PIEZO1 activation and signalling is associated with inflammation permissive phenocopy of synovial resident macrophages in early arthritis

Synovial resident macrophages in humans play a key role in maintaining tissue and synovial fluid homeostasis and typically exhibit protective phenotypes. However, at the start of rheumatoid arthritis, macrophages can adopt inflammation-permissive phenotypes. The factors that influence macrophage phenotype during the early stages of arthritis are not yet fully understood. Interestingly, in mice, mechanical stress is required for the localization of joint inflammation. Given the critical roles of macrophages, we investigated whether mechanosensation contributes to the regulation of macrophage phenotypes. PIEZO1, a well-established mechanosensor expressed on macrophages, regulates various macrophage functions, including migration and phagocytosis. We found that PIEZO1 activation in macrophages shifted their phenotype to a tissue-resident (MERTKhigh, CD36high), inflammation-permissive phenocopy (TREM2low, VSIG4low), with elevated MHC class II and cytokine secretion. Additionally, in vivo activation of PIEZO1 in mice produced similar effects in synovial tissue macrophages (STMs) and led to increased recruitment of monocytes and neutrophils. By dissecting the PIEZO1 signalling pathway, we were able to restore the protective macrophage phenotype by inhibiting calpain signalling and knocking-out HIF1a, both downstream mediators of PIEZO1 activation. Furthermore, we observed that PIEZO1 expression is present in synovial macrophages from patients with active rheumatoid arthritis and decreases during remission, suggesting a link between macrophage PIEZO1 expression and disease activity