Evidence for synthesis of scrapie prion proteins in the endocytic pathway

Transmissible spongiform encephalopathies

Scrapie, bovine spongiform encephalopathy (BSE), and the Creutzfeldt-Jakob disease (CJD) belong to a group of lethal neurodegenerative disorders in mammals. Prion diseases or transmissible spongiform encephalopathies (TSEs) are characterized by the accumulation of an abnormal isoform (PrPSc) of the host-encoded cellular prion protein (PrPC) in the brain. The infectious agent, the 'prion,' is believed to be devoid of informational nucleic acid and to consist largely, if not entirely, of PrPSc. The PrP isoforms contain identical amino acid sequences yet differ in their overall secondary structure with the PrPSc isoform possessing a higher beta-sheet and lower alpha-helix content than PrPC. Elucidation of the three-dimensional structure of PrPC has provided important clues on the molecular basis of inherited human TSEs and on the species barrier phenomenon of TSEs. Nevertheless, the molecular mechanism of the conformational rearrangement of PrPC into PrPSc is still unknown, mainly due to the lack of detailed structural information on PrPSc. Within the framework of the 'protein only' hypothesis, two plausible models for the self-replication of prions have been suggested, the conformational model and the nucleation-dependent polymerization model.

A scrapie-like unfolding intermediate of the prion protein domain PrP(121-231) induced by acidic pH

The infectious agent of transmissible spongiform encephalopathies is believed to consist of an oligomeric isoform, PrPSc, of the monomeric cellular prion protein, PrPC. The conversion of PrPC to PrPSc is characterized by a decrease in alpha-helical structure, an increase in beta-sheet content, and the formation of PrPSc amyloid. Whereas the N-terminal part of PrPC comprising residues 23-120 is flexibly disordered, its C-terminal part, PrP(121-231), forms a globular domain with three alpha-helices and a small beta-sheet. Because the segment of residues 90-231 is protease-resistant in PrPSc, it is most likely structured in the PrPSc form. The conformational change of the segment containing residues 90-120 thus constitutes the minimal structural difference between PrPC and a PrPSc monomer. To test whether PrP(121-231) is also capable to undergo conformational transitions, we analyzed its urea-dependent unfolding transitions at neutral and acidic pH. We identified an equilibrium unfolding intermediate of PrP(121-231) that is exclusively populated at acidic pH and shows spectral characteristics of a beta-sheet protein. The intermediate is in rapid equilibrium with native PrP(121-231), significantly populated in the absence of urea at pH 4.0, and may have important implications for the presumed formation of PrPSc during endocytosis.

A prion protein fragment primes type 1 astrocytes to proliferation signals from microglia

Giliosis is a hallmark of prion disease. A neurotoxic prion peptide (PrP106-126) induces astrocyte proliferation in the presence of microglia. This peptide also directly enhances microglial proliferation in culture. We have investigated this further to understand the method by which factors released by microglia and PrP106-126 work together to enhance astrocyte proliferation. PrP106-126 in the presence of microglia specifically enhanced type 1 astrocyte proliferation but not Type 2. Astrocytes that do not express the prion protein were more sensitive to oxidative stress and the toxicity of cytosine arabinoside. In the presence of cytosine arabinoside, PrP106-126 was toxic to pure astrocyte cultures. Using conditioned medium from microglia we have shown that PrPc-expressing astrocytes proliferate in response to factors released by microglia stimulated by granulocyte/macrophage colony-stimulating factor. This response is enhanced in the presence of PrP106-126. PrPc-deficient astrocytes do not show this response. These results suggest that astrocytes are primed by PrP106-126 to respond more to factors released by proliferating microglia. Astrocytes may proliferate in this system to escape entering the cell suicide pathway.

Chaperone-supervised conversion of prion protein to its protease-resistant form

Transmissible spongiform encephalopathies (TSEs) are lethal, infectious disorders of the mammalian nervous system. A TSE hallmark is the conversion of the cellular protein PrPC to disease-associated PrPSc (named for scrapie, the first known TSE). PrPC is protease-sensitive, monomeric, detergent soluble, and primarily alpha-helical; PrPSc is protease-resistant, polymerized, detergent insoluble, and rich in beta-sheet. The "protein-only" hypothesis posits that PrPSc is the infectious TSE agent that directly converts host-encoded PrPC to fresh PrPSc, harming neurons and creating new agents of infection. To gain insight on the conformational transitions of PrP, we tested the ability of several protein chaperones, which supervise the conformational transitions of proteins in diverse ways, to affect conversion of PrPC to its protease-resistant state. None affected conversion in the absence of pre-existing PrPSc. In its presence, only two, GroEL and Hsp104 (heat shock protein 104), significantly affected conversion. Both promoted it, but the reaction characteristics of conversions with the two chaperones were distinct. In contrast, chemical chaperones inhibited conversion. Our findings provide new mechanistic insights into nature of PrP conversions, and provide a new set of tools for studying the process underlying TSE pathogenesis.

Complementary hydropathy identifies a cellular prion protein receptor

Prions, the etiological agents for infectious degenerative encephalopathies, act by entering the cell and inducing conformational changes in PrPC (a normal cell membrane sialoglycoprotein), which result in cell death. A specific cell-surface receptor to mediate PrPC and prion endocytosis has been predicted. Complementary hydropathy let us generate a hypothetical peptide mimicking the receptor binding site. Antibodies raised against this peptide stain the surface of mouse neurons and recognize a 66-kDa membrane protein that binds PrPC both in vitro and in vivo. Furthermore, both the complementary prion peptide and antiserum against it inhibit the toxicity of a prion-derived peptide toward neuronal cells in culture. Such reagents might therefore have therapeutic applications.

pH-dependent stability and conformation of the recombinant human prion protein PrP(90-231)

A recombinant protein corresponding to the human prion protein domain encompassing residues 90-231 (huPrP(90-231)) was expressed in Escherichia coli in a soluble form and purified to homogeneity. Spectroscopic data indicate that the conformational properties and the folding pathway of huPrP(90-231) are strongly pH-dependent. Acidic pH induces a dramatic increase in the exposure of hydrophobic patches on the surface of the protein. At pH between 7 and 5, the unfolding of hPrP(90-231) in guanidine hydrochloride occurs as a two-state transition. This contrasts with the unfolding curves at lower pH values, which indicate a three-state transition, with the presence of a stable protein folding intermediate. While the secondary structure of the native huPrP(90-231) is largely alpha-helical, the stable intermediate is rich in beta-sheet structure. These findings have important implications for understanding the initial events on the pathway toward the conversion of the normal into the pathological forms of prion protein.

Molecular properties of complexes formed between the prion protein and synthetic peptides

Complexes of the Syrian hamster cellular prion protein (PrPC) and synthetic Syrian hamster PrP peptides were found to mimic many of the characteristics of the scrapie PrP isoform (PrPSc). Either PrPC expressed in chinese hamster ovary (CHO) cells or a C-terminal fragment of 142 residues of recombinant PrP protein (rPrP) produced in Escherichia coli was mixed with an excess of a synthetic 56 amino acid peptide, denoted PrP(90-145). Complex formation required PrPC or rPrP to be destabilized by guanidine hydrochloride (GdnHCl) or urea and PrP(90-145) to be in a coil conformation; it was enhanced by an acidic environment, salt and detergent. If PrP(90-145) was in a beta-sheet conformation, then no complexes were formed. While complex formation was rapid, acquisition of protease resistance was a slow process. Amorphous aggregates with a PrPC/PrP(90-145) ratio of 1:1 were formed in phosphate buffer, whereas fibrils with a diameter of approximately 10 nm and a PrPC/PrP(90-145) ratio of 1:5 were formed in Tris buffer. The complexes were stable only in the presence of excess peptide in either the coil or beta-sheet conformation; they dissociated rapidly after centrifugation and resuspension in buffer without peptide. Neither a peptide having a similar hydrophobicity profile/charge distribution to PrP(90-145) nor a scrambled version, denoted hPrP(90-145) and sPrP(90-145), respectively, were able to induce complex formation. Although hPrP(90-145) could stabilize the PrPC/PrP(90-145) complexes, sPrP(90-145) could not. Studies of PrPC/peptide complexes may provide insights into how PrPC interacts with PrPSc during the formation of a nascent PrPSc molecule and into the process by which PrPC is converted into PrPSc.

Identification of intermediate steps in the conversion of a mutant prion protein to a scrapie-like form in cultured cells

The central causative event in infectious, familial, and sporadic forms of prion disease is thought to be a conformational change that converts the cellular isoform of the prion protein (PrPC) into the scrapie isoform (PrPSc) that is the primary constituent of infectious prion particles. To provide a model system for analyzing the mechanistic details of this critical transformation, we have previously prepared cultured Chinese hamster ovary cells that stably express mouse PrP molecules carrying mutations homologous to those seen in familial prion diseases of humans. In the present work, we have analyzed the kinetics with which a PrP molecule containing an insertional mutation associated with Creutzfeldt-Jakob disease acquires several biochemical properties characteristic of PrPSc. Within 10 min of pulse labeling, the mutant protein undergoes a molecular alteration that is detectable by a change in Triton X-114 phase partitioning and phenyl-Sepharose binding. After 30 min of labeling, a detergent-insoluble and protease-sensitive form of the protein appears. After a chase period of several hours, the protein becomes protease-resistant. Incubation of cells at 18 degrees C or treatment with brefeldin A inhibits acquisition of detergent insolubility and protease resistance but does not affect Triton X-114 partitioning and phenyl-Sepharose binding. Our results support a model in which conversion of mutant PrPs to a PrPSc-like state proceeds in a stepwise fashion via a series of identifiable biochemical intermediates, with the earliest step occurring during or very soon after synthesis of the polypeptide in the endoplasmic reticulum.

Conformational properties of the prion octa-repeat and hydrophobic sequences

We have used circular dichroism to study synthetic peptides from two important regions of the prion protein: the N-terminal octa-repeat domain and a highly conserved hydrophobic section. Our results show that the octa-repeat sequence in free solution can adopt a non-random, extended conformation with properties similar to the poly-L-proline type II left-handed helix. We also show that the conformation can be changed by temperature, organic solvents (e.g. acetonitrile) and on binding to phospholipid vesicles. We compared CD data from two peptides corresponding to the hydrophobic region between residues 106 and 136 which contained either methionine or valine at position 129. This variation represents a common polymorphism in humans which has been shown to influence predisposition towards iatrogenic and sporadic CJD. There was no detectable difference between the CD spectra of these peptides irrespective of the solvent conditions we used.

COOH-terminal sequence of the cellular prion protein directs subcellular trafficking and controls conversion into the scrapie isoform

Efficient formation of scrapie isoform of prion protein (PrP(Sc)) requires targeting PrP(Sc) by glycophosphatidyl inositol (GPI) anchors to caveolae-like domains (CLDs). Redirecting the cellular isoform of prion protein (PrP(C)) to clathrin-coated pits by creating chimeric PrP molecules with four different COOH-terminal transmembrane domains prevented the formation of PrP(Sc). To determine if these COOH-terminal transmembrane segments prevented PrP(C) from refolding into PrP(Sc) by altering the structure of the polypeptide, we fused the 28-aa COOH termini from the Qa protein. Two COOH-terminal Qa segments differing by a single residue direct the transmembrane protein to clathrin-coated pits or the GPI form to CLDs; PrP(Sc) was formed from GPI-anchored PrP(C) but not from transmembrane PrP(C). Our findings argue that PrP(Sc) formation is restricted to a specific subcellular compartment and as such, it is likely to involve auxiliary macromolecules found within CLDs.

Characterization of detergent-insoluble complexes containing the cellular prion protein and its scrapie isoform

Cells infected with prions contain both prion protein isoforms cellular prion protein (PrPC) and scrapie prion protein (PrPSc). PrPSc is formed posttranslationally through the pathological refolding of PrPC. In scrapie-infected ScN2a cells, the metabolism of both PrP isoforms involves cholesterol-dependent pathways. We show here that both PrPC and PrPSc are attached to Triton X-100-insoluble, low-density complexes or "rafts." These complexes are sensitive to saponin and thus probably contain cholesterol. This finding suggests that the transformation PrPC --> PrPSc occurs within rafts. It also reveals the existence of rafts in late compartments of the endocytic pathway, where most PrPSc resides. When Triton X-100 lysates of cells were incubated at 37 degrees C prior to density analysis, PrPC was still found in buoyant complexes, although it now failed to sediment at high speed. This property was shared by another glycophosphatidyl inositol protein, Thy-1, and also by the raft resident GM1. In one ScN2a clone and in the brain of a Syrian hamster with scrapie, Triton X-100 extraction at 37 degrees C permitted resolution of PrPC and PrPSc into two distinct peaks of different densities. This suggests that there are two populations of PrP-containing rafts and may permit isolation of PrPC-specific rafts from those containing PrPSc. Our findings reinforce the contention that rafts are involved in various aspects of PrP metabolism and in the "life cycle" of prions.

Cell surface dynamics of GPI-anchored proteins

Several cell surface eukaryotic proteins have a glycosylphosphoinositol lipid (GPI) modification at the carboxy-terminal end that serves as their sole means of membrane anchoring. In this report we review recent observations regarding the surface dynamics of GPI-anchored proteins. We discuss the association of GPI-anchored proteins with caveolae at the cell surface and their role in signal transduction as determined by the ability of GPI-anchored proteins to form detergent-insoluble complexes enriched in several cytoplasmic proteins including non-receptor type tyrosine kinases and caveolin/VIP-21, a component of the striated coat of caveolae. We have shown by immunofluorescence and electron microscopy that GPI-anchored proteins are not constitutively concentrated in caveolae but may be enriched in these structures only after cross-linking. While caveolae occupy only a small fraction of the cell surface (< 4%) almost all of the GPI-anchored protein at the cell surface becomes incorporated into detergent-insoluble low-density complexes, suggesting that these proteins are intrinsically detergent-insoluble in the milieu of the plasma membrane, and their co-purification with caveolin is not reflective of their native distribution. The finding that GPI-anchored proteins are not normally clustered over caveolae raised questions about the involvement of caveolae in the internalization of GPI-anchored proteins. In recent studies we have found that GPI-anchored proteins are internalized into bona fide endosomes wherein they appear to be sorted from bulk membrane components. The implications of these observations on the biology of GPI-anchored proteins are discussed.

Subcellular colocalization of the cellular and scrapie prion proteins in caveolae-like membranous domains

Results of transgenetic studies argue that the scrapie isoform of the prion protein (PrPSc) interacts with the substrate cellular PrP (PrPC) during conversion into nascent PrPSc. While PrPSc appears to accumulate primarily in lysosomes, caveolae-like domains (CLDs) have been suggested to be the site where PrPC is converted into PrPSc. We report herein that CLDs isolated from scrapie-infected neuroblastoma (ScN2a) cells contain PrPC and PrPSc. After lysis of ScN2a cells in ice-cold Triton X-100, both PrP isoforms and an N-terminally truncated form of PrPC (PrPC-II) were found concentrated in detergent-insoluble complexes resembling CLDs that were isolated by flotation in sucrose gradients. Similar results were obtained when CLDs were purified from plasma membranes by sonication and gradient centrifugation; with this procedure no detergents are used, which minimizes artifacts that might arise from redistribution of proteins among subcellular fractions. The caveolar markers ganglioside GM1 and H-ras were found concentrated in the CLD fractions. When plasma membrane proteins were labeled with the impermeant reagent sulfo-N-hydroxysuccinimide-biotin, both PrPC and PrPSc were found biotinylated in CLD fractions. Similar results on the colocalization of PrPC and PrPSc were obtained when CLDs were isolated from Syrian hamster brains. Our findings demonstrate that both PrPC and PrPSc are present in CLDs and, thus, support the hypothesis that the PrPSc formation occurs within this subcellular compartment.

Prion protein PrPc interacts with molecular chaperones of the Hsp60 family

Prions mediate the pathogenesis of certain neurodegenerative diseases, including bovine spongiform encephalopathy in cattle and Creutzfeldt-Jakob disease in humans. The prion particle consists mainly, if not entirely, of PrPSc, a posttranslationally modified isoform of the cellular host-encoded prion protein (PrPc). It has been suggested that additional cellular factors might be involved in the physiological function of PrPc and in the propagation of PrPSc. Here we employ a Saccharomyces cerevisiae two-hybrid screen to search for proteins which interact specifically with the Syrian golden hamster prion protein. Screening of a HeLa cDNA library identified heat shock protein 60 (Hsp60), a cellular chaperone as a major interactor for PrPc. The specificity of the interaction was confirmed in vitro for the recombinant proteins PrPc23-231 and rPrP27-30 fused to glutathione S-transferase with recombinant human Hsp60 as well as the bacterial GroEL. The interaction site for recombinant Hsp60 and GroEL proteins was mapped between amino acids 180 and 210 of the prion protein by screening with a set of recombinant PrPc fragments. The binding of Hsp60 and GroEL occurs within a region which contains parts of the putative alpha-helical domains H3 and H4 of the prion protein.

Prion protein amyloidosis

The prion protein (PrP) plays an essential role in the pathogenesis of a group of sporadic, genetically determined and infectious fatal degenerative diseases, referred to as "prion diseases", affecting the central nervous system of humans and other mammals. The cellular PrP is encoded by a single copy gene, highly conserved across mammalian species. In prion diseases, PrP undergoes conformational changes involving a shift from alpha-helix to beta-sheet structure. This conversion is important for PrP amyloidogenesis, which occurs to the highest degree in the genetically determined Gerstmann-Sträussler-Scheinker disease (GSS) and prion protein cerebral amyloid angiopathy (PrP-CAA), while it is less frequently seen in other prion diseases. GSS and PrP-CAA are associated with point mutations of the prion protein gene (PRNP); these conditions show a broad spectrum of clinical presentation, the main signs being ataxia, spastic paraparesis, extrapyramidal signs and dementia. In GSS, parenchymal amyloid may be associated with spongiform changes or neurofibrillary lesions; in PrP-CAA, vascular amyloid is associated with neurofibrillary lesions. A major component of the amyloid fibrils in the two diseases is a 7 kDa peptide, spanning residues 81-150 of PrP.

Mutant and infectious prion proteins display common biochemical properties in cultured cells

Prion diseases are unusual neurodegenerative disorders that can be both infectious and inherited. Both forms are hypothesized to result from a posttranslational structural alteration in the cell surface glycoprotein PrPc (cellular isoform of the prion protein) that converts it into the protease-resistant isoform PrPSc (scrapie isoform of the prion protein). However, a direct comparison of molecular events underlying these two manifestations of prion diseases has not been possible, because there has been no cell culture model for the familial forms. We report here that when mutant prion proteins associated with three different inherited prion disorders of humans are expressed as their murine homologues in cultured Chinese hamster ovary cells, the proteins are protease-resistant and detergent-insoluble, two biochemical properties characteristic of infectious PrPSc. In addition, each mutant protein remains tightly associated with the plasma membrane after enzymatic cleavage of its glycosylphosphatidylinositol anchor, a property that we now show is also typical of infectious PrPSc. The cell culture system described here is the first in vitro model for familial prion diseases and provides compelling evidence that infectious and genetic cases share common molecular features.

Endosome-lysosomes, ubiquitin and neurodegeneration

Before the advent of ubiquitin immunochemistry and immunogold electron microscopy, there was no known intracellular molecular commonality between neurodegenerative diseases. The application of antibodies which primarily detect ubiquitin protein conjugates has shown that all of the human and animal idiopathic and transmissible chronic neurodegenerative diseases, (including Alzheimer's disease (AD), Lewy body disease (LBD), amyotrophic lateral sclerosis (ALS), Creutzfeldt-Jakob disease (CJD) and scrapie) are related by some form of intraneuronal inclusion which contains ubiquitin protein conjugates. In addition, disorders such as Alzheimer's disease, CJD and sheep scrapie, are characterised by deposits of amyloid, arising through incomplete breakdown of membrane proteins which may be associated with cytoskeletal reorganisation. Although our knowledge about these diseases is increasing, they remain largely untreatable. Recently, attention has focused on the mechanisms of production of different types of amyloid and the likely involvement within cells of the endosome-lysosome system, organelles which are immuno-positive for ubiquitin protein conjugates. These organelles may be 'bioreactor' sites for the unfolding and partial degradation of membrane proteins to generate the amyloid materials or their precursors which subsequently become expelled from the cell, or are released from dead cells, and accumulate as pathological entities. Such common features of the disease processes give new direction to therapeutic intervention.

Sulfated glycans stimulate endocytosis of the cellular isoform of the prion protein, PrPC, in cultured cells

There is currently no effective therapy for human prion diseases. However, several polyanionic glycans, including pentosan sulfate and dextran sulfate, prolong the incubation time of scrapie in rodents, and inhibit the production of the scrapie isoform of the prion protein (PrPSc), the major component of infectious prions, in cultured neuroblastoma cells. We report here that pentosan sulfate and related compounds rapidly and dramatically reduce the amount of PrPC, the non-infectious precursor of PrPSc, present on the cell surface. This effect results primarily from the ability of these agents to stimulate endocytosis of PrPC, thereby causing a redistribution of the protein from the plasma membrane to the cell interior. Pentosan sulfate also causes a change in the ultrastructural localization of PrPC, such that a portion of the protein molecules are shifted into late endosomes and/or lysosomes. In addition, we demonstrate, using PrP-containing bacterial fusion proteins, that cultured cells express saturable and specific surface binding sites for PrP, many of which are glycosaminoglycan molecules. Our results raise the possibility that sulfated glycans inhibit prion production by altering the cellular localization of PrPC precursor, and they indicate that endogenous proteoglycans are likely to play an important role in the cellular metabolism of both PrPC and PrPSc.

A mutant prion protein displays an aberrant membrane association when expressed in cultured cells

Inherited forms of prion disease have been linked to mutations in the gene encoding PrP, a neuronal and glial protein that is attached to the plasma membrane by a glycosyl-phosphatidylinositol (GPI) anchor. One familial form of Creutzfeldt-Jakob disease is associated with a mutant PrP containing six additional octapeptide repeats. We report here our analysis of cultured Chinese hamster ovary cells expressing a murine homologue of this mutant PrP. We find that, like wild-type PrP, the mutant protein is glycosylated, GPI-anchored, and expressed on the cell surface. Surprisingly, however, cleavage of the GPI anchor using phosphatidylinositol-specific phospholipase C fails to release the mutant PrP from the surface of intact cells, suggesting that it has an additional mode of membrane attachment. The phospholipase-treated protein is hydrophobic, since it partitions into the detergent phase of Triton X-114 lysates; and it is tightly membrane-associated, since it is not extractable in carbonate buffer at pH 11.5. Whether membrane attachment of the mutant PrP involves integration of the polypeptide into the lipid bilayer, self-association, or binding to other membrane proteins remains to be determined. Our results suggest that alterations in the membrane association of PrP may be an important feature of prion diseases.

Truncated forms of the human prion protein in normal brain and in prion diseases

The cellular form of the prion protein (PrPc) is a glycoprotein anchored to the cell membrane by a glycosylphosphatidylinositol moiety. An aberrant form of PrPc that is partially resistant to proteases, PrPres, is a hallmark of prion diseases, which in humans include Cruetzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome, and fatal familial insomnia. We have characterized the major forms of PrP in normal and pathological human brains. A COOH-terminal fragment of PrPc, designated C1, is abundant in normal and CJD brains as well as in human neuroblastoma cells. Sequence analysis revealed that C1 contains alternative NH2 termini starting at His-111 or Met-112. Like PrPc, C1 is glycosylated, anchored to the cell membrane, and is heat-stable. Consistent with the lack of the NH2-terminal region of PrPc, C1 is more acidic than PrPc and does not bind heparin. An additional fragment longer than C1, designated C2, is present in substantial amounts in CJD brains. Like PrPres, C2 is resistant to proteases and is detergent-insoluble. Our data indicate that C1 is a major product of normal PrPc metabolism, generated by a cleavage that disrupts the neurotoxic and amyloidogenic region of PrP comprising residues 106-126. This region remains intact in C2, suggesting a role for C2 in prion diseases.

The abnormal isoform of the prion protein accumulates in late-endosome-like organelles in scrapie-infected mouse brain

The prion encephalopathies are characterized by accumulation in the brain of the abnormal form PrPsc of a normal host gene product PrPc. The mechanism and site of formation of PrPsc from PrPc are currently unknown. In this study, ME7 scrapie-infected mouse brain was used to show, both biochemically and by double-labelled immunogold electron microscopy, that proteinase K-resistant PrPsc is enriched in subcellular structures which contain the cation-independent mannose 6-phosphate receptor, ubiquitin-protein conjugates, beta-glucuronidase, and cathepsin B, termed late endosome-like organelles. The glycosylinositol phospholipid membrane-anchored PrPc will enter such compartment for normal degradation and the organelles may therefore act as chambers for the conversion of PrPc into infectious PrPsc in this murine model of scrapie.

The N-terminal domain of a glycolipid-anchored prion protein is essential for its endocytosis via clathrin-coated pits

The cellular prion protein (PrPC) is a glycolipid-anchored protein that is involved in the pathogenesis of fatal spongiform encephalopathies. We have shown previously that, in contrast to several other glycolipid-anchored proteins, chPrP, the chicken homologue of mammalian PrPC, is endocytosed via clathrin-coated pits in cultured neuroblastoma cells, as well as in embryonic neurons and glia (Shyng, S.-L., Heuser, J. E., and Harris, D. A. (1994) J. Cell Biol. 125, 1239-1250). In this study, we have determined that the N-terminal half of the chPrP polypeptide chain is essential for its endocytosis. Deletions within this region reduce the amount of chPrP internalized, as measured by surface iodination or biotinylation, and decrease its concentration in clathrin-coated pits, as determined by quantitative electron microscopic immunogold labeling. Mouse PrP, as well as two mouse PrP/chPrP chimeras, are internalized as efficiently as chPrP, suggesting that conserved features of secondary and tertiary structure are involved in interaction with the endocytic machinery. Our results indicate that the ectodomain of a protein can contain endocytic targeting information, and they strongly support a model in which the polypeptide chain of PrPC binds to the extracellular domain of a transmembrane protein that contains a coated pit localization signal in its cytoplasmic tail.

Theoretical studies of sequence effects on the conformational properties of a fragment of the prion protein: implications for scrapie formation

BACKGROUND

Prion diseases are neurodegenerative disorders that appear to be due to a conformational change, involving the conversion of alpha-helices in the normal, cellular isoform of the prion protein (PrPC) to beta-structure in the infectious scrapie form (PrPSc). One form of Gerstmann-Sträussler-Scheinker syndrome (GSS), an inherited prion disease, is caused by mutation of Ala117 of PrPC to Val. We therefore set out to evaluate the effects of this mutation on the stability of the PrPC form.

RESULTS

We have performed molecular dynamics simulations of a portion of the PrPC sequence (residues 109-122, termed H1) that is proposed to figure prominently in the conversion of PrPC to PrPSc. In particular, beginning with H1 in the alpha-helical state, the conformational consequences of sequence changes at position 117 were investigated for six hydrophobic mutations. Of these, only the Val mutation was helix-destabilizing. Portions of this mutant peptide adopted and retained an extended conformation during a 2 ns simulation of the peptide in water.

CONCLUSIONS

The conformational transitions and structures observed in the simulation of the mutant peptide with Val at position 117 provide insight into the possible early steps in the conversion of PrPC to PrPSc.

The chemistry of scrapie infection: implications of the 'ice 9' metaphor

The transmissible spongiform encephalopathies pose an increasing problem for animal, and perhaps human, health. The infectious agent seems to lack a nucleic acid component, posing the question of how it can reproduce. A model of reproduction by nucleated polymerization suggests a number of novel approaches to the problem.

Scrapie-associated PrP accumulation and its inhibition: revisiting the amyloid-glycosaminoglycan connection

An abnormal protease-resistant isoform of the protein PrP accumulates in the brain of hosts with transmissible spongiform encephalopathies (TSEs) and appears to be centrally involved in TSE pathogenesis. Studies with scrapie-infected tissue culture cells have indicated that this abnormal PrP is formed from an apparently normal precursor on the plasma membrane or along an endocytic pathway to the lysosomes. Inhibitors of protease-resistant PrP accumulation might serve as tools for studying the basic mechanism of protease-resistant PrP formation and as potential drugs for TSE therapy. Using scrapie-infected neuroblastoma cells to screen for such compounds in vitro, we found that the amyloid binding dye Congo red and certain sulfated glycans potently inhibited the accumulation of protease-resistant PrP in scrapie-infected cells without apparent effects on the metabolism of the normal isoform. The relative potencies of the sulfated glycans corresponded with their previously determined anti-scrapie activities in vivo, suggesting that the prophylactic effects of sulfated polyanions may be due to inhibition of protease-resistant PrP accumulation. Since protease-resistant PrP amyloid is known to contain sulfated glycosaminoglycans, as do other naturally derived amyloids, we hypothesize that these sulfated inhibitors competitively block binding between PrP and endogenous glycosaminoglycans that is important for its accumulation in a protease-resistant, potentially amyloidogenic state. Drugs which interfere with this (pre)amyloid-glycosaminoglycan interaction may be useful for treating a variety of amyloidoses.

Expression of polyubiquitin and heat-shock protein 70 genes increases in the later stages of disease progression in scrapie-infected mouse brain

We have shown by northern analyses that the expression of the mouse polyubiquitin C gene is increased severalfold in the brains of mice infected with both the ME7 and 87V strains of scrapie. Expression of the polyubiquitin gene does not change significantly, compared with controls, until the later stages of disease progression when there is a 2.5-fold increase in ME7-infected brains and a 1.8-fold increase in 87V-infected brains. The patterns of changes of expression of the polyubiquitin genes in brains infected with the two strains of scrapie resemble those of accumulation of ubiquitin-conjugate-positive structures in the brain that are detected immunohistochemically. A similar increase in the expression of a heat-shock protein 70 gene also occurs.

Endosome-lysosomes and neurodegeneration

A number of the major human and animal neurodegenerative diseases, such as Alzheimer's disease and sheep scrapie, are characterised by deposits of amyloid, arising through incomplete breakdown of membrane proteins. Although our knowledge concerning these diseases is increasing, they remain largely untreatable. Recently, attention has focussed on the mechanisms of production of different types of amyloid and the likely involvement within cells of acid compartments called endosome-lysosomes. These organelles may be 'bioreactor' sites for the unfolding and partial degradation of membrane proteins to generate the amyloid materials. These subsequently become expelled from the cell, or are released from dead cells, and accumulate as pathological entities. Common features of the disease processes give new direction to therapeutic intervention.

Identification of putative internalization signals in prion proteins

Prion proteins were found to contain regions in their cytoplasmic domains that have significant structural homologies to molecular trafficking signals for internalization of membrane proteins. These regions may facilitate the endocytosis of prion proteins, which appears to be a first step in their conversion to the abnormal, amyloidogenic form.

Cytoprotective effect of NMDA receptor antagonists on prion protein (PrionSc)-induced toxicity in rat cortical cell cultures

Rat cortical cells were incubated with the Scrapie prion protein, PrionSc. At concentrations of 3 ng/ml of PrionSc and higher, the viability of the cells decreased significantly after a 12-h incubation period. Simultaneously, the degree of DNA fragmentation increased. In control experiments with antibodies against PrionSc, PrionSc lost its deleterious effect on neurons. PrionSc did not affect the viability of astrocytes. Drugs known to block NMDA receptor channels, such as memantine (1-amino-3,5-dimethyl-adamantane) (Mem), its analogue 1-N-methylamino-3,5-dimethyl-adamantane as well as (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) prevented the effect of PrionSc. Production of PrionSc in the Scrapie prion-infected subclone of N2 a cells (ScN2 a cells) was not affected by memantine. We conclude that antagonists of the NMDA receptor-channel complex (i) abolish the PrionSc-induced neuronal injury in vitro, and (ii) display no influence on the synthesis and/or the processing of PrionSc.

Structural studies of the scrapie prion protein using mass spectrometry and amino acid sequencing

The only component of the infectious scrapie prion identified to date is a protein designated PrPSc. A posttranslational process converts the cellular PrP isoform (PrPC) into PrPSc. Denatured PrPSc was digested with endoproteases, and the resulting fragments were isolated by HPLC. By both mass spectrometry and Edman sequencing, the primary structure of PrPSc was found to be the same as that deduced from the PrP gene sequence, arguing that neither RNA editing nor protein splicing feature in the synthesis of PrPSc. Mass spectrometry also was used to search for posttranslational chemical modifications other than the glycosylinositol phospholipid anchor attached to the C-terminus and two Asn-linked oligosaccharides already known to occur on both PrPSc and PrPC. These results contend that PrPSc molecules do not differ from PrPC at the level of an amino acid substitution or a posttranslational chemical modification; however, we cannot eliminate the possibility that a small fraction of PrPSc is modified by an as yet unidentified posttranslational process or that PrPC carries a modification that is removed in the formation of PrPSc. It seems likely that PrPSc differs from PrPC in its secondary and tertiary structure, but the possibility of a tightly bound, disease-specific molecule which purifies with PrPSc must also be considered.

The biology and molecular biology of scrapie-like diseases

The transmissible spongiform encephalopathies (TSE's) are degenerative diseases of the central nervous system which naturally affect man (Creutzfeldt-Jakob disease [CJD], Gerstmann-Sträussler syndrome [GSS], kuru), sheep and goats (scrapie), cattle (bovine spongiform encephalopathy [BSE]), mink (transmissible mink encephalopathy), mule deer, elk and antelope (chronic wasting disease). Spongiform encephalopathies have also been diagnosed in captive species of zoo antelope and in domestic cats. Much has been written about these maladies in the wake of the BSE outbreak, the tragic cases of CJD in recipients of cadaver-derived human growth hormone, sex hormones or dura mater and this has stimulated a continuing public health debate about the transmissibility, prevalence and clinical variability of scrapie, CJD and related ("prion") diseases. Prions (Weissmann, Liautard, this volume) and the human (Kretzschmar, this volume) and cattle (Wilesmith, Marsh, this volume) diseases are described in more detail elsewhere. This article presents a brief overview of the biology and molecular cell biology of scrapie and rodent models of these diseases.

Natural and experimental prion diseases of humans and animals

Prions cause transmissible and genetic neurodegenerative diseases. Infectious prion particles are composed largely, if not entirely, of an abnormal isoform of the prion protein (PrPSc), which is encoded by a chromosomal gene. Although the PrP gene is single copy, transgenic mice with both alleles of the PrP gene ablated develop normally. A post-translational process, as yet unidentified, converts the cellular prion protein (PrPC) into PrPSc. Scrapie incubation times, neuropathology and prion synthesis in transgenic mice are controlled by the PrP gene. Mutations in this gene are genetically linked to the development of neurodegeneration. Transgenic mice expressing mutant PrP spontaneously develop neurological dysfunction and spongiform neuropathology. Future investigations of prion diseases using molecular biological and genetic approaches promise to yield much new information about these once enigmatic disorders.