Inflammatory mediators increase vascular permeability primarily by formation of intercellular gaps between endothelial cells of post-capillary venules. Under these conditions, endothelial cell-cell contacts such as adherens and tight junctions open to allow paracellular fluid passage. Small guanosine triphosphatases (GTPases) from the ras superfamily, primarily Rho GTPases (RhoA, Rac1, Cdc42) or Rap1 are known to regulate cell adhesion, in part by reorganization of the junction-associated cortical actin cytoskeleton. In this review, we will discuss the role of small GTPases for the maintenance of microvascular barrier functions under resting conditions as well as under conditions of increased permeability and their involvement in signalling pathways downstream of both barrier-stabilizing and inflammatory mediators. Rac1 and Cdc42 are the main GTPases required for barrier maintenance and stabilization, whereas RhoA negatively regulates barrier properties under both resting and inflammatory conditions. For Rac1 and RhoA, contrary functions under certain conditions have also been described. However, Rac1-mediated barrier destabilization in microvascular endothelium appears to be largely restricted to conditions of enhanced endothelial cell migration and thus to be more closely related to angiogenesis rather than to inflammation. Recent studies revealed that cAMP signalling, which is well known to be barrier protective, enhances barrier functions in part via Rap1-mediated activation of Rac1 and Cdc42 as well as by inhibition of RhoA. Moreover, barrier-stabilizing mediators directly activate Rac1 and Cdc42 or increase cAMP levels. On the other hand, several barrier-disruptive components appear to increase permeability by reduced formation of cAMP, leading to both inactivation of Rac1 and activation of RhoA.

The autoimmune blistering skin diseases pemphigus vulgaris (PV) and pemphigus foliaceus (PF) are mainly caused by autoantibodies against desmosomal cadherins. In this study, we provide evidence that PV–immunoglobulin G (IgG) and PF-IgG induce skin blistering by interference with Rho A signaling. In vitro, pemphigus IgG caused typical hallmarks of pemphigus pathogenesis such as epidermal blistering in human skin, cell dissociation, and loss of desmoglein 1 (Dsg 1)–mediated binding probed by laser tweezers. These changes were accompanied by interference with Rho A activation and reduction of Rho A activity. Pemphigus IgG–triggered keratinocyte dissociation and Rho A inactivation were p38 mitogen-activated protein kinase dependent. Specific activation of Rho A by cytotoxic necrotizing factor-y abolished all pemphigus-triggered effects, including keratin retraction and release of Dsg 3 from the cytoskeleton. These data demonstrate that Rho A is involved in the regulation of desmosomal adhesion, at least in part by maintaining the cytoskeletal anchorage of desmosomal proteins. This may open the possibility of pemphigus treatment with the epidermal application of Rho A agonists.

Tumor necrosis factor (TNF) elicits its biological activities by stimulation of two receptors, TNFR1 and TNFR2, both belonging to the TNF receptor superfamily. Whereas TNFR1-mediated signal transduction has been intensively studied and is understood in detail, especially with respect to activation of the classical NFB pathway, cell death induction, and MAP kinase signaling, TNFR2-associated signal transduction is poorly defined. Here, we demonstrate in various tumor cell lines and primary T-cells that TNFR2, but not TNFR1, induces activation of the alternative NFB pathway. In accord with earlier findings demonstrating that only membrane TNF, but not soluble TNF, properly activates TNFR2, we further show by use of TNFR1-and TNFR2-specific mutants of soluble TNF and membrane TNF that soluble ligand trimers fail to activate the alternative NFB pathway. In accord with the known inhibitory role of TRAF2 in the alternative NFB pathway, TNFR2-, but not TNFR1-specific TNF induced depletion of cytosolic TRAF2. Thus, we identified activation of the alternative NFB pathway as a TNF signaling effect that can be specifically assigned to TNFR2 and membrane TNF. Tumor necrosis factor (TNF)2 is a highly pleiotropic cytokine and the prototypic member of the phylogenetically conserved TNF ligand family (1, 2). TNF, as well as other members of this cytokine family, is a type II transmembrane protein, which selfassembles into noncovalently bound trimers (1, 2). TNF occurs also as a soluble trimeric protein, which is derived from the transmembrane form by limited proteolysis (1, 2). TNF is mainly released from activated macrophages and T-cells, but it can also be produced by a variety of other cell types, especially after contact with bacterial products. TNF interacts with two receptors, TNFR1 and TNFR2, which both belong to the TNF receptor superfamily. Whereas TNFR1 is constitutively expressed in most cell types, TNFR2 is typically found on immune and endothelial cells (3). Remarkably, soluble and transmembrane TNF differ in their capability to stimulate signaling via TNFR1 and TNFR2. Whereas transmembrane TNF (memTNF) triggers signaling potently via both TNF receptors, soluble TNF trimers (sTNF) only activate TNFR1 robustly and show none or only limited activity on TNFR2 (4).TNFR1 contains a death domain (DD) and utilizes this protein-protein interaction domain to recruit intracellular signaling proteins involved in the activation of proinflammatory pathways, but also in cell death induction. For example, activation of the classical NFB pathway and the various MAP kinases by TNFR1 rely on recruitment of the DD-containing serine-threonine kinase RIP, the DD-containing adaptor protein TRADD, and a complex of the TRADD-interacting TRAF2 protein with cIAP1 and cIAP2 (3,5,6). Notably, RIP has also been implicated in TNFR1-induced necrosis, and TRADD, together with FADD and caspase-8, is crucially involved in TNFR1-mediated apoptosis (3). In general, the signaling mechanisms utilized by TNFR1 are biochemically well understood and the i...

Taken together, we provide first evidence for positive adhesiotropy as a new cardiac function of sympathetic signaling. Positive adhesiotropy is dependent on Pg phosphorylation at S665 by protein kinase A. This mechanism may be of high medical relevance because loss of junctional Pg is a hallmark of arrhythmogenic cardiomyopathy.

In pemphigus vulgaris, a life-threatening autoimmune skin disease, epidermal blisters are caused by autoantibodies primarily targeting desmosomal cadherins desmoglein 3 (DSG3) and DSG1, leading to loss of keratinocyte cohesion. Due to limited insights into disease pathogenesis, current therapy relies primarily on nonspecific long-term immunosuppression. Both direct inhibition of DSG transinteraction and altered intracellular signaling by p38 MAPK likely contribute to the loss of cell adhesion. Here, we applied a tandem peptide (TP) consisting of 2 connected peptide sequences targeting the DSG adhesive interface that was capable of blocking autoantibody-mediated direct interference of DSG3 transinteraction, as revealed by atomic force microscopy and optical trapping. Importantly, TP abrogated autoantibody-mediated skin blistering in mice and was effective when applied topically. Mechanistically, TP inhibited both autoantibody-induced p38 MAPK activation and its association with DSG3, abrogated p38 MAPK-induced keratin filament retraction, and promoted desmosomal DSG3 oligomerization. These data indicate that p38 MAPK links autoantibody-mediated inhibition of DSG3 binding to skin blistering. By limiting loss of DSG3 transinteraction, p38 MAPK activation, and keratin filament retraction, which are hallmarks of pemphigus pathogenesis, TP may serve as a promising treatment option.