ABSTRACT: Osteoclast lineage commitment and differentiation have been studied extensively, although the mechanism by which transcription factor(s) control osteoclast terminal differentiation, activation, and function remains unclear. CCAAT/enhancer-binding protein ? (C/ebp?) has been reported to be a key regulator of osteoclast cell lineage commitment, yet C/ebp?'s roles in osteoclast terminal differentiation, activation and function, and bone homeostasis, under physiological or pathological conditions, have not been studied because newborn C/ebp?-null mice die within several hours after birth. Furthermore, the function of C/ebp? in osteoclast terminal differentiation, activation, and function is largely unknown. Herein, we generated and analyzed an osteoclast-specific C/ebp? conditional knockout (CKO) mouse model via Ctsk-Cre mice and found that C/ebp?-deficient mice exhibited a severe osteopetrosis phenotype due to impaired osteoclast terminal differentiation, activation, and function, including mildly reduced osteoclast number, impaired osteoclast polarization, actin formation, and bone resorption, which demonstrated the novel function of C/ebp? in cell function and terminal differentiation. Interestingly, C/ebp? deficiency did not affect bone formation or monocyte/macrophage development. Our results further demonstrated that C/ebp? deficiency suppressed the expression of osteoclast functional genes, e.g. encoding cathepsin K (Ctsk), Atp6i (Tcirg1), and osteoclast regulator genes, e.g. encoding c-fos (Fos), and nuclear factor of activated T-cells 1 (Nfatc1), while having no effect on Pu.1 (Spi1) expression. Promoter activity mapping and ChIP assay defined the critical cis-regulatory element (CCRE) in the promoter region of Nfatc1, and also showed that the CCREs were directly associated with C/ebp?, which enhanced the promoter's activity. The deficiency of C/ebp? in osteoclasts completely blocked ovariectomy-induced bone loss, indicating that C/ebp? is a promising new target for the treatment of osteolytic diseases. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.
