Abstract
Hyperpolarization- activated cyclic nucleotide-modulated (HCN) channels play fundamental roles in excitable cells, including central neurons and cardiac myocytes. However, whether HCN functions in non-excitable cells remains to be solved. In this study, we investigated localization of and roles of HCN in murine bone cells in vitro and in vivo. All four HCN subtypes (HCN1-4) were found in bone tissue, but their expression altered among different cell types. Immunoreactivity for HCN1 was abundant in osteoclasts, specifically at the sealing zone and ruffled border that are faced to the bone surface and responsible for acid secretion. HCN2 and HCN3 were expressed in bone marrow cells and osteoblasts respectively. The whole-cell clamp recordings revealed HCN-like currents (Ih), featured by activation at hyperpolarization and slow gating kinetics, in cultured osteoclasts. Extracellular acidification shifted the activation voltage towards more positive voltages, indicating that HCN channels would respond to changes in local pH caused by proton secretion from osteoclasts during bone resorption. An HCN blocker (ZD7288) inhibited differentiation of osteoclasts and osteoblasts from their precursors of the bone marrow. HCN2 knockout mice had low bone mass, fragile bone structure and weak bone strength. These findings suggest that HCN channels are essential in regulation of both bone formation and resorption. [J Physiol Sci. 2008;58 Suppl:S71]
