Rab11 Helps Maintain Apical Crumbs and Adherens Junctions in the Drosophila Embryonic Ectoderm

BackgroundTissue morphogenesis and organogenesis require that cells retain stable cell-cell adhesion while changing shape and moving. One mechanism to accommodate this plasticity in cell adhesion involves regulated trafficking of junctional proteins.
Methodology/Principal FindingsHere we explored trafficking of junctional proteins in two well-characterized model epithelia, the Drosophila embryonic ectoderm and amnioserosa. We find that DE-cadherin, the transmembrane protein of adherens junctions, is actively trafficked through putative vesicles, and appears to travel through both Rab5-positive and Rab11-positive structures. We manipulated the functions of Rab11 and Rab5 to examine the effects on junctional stability and morphogenesis. Reducing Rab11 function, either using a dominant negative construct or loss of function alleles, disrupts integrity of the ectoderm and leads to loss of adherens junctions. Strikingly, the apical junctional regulator Crumbs is lost before AJs are destabilized, while the basolateral protein Dlg remains cortical. Altering Rab5 function had less dramatic effects, not disrupting adherens junction integrity but affecting dorsal closure.
Conclusions/SignificanceWe contrast our results with what others saw when disrupting other trafficking regulators, and when disrupting Rab function in other tissues; together these data suggest distinct mechanisms regulate junctional stability and plasticity in different tissues.

背景 组织形态发生与器官发生过程中，细胞在改变形态并发生迁移的同时，仍需维持稳定的细胞间黏附。调控连接蛋白的定向运输，是适配细胞黏附可塑性的重要机制之一。

方法与主要结果 本研究以两种特征明确的经典上皮模型——果蝇胚胎外胚层与羊浆膜为对象，探究了连接蛋白的囊泡运输过程。研究发现，作为黏着连接（adherens junctions, AJs）跨膜蛋白的DE-钙粘蛋白（DE-cadherin），可通过推定囊泡进行主动运输，且似乎可经Rab5阳性（Rab5-positive）与Rab11阳性（Rab11-positive）结构完成转运。本研究通过操纵Rab11与Rab5的功能，探究其对连接稳定性与形态发生的影响。通过显性负性构建体或功能缺失等位基因降低Rab11功能后，外胚层完整性被破坏，黏着连接出现丢失。值得注意的是，顶端连接调控蛋白Crumbs在黏着连接失稳前即出现丢失，而基底外侧蛋白Dlg仍保留皮层定位。改变Rab5功能仅产生较为温和的效应，虽未破坏黏着连接完整性，但会影响背闭合过程。

结论与意义 本研究将所得结果与其他学者在干扰其他运输调控因子、以及在其他组织中干扰Rab功能时的观测结果进行对比，综合这些数据表明，不同组织中调控连接蛋白稳定性与可塑性的机制存在显著差异。