Modulation of serotonergic receptor signaling and cross-talk by prion protein

A roadmap for investigating the role of the prion protein in depression associated with neurodegenerative disease

The physiological properties of the native, endogenous prion protein (PrP(C)) is a matter of concern, due to its pleiotropic functions and links to neurodegenerative disorders and cancer. In line with our hypothesis that the basic function of PrP(C) is to serve as a cell surface scaffold for the assembly of signaling modules, multiple interactions have been identified of PrP(C) with signaling molecules, including neurotransmitter receptors. We recently reported evidence that PrP(C) may modulate monoaminergic neurotransmission, as well as depressive-like behavior in mice. Here, we discuss how those results, together with a number of other studies, including our previous demonstration that both inflammatory and behavioral stress modulate PrP(C) content in neutrophils, suggest a distributed role of PrP(C) in clinical depression and inflammation associated with neurodegenerative diseases. An overarching understanding of the multiple interventions of PrP(C) upon physiological events may both shed light on the pathogenesis of, as well as help the identification of novel therapeutic targets for clinical depression, Prion and Alzheimer's Diseases.

Metals and Circadian Rhythms

Circadian rhythms describe the behavioral and physiological changes that occur in living organisms in order to attune to a 24 hour cycle of day and night. The most striking aspect of circadian function is the sleep-wake cycle, however many other physiological processes are regulated in 24 hour oscillations, including blood pressure, body temperature, appetite, urine production, and the transcription and translation of thousands of circadian dependent genes. Circadian disruption and sleep disorders are strongly connected to neurodegenerative diseases including Parkinson's disease, Alzheimer's disease, and Huntington's disease as well as others. Metal exposures have been implicated in neurodegenerative diseases, in some cases involving metals that are essential micronutrients but are toxic at high levels of exposure (such as manganese, copper, and zinc), and in other cases involving metals that have no biological role but are toxic to living systems (such as lead, mercury, and aluminum). In this review, we examine the evidence for circadian and sleep disorders with exposures to these metals and review the literature for possible mechanisms. We suggest that giving the aging population, the prevalence of environmental exposures to metals, and the increasing prevalence of neurodegenerative disease in the aged, more research into the mechanisms of circadian disruption subsequent to metal exposures is warranted.

Cell Biology of Prion Protein

Cellular prion protein (PrP^C) is a mammalian glycoprotein which is usually found anchored to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. The precise function of PrP^C remains elusive but may depend upon its cellular localization. PrP^C misfolds to a pathogenic isoform PrP^Sc, the causative agent of neurodegenerative prion diseases. Nonetheless some forms of prion disease develop in the apparent absence of infectious PrP^Sc, suggesting that molecular species of PrP distinct from PrP^Sc may represent the primary neurotoxic culprits. Indeed, in some inherited cases of human prion disease, the predominant form of PrP detectable in the brain is not PrP^Sc but rather ^CtmPrP, a transmembrane form of the protein. The relationship between the neurodegeneration occurring in prion diseases involving PrP^Sc and that associated with ^CtmPrP remains unclear. However, the different membrane topology of the PrP mutants, as well as the presence of the GPI anchor, could influence both the function and the intracellular localization and trafficking of the protein, all being potentially very important in the pathophysiological mechanism that ultimately causes the disease. Here, we review the latest findings on the fundamental aspects of prions biology, from the PrP^C biosynthesis, function, and structure up to its intracellular traffic and analyze the possible roles of the different topological isoforms of the protein, as well as the GPI anchor, in the pathogenesis of the disease.

Functions of the Prion Protein

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 unraveling 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 signaling 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.

Regulation of Amyloid β Oligomer Binding to Neurons and Neurotoxicity by the Prion Protein-mGluR5 Complex

The prion protein (PrP^C) has been suggested to operate as a scaffold/receptor protein in neurons, participating in both physiological and pathological associated events. PrP^C, laminin, and metabotropic glutamate receptor 5 (mGluR5) form a protein complex on the plasma membrane that can trigger signaling pathways involved in neuronal differentiation. PrP^C and mGluR5 are co-receptors also for β-amyloid oligomers (AβOs) and have been shown to modulate toxicity and neuronal death in Alzheimer's disease. In the present work, we addressed the potential crosstalk between these two signaling pathways, laminin-PrP^C-mGluR5 or AβO-PrP^C-mGluR5, as well as their interplay. Herein, we demonstrated that an existing complex containing PrP^C-mGluR5 has an important role in AβO binding and activity in neurons. A peptide mimicking the binding site of laminin onto PrP^C (Ln-γ1) binds to PrP^C and induces intracellular Ca²⁺ increase in neurons via the complex PrP^C-mGluR5. Ln-γ1 promotes internalization of PrP^C and mGluR5 and transiently decreases AβO biding to neurons; however, the peptide does not impact AβO toxicity. Given that mGluR5 is critical for toxic signaling by AβOs and in prion diseases, we tested whether mGlur5 knock-out mice would be susceptible to prion infection. Our results show mild, but significant, effects on disease progression, without affecting survival of mice after infection. These results suggest that PrP^C-mGluR5 form a functional response unit by which multiple ligands can trigger signaling. We propose that trafficking of PrP^C-mGluR5 may modulate signaling intensity by different PrP^C ligands.

Prion Protein Modulates Monoaminergic Systems and Depressive-like Behavior in Mice

We sought to examine interactions of the prion protein (PrP(C)) with monoaminergic systems due to: the role of PrP(C) in both Prion and Alzheimer diseases, which include clinical depression among their symptoms, the implication of monoamines in depression, and the hypothesis that PrP(C) serves as a scaffold for signaling systems. To that effect we compared both behavior and monoaminergic markers in wild type (WT) and PrP(C)-null (PrP(-/-)) mice. PrP(-/-) mice performed poorly when compared with WT in forced swimming, tail suspension, and novelty suppressed feeding tests, typical of depressive-like behavior, but not in the control open field nor rotarod motor tests; cyclic AMP responses to stimulation of D1 receptors by dopamine was selectively impaired in PrP(-/-) mice, and responses to serotonin, but not to norepinephrine, also differed between genotypes. Contents of dopamine, tyrosine hydroxylase, and the 5-HT5A serotonin receptor were increased in the cerebral cortex of PrP(-/-), as compared with WT mice. Microscopic colocalization, as well as binding in overlay assays were found of PrP(C) with both the 5HT5A and D1, but not D4 receptors. The data are consistent with the scaffolding of monoaminergic signaling modules by PrP(C), and may help understand the pathogenesis of clinical depression and neurodegenerative disorders.

Regulation of PrP(C) signaling and processing by dimerization

The cellular prion protein (PrP^C) is a glycosylphosphatidylinositol (GPI)-anchored protein present at the cell surface. PrP^C N-terminal moiety is intrinsically disordered and is able to interact with a variety of ligands. Physiological ligands have neurotrophic activity, whilst others, including protein toxic oligomers, have neurotoxic functions. These two opposite activities involve different interacting partners and result from different PrP^C-activated signaling pathways. Remarkably, PrP^C may be inactivated either by physiological endoproteolysis and release of the N-terminal domain, or by ectodomain shedding. Ligand-induced PrP^C dimerization or enforced dimerization of PrP^C indicate that PrP^C dimerization represents an important molecular switch for both intracellular signaling and inactivation by the release of PrP^C N-terminal domain or shedding. In this review, we summarize evidence that cell surface receptor activity of PrP^C is finely regulated by dimerization.

PrP(C) signalling in neurons: from basics to clinical challenges

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, GPI-anchored 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 well-established 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 Aβ oligomers, whose accumulation is associated with Alzheimer's disease (AD), were shown to similarly instigate toxic events by deviating PrP(C)-mediated 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 Aβ oligomers contributes to TSE or AD neuropathogenesis and discuss the ensuing clinical implications.

A PrP(C)-caveolin-Lyn complex negatively controls neuronal GSK3β and serotonin 1B receptor

The cellular prion protein, PrP^C, is a glycosylphosphatidylinositol-anchored protein, abundant in lipid rafts and highly expressed in the brain. While PrP^C is much studied for its involvement under its abnormal PrP^Sc isoform in Transmissible Spongiform Encephalopathies, its physiological role remains unclear. Here, we report that GSK3β, a multifunctional kinase whose inhibition is neuroprotective, is a downstream target of PrP^C signalling in serotonergic neuronal cells. We show that the PrP^C-dependent inactivation of GSK3β is relayed by a caveolin-Lyn platform located on neuronal cell bodies. Furthermore, the coupling of PrP^C to GSK3β potentiates serotonergic signalling by altering the distribution and activity of the serotonin 1B receptor (5-HT_1BR), a receptor that limits neurotransmitter release. In vivo, our data reveal an increased GSK3β kinase activity in PrP-deficient mouse brain, as well as sustained 5-HT_1BR activity, whose inhibition promotes an anxiogenic behavioural response. Collectively, our data unveil a new facet of PrP^C signalling that strengthens neurotransmission.

Hijacking PrP(c)-dependent signal transduction: when prions impair Aβ clearance

The cellular prion protein PrP^c is the normal counterpart of the scrapie prion protein PrP ^Sc, the main component of the infectious agent of transmissible spongiform encephalopathies. The recent discovery that PrP ^c can serve as a receptor for the amyloid beta (Aβ) peptide and relay its neurotoxicity is sparking renewed interest on this protein and its involvement in signal transduction processes. Disease-associated PrP ^Sc shares with Aβ the ability to hijack PrP ^c-dependent signaling cascades, and thereby instigate pathogenic events. Among these is an impairment of Aβ clearance, uncovered in prion-infected neuronal cells. These findings add another facet to the intricate interplay between PrP ^c and Aβ. Here, we summarize the connection between PrP-mediated signaling and Aβ clearance and discuss its pathological implications.

Bovine Spongiform Encephalopathy in Japan: History and Recent Studies on Oxidative Stress in Prion Diseases

With the respect to BSE and vCJD, compliance with the following three rules should strictly be observed: (i) Identification and destruction of all clinically affected cattle; (ii) destruction of all mammalian proteins used in feeding ruminant livestock; and (iii) destruction of all high-risk tissues for use in human consumption. Scrapie in sheep has been documented in the 18th century in the United Kingdom. Through studies of brain-to-brain transmission in the same species in 1935, Cuille et al. successfully isolated the culprit protein from the sheep brain. To transmit said protein from an animal to another, intracerebral inoculation was much more efficient than intraperitoneal or oral route in certain species; i.e. the hamster and mouse. Since discovery of the more efficacious infection route, studies and development of prion research have undergone 4 developmental phases. Phase I depicted discoveries of the pathological features of Creutzfeldt-Jakob disease (CJD) and scrapie with typical lesions of spongiform encephalopathy, while Phase II revealed individual-to-individual (or cross-species) transmissions of CJD, kuru and scrapie in animals. Phases I and II suggested the possible participation of a slow virus in the infection process. In Phase III, Prusiner et al. proposed the 'prion' theory in 1982, followed by the milestone development of the transgenic or gene-targeted mouse in prion research in Phase IV. By strain-typing of prions, CJD has been classified as type 2 or 4 by Parchi et al. and Wadsworth as type-2 or -4 and type-1 or -2, respectively. Wadsworth type 1 is detected in the cerebellum, while Wadsworth type 2 was detected in the prefrontal cortex of 10% of sporadic CJD patients. In 1999, Puoti et al. have reported the co-existence of two types of PrP(res) in a same patient. These reports indicated that PrP(res)-typing is a quantitative rather than a qualitative process, and the relationship between the molecular type and the prion strain is rather complex. In fact, previous findings of Truchot have correlated type-1 distribution with synaptic deposits, and type-2 with arrangement of diffuse deposits in neurons. Although the normal function of PrP(C) has not been fully understood, recent studies have shown that PrP(C) plays a role in copper metabolism, signal transduction, neuroprotection and cell maturation. Further search of PrP(C)-interacting molecules and detailed studies using Prnp(-/-) mice and various type of Prnp(-/-) cell lines under various conditions are the prerequisites in elucidating PrP functions. In the pathogenesis of prion diseases, present results support the hypothesis that 'loss-of-function' of PrP(C) decreases resistance to oxidative stress, and 'gain-of-function' of PrP(Sc) increases oxidative stress. The mechanisms of (i) the 'loss-of-function' of PrP(C) in enhanced susceptibility to oxidative stress and (ii) the 'gain-of-function' of PrP(Sc) in generation of oxidative stress remain to be elucidated, although their mechanisms of action, at least in part, involve the decrease and increase in SOD activity, respectively.

PrP octarepeats region determined the interaction with caveolin-1 and phosphorylation of caveolin-1 and Fyn

Caveolin-1 is one of the major constituents of caveolae. Both Cav-1 and PrP are plasma membrane proteins, which show active capacities for molecular interactions with many other proteins or agents, including themselves. Using yeast two-hybrid system and immunoprecipitation, we reconfirmed the molecular interaction between human Cav-1 and PrP. With co-immunoprecipitation tests, PrP(C)-Cav-1 and PrP(Sc)-Cav-1 complexes were identified in the brain homogenates of normal and scrapie agent 263K-infected hamsters, respectively. Transient expression of wild-type PrP (PrP-PG5) in HEK293 cells did not change the situation of Cav-1 and subsequent signal transduction pathways, while cross-linking of the expressed PrP with specific antibody induced remarkable colocalization of PrP and Cav-1 on the plasma membrane and significant increases of phosphorylated Cav-1 and phosphorylated Fyn. With deleted and inserted PrP mutants within octarepeat region, we observed obvious octarepeat-associated phenomena, including lower binding capacity with Cav-1 in vitro, unable to co-localize with Cav-1 in the cells and to induce up-regulation of p-Cav-1 and p-Fyn when removal of octarepeats in the context of full-length PrP. Moreover, we found that treatment on HEK293 cells with fibrous form of recombinant PrP protein led to up-regulating the levels of p-Cav-1 and p-Fyn. Our data here provide strong evidence that octarepeats of PrP are critical for the interaction between PrP and Cav-1. Significant alterations in the cultured cells, either the distributions of PrP and Cav-1 morphologically or the up-regulations of p-Cav-1 and p-Fyn, induced by antibody-mediated cross-linking or fibrous forms of PrP may suggest a possible internalization process of PrP(Sc).

Proteomic analysis of prion diseases: creating clarity or causing confusion?

Prion diseases, or transmissible spongiform encephalopathies, are progressive, fatal neurodegenerative diseases. There are both human and animal forms of the disease and all are associated with the conversion of a normal host-coded cellular prion protein (PrP(C) ) into an abnormal protease-resistant isoform (PrP(Sc) ). Although methodologies are sensitive and specific for postmortem disease diagnosis, the use of PrP(Sc) as a preclinical or general biomarker for surveillance is difficult, due to the fact that it is present in extremely small amounts in accessible tissues or body fluids such as blood, urine, saliva, and cerebrospinal fluid. Recently, amplification techniques have been developed, which have enabled increased sensitivity for PrP(Sc) detection. However, it has recently been reported that proteinase K sensitive, pathological isoforms of PrP may have a significant role in the pathogenesis of some prion diseases. Accordingly, the development of new diagnostic tests that do not rely on PrP(Sc) and proteinase K digestion is desirable. The search for biomarkers (other than PrP(Sc) ) as tools for diagnosis of prion diseases has a long history. Ideally biomarkers able to detect all transmissible spongiform encephalopathies, even at preclinical stages of infection are desirable but not yet possible due to the heterogeneity of the disease and lengthy disease progression. Recent advances in neuroproteomics have led to an overwhelming amount of information, which may offer insight on protein-protein interactions. While the amount of data obtained is impressive, the ability to relate it to the disease and validating its usefulness in diagnostic biomarker development remains a formidable challenge.

The Cellular Prion Protein Prevents Copper-Induced Inhibition of P2X4Receptors

Although the physiological function of the cellular prion protein (PrP^C) remains unknown, several evidences support the notion of its role in copper homeostasis. PrP^C binds Cu²⁺ through a domain composed by four to five repeats of eight amino acids. Previously, we have shown that the perfusion of this domain prevents and reverses the inhibition by Cu²⁺ of the adenosine triphosphate (ATP)-evoked currents in the P2X₄ receptor subtype, highlighting a modulatory role for PrP^C in synaptic transmission through regulation of Cu²⁺ levels. Here, we study the effect of full-length PrP^C in Cu²⁺ inhibition of P2X₄ receptor when both are coexpressed. PrP^C expression does not significantly change the ATP concentration-response curve in oocytes expressing P2X₄ receptors. However, the presence of PrP^C reduces the inhibition by Cu²⁺ of the ATP-elicited currents in these oocytes, confirming our previous observations with the Cu²⁺ binding domain. Thus, our observations suggest a role for PrP^C in modulating synaptic activity through binding of extracellular Cu²⁺.

Role of alpha7 nicotinic acetylcholine receptor in calcium signaling induced by prion protein interaction with stress-inducible protein 1

The prion protein (PrP(C)) is a conserved glycosylphosphatidylinositol-anchored cell surface protein expressed by neurons and other cells. Stress-inducible protein 1 (STI1) binds PrP(C) extracellularly, and this activated signaling complex promotes neuronal differentiation and neuroprotection via the extracellular signal-regulated kinase 1 and 2 (ERK1/2) and cAMP-dependent protein kinase 1 (PKA) pathways. However, the mechanism by which the PrP(C)-STI1 interaction transduces extracellular signals to the intracellular environment is unknown. We found that in hippocampal neurons, STI1-PrP(C) engagement induces an increase in intracellular Ca(2+) levels. This effect was not detected in PrP(C)-null neurons or wild-type neurons treated with an STI1 mutant unable to bind PrP(C). Using a best candidate approach to test for potential channels involved in Ca(2+) influx evoked by STI1-PrP(C), we found that α-bungarotoxin, a specific inhibitor for α7 nicotinic acetylcholine receptor (α7nAChR), was able to block PrP(C)-STI1-mediated signaling, neuroprotection, and neuritogenesis. Importantly, when α7nAChR was transfected into HEK 293 cells, it formed a functional complex with PrP(C) and allowed reconstitution of signaling by PrP(C)-STI1 interaction. These results indicate that STI1 can interact with the PrP(C)·α7nAChR complex to promote signaling and provide a novel potential target for modulation of the effects of prion protein in neurodegenerative diseases.

Serotonergic 5-HT(2B) receptor controls tissue-nonspecific alkaline phosphatase activity in osteoblasts via eicosanoids and phosphatidylinositol-specific phospholipase C

In previous studies, we observed that mice knocked out for the serotonin-2B receptor (5-HT(2B)R) show defects in bone homeostasis. The present work focuses on the downstream targets relaying the anabolic function of this receptor in osteoblasts. A functional link between the 5-HT(2B)R and the activity of the tissue-nonspecific alkaline phosphatase (TNAP) is established using the C1 osteoprogenitor cell line. During C1 osteogenic differentiation, both 5-HT(2B)R and TNAP mRNA translations are delayed with respect to extracellular matrix deposition. Once the receptor is expressed, it constitutively controls TNAP activity at a post-translational level along the overall period of mineral deposition. Indeed, pharmacological inhibition of the 5-HT(2B)R intrinsic activity or shRNA-mediated 5-HT(2B)R knockdown prevents TNAP activation, but not its mRNA translation. In contrast, agonist stimulation of the receptor further increases TNAP activity during the initial mineralization phase. Building upon our previous observations that the 5-HT(2B)R couples with the phospholipase A2 pathway and prostaglandin production at the beginning of mineral deposition, we show that the 5-HT(2B)R controls leukotriene synthesis via phospholipase A2 at the terminal stages of C1 differentiation. These two 5-HT(2B)R-dependent eicosanoid productions delineate distinct time windows of TNAP regulation during the osteogenic program. Finally, prostaglandins or leukotrienes are shown to relay the post-translational activation of TNAP via stimulation of the phosphatidylinositol-specific phospholipase C. In agreement with the above findings, primary calvarial osteoblasts from 5-HT(2B)R-null mice exhibit defects in TNAP activity.

Prion protein: From physiology to cancer biology

Prion protein (PrPc) was originally viewed solely as being involved in prion disease, but now several intriguing lines of evidence have emerged indicating that it plays a fundamental role not only in the nervous system, but also throughout the human body. PrPc is expressed most abundantly in the brain, but has also been detected in other non-neuronal tissues as diverse as lymphoid cells, lung, heart, kidney, gastrointestinal tract, muscle, and mammary glands. Recent data indicate that PrPc may be implicated in biology of glioblastoma, breast cancer, prostate and gastric cancer. Over expression of PrPc is correlated to the acquisition by tumor cells of a phenotype for resistance to cell death induced by TNF alpha and TRAIL or antitumor drugs such as paclitaxel and anthracyclines. PrPc may promote tumorigenesis, proliferation and G1/S transition in gastric cancer cells. This review revisits the physiological functions of PrPc, and its possible implications for cancer biology.

The cellular prion protein interacts with the tissue non-specific alkaline phosphatase in membrane microdomains of bioaminergic neuronal cells

Background

The cellular prion protein, PrP^C, is GPI anchored and abundant in lipid rafts. The absolute requirement of PrP^C in neurodegeneration associated to prion diseases is well established. However, the function of this ubiquitous protein is still puzzling. Our previous work using the 1C11 neuronal model, provided evidence that PrP^C acts as a cell surface receptor. Besides a ubiquitous signaling function of PrP^C, we have described a neuronal specificity pointing to a role of PrP^C in neuronal homeostasis. 1C11 cells, upon appropriate induction, engage into neuronal differentiation programs, giving rise either to serotonergic (1C11^5-HT) or noradrenergic (1C11^NE) derivatives.

Methodology/Principal Findings

The neuronal specificity of PrP^C signaling prompted us to search for PrP^C partners in 1C11-derived bioaminergic neuronal cells. We show here by immunoprecipitation an association of PrP^C with an 80 kDa protein identified by mass spectrometry as the tissue non-specific alkaline phosphatase (TNAP). This interaction occurs in lipid rafts and is restricted to 1C11-derived neuronal progenies. Our data indicate that TNAP is implemented during the differentiation programs of 1C11^5-HT and 1C11^NE cells and is active at their cell surface. Noteworthy, TNAP may contribute to the regulation of serotonin or catecholamine synthesis in 1C11^5-HT and 1C11^NE bioaminergic cells by controlling pyridoxal phosphate levels. Finally, TNAP activity is shown to modulate the phosphorylation status of laminin and thereby its interaction with PrP.

Conclusion/Significance

The identification of a novel PrP^C partner in lipid rafts of neuronal cells favors the idea of a role of PrP in multiple functions. Because PrP^C and laminin functionally interact to support neuronal differentiation and memory consolidation, our findings introduce TNAP as a functional protagonist in the PrP^C-laminin interplay. The partnership between TNAP and PrP^C in neuronal cells may provide new clues as to the neurospecificity of PrP^C function.

PrPc activation induces neurite outgrowth and differentiation in PC12 cells: role for caveolin-1 in the signal transduction pathway

Cellular prion protein (PrP(c)) is a ubiquitous glycoprotein, whose physiological role is poorly characterized. It has been suggested that PrP(c) participates in neuritogenesis, neuroprotection, copper metabolism, and signal transduction. In this study we detailed the intracellular events induced by PrP(c) antibody-mediated cross-linking in PC12 cells. We found a Fyn-dependent activation of the Ras-Raf pathway, which leads to a rapid and transient phosphorylation of extracellular regulated kinases. In addition, this activation cascade relies on the engagement of integrins, and involves focal adhesion kinase activation. We demonstrated the tyrosine phosphorylation of caveolin-1 as a consequence of PrP(c) stimulation, and showed that phosphocaveolin-1 scaffolds and coordinates protein complexes involved in PrP(c)-dependent signaling. Moreover, we found that caveolin-1 phosphorylation, is a mechanism for recruiting the C-terminal Src kinase and inactivating Fyn, so as to terminate cell signaling. Furthermore our data support a significant role for PrP(c) as a response mediator in neuritogenesis and cell differentiation.

Cryo-immunogold electron microscopy for prions: toward identification of a conversion site

Prion diseases are caused by accumulation of an abnormally folded isoform (PrP(Sc)) of the cellular prion protein (PrP(C)). The subcellular distribution of PrP(Sc) and the site of its formation in brain are still unclear. We performed quantitative cryo-immunogold electron microscopy on hippocampal sections from mice infected with the Rocky Mountain Laboratory strain of prions. Two antibodies were used: R2, which recognizes both PrP(C) and PrP(Sc); and F4-31, which only detects PrP(C) in undenatured sections. At a late subclinical stage of prion infection, both PrP(C) and PrP(Sc) were detected principally on neuronal plasma membranes and on vesicles resembling early endocytic or recycling vesicles in the neuropil. The R2 labeling was approximately six times higher in the infected than the uninfected hippocampus and gold clusters were only evident in infected tissue. The biggest increase in labeling density (24-fold) was found on the early/recycling endosome-like vesicles of small-diameter neurites, suggesting these as possible sites of conversion. Trypsin digestion of infected hippocampal sections resulted in a reduction in R2 labeling of >85%, which suggests that a high proportion of PrP(Sc) may be oligomeric, protease-sensitive PrP(Sc).

A deleted prion protein that is neurotoxic in vivo is localized normally in cultured cells

The prion protein (PrP) possesses sequence-specific domains that endow the molecule with neuroprotective and neurotoxic activities, and that may contribute to the pathogenesis of prion diseases. To further define critical neurotoxic determinants within PrP, we previously generated Tg(DeltaCR) mice that express a form of PrP harboring a deletion of 21 amino acids within the central domain of the protein [Li et al., EMBO J. 26 (2007), 548]. These animals exhibit a neonatal lethal phenotype that is dose-dependently rescued by co-expression of wild-type PrP. In this study, we examined the localization and cell biological properties of the PrP(DeltaCR) protein in cultured cells to further understand the mechanism of PrP(DeltaCR) neurotoxicity. We found that the distribution of PrP(DeltaCR) was identical to that of wild-type PrP in multiple cell lines of both neuronal and non-neuronal origin, and that co-expression of the two proteins did not alter the localization of either one. Both proteins were found in lipid rafts, and both were localized to the apical surface in polarized epithelial cells. Taken together, our results suggest that PrP(DeltaCR) toxicity is not a result of mislocalization or aggregation of the protein, and more likely stems from altered binding interactions leading to the activation of deleterious signaling pathways.

Glycosylation-related genes are variably expressed depending on the differentiation state of a bioaminergic neuronal cell line: implication for the cellular prion protein

A striking feature of the cellular prion protein (PrP(C)) is the heterogeneity of its glycoforms, whose contribution to PrP(C) function has yet to be defined. Using the 1C11 neuronal bioaminergic differentiation model and a glycomics approach, we show here a correlation between differential PrP(C) N-glycosylations in 1C11(5-HT) serotonergic and 1C11(NE) noradrenergic cells compared to their 1C11 precursor cells and a variation of the glycogenome expression status in these cells. In particular, expression of genes involved in N-glycan synthesis or in the modeling of chondroitin and heparan sulfate proteoglycans appeared to be modulated. Our results highlight that, the expression of glycosylation-related genes is regulated during bioaminergic neuronal differentiation, consistent with a participation of glycoconjugates in neuronal development and plasticity. A neuronal regulation of glycosylation processes may have direct implications on some neurospecific functions of PrP(C) and may participate in specific brain targeting of prion strains.

Prions impair bioaminergic functions through serotonin- or catecholamine-derived neurotoxins in neuronal cells

The conversion of the cellular prion protein, PrP(C), to an abnormal isoform, PrP(Sc), is a central event leading to neurodegeneration in prion diseases. Deciphering the molecular and cellular changes imparted by PrP(Sc) accumulation remains an arduous task due to the small number of cell lines supporting prion replication. Here we introduce the 1C11 cell line as a new in vitro model to investigate prion pathogenesis. This cell line is a committed neuroectodermal progenitor able to differentiate into fully functional serotonergic or catecholaminergic neurons. 1C11 cells, which naturally express PrP(C) from the undifferentiated state, can be chronically infected with various prion strains. Prion infection does not promote any noticeable phenotypic change in the progenitor cells nor prevent the onset of the serotonergic and catecholaminergic differentiation programs. Pathogenic prions, however, deviate the overall neurotransmitter-metabolism in both pathways by decreasing bioamine synthesis, storage, and transport, and enhancing catabolism. Noteworthy, oxidized derivatives of both serotonin and catecholamines are selectively detected in the differentiated progenies of infected cells and contribute to irreversible impairment in bioamine synthesis. Finally, the level of PrP(Sc) accumulation, that of infectivity, and the extent of all prion-induced changes in infected cells appear to be correlated. The report of such specific effects of infection on neuronal functions provides a foundation for dissecting the events underlying loss of neuronal homeostasis in prion diseases.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

Copper induced oxidation of serotonin: analysis of products and toxicity

Serotonin is a major neurotransmitter that controls many functions, ranging from mood and behaviour through to sleep and motor functions. The non-enzymatic oxidation of serotonin is of significant importance as some oxidation products are considered to be neurotoxic. An interaction between copper and serotonin has been suggested by symptoms observed in a number of neurodegenerative diseases such as Wilson's and Prion diseases. Using PC12 cells as a model of neuronal cells, we show that the interaction between copper and serotonin is toxic to undifferentiated cells. The toxicity is largely due to reactive oxygen species as cell death is significantly reduced in the presence of the antioxidant mannitol. Differentiation of the PC12 cells also confers resistance to the oxidative process. In vitro oxidation of serotonin by copper results in the eventual formation of a coloured pigment, thought to be a melanin-like polymeric species. Using spectroscopic methods we provide evidence for the formation of a single intermediate product. This dimeric intermediate was identified and characterized as 5,5'-dihydroxy-4,4'-bitryptamine. These results indicate that copper structurally alters serotonin and this process may play a role in copper related neurodegenerative diseases.

The cellular prion protein (PrP(C)): its physiological function and role in disease

Prion diseases are caused by conversion of a normal cell-surface glycoprotein (PrP(C)) into a conformationally altered isoform (PrP(Sc)) that is infectious in the absence of nucleic acid. Although a great deal has been learned about PrP(Sc) and its role in prion propagation, much less is known about the physiological function of PrP(C). In this review, we will summarize some of the major proposed functions for PrP(C), including protection against apoptotic and oxidative stress, cellular uptake or binding of copper ions, transmembrane signaling, formation and maintenance of synapses, and adhesion to the extracellular matrix. We will also outline how loss or subversion of the cytoprotective or neuronal survival activities of PrP(C) might contribute to the pathogenesis of prion diseases, and how similar mechanisms are probably operative in other neurodegenerative disorders.

Involvement of cellular prion protein in the nociceptive response in mice

The role of the cellular prion protein (PrP(c)) in neuronal functioning includes neuronal excitability, cellular adhesion, neurite outgrowth and maintenance. Here we investigated the putative involvement of the PrP(c) function on the nociceptive response using PrP(c) null (Prnp(0/0)) and wild-type (Prnp(+/+)) mice submitted to thermal and chemical models of nociception. PrP(c) null mice were more resistant than wild-type mice to thermal nociception of the tail-flick test. However, no significant difference was found on the hot plate test. In the acetic acid-induced visceral nociception, PrP(c) null mice showed an enhanced response when compared to wild-type mice. However, there was no difference between Prnp(0/0) and wild-type mice on glutamate- and formalin-induced licking behaviour and Freund's Complete Adjuvant (FCA)-induced mechanical allodynia. PrP(c) null mice developed significantly lower paw edema than wild-type mice. In addition, the visceral conditioning stimuli produced by a previous injection of acetic acid (20 days before testing) significantly reduced early and late phases of formalin-induced nociception in wild-type mice. In contrast, the same pre-treatment did not alter the formalin response in PrP(c) null mice. These results indicate a role of PrP(c) in the nociceptive transmission, including the thermal tail-flick test and visceral inflammatory nociception (acetic acid-induced abdominal constriction). Our findings show that PrP(c) is involved with a response mediated by inflammation (paw edema) and by visceral conditioning stimuli.

Cellular Prion Protein Signaling in Serotonergic Neuronal Cells

The cellular prion protein PrP(C) is the normal counterpart of the scrapie prion protein PrP(Sc), the main component of the infectious agent of transmissible spongiform encephalopathies (TSEs). It is a ubiquitous cell-surface glycoprotein, abundantly expressed in neurons, which constitute the targets of TSE pathogenesis. Taking advantage of the 1C11 neuroectodermal cell line, endowed with the capacity to convert into 1C11(5-HT) serotonergic or 1C11(NE) noradrenergic neuronal cells, allowed us to ascribe a signaling function to PrP(C). Antibody-mediated ligation of PrP(C) recruits transduction pathways, which involve nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent reactive oxygen species production and target the extracellular-regulated kinases ERK1/2. In fully differentiated cells only, these effectors are under the control of a PrP(C)-caveolin-Fyn platform, located on neuritic extensions. In addition to its proper signaling activity, PrP(C) modulates the agonist-induced response of the three serotonergic G protein-coupled receptors present on the 1C11(5-HT) differentiated cells. The impact of PrP(C) ligation on the receptor couplings depends on the receptor subtype and the pathway considered. The implementation of the PrP(C)-caveolin complex again is mandatory for PrP(C) to exert its action on 5-HT receptor signaling. Our current data argue that PrP(C) interferes with the intensities and/or dynamics of G protein activation by agonist-bound 5-HT receptors. By mobilizing transduction cascades controlling the cellular redox state and the ERK1/2 kinases and by altering 5-HT receptor-mediated intracellular response, PrP(C) takes part in the homeostasis of serotonergic neuronal cells. These findings may have implications for future research aiming at understanding the fate of serotonergic neurons in prion diseases.

Novel Aspects of Prions, Their Receptor Molecules, and Innovative Approaches for TSE Therapy

1. Prion diseases are a group of rare, fatal neurodegenerative diseases, also known as transmissible spongiform encephalopathies (TSEs), that affect both animals and humans and include bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep, chronic wasting disease (CWD) in deer and elk, and Creutzfeldt-Jakob disease (CJD) in humans. TSEs are usually rapidly progressive and clinical symptoms comprise dementia and loss of movement coordination due to the accumulation of an abnormal isoform (PrP(Sc)) of the host-encoded prion protein (PrP(c)). 2. This article reviews the current knowledge on PrP(c) and PrP(Sc), prion replication mechanisms, interaction partners of prions, and their cell surface receptors. Several strategies, summarized in this article, have been investigated for an effective antiprion treatment including development of a vaccination therapy and screening for potent chemical compounds. Currently, no effective treatment for prion diseases is available. 3. The identification of the 37 kDa/67 kDa laminin receptor (LRP/LR) and heparan sulfate as cell surface receptors for prions, however, opens new avenues for the development of alternative TSE therapies.

Control of Bioamine Metabolism by 5-HT2Band α1DAutoreceptors through Reactive Oxygen Species and Tumor Necrosis Factor-α Signaling in Neuronal Cells

Homeostasis of the central nervous system relies on the proper integration of cell-signaling pathways recruited by a variety of neuronal and non-neuronal factors, with the aim of tightly controlling neurotransmitter metabolism, storage, and transport. We took advantage of the 1C11 neuroectodermal cell line, endowed with the capacity to selectively differentiate into serotonergic (1C11(5-HT)) or noradrenergic (1C11(NE)) neurons, to identify functional targets of serotonin (5-hydroxytryptamine [5-HT]) and norepinephrine (NE) autoreceptors possibly involved in the control of neuronal functions. We demonstrate that 5-HT(2B) and adreno alpha(1D) receptors are coupled to reactive oxygen species (ROS) production through NADPH oxidase activation in 1C11(5-HT) and 1C11(NE) neuronal cells, respectively. In the signaling cascade linking 5-HT(2B) receptors to NADPH oxidase, phospholipase A2-mediated arachidonic acid production is required for ROS synthesis. ROS, in turn, act as second message signals and control the activation of TACE (TNF-alpha converting enzyme), a member of a disintegrin and metalloproteinase family. 5-HT(2B) and alpha(1D) receptor stimulation triggers TACE-dependent TNF-alpha shedding in the surrounding milieu of 1C11(5-HT) and 1C11(NE) cells. In these cells, shed TNF-alpha triggers degradation of 5-HT and NE into 5-HIAA and MHPG, respectively. Finally, we observe that 5-HT(2B) and alpha(1D) receptor couplings to the NADPH oxidase-TACE cascade are strictly restricted to 1C11-derived progenies that have implemented a complete neuronal phenotype. Altogether, our data indicate that couplings of 5-HT(2B) and alpha(1D) autoreceptors to ROS and TNF-alpha signaling control neurotransmitter metabolism in 1C11-derived neuronal cells. Eventually, we might explain the origin of oxidative stress and high level of TNF-alpha in neurodegenerative diseases as a consequence of deviation of normal signaling pathways coupled to neurotransmitters.

Unaltered susceptibility to scrapie in serotonin transporter deficient mice

The serotonergic system has been hypothesized to play an important role in prion diseases. Specifically, hyperactivity of the serotonergic system in prion diseases is suggested by an increase in the turnover rate of the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) in human and experimental prion diseases. The 5-HT transporter (5-HTT) determines the duration of serotonergic neurotransmission by way of reuptake of 5-HT from the extracellular space. 5-HTT availability is reduced in brains of patients with the human prion disease familial fatal insomnia. To further clarify a possible role of the 5-HTT in prion diseases we investigated whether mice lacking the 5-HTT display an altered susceptibility to experimental scrapie infection. Surprisingly, 5-HTT knockout mice developed mouse scrapie in a time course similar to wildtype control mice with accumulation of the pathological prion protein, PrP(Sc) and with typical pathological hallmarks of the disease. These findings argue against a major role of the 5-HTT in the pathogenesis of prion diseases in mice.

The prion protein and lipid rafts

Prions are the causative agent of the transmissible spongiform encephalopathies, such as Creutzfeldt-Jakob disease in humans. In these prion diseases the normal cellular form of the prion protein (PrP(C)) undergoes a post-translational conformational conversion to the infectious form (PrP(Sc)). PrP(C) associates with cholesterol- and glycosphingolipid-rich lipid rafts through association of its glycosyl-phosphatidylinositol (GPI) anchor with saturated raft lipids and through interaction of its N-terminal region with an as yet unidentified raft associated molecule. PrP(C) resides in detergent-resistant domains that have different lipid and protein compositions to the domains occupied by another GPI-anchored protein, Thy-1. In some cells PrP(C) may endocytose through caveolae, but in neuronal cells, upon copper binding to the N-terminal octapeptide repeats, the protein translocates out of rafts into detergent-soluble regions of the plasma membrane prior to endocytosis through clathrin-coated pits. The current data suggest that the polybasic region at its N-terminus is required to engage PrP(C) with a transmembrane adaptor protein which in turn links with the clathrin endocytic machinery. PrP(C) associates in rafts with a variety of signalling molecules, including caveolin-1 and Fyn and Src tyrosine kinases. The clustering of PrP(C) triggers a range of signal transduction processes, including the recruitment of the neural cell adhesion molecule to rafts which in turn promotes neurite outgrowth. Lipid rafts appear to be involved in the conformational conversion of PrP(C) to PrP(Sc), possibly by providing a favourable environment for this process to occur and enabling disease progression.