IGF‐I Signaling in Osterix‐Expressing Cells Regulates Secondary Ossification Center Formation,Growth Plate Maturation,and Metaphyseal Formation During Postnatal Bone Development

To investigate the role of IGF‐I signaling in osterix (OSX)‐expressing cells in the skeleton, we generated IGF‐I receptor (IGF‐IR) knockout mice (^OSXIGF‐IRKO) (floxed‐IGF‐IR mice × OSX promoter‐driven GFP‐labeled cre‐recombinase [^OSXGFPcre]), and monitored postnatal bone development. At day 2 after birth (P2), ^OSXGFP‐cre was highly expressed in the osteoblasts in the bone surface of the metaphysis and in the prehypertrophic chondrocytes (PHCs) and inner layer of perichondral cells (IPCs). From P7, ^OSXGFP‐cre was highly expressed in PHCs, IPCs, cartilage canals (CCs), and osteoblasts (OBs) in the epiphyseal secondary ossification center (SOC), but was only slightly expressed in the OBs in the metaphysis. Compared with the control mice, the IPC proliferation was decreased in the ^OSXIGF‐IRKOs. In these mice, fewer IPCs invaded into the cartilage, resulting in delayed formation of the CC and SOC. Immunohistochemistry indicated a reduction of vessel number and lower expression of VEGF and ephrin B2 in the IPCs and SOC of ^OSXIGF‐IRKOs. Quantitative real‐time PCR revealed that the mRNA levels of the matrix degradation markers, MMP‐9, 13 and 14, were decreased in the ^OSXIGF‐IRKOs compared with the controls. The ^OSXIGF‐IRKO also showed irregular morphology of the growth plate and less trabecular bone in the tibia and femur from P7 to 7 weeks, accompanied by decreased chondrocyte proliferation, altered chondrocyte differentiation, and decreased osteoblast differentiation. Our data indicate that during postnatal bone development, IGF‐I signaling in OSX‐expressing IPCs promotes IPC proliferation and cartilage matrix degradation and increases ephrin B2 production to stimulate vascular endothelial growth factor (VEGF) expression and vascularization. These processes are required for normal CC formation in the establishment of the SOC. Moreover, IGF‐I signaling in the OSX‐expressing PHC is required for growth plate maturation and osteoblast differentiation in the development of the metaphysis. © 2015 American Society for Bone and Mineral Research.     

Suppression of p38α MAPK Signaling in Osteoblast Lineage Cells Impairs Bone Anabolic Action of Parathyroid Hormone

Intermittent parathyroid hormone administration (iPTH) increases bone mass and strength by stimulating osteoblast number and activity. PTH exerts its anabolic effects through cAMP/protein kinase A (PKA) signaling pathway in mature osteoblasts and osteocytes. Here, we show that inactivation of the p38α MAPK‐encoding gene with the use of an osteocalcin‐cre transgene prevents iPTH bone anabolic action. Indeed, iPTH fails to increase insulin‐like growth factor 1 expression, osteoblast number and activity, and bone formation in mice lacking p38α in osteoblasts and osteocytes. Moreover, iPTH‐induced expression of receptor activator of NF‐κB ligand (RANKL) and subsequent increased bone resorption are suppressed in those mice. Finally, we found that PTH activates p38α MAPK downstream of cAMP/PKA signaling pathway in mature osteoblasts. Our findings identify p38α MAPK as a key component of PTH signaling in osteoblast lineage cells and highlight its requirement in iPTH osteoanabolic activity. © 2015 American Society for Bone and Mineral Research.