An alternative mode of epithelial polarity in the Drosophila midgut

Chen J et al. (2018) PLoS Biology 16. DOI: 10.1371/journal.pbio.3000041.

Author summary 

The Drosophila midgut is lined by a single-layered epithelium that acts as a barrier to the environment while allowing for nutrient uptake and related physiological processes. To fulfill these roles, midgut epithelial cells are highly polarised, with a pronounced asymmetric distribution of cellular components.Previous work in Drosophila revealed a conserved set of factors governing cell polarity, and it is thought that this network of proteins underlies all examples of polarity in Drosophila (and other organisms).

Here, we demonstrate that the epithelial cells in the Drosophila midgut are not polarised by these canonical polarity factors but instead rely on the integrin adhesion complex. Thus, there are two types of epithelia in Drosophila that polarise using fundamentally different mechanisms.

A section of the midgut epithelium stained for F-actin (red) and DNA (blue) containing an aPKC null mutant clone marked by GFP expression (green). The mutant cells polarise normally and show no obvious phenotype. 

Abstract from the paper

Apical–basal polarity is essential for the formation and function of epithelial tissues, whereas loss of polarity is a hallmark of tumours. Studies in Drosophila have identified conserved polarity factors that define the apical (Crumbs, Stardust, Par-6, atypical protein kinase C [aPKC]), junctional (Bazooka [Baz]/Par-3), and basolateral (Scribbled [Scrib], Discs large [Dlg], Lethal [2] giant larvae [Lgl]) domains of epithelial cells. Because these conserved factors mark equivalent domains in diverse types of vertebrate and invertebrate epithelia, it is generally assumed that this system underlies polarity in all epithelia.

Here, we show that this is not the case, as none of these canonical factors are required for the polarisation of the endodermal epithelium of the Drosophila adult midgut. Furthermore, like vertebrate epithelia but not other Drosophila epithelia, the midgut epithelium forms occluding junctions above adherens junctions (AJs) and requires the integrin adhesion complex for polarity.

Thus, Drosophila contains two types of epithelia that polarise by fundamentally different mechanisms. This diversity of epithelial types may reflect their different developmental origins, junctional arrangement, or whether they polarise in an apical–basal direction or vice versa. Since knock-outs of canonical polarity factors in vertebrates often have little or no effect on epithelial polarity and the Drosophila midgut shares several common features with vertebrate epithelia, this diversity of polarity mechanisms is likely to be conserved in other animals.

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