The adherens junction–associated LIM domain protein Smallish regulates epithelial morphogenesis

The regulation of cell–cell adhesion between epithelial cells is crucial for the control of morphogenetic movements during development (Haigo et al., 2003; Gumbiner, 2005; Lecuit and Yap, 2015). A major driving force for cell shape changes during morphogenesis is the contraction of the actomyosin network anchored at the belt-shaped adherens junction (AJ), the zonula adherens (ZA; Simões et al., 2014; Murrell et al., 2015; Siedlik and Nelson, 2015; Harris, 2017; Umetsu and Kuranaga, 2017). Links between the actomyosin network and the cell adhesion molecules of the ZA, the cadherins, are provided by actin-binding proteins that associate with the cytoplasmic tails of cadherins (Simões et al., 2010; Leckband and de Rooij, 2014; Takeichi, 2014). Among these linker proteins are α-catenin, vinculin, and afadin (Canoe [Cno] in Drosophila melanogaster; Sawyer et al., 2009). The contractility of actomyosin is regulated via phosphorylation of the regulatory light chain of nonmuscle myosin II (Spaghetti squash [Sqh] in Drosophila) by Rho-associated coiled-coil containing kinase (Rho kinase [Rok] in Drosophila; Lilien and Balsamo, 2005; Julian and Olson, 2014) and its binding partner Shroom (Shrm; Hildebrand and Soriano, 1999). Overexpression of Shrm induces apical constriction of polarized epithelial cells (Haigo et al., 2003; Plageman et al., 2011). In Drosophila, Shrm binds to Rok and is required for its planar polarization during convergent extension movements of the epidermis (Simões et al., 2014). Among the phosphorylation targets of Rok is the polarity regulator Bazooka/Par-3 (Baz), which loses its planar polarization upon mutation or inhibition of Rok (Simões et al., 2010). In Drosophila embryonic morphogenesis, Baz apparently has several key functions, as it is required for apical-basal polarity, planar cell polarity (PCP), and formation of the ZA in the neuroectodermal epithelium during germ band extension (Müller and Wieschaus, 1996; Bilder et al., 2003; Harris and Peifer, 2004; Zallen and Wieschaus, 2004). How these functions are coordinated at the molecular level is not well understood so In epithelia, cells adhere to each other in a dynamic fashion, allowing the cells to change their shape and move along each other during morphogenesis. The regulation of adhesion occurs at the belt-shaped adherens junction, the zonula adherens (ZA). Formation of the ZA depends on components of the Par–atypical PKC (Par-aPKC) complex of polarity regulators. We have identified the Lin11, Isl-1, Mec-3 (LIM) protein Smallish (Smash), the orthologue of vertebrate LMO7, as a binding partner of Bazooka/Par-3 (Baz), a core component of the Par-aPKC complex. Smash also binds to Canoe/Afadin and the tyrosine kinase Src42A and localizes to the ZA in a planar polarized fashion. Animals lacking Smash show loss of planar cell polarity (PCP) in the embryonic epidermis and reduced cell bond tension, leading to severe defects during embryonic morphogenesis of epithelial tissues and organs. Overexpression of Smash causes apical constriction of epithelial cells. We propose that Smash is a key regulator of morphogenesis coordinating PCP and actomyosin contractility at the ZA. The adherens junction–associated LIM domain protein Smallish regulates epithelial morphogenesis