Gtf2ird1-Dependent Mohawk Expression Regulates Mechanosensing Properties of the Tendon

Abstract

Mechanoforces experienced by an organ are translated into biological information for cellular sensing and response. In mammals, the tendon connective tissue experiences and resists physical forces, with tendon-specific mesenchymal cells called tenocytes orchestrating extracellular matrix (ECM) turnover. We show that Mohawk (Mkx), a tendon-specific transcription factor, is essential in mechanoresponsive tenogenesis through regulation of its downstream ECM genes such as type I collagens and proteoglycans such as fibromodulin both in vivo and in vitro. Wild-type (WT) mice demonstrated an increase in collagen fiber diameter and density in response to physical treadmill exercise, whereas in Mkx−/− mice, tendons failed to respond to the same mechanical stimulation. Furthermore, functional screening of the Mkx promoter region identified several upstream transcription factors that regulate Mkx. In particular, general transcription factor II-I repeat domain-containing protein 1 (Gtf2ird1) that is expressed in the cytoplasm of unstressed tenocytes translocated into the nucleus upon mechanical stretching to activate the Mkx promoter through chromatin regulation. Here, we demonstrate that Gtf2ird1 is essential for Mkx transcription, while also linking mechanical forces to Mkx-mediated tendon homeostasis and regeneration.

Gene targeting of the transcription factor Mohawk in rats causes heterotopic ossification of Achilles tendon via failed tenogenesis

“…It is also reported that mechanical strain promotes the differentiation of MSCs into tenocytes in vitro (21,42). In this regard, we recently found that Gtf2ird1 translocates into the nucleus in response to mechanical strain and activates the Mkx promoter through chromatin regulation (43). Here, our mechanical stretch experiments with TDCs stimulation.…”

Mkx-Deficient Mice Exhibit Hedgehog Signaling–Dependent Ectopic Ossification in the Achilles Tendons

“…Given that both Mkx –/– rats as well as Mkx +/– and Mkx –/– mice develop ectopic bones in the Achilles tendons, it is possible that rare variants in the MKX gene might be a genetic predisposing factor for Achilles tendon ossification in humans. Recent studies have shown that Mkx expression in tendon cells is regulated by mechanical strain through the Gtf2ird1 transcription factor and that mechanical stimulation caused cultured Mkx –/– mutant, but not wild‐type, tendon cells to undergo chondrogenic differentiation . Furthermore, it has been shown that the inflammatory cytokine IL‐1β strongly suppressed MKX gene expression in cultured human anterior cruciate ligament (ACL)‐derived cells and that MKX expression was significantly reduced in degenerated ACL in osteoarthritis patients .…”

Tendons and Ligaments: Connecting Developmental Biology to Musculoskeletal Disease Pathogenesis

“…Tenocytes respond to mechanical strain by increasing collagen production, and mechanical signals promote MSC differentiation into tenocytes . Mechanical stimulation also promotes collagen fiber thickening along with increased fiber density …”

“…Mkx is also responsive to mechanical stimulation both in vitro and in vivo. In moderate treadmill exercise, the effects of mechanical loading on collagen fiber thickening are not observed in Mkx‐deficient mice, suggesting that Mkx is critical in the response to mechanical stimuli . Interestingly, general transcription factor II‐I repeat domain‐containing protein 1 (Gtf2ird1) was found to play an important role in translating mechanical stress to Mkx expression .…”

“…In moderate treadmill exercise, the effects of mechanical loading on collagen fiber thickening are not observed in Mkx‐deficient mice, suggesting that Mkx is critical in the response to mechanical stimuli . Interestingly, general transcription factor II‐I repeat domain‐containing protein 1 (Gtf2ird1) was found to play an important role in translating mechanical stress to Mkx expression . Gtf2ird1 is localized in the cytoplasm of unstressed tenocytes and translocated into the nucleus upon mechanical stretching to activate the Mkx expression, thereby acting as a mechano‐sensor .…”