Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, Canada.
PLoS One. 2009 Nov 30;4(11):e8097. doi: 10.1371/journal.pone.0008097.
The axial and appendicular skeleton of vertebrates develops by endochondral ossification, in which skeletogenic tissue is initially cartilaginous and the differentiation of chondrocytes via the hypertrophic pathway precedes the differentiation of osteoblasts and the deposition of a definitive bone matrix. Results from both loss-of-function and misexpression studies have implicated the related homeobox genes Dlx5 and Dlx6 as partially redundant positive regulators of chondrocyte hypertrophy. However, experimental perturbations of Dlx expression have either not been cell type specific or have been done in the context of endogenous Dlx5 expression. Thus, it has not been possible to conclude whether the effects on chondrocyte differentiation are cell autonomous or whether they are mediated by Dlx expression in adjacent tissues, notably the perichondrium. To address this question we first engineered transgenic mice in which Dlx5 expression was specifically restricted to immature and differentiating chondrocytes and not the perichondrium. Col2a1-Dlx5 transgenic embryos and neonates displayed accelerated chondrocyte hypertrophy and mineralization throughout the endochondral skeleton. Furthermore, this transgene specifically rescued defects of chondrocyte differentiation characteristic of the Dlx5/6 null phenotype. Based on these results, we conclude that the role of Dlx5 in the hypertrophic pathway is cell autonomous. We further conclude that Dlx5 and Dlx6 are functionally equivalent in the endochondral skeleton, in that the requirement for Dlx5 and Dlx6 function during chondrocyte hypertrophy can be satisfied with Dlx5 alone.
脊椎动物的轴性和附肢骨骼通过软骨内骨化发育，其中骨骼生成组织最初是软骨的，并且软骨细胞通过肥大途径的分化先于成骨细胞的分化和确定的骨基质的沉积。功能丧失和异位表达研究的结果表明，相关同源盒基因 Dlx5 和 Dlx6 是软骨细胞肥大的部分冗余正调节因子。然而，对 Dlx 表达的实验干扰要么不是细胞类型特异性的，要么是在内源性 Dlx5 表达的背景下进行的。因此，不可能得出关于软骨细胞分化的影响是否是细胞自主的结论，或者它们是否是由相邻组织（特别是软骨膜）中的 Dlx 表达介导的。为了解决这个问题，我们首先构建了专门将 Dlx5 表达限制在未成熟和分化的软骨细胞而不是软骨膜的转基因小鼠。Col2a1-Dlx5 转基因胚胎和新生小鼠表现出整个软骨内骨骼中软骨细胞肥大和矿化的加速。此外，这种转基因专门挽救了 Dlx5/6 缺失表型特征性的软骨细胞分化缺陷。基于这些结果，我们得出结论，Dlx5 在肥大途径中的作用是细胞自主的。我们进一步得出结论，Dlx5 和 Dlx6 在软骨内骨骼中具有功能等同性，即在软骨细胞肥大过程中 Dlx5 和 Dlx6 功能的要求可以仅由 Dlx5 满足。
