Interrogation of Rab8 as a therapeutic target for Huntington’s disease in Drosophila melanogaster

Huntington’s disease (HD) is a familial neurodegenerative disorder largely caused by atrophy in the striatal and cortical regions of the brain. At the molecular level HD is triggered by a trinucleotide CAG repeat expansion in the Huntingtin gene (HTT), which encodes a poly glutamine (polyQ) stretch, and ultimately leads to the production of the toxic, aggregationprone protein mutant HTT (mHTT). Upon expression of mHTT, several cellular pathways are either disrupted or impaired, including the vesicle trafficking directed by the Rab GTPase family. Here, I focused on Rab8, a protein involved in the secretory traffic from the trans-Golgi network to the plasma membrane, whose down-regulation has been shown to worsen HDrelated phenotypes in mammalian cells. My results show that pan-neuronal expression of Drosophila Rab8 (dRAB8) provides neuroprotection against HD-relevant phenotypes in Drosophila by reducing degeneration of the eye photoreceptors, ameliorating the rate of fly emergence from the pupal case and increasing average lifespan of adult flies. Notably, this rescue depends on the nucleotide-binding state of the GTPase. The protective role of dRAB8 was also validated in a subset of circadian clock cells, the Pigment Dispersing Factor (PDF) neurons. mHTT triggered arrhythmic locomotor behaviour in constant darkness and progressive death of a cluster of PDF neurons, the small lateral neurons ventral (s-LNvs), phenotypes which were partially or completely rescued by dRAB8 overexpression. The levels of aggregated mHTT are increased upon dRAB8 co-expression in flies and experiments performed in HEK293T cells suggest that the interaction dynamics between mHTT and dRAB8 increase in a polyQ dependent manner. Aggregation has been shown to be neuroprotective against toxic soluble mHTT species in several HD model organisms and might underlie the mechanism of dRAB8 rescue. In summary, this study validates Rab8 as a modifier of HD in Drosophila, provides insight into its mechanism of action, and may ultimately inform novel therapeutic approaches for HD.