Project Summary Neuronal function of huntingtin-associated protein Huntingtin-associated protein-1 (Hap1) was first identified as a neuronal protein that interacts with the Huntington's disease (HD) protein, huntingtin (Htt). Hap1's binding to Htt is enhanced by expanded polyglutamine repeats in the N-terminal region of Htt. Moreover, unlike Htt, which is ubiquitously expressed, Hap1 is expressed primarily in neuronal cells, suggesting that it is a good candidate for involvement in the selective neurodegeneration in HD. Extensive studies have shown that Hap1 and Htt associate with each other in the intracellular trafficking of various vesicles and proteins. Also, like Htt, Hap1 is essential for early development and neurogenesis. Our recent studies showed that the function of both Hap1 and Htt is cell type and age dependent. We also know that neurogenesis is important for early brain development and the repair of neuronal damage in the adult brain. Although both Htt and Hap1 participate in neurogenesis, and impaired neurogenesis is seen in HD brains, whether Hap1 and Htt work together to regulate neurogenesis and whether mutant Htt affects neurogenesis via its interaction with Hap1 remain unknown. We hypothesize that Hap1 and Htt participate in age- and environmental stress-dependent neurogenesis and that mutant Htt affects this function by its interaction with Hap1. To test this hypothesis, we will use Hap1 KO and Htt KO mice as well as HD140Q knock-in mice to examine the role of the interaction of Hap1 and Htt in postnatal and adult neurogenesis. We will focus on neurogenesis in the hypothalamus and hippocampus, as loss of Hap1 in these two regions is found to cause body weight loss and depression, two well-known phenotypes that also occur in HD patients. We will use CRISPR/Cas9 to selectively deplete Htt expression or to increase the production of N-terminal mutant Htt in HD140Q KI mice to examine whether they have any effects on neurogenesis. Using these approaches, we propose two aims in this application. Aim 1 will investigate whether Hap1 and Htt work together to promote neurogenesis. Aim 2 will explore whether mutant Htt affects neurogenesis via its abnormal interaction with Hap1. The studies seek to provide new insight into the cell type- and age-dependent function of Hap1 and Htt, as well as the selective neuropathology of HD. Findings from these studies will also help us uncover therapeutic strategies for the specific neuropathology and phenotypes in HD. !