Distinct glycosyltransferases synthesize E-selectin ligands in human vs. mouse leukocytes

Abstract

The binding of selectins to carbohydrate epitopes expressed on leukocytes is the first step in a multi-step cell adhesion cascade that controls the rate of leukocyte recruitment at sites of inflammation. The glycans that function as selectin-ligands are post-translationally synthesized by the serial action of Golgi resident enzymes called glycosyltransferases (glycoTs). Whereas much of our current knowledge regarding the role of glycoTs in constructing selectin-ligands comes from reconstituted biochemical investigations or murine models, tools to assess the impact of these enzymes on the human ligands are relatively underdeveloped. This is significant since the selectin-ligands, particularly those that bind E-selectin, vary between different leukocyte cell populations and they are also different in humans compared with mice. To address this shortcoming, a recent study by Buffone et al. (2013) outlines a systematic strategy to knockdown upto three glycoTs simultaneously in human leukocytes. The results suggest that the fucosyltransferases (FUTs) regulating selectin-ligand synthesis may be species-specific. In particular, they demonstrate that FUT9 plays a significant role during human, but not mouse, leukocyte-endothelial interactions. Overall, this article discusses the relative roles of the FUTs during human L-, E-, and P-selectin-ligand biosynthesis, and the potential that the knockdown strategy outlined here may assess the role of other glycoTs in human leukocytes also.

ST3Gal-4 is the primary sialyltransferase regulating the synthesis of E-, P-, and L-selectin ligands on human myeloid leukocytes

“…These are potentially expressed on CD43, CD44, L-selectin, CD18-integrin Mac-1, and gangliosides. 4,7 For example, in the case of GSLs, E-selectin binds gangliosides bearing non-sLe X epitopes. 45 Regardless of the scaffold protein/lipid, our data suggest that a single enzyme ST3Gal-4 may a(2,3)sialylate a majority of these physiologically relevant human E-selectin ligands.…”

“…4,7 This is particularly relevant for the E-selectin ligands because P-selectin glycoprotein ligand-1 (PSGL-1, CD162) is the major ligand for L-and P-selectin in both humans and mice, and the glycoTs constructing functional selectin ligand(s) on this glycoprotein are similar in both species. 7,8 With regard to E-selectin, however, human but not mouse granulocyte rolling on this selectin is insensitive to pronase treatment. 9,10 Thus, protease-insensitive gangliosides may be physiological E-selectin ligands that are unique to humans.…”

Glycosphingolipids on Human Myeloid Cells Stabilize E-Selectin–Dependent Rolling in the Multistep Leukocyte Adhesion Cascade

“…P- (CD62P), E- (CD62E) and L-selectin (CD62L) constitute a family of C-type lectins that initiate the multi-step process of leukocyte recruitment at sites of inflammation 1-3 . Among these, P- and E-selectin are expressed on stimulated endothelial cells and they mediate the direct capture of leukocytes to the inflamed vessel.…”

“…Whereas only 2% of the overall O-glycans of human PSGL-1 contain the sLe X core-2 O-glycan 10 , in myeloid cells, this structure constitutes ~20% of the carbohydrates at Thr-57 on the N-terminus of this glycoprotein 11 . With regard to E-selectin, the physiological carbohydrate ligands likely differ between mice and humans 1, 3 . Notably, the rolling of mouse neutrophils on E-selectin is protease sensitive but this is not the case for human leukocytes 12, 13 .…”

“…Notably, the rolling of mouse neutrophils on E-selectin is protease sensitive but this is not the case for human leukocytes 12, 13 . Further, while the E-selectin ligands in mouse neutrophils have largely been identified to include PSGL-1, E-selectin Ligand-1 (ESL-1) and CD44 14 , the major players in humans remain unknown 1, 3 . In this regard, PSGL-1 is a relatively minor ligand for human E-selectin and there is no homolog for ESL-1 in humans 1, 15 .…”