The regulation of autophagosome dynamics by huntingtin and HAP1 is disrupted by expression of mutant huntingtin, leading to defective cargo degradation.

ABSTRACT

Autophagy is an essential cellular pathway for degrading defective organelles and aggregated proteins. Defects in autophagy have been implicated in the neurodegenerative disorder Huntington's disease (HD), in which polyglutamine-expanded huntingtin (polyQ-htt) is predominantly cleared by autophagy. In neurons, autophagosomes form constitutively at the axon tip and undergo robust retrograde axonal transport toward the cell body, but the factors regulating autophagosome dynamics and autophagosome maturation are not well understood. Here, we show that both huntingtin (htt) and its adaptor protein huntingtin-associated protein-1 (HAP1) copurify and colocalize with autophagosomes in neurons. We use live-cell imaging and RNAi in primary neurons from GFP-LC3 transgenic mice to show that htt and HAP1 control autophagosome dynamics, regulating dynein and kinesin motors to promote processive transport. Expression of polyQ-htt in either primary neurons or striatal cells from HD knock-in mice is sufficient to disrupt the axonal transport of autophagosomes. Htt is not required for autophagosome formation or cargo loading. However, the defective autophagosome transport observed in both htt-depleted neurons and polyQ-htt-expressing neurons is correlated with inefficient degradation of engulfed mitochondrial fragments. Together, these studies identify htt and HAP1 as regulators of autophagosome transport in neurons and suggest that misregulation of autophagosome transport in HD leads to inefficient autophagosome maturation, potentially due to inhibition of autophagosome/lysosome fusion along the axon. The resulting defective clearance of both polyQ-htt aggregates and dysfunctional mitochondria by neuronal autophagosomes may contribute to neurodegeneration and cell death in HD.