α-Cleavage of cellular prion protein

Grp78 destabilization of infectious prions is strain-specific and modified by multiple factors including accessory chaperones and pH

Lethal neurodegenerative prion diseases result from the continuous accumulation of infectious and variably protease-resistant prion protein aggregates (PrPD) which are misfolded forms of the normally detergent soluble and protease-sensitive cellular prion protein. Molecular chaperones like Grp78 have been found to reduce the accumulation of PrPD, but how different cellular environments and other chaperones influence the ability of Grp78 to modify PrPD is poorly understood. In this work, we investigated how pH and protease-mediated structural changes in PrPD from two mouse-adapted scrapie prion strains, 22L and 87V, influenced processing by Grp78 in the presence or absence of chaperones Hsp90, DnaJC1, and Stip1. We developed a cell-free in vitro system to monitor chaperone-mediated structural changes to, and disaggregation of, PrPD. For both strains, Grp78 was most effective at structurally altering PrPD at low pH, especially when additional chaperones were present. While Grp78, DnaJC1, Stip1, and Hsp90 were unable to disaggregate the majority of PrPD from either strain, pretreatment of PrPD with proteases increased disaggregation of 22L PrPD compared to 87V, indicating strain-specific differences in aggregate structure were impacting chaperone activity. Hsp90 also induced structural changes in 87V PrPD as indicated by an increase in the susceptibility of its n-terminus to proteases. Our data suggest that, while chaperones like Grp78, DnaJC1, Stip1, and Hsp90 disaggregate only a small fraction of PrPD, they may still facilitate its clearance by altering aggregate structure and sensitizing PrPD to proteases in a strain and pH-dependent manner.

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.

Prion Protein Octarepeat Domain Forms Transient β-Sheet Structures upon Residue-Specific Binding to Cu(II) and Zn(II) Ions

Misfolding of the cellular prion protein (PrP^C) is associated with the development of fatal neurodegenerative diseases called transmissible spongiform encephalopathies (TSEs). Metal ions appear to play a crucial role in PrP^C misfolding. PrP^C is a combined Cu(II) and Zn(II) metal-binding protein, where the main metal-binding site is located in the octarepeat (OR) region. Thus, the biological function of PrP^C may involve the transport of divalent metal ions across membranes or buffering concentrations of divalent metal ions in the synaptic cleft. Recent studies have shown that an excess of Cu(II) ions can result in PrP^C instability, oligomerization, and/or neuroinflammation. Here, we have used biophysical methods to characterize Cu(II) and Zn(II) binding to the isolated OR region of PrP^C. Circular dichroism (CD) spectroscopy data suggest that the OR domain binds up to four Cu(II) ions or two Zn(II) ions. Binding of the first metal ion results in a structural transition from the polyproline II helix to the β-turn structure, while the binding of additional metal ions induces the formation of β-sheet structures. Fluorescence spectroscopy data indicate that the OR region can bind both Cu(II) and Zn(II) ions at neutral pH, but under acidic conditions, it binds only Cu(II) ions. Molecular dynamics simulations suggest that binding of either metal ion to the OR region results in the formation of β-hairpin structures. As the formation of β-sheet structures can be a first step toward amyloid formation, we propose that high concentrations of either Cu(II) or Zn(II) ions may have a pro-amyloid effect in TSE diseases.

An optimized Western blot assay provides a comprehensive assessment of the physiological endoproteolytic processing of the prion protein

The prion protein (PrP^C) is subjected to several conserved endoproteolytic events producing bioactive fragments that are of increasing interest for their physiological functions and their implication in the pathogenesis of prion diseases and other neurodegenerative diseases. However, systematic and comprehensive investigations on the full spectrum of PrP^C proteoforms have been hampered by the lack of methods able to identify all PrP^C-derived proteoforms. Building on previous knowledge of PrP^C endoproteolytic processing, we thus developed an optimized Western blot assay able to obtain the maximum information about PrP^C constitutive processing and the relative abundance of PrP^C proteoforms in a complex biological sample. This approach led to the concurrent identification of the whole spectrum of known endoproteolytic-derived PrP^C proteoforms in brain homogenates, including C-terminal, N-terminal and, most importantly, shed PrP^C-derived fragments. Endoproteolytic processing of PrP^C was remarkably similar in the brain of widely used wild type and transgenic rodent models, with α-cleavage-derived C1 representing the most abundant proteoform and ADAM10-mediated shedding being an unexpectedly prominent proteolytic event. Interestingly, the relative amount of shed PrP^C was higher in WT mice than in most other models. Our results indicate that constitutive endoproteolytic processing of PrP^C is not affected by PrP^C overexpression or host factors other than PrP^C but can be impacted by PrP^C primary structure. Finally, this method represents a crucial step in gaining insight into pathophysiological roles, biomarker suitability, and therapeutic potential of shed PrP^C and for a comprehensive appraisal of PrP^C proteoforms in therapies, drug screening, or in the progression of neurodegenerative diseases.

Role of redox-sensitive catalytic interaction with ADAM10 in mutant-selective extracellular shedding of prion protein

Misfolded glycosylphosphatidylinositol-anchored prion protein (PrP) is primarily degraded in lysosomes but is often rapidly removed from the cell surface before endocytosis in a preemptive manner. However, this mechanism is poorly understood. In this study, we discovered a disease-causing prion mutation (Q212P) that exceptionally promoted the extracellular release of PrP. Spatiotemporal analyses combined with genome editing identified the role of sheddase ADAM10 in Q212P shedding from the cell surface. ADAM10 was observed to catalytically interacts with Q212P but non-catalytically with wild-type PrP (wtPrP). This intrinsic difference in the interaction of ADAM10 between Q212P and wtPrP allowed Q212P to selectively access the sheddase activity of ADAM10 in a redox-sensitive manner. In addition, redox perturbation instigated the latent misfolding propensity of Q212P and disrupted the catalytic interaction between PrP and ADAM10, resulting in the accumulation of misfolded PrP on the cell surface. Upon recovery, active ADAM10 was able to reversibly release the surface Q212P. However, it might prove detrimental if unregulated resulting in unexpected proteotoxicity. This study provides a molecular basis of the mutant-selective shedding of PrP by demonstrating the catalytic interaction of ADAM10 with Q212P.

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Pathogenic Q212P mutation provides a unique pattern of PrP metabolism.

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Q212P mutation promotes the extracellular release of surface PrP.

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Q212P shedding is catalyzed by ADAM10.

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ADAM10-mediated Q212P shedding is redox-sensitive.

Prion protein with a mutant N-terminal octarepeat region undergoes cobalamin-dependent assembly into high-molecular weight complexes

The cellular prion protein (PrP^C) has a C-terminal globular domain and a disordered N-terminal region encompassing five octarepeats (ORs). Encounters between Cu(II) ions and four OR sites produce interchangeable binding geometries; however, the significance of Cu(II) binding to ORs in different combinations is unclear. To understand the impact of specific binding geometries, OR variants were designed that interact with multiple or single Cu(II) ions in specific locked coordinations. Unexpectedly, we found that one mutant produced detergent-insoluble, protease-resistant species in cells in the absence of exposure to the infectious prion protein isoform, scrapie-associated prion protein (PrP^Sc). Formation of these assemblies, visible as puncta, was reversible and dependent upon medium formulation. Cobalamin (Cbl), a dietary cofactor containing a corrin ring that coordinates a Co³⁺ ion, was identified as a key medium component, and its effect was validated by reconstitution experiments. Although we failed to find evidence that Cbl interacts with Cu-binding OR regions, we instead noted interactions of Cbl with the PrP^C C-terminal domain. We found that some interactions occurred at a binding site of planar tetrapyrrole compounds on the isolated globular domain, but others did not, and N-terminal sequences additionally had a marked effect on their presence and position. Our studies define a conditional effect of Cbl wherein a mutant OR region can act in cis to destabilize a globular domain with a wild type sequence. The unexpected intersection between the properties of PrP^Sc's disordered region, Cbl, and conformational remodeling events may have implications for understanding sporadic prion disease that does not involve exposure to PrP^Sc.

Anchorless risk or released benefit? An updated view on the ADAM10-mediated shedding of the prion protein

The prion protein (PrP) is a broadly expressed glycoprotein linked with a multitude of (suggested) biological and pathological implications. Some of these roles seem to be due to constitutively generated proteolytic fragments of the protein. Among them is a soluble PrP form, which is released from the surface of neurons and other cell types by action of the metalloprotease ADAM10 in a process termed 'shedding'. The latter aspect is the focus of this review, which aims to provide a comprehensive overview on (i) the relevance of proteolytic processing in regulating cellular PrP functions, (ii) currently described involvement of shed PrP in neurodegenerative diseases (including prion diseases and Alzheimer's disease), (iii) shed PrP's expected roles in intercellular communication in many more (patho)physiological conditions (such as stroke, cancer or immune responses), (iv) and the need for improved research tools in respective (future) studies. Deeper mechanistic insight into roles played by PrP shedding and its resulting fragment may pave the way for improved diagnostics and future therapeutic approaches in diseases of the brain and beyond.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

The unique ability to adapt and thrive in inhospitable, stressful tumor microenvironments (TME) also renders cancer cells resistant to traditional chemotherapeutic treatments and/or novel pharmaceuticals. Cancer cells exhibit extensive metabolic alterations involving hypoxia, accelerated glycolysis, oxidative stress, and increased extracellular ATP that may activate ancient, conserved prion adaptive response strategies that exacerbate multidrug resistance (MDR) by exploiting cellular stress to increase cancer metastatic potential and stemness, balance proliferation and differentiation, and amplify resistance to apoptosis. The regulation of prions in MDR is further complicated by important, putative physiological functions of ligand-binding and signal transduction. Melatonin is capable of both enhancing physiological functions and inhibiting oncogenic properties of prion proteins. Through regulation of phase separation of the prion N-terminal domain which targets and interacts with lipid rafts, melatonin may prevent conformational changes that can result in aggregation and/or conversion to pathological, infectious isoforms. As a cancer therapy adjuvant, melatonin could modulate TME oxidative stress levels and hypoxia, reverse pH gradient changes, reduce lipid peroxidation, and protect lipid raft compositions to suppress prion-mediated, non-Mendelian, heritable, but often reversible epigenetic adaptations that facilitate cancer heterogeneity, stemness, metastasis, and drug resistance. This review examines some of the mechanisms that may balance physiological and pathological effects of prions and prion-like proteins achieved through the synergistic use of melatonin to ameliorate MDR, which remains a challenge in cancer treatment.

Cellular Prion Protein Is Essential for Myocardial Regeneration but Not the Recovery of Left Ventricular Function from Apical Ballooning

This study tested the hypothesis that cellular prion protein (PrP^C) played an essential role in myocardial regeneration and recovery of left ventricular ejection fraction (LVEF) from apical takotsubo cardiomyopathy (TCM) induced by transaortic constriction (TAC). In vitro study was categorized into G1 (H9C2), G2 (H9C2-overexpression-PrP^C), G3 (H9C2-overexpression-PrP^C + Stelazine/1 uM), and G4 (H9C2 + siRNA-PrP^C), respectively. The results showed that the protein expressions of PrP^C, cell-stress signaling (p-PI3K/p-Akt/p-m-TOR) and signal transduction pathway for cell proliferation/division (RAS/c-RAF/p-MEK/p-ERK1/2) were lowest in G1, highest in G2, significantly higher in G3 than in G4 (all p < 0.001). Adult-male B6 mice (n = 30) were equally categorized in group 1 (sham-control), group 2 (TAC) for 14 days, then relieved the knot and administered BrdU (50 ug/kg/intravenously/q.6.h for two times from day-14 after TAC) and group 3 (TAC + Stelazine/20 mg/kg/day since day 7 after TAC up to day 21 + BrdU administered as group 2), and animals were euthanized at day 28. The results showed that by day 28, the LVEF was significantly higher in group 1 than in groups 2/3 and significantly higher in group 3 than in group 2, whereas the LV chamber size exhibited an opposite pattern of LVEF (all p < 0.0001). The protein expressions of PrP^C/p-PI3K/p-Akt/p-m-TOR/cyclin D/cyclin E and cellular-proliferation biomarkers (Ki67/PCNA/BrdU) exhibited an opposite pattern of LVEF (all p < 0.0001) among the three groups, whereas the protein expressions of RAS/c-RAF/p-MEK/p-ERK1/2 were significantly and progressively increased from groups 1 to 3 (all p < 0.0001). In conclusion, PrP^C participated in regulating the intrinsic response of cell-stress signaling and myocardial regeneration but did not offer significant benefit on recovery of the heart function in the setting of TCM.

PrP C as a Transducer of Physiological and Pathological Signals

After the discovery of prion phenomenon, the physiological role of the cellular prion protein (PrP ^C ) remained elusive. In the past decades, molecular and cellular analysis has shed some light regarding interactions and functions of PrP ^C in health and disease. PrP ^C , which is located mainly at the plasma membrane of neuronal cells attached by a glycosylphosphatidylinositol (GPI) anchor, can act as a receptor or transducer from external signaling. Although the precise role of PrP ^C remains elusive, a variety of functions have been proposed for this protein, namely, neuronal excitability and viability. Although many issues must be solved to clearly define the role of PrP ^C , its connection to the central nervous system (CNS) and to several misfolding-associated diseases makes PrP ^C an interesting pharmacological target. In a physiological context, several reports have proposed that PrP ^C modulates synaptic transmission, interacting with various proteins, namely, ion pumps, channels, and metabotropic receptors. PrP ^C has also been implicated in the pathophysiological cell signaling induced by β-amyloid peptide that leads to synaptic dysfunction in the context of Alzheimer's disease (AD), as a mediator of Aβ-induced cell toxicity. Additionally, it has been implicated in other proteinopathies as well. In this review, we aimed to analyze the role of PrP ^C as a transducer of physiological and pathological signaling.

β-cleavage of the human prion protein impacts Cu(II) coordination at its non-octarepeat region

The cellular prion protein (PrP^C) is a membrane-anchored copper binding protein that undergoes proteolytic processing. β-cleavage of PrP^C is associated with a pathogenic condition and it yields two fragments: N2 with residues 23-89, and C2 including residues 90-231. The membrane-bound C2 fragment retains the Cu binding sites at His96 and His111, but it also has a free N-terminal NH₂ group. In this study, the impact of β-cleavage of PrP^C in its Cu(II) binding properties was evaluated, using the peptide of the human prion protein hPrP(90-115) as a model for the C2 fragment. The Cu(II) coordination properties of hPrP(90-115) were studied using circular dichroism (CD) and electron paramagnetic resonance (EPR); while the H96A and H111A substitutions and its acetylated variants were also studied. Cu binding to hPrP(90-115) is dependent on metal ion concentration: At low copper concentrations the participation of His96 and free NH₂-terminus is evident, while at high copper concentrations the His111 site is populated without participation of the N-terminal NH₂ group. The presence of a free NH₂-terminal group in the C2 fragment significantly impacts the Cu(II) coordination properties of the His96 site, where the NH₂ group also anchors the metal ion. This study provides further insights into the impact of proteolytic processing of PrP^C in the Cu binding properties of this important neuronal protein.

Transcriptomic analysis of zebrafish prion protein mutants supports conserved cross-species function of the cellular prion protein

Cellular Prion Protein (PrPC) is a well-studied protein as the substrate for various progressive untreatable neurodegenerative diseases. Normal functions of PrPC are poorly understood, though recent proteomic and transcriptomic approaches have begun to reveal common themes. We use our compound prp1 and prp2 knockout mutant zebrafish at three days post fertilization to take a transcriptomic approach to investigating potentially conserved PrPC functions during development. Gene ontology analysis shows the biological processes with the largest changes in gene expression include redox processing, transport and cell adhesion. Within these categories several different gene families were prevalent including the solute carrier proteins, cytochrome p450 enzymes and protocadherins. Continuing from previous studies identifying cell adhesion as an important function of PrPC we found that in addition to the protocadherins there was a significant reduction in transcript abundance of both ncam1a and st8sia2. These two genes are involved in the early development of vertebrates. The alterations in cell adhesion transcripts were consistent with past findings in zebrafish and mouse prion protein mutants; however E-cadherin processing after prion protein knockdown failed to reveal any differences compared with wild type in either our double prp1/prp2 mutant fish or after prp1 morpholino knockdown. Our data supports a cross species conserved role for PrPC in the development and maintenance of the central nervous system, particularly by regulating various and important cell adhesion processes.

Prion Protein Biology Through the Lens of Liquid-Liquid Phase Separation

Conformational conversion of the α-helix-rich cellular prion protein into the misfolded, β-rich, aggregated, scrapie form underlies the molecular basis of prion diseases that represent a class of invariably fatal, untreatable, and transmissible neurodegenerative diseases. However, despite the extensive and rigorous research, there is a significant gap in the understanding of molecular mechanisms that contribute to prion pathogenesis. In this review, we describe the historical perspective of the development of the prion concept and the current state of knowledge of prion biology including structural, molecular, and cellular aspects of the prion protein. We then summarize the putative functional role of the N-terminal intrinsically disordered segment of the prion protein. We next describe the ongoing efforts in elucidating the prion phase behavior and the emerging role of liquid-liquid phase separation that can have potential functional relevance and can offer an alternate non-canonical pathway involving conformational conversion into a disease-associated form. We also attempt to shed light on the evolutionary perspective of the prion protein highlighting the potential role of intrinsic disorder in prion protein biology and summarize a few important questions associated with the phase transitions of the prion protein. Delving deeper into these key aspects can pave the way for a detailed understanding of the critical molecular determinants of the prion phase transition and its relevance to physiology and neurodegenerative diseases.

β-Cleavage of the prion protein in the human eye: Implications for the spread of infectious prions and human ocular disorders

Recently, we reported β-cleavage of the prion protein (PrP^C) in human ocular tissues. Here, we explored whether this is unique to the human eye, and its functional implications. A comparison of the cleavage pattern of PrP^C in human ocular tissues with common nocturnal and diurnal animals revealed mainly β-cleavage in humans, and mostly full-length PrP^C in animal retinas. Soluble FL PrP^C and N-terminal fragment (N2) released from β-cleavage was observed in the aqueous and vitreous humor (AH & VH). Expression of human PrP^C in ARPE-19 cells, a human retinal pigmented epithelial cell line, also showed β-cleaved PrP^C. Surprisingly, β-cleavage was not altered by a variety of insults, including oxidative stress, suggesting a unique role of this cleavage in the human eye. It is likely that β-cleaved C- or N-terminal fragments of PrP^C protect from various insults unique to the human eye. On the contrary, β-cleaved C-terminus of PrP^C is susceptible to conversion to the pathological PrP-scrapie form, and includes the binding sites for β1-integrin and amyloid-β, molecules implicated in several ocular disorders. Considering the species and tissue-specific cleavage of PrP^C, our data suggest re-evaluation of prion infectivity and other ocular disorders of the human eye conducted in mouse models.

Neuroprotective effect and potential of cellular prion protein and its cleavage products for treatment of neurodegenerative disorders part II: strategies for therapeutics development

Introduction: The cellular prion protein (PrP^C), some of its derivatives (especially PrP N-terminal N1 peptide and shed PrP), and PrP^C-containing exosomes have strong neuroprotective activities, which have been reviewed in the companion article (Part I) and are briefly summarized here.Areas covered: We propose that elevating the extracellular levels of a protective PrP form using gene therapy and other approaches is a very promising novel avenue for prophylactic and therapeutic treatments against prion disease, Alzheimer's disease, and several other neurodegenerative diseases. We will dissect the pros and cons of various potential PrP-based treatment options and propose a few strategies that are more likely to succeed. The cited references were obtained from extensive PubMed searches of recent literature, including peer-reviewed original articles and review articles.Expert opinion: Concurrent knockdown of celllular PrP expression and elevation of the extracellular levels of a neuroprotective PrP N-terminal peptide via optimized gene therapy vectors is a highly promising broad-spectrum prophylactic and therapeutic strategy against several neurodegenerative diseases, including prion diseases, Alzheimer's disease and Parkinson's disease.

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 aminoglycoside G418 hinders de novo prion infection in cultured cells

The study of prions and the discovery of candidate therapeutics for prion disease have been facilitated by the ability of prions to replicate in cultured cells. Paradigms in which prion proteins from different species are expressed in cells with low or no expression of endogenous prion protein (PrP) have expanded the range of prion strains that can be propagated. In these systems, cells stably expressing a PrP of interest are typically generated via coexpression of a selectable marker and treatment with an antibiotic. Here, we report the unexpected discovery that the aminoglycoside G418 (Geneticin) interferes with the ability of stably transfected cultured cells to become infected with prions. In G418-resistant lines of N2a or CAD5 cells, the presence of G418 reduced levels of protease-resistant PrP following challenge with the RML or 22L strains of mouse prions. G418 also interfered with the infection of cells expressing hamster PrP with the 263K strain of hamster prions. Interestingly, G418 had minimal to no effect on protease-resistant PrP levels in cells with established prion infection, arguing that G418 selectively interferes with de novo prion infection. As G418 treatment had no discernible effect on cellular PrP levels or its localization, this suggests that G418 may specifically target prion assemblies or processes involved in the earliest stages of prion infection.

Oligodendrocyte myelin glycoprotein as a novel target for pathogenic autoimmunity in the CNS

Autoimmune disorders of the central nervous system (CNS) comprise a broad spectrum of clinical entities. The stratification of patients based on the recognized autoantigen is of great importance for therapy optimization and for concepts of pathogenicity, but for most of these patients, the actual target of their autoimmune response is unknown. Here we investigated oligodendrocyte myelin glycoprotein (OMGP) as autoimmune target, because OMGP is expressed specifically in the CNS and there on oligodendrocytes and neurons. Using a stringent cell-based assay, we detected autoantibodies to OMGP in serum of 8/352 patients with multiple sclerosis, 1/28 children with acute disseminated encephalomyelitis and unexpectedly, also in one patient with psychosis, but in none of 114 healthy controls. Since OMGP is GPI-anchored, we validated its recognition also in GPI-anchored form. The autoantibodies to OMGP were largely IgG1 with a contribution of IgG4, indicating cognate T cell help. We found high levels of soluble OMGP in human spinal fluid, presumably due to shedding of the GPI-linked OMGP. Analyzing the pathogenic relevance of autoimmunity to OMGP in an animal model, we found that OMGP-specific T cells induce a novel type of experimental autoimmune encephalomyelitis dominated by meningitis above the cortical convexities. This unusual localization may be directed by intrathecal uptake and presentation of OMGP by meningeal phagocytes. Together, OMGP-directed autoimmunity provides a new element of heterogeneity, helping to improve the stratification of patients for diagnostic and therapeutic purposes.

Characterization of brain-derived extracellular vesicles reveals changes in cellular origin after stroke and enrichment of the prion protein with a potential role in cellular uptake

Extracellular vesicles (EVs) are important means of intercellular communication and a potent tool for regenerative therapy. In ischaemic stroke, transient blockage of a brain artery leads to a lack of glucose and oxygen in the affected brain tissue, provoking neuronal death by necrosis in the core of the ischaemic region. The fate of neurons in the surrounding penumbra region depends on the stimuli, including EVs, received during the following hours. A detailed characterization of such stimuli is crucial not only for understanding stroke pathophysiology but also for new therapeutic interventions. In the present study, we characterize the EVs in mouse brain under physiological conditions and 24 h after induction of transient ischaemia in mice. We show that, in steady-state conditions, microglia are the main source of small EVs (sEVs), whereas after ischaemia the main sEV population originates from astrocytes. Brain sEVs presented high amounts of the prion protein (PrP), which were further increased after stroke. Moreover, EVs were enriched in a proteolytically truncated PrP fragment (PrP-C1). Because of similarities between PrP-C1 and certain viral surface proteins, we studied the cellular uptake of brain-derived sEVs from mice lacking (PrP-KO) or expressing PrP (WT). We show that PrP-KO-sEVs are taken up significantly faster and more efficiently than WT-EVs by primary neurons. Furthermore, microglia and astrocytes engulf PrP-KO-sEVs more readily than WT-sEVs. Our results provide novel information on the relative contribution of brain cell types to the sEV pool in murine brain and indicate that increased release of sEVs by astrocytes together with elevated levels of PrP in sEVs may play a role in intercellular communication at early stages after stroke. In addition, amounts of PrP (and probably PrP-C1) in brain sEVs seem to contribute to regulating their cellular uptake.

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.

Intrinsic toxicity of the cellular prion protein is regulated by its conserved central region

The conserved central region (CR) of PrP^C has been hypothesized to serve as a passive linker connecting the protein's toxic N-terminal and globular C-terminal domains. Yet, deletion of the CR causes neonatal fatality in mice, implying the CR possesses a protective function. The CR encompasses the regulatory α-cleavage locus, and additionally facilitates a regulatory metal ion-promoted interaction between the PrP^C N- and C-terminal domains. To elucidate the role of the CR and determine why CR deletion generates toxicity, we designed PrP^C constructs wherein either the cis-interaction or α-cleavage are selectively prevented. These constructs were interrogated using nuclear magnetic resonance, electrophysiology, and cell viability assays. Our results demonstrate the CR is not a passive linker and the native sequence is crucial for its protective role over the toxic N-terminus, irrespective of α-cleavage or the cis-interaction. Additionally, we find that the CR facilitates homodimerization of PrP^C , attenuating the toxicity of the N-terminus.

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.

Metal homeostasis disturbances in neurodegenerative disorders, with special emphasis on Creutzfeldt-Jakob disease - Potential pathogenetic mechanism and therapeutic implications

Creutzfeldt-Jakob disease (CJD) is characterized by a rapidly progressive dementia often accompanied by myoclonus and other signs of brain dysfunction, relying on the conversion of the normal cellular form of the prion protein (PrPC) to a misfolded form (PrPSc). The neuropathological changes include spongiform degeneration, neuronal loss, astrogliosis, and deposition of PrPSc. It is still unclear how these pathological changes correlate with the development of CJD symptoms because few patients survive beyond 2 years after diagnosis. Inasmuch as the symptoms of CJD overlap some of those observed in Alzheimer's, Parkinson's, and Huntington's diseases, there may be some underlying pathologic mechanisms associated with CJD that may contribute to the symptoms of non-prion neurodegenerative diseases as well. Data suggest that imbalance of metals, including copper, zinc, iron, and manganese, induces abnormalities in processing and degradation of prion proteins that are accompanied by self-propagation of PrPSc. These events appear to be responsible for glutamatergic synaptic dysfunctions, neuronal death, and PrPSc aggregation. Given that the prodromal symptoms of CJD such as sleep disturbances and mood disorders are associated with brain stem and limbic system dysfunction, the pathological changes may initially occur in these brain regions, then spread throughout the entire brain. Alterations in cerebrospinal fluid homeostasis, which may be linked to imbalance of these metals, seem to be more important than neuroinflammation in causing the cell death. It is proposed that metal dyshomeostasis could be responsible for the initiation and progression of the pathological changes associated with symptoms of CJD and other neurodegenerative disorders.

The cellular prion protein and its derived fragments in human prion diseases and their role as potential biomarkers

Introduction: Human prion diseases are a heterogeneous group of incurable and debilitating conditions characterized by a progressive degeneration of the central nervous system. The conformational changes of the cellular prion protein and its formation into an abnormal isoform, spongiform degeneration, neuronal loss, and neuroinflammation are central to prion disease pathogenesis. It has been postulated that truncated variants of aggregation-prone proteins are implicated in neurodegenerative mechanisms. An increasing body of evidence indicates that proteolytic fragments and truncated variants of the prion protein are formed and accumulated in the brain of prion disease patients. These prion protein variants provide a high degree of relevance to disease pathology and diagnosis. Areas covered: In the present review, we summarize the current knowledge on the occurrence of truncated prion protein species and their potential roles in pathophysiological states during prion diseases progression. In addition, we discuss their usability as a diagnostic biomarker in prion diseases. Expert opinion: Either as a primary factor in the formation of prion diseases or as a consequence from neuropathological affection, abnormal prion protein variants and fragments may provide independent information about mechanisms of prion conversion, pathological states, or disease progression.

Proteolysis: a double-edged sword for the development of amyloidoses

The yeast Saccharomyces cerevisiae has proven to be a useful model system to investigate the mechanism of prion generation and inheritance, to which studies in Sup35 made a great contribution. Recent studies demonstrated that 'protein misfolding and aggregation' (i.e. amyloidogenesis) is a common principle underlying the pathogenesis of neurodegenerative diseases including prion, amyotrophic lateral sclerosis (ALS), Perkinson's (PD), Alzheimer's (AD) diseases and polyglutamine (polyQ) diseases such as spinocerebellar ataxia (SCA) and Hantington's disease (HD). By these findings, the yeast has again been drawing increased attention as a useful system for studying neurodegenerative proteinopathies. So far, it has been reported that proteolytic cleavage of causative amyloidogenic proteins might affect the pathogenesis of the respective neurodegenerative diseases. Although those reports provide a clear phenomenological description, in the majority of cases, it has remained elusive if proteolysis is directly involved in the pathogenesis of the diseases. Recently, we have demonstrated in yeast that proteolysis suppresses prion generation. The yeast-based strategy might make a breakthrough to the unsolved issues.

Cellular Prion Protein Enhances Drug Resistance of Colorectal Cancer Cells via Regulation of a Survival Signal Pathway

Anti-cancer drug resistance is a major problem in colorectal cancer (CRC) research. Although several studies have revealed the mechanism of cancer drug resistance, molecular targets for chemotherapeutic combinations remain elusive. To address this issue, we focused on the expression of cellular prion protein (PrP^C) in 5-FU-resistant CRC cells. In 5-FU-resistant CRC cells, PrP^C expression is significantly increased, compared with that in normal CRC cells. In the presence of 5-FU, PrP^C increased CRC cell survival and proliferation by maintaining the activation of the PI3K-Akt signaling pathway and the expression of cell cycle-associated proteins, including cyclin E, CDK2, cyclin D1, and CDK4. In addition, PrP^C inhibited the activation of the stress-associated proteins p38, JNK, and p53. Moreover, after treatment of 5-FU-resistant CRC cells with 5-FU, silencing of PrP^C triggered apoptosis via the activation of caspase-3. These results indicate that PrP^C plays a key role in CRC drug resistance. The novel strategy of combining chemotherapy with PrP^C targeting may yield efficacious treatments of colorectal cancer.

Structural and mechanistic aspects influencing the ADAM10-mediated shedding of the prion protein

Background

Proteolytic processing of the prion protein (PrPC) by endogenous proteases generates bioactive membrane-bound and soluble fragments which may help to explain the pleiotropic roles of this protein in the nervous system and in brain diseases. Shedding of almost full-length PrPC into the extracellular space by the metalloprotease ADAM10 is of peculiar relevance since soluble PrP stimulates axonal outgrowth and is protective in neurodegenerative conditions such as Alzheimer’s and prion disease. However, molecular determinates and mechanisms regulating the shedding of PrP are entirely unknown.

Methods

We produced an antibody recognizing the neo-epitope of shed PrP generated by ADAM10 in biological samples and used it to study structural and mechanistic aspects affecting the shedding. For this, we investigated genetically modified cellular and murine models by biochemical and morphological approaches.

Results

We show that the novel antibody specifically detects shed PrP in cell culture supernatants and murine brain. We demonstrate that ADAM10 is the exclusive sheddase of PrPC in the nervous system and reveal that the glycosylation state and type of membrane-anchorage of PrPC severely affect its shedding. Furthermore, we provide evidence that PrP shedding can be modulated by pharmacological inhibition and stimulation and present data suggesting that shedding is a relevant part of a compensatory network ensuring PrPC homeostasis of the cell.

Conclusions

With the new antibody, our study introduces a new tool to reliably investigate PrP-shedding. In addition, this study provides novel and important insight into the regulation of this cleavage event, which is likely to be relevant for diagnostic and therapeutic approaches even beyond neurodegeneration.

Neuroprotective alpha-cleavage of the human prion protein significantly impacts Cu(ii) coordination at its His111 site

Alpha-cleavage proteolytic processing of human prion protein significantly impacts its Cu(ii) coordination properties at the His111 site.

Classic and atypical scrapie - a genetic perspective

Scrapie was the first prion disease to be recognised and the study of this disease in sheep and goats has provided a wealth of information not only for scrapie but also for the other prion diseases. All prion diseases are under strong genetic control of the prion gene PRNP, independent of whether they are typical or atypical scrapie and which of the different prion strains is causing infection. Decades of studies using experimental disease challenges and field surveys have established disease association models, in which species-specific amino acid variations in the prion or PrP protein, encoded by the PRNP gene, can predict disease susceptibility or resistance. PRNP genetics represents an important and successful basis for implementing scrapie eradication strategies in sheep and goats. In general terms these studies have revealed that there appear to be many more amino acid changes in PrP leading to increased resistance than to higher susceptibility. Most changes are in the globular part of PrP protein and three regions appear to have major influence. This knowledge can be transferred into prion diseases of other species to facilitate genetic control strategies. However, an obstacle remains with the lack of fully understanding the underlying molecular mechanism, impeding our ability to deal with the difference in the genetic control between typical and atypical forms of scrapie or to predict association in newly infected species. This chapter will discuss the advances in both typical and atypical scrapie from a genetic perspective.

Effects of safflower yellow on beta-amyloid deposition and activation of astrocytes in the brain of APP/PS1 transgenic mice

Safflower yellow (SY), one of traditional Chinese medicine extracted from safflower, has been shown to have neuroprotective effects on animal models of vascular dementia and Alzheimer's diseases (AD), by inhibiting oxidative injury, neuronal apoptosis and tau hyperphosphorylation. In this study, we investigated whether safflower yellow (SY) can improve cognitive function, decrease Amyloid β (Aβ) accumulation and overactivation of astrocytes in AD mouse model. We found that SY treatment significantly ameliorated the learning and memory deficits of APP/PS1 mice. By hematoxylin-eosin staining, we found that the neuronal loss and death in APP/PS1 mice was decreased by SY treatment. Immunohistochemical staining showed that SY treatment dramatically down-regulated Aβ_1-42 deposition and glial fibrillary acidic protein (GFAP) level in APP/PS1 mice. Biochemical analysis also showed that SY treatment reduced soluble and insoluble Aβ_1-42 level in the cortex and soluble Aβ_1-42 level in the hippocampus of APP/PS1 mice. Moreover, we found that SY treatment decreased the expression of proteins related to generation of Aβ, and markedly increased expression of enzymes associated with clearance of Aβ in the brain of APP/PS1 mice. These results indicate that the SY can serve as a promising therapeutic approach for the treatment of AD.

Biological and biochemical characterization of M2B cells: Classical BSE prion is conserved in transgenic mice overexpressing bovine prion protein gene

M2B cells with persistent classical bovine spongiform encephalopathy (C-BSE) have been established previously. In this study, we performed strain characterization of the M2B cell line in bovine PrP^C overexpressing mice (Tg 1896). Mice intracranially inoculated with M2B cells and C-BSE survived for 451 ± 7 and 465 ± 31 d post inoculation, respectively. Although biochemical properties, including deglycosylation and conformational stability, differed between M2B cells and C-BSE, inoculation with M2B cell lysate and C-BSE resulted in comparable phenotypes. Comparable vacuolation scores and PrP^Sc depositions were observed in the brain of Tg 1896 inoculated with both M2B cell lysate and C-BSE. Our results show that biochemical and biological characteristics of M2B cells and C-BSE are classifiable in the same strain.

Diverse functions of the prion protein - Does proteolytic processing hold the key?

Proteolytic processing of the cellular and disease-associated form of the prion protein leads to generation of bioactive soluble prion protein fragments and modifies the structure and function of its cell-bound form. The nature of proteases responsible for shedding, α-, β-, and γ-cleavage of the prion protein are only partially identified and their regulation is largely unknown. Here, we provide an overview of the increasingly multifaceted picture of prion protein proteolysis and shed light on physiological and pathological roles associated with these cleavages. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

Prions amplify through degradation of the VPS10P sorting receptor sortilin

Prion diseases are a group of fatal neurodegenerative disorders caused by prions, which consist mainly of the abnormally folded isoform of prion protein, PrP^Sc. A pivotal pathogenic event in prion disease is progressive accumulation of prions, or PrP^Sc, in brains through constitutive conformational conversion of the cellular prion protein, PrP^C, into PrP^Sc. However, the cellular mechanism by which PrP^Sc is progressively accumulated in prion-infected neurons remains unknown. Here, we show that PrP^Sc is progressively accumulated in prion-infected cells through degradation of the VPS10P sorting receptor sortilin. We first show that sortilin interacts with PrP^C and PrP^Sc and sorts them to lysosomes for degradation. Consistently, sortilin-knockdown increased PrP^Sc accumulation in prion-infected cells. In contrast, overexpression of sortilin reduced PrP^Sc accumulation in prion-infected cells. These results indicate that sortilin negatively regulates PrP^Sc accumulation in prion-infected cells. The negative role of sortilin in PrP^Sc accumulation was further confirmed in sortilin-knockout mice infected with prions. The infected mice had accelerated prion disease with early accumulation of PrP^Sc in their brains. Interestingly, sortilin was reduced in prion-infected cells and mouse brains. Treatment of prion-infected cells with lysosomal inhibitors, but not proteasomal inhibitors, increased the levels of sortilin. Moreover, sortilin was reduced following PrP^Sc becoming detectable in cells after infection with prions. These results indicate that PrP^Sc accumulation stimulates sortilin degradation in lysosomes. Taken together, these results show that PrP^Sc accumulation of itself could impair the sortilin-mediated sorting of PrP^C and PrP^Sc to lysosomes for degradation by stimulating lysosomal degradation of sortilin, eventually leading to progressive accumulation of PrP^Sc in prion-infected cells.

Once prions consisting mainly of PrP^Sc infect hosts, they constitutively propagate in their brains. Progressive production of PrP^Sc through the constitutive conformational conversion of PrP^C into PrP^Sc underlies prion propagation. However, the mechanism enabling progressive production of PrP^Sc in prion-infected cells remains unknown. We here found that the VPS10P sorting receptor sortilin is involved in degradation of PrP^C and PrP^Sc in infected cells by binding to both molecules and subsequently trafficking them to the lysosomal protein degradation pathway. Interestingly, we also found that degradation of sortilin was stimulated in lysosomes in prion-infected cells possibly as a result of the sortilin-PrP^C or -PrP^Sc complexes being trafficked to lysosomes. Our findings indicate that PrP^Sc itself impairs the sortilin-mediated degradation of PrP^C and PrP^Sc by stimulating degradation of sortilin in lysosomes. This eventually results in progressive production of PrP^Sc in prion-infected cells by increasing the opportunity of PrP^C to convert into PrP^Sc and by accumulating the already produced PrP^Sc. This mechanism was confirmed in sortilin-KO mice infected with prions. The mice had exacerbated prion disease with earlier accumulation of PrP^Sc in their brains.

Neuroprotective properties of the PrP-like Shadoo glycoprotein assessed in the middle cerebral artery occlusion model of ischemia

Biochemical similarities have been noted between the natively unstructured region of the cellular prion protein, PrP^C, and a GPI-linked glycoprotein called Shadoo (Sho); these proteins are encoded by the Prnp and Sprn genes, respectively. Both proteins are expressed in the adult central nervous system and they share overlapping partners, including each other, in interactome studies. As prior studies have ascribed neuroprotective properties to the N-terminal region of PrP^C, specifically the octarepeat region, we investigated Sho's neuroprotective properties. To this end we assessed Sho-null (Sprn^0/0) and hemizygous (Sprn^0/+) mice in the middle cerebral artery occlusion (MCAO) model versus wild type mice and also vs. transgene-rescued Sprn^0/0-TgSprn mice. Sprn^0/0 mice had a tendency to greater fragility in reaching endpoint and deficits in parameters including infarct volume and neurogenesis, with a reciprocal trend noted in transgene-rescued mice; however these effects did not reach significance. Loss of both PrP^C and Sho immunostaining occurred in parallel to neuronal loss on the ipsilateral side of MCAO-lesioned animals; while focal elevations in immunostaining in the penumbra region were sometimes evident for PrP^C, they were not noted for Sho. Our studies argue against discernible neuroprotective action of Sho in the genetic backgrounds used for this MCAO paradigm. Whether or not the positively charged N-terminal regions in Sho and PrP^C fulfil different roles in vivo remains to be determined.

Prion diseases: New considerations

The transmissible spongiform encephalopathies, which include Creutzfeldt-Jakob disease, are fatal neurodegenerative disorders caused by the pathological accumulation of abnormal prion protein. The diagnosis of Creutzfeldt-Jakob disease is complex. The electroencephalogram, magnetic resonance imaging, lumbar puncture and genetic testing findings can help in the differential diagnosis of rapidly progressive dementia. There has recently been considerable debate as to whether proteins involved in the development of neurodegenerative diseases should be regarded as prions or only share prion-like mechanisms. Two recent reports described the detection of abnormal prion protein in the nasal mucosa and urine of patients with Creutzfeldt-Jakob disease. These findings raise major health concerns regarding the transmissibility of human prion diseases. We set out to address this neurological hot topic and to draw conclusions on the basis of what is known in the literature thus far.

PrPC Undergoes Basal to Apical Transcytosis in Polarized Epithelial MDCK Cells

The Prion Protein (PrP) is an ubiquitously expressed glycosylated membrane protein attached to the external leaflet of the plasma membrane via a glycosylphosphatidylinositol anchor (GPI). While the misfolded PrPSc scrapie isoform is the infectious agent of prion disease, the cellular isoform (PrPC) is an enigmatic protein with unclear function. Of interest, PrP localization in polarized MDCK cells is controversial and its mechanism of trafficking is not clear. Here we investigated PrP traffic in MDCK cells polarized on filters and in three-dimensional MDCK cysts, a more physiological model of polarized epithelia. We found that, unlike other GPI-anchored proteins (GPI-APs), PrP undergoes basolateral-to-apical transcytosis in fully polarized MDCK cells. Following this event full-length PrP and its cleavage fragments are segregated in different domains of the plasma membrane in polarized cells in both 2D and 3D cultures.

Prion Protein and Stage Specific Embryo Antigen 1 as Selection Markers to Enrich the Fraction of Murine Embryonic Stem Cell-Derived Cardiomyocytes

BACKGROUND

The prion protein (PrP) might be useful as a tool to collect cardiac progenitor cells derived from embryonic stem (ES) cells. It is also possible that PrP(+) cells include undifferentiated cells with a capacity to develop into tumors.

METHODS

PrP(+) cells isolated from embryoid bodies (EB) formed by mouse AB1 ES cells were examined using RT-PCR analysis and clonogeneic cell assay. To assess their potential to differentiate into cardiomyocytes, Nkx2.5(GFP/+) (hcgp7) cells, another ES cell line that carries the GFP reporter gene in the Nkx2.5 loci, were used.

RESULTS

PrP(+) cells isolated from EB of day 7 and 14 did not express pluripotency markers, but expressed cardiac cell markers, while PrP(+) cells isolated from EB of day 21 expressed pluripotency markers. Cultured PrP(+) cells isolated from EB of day 21 expressed pluripotency markers to form colonies, whereas those isolated from EB of day 7 and 14 did not. To exclude proliferating cells from PrP(+) cells, stage specific embryo antigen 1 (SSEA1) was employed as a second marker. PrP(+)/SSEA1(-) cells did not proliferate and expressed cardiac cell markers, while PrP(+)/SSEA1(+) did proliferate.

CONCLUSION

PrP(+) cells isolated from EB included undifferentiated cells in day 21. PrP(+)/SSEA1(-) cells included cardiomyoctes, suggesting PrP and SSEA1 may be useful as markers to enrich the fraction of cardiomyocytes.

Shedding light on prion disease

Proteolytic processing regulates key processes in health and disease. The cellular prion protein (PrP(C)) is subject to at least 3 cleavage events, α-cleavage, β-cleavage and shedding. In contrast to α- and β-cleavage where there is an ongoing controversy on the identity of relevant proteases, the metalloprotease ADAM10 represents the only relevant PrP sheddase. Here we focus on the roles that ADAM10-mediated shedding of PrP(C) and its pathogenic isoform (PrP(Sc)) might play in regulating their physiological and pathogenic functions, respectively. As revealed by our recent study using conditional ADAM10 knockout mice (Altmeppen et al., 2015), shedding of PrP seems to be involved in key processes of prion diseases. These aspects and several open questions arising from them are discussed. Increased knowledge on this topic can shed new light on prion diseases and other neurodegenerative conditions as well.

New insights into structural determinants of prion protein folding and stability

Prions are the etiological agent of fatal neurodegenerative diseases called prion diseases or transmissible spongiform encephalopathies. These maladies can be sporadic, genetic or infectious disorders. Prions are due to post-translational modifications of the cellular prion protein leading to the formation of a β-sheet enriched conformer with altered biochemical properties. The molecular events causing prion formation in sporadic prion diseases are still elusive. Recently, we published a research elucidating the contribution of major structural determinants and environmental factors in prion protein folding and stability. Our study highlighted the crucial role of octarepeats in stabilizing prion protein; the presence of a highly enthalpically stable intermediate state in prion-susceptible species; and the role of disulfide bridge in preserving native fold thus avoiding the misfolding to a β-sheet enriched isoform. Taking advantage from these findings, in this work we present new insights into structural determinants of prion protein folding and stability.

Inhibition of the FKBP family of peptidyl prolyl isomerases induces abortive translocation and degradation of the cellular prion protein

Prion diseases are fatal neurodegenerative disorders for which there is no effective treatment. Because the cellular prion protein (PrP(C)) is required for propagation of the infectious scrapie form of the protein, one therapeutic strategy is to reduce PrP(C) expression. Recently FK506, an inhibitor of the FKBP family of peptidyl prolyl isomerases, was shown to increase survival in animal models of prion disease, with proposed mechanisms including calcineurin inhibition, induction of autophagy, and reduced PrP(C) expression. We show that FK506 treatment results in a profound reduction in PrP(C) expression due to a defect in the translocation of PrP(C) into the endoplasmic reticulum with subsequent degradation by the proteasome. These phenotypes could be bypassed by replacing the PrP(C) signal sequence with that of prolactin or osteopontin. In mouse cells, depletion of ER luminal FKBP10 was almost as potent as FK506 in attenuating expression of PrP(C). However, this occurred at a later stage, after translocation of PrP(C) into the ER. Both FK506 treatment and FKBP10 depletion were effective in reducing PrP(Sc) propagation in cell models. These findings show the involvement of FKBP proteins at different stages of PrP(C) biogenesis and identify FKBP10 as a potential therapeutic target for the treatment of prion diseases.

Loss of Cellular Sialidases Does Not Affect the Sialylation Status of the Prion Protein but Increases the Amounts of Its Proteolytic Fragment C1

The central molecular event underlying prion diseases involves conformational change of the cellular form of the prion protein (PrPC), which is a sialoglycoprotein, into the disease-associated, transmissible form denoted PrPSc. Recent studies revealed a correlation between the sialylation status of PrPSc and incubation time to disease and introduced a new hypothesis that progression of prion diseases could be controlled or reversed by altering the sialylation level of PrPC. Of the four known mammalian sialidases, the enzymes that cleave off sialic acid residues, only NEU1, NEU3 and NEU4 are expressed in the brain. To test whether cellular sialidases control the steady-state sialylation level of PrPC and to identify the putative sialidase responsible for desialylating PrPC, we analyzed brain-derived PrPC from knockout mice deficient in Neu1, Neu3, Neu4, or from Neu3/Neu4 double knockouts. Surprisingly, no differences in the sialylation of PrPC or its proteolytic product C1 were noticed in any of the knockout mice tested as compared to the age-matched controls. However, significantly higher amounts of the C1 fragment relative to full-length PrPC were detected in the brains of Neu1 knockout mice as compared to WT mice or to the other knockout mice. Additional experiments revealed that in neuroblastoma cell line the sialylation pattern of C1 could be changed by an inhibitor of sialylatransferases. In summary, this study suggests that targeting cellular sialidases is apparently not the correct strategy for altering the sialylation levels of PrPC, whereas modulating the activity of sialylatransferases might offer a more promising approach. Our findings also suggest that catabolism of PrPC involves its α-cleavage followed by desialylation of the resulting C1 fragments by NEU1 and consequent fast degradation of the desialylated products.

The pathogenesis of soluble PrP fragments containing Aβ binding sites

Prion protein (PrP) has proven to bind amyloid beta (Aβ) oligomers with high affinity, changing our understanding of both prion diseases (PD) and Alzheimer's disease (AD) at the molecular and phenotypic levels, although the latter currently lacks sufficient attentions. Transgenic mice expressing anchorless PrP developed unusual diseases reminiscent of AD with tremendous amyloid plaque formation. In this review, we described two interesting observations at the phenotypic level. First, common pathogenic mutations of the PRNP gene in Gerstmann-Sträussler-Scheinker (GSS) syndrome were clustered at PrP95-105. Meanwhile, all nonsense PRNP mutations that generated soluble PrP 95-105 exhibited phenotypes with abundant amyloid formations. We speculate that PrP-Aβ oligomers binding might be the underlying mechanism of the predominant amyloid phenotypes. Second, soluble PrP-Aβ oligomer complexes might exist in the extracellular space at the beginning of both PD and AD and subserve an initial neuroprotective function. Thus, the diseases would only present after long-term accumulation. This might be the central common pathogenic event of both PD and AD.

The GPI-anchoring of PrP: implications in sorting and pathogenesis

The cellular prion protein (PrP(C)) is an N-glycosylated GPI-anchored protein usually present in lipid rafts with numerous putative functions. When it changes its conformation to a pathological isoform (then referred to as PrP(Sc)), it is an essential part of the prion, the agent causing fatal and transmissible neurodegenerative prion diseases. There is growing evidence that toxicity and neuronal damage on the one hand and propagation/infectivity on the other hand are two distinct processes of the disease and that the GPI-anchor attachment of PrP(C) and PrP(Sc) plays an important role in protein localization and in neurotoxicity. Here we review how the signal sequence of the GPI-anchor matters in PrP(C) localization, how an altered cellular localization of PrP(C) or differences in GPI-anchor composition can affect prion infection, and we discuss through which mechanisms changes on the anchorage of PrP(C) can modify the disease process.

The Ubiquitin-Proteasome System and Molecular Chaperone Deregulation in Alzheimer's Disease

One of the shared hallmarks of neurodegenerative diseases is the accumulation of misfolded proteins. Therefore, it is suspected that normal proteostasis is crucial for neuronal survival in the brain and that the malfunction of this mechanism may be the underlying cause of neurodegenerative diseases. The accumulation of amyloid plaques (APs) composed of amyloid-beta peptide (Aβ) aggregates and neurofibrillary tangles (NFTs) composed of misfolded Tau proteins are the defining pathological markers of Alzheimer's disease (AD). The accumulation of these proteins indicates a faulty protein quality control in the AD brain. An impaired ubiquitin-proteasome system (UPS) could lead to negative consequences for protein regulation, including loss of function. Another pivotal mechanism for the prevention of misfolded protein accumulation is the utilization of molecular chaperones. Molecular chaperones, such as heat shock proteins (HSPs) and FK506-binding proteins (FKBPs), are highly involved in protein regulation to ensure proper folding and normal function. In this review, we elaborate on the molecular basis of AD pathophysiology using recent data, with a particular focus on the role of the UPS and molecular chaperones as the defensive mechanism against misfolded proteins that have prion-like properties. In addition, we propose a rational therapy approach based on this mechanism.

Prion Protein Signaling in the Nervous System—A Review and Perspective

Prion protein (PrPC) was originally known as the causative agent of transmissible spongiform encephalopathy (TSE) but with recent research, its true function in cells is becoming clearer. It is known to act as a scaffolding protein, binding multiple ligands at the cell membrane and to be involved in signal transduction, passing information from the extracellular matrix (ECM) to the cytoplasm. Its role in the coordination of transmitters at the synapse, glyapse, and gap junction and in short- and long-range neurotrophic signaling gives PrPC a major part in neural transmission and nervous system signaling. It acts to regulate cellular function in multiple targets through its role as a controller of redox status and calcium ion flux. Given the importance of PrPC in cell physiology, this review considers its potential role in disease apart from TSE. The putative functions of PrPC point to involvement in neurodegenerative disease, neuropathic pain, chronic headache, and inflammatory disease including neuroinflammatory disease of the nervous system. Potential targets for the treatment of disease influenced by PrPC are discussed.

Structural determinants in prion protein folding and stability

Prions are responsible for a heterogeneous group of fatal neurodegenerative diseases, involving post-translational modifications of the cellular prion protein. Epidemiological studies on Creutzfeldt-Jakob disease, a prototype prion disorder, show a majority of cases being sporadic, while the remaining occurrences are either genetic or iatrogenic. The molecular mechanisms by which PrP(C) is converted into its pathological isoform have not yet been established. While point mutations and seeds trigger the protein to cross the energy barriers, thus causing genetic and infectious transmissible spongiform encephalopathies, respectively, the mechanism responsible for sporadic forms remains unclear. Since prion diseases are protein-misfolding disorders, we investigated prion protein folding and stability as functions of different milieus. Using spectroscopic techniques and atomistic simulations, we dissected the contribution of major structural determinants, also defining the energy landscape of prion protein. In particular, we elucidated (i) the essential role of the octapeptide region in prion protein folding and stability, (ii) the presence of a very enthalpically stable intermediate in prion-susceptible species, and (iii) the role of the disulfide bridge in prion protein folding.