Rapid lamellipodial responses by neighbor cells drive epithelial sealing in response to pyroptotic cell death.

Cell injury poses a substantial challenge for epithelia homeostasis. Several cellular processes preserve epithelial barriers in response to apoptosis, but less is known about other forms of cell death, such as pyroptosis. Here we use an inducible caspase-1 system to analyze how colon epithelial monolayers respond to pyroptosis. We confirm that sporadic pyroptotic cells are physically eliminated from confluent monolayers by apical extrusion. This is accompanied by a transient defect in barrier function at the site of the pyroptotic cells. By visualizing cell shape changes and traction patterns in combination with cytoskeletal inhibitors, we show that rapid lamellipodial responses in the neighbor cells are responsible for correcting the leakage and resealing the barrier. Cell contractility is not required for this resealing response, in contrast to the response to apoptosis. Therefore, pyroptosis elicits a distinct homeostatic response from the epithelium that is driven by the stimulation of lamellipodia in neighbor cells.

Enhanced RhoA signalling stabilizes E-cadherin in migrating epithelial monolayers.

Epithelia migrate as physically coherent populations of cells. Previous studies have revealed that mechanical stress accumulates in these cellular layers as they move. These stresses are characteristically tensile in nature and have often been inferred to arise when moving cells pull upon the cell-cell adhesions that hold them together. We now report that epithelial tension at adherens junctions between migrating cells also increases due to an increase in RhoA-mediated junctional contractility. We found that active RhoA levels were stimulated by p114 RhoGEF (also known as ARHGEF18) at the junctions between migrating MCF-7 monolayers, and this was accompanied by increased levels of actomyosin and mechanical tension. Applying a strategy to restore active RhoA specifically at adherens junctions by manipulating its scaffold, anillin, we found that this junctional RhoA signal was necessary to stabilize junctional E-cadherin (CDH1) during epithelial migration and promoted orderly collective movement. We suggest that stabilization of E-cadherin by RhoA serves to increase cell-cell adhesion to protect against the mechanical stresses of migration. This article has an associated First Person interview with the first author of the paper.

How adherens junctions move cells during collective migration.

In this review, we consider how the association between adherens junctions and the actomyosin cytoskeleton influences collective cell movement. We focus on recent findings which reveal different ways for adherens junctions to promote the locomotion of cells within tissues: through lamellipodia and junctional contraction. These contributions reflect how classic cadherins establish sites of cortical actin assembly and how adherens junctions couple to contractile actomyosin, respectively. The diverse interplay between cadherin adhesion and the cytoskeleton thus provides different ways for adherens junctions to support epithelial locomotion.

Investigating the nature of active forces in tissues reveals how contractile cells can form extensile monolayers.

Actomyosin machinery endows cells with contractility at a single-cell level. However, within a monolayer, cells can be contractile or extensile based on the direction of pushing or pulling forces exerted by their neighbours or on the substrate. It has been shown that a monolayer of fibroblasts behaves as a contractile system while epithelial or neural progentior monolayers behave as an extensile system. Through a combination of cell culture experiments and in silico modelling, we reveal the mechanism behind this switch in extensile to contractile as the weakening of intercellular contacts. This switch promotes the build-up of tension at the cell-substrate interface through an increase in actin stress fibres and traction forces. This is accompanied by mechanotransductive changes in vinculin and YAP activation. We further show that contractile and extensile differences in cell activity sort cells in mixtures, uncovering a generic mechanism for pattern formation during cell competition, and morphogenesis.

Hands and feet: Closer than you think in epithelial migration.

Epithelial migration requires that substrate-based motility be coordinated with cell-cell adhesion. In this issue, Ozawa et al. (2020. J. Cell Biol.https://doi.org/10.1083/jcb.202006196) identify a central role for actin assembly at adherens junctions that contributes to both of these processes.

A Mechanosensitive RhoA Pathway that Protects Epithelia against Acute Tensile Stress.

Adherens junctions are tensile structures that couple epithelial cells together. Junctional tension can arise from cell-intrinsic application of contractility or from the cell-extrinsic forces of tissue movement. Here, we report a mechanosensitive signaling pathway that activates RhoA at adherens junctions to preserve epithelial integrity in response to acute tensile stress. We identify Myosin VI as the force sensor, whose association with E-cadherin is enhanced when junctional tension is increased by mechanical monolayer stress. Myosin VI promotes recruitment of the heterotrimeric Gα12 protein to E-cadherin, where it signals for p114 RhoGEF to activate RhoA. Despite its potential to stimulate junctional actomyosin and further increase contractility, tension-activated RhoA signaling is necessary to preserve epithelial integrity. This is explained by an increase in tensile strength, especially at the multicellular vertices of junctions, that is due to mDia1-mediated actin assembly.

Author Correction: Tyrosine dephosphorylated cortactin downregulates contractility at the epithelial zonula adherens through SRGAP1.

A correction to this article has been published and is linked from the HTML version of this article.

Tyrosine dephosphorylated cortactin downregulates contractility at the epithelial zonula adherens through SRGAP1.

Contractile adherens junctions support cell-cell adhesion, epithelial integrity, and morphogenesis. Much effort has been devoted to understanding how contractility is established; however, less is known about whether contractility can be actively downregulated at junctions nor what function this might serve. We now identify such an inhibitory pathway that is mediated by the cytoskeletal scaffold, cortactin. Mutations of cortactin that prevent its tyrosine phosphorylation downregulate RhoA signaling and compromise the ability of epithelial cells to generate a contractile zonula adherens. This is mediated by the RhoA antagonist, SRGAP1. We further demonstrate that this mechanism is co-opted by hepatocyte growth factor to promote junctional relaxation and motility in epithelial collectives. Together, our findings identify a novel function of cortactin as a regulator of RhoA signaling that can be utilized by morphogenetic regulators for the active downregulation of junctional contractility.Epithelial cell-cell adhesions are contractile junctions, but whether contractility can be down-regulated is not known. Here the authors report how tyrosine dephosphorylation of the cytoskeletal scaffold, cortactin, recruits the RhoA antagonist SRGAP1 to relax adherens junctions in response to HGF.

Smart "lanthano" proteins for phospholipid sensing.

Metal-ion-mediated interactions between calcium-binding peripheral proteins and membrane phospholipids is a key feature of multiple cell signaling processes. The molecular basis for the interaction involves the displacement of inner-sphere water molecules on calcium ions by phosphate groups of the phospholipids. On the basis of this fundamental mechanism, we have devised a novel "turn-on" optical sensing strategy for anionic phospholipids by using a lanthanide reconstituted protein. The "lanthano" protein turns on selectively in the presence of a crucial signaling phospholipid, phosphatidylserine, by affording a 6 times enhancement in lanthanide luminescence. The "turn-on" sensing strategy was distinctly validated by direct evidence for the water-displacement mechanism via lifetime measurements.