Application of N-Terminal Labeling Methods Provide Novel Insights into Endoproteolysis of the Prion Protein in Vivo

Alternative α- and β-cleavage events in the cellular prion protein (PrPC) central region generate fragments with distinct biochemical features that affect prion disease pathogenesis, but the assignment of precise cleavage positions has proven challenging. Exploiting mouse transgenic models expressing wild-type (WT) PrPC and an octarepeat region mutant allele (S3) with increased β-fragmentation, cleavage sites were defined using LC-MS/MS in conjunction with N-terminal enzymatic labeling and chemical in-gel acetylation. Our studies profile the net proteolytic repertoire of the adult brain, as deduced from defining hundreds of proteolytic events in other proteins, and position individual cleavage events in PrPC α- and β-target areas imputed from earlier, lower resolution methods; these latter analyses established site heterogeneity, with six cleavage sites positioned in the β-cleavage region of WT PrPC and nine positions for S3 PrPC. Regarding α-cleavage, aside from reported N-termini at His110 and Val111, we identified a total of five shorter fragments in the brain of both mice lines. We infer that aminopeptidase activity in the brain could contribute to the ragged N-termini observed around PrPC's α- and β-cleavage sites, with this work providing a point of departure for further in vivo studies of brain proteases.

Two mouse models of Alzheimer's disease accumulate amyloid at different rates and have distinct Aβ oligomer profiles unaltered by ablation of cellular prion protein

Oligomers formed from monomers of the amyloid β-protein (Aβ) are thought to be central to the pathogenesis of Alzheimer's disease (AD). Unsurprisingly for a complex disease, current mouse models of AD fail to fully mimic the clinical disease in humans. Moreover, results obtained in a given mouse model are not always reproduced in a different model. Cellular prion protein (PrPC) is now an established receptor for Aβ oligomers. However, studies of the Aβ-PrPC interaction in different mouse models have yielded contradictory results. Here we performed a longitudinal study assessing a range of biochemical and histological features in the commonly used J20 and APP-PS1 mouse models. Our analysis demonstrated that PrPC ablation had no effect on amyloid accumulation or oligomer production. However, we found that APP-PS1 mice had higher levels of oligomers, that these could bind to recombinant PrPC, and were recognised by the OC antibody which distinguishes parallel, in register fibrils. On the other hand, J20 mice had a lower level of Aβ oligomers, which did not interact with PrPC when tested in vitro and were OC-negative. These results suggest the two mouse models produce diverse Aβ assemblies that could interact with different targets, highlighting the necessity to characterise the conformation of the Aβ oligomers concomitantly with the toxic cascade elicited by them. Our results provide an explanation for the apparent contradictory results found in APP-PS1 mice and the J20 mouse line in regards to Aβ toxicity mediated by PrPC.

Emerging roles of the cellular prion protein (PrPC) and 37/67 kDa laminin receptor (RPSA) interaction in cancer biology

The cellular prion protein (PrPC) is well-known for its involvement, under its pathogenic protease-resistant form (PrPSc), in a group of neurodegenerative diseases, known as prion diseases. PrPC is expressed in nervous system, as well as in other peripheral organs, and has been found overexpressed in several types of solid tumors. Notwithstanding, studies in recent years have disclosed an emerging role for PrPC in various cancer associated processes. PrPC has high binding affinity for 37/67 kDa laminin receptor (RPSA), a molecule that acts as a key player in tumorigenesis, affecting cell growth, adhesion, migration, invasion and cell death processes. Recently, we have characterized at cellular level, small molecules able to antagonize the direct PrPC binding to RPSA and their intracellular trafficking. These findings are very crucial considering that the main function of RPSA is to modulate key events in the metastasis cascade. Elucidation of the role played by PrPC/RPSA interaction in regulating tumor development, progression and response to treatment, represents a very promising challenge to gain pathogenetic information and discover novel specific biomarkers and/or therapeutic targets to be exploited in clinical settings. This review attempts to convey a detailed description of the complexity surrounding these multifaceted proteins from the perspective of cancer hallmarks, but with a specific focus on the role of their interaction in the control of proliferation, migration and invasion, genome instability and mutation, as well as resistance to cell death controlled by autophagic pathway.

A Soluble PrPC Derivative and Membrane-Anchored PrPC in Extracellular Vesicles Attenuate Innate Immunity by Engaging the NMDA-R/LRP1 Receptor Complex

Nonpathogenic cellular prion protein (PrPC) demonstrates anti-inflammatory activity; however, the responsible mechanisms are incompletely defined. PrPC exists as a GPI-anchored membrane protein in diverse cells; however, PrPC may be released from cells by ADAM proteases or when packaged into extracellular vesicles (EVs). In this study, we show that a soluble derivative of PrPC (S-PrP) counteracts inflammatory responses triggered by pattern recognition receptors in macrophages, including TLR2, TLR4, TLR7, TLR9, NOD1, and NOD2. S-PrP also significantly attenuates the toxicity of LPS in mice. The response of macrophages to S-PrP is mediated by a receptor assembly that includes the N-methyl-d-aspartate receptor (NMDA-R) and low-density lipoprotein receptor-related protein-1 (LRP1). PrPC was identified in EVs isolated from human plasma. These EVs replicated the activity of S-PrP, inhibiting cytokine expression and IκBα phosphorylation in LPS-treated macrophages. The effects of plasma EVs on LPS-treated macrophages were blocked by PrPC-specific Ab, by antagonists of LRP1 and the NMDA-R, by deleting Lrp1 in macrophages, and by inhibiting Src family kinases. Phosphatidylinositol-specific phospholipase C dissociated the LPS-regulatory activity from EVs, rendering the EVs inactive as LPS inhibitors. The LPS-regulatory activity that was lost from phosphatidylinositol-specific phospholipase C-treated EVs was recovered in solution. Collectively, these results demonstrate that GPI-anchored PrPC is the essential EV component required for the observed immune regulatory activity of human plasma EVs. S-PrP and EV-associated PrPC regulate innate immunity by engaging the NMDA-R/LRP1 receptor system in macrophages. The scope of pattern recognition receptors antagonized by S-PrP suggests that released forms of PrPC may have broad anti-inflammatory activity.

The protease-sensitive N-terminal polybasic region of prion protein modulates its conversion to the pathogenic prion conformer

Conversion of normal prion protein (PrP^C) to the pathogenic PrP^Sc conformer is central to prion diseases such as Creutzfeldt-Jakob disease and scrapie; however, the detailed mechanism of this conversion remains obscure. To investigate how the N-terminal polybasic region of PrP (NPR) influences the PrP^C-to-PrP^Sc conversion, we analyzed two PrP mutants: ΔN6 (deletion of all six amino acids in NPR) and Met4-1 (replacement of four positively charged amino acids in NPR with methionine). We found that ΔN6 and Met4-1 differentially impacted the binding of recombinant PrP (recPrP) to the negatively charged phospholipid 1-palmitoyl-2-oleoylphosphatidylglycerol, a nonprotein cofactor that facilitates PrP conversion. Both mutant recPrPs were able to form recombinant prion (recPrP^Sc) in vitro, but the convertibility was greatly reduced, with ΔN6 displaying the lowest convertibility. Prion infection assays in mammalian RK13 cells expressing WT or NPR-mutant PrPs confirmed these differences in convertibility, indicating that the NPR affects the conversion of both bacterially expressed recPrP and post-translationally modified PrP in eukaryotic cells. We also found that both WT and mutant recPrP^Sc conformers caused prion disease in WT mice with a 100% attack rate, but the incubation times and neuropathological changes caused by two recPrP^Sc mutants were significantly different from each other and from that of WT recPrP^Sc. Together, our results support that the NPR greatly influences PrP^C-to-PrP^Sc conversion, but it is not essential for the generation of PrP^Sc. Moreover, the significant differences between ΔN6 and Met4-1 suggest that not only charge but also the identity of amino acids in NPR is important to PrP conversion.

Potential for transmission of sporadic Creutzfeldt-Jakob disease through peripheral routes

Five sporadic Creutzfeldt-Jakob disease (CJD) strains have been identified to date, based on differences in clinicopathological features of the patients, the biochemical properties of abnormal prion proteins, and transmission properties. Recent advances in our knowledge about iatrogenic transmission of sporadic CJD have raised the possibility that the infectivity of sporadic CJD strains through peripheral routes is different from that of intracranial infection. To test this possibility, here we assessed systematically the infectivity of sporadic CJD strains through the peripheral route for the first time using a mouse model expressing human prion protein. Although the infectivity of the V2 and M1 sporadic CJD strains is almost the same in intracerebral transmission studies, the V2 strain infected more efficiently than the M1 strain through the peripheral route. The other sporadic CJD strains examined lacked infectivity. Of note, both the V2 and M1 strains showed preference for mice with the valine homozygosity at the PRNP polymorphic codon. These results indicate that the V2 strain is the most infectious sporadic CJD strain for infection through peripheral routes. In addition, these findings raise the possibility that individuals with the valine homozygosity at the PRNP polymorphic codon might have higher risks of infection through peripheral routes compared with the methionine homozygotes. Thus, preventive measures against the transmission of the V2 sporadic CJD strain will be important for the eradication of iatrogenic CJD transmission through peripheral routes.

Altered cellular localisation and expression, together with unconventional protein trafficking, of prion protein, PrPC, in type 1 diabetes

AIMS/HYPOTHESIS

Normal cellular prion protein (PrP^C) is a conserved mammalian glycoprotein found on the outer plasma membrane leaflet through a glycophosphatidylinositol anchor. Although PrP^C is expressed by a wide range of tissues throughout the body, the complete repertoire of its functions has not been fully determined. The misfolded pathogenic isoform PrP^Sc (the scrapie form of PrP) is a causative agent of neurodegenerative prion diseases. The aim of this study is to evaluate PrP^C localisation, expression and trafficking in pancreases from organ donors with and without type 1 diabetes and to infer PrP^C function through studies on interacting protein partners.

METHODS

In order to evaluate localisation and trafficking of PrP^C in the human pancreas, 12 non-diabetic, 12 type 1 diabetic and 12 autoantibody-positive organ donor tissue samples were analysed using immunofluorescence analysis. Furthermore, total RNA was isolated from 29 non-diabetic, 29 type 1 diabetic and 24 autoantibody-positive donors to estimate PrP^C expression in the human pancreas. Additionally, we performed PrP^C-specific immunoblot analysis on total pancreatic protein from non-diabetic and type 1 diabetic organ donors to test whether changes in PrP^C mRNA levels leads to a concomitant increase in PrP^C protein levels in human pancreases.

RESULTS

In non-diabetic and type 1 diabetic pancreases (the latter displaying both insulin-positive [INS(+)] and -negative [INS(-)] islets), we found PrP^C in islets co-registering with beta cells in all INS(+) islets and, strikingly, unexpected activation of PrP^C in alpha cells within diabetic INS(-) islets. We found PrP^C localised to the plasma membrane and endoplasmic reticulum (ER) but not the Golgi, defining two cellular pools and an unconventional protein trafficking mechanism bypassing the Golgi. We demonstrate PrP^C co-registration with established protein partners, neural cell adhesion molecule 1 (NCAM1) and stress-inducible phosphoprotein 1 (STI1; encoded by STIP1) on the plasma membrane and ER, respectively, linking PrP^C function with cyto-protection, signalling, differentiation and morphogenesis. We demonstrate that both PRNP (encoding PrP^C) and STIP1 gene expression are dramatically altered in type 1 diabetic and autoantibody-positive pancreases.

CONCLUSIONS/INTERPRETATION

As the first study to address PrP^C expression in non-diabetic and type 1 diabetic human pancreas, we provide new insights for PrP^C in the pathogenesis of type 1 diabetes. We evaluated the cell-type specific expression of PrP^C in the human pancreas and discovered possible connections with potential interacting proteins that we speculate might address mechanisms relevant to the role of PrP^C in the human pancreas.

Direct interaction of HIV gp120 with neuronal CXCR4 and CCR5 receptors induces cofilin-actin rod pathology via a cellular prion protein- and NOX-dependent mechanism

Nearly 50% of individuals with long-term HIV infection are affected by the onset of progressive HIV-associated neurocognitive disorders (HAND). HIV infiltrates the central nervous system (CNS) early during primary infection where it establishes persistent infection in microglia (resident macrophages) and astrocytes that in turn release inflammatory cytokines, small neurotoxic mediators, and viral proteins. While the molecular mechanisms underlying pathology in HAND remain poorly understood, synaptodendritic damage has emerged as a hallmark of HIV infection of the CNS. Here, we report that the HIV viral envelope glycoprotein gp120 induces the formation of aberrant, rod-shaped cofilin-actin inclusions (rods) in cultured mouse hippocampal neurons via a signaling pathway common to other neurodegenerative stimuli including oligomeric, soluble amyloid-β and proinflammatory cytokines. Previous studies showed that synaptic function is impaired preferentially in the distal proximity of rods within dendrites. Our studies demonstrate gp120 binding to either chemokine co-receptor CCR5 or CXCR4 is capable of inducing rod formation, and signaling through this pathway requires active NADPH oxidase presumably through the formation of superoxide (O2-) and the expression of cellular prion protein (PrPC). These findings link gp120-mediated oxidative stress to the generation of rods, which may underlie early synaptic dysfunction observed in HAND.

Neurometals in the Pathogenesis of Prion Diseases

Prion diseases are progressive and transmissive neurodegenerative diseases. The conformational conversion of normal cellular prion protein (PrP^C) into abnormal pathogenic prion protein (PrP^Sc) is critical for its infection and pathogenesis. PrP^C possesses the ability to bind to various neurometals, including copper, zinc, iron, and manganese. Moreover, increasing evidence suggests that PrP^C plays essential roles in the maintenance of homeostasis of these neurometals in the synapse. In addition, trace metals are critical determinants of the conformational change and toxicity of PrP^C. Here, we review our studies and other new findings that inform the current understanding of the links between trace elements and physiological functions of PrP^C and the neurotoxicity of PrP^Sc.

Strain-Dependent Prion Infection in Mice Expressing Prion Protein with Deletion of Central Residues 91-106

Conformational conversion of the cellular prion protein, PrP^C, into the abnormally folded isoform, PrP^Sc, is a key pathogenic event in prion diseases. However, the exact conversion mechanism remains largely unknown. Transgenic mice expressing PrP with a deletion of the central residues 91–106 were generated in the absence of endogenous PrP^C, designated Tg(PrP∆91–106)/Prnp^0/0 mice and intracerebrally inoculated with various prions. Tg(PrP∆91–106)/Prnp^0/0 mice were resistant to RML, 22L and FK-1 prions, neither producing PrP^Sc∆91–106 or prions in the brain nor developing disease after inoculation. However, they remained marginally susceptible to bovine spongiform encephalopathy (BSE) prions, developing disease after elongated incubation times and accumulating PrP^Sc∆91–106 and prions in the brain after inoculation with BSE prions. Recombinant PrP∆91-104 converted into PrP^Sc∆91–104 after incubation with BSE-PrP^Sc-prions but not with RML- and 22L–PrP^Sc-prions, in a protein misfolding cyclic amplification assay. However, digitonin and heparin stimulated the conversion of PrP∆91–104 into PrP^Sc∆91–104 even after incubation with RML- and 22L-PrP^Sc-prions. These results suggest that residues 91–106 or 91–104 of PrP^C are crucially involved in prion pathogenesis in a strain-dependent manner and may play a similar role to digitonin and heparin in the conversion of PrP^C into PrP^Sc.

Transgenic Overexpression of the Disordered Prion Protein N1 Fragment in Mice Does Not Protect Against Neurodegenerative Diseases Due to Impaired ER Translocation

The structurally disordered N-terminal half of the prion protein (PrP^C) is constitutively released into the extracellular space by an endogenous proteolytic cleavage event. Once liberated, this N1 fragment acts neuroprotective in ischemic conditions and interferes with toxic peptides associated with neurodegenerative diseases, such as amyloid-beta (Aβ) in Alzheimer’s disease. Since analog protective effects of N1 in prion diseases, such as Creutzfeldt-Jakob disease, have not been studied, and given that the protease releasing N1 has not been identified to date, we have generated and characterized transgenic mice overexpressing N1 (TgN1). Upon intracerebral inoculation of TgN1 mice with prions, no protective effects were observed at the levels of survival, clinical course, neuropathological, or molecular assessment. Likewise, primary neurons of these mice did not show protection against Aβ toxicity. Our biochemical and morphological analyses revealed that this lack of protective effects is seemingly due to an impaired ER translocation of the disordered N1 resulting in its cytosolic retention with an uncleaved signal peptide. Thus, TgN1 mice represent the first animal model to prove the inefficient ER translocation of intrinsically disordered domains (IDD). In contrast to earlier studies, our data challenge roles of cytoplasmic N1 as a cell penetrating peptide or as a potent “anti-prion” agent. Lastly, our study highlights both the importance of structured domains in the nascent chain for proteins to be translocated and aspects to be considered when devising novel N1-based therapeutic approaches against neurodegenerative diseases.

In vitro detection of haematogenous prions in white-tailed deer orally dosed with low concentrations of chronic wasting disease

Infectivity associated with prion disease has been demonstrated in blood throughout the course of disease, yet the ability to detect blood-borne prions by in vitro methods remains challenging. We capitalized on longitudinal pathogenesis studies of chronic wasting disease (CWD) conducted in the native host to examine haematogenous prion load by real-time quaking-induced conversion (RT-QuIC) and protein misfolding cyclic amplification. Our study demonstrated in vitro detection of amyloid seeding activity (prions) in buffy-coat cells harvested from deer orally dosed with low concentrations of CWD positive (+) brain (1 gr and 300 ng) or saliva (300 ng RT-QuIC equivalent). These findings make possible the longitudinal assessment of prion disease and deeper investigation of the role haematogenous prions play in prion pathogenesis.

Role of donor genotype in RT-QuIC seeding activity of chronic wasting disease prions using human and bank vole substrates

Chronic wasting disease is a transmissible spongiform encephalopathy of cervids. This fatal neurodegenerative disease is caused by misfolding of the cellular prion protein (PrPC) to pathogenic conformers (PrPSc), and the pathogenic forms accumulate in the brain and other tissues. Real-time Quaking Induced Conversion (RT-QuIC) can be used for the detection of prions and for prion strain discrimination in a variety of biological tissues from humans and animals. In this study, we evaluated how either PrPSc from cervids of different genotypes or PrPSc from different sources of CWD influence the fibril formation of recombinant bank vole (BV) or human prion proteins using RT-QuIC. We found that reaction mixtures seeded with PrPSc from different genotypes of white-tailed deer or reindeer brains have similar conversion efficiency with both substrates. Also, we observed similar results when assays were seeded with different sources of CWD. Thus, we conclude that the genotypes of all sources of CWD used in this study do not influence the level of conversion of PrPC to PrPSc.

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

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

Mutations in Prion Protein Gene: Pathogenic Mechanisms in C-Terminal vs. N-Terminal Domain, a Review

Inherited mutations in the Prion protein (PrP), encoded by the PRNP gene, have been associated with autosomal dominant neurodegenerative disorders, such as Creutzfeldt-Jacob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome (GSS), and Fatal Familial Insomnia (FFI). Notably, PRNP mutations have also been described in clinical pictures resembling other neurodegenerative diseases, such as frontotemporal dementia. Regarding the pathogenesis, it has been observed that these point mutations are located in the C-terminal region of the PRNP gene and, currently, the potential significance of the N-terminal domain has largely been underestimated. The purpose of this report is to review and provide current insights into the pathogenic mechanisms of PRNP mutations, emphasizing the differences between the C- and N-terminal regions and focusing, in particular, on the lesser-known flexible N-terminal, for which recent biophysical evidence has revealed a physical interaction with the globular C-terminal domain of the cellular prion protein (PrPC).

PrPC knockdown by liposome-siRNA-peptide complexes (LSPCs) prolongs survival and normal behavior of prion-infected mice immunotolerant to treatment

Prion diseases are members of neurodegenerative protein misfolding diseases (NPMDs) that include Alzheimer's, Parkinson's and Huntington diseases, amyotrophic lateral sclerosis, tauopathies, traumatic brain injuries, and chronic traumatic encephalopathies. No known therapeutics extend survival or improve quality of life of humans afflicted with prion disease. We and others developed a new approach to NPMD therapy based on reducing the amount of the normal, host-encoded protein available as substrate for misfolding into pathologic forms, using RNA interference, a catabolic pathway that decreases levels of mRNA encoding a particular protein. We developed a therapeutic delivery system consisting of small interfering RNA (siRNA) complexed to liposomes and addressed to the central nervous system using a targeting peptide derived from rabies virus glycoprotein. These liposome-siRNA-peptide complexes (LSPCs) cross the blood-brain barrier and deliver PrP siRNA to neuronal cells to decrease expression of the normal cellular prion protein, PrPC, which acts as a substrate for prion replication. Here we show that LSPCs can extend survival and improve behavior of prion-infected mice that remain immunotolerant to treatment. LSPC treatment may be a viable therapy for prion and other NPMDs that can improve the quality of life of patients at terminal disease stages.

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.

A Quick Method to Evaluate the Effect of the Amino Acid Sequence in the Misfolding Proneness of the Prion Protein

Prion diseases or transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative diseases where the misfolding of the prion protein (PrP) is a crucial event. Based on studies in TSE-affected humans and the generation of transgenic mouse models overexpressing different mutated versions of the PrP, we conclude that both wild-type and mutated PrPs exhibit differential propensity to misfold in vivo. Here, we describe a new method in vitro to assess and quantify the PrP misfolding phenomenon in order to better understand the molecular mechanisms involved in this process.

Prion protein is essential for the RE1 silencing transcription factor (REST)-dependent developmental switch in synaptic NMDA receptors

It is important that the correct amounts of GluN2 subunits are maintained, as they determine NMDAR functional properties, which are crucial to neuronal communication, synaptogenesis and cognitive function. The transcriptional repressor RE1 silencing transcription factor (REST) is critical for the postnatal developmental switch in NMDARs. However, the mechanisms triggering REST and the link between NMDARs and REST are unclear. Here we show a new physiological essential role for cellular prion protein (PrPC) in REST-dependent homeostasis and the developmental switch of NMDARs. REST and REST-associated proteins were overactivated in the hippocampi of Prnp knockout mice (Prnp 0/0 ) compared with wild-type Prnp (Prnp +/+ ) mice. This coincided with the disruption of the normal developmental switch from GluN2B-to-GluN2A in vivo. PrPC co-located with REST under physiological environments and mediated the translocation of REST in conditioners of NMDARs in vitro in Prnp +/+ hippocampal neurons. Regardless of whether REST was knocked down or overexpressed, deletion of PrPC not only disrupted REST-mediated distribution of mitochondria, but also prevented REST-regulated expression of GluN2B and GluN2A in Prnp 0/0 . Importantly, these effects were rescued after overexpression of full-length PrPC through restoration of NMDAR2 subunits and their distributions in dendritic processes in Prnp 0/0 . Consistently, knockdown of PrPC in Prnp +/+ had a similar effect on Prnp 0/0 . Furthermore, PrPC colocalized with both GluN2B and GluN2A in Prnp +/+ . For the first time, we demonstrate that PrPC is essential for REST-regulated NMDARs. Confirming the regulation of NMDAR-modulating mechanisms could provide novel therapeutic targets against dysfunctions of glutamatergic transmission in the nervous system.

The cellular prion protein promotes olfactory sensory neuron survival and axon targeting during adult neurogenesis

The cellular prion protein (PrP^C) has been associated with diverse biological processes including cell signaling, neurogenesis, and neuroprotection, but its physiological function(s) remain ambiguous. Here we determine the role of PrP^C in adult neurogenesis using the olfactory system model in transgenic mice. Olfactory sensory neurons (OSNs) within the olfactory sensory epithelium (OSE) undergo neurogenesis, integration, and turnover even into adulthood. The neurogenic processes of proliferation, differentiation/maturation, and axon targeting were evaluated in wild type, PrP-overexpressing, and PrP-null transgenic mice. Our results indicate that PrP^C plays a role in maintaining mature OSNs within the epithelium: overexpression of PrP^C resulted in greater survival of mitotically active cells within the OSE, whereas absence of prion protein resulted in fewer cells being maintained over time. These results are supported by both quantitative PCR analysis of gene expression and protein analysis characteristic of OSN differentiation. Finally, evaluation of axon migration determined that OSN axon targeting in the olfactory bulb is PrP^C dose-dependent. Together, these findings provide new mechanistic insight into the neuroprotective role for PrP^C in adult OSE neurogenesis, whereby more mature neurons are stably maintained in animals expressing PrP^C.

Asparagine and glutamine ladders promote cross-species prion conversion

Prion transmission between species is governed in part by primary sequence similarity between the infectious prion aggregate, PrP^Sc, and the cellular prion protein of the host, PrP^C A puzzling feature of prion formation is that certain PrP^C sequences, such as that of bank vole, can be converted by a remarkably broad array of different mammalian prions, whereas others, such as rabbit, show robust resistance to cross-species prion conversion. To examine the structural determinants that confer susceptibility or resistance to prion conversion, we systematically tested over 40 PrP^C variants of susceptible and resistant PrP^C sequences in a prion conversion assay. Five key residue positions markedly impacted prion conversion, four of which were in steric zipper segments where side chains from amino acids tightly interdigitate in a dry interface. Strikingly, all five residue substitutions modulating prion conversion involved the gain or loss of an asparagine or glutamine residue. For two of the four positions, Asn and Gln residues were not interchangeable, revealing a strict requirement for either an Asn or Gln residue. Bank voles have a high number of Asn and Gln residues and a high Asn:Gln ratio. These findings suggest that a high number of Asn and Gln residues at specific positions may stabilize β-sheets and lower the energy barrier for cross-species prion transmission, potentially because of hydrogen bond networks from side chain amides forming extended Asn/Gln ladders. These data also suggest that multiple PrP^C segments containing Asn/Gln residues may act in concert along a replicative interface to promote prion conversion.

Inhibition of group-I metabotropic glutamate receptors protects against prion toxicity

Prion infections cause inexorable, progressive neurological dysfunction and neurodegeneration. Expression of the cellular prion protein PrPC is required for toxicity, suggesting the existence of deleterious PrPC-dependent signaling cascades. Because group-I metabotropic glutamate receptors (mGluR1 and mGluR5) can form complexes with the cellular prion protein (PrPC), we investigated the impact of mGluR1 and mGluR5 inhibition on prion toxicity ex vivo and in vivo. We found that pharmacological inhibition of mGluR1 and mGluR5 antagonized dose-dependently the neurotoxicity triggered by prion infection and by prion-mimetic anti-PrPC antibodies in organotypic brain slices. Prion-mimetic antibodies increased mGluR5 clustering around dendritic spines, mimicking the toxicity of Aβ oligomers. Oral treatment with the mGluR5 inhibitor, MPEP, delayed the onset of motor deficits and moderately prolonged survival of prion-infected mice. Although group-I mGluR inhibition was not curative, these results suggest that it may alleviate the neurological dysfunctions induced by prion diseases.

Common therapeutic strategies for prion and Alzheimer's diseases

A number of unexpected pathophysiological connections linking different neurodegenerative diseases have emerged over the past decade. An example is provided by prion and Alzheimer's diseases. Despite being distinct pathologies, these disorders share several neurotoxic mechanisms, including accumulation of misfolded protein isoforms, stress of the protein synthesis machinery, and activation of a neurotoxic signaling mediated by the cellular prion protein. Here, in addition to reviewing these mechanisms, we will discuss the potential therapeutic interventions for prion and Alzheimer's diseases that are arising from the comprehension of their common neurodegenerative pathways.

Loss of prion protein induces a primed state of type I interferon-responsive genes

The cellular prion protein (PrPC) has been extensively studied because of its pivotal role in prion diseases; however, its functions remain incompletely understood. A unique line of goats has been identified that carries a nonsense mutation that abolishes synthesis of PrPC. In these animals, the PrP-encoding mRNA is rapidly degraded. Goats without PrPC are valuable in re-addressing loss-of-function phenotypes observed in Prnp knockout mice. As PrPC has been ascribed various roles in immune cells, we analyzed transcriptomic responses to loss of PrPC in peripheral blood mononuclear cells (PBMCs) from normal goat kids (n = 8, PRNP+/+) and goat kids without PrPC (n = 8, PRNPTer/Ter) by mRNA sequencing. PBMCs normally express moderate levels of PrPC. The vast majority of genes were similarly expressed in the two groups. However, a curated list of 86 differentially expressed genes delineated the two genotypes. About 70% of these were classified as interferon-responsive genes. In goats without PrPC, the majority of type I interferon-responsive genes were in a primed, modestly upregulated state, with fold changes ranging from 1.4 to 3.7. Among these were ISG15, DDX58 (RIG-1), MX1, MX2, OAS1, OAS2 and DRAM1, all of which have important roles in pathogen defense, cell proliferation, apoptosis, immunomodulation and DNA damage response. Our data suggest that PrPC contributes to the fine-tuning of resting state PBMCs expression level of type I interferon-responsive genes. The molecular mechanism by which this is achieved will be an important topic for further research into PrPC physiology.

Involvement of Cellular Prion Protein in α-Synuclein Transport in Neurons

The cellular prion protein, encoded by the gene Prnp, has been reported to be a receptor of β-amyloid. Their interaction is mandatory for neurotoxic effects of β-amyloid oligomers. In this study, we aimed to explore whether the cellular prion protein participates in the spreading of α-synuclein. Results demonstrate that Prnp expression is not mandatory for α-synuclein spreading. However, although the pathological spreading of α-synuclein can take place in the absence of Prnp, α-synuclein expanded faster in PrP^C-overexpressing mice. In addition, α-synuclein binds strongly on PrP^C-expressing cells, suggesting a role in modulating the effect of α-synuclein fibrils.

The Effects of Ca2+ Concentration and E200K Mutation on the Aggregation Propensity of PrPC: A Computational Study

The propensity of cellular prion protein to aggregation is reputed essential for the initiation of the amyloid cascade that ultimately lead to the accumulation of neurotoxic aggregates. In this paper, we extended and applied an already reported computational workflow [Proteins 2015; 83: 1751–1765] to elucidate in details the aggregation propensity of PrP protein systems including wild type, wild type treated at different [Ca²⁺] and E200K mutant. The application of the computational procedure to two segments of PrP^C, i.e. 125–228 and 120–231, allowed to emphasize how the inclusion of complete C-terminus and last portion (120–126) of the neurotoxic segment 106–126 may be crucial to unveil significant and unexpected interaction properties. Indeed, the anchoring of N-terminus on H2 domain detected in the wild type resulted to be disrupted upon either E200K mutation or Ca²⁺ binding, and to unbury hydrophobic spots on the PrP^C surface. A peculiar dinuclear Ca²⁺ binding motif formed by the C-terminus and the S2-H2 loop was detected for [Ca²⁺] > 5 mM and showed similarities with binding motifs retraced in other protein systems, thus suggesting a possible functional meaning for its formation. Therefore, we potentiated the computational procedure by including a tool that clusterize the minima of molecular interaction fields of a proteinand delimit the regions of space with higher hydrophobic or higher hydrophilic character, hence, more likely involved in the self-assembly process. Plausible models for the self-assembly of either the E200K mutated or Ca²⁺-bound PrP^C were sketched and discussed. The present investigation provides for structure-based information and new prompts that may represent a starting point for future experimental or computational works on the PrP^C aggregation.

Elevated PrPC Expression Predisposes to Increased HSV-1 Pathogenicity

PrPC is a ubiquitously expressed glycophos-phatidylinositol-linked cell-surface glycoprotein found primarily in neural tissue. Although its normal function has not been established, there is evidence suggesting that PrPC is involved in cell signalling and cellular homeostasis. This suggests that variation in neuronal expression levels of this protein contributes towards pathogenicity induced by neurotropic agents. We have investigated the pathological response to infection with herpes simplex virus type-1 (HSV-1) in strains of mice that express different levels of PrPC. Prnp−/− mice fail to express PrPC due to an interruption in the open reading frame of the Prnp gene, whilst tg19 and tga20 mice express approximately 5 and 10 times more PrPC protein, respectively, than wild-type animals. Mice that express normal or increased levels of PrPC protein were more susceptible to acute HSV-1 infection than Prnp−/− mice. Following ear pinna inoculation with HSV-1 SC16, the order of susceptibility was tga20>tg19>wild-type> Prnp−/−. This trend was reversed when latent virus was assessed. Prnp−/− mice expressed significantly higher levels of latency-associated transcript-positive neurons in various tissues when compared with wild-type, tg19 and tga20 mice. Collectively, our data show that acute HSV-1 replication proceeds more efficiently in neuronal tissue that expresses PrPC protein and lends support to the view that this protein is involved in regulation of neurotropic viral pathogenesis. This suggests that interference of PrPC expression, or possible biochemical pathways associated with its function, may serve as an effective means of limiting the pathogenesis of acute HSV-1 infection.

The Distribution of Prion Protein Allotypes Differs Between Sporadic and Iatrogenic Creutzfeldt-Jakob Disease Patients

Sporadic Creutzfeldt-Jakob disease (sCJD) is the most prevalent of the human prion diseases, which are fatal and transmissible neurodegenerative diseases caused by the infectious prion protein (PrP^Sc). The origin of sCJD is unknown, although the initiating event is thought to be the stochastic misfolding of endogenous prion protein (PrP^C) into infectious PrP^Sc. By contrast, human growth hormone-associated cases of iatrogenic CJD (iCJD) in the United Kingdom (UK) are associated with exposure to an exogenous source of PrP^Sc. In both forms of CJD, heterozygosity at residue 129 for methionine (M) or valine (V) in the prion protein gene may affect disease phenotype, onset and progression. However, the relative contribution of each PrP^C allotype to PrP^Sc in heterozygous cases of CJD is unknown. Using mass spectrometry, we determined that the relative abundance of PrP^Sc with M or V at residue 129 in brain specimens from MV cases of sCJD was highly variable. This result is consistent with PrP^C containing an M or V at residue 129 having a similar propensity to misfold into PrP^Sc thus causing sCJD. By contrast, PrP^Sc with V at residue 129 predominated in the majority of the UK human growth hormone associated iCJD cases, consistent with exposure to infectious PrP^Sc containing V at residue 129. In both types of CJD, the PrP^Sc allotype ratio had no correlation with CJD type, age at clinical onset, or disease duration. Therefore, factors other than PrP^Sc allotype abundance must influence the clinical progression and phenotype of heterozygous cases of CJD.

In Creutzfeldt-Jakob disease (CJD), heterozygosity at residue 129 for methionine or valine in normal prion protein may affect disease phenotype, onset and progression. However, the relative contribution of each prion protein allotype to the infectious, disease associated form of prion protein (PrP^Sc) is unknown. Here we report the novel observation that in heterozygous cases of sporadic CJD the PrP^Sc allotype ratio is highly variable. This case-by-case variability is consistent with the origin of sporadic CJD being the spontaneous, but random, misfolding of either host prion protein allotype into infectious PrP^Sc. By contrast, in heterozygous cases of iatrogenic CJD in the United Kingdom resulting from exposure to contaminated human growth hormone, the PrP^Sc allotype ratio is much more homogeneous and consistent with exposure to infectious PrP^Sc containing valine at residue 129. Surprisingly, the PrP^Sc allotype ratio did not correlate with disease onset or duration in either disease type. Thus, factors other than PrP^Sc allotype ratio likely influence the clinical progression of heterozygous cases of CJD. Moreover, our results suggest that the ratio of methionine to valine in PrP^Sc may be a means of determining the origin of prion infection.

Neuroprotective effect of cellular prion protein (PrPC) is related with activation of alpha7 nicotinic acetylcholine receptor (α7nAchR)-mediated autophagy flux

Activation of the alpha7 nicotinic acetylcholine receptor (α7nAchR) is regulated by prion protein (PrPC) expression and has a neuroprotective effect by modulating autophagic flux. In this study, we hypothesized that PrPC may regulate α7nAchR activation and that may prevent prion-related neurodegenerative diseases by regulating autophagic flux. PrP(106-126) treatment decreased α7nAchR expression and activation of autophagic flux. In addition, the α7nAchR activator PNU-282987 enhanced autophagic flux and protected neuron cells against PrP(106-126)-induced apoptosis. However, activation of autophagy and the protective effects of PNU-282987 were inhibited in PrPC knockout hippocampal neuron cells. In addition, PrPC knockout hippocampal neuron cells showed decreased α7nAchR expression levels. Adenoviral overexpression of PrPC in PrPC knockout hippocampal neuron cells resulted in activation of autophagic flux and inhibition of prion peptide-mediated cell death via α7nAchR activation. This is the first report demonstrating that activation of α7nAchR-mediated autophagic flux is regulated by PrPC, and that activation of α7nAchR regulated by PrPC expression may play a pivotal role in protection of neuron cells against prion peptide-induced neuron cell death by autophagy. These results suggest that α7nAchR-mediated autophagic flux may be involved in the pathogenesis of prion-related diseases and may be a therapeutic target for prion-related neurodegenerative diseases.

Prion protein prevents heavy metals overloading of cells and thus protects them against their toxicity

Physiological function of a prion protein (PrP) is not known yet. Regarding the relation of PrP to heavy metals it is known that PrP is able to bind divalent ions of copper, zinc, manganese and nickel through its octarepeat region. It has been hypothesized but not yet confirmed that PrP could play a role in copper metabolism. In this study, cells expressing human full-length PrP (HuPrP1) and PrP-knockout (PrP0/0/1) cells were incubated with various concentrations of copper, zinc, manganese and nickel for 4 days and then were assayed for intracellular content of these metals and cell viability. The results showed that HuPrP1 cells accumulated less heavy metals than PrP0/0/1 cells when concentrations of heavy metals exceeded physiological level. In conclusion, HuPrP1 cells are more resistant to chronic overload with copper, manganese, zinc or nickel than PrP0/0/1 cells. The resistance to metals overload is caused solely by the presence of PrP, since HuPrP1 and PrP0/0/1 cells differ only in the expression of PrP. These results indicate that one of the functions of PrP can be the modulation of trace heavy metal concentrations in cells and protection of cells against heavy metals overload and subsequent oxidative stress.

Transgenic Rabbits Expressing Ovine PrP Are Susceptible to Scrapie

Transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative diseases affecting a wide range of mammalian species. They are caused by prions, a proteinaceous pathogen essentially composed of PrPSc, an abnormal isoform of the host encoded cellular prion protein PrPC. Constrained steric interactions between PrPSc and PrPC are thought to provide prions with species specificity, and to control cross-species transmission into other host populations, including humans. Transgenetic expression of foreign PrP genes has been successfully and widely used to overcome the recognized resistance of mouse to foreign TSE sources. Rabbit is one of the species that exhibit a pronounced resistance to TSEs. Most attempts to infect experimentally rabbit have failed, except after inoculation with cell-free generated rabbit prions. To gain insights on the molecular determinants of the relative resistance of rabbits to prions, we generated transgenic rabbits expressing the susceptible V136R154Q171 allele of the ovine PRNP gene on a rabbit wild type PRNP New Zealand background and assessed their experimental susceptibility to scrapie prions. All transgenic animals developed a typical TSE 6-8 months after intracerebral inoculation, whereas wild type rabbits remained healthy more than 700 days after inoculation. Despite the endogenous presence of rabbit PrPC, only ovine PrPSc was detectable in the brains of diseased animals. Collectively these data indicate that the low susceptibility of rabbits to prion infection is not enciphered within their non-PrP genetic background.

Amyloid-β and proinflammatory cytokines utilize a prion protein-dependent pathway to activate NADPH oxidase and induce cofilin-actin rods in hippocampal neurons

Neurites of neurons under acute or chronic stress form bundles of filaments (rods) containing 1∶1 cofilin∶actin, which impair transport and synaptic function. Rods contain disulfide cross-linked cofilin and are induced by treatments resulting in oxidative stress. Rods form rapidly (5-30 min) in >80% of cultured hippocampal or cortical neurons treated with excitotoxic levels of glutamate or energy depleted (hypoxia/ischemia or mitochondrial inhibitors). In contrast, slow rod formation (50% of maximum response in ∼6 h) occurs in a subpopulation (∼20%) of hippocampal neurons upon exposure to soluble human amyloid-β dimer/trimer (Aβd/t) at subnanomolar concentrations. Here we show that proinflammatory cytokines (TNFα, IL-1β, IL-6) also induce rods at the same rate and within the same neuronal population as Aβd/t. Neurons from prion (PrP(C))-null mice form rods in response to glutamate or antimycin A, but not in response to proinflammatory cytokines or Aβd/t. Two pathways inducing rod formation were confirmed by demonstrating that NADPH-oxidase (NOX) activity is required for prion-dependent rod formation, but not for rods induced by glutamate or energy depletion. Surprisingly, overexpression of PrP(C) is by itself sufficient to induce rods in over 40% of hippocampal neurons through the NOX-dependent pathway. Persistence of PrP(C)-dependent rods requires the continuous activity of NOX. Removing inducers or inhibiting NOX activity in cells containing PrP(C)-dependent rods causes rod disappearance with a half-life of about 36 min. Cofilin-actin rods provide a mechanism for synapse loss bridging the amyloid and cytokine hypotheses for Alzheimer disease, and may explain how functionally diverse Aβ-binding membrane proteins induce synaptic dysfunction.

β-hairpin-mediated formation of structurally distinct multimers of neurotoxic prion peptides

Protein misfolding disorders are associated with conformational changes in specific proteins, leading to the formation of potentially neurotoxic amyloid fibrils. During pathogenesis of prion disease, the prion protein misfolds into β-sheet rich, protease-resistant isoforms. A key, hydrophobic domain within the prion protein, comprising residues 109-122, recapitulates many properties of the full protein, such as helix-to-sheet structural transition, formation of fibrils and cytotoxicity of the misfolded isoform. Using all-atom, molecular simulations, it is demonstrated that the monomeric 109-122 peptide has a preference for α-helical conformations, but that this peptide can also form β-hairpin structures resulting from turns around specific glycine residues of the peptide. Altering a single amino acid within the 109-122 peptide (A117V, associated with familial prion disease) increases the prevalence of β-hairpin formation and these observations are replicated in a longer peptide, comprising residues 106-126. Multi-molecule simulations of aggregation yield different assemblies of peptide molecules composed of conformationally-distinct monomer units. Small molecular assemblies, consistent with oligomers, comprise peptide monomers in a β-hairpin-like conformation and in many simulations appear to exist only transiently. Conversely, larger assemblies are comprised of extended peptides in predominately antiparallel β-sheets and are stable relative to the length of the simulations. These larger assemblies are consistent with amyloid fibrils, show cross-β structure and can form through elongation of monomer units within pre-existing oligomers. In some simulations, assemblies containing both β-hairpin and linear peptides are evident. Thus, in this work oligomers are on pathway to fibril formation and a preference for β-hairpin structure should enhance oligomer formation whilst inhibiting maturation into fibrils. These simulations provide an important new atomic-level model for the formation of oligomers and fibrils of the prion protein and suggest that stabilization of β-hairpin structure may enhance cellular toxicity by altering the balance between oligomeric and fibrillar protein assemblies.

Differential responses of neuronal and spermatogenic cells to the doppel cytotoxicity

Although structurally and biochemically similar to the cellular prion (PrP(C)), doppel (Dpl) is unique in its biological functions. There are no reports about any neurodegenerative diseases induced by Dpl. However the artificial expression of Dpl in the PrP-deficient mouse brain causes ataxia with Purkinje cell death. Abundant Dpl proteins have been found in testis and depletion of the Dpl gene (Prnd) causes male infertility. Therefore, we hypothesize different regulations of Prnd in the nerve and male productive systems. In this study, by electrophoretic mobility shift assays we have determined that two different sets of transcription factors are involved in regulation of the Prnd promoter in mouse neuronal N2a and GC-1 spermatogenic (spg) cells, i.e., upstream stimulatory factors (USF) in both cells, Brn-3 and Sp1 in GC-1 spg cells, and Sp3 in N2a cells, leading to the expression of Dpl in GC-1 spg but not in N2a cells. We have further defined that, in N2a cells, Dpl induces oxidative stress and apoptosis, which stimulate ataxia-telangiectasia mutated (ATM)-modulating bindings of transcription factors, p53 and p21, to Prnp promoter, resulting the PrP(C) elevation for counteraction of the Dpl cytotoxicity; in contrast, in GC-1 spg cells, phosphorylation of p21 and N-terminal truncated PrP may play roles in the control of Dpl-induced apoptosis, which may benefit the physiological function of Dpl in the male reproduction system.

Fusion of Doppel to Octapeptide Repeat and N-Terminal Half of Hydrophobic Region of Prion Protein Confers Resistance to Serum Deprivation

Our previous studies have shown an essential role played by the octapeptide repeat region (OR) and the N-terminal half of hydrophobic region (HR) in the anti-apoptotic activity of prion protein (PrP). As PrP-like protein Doppel (Dpl), which structurally resembles an N-terminally truncated PrP, did not show any anti-apoptotic activity, we examined apoptosis of HpL3-4 cells expressing Dpl fused to various lengths of the N-terminal region of PrP to investigate whether the PrP/Dpl fusion proteins retain anti-apoptotic function. HpL3-4 cells expressing Dpl fused to PrP(1-124) with the OR and N-terminal half of HR of PrP showed anti-apoptotic function, whereas Dpl fused to PrP(1-95) with OR did not rescue cells from apoptotic cell death induced by serum deprivation. These results indicate that the OR and N-terminal half of HR of PrP retains anti-apoptotic activity similar to full-length PrP.

Hsp104 as a key modulator of prion-mediated oxidative stress in Saccharomyces cerevisiae

Maintenance of cellular redox homoeostasis forms an important part of the cellular defence mechanism and continued cell viability. Despite extensive studies, the role of the chaperone Hsp104 (heat-shock protein of 102 kDa) in propagation of misfolded protein aggregates in the cell and generation of oxidative stress remains poorly understood. Expression of RNQ1-RFP in Saccharomyces cerevisiae cells led to the generation of the prion form of the protein and increased oxidative stress. In the present study, we show that disruption of Hsp104 in an isogenic yeast strain led to solubilization of RNQ1-RFP. This reduced the oxidative stress generated in the cell. The higher level of oxidative stress in the Hsp104-containing (parental) strain correlated with lower activity of almost all of the intracellular antioxidant enzymes assayed. Surprisingly, this did not correspond with the gene expression analysis data. To compensate for the decrease in protein translation induced by a high level of reactive oxygen species, transcriptional up-regulation takes place. This explains the discrepancy observed between the transcription level and functional enzymatic product. Our results show that in a ΔHsp104 strain, due to lower oxidative stress, no such mismatch is observed, corresponding with higher cell viability. Thus Hsp104 is indirectly responsible for enhancing the oxidative stress in a prion-rich environment.

Gene expression resulting from PrPC ablation and PrPC overexpression in murine and cellular models

The cellular prion protein (PrP(C)) plays a key role in prion diseases when it converts to the pathogenic form scrapie prion protein. Increasing knowledge of its participation in prion infection contrasts with the elusive and controversial data regarding its physiological role probably related to its pleiotropy, cell-specific functions, and cellular-specific milieu. Multiple approaches have been made to the increasing understanding of the molecular mechanisms and cellular functions modulated by PrP(C) at the transcriptomic and proteomic levels. Gene expression analyses have been made in several mouse and cellular models with regulated expression of PrP(C) resulting in PrP(C) ablation or PrP(C) overexpression. These analyses support previous functional data and have yielded clues about new potential functions. However, experiments on animal models have shown moderate and varied results which are difficult to interpret. Moreover, studies in cell cultures correlate little with in vivo counterparts. Yet, both animal and cell models have provided some insights on how to proceed in the future by using more refined methods and selected functional experiments.

The PrP(C) C1 fragment derived from the ovine A136R154R171PRNP allele is highly abundant in sheep brain and inhibits fibrillisation of full-length PrP(C) protein in vitro

Expression of the cellular prion protein (PrP(C)) is crucial for the development of prion diseases. Resistance to prion diseases can result from reduced availability of the prion protein or from amino acid changes in the prion protein sequence. We propose here that increased production of a natural PrP α-cleavage fragment, C1, is also associated with resistance to disease. We show, in brain tissue, that ARR homozygous sheep, associated with resistance to disease, produced PrP(C) comprised of 25% more C1 fragment than PrP(C) from the disease-susceptible ARQ homozygous and highly susceptible VRQ homozygous animals. Only the C1 fragment derived from the ARR allele inhibits in-vitro fibrillisation of other allelic PrP(C) variants. We propose that the increased α-cleavage of ovine ARR PrP(C) contributes to a dominant negative effect of this polymorphism on disease susceptibility. Furthermore, the significant reduction in PrP(C) β-cleavage product C2 in sheep of the ARR/ARR genotype compared to ARQ/ARQ and VRQ/VRQ genotypes, may add to the complexity of genetic determinants of prion disease susceptibility.

CD44-positive cancer stem cells expressing cellular prion protein contribute to metastatic capacity in colorectal cancer

Cancer stem cells are implicated in tumor progression, metastasis, and recurrence, although the exact mechanisms remain poorly understood. Here, we show that the expression of cellular prion protein (PrPc, PRNP) is positively correlated with an increased risk of metastasis in colorectal cancer. PrPc defines a subpopulation of CD44-positive cancer stem cells that contributes to metastatic capacity. PrPc(+)CD44(+) colorectal cancer stem cells displayed high liver metastatic capability, unlike PrPc(-)CD44(+) stem cells, that was inhibited by RNAi-mediated attenuation of PrPc. Notably, administration of PrPc monoclonal antibodies significantly inhibited tumorigenicity and metastasis of colorectal cancer stem cells in mouse models of orthotopic metastasis. PrPc promoted epithelial to mesenchymal transition (EMT) via the ERK2 (MAPK1) pathway, thereby conferring high metastatic capacity. Our findings reveal the function of PrPc in regulating EMT in cancer stem cells, and they identify PrPc as candidate therapeutic target in metastatic colorectal cancer.

Prion protein conversion induced by trivalent iron in vesicular trafficking

Iron dyshomeostasis has been observed in prion diseases; however, little is known regarding the contribution of the oxidation state of iron to prion protein (PrP) conversion. In this study, PrP(C)-deficient HpL3-4 cells were exposed to divalent [Fe(II)] or trivalent [Fe(III)] iron, followed by exogenous recombinant PrP (rPrP) treatment. We then analyzed the accumulation of internalized rPrP and its biochemical properties, including its resistance to both proteinase K (PK) digestion and detergent solubility. Fe(III), but not Fe(II), induced the accumulation of internalized rPrP, which was partially converted to detergent-insoluble and PK-resistant PrP (PrP(res)). The Fe(III)-induced PrP(res) generation required an intact cell structure, and it was hindered by U18666A, an inhibitor of vesicular trafficking, but not by NH4Cl, an inhibitor of endolysosomal acidification. These observations implicated that the Fe(III)-mediated PrP(res) conversion likely occurs during endosomal vesicular trafficking rather than in the acidic environment of lysosomes.

Targeted knock-down of cellular prion protein expression in myelinating Schwann cells does not alter mouse prion pathogenesis

In naturally acquired transmissible spongiform encephalopathies, the pathogenic agents or prions spread from the sites of initial peripheral uptake or replication to the brain where they cause progressive and fatal neurodegeneration. Routing via the peripheral nervous system is considered to be one of the main pathways to the central nervous system. Replication of prions in Schwann cells is viewed as a potentially important mechanism for efficient prion spread along nerves. Here we used a Cre-loxP mouse transgenetic approach to disrupt host-encoded prion protein (PrP(C)) specifically in myelinating Schwann cells. Despite the use of infection routes targeting highly myelinated nerves, there was no alteration in mouse prion pathogenesis, suggesting that conversion-dependent, centripetal spread of prions does not crucially rely on PrP(C) expressed by myelinating Schwann cells.

Snord 3A: a molecular marker and modulator of prion disease progression

Since preventive treatments for prion disease require early identification of subjects at risk, we searched for surrogate peripheral markers characterizing the asymptomatic phases of such conditions. To this effect, we subjected blood mRNA from E200K PrP CJD patients and corresponding family members to global arrays and found that the expression of Snord3A, a non-coding RNA transcript, was elevated several times in CJD patients as compared to controls, while asymptomatic carriers presented intermediate Snord3A levels. In the brains of TgMHu2ME199K mice, a mouse model mimicking for E200K CJD, Snord 3A levels were elevated in an age and disease severity dependent manner, as was the case for brains of these mice in which disease was exacerbated by copper administration. Snord3A expression was also elevated in scrapie infected mice, but not in PrP^0/0 mice, indicating that while the expression levels of this transcript may reflect diverse prion etiologies, they are not related to the loss of PrP^C’s function. Elevation of Snord3A was consistent with the activation of ATF6, representing one of the arms of the unfolded protein response system. Indeed, SnoRNAs were associated with reduced resistance to oxidative stress, and with ER stress in general, factors playing a significant role in this and other neurodegenerative conditions. We hypothesize that in addition to its function as a disease marker, Snord3A may play an important role in the mechanism of prion disease manifestation and progression.

[Study on the correlation between membrane protein Flotillin-1 and PrPc endocytosis]

OBJECTIVE

To explore whether the membrane-associated protein Flotillin-1 has relationship with endocytosis of PrPc.

METHODS

The expression of Flotillin-1 in different cell lines was detected with the method of Western Blot; the interaction between Flotillin-1 and PrPc in Cells which were treated with copper ions was observed using immunoprecipitation method.

RESULTS

(1) Flotillin-1 was widely expressed in many cell lines without significant difference in the amounts of expression level; (2) Only in the appearance of copper ions, the protein complexes of PrPc and Flotillin-1 can be detected with the method of IP, which were related to copper ions concentration and processing time.

CONCLUSION

The membrane-associated protein Flotillin-1 has the relationship with the endocytosis of PrPc.

Role of the cellular prion protein in oligodendrocyte precursor cell proliferation and differentiation in the developing and adult mouse CNS

There are numerous studies describing the signaling mechanisms that mediate oligodendrocyte precursor cell (OPC) proliferation and differentiation, although the contribution of the cellular prion protein (PrP^c) to this process remains unclear. PrP^c is a glycosyl-phosphatidylinositol (GPI)-anchored glycoprotein involved in diverse cellular processes during the development and maturation of the mammalian central nervous system (CNS). Here we describe how PrP^c influences oligodendrocyte proliferation in the developing and adult CNS. OPCs that lack PrP^c proliferate more vigorously at the expense of a delay in differentiation, which correlates with changes in the expression of oligodendrocyte lineage markers. In addition, numerous NG2-positive cells were observed in cortical regions of adult PrP^c knockout mice, although no significant changes in myelination can be seen, probably due to the death of surplus cells.

Resolution-enhanced native acidic gel electrophoresis: a method for resolving, sizing, and quantifying prion protein oligomers

The formation of β-sheet-rich prion protein (PrP(β)) oligomers from native or cellular PrP(c) is thought to be a key step in the development of prion diseases. To assist in this characterization process we have developed a rapid and remarkably high resolution gel electrophoresis technique called RENAGE (resolution-enhanced native acidic gel electrophoresis) for separating, sizing, and quantifying oligomeric PrP(β) complexes. PrP(β) oligomers formed via either urea/salt or acid conversion can be resolved by RENAGE into a clear set of oligomeric bands differing by just one subunit. Calibration of the size of the PrP(β) oligomer bands was made possible with a cross-linked mouse PrP(90-232) ladder (1- to 11-mer) generated using ruthenium bipyridyl-based photoinduced cross-linking of unmodified proteins (PICUP). This PrP PICUP ladder allowed the size and abundance of PrP(β) oligomers formed from urea/salt and acid conversion to be determined. This distribution consists of 7-, 8-, 9-, 10-, and 11-mers, with the most abundant species being the 8-mer. The high-resolution separation afforded by RENAGE has allowed us to investigate distinctive size and population changes in PrP(β) oligomers formed under various conversion conditions, with various construct lengths, from various species or in the presence of anti-prion compounds.

Cellular prion protein participates in amyloid-β transcytosis across the blood-brain barrier

The blood-brain barrier (BBB) facilitates amyloid-β (Aβ) exchange between the blood and the brain. Here, we found that the cellular prion protein (PrP(c)), a putative receptor implicated in mediating Aβ neurotoxicity in Alzheimer's disease (AD), participates in Aβ transcytosis across the BBB. Using an in vitro BBB model, [(125)I]-Aβ(1-40) transcytosis was reduced by genetic knockout of PrP(c) or after addition of a competing PrP(c)-specific antibody. Furthermore, we provide evidence that PrP(c) is expressed in endothelial cells and, that monomeric Aβ(1-40) binds to PrP(c). These observations provide new mechanistic insights into the role of PrP(c) in AD.

The toxicity of a mutant prion protein is cell-autonomous, and can be suppressed by wild-type prion protein on adjacent cells

Insight into the normal function of PrP^C, and how it can be subverted to produce neurotoxic effects, is provided by PrP molecules carrying deletions encompassing the conserved central region. The most neurotoxic of these mutants, Δ105–125 (called ΔCR), produces a spontaneous neurodegenerative illness when expressed in transgenic mice, and this phenotype can be dose-dependently suppressed by co-expression of wild-type PrP. Whether the toxic activity of ΔCR PrP and the protective activity or wild-type PrP are cell-autonomous, or can be exerted on neighboring cells, is unknown. To investigate this question, we have utilized co-cultures of differentiated neural stem cells derived from mice expressing ΔCR or wild-type PrP. Cells from the two kinds of mice, which are marked by the presence or absence of GFP, are differentiated together to yield neurons, astrocytes, and oligodendrocytes. As a surrogate read-out of ΔCR PrP toxicity, we assayed sensitivity of the cells to the cationic antibiotic, Zeocin. In a previous study, we reported that cells expressing ΔCR PrP are hypersensitive to the toxic effects of several cationic antibiotics, an effect that is suppressed by co-expression of wild type PrP, similar to the rescue of the neurodegenerative phenotype observed in transgenic mice. Using this system, we find that while ΔCR-dependent toxicity is cell-autonomous, the rescuing activity of wild-type PrP can be exerted in trans from nearby cells. These results provide important insights into how ΔCR PrP subverts a normal physiological function of PrP^C, and the cellular mechanisms underlying the rescuing process.

Alpha- and beta- cleavages of the amino-terminus of the cellular prion protein

It is commonly assumed that the physiological isoform of prion protein, PrP(C), is cleaved during its normal processing between residues 111/112, whereas the pathogenic isoform, PrP(Sc), is cleaved at an alternate site in the octapeptide repeat region around position 90. Here we demonstrated both in cultured cells and in vivo, that PrP(C) is subject to a complex set of post-translational processing with the molecule being cleaved upstream of position 111/112, in the octapeptide repeat region or at position 96. PrP has therefore two main cleavage sites that we decided to name alpha and beta. Cleavage of PrP(C) at these sites leads us to re-evaluate the function of both N- and C-terminus fragments thus generated.

PrPC displays an essential protective role from oxidative stress in an astrocyte cell line derived from PrPC knockout mice

The PrP(C) protein, which is especially present in the cellular membrane of nervous system cells, has been extensively studied for its controversial antioxidant activity. In this study, we elucidated the free radical scavenger activity of purified murine PrP(C) in solution and its participation as a cell protector in astrocytes that were subjected to treatment with an oxidant. In vitro and using an EPR spin-trapping technique, we observed that PrP(C) decreased the oxidation of the DMPO trap in a Fenton reaction system (Cu(2+)/ascorbate/H(2)O(2)), which was demonstrated by approximately 70% less DMPO/OH(). In cultured PrP(C)-knockout astrocytes from mice, the absence of PrP(C) caused an increase in intracellular ROS (reactive oxygen species) generation during the first 3h of H(2)O(2) treatment. This rapid increase in ROS disrupted the cell cycle in the PrP(C)-knockout astrocytes, which increased the population of cells in the sub-G1 phase when compared with cultured wild-type astrocytes. We conclude that PrP(C) in solution acts as a radical scavenger, and in astrocytes, it is essential for protection from oxidative stress caused by an external chemical agent, which is a likely condition in human neurodegenerative CNS disorders and pathological conditions such as ischemia.