Osteoblast-Specific γ-Glutamyl Carboxylase-Deficient Mice Display Enhanced Bone Formation with Aberrant Mineralization

Abstract

Vitamin K is a fat-soluble vitamin that is necessary for blood coagulation. In addition, it has bone-protective effects. Vitamin K functions as a cofactor of γ-glutamyl carboxylase (GGCX), which activates its substrates by carboxylation. These substrates are found throughout the body and examples include hepatic blood coagulation factors. Furthermore, vitamin K functions as a ligand of the nuclear receptor known as steroid and xenobiotic receptor (SXR) and its murine ortholog, pregnane X receptor (PXR). We have previously reported on the bone-protective role of SXR/PXR signaling by demonstrating that systemic Pxr-knockout mice displayed osteopenia. Because systemic Ggcx-knockout mice die shortly after birth from severe hemorrhage, the GGCX-mediated effect of vitamin K on bone metabolism has been difficult to evaluate. In this work, we utilized Ggcx-floxed mice to generate osteoblast-specific GGCX-deficient (GgcxΔobl/Δobl) mice by crossing them with Col1-Cre mice. The bone mineral density (BMD) of GgcxΔobl/Δobl mice was significantly higher than that of control Col1-Cre (Ggcx+/+) mice. Histomorphometrical analysis of trabecular bones in the proximal tibia showed increased osteoid volume and a higher rate of bone formation in GgcxΔobl/Δobl mice. Histomorphometrical analysis of cortical bones revealed a thicker cortical width and a higher rate of bone formation in GgcxΔobl/Δobl mice. Electron microscopic examination revealed disassembly of mineralized nodules and aberrant calcification of collagen fibers in GgcxΔobl/Δobl mice. The mechanical properties of bones from GgcxΔobl/Δobl mice tended to be stronger than those from control Ggcx+/+ mice. These results suggest that GGCX in osteoblasts functions to prevent abnormal mineralization in bone formation, although this function may not be a prerequisite for the bone-protective effect of vitamin K.