Cellular heparan sulfate participates in the metabolism of prions

Glypican-1 mediates both prion protein lipid raft association and disease isoform formation

In prion diseases, the cellular form of the prion protein, PrP^C, undergoes a conformational conversion to the infectious isoform, PrP^Sc. PrP^C associates with lipid rafts through its glycosyl-phosphatidylinositol (GPI) anchor and a region in its N-terminal domain which also binds to heparan sulfate proteoglycans (HSPGs). We show that heparin displaces PrP^C from rafts and promotes its endocytosis, suggesting that heparin competes with an endogenous raft-resident HSPG for binding to PrP^C. We then utilised a transmembrane-anchored form of PrP (PrP-TM), which is targeted to rafts solely by its N-terminal domain, to show that both heparin and phosphatidylinositol-specific phospholipase C can inhibit its association with detergent-resistant rafts, implying that a GPI-anchored HSPG targets PrP^C to rafts. Depletion of the major neuronal GPI-anchored HSPG, glypican-1, significantly reduced the raft association of PrP-TM and displaced PrP^C from rafts, promoting its endocytosis. Glypican-1 and PrP^C colocalised on the cell surface and both PrP^C and PrP^Sc co-immunoprecipitated with glypican-1. Critically, treatment of scrapie-infected N2a cells with glypican-1 siRNA significantly reduced PrP^Sc formation. In contrast, depletion of glypican-1 did not alter the inhibitory effect of PrP^C on the β-secretase cleavage of the Alzheimer's amyloid precursor protein. These data indicate that glypican-1 is a novel cellular cofactor for prion conversion and we propose that it acts as a scaffold facilitating the interaction of PrP^C and PrP^Sc in lipid rafts.

The prion diseases are unique in that their infectious nature is not dependent on nucleic acid but is instead attributed to a misfolded protein, the prion protein. This misfolded prion protein is capable of inducing the misfolding of the normal form of the prion protein that is present on the surface of neurons and other cells in the body. However, the site in the cell at which this misfolding occurs and whether other proteins are involved remains controversial. We have addressed these questions by investigating how the normal form of the prion protein is targeted to specialised domains on the plasma membrane termed cholesterol-rich lipid rafts. We show that targeting is due, in part, to a particular heparin sulfate proteoglycan called glypican-1. Significantly, reducing the levels of glypican-1 in an infected cell line reduced the accumulation of misfolded prion protein. We propose that glypican-1 acts as a scaffold facilitating the favourable interaction of the misfolded, infectious form of the prion protein with the normal cellular form within cholesterol-rich lipid rafts. Our results indicate that glypican-1 is intimately involved in the misfolding of the prion protein, the critical event in the pathogenesis of prion diseases such as Creutzfeldt-Jakob disease in humans.

Structure of the flexible amino-terminal domain of prion protein bound to a sulfated glycan

The intrinsically disordered amino-proximal domain of hamster prion protein (PrP) contains four copies of a highly conserved octapeptide sequence, PHGGGWGQ, that is flanked by two polycationic residue clusters. This N-terminal domain mediates the binding of sulfated glycans, which can profoundly influence the conversion of PrP to pathological forms and the progression of prion disease. To investigate the structural consequences of sulfated glycan binding, we performed multidimensional heteronuclear ((1)H, (13)C, (15)N) NMR (nuclear magnetic resonance), circular dichroism (CD), and fluorescence studies on hamster PrP residues 23-106 (PrP 23-106) and fragments thereof when bound to pentosan polysulfate (PPS). While the majority of PrP 23-106 remain disordered upon PPS binding, the octarepeat region adopts a repeating loop-turn structure that we have determined by NMR. The beta-like turns within the repeats are corroborated by CD data demonstrating that these turns are also present, although less pronounced, without PPS. Binding to PPS exposes a hydrophobic surface composed of aligned tryptophan side chains, the spacing and orientation of which are consistent with a self-association or ligand binding site. The unique tryptophan motif was probed by intrinsic tryptophan fluorescence, which displayed enhanced fluorescence of PrP 23-106 when bound to PPS, consistent with the alignment of tryptophan side chains. Chemical-shift mapping identified binding sites on PrP 23-106 for PPS, which include the octarepeat histidine and an N-terminal basic cluster previously linked to sulfated glycan binding. These data may in part explain how sulfated glycans modulate PrP conformational conversions and oligomerizations.

Interactome analyses identify ties of PrP and its mammalian paralogs to oligomannosidic N-glycans and endoplasmic reticulum-derived chaperones

The physiological environment which hosts the conformational conversion of the cellular prion protein (PrP(C)) to disease-associated isoforms has remained enigmatic. A quantitative investigation of the PrP(C) interactome was conducted in a cell culture model permissive to prion replication. To facilitate recognition of relevant interactors, the study was extended to Doppel (Prnd) and Shadoo (Sprn), two mammalian PrP(C) paralogs. Interestingly, this work not only established a similar physiological environment for the three prion protein family members in neuroblastoma cells, but also suggested direct interactions amongst them. Furthermore, multiple interactions between PrP(C) and the neural cell adhesion molecule, the laminin receptor precursor, Na/K ATPases and protein disulfide isomerases (PDI) were confirmed, thereby reconciling previously separate findings. Subsequent validation experiments established that interactions of PrP(C) with PDIs may extend beyond the endoplasmic reticulum and may play a hitherto unrecognized role in the accumulation of PrP(Sc). A simple hypothesis is presented which accounts for the majority of interactions observed in uninfected cells and suggests that PrP(C) organizes its molecular environment on account of its ability to bind to adhesion molecules harboring immunoglobulin-like domains, which in turn recognize oligomannose-bearing membrane proteins.

Getting a grip on prions: oligomers, amyloids, and pathological membrane interactions

The prion (infectious protein) concept has evolved with the discovery of new self-propagating protein states in organisms as diverse as mammals and fungi. The infectious agent of the mammalian transmissible spongiform encephalopathies (TSE) has long been considered the prototypical prion, and recent cell-free propagation and biophysical analyses of TSE infectivity have now firmly established its prion credentials. Other disease-associated protein aggregates, such as some amyloids, can also have prion-like characteristics under certain experimental conditions. However, most amyloids appear to lack the natural transmissibility of TSE prions. One feature that distinguishes the latter from the former is the glycophosphatidylinositol membrane anchor on prion protein, the molecule that is corrupted in TSE diseases. The presence of this anchor profoundly affects TSE pathogenesis, which involves major membrane distortions in the brain, and may be a key reason for the greater neurovirulence of TSE prions relative to many other autocatalytic protein aggregates.

Antiprion action of new cyclodextrin analogues

BACKGROUND

Prion disorders are characterised by the accumulation of a misfolded isoform (PrPSc) of the host encoded prion protein (PrPC). This paper examines the antiprion potential of cyclodextrin (CD) analogues and it identifies sulphated-beta-cyclodextrin, with a half-maximal inhibitory concentration (IC50) of 2.4 microM, as having 31-fold greater antiprion activity than that previously reported for beta-cyclodextrin (betaCD).

METHODS

Scrapie infected cells were treated with a range of betaCD analogues. This enabled a CD structure to antiprion activity analysis to be carried out. The metachromatic activity of each of the cyclodextrins was determined, this test is employed to mimic complexation of glycosaminogylcans to a cell membrane.

RESULTS

Sulphated-betaCD had an IC50 of 2.4 microM and it was the only CD found to have metachromatic activity. Its activity was equivalent to that of heparin and heparin sulphate, this may account for sulphated-betaCD's superior antiprion action.

GENERAL SIGNIFICANCE

In solution heparin can form a helical structure with a hydrophobic interior, the hydrophobic interior of cyclic CDs is vital for CD molecule encapsulation. The controlled CD structure, however, restricts degradation by human enzymes; consequently sulphated-CDs could be ideal candidates in the search for prion therapeutics. Sulphated-CDs may open up avenues for the treatment of TSEs.

Recent advances in prion chemotherapeutics

The transmissible spongiform encephalopathies are rapidly progressive and invariably fatal neurodegenerative diseases for which there are no proven efficacious treatments. Many approaches have been undertaken to find ways to prevent, halt, or reverse these prion diseases, with limited success to date. However, as both our understanding of pathogenesis and our ability to detect early disease increases, so do our potential therapeutic targets and our chances of finding effective drugs. There is increasing pressure to find effective decontaminants for blood supplies, as variant Creutzfeldt Jakob Disease (vCJD) has been shown to be transmissible by blood, and to find non-toxic preventative therapies, with ongoing cases of Bovine Spongiform Encephalopathy (BSE) and the spread of Chronic Wasting Disease (CWD). Within the realm of chemotherapeutic approaches, much research has focussed on blocking the conversion of the normal form of prion protein (PrP(c)) to its abnormal counterpart (PrP(res)). Structurally, these chemotherapeutic agents are often polyanionic or polycyclic and may directly bind PrP(c) or PrP(res), or act by redistributing, sequestering, or down-regulating PrP(c), thus preventing its conversion. There are also some polycationic compounds which proport to enhance the clearance of PrP(res). Other targets include accessory molecules such as the laminin receptor precursor which influences conversion, or cell signalling molecules which may be required for pathogenesis. Of recent interest are the possible neuroprotective effects of some drugs. Importantly, there is evidence that combining compounds may provide synergistic responses. This review provides an update on current testing methods, therapeutic targets, and promising candidates for chemical-based therapy.

A systems approach to prion disease

Prions cause transmissible neurodegenerative diseases and replicate by conformational conversion of normal benign forms of prion protein (PrP^C) to disease-causing PrP^Sc isoforms. A systems approach to disease postulates that disease arises from perturbation of biological networks in the relevant organ. We tracked global gene expression in the brains of eight distinct mouse strain–prion strain combinations throughout the progression of the disease to capture the effects of prion strain, host genetics, and PrP concentration on disease incubation time. Subtractive analyses exploiting various aspects of prion biology and infection identified a core of 333 differentially expressed genes (DEGs) that appeared central to prion disease. DEGs were mapped into functional pathways and networks reflecting defined neuropathological events and PrP^Sc replication and accumulation, enabling the identification of novel modules and modules that may be involved in genetic effects on incubation time and in prion strain specificity. Our systems analysis provides a comprehensive basis for developing models for prion replication and disease, and suggests some possible therapeutic approaches.

The propagation of hamster-adapted scrapie PrPSc can be enhanced by reduced pyridine nucleotide in vitro

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are fatal neurodegenerative disorders caused by an infectious agent termed a prion, which can convert normal cellular prion protein (PrP(C)) into a pathologically misfolded isoform (PrP(Sc)). Taking advantage of protein misfolding cyclic amplification (PMCA), a series of experiments was conducted to investigate the possible influences of pyridine nucleotides on the propagation activities of hamster-adapted scrapie agents 263K and 139A in vitro using normal hamster brain homogenates and recombinant hamster PrP as the substrates. The results showed that PrP(Sc) from both scrapie agent 263K- and 139A-infected brains propagated more efficiently in PMCA with the addition of reduced NADPH, showing an obvious dose-dependent enhancement. Reduced NADH also prompted PrP(Sc) propagation, whereas NADP, NAD and vitamin C failed. Moreover, following incubation with NADPH, recombinant hamster PrP could be efficiently converted into the proteinase K-resistant form when exposed to the trace of PrP(Sc) from infected hamsters. Our data provide evidence that the reduced pyridine nucleotide plays an important role in the propagation of prion and this process seems to target PrP(C) molecules.

Ligand binding promotes prion protein aggregation--role of the octapeptide repeats

Aggregation of the normal cellular prion protein, PrP, is important in the pathogenesis of prion disease. PrP binds glycosaminoglycan (GAG) and divalent cations, such as Cu(2+) and Zn(2+). Here, we report our findings that GAG and Cu(2+) promote the aggregation of recombinant human PrP (rPrP). The normal cellular prion protein has five octapeptide repeats. In the presence of either GAG or Cu(2+), mutant rPrPs with eight or ten octapeptide repeats are more aggregation prone, exhibit faster kinetics and form larger aggregates than wild-type PrP. When the GAG-binding motif, KKRPK, is deleted the effect of GAG but not that of Cu(2+) is abolished. By contrast, when the Cu(2+)-binding motif, the octapeptide-repeat region, is deleted, neither GAG nor Cu(2+) is able to promote aggregation. Therefore, the octapeptide-repeat region is critical in the aggregation of rPrP, irrespective of the promoting ligand. Furthermore, aggregation of rPrP in the presence of GAG is blocked with anti-PrP mAbs, whereas none of the tested anti-PrP mAbs block Cu(2+)-promoted aggregation. However, a mAb that is specific for an epitope at the N-terminus enhances aggregation in the presence of either GAG or Cu(2+). Therefore, although binding of either GAG or Cu(2+) promotes the aggregation of rPrP, their aggregation processes are different, suggesting multiple pathways of rPrP aggregation.

Opposing effects of ERK and p38-JNK MAP kinase pathways on formation of prions in GT1-1 cells

Brain-derived neurotrophic factor, which activates the extracellular regulated kinase (ERK) pathway, increases formation of prions in scrapie-infected gonadotropin-releasing hormone (GT1-1) cells. This indicates that conversion of the cellular prion protein PrP(C) to its pathogenic isoform, PrP(Sc), can be regulated by physiological stimuli acting on specific signal transduction pathways. In the present study, we examined the involvement of different mitogen-activated protein (MAP) kinase cascades and the cAMP-PKA pathway in formation of proteinase K-resistant PrP(Sc) (rPrP(Sc)). Long-term depolarization of GT1-1 cells infected with the Rocky Mountain Laboratory strain of scrapie increased the formation of rPrP(Sc). This effect was associated to ERK activation and was blocked by the MAPK/ERK kinase (MEK) inhibitor U0126. Treatment with forskolin caused a similar increase in rPrP(Sc) formation that was prevented by the protein kinase A (PKA) inhibitor H89. Both depolarization and forskolin treatment were accompanied by increased phosphorylation of the S6 ribosomal protein, while phosphorylation of histone H3 occurred only after forskolin treatment. Inhibitors of p38- and c-Jun NH(2)-terminal kinase (JNK) promoted the formation of rPrP(Sc), in contrast to the clearance of rPrP(Sc) produced by inhibitors of the ERK pathway. Thus, the ERK and the p38-JNK MAP kinase pathways appear to exert opposing effects on rPrP(Sc) formation, suggesting that balances between these intracellular signaling cascades may regulate replication of prions.

Structure, biological activity, and enzymatic transformation of fucoidans from the brown seaweeds

Recent advances in the study of fucoidans, biologically active sulfated alpha-L-fucans of diverse structures and synthesized exclusively by marine organisms, are overviewed. Their structure, biological activity, the products of their enzymatic degradation and the different enzymes of degradation and modification are considered.

LRP1 controls biosynthetic and endocytic trafficking of neuronal prion protein

The trafficking of normal cellular prion protein (PrPC) is believed to control its conversion to the altered conformation (designated PrPSc) associated with prion disease. Although anchored to the membrane by means of glycosylphosphatidylinositol (GPI), PrPC on neurons is rapidly and constitutively endocytosed by means of coated pits, a property dependent upon basic amino acids at its N-terminus. Here, we show that low-density lipoprotein receptor-related protein 1 (LRP1), which binds to multiple ligands through basic motifs, associates with PrPC during its endocytosis and is functionally required for this process. Moreover, sustained inhibition of LRP1 levels by siRNA leads to the accumulation of PrPC in biosynthetic compartments, with a concomitant lowering of surface PrPC, suggesting that LRP1 expedites the trafficking of PrPC to the neuronal surface. PrPC and LRP1 can be co-immunoprecipitated from the endoplasmic reticulum in normal neurons. The N-terminal domain of PrPC binds to purified human LRP1 with nanomolar affinity, even in the presence of 1 μM of the LRP-specific chaperone, receptor-associated protein (RAP). Taken together, these data argue that LRP1 controls both the surface, and biosynthetic, trafficking of PrPC in neurons.

Structural and functional analysis of the ovine laminin receptor gene (RPSA): Possible involvement of the LRP/LR protein in scrapie response

Scrapie is a prion disease affecting sheep and goats. Susceptibility to this neurodegenerative disease shows polygenic variance. The involvement of the laminin receptor (LRP/LR) in the metabolism and propagation of prions has previously been demonstrated. In the present work, the ovine laminin receptor gene (RPSA) was isolated, characterized, and mapped to ovine chromosome OAR19q13. Real-time RT-PCR revealed a significant decrease in RPSA mRNA in cerebellum after scrapie infection. Conversely, no differences were detected in other brain regions such as diencephalon and medulla oblongata. Association analysis showed that a polymorphism reflecting the presence of a RPSA pseudogene was overrepresented in a group of sheep resistant to scrapie infection. No amino acid change in the LRP/LR protein was found in the 126 sheep analyzed. However, interesting amino acid positions (241, 272, and 290), which could participate in the species barrier to scrapie and maybe to other transmissible spongiform encephalopathies, were identified by comparing LRP/LR sequences from various mammals with variable levels of resistance to scrapie.

Cell culture models to unravel prion protein function and aberrancies in prion diseases

From an early stage of prion research, tissue cultures that could support and propagate the scrapie agent were sought after. The earliest attempts were explants from brains of infected mice, and their growth and morphological characteristics were compared with those from uninfected mice. Using the explant technique, several investigators reported increased cell growth in cultures established from scrapie-sick brain compared with cultures from normal mice. These are odd findings in the light of the massive neuronal cell death known to occur in scrapie-infected brains; however, the cell types responsible for the increased cell growth in the scrapie-explants most probably were not neuronal. The first successful cell culture established in this way, in which the scrapie agent was serially and continuously passaged beyond the initial explant, was in the scrapie mouse brain culture, which is still used today. This chapter describes the generation and use of chronically prion-infected cell lines as cell culture models of prion diseases. These cell lines have been crucial for the current understanding of the cell biology of both the normal (PrP(C)) and the pathogenic isoform (PrP(Sc)) of the prion protein. They also have been useful in the development of antiprion drugs, prospectively used for therapy of prion diseases, and they offer an alternative approach for transmission/infectivity assays normally performed by mouse bioassay. Cell culture models also have been used to study prion-induced cytopathological changes, which could explain the typical spongiform neurodegeneration in prion diseases.

Cell models of prion infection

Due to recent renewal of interest and concerns in prion diseases, a number of cell systems permissive to prion multiplication have been generated in the last years. These include established cell lines, neuronal stem cells and primary neuronal cultures. While most of these models are permissive to experimental, mouse-adapted strains of prions, the propagation of natural field isolates from sheep scrapie and chronic wasting disease has been recently achieved. These models have improved our knowledge on the molecular and cellular events controlling the conversion of the PrP(C) protein into abnormal isoforms and on the cell-to-cell spreading of prions. Infected cultured cells will also facilitate investigations on the molecular basis of strain identity and on the mechanisms that lead to neurodegeneration. The ongoing development of new cell models with improved characteristics will certainly be useful for a number of unanswered critical issues in the prion field.

Selective incorporation of polyanionic molecules into hamster prions

The central pathogenic event of prion disease is the conformational conversion of a host protein, PrPC, into a pathogenic isoform, PrPSc. We previously showed that the protein misfolding cyclic amplification (PMCA) technique can be used to form infectious prion molecules de novo from purified native PrPC molecules in an autocatalytic process requiring accessory polyanions (Deleault, N. R., Harris, B. T., Rees, J. R., and Supattapone, S. (2007) Proc. Natl. Acad. Sci. U. S. A. 104, 9741-9746). Here we investigated the molecular mechanism by which polyanionic molecules facilitate infectious prion formation in vitro. Ina PMCA reaction lacking PrPSc template seed, synthetic polyA RNA molecules induce hamster HaPrPC to adopt a protease-sensitive, detergent-insoluble conformation reactive against antibodies specific for PrPSc. During PMCA, labeled nucleic acids form nuclease-resistant complexes with HaPrP molecules. Strikingly, purified HaPrPC molecules subjected to PMCA selectively incorporate an approximately 1-2.5-kb subset of [32P]polyA RNA molecules from a heterogeneous mixture ranging in size from approximately 0.1 to >6 kb. Neuropathological analysis of scrapie-infected hamsters using the fluorescent dye acridine orange revealed that RNA molecules co-localize with large extracellular HaPrP aggregates. These findings suggest that polyanionic molecules such as RNA may become selectively incorporated into stable complexes with PrP molecules during the formation of native hamster prions.

Copaxone interferes with the PrP Sc-GAG interaction

The hallmark of prion disease-induced neurodegeneration is the accumulation of PrP(Sc), a misfolded form of PrP(C). In addition, several lines of evidence indicate a role for the immune system and, in particular, inflammation in prion disease pathogenesis. In this work, we tested whether Copaxone, an immunomodulatory agent currently used for the treatment of multiple sclerosis, can affect prion disease manifestation in scrapie-infected hamsters. We show here that Copaxone exerted no effect on prion disease incubation time when treatment commenced 2 weeks after i.p. prion infection. However, when Copaxone was mixed with the initial prion inoculum or administered to hamsters weekly starting on the day of infection, prion disease incubation time was prolonged by 30 days. This suggests that Copaxone may affect the initial infection process. In vitro experiments indicate that Copaxone significantly reduced PrP(Sc) binding to both Chinese hamster ovary (CHO) cells and heparin beads and also binds to heparin by itself. Interestingly, Copaxone also abolished PrP(Sc) accumulation in scrapie-infected cells. We propose that Copaxone delays prion infection by competing with the PrP(Sc)-glycosaminoglycans interaction. Whether the immunomodulating activity of Copaxone is related to its heparin binding and anti-prion properties remains to be established.

Heparanase induces Akt phosphorylation via a lipid raft receptor

The endoglycosidase heparanase is the predominant enzyme that degrades heparan sulfate side chains of heparan sulfate proteoglycans, activity that is strongly implicated in tumor metastasis. Apart of its well characterized enzymatic activity, heparanase was noted to exert also enzymatic-independent functions. Among these is the induction of Akt/PKB phosphorylation noted in endothelial- and tumor-derived cells. Protein domains of heparanase required for signaling were not identified to date, nor were identified heparanase binding proteins/receptors capable of transmitting heparanase signals. Here, we examined the possible function of mannose 6-phosphate receptor (MPR) and low-density lipoprotein-receptor related protein (LRP), recently implicated in cellular uptake of heparanase, as heparanase receptors mediating Akt phosphorylation. We found that heparanase addition to MPR- and LRP-deficient fibroblasts elicited Akt activation indistinguishable from control fibroblasts. In contrast, disruption of lipid rafts abrogated Akt/PKB phosphorylation following heparanase addition. These results suggest that lipid raft-resident receptor mediates heparanase signaling.

Branched chain mechanism of polymerization and ultrastructure of prion protein amyloid fibrils

The discovery of prion disease and the establishment of the protein only hypothesis of prion propagation raised substantial interest in the class of maladies referred to as conformational diseases. Although significant progress has been made in elucidating the mechanisms of polymerization for several amyloidogenic proteins and peptides linked to conformational disorders and solving their fibrillar 3D structures, studies of prion protein amyloid fibrils and their polymerization mechanism have proven to be very difficult. The present minireview introduces the mechanism of branched-chain reaction for describing the peculiar kinetics of prion polymerization and summarizes our current knowledge about the substructure of prion protein amyloid fibrils.

Formation of native prions from minimal components in vitro

The conformational change of a host protein, PrP(C), into a disease-associated isoform, PrP(Sc), appears to play a critical role in the pathogenesis of prion diseases such as Creutzfeldt-Jakob disease and scrapie. However, the fundamental mechanism by which infectious prions are produced in neurons remains unknown. To investigate the mechanism of prion formation biochemically, we conducted a series of experiments using the protein misfolding cyclic amplification (PMCA) technique with a preparation containing only native PrP(C) and copurified lipid molecules. These experiments showed that successful PMCA propagation of PrP(Sc) molecules in a purified system requires accessory polyanion molecules. In addition, we found that PrP(Sc) molecules could be formed de novo from these defined components in the absence of preexisting prions. Inoculation of samples containing either prion-seeded or spontaneously generated PrP(Sc) molecules into hamsters caused scrapie, which was transmissible on second passage. These results show that prions able to infect wild-type hamsters can be formed from a minimal set of components including native PrP(C) molecules, copurified lipid molecules, and a synthetic polyanion.

Human prion proteins with pathogenic mutations share common conformational changes resulting in enhanced binding to glycosaminoglycans

Mutation in the prion gene PRNP accounts for 10-15% of human prion diseases. However, little is known about the mechanisms by which mutant prion proteins (PrPs) cause disease. Here we investigated the effects of 10 different pathogenic mutations on the conformation and ligand-binding activity of recombinant human PrP (rPrP). We found that mutant rPrPs react more strongly with N terminus-specific antibodies, indicative of a more exposed N terminus. The N terminus of PrP contains a glycosaminoglycan (GAG)-binding motif. Binding of GAG is important in prion disease. Accordingly, all mutant rPrPs bind more GAG, and GAG promotes the aggregation of mutant rPrPs more efficiently than wild-type recombinant normal cellular PrP (rPrP(C)). Furthermore, point mutations in PRNP also cause conformational changes in the region between residues 109 and 136, resulting in the exposure of a second, normally buried, GAG-binding motif. Importantly, brain-derived PrP from transgenic mice, which express a pathogenic mutant with nine extra octapeptide repeats, also binds more strongly to GAG than wild-type PrP(C). Thus, several rPrPs with distinct pathogenic mutations have common conformational changes, which enhance binding to GAG. These changes may contribute to the pathogenesis of inherited prion diseases.

Evaluation of fluorescently labeled xylopyranosides as probes for proteoglycan biosynthesis

A new fluorescent analog to the antiproliferative 2-(6-hydroxynaphthyl)-beta-d-xylopyranoside has been synthesized and tested on a T24 cell line. The new analog was efficiently uptaken by the T24 cells but did not initiate priming of GAG chains. The results are similar to other fluorescently labeled analogs and we propose that these compounds are too large and unpolar to efficiently function as GAG-primers.

Comparison of CR36, a new heparan mimetic, and pentosan polysulfate in the treatment of prion diseases

Sulfated polyanions, including pentosan polysulfate (PPS) and heparan mimetics, number among the most effective drugs that have been used in experimental models of prion disease and are presumed to act in competition with endogenous heparan sulfate proteoglycans as co-receptors for prion protein (PrP) on the cell surface. PPS has been shown to prolong the survival of animals after intracerebral perfusion and is in limited use for the experimental treatment of human transmissible spongiform encephalopathies (TSEs). Here, PPS is compared with CR36, a new heparan mimetic. Ex vivo, CR36 was more efficient than PPS in reducing PrPres in scrapie-infected cell cultures and showed long-lasting activity. In vivo, CR36 showed none of the acute toxicity observed with PPS and reduced PrPres accumulation in spleens, but had only a marginal effect on the survival time of mice infected with bovine spongiform encephalopathy. In contrast, mice treated with PPS that survived the initial toxic mortality had no detectable PrPres in the spleens and lived 185 days longer than controls (+55 %). These results show, once again, that anti-TSE drugs cannot be encouraged for human therapeutic trials solely on the basis of in vitro or ex vivo observations, but must first be subjected to in vivo animal studies.

Binding of bovine prion protein to heparin: a fluorescence polarization study

Glycosaminoglycans (GAGs) are believed to be associated with prion disease pathology and also with metabolism of the prion protein. Fluorescence polarization assay (FPA) of binding between bovine recombinant prion protein (brecPrP) and heparin labelled with AlexaFluor488 was used in model experiments to study glycosaminoglycan-prion protein interaction. Heparin binding to brecPrP was a rapid reversible event which occurred under defined conditions. The interaction of brecPrP with fluorophore-labelled heparin was inhibited by the presence of Cu(2+) ions and was sensitive to competition with heparin, heparan sulphate, and dextran. The dissociation constant of the heparin-brecPrP complex was 73.4+/-3.7 nM. Circular dichroism (CD) experiments indicated that the structure of brecPrP was less helical in the presence of heparin.

Prion infection-impaired functional blocks identified by proteomics enlighten the targets and the curing pathways of an anti-prion drug

Prion-induced neurodegeneration results from multiple cellular alterations among which the accumulation of a modified form of the host protein PrP is but a hallmark. Drug treatments need understanding of underlying mechanisms. Proteomics allows getting a comprehensive view of perturbations leading to neuronal death. Heparan sulfate mimetics has proved to be efficient to clear scrapie protein in cultured cells and in animals. To investigate the mechanisms of drug attack, protein profiles of the neuronal cell line GT1 and its chronically Chandler strain infected counterpart were compared, either in steady state cultures or after a 4-day drug treatment. Differentially expressed proteins were associated into functional blocks relevant to neurodegenerative diseases. Protein structure repair and modification, proteolysis, cell shape and energy/oxidation players were affected by infection, in agreement with prion biology. Unexpectedly, novel affected blocks related to translation, nucleus structure and DNA replication were unravelled displaying commonalities with proliferative processes. The drug had a double action in infected cells by reversing protein levels back to normal in some blocks and by heightening survival functions in others. This study emphasizes the interest of a proteomic approach to unravel novel networks involved in prion infection and curing.

Prions and their partners in crime

Prions, the infectious agents of transmissible spongiform encephalopathies (TSEs), have defied full characterization for decades. The dogma has been that prions lack nucleic acids and are composed of a pathological, self-inducing form of the host's prion protein (PrP). Recent progress in propagating TSE infectivity in cell-free systems has effectively ruled out the involvement of foreign nucleic acids. However, host-derived nucleic acids or other non-PrP molecules seem to be crucial. Interactions between TSE-associated PrP and its normal counterpart are also pathologically important, so the physiological functions of normal PrP and how they might be corrupted by TSE infections have been the subject of recent research.

The intriguing prion disorders

Prion diseases are among the most intriguing illnesses. Despite their rare incidence, they have captured enormous attention from the scientific community and general public. One of the most hotly debated issues in these diseases is the nature of the infectious material. In recent years increasing evidence has emerged supporting the protein-only hypothesis of prion transmission. In this model PrPSc (the pathological isoform of the prion protein, PrPC) represents the sole component of the infectious particle. However, uncertainties about possible additional factors involved in the conversion of PrPC into PrPSc remain despite extensive attempts to isolate and characterize these elusive components. In this article, we review recent developments concerning the protein-only hypothesis as well as the possible involvement of cellular factors in PrPC to PrPSc conformational change and their influence on the pathogenesis of prion diseases.

Prion strain-dependent differences in conversion of mutant prion proteins in cell culture

Although the protein-only hypothesis proposes that it is the conformation of abnormal prion protein (PrP(Sc)) that determines strain diversity, the molecular basis of strains remains to be elucidated. In the present study, we generated a series of mutations in the normal prion protein (PrP(C)) in which a single glutamine residue was replaced with a basic amino acid and compared their abilities to convert to PrP(Sc) in cultured neuronal N2a58 cells infected with either the Chandler or 22L mouse-adapted scrapie strain. In mice, these strains generate PrP(Sc) of the same sequence but different conformations, as judged by infrared spectroscopy. Substitutions at codons 97, 167, 171, and 216 generated PrP(C) that resisted conversion and inhibited the conversion of coexpressed wild-type PrP in both Chandler-infected and 22L-infected cells. Interestingly, substitutions at codons 185 and 218 gave strain-dependent effects. The Q185R and Q185K PrP were efficiently converted to PrP(Sc) in Chandler-infected but not 22L-infected cells. Conversely, Q218R and Q218H PrP were converted only in 22L-infected cells. Moreover, the Q218K PrP exerted a potent inhibitory effect on the conversion of coexpressed wild-type PrP in Chandler-infected cells but had little effect on 22L-infected cells. These results show that two strains with the same PrP sequence but different conformations have differing abilities to convert the same mutated PrP(C).

Structural insights into the interaction between prion protein and nucleic acid

The infectious agent of transmissible spongiform encephalopathies (TSE) is believed to comprise, at least in part, the prion protein (PrP). Other molecules can modulate the conversion of the normal PrP(C) into the pathological conformer (PrP(Sc)), but the identity and mechanisms of action of the key physiological factors remain unclear. PrP can bind to nucleic acids with relatively high affinity. Here, we report small-angle X-ray scattering (SAXS) and nuclear magnetic resonance spectroscopy measurements of the tight complex of PrP with an 18 bp DNA sequence. This double-stranded DNA sequence (E2DBS) binds with nanomolar affinity to the full-length recombinant mouse PrP. The SAXS data show that formation of the rPrP-DNA complex leads to larger values of the maximum dimension and radius of gyration. In addition, the SAXS studies reveal that the globular domain of PrP participates importantly in the formation of the complex. The changes in NMR HSQC spectra were clustered in two major regions: one in the disordered portion of the PrP and the other in the globular domain. Although interaction is mediated mainly through the PrP globular domain, the unstructured region is also recruited to the complex. This visualization of the complex provides insight into how oligonucleotides bind to PrP and opens new avenues to the design of compounds against prion diseases.

N-terminal peptides from unprocessed prion proteins enter cells by macropinocytosis

A peptide derived from the N-terminus of the unprocessed bovine prion protein (bPrPp), incorporating the hydrophobic signal sequence (residues 1-24) and a basic domain (KKRPKP, residues 25-30), internalizes into mammalian cells, even when coupled to a sizeable cargo, and therefore functions as a cell-penetrating peptide (CPP). Confocal microscopy and co-localization studies indicate that the internalization of bPrPp is mainly through macropinocytosis, a fluid-phase endocytosis process, initiated by binding to cell-surface proteoglycans. Electron microscopy studies show internalized bPrPp-DNA-gold complexes residing in endosomal vesicles. bPrPp induces expression of a complexed luciferase-encoding DNA plasmid, demonstrating the peptide's ability to transport the cargo across the endosomal membrane and into the cytosol and nucleus. The novel CPP activity of the unprocessed N-terminal domain of PrP could be important for the retrotranslocation of partly processed PrP and for PrP trafficking inside or between cells, with implications for the infectivity associated with prion diseases.

Prions and transmissible spongiform encephalopathy (TSE) chemotherapeutics: A common mechanism for anti-TSE compounds?

No validated treatments exist for transmissible spongiform encephalopathies (TSEs or prion diseases) in humans or livestock. The search for TSE therapeutics is complicated by persistent uncertainties about the nature of mammalian prions and their pathogenic mechanisms. In pursuit of anti-TSE drugs, we and others have focused primarily on blocking conversion of normal prion protein, PrP(C), to the TSE-associated isoform, PrP(Sc). Recently developed high-throughput screens have hastened the identification of new inhibitors with strong in vivo anti-TSE activities such as porphyrins, phthalocyanines, and phosphorthioated oligonucleotides. New routes of administration have enhanced beneficial effects against established brain infections. Several different classes of TSE inhibitors share structural similarities, compete for the same site(s) on PrP(C), and induce the clustering and internalization of PrP(C) from the cell surface. These activities may represent a common mechanism of action for these anti-TSE compounds.

Interaction of Prion Protein with Small Highly Structured RNAs: Detection and Characterization of PrP-Oligomers

Conformational modification of normal prion protein (PrP(c)) to protease-resistant, beta-sheet rich, aggregates (PrP(sc)) is commonly accepted cause for prion diseases. On the other hand, several studies in recent years implicate soluble, protease-sensitive, oligomers of PrP(c) in neuronal damage. Previously, our group has shown that small, highly structured RNAs (shsRNAs), in conjunction with a serum factor, facilitated the conversion of hrPrP to a protease resistant, high molecular weight isoform. In the current study we demonstrate that shsRNAs, in the absence of the serum factor, generate soluble, protease-sensitive, and potentially toxic oligomers of ovrPrP. We have isolated a 500 kD oligomer by size exclusion chromatography of the reaction mixture and identified the accessible epitopes. The soluble PrP-oligomers were present in enhanced amounts in scrapie infected sheep brain and treating extracts of normal sheep brain with shsRNA resulted in oligomerization of endogenous PrP. Isolation, characterization of PrP-oligomers and their possible implication in prion diseases is discussed.

Potent antiscrapie activities of degenerate phosphorothioate oligonucleotides

Although transmissible spongiform encephalopathies (TSEs) are incurable, a key therapeutic approach is prevention of conversion of the normal, protease-sensitive form of prion protein (PrP-sen) to the disease-specific protease-resistant form of prion protein (PrP-res). Here degenerate phosphorothioate oligonucleotides (PS-ONs) are introduced as low-nM PrP-res conversion inhibitors with strong antiscrapie activities in vivo. Comparisons of various PS-ON analogs indicated that hydrophobicity and size were important, while base composition was only minimally influential. PS-ONs bound avidly to PrP-sen but could be displaced by sulfated glycan PrP-res inhibitors, indicating the presence of overlapping binding sites. Labeled PS-ONs also bound to PrP-sen on live cells and were internalized. This binding likely accounts for the antiscrapie activity. Prophylactic PS-ON treatments more than tripled scrapie survival periods in mice. Survival times also increased when PS-ONs were mixed with scrapie brain inoculum. With these antiscrapie activities and their much lower anticoagulant activities than that of pentosan polysulfate, degenerate PS-ONs are attractive new compounds for the treatment of TSEs.

Regioselective Reductive Openings of Acetals; Mechanistic Details and Synthesis of Fluorescently Labeled Compounds

Regioselective reductive openings of mixed phenolic-benzylic acetals, using BH3.NMe3-AlCl3, was investigated, and a mechanism where the outcome is directed by the electrostatic potential of the two oxygen atoms is presented. The regioselective acetal opening was used in the synthesis of a fluorescently labeled analogue to antiproliferative xylosides. The fluorescently labeled xyloside was tested for uptake, antiproliferative activity, and glycosaminoglycan priming in different cell lines. The xyloside was taken up by all cell lines but did not initiate glycosaminoglycan biosynthesis.

Inhibitors of the mitogen-activated protein kinase kinase 1/2 signaling pathway clear prion-infected cells from PrPSc

Prions represent a unique class of infectious agents in which the normal cellular prion protein (PrPC) is converted to an abnormal isoform (PrPSc), which accumulates in the brain and constitutes the major, if not the only, component of the infectious particle. Factors that still remain to be identified may facilitate the conversion of PrPC to PrPSc. In the present study, we first demonstrated that a growth factor of the neurotrophin family, brain-derived neurotrophic factor (BDNF), stimulates the formation of PrPSc in a gonadotropin-releasing hormone-secreting neuronal cell line (GT1-1 cells) infected with the Rocky Mountain Laboratory (RML) strain of scrapie as determined by Western blot analysis. We then observed that the prion-infected cells can be cleared from PrPSc by treatment with three inhibitors of mitogen-activated protein kinase kinase 1/2 (MEK1/2) [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene and 2-(2-amino-3-methyoxyphenyl)-4H-1-benzopyran-4-one, as well as alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl) benzeneacetonitrile, which passes the blood-brain barrier], a component of one of the intracellular signaling pathways activated by BDNF. The MEK1/2 inhibitors were also efficient in clearing PrPSc from prion-infected GT1-1 cells stimulated to accumulate high levels of PrPSc by enhanced serum concentrations in the medium or by the use of a serum-free neuron-specific neurobasal medium. PrPSc did not reappear in the cultures within 5 weeks after completion of treatment. We conclude that inhibitors of the MEK1/2 pathway can efficiently and probably irreversibly clear PrP(Sc) from prion-infected cells. The MEK pathway may therefore be a suitable target for therapeutic intervention in prion diseases.

A bovine glucuronidase for assembly of β-d-glucuronyl-(1–3)-6-O-sulfo-β-d-gluco- and galacto-pyranosyl linkages

Glucuronidase-catalyzed transglycosylation was examined by using 4-nitrophenyl beta-D-glucuronide (D-GlcA-O-pNP) as the glycosyl donor; when pNP 6-O-sulfo-beta-D-gluco- and D-galacto-pyranosides were used as the acceptors, a bovine enzyme was found to construct beta-D-GlcA-(1-3)-linkages with the 6-O-sulfo-sugars in both a site- and beta-selective way.

The metabolism of glycosaminoglycans is impaired in prion diseases

It is well established that the conversion of PrP(C) to PrP(Sc) is the key event in prion disease biology. In addition, several lines of evidence suggest that glycosaminoglycans (GAGs) and in particular heparan sulfate (HS) may play a role in the PrP(C) to PrP(Sc) conversion process. It has been proposed that PrP(Sc) accumulation in prion diseases may induce aberrant activation of lysosomal activity, which has been shown to result in neurodegeneration in a number of diseases, especially lysosomal storage disorders. Among such diseases, only the ones resulting from defects in GAGs degradation are accompanied by secretion of large amounts of GAG metabolites in urine. In this work, we show that GAGs are secreted in the urine of prion-infected animals and humans, and surprisingly, also in the urine of mice ablated for the PrP gene. We hypothesize that both the presence of PrP(Sc) or the absence of PrP(C) may alter the metabolism of GAGs.

Membrane perturbation effects of peptides derived from the N-termini of unprocessed prion proteins

Peptides derived from the unprocessed N-termini of mouse and bovine prion proteins (mPrPp and bPrPp, respectively), comprising hydrophobic signal sequences followed by charged domains (KKRPKP), function as cell-penetrating peptides (CPPs) with live cells, concomitantly causing toxicity. Using steady-state fluorescence techniques, including calcein leakage and polarization of a membrane probe (diphenylhexatriene, DPH), as well as circular dichroism, we studied the membrane interactions of the peptides with large unilamellar phospholipid vesicles (LUVs), generally with a 30% negative surface charged density, comparing the effects with those of the CPP penetratin (pAntp) and the pore-forming peptide melittin. The prion peptides caused significant calcein leakage from LUVs concomitant with increased membrane ordering. Fluorescence correlation spectroscopy (FCS) studies of either rhodamine-entrapping (REVs) or rhodamine-labeled (RLVs) vesicles, showed that addition of the prion peptides resulted in significant release of rhodamine from the REVs without affecting the overall integrity of the RLVs. The membrane leakage effects due to the peptides had the following order of potency: melittin>mPrPp>bPrPp>pAntp. The membrane perturbation effects of the N-terminal prion peptides suggest that they form transient pores (similar to melittin) causing toxicity in parallel with their cellular trafficking.

Semiautomated cell-free conversion of prion protein: applications for high-throughput screening of potential antiprion drugs

Transmissible spongiform encephalitis (TSE) is a lethal illness with no known treatment. Conversion of the cellular prion protein (PrP(C)) into the infectious isoform (PrP(Sc)) is believed to be the central event in the development of this disease. Recombinant PrP (rPrP) protein folded into the amyloid conformation was shown to cause the transmissible form of prion disease in transgenic mice and can be used as a surrogate model for PrP(Sc). Here, we introduced a semiautomated assay of in vitro conversion of rPrP protein to the amyloid conformation. We have examined the effect of known inhibitors of prion propagation on this conversion and found good correlation between their activity in this assay and that in other in vitro assays. We thus propose that the conversion of rPrP to the amyloid isoform can serve as a high-throughput screen for possible inhibitors of PrP(Sc) formation and potential anti-TSE drugs.

Anchorless prion protein results in infectious amyloid disease without clinical scrapie

In prion and Alzheimer's diseases, the roles played by amyloid versus nonamyloid deposits in brain damage remain unresolved. In scrapie-infected transgenic mice expressing prion protein (PrP) lacking the glycosylphosphatidylinositol (GPI) membrane anchor, abnormal protease-resistant PrPres was deposited as amyloid plaques, rather than the usual nonamyloid form of PrPres. Although PrPres amyloid plaques induced brain damage reminiscent of Alzheimer's disease, clinical manifestations were minimal. In contrast, combined expression of anchorless and wild-type PrP produced accelerated clinical scrapie. Thus, the PrP GPI anchor may play a role in the pathogenesis of prion diseases.

Protease-resistant prion protein amplification reconstituted with partially purified substrates and synthetic polyanions

Little is currently known about the biochemical mechanism by which induced prion protein (PrP) conformational change occurs during mammalian prion propagation. In this study, we describe the reconstitution of PrPres amplification in vitro using partially purified and synthetic components. Overnight incubation of purified PrP27-30 and PrPC molecules at a molar ratio of 1:250 yielded approximately 2-fold baseline PrPres amplification. Addition of various polyanionic molecules increased the level of PrPres amplification to approximately 10-fold overall. Polyanionic compounds that stimulated purified PrPres amplification to varying degrees included synthetic, homopolymeric nucleic acids such as poly(A) and poly(dT), as well as non-nucleic acid polyanions, such as heparan sulfate proteoglycan. Size fractionation experiments showed that synthetic poly(A) polymers must be >0.2 kb in length to stimulate purified PrPres amplification. Thus, one possible set of minimal components for efficient conversion of PrP molecules in vitro may be surprisingly simple, consisting of PrP27-30, PrPC, and a stimulatory polyanionic compound.

In vivo fragmentation of heparan sulfate by heparanase overexpression renders mice resistant to amyloid protein A amyloidosis

Amyloid diseases encompass >20 medical disorders that include amyloid protein A (AA) amyloidosis, Alzheimer's disease, and type 2 diabetes. A common feature of these conditions is the selective organ deposition of disease-specific fibrillar proteins, along with the sulfated glycosaminoglycan, heparan sulfate. We have generated transgenic mice that overexpress human heparanase and have tested their susceptibility to amyloid induction. Drastic shortening of heparan sulfate chains was observed in heparanase-overproducing organs, such as liver and kidney. These sites selectively escaped amyloid deposition on experimental induction of inflammation-associated AA amyloidosis, as verified by lack of material staining with Congo Red, as well as lack of associated polysaccharide, whereas the same tissues from control animals were heavily infiltrated with amyloid. By contrast, the spleens of transgenic mice that failed to significantly overexpress heparanase contained heparan sulfate chains similar in size to those of control spleen and remained susceptible to amyloid deposition. Our findings provide direct in vivo evidence that heparan sulfate is essential for the development of amyloid disease.

Glycosylation-related Gene Expression in Prion Diseases

Several lines of evidence indicate that some glycoconjugates are efficient effectors of the cellular prion protein (PrP(C)) conversion into its pathogenic (PrP(Sc)) isoform. To assess how glycoconjugate glycan moieties participate in the biogenesis of PrP(Sc), an exhaustive comparative analysis of the expression of about 200 glycosylation-related genes was performed on prion-infected or not, hypothalamus-derived GT1 cells by hybridization of DNA microarrays, semiquantitative RT-PCR, and biochemical assays. A significant up- (30-fold) and down- (17-fold) regulation of the expression of the ChGn1 and Chst8 genes, respectively, was observed in prion-infected cells. ChGn1 and Chst8 are involved in the initiation of the synthesis of chondroitin sulfate and in the 4-O-sulfation of non-reducing N-acetylgalactosamine residues, respectively. A possible role for a hyposulfated chondroitin in PrP(Sc) accumulation was evidenced at the protein level and by determination of chondroitin and heparan sulfate amounts. Treatment of Sc-GT1 cells with a heparan mimetic (HM2602) induced an important reduction of the amount of PrP(Sc), associated with a total reversion of the transcription pattern of the N-acetylgalactosamine-4-O-sulfotransferase 8. It suggests a link between the genetic control of 4-O-sulfation and PrP(Sc) accumulation.

Cationic Polysaccharides as Antiprion Agents

Cationic polysaccharides were synthesized by conjugation of various oligoamines to oxidized polysaccharides by reductive amination and tested for antiprion activity. Polycations of dextran, pullulan and arabinogalactan grafted with oligoamines of 2 to 4 amino groups were investigated for their ability to eliminate PrP(Sc), the protease-resistant isoform of the prion protein, from chronically infected neuroblastoma cells, ScN2a-M. The proteinase K (PK)-resistant PrP elimination depends on both the concentration of the reagent and the duration of exposure. The most potent compound was found to be dextran-spermine that caused depletion of PrP(Sc) to undetectable levels at concentration of 31 ng/mL after 4 days of exposure. Activity analysis revealed that grafted oligoamine indentity of the polycation plays a significant role in elimination of PK-resistant PrP from chronically infected N2a-M cells, regardless of the polysaccharide used. Dextran-spermine conjugates were modified with oleic acid and with methoxypoly(ethylene glycol) (MPEG) at various degrees of substitution for further studies and their antiprion activity was examined. Substitution of dextran-spermine with MPEG or oleic acid slightly decreases its activity as a function of MPEG/oleic acid content. These findings confirm previous reports that polycations are effective in eliminating PrP(Sc) in vitro.

Heparan sulfate is a cellular receptor for purified infectious prions

Prions replicate in the host cell by the self-propagating refolding of the normal cell surface protein, PrP(C), into a beta-sheet-rich conformer, PrP(Sc). Exposure of cells to prion-infected material and subsequent endocytosis can sometimes result in the establishment of an infected culture. However, the relevant cell surface receptors have remained unknown. We have previously shown that cellular heparan sulfates (HS) are involved in the ongoing formation of scrapie prion protein (PrP(Sc)) in chronically infected cells. Here we studied the initial steps in the internalization of prions and in the infection of cells. Purified prion "rods" are arguably the purest prion preparation available. The only proteinaceous component of rods is PrP(Sc). Mouse neuroblastoma N2a, hypothalamus GT1-1, and Chinese hamster ovary cells efficiently bound both hamster and mouse prion rods (at 4 degrees C) and internalized them (at 37 degrees C). Treating cells with bacterial heparinase III or chlorate (a general inhibitor of sulfation) strongly reduced both binding and uptake of rods, whereas chondroitinase ABC was inactive. These results suggested that the cell surface receptor of prion rods involves sulfated HS chains. Sulfated glycans inhibited both binding and uptake of rods, probably by competing with the binding of rods to cellular HS. Treatments that prevented endocytosis of rods also prevented the de novo infection of GT1-1 cells when applied during their initial exposure to prions. These results indicate that HS are an essential part of the cellular receptor used both for prion uptake and for cell infection. Cellular HS thus play a dual role in prion propagation, both as a cofactor for PrP(Sc) synthesis and as a receptor for productive prion uptake.

PrPSc incorporation to cells requires endogenous glycosaminoglycan expression

Many lines of evidence suggest an interaction between glycosaminoglycans (GAGs) and the PrP proteins as well as a possible role for GAGs in prion disease pathogenesis. In this work, we sought to determine whether the PrP-GAG interaction affects the incorporation of PrP(Sc) (the scrapie isoform of PrP) to normal cells. This may be the first step in prion disease pathogenesis. To this effect, we incubated proteinase K-digested hamster scrapie brain homogenates with several lines of Chinese hamster ovary (CHO) cells in the presence or absence of heparin. Our results show that over a large range of PrP(Sc) concentrations the binding of PrP(Sc) to wild type CHO cells, which do not express detectable PrP, was equivalent to the binding of PrP(Sc) to CHO cells overexpressing PrP. A significant part of PrP(Sc) binding to both lines could be inhibited by heparin. Additional evidence that PrP(Sc) binding to cells was dependent on the presence of GAGs could be concluded from the fact that the binding of PrP(Sc) to CHO cells missing GAGs on the cell surface was significantly reduced. Interestingly, preincubation of scrapie brain homogenate with heparin before intraperitoneal inoculation into normal hamsters resulted in a significant delay in prion disease manifestation.

Synthetic human prion protein octapeptide repeat binds to the proteinase K active site

Proteinase K is widely used in tests for the presence of infectious prion protein causing fatal spongiform encephalopathies. To investigate possible interactions between the enzyme and the functionally important N-terminal prion domain, we crystallized mercury-inhibited proteinase K in the presence of the synthetic peptides GGGWGQPH and HGGGW. The octapeptide sequence is identical to that of a single octapeptide repeat (OPR) from the physiologically important OPR region. Here, we present the first direct evidence for the complex formation between a proteolytic enzyme and a segment of human prion molecule. The X-ray structures of the complexes at 1.4 and 1.8A resolution, respectively, revealed that in both cases the segment GGG is strongly bound as a real substrate at the substrate recognition site of the proteinase forming an antiparallel beta-strand between the two parallel strands of Asn99-Tyr104 and Ser132-Gly136. The complex is stabilized through an extended H-bonding network.

RNA mutagenesis and sporadic prion diseases

The extremely low incidence of sporadic prion diseases suggests that they may arise as a rare stochastic event in otherwise healthy animals or humans. Current hypotheses for sporadic prion disease include horizontal transmission, spontaneous conversion of PrpC into PrpSc, and somatic mutation of the Prp gene. Here, we suggest RNA mutation as a possible initial event in the etiology of sporadic prion disease. The proposed model is based on (i) the fact that in Prp-expressing cells, mutations are statistically more likely to occur in the Prp mRNA population than in the corresponding two copies of the Prp gene, and (ii) the absence of RNA repair mechanisms analogous to those found for DNA mismatch correction resulting in a relatively higher rate of RNA mutations. Here, we suggest that translation of mutated Prp mRNA could lead to the synthesis of transient Prp(Sc) which results in the conversion of PrpC into PrpSc and the propagation of a disease-associated isoform. This model points to RNA mutation as a possible mechanism for the generation of sporadic prion diseases and other pathological disorders in which infectious proteins other than PrpSc might be implicated.

Prion diseases — close to effective therapy?

The transmissible spongiform encephalopathies could represent a new mode of transmission for infectious diseases--a process more akin to crystallization than to microbial replication. The prion hypothesis proposes that the normal isoform of the prion protein is converted to a disease-specific species by template-directed misfolding. Therapeutic and prophylactic strategies to combat these diseases have emerged from immunological and chemotherapeutic approaches. The lessons learned in treating prion disease will almost certainly have an impact on other diseases that are characterized by the pathological accumulation of misfolded proteins.