Glycan-controlled epitopes of prion protein include a major determinant of susceptibility to sheep scrapie

Further Characterization of Glycoform-Selective Prions of Variably Protease-Sensitive Prionopathy

Prion is an infectious protein (PrP^Sc) that is derived from a cellular glycoprotein (PrP^C) through a conformational transition and associated with a group of prion diseases in animals and humans. Characterization of proteinase K (PK)-resistant PrP^Sc by western blotting has been critical to diagnosis and understanding of prion diseases including Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker (GSS) disease in humans. However, formation as well as biochemical and biological properties of the glycoform-selective PrP^Sc in variably protease-sensitive prionopathy (VPSPr) remain poorly understood. Here we reveal that formation of the ladder-like PrP^Sc in VPSPr is a PK-dependent two-step process, which is enhanced by basic pH. Two sets of PrP^Sc fragments can be identified with antibodies directed against an intermediate or a C-terminal domain of the protein. Moreover, antibodies directed against specific PrP glycoforms reveal faster electrophoretic migrations of PrP fragments mono-glycosylated at residue 181 and 197 in VPSPr than those in sporadic CJD (sCJD). Finally, RT-QuIC assay indicates that PrP^Sc-seeding activity is lower and its lag time is longer in VPSPr than in sCJD. Our results suggest that the glycoform-selective PrP^Sc in VPSPr is associated with altered glycosylation, resulting in different PK-truncation and aggregation seeding activity compared to PrP^Sc in sCJD.

Incomplete glycosylation during prion infection unmasks a prion protein epitope that facilitates prion detection and strain discrimination

The causative factors underlying conformational conversion of cellular prion protein (PrP^C) into its infectious counterpart (PrP^Sc) during prion infection remain undetermined, in part because of a lack of monoclonal antibodies (mAbs) that can distinguish these conformational isoforms. Here we show that the anti-PrP mAb PRC7 recognizes an epitope that is shielded from detection when glycans are attached to Asn-196. We observed that whereas PrP^C is predisposed to full glycosylation and is therefore refractory to PRC7 detection, prion infection leads to diminished PrP^Sc glycosylation at Asn-196, resulting in an unshielded PRC7 epitope that is amenable to mAb recognition upon renaturation. Detection of PRC7-reactive PrP^Sc in experimental and natural infections with various mouse-adapted scrapie strains and with prions causing deer and elk chronic wasting disease and transmissible mink encephalopathy uncovered that incomplete PrP^Sc glycosylation is a consistent feature of prion pathogenesis. We also show that interrogating the conformational properties of the PRC7 epitope affords a direct means of distinguishing different prion strains. Because the specificity of our approach for prion detection and strain discrimination relies on the extent to which N-linked glycosylation shields or unshields PrP epitopes from antibody recognition, it dispenses with the requirement for additional standard manipulations to distinguish PrP^Sc from PrP^C, including evaluation of protease resistance. Our findings not only highlight an innovative and facile strategy for prion detection and strain differentiation, but are also consistent with a mechanism of prion replication in which structural instability of incompletely glycosylated PrP contributes to the conformational conversion of PrP^C to PrP^Sc.

Full restoration of specific infectivity and strain properties from pure mammalian prion protein

The protein-only hypothesis predicts that infectious mammalian prions are composed solely of PrPSc, a misfolded conformer of the normal prion protein, PrPC. However, protein-only PrPSc preparations lack significant levels of prion infectivity, leading to the alternative hypothesis that cofactor molecules are required to form infectious prions. Here, we show that prions with parental strain properties and full specific infectivity can be restored from protein-only PrPSc in vitro. The restoration reaction is rapid, potent, and requires bank vole PrPC substrate, post-translational modifications, and cofactor molecules. To our knowledge, this represents the first report in which the essential properties of an infectious mammalian prion have been restored from pure PrP without adaptation. These findings provide evidence for a unified hypothesis of prion infectivity in which the global structure of protein-only PrPSc accurately stores latent infectious and strain information, but cofactor molecules control a reversible switch that unmasks biological infectivity.

In Vitro Seeding Activity of Glycoform-Deficient Prions from Variably Protease-Sensitive Prionopathy and Familial CJD Associated with PrPV180I Mutation

Both sporadic variably protease-sensitive prionopathy (VPSPr) and familial Creutzfeldt-Jakob disease linked to the prion protein (PrP) V180I mutation (fCJD^V180I) have been found to share a unique pathological prion protein (PrP^Sc) that lacks the protease-resistant PrP^Sc glycosylated at residue 181 because two of four PrP glycoforms are apparently not converted into the PrP^Sc from their cellular PrP (PrP^C). To investigate the seeding activity of these unique PrP^Sc molecules, we conducted in vitro prion conversion experiments using serial protein misfolding cyclic amplification (sPMCA) and real-time quaking-induced conversion (RT-QuIC) assays with different PrP^C substrates. We observed that the seeding of PrP^Sc from VPSPr or fCJD^V180I in the sPMCA reaction containing normal human or humanized transgenic (Tg) mouse brain homogenates generated PrP^Sc molecules that unexpectedly exhibited a dominant diglycosylated PrP isoform along with PrP monoglycosylated at residue 181. The efficiency of PrP^Sc amplification was significantly higher in non-CJDMM than in non-CJDVV human brain homogenate, whereas it was higher in normal TgVV than in TgMM mouse brain homogenate. PrP^C from the mixture of normal TgMM and Tg mouse brain expressing PrP^V180I mutation (Tg180) but not TgV180I alone was converted into PrP^Sc by seeding with the VPSPr or fCJD^V180I. The RT-QuIC seeding activity of PrP^Sc from VPSPr and fCJD^V180I was significantly lower than that of sCJD. Our results suggest that the formation of glycoform-selective prions may be associated with an unidentified factor in the affected brain and the glycoform-deficiency of PrP^Sc does not affect the glycoforms of in vitro newly amplified PrP^Sc.

In silico investigation of the prion protein glycosylation profiles in relation to scrapie disease resistance in domestic sheep (Ovis aries)

The prion protein is a membrane-bound glycoprotein which consists mainly α-helix structure. In contrast, the infectious prion protein shows the beta-sheet structure. The prion-associated diseases are all lethal neurodegenerative abnormalities, called transmissible spongiform encephalopathies. Scrapie is the most common type of these illnesses affecting sheep, goats, and moufflon. The VRQ, AHQ, ARR and N146S polymorphisms in the sheep prion gene have been found to be associated with resistance to scrapie disease. So far, the relationship of polymorphisms to three-dimensional protein structures, post-translational modifications, and scrapie resistance has not been studied. In this study, the potential N- and O-glycosylation positions of sheep prion protein polymorphisms were analyzed, the secondary and three-dimensional protein structure models were predicted, three-dimensional glycoprotein models were constructed and the role of glycosylation positions in protein interactions was investigated. Here, we found that protein secondary and three-dimensional structures vary among polymorphisms. Moreover, we found wild-type prion and all polymorphic variants show N-glycosylation at Asn¹⁸⁴ and Asn²⁰⁰ positions, while O-glycosylation profiles are variant-specific. We also found that structural changes among prion polymorphisms leads to the formation of variant spesific O-glycosylation profiles and these positions are associated with protein interactions. Based on these findings, we suggest that O-glycosylation may be effective on resistance/susceptibility of sheep prion polymorphisms to scrapie disease.

Octarepeat region flexibility impacts prion function, endoproteolysis and disease manifestation

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

Prion disease tempo determined by host-dependent substrate reduction

The symptoms of prion infection can take years or decades to manifest following the initial exposure. Molecular markers of prion disease include accumulation of the misfolded prion protein (PrPSc), which is derived from its cellular precursor (PrPC), as well as downregulation of the PrP-like Shadoo (Sho) glycoprotein. Given the overlapping cellular environments for PrPC and Sho, we inferred that PrPC levels might also be altered as part of a host response during prion infection. Using rodent models, we found that, in addition to changes in PrPC glycosylation and proteolytic processing, net reductions in PrPC occur in a wide range of prion diseases, including sheep scrapie, human Creutzfeldt-Jakob disease, and cervid chronic wasting disease. The reduction in PrPC results in decreased prion replication, as measured by the protein misfolding cyclic amplification technique for generating PrPSc in vitro. While PrPC downregulation is not discernible in animals with unusually short incubation periods and high PrPC expression, slowly evolving prion infections exhibit downregulation of the PrPC substrate required for new PrPSc synthesis and as a receptor for pathogenic signaling. Our data reveal PrPC downregulation as a previously unappreciated element of disease pathogenesis that defines the extensive, presymptomatic period for many prion strains.

Prions in variably protease-sensitive prionopathy: an update

Human prion diseases, including sporadic, familial, and acquired forms such as Creutzfeldt-Jakob disease (CJD), are caused by prions in which an abnormal prion protein (PrP^Sc) derived from its normal cellular isoform (PrP^C) is the only known component. The recently-identified variably protease-sensitive prionopathy (VPSPr) is characterized not only by an atypical clinical phenotype and neuropathology but also by the deposition in the brain of a peculiar PrP^Sc. Like other forms of human prion disease, the pathogenesis of VPSPr also currently remains unclear. However, the findings of the peculiar features of prions from VPSPr and of the possible association of VPSPr with a known genetic prion disease linked with a valine to isoleucine mutation at residue 180 of PrP reported recently, may be of great importance in enhancing our understanding of not only this atypical human prion disease in particular, but also other prion diseases in general. In this review, we highlight the physicochemical and biological properties of prions from VPSPr and discuss the pathogenesis of VPSPr including the origin and formation of the peculiar prions.

Glycoform-selective prion formation in sporadic and familial forms of prion disease

The four glycoforms of the cellular prion protein (PrP(C)) variably glycosylated at the two N-linked glycosylation sites are converted into their pathological forms (PrP(Sc)) in most cases of sporadic prion diseases. However, a prominent molecular characteristic of PrP(Sc) in the recently identified variably protease-sensitive prionopathy (VPSPr) is the absence of a diglycosylated form, also notable in familial Creutzfeldt-Jakob disease (fCJD), which is linked to mutations in PrP either from Val to Ile at residue 180 (fCJD(V180I)) or from Thr to Ala at residue 183 (fCJD(T183A)). Here we report that fCJD(V180I), but not fCJD(T183A), exhibits a proteinase K (PK)-resistant PrP (PrP(res)) that is markedly similar to that observed in VPSPr, which exhibits a five-step ladder-like electrophoretic profile, a molecular hallmark of VPSPr. Remarkably, the absence of the diglycosylated PrP(res) species in both fCJD(V180I) and VPSPr is likewise attributable to the absence of PrP(res) glycosylated at the first N-linked glycosylation site at residue 181, as in fCJD(T183A). In contrast to fCJD(T183A), both VPSPr and fCJD(V180I) exhibit glycosylation at residue 181 on di- and monoglycosylated (mono181) PrP prior to PK-treatment. Furthermore, PrP(V180I) with a typical glycoform profile from cultured cells generates detectable PrP(res) that also contains the diglycosylated PrP in addition to mono- and unglycosylated forms upon PK-treatment. Taken together, our current in vivo and in vitro studies indicate that sporadic VPSPr and familial CJD(V180I) share a unique glycoform-selective prion formation pathway in which the conversion of diglycosylated and mono181 PrP(C) to PrP(Sc) is inhibited, probably by a dominant-negative effect, or by other co-factors.

Characterization of conformation-dependent prion protein epitopes

Whereas prion replication involves structural rearrangement of cellular prion protein (PrP(C)), the existence of conformational epitopes remains speculative and controversial, and PrP transformation is monitored by immunoblot detection of PrP(27-30), a protease-resistant counterpart of the pathogenic scrapie form (PrP(Sc)) of PrP. We now describe the involvement of specific amino acids in conformational determinants of novel monoclonal antibodies (mAbs) raised against randomly chimeric PrP. Epitope recognition of two mAbs depended on polymorphisms controlling disease susceptibility. Detection by one, referred to as PRC5, required alanine and asparagine at discontinuous mouse PrP residues 132 and 158, which acquire proximity when residues 126-218 form a structured globular domain. The discontinuous epitope of glycosylation-dependent mAb PRC7 also mapped within this domain at residues 154 and 185. In accordance with their conformational dependence, tertiary structure perturbations compromised recognition by PRC5, PRC7, as well as previously characterized mAbs whose epitopes also reside in the globular domain, whereas conformation-independent epitopes proximal or distal to this region were refractory to such destabilizing treatments. Our studies also address the paradox of how conformational epitopes remain functional following denaturing treatments and indicate that cellular PrP and PrP(27-30) both renature to a common structure that reconstitutes the globular domain.

Integrity of helix 2-helix 3 domain of the PrP protein is not mandatory for prion replication

The process of prion conversion is not yet well understood at the molecular level. The regions critical for the conformational change of PrP remain mostly debated and the extent of sequence change acceptable for prion conversion is poorly documented. To achieve progress on these issues, we applied a reverse genetic approach using the Rov cell system. This allowed us to test the susceptibility of a number of insertion mutants to conversion into prion in the absence of wild-type PrP molecules. We were able to propagate several prions with 8 to 16 extra amino acids, including a polyglycine stretch and His or FLAG tags, inserted in the middle of the protease-resistant fragment. These results demonstrate the possibility to increase the length of the loop between helices H2 and H3 up to 4-fold, without preventing prion replication. They also indicate that this loop probably remains unstructured in PrP(Sc). We also showed that bona fide prions can be produced following insertion of octapeptides in the two C-terminal turns of H2. These insertions do not interfere with the overall fold of the H2-H3 domain indicating that the highly conserved sequence of the terminal part of H2 is not critical for the conversion. Altogether these data showed that the amplitude of modifications acceptable for prion conversion in the core of the globular domain of PrP is much greater than one might have assumed. These observations should help to refine structural models of PrP(Sc) and elucidate the conformational changes underlying prions generation.

Characterization of spontaneously generated prion-like conformers in cultured cells

A distinct conformational transition from the α-helix-rich cellular prion protein (PrP^C) into its β-sheet-rich pathological isoform (PrP^Sc) is the hallmark of prion diseases, a group of fatal transmissible encephalopathies that includes spontaneous and acquired forms. Recently, a PrP^Sc-like intermediate form characterized by the formation of insoluble aggregates and protease-resistant PrP species termed insoluble PrP^C (iPrP^C) has been identified in uninfected mammalian brains and cultured neuronal cells, providing new insights into the molecular mechanism(s) of these diseases. Here, we explore the molecular characteristics of the spontaneously formed iPrP^C in cultured neuroblastoma cells expressing wild-type or mutant human PrP linked to two familial prion diseases. We observed that although PrP mutation at either residue 183 from Thr to Ala (PrP^T183A) or at residue 198 from Phe to Ser (PrP^F198S) affects glycosylation at both N-linked glycosylation sites, the T183A mutation that results in intracellular retention significantly increased the formation of iPrP^C. Moreover, while autophagy is increased in F198S cells, it was significantly decreased in T183A cells. Our results indicate that iPrP^C may be formed more readily in an intracellular compartment and that a significant increase in PrP^T183A aggregation may be attributable to the inhibition of autophagy.

Microsecond unfolding kinetics of sheep prion protein reveals an intermediate that correlates with susceptibility to classical scrapie

The microsecond folding and unfolding kinetics of ovine prion proteins (ovPrP) were measured under various solution conditions. A fragment comprising residues 94-233 of the full-length ovPrP was studied for four variants with differing susceptibilities to classical scrapie in sheep. The observed biexponential unfolding kinetics of ovPrP provides evidence for an intermediate species. However, in contrast to previous results for human PrP, there is no evidence for an intermediate under refolding conditions. Global analysis of the kinetic data, based on a sequential three-state mechanism, quantitatively accounts for all folding and unfolding data as a function of denaturant concentration. The simulations predict that an intermediate accumulates under both folding and unfolding conditions, but is observable only in unfolding experiments because the intermediate is optically indistinguishable from the native state. The relative population of intermediates in two ovPrP variants, both transiently and under destabilizing equilibrium conditions, correlates with their propensities for classical scrapie. The variant susceptible to classical scrapie has a larger population of the intermediate state than the resistant variant. Thus, the susceptible variant should be favored to undergo the PrP(C) to PrP(Sc) conversion and oligomerization.

Prion propagation in cells expressing PrP glycosylation mutants

Infection by prions involves conversion of a host-encoded cell surface protein (PrP(C)) to a disease-related isoform (PrP(Sc)). PrP(C) carries two glycosylation sites variably occupied by complex N-glycans, which have been suggested by previous studies to influence the susceptibility to these diseases and to determine characteristics of prion strains. We used the Rov cell system, which is susceptible to sheep prions, to generate a series of PrP(C) glycosylation mutants with mutations at one or both attachment sites. We examined their subcellular trafficking and ability to convert into PrP(Sc) and to sustain stable prion propagation in the absence of wild-type PrP. The susceptibility to infection of mutants monoglycosylated at either site differed dramatically depending on the amino acid substitution. Aglycosylated double mutants showed overaccumulation in the Golgi compartment and failed to be infected. Introduction of an ectopic glycosylation site near the N terminus fully restored cell surface expression of PrP but not convertibility into PrP(Sc), while PrP(C) with three glycosylation sites conferred cell permissiveness to infection similarly to the wild type. In contrast, predominantly aglycosylated molecules with nonmutated N-glycosylation sequons, produced in cells expressing glycosylphosphatidylinositol-anchorless PrP(C), were able to form infectious PrP(Sc). Together our findings suggest that glycosylation is important for efficient trafficking of anchored PrP to the cell surface and sustained prion propagation. However, properly trafficked glycosylation mutants were not necessarily prone to conversion, thus making it difficult in such studies to discern whether the amino acid changes or glycan chain removal most influences the permissiveness to prion infection.

The physical relationship between infectivity and prion protein aggregates is strain-dependent

Prions are unconventional infectious agents thought to be primarily composed of PrP(Sc), a multimeric misfolded conformer of the ubiquitously expressed host-encoded prion protein (PrP(C)). They cause fatal neurodegenerative diseases in both animals and humans. The disease phenotype is not uniform within species, and stable, self-propagating variations in PrP(Sc) conformation could encode this 'strain' diversity. However, much remains to be learned about the physical relationship between the infectious agent and PrP(Sc) aggregation state, and how this varies according to the strain. We applied a sedimentation velocity technique to a panel of natural, biologically cloned strains obtained by propagation of classical and atypical sheep scrapie and BSE infectious sources in transgenic mice expressing ovine PrP. Detergent-solubilized, infected brain homogenates were used as starting material. Solubilization conditions were optimized to separate PrP(Sc) aggregates from PrP(C). The distribution of PrP(Sc) and infectivity in the gradient was determined by immunoblotting and mouse bioassay, respectively. As a general feature, a major proteinase K-resistant PrP(Sc) peak was observed in the middle part of the gradient. This population approximately corresponds to multimers of 12-30 PrP molecules, if constituted of PrP only. For two strains, infectivity peaked in a markedly different region of the gradient. This most infectious component sedimented very slowly, suggesting small size oligomers and/or low density PrP(Sc) aggregates. Extending this study to hamster prions passaged in hamster PrP transgenic mice revealed that the highly infectious, slowly sedimenting particles could be a feature of strains able to induce a rapidly lethal disease. Our findings suggest that prion infectious particles are subjected to marked strain-dependent variations, which in turn could influence the strain biological phenotype, in particular the replication dynamics.

Endogenous proteolytic cleavage of disease-associated prion protein to produce C2 fragments is strongly cell- and tissue-dependent

The abnormally folded form of the prion protein (PrP(Sc)) accumulating in nervous and lymphoid tissues of prion-infected individuals can be naturally cleaved to generate a N-terminal-truncated fragment called C2. Information about the identity of the cellular proteases involved in this process and its possible role in prion biology has remained limited and controversial. We investigated PrP(Sc) N-terminal trimming in different cell lines and primary cultured nerve cells, and in the brain and spleen tissue from transgenic mice infected by ovine and mouse prions. We found the following: (i) the full-length to C2 ratio varies considerably depending on the infected cell or tissue. Thus, in primary neurons and brain tissue, PrP(Sc) accumulated predominantly as untrimmed species, whereas efficient trimming occurred in Rov and MovS cells, and in spleen tissue. (ii) Although C2 is generally considered to be the counterpart of the PrP(Sc) proteinase K-resistant core, the N termini of the fragments cleaved in vivo and in vitro can actually differ, as evidenced by a different reactivity toward the Pc248 anti-octarepeat antibody. (iii) In lysosome-impaired cells, the ratio of full-length versus C2 species dramatically increased, yet efficient prion propagation could occur. Moreover, cathepsin but not calpain inhibitors markedly inhibited C2 formation, and in vitro cleavage by cathepsins B and L produced PrP(Sc) fragments lacking the Pc248 epitope, strongly arguing for the primary involvement of acidic hydrolases of the endolysosomal compartment. These findings have implications on the molecular analysis of PrP(Sc) and cell pathogenesis of prion infection.

Glycosylation-related genes are variably expressed depending on the differentiation state of a bioaminergic neuronal cell line: implication for the cellular prion protein

A striking feature of the cellular prion protein (PrP(C)) is the heterogeneity of its glycoforms, whose contribution to PrP(C) function has yet to be defined. Using the 1C11 neuronal bioaminergic differentiation model and a glycomics approach, we show here a correlation between differential PrP(C) N-glycosylations in 1C11(5-HT) serotonergic and 1C11(NE) noradrenergic cells compared to their 1C11 precursor cells and a variation of the glycogenome expression status in these cells. In particular, expression of genes involved in N-glycan synthesis or in the modeling of chondroitin and heparan sulfate proteoglycans appeared to be modulated. Our results highlight that, the expression of glycosylation-related genes is regulated during bioaminergic neuronal differentiation, consistent with a participation of glycoconjugates in neuronal development and plasticity. A neuronal regulation of glycosylation processes may have direct implications on some neurospecific functions of PrP(C) and may participate in specific brain targeting of prion strains.

A real-time polymerase chain reaction assay to detect single nucleotide polymorphisms at codon 171 in the prion gene for the genotyping of scrapie susceptibility in sheep

The objective of this study was to report a reliable real-time polymerase chain reaction assay compatible with the Roche LightCycler 2.0 capable of genotyping sheep for scrapie susceptibility at codon 171. The single nucleotide polymorphisms (SNPs) in the prion protein gene in sheep that may govern resistance to scrapie at codon 171 encode for lysine (K), histidine (H), glutamine (Q), and arginine (R). A modified proteinase K method for leukocytes or whole blood was used to isolate genomic DNA from sheep blood. Fluoresentric developed and optimized primers and probes for the codon 171 SNP assay. The assay was initially validated using 218 determinations from whole blood of known genotypes with 100% correct identity. The assay was further validated through a whole-blood check test provided annually by the National Veterinary Services Laboratory with a correct identification rate of 100%. From January 2005 to December 2006, 3,672 samples from blood were genotyped at codon 171. The genotypes were QR(171) (n = 1,838, 50.05%), RR(171) (n = 1,423, 38.75%), QQ(171) (n = 407, 11.08%), HR(171) (n = 2, 0.05%), and HQ(171) (n = 2, 0.05%). The combination of this simple extraction method and the novel Fluoresentric assay is very accurate, is capable of identifying all 4 SNPs at codon 171 in one reaction, and has proven to be a useful tool for producers in their selective breeding programs.

Discrimination of sheep susceptible and resistant to transmissible spongiform encephalopathies by an haplotype specific monoclonal antibody

In the present report, the selective detection of sheep PrP haplotypes by monoclonal antibody 2A11 is described. It is showed that the substitution of glutamine by arginine but not by histidine at ovine PrP position 171 abolishes completely the recognition of either PrP(c) or PrP(d) by mAb 2A11, in such a way that the application of this antibody allows the unambiguous discrimination of R(171) homozygotes. On the basis of the high resistance to classical scrapie and bovine spongiform encephalophaty (BSE) infection associated to the R(171) PrP haplotype, animals bearing the ARR allele are currently selected within the scrapie national plan initiated in Great Britain. A 2A11-based immuno enzymatic test have been developed and evaluated using a panel of plasma and sera from sheep of different PrP genotypes and breeds. The test allows the efficient discrimination of R(171) homozygotes, R(171) heterozygotes and non-R(171) carriers, therefore offering a rapid, cheap and easy to use alternative method to select sheep for their resistance to scrapie.

Glycan chains modulate prion protein binding to immobilized metal ions

PrP(c) is the normal isoform of the prion protein which can be converted into PrP(Sc), the pathology-associated conformer in prion diseases. It contains two N-linked glycan chains attached to the C-proximal globular domain. While the biological functions of PrP(c) are still unknown, its ability to bind Cu(2+) is well documented. The main Cu(2+)-binding sites are located in the N-proximal, unstructured region of the molecule. Here we report that PrP(c) glycans influence the capacity of PrP(c) from sheep brain or cultured Rov cells to bind IMAC columns loaded with Cu(2+) or Co(2+). Using different anti-PrP antibodies and PrP(c) glycosylation mutants, we show that the full length non-glycosylated form of PrP(c) has a higher binding efficiency for column-bound Cu(2+) and Co(2+) than the corresponding glycosylated form. Our findings raise the possibility that the accessibility of the PrP(c) metal ion-binding sites might be controlled by the glycan chains.

Prion proteins from susceptible and resistant sheep exhibit some distinct cell biological features

It is well established that natural polymorphisms in the coding sequence of the PrP protein can control the expression of prion disease. Studies with a cell model of sheep prion infection have shown that ovine PrP allele associated with resistance to sheep scrapie may confer resistance by impairing the multiplication of the infectious agent. To further explore the biochemical and cellular mechanisms underlying the genetic control of scrapie susceptibility, we established permissive cells expressing two different PrP variants. In this study, we show that PrP variants with opposite effects on prion multiplication exhibit distinct cell biological features. These findings indicate that cell biological properties of ovine PrP can vary with natural polymorphisms and raise the possibility that differential interactions of PrP variants with the cellular machinery may contribute to permissiveness or resistance to prion multiplication.

Prion protein glycosylation

The transmissible spongiform encephalopathies (TSE), or prion diseases are a group of transmissible neurodegenerative disorders of humans and animals. Although the infectious agent (the 'prion') has not yet been formally defined at the molecular level, much evidence exists to suggest that the major or sole component is an abnormal isoform of the host encoded prion protein (PrP). Different strains or isolates of the infectious agent exist, which exhibit characteristic disease phenotypes when transmitted to susceptible animals. In the absence of a nucleic acid genome it has been hard to accommodate the existence of TSE strains within the protein-only model of prion replication. Recent work examining the conformation and glycosylation patterns of disease-associated PrP has shown that these post-translational modifications show strain-specific properties and contribute to the molecular basis of TSE strain variation. This article will review the role of glycosylation in the susceptibility of cellular PrP to conversion to the disease-associated conformation and the role of glycosylation as a marker of TSE strain type.

Analysis of mammalian scrapie protein by novel monoclonal antibodies recognizing distinct prion protein glycoforms: an immunoblot and immunohistochemical study at the light and electron microscopic levels

The availability of specific monoclonal antibodies (mAbs) recognizing the aberrant form (PrP(Sc)) of the cellular prion protein (PrP(C)) in different mammalian species is important for molecular diagnostics, PrP(Sc) typing and future immunotherapy. We obtained a panel of anti-PrP monoclonal antibodies in PrP(0/0) knock-out mice immunized with recombinant human PrP(23-231). Two mAbs, recognizing PrP epitopes in the alpha-helix 1 (mAb SA65) and alpha-helix 2 (mAb SA21) regions, immunoreacted with PrP(C) and PrP(Sc) and its proteolytic product, PrP27-30, from human, murine, bovine, caprine and ovine brains by Western blot. Remarkably, mAb SA21 recognized unglycosylated and monoglycosylated PrP with the second site occupied by glycan moieties, but not monoglycosylated PrP with the first consensus site occupied or highly glycosylated species. Immunoblots with mAb SA21 disclosed that PrP glycosylated at the second site accounted for the slower migrating form of the customary monoglycosylated PrP doublet. mAb SA65 immunolabelled all PrP glycoforms by Western blot and was highly efficient in detecting tissue PrP by immunohistochemistry in light microscopy and in immunoelectron microscopy. These novel anti-PrP mAbs provide tools to investigate the subcellular site of PrP deposition in mammalian prion diseases and may also contribute to assess the role of different PrP glycoforms in human and animal prion diseases.

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.