, C, II, d: Thermophoresis experiment).

Figure 35

 35 Figure 35 tRNA distribution in E. coli measured by deep sequencing and MST experiments

  TrmH, TrmJ or TrmL are not available, so this hypothesis cannot be easily verified by simple Western blotting. Nevertheless, one can envisage to measure levels of mRNAs encoding TrmH, TrmJ and TrmL by RT-qPCR.In contrast, the decrease of tRNA modification steady-state level can be only explained by accelerated turnover of modified tRNA under stress and their replacement by newly synthetized and poorly modified counterparts.We observed that different locations of 2'O-methylation in tRNA seem to be modulated by different stress and may be involved in different cell regulation mechanisms. It has been demonstrated that dynamic regulation of tRNA modification at the wobble base (position 34)

  already described in the discussion section of the chapter II of this PhD thesis manuscript, TLR recognize PAMP (pathogens-associated molecular patterns) and therefore generate a cascade of activations leading to the production of NF-KB, interferons and other inflammatory factors and to the release of cytokines outside the cell. Thus, pathogens are able to play with TLR recognition pattern and modulate PAMP. If TLR activation is prevented, therefore downstream pathways leading to the production of cytokines are inhibited. Secondly, microorganisms can directly act and attack/weaken the host immune cells. They are able to secrete virulence molecules with different functions. These immune modulators can paralyse phagocytose phenomenon, avoid phagolysosomal fusion, dysregulate natural killer cells or mediate the production of reactive oxygen species and nitric oxide components. They obviously include a multitude of toxins that directly kill host cells.Finally, the last strategy is to manipulate the host systems in order to spread themselves in the organism. Pathogens secrete immune modulators that mediate bacterial uptake or reprogram vesicular trafficking to enhance intracellular parasitism. They are also able to manipulate apoptosis, by suppressing apoptotic death of host cells, preferring cytotoxic death and thus neutralizing a variety of host cells.Both projects I've conducted during my phD thesis enter exactly in this domain of research and contribute to understand strategies employed by bacteria to invade their host. On one side, I've demonstrated that under conditions, partially mimicking infection/treatment for bacterial invasion, bacterial tRNA modification gains higher relative proportion of immunosuppressive Gm18 residues. The consequence is a global decrease of immunostimulation capacity of bacterial RNA and even if RNA contributes only moderately to global immunostimulation by living bacteria (376-379), RNA (and particularly tRNA) are particularly stable in acidic endosomal environment, and such reduced TLR7-driven immunostimulation may contribute to attenuated immune response and thus facilitate establishment of bacterial infection.

  TGG GGG AAG GAT TCG AAC CTT CGA AGT CGA TGA CGG CAG ATT TAC AGT CTG CTC CCT TTG GCC GCT CGG GAA CCC CAC C Leu )AA AAA TGG AGC GGG CGA AGG GAA TCG AAC CCT CGT ATA GAG CTT GGG AAG CTC TCG TTC TAC CAT TGA ACT ACG CCC GC Phe GAA AAA TGG TTG CGG GGG CCG GAT TTG AAC CGA CGA TCT TCG GGT TAT GAG CCC GAC GAG CTA CCA GGC TGC TCC ACC CCG CG Ser UGA AAA TGG TGG GTC GTG CAG GAT TCG AAC CTG CGA CCA ATT GAT TAA AAG TCA ACT GCT CTA CCA ACT GAG CTA ACG ACC C Trp CCA AAA TGG CGG TGA GGC GGG GAT TCG AAC CCC GGA TGC AGC TTT TGA CCG CAT ACT CCC TTA GCA GGG GAG CGC CTT CAG CCT CTC GGC CAC CTC ACC Tyr QUA AAA TGG TGC TGA TAG GCA GAT TCG AAC TGC CGA CCT CAC CCT TAC CAA GGG TGC GCT CTA CCA ACT GAG CTA TAT CAG C Ala VGC AAA T GGT GGG TGA TGA CGG GAT CGA ACC GCC GAC CCC CTC CTT GTA AGG GAG GTG CTC TCC CAG CTG AGC TAA TCA CCC Arg ICG AAA TGG CGG AGA GAG GGG GAT TTG AAC CCC CGG TAG AGT TGC CCC TAC TCC GGT TTT CGA GAC CGG TCC GTT CAG CCG CTC CGG CAT CTC TCC Arg CCG AAA TGG AGG CGC GTT CCG GAG TCG AAC CGG ACT AGA CGG ATT TGC AAT CCG CTA CAT AAC CGC TTT GTT AAC GCG CC Arg {CU1 AAA TGG TAC CCG GAG CGG GAC TTG AAC CCG CAC AGC GCG AAC GCC GAG GGA TTT TAA ATC CCT TGT GTC TAC CGA TTC CAC CAT CCG GGC Arg {CU2 AAA TGG TGC CCG GAC TCG GAA TCG AAC CAA GGA CAC GGG GAT TTT CAA TCC CCT GCT CTA CCG ACT GAG CTA TCC GGG C Asn QUU AAA TGG CGG AAG CGC AGA GAT TCG AAC TCT GGA ACC CTT TCG GGT CGC CGG TTT TCA AGA CCG GTG CCT TCA ACC GCT CGG CCA CAC TTC C GGG CGG AGA GAC TCG AAC TCC CAA CAC CCG GTT TTG GAG ACC GGT GCT CTA CCA ATT GAA CTA CGC CCC T Gln CUG AAA TGG TGG AGC TAT GCG GGA TCG AAC CGC AGA CCT CCT GCG TGC AAA GCA GGC GCT CTC CCA GCT GAG CTA TAG CCC C Glu SUC AAA TGG TGC ATC CGG GAG GAT TCG AAC CTC CGA CCG CTC GGT TCG TAG CCG AGT ACT CTA TCC AGC TGA GCT ACG GAT GC Gly CCC AAA TGG CGC GCC CGA CAG GAT TCG AAC CTG AGA CCT CTG CCT CCG GAG GGC AGC GCT CTA TCC AGC TGA GCT ACG GGC GC Gly GCC AAA TGG CGC GCC CTG CAG GAT TCG AAC CTG CGG CCC ACG ACT TAG AAG GTC GTT GCT CTA TCC AAC TGA GCT AAG GGC GC Gly {CC AAA TGG TGT CCC CTG CAG GAA TCG AAC CTG CAA TTA GCC CTT AGA AGG GGC TCG TTA TAT CCA TTT AAC TAA GAG GAC His QUG AAA TGG CTC CTC TGA CTG GAC TCG AAC CAG TGA CAT ACG GAT TAA CAG TCC GCC GTT CTA CCG ACT GAA CTA CAG AGG A Ile GAU AAA TGG CGG AAC GGA CGG GAC TCG AAC CCG CGA CCC CCT GCG TGA CAG GCA GGT ATT CTA ACC GAC TGA ACT ACC GCT CC Ile }AU AAA TGG CTG GGG TAC GAG GAT TCG AAC CTC GGA ATG CCG GAA TCA GAA TCC GGT GCC TTA CCG CTT GGC GAT ACC CCA Ini CAU AAA TGG CGT CCC CTA GGG GAT TCG AAC CCC TGT TAC CGC CGT GAA AGG GCG GTG TCC TGG GCC TCT AGA CGA AGG GGA C Leu BAA AAA TGG AGC GGG AAA CGA GAC TCG AAC TCG CGA CCC CGA CCT TGG CAA GGT CGT GCT CTA CCA ACT GAG CTA TTC CCG C Leu CAG AAA TGG AGC GGG CAG CGG GAA TCG AAC CCG CAT CAT CAG CTT GGA AGG CTG AGG TAA TAG CCA TTA TAC GAT GCC CGC

  order to use again the flow through for other individual tRNA isolation, residual probe need to be removed via DNase I digestion. Because 5X SCC prevent DNase I activity, samples were first precipitated (see the Method section Chapter II, C, II, b, 3: section RNA/DNA precipitation) and resuspended in 44µL. Then 100 U DNase I and 10X DNase I buffer were added to a final concentration of 1X DNase I buffer and incubated for 1 hour at 37 °C. Enzymes and buffer were finally removed by the MEGAclear™ Kit according to manufacturer's instruction. Only the volume of Ethanol during RNA binding step was increased to 675 µL in order to keep RNA lower than 100 nt. Sample could then be used for another individual tRNA isolation.

Fluorescein

  signal was directly visualized on the Typhoon 9400 device. For nucleic acid signal visualization, gels were stirred for 20 minutes in GelRed TM 1X staining solution diluted in 1X TBE and the signal was measured on the Typhoon 9400 device. The parameters for Fluorescein signal detection are: excitation 532 nm, emission filter 526 SP while the one for GelRed TM signal measurement are excitation 532 nm, emission filter 610 BP30. individual tRNA samples were digested into nucleosides thanks to an enzyme cocktail. 150-300 ng of RNA were incubated overnight at 37°C in 25 mM NH4OAc (pH 7.5) with the following enzymes: 10 U of Benzonase nuclease, 2 U of thermosensitive Alkaline Phosphatase (Fast AP), 0.6 U of nuclease P1 (NP1), 0.2 U of snake venom phosphodiesterase (PDE) and 200 ng of Pentostatin (PS) and 500 ng of tetrahydrouridine (THU) which are adenosine and cytidine deaminase inhibitors respectively. b. LC-MS/MS 40 ng of digested RNA were mixed with 10 ng of 13 C S. cerevisiae total tRNA digest: internal standard (ISTD) and used for LC-MS/MS measurement in order to quantify the level of 2'Omethylations in the samples. One sample contained only 10 ng of ISTD for subsequent normalization during analyses. Analysis of the samples was performed with an Agilent 1260 Infinity system with binary pump in combination with an Agilent 6460 triple quadrupole (QQQ) mass spectrometer with an electrospray ion source (ESI). In the LC-part, a C18 reverse phase HPLC column (Synergi™ 4 µm Fusion-RP 80 Å, LC Column 250 x 2 mm, Phenomenex) was used at a temperature of 35 °C. A rising gradient of 100% LC-MS grade acetonitrile and a 5 mM NH4OAc buffer (pH 5.3, adjusted with acetic acid)

Figure 37

 37 Figure 37 LC gradient The four main nucleosides were detected photometrically at 254 nm whereas detection of the RNA modifications was conducted via QQQ in the positive ion mode. The parameters of the electrospray ion source (Agilent Jet Stream ESI) are: gas temperature 350°C, gas flow 8 L/min, nebulizer pressure 50 psi, sheath gas temperature 350°C, sheath gas flow 12 L/min, capillary voltage 3,000 V, nozzle voltage 500 V.

  12𝐶)÷𝑑𝑀𝑅𝑀(13𝐶) 𝑈𝑉(𝐴)-𝑈𝑉(𝐴)𝐼𝑆𝑇𝐷. dMRM ( 12 C) or ( 13 C) correspond to values for the area under the curve for 12 C (Sample) and13 C molecules (ISTD), UV (A) and UV(A) ISTD corresponds to the peak area of Adenosine in the UV chromatogram of the analysed sample and alone ISTD sample respectively. For comparison between control and stress conditions, control values were reported to 100 % and stress values were subjected to this calculation:

  . Samples were denaturated 3 min at 90°C and hybridized for 10 min at 65°C. Measurement was performed on a MonolithTM NT.115 instrument (NanoTemper Technologies) using Monolith NT.115 Capillaries where samples where added. Experiments were performed at 25°C with the blue laser. MST setting were: LED power 60% (the power of the measuring laser), MST Power 40% (the power of the infrared laser heating the sample), Fluorescence before active laser 5s, MST on 45s (the time the infrared laser was active), Fluorescence after active laser 15s and Delay 25s (the time between measurements).

Figure 39

 39 Figure 39 Composition des exosomesLes exosomes sont composés d'une bicouche lipidique contenant des protéines transmembranaires telles que les tetraspanines, les intégrines and plusieurs composant lipidiques (céramide, cholestérol ou sphingomyéline). A l'intérieur des exosomes on retrouve les composants du ESCRT comme Tsg101 et Alix, des protéines du cytosquelette ou de transduction du signal.

Figure 42 :

 42 Figure 42: Structure des différents isoformes de l'ARN humain Y Dans les cellules humaines, l'ARN Y possède quatre isoformes, hY1, hY3, hY4 et hY5, composées d'un domaine de boucle conservé, d'une tige supérieure impliquée dans la réplication de l'ADN, d'une tige inférieure et d'une queue de polyuridine impliquée dans l'interaction avec les protéines Ro60 et La jouant un rôle dans la stabilité et le contrôle de la qualité des ARN.

Figure 43

 43 Figure 43 Modification de l'ARNt de E. coli : positions et enzymes impliquées k. Focus sur les 2'O-méthylations

  Chez la levure, m5 C et m 7 G sont des modifications impliquées dans la réponse au stress oxydatif (100-102). Le laboratoire de Peter Dedon a indiqué, avec une méthode élégante de détection et de quantification de la modification de l'ARNt par LC-MS/MS, que S. cerevisiae Cm, m 5 C et m 2 2G augmentaient après exposition à l'agent H2O2 mais pas en réponse à NaOCl: un autre agent oxydant induisant un stress (103). De plus, lorsque la souche est déplétée de l'enzyme responsable des modifications impliquées dans la réponse au stress oxydatif, on retrouve une hypersensibilité cytotoxique à ce stress. Ainsi, les modifications de l'ARNt sont impliquées dans la survie des cellules après le stress des espèce réactives à l'oxygène en jouant un rôle dans les voies de mécanismes spécifiques. Au-delà du stress oxydatif, cette méthode très intéressante à haut débit a signalé un effet sur le niveaude 23 modifications sur 25 analysées en réponse aux substances toxiques utilisées telles que NaAsO2 (toxicité de l'arsénite) ou MMS (dommages causés par l'ADN) (103). Ceci démontre clairement l'importance de la modification des ARNt en réponse à un événement de stress. Lorsque le dynamisme de modification est observé dans la boucle anticodon de l'ARNt et plus particulièrement à la 34e position (« wobble base »), telle que m 5 C et mcm 5 U en condition de stress chez S. cerevisiae, il a été démontré que ce comportement était crucial pour la traduction biaisée par les codons. L'idée est que des modifications spécifiques régulent l'expression des gènes. En effet, ces MoTT (« modification tunable transcripts » modification de transcrits accordables) peuvent influencer les schémas d'utilisation des codons. Le laboratoire de Peter Dedon a démontré que les modifications au niveau de la « wobble base » sont coordonnées avec une reprogrammation spécifique au stress conduisant à la traduction des protéines impliquées dans la réponse au stress (figure

Figure 45

 45 Figure 45 Modèle pour la régulation de la traduction par les MoTTLorsque les transcrits sont modifiés, un pool de protéines est traduit. Si les ARNt sont des moTT (transcrits modifiés accordables), un autre pool de protéines est traduit. En condition de stress, ces modifications peuvent être augmentées et conduire à la production plus importante des protéines correspondantes. Sans modification de l'ARNt sur les moTT, moins de protéines correspondantes sont produites. Ainsi, en fonction du niveau de modification sur les moTT, la traduction est régulée.XIII. Objectifs

Figure 46

 46 Figure 46 Heatmap global représentant les Methscores pour chaque 2'O-méthylation étudiée en conditions de stress : privation de nutriments et en conditions antibiotiques en réplicats biologiques

Figure 47

 47 Figure 47 Heatmap représentant le rétablissement du phénotype contrôle

Figure 48

 48 Figure 48 Quantification relative des 2'O-méthylations et pseudouridines par spectrométrie de masse sur échantillons d'ARNt totaux Les valeurs MS ont été rapportées entre 0 et 1. 1 représente la valeur de MS la plus élevée. Les cercles, triangles et rectangles noirs représentent respectivement les 1ers, 2 e et 3 e réplicats biologiques. Chaque échantillon a été réalisé en duplicat technique.

Figure 49

 49 Figure 49 Comparaison des proportion relatives entre expériences de spectrométrie de masse et de séquençage à haut débit sur plusieurs 2'O-méthylations montrant un dynamisme en conditions de stress Le panneau de gauche représente les proportions relatives des données de spectrométrie de masse tandis que celui de droite représentes les données de séquençage à haut débit. Chaque point noir correspond à une réplicat biologique.

Table of content:

Table 6

 6 Yield of individual tRNA purified by biotin/streptavidin isolation protocol ..............

Table 7

 7 Individual tRNA yield in NH2/NHS isolation method ................................................Table 8 tRNA distribution in Sequencing and MST experiment for four given tRNA ...........

Table 9

 9 Chemicals, reagents, enzymes and ready-to-use buffers.............................................

Table 10

 10 Buffers and medium ..................................................................................................

Table 11

 11 Oligonucleotides used for individual tRNA isolation and relative quantifications by

Table 1

 1 Comparison between sequencing by ligation, sequencing by nucleotide addition and Illumina sequencing

Whatever the technique used for 2 nd generation of sequencing, reads are short (below 300 bp) and an amplification step is required. This amplification can lead to several uncontrolled biases due to polymerase errors or unequal amplification of fragments. That is why, it is crucial to develop new methods in this field in order to improve sequencing in term of accuracy, efficiency, sensitivity and cost.

Table 2

 2 Published studies of human blood exRNA characterization.Twelve studies on human blood exRNA characterization were compared by different parameters: type of sample and treatment, RNA extraction method, RNA sequencing and analysis method.

	Galvanin et al, Biochimie. 2019 (208)

Sample type Blood treatment RNA extraction methods RNA seq methods Studied RNA Majors species found Comments/Potential issues Reference

	Exosomes from	Exoquick	Qiagen miRNeasy	Illumina sequencing	miRNA,	miRNA,	Kits based on precipitation	Huang et al,
	plasma	(System	micro kit	(NEBNext® Multiplex Small RNA	rRNA, lncRNA, DNA,tRNA,	rRNA	include a lot of soluble	2013 (197)
		Biosciences)		Library Prep Set for Illumina and	miscRNA, piRNA, mRNA, snRNA,		extracellular RNA
		+/-RNase A		NEXTflex)	snoRNA, other
		treatment		-RNA sequencing kit (Bioo Scientific)			RNA from RNP or
				-TruSeq Small RNA sample preparation			lipoproteins are not
				kit (Illumina)			sensitive to RNase A
				Size selection of the libraries 140-160 bp
	Exosomes from	Total exosome	Total exosome RNA	Ion proton system (Ion Total RNA-Seq	miRNA, tRNA, rRNA, mRNA,	miRNA,	Kits based on precipitation	Li et al, 2014
	serum	isolation (from	and protein isolation kit	Kit v2 [Thermofischer])	snRNA, scaRNA, snoRNA, piRNA	mRNA, tRNA	include a lot of soluble	(193)
		serum) reagent	(Invitrogen)				extracellular RNA
		(Invitrogen)
	Plasma	N.A.	Exiqon miRCURY	Ion proton system (Ion Total RNAseq kit	rRNA removal	miRNA,		Freedman et
			RNA Isolation Kit-	v2 [Thermofischer])		piRNA,		al, 2016 (195)
			biofluids		miRNA, tRNA, snoRNA, piRNA,	snoRNA
					and other RNA RFam database
	Exosomes from	Exoquick	Qiagen miRNeasy	Illumina sequencing (NEBNext®	miRNA, piRNA, siRNA, lncRNA,	miRNA,	Kits based on precipitation	Yuan et al,
	plasma	(System	micro kit	Multiplex Small RNA Library Prep Set	mRNA, pseudogenes, antisense	piRNA	include a lot of soluble	2016 (192)
		Bioscience)		for Illumina)	RNA, rRNA, snRNA, snoRNA,		extracellular RNA
		+ RNase A		Size selection of the libraries 140-160 bp	misc RNA other transcribed RNA,
		treatment			FLJ human cDNA and predicted		RNA from RNP or
					RNA		lipoproteins are not
							sensitive to RNase A
	Serum	N.A.	-Qiagen Circulating	Illumina sequencing (TruSeq Small RNA	miRNAs, isomiRs, tDRs and	osRNAs,	Serum: include RNA from	Guo et al,
			Nucleic Acid Kit,	sample preparation kit)	osRNAs	tDRs, miRNA	platelets secretion: affects	2017 (194)
			-TRIzol LS Reagent,	Size selection of the libraries 5-40 bp		(including	the composition of exRNA
			-Qiagen miRNeasy			isomiR)	originally present in
			Serum/Plasma kit,				circulating blood
			-QiaSymphony RNA
			extraction kit
			-Exiqon miRCURY
			RNA Isolation Kit-
			biofluids

Sample type Blood treatment RNA extraction methods RNA seq methods Studied RNA Majors species found Comments/Potential issues

								Reference
	EV from plasma Differential	Total RNA purification	Ion proton system (Ion Total RNA-Seq	rRNA removal	mRNA	Ultracentrifugation on	Amorim et al,
		centrifugation,	kit (Norgen Biotek)	Kit v2 [Thermofisher])			viscous plasma procedure	2017 (170)
		ultracentrifugat			pseudogenes, mRNA, lncRNA,		can co-pellet soluble
		ion	RNA fragmentation by		miRNA, tRNA, misc RNA, other		extracellular RNA
			RNase III
	Whole platelet	Differential	TRIzol reagent (Life	SOLiD sequencing (SOLiD total RNA-	Mitochondrial RNA, human	rRNA,	Ultracentrifugation on	Savelyeva et
	poor plasma,	centrifugation	Technologies)	Seq kit [Life Technologies])	genomic repeat consensus	mRNA,	viscous plasma procedure	al, 2017 (94)
	16,000xg	procedure			sequences, tRNA, rRNA, snRNA,	mitochondrial	can co-pellet soluble
	vesicles,		dephosphorylation/		small cytoplasmic RNA	RNA (for	extracellular RNA
	160,000xg		phosphorylation			plasma and
	vesicles and		treatment			16,000xg
	particles					vesicles only)
	depleted in		size selection on a gel
	plasma		for RNA > 19nt
	Plasma	N.A.	Thermofisher mirVana	Illumina sequencing (TruSeq Small RNA	rRNA,	Y RNA,		Yeri et al,
			miRNA Isolation kit	sample preparation kit)	miRNA, tRNA, piRNA, RNA from	miRNA		2017 (191)
				Gel purification: removal of dimers	ENSEMBL75 database, non-human
					RNA
	Plasma	+/-Proteinase	Exiqon miRCURY	Illumina sequencing (NEBNext®	rRNA,	rRNA,		Danielson et
		K treatment	RNA Isolation Kit-	Multiplex Small RNA Library Prep Set	miRNA, tRNA, snoRNA, piRNA	miRNA,		al, 2017 (199)
			biofluids	for Illumina)		tRNA
				Size selection of the libraries: RNA of
				21-40 nt
	Plasma and	Detergent and	Personalized	Illumina sequencing (personalized library	miRNA, rRNA, tRNA, small	Plasma:		Max et al,
	serum	Proteinase K	phenol/chloroform	preparation including RNA size selection)	cytoplasmic RNA, other	miRNA,		2018 (196)
		treatment	procedure		cytoplasmic RNA, mRNA	mRNA, rRNA
						Serum:
						miRNA,
						scRNA, rRNA
	Abbreviations:

isomiRs-micro

RNA isoforms, lncRNAlong non-coding RNA, miRNAmicro RNA, miscRNAmiscellaneous RNA, mRNAmessenger RNA, osRNAsother miscellaneous RNA, piRNApiwi RNA, rRNAribosomal RNA, siRNAsmall interfering RNA, snoRNAsmall nucleolar RNA, snRNAsmall nuclear RNA, tDRstransfer-derived small RNA, tRNAtransfer RNA

  Crystal structures of TrmH bound or not to SAM have been established by extensive mutagenesis studies. Catalytic site and enzyme reaction mechanism have also been characterized. Four amino acids of the catalytic site (Asn35, Arg41, Glu124 and Asn152) are involved in tRNA binding. In addition, Asn35 is also responsible for SAM release(278,279,282).

	TrmJ (or YfhQ or TrMet5Xm32) is involved in the formation of Cm32 in tRNA Ser 1 and
	tRNA Gln 2 (283). This enzyme also belongs to the SPOUT class of MTases (284). The 3D
	structure and catalytic site of E. coli TrmJ have been characterized. tRNA recognition by TrmJ
	involves not only SPOUT domain, but also additional parts of the protein (285).
	TrmL (yibK) is responsible for 2'-O-methylation at position 34 in two E. coli tRNA Leu
	(anticodons CAA and UAA). Cm34 is formed in tRNA Leu (CAA) and Um34 is introduced in an
	hypermodified nucleotide in another isoacceptor tRNA Leu (cmnm5UmAA) (286). TrmL
	methylates only pyrimidine residues at position 34 and requires 2-methylthio-N 6 -
	isopentenyadenosine (ms 2 i 6 A) at position 37 for activity (287).

2'O-methylation affects the physico-chemical properties of the modified nucleoside. It does not change the base-pairing even if stabilization of the typical A-type RNA helix is observed (288,289) but it increases the stability of the nucleotide against alkaline (and even enzymatic) hydrolysis (290,291). Indeed, methylation at the 2'OH of the ribose prevent one hydrogen bound and abolishes de nucleophilic property of the O2'. In addition, tRNA modification are important for specific recognition between the tRNA acceptor and its cognate aminoacyl-tRNA synthetase (aaRS) (264). Another function for 2'O-methylation in position 18 in most of gramnegative bacteria, is its property of immunosuppressor. Indeed, Gm18 has been demonstrated to escape the innate immune system by inactivating Toll-like receptor (TLR) that are involved in the innate immune response

(268)

Table 3

 3 Stress conditions for the first screeningNumber(1,2,3) represent replicates for the same stress/ Letter (A or B) corresponds to two different protocols for a given stress/ H represents the conditions with a decrease level of oxygen (Hypoxia like condition)

b.

tRNA isolation and RiboMethSeq protocol

  To measure the level of 2'O-methylation in E. coli tRNA by high-throughput sequencing RiboMethSeq protocol, appropriate total tRNA isolation procedure is essential. In order to provide the optimized total tRNA isolation protocol we performed the comparative analysis of In this work we demonstrated that the best method for isolation of total E. coli tRNA preparations compatible with downstream NGS applications is the TRIzol protocol on harvested bacterial cells. However, this method is not suitable neither for human tRNA, for which it gives a total RNA (not just tRNA) extraction, nor for S. cerevisiae tRNAs, since a highly biased composition of tRNA fraction was observed. TRIzol tRNA extraction from E. coli cells gives high purity and unbiased tRNA composition with a low cost. Indeed, the global proportions of individual tRNA species are not affected compared to a classical total RNA extraction protocol, such as acid phenol extraction.

	this study were published in the manuscript entitled "Mapping and Quantification of tRNA 2'-
	O-Methylation by RiboMethSeq" appeared in Methods in Molecular Bioliology (Springer) in
	2019 (364) (annexe 2).
	Sample	Type of Stress		Seeding OD600nm	Growing time	Final OD600nm	Comments
	LB 37°C control 1				0.05	3h	1.3
	LB 37°C control 2	/			0.05	3h	1.3
	LB 37°C control 3				0.05	3h	1.3
	LB 20°C	Temperature: 20°C		0.05	24h	1.4
	LB 42°C	Temperature: 42°C		0.05	3h	0.83
	M9 37°C	Nutrient level: minimal medium	0.05	24h	1.3
	M9 20°C	Nutrient level: minimal medium Temperature: 42°C	0.05	48h	1.26
	LB 37 H A LB 37 H 1 B LB 37 H 2 B LB 37 H 3 B	Hypoxia like condition		0.05 0.02 0.02 0.02	3h 24h 24h 24h	0.83 0.35 0.36 0.37	E. coli was grown in a closed bottle under agitation E. coli was grown in a closed full falcon without any agitation
	LB 20 H A	Temperature: 20°C		0.05	26h	1.6	E. coli was grown in a closed bottle under agitation
	LB 20 H 1 B	Hypoxia like condition		0.02	24h	0.6	E. coli was grown in a closed full
	LB 20 H 2 B				0.02	24h	0.63	falcon without any agitation
		Nutrient level: minimal
	M9 37°C H A	medium	Hypoxia	like	0.05	24h	0.8
		condition						E. coli was grown in a closed bottle
		Nutrient level: minimal				under agitation
	M9 20°C H A	medium Temperature: 20°C	0.05	48h	1.06
		Hypoxia like condition
	Starvation 1				0.05	3h	1.2	E. coli was grown in LB37°C until
	Starvation 2	Nutrient level: starvation		0.01	7h	1.4	exponential phase, harvest and pellet was resuspended in PBS for 24h
	Streptomycin 1 A				0.05	3h	0.8
	Streptomycin 2 A				0.01	7h	1.4	E. coli was seeded at 0.01 or 0.05
	Spectinomycin 1 A				0.05	3h	0.76	OD600nm in antibiotic concentration leading to growth delay of around
	Spectinomycin 2 A	Antibiotic			0.01	7h	1.6	30%
	Streptomycin B				0.01	5h30	0.9 → 2.6	E. coli was grown in control condition until exponential phase,
								antibiotic was then added in a final
	Spectinomycin B				0.01	5h30	0.82 → 2	concentration leading to a growth
								delay of around 90%

available methods dedicated to tRNA isolation. We compared these approached in term of overall yield, purity and cost for E. coli, S. cerevisiae and human tRNA samples. The results of After tRNA extraction, samples are submitted to RiboMethSeq protocol in order to measure 2'O-methylation level under control and stress conditions. The RiboMethSeq principle was explained in the section (chapter II, A, III, b : a. Specific detection of 2'O-methylation) and is also described in this article as well as in

Marchand et al, 2017 (357)

.

Table 4

 4 Stress conditions for biological replicates

Table 5

 5 Determined DNA/RNA ratios for the four studied tRNA

	tRNA	Tot tRNA amount for 100% hybridization
		with 2 pmol of oligonucleotide
	tRNA Leu (cmnm 5 UmAA)	3,5 µg
	tRNA Ser (cmo 5 UGA)	3,5 µg
	tRNA Leu (CmAA)	6 µg
	tRNA Ser (CGA)	> 12 µg

Table 6

 6 Yield of individual tRNA purified by biotin/streptavidin isolation protocol

	tRNA	Amount of individual tRNA isolated from 100
		µg of total tRNA pool
	tRNA Leu (cmnm 5 UmAA)	600-700 ng
	tRNA Ser (cmo 5 UGA)	700-800 ng
	tRNA Leu (CmAA)	425-450 ng
	tRNA Ser (CGA)	90-225 ng
	1. Setting up DNA-NH2/NHS isolation protocol

Table 7

 7

	Individual tRNA yield in NH2/NHS isolation method
	tRNA	Amount of individual tRNA isolated from 1
		mg of total tRNA pool
	tRNA Leu cmnm5UmAA	400-500 ng
	tRNA Ser cmo5UGA	250-350 ng
	tRNA Leu CmAA	350-700 ng
	tRNA Ser CGA	250-400 ng

  in the total tRNA pool) measured by deep sequencing and by MST. From the first glance, it is clear that correlation is very limited for MST data compared to deep sequencing. Some tRNA are under-represented while other are over-represented, which is expected results since the two methods are different. Moreover, similar results were described inJacob et al, 2019 (367) for analysis of S. cerevisiae total tRNA fractions. There are several reasons for such discrepancies. First, library preparation certainly induces biases in measured tRNA distribution by differential amplification efficiency according to tRNA species

	1. Global tRNA distribution in E. coli
	First, we compared two techniques, deep sequencing and MST to measure the global tRNA
	distribution in E. coli strain grown under normal conditions (LB 37°C). Culture, total tRNA
	isolation and MST experiments were performed by Christina Dal Malgro and Felix Green from

Pr Helm's group, while I prepared sequencing libraries from the same samples, the NGS core facility UMS 2008 IBSLor performed sequencing and Pr Yuri Motorin extracted raw tRNA counts data for further analysis. The figure 35 represents the tRNA proportion (% of each individual tRNA

Table 8

 8 tRNA distribution in Sequencing and MST experiment for four given tRNA Experiment were performed with three biological triplicates, the average is represented here. The same samples were used for both sequencing and MST experiment.

		Sequencing		MST
	tRNA	control spectinomycin Fold change tRNA %	control spectinomycin Fold change tRNA %
	Leu cmnm 5 UmAA	1.9	4.6	2.42	2.05	4.23	2.06
	Ser cmo 5 UGA	1.9	1.1	0.58	1.63	0.74	0.45
	Leu CmAA	0.98	1.1	1.12	0.61	0.69	1.13
	Ser CGA	0.22	0.15	0.68	0.42	0.25	0.59

Table 9

 9 Chemicals, reagents, enzymes and ready-to-use buffers

	Product	Manufacturer
	10 X TBE buffer	Carl Roth
	Acetonitrile	Honeywell
	Benzonase® Nuclease	Sigma
	Chloramphenicol	Euromedex
	Fast AP	Thermo Fisher Scientific
	Gel buffer concentrate	Carl Roth
	Gel concentrate	Carl Roth
	Gel diluent	Carl Roth
	Gentamycin	Sigma
	InSolution Tetrahydro uridine (THU)	Merck
	N,N,N,N'-Tetramethyl ethylenediamine (TEMED)	Carl Roth
	Nuclease P1	Sigma
	Pentostatine >95%	Sigma
	Snake Venom Phosphodiesterase	Worthington Biochemical Corporation
	Spectinomycin	Sigma
	Streptomycin	Sigma

Table 10

 10 Buffers and medium

	Product	Composition
	1X B&W buffer	5 mM Tris-HCl (pH 7,5), 0.5 mM EDTA, 1 M NaCl
	1X PAGE	Na 2 HPO4*12H 2 O 8.1 mM; KH 2 PO4 1.47 mM, NaCl 137 mM; KCl 2.7 mM; pH 7.2-7.4
	20X SSC	3M NaCl, 300 mM trisodiumcitrate (pH 7,0)
	APS	10% APS (m/v) in Milli-Q water
	Buffer A/ blocking buffer	ethanolamine 0.5 M, NaCl 0.5 M, pH 8.3
	Buffer B/ wash buffer	0.1 M sodium acetate, 0.5 M NaCl, pH 4 with acetic acid
	Chloramphenicol	stock solution 10 mg/mL in ethanol
	Gel Elution buffer	NH4oAc 0.5M pH 5.3
	Gentamycin	stock solution 10 mg/mL in water
	LB medium	Tryptone 10 g/L, Yeast extract 5 g/L, NaCl 10 g/L
	Loading dye	90% (v/v) formamide in 1X TBE
		Na2HPO4 33.7mM, KH2PO4 22.0 mM, NaCl 8.55 mM, NH4Cl 9.35 mM, glucose
	M9 medium	0.4%, MgSO4 1mM, CaCl2 2.3 mM, thiamine 1 µg/mL, FeCl3 1mM, MnCl2 1mM, ZnSO4 1mM, CoCl2 0.2 mM, CuCl2 0.2 mM, NuCl2 0.2 mM, Na2Mo 0.2 mM,
		Na2Mo 0.2 mM, Na2Se 0.2 mM, H3BO3 0.2 mM
	Oligonucleotide Coupling buffer	0.2 M NaHCO3, 0.5 M NaCl pH 8.3
	Spectinomycin	stock solution 10 mg/mL in water
	Storage buffer/phosphate buffer	50 mM Na2HPO4 pH 7.1, NaN3 0,1%
	Streptomycin	stock solution 10 mg/mL in water
	tRNA binding buffer	1.2 M NaCl, 30mM Hepes KOH pH 7.4
	tRNA elution buffer	1 mM EDTA, 1 mM HEPES
	wash buffer	10 mM HEPES, 100 mM NaCl

Table 11

 11 Oligonucleotides used for individual tRNA isolation and relative quantifications by MST experimentTable 12 Oligonucleotides used for global tRNA distributin determination by MST experiments

Table 13

 13

	QQQ settings
	Modification	Precursor ion (m/z)	Product ion (m/z)	Retention time (min)	Retention time window (min)	Fragmentor voltage (V)	Collision energy (V)	Cell accelerator voltage (V)
	Cm	258	112.1	9.3	3	60	9	2
	Cm 13C	268	116.1	9.3	3	60	9	2
	Gm	298	152	12.4	3	72	5	2
	Gm 13C	309	157	12.4	3	72	5	2
	Um	259	113	11.3	2	66	5	2
	Um 13C	269	117	11.3	2	66	5	2
	Pseudouridine	245	209	3.8	2	81	5	2
	Pseudouridine 13C	254	218	3.8	2	81	5	2

Table 14

 14 Parameters for the preparation of samples for MST experiment

	tRNA	Probe amount (pmol)	Tot tRNA amount (pmol)
	All individual tRNA in DH5α strain for global tRNA distribution experiment	0.5	0.004-20
	Leu cmnm 5 UmAA	0.4	0.008-4
	Ser cmo 5 UGA	0.4	0.008-4
	Leu CmAA	0.2	0.008-4
	Ser CGA	0.1	0.008 -10

  Depuis une petite dizaine d'années, le séquençage à haut débit est devenu une technique très puissante, sensible et précise pour l'étude des acides ribonucléiques (ARN). Pendant ma thèse de doctorat, nous avons utilisé cette technologie pour la caractérisation en profondeur des ARN extracellulaire (ARNex) provenant du plasma humain. Les ARNex du plasma peuvent être retrouvés soit à l'état « soluble » sous forme de complexes ribonucléoprotéiques (RNP) ou encapsulés au sein de vésicules extracellulaires (VE) de diverses origines (exosomes, microvesicles, …). Dans ce projet, j'ai démontré que le plasma entier contenait principalement des micro ARN et le fragment hY4 ainsi que des ARN ribosomiques dégradés. Par ailleurs, suite à une stratégie rigoureuse via chromatographie à exclusion de tailles ou par traitement consécutif à la protéinase K et à la RNase A, des EV hautement purifiés peuvent être obtenus. Les Micro ARN et le fragment d'ARN hY4 ne sont pas présents en majorité dans ces échantillons, démontrant une grande différence de composition entre les ARNex solubles et ceux des vésicules purifiées. Le contenu ARN de ces VE reflète principalement une composition en ARN du microbiote humain. Par ailleurs, j'ai également effectué une étude comparative de kits commerciaux pour l'enrichissement des exosomes qui sont supposés purifier les exosomes par précipitation. La composition en ARN de ces fractions est très similaire au plasma humain total, montrant une forte contamination non contrôlée par les RNP solubles. Sur la base de cette étude, nous sommes en mesure de proposer un protocole pour l'études des ARNex dans le cadre de biopsies liquides avec des échantillons cliniques afin de découvrir de potentiels biomarqueurs de diagnostic.Au-delà de la caractérisation d'ARN, le séquençage à haut-débit peut être utilisé pour la détection et quantification des modifications post-transcriptionnelles. Pendant ma thèse, j'ai utilisé le séquençage en profondeur pour l'analyse des 2'Ométhylation des ARN de transfert (ARNt) chez le model bactérien E. coli par RiboMethSeq. Sous plusieurs conditions de stress telles que le manque de nutriments ou des concentrations non létales d'antibiotiques, certaines 2'O-méthylations montrent une réponse adaptative. Alors que plus de la moitié des 2'O-méthylation en position 18 sont augmentées dans toutes les conditions de stress étudiées, les résidus 2'O-methylés à la positon 34 eux montrent un effet opposé avec une diminution dans certains stress comme le chloramphénicol et la streptomycine. Chacun de ces deux comportements dynamiques peut être relié à un phénomène de régulation cellulaire en réponse au stress. Un changement au niveau de la « wobble base » (position 34) pourrait être un moyen de réguler la traduction en modifiant l'usage des codons. En ce qui concerne la modification Gm18, son rôle dans l'évasion du système immunitaire inné lors de l'invasion d'un hôte est en cours d'élucidation.

	Keywords: high-throughput sequencing, extracellular RNA, extracellular vesicles, tRNA modifications; 2'O-methylation
	Résumé :

  . Les techniques utilisées pour la caractérisation des ARNex étaient la « microarray » mais cette méthode nécessite le design de sondes spécifiques pour des cibles connues. Il n'est pas possible de déterminer la présence d'ARN non attendus. De nos jours, le séquençage à haut débit permet de pallier ce problème par le séquençage et l'identification de tous les ARN présents dans l'échantillon. Depuis une dizaine d'année, plus d'une douzaine d'articles ont décrit le contenuen ARNex du plasma/sérum ou de leur respectives VE(24,(37)(38)(39)(40)(41)(42)(43)(44)(45)(46). Utilisant le séquençage en profondeur, quasiment toutes les espèces d'ARN sont retrouvées dans le sang et leurs VE :ARNm, ARNr, ARNt, miARN, piwi ARN, ARN Y, … Cependant, leur proportion peut varier dramatiquement d'une étude à une autre (tableau 1). Un exemple parmi d'autre est la différence de proportion pour les ARNm et miARN qui sont retrouvés à 24,5% et 8,8% respectivement (

VII. Design de l'étude g. Source du liquide biologique

  Tableau 1 Etudes publiées sur la caractérisation des ARNex humains du plasma/sérum ou leur respective EV Douze études sur la caractérisation des ARNex du plasma/Sérum ont été comparée selon plusieurs critères : type d'échantillon et traitements, méthode d'extraction des ARN, séquençage des ARN et analyse ARNlnc -ARN long non codant, miARNmicro ARN, miscARNmiscellaneous ARN, ARNm -ARN messager, osARN -« other miscellaneous » ARN, ARNr -ARN ribosomiques, siARN -« small interfering » ARN, snoARN -« small nucleolar » ARN, snARN -« small nuclear » ARN, tDR -« transfer-derived » petits ARN, ARNt -ARN de transfertVI. ObjectifsComme mentionné précédemment, les ARNex peuvent être retrouvés dans tous les liquides biologiques et montrent un fort intérêt dans le domaine des biopsies liquides. Nous avons décidé Les méthodes de prétraitement du sang les plus courantes consistent à préparer des fractions plasmatiques ou sériques. Le plasma est obtenu par centrifugation en présence d'anticoagulants, tandis que le sérum est préparé par centrifugation d'échantillons coagulés. Techniquement, travailler avec du sérum est plus facile, mais il est connu que les plaquettes libèrent un grand nombre de miARN contenant des VE à la suite de la coagulation(51), ce qui a un effet considérable sur la composition des ARNex initialement présent dans le sang en circulation. Cependant, les produits de dégradation probablement présents dans le plasma entier ou dans les exosomes ont des extrémités 5'-OH et 3'-P (ou 3'-cyclophosphate). Ainsi, nous avons décidé d'inclure ces espèces d'ARN dans l'analyse en ajoutant une étape préliminaire de déphosphorylation/phosphorylation conduisant aux extrémités adéquates pour tous les ARN présents dans l'échantillon. Sans traitement, seuls les produits de maturation sont convertis en librairies et séquencés, tandis que les produits de dégradation ne sont inclus qu'après cette étape facultative. L'objectif est de réaliser l'analyse avec ou sans étape de déphosphorylation/phosphorylation afin de comparer les produits de maturation et de dégradation sur le pool d'ARNex.Néanmoins, nous devons garder à l'esprit que les ARN eucaryotes ne sont pas seulement 5'P / 3'OH ou vice versa. Alors que l'ARNr, l'ARNt, les petits ARN impliqués dans la régulation (miARN, piwi ARN), la plupart des snoARN et des scaARN portent ces extrémités, ce n'est pas le cas pour l'ARNm, le snoARN U3 et la plupart des ARNlnc. Même si les ARNex sont

	Type d'échantillon Exosomes du plasma Type d'échantillon VE du plasma isomiARN-micro ARN isoforme , de se concentrer sur la caractérisation des ARNex du sang dû à leur fort intérêt potentiel Traitement du sang Méthode d'extraction des ARN Méthode de séquençage des ARN ARN étudiés Exoquick (System Biosciences) Qiagen miRNeasy micro kit Illumina sequencing (NEBNext® Multiplex Small RNA Library Prep Set for miARN, ARNr, ARNlnc, ADN,ARNt, Traitement du sang Méthode d'extraction des ARN Méthode de séquençage des ARN ARN étudiés Centrifugation différentielle, Total RNA purification kit (Norgen Biotek) Ion proton system (Ion Total RNA-Seq Kit v2 [Thermofisher]) Suppression des ARNr B. Résultats	Principales espèce retrouvées miARN, ARNr Principales espèce Commentaires/ problème potentiels Kits basés sur la précipitation incluent beaucoup d'ARNex solubles retrouvées Commentaires/ problème potentiels ARNm Ultracentrifugation sur le plasma visqueux peut	Référence Huang et al, 2013 (43) Référence Amorim et al, 2017
	+/-RNase A traitement ultracentrifugat ion clinique. En ayant une meilleure connaissance des ARNex, nous pourrions déterminer de Illumina and NEXTflex) -RNA sequencing kit (Bioo miscARN, piwi ARN, ARNm, snARN, Fragmentation des Pseudogènes, ARNm, ARNlnc, miARN, ARNt,			ARN des RNP ou lipoprotéines entrainer la coprécipitation d'ARNex solubles	(44)
	Scientific) -TruSeq Small RNA sample SOLiD sequencing (SOLiD total RNA-nouveaux biomarqueurs de diagnostic pour les cancers, maladies cardiovasculaires ou même snoARN, autres ARN par la RNase III miscARN, autres Plasma pauvre Centrifugation TRIzol reagent (Life ARN mitochondriaux,		ARNr, ARNm,	ne sont pas sensibles à la RNase A Ultracentrifugation sur le	Savelyeva et
	en plaquettes, 16,000xg des infections par certaines virus. différentielle Technologies)	preparation kit (Illumina) Sélection de taille des banques Seq kit [Life Technologies])	séquences consensus répétées du génome humain, ARNt,	ARN mitochondriaux	plasma visqueux peut entrainer la coprécipitation	al, 2017 (24)
	vésicules,		dephosphorylation/		140-160 bp	ARNr, snARN, petits ARN		(pour plasma et	d'ARNex solubles
	Exosomes du sérum 160,000xg vésicules et Dans ce projet, nous avons abordé la caractérisation exhaustive des ARNex à partir de plasma Total exosome isolation (from Total exosome RNA and protein isolation kit (Invitrogen) Ion proton system (Ion Total RNA-Seq Kit v2 miARN, ARNt, ARNr, ARNm, phosphorylation traitement cytoplasmiques	miARN, ARNm, ARNt 16000xg vésicules Kits basés sur la précipitation incluent beaucoup d'ARNex uniquement)	Li et al, 2014 (39)
	serum) reagent (Invitrogen) humain sain en utilisant une combinaison de méthodes d'extraction/caractérisation. Nous avons [Thermofischer]) snARN, scaARN, snoARN, piwi ARN particules déplétées dans Sélection de taille par			solubles
	Plasma le plasma Plasma déterminé la composition des ARNex à partir de plasma entier ou d'exosomes enrichis ou N.A. Exiqon miRCURY RNA Isolation Kit-biofluids Ion proton system (Ion Total RNAseq kit v2 Suppression des ARNr gel pour ARN > 19 nt N.A. Thermofisher mirVana Illumina sequencing (TruSeq Small RNA ARNr,	miARN, piwi ARN, snoARN ARN Y, miARN	Freedman et al, 2016 Yeri et al,
	purifiés.	miARN Isolation kit	[Thermofischer]) sample preparation kit) Sélection de taille des banques : enlever	miARN, ARNt, miARN, ARNt, piwi ARN, ARN de la banque de donnée			(41) 2017 (37)
				les dimères		snoARN, piwi ARN ENSEMBL75, ARN non
						et autres ARN de la humains
	Plasma	+/-Proteinase	Exiqon miRCURY	Illumina sequencing (NEBNext®	banque de donnée ARNr,		ARNr, miARN,	Danielson et
		K traitement	RNA Isolation Kit-	Multiplex Small RNA Library Prep Set	RFam miARN, ARNt, snoARN,		ARNt	al, 2017
	Exosomes du	Exoquick	Qiagen miRNeasy micro kit biofluids for Illumina)	Illumina sequencing	miARN, piwi ARN, piwi ARN	miARN, piwi	Kits basés sur la précipitation	Yuan et al, (46)
	plasma	(System		(NEBNext® Multiplex Small Sélection de taille des banques 21-40 nt	siRNA, ARNlnc,	ARN	incluent beaucoup d'ARNex	2016 (38)
	Bioscience) Plasma et sérum Détergent et	Procédure	RNA Library Prep Set for Illumina sequencing (préparation des	ARNm, pseudogènes, miARN, ARNr, ARNt, petits		Plasma: miARN,	solubles	Max et al,
		+ RNase A Proteinase K	personnalisée de	Illumina) banques personnalisée incluant une	ARN antisens, ARNr, ARN cytoplasmiques, autres		ARNm, ARNr	2018 (42)
		traitement traitement	phénol/chloroforme	Sélection de taille des banques sélection de taille)	snARN, snoARN, ARN cytoplasmiques, ARNm		ARN des RNP ou lipoprotéines
					140-160 bp	miscARN et autres		Sérum: miARN,	ne sont pas sensibles à la RNase
						transcrits,		scRNA, ARNr	A
	Sérum	N.A.	-Qiagen Circulating Nucleic Acid	Illumina sequencing (TruSeq	miARN, isomiARN,	osRNA, tDR,	Sérum : inclue les ARN sécrétées
			Kit,		Small RNA sample preparation	tDR et osARN	miARN (incluant	par les plaquettes et peut
			-TRIzol LS Reagent,		kit)		isomiARN)	influencer la composition des
	Abréviations :	-Qiagen miRNeasy Serum/Plasma kit,	Sélection de taille des banques 5-40 bp				ARNex originairement présents dans le sang
			-QiaSymphony RNA extraction kit
			-Exiqon miRCURY RNA Isolation
			Kit-biofluids
								Tableau adapté de Galvanin et al, Biochimie. 2019 (50)
		i. Inclusion des produits de dégradation
	Au cours des dernières décennies, on pensait que les exosomes n'étaient que les sacs poubelles
	de la cellule, mais cette idée a maintenant disparu au profit de la fonction de communication
	intercellulaire pour les VE. Cependant, la plupart des études réalisées sur la composition des
	ARNex par séquençage en profondeur n'incluent que des produits de maturation. En effet,

C'est la raison pour laquelle le plasma est la source privilégiée lors de l'étude des ARNex comme biomarqueurs potentiels

(52)

.

h. Traitement du plasma

Le plasma étant un fluide biologique visqueux et complexe, nous avons d'abord dû trouver la meilleure méthode pour isoler les exosomes, car de nombreux contaminants sont couramment détectés dans les procédures de kits d'ultracentrifugation ou de précipitation

(53,54)

. La première partie de ce projet consistait à comparer plusieurs méthodes d'isolement, notamment : des kits basés sur la précipitation, l'ultracentrifugation, la chromatographie par exclusion de taille. Nous avons également développé une autre méthode qui consiste à dégrader les protéines solubles dans le plasma avec un traitement à la protéinase K. Par conséquent, les ARN des RNP sont vulnérables, tandis que ceux de VE sont toujours protégés par la bicouche lipidique. En effet, la protéinase K n'est pas capable de rendre les VE perméables. Ensuite, les ARN non protégés sont dégradés par le traitement à la RNase A. Cette méthode devrait permettre une bonne pureté d'exosomes en éliminant les protéines et les contaminants RNP

(55)

. lorsque vous travaillez avec des ARNex dont la longueur est généralement inférieure à 200 nt, les kits de préparation de librairies nécessitent des extrémités 5'-P et 3'-OH. composés majoritairement de petits transcrits (<200 nt), cela ne signifie pas qu'il n'y a pas d'ARN de taille plus longue. Ainsi, nous devons être conscients que les ARNm, snoARN U3 et la plupart des ARNlnc extracellulaires potentiels sont exclus de l'analyse.

  Les espèces les plus fréquentes étaient celles du phylum des Protéobactéries, de la classe Gammaproteibacteria, suivies des Bactéroïdes et des Firmicutes. Nous avons également tenté d'aligner des séquences encore non identifiées sur une base de données ARN/ADN virale/phage afin de mieux comprendre le microbiote viral potentiel. Des séquences correspondant à des virus humains et des phages ciblant des bactéries commensales ont été détectées, les plus fréquentes étant Pandora virus et Herpes virus. Il est intéressant de noter que les ARNex de plasma total sont relativement enrichis en séquences virales humaines,

	alors que les VE hautement purifiées (chromatographie à exclusion de taille traitement
	protéinase K/ RNase A) contenaient principalement des espèces cartographiées sur des phages
	bactériens.
	IX. Discussion et conclusion
	Toutes les données concernant ce premier projet sont rapportées dans l'article intitulé «
	Diversity and heterogeneity of extracellular RNA in human plasma » publié dans BIOCHIMIE
	en mai 2019 (50). Les méthodes d'ultracentrifugation ou de précipitation montrent un profil similaire au niveau
	Dans cette étude, nous avons démontré que les ARNex étaient présents en faible quantité (entre de la taille des fragments et aussi au niveau de la composition en ARN par rapport au plasma
	1 et 8 ng d'ARN/mL de plasma de départ) et présentaient un profil similaire sur Bioanalyzer entier. La seule différence est l'augmentation de la proportion des ARN non humains qui est
	quel que soit leur traitement. En fait, les ARNex sont en majorité inférieurs à 200 nt et même inférieurs à 70 nt (figure 3). assez importante pour certains kits tels que le kit Exiqon. Ces données démontrent l'inefficacité exosomes ont été isolés par ultracentrifugation. Clairement, ceci démontre une méthode de de ces méthodes de purification des exosomes et sont en corrélation avec la littérature décrivant Ces données ont démontré que les fraction de VE hautement purifiées contiennent purification de VE inappropriée dans laquelle d'autres ARN non exosomaux ont également été comment elles contribuent à la contamination d'autres ARN et protéines non exosomaux (figure principalement des ARN non humains et que sa composition est radicalement différente de celle purifiés. Dans la littérature, plusieurs études comparatives ont mis en évidence un fort contenu 4 partie du milieu). obtenue par ultracentrifugation ou précipitation. différentiel en ARN des exosomes lorsque différentes méthodes d'isolement des vésicules
	Cependant, le profil des ARNex soumis au traitement par chromatographie à exclusion de taille extracellulaires sont utilisées sur le même échantillon biologique (58-61). Ensemble, cela
	ou traitement à la protéinase K/RNase A est complètement différent, ce qui conduit clairement L'analyse des ARN non humain a révélé la présence d'ARN bactériens et eucaryotes provenant montre la priorité d'établir une pipeline optimisée. Pour caractériser les ARNex en limitant le
	à un autre type d'ARNex qui proviennent d'exosomes. Ici, la taille des fragments d'ARN est de microbiote et même de nourriture (espèce de poissons de riz ou encore de levure de plus possible les biais, nous avons conçu chaque étape pour établir un équilibre entre les

Adapté de

Galvanin et al, May 2019, Biochimie (50)

Figure

40

Profil ARN des échantillons de plasma avec ou sans traitement particulier Les échantillons d'ARN proviennent soit de plasma humain sans traitement ou traité par des kits basés sur la précipitation des exosomes. Les échantillons ont également été traités à la protéinase K puis RNase A ou par chromatographie à exclusion de taille.

L'analyse de la taille des fragments dans le plasma entier sans traitement supplémentaire a mis en évidence la présence de deux pics majeurs à 21 et 33 nt, correspondant respectivement aux fragments des miARN et de l'ARN hY4. Lorsque les produits de dégradation sont inclus, le profil change avec un large pic de fragments de petite taille correspondent principalement aux fragments d'ARNr (figure 4 partie de gauche). généralement supérieure à 50 nt sans aucune signature de miARN ou ARN hY4. De plus, la proportion d'ARN non humain augmente considérablement par rapport au plasma total non traité (figure 4 partie de droite). Adapté de Galvanin et al, May 2019, Biochimie (50) Figure 41 Vue d'ensemble de la composition en ARNex du plasma total ou de VE enrichies ou purifiées La partie de gauche correspond aux échantillons d'ARNex de plasma entier sans traitement, la partie du milieu aux échantillons traités par différents kits de purification d'exosomes basés sur leur précipitation alors que la partie de droite correspond aux échantillons traités par chromatographie à exclusion de taille ou traitement consécutif à la protéinase K/ RNase A. Les graphes dans la partie haute de la figure représentent la distribution de taille des séquences étudiées tandis que la partie basse représentes les types d'ARN retrouvés et leur proportion. boulanger). La composition d'ARN bactériens était relativement similaire dans différents échantillons de plasma, reflétant probablement la composition du microbiote humain. Dans cette étude, nous avons montré la complexité de l'analyse des ARNex du plasma. Ceci est lié à l'extrême diversité des sous-populations d'ARNex et à la présence d'autres composants moléculaires dans ce fluide biologique très complexe. De plus, il n'existe pas de méthodes communément acceptées et normalisées pour la préparation, l'analyse et la validation des échantillons, ce qui entraîne une confusion générale dans la littérature. La comparaison d'études sur les ARNex est compromise en raison de nombreux biais biologiques et techniques. À titre d'exemple, plusieurs études ont publié des données sur les ARNex de VE révélant d'importantes différences dans la composition de la fraction d'ARNex (proportions d'ARNr, de miARN, etc.)

(38,39,43,44,56)

(voir le tableau 1). De plus, si on se concentre sur un type particulier d'ARN tel que les miARN, qui sont largement étudiés pour la détermination de biomarqueurs, Drirh

Khare et al, 2017 (57)

n'ont démontré aucune différence significative entre le plasma entier et les exosomes de 10 volontaires en bonne santé. Dans cette étude, les avantages et les inconvénients identifiés pour chaque procédure, mais il est également important de prendre en compte les problèmes restants connus et inconnus.

analyse du dynamisme des 2'O- méthylations des ARNt de E. coli en conditions de stress A. Revue de la littérature X. ARNt

  Selon la question posée, on peut utiliser le plasma entier ou une sous-population via un traitement spécifique (chromatographie à exclusion de taille ou traitement à la protéinase K / RNase A). Par exemple, le plasma entier convient bien au diagnostic de cancers mais moins avec les exosomes. Au contraire, des échantillons enrichis par chromatographie à exclusion de taille ou traitement à la protéinase K / RNase A peuvent être utilisés pour des analyses détaillées d'ARNex provenant du microbiote. Après leur transcription et leur maturation, les ARNt sont fortement modifiés, puis aminoacylés. La structure globale d'un ARNt consiste en une extrémité 5' monophosphate, suivie d'une tige de l'accepteur, de la boucle D, de la boucle anticodon, d'une boucle variable et de la boucle TΨ. Un ARNt se termine par le deuxième brin de la tige de l'accepteur et par une extrémité 3'CCA.

	Les ARNt sont des ARN non codants jouant un rôle crucial dans la traduction. Les ARNt
	individuels sont des adaptateurs spécifiques qui entraînent le décodage productif de l'ARN
	messager en protéine. Ils transportent l'acide aminé approprié vers la chaîne polypeptidique en
	élongation associée au ribosome.
	Dans ce projet, nous avons apporté notre originalité en effectuant un prétraitement sur les ARN
	avant la préparation des banques pour le séquençage. Ce traitement de deP/P permet d'inclure
	des ARN qui n'ont pas naturellement les extrémités 5'P et 3'OH comme les produits de
	dégradation. En fait, la plupart des études n'effectuent pas cette étape supplémentaire et c'est
	pourquoi les miARN (avec les extrémités adaptées) sont l'espèce la plus retrouvée dans le
	plasma. Enfin, il s'agit d'un biais technique important, car les kits de préparation de banques
	ne tiennent pas compte des produits de dégradation, ni de tous les acides nucléiques sans 5'P,
	tels que les ARN coiffés (ARNm, snARN…).
	Enfin, étudier les ARNex à partir du plasma est vraiment compliqué et il faut se rappeler que la
	conception de l'étude est essentielle car peu de changements dans la stratégie d'analyse peuvent
	faire varier fortement la composition en ARNex. L'obtention d'une caractérisation exhaustive

d'ARNex à partir de plasma entier ou de sous-population (exosomes) constitue un véritable défi.

Cependant, son utilisation pour la determination de biomarqueurs de diagnostic dans le domaine des biopsies liquides est très excitante et prometteuse.

CHAPITRE II :

  position 34 dans deux ARNt Leu de E. coli (anticodons CAA et UAA). Un Cm34 est formé dans l'ARNt Leu (CAA) et un Um34 est introduit dans un nucléotide hypermodifié au sein de l'anticodon de l'ARNt Leu (cmnm 5 UmAA)(84). TrmL ne méthyle que des pyrimidines en position 34 et nécessite la modification 2-méthylthio-N 6 -isopentényadénosine (ms 2 I 6 A) en position 37 pour son activité ARNt est importante pour la reconnaissance spécifique entre l'accepteur de l'ARNt et son aminoacyl-ARNt synthétase (aaRS) apparentée(67). Une autre fonction de la 2'O-méthylation en position 18 dans la plupart des bactéries à Gram négatif est sa propriété immunosuppressive.

	(85).
	Les 2'O-méthylations affectent les propriétés physico-chimiques du nucléoside modifié. Elles
	ne modifient pas l'appariement de bases même si une stabilisation de l'hélice d'ARN de type A
	typique est observée (86,87), mais elles augmentent la stabilité du nucléotide contre l'hydrolyse
	alcaline (et même enzymatique) (88,89). En effet, la méthylation au 2'OH du ribose empêche
	une liaison hydrogène et supprime la propriété nucléophile de l'O2 '. De plus, la modification
	de l'
	TrmH (ou SpoU) est responsable de Gm18 (78,79). Les structures cristallines de TrmH lié ou
	non à SAM ont été établies par des études de mutagenèse approfondies, le site catalytique et le
	mécanisme de réaction enzymatique ont été caractérisés. Quatre acides aminés du site
	catalytique (Asn35, Arg41, Glu124 et Asn152) sont impliqués dans la liaison de l'ARNt. De
	plus, Asn35 est également responsable de la libération de SAM (78,80,81). TrmJ est impliqué
	dans la formation de Cm32 pour les ARNt Ser 1 et ARNt Gln 2 (82). La structure 3D et le site

). Le donneur de méthyle presque universel est S-adénosyl-L-méthionine (SAM ou AdoMet). La plupart des 2'-OMTases appartiennent à la superfamille des SPOUT MTases. Les enzymes impliquées dans la ribmométhylation des ARNt bactériens sont assez bien étudiées chez E. coli. Chaque position connue est formée par une ARNt MTase spécifique. catalytique de TrmJ chez E. coli ont été caractérisés. La reconnaissance de l'ARNt par TrmJ implique non seulement le domaine SPOUT, mais également des parties supplémentaires de la protéine

(83)

. TrmL est responsable de la 2'-O-méthylation en

Mots clés : Séquençage à haut débit, ARN extracellulaires, vésicules extracellulaires, modification des ARNt, 2'O-méthylations

En ce qui concerne les ARNex solubles dans le plasma, les principales espèces sont les miARN au sein de RNP via l'interaction avec la protéine ago2(62). Ces ARN représentent une énorme proportion d'ARNex et nos données entrent en corrélation totale. Il pourrait être intéressant d'isoler cette population grâce à une immunoprécipitation ciblant ago2 et de comparer leur profil avec les données obtenues avec du plasma entier et des échantillons traités avec des kits d'ultracentrifugation et de précipitation. Nous devrions obtenir des résultats similaires sans le pic correspondant au fragment d'ARN hY4. En ce qui concerne l'ARN Y4, son rôle en dehors de la cellule n'est pas bien étudié. Cet ARN existe en 4 isoformes de 83 à 112 nt de long (figure 5) et est composé d'un domaine tige boucle, d'une tige supérieure et inférieure et d'une queue de polyuridine. Dans la cellule, l'ARN Y a deux fonctions. La tige supérieure est impliquée dans la réplication de l'ADN. La tige inférieure et la queue de polyuridine se lient respectivement aux protéines Ro60 et La et participent au contrôle qualité et à la stabilité de l'ARN(63). Cependant, dans le plasma, l'ARN-Y n'est pas trouvé dans sa séquence entière, mais surtout en tant que fragments de 31 nt de longueur dérivés de l'extrémité 5 '. Ces fragments sont générés par clivage dans une boucle interne prédite(64).Jusqu'à présent, la fonction des fragments 5' de l'ARN Y est inconnue, mais il a été démontré que les isoformes hY1 et hY3 sont surexprimés dans des tumeurs telles que les carcinomes, tandis que hY4 est l'isoforme le plus abondant dans les cellules saines(65). Ainsi, les fragments d'ARN Y peuvent être des candidats potentiels pour les biomarqueurs du cancer.

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IV. ARN extracellulaires

En dehors des ARN retrouvés à l'intérieur de la cellule, il est maintenant établi qu'il existe une multitude d'ARN capable de circuler en dehors de la cellule. Ils sont sécrétés par diverses cellules selon des mécanismes qui ne sont pas encore entièrement connus. Pour rester stable dans un environnement extracellulaire peu favorable car riche en ribonucléases (RNases), les ARN extracellulaires doivent se protéger des dégradation nucléolytiques. Cette protection est assurée par deux différents moyens.