alpha-cleavage of the prion protein occurs in a late compartment of the secretory pathway and is independent of lipid rafts

What is the role of lipids in prion conversion and disease?

The molecular cause of transmissible spongiform encephalopathies (TSEs) involves the conversion of the cellular prion protein (PrP^C) into its pathogenic form, called prion scrapie (PrP^Sc), which is prone to the formation of amorphous and amyloid aggregates found in TSE patients. Although the mechanisms of conversion of PrP^C into PrP^Sc are not entirely understood, two key points are currently accepted: (i) PrP^Sc acts as a seed for the recruitment of native PrP^C, inducing the latter's conversion to PrP^Sc; and (ii) other biomolecules, such as DNA, RNA, or lipids, can act as cofactors, mediating the conversion from PrP^C to PrP^Sc. Interestingly, PrP^C is anchored by a glycosylphosphatidylinositol molecule in the outer cell membrane. Therefore, interactions with lipid membranes or alterations in the membranes themselves have been widely investigated as possible factors for conversion. Alone or in combination with RNA molecules, lipids can induce the formation of PrP in vitro-produced aggregates capable of infecting animal models. Here, we discuss the role of lipids in prion conversion and infectivity, highlighting the structural and cytotoxic aspects of lipid-prion interactions. Strikingly, disorders like Alzheimer's and Parkinson's disease also seem to be caused by changes in protein structure and share pathogenic mechanisms with TSEs. Thus, we posit that comprehending the process of PrP conversion is relevant to understanding critical events involved in a variety of neurodegenerative disorders and will contribute to developing future therapeutic strategies for these devastating conditions.

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.

ER entry pathway and glycosylation of GPI-anchored proteins are determined by N-terminal signal sequence and C-terminal GPI-attachment sequence

Newly synthesized proteins in the secretory pathway, including glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs), need to be correctly targeted and imported into the endoplasmic reticulum (ER) lumen. GPI-APs are synthesized in the cytosol as preproproteins, which contain an N-terminal signal sequence (SS), mature protein part, and C-terminal GPI-attachment sequence (GPI-AS), and translocated into the ER lumen where SS and GPI-AS are removed, generating mature GPI-APs. However, how various GPI-APs are translocated into the ER lumen in mammalian cells is unclear. Here, we investigated the ER entry pathways of GPI-APs using a panel of KO cells defective in each signal recognition particle–independent ER entry pathway—namely, Sec62, GET, or SND pathway. We found GPI-AP CD59 largely depends on the SND pathway for ER entry, whereas prion protein (Prion) and LY6K depend on both Sec62 and GET pathways. Using chimeric Prion and LY6K constructs in which the N-terminal SS or C-terminal GPI-AS was replaced with that of CD59, we revealed that the hydrophobicity of the SSs and GPI-ASs contributes to the dependence on Sec62 and GET pathways, respectively. Moreover, the ER entry route of chimeric Prion constructs with the C-terminal GPI-ASs replaced with that of CD59 was changed to the SND pathway. Simultaneously, their GPI structures and which oligosaccharyltransferase isoforms modify the constructs were altered without any amino acid change in the mature protein part. Taking these findings together, this study revealed N- and C-terminal sequences of GPI-APs determine the selective ER entry route, which in turn regulates subsequent maturation processes of GPI-APs.

Prion Protein: The Molecule of Many Forms and Faces

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

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.

A seven-residue deletion in PrP leads to generation of a spontaneous prion formed from C-terminal C1 fragment of PrP

Prions result from a drastic conformational change of the host-encoded cellular prion protein (PrP), leading to the formation of β-sheet-rich, insoluble, and protease-resistant self-replicating assemblies (PrP^Sc). The cellular and molecular mechanisms involved in spontaneous prion formation in sporadic and inherited human prion diseases or equivalent animal diseases are poorly understood, in part because cell models of spontaneously forming prions are currently lacking. Here, extending studies on the role of the H2 α-helix C terminus of PrP, we found that deletion of the highly conserved ¹⁹⁰HTVTTTT¹⁹⁶ segment of ovine PrP led to spontaneous prion formation in the RK13 rabbit kidney cell model. On long-term passage, the mutant cells stably produced proteinase K (PK)-resistant, insoluble, and aggregated assemblies that were infectious for naïve cells expressing either the mutant protein or other PrPs with slightly different deletions in the same area. The electrophoretic pattern of the PK-resistant core of the spontaneous prion (Δ^Spont) contained mainly C-terminal polypeptides akin to C1, the cell-surface anchored C-terminal moiety of PrP generated by natural cellular processing. RK13 cells expressing solely the Δ190-196 C1 PrP construct, in the absence of the full-length protein, were susceptible to Δ^Spont prions. Δ^Spont infection induced the conversion of the mutated C1 into a PK-resistant and infectious form perpetuating the biochemical characteristics of Δ^Spont prion. In conclusion, this work provides a unique cell-derived system generating spontaneous prions and provides evidence that the 113 C-terminal residues of PrP are sufficient for a self-propagating prion entity.

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.

Structural Consequences of Copper Binding to the Prion Protein

Prion, or PrP^Sc, is the pathological isoform of the cellular prion protein (PrP^C) and it is the etiological agent of transmissible spongiform encephalopathies (TSE) affecting humans and animal species. The most relevant function of PrP^C is its ability to bind copper ions through its flexible N-terminal moiety. This review includes an overview of the structure and function of PrP^C with a focus on its ability to bind copper ions. The state-of-the-art of the role of copper in both PrP^C physiology and in prion pathogenesis is also discussed. Finally, we describe the structural consequences of copper binding to the PrP^C structure.

Secreted cellular prion protein binds doxorubicin and correlates with anthracycline resistance in breast cancer

Anthracyclines are amongst the most effective chemotherapeutics ever developed, but they produce grueling side-effects, serious adverse events and resistance often develops over time. We found that these compounds can be sequestered by secreted cellular Prion protein (PrPC), blocking their cytotoxic activity. This effect was dose-dependent using either cell line-conditioned medium or human serum as a source of PrPC. Genetic depletion of PrPC or inhibition of binding via chelation of ionic copper prevented the interaction and restored cytotoxic activity. This was more pronounced for doxorubicin than its epimer, epirubicin. Investigating the relevance to breast cancer management, we found that the levels of PRNP transcript in pre-treatment tumor biopsies stratified relapse-free survival after neoadjuvant treatment with anthracyclines, particularly amongst doxorubicin-treated patients with residual disease at surgery (p=2.8E-08). These data suggest that local sequestration could mediate treatment resistance. Consistent with this, tumor cell expression of PrPC protein correlated with poorer response to doxorubicin but not epirubicin in an independent cohort analyzed by immunohistochemistry, particularly soluble isoforms released into the extracellular environment by shedding (p=0.015). These findings have important potential clinical implications for frontline regimen decision-making. We suggest there is warranted utility for prognostic PrPC/PRNP assays to guide chemo-sensitization strategies that exploit an understanding of PrPC-anthracycline-copper ion complexes.

Cellular and Molecular Mechanisms of Prion Disease

Prion diseases are rapidly progressive, incurable neurodegenerative disorders caused by misfolded, aggregated proteins known as prions, which are uniquely infectious. Remarkably, these infectious proteins have been responsible for widespread disease epidemics, including kuru in humans, bovine spongiform encephalopathy in cattle, and chronic wasting disease in cervids, the latter of which has spread across North America and recently appeared in Norway and Finland. The hallmark histopathological features include widespread spongiform encephalopathy, neuronal loss, gliosis, and deposits of variably sized aggregated prion protein, ranging from small, soluble oligomers to long, thin, unbranched fibrils, depending on the disease. Here, we explore recent advances in prion disease research, from the function of the cellular prion protein to the dysfunction triggering neurotoxicity, as well as mechanisms underlying prion spread between cells. We also highlight key findings that have revealed new therapeutic targets and consider unanswered questions for future research.

The cellular and pathologic prion protein

The cellular prion protein, PrP^C, is a small, cell surface glycoprotein with a function that is currently somewhat ill defined. It is also the key molecule involved in the family of neurodegenerative disorders called transmissible spongiform encephalopathies, which are also known as prion diseases. The misfolding of PrP^C to a conformationally altered isoform, designated PrP^TSE, is the main molecular process involved in pathogenesis and appears to precede many other pathologic and clinical manifestations of disease, including neuronal loss, astrogliosis, and cognitive loss. PrP^TSE is also believed to be the major component of the infectious "prion," the agent responsible for disease transmission, and preparations of this protein can cause prion disease when inoculated into a naïve host. Thus, understanding the biochemical and biophysical properties of both PrP^C and PrP^TSE, and ultimately the mechanisms of their interconversion, is critical if we are to understand prion disease biology. Although entire books could be devoted to research pertaining to the protein, herein we briefly review the state of knowledge of prion biochemistry, including consideration of prion protein structure, function, misfolding, and dysfunction.

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.

Interaction of Peptide Aptamers with Prion Protein Central Domain Promotes α-Cleavage of PrPC

Prion diseases are infectious and fatal neurodegenerative diseases affecting humans and animals. Transmission is possible within and between species with zoonotic potential. Currently, no prophylaxis or treatment exists. Prions are composed of the misfolded isoform PrP^Sc of the cellular prion protein PrP^C. Expression of PrP^C is a prerequisite for prion infection, and conformational conversion of PrP^C is induced upon its direct interaction with PrP^Sc. Inhibition of this interaction can abrogate prion propagation, and we have previously established peptide aptamers (PAs) binding to PrP^C as new anti-prion compounds. Here, we mapped the interaction site of PA8 in PrP and modeled the complex in silico to design targeted mutations in PA8 which presumably enhance binding properties. Using these PA8 variants, we could improve PA-mediated inhibition of PrP^Sc replication and de novo infection of neuronal cells. Furthermore, we demonstrate that binding of PA8 and its variants increases PrP^C α-cleavage and interferes with its internalization. This gives rise to high levels of the membrane-anchored PrP-C1 fragment, a transdominant negative inhibitor of prion replication. PA8 and its variants interact with PrP^C at its central and most highly conserved domain, a region which is crucial for prion conversion and facilitates toxic signaling of Aβ oligomers characteristic for Alzheimer's disease. Our strategy allows for the first time to induce α-cleavage, which occurs within this central domain, independent of targeting the responsible protease. Therefore, interaction of PAs with PrP^C and enhancement of α-cleavage represent mechanisms that can be beneficial for the treatment of prion and other neurodegenerative diseases.

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.

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.

The biological function of the cellular prion protein: an update

The misfolding of the cellular prion protein (PrP^C) causes fatal neurodegenerative diseases. Yet PrP^C is highly conserved in mammals, suggesting that it exerts beneficial functions preventing its evolutionary elimination. Ablation of PrP^C in mice results in well-defined structural and functional alterations in the peripheral nervous system. Many additional phenotypes were ascribed to the lack of PrP^C, but some of these were found to arise from genetic artifacts of the underlying mouse models. Here, we revisit the proposed physiological roles of PrP^C in the central and peripheral nervous systems and highlight the need for their critical reassessment using new, rigorously controlled animal models.

Physiological Functions of the Cellular Prion Protein

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

Dual role of cellular prion protein in normal host and Alzheimer's disease

Using PrPC-knockout cell lines, it has been shown that the inhibition of apoptosis through STI1 is mediated by PrPC-dependent SOD activation. Antioxidant PrPC may contribute to suppression of inflammasome activation. PrPC is functionally involved in copper metabolism, signal transduction, neuroprotection, and cell maturation. Recently several reports have shown that PrPC participates in trans-membrane signaling processes associated with hematopoietic stem cell replication and neuronal differentiation. In another role, PrPC also tends to function as a neurotoxic protein. Aβ oligomer, which is associated with neurodegeneration in Alzheimer's disease (AD), has also been reported to act as a ligand of PrPC. However, the physiological role of PrPC as an Aβ42-binding protein is not clear. Actually, PrPC is critical in Aβ42-mediated autophagy in neurons. PrPC shows a beneficial role in lipid rafts to promote autophagy. Further search for PrPC-interaction molecules using Prnp-/- mice and various types of Prnp-/- cell lines under various conditions may elucidate other important PrPC important functions.

Prion Diseases

Prion diseases are a group of invariably fatal and transmissible neurodegenerative disorders that are associated with the misfolding of the normal cellular prion protein, with the misfolded conformers constituting an infectious unit referred to as a "prion". Prions can spread within an affected organism by directly propagating this misfolding within and between cells and can transmit disease between animals of the same and different species. Prion diseases have a range of clinical phenotypes in humans and animals, with a principle determinant of this attributed to different conformations of the misfolded protein, referred to as prion strains. This chapter will describe the different clinical manifestations of prion diseases, the evidence that these diseases can be transmitted by an infectious protein and how the misfolding of this protein causes disease.

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.

Secretory pathway retention of mutant prion protein induces p38-MAPK activation and lethal disease in mice

Misfolding of proteins in the biosynthetic pathway in neurons may cause disturbed protein homeostasis and neurodegeneration. The prion protein (PrP^C) is a GPI-anchored protein that resides at the plasma membrane and may be misfolded to PrP^Sc leading to prion diseases. We show that a deletion in the C-terminal domain of PrP^C (PrPΔ214–229) leads to partial retention in the secretory pathway causing a fatal neurodegenerative disease in mice that is partially rescued by co-expression of PrP^C. Transgenic (Tg(PrPΔ214–229)) mice show extensive neuronal loss in hippocampus and cerebellum and activation of p38-MAPK. In cell culture under stress conditions, PrPΔ214–229 accumulates in the Golgi apparatus possibly representing transit to the Rapid ER Stress-induced ExporT (RESET) pathway together with p38-MAPK activation. Here we describe a novel pathway linking retention of a GPI-anchored protein in the early secretory pathway to p38-MAPK activation and a neurodegenerative phenotype in transgenic mice.

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

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

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 Cellular Prion Protein: A Player in Immunological Quiescence

Despite intensive studies since the 1990s, the physiological role of the cellular prion protein (PrP(C)) remains elusive. Here, we present a novel concept suggesting that PrP(C) contributes to immunological quiescence in addition to cell protection. PrP(C) is highly expressed in diverse organs that by multiple means are particularly protected from inflammation, such as the brain, eye, placenta, pregnant uterus, and testes, while at the same time it is expressed in most cells of the lymphoreticular system. In this paradigm, PrP(C) serves two principal roles: to modulate the inflammatory potential of immune cells and to protect vulnerable parenchymal cells against noxious insults generated through inflammation. Here, we review studies of PrP(C) physiology in view of this concept.

PrP charge structure encodes interdomain interactions

Almost all proteins contain charged residues, and their chain distribution is tailored to fulfill essential ionic interactions for folding, binding and catalysis. Among proteins, the hinged two-domain chain of the cellular prion protein (PrP^C) exhibits a peculiar charge structure with unclear consequences in its structural malleability. To decipher the charge design role, we generated charge-reverted mutants for each domain and analyzed their effect on conformational and metabolic features. We found that charges contain the information for interdomain interactions. Use of dynamic light scattering and thermal denaturation experiments delineates the compaction of the α-fold by an electrostatic compensation between the polybasic 23–30 region and the α3 electronegative surface. This interaction increases stability and disfavors fibrillation. Independently of this structural effect, the N-terminal electropositive clusters regulate the α-cleavage efficiency. In the fibrillar state, use of circular dichroism, atomic-force and fluorescence microscopies reveal that the N-terminal positive clusters and the α3 electronegative surface dictate the secondary structure, the assembly hierarchy and the growth length of the fibril state. These findings show that the PrP charge structure functions as a code set up to ensure function and reduce pathogenic routes.

Ubiquitin-specific protease 14 modulates degradation of cellular prion protein

Prion diseases are fatal neurodegenerative disorders characterized by the accumulation of prion protein (PrP^C). To date, there is no effective treatment for the disease. The accumulated PrP, termed PrP^Sc, forms amyloid fibrils and could be infectious. It has been suggested that PrP^Sc is abnormally folded and resistant to proteolytic degradation, and also inhibits proteasomal functions in infected cells, thereby inducing neuronal death. Recent work indicates that the ubiquitin-proteasome system is involved in quality control of PrP^C. To reveal the significance of prion protein ubiqitination, we focused on ubiquitin-specific protease 14 (USP14), a deubiqutinating enzyme that catalyzes trimming of polyubiquitin chains and plays a role in regulation of proteasomal processes. Results from the present study showed that treatment with a selective inhibitor of USP14 reduced PrP^C, as well as PrP^Sc, levels in prion-infected neuronal cells. Overexpression of the dominant negative mutant form of USP14 reduced PrP^Sc, whereas wildtype USP14 increased PrP^Sc in prion-infected cells. These results suggest that USP14 prevents degradation of both normal and abnormal PrP. Collectively, a better understanding about the regulation of PrP^Sc clearance caused by USP14 might contribute greatly to the development of therapeutic strategies for prion diseases.

Octarepeat region flexibility impacts prion function, endoproteolysis and disease manifestation

The cellular prion protein (PrP(C)) comprises a natively unstructured N-terminal domain, including a metal-binding octarepeat region (OR) and a linker, followed by a C-terminal domain that misfolds to form PrP(S) (c) in Creutzfeldt-Jakob disease. PrP(C) β-endoproteolysis to the C2 fragment allows PrP(S) (c) formation, while α-endoproteolysis blocks production. To examine the OR, we used structure-directed design to make novel alleles, 'S1' and 'S3', locking this region in extended or compact conformations, respectively. S1 and S3 PrP resembled WT PrP in supporting peripheral nerve myelination. Prion-infected S1 and S3 transgenic mice both accumulated similar low levels of PrP(S) (c) and infectious prion particles, but differed in their clinical presentation. Unexpectedly, S3 PrP overproduced C2 fragment in the brain by a mechanism distinct from metal-catalysed hydrolysis reported previously. OR flexibility is concluded to impact diverse biological endpoints; it is a salient variable in infectious disease paradigms and modulates how the levels of PrP(S) (c) and infectivity can either uncouple or engage to drive the onset of clinical disease.

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.

Neutron reflectometry studies define prion protein N-terminal peptide membrane binding

The prion protein (PrP), widely recognized to misfold into the causative agent of the transmissible spongiform encephalopathies, has previously been shown to bind to lipid membranes with binding influenced by both membrane composition and pH. Aside from the misfolding events associated with prion pathogenesis, PrP can undergo various posttranslational modifications, including internal cleavage events. Alpha- and beta-cleavage of PrP produces two N-terminal fragments, N1 and N2, respectively, which interact specifically with negatively charged phospholipids at low pH. Our previous work probing N1 and N2 interactions with supported bilayers raised the possibility that the peptides could insert deeply with minimal disruption. In the current study we aimed to refine the binding parameters of these peptides with lipid bilayers. To this end, we used neutron reflectometry to define the structural details of this interaction in combination with quartz crystal microbalance interrogation. Neutron reflectometry confirmed that peptides equivalent to N1 and N2 insert into the interstitial space between the phospholipid headgroups but do not penetrate into the acyl tail region. In accord with our previous studies, interaction was stronger for the N1 fragment than for the N2, with more peptide bound per lipid. Neutron reflectometry analysis also detected lengthening of the lipid acyl tails, with a concurrent decrease in lipid area. This was most evident for the N1 peptide and suggests an induction of increased lipid order in the absence of phase transition. These observations stand in clear contrast to the findings of analogous studies of Ab and ?-synuclein and thereby support the possibility of a functional role for such N-terminal fragment-membrane interactions.

Role of lipid rafts and GM1 in the segregation and processing of prion protein

The prion protein (PrP^C) is highly expressed within the nervous system. Similar to other GPI-anchored proteins, PrP^C is found in lipid rafts, membrane domains enriched in cholesterol and sphingolipids. PrP^C raft association, together with raft lipid composition, appears essential for the conversion of PrP^C into the scrapie isoform PrP^Sc, and the development of prion disease. Controversial findings were reported on the nature of PrP^C-containing rafts, as well as on the distribution of PrP^C between rafts and non-raft membranes. We investigated PrP^C/ganglioside relationships and their influence on PrP^C localization in a neuronal cellular model, cerebellar granule cells. Our findings argue that in these cells at least two PrP^C conformations coexist: in lipid rafts PrP^C is present in the native folding (α-helical), stabilized by chemico-physical condition, while it is mainly present in other membrane compartments in a PrP^Sc-like conformation. We verified, by means of antibody reactivity and circular dichroism spectroscopy, that changes in lipid raft-ganglioside content alters PrP^C conformation and interaction with lipid bilayers, without modifying PrP^C distribution or cleavage. Our data provide new insights into the cellular mechanism of prion conversion and suggest that GM1-prion protein interaction at the cell surface could play a significant role in the mechanism predisposing to pathology.

C-terminal peptides modelling constitutive PrPC processing demonstrate ameliorated toxicity predisposition consequent to α-cleavage

C-terminal fragments generated through α- and β-cleavage of PrPC appear to harbour different pathogenic potential for host cells. Most significantly, α-cleavage produces a C-terminal fragment that is resistant to folding into soluble β-strand-rich toxic isoforms.

Association of prion protein genotype and scrapie prion protein type with cellular prion protein charge isoform profiles in cerebrospinal fluid of humans with sporadic or familial prion diseases

The present study investigates whether posttranslational modifications of cellular prion protein (PrP(C)) in the cerebrospinal fluid (CSF) of humans with prion diseases are associated with methionine (M) and/or valine (V) polymorphism at codon 129 of the prion protein gene (PRNP), scrapie prion protein (PrP(Sc)) type in sporadic Creutzfeldt-Jakob disease (sCJD), or PRNP mutations in familial Creutzfeldt-Jakob disease (fCJD/E200K), and fatal familial insomnia (FFI). We performed comparative 2-dimensional immunoblotting of PrP(C) charge isoforms in CSF samples from cohorts of diseased and control donors. Mean levels of total PrP(C) were significantly lower in the CSF from fCJD patients than from those with sCJD or FFI. Of the 12 most abundant PrP(C) isoforms in the examined CSF, one (IF12) was relatively decreased in (1) sCJD with VV (vs. MM or MV) at PRNP codon 129; (2) in sCJD with PrP(Sc) type 2 (vs. PrP(Sc) type 1); and (3) in FFI versus sCJD or fCJD. Furthermore, truncated PrP(C) species were detected in sCJD and control samples without discernible differences. Finally, serine 43 of PrP(C) in the CSF and brain tissue from CJD patients showed more pronounced phosphorylation than in control donors.

Effects of FlAsH/tetracysteine (TC) Tag on PrP proteolysis and PrPres formation by TC-scanning

Protein-protein interactions associated with proteolytic processing and aggregation are integral to normal and pathological aspects of prion protein (PrP) biology. Characterization of these interactions requires the identification of amino acid residues involved. The FlAsH/tetracysteine (FlAsH/TC) tag is a small fluorescent tag amenable to insertion at internal sites in proteins. In this study, we used serial FlAsH/TC insertions (TC-scanning) as a probe to characterize sites of protein-protein interaction between PrP and other molecules. To explore this application in the context of substrate-protease interactions, we analyzed the effect of FlAsH/TC insertions on proteolysis of cellular prion protein (PrPsen) in in vitro reactions and generation of the C1 metabolic fragment of PrPsen in live neuroblastoma cells. The influence of FlAsH/TC insertion was evaluated by TC-scanning across the cleavage sites of each protease. The results showed that FlAsH/TC inhibited protease cleavage only within limited ranges of the cleavage sites, which varied from about one to six residues in width, depending on the protease, providing an estimate of the PrP residues interacting with each protease. TC-scanning was also used to probe a different type of protein-protein interaction: the conformational conversion of FlAsH-PrPsen to the prion disease-associated isoform, PrPres. PrP constructs with FlAsH/TC insertions at residues 90-96 but not 97-101 were converted to FlAsH-PrPres, identifying a boundary separating loosely versus compactly folded regions of PrPres. Our observations demonstrate that TC-scanning with the FlAsH/TC tag can be a versatile method for probing protein-protein interactions and folding processes.

Increased prion protein processing and expression of metabotropic glutamate receptor 1 in a mouse model of Alzheimer's disease

Prion protein (PrP(C) ), a glycosylphosphatidylinositol-anchored protein corrupted in prion diseases, has been shown recently to interact with group I metabotropic glutamate receptors (mGluRs). Moreover, both PrP(C) and mGluRs were proposed to function as putative receptors for β-amyloid in Alzheimer's disease. PrP(C) can be processed in neurons via α or β-cleavage to produce PrP(C) fragments that are neuroprotective or toxic, respectively. We found PrP(C) α-cleavage to be 2-3 times higher in the cortex of APPswe/PS1dE9 mice, a mouse model of Alzheimer's disease. A similar age-dependent increase was observed for PrP(C) β-cleavage. Moreover, we observed considerable age-dependent increase in cortical expression of mGluR1, but not mGluR5. Exposure of cortical neuronal cultures to β-amyloid oligomers upregulated mGluR1 and PrP(C) α-cleavage, while activation of group I mGluRs increased PrP(C) shedding from the membrane, likely due to increased levels of a disintegrin and metalloprotease10, a key disintegrin for PrP(C) shedding. Interestingly, a similar increase in a disintegrin and metalloprotease10 was detected in the cortex of 9-month-old APPswe/PS1dE9 animals. Our experiments reveal novel and complex processing of PrP(C) in connection with mGluR overexpression that seems to be triggered by β-amyloid peptides. Prion protein (PrP(C) ) and metabotropic glutamate receptors (mGluR) are implicated in Alzheimer's disease (AD). We found age-dependent increase in PrP(C) processing, ADAM10 and mGluR1 levels in AD mouse model. These changes could be reproduced in cultured cortical neurons treated with Aβ peptide. Our findings suggest that increased levels of Aβ can trigger compensatory responses that may affect neuronal toxicity.

Homodimerization as a molecular switch between low and high efficiency PrP C cell surface delivery and neuroprotective activity

PrP (C) is associated with a variety of functions, and its ability to interact with a multitude of partners, including itself, may largely explain PrP multifunctionality and the lack of consensus on the genuine physiological function of the protein in vivo. In contrast, there is a consensus in the literature that alterations in PrP (C) trafficking and intracellular retention result in neuronal degeneration. In addition, a proteolytic modification in the late secretory pathway termed the α-cleavage induces the secretion of PrPN1, a PrP (C) -derived metabolite with fascinating neuroprotective activity against toxic oligomeric Aβ molecules implicated in Alzheimer disease. Thus, studies focusing on understanding the regulation of PrP (C) trafficking to the cell surface and the modulation of α-cleavage are essential. The objective of this commentary is to highlight recent evidences that PrP (C) homodimerization stimulates trafficking of the protein to the cell surface and results in high levels of PrPN1 secretion. We also discuss a hypothetical model for these results and comment on future challenges and opportunities.

Cellular aspects of prion replication in vitro

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders in mammals that are caused by unconventional agents predominantly composed of aggregated misfolded prion protein (PrP). Prions self-propagate by recruitment of host-encoded PrP into highly ordered β-sheet rich aggregates. Prion strains differ in their clinical, pathological and biochemical characteristics and are likely to be the consequence of distinct abnormal prion protein conformers that stably replicate their alternate states in the host cell. Understanding prion cell biology is fundamental for identifying potential drug targets for disease intervention. The development of permissive cell culture models has greatly enhanced our knowledge on entry, propagation and dissemination of TSE agents. However, despite extensive research, the precise mechanism of prion infection and potential strain effects remain enigmatic. This review summarizes our current knowledge of the cell biology and propagation of prions derived from cell culture experiments. We discuss recent findings on the trafficking of cellular and pathologic PrP, the potential sites of abnormal prion protein synthesis and potential co-factors involved in prion entry and propagation.

PrP(C) homodimerization stimulates the production of PrPC cleaved fragments PrPN1 and PrPC1

An endoproteolytic cleavage termed α-cleavage between residues 111/112 is a characteristic feature of the cellular prion protein (PrP(C)). This cleavage generates a soluble N-terminal fragment (PrPN1) and a glycosylphosphatidylinositol-anchored C-terminal fragment (PrPC1). Independent studies demonstrate that modulating PrP(C) α-cleavage represents a potential therapeutic strategy in prion diseases. The regulation of PrP(C) α-cleavage is unclear. The only known domain that is essential for the α-cleavage to occur is a hydrophobic domain (HD). Importantly, the HD is also essential for the formation of PrP(C) homodimers. To explore the role of PrP(C) homodimerization on the α-cleavage, we used a well described inducible dimerization strategy whereby a chimeric PrP(C) composed of a modified FK506-binding protein (Fv) fused with PrP(C) and termed Fv-PrP is incubated in the presence of a dimerizer AP20187 ligand. We show that homodimerization leads to a considerable increase of PrP(C) α-cleavage in cultured cells and release of PrPN1 and PrPC1. Interestingly, enforced homodimerization increased PrP(C) levels at the plasma membrane, and preventing PrP(C) trafficking to the cell surface inhibited dimerization-induced α-cleavage. These observations were confirmed in primary hippocampal neurons from transgenic mice expressing Fv-PrP. The proteases responsible for the α-cleavage are still elusive, and in contrast to initial studies we confirm more recent investigations that neither ADAM10 nor ADAM17 are involved. Importantly, PrPN1 produced after PrP(C) homodimerization protects against toxic amyloid-β (Aβ) oligomers. Thus, our results show that PrP(C) homodimerization is an important regulator of PrP(C) α-cleavage and may represent a potential therapeutic avenue against Aβ toxicity in Alzheimer's disease.

Separate mechanisms act concurrently to shed and release the prion protein from the cell

The cellular prion protein (PrP (C) ) is attached to the cell membrane via its glycosylphosphatidylinositol (GPI)-anchor and is constitutively shed into the extracellular space. Here, three different mechanisms are presented that concurrently shed PrP (C) from the cell. The fast α-cleavage released a N-terminal fragment (N1) into the medium and the extreme C-terminal cleavage shed soluble full-length (FL-S) PrP and C-terminally cleaved (C1-S) fragments outside the cell. Also, a slow exosomal release of full-length (FL) and C1-fragment (C1) was demonstrated. The three separate mechanisms acting simultaneously, but with different kinetics, have to be taken into consideration when elucidating functional roles of PrP (C) and also when processing of PrP (C) is considered as a target for intervention in prion diseases. Further, in this study it was shown that metalloprotease inhibitors affected the extreme C-terminal cleavage and shedding of PrP (C) . The metalloprotease inhibitors did not influence the α-cleavage or the exosomal release. Taken together, these results are important for understanding the different mechanisms acting in parallel in the shedding and cleavage of PrP (C) .

α-Cleavage of cellular prion protein

The cellular prion protein (PrP^C) is subjected to various processing under physiological and pathological conditions, of which the α-cleavage within the central hydrophobic domain not only disrupts a region critical for both PrP toxicity and PrP^C to PrP^Sc conversion but also produces the N1 fragment that is neuroprotective and the C1 fragment that enhances the pro-apoptotic effect of staurosporine in one report and inhibits prion in another. The proteases responsible for the α-cleavage of PrP^C are controversial. The effect of ADAM10, ADAM17, and ADAM9 on N1 secretion clearly indicates their involvement in the α-cleavage of PrP^C, but there has been no report of direct PrP^C α-cleavage activity with any of the three ADAMs in a purified protein form. We demonstrated that, in muscle cells, ADAM8 is the primary protease for the α-cleavage of PrP^C, but another unidentified protease(s) must also play a minor role. We also found that PrP^C regulates ADAM8 expression, suggesting that a close examination on the relationships between PrP^C and its processing enzymes may reveal novel roles and underlying mechanisms for PrP^C in non-prion diseases such as asthma and cancer.

LIV-1 ZIP ectodomain shedding in prion-infected mice resembles cellular response to transition metal starvation

We recently documented the co-purification of members of the LIV-1 subfamily of ZIP (Zrt-, Irt-like Protein) zinc transporters (LZTs) with the cellular prion protein (PrP(C)) and, subsequently, established that the prion gene family descended from an ancestral LZT gene. Here, we begin to address whether the study of LZTs can shed light on the biology of prion proteins in health and disease. Starting from an observation of an abnormal LZT immunoreactive band in prion-infected mice, subsequent cell biological analyses uncovered a surprisingly coordinated biology of ZIP10 (an LZT member) and prion proteins that involves alterations to N-glycosylation and endoproteolysis in response to manipulations to the extracellular divalent cation milieu. Starving cells of manganese or zinc, but not copper, causes shedding of the N1 fragment of PrP(C) and of the ectodomain of ZIP10. For ZIP10, this posttranslational biology is influenced by an interaction between its PrP-like ectodomain and a conserved metal coordination site within its C-terminal multi-spanning transmembrane domain. The transition metal starvation-induced cleavage of ZIP10 can be differentiated by an immature N-glycosylation signature from a constitutive cleavage targeting the same site. Data from this work provide a first glimpse into a hitherto neglected molecular biology that ties PrP to its LZT cousins and suggest that manganese or zinc starvation may contribute to the etiology of prion disease in mice.

Cellular prion protein regulates its own α-cleavage through ADAM8 in skeletal muscle

The ubiquitously expressed cellular prion protein (PrP(C)) is subjected to the physiological α-cleavage at a region critical for both PrP toxicity and the conversion of PrP(C) to its pathogenic prion form (PrP(Sc)), generating the C1 and N1 fragments. The C1 fragment can activate caspase 3 while the N1 fragment is neuroprotective. Recent articles indicate that ADAM10, ADAM17, and ADAM9 may not play a prominent role in the α-cleavage of PrP(C) as previously thought, raising questions on the identity of the responsible protease(s). Here we show that, ADAM8 can directly cleave PrP to generate C1 in vitro and PrP C1/full-length ratio is greatly decreased in the skeletal muscles of ADAM8 knock-out mice; in addition, the PrP C1/full-length ratio is linearly correlated with ADAM8 protein level in myoblast cell line C2C12 and in skeletal muscle tissues of transgenic mice. These results indicate that ADAM8 is the primary protease responsible for the α-cleavage of PrP(C) in muscle cells. Moreover, we found that overexpression of PrP(C) led to up-regulation of ADAM8, suggesting that PrP(C) may regulate its own α-cleavage through modulating ADAM8 activity.

Prion protein facilitates uptake of zinc into neuronal cells

Zinc is released into the synaptic cleft upon exocytotic stimuli, although the mechanism for its reuptake into neurons is unresolved. Here we show that the cellular prion protein enhances the uptake of zinc into neuronal cells. This prion-protein-mediated zinc influx requires the octapeptide repeats and amino-terminal polybasic region in the prion protein, but not its endocytosis. Selective antagonists of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors block the prion protein-mediated zinc uptake, and the prion protein co-immunoprecipitates with both GluA1 and GluA2 AMPA receptor subunits. Zinc-sensitive intracellular tyrosine phosphatase activity is decreased in cells expressing prion protein and increased in the brains of prion-protein-null mice, providing evidence of a physiological consequence of this process. Prion protein-mediated zinc uptake is ablated in cells expressing familial associated mutants of the protein and in prion-infected cells. These data suggest that alterations in the cellular prion protein-mediated zinc uptake may contribute to neurodegeneration in prion and other neurodegenerative diseases.

A naturally occurring C-terminal fragment of the prion protein (PrP) delays disease and acts as a dominant-negative inhibitor of PrPSc formation

The cellular prion protein (PrPC) undergoes constitutive proteolytic cleavage between residues 111/112 to yield a soluble N-terminal fragment (N1) and a membrane-anchored C-terminal fragment (C1). The C1 fragment represents the major proteolytic fragment of PrPC in brain and several cell types. To explore the role of C1 in prion disease, we generated Tg(C1) transgenic mice expressing this fragment (PrP(Δ23-111)) in the presence and absence of endogenous PrP. In contrast to several other N-terminally deleted forms of PrP, the C1 fragment does not cause a spontaneous neurological disease in the absence of endogenous PrP. Tg(C1) mice inoculated with scrapie prions remain healthy and do not accumulate protease-resistant PrP, demonstrating that C1 is not a substrate for conversion to PrPSc (the disease-associated isoform). Interestingly, Tg(C1) mice co-expressing C1 along with wild-type PrP (either endogenous or encoded by a second transgene) become ill after scrapie inoculation, but with a dramatically delayed time course compared with mice lacking C1. In addition, accumulation of PrPSc was markedly slowed in these animals. Similar effects were produced by a shorter C-terminal fragment of PrP(Δ23-134). These results demonstrate that C1 acts as dominant-negative inhibitor of PrPSc formation and accumulation of neurotoxic forms of PrP. Thus, C1, a naturally occurring fragment of PrPC, might play a modulatory role during the course of prion diseases. In addition, enhancing production of C1, or exogenously administering this fragment, represents a potential therapeutic strategy for the treatment of prion diseases.

Lack of a-disintegrin-and-metalloproteinase ADAM10 leads to intracellular accumulation and loss of shedding of the cellular prion protein in vivo

Background

The cellular prion protein (PrP^C) fulfils several yet not completely understood physiological functions. Apart from these functions, it has the ability to misfold into a pathogenic scrapie form (PrP^Sc) leading to fatal transmissible spongiform encephalopathies. Proteolytic processing of PrP^C generates N- and C-terminal fragments which play crucial roles both in the pathophysiology of prion diseases and in transducing physiological functions of PrP^C. A-disintegrin-and-metalloproteinase 10 (ADAM10) has been proposed by cell culture experiments to be responsible for both shedding of PrP^C and its α-cleavage. Here, we analyzed the role of ADAM10 in the proteolytic processing of PrP^C in vivo.

Results

Using neuron-specific Adam10 knockout mice, we show that ADAM10 is the sheddase of PrP^C and that its absence in vivo leads to increased amounts and accumulation of PrP^C in the early secretory pathway by affecting its posttranslational processing. Elevated PrP^C levels do not induce apoptotic signalling via p53. Furthermore, we show that ADAM10 is not responsible for the α-cleavage of PrP^C.

Conclusion

Our study elucidates the proteolytic processing of PrP^C and proves a role of ADAM10 in shedding of PrP^C in vivo. We suggest that ADAM10 is a mediator of PrP^C homeostasis at the plasma membrane and, thus, might be a regulator of the multiple functions discussed for PrP^C. Furthermore, identification of ADAM10 as the sheddase of PrP^C opens the avenue to devising novel approaches for therapeutic interventions against prion diseases.

Characterization of the prion protein in human urine

The presence of the prion protein (PrP) in normal human urine is controversial and currently inconclusive. This issue has taken a special relevance because prion infectivity has been demonstrated in urine of animals carrying experimental or naturally occurring prion diseases, but the actual presence and tissue origin of the infectious prion have not been determined. We used immunoprecipitation, one- and two-dimensional electrophoresis, and mass spectrometry to prove definitely the presence of PrP in human urine and its post-translational modifications. We show that urinary PrP (uPrP) is truncated mainly at residue 112 but also at other residues up to 122. This truncation makes uPrP undetectable with some commonly used antibodies to PrP. uPrP is glycosylated and carries an anchor which, at variance with that of cellular PrP, lacks the inositol-associated phospholipid moiety, indicating that uPrP is probably shed from the cell surface. The detailed characterization of uPrP reported here definitely proves the presence of PrP in human urine and will help determine the origin of prion infectivity in urine.

Codon 129 polymorphism and the E200K mutation do not affect the cellular prion protein isoform composition in the cerebrospinal fluid from patients with Creutzfeldt-Jakob disease

The cellular prion protein (PrP(c)) is a multifunctional, highly conserved and ubiquitously expressed protein. It undergoes a number of modifications during its post-translational processing, resulting in different PrP(c) glycoforms and truncated PrP(c) fragments. Limited data are available in humans on the expression and cleavage of PrP(c). In this study we investigated the PrP(c) isoform composition in the cerebrospinal fluid from patients with different human prion diseases. The first group of patients was affected by sporadic Creutzfeldt-Jakob disease exhibiting different PrP codon 129 genotypes. The second group contained patients with a genetic form of Creutzfeldt-Jakob disease (E200K). The third group consisted of patients with fatal familial insomnia and the last group comprised cases with the Gerstmann-Sträussler-Scheinker syndrome. We examined whether the PrP codon 129 polymorphism in sporadic Creutzfeldt-Jakob disease as well as the type of prion disease in human patients has an impact on the glycosylation and processing of PrP(c). Immunoblotting analyses using different monoclonal PrP(c) antibodies directed against various epitopes of PrP(c) revealed, for all examined groups of patients, a consistent predominance of the glycosylated PrP(c) isoforms as compared with the unglycosylated form. In addition, the antibody SAF70 recognized a variety of PrP(c) fragments with sizes of 21, 18, 13 and 12 kDa. Our findings indicate that the polymorphisms at PrP codon 129, the E200K mutation at codon 200 or the examined types of human transmissible spongiform encephalopathies do not exert a measurable effect on the glycosylation and processing of PrP(c) in human prion diseases.

Unexpected tolerance of alpha-cleavage of the prion protein to sequence variations

The cellular form of the prion protein, PrP^C, undergoes extensive proteolysis at the α site (109K↓H110). Expression of non-cleavable PrP^C mutants in transgenic mice correlates with neurotoxicity, suggesting that α-cleavage is important for PrP^C physiology. To gain insights into the mechanisms of α-cleavage, we generated a library of PrP^C mutants with mutations in the region neighbouring the α-cleavage site. The prevalence of C1, the carboxy adduct of α-cleavage, was determined for each mutant. In cell lines of disparate origin, C1 prevalence was unaffected by variations in charge and hydrophobicity of the region neighbouring the α-cleavage site, and by substitutions of the residues in the palindrome that flanks this site. Instead, α-cleavage was size-dependently impaired by deletions within the domain 106–119. Almost no cleavage was observed upon full deletion of this domain. These results suggest that α-cleavage is executed by an α-PrPase whose activity, despite surprisingly limited sequence specificity, is dependent on the size of the central region of PrP^C.