Théo Z Hirsch

Julia Hernandez-Rapp

Séverine Martin-Lannerée

Jean-Marie Launay

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

PrP C signalling in neurons: from basics to clinical challenges

Keywords: Prion protein, cell signalling, neurons, A, prion diseases, Alzheimer's disease

The cellular prion protein PrP C was identified over twenty-five years ago as the normal counterpart of the scrapie prion protein PrP Sc , itself the main if not the sole component of the infectious agent at the root of Transmissible Spongiform Encephalopathies (TSEs). PrP C is a ubiquitous cell surface protein, abundantly expressed in neurons, which constitute the targets of PrP Sc -mediated toxicity. Converging evidence have highlighted that neuronal, GPIanchored PrP C is absolutely required for prion-induced neuropathogenesis, which warrants investigating into the normal function exerted by PrP C in a neuronal context. It is now wellestablished that PrP C can serve as a cell signalling molecule, able to mobilize transduction cascades in response to interactions with partners. This function endows PrP C with the capacity to participate in multiple neuronal processes, ranging from survival to synaptic plasticity. A diverse array of data have allowed to shed light on how this function is corrupted by PrP Sc . Recently, amyloid A oligomers, whose accumulation is associated with Alzheimer's disease (AD), were shown to similarly instigate toxic events by deviating PrP Cmediated signalling. Here, we provide an overview of the various signal transduction cascades ascribed to PrP C in neurons, summarize how their subversion by PrP Sc or A oligomers contributes to TSE or AD neuropathogenesis and discuss the ensuing clinical implications.

Abbreviations: AD, Alzheimer's disease; APP, amyloid precursor protein; cPLA2, cytosolic phospholipase A2; GPI, glycosylphosphatidylinositol; GPCR, G-protein coupled receptor; GSK3, glycogen synthase kinase 3 ; Ln1, laminin 1 chain; LRP-1, lowdensity lipoprotein-related protein 1; LTD, long-term depression; LTP, long-term potentiation; mTOR, mammalian target of rapamycin; NCAM, neural cell adhesion molecule; PDK1, phosphoinositide-dependent kinase 1; PI3K, phosphoinositide 3 kinase; PKA, protein kinase A; PLC, phospholipase C; PrP C , cellular prion protein; PrP Sc , scrapie prion protein; PSD, postsynaptic density; ROS, reactive oxygen species; STI-1, stressinducible protein 1; TACE, TNF-converting enzyme; TSE, Transmissible Spongiform Encephalopathies; UPR: unfolded protein response;

Introduction

The cellular prion protein PrP C has received considerable attention due to its central role in the development of Transmissible Spongiform Encephalopathies (TSEs). These fatal neurodegenerative disorders arise from the autocatalytic conversion of PrP C into its aggregate-prone conformational variant PrP Sc , standing for scrapie isoform of the prion protein [START_REF] Aguzzi | Prions: protein aggregation and infectious diseases[END_REF]. PrP Sc actually constitutes the only identified component of the infectious TSE agent, which propagates by acting as a seed to promote the templatedirected misfolding of endogenous PrP C [START_REF] Aguzzi | Prions: protein aggregation and infectious diseases[END_REF]. Accordingly, PrP-null mice fail to develop TSE upon prion inoculation [START_REF] Bueler | Mice devoid of PrP are resistant to scrapie[END_REF]. PrP C is a normal host-encoded protein of about 230 residues, present in all cell types and most abundantly found in neurons [START_REF] Linden | Physiology of the prion protein[END_REF]. PrP C possesses a glycosylphosphatidylinositol (GPI) anchor that sustains its attachment to the plasma membrane and can be hydrolyzed by endogenous phospholipases to yield secreted PrP C (see Fig. 1) [START_REF] Parkin | Dual mechanisms for shedding of the cellular prion protein[END_REF][START_REF] Puig | The GPI-anchoring of PrP: Implications in sorting and pathogenesis[END_REF]. Its main features include the presence of octapeptide repeats in the unstructured N-terminal region, involved in the binding of divalent cations such as copper [START_REF] Haigh | Copper, constitutive endoproteolytic processing of the prion protein and cell signalling[END_REF] or zinc [START_REF] Watt | The prion protein and neuronal zinc homeostasis[END_REF], as well as a disulfide bond encompassing two asparagine residues that can be potentially N-glycosylated [START_REF] Ermonval | Evolving views in prion glycosylation: functional and pathological implications[END_REF]. PrP C can undergo several proteolytic cleavages: (i) in the octarepeat region (beta-cleavage), (ii) at position 111/112 (alpha-cleavage) or (iii) in the extreme C-terminus to generate an anchorless isoform (reviewed in [START_REF] Puig | The GPI-anchoring of PrP: Implications in sorting and pathogenesis[END_REF][START_REF] Checler | Alzheimer's and prion diseases: distinct pathologies, common proteolytic denominators[END_REF], [START_REF] Mcdonald | A new paradigm for enzymatic control of alpha-cleavage and beta-cleavage of the prion protein[END_REF]). The so-called alpha cleavage, giving rise to the N1 Nterminal and the C1 C-terminal fragments, has been most extensively studied as it occurs physiologically, notably in the brain [START_REF] Laffont-Proust | The N-terminal cleavage of cellular prion protein in the human brain[END_REF] and because it precludes the conversion into pathological PrP Sc [START_REF] Westergard | A naturally occurring C-terminal fragment of the prion protein (PrP) delays disease and acts as a dominant-negative inhibitor of PrPSc formation[END_REF]. Thus, while often referred to as a unique entity, PrP C actually encompasses a great diversity of isoforms, whose distribution may vary according to the cell type and whose respective functions await further investigation [START_REF] Linden | Physiology of the prion protein[END_REF].

Like many GPI-anchored protein, PrP C segregates in sphingolipid-rich membrane microdomains known as "lipid-rafts" (reviewed in [START_REF] Lewis | The role of lipid rafts in prion protein biology[END_REF]). These specialized subcellular compartments are enriched in receptors, second messengers and effectors and are considered as "signal transduction centres" [START_REF] Parton | Caveolae as plasma membrane sensors, protectors and organizers[END_REF]. Over the past fifteen years, evidence has accumulated that PrP C can act as a cell surface receptor, co-receptor or ligand, able to recruit downstream signal transduction pathways. Such function is thought to be driven by the interaction of PrP C with promiscuous partners, within multimolecular complexes [START_REF] Linden | Physiology of the prion protein[END_REF]. Among the vast array of PrP C interactors and effectors in different cell types and tissues, special interest is devoted to those that may play a role in prioninduced neurotoxicity. Indeed, it is now well-acknowledged that pathogenic prions exert their deleterious action by subverting the normal function of PrP C in the brain [START_REF] Harris | New insights into prion structure and toxicity[END_REF].

Their toxicity is fully dependent on the expression of PrP C in neurons [START_REF] Brandner | Normal host prion protein (PrPC) is required for scrapie spread within the central nervous system[END_REF][START_REF] Mallucci | Depleting neuronal PrP in prion infection prevents disease and reverses spongiosis[END_REF] and requires that it is GPI-anchored [START_REF] Chesebro | Anchorless prion protein results in infectious amyloid disease without clinical scrapie[END_REF]. Deciphering how PrP Sc deviates PrP C -dependent signalling cascades is attracting all the more attention since this pathogenic mechanism appears to be also at play in Alzheimer's disease (AD), with amyloid A oligomers hijacking PrP C -dependent pathways to instigate toxic events (reviewed in [START_REF] Lauren | Cellular prion protein as a therapeutic target in Alzheimer's disease[END_REF]).

In the present review, we summarize the current knowledge of PrP C -dependent signalling in neurons, its pathophysiological implications and highlight some therapeutic prospects to alleviate TSE and AD-related pathogenesis.

How is PrP C signalling initiated?

A current view is that the mobilization of PrP C -mediated cell signalling events occurs in response to the association of this protein with various partners at the plasma membrane [START_REF] Linden | Physiology of the prion protein[END_REF], a typical scheme for GPI-anchored proteins [START_REF] Parton | Caveolae as plasma membrane sensors, protectors and organizers[END_REF]. Indeed, in neurons as in other cell types, the majority of PrP C molecules are GPI-anchored, which drives their subcellular localization in lipid rafts [START_REF] Lewis | The role of lipid rafts in prion protein biology[END_REF]. Two transmembrane variants of PrP C have further been described that are associated with neurodegeneration and whose physiological relevance remains elusive [START_REF] Biasini | Prion protein at the crossroads of physiology and disease[END_REF]. Beyond rafts, PrP C is also found in postsynaptic densities (PSDs) [START_REF] Collins | Molecular characterization and comparison of the components and multiprotein complexes in the postsynaptic proteome[END_REF], which may associate with postsynaptic membrane rafts [START_REF] Suzuki | Association of membrane rafts and postsynaptic density: proteomics, biochemical, and ultrastructural analyses[END_REF]. The special composition of these compartments allows the spatial segregation of receptors and signal transduction effectors and favours their effective interaction [START_REF] Lingwood | Lipid rafts as a membrane-organizing principle[END_REF]. Classically, the mobilization of signalling cascades by GPI-anchored proteins is induced by their clustering in response to ligand binding [START_REF] Lingwood | Lipid rafts as a membrane-organizing principle[END_REF]. Such clustering can be triggered by antibody-mediated ligation, an experimental paradigm that allowed our laboratory to unveil various PrP C -dependent cascades in neuronal cells [START_REF] Rambold | Stressprotective signalling of prion protein is corrupted by scrapie prions[END_REF][START_REF] Beland | PrP(C) homodimerization stimulates the production of PrPC cleaved fragments PrPN1 and PrPC1[END_REF][START_REF] Beland | Taking advantage of physiological proteolytic processing of the prion protein for a therapeutic perspective in prion and Alzheimer diseases[END_REF][START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF]. Data from the Taztelt laboratory further established that the recruitment of neuroprotective signals by PrP C necessitates its dimerization via its hydrophobic domain (residues 113-133) [START_REF] Rambold | Stressprotective signalling of prion protein is corrupted by scrapie prions[END_REF]. Interestingly, PrP C dimerization appears to enhance its alpha-cleavage and production of the N1 fragment [START_REF] Beland | PrP(C) homodimerization stimulates the production of PrPC cleaved fragments PrPN1 and PrPC1[END_REF] as well as its shedding [START_REF] Beland | Taking advantage of physiological proteolytic processing of the prion protein for a therapeutic perspective in prion and Alzheimer diseases[END_REF]. PrP C cleavage may additionally be influenced by the binding of copper [START_REF] Mcdonald | A new paradigm for enzymatic control of alpha-cleavage and beta-cleavage of the prion protein[END_REF], which also impacts on PrP C trafficking and signalling activity [START_REF] Haigh | Copper, constitutive endoproteolytic processing of the prion protein and cell signalling[END_REF][START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF]. Shed PrP C may act in cis or in trans to homodimerize [START_REF] Hundt | PrP dimerization in the yeast two-hybrid system[END_REF], interact with the neural cell adhesion molecule NCAM [START_REF] Santuccione | Prion protein recruits its neuronal receptor NCAM to lipid rafts to activate p59fyn and to enhance neurite outgrowth[END_REF] or putatively other yet-to-be-identified cell surface molecules [START_REF] Chen | Prion protein as transinteracting partner for neurons is involved in neurite outgrowth and neuronal survival[END_REF][START_REF] Kanaani | Recombinant prion protein induces rapid polarization and development of synapses in embryonic rat hippocampal neurons in vitro[END_REF]. Besides, physiologically relevant interactions of PrP C with several extracellular molecules, acting as ligands, have been documented and are summarized in (Fig. 1). These include the matrix components vitronectin and laminin or the extracellular chaperone stressinducible protein 1 (STI-1), whose binding to PrP C causes reciprocal conformational changes [START_REF] Romano | Reciprocal remodeling upon binding of the prion protein to its signaling partner hop/STI1[END_REF]. The repertoire of PrP C interactors also includes the amyloid precursor protein APP [START_REF] Kaiser | Amyloid beta precursor protein and prion protein have a conserved interaction affecting cell adhesion and CNS development[END_REF][START_REF] Schmitt-Ulms | Time-controlled transcardiac perfusion cross-linking for the study of protein interactions in complex tissues[END_REF] and its aggregation-prone cleavage product A [START_REF] Lauren | Cellular prion protein as a therapeutic target in Alzheimer's disease[END_REF][START_REF] Lauren | Cellular prion protein mediates impairment of synaptic plasticity by amyloid-beta oligomers[END_REF] (see Fig. 1), whose unbalanced production lies at the root of AD, according to the amyloid cascade hypothesis [START_REF] Haass | Soluble protein oligomers in neurodegeneration: lessons from the Alzheimer's amyloid beta-peptide[END_REF]. While it is unclear whether the PrP C -APP interaction instructs downstream cell signalling events or whether the binding of PrP C to A may occur under physiological conditions, it is now well-established that A oligomers bind to PrP C in AD brain [START_REF] Dohler | High molecular mass assemblies of amyloidbeta oligomers bind prion protein in patients with Alzheimer's disease[END_REF] and subvert PrP C -dependent signalling cascades to affect various neuronal processes (reviewed in [START_REF] Lauren | Cellular prion protein as a therapeutic target in Alzheimer's disease[END_REF]) (see below).

The multiplicity of PrP C interactors on the one hand and isoforms on the other hand raises the issue as to which PrP C entities interact with a given partner and whether several ligands may compete for binding to a given PrP C entity, as recently suggested for STI-1 and A oligomers [START_REF] Ostapchenko | The prion protein ligand, stress-inducible phosphoprotein 1, regulates amyloid-beta oligomer toxicity[END_REF].

PrP C signalling in neuronal cells: a plethora of effectors

The ability of PrP C to trigger cell signalling events, initially suspected from its location in lipid rafts (reviewed in [START_REF] Lewis | The role of lipid rafts in prion protein biology[END_REF]) and the reported ability of GPI-anchored proteins to activate src kinases [START_REF] Stefanova | GPI-anchored cellsurface molecules complexed to protein tyrosine kinases[END_REF], has been examined using multiple experimental paradigms. A well-characterized proximal effector of PrP C is Fyn, a tyrosine kinase of the src family that we first identified as a neuronal-restricted target of PrP C using the inducible neuronal 1C11 cell line [START_REF] Mouillet-Richard | Signal transduction through prion protein[END_REF], and that associates with PrP C to form a signalling complex [START_REF] Pantera | PrPc activation induces neurite outgrowth and differentiation in PC12 cells: role for caveolin-1 in the signal transduction pathway[END_REF]. In 1C11-derived differentiated cells, the PrP C -Fyn coupling is relayed by the membrane protein caveolin [START_REF] Mouillet-Richard | Signal transduction through prion protein[END_REF], now recognized as an important regulator of neuronal intracellular signalling [START_REF] Stern | Caveolin regulation of neuronal intracellular signaling[END_REF]. The PrP C -dependent activation of Fyn was subsequently shown to be involved in NCAM-dependent neurite outgrowth in primary hippocampal neurons [START_REF] Santuccione | Prion protein recruits its neuronal receptor NCAM to lipid rafts to activate p59fyn and to enhance neurite outgrowth[END_REF]. Apart from caveolin and NCAM, the natures of the PrP C partners that may contribute to Fyn activation in a physiological context remain to be identified. From a pathological point of view, the PrP C -Fyn platform has received renewed interest in the last few years as it is hijacked both by PrP Sc [START_REF] Pradines | Pathogenic prions deviate PrP(C) signaling in neuronal cells and impair A-beta clearance[END_REF] and by A oligomers [START_REF] Lauren | Cellular prion protein as a therapeutic target in Alzheimer's disease[END_REF][START_REF] Um | Alzheimer amyloid-beta oligomer bound to postsynaptic prion protein activates Fyn to impair neurons[END_REF] to transduce their toxicity (see below).

In a recent study, we documented that neuronal PrP C has the capacity to recruit another member of the src kinase family, the Lyn kinase [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF], previously found to be present in the plasma microenvironment of PrP C in primary cerebellar neurons [START_REF] Loberto | The membrane environment of endogenous cellular prion protein in primary rat cerebellar neurons[END_REF]. This signal, which can be revealed in response to antibody-mediated ligation of PrP C , also occurs upon binding of PrP C to its endogenous ligand STI-1 [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF]. Noteworthy, like Fyn, the activation of Lyn driven by PrP C is transduced via caveolin [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF], further supporting a central role for caveolin in PrP C -dependent signalling.

Other reported intracellular effectors of PrP C signalling include the phosphoinositide 3 kinase (PI3K) [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF][START_REF] Vassallo | Activation of phosphatidylinositol 3-kinase by cellular prion protein and its role in cell survival[END_REF], Akt [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF][START_REF] Roffe | Prion protein interaction with stressinducible protein 1 enhances neuronal protein synthesis via mTOR[END_REF], protein kinase A (PKA) [START_REF] Caiati | PrPC controls via protein kinase A the direction of synaptic plasticity in the immature hippocampus[END_REF][START_REF] Chiarini | Cellular prion protein transduces neuroprotective signals[END_REF][START_REF] Lopes | Interaction of cellular prion and stress-inducible protein 1 promotes neuritogenesis and neuroprotection by distinct signaling pathways[END_REF], the reactive oxygen species (ROS) generating enzyme NADPH oxidase [START_REF] Schneider | NADPH oxidase and extracellular regulated kinases 1/2 are targets of prion protein signaling in neuronal and nonneuronal cells[END_REF], the MAP kinases ERK1/2 [START_REF] Chiarini | Cellular prion protein transduces neuroprotective signals[END_REF][START_REF] Lopes | Interaction of cellular prion and stress-inducible protein 1 promotes neuritogenesis and neuroprotection by distinct signaling pathways[END_REF][START_REF] Schneider | NADPH oxidase and extracellular regulated kinases 1/2 are targets of prion protein signaling in neuronal and nonneuronal cells[END_REF][START_REF] Caetano | Endocytosis of prion protein is required for ERK1/2 signaling induced by stress-inducible protein 1[END_REF], the glycogen synthase kinase GSK3 [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF], the TNF-converting enzyme (TACE) metalloprotease [START_REF] Pradines | Cellular prion protein coupling to TACE-dependent TNF-alpha shedding controls neurotransmitter catabolism in neuronal cells[END_REF], the mammalian target of rapamycin mTOR [START_REF] Roffe | Prion protein interaction with stressinducible protein 1 enhances neuronal protein synthesis via mTOR[END_REF], as well as several transcription factors: CREB, Egr-1 and c-fos [START_REF] Pradines | CREB-dependent gene regulation by prion protein: impact on MMP-9 and betadystroglycan[END_REF]. Links with sphingolipid [START_REF] Schmalzbauer | Evidence for an association of prion protein and sphingolipid-mediated signaling[END_REF] and calcium signalling [START_REF] Beraldo | Metabotropic glutamate receptors transduce signals for neurite outgrowth after binding of the prion protein to laminin gamma1 chain[END_REF][START_REF] Beraldo | Role of alpha7 nicotinic acetylcholine receptor in calcium signaling induced by prion protein interaction with stress-inducible protein 1[END_REF][START_REF] Fuhrmann | Loss of the cellular prion protein affects the Ca2+ homeostasis in hippocampal CA1 neurons[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] have also been depicted. The diverse array of PrP C signalling effectors is summarized in Table 1. Some of these effectors have been shown to be under the control of Fyn (e.g. NADPH oxidase) or Lyn (e.g. GSK3). Some other pathways, such as those featuring PKA, remain to be delineated. More generally, an important challenge for the next few years will be to translate the repertoire of PrP C targets into transduction cascades in relation with the initial triggering signal (e.g. STI-1 or laminin), cell type and biological response, as outlined in Fig. 2. Altogether, achieving this goal appears as a prerequisite to ultimately reach a global view of the various PrP C -dependent signalling platforms and pathways, their potential crosstalks, as well as their integrated cellular outcome.

An intimate connection with neurotransmitter receptors

Another important class of lipid raft residents in neurons is neurotransmitter receptors [START_REF] Allen | Lipid raft microdomains and neurotransmitter signalling[END_REF]. A number of studies have unveiled functional interactions between PrP C and diverse ionotropic or G-protein coupled neurotransmitter receptors, which are summarized in Table 2. Focusing on serotonergic receptors, our laboratory reported on the PrP C -dependent modulation of the signalling activity of 5-HT1B, 5-HT2A and 5-HT2B receptors, all coupled to G-proteins, in serotonergic 1C11 5-HT neuronal cells [START_REF] Mouillet-Richard | Modulation of serotonergic receptor signaling and cross-talk by prion protein[END_REF]. Interestingly, we observed both positive and negative actions of PrP C , depending on the G-protein and pathway considered, and that the modulatory effect of PrP C depends on caveolin [START_REF] Mouillet-Richard | Modulation of serotonergic receptor signaling and cross-talk by prion protein[END_REF]. Very recently, we delineated an additional link between PrP C and the 5-HT1B receptor, by showing that the PrP C -dependent inactivation of GSK3 interferes with the subcellular localization and functional activity of this receptor and thereby potentiates neurotransmitter release [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF]. Metabotropic glutamate receptors constitute another class of G-protein coupled neurotransmitter receptors (GPCR) that has captured much attention in relation to PrP C function. PrP C has the capacity to interact with mGlu1AR, mGlu1BR as well as mGlu5R [START_REF] Beraldo | Metabotropic glutamate receptors transduce signals for neurite outgrowth after binding of the prion protein to laminin gamma1 chain[END_REF], which all belong to the Group I of metabotropic glutamate receptors and are coupled to Gq-mediated phospholipase C (PLC) activation [START_REF] Willard | Glutamate, glutamate receptors, and downstream signaling pathways[END_REF]. Further, the recruitment of the PrP C -mGluR signalling complex by the laminin 1 chain (Ln1) was shown to trigger calcium release, ERK1/2 activation and to sustain neurite outgrowth [START_REF] Beraldo | Metabotropic glutamate receptors transduce signals for neurite outgrowth after binding of the prion protein to laminin gamma1 chain[END_REF]. In a pathological context, mGlu5R receptors have been reported to relay the activation of Fyn [START_REF] Um | Metabotropic glutamate receptor 5 is a coreceptor for Alzheimer abeta oligomer bound to cellular prion protein[END_REF] and to mediate the facilitation of long term depression (LTD) [START_REF] Hu | mGlu5 receptors and cellular prion protein mediate amyloid-beta-facilitated synaptic long-term depression in vivo[END_REF] in response to the binding of A oligomers to PrP C (see below).

The capacity of PrP C to interact with neurotransmitter receptors is not restricted to GPCRs. Indeed, it may form a complex with the ionotropic nicotinic acetylcholine receptor 7nAChR, whose mobilization in the hippocampus by the STI-1 ligand promotes intracellular calcium release [START_REF] Beraldo | Role of alpha7 nicotinic acetylcholine receptor in calcium signaling induced by prion protein interaction with stress-inducible protein 1[END_REF]. Finally, multiple links with NMDA receptors have been depicted, in relation with the critical involvement of these receptors in longterm potentiation and synaptic plasticity [START_REF] Collingridge | Receptor trafficking and synaptic plasticity[END_REF]. For instance, PrP C was reported to exert a negative regulation on GluN2D-containing NMDA receptors, thereby providing a neuroprotective mechanism against glutamate excitotoxicity [START_REF] Khosravani | Prion protein attenuates excitotoxicity by inhibiting NMDA receptors[END_REF]. It also appears to sustain the internalization of GluN2B-containing NMDA receptors via Fyn-mediated phosphorylation in response to ethanol exposure [START_REF] Petit-Paitel | Prion protein is a key determinant of alcohol sensitivity through the modulation of Nmethyl-D-aspartate receptor (NMDAR) activity[END_REF]. Of note, several lines of evidence indicate that the PrP C -dependent modulation of NMDA receptors is lost in the presence of toxic oligomers, including A, leading to reduced receptor desensitization [START_REF] Um | Alzheimer amyloid-beta oligomer bound to postsynaptic prion protein activates Fyn to impair neurons[END_REF][START_REF] Resenberger | The cellular prion protein mediates neurotoxic signalling of beta-sheet-rich conformers independent of prion replication[END_REF][START_REF] You | Abeta neurotoxicity depends on interactions between copper ions, prion protein, and N-methyl-D-aspartate receptors[END_REF].

Altogether, these observations substantiate an important role for PrP C in the regulation of diverse neurotransmitter receptors, which may account for the subtle behavioural and / or cognitive deficits reported in PrP-null mice [START_REF] Schmitz | Loss of Prion Protein Leads to Age-Dependent Behavioral Abnormalities and Changes in Cytoskeletal Protein Expression[END_REF]. Refining our current knowledge of the interplay between PrP C and neurotransmitter receptors has obvious implications for unravelling the global impact of prions or A oligomers on neuronal activity and homeostasis, as will be discussed below.

PrP C signalling partners at the membrane

Lipid rafts typically allow a tight spatial and temporal control of cell signalling events [START_REF] Golub | Spatial and temporal control of signaling through lipid rafts[END_REF]. As for PrP C -dependent signalling cascades, multiple regulatory mechanisms have been highlighted. First, the interaction of PrP C with caveolin [START_REF] Mouillet-Richard | Signal transduction through prion protein[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], likely sustains the compartmentation of PrP C with appropriate signalling partners, e.g. src kinases or neurotransmitter receptors [START_REF] Stern | Caveolin regulation of neuronal intracellular signaling[END_REF]. Adding further complexity to the picture, lipid rafts may vary in their composition [START_REF] Mishra | Lipid raft heterogeneity: an enigma[END_REF]. Such raft heterogeneity may account for the spatial restriction of the PrP C -caveolin-Lyn complex to neuronal cell bodies, to the opposite of the PrP C -caveolin-Fyn complex, which is found in neuronal processes [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF]. Whether each platform involves distinct subsets of PrP C molecules remains to be investigated.

The nature of potential transmembrane partners contributing to the spatial control of PrP C -signalling complexes also deserves attention. Both NCAM [START_REF] Santuccione | Prion protein recruits its neuronal receptor NCAM to lipid rafts to activate p59fyn and to enhance neurite outgrowth[END_REF] and mGlu5R [START_REF] Um | Metabotropic glutamate receptor 5 is a coreceptor for Alzheimer abeta oligomer bound to cellular prion protein[END_REF] have been reported to transduce PrP C -dependent Fyn activation. Recently, we highlighted the contribution of the transmembrane low-density receptor protein 1 (LRP-1) to the recruitment of effectors downstream from the PrP C -caveolin-Lyn platform [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF].

LRP-1 was previously identified as a PrP C partner that regulates its internalization [START_REF] Taylor | The low-density lipoprotein receptor-related protein 1 (LRP1) mediates the endocytosis of the cellular prion protein[END_REF], and whose interaction with PrP C is enhanced by copper [START_REF] Singh | Low levels of copper disrupt brain amyloid-beta homeostasis by altering its production and clearance[END_REF]. Among its multiple ligands, LRP-1 is known to interact with A, and recent data indicate that LRP-1 is mandatory for the PrP C -dependent toxicity of A [START_REF] Rushworth | Prion protein-mediated toxicity of amyloid-beta oligomers requires lipid rafts and the transmembrane LRP1[END_REF]. Beyond favouring the interaction with LRP-1, the binding of copper to PrP C was also shown to enhance its endocytosis together with Glypican-1 [START_REF] Cheng | Copper-dependent cointernalization of the prion protein and glypican-1[END_REF], a GPI-anchored heparan sulfate proteoglycan that mediates the association of PrP C with lipid rafts and, in a pathological context, favours the formation of PrP Sc [START_REF] Taylor | Glypican-1 mediates both prion protein lipid raft association and disease isoform formation[END_REF]. While it may act as a cofactor for prion conversion, whether Glypican-1 modulates interactions of PrP C with partners and downstream cell signalling events remains to be investigated. The same question applies to the 37kDa/67kDa laminin receptor, reported to promote the internalization of PrP C [START_REF] Gauczynski | The 37-kDa/67-kDa laminin receptor acts as the cell-surface receptor for the cellular prion protein[END_REF] and to sustain PrP Sc propagation in prion-infected cells [START_REF] Leucht | The 37 kDa/67 kDa laminin receptor is required for PrP(Sc) propagation in scrapie-infected neuronal cells[END_REF]. Whatever the partners involved, PrP C endocytosis appears to be mandatory for the recruitment of some effectors, such as the activation of ERK1/2 in response to STI-1 [START_REF] Caetano | Endocytosis of prion protein is required for ERK1/2 signaling induced by stress-inducible protein 1[END_REF]. Finally, as will be emphasized below, alterations in the dynamics of PrP C interactions with its partners, such as its increased cell-surface localization upon binding to A oligomers [START_REF] Caetano | Amyloid-beta oligomers increase the localization of prion protein at the cell surface[END_REF], may disrupt the strength and duration of PrP C -related signalling and instigate toxic cascades.

Translating PrP C -mediated signalling pathways in neurons into a biological response

Overall, through the various signalling cascades brought to light in a neuronal context, PrP C appears to be connected with nearly all aspects of neuronal physiology. Its contribution to neurite outgrowth may be sustained by its interaction with NCAM and downstream Fyn activation [START_REF] Santuccione | Prion protein recruits its neuronal receptor NCAM to lipid rafts to activate p59fyn and to enhance neurite outgrowth[END_REF], vitronectin [START_REF] Hajj | Cellular prion protein interaction with vitronectin supports axonal growth and is compensated by integrins[END_REF], Ln1 [START_REF] Graner | Cellular prion protein binds laminin and mediates neuritogenesis[END_REF][START_REF] Machado | Disease-associated mutations in the prion protein impair laminininduced process outgrowth and survival[END_REF] or STI-1 [START_REF] Lopes | Interaction of cellular prion and stress-inducible protein 1 promotes neuritogenesis and neuroprotection by distinct signaling pathways[END_REF] and may also involve an interplay with 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][START_REF] Loubet | Neuritogenesis: the prion protein controls beta1 integrin signaling activity[END_REF]. Of note, a synergistic action of Ln1 and STI-1 in triggering PrP C -dependent calcium signalling and neuritogenesis has been described [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]. Beraldo et al. also highlighted the requirement of mGlu1R to transduce the PrP C -Ln1-mediated induction of process outgrowth [START_REF] Beraldo | Metabotropic glutamate receptors transduce signals for neurite outgrowth after binding of the prion protein to laminin gamma1 chain[END_REF]. In vivo, the PrP C -Ln1 interaction was shown to modulate memory consolidation, with a suggested implication of PKA and ERK1/2 [START_REF] Coitinho | The interaction between prion protein and laminin modulates memory consolidation[END_REF]. This property is shared by the PrP C -STI-1 duo [START_REF] Coitinho | Short-term memory formation and long-term memory consolidation are enhanced by cellular prion association to stressinducible protein 1[END_REF], which also sustains protein synthesis through the mobilization of mTOR [START_REF] Roffe | Prion protein interaction with stressinducible protein 1 enhances neuronal protein synthesis via mTOR[END_REF]. Interestingly, mTOR also emerges has an important player in synaptic plasticity [START_REF] Costa-Mattioli | mTOR complexes in neurodevelopmental and neuropsychiatric disorders[END_REF], like other PrP C targets such as ERK1/2, CREB, or egr-1 [START_REF] Thomas | MAPK cascade signalling and synaptic plasticity[END_REF]. Another possible signalling intermediate in these processes is the GSK3 kinase, which we recently identified as a downstream target of the PrP C -STI-1 coupling [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF] and has been shown both to play a pivotal role in synaptic plasticity and long-term potentiation (LTP) [START_REF] Bradley | A pivotal role of GSK-3 in synaptic plasticity[END_REF] and to regulate mTOR activity [START_REF] Inoki | TSC2 integrates Wnt and energy signals via a coordinated phosphorylation by AMPK and GSK3 to regulate cell growth[END_REF]. Tight regulation of the GSK3 kinase is also critical to neuronal cell survival [START_REF] Kaidanovich-Beilin | GSK-3: Functional Insights from Cell Biology and Animal Models[END_REF], and we may suspect that inhibition of this kinase instigated by the interaction of PrP C with STI-1 also contributes to the well-established neuroprotective action of these two molecules [START_REF] Chiarini | Cellular prion protein transduces neuroprotective signals[END_REF][START_REF] Resenberger | Neuroprotective and neurotoxic signaling by the prion protein[END_REF][START_REF] Roucou | Cellular prion protein neuroprotective function: implications in prion diseases[END_REF][START_REF] Zanata | Stress-inducible protein 1 is a cell surface ligand for cellular prion that triggers neuroprotection[END_REF].

Multiple links between PrP C and neurotransmitter-associated functions have further been uncovered. For instance, through a ROS-TACE-TNF pathway, PrP C promotes bioamine degradation [START_REF] Pradines | Cellular prion protein coupling to TACE-dependent TNF-alpha shedding controls neurotransmitter catabolism in neuronal cells[END_REF], an observation which has to be brought together with the contribution of bioamine turnover to memory consolidation [START_REF] Haider | Enhanced serotonergic neurotransmission in the hippocampus following tryptophan administration improves learning acquisition and memory consolidation in rats[END_REF][START_REF] Kobayashi | Genetic evidence for noradrenergic control of longterm memory consolidation[END_REF]. Our recent work further established that the negative regulation of the serotonergic 5-HT1B receptor activity downstream from the PrP C -Lyn-GSK3 cascade potentiates neurotransmitter release [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF]. Adding to this view is the interplay between PrP C and diverse neurotransmitter receptors, as summarized above. Finally, we may note that the multifaceted contribution of PrP C to neuronal homeostasis and activity is in keeping with the wide array of alterations depicted in the context of prion infection [START_REF] Kovacs | Prion diseases: from protein to cell pathology[END_REF][START_REF] Ledoux | Effects on the serotoninergic system in sub-acute transmissible spongiform encephalopathies: current data, hypotheses, suggestions for experimentation[END_REF][START_REF] Vidal | Early dysfunction of central 5-HT system in a murine model of bovine spongiform encephalopathy[END_REF] or after binding of A oligomers to PrP C (reviewed in [START_REF] Lauren | Cellular prion protein as a therapeutic target in Alzheimer's disease[END_REF]).

Causes and consequences of the subversion of PrP C signalling in neurons

That the deviation of PrP C -dependent-signalling may contribute to neurodegeneration is now well established in the case of prion infection and also in Alzheimer's disease.

Prions fail to induce any pathogenesis when endogenous PrP C is selectively knocked-out in neurons [START_REF] Mallucci | Depleting neuronal PrP in prion infection prevents disease and reverses spongiosis[END_REF] or is exclusively anchorless [START_REF] Chesebro | Anchorless prion protein results in infectious amyloid disease without clinical scrapie[END_REF]. These pioneering studies have set the stage for investigating how PrP Sc may usurp PrP C normal signalling function to exert its toxic action. For instance, pathological PrP conformers were shown to induce a switch from anti-apoptotic to pro-apoptotic signalling of PrP C [START_REF] Rambold | Stressprotective signalling of prion protein is corrupted by scrapie prions[END_REF]. PrP Sc aggregation was further reported to trigger the clustering of its GPI anchors at high densities and thereby cause alterations in membrane composition, pathological activation of cytosolic phospholipase A2 (cPLA2) and synaptic damage [START_REF] Bate | Neurodegeneration induced by clustering of sialylated glycosylphosphatidylinositols of prion proteins[END_REF]. In a recent work, we documented that pathogenic prions hijack the PrP C -Fyn platform to constitutively recruit downstream effectors, including NADPH oxidase, and cause impairment in the clearance of amyloid A [START_REF] Pradines | Pathogenic prions deviate PrP(C) signaling in neuronal cells and impair A-beta clearance[END_REF] (Fig. 3). The former observation recalls the central role established for the NADPH oxidase subunit NOX2 in prion-induced neuronal damage [START_REF] Falsig | Prion pathogenesis is faithfully reproduced in cerebellar organotypic slice cultures[END_REF][START_REF] Sonati | The toxicity of antiprion antibodies is mediated by the flexible tail of the prion protein[END_REF]. Altogether, these data support the notion that the conversion of PrP C into PrP Sc may promote the constitutive recruitment of proximal downstream effectors in lipid rafts. Overactivation of PrP C downstream targets may similarly account for the development of a spontaneous neurodegenerative illness in transgenic mice overexpressing PrP C by 5 to 10 fold [START_REF] Chiesa | Aggregated, wild-type prion protein causes neurological dysfunction and synaptic abnormalities[END_REF]. Besides, the accumulation of PrP C in lipid raft during aging, proposed to explain the occurrence of sporadic prion diseases [START_REF] Agostini | Prion protein accumulation in lipid rafts of mouse aging brain[END_REF], might also amplify the activation of downstream cascades. Importantly, the pathological deviation of PrP C -mediated signalling may be further self-sustained by various subsequent alterations such as aberrant distribution of caveolin-1 [START_REF] Russelakis-Carneiro | Prion replication alters the distribution of synaptophysin and caveolin 1 in neuronal lipid rafts[END_REF], impaired cholesterol metabolism [START_REF] Cui | Prion infection impairs cholesterol metabolism in neuronal cells[END_REF], decreased protective alpha-cleavage [START_REF] Pietri | PDK1 decreases TACE-mediated alpha-secretase activity and promotes disease progression in prion and Alzheimer's diseases[END_REF] or reduced clearance of A [START_REF] Pradines | Pathogenic prions deviate PrP(C) signaling in neuronal cells and impair A-beta clearance[END_REF][START_REF] Hernandez-Rapp | Hijacking PrP(c)-dependent signal transduction: when prions impair Abeta clearance[END_REF].

The pathological implications of the subversion of PrP C function are not restricted to prion infection since PrP C can convey neurotoxic signals after binding to various types of beta-sheet rich conformers, including A [START_REF] Resenberger | The cellular prion protein mediates neurotoxic signalling of beta-sheet-rich conformers independent of prion replication[END_REF]. Regarding the AD connection, converging data gathered in the last 5 years demonstrate that PrP C can act as a cell-surface receptor for oligomeric A (reviewed in [START_REF] Lauren | Cellular prion protein as a therapeutic target in Alzheimer's disease[END_REF]). Multiple deleterious outcomes of the interaction of A with PrP C have been depicted, including synaptic dysfunction and dendritic spine retraction [START_REF] Um | Alzheimer amyloid-beta oligomer bound to postsynaptic prion protein activates Fyn to impair neurons[END_REF], neuronal death [START_REF] Kudo | Cellular prion protein is essential for oligomeric amyloid-beta-induced neuronal cell death[END_REF], inhibition of LTP [START_REF] Lauren | Cellular prion protein mediates impairment of synaptic plasticity by amyloid-beta oligomers[END_REF], exacerbation of LTD [START_REF] Hu | mGlu5 receptors and cellular prion protein mediate amyloid-beta-facilitated synaptic long-term depression in vivo[END_REF],

axonal degeneration [START_REF] Gimbel | Memory impairment in transgenic Alzheimer mice requires cellular prion protein[END_REF], as well as induction of autophagy [START_REF] Nah | BECN1/Beclin 1 is recruited into lipid rafts by prion to activate autophagy in response to amyloid beta 42[END_REF]. From a mechanistic point of view, A oligomers bind to PrP C at different sites within the N-terminal region (see Fig. 1) and recruit downstream effectors (Fig. 3). Among these, Fyn has captured the most attention since this kinase has long been known to be mandatory for Amediated toxicity [START_REF] Williamson | Rapid tyrosine phosphorylation of neuronal proteins including tau and focal adhesion kinase in response to amyloid-beta peptide exposure: involvement of Src family protein kinases[END_REF][START_REF] Williamson | Membrane-bound betaamyloid oligomers are recruited into lipid rafts by a fyn-dependent mechanism[END_REF]. In recent years, several outstanding observations have been reported, including the central role of Fyn in promoting phosphorylation of the GluN2B subunit of NMDA receptors and dendritic spine loss [START_REF] Um | Alzheimer amyloid-beta oligomer bound to postsynaptic prion protein activates Fyn to impair neurons[END_REF], as well as tau hyperphosphorylation [START_REF] Larson | The complex PrP(c)-Fyn couples human oligomeric Abeta with pathological tau changes in Alzheimer's disease[END_REF] after engagement of A oligomers with PrP C . mGlu5Rs were further shown to act as co-receptors for PrP C -bound A oligomers to promote Fyn activation [START_REF] Um | Metabotropic glutamate receptor 5 is a coreceptor for Alzheimer abeta oligomer bound to cellular prion protein[END_REF]. Whether this property is shared by other metabotropic or ionotropic receptors deserves to be investigated, in view of the broad implication of neurotransmitter receptors in AD pathology [START_REF] Thathiah | The role of G protein-coupled receptors in the pathology of Alzheimer's disease[END_REF]. Adding to the above data, PrP C was also shown to relay the A-dependent activation of cPLA2 [START_REF] Bate | Amyloid-beta-induced synapse damage is mediated via cross-linkage of cellular prion proteins[END_REF], caspase 3 [START_REF] Kudo | Cellular prion protein is essential for oligomeric amyloid-beta-induced neuronal cell death[END_REF], ERK1/2 [START_REF] Caetano | Amyloid-beta oligomers increase the localization of prion protein at the cell surface[END_REF], NADPH oxidase [START_REF] Walsh | Amyloid-beta and Proinflammatory Cytokines Utilize a Prion Protein-Dependent Pathway to Activate NADPH Oxidase and Induce Cofilin-Actin Rods in Hippocampal Neurons[END_REF] and phosphoinositide-dependent kinase-1 (PDK1), itself leading to decreased TACE activity [START_REF] Pietri | PDK1 decreases TACE-mediated alpha-secretase activity and promotes disease progression in prion and Alzheimer's diseases[END_REF]. Further studies are needed to explore the relationship between these diverse events and Fyn activation. Finally, from a therapeutic perspective, it is worth noting that some studies have addressed the relevance of observations gained in vitro or in mice and have for instance established that A oligomers and PrP C interact in the brain of AD patients but not healthy controls [START_REF] Dohler | High molecular mass assemblies of amyloidbeta oligomers bind prion protein in patients with Alzheimer's disease[END_REF]. The overactivation of PDK1 found in AD mouse models to be dependent on PrP C expression was corroborated in AD patients as well [START_REF] Pietri | PDK1 decreases TACE-mediated alpha-secretase activity and promotes disease progression in prion and Alzheimer's diseases[END_REF]. Overall, while PrP C certainly is not the sole relay of A-mediated toxicity [START_REF] Benilova | Neuroscience. Promiscuous Alzheimer's amyloid: yet another partner[END_REF], these cumulative data provide several promising avenues for therapeutic intervention in prion and Alzheimer's diseases, which are discussed below.

Therapeutic prospects

At present, TSEs remain fatal diseases with no effective treatment despite numerous trials (see [START_REF] Haik | Doxycycline in Creutzfeldt-Jakob disease: a phase 2, randomised, double-blind, placebocontrolled trial[END_REF] for example). This also holds true for AD, which represents a major health burden with an estimated 25 million people affected worldwide. The development of effective therapies for prion and Alzheimer's diseases faces multiple challenges: patients are diagnosed at a stage when brain damage has already occurred, drugs need to cross the blood-brain-barrier, and last but not least, the normal functions of the targeted proteins need to be preserved. Beyond strategies aiming at limiting the production and / or accumulation of PrP Sc , which will not be addressed here, one major axis followed for the treatment of TSEs is that of immunotherapy (reviewed in [START_REF] Roettger | Immunotherapy in prion disease[END_REF]). Some passive immunotherapy studies have notably demonstrated a substantial benefit for a panel of anti-PrP C antibodies in delaying onset of disease, although whether this strategy can still prove useful when treatment is initiated after the appearance of clinical symptoms has yet to be demonstrated (reviewed in [START_REF] Roettger | Immunotherapy in prion disease[END_REF]). More recently, a subset of the same antibodies has been tested for their ability to counteract the interaction of A oligomers with PrP C . In vitro, the binding of A oligomers to PrP C was successfully antagonized using 6D11 [START_REF] Lauren | Cellular prion protein mediates impairment of synaptic plasticity by amyloid-beta oligomers[END_REF][START_REF] Um | Alzheimer amyloid-beta oligomer bound to postsynaptic prion protein activates Fyn to impair neurons[END_REF][START_REF] Kudo | Cellular prion protein is essential for oligomeric amyloid-beta-induced neuronal cell death[END_REF][START_REF] Larson | The complex PrP(c)-Fyn couples human oligomeric Abeta with pathological tau changes in Alzheimer's disease[END_REF], ICSM35 [START_REF] Freir | Interaction between prion protein and toxic amyloid beta assemblies can be therapeutically targeted at multiple sites[END_REF] or Sha31 [START_REF] Alier | Abeta inhibition of ionic conductance in mouse basal forebrain neurons is dependent upon the cellular prion protein PrPC[END_REF] anti-PrP C antibodies. The 6D11 antibody was further shown to protect against cognitive deficits in a mouse model of AD [START_REF] Chung | Anti-PrPC monoclonal antibody infusion as a novel treatment for cognitive deficits in an Alzheimer's disease model mouse[END_REF]. Finally, brain-derived A-mediated impairment of LTP could be efficiently prevented following immunotargeting of PrP C using ISCM35 or ISCM18 antibodies or the D13 antibody fragment (Fab) [START_REF] Barry | Alzheimer's disease brain-derived amyloid-beta-mediated inhibition of LTP in vivo is prevented by immunotargeting cellular prion protein[END_REF].

To the opposite of these studies, some antibodies targeting PrP C exhibit neurotoxic potential. Beyond some conflicting results [START_REF] Klohn | PrP antibodies do not trigger mouse hippocampal neuron apoptosis[END_REF][START_REF] Solforosi | Cross-linking cellular prion protein triggers neuronal apoptosis in vivo[END_REF], an extensive study from the Aguzzi laboratory has allowed to map the epitopes of a panel of toxicity-inducing anti-PrP C antibodies [START_REF] Sonati | The toxicity of antiprion antibodies is mediated by the flexible tail of the prion protein[END_REF]. These neurotoxic antibodies were shown to systematically target the globular domain (residues 124-230) -although some antibodies recognizing this region remained innocuous -and to promote neurodegeneration via calpain and NOX2 activation [START_REF] Sonati | The toxicity of antiprion antibodies is mediated by the flexible tail of the prion protein[END_REF], in a manner similar to infectious prions [START_REF] Falsig | Prion pathogenesis is faithfully reproduced in cerebellar organotypic slice cultures[END_REF]. Since NADPH oxidase is a downstream effector of PrP C signalling [START_REF] Schneider | NADPH oxidase and extracellular regulated kinases 1/2 are targets of prion protein signaling in neuronal and nonneuronal cells[END_REF] and is overactivated in prion infection [START_REF] Pradines | Pathogenic prions deviate PrP(C) signaling in neuronal cells and impair A-beta clearance[END_REF], NOX2, as suggested above [START_REF] Falsig | Prion pathogenesis is faithfully reproduced in cerebellar organotypic slice cultures[END_REF][START_REF] Sonati | The toxicity of antiprion antibodies is mediated by the flexible tail of the prion protein[END_REF], and putatively other NADPH oxidase subunits, may prove valuable targets to combat TSEs. The indispensable role of NOX2 in host defence would, however, need to be taken into account, should its inhibition be further considered for therapeutic purposes [START_REF] Gao | NADPH oxidases: novel therapeutic targets for neurodegenerative diseases[END_REF].

Further in relation with the above-mentioned subversion of PrP C -mediated pathways by PrP Sc or A, the potential benefit of targeting Fyn with the small molecule inhibitor saracatinib is now being evaluated within a clinical trial focusing on AD [START_REF] Nygaard | Fyn kinase inhibition as a novel therapy for Alzheimer's disease[END_REF]. In view of the central role exerted by Fyn in the recruitment of pathogenic cascades by prions [START_REF] Pradines | Pathogenic prions deviate PrP(C) signaling in neuronal cells and impair A-beta clearance[END_REF], whether the same strategy may apply to TSEs seems worth investigating. Likewise, PDK1 overactivation was recently identified as another common feature of TSEs and AD, whose inhibition allows alleviating both diseases [START_REF] Pietri | PDK1 decreases TACE-mediated alpha-secretase activity and promotes disease progression in prion and Alzheimer's diseases[END_REF]. Of note, although further work is needed to develop inhibitory molecules devoid of intrinsic toxicity, treatment was shown to be effective while starting at a relatively late stage [START_REF] Pietri | PDK1 decreases TACE-mediated alpha-secretase activity and promotes disease progression in prion and Alzheimer's diseases[END_REF]. Besides, the observation that PDK1 overactivation in AD can be antagonized through silencing of PrP C [START_REF] Pietri | PDK1 decreases TACE-mediated alpha-secretase activity and promotes disease progression in prion and Alzheimer's diseases[END_REF] points to an interesting link between PrP C signalling and PDK1, which deserves further investigation.

The group of Mallucci adopted a different approach based on targeting a generic pathway sustaining neurodegeneration. A commonality between TSEs, AD and Parkinson's disease is the impairment in the unfolded protein response (UPR) [START_REF] Halliday | Targeting the unfolded protein response in neurodegeneration: A new approach to therapy[END_REF].

One effector of this pathway is the eIF2 elongation factor, whose phosphorylation halts protein synthesis. Following the demonstration that translational repression by phosphorylated eIF2 mediates prion neurodegeneration [START_REF] Moreno | Sustained translational repression by eIF2alpha-P mediates prion neurodegeneration[END_REF], Moreno et al. documented that inhibition of the UPR using an orally-administered pharmacological compound could abrogate the development of clinical disease in prion-infected mice, even when starting treatment after the emergence of behavioural signs [START_REF] Moreno | Oral treatment targeting the unfolded protein response prevents neurodegeneration and clinical disease in prion-infected mice[END_REF]. Whether the advantage of this treatment is comparable to that afforded by others such as PDK1 inhibition is unclear at present. In this respect, the development of standardized methods using defined mouse models and prion strains would help launch comparative studies to assess the relative benefit of various therapeutic approaches since the onset and progression of prion pathogenesis is highly synchronous in inoculated mice, contrary to AD mouse models. Notwithstanding, it is worthy to note that the suppression of eIF2phosphorylation was similarly shown to be beneficial in a mouse model of AD [START_REF] Ma | Suppression of eIF2alpha kinases alleviates Alzheimer's disease-related plasticity and memory deficits[END_REF], suggesting that targeting the UPR may represent a powerful strategy to combat various neurodegenerative disorders [START_REF] Halliday | Targeting the unfolded protein response in neurodegeneration: A new approach to therapy[END_REF]. Because transient phosphorylation of eIF2is involved in proteostasis, one important challenge for the next years would be to develop UPR-targeting therapeutic approaches that preserve the beneficial function of eIF2 as recently outlined [START_REF] Halliday | Targeting the unfolded protein response in neurodegeneration: A new approach to therapy[END_REF]Some progress towards this goal may be achieved through further delineating the link between PrP C signalling and eIF2which has been but barely addressed until now [START_REF] Roffe | Prion protein interaction with stressinducible protein 1 enhances neuronal protein synthesis via mTOR[END_REF].

Finally, an emerging axis focuses on the protective properties of the N1 PrP C fragment.

Multiple studies have ascribed the neuroprotective role of PrP C to the N-terminal domain and have suggested that this action is, at least partly, supported by its soluble N1 fragment [START_REF] Resenberger | Neuroprotective and neurotoxic signaling by the prion protein[END_REF][START_REF] Guillot-Sestier | alpha-Secretase-derived cleavage of cellular prion yields biologically active catabolites with distinct functions[END_REF]. Recently, building upon the mapping of the binding sites of A oligomers to the N-terminal region of PrP C (see Fig. 1), several groups have documented that soluble N1 is endowed with the capacity to trap A oligomers and thereby protect against A-induced neurotoxicity in various paradigms [START_REF] Beland | PrP(C) homodimerization stimulates the production of PrPC cleaved fragments PrPN1 and PrPC1[END_REF][START_REF] Fluharty | An N-terminal fragment of the prion protein binds to amyloid-beta oligomers and inhibits their neurotoxicity in vivo[END_REF][START_REF] Guillot-Sestier | alpha-Secretase-derived fragment of cellular prion, N1, protects against monomeric and oligomeric amyloid beta (Abeta)-associated cell death[END_REF][START_REF] Nieznanski | Soluble prion protein inhibits amyloid-beta (Abeta) fibrillization and toxicity[END_REF]. The challenge now is to translate these observations into therapeutic strategies [START_REF] Beland | Taking advantage of physiological proteolytic processing of the prion protein for a therapeutic perspective in prion and Alzheimer diseases[END_REF].

Conclusion

The recent array of convergent data supporting the view that the subversion of PrP Cdependent signalling cascades is a pathogenic trigger in both prion and Alzheimer's diseases have sparked renewed interest on the physiological neuronal function exerted by PrP C . While considerable progress has been achieved in deciphering the signalling events instructed by PrP C , the challenge now is to reach a global view of the pathways orchestrated by PrP C according to the ligands at the play and the neuronal subtype considered, as well as their potential interconnections. Besides, further increasing our knowledge of PrP C -dependent signalling cascades may allow the identification of additional effectors, a subset of which may represent potential targets according to their status in disease. Overall, multiple strategies are now being exploited to counteract the toxic cascades arising from the deviation of PrP C -mediated signalling and we may hope some will reach success in the near future. [START_REF] Beraldo | Role of alpha7 nicotinic acetylcholine receptor in calcium signaling induced by prion protein interaction with stress-inducible protein 1[END_REF], protein synthesis [START_REF] Roffe | Prion protein interaction with stressinducible protein 1 enhances neuronal protein synthesis via mTOR[END_REF], potentiation of neurotransmitter release [START_REF] Hernandez-Rapp | A PrPC-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor[END_REF], axonal growth [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] and neurite outgrowth [START_REF] Beraldo | Metabotropic glutamate receptors transduce signals for neurite outgrowth after binding of the prion protein to laminin gamma1 chain[END_REF]. mRNAs and activity, which drives impairment in Aβ clearance, causing Aβ accumulation [START_REF] Pradines | Pathogenic prions deviate PrP(C) signaling in neuronal cells and impair A-beta clearance[END_REF]. In Alzheimer's disease, the Aβ-PrP C -Fyn cascade promotes Tau hyperphosphorylation [START_REF] Larson | The complex PrP(c)-Fyn couples human oligomeric Abeta with pathological tau changes in Alzheimer's disease[END_REF] as well as impairment of the NMDA receptors surface localization [START_REF] Um | Alzheimer amyloid-beta oligomer bound to postsynaptic prion protein activates Fyn to impair neurons[END_REF]. PrP C is also involved in the deleterious Aβ-dependent overactivation of PDK1 [START_REF] Pietri | PDK1 decreases TACE-mediated alpha-secretase activity and promotes disease progression in prion and Alzheimer's diseases[END_REF].

Legends to figures

Figure 1 :

 1 Figure 1: Secondary structure of mouse PrP C . The alpha-cleavage (green) occurs at

Figure 2 :

 2 Figure 2: Summary of the signalling cascades mobilized by PrP C in response to the

Figure 3 :

 3 Figure 3: Schematic representation of the corruption of PrP C -mediated signalling in

Figure 1 Click here to download high resolution image Figure 2 Click here to download high resolution image Figure 3

 123 Figure 1 Click here to download high resolution image

Table 1 :

 1 Downstream targets of PrP C -mediated signalling in a neuronal context (IEG: immediate early genes; SMase: sphingomyelinase)

	Targets	Cell type / tissue	Physiological response	Reference
	Src kinases
	Fyn	1C11-derived bioaminergic neuronal cells		[40]
		Primary hippocampal neurons	Neurite outgrowth	[29]
		PC12 neuronal cells	Neurite outgrowth	[41]
	Lyn	1C11-derived serotonergic neuronal cells	Potentiation of neurotransmitter release	[27]
		Raphe	Potentiation of neurotransmitter release	[27]
	PI3K/Akt	N2a neuroblastoma cells	Cell survival	[46]
		Primary hippocampal neurons	Protein synthesis	[47]
		1C11-derived serotonergic neuronal cells	Potentiation of neurotransmitter release	[27]
	PKA	Rat retina	Neuroprotection	[49]
		Primary hippocampal neurons	Neuroprotection	[50]
		Hippocampal slices	Synaptic plasticity	[48]
	NADPH oxidase	1C11-derived bioaminergic neuronal cells		[51]
	ERK1/2	Rat retina	Neuroprotection	[49]
		SN56 cells		[52]
		Primary hippocampal neurons	Neurite outgrowth	[50]
		1C11-derived bioaminergic neuronal cells		[51]
		Primary hippocampal neurons	Neurite outgrowth	[56]
	GSK3	1C11-derived serotonergic neuronal cells	Potentiation of neurotransmitter release	[27]
		Raphe	Potentiation of neurotransmitter release	[27]
	TACE	1C11-derived bioaminergic neuronal cells	Neurotransmitter catabolism	[53]
	mTOR	Primary hippocampal neurons	Protein synthesis	[47]
	CREB	1C11-derived bioaminergic neuronal cells	Transcription of IEG	[54]
	SMase	Hippocampal cell lines, mouse brain		[55]
	Ca2+	Hippocampal neurons		[58]
		Primary hippocampal neurons	Neuroprotection, neuronal survival	[57]
		Primary hippocampal neurons	Neurite outgrowth	[56]
		Dorsal root ganglia neurons	Axonal growth	[59]

Table 2 :

 2 Functional interactions between PrP C -mediated signalling with neurotransmitter receptors in physiological or pathological contexts Receptor

Cell type / tissue Link with PrP C signalling

				References
	Serotonergic
	5-HT1BR, 5-HT2AR, 5-HT2BR	1C11-derived serotonergic neuronal cells	Modulation of couplings	[40]
	5-HT1BR	1C11-derived serotonergic neuronal cells	Relocalization of receptor, blockade of activity	[27]
	Glutamatergic metabotropic
	mGlu1R, mGluR5	Primary hippocampal neurons	LN1-induced neurite outgrowth	[56]
	mGlu5R	Cortical neurons	A-induced alteration of neuronal processes	[63]
	mGlu5R	Hippocampus	A-induced facilitation of LTD	[64]
	Glutamatergic ionotropic
	GluN2DR	Hippocampal slices	Negative regulation	[66]
	GluN2BR	Hippocampal slices	Internalization	[67]
	NMDAR	SH-SY5Y cells	A-induced neurotoxicity	[68]
	GluN2BR	Cortical neurons	A-induced Internalization	[44]
	NMDAR	Hippocampal neurons	A-induced neurotoxicity	[69]
	Cholinergic ionotropic
	7nAChR	Hippocampal neurons	Neuroprotection, neuronal survival	[57]

Acknowledgements

We apologize to colleagues in the field whose work we were unable to cite owing to space limitations. We acknowledge financial support from the ANR, INSERM, as well as the Region Ile de France (DIM-Stem Pôle).

The authors declare no conflict of interest.


Emilia Neag
email: emilia.neag@icia.ro

Anamaria Iulia Török

Zamfira Dincă

Marin Senila

Cerasel Varaticeanu

Erika Andrea Levei

Ekaterina Shilova

Francoise Bodenan

OPTIMIZATION OF GOLD SORPTION FROM AMMONIACAL THIOSULPHATE SOLUTION ON ANION EXCHANGE FIBER USING TAGUCHI EXPERIMENTAL DESIGN

Keywords: gold sorption, anion exchange fiber, Taguchi experimental design, ANOVA

Taguchi experimental design was used to optimize the experimental parameters for Au sorption from ammoniacal thiosulfate solutions containing 2 mg/L Cu complexed as cuprous thiosulfate [Cu(S2O3)3] 5- onto anion exchange fiber (METALICAPT® MFH11). An L9 orthogonal array was chosen to study the effect of three parameters, namely Au initial concentration (10-30 mg/L), fiber quantity (0.05-0.55 g) and temperature (20-30°C). The sorption tests were carried out in batch mode at a stirring rate of 500 rpm and pH of 9.3. The experimental data were processed using signal-to-noise ratio and the analysis of variance was used to determine the most significant parameters. The optimal conditions for Au sorption were found to be 30 mg/L Au concentration, 0.05 g of fiber and temperature of 20°C. The obtained results indicated fiber quantity as the most influential parameter on Au sorption with a contribution of 66%, followed by Au initial concentration with a contribution of 33%. In the optimal conditions, the sorption capacity of the fiber was found to be 16.8 mg/g, in good agreement with the predicted result by Taguchi method (16.7 mg/g). Further investigations are necessary to examine the elution behavior of gold from the fiber.

INTRODUCTION

The technologies for precious metal recovery have gained considerable interest, as gold (Au) resources are limited while Au is frequently used in various fields due to its outstanding thermal and electrical conductivity, high catalytic performance and photothermal conversion efficiency [1,2].

Gold leaching using thiosulfate solutions has emerged as an innovative and eco-friendly alternative to cyanide leaching, for gold containing wastes. However, the recovery of gold thiosulfate complex from pregnant leachate solution proved to be challenging. Thus, development of appropriate methods to effectively recover gold thiosulfate complex are still unavailable [3]. Although there are several papers on gold leaching from wastes, only few studies reported the recovery of Au from pregnant thiosulfate leaching solution. Literature data suggest that activated carbon adsorption, cementation, electrowinning and solvent extraction are likely inapplicable for the separation of gold from thiosulphate solutions, especially in case of the presence of competitive metal ions. Activated carbon presents low affinity for [Au(S 2 O 3 ) 2 ] 3- probably due to weak interaction between carbon active sites and S 2 O 3 2-. The recovery of gold using activated carbon can be improved by the addition of cyanide, which is not environmentally friendly. Through cementation process, gold can be precipitated, yet the recycling solution is difficult. The main disadvantages of electrowinning process for gold recovery are the high energy consumption and the low efficiency. Solvent extraction is a time-consuming process and expensive to achieve a complete solid-liquid separation [4]. Compared to other separation techniques, like electrolysis, solvent extraction and membrane separation, adsorption using ion exchange resins presents several advantages, like fast adsorption, high loading capacity, simultaneous elution and regeneration as well as operation at ambient temperature [4,[START_REF] Mohebbi | Izadi Ion Exchange Resin Technology in Recovery of Precious and Noble Metals[END_REF]. Moreover, it generates fewer toxic by-products and the adsorbent can be regenerated and reused several times [2]. Although, few information related to resin regeneration is available [START_REF] Azizitorghabeh | [END_REF].

The efficiency of gold recovery is dependent on physical and chemical properties of the adsorbent and is influenced by the sorption conditions, such as pH of the solution, contact time, the quantity and initial gold concentration. Weak and strong base anion exchange resins are presently used in gold recovery [START_REF] Mohebbi | Izadi Ion Exchange Resin Technology in Recovery of Precious and Noble Metals[END_REF]. In general, the pH of ammoniacal thiosulfate solution used for gold leaching is in the range of 9-11, thus at this pH both the thiosulfate and copper ammonia complexes formed during gold leaching are simultaneously stabilized [4]. Consequently, the weak-base resins are not adequate to recover Au from ammoniacal thiosulfate solution since their ion-exchange ability is damaged at a pH higher than 8. The strong base anion exchange resins are more resistant to the competing anions and aurothiosulfate complex can be efficiently removed at low concentrations. Nonetheless, these resins have a poor selectivity, [Cu(S 2 O 3 ) 3 ] 5-will strongly compete with [Au(S 2 O 3 ) 2 ] 3- for the active sites on resin surface [4]. The elution of gold and copper from the adsorbent is a two-stage process: in the first stage, Cu is selectively eluted by a solution containing oxygenated ammonia-ammonium sulfate or ammonium thiosulfate and in the second stage, Au is effectively eluted by thiocyanate, polythionates, perchlorate or nitrate containing solutions or by mixtures of thiourea and sulfuric acid or sulfite and chloride [4].

Taguchi method is a simple and effective experimental design technique used for optimizing process parameters [7]. The optimum experimental conditions are determined using an orthogonal array which aims to maximize the experimental results using a small number of experiments. Consequently, the signal-to-noise (S/N) ratio is used to analyze the experimental results and to determine the optimal levels of each parameter [8,9]. S/N represents the ratio of desirable results (signal) to undesirable results (noise) obtained [10]. Thus, using S/N ratio, the control factors that reduce the variability of the Au removal by minimizing the effects of uncontrollable factors can be identified [11]. Through Taguchi method, the effects of multiple parameters and their interaction can be investigated in a time-and cost-effective manner [12].

The objective of the present study was to maximize the total amount of Au as [Au(S 2 O 3 ) 2 ] 3-adsorbed on METALICAPT® MFH11 fiber from a synthetic aqueous solution containing ammonium thiosulfate (0.1 M) and 2 mg/L Cu complexed as [Cu(S 2 O 3 ) 3 ] 5-by optimizing the selected process parameters using Taguchi method. The METALICAPT® MFH11 fiber is a newly developed anion exchange fiber containing ammonium (-NR 3 + ) and tertiary amine (-NR 2 ) functional groups that has not been previously studied for Au sorption from ammoniacal thiosulphate solution. The research was carried out in four steps. In the first step, three parameters, namely Au initial concentration, fiber quantity and temperature were selected to be optimized in batch experiments due to their significant effect on aurothiosulfate complex sorption. An appropriate orthogonal array was chosen based on the considered parameters and their levels. In the second step, the experiments were performed using the selected 9 experimental trials. For each experiment conducted, the total amount of Au as [Au(S 2 O 3 ) 2 ] 3-sorbed on METALICAPT® MFH11 fiber was calculated. In the third step, the obtained results were processed with larger-the-better equation (S/N output) to determine the optimum conditions for the considered process and to maximize the response in terms of the total amount of [Au(S 2 O 3 ) 2 ] 3- sorbed on METALICAPT® MFH11 fiber. Finally, in the fourth step, confirmation experiments were carried out to verify the experimental conclusions.

RESULTS AND DISCUSSION

Taguchi Experimental Design

The experimental results express in terms of sorption capacity and removal efficiency obtained based on Taguchi's orthogonal array are presented in Table 1. Based on the obtained results, it can be seen that as initial Au concentration increased the q e values increased and the removal efficiency decreased. The initial Au concentration offers the necessary driving force to overcome the mass transfer resistance of adsorbate [13]. The q e decreases as the fiber quantity increases because more sorption sites are available. As the temperature increases, q e decreased suggesting an exothermic process for Au sorption [12]. The Minitab software was used to calculated the equivalent S/N ratio of the experiments based on the obtained q e values using largerthe-better equation to maximize the response [11]. The S/N ratio response (the average value of S/N ratio of each parameter at levels 1, 2 and 3) for Au sorption onto METALICAPT® MFH11 fiber is given in Figure 1. The greatest S/N ratio represents the optimal level of the considered parameter. Based on the difference between the highest S/N and lowest S/N for each parameter, the delta value can be calculated and compared. The following series was depicted based on the obtained delta values: fiber quantity (13.15) > Au concentration (9.12) > temperature (0.65). Thus, fiber quantity has a strong influence on Au sorption, followed by concentration and temperature has a low influence on Au sorption onto METALICAPT® MFH11 fiber. Consequently, concentration has the highest effect on the Au sorption at level 3 and fiber quantity and temperature had the highest effect at level 1. The optimum conditions for Au sorption were found to be initial Au concentration of 30 mg/L, fiber quantity of 0.05 g and temperature of 20°C.

Figure 1. The effects of process parameters on Au sorption onto METALICAPT® MFH11 fiber

Analysis of variance (ANOVA)

ANOVA is one of the statistical models used to distinguish the performance of parameters and their interactions. ANOVA analysis was used to identify the significant parameters for Au sorption onto METALICAPT® MFH11 fiber by the sum of the squared deviations from the total mean of S/N ratio [11].

The degrees of freedom (DF), sum of squares (SS), mean square (MS), distribution of the ratio (F) and p value are shown in Table 2. Further, the percent contribution (PC) of each parameter [14] was calculated and the results are given in Table 2. The p value for concentration and fiber quantity is very small which means that there is significant evidence against null hypothesis (H 0 ). The most significant parameters (p<0.01) for Au sorption are initial Au concentration and fiber quantity (Table 2). The p value for temperature was 0.512, thus this parameter can be ignored as it is insignificant for Au sorption. Fiber quantity (PC% = 66.25) was the most important parameter on Au sorption, followed by Au initial concentration (PC% = 33.42). The mean response for larger-thebetter used to predict the optimum q e value is given in Table 3. Three experiments were carried out to validate the obtained results using the levels of the optimal process parameters for Au sorption onto anion exchange fiber . The results obtained are presented in Table 4. As it can be seen in Table 4 the predicted optimal q e value and the average of confirmation experiment was 16.7 mg Au/g fiber and 16.8 mg/g, respectively. As it can be observed the obtained values are very close. Thus, the results predicted by Taguchi method were validated by the confirmation experiments. Further, the fiber can be burned to obtain Au. Also, the purity of the Au depends on the concentration of other concurrent anions present in the solution subjected to treatment.

CONCLUSIONS

In the current study, the Taguchi method was used to investigate the effects of Au initial concentration, fiber quantity and temperature on Au sorption from ammoniacal thiosulfate solutions containing 2 mg/L Cu onto an anion exchange fiber. The experimental results were analyzed using the signal to noise ratio and ANOVA to identify the optimum and to most significant parameters. The most prominent parameter affecting the Au sorption was fiber quantity, followed by initial concentration. The data suggested that temperature was insignificant for Au sorption. The predicted optimal value was confirmed through the confirmation experiments performed using the optimal conditions identified by Taguchi method.

EXPERIMENTAL SECTION

Description and Characteristics of the Sorbent METALICAPT® MFH11, an anion exchange fiber supplied by AJELIS SAS was used for Au sorption. Its characteristics are given in Table 5.

Table 5. Characteristics of the METALICAPT® MFH11 fiber

Ionic form

Chloride (Cl -) Functional group Ammonium (-NR3 + ), Tertiary amine (-NR2) Total exchange capacity (eq /L)

1.25-2.5 (-NR3 + ), 1-1.

Reagents and solutions

Analytical grade reagents, such as copper (II) sulfate pentahydrate (CuSO 4 •5H 2 O), ammonium thiosulfate (NH 4 ) 2 S 2 O 3 and sodium hydroxide (NaOH) were used for the preparation of the solutions. All used chemicals were of analytical purity and purchased from Merck (Darmstadt, Germany). Au as AuCl 4 -was introduced in 0.1 M ammoniacal thiosulfate solutions, which was reduced to Au(S 2 O 3 ) 2 3-. The pH of the solution was adjusted to 9.3 with NaOH, and then Cu (2 mg/L) was added to simulate the effluent obtained after leaching of gold from ores.

Taguchi experimental design

Three parameters with three levels each (Table 6) were taken into consideration to identify the optimum conditions for Au sorption using METALICAPT® MFH11 fiber. Based on the considered parameters and their levels, the Minitab software generated the L 9 orthogonal array (Table 7).

𝐸(%) = 𝐶 -𝐶 𝐶 • 100 (2)
where, C o , C e are the initial and equilibrium Au concentrations (mg/L); V is the volume of Au solution (mL) and m is the mass of the fiber (g).

Based on the obtained q e values, the S/N ratio (using the larger-thebetter equation) was calculated to analyze the combined effects of control and noise factors [14]. The larger-the-better equation to maximize the response is given below [11,14].

𝑆/𝑁 = -10 𝑙𝑜𝑔 1 𝑌 /𝑛 (3)
where, Y is the value of the response and n is the total number of repetitions in a trial [14]. Further, the ANOVA analysis was conducted to identify the significant parameters of the sorption process. Three confirmation experiment were conducted using the optimal levels of the process parameters. The average value was compared with the predicted value. After determination of the optimum conditions, the mean (μ) was estimated from the significant parameters identified by ANOVA analysis [12,16].

μ = Ū + (A 3 -Ū) + (B 1 -Ū) (4)
where, Ū is the overall mean of the response (mean of q e value of the experiments performed according to L 9 ) and A 3 , B 1 represent average values of response at their optimum levels. The predicted optimum value of q e was calculated as follows:

μ fiber = Ū fiber + (A 3 -Ū fiber ) + (B 1 -Ū fiber ) + (C 1 -Ū fiber ) [START_REF] Mohebbi | Izadi Ion Exchange Resin Technology in Recovery of Precious and Noble Metals[END_REF] where, μ fiber is the predicted optimum value of q e for the anion exchange fiber, Ū fiber is the overall mean of q e , and A 3, B 1, C 1, are the average q e values at their optimum levels [12,16].

  EMILIA NEAG, ANAMARIA IULIA TÖRÖK, ZAMFIRA DINCĂ, MARIN SENILA, CERASEL VARATICEANU, ERIKA ANDREA LEVEI, EKATERINA SHILOVA, FRANCOISE BODENAN 154

Table 1 .

 1 The experimental results obtained based on Taguchi's orthogonal array and the corresponding S/N ratio for each experiment

	Experiment	Experimental result	S/N ratio
	number	qe (mg/g)	Au removal (%)
	1	8.1	40.8	18.12
	2	2.9	73.3	9.24
	3	1.6	88.0	3.98
	4	16.8	37.6	23.96
	5	6.5	73.0	16.12
	6	3.5	87.2	10.83
	7	19.9	31.8	25.92
	8	8.9	71.5	19.02
	9	4.9	85.7	13.75

Table 2 .

 2 Analysis of Variance for S/N ratio

	Parameter	DF	SS	MS	F	p	PC
	Concentration	2	132.48	66.24	197.95	0.005	33.42
	Fiber quantity	2	262.65	131.32	392.46	0.003	66.25
	Temperature	2	0.64	0.32	0.95	0.512	0.16
	Residual Error	2	0.67	0.34			0.17
	Total	8	396.43

Table 3 .

 3 The mean response for larger-the-better (average qe values)

	Level	Concentration Fiber quantity Temperature
	1	3.51	14.24	6.17
	2	8.93	6.10	8.19
	3	11.22	3.32	9.30
	Delta	7.71	10.92	3.14
	Rank	2	1	3

Table 4 .

 4 Predicted optimal qe values and the results of confirmation experiment

	Adsorbent	Predicted	Mean	Average of
		optimal q e value	predicted	confirmation experiment (q e )
		mg/g	mg/g	mg/g
	METALICAPT®	16.7	18.0	16.8
	MFH11

Table 6 .

 6 Selected parameters and their levels used in the Taguchi experimental design

	Symbol	Parameters	Units	1	Levels 2	3
	A	Au initial concentration	mg/L	10	20	30
	B	Fiber quantity	g	0.05	0.25	0.55
	C	Temperature	°C	20	25	30

Table 7 .

 7 Design of experiments using Taguchi orthogonal array

	Experiment	Experiment Design
	number	Au concentration (mg/L)	Fiber quantity (g)	Temperature (°C)
	1	10	0.05	20
	2	10	0.25	25
	3	10	0.55	30
	4	20	0.05	25
	5	20	0.25	30
	6	20	0.55	20
	7	30	0.05	30
	8	30	0.25	20
	9	30	0.55	25

Scientific Research and Innovation, CCCDI-UEFISCDI, project number 52/2018, COFUND-ERANET-ERAMIN-MINTECO-2, within PNCDI III.

ACKNOWLEDGMENTS

This research was funded by a grant of the Romanian National Authority for

Sorption and confirmation experiments

The optimization of gold sorption on METALICAPT® MFH11 fiber using Taguchi experimental design is detailed in Figure 2. The sorption experiments were performed in triplicate according to Table 7 for 240 min at a stirring rate of 500 rpm. Preliminary tests showed that equilibrium was reached up to 240 min. The Au concentration was analyzed using an Optima 5300 DV Perkin-Elmer inductively coupled plasma optical emission spectrometer. At the end of each experiment, the sorption capacity of the fiber (q e ) and removal efficiency (percent of sorption, E) were calculated [15] according to Equation 1 and 2, respectively.