The profiling of extracellular vesicle subtypes in Huntington's disease brains identifies Alix as a novel marker of neuropathology.

BACKGROUND: Huntington's disease (HD) is the most frequent autosomal dominant neurodegenerative disorder, which is caused by a CAG repeat expansion in the HTT gene. Despite its well-defined genetic origin, there is currently no cure, and reliable biomarkers for disease progression and pathophysiology remain limited. Mutant huntingtin protein accumulates in endosomal compartments, disrupting endosomal trafficking and potentially affecting the biogenesis, release, and cargo of exosomes-extracellular vesicles (EVs) derived from the endosomal pathway. However, the role of exosomes in HD pathogenesis and their potential as biomarkers has been underexplored. In this work, we investigated whether the levels and content of small EV subpopulations, including exosomes, are altered in the brains of HD patients.

METHODS: We analyzed two distinct subpopulations of small EVs from the striatum and cortex of postmortem HD brains at early and advanced neuropathological stages, as well as from age-matched controls. EVs were isolated by differential ultracentrifugation and high-resolution iodixanol density gradient centrifugation, and analyzed by Western blotting, electron microscopy, NTA, and proteomics using mass spectrometry. EV secretion was also analyzed in primary fibroblasts derived from HD patients and healthy controls.

RESULTS: Mass spectrometry data revealed HD-associated alterations in EV protein content, particularly proteins related to the endosomal system. Our data also indicate that the level of ectosomes increased in the HD cortex, whereas exosomes were reduced in the HD striatum compared to controls. In terms of EV content, EVs from HD brains showed increased levels of Annexin A2 and decreased levels of Alix, a key component of the endosomal sorting complex required for transport (ESCRT). Alix depletion in EVs mirrored a progressive reduction of Alix in brain tissue, correlating with disease severity based on Vonsattel staging. In vitro, HD fibroblasts secreted EVs with reduced Alix content, despite no significant difference in cellular Alix levels compared to controls.

CONCLUSIONS: These findings highlight disease-specific changes in EV populations and cargo in HD, and identify Alix as a potential neuropathological marker. This study advances our understanding of the role of brain-derived EVs in HD and underscores their potential utility in biomarker discovery.