Mmp13 deletion in mesenchymal cells increases bone mass and may attenuate the cortical bone loss caused by estrogen deficiency

The protective effect of estrogens against cortical bone loss is mediated via direct actions on mesenchymal cells, but functional evidence for the mediators of these effects has only recently begun to emerge. We report that the matrix metalloproteinase 13 (MMP13) is the highest up-regulated gene in mesenchymal cells from mice lacking the estrogen receptor alpha (ERα). In sham-operated female mice with conditional Mmp13 deletion in Prrx1 expressing cells (Mmp13^ΔPrrx1), the femur and tibia length was lower as compared to control littermates (Mmp13f.^/f). Additionally, in the sham-operated female Mmp13^ΔPrrx1 mice cortical thickness and trabecular bone volume in the femur and tibia were higher and osteoclast number at the endocortical surfaces was lower, whereas bone formation rate was unaffected. Notably, the decrease of cortical thickness caused by ovariectomy (OVX) in the femur and tibia of Mmp13f.^/f mice was attenuated in the Mmp13^ΔPrrx1 mice; but the decrease of trabecular bone caused by OVX was not affected. These results reveal that mesenchymal cell–derived MMP13 may regulate osteoclast number and/or activity, bone resorption, and bone mass. And increased production of mesenchymal cell-derived factors may be important mediators of the adverse effect of estrogen deficiency on cortical, but not trabecular, bone.