線蟲神經分支發育的遺傳分析

Abstract

在神經系統中，神經細胞之間精確的連結是建構複雜的神經迴路中最重要結構性基礎，確保感覺，運動和認知等功能的正確運作。神經元的軸突分支可同時連結許多目標，這樣精確的分支受到許多內在與外源性因子的調控。當軸突分支形成後，由於絕大多數神經元不再進行更替，意味著軸突分支必須長時間保持結構上的穩定。當神經連結受到不同來源的損傷時，神經細胞通常會移除受損的部分，並且刺激留存的分支或主軸突生長，藉此重塑神經迴路的連結。利用秀麗桿狀線蟲做為模式研究生物，我進行了一系列遺傳學及細胞生物學的實驗，想要回答兩個問題：第一，神經軸突側分支在主軸突上的生長點中，如何接受外源性生長或導引因子的調控而促進神經迴路連結的精確性? 第二，神經細胞如何感知結構或功能上的損害，並且引發重塑作用以回復或建立新的神經連結? 在本論文的第一部分，我發現了醣蛋白Wnt可以藉由限制絲狀Actin (F-Actin)在軸突內的聚合，使線蟲觸覺神經PLM的側分支從一定的位置生長而出。依據所分布的濃度梯度，Wnt可以導引PLM側分支的生長位置，並透過Wnt的受器MIG-1/Frizzled和平面細胞極性蛋白VANG-1來傳遞訊息，藉此將F-Actin的聚合局限在PLM神經突起的遠端位，從而決定PLM神經分支的生長位置。我發現VANG-1可以透過胞吞作用幫助MIG-1從細胞膜進入早期胞內小泡中，此胞吞作用對於Wnt-Frizzled訊息的傳遞至為重要。 本論文第二部分的研究成果裡，我發現當線蟲的PLM神經中的微管蛋白被破壞後，側分支的突觸前膨大構造(Presynaptic varicosity)會萎縮，引發PLM側分枝的退化，並刺激PLM主突起的重新生長和延伸。在此PLM結構的重塑過程中扮演關鍵角色的是一含有PDZ domain的Rho家族鳥糞嘌呤核苷酸交換因子(PDZ-Rho guanine nucleotide exchange factor) RHGF-1。 RHGF-1蛋白原先與微管結合並受到抑制；當微管因各種物理、化學或遺傳性的侵擾而被破壞後，RHGF-1會被釋放、激活，並促進下游分子LET-502/ROCK以及反向運輸的MAPK訊息路徑，將神經損傷的訊息轉變成結構重塑的啟動訊號。因此，我的研究闡明Wnt蛋白如何透過指引神經分支的位置來建立神經迴路的連結，並建立了神經結構重塑的分子機制，可解釋神經細胞如何感知外在或內在傷害，並透過一系列的訊息傳遞，除去受損部分並透過增長其他部位來彌補已經失去的連結。

Precise connections between neurons in the nervous system are the most important structural basis for complex circuits that execute diverse sensory, motor and cognitive functions. As part of such exquisite connectivity, neurons form elaborate neurite branches to connect with multiple targets, and stably maintain these branches over their long postmitotic lifetime. Upon various insults that disrupt neural circuits, neurons often initiate structural remodeling responses aiming to resume circuit connectivity, by coordinated removal of the damaged compartment and compensatory growth of the remaining neurites. Using the nematode Caenorhabditis elegans as a genetic model, I have carried out a series of genetic and molecular studies to tackle the following two questions: First, how are specific neurite branching patterns established? Second, how do neurons sense various insults and remodel circuit connectivity accordingly? In the first study, I found that the conserved Wnt morphogens spatially control neurite branching patterns by restricting F-actin assembly at define locations on the primary neurite of the PLM touch neurons. Distinct Wnts functioned either permissively or instructively, through the Frizzled receptor MIG-1 and VANG-1, a planar cell polarity (PCP) transmembrane protein. Wnts, MIG-1 and VANG-1 shared similar functions in restricting F-actin to the distal PLM neurite. I found that VANG-1 facilitated MIG-1 endocytosis, which is a critical step for MIG-1 to transduce Wnt signals. I also demonstrated that Netrin functions orthogonally to the Wnts in the dorsal-ventral axis to guide ventral projection of the PLM branch. In the second study, I found that in response to microtubule disassembly, the PLM neuron remodeled by retracting its synaptic branch and overextended the primary neurite. This remodeling required RHGF-1, a PDZ-Rho guanine nucleotide exchange factor (GEF) that was associated with and inhibited by microtubules. Independent of the myosin light chain activation, RHGF-1 acted through the Rho-dependent kinase LET-502/ROCK and activated a conserved, retrograde DLK-1 MAPK pathway, which triggered synaptic branch retraction and overgrowth of the PLM neurite in a dose-dependent manner. Together these results demonstrate how diffusible cues instruct neurite branching to sculpt circuit connectivity in the nervous system, and they also present a neuronal remodeling paradigm during development by which neurons reshape their structures in response to microtubule disruption.