Integration of Canonical and Noncanonical Wnt Signaling Pathways Patterns the Neuroectoderm Along the Anterior–Posterior Axis of Sea Urchin Embryos

Patterning the neuroectoderm along the anterior–posterior (AP) axis is a critical event in the early development of deuterostome embryos. However, the mechanisms that regulate the specification and patterning of the neuroectoderm are incompletely understood. Remarkably, the anterior neuroectoderm (ANE) of the deuterostome sea urchin embryo expresses many of the same transcription factors and secreted modulators of Wnt signaling, as does the early vertebrate ANE (forebrain/eye field). Moreover, as is the case in vertebrate embryos, confining the ANE to the anterior end of the embryo requires a Wnt/β-catenin-dependent signaling mechanism. Here we use morpholino- or dominant negative-mediated interference to demonstrate that the early sea urchin embryo integrates information not only from Wnt/β-catenin but also from Wnt/Fzl5/8-JNK and Fzl1/2/7-PKC pathways to provide precise spatiotemporal control of neuroectoderm patterning along its AP axis. Together, through the Wnt1 and Wnt8 ligands, they orchestrate a progressive posterior-to-anterior wave of re-specification that restricts the initial, ubiquitous, maternally specified, ANE regulatory state to the most anterior blastomeres. There, the Wnt receptor antagonist, Dkk1, protects this state through a negative feedback mechanism. Because these different Wnt pathways converge on the same cell fate specification process, our data suggest they may function as integrated components of an interactive Wnt signaling network. Our findings provide strong support for the idea that the sea urchin ANE regulatory state and the mechanisms that position and define its borders represent an ancient regulatory patterning system that was present in the common echinoderm/vertebrate ancestor.沿前后轴（anterior–posterior axis, AP）对神经外胚层（neuroectoderm）进行模式建成，是后口动物（deuterostome）胚胎早期发育过程中的关键事件。然而，调控神经外胚层特化与模式建成的分子机制目前尚未被完全阐明。值得注意的是，后口动物海胆胚胎的前神经外胚层（anterior neuroectoderm, ANE）所表达的转录因子与Wnt信号分泌调控因子（secreted modulators of Wnt signaling），与早期脊椎动物前神经外胚层（前脑/眼域）的同类分子高度重合。此外，与脊椎动物胚胎的情况一致，将前神经外胚层限定在胚胎前端，依赖于Wnt/β-连环蛋白（Wnt/β-catenin）信号通路。本研究通过吗啉代寡核苷酸（morpholino）或显性负效（dominant negative）介导的干扰实验，证明早期海胆胚胎不仅整合了Wnt/β-连环蛋白信号通路的信息，还同时接收Wnt/Fzl5/8-JNK与Fzl1/2/7-PKC通路的信号，从而对神经外胚层沿前后轴的模式建成实现精准的时空调控。这些通路通过Wnt1与Wnt8配体，共同介导了一场从后向前逐步推进的重特化波，将最初广泛存在的、由母源特化的前神经外胚层调控状态，仅限定在最前端的卵裂球（blastomeres）中。在该区域内，Wnt受体拮抗剂（Wnt receptor antagonist）Dkk1通过负反馈调控机制维持这一调控状态。由于这些不同的Wnt信号通路均汇聚于同一细胞命运特化过程，本研究数据表明它们可作为交互式Wnt信号网络的整合组分发挥功能。本研究结果有力支持了以下观点：海胆前神经外胚层调控状态及其定位与边界界定机制，是存在于棘皮动物（echinoderm）与脊椎动物（vertebrate）共同祖先中的古老调控模式建成系统。