PROJECT SUMMARY Osteoarthritis (OA), the loss of cartilage lining articular joints, is the leading cause of pain and disability worldwide. There are no existing treatments for OA and pain management strategies are inadequate. OA was historically dismissed as a consequence of mechanical overloading, but overloading does not explain the burden of OA. Furthermore, approaches to study this painful disease have often focused on evaluating single tissues in isolation, like cartilage, and assume that OA pathology is focused solely within the joint organ system. However, the OA population disproportionally is comprised of individuals with obesity and other co- morbidities. We and others have contributed to a paradigm shift in OA, demonstrating the surprising finding that OA may have systemic origins involving adipose tissue.. To determine the individual mechanistic influences of systemic adipose tissue on the joint, we developed a model of fat free mice, that completely lack fat, but are protected from OA. We can reverse this protection by implanting a small fat graft, which illustrates a line of communication between fat and cartilage that is separable from other factors like insulin resistance and liver damage, which are incompletely rescued. Consistent with this hypothesis, we are changing the prevalent view in OA and propose that OA is a whole-body disease of pain and loss of physical function. Here we aim to define systemic mediators of OA, which may enable the generation of first-in-class disease modifying treatments that would chart the course for improved whole-body health and longevity. By studying OA, this proposal will provide insight into a critical unanswered question in multi- organ aging: Is OA a systemic disease that generates symptoms in the joint? Our objective is to determine how adipose tissue communicates with the joint. We will interrogate three potential direct interorgan crosstalk mechanisms using a comprehensive and ambitious approach: (1) signaling through soluble secreted factors, (2) cell migration or cell-to-cell mediated, and (3) nervous system mediated adipose-joint crosstalk. Our previous work suggests that all three mechanisms occur in OA, however their relative contribution and hierarchy are unknown. In the first study, we will use leptin signaling, a factor widely implicated in OA pathogenesis, to understand joint tissue-specific responses and signal transduction between fat and the knee joint. Then, we will use heterochronic parabiosis as a model to understand cell migration to knee joint injury. Finally, we will consider crosstalk between adipose and joint tissues through sensory neurons that innervate the knee joint. Mouse models of OA typically focus on structural assessments using histology or imaging alone, however our lab incorporates pain and structural damage assessments in all our OA models. Our lab is uniquely positioned to interrogate these adipose-knee joint crosstalk mechanisms, as we have the tools and expertise to dissect and test each. From these studies, we will delineate pathological mechanisms of OA to develop much needed first-in-class therapeutic targets.

PROJECT NARRATIVE Our work, indicating that fat is a source of systemic inflammation contributing to the loss of cartilage in articular joints, has shifted the historical paradigm that osteoarthritis is a wear and tear disease. We posit that the complex interorgan crosstalk between fat and musculoskeletal tissues is key to unlocking a mechanistic understanding of disease progression and pain. The goal of this proposal is to define whether cell signaling, cell migration, or the nervous system are facilitating fat-joint crosstalk to enable the development of first-in- class OA therapeutics targeting the multi-organ interface to improve whole-body health.