CONNEXIN CHANNELS IN TRANSDUCING MECHANICAL SIGNALS IN BONE
Mechanical loading experienced by skeletal tissues plays an important role in bone formation and remodeling.Osteocytes are the most abundant bone cell type and the major mechanosensory cells of the bone. Theyorchestrate bone remodeling from their location throughout bone matrix by coordinating osteoblastic formationand osteoclastic resorption. Osteocytes are connected to neighboring osteocytes and other bone cells via gapjunction channels and to extracellular environments via hemichannels. Both types of channels are formed byconnexin (Cx) 43. The involvement of Cx43 in response to mechanical stimulation of bone tissue has beenshown in gene knockout models; however, the distinct functions of gap junction channels and hemichannels inosteocytes, as well as the mechanism underlying the physiological roles of these channels during mechanicalloading remain largely unknown. To dissect the physiological roles of these two types of channels, we haverecently developed two transgenic mouse models expressing Cx43 dominant negative mutants predominantlyin osteocytes. We are also generating a Cx43 mutant gene knockin mouse model to complement ourtransgenic models. These mutations impair osteocytic gap junction channels and/or hemichannels. In addition,we have generated antibodies that specifically inhibit Cx43 hemichannels, but not gap junction channels.Moreover, we have developed novel approaches that allow us to assess osteocytic hemichannel activity in situ.Preliminary data show that impairment of Cx43 hemichannels attenuates the anabolic effect of mechanicalloading on the bone. We and others have also shown that osteocytic Cx43 hemichannels are opened bymechanical stress, releasing small bone anabolic factors including prostaglandins in vitro. The objective is todetermine the specific mechanistic role of Cx43 hemichannels in mediating the anabolic effect of mechanicalloading on the skeletal tissues. Three specific aims are proposed: 1) To test the hypothesis that osteocyticCx43 hemichannels play a crucial role in mediating anabolic function of mechanical loading on skeletal tissue.2) To test the hypothesis that osteocytic Cx43 hemichannels mediate anabolic function of mechanical loadingvia PGE2 release, and activation of PGE2 and Wnt signaling. 3) To test the hypothesis that activation andinactivation of Cx43 hemichannels are specifically regulated by integrin activation/cytoskeleton and MAPKsignaling, respectively. The proposed studies are expected to have a major positive impact by defining themechanical transduction mechanism and its regulation in bone tissue, constituting potential, significantcontributions toward the development of new therapeutic agents for the treatment of osteoporosis and boneloss.