PROJECT SUMMARY: Obesity is a growing public health crisis associated with life-threatening comorbidities such as hypertension, type 2 diabetes, and coronary heart disease. More than two-thirds of Americans are overweight or obese, and one-sixth of children are obese. The economic impact associated with obesity is substantial, costing approximately $300 billion annually in medical costs. Obesity is a complex disease involving environmental, psychological, metabolic, and genetic factors. A class of genetic disorders that present with severe obesity are ciliopathies due to disruptions to the cilia or ciliary genes. Mouse models of neuronal cilia dysfunction cause hyperphagia-associated obesity as hunger and satiety signals that control feeding behavior are regulated by neurons in the hypothalamus. Multiple G-protein coupled receptors (GPCRs) highly enriched in neuronal cilia and involved in regulating energy homeostasis mislocalize in human and animal models of cilia- associated obesity, indicating a critical role for ciliary signaling in the hypothalamic control of feeding behavior. Understanding how ciliary signaling is involved in regulating feeding behavior and energy balance is integral to developing new treatment strategies. Cilia, small microtubule projections emanating from almost every cell, are critical signaling centers. Delineating ciliary signaling pathways in vivo is especially challenging with current genetic models as loss of cilia ablates all ciliary signaling, while genetic deletion of ciliary proteins removes both ciliary and cellular protein. Our lab genetically engineered a cilia-excluded mutant mouse, Arl13bV358A, where the ARL13BV358A protein is excluded from the cilia but retains all its known functions. ARL13B is a regulatory GTPase highly enriched in cilia and known for its role in ciliogenesis. Arl13bV358A/V358A mice are viable and fertile; they are also hyperphagic and obese and display defects in glucose and insulin metabolism. Our findings identify that ARL13B’s function within the cilia controls normal feeding behavior and that our cilia-excluded mouse model is well suited to isolate ARL13B’s ciliary role in obesity. The long-term goal of this proposal is to dissect the cilia- specific function of ARL13B as a regulator of energy homeostasis. I will test my central hypothesis that ciliary ARL13B is required to regulate energy homeostasis. In Aim 1, I will determine the impact of ciliary ARL13B in specific neuronal populations in regulating energy homeostasis. In Aim 2, I will test whether ciliary ARL13B in the same neuronal populations is sufficient to regulate energy homeostasis. In Aim 3, I will investigate whether ciliary ARL13B is continuously required by introducing ciliary ARL13B before and after the obesity phenotype. Successful completion of these aims will (1) determine the ciliary contribution of ARL13B in specific neuronal populations to regulate energy homeostasis and (2) delineate to what extent ciliary AR...