Project Summary Huntington's disease (HD) is a neurodegenerative disorder that presents with progressive motor, psychiatric, and cognitive symptoms leading to early disability and mortality. Although significant advances have been made in identifying pathogenic pathways and screening of potential drug targets, no treatments to delay the onset or slow the disease progression exist yet. Thus, there is a need for fresh perspectives on the disease pathogenesis to discover novel therapeutic targets and to facilitate treatment development for HD. This proposal’s foundation is rooted in the discovery of impairment of the “glymphatic system” in HD brain. Glymphatic system is a brain-wide perivascular network that facilitates the exchange of interstitial fluid and cerebrospinal fluid and clears waste products from the brain. This drainage system is supported by aquaporin- 4 (AQP4) water channels which present with high density in perivascular astrocytic endfeet membranes: termed AQP4 polarization. AQP4 polarization requires presence of a functional protein complex composed of a key protein, α1-syntrophin (SNTA1). While multiple independent studies have speculated that the glymphatic system may play a role in the clearance of neurodegenerative disease-relevant proteins, there is limited direct evidence available on how this system is altered in HD, and whether its disruption contributes to HD pathology and disease manifestation. We developed a molecular MRI technique, dynamic glucose-enhanced MR imaging. Using this MRItechnique, we discovered that D-glucose clearance is significantly reduced in a mouse model of HD, and glymphatic clearance is impaired prior to brain pathology and motor deficits. We also found that AQP4 loses its polarization in the HD brain and SNTA1 protein levels were reduced in HD brains. Based on these findings, we hypothesize that loss of perivascular AQP4 polarization impairs glymphatic function, consequently preventing mutant HTT clearance and accelerating HD neuropathology and disease progression. Aim 1 is to define whether glymphatic impairment precedes the development of pathology and behavioral deficits in HD mice. Aim 2 is to determine whether loss of perivascular Aqp4 polarization by Snta1 knockdown accelerates HD-like neuropathology and behavioral deficits in HD mice. Aim 3 is to evaluate whether overexpressing Snta1 or combined with Aqp4 improves glymphatic function in HD mice and attenuates mHTT accumulation and rescue HD manifestation. This project will reveal a mechanistic basis for identifying new therapeutics as well as potential biomarkers for HD.