An insight into the cells’ glycans and lectin-glycosensing sites

Abstract

Glycosylation produces a diverse and abundant repertoire of glycans that help protein folding and cellular secretion [1]. Glycoproteins often sprout sialic acid (alias N-acetyl neuraminic acid, abbreviated as Neu5Ac) at the termini of their Nand O-glycans in a process called sialylation [2]. Alterations to the normal function of the glycosylation machinery, including increased branching of N-linked glycans and sialylation, are major hallmarks of cancer progression. Particularly, increased sialylation on the cell surface induces tumor proliferation by promoting cell detachment from primary tumors through charge repulsion [3]. Despite the potential of increased or altered sialylation as diagnostic and prognostic biomarkers, there is still no early glycosensing platform that can be ubiquitously applied to detect glycans sialylation in diverse bio-samples with high selectivity and sensitivity. Further analyses have been recently found that the metabolic control of glycan’s sialylation is regulated by two factors: (1) the rate of flux through the sialic acid pathway and (2) the expression of an appropriate complement of sialyltransferase. However, none of these factors have been exploited with sufficient precision to achieve specific endpoints. Metabolic glycoengineering has been established as a very useful technique for the modulation of glycan sialylation structures on cell surfaces, and various constituents of the target sialoglycans are studied by lectins, a famous class of glycan-binding proteins [4]. Structurally, lectins have small or shallow binding sites, which results in relatively weak and non-covalent glycan interactions, often with binding constants in the millimolar range. However, the weak binding affinity and non-covalent characteristics of the lectin glyco-interactive sites can be enhanced dramatically by multivalent binding approaches [5]. Among these, the most powerful one is sensing the structural motifs and intensity of the accessible cell surface sialylation constellations [6, 7]. Djansugurova group from Kazakh Institute of General Genetics and Cytology, in collaboration with Ahmed group in the University of Maryland, reported in a preliminary communication the restoration of surface sialylation sites in a T47D breast cancer line compared to HB4A normal cells after sialic acid supplementation under conditions of nutrient deprivation [8]. Looking forward, Ahmed’s group [9] focused further efforts on modulating cell surface glycan sialylation and found increases in cell surface binding for wheat germ agglutinin (WGA), a lectin that binds to both 2,3and 2,6-linked sialic acid with slightly greater enhancement in the cancer cells (T47D, MCF7 and MDA MB231) when compared to the normal lines (MCF10A and HB4A). However, upon using plant lectins that discriminate between 2,3and 2,6-linkages, the Sambucus nigra lectin (SNA) binding to 2,6-sialic acids revealed roughly proportional increases in all lines, whereas Maackia amurensis lectin I (MAL-I) binding to 2,3-linked sialic acid showed greater enhancement in the malignant lines, compared to the normal cells. These research results highlight, for the first time, a detailed differential lectin-glycosensing site approach for the application of cell surface sialylation detection, specifically the highly dense clustered patches of sialo-epitopes NeuAc(2–3)bGal(1–4)bGlcNAc/Glc, which are displayed on the mammal cell surface envelope, enabling the detection of targeted sialoglycans in a highly sensitive and cost-effective manner [10]. More work along this line will potentially be useful for both cytochemical C.-Z. Li (&) Nanobioengineering and Bioelectronics Lab, Department of Biomedical Engineering, Florida International University, Miami, FL 33174, USA e-mail: licz@fiu.edu