PROJECT SUMMARY A heterozygous missense mutation OtpQ153R/+ has recently been discovered in a cohort of individuals with severe, early-onset obesity. Like many other obesity-associated variants, despite a strong association, a causal relationship has yet been established. Otp encodes a transcription factor that is highly conserved across multiple species. Importantly, mice and humans share the identical amino acid sequence of Otp. To study the functional impact of OtpQ153R/+, we have generated new knock-in mice that carry the same human mutation. Similar to the human subjects, we found that mice heterozygous for OtpQ153R (OtpQ153R/+) survive through adulthood but develop obesity and glucose intolerance. These findings, therefore, strongly support a causal role for OtpQ153R/+ in human obesity. We propose to investigate the mechanisms behind OtpQ153R-induced obesity and glucose deficits. Otp is broadly distributed in the central nervous system. To determine the brain site where Otp deficiency impairs energy and glucose balance, we generated and characterized a floxed Otp allele (Otpflox). Our new preliminary studies show that selective loss of Otp in forebrain Sim1-Cre-expressing neurons reproduces lethality seen in Otp null mice, whereas its haploinsufficiency in these neurons results in obesity. Furthermore, we find that Otp is transiently expressed in a subset of immature POMC neurons in the arcuate nucleus of the hypothalamus (ARH) and is required for the POMC→NPY/AgRP fate switch during development. Selective deletion of Otp in these neurons leads to a significant loss of POMC-derived NPY/AgRP neuron identity. Collectively, our new findings suggest that Otp plays critical roles in two distinct populations of hypothalamic neurons to regulate energy and glucose metabolism. In summary, the overarching goals of these studies are to better understand OtpQ153R-induced pathophysiology and develop mechanism-based therapeutics to mitigate metabolic syndrome in human OtpQ153R/+ patients.