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Mechanical forces are generated during epithelial morphogenesis, but how cells maintain adhesion when exposed to these forces is poorly understood. Razzell et al. show that the LIM domain protein Ajuba localizes to adherens junctions under tension in the Drosophila embryo and is required to maintain cell adhesion during epithelial remodeling., The reorganization of cells in response to mechanical forces converts simple epithelial sheets into complex tissues of various shapes and dimensions. Epithelial integrity is maintained throughout tissue remodeling, but the mechanisms that regulate dynamic changes in cell adhesion under tension are not well understood. In Drosophila melanogaster, planar polarized actomyosin forces direct spatially organized cell rearrangements that elongate the body axis. We show that the LIM-domain protein Ajuba is recruited to adherens junctions in a tension-dependent fashion during axis elongation. Ajuba localizes to sites of myosin accumulation at adherens junctions within seconds, and the force-sensitive localization of Ajuba requires its N-terminal domain and two of its three LIM domains. We demonstrate that Ajuba stabilizes adherens junctions in regions of high tension during axis elongation, and that Ajuba activity is required to maintain cell adhesion during cell rearrangement and epithelial closure. These results demonstrate that Ajuba plays an essential role in regulating cell adhesion in response to mechanical forces generated by epithelial morphogenesis.

Collective cell migration is absolutely essential for a wide variety of physiological episodes including the re-epithelialisation component of tissue repair. However, the investigation of such processes has been frustrated by difficulties in quantitatively analysing the behaviours of a large body of cells within a migrating epithelial sheet, which previously required manually tracking a large number of individual cells, or using advanced computational techniques. Here, we describe a novel and simpler image subtraction method with which we can visualise and quantify collective cell mobilisation as a ‘white wave’ that propagates back from the leading edge of a scratch-wounded monolayer of cultured epithelial cells. Using this technique, we show that actomyosin constriction negatively regulates cell mobilisation and that the advancement of cell sheets and the mobilisation of rows of cells behind their leading edges are independently regulated. We also show that there is a finite limit to the number of rows of cells mobilised after wounding. Moreover, our data suggest that enhancing cell mobilisation, by release from myosin II contractility, accelerates the healing of large wounds in the long term, thus raising the possibility that the cell mobilisation ‘wave’ we reveal here might be a therapeutic target for improving wound healing.

Summary A crucial early wound response is the recruitment of inflammatory cells drawn by danger cues released by the damaged tissue. Hydrogen peroxide (H2O2) has recently been identified as the earliest wound attractant in Drosophila embryos and zebrafish larvae [1, 2]. The H2O2 signal is generated by activation of an NADPH oxidase, DUOX, and as a consequence, the first inflammatory cells are recruited to the wound within minutes. To date, nothing is known about how wounding activates DUOX. Here, we show that laser wounding of the Drosophila embryo epidermis triggers an instantaneous calcium flash, which travels as a wave via gap junctions several cell rows back from the wound edge. Blocking this calcium flash inhibits H2O2 release at the wound site and leads to a reduction in the number of immune cells migrating to the wound. We suggest that the wound-induced calcium flash activates DUOX via an EF hand calcium-binding motif and thus triggers the production of the attractant damage cue H2O2. Therefore, calcium represents the earliest signal in the wound inflammatory response., Highlights ► Wounding the Drosophila embryo epidermis leads to a rapid intracellular Ca2+ wave ► Blocking the Ca2+ response leads to reduced H2O2 and hemocytes at wounds ► DUOX’s EF hand domain is indispensible to interpret wound-induced Ca2+ flashes

Apical constriction changes cell shapes, driving critical morphogenetic events including gastrulation in diverse organisms and neural tube closure in vertebrates. Apical constriction is thought to be triggered by contraction of apical actomyosin networks. We found that apical actomyosin contractions began before cell shape changes in both C. elegans and Drosophila. In C. elegans, actomyosin networks were initially dynamic, contracting and generating cortical tension without significant shrinking of apical surfaces. Apical cell-cell contact zones and actomyosin only later moved increasingly in concert, with no detectable change in actomyosin dynamics or cortical tension. Thus, apical constriction appears to be triggered not by a change in cortical tension but by dynamic linking of apical cell-cell contact zones to an already contractile apical cortex.

All embryos, from worms to humans, are shaped during development by morphogenetic steps that tug, bend, fold, and sculpt epithelial sheets into forms that resemble, or are the precursors of, the final adult structure ( 1 ). Most of these changes are the consequence of constrictions of the apical surfaces of epithelial cells that are powered by pulsatile contracting cytoskeletal (actomyosin) networks. On page 1232 of this issue, Roh-Johnson et al. ( 2 ) show that, just as in a car where the power of the engine is linked to forward movement by means of a clutch, clutch control is also the rate-limiting step for contracting cells in tissues.