IntroductionPlasma membrane contains small organized microdomains (lipid rafts) in which restricted repertoires of proteins are arranged together. 1,2 In resting cells, lipid rafts are estimated to be around 100 nm in diameter, including a few dozen proteins, and are distributed randomly on the cell surface, covering up to 50% of the plasma membrane. Upon cell activation, raft domains coalesce, recruiting and excluding different receptors, and allowing the proper organization of signaling complexes for efficient signal transduction. 1,2 Tetraspanins comprise a large number of small palmitoylated polypeptides that span the plasma membrane 4 times, [3][4][5][6] and form microdomains that contain a restricted repertoire of proteins. Biochemically, they share some properties with lipid rafts, but tetraspanin microdomains are based on protein-protein interactions. [7][8][9][10] Tetraspanins have a highly conserved structure with a short and a large extracellular loop (LEL) where 2 or 3 disulfide bonds can be formed. 11 This large loop interacts noncovalently with other tetraspanins and transmembrane proteins, including integrins and adhesion receptors of the immunoglobulin (Ig) superfamily. Although all mammalian cells express different tetraspanins, genetic approaches have been elusive and their function has not yet been fully elucidated. However, their role in antigen presentation and sperm-egg binding has been recently underscored. [12][13][14][15][16][17][18][19][20] The association of certain plasma membrane proteins to the cortical actin cytoskeleton is critical for their proper localization and function. Thus, the concentration of selectins and their ligands on the tip of microvilli 21,22 both at the leukocyte and the apical surface of endothelial cells favors their interaction during the rolling phase of leukocyte extravasation. Likewise, vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1), which are relevant in the subsequent leukocyte firm adhesion step, are also displayed anchored to actin through ezrin-radixin-moesin proteins (ERMs) 23,24 at the apical surface on endothelial cells. Upon leukocyte firm adhesion, the engagement of VCAM-1 and ICAM-1 triggers the reorganization of the endothelial cortical actin cytoskeleton, building up a 3-dimensional docking structure that prevents the detachment of leukocytes by shear stress. 22,23 Here, we show that ICAM-1 and VCAM-1 are included in tetraspanin microdomains that regulate their membrane expression and the efficient adhesive function necessary for proper leukocyte transendothelial migration under flow conditions. Materials and methods Cells and cell culturesHuman umbilical vein endothelial cells (HUVECs) were obtained and cultured as previously described. 25 Cells were used up to the third passage in all assays. To activate HUVECs, tumor necrosis factor-␣ (TNF-␣; 20 ng/mL)(R&D Systems, Minneapolis, MN) was added to the culture media 20 hours before the assays were performed. T lymphoblasts were derived The online vers...

The immunological synapse (IS) is a cell-cell junction formed between CD4(+) T cells and dendritic cells (DCs). Here we show in vitro and in vivo that IS formation inhibits apoptosis of DCs. Consistent with these results, IS formation induced antiapoptotic signaling events, including activation of the kinase Akt1 and localization of the prosurvival transcription factor NF-kappaB and the proapoptotic transcription factor FOXO1 to the nucleus and cytoplasm, respectively. Inhibition of phosphatidylinositol 3-OH kinase and Akt1 partially prevented the antiapoptotic effects of IS formation. Direct stimulation of the IS component CD40 on DCs leads to the activation of Akt1, suggesting the involvement of this receptor in the antiapoptotic effects observed upon IS formation.

ADAM17/TACE is a metalloproteinase responsible for the shedding of the proinflammatory cytokine TNF-α and many other cell surface proteins involved in development, cell adhesion, migration, differentiation, and proliferation. Despite the important biological function of ADAM17, the mechanisms of regulation of its metalloproteinase activity remain largely unknown. We report here that the tetraspanin CD9 and ADAM17 partially co-localize on the surface of endothelial and monocytic cells. In situ proximity ligation, co-immunoprecipitation, crosslinking, and pull-down experiments collectively demonstrate a direct association between these molecules. Functional studies reveal that treatment with CD9-specific antibodies or neoexpression of CD9 exert negative regulatory effects on ADAM17 sheddase activity. Conversely, CD9 silencing increased the activity of ADAM17 against its substrates TNF-α and ICAM-1. Taken together, our results show that CD9 associates with ADAM17 and, through this interaction, negatively regulates the sheddase activity of ADAM17.