Insulin regulates activity-dependent mitochondrial depolarization and calcium dynamics in dentate granule cells

Systemic insulin resistance is associated with reduced hippocampal energy metabolism and cognitive impairment both in humans and animal models. However, whether insulin rapidly regulates neuronal metabolic stability during activity remains unclear. Here, we examined insulin signaling and queried the corresponding mitochondrial responses in dentate granule cells (DGCs) of adult female mice. This layer of excitatory neurons within the hippocampus plays an essential role in learning and memory. Insulin robustly increased Akt phosphorylation in dentate gyrus tissue when insulin degradation was inhibited, and this response was attenuated by genetic deletion of the insulin receptor in DGCs, consistent with cell-autonomous signaling. We found that insulin did not alter mitochondrial membrane potential at rest, but significantly limited activity-evoked mitochondrial depolarization induced by neuronal stimulation. However, this change had no measurable impact on cytosolic ATP availability and was instead associated with Ca 2+ dynamics. Removing Ca 2+ ions from the extracellular solution, or disrupting mitochondrial Ca 2+ uptake, prevented insulin modulation of the mitochondrial membrane potential. Furthermore, insulin limited cytosolic Ca 2+ accumulation through an insulin receptor-dependent mechanism that persisted despite Akt inhibition. Insulin also accelerated the clearance of mitochondrial Ca 2+ transients in active neurons, which occurred without changes in either neuronal excitability or pyruvate dehydrogenase phosphorylation. Together, these findings identify endogenous insulin degradation as a key regulator of hippocampal insulin signaling, and reveal a previously unrecognized interaction between insulin and Ca 2+ signaling in stimulated DGCs. This work also establishes a direct link between central insulin signaling and the maintenance of mitochondrial electrochemical homeostasis in the adult brain. Significance Statement Insulin signaling is known to influence synaptic plasticity, yet its direct effects on neuronal metabolism remain poorly defined. Here, we show that endogenous insulin degradation limits insulin signaling in adult hippocampal tissue. This finding suggests that inhibitors of the insulin degrading enzyme may hold the potential to improve the efficacy of insulin-based interventions aimed at improving learning and memory. Acute insulin signaling does not modify neuronal excitability, or cytosolic ATP availability. Instead, it stabilizes mitochondrial membrane potential and reduces calcium accumulation in active dentate granule cells. By identifying an insulin-dependent mechanism that regulates the electrochemical homeostasis of mitochondria in active neurons, this work provides insight into how brain insulin resistance may contribute to cognitive dysfunction in aging and neurometabolic disorders.