Novel transcriptional regulators of synovial joint and bone development

Project Summary Osteoarthritis (OA) is the most common form of arthritis, affecting 1 in 8 adults in the United States. It is characterized by pain, stiffness, and swelling in the hands, hips, or knees, significantly impacting daily functioning and overall quality of life. Current treatment approaches primarily focus on pain management rather than addressing the underlying cause of OA: the degradation of articular cartilage (AC). AC is a smooth and protective layer of hyaline cartilage that covers the ends of bones in synovial joints that is actively maintained by long-lived articular cartilage chondrocytes (ACC). However, ACCs have limited regenerative capacity and fail to repair AC after damage. Ultimately, AC damage leads to improper joint loading and further pathological remodeling of the synovial joint, including AC ossification. AC ossification occurs through the inappropriate activation of endochondral ossification-like programs, similar to the development of growth plate chondrocytes (GPC) during embryogenesis. Interestingly, both ACCs and GPCs originate from chondrocyte progenitors (CPs) in the developing limb. As skeletal elements mature, CPs can differentiate into hypertrophic chondrocytes, which eventually form bone. In contrast, at the prospective synovial joint site, CPs undergo a process of "dedifferentiation," downregulating Sox9 and Col2a1 to form a structure called the interzone (IZ). Cells within and around the IZ later develop into ACCs and other components of synovial joints. To investigate the critical decision between GPC and IZ/ACC fate, we employed gene regulatory network (GRN) analysis and identified the transcription factors PITX1, ATF4, and SOX4 as regulators of CP cell fate before interzone development. Notably, PITX1 and ATF4 have each been shown to bind the Sox9 promoter, but PITX1 and ATF4 are associated with different lineage biases (ACC and GPC, respectively). The mechanisms underlying the maintenance of CP multipotency and the reprogramming of CPs to commit to ACC differentiation remain unclear. In our proposed project, we will utilize murine Sox9 lineage tracing models to characterize changes in cell identity during differentiation. The objectives of the study are to determine: 1) the extent to which candidate CP transcription factors, such as PITX1 and ATF4 exhibit mutually antagonistic effects and promote the expression of ACC and GPC genes and 2) the requirement of CP reprogramming during IZ development for ACC fate. We foresee that the completion of this project will reveal novel properties of synovial joint development for potential therapeutic targeting.