Restoring Degradative Capacity as a Therapeutic Strategy for Treating Prion Disease

Dystrophic (swollen) axons containing intracellular accumulations of misfolded protein aggregates are a hallmark of virtually all neurodegenerative disorders. These swellings are observed in human brains at early disease stages and are linked to neuronal impairment. How swellings containing aggregates form in axons and how they impair axonal function, is not well understood. We identified an endolysosomal pathway in mammalian axons, that drives the formation of toxic aggregates by mutant prion proteins (PrP) involved in human prionopathies. In this axonal rapid endosomal sorting and transport-dependent aggregation (ARESTA) pathway, post-Golgi-derived membrane endosomes containing misfolded mutant PrP are translocated by active kinesin-1 into the axon, where they fuse to generate enlarged endolysosomes within which mutant PrP aggregates form termed “endoggresomes” (aggregates within endolysosomes). Quantitative live imaging using high-resolution light microscopy of localized swollen micro-domains along axons revealed selective impairment of the microtubule cytoskeleton, and impaired organelle (mitochondrial) function via the accumulation within swellings of mitochondria, endosomes, and molecular motors. We used focused ion beam (FIB)/SEM of neurons to obtain longitudinal 8nm³ voxel CLEM sections of axons expressing mutant PrP, and generated the first in situ 3D ultrastructure model of axonal mutant PrP aggregate swellings. Pharmacologically activation of autophagy with newly identified small molecules efficiently clears axonal endoggresomes and ameliorates neuronal toxicity (pathological lesions) in cultured neurons and in mouse models of mutant PrP disease. These data strongly suggest that ARESTA pathway and the autophagy/lysosomal trafficking pathways are anti-aggregation targets amenable to therapeutic modulation in the prionopathies.