Mutant Prion Protein Endoggresomes are Hubs for Local Axonal Organelle-Cytoskeletal Remodeling

Abstract

Dystrophic axons comprising misfolded mutant prion protein (PrP) aggregates are a characteristic pathological feature in the prionopathies. These aggregates form inside endolysosomes -called endoggresomes-, within swellings that line up the length of axons of degenerating neurons. The pathways impaired by endoggresomes that result in failed axonal and consequently neuronal health, remain undefined. Here, we dissect the local subcellular impairments that occur within individual mutant PrP endoggresome swelling sites in axons. Quantitative high-resolution light and electron microscopy revealed the selective impairment of the acetylated vs tyrosinated microtubule cytoskeleton, while micro-domain image analysis of live organelle dynamics within swelling sites revealed deficits uniquely to the MT-based active transport system that translocates mitochondria and endosomes toward the synapse. Cytoskeletal and defective transport results in the retention of mitochondria, endosomes, and molecular motors at swelling sites, enhancing mitochondria-Rab7 late endosome contacts that induce mitochondrial fission via the activity of Rab7, and render mitochondria dysfunctional. Our findings point to mutant Pr Pendoggresome swelling sites as selective hubs of cytoskeletal deficits and organelle retention that drive the remodeling of organelles along axons. We propose that the dysfunction imparted locally within these axonal micro-domains spreads throughout the axon over time, leading to axonal dysfunction in prionopathies.

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“…159,160 These aggregates are often located within dystrophic axonal domains, where accumulation of organelles (mitochondria), neurofilaments, and endosomal vesicles including enlarged lysosome-like structures that contain incompletely digested material, also occurs. 161–165 A mutant prion protein (PrP) fusion construct containing an expanded 14 octapeptide repeat (PrP PG14 ; there is a 9 octapeptide insertion in wild-type PrP (PrP WT ); see Figure S5A) is associated with Gerstmann-Sträussler-Scheinker (GSS), a human familial prion disorder. 166 Previous work showed that expression of PrP PG14 in cultured murine hippocampal neurons results in the formation of enlarged endolysosomes along axons containing PrP PG14 aggregates, known as endoggresomes (aggregates within endosomes) (Figure S5A, bottom panels).…”

“…166 Previous work showed that expression of PrP PG14 in cultured murine hippocampal neurons results in the formation of enlarged endolysosomes along axons containing PrP PG14 aggregates, known as endoggresomes (aggregates within endosomes) (Figure S5A, bottom panels). 165,167 Mutant and WT PrP sequences contain a PrP signal sequence (SS) and PrP glycosylphosphatidylinositol (GPI)-anchoring sequence to ensure proper transport through the secretory pathway within the lumen of vesicles and compartments. 167 Endoggresomes form within axonal dystrophic swellings following the active transport of PrP PG14 -carrying Golgi-derived vesicles into the axon by their association with the small endolysosomal GTPase ARL8b, which recruits kinesin-1 and the multiunit homotypic fusion and protein sorting (HOPS) complex subunit VPS41 onto PrP PG14 -carrying endosomes.…”