The role of cGMP/PKG1 signaling in osteogenic differentiation

Abstract

Nitric oxide has important effects on bone cell function. NO stimulates cGMP synthesis, activation of protein kinase G (PKGs) and improves proliferation, differentiation, and survival of bone forming cells. In bone resorbing osteoclasts, NO regulates their differentiation and activity (effects are only partly cGMP-independent). Mice deficient in NO synthesis show severe deficiencies in bone repair, but the mechanism(s) whereby NO affects fracture healing are unknown. Mice with osteoblast-specific knockout of type 1 PKG (Prkg1 OB-KO) were generated to investigate the role of PKG in healing of a bone defect. Skeletal phenotypes in NO synthase- and PKG-deficient mice as well as NO/cGMP effects on bone remodelling were reviewed. <br> Prkg1 OB-KO mice have normal osteoblast and osteoclast numbers, and PKG1 deficient osteoblasts proliferate normally <em>in vitro</em>. Despite evidence of reduced osteoblast activity <em>in vivo</em>, by decreased bone formation rates and osteoblastic marker gene expression, Prkg1 OB-KO mice showed normal bone micro-architecture under basal conditions. However, bone regeneration in a mono-cortical defect model was severely reduced in Prkg1 OB-KO mice compared to control litter mates, and the knockout mice exhibit decreased capillary density and evidence of reduced BMP signaling in the region of the bone defect. Primary osteoblasts and tibiae from Prkg1 OB-KO mice contained less transcripts encoding bone morphogenic protein (BMP)-2/4, vascular endothelial growth factor (VEGF)-A, and their receptors BMPR2 and VEGFR1 than bone samples and cells from control littermates. PKG1 was required for cGMP-stimulated expression of these genes, and for BMP-induced Smad phosphorylation in osteoblasts. Since osteoblast-derived BMP-2 and VEGF are essential for fracture healing, these data suggest that PKG1 controls bone regeneration by regulating BMP and VEGF production in osteoblasts.