Dscam2訊號機制對限制視覺神經樹突場尺寸之研究

Abstract

過去研究認為Down syndrome cell adhesion molecule 2 (Dscam2)通過介導神經突之間的同型交互作用，來指引神經迴路的組裝。然而，此訊號傳遞機制尚不明確。在本篇論文中，我們使用果蠅視覺神經作為模型，證明Dscam2對於髓質投射神經元Tm20的樹突場尺寸擴展與正確的平面投射方向不可或缺。這些Dscam2的細胞自主功能具異構特異性:Dscam2 10B異構體可限制樹突場尺寸，而10A和10B皆會調控平面投射的方向。除了Dscam2細胞膜外域的調控機制，Dscam2細胞質域的訊號傳遞對於Tm20形成正確樹突型態也是必要的。接著我們描繪Dscam2在細胞內的訊號途徑，並發現非受體酪胺酸激酶Src2，和Dscam2有遺傳交互作用，參與指示應有的樹突型態。通過篩選候選基因，我們認為Rac1是Src在Dscam2路徑的下游的潛在標的，可被核甘酸轉換因子Trio調控，推測它會調節樹突發育過程中的肌動蛋白骨架。除此之外，我們發現與先前認知不同，PI3K並非Dscam2下游限制樹突發展的因子。我們的研究為更深入了解Dscam2訊號路徑建立基礎。

It has been proposed that the Down syndrome cell adhesion molecule 2 (Dscam2) mediates homotypic interactions between neurites to guide neural circuit assembly. However, its signaling mechanism is unknown. In this thesis, using the Drosophila visual neurons as a model, we demonstrate that Dscam2 is required in the medulla projection neurons, Tm20, for proper expansion of dendritic field sizes and correct planar projection. These cell-autonomous functions of Dscam2 are isoform-specific: Dscam2 10B isoform restricts dendritic field sizes while both 10A and 10B isoforms regulate the direction of dendritic planar projection. Aside the mechanism utilized by the ectodomains, Dscam2’s cytoplasmic domain is essential to elicit signal transduction for proper dendritic patterning of Tm20 neurons. We then, delineated Dscam2’s intracellular signaling pathway and discovered the involvement of the non-receptor tyrosine kinase, Src42 which genetically interacts with Dscam2 to instruct appropriate dendritic arbor of Tm20 neurons. By screening candidates, we identified Rac1 as the potential target of Src in Dscam2 pathway, which itself is regulated by its nucleotide exchanger Trio, presumably, to modulate actin cytoskeleton during dendrite development. In addition, we determined that unlike previous suggestion, PI3K, does not act downstream of Dscam2 to restrict dendritic elaboration. Our findings pave the path to elucidating the signaling pathway of Dscam2.