The N-terminal cleavage of cellular prion protein in the human brain

Cleavage site-directed antibodies reveal the prion protein in humans is shed by ADAM10 at Y226 and associates with misfolded protein deposits in neurodegenerative diseases

Proteolytic cell surface release ('shedding') of the prion protein (PrP), a broadly expressed GPI-anchored glycoprotein, by the metalloprotease ADAM10 impacts on neurodegenerative and other diseases in animal and in vitro models. Recent studies employing the latter also suggest shed PrP (sPrP) to be a ligand in intercellular communication and critically involved in PrP-associated physiological tasks. Although expectedly an evolutionary conserved event, and while soluble forms of PrP are present in human tissues and body fluids, for the human body neither proteolytic PrP shedding and its cleavage site nor involvement of ADAM10 or the biological relevance of this process have been demonstrated thus far. In this study, cleavage site prediction and generation (plus detailed characterization) of sPrP-specific antibodies enabled us to identify PrP cleaved at tyrosin 226 as the physiological and apparently strictly ADAM10-dependent shed form in humans. Using cell lines, neural stem cells and brain organoids, we show that shedding of human PrP can be stimulated by PrP-binding ligands without targeting the protease, which may open novel therapeutic perspectives. Site-specific antibodies directed against human sPrP also detect the shed form in brains of cattle, sheep and deer, hence in all most relevant species naturally affected by fatal and transmissible prion diseases. In human and animal prion diseases, but also in patients with Alzheimer`s disease, sPrP relocalizes from a physiological diffuse tissue pattern to intimately associate with extracellular aggregated deposits of misfolded proteins characteristic for the respective pathological condition. Findings and research tools presented here will accelerate novel insight into the roles of PrP shedding (as a process) and sPrP (as a released factor) in neurodegeneration and beyond.

Endoproteolysis of cellular prion protein by plasmin hinders propagation of prions

Many questions surround the underlying mechanism for the differential metabolic processing observed for the prion protein (PrP) in healthy and prion-infected mammals. Foremost, the physiological α-cleavage of PrP interrupts a region critical for both toxicity and conversion of cellular PrP (PrP ^C ) into its misfolded pathogenic isoform (PrP ^Sc ) by generating a glycosylphosphatidylinositol (GPI)-anchored C1 fragment. During prion diseases, alternative β-cleavage of PrP becomes prominent, producing a GPI-anchored C2 fragment with this particular region intact. It remains unexplored whether physical up-regulation of α-cleavage can inhibit disease progression. Furthermore, several pieces of evidence indicate that a disintegrin and metalloproteinase (ADAM) 10 and ADAM17 play a much smaller role in the α-cleavage of PrP ^C than originally believed, thus presenting the need to identify the primary protease(s) responsible. For this purpose, we characterized the ability of plasmin to perform PrP α-cleavage. Then, we conducted functional assays using protein misfolding cyclic amplification (PMCA) and prion-infected cell lines to clarify the role of plasmin-mediated α-cleavage during prion propagation. Here, we demonstrated an inhibitory role of plasmin for PrP ^Sc formation through PrP α-cleavage that increased C1 fragments resulting in reduced prion conversion compared with non-treated PMCA and cell cultures. The reduction of prion infectious titer in the bioassay of plasmin-treated PMCA material also supported the inhibitory role of plasmin on PrP ^Sc replication. Our results suggest that plasmin-mediated endoproteolytic cleavage of PrP may be an important event to prevent prion propagation.

The prion protein and its ligands: Insights into structure-function relationships

The prion protein is a multifunctional protein that exists in at least two different folding states. It is subject to diverse proteolytic processing steps that lead to prion protein fragments some of which are membrane-bound whereas others are soluble. A multitude of ligands bind to the prion protein and besides proteinaceous binding partners, interaction with metal ions and nucleic acids occurs. Although of great importance, information on structural and functional consequences of prion protein binding to its partners is limited. Here, we will reflect on the structure-function relationship of the prion protein and its binding partners considering the different folding states and prion protein fragments.

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.

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.

Biological Functions of the Intrinsically Disordered N-Terminal Domain of the Prion Protein: A Possible Role of Liquid-Liquid Phase Separation

The mammalian prion protein (PrPC) is composed of a large intrinsically disordered N-terminal and a structured C-terminal domain, containing three alpha-helical regions and a short, two-stranded beta-sheet. Traditionally, the activity of a protein was linked to the ability of the polypeptide chain to adopt a stable secondary/tertiary structure. This concept has been extended when it became evident that intrinsically disordered domains (IDDs) can participate in a broad range of defined physiological activities and play a major functional role in several protein classes including transcription factors, scaffold proteins, and signaling molecules. This ability of IDDs to engage in a variety of supramolecular complexes may explain the large number of PrPC-interacting proteins described. Here, we summarize diverse physiological and pathophysiological activities that have been described for the unstructured N-terminal domain of PrPC. In particular, we focus on subdomains that have been conserved in evolution.

Amyloid β Perturbs Cu(II) Binding to the Prion Protein in a Site-Specific Manner: Insights into Its Potential Neurotoxic Mechanisms

Amyloid β (Aβ) is a Cu-binding peptide that plays a key role in the pathology of Alzheimer's disease. A recent report demonstrated that Aβ disrupts the Cu-dependent interaction between cellular prion protein (PrP^C) and N-methyl-d-aspartate receptor (NMDAR), inducing overactivation of NMDAR and neurotoxicity. In this context, it has been proposed that Aβ competes for Cu with PrP^C; however, there is no spectroscopic evidence to support this hypothesis. Prion protein (PrP) can bind up to six Cu(II) ions: from one to four at the octarepeat (OR) region, producing low- and high-occupancy modes, and two at the His96 and His111 sites. Additionally, PrP^C is cleaved by α-secretases at Lys110/His111, yielding a new Cu(II)-binding site at the α-cleaved His111. In this study, the competition for Cu(II) between Aβ(1-16) and peptide models for each Cu-binding site of PrP was evaluated using circular dichroism and electron paramagnetic resonance. Our results show that the impact of Aβ(1-16) on Cu(II) coordination to PrP is highly site-specific: Aβ(1-16) cannot effectively compete with the low-occupancy mode at the OR region, whereas it partially removes the metal ion from the high-occupancy modes and forms a ternary OR-Cu(II)-Aβ(1-16) complex. In contrast, Aβ(1-16) removes all Cu(II) ions from the His96 and His111 sites without formation of ternary species. Finally, at the α-cleaved His111 site, Aβ(1-16) yields at least two different ternary complexes depending on the ratio of PrP/Cu(II)/Aβ. Altogether, our spectroscopic results indicate that only the low-occupancy mode at the OR region resists the effect of Aβ, while Cu(II) coordination to the high-occupancy modes and all other tested sites of PrP is perturbed, by either removal of the metal ion or formation of ternary complexes. These results provide important insights into the intricate effect of Aβ on Cu(II) binding to PrP and the potential neurotoxic mechanisms through which Aβ might affect Cu-dependent functions of PrP^C, such as NMDAR modulation.

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.

Regulation of the α-secretase ADAM10 at transcriptional, translational and post-translational levels

A tremendous gain of interest in the biology of ADAM10 emerged during the past 15 years when it has first been shown that this protease was able to target the α-site of the β-amyloid precursor protein (βAPP) and later confirmed as the main physiological α-secretase activity. However, beside its well-established implication in the so-called non-amyloidogenic processing of βAPP and its probable protective role against Alzheimer's disease (AD), this metalloprotease also cleaves many other substrates, thereby being implicated in various physiological as well as pathological processes such as cancer and inflammation. Thus, in view of possible effective therapeutic interventions, a full comprehension of how ADAM10 is up and down regulated is required. This review discusses our current knowledge concerning the implication of this enzyme in AD as well as its more recently established roles in other brain disorders and provides a detailed up-date on its various transcriptional, translational and post-translational modulations.

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.

PrP(C) signalling in neurons: from basics to clinical challenges

The cellular prion protein PrP(C) was identified over twenty-five years ago as the normal counterpart of the scrapie prion protein PrP(Sc), itself the main if not the sole component of the infectious agent at the root of Transmissible Spongiform Encephalopathies (TSEs). PrP(C) is a ubiquitous cell surface protein, abundantly expressed in neurons, which constitute the targets of PrP(Sc)-mediated toxicity. Converging evidence have highlighted that neuronal, GPI-anchored PrP(C) is absolutely required for prion-induced neuropathogenesis, which warrants investigating into the normal function exerted by PrP(C) in a neuronal context. It is now well-established that PrP(C) can serve as a cell signalling molecule, able to mobilize transduction cascades in response to interactions with partners. This function endows PrP(C) with the capacity to participate in multiple neuronal processes, ranging from survival to synaptic plasticity. A diverse array of data have allowed to shed light on how this function is corrupted by PrP(Sc). Recently, amyloid Aβ oligomers, whose accumulation is associated with Alzheimer's disease (AD), were shown to similarly instigate toxic events by deviating PrP(C)-mediated signalling. Here, we provide an overview of the various signal transduction cascades ascribed to PrP(C) in neurons, summarize how their subversion by PrP(Sc) or Aβ oligomers contributes to TSE or AD neuropathogenesis and discuss the ensuing clinical implications.

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.

Genetic studies in human prion diseases

Human prion diseases are fatal neurodegenerative disorders that are characterized by spongiform changes, astrogliosis, and the accumulation of an abnormal prion protein (PrP(Sc)). Approximately 10%-15% of human prion diseases are familial variants that are caused by pathogenic mutations in the prion protein gene (PRNP). Point mutations or the insertions of one or more copies of a 24 bp repeat are associated with familial human prion diseases including familial Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome, and fatal familial insomnia. These mutations vary significantly in frequency between countries. Here, we compare the frequency of PRNP mutations between European countries and East Asians. Associations between single nucleotide polymorphisms (SNPs) of several candidate genes including PRNP and CJD have been reported. The SNP of PRNP at codon 129 has been shown to be associated with sporadic, iatrogenic, and variant CJD. The SNPs of several genes other than PRNP have been showed contradictory results. Case-control studies and genome-wide association studies have also been performed to identify candidate genes correlated with variant and/or sporadic CJD. This review provides a general overview of the genetic mutations and polymorphisms that have been analyzed in association with human prion diseases to date.

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.

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.

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

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

Genetic predictions of prion disease susceptibility in carnivore species based on variability of the prion gene coding region

Mammalian species vary widely in their apparent susceptibility to prion diseases. For example, several felid species developed prion disease (feline spongiform encephalopathy or FSE) during the bovine spongiform encephalopathy (BSE) epidemic in the United Kingdom, whereas no canine BSE cases were detected. Whether either of these or other groups of carnivore species can contract other prion diseases (e.g. chronic wasting disease or CWD) remains an open question. Variation in the host-encoded prion protein (PrP^C) largely explains observed disease susceptibility patterns within ruminant species, and may explain interspecies differences in susceptibility as well. We sequenced and compared the open reading frame of the PRNP gene encoding PrP^C protein from 609 animal samples comprising 29 species from 22 genera of the Order Carnivora; amongst these samples were 15 FSE cases. Our analysis revealed that FSE cases did not encode an identifiable disease-associated PrP polymorphism. However, all canid PrPs contained aspartic acid or glutamic acid at codon 163 which we propose provides a genetic basis for observed susceptibility differences between canids and felids. Among other carnivores studied, wolverine (Gulo gulo) and pine marten (Martes martes) were the only non-canid species to also express PrP-Asp163, which may impact on their prion diseases susceptibility. Populations of black bear (Ursus americanus) and mountain lion (Puma concolor) from Colorado showed little genetic variation in the PrP protein and no variants likely to be highly resistant to prions in general, suggesting that strain differences between BSE and CWD prions also may contribute to the limited apparent host range of the latter.

Prion processing: a double-edged sword?

The events leading to the degradation of the endogenous PrP(C) (normal cellular prion protein) have been the subject of numerous studies. Two cleavage processes, α-cleavage and β-cleavage, are responsible for the main C- and N-terminal fragments produced from PrP(C). Both cleavage processes occur within the N-terminus of PrP(C), a region that is significant in terms of function. α-Cleavage, an enzymatic event that occurs at amino acid residues 110 and 111 on PrP(C), interferes with the conversion of PrP(C) into the prion disease-associated isoform, PrP(Sc) (abnormal disease-specific conformation of prion protein). This processing is seen as a positive event in terms of disease development. The study of β-cleavage has taken some surprising turns. β-Cleavage is brought about by ROS (reactive oxygen species). The C-terminal fragment produced, C2, may provide the seed for the abnormal conversion process, as it resembles in size the fragments isolated from prion-infected brains. There is, however, strong evidence that β-cleavage provides an essential process to reduce oxidative stress. β-Cleavage may act as a double-edged sword. By β-cleavage, PrP(C) may try to balance the ROS levels produced during prion infection, but the C2 produced may provide a PrP(Sc) seed that maintains the prion conversion process.

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.

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.

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.

Biochemical and strain properties of CJD prions: complexity versus simplicity

Prions, the agents responsible for transmissible spongiform encephalopathies, are infectious proteins consisting primarily of scrapie prion protein (PrP(Sc)), a misfolded, β-sheet enriched and aggregated form of the host-encoded cellular prion protein (PrP(C)). Their propagation is based on an autocatalytic PrP conversion process. Despite the lack of a nucleic acid genome, different prion strains have been isolated from animal diseases. Increasing evidence supports the view that strain-specific properties may be enciphered within conformational variations of PrP(Sc). In humans, sporadic Creutzfeldt-Jakob disease (sCJD) is the most frequent form of prion diseases and has demonstrated a wide phenotypic and molecular spectrum. In contrast, variant Creutzfeldt-Jakob disease (vCJD), which results from oral exposure to the agent of bovine spongiform encephalopathy, is a highly stereotyped disease, that, until now, has only occurred in patients who are methionine homozygous at codon 129 of the PrP gene. Recent research has provided consistent evidence of strain diversity in sCJD and also, unexpectedly enough, in vCJD. Here, we discuss the puzzling biochemical/pathological diversity of human prion disorders and the relationship of that diversity to the biological properties of the agent as demonstrated by strain typing in experimental models.

Endogenous prion protein attenuates experimentally induced colitis

Although the cellular prion protein (PrP(C)) is expressed in the enteric nervous system and lamina propria, its function(s) in the gut is unknown. Because PrP(C) may exert a cytoprotective effect in response to various physiologic stressors, we hypothesized that PrP(C) expression levels might modulate the severity of experimental colitis. We evaluated the course of dextran sodium sulfate (DSS)-induced colitis in hemizygous Tga20 transgenic mice (approximately sevenfold overexpression of PrP(C)), Prnp(-/-) mice, and wild-type mice. On day 7, colon length, disease severity, and histologic activity indices were determined. Unlike DSS-treated wild-type and Prnp(-/-) animals, PrP(C) overexpressing mice were resistant to colitis induction, exhibited much milder histopathologic features, and did not exhibit weight loss or colonic shortening. In keeping with these results, pro-survival molecule expression and/or phosphorylation levels were elevated in DSS-treated Tga20 mice, whereas pro-inflammatory cytokine production and pSTAT3 levels were reduced. In contrast, DSS-treated Prnp(-/-) mice exhibited increased BAD protein expression and a cytokine expression profile predicted to favor inflammation and differentiation. PrP(C) expression from both the endogenous Prnp locus or the Tga20 transgene was increased in the colons of DSS-treated mice. Considered together, these findings demonstrate that PrP(C) has a previously unrecognized cytoprotective and/or anti-inflammatory function within the murine colon.

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.

The extracellular regulated kinase-1 (ERK1) controls regulated alpha-secretase-mediated processing, promoter transactivation, and mRNA levels of the cellular prion protein

The α-secretases A disintegrin and metalloprotease 10 (ADAM10) and ADAM17 trigger constitutive and regulated processing of the cellular prion protein (PrP(c)) yielding N1 fragment. The latter depends on protein kinase C (PKC)-coupled M1/M3 muscarinic receptor activation and subsequent phosphorylation of ADAM17 on its intracytoplasmic threonine 735. Here we show that regulated PrP(c) processing and ADAM17 phosphorylation and activation are controlled by the extracellular-regulated kinase-1/MAP-ERK kinase (ERK1/MEK) cascade. Thus, reductions of ERK1 or MEK activities by dominant-negative analogs, pharmacological inhibition, or genetic ablation all impair N1 secretion, whereas constitutively active proteins increase N1 recovery in the conditioned medium. Interestingly, we also observed an ERK1-mediated enhanced expression of PrP(c). We demonstrate that the ERK1-associated increase in PrP(c) promoter transactivation and mRNA levels involve transcription factor AP-1 as a downstream effector. Altogether, our data identify ERK1 as an important regulator of PrP(c) cellular homeostasis and indicate that this kinase exerts a dual control of PrP(c) levels through transcriptional and post-transcriptional mechanisms.

Upregulation of the alpha-secretase ADAM10--risk or reason for hope?

A decade ago, a disintegrin and metalloproteinase 10 (ADAM10) was identified as an alpha-secretase and as a key proteinase in the processing of the amyloid precursor protein. Accordingly, the important role that it plays in Alzheimer's disease was manifested. Animal models with an overexpression of ADAM10 revealed a beneficial profile of the metalloproteinase with respect to learning and memory, plaque load and synaptogenesis. Therefore, ADAM10 presents a worthwhile target with respect to the treatment of a neurodegenerative disease such as Morbus Alzheimer. Initially, ADAM10 was suggested to be an enzyme, shaping the extracellular matrix by cleavage of collagen type IV, or to be a tumour necrosis factor alpha convertase. In a relatively short time, a wide variety of additional substrates (with amyloid precursor protein probably being the most prominent) has been identified and the search is still ongoing. Hence, any side effects concerning the therapeutic enhancement of ADAM10 alpha-secretase activity have to be considered. The present review summarizes our knowledge about the structure and function of ADAM10 and highlights the opportunities for enhancing the expression and/or activity of the alpha-secretase as a therapeutic target.

Influence of ADAM10 on prion protein processing and scrapie infectiosity in vivo

Both the cellular prion protein (PrP(c)) and the amyloid precursor protein (APP) are physiologically subjected to complex proteolytic processing events. While for APP the proteinases involved--alpha-, beta- and gamma-secretase--have been identified in vitro and in vivo, the cleavage of PrP(c) by now has been linked only to the shedding activity of the metalloproteinase ADAM10 and/or ADAM17 in cell culture. Here we show that neuronal overexpression of the alpha-secretase ADAM10 in mice reduces all PrP(c) species detected in the brain instead of leading to enhanced amounts of specific cleavage products of PrP(c). Additionally, the incubation time of mice after scrapie infection is significantly increased in mice moderately overexpressing ADAM10. This indicates that overexpression of ADAM10 rather influences the amount of the cellular prion protein than its processing in vivo.

A potential mechanism for Cu2+ reduction, beta-cleavage, and beta-sheet initiation within the N-terminal domain of the prion protein: insights from density functional theory and molecular dynamics calculations

The N-terminal region of the native human prion protein encompasses four highly conserved octarepeats that each contain a single His, Pro, Gln, and Trp residue as well as several Gly residues. At neutral pH these repeats are capable of individually binding copper (Cu(2+)) ions, involving the His side chain and the backbone amide of the Gly residues. In addition, the two His residues at positions 96 and 111 are also capable of binding Cu(2+). At low concentrations of the metal ion or at low pH, one Cu(2+) may be bound by multiple His residues of the four octarepeats. This complex is known to be redox active, while none of the other Cu(2+)-bound complexes are. Using density functional theory and molecular dynamics calculations data demonstrated how this form of the protein could reduce Cu(2+), through a process involving electron transfer from the Trp side chain. The reduced Cu gives rise to reactive oxygen species (ROS), which can lead to beta-cleavage of the prion protein chain at any of the Gly residues around position 90. Protein fragments of lengths similar to those arising from beta-cleavage are predominantly found in both healthy and Creutzfeldt-Jakob disease (CJD)-affected brains. Models of Cu binding to the His96 and His111 residues also indicate that different modes of Cu(2+) binding result in formation of stable beta-hairpin structures in this region of the protein. It is postulated that through interactions with the C-terminal part of the protein these hairpins may initiate misfolding and yield more stable beta-sheet structures that might associate in the same fashion with additional prion proteins.

Molecular characterization of chicken prion proteins by C-terminal-specific monoclonal antibodies

We obtained seven monoclonal antibodies (mAbs) against chicken cellular prion protein (ChPrP(C)) by immunizing BALB/c mice with recombinant prion protein (rChPrP). Of the seven mAbs, two mAbs (6 and 26) could recognize rChPrP, but not ChPrP(C), in chicken brain lysate via Western blot (WB) analysis. Three C-terminal linear epitopes (AAANQTEVEM, RWWS and SPVPQD) were identified in ChPrP amino acids by pepspot analysis with five mAbs. The mAbs recognizing the C-terminal epitopes in ChPrP(C) predominantly reacted with the N-terminal truncated ChPrP(C) in WB analysis, which differed from the reaction with N-terminal proline/glycine-rich repeats recognizing rabbit polyclonal antibody. These mAbs will soon be available as a useful tool to characterize the biology of ChPrP(C) in birds.

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

Endoproteolysis of the cellular prion protein (PrP(C)) modulates both the normal function of the protein and the pathogenesis of the neurodegenerative prion diseases. PrP(C) undergoes alpha-cleavage to generate the N-terminally truncated fragment C1. Utilizing various constructs of PrP(C) expressed in human neuroblastoma cells we investigated the subcellular compartment where alpha-cleavage occurs. C1 was detected at the cell surface and the generation of C1 occurred in mutants of PrP(C) incapable of Cu2+-mediated endocytosis. A transmembrane-anchored form that is not lipid raft-localised, as well as a secreted construct lacking the glycosyl-phosphatidylinositol membrane anchor, were also subject to alpha-cleavage. However, when this transmembrane-anchored form was modified with an endoplasmic reticulum retention motif, C1 was not formed. Inhibition of protein export from the Golgi by temperature block increased the amount of C1. Our data thus demonstrate that the alpha-cleavage of PrP(C) occurs predominantly in a raft-independent manner in a late compartment of the secretory pathway.

Isoform-specific contribution of protein kinase C to prion processing

The cellular prion protein (PrP(c)) undergoes a physiological cleavage between amino acids 111 and 112, thereby leading to the secretion of an amino-terminal fragment referred to as N1. This proteolytic event is either constitutive or regulated by protein kinase C (PKC) and is operated by the disintegrins ADAM9/ADAM10 or ADAM17 respectively. We recently showed that the stimulation of the M1/M3 muscarinic receptors potentiates this cleavage via the phosphorylation and activation of ADAM17. We have examined the contribution of various PKC isoforms in the regulated processing of PrP(c). First we show that the PDBu- and carbachol-stimulated N1 secretions are blocked by the general PKC inhibitor GF109203X. We establish that HEK293 and human-derived rhabdhomyosarcoma cells over-expressing constitutively active PKCalpha, PKCdelta or PKCepsilon, but not PKCzeta, produce increased amounts of N1 and harbor enhanced ability to hydrolyze the fluorimetric substrate of ADAM17, JMV2770. Conversely, over-expression of the corresponding dominant negative proteins abolishes PDBU-stimulated N1 secretion and restores N1 to levels comparable to constitutive production. Moreover, deletion of PKCalpha lowers N1 recovery in primary cultured fibroblasts. Importantly, mutation of threonine 735 of ADAM17 significantly lowers the PDBu-induced N1 formation while transient over-expression of constitutively active PKCalpha, PKCdelta or PKCepsilon, but not PKCzeta, induced both the phosphorylation of ADAM17 on its threonine residues and N1 secretion. As a corollary, T735A mutation concomitantly reversed PKCalpha-, PKCdelta- and PKCepsilon-induced ADAM17 phosphorylation and N1 recovery. Finally, we established that PKCepsilon-dependent N1 production is fully prevented by ADAM17 deficiency. Altogether, the present results provide strong evidence that the activation of PKCalpha, delta and epsilon, but not zeta, isoforms leads to increased N1 secretion via the phosphorylation and activation of ADAM17, a process that likely accounts for M1/M3 muscarinic receptors-mediated control of N1 production.

Anterograde axonal transport of chicken cellular prion protein (PrPc) in vivo requires its N-terminal part

The cellular isoform of prion protein (PrP(c)) can exist in membrane-bound and secreted forms. Both forms of PrP(c) can be transported by retinal ganglion cell (RGC) axons along the optic nerve in the anterograde direction. In this study we determined which part of chicken PrP(c) is required for its anterograde axonal transport within the optic nerve of embryonic chicken. We intraocularly injected radio-iodinated fragments of recombinant chicken PrP(c) and then examined their anterograde axonal transport from retina into optic tectum. Using gamma-counting and different autoradiographic techniques we quantified anterograde axonal transport of the N-terminal part of chicken PrP(c) (amino acid residues 1-116) in this model system. The transport of the N-terminal part has similar properties as the anterograde transport of full-length chicken PrP(c) (Butowt et al., 2006) described previously (e.g., has similar efficiency, is microtubule-dependent, and is saturable). Moreover, the pattern of ultrastructural distribution of the N-terminal fragment within RGCs is similar to the distribution of full-length PrP(c). The C-terminal fragment of chicken PrP(c) (residues 118-246) and different PrP-derived peptides were not transported. Moreover, PrP(c)-derived peptides were sorted into different endocytotic pathways in neurons, indicating that they cannot substitute for full-length PrP(c) to study its internalization and trafficking. These data indicate that the N-terminal half of chicken PrP(c) contains the necessary information to drive the internalization and subsequent sorting of extracellular PrP(c) in RGCs soma into the anterograde axonal transport pathway.

Developmental changes in cellular prion protein in primate visual cortex

Cellular prion protein (PrP(c)) is a cell surface glycoprotein highly expressed in neurons, and a protease-resistant conformer of the protein accumulates in the brain parenchyma in prion diseases. In human prion diseases, visual cortex and visual function can be affected. We examined both the levels and the localization of PrP(c) in developing visual cortex of the common marmoset. Western blot analysis showed that PrP(c) increased from the day of birth through adulthood, and this increase correlated with the progression of synapse formation. Immunohistochemistry showed that PrP(c) was present in fiber tracts of the neonate, and this immunoreactivity was lost with maturation. Within the neuropil, the laminar distribution of PrP(c) changed with age. In the neonate, PrP(c) immunoreactivity was strongest in layer 1, where the earliest synapses form. At the end of the first postnatal week, layer 4C, as identified by its strong cytochrome oxidase activity, was noticeably lighter in terms of PrP(c) immunoreactivity than the adjacent layers. The contrast between the strong immunoreactivity in both supragranular and infragranular layers and weak immunoreactivity in layer 4C increased with age. Layers 2/3 and 5 contained more intense PrP(c) immunoreactivity; these layers receive thalamic input from the koniocellular division of the LGN, and these layers of the LGN also had strong PrP(c) immunoreactivity. Together, these results provide evidence for PrP(c) localization in an identified functional pathway and may shed some light on prion disease pathogenesis.

M1 and M3 muscarinic receptors control physiological processing of cellular prion by modulating ADAM17 phosphorylation and activity

The cellular prion protein (PrP(c)) undergoes a physiological processing yielding the N-terminal fragment referred to as N1, the production of which can be constitutive or protein kinase C regulated. We show that activation of endogenous muscarinic receptors by carbachol and by the M1-selective agonist AF267B increases N1 recovery in an atropine-sensitive manner, in mouse embryonic primary neurons. To identify the muscarinic receptor subtype involved, we used human embryonic kidney HEK293 (HEK) cells stably overexpressing M1, M2, M3, or M4 receptor subtype. Carbachol and the selective M1 agonist AF267B dose dependently increased N1 release by HEK-M3 and HEK-M1 cells, respectively, whereas carbachol did not modify N1 production by HEK-M2 or HEK-M4 cells. We demonstrate that the increase of N1 was not attributable to modified trafficking to the membrane of either PrP(c) or the disintegrin metalloproteases ADAM10 or ADAM17. Furthermore, we establish that carbachol affects the overall phosphorylation of ADAM17 on its threonine and tyrosine but not serine residues, whereas levels of phosphorylated ADAM9 were not affected. Interestingly, carbachol also increases the hydrolysis of the fluorimetric substrate JMV2770, which mimicked the sequence encompassing the N1 site cleavage and was shown previously to behave as an ADAM protease substrate. Mutations of threonine 735 but not of tyrosine 702 of the ADAM17 cytoplasmic tail abolishes the carbachol-induced increase of N1, ADAM17 phosphorylation, and JMV2770-hydrolyzing activity in M1- and M3-expressing HEK293 cells. Thus, our data provide strong evidence that muscarinic receptor activation increases the physiological processing of PrP(c) by upregulating the phosphorylation state and activity of ADAM17 protease.

Regional and phenotype heterogeneity of cellular prion proteins in the human brain

Transmissible spongiform encephalopathies (TSEs) are neurological disorders that include genetic, infectious and sporadic forms of human Creutzfeldt-Jakob disease (CJD). The pathogenic agent is the prion protein that is composed of an abnormal isoform (PrP(Sc)) of a host-encoded protein (PrP(C)). Analysis of the relative amounts of PrP(Sc) glycoforms has been used to discriminate between various agents involved in TSE. The distribution and efficiency of conversion to PrP(Sc) can be influenced by differences in the expression of PrP(C). However, little attention has been given so far to the banding patterns of PrP(C). Using four different antibodies recognizing amino- and carboxyl-terminal PrP sequences we analysed the glycoforms of PrP(C) in seven regions of the human brain using brains obtained from six subjects. For determination of the staining intensities, signals were quantified by densitometry and reproducible patterns were accomplished by many repeated immunoblot analyses. When amino-terminal binding antibodies were used for detection, PrP(C) in the frontal neocortex, nucleus lentiformis, thalamus, hippocampus and cerebellum displayed a glycotype with high staining of the diglycosylated isoforms. This was different from patterns in the pons and medulla oblongata, which showed a high intensity of the nonglycosylated isoform, and PrP(C) proteins, approximately 27 kDa in size, exhibited high staining using the carboxyl-terminal binding antibodies. This intense staining followed from an overlay of full-length and truncated PrP(C) isoforms. Furthermore, we found marked differences in the expression of PrP(C). Variations in the processing of PrP(C) may lead to interregional differences in the glycoform composition of PrP(Sc) in human brains.

Prion protein in milk

Background

Prions are known to cause transmissible spongiform encephalopathies (TSE) after accumulation in the central nervous system. There is increasing evidence that prions are also present in body fluids and that prion infection by blood transmission is possible. The low concentration of the proteinaceous agent in body fluids and its long incubation time complicate epidemiologic analysis and estimation of spreading and thus the risk of human infection. This situation is particularly unsatisfactory for food and pharmaceutical industries, given the lack of sensitive tools for monitoring the infectious agent.

Methodology/Principal Findings

We have developed an adsorption matrix, Alicon PrioTrap®, which binds with high affinity and specificity to prion proteins. Thus we were able to identify prion protein (PrP^C)–the precursor of prions (PrP^Sc)–in milk from humans, cows, sheep, and goats. The absolute amount of PrP^C differs between the species (from µg/l range in sheep to ng/l range in human milk). PrP^C is also found in homogenised and pasteurised off-the-shelf milk, and even ultrahigh temperature treatment only partially diminishes endogenous PrP^C concentration.

Conclusions/Significance

In view of a recent study showing evidence of prion replication occurring in the mammary gland of scrapie infected sheep suffering from mastitis, the appearance of PrP^C in milk implies the possibility that milk of TSE-infected animals serves as source for PrP^Sc.

The C-terminal products of cellular prion protein processing, C1 and C2, exert distinct influence on p53-dependent staurosporine-induced caspase-3 activation

The cellular prion protein (PrP(c)) undergoes various endopro-teolytic attacks within its N-terminal domain, leading to the production of C-terminal fragments (C) tethered to the plasma membrane and soluble N-terminal peptides (N). One of these cleavages occurs at position 110/111, thereby generating C1 and N1 products. We have reported that disintegrins ADAM-10, -9, and -17 participate either directly or indirectly to this proteolytic event. An alternative proteolytic event taking place around residue 90 yields C2 and N2 fragments. The putative function of these proteolytic fragments remained to be established. We have set up two novel human embryonic kidney 293 cell lines stably overexpressing either C1 or C2. We show that C1 potentiates staurosporine-induced caspase-3 activation through a p53-dependent mechanism. Thus, C1 positively controls p53 transcription and mRNA levels and increases p53-like immunoreactivity and activity. C1-induced caspase-3 activation remained unaffected by the blockade of endocytosis in HEK 293 cells and was abolished in p53-deficient fibroblasts. Conversely, overexpression of the C2 fragment did not significantly sensitize HEK 293 cells to apoptotic stimuli and did not modify p53 mRNA levels or activity. Therefore, the nature of the proteolytic cleavage taking place on PrP(c) yielded C-terminal catabolites with distinct function and could be seen as a switch mechanism controlling the function of the PrP(c) in cell survival.

Delineating common molecular mechanisms in Alzheimer's and prion diseases

The structure of the infectious agent responsible for prion diseases has not been fully characterized, but evidence points to a beta-rich conformer of the host-encoded prion protein. Amyloid-beta peptide (Abeta), a proteolytic fragment generated from the amyloid precursor protein, has been implicated as the toxic molecule involved in the pathogenesis of Alzheimer's disease. The mechanism of Abeta toxicity might be mediated through the coordination of redox-active transition-metal ions such as copper leading to the generation of reactive oxygen species, coupled with the propensity to interact with lipid bilayers. Key sequence and chemical similarities between prion protein (PrP) and Abeta indicate that similar therapeutic strategies might be applicable for the treatment of Alzheimer's and prion diseases.

Design and characterization of a novel cellular prion-derived quenched fluorimetric substrate of alpha-secretase

Under normal conditions, the cellular prion protein (PrP(c)) undergoes a proteolytic attack between amino acids 111 and 112 which gives rise to the N-terminal secreted N1 fragment and its C-terminal membrane-tethered counterpart C1. Importantly, this cleavage precludes the integrity of the neurotoxic 106-126 sequence. Here, we describe an original and reliable assay based on a quenched fluorimetric substrate (JMV2770) encompassing the 111/112 sequence of PrP(c). In whole brain homogenate, the JMV2770-hydrolysing activity is optimal at neutral pH and sensitive to the metalloprotease inhibitor BB3103 but not to acidic and serine protease blockers. JMV2770 is efficiently cleaved by intact HEK293 cells and fibroblasts in culture, consistent with an hydrolysis by a typical ectoprotease. Overexpressions of alpha-secretases a disintegrin and metalloprotease-9 (ADAM9), ADAM10 or TACE (ADAM17) in human cells increase BB3103-sensitive JMV2770 hydrolysis, while invalidation of ADAM10 and TACE or reduced expression of ADAM9 by an antisense approach significantly reduced its cleavage. Finally, analysis of JMV2770 hydrolysis following transient transfection of ADAM10 or ADAM9 cDNA in ADAM10(-/-) fibroblasts allowed to confirm our previous data establishing that ADAM9 does not behave as a genuine alpha-secretase but rather acts as an important upstream regulator of ADAM10 activity.

Truncated PrP(c) in mammalian brain: interspecies variation and location in membrane rafts

A key molecular event in prion diseases is the conversion of cellular prion protein (PrP(c)) into an abnormal misfolded conformer (PrP(sc)). The PrP(c) N-terminal domain plays a central role in PrP(c) functions and in prion propagation. Because mammalian PrP(c) is found as a full-length and N-terminally truncated form, we examined the presence and amount of PrP(c) C-terminal fragment in the brain of different species. We found important variations between primates and rodents. In addition, our data show that the PrP(c) fragment is present in detergent-resistant raft domains, a membrane domain of critical importance for PrP(c) functions and its conversion into PrP(sc).