Glycan analysis of human neutrophil granules implicates a maturation-dependent glycosylation machinery

Abstract

Protein glycosylation is essential to trafficking and immune functions of human neutrophils. During granulopoiesis in the bone marrow, distinct neutrophil granules are successively formed. Distinct receptors and effector proteins, many of which are glycosylated, are targeted to each type of granule according to their time of expression, a process called "targeting by timing." Therefore, these granules are time capsules reflecting different times of maturation that can be used to understand the glycosylation process during granulopoiesis. Herein, neutrophil subcellular granules were fractionated by Percoll density gradient centrifugation, and N- and O-glycans present in each compartment were analyzed by LC-MS. We found abundant paucimannosidic N-glycans and lack of O-glycans in the early-formed azurophil granules, whereas the later-formed specific and gelatinase granules and secretory vesicles contained complex N- and O-glycans with remarkably elongated N-acetyllactosamine repeats with Lewis epitopes. Immunoblotting and histochemical analysis confirmed the expression of Lewis X and sialyl-Lewis X in the intracellular granules and on the cell surface, respectively. Many glycans identified are unique to neutrophils, and their complexity increased progressively from azurophil granules to specific granules and then to gelatinase granules, suggesting temporal changes in the glycosylation machinery indicative of "glycosylation by timing" during granulopoiesis. In summary, this comprehensive neutrophil granule glycome map, the first of its kind, highlights novel granule-specific glycosylation features and is a crucial first step toward a better understanding of the mechanisms regulating protein glycosylation during neutrophil granulopoiesis and a more detailed understanding of neutrophil biology and function.

Keywords: LacNAc; Lewis epitope; N-glycan; N-linked glycosylation; O-glycan; glycosylation; granule; liquid chromatography; mass spectrometry (MS); neutrophil; plasma membrane; targeting by timing.