Unexpected enhanced response to mechanical loading of mice lacking Cx43 exclusively in osteocytes.

BIVI N; FARLOW N; BRUN LR; BENSON JD; CONDON KW; ROBLING AG; BELLIDO T; PLOTKIN LI

San Diego

Congreso; American Society for Bone and Mineral Research (ASBMR) Annual Meeting 2011; 2011

American Society for Bone and Mineral Research

The osteocyte network is crucial for the response of bone to mechanical force. Within this network, connexin 43 (Cx43) mediates the communication of osteocytes and osteoblasts among themselves and with the extracellular milieu. Although Cx43 appears to be important for the response of osteocytic cells to mechanical stimulation in vitro, the contribution of Cx43 expression in osteocytes to mechanotransduction in vivo is not known. To this end, we examined the anabolic response to loading of mice lacking Cx43 in osteocytes (Cx43OT) generated by crossing Cx43fl/fl mice with DMP1-8kb-Cre mice. Unlike the 10kb fragment that directs gene expression to osteocytes and some osteoblasts, the 8kb fragment of the DMP1 promoter targets gene expression only to osteocytes. Indeed, Cre recombinase mRNA was detected only in green fluorescent protein (GFP) positive cells (osteocytes) and not in GFP negative cells (osteoblasts) isolated from calvaria of DMP1-8kb-Cre mice crossed with DMP1-8kb-GFP mice. Immunohistochemistry confirmed lack of Cx43 expression only in osteocytes in Cx43OT bone sections. Moreover, Cx43 mRNA was reduced by 88% in osteocyte-enriched cortical bone of Cx43OT mice compared to control Cx43fl/fl littermates. Right ulnae of control and Cx43OT mice were subjected to axial loading at equivalent low, medium, and high strain magnitudes during 1min/d for 3 consecutive days; left ulnae were used as non-loaded controls. Bone formation rate (BFR/BS) was quantified 14d later. Loading induced strain-dependent increases in periosteal BFR/BS of 1.4-, 1.8-, and 2.2-fold in control animals. To our surprise, loading induced a greater response in Cx43OT mice (2.5-, 4.3- and 4.9-fold), due to a combination of higher mineralizing surface covered by osteoblasts (MS/BS) and higher rate of mineral apposition by individual osteoblasts (MAR). These results suggest that the absence of Cx43 in osteocytes unleashes bone formation by periosteal osteoblasts and are consistent with previously reported higher basal periosteal bone formation in femora from Cx43OT mice. The apparent discrepancy with earlier evidence showing decreased endosteal BFR/BS induced by tibia loading in mice lacking Cx43 in both osteocytes and osteoblasts (col1-2.3kb) can be explained by the fact that, in the latter model, osteoblasts are unable to function optimally. We conclude that the intrinsic function of Cx43 in osteocytes is to restrain the response of osteoblasts to mechanical loading.