Nuclear receptors (NRs) influence development and physiology by controlling patterns of gene expression via ligand-directed chromatin modifications. The resultant induced epigenomic state mobilizes networks of genes to engender unique cell functions and physiology. This proposal extends on previous studies that elucidated the roles of the NRs PPARd and ERRa/g in context-specific transcriptional activities that reprogram skeletal muscle metabolism and physiology to increase endurance performance. A central focus of exercise physiology has been the functional association of induced transcriptional programs with changes in histone acetylation, including understanding how the activation of PPARd and ERRa/g and their coactivator PGC1a drive oxidative metabolic programs that lead to increased endurance performance. In contrast, little is known about the potential contributions of repressive programs and the expected histone deacetylation. BCL6 was identified as a novel, exercise-responsive transcriptional repressor in both humans and mice, however its importance in muscle physiology has not been explored. Interestingly, an interaction between BCL6 and PPARd, the master regulator of fatty acid substrate utilization, has been shown in other cell types suggesting that BCL6 may have dual roles in skeletal muscle; effecting transcriptional outcomes through its direct chromatin binding as well as indirectly by modulating the PPARd transcriptional program. This proposal utilizes pharmacologic strategies and muscle-specific genetic knockout mouse models in combination with epigenetic and transcriptional mapping techniques to delineate BCL6’s function in exercise physiology. In Aim 1, the BCL6 transcriptional complex will be characterized using state-of-the-art APEX2 proximity labeling techniques in WT and PPARd KO primary skeletal muscle myotubes with and without pharmacologic agents targeting BCL6. Changes in the BCL6 complex in response to ligands will be correlated with altered epigenomic and gene expression states to associate specific complexes with functional outcomes. In Aim 2, the contributions of BCL6 to exercise-induced physiologic and transcriptional changes will be determined using genetic (muscle-specific Bcl6 knockout mice) and pharmacologic (BCL6 inhibitor) loss-of-function mouse models, while related studies in muscle-specific Ppard knockout mice will distinguish the PPARd dependent and independent effects. Lastly, Aim 3 will exploit the advances of the CUT&RUN technology to map the BCL6 chromatin binding sites in sedentary and exercised skeletal muscle. Combined with the changes induced in PPARd and ERRg binding, in addition to alterations in histone regulatory marks, these studies will molecularly dissect the competing repressive and activating transcriptional programs in muscle. Moreover, associating these findings with the transcriptional complexes identified in Aim 1 and the transcriptomics data from Aim 2 will provide a comprehensive view...