Sequential Notch Signalling at the Boundary of Fringe Expressing and Non-Expressing Cells

Wing development in Drosophila requires the activation of Wingless (Wg) in a small stripe along the boundary of Fringe (Fng) expressing and non-expressing cells (FB), which coincides with the dorso-ventral (D/V) boundary of the wing imaginal disc. The expression of Wg is induced by interactions between dorsal and ventral cells mediated by the Notch signalling pathway. It appears that mutual signalling from dorsal to ventral and ventral to dorsal cells by the Notch ligands Serrate (Ser) and Delta (Dl) respectively establishes a symmetric domain of Wg that straddles the D/V boundary. The directional signalling of these ligands requires the modification of Notch in dorsal cells by the glycosyltransferase Fng and is based on the restricted expression of the ligands with Ser expression to the dorsal and that of Dl to the ventral side of the wing anlage. In order to further investigate the mechanism of Notch signalling at the FB, we analysed the function of Fng, Ser and Dl during wing development at an ectopic FB and at the D/V boundary. We find that Notch signalling is initiated in an asymmetric fashion on only one side of the FB. During this initial asymmetric phase, only one ligand is required, with Ser initiating Notch-signalling at the D/V and Dl at the ectopic FB. Furthermore, our analysis suggests that Fng has also a positive effect on Ser signalling. Because of these additional properties, differential expression of the ligands, which has been a prerequisite to restrict Notch activation to the FB in the current model, is not required to restrict Notch signalling to the FB.果蝇（Drosophila）翅膀发育需要在表达Fringe蛋白（Fringe, Fng）与不表达该蛋白的细胞边界（FB）处的窄条区域中激活无翅基因（Wingless, Wg），该边界与翅成虫盘的背腹（D/V）边界重合。Wg的表达由背侧与腹侧细胞间的相互作用诱导，该过程由Notch信号通路（Notch signalling pathway）介导。现有研究表明，Notch配体锯齿蛋白（Serrate, Ser）和Delta蛋白（Delta, Dl）分别介导背侧向腹侧、腹侧向背侧的双向信号传导，由此构建出横跨D/V边界的对称Wg表达域。这些配体的定向信号传导需要糖基转移酶Fng对背侧细胞中的Notch进行修饰，且依赖于配体的限制性表达：Ser仅在翅膀原基的背侧表达，而Dl则定位于腹侧。为进一步探究FB处的Notch信号传导机制，我们在异位FB与天然D/V边界处分析了Fng、Ser及Dl在翅膀发育过程中的功能。研究发现，Notch信号传导仅在FB的单侧以不对称方式启动。在这一初始不对称阶段，仅需单一配体即可完成信号启动：Ser介导D/V边界处的Notch信号激活，而Dl则负责异位FB处的信号启动。此外，我们的分析显示Fng对Ser介导的信号传导亦具有正向调控作用。基于上述新发现的特性，当前模型中作为将Notch激活限定于FB的必要条件——配体的差异表达——已不再是限定Notch信号传导至FB的必需条件。