Research Highlights

Leukocyte recruitment: Selectin the way forward

Fluorescent immunohistochemistry of rat kidney sections showing E-selectin expression after treatment with a proinflammatory agent.

Selectins (see here and here) support leukocyte rolling along endothelial cells towards lymphatic organs and sites of inflammation by binding to oligosaccharide ligands. The selectin cell adhesion molecules are classified according to the cell type that predominantly expresses them; thus, activated endothelial cells express E- and P-selectin, activated platelets express P-selectin, and leukocytes express L-selectin. P-selectin is known to cluster in clathrin-coated pits on the cell surface leading to locally increased P-selectin concentrations that cause slower rolling on endothelial cells. Furthermore, as endocytotic vesicles can readily be formed from clathrin-coated pits, inflammatory processes are quickly limited by the switch from P-selectin clustering to internalization. It was thought that E-selectin associates primarily with lipid rafts; and now in Blood, Setiadi and McEver elucidate in detail the partitioning of E-selectin membrane subdomains.

The authors used human umbilical vein endothelial cells (HUVEC) and Chinese hamster ovary cells expressing E-selectin (E-CHOCs) to investigate E-selectin membrane distribution and leukocyte rolling. E-selectin was rapidly internalized from the surfaces of both E-CHOCs and HUVECs. Both endocytosis of E-selectin and the number of rolling leukocytes were markedly reduced on E-CHOCs expressing E-selectin that lacked a cytoplasmic tail. Both effects also occurred upon treatment of the cells with hypertonic medium that disassembles clathrin lattices. These findings indicate that E-selectin is internalized via clathrin-coated vesicles, and that E-selectin clustering in clathrin-coated pits is necessary for leukocyte rolling; furthermore, both processes require the cytoplasmic domain of E-selectin.

Interestingly, Setiadi and McEver observed residual leukocyte rolling on HUVECs in hypertonic medium. When they fractionated the cell membranes, the authors found E-selectin in lipid rafts from HUVEC but not from E-CHOCs. E-selectin colocalized with the lipid-raft components flotillin and ganglioside GM1, but not with caveolin 1. The hypothesis that lipid raft E-selectin contributes to leukocyte rolling on endothelial cells was confirmed by an observed reduction in the quantity of leukocytes rolling on HUVECs treated with methyl--cyclodextrin, which disturbs lipid rafts. However, E-selectin is not endocytosed by lipid rafts as endocytosis was eliminated by hypertonic medium, which disrupts clathrin lattices but not lipid rafts.

This study greatly enhances our knowledge about leukocyte rolling regulation and shows that leukocyte rolling along E-selectin on endothelial cells is regulated akin to P-selectin-governed leukocyte rolling. It remains an open question as to whether selectins interact in cis with glycans on the cell surface (see here), which may further contribute to rolling regulation.

Author: Mirko von Elstermann