Armes N A, Smith J C
Division of Developmental Biology, National Institute for Medical Research, The Ridgeway, London, UK.
Development. 1997 Oct;124(19):3797-804. doi: 10.1242/dev.124.19.3797.
The TGFbeta family member activin induces different mesodermal cell types in a dose-dependent fashion in the Xenopus animal cap assay. High concentrations of activin induce dorsal and anterior cell types such as notochord and muscle, while low concentrations induce ventral and posterior tissues such as mesenchyme and mesothelium. In this paper we investigate whether this threshold phenomenon involves the differential effects of the two type I activin receptors ALK-2 and ALK-4. Injection of RNA encoding constitutively active forms of the receptors (here designated ALK-2* and ALK-4*) reveals that ALK-4* strongly induces the more posterior mesodermal marker Xbra and the dorsoanterior marker goosecoid in animal cap explants. Maximal levels of Xbra expression are attained using lower concentrations of RNA than are required for the strongest activation of goosecoid, and at the highest doses of ALK-4*, levels of Xbra transcription decrease, as is seen with high concentrations of activin. By contrast, the ALK-2* receptor activates Xbra but fails to induce goosecoid to significant levels. Analysis at later stages reveals that ALK-4* signalling induces the formation of a variety of mesodermal derivatives, including dorsal cell types, in a dose-dependent fashion, and that high levels also induce endoderm. By contrast, the ALK-2* receptor induces only ventral mesodermal markers. Consistent with these observations, ALK-4* is capable of inducing a secondary axis when injected into the ventral side of 32-cell stage embryos whilst ALK-2* cannot. Co-injection of RNAs encoding constitutively active forms of both receptors reveals that ventralising signals from ALK-2* antagonise the dorsal mesoderm-inducing signal derived from ALK-4*, suggesting that the two receptors use distinct and interfering signalling pathways. Together, these results show that although ALK-2* and ALK-4* transduce distinct signals, the threshold responses characteristic of activin cannot be due to interactions between these two pathways; rather, thresholds can be established by ALK-4* alone. Furthermore, the effects of ALK-2* signalling are at odds with it behaving as an activin receptor in the early Xenopus embryo.
在非洲爪蟾动物帽实验中，转化生长因子β（TGFbeta）家族成员激活素以剂量依赖方式诱导不同的中胚层细胞类型。高浓度的激活素诱导背侧和前侧细胞类型，如脊索和肌肉，而低浓度则诱导腹侧和后侧组织，如间充质和间皮。在本文中，我们研究了这种阈值现象是否涉及两种I型激活素受体ALK - 2和ALK - 4的不同作用。注射编码受体组成型活性形式的RNA（此处称为ALK - 2和ALK - 4）表明，ALK - 4在动物帽外植体中强烈诱导更后侧的中胚层标记物Xbra和背前侧标记物goosecoid。与最强激活goosecoid所需的RNA浓度相比，使用较低浓度的RNA就能达到Xbra表达的最大水平，并且在最高剂量的ALK - 4时，Xbra转录水平下降，这与高浓度激活素的情况相同。相比之下，ALK - 2受体激活Xbra，但不能将goosecoid诱导到显著水平。后期分析表明，ALK - 4信号以剂量依赖方式诱导多种中胚层衍生物的形成，包括背侧细胞类型，并且高水平还诱导内胚层。相比之下，ALK - 2受体仅诱导腹侧中胚层标记物。与这些观察结果一致，当注射到32细胞期胚胎的腹侧时，ALK - 4能够诱导形成次级轴，而ALK - 2则不能。共同注射编码两种受体组成型活性形式的RNA表明，来自ALK - 2的腹侧化信号拮抗来自ALK - 4的背侧中胚层诱导信号，这表明两种受体使用不同且相互干扰的信号通路。总之，这些结果表明，尽管ALK - 2和ALK - 4转导不同的信号，但激活素特有的阈值反应并非由于这两条途径之间的相互作用；相反，阈值可以仅由ALK - 4建立。此外，ALK - 2*信号的作用与它在早期非洲爪蟾胚胎中作为激活素受体的行为不一致。
