Mehdi Azouz

Mathilde Gonin

Sebastian Fiedler

Jonathan Faherty

Marion Decossas

Christophe Cullin

Sandrine Villette

Michel Lafleur

Isabel D Alves

Sophie Lecomte

Alexandre Ciaccafava

Microfluidic diffusional sizing probes lipid nanodiscs formation

Keywords: Polymer nanodiscs, SMALP, Lipids, Peptide, Microfluidic Diffusional Sizing

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The plasma membrane is a complex supramolecular assembly composed of various lipids, sugars and membrane proteins (MPs) working together to modulate essential biological processes such as generation of electrochemical gradient, signalling pathways, ligand-receptor interactions and energy conversion processes {Cournia, 2015 #1}. Despite the importance of this key interface crucial for life, many aspects remain obscure and rather poorly investigated. Among them, the characterisation of MPs in natural membranes remains a major challenge. Indeed the proper functionality of a MP is intimately linked to its native environment: the lipid membrane {Phillips, 2009 #865}. One of the main bottlenecks lies in the ability to extract MPs from their native environment while preserving their biological properties. A paradox that has considerably held back researches, as reflected by the small amount of structural data available for MPs (<1% in the Protein Data Bank (PDB)) {Scott, 2013 #4}. Traditionally detergents are used to solubilise biological membranes and ultimately extract MPs prior to biophysical characterisation {Champeil, 2016 #863;Sadaf, 2015 #864}. However in many cases, solubilisation with detergents leads to inactivation of the MPs. Among the reasons invocated for such a loss, the dissociation character of detergent leading to delipidation of the MPs is often pointed out.

As valuable alternative to detergent solubilisation, tailored amphipathic polymer (amphipols) have succeeded in solubilising membranes while maintaining the functionality of a wide range of MPs {Kleinschmidt, 2014 #862}. However, those methods fail to keep native lipids around the MP. In order to provide closer physiological conditions, reconstitution methods of MPs into biomimetic lipid bilayers of controlled compositions have been developed. Detergent solubilised MPs can be reconstituted into spherical lipid bilayer system (liposomes) or planar ring shaped lipid bilayer surrounded by a rim of short detergent molecules (bicelles) {Dürr, 2013 #866}. Bicelles are particularly suited for NMR studies due to their ability to align in magnetic field {Marcotte, 2005 #909}. Closely related to bicelles, membrane scaffold protein (MSP) lipid nanodiscs are constituted of a disc-shaped lipid bilayer sizing around 10 nm diameter surrounded by a belt of MSP {Ritchie, 2009 #867}. Unfortunately, the MSP allowing the formation of nanodiscs often interferes with spectroscopic investigations due to spectral overlap with the targeted MP.

While the aforementioned methods represent a considerable improvement, they however require the MP to be first solubilised with detergent before reconstitution into those mimicking systems. This transient state might induce irreversible structural changes and loss of essential lipids, which could act as cofactors {Kern, 2011 #890}. Recently a new class of nanodiscs has emerged, characterised by the substitution of MSP by amphiphilic copolymers forming a belt around lipid bilayers {Overduin, 2018 #905}. Styrene Maleic -Acid (SMA) was the first copolymer to be used, with styrene as a hydrophobic moiety and maleic acid as the hydrophilic part {Knowles, 2009 #868}. This novel class of nanodiscs referred as SMALPs (SMA lipid particles) offers the unique feature of extracting MPs directly from the native membrane with their intimate surrounding lipids. Typically, 100 -200 lipids are co-extracted {Rajesh, 2011 #913;Bayburt, 2010 #908}. With such approaches, MPs are solubilised in a single step without any detergent. Moreover, thanks to their unique ability the use of SMALPs for MPs extraction and their analysis by lipidomics have revealed very distinct environments around different MPs {Teo, 2019 #873;Schmidt, 2019 #874}. SMA copolymer have been shown to efficiently solubilise various lipid compositions including prokaryotic and eukaryotic membranes {Karlova, 2019 #872;Swainsbury, 2017 #881;Scheidelaar, 2015 #871}. While SMALPs open the road to a wide range of biophysical characterisation techniques, they also suffer from inherent limitations. The first one is the size of the MP that can be extracted, which is directly linked to the maximal size of the nanodisc. Typically, SMALPs have a 10 nm diameter but reported values range from 5 to 30 nm depending on the preparation and the MP extracted {Dörr, 2016 #887}. So far, the largest MP trapped inside a SMALP is the alternative complex III associated with a cytochrome c oxidase constituting a supercomplex of 464 kDa and 48 transmembrane helix {Sun, 2018 #886}. In this supramolecular assembly, only 11 lipid molecules were observed, reflecting somehow a very tight packing. The second limitation is the pH range of use since SMA tends to aggregate at pH below 6.5 {Scheidelaar, 2016 #841}. SMA copolymer also chelates divalent cations impeding its use under these conditions. Finally, the styrene group of SMA absorbs light at 260 nm, which can be advantageously used to quantify its concentration, but unfortunately also overlaps with protein absorption signals.

In this respect, several variants of SMA have been designed to overcome those limitations but also to broaden their potential applications {Scheidelaar, 2016 #841}. Among them, tolerance to divalent cations has been largely improved (up to 20mM) thanks to the substitution of the styrene ring by a diisobutylene moiety (DIBMA) {Oluwole, 2017 #888}. Thanks to the absence of the styrene ring, DIBMA copolymer and a methacrylate copolymer were shown to be compatible with UV-vis spectroscopy for protein characterisation{Yasuhara, 2017 #878}. The pH range of use has been extended down to 2.5 and up to 10 thanks to the addition of an ammonium derivative on the maleic acid part. Moreover, this modification allowed a size tunability of the nanodisc diameter from 10 to 30 nm depending on the copolymer / lipid ratio {Ravula, 2018 #877}. Furthermore, SMA copolymers were modified to incorporate a thiol function for click chemistry, paving the way for many applications including surface immobilisation and labelling {Lindhoud, 2016 #884}.

Another bottleneck that needs further investigation regards the SMALP formation process. Indeed, only a handful of studies have addressed the problematic of SMALP formation. Among them, a systematic biophysical study has investigated SMA copolymer solubilisation of lipid bilayers of various composition, leading to the proposal of a model for SMA-driven membrane solubilisation mode of action {Scheidelaar, 2015 #871}. This experimental work was further supported by coarse-grained molecular dynamics {Orekhov, 2019 #937;Xue, 2018 #876}. Together, those data are giving a clearer picture of SMALP formation mechanism. The general mechanism of membrane solubilisation by copolymer can be compared to the one followed by detergent. Thus the formation of nanodiscs follows the so called three-stage model and can be depicted with the use of two dimensional composition-phase diagrams as detergent/lipid mixtures {Lichtenberg, 2013 #893;Lichtenberg, 2013 #922;Helenius, 1975 #921}. In the stage I, the copolymer is added stepwise to the bilayers (generally in the form of large unilamellar vesicles (LUVs)) and starts interacting with the surface mostly through hydrophobic and to a lesser extent electrostatic forces. After this initial surface binding, SMA copolymer inserts into the hydrophobic membrane core in a process modulated by the alkyl chain packing. Continuous polymer addition leads to a saturation limit, as the bilayer can no longer incorporate the copolymer, and nanodiscs start being expelled from the membrane. The copolymer to lipid molar ratio at which this transition occurs is generally called saturation (Rsat) and marks the beginning of stage II. When reported on a diagram, this value represents the phase boundary, at a precise ratio, between the mixed lamellar vesicle domain and a range of coexistent species, where nanodiscs are in equilibrium with copolymer-containing vesicles. As more copolymer is added to the vesicles, complete solubilisation occurs at a higher ratio called solubilisation (Rsol) corresponding to the beginning of stage III, marking the phase boundary between the coexistence range and the domain of pure nanodiscs.

Many techniques are available to follow the copolymer-induced solubilisation of lamellar vesicles. Among them, one can cite DLS, solid state 31 P NMR, fluorescence spectroscopy, Cryo TEM and ITC (See {Lichtenberg, 2013 #893} for a thorough review in the context of detergent solubilisation). In this work, we propose to evaluate the potential of the Microfluidic Diffusional Sizing (MDS) technology as an alternative method to follow SMALPs formation. Originally, MDS was designed to determine the size and concentration of protein samples for quality control purposes {Yates, 2015 #894}. Soon after, other applications have emerged and the method has been applied to evaluate protein/ligand and protein/lipid interactions {Gang, 2018 #897;Zhang, 2016 #895;Herling, 2016 #896}. We have chosen to investigate the thoroughly characterised phosphatidylcholine (PC), a major lipid component of mammalian membranes. In particular, we have focused on POPC and DMPC for straightforward comparison with literature. We present a specific protocol for MDS detection of lipid self-assemblies instead of proteins and we discuss the information extracted by MDS on SMALP formation. Our approach is supported by DLS and transmission electron microscopy (TEM) measurements. Moreover, we have used MDS along with SMALPs as lipid model system to investigate the role of a specific lipid in promoting membrane/peptide interactions in a case study. We have used K18 peptide as an example, corresponding to the aggregative domain of Tau protein, and demonstrated that MDS can highlight the role of PIP2 in triggering such interaction.

Materials and methods

Materials:

1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero- 3-phosphoethanolamine (POPE), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPE), 1,2- dimyristoyl-sn-glycero-3-phosphocholine (DMPC) L-α-phosphoinositol-4
,5-biphosphate (PIP2, purified from porcine brain) were purchased from Avanti Polar Lipids (Alabaster, AL) and were received as organic solutions (chloroform and chloroform/methanol/water for PIP2 (20:(9:1) vol/vol)). 4-(2-Hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) and sodium chloride were obtained from Sigma (Steinhem, Germany). All the solutions were prepared with ultra-pure water (18 MΩ cm).

Preparation of SMA (3:1) stock solution:

SMA (3:1) (XIRAN® SL25010 P20) was a kind gift from Polyscope. SMA (3:1) in HEPES 50 mM pH 7.0 was prepared trough dialysis as described before {Cuevas Arenas, 2016 #898}. The final concentration was determined by UV-vis spectroscopy (Jasco V630 spectrophotometer) by measurement of the absorbance at 260 nm and using a molar extinction coefficient ε260 = 6989 L. mol -1 .cm -1 obtained elsewhere {Oluwole, 2017 #888}.

Preparation of lipid suspensions:

Lipid solutions were mixed to obtain the desired molar ratio (POPC/PIP2, 80:20 % mol), or weight ratio (POPC/POPE (9:1), DMPC/DMPE (9:1) w:w. Organic solvents were then evaporated under a gentle stream of nitrogen to form lipid films. In case of mixtures, evaporation was carried out at 50 °C to ensure lipids miscibility. To remove residual solvent traces, the lipid films were further dried in vacuum at room temperature for at least 16 h. The films were then dissolved into a buffer (HEPES 20 mM, NaCl 140 mM, pH 7.4, or 50 mM pH 7.0) to a concentration of 1 mg/mL and were thoroughly agitated at 50 °C for 30 minutes. The resulting multilamellar vesicle suspensions (MLVs) were then submitted to three freeze and thaw cycles (from liquid nitrogen to 50 °C). In the case of POPC/POPE (9:1) and DMPC/DMPE (9:1) mixtures, MLVs were extruded (21 times) through a polycarbonate filter (100 nm) using a mini-extruder device (Avanti Polar Lipids) to obtain Large Unilamellar Vesicles (LUVs). Finally, lipid concentrations were determined by phosphate quantification {Rouser, 1970 #903}

Preparation of nanodiscs suspensions:

In the case of POPC and POPC/PIP2 (80:20 % mol) nanodisc suspensions, an appropriate amount of SMA (3:1) was added to reach a final volume of 300 µL. The complete solubilisation of the membranes was verified by DLS and was confirmed when the suspensions gave rise to a monodisperse population with a size of 10 -20 nm. All nanodiscs suspensions were stored in 1.5 mL Eppendorf tubes at 4 °C until use.

Production and purification of K18 peptide:

pNG2 K18 (kindly given by Pr. E. Mandelkow) was used to transform E.coli C41 (DE3) (F-ompT hsdSB (rB-mB-) gal dcm (DE3)). Several transformants were grown on 120 mL LB + 1% dextrose, 100 mg/L ampicillin. When the culture reached an OD650= 5.2, 10 mL was added to 990 mL of ZYM 5052 medium (1% N-Z-amine, 0.5% yeast extract, 25 mM Na2HPO4, 25 mM KH2PO4, 50 mM NH4Cl, 5 mM Na2SO4, 2 mM MgSO4, 0.5 % glycerol, 0.05 % dextrose, 0.2 % lactose) containing 100 mg/L ampicillin and incubated overnight at 37°C. After centrifugation, cell pellets (corresponding to 500 mL OD650=10) were suspended in 50 mL of MES pH 6.8 (20 mM), NaCl (500 mM), EDTA (1 mM) PMSF (1 mM), benzamidine (2 mM) and DTT (5 mM), sonicated four times (1 min cycle on ice; output 5, 50 % duty cycle) and then heated at 80°C for 20 min. After centrifugation (30 min at 15 000 g) the supernatant was dialyzed for at least 16 h at 4 °C against cation exchange buffer A (20 mM MES pH 6.8, EDTA 1 mM, NaCl 50 mM, DTT 1 mM) with a Spectra/Por Dialysis Membrane (MWCO 3 500). The dialysate was then cleared (30 min, 15 000 g), filtered through a 0.22 µm membrane and applied on a HiTrap SP column (GE Healthcare) equilibrated with the cation exchange buffer A. After washing with 25 mL of the same buffer, the protein was eluted with 25 mL of Buffer B (MES 20 mM pH 6.8, EDTA 1 mM, NaCl 150 mM) and the 5 mL fractions containing most of K18 were pooled and concentrated by ultrafiltration devices (e.g., Ultrafree, Millipore 5 kDa MWCO) to a final volume of 0.5 to 1 mL. Finally, the concentrated protein was applied onto a gel filtration column (Superdex-75) equilibrated in 100 mM ammonium acetate containing 2-mercaptoethanol (0.1 %). The fractions containing the pure protein were pooled, aliquoted and lyophilized.

Dynamic Light Scattering:

DLS measurements were performed on a Vasco Nanoparticle Size Analyser (Cordouan Technologies, France) working with a He-Ne laser (633 nm) and a detection angle of 135 °.

Negative staining for transmission electron microscopy

For EM grid preparations, the sample suspension diluted at 50 µg/mL for LUVs or 2 µg/mL for nanodiscs in HEPES buffer (50 mM pH 7.0) was applied to a glow-discharged carbon-coated copper 300 mesh grids and stained with 2% uranyl acetate (w/v) solution. Images were recorded under lowdose conditions on transmission electron microscope (Tecnai F20 or CM120, ThermoFischer) using a ThermoFisher Eagle 4k_4k or a GATAN UltraScan 1000 2k_k camera. Images were analysed with ImageJ.

Microfluidic diffusional Sizing:

Concentrations of POPC/POPE (9:1) and DMPC/DMPE (9:1) LUVs solution after extrusion procedure ranged from 0.2 to 1 mM. Aliquots of SMA (3:1) stock solution (11 mM) or diluted one (1.1 mM) were added to the required volume of LUVs to produce the appropriate molar ratio (nSMA3:1/nLipids) typically ranging from 0.001 to 100. The mixtures were incubated for at least 16 h at 25°C for POPC/POPE (9:1) and 30°C for DMPC/DMPE (9:1). A volume of 7 µL of SMA (3:1) / LUVs mixture was pipetted onto microfluidic chips, which were then loaded into the Microfluidic Diffusional Sizing (MDS) device Fluidity-One (Fluidic Analytics, Cambridge, UK) for measurements. The device was set to the 2-20 nm size-range setting resulting in a measurement time of 15 min.

Peptide/nanodiscs interactions:

The concentration of K18 was kept constant at 1 µM. Aliquots of nanodisc solution were added to obtain lipid concentrations ranging from 1 to 50 µM into a total volume of 200 µL. The tubes were thoroughly vortexed for 30 seconds at room temperature and an aliquot of 7 µL of the mixture was then applied onto the microfluidic chip for size measurements by MDS.

Results & Discussion

Protocol setup for lipid detection by MDS

In this work, we propose to use MDS to follow SMALP formation by solubilisation of lipid bilayers and to evaluate to which extent we can obtain information on such process. It is worth noticing that, MDS was designed to detect and measure protein hydrodynamic radii, in consequence, the experimental protocol must be adapted. Indeed, with MDS, protein size and concentration are determined through diffusion of the sample under laminar flow in a microfluidic chip. Briefly, the sample protein flows in channel A (ChA) running alongside a channel B (ChB) filled with water. Since no convection occurs at the interface between both channels, the protein can only migrate from channel A to channel B through diffusion, which depends on the protein size. At the end of the run, both channels are split and the protein that is left in channel A and the protein that has migrated in channel B are quantified thanks to a latent labelling specific of primary amine functions (Fig. 1). The length of the diffusion channel on the chips used for protein sample diffusion has been tuned to detect species with a hydrodynamic radius (RH) ranging from 0.3 to 20 nm. Thus, species with a RH > 20 nm are not expected to be detected. In order to follow SMALP formation by MDS in a rigorous manner, we have determined four requirements that the studied system must fulfil. (1) In order to avoid underestimation of the copolymer concentration for the onset of solubilisation and overestimation for the complete solubilisation we have chosen to solubilise LUVs of homogenous diameter over MLVs {Lichtenberg, 2013 #893}. Indeed, due to their onion-like concentric lipid bilayers, not all MLVs lipids are simultaneously available for solubilisation by the copolymer causing misestimations (2) To be detectable, the sample must switch from a non-diffusive to a diffusive behaviour in the microfluidic channel upon SMA(3:1) copolymer addition (note that we will refer later to diffusive and non-diffusive behaviour regarding diffusion along the microfluidic channel). In this respect, LUVs of a mean hydrodynamic radius much larger than 20 nm, are not expected to diffuse whereas the smaller nanodiscs species with expected RH in a range between 5 and 15 nm will be detectable. (3) Again, to be detectable, the measured sample must carry a free primary amine function. Usually lysine residues or N-terminal amine functions are used in protein samples. In our case we have doped POPC and DMPC lipids with their amino-terminated counterparts POPE and DMPE (at a mass ratio POPC/POPE and DMPC/DMPE of (9:1) w:w) in order to apply the same instrument capability to detect lipids that was used for proteins. Note that this protocol also works with POPS, (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine) (Fig. S1) (4) Since we no longer deal with pure lipid composition but with mixtures, we have to make sure that the copolymer will solubilise either lipids without preference. Indeed, SMA (3:1) has been shown to solubilise lipids in a non-selective manner allowing the use of lipid mixtures and ensuring a non-biased detection {Scheidelaar, 2016 #841;Cuevas Arenas, 2016 #898}.

Validation of lipid detection by MDS

With those requirements in mind, we have performed several control experiments to validate this novel application of the MDS technology. POPC/POPE (9:1) and DMPC/DMPE (9:1) MLVs were prepared at a concentration of 1 mg/mL in HEPES buffer 50 mM, pH 7.0. From those lipid suspensions, we prepared LUVs through extrusion with 100 nm diameter filter to generate homogeneous species expected to have a non-diffusive behaviour. The mean diameter of these extruded LUVs of POPC/POPE (9:1) and DMPC/DMPE (9:1) was measured by DLS, giving values of 140 and 150 nm respectively.

As expected, due to their large RH of about 70 -75 nm, twice the MDS detection limit (RH > 20 nm), only a very weak diffusion signal was observed in ChB (Fig. 2A). Accordingly, the MDS instrument does not provide a value for RH of the LUV sample as the experimental errors would be too large (note that the same experiment with MLVs, which diameter is around 700 nm, showed no diffusion signal at all, see Fig. S1). When a large excess of SMA (3:1) was added to the LUVs (nSMA3:1/nLipids > 5) and left to react for at least 16 h, a marked diffusion signal was observed in ChB, demonstrating that diffusion occurred for lipid particles with RH lower than 20 nm (Fig. 2B). In order to support the results obtained with MDS it is highly desirable to correlate them with a visual record of the lipid samples. In this respect, the samples were observed by transmission electron microscopy. The LUVs exhibited dimensions with an average diameter of 150 ± 40 nm and 150 ± 12 nm for POPC/POPE (9:1) and DMPC/DMPE (9:1), respectively, in agreement with DLS measurements (Fig. 3A andB). After exposition to excess of SMA (3:1) overnight at 25 °C for POPC/POPE (9:1) and 30 °C for DMPC/DMPE (9:1), LUVs were solubilised into nanodisc shaped particles with a diameter of 10.5 ± 0.7 nm and 11.7 ± 0.9 nm for each composition respectively (Fig. 3C). These values are in good agreement with their typical diameter of ~10 nm reported in the literature {Lindhoud, 2016 #884;Cuevas Arenas, 2016 #898;Jamshad, 2015 #900}. Importantly, the observed mean diameters of the objects correlate perfectly both with their diffusive behaviour (nanodiscs) or non-diffusive behaviour (LUVs) observed by MDS. This set of control experiments following our established protocol proved that MDS can (1) detect lipids in addition to protein and (2) can distinguish between LUVs and nanodiscs due to their different size range.

Solubilisation of POPC/POPE (9:1) and DMPC/DMPE (9:1) LUVs by SMA (3:1)

The validation of this approach following the results described above invited us to further explore the SMALPs formation by MDS. We asked ourselves the questions if, beyond the extraction of nanodiscs size, it would be possible to access the concentration of copolymer required for the onset of solubilisation, Rsat, and for the complete solubilisation, Rsol, of lipids. Thus, we have explored the solubilisation of LUVs composed of POPC/POPE (9:1) (long alkyl chains 16:0-18:1) and DMPC/DMPE (9:1) (short alkyl chains 14:0) induced by SMA (3:1) copolymer. Our objective, besides further exploring MDS capabilities was to determine whether lipid chain unsaturation could affect the values of Rsat and Rsol. As LUVs were prepared by extrusion, we have systematically determined the lipid concentrations with phosphorus assays {Rouser, 1970 #903}. Typically, LUVs concentrations ranging between 0.2 -1 mM were measured. Increasing amounts of SMA (3:1) copolymer were added to the required volume of LUVs solutions to obtain the desired ratio. Samples were incubated for at least 16 h at 25 °C for POPC/POPE (9:1) and 30°C for DMPC/DMPE (9:1), to avoid impact of membrane fluidity changes on the process.

The molar ratio nSMA3:1/nLipids was explored extensively from values as low as 0.001 up to 100. After MDS measurements, the diffusion profiles of ChA and ChB were extracted and their maxima of intensity were used to define a diffusion ratio equal to Imax(ChB)/(Imax(ChA) + Imax(ChB)). This so-defined diffusion ratio can vary between two extrema, 0 and 0.5. The absence of diffusion corresponds to a value of 0 while a value of 0.5 means that equilibrium has been reached between both channels and the samples have fully diffused. Measurements for each molar ratio were at least duplicated (n≥2). For each lipid mixture, the diffusion ratio Imax(ChB)/(Imax(ChA) + Imax(ChB)) has been plotted against the molar ratio nSMA3:1/nLipids (Fig. 4A andB ). Rsat and Rsol values were determined by linear fitting of Imax(ChB)/(Imax(ChA) + Imax(ChB)) vs nSMA3:1/nLipids following a three stages solubilisation process as described in {Grethen, 2017 #840;Cuevas Arenas, 2016 #898}. Briefly, in stage I the diffusion ratio remains constant before Rsat. In stage II, the diffusion ratio increases from Rsat with a steep slope until reaching a plateau marking Rsol and the beginning of stage III, see supplementary data (Fig. S2). In the case of POPC/POPE (9:1) mixture, for molar ratios below 0.070, the diffusion ratio has a constant value around 0.1, similar to the diffusion ratio observed in the absence of SMA (3:1). This means that for molar ratios ≤ 0.070, the diffusion behaviour remained unaffected, corresponding to stage I in the three-stage solubilisation model where LUVs start to be loaded with copolymer molecules. For molar ratio ≥ 0.070, the diffusion ratio started to increase with a steep slope. Interestingly this molar ratio matched quite well the saturation ratio (Rsat = 0.108) for pure POPC LUVs solubilised by SMA (3:1) determined by the Keller's group using 31 P NMR {Cuevas Arenas, 2016 #898} (Table.1). In the case of DMPC/DMPE (9:1) mixture, the diffusion ratio remained constant at a slightly higher value of 0.12 than POPC/POPE (9:1) for molar ratio below 0.043. Above 0.043, the diffusion ratio started to increase significantly. Again this value is in good agreement with the Rsat = 0.078 reported for pure DMPC {Cuevas Arenas, 2016 #898} (Table .1). In this respect, the increase of diffusion ratios for both lipid mixtures marked the onset of solubilisation and the beginning of stage II. Interestingly, MDS was able to distinguish between the solubilisation of saturated (DMPC/DMPE) and unsaturated (POPC/POPE) lipids giving distinct Rsat values of 0.043 vs 0.070 respectively. For POPC/POPE 9:1, a further increase in SMA (3:1) concentration led to a plateau at a diffusion ratio value of 0.45 close to the theoretical equilibrium value of 0.5. The plateau appeared at molar ratio ≥ 1.03. This value is very different from the Rsol reported previously for POPC (Rsol = 0.167) (Table .1). For DMPC/DMPE 9:1, increasing further the molar ratio nSMA3:1/nLipids allowed to reach a plateau for molar ratio > 0.42, which is once again much higher than the reported Rsol for pure DMPC (Rsol = 0.144) {Cuevas Arenas, 2016 #898} (Table .1). These values, that could have been anticipated as a direct reading of the complete solubilisation ratio at the beginning of stage III were however larger than Rsol values reported in the literature (Table .1). Thus, we will refer later to these noticeably high values as "apparent Rsol". Our results indicate that the approach used apparently slightly underestimate Rsat values whereas they largely overestimate Rsol values. Two reasons can explain these observed discrepancies. First, Rsat and Rsol values were compared to the ones determined for pure POPC or DMPC LUVs whereas we have introduced a small fraction of POPE or DMPE to enable sample detection. The introduction of such lipids is not without consequence. Due to their smaller phosphoethanolamine headgroups, POPE and DMPE present intrinsic negative curvature, inducing an increased lateral pressure in the bilayer. The introduction of lipids with negative curvature has been shown to greatly influence the solubilisation by SMA copolymer. In the case of DOPC, the introduction of a fraction of DOPE seems to facilitate the beginning of solubilisation, but prevents complete solubilisation {Scheidelaar, 2015 #871}. This observation was further complemented by another study, demonstrating that the introduction of POPE in POPC bilayers tends to decrease Rsat values while at the same time increasing Rsol values, meaning that more SMA (3:1) copolymer is required to reach complete solubilisation {Cuevas Arenas, 2016 #898}. Importantly, no bias was observed in the solubilisation of POPE over POPC, indicating that the global state of the bilayer governs the solubilisation rather than the individual properties of each lipid component. Indeed, thanks to the quantitative results obtained in the latter study, the expected values for Rsat end Rsol for the solubilisation of POPC/POPE (9:1) by SMA (3:1) can be estimated around ~ 0.1 (0.108 without POPE) and ~ 0.175 (0.167 without POPE) respectively. Consequently, the presence of PE in our systems supports the slightly lower values of Rsat that we have observed, but is hardly compatible with the large increase that we have measured for Rsol values.

In this respect, a second observation could explain our apparent overestimation of the Rsol values. Indeed, to reach complete solubilisation, more and more copolymer molecules were added to the solution competing for a finite amount of lipids. It has been shown that increasing amount of copolymer tends to decrease the size of nanodiscs {Grethen, 2017 #840;Cuevas Arenas, 2016 #898}. Indeed, we have determined apparent Rsol from the maximum values of the diffusion ratio, which depended on the size of the nanodisc. Therefore, the size reduction of SMALPs diameter as a function of the copolymer concentration could explain the overestimation of the Rsol value. In this respect, we have plotted the diameters of diffusing particles detected by MDS against the molar ratio (Fig. 4A andB). Notably, the following interpretation of Rsol values is based on the assumption that all LUVs have been converted into nanodiscs, an information that MDS is unfortunately not able to provide. Interestingly, our results show that the usual size range of nanodiscs of 10 -30 nm is reached for molar ratios preceding the beginning of the plateau of the diffusion ratio curve, in other words before the apparent Rsol. Typically, SMALPs with diameters of 30 nm are obtained for molar ratios of ~ 0.36 for POPC/POPE (9:1) and ~ 0.17 for DMPC/DMPE (9:1). Those latter values reported as Rsol 30 nm in Table .1 are much closer to the Rsol values reported in the literature. Diameters of 10 nm are observed for larger molar ratio values (Rsol 10 nm ) of 1 and 0.72 for POPC/POPE (9:1) and DMPC/DMPE (9:1) respectively. For both lipid mixtures, increasing further the molar ratio leads to a plateau with a lower limit at ~ 6 nm diameter which seems to be the lowest achievable diameter for SMA (3:1) based SMALPs.

Additionally, DLS measurement were performed to compare with MDS data. Despite a slight overestimation, the diameters determined by DLS were in good agreement with the ones measured by MDS (Fig. 4A andB). While DLS is known to be biased toward large particles because of the 10 6 factor between the light diffusion and particle size, our measurements did not detect large particles (diameter > 100 nm) in the molar ratio range where MDS is able to determine particle size . This observation tends to support: (1) the assumption that all LUVs were converted into nanodiscs (2) the observation that the size of nanodiscs decreased with increasing copolymer concentration. Consequently, MDS is efficient in the determination of Rsat, namely the onset of solubilisation, with values that are very similar to those reported in the literature. However, regarding Rsol determination, the dependence of the particle size appears to lead to larger values than those obtained by 31 P-NMR. Nevertheless, MDS provides a relevant range for Rsol values and offers the opportunity to carefully control SMALPs size once Rsol is reached by fine-tuning the nSMA3:1/nLipids ratio.

Peptide/nanodisc interactions:

As an additional application of MDS to illustrate its potential, we studied the interaction of a peptide with SMALPs of different compositions. In this case, the lipids used (POPC and PIP2) do not contain primary amines, and the technique specifically follows peptide diffusion. Aggregation of phosphorylated Tubulin-associated unit (Tau) protein is involved in the neurodegeneration process observed in Alzheimer's disease {Virginia M-Y Lee, 2001 #915}. This phenomenon appears as a pathological event and needs to be investigated. In particular, negative lipids have been highlighted to induce Tau protein aggregation thus suggesting the importance of such lipids in the interaction {Mari, 2018 #918; Ait-Bouziad, 2017 #912;Elbaum-Garfinkle, 2010 #911;Chirita, 2003 #914}. This phenomenon stems from the aggregative domain (K18) of Tau, known as the microtubule-binding domain {Crowther, 1989 #916}. In a recent study, we have demonstrated that PIP2, a negatively charged lipid found in the inner leaflet of membranes, promoted K18 peptide aggregation and fibrillation {Talaga, 2018 #907}. To investigate the specific interaction with PIP2, we prepared POPC and POPC/PIP2 (80/20 %mol) SMALPs with sizes of 9.3 nm and 10.5 nm respectively (determined by DLS). Without lipids, K18 exhibited a mean native hydrodynamic radius of 2.9 ± 0.1 nm as measured by MDS. In aqueous solution, the peptide is supposed to be unfolded {von Bergen, 2001 #920}. Pure zwitterionic lipids such as POPC have been shown to be unable to promote K18 aggregation {Talaga, 2018 #907}. Yet, we observed a progressive increase in size of the peptide hydrodynamic radius up to 12.2 ± 0.2 nm in the presence of 50 µM of lipids (Fig. 5). In addition, measurements of K18 with SMA (3:1) copolymer in absence of lipids (to a copolymer concentration equivalent to 25 µM of SMALPs) showed an increase of K18 radius to 5.2 nm. Such results are pointing to an interaction between K18 and the copolymer molecules. The peptide is positively charged at physiological pH {Elbaum-Garfinkle, 2010 #911} and can establish electrostatic interactions with the maleic acid moieties of the copolymer that are partially deprotonated at pH 7.4 {Scheidelaar, 2016 #841}. By contrast, in presence of PIP2-containing nanodiscs, K18 size evolution was different as the diameter increase was much more pronounced for same lipid concentrations (above 10 µM). A maximum hydrodynamic radius of 19.2 ± 0.9 nm was reached at 50 µM lipid POPC/PIP2 concentration. Two co-existing phenomena could be responsible for this substantial increase in the hydrodynamic radius of K18: (1) K18 interactions with PIP2-containing nanodiscs, (2) aggregation of K18 peptides promoted by the presence of PIP2. This phenomenon was observed within few minutes after mixing the peptide and the nanodiscs, a short time scale relative to the onset of fibrillation of K18, which was reported to be few hours of incubation in the presence of lipids {Talaga, 2018 #907}. The technique may therefore probe preliminary events that eventually lead to the formation of fibers induced by negatively charged lipids such as PIP2. The formation of the first aggregated objects (a few monomers) constitute a crucial event that is generally not temporally characterised as they are not detected with classical methods such as fluorescence assays. The formation of small peptide aggregates is of primary importance, as it may trigger nucleation and lead to the formation of fibers.

Conclusions:

With their undeniable assets in the study of membrane proteins over conventional detergents, SMALPs draw more and more attention from structural biologists, biophysicists and chemists. With the idea of pushing forward the boundaries set by the current inherent limitations of SMALPs, many variants have been developed. Such an emulation calls for better characterisation and control of SMALPs formation, a process that remains to be understood in detail. In this work, we have used a novel method based on microfluidic diffusional sizing to follow SMALPs formation from SMA (3:1)-driven solubilisation of LUVs composed of POPC/POPE (9:1) and DMPC/DMPE (9:1) mixtures. While this technology was originally developed to detect and measure protein hydrodynamic radius mostly for quality control purposes, herein we have set up a protocol to follow SMALPs formation. We have demonstrated that the introduction of a small fraction of amino-containing lipids enabled the detection of lipid particles. This opens the road to detect virtually any primary amine-containing molecule, emphasizing the versatility of the technique. Consequently, using minute amount of sample (less than 10 µL) and in a relative short time (t < 15min) MDS was able to determine the diameter of SMALPs with high precision as confirmed by supportive DLS data. Taking advantage of MDS diffusion profiles derived from the titration of LUVs by SMA (3:1) copolymer, we were able to construct saturation curves providing a direct reading of the solubilisation process. The onset of solubilisation (Rsat) can be obtained directly from the curve. Interestingly, MDS is also sensitive to the subtle effect of chain unsaturation revealing slightly different Rsat values for POPC and DMPC in agreement with the literature. However, since SMALPs diameter keeps decreasing after complete solubilisation upon SMA (3:1) addition, MDS profiles still evolved even after complete bilayer solubilisation, causing an apparent overestimation of Rsol. Once this phenomenon is understood, Rsol values can be derived for specific SMALPs size using particle diameter vs molar ratio curves. In addition, once complete solubilisation is reached, MDS allows to control nanodiscs size by fine-tuning nSMA3:1/nLipids ratio. MDS could potentially be a good tool to investigate the first stages of peptide aggregation, where other methods as ThT seem to fail.

Supplementary data:

Fig. 1 :

 1 Fig.1: Scheme representing the flow of sample through a microfluidic chip during MDS analysis on a Fluidity One instrument.

Fig. 2 :

 2 Fig.2: A) MDS measurement of POPC/POPE (9:1) LUVs. B) MDS measurement of POPC/POPE (9:1) LUVs after solubilisation with excess SMA (3:1) (nSMA3:1/nLipids > 5). In blue, Channel A corresponds to the channel where the sample has been introduced while in red, channel B is the one into which the sample has diffused.

Fig. 3 :

 3 Fig.3: A and B) TEM images of LUVs of POPC/POPE (9:1) and C) after addition of solubilising excess SMA (3:1) (nSMA3:1/nLipids > 5).

Fig. 4 :

 4 Fig.4: Particle diffusion ratio (blue full circles) and diameter (red full circles for MDS and empty circles for DLS) as a function of the molar ratio for A) POPC/POPE (9:1) and B) DMPC/DMPE (9:1). Horizontal blue and red error bars represent variations in lipid concentration determination. Vertical blue and red bars represent the standard deviation on the measuments (n ≥ 2). Particle diameters were calculated by multiplying by two RH values obtained by MDS.

Fig. 5 :

 5 Fig. 5: Hydrodynamic radius (RH) changes of K18 (1 µM) following a titration with increasing concentrations of POPC (black) and POPC/PIP2 (80/20 mol%) (red) SMALPs. Each experiment was repeated three times to get statistical insight about the size variation upon SMALP interaction.

Fig

  Fig. Supp.1: MDS measurement of POPC/POPS (9:1) MLVs diffusion. In blue, Channel A corresponds to the channel where the sample has been introduced while in red, channel B is the one where the sample will diffuse.

Fig

  Fig. Supp.2: Particle diffusion ratio (blue full circles) as a function of the molar ratio for A) POPC/POPE (9:1) and B) DMPC/DMPE (9:1). Linear fits (blue lines) have been determined according to the three-stage solubilisation process.

Table . 1

 . : Comparative table of Rsat and Rsol values for SMA (3:1) driven solubilisation of different lipids or mixtures.

		Rsat		Rsol		Rsol 30nm***
		31 P NMR MDS 31 P NMR	MDS	31 P NMR MDS
	POPC	0.108*	-	0.167*	-	-
	DMPC	0.078*	-	0.144*	-	-
	POPC/POPE (9:1)	0.1*	0.07	0.175*	1.03**	-	0.36
	DMPC/DMPE (9:1)	-	0.043	-	0.42**	-	0.17

*: values taken or extrapolated from {Cuevas Arenas, 2016 #898} **: apparent Rsol ***: nSMA3:1/nLipids ratio for which nanodiscs of 30 nm diameter were observed.

Acknowledgments:

The authors are thankful to Sandro Keller for fruitful exchanges. M.A. is gratefully acknowledges the financial support of the Faculté des Etudes Supérieures et Post-doctorales (FESP) from Université de Montréal, of the Natural Sciences and Engineering Research Council (NSERC), of Canada, the Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT), through its excellence network program supporting the Québec Centre for Advanced Materials, Université de Bordeaux's Initiative d'Excellence (IdEx) and Mitacs Globalink.