A genetic polymorphism in the human gene encoding connexin37 (CX37, encoded by GJA4, also known as CX37) has been reported as a potential prognostic marker for atherosclerosis. The expression of this gap-junction protein is altered in mouse and human atherosclerotic lesions: it disappears from the endothelium of advanced plaques but is detected in macrophages recruited to the lesions. The role of CX37 in atherogenesis, however, remains unknown. Here we have investigated the effect of deleting the mouse connexin37 (Cx37) gene (Gja4, also known as Cx37) on atherosclerosis in apolipoprotein E-deficient (Apoe(-/-)) mice, an animal model of this disease. We find that Gja4(-/-)Apoe(-/-) mice develop more aortic lesions than Gja4(+/+)Apoe(-/-) mice that express Cx37. Using in vivo adoptive transfer, we show that monocyte and macrophage recruitment is enhanced by eliminating expression of Cx37 in these leukocytes but not by eliminating its expression in the endothelium. We further show that Cx37 hemichannel activity in primary monocytes, macrophages and a macrophage cell line (H36.12j) inhibits leukocyte adhesion. This antiadhesive effect is mediated by release of ATP into the extracellular space. Thus, Cx37 hemichannels may control initiation of the development of atherosclerotic plaques by regulating monocyte adhesion. H36.12j macrophages expressing either of the two CX37 proteins encoded by a polymorphism in the human GJA4 gene show differential ATP-dependent adhesion. These results provide a potential mechanism by which a polymorphism in CX37 protects against atherosclerosis.

IntroductionThe immune surveillance of the body is carried out by lymphocytes constantly circulating from the blood to lymphoid or peripheral organs and back to the blood. Most lymphocytes extravasate through specialized vascular endothelium created by tissue microenvironment or inflammation. This suggests that a regional specialization of endothelial cells may control lymphocyte traffic. Postcapillary high endothelial venules (HEVs) are specialized sites along vessels where the migration of lymphocytes to lymphoid organs occurs. 1,2 Migration occurs in a multistep process involving rolling of lymphocytes along the vessel wall, adhesion, and transmigration. 3 The first and the second steps are well described and involve selectins, mucins, immunoglobulin (Ig) superfamily molecules, and integrins. 4,5 The last transmigration step is less well understood and may occur through a transcellular pathway or by the paracellular route. 6 Evidence to support the latter model comes from numerous studies suggesting that the transmigration of leukocytes involves the disruption of interendothelial junctions in a specific and localized manner. [7][8][9] Nevertheless, the molecular events underlying the transmigration process are still controversial because other studies have demonstrated that transendothelial migration may occur without a widespread disruption of tight or adherens junctions. 10,11 In this context, the molecules participating specifically in intercellular junctional complexes of endothelial cells may play a central role in regulating leukocyte transmigration and vascular functions. 12,13 Although all endothelial cells are involved in exchanges of material from blood to tissue, there is a great degree of tissue-specific specialization of vascular junctions. This heterogeneity was described more than 20 years ago when the first electron microscopy studies pointed out structural differences in junctional complexes in different endothelial cells. 14,15 Recently, the molecular characterization of proteins participating in intercellular junctional complexes has increased our understanding of vascular junction heterogeneity. The interendothelial adhesive structures include tight, adherens and gap junctions in which surface proteins such as occludin, claudins, cadherins, or connexins are specifically incorporated. [16][17][18][19] Interestingly, some members of these protein families-among them Claudin-5 and VE-cadherinhave been found to be specifically expressed by endothelial cells. 20,21 Both molecules are involved in vascular integrity and normal vascular function. 22,23 In addition, junctional proteins normally found outside the vascular system may participate to interendothelial junctions of specialized vessels in a tissue-specific manner. This is obvious for occludin, which is highly expressed by the brain vascular bed, but it is barely detectable in other interendothelial junctions. 24 These results led to the concept that the tissue-specific specialization of blood vessels may be mediated by the molecular archi...

PECAM-1/CD31 is a cell adhesion and signaling molecule that is enriched at the endothelial cell junctions. Alternative splicing generates multiple PECAM-1 splice variants, which differ in their cytoplasmic domains. It has been suggested that the extracellular ligand-binding property, homophilic versus heterophilic, of these isoforms is controlled by their cytoplasmic tails. To determine whether the cytoplasmic domains also regulate the cell surface distribution of PECAM-1 splice variants, we examined the distribution of CD31-EGFPs (PECAM-1 isoforms tagged with the enhanced green fluorescent protein) in living Chinese hamster ovary cells and in PECAM-1-deficient endothelial cells. Our results indicate that the extracellular, rather than the cytoplasmic domain, directs PECAM-1 to the cell-cell borders. Furthermore, coculturing PECAM-1 expressing and deficient cells along with transfection of CD31-EGFP cDNAs into PECAM-1 deficient cells reveal that this PECAM-1 localization is mediated by homophilic interactions. Although the integrin ␣v␤3 has been shown to interact with PECAM-1, this trans-heterophilic interaction was not detected at the borders of endothelial cells. However, based on cocapping experiments performed on proT cells, we provide evidence that the integrin ␣v␤3 associates with PECAM-1 on the same cell surface as in a cis manner.

No abstract

Gap junctions, formed by the connexin (Cx) protein family, are intercellular channels that permit the cytoplasmic exchange of ions and small metabolites between neighboring cells, a process called gap junction intercellular communication (GJIC). These channels possess unique properties, including distinctive permeabilities for various signaling molecules, which depend on the connexin member(s) that form them. Importantly, GJIC must be properly controlled as its misregulation might contribute to diseases. Morphological and functional studies have revealed 'gap junction-like' structures and cell-to-cell communication involving cells of the immune system. The connexins involved in such contacts have been partially identified in recent years. This review focuses on the potential physiological roles of gap junctions in the development and recruitment of leukocytes as well as in the regulation of the immune response. Furthermore, the importance of GJIC in immuno-inflammatory pathologies is illustrated in atherosclerosis.