The Roles of Huntingtin Associated Protein 40 in Huntingtin Functions and Huntington's Disease Pathogenesis - Huntington's disease (HD) is caused by an abnormal expansion of the glutamine tract (polyQ) in Huntingtin
(HTT). A clear understanding on how endogenous HTT is regulated in vivo is critical both for elucidating HD
etiology and for identifying effective drug targets. HTT has numerous reported HTT associated partners (HAPs)
and is functionally implicated in a growing list of cellular processes. However, little is known how HTT itself is
regulated and whether such regulation is altered in HD. We previously characterized the HTT homolog (dHtt)
in model organism Drosophila. Given the significant functional conservation of HTT from the fly to mammals,
we hypothesized that the core regulators of HTT likely are among the numerous known HAPs and should also
be conserved in Drosophila. In a proteomic study for such conserved central regulators of HTT in Drosophila,
we isolated dHap40, the fly homolog of HAP40, as the strongest dHtt interactor. Importantly, converging
evidence from studies in multiple species all support that in vivo HTT protein normally exists in a complex with
HAP40, and HAP40 binding stabilizes the conformation of HTT. Further, in samples from HD patients, a ~10-
fold increase of the levels of endogenous HAP40 were observed as compared to controls. However, despite
these findings, by now there is no reported functional study of HAP40 in any physiological settings, and its
effect on HTT's normal functions and mutant HTT toxicity remains unclear. Our preliminary studies support the
significantly conserved physical and functional interactions between HTT and HAP40, implying a highly
important regulatory relationship that constrains their co-evolution from flies to humans. Our findings not only
establish Drosophila as a relevant genetic model to study the physiological roles of HAP40, but also lead to our
hypothesis that HAP40 is a conserved central regulator of HTT and potentially a critical modulator of mutant
HTT toxicity. Using established assays and HD models in Drosophila and cultured mammalian cells, we will
systematically test this hypothesis. In Aim 1, we will carry out a comprehensive phenotypic analyses of dhap40
gene and test its genetic interactions with dhtt, so as to obtain a first systematic evaluation of HAP40 in a
physiological setting and clarify its relationship with HTT at whole-animal level. In Aim 2, we will systematically
test whether HAP40 is a central regulator of HTT's subcellular dynamics and its diverse cellular functions, so
as to elucidate its relationship with HTT at molecular and cellular levels. In Aim 3, taking advantage of the well-
established HD models in Drosophila and mammalian neurons, we will rigorously interrogate the role of HAP40
on mutant HTT toxicity. From these multidisciplinary studies, we will obtain a first comprehensive evaluation on
the physiological functions of HAP40, its effect on endogenous HTT functions and on HD pathogenesis. The
results potentially lay foundation on novel therapeutic avenues against HD via HAP40.
