Prion diseases

Potential Prion Involvement in Long COVID-19 Neuropathology, Including Behavior

Prion' is a term used to describe a protein infectious particle responsible for several neurodegenerative diseases in mammals, e.g., Creutzfeldt-Jakob disease. The novelty is that it is protein based infectious agent not involving a nucleic acid genome as found in viruses and bacteria. Prion disorders exhibit, in part, incubation periods, neuronal loss, and induce abnormal folding of specific normal cellular proteins due to enhancing reactive oxygen species associated with mitochondria energy metabolism. These agents may also induce memory, personality and movement abnormalities as well as depression, confusion and disorientation. Interestingly, some of these behavioral changes also occur in COVID-19 and mechanistically include mitochondrial damage caused by SARS-CoV-2 and subsequenct production of reactive oxygen species. Taken together, we surmise, in part, long COVID may involve the induction of spontaneous prion emergence, especially in individuals susceptible to its origin may thus explain some of its manesfestions post-acute viral infection.

Antler cannibalism in reindeer

Prion diseases constitute a class of invariably fatal and degenerative encephalopathies. Chronic Wasting Disease (CWD) is a contagious prion disease among cervids, which is spreading and causing marked population declines in USA and Canada. The first outbreak of CWD in Europe was discovered in a reindeer population in Norway in 2016. In the worst-case scenario with continental-wide spreading of CWD in Eurasia, an annual harvest of around 4 million cervids is at stake only in Europe, with huge economic and cultural significance. An in situ origin of CWD was suspected, and it appear urgent to identify the likely cause to prevent future emergences. Here, we document the novel phenomenon of extensive antler cannibalism prior to shedding among reindeer in the CWD-infected population. The extent of antler cannibalism increased over the last decades when CWD emerged, and included ingestion of vascularized antlers. Ingestion of tissues from conspecifics is a risk factor for the emergence of prion diseases, where the presence of extensive antler cannibalism opens the intriguing possibility of a ‘Kuru-analogue’ origin of CWD among the reindeer in Europe. Based on general insight on pathology of prion diseases and strain selection processes, we propose an hypothesis for how contagious CWD may emerge from sporadic CWD under the unique epidemiological conditions we document here. More research is required to document the presence of prions in reindeer antlers, and whether antler cannibalism actually led to a strain selection process and the emergence of a contagious form of CWD from a sporadic form of CWD.

Prion-like mechanisms in Alzheimer disease

Senile plaques and neurofibrillary tangles are the principal histopathologic hallmarks of Alzheimer disease. The essential constituents of these lesions are structurally abnormal variants of normally generated proteins: Aβ protein in plaques and tau protein in tangles. At the molecular level, both proteins in a pathogenic state share key properties with classic prions, i.e., they consist of alternatively folded, β-sheet-rich forms of the proteins that autopropagate by the seeded corruption and self-assembly of like proteins. Other similarities with prions include the ability to manifest as polymorphic and polyfunctional strains, resistance to chemical and enzymatic destruction, and the ability to spread within the brain and from the periphery to the brain. In Alzheimer disease, current evidence indicates that the pathogenic cascade follows from the endogenous, sequential corruption of Aβ and then tau. Therapeutic options include reducing the production or multimerization of the proteins, uncoupling the Aβ-tauopathy connection, or promoting the inactivation or removal of anomalous assemblies from the brain. Although aberrant Aβ appears to be the prime mover of Alzheimer disease pathogenesis, once set in motion by Aβ, the prion-like propagation of tauopathy may proceed independently of Aβ; if so, Aβ might be solely targeted as an early preventive measure, but optimal treatment of Alzheimer disease at later stages of the cascade could require intervention in both pathways.

Proteopathic Strains and the Heterogeneity of Neurodegenerative Diseases

Most age-related neurodegenerative diseases are associated with the misfolding and aberrant accumulation of specific proteins in the nervous system. The proteins self-assemble and spread by a prion-like process of corruptive molecular templating, whereby abnormally folded proteins induce the misfolding and aggregation of like proteins into characteristic lesions. Despite the apparent simplicity of this process at the molecular level, diseases such as Alzheimer's, Parkinson's, Creutzfeldt-Jakob, and others display remarkable phenotypic heterogeneity, both clinically and pathologically. Evidence is growing that this variability is mediated, at least in part, by the acquisition of diverse molecular architectures by the misfolded proteins, variants referred to as proteopathic strains. The structural and functional diversity of the assemblies is influenced by genetic, epigenetic, and local contextual factors. Insights into proteopathic strains gleaned from the classical prion diseases can be profitably incorporated into research on other neurodegenerative diseases. Their potentially wide-ranging influence on disease phenotype also suggests that proteopathic strains should be considered in the design and interpretation of diagnostic and therapeutic approaches to these disorders.

Prions in dentistry: A need to be concerned and known

Prion diseases were first discovered by Stanley B. Prusiner who defined prions as infectious, transmissible proteinaceous particles that lack nucleic acid and are composed exclusively of a modified isoform of the noninfectious cellular prion protein (PrPC). These are incurable neurodegenerative conditions affecting both animals and humans. They may be sporadic, infectious or inherited in origin. Human prion diseases include Creutzfeldt–Jakob desease (CJD), Gerstmann– Straussler–Scheinker disease, Kuru and Fatal familial insomnia. Prions resist the conventional sterilization procedures and hence the dentists must be aware of such diseases so as to opt standard methods of infection control and decontamination for such infectious agents. This review article divulge the dentists with a brief overview of the characteristics of prions, the risk of transmission and the implications for infection control in dentist.

Treatment of Prion Disease with Heterologous Prion Proteins

Prion diseases such as Creutzfeldt-Jakob disease in humans, bovine spongiform encephalopathy in cattle, and scrapie in sheep are fatal neurodegenerative diseases for which there is no effective treatment. The pathology of these diseases involves the conversion of a protease sensitive form of the cellular prion protein (PrP^C) into a protease resistant infectious form (PrP^sc or PrP^res). Both in vitro (cell culture and cell free conversion assays) and in vivo (animal) studies have demonstrated the strong dependence of this conversion process on protein sequence homology between the initial prion inoculum and the host’s own cellular prion protein. The presence of non-homologous (heterologous) proteins is often inhibitory to this conversion process. We hypothesize that the presence of heterologous prion proteins from one species might therefore constitute an effective treatment for prion disease in another species. To test this hypothesis, we infected mice intracerebrally with murine adapted RML-Chandler scrapie and treated them with heterologous prion protein (purified bacterially expressed recombinant hamster prion protein) or vehicle alone. Treated animals demonstrated reduced disease associated pathology, decreased accumulation of protease-resistant disease-associated prion protein, with delayed onset of clinical symptoms and motor deficits. This was concomitant with significantly increased survival times relative to mock-treated animals. These results provide proof of principle that recombinant hamster prion proteins can effectively and safely inhibit prion disease in mice, and suggest that hamster or other non-human prion proteins may be a viable treatment for prion diseases in humans.

A Short Introduction to Disease Emergence

The purpose of this introductory chapter is to provide the reader with a brief overview of the factors that drive disease emergence in order to set the scene for the more detailed chapters that follow. The chapter is divided into three parts. The first deals with the activities of humans that drive disease emergence. This ranges from historical factors such as animal domestication to the impact of new technologies such as air travel and agricultural intensification in response to population growth. The second section deals with virus properties that enable them to adapt to new hosts, particularly jumping between species. The final section deals with the activities of animals themselves that contribute to disease emergence.

Reconstructing the fungal tree of life using phylogenomics and a preliminary investigation of the distribution of yeast prion-like proteins in the fungal kingdom

We have used three independent phylogenomic approaches (concatenated alignments, single-, and multi-gene supertrees) to reconstruct the fungal tree of life (FTOL) using publicly available fungal genomes. This is the first time multi-gene families have been used in fungal supertree reconstruction and permits us to use up to 66% of the 1,001,217 genes in our fungal database. Our analyses show that different phylogenomic datasets derived from varying clustering criteria and alignment orientation do not have a major effect on phylogenomic supertree reconstruction. Overall the resultant phylogenomic trees are relatively congruent with one another and successfully recover the major fungal phyla, subphyla and classes. We find that where incongruences do occur, the inferences are usually poorly supported. Within the Ascomycota phylum, our phylogenies reconstruct monophyletic Saccharomycotina and Pezizomycotina subphyla clades and infer a sister group relationship between these to the exclusion of the Taphrinomycotina. Within the Pezizomycotina subphylum, all three phylogenies infer a sister group relationship between the Leotiomycetes and Sordariomycetes classes. However, there is conflict regarding the relationships with the Dothideomycetes and Eurotiomycetes classes. Within the Basidiomycota phylum, supertrees derived from single- and multi-gene families infer a sister group relationship between the Pucciniomycotina and Agaricomycotina subphyla while the concatenated phylogeny infers a poorly supported relationship between the Agaricomycotina and Ustilagomycotina. The reconstruction of a robust FTOL is important for future fungal comparative analyses. We illustrate this point by performing a preliminary investigation into the phyletic distribution of yeast prion-like proteins in the fungal kingdom.

CWD prevalence, perceived human health risks, and state influences on deer hunting participation

This study examined factors predicted by previous research to influence hunters' decisions to stop hunting deer in a state. Data were obtained from mail surveys of resident and nonresident deer hunters in Arizona, North Dakota, South Dakota, and Wisconsin (n = 3,518). Hunters were presented with six scenarios depicting hypothetical CWD prevalence levels and human health risks from the disease (e.g., death), and asked if they would continue or stop hunting deer in the state. Bivariate analyses examined the influence of five predictor variables: (a) CWD prevalence, (b) hypothetical human death from CWD, (c) perceived human health risks from CWD, (d) state, and (e) residency. In the bivariate analyses, prevalence was the strongest predictor of quitting hunting in the state followed by hypothetical human death and perceived risk. The presence of CWD in a state and residency were weak, but statistically significant, predictors. Interactions among these predictors increased the potential for stopping hunting in the state. Multivariate analyses suggested that 64% of our respondents would quit hunting in the worst-case scenario.

Crystallization and preliminary X-ray diffraction analysis of a specific VHH domain against mouse prion protein

Prion disorders are infectious diseases that are characterized by the conversion of the cellular prion protein PrPC into the pathogenic isoform PrPSc. Specific antibodies that interact with the cellular prion protein have been shown to inhibit this transition. Recombinant VHHs (variable domain of dromedary heavy-chain antibodies) or nanobodies are single-domain antibodies, making them the smallest antigen-binding fragments. A specific nanobody (Nb_PrP_01) was raised against mouse PrPC. A crystallization condition for this recombinant nanobody was identified using high-throughput screening. The crystals were optimized using streak-seeding and the hanging-drop method. The crystals belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a=30.04, b=37.15, c=83.00 Å, and diffracted to 1.23 Å resolution using synchrotron radiation. The crystal structure of this specific nanobody against PrPC together with the known PrPC structure may help in understanding the PrPC/PrPSc transition mechanism.

Induction of cerebral beta-amyloidosis: intracerebral versus systemic Abeta inoculation

Despite the importance of the aberrant polymerization of Abeta in the early pathogenic cascade of Alzheimer's disease, little is known about the induction of Abeta aggregation in vivo. Here we show that induction of cerebral beta-amyloidosis can be achieved in many different brain areas of APP23 transgenic mice through the injection of dilute Abeta-containing brain extracts. Once the amyloidogenic process has been exogenously induced, the nature of the induced Abeta-deposition is determined by the brain region of the host. Because these observations are reminiscent of a prion-like mechanism, we then investigated whether cerebral beta-amyloidosis also can be induced by peripheral and systemic inoculations or by the intracerebral implantation of stainless steel wires previously coated with minute amounts of Abeta-containing brain extract. Results reveal that oral, intravenous, intraocular, and intranasal inoculations yielded no detectable induction of cerebral beta-amyloidosis in APP23 transgenic mice. In contrast, transmission of cerebral beta-amyloidosis through the Abeta-contaminated steel wires was demonstrated. Notably, plasma sterilization, but not boiling of the wires before implantation, prevented the induction of beta-amyloidosis. Our results suggest that minute amounts of Abeta-containing brain material in direct contact with the CNS can induce cerebral beta-amyloidosis, but that systemic cellular mechanisms of prion uptake and transport to the CNS may not apply to Abeta.

The octarepeat region of prion protein, but not the TM1 domain, is important for the antioxidant effect of prion protein

The cellular prion protein (PrP(c)) plays a crucial role in the pathogenesis of prion diseases, but its physiological function is far from understood. Several candidate functions have been proposed including binding and internalization of metal ions, a superoxide dismutase-like activity, regulation of cellular antioxidant activities, and signal transduction. The transmembrane (TM1) region of PrP(c) (residues 110-135) is particularly interesting because of its very high evolutionary conservation. We investigated a possible role of TM1 in the antioxidant defense, by assessing the impact of overexpressing wt-PrP or deletion mutants in N(2)A mouse neuroblastoma cells on intracellular reactive oxygen species (ROS) levels. Under conditions of oxidative stress, intracellular ROS levels were significantly lowered in cells overexpressing either wild-type PrP(c) (wt-PrP) or a deletion mutant affecting TM1 (Delta8TM1-PrP), but, as expected, not in cultures overexpressing a deletion mutant lacking the octapeptide region (Deltaocta-PrP). Overexpression of wt-PrP, Delta8TM1-PrP, or Deltaocta-PrP did not affect basal ROS levels. Interestingly, the mitochondrial membrane potential was significantly lowered in Deltaocta-PrP-transfected cultures in the absence of oxidative stress. We conclude that the protective effect of PrP(c) against oxidative stress involves the octarepeat region but not the TM1 domain nor the high-affinity copper binding site described for human residues His96/His111.

Opposing roles of prion protein in oxidative stress- and ER stress-induced apoptotic signaling

Although the prion protein is abundantly expressed in the CNS, its biological functions remain unclear. To determine the endogenous function of the cellular prion protein (PrP(c)), we compared the effects of oxidative stress and endoplasmic reticulum (ER) stress inducers on apoptotic signaling in PrP(c)-expressing and PrP(ko) (knockout) neural cells. H(2)O(2), brefeldin A (BFA), and tunicamycin (TUN) induced increases in caspase-9 and caspase-3, PKCdelta proteolytic activation, and DNA fragmentation in PrP(c) and PrP(ko) cells. Interestingly, ER stress-induced activation of caspases, PKCdelta, and apoptosis was significantly exacerbated in PrP(c) cells, whereas H(2)O(2)-induced proapoptotic changes were suppressed in PrP(c) compared to PrP(ko) cells. Additionally, caspase-12 and caspase-8 were activated only in the BFA and TUN treatments. Inhibitors of caspase-9, caspase-3, and PKCdelta significantly blocked H(2)O(2)-, BFA-, and TUN-induced apoptosis, whereas the caspase-8 inhibitor attenuated only BFA- and TUN-induced cell death, and the antioxidant MnTBAP blocked only H(2)O(2)-induced apoptosis. Overexpression of the kinase-inactive PKCdelta(K376R) or the cleavage site-resistant PKCdelta(D327A) mutant suppressed both ER and oxidative stress-induced apoptosis. Thus, PrP(c) plays a proapoptotic role during ER stress and an antiapoptotic role during oxidative stress-induced cell death. Together, these results suggest that cellular PrP enhances the susceptibility of neural cells to impairment of protein processing and trafficking, but decreases the vulnerability to oxidative insults, and that PKCdelta is a key downstream mediator of cellular stress-induced neuronal apoptosis.

Experimental scrapie in 'plt' mice: an assessment of the role of dendritic-cell migration in the pathogenesis of prion diseases

Peripherally acquired transmissible spongiform encephalopathies display strikingly long incubation periods, during which increasing amounts of prions can be detected in lymphoid tissues. While precise sites of peripheral accumulation have been described, the mechanisms of prion transport from mucosa and skin to lymphoid and nervous tissues remain unknown. Because of unique functional abilities, dendritic cells (DCs) have been suspected to participate in prion pathogenesis. In mice inoculated subcutaneously with scrapie-infected DCs, the incubation was shorter when cells were alive as compared with killed cells, suggesting that DC functions may facilitate prion neuroinvasion. However, early propagation in lymphoid tissues seemed not importantly affected by DC vitality. Mutant (plt) mice that have deficient CCL19/CCL21 expression and DC migration displayed similar infection of secondary lymphoid organs as normal mice, regardless of the route of inoculation and scrapie strain. Under certain conditions of transcutaneous inoculation, the incubation and duration of disease were moderately prolonged in plt mice. This was not related to a milder neuropathogenesis, since plt and normal mice were equally susceptible to intracerebral prion challenge. We conclude that peripheral spreading of prions appears poorly dependent on cell migration through the chemokine/receptor system CCL19/CCL21/CCR7, although DCs might be able to help prions reach sites of neuroinvasion.

Strain fidelity of chronic wasting disease upon murine adaptation

Chronic wasting disease (CWD), a prion disease of deer and elk, is highly prevalent in some regions of North America. The establishment of mouse-adapted CWD prions has proven difficult due to the strong species barrier between mice and deer. Here we report the efficient transmission of CWD to transgenic mice overexpressing murine PrP. All mice developed disease 500 +/- 62 days after intracerebral CWD challenge. The incubation period decreased to 228 +/- 103 days on secondary passage and to 162 +/- 6 days on tertiary passage. Mice developed very large, radially structured cerebral amyloid plaques similar to those of CWD-infected deer and elk. PrP(Sc) was detected in spleen, indicating that murine CWD was lymphotropic. PrP(Sc) glycoform profiles maintained a predominantly diglycosylated PrP pattern, as seen with CWD in deer and elk, across all passages. Therefore, all pathological, biochemical, and histological strain characteristics of CWD appear to persist upon repetitive serial passage through mice. These findings indicate that the salient strain-specific properties of CWD are encoded by agent-intrinsic components rather than by host factors.

GroEL-mediated protein folding: making the impossible, possible

Protein folding is a spontaneous process that is essential for life, yet the concentrated and complex interior of a cell is an inherently hostile environment for the efficient folding of many proteins. Some proteins-constrained by sequence, topology, size, and function-simply cannot fold by themselves and are instead prone to misfolding and aggregation. This problem is so deeply entrenched that a specialized family of proteins, known as molecular chaperones, evolved to assist in protein folding. Here we examine one essential class of molecular chaperones, the large, oligomeric, and energy utilizing chaperonins or Hsp60s. The bacterial chaperonin GroEL, along with its co-chaperonin GroES, is probably the best-studied example of this family of protein-folding machine. In this review, we examine some of the general properties of proteins that do not fold well in the absence of GroEL and then consider how folding of these proteins is enhanced by GroEL and GroES. Recent experimental and theoretical studies suggest that chaperonins like GroEL and GroES employ a combination of protein isolation, unfolding, and conformational restriction to drive protein folding under conditions where it is otherwise not possible.

Inducible proteopathies

Numerous degenerative diseases are characterized by the aberrant polymerization and accumulation of specific proteins. These proteopathies include neurological disorders such as Alzheimer's disease, Parkinson's disease, Huntington's disease and the prion diseases, in addition to diverse systemic disorders, particularly the amyloidoses. The prion diseases have been shown to be transmissible by an alternative conformation of the normal cellular prion protein. Other proteopathies have been thought to be non-transmissible, but there is growing evidence that some systemic and cerebral amyloidoses can be induced by exposure of susceptible hosts to cognate molecular templates. As we review here, the mechanistic similarities among these diseases provide unprecedented opportunities for elucidating the induction of protein misfolding and assembly in vivo, and for developing an integrated therapeutic approach to degenerative proteopathies.

Copper induces conformational changes in the N-terminal part of cell-surface PrPC

Prion diseases are caused by misfolding of the cellular prion protein, PrPC. In vitro studies have shown that PrP binds copper via the octarepeat region lying within the unstructured N-terminal segment of the protein, but the significance of copper in PrP metabolism remains unclear. Here, six specific antibodies recognizing different epitope regions of PrP were used to measure the effect of copper on the conformation of the molecule at the cell surface. Binding of an antibody, E149, to an epitope within the octarepeat domain of PrP is halved in the presence of copper, whereas binding of antibodies recognizing epitope motifs C-terminal to residue 90 of PrP remain relatively unaltered under equivalent conditions. These experiments strongly suggest that copper induces localized conformational change within the N-terminal portion of cell-surface PrPC.

Rapid presymptomatic detection of PrPSc via conformationally responsive palindromic PrP peptides

Structurally unique, synthetic prion peptides provide the basis of a simple assay to serve as both a detection and signal amplification system that distinguishes the normal prion protein, PrPC, from the misfolded prion protein, PrPSc, that is associated with the occurrence of transmissible spongiform encephalopathies (TSE). Proof-of-principle has been shown on brain samples from an experimental scrapie hamster model. The assay demonstrates very sensitive detection of PrPSc in animal brain tissue with potential application for early presymptomatic detection in animal screening. Furthermore, the sensitivity of the assay could enable blood tests for this TSE disease as well as other amyloid and/or misfolded protein diseases.

Polymorphisms at codons 56 and 174 of the prion-like protein gene (PRND) are not associated with sporadic Creutzfeldt-Jakob disease

Association between sporadic Creutzfeldt-Jakob disease (CJD) and the prion-like protein gene (PRND) has been reported in the German population. To investigate whether the PRND polymorphisms are associated with an increased risk for developing sporadic CJD in the Korean population, we compared the genotype and allele frequencies of PRND polymorphisms in 110 sporadic CJD patients with those in 102 healthy Koreans. Two polymorphisms (P56L, T174 M) in Koreans were found in the open reading frame (ORF) of PRND. One heterozygote of P56L was observed in normal controls but not in sporadic CJD patients. A strong significant difference of PRND genotype frequency at codon 174 was found between the normal Korean population and various European populations. In contrast to results in the German population, our study did not show a significant difference in PRND genotype or allele frequency at codon 174 between sporadic CJD and normal controls. This was the first genetic association study of the ORF of PRND in an Asian CJD population.

RNA mutagenesis and sporadic prion diseases

The extremely low incidence of sporadic prion diseases suggests that they may arise as a rare stochastic event in otherwise healthy animals or humans. Current hypotheses for sporadic prion disease include horizontal transmission, spontaneous conversion of PrpC into PrpSc, and somatic mutation of the Prp gene. Here, we suggest RNA mutation as a possible initial event in the etiology of sporadic prion disease. The proposed model is based on (i) the fact that in Prp-expressing cells, mutations are statistically more likely to occur in the Prp mRNA population than in the corresponding two copies of the Prp gene, and (ii) the absence of RNA repair mechanisms analogous to those found for DNA mismatch correction resulting in a relatively higher rate of RNA mutations. Here, we suggest that translation of mutated Prp mRNA could lead to the synthesis of transient Prp(Sc) which results in the conversion of PrpC into PrpSc and the propagation of a disease-associated isoform. This model points to RNA mutation as a possible mechanism for the generation of sporadic prion diseases and other pathological disorders in which infectious proteins other than PrpSc might be implicated.

Creutzfeldt-Jakob disease in Slovenia from 1985 to 2003

AIM

The epidemic of bovine spongiform encephalopathy and subsequent emergence of a new variant of Creutzfeldt-Jakob disease have raised great public concern, initiating improved and prospective surveillance of human prion diseases in Europe and all over the world. This report briefly presents the epidemiology, clinical data, neuropathology, immunohistochemistry, biochemistry, and prion-protein gene analysis of Slovenian cases of Creutzfeldt-Jakob disease from January 1985 to the end of 2003.

MATERIALS, METHODS AND RESULTS

During the 19-year period, 39 suspected cases of Creutzfeldt-Jakob disease were referred and 22 were confirmed. The prion-protein gene was analyzed in 12 of the confirmed cases and the protein glycosylation pattern in 11. There was a low average incidence of Creutzfeldt-Jakob disease (0.5/million) throughout the surveillance period, but a pronounced increase between January 2001 and December 2003 (to 1.9/million/year). A high female to male ratio (2.5/1) was noted. All of the confirmed cases were defined as sporadic Creutzfeldt-Jakob disease based on the clinical data, neuropathological findings, glycosylation pattern, and gene analysis. All tested cases had a type-2 glycosylation pattern; eleven of the twelve tested patients were homozygous at codon 129 of the prion-protein gene (1 VV and 10 MM) and one was heterozygous.

CONCLUSION

The small number of Slovenian cases of sporadic Creutzfeldt-Jakob disease during the last 19 years has shown a pronounced increase in incidence, reflecting improved surveillance, and a high female to male ratio, where female cases are more than twice as numerous as male cases.

Infectious agents and age-related neurodegenerative disorders

chlamdAs with other organ systems, the vulnerability of the nervous system to infectious agents increases with aging. Several different infectious agents can cause neurodegenerative conditions, with prominent examples being human immunodeficiency virus (HIV-1) dementia and prion disorders. Such infections of the central nervous system (CNS) typically have a relatively long incubation period and a chronic progressive course, and are therefore increasing in frequency as more people live longer. Infectious agents may enter the central nervous system in infected migratory macrophages, by transcytosis across blood-brain barrier cells or by intraneuronal transfer from peripheral nerves. Synapses and lipid rafts are important sites at which infectious agents may enter neurons and/or exert their cytotoxic effects. Recent findings suggest the possibility that infectious agents may increase the risk of common age-related neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS) and stroke. While scenarios can be envisioned whereby viruses such as Chlamydia pneumoniae, herpes simplex and influenza promote damage to neurons during aging, there is no conclusive evidence for a major role of these pathogens in neurodegenerative disorders. In the case of stroke, blood vessels may be adversely affected by bacteria or viruses resulting in atherosclerosis.

Destabilizing Interactions Among [PSI +] and [PIN +] Yeast Prion Variants

The yeast Sup35 and Rnq1 proteins can exist in either the noninfectious soluble forms, [psi–]or[pin–], respectively, or the multiple infectious amyloid-like forms called [PSI+]or[PIN+] prion variants (or prion strains). It was previously shown that [PSI+] and [PIN+] prions enhance one another's de novo appearance. Here we show that specific prion variants of [PSI+] and [PIN+] disrupt each other's stable inheritance. Acquiring [PSI+] often impedes the inheritance of particular [PIN+] variants. Conversely, the presence of some [PIN+] variants impairs the inheritance of weak [PSI+] but not strong [PSI+] variants. These same [PIN+] variants generate a single-dot fluorescence pattern when a fusion of Rnq1 and green fluorescent protein is expressed. Another [PIN+] variant, which forms a distinctly different multiple-dot fluorescence pattern, does not impair [PSI+] inheritance. Thus, destabilization of prions by heterologous prions depends upon the variants involved. These findings may have implications for understanding interactions among other amyloid-forming proteins, including those associated with certain human diseases.

The Ubiquitin Proteasome System in Neurodegenerative Diseases

The ubiquitin-proteasome system targets numerous cellular proteins for degradation. In addition, modifications by ubiquitin-like proteins as well as proteins containing ubiquitin-interacting and -associated motifs modulate many others. This tightly controlled process involves multiple specific and general enzymes of the system as well as many modifying and ancillary proteins. Thus, it is not surprising that ubiquitin-mediated degradation/processing/modification regulates a broad array of basic cellular processes. Moreover, aberrations in the system have been implicated, either as a primary cause or secondary consequence, in the pathogenesis of both inherited and acquired neurodegenerative diseases. Recent findings indicate that the system is involved in the pathogenesis of Parkinson's, Alzheimer's, Huntington's, and Prion diseases as well as amyotrophic lateral sclerosis. This raises hopes for a better understanding of the pathogenetic mechanisms involved in these diseases and for the development of novel, mechanism-based therapeutic modalities.