Mesenchymal-to-epithelial transitions require tissue-specific interactions with distinct laminins

Mesenchymal-to-epithelial transition (MET) converts cells from migratory mesenchymal to polarized epithelial states. Despite its importance for both normal and pathological processes, very little is known about the regulation of MET in vivo. Here we exploit midgut morphogenesis in Drosophila melanogaster to investigate the mechanisms underlying MET. We show that down-regulation of the EMT transcription factor Serpent is required for MET, but not sufficient, as interactions with the surrounding mesoderm are also essential. We find that midgut MET relies on the secretion of specific laminins via the CopII secretory pathway from both mesoderm and midgut cells. We show that secretion of the laminin trimer containing the Wingblister α-subunit from the mesoderm is an upstream cue for midgut MET, leading to basal polarization of αPS1 integrin in midgut cells. Polarized αPS1 is required for the formation of a monolayered columnar epithelium and for the apical polarization of αPS3, Baz, and E-Cad. Secretion of a distinct LamininA-containing trimer from midgut cells is required to reinforce the localization of αPS1 basally, and αPS3 apically, for robust repolarization. Our data suggest that targeting these MET pathways, in conjunction with therapies preventing EMT, may present a two-pronged strategy toward blocking metastasis in cancer.