Huntingtin expression stimulates endosomal-lysosomal activity, endosome tubulation, and autophagy

An expansion of polyglutamines in the N terminus of huntingtin causes Hunti ngton's disease (HD) and results in the accrual of mutant protein in the nu cleus and cytoplasm of affected neurons. How mutant huntingtin causes neuro ns to die is unclear, but some recent observations suggest that an autophag ic process may occur. We showed previously that huntingtin markedly accumul ates in endosomal-lysosomal organelles of affected HD neurons and, when exo genously expressed in clonal striatal neurons, huntingtin appears in cytopl asmic vacuoles causing cells to shrink. Here we show that the huntingtin-en riched cytoplasmic vacuoles formed in vitro internalized the lysosomal enzy me cathepsin D in proportion to the polyglutamine-length in huntingtin. Hun tingtin-labeled vacuoles displayed the ultrastructural features of early an d late autophagosomes (autolysosomes), had little or no overlap with ubiqui tin, proteasome, and heat shock protein 70/heat shock cognate 70 immunoreac tivities, and altered the arrangement of Golgi membranes, mitochondria, and nuclear membranes. Neurons with excess cytoplasmic huntingtin also exhibit ed 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-termina l huntingtin products were found only in membrane fractions that contained lysosomal organelles. We speculate that mutant huntingtin accumulation in H D activates the endosomal-lysosomal system, which contributes to huntingtin proteolysis and to an autophagic process of cell death.