Deficiency of the G-protein α-Subunit Gsα in Osteoblasts Leads to Differential Effects on Trabecular and Cortical Bone

Abstract

The G-protein ␣-subunit G s ␣ is required for the intracellular cAMP responses to hormones and other agonists. G s ␣ is known to mediate the cAMP response to parathyroid hormone and other hormones and cytokines in bone and cartilage. To analyze the in vivo role of G s ␣ signaling in osteoblasts, we developed mice with osteoblast/osteocyte-specific G s ␣ deficiency (BGsKO) by mating G s ␣-floxed mice with collagen I␣1 promoter-Cre recombinase transgenic mice. Early skeletal development was normal in BGsKO mice, because formation of the initial cartilage template and bone collar was unaffected. The chondrocytic zones of the growth plates also appeared normal in BGsKO mice. BGsKO mice had a defect in the formation of the primary spongiosa with reduced immature osteoid (new bone formation) and overall length, which led to reduced trabecular bone volume. In contrast, cortical bone was thickened with narrowing of the bone marrow cavity. This was probably due to decreased cortical bone resorption, because osteoclasts were markedly reduced on the endosteal surface of cortical bone. In addition, the expression of alkaline phosphatase, an early osteoblastic differentiation marker, was normal, whereas the expression of the late osteoblast differentiation markers osteopontin and osteocalcin was reduced, suggesting that the number of mature osteoblasts in bone is reduced. Expression of the osteoclast-stimulating factor receptor activator of NF-B ligand was also reduced. Overall, our findings have similarities to parathyroid hormone null mice and confirm that the differential effects of parathyroid hormone on trabecular and cortical bone are primarily mediated via G s ␣ in osteoblasts. Osteoblast-specific G s ␣ deficiency leads to reduced bone turnover.1 is a ubiquitously expressed G-protein ␣-subunit that couples receptors to adenylyl cyclase and is required for receptor-stimulated cAMP generation (1). cAMP is an important second messenger in osteoblasts for several hormones and other extracellular factors, such as parathyroid hormone (PTH) and prostaglandin E 2 . Both PTH and PTH-related peptide (PTHrP), a paracrine regulator of chondrocyte differentiation in growth plates, activate a common receptor (PTH/PTHrP receptor, PPR), which activates G s ␣ as well as other G-proteins (2-4). PTH stimulates bone formation and bone resorption, resulting in high bone remodeling (5).Different bones are formed by one of two different mechanisms, either intramembranous or endochondral ossification. Intramembranous ossification, which leads to formation of many of the flat bones, occurs when pluripotent mesenchymal cells directly enter an osteoblast lineage and differentiate into osteoblastic cells. Long bones develop by endochondral ossification in which a cartilage template is formed, and chondrocytes undergo hypertrophic differentiation and are then replaced by osteoblasts that form the primary spongiosa, which eventually forms the trabecular bone. Simultaneously, perichondrial cells surrounding the hypertrophic chondrocytes differentia...

Gender‐dependent association of the GNAS1 T393C polymorphism with early aseptic loosening after total hip arthroplasty

“…15 Third, it has to be mentioned that not only activated or increased Gas but also deficient or decreased Gas can be associated with malformation of skeletal bone and reduced bone mass turnover. 13 All these considerations make it clear that additional experiments are required to better understand the apparently contradictory results with regard to the GNAS1 T393C polymorphism in women and men. The present study suggests a significant role of the T393C polymorphism in aseptic loosening.…”

“…Second, both activating (McCune-Albright syndrome) and inactivating (Albright hereditary osteodystrophy) mutations of Gas lead to typical phenotypes, especially malformations of the bone as well as functional defects of osteoblasts. 12,13 Third, Gas/cAMP are key regulators of interleukin-6 (IL-6) in osteoblasts 14 and in macrophages, 15 and IL-6 is a potent activator of osteoclasts and a promoter of aseptic loosening. 16 GNAS1, the gene coding for the stimulatory Gas subunit, is located on chromosome 20q13.2-13.3.…”

Osteoblast expression of an engineered G s -coupled receptor dramatically increases bone mass

“…103580). Mouse models of AHO with chondrocyte-or osteoblast-specific inactivation of GNAS show severe alterations in chondrocyte maturation (7) or cortical bone formation (8), respectively.…”

Continuous Activation of Gαq in Osteoblasts Results in Osteopenia through Impaired Osteoblast Differentiation

“…Maternally inherited mutations lead to pseudohypoparathyroidism type 1A, and paternally inherited mutations lead to Albright hereditary osteodystrophy alone, and pseudohypoparathyroidism type 1B is almost always associated with a GNAS imprinting defect in which both alleles have a paternal specific imprinting pattern on both parental alleles. Moreover, mice with osteoblast-specific G␣ s deficiency was generated recently, and this deficiency led to a defect in the formation of the primary spongiosa with reduced immature osteoid and thickened cortical bone with narrowing of the bone marrow cavity, implying a strong involvement of the G␣ s signal in bone development (7). On the other hand, G␣ q mutations have not been reported in human disease, perhaps because the functional redundancy of G␣ q and G␣ 11 would compensate for a loss-of-function mutation, and little is known regarding the G␣ q signaling role in bone cells.…”