Huntingtin Expression Stimulates Endosomal–Lysosomal Activity, Endosome Tubulation, and Autophagy

TLDR

Polyglutamine expansion in huntingtin causes Huntington’s disease, leading to mutant protein accumulation in neuronal nuclei and cytoplasm, though the mechanism of neuronal death remains unclear and may involve autophagy. The study proposes that mutant huntingtin accumulation activates the endosomal–lysosomal system, promoting proteolysis and an autophagic cell‑death pathway. Huntingtin localizes to endosomal–lysosomal organelles, forming cytoplasmic vacuoles that internalize cathepsin D in a polyglutamine‑length–dependent manner, display autophagosome features, disrupt organelle architecture, and increase endosomal tubulation, while wild‑type and mutant forms co‑localize with endogenous huntingtin in soluble and membrane fractions.

Abstract

An expansion of polyglutamines in the N terminus of huntingtin causes Huntington9s disease (HD) and results in the accrual of mutant protein in the nucleus and cytoplasm of affected neurons. How mutant huntingtin causes neurons to die is unclear, but some recent observations suggest that an autophagic process may occur. We showed previously that huntingtin markedly accumulates in endosomal–lysosomal organelles of affected HD neurons and, when exogenously expressed in clonal striatal neurons, huntingtin appears in cytoplasmic vacuoles causing cells to shrink. Here we show that the huntingtin-enriched cytoplasmic vacuoles formed <i>in vitro</i> internalized the lysosomal enzyme cathepsin D in proportion to the polyglutamine-length in huntingtin. Huntingtin-labeled vacuoles displayed the ultrastructural features of early and late autophagosomes (autolysosomes), had little or no overlap with ubiquitin, proteasome, and heat shock protein 70/heat shock cognate 70 immunoreactivities, and altered the arrangement of Golgi membranes, mitochondria, and nuclear membranes. Neurons with excess cytoplasmic huntingtin also exhibited increased tubulation of endosomal membranes. Exogenously expressed human full-length wild-type and mutant huntingtin codistributed with endogenous mouse huntingtin in soluble and membrane fractions, whereas human N-terminal huntingtin products were found only in membrane fractions that contained lysosomal organelles. We speculate that mutant huntingtin accumulation in HD activates the endosomal–lysosomal system, which contributes to huntingtin proteolysis and to an autophagic process of cell death.