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-cadherinHMR−1 in combination with β-integrinPAT−3 had no impact on the onset or formation of the arcade cell epithelium, nor the epidermis or digestive tract. Moreover, ß-integrinPAT-3 was not enriched at the basal surface of the arcades, and the candidate PAT-3 binding partner β-lamininLAM−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.