Role of Polarity Proteins in the Generation and Organization of Apical Surface Protrusions

Abstract

Protruding from the apical surfaces of epithelial cells are specialized structures, including cilia, microplicae, microvilli, and stereocilia. These contribute to epithelial function by cushioning the apical surface, by amplifying its surface area to facilitate nutrient absorption, and by promoting sensory transduction and barrier function. Despite these important roles, and the diseases that result when their formation is perturbed, there remain significant gaps in our understanding of the biogenesis of apical protrusions, or the pathways that promote their organization and orientation once at the apical surface. Here, I review some general aspects of these apical structures, and then discuss our current understanding of their formation and organization with respect to proteins that specify apicobasolateral polarity and planar cell polarity.

Apical cytoskeletons and junctional complexes as a combined system in epithelial cell sheets

“…). One is associated with apical membrane protrusions, such as microvilli, cilia, and stereocilia, or apical budding processes, a theme that has been well covered in other reviews . Here, we focus on the other type of apical cytoskeleton, which is not associated with apical protrusions …”

“…Later, the terminal web was found to be composed of actin, myosin 2, spectrins, tropomyosins, caldesmon, and cytokeratins . In intestinal epithelial cells, the core actin filaments of the microvilli, which are associated with villin, fimbrin, plastin‐1, and espin, terminating as rootlets, are connected to the terminal web by myosin 2 and spectrin to form the basic structural framework for the brush border system . In addition, various actin‐related proteins, including myosins, protocadherins, and ERM proteins, are involved in the brush border , which includes terminal webs, microvilli, and related structures in the apical region of intestinal epithelial cells, suggesting that these actin systems are more complex.…”

“…The Crumbs complex is associated with the PAR complex through PAR6, and both complexes promote the establishment of the mature apical compartment, including the apical membrane and TJs (apical maturation). 22,23 Ezrin, a FERM domain-containing apical membrane scaffolding protein, 24-27 interacts with a FERM-binding motif on Crumbs, suggesting that Crumbs is involved in recruiting ezrin to the apical membrane. In support of this mechanism, knockout of the Crumbs gene results in defects in epithelial morphogenesis.…”

“…One is associated with apical membrane protrusions, such as microvilli, cilia, and stereocilia, or apical budding processes, a theme that has been well covered in other reviews. 23 Here, we focus on the other type of apical cytoskeleton, which is not associated with apical protrusions. 23,50,51 The terminal web as the apical cytoskeleton of epithelial cell sheets Just beneath the apical membrane of the cells that make up many simple epithelia, including the enterocytes of the intestine, a fibrous layer extends between the terminal bars, a histological term for the unresolved group of cell-cell junctions including the TJs, AJs, and desmosomes.…”

“…23 Here, we focus on the other type of apical cytoskeleton, which is not associated with apical protrusions. 23,50,51 The terminal web as the apical cytoskeleton of epithelial cell sheets Just beneath the apical membrane of the cells that make up many simple epithelia, including the enterocytes of the intestine, a fibrous layer extends between the terminal bars, a histological term for the unresolved group of cell-cell junctions including the TJs, AJs, and desmosomes. 52,53 This layer of fibrils was classically defined as the terminal web.…”