Conformational variation between allelic variants of cell-surface ovine prion protein

The cellular and pathologic prion protein

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

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.

Effects of polymorphisms in ovine and caprine prion protein alleles on cell-free conversion

In sheep polymorphisms of the prion gene (PRNP) at the codons 136, 154 and 171 strongly influence the susceptibility to scrapie and bovine spongiform encephalopathy (BSE) infections. In goats a number of other gene polymorphisms were found which are suspected to trigger similar effects. However, no strong correlation between polymorphisms and TSE susceptibility in goats has yet been obtained from epidemiological studies and only a low number of experimental challenge data are available at present. We have therefore studied the potential impact of these polymorphisms in vitro by cell-free conversion assays using mouse scrapie strain Me7. Mouse scrapie brain derived PrP^Sc served as seeds and eleven recombinant single mutation variants of sheep and goat PrP^C as conversion targets. With this approach it was possible to assign reduced conversion efficiencies to specific polymorphisms, which are associated to low frequency in scrapie-affected goats or found only in healthy animals. Moreover, we could demonstrate a dominant-negative inhibition of prion polymorphisms associated with high susceptibility by alleles linked to low susceptibility in vitro.

Perturbation of T-cell development by insertional mutation of a PrP transgene

The normal cellular form of the prion protein PrP(C) is a glycosylphosphatidylinositol-linked cell-surface glycoprotein expressed primarily by cells of the nervous and immune systems. There is evidence to suggest that PrP(C) is involved in cell signalling and cellular homeostasis. We have investigated the immune composition of peripheral lymphoid tissue in PrP-/-, wild-type, tg19 and tga20 strains of mice, which express 0, 1-, 3-5- and 4-7-fold higher levels of PrP(C), respectively, relative to wild-type mice. Our data show that tga20 mice have a reduced number of spleen T-cell receptor (TCR)-alphabeta(+) T cells and an increased number of TCR-gammadelta(+) T cells compared with wild-type mice. This was not seen in tg19 mice, which also express elevated levels of PrP(C). In addition, we have found that the Prnp transgene in the tga20 genome is located centrally on chromosome 17, in or around genes involved in T-cell development. Significantly, mRNA transcripts from pre-TCR-alpha (pTalpha), a T-cell development gene located on mouse chromosome 17, are drastically reduced in tga20 mice, indicative of a perturbation in pTalpha gene regulation. We propose that the immune cell phenotype of tga20 mice may be caused by the insertional mutation of the Prnp transgene into the pTalpha gene or its regulatory elements.

Molecular and transmission characteristics of primary-passaged ovine scrapie isolates in conventional and ovine PrP transgenic mice

A more complete assessment of ovine prion strain diversity will be achieved by complementing biological strain typing in conventional and ovine PrP transgenic mice with a biochemical analysis of the resultant PrPSc. This will provide a correlation between ovine prion strain phenotype and the molecular nature of different PrP conformers associated with particular prion strains. Here, we have compared the molecular and transmission characteristics of ovine ARQ/ARQ and VRQ/VRQ scrapie isolates following primary passage in tg338 (VRQ) and tg59 (ARQ) ovine PrP transgenic mice and the conventional mouse lines C57BL/6 (Prnp(a)), RIII (Prnp(a)), and VM (Prnp(b)). Our data show that these different genotypes of scrapie isolates display similar incubation periods of >350 days in conventional and tg59 mice. Facilitated transmission of sheep scrapie isolates occurred in tg338 mice, with incubation times reduced to 64 days for VRQ/VRQ inocula and to </=210 days for ARQ/ARQ samples. Distinct genotype-specific lesion profiles were seen in the brains of conventional and tg59 mice with prion disease, which was accompanied by the accumulation of more conformationally stable PrPSc, following inoculation with ARQ/ARQ compared to VRQ/VRQ scrapie isolates. In contrast, the lesion profiles, quantities, and stability of PrPSc induced by the same inocula in tg338 mice were more similar than in the other mouse lines. Our data show that primary transmission of different genotypes of ovine prions is associated with the formation of different conformers of PrPSc with distinct molecular properties and provide the basis of a molecular approach to identify the true diversity of ovine prion strains.

Polymorphism of prion protein gene in Arctic fox (Vulpes lagopus)

Prion diseases are fatal neurodegenerative disorders of humans and certain other mammals. Prion protein gene (Prnp) is associated with susceptibility and species barrier to prion diseases. No natural and experimental prion diseases have been documented to date in Arctic fox. In the present study, coding region of Prnp from 135 Arctic foxes were cloned and screened for polymorphisms. Our results indicated that the Arctic fox Prnp open reading frame (ORF) contains 771 nucleotides encoding 257 amino acids. Four single nucleotide polymorphisms (SNPs) (G312C, A337G, C541T, and A723G) were identified. SNPs G312C and A723G produced silent mutations, but SNPs A337G and C541T resulted in a M-V change at codon 113 and R-C at codon 181, respectively. The Arctic fox Prnp amino acid sequence was similar to that of the dog (XM 542906). In short, this study provides preliminary information about genotypes of Prnp in Arctic fox.

Mouse-adapted ovine scrapie prion strains are characterized by different conformers of PrPSc

The agent responsible for prion disease may exist in different forms, commonly referred to as strains, with each carrying the specific information that determines its own distinct biological properties, such as incubation period and lesion profile. Biological strain typing of ovine scrapie isolates by serial passage in conventional mice has shown some diversity in ovine prion strains. However, this biological diversity remains poorly supported by biochemical prion strain typing. The protein-only hypothesis predicts that variation between different prion strains in the same host is manifest in different conformations adopted by PrPSc. Here we have investigated the molecular properties of PrPSc associated with two principal Prnp(a) mouse-adapted ovine scrapie strains, namely, RML and ME7, in order to establish biochemical prion strain typing strategies that may subsequently be used to discriminate field cases of mouse-passaged ovine scrapie isolates. We used a conformation-dependent immunoassay and a conformational stability assay, together with Western blot analysis, to demonstrate that RML and ME7 PrPSc proteins show distinct biochemical and physicochemical properties. Although RML and ME7 PrPSc proteins showed similar resistance to proteolytic digestion, they differed in their glycoform profiles and levels of proteinase K (PK)-sensitive and PK-resistant isoforms. In addition, the PK-resistant core (PrP27-30) of ME7 was conformationally more stable following exposure to guanidine hydrochloride or Sarkosyl than was RML PrP27-30. Our data show that mouse-adapted ovine scrapie strains can be discriminated by their distinct conformers of PrPSc, which provides a basis to investigate their diversity at the molecular level.

Immunological differences between susceptible and resistant sheep during the preclinical phase of scrapie infection

In order to investigate the relationship between the immune response to scrapie infection and genetic susceptibility to the disease in sheep, immune cell subsets and prion protein (PrP) expression were determined in susceptible and resistant Suffolk sheep in the preclinical phase of infection. At 6 months of age, 12 ARQ/ARQ (susceptible) and nine ARR/ARR (resistant) scrapie-free Suffolk lambs were challenged subcutaneously with scrapie inoculum. Prefemoral lymphadenectomies were carried out at 14 and 180 days post-inoculation (p.i.) and serial bleeds were collected at monthly intervals for up to 1 year p.i. An indirect double-labelling procedure was carried out on peripheral blood mononuclear cells (PBMCs) and lymph node cell preparations and analysed using flow cytometry. Prior to scrapie challenge, significantly more PrP+cells were detected in PBMCs from the susceptible sheep. Furthermore, following challenge, significantly more CD8+andγΔ+T cells were detected in the PBMCs of the resistant sheep. However, at both 14 and 180 days p.i, CD21+cell expression was significantly higher in the lymph node preparations of the susceptible sheep. In contrast, more CD4+cells were detected in the lymph nodes of the resistant sheep at both time points. It was concluded that significant differences in immune cell subsets and PrP expression occur between ARQ/ARQ and ARR/ARR Suffolk sheep in the preclinical phase of infection.

Three-colour flow cytometric detection of PrP in ovine leukocytes

PrP(c) (cellular prion protein, CD230) expression by subpopulations of lymphoid cells has been widely investigated in a variety of species, possibly because of the possible link between transmissible spongiform encephalopathies (TSE) transmission and blood transfusion. However, the role of the immune cells in the transmission of the disease is still unclear. Here we describe the optimisation and standardisation of a three-colour staining procedure to detect PrP in association with phenotypic and activation markers in ovine immune cells. We demonstrate a reproducible, flexible and sensitive method and that the combination of isotype-specific antibodies and Fab fragments is feasible. To our knowledge, this is the first report of such labelling of ovine cells. Using this method, we were able to detect differences in levels of PrP expression between blood and lymph node cells of the same animal, and considerable variability between animals. Moreover, we were able to explore possible associations between PrP expression and cellular activation and to identify cell subsets with different labelling patterns. We are currently employing this approach to evaluate variations in immunological parameters during experimental infection in sheep.

Proteinase K-sensitive disease-associated ovine prion protein revealed by conformation-dependent immunoassay

PrPSc [abnormal disease-specific conformation of PrP (prion-related protein)] accumulates in prion-affected individuals in the form of amorphous aggregates. Limited proteolysis of PrPSc results in a protease-resistant core of PrPSc of molecular mass of 27-30 kDa (PrP27-30). Aggregated forms of PrP co-purify with prion infectivity, although infectivity does not always correlate with the presence of PrP27-30. This suggests that discrimination between PrPC (normal cellular PrP) and PrPSc by proteolysis may underestimate the repertoire and quantity of PrPSc subtypes. We have developed a CDI (conformation-dependent immunoassay) utilizing time-resolved fluorescence to study the conformers of disease-associated PrP in natural cases of sheep scrapie, without using PK (proteinase K) treatment to discriminate between PrPC and PrPSc. The capture-detector CDI utilizes N-terminal- and C-terminal-specific anti-PrP monoclonal antibodies that recognize regions of the prion protein differentially buried or exposed depending on the extent of denaturation of the molecule. PrPSc was precipitated from scrapie-infected brain stem and cerebellum tissue following sarkosyl extraction, with or without the use of sodium phosphotungstic acid, and native and denatured PrPSc detected by CDI. PrPSc was detectable in brain tissue from homozygous VRQ (V136 R154 Q171) and ARQ (A136 R154 Q171) scrapie-infected sheep brains. The highest levels of PrPSc were found in homozygous VRQ scrapie-infected brains. The quantity of PrPSc was significantly reduced, up to 90% in some cases, when samples were treated with PK prior to the CDI. Collectively, our results show that the level of PrPSc in brain samples from cases of natural scrapie display genotypic differences and that a significant amount of this material is PK-sensitive.

Ovine plasma prion protein levels show genotypic variation detected by C-terminal epitopes not exposed in cell-surface PrPC

Ovine PBMCs (peripheral blood mononuclear cells) express PrP(C) [cellular PrP (prion-related protein)] and have the potential to harbour and release disease-associated forms of PrP during scrapie in sheep. Cell-surface PrP(C) expression by PBMCs, together with plasma PrP(C) levels, may contribute to the regulatory mechanisms that determine susceptibility and resistance to natural scrapie in sheep. Here, we have correlated cell-surface PrP(C) expression on normal ovine PBMCs by FACS with the presence of PrP(C) in plasma measured by capture-detector immunoassay. FACS showed similar levels of cell-surface PrP(C) on homozygous ARR (Ala136-Arg154-Arg171), ARQ (Ala136-Arg154-Gln171) and VRQ (Val136-Arg154-Gln171) PBMCs. Cell-surface ovine PrP(C) showed modulation of N-terminal epitopes, which was more evident on homozygous ARR cells. Ovine plasma PrP(C) levels showed genotypic variation and the protein displayed C-terminal epitopes not available in cell-surface PrP(C). Homozygous VRQ sheep showed the highest plasma PrP(C) level and homozygous ARR animals the lowest. For comparison, similar analyses were performed on normal bovine PBMCs and plasma. PrP(C) levels in bovine plasma were approx. 4-fold higher than ovine homozygous ARQ plasma despite similar levels of PBMC cell-surface PrP(C) expression. Immunoassays using C-terminal-specific anti-PrP monoclonal antibodies as capture and detector reagents revealed the highest level of PrP(C) in both ovine and bovine plasma, whilst lower levels were detected using N-terminal-specific monoclonal antibody FH11 as the capture reagent. This suggested that a proportion of plasma PrP(C) was N-terminally truncated. Our results indicate that the increased susceptibility to natural scrapie displayed by homozygous VRQ sheep correlates with a higher level of plasma PrP(C).

Structural differences between allelic variants of the ovine prion protein revealed by molecular dynamics simulations

We have modeled ovine prion protein (residues 119-233) based on NMR structures of PrP from other mammalian species. Modeling of the C-terminal domain of ovine PrP predicts three helices: helix-1 (residues 147-155), flanked by two short beta-strands; helix-2 (residues 176-197), and helix-3 (residues 203-229). Molecular dynamics simulations on this model of ovine PrP have determined structural differences between allelic variants. At neutral pH, limited root mean-squared (RMS) fluctuations were seen in the region of helix-1; between beta-strand-2 and residue 171, and the loop connecting helix-2 and helix-3. At low pH, these RMS fluctuations increased and showed allelic variation. The extent of RMS fluctuation between beta-strand 2 and residue 171 was ARR > ARQ > VRQ. This order was reversed for the loop region connecting helix-2 and helix-3. Although all three variants have the potential to display an extended helix at the C-terminal region of helix-1, the major influence of the VRQ allele was to restrict the conformations of the Asn162 and Arg139 side-chains. Variations observed in the simulations in the vicinity of helix-1 correlated with reactivity of C-terminal specific anti-PrP monoclonal antibodies with peripheral blood cells from scrapie-susceptible and -resistant genotypes of sheep: cells from VRQ homozygous sheep showed uniform reactivity, while cells from ARQ and ARR homozygous sheep showed variable binding. Our data show that molecular dynamics simulations can be used to determine structural differences between allelic variants of ovine PrP. The binding of anti-PrP monoclonal antibodies to ovine blood cells may validate these structural predictions.

Modification of blood cell PrP epitope exposure during prion disease

PrPC [normal cellular PrP (prion-related protein)] is a glycosylphosphatidylinositol-linked cell-surface glycoprotein that is expressed primarily by cells of the central and peripheral nervous system and the lymphoreticular system. During prion disease, PrPC undergoes structural modification to PrPSc (abnormal disease-specific conformation of PrP). The appearance of prion infectivity and PrPSc within different peripheral lymphoid tissue sites during natural scrapie infection in sheep is suggestive of haematogenic dissemination. For this to occur, blood cells may harbour or carry disease-associated PrP and in doing so present altered conformations of PrP on their cell-surface. In the present study, we show that changes in PrP epitope expression, or accessibility, can be detected on peripheral blood mononuclear cells during the course of experimental scrapie in susceptible sheep. Peripheral blood mononuclear cells isolated from VRQ homozygous lambs inoculated orally with scrapie were probed with either N- or C-terminal-specific anti-PrP monoclonal antibodies and analysed by flow cytometry. During the progression of scrapie, significant alterations were seen in the exposure of particular cell-surface PrP epitopes. These modifications included increased accessibility to N-terminal regions of the PrP molecule, to the region between beta-strand-2 and residue 171, and to the C-terminal region of helix-3. Increased accessibility in the globular C-terminal domain of PrP occurred in the vicinity of tyrosine dimers, which are believed to have increased solvent exposure in disease-associated PrP. We suggest that the alterations in anti-PrP monoclonal antibody recognition of cell-surface PrP on blood cells from scrapie-infected sheep are indicative of structural changes within this molecule that may be relevant to prion disease.

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.

Expression of PrPC on cellular components of sheep blood

PrPC, a glycosylphosphatidylinositol-linked glycoprotein, plays a central role in the pathogenesis of transmissible spongiform encephalopathies (TSEs), undergoing a conformational alteration to the disease-associated isoform, commonly designated PrPSc. PrPC is expressed in many tissues other than the nervous system, although its precise function(s) remains unclear. It has previously been demonstrated that TSEs can be transmitted by blood transfusion in sheep. The aim of this work was to identify which components of blood carried the infection. As an initial step, the distribution of PrPC on cellular components of sheep blood was examined to identify potential targets for infection. Cell-surface expression of PrPC was found only on peripheral blood mononuclear cells (PBMCs); however, platelets also contained significant amounts of intracellular PrPC. The level of PrPC expressed on the cell surface of PBMCs was influenced by PrP genotype, with the highest levels found in scrapie-susceptible VRQ/VRQ sheep and the lowest levels in scrapie-resistant ARR/ARR sheep. In susceptible sheep, PrPC was expressed at varying levels on all major subsets of PBMCs, with the highest levels on the CD21+ subset of B cells, and PrP expression was upregulated dramatically on CD21+ B cells in some scrapie-infected sheep.