Obesity is a major health and economic burden with approximately 35% of United States citizens classified as either overweight or obese, and medical spending for treating obesity and its comorbidities exceeds $200 billion annually. The stomach-derived orexigenic hormone ghrelin is a key mediator of energy homeostasis and adiposity in humans due to its regulation of food intake, gut motility, energy expenditure, nutrient partitioning, glycemia, and body temperature. The ghrelin receptor, growth hormone secretagogue receptor 1a (GHSR), is widely expressed in the brain and on gastrointestinal vagal sensory neurons, and neuronal GHSR knockout results in a profoundly beneficial metabolic profile and high fat diet (HFD)-induced obesity resistance. Moreover, ghsr knockout mice have impaired metabolic regulation during energetic challenges, and ghsr restoration in the brain does not fully restore ghrelin?s effects suggesting peripheral ghrelin signaling is critical for metabolic control. 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 expression is markedly upregulated by energetic challenges suggesting a novel mechanism mediating ghrelin?s effects on energy homeostasis. As this is the first time these receptors have been identified on DRGs, their function and phenotype (i.e. whether they also detect gastric stretch or nutrients) is completely unknown. Thus, the overall goal of this project is to identify the phenotype and organs innervated by GHSR-containing DRGs, and to elucidate the overall necessity of DRG and vagal sensory neuron ghrelin signaling. A ghsrGFP reporter mouse will be used in conjunction with neuronal retrograde tracers to identify the organs innervated by GHSR-containing DRGs. In addition, immunohistochemical analysis measuring neuronal activity in response to peripheral manipulations designed to activate GHSR-GFP sensory neurons will elucidate the phenotype of these neurons. To test the necessity of sensory neuron ghrelin signaling for regulating metabolic homeostasis, we will generate a novel, sensory neuron-specific GHSR knockout mouse by crossing our ghsrloxP/loxP mouse line (that allow for Cre-recombinase dependent deletion of the ghsr gene) with our sensory neuron-specific Cre driver mice Advillin- Cre. With the resulting offspring, we will test the overall phenotype, ability to maintain metabolic homeostasis when energetically challenged, and susceptibility to diet-induced obesity. These overarching goals are in line with the National Institute of Diabetes and Digestive and Kidney Diseases mission to investigate the fundamental causes of obesity and other metabolic diseases including diabetes. Moreover, the findings from this study will greatly expand our knowledge of the neuroendocrine regulation of energy homeostasis, and will provide another point of attack for behavioral and/or pharmacological interventions to combat obesity.
The obesity epidemic is a major health and economic burden in the United States with approximately 35% of the population classified as overweight or obese (Flegal et al., 2012). Previous research has illustrated a complex neuroendocrine framework within the brain controlling energy intake, yet the continued rise in the global incidence of obesity necessitates a comprehensive study investigating the peripheral mechanisms controlling energy homeostasis and may provide another point of attack for pharmacological and/or behavioral intervention. The proposed investigation identifies and characterizes a novel mechanism in which the appetite- stimulating hormone ghrelin signals through peripheral sensory neurons to regulate energy homeostasis.
