P-selectin is an endothelial cell adhehion molecule which mediates the binding of neutrophils and monocytes. Its appearance at the c d l surface can be induced within minutes by histamine and thrombin which rapidly stimul'3te the transport of 1'-selectin from intracel-M a r storage granules to the plasma nie~nbr~ine. We have recently found c3 second regulation mechanism for 1'-selectin o n moiisc cniiothelioma cells. Like E-selectin, 1'-selectin is also regulated at the level of transcription. Both selectins are induced by 1-PS or TNF-n with a maximal expression level at thc c c 4 s u r f x e 3-4 h after stimulation. Here, we report that this up-regulation of the synthesis ot P -s c k t i n also occurs iri i1i710 i n endothelium of the mouse. Analysing brain tissue, which is devoid of constitutive expression o f 1'-selectin, we found that LPS and also TNF-(L strongly induce the expression of 1'-selectin on all venular endothelial cells of the 1eptomcmingt.s dnd, at a weaker level, on some blood vessels of the brain parenchyma. Induction ot P-selectin expression could also be observed in tissues, such a s the tongue, where P-sclectin is constitutively expressed on smnll venules but only rarely on larger venules. Strong staining for P-selectin on endotheiium of all large venules was observed in tissues of LI'S m d TNF-u treated animals and staining for this newly synthesized P-selectin was enriched at the luminal surface o f these cells. Comparison of the expression pattern of LPS-induced 1'-selectin and E-selectin on blood vessels of the leptomeninges revealed that both selectins were co-induced on venular endothelium while on most arterial endothelial cells only E-selectin and not P-selectin was induced. Thus, in vivo, endothelial cells can differ in their capacity to upregulate the expression of the two endothelial selectins

An early step in the formation of the extraembryonic and intraembryonic vasculature is endothelial cell differentiation and organization in blood islands and vascular structures. This involves the expression and function of specific adhesive molecules at cell-to-cell junctions. Previous work showed that endothelial cells express a cell-specific cadherin (vascular endothelial [VE]-cadherin, or 7B4/cadherin-5) that is organized at cell-to-cell contacts in cultured cells and is able to promote intercellular adhesion. In this study, we investigated whether VE-cadherin could be involved in early cardiovascular development in the mouse embryo. We first cloned and sequenced the mouse VE-cadherin cDNA. At the protein level, murine VE-cadherin presented 75% identity (90%, considering conservative amino acid substitutions) with the human homologue. Transfection of murine VE-cadherin cDNA in L cells induced Ca(++)-dependent cell-to-cell aggregation and reduced cell detachment from monolayers. In situ hybridization of adult tissues showed that the murine molecule is specifically expressed by endothelial cells. In mouse embryos, VE-cadherin transcripts were detected at the very earliest stages of vascular development (E7.5) in mesodermal cells of the yolk sac mesenchyme. At E9.5, expression of VE-cadherin was restricted to the peripheral cell layer of blood islands that gives rise to endothelial cells. Hematopoietic cells in the center of blood islands were not labeled. At later embryonic stages, VE-cadherin transcripts were detected in vascular structures of all organs examined, eg, in the ventricle of the heart, the inner cell lining of the atrium and the dorsal aorta, in intersomitic vessels, and in the capillaries of the developing brain. A comparison with flk-1 expression during brain angiogenesis revealed that brain capillaries expressed relatively low amounts of VE-cadherin. In the adult brain, the level of VE-cadherin transcript was further reduced. By immunohistochemistry, murine VE-cadherin protein was detected at cell-to-cell junctions of endothelial cells. Overall, these data demonstrate that VE-cadherin is an early, constitutive, and specific marker of endothelial cells. This distinguishes this molecule from other cadherins and suggests that its expression is associated with the early assembly of vascular structures.

Neutrophils enter sites of inflammation by crossing the endothelial lining of the blood vessel wall. VE-cadherin is an endothelial specific, homophilic adhesion molecule located at the lateral cell surface. We have generated a monoclonal antibody against mouse VE-cadherin which inhibits electrical resistance of endothelial cell monolayers in vitro as well as aggregation of VE-cadherin transfected cells. In vivo, this antibody was found to increase vascular permeability and to accelerate the entry of neutrophils into chemically inflamed mouse peritoneum. Thus, VE-cadherin is essential for the integrity of the endothelial barrier in vivo. Our data suggest that opening of VE-cadherin mediated endothelial cell contacts may be a relevant step during neutrophil extravasation.