Laminin-induced PC-12 cell differentiation is inhibited following laser inactivation of cellular prion protein

In Vitro and In Vivo Evidence towards Fibronectin's Protective Effects against Prion Infection

A distinctive signature of the prion diseases is the accumulation of the pathogenic isoform of the prion protein, PrPSc, in the central nervous system of prion-affected humans and animals. PrPSc is also found in peripheral tissues, raising concerns about the potential transmission of pathogenic prions through human food supplies and posing a significant risk to public health. Although muscle tissues are considered to contain levels of low prion infectivity, it has been shown that myotubes in culture efficiently propagate PrPSc. Given the high consumption of muscle tissue, it is important to understand what factors could influence the establishment of a prion infection in muscle tissue. Here we used in vitro myotube cultures, differentiated from the C2C12 myoblast cell line (dC2C12), to identify factors affecting prion replication. A range of experimental conditions revealed that PrPSc is tightly associated with proteins found in the systemic extracellular matrix, mostly fibronectin (FN). The interaction of PrPSc with FN decreased prion infectivity, as determined by standard scrapie cell assay. Interestingly, the prion-resistant reserve cells in dC2C12 cultures displayed a FN-rich extracellular matrix while the prion-susceptible myotubes expressed FN at a low level. In agreement with the in vitro results, immunohistopathological analyses of tissues from sheep infected with natural scrapie demonstrated a prion susceptibility phenotype linked to an extracellular matrix with undetectable levels of FN. Conversely, PrPSc deposits were not observed in tissues expressing FN. These data indicate that extracellular FN may act as a natural barrier against prion replication and that the extracellular matrix composition may be a crucial feature determining prion tropism in different tissues.

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

Harnessing the Physiological Functions of Cellular Prion Protein in the Kidneys: Applications for Treating Renal Diseases

A cellular prion protein (PrP^C) is a ubiquitous cell surface glycoprotein, and its physiological functions have been receiving increased attention. Endogenous PrP^C is present in various kidney tissues and undergoes glomerular filtration. In prion diseases, abnormal prion proteins are found to accumulate in renal tissues and filtered into urine. Urinary prion protein could serve as a diagnostic biomarker. PrP^C plays a role in cellular signaling pathways, reno-protective effects, and kidney iron uptake. PrP^C signaling affects mitochondrial function via the ERK pathway and is affected by the regulatory influence of microRNAs, small molecules, and signaling proteins. Targeting PrP^C in acute and chronic kidney disease could help improve iron homeostasis, ameliorate damage from ischemia/reperfusion injury, and enhance the efficacy of mesenchymal stem/stromal cell or extracellular vesicle-based therapeutic strategies. PrP^C may also be under the influence of BMP/Smad signaling and affect the progression of TGF-β-related renal fibrosis. PrP^C conveys TNF-α resistance in some renal cancers, and therefore, the coadministration of anti-PrP^C antibodies improves chemotherapy. PrP^C can be used to design antibody-drug conjugates, aptamer-drug conjugates, and customized tissue inhibitors of metalloproteinases to suppress cancer. With preclinical studies demonstrating promising results, further research on PrP^C in the kidney may lead to innovative PrP^C-based therapeutic strategies for renal disease.

The Cellular Prion Protein: A Promising Therapeutic Target for Cancer

Studies on the cellular prion protein (PrP^C) have been actively conducted because misfolded PrP^C is known to cause transmissible spongiform encephalopathies or prion disease. PrP^C is a glycophosphatidylinositol-anchored cell surface glycoprotein that has been reported to affect several cellular functions such as stress protection, cellular differentiation, mitochondrial homeostasis, circadian rhythm, myelin homeostasis, and immune modulation. Recently, it has also been reported that PrP^C mediates tumor progression by enhancing the proliferation, metastasis, and drug resistance of cancer cells. In addition, PrP^C regulates cancer stem cell properties by interacting with cancer stem cell marker proteins. In this review, we summarize how PrP^C promotes tumor progression in terms of proliferation, metastasis, drug resistance, and cancer stem cell properties. In addition, we discuss strategies to treat tumors by modulating the function and expression of PrP^C via the regulation of HSPA1L/HIF-1α expression and using an anti-prion antibody.

Cellular Prion Protein (PrPc): Putative Interacting Partners and Consequences of the Interaction

Cellular prion protein (PrPc) is a small glycosylphosphatidylinositol (GPI) anchored protein most abundantly found in the outer leaflet of the plasma membrane (PM) in the central nervous system (CNS). PrPc misfolding causes neurodegenerative prion diseases in the CNS. PrPc interacts with a wide range of protein partners because of the intrinsically disordered nature of the protein’s N-terminus. Numerous studies have attempted to decipher the physiological role of the prion protein by searching for proteins which interact with PrPc. Biochemical characteristics and biological functions both appear to be affected by interacting protein partners. The key challenge in identifying a potential interacting partner is to demonstrate that binding to a specific ligand is necessary for cellular physiological function or malfunction. In this review, we have summarized the intracellular and extracellular interacting partners of PrPc and potential consequences of their binding. We also briefly describe prion disease-related mutations at the end of this review.

Sleep deprivation regulates availability of PrPC and Aβ peptides which can impair interaction between PrPC and laminin and neuronal plasticity

PrP^C is a glycoprotein capable to interact with several molecules and mediates diverse signaling pathways. Among numerous ligands, laminin (LN) is known to promote neurite outgrowth and memory consolidation, while amyloid‐beta oligomers (Aβo) trigger synaptic dysfunction. In both pathways, mGluR1 is recruited as co‐receptor. The involvement of PrP^C/mGluR1 in these opposite functions suggests that this complex is a key element in the regulation of synaptic activity. Considering that sleep‐wake cycle is important for synaptic homeostasis, we aimed to investigate how sleep deprivation affects the expression of PrP^C and its ligands, laminin, Aβo, and mGluR1, a multicomplex that can interfere with neuronal plasticity. To address this question, hippocampi of control (CT) and sleep deprived (SD) C57BL/6 mice were collected at two time points of circadian period (13 hr and 21 hr). We observed that sleep deprivation reduced PrP^C and mGluR1 levels with higher effect in active state (21 hr). Sleep deprivation also caused accumulation of Aβ peptides in rest period (13 hr), while laminin levels were not affected. In vitro binding assay showed that Aβo can compete with LN for PrP^C binding. The influence of Aβo was also observed in neuritogenesis. LN alone promoted longer neurite outgrowth than non‐treated cells in both Prnp^+/+ and Prnp^0/0 genotypes. Aβo alone did not show any effects, but when added together with LN, it attenuated the effects of LN only in Prnp^+/+ cells. Altogether, our findings indicate that sleep deprivation regulates the availability of PrP^C and Aβ peptides, and based on our in vitro assays, these alterations induced by sleep deprivation can negatively affect LN–PrP^C interaction, which is known to play roles in neuronal plasticity.

Emerging Role of Cellular Prion Protein in the Maintenance and Expansion of Glioma Stem Cells

Cellular prion protein (PrP^C) is a membrane-anchored glycoprotein representing the physiological counterpart of PrP scrapie (PrP^Sc), which plays a pathogenetic role in prion diseases. Relatively little information is however available about physiological role of PrP^C. Although PrP^C ablation in mice does not induce lethal phenotypes, impairment of neuronal and bone marrow plasticity was reported in embryos and adult animals. In neurons, PrP^C stimulates neurite growth, prevents oxidative stress-dependent cell death, and favors antiapoptotic signaling. However, PrP^C activity is not restricted to post-mitotic neurons, but promotes cell proliferation and migration during embryogenesis and tissue regeneration in adult. PrP^C acts as scaffold to stabilize the binding between different membrane receptors, growth factors, and basement proteins, contributing to tumorigenesis. Indeed, ablation of PrP^C expression reduces cancer cell proliferation and migration and restores cell sensitivity to chemotherapy. Conversely, PrP^C overexpression in cancer stem cells (CSCs) from different tumors, including gliomas—the most malignant brain tumors—is predictive for poor prognosis, and correlates with relapses. The mechanisms of the PrP^C role in tumorigenesis and its molecular partners in this activity are the topic of the present review, with a particular focus on PrP^C contribution to glioma CSCs multipotency, invasiveness, and tumorigenicity.

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.

Physiological Functions of the Cellular Prion Protein

The prion protein, PrP^C, is a small, cell-surface glycoprotein notable primarily for its critical role in pathogenesis of the neurodegenerative disorders known as prion diseases. A hallmark of prion diseases is the conversion of PrP^C into an abnormally folded isoform, which provides a template for further pathogenic conversion of PrP^C, allowing disease to spread from cell to cell and, in some circumstances, to transfer to a new host. In addition to the putative neurotoxicity caused by the misfolded form(s), loss of normal PrP^C function could be an integral part of the neurodegenerative processes and, consequently, significant research efforts have been directed toward determining the physiological functions of PrP^C. In this review, we first summarise important aspects of the biochemistry of PrP^C before moving on to address the current understanding of the various proposed functions of the protein, including details of the underlying molecular mechanisms potentially involved in these functions. Over years of study, PrP^C has been associated with a wide array of different cellular processes and many interacting partners have been suggested. However, recent studies have cast doubt on the previously well-established links between PrP^C and processes such as stress-protection, copper homeostasis and neuronal excitability. Instead, the functions best-supported by the current literature include regulation of myelin maintenance and of processes linked to cellular differentiation, including proliferation, adhesion, and control of cell morphology. Intriguing connections have also been made between PrP^C and the modulation of circadian rhythm, glucose homeostasis, immune function and cellular iron uptake, all of which warrant further investigation.

Regulation of RhoA activity by the cellular prion protein

The cellular prion protein (PrP^C) is a highly conserved glycosylphosphatidylinositol (GPI)-anchored membrane protein that is involved in the signal transduction during the initial phase of neurite outgrowth. The Ras homolog gene family member A (RhoA) is a small GTPase that is known to have an essential role in regulating the development, differentiation, survival, and death of neurons in the central nervous system. Although recent studies have shown the dysregulation of RhoA in a variety of neurodegenerative diseases, the role of RhoA in prion pathogenesis remains unclear. Here, we investigated the regulation of RhoA-mediated signaling by PrP^C using both in vitro and in vivo models and found that overexpression of PrP^C significantly induced RhoA inactivation and RhoA phosphorylation in hippocampal neuronal cells and in the brains of transgenic mice. Using siRNA-mediated depletion of endogenous PrP^C and overexpression of disease-associated mutants of PrP^C, we confirmed that PrP^C induced RhoA inactivation, which accompanied RhoA phosphorylation but reduced the phosphorylation levels of LIM kinase (LIMK), leading to cofilin activation. In addition, PrP^C colocalized with RhoA, and the overexpression of PrP^C significantly increased neurite outgrowth in nerve growth factor-treated PC12 cells through RhoA inactivation. However, the disease-associated mutants of PrP^C decreased neurite outgrowth compared with wild-type PrP^C. Moreover, inhibition of Rho-associated kinase (ROCK) substantially facilitated neurite outgrowth in NGF-treated PC12 cells, similar to the effect induced by PrP^C. Interestingly, we found that the induction of RhoA inactivation occurred through the interaction of PrP^C with RhoA and that PrP^C enhanced the interaction between RhoA and p190RhoGAP (a GTPase-activating protein). These findings suggest that the interactions of PrP^C with RhoA and p190RhoGAP contribute to neurite outgrowth by controlling RhoA inactivation and RhoA-mediated signaling and that disease-associated mutations of PrP^C impair RhoA inactivation, which in turn leads to prion-related neurodegeneration.

Mechanism of Unfolding of Human Prion Protein

Misfolding and aggregation of prion proteins are associated with several neurodegenerative diseases. Therefore, understanding the mechanism of the misfolding process is of enormous interest in the scientific community. It has been speculated and widely discussed that the native cellular prion protein (PrP^C) form needs to undergo substantial unfolding to a more stable PrP^C* state, which may further oligomerize into the toxic scrapie (PrP^Sc) form. Here, we have studied the mechanism of the unfolding of the human prion protein (huPrP) using a set of extensive well-tempered metadynamics simulations. Through multiple microsecond-long metadynamics simulations, we find several possible unfolding pathways. We show that each pathway leads to an unfolded state of lower free energy than the native state. Thus, our study may point to the signature of a PrP^C* form that corresponds to a global minimum on the conformational free-energy landscape. Moreover, we find that these global minima states do not involve an increased β-sheet content, as was assumed to be a signature of PrP^Sc formation in previous simulation studies. We have further analyzed the origin of metastability of the PrP^C form through free-energy surfaces of the chopped helical segments to show that the helices, particularly H2 and H3 of the prion protein, have the tendency to form either a random coil or a β-structure. Therefore, the secondary structural elements of the prion protein are only weakly stabilized by tertiary contacts and solvation forces so that relatively weak perturbations induced by temperature, pressure, pH, and so forth can lead to substantial unfolding with characteristics of intrinsically disordered proteins.

Almost a century of prion protein(s): From pathology to physiology, and back to pathology

Prions are one of the few pathogens whose name is renowned at all population levels, after the dramatic years pervaded by the fear of eating prion-infected food. If now this, somehow irrational, scare of bovine meat inexorably transmitting devastating brain disorders is largely subdued, several prion-related issues are still unsolved, precluding the design of therapeutic approaches that could slow, if not halt, prion diseases. One unsolved issue is, for example, the role of the prion protein (PrP^C), whole conformational misfolding originates the prion but whose physiologic reason d'etre in neurons, and in cells at large, remains enigmatic. Preceded by a historical outline, the present review will discuss the functional pleiotropicity ascribed to PrP^C, and whether this aspect could fall, at least in part, into a more concise framework. It will also be devoted to radically different perspectives for PrP^C, which have been recently brought to the attention of the scientific world with unexpected force. Finally, it will discuss the possible reasons allowing an evolutionary conserved and benign protein, as PrP^C is, to turn into a high affinity receptor for pathologic misfolded oligomers, and to transmit their toxic message into neurons.

Prion Protein Protects against Renal Ischemia/Reperfusion Injury

The cellular prion protein (PrP^C), a protein most noted for its link to prion diseases, has been found to play a protective role in ischemic brain injury. To investigate the role of PrP^C in the kidney, an organ highly prone to ischemia/reperfusion (IR) injury, we examined wild-type (WT) and PrP^C knockout (KO) mice that were subjected to 30-min of renal ischemia followed by 1, 2, or 3 days of reperfusion. Renal dysfunction and structural damage was more severe in KO than in WT mice. While PrP was undetectable in KO kidneys, Western blotting revealed an increase in PrP in IR-injured WT kidneys compared to sham-treated kidneys. Compared to WT, KO kidneys exhibited increases in oxidative stress markers heme oxygenase-1, nitrotyrosine, and N^ε-(carboxymethyl)lysine, and decreases in mitochondrial complexes I and III. Notably, phosphorylated extracellular signal-regulated kinase (pERK) staining was predominantly observed in tubular cells from KO mice following 2 days of reperfusion, a time at which significant differences in renal dysfunction, histological changes, oxidative stress, and mitochondrial complexes between WT and KO mice were observed. Our study provides the first evidence that PrP^C may play a protective role in renal IR injury, likely through its effects on mitochondria and ERK signaling pathways.

Functional cross-talk between the cellular prion protein and the neural cell adhesion molecule is critical for neuronal differentiation of neural stem/precursor cells

Cellular prion protein (PrP) is prominently expressed in brain, in differentiated neurons but also in neural stem/precursor cells (NPCs). The misfolding of PrP is a central event in prion diseases, yet the physiological function of PrP is insufficiently understood. Although PrP has been reported to associate with the neural cell adhesion molecule (NCAM), the consequences of concerted PrP-NCAM action in NPC physiology are unknown. Here, we generated NPCs from the subventricular zone (SVZ) of postnatal day 5 wild-type and PrP null (-/-) mice and observed that PrP is essential for proper NPC proliferation and neuronal differentiation. Moreover, we found that PrP is required for the NPC response to NCAM-induced neuronal differentiation. In the absence of PrP, NCAM not only fails to promote neuronal differentiation but also induces an accumulation of doublecortin-positive neuronal progenitors at the proliferation stage. In agreement, we noted an increase in cycling neuronal progenitors in the SVZ of PrP-/- mice compared with PrP+/+ mice, as evidenced by double labeling for the proliferation marker Ki67 and doublecortin as well as by 5-bromo-2'-deoxyuridine incorporation experiments. Additionally, fewer newly born neurons were detected in the rostral migratory stream of PrP-/- mice. Analysis of the migration of SVZ cells in microexplant cultures from wild-type and PrP-/- mice revealed no differences between genotypes or a role for NCAM in this process. Our data demonstrate that PrP plays a critical role in neuronal differentiation of NPCs and suggest that this function is, at least in part, NCAM-dependent.

Roles of the cellular prion protein in the regulation of cell-cell junctions and barrier function

The cellular prion protein was historically characterized owing to its misfolding in prion disease. Although its physiological role remains incompletely understood, PrP(C) has emerged as an evolutionary conserved, multifaceted protein involved in a wide-range of biological processes. PrP(C) is a GPI-anchored protein targeted to the plasma membrane, in raft microdomains, where its interaction with a repertoire of binding partners, which differ depending on cell models, mediates its functions. Among identified PrP(C) partners are cell adhesion molecules. This review will focus on the multiple implications of PrP(C) in cell adhesion processes, mainly the regulation of cell-cell junctions in epithelial and endothelial cells and the consequences on barrier properties. We will show how recent findings argue for a role of PrP(C) in the recruitment of signaling molecules, which in turn control the targeting or the stability of adhesion complexes at the plasma membrane.

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.

PrPc is temporospatially expressed in molar development of rats

Odontogenesis, tooth development, is derived from two tissue components: ectoderm and neural crest-derived mesenchyme. Cyto-differentiation of odontogenic cells during development involves time-dependent and sequential regulation of genetic programs. This study was conducted to detect molecules implicated in cyto-differentiation of developing molar germs of rats. Differential display-PCR revealed that PrP(c) was differentially expressed between cap/early bell-staged germs (maxillary 3rd molar germs) and root formation-staged germs (maxillary 2nd molar germs) at postnatal day 9. Both levels of PrP(c) mRNA and protein expression were higher in the root formation stage than the cap/early bell stage and increased in a time-dependent manner. Immunofluorescence revealed for the first time that PrP(c) was not localized in the enamel organ, but localized in dental follicular cells for the development of the periodontal ligament and cementum as well as odontoblasts, both of which are of neural crest origin. These results suggest that the physiological functions of the PrP(c) in tooth development may be implicated in the differentiation of neural crest-derived mesenchyme including the periodontal tissues for root formation rather than epithelial tissue.

Laminin-γ1 chain and stress inducible protein 1 synergistically mediate PrPC-dependent axonal growth via Ca2+ mobilization in dorsal root ganglia neurons

Prion protein (PrP(C)) is a cell surface glycoprotein that is abundantly expressed in nervous system. The elucidation of the PrP(C) interactome network and its significance on neural physiology is crucial to understanding neurodegenerative events associated with prion and Alzheimer's diseases. PrP(C) co-opts stress inducible protein 1/alpha7 nicotinic acetylcholine receptor (STI1/α7nAChR) or laminin/Type I metabotropic glutamate receptors (mGluR1/5) to modulate hippocampal neuronal survival and differentiation. However, potential cross-talk between these protein complexes and their role in peripheral neurons has never been addressed. To explore this issue, we investigated PrP(C)-mediated axonogenesis in peripheral neurons in response to STI1 and laminin-γ1 chain-derived peptide (Ln-γ1). STI1 and Ln-γ1 promoted robust axonogenesis in wild-type neurons, whereas no effect was observed in neurons from PrP(C) -null mice. PrP(C) binding to Ln-γ1 or STI1 led to an increase in intracellular Ca(2+) levels via distinct mechanisms: STI1 promoted extracellular Ca(2+) influx, and Ln-γ1 released calcium from intracellular stores. Both effects depend on phospholipase C activation, which is modulated by mGluR1/5 for Ln-γ1, but depends on, C-type transient receptor potential (TRPC) channels rather than α7nAChR for STI1. Treatment of neurons with suboptimal concentrations of both ligands led to synergistic actions on PrP(C)-mediated calcium response and axonogenesis. This effect was likely mediated by simultaneous binding of the two ligands to PrP(C). These results suggest a role for PrP(C) as an organizer of diverse multiprotein complexes, triggering specific signaling pathways and promoting axonogenesis in the peripheral nervous system.

The role of RNA in mammalian prion protein conversion

Prion diseases remain a challenge to modern science in the 21st century because of their capacity for transmission without an encoding nucleic acid. PrP(Sc), the infectious and alternatively folded form of the PrP prion protein, is capable of self-replication, using PrP(C), the properly folded form of PrP, as a template. This process is associated with neuronal death and the clinical manifestation of prion-based diseases. Unfortunately, little is known about the mechanisms that drive this process. Over the last decade, the theory that a nucleic acid, such as an RNA molecule, might be involved in the process of prion structural conversion has become more widely accepted; such a nucleic acid would act as a catalyst rather than encoding genetic information. Significant amounts of data regarding the interactions of PrP with nucleic acids have created a new foundation for understanding prion conversion and the transmission of prion diseases. Our knowledge has been enhanced by the characterization of a large group of RNA molecules known as non-coding RNAs, which execute a series of important cellular functions, from transcriptional regulation to the modulation of neuroplasticity. The RNA-binding properties of PrP along with the competition with other polyanions, such as glycosaminoglycans and nucleic acid aptamers, open new avenues for therapy.

Prion protein expression and functional importance in skeletal muscle

Skeletal muscle expresses prion protein (PrP) that buffers oxidant activity in neurons.

AIMS

We hypothesize that PrP deficiency would increase oxidant activity in skeletal muscle and alter redox-sensitive functions, including contraction and glucose uptake. We used real-time polymerase chain reaction and Western blot analysis to measure PrP mRNA and protein in human diaphragm, five murine muscles, and muscle-derived C2C12 cells. Effects of PrP deficiency were tested by comparing PrP-deficient mice versus wild-type mice and morpholino-knockdown versus vehicle-treated myotubes. Oxidant activity (dichlorofluorescin oxidation) and specific force were measured in murine diaphragm fiber bundles.

RESULTS

PrP content differs among mouse muscles (gastrocnemius>extensor digitorum longus, EDL>tibialis anterior, TA; soleus>diaphragm) as does glycosylation (di-, mono-, nonglycosylated; gastrocnemius, EDL, TA=60%, 30%, 10%; soleus, 30%, 40%, 30%; diaphragm, 30%, 30%, 40%). PrP is predominantly di-glycosylated in human diaphragm. PrP deficiency decreases body weight (15%) and EDL mass (9%); increases cytosolic oxidant activity (fiber bundles, 36%; C2C12 myotubes, 7%); and depresses specific force (12%) in adult (8-12 mos) but not adolescent (2 mos) mice.

INNOVATION

This study is the first to directly assess a role of prion protein in skeletal muscle function.

CONCLUSIONS

PrP content varies among murine skeletal muscles and is essential for maintaining normal redox homeostasis, muscle size, and contractile function in adult animals.

Amyloid-beta oligomers increase the localization of prion protein at the cell surface

In Alzheimer's disease, the amyloid-β peptide (Aβ) interacts with distinct proteins at the cell surface to interfere with synaptic communication. Recent data have implicated the prion protein (PrP(C)) as a putative receptor for Aβ. We show here that Aβ oligomers signal in cells in a PrP(C)-dependent manner, as might be expected if Aβ oligomers use PrP(C) as a receptor. Immunofluorescence, flow cytometry and cell surface protein biotinylation experiments indicated that treatment with Aβ oligomers, but not monomers, increased the localization of PrP(C) at the cell surface in cell lines. These results were reproduced in hippocampal neuronal cultures by labeling cell surface PrP(C). In order to understand possible mechanisms involved with this effect of Aβ oligomers, we used live cell confocal and total internal reflection microscopy in cell lines. Aβ oligomers inhibited the constitutive endocytosis of PrP(C), but we also found that after Aβ oligomer-treatment PrP(C) formed more clusters at the cell surface, suggesting the possibility of multiple effects of Aβ oligomers. Our experiments show for the first time that Aβ oligomers signal in a PrP(C)-dependent way and that they can affect PrP(C) trafficking, increasing its localization at the cell surface.

Gene expression in the medulla following oral infection of cattle with bovine spongiform encephalopathy

The identification of variations in gene expression in response to bovine spongiform encephalopathy (BSE) may help to elucidate the mechanisms of neuropathology and prion replication and discover biomarkers for disease. In this study, genes that are differentially expressed in the caudal medulla tissues of animals infected with different doses of PrP(BSE) at 12 and 45 mo post infection were compared using array containing 24,000 oligonucleotide probes. Data analysis identified 966 differentially expressed (DE) genes between control and infected animals. Genes identified in at least two of four experiments (control versus 1-g infected animals at 12 and 45-mo; control versus 100-g infected animals at 12 and 45 mo) were considered to be the genes that may be associated with BSE disease. From the 176 DE genes associated with BSE, 84 had functions described in the Gene Ontology (GO) database. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of 14 genes revealed that prion infection may cause dysfunction of several different networks, including extracellular matrix (ECM), cell adhesion, neuroactive ligand-receptor interaction, complement and coagulation cascades, MAPK signaling, neurodegenerative disorder, SNARE interactions in vesicular transport, and the transforming growth factor (TGF) beta signaling pathways. The identification of DE genes will contribute to a better understanding of the molecular mechanisms of neuropathology in bovine species. Additional studies on larger number of animals are in progress in our laboratory to investigate the roles of these DE genes in pathogenesis of BSE.

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.

Treatment with normal prion protein delays differentiation and helps to maintain high proliferation activity in human embryonic stem cells

The normal cellular form of prion protein (PrP(C)) has been shown to exhibit a diverse range of biological activities. Several recent studies highlighted potential involvement of PrP(C) in embryogenesis or in regulating stem cell self-renewal and proliferation. In the current study, we employed human embryonic stem cells (hESCs) for assessing the potential role of prion protein in early human development. Here, we showed that treatment of hESCs with full-length recombinant PrP folded into an alpha-helical conformation similar to that of PrP(C) delayed the spontaneous differentiation of hESCs and helped to maintain their high proliferation activity during spontaneous differentiation. Considering that administration of alpha-rPrP was also found to down-regulate the expression of endogenous PrP(C), the effects of alpha-rPrP were likely to be indirect, i.e. executed by endogenous PrP(C). Together with previous observations, these work support the hypothesis that PrP(C) is involved in regulating self-renewal/differentiation status of stem cells including hESCs.

De novo mammalian prion synthesis

Prions are responsible for a heterogeneous group of fatal neurodegenerative diseases. They can be sporadic, genetic, or infectious disorders involving post-translational modifications of the cellular prion protein (PrP(C)). Prions (PrP(Sc)) are characterized by their infectious property and intrinsic ability to convert the physiological PrP(C) into the pathological form, acting as a template. The "protein-only" hypothesis, postulated by Stanley B. Prusiner, implies the possibility to generate de novo prions in vivo and in vitro. Here we describe major milestones towards proving this hypothesis, taking into account physiological environment/s, biochemical properties and interactors of the PrP(C).

Is, indeed, the prion protein a Harlequin servant of "many" masters?

Tens of putative interacting partners of the cellular prion protein (PrP(C)) have been identified, yet the physiologic role of PrP(C) remains unclear. For the first time, however, a recent paper has demonstrated that the absence of PrP(C) produces a lethal phenotype. Starting from this evidence, here we discuss the validity of past and more recent literature supporting that, as part of protein platforms at the cell surface, PrP(C) may bridge extracellular matrix molecules and/or membrane proteins to intracellular signaling pathways.

Therapeutic potential of olfactory ensheathing cells in neurodegenerative diseases

The regenerative capacity of the olfactory system has generated interest in potential clinical application of cells from the olfactory epithelium in the treatment of neurodegenerative diseases. Experimental evidence from animal models and clinical studies suggest that transplantation of olfactory ensheathing cells (OEC), specialized glia in the olfactory system, may be therapeutically useful in neurodegenerative diseases such as spinal cord injury and stroke. This review article describes the different experimental approaches in OEC transplantation. We also discuss the possible effects of OEC implantation on the underlying pathophysiology in neurological disease, including neuroplasticity. Our recent study of this particular population of cells has disclosed some of the molecular basis of the regenerative mechanism of OECs. In summary OECs produce several neurotrophic factors such as stromal cell-derived factor 1alpha and brain-derived neurotrophic factor and enhance axonal regeneration to promote neuroplasticity in neurodegenerative diseases.

Prion protein ablation increases cellular aggregation and embolization contributing to mechanisms of metastasis

Cellular Prion Protein (PrP(C)) is a cell surface protein highly expressed in the nervous system, and to a lesser extent in other tissues. PrP(C) binds to the extracellular matrix laminin and vitronectin, to mediate cell adhesion and differentiation. Herein, we investigate how PrP(C) expression modulates the aggressiveness of transformed cells. Mesenchymal embryonic cells (MEC) from wild-type (Prnp(+/+)) and PrP(C)-null (Prnp(0/0)) mice were immortalized and transformed by co-expression of ras and myc. These cells presented similar growth rates and tumor formation in vivo. When injected in the tail vein, Prnp(0/0)ras/myc cells exhibited increased lung colonization compared with Prnp(+/+)ras/myc cells. Additionally, Prnp(0/0)ras/myc cells form more aggregates with blood components than Prnp(+/+)ras/myc cells, facilitating the arrest of Prnp(0/0)ras/myc cells in the lung vasculature. Integrin alpha(v)beta(3) is more expressed and activated in MEC and in transformed Prnp(0/0) cells than in the respective Prnp(+/+) cells. The blocking of integrin alpha(v)beta(3) by RGD peptide reduces lung colonization in transformed Prnp(0/0) cells to similar levels of those presented by transformed Prnp(+/+) cells. Our data indicate that PrP(C) negatively modulates the expression and activation of integrin alpha(v)beta(3) resulting in a more aggressive phenotype. These results indicate that PrP(C) may have main implications in modulating metastasis formation.

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.

Implantation of olfactory ensheathing cells promotes neuroplasticity in murine models of stroke

Murine olfactory ensheathing cells (OECs) promote central nervous system axonal regeneration in models of spinal cord injury. We investigated whether OECs could induce a neuroplastic effect to improve the neurological dysfunction caused by hypoxic/ischemic stress. In this study, human OECs/olfactory nerve fibroblasts (hOECs/ONFs) specifically secreted trophic factors including stromal cell-derived factor-1alpha (SDF-1alpha). Rats with intracerebral hOEC/ONF implantation showed more improvement on behavioral measures of neurological deficit following stroke than control rats. [18F]fluoro-2-deoxyglucose PET (FDG-PET) showed increased glucose metabolic activity in the hOEC/ONF-treated group compared with controls. In mice, transplanted hOECs/ONFs and endogenous homing stem cells including intrinsic neural progenitor cells and bone marrow stem cells colocalized with specific neural and vascular markers, indicating stem cell fusion. Both hOECs/ONFs and endogenous homing stem cells enhanced neuroplasticity in the rat and mouse ischemic brain. Upregulation of SDF-1alpha and CXCR4 in hOECs/ONFs promoted neurite outgrowth of cocultured primary cortical neurons under oxygen glucose deprivation conditions and in stroke animals through upregulation of cellular prion protein (PrP C) expression. Therefore, the upregulation of SDF-1alpha and the enhancement of CXCR4 and PrP C interaction induced by hOEC/ONF implantation mediated neuroplastic signals in response to hypoxia and ischemia.

CREB-dependent gene regulation by prion protein: impact on MMP-9 and beta-dystroglycan

Corruption of the normal function of the cellular prion protein (PrP(C)) by the scrapie isoform (PrP(Sc)) emerges as a critical causal event in Transmissible Spongiform Encaphalopathies (TSE) pathogenesis. However, PrP(C) physiological role remains unclear. By exploiting the properties of the 1C11 neuroectodermal cell line, able to convert into 1C11(5-HT) serotonergic or 1C11(NE) noradrenergic neuronal cells, we assigned a signaling function to PrP(C). Here, we establish that antibody-mediated PrP(C) ligation promotes the recruitment of the cAMP responsive element binding protein (CREB) transcription factor downstream from the MAPK ERK1/2, in 1C11 precursor cells and their 1C11(5-HT) and 1C11(NE) neuronal progenies. Whatever the differentiation state of 1C11 cells, the PrP(C)-dependent CREB activation triggers Egr-1 and c-fos transcription, two immediate early genes that relay CREB's role in cell survival and proliferation as well as in neuronal plasticity. Furthermore, in 1C11-derived neuronal cells, we draw a link between the PrP(C)-CREB coupling and a transcriptional regulation of the metalloproteinase MMP-9 and its inhibitor TIMP-1, which play pivotal roles in neuronal pathophysiology. Finally, the PrP(C)-dependent control on MMP-9 impacts on the processing of the transmembrane protein, beta-dystroglycan. Taken together, our data define molecular mechanisms that likely mirror PrP(C) ubiquitous contribution to cytoprotection and its involvement in neuronal plasticity.

The 67 kDa laminin receptor: structure, function and role in disease

The 67LR (67 kDa laminin receptor) is a cell-surface receptor with high affinity for its primary ligand. Its role as a laminin receptor makes it an important molecule both in cell adhesion to the basement membrane and in signalling transduction following this binding event. The protein also plays critical roles in the metastasis of tumour cells. Isolation of the protein from either normal or cancerous cells results in a product with an approx. molecular mass of 67 kDa. This protein is believed to be derived from a smaller precursor, the 37LRP (37 kDa laminin receptor precursor). However, the precise mechanism by which cytoplasmic 37LRP becomes cell-membrane-embedded 67LR is unclear. The process may involve post-translational fatty acylation of the protein combined with either homo- or hetero-dimerization, possibly with a galectin-3-epitope-containing partner. Furthermore, it has become clear that acting as a receptor for laminin is not the only function of this protein. 67LR also acts as a receptor for viruses, such as Sindbis virus and dengue virus, and is involved with internalization of the prion protein. Interestingly, unmodified 37LRP is a ribosomal component and homologues of this protein are found in all five kingdoms. In addition, it appears to be strongly associated with histones in the eukaryotic cell nucleus, although the precise role of these interactions is not clear. Here we review the current understanding of the structure and function of this molecule, as well as highlighting areas requiring further research.

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.

Anterograde axonal transport of chicken cellular prion protein (PrPc) in vivo requires its N-terminal part

The cellular isoform of prion protein (PrP(c)) can exist in membrane-bound and secreted forms. Both forms of PrP(c) can be transported by retinal ganglion cell (RGC) axons along the optic nerve in the anterograde direction. In this study we determined which part of chicken PrP(c) is required for its anterograde axonal transport within the optic nerve of embryonic chicken. We intraocularly injected radio-iodinated fragments of recombinant chicken PrP(c) and then examined their anterograde axonal transport from retina into optic tectum. Using gamma-counting and different autoradiographic techniques we quantified anterograde axonal transport of the N-terminal part of chicken PrP(c) (amino acid residues 1-116) in this model system. The transport of the N-terminal part has similar properties as the anterograde transport of full-length chicken PrP(c) (Butowt et al., 2006) described previously (e.g., has similar efficiency, is microtubule-dependent, and is saturable). Moreover, the pattern of ultrastructural distribution of the N-terminal fragment within RGCs is similar to the distribution of full-length PrP(c). The C-terminal fragment of chicken PrP(c) (residues 118-246) and different PrP-derived peptides were not transported. Moreover, PrP(c)-derived peptides were sorted into different endocytotic pathways in neurons, indicating that they cannot substitute for full-length PrP(c) to study its internalization and trafficking. These data indicate that the N-terminal half of chicken PrP(c) contains the necessary information to drive the internalization and subsequent sorting of extracellular PrP(c) in RGCs soma into the anterograde axonal transport pathway.

Cell culture models to unravel prion protein function and aberrancies in prion diseases

From an early stage of prion research, tissue cultures that could support and propagate the scrapie agent were sought after. The earliest attempts were explants from brains of infected mice, and their growth and morphological characteristics were compared with those from uninfected mice. Using the explant technique, several investigators reported increased cell growth in cultures established from scrapie-sick brain compared with cultures from normal mice. These are odd findings in the light of the massive neuronal cell death known to occur in scrapie-infected brains; however, the cell types responsible for the increased cell growth in the scrapie-explants most probably were not neuronal. The first successful cell culture established in this way, in which the scrapie agent was serially and continuously passaged beyond the initial explant, was in the scrapie mouse brain culture, which is still used today. This chapter describes the generation and use of chronically prion-infected cell lines as cell culture models of prion diseases. These cell lines have been crucial for the current understanding of the cell biology of both the normal (PrP(C)) and the pathogenic isoform (PrP(Sc)) of the prion protein. They also have been useful in the development of antiprion drugs, prospectively used for therapy of prion diseases, and they offer an alternative approach for transmission/infectivity assays normally performed by mouse bioassay. Cell culture models also have been used to study prion-induced cytopathological changes, which could explain the typical spongiform neurodegeneration in prion diseases.

Gene expression profile of quinacrine-cured prion-infected mouse neuronal cells

Prion diseases are transmissible fatal neurodegenerative diseases of humans and animals, characterised by the presence of an abnormal isoform (scrapie prion protein; PrP(Sc)) of the endogenous cellular prion protein (PrP(C)). The pathological mechanisms at the basis of prion diseases remain elusive, although the accumulation of PrP(Sc) has been linked to neurodegeneration. Different genomic approaches have been applied to carry out large-scale expression analysis in prion-infected brains and cell lines, in order to define factors potentially involved in pathogenesis. However, the general lack of overlap between the genes found in these studies prompted us to carry an analysis of gene expression using an alternative approach. Specifically, in order to avoid the complexities of shifting gene expression in a heterogeneous cell population, we used a single clone of GT1 cells that was de novo infected with mouse prion-infected brain homogenate and then treated with quinacrine to clear PrP(Sc). By comparing the gene expression profiles of about 15 000 genes in quinacrine-cured and not cured prion-infected GT1 cells, we investigated the influence of the presence or the absence of PrP(Sc). By real-time PCR, we confirmed that the gene encoding for laminin was down-regulated as a consequence of the elimination of PrP(Sc) by the quinacrine treatment. Thus, we speculate that this protein could be a specific candidate for further analysis of its role in prion infection and pathogenesis.

Neuronal and astrocytic cells, obtained after differentiation of human neural GFAP-positive progenitors, present heterogeneous expression of PrPc

PrP(c) is a cellular isoform of the prion protein with an unknown normal function. One of the putative physiological roles of this protein is its involvement in cell differentiation. Recently, in vitro and in vivo studies showed that GFAP-positive cells have characteristics of stem/progenitor cells that generate neurons and glia. We used an in vitro model of human neurogenesis from GFAP-positive progenitor cells to study the expression of PrP(c) during neural differentiation. Semi-quantitative multiplex-PCR assay and Western blot analysis revealed a significant increase of PRNP expression level in differentiated cells compared to undifferentiated cell population. As determined by immunocytochemistry followed by a quantitative image analysis, the PrP(c) level increased significantly in neuronal cells and did not increase significantly in glial cells. Of note, glial and neuronal cells showed a very large heterogeneity of PrP(c) expression. Our results provide the basis for studying the role of PrP(c) in cell differentiation and neurogenesis from human GFAP-positive progenitor cells.

Cellular prion protein expression in astrocytes modulates neuronal survival and differentiation

The functions of cellular prion protein (PrP(C)) are under intense debate and PrP(C) loss of function has been implicated in the pathology of prion diseases. Neuronal PrP(C) engagement with stress-inducible protein-1 and laminin (LN) plays a key role in cell survival and differentiation. The present study evaluated whether PrP(C) expression in astrocytes modulates neuron-glia cross-talk that underlies neuronal survival and differentiation. Astrocytes from wild-type mice promoted a higher level neuritogenesis than astrocytes obtained from PrP(C)-null animals. Remarkably, neuritogenesis was greatly diminished in co-cultures combining PrP(C)-null astrocytes and neurons. LN secreted and deposited at the extracellular matrix by wild-type astrocytes presented a fibrillary pattern and was permissive for neuritogenesis. Conversely, LN coming from PrP(C)-null astrocytes displayed a punctate distribution, and did not support neuronal differentiation. Additionally, secreted soluble factors from PrP(C)-null astrocytes promoted lower levels of neuronal survival than those secreted by wild-type astrocytes. PrP(C) and stress-inducible protein-1 were characterized as soluble molecules secreted by astrocytes which participate in neuronal survival. Taken together, these data indicate that PrP(C) expression in astrocytes is critical for sustaining cell-to-cell interactions, the organization of the extracellular matrix, and the secretion of soluble factors, all of which are essential events for neuronal differentiation and survival.

Fonctions de la protéine prion PrPc

It is now well established that both normal and pathological (or scrapie) isoforms of prion protein, PrPc and PrPsc respectively, are involved in the development and progression of various forms of neurodegenerative diseases, including scrapie in sheep, bovine spongiform encephalopathy (or "mad cow disease") and Creutzfeldt-Jakob disease in human, collectively known as prion diseases. The protein PrPc is highly expressed in the central nervous system in neurons and glial cells, and also present in non-brain cells, such as immune cells or epithelial and endothelial cells. Identification of the physiological functions of PrPc in these different cell types thus appears crucial for understanding the progression of prion diseases. Recent studies highlighted several major roles for PrPc that may be considered in two major domains : (1) cell survival (protection against oxidative stress and apoptosis) and (2) cell adhesion. In association with cell adhesion, distinct functions of PrPc were observed, depending on cell types : neuronal differentiation, epithelial and endothelial barrier integrity, transendothelial migration of monocytes, T cell activation. These observations suggest that PrPc functions may be particularly relevant to cellular stress, as well as inflammatory or infectious situations.

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.

Cellular prion protein interaction with vitronectin supports axonal growth and is compensated by integrins

The physiological functions of the cellular prion protein, PrP(C), as a cell surface pleiotropic receptor are under debate. We report that PrP(C) interacts with vitronectin but not with fibronectin or collagen. The binding sites mediating this PrP(C)-vitronectin interaction were mapped to residues 105-119 of PrP(C) and the residues 307-320 of vitronectin. The two proteins were co-localized in embryonic dorsal root ganglia from wild-type mice. Vitronectin addition to cultured dorsal root ganglia induced axonal growth, which could be mimicked by vitronectin peptide 307-320 and abrogated by anti-PrP(C) antibodies. Full-length vitronectin, but not the vitronectin peptide 307-320, induced axonal growth of dorsal root neurons from two strains of PrP(C)-null mice. Functional assays demonstrated that relative to wild-type cells, PrP(C)-null dorsal root neurons were more responsive to the Arg-Gly-Asp peptide (an integrin-binding site), and exhibited greater alphavbeta3 activity. Our findings indicate that PrP(C) plays an important role in axonal growth, and this function may be rescued in PrP(C)-knockout animals by integrin compensatory mechanisms.

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.

Comparison of CR36, a new heparan mimetic, and pentosan polysulfate in the treatment of prion diseases

Sulfated polyanions, including pentosan polysulfate (PPS) and heparan mimetics, number among the most effective drugs that have been used in experimental models of prion disease and are presumed to act in competition with endogenous heparan sulfate proteoglycans as co-receptors for prion protein (PrP) on the cell surface. PPS has been shown to prolong the survival of animals after intracerebral perfusion and is in limited use for the experimental treatment of human transmissible spongiform encephalopathies (TSEs). Here, PPS is compared with CR36, a new heparan mimetic. Ex vivo, CR36 was more efficient than PPS in reducing PrPres in scrapie-infected cell cultures and showed long-lasting activity. In vivo, CR36 showed none of the acute toxicity observed with PPS and reduced PrPres accumulation in spleens, but had only a marginal effect on the survival time of mice infected with bovine spongiform encephalopathy. In contrast, mice treated with PPS that survived the initial toxic mortality had no detectable PrPres in the spleens and lived 185 days longer than controls (+55 %). These results show, once again, that anti-TSE drugs cannot be encouraged for human therapeutic trials solely on the basis of in vitro or ex vivo observations, but must first be subjected to in vivo animal studies.

Vaccination with prion peptide-displaying papillomavirus-like particles induces autoantibodies to normal prion protein that interfere with pathologic prion protein production in infected cells

Prion diseases are fatal neurodegenerative disorders caused by proteinaceous infectious pathogens termed prions (PrP(Sc)). To date, there is no prophylaxis or therapy available for these transmissible encephalopathies. Passive immunization with monclonal antibodies recognizing the normal host-encoded prion protein (PrP(C)) has been reported to abolish PrP(Sc) infectivity and to delay onset of disease. Because of established immunologic tolerance against the widely expressed PrP(C), active immunization appears to be difficult to achieve. To overcome this limitation, papillomavirus-like particles were generated that display a nine amino acid B-cell epitope, DWEDRYYRE, of the murine/rat prion protein in an immunogenic capsid surface loop, by insertion into the L1 major capsid protein of bovine papillomavirus type 1. The PrP peptide was selected on the basis of its previously suggested central role in prion pathogenesis. Immunization with PrP-virus-like particles induced high-titer antibodies to PrP in rabbit and in rat, without inducing overt adverse effects. As determined by peptide-specific ELISA, rabbit immune sera recognized the inserted murine/rat epitope and also cross-reacted with the homologous rabbit/human epitope differing in one amino acid residue. In contrast, rat immune sera recognized the murine/rat peptide only. Sera of both species reacted with PrP(C) in its native conformation in mouse brain and on rat pheochromocytoma cells, as determined by immunoprecipitation and fluorescence-activated cell sorting analysis. Importantly, rabbit anti-PrP serum contained high-affinity antibody that inhibited de novo synthesis of PrP(Sc) in prion-infected cells. If also effective in vivo, PrP-virus-like particle vaccination opens a unique possibility for immunologic prevention of currently fatal and incurable prion-mediated diseases.