The Role of the Huntingtin N17 Domain and its Modifications in Modulating Protein-Protein Interactions and Huntington's Disease Pathogenesis
Huntington's disease (HD) is an autosomal dominant, fatal neurodegenerative disorder characterized by movement disorders, cognitive impairment, and psychiatric deficits. Despite the discovery of the huntingtin (htt) gene more than 19 years ago, the biological function of normal and mutant huntingtin (Htt) protein still remain unclear in HD pathogenesis. We performed a spatiotemporal proteomic profiling of proteins co-immunoprecipitated with full-length Htt in BACHD and wildtype mouse brains to identify 747 Htt-interacting proteins that are highly enriched with known Htt interactors. In order to obtain an unbiased interpretation of this Htt interactome, we applied Weighted Gene Correlation Network Analyses (WGCNA) to define protein correlation relationships among the Htt interactors across 30 independent tissue (cortex, striatum, cerebellum), genotype (Wt vs BACHD), and age (2m vs 12m) conditions. The analysis established a verifiable rank of proteins based on their correlation strength with Htt, and also revealed the organization of Htt-interacting protein networks, with one such network containing Htt itself, 19 known Htt interactors, and 12 novel genetic modifiers of HD. Our study demonstrates a novel systems biology approach to interpret in vivo proteomic interactomes derived from complex tissues such as the brain, and the value of such an approach in illuminating known and novel molecular targets for HD.
Recent studies suggest that the N-terminal 17 amino acids of Htt (N17 domain) immediately preceding the polyQ domain may constitute a critical domain for Htt function and HD pathogenesis. Biochemical characterization, proteomic profiling, and WGCNA network analysis of the full-length Htt interactomes from novel BAC transgenic mouse models of HD expressing human huntingtin with 97 glutamines, with 1) phosphomimetic Ser13 and Ser16 mutations (SD mice), 2) phosphoresisant Ser13 and Ser16 mutations (SA mice), and 3) deletion of Htt aa 2-16 (BACHD-∆N17 mice) reveal critical insights into how this domain regulates Htt nuclear toxicity, protein aggregation, and protein-protein interactions.