Lack of prion protein expression results in a neuronal phenotype sensitive to stress

Parkinson's Spectrum Mechanisms in Pregnancy: Exploring Hypothetical Scenarios for MSA in the Era of ART

Multiple System Atrophy (MSA) is a rare, rapidly progressive neurodegenerative disorder marked by autonomic dysfunction, parkinsonism, and cerebellar ataxia. While predominantly affecting individuals in their fifth or sixth decade, advancements in assisted reproductive technologies (ART) have created new clinical scenarios involving pregnancies in women within MSA's typical onset range. Given the scarcity of documented MSA pregnancies, this review leverages insights from related Parkinson's spectrum mechanisms to explore hypothetical scenarios for how pregnancy-induced physiological changes might influence MSA progression. Pregnancy-induced hormonal fluctuations, including elevated estrogen and progesterone levels, may modulate α-synuclein aggregation and neuroinflammatory pathways. Immune adaptations, such as fetal microchimerism and Th2-biased immune profiles, introduce additional complexities, particularly in donor embryo pregnancies involving complex microchimerism. Metabolic demands and oxidative stress further intersect with these mechanisms, potentially accelerating disease progression. We analyze existing literature and theoretical models, emphasizing the need for interdisciplinary research. Clinical implications are discussed to propose evidence-based strategies for optimizing maternal-fetal outcomes. This paper identifies critical knowledge gaps and proposes avenues for future investigation to optimize maternal-fetal outcomes in this unique and underexplored clinical intersection.

Perspectives on CRISPR Genome Editing to Prevent Prion Diseases in High-Risk Individuals

Prion diseases are neurodegenerative disorders caused by misfolded prion proteins. Although rare, the said diseases are always fatal; they commonly cause death within months of developing clinical symptoms, and their diagnosis is exceptionally difficult pre-mortem. There are no known cures or treatments other than symptomatic care. Given the aggressiveness of prion diseases on onset, therapies after disease onset could be challenging. Prevention to reduce the incidence or to delay the disease onset has been suggested to be a more feasible approach. In this perspective article, we summarize our current understandings of the origin, risk factors, and clinical manifestations of prion diseases. We propose a PCR testing of the blood to identify PRNP gene polymorphisms at codons 129 and 127 in individuals with familial PRNP mutations to assess the risk. We further present the CRISPR/Cas9 gene editing strategy as a perspective preventative approach for these high-risk individuals to induce a polymorphic change at codon 127 of the PRNP gene, granting immunity to prion diseases in selected high-risk individuals, in particular, in individuals with familial PRNP mutations.

Evidence that Alzheimer's Disease Is a Disease of Competitive Synaptic Plasticity Gone Awry

 Mounting evidence indicates that a physiological function of amyloid-β (Aβ) is to mediate neural activity-dependent homeostatic and competitive synaptic plasticity in the brain. I have previously summarized the lines of evidence supporting this hypothesis and highlighted the similarities between Aβ and anti-microbial peptides in mediating cell/synapse competition. In cell competition, anti-microbial peptides deploy a multitude of mechanisms to ensure both self-protection and competitor elimination. Here I review recent studies showing that similar mechanisms are at play in Aβ-mediated synapse competition and perturbations in these mechanisms underpin Alzheimer's disease (AD). Specifically, I discuss evidence that Aβ and ApoE, two crucial players in AD, co-operate in the regulation of synapse competition. Glial ApoE promotes self-protection by increasing the production of trophic monomeric Aβ and inhibiting its assembly into toxic oligomers. Conversely, Aβ oligomers, once assembled, promote the elimination of competitor synapses via direct toxic activity and amplification of "eat-me" signals promoting the elimination of weak synapses. I further summarize evidence that neuronal ApoE may be part of a gene regulatory network that normally promotes competitive plasticity, explaining the selective vulnerability of ApoE expressing neurons in AD brains. Lastly, I discuss evidence that sleep may be key to Aβ-orchestrated plasticity, in which sleep is not only induced by Aβ but is also required for Aβ-mediated plasticity, underlining the link between sleep and AD. Together, these results strongly argue that AD is a disease of competitive synaptic plasticity gone awry, a novel perspective that may promote AD research.

Cellular Prion Protein Attenuates OGD/R-Induced Damage by Skewing Microglia toward an Anti-inflammatory State via Enhanced and Prolonged Activation of Autophagy

Modulation of microglial pro/anti-inflammatory states and autophagy are promising new therapies for ischemic stroke, but the underlying mechanisms remain largely unexplored. The objective of the study is to determine the intrinsic role of PrP^C (cellular prion protein) in the regulation of microglial inflammatory states and autophagy in ischemic stroke. PrP^C was expressed in murine microglia, and an in vitro oxygen-glucose deprivation/reperfusion (OGD/R) model was established in microglia of different PRNP genotypes. During reperfusion following OGD, wild-type (WT) microglia had significantly increased pro/anti-inflammatory microglial percentages and related cytokine [interleukin [IL]-6, IL-10, IL-4, tumor necrosis factor, and interferon-gamma] release at reperfusion after 48 or 72 h. WT microglia also showed greater accumulation of the autophagy markers LC3B-II/I (microtubule-associated protein B-light chain 3), but not of p62 or LAMP1 (lysosome-associated membrane protein) at reperfusion after 24 h and 48 h. Inhibition of autophagy using 3-methyladenine or bafilomycin A1 aggravated the OGD/R-induced pro-inflammatory state, and the effect of 3-methyladenine was significantly stronger than that of bafilomycin A1. Concomitantly, PRNP knockout shortened the accumulation of LC3B-II/I, suppressed microglial anti-inflammatory states, and further aggravated the pro-inflammatory states. Conversely, PRNP overexpression had the opposite effects. Bafilomycin A1 reversed the effect of PrP^C on microglial inflammatory state transformation. Moreover, microglia with PRNP overexpression exhibited higher levels of LAMP1 expression in the control and OGD/R groups and delayed the OGD/R-induced decrease of LAMP1 to reperfusion after 48 h. PrP^C attenuates OGD/R-induced damage by skewing microglia toward an anti-inflammatory state via enhanced and prolonged activation of autophagy.

Melatonin-Assisted Cisplatin Suppresses Urinary Bladder Cancer Cell Proliferation and Growth through Inhibiting PrPC-Regulated Cell Stress and Cell Proliferation Signaling

This study investigated whether melatonin (Mel) would promote cisplatin to suppress the proliferation and growth of bladder cancer (BC) cells by inhibiting cellular prion protein (PrP^C)-mediated cell stress and cell proliferation signaling. An immunohistochemical staining of tissue arrays from BC patients demonstrated that the PrP^C expression was significantly upregulated from stage I to III BC (p < 0.0001). The BC cellline of T24 was categorized into G1 (T24), G2 (T24 + Mel/100 μM), G3 (T24+cisplatin/6 μM), G4 (PrP^C overexpression in T24 (i.e., PrP^C-OE-T24)), G5 (PrP^C-OE-T24+Mel), and G6 (PrP^C-OE-T24+cisplatin). When compared with a human uroepithelial cell line (SV-HUC-1), the cellular viability/wound healing ability/migration rate were significantly increased in T24 cells (G1) and further significantly increased in PrP^C-OE-T24 cells (G4); and they were suppressed in Mel (G2/G5) or cisplatin (G3/G6) treatment (all p < 0.0001). Additionally, the protein expressions of cell proliferation (PI3K/p-Akt/p-m-TOR/MMP-9/PrP^C), cell cycle/mitochondrial functional integrity (cyclin-D1/clyclin-E1/ckd2/ckd4/mitochondrial-cytochrome-C/PINK1), and cell stress (RAS/c-RAF/p-MEK1/2, p-ERK1/2) markers showed a similar pattern of cell viability among the groups (all p < 0.001). After the BC cell line of UMUC3 was implanted into nude mouse backs, by day 28 mthe BC weight/volume and the cellular levels of PrP^C/MMP-2/MMP-9 were significantly, gradually reduced from groups one to four (all p < 0.0001). The protein expressions of cell proliferation (PI3K/p-Akt/p-m-TOR/MMP-9/PrP^C), cell cycle/mitophagy (cyclin-D1/clyclin-E1/ckd2/ckd4/PINK1), and cell stress (RAS/c-RAF/p-MEK1,2/p-ERK1,2) signaling were significantly, progressively reduced from groups one to four, whereas the protein expressions of apoptotic (Mit-Bax/cleaved-caspase-3/cleaved-PARP) and oxidative stress/mitochondrial damaged (NOX-1/NOX-2/cytosolic-cytochrome-C/p-DRP1) markers expressed an opposite pattern of cell proliferation signaling among the groups (all p < 0.0001). Mel-cisplatin suppressed BC cell growth/proliferation via inhibiting the PrP^C in upregulating the cell proliferation/cell stress/cell cycle signaling.

Rejuvenated endothelial progenitor cells through overexpression of cellular prion protein effectively salvaged the critical limb ischemia in rats with preexisting chronic kidney disease

BACKGROUND

This study tested the hypothesis that overexpression of cellular prion protein in endothelial progenitor cells (PrPc^OE-EPCs), defined as "rejuvenated EPCs," was superior to EPCs for salvaging the critical limb ischemia (CLI) induced after 28-day chronic kidney disease (CKD) induction in rat.

METHODS AND RESULTS

Cell viability and flow cytometric analyses of early/late apoptosis/total-intracellular ROS/cell cycle (sub-G1, G2/M phase) were significantly higher in EPCs + H₂O₂ than in EPCs that were significantly reversed in PrPc^OE-EPCs + H₂O₂ (all p < 0.001). The protein expressions of inflammation (IL-1ß/IL-6/MMP-9/p-NF-κB) were significantly increased in EPC + TNF-α than in EPCs that were significantly reversed in PrPc^OE-EPCs + TNF-α (all p < 0.001). Adult-male SD rats (n = 8/each group) were categorized into group 1 (sham-operated control), group 2 (CKD + CLI), group 3 [CKD + CLI + EPCs by intravenous (0.6 × 10⁵)/intra-muscular (0.6 × 10⁵) injections at 3 h after CLI induction], group 4 (CKD + CLI + PrPc^OE-EPCs/dose-administration as group 3) and group 5 (CKD + CLI + siPrnp-EPCs/dose-administration as group 3). By day 14 after CLI induction, the ratio of ischemia to normal blood flow (INBF) in CLI area was highest in group 1/lowest in group 2/significantly higher in group 4 than in groups 3/5 and significantly higher in group 3 than in group 5 (all p < 0.0001). Histopathology demonstrated that the angiogenesis (number of small vessels/CD31 + cells) exhibited a similar trend, whereas the fibrosis/kidney injury score exhibited an opposite pattern of INBF among the groups (all p < 0.0001). The protein expressions of angiogenesis (SDF-1α/VEGF/CXCR4)/cell-stress signaling (p-PI3K/p-Akt/p-m-TOR) were significantly and progressively increased from groups 1-4 that were reversed in group 5 (all p < 0.0001). The protein expressions of fibrotic (p-Smad3/TGF-ß)/oxidative-stress (NOX-1/NOX-2/oxidized-protein)/apoptotic (mitochondrial-Bax/cleaved caspase3/cleaved PARP)/mitochondrial-damaged (cytosolic-cytochrome-C) biomarkers displayed an opposite pattern of INBF among the groups (all p < 0.0001).

CONCLUSION

PrPc^OE-EPCs were superior to EPCs only therapy for salvaging the CLI.

Understanding the Mechanism of Action of Melatonin, Which Induces ROS Production in Cancer Cells

Reactive oxygen species (ROS) constitute a group of highly reactive molecules that have evolved as regulators of important signaling pathways. In this context, tumor cells have an altered redox balance compared to normal cells, which can be targeted as an antitumoral therapy by ROS levels and by decreasing the capacity of the antioxidant system, leading to programmed cell death. Melatonin is of particular importance in the development of innovative cancer treatments due to its oncostatic impact and lack of adverse effects. Despite being widely recognized as a pro-oxidant molecule in tumor cells, the mechanism of action of melatonin remains unclear, which has hindered its use in clinical treatments. The current review aims to describe and clarify the proposed mechanism of action of melatonin inducing ROS production in cancer cells in order to propose future anti-neoplastic clinical applications.

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.

Prion Protein: The Molecule of Many Forms and Faces

Cellular prion protein (PrP^C) is a glycosylphosphatidylinositol (GPI)-anchored protein most abundantly found in the outer membrane of neurons. Due to structural characteristics (a flexible tail and structured core), PrP^C interacts with a wide range of partners. Although PrP^C has been proposed to be involved in many physiological functions, only peripheral nerve myelination homeostasis has been confirmed as a bona fide function thus far. PrP^C misfolding causes prion diseases and PrP^C has been shown to mediate β-rich oligomer-induced neurotoxicity in Alzheimer’s and Parkinson’s disease as well as neuroprotection in ischemia. Upon proteolytic cleavage, PrP^C is transformed into released and attached forms of PrP that can, depending on the contained structural characteristics of PrP^C, display protective or toxic properties. In this review, we will outline prion protein and prion protein fragment properties as well as overview their involvement with interacting partners and signal pathways in myelination, neuroprotection and neurodegenerative diseases.

Valsartan- and melatonin-supported adipose-derived mesenchymal stem cells preserve renal function in chronic kidney disease rat through upregulation of prion protein participated in promoting PI3K-Akt-mTOR signaling and cell proliferation

This study tested the hypothesis that valsartan (Val) and melatonin (Mel)-assisted adipose-derived mesenchymal stem cells (ADMSCs) preserved the residual renal function in chronic kidney disease (CKD) rat through promoting cellular-prior-protein (PrP^C) to upregulate PI3K/Akt/mTOR signaling and cell proliferation. In vitro study demonstrated that as compared with CKD-derived-ADMSCs, Val/Mel/overexpression of PrP^C-treated CKD derived-ADMSCs significantly upregulated cell proliferation and protein expressions of PrP^C and phosphorylated (p)-PI3K/p-Akt/p-mTOR, and downregulated oxidative stress (all p < 0.001). Rats (n = 42) were categorized into group 1 (sham-operated-control), group 2 (CKD), group 3 (CKD + ADMSCs/1.2 ×10⁶ cells) + Mel/20 mg/kg/day), group 4 (CKD + siRNA-PrP^C-ADMSCs/1.2 ×10⁶ cells), group 5 (CKD + ADMSCs/1.2 ×10⁶ cells + Val/20 mg/kg/day) and group 6 (CKD + Val + Mel). By day 35, the kidney specimens were harvested and the result showed that the protein expression of PrP^C was highest in group 1, lowest in groups 2/4 and significantly lower in group 6 than in groups 3/5, but it was similar in groups 3/5 (all p < 0.0001). The protein expressions of cell-stress-signaling (p-PI3K/p-Akt/p-mTOR) and cell-cycle activity (cyclin-D1/clyclin-E2/Cdk2/Cdk4) exhibited an identical pattern, whereas the protein expressions of oxidative-stress (NOX-1/NOX-2)/mitochondrial fission (PINK1/DRP1)/apoptosis (cleaved-capsase3/cleaved-PARP) and fibrosis (TFG-ß/Smad3) as well as creatinine/BUN levels, ratio of urine-protein to urine-creatine and kidney-injured score exhibited an opposite pattern of PrP^C among the groups (all p < 0.0001). In conclusion, Mel/Val facilitated-ADMSCs preserved renal architecture and function in CKD rat through promoting PrP^C to regulate the cell proliferation/oxidative-stress/cell-stress signalings.

Neuroprotective effect and potential of cellular prion protein and its cleavage products for treatment of neurodegenerative disorders part I. a literature review

INTRODUCTION

The cellular prion protein (PrP^C) is well known for its pathogenic roles in prion diseases, several other neurodegenerative diseases (such as Alzheimer's disease), and multiple types of cancer, but the beneficial aspects of PrP^C and its cleavage products received much less attention.

AREAS COVERED

Here the authors will systematically review the literatures on the negative as well as protective aspects of PrP^C and its derivatives (especially PrP N-terminal N1 peptide and shed PrP). The authors will dissect the current findings on N1 and shed PrP, including evidence for their neuroprotective effects, the categories of PrP^C cleavage, and numerous cleavage enzymes involved. The authors will also discuss the protective effects and therapeutic potentials of PrP^C-rich exosomes. The cited articles were obtained from extensive PubMed searches of recent literature, including peer-reviewed original articles and review articles.

EXPERT OPINION

PrP and its N-terminal fragments have strong neuroprotective activities that should be explored for therapeutics and prophylactics development against prion disease, Alzheimer's disease and a few other neurodegenerative diseases. The strategies to develop PrP-based therapeutics and prophylactics for these neurodegenerative diseases will be discussed in a companion article (Part II).

The Link between Oxidative Stress, Redox Status, Bioenergetics and Mitochondria in the Pathophysiology of ALS

Amyotrophic lateral sclerosis (ALS) is the most common neurodegenerative disease of the motor system. It is characterized by the degeneration of both upper and lower motor neurons, which leads to muscle weakness and paralysis. ALS is incurable and has a bleak prognosis, with median survival of 3-5 years after the initial symptomatology. In ALS, motor neurons gradually degenerate and die. Many features of mitochondrial dysfunction are manifested in neurodegenerative diseases, including ALS. Mitochondria have shown to be an early target in ALS pathophysiology and contribute to disease progression. Disruption of their axonal transport, excessive generation of reactive oxygen species, disruption of the mitochondrial structure, dynamics, mitophagy, energy production, calcium buffering and apoptotic triggering have all been directly involved in disease pathogenesis and extensively reported in ALS patients and animal model systems. Alterations in energy production by motor neurons, which severely limit their survival capacity, are tightly linked to the redox status and mitochondria. The present review focuses on this link. Placing oxidative stress as a main pathophysiological mechanism, the molecular interactions and metabolic flows involved are analyzed. This leads to discussing potential therapeutic approaches targeting mitochondrial biology to slow disease progression.

Mechanistic insights into TNFR1/MADD death domains in Alzheimer's disease through conformational molecular dynamic analysis

Proteins are tiny players involved in the activation and deactivation of multiple signaling cascades through interactions in cells. The TNFR1 and MADD interact with each other and mediate downstream protein signaling pathways which cause neuronal cell death and Alzheimer’s disease. In the current study, a molecular docking approach was employed to explore the interactive behavior of TNFR1 and MADD proteins and their role in the activation of downstream signaling pathways. The computational sequential and structural conformational results revealed that Asp400, Arg58, Arg59 were common residues of TNFR1 and MADD which are involved in the activation of downstream signaling pathways. Aspartic acid in negatively charged residues is involved in the biosynthesis of protein. However, arginine is a positively charged residue with the potential to interact with oppositely charged amino acids. Furthermore, our molecular dynamic simulation results also ensured the stability of the backbone of TNFR1 and MADD death domains (DDs) in binding interactions. This DDs interaction mediates some conformational changes in TNFR1 which leads to the activation of mediators proteins in the cellular signaling pathways. Taken together, a better understanding of TNFR1 and MADD receptors and their activated signaling cascade may help treat Alzheimer’s disease. The death domains of TNFR1 and MADD could be used as a novel pharmacological target for the treatment of Alzheimer’s disease by inhibiting the MAPK pathway.

Tau Exon 10 Inclusion by PrPC through Downregulating GSK3β Activity

Tau protein is largely responsible for tauopathies, including Alzheimer’s disease (AD), where it accumulates in the brain as insoluble aggregates. Tau mRNA is regulated by alternative splicing, and inclusion or exclusion of exon 10 gives rise to the 3R and 4R isoforms respectively, whose balance is physiologically regulated. In this sense, one of the several factors that regulate alternative splicing of tau is GSK3β, whose activity is inhibited by the cellular prion protein (PrP^C), which has different physiological functions in neuroprotection and neuronal differentiation. Moreover, a relationship between PrP^C and tau expression levels has been reported during AD evolution. For this reason, in this study we aimed to analyze the role of PrP^C and the implication of GSK3β in the regulation of tau exon 10 alternative splicing. We used AD human samples and mouse models of PrP^C ablation and tau overexpression. In addition, we used primary neuronal cultures to develop functional studies. Our results revealed a paralleled association between PrP^C expression and tau 4R isoforms in all models analyzed. In this sense, reduction or ablation of PrP^C levels induces an increase in tau 3R/4R balance. More relevantly, our data points to GSK3β activity downstream from PrP^C in this phenomenon. Our results indicate that PrP^C plays a role in tau exon 10 inclusion through the inhibitory capacity of GSK3β.

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.

Methionine oxidation within the prion protein

Prion diseases are characterized by the self-templated misfolding of the cellular prion protein (PrPC) into infectious aggregates (PrPSc). The detailed molecular basis of the misfolding and aggregation of PrPC remains incompletely understood. It is believed that the transient misfolding of PrPC into partially structured intermediates precedes the formation of insoluble protein aggregates and is a critical component of the prion misfolding pathway. A number of environmental factors have been shown to induce the destabilization of PrPC and promote its initial misfolding. Recently, oxidative stress and reactive oxygen species (ROS) have emerged as one possible mechanism by which the destabilization of PrPC can be induced under physiological conditions. Methionine residues are uniquely vulnerable to oxidation by ROS and the formation of methionine sulfoxides leads to the misfolding and subsequent aggregation of PrPC. Here, we provide a review of the evidence for the oxidation of methionine residues in PrPC and its potential role in the formation of pathogenic prion aggregates.

The cellular prion protein promotes neuronal regeneration after acute nasotoxic injury

Adult neurogenesis, analogous to early development, is comprised of several, often concomitant, processes including proliferation, differentiation, and formation of synaptic connections. However, due to continual, asynchronous turn-over, newly-born adult olfactory sensory neurons (OSNs) must integrate into existing circuitry. Additionally, OSNs express high levels of cellular prion protein (PrPC), particularly in the axon, which implies a role in this cell type. The cellular prion has been shown to be important for proper adult OSN neurogenesis primarily by stabilizing mature olfactory neurons within this circuitry. However, the role of PrPC on each specific adult neurogenic processes remains to be investigated in detail. To tease out the subtle effects of prion protein expression level, a large population of regenerating neurons must be investigated. The thyroid drug methimazole (MTZ) causes nearly complete OSN loss in rodents and is used as a model of acute olfactory injury, providing a mechanism to induce synchronized OSN regeneration. This study investigated the effect of PrPC on adult neurogenesis after acute nasotoxic injury. Altered PrPC levels affected olfactory sensory epithelial (OSE) regeneration, cell proliferation, and differentiation. Attempts to investigate the role of PrPC level on axon regeneration did not support previous studies, and glomerular targeting did not recover to vehicle-treated levels, even by 20 weeks. Together, these studies demonstrate that the cellular prion protein is critical for regeneration of neurons, whereby increased PrPC levels promote early neurogenesis, and that lack of PrPC delays the regeneration of this tissue after acute injury.

Show Me Your Friends and I Tell You Who You Are: The Many Facets of Prion Protein in Stroke

Ischemic stroke belongs to the leading causes of mortality and disability worldwide. Although treatments for the acute phase of stroke are available, not all patients are eligible. There is a need to search for therapeutic options to promote neurological recovery after stroke. The cellular prion protein (PrPC) has been consistently linked to a neuroprotective role after ischemic damage: it is upregulated in the penumbra area following stroke in humans, and animal models of stroke have shown that lack of PrPC aggravates the ischemic damage and lessens the functional outcome. Mechanistically, these effects can be linked to numerous functions attributed to PrPC: (1) as a signaling partner of the PI3K/Akt and MAPK pathways, (2) as a regulator of glutamate receptors, and (3) promoting stem cell homing mechanisms, leading to angio- and neurogenesis. PrPC can be cleaved at different sites and the proteolytic fragments can account for the manifold functions. Moreover, PrPC is present on extracellular vesicles (EVs), released membrane particles originating from all types of cells that have drawn attention as potential therapeutic tools in stroke and many other diseases. Thus, identification of the many mechanisms underlying PrPC-induced neuroprotection will not only provide further understanding of the physiological functions of PrPC but also new ideas for possible treatment options after ischemic stroke.

Identification of intracerebral hemorrhage in the early-phase of MM1+2C-type sporadic Creutzfeldt-Jakob disease: A case report

We report a case of early-phase sporadic Creutzfeldt-Jakob disease (sCJD) complicated by intracerebral hemorrhage (ICH), classified as MM1 + 2C-type based on autopsy. A 61-year-old Japanese man presented to our hospital with speaking difficulties including repeated usage of the same words. He was hospitalized on the seventh day after symptom onset, and diffusion-weighted images on magnetic resonance imaging showed hyperintense regions in the frontal cortex and caudate nucleus. On the 11th day after symptom onset, head computed tomography revealed ICH in the right occipital and parietal lobes. Routine laboratory evaluations and angiography revealed no cause of ICH. Myoclonus of the extremities and drowsiness were observed on the 15th day after symptom onset. He reached the state of akinetic mutism approximately two months after symptom onset. The cerebrospinal fluid test revealed positive real-time quaking-induced conversion and 14-3-3 protein. Electroencephalography revealed periodic sharp wave complexes. A clinical diagnosis of probable Creutzfeldt-Jakob disease was made according to the diagnostic criteria. After a relapse of pneumonia, he passed away on the 103rd day after symptom onset. Postmortem examination revealed ICH in the right posterior cingulate gyrus. No pathological change that might have caused ICH was obtained. Although the effect of sCJD on the onset of ICH is undeniable, the cause of ICH was unknown. Prion protein immunohistochemistry revealed the following results: (1) weak synaptic-type deposits in the tissue rarefacted by ICH; (2) synaptic-type deposits in the cerebral cortex, which showed fine vacuoles; and (3) perivacuolar-type deposits in the inferior temporal gyrus and lingual gyrus, which showed frequent large confluent vacuoles. Although it could be considered MM1-type sCJD clinically, this case was neuropathologically diagnosed as having MM1 + 2C-type sCJD. It was shown that ICH may occur in early-phase sCJD. To improve sCJD prognosis, treatment of complications and careful follow up are important. Furthermore, pathological diagnosis is indispensable for sCJD type diagnosis.

The Quest for Cellular Prion Protein Functions in the Aged and Neurodegenerating Brain

Cellular (also termed ‘natural’) prion protein has been extensively studied for many years for its pathogenic role in prionopathies after misfolding. However, neuroprotective properties of the protein have been demonstrated under various scenarios. In this line, the involvement of the cellular prion protein in neurodegenerative diseases other than prionopathies continues to be widely debated by the scientific community. In fact, studies on knock-out mice show a vast range of physiological functions for the protein that can be supported by its ability as a cell surface scaffold protein. In this review, we first summarize the most commonly described roles of cellular prion protein in neuroprotection, including antioxidant and antiapoptotic activities and modulation of glutamate receptors. Second, in light of recently described interaction between cellular prion protein and some amyloid misfolded proteins, we will also discuss the molecular mechanisms potentially involved in protection against neurodegeneration in pathologies such as Alzheimer’s, Parkinson’s, and Huntington’s diseases.

Nuclear factor-kappa β as a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is a typical progressive, chronic neurodegenerative disorder with worldwide prevalence. Its clinical manifestation involves the presence of extracellular plaques and intracellular neurofibrillary tangles (NFTs). NFTs occur in brain tissues as a result of both Aβ agglomeration and Tau phosphorylation. Although there is no known cure for AD, research into possible cures and treatment options continues using cell-cultures and model animals/organisms. The nuclear factor-kappa β (NF-κβ) plays an active role in the progression of AD. Impairment to this signaling module triggers undesirable phenotypic changes such as neuroinflammation, activation of microglia, oxidative stress related complications, and apoptotic cell death. These imbalances further lead to homeostatic abnormalities in the brain or in initial stages of AD essentially pushing normal neurons toward the degeneration process. Interestingly, the role of NF-κβ signaling associated receptor-interacting protein kinase is currently observed in apoptotic and necrotic cell death, and has been reported in brains. Conversely, the NF-κβ signaling pathway has also been reported to be involved in normal brain functioning. This pathway plays a crucial role in maintaining synaptic plasticity and balancing between learning and memory. Since any impairment in the pathways associated with NF-κβ signaling causes altered neuronal dynamics, neurotherapeutics using compounds including, antioxidants, bioflavonoids, and non-steroidal anti-inflammatory drugs against such abnormalities offer possibilities to rectify aberrant excitatory neuronal activity in AD. In this review, we have provided an extensive overview of the crucial role of NF-κβ signaling in normal brain homeostasis. We have also thoroughly outlined several established pathomechanisms associated with NF-κβ pathways in AD, along with their respective therapeutic approaches.

TUDCA-treated chronic kidney disease-derived hMSCs improve therapeutic efficacy in ischemic disease via PrPC

Although autologous human mesenchymal stem cells (hMSCs) are a promising source for regenerative stem cell therapy in chronic kidney disease (CKD), the barriers associated with pathophysiological conditions limit therapeutic applicability to patients. We confirmed that level of cellular prion protein (PrP^C) in serum was decreased and mitochondria function of CKD-derived hMSCs (CKD-hMSCs) was impaired in patients with CKD. We proved that treatment of CKD-hMSCs with tauroursodeoxycholic acid (TUDCA), a bile acid, enhanced the mitochondrial function of these cells through regulation of PINK1-PrP^C-dependent pathway. In a murine hindlimb ischemia model with CKD, tail vein injection of TUDCA-treated CKD-hMSCs improved the functional recovery, including kidney recovery, limb salvage, blood perfusion ratio, and vessel formation along with restored expression of PrP^C in the blood serum of the mice. These data suggest that TUDCA-treated CKD-hMSCs are a promising new autologous stem cell therapeutic intervention that dually treats cardiovascular problems and CKD in patients.

TUDCA-Treated Mesenchymal Stem Cells Protect against ER Stress in the Hippocampus of a Murine Chronic Kidney Disease Model

Chronic kidney disease (CKD) leads to the loss of kidney function, as well as the dysfunction of several other organs due to the release of uremic toxins into the system. In a murine CKD model, reactive oxygen species (ROS) generation and endoplasmic reticulum (ER) stress are increased in the hippocampus. Mesenchymal stem cells (MSCs) are one of the candidates for cell-based therapy for CKD; however severe pathophysiological conditions can decrease their therapeutic potential. To address these issues, we established tauroursodeoxycholic acid (TUDCA)-treated MSCs using MSCs isolated from patients with CKD (CKD-hMSCs) and assessed the survival and ROS generation of neural cell line SH-SY5Y cells by co-culturing with TUDCA-treated CKD-hMSCs. In the presence of the uremic toxin P-cresol, the death of SH-SY5Y cells was induced by ROS-mediated ER stress. Co-culture with TUDCA-treated CKD-hMSCs increased anti-oxidant enzyme activities in SH-SY5Y cells through the upregulation of the cellular prion protein (PrP^C) expression. Upregulated PrP^C expression in SH-SY5Y cells protected against CKD-mediated ER stress and apoptosis. In an adenine-induced murine CKD model, injection with TUDCA-treated CKD-hMSCs suppressed ROS generation and ER stress in the hippocampus. These results indicate that TUDCA-treated CKD-hMSCs prevent the CKD-mediated cell death of SH-SY5Y cells by inhibiting ER stress. Our study suggests that treatment with TUDCA could be a powerful strategy for developing autologous MSC-based therapeutics for patients with CKD, and that PrP^C might be a pivotal target for protecting neural cells from CKD-mediated ER stress.

Cellular prion protein-mediated protection against influenza A virus infection

The cellular prion protein, termed PrPC, is a glycoprotein abundantly expressed in brains and to a lesser extent in non-neuronal tissues including lungs. It was reported that PrPC is expressed by lung epithelial cells in mice, and that it may play a protective role against lethal infection with influenza A viruses (IAVs). This may occur by regulating Cu content and  superoxide dismutase (SOD) activity, eventually reducing oxidative stress in infected lungs. Antioxidative therapeutics have been demonstrated to protect mice from lethal infection with IAVs. Therefore, PrPC might be a new target molecule for development of IAV therapeutics. Here, we introduce the antiviral mechanism of PrPC against IAV infection and discuss perspectives of PrPC-targeting therapeutics against IAV infection.

Prion protein cleavage fragments regulate adult neural stem cell quiescence through redox modulation of mitochondrial fission and SOD2 expression

Neurogenesis continues in the post-developmental brain throughout life. The ability to stimulate the production of new neurones requires both quiescent and actively proliferating pools of neural stem cells (NSCs). Actively proliferating NSCs ensure that neurogenic demand can be met, whilst the quiescent pool makes certain NSC reserves do not become depleted. The processes preserving the NSC quiescent pool are only just beginning to be defined. Herein, we identify a switch between NSC proliferation and quiescence through changing intracellular redox signalling. We show that N-terminal post-translational cleavage products of the prion protein (PrP) induce a quiescent state, halting NSC cellular growth, migration, and neurite outgrowth. Quiescence is initiated by the PrP cleavage products through reducing intracellular levels of reactive oxygen species. First, inhibition of redox signalling results in increased mitochondrial fission, which rapidly signals quiescence. Thereafter, quiescence is maintained through downstream increases in the expression and activity of superoxide dismutase-2 that reduces mitochondrial superoxide. We further observe that PrP is predominantly cleaved in quiescent NSCs indicating a homeostatic role for this cascade. Our findings provide new insight into the regulation of NSC quiescence, which potentially could influence brain health throughout adult life.

Prion protein protects mice from lethal infection with influenza A viruses

The cellular prion protein, designated PrP^C, is a membrane glycoprotein expressed abundantly in brains and to a lesser extent in other tissues. Conformational conversion of PrP^C into the amyloidogenic isoform is a key pathogenic event in prion diseases. However, the physiological functions of PrP^C remain largely unknown, particularly in non-neuronal tissues. Here, we show that PrP^C is expressed in lung epithelial cells, including alveolar type 1 and 2 cells and bronchiolar Clara cells. Compared with wild-type (WT) mice, PrP^C-null mice (Prnp^0/0) were highly susceptible to influenza A viruses (IAVs), with higher mortality. Infected Prnp^0/0 lungs were severely injured, with higher inflammation and higher apoptosis of epithelial cells, and contained higher reactive oxygen species (ROS) than control WT lungs. Treatment with a ROS scavenger or an inhibitor of xanthine oxidase (XO), a major ROS-generating enzyme in IAV-infected lungs, rescued Prnp^0/0 mice from the lethal infection with IAV. Moreover, Prnp^0/0 mice transgenic for PrP with a deletion of the Cu-binding octapeptide repeat (OR) region, Tg(PrPΔOR)/Prnp^0/0 mice, were also highly susceptible to IAV infection. These results indicate that PrP^C has a protective role against lethal infection with IAVs through the Cu-binding OR region by reducing ROS in infected lungs. Cu content and the activity of anti-oxidant enzyme Cu/Zn-dependent superoxide dismutase, SOD1, were lower in Prnp^0/0 and Tg(PrPΔOR)/Prnp^0/0 lungs than in WT lungs. It is thus conceivable that PrP^C functions to maintain Cu content and regulate SOD1 through the OR region in lungs, thereby reducing ROS in IAV-infected lungs and eventually protecting them from lethal infection with IAVs. Our current results highlight the role of PrP^C in protection against IAV infection, and suggest that PrP^C might be a novel target molecule for anti-influenza therapeutics.

Influenza A virus (IAV) is an enveloped, negative sense, single-stranded RNA virus, causing seasonal epidemic outbreaks of influenza. Anti-influenza agents targeting viral molecules, such as neuraminidase inhibitors, are currently available. However, these agents have accelerated emergence of mutant IAVs that are resistant to these agents among human populations. Development of new types of anti-influenza agents is awaited. We show that the cellular prion protein PrP^C has a protective role against lethal infection with IAVs through the octapeptide repeat (OR) region by abrogating lung epithelial cell apoptosis induced by reactive oxygen species (ROS) in infected lungs. We also show that PrP^C could reduce ROS in IAV-infected lungs through the OR region by maintaining Cu ion homeostasis and thereby activating Cu/Zn-dependent superoxide dismutase, SOD1. These results highlight the protective role of PrP^C in IAV infection. Elucidation of the exact mechanism underlying the PrP^C-mediated protection against IAV infection would be important for further understanding the pathogenesis of IAV infection and could be useful for development of new types of anti-influenza therapeutics.

Implantation of Neural Probes in the Brain Elicits Oxidative Stress

Clinical implantation of intracortical microelectrodes has been hindered, at least in part, by the perpetual inflammatory response occurring after device implantation. The neuroinflammatory response observed after device implantation has been correlated to oxidative stress that occurs due to neurological injury and disease. However, there has yet to be a definitive link of oxidative stress to intracortical microelectrode implantation. Thus, the objective of this study is to give direct evidence of oxidative stress following intracortical microelectrode implantation. This study also aims to identify potential molecular targets to attenuate oxidative stress observed postimplantation. Here, we implanted adult rats with silicon non-functional microelectrode probes for 4 weeks and compared the oxidative stress response to no surgery controls through postmortem gene expression analysis and qualitative histological observation of oxidative stress markers. Gene expression analysis results at 4 weeks postimplantation indicated that EH domain-containing 2, prion protein gene (Prnp), and Stearoyl-Coenzyme A desaturase 1 (Scd1) were all significantly higher for animals implanted with intracortical microelectrode probes compared to no surgery control animals. To the contrary, NADPH oxidase activator 1 (Noxa1) relative gene expression was significantly lower for implanted animals compared to no surgery control animals. Histological observation of oxidative stress showed an increased expression of oxidized proteins, lipids, and nucleic acids concentrated around the implant site. Collectively, our results reveal there is a presence of oxidative stress following intracortical microelectrode implantation compared to no surgery controls. Further investigation targeting these specific oxidative stress linked genes could be beneficial to understanding potential mechanisms and downstream therapeutics that can be utilized to reduce oxidative stress-mediated damage following microelectrode implantation.

Stress Resilience of Spermatozoa and Blood Mononuclear Cells without Prion Protein

The cellular prion protein PrP^C is highly expressed in neurons, but also present in non-neuronal tissues, including the testicles and spermatozoa. Most immune cells and their bone marrow precursors also express PrP^C. Clearly, this protein operates in highly diverse cellular contexts. Investigations into putative stress-protective roles for PrP^C have resulted in an array of functions, such as inhibition of apoptosis, stimulation of anti-oxidant enzymes, scavenging roles, and a role in nuclear DNA repair. We have studied stress resilience of spermatozoa and peripheral blood mononuclear cells (PBMCs) derived from non-transgenic goats that lack PrP^C (PRNP^Ter/Ter) compared with cells from normal (PRNP^+/+) goats. Spermatozoa were analyzed for freeze tolerance, DNA integrity, viability, motility, ATP levels, and acrosome intactness at rest and after acute stress, induced by Cu²⁺ ions, as well as levels of reactive oxygen species (ROS) after exposure to FeSO₄ and H₂O₂. Surprisingly, PrP^C-negative spermatozoa reacted similarly to normal spermatozoa in all read-outs. Moreover, in vitro exposure of PBMCs to Doxorubicin, H₂O₂ and methyl methanesulfonate (MMS), revealed no effect of PrP^C on cellular survival or global accumulation of DNA damage. Similar results were obtained with human neuroblastoma (SH-SY5Y) cell lines stably expressing varying levels of PrP^C. RNA sequencing of PBMCs (n = 8 of PRNP^+/+ and PRNP^Ter/Ter) showed that basal level expression of genes encoding DNA repair enzymes, ROS scavenging, and antioxidant enzymes were unaffected by the absence of PrP^C. Data presented here questions the in vitro cytoprotective roles previously attributed to PrP^C, although not excluding such functions in other cell types or tissues during inflammatory stress.

The Role of the Mammalian Prion Protein in the Control of Sleep

Sleep disruption is a prevalent clinical feature in many neurodegenerative disorders, including human prion diseases where it can be the defining dysfunction, as in the case of the "eponymous" fatal familial insomnia, or an early-stage symptom as in certain types of Creutzfeldt-Jakob disease. It is important to establish the role of the cellular prion protein (PrP^C), the key molecule involved in prion pathogenesis, within the sleep-wake system in order to understand fully the mechanisms underlying its contribution to both healthy circadian rhythmicity and sleep dysfunction during disease. Although severe disruption to the circadian rhythm and melatonin release is evident during the pathogenic phases of some prion diseases, untangling whether PrP^C plays a role in circadian rhythmicity, as suggested in mice deficient for PrP^C expression, is challenging given the lack of basic experimental research. We provide a short review of the small amount of direct literature focused on the role of PrP^C in melatonin and circadian rhythm regulation, as well as suggesting mechanisms by which PrP^C might exert influence upon noradrenergic and dopaminergic signaling and melatonin synthesis. Future research in this area should focus upon isolating the points of dysfunction within the retino-pineal pathway and further investigate PrP^C mediation of pinealocyte GPCR activity.

Gene Targeted Transgenic Mouse Models in Prion Research

The production of transgenic mice expressing different forms of the prion protein (PrP) or devoid of PrP has enabled researchers to study the role of PrP in the infectious process of a prion disease and its normal function in the healthy individual. A wide range of transgenic models have been produced ranging from PrP null mice, normal expression levels to overexpression models, models expressing different species of the Prnp gene and different mutations and polymorphisms within the gene. Using this range of transgenic models has allowed us to define the influence of PrP expression on disease susceptibility and transmission, assess zoonotic potential, define strains of human prion diseases, elucidate the function of PrP, and start to unravel the mechanisms involved in chronic neurodegeneration. This chapter focuses mainly on the use of the gene targeted transgenic models and summarizes the ways in which they have allowed us to study the role of PrP in prion disease and the insights they have provided into the mechanisms of neurodegenerative diseases.

Engagement of cellular prion protein with the co-chaperone Hsp70/90 organizing protein regulates the proliferation of glioblastoma stem-like cells

Background

Glioblastoma (GBM), a highly aggressive brain tumor, contains a subpopulation of glioblastoma stem-like cells (GSCs) that play roles in tumor maintenance, invasion, and therapeutic resistance. GSCs are therefore a promising target for GBM treatment. Our group identified the cellular prion protein (PrP^C) and its partner, the co-chaperone Hsp70/90 organizing protein (HOP), as potential target candidates due to their role in GBM tumorigenesis and in neural stem cell maintenance.

Methods

GSCs expressing different levels of PrP^C were cultured as neurospheres with growth factors, and characterized with stem cells markers and adhesion molecules markers through immunofluorescence and flow cytometry. We than evaluated GSC self-renewal and proliferation by clonal density assays and BrdU incorporation, respectively, in front of recombinant HOP treatment, combined or not with a HOP peptide which mimics the PrP^C binding site. Stable silencing of HOP was also performed in parental and/or PrP^C-depleted cell populations, and proliferation in vitro and tumor growth in vivo were evaluated. Migration assays were performed on laminin-1 pre-coated glass.

Results

We observed that, when GBM cells are cultured as neurospheres, they express specific stemness markers such as CD133, CD15, Oct4, and SOX2; PrP^C is upregulated compared to monolayer culture and co-localizes with CD133. PrP^C silencing downregulates the expression of molecules associated with cancer stem cells, upregulates markers of cell differentiation and affects GSC self-renewal, pointing to a pivotal role for PrP^C in the maintenance of GSCs. Exogenous HOP treatment increases proliferation and self-renewal of GSCs in a PrP^C-dependent manner while HOP knockdown disturbs the proliferation process. In vivo, PrP^C and/or HOP knockdown potently inhibits the growth of subcutaneously implanted glioblastoma cells. In addition, disruption of the PrP^C-HOP complex by a HOP peptide, which mimics the PrP^C binding site, affects GSC self-renewal and proliferation indicating that the HOP-PrP^C complex is required for GSC stemness. Furthermore, PrP^C-depleted GSCs downregulate cell adhesion-related proteins and impair cell migration indicating a putative role for PrP^C in the cell surface stability of cell adhesion molecules and GBM cell invasiveness, respectively.

Conclusions

In conclusion, our results show that the modulation of HOP-PrP^C engagement or the decrease of PrP^C and HOP expression may represent a potential therapeutic intervention in GBM, regulating glioblastoma stem-like cell self-renewal, proliferation, and migration.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0518-1) contains supplementary material, which is available to authorized users.

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.

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.

Functions of the cellular prion protein, the end of Moore's law, and Ockham's razor theory

Since its discovery the cellular prion protein (encoded by the Prnp gene) has been associated with a large number of functions. The proposed functions rank from basic cellular processes such as cell cycle and survival to neural functions such as behavior and neuroprotection, following a pattern similar to that of Moore's law for electronics. In addition, particular interest is increasing in the participation of Prnp in neurodegeneration. However, in recent years a redefinition of these functions has begun, since examples of previously attributed functions were increasingly re-associated with other proteins. Most of these functions are linked to so-called "Prnp-flanking genes" that are close to the genomic locus of Prnp and which are present in the genome of some Prnp mouse models. In addition, their role in neuroprotection against convulsive insults has been confirmed in recent studies. Lastly, in recent years a large number of models indicating the participation of different domains of the protein in apoptosis have been uncovered. However, after more than 10 years of molecular dissection our view is that the simplest mechanistic model in PrP(C)-mediated cell death should be considered, as Ockham's razor theory suggested.

Tauroursodeoxycholic acid reduces ER stress by regulating of Akt-dependent cellular prion protein

Although mesenchymal stem cells (MSCs) are a promising cell source for regenerative medicine, ischemia-induced endoplasmic reticulum (ER) stress induces low MSC engraftment and limits their therapeutic efficacy. To overcome this, we investigated the protective effect of tauroursodeoxycholic acid (TUDCA), a bile acid, on ER stress in MSCs in vitro and in vivo. In ER stress conditions, TUDCA treatment of MSCs reduced the activation of ER stress-associated proteins, including GRP78, PERK, eIF2α, ATF4, IRE1α, JNK, p38, and CHOP. In particular, TUDCA inhibited the dissociation between GRP78 and PERK, resulting in reduced ER stress-mediated cell death. Next, to explore the ER stress protective mechanism induced by TUDCA treatment, TUDCA-mediated cellular prion protein (PrP^C) activation was assessed. TUDCA treatment increased PrP^C expression, which was regulated by Akt phosphorylation. Manganese-dependent superoxide dismutase (MnSOD) expression also increased significantly in response to signaling through the TUDCA-Akt axis. In a murine hindlimb ischemia model, TUDCA-treated MSC transplantation augmented the blood perfusion ratio, vessel formation, and transplanted cell survival more than untreated MSC transplantation did. Augmented functional recovery following MSC transplantation was blocked by PrP^C downregulation. This study is the first to demonstrate that TUDCA protects MSCs against ER stress via Akt-dependent PrP^C and Akt-MnSOD pathway.

Prion protein "gamma-cleavage": characterizing a novel endoproteolytic processing event

The cellular prion protein (PrP(C)) is a ubiquitously expressed protein of currently unresolved but potentially diverse function. Of putative relevance to normal biological activity, PrP(C) is recognized to undergo both α- and β-endoproteolysis, producing the cleavage fragment pairs N1/C1 and N2/C2, respectively. Experimental evidence suggests the likelihood that these processing events serve differing cellular needs. Through the engineering of a C-terminal c-myc tag onto murine PrP(C), as well as the selective use of a far-C-terminal anti-PrP antibody, we have identified a new PrP(C) fragment, nominally 'C3', and elaborating existing nomenclature, 'γ-cleavage' as the responsible proteolysis. Our studies indicate that this novel γ-cleavage event can occur during transit through the secretory pathway after exiting the endoplasmic reticulum, and after PrP(C) has reached the cell surface, by a matrix metalloprotease. We found that C3 is GPI-anchored like other C-terminal and full length PrP(C) species, though it does not localize primarily at the cell surface, and is preferentially cleaved from an unglycosylated substrate. Importantly, we observed that C3 exists in diverse cell types as well as mouse and human brain tissue, and of possible pathogenic significance, γ-cleavage may increase in human prion diseases. Given the likely relevance of PrP(C) processing to both its normal function, and susceptibility to prion disease, the potential importance of this previously underappreciated and overlooked cleavage event warrants further consideration.

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.

Involvement of PrP(C) in kainate-induced excitotoxicity in several mouse strains

The cellular prion protein (PrP(C)) has been associated with a plethora of cellular functions ranging from cell cycle to neuroprotection. Mice lacking PrP(C) show an increased susceptibility to epileptic seizures; the protein, then, is neuroprotective. However, lack of experimental reproducibility has led to considering the possibility that other factors besides PrP(C) deletion, such as the genetic background of mice or the presence of so-called "Prnp flanking genes", might contribute to the reported susceptibility. Here, we performed a comparative analysis of seizure-susceptibility using characterized Prnp(+/+) and Prnp(0/0) mice of B6129, B6.129, 129/Ola or FVB/N genetic backgrounds. Our study indicates that PrP(C) plays a role in neuroprotection in KA-treated cells and mice. For this function, PrP(C) should contain the aa32-93 region and needs to be linked to the membrane. In addition, some unidentified "Prnp-flanking genes" play a role parallel to PrP(C) in the KA-mediated responses in B6129 and B6.129 Prnp(0/0) mice.

Downregulation of β-Soluble N-Ethylmaleimide-Sensitive Factor Attachment Protein: Proteomics-Based Identification in Early-Stage Prion Disease

Prion diseases are known as neurodegenerative diseases of the central nervous system with a long incubation period. Alzheimer's disease (AD) and prion diseases share the hallmark of severe neuronal loss, although their pathogenic mechanisms are similarly incomplete. It appears that these two neurodegenerative diseases share a complex deterioration of function involved in the onset of neuronal loss. To investigate presymptomatic biochemical changes indicative of the initial stage of prion diseases and decipher the pathophysiological mechanisms of these two neurodegenerative diseases, we performed a differential proteomic analysis on brain tissues of 263K-infected hamsters during the presymptomatic period and transgenic APPSWE, PSEN1dE9 mice (a mouse model of AD). We identified 7 differentially expressed proteins including the β-soluble N-ethylmaleimide-sensitive factor attachment protein (β-SNAP) by 2-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The β-SNAP expression patterns in the brains of cases and controls were further quantified by Western blotting. β-SNAP showed an early decrease followed by a progressive depletion. The expression of β-SNAP was also significantly downregulated in the mouse model of AD. β-SNAP is brain-specific and known to bind to the SNAP receptors and is therefore involved in the control of neurotransmitter release as well as in constitutive vesicular transport. Our results suggest that presynaptic failure and abnormalities in neurotransmission may be early events in the development of neuronal dysfunction.

Review of studies that have used knockout mice to assess normal function of prion protein under immunological or pathophysiological stress

Deletion of cellular isoform of prion protein (PrP(C)) increases neuronal predisposition to damage by modulating apoptosis and the negative consequences of oxidative stress. In vivo studies have demonstrated that PrP(C)-deficient mice are more prone to seizure, depression, and induction of epilepsy and experience extensive cerebral damage following ischemic challenge or viral infection. In addition, adenovirus-mediated overexpression of PrP(C) reduces brain damage in rat models of cerebral ischemia. In experimental autoimmune encephalomyelitis, PrP(C)-deficient mice reportedly have a more aggressive disease onset and less clinical improvement during the chronic phase than wild-type mice mice. In mice given oral dextran sulfate, PrP(C) has a potential protective role against inflammatory bowel disease. PrP(C)-deficient mice demonstrate significantly greater increases in blood glucose concentrations after intraperitoneal injection of glucose than wild-type mice. Further in vivo challenges to PrP gene-deficient models and conditional knockout models with siRNA and in vivo administration of PrP-ligating agents may assist in refining knowledge of the lymphoid function of PrP(C) and predicting the effects of anti-PrP treatment on the immune system. Together, these findings indicate that PrP(C) may have multiple neuroprotective and anti-inflammatory roles, which explains why this protein is so widely expressed.

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.

The prion protein is critical for DNA repair and cell survival after genotoxic stress

The prion protein (PrP) is highly conserved and ubiquitously expressed, suggesting that it plays an important physiological function. However, despite decades of investigation, this role remains elusive. Here, by using animal and cellular models, we unveil a key role of PrP in the DNA damage response. Exposure of neurons to a genotoxic stress activates PRNP transcription leading to an increased amount of PrP in the nucleus where it interacts with APE1, the major mammalian endonuclease essential for base excision repair, and stimulates its activity. Preventing the induction of PRNP results in accumulation of abasic sites in DNA and impairs cell survival after genotoxic treatment. Brains from Prnp^−/− mice display a reduced APE1 activity and a defect in the repair of induced DNA damage in vivo. Thus, PrP is required to maintain genomic stability in response to genotoxic stresses.

Relationships between cobalamin, epidermal growth factor, and normal prions in the myelin maintenance of central nervous system

Cobalamin (Cbl), epidermal growth factor (EGF), and prions (PrPs) are key molecules for myelin maintenance in the central and peripheral nervous systems. Cbl and EGF increase normal prion (PrP(C)) synthesis and PrP(C) levels in rat spinal cord (SC) and elsewhere. Cbl deficiency increases PrP(C) levels in rat SC and cerebrospinal fluid (CSF), and decreases PrP(C)-mRNA levels in rat SC. The administration of anti-octapeptide repeat PrP(C) region antibodies (Abs) to Cbl-deficient (Cbl-D) rats prevents SC myelin lesions and a local increase in tumor necrosis factor (TNF)-α levels, whereas anti-TNF-α Abs prevent SC myelin lesions and the increase in SC and CSF PrP(C) levels. As it is known that both Cbl and EGF regulate SC PrP(C) synthesis independently, and that Cbl regulates SC EGF synthesis, EGF may play both Cbl-independent and Cbl-dependent roles. When Cbl-D rats undergo Cbl replacement therapy, SC PrP(C) levels are similar to those observed in Cbl-D rats. In rat frontal cortex (which is marginally affected by Cbl deficiency in histological terms), Cbl deficiency decreases PrP(C) levels and the increase induced by Cbl replacement leads to their normalization. Increased nerve PrP(C) levels are detected in the myelin lesions of the peripheral neuropathy of Cbl-D rats, and CSF PrP(C) levels are also increased in Cbl-D patients (but not in patients with Cbl-unrelated neurological diseases). Various common steps in the downstream signaling pathway of Cbl, EGF, and PrP(C) underlines the close relationship between the three molecules in keeping myelin normal.

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.

Prion protein and cancers

The normal cellular prion protein, PrP(C) is a highly conserved and widely expressed cell surface glycoprotein in all mammals. The expression of PrP is pivotal in the pathogenesis of prion diseases; however, the normal physiological functions of PrP(C) remain incompletely understood. Based on the studies in cell models, a plethora of functions have been attributed to PrP(C). In this paper, we reviewed the potential roles that PrP(C) plays in cell physiology and focused on its contribution to tumorigenesis.

Accelerated clinical course of prion disease in mice compromised in repair of oxidative DNA damage

The detailed mechanisms of prion-induced neurotoxicity are largely unknown. Here, we have studied the role of DNA damage caused by reactive oxygen species in a mouse scrapie model by characterizing prion disease in the ogg1(-/-)mutyh(-/-) double knockout, which is compromised in oxidative DNA base excision repair. Ogg1 initiates removal of the major oxidation product 8-oxoguanine (8-oxoG) in DNA, and Mutyh initiates removal of adenine that has been misincorporated opposite 8-oxoG. Our data show that the onset of clinical signs appeared unaffected by Mutyh and Ogg1 expression. However, the ogg1(-/-)mutyh(-/-) mice displayed a significantly shorter clinical phase of the disease. Thus, accumulation of oxidative DNA damage might be of particular importance in the terminal clinical phase of prion disease. The prion-induced pathology and lesion profile were similar between knockout mice and controls. The fragmentation pattern of protease-resistant PrP as revealed in Western blots was also identical between the groups. Our data show that the fundamentals of prion propagation and pathological manifestation are not influenced by the oxidative DNA damage repair mechanisms studied here, but that progressive accumulation of oxidative lesions may accelerate the final toxic phase of prion disease.