Prion protein (PrP) synthetic peptides induce cellular PrP to acquire properties of the scrapie isoform

Nucleic acid actions on abnormal protein aggregation, phase transitions and phase separation

Liquid-liquid phase separation (LLPS) and phase transitions (PT) of proteins, which include the formation of gel- and solid-like species, have been characterized as physical processes related to the pathology of conformational diseases. Nucleic acid (NA)-binding proteins related to neurodegenerative disorders and cancer were shown by us and others to experience PT modulated by different NAs. Herein, we discuss recent work on phase separation and phase transitions of two amyloidogenic proteins, i.e. the prion protein (PrP) and p53, which undergo conformational changes and aggregate upon NA interaction. The role of different NAs in these processes is discussed to shed light on the relevance of PSs and PTs for both the functional and pathological roles of these mammalian proteins.

Membrane composition and lipid to protein ratio modulate amyloid kinetics of yeast prion protein

Understanding of prion aggregation in a membrane environment may help to ameliorate neurodegenerative complications caused by the amyloid forms of prions. Here, we investigated the membrane binding-induced aggregation of yeast prion protein Sup35. Using the combination of fluorescence correlation spectroscopy (FCS) at single molecule resolution and other biophysical studies, we establish that lipid composition and lipid/protein ratio are key modulators of the aggregation kinetics of Sup35. In the presence of a zwitterionic membrane (DMPC), Sup35 exhibited novel biphasic aggregation kinetics at lipid/protein ratios ranging between 20 : 1 and 70 : 1 (termed here as the optimum lipid concentration, OLC). In ratios below (low lipid concentration, LLC) and above (ELC, excess lipid concentration) that range, the aggregation was found to be monophasic. In contrast, in the presence of negatively charged membranes, we did not observe any bi-phasic aggregation kinetics in the entire range of protein to lipid ratios. Our results provide a mechanistic description of the role that membrane concentration/composition-modulated aggregation may play in neurodegenerative diseases.

Neurodegenerative Disease Transmission and Transgenesis in Mice

Although the discovery of the prion protein (PrP) resulted from its co-purification with scrapie infectivity in Syrian hamsters, work with genetically defined and genetically modified mice proved crucial for understanding the fundamental processes involved not only in prion diseases caused by PrP misfolding, aggregation, and spread but also in other, much more common, neurodegenerative brain diseases. In this review, we focus on methodological and conceptual approaches used to study scrapie and related PrP misfolding diseases in mice and how these approaches have advanced our understanding of related disorders including Alzheimer's and Parkinson's disease.

Polymorphisms at amino acid residues 141 and 154 influence conformational variation in ovine PrP

Polymorphisms in ovine PrP at amino acid residues 141 and 154 are associated with susceptibility to ovine prion disease: Leu141Arg154 with classical scrapie and Phe141Arg154 and Leu141His154 with atypical scrapie. Classical scrapie is naturally transmissible between sheep, whereas this may not be the case with atypical scrapie. Critical amino acid residues will determine the range or stability of structural changes within the ovine prion protein or its functional interaction with potential cofactors, during conversion of PrPC to PrPSc in these different forms of scrapie disease. Here we computationally identified that regions of ovine PrP, including those near amino acid residues 141 and 154, displayed more conservation than expected based on local structural environment. Molecular dynamics simulations showed these conserved regions of ovine PrP displayed genotypic differences in conformational repertoire and amino acid side-chain interactions. Significantly, Leu141Arg154 PrP adopted an extended beta sheet arrangement in the N-terminal palindromic region more frequently than the Phe141Arg154 and Leu141His154 variants. We supported these computational observations experimentally using circular dichroism spectroscopy and immunobiochemical studies on ovine recombinant PrP. Collectively, our observations show amino acid residues 141 and 154 influence secondary structure and conformational change in ovine PrP that may correlate with different forms of scrapie.

Heparin binding confers prion stability and impairs its aggregation

The conversion of the prion protein (PrP) into scrapie PrP (PrP(Sc)) is a central event in prion diseases. Several molecules work as cofactors in the conversion process, including glycosaminoglycans (GAGs). GAGs exhibit a paradoxical effect, as they convert PrP into protease-resistant PrP (PrP-res) but also exert protective activity. We compared the stability and aggregation propensity of PrP and the heparin-PrP complex through the application of different in vitro aggregation approaches, including real-time quaking-induced conversion (RT-QuIC). Transmissible spongiform encephalopathy-associated forms from mouse and hamster brain homogenates were used to seed RT-QuIC-induced fibrillization. In our study, interaction between heparin and cellular PrP (PrP(C)) increased thermal PrP stability, leading to an 8-fold decrease in temperature-induced aggregation. The interaction of low-molecular-weight heparin (LMWHep) with the PrP N- or C-terminal domain affected not only the extent of PrP fibrillization but also its kinetics, lowering the reaction rate constant from 1.04 to 0.29 s(-1) and increasing the lag phase from 12 to 19 h in RT-QuIC experiments. Our findings explain the protective effect of heparin in different models of prion and prion-like neurodegenerative diseases and establish the groundwork for the development of therapeutic strategies based on GAGs.

Prion protein self-interaction in prion disease therapy approaches

Transmissible spongiform encephalopathies (TSEs) or prion diseases are unique disorders that are not caused by infectious micro-organisms (bacteria or fungi), viruses or parasites, but rather seem to be the result of an infectious protein. TSEs are comprised of fatal neurodegenerative disorders affecting both human and animals. Prion diseases cause sponge-like degeneration of neuronal tissue and include (among others) Creutzfeldt-Jacob disease in humans, bovine spongiform encephalopathy (BSE) in cattle and scrapie in sheep. TSEs are characterized by the formation and accumulation of transmissible (infectious) disease-associated protease-resistant prion protein (PrP(Sc)), mainly in tissues of the central nervous system. The exact molecular processes behind the conversion of PrP(C) into PrP(Sc) are not clearly understood. Correlations between prion protein polymorphisms and disease have been found, however in what way these polymorphisms influence the conversion processes remains an enigma; is stabilization or destabilization of the prion protein the basis for a higher conversion propensity? Apart from the disease-associated polymorphisms of the prion protein, the molecular processes underlying conversion are not understood. There are some notions as to which regions of the prion protein are involved in refolding of PrP(C) into PrP(Sc) and where the most drastic structural changes take place. Direct interactions between PrP(C) molecules and/or PrP(Sc) are likely at the basis of conversion, however which specific amino acid domains are involved and to what extent these domains contribute to conversion resistance/sensitivity of the prion protein or the species barrier is still unknown.

Chemical biology of prion protein: tools to bridge the in vitro/vivo interface

Research on prion protein (PrP) and pathogenic prion has been very intensive because of its importance as model system for neurodegenerative diseases. One important aspect of this research has been the application of chemical biology tools. In this review we describe new developments like native chemical ligation (NCL) and expressed protein ligation (EPL) for the synthesis and semisynthesis of proteins in general and PrP in particular. These techniques allow the synthesis of designed tailor made analogs which can be used in conjunction with modern biophysical methods like fluorescence spectroscopy, solid state Nuclear Magnetic Resonance (ssNMR), and Electron Paramagnetic Resonance (EPR). Another aspect of prion research is concerned with the interaction of PrP with small organic molecules and metals. The results are critically reviewed and put into perspective of their implication for PrP function.

Unique structural characteristics of the rabbit prion protein

Rabbits are one of the few mammalian species that appear to be resistant to transmissible spongiform encephalopathies due to the structural characteristics of the rabbit prion protein (RaPrP(C)) itself. Here, we determined the solution structures of the recombinant protein RaPrP(C)-(91-228) and its S173N variant and detected the backbone dynamics of their structured C-terminal domains-(121-228). In contrast to many other mammalian PrP(C)s, loop 165-172, which connects β-sheet-2 and α-helix-2, is well-defined in RaPrP(C). For the first time, order parameters S(2) are obtained for residues in this loop region, indicating that loop 165-172 of RaPrP(C) is highly ordered. Compared with the wild-type RaPrP(C), less hydrogen bonds form in the S173N variant. The NMR dynamics analysis reveals a distinct increase in the structural flexibility of loop 165-172 and helix-3 after the S173N substitution, implying that the S173N substitution disturbs the long range interaction of loop 165-172 with helix-3, which further leads to a marked decrease in the global conformational stability. Significantly, RaPrP(C) possesses a unique charge distribution, carrying a continuous area of positive charges on the surface, which is distinguished from other PrP(C)s. The S173N substitution causes visible changes of the charge distribution around the recognition sites for the hypothetical protein X. Our results suggest that the ordered loop 165-172 and its interaction with helix-3, together with the unique distribution of surface electrostatic potential, significantly contribute to the unique structural characteristics of RaPrP(C).

Antibody-based immunotherapeutic attempts in experimental animal models of prion diseases

BACKGROUND

There has been a dramatic decrease in the risk of transmission of bovine spongiform encephalopathy to humans. In contrast, the risk of human-to-human transmission of variant Creutzfeldt-Jakob disease (vCJD) via medical treatments became potentially high since 4 vCJD cases were reported to be possibly transmitted through blood transfusion in the UK. However, no treatments are yet available for curing prion diseases.

OBJECTIVE

Conversion of the normal prion protein, PrP(C), to the amyloidogenic PrP, PrP(Sc), plays a pivotal role in the pathogenesis. Recently, certain anti-PrP or anti-37/67-kDa laminin receptor (LRP/LR) antibodies were shown to have the potential to cure chronically infected cells, clearing PrP(Sc) from the cells. This has raised the possibility of antibody based-immunotherapy for prion diseases. This article aims to introduce and discuss the recently published attempts of immunotherapy in prion diseases.

METHODS

Bibliographic research was carried out using the PubMed database. Patent literature was searched using the UK Intellectual Property Office website.

RESULTS/CONCLUSION

No satisfying consequences in animals could be detected with anti-PrP antibodies directly infused into the brains of animals by the intraventricular route or by anti-PrP or anti-LRP/LR single chain fragment antibodies directly delivered into the brain by virus vector-mediated gene transfer. This is probably because such delivery systems failed to deliver the antibodies to the neurons relevant for the treatments.

Effect of intraventricular infusion of anti-prion protein monoclonal antibodies on disease progression in prion-infected mice

It is well known that anti-prion protein (PrP) monoclonal antibodies (mAbs) inhibit abnormal isoform PrP (PrPSc) formation in cell culture. Additionally, passive immunization of anti-PrP mAbs protects the animals from prion infection via peripheral challenge when mAbs are administered simultaneously or soon after prion inoculation. Thus, anti-PrP mAbs are candidates for the treatment of prion diseases. However, the effects of mAbs on disease progression in the middle and late stages of the disease remain unclear. This study carried out intraventricular infusion of mAbs into prion-infected mice before and after clinical onset to assess their ability to delay disease progression. A 4-week infusion of anti-PrP mAbs initiated at 120 days post-inoculation (p.i.), which is just after clinical onset, reduced PrPSc levels to 70-80 % of those found in mice treated with a negative-control mAb. Spongiform changes, microglial activation and astrogliosis in the hippocampus and thalamus appeared milder in mice treated with anti-PrP mAbs than in those treated with a negative-control mAb. Treatment with anti-PrP mAb prolonged the survival of mice infected with Chandler or Obihiro strain when infusion was initiated at 60 days p.i., at which point PrPSc is detectable in the brain. In contrast, infusion initiated after clinical onset prolonged the survival time by about 8 % only in mice infected with the Chandler strain. Although the effects on survival varied for different prion strains, the anti-PrP mAb could partly prevent disease progression, even after clinical onset, suggesting immunotherapy as a candidate for treatment of prion diseases.

Profiling the culprit in Alzheimer's disease (AD): bacterial toxic proteins - Will they be significant for the aetio-pathogenesis of AD and the transmissible spongiform encephalopathies?

The aetiology of Alzheimer's disease (AD) and the transmissible spongiform encephalopathies (tSEs) is still elusive. The concept that prion protein (PrP(Sc)) is the aetiological agent (infectious protein) in the tSEs has recently been questioned. In AD, the cause of the aberrant cleavage of the beta-amyloid precursor protein (APP), resulting in the production of amyloidogenic Abeta fragments, has yet remained obscure. Moreover, the amyloid hypothesis of AD has been seriously challenged. In both AD and the tSEs, pathogens of various nature, including bacteria, have been discussed as possible causal factors. However, aetiological considerations have completely neglected microbial products such as the bacterial toxic proteins (BTPs). The present paper is aimed at drawing a "culprit profile" of these toxic molecules that can exert, at low-dosage, neuro-degeneration through various effects. Clearly, BTPs may affect cell-surface receptors including modulatory amine transmitter receptor expression, block neuro-transmitter release, increase intra-cellular Ca(2+) levels, affect intra-cellular signal transduction, change cyto-skeletal processing, alter synaptic transmission, influence APP proteolysis, interact with cell surface proteins like PrP(C) or their GPI anchors, act as chaperones inducing conformational change in proteins (e.g., PrP(C) to PrP(Sc)), alter lipid membrane integrity by affecting phospholipases or forming pores and channels, induce vacuolar (spongiform) change and elicit inflammatory reactions with cytokine production including cytokines that were demonstrated in the AD brain. Like PrP(Sc), BTPs can be heat-stable and acid-resistant. BTPs can meet the key-proteins of AD and tSEs in the lipid-rich domains of the plasma membrane called rafts. Basically, this might enable them to initiate a large variety of unfavourable molecular events, eventually resulting in pathogenetic cascades as in AD and the tSEs. All in all, their profile lends support to the hypothesis that BTPs might represent relevant culprits capable to cue and/or promote neuro-degeneration in both AD and the tSEs.

Quantifying the sequence-dependent species barrier between hamster and mouse prions

Prion diseases are transmissible neurodegenerative disorders. It is widely accepted that prions are the infectious agents responsible for disease transmission, and the sequence homology between the infectious prion and the host prion protein determines its transmission efficiency across species. However, previous studies have often reported different results regarding seeding efficiency, the efficiency of initiating amyloid propagation by adding pre-existing amyloid fibrils as seed. In the present study, we used synthetic peptides as a simple system to determine the sequence-dependent transmission barrier between hamster and mouse. We found that the heterologous seeding efficiency of hamster and mouse prion peptides was 4 times less than that of homologous seeding. Moreover, residue 139 was not the only residue in determining seeding efficiency. When the seed had Ile at this position, the homology at this position between seed and monomer determined the seeding efficiency. When the seed had Met at this position, homology at residues 109 and 112 determined the seeding efficiency.

Investigation of Laser‐Microdissected Inclusion Bodies

We established a novel combinatorial method of laser microdissection system and immunoblot analysis in combination with a novel unfolding chaperone (oligomeric Aip2p/Dld2p) that enables us to examine the molecular profile of proteins in the microscopic regions of interest. As a model system for analyzing inclusion bodies associated with various diseases such as Alzheimer's disease, Parkinson's disease, and prion diseases including bovine spongiform encephalopathy (BSE), we applied this novel method to examine brain samples of patients with Pick's disease, a type of progressive presenile dementia with intraneuronal lesions denoted as Pick bodies (PBs) whose major structural components are tau proteins. After boiling in Laemmli's sample buffer according to the established immunoblotting procedures, 500-2000 PBs were initially applied onto SDS-PAGE gels; however, only faint signals were obtained. Remarkably, only one Pick body was sufficient to illustrate an immunoblot signal; this indicates that pretreatment with oligomeric Aip2p/Dld2p enhances the immunoblot sensitivity by more than a 100-fold. This unprecedented property of laser microdissection combined with oligomeric Aip2p/Dld2p may have further potential applications. For example, a number of proteomic strategies for such inclusion bodies depend on liquid chromatography-tandem mass spectrometry (LC-MS/MS); however, sample preparation methods typically involve the use of detergents and chaotropic agents that often interfere with chromatographic separation and/or electrospray ionization. However, the use of oligomeric Aip2p/Dld2p would not interfere with the LC-MS/MS procedures. Therefore, it might significantly facilitate nanoscale analysis, which is often hindered by the aggregation property of the target proteins present under various analytical conditions, particularly, when the sample protein is present in minor quantities.

More than a 100-fold increase in immunoblot signals of laser-microdissected inclusion bodies with an excessive aggregation property by oligomeric actin interacting protein 2/d-lactate dehydrogenase protein 2

We established a histobiochemical approach targeting micron-order inclusion bodies possessing extensive aggregation properties in situ by using a nonchemical denaturant (oligomeric actin interacting protein 2/d-lactate dehydrogenase protein 2 [Aip2p/Dld2p]) with the combinatorial method of laser-microdissection and immunoblot analysis. As a model, pick bodies were chosen and laser-microdissected from three different brain regions of two patients with Pick's disease. Initially, 500 to 2000 pick bodies were applied onto SDS-PAGE gels after boiling in Laemmli's sample buffer according to established immunoblotting procedures; however, only faint signals were obtained. Following negative results with chemical denaturants or detergent, including 6 M guanidine hydrochloride, 8 M urea, and 2% SDS, the laser-microdissected pick bodies were pretreated with oligomeric Aip2p/Dld2p, which possesses robust protein unfolding activity under biological conditions. Strikingly, only one pick body was sufficient to illustrate an immunoblot signal, indicating that pretreatment with oligomeric Aip2p/Dld2p enhanced the immunoblot sensitivity by more than 100-fold. Pretreatment with oligomeric Aip2p/Dld2p also allowed us to quantify the total protein content of pick bodies. Thus, use of oligomeric Aip2p/Dld2p significantly contributed toward the acquisition of information pertaining to the molecular profile of proteins possessing an extensive aggregation property, particularly in small amounts.

Assembly of natural and recombinant prion protein into fibrils

The conversion of the alpha-helical, cellular isoform of the prion protein (PrP C ) to the insoluble, beta-sheet-rich, infectious, disease-causing isoform (PrP Sc ) is the fundamental event in the prion diseases. The C-terminal fragment of PrP Sc (PrP 27-30) is formed by limited proteolysis and retains infectivity. Unlike full-length PrP Sc , PrP 27-30 polymerizes into rod-shaped structures with the ultra-structural and tinctorial properties of amyloid. To study the folding of PrP, both with respect to the formation of PrP Sc from PrP C and the assembly of rods from PrP 27-30, we solubilized Syrian hamster (sol SHa) PrP 27-30 in low concentrations (0.2%) of sodium dodecyl sulfate (SDS) under conditions previously used to study the structural transitions of this protein. Sol SHaPrP 27-30 adopted a beta-sheet-rich structure at SDS concentrations between 0.02% and 0.04% and remained soluble. Here we report that NaCl stabilizes SHaPrP 27-30 in a soluble, beta-sheet-rich state that allows fibril assembly to proceed over several weeks. Under these conditions, fibril formation occurred not only with sol PrP 27-30, but also with native SHaPrP C . Addition of sphingolipids seems to increase fibril growth. When recombinant (rec) SHaPrP(90-231) was exposed to low concentrations of SDS, similar to those used to polymerize sol SHaPrP 27-30 in the presence of 250 mM NaCl, fibril formation occurred regularly. When fibrils formed from PrP 27-30 or PrP C were bioassayed in transgenic mice overexpressing full-length SHaPrP, no infectivity was obtained, whereas amyloid fibrils formed of rec mouse PrP(89-230) were infectious. At present, it cannot be determined whether the lack of infectivity is caused by a difference in the structure of the fibrils or in the bioassay conditions.

Prion protein with Y145STOP mutation induces mitochondria-mediated apoptosis and PrP-containing deposits in vitro

A pathogenic truncation of an amber mutation at codon 145 (Y145STOP) in Gerstmann-Straussler-Scheinker disease (GSS) was investigated through the real-time imaging in living cells, by utilizing GFP-PrP constructs. GFP-PrP(1-144) exhibited an aberrant localization to mitochondria in mouse neuroblastoma neuro2a (N2a) and HpL3-4 cells, a hippocampal cell line established from prnp gene-ablated mice, whereas full-length GFP-PrP did not. The aberrant mitochondrial localization was also confirmed by Western blot analysis. Since GFP-PrP(1-121), as previously reported, and full-length GFP-PrP do not exhibit such mitochondrial localization, the mitochondrial localization of GFP-PrP(1-144) requires not only PrP residues 121-144 (in human sequence) but also COOH-terminal truncation in the current experimental condition. Subsequently, the GFP-PrP(1-144) induced a change in the mitochondrial innermembrane potential (DeltaPsi(m)), release of cytochrome c from the intermembrane space into the cytosol, and DNA fragmentation in these cells. Non-fluorescent PrP(1-144) also induced the DNA fragmentation in N2a and HpL3-4 cells after the proteasomal inhibition. These data may provide clues as to the molecular mechanism of the neurotoxic property of Y145STOP mutation. Furthermore, immunoelectron microscopy revealed numerous electron-dense deposits in mitochondria clusters of GFP-PrP(1-144)-transfected N2a cells, whereas no deposit was detected in the cells transfected with full-length GFP-PrP. Co-localization of GFP/PrP-immunogold particles with porin-immunogold particles as a mitochondrial marker was observed in such electron-dense vesicular foci, resembling those found in autophagic vacuoles forming secondary lysosomes. Whether such electron-dense deposits may serve as a seed for the growth of amyloid plaques, a characteristic feature of GSS with Y145STOP, awaits further investigations.

The Role of Electrostatic Interaction in Triggering the Unraveling of Stable Helix 1 in Normal Prion Protein. A Molecular Dynamics Simulation Investigation

The conversion of normal prion protein (PrPC) into scrapie isoform (PrPSc) is a key event in the pathogenesis of prion diseases. However, the conversion mechanism has given rise to much controversy. For instance, there is much debate on the behavior of helix 1 (H1) in the conversion. A series of experiments demonstrated that H1 in isolated state was very stable under a variety of conditions. But, other experiments indicated that helices 2 and 3 rather than H1 were retained in PrPSc. In this paper, molecular dynamics (MD) simulation is employed to investigate the dynamic behavior of H1. It is revealed that although the helix 1 of Human PrPC (HuPrPC) is very stable in the isolated state, it becomes unstable when incorporated into native HuPrPC, which likely results from the long-range electrostatic interaction between Asp147 and Arg208 located in the helices 1 and 3, respectively. This explanation is supported by experimental evaluation and MD simulation on D147N mutant of HuPrPC that the mutant becomes a little more stable than the wild type HuPrPC. This finding not only help to reconcile the existing debate on the role of helix 1 in the PrPC-->PrPSc transition, but also reveals a possible mechanism for triggering the PrPC-->PrPSc conversion.

The neurotoxicity of prion protein (PrP) peptide 106–126 is independent of the expression level of PrP and is not mediated by abnormal PrP species

A synthetic peptide homologous to region 106-126 of the prion protein (PrP) is toxic to cells expressing PrP, but not to PrP knockout neurons, arguing for a specific role of PrP in mediating the peptide's activity. Whether this is related to a gain of toxicity or a loss of function of PrP is not clear. We explored the possibility that PrP106-126 triggered formation of PrP(Sc) or other neurotoxic PrP species. We found that PrP106-126 did not induce detergent-insoluble and protease-resistant PrP, nor did it alter its membrane topology or cellular distribution. We also found that neurons expressing endogenous or higher level of either wild-type PrP or a nine-octapeptide insertional mutant were equally susceptible to PrP106-126, and that sub-physiological PrP expression was sufficient to restore vulnerability to the peptide. These results indicate that PrP106-126 interferes with a PrP function that requires only low protein levels, and is not impaired by a pathogenic insertion in the octapeptide region.

Cell-surface retention of PrPC by anti-PrP antibody prevents protease-resistant PrP formation

The C-terminal portion of the prion protein (PrP), corresponding to a protease-resistant core fragment of the abnormal isoform of the prion protein (PrPSc), is essential for prion propagation. Antibodies to the C-terminal portion of PrP are known to inhibit PrPScaccumulation in cells persistently infected with prions. Here it was shown that, in addition to monoclonal antibodies (mAbs) to the C-terminal portion of PrP, a mAb recognizing the octapeptide repeat region in the N-terminal part of PrP that is dispensable for PrPScformation reduced PrPScaccumulation in cells persistently infected with prions. The 50 % effective dose was as low as ∼1 nM, and, regardless of their epitope specificity, the inhibitory mAbs shared the ability to bind cellular prion protein (PrPC) expressed on the cell surface. Flow cytometric analysis revealed that mAbs that bound to the cell surface during cell culture were not internalized even after their withdrawal from the growth medium. Retention of the mAb–PrPCcomplex on the cell surface was also confirmed by the fact that internalization was enhanced by treatment of cells with dextran sulfate. These results suggested that anti-PrP mAb antagonizes PrPScformation by interfering with the regular PrPCdegradation pathway.

β-Hairpin conformation of fibrillogenic peptides: Structure and α-β transition mechanism revealed by molecular dynamics simulations

Understanding the conformational transitions that trigger the aggregation and amyloidogenesis of otherwise soluble peptides at atomic resolution is of fundamental relevance for the design of effective therapeutic agents against amyloid-related disorders. In the present study the transition from ideal alpha-helical to beta-hairpin conformations is revealed by long timescale molecular dynamics simulations in explicit water solvent, for two well-known amyloidogenic peptides: the H1 peptide from prion protein and the Abeta(12-28) fragment from the Abeta(1-42) peptide responsible for Alzheimer's disease. The simulations highlight the unfolding of alpha-helices, followed by the formation of bent conformations and a final convergence to ordered in register beta-hairpin conformations. The beta-hairpins observed, despite different sequences, exhibit a common dynamic behavior and the presence of a peculiar pattern of the hydrophobic side-chains, in particular in the region of the turns. These observations hint at a possible common aggregation mechanism for the onset of different amyloid diseases and a common mechanism in the transition to the beta-hairpin structures. Furthermore the simulations presented herein evidence the stabilization of the alpha-helical conformations induced by the presence of an organic fluorinated cosolvent. The results of MD simulation in 2,2,2-trifluoroethanol (TFE)/water mixture provide further evidence that the peptide coating effect of TFE molecules is responsible for the stabilization of the soluble helical conformation.

Effects of beta-sheet breaker peptide polymers on scrapie-infected mouse neuroblastoma cells and their affinities to prion protein fragment PrP(81-145)

The effects of Soto's 'beta-sheet breaker peptide' and its polymer on PrPSc formation in ScN2a cells were investigated. Surface plasmon resonance study indicated that direct binding between PrP(81-145) and the 'beta-sheet breaker peptide' is not specific and may not play a major role in the inhibition of PrPSc formation.

The role of helix 1 aspartates and salt bridges in the stability and conversion of prion protein

A key event in the pathogenesis of transmissible spongiform encephalopathies is the conversion of PrP-sen to PrP-res. Morrissey and Shakhnovich (Morrissey, M. P., and Shakhnovich, E. I. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 11293-11298) proposed that the conversion mechanism involves critical interactions at helix 1 (residues 144-153) and that the helix is stabilized on PrP-sen by intra-helix salt bridges between two aspartic acid-arginine ion pairs at positions 144 and 148 and at 147 and 151, respectively. Mutants of the hamster prion protein were constructed by replacing the aspartic acids with either asparagines or alanines to destabilize the proposed helix 1 salt bridges. Thermal and chemical denaturation experiments using circular dichroism spectroscopy indicated the overall structures of the mutants are not substantially destabilized but appear to unfold differently. Cell-free conversion reactions performed using ionic denaturants, detergents, and salts (conditions unfavorable to salt bridge formation) showed no significant differences between conversion efficiencies of mutant and wild type proteins. Using conditions more favorable to salt bridge formation, the mutant proteins converted with up to 4-fold higher efficiency than the wild type protein. Thus, although spectroscopic data indicate the salt bridges do not substantially stabilize PrP-sen, the cell-free conversion data suggest that Asp-144 and Asp-147 and their respective salt bridges stabilize PrP-sen from converting to PrP-res.

In vitro conversion of normal prion protein into pathologic isoforms

The in vitro conversion techniques in cell-free and cell culture systems have provided tools to adequately study the underlying mechanism of TSEs, namely PrP conversion. These systems also have provided tools that make it easier to study the interspecies and intraspecies transmissibilities of TSEs. Finally, these systems also may assist in the discovery of TSE therapeutic strategies and in the development of extremely sensitive TSE detection techniques. In vivo TSE transmission studies are limited to (transgenic) animals (mostly mice). Although the cell culture systems also are restricted in their species-range (mostly mouse), the currently used cell-free systems. Allow studying almost all possible species barriers (including the potential transmission of various TSEs to humans). One advantage of the cell culture systems, however, is that they generate do novo TSE infectivity. Studies using cell cultures also take into account several cofactors in addition to PrP that might be involved in replication the TSE agent. Although the in vitro systems provide accurate tools to study TSE agent parameters, they mainly or only focus on the molecular processes of PrP conversion. Other factors (i.e., host genetic factors [99]) that, for example, determine the differential uptake of the TSE agent from the environment, might play an additional role in determining the susceptibility of hosts for TSEs and on the transmission of the disease among individuals.

Competing intrachain interactions regulate the formation of beta-sheet fibrils in bovine PrP peptides

At the heart of the pathogenesis of transmissible spongiform encephalopathies (TSEs), such as BSE, scrapie, and Creutzfeldt-Jakob disease, lies a poorly understood structural rearrangement of PrP, an abundant glycoprotein of the nervous and lymphoid systems. The normal form (PrP(C)), rich in alpha-helix, converts into an aberrant beta-sheet-dominated form (PrP(Sc)), which seems to be at the center of the pathotoxic symptoms observed in TSEs. To understand this process better at a molecular level, we have studied the interactions between different peptides derived from bovine PrP and their structural significance. We show that two unstructured peptides derived from the central region of bovine PrP, residues 115-133 and 140-152, respectively, interact stoichiometrically under physiological conditions to generate beta-sheet-dominated fibrils. However, when both peptides are incubated in the presence of a third peptide derived from an adjoining alpha-helical region (residues 153-169), the formation of beta-sheet-rich fibrils is abolished. These data indicate that native PrP(C) helix 1 might inhibit the strong intrinsic beta-sheet-forming propensity of sequences immediately N-terminal to the globular core of PrP(C), by keeping in place intrachain interactions that would prevent these amyloidogenic regions from triggering aggregation. Moreover, these results indicate new ways in which PrP(Sc) formation could be prevented.

One O-linked sugar can affect the coil-to-beta structural transition of the prion peptide

It has been known that the structural transition from PrP(C) to PrP(Sc) leads to the prion formation. This putative conformational change challenges the central dogma of the protein folding theory-"one sequence, one structure." Generally, scientists believe that there must be either a posttranslational modification or environmental factors involved in this event. However, all of the efforts to solve the mystery of the PrP(C) to PrP(Sc) transition have ended in vain so far. Here we provide evidence linking O-linked glycosylation to the structural transition based on prion peptide studies. We find that the O-linked alpha-GalNAc at Ser-135 suppresses the formation of amyloid fibril formation of the prion peptide at physiological salt concentrations, whereas the peptide with the same sugar at Ser-132 shows the opposite effect. Moreover, this effect is sugar specific. Replacing alpha-GalNAc with beta-GlcNAc does not yield the same effect.

Cell surface accumulation of a truncated transmembrane prion protein in Gerstmann-Straussler-Scheinker disease P102L

A familial prion disorder with a proline to leucine substitution at residue 102 of the prion protein (PrP(102L)) is typically associated with protease-resistant PrP fragments (PrP(Sc)) in the brain parenchyma that are infectious to recipient animals. When modeled in transgenic mice, a fatal neurodegenerative disease develops, but, unlike the human counterpart, PrP(Sc) is lacking and transmission to recipient animals is questionable. Alternate mice expressing a single copy of PrP(102L) (mouse PrP(101L)) do not develop spontaneous disease, but show dramatic susceptibility to PrP(Sc) isolates from different species. To understand these discrepant results, we studied the biogenesis of human PrP(102L) in a cell model. Here, we report that cells expressing PrP(102L) show decreased expression of the normal 18-kDa fragment on the plasma membrane. Instead, a 20-kDa fragment, probably derived from transmembrane PrP ((Ctm)PrP), accumulates on the cell surface. Because the 20-kDa fragment includes an amyloidogenic region of PrP that is disrupted in the 18-kDa form, increased surface expression of 20-kDa fragment may enhance the susceptibility of these cells to PrP(Sc) infection by providing an optimal substrate, or by amplifying the neurotoxic signal of PrP(Sc). Thus, altered susceptibility of PrP(101L) mice to exogenous PrP(Sc) may be mediated by the 20-kDa (Ctm)PrP fragment, rather than PrP(102L) per se.

Amyloid aggregates of the HET-s prion protein are infectious

The [Het-s] infectious element of the filamentous fungus Podospora anserina is a prion. We have recently reported that recombinant HET-s protein aggregates in vitro into amyloid fibers. In vivo, the protein aggregates specifically in the [Het-s] prion strains. Here, we show that biolistic introduction of aggregated recombinant HET-s protein into fungal cells induces emergence of the [Het-s] prion with a high frequency. Thus, we demonstrate that prion infectivity can be created de novo, in vitro from recombinant protein in this system. Although the amyloid filaments formed from HET-s could transmit [Het-s] efficiently, neither the soluble form of the protein nor amorphous aggregates would do so. In addition, we have found that (i) [Het-s] infectivity correlates with the ability to convert HET-s to amyloids in vitro, (ii) [Het-s] infectivity is resistant to proteinase K digestion, and (iii) HET-s aggregates formed in vivo in [Het-s] strains have the ability to convert the recombinant protein to aggregates. Together, our data designate the HET-s amyloids as the molecular basis of [Het-s] prion propagation.

Prion peptide 106-126 modulates the aggregation of cellular prion protein and induces the synthesis of potentially neurotoxic transmembrane PrP

In infectious and familial prion disorders, neurodegeneration is often seen without obvious deposits of the scrapie prion protein (PrP(Sc)), the principal cause of neuronal death in prion disorders. In such cases, neurotoxicity must be mediated by alternative pathways of cell death. One such pathway is through a transmembrane form of PrP. We have investigated the relationship between intracellular accumulation of prion protein aggregates and the consequent up-regulation of transmembrane prion protein in a cell model. Here, we report that exposure of neuroblastoma cells to the prion peptide 106-126 catalyzes the aggregation of cellular prion protein to a weakly proteinase K-resistant form and induces the synthesis of transmembrane prion protein, the proposed mediator of neurotoxicity in certain prion disorders. The N terminus of newly synthesized transmembrane prion protein is cleaved spontaneously on the cytosolic face of the endoplasmic reticulum, and the truncated C-terminal fragment accumulates on the cell surface. Our results suggest that neurotoxicity in prion disorders is mediated by a complex pathway involving transmembrane prion protein and not by deposits of aggregated and proteinase K-resistant PrP alone.

Biochemical and structural studies of the prion protein polymorphism

A hallmark event in transmissible spongiform encephalopathies is the conversion of the physiological prion protein into the disease-associated isoform. A natural polymorphism at codon 129 of the human prion gene, resulting in either methionine or valine, has profound influence on susceptibility and phenotypic expression of the disease in humans. In this study, we investigated the local propensity of synthetic peptides, corresponding to the region of the polymorphism and containing either methionine or valine, to adopt a beta-sheet-rich structure similar to the pathological protein. Circular dichroism studies showed that the methionine-containing peptide has a greater propensity to adopt a beta-sheet conformation in a variety of experimental conditions. The higher beta-sheet tendency of this peptide was also associated with an increased ability to aggregate into amyloid-like fibrils. These results suggest that methionine at position 129 of the prion protein increases its susceptibility to switch to the abnormal conformation, in comparison with the presence of valine at the same position.

PrPC directly interacts with proteins involved in signaling pathways

The cellular prion protein (PrP(C)) is a conserved glycoprotein predominantly expressed in neuronal cells. Its purpose in living cells is still enigmatic. To elucidate on its cellular function, we performed a yeast two-hybrid screen for interactors. We used murine PrP(C) (amino acids 23-231) as bait to search a mouse brain cDNA expression library. Several interaction partners were identified. Three of them with a high homology to known sequences were further characterized. These candidates were the neuronal phosphoprotein synapsin Ib, the adaptor protein Grb2, and the still uncharacterized prion interactor Pint1. The in vivo interaction of the three proteins with PrP(C) was confirmed by co-immunoprecipitation assays with recombinant and authentic proteins in mammalian cells. The binding regions were mapped using truncated PrP constructs. As both synapsin Ib and Grb2 are implicated in neuronal signaling processes, our findings further strengthen the putative role of the prion protein in signal transduction.

Engineering the prion protein using chemical synthesis

In recent years, the technology of solid-phase peptide synthesis (SPPS) has improved to the extent that chemical synthesis of small proteins may be a viable complementary strategy to recombinant expression. We have prepared several modified and wild-type prion protein (PrP) polypeptides, of up to 112 residues, that demonstrate the flexibility of a chemical approach to protein synthesis. The principal event in prion disease is the conformational change of the normal, alpha-helical cellular protein (PrPc) into a beta-sheet-rich pathogenic isoform (PrP(Sc)). The ability to form PrP(Sc) in transgenic mice is retained by a 106 residue 'mini-prion' (PrP106), with the deletions 23-88 and 141-176. Synthetic PrP106 (sPrP106) and a His-tagged analog (sPrP106HT) have been prepared successfully using a highly optimized Fmoc chemical methodology involving DCC/HOBt activation and an efficient capping procedure with N-(2-chlorobenzyloxycarbonyloxy) succinimide. A single reversed-phase purification step gave homogeneous protein, in excellent yield. With respect to its conformational and aggregational properties and its response to proteinase digestion, sPrP106 was indistinguishable from its recombinant analog (rPrP106). Certain sequences that proved to be more difficult to synthesize using the Fmoc approach, such as bovine (Bo) PrP(90-200), were successfully prepared using a combination of the highly activated coupling reagent HATU and t-Boc chemistry. To mimic the glycosylphosphatidyl inositol (GPI) anchor and target sPrP to cholesterol-rich domains on the cell surface, where the conversion of PrPc is believed to occur, a lipophilic group or biotin, was added to an orthogonally side-chain-protected Lys residue at the C-terminus of sPrP sequences. These groups enabled sPrP to be immobilized on either the cell surface or a streptavidin-coated ELISA plate, respectively, in an orientation analogous to that of membrane-bound, GPI-anchored PrPc. The chemical manipulation of such biologically relevant forms of PrP by the introduction of point mutations or groups that mimic post-translational modifications should enhance our understanding of the processes that cause prion diseases and may lead to the chemical synthesis of an infectious agent.

Conformational change, aggregation and fibril formation induced by detergent treatments of cellular prion protein

The conversion of protease-sensitive prion protein (PrP-sen) to a high beta-sheet, protease-resistant and often fibrillar form (PrP-res) is a central event in transmissible spongiform encephalopathies (TSE) or prion diseases. This conversion can be induced by PrP-res itself in cell-free conversion reactions. The detergent sodium N-lauroyl sarkosinate (sarkosyl) is a detergent that is widely used in PrP-res purifications and is known to stimulate the PrP-res-induced conversion reaction. Here we report effects of sarkosyl and other detergents on recombinant hamster PrP-sen purified from mammalian cells under oxidizing conditions that maintain the single native disulfide bond. Low concentrations of sarkosyl (0.001-0.1%) induced aggregation of PrP-sen molecules, increased light scattering, altered fluorescence excitation and emission spectra, and enhanced the proportion of beta-sheet secondary structure according to circular dichroism and infrared spectroscopies. An enhancement of beta-sheet content was also seen with 0.001% sodium dodecyl sulfate (SDS) but not several other types of detergents. Electron microscopy revealed that sarkosyl induced the formation of both amorphous and fibrillar aggregates. The fibrils appeared to be constructed from spherical bead-like protofibrils. Neither TSE infectivity nor the characteristic partial proteinase K resistance of PrP-res was detected in the sarkosyl-induced PrP aggregates. We conclude that certain anionic detergents can disrupt the conformation of PrP-sen and induce high beta-sheet aggregates that are distinct from scrapie-associated PrP-res in terms of protease-resistance, infrared spectrum and infectivity. These results reinforce the idea that not all high-beta aggregates of PrP are equivalent to the pathologic form, PrP-res.

Cryptic epitopes in N-terminally truncated prion protein are exposed in the full-length molecule: dependence of conformation on pH

Prion diseases are diseases of protein conformation. Structure-dependent antibodies have been sought to probe conformations of the prion protein (PrP) resulting from environmental changes, such as differences in pH. Despite the absence of such antibodies for full-length PrP, a recombinant Fab (D13) and a Fab derived from mAb 3F4 showed pH-dependent reactivity toward epitopes within the N-terminus of N-terminally truncated PrP(90-231). Refolding and maintaining this protein at pH > or =5.2 before immobilization on an ELISA plate inhibited reactivity relative to protein exposed to pH < or =4.7. The reactivity was not affected by pH changes after immobilization, showing retention of conformation after binding to the plate surface, although guanidine hydrochloride at 1.5-2 M was able to expose the cryptic epitopes after immobilization at pH > or =5.2. The alpha-helical CD spectrum of PrP(90-231) refolded at pH 5.5 was reduced somewhat by these pH changes, with a minor shift toward beta-sheet at pH 4 and then toward coil at pH 2. No covalent changes were caused by the pH differences. This pH dependence suggests titration of an acidic region that might inhibit the N-terminal epitopes. A similar pH dependence for a monoclonal antibody reactive to the central region identified an acidic region incorporating Glu152 as a significant participant.

Inhibition of interactions and interconversions of prion protein isoforms by peptide fragments from the C-terminal folded domain

The formation of protease-resistant prion protein (PrP-res or PrP(Sc)) involves selective interactions between PrP-res and its normal protease-sensitive counterpart, PrP-sen or PrP(C). Previous studies have shown that synthetic peptide fragments of the PrP sequence corresponding to residues 119-136 of hamster PrP (Ha119-136) can selectively block PrP-res formation in cell-free systems and scrapie-infected tissue culture cells. Here we show that two other peptides corresponding to residues 166-179 (Ha166-179) and 200-223 (Ha200-223) also potently inhibit the PrP-res induced cell-free conversion of PrP-sen to the protease-resistant state. In contrast, Ha121-141, Ha180-199, and Ha218-232 were much less effective as inhibitors. Mechanistic analyses indicated that Ha166-179, Ha200-223, and peptides containing residues 119-136 inhibit primarily by binding to PrP-sen and blocking its binding to PrP-res. Circular dichroism analyses indicated that Ha117-141 and Ha200-223, but not non-inhibitory peptides, readily formed high beta-sheet structures when placed under the conditions of the conversion reaction. We conclude that these inhibitory peptides may mimic contact surfaces between PrP-res and PrP-sen and thereby serve as models of potential therapeutic agents for transmissible spongiform encephalopathies.

Reconstitution of prion infectivity from solubilized protease-resistant PrP and nonprotein components of prion rods

The scrapie isoform of the prion protein, PrP(Sc), is the only identified component of the infectious prion, an agent causing neurodegenerative diseases such as Creutzfeldt-Jakob disease and bovine spongiform encephalopathy. Following proteolysis, PrP(Sc) is trimmed to a fragment designated PrP 27-30. Both PrP(Sc) and PrP 27-30 molecules tend to aggregate and precipitate as amyloid rods when membranes from prion-infected brain are extracted with detergents. Although prion rods were also shown to contain lipids and sugar polymers, no physiological role has yet been attributed to these molecules. In this work, we show that prion infectivity can be reconstituted by combining Me(2)SO-solubilized PrP 27-30, which at best contained low prion infectivity, with nonprotein components of prion rods (heavy fraction after deproteination, originating from a scrapie-infected hamster brain), which did not present any infectivity. Whereas heparanase digestion of the heavy fraction after deproteination (originating from a scrapie-infected hamster brain), before its combination with solubilized PrP 27-30, considerably reduced the reconstitution of infectivity, preliminary results suggest that infectivity can be greatly increased by combining nonaggregated protease-resistant PrP with heparan sulfate, a known component of amyloid plaques in the brain. We submit that whereas PrP 27-30 is probably the obligatory template for the conversion of PrP(C) to PrP(Sc), sulfated sugar polymers may play an important role in the pathogenesis of prion diseases.

Neurotoxicity but not infectivity of prion proteins can be induced reversibly in vitro

Prion diseases include Creutzfeldt-Jakob disease in humans, scrapie in sheep and bovine spongiform encephalopathy. The hallmark of prion diseases is the accumulation of an abnormal isoform (PrP(Sc)) of the cellular prion protein accompanied by neuronal cell death and astroglial proliferation. To characterize the correlation between PrP secondary and quarternary structure and their biological effects we assayed soluble and aggregated forms of PrP 27-30, the N-terminal truncated form of PrP(Sc), as well as the corresponding recombinant PrP(90-231) for their neurotoxicity and infectivity. PrP was kept soluble in 0.2% SDS and subsequently re-aggregated either by diluting the SDS or by adding acetonitril. The neurotoxicity of the re-aggregated states were comparable to that of prion rods (PrP 27-30) whereas the soluble forms had no neurotoxic effects. The solubilized PrP 27-30 showed no significant infection upon re-aggregation as determined by bioassays in Syrian golden hamsters. The recombinant PrP did not exhibit infectivity in any state.

Structural intermediates in the putative pathway from the cellular prion protein to the pathogenic form

The conversion of the alpha-helical, protease sensitive and noninfectious form of the prion protein (PrP(C)) into an insoluble, protease resistant, predominantly beta-sheeted and infectious form (PrP(Sc)) is the fundamental event in prion formation. In the present work, two soluble and stable intermediate structural states are newly identified for recombinant Syrian hamster PrP(90-231) (recPrP), a dimeric alpha-helical state and a tetra- or oligomeric, beta-sheet rich state. In 0.2% SDS at room temperature, recPrP is soluble and exhibits alpha-helical and random coil secondary structure as determined by circular dichroism. Reduction of the SDS concentration to 0.06% leads first to a small increase in alpha-helical content, whereas further dilution to 0.02% results in the aquisition of beta-sheet structure. The reversible transition curve is sigmoidal within a narrow range of SDS concentrations (0.04 to 0.02%). Size exclusion chromatography and chemical crosslinking revealed that the alpha-helical form is dimeric, while the beta-sheet rich form is tetra- or oligomeric. Both the alpha-helical and beta-sheet rich intermediates are soluble and stable. Thus, they should be accessible to further structural and mechanistic studies. At 0.01% SDS, the oligomeric intermediates aggregated into large, insoluble structures as observed by fluorescence correlation spectroscopy. Our results are discussed with respect to the mechanism of PrP(Sc) formation and the propagation of prions.

A protease-resistant 61-residue prion peptide causes neurodegeneration in transgenic mice

An abridged prion protein (PrP) molecule of 106 amino acids, designated PrP106, is capable of forming infectious miniprions in transgenic mice (S. Supattapone, P. Bosque, T. Muramoto, H. Wille, C. Aagaard, D. Peretz, H.-O. B. Nguyen, C. Heinrich, M. Torchia, J. Safar, F. E. Cohen, S. J. DeArmond, S. B. Prusiner, and M. Scott, Cell 96:869-878, 1999). We removed additional sequences from PrP106 and identified a 61-residue peptide, designated PrP61, that spontaneously adopted a protease-resistant conformation in neuroblastoma cells. Synthetic PrP61 bearing a carboxy-terminal lipid moiety polymerized into protease-resistant, beta-sheet-enriched amyloid fibrils at a physiological salt concentration. Transgenic mice expressing low levels of PrP61 died spontaneously with ataxia. Neuropathological examination revealed accumulation of protease-resistant PrP61 within neuronal dendrites and cell bodies, apparently causing apoptosis. PrP61 may be a useful model for deciphering the mechanism by which PrP molecules acquire protease resistance and become neurotoxic.

A monomer-dimer equilibrium of a cellular prion protein (PrPC) not observed with recombinant PrP

Both the purified normal (protease-sensitive) isoform of the prion protein (PrP(C)) (Pergami, P., Jaffe, H., and Safar, J. (1996) Anal. Biochem. 236, 63-73) and recombinant prion protein (PrP) have been found to be in monomeric form (Mehlhorn, I., Groth, D., Stockel, J., Moffat, B., Reilly, D., Yansura, D., Willet, W. S., Baldwin, M., Fletterick, R., Cohen, F. E., Vandlen, R., Henner, D., and Prusiner, S. B. (1996) Biochemistry 35, 5528-5537; and this paper), and therefore PrP(C)-PrP(C) interactions were previously unknown. In this report we confirm recombinant PrP to be a monomer by analytical ultracentrifugation. However, by three lines of evidence (enzyme-linked immunosorbent assay (ELISA), cross-linking experiments, and size exclusion chromatography) we could also demonstrate that, under native conditions, at least part of the native bovine PrP(C) exists as a monomer-dimer equilibrium. A bovine PrP(C)-specific immuno-sandwich ELISA was developed and calibrated with recombinant PrP (Meyer, R. K., Oesch, B., Fatzer, R., Zurbriggen, A., and Vandevelde, M. (1999) J. Virol. 73, 9386-9392). By this ELISA we identified a distinct PrP(C) fraction and partially purified this protein. When serial dilutions of brain homogenate or partially purified PrP(C) were measured, using the peptide antibody C15S, a nonlinear dose-response curve was obtained. This nonlinearity was shown not to be due to an artifact of the procedure but to a monomer-dimer equilibrium of PrP(C) with preferential binding of the antibody to the dimer. From the curvature we could deduce the association constant (3.9 x 10(8) M(-1) at 37 degrees C). Accordingly, DeltaG degrees of the reaction was calculated (-48.6 kJ M(-1)), and DeltaH degrees (9.5 kJ M(-1)) as well as DeltaS degrees (0.2 kJ K(-1) M(-1)) were extrapolated from the van't Hoff plot. When serial dilutions of monomeric recombinant PrP were tested, only a straight line was obtained, supporting our hypothesis. Additional evidence of dimer formation was revealed by Western blotting of partially purified PrP(C) cross-linked by the homobifunctional cross-linker BS(3). Finally, size exclusion chromatography of partially purified PrP(C) fractions revealed an additional shoulder not observed with recombinant PrP. The difference in respect of dimer formation between native PrP(C) and recombinant PrP could be explained by the lack of glycosylation of the latter.

PrPSc-like prion protein peptide inhibits the function of cellular prion protein

Mice lacking expression of the prion protein are protected against infection with prion disease. Neurodegeneration in prion disease requires the formation of the abnormal isoform of the prion protein (PrP(Sc)) from host prion protein. Therefore expression of normal host prion protein is necessary for prion disease. In the present investigation, it was demonstrated that PrP(Sc) and a peptide resembling PrP(Sc), PrP106-126, both bind to cellular prion protein at amino acid residues 112-119. Interaction between PrP106-126 and the prion protein strips the prion protein from cells. Direct interaction of PrP106-126 with the prion protein was found to make cells more susceptible to copper toxicity, inhibited copper uptake into cells and inhibited the superoxide dismutase-like activity of the prion protein. Direct inhibition of prion protein function by PrP(Sc) may be necessary for neurodegeneration in prion disease.

Design of peptides undergoing self-catalytic alpha-to-beta transition and amyloidogenesis

Improved understanding of amyloidogenic peptides and proteins such as prion proteins and Alzheimer's beta peptides has attracted much attention to the elucidation of the molecular mechanisms of such amyloidogenesis. As a representative, in the prion protein, the conformational transitions from alpha-helix to beta-structure undergo along with the amyloidogenesis in a self-catalytic manner. Moreover, recent studies by the de novo design of peptides and proteins as well as the amyloidogenesis of peptides and proteins including pathogenic protein mutants have provided insight into the conformational changes essential to amyloidogenesis and correct folding.