Role of Osteoblast Gi Signaling in Age-Related Bone Loss in Female Mice

Age-related bone loss is an important risk factor for fractures in the elderly; it results from an imbalance in bone remodeling mainly due to decreased bone formation. We have previously demonstrated that endogenous G protein-coupled receptor (GPCR)-driven Gi signaling in osteoblasts (Obs) restrains bone formation in mice during growth. Here, we launched a longitudinal study to test the hypothesis that Gi signaling in Obs restrains bone formation in aging mice, thereby promoting bone loss. Our approach was to block Gi signaling in maturing Obs by the induced expression of the catalytic subunit of pertussis toxin (PTX) after the achievement of peak bone mass. In contrast to the progressive cancellous bone loss seen in aging sex-matched littermate control mice, aging female Col1(2.3)+/PTX+ mice showed an age-related increase in bone volume. Increased bone volume was associated with increased bone formation at both trabecular and endocortical surfaces as well as increased bending strength of the femoral middiaphyses. In contrast, male Col1(2.3)+/PTX+ mice were not protected from age-related bone loss. Our results indicate that Gi signaling markedly restrains bone formation at cancellous and endosteal bone surfaces in female mice during aging. Blockade of the relevant Gi-coupled GPCRs represents an approach for the development of osteoporosis therapies-at least in the long bones of aging women.