Figure 2. Dual regulation of tension by Rap1. (A) Rap1 regulates endothelial barrier by a two-fold regulation of cytoskeletal tension. First, Rap1 relieves tension on radial stress fibers, thereby decreasing the radial tension on the junction (upper cartoon). Second, Rap1 induces tension in linear actin cables along the cell–cell contact (junctional tension, lower cartoon). The middle cartoon is a normally non-existent, intermediate state (winding actin) when no tension is present, which can be induced by inhibiting Myosin. Cell–cell contacts are shown in gray, actin cables are shown in green. (B) Immunofluorescence pictures showing the different actin morphologies depicted in 2A, which were induced by ArhGAP29 depletion (upper box), blebbistatin treatment (100 µM, 30 min) (middle box), or 007 treatment (1 µM, 10 min) (lower box). Actin in shown in green, whereas the AJ marker β-catenin is shown in red. (C) Molecular mechanisms of tension regulation employed by Rap1 to regulate endothelial barrier function, comprising both regulation of radial tension (left box, green) and regulation of junctional tension (right box, red). See text for details.