Abstract
The integrity and function of the epithelial barrier is dependent on the apical junctional complex (AJC) composed of tight and adherens junctions and regulated by the underlying actin filaments. A major F-actin motor, myosin II, was previously implicated in regulation of the AJC, however direct evidence of the involvement of myosin II in AJC dynamics are lacking and the molecular identity of the myosin II motor that regulates formation and disassembly of apical junctions in mammalian epithelia is unknown. We investigated the role of nonmuscle myosin II (NMMII) heavy chain isoforms, A, B, and C in regulation of epithelial AJC dynamics and function. Expression of the three NMMII isoforms was observed in model intestinal epithelial cell lines, where all isoforms accumulated within the perijunctional F-actin belt. siRNA-mediated downregulation of NMMIIA, but not NMMIIB or NMMIIC expression in SK-CO15 colonic epithelial cells resulted in profound changes of cell morphology and cell-cell adhesions. These changes included acquisition of a fibroblast-like cell shape, defective paracellular barrier, and substantial attenuation of the assembly and disassembly of both adherens and tight junctions. Impaired assembly of the AJC observed after NMMIIA knock-down involved dramatic disorganization of perijunctional actin filaments. These findings provide the first direct non-pharmacological evidence of myosin II-dependent regulation of AJC dynamics in mammalian epithelia and highlight a unique role of NMMIIA in junctional biogenesis.
Highlights
The apical junctional complex (AJC) represents one of the most characteristic cellular structures of differentiated simple epithelia [1,2,3]
Expression and localization of nonmuscle myosin II (NMMII) isoforms in human intestinal epithelial cells In order to examine roles of different myosin II heavy chain isoforms in the regulation of AJC in simple polarized epithelia, we used SKCO15, Caco-2 and T84 human colonic epithelial cell lines
These results demonstrate that well differentiated colonic epithelial cells coexpress all NMMII isoforms, relative expression of different NMMII heavy chains is likely to be variable depending on cell type
Summary
The apical junctional complex (AJC) represents one of the most characteristic cellular structures of differentiated simple epithelia [1,2,3]. It is positioned at the apical-most aspect of the lateral plasma membrane and regulates the integrity of epithelial monolayers, formation of paracellular barrier and cell polarity [1,2,3]. The AJC is composed of two multiprotein complexes known as the tight junction (TJ), and adherens junction (AJ) [1,2,3,4,5] These complexes are regulated by three major types of proteinprotein interactions. The third type of interactions is mediated by actin-binding proteins such as members of ‘‘zonula occludens’’ (ZO) family, afadin, a-catenin and vinculin that may physically link the AJC to actin microfilaments [9,10,11]
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