Dendrite morphogenesis plays an essential role in establishing the connectivity and receptive fields of neurons during the development of the nervous system. To generate the diverse morphologies of branched dendrites, neurons use external cues and cell surface receptors to coordinate intracellular cytoskeletal organization; however, the molecular mechanisms of how this signaling forms branched dendrites are not fully understood. We performed in vivo time-lapse imaging of the PVD neuron in C. elegans in several mutants of actin regulatory proteins, such as the WAVE Regulatory Complex (WRC) and UNC-34 (homolog of Enabled/Vasodilator-stimulated phosphoprotein (Ena/VASP)). We examined the direct interaction between the WRC and UNC-34 and analyzed the localization of UNC-34 in vivo using transgenic worms expressing UNC-34 fused to GFP. We identify a stereotyped sequence of morphological events during dendrite outgrowth in the PVD neuron in C. elegans. Specifically, local increases in width (“swellings”) give rise to filopodia to facilitate a “rapid growth and pause” mode of growth. In unc-34 mutants, filopodia fail to form but swellings are intact. In WRC mutants, dendrite growth is largely absent, resulting from a lack of both swelling and filopodia formation. We also found that UNC-34 can directly bind to the WRC. Disrupting this binding by deleting the UNC-34 EVH1 domain prevented UNC-34 from localizing to swellings and dendrite tips, resulting in a stunted dendritic arbor and reduced filopodia outgrowth. We propose that regulators of branched and linear F-actin cooperate to establish dendritic branches. By combining our work with existing literature, we propose that the dendrite guidance receptor DMA-1 recruits the WRC, which polymerizes branched F-actin to generate “swellings” on a mother dendrite. Then, WRC recruits the actin elongation factor UNC-34/Ena/VASP to initiate growth of a new dendritic branch from the swelling, with the help of the actin-binding protein UNC-115/abLIM. Extension of existing dendrites also proceeds via swelling formation at the dendrite tip followed by UNC-34-mediated outgrowth. Following dendrite initiation and extension, the stabilization of branches by guidance receptors further recruits WRC, resulting in an iterative process to build a complex dendritic arbor.

中文翻译：

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两步肌动蛋白聚合机制驱动枝晶分支

在神经系统发育过程中，树突形态发生在建立神经元的连接性和感受野方面起着至关重要的作用。为了产生不同形态的分支树突，神经元使用外部信号和细胞表面受体来协调细胞内的细胞骨架组织；然而，这种信号如何形成分支树突的分子机制尚不完全清楚。我们对秀丽隐杆线虫的几个肌动蛋白调节蛋白突变体中的 PVD ​​神经元进行了体内延时成像，例如 WAVE 调节复合物 (WRC) 和 UNC-34（启用/血管扩张剂刺激的磷蛋白 (Ena/VASP) 的同源物））。我们检查了 WRC 和 UNC-34 之间的直接相互作用，并使用表达 UNC-34 与 GFP 融合的转基因蠕虫分析了 UNC-34 在体内的定位。我们确定了秀丽隐杆线虫 PVD ​​神经元树突生长过程中形态学事件的定型序列。具体来说，宽度的局部增加（“肿胀”）会导致丝状伪足，以促进“快速生长和暂停”的生长模式。在 unc-34 突变体中，丝状伪足无法形成，但肿胀完好无损。在 WRC 突变体中，由于缺乏肿胀和丝状伪足形成，树突生长基本上不存在。我们还发现 UNC-34 可以直接绑定到 WRC。通过删除 UNC-34 EVH1 域来破坏这种结合阻止了 UNC-34 定位到肿胀和树突尖端，导致树突状乔木发育不良和丝状伪足生长减少。我们建议分支和线性 F-肌动蛋白的调节剂合作建立树突状分支。通过将我们的工作与现有文献相结合，我们建议树突引导受体 DMA-1 招募 WRC，WRC 聚合支链 F-肌动蛋白以在母树突上产生“肿胀”。然后，在肌动蛋白结合蛋白 UNC-115/abLIM 的帮助下，WRC 招募肌动蛋白延伸因子 UNC-34/Ena/VASP 从肿胀中开始新的树突分支的生长。现有树突的延伸也通过在树突尖端形成膨胀，然后是 UNC-34 介导的生长而进行。在树突起始和延伸之后，引导受体对分支的稳定进一步招募了 WRC，从而导致构建复杂树突乔木的迭代过程。在肌动蛋白结合蛋白 UNC-115/abLIM 的帮助下，WRC 招募肌动蛋白延伸因子 UNC-34/Ena/VASP 从肿胀中开始生长新的树突分支。现有树突的延伸也通过在树突尖端形成膨胀，然后是 UNC-34 介导的生长而进行。在树突起始和延伸之后，引导受体对分支的稳定进一步招募了 WRC，从而导致构建复杂树突乔木的迭代过程。在肌动蛋白结合蛋白 UNC-115/abLIM 的帮助下，WRC 招募肌动蛋白延伸因子 UNC-34/Ena/VASP 从肿胀中开始生长新的树突分支。现有树突的延伸也通过在树突尖端形成膨胀，然后是 UNC-34 介导的生长而进行。在树突起始和延伸之后，引导受体对分支的稳定进一步招募了 WRC，从而导致构建复杂树突乔木的迭代过程。