The polyglutamine expansion in the huntingtin (mHTT) causes HD. Before HD onset, striatal atrophy occurs, with subsequent loss of cortico-striatal white matter (WM) connections in layer V. As HD progresses, it affects other parts of the brain and the peripheral tissues. The mechanisms underlying the neuronal vulnerability in HD are not entirely understood and likely several, including aggregation, transcriptional dysregulation, energy metabolism deficits, synaptic dystrophy, oxidative stress, inflammation, and others. We demonstrated that striatal enriched protein Rhes interacts with mHTT via the farnesylation domain and promotes cellular toxicity by increasing the soluble forms of mHTT via small ubiquitin-like modifier (SUMO)-1 modification. However, the mechanisms by which the Rhes-SUMO pathway mediates neuronal vulnerability in HD are unclear. We hypothesize that Rhes and SUMO1 signaling circuitry orchestrate striatal vulnerability and HD progression by altering mHTT levels and promoting the spread of mHTT in the brain. This hypothesis is formulated based on the following findings: a) Rhes mediates mHTT transmission between cultured neurons involving actin-based tunneling nanotube-like (TNT-like) membranous protrusions, b) SUMO depletion or SUMO-defective mHTT diminishes transport of mHTT via Rhes–TNTs. c) SUMO1 deletion enhances autophagy activities and decreases mHTT abundance in the striatum and prevents HD-related behavioral and anatomical deficits in Q175HD KI mice, and d) Rhes transits within striatum and from the striatum to the cortex (V, VI, layers) and spread mHTT. These data indicate an intricate link between Rhes and SUMO pathways in HD pathogenesis. But the detailed mechanism of action(s) of Rhes–SUMO1 pathways in HD is unclear. This proposal addresses two distinct yet interrelated Aims: Aim 1. To uncover the role and mechanisms of Rhes-mediated mHTT spreading in the brain. Here will test the hypothesis that Rhes spreads mHTT and promotes neuropathology involving posttranslational mechanisms and TNT-like routes. Aim 2. To identify the mechanisms of SUMO1- mediated HD pathogenesis. Here we test the hypothesis that SUMO1–mHTT inhibits autophagy, thereby allowing accumulation and spread of mHTT from the striatum to the cortex. This project will provide novel mechanistic insights into SUMO-mediated autophagy deficits in HD and SUMO roles in Rhes-mediated mHTT transport between brain regions while also potentially identifying novel therapeutic targets that can limit mHTT spread in the HD brain.