[Gap junctions and bone remodeling].

Gap junctions are specialized plasma membrane structures consisting of transmembrane channels that directly link the cytoplasms of adjoining cells and mediate the reciprocal exchange of ions and low molecular weight molecules (<1200 Da). Structural studies have demonstrated that each gap junctional channel is formed by the extracellular interaction of two hemi-channels (connexons). Each connexon is a hexameric assembly of protein subunits (connexins), which delineate an aqueous pore. Connexins are homologous proteins encoded by a multigene family and are named according to their predicted molecular weight. Connexin 43, widely distributed in different cell types, is the main gap junction protein expressed in human bone cells, although Cx45 and Cx46 have been reported to be expressed as well. Bone remodeling requires coordinated activity among osteoblasts and osteoclasts. Osteoblasts (bone forming cells) are derived from mesenchymal stem cells, and osteoclasts (bone resorbing cells), are multinucleated cells of monocyte/macrophage origin. Here, we review what is known regarding the structure of gap junctions and the mechanisms regulating bone remodeling, and discuss the evidence suggesting that gap junctional intercellular communication contributes to the bone remodeling.