Sakamoto Akio, Chen Min, Nakamura Takashi, Xie Tao, Karsenty Gerard, Weinstein Lee S
Metabolic Diseases Branch, NIDDK, National Institutes of Health, Bethesda, Maryland 20892, USA.
J Biol Chem. 2005 Jun 3;280(22):21369-75. doi: 10.1074/jbc.M500346200. Epub 2005 Mar 28.
The G-protein alpha-subunit G(s)alpha is required for the intracellular cAMP responses to hormones and other agonists. G(s)alpha 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)alpha signaling in osteoblasts, we developed mice with osteoblast/osteocyte-specific G(s)alpha deficiency (BGsKO) by mating G(s)alpha-floxed mice with collagen Ialpha1 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-kappaB 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)alpha in osteoblasts. Osteoblast-specific G(s)alpha deficiency leads to reduced bone turnover.
G蛋白α亚基G(s)α是细胞内对激素和其他激动剂产生环磷酸腺苷(cAMP)反应所必需的。已知G(s)α介导骨和软骨中对甲状旁腺激素以及其他激素和细胞因子的cAMP反应。为了分析G(s)α信号在成骨细胞中的体内作用,我们通过将G(s)α基因 floxed小鼠与Iα1型胶原启动子-Cre重组酶转基因小鼠交配,培育出了成骨细胞/骨细胞特异性G(s)α缺陷小鼠(BGsKO)。BGsKO小鼠的早期骨骼发育正常,因为初始软骨模板和骨环的形成未受影响。BGsKO小鼠生长板的软骨细胞区也看起来正常。BGsKO小鼠在初级骨小梁形成方面存在缺陷,未成熟类骨质(新骨形成)减少且总体长度缩短,这导致骨小梁骨体积减少。相比之下,皮质骨增厚,骨髓腔变窄。这可能是由于皮质骨吸收减少所致,因为皮质骨内表面的破骨细胞明显减少。此外,早期成骨细胞分化标志物碱性磷酸酶的表达正常,而晚期成骨细胞分化标志物骨桥蛋白和骨钙素的表达减少,这表明骨中成熟成骨细胞的数量减少。核因子κB受体激活蛋白配体(一种破骨细胞刺激因子)的表达也减少。总体而言,我们的研究结果与甲状旁腺激素基因敲除小鼠相似,并证实甲状旁腺激素对骨小梁和皮质骨的不同作用主要是通过成骨细胞中的G(s)α介导的。成骨细胞特异性G(s)α缺陷导致骨转换减少。