Cell Biology

A New Way to Cluster

Science Signaling  11 Nov 2008:
Vol. 1, Issue 45, pp. ec381
DOI: 10.1126/scisignal.145ec381

The organization of molecules in the plasma membrane plays important roles in mediating signals from the extracellular environment to cytoplasmic compartments. Lipid rafts, which are enriched in cholesterol and sphingolipids, represent one means of grouping signaling molecules at the membrane. Now, Barreiro et al. provide evidence for a functional role for a distinct microdomain called tetraspanin-enriched microdomains (TEMs) in endothelial cell-leukocyte adhesion. Tetraspanins are small transmembrane proteins that associate with various membrane proteins and with cytoplasmic signaling molecules. A TEM on endothelial cells that contained the tetraspanins, vascular cell adhesion molecule-1 (VCAM-1), and intercellular cell adhesion molecule-1 (ICAM-1) was involved in the initial interaction between leukocytes and endothelial cells. In blood vessels, leukocytes adhere to endothelial cells and then can extravasate into the tissues. ICAM-1 and VCAM-1 are adhesion molecules on the endothelial cells that interact with ligands on the leukocytes. Immunocytochemistry experiments indicated that when leukocytes expressing one or the other ligand for these two adhesion receptors were introduced to endothelial cell cultures, both adhesion receptors were recruited to docking structures on endothelial cells. Fluorescence resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) experiments suggested that direct interactions between VCAM-1 and ICAM-1 did not appear to be required for clustering. Instead, it appeared that tetraspanins recruited the adhesion receptors, as the addition of beads coated with CD9 or CD151 antibodies was sufficient to trigger formation of clusters containing both adhesion receptors and both tetraspanins, even in the absence of leukocytes. Dubbing these microdomains endothelial adhesive platforms (EAPs), the authors used the FRET and FLIM techniques to show selectivity in the interactions between tetraspanins and adhesion receptors. CD9 preferentially interacted with ICAM-1, and CD151 interacted primarily with VCAM-1. In addition, CD9 and CD151 could interact in a homophilic or heterophilic manner as CD9-CD9, CD151-CD151, and CD9-CD151 pairs were detected. Scanning electron microscopy revealed that blocking CD9 function increased receptor spacing and reduced the percentage of clustered receptors, which the authors speculated could decrease leukocyte adhesiveness. Scanning electron microscopy suggests that EAPs were preformed nanoclusters that were organized around the docking structures at the site of adhering leukocytes. In their commentary, Ley and Zhang note that EAP formation observed by the authors may differ when shear stress is present during leukocyte migration, such as when leukocytes roll before adhering. In addition, they speculate that different models of transmigration, including ones that involve EAPs, may be relevant to distinct combinations of endothelial cell and leukocyte subtypes and vascular beds.

O. Barreiro, M. Zamai, M. Yáñez-Mó, E. Tejera, P. López-Romero, P. N. Monk, E. Gratton, V. R. Caiolfa, F. Sánchez-Madrid, Endothelial adhesion receptors are recruited to adherent leukocytes by inclusion in preformed tetraspanin nanoplatforms. J. Cell Biol. 183, 527-542 (2008). [Abstract] [Full Text]

K. Ley, H. Zhang, Dances with leukocytes: How tetraspanin-enriched microdomains assemble to form endothelial adhesive platforms. J. Cell Biol. 183, 375-376 (2008). [Abstract] [Full Text]