Huntington disease (HD) is an autosomal-dominant neurodegenerative disorder caused by the pathogenic expansion of the poly-glutamine (polyQ) N-terminal stretch in the huntingtin protein (HTT; encoded by HTT). HD is characterized by the dysfunction and death of neurons in the brain, leading to devastating cognitive, psychiatric, and motor symptoms in patients. Studies in multiple cell and animal model systems support the notion that polyQ expansion in mutant HTT leads to the gain of new toxic functions and loss of the neuroprotective functions of the wild-type HTT. During my thesis, I focused on the description and functional validation of a new tool to study HTT: pARIS-htt. pARIS-htt is a synthetic gene built to facilitate cloning and tagging of full-length HTT. Using different cellular approaches, we showed that pARIS-htt can replace endogenous HTT in the transport of Golgi and brain derived neurotrophic factor (BDNF) containing vesicles. pARIS-htt mutant version could not restore vesicular transport when endogenous HTT was knocked-down. Moreover, we generated pARIS-htt deletion mutants for HTT interaction domain with dynein, a minus-end directed motor protein, and huntingtin associated protein 1 (HAP1), a HTT interactor. Both deletion mutants failed to restore vesicular transport in gene replacement assays. Another aspect of my thesis was the study of HTT during mitosis. We showed that HTT monitors spindle orientation though its interaction with diverse proteins involved in cell division. This function is lost when HTT is mutated and can be reverted by Akt phosphorylation at serine 421. The control of spindle orientation is particularly important during neurogenesis since spindle orientation and the mode of division of apical progenitors are implicated in the determination of cell fate. This function of HTT is conserved in D. melanogaster. This study contributes to the understanding of HTT functions and suggests new therapeutical approaches to treat HD.
