Department of Genetics, University of Melbourne, VIC, 3010, Australia.
The University of Adelaide, Adelaide, SA 5005, Australia.
Biol Open. 2015 Jan 23;4(2):233-43. doi: 10.1242/bio.201410827.
Mesenchymal-epithelial transitions (METs) are important in both development and the growth of secondary tumours. Although the molecular basis for epithelial polarity is well studied, less is known about the cues that induce MET. Here we show that Netrins, well known as chemotropic guidance factors, provide a basal polarising cue during the Drosophila midgut MET. Both netrinA and netrinB are expressed in the visceral mesoderm, the substrate upon which midgut cells migrate, while their receptor frazzled (fra) is expressed in midgut cells. Netrins are required to polarise Fra to the basal surface, and Netrins and Fra undergo mutually-dependent endocytosis, with Fra subsequently trafficking to late endosomes. Mutations to fra and netrins affect both migration and MET but to different degrees. Loss of fra strongly delays migration, midgut cells fail to extend protrusions, and apico-basal polarisation of proteins and epithelium formation is inhibited. In netrin mutants, the migration phenotype is weaker and cells still extend protrusions. However, apico-basal polarisation of proteins, including Fra, and FActin is greatly disrupted and a monolayer fails to form. Delocalised accumulations of FActin are prevalent in netrin mutants but not fra mutants suggesting delocalised Fra may disrupt the MET. βPS localisation is also affected in netrin mutants in that a basal gradient is reduced while localisation to the midgut/VM interface is increased. Since a similar effect is seen when endocytosis is inhibited, Netrin and Fra may regulate Integrin turnover. The results suggest Netrin-dependent basal polarisation of Fra is critical for the formation of an epithelium.
间质上皮转化 (MET) 在发育和次级肿瘤的生长中都很重要。尽管上皮极性的分子基础研究得很好，但诱导 MET 的线索知之甚少。在这里，我们表明 Netrins，作为众所周知的趋化性导向因子，在果蝇中肠 MET 期间提供了一个基底极化的线索。netrinA 和 netrinB 都在内脏中胚层表达，这是中肠细胞迁移的基底，而它们的受体 frazzled（fra）在中肠细胞中表达。Netrins 被要求将 Fra 极化到基底表面，并且 Netrins 和 Fra 经历相互依赖的内吞作用，随后 Fra 运输到晚期内体。fra 和 netrins 的突变会影响迁移和 MET，但程度不同。fra 的缺失强烈延迟迁移，中肠细胞不能延伸突起，并且蛋白质和上皮形成的顶底极性被抑制。在 netrin 突变体中，迁移表型较弱，细胞仍然延伸突起。然而，蛋白质的顶底极性，包括 Fra 和 FActin，受到严重破坏，单层不能形成。在 netrin 突变体中，FActin 的定位也受到影响，即基底梯度降低，而定位到中肠/VM 界面增加。由于当抑制内吞作用时也会出现类似的效果，因此 Netrin 和 Fra 可能调节整合素周转。结果表明，Fra 的 Netrin 依赖性基底极化对于上皮的形成至关重要。
