Center for Molecular Medicine, Maine Medical Center Research Institute, Scarborough, ME, USA.
J Bone Miner Res. 2013 Jun;28(6):1412-21. doi: 10.1002/jbmr.1871.
Endochondral bone formation is a multistep process during which a cartilage primordium is replaced by mineralized bone. Several genes involved in cartilage and bone development have been identified as target genes for the Snail family of zinc finger transcriptional repressors, and a gain-of-function study has demonstrated that upregulation of Snai1 activity in mouse long bones caused a reduction in bone length. However, no in vivo loss-of-function studies have been performed to establish whether Snail family genes have an essential, physiological role during normal bone development. We demonstrate here that the Snai1 and Snai2 genes function redundantly during embryonic long bone development in mice. Deletion of the Snai2 gene, or limb bud-specific conditional deletion of the Snai1 gene, did not result in obvious defects in the skeleton. However, limb bud-specific Snai1 deletion on a Snai2 null genetic background resulted in substantial defects in the long bones of the limbs. Long bones of the Snai1/Snai2 double mutants exhibited defects in chondrocyte morphology and organization, inhibited trabecular bone formation, and delayed ossification. Chondrocyte proliferation was markedly reduced, and transcript levels of genes encoding cell cycle regulators, such as p21(Waf1/Cip1) , were strikingly upregulated in the Snai1/Snai2 double mutants, suggesting that during chondrogenesis Snail family proteins act to control cell proliferation by mediating expression of cell-cycle regulators. Snai2 transcript levels were increased in Snai1 mutant femurs, whereas Snai1 transcript levels were increased in Snai2 mutant femurs. In addition, in the mutant femurs the Snai1 and Snai2 genes compensated for each other's loss not only quantitatively, but also by expanding their expression into the other genes' normal expression domains. These results demonstrate that the Snai1 and Snai2 genes transcriptionally compensate temporally, spatially, and quantitatively for each other's loss, and demonstrate an essential role for Snail family genes during chondrogenesis in mice.
软骨内成骨是一个多步骤的过程，在此过程中，软骨原基被矿化的骨所取代。已经鉴定出一些参与软骨和骨骼发育的基因是锌指转录抑制因子Snail 家族的靶基因，功能获得研究表明，在小鼠长骨中上调 Snai1 活性会导致骨长度减少。然而，尚未进行体内功能丧失研究以确定 Snail 家族基因在正常骨骼发育中是否具有必需的生理作用。我们在此证明，Snail1 和 Snai2 基因在小鼠胚胎长骨发育中具有冗余功能。Snail2 基因缺失或肢芽特异性条件性缺失 Snai1 基因不会导致骨骼明显缺陷。然而，肢芽特异性 Snai1 缺失在 Snai2 缺失遗传背景下导致肢体长骨严重缺陷。Snail1/Snai2 双突变体的长骨表现出软骨细胞形态和组织缺陷，抑制小梁骨形成，并延迟骨化。软骨细胞增殖明显减少，编码细胞周期调节剂的基因的转录水平，如 p21(Waf1/Cip1) ，在 Snai1/Snai2 双突变体中显著上调，表明在软骨生成过程中，Snail 家族蛋白通过调节细胞周期调节剂的表达来控制细胞增殖。在 Snai1 突变股骨中 Snai2 转录本水平增加，而在 Snai2 突变股骨中 Snai1 转录本水平增加。此外，在突变股骨中，Snail1 和 Snai2 基因不仅在数量上，而且在表达量上都相互补偿，将其表达扩展到另一个基因的正常表达区域。这些结果表明，Snail1 和 Snai2 基因在时间、空间和数量上相互转录补偿，并且证明了 Snail 家族基因在小鼠软骨生成中具有必需的作用。
