Autophagy and glutamate signaling in the onset of Huntington's disease

Neuronal cell death is often caused by an excess of glutamate that is maintained and physiological level by an equilibrium of signaling between glia and neurons, also called the glutamate-glutamine cycle. Glutamate removal from the synaptic cleft by neuroglia is reduced in a mouse model of Huntington's disease (HD) and in HD patients, suggesting that glial cells actively participate in the survival of neurons cells in HD. But is the glutamate-glutamine cycle contributing to neuronal death in HD? To answer this question we are taking advantage of a Drosophila model that has been successfully used to dissect the cellular and molecular events of neurodegenerative disorders including HD. HD, an inherited disease caused by expansion of CAG trinucleotide, which results in translation of a protein containing an enlarged polyglutamine (polyQ) domain. HD is characterized by the progressive degeneration of neurons, resulting in involuntary movement and death. Our preliminary data show that modulation of the enzyme GS1 that converts glutamate into glutamine, in Drosophila’s neurons expressing the human mutant HttQ93 ameliorate animal motility. On the contrary, using enzymes that increase the concentration of glutamate has the opposite effect. Growth factor signaling play a dual role in the onset of HD and both in flies and mammals it was shown that while activation of IGF/Akt signaling in neuroglia ameliorates neuronal degeneration induced by HD, the opposite was seen when autophagy was inhibited by activation of the amino TOR signaling pathway. We would like to better understand how components of the TOR pathways act in the regulation of the glutamate-glutamine cycle, and the potential new role of components of the cycle in controlling autophagy in neuronal and glial cells. With this approach we hope to provide novel insights into the processes that cause neuronal degeneration not only in Huntington's disease but also in other neuronal pathologies.