The precise development of neuronal morphologies is crucial to the establishment of synaptic circuits and, ultimately, proper brain function. Signaling by the axon guidance cue semaphorin 3A (Sema3A) and its receptor complex of neuropilin-1 and plexin-A4 has multifunctional outcomes in neuronal morphogenesis. Downstream activation of the RhoGEF FARP2 through interaction with the lysine-arginine-lysine motif of plexin-A4 and consequent activation of the small GTPase Rac1 promotes dendrite arborization, but this pathway is dispensable for axon repulsion. Here, we investigated the interplay of small GTPase signaling mechanisms underlying Sema3A-mediated dendritic elaboration in mouse layer V cortical neurons in vitro and in vivo. Sema3A promoted the binding of the small GTPase Rnd1 to the amino acid motif lysine-valine-serine (LVS) in the cytoplasmic domain of plexin-A4. Rnd1 inhibited the activity of the small GTPase RhoA and the kinase ROCK, thus supporting the activity of the GTPase Rac1, which permitted the growth and branching of dendrites. Overexpression of a dominant-negative RhoA, a constitutively active Rac1, or the pharmacological inhibition of ROCK activity rescued defects in dendritic elaboration in neurons expressing a plexin-A4 mutant lacking the LVS motif. Our findings provide insights into the previously unappreciated balancing act between Rho and Rac signaling downstream of specific motifs in plexin-A4 to mediate Sema3A-dependent dendritic elaboration in mammalian cortical neuron development.

中文翻译：

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plexin-A4信号基序下游小GTP酶的平衡作用促进哺乳动物皮质神经元树突的形成

神经元形态的精确发育对于突触回路的建立以及最终的正常大脑功能至关重要。轴突引导信号信号蛋白 3A (Sema3A) 及其神经毡蛋白-1 和丛蛋白-A4 受体复合物的信号传导在神经元形态发生中具有多功能结果。RhoGEF FARP2 通过与 plexin-A4 的赖氨酸-精氨酸-赖氨酸基序相互作用而下游激活，并随后激活小 GTPase Rac1，促进树突分枝，但该途径对于轴突排斥来说是可有可无的。在这里，我们在体外和体内研究了小鼠 V 层皮质神经元中 Sema3A 介导的树突细化背后的小 GTPase 信号传导机制的相互作用。Sema3A 促进小 GTPase Rnd1 与 plexin-A4 胞质结构域中的氨基酸基序赖氨酸-缬氨酸-丝氨酸 (LVS) 的结合。Rnd1 抑制小 GTPase RhoA 和激酶 ROCK 的活性，从而支持 GTPase Rac1 的活性，从而允许树突的生长和分支。显性失活的 RhoA、组成型活性 Rac1 的过表达或 ROCK 活性的药理抑制可以挽救表达缺乏 LVS 基序的 plexin-A4 突变体的神经元中树突加工的缺陷。我们的研究结果提供了对 plexin-A4 中特定基序下游 Rho 和 Rac 信号传导之间先前未被重视的平衡行为的见解，以介导哺乳动物皮质神经元发育中 Sema3A 依赖性树突的形成。