Monocyte-Specific Knockout of C/ebpα Results in Osteopetrosis Phenotype, Blocks Bone Loss in Ovariectomized Mice, and Reveals an Important Function of C/ebpα in Osteoclast Differentiation and Function

Abstract

CCAAT/enhancer-binding protein α (C/ebpα) is critical for osteoclastogenesis by regulating osteoclast (OC) lineage commitment and is also important for OC differentiation and function in vitro. However, the role of C/ebpα in postnatal skeletal development has not been reported owing to lethality in C/ebpα−/− mice from hypoglycemia within 8 hours after birth. Herein, we generated conditional knockout mice by deleting the C/ebpα gene in monocyte via LysM-Cre to examine its role in OC differentiation and function. C/ebpαf/fLysM-Cre mice exhibited postnatal osteopetrosis due to impaired osteoclastogenesis, OC lineage priming defects, as well as defective OC differentiation and activity. Furthermore, our ex-vivo analysis demonstrated that C/ebpα conditional deletion significantly reduced OC differentiation, maturation, and activity while mildly repressing macrophage development. At the molecular level, C/ebpα deficiency significantly suppresses the expressions of OC genes associated with early stages of osteoclastogenesis as well as genes associated with OC differentiation and activity. We also identified numerous C/ebpα critical cis-regulatory elements on the Cathepsin K promoter which allow C/ebpα to significantly upregulate Cathepsin K expression during OC differentiation and activity. In pathologically induced mouse model of osteoporosis, C/ebpα deficiency can protect mice against ovariectomy-induced bone loss, uncovering a central role for C/ebpα in osteolytic diseases. Collectively, our findings have further established C/ebpα as a promising therapeutic target for bone loss by concurrently targeting OC lineage priming, differentiation and activity.

The Macrophage-Osteoclast Axis in Osteoimmunity and Osteo-Related Diseases

“…In osteoporosis and other bone loss inflammatory conditions, the macrophage-osteoclast axis plays a vital role. The deletion of the C/ebpα gene in monocytes causes osteopetrosis and inhibits bone resorption in ovariectomized mice, which indicates that the C/ebpα gene is involved in regulating the function of osteoclasts ( 165 ). The typical features of osteopetrosis are the obstruction of the formation of osteoclasts and functional defects.…”

Angiogenesis stimulated by elevated PDGF-BB in subchondral bone contributes to osteoarthritis development

“…Osteoclasts are derived from bone marrow monocytes/macrophages. Under physiological conditions, monocytes/macrophages first commit to macrophage colony-stimulating factor receptor-positive osteoclast precursor cells and then differentiate into receptor activator of nuclear factor κB-positive (RANK + ) tartrate-resistant acid phosphatase (TRAP + ) mononuclear preosteoclasts and eventually fuse to form mature multinuclear osteoclasts (39)(40)(41)(42)(43)(44)(45). Serving as precursors for osteoclasts, preosteoclasts have limited bone-resorbing activity.…”

F2r negatively regulates osteoclastogenesis through inhibiting the Akt and NFκB signaling pathways

“…Mouse bone marrow cells (MBMs) were isolated from long bones of 8-weeks old wild type mice as described previously [5,24]. Then, MBMs were cultured in α-minimal essential medium (α-MEM, Invitrogen, USA) with 10% FBS (Invitrogen) containing macrophage colony-stimulating factor (M-CSF, 20 ng/ml, R&D Systems, USA).…”

“…We used 10% sodium dodecyl sulfatepolyacrylamide gel electrophoresis (SDS-PAGE) to perform western blot as described [24,28,29]. MBMs were transfected lentivirus and induced to 5 days for western blot analysis.…”