A cell atlas of human and mouse synovium from early and advanced stages of knee osteoarthritis: BHLHE40 regulates fibroblast activation

Osteoarthritis (OA) is a destructive joint disease affecting multiple tissues, including synovium. Previous studies have identified some distinct fibroblast subtypes within synovium; however, the characterization of fibroblast subsets during distinct stages of knee (K)OA disease, and their contributions to the endogenous mechanisms that drive synovial fibrosis during KOA, are not well characterized. Here we profile synovium from early- (KL I) and advanced- (KL III/IV) stages of radiographic KOA. First, bulk-RNA sequencing of early- and advanced-staged KOA synovial tissue revealed transcriptomic differences between the two disease stages. Using single-nuclei RNA sequencing (snRNA-seq) and flow cytometry, we identified distinct fibroblast subsets and uncovered an endotypic shift in fibroblast subsets during KOA pathogenesis, transitioning from DPP4+ in early-stage to ITGB8+ in advanced-stages. SnRNA-seq of synovium from mice with experimental KOA revealed analogous populations of Dpp4+ and Itgb8+ fibroblasts in tissue from early and advanced model stages. Human advanced-stage KOA synovial tissue had stronger expression of matrisome-annotated genes compared to early-stage tissue. BHLHE40, a crucial transcriptional regulator of ECM related genes, was identified as upregulated in ITGB8+ fibroblasts compared to DPP4+ fibroblasts. Using primary human OA fibroblasts in vitro, and conditional knock out mice in vivo, we found that fibroblast-intrinsic loss of BHLHE40 increased fibrosis-related gene expression, enhanced fibroblast activation and induced severe synovial fibrosis in vivo. In contrast, overexpression of BHLHE40 in vitro was able to suppress TGF-{beta}-induced fibroblast activation. Overall, this study provides a comprehensive cellular atlas of KOA synovium and has identified BHLHE40 as a crucial regulator of fibroblast-mediated synovial fibrosis.