PAR-6, but not E-cadherin and β-integrin, is necessary for epithelial polarization in C. elegans

Cell polarity is a fundamental characteristic of epithelial cells. Classical cell biological studies have suggested that establishment and orientation of polarized epithelia depend on outside-in cues that derive from interactions with either neighboring cells or the substratum ( Akhtar and Streuli, 2013; Chen and Zhang, 2013; Chung and Andrew, 2008; McNeill et al., 1990; Nejsum and Nelson, 2007; Nelson et al., 2013; Ojakian and Schwimmer, 1994; Wang et al., 1990; Yu et al., 2005 ). This paradigm has been challenged by examples of epithelia generated in the absence of molecules that mediate cell-cell or cell-matrix interactions, notably E-cadherin and integrins ( Baas et al., 2004; Choi et al., 2013; Costa et al., 1998; Harris and Peifer, 2004; Raich et al., 1999; Roote and Zusman, 1995; Vestweber et al., 1985; Williams and Waterston, 1994; Wu et al., 2009 ). Here we explore an alternative hypothesis, that cadherins and integrins function redundantly to substitute for one another during epithelium formation ( Martinez-Rico et al., 2010; Ojakian et al., 2001; Rudkouskaya et al., 2014; Weber et al., 2011 ). We use C. elegans, which possesses a single E-cadherin ( Costa et al., 1998; Hardin et al., 2013; Tepass, 1999 ) and a single β-integrin ( Gettner et al., 1995; Lee et al., 2001 ), and analyze the arcade cells, which generate an epithelium late in embryogenesis ( Portereiko and Mango, 2001; Portereiko et al., 2004 ), after most maternal factors are depleted. Loss of E-cadherin HMR−1 in combination with β-integrin PAT−3 had no impact on the onset or formation of the arcade cell epithelium, nor the epidermis or digestive tract. Moreover, ß-integrin PAT-3 was not enriched at the basal surface of the arcades, and the candidate PAT-3 binding partner β-laminin LAM−1 was not detected until after arcade cell polarity was established and exhibited no obvious polarity defect when mutated. Instead, the polarity protein par-6 ( Chen and Zhang, 2013; Watts et al., 1996 ) was required to polarize the arcade cells, and par-6 mutants exhibited mislocalized or absent apical and junctional proteins. We conclude that the arcade cell epithelium polarizes by a PAR-6-mediated pathway that is independent of E-cadherin, β-integrin and β-laminin.