Mechanisms of increased endothelial permeability.

The increase in endothelial permeability in response to inflammatory mediators such as thrombin and histamine is accompanied by reversible cell rounding and interendothelial gap formation, suggesting that the predominant transport pathway is a diffusive one (i.e., via cellular junctions (paracellular transport)). However, vesicle-mediated transport (i.e., via albumin-binding protein gp60) may also contribute significantly to the overall increase in permeability. Regulation of paracellular transport in endothelial cells is associated with modulation of actin-based systems, which anchor the cell to its neighbor or extracellular matrix, thus maintaining endothelial integrity. At the cell-cell junctions, actin is linked indirectly to the plasma membrane by linking proteins (e.g., vinculin, catenins, alpha-actinin) to cadherins, which function in homophilic intercellular adhesion. At endothelial focal contacts, the transmembrane receptors (integrins) for matrix proteins are linked to actin via linking proteins (i.e., vinculin, talin, alpha-actinin). In response to inflammatory mediators, second messengers signal two regulatory pathways, which modulate the actin-based systems, and can thus lead to impairment of the endothelial barrier integrity. One critical signal may be based on protein kinase C isoenzyme specific phosphorylation of linking proteins at the cell-cell and cell-matrix junctions. The increased phosphorylation is associated with actin reorganization, cell rounding, and increased paracellular transport. Another important event is the activation of myosin light chain kinase (MLCK), which causes an actin-myosin-based contraction that may lead to centripetal retraction of endothelial cells. Current research is being conducted at identification of protein substrates of protein kinase C isoenzymes, the specific role of their phosphorylation in barrier function, and determination of the precise role of MLCK in modulation of endothelial barrier function. Since mechanisms by which the increased permeability is returned to normal may be regulated at multiple levels (e.g., receptor desensitization, protein kinase C mediated negative feedback pathways, activation of protein phosphatases), it is also important to determine these cellular "off-switch" mechanisms.