An important molecular mechanism underlying transcellular permeability is macromolecule transcytosis via caveoli – specialised plasmalemmal vesicles containing caveolin-1. The involvement of caveolin-1 in regulating cardiovascular functions associated with endothelial barrier properties has been demonstrated through studies using transgenic and knockout animals (Refs 1
). Upon binding to plasma proteins (of size >3 nm, e.g. albumin), the cell-surface docking protein gp60 interacts with caveolin-1 and signalling intermediaries, including a G protein and Src-family tyrosine kinase. This cascade results in the formation and release of albumin- and solute-containing caveolae from the apical membrane (Ref. 5
). The vesicles are subsequently transported to the basal membrane and release their contents through exocytosis (Ref. 5
). Recent ultrastructural evidence suggests that caveolae-like vesiculovacuolar organelles can interconnect to each other, forming secondary, grape-like structures that function as transmembrane channels for molecular trafficking across the cell (Ref. 6
). Receptors of endogenous permeability-enhancing agents have been identified on the surface of these channels, indicating the possibility of their participation in transcytosis-related endothelial permeability (Refs 7
Despite the potential contribution of transcytosis to the basal permeability of the endothelium, paracellular flux of plasma fluid and proteins through endothelial cell–cell junctions has been emphasised for its pathophysiological importance in vascular inflammation during disease and injury. Among the different types of junction structures in the vascular endothelium, the tight junction and adherens junction are the best characterised with respect to their function in mediating cell–cell adhesion and thus barrier properties. Briefly, the tight junction is a zipper-like structure formed at the cell–cell contact area by a group of transmembrane proteins primarily expressed in the blood–brain barrier and retinal microvasculature, including claudins, occludins and zonular occludins (ZO-1 and ZO-2). The adherens junction has been identified in nearly all types of vascular beds, especially in the peripheral microvasculature (). Its molecular structure is based on VE-cadherin (vascular endothelial cadherin), a transmembrane receptor whose extracellular domain homophilically binds to the extracellular domain of another VEcadherin molecule from an adjacent cell and whose intracellular domain is anchored to the cell cytoskeleton via a family of actin-binding proteins called catenins (α, β, γ and p120 catenins). The catenins not only serve as a structural linkage between VE-cadherin and the cytoskeleton, but also transduce biochemical signals for cell–cell communications (Refs 9
). Moreover, endothelial cells are tethered to the extracellular matrix through focal adhesions, which consist mainly of integrin transmembrane proteins and a family of actin-linking proteins including focal adhesion kinase (FAK), talin and paxillin (Ref. 5
) (). The stability of this junction–cytoskeleton complex withstands fluid shear stress and is essential in maintaining the endothelial barrier function.
Schematic diagram of microvascular endothelial barrier structure
Many inflammatory mediators are capable of disrupting the interendothelial junction assembly, thereby causing endothelial hyperpermeability. More in-depth molecular analyses suggest that the mechanism underlying inflammation-induced endothelial paracellular hyperpermeability involves phosphorylation, internalisation or degradation of the junctional molecules (Ref. 11
). In addition, the junction–cytoskeleton complex participates in other cellular processes including molecular scaffolding, intracellular trafficking, transcription and apoptosis that may directly or indirectly alter vascular barrier function (Ref. 12
). Regardless of the molecular details, however, essentially all permeability responses in the vascular endothelium are initiated with receptor occupancy followed by a series of intracellular signalling cascades, some of which are described below ().
Signal transduction in endothelial hyperpermeability