# Nonmuscle Myosin-2B Regulates Apical Cortical Mechanics, ZO-1 Dynamics and Cell Size in MDCK Epithelial Cells

## TL;DR

This study shows that nonmuscle myosin-2B (NM2B) affects the mechanical properties of the apical cortex, ZO-1 dynamics, and cell size in MDCK epithelial cells.

## Contribution

The paper reveals a novel role of NM2B in regulating apical cortical mechanics and cell size independently of cingulin.

## Key findings

- Loss of NM2B decreases apical cortex stiffness and increases its fluidity.
- KO of NM2B increases ZO-1 dynamics and reduces its accumulation at tight junctions.
- NM2B deficiency increases cell size in both 2D and 3D cultures without affecting lumen morphogenesis.

## Abstract

In epithelial cells, nonmuscle myosin-2B (NM2B) shows a cortical localization and is tethered to tight junctions (TJs) and adherens junctions (AJs) by the junctional adaptor proteins cingulin and paracingulin. MDCK cells knock-out (KO) for cingulin show decreased apical membrane cortex stiffness and decreased TJ membrane tortuosity, and the rescue of these phenotypes requires the myosin-binding region of cingulin. Here, we investigated whether NM2B contributes to these phenotypes independently of cingulin by generating and characterizing clonal lines of MDCK cells KO for NM2B. The loss of NM2B resulted in decreased stiffness and increased fluidity of the apical cortex and reduced accumulation of E-cadherin and phalloidin-labeled actin filaments at junctions but had no significant effect on TJ membrane tortuosity. Fluorescence recovery after photobleaching (FRAP) showed that the KO of NM2B increased the dynamics of the TJ scaffold protein ZO-1, correlating with decreased ZO-1 accumulation at TJs. Finally, the KO of NM2B increased cell size in cells grown both in 2D and 3D but did not alter lumen morphogenesis of cysts. These results extend our understanding of the functions of NM2B by describing its role in the regulation of the mechanical properties of the apical membrane cortex and cell size and validate our model about the role of cingulin–NM2B interaction in the regulation of ZO-1 dynamics.