Binding of prion proteins to lipid membranes

Structural characterization of a neurotoxic threonine-rich peptide corresponding to the human prion protein alpha 2-helical 180-195 segment, and comparison with full-length alpha 2-helix-derived peptides

The 173-195 segment corresponding to the helix 2 of the globular PrP domain is a good candidate to be one of the several 'spots' of intrinsic structural flexibility, which might induce local destabilization and concur to protein transformation, leading to aggregation-prone conformations. Here, we report CD and NMR studies on the alpha2-helix-derived peptide of maximal length (hPrP[180-195]) that is able to exhibit a regular structure different from the prevalently random arrangement of other alpha2-helix-derived peptides. This peptide, which has previously been shown to be affected by buffer composition via the ion charge density dependence typical of Hofmeister effects, corresponds to the C-terminal sequence of the PrP(C) full-length alpha2-helix and includes the highly conserved threonine-rich 188-195 segment. At neutral pH, its conformation is dominated by beta-type contributions, which only very strong environmental modifications are able to modify. On TFE addition, an increase of alpha-helical content can be observed, but a fully helical conformation is only obtained in neat TFE. However, linking of the 173-179 segment, as occurring in wild-type and mutant peptides corresponding to the full-length alpha2-helix, perturbs these intrinsic structural propensities in a manner that depends on whether the environment is water or TFE. Overall, these results confirm that the 180-195 parental region in hPrP(C) makes a strong contribution to the chameleon conformational behavior of the segment corresponding to the full-length alpha2-helix, and could play a role in determining structural rearrangements of the entire globular domain.

Deep membrane insertion of prion protein upon reduction of disulfide bond

The membrane may play a role in the pathogenesis of the prion protein (PrP). Cytoplasmic expression of PrP causes the conversion of PrP to a self-perpetuating PrP(Sc)-like conformation and the interaction of polypeptide chain with the hydrophobic core of the membrane is believed to be closely correlated with neurodegeneration. However, it is still elusive what factors govern the membrane interaction of PrP. Here, we show that PrP penetrates deeply into the membrane when the single disulfide bond is reduced, which results in membrane disruption and leakage. The proteinase K treatment and the fluorescence quenching assays showed that a predicted transmembrane domain of PrP penetrates into the membrane when the disulfide bond was reduced. Therefore, the oxidation state of PrP might be an important factor that influences its neurotoxicity or pathogenesis.

Misfolding of the prion protein: linking biophysical and biological approaches

Prion diseases are a group of neurodegenerative diseases that can arise spontaneously, be inherited, or acquired by infection in mammals. The propensity of the prion protein to adopt different structures is a clue to its pathological and perhaps biological role too. While the normal monomeric PrP is well characterized, the misfolded conformations responsible for neurodegeneration remain elusive despite progress in this field. Both structural dynamics and physico-chemical approaches are thus fundamental for a better knowledge of the molecular basis of this pathology. Indeed, multiple misfolding pathways combined with extensive posttranslational modifications of PrP and probable interaction(s) with cofactors call for a combination of approaches. In this review, we outline the current physico-chemical knowledge explaining the conformational diversities of PrP in relation with postulated or putative cellular partners such as proteic or non-proteic ligands.

Analysis of PrP conformation using circular dichroism

The availability of recombinant prion proteins (recPrP) has been exploited as a model system to study PrP-mediated toxicity, conversion, and infectivity. According to the protein only hypothesis, the central event in the pathogenesis of prion diseases is the conversion of PrP(C) to PrP(Sc). This involves a dramatic increase in beta sheet conformation as PrP(C) is converted to PrP(Sc), and it is widely believed that this conformational change affects the as-yet undefined function of PrP(C). Although there are many methods available to monitor for the changes in the structural makeup of PrP mutants and oligomers formed with respect to disease relevance, circular dichroism is one of the most popular methods used. In this chapter, we discuss the fundamental principles of circular dichroism and its current role and applications in prion disease research.

Selective incorporation of polyanionic molecules into hamster prions

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

Role of lipid rafts in the processing of the pathogenic prion and Alzheimer's amyloid-beta proteins

The conformational conversion of the cellular form of the prion protein (PrP C) into the infectious form (PrP Sc) and the proteolytic processing of the amyloid-beta (Abeta) peptide are central pathogenetic events in the prion diseases and Alzheimer's disease, respectively. Cholesterol- and sphingolipid-rich lipid rafts have emerged as important sites for the conversion of PrP C into PrP Sc, and for the proteolytic production, degradation and aggregation of Abeta. Here, we discuss these findings and their implications for our understanding of these disease processes. In addition, the potential for rafts as sites for therapeutic intervention in prion diseases and Alzheimer's disease is considered.

Semisynthetic Murine Prion Protein Equipped with a GPI Anchor Mimic Incorporates into Cellular Membranes

Conversion of cellular prion protein (PrP(C)) into the pathological conformer (PrP(Sc)) has been studied extensively by using recombinantly expressed PrP (rPrP). However, due to inherent difficulties of expressing and purifying posttranslationally modified rPrP variants, only a limited amount of data is available for membrane-associated PrP and its behavior in vitro and in vivo. Here, we present an alternative route to access lipidated mouse rPrP (rPrP(Palm)) via two semisynthetic strategies. These rPrP variants studied by a variety of in vitro methods exhibited a high affinity for liposomes and a lower tendency for aggregation than rPrP. In vivo studies demonstrated that double-lipidated rPrP is efficiently taken up into the membranes of mouse neuronal and human epithelial kidney cells. These latter results enable experiments on the cellular level to elucidate the mechanism and site of PrP-PrP(Sc) conversion.

Antiprion activity of cholesterol esterification modulators: a comparative study using ex vivo sheep fibroblasts and lymphocytes and mouse neuroblastoma cell lines

Our studies on the role of cholesterol homeostasis in the pathogenesis of scrapie revealed abnormal accumulation of cholesterol esters in ex vivo peripheral blood mononuclear cells (PBMCs) and skin fibroblasts from healthy and scrapie-affected sheep carrying a scrapie-susceptible genotype compared to sheep with a resistant genotype. Similar alterations were observed in mouse neuroblastoma N2a cell lines persistently infected with mouse-adapted 22L and RML strains of scrapie that showed up to threefold-higher cholesterol ester levels than parental N2a cells. We now report that proteinase K-resistant prion protein (PrPres)-producing cell populations of subclones from scrapie-infected cell lines were characterized by higher cholesterol ester levels than clone populations not producing PrPres. Treatments with a number of drugs known to interfere with different steps of cholesterol metabolism strongly reduced the accumulation of cholesterol esters in ex vivo PBMCs and skin fibroblasts from scrapie-affected sheep but had significantly less or no effect in their respective scrapie-resistant or uninfected counterparts. In scrapie-infected N2a cells, inhibition of cholesterol esters was associated with selective antiprion activity. Effective antiprion concentrations of cholesterol modulators (50% effective concentration [EC(50)] range, 1.4 to 40 microM) were comparable to those of antiprion reference compounds (EC(50) range, 0.6 to 10 microM). These data confirm our hypothesis that abnormal accumulation of cholesterol esters may represent a biological marker of susceptibility to prion infection/replication and a novel molecular target of potential clinical importance.

Canadian Association of Neurosciences Review: prion protein and prion diseases: the good and the bad

In the 1700's a strange new disease affecting sheep was recognized in Europe. The disease later became known as "Scrapie" and was the first of a family of similar diseases affecting a number of species that are now known as the Transmissible Spongiform Encephalopathies (TSEs). The appearance of a new disease in humans linked to the consumption of meat products from infected cattle has stimulated widespread public concern and scientific interest in the prion protein and related diseases. Nearly 300 years after the first report, these diseases still merit the descriptor "strange". This family of diseases is characterized by a unique profile of histological changes, can be transmitted as inherited or acquired diseases, as well as apparent sporadic spontaneous generation of the disease. These diseases are believed by many, to be caused by a unique protein only infectious agent. The "prion protein" (PrPC), a term first coined by Stanley Prusiner in 1982 is crucial to the development of these diseases, apparently by acting as a substrate for an abnormal disease associated form. However, aside from being critical to the pathogenesis of the disease, the function of PrPC, which is expressed in all mammals, has defied definitive description. Several roles have been proposed on the basis of in vitro studies, however, thus far, in vivo confirmation has not been forthcoming. The biological features of PrPC also seem to be unusual. Numerous mouse models have been generated in an attempt to understand the pathogenesis of these diseases. This review summarizes the current state of histological features, the etiologic agent, the normal metabolism and the function of the prion protein, as well as the limitations of the mouse models.

Formation of native prions from minimal components in vitro

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

The infectivity of transmissible spongiform encephalopathy agent at low doses: the importance of phospholipid

The issue of whether the mechanism of infection is independent or co-operative for low doses of transmissible spongiform encephalopathy (TSE) agent is critical for risk assessment. The susceptibility (and hence ID(50)) of individuals with the same prion protein (PrP) genotype may vary considerably with a small proportion being very susceptible. Assuming independent action, the incubation period (IP) would continue to increase until the dose is below the ID(50) of the most susceptible individuals in the experiment, at which point it would become constant. This may explain the observed increase in IP with decreasing dose below the apparent ID(50) in experiments with untreated TSE agent. In contrast, IPs for autoclaved or NaOH-treated TSE agent increase greatly at doses <ID(50) suggesting strong co-operative action, or even a threshold. It is proposed here that the unit of infectivity for prion disease is a nucleation seed comprised of PrP and host phospholipid (PL). PL would play a structural role through mediating protein/lipid interactions with PrP. Heating or alkali treatment destroys the PL breaking up the nucleation seeds, which require long IPs to reform at low doses. Replenishing those inactivated PLs with host PL would explain how the phenotypic effect of long IP at low dose is lost on subpassage. It is proposed here that strain thermostability is controlled by the nature and strength of the PrP/PL interactions, which are independent of the host PrP genotype. Although repeated oral exposure to low doses of scrapie is less harmful than a single large exposure, this effect may reflect interference by competition rather than diminished risks due to a co-operative effect, and is of little importance for 'one-off' low-dose environmental exposures.

Interaction of the cellular prion protein with raft-like lipid membranes

Conversion of the cellular isoform of the prion protein (PrP(C)) into the disease-associated isoform (PrP(Sc)) plays a key role in the development of prion diseases. Within its cellular pathway, PrP(C) undergoes several posttranslational modifications, i.e., the attachment of two N-linked glycans and a glycosyl phosphatidyl inositol (GPI) anchor, by which it is linked to the plasma membrane on the exterior cell surface. To study the interaction of PrP(C) with model membranes, we purified posttranslationally modified PrP(C) from transgenic Chinese hamster ovary (CHO) cells. The mono-, di- and oligomeric states of PrP(C) free in solution were analyzed by analytical ultracentrifugation. The interaction of PrP(C) with model membranes was studied using both lipid vesicles in solution and lipid bilayers bound to a chip surface. The equilibrium and mechanism of PrP(C) association with the model membranes were analyzed by surface plasmon resonance. Depending on the degree of saturation of binding sites, the concentration of PrP(C) released from the membrane into aqueous solution was estimated at between 10(-9) and 10(-7) M. This corresponds to a free energy of the insertion reaction of -48 kJ/mol. Consequences for the conversion of PrP(C) to PrP(Sc) are discussed.

The reconstitution of mammalian prion infectivity de novo

The discovery of prion disease transmission in mammals, as well as a non-Mendelian type of inheritance in yeast, has led to the establishment of a new concept in biology, the prion hypothesis. The prion hypothesis postulates that an abnormal protein conformation propagates itself in an autocatalytic manner using the normal isoform of the same protein as a substrate and thereby acts either as a transmissible agent of disease (in mammals), or as a heritable determinant of phenotype (in yeast and fungus). While the prion biology of yeast and fungus supports this idea strongly, the direct proof of the prion hypothesis in mammals, specifically the reconstitution of the disease-associated isoform of the prion protein (PrP(Sc)) in vitro de novo from noninfectious prion protein, has been difficult to achieve despite many years of effort. The present review summarizes our current knowledge about the biochemical nature of the prion infectious agent and structure of PrP(Sc), describes potential strategies for generating prion infectivity de novo and provides some insight on why the reconstitution of infectivity has been difficult to achieve in vitro. Several hypotheses are proposed to explain the apparently low infectivity of the first generation of recently reported synthetic mammalian prions.

The role of rafts in the fibrillization and aggregation of prions

A key molecular event in prion diseases is the conversion of the prion protein (PrP) from its normal cellular form (PrP(C)) to the disease-specific form (PrP(Sc)). The transition from PrP(C) to PrP(Sc) involves a major conformational change, resulting in amorphous aggregates and/or fibrillar amyloid deposits. Here several lines of evidence implicating membranes in the conversion of PrP are reviewed with a particular emphasis on the role of lipid rafts in the conformational transition of prion proteins. New correlations between in vitro biophysical studies and findings from cell biology work on the role of rafts in prion conversion are highlighted and a mechanism for the role of rafts in prion conversion is proposed.

Surface plasmon resonance in protein-membrane interactions

Surface plasmon resonance (SPR) has become one of the most important techniques for studying macromolecular interactions. The most obvious advantages of SPR over other techniques are: direct and rapid determination of association and dissociation rates of binding process, no need for labelling of protein or lipids, and small amounts of sample used in the assay (often nM concentrations of proteins). In biochemistry, SPR is used mainly to study protein-protein interactions. On the other hand, protein-membrane interactions, although crucial for many cell processes, are less well studied. Recent advances in the preparation of stable membrane-like surfaces and the commercialisation of sensor chips has enabled widespread use of SPR in protein-membrane interactions. One of the most popular is Biacore's L1 sensor chip that allows capture of intact liposomes or even subcellular preparations. Lipid specificity of protein-membrane interactions can, therefore, be easily studied by manipulating the lipid composition of the immobilised membrane. The number of published papers has increased steadily in the last few years and the examples include domains or proteins that participate in cell signalling, pore-forming proteins, membrane-interacting peptides, coagulation factors, enzymes, amyloidogenic proteins, prions, etc. This paper gives a brief overview of different membrane-mimetic surfaces that can be prepared on the surface of SPR chips, properties of liposomes on the surface of L1 chips and some selected examples of protein-membrane interactions studied with such system.

The interaction between cytoplasmic prion protein and the hydrophobic lipid core of membrane correlates with neurotoxicity

Prion protein (PrP), normally a cell surface protein, has been detected in the cytosol of a subset of neurons. The appearance of PrP in the cytosol could result from either retro-translocation of misfolded PrP from the endoplasmic reticulum (ER) or impaired import of PrP into the ER. Transgenic mice expressing cytoplasmic PrP (cyPrP) developed neurodegeneration in cerebellar granular neurons, although no detectable pathology was observed in other brain regions. In order to understand why granular neurons in the cerebellum were most susceptible to cyPrP-induced degeneration, we investigated the subcellular localization of cyPrP. Interestingly, we found that cyPrP is membrane-bound. In transfected cells, it binds to the ER and plasma/endocytic vesicular membranes. In transgenic mice, it is associated with synaptic and microsomal membranes. Furthermore, the cerebellar neurodegeneration in transgenic mice correlates with the interaction between cyPrP and the hydrophobic lipid core of the membrane but not with either the aggregation status or the dosage of cyPrP. These results suggest that lipid membrane perturbation could be a cellular mechanism for cyPrP-induced neurotoxicity and explain the seemingly conflicting results concerning cyPrP.

Synthesis and structural characterization of a mimetic membrane-anchored prion protein

During pathogenesis of transmissible spongiform encephalopathies (TSEs) an abnormal form (PrP(Sc)) of the host encoded prion protein (PrP(C)) accumulates in insoluble fibrils and plaques. The two forms of PrP appear to have identical covalent structures, but differ in secondary and tertiary structure. Both PrP(C) and PrP(Sc) have glycosylphospatidylinositol (GPI) anchors through which the protein is tethered to cell membranes. Membrane attachment has been suggested to play a role in the conversion of PrP(C) to PrP(Sc), but the majority of in vitro studies of the function, structure, folding and stability of PrP use recombinant protein lacking the GPI anchor. In order to study the effects of membranes on the structure of PrP, we synthesized a GPI anchor mimetic (GPIm), which we have covalently coupled to a genetically engineered cysteine residue at the C-terminus of recombinant PrP. The lipid anchor places the protein at the same distance from the membrane as does the naturally occurring GPI anchor. We demonstrate that PrP coupled to GPIm (PrP-GPIm) inserts into model lipid membranes and that structural information can be obtained from this membrane-anchored PrP. We show that the structure of PrP-GPIm reconstituted in phosphatidylcholine and raft membranes resembles that of PrP, without a GPI anchor, in solution. The results provide experimental evidence in support of previous suggestions that NMR structures of soluble, anchor-free forms of PrP represent the structure of cellular, membrane-anchored PrP. The availability of a lipid-anchored construct of PrP provides a unique model to investigate the effects of different lipid environments on the structure and conversion mechanisms of PrP.

Detergent-resistant membrane domains but not the proteasome are involved in the misfolding of a PrP mutant retained in the endoplasmic reticulum

Inherited prion diseases are neurodegenerative pathologies related to genetic mutations in the prion protein (PrP) gene, which favour the conversion of PrP(C) into a conformationally altered pathogenic form, PrP(Sc). The molecular basis of PrP(C)/PrP(Sc) conversion, the intracellular compartment where it occurs and how this process leads to neurological dysfunction are not yet known. We have studied the intracellular synthesis, degradation and localization of a PrP mutant associated with a genetic form of Creutzfeldt-Jakob disease (CJD), PrPT182A, in transfected FRT cells. PrPT182A is retained in the endoplasmic reticulum (ER), is mainly associated with detergent-resistant microdomains (DRMs) and is partially resistant to proteinase K digestion. Although an untranslocated form of this mutant is polyubiquitylated and undergoes ER-associated degradation, the proteasome is not responsible for the degradation of its misfolded form, suggesting that it does not have a role in the pathogenesis of inherited diseases. On the contrary, impairment of PrPT182A association with DRMs by cholesterol depletion leads to its accumulation in the ER and substantially increases its misfolding. These data support the previous hypothesis that DRMs are important for the correct folding of PrP and suggest that they might have a protective role in pathological scrapie-like conversion of PrP mutants.

The prion protein and lipid rafts

Prions are the causative agent of the transmissible spongiform encephalopathies, such as Creutzfeldt-Jakob disease in humans. In these prion diseases the normal cellular form of the prion protein (PrP(C)) undergoes a post-translational conformational conversion to the infectious form (PrP(Sc)). PrP(C) associates with cholesterol- and glycosphingolipid-rich lipid rafts through association of its glycosyl-phosphatidylinositol (GPI) anchor with saturated raft lipids and through interaction of its N-terminal region with an as yet unidentified raft associated molecule. PrP(C) resides in detergent-resistant domains that have different lipid and protein compositions to the domains occupied by another GPI-anchored protein, Thy-1. In some cells PrP(C) may endocytose through caveolae, but in neuronal cells, upon copper binding to the N-terminal octapeptide repeats, the protein translocates out of rafts into detergent-soluble regions of the plasma membrane prior to endocytosis through clathrin-coated pits. The current data suggest that the polybasic region at its N-terminus is required to engage PrP(C) with a transmembrane adaptor protein which in turn links with the clathrin endocytic machinery. PrP(C) associates in rafts with a variety of signalling molecules, including caveolin-1 and Fyn and Src tyrosine kinases. The clustering of PrP(C) triggers a range of signal transduction processes, including the recruitment of the neural cell adhesion molecule to rafts which in turn promotes neurite outgrowth. Lipid rafts appear to be involved in the conformational conversion of PrP(C) to PrP(Sc), possibly by providing a favourable environment for this process to occur and enabling disease progression.

Thermotropic phase behaviour of the pseudobinary mixtures of DPPC/C12E5 and DMPC/C12E5 determined by differential scanning calorimetry and ultrasonic velocimetry

The present paper reports on the phase behaviour of the pseudobinary aqueous mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/pentaethylene glycol monododecyl ether (C12E5) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine monohydrate (DMPC)/C12E5. Both systems exhibit a variety of mesophases, such as lamellar gel, liquid crystalline and micellar phases. The phase diagrams show peritectic and eutectic behaviours. The existence of a compound complex is established. From the phase diagrams, the temperature dependence of the solubilisation parameters is obtained. The phase diagrams, especially with respect to the solubilisation process were qualitatively explained assuming that the packing of the constituents plays a dominating role. Finally, differential scanning calorimetry and ultrasonic velocimetry are compared concerning their potentials to determine characteristics of phase transitions in pseudobinary phospholipid/surfactant mixtures.

Gerstmann-Sträussler-Scheinker Disease Amyloid Protein Polymerizes According to the “Dock-and-Lock” Model

Prion protein (PrP) amyloid formation is a central feature of genetic and acquired prion diseases such as Gerstmann-Sträussler-Scheinker disease (GSS) and variant Creutzfeldt-Jakob disease. The major component of GSS amyloid is a PrP fragment spanning residues approximately 82-146, which when synthesized as a peptide, readily forms fibrils featuring GSS amyloid. The present study employed surface plasmon resonance (SPR) to characterize the binding events underlying PrP82-146 oligomerization at the first stages of fibrillization, according to evidence suggesting a pathogenic role of prefibrillar oligomers rather than mature amyloid fibrils. We followed in real time the binding reactions occurring during short term (seconds) addition of PrP82-146 small oligomers (1-5-mers, flowing species) onto soluble prefibrillar PrP82-146 aggregates immobilized on the sensor surface. SPR data confirmed very efficient aggregation/elongation, consistent with the hypothesis of nucleation-dependent polymerization process. Much lower binding was observed when PrP82-146 flowed onto the scrambled sequence of PrP82-146 or onto prefibrillar Abeta42 aggregates. As previously found with Abeta40, SPR data could be adequately fitted by equations modeling the "dock-and-lock" mechanism, in which the "locking" step is due to sequential conformational changes, each increasing the affinity of the monomer for the fibril until a condition of irreversible binding is reached. However, these conformational changes (i.e. the locking steps) appear to be faster and easier with PrP82-146 than with Abeta40. Such differences suggest that PrP82-146 has a greater propensity to polymerize and greater stability of the aggregates.

Correlative studies support lipid peroxidation is linked to PrPres propagation as an early primary pathogenic event in prion disease

To assess whether heightened oxidative stress plays an early and primary pathogenic role in transmissible spongiform encephalopathies (TSE), we undertook detailed correlative studies using a mouse-adapted model of human disease. The spatio-temporal evolution of the abnormal, protease-resistant isoform of the prion protein (PrP(res)) and neuropathological changes were correlated with the occurrence and type of oxidative stress. Heightened oxidative stress was demonstrated, but restricted to elevated levels of free aldehydic breakdown products of lipid peroxidation, affecting all brain regions to varying extents. The increase in lipid peroxidation was highest over the mid-incubation period, with the onset showing close temporal and general topographical concordance with the first detection of PrP(res) with both pre-empting the typical neuropathological changes of spongiform change, gliosis and neuronal loss. Further, prion propagation over the disease course was assessed using murine bioassay. This revealed that the initial rapid increase in infectivity titres was contemporaneous with the abrupt onset and maximisation of lipid peroxidation. The present results are an important extension to previous studies, showing that heightened oxidative stress in the form of lipid peroxidation is likely to constitute an early primary pathogenic event in TSE, associated temporally with the integral disease processes of prion propagation and PrP(res) formation, and consistent with causal links between these events and subsequent typical neuropathological changes.

Fate of Prions in Soil: Adsorption Kinetics of Recombinant Unglycosylated Ovine Prion Protein onto Mica in Laminar Flow Conditions and Subsequent Desorption

Prions can be disseminated in soils. Their interaction with soil minerals is a key factor for the assessment of risks associated with the transport of their infectivity. We did not examine here the infectivity itself but the adsorption kinetics of an ovine recombinant prion protein (ovine PrPrec), as a noninfectious model protein, on muscovite mica, a phyllosilicate with surface properties analogous to soil clays, in conditions of laminar flow through a channel. The protein was labeled with (125)I, and its adsorption examined between pH 4.0 and 9.0. At wall shear rate 100 s(-1), we found the process to be controlled mainly by transport at the beginning of the adsorption process. Additional experiments at 1000 s(-1) (pH 5 and 6) determined that the diffusion coefficient was in accordance with the hydrodynamic radius measured by size exclusion chromatography. The pseudo-plateau of the interfacial concentration with time was compatible with more than a monolayer and suggests the presence of aggregates. Desorption was not observed in the presence of buffer between pH 4 and 9 and sheep plasma, while the addition of alkaline detergent or 10(-1) M NaOH allowed an almost complete removal from the interface. The ensemble of results suggests that the largely irreversible adsorption of the ovine PrPrec onto mica is mainly due to electrostatic attraction between the protein and the highly negatively charged mica surface. Possible consequences for the mode of dissemination of prion proteins in soils are indicated.

Local environmental effects on the structure of the prion protein

Prion diseases are neurodegenerative diseases causally linked to the partial unfolding and subsequent misfolding and aggregation of the prion protein (PrP). While most proteins fold into a single low energy state, PrP can fold into two distinct isoforms. In its innocuous state, denoted as PrPC, the protein has predominantly alpha-helical secondary structure, however, PrPC can misfold into an isoform rich in extended structure capable of forming toxic and infectious aggregates. While prion disease is believed to be a protein-only disease, one not requiring any non-protein elements for propagation, the different environments the protein finds itself in vivo likely influence its ability to misfold and aggregate. In this review we will examine various molecules, covalent modifications and environments PrP faces in vivo and the effect they have on PrP's local environment and, potentially, conformation. Included in this discussion are: (1) pH, (2) carbohydrates, (3) lipid membranes, (4) metal ions, and (5) small molecules.

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

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

An unusual soluble beta-turn-rich conformation of prion is involved in fibril formation and toxic to neuronal cells

A key molecular event in prion diseases is the conversion of the prion protein (PrP) from its normal cellular form (PrPC) to the disease-specific form (PrPSc). The transition from PrPC to PrPSc involves a major conformational change, resulting in amorphous protein aggregates and fibrillar amyloid deposits with increased beta-sheet structure. Using recombinant PrP refolded into a beta-sheet-rich form (beta-PrP) we have studied the fibrillization of beta-PrP both in solution and in association with raft membranes. In low ionic strength thick dense fibrils form large networks, which coexist with amorphous aggregates. High ionic strength results in less compact fibrils, that assemble in large sheets packed with globular PrP particles, resembling diffuse aggregates found in ex vivo preparations of PrPSc. Here we report on the finding of a beta-turn-rich conformation involved in prion fibrillization that is toxic to neuronal cells in culture. This is the first account of an intermediate in prion fibril formation that is toxic to neuronal cells. We propose that this unusual beta-turn-rich form of PrP may be a precursor of PrPSc and a candidate for the neurotoxic molecule in prion pathogenesis.

Survey of the year 2004 commercial optical biosensor literature

The year 2004 represents a milestone for the biosensor research community: in this year, over 1000 articles were published describing experiments performed using commercially available systems. The 1038 papers we found represent an approximately 10% increase over the past year and demonstrate that the implementation of biosensors continues to expand at a healthy pace. We evaluated the data presented in each paper and compiled a 'top 10' list. These 10 articles, which we recommend every biosensor user reads, describe well-performed kinetic, equilibrium and qualitative/screening studies, provide comparisons between binding parameters obtained from different biosensor users, as well as from biosensor- and solution-based interaction analyses, and summarize the cutting-edge applications of the technology. We also re-iterate some of the experimental pitfalls that lead to sub-optimal data and over-interpreted results. We are hopeful that the biosensor community, by applying the hints we outline, will obtain data on a par with that presented in the 10 spotlighted articles. This will ensure that the scientific community at large can be confident in the data we report from optical biosensors.

Characterization of recombinant, membrane-attached full-length prion protein

An abnormal isoform, PrP(Sc), of the normal cellular prion protein (PrP(C)) is the major component of the causative agent of prion diseases. Both isoforms were found to possess the same covalent structures, including a C-terminal glycosylphosphatidylinositol anchor, but different secondary and tertiary structures. In this study, a variant of full-length PrP with an unpaired cysteine at the C terminus was recombinantly produced in Escherichia coli, covalently coupled to a thiol-reactive phospholipid, and incorporated into liposomes to serve as a model for studying possible changes in structure and stability of recombinant PrP upon membrane attachment. Covalent coupling of PrP to liposomes did not result in significant structural changes observable by far-UV circular dichroism. Moreover, limited proteolysis experiments failed to detect changes in the stability of liposome-bound PrP relative to soluble PrP. These data suggest that the requirement of raft localization for the PrP(C) to PrP(Sc) conversion, observed previously in cell culture models, is not because of a direct influence of raft lipids on the structure and stability of membranebound PrP(C) but caused by other factors, e.g. increased local PrP concentrations or high effective concentrations of membrane-associated conversion factors. The availability of recombinant PrP covalently attached to liposomes provides the basis for systematic in vitro conversion assays with recombinant PrP on the surface of membranes. In addition, our results indicate that the three-dimensional structure of mammalian PrP(C) in membranes is identical to that of recombinant PrP in solution.