Metabolic Regulation by Microglial Inflammatory Signaling

Project Summary Diabetes and obesity are interconnected diseases with significant public health ramifications. Overnutrition triggers immune cell activation in the brain that promotes weight gain and was therefore presumed to worsen glucose tolerance. On the contrary, we recently discovered a marked dissociation between the regulation of energy balance and glucose homeostasis by microglia. Mice with microglia-specific deletion of inflammatory signaling are protected from diet-induced obesity but show glycemic dysregulation when compared with weight-matched controls. Similarly, mice with impaired prostaglandin signaling in microglia show unaltered inflammatory responses but less diet-induced phagocytic activity, resulting in leaner mice but without a glucose benefit. In contrast, genetically increasing microglial inflammatory signaling triggers weight gain even on a chow diet but nevertheless improves glucose tolerance relative to the leaner control mice. Thus, there is a bidirectional relationship between microglial activation and glucose tolerance. To explore mechanism, we developed a mouse model with the Gq-coupled DREADD receptor hM3D expressed in microglia. Chemogenetic microglial activation evokes an acute improvement in glucose tolerance even in the setting of HFD feeding. The mechanism involves a TNF-dependent pathway by which glucose- triggered neuronal activity is increased in melanocortin neurons, ultimately leading to parasympathetic enhancement of insulin secretion. Importantly, Gi-coupled DREADD activation worsens glucose tolerance, providing a rationale to test Gi-GPCR antagonists for anti-diabetic efficacy. Our preliminary data show that central blockade of the P2Y12 receptor, a major microglia-specific Gi-GPCR, markedly improves glucose tolerance, supporting the viability of this approach. In this proposal, we will further explore the therapeutic potential of this system in preclinical models of diabetes and explore mechanisms contributing to the glycemic benefits. In Aim 1, we will perform the first studies measuring neuronal activity in response to a direct cell- specific manipulation of microglial activation state. Specifically, we will use electrophysiology and fiber photometry to determine how microglia modulate melanocortin neuron glucose sensing and electrical activity. In Aim 2, we will assess the natural history of microglial state changes during the development of DIO and DM2 along with the impact of chronic microglial activation. Further, we will assess the contribution of microglial phagocytic capacity to metabolic regulation using tools related to the key regulatory protein MerTK. Finally, we will test the participation of the P2Y12 receptor in the regulation of glucose homeostasis by microglia. Together, these studies will help test the hypothesis that microglial inflammatory signaling and phagocytic capacity function to offset obesity-associated glucose intolerance via alterations to hypothalamic glucose sensing, a mechanism that can be harnessed to improve glycemia in the setting of T2D.