Abstract/Project Summary Mice physiology is deeply affected by brown fat through its control of metabolic homeostasis and production of signaling molecules. However, the vast majority of humans do not have brown fat. This deeply bias the use of mice as a model of human physiology. A mouse model without BAT would allow us to undoubtably determine the putative roles of brown fat in metabolism, in other organ systems and to have a better, humanized, model for preclinical research. Following our mechanistic exploration of the role of growth factors/PI3K/mTOR signaling to understand brown fat formation and metabolism, we unexpectedly generated a mouse model with no discernable classic brown fat, which we call BAT-less mice. The BAT-less mice are born at normal mendelian ratios, fully viable, with normal reproductive capacity and the effects are 100% penetrant. Based on our exciting preliminary data, the central hypothesis for this application is that the complete lack of UCP1 expressing cells in BAT-less mice lead to distinct obesogenic effects compared to UCP1 knockouts. To test this hypothesis, we have developed a toolkit of unique in vivo models including new reporter mouse models and a multidisciplinary approach using whole mouse and tissue-clearing lineage tracing techniques, genomics and state of the art metabolic phenotyping techniques. In Aim 1, we will critically and unequivocally determine if all brown fat depots are lost in BAT-less at all developmental stages using a multi-reporter system tracking UCP1 expressing cells. In Aim 2, we will employ the BAT-less mice to test the contribution of BAT energy consumption to whole body metabolism and obesity propensity independently of UCP1. The BAT-less mice are anticipated to be useful to scientists in an array of fields with implications in multiple NIH institutes. Additionally, the BAT-less mice will be a step forward towards a more humanized, broadly useful, mouse model.