Identification of FOXO1 as a geroprotector in human synovium through single-nucleus transcriptomic profiling.

The synovium, a thin layer of tissue that adjacent to the joints and secretes synovial fluid, undergoes changes in aging that contribute to intense shoulder pain and other joint diseases. However, the mechanism underlying human synovial aging remains poorly characterized. Here, we generated a comprehensive profile of synovial cell types present in subacromial synovium from young and aged individuals. By delineating aging-related transcriptomic changes across cell types and their associated regulatory networks, we identified two subsets of mesenchymal stromal cell (MSC) in human synovium, which are lining and sublining MSCs, and found that angiogenesis and fibrosis-associated genes were upregulated whereas genes associated with cell adhesion and cartilage development were downregulated during aging. Moreover, the specific cell-cell communications in aged synovium mirrors that of aging-related inflammation and tissue remodeling, including vascular hyperplasia and tissue fibrosis. In particular, we identified Forkhead box O1 (FOXO1) as one of the major regulons for aging DEGs of synovium MSCs, and validated its downregulation in both lining and sublining MSC populations of the aged synovium. In human FOXO1-depleted MSCs derived from human embryonic stem cells, we recapitulated the senescent phenotype observed in the subacromial synovium of aged donors. These data indicate the important role for FOXO1 in the regulation of human synovial aging. Overall, our study improves upon our understanding of synovial aging during joint degeneration, thereby informing development of new treatments aimed at rejuvenating the aged joint.