Project Summary Obesity is a major risk factor for several metabolic diseases most notably diabetes and cardiovascular disease. During diet-induced obesity, visceral white adipose tissue (vWAT) responds to caloric excess through the recruitment of new fat cells (hyperplasia) and enlargement of existing white adipocytes (hypertrophy). To facilitate this adipocyte expansion, the stromal vascular compartment consisting of mesenchymal stromal cells (MSC) and immune cells undergo dramatic changes in cell number and gene expression leading to WAT fibrosis and inflammation, thereby exacerbating metabolic dysfunction. To investigate the mechanisms controlling hyperplasia, we applied single cell RNA sequencing (scRNAseq) to define the cellular landscape of the stroma and examine how it changes during diet-induced obesity. We observed an extensive increase in macrophages including CD9+ senescent macrophages which secreted profibrogenic factors most notably osteopontin. We also identified four distinct populations of MSCs and an early adipocyte progenitor cluster expressing both PDGFRα and PDGFRβ. In an obese state, these cells enhanced their ECM production thereby contributing to fibrosis. In the case of hypertrophy, adipocytes progressively increase their volume as they take up and store excess calories. This expansion along with reduced angiogenesis causes local hypoxia that induces the deposition of pericellular ECM. The combination of ECM production from MSCs and adipocytes leads to formation of a stiff and inflexible barrier against further adipocyte expansion and adipocyte dysfunction. To identify mechanisms responsible for the dysfunction, we performed a RNAseq analysis of isolated adipocytes to assess global pathway changes occurring during a prolonged HFD. The studies revealed that adipocytes respond dramatically to the diet by enhancing expression of genes encoding ECM, focal adhesion, and cytoskeletal proteins. In fact, the transcriptional signature of the resultant pathological adipocytes was similar to fibroblastic-like or progenitor cells. The transcriptional mechanisms regulating both hyperplasia and hypertrophy are still poorly defined. Our earlier studies discovered that the transcriptional cofactor myocardin-related transcription factor, MRTFA contributes to diet-induced metabolic disruption of adipose tissue by regulating the fate of MSCs to favor fibrogenesis over adipogenesis. We hypothesize that MRTFA integrates multiple fibrogenic signals leading to ECM production by MSCs producing mechanical stress on adipocytes leading to their fibroblastic-like phenotype and adipose tissue dysfunction. We propose three aims:1: Determine the effect of conditional deletion of MRTFA in adipogenic progenitors on the cellular composition and function of adipose tissue in obese mice. 2: Define the signaling pathways regulating adipocyte progenitor cell fate in response to diet. 3: Identify transcriptional mechanisms regulating the fate of vascular proge...