Obesity has reached epidemic proportions in the US and plays a major role in the development of type 2 diabetes and cardiovascular disease. There remains a very significant need for better treatments. While current weight loss agents act by suppressing appetite, strategies that can safely enhance energy expenditure have the potential to effectively treat obesity. Brown adipose tissue (BAT) is a thermogenic tissue that uniquely expresses mitochondrial UnCoupling Protein-1 (UCP1). This protein dissipates, in a regulated fashion, the electrochemical gradient in the mitochondria of brown adipocytes as heat, and thus plays an important role in the maintenance of body temperature and energy balance. BAT is a flexible tissue that normally enlarges or atrophies over time depending on environmental temperature. In numerous animal models, enhancement of BAT mass has been shown to cause weight loss and diabetes resistance. While BAT was until recently thought to be insignificant in adult humans, data obtained in the past several years with PET imaging show that adults in fact have significant amounts of functional BAT, and that the amount of BAT in individuals is strongly correlated with leanness. Furthermore, the genetic locus most tightly linked with human obesity (FTO) was recently shown to cause defective recruitment of new brown adipocytes. Until recently no brown adipocyte stem cell had been identified. We discovered human skeletal muscle- resident brown adipocyte progenitor cells that under appropriate conditions become fully functional brown adipocytes, expressing high levels of UCP1 and having a very high metabolic rate. Using these progenitors we identified a protein factor that is secreted by brown adipocytes and in turn promotes the recruitment of additional brown adipocytes in culture. A slightly truncated version of this protein with a somewhat longer circulating half-life, which is already marketed for an unrelated acute indication, was tested in Diet-Induced Obese (DIO) mice, a highly predictive animal model for human obesity and insulin resistance. Marked decreases in body weight and body fat, as well as improvements in glucose metabolism were observed in a 28-day study. However, the still relatively short plasma half-life of the tested agent requires that it be injected at least every other day, a major challenge for a chronic metabolic disease indication. In the proposed work, we therefore aim to generate protein analogs with significantly extended plasma half- life that show similar or better efficacy in DIO mice. Such a compound could be administered once weekly or less frequently. We will evaluate protein yield (manufacturability), confirm the activity of the analogs in vitro, and determine their plasma half-life in mice. Those compounds with the best profile will be tested for efficacy in DIO mice. If this work is successful, we plan to rapidly move to select a lead and backup compound and initiate IND-enabling studies with a novel, brown fat-recruiting product candidate for the treatment of obesity and diabetes.