Objectives: Our objective is to delineate proximate mechanisms of neurodegeneration in Huntington disease (HD), an uncommon, adult-onset, autosomal neurodegenerative disorder. HD is caused by an expanded CAG repeat, translated as polyglutamine (polyQ), dominant in the huntingtin locus. Several different proximate mechanisms of neuronal death are hypothesized to cause neurodegeneration in HD. We are focused on critically evaluating, in vivo, the hypothesized mechanisms of excitotoxic neuronal injury and mitochondrial dysfunction. We are also addressing the important issue of cell autonomous versus non-cell autonomous causes of neurodegeneration. In our prior funding period, we characterized an excellent murine genetic model of HD and provided strong in vivo evidence for excitotoxic neuronal injury as a proximate cause of neurodegeneration in HD. Research Plan: We have 3 primary experiments. We will cross a well validated knockin murine genetic model of HD with 3 other lines carrying mutations that allow us to test the hypotheses of excitotoxic neuronal injury, cell autonomous versus cell non-autonomous causes of neurodegeneration, and mitochondrial dysfunction. Each cross embodies a specific prediction based on each hypothesis, allowing falsification or verification of each hypothesis. In the first cross, we will breed HD-like mice with mice carrying a forebrain deletion of the NR2B subunit of the NMDA receptor. In the second experiment, we will breed HD-like mice with mice carrying a striatal specific deletion of the NR2B subunit of the NMDA receptor. In the third experiment, we will breed HD-like mice with a mitochondrial deficiency mutant. In all experiments, prospective evaluations of behavior and pathology will be used to assess outcomes of these crosses. Methods: The same set of methods are used across all experiments. HD-like and other mouse lines are intercrossed to generate bigenic mutants embodying predictions derived from each hypothesis. The resulting mice and control animals are evaluated prospectively with a standard battery of behavioral tests up to 2 years of age. Pathologic changes are assessed with stereology, immunohistochemistry, and receptor binding methods. All methods are well established in our laboratory. Clinical Relevance (if basic science study): Neurodegenerative disorders are common problems among the elderly and in the US Veteran population. The mechanisms of neurodegeneration are understood poorly and improved treatment requires understanding mechanisms of neurodegeneration. Better understanding of HD may lead to understanding of mechanisms of neurodegeneration in more common neurodegenerative disorders. We are investigating 2 proximate mechanisms, excitotoxicity and mitochondrial dysfunction, thought to play a major role in several neurodegenerative disorders.