埃及斑蚊Wnt對於調節卵黃生成之重要作用

Abstract

Wnt-Frizzled訊號傳遞路徑對於維持組織、器官的發育方面扮演很重要的角色，其中在Wnt、Frizzled的家族基因裡，最早都是從Drosophila中發現並且對於其功能加以描述之。有趣的是，許多的研究報告也顯示出在哺乳動物系統中，Wnt signaling與誘導NF-κB所調節的轉錄作用(NF-κB-mediated transcription)有關聯。此外，Wnt signaling也被證明出會影響人類的腫瘤形成。然而，Wnt signaling在蚊子中的作用仍是未知地。到目前為止，我們已經知道在登革熱的主要媒介-埃及斑蚊中有9個Wnt ligands和1個Frizzled receptor。其中，這些Wnt ligands都具有能醣基化(glycosylation)和棕櫚酸酯化(palmitoylation)的位置，這也許是Wnt訊號能有效傳遞的原因，而Frizzled receptor則是由7個疏水性的α-helix所組成，並橫跨於雙層磷脂質中。先前實驗室的研究結果顯示出Frizzled (Fz)於餵血後6小時會在脂肪體(Fatbody)中大量的表現，而將Fz silence則會造成埃及斑蚊的產卵量下降。因此，在本次研究中我們試圖去釐清Wnt在埃及斑蚊卵黃生成中所扮演的角色。從我們的實驗結果得知，不論是mRNA或蛋白質，Wnt皆大量的表現在卵巢(ovary)中。而Wnt silence同樣地會造成埃及斑蚊的產卵量下降。特別的是，在餵血前以及餵血後的12小時內，Wnt會分佈於follicle cells中的oocyte。而若將Fz silence也會導致卵巢中caspase-positvie follice cells的數目增加。除此之外，我們透過RNAi方式將destruction complex中的GSK-3抑制，使Wnt signaling pathway可以in vivo活化，結果顯示出TOR的downstream mediator S6K在pre-vitellogenic stage有磷酸化的現象。另外，Wnt也參與調控埃及斑蚊的羽化過程(Metamorphosis)。總之，本實驗的研究結果除了帶給conserved Wnt signaling新的觀點外，並提供致力於研究病媒蚊的傳播能力和控制方法一個新的方向。

Signaling transduction of Wnt-Frizzled is an important aspect of tissue/organ development and maintenance. Wnt and Frizzled family genes were first cloned and characterized in Drosophila as a key event in development. Interestingly, several reports have accordingly suggested the crosstalk between Wnt signaling and induction of the NF-κB-mediated transcription in mammalian system. In addition, Wnt signaling was shown to be essential to human tumorigenesis. However, the roles of Wnt signaling in the mosquito remain largely unknown. Recently, we identified nine Wnt ligands and one Frizzled receptor in the mosquito Aedes aegypti, the major vector of dengue virus. All of these Wnt ligands harbor potential sites for glycosylation and palmitoylation, which appear to be useful for signal transduction. The only Frizzled receptor identified in the mosquito Ae. aegypti comprises seven hydrophobic α-helix that span the lipid bilayers. Our previous results demonstrated that Frizzled (Fz) is expressed in the mosquito fatbody at 6 hr after a blood meal. Silence of Fz reduced the oviposition rate in the mosquito. In this study, we attempt to characterize the role of the Wnt in the yellow fever mosquito, Ae. aegypti. Our results showed that Wnt is highly expressed in the mosquito ovary in terms of transcriptional and translational level. Also, silence of Wnt significantly reduced the egg production. Intriguingly, Wnt is specifically expressed in the oocyte of the follicle cells before a blood meal and only shortly after the blood meal. Furthermore, Wnt also affect the metamorphosis in Aedes aegypti. Activation of Wnt signaling by RNAi-mediated silence of GSK-3 revealed that the phosphorylation of S6K at pre-vitellogenic stage. In addition, silence of Fz showed a significant increase number of caspase-positive follicle cells in the mosquito ovary. Detail characterization of mosquito Wnt signaling in this study will provide new insights into the evolutionary conserved features of Wnt signaling. Data revealed by this proposal will be crucial for future studies on vector competence and vector control in the field.