Tracking N-Acetyllactosamine on Cell-Surface Glycans In Vivo

摘要

The glycome, the totality of glycans produced by a cell, is a dynamic indicator of the cell's physiology. Changes in the glycome reflect a cell's developmental stage and the transformation state of a cell. Recently, imaging of glycans in vivo has been enabled using a bioorthogonal chemical reporter strategy by treating cells or organisms with azide- or alkynetagged monosaccharide precursors. The modified monosaccharides, when taken up by cells, are activated in the cytoplasm to form nucleotide sugars, substrates of glycosyl-transferases that generate complex glycans in the endoplasmic reticulum and Golgi. Once incorporated into cell-surface glycoconjugates, the bioorthogonal chemical tags allow covalent conjugation with fluorescent probes for visualization or with affinity probes for enrichment and glycomic analysis. This approach has been successfully used for the detection and imaging of mucin O-linked glycans, sialylated and fucosylated glycans, and cytosolic O-GlcNAcylated proteins (GlcNAc = N-acetylglucosamine). However, only monosaccharides are tracked by this strategy, and each monosaccharide is usually found on a plethora of glycans. Higher-order glycans, such as disaccharides or trisaccharides, of specific composition cannot be uniquely labeled by hijacking their biosynthetic pathways with nonnatural monosaccharides (Figure la). Herein, we report a rapid and highly specific chemoenzymatic method for labeling cell-surface glycans bearing a ubiquitous disaccharide—N-acetyllactosamine (LacNAc, Galβ1,4GlcNAc)—with biophysical probes for imaging or glycomic analysis.