Key Points• Endothelial endoglin has a regulatory role in leukocyte trafficking through vascular endothelia.• Leukocytes and endothelial cells interact via integrin receptors and endoglin, being this cell adhesion process stimulated by inflammatory stimuli.Human endoglin is an RGD-containing transmembrane glycoprotein identified in vascular endothelial cells. Although endoglin is essential for angiogenesis and its expression is up-regulated in inflammation and at sites of leukocyte extravasation, its role in leukocyte trafficking is unknown. This function was tested in endoglin heterozygous mice (Eng ؉/؊ ) and their wild-type siblings Eng ؉/؉ treated with carrageenan or LPS as inflammatory agents. Both stimuli showed that inflammation-induced leukocyte transendothelial migration to peritoneum or lungs was significantly lower in Eng ؉/؊ than in Eng ؉/؉ mice. Leukocyte transmigration through cell monolayers of endoglin transfectants was clearly enhanced in the presence of endoglin. Coating transwells with the RGD-containing extracellular domain of endoglin, enhanced leukocyte transmigration, and this increased motility was inhibited by soluble endoglin. Leukocytes stimulated with CXCL12, a chemokine involved in inflammation, strongly adhered to endoglincoated plates and to endoglin-expressing endothelial cells. This endoglin-dependent adhesion was abolished by soluble endoglin, RGD peptides, the anti-integrin ␣5␤1 inhibitory antibody LIA1/2 and the chemokine receptor inhibitor AMD3100. These results demonstrate for the first time that endothelial endoglin interacts with leukocyte integrin ␣5␤1 via its RGD motif, and this adhesion process is stimulated by the inflammatory chemokine CXCL12, suggesting a regulatory role for endoglin in transendothelial leukocyte trafficking. (Blood. 2013;121(2):403-415) IntroductionThe vascular endothelium controls the transit of white blood cells into and out of the bloodstream. The migration of leukocytes involves the adhesive interaction of cell surface receptors with ligands expressed on endothelial cells in a process regulated by inflammatory stimuli. Stromal-derived factor 1 (SDF1␣), renamed CXCL12, is a potent chemoattractant for a variety of cells including lymphocytes, monocytes, dendritic cells, and hematopoietic stem cells. 1 CXCL12 and its receptor CXCR4 play relevant roles in immune and inflammatory responses, including leukocyte migration and recruitment, as well as integrin-dependent adhesion and transendothelial migration. 1 CXCL12 is a critical activator of endothelial progenitors by inducing a proangiogenic phenotype and increasing migration and rolling mediated by ␣4 and ␣M integrin subunits. 2 The process of leukocyte migration through the endothelial cell monolayer involves an interaction between leukocytes' integrins and endothelial-cell receptors, both acting as adhesion molecules. Integrins most relevant to leukocytes belong to the ␤ 1 -integrin and the ␤ 2 -integrin subfamilies. Classic chemoattractants and chemokines are the most powerful physiologic activat...

Endoglin is a transmembrane glycoprotein that acts as an auxiliary receptor for transforming growth factor-b (TGF-b) and modulates cellular responses to this pleiotropic cytokine. Endoglin is strongly expressed in endothelial cells, where it appears to exert a crucial role in vascular development and angiogenesis. Two endoglin isoforms (L and S), differing in their cytoplasmic domains, have been previously characterized in human tissues. We now demonstrate the existence of similar L-and Sendoglin variants in murine tissues with 47 and 35 amino acids, respectively, in their cytoplasmic tail. RT-PCR analysis showed that L is the predominant endoglin isoform expressed in mouse tissues, although S-endoglin mRNA is significantly expressed in liver and lung, as well as in endothelial cell lines. Furthermore, a protein of size equivalent to recombinant S-endoglin expressed in mammalian cells was detected in mouse endothelial cells by Western blot analysis. L-and S-endoglin isoforms can form disulfide-linked heterodimers, as demonstrated by cotransfection of L-and S-endoglin constructs. To address the role of S-endoglin in vivo, an S-Eng þ transgenic mouse model that targets S-endoglin expression to the endothelium was generated. The lethal phenotype of endoglin-null (Eng À/À ) mice was not rescued by breeding S-Eng þ transgenic mice into the endoglin-null background. S-Eng þ mice exhibited reduced tumor growth and neovascularization after transplantation of Lewis lung carcinoma cells. In addition, S-Eng þ mice showed a drastic inhibition of benign papilloma formation when subjected to two-stage chemical skin carcinogenesis. These results point to S-endoglin as an antiangiogenic molecule, in contrast to L-endoglin which is proangiogenic.

Abstract-The goal of the present study was to evaluate the role of endoglin, a transforming growth factor-␤1 (TGF-␤1) accessory receptor, in the pathogenesis of renal fibrosis. This was achieved by testing a model of tubulo-interstitial fibrosis induced by unilateral ureteral obstruction in endoglin heterozygous (Eng ϩ/-) mice. Northern and Western blot analysis revealed that endoglin expression in kidneys of these mice was significantly reduced compared with Eng ϩ/ϩ littermates. Pronounced interstitial fibrosis induced by ureteral obstruction was confirmed histologically by Masson's trichromic staining and by increased immunostaining for fibronectin and laminin without significant differences between Eng ϩ/-and Eng ϩ/ϩ mice. Ureteral obstruction induced significant increases in ␣2(I) and ␣1(IV) collagen, fibronectin, and TGF-␤1 mRNA levels, as well as in total kidney collagen but changes were similar in Eng ϩ/-and Eng ϩ/ϩ mouse kidneys. Ureteral obstruction also induced a 2-fold increase in endoglin mRNA levels in both Eng ϩ/ϩ mice and Eng ϩ/-mice, which was confirmed by Western blot analysis. Thus, the present study provides clear evidence that endoglin is upregulated in the kidneys of mice with interstitial fibrosis induced by unilateral ureteral ligation. However, Eng ϩ/-mice do not show any changes in the severity of renal disease induced in this model when compared with normal mice, suggesting that the absolute level of endoglin is not critical for the effects of TGF-␤1 in the renal fibrosis process.