Antibody to DNA detects scrapie but not normal prion protein

Seeding Activity of Skin Misfolded Tau as a Biomarker for Tauopathies

Background

Tauopathies are a group of age-related neurodegenerative diseases characterized by the accumulation of pathologically phosphorylated tau protein in the brain, leading to prion-like propagation and aggregation. They include Alzheimer's disease (AD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Pick's disease (PiD). Currently, reliable diagnostic biomarkers that directly reflect the capability of propagation and spreading of misfolded tau aggregates in peripheral tissues and body fluids are lacking.

Methods

We utilized the seed-amplification assay (SAA) employing ultrasensitive real-time quaking-induced conversion (RT-QuIC) to assess the prion-like seeding activity of pathological tau in the skin of cadavers with neuropathologically confirmed tauopathies, including AD, PSP, CBD, and PiD, compared to normal controls.

Results

We found that the skin prion-SAA demonstrated a significantly higher sensitivity (75-80%) and specificity (95-100%) for detecting tauopathy, depending on the tau substrates used. Moreover, increased tau-seeding activity was also observed in biopsy skin samples from living AD and PSP patients examined. Analysis of the end products of skin-tau SAA confirmed that the increased seeding activity was accompanied by the formation of tau aggregates with different physicochemical properties related to two different tau substrates used.

Conclusions

Overall, our study provides proof-of-concept that the skin tau-SAA can differentiate tauopathies from normal controls, suggesting that the seeding activity of misfolded tau in the skin could serve as a diagnostic biomarker for tauopathies.

Phase separation of the mammalian prion protein: physiological and pathological perspectives

Abnormal phase transitions have been implicated in the occurrence of proteinopathies. Disordered proteins with nucleic acid binding ability drive the formation of reversible micron-sized condensates capable of controlling nucleic acid processing/transport. This mechanism, achieved via liquid-liquid phase separation (LLPS), underlies the formation of long-studied membraneless organelles (e.g., nucleolus) and various transient condensates formed by driver proteins. The prion protein (PrP) is not a classical nucleic acid-binding protein. However, it binds nucleic acids with high affinity, undergoes nucleocytoplasmic shuttling, contains a long intrinsically disordered region rich in glycines and evenly spaced aromatic residues, among other biochemical/biophysical properties of bona fide drivers of phase transitions. Because of this, our group and others have characterized LLPS of recombinant PrP. In vitro phase separation of PrP is modulated by nucleic acid aptamers, and, depending on the aptamer conformation, the liquid droplets evolve to solid-like species. Herein we discuss recent studies and previous evidence supporting PrP phase transitions. We focus on the central role of LLPS related to PrP physiology and pathology, with a special emphasis on the interaction of PrP with different ligands, such as proteins and nucleic acids, which can play a role in prion disease pathogenesis. Finally, we comment on therapeutic strategies directed at the nonfunctional phase separation that could potentially tackle prion diseases or other protein misfolding disorders.

Intrinsic disorder and phase transitions: Pieces in the puzzling role of the prion protein in health and disease

After four decades of prion protein research, the pressing questions in the literature remain similar to the common existential dilemmas. Who am I? Some structural characteristics of the cellular prion protein (PrP^C) and scrapie PrP (PrP^Sc) remain unknown: there are no high-resolution atomic structures for either full-length endogenous human PrP^C or isolated infectious PrP^Sc particles. Why am I here? It is not known why PrP^C and PrP^Sc are found in specific cellular compartments such as the nucleus; while the physiological functions of PrP^C are still being uncovered, the misfolding site remains obscure. Where am I going? The subcellular distribution of PrP^C and PrP^Sc is wide (reported in 10 different locations in the cell). This complexity is further exacerbated by the eight different PrP fragments yielded from conserved proteolytic cleavages and by reversible post-translational modifications, such as glycosylation, phosphorylation, and ubiquitination. Moreover, about 55 pathological mutations and 16 polymorphisms on the PrP gene (PRNP) have been described. Prion diseases also share unique, challenging features: strain phenomenon (associated with the heterogeneity of PrP^Sc conformations) and the possible transmissibility between species, factors which contribute to PrP undruggability. However, two recent concepts in biochemistry-intrinsically disordered proteins and phase transitions-may shed light on the molecular basis of PrP's role in physiology and disease.

Decrease in Skin Prion-Seeding Activity of Prion-Infected Mice Treated with a Compound Against Human and Animal Prions: a First Possible Biomarker for Prion Therapeutics

Previous studies have revealed that the infectious scrapie isoform of prion protein (PrP^Sc) harbored in the skin tissue of patients or animals with prion diseases can be amplified and detected through the serial protein misfolding cyclic amplification (sPMCA) or real-time quaking-induced conversion (RT-QuIC) assays. These findings suggest that skin PrP^Sc-seeding activity may serve as a biomarker for the diagnosis of prion diseases; however, its utility as a biomarker for prion therapeutics remains largely unknown. Cellulose ethers (CEs, such as TC-5RW), widely used as food and pharmaceutical additives, have recently been shown to prolong the lifespan of prion-infected mice and hamsters. Here we report that in transgenic (Tg) mice expressing hamster cellular prion protein (PrP^C) infected with the 263K prion, the prion-seeding activity becomes undetectable in the skin tissues of TC-5RW-treated Tg mice by both sPMCA and RT-QuIC assays, whereas such prion-seeding activity is readily detectable in the skin of untreated mice. Notably, TC-5RW exhibits an inhibitory effect on the in vitro amplification of PrP^Sc in both skin and brain tissues by sPMCA and RT-QuIC. Moreover, we reveal that TC-5RW is able to directly decrease protease-resistant PrP^Sc and inhibit the seeding activity of PrP^Sc from chronic wasting disease and various human prion diseases. Our results suggest that the level of prion-seeding activity in the skin may serve as a useful biomarker for assessing the therapeutic efficacy of compounds in a clinical trial of prion diseases and that TC-5RW may have the potential for the prevention/treatment of human prion diseases.

A novel mechanism of phenotypic heterogeneity in Creutzfeldt-Jakob disease

One of remarkable features of sporadic Creutzfeldt-Jakob disease (sCJD) is the great phenotypic variability. Understanding the molecular basis of this variability has important implications for the development of therapeutic approaches. It is well established that, in many cases, phenotypic heterogeneity of sCJD is under control of two determinants: the genotype at the methionine (M)/valine (V) polymorphic codon 129 of the human prion protein gene and the type, 1 or 2, of the pathogenic and disease-related form of the prion protein, PrP^D. However, this scenario fails to explain the existence of distinct heterozygous sCJDMV2 subtypes, where heterogeneity occurs without any variation of the 129 allotype and PrP^D type. One of these subtypes, denoted sCJDMV2C, associated with PrP^D type 2, is characterized by widespread spongiform degeneration of the cerebral cortex (C). The second variant, denoted sCJDMV2K, features prominent deposition of PrP^D amyloid forming kuru type (K) plaques. Here we used a mass spectrometry based approach to test the hypothesis that phenotypic variability within the sCJDMV2 subtype is at least partly determined by the abundance of 129 M and 129 V polymorphic forms of proteinase K-resistant PrP^D (resPrP^D). Consistent with this hypothesis, our data demonstrated a strong correlation of the MV2C and MV2K phenotypes with the relative populations of protease-resistant forms of the pathogenic prion proteins, resPrP^D-129 M and resPrP^D-129 V, where resPrP^D-129 M dominated in the sCJDMV2C variant and resPrP^D-129 V in the sCJDMV2K variant. This finding suggests an important, previously unrecognized mechanism for phenotypic determination in human prion diseases.

Prion protein PrP nucleic acid binding and mobilization implicates retroelements as the replicative component of transmissible spongiform encephalopathy

The existence of more than 30 strains of transmissible spongiform encephalopathy (TSE) and the paucity of infectivity of purified PrP^Sc, as well as considerations of PrP structure, are inconsistent with the protein-only (prion) theory of TSE. Nucleic acid is a strong contender as a second component. We juxtapose two key findings: (i) PrP is a nucleic-acid-binding antimicrobial protein that is similar to retroviral Gag proteins in its ability to trigger reverse transcription. (ii) Retroelement mobilization is widely seen in TSE disease. Given further evidence that PrP also mediates nucleic acid transport into and out of the cell, a strong case is to be made that a second element – retroelement nucleic acid – bound to PrP constitutes the second component necessary to explain the multiple strains of TSE.

PMCA-replicated PrPD in urine of vCJD patients maintains infectivity and strain characteristics of brain PrPD: Transmission study

The presence of abnormal, disease-related prion protein (PrP^D) has recently been demonstrated by protein misfolding cyclic amplification (PMCA) in urine of patients affected with variant Creutzfeldt-Jakob disease (vCJD), a prion disease typically acquired from consumption of prion contaminated bovine meat. The complexity and multistage process of urine excretion along with the obligatory use of PMCA raise the issue of whether strain characteristics of the PrP^D present in vCJD brains, such as infectivity and phenotype determination, are maintained in urine excreted PrP^D and following amplification by PMCA. We inoculated transgenic mice expressing normal human PrP with amplified urine and brain homogenate achieving the same 100% attack rate, similar incubation periods (in both cases extremely long) and histopathological features as for type and severity of the lesions. Furthermore, PrP^D characteristics analyzed by immunoblot and conformational stability immunoassay were indistinguishable. Inoculation of raw vCJD urine caused no disease, confirming the extremely low concentration of PrP^D in vCJD urine. These findings show that strain characteristics of vCJD brain PrP^D, including infectivity, are preserved in PrP^D present in urine and are faithfully amplified by means of PMCA; moreover, they suggest that the PrP^D urine test might allow for the diagnosis and identification of disease subtype also in sporadic CJD.

Co-occurrence of chronic traumatic encephalopathy and prion disease

Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease associated with repetitive traumatic brain injury (TBI). CTE is generally found in athletes participating in contact sports and military personnel exposed to explosive blasts but can also affect civilians. Clinically and pathologically, CTE overlaps with post-traumatic stress disorder (PTSD), a term mostly used in a clinical context. The histopathology of CTE is defined by the deposition of hyperphosphorylated tau protein in neurons and astrocytes preferentially with perivascular distribution and at the depths of the cortical sulci. In addition to hyperphosphorylated tau, other pathologic proteins are deposited in CTE, including amyloid β (Aβ), transactive response (TAR) DNA-binding protein 43 kDa (TDP-43) and α-synuclein. However, the coexistence of prion disease in CTE has not been observed. We report three cases of histopathologically validated CTE with co-existing sporadic prion disease. Two were identified in a cohort of 55 pathologically verified cases of CTE submitted to the CTE Center of Boston University. One was identified among brain tissues submitted to the National Prion Disease Pathology Surveillance Center of Case Western Reserve University. The histopathological phenotype and properties of the abnormal, disease-related prion protein (PrPD) of the three CTE cases were examined using lesion profile, immunohistochemistry, electrophoresis and conformational tests. Subjects with sporadic Creutzfeldt-Jakob disease (sCJD) matched for age, PrP genotype and PrPD type were used as controls. The histopathology phenotype and PrPD properties of the three CTE subjects showed no significant differences from their respective sCJD controls suggesting that recurring neurotrauma or coexisting CTE pathology did not detectably impact the prion disease phenotype and PrPD conformational characteristics. Based on the reported incidence of sporadic prion disease, the detection of two cases with sCJD in the CTE Center series of 55 CTE cases by chance alone would be highly unlikely (p = 8.93*10- 6). Nevertheless, examination of a larger cohort of CTE is required to conclusively determine whether the risk of CJD is significantly increased in patients with CTE.

Prion Protein PRNP: A New Player in Innate Immunity? The Aβ Connection

The prion protein PRNP has been centrally implicated in the transmissible spongiform encephalopathies (TSEs), but its normal physiological role remains obscure. We highlight emerging evidence that PRNP displays antimicrobial activity, inhibiting the replication of multiple viruses, and also interacts directly with Alzheimer's disease (AD) amyloid-β (Aβ) peptide whose own antimicrobial role is now increasingly secure. PRNP and Aβ share share membrane-penetrating, nucleic acid binding, and antiviral properties with classical antimicrobial peptides such as LL-37. We discuss findings that binding of abnormal nucleic acids to PRNP leads to oligomerization of the protein, and suggest that this may be an entrapment and sequestration process that contributes to its antimicrobial activity. Some antimicrobial peptides are known to be exploited by infectious agents, and we cover evidence that PRNP is usurped by herpes simplex virus (HSV-1) that has evolved a virus-encoded 'anti-PRNP'.unction. These findings suggest that PRNP, like LL-37 and Aβ, is likely to be a component of the innate immune system, with implications for the pathoetiology of both AD and TSE.

Quiescin-sulfhydryl oxidase inhibits prion formation in vitro

Prions are infectious proteins that cause a group of fatal transmissible diseases in animals and humans. The scrapie isoform (PrP^Sc) of the cellular prion protein (PrP^C) is the only known component of the prion. Several lines of evidence have suggested that the formation and molecular features of PrP^Sc are associated with an abnormal unfolding/refolding process. Quiescin-sulfhydryl oxidase (QSOX) plays a role in protein folding by introducing disulfides into unfolded reduced proteins. Here we report that QSOX inhibits human prion propagation in protein misfolding cyclic amplification reactions and murine prion propagation in scrapie-infected neuroblastoma cells. Moreover, QSOX preferentially binds PrP^Sc from prion-infected human or animal brains, but not PrP^C from uninfected brains. Surface plasmon resonance of the recombinant mouse PrP (moPrP) demonstrates that the affinity of QSOX for monomer is significantly lower than that for octamer (312 nM vs 1.7 nM). QSOX exhibits much lower affinity for N-terminally truncated moPrP (PrP89-230) than for the full-length moPrP (PrP23-231) (312 nM vs 2 nM), suggesting that the N-terminal region of PrP is critical for the interaction of PrP with QSOX. Our study indicates that QSOX may play a role in prion formation, which may open new therapeutic avenues for treating prion diseases.

Unraveling Prion Protein Interactions with Aptamers and Other PrP-Binding Nucleic Acids

Transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative disorders that affect humans and other mammals. The etiologic agents common to these diseases are misfolded conformations of the prion protein (PrP). The molecular mechanisms that trigger the structural conversion of the normal cellular PrP (PrP^C) into the pathogenic conformer (PrP^Sc) are still poorly understood. It is proposed that a molecular cofactor would act as a catalyst, lowering the activation energy of the conversion process, therefore favoring the transition of PrP^C to PrP^Sc. Several in vitro studies have described physical interactions between PrP and different classes of molecules, which might play a role in either PrP physiology or pathology. Among these molecules, nucleic acids (NAs) are highlighted as potential PrP molecular partners. In this context, the SELEX (Systematic Evolution of Ligands by Exponential Enrichment) methodology has proven extremely valuable to investigate PrP–NA interactions, due to its ability to select small nucleic acids, also termed aptamers, that bind PrP with high affinity and specificity. Aptamers are single-stranded DNA or RNA oligonucleotides that can be folded into a wide range of structures (from harpins to G-quadruplexes). They are selected from a nucleic acid pool containing a large number (10¹⁴–10¹⁶) of random sequences of the same size (~20–100 bases). Aptamers stand out because of their potential ability to bind with different affinities to distinct conformations of the same protein target. Therefore, the identification of high-affinity and selective PrP ligands may aid the development of new therapies and diagnostic tools for TSEs. This review will focus on the selection of aptamers targeted against either full-length or truncated forms of PrP, discussing the implications that result from interactions of PrP with NAs, and their potential advances in the studies of prions. We will also provide a critical evaluation, assuming the advantages and drawbacks of the SELEX (Systematic Evolution of Ligands by Exponential Enrichment) technique in the general field of amyloidogenic proteins.

Novel strain properties distinguishing sporadic prion diseases sharing prion protein genotype and prion type

In most human sporadic prion diseases the phenotype is consistently associated with specific pairings of the genotype at codon 129 of the prion protein gene and conformational properties of the scrapie PrP (PrP^Sc) grossly identified types 1 and 2. This association suggests that the 129 genotype favours the selection of a distinct strain that in turn determines the phenotype. However, this mechanism cannot play a role in the phenotype determination of sporadic fatal insomnia (sFI) and a subtype of sporadic Creutzfeldt-Jakob disease (sCJD) identified as sCJDMM2, which share 129 MM genotype and PrP^Sc type 2 but are associated with quite distinct phenotypes. Our detailed comparative study of the PrP^Sc conformers has revealed major differences between the two diseases, which preferentially involve the PrP^Sc component that is sensitive to digestion with proteases (senPrP^Sc) and to a lesser extent the resistant component (resPrP^Sc). We conclude that these variations are consistent with two distinct strains in sFI and sCJDMM2, and that the rarer sFI is the result of a variant strain selection pathway that might be favoured by a different brain site of initial PrP^Sc formation in the two diseases.

Prions: Beyond a Single Protein

Since the term protein was first coined in 1838 and protein was discovered to be the essential component of fibrin and albumin, all cellular proteins were presumed to play beneficial roles in plants and mammals. However, in 1967, Griffith proposed that proteins could be infectious pathogens and postulated their involvement in scrapie, a universally fatal transmissible spongiform encephalopathy in goats and sheep. Nevertheless, this novel hypothesis had not been evidenced until 1982, when Prusiner and coworkers purified infectious particles from scrapie-infected hamster brains and demonstrated that they consisted of a specific protein that he called a "prion." Unprecedentedly, the infectious prion pathogen is actually derived from its endogenous cellular form in the central nervous system. Unlike other infectious agents, such as bacteria, viruses, and fungi, prions do not contain genetic materials such as DNA or RNA. The unique traits and genetic information of prions are believed to be encoded within the conformational structure and posttranslational modifications of the proteins. Remarkably, prion-like behavior has been recently observed in other cellular proteins-not only in pathogenic roles but also serving physiological functions. The significance of these fascinating developments in prion biology is far beyond the scope of a single cellular protein and its related disease.

PrPSc-Specific Antibody Reveals C-Terminal Conformational Differences between Prion Strains

Understanding the structure of PrP(Sc) and its strain variation has been one of the major challenges in prion disease biology. To study the strain-dependent conformations of PrP(Sc), we purified proteinase-resistant PrP(Sc) (PrP(RES)) from mouse brains with three different murine-adapted scrapie strains (Chandler, 22L, and Me7) and systematically tested the accessibility of epitopes of a wide range of anti-PrP and anti-PrP(Sc) specific antibodies by indirect enzyme-linked immunosorbent assay (ELISA). We found that epitopes of most anti-PrP antibodies were hidden in the folded structure of PrP(RES), even though these epitopes are revealed with guanidine denaturation. However, reactivities to a PrP(Sc)-specific conformational C-terminal antibody showed significant differences among the three different prion strains. Our results provide evidence for strain-dependent conformational variation near the C termini of molecules within PrP(Sc) multimers.

IMPORTANCE

It has long been apparent that prion strains can have different conformations near the N terminus of the PrP(Sc) protease-resistant core. Here, we show that a C-terminal conformational PrP(Sc)-specific antibody reacts differently to three murine-adapted scrapie strains. These results suggest, in turn, that conformational differences in the C terminus of PrP(Sc) also contribute to the phenotypic distinction between prion strains.

Comparative Study of Prions in Iatrogenic and Sporadic Creutzfeldt-Jakob Disease

Differentiating iatrogenic Creutzfeldt-Jakob disease (iCJD) from sporadic CJD (sCJD) would be useful for the identification and prevention of human-to-human prion transmission. Currently, the diagnosis of iCJD depends on identification of a recognized source of contamination to which patients have been exposed, in addition to fulfilling basic requirements for the establishment of diagnosis of CJD. Attempts to identify differences in clinical manifestations, neuropathological changes and pathological prion protein (PrPSc) between iCJD and sCJD have been unsuccessful. In the present study, using a variety of more sophisticated methods including sucrose step gradient sedimentation, conformational stability immunoassay, protein misfolding cyclic amplification (PMCA), fragment-mapping, and transmission study, we show no significant differences in gel profiles, oligomeric state, conformational stability and infectivity of PrPSc between iCJD and sCJD. However, using PMCA, we find that convertibility and amplification efficiency of PrPSc is greater in iCJD than in sCJD in a polymorphism-dependent manner. Moreover, two protease-resistant PrP C-terminal fragments (termed PrP-CTF12/13) were detected in all 9 cases of sCJD but not in 6 of 8 cases of iCJD tested in this study. The use of fragment mapping- and PMCA-based assays thus provides a means to distinguish most cases of iCJD from sCJD.

Advances in screening test development for transmissible spongiform encephalopathies

The blood of patients with transmissible spongiform encephalopathy or prion disease can no longer be considered free of infectivity. There have been two recent reports of highly probable transfusion-associated iatrogenic variant Creutzfeldt-Jakob disease infections, and there is supporting experimental evidence of scrapie transmission by the transfusion of blood from sheep with naturally occurring disease. In the absence of a preclinical diagnostic test for transmissible spongiform encephalopathy, the main precautionary measures undertaken by blood agencies employ donor exclusion criteria, ensuring that the number of any further iatrogenic cases will be small. The development of a sensitive, specific and reliable diagnostic test is urgently needed for early identification of infected individuals in order to ensure the safety of blood supplies. During the past 5 years, significant progress has been made in improving the sensitivity and specificity of tests using brain and lymphoreticular tissues to identify Creutzfeldt-Jakob disease-infected individuals. However, the quest for a blood test is still in its infancy and requires extensive further research.

Recombinant human prion protein inhibits prion propagation in vitro

Prion diseases are associated with the conformational conversion of the cellular prion protein (PrP^C) into the pathological scrapie isoform (PrP^Sc) in the brain. Both the in vivo and in vitro conversion of PrP^C into PrP^Sc is significantly inhibited by differences in amino acid sequence between the two molecules. Using protein misfolding cyclic amplification (PMCA), we now report that the recombinant full-length human PrP (rHuPrP23-231) (that is unglycosylated and lacks the glycophosphatidylinositol anchor) is a strong inhibitor of human prion propagation. Furthermore, rHuPrP23-231 also inhibits mouse prion propagation in a scrapie-infected mouse cell line. Notably, it binds to PrP^Sc, but not PrP^C, suggesting that the inhibitory effect of recombinant PrP results from blocking the interaction of brain PrP^C with PrP^Sc. Our findings suggest a new avenue for treating prion diseases, in which a patient's own unglycosylated and anchorless PrP is used to inhibit PrP^Sc propagation without inducing immune response side effects.

Protease-sensitive prions with 144-bp insertion mutations

Insertion of 144-base pair (bp) containing six extra octapeptide repeats between residues 51 and 91 of prion protein (PrP) gene is associated with inherited prion diseases. Most cases linked to this insertion examined by Western blotting showed detectable proteinase K-resistant PrPSc (rPrPSc) resembling PrPSc type 1 and type 2 in sporadic Creutzfeldt-Jakob disease (sCJD), or PrP7-8 in Gerstmann-Sträussler-Scheinker disease. However, cases lacking detectable rPrPSc also have been reported. Which PrP conformer is associated with neuropathological changes in the cases without detectable rPrPSc remains to be determined. Here we report that while all six but one subjects with the 144-bp insertion mutations examined display the pathognomonic PrP patches in the cerebellum, one of them exhibits no detectable typical rPrPSc even in PrPSc-enriched preparations. Instead, a large amount of abnormal PrP is captured from this case by gene 5 protein and sodium phosphotungstate, reagents that have been proved to specifically capture abnormal PrP. All captured abnormal PrP from the cerebellum and other brain regions is virtually sensitive to PK-digestion (termed sPrPSc). The presence of the predominant sPrPSc but absence of rPrPSc in this 144-bp insertion-linked inherited CJD case suggests that mutant sPrPSc is the main component of the PrP deposit patches and sPrPSc is sufficient to cause neurotoxicity and prion disease.

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.

Isolation of soluble and insoluble PrP oligomers in the normal human brain

The central event in the pathogenesis of prion diseases involves a conversion of the host-encoded cellular prion protein PrP(C) into its pathogenic isoform PrP(Sc 1). PrP(C) is detergent-soluble and sensitive to proteinase K (PK)-digestion, whereas PrP(Sc) forms detergent-insoluble aggregates and is partially resistant to PK(2-6). The conversion of PrP(C) to PrP(Sc) is known to involve a conformational transition of α-helical to β-sheet structures of the protein. However, the in vivo pathway is still poorly understood. A tentative endogenous PrP(Sc), intermediate PrP* or "silent prion", has yet to be identified in the uninfected brain(7). Using a combination of biophysical and biochemical approaches, we identified insoluble PrP(C) aggregates (designated iPrP(C)) from uninfected mammalian brains and cultured neuronal cells(8, 9). Here, we describe detailed procedures of these methods, including ultracentrifugation in detergent buffer, sucrose step gradient sedimentation, size exclusion chromatography, iPrP enrichment by gene 5 protein (g5p) that specifically bind to structurally altered PrP forms(10), and PK-treatment. The combination of these approaches isolates not only insoluble PrP(Sc) and PrP(C) aggregates but also soluble PrP(C) oligomers from the normal human brain. Since the protocols described here have been used to isolate both PrP(Sc) from infected brains and iPrP(C) from uninfected brains, they provide us with an opportunity to compare differences in physicochemical features, neurotoxicity, and infectivity between the two isoforms. Such a study will greatly improve our understanding of the infectious proteinaceous pathogens. The physiology and pathophysiology of iPrP(C) are unclear at present. Notably, in a newly-identified human prion disease termed variably protease-sensitive prionopathy, we found a new PrP(Sc) that shares the immunoreactive behavior and fragmentation with iPrP(C 11, 12). Moreover, we recently demonstrated that iPrP(C) is the main species that interacts with amyloid-β protein in Alzheimer disease(13). In the same study, these methods were used to isolate Abeta aggregates and oligomers in Alzheimer's disease(13), suggesting their application to non-prion protein aggregates involved in other neurodegenerative disorders.

Sporadic human prion diseases: molecular insights and diagnosis

Human prion diseases can be sporadic, inherited, or acquired by infection. Distinct clinical and pathological characteristics separate sporadic diseases into three phenotypes: Creutzfeldt-Jakob disease (CJD), fatal insomnia, and variably protease-sensitive prionopathy. CJD accounts for more than 90% of all cases of sporadic prion disease; it is commonly categorised into five subtypes that can be distinguished according to leading clinical signs, histological lesions, and molecular traits of the pathogenic prion protein. Three subtypes affect prominently cognitive functions whereas the other two impair cerebellar motor activities. An accurate and timely diagnosis depends on careful clinical examination and early performance and interpretation of diagnostic tests, including electroencephalography, quantitative assessment of the surrogate markers 14-3-3, tau, and of the prion protein in the CSF, and neuroimaging. The reliability of CSF tests is improved when these tests are interpreted alongside neuroimaging data.

Insoluble cellular prion protein and its association with prion and Alzheimer diseases

The soluble cellular prion protein (PrP(C)) is best known for its association with prion disease (PrD) through its conversion to a pathogenic insoluble isoform (PrP(Sc)). However, its deleterious effects independent of PrP(Sc) have recently been observed not only in PrD but also in Alzheimer disease (AD), two diseases which mainly affect cognition. At the same time, PrP(C) itself seems to have broad physiologic functions including involvement in cognitive processes. The PrP(C) that is believed to be soluble and monomeric has so far been the only PrP conformer observed in the uninfected brain. In 2006, we identified an insoluble PrP(C) conformer (termed iPrP(C) ) in uninfected human and animal brains. Remarkably, the PrP(Sc) -like iPrPC shares the immunoreactivity behavior and fragmentation with a newly-identified PrP(Sc) species in a novel human PrD termed variably protease-sensitive prionopathy. Moreover, iPrP(C) has been observed as the major PrP species that interacts with amyloid β (Aβ) in AD. This article highlights evidence of PrP involvement in two putatively beneficial and deleterious PrP-implicated pathways in cognition, and hypothesizes first, that beneficial and deleterious effects of PrP(C) are attributable to the chameleon-like conformation of the protein and second, that the iPrP(C) conformer is associated with PrD and AD.

Selection and characterization of DNA aptamers against PrP(Sc)

Transmissible spongiform encephalopathies (TSEs) are a group of zoonotic and fatal neurodegenerative disorders that affect humans and animals. The pathogenesis of TSEs involves a conformational conversion of the cellular prion protein (PrP) into abnormal isoforms. Currently, cellular and pathological forms of PrP are differentiated using specific antibody-based analyses that are resource intensive and not applicable to all species and strains. Thus, there is an urgent need for sensitive and efficient assays that can detect pathological forms of PrP. Using systematic evolution of ligands by exponential enrichment, we developed DNA aptamers that can differentiate normal and abnormal PrP isoforms. These aptamers represent the first reagents that can identify pathological isoforms of PrP across multiple host species. Second, they are able to distinguish different strains of prions. Third, they can be used to detect prions in peripheral blood cells, which are otherwise undetectable using conventional antibody-based detection methods. Thus, DNA aptamers offer promise for the development of presymptomatic screens of tissue, blood and other body fluids for prion contamination.

Amyloid-beta42 interacts mainly with insoluble prion protein in the Alzheimer brain

The prion protein (PrP) is best known for its association with prion diseases. However, a controversial new role for PrP in Alzheimer disease (AD) has recently emerged. In vitro studies and mouse models of AD suggest that PrP may be involved in AD pathogenesis through a highly specific interaction with amyloid-β (Aβ42) oligomers. Immobilized recombinant human PrP (huPrP) also exhibited high affinity and specificity for Aβ42 oligomers. Here we report the novel finding that aggregated forms of huPrP and Aβ42 are co-purified from AD brain extracts. Moreover, an anti-PrP antibody and an agent that specifically binds to insoluble PrP (iPrP) co-precipitate insoluble Aβ from human AD brain. Finally, using peptide membrane arrays of 99 13-mer peptides that span the entire sequence of mature huPrP, two distinct types of Aβ binding sites on huPrP are identified in vitro. One specifically binds to Aβ42 and the other binds to both Aβ42 and Aβ40. Notably, Aβ42-specific binding sites are localized predominantly in the octapeptide repeat region, whereas sites that bind both Aβ40 and Aβ42 are mainly in the extreme N-terminal or C-terminal domains of PrP. Our study suggests that iPrP is the major PrP species that interacts with insoluble Aβ42 in vivo. Although this work indicated the interaction of Aβ42 with huPrP in the AD brain, the pathophysiological relevance of the iPrP/Aβ42 interaction remains to be established.

Conditional modulation of membrane protein expression in cultured cells mediated by prion protein recognition of short phosphorothioate oligodeoxynucleotides

We demonstrate that the levels of native as well as transfected prion protein (PrP) are lowered in various cell lines exposed to phosphorothioate oligodeoxynucleotides (PS-DNA) and can be rapidly reverted to their normal amounts by removal of PS-DNA. This transient modulation was independent of the glycosylation state of PrP, and in addition, all three PrP glycoforms were susceptible to PS-DNA treatment. Deletion of the N-terminal domain (amino acids 23-99), but not of the other domains of PrP, abrogated its PS-DNA-mediated down-regulation. PrP versions localized in the mitochondria, cytoplasm, or nucleus were not modulated by PS-DNA, indicating that PrP surface exposure is required for executing this effect. Proteins that in their native forms were not responsive to PS-DNA, such as thymocyte antigen 1 (Thy1), Doppel protein (Dpl), green fluorescent protein (GFP), and cyan fluorescent protein (CFP), became susceptible to PS-DNA-mediated down-regulation following introduction of the N terminus of PrP into their sequence. These observations demonstrate the essential role of the N-terminal domain for promoting oligonucleotide-mediated reduction of the PrP level and suggest that transient treatment of cultured cells with PS-DNA may provide a general method for targeted modulation of the levels of desired surface proteins in a conditional and reversible manner.

Experimental approaches to the interaction of the prion protein with nucleic acids and glycosaminoglycans: Modulators of the pathogenic conversion

The concept that transmissible spongiform encephalopathies (TSEs) are caused only by proteins has changed the traditional paradigm that disease transmission is due solely to an agent that carries genetic information. The central hypothesis for prion diseases proposes that the conversion of a cellular prion protein (PrP(C)) into a misfolded, β-sheet-rich isoform (PrP(Sc)) accounts for the development of (TSE). There is substantial evidence that the infectious material consists chiefly of a protein, PrP(Sc), with no genomic coding material, unlike a virus particle, which has both. However, prions seem to have other partners that chaperone their activities in converting the PrP(C) into the disease-causing isoform. Nucleic acids (NAs) and glycosaminoglycans (GAGs) are the most probable accomplices of prion conversion. Here, we review the recent experimental approaches that have been employed to characterize the interaction of prion proteins with nucleic acids and glycosaminoglycans. A PrP recognizes many nucleic acids and GAGs with high affinities, and this seems to be related to a pathophysiological role for this interaction. A PrP binds nucleic acids and GAGs with structural selectivity, and some PrP:NA complexes can become proteinase K-resistant, undergoing amyloid oligomerization and conversion to a β-sheet-rich structure. These results are consistent with the hypothesis that endogenous polyanions (such as NAs and GAGs) may accelerate the rate of prion disease progression by acting as scaffolds or lattices that mediate the interaction between PrP(C) and PrP(Sc) molecules. In addition to a still-possible hypothesis that nucleic acids and GAGs, especially those from the host, may modulate the conversion, the recent structural characterization of the complexes has raised the possibility of developing new diagnostic and therapeutic strategies.

A new method for the characterization of strain-specific conformational stability of protease-sensitive and protease-resistant PrPSc

Although proteinacious in nature, prions exist as strains with specific self-perpetuating biological properties. Prion strains are thought to be associated with different conformers of PrP^Sc, a disease-associated isoform of the host-encoded cellular protein (PrP^C). Molecular strain typing approaches have been developed which rely on the characterization of protease-resistant PrP^Sc. However, PrP^Sc is composed not only of protease-resistant but also of protease-sensitive isoforms. The aim of this work was to develop a protocol for the molecular characterization of both, protease-resistant and protease-sensitive PrP^Sc aggregates. We first set up experimental conditions which allowed the most advantageous separation of PrP^C and PrP^Sc by means of differential centrifugation. The conformational solubility and stability assay (CSSA) was then developed by measuring PrP^Sc solubility as a function of increased exposure to GdnHCl. Brain homogenates from voles infected with human and sheep prion isolates were analysed by CSSA and showed strain-specific conformational stabilities, with mean [GdnHCl]_1/2 values ranging from 1.6 M for MM2 sCJD to 2.1 for scrapie and to 2.8 M for MM1/MV1 sCJD and E200K gCJD. Interestingly, the rank order of [GdnHCl]_1/2 values observed in the human and sheep isolates used as inocula closely matched those found following transmission in voles, being MM1 sCJD the most resistant (3.3 M), followed by sheep scrapie (2.2 M) and by MM2 sCJD (1.6 M). In order to test the ability of CSSA to characterise protease-sensitive PrP^Sc, we analysed sheep isolates of Nor98 and compared them to classical scrapie isolates. In Nor98, insoluble PrP^Sc aggregates were mainly protease-sensitive and showed a conformational stability much lower than in classical scrapie. Our results show that CSSA is able to reveal strain-specified PrP^Sc conformational stabilities of protease-resistant and protease-sensitive PrP^Sc and that it is a valuable tool for strain typing in natural hosts, such as humans and sheep.

Non-infectious aggregates of the prion protein react with several PrPSc-directed antibodies

The key event in the pathogenesis of prion diseases is the conformational conversion of the normal prion protein (PrP) (PrP(C)) into an infectious, aggregated isoform (PrP(Sc)) that has a high content of beta-sheet. Historically, a great deal of effort has been devoted to developing antibodies that specifically recognize PrP(Sc) but not PrP(C), as such antibodies would have enormous diagnostic and experimental value. A mouse monoclonal IgM antibody (designated 15B3) and three PrP motif-grafted monoclonal antibodies (referred to as IgG 19-33, 89-112, and 136-158) have been previously reported to react specifically with infectious PrP(Sc) but not PrP(C). In this study, we extend the characterization of these four antibodies by testing their ability to immunoprecipitate and immunostain infectious and non-infectious aggregates of wild-type, mutant, and recombinant PrP. We find that 15B3 as well as the motif-grafted antibodies recognize multiple types of aggregated PrP, both infectious and non-infectious, including forms found in brain, in transfected cells, and induced in vitro from purified recombinant protein. These antibodies are exquisitely selective for aggregated PrP, and do not react with soluble PrP even when present in vast excess. Our results suggest that 15B3 and the motif-grafted antibodies recognize structural features common to both infectious and non-infectious aggregates of PrP. Our study extends the utility of these antibodies for diagnostic and experimental purposes, and it provides new insight into the structural changes that accompany PrP oligomerization and prion propagation.

Characterization of the prion protein in human urine

The presence of the prion protein (PrP) in normal human urine is controversial and currently inconclusive. This issue has taken a special relevance because prion infectivity has been demonstrated in urine of animals carrying experimental or naturally occurring prion diseases, but the actual presence and tissue origin of the infectious prion have not been determined. We used immunoprecipitation, one- and two-dimensional electrophoresis, and mass spectrometry to prove definitely the presence of PrP in human urine and its post-translational modifications. We show that urinary PrP (uPrP) is truncated mainly at residue 112 but also at other residues up to 122. This truncation makes uPrP undetectable with some commonly used antibodies to PrP. uPrP is glycosylated and carries an anchor which, at variance with that of cellular PrP, lacks the inositol-associated phospholipid moiety, indicating that uPrP is probably shed from the cell surface. The detailed characterization of uPrP reported here definitely proves the presence of PrP in human urine and will help determine the origin of prion infectivity in urine.

PrP conformational transitions alter species preference of a PrP-specific antibody

The epitope of the 3F4 antibody most commonly used in human prion disease diagnosis is believed to consist of residues Met-Lys-His-Met (MKHM) corresponding to human PrP-(109-112). This assumption is based mainly on the observation that 3F4 reacts with human and hamster PrP but not with PrP from mouse, sheep, and cervids, in which Met at residue 112 is replaced by Val. Here we report that, by brain histoblotting, 3F4 did not react with PrP of uninfected transgenic mice expressing elk PrP; however, it did show distinct immunoreactivity in transgenic mice infected with chronic wasting disease. Compared with human PrP, the 3F4 reactivity with the recombinant elk PrP was 2 orders of magnitude weaker, as indicated by both Western blotting and surface plasmon resonance. To investigate the molecular basis of these species- and conformer-dependent preferences of 3F4, the epitope was probed by peptide membrane array and antigen competition experiments. Remarkably, the 3F4 antibody did not react with MKHM but reacted strongly with KTNMK (corresponding to human PrP-(106-110)), a sequence that is also present in cervids, sheep, and cattle. 3F4 also reacted with elk PrP peptides containing KTNMKHV. We concluded that the minimal sequence for the 3F4 epitope consists of residues KTNMK, and the species- and conformer-dependent preferences of 3F4 arise largely from the interactions between Met(112) (human PrP) or Val(115) (cervid PrP) and adjacent residues.

Co-existence of scrapie prion protein types 1 and 2 in sporadic Creutzfeldt-Jakob disease: its effect on the phenotype and prion-type characteristics

Five phenotypically distinct subtypes have been identified in sporadic Creutzfeldt-Jakob disease (sCJD), based on the methionine/valine polymorphic genotype of codon 129 of the prion protein (PrP) gene and the presence of either one of the two protease K-resistant scrapie prion protein (PrP(Sc)) types identified as 1 and 2. The infrequent co-existence of both PrP(Sc) types in the same case has been known for a long time. Recently, it has been reported, using type-specific antibodies, that the PrP(Sc) type 1 is present in all cases of sCJD carrying PrP(Sc) type 2. The consistent co-occurrence of both PrP(Sc) types complicates the diagnosis and the current classification of sCJD, and has implications for the pathogenesis of naturally occurring prion diseases. In the present study, we investigated the prevalence of PrP(Sc) types 1 and 2 co-occurrence, along with its effects on the disease phenotype and PrP(Sc) strain characteristics, comparatively analysing 34 cases of sCJD, all methionine homozygous at codon 129 of the PrP gene (sCJDMM). To minimize overestimating the prevalence of the sCJDMM cases carrying PrP(Sc) types 1 and 2 (sCJDMM1-2), we used proteinase K concentrations designed to hydrolyse all fragments resulting from an incomplete digestion, while preserving the protease-resistant PrP(Sc) core. Furthermore, we used several antibodies to maximize the detection of both PrP(Sc) types. Our data show that sCJDMM cases associated exclusively with either PrP(Sc) type 1 (sCJDMM1) or PrP(Sc) type 2 (sCJDMM2) do exist; we estimate that they account for approximately 56% and 5% of all the sCJDMM cases, respectively; while in 39% of the cases, both PrP(Sc) types 1 and 2 are present together (sCJDMM1-2) either mixed in the same anatomical region or separate in different regions. Clinically, sCJDMM1-2 had an average disease duration intermediate between the other two sCJDMM subtypes. The histopathology was also intermediate, except for the cerebellum where it resembled that of sCJDMM1. These features, along with the PrP immunostaining pattern, offer a diagnostic clue. We also observed a correlation between the disease duration and the prevalence of PrP(Sc) type 2 and sCJDMM2 phenotypes. The use of different antibodies and of the conformational stability immunoassay indicated that the co-existence of types 1 and 2 in the same anatomical region may confer special conformational characteristics to PrP(Sc) types 1 and 2. All of these findings indicate that sCJDMM1-2 should be considered as a separate entity at this time.

Cryptic peptides of the kringle domains preferentially bind to disease-associated prion protein

Prion diseases are a group of fatal neurodegenerative disorders characterized by the accumulation of a misfolded form (PrP(Sc)) of the cellular prion protein (PrP(C)) in the brains of affected individuals. The conversion of PrP(C) to PrP(Sc) is thought to involve a change in protein conformation from a normal, primarily alpha-helical structure into a beta-sheet conformer. Few proteins have been identified that differentially interact with the two forms of PrP. It has been reported that plasminogen binds to PrP(Sc) from a variety of prion phenotypes. We have examined potential motifs within the kringle region that may be responsible for binding to PrP. We synthesized 12-15-mer peptides that contain small, repetitive stretches of amino acid residues found within the kringle domains of plasminogen. These synthetic peptides were found to capture PrP(Sc) from the brain homogenates of bovine spongiform encephalopathy affected cattle, chronic wasting disease affected elk, experimental scrapie of hamsters and that of subjects affected by Creutzfeldt-Jakob disease, without binding to PrP(C) in unaffected controls. Therefore, we have identified critical peptide motifs that may be important for protein-protein interactions in prion disease pathogenesis. The ability of these synthetic peptides to bind preferentially to PrP(Sc) suggests a potential application in the diagnosis of prion diseases.

Failure to detect the presence of prions in the uterine and gestational tissues from a Gravida with Creutzfeldt-Jakob disease

The vertical transmission of a prion disease from infected mothers to their offspring is believed to be one of the routes for the natural spread of animal prion diseases. Supporting this notion is the observation that prion infectivity occurs in the placenta of infected ewes. Furthermore, the prion protein (PrP), both in its cellular form (PrP(C)) and its pathological isoform (PrP(Sc)), has been observed at the fetal-maternal interface of scrapie-infected sheep. However, whether these features of prion infectivity also hold true for human prion diseases is currently unknown. To begin to address such an important question, we examined PrP in the uterus as well as gestational tissues, including the placenta and amniotic fluid, in a pregnant woman with sporadic Creutzfeldt-Jakob disease (CJD). Although the proteinase K (PK)-resistant prion protein, PrP27-30, was present in the brain tissues of the mother, the PrP detected in the uterus, placenta, and amniotic fluid was sensitive to PK digestion. Unlike PrP(C) in the brain and adjacent cerebrospinal fluid, the predominant PrP species in the reproductive and gestational tissues were N-terminally truncated, similar to urine PrP. Our study did not detect abnormal PrP in the reproductive and gestational tissues in this case of CJD. Nevertheless, examination by a highly sensitive bioassay is ongoing to ascertain possible prion infectivity from CJD in the amniotic fluid.

Small-ruminant lentivirus enhances PrPSc accumulation in cultured sheep microglial cells

Sheep scrapie is the prototypical transmissible spongiform encephalopathy (prion disease), which has a fundamental pathogenesis involving conversion of normal cellular prion protein (PrP(C) [C superscript stands for cellular]) to disease-associated prion protein (PrP(Sc) [Sc superscript stands for sheep scrapie]). Sheep microglial cell cultures, derived from a prnp 136VV/171QQ near-term fetal brain, were developed to study sheep scrapie in the natural host and to investigate potential cofactors in the prion conversion process. Two culture systems, a primary cell culture and a cell line transformed with the large T antigen of simian virus 40, were developed, and both were identified as microglial in origin as indicated by expression of several microglial phenotype markers. Following exposure to PrP(Sc), sheep microglial cells demonstrated relatively low levels (transformed cell line) to high levels (primary cell line) of PrP(Sc) accumulation over time. The accumulated PrP(Sc) demonstrated protease resistance, an inferred beta-sheet conformation (as determined by a commercial enzyme-linked immunosorbent assay), specific inhibition by anti-PrP antibodies, and was transmissible in a dose-dependent manner. Primary microglia coinfected with a small-ruminant lentivirus (caprine arthritis encephalitis virus-Cork strain) and PrP(Sc) demonstrated an approximately twofold increase in PrP(Sc) accumulation compared to that of primary microglia infected with PrP(Sc) alone. The results demonstrate the in vitro utility of PrP(Sc)-permissive sheep microglial cells in investigating the biology of natural prion diseases and show that small-ruminant lentiviruses enhance prion conversion in cultured sheep microglia.

A novel human disease with abnormal prion protein sensitive to protease

OBJECTIVE

To report a novel prion disease characterized by distinct histopathological and immunostaining features, and associated with an abnormal isoform of the prion protein (PrP) that, contrary to the common prion diseases, is predominantly sensitive to protease digestion.

METHODS

Eleven subjects were investigated at the National Prion Disease Pathology Surveillance Center for clinical, histopathological, immunohistochemical, genotypical, and PrP characteristics.

RESULTS

Patients presented with behavioral and psychiatric manifestations on average at 62 years, whereas mean disease duration was 20 months. The type of spongiform degeneration, the PrP immunostaining pattern, and the presence of microplaques distinguished these cases from those with known prion diseases. Typical protease-resistant PrP was undetectable in the cerebral neocortex with standard diagnostic procedures. After enrichment, abnormal PrP was detected at concentrations 16 times lower than common prion diseases; it included nearly 4 times less protease-resistant PrP, which formed a distinct electrophoretic profile. The subjects examined comprised about 3% of sporadic cases evaluated by the National Prion Disease Pathology Surveillance Center. Although several subjects had family histories of dementia, no mutations were found in the PrP gene open reading frame.

INTERPRETATION

The distinct histopathological, PrP immunohistochemical, and physicochemical features, together with the homogeneous genotype, indicate that this is a previously unidentified type of disease involving the PrP, which we designated "protease-sensitive prionopathy" (or PSPr). Protease-sensitive prionopathy is not rare among prion diseases, and it may be even more prevalent than our data indicate because protease-sensitive prionopathy cases are likely also to be classified within the group of non-Alzheimer's dementias.

Prion protein complexed to N2a cellular RNAs through its N-terminal domain forms aggregates and is toxic to murine neuroblastoma cells

Conversion of the cellular prion protein (PrP^C) into its altered conformation, PrP^Sc, is believed to be the major cause of prion diseases. Although PrP is the only identified agent for these diseases, there is increasing evidence that other molecules can modulate the conversion. We have found that interaction of PrP with double-stranded DNA leads to a protein with higher β-sheet content and characteristics similar to those of PrP^Sc. RNA molecules can also interact with PrP and potentially modulate PrP^C to PrP^Sc conversion or even bind differentially to both PrP isoforms. Here, we investigated the interaction of recombinant murine PrP with synthetic RNA sequences and with total RNA extracted from cultured neuroblastoma cells (N2aRNA). We found that PrP interacts with N2aRNA with nanomolar affinity, aggregates upon this interaction, and forms species partially resistant to proteolysis. RNA does not bind to N-terminal deletion mutants of PrP, indicating that the N-terminal region is important for this process. Cell viability assays showed that only the N2aRNA extract induces PrP-RNA aggregates that can alter the homeostasis of cultured cells. Small RNAs bound to PrP give rise to nontoxic small oligomers. Nuclear magnetic resonance measurements of the PrP-RNA complex revealed structural changes in PrP, but most of its native fold is maintained. These results indicate that there is selectivity in the species generated by interaction with different molecules of RNA. The catalytic effect of RNA on the PrP^C→PrP^Sc conversion depends on the RNA sequence, and small RNA molecules may exert a protective effect.

Progress and limits of TSE diagnostic tools

Following the two "mad cow" crises of 1996 and 2000, there was an urgent need for rapid and sensitive diagnostic methods to identify animals infected with the bovine spongiform encephalopathy (BSE) agent. This stimulated research in the field of prion diagnosis and led to the establishment of numerous so-called "rapid tests" which have been in use in Europe since 2001 for monitoring at-risk populations (rendering plants) and animals slaughtered for human consumption (slaughterhouse). These rapid tests have played a critical role in the management of the mad cow crisis by allowing the removal of prion infected carcasses from the human food chain, and by allowing a precise epidemiological monitoring of the BSE epizootic. They are all based on the detection of the abnormal form of the prion protein (PrP(Sc) or PrP(res)) in brain tissues and consequently are only suitable for post-mortem diagnosis. Since it is now very clear that variant Creutzfeldt-Jakob disease (vCJD) can be transmitted by blood transfusion, the development of a blood test for the diagnosis of vCJD is a top priority. Although significant progress has been made in this direction, including the development of the protein misfolding cyclic amplification (PMCA) technology, at the time this paper was written, this objective had not yet been achieved. This is the most important challenge for the years to come in this field of prion research.

Controlled proteolysis of normal and pathological prion protein in a microfluidic chip

A microreactor for proteinase K (PK)-mediated protein digestion was developed as a step towards the elaboration of a fully integrated microdevice for the detection of pathological prion protein (PrP). PK-grafted magnetic beads were immobilized inside a polydimethylsiloxane (PDMS) microchannel using a longitudinal magnetic field parallel to the flow direction and a magnetic field gradient, thereby forming a matrix for enzymatic digestion. This self-organization provided uniform pore sizes, a low flow resistance and a strong reaction efficiency due to a very thin diffusion layer. The microreactor's performance was first evaluated using a model substrate, succinyl-ala-ala-ala-paranitroanilide (SAAAP). Reaction kinetics were typically accelerated a hundred-fold as compared to conventional batch reactions. Reproducibility was around 98% for on-chip experiments. This microsystem was then applied to the digestion of prion protein from brain tissues. Controlled proteolysis could be obtained by varying the on-chip flow rate, while a complete proteolysis of normal protein was achieved in only three minutes. Extracts from normal and pathological brain homogenates were finally compared and strong discrimination between normal and pathological samples was demonstrated.

A novel real-time ultrasonic method for prion protein detection using plasminogen as a capture molecule

Background

High resolution ultrasonography (HR-US) can monitor the molecular changes and biochemical interactions between proteins in real-time. The aim of this study was to use HR-US to characterize the real-time interactions between plasminogen coated beads and PrP^Sc and to determine if this approach could be applied to the identification of animals affected by prion diseases. Plasminogen, immobilized to beads, was used as a capturing tool for PrP^Sc in brain homogenates from scrapie affected sheep and the binding reaction was monitored in real-time in an ultrasonic cell.

Results

Changes in the ultrasonic parameters suggested that three processes occurred during the incubation: binding, protein-protein network formation and precipitation and that these processes occurred in a concentration dependent manner. Conversely, when homogenates from normal sheep were similarly examined, no evidence for the occurrence of these processes was found indicating the specificity of the interaction between the plasminogen coated beads and PrP^Sc.

Conclusion

These results indicate firstly, that the plasminogen coated beads binded selectively to PrP^Sc and secondly, that a HR-US system can discriminate between scrapie affected and non-affected samples and thus has potential as a tool for the rapid diagnosis for prion diseases. This approach has the significant advantage of not requiring a proteinase K pre-digestion step, which is routinely used in current PrP^Sc detection assays.