Defenders and Challengers of Endothelial Barrier Function

Abstract

Regulated vascular permeability is an essential feature of normal physiology and its dysfunction is associated with major human diseases ranging from cancer to inflammation and ischemic heart diseases. Integrity of endothelial cells also play a prominent role in the outcome of surgical procedures and organ transplant. Endothelial barrier function and integrity are regulated by a plethora of highly specialized transmembrane receptors, including claudin family proteins, occludin, junctional adhesion molecules (JAMs), vascular endothelial (VE)-cadherin, and the newly identified immunoglobulin (Ig) and proline-rich receptor-1 (IGPR-1) through various distinct mechanisms and signaling. On the other hand, vascular endothelial growth factor (VEGF) and its tyrosine kinase receptor, VEGF receptor-2, play a central role in the destabilization of endothelial barrier function. While claudins and occludin regulate cell-cell junction via recruitment of zonula occludens (ZO), cadherins via catenin proteins, and JAMs via ZO and afadin, IGPR-1 recruits bullous pemphigoid antigen 1 [also called dystonin (DST) and SH3 protein interacting with Nck90/WISH (SH3 protein interacting with Nck)]. Endothelial barrier function is moderated by the function of transmembrane receptors and signaling events that act to defend or destabilize it. Here, I highlight recent advances that have provided new insights into endothelial barrier function and mechanisms involved. Further investigation of these mechanisms could lead to the discovery of novel therapeutic targets for human diseases associated with endothelial dysfunction.

Keywords: adherens junctions; cell adhesion molecules; endothelial dysfunction; gap junctions; vascular endothelial growth factor A; vascular permeability.

Figure 1

Mechanisms linked to endothelial dysfunction: several key mechanisms that promote endothelial dysfunction and vascular damage are shown. Also, shown are the major endothelial responses triggered by these factors Nitric oxide (NO).

Figure 2

Transmembrane receptors involved in the endothelial cell–cell junction and their key cytoplasmic-binding partners. Claudin family proteins, occludin, junctional adhesion molecules (JAMs), vascular endothelial (VE)-cadherin an immunoglobulin (Ig) and proline-rich receptor-1 (IGPR-1) are major receptors in endothelial cells that regulate endothelial cell–cell junctions and barrier. The core mechanism associated with the function of these receptors involves with their ability to recruit specific signaling proteins that signal to strength the cell–cell junctions. Zonula occludens (ZO 1–3), catenin proteins (α-catenin, β-catenin, and γ-catenin), polarity protein-3 and 6 (PAR3/6), afadin (AF6), bullous pemphigoid antigen 1 [BPAG1 also called dystonin and SH3 protein interacting with Nck90 (SPIN90)/WISH (SH3 protein interacting with Nck)] are key substrates involved with these receptors.

Figure 3

Regulation of occludin mediated cell–cell junction assembly and organization. Occludin via its coiled-coil domain recognizes guanylate kinase (GUK) domain, PDZ, and SH3 domain (43), that further recruits Cingulin and ZONAB that participate in the formation and regulation of the tight junction and paracellular permeability barrier. C-terminal of occludin is subject to phosphorylation at serine/threonine and tyrosine residues by multiple serine/threonine kinases and tyrosine kinases, which in part regulate occludin binding with ZO proteins and its tight junctional function (see the text).

Figure 4

Vascular endothelial growth factor (VEGF)-induced VEGF receptor-2 (VEGFR-2) activation signal transduction that leads to destabilization of cell–cell junctions. Stimulation of VEGFR-2 by VEGF results in the kinase activation of VEGFR-2 and recruitment of diverse signaling proteins to VEGFR-2. The key VEGFR-2 signaling proteins whose activity are linked to vascular permeability include Src family kinases, phosphoinositide 3-kinase (PI3 kinase), and phospholipase Cγ1 (PLCγ1). Src kinases in turn can phosphorylate VE-cadherin and VE-cadherin-associated proteins such as β-catenin and p120 leading destabilization of VE-cadherin mediated endothelial barrier function. Activation of PI3 kinase and PLCγ1 can lead to phosphorylation of eNOS and production of nitric oxide (NO) that leads to interruption of endothelial junctions.

Figure 5

Proposed model of immunoglobulin (Ig) and proline-rich receptor-1 (IGPR-1) mediated regulation of endothelial barrier. (A) Trans-dimeric IGPR-1 undergoes serine phosphorylation at multiple sites. IGPR-1 through its proline-rich motifs recruits SH3 containing proteins, bullous pemphigoid antigen 1 (BPAG1), and SH3 protein interacting with Nck90 (SPIN90)/WISH (SH3 protein interacting with Nck). Interaction of IGPR-1 with BPAG1 and SPIN90 links IGPR-1 to actin fibril assembly, intermediate filament formation, microtubule cytoskeleton networks, and vimentin filament assembly (see text). (B) The conventional proline-rich motifs and the proline-rich motifs on the cytoplasmic domain of IGPR-1 are shown.

Figure 6

Bullous pemphigoid antigen 1 (BPAG1) is a multidomain protein with various alternatively spliced variants. BPAG1 is a large protein with 7,570 amino acids with multiple domains including N-terminus actin-binding domain, followed by plakin domain which consists of 4–8 spectrin repeats interrupted by a Src-homology3 (SH3) domain. The C-terminal of BPAG1 is composed of additional plakin repeat domains and intermediate filaments binding domain. Various alternatively spliced variants of BPAG1 are also shown.