新的果蠅基因taroid在Hedgehog資訊傳遞的發育遺傳分析

Abstract

Hedgehog（Hh）蛋白質是一種分泌性的傳訊分子，在脊椎或無脊椎動物的發育及體制形成上扮演著極為重要的角色。果蠅（Drosophila melanogaster）的hh突變體會導致胚胎腹部體節的方向極性受損，形成所謂的「節向性狀」（segment polarity phenotype），而已知的節向基因多為Wingless（Wg）或Hh資訊傳遞系統的成員。本論文報導了一個新的果蠅基因taroid(tr)的遺傳分析以及其發育功能的探討。此新基因乃篩選自221株以EMS引致突變的致死突變株，透過種細胞遺傳重組技術（the FLP-DFS technique）分析發現，缺少tr基因的母源成分的tr突變胚胎呈現出所謂的lawn-of-denticles節向性狀。在tr突變胚胎中，分別偵測Wg信息的標的engrailed(en)與Hh信息的標的wg在表皮細胞的蛋白質表現，得知Wg的分解消失比En來的早，顯示Hh資訊傳遞需要在tr的功能正常時才能進行。 進一步分析tr對過量表現的Wg和Hh的遺傳上位關係（epistatic relationship），Wg異位表現的效應在tr突變胚胎中不會被抑制，反之Hh異位表現則會被tr突變所抑制，顯示tr的確作用在Hh下游而非Wg下游。然而，tr突變不會影響Hh資訊的標的基因decapentaplegic在imaginal discs上的表現，只會使眼睛分化過程中morphogenetic furrow的推進略微延遲，因此我們推測可能有重複的資訊路徑與tr的路徑平行存在，能補償缺少tr的效應。 未來，我們將繼續分析tr與其他Hh傳訊成員的遺傳上位關係，同時進行tr基因的分子選殖，以確切瞭解其作用機制。本研究除了希望能探索Hh資訊傳遞路徑的未知領域，也期盼能在若干因Hh資訊失調而導致的發育不正常及癌症發生的研究中做出貢獻。

The hedgehog (hh) gene family encode intercellular signaling molecules that play key roles in the pattern formation of both vertebrate and invertebrate animals. In Drosophila melanogaster, mutation of hh gene disrupts the polarity within each segment in the abdomen of embryo and is thereby belonged to the segment polarity mutation. Most of this class of mutations have been identified to play roles in either Wingless (Wg) or Hh signaling pathways. This thesis reports the recovery of a novel segment polarity gene, taroid (tr). With the efficient FLP-DFS technique, the tr1094 allele was recovered from 221 EMS-induced zygotic lethal mutations. The homozygous mutant embryos derived from tr1094 germline clones exhibited the so-called lawn-of-denticles segment polarity phenotype. Analyzing the expression of Wg and Hh target genes, Engrailed (En) and Wg, respectively, in epidermal cells of tr mutant embryos, the decay of Wg protein is earlier and more severely than that of En protein. It suggests it is more likely the requirement of tr in Hh signaling that targeted Wg expression. The epistatic interaction of tr mutant to overexpressed Wg and Hh consistently suggests that tr acts downstream of Hh but not of Wg. The effect of ectopically expressed Wg can be seen in tr null embryos while that of ectopically expressed Hh cannot. However, hh-dependent Dpp expression on imaginal discs is not affected in cells lacking tr function while the morphogenetic furrow progression of eye discs in these tr mutant cells is slightly retarded. The existence of parallel pathways that may compensate the tr function is thus hypothesized. Further investigation will focus on the verification of the epistatic relationship between tr and other known components in the Hh signaling and the molecular characterization of products encoded by the tr gene. The study of tr is intended not only to increase our realization of Hh signaling pathway but also to improve our knowledge in the mechanism of certain developmental disorders and human cancers that were recently indicated to be caused by the disruption of Hh signaling.