Mechanisms of Mutant Prion Protein Aggregation Within Endolysosomal Pathways

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Ultrastructural studies of human patient brains with prion disease have revealed the accumulation of misfolded PrP within dystrophic neurites inside endolysosomes. Other complex structures co- exist within these dystrophies and are now considered common features of prion pathologies, including autophagic vacuole-like membrane-bound organelles, lysosomal electron-dense bodies, and enlarged endolysosomes. The mechanisms leading to the formation of these structures remain unknown. Our studies have identified an endolysosomal pathway in mammalian neurons that we call axonal rapid endosomal sorting and transport-dependent aggregation (ARESTA), that drives the formation of neurotoxic axonal aggregates of a misfolded mutant prion protein (PrP) inside endo-membrane structures that we call “endoggresomes”. The long-term goal of this proposal is to characterize the endocytic pathways that play a role in the pathophysiology of prion diseases, Alzheimer’s disease, and of Alzheimer’s disease related dementias, that will provide actionable targets for their pharmacological treatment. The objectives of this proposal are (i) to determine the generality of the ARESTA pathway in formation of endoggresomes in axons of neurons expressing various familial PrP mutations, and to characterize the molecular and ultrastructural architecture of neurotoxic endoggresomes; (ii) to determine the mechanisms of mutant PrP endoggresome-mediated axonal impairments; and (iii) to determine how the endolysosomal ARESTA pathway modulates the formation of axonal mutant PrP aggregates in vivo. The central hypotheses are (i) ARESTA drives the formation of endoggresomes in various familial prion diseases by interactions with co-factors within endocytic routes; (ii) mutant PrP endoggresome-induced pathologies act as axonotoxicity hubs that inhibit neuronal function by impairing local axonal cytoskeletal-organelle interactions, and (iii) endolysosomal pathways modulate the formation and pathology of mutant PrP aggregates in vivo. The proposed research is innovative because it provides a conceptual framework for developing models that include novel endolysosomal pathway-mediated mechanisms to explain how intra-axonal aggregates form and impair neuronal function in the prionopathies. The proposed research is significant because it identifies the endocytic pathway and specifically ARESTA and endoggresomes, as anti- aggregation targets for therapies to inhibit aggregate formation and reverse related pathologies. As amyloid-b peptides, tau, and most proteins that misfold in neurodegeneration transit within endocytic routes at some point in their processing routes, our findings are expected to be relevant to Alzheimer’s disease and Alzheimer’s disease related dementias.