miR-221/222 drive synovial fibroblast expansion and pathogenesis of TNF-mediated arthritis

MicroRNAs (miRNAs) constitute fine tuners of gene expression and are implicated in a variety of diseases spanning from inflammation to cancer. miRNA expression is deregulated in rheumatoid arthritis (RA), however, their specific role in key arthritogenic cells such as the synovial fibroblast (SF) remains elusive. We have shown in the past that the expression of the miR-221/222 cluster is upregulated in RA SFs. Here, we demonstrate that miR-221/222 activation is downstream of major inflammatory cytokines, such as TNF and IL-1β, which promote miR-221/222 expression independently. miR-221/222 expression in SFs from the huTNFtg mouse model of arthritis correlates with disease progression. Targeted transgenic overexpression of miR-221/222 in SFs of the huTNFtg mouse model led to further expansion of synovial fibroblasts and disease exacerbation. miR-221/222 overexpression altered the transcriptional profile of SFs igniting pathways involved in cell cycle progression and ECM regulation. Validated targets of miR-221/222 included p27 and p57 cell cycle inhibitors, as well as Smarca1 (a chromatin remodeling component). In contrast, complete genetic ablation of miR-221/222 in arthritic mice led to decreased proliferation of fibroblasts, reduced synovial expansion and attenuated disease. scATAC-seq data analysis revealed increased miR-221/222 gene activity in the pathogenic and activated clusters of the intermediate and lining compartment. Taken together, our results establish an SF-specific pathogenic role of the miR-221/222 cluster in arthritis and suggest that its therapeutic targeting in specific subpopulations should inform the design of novel fibroblast-targeted therapies for human disease.