Snail induces epithelial cell extrusion through transcriptional control of RhoA contractile signaling and cell matrix adhesion

Cell extrusion is a morphogenetic process that is implicated in epithelial homeostasis and highly dependent on the cellular insult and context. Minorities of cells expressing HRas ^V12 or undergoing apoptosis are typically extruded apically in vertebrate cells. However, basal extrusion (delamination) predominates when mammalian cells express oncogenic KRas and during development in Drosophila. To explore if the morphogenetic transcription factor, Snail, induces extrusion, we inducibly expressed a metabolically stabilized Snail ^6SA transgene mosaically in confluent MCF-7 monolayers. We found that the morphogenetic impact of Snail ^6SA was critically influenced by the proportion of cells in which it was expressed. When expressed in small clusters (<3 cells) within confluent monolayers, Snail ^6SA expression induced apical cell extrusion. In contrast, confluent cultures of Snail ^6SA expressing cells were retained in the monolayer to eventually undergo basal extrusion (delamination). Transcriptomic profiling revealed that Snail ^6SA did not substantively alter the balance of epithelial:mesenchymal genes. However, we identified a gene transcriptional network that led to the upregulation of RhoA signalling, actomyosin contractility and reduced basal adhesion in Snail ^6SA expressing cells. We show that this was necessary to drive both apical extrusion and basal delamination. Thus, contractile upregulation by RhoA along with weakened basal adhesion provides a pathway for Snail to influence epithelial morphogenesis independently of classic Epithelial to Mesenchymal Transition (EMT).