Osteoporosis is a skeletal disease characterized by low bone mass that often results in fracture. Mechanical loading of the skeleton is a promising approach to maintain or recover bone mass. Mouse models of in vivo loading differentially increase bone mass in cortical and cancellous sites. The molecular mechanisms behind this anabolic response to mechanical loading need to be determined and compared between cortical and cancellous bone. This knowledge could enhance the development of drug therapies to increase bone formation in osteoporotic patients.

After developing a method to isolate high-quality RNA from marrow-free mouse cortical and cancellous bone, differences in gene transcription were determined at baseline and at two time points following mechanical loading of wild-type mice. Cortical and cancellous bone exhibited different transcriptional profiles at baseline and in response to mechanical loading. Enhanced Wnt signaling dominated the response in cortical bone at both time points, but in cancellous bone only at the early time point. In cancellous bone at the later time point, many muscle-related genes were downregulated.

Decreased bioavailable estrogen levels are a major cause of bone loss in postmenopausal women. Estrogen signaling through estrogen receptor alpha (ERa) has been found to be particularly important in regulating bone mass and the skeletal response to mechanical loading. We recently showed that mice lacking ERa in osteoblasts and osteocytes (pOC-ERaKO) had an increased adaptive response to mechanical loading, particularly in cancellous bone. The molecular mechanisms of functional adaptation to load in the context of decreased estrogen signaling are not fully elucidated, particularly in cortical versus cancellous sites. We examined transcription in cortical versus cancellous bone of tibiae from littermate control (LC) and pOCERaKO mice. pOC-ERaKO mice had a blunted transcriptional response to mechanical loading in cortical bone compared to littermate controls, but had an increased response in cancellous bone. This work demonstrates the importance of examining cortical and cancellous bone separately, and that next-generation sequencing is a powerful tool for discovering the complete transcriptional mechanisms responsible for mechanical loading-related bone anabolism.