Tucker A S, Headon D J, Schneider P, Ferguson B M, Overbeek P, Tschopp J, Sharpe P T
MRC Centre for Developmental Neurobiology, King's College, Guy's Hospital, London Bridge, London SE1 1UL, UK.
Development. 2000 Nov;127(21):4691-700. doi: 10.1242/dev.127.21.4691.
tabby and downless mutant mice have apparently identical defects in teeth, hair and sweat glands. Recently, genes responsible for these spontaneous mutations have been identified. downless (Dl) encodes Edar, a novel member of the tumour necrosis factor (TNF) receptor family, containing the characteristic extracellular cysteine rich fold, a single transmembrane region and a death homology domain close to the C terminus. tabby (Ta) encodes ectodysplasin-A (Eda) a type II membrane protein of the TNF ligand family containing an internal collagen-like domain. As predicted by the similarity in adult mutant phenotype and the structure of the proteins, we demonstrate that Eda and Edar specifically interact in vitro. We have compared the expression pattern of Dl and Ta in mouse development, taking the tooth as our model system, and find that they are not expressed in adjacent cells as would have been expected. Teeth develop by a well recorded series of epithelial-mesenchymal interactions, similar to those in hair follicle and sweat gland development, the structures found to be defective in tabby and downless mice. We have analysed the downless mutant teeth in detail, and have traced the defect in cusp morphology back to initial defects in the structure of the tooth enamel knot at E13. Significantly, the defect is distinct from that of the tabby mutant. In the tabby mutant, there is a recognisable but small enamel knot, whereas in the downless mutant the knot is absent, but enamel knot cells are organised into a different shape, the enamel rope, showing altered expression of signalling factors (Shh, Fgf4, Bmp4 and Wnt10b). By adding a soluble form of Edar to tooth germs, we were able to mimic the tabby enamel knot phenotype, demonstrating the involvement of endogenous Eda in tooth development. We could not, however, reproduce the downless phenotype, suggesting the existence of yet another ligand or receptor, or of ligand-independent activation mechanisms for Edar. Changes in the structure of the enamel knot signalling centre in downless tooth germs provide functional data directly linking the enamel knot with tooth cusp morphogenesis. We also show that the Lef1 pathway, thought to be involved in these mutants, functions independently in a parallel pathway.
虎斑和无毛突变小鼠在牙齿、毛发和汗腺方面显然存在相同的缺陷。最近，已鉴定出导致这些自发突变的基因。无毛（Dl）编码Edar，它是肿瘤坏死因子（TNF）受体家族的一个新成员，具有特征性的富含细胞外半胱氨酸的结构域、单个跨膜区域以及靠近C端的死亡同源结构域。虎斑（Ta）编码外胚层发育不良蛋白A（Eda），它是TNF配体家族的一种II型膜蛋白，含有一个内部胶原样结构域。正如成年突变体表型和蛋白质结构的相似性所预测的那样，我们证明Eda和Edar在体外特异性相互作用。我们以牙齿为模型系统，比较了Dl和Ta在小鼠发育过程中的表达模式，发现它们并非如预期那样在相邻细胞中表达。牙齿通过一系列有详细记录的上皮 - 间充质相互作用发育，类似于毛囊和汗腺的发育过程，而毛囊和汗腺正是在虎斑和无毛小鼠中发现有缺陷的结构。我们详细分析了无毛突变小鼠的牙齿，并将牙尖形态的缺陷追溯到E13时牙釉质结结构的初始缺陷。值得注意的是，该缺陷与虎斑突变体的不同。在虎斑突变体中，有一个可识别但较小的釉结，而在无毛突变体中，釉结不存在，但釉结细胞被组织成不同的形状，即釉索，显示出信号因子（Shh、Fgf4、Bmp4和Wnt10b）表达的改变。通过向牙胚添加可溶性形式的Edar，我们能够模拟虎斑釉结表型，证明内源性Eda参与牙齿发育。然而，我们无法重现无毛表型，这表明可能存在另一种配体或受体，或者存在Edar的非配体依赖性激活机制。无毛牙胚中釉结信号中心结构的变化提供了直接将釉结与牙尖形态发生联系起来的功能数据。我们还表明，被认为与这些突变体有关的Lef1途径在平行途径中独立发挥作用。
