Abstract
DAP12 and or FcRγ ITAM signals are required during normal osteoclast (OC) development in vivo and in vitro. To determine whether ITAM deficient mice are resistant to bone loss induced by estrogen deficiency, we performed ovariectomy (OVX) or SHAM surgery on 10–12 week female DAP12−/−, FcRγ−/−, DAP12−/−FcRγ−/− and control C57BL/6 (B6) mice. MicroCT of the distal femur showed statistically significant losses of trabecular bone in DAP12−/−FcRγ−/−, DAP12−/−, FcRγ−/− and B6 OVX groups compared to SHAM groups, BV/TV dropping from 56% to 35% (p<0.01), 21% to 12% (p<0.01), 12% to 7% (p<0.01), and 6% to 3% (p<0.05) respectively. Bone turnover measured by serum osteocalcin and urinary DPD increased at 4 weeks post-OVX in all four strains. In vitro OC bone resorption assays revealed a 2.7 or 3.6 fold increase in OC resorption from B6 and FcRγ−/− OVX mice that were not seen in DAP12−/− and DAP12−/−FcRγ−/− OVX OC. FSH, reported to stimulate postmenopausal osteoporosis, combined with RANKL could not rescue the in vitro OC defect in DAP12−/− and DAP12−/−FcRγ−/− cultures. TGB-β followed by TNF-α stimulation induced OC differentiation in B6 cells in vitro without RANKL; however, DAP12−/− and DAP12−/−FcRγ−/− OC cultures were not rescued. We conclude that OC differentiation during estrogen deficiency induced osteoporosis bypasses the requirement of ITAM signaling and the unidentified novel effect is microenvironment dependent.
