TRPM7 channels mediate spontaneous Ca 2+ fluctuations in growth plate chondrocytes that promote bone development

Abstract

During endochondral ossification of long bones, the proliferation and differentiation of chondrocytes cause them to be arranged into layered structures constituting the epiphyseal growth plate, where they secrete the cartilage matrix that is subsequently converted into trabecular bone. Ca 2+ signaling has been implicated in chondrogenesis in vitro. Through fluorometric imaging of bone slices from embryonic mice, we demonstrated that live growth plate chondrocytes generated small, cell-autonomous Ca 2+ fluctuations that were associated with weak and intermittent Ca 2+ influx. Several genes encoding Ca 2+ -permeable channels were expressed in growth plate chondrocytes, but only pharmacological inhibitors of transient receptor potential cation channel subfamily M member 7 (TRPM7) reduced the spontaneous Ca 2+ fluctuations. The TRPM7-mediated Ca 2+ influx was likely activated downstream of basal phospholipase C activity and was potentiated upon cell hyperpolarization induced by big-conductance Ca 2+ -dependent K + channels. Bones from embryos in which Trpm7 was conditionally knocked out during ex vivo culture exhibited reduced outgrowth and displayed histological abnormalities accompanied by insufficient autophosphorylation of Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) in the growth plate. The link between TRPM7-mediated Ca 2+ fluctuations and CaMKII-dependent chondrogenesis was further supported by experiments with chondrocyte-specific Trpm7 knockout mice. Thus, growth plate chondrocytes generate spontaneous, TRPM7-mediated Ca 2+ fluctuations that promote self-maturation and bone development.