Abstract
Apico-basal polarity is the defining characteristic of epithelial cells. In Drosophila, apical membrane identity is established and regulated through interactions between the highly conserved Par complex (Bazooka/Par3, atypical protein kinase C and Par6), and the Crumbs complex (Crumbs, Stardust and PATJ). It has been proposed that Bazooka operates at the top of a genetic hierarchy in the establishment and maintenance of apico-basal polarity. However, there is still ambiguity over the correct sequence of events and cross-talk with other pathways during this process. In this study, we reassess this issue by comparing the phenotypes of the commonly used baz4 and baz815-8 alleles with those of the so far uncharacterized bazXR11 and bazEH747 null alleles in different Drosophila epithelia. While all these baz alleles display identical phenotypes during embryonic epithelial development, we observe strong discrepancies in the severity and penetrance of polarity defects in the follicular epithelium: polarity is mostly normal in bazEH747 and bazXR11 while baz4 and baz815-8 show loss of polarity, severe multilayering and loss of epithelial integrity throughout the clones. Further analysis reveals that the chromosomes carrying the baz4 and baz815-8 alleles may contain additional mutations that enhance the true baz loss-of-function phenotype in the follicular epithelium. This study clearly shows that Baz is dispensable for the regulation of polarity in the follicular epithelium, and that the requirement for key regulators of cell polarity is highly dependent on developmental context and cell type.
Highlights
The defining characteristic of epithelial cells in all metazoans is apico-basal polarity
Baz posterior follicular cell (PFC) mutant clones were reported to show strong multilayering (Abdelilah-Seyfried et al, 2003). Our results confirmed these findings with regards to baz4 and baz815-8 follicular epithelium (FE) clones which often form large holes in the lateral FE and display strong multilayering of the PFCs (Figs 1 and 2)
Discrepancies between baz alleles and elucidation of the true baz loss-of-function FE phenotype Previous studies using the baz4 and baz815-8 alleles reported that baz is essential for polarization of the FE. baz4 and baz815-8 clones show large holes in the FE and strong multilayering of the PFCs
Summary
The defining characteristic of epithelial cells in all metazoans is apico-basal polarity. Apico-basal polarity in epithelial cells manifests upon the assembly of polarized cytoskeletal networks, polarized trafficking of vesicles and cargo and the localization of adhesion complexes to specific cortical locations. This causes division of the epithelial plasma membrane into distinct apical, basal, and lateral domains, typically enriched for specific phospholipid components and distinct, yet highly conserved, protein complexes (Johnson and Wodarz, 2003; McCaffrey and Macara, 2011; Tepass, 2012; Rodriguez-Boulan and Macara, 2014)
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