Structural evidence for the critical role of the prion protein hydrophobic region in forming an infectious prion

Prion or PrPSc is the proteinaceous infectious agent causing prion diseases in various mammalian species. Despite decades of research, the structural basis for PrPSc formation and prion infectivity remains elusive. To understand the role of the hydrophobic region in forming infectious prion at the molecular level, we report X-ray crystal structures of mouse (Mo) prion protein (PrP) (residues 89-230) in complex with a nanobody (Nb484). Using the recombinant prion propagation system, we show that the binding of Nb484 to the hydrophobic region of MoPrP efficiently inhibits the propagation of proteinase K resistant PrPSc and prion infectivity. In addition, when added to cultured mouse brain slices in high concentrations, Nb484 exhibits no neurotoxicity, which is drastically different from other neurotoxic anti-PrP antibodies, suggesting that the Nb484 can be a potential therapeutic agent against prion disease. In summary, our data provides the first structure-function evidence supporting a crucial role of the hydrophobic region of PrP in forming an infectious prion.

Evaluation of a combinatorial approach to prion inactivation using an oxidizing agent, SDS, and proteinase K

BACKGROUND

Prions demonstrate an unusual resistance to methods effective at inactivating conventional microorganisms. This has resulted in a very tangible and difficult infection control challenge to the medical and veterinary communities, as well as animal agriculture and related industries. Currently accepted practices of harsh chemical treatments such as prolonged exposure to sodium hydroxide or sodium hypochlorite, or autoclaving are not suitable in many situations. Less caustic and more readily applicable treatments to contaminated environments are therefore desirable. We recently demonstrated that exposure of the RML scrapie agent to a commercial product containing sodium percarbonate (SPC-P) with or without sodium dodecyl sulfate (SDS) rendered PrP(Sc) sensitive to proteinase K (PK), but did not eliminate infectivity. The current study was designed to evaluate the efficacy of a combinatorial approach to inactivating prions by exposing RML-positive brain homogenate to SPC-P and SDS followed by PK. Treated samples were evaluated for PrP(Sc)-immunoreactivity by western blot, and residual infectivity by mouse bioassay.

RESULTS

Treatment of infected brain homogenate with SPC-P and SDS followed by PK exposure resulted in a 4-5 log10 reduction in infectivity when bioassayed in tga20 mice.

CONCLUSIONS

This study demonstrates that exposure of the RML scrapie agent to SPC-P and SDS followed by PK markedly reduces, but does not eliminate infectivity. The results of this study encourage further investigation into whether consecutive or concomitant exposure to sodium percarbonate, SDS, and a protease may serve as a viable and non-caustic option for prion inactivation.

Kinetics of ozone inactivation of infectious prion protein

The kinetics of ozone inactivation of infectious prion protein (PrP(Sc), scrapie 263K) was investigated in ozone-demand-free phosphate-buffered saline (PBS). Diluted infectious brain homogenates (IBH) (0.01%) were exposed to a predetermined ozone dose (10.8 ± 2.0 mg/liter) at three pHs (pH 4.4, 6.0, and 8.0) and two temperatures (4°C and 20°C). The inactivation of PrP(Sc) was quantified by determining the in vitro destruction of PrP(Sc) templating properties using the protein misfolding cyclic amplification (PMCA) assay and bioassay, which were shown to correlate well. The inactivation kinetics were characterized by both Chick-Watson (CW) and efficiency factor Hom (EFH) models. It was found that the EFH model fit the experimental data more appropriately. The efficacy of ozone inactivation of PrP(Sc) was both pH and temperature dependent. Based on the EFH model, CT (disinfectant concentration multiplied by contact time) values were determined for 2-log10, 3-log10, and 4-log10 inactivation at the conditions under which they were achieved. Our results indicated that ozone is effective for prion inactivation in ozone-demand-free water and may be applied for the inactivation of infectious prion in prion-contaminated water and wastewater.

Exposure of RML scrapie agent to a sodium percarbonate-based product and sodium dodecyl sulfate renders PrPSc protease sensitive but does not eliminate infectivity

BACKGROUND

Prions, the causative agents of the transmissible spongiform encephalopathies, are notoriously difficult to inactivate. Current decontamination recommendations by the World Health Organization include prolonged exposure to 1 N sodium hydroxide or > 20,000 ppm sodium hypochlorite, or autoclaving. For decontamination of large stainless steel surfaces and equipment as in abattoirs, for example, these methods are harsh or unsuitable. The current study was designed to evaluate the effectiveness of a commercial product containing sodium percarbonate to inactivate prions. Samples of mouse brain infected with a mouse-adapted strain of the scrapie agent (RML) were exposed to a sodium percarbonate-based product (SPC-P). Treated samples were evaluated for abnormal prion protein (PrPSc)-immunoreactivity by western blot analysis, and residual infectivity by mouse bioassay.

RESULTS

Exposure to a 21% solution of SPC-P or a solution containing either 2.1% or 21% SPC-P in combination with sodium dodecyl sulfate (SDS) resulted in increased proteinase K sensitivity of PrPSc. Limited reductions in infectivity were observed depending on treatment condition. A marginal effect on infectivity was observed with SPC-P alone, but an approximate 2-3 log10 reduction was observed with the addition of SDS, though exposure to SDS alone resulted in an approximate 2 log10 reduction.

CONCLUSIONS

This study demonstrates that exposure of a mouse-adapted scrapie strain to SPC-P does not eliminate infectivity, but does render PrPSc protease sensitive.

In vitro conversion and seeded fibrillization of posttranslationally modified prion protein

The conversion of the cellular isoform of the prion protein (PrP(C)) into the pathologic isoform (PrP(Sc)) is the key event in prion diseases. To study the conversion process, an in vitro system based on varying the concentration of low amounts of sodium dodecyl sulfate (SDS) has been employed. In the present study, the conversion of full-length PrP(C) isolated from Chinese hamster ovary cells (CHO-PrP(C)) was examined. CHO-PrP(C) harbors native, posttranslational modifications, including the GPI anchor and two N-linked glyco-sylation sites. The properties of CHO-PrP(C) were compared with those of full-length and N-terminally truncated recombinant PrP. As shown earlier with recombinant PrP (recPrP90-231), transition from a soluble α-helical state as known for native PrP(C) into an aggregated, β-sheet-rich PrP(Sc)-like state could be induced by dilution of SDS. The aggregated state is partially proteinase K (PK)-resistant, exhibiting a cleavage site similar to that found with PrP(Sc). Compared to recPrP (90-231), fibril formation with CHO-PrP(C) requires lower SDS concentrations (0.0075%), and can be drastically accelerated by seeding with PrP(Sc) purified from brain homogenates of terminally sick hamsters. Our results show that recPrP 90-231 and CHO-PrPC behave qualitatively similar but quantitatively different. The in vivo situation can be simulated closer with CHO-PrP(C) because the specific PK cleave site could be shown and the seed-assisted fibrillization was much more efficient.

Heparin enhances the cell-protein misfolding cyclic amplification efficiency of variant Creutzfeldt-Jakob disease

Highly sensitive in vitro screening tests are required to prevent the iatrogenic spread of variant Creutzfeldt-Jakob disease (vCJD). Protein misfolding cyclic amplification (PMCA) is a candidate for such a test, but the sensitivity of this method is insufficient. Polyanions were reported to enhance PMCA efficiency, but their effects on vCJD are unclear. We developed a cell-PMCA of vCJD, wherein cell lysate containing exogenously expressed human PrP was used as substrates, to investigate the effects of various sulfated polysaccharides on amplification efficiency. PrP(res) amounts after cell-PMCA were analyzed by western blotting. Heparin, dermatan sulfate, and dextran sulfate (average molecular weight [MW] 1400kDa) enhanced efficiency, but dextran sulfate (average MW 8kDa) and a heparin pentasaccharide analog had no effect. Pentosan polysulfate inhibited cell-PMCA reaction. The amplification efficiency of cell-PMCA of vCJD increased to >100-fold per round with heparin. The enhancing effects of heparin on cell-PMCA were seed dependent: it was high for vCJD, low for sporadic Creutzfeldt-Jakob disease, and low to negligible for hamster-adapted scrapie-derived 263K. In multi-round PMCA, signals were detected at earlier rounds with heparin than without heparin, and PrP(Sc) in 10(-10) diluted vCJD brain was detected by the sixth round. Heparin-assisted cell-PMCA of vCJD represents a significant step toward detecting very minute amounts of PrP(Sc) in the body fluids of asymptomatic vCJD patients.

Prion strain- and species-dependent effects of antiprion molecules in primary neuronal cultures

Transmissible spongiform encephalopathies (TSE) arise as a consequence of infection of the central nervous system by prions and are incurable. To date, most antiprion compounds identified by in vitro screening failed to exhibit therapeutic activity in animals, thus calling for new assays that could more accurately predict their in vivo potency. Primary nerve cell cultures are routinely used to assess neurotoxicity of chemical compounds. Here, we report that prion strains from different species can propagate in primary neuronal cultures derived from transgenic mouse lines overexpressing ovine, murine, hamster, or human prion protein. Using this newly developed cell system, the activity of three generic compounds known to cure prion-infected cell lines was evaluated. We show that the antiprion activity observed in neuronal cultures is species or strain dependent and recapitulates to some extent the activity reported in vivo in rodent models. Therefore, infected primary neuronal cultures may be a relevant system in which to investigate the efficacy and mode of action of antiprion drugs, including toward human transmissible spongiform encephalopathy agents.

Enzymatic detergent treatment protocol that reduces protease-resistant prion protein load and infectivity from surgical-steel monofilaments contaminated with a human-derived prion strain

The unconventional nature of the infectious agent of prion diseases poses a challenge to conventional infection control methodologies. The extraneural tissue distribution of variant and sporadic Creutzfeldt–Jakob disease has increased concern regarding the risk of prion disease transmission via general surgical procedures and highlighted the need for decontamination procedures that can be incorporated into routine processing. In this study, the ability of preparations of enzymatic medical instrument cleaners to reduce the infectivity associated with a rodent-adapted strain of human prion disease, previously reported to be resistant to decontamination, was tested. Efficient degradation of the disease-associated prion protein by enzymatic cleaning preparations required high treatment temperatures (50–60 °C). Standard decontamination methods (1 M NaOH for 1 h or autoclaving at 134 °C for 18 min) reduced infectivity associated with the human-derived prion strain by less than 3 log10 LD50. In contrast, a 30 min treatment with the optimized enzymatic cleaning preparation protocols reduced infectivity by more than 3 log10 LD50 and when used in conjunction with autoclave cycles eliminated detectable levels of infectivity. The development of prion decontamination procedures that are compatible with routine cleaning and sterilization of medical and surgical instruments may reduce the risk of the transmission of prion disease in general surgery.

Copaxone interferes with the PrP Sc-GAG interaction

The hallmark of prion disease-induced neurodegeneration is the accumulation of PrP(Sc), a misfolded form of PrP(C). In addition, several lines of evidence indicate a role for the immune system and, in particular, inflammation in prion disease pathogenesis. In this work, we tested whether Copaxone, an immunomodulatory agent currently used for the treatment of multiple sclerosis, can affect prion disease manifestation in scrapie-infected hamsters. We show here that Copaxone exerted no effect on prion disease incubation time when treatment commenced 2 weeks after i.p. prion infection. However, when Copaxone was mixed with the initial prion inoculum or administered to hamsters weekly starting on the day of infection, prion disease incubation time was prolonged by 30 days. This suggests that Copaxone may affect the initial infection process. In vitro experiments indicate that Copaxone significantly reduced PrP(Sc) binding to both Chinese hamster ovary (CHO) cells and heparin beads and also binds to heparin by itself. Interestingly, Copaxone also abolished PrP(Sc) accumulation in scrapie-infected cells. We propose that Copaxone delays prion infection by competing with the PrP(Sc)-glycosaminoglycans interaction. Whether the immunomodulating activity of Copaxone is related to its heparin binding and anti-prion properties remains to be established.

A chimeric ligand approach leading to potent antiprion active acridine derivatives: design, synthesis, and biological investigations

Human transmissible neurodegenerations including Creutzfeldt-Jakob disease are unique, since they are caused by prions, an infectious agent that replicates without nucleic acids but instead by inducing conversion of a host-resident normal prion protein to a misfolded conformational isoform. For pharmacotherapy of these unusual diseases, tricyclic heterocyclic compounds such as quinacrine have been considered, but with ambiguous success in vivo, so far. On the basis of the synergistic antiprion effects of quinacrine and iminodibenzyl derivatives, we introduce a novel class of potential pharmaceuticals representing structural chimeras of quinacrine and imipramine analogues. We describe the chemical synthesis and bioassays of a focused library of these compounds. The most potent target compound 2a revealed an EC(50) value of 20 nM determined with a cell model of prion disease, thus substantially improving the antiprion efficacy of quinacrine.

Titanium dioxide photocatalytic inactivation of prions

Prions are postulated to be the infectious agents of a family of transmissible, fatal, neurodegenerative disorders affecting both humans and animals. The possibility of prion transmission constitutes a public-health risk that confronts regulatory authorities everywhere. The main problem in handling prions is the fact that they are extremely resistant to standard decontamination methods. Thus, the use of harsh and expensive practices to destroy prions is inevitable. The development of applicable and efficient prion-inactivation practices is still highly important for the prevention of accidental transmission. In the search for effective and environmentally friendly methods to eliminate organic compounds and bacteria, much attention has been focused on the so-called advanced oxidation processes. These are based on the formation of hydroxyl radicals, which are known to possess a high reductive potential. This study tested the potential of titanium dioxide, an inexpensive and completely inert reagent, to inactivate prions in a heterogeneous photocatalytic process. Initial in vitro experiments were followed by a bioassay with the scrapie strain 263K in Syrian hamsters. The results obtained from this study indicate that titanium dioxide photocatalytic treatment of scrapie-infected brain homogenates reduces infectivity titres significantly.

Activity of an alkaline 'cleaner' in the inactivation of the scrapie agent

The capacity of a routinely available alkaline cleaner for medical devices to inactivate the causative agent of a transmissible spongiform encephalopathy (TSE) was tested. The co-incubation of brain homogenates, prepared from terminally ill scrapie-infected hamsters, with the cleaner led to the denaturation of misfolded protein as the proteinase K-resistant prion protein was no longer detectable after such treatment. In addition, intra-cerebral inoculation of hamsters with the alkaline cleaner-treated and subsequently neutralized samples reduced the level of infectivity of the material below the limit of detection. This report shows the possibility that a routinely available alkaline cleaner could reduce the infectivity of TSE agents and so minimize the risk of iatrogenic transmission of TSEs by asymptomatic carriers. This study is intended to encourage further investigations in this field.

Acid inactivation of prions: efficient at elevated temperature or high acid concentration

Scrapie prion rods isolated from hamster and non-infectious aggregates of the corresponding recombinant protein rPrP(90–231) were incubated with hydrochloric acid. The amount of PrP and of infectivity that survived incubation in HCl at varying times, acid concentrations and temperatures was quantified by Western blot densitometry and bioassays, respectively. Prion rods and rPrP aggregates showed similar HCl hydrolysis kinetics of PrP, indicating structural homology. For 1 M HCl and 25 °C, the rate of PrP hydrolysis follows first-order kinetics at 0·014 h−1; the rate of infectivity inactivation is 0·54 h−1. Hydrolysis for 1 h at 25 °C was only slightly proportional to HCl concentration up to 5 M, but complete loss of infectivity and PrP reduction to <2 % was observed at 8 M HCl. The temperature dependence of unhydrolysed PrP, as well as infectivity at 1 M HCl for 1 h, showed a slight decrease up to 45 °C, but a sigmoidal decrease by several orders of magnitude at higher temperatures. The slow hydrolysis of PrP and inactivation of infectivity by acid treatment at room temperature are attributed to solvent inaccessibility of prion rods and rPrP aggregates, respectively. The more effective hydrolysis and inactivation at temperatures above 45 °C are interpreted as thermally induced disaggregation with an activation energy of 50–60 kJ mol−1. Most importantly, infectivity was always inactivated faster or to a higher extent than PrP was hydrolysed at several incubation times, HCl concentrations and temperatures.

Clearance and prevention of prion infection in cell culture by anti-PrP antibodies

Prion diseases are transmissible and invariably fatal neurodegenerative disorders associated with a conformational transformation of the cellular prion protein (PrP(C)) into a self-replicating and proteinase K (PK)-resistant conformer, scrapie PrP (PrP(Sc)). Humoral immunity may significantly prolong the incubation period and even prevent disease in murine models of prionoses. However, the mechanism(s) of action of anti-PrP monoclonal antibodies (Mabs) remain(s) obscure. The murine neuroblastoma N2a cell line, infected with the 22L mouse-adapted scrapie strain, was used to screen a large library of Mabs with similar binding affinities to PrP, to identify those antibodies which could clear established infection and/or prevent infection de novo. Three Mabs were found capable of complete and persistent clearing of already-infected N2a cells of PrP(Sc). These antibodies were 6D11 (generated to PK-resistant PrP(Sc) and detecting PrP residues 93-109), and 7H6 and 7A12, which were raised against recombinant PrP and react with neighbouring epitopes of PrP residues 130-140 and 143-155, respectively. Mabs were found to interact with PrP(Sc) formation both on the cell surface and after internalization in the cytosol. Treatment with Mabs was not associated with toxicity nor did it result in decreased expression of PrP(C). Both preincubation of N2a cells with Mabs prior to exposure to 22L inoculum and preincubation of the inoculum with Mabs prior to infecting N2a cells resulted in a significant reduction in PrP(Sc) levels. Information provided in these studies is important for the rational design of humoral immune therapy for prion infection in animals and eventually in humans.

Comparative study of the effects of several chemical and physical treatments on the activity of protease resistance and infectivity of scrapie strain 263K

To study the influences of chemical and physical factors on the protease resistant activity in vitro and the infectivity in vivo of scrapie strain 263K, PrPSc from the hamsters infected intracerebrally with scrapie strain 263K were treated with several commonly used disinfection methods, including sodium hydroxide (NaOH), sodium hypochlorite (NaOCl), heating or autoclaving at 80, 100, 121 and 134 degrees C in the solutions with or without 3% sodium dodecyl sulphate (SDS). The protease resistance of PrPSc was analysed by a proteinase K (PK) digesting Western blot and the infectivity of PrPSc was analysed by intracerebral (i.c.) inoculation into experimental hamsters. The results showed that PrPSc signals were removed in the preparations treated with NaOH higher than 0.05 mol/l, NaOCl higher than 0.1%, autoclaved over 121 degrees C, or heated over 80 degrees C in the presence of 3% SDS. Animal challenges revealed that mixing with 2 mol/l NaOH or 2% NaOCl, autoclaving at 134 degrees C, as well as heating at 100 degrees C or autoclaving at 121 degrees C in the solutions with 3% SDS completely blocks the transmission of scrapie 263K in this experimental situation. It is obvious that the removal of PK resistance of PrPSc happened at relatively lower concentration chemicals or lower temperature, while elimination of the infectivity needs more vigorous conditions. Our data provide the useful evidences for several commonly used methods to inactivate TSEs agent and suggest that it is inappropriate to use PrPSc as a surrogate for TSEs infectivity in inactivation experiments.

Influence of guanidine on proteinase K resistance in vitro and infectivity of scrapie prion protein PrP(Sc)

As the scrapie prion protein PrP(Sc) is rich in beta-sheets it aggregates into prion rods, which show infectivity and proteinase K (PK) resistance. Consequently, dissociation of prion rods and breakdown of beta-sheets in PrP(Sc) by denaturation results in loss of both infectivity and PK-sensitivity. In this study, the effects of guanidine (Gdn), which solubilizes and denatures proteins by breaking down their higher structure, on the solubility, the PK-resistance in vitro and the infectivity of PrP(Sc) of scrapie strain 263K was examined. The infectivity was assayed by intracerebral inoculation into hamsters. Brain tissues of scrapie-infected hamsters were used for preparation of homogenates and crude extracts of PrP(Sc). A treatment of PrP(Sc) with Gdn enhanced its PK-sensitivity in a dose-dependent manner. The PK-resistance in vitro of PrP(Sc) denatured with lower concentrations of Gdn (<2.5 mol/l) could partially resume by renaturation. Gdn markedly reduced or, at higher concentrations, even destroyed the infectivity of PrP(Sc). On the other hand, the infectivity of PrP(Sc) inactivated by denaturation could be partially restored by renaturation. These results confirmed our assumption that all the alternations in the PK-resistance and the infectivity of PrP(Sc) caused by Gdn resulted from changes in its higher structure. However, it should be emphasized that a complete loss of PK-resistance of PrP(Sc) may not necessarily mean its full non-infectivity.

Distribution of a bovine spongiform encephalopathy-derived agent over ion-exchange chromatography used in the preparation of concentrates of fibrinogen and factor VIII

BACKGROUND AND OBJECTIVES

The risk of haemophiliacs contracting variant Creutzfeldt-Jakob disease (vCJD) via treatment with factor VIII concentrates is not known. Therefore, in order to determine the extent to which the vCJD agent might be removed during the preparation of factor VIII concentrate, the partitioning of a bovine spongiform encephalopathy (BSE)-derived agent was measured over the main purification step used to prepare the Scottish National Blood Transfusion Service high-purity factor VIII concentrate (Liberate).

MATERIALS AND METHODS

Murine-passaged BSE (strain 301V), in the form of a microsomal fraction prepared from infected brain, was used to 'spike' a solution of factor VIII of intermediate purity. The 'spiked' starting material was subjected to solvent-detergent treatment and then to anion-exchange chromatography with Toyopearl DEAE-650M. All fractions were tested for 301V infectivity using a murine bioassay, including the procedures used to clean the ion-exchange media after use.

RESULTS

BSE 301V infectivity was reduced by 2.9 log(10) in the fibrinogen fraction and by 2.7 log(10) in the factor VIII fraction. Over 99% of the added 301V infectivity remained bound to the ion-exchange column after elution of factor VIII. A large quantity of infectivity was subsequently removed by washing the ion-exchange media with 2 m NaCl. No further BSE 301V infectivity was detected in column eluates after treatment with 0.1 m NaOH or a second wash with 2 m NaCl.

CONCLUSIONS

Results using a BSE-derived agent suggest that vCJD infectivity would be substantially removed by the ion-exchange process used in the preparation of fibrinogen and factor VIII concentrate. Although 301V infectivity remained bound to the ion-exchange matrix following elution of factor VIII, this appeared to be eliminated by the procedure used for cleaning the ion-exchange media after each use.

The tyrosine kinase inhibitor STI571 induces cellular clearance of PrPSc in prion-infected cells

The conversion of the cellular prion protein (PrP(c)) into pathologic PrP(Sc) and the accumulation of aggregated PrP(Sc) are hallmarks of prion diseases. A variety of experimental approaches to interfere with prion conversion have been reported. Our interest was whether interference with intracellular signaling events has an impact on this conversion process. We screened approximately 50 prototype inhibitors of specific signaling pathways in prion-infected cells for their capacity to affect prion conversion. The tyrosine kinase inhibitor STI571 was highly effective against PrP(Sc) propagation, with an IC(50) of < or =1 microM. STI571 cleared prion-infected cells in a time- and dose-dependent manner from PrP(Sc) without influencing biogenesis, localization, or biochemical features of PrP(c). Interestingly, this compound did not interfere with the de novo formation of PrP(Sc) but activated the lysosomal degradation of pre-existing PrP(Sc), lowering the half-life of PrP(Sc) from > or =24 h to <9 h. Our data indicate that among the kinases known to be inhibited by STI571, c-Abl is likely responsible for the observed anti-prion effect. Taken together, we demonstrate that treatment with STI571 strongly activates the lysosomal degradation of PrP(Sc) and that substances specifically interfering with cellular signaling pathways might represent a novel class of anti-prion compounds.

Scrapie protein degradation by cysteine proteases in CD11c+ dendritic cells and GT1-1 neuronal cells

Dendritic cells (DC) of the CD11c(+) myeloid phenotype have been implicated in the spread of scrapie in the host. Previously, we have shown that CD11c(+) DC can cause a rapid degradation of proteinase K-resistant prion proteins (PrP(Sc)) in vitro, indicating a possible role of these cells in the clearance of PrP(Sc). To determine the mechanisms of PrP(Sc) degradation, CD11c(+) DC that had been exposed to PrP(Sc) derived from a neuronal cell line (GT1-1) infected with scrapie (ScGT1-1) were treated with a battery of protease inhibitors. Following treatment with the cysteine protease inhibitors (2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane (E-64c), its ethyl ester (E-64d), and leupeptin, the degradation of PrP(Sc) was inhibited, while inhibitors of serine and aspartic and metalloproteases (aprotinin, pepstatin, and phosphoramidon) had no effect. An endogenous degradation of PrP(Sc) in ScGT1-1 cells was revealed by inhibiting the expression of cellular PrP (PrP(C)) by RNA interference, and this degradation could also be inhibited by the cysteine protease inhibitors. Our data show that PrP(Sc) is proteolytically cleaved preferentially by cysteine proteases in both CD11c(+) DC and ScGT1-1 cells and that the degradation of PrP(Sc) by proteases is different from that of PrP(C). Interference by protease inhibitors with DC-induced processing of PrP(Sc) has the potential to modify prion spread, clearance, and immunization in a host.

Misfolding of the Prion Protein at the Plasma Membrane Induces Endocytosis, Intracellular Retention and Degradation

Suramin induces misfolding of the cellular prion protein (PrP(C)) and interferes with the propagation of infectious scrapie prions. A mechanistic analysis of this effect revealed that suramin-induced misfolding occurs at the plasma membrane and is dependent on the proximal region of the C-terminal domain (aa 90-158) of PrP(C). The conformational transition induces rapid internalization, mediated by the unstructured N-terminal domain, and subsequent intracellular degradation of PrP(C). As a consequence, PrP Delta N adopts a misfolded conformation at the plasma membrane; however, internalization is significantly delayed. We also found that misfolding and intracellular retention of PrP(C) can be induced by copper and that, moreover, copper interferes with the propagation of the pathogenic prion protein (PrP(Sc)) in scrapie-infected N2a cells. Our study revealed a quality control pathway for aberrant PrP conformers present at the plasma membrane and identified distinct PrP domains involved.

Quinoline derivatives are therapeutic candidates for transmissible spongiform encephalopathies

We previously reported that quinacrine inhibited the formation of an abnormal prion protein (PrPres), a key molecule in the pathogenesis of transmissible spongiform encephalopathy, or prion disease, in scrapie-infected neuroblastoma cells. To elucidate the structural aspects of its inhibiting action, various chemicals with a quinoline ring were screened in the present study. Assays of the scrapie-infected neuroblastoma cells revealed that chemicals with a side chain containing a quinuclidine ring at the 4 position of a quinoline ring (represented by quinine) inhibited the PrPres formation at a 50% inhibitory dose ranging from 10(-1) to 10(1) micro M. On the other hand, chemicals with a side chain at the 2 position of a quinoline ring (represented by 2,2'-biquinoline) more effectively inhibited the PrPres formation at a 50% inhibitory dose ranging from 10(-3) to 10(-1) micro M. A metabolic labeling study revealed that the action of quinine or biquinoline was not due to any alteration in the biosynthesis or turnover of normal prion protein, whereas surface plasmon resonance analysis showed a strong binding affinity of biquinoline with a recombinant prion protein. In vivo studies revealed that 4-week intraventricular infusion of quinine or biquinoline was effective in prolonging the incubation period in experimental mouse models of intracerebral infection. The findings suggest that quinoline derivatives with a nitrogen-containing side chain have the potential of both inhibiting PrPres formation in vitro and prolonging the incubation period of infected animals. These chemicals are new candidates for therapeutic drugs for use in the treatment of transmissible spongiform encephalopathies.

Copper binding to PrPC may inhibit prion disease propagation

Although it has been well established that PrP(C), the normal isoform of PrP(Sc), is a copper-binding protein, the role of this metal in the function of PrP(C) as well as in prion disease pathology remains unclear. Here, we show that when scrapie-infected neuroblastoma cells were cultured in the presence of copper, the accumulation of PrP(Sc) in these cells was markedly reduced. In addition, our results indicate that when normal neuroblastoma cells were cultured in the presence of copper ions, they could no longer bind and internalize PrP(Sc). In another set of experiments, copper was added to the drinking water of normal and scrapie-infected hamsters. Our results show that administration of copper to normal hamsters induced cerebellar PrP(C) accumulation. Most important, a significant delay in prion disease onset was observed when scrapie-infected hamsters were treated with copper. As shown before for neuroblastoma cells, also in vivo most of the copper-induced accumulation of PrP(C) was intracellular. We hypothesized that PrP(C) internalization by copper may hinder PrP(Sc) interaction with this molecule, and thereby affect prion disease propagation.

Scrapie-infected GT1-1 cells show impaired function of voltage-gated N-type calcium channels (Cav 2.2) which is ameliorated by quinacrine treatment

Prions are transmissible pathogens that cause neurodegenerative diseases, although the mechanisms behind the nervous system dysfunctions are unclear. To study the effects of a prion infection on voltage-gated calcium channels, scrapie-infected gonadotropin-releasing hormone neuronal cells (ScGT1-1) in culture were depolarized by KCl and calcium responses recorded. Lower calcium responses were observed in infected compared to uninfected cells. This effect was still observed when L-type calcium channels were blocked by nimodipine. After inhibition of N-type calcium channels with omega-conotoxin GVIA, there was no difference in calcium responses. The calcium responses after nimodipine treatment became progressively lower during infection, but there was no major loss of the cellular prion protein (PrP(C)) or marked increase in accumulation of the abnormal prion protein (PrP(Sc)) in the cultures. These results indicate that scrapie infection causes a dysfunction of voltage-gated N-type calcium channels, which is exacerbated slowly over time. Quinacrine treatment cleared PrP(Sc) and restored calcium responses in the ScGT1-1 cultures.

Novel heparan mimetics potently inhibit the scrapie prion protein and its endocytosis

During prion diseases the normal prion protein PrP(C) is refolded into an abnormal conformer PrP(Sc). We have studied the PrP(Sc) inhibiting activity of a library of synthetic heparan mimetic (HM) biopolymers. HMs are chemically derived dextrans obtained by successive substitutions with carboxymethyl, benzylamide, and sulfate groups on glucose residues. Some HMs eliminated PrP(Sc) from prion-infected cells after a 5 day course at 100 ng/ml and were 15 x potent than pentosan sulfate in this system. The anti-PrP(Sc) activity of HMs correlated with the degree of sulfation but was increased by benzylamidation. HMs did not reduce the synthesis of PrP(C) nor its attachment to lipid rafts, but instead blocked its conversion into PrP(Sc). The anti-PrP(Sc) HMs also prevented the uptake of prion rods by cultured cells. HMs may thus block the interaction of PrP(Sc) with a putative cellular receptor, possibly heparan sulfate. HMs provide an attractive chemical approach for the synthesis of TSE therapeutic and prophylactic reagents.

Copper Chelation Delays the Onset of Prion Disease

The prion protein (PrP) binds copper and under some conditions copper can facilitate its folding into a more protease resistant form. Hence, copper levels may influence the infectivity of the scrapie form of prion protein (PrPSc). To determine the feasibility of copper-targeted therapy for prion disease, we treated mice with a copper chelator, D-(-)-penicillamine (D-PEN), starting immediately following intraperitoneal scrapie inoculation. D-PEN delayed the onset of prion disease in the mice by about 11 days (p = 0.002), and reduced copper levels in brain by 29% (p < 0.01) and in blood by 22% (p = 0.03) compared with control animals. Levels of other metals were not significantly altered in the blood or brain. Modest correlation was observed between incubation period and levels of copper in brain (p = 0.08) or blood (p = 0.04), indicating that copper levels are only one of many factors that influence the rate of progression of prion disease. In vitro, copper dose-dependently enhanced the proteinase K resistance of the prion protein, and this effect was counteracted in a dose-dependent manner by co-incubation with D-PEN. Overall, these findings indicate that copper levels can influence the conformational state of PrP, thereby enhancing its infectivity, and this effect can be attenuated by chelator-based therapy.

Sodium hydroxide renders the prion protein PrPSc sensitive to proteinase K

Sodium hydroxide (NaOH) solutions are widely used for the purification of contaminated equipment, as they are known to inactivate a variety of pathogens. However, information about their effect on agents causing transmissible spongiform encephalopathy (TSE) is sparse and contradictory. Scrapie hamster brain homogenate, containing the disease-associated form of the prion protein (PrP(Sc)), was exposed to NaOH. Kinetics studies showed that treatment of brain homogenate with millimolar concentrations of NaOH rapidly abolished the proteinase K-resistant form of the prion protein (PrP(res)). NaOH treatment converted PrP(Sc) into a protease-sensitive form, either in solution or when adsorbed to a metallic surface. If infectivity of TSEs is linked with PrP(res), the results imply that inactivation of TSE occurs more efficiently than currently assumed.

Evaluation of quinacrine treatment for prion diseases

Based on in vitro observations in scrapie-infected neuroblastoma cells, quinacrine has recently been proposed as a treatment for Creutzfeldt-Jakob disease (CJD), including a new variant CJD which is linked to contamination of food by the bovine spongiform encephalopathy (BSE) agent. The present study investigated possible mechanisms of action of quinacrine on prions. The ability of quinacrine to interact with and to reduce the protease resistance of PrP peptide aggregates and PrPres of human and animal origin were analyzed, together with its ability to inhibit the in vitro conversion of the normal prion protein (PrPc) to the abnormal form (PrPres). Furthermore, the efficiencies of quinacrine and chlorpromazine, another tricyclic compound, were examined in different in vitro models and in an experimental murine model of BSE. Quinacrine efficiently hampered de novo generation of fibrillogenic prion protein and PrPres accumulation in ScN2a cells. However, it was unable to affect the protease resistance of preexisting PrP fibrils and PrPres from brain homogenates, and a "curing" effect was obtained in ScGT1 cells only after lengthy treatment. In vivo, no detectable effect was observed in the animal model used, consistent with other recent studies and preliminary observations in humans. Despite its ability to cross the blood-brain barrier, the use of quinacrine for the treatment of CJD is questionable, at least as a monotherapy. The multistep experimental approach employed here could be used to test new therapeutic regimes before their use in human trials.

Inhibition of complex glycosylation increases the formation of PrPsc

N-linked glycans with complex structure have a major role in the biological activity of a wide variety of cell surface and secreted glycoproteins. Here, we show that geldanamycin, an inhibitor of Hsp90, interferes with the formation of complex glycosylated mammalian prion protein (PrPC). Similarly to inhibitors of alpha-mannosidases, geldanamycin stabilized a high mannose PrPC glycoform and prevented the subsequent processing into complex structures. Moreover, a PrP/Grp94 complex could be isolated from geldanamycin-treated cells, suggesting that Grp94 might play a role in the processing of PrPC in the endoplasmic reticulum. Inhibition of complex glycosylation did not interfere with the glycosylphosphatidylinositol (GPI) anchor attachment and cellular trafficking of high mannose PrPC to the outer leaflet of the plasma membrane. In scrapie-infected neuroblastoma cells, however, high mannose PrPC glycoforms were preferred substrates for the formation of PrP-scrapie (PrPSc). Our study reveals that complex glycosylation is dispensable for the cellular trafficking of PrPC, but modulates the formation of PrPSc.

Potent inhibition of scrapie prion replication in cultured cells by bis-acridines

Prion diseases are characterized by an accumulation of PrP(Sc), a misfolded isoform of the normal cellular prion protein, PrP(C). We previously reported the bioactivity of acridine-based compounds against PrP(Sc) replication in scrapie-infected neuroblastoma cells and now report the improved potency of bis-acridine compounds. Bis-acridines are characterized by a dimeric motif, comprising two acridine heterocycles tethered by a linker. A library of bis-(6-chloro-2-methoxy-acridin-9-yl) and bis-(7-chloro-2-methoxy-benzo[b][1,5]naphthyridin-10-yl) analogs was synthesized to explore the effect of structurally diverse linkers on PrP(Sc) replication in scrapie-infected neuroblastoma cells. Structure-activity analysis revealed that linker length and structure are important determinants for inhibition of prion replication in cultured scrapied cells. Three bis-acridine analogs, (6-chloro-2-methoxy-acridin-9-yl)-(3-[4-[3-(6-chloro-2-methoxy-acridin-9-ylamino)-propyl]-piperazin-1-yl]-propyl)-amine, N,N'-bis-(6-chloro-2-methoxy-acridin-9-yl)-1,8-diamino-3,6-dioxaoctane, and (1-[[4-(6-chloro-2-methoxy-acridin-9-ylamino)-butyl]-[3-(6-chloro-2-methoxy-acridin-9-ylamino)-propyl]-carbamoyl]-ethyl)-carbamic acid tert-butyl ester, showed half-maximal inhibition of PrP(Sc) formation at 40, 25, and 30 nM, respectively, and were not cytotoxic to uninfected neuroblastoma cells at concentrations of 500 nM. Our data suggest that bis-acridine analogs may provide a potent alternative to the acridine-based compound quinacrine, which is currently under clinical evaluation for the treatment of prion disease.

Therapeutic approaches to prion diseases

Prion diseases are unique in that they comprise sporadic, genetic, and iatrogenically or environmentally acquired forms. When disease is acquired by peripheral route, neuroinvasion occurs via at least two different neural pathways (vague and splanchnic nerves) and is usually preceded by prion propagation in secondary lymphoid organs. Conversely, in the other etiologic forms, PrPSc formation occurs within, and is apparently limited to, the CNS. Longitudinal studies on experimental scrapie indicate that substantial neuropathologic changes (i.e., glial activation and nerve cell degeneration) already are present before the onset of symptoms and are topographically related to PrPSc deposits. Accordingly, any effective intervention should start during the preclinical stage of disease, and be aimed at preventing neuroinvasion or PrPSc propagation in the CNS. Unfortunately, no tests are available currently to detect presymptomatic individuals, except for carriers of pathogenic mutations of the PRNP gene. Inhibition of PrPSc formation can be achieved through (1) abrogation of PrPC synthesis or prevention of its transport to the cell surface; (2) stabilization of the PrPC structure to make its conformational change unfavorable; (3) sequestration of PrPSc; (4) reversion of PrPSc to a protease-sensitive form; or (5) interference with the interaction between PrPC, PrPSc, and other macromolecules that feature in the conversion process. The compounds that have some effectiveness in in vitro, cell culture, or animal models of prion disease seem to operate through one of these mechanisms (see Table 1); however, even the most effective drugs only work when administered at the time of infection or very short thereafter, and these conditions are incurable at present. The heterogeneity and complexity of the etiopathogenesis of prion diseases suggest that various strategies and a combination of several compounds with different modes of actions are likely necessary for prevention and treatment. Major efforts should be focused on the development of preclinical diagnostic tests in conjunction with immunization strategies for diseases acquired by peripheral route and identification of more effective compounds for the other etiological forms.

Tetracyclines affect prion infectivity

Prion diseases are transmissible neurodegenerative disorders of humans and animals for which no effective treatment is available. Conformationally altered, protease-resistant forms of the prion protein (PrP) termed PrP(Sc) are critical for disease transmissibility and pathogenesis, thus representing a primary target for therapeutic strategies. Based on previous findings that tetracyclines revert abnormal physicochemical properties and abolish neurotoxicity of PrP peptides in vitro, we tested the ability of these compounds to interact with PrP(Sc) from patients with the new variant of Creutzfeldt-Jakob disease (vCJD) and cattle with bovine spongiform encephalopathy (BSE). The incubation with tetracycline hydrochloride or doxycycline hyclate at concentrations ranging from 10 microM to 1 mM resulted in a dose-dependent decrease in protease resistance of PrP(Sc). This finding prompted us to investigate whether tetracyclines affect prion infectivity by using an animal model of disease. Syrian hamsters were injected intracerebrally with 263K scrapie-infected brain homogenate that was coincubated with 1 mM tetracycline hydrochloride, 1 mM doxycycline hyclate, or vehicle solution before inoculation. Hamsters injected with tetracycline-treated inoculum showed a significant delay in the onset of clinical signs of disease and prolonged survival time. These effects were paralleled by a delay in the appearance of magnetic-resonance abnormalities in the thalamus, neuropathological changes, and PrP(Sc) accumulation. When tetracycline was preincubated with highly diluted scrapie-infected inoculum, one third of hamsters did not develop disease. Our data suggest that these well characterized antibiotics reduce prion infectivity through a direct interaction with PrP(Sc) and are potentially useful for inactivation of BSE- or vCJD-contaminated products and prevention strategies.

Conversion of raft associated prion protein to the protease-resistant state requires insertion of PrP-res (PrP(Sc)) into contiguous membranes

Prion protein (PrP) is usually attached to membranes by a glycosylphosphatidylinositol-anchor that associates with detergent-resistant membranes (DRMs), or rafts. To model the molecular processes that might occur during the initial infection of cells with exogenous transmissible spongiform encephalopathy (TSE) agents, we examined the effect of membrane association on the conversion of the normal protease-sensitive PrP isoform (PrP-sen) to the protease-resistant isoform (PrP-res). A cell-free conversion reaction approximating physiological conditions was used, which contained purified DRMs as a source of PrP-sen and brain microsomes from scrapie-infected mice as a source of PrP-res. Interestingly, DRM-associated PrP-sen was not converted to PrP-res until the PrP-sen was either released from DRMs by treatment with phosphatidylinositol-specific phospholipase C (PI-PLC), or the combined membrane fractions were treated with the membrane-fusing agent polyethylene glycol (PEG). PEG-assisted conversion was optimal at pH 6--7, and acid pre-treating the DRMs was not sufficient to permit conversion without PI-PLC or PEG, arguing against late endosomes/lysosomes as primary compartments for PrP conversion. These observations raise the possibility that generation of new PrP-res during TSE infection requires (i) removal of PrP-sen from target cells; (ii) an exchange of membranes between cells; or (iii) insertion of incoming PrP-res into the raft domains of recipient cells.

Interventional strategies against prion diseases

Only a few years ago, the idea that transmissible spongiform encephalopathies could be treated pharmacologically would have met with considerable scepticism. Even now, there is no way to cure a patient or animal suffering from a manifest prion disease. But recent, exciting developments seem to indicate that immunological and pharmacological interventions could have some potential for the pre-exposure and post-exposure prophylaxis of prion diseases. Although it is unlikely that we will be able to cure the clinically overt stages of prion diseases in the foreseeable future, palliative and even life-prolonging interventions might no longer be confined to the realm of science fiction.

Acridine and phenothiazine derivatives as pharmacotherapeutics for prion disease

Prion diseases in humans and animals are invariably fatal. Prions are composed of a disease-causing isoform (PrP(Sc)) of the normal host prion protein (PrP(C)) and replicate by stimulating the conversion of PrP(C) into nascent PrP(Sc). We report here that tricyclic derivatives of acridine and phenothiazine exhibit half-maximal inhibition of PrP(Sc) formation at effective concentrations (EC(50)) between 0.3 microM and 3 microM in cultured cells chronically infected with prions. The EC(50) for chlorpromazine was 3 microM, whereas quinacrine was 10 times more potent. A variety of 9-substituted, acridine-based analogues of quinacrine were synthesized, which demonstrated variable antiprion potencies similar to those of chlorpromazine and emphasized the importance of the side chain in mediating the inhibition of PrP(Sc) formation. Thus, our studies show that tricyclic compounds with an aliphatic side chain at the middle ring moiety constitute a new class of antiprion reagents. Because quinacrine and chlorpromazine have been used in humans for many years as antimalarial and antipsychotic drugs, respectively, and are known to pass the blood-brain barrier, we suggest that they are immediate candidates for the treatment of Creutzfeldt-Jakob disease and other prion diseases.

Inhibition of formation of protease-resistant prion protein by Trypan Blue, Sirius Red and other Congo Red analogs

Five compounds related to Congo Red were found to inhibit generation of protease-resistant prion protein in a cell-free system. In this assay Trypan Blue, Evans Blue, Sirius Red F3B, Primuline and Thioflavin-S were all more inhibitory than Congo Red itself. In scrapie-infected mouse neuroblastoma cells one compound, Sirius Red F3B, was capable of blocking the formation of protease-resistant prion protein to a similar extent as Congo Red; however, the other four compounds were less effective. Some of these compounds should be considered for testing in TSE disease models in live animals.

Characterization of the murine BSE infectious agent

Bovine spongiform encephalopathy (BSE) is a prion-associated disease where the infectious agent is thought to be a host-encoded protein with a protease-resistant conformation (PrP(Sc)). Here, data are presented on the solubilization of purified murine BSE material, using guanidine-HCl as a denaturing agent. This treatment led to loss of infectivity, which was partially recovered on renaturation after dialysis to remove the chaotropic agent. The renatured product was then fractionated on an isopycnic sucrose-density gradient and the fractions were analysed for the presence of PrP(Sc), nucleic acids and infectivity. It was found that the major part of PrP(Sc) (>90%) and the endogenous nucleic acids did not contribute towards the formation of infectious particles on renaturation. Infectivity was distributed in the top three, low-density fractions. Among these, the presence of considerable infectivity in the fraction of lowest density, with barely detectable PrP(Sc), is of particular interest.

Lysosomotropic agents and cysteine protease inhibitors inhibit scrapie-associated prion protein accumulation

We report that lysosomotropic agents and cysteine protease inhibitors inhibited protease-resistant prion protein accumulation in scrapie-infected neuroblastoma cells. The inhibition occurred without either apparent effects on normal prion protein biosynthesis or turnover or direct interactions with prion protein molecules. The findings introduce two new classes of inhibitors of the formation of protease-resistant prion protein.

Porphyrin and phthalocyanine antiscrapie compounds

The transmissible spongiform encephalopathies (TSEs) are fatal, neurodegenerative diseases for which no effective treatments are available. The likelihood that a bovine form of TSE has crossed species barriers and infected humans underscores the urgent need to identify anti-TSE drugs. Certain cyclic tetrapyrroles (porphyrins and phthalocyanines) have recently been shown to inhibit the in vitro formation of PrP-res, a protease-resistant protein critical for TSE pathogenesis. We now report that treatment of TSE-infected animals with three such compounds increased survival time from 50 to 300%. The significant inhibition of TSE disease by structurally dissimilar tetrapyrroles identifies these compounds as anti-TSE drugs.

Immune surveillance and antigen conformation determines humoral immune response to the prion protein immunogen

Transmissible spongiform encephalopathies (TSE) are progressive degenerative disorders of the central nervous system. PrP(Sc) is a TSE-specific marker derived from the host-encoded glycoprotein, PrPc. The generation of antibodies to PrP plays an important role in the diagnosis of these diseases. In this study the role of the PrP immunogen and the species being immunized was examined in relation to specific epitopes. Various mammals (mice, hamsters, rabbits and PrP null mice) were immunized with formic acid-treated PrP(Sc) isolated from mice, hamsters and sheep. Both the species being immunized and the source of immunogen played an important role in the antibody response. Response to a limited number of linear epitopes was seen among the various immunized animals. One region in the C-terminal portion of PrP appeared highly immunogenic in all species. Comparison of immunoreactivity and the pepscan-defined linear epitope sites suggests both linear and conformational directed responses in many of the animals. Information on the forces directing immune responses to PrP will lead to a better understanding of host-PrP interactions. It will also assist in the development of new strategies for generating additional tools for immunodiagnosis.

Survival of scrapie agent after exposure to sodium dodecyl sulphate and heat

Fifty mg aliquots of macerated mouse-brain infected with the 22A strain of scrapie agent were treated by exposing them without mechanical mixing to (a) distilled water for 2 h, (b) 5% sodium dodecyl sulphate (SDS) for 2 h, (c) autoclaving at 121 degrees C for 15 min in distilled water, (d) autoclaving at 121 degrees C for 15 min in 5% SDS, or (e) boiling in 5% SDS for 15 min. Prior to injection into mice, all samples were washed by a procedure that is described and was shown not to reduce infectivity titres. Although the infectivity titre of the sample that was autoclaved in SDS was reduced considerably, infectivity was present in all of the samples exposed to cold or hot SDS.

MS-8209, a water-soluble amphotericin B derivative, affects both scrapie agent replication and PrPres accumulation in Syrian hamster scrapie

Amphotericin B (AmB) has been shown to delay hamster scrapie. Infectivity studies have been performed previously using AmB in order to understand the relationship between the accumulation of an abnormal isoform (PrPres) of the prion protein and 263K scrapie agent replication in the brain. The first study reported that AmB had no effect upon agent replication, although it delayed the development of both clinical signs and PrPres accumulation. However, subsequent experiments using the same model showed a significant effect both on agent replication and PrPres accumulation early in infection. This fundamental discrepancy was assumed to be linked to differences in experimental protocols. In order to unravel the issue, a new experiment has been performed encompassing different protocols and using an AmB derivative, MS-8209, that can be used at higher doses because of its lower toxicity. The findings of this study exclude the suspected differences in the protocols as the reason for previous conflicting results, and suggest strongly that these discrepancies were due to a low dose of AmB causing a 'threshold effect'. Overall, this study indicates that, in this model, PrPres cannot be dissociated from infectivity by polyene antibiotics.

Contribution of a peroxide adduct of copper(II)-peptide complex to modify the secondary structure of albumin

We have found that copper(II) compounds containing a peptide group in the chelate exhibit high activity for modification or degradation of albumin in the presence of hydrogen peroxide, whereas no activity was detected for the copper(II) compounds without an amide-group. It is suggested that presence of the amide-group in the ligand may play an important role in the formation of a peroxide adduct and in activation of the peroxide ion, leading to cleavage of the peptide bond of a neighboring protein. It is implied that conversion of normal cellular prion protein PrPC into a disease-causing isoform, PrPSc is attributed to the activated peroxide ion coordinated to a copper(II) captured in the NH2-terminal domain of the PrPC.

Scrapie infectivity correlates with converting activity, protease resistance, and aggregation of scrapie-associated prion protein in guanidine denaturation studies

Denaturation studies with guanidine HCl (GdnHCl) were performed to test the relationship between scrapie infectivity and properties of scrapie-associated prion protein (PrP(Sc)). Large GdnHCl-induced reductions in infectivity were associated with the irreversible elimination of both the proteinase K resistance and apparent self-propagating converting activity of PrP(Sc). In intermediate GdnHCl concentrations that stimulate converting activity and partially disaggregate PrP(Sc), both scrapie infectivity and converting activity were associated with residual partially protease-resistant multimers of PrP(Sc).

Identification of intermediate steps in the conversion of a mutant prion protein to a scrapie-like form in cultured cells

The central causative event in infectious, familial, and sporadic forms of prion disease is thought to be a conformational change that converts the cellular isoform of the prion protein (PrPC) into the scrapie isoform (PrPSc) that is the primary constituent of infectious prion particles. To provide a model system for analyzing the mechanistic details of this critical transformation, we have previously prepared cultured Chinese hamster ovary cells that stably express mouse PrP molecules carrying mutations homologous to those seen in familial prion diseases of humans. In the present work, we have analyzed the kinetics with which a PrP molecule containing an insertional mutation associated with Creutzfeldt-Jakob disease acquires several biochemical properties characteristic of PrPSc. Within 10 min of pulse labeling, the mutant protein undergoes a molecular alteration that is detectable by a change in Triton X-114 phase partitioning and phenyl-Sepharose binding. After 30 min of labeling, a detergent-insoluble and protease-sensitive form of the protein appears. After a chase period of several hours, the protein becomes protease-resistant. Incubation of cells at 18 degrees C or treatment with brefeldin A inhibits acquisition of detergent insolubility and protease resistance but does not affect Triton X-114 partitioning and phenyl-Sepharose binding. Our results support a model in which conversion of mutant PrPs to a PrPSc-like state proceeds in a stepwise fashion via a series of identifiable biochemical intermediates, with the earliest step occurring during or very soon after synthesis of the polypeptide in the endoplasmic reticulum.

Probing the dynamics of prion diseases with amphotericin B

Amphotericin B (AmB) is one of the rare drugs that affect the course of experimental prion diseases and modify the kinetics of abnormal prion protein accumulation in the central nervous system. Therefore, AmB could be used as a pharmacological tool to contribute to our understanding of the pathogenic mechanisms involved in these neurodegenerative disorders.

Chemical chaperones interfere with the formation of scrapie prion protein

The fundamental event in prion diseases involves a conformational change in one or more of the alpha-helices of the cellular prion protein (PrP(C)) as they are converted into beta-sheets during the formation of the pathogenic isoform (PrP(Sc)). Here, we show that exposure of scrapie-infected mouse neuroblastoma (ScN2a) cells to reagents known to stabilize proteins in their native conformation reduced the rate and extent of PrP(Sc) formation. Such reagents include the cellular osmolytes glycerol and trimethylamine N-oxide (TMAO) and the organic solvent dimethylsulfoxide (DMSO), which we refer to as 'chemical chaperones' because of their influence on protein folding. Although the chemical chaperones did not appear to affect the existing population of PrP(Sc) molecules in ScN2a cells, they did interfere with the formation of PrP(Sc) from newly synthesized PrP(C). We suggest that the chemical chaperones act to stabilize the alpha-helical conformation of PrP(C) and thereby prevent the protein from undergoing a conformational change to produce PrP(Sc). These observations provide further support for the idea that prions arise due to a change in protein conformation and reveal potential strategies for preventing PrP(Sc) formation.

Exposure to autoclaving or sodium hydroxide extends the dose-response curve of the 263K strain of scrapie agent in hamsters

An analysis was made of incubation period data from experiments in which samples of brain-tissue infected with the 263K strain of scrapie agent were injected intracerebrally into hamsters following exposure of the tissue to autoclaving or sodium hydroxide. Where there was survival of infectivity, this often produced extended mean incubation periods compared with the maximal incubation periods in controls injected with untreated agent. These results confirmed that, after chemical or physical treatment, infectivity titre should not be calculated by comparing the incubation period from a single dilution-group against a standard dose-response curve for untreated agent.

Congo red prolongs the incubation period in scrapie-infected hamsters

In scrapie-infected cells, Congo red inhibits both the replication of the infectious agent and accumulation of the protease-resistant form of PrP (PrP-res). In this report, we show that Congo red prolongs the incubation periods of hamsters experimentally infected with two different strains of scrapie.

Amphotericin B delays both scrapie agent replication and PrP-res accumulation early in infection

Amphotericin B delays the onset of clinical symptoms in hamsters infected with scrapie agent strain 263K. Here we show that accumulation of a scrapie-specific isoform of the prion protein (PrP-res) and agent replication were delayed early in amphotericin B-treated animals. By 8 weeks postinfection, only untreated animals exhibited clinical symptoms of scrapie infection whereas PrP-res levels and titers were similar in treated and untreated animals. This suggests that although PrP-res accumulation and agent replication are linked, they are not the sole factors required for the onset of clinical disease.