Abstract
Huntington’s disease is a progressive autosomal dominant neurodegenerative disorder caused by the expansion of a polyglutamine tract at the N-terminus of a large cytoplasmic protein. The Drosophila huntingtin (htt) gene is widely expressed during all developmental stages from embryos to adults. However, Drosophila htt mutant individuals are viable with no obvious developmental defects. We asked if such defects could be detected in htt mutants in a background that had been genetically sensitized to reveal cryptic developmental functions. Amyloid precursor protein (APP) is linked to Alzheimer’s disease. Appl is the Drosophila APP ortholog and Appl signaling modulates axon outgrowth in the mushroom bodies (MBs), the learning and memory center in the fly, in part by recruiting Abl tyrosine kinase. Here, we find that htt mutations suppress axon outgrowth defects of αβ neurons in Appl mutant MB by derepressing the activity of Abl. We show that Abl is required in MB αβ neurons for their axon outgrowth. Importantly, both Abl overexpression and lack of expression produce similar phenotypes in the MBs, indicating the necessity of tightly regulating Abl activity. We find that Htt behaves genetically as a repressor of Abl activity, and consistent with this, in vivo FRET-based measurements reveal a significant increase in Abl kinase activity in the MBs when Htt levels are reduced. Thus, Appl and Htt have essential but opposing roles in MB development, promoting and suppressing Abl kinase activity, respectively, to maintain the appropriate intermediate level necessary for axon growth.
Understanding the normal physiological roles of proteins involved in neurodegenerative diseases can provide significant insight into disease mechanisms. Drosophila offers a powerful system in which to ask these fundamental questions. Both Htt, related to Huntington’s disease, and Appl, related to Alzheimer’s disease, have well-conserved single orthologs in the fly genome. Appl has been shown to be a conserved modulator of a Wnt-PCP signaling pathway required for axon outgrowth in the mushroom body (MB) in the Drosophila brain. However, roles for Htt in fly brain development have not been reported. Unexpectedly, we found that htt mutations suppress the axon outgrowth defects of Appl mutants in the MB, indicating a link between these two neurodegenerative proteins and a cryptic role of Htt during development. Abl tyrosine kinase is a downstream effector of the Appl receptor, and we show here that Abl is also required for MB axon outgrowth. Importantly, Abl activity must be tightly regulated as evidenced by our observations that both under and overexpression of Abl result in similar axonal defects. We demonstrate that Htt is an inhibitor of Abl activity and provide evidence that the phenotypic rescue of αβ axons in Appl mutants by reducing htt is mediated by the restoration of proper levels of Abl signaling. These data, therefore, suggest that Appl and Htt act antagonistically to maintain an optimal balance of activation and inhibition of Abl, and thereby promote the growth of MB αβ axons.
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