Connexin 43 (Cx43) mediates osteocyte communication with other cells and with the extracellular milieu and regulates osteoblastic cell signaling and gene expression. We now report that mice lacking Cx43 in osteoblasts/osteocytes or only in osteocytes (Cx43^ΔOt mice) exhibit increased osteocyte apoptosis, endocortical resorption, and periosteal bone formation, resulting in higher marrow cavity and total tissue areas measured at the femoral mid‐diaphysis. Blockade of resorption reversed the increased marrow cavity but not total tissue area, demonstrating that endocortical resorption and periosteal apposition are independently regulated. Anatomical mapping of apoptotic osteocytes, osteocytic protein expression, and resorption and formation suggests that Cx43 controls osteoclast and osteoblast activity by regulating osteoprotegerin and sclerostin levels, respectively, in osteocytes located in specific areas of the cortex. Whereas empty lacunae and living osteocytes lacking osteoprotegerin were distributed throughout cortical bone in Cx43^ΔOt mice, apoptotic osteocytes were preferentially located in areas containing osteoclasts, suggesting that osteoclast recruitment requires active signaling from dying osteocytes. Furthermore, Cx43 deletion in cultured osteocytic cells resulted in increased apoptosis and decreased osteoprotegerin expression. Thus, Cx43 is essential in a cell‐autonomous fashion in vivo and in vitro for osteocyte survival and for controlling the expression of osteocytic genes that affect osteoclast and osteoblast function. © 2012 American Society for Bone and Mineral Research

Connexin (Cx)43 is required for inhibition of osteocyte and osteoblast apoptosis by bisphosphonates in vitro. Herein, we evaluated its requirement for the in vivo actions of bisphosphonates using mice in which Cx43 was deleted specifically from osteocytes and osteoblasts (Cx43ΔOb−Ot/− mice). Effective removal of Cx43 was confirmed by the presence of the deleted form of the gene and by reduced mRNA and protein expression in osteoblastic cells and bones obtained from Cx43ΔOb−Ot/− mice. The amino-bisphosphonate alendronate (2.3 μmol/kg/d) was injected daily into 5-mo-old female mice (n = 6–11) for 31 days, starting 3 days before implantation of pellets releasing the glucocorticoid prednisolone (2.1 mg/kg/d). Cx43ΔOb−Ot/− mice and their littermates (Cx43fl/−, Cx43ΔOb−Ot/+, and Cx43fl/+) gained bone with similar kinetics and exhibited identical bone mass from 2 to 4.5 mo of age, indicating that Cx43 deletion from osteocytes and mature osteoblasts does not impair bone acquisition. In addition, prednisolone induced a similar increase in osteocyte and osteoblast apoptosis in Cx43ΔOb−Ot/− or in control Cx43fl/− littermates. However, whereas alendronate prevented prednisolone-induced apoptosis in control Cx43fl/− mice, it was ineffective in Cx43ΔOb−Ot/− mice. In contrast, alendronate inhibited glucocorticoid-induced bone loss in both type of animals, suggesting that inhibition of resorption is the predominant effect of alendronate against the early phase of glucocorticoid-induced bone loss. Taken together with earlier in vitro evidence, these findings show that Cx43 is required for the anti-apoptotic effect of bisphosphonates on osteocytes and osteoblasts.