Classical cadherins accumulate at cell-cell contacts as a characteristic response to productive adhesive ligation. Such local accumulation of cadherins is a developmentally regulated process that supports cell adhesiveness and cell-cell cohesion. Yet the molecular effectors responsible for cadherin accumulation remain incompletely understood. We now report that Myosin 2 is critical for cells to concentrate E-cadherin at cell-cell contacts. Myosin 2 is found at cadherin-based cell-cell contacts and its recruitment requires E-cadherin activity. Indeed, both Myosin 2 recruitment and its activation were stimulated by E-cadherin homophilic ligation alone. Inhibition of Myosin 2 activity by blebbistatin or ML-7 rapidly impaired the ability of cells to concentrate E-cadherin at adhesive contacts, accompanied by decreased cadherin-based cell adhesiveness. The total surface expression of cadherins was unaffected, suggesting that Myosin 2 principally regulates the regional distribution of cadherins at the cell surface. The recruitment of Myosin 2 to cadherin contacts, and its activation, required Rho kinase; furthermore, inhibition of Rho kinase signaling effectively phenocopied the effects of Myosin 2 inhibition. We propose that Myosin 2 is a key effector of Rho-Rho kinase signaling that regulates cell-cell adhesion by determining the ability of cells to concentrate cadherins at contacts in response to homophilic ligation.

Cooperation between cadherins and the actin cytoskeleton controls many aspects of epithelial biogenesis. We report here that myosin VI critically regulates the morphogenesis of epithelial cell–cell contacts. As epithelial monolayers mature in culture, discontinuous cell–cell contacts are initially replaced by continuous (cohesive) contacts. Myosin VI is recruited to cell contacts as they become linear and cohesive, where it forms a biochemical complex with epithelial cadherin (E-cadherin). Myosin VI is necessary for strong cadherin adhesion, for cells to form cohesive linear contacts, and for the integrity of the apical junctional complex. We find that vinculin mediates this effect of myosin VI. Myosin VI is necessary for vinculin and E-cadherin to interact. A combination of gain and loss of function approaches identifies vinculin as a downstream effector of myosin VI that is necessary for the integrity of intercellular contacts. We propose that myosin VI and vinculin form a molecular apparatus that generates cohesive cell–cell contacts in cultured mammalian epithelia.

RhoA-inhibitory bacterial toxins, such as Staphylococcus aureus EDIN toxin, induce large transendothelial cell macroaperture (TEM) tunnels that rupture the host endothelium barrier and promote bacterial dissemination. Host cells repair these tunnels by extending actin-rich membrane waves from the TEM edges. We reveal that cyclic-AMP signaling produced by Bacillus anthracis edema toxin (ET) also induces TEM formation, which correlates with increased vascular permeability. We show that ET-induced TEM formation resembles liquid dewetting, a physical process of nucleation and growth of holes within a thin liquid film. We also identify the cellular mechanisms of tunnel closure and reveal that the I-BAR domain protein Missing in Metastasis (MIM) senses de novo membrane curvature generated by the TEM, accumulates at the TEM edge, and triggers Arp2/3-dependent actin polymerization, which induces actin-rich membrane waves that close the TEM. Thus, the balance between ET-induced TEM formation and resealing likely determines the integrity of the host endothelium barrier.

Oriented cell division is a fundamental determinant of tissue organization. Simple epithelia divide symmetrically in the plane of the monolayer to preserve organ structure during epithelial morphogenesis and tissue turnover. For this to occur, mitotic spindles must be stringently oriented in the Z-axis, thereby establishing the perpendicular division plane between daughter cells. Spatial cues are thought to play important roles in spindle orientation, notably during asymmetric cell division. The molecular nature of the cortical cues that guide the spindle during symmetric cell division, however, is poorly understood. Here we show directly for the first time that cadherin adhesion receptors are required for planar spindle orientation in mammalian epithelia. Importantly, spindle orientation was disrupted without affecting tissue cohesion or epithelial polarity. This suggests that cadherin receptors can serve as cues for spindle orientation during symmetric cell division. We further show that disrupting cadherin function perturbed the cortical localization of APC, a microtubule-interacting protein that was required for planar spindle orientation. Together, these findings establish a novel morphogenetic function for cadherin adhesion receptors to guide spindle orientation during symmetric cell division.

Pathogenic bacteria can cross from blood vessels to host tissues by opening transendothelial cell macroapertures (TEMs). To induce TEM opening, bacteria intoxicate endothelial cells with proteins that disrupt the contractile cytoskeletal network. Cell membrane tension is no longer resisted by contractile fibers, leading to the opening of TEMs. Here we model the opening of TEMs as a new form of dewetting. While liquid dewetting is irreversible, we show that cellular dewetting is transient. Our model predicts the minimum radius for hole nucleation, the maximum TEM size, and the dynamics of TEM opening, in good agreement with experimental data. The physical model is then coupled with biological experimental data to reveal that the protein missing in metastasis (MIM) controls the line tension at the rim of the TEM and opposes its opening.

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Inflammasomes are inflammatory signaling complexes that provide molecular platforms to activate the protease function of inflammatory caspases. Caspases-1, -4, -5, and -11 are inflammatory caspases activated by inflammasomes to drive lytic cell death and inflammatory mediator production, thereby activating host-protective and pathological immune responses. Here, we comprehensively review the mechanisms that govern the activity of inflammatory caspases. We discuss inflammatory caspase activation and deactivation mechanisms, alongside the physiological importance of caspase activity kinetics. We also examine mechanisms of caspase substrate selection and how inflammasome and cell identities influence caspase activity and resultant inflammatory and pyroptotic cellular programs. Understanding how inflammatory caspases are regulated may offer new strategies for treating infection and inflammasome-driven disease. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Small GTPases of the Rho protein family are master regulators of the actin cytoskeleton and are targeted by potent virulence factors of several pathogenic bacteria. Their dysfunctional regulation can lead to severe human pathologies. Both host and bacterial factors can activate or inactivate Rho proteins by direct post-translational modifications: such as deamidation and transglutamination for activation, or ADP-ribosylation, glucosylation, adenylylation and phosphorylation for inactivation. We review and compare these unconventional ways in which both host cells and bacterial pathogens regulate Rho proteins.