Enhanced Ca2 +-dependent glutamate release from astrocytes of the BACHD Huntington's disease mouse model

• Mutant huntingtin affects levels of glutamate released from cultured astrocytes.
• There is no change in vesicular glutamate transporter 3 trafficking/levels in mhtt astrocytes.
• Glutamine synthetase levels are normal in cultured mhtt astrocytes.
• Elevated pyruvate carboxylase in mhtt astrocytes could lead to increased glutamate release.

Huntington's disease (HD) causes preferential loss of a subset of neurons in the brain although the huntingtin protein is expressed broadly in various neural cell types, including astrocytes. Glutamate-mediated excitotoxicity is thought to cause selective neuronal injury, and brain astrocytes have a central role in regulating extracellular glutamate. To determine whether full-length mutant huntingtin expression causes a cell-autonomous phenotype and perturbs astrocyte gliotransmitter release, we studied cultured cortical astrocytes from BACHD mice. Here, we report augmented glutamate release through Ca2 +-dependent exocytosis from BACHD astrocytes. Although such release is usually dependent on cytosolic Ca2 + levels, surprisingly, we found that BACHD astrocytes displayed Ca2 + dynamics comparable to those in wild type astrocytes. These results point to a possible involvement of other factors in regulating Ca2 +-dependent/vesicular release of glutamate from astrocytes. We found a biochemical footprint that would lead to increased availability of cytosolic glutamate in BACHD astrocytes: i) augmented de novo glutamate synthesis due to an increase in the level of the astrocyte specific mitochondrial enzyme pyruvate carboxylase; and ii) unaltered conversion of glutamate to glutamine, as there were no changes in the expression level of the astrocyte specific enzyme glutamine synthetase. This work identifies a new mechanism in astrocytes that could lead to increased levels of extracellular glutamate in HD and thus may contribute to excitotoxicity in this devastating disease.