Osteoarthritis: articular chondrocyte inflammatory signaling leads to enhanced gene expression and function of mechanotransduction channel Piezo1 as a pathogenic feedforwardmechanism
Osteoarthritis (OA) is a painful and debilitating condition of synovial joints without any disease-modifying therapies (1, 2). We previously identified mechanosensitive PIEZO channels, PIEZO1 and PIEZO2, both expressed in articular cartilage, to function in chondrocyte mechanotransduction in response to injury (3, 4). We therefore asked whether interleukin-1-mediated inflammatory signaling, as occurs in OA, influences Piezo gene expression and channel function, thus indicative of maladaptive reprogramming that can be rationally targeted. Primary porcine chondrocyte culture and human osteoarthritic cartilage tissue were studied. We found that interleukin-1 (IL-1) upregulated Piezo1 in porcine chondrocytes. Piezo1 expression was significantly increased in human osteoarthritic cartilage. Increased Piezo1 expression in chondrocytes resulted in a feed-forward pathomechanism whereby increased function of Piezo1 induced excess intracellular Ca2+, at baseline and in response to mechanical deformation. Elevated resting state Ca2+ in turn rarefied the F-actin cytoskeleton and amplified mechanically-induced deformation-microtrauma. As intracellular substrates of this novel OA-related inflammatory pathomechanism, in porcine articular chondrocytes exposed to IL-1 we discovered that enhanced Piezo1 expression depended on p38 MAP-kinase and transcription factors HNF4 and ATF2/CREBP1. CREBP1 directly bound to the proximal PIEZO1 gene promoter. In ensemble, these signaling and genetic re-programming events represent a novel and detrimental Ca2+-driven feed-forward mechanism that can be rationally targeted to stem the progression of OA. Significance StatementOsteoarthritis affecting weight-bearing joints is a global health problem, causing loss of mobility and enormous healthcare costs. Disease-modifying approaches are lacking. Here, we report a new cellular mechanism of inflammatory signaling in chondrocytes, the cellular substrate of cartilage. We show how osteoarthritis-relevant levels of interleukin-1 reprogram articular chondrocytes so that they become more susceptible to mechanical trauma, which chondrocytes sense via Piezo1/2 mechanosensitive ion channels. We uncover that IL-1 enhances gene expression of Piezo1 in primary articular chondrocytes underlying Piezo1 gain-of-function. We elucidate the new signaling pathway, from membrane to nucleus, including transcription factors that enhance Piezo1-expression. We also define detrimental effects of gain-of-function of Piezo1, for mechanotransduction and at-rest, that suggest this new reprogramming mechanism to contribute to osteoarthritis pathogenesis.