Astrocyte-derived signals for neuronal and behavioral modulation and its implications in mental illness.

Project Summary/Abstract: Major depressive disorder (MDD) is a common but serious mental illness that negatively affects emotion, cognition, physical activity, and increases mortality. In general, the manifestation of depression is thought to be due to the imbalance of neurotransmitters in the brain. These neurotransmitters include glutamate, GABA, and especially a class of monoamines, such as serotonin, dopamine, and norepinephrine. Many recent studies have demonstrated the importance of dopamine homeostasis and dynamics on reward and motivation especially after exposure to chronic stress. Our previous studies have identified a novel astrocyte-dependent modulatory mechanism for the dopamine system, in which the hormone insulin regulates ATP release in astrocytes, which in turn contributes to the modulation of dopamine release and depressive-like behavior in mice. These exciting findings provide a novel and potentially important molecular basis for the etiology of depressive disorders, given the epidemiological link between diabetes and major depression. How insulin mediates astrocytic ATP release has yet to be elucidated, although exocytosis from secretory lysosomes has been suggested as a major route of ATP release by astrocytes. Therefore, in the proposed research, we aim to further investigate the molecular mechanism and functional relevance of the astrocytic insulin action and ATP release on dopaminergic signaling under chronic stress. We hypothesize that insulin regulates the exocytosis of ATP from secretory lysosomes in astrocytes to contribute to dopamine release, and impairment of this astrocyte-initiated pathway will negatively impact dopamine release and exacerbate deficits in reward in mice exposed to chronic stress. To test this central hypothesis, we have developed a new genetic mouse model, in which we can specifically delete astrocytic vesicular nucleotide transporters (VNUTs) that are required for loading ATP into secretory lysosomes. Consistent with our original findings, preliminary analyses show that astrocyte-specific VNUTKO mice display increased depressive-like behavior and decreased activation of medium spiny neurons in the nucleus accumbens, indicating decreased dopamine signaling. Moving forward, in Aim 1, we will continue to use astrocyte-specific VNUTKO mice to determine the role of ATP exocytosis by astrocyte on dopamine signaling and reward in these mice after exposure to both unpredictable chronic mild stress (CMS) and chronic social defeat stress (CSDS). In Aim 2, taking advantage of the highly sensitive luciferase-based ATP quantification and the super-resolution living imaging by Nanoimager S, we will quantify the insulin-induced lysosomal trafficking and exocytosis of ATP in cultured astrocytes. Further in vitro and in vivo applications of pharmacological inhibitors and viral expression of mutations of key signaling molecules will dissect the molecular mechanisms of insulin-stimulated ATP release by astrocytes. Together, the proposed research will expand our understanding of the etiology of depressive disorders from a novel perspective of insulin action in astrocytes and may reveal new therapeutic approaches for depression.