Alpha-synuclein-immunoreactive deposits in human and animal prion diseases

Cross-talk between α-synuclein and the microtubule cytoskeleton in neurodegeneration

Looking at the puzzle that depicts the molecular determinants in neurodegeneration, many pieces are lacking and multiple interconnections among key proteins and intracellular pathways still remain unclear. Here we focus on the concerted action of α-synuclein and the microtubule cytoskeleton, whose interplay, indeed, is emerging but remains largely unexplored in both its physiology and pathology. α-Synuclein is a key protein involved in neurodegeneration, underlying those diseases termed synucleinopathies. Its propensity to interact with other proteins and structures renders the identification of neuronal death trigger extremely difficult. Conversely, the unbalance of microtubule cytoskeleton in terms of structure, dynamics and function is emerging as a point of convergence in neurodegeneration. Interestingly, α-synuclein and microtubules have been shown to interact and mediate cross-talks with other intracellular structures. This is supported by an increasing amount of evidence ranging from their direct interaction to the engagement of in-common partners and culminating with their respective impact on microtubule-dependent neuronal functions. Last, but not least, it is becoming even more clear that α-synuclein and tubulin work synergically towards pathological aggregation, ultimately resulting in neurodegeneration. In this respect, we supply a novel perspective towards the understanding of α-synuclein biology and, most importantly, of the link between α-synuclein with microtubule cytoskeleton and its impact for neurodegeneration and future development of novel therapeutic strategies.

Spatiotemporal modulations in heterotypic condensates of prion and α-synuclein control phase transitions and amyloid conversion

Biomolecular condensation via liquid-liquid phase separation of proteins and nucleic acids is associated with a range of critical cellular functions and neurodegenerative diseases. Here, we demonstrate that complex coacervation of the prion protein and α-synuclein within narrow stoichiometry results in the formation of highly dynamic, reversible, thermo-responsive liquid droplets via domain-specific electrostatic interactions between the positively-charged intrinsically disordered N-terminal segment of prion and the acidic C-terminal tail of α-synuclein. The addition of RNA to these coacervates yields multiphasic, vesicle-like, hollow condensates. Picosecond time-resolved measurements revealed the presence of transient electrostatic nanoclusters that are stable on the nanosecond timescale and can undergo breaking-and-making of interactions on slower timescales giving rise to a liquid-like behavior in the mesoscopic regime. The liquid-to-solid transition drives a rapid conversion of complex coacervates into heterotypic amyloids. Our results suggest that synergistic prion-α-synuclein interactions within condensates provide mechanistic underpinnings of their physiological role and overlapping neuropathological features.

Accumulation of Prion and Abnormal Prion Protein Induces Hyperphosphorylation of α-Synuclein in the Brain Tissues from Prion Diseases and in the Cultured Cells

Prion disease (PrD) and Parkinson's disease (PD) are neurodegenerative diseases characterized by aggregation of misfolded proteins in brain tissues, including protease-resistant prion protein (PrP^Sc) in PrD and α-synuclein in PD. In recent years, overlap of these two proteins has attracted increased attention, and cross-seeding of prion proteins by aggregated α-synuclein has been proposed. However, the changes in α-synuclein after prion infection are still unclear. In this study, we showed that α-synuclein expression was significantly decreased in the brains of prion-infected rodent models, in the SMB-S15 cell line, which exhibits persistent prion replication, and in the brains of humans with PrDs. Meanwhile, α-synuclein phosphorylated at serine 129(p(S129)-α-synuclein) was significantly increased in the brains of scrapie-infected mice and prion-infected SMB-S15 cells. The increased p(S129)-α-synuclein colocalized with GFAP- and NeuN-positive cells in the brains of scrapie-infected mice. p(S129)-α-synuclein was also observed in the cytoplasm of SMB-S15 and HEK-293 cells transiently expressing an abnormal form of prion protein (Cyto-PrP). Molecular interactions between PrP and α-synuclein were detected in recombinant proteins, normal and prion-infected brain tissues, and cultured cells. The increased p(S129)-α-synuclein colocalized with PrP signals from prion-infected SMB-S15 and HEK-293 cells expressing Cyto-PrP. Moreover, increased morphological colocalization of p(S129)-α-synuclein with mitochondrial markers was also detected in the two cell types. Our results indicate that prion replication and accumulation in cells and brains induce hyperphosphorylation of α-synuclein, particularly at S129, which may aggravate mitochondrial damage and facilitate α-synuclein aggregation in the central nervous system tissues from PrDs.

Understanding Creutzfeldt-Jackob disease from a viewpoint of amyloidogenic evolvability

Creutzfeldt-Jackob disease (CJD), the most common human prion disorder, is frequently accompanied by ageing-associated neurodegenerative conditions, such as Alzheimer’s disease and Parkinson’s disease. Although cross-seeding of amyloidogenic proteins (APs), including amyloid β and α-synuclein, may be critical in the co-morbidity of neurodegenerative disorders, the direct interaction of APs with prion protein (PrP), the central molecule involved in the pathogenesis of CJD, is unlikely. Currently, the nature of this biological interaction and its significance remain obscure. In this context, the objective of the present study is to discuss such interactions from the perspective of amyloidogenic evolvability, a putative function of APs. Hypothetically, both hereditary- and sporadic CJD might be attributed to the role of PrP in evolvability against multiple stressors, such as physical stresses relevant to concussions, which might be manifest through the antagonistic pleiotropy mechanism in ageing. Furthermore, accumulating evidence suggests that PrP- and other APs evolvability may negatively regulate each other. Provided that increased APs evolvability might be beneficial for acquired CJD in young adults, a dose-reduction of α-synuclein, a natural inhibitor of αS aggregation, might be therapeutically effective in upregulating APs evolvability. Collectively, a better understanding of amyloidogenic evolvability may lead to the development of novel therapies for CJD.

Is Multiple System Atrophy a Prion-like Disorder?

Multiple system atrophy (MSA) is a rapidly progressive, fatal neurodegenerative disease of uncertain aetiology that belongs to the family of α-synucleinopathies. It clinically presents with parkinsonism, cerebellar, autonomic, and motor impairment in variable combinations. Pathological hallmarks are fibrillary α-synuclein (αSyn)-rich glial cytoplasmic inclusions (GCIs) mainly involving oligodendroglia and to a lesser extent neurons, inducing a multisystem neurodegeneration, glial activation, and widespread demyelinization. The neuronal αSyn pathology of MSA has molecular properties different from Lewy bodies in Parkinson's disease (PD), both of which could serve as a pool of αSyn (prion) seeds that could initiate and drive the pathogenesis of synucleinopathies. The molecular cascade leading to the "prion-like" transfer of "strains" of aggregated αSyn contributing to the progression of the disease is poorly understood, while some presented evidence that MSA is a prion disease. However, this hypothesis is difficult to reconcile with postmortem analysis of human brains and the fact that MSA-like pathology was induced by intracerebral inoculation of human MSA brain homogenates only in homozygous mutant 53T mice, without production of disease-specific GCIs, or with replication of MSA prions in primary astrocyte cultures from transgenic mice expressing human αSyn. Whereas recent intrastriatal injection of Lewy body-derived or synthetic human αSyn fibrils induced PD-like pathology including neuronal αSyn aggregates in macaques, no such transmission of αSyn pathology in non-human primates by MSA brain lysate has been reported until now. Given the similarities between αSyn and prions, there is a considerable debate whether they should be referred to as "prions", "prion-like", "prionoids", or something else. Here, the findings supporting the proposed nature of αSyn as a prion and its self-propagation through seeding as well as the transmissibility of neurodegenerative disorders are discussed. The proof of disease causation rests on the concordance of scientific evidence, none of which has provided convincing evidence for the classification of MSA as a prion disease or its human transmission until now.

Biophysical processes underlying cross-seeding in amyloid aggregation and implications in amyloid pathology

Abnormal aggregation of proteins into filamentous aggregates commonly associates with many diseases, such as Alzheimer's disease, Parkinson's disease and type-2 diabetes. These filamentous aggregates, also known as amyloids, can propagate their abnormal structures to either the same precursor molecules (seeding) or other protein monomers (cross-seeding). Cross-seeding has been implicated in the abnormal protein aggregation and has been found to facilitate the formation of physiological amyloids. It has risen to be an exciting area of research with a high volume of published reports. In this review article, we focus on the biophysical processes underlying the cross-seeding for some of the most commonly studied amyloid proteins. Here we will discuss the relevant literature related to cross-seeded polymerization of amyloid-beta, human islet amyloid polypeptide (hIAPP, or also known as amylin) and alpha-synuclein. SEVI (semen-derived enhancer of viral infection) amyloid formation by the cross-seeding between the bacterial curli protein and PAP248-286 is also briefly discussed.

Beyond the synucleinopathies: alpha synuclein as a driving force in neurodegenerative comorbidities

The fundamental role that alpha-synuclein (aSyn) plays in the pathogenesis of neurodegenerative synucleinopathies, including Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy, is a well-accepted fact. A wealth of experimental evidence has linked this relatively small but ubiquitously expressed protein to a plethora of cytopathologic mechanisms and suggests that aSyn may be capable of seeding the progressive spread of synucleinopathy throughout the brain. Beyond the synucleinopathies, the abnormal deposition of aSyn is frequently seen in a variety of other neurodegenerative proteinopathies including Alzheimer’s disease. In spite of the fact that the frequency of concomitant aSyn pathology in these disorders is such that it can be considered the rule rather than the exception, the potential role that aSyn may have in these disorders has received relatively little attention.

In this article we postulate that aSyn may in fact be a key protein in driving the pathogenic processes in neurodegenerative comorbidities. In addition to reviewing the frequency of concomitant deposition of aSyn in the neurodegenerative proteinopathies, we also consider our current understanding of the interaction of aSyn with other neurodegenerative disease-associated proteins, including tau, TDP-43, amyloid-β and prion protein, in the context of neuropathologic studies describing the anatomical sites of potential concomitant pathology. We conclude that a growing body of evidence, encompassing neuropathology studies in human brain, animal models of concomitant proteinopathies and studies employing sophisticated methods of probing protein-protein interaction, cumulatively suggest that aSyn is well positioned to exert a strong influence on the pathogenesis of the neurodegenerative comorbidities.

We hope to stimulate research in this emerging field and consider that future studies exploring the contribution of aSyn to the pathogenic processes in neurodegenerative comorbidities may provide critical information pertaining to diagnosis and the development of vital disease modifying treatments for these devastating diseases.

Prion Efficiently Replicates in α-Synuclein Knockout Mice

Prion diseases are a group of neurodegenerative disorders associated with the conformational conversion of the cellular prion protein (PrP^C) into an abnormal misfolded form named PrP^Sc. Other than accumulating in the brain, PrP^Sc can bind PrP^C and force it to change conformation to PrP^Sc. The exact mechanism which underlies the process of PrP^C/PrP^Sc conversion still needs to be defined and many molecules or cofactors might be involved. Several studies have documented an important role of PrP^C to act as receptor for abnormally folded forms of α-synuclein which are responsible of a group of diseases known as synucleinopathies. The presence of PrP^C was required to promote efficient internalization and spreading of abnormal α-synuclein between cells. In this work, we have assessed whether α-synuclein exerts any role in PrP^Sc conversion and propagation either in vitro or in vivo. Indeed, understanding the mechanism of PrP^C/PrP^Sc conversion and the identification of cofactors involved in this process is crucial for developing new therapeutic strategies. Our results showed that PrP^Sc was able to efficiently propagate in the brain of animals even in the absence of α-synuclein thus suggesting that this protein did not act as key modulator of prion propagation. Thus, α-synuclein might take part in this process but is not specifically required for sustaining prion conversion and propagation.

The role of the prion protein in the internalization of α-synuclein amyloids

Synucleinopathies are a group of neurodegenerative diseases characterized by the accumulation of α-synuclein amyloids in several regions of the brain. α-Synuclein fibrils are able to spread via cell-to-cell transfer, and once inside the cells, they can template the misfolding and aggregation of the endogenous α-synuclein. Multiple mechanisms have been shown to participate in the process of propagation: endocytosis, tunneling nanotubes and macropinocytosis. Recently, we published a research showing that the cellular form of the prion protein (PrP^C) acts as a receptor for α-synuclein amyloid fibrils, facilitating their internalization through and endocytic pathway. This interaction occurs by a direct interaction between the fibrils and the N-terminal domain of PrP^C. In cell lines expressing the pathological form of PrP (PrP^Sc), the binding between PrP^C and α-synuclein fibrils prevents the formation and accumulation of PrP^Sc, since PrP^C is no longer available as a substrate for the pathological conversion templated by PrP^Sc. On the contrary, PrP^Sc deposits are cleared over passages, probably due to the increased processing of PrP^C into the neuroprotective fragments N1 and C1. Starting from these data, in this work we present new insights into the role of PrP^C in the internalization of protein amyloids and the possible therapeutic applications of these findings.

α-Synuclein Amyloids Hijack Prion Protein to Gain Cell Entry, Facilitate Cell-to-Cell Spreading and Block Prion Replication

The precise molecular mechanism of how misfolded α-synuclein (α-Syn) accumulates and spreads in synucleinopathies is still unknown. Here, we show the role of the cellular prion protein (PrP^C) in mediating the uptake and the spread of recombinant α-Syn amyloids. The in vitro data revealed that the presence of PrP^C fosters the higher uptake of α-Syn amyloid fibrils, which was also confirmed in vivo in wild type (Prnp^+/+) compared to PrP knock-out (Prnp^−/−) mice. Additionally, the presence of α-Syn amyloids blocked the replication of scrapie prions (PrP^Sc) in vitro and ex vivo, indicating a link between the two proteins. Indeed, whilst PrP^C is mediating the internalization of α-Syn amyloids, PrP^Sc is not able to replicate in their presence. This observation has pathological relevance, since several reported case studies show that the accumulation of α-Syn amyloid deposits in Creutzfeldt-Jakob disease patients is accompanied by a longer disease course.

Cross-seeding of prions by aggregated α-synuclein leads to transmissible spongiform encephalopathy

Aggregation of misfolded proteins or peptides is a common feature of neurodegenerative diseases including Alzheimer's, Parkinson's, Huntington's, prion and other diseases. Recent years have witnessed a growing number of reports of overlap in neuropathological features that were once thought to be unique to only one neurodegenerative disorder. However, the origin for the overlap remains unclear. One possibility is that diseases with mixed brain pathologies might arise from cross-seeding of one amyloidogenic protein by aggregated states of unrelated proteins. In the current study we examined whether prion replication can be induced by cross-seeding by α-synuclein or Aβ peptide. We found that α-synuclein aggregates formed in cultured cells or in vitro display cross-seeding activity and trigger misfolding of the prion protein (PrPC) in serial Protein Misfolding Cyclic Amplification reactions, producing self-replicating PrP states characterized by a short C-terminal proteinase K (PK)-resistant region referred to as PrPres. Non-fibrillar α-synuclein or fibrillar Aβ failed to cross-seed misfolding of PrPC. Remarkably, PrPres triggered by aggregated α-synuclein in vitro propagated in animals and, upon serial transmission, produced PrPSc and clinical prion disease characterized by spongiosis and astrocytic gliosis. The current study demonstrates that aggregated α-synuclein is potent in cross-seeding of prion protein misfolding and aggregation in vitro, producing self-replicating states that can lead to transmissible prion diseases upon serial passaging in wild type animals. In summary, the current work documents direct cross-seeding between unrelated amyloidogenic proteins associated with different neurodegenerative diseases. This study suggests that early interaction between unrelated amyloidogenic proteins might underlie the etiology of mixed neurodegenerative proteinopathies.

Multiple system atrophy: the application of genetics in understanding etiology

Classically defined phenotypically by a triad of cerebellar ataxia, parkinsonism, and autonomic dysfunction in conjunction with pyramidal signs, multiple system atrophy (MSA) is a rare and progressive neurodegenerative disease affecting an estimated 3-4 per every 100,000 individuals among adults 50-99 years of age. With a pathological hallmark of alpha-synuclein-immunoreactive glial cytoplasmic inclusions (GCIs; Papp-Lantos inclusions), MSA patients exhibit marked neurodegenerative changes in the striatonigral and/or olivopontocerebellar structures of the brain. As a member of the alpha-synucleinopathy family, which is defined by its well-demarcated alpha-synuclein-immunoreactive inclusions and aggregation, MSA's clinical presentation exhibits several overlapping features with other members including Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Given the extensive fund of knowledge regarding the genetic etiology of PD revealed within the past several years, a genetic investigation of MSA is warranted. While a current genome-wide association study is underway for MSA to further clarify the role of associated genetic loci and single-nucleotide polymorphisms, several cases have presented solid preliminary evidence of a genetic etiology. Naturally, genes and variants manifesting known associations with PD (and other phenotypically similar neurodegenerative disorders), including SNCA and MAPT, have been comprehensively investigated in MSA patient cohorts. More recently variants in COQ2 have been linked to MSA in the Japanese population although this finding awaits replication. Nonetheless, significant positive associations with subsequent independent replication studies have been scarce. With very limited information regarding genetic mutations or alterations in gene dosage as a cause of MSA, the search for novel risk genes, which may be in the form of common variants or rare variants, is the logical nexus for MSA research. We believe that the application of next generation genetic methods to MSA will provide valuable insight into the underlying causes of this disease, and will be central to the identification of etiologic-based therapies.

Parkinson's disease as a member of Prion-like disorders

Parkinson's disease is one of several neurodegenerative diseases associated with a misfolded, aggregated and pathological protein. In Parkinson's disease this protein is alpha-synuclein and its neuronal deposits in the form of Lewy bodies are considered a hallmark of the disease. In this review we describe the clinical and experimental data that have led to think of alpha-synuclein as a prion-like protein and we summarize data from in vitro, cellular and animal models supporting this view.

Is tau ready for admission to the prion club?

Aggregation-prone proteins associated with neurodegenerative disease, such as α synuclein and β amyloid, now appear to share key prion-like features with mammalian prion protein, such as the ability to recruit normal proteins to aggregates and to translocate between neurons. These features may shed light on the genesis of stereotyped lesion development patterns in conditions such as Alzheimer disease and Lewy Body dementia. We discuss the qualifications of tau protein as a possible "prionoid" mediator of lesion spread based on recent characterizations of the secretion, uptake and transneuronal transfer of human tau isoforms in a variety of tauopathy models, and in human patients. In particular, we consider (1) the possibility that prionoid behavior of misprocessed tau in neurodegenerative disease may involve other aggregation-prone proteins, including PrP itself, and (2) whether "prionlike" tau lesion propagation might include mechanisms other than protein-protein templating.

Gene expression alterations in Rocky Mountain elk infected with chronic wasting disease

Chronic wasting disease (CWD) is an invariably fatal neurologic disease that naturally infects mule deer, white tailed deer and elk. The understanding of CWD neurodegeneration at a molecular level is very limited. In this study, microarray analysis was performed to determine changes in the gene expression profiles in six different tissues including brain, midbrain, thalamus, spleen, RPLN and tonsil of CWD-infected elk in comparison to non-infected healthy elk, using 24,000 bovine specific oligo probes. In total, 329 genes were found to be differentially expressed (> 2.0-fold) between CWD negative and positive brain tissues, with 132 genes upregulated and 197 genes downregulated. There were 249 DE genes in the spleen (168 up- and 81 downregulated), 30 DE genes in the retropharyngeal lymph node (RPLN) (18 up- and 12 downregulated), and 55 DE genes in the tonsil (21 up- and 34 downregulated). Using Gene Ontology (GO), the DE genes were assigned to functional groups associated with cellular process, biological regulation, metabolic process, and regulation of biological process. For all brain tissues, the highest ranking networks for DE genes identified by Ingenuity Pathway Analysis (IPA) were associated with neurological disease, cell morphology, cellular assembly and organization. Quantitative real-time PCR (qRT-PCR) validated the expression of DE genes primarily involved in different regulatory pathways, including neuronal signaling and synapse function, calcium signaling, apoptosis and cell death and immune cell trafficking and inflammatory response. This is the first study to evaluate altered gene expression in multiple organs including brain from orally infected elk and the results will improve our understanding of CWD neurodegeneration at the molecular level.

Prion infection promotes extensive accumulation of α-synuclein in aged human α-synuclein transgenic mice

In neurodegenerative disorders of the aging population, misfolded proteins, such as PrP(Sc), α-synuclein, amyloid β protein and tau, can interact resulting in enhanced aggregation, cross seeding and accelerated disease progression. Previous reports have shown that in Creutzfeldt-Jakob disease and scrapie, α-synuclein accumulates near PrP(Sc) deposits. However, it is unclear if pre-existing human α-synuclein aggregates modified prion disease pathogenesis, or if PrP(Sc) exacerbates the α-synuclein pathology. Here, we inoculated infectious prions into aged α-synuclein transgenic (tg) and non-transgenic littermate control mice by the intracerebral route. Remarkably, inoculation of RML and mNS prions into α-synuclein tg mice resulted in more extensive and abundant intraneuronal and synaptic α-synuclein accumulation. In addition, infectious prions led to the formation of perineuronal α-synuclein deposits with a neuritic plaque-like appearance. Prion pathology was unmodified by the presence of α-synuclein. However, with the mNS prion strain there was a modest but significant acceleration in the time to terminal prion disease in mice having α-synuclein aggregates as compared with non-tg mice. Taken together, these studies support the notion that PrP(Sc) directly or indirectly promotes α-synuclein pathology.

Genesis of mammalian prions: from non-infectious amyloid fibrils to a transmissible prion disease

The transmissible agent of prion disease consists of a prion protein in its abnormal, β-sheet rich state (PrP^Sc), which is capable of replicating itself according to the template-assisted mechanism. This mechanism postulates that the folding pattern of a newly recruited polypeptide chain accurately reproduces that of a PrP^Sc template. Here we report that authentic PrP^Sc and transmissible prion disease can be generated de novo in wild type animals by recombinant PrP (rPrP) amyloid fibrils, which are structurally different from PrP^Sc and lack any detectable PrP^Sc particles. When induced by rPrP fibrils, a long silent stage that involved two serial passages preceded development of the clinical disease. Once emerged, the prion disease was characterized by unique clinical, neuropathological, and biochemical features. The long silent stage to the disease was accompanied by significant transformation in neuropathological properties and biochemical features of the proteinase K-resistant PrP material (PrPres) before authentic PrP^Sc evolved. The current work illustrates that transmissible prion diseases can be induced by PrP structures different from that of authentic PrP^Sc and suggests that a new mechanism different from the classical templating exists. This new mechanism designated as “deformed templating” postulates that a change in the PrP folding pattern from the one present in rPrP fibrils to an alternative specific for PrP^Sc can occur. The current work provides important new insight into the mechanisms underlying genesis of the transmissible protein states and has numerous implications for understanding the etiology of neurodegenerative diseases.

The transmissible agent of prion disease consists of a prion protein in its abnormal conformation (PrP^Sc), which replicates itself according to the template-assisted mechanism. This mechanism postulates that the folding pattern of a newly recruited polypeptide chain accurately reproduces that of a PrP^Sc. The current study reports that infectious prions and transmissible prion disease can be triggered in wild type animals by amyloid fibrils produced from recombinant prion prtotein, which are structurally different from PrP^Sc and lacks any detectable PrP^Sc particles. This work introduces a new hypothesis that transmissible prion diseases can be induced by prion protein structures different from that of authentic PrP^Sc and suggests that a new mechanism for triggering PrP^Sc formation different from the classical templating exists. The current work provides important new insight into the mechanisms underlying genesis and evolution of the transmissible states of the prion protein and has numerous implications for understanding the etiology of prion and other neurodegenerative diseases.

Transmission of prion strains in a transgenic mouse model overexpressing human A53T mutated α-synuclein

There is a growing interest in the potential roles of misfolded protein interactions in neurodegeneration. To investigate this issue, we inoculated 3 prion strains intracerebrally into transgenic (TgM83) mice that overexpress human A53T α-synuclein. In comparison to nontransgenic controls, there was a striking decrease in the incubation periods of scrapie, classic and H-type bovine spongiform encephalopathies(C-BSE and H-BSE), with conservation of the histopathologic and biochemical features characterizing these 3 prion strains. TgM83 mice died of scrapie or C-BSE prion diseases before accumulating the insoluble and phosphorylated forms of α-synuclein specific to late stages of synucleinopathy. In contrast, the median incubation time for TgM83 mice inoculated with H-BSE was comparable to that observed when these mice were uninfected, thereby allowing the development of molecular alterations of α-synuclein. The last 4 mice of this cohort exhibited early accumulations of H-BSE prion protein along with α-synuclein pathology. The results indicate that a prion disease was triggered concomitantly with an overt synucleinopathy in some transgenic mice overexpressing human A53T α-synuclein after intracerebral inoculation with an H-BSE prion strain.

Genetic Creutzfeldt-Jakob disease associated with the E200K mutation: characterization of a complex proteinopathy

The E200K mutation is the most frequent prion protein gene (PRNP) mutation detected worldwide that is associated with Creutzfeldt-Jakob disease (CJD) and thought to have overlapping features with sporadic CJD, yet detailed neuropathological studies have not been reported. In addition to the prion protein, deposition of tau, α-synuclein, and amyloid-β has been reported in human prion disease. To describe the salient and concomitant neuropathological alterations, we performed a systematic clinical, neuropathological, and biochemical study of 39 individuals carrying the E200K PRNP mutation originating from different European countries. The most frequent clinical symptoms were dementia and ataxia followed by myoclonus and various combinations of further symptoms, including vertical gaze palsy and polyneuropathy. Neuropathological examination revealed relatively uniform anatomical pattern of tissue lesioning, predominating in the basal ganglia and thalamus, and also substantia nigra, while the deposition of disease-associated PrP was more influenced by the codon 129 constellation, including different or mixed types of PrP(res) detected by immunoblotting. Unique and prominent intraneuronal PrP deposition involving brainstem nuclei was also noted. Systematic examination of protein depositions revealed parenchymal amyloid-β in 53.8%, amyloid angiopathy (Aβ) in 23.1%, phospho-tau immunoreactive neuritic profiles in 92.3%, neurofibrillary degeneration in 38.4%, new types of tau pathology in 33.3%, and Lewy-type α-synuclein pathology in 15.4%. TDP-43 and FUS immunoreactive protein deposits were not observed. This is the first demonstration of intensified and combined neurodegeneration in a genetic prion disease due to a single point mutation, which might become an important model to decipher the molecular interplay between neurodegeneration-associated proteins.

Protein coding of neurodegenerative dementias: the neuropathological basis of biomarker diagnostics

Neuropathological diagnosis of neurodegenerative dementias evolved by adapting the results of neuroanatomy, biochemistry, and cellular and molecular biology. Milestone findings of intra- and extracellular argyrophilic structures, visualizing protein deposition, initiated a protein-based classification. Widespread application of immunohistochemical and biochemical investigations revealed that (1) there are modifications of proteins intrinsic to disease (species that are phosphorylated, nitrated, oligomers, proteinase-resistant, with or without amyloid characteristics; cleavage products), (2) disease forms characterized by the accumulation of a single protein only are rather the exception than the rule, and (3) some modifications of proteins elude present neuropathological diagnostic procedures. In this review, we summarize how neuropathology, together with biochemistry, contributes to disease typing, by demonstrating a spectrum of disorders characterized by the deposition of various modifications of various proteins in various locations. Neuropathology may help to elucidate how brain pathologies alter the detectability of proteins in body fluids by upregulation of physiological forms or entrapment of different proteins. Modifications of at least the five most relevant proteins (amyloid-beta, prion protein, tau, alpha-synuclein, and TDP-43), aided by analysis of further "attracted" proteins, are pivotal to be evaluated simultaneously with different methods. This should complement the detection of biomarkers associated with pathogenetic processes, and also neuroimaging and genetic analysis, in order to obtain a highly personalized diagnostic profile. Defining clusters of patients based on the patterns of protein deposition and immunohistochemically or biochemically detectable modifications of proteins ("codes") may have higher prognostic predictive value, may be useful for monitoring therapy, and may open new avenues for research on pathogenesis.

Molecular pathology of human prion diseases

Prion diseases are fatal neurodegenerative conditions in humans and animals. In this review, we summarize the molecular background of phenotypic variability, relation of prion protein (PrP) to other proteins associated with neurodegenerative diseases, and pathogenesis of neuronal vulnerability. PrP exists in different forms that may be present in both diseased and non-diseased brain, however, abundant disease-associated PrP together with tissue pathology characterizes prion diseases and associates with transmissibility. Prion diseases have different etiological background with distinct pathogenesis and phenotype. Mutations of the prion protein gene are associated with genetic forms. The codon 129 polymorphism in combination with the Western blot pattern of PrP after proteinase K digestion serves as a basis for molecular subtyping of sporadic Creutzfeldt-Jakob disease. Tissue damage may result from several parallel, interacting or subsequent pathways that involve cellular systems associated with synapses, protein processing, oxidative stress, autophagy, and apoptosis.

PRNP M129V homozygosity in multiple system atrophy vs. Parkinson's disease

Multiple system atrophy (MSA) is a neurodegenerative disorder of unknown etiology characterized by extrapyramidal, pyramidal, cerebellar, and autonomic dysfunction in any combination. We report a patient with a 4-year history of MSA who developed dementia associated with sporadic Creutzfeldt-Jakob disease (CJD). Our proband was MM homozygous for the M129V polymorphism within the prion protein gene (PRNP), a known risk factor for CJD. We conducted a case-control study to test the hypothesis that homozygosity for the M129V polymorphism of PRNP occurs more frequently in MSA in comparison to Parkinson's disease and healthy volunteers. A total of 63 patients with MSA, 54 age-, race- and gendermatched controls with Parkinson's disease, and 126 matched healthy volunteers were studied. The genotype analysis revealed no significant difference in the codon 129 genotype distribution in MSA as compared to controls. Nonetheless, the frequencies of the MM and VV genotypes were higher in MSA than in Parkinson's disease. Thus, homozygosity, particularly VV homozygosity, at codon 129 of PRNP is associated with MSA compared to a clinically related but pathophysiologically distinct alpha-synucleinopathy. Considering the possibility that the prion protein contributes to the pathogenesis of MSA would require confirmation of these findings in an independent patient population.

Neuronal and glial accumulation of alpha- and beta-synucleins in human lipidoses

A number of the lysosomal storage diseases that have now been characterized are associated with intra-lysosomal accumulation of lipids, caused by defective lysosomal enzymes. We have previously reported neuronal accumulation of both alpha- and beta-synucleins in brain tissue of a GM2 gangliosidosis mouse model. Although alpha-synuclein has been implicated in several neurodegenerative disorders including Parkinson's disease, dementia with Lewy bodies and multiple system atrophy, its functions remain largely unclear. In our present study, we have examined a cohort of human lipidosis cases, including Sandhoff disease, Tay-Sachs disease, metachromatic leukodystrophy, beta-galactosialidosis and adrenoleukodystrophy, for the expression of alpha- and beta-synucleins and the associated lipid storage levels. The accumulation of alpha-synuclein was found in brain tissue in not only cases of lysosomal storage diseases, but also in instances of adrenoleukodystrophy, which is a peroxisomal disease. alpha-synuclein was detected in both neurons and glial cells of patients with these two disorders, although its distribution was found to be disease-dependent. In addition, alpha-synuclein-positive neurons were also found to be NeuN-positive, whereas NeuN-negative neurons did not show any accumulation of this protein. By comparison, the accumulation of beta-synuclein was detectable only in the pons of Sandhoff disease cases. This differential accumulation of alpha- and beta-synucleins in human lipidoses may be related to functional differences between these two proteins. In addition, the accumulation of alpha-synuclein may also be a condition that is common to lysosomal storage diseases and adrenoleukodystrophies that show an enhanced expression of this protein upon the elevation of stored lipids.

Highly promiscuous nature of prion polymerization

The primary structure of the prion protein (PrP) is believed to be the key factor in regulating the species barrier of prion transmission. Because the strength of the species barrier was found to be affected by the prion strain, the extent to which the barrier can indeed be attributed to differences in the PrP primary structures of either donor and acceptor species remains unclear. In this study, we exploited the intrinsic property of PrP to polymerize spontaneously into disease-related amyloid conformations in the absence of a strain-specified template and analyzed polymerization of mouse and hamster full-length recombinant PrPs. Unexpectedly, we found no evidence of species specificity in cross-seeding polymerization assays. Even when both recombinant PrP variants were present in mixtures, preformed mouse or hamster fibrils displayed no selectivity in elongation reactions and consumed equally well both homologous and heterologous substrates. Analysis of individual fibrils revealed that fibrils can elongate in a bidirectional or unidirectional manner. Our work revealed that, in the absence of a cellular environment, post-translational modifications, or strain-specified conformational constraints, PrP fibrils are intrinsically promiscuous and capable of utilizing heterologous PrP variants as a substrate in a highly efficient manner. This study suggests that amyloid structures are capable of accommodating local perturbations arising because of a mismatch in amino acid sequences and highlights the promiscuous nature of the self-propagating activity of amyloid fibrils.

Alpha-synuclein Accumulates in the Brain of Scrapie-affected Sheep and Goats

Immunohistochemical examination demonstrated widespread granular deposits of alpha-synuclein (alphaSN) in the brains of sheep and goats with natural scrapie, especially in the cornu ammonis and subiculum of the hippocampus; this contrasted with the diffuse and non-granular immunolabelling seen in healthy controls. There was non-regular "co-localization" of PrP(Sc) and alphaSN. The findings resembled those reported in Creutzfeldt-Jakob disease and in experimental prion disease in hamsters and mice. The results suggest that perturbation of alphaSN metabolism plays a role in human and animal prion diseases.

Glial degeneration and reactive gliosis in alpha-synucleinopathies: the emerging concept of primary gliodegeneration

The concept of gliodegenerative diseases has not been widely established although there is accumulating evidence that glial cells may represent a primary target of degenerative disease processes. In the central nervous system (CNS), examples that provide a "proof of concept" include at least one alpha-synucleinopathy, multiple system atrophy (MSA), but this disease is conventionally discussed under the heading of "neurodegeneration". Additional evidence in support of primary glial affection has been reported in neurodegenerative disorders such as Parkinson's disease, Alzheimer's disease and transmissible spongiform encephalopathies. Based on biochemical, genetic and transcriptomic studies it is also becoming increasingly clear that the molecular changes measured in whole tissue extracts, e.g. obtained from Parkinson's disease brain, are not based on a purely neuronal contribution. This important evidence has been missed in cell culture or laser capture work focusing on the neuronal cell population. Studies of animal and in vitro models of disease pathogenesis additionally suggest glial accountability for some CNS degenerative processes. This review provides a critical analysis of the evidence available to date in support of the concept of gliodegeneration, which we propose to represent an essential although largely disregarded component of the spectrum of classical "neurodegeneration". Examples from the spectrum of alpha-synucleinopathies are presented.

The human prion gene M129V polymorphism is not associated with idiopathic Parkinson's disease in three distinct populations

Coexistence of prion disease and idiopathic Parkinson's disease (IPD) has been previously described. It remains unclear whether this relationship may reflect the high incidence of IPD or whether both prion and IPD share common pathogenetic mechanisms. For this reason, we investigated the genotype distribution of the M129V polymorphism of the human prion gene for association with IPD (controls: n = 398, IPD cases: n = 400). No association between genotypes in codon 129 and IPD was detected in three distinct populations, suggesting that this PRNP polymorphism has no direct influence on the susceptibility to IPD.

Analysis of the polymorphic prion protein gene codon 129 in idiopathic Parkinson's disease

Idiopathic Parkinson's disease (IPD) is a neurodegenerative disorder of unknown aetiology. Histopathological similarities between IPD and Creutzfeldt-Jakob prion disease (CJD) have been suggested. Homozygosity at polymorphic prion protein gene codon 129 (PRNP129) is a risk factor for developing CJD. Therefore we investigated a putative genetic link between CJD and IPD by studying PRNP129 genotype segregation in 81 patients with IPD. We did not ascertain a different PRNP129 genotype distribution in IPD patients compared to healthy Germans. We found a significant difference in PRNP129 genotype in dependence of the clinical predominance type of IPD. Patients with tremor-dominant IPD presented less frequent a methionine homozygosis at PRNP129 than hypokinetic-rigid IPD patients (30% versus 62.5%; p<0.033). In conclusion, genotype distribution at codon 129 is obviously not essential in determining IPD. But our results may provide first evidence of an association between certain PRNP129 polymorphisms and the clinical presentation of IPD.

Neuronal accumulation of alpha- and beta-synucleins in the brain of a GM2 gangliosidosis mouse model

Sandhoff disease (SD) is a heritable lysosomal storage disease resulting from impaired degradation of GM2 ganglioside. The hallmark pathology of the SD model mouse brain is GM2 ganglioside accumulation in neurons. In the present study, we immunohistochemically investigated the neuronal pathology in SD mouse brains, and demonstrated neuronal accumulation of alpha- and beta-synucleins in addition to GM2 ganglioside. Synuclein-positive neurons were extensively observed throughout SD mouse brains, although the distribution of beta-synuclein was less extensive than that of alpha-synuclein. Synuclein-positive neurons were negative to ubiquitin and PHF-tau. These findings suggest that neuronal synucleins may accumulate secondarily to GM2 ganglioside in SD mouse brains, and that neuronal accumulation of synucleins may be more critical than that of GM2 ganglioside for SD mice.

The prion protein in human neurodegenerative disorders

We evaluate cellular prion protein (PrP(C)) immunoreactivity (IR) in Alzheimer's, Parkinson's, diffuse Lewy body, and motor neuron diseases (MND), progressive supranuclear palsy, and multiple system atrophy. We use immunohistochemistry for PrP, including five monoclonal antibodies against different epitopes and three different pretreatments, alpha-synuclein, phosphorylated tau, beta-amyloid, and ubiquitin. Disease-specific inclusions are devoid of PrP(C) IR. Using double immunofluorescence and confocal laser microscopy we observe focal overlapping of PrP(C) with tau and with alpha-synuclein in early, but not in fully developed inclusions. However, PrP(C) IR neurons may contain abnormal tau or alpha-synuclein aggregates. Additionally, we observe a loss of PrP(C) IR in anterior horn neurons in MND. Our results suggest that expression of PrP(C) reflects a general response to cellular stress rather than specific co-operation in aggregation of other proteins.