Summary Huntington's disease (HD) is caused by the expansion of a polyglutamine (polyQ) domain in the N-terminal region of the protein huntingtin (htt). While mutant htt is expressed ubiquitously in most types of cells, it leads to selective neurodegeneration that is characterized by severe neuronal loss in the brain striatum of HD patients. Glutamate excitotoxicity has been a long- standing theory to account for the selective degeneration of medium spiny neurons (MSNs) in the striatum. MSNs receive abundant glutamatergic input and are hypersensitive to glutamate stimulation. Since extracellular glutamate is largely removed by astrocytes, a major type of glial cell that supports neuronal survival and protects against neuronal exictotoxicity, any reduction in the ability of astrocytes to remove extracellular glutamate may increase the vulnerability of MSNs to glutamate toxicity. Yet the role of glia in HD neuropathology remains to be fully characterized. Our recent studies show that mutant htt is expressed in astrocytes, forming aggregates in their nuclei. We also found that mutant htt in astrocytes reduces both the expression of the glutamate transporter GLT-1 and the uptake of glutamate. Furthermore, neuron-glia cocultures revealed that normal glia protect against htt-mediated neurotoxicity, whereas glia expressing mutant htt promote neuronal vulnerability to glutamate excitotoxicity. We hypothesize that the expression of mutant htt in glia affects glial function, which leads to altered neurotransmitter uptake and contributes to neuropathology. To test this hypothesis, we will focus on the effects of mutant htt in astrocytes and examine whether mutant htt impedes the important functions of astrocytes in vitro and in vivo. In Aim 1, we will use cultured primary glial cells to examine how mutant htt affects glutamate uptake and neuronal function in glial cells. In Aim 2, we will use transgenic mice expressing mutant htt in astrocytes to study how mutant htt in astrocytes contributes to neuronal excitotoxicity.