Cellular Prion Protein Mediates α-Synuclein Uptake, Localization, and Toxicity In Vitro and In Vivo

PrP meets alpha-synuclein: Molecular mechanisms and implications for disease

The discovery of prions has challenged dogmas and has revolutionized our understanding of protein-misfolding diseases. The concept of self-propagation via protein conformational changes, originally discovered for the prion protein (PrP), also applies to other proteins that exhibit similar behavior, such as alpha-synuclein (aSyn), a central player in Parkinson's disease and in other synucleinopathies. aSyn pathology appears to spread from one cell to another during disease progression, and involves the misfolding and aggregation of aSyn. How the transfer of aSyn between cells occurs is still being studied, but one important hypothesis involves receptor-mediated transport. Interestingly, recent studies indicate that the cellular prion protein (PrPC) may play a crucial role in this process. PrPC has been shown to act as a receptor/sensor for protein aggregates in different neurodegenerative disorders, including Alzheimer's disease and amyotrophic lateral sclerosis. Here, we provide a comprehensive overview of the current state of knowledge regarding the interaction between aSyn and PrPC and discuss its role in synucleinopathies. We examine the properties of PrP and aSyn, including their structure, function, and aggregation. Additionally, we discuss the current understanding of PrPC's role as a receptor/sensor for aSyn aggregates and identify remaining unanswered questions in this area of research. Ultimately, we posit that exploring the interaction between aSyn and PrPC may offer potential treatment options for synucleinopathies.

Efficient characterization of multiple binding sites of small molecule imaging ligands on amyloid-beta, tau and alpha-synuclein

PURPOSE

There is an unmet need for compounds to detect fibrillar forms of alpha-synuclein (αSyn) and 4-repeat tau, which are critical in many neurodegenerative diseases. Here, we aim to develop an efficient surface plasmon resonance (SPR)-based assay to facilitate the characterization of small molecules that can bind these fibrils.

METHODS

SPR measurements were conducted to characterize the binding properties of fluorescent ligands/compounds toward recombinant amyloid-beta (Aβ)42, K18-tau, full-length 2N4R-tau and αSyn fibrils. In silico modeling was performed to examine the binding pockets of ligands on αSyn fibrils. Immunofluorescence staining of postmortem brain tissue slices from Parkinson's disease patients and mouse models was performed with fluorescence ligands and specific antibodies.

RESULTS

We optimized the protocol for the immobilization of Aβ42, K18-tau, full-length 2N4R-tau and αSyn fibrils in a controlled aggregation state on SPR-sensor chips and for assessing their binding to ligands. The SPR results from the analysis of binding kinetics suggested the presence of at least two binding sites for all fibrils, including luminescent conjugated oligothiophenes, benzothiazole derivatives, nonfluorescent methylene blue and lansoprazole. In silico modeling studies for αSyn (6H6B) revealed four binding sites with a preference for one site on the surface. Immunofluorescence staining validated the detection of pS129-αSyn positivity in the brains of Parkinson's disease patients and αSyn preformed-fibril injected mice, 6E10-positive Aβ in arcAβ mice, and AT-8/AT-100-positivity in pR5 mice.

CONCLUSION

SPR measurements of small molecules binding to Aβ42, K18/full-length 2N4R-tau and αSyn fibrils suggested the existence of multiple binding sites. This approach may provide efficient characterization of compounds for neurodegenerative disease-relevant proteinopathies.

α-Synuclein oligomers and fibrils: partners in crime in synucleinopathies

The misfolding and aggregation of α-synuclein is the general hallmark of a group of devastating neurodegenerative pathologies referred to as synucleinopathies, such as Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. In such conditions, a range of different misfolded aggregates, including oligomers, protofibrils, and fibrils, are present both in neurons and glial cells. Growing experimental evidence supports the proposition that soluble oligomeric assemblies, formed during the early phases of the aggregation process, are the major culprits of neuronal toxicity; at the same time, fibrillar conformers appear to be the most efficient at propagating among interconnected neurons, thus contributing to the spreading of α-synuclein pathology. Moreover, α-synuclein fibrils have been recently reported to release soluble and highly toxic oligomeric species, responsible for an immediate dysfunction in the recipient neurons. In this review, we discuss the current knowledge about the plethora of mechanisms of cellular dysfunction caused by α-synuclein oligomers and fibrils, both contributing to neurodegeneration in synucleinopathies.

Sigma-2 Receptors—From Basic Biology to Therapeutic Target: A Focus on Age-Related Degenerative Diseases

There is a large unmet medical need to develop disease-modifying treatment options for individuals with age-related degenerative diseases of the central nervous system. The sigma-2 receptor (S2R), encoded by TMEM97, is expressed in brain and retinal cells, and regulates cell functions via its co-receptor progesterone receptor membrane component 1 (PGRMC1), and through other protein–protein interactions. Studies describing functions of S2R involve the manipulation of expression or pharmacological modulation using exogenous small-molecule ligands. These studies demonstrate that S2R modulates key pathways involved in age-related diseases including autophagy, trafficking, oxidative stress, and amyloid-β and α-synuclein toxicity. Furthermore, S2R modulation can ameliorate functional deficits in cell-based and animal models of disease. This review summarizes the current evidence-based understanding of S2R biology and function, and its potential as a therapeutic target for age-related degenerative diseases of the central nervous system, including Alzheimer’s disease, α-synucleinopathies, and dry age-related macular degeneration.

Probing the interactions between amyloidogenic proteins and bio-membranes

Protein misfolding diseases (PMDs) in humans are characterized by the deposition of protein aggregates in tissues, including Alzheimer's disease, Parkinson's disease, type 2 diabetes, and amyotrophic lateral sclerosis. Misfolding and aggregation of amyloidogenic proteins play a central role in the onset and progression of PMDs, and these processes are regulated by multiple factors, especially the interaction between proteins and bio-membranes. Bio-membranes induce conformational changes in amyloidogenic proteins and affect their aggregation; on the other hand, the aggregates of amyloidogenic proteins may cause membrane damage or dysfunction leading to cytotoxicity. In this review, we summarize the factors that affect the binding of amyloidogenic proteins and membranes, the effects of bio-membranes on the aggregation of amyloidogenic proteins, mechanisms of membrane disruption by amyloidogenic aggregates, technical approaches for detecting these interactions, and finally therapeutic strategies targeting membrane damage caused by amyloidogenic proteins.

How Well Do Rodent Models of Parkinson's Disease Recapitulate Early Non-Motor Phenotypes? A Systematic Review

The prodromal phase of Parkinson's disease (PD) is characterised by many non-motor symptoms, and these have recently been posited to be predictive of later diagnosis. Genetic rodent models can develop non-motor phenotypes, providing tools to identify mechanisms underlying the early development of PD. However, it is not yet clear how reproducible non-motor phenotypes are amongst genetic PD rodent models, whether phenotypes are age-dependent, and the translatability of these phenotypes has yet to be explored. A systematic literature search was conducted on studies using genetic PD rodent models to investigate non-motor phenotypes; cognition, anxiety/depressive-like behaviour, gastrointestinal (GI) function, olfaction, circadian rhythm, cardiovascular and urinary function. In total, 51 genetic models of PD across 150 studies were identified. We found outcomes of most phenotypes were inconclusive due to inadequate studies, assessment at different ages, or variation in experimental and environmental factors. GI dysfunction was the most reproducible phenotype across all genetic rodent models. The mouse model harbouring mutant A53T, and the wild-type hα-syn overexpression (OE) model recapitulated the majority of phenotypes, albeit did not reliably produce concurrent motor deficits and nigral cell loss. Furthermore, animal models displayed different phenotypic profiles, reflecting the distinct genetic risk factors and heterogeneity of disease mechanisms. Currently, the inconsistent phenotypes within rodent models pose a challenge in the translatability and usefulness for further biomechanistic investigations. This review highlights opportunities to improve phenotype reproducibility with an emphasis on phenotypic assay choice and robust experimental design.

Mechanisms of prion-induced toxicity

Prion diseases are devastating neurodegenerative diseases caused by the structural conversion of the normally benign prion protein (PrP^C) to an infectious, disease-associated, conformer, PrP^Sc. After decades of intense research, much is known about the self-templated prion conversion process, a phenomenon which is now understood to be operative in other more common neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. In this review, we provide the current state of knowledge concerning a relatively poorly understood aspect of prion diseases: mechanisms of neurotoxicity. We provide an overview of proposed functions of PrP^C and its interactions with other extracellular proteins in the central nervous system, in vivo and in vitro models used to delineate signaling events downstream of prion propagation, the application of omics technologies, and the emerging appreciation of the role played by non-neuronal cell types in pathogenesis.

The multiple functions of PrPC in physiological, cancer, and neurodegenerative contexts

Cellular prion protein (PrP^C) is a highly conserved glycoprotein, present both anchored in the cell membrane and soluble in the extracellular medium. It has a diversity of ligands and is variably expressed in numerous tissues and cell subtypes, most notably in the central nervous system (CNS). Its importance has been brought to light over the years both under physiological conditions, such as embryogenesis and immune system homeostasis, and in pathologies, such as cancer and neurodegenerative diseases. During development, PrP^C plays an important role in CNS, participating in axonal growth and guidance and differentiation of glial cells, but also in other organs such as the heart, lung, and digestive system. In diseases, PrP^C has been related to several types of tumors, modulating cancer stem cells, enhancing malignant properties, and inducing drug resistance. Also, in non-neoplastic diseases, such as Alzheimer's and Parkinson's diseases, PrP^C seems to alter the dynamics of neurotoxic aggregate formation and, consequently, the progression of the disease. In this review, we explore in detail the multiple functions of this protein, which proved to be relevant for understanding the dynamics of organism homeostasis, as well as a promising target in the treatment of both neoplastic and degenerative diseases.

Alpha-Synuclein: The Spark That Flames Dopaminergic Neurons, In Vitro and In Vivo Evidence

Mitochondria, α-syn fibrils and the endo-lysosomal system are key players in the pathophysiology of Parkinson’s disease. The toxicity of α-syn is amplified by cell-to-cell transmission and aggregation of endogenous species in newly invaded neurons. Toxicity of α-syn PFF was investigated using primary cultures of dopaminergic neurons or on aged mice after infusion in the SNpc and combined with mild inhibition of GBA. In primary dopaminergic neurons, application of α-syn PFF induced a progressive cytotoxicity associated with mitochondrial dysfunction, oxidative stress, and accumulation of lysosomes suggesting that exogenous α-syn reached the lysosome (from the endosome). Counteracting the α-syn endocytosis with a clathrin inhibitor, dopaminergic neuron degeneration was prevented. In vivo, α-syn PFF induced progressive neurodegeneration of dopaminergic neurons associated with motor deficits. Histology revealed progressive aggregation of α-syn and microglial activation and accounted for the seeding role of α-syn, injection of which acted as a spark suggesting a triggering of cell-to-cell toxicity. We showed for the first time that a localized SNpc α-syn administration combined with a slight lysosomal deficiency and aging triggered a progressive lesion. The cellular and animal models described could help in the understanding of the human disease and might contribute to the development of new therapies.

Oligomeropathies, inflammation and prion protein binding

The central role of oligomers, small soluble aggregates of misfolded proteins, in the pathogenesis of neurodegenerative disorders is recognized in numerous experimental conditions and is compatible with clinical evidence. To underline this concept, some years ago we coined the term oligomeropathies to define the common mechanism of action of protein misfolding diseases like Alzheimer, Parkinson or prion diseases. Using simple experimental conditions, with direct application of synthetic β amyloid or α-synuclein oligomers intraventricularly at micromolar concentrations, we could detect differences and similarities in the biological consequences. The two oligomer species affected cognitive behavior, neuronal dysfunction and cerebral inflammatory reactions with distinct mechanisms. In these experimental conditions the proposed mediatory role of cellular prion protein in oligomer activities was not confirmed. Together with oligomers, inflammation at different levels can be important early in neurodegenerative disorders; both β amyloid and α-synuclein oligomers induce inflammation and its control strongly affects neuronal dysfunction. This review summarizes our studies with β-amyloid or α-synuclein oligomers, also considering the potential curative role of doxycycline, a well-known antibiotic with anti-amyloidogenic and anti-inflammatory activities. These actions are analyzed in terms of the therapeutic prospects.

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.

The authors show that prion protein and α-synuclein undergo phase separation through domain-specific electrostatic interactions. These complex coacervates possess electrostatic nanoclusters and can convert into multiphasic condensates and amyloids.

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.