Zhang Jishuai, Tan Xiaohong, Li Wenlong, Wang Youliang, Wang Jian, Cheng Xuan, Yang Xiao
Genetic Laboratory of Development and Diseases, Institute of Biotechnology, Beijing 100071, P.R. China.
Dev Biol. 2005 Aug 15;284(2):311-22. doi: 10.1016/j.ydbio.2005.05.036.
Smad4 is the central intracellular mediator of transforming growth factor-beta (TGF-beta) signals. To study the role of Smad4 in skeletal development, we introduced a conditional mutation of the gene in chondrocytes using Cre--loxP system. We showed that Smad4 was expressed strongly in prehypertrophic and hypertrophic chondrocytes. The abrogation of Smad4 in chondrocytes resulted in dwarfism with a severely disorganized growth plate characterized by expanded resting zone of chondrocytes, reduced chondrocyte proliferation, accelerated hypertrophic differentiation, increased apoptosis and ectopic bone collars in perichondrium. Meanwhile, Smad4 mutant mice exhibited decreased expression of molecules in Indian hedgehog/parathyroid hormone-related protein (Ihh/PTHrP) signaling. The cultured mutant metatarsal bones failed to response to TGF-beta1, while the hypertrophic differentiation was largely inhibited by Sonic hedgehog (Shh). This indicated that Ihh/PTHrP inhibited the hypertrophic differentiation of chondrocytes independent of the Smad4-mediated TGF-beta signals. All these data provided the first genetic evidence demonstrating that Smad4-mediated TGF-beta signals inhibit the chondrocyte hypertrophic differentiation, and are required for maintaining the normal organization of chondrocytes in the growth plate.
Smad4是转化生长因子-β（TGF-β）信号的核心细胞内介质。为了研究Smad4在骨骼发育中的作用，我们使用Cre-loxP系统在软骨细胞中引入了该基因的条件性突变。我们发现Smad4在肥大前和肥大软骨细胞中强烈表达。软骨细胞中Smad4的缺失导致侏儒症，生长板严重紊乱，其特征为软骨细胞静止区扩大、软骨细胞增殖减少、肥大分化加速、细胞凋亡增加以及软骨膜中出现异位骨环。同时，Smad4突变小鼠中印度刺猬因子/甲状旁腺激素相关蛋白（Ihh/PTHrP）信号通路中的分子表达降低。培养的突变跖骨对TGF-β1无反应，而音猬因子（Shh）在很大程度上抑制了肥大分化。这表明Ihh/PTHrP独立于Smad4介导的TGF-β信号抑制软骨细胞的肥大分化。所有这些数据提供了首个遗传学证据，证明Smad4介导的TGF-β信号抑制软骨细胞肥大分化，并且是维持生长板中软骨细胞正常组织结构所必需的。
