Age-Related Low Bone Mineral Density in C57BL/6 Mice Is Reflective of Aberrant Bone Morphogenetic Protein-2 Signaling Observed in Human Patients Diagnosed with Osteoporosis.

Osteoporosis (OP) is a bone disorder characterized by decreased bone mineral density (BMD). Bone Morphogenetic Protein-2 (BMP-2) injections are used to promote bone formation in OP patients. However, patients are unresponsive to BMP-2 while displaying an upregulation of BMP Receptor Type 1a (BMPRIa) and protein kinase CK2α (CK2α). A synthetically produced peptide named casein kinase 2.3 (CK2.3) utilizes the BMP-signaling pathway as it enhances osteogenesis of primary osteoblasts isolated from OP patients, whereas BMP-2 does not. Although shown in OP patients, there is currently no reliable mouse model to study BMP-2 and CK2.3 signaling. In this publication, we show that BMPRIa was required for CK2.3-mediated osteogenesis in C2C12 cells with a CRISPR-Cas9-mediated gene knockout for BMPRIa. We utilized the C57BL/6 (B6) mouse strain as an aging-model to study aberrant BMP-2 signaling, demonstrating that, like OP patients, in 15 and 20-month mice, BMP-2 did not increase bone growth and displayed upregulated BMPRIa and CK2α protein expression. Furthermore, CK2.3 enhanced osteogenesis and decreased osteoclastogenesis in all age groups, whereas BMP-2 only increased mineralization in 6-month mice while increasing osteoclast formation in all age groups. These data demonstrated that aging B6 mice were a reliable model and mimicked data obtained from OP patients.

Differentiation of Cells Isolated from Human Femoral Heads into Functional Osteoclasts.

Proper formation of the skeleton during development is crucial for the mobility of humans and the maintenance of essential organs. The production of bone is regulated by osteoblasts and osteoclasts. An imbalance of these cells can lead to a decrease in bone mineral density, which leads to fractures. While many studies are emerging to understand the role of osteoblasts, less studies are present about the role of osteoclasts. This present study utilized bone marrow cells isolated directly from the bone marrow of femoral heads obtained from osteoarthritic (OA) patients after undergoing hip replacement surgery. Here, we used tartrate resistant acid phosphatase (TRAP) staining, Cathepsin K, and nuclei to identity osteoclasts and their functionality after stimulation with macrophage-colony stimulation factor (M-CSF) and receptor activator of nuclear factor kappa-ß ligand (RANKL). Our data demonstrated that isolated cells can be differentiated into functional osteoclasts, as indicated by the 92% and 83% of cells that stained positive for TRAP and Cathepsin K, respectively. Furthermore, isolated cells remain viable and terminally differentiate into osteoclasts when stimulated with RANKL. These data demonstrate that cells isolated from human femoral heads can be differentiated into osteoclasts to study bone disorders during development and adulthood.