Department of Biology and Biochemistry, University of Houston, Houston, TX, 77204-5001, USA.
Department of Biology, Texas A&M University, College Station, TX, 77843-3258, USA.
Dev Biol. 2020 Jun 15;462(2):152-164. doi: 10.1016/j.ydbio.2020.03.016. Epub 2020 Mar 31.
The process that partitions the nascent vertebrate central nervous system into forebrain, midbrain, hindbrain, and spinal cord after neural induction is of fundamental interest in developmental biology, and is known to be dependent on Wnt/β-catenin signaling at multiple steps. Neural induction specifies neural ectoderm with forebrain character that is subsequently posteriorized by graded Wnt signaling: embryological and mutant analyses have shown that progressively higher levels of Wnt signaling induce progressively more posterior fates. However, the mechanistic link between Wnt signaling and the molecular subdivision of the neural ectoderm into distinct domains in the anteroposterior (AP) axis is still not clear. To better understand how Wnt mediates neural AP patterning, we performed a temporal dissection of neural patterning in response to manipulations of Wnt signaling in zebrafish. We show that Wnt-mediated neural patterning in zebrafish can be divided into three phases: (I) a primary AP patterning phase, which occurs during gastrulation, (II) a mes/r1 (mesencephalon-rhombomere 1) specification and refinement phase, which occurs immediately after gastrulation, and (III) a midbrain-hindbrain boundary (MHB) morphogenesis phase, which occurs during segmentation stages. A major outcome of these Wnt signaling phases is the specification of the major compartment divisions of the developing brain: first the MHB, then the diencephalic-mesencephalic boundary (DMB). The specification of these lineage divisions depends upon the dynamic changes of gene transcription in response to Wnt signaling, which we show primarily involves transcriptional repression or indirect activation. We show that otx2b is directly repressed by Wnt signaling during primary AP patterning, but becomes resistant to Wnt-mediated repression during late gastrulation. Also during late gastrulation, Wnt signaling becomes both necessary and sufficient for expression of wnt8b, en2a, and her5 in mes/r1. We suggest that the change in otx2b response to Wnt regulation enables a transition to the mes/r1 phase of Wnt-mediated patterning, as it ensures that Wnts expressed in the midbrain and MHB do not suppress midbrain identity, and consequently reinforce formation of the DMB. These findings integrate important temporal elements into our spatial understanding of Wnt-mediated neural patterning and may serve as an important basis for a better understanding of neural patterning defects that have implications in human health.
神经诱导后将新生脊椎动物中枢神经系统分割成前脑、中脑、后脑和脊髓的过程在发育生物学中具有根本意义，并且已知在多个步骤中依赖于 Wnt/β-连环蛋白信号传导。神经诱导指定具有前脑特征的神经外胚层，随后通过分级 Wnt 信号向后极化：胚胎学和突变分析表明，Wnt 信号的水平逐渐升高，诱导的后命运逐渐增加。然而，Wnt 信号与神经外胚层在前后 (AP) 轴上的分子细分之间的机制联系仍然不清楚。为了更好地理解 Wnt 如何介导神经 AP 模式形成，我们在斑马鱼中对 Wnt 信号转导的操作进行了时间解析，以研究神经模式形成。我们表明，斑马鱼中 Wnt 介导的神经模式形成可以分为三个阶段：（I）初级 AP 模式形成阶段，发生在原肠胚形成过程中，（II）中脑-rhombomere1（mesencephalon-rhombomere 1）的指定和细化阶段，发生在原肠胚形成之后，以及（III）中脑-后脑边界 (MHB) 形态发生阶段，发生在分段阶段。这些 Wnt 信号阶段的主要结果是指定正在发育的大脑的主要隔室划分：首先是 MHB，然后是背侧中脑边界 (DMB)。这些谱系分裂的指定取决于基因转录的动态变化以响应 Wnt 信号，我们表明这主要涉及转录抑制或间接激活。我们表明，otx2b 在初级 AP 模式形成过程中直接受到 Wnt 信号的抑制，但在晚期原肠胚形成过程中对 Wnt 介导的抑制具有抗性。同样在晚期原肠胚形成过程中，Wnt 信号对于 mes/r1 中的 wnt8b、en2a 和 her5 的表达既是必要的也是充分的。我们认为，otx2b 对 Wnt 调节的反应变化使得向 Wnt 介导的模式形成的 mes/r1 阶段过渡成为可能，因为它确保了在中脑和 MHB 中表达的 Wnts 不会抑制中脑特征，并且因此加强了 DMB 的形成。这些发现将重要的时间元素整合到我们对 Wnt 介导的神经模式形成的空间理解中，并可能成为更好地理解对人类健康有影响的神经模式形成缺陷的重要基础。
