Crosstalk between sensory ghrelin signaling and adipose tissue sympathetic outflow regulates metabolic homeostasis

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Obesity has become a world-wide epidemic problem, and is an independent risk factor for a panel of metabolic diseases, including insulin resistance/type 2 diabetes, dyslipidemia, cardiovascular diseases, and cancer. The stomach-derived orexigenic hormone ghrelin acting through its receptor growth hormone secretagogue receptor 1a (GHSR) is a key mediator of energy homeostasis. GHSR is widely expressed in the brain and on gastrointestinal vagal sensory neurons. In the current proposal, we discovered that in addition to the well characterized vagal GHSRs, gastrointestinal sensory neurons emanating from spinal dorsal root ganglia (DRG) robustly express GHSRs. In addition, DRG GHSR-containing neurons innervates the gastrointestinal tract, and DRG GHSR is markedly induced by energetic challenges, including food deprivation and cold exposure, suggesting DRG ghrelin/GHSR signaling is important in regulating energy homeostasis. Interestingly, sensory GHSR deletion protects against diet-induced obesity primarily due to increased energy expenditure via increased adipose tissue sympathetic outflow and thermogenesis. Thus, we have identified a novel pathway in which gut non-vagal sensory GHSR neurons negatively regulate energy homeostasis via suppression of adipose tissue sympathetic outflow, thermogenesis and whole body energy expenditure. The overall goal of this project is to test the importance of ghrelin/GHSR signaling on peripheral sensory neurons in regulating whole body metabolic homeostasis, and to investigate the neuronal mechanisms by which sensory GHSR regulates adipose tissue sympathetic outflow. In Specific Aim 1, the importance of sensory GHSR in regulating energy homeostasis will be tested using DRG sensory neuron-specific deletion or re-activation of GHSR. In Specific Aim 2, the neuronal circuitry connecting gut sensory GHSR signaling to adipose tissue sympathetic outflow will be investigated via viral tract tracer mapping. In addition, the possible involvement of central melanocortin signaling pathways as a downstream target connecting sensory GHSR signaling to adipose tissue sympathetic outflow will be tested. Completing this project will greatly expand our current knowledge of the neuroendocrine regulation of energy homeostasis, and will provide novel therapeutic targets in the preventing and treatment of obesity.