FMI-1/Flamingo 調控線蟲樹突自我迴避之分子機轉

Abstract

精密神經迴路的構成使得神經細胞之間得以正確的傳遞與解讀感覺訊息。「神經自我迴避」是一個廣泛見於不同物種之間的神經發育現象，這個機制防止同一個神經細胞的分支之間相互碰觸與交疊在一起，使神經最大化其感覺領域，並且同時避免接收感覺訊息時的衝突與混淆。其中非典型鈣黏蛋白又名黏著性G蛋白偶聯受體Flamingo被發現參與調控果蠅感覺神經元樹突的自我迴避機制中，但是對於Flamingo如何去達成這種自我迴避的機制目前仍欠缺充分的了解。我們在本篇研究中發現，當線蟲中缺乏Flamingo的同源基因fmi-1的情況之下，會造成蟲體中傷害性感受神經元PVD的樹突產生相互碰觸或交疊的情形，顯示出自我迴避機制的受損。我們的遺傳分析結果更進一步顯示Flamingo可能透過肌動蛋白調控因子N-WASP (Neural Wiskott-Aldrich syndrome protein)的線蟲同源基因wsp-1來調控局部纖維狀肌動蛋白的形成，這樣使得兩個相互碰觸的樹突得以進行排斥。此外，我們的研究意外地發現當PVD失去平面極性蛋白VANG-1/Strabismus/Vangl2時，可以抑制fmi-1突變所造成的神經樹突自我迴避缺陷。不僅如此，我們也發現FMI-1在PVD中能夠抑制VANG-1的表現量，進而調控VANG-1在PVD上的作用。除此之外，我們實驗室先前的研究顯示出VANG-1在線蟲的觸覺神經元PLM中具有抑制局部纖維狀肌動蛋白形成的作用，結合這兩項研究，我們認為Flamingo透過調節不同的肌動蛋白調控因子來達成局部充分纖維狀肌動蛋白形成，進而使得兩個相互碰觸的同源分枝得以迅速分開，確保了神經自我迴避的產生。

Accurate encoding and processing of sensory information relies on precise wiring of neuronal circuitries. Dendrite self-avoidance prevents crossing and overlapping of sister branches from the same neuron and is a highly conserved developmental phenomenon among diverse species. Flamingo, an atypical cadherin with seven transmembrane domains also classified as an adhesive G protein-coupled receptor (GPCR), had been previously shown to control dendrite self-avoidance in Drosophila sensory neurons, but the mechanisms by which Flamingo prevents dendrite self-crossing remain largely elusive. Here we report that loss-of-function mutations of fmi-1, the C. elegans flamingo homolog, caused dendrite self-avoidance defects in the multidendritic nociceptive neuron, PVD. Our genetic data indicate that FMI-1 likely acts through WSP-1, the homolog of N-WASP (Neural Wiskott-Aldrich syndrome protein) in C. elegans, and engages F-actin cytoskeleton to mediate repulsion between contacting PVD sister dendrites. Unexpectedly, we found that mutations in vang-1, which encodes the planar cell polarity (PCP) protein Strabismus/Vangl2, suppressed self-avoidance defects of the fmi-1 mutant. Furthermore, we observed that FMI-1 inhibits VANG-1 function by downregulating its protein level in PVD. Together with our lab’s previous study that VANG-1 functions to restrict the formation of F-actin foci in C. elegans neurons. These results suggest that Flamingo regulates dendrite self-avoidance by orchestrating distinct actin regulators to facilitate F-actin assembly.