Reprogramming of lipid secretome in senescent synovial fibroblasts ameliorate osteoarthritis development

Osteoarthritis (OA) is one of the most common causes of physical disability among older people and its incidence increases with age. Removal of the senescent cells (SNCs) delays OA pathologies, but little is known about the SNCs in OA synovium and their roles in OA pathogenesis. Here, we reported that RCAN1+IL1+ double-positive cells represent a senescent subset of synovial fibroblasts which accelerate cartilage degeneration via saturated fatty acids (SFAs). Using single-cell RNA sequencing and synovial organoids, we found that IL1+ senescent synovial fibroblasts mainly located at lining layer of human OA synovium and promote cartilage degeneration. We performed genome-wide CRISPR/Cas9 screens to identify novel regulators of cell-surface bounded IL1 in senescent cells. RCAN1, which was a hit as positive regulator of IL1, governs the cellular senescence and pro-degenerative senescence-associated secretory phenotype (SASP). Mechanistically, we found that RCAN1 mediated the secretion of SFAs from senescent synovial fibroblasts, which promote chondrocytes senescence and cartilage matrix degradation. Intra-articular delivery of synovium-targeted anti-Rcan1 siRNA ameliorates the development of post-traumatic OA in mice, while reducing of SNCs accumulation in synovium and increasing cartilage regeneration. Last, co-culture experiments of human OA cartilage explant with synovial organoids confirmed that RCAN1 silencing in human synovial fibroblasts suppressed chondrocyte senescence and cartilage degradation. Thus, our study revealed a pro-degenerative interaction between RCAN1+IL1+ senescent synovial fibroblasts and chondrocytes through secreted lipid mediators in OA progression. And targeted RCAN1 inhibition in senescent synovium is a promising approach for restore the joint homeostasis.