Osteoarthritis (OA) disproportionately affects veterans, resulting in more pain and functional limitations compared to the general population. No disease-modifying treatments exist for OA, and current pain medications (e.g., opioids and NSAIDs) have limited long-term efficacy and adverse side effects. Unresolved cellular and molecular joint inflammation is recognized as the central mechanism of OA progression. However, a barrier to progress in the field is identifying the causes of chronic OA inflammation and how to resolve them. The applicant's long-term goal for overcoming this barrier is to understand the molecular mechanisms of how exercise therapy reduces OA inflammation and pain so that synergistic drug targets can be identified and developed for therapeutic use. The premise of this application is that macrophages depend on lipid metabolism reprogramming to complete anti-inflammatory alternative activation. The objective here is to determine how intra-articular adipose tissue lipolysis modifies joint inflammation by regulating anti-inflammatory macrophage polarization. The central hypothesis is that the resolution of joint inflammation requires the temporal coupling of infra-patellar fat pad (IFP) lipolysis with macrophage lipid uptake and fatty acid metabolism to drive alternative activation. This hypothesis has been developed based on the applicant's exciting preliminary data showing that exercise triggers a transient induction of pro-inflammatory cytokines and macrophages in the knee synovium and IFP, which fully resolves by day 14 of running. Notably, the induction and resolution of inflammation occurs in parallel with a transient cycle of IFP lipolysis, fibrosis, and lipogenesis. The rationale for the proposed research is that an understanding of the causal relationship between joint tissue metabolites and cellular inflammatory mediators has the potential to generate new therapeutic opportunities by advancing fundamental knowledge about how joint inflammation is regulated. With strong preliminary data and expertise in small animal exercise, metabolism, and OA studies, the applicant will test the hypothesis by pursuing three specific aims: 1) Determine how intra-articular adipose tissue lipolysis mediates macrophage activation, joint inflammation, and post-traumatic OA; 2) Determine the effect of macrophage lipid uptake and fatty acid oxidation on joint inflammation and the development of post-traumatic OA; and 3) Develop a combined physical and biologic intervention strategy targeting lipid metabolism to reduce joint inflammation and pain in a pre-clinical model of chronic knee OA. Aims 1 and 2 will be tested in mouse models of resolving and non- resolving joint inflammation using wheel running and destabilization of the medial meniscus (DMM) models, respectively. The models, which have been established as feasible in the applicant's hands, will be used to test causal mechanisms that establish the pro- or anti-resolving effects of intra-articular lipids on joint inflammation. In aim 1, these include an inducible genetic approach to block lipolysis in the joint or a pharmacologic approach to enhance lipolysis. In the second aim, an inducible genetic approach will be used to inhibit peroxisome proliferator activated receptor-? (PPAR?) in macrophages or stimulate PPAR? pharmacologically. The third aim combines physical and PPAR? pharmacologic treatments in the DMM model to test for synergistic interactions that improve pain and function more than exercise alone. By focusing on the cellular and molecular transducers of OA exercise therapy, the proposed research tests new, innovative paradigms for designing drugs to potentiate the therapeutic effects of OA exercise therapy. The proposed research is significant because it will initiate the systematic study of how synovial joint metabolism may be manipulated to promote the resolution of joint inflammation. This knowledge is also expected to be important for other OA therapies, such as optimizing the joint environment to support stem cell and tissue-engineering-based regenerative medicine strategies.