Théo Z Hirsch

Séverine Martin-Lannerée

Sophie Mouillet-Richard
email: sophie.mouillet-richard@parisdescartes.fr

FUNCTIONS OF THE PRION PROTEIN

Keywords: CNS function, cell fate and differentiation, protection against stress, cell adhesion, cell signalling

Although initially disregarded compared to prion pathogenesis, the functions exerted by the cellular prion protein PrP C have gained much interest over the past two decades.

Research aiming at unravelling PrP C functions started to intensify when it became appreciated that it would give clues as to how it is subverted in the context of prion infection, and more recently, in the context of Alzheimer's disease. It must now be admitted that PrP C is implicated in an incredible variety of biological processes, including neuronal homeostasis, stem cell fate, protection against stress, or cell adhesion. It appears that these diverse roles can all be fulfilled through the involvement of PrP C in cell signalling events. Our aim here is to provide an overview of our current understanding of PrP C functions from the animal to the molecular scale and to highlight some of the remaining gaps that should be addressed in future research.

Introduction

Historically, the cellular prion protein PrP C was discovered as the normal host counterpart of the pathogenic scrapie prion protein (PrP Sc ), itself the main component of the infectious agent responsible for Transmissible Spongiform Encephalopathies 1 . In the early years of prion research, unravelling PrP C normal function seemed not as much a priority as grasping the properties of PrP Sc and understanding TSE-associated pathogenesis. This view was reversed when it came to light that unravelling PrP C function would illuminate how it is corrupted by pathogenic prions to promote neurodegeneration. The first generation of PrP-knockout mice in the early 90's 2 set the stage for studying PrP C physiological role. Although somehow disappointing at first glance since these mice develop with no overt phenotype, they have actually allowed bringing to light subtle defects and remain a major tool to dissect PrP C function. At the molecular scale, much insight has been gained through the identification of PrP Cinteracting molecules, as well as PrP C -dependent signalling events. In addition, in more recent years, our view of PrP C has evolved from a protein majorly involved in neuronal function to a broad player in cellular homeostasis, in various tissues and at various stages of life. Another equally important concept that has emerged is that PrP C exerts a broad protective role against various types of stresses, which has notably been exemplified upon challenging PrP-null animals. In this chapter, we will address these different points with the aim to provide a comprehensive view of our current knowledge of PrP C functions.

The basics of PrP C

PrP C from gene to function

PrP C is encoded by a single gene denoted Prnp, which is located on chromosome 20 in man and 2 in mouse. It possesses two paralogues, Prnd and Sprn, encoding the Doppel and Shadoo proteins respectively, whose properties have been reviewed recently [START_REF] Ciric | Biochemical insight into the prion protein family[END_REF] and will not be discussed further in the present chapter. According to species, the Prnp gene contains 2 or 3 exons, the entire coding region being contained in the last exon, excluding the possibility of alternative splicing [START_REF] Mastrangelo | The prion gene complex encoding PrPC and Doppel: insights from mutational analysis[END_REF] . The resulting native protein of 253 amino acids (human numbering) is processed to generate a mature form of 208 residues lacking its N-terminal-most signal peptide and modified in its C-terminus with the attachment of a glycosyl-phosphatidyl (GPI) moiety allowing for its anchoring to the extracellular plasma membrane [START_REF] Stahl | Glycosylinositol phospholipid anchors of the scrapie and cellular prion proteins contain sialic acid[END_REF] . Two topological transmembrane variants with either their N-terminus (CtmPrP) or C-terminus (NtmPrP) portions in the cytosol have been described [START_REF] Harris | Trafficking, turnover and membrane topology of PrP[END_REF] , and may account for the interaction of PrP C with several cytosolic partners.

In terms of secondary/tertiary structure, it can be globally divided into a N-terminally intrinsically disordered domain -often referred to as "flexible tail"-, a central hydrophobic domain and a C-terminal globular domain (Fig. 1). The latter region contains two Asparagine residues that can be N-glycosylated, flanked by two Cysteine residues forming a disulfide bridge. In addition to the full-length isoforms, PrP C can undergo proteolytic processing at various positions [START_REF] Mcdonald | A new paradigm for enzymatic control of alpha-cleavage and beta-cleavage of the prion protein[END_REF] . The best characterized and most conserved cleavage site lies in the central domain (alpha-cleavage site) and leads to the production of the N1 soluble fragment and the GPI-anchored C1 fragment (Fig. 1).

Finally, PrP C may exist under soluble full-length isoforms, resulting either from a phospholipase-dependent GPI cleavage or from proteolytic cleavage at residues 228/229 [START_REF] Mcdonald | A new paradigm for enzymatic control of alpha-cleavage and beta-cleavage of the prion protein[END_REF][START_REF] Parkin | Dual mechanisms for shedding of the cellular prion protein[END_REF] . Thus, the various post-translational modifications that can impact on PrP C give rise to a plethora of isoforms, which add to the complexity when studying PrP C function.

From an evolutionary perspective, it is noteworthy that the structural features of PrP C are conserved in vertebrates [START_REF] Aguzzi | The prion's elusive reason for being[END_REF] , most notably in mammals 10 , underscoring their functional importance. Interestingly, bioinformatics analyses suggest that the Prnp gene family may originate from a common ancestor of the ZIP family of metal ion transporter [START_REF] Schmitt-Ulms | Evolutionary descent of prion genes from the ZIP family of metal ion transporters[END_REF] . In this scheme, the metal binding capacity of PrP C (see below) would represent one of its ancestral functions.

PrP C expression and localization

Delineating the functions of PrP C necessitates taking into account when and where it is expressed. During embryonic development, Prnp mRNA expression was reported to start around embryonic day 6.5 (E6.5) in extra-embryonic tissues [START_REF] Manson | The prion protein gene: a role in mouse embryogenesis?[END_REF] , followed by the cardiac mesoderm at E7.5 [START_REF] Hidaka | The cellular prion protein identifies bipotential cardiomyogenic progenitors[END_REF] and the developing CNS at E8 14 . Around E9, PrP C is detected in the developing nervous and cardiovascular systems as well as in extraembryonic regions [START_REF] Halliez | Prion protein localizes at the ciliary base during neural and cardiovascular development and its depletion affects α-tubulin post-translational modifications[END_REF] , in line with its role in stem cell self-renewal (see below). The observation that PrP C expression in extra embryonic annexes is maintained at early differentiation stages fits in with an important -albeit little explored-role for this protein in the physiology of the placenta (reviewed in [START_REF] Makzhami | The prion protein family: a view from the placenta[END_REF] ) and in angiogenesis [START_REF] Alfaidy | Prion protein expression and functional importance in developmental angiogenesis: role in oxidative stress and copper homeostasis[END_REF][START_REF] Doeppner | Cellular prion protein promotes post-ischemic neuronal survival, angioneurogenesis and enhances neural progenitor cell homing via proteasome inhibition[END_REF] .

During neurodevelopment, PrP C remains abundant in sites of active neurogenesis such as the ventricular zone, while it is also highly expressed in regions where maturing neurons migrate such as the hippocampus and the thalamic and hypothalamic regions, consistent with its involvement in neuronal differentiation [START_REF] Benvegnù | Neurodevelopmental expression and localization of the cellular prion protein in the central nervous system of the mouse[END_REF] . Analysis of PrP C expression during forebrain development in human also indicates that synapses exhibit increasing amounts of PrP C throughout synaptogenesis [START_REF] Adle-Biassette | Immunohistochemical expression of prion protein (PrPC) in the human forebrain during development[END_REF] . In the adult, while PrP C still is enriched in the brain not only in neurons but also in glia and endothelial cells of the blood brain barrier [START_REF] Viegas | Junctional expression of the prion protein PrPC by brain endothelial cells: a role in transendothelial migration of human monocytes[END_REF] , it is also widely expressed in all tissues.

At the sub-cellular scale, most PrP C is found at the membrane, and is notably enriched in lipid rafts, or at the synapse in neurons [START_REF] Collins | Molecular characterization and comparison of the components and multiprotein complexes in the postsynaptic proteome[END_REF][START_REF] Moya | Immunolocalization of the cellular prion protein in normal brain[END_REF] . This plasma membrane location led to the assumption that it may act as a cell surface receptor or co-receptor and instruct signal transduction events. PrP C -dependent signalling now represents one of the most fertile areas of research related to PrP C function, and will be addressed in detail in section 8.

Beyond the membrane, a less studied spatial location is the cytosol, where PrP C seems to accumulate in conditions of stress [START_REF] Dubnikov | PrP-containing aggresomes are cytosolic components of an ER quality control mechanism[END_REF] . Other locations include the base of the primary cilium [START_REF] Halliez | Prion protein localizes at the ciliary base during neural and cardiovascular development and its depletion affects α-tubulin post-translational modifications[END_REF] , the nucleus [START_REF] Strom | Cellular prion protein localizes to the nucleus of endocrine and neuronal cells and interacts with structural chromatin components[END_REF] or extracellular vesicles such as exosomes [START_REF] Vella | Packaging of prions into exosomes is associated with a novel pathway of PrP processing[END_REF] . At present, the mechanisms sustaining the trafficking of PrP C to the cilia or its shuttling to the nucleus remain enigmatic.

Lessons from knockout models

In the early 90's, several groups reported on the generation of PrP-null mice. Beyond a drastic neurodegenerating phenotype due to the accidental upregulation of the PrP paralogue Doppel, whose gene is located downstream from Prnp 27 and a mis-attributed regulation of phagocytosis due to confounding effects of the flanking gene Sirpa 28 , these various models have revealed that PrP C deficiency is associated with a variety of neurological defects. These include alterations in memory and locomotor activity [START_REF] Coitinho | Cellular prion protein ablation impairs behavior as a function of age[END_REF] , circadian rythms [START_REF] Tobler | Altered circadian activity rhythms and sleep in mice devoid of prion protein[END_REF][START_REF] Vidal | Early dysfunction of central 5-HT system in a murine model of bovine spongiform encephalopathy[END_REF] , olfaction [START_REF] Pichon | Olfactory behavior and physiology are disrupted in prion protein knockout mice[END_REF] , pain sensitivity [START_REF] Vidal | Early dysfunction of central 5-HT system in a murine model of bovine spongiform encephalopathy[END_REF][START_REF] Gadotti | Cellular prion protein protects from inflammatory and neuropathic pain[END_REF] , and a depressive-like behaviour [START_REF] Gadotti | Depressive-like behaviour of mice lacking cellular prion protein[END_REF] . Interestingly, some defects appear to worsen with age [START_REF] Schmitz | Loss of Prion Protein Leads to Age-Dependent Behavioral Abnormalities and Changes in Cytoskeletal Protein Expression[END_REF] , suggesting that the lack of PrP C may be more detrimental at defined periods of life [START_REF] Gasperini | Prion protein and aging[END_REF] . Prnp-null mice have also been instrumental in revealing that PrP C is necessary for the long-term maintenance of hematopoietic stem cells (HSCs) [START_REF] Zhang | Prion protein is expressed on longterm repopulating hematopoietic stem cells and is important for their self-renewal[END_REF] and for the proliferation of neural progenitors [START_REF] Steele | Prion protein (PrPc) positively regulates neural precursor proliferation during developmental and adult mammalian neurogenesis[END_REF] , opening the question of the role exerted by this protein in stem cell biology (see below and [START_REF] Martin-Lannerée | PrPC from stem cells to cancer[END_REF] ). Another set of observations has exemplified an increased vulnerability of PrPnull mice to various types of stresses: ischemia [START_REF] Mclennan | Prion protein accumulation and neuroprotection in hypoxic brain damage[END_REF] , behavioural stresses [START_REF] Nico | Altered behavioural response to acute stress in mice lacking cellular prion protein[END_REF] or inflammatory conditions (reviewed in [START_REF] Bakkebø | The Cellular Prion Protein: A Player in Immunological Quiescence[END_REF] ). Perhaps the best understood phenotype at present is the demyelinating disease not only observed in first-generation PrP knockout mice [START_REF] Bremer | Axonal prion protein is required for peripheral myelin maintenance[END_REF] but also confirmed in Prnp-ablated mice generated through genome editing [START_REF] Nuvolone | Strictly co-isogenic C57BL/6J-Prnp-/mice: A rigorous resource for prion science[END_REF] .

Indeed, molecular studies have shown that the PrP C N-terminal acts as an agonistic ligand of the Adgrg6 receptor, whose activation is necessary for myelin maintenance [START_REF] Küffer | The prion protein is an agonistic ligand of the G protein-coupled receptor Adgrg6[END_REF] .

Finally, in contrast to the mild phenotypes observed in mice invalidated for PrP C , a drastic impact of the knockdown of the fish Prnp orthologue PrP-1 was reported in zebrafish, with a lethal phenotype due to cell adhesion and gastrulation defects [START_REF] Malaga-Trillo | Regulation of embryonic cell adhesion by the prion protein[END_REF] .

Interestingly, the phenotype of PrP-1 morphants could be rescued using mouse PrP C , highlighting an evolutionary conserved function [START_REF] Malaga-Trillo | Regulation of embryonic cell adhesion by the prion protein[END_REF] . Readers are referred to the recent review by Halliez et al. for a comprehensive view of the consequences of PrP C invalidation [START_REF] Halliez | To develop with or without the prion protein[END_REF] .

Miscellaneous roles in CNS function

Cognitive and behavioural functions

Because TSE are neurodegenerative diseases and because PrP C is most abundantly expressed in the brain, PrP C function has been the most studied in the context of neurons. The notion that pathogenic prions exert their toxicity by deviating the normal function of PrP C in neurons [START_REF] Westergard | The cellular prion protein (PrP(C)): its physiological function and role in disease[END_REF] has further boosted research on the physiological role exerted by PrP C in these cells. As mentioned above, synaptic deficits were among the first defects to be described in PrP-null mice [START_REF] Collinge | Prion protein is necessary for normal synaptic function[END_REF] , although initially questioned [START_REF] Lledo | Mice deficient for prion protein exhibit normal neuronal excitability and synaptic transmission in the hippocampus[END_REF] . Further studies have consistently reported on alterations of synaptic activity in absence of PrP C 51,52 , including in the context where PrP C is ablated at a post-natal stage [START_REF] Mallucci | Post-natal knockout of prion protein alters hippocampal CA1 properties, but does not result in neurodegeneration[END_REF] . Incidentally, the notion that PrP C may contribute to memory is also indirectly supported by the identification of SNPs in the human PRNP gene associated with enhanced long-term memory [START_REF] Papassotiropoulos | The prion gene is associated with human long-term memory[END_REF] and the demonstration that PRNP is a target gene of FOXP2, a key transcription factor involved in language development [START_REF] Konopka | Human-specific transcriptional regulation of CNS development genes by FOXP2[END_REF] . The regulation of Prnp gene expression by the E4BP4 transcription factor [START_REF] Burgess | Identification of adjacent binding sites for the YY1 and E4BP4 transcription factors in the ovine PrP (Prion) gene promoter[END_REF] , which is involved in the regulation of the circadian clock [START_REF] Ben-Moshe | The light-induced transcriptome of the zebrafish pineal gland reveals complex regulation of the circadian clockwork by light[END_REF] , may on another hand accommodate the link between PrP C and circadian rhythms [START_REF] Tobler | Altered circadian activity rhythms and sleep in mice devoid of prion protein[END_REF][START_REF] Vidal | Early dysfunction of central 5-HT system in a murine model of bovine spongiform encephalopathy[END_REF] and contribute to the induction of PrP C mRNAs in response to melatonin [START_REF] Dupré | Identification of melatonin-regulated genes in the ovine pituitary pars tuberalis, a target site for seasonal hormone control[END_REF] .

Neuritogenesis and neuronal differentiation

From a mechanistic point of view, the contribution of PrP C to synaptic function may operate at different levels. A first axis relates to neuronal differentiation and brain wiring. The contribution of PrP C to neuronal differentiation has been mostly investigated from a cell architecture point of view, although the implementation of neuronal polarity and that of neurotransmitter-associated functions are intimately connected [START_REF] Jung | Axonal mRNA localization and local protein synthesis in nervous system assembly, maintenance and repair[END_REF] , and are both altered in the context of prion infection [START_REF] Alleaume-Butaux | Double-Edge Sword of Sustained ROCK Activation in Prion Diseases through Neuritogenesis Defects and Prion Accumulation[END_REF] . Several studies indeed have exemplified a participation of PrP C to neurite elongation and synapse development [START_REF] Amin | Characterization of prion protein function by focal neurite stimulation[END_REF][START_REF] Chen | Prion protein as transinteracting partner for neurons is involved in neurite outgrowth and neuronal survival[END_REF][START_REF] Graner | Cellular prion protein binds laminin and mediates neuritogenesis[END_REF][START_REF] Kanaani | Recombinant prion protein induces rapid polarization and development of synapses in embryonic rat hippocampal neurons in vitro[END_REF][START_REF] Loubet | Neuritogenesis: the prion protein controls beta1 integrin signaling activity[END_REF][START_REF] Santuccione | Prion protein recruits its neuronal receptor NCAM to lipid rafts to activate p59fyn and to enhance neurite outgrowth[END_REF] . This positive action of PrP C on neuritogenesis may depend on the functional interaction of PrP C with various partners: the neural cell adhesion molecule NCAM [START_REF] Amin | Characterization of prion protein function by focal neurite stimulation[END_REF][START_REF] Santuccione | Prion protein recruits its neuronal receptor NCAM to lipid rafts to activate p59fyn and to enhance neurite outgrowth[END_REF] , laminin [START_REF] Graner | Cellular prion protein binds laminin and mediates neuritogenesis[END_REF][START_REF] Beraldo | Metabotropic glutamate receptors transduce signals for neurite outgrowth after binding of the prion protein to laminin gamma1 chain[END_REF] , STI1 [START_REF] Lopes | Interaction of cellular prion and stressinducible protein 1 promotes neuritogenesis and neuroprotection by distinct signaling pathways[END_REF][START_REF] Santos | Laminin-gamma1 chain and stress inducible protein 1 synergistically mediate PrPC-dependent axonal growth via Ca2+ mobilization in dorsal root ganglia neurons[END_REF] , integrins [START_REF] Loubet | Neuritogenesis: the prion protein controls beta1 integrin signaling activity[END_REF][START_REF] Hajj | Cellular prion protein interaction with vitronectin supports axonal growth and is compensated by integrins[END_REF][START_REF] Pantera | PrPc activation induces neurite outgrowth and differentiation in PC12 cells: role for caveolin-1 in the signal transduction pathway[END_REF] , vitronectin [START_REF] Hajj | Cellular prion protein interaction with vitronectin supports axonal growth and is compensated by integrins[END_REF] or itself [START_REF] Amin | Characterization of prion protein function by focal neurite stimulation[END_REF] and the recruitment of signal transduction cascades, as will be described below. On another hand, PrP C was shown to potentiate the inhibitory effect of the contactin-associated protein (Caspr) on neurite outgrowth in a Reelin-dependent manner [START_REF] Devanathan | Cellular form of prion protein inhibits Reelin-mediated shedding of Caspr from the neuronal cell surface to potentiate Caspr-mediated inhibition of neurite outgrowth[END_REF] . In mature neurons, PrP C may further contribute to the structural maintenance of the synapse by regulating cytoskeletal components such as cofilin or local protein translation, two processes that are impaired under prion infection [START_REF] Alleaume-Butaux | Double-Edge Sword of Sustained ROCK Activation in Prion Diseases through Neuritogenesis Defects and Prion Accumulation[END_REF][START_REF] Moreno | Sustained translational repression by eIF2alpha-P mediates prion neurodegeneration[END_REF] .

Balancing neuroprotection and neurotoxicity

Given the beneficial functions exerted by PrP C in neuronal physiology (see also section 6 for the protective role of PrP C against stress), it came as a surprise that antibodymediated crosslinking of PrP C caused extensive neuronal apoptosis in vivo [START_REF] Solforosi | Cross-linking cellular prion protein triggers neuronal apoptosis in vivo[END_REF] , hinting at a pro-apoptotic role of this protein. Several observations obtained in mice expressing mutant forms of PrP C have additionally brought insight into the dichotomous role of PrP C as a trigger of pro-survival or pro-apoptotic cascades in neurons. For instance, mice expressing a N-terminally truncated form of PrP C (∆32-121 or ∆32-134) were shown to suffer from ataxia and cerebellar degeneration [START_REF] Shmerling | Expression of amino-terminally truncated PrP in the mouse leading to ataxia and specific cerebellar lesions[END_REF] , pointing to a protective role of the Nterminal flexible tail of PrP C . Deletions of the central hydrophobic region (∆94-134 or ∆105-125) also caused drastic spontaneous cerebellar disorders [START_REF] Baumann | Lethal recessive myelin toxicity of prion protein lacking its central domain[END_REF][START_REF] Li | Neonatal lethality in transgenic mice expressing prion protein with a deletion of residues 105-125[END_REF] . Deleting the central domain in PrP C notably obliterates the alpha-cleavage site and precludes the generation of the soluble N1 fragment, raising the possibility that these severe phenotypes are caused -at least partly -by the lack of N1.

More recently, several elegant studies lead by the Aguzzi's group and exploiting a panel of antibodies targeting various epitopes of PrP C have shed further light on the respective roles of the N-terminal and C-terminal regions of PrP C . One outstanding conclusion of these works is that antibodies targeting the globular domain exert a neurotoxic effect in mice or cerebellar organotypic slices and seem to mimic the effect of prion infection [START_REF] Herrmann | Prion infections and anti-PrP antibodies trigger converging neurotoxic pathways[END_REF] , while those directed against the flexible tail are innocuous [START_REF] Sonati | The toxicity of antiprion antibodies is mediated by the flexible tail of the prion protein[END_REF] . Of note, the latter protect against the deleterious effects of antibodies to the globular domain [START_REF] Herrmann | Prion infections and anti-PrP antibodies trigger converging neurotoxic pathways[END_REF] , as well as against the toxicity of PrP C mutants bearing a deletion of the central domain (PrP∆94-134) [START_REF] Sonati | The toxicity of antiprion antibodies is mediated by the flexible tail of the prion protein[END_REF] .

These observations hence corroborate the notion that the flexible tail of PrP C and its globular domain exert distinct action. Since the flexible tail of PrP C was recently identified as an agonistic ligand of the Adgrg6 receptor to sustain myelin maintenance [START_REF] Küffer | The prion protein is an agonistic ligand of the G protein-coupled receptor Adgrg6[END_REF] , we may envision that it exerts broader agonistic effects on yet-to-be-identified receptor(s) to maintain brain homeostasis. Other members of the adhesion GPCR family beyond Adgrg6 itself clearly represent candidates for relaying the protective action of PrP C N-terminal tail in view of their conspicuous roles in nervous system development and function [START_REF] Langenhan | Adhesion G protein-coupled receptors in nervous system development and disease[END_REF] .

Beyond neurons: PrP C in glia

It has recently emerged that neural circuits are also shaped by complex neuron-glia interactions [START_REF] Clarke | Emerging roles of astrocytes in neural circuit development[END_REF][START_REF] Wake | Microglia: actively surveying and shaping neuronal circuit structure and function[END_REF] . In this scheme, the contribution of PrP C to proper brain function may involve dialogues between neurons and other cell types, as described for the PrP C -Adgrg6 duo at the interface between neurons and Schwann cells [START_REF] Küffer | The prion protein is an agonistic ligand of the G protein-coupled receptor Adgrg6[END_REF] . For instance, PrP C from astrocytes was shown to promote neuritogenesis and neuronal survival [START_REF] Lima | Cellular prion protein expression in astrocytes modulates neuronal survival and differentiation[END_REF] . More generally, PrP C from astrocytes appears to play a protective role under stress conditions, both on astrocytes themselves [START_REF] Bertuchi | PrPC displays an essential protective role from oxidative stress in an astrocyte cell line derived from PrPC knockout mice[END_REF] , but also on neighbouring neurons [START_REF] Guitart | Improvement of neuronal cell survival by astrocyte-derived exosomes under hypoxic and ischemic conditions depends on prion protein[END_REF] . In the latter study, PrP C was shown to be transferred from astrocytes to neurons via exosomes [START_REF] Guitart | Improvement of neuronal cell survival by astrocyte-derived exosomes under hypoxic and ischemic conditions depends on prion protein[END_REF] .

Adding yet another layer of complexity to the picture, PrP C appears to regulate the secretion of exosomes itself [START_REF] Dias | PRNP/prion protein regulates the secretion of exosomes modulating CAV1/caveolin-1-suppressed autophagy[END_REF] . We may hence suspect that the absence of PrP C may have broader impact on brain extracellular vesicles, which represents an important means of intercellular communication [START_REF] Budnik | Extracellular vesicles round off communication in the nervous system[END_REF] . Other consequences of PrP deficiency in astrocytes include defects in lactate transport [START_REF] Kleene | Prion protein regulates glutamate-dependent lactate transport of astrocytes[END_REF] , as well as altered excitatory amino acid transporter activity [START_REF] Guitart | Prion protein regulates glutathione metabolism and neural glutamate and cysteine uptake via excitatory amino acid transporter 3[END_REF][START_REF] Pathmajeyan | Increased excitatory amino acid transport into murine prion protein knockout astrocytes cultured in vitro[END_REF] .

The metal connection

The involvement of PrP C in neuronal functions may further arise from its ability to bind divalent cations in its N-terminal region. Following the first report that PrP C binds copper in vivo [START_REF] Brown | The cellular prion protein binds copper in vivo[END_REF] , several studies have suggested that PrP C may be involved in the regulation of copper concentration at the synapse (see [START_REF] Vassallo | Cellular prion protein function in copper homeostasis and redox signalling at the synapse[END_REF] for review). Interestingly, copper appears to influence some PrP C -mediated cell signalling events [START_REF] Haigh | Copper, constitutive endoproteolytic processing of the prion protein and cell signalling[END_REF][START_REF] Hernandez-Rapp | A PrP(C)-caveolin-Lyn complex negatively controls neuronal GSK3beta and serotonin 1B receptor[END_REF] . More recently, Gasperini et al found that PrP C cooperates with copper to induce Snitrosylation of the NMDA receptor [START_REF] Gasperini | Prion protein and copper cooperatively protect neurons by modulating NMDA receptor through Snitrosylation[END_REF] . Beyond copper, PrP C also binds zinc ions and facilitate their uptake in neuronal cells via AMPA receptors [START_REF] Watt | Prion protein facilitates uptake of zinc into neuronal cells[END_REF] . Altogether, several reports converge onto a protective role against glutamate or AMPA/kainate-induced excitotoxicity [START_REF] Gasperini | Prion protein and copper cooperatively protect neurons by modulating NMDA receptor through Snitrosylation[END_REF][START_REF] Black | Cellular prion protein and NMDA receptor modulation: protecting against excitotoxicity[END_REF][START_REF] Carulla | Involvement of PrP(C) in kainateinduced excitotoxicity in several mouse strains[END_REF][START_REF] Rambold | Stressprotective signalling of prion protein is corrupted by scrapie prions[END_REF] . It is noteworthy that this protective role with respect to neuron excitability is conserved in fish 100 .

From neurotransmitter receptors to neurotransmitter-associated functions

The above studies further point to crosstalks between PrP C and neurotransmitter receptors. These functional interactions, which have been reviewed recently 101 , extend beyond NMDA and AMPA receptors and also include serotonergic [START_REF] Hernandez-Rapp | A PrP(C)-caveolin-Lyn complex negatively controls neuronal GSK3beta and serotonin 1B receptor[END_REF]102,103 or cholinergic 104,105 receptors. An interaction between PrP C and the purinergic P2X4R has also been recently described 106 . A related emerging theme is the broader implication of PrP C in the fine-tuning of neurotransmitter-associated functions. Indeed, at the animal scale, the absence of PrP C appears to induce defects in the dopaminergic 107,108 , cholinergic 109 or serotonergic [START_REF] Vidal | Early dysfunction of central 5-HT system in a murine model of bovine spongiform encephalopathy[END_REF]107 systems. As for the latter, PrP C contributes to the regulation of monoamine synthesis 110 , catabolism 111 , or release [START_REF] Hernandez-Rapp | A PrP(C)-caveolin-Lyn complex negatively controls neuronal GSK3beta and serotonin 1B receptor[END_REF] through various cell-signalling dependent mechanisms. Interestingly, a modulatory role of PrP C on synaptic release was recently exemplified at the Drosophila neuro-muscular junction 112 , further adding support to a potentiating role of PrP C in synaptic transmission 113 (for review).

Cell fate and differentiation

Stem cell self-renewal and expansion

Interest for a potential contribution of PrP C to the biology of stem cells arose with the discovery that it is abundantly expressed in HSCs, including the most immature Lin - Sca + Endoglin + subpopulation, and that it is necessary for the long-term maintenance of these cells [START_REF] Zhang | Prion protein is expressed on longterm repopulating hematopoietic stem cells and is important for their self-renewal[END_REF] . PrP C was subsequently shown to be highly expressed in neural [START_REF] Steele | Prion protein (PrPc) positively regulates neural precursor proliferation during developmental and adult mammalian neurogenesis[END_REF] , cardiac [START_REF] Hidaka | The cellular prion protein identifies bipotential cardiomyogenic progenitors[END_REF] , mammary 114 , skeletal muscle 115 or mesenchymal 116 progenitors. Thus, high PrP C expression appears as a generic feature of various types of stem cells. In line with this, PrP C was shown to be upregulated following the induction of multipotential embryonic stem (ES) or carcinoma (EC) cells towards a more restricted neural 117,118 or cardiac [START_REF] Hidaka | The cellular prion protein identifies bipotential cardiomyogenic progenitors[END_REF] fate, or during their spontaneous differentiation 119,120 . Furthermore, expressing PrP C in ES cells grown under self-renewal conditions was reported to promote their induction towards lineages of the three germ layers 121 . Altogether, these observations posit PrP C as an important player in stem cell fate determination and raise the question as to the cellular processes to which it may participate in a stem cell context. One recurring theme is its contribution to proliferation. For instance, PrP C levels correlate with the proliferation rate of neural progenitors in two neurogenic regions in adult mice: the subventricular zone (SVZ) and the dentate gyrus (DG) [START_REF] Steele | Prion protein (PrPc) positively regulates neural precursor proliferation during developmental and adult mammalian neurogenesis[END_REF] . Ex vivo, neurospheres from fetal 122 or adult 123 brain further proliferate less efficiently when derived from Prnp knockout versus wild type mice. These defects may notably be explained by a lack of response to the PrP C ligand STI1 122 . The contribution of PrP C to stem cell proliferation also holds true for mesenchymal progenitors 116 . Interestingly, PrP C expression was recently shown to be induced after hypoxia preconditioning and to enhance the survival and proliferation of transplanted mesenchymal stem cells (MSCs) 124 . That PrP C may strengthen tissue repair was also exemplified in the context of HSC transplantation [START_REF] Zhang | Prion protein is expressed on longterm repopulating hematopoietic stem cells and is important for their self-renewal[END_REF] or muscle regeneration 115 . Another notable example is the participation of PrP C to recovery from ischemia [START_REF] Doeppner | Cellular prion protein promotes post-ischemic neuronal survival, angioneurogenesis and enhances neural progenitor cell homing via proteasome inhibition[END_REF][START_REF] Mclennan | Prion protein accumulation and neuroprotection in hypoxic brain damage[END_REF]125,126 , possibly after mobilization by its ligand STI1 127 .

Finally, because PrP C remains highly expressed in adult neurogenic zones, it is likely to take part to brain repair following stress or injury, a process that appears to be defective under prion infection 128 .

A hub in developmental signalling pathways

At a mechanistic level, we reported on altered cell cycling and reduced expression of Cyclin D1 after knockdown of PrP C in a neuronal progenitor cell line [START_REF] Halliez | Prion protein localizes at the ciliary base during neural and cardiovascular development and its depletion affects α-tubulin post-translational modifications[END_REF] . These changes may notably be accounted for by defects in the Hedgehog pathway, that we identified in vitro and further confirmed in vivo within a narrow time-window during embryonic development [START_REF] Halliez | Prion protein localizes at the ciliary base during neural and cardiovascular development and its depletion affects α-tubulin post-translational modifications[END_REF] . Another important clue to the PrP C -Hedgehog connection is the location of PrP C at the base of the primary cilium, where Hedgehog signalling operates 129 , highlighting a potential role for PrP C in primary cilium turnover [START_REF] Halliez | Prion protein localizes at the ciliary base during neural and cardiovascular development and its depletion affects α-tubulin post-translational modifications[END_REF] . It is of note that PrP C was found in the proteome of the primary cilium in three independent studies [130][131][132] , calling for a better knowledge of the function exerted by PrP C in this specific organelle.

Beyond the Hedgehog pathway, we recently added another piece to the PrP-stem cell puzzle by uncovering a connection between PrP C and the Notch pathway 133 . We found that the absence of PrP C in neural progenitors is associated with a marked reduction in the expression of various effectors of the Notch pathway, which plays an overriding role in neural stem cell maintenance 134 . This correlates with subtle differences in the developing nervous system of Prnp knockout mice, including transient decreases in the expression of the neural stem cell marker Nestin, and altered distribution of N-cadherin, which is important for the maintenance of neural progenitors in the ventricular zone 135 .

The latter observation also fits in with the proposed role of PrP C in cell adhesion, notably during embryonic development (see section 7). Our study further allowed delineating a gene regulatory network involving PrP C , Notch and the EGF receptor (EGFR), where PrP C positively controls the expression of EGFR via Notch while EGFR exerts a negative feedback on both PrP C and Notch 133 . Adding another layer to this network, we documented a positive feedback regulation of the Notch pathway on PrP C . Incidentally, the Notch target gene Hes1 was reported to bind to the Prnp promoter 136 and thus represents a good candidate for relaying this process. Finally, because PrP C , Notch and EGFR all regulate HSC self-renewal in the bone-marrow niche 137,138 , we may speculate that this network also operates in HSCs.

Choice of cell fate and differentiation

A corollary to the reduced self-renewal of neural progenitors in absence of PrP C would be a precocious engagement into a more mature state. This notion is supported by the increase in Doublecortin-positive neuroblasts in Prnp-null mice 123 . The absence of PrP C further promotes an increase in NG2 + Olig2 + oligodendrocyte precursor cells, both during embryonic development and in the adult brain 139 . In line with the notion that Notch signalling negatively regulates Olig2 expression during brain patterning 140 , we proposed that Notch pathway alterations may account for the increased Olig2 expression in the neural tube of PrP-null embryos 133 . Curiously, while PrP C is necessary for the maintenance of neural progenitor cells in an immature state, it is on another hand required for the early steps of neurite elongation [START_REF] Loubet | Neuritogenesis: the prion protein controls beta1 integrin signaling activity[END_REF] and subsequent neuronal differentiation (see section 4.2). One explanation for this apparent discrepancy would be the multiplicity of PrP C isoforms, likely endowed with specific -or even oppositefunctions. We may also suspect the involvement of defined partners, which would engage preferentially with a subset of PrP C isoforms. Although much speculative at present, such a view is nevertheless supported by the reported competitive action of the two PrP C ligands STI1 and A for the recruitment of protective and toxic pathways, respectively 141 . Apart from neurons, high levels of PrP C were shown to enhance the development of astrocytes 142 . Finally, PrP C is differentially regulated according to cell fate along the hematopoietic lineage as well (reviewed in [START_REF] Martin-Lannerée | PrPC from stem cells to cancer[END_REF][START_REF] Bakkebø | The Cellular Prion Protein: A Player in Immunological Quiescence[END_REF] ). For instance, PrP C expression is decreased along erythroid differentiation 143 . In this context, it is noteworthy that mutant goats naturally devoid of PrP C exhibit elevated red blood cell number 144 .

A broad protective role against stress

PrP C is induced under stress

Dissecting the regulation of a given gene can teach us about the function of the corresponding protein. In the case of PrP C , several analyses of the Prnp promoter have notably revealed binding sites for various transcription factors, including Sp1 and AP-1 145 , p53 146 , the heat shock factor HSF1 147 , sXBP1 148 or FOXO3A 149 . Interestingly, Qin et al. 150 reported on the induction of PrP C expression by Sp1 and p53 following the activation of the ATM kinase under oxidative stress conditions. In line with this, the PrP C protein content was shown to correlate with altered redox states in prostate spheroids 151 . The Sp1 transcription factor, which is positively regulated by the hypoxia-inducible factor HIF was also proposed to account for the induction of PrP C under hypoxic conditions 152 . The upregulation of PrP C in response to hypoxia has been further corroborated by diverse studies [START_REF] Doeppner | Cellular prion protein promotes post-ischemic neuronal survival, angioneurogenesis and enhances neural progenitor cell homing via proteasome inhibition[END_REF]153 . The regulation of the Prnp gene by sXBP1 was on another hand shown to promote PrP C expression under ER stress conditions 148,154 . As for the control of Prnp gene expression by FOXO3A, it may notably have relevance as to contribution of PrP C to the long-term maintenance of HSCs since FOXO3A is essential for this process 155 . A FOXO3A-dependent induction of PrP C could also possibly account for the protection afforded by FOXO3A in HSCs against oxidative DNA damage 156 . In addition to these findings, an interesting observation is the upregulation of PrP C in neutrophils after systemic exposure to LPS, which was shown to involve TGF and glucocorticoids 157 . The latter study hints at a potential role of PrP C in the response to neuroimmunoendocrine stress. In agreement, PrP C was recently shown to mitigate the inflammatory response to LPS 158 .

Protection against oxidative stress

That PrP C protects against oxidative stress is a recurring theme in PrP C function and has been addressed in several reviews [START_REF] Aguzzi | The prion's elusive reason for being[END_REF][START_REF] Vassallo | Cellular prion protein function in copper homeostasis and redox signalling at the synapse[END_REF]159,160 . A global notion is that PrP C may take part to antioxidant defence either by directly buffering ROS 161 , undergoing processing within its N-terminal region (so-called beta-cleavage) 162,163 or by promoting the induction of antioxidant enzyme activities and increasing glutathione levels [START_REF] Guitart | Prion protein regulates glutathione metabolism and neural glutamate and cysteine uptake via excitatory amino acid transporter 3[END_REF]124,164 . These actions are directly connected to the copper-binding capacity of PrP C and depend on its Nterminal domain 160 . The protective action of PrP C against oxidative stress is not restricted to neurons, but was also reported in various cell contexts including astrocytes [START_REF] Bertuchi | PrPC displays an essential protective role from oxidative stress in an astrocyte cell line derived from PrPC knockout mice[END_REF] , MSCs 124 or the placenta [START_REF] Alfaidy | Prion protein expression and functional importance in developmental angiogenesis: role in oxidative stress and copper homeostasis[END_REF] . This function underlies, at least partly, the beneficial role exerted by PrP C under hypoxic conditions [START_REF] Doeppner | Cellular prion protein promotes post-ischemic neuronal survival, angioneurogenesis and enhances neural progenitor cell homing via proteasome inhibition[END_REF]124 .

Protection against genotoxic stress

The beneficial action of PrP C against oxidative stress extends to the protection against reactive oxygen species-induced DNA damage 165,166 and also involves the N-terminal domain of the protein 166 . Recently, Bravard et al. brought further insight into the relationship between PrP C and the DNA damage response by demonstrating that PRNP gene transcription is upregulated in neurons after exposure to a genotoxic stress 167 . The resulting increased PrP C was shown to accumulate in the nucleus, where it interacts with the endonuclease APE1 and stimulates its base excision repair activity. The involvement of PrP C in protection against genotoxic stress is also supported by its positive regulation by the ATM kinase 150 , a major guardian of genome integrity 168 .

Protection against apoptosis

The protective role exerted by PrP C against other types of deleterious inputs has been extensively exemplified in various settings (see 159,169 for review). The pro-survival action of PrP C has notably been documented in the context of serum deprivation 170 , caspase activation by staurosporine [171][172][173] , protein synthesis inhibition 174 , calcium overload 172 , ischemia [START_REF] Doeppner | Cellular prion protein promotes post-ischemic neuronal survival, angioneurogenesis and enhances neural progenitor cell homing via proteasome inhibition[END_REF][START_REF] Mclennan | Prion protein accumulation and neuroprotection in hypoxic brain damage[END_REF]125,126 or excitotoxicity [START_REF] Gasperini | Prion protein and copper cooperatively protect neurons by modulating NMDA receptor through Snitrosylation[END_REF][START_REF] Black | Cellular prion protein and NMDA receptor modulation: protecting against excitotoxicity[END_REF][START_REF] Carulla | Involvement of PrP(C) in kainateinduced excitotoxicity in several mouse strains[END_REF][START_REF] Rambold | Stressprotective signalling of prion protein is corrupted by scrapie prions[END_REF] . From a mechanistic point of view, the cytoprotective role of PrP C is imparted by its N-terminal domain [START_REF] Rambold | Stressprotective signalling of prion protein is corrupted by scrapie prions[END_REF]125,171,175 (see also section 4.3). It may relate to its mobilization by STI1 [START_REF] Lopes | Interaction of cellular prion and stressinducible protein 1 promotes neuritogenesis and neuroprotection by distinct signaling pathways[END_REF]127,176,177 , and the downstream activation of PI3K/Akt 126,175 or PKA [START_REF] Lopes | Interaction of cellular prion and stressinducible protein 1 promotes neuritogenesis and neuroprotection by distinct signaling pathways[END_REF] . An alternative mode of action relies on the shedding of the soluble N1 fragment and the downregulation of p53 171 .

An immune privilege molecule ?

These well-established cytoprotective actions of PrP C together with its high and regulated expression in the immune system led to the proposal that it may contribute to immune quiescence [START_REF] Bakkebø | The Cellular Prion Protein: A Player in Immunological Quiescence[END_REF] . Immune quiescence, also termed immune privilege, encompasses a combination of anatomical, physical and immunoregulatory processes that restrain immune-mediated inflammation in sites where it may cause irreversible damage 178 . The PrP C -immune privilege connection is supported by the abundant expression of PrP C in tissues with immune privilege, such as the brain and the placenta, together with a set of observations highlighting that the absence of PrP C aggravates pathology in various experimental inflammatory settings [START_REF] Bakkebø | The Cellular Prion Protein: A Player in Immunological Quiescence[END_REF] . The concept that PrP C may be considered as an immune privilege molecule provides a new angle for investigating PrP C function and will certainly constitute an interesting area for future research.

Involvement in proteostasis

Proteostasis refers to the global network allowing for cellular proteins to maintain a proper biological activity according to the cell environment 179 . A strong indicator that PrP C may contribute to the proteostatic network is the regulation of Prnp gene transcription by various factors of this network 179 , such as HSF1 147 , sXBP1 148 or FOXO3A 149 . In addition, in terms of protein translation, one interesting observation is that in the course of TSEs, while activation of the PERK pathway halts the translation of most proteins, including synaptic constituents, Prnp transcripts escape this negative regulation [START_REF] Moreno | Sustained translational repression by eIF2alpha-P mediates prion neurodegeneration[END_REF] . From a functional point of view, PrP C may take part to the proteostatic network at several levels: by promoting protein synthesis via mTOR 180 , activating autophagy by recruiting Beclin1 181 or mediating the action of the cholinergic alpha7nAchR 105 , organizing the interplay between exosome secretion and autophagosome formation [START_REF] Dias | PRNP/prion protein regulates the secretion of exosomes modulating CAV1/caveolin-1-suppressed autophagy[END_REF] or protecting against ER stress 182 . Finally, the location of PrP C at the base of the primary cilium and its potential impact on ciliary turnover 15 may also have relevance as to this issue in view of the emerging relationship between cellular proteostasis and ciliogenesis 183 .

Cell adhesion and ECM contacts

Multiple adhesion partners

The involvement of PrP C in cell adhesion was first supported by the demonstration that it interacts with several proteins relevant to this process such as the ECM molecules laminin [START_REF] Graner | Cellular prion protein binds laminin and mediates neuritogenesis[END_REF]184 or vitronectin [START_REF] Hajj | Cellular prion protein interaction with vitronectin supports axonal growth and is compensated by integrins[END_REF] , or several transmembrane proteins such as the laminin receptor precursor 185 or NCAM 186 . PrP C has further been shown to interact with 1 integrins [START_REF] Pantera | PrPc activation induces neurite outgrowth and differentiation in PC12 cells: role for caveolin-1 in the signal transduction pathway[END_REF]187 and to modulate their activity [START_REF] Loubet | Neuritogenesis: the prion protein controls beta1 integrin signaling activity[END_REF]188 . This interplay with integrins may endow PrP C with the capacity to regulate the turnover of focal adhesions [START_REF] Loubet | Neuritogenesis: the prion protein controls beta1 integrin signaling activity[END_REF]189 . The contribution of PrP C to cell adhesion processes may further involve homophilic interactions, as suggested for instance in the case of brain endothelial cells 21 .

Special links with cadherins

Cadherins represent another class of adhesion molecules sustaining cell-cell contacts whose functional interactions with PrP C have received much interest. In intestinal cells, which form a highly polarized epithelium, PrP C was found to co-localize with E-cadherin at cell-cell junctions 190 . While E-cadherin did not co-precipitate with PrP C in that study, it is noteworthy that PrP C was however found in the interactome of E-cadherin 191 . Subsequent reports suggested that, according to cell density, PrP C may shuttle between the membrane, where it interacts with desmosomal proteins, and the nucleus, where it regulates the transcriptional activity of -catenin 192,193 . These observations are consistent with a role for PrP C in the balance between proliferation and differentiation of intestinal epithelial cells. They are further in line with the reported alteration in the intestinal epithelial barrier in PrP null mice and the decreased levels of PrP C in the intestine of patients with inflammatory bowel diseases 194 . Besides, in the context of embryonic development, the seminal study by Malaga-Trillo demonstrated that the lethal phenotype obtained upon knockdown of PrP-1 in zebrafish correlated with impaired delivery of E-cadherin at the cell surface [START_REF] Malaga-Trillo | Regulation of embryonic cell adhesion by the prion protein[END_REF] . PrP C actually appears to broadly affect the cycling of E-cadherin since it also negatively controls its endocytosis via src kinases 195 . A second member of the cadherin family, namely N-cadherin, may also be influenced by PrP C , in terms of trafficking to the neuronal cell surface 196 or distribution in the developing neuroepithelium 133 .

Modulating the ECM composition

Finally, PrP C may impact on the composition of the ECM itself, as described in astrocytes [START_REF] Lima | Cellular prion protein expression in astrocytes modulates neuronal survival and differentiation[END_REF] or neuronal progenitor cells [START_REF] Loubet | Neuritogenesis: the prion protein controls beta1 integrin signaling activity[END_REF] , or on its remodelling, for instance by controlling the activity of the metalloprotease MMP9 197 . Another context where we may suspect PrP C to act on the ECM composition is in dental tissue in view of the defects in dentin structure reported in Prnp-knockout mice 198 . The involvement of PrP C in such a broad range of biological processes raises the question as to how these heterogeneous roles can be unified. A consensual view is to consider PrP C from a cell communication point of view: its interaction with a given set of partners within a given cell environment will mobilize signal transduction cascades. In this paradigm, PrP C will serve as a "hub", connecting an input to cell adaptation. This function may operate according to different modes of action: in some instances PrP C (or its cleavage products) would serve as a ligand, as described for the activation for Adgrg6 [START_REF] Küffer | The prion protein is an agonistic ligand of the G protein-coupled receptor Adgrg6[END_REF] , while in others it may act as a cell-surface receptor or co-receptor, as reported in the case of the A-PrP C -mGluR5 complex 199 . Finally, PrP C may also indirectly regulate the activity of various signalling molecules such as 1 integrins [START_REF] Loubet | Neuritogenesis: the prion protein controls beta1 integrin signaling activity[END_REF] , E-cadherin [START_REF] Malaga-Trillo | Regulation of embryonic cell adhesion by the prion protein[END_REF] or EGFR 200 by impinging on their trafficking.

From partners to cell signalling

A great variety of partners

A plethora of PrP C partners have been identified through various methods (see [START_REF] Aguzzi | The prion's elusive reason for being[END_REF]201 for review), including using proteomics-based strategies 187,202,203 . Among those, a subset whose interaction with PrP C was shown to have functional relevance is listed in Table 1.

Two of the most studied PrP C interacting molecules until now are the extracellular chaperone STI1 and the amyloid peptide A, involved in Alzheimer's disease. STI1 was initially identified as a PrP C ligand through complementary hydropathy 204 . Subsequent studies have highlighted an involvement of the PrP C -STI1 duo in multiple biological processes including neural stem cell self renewal 122 , neuronal differentiation [START_REF] Lopes | Interaction of cellular prion and stressinducible protein 1 promotes neuritogenesis and neuroprotection by distinct signaling pathways[END_REF][START_REF] Santos | Laminin-gamma1 chain and stress inducible protein 1 synergistically mediate PrPC-dependent axonal growth via Ca2+ mobilization in dorsal root ganglia neurons[END_REF] , neuronal survival [START_REF] Lopes | Interaction of cellular prion and stressinducible protein 1 promotes neuritogenesis and neuroprotection by distinct signaling pathways[END_REF]177 , neuronal protein synthesis 180 , memory consolidation 205 , potentiation of neurotransmitter release [START_REF] Hernandez-Rapp | A PrP(C)-caveolin-Lyn complex negatively controls neuronal GSK3beta and serotonin 1B receptor[END_REF] , or protection against ischemia 127,176 . As mentioned above, STI1 may further protect against the toxicity of the PrP C alternative ligand A 141 . A has entered the PrP C field more recently, with the seminal report by Lauren et al. that PrP C serves as a receptor for A-derived diffusible ligands (ADDLs) and partially relays their toxicity 206 . The A-PrP C interplay now represents an area of intense research and has been the topic of several recent reviews [207][208][209] . Of note, some studies anterior to the Lauren paper had identified the A precursor protein APP or its paralogues APLP1 and APLP2 as PrP C partners 203,210 . From a functional point of view, the interaction of APP with PrP C was shown to be conserved from zebrafish to human and to contribute to cell adhesion and CNS development 211 . Another central PrP C partner that deserves special attention is the scaffold protein caveolin-1. Caveolin-1 not only serves as a relay for the activation of src kinases downstream from PrP C 71,94,212 , but is also involved in regulating the balance between exosomes and autophagosomes [START_REF] Dias | PRNP/prion protein regulates the secretion of exosomes modulating CAV1/caveolin-1-suppressed autophagy[END_REF] .

Caveolin may actually act as a bridging molecule between PrP C and several of its binding partners: cell signalling effectors such as src kinases [START_REF] Pantera | PrPc activation induces neurite outgrowth and differentiation in PC12 cells: role for caveolin-1 in the signal transduction pathway[END_REF][START_REF] Hernandez-Rapp | A PrP(C)-caveolin-Lyn complex negatively controls neuronal GSK3beta and serotonin 1B receptor[END_REF]212 or -catenin 192,213 , integrins [START_REF] Pantera | PrPc activation induces neurite outgrowth and differentiation in PC12 cells: role for caveolin-1 in the signal transduction pathway[END_REF] , cytoskeletal components such as actin 193,214 . The reader is referred to Table 1 for other relevant PrP C interacting molecules.

Overview of PrP C -dependent signal transduction

The involvement of PrP C in cell signalling events has been addressed in several recent reviews, with special focus on neuronal 101,215 or immune [START_REF] Bakkebø | The Cellular Prion Protein: A Player in Immunological Quiescence[END_REF] contexts. We further refer the reader to these reviews for a list of PrP C downstream targets. Mechanistically, it is now generally accepted that the mobilization of signalling cascades by GPI-anchored PrP C relies on the formation of multimolecular complexes in specific subdomains of the plasma membrane known as lipid rafts. The participation of raft constituents such as caveolin [START_REF] Pantera | PrPc activation induces neurite outgrowth and differentiation in PC12 cells: role for caveolin-1 in the signal transduction pathway[END_REF][START_REF] Hernandez-Rapp | A PrP(C)-caveolin-Lyn complex negatively controls neuronal GSK3beta and serotonin 1B receptor[END_REF]212 or flotillin 216 has been documented in some studies and is suspected in some others, where raft integrity is an important parameter 217,218 . In this line, it is noteworthy that the removal of sialic acid within the GPI anchor interferes with the targeting of PrP C to the synapse 219 , directs PrP C to a subset of rafts distinct from those where it normally segregates and hinders its activation of the cytosolic form of phospholipase A2 220 . Thus, compartmentalization represents an important parameter with respect to PrP C signalling, which may provide spatial proximity or, to the opposite, insulation between PrP C and its effectors. This notion is all the more important since prion infection may alter the spatial organisation and composition of rafts 221,222 .

Endocytosis represents a connected issue, with relevance to signal duration and formation of signalling endosomes 223 . Although little explored until now, PrP C endocytosis was shown to be important for the recruitment of ERK1/2 in response to STI1 224 . The cell surface residency of PrP C was further reported to be increased in cells exposed to A 225 , while decreased in prion infected cells 226 . Another interesting point is the notion that PrP C -dependent signalling may necessitate the formation of PrP C dimers [START_REF] Rambold | Stressprotective signalling of prion protein is corrupted by scrapie prions[END_REF]227 and in turn favour proteolytic processing [START_REF] Haigh | Copper, constitutive endoproteolytic processing of the prion protein and cell signalling[END_REF]228 . Metal and ligand binding, internalisation and cleavage may actually be intertwined processes that clearly deserve further investigation.

Finally, despite tremendous progress in the PrP C signalling field, several grey areas remain. One unresolved question relates to the subset of PrP C isoforms that specifically engages into a given signalling platform, according to the cell type or even according to the sub-cellular location, as in the case of neuronal cell bodies versus neurites [START_REF] Hernandez-Rapp | A PrP(C)-caveolin-Lyn complex negatively controls neuronal GSK3beta and serotonin 1B receptor[END_REF]212 .

Delineating the mechanisms imparting spatial and temporal control on PrP C signalling also feature among the priority axes for future investigations, since their deregulation is likely to contribute to the deleterious action of pathogenic prions 229 or other amyloid peptides such as Aβ (see 230 for review).

Concluding remarks

Because it fulfils myriads of functions and at the same time appears to be dispensable Activation of src kinases downstream from PrP C 71,94,212

-catenin

Transcriptional activity of the catenin/TCF7L2 complex 192

Legend to Figure 1:

Secondary structure of mouse PrP C emphasizing the N-terminal flexible tail and the Cterminal globular domain. N1 and C1 fragments are generated following alpha-cleavage.

N-glycosylation sites and cysteine disulfide bridge formation are indicated. Beta sheets and alpha helices are highlighted in red and green, respectively, while the hydrophobic domain is highlighted in purple.

8. 1 .

 1 Cell signalling: the unifying function of PrP C ?

  for normal development and most physiologic processes, PrP C is both a tricky and fascinating molecule. Recent years have witnessed important advance in our understanding of the cellular and molecular pathways to which PrP C contributes and we may expect exciting new discoveries in the coming years. Achieving a comprehensive picture of PrP C biological role goes well beyond curiosity as it may have implications for the development of drugs aimed at restoring PrP C functions in TSEs and other pathological settings.

Table 1

 1

	Protein name	Biological outcome
	Extracellular ligands
	STI1	Neural stem cell self renewal 122 ,
		Neuronal differentiation 68,69 ,
		Neuronal survival 68,177 ,
		Neuronal protein synthesis 180 ,
		Memory consolidation 205 ,
		Potentiation of neurotransmitter release 94 ,
		Protection against ischemia 127,176 .
	Aβ	Neurotoxicity 206 ,
		NMDA turnover 231 ,
		Tau phosphorylation 232
	ECM proteins
	Laminin	Neurite outgrowth 63,67,69 ,
		Memory 233 .
	Vitronectin	Neurite outgrowth 70
	Scaffold proteins
	Caveolin-1

  , Balance between exosomes and autophagosomes 86 Flotillin Trafficking of N-cadherin 196 , Trafficking of EGFR 200 .

		Potentiation of neurotransmitter release 94 .
	Adgrg6	Peripheral myelin maintenance 45
	Na + /K + -ATPase	Astrocytic lactate transport 88
	Glypican-1	Raft association 240 ,
	TNAP	Laminin dephosphorylation 110
	BACE1	APP cleavage 241
	Miscellaneous
	intracellular
	proteins
	Members of the Rab	PrP C trafficking 242
	family of small
	GTPases
	Argonaute	MicroRNA processing 243
	Lactate	Activation of LDHA 244
	dehydrogenase A
	(LDHA)
	Cell adhesion
	receptors
	APP	Cell adhesion and CNS development 211 .
	Integrin β1	Neuritogenesis 65,70,71 .
	37kDa/67kDa Laminin	Cell survival 234 .
	receptor
	NCAM	Neurite outgrowth 66 ,
		Neuronal differentiation 123
	Growth factor
	receptors
	EGFR	Cell proliferation 235 .
	Focal adhesion
	components
	Desmosomal proteins Maintenance of epithelial polarity 193
	Neurotransmitter
	receptors
	mGluR1	Neurite outgrowth 67
	mGluR2	Astrocytic lactate transport 88
	mGluR5	Neurite outgrowth 67
		Long-term depression 236 ,
		A-dependent neurotoxicity 199
	mGluR6/7	Protection against excitotoxicity 237
	GluA1, GluA2	Neuronal zinc transport 96
	GluN2D	Protection against excitotoxicity 238
	7nAchR	Regulation of autophagy 105
		Calcium signalling 104
	Miscellaneous cell
	surface proteins
	LRP1	Endocytosis 239 ,