Huntingtin and HAP1 Regulate Microtubule Motors during Autophagosome Transport

Abstract

Autophagy is a critical homeostatic process whereby autophagosomes engulf and degrade cellular cargo. Live cell imaging assays in neurons recently demonstrated autophagosome formation is restricted to the axon tip, followed by robust retrograde transport of the autophagosome towards the cell body. While >75% of autophagosome motility is directed towards microtubule minus ends, both the minus end-directed motor dynein and the plus end-directed motor kinesin colocalize to autophagosomes in neurons and copurify with isolated autophagosomes. Thus, it still remains unclear what mechanisms promote autophagosome transport, activating the retrograde motor dynein while inactivating the anterograde motor kinesin. One key candidate for regulating autophagosome transport is huntingtin (htt), which directly binds to dynein and binds to kinesin and dynactin through the adaptor HAP1 (huntingtin associated protein 1). Using live cell imaging in primary neurons, we found that depleting endogenous htt disrupted the retrograde motility of autophagosomes resulting in shorter run lengths and slower velocities. While these defects could be rescued by expression of full-length htt, deletion constructs of htt that could not bind either dynein or HAP1 were unable to rescue the defects in autophagosome motility upon depletion of endogenous htt. To further test HAP1's role in regulating autophagosome transport, we depleted endogenous HAp1 and observed a similar disruption in the retrograde motility of autophagosomes. Finally, we found that autophagosome transport is disrupted in a neuronal model of Huntington's disease, a genetic neurodegenerative disorder characterized by polyglutamine expansions in htt. Our results suggest a model where htt regulates autophagosome transport by binding to dynein and enhancing motor processivity, as well as to HAP1 whose interactions with kinesin and dynactin may further promote retrograde autophagosome transport. Further, out data suggest that misregulation of this transport may contribute to the pathogenesis of Huntington's disease.