From cell protection to death: May Ca2+ signals explain the chameleonic attributes of the mammalian prion protein?

The Link of the Prion Protein with Ca2+ Metabolism and ROS Production, and the Possible Implication in Aβ Toxicity

The cellular prion protein (PrP^C) is an ubiquitous cell surface protein mostly expressed in neurons, where it localizes to both pre- and post-synaptic membranes. PrP^C aberrant conformers are the major components of mammalian prions, the infectious agents responsible for incurable neurodegenerative disorders. PrP^C was also proposed to bind aggregated misfolded proteins/peptides, and to mediate their neurotoxic signal. In spite of long-lasting research, a general consensus on the precise pathophysiologic mechanisms of PrP^C has not yet been reached. Here we review our recent data, obtained by comparing primary neurons from PrP-expressing and PrP-knockout mice, indicating a central role of PrP^C in synaptic transmission and Ca²⁺ homeostasis. Indeed, by controlling gene expression and signaling cascades, PrP^C is able to optimize glutamate secretion and regulate Ca²⁺ entry via store-operated channels and ionotropic glutamate receptors, thereby protecting neurons from threatening Ca²⁺ overloads and excitotoxicity. We will also illustrate and discuss past and unpublished results demonstrating that Aβ oligomers perturb Ca²⁺ homeostasis and cause abnormal mitochondrial accumulation of reactive oxygen species by possibly affecting the PrP-dependent downregulation of Fyn kinase activity.

The Prion Protein Regulates Synaptic Transmission by Controlling the Expression of Proteins Key to Synaptic Vesicle Recycling and Exocytosis

The cellular prion protein (PrP^C), whose misfolded conformers are implicated in prion diseases, localizes to both the presynaptic membrane and postsynaptic density. To explore possible molecular contributions of PrP^C to synaptic transmission, we utilized a mass spectrometry approach to quantify the release of glutamate from primary cerebellar granule neurons (CGN) expressing, or deprived of (PrP-KO), PrP^C, following a depolarizing stimulus. Under the same conditions, we also tracked recycling of synaptic vesicles (SVs) in the two neuronal populations. We found that in PrP-KO CGN these processes decreased by 40 and 60%, respectively, compared to PrP^C-expressing neurons. Unbiased quantitative mass spectrometry was then employed to compare the whole proteome of CGN with the two PrP genotypes. This approach allowed us to assess that, relative to the PrP^C-expressing counterpart, the absence of PrP^C modified the protein expression profile, including diminution of some components of SV recycling and fusion machinery. Subsequent quantitative RT-PCR closely reproduced proteomic data, indicating that PrP^C is committed to ensuring optimal synaptic transmission by regulating genes involved in SV dynamics and neurotransmitter release. These novel molecular and cellular aspects of PrP^C add insight into the underlying mechanisms for synaptic dysfunctions occurring in neurodegenerative disorders in which a compromised PrP^C is likely to intervene.

α-synuclein interacts with PrPC to induce cognitive impairment through mGluR5 and NMDAR2B

Synucleinopathies, such as Parkinson's disease and dementia with Lewy bodies, are neurodegenerative disorders that are characterized by the accumulation of α-synuclein (aSyn) in intracellular inclusions known as Lewy bodies. Prefibrillar soluble aSyn oligomers, rather than larger inclusions, are currently considered to be crucial species underlying synaptic dysfunction. We identified the cellular prion protein (PrP^C) as a key mediator in aSyn-induced synaptic impairment. The aSyn-associated impairment of long-term potentiation was blocked in Prnp null mice and rescued following PrP^C blockade. We found that extracellular aSyn oligomers formed a complex with PrP^C that induced the phosphorylation of Fyn kinase via metabotropic glutamate receptors 5 (mGluR5). aSyn engagement of PrP^C and Fyn activated NMDA receptor (NMDAR) and altered calcium homeostasis. Blockade of mGluR5-evoked phosphorylation of NMDAR in aSyn transgenic mice rescued synaptic and cognitive deficits, supporting the hypothesis that a receptor-mediated mechanism, independent of pore formation and membrane leakage, is sufficient to trigger early synaptic damage induced by extracellular aSyn.

The prion protein regulates glutamate-mediated Ca2+ entry and mitochondrial Ca2+ accumulation in neurons

The cellular prion protein (PrP^C) whose conformational misfolding leads to the production of deadly prions, has a still-unclarified cellular function despite decades of intensive research. Following our recent finding that PrP^C limits Ca²⁺ entry via store-operated Ca²⁺ channels in neurons, we investigated whether the protein could also control the activity of ionotropic glutamate receptors (iGluRs). To this end, we compared local Ca²⁺ movements in primary cerebellar granule neurons and cortical neurons transduced with genetically encoded Ca²⁺ probes and expressing, or not expressing, PrP^C Our investigation demonstrated that PrP^C downregulates Ca²⁺ entry through each specific agonist-stimulated iGluR and after stimulation by glutamate. We found that, although PrP-knockout (KO) mitochondria were displaced from the plasma membrane, glutamate addition resulted in a higher mitochondrial Ca²⁺ uptake in PrP-KO neurons than in their PrP^C-expressing counterpart. This was because the increased Ca²⁺ entry through iGluRs in PrP-KO neurons led to a parallel increase in Ca²⁺-induced Ca²⁺ release via ryanodine receptor channels. These data thus suggest that PrP^C takes part in the cell apparatus controlling Ca²⁺ homeostasis, and that PrP^C is involved in protecting neurons from toxic Ca²⁺ overloads.

Recent trends in the transdermal delivery of therapeutic agents used for the management of neurodegenerative diseases

With the increasing proportion of the global geriatric population, it becomes obvious that neurodegenerative diseases will become more widespread. From an epidemiological standpoint, it is necessary to develop new therapeutic agents for the management of Alzheimer's disease, Parkinson's disease, multiple sclerosis and other neurodegenerative disorders. An important approach in this regard involves the use of the transdermal route. With transdermal drug delivery systems (TDDS), it is possible to modulate the pharmacokinetic profiles of these medications and improve patient compliance. Transdermal drug delivery has also been shown to be useful for drugs with short half-life and low or unpredictable bioavailability. In this review, several transdermal drug delivery enhancement technologies are being discussed in relation to the delivery of medications used for the management of neurodegenerative disorders.

The prion protein constitutively controls neuronal store-operated Ca(2+) entry through Fyn kinase

The prion protein (PrP^C) is a cell surface glycoprotein mainly expressed in neurons, whose misfolded isoforms generate the prion responsible for incurable neurodegenerative disorders. Whereas PrP^C involvement in prion propagation is well established, PrP^C physiological function is still enigmatic despite suggestions that it could act in cell signal transduction by modulating phosphorylation cascades and Ca²⁺ homeostasis. Because PrP^C binds neurotoxic protein aggregates with high-affinity, it has also been proposed that PrP^C acts as receptor for amyloid-β (Aβ) oligomers associated with Alzheimer’s disease (AD), and that PrP^C-Aβ binding mediates AD-related synaptic dysfunctions following activation of the tyrosine kinase Fyn. Here, use of gene-encoded Ca²⁺ probes targeting different cell domains in primary cerebellar granule neurons (CGN) expressing, or not, PrP^C, allowed us to investigate whether PrP^C regulates store-operated Ca²⁺ entry (SOCE) and the implication of Fyn in this control. Our findings show that PrP^C attenuates SOCE, and Ca²⁺ accumulation in the cytosol and mitochondria, by constitutively restraining Fyn activation and tyrosine phosphorylation of STIM1, a key molecular component of SOCE. This data establishes the existence of a PrP^C-Fyn-SOCE triad in neurons. We also demonstrate that treating cerebellar granule and cortical neurons with soluble Aβ_(1–42) oligomers abrogates the control of PrP^C over Fyn and SOCE, suggesting a PrP^C-dependent mechanizm for Aβ-induced neuronal Ca²⁺ dyshomeostasis.

Nrf-2 regulation of prion protein expression is independent of oxidative stress

Cellular expression of host prion protein (PrP) is essential to infection with prion disease. Understanding the mechanisms that regulate prion protein expression at both the transcriptional and translational levels is therefore an important goal. The cellular prion protein has been associated with resistance to oxidative, and its expression is also increased by oxidative stress. The transcription factor Nrf-2 is associated with cellular responses to oxidative stress and is known to induce upregulation of antioxidant defense mechanisms. We have identified an Nrf-2 binding site in the prion protein promoter (Prnp) and shown that Nrf-2 downregulated PrP expression. However, this effect is independent of oxidative stress as oxidative stress can up-regulate PrP expression regardless of the level of Nrf-2 expression. Furthermore, Nrf-2 has no impact on PrP expression when cells are infected with scrapie. These findings highlight that Nrf-2 can regulate PrP expression, but that this regulation becomes uncoupled during cellular stress.

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.

Genetic variability of the gene cluster CALHM 1-3 in sporadic Creutzfeldt-Jakob disease

Perturbations of calcium homeostasis have been associated with several neurodegenerative disorders. A common polymorphism (rs2986017) in the CALHM1 gene, coding for a regulator of calcium homeostasis, is a genetic risk factor for the development of Alzheimer disease (AD). Although some authors failed to confirm these results, a meta-analysis has shown that this polymorphism modulates the age at disease onset. Furthermore, a recent association study has explored the genetic variability of CALHM1 gene and two adjacent paralog genes (CALHM3 and CALHM2) in an Asian population. Since several lines of evidence suggest that AD and prion diseases share pathophysiologic mechanisms, we investigated for the first time the genetic variability of the gene cluster formed by CALHM1 and its paralogs in a series of 235 sporadic Creutzfeldt-Jakob disease (sCJD) patients, and compared the genotypic and allelic frequencies with those presented in 329 controls from the same ancestry. As such, this work also represents the first association analysis of CALHM genes in sCJD. Sequencing analysis of the complete coding regions of the genes demonstrated the presence of 10 single nucleotide polymorphisms (SNP) within the CALHM genes. We observed that rs4918016-rs2986017-rs2986018 and rs41287502-rs41287500 polymorphic sites at CALHM1 were in linkage disequilibrium. We found marginal associations for sCJD risk at CALHM1 polymorphic sites rs41287502 and rs41287500 [coding for two linked missense mutations (p.(Met323Ile); (Gly282Cys)], and rs2986017 [p.(Leu86Pro)]. Interestingly, a TGG haplotype defined by the rs4918016-rs2986017-rs2986018 block was associated with sCJD. These findings underscore the need of future multinational collaborative initiatives in order to corroborate these seminal data.

Calcium binding promotes prion protein fragment 90-231 conformational change toward a membrane destabilizing and cytotoxic structure

The pathological form of prion protein (PrP(Sc)), as other amyloidogenic proteins, causes a marked increase of membrane permeability. PrP(Sc) extracted from infected Syrian hamster brains induces a considerable change in membrane ionic conductance, although the contribution of this interaction to the molecular mechanism of neurodegeneration process is still controversial. We previously showed that the human PrP fragment 90-231 (hPrP₉₀₋₂₃₁) increases ionic conductance across artificial lipid bilayer, in a calcium-dependent manner, producing an alteration similar to that observed for PrP(Sc). In the present study we demonstrate that hPrP₉₀₋₂₃₁, pre-incubated with 10 mM Ca⁺⁺ and then re-suspended in physiological external solution increases not only membrane conductance but neurotoxicity as well. Furthermore we show the existence of a direct link between these two effects as demonstrated by a highly statistically significant correlation in several experimental conditions. A similar correlation between increased membrane conductance and cell degeneration has been observed assaying hPrP₉₀₋₂₃₁ bearing pathogenic mutations (D202N and E200K). We also report that Ca⁺⁺ binding to hPrP₉₀₋₂₃₁ induces a conformational change based on an alteration of secondary structure characterized by loss of alpha-helix content causing hydrophobic amino acid exposure and proteinase K resistance. These features, either acquired after controlled thermal denaturation or induced by D202N and E200K mutations were previously identified as responsible for hPrP₉₀₋₂₃₁ cytotoxicity. Finally, by in silico structural analysis, we propose that Ca⁺⁺ binding to hPrP₉₀₋₂₃₁ modifies amino acid orientation, in the same way induced by E200K mutation, thus suggesting a pathway for the structural alterations responsible of PrP neurotoxicity.

Detection and control of prion diseases in food animals

Transmissible spongiform encephalopathies (TSEs), or prion diseases, represent a unique form of infectious disease based on misfolding of a self-protein (PrP^C) into a pathological, infectious conformation (PrP^Sc). Prion diseases of food animals gained notoriety during the bovine spongiform encephalopathy (BSE) outbreak of the 1980s. In particular, disease transmission to humans, to the generation of a fatal, untreatable disease, elevated the perspective on livestock prion diseases from food production to food safety. While the immediate threat posed by BSE has been successfully addressed through surveillance and improved management practices, another prion disease is rapidly spreading. Chronic wasting disease (CWD), a prion disease of cervids, has been confirmed in wild and captive populations with devastating impact on the farmed cervid industries. Furthermore, the unabated spread of this disease through wild populations threatens a natural resource that is a source of considerable economic benefit and national pride. In a worst-case scenario, CWD may represent a zoonotic threat either through direct transmission via consumption of infected cervids or through a secondary food animal, such as cattle. This has energized efforts to understand prion diseases as well as to develop tools for disease detection, prevention, and management. Progress in each of these areas is discussed.

Manganese upregulates cellular prion protein and contributes to altered stabilization and proteolysis: relevance to role of metals in pathogenesis of prion disease

Prion diseases are fatal neurodegenerative diseases resulting from misfolding of normal cellular prion (PrP(C)) into an abnormal form of scrapie prion (PrP(Sc)). The cellular mechanisms underlying the misfolding of PrP(C) are not well understood. Since cellular prion proteins harbor divalent metal-binding sites in the N-terminal region, we examined the effect of manganese on PrP(C) processing in in vitro models of prion disease. Exposure to manganese significantly increased PrP(C) levels both in cytosolic and in membrane-rich fractions in a time-dependent manner. Manganese-induced PrP(C) upregulation was independent of messenger RNA transcription or stability. Additionally, manganese treatment did not alter the PrP(C) degradation by either proteasomal or lysosomal pathways. Interestingly, pulse-chase analysis showed that the PrP(C) turnover rate was significantly altered with manganese treatment, indicating increased stability of PrP(C) with the metal exposure. Limited proteolysis studies with proteinase-K further supported that manganese increases the stability of PrP(C). Incubation of mouse brain slice cultures with manganese also resulted in increased prion protein levels and higher intracellular manganese accumulation. Furthermore, exposure of manganese to an infectious prion cell model, mouse Rocky Mountain Laboratory-infected CAD5 cells, significantly increased prion protein levels. Collectively, our results demonstrate for the first time that divalent metal manganese can alter the stability of prion proteins and suggest that manganese-induced stabilization of prion protein may play a role in prion protein misfolding and prion disease pathogenesis.

Involvement of peptidylarginine deiminase-mediated post-translational citrullination in pathogenesis of sporadic Creutzfeldt-Jakob disease

Peptidylarginine deiminases (PADs)-mediated post-translational citrullination processes play key roles in protein functions and structural stability through the conversion of arginine to citrulline in the presence of excessive calcium concentrations. In brain, PAD2 is abundantly expressed and can be involved in citrullination in disease. Recently, we have reported pathological characterization of PAD2 and citrullinated proteins in scrapie-infected mice, but the implication of protein citrullination in the pathophysiology in human prion disease is not clear. In the present study, we explored the molecular and biological involvement of PAD2 and the pathogenesis of citrullinated proteins in frontal cortex of patients with sporadic Creutzfeldt-Jakob disease (sCJD). We found increased expression of PAD2 in reactive astrocytes that also contained increased levels of citrullinated proteins. In addition, PAD activity was significantly elevated in patients with sCJD compared to controls. From two-dimensional gel electrophoresis and MALDI-TOF mass analysis, we found various citrullinated candidates, including cytoskeletal and energy metabolism-associated proteins such as vimentin, glial fibrillary acidic protein, enolase, and phosphoglycerate kinase. Based on these findings, our investigations suggest that PAD2 activation and aberrant citrullinated proteins could play a role in pathogenesis and have value as a marker for the postmortem classification of neurodegenerative diseases.

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.