Clickable Galactose Analogues for Imaging Glycans in Developing Zebrafish.

Galactose is one of only nine monosaccharide precursors used to build complex glycans in vertebrates. Defects in galactose metabolism cause galactosemia and lysosomal storage diseases, and the ability to visualize metabolic flux through these pathways would help to understand mechanisms underlying disease pathogenesis. Bioorthogonal metabolic reporters are widely used tools to image glycan biosynthesis; however, to date, no galactose analogues have capitalized on this strategy. We demonstrate that the galactose salvage pathway is remarkably intolerant of unnatural galactose and galactose-1-phosphate analogues. Subtle modifications to uridine diphosphate galactose (UDP-Gal), which is the universal donor for galactosyltransferases, however, yielded effective metabolic probes for labeling glycans in vivo. We applied 6-alkynyl UDP-Gal, followed by click chemistry tagging, to visualize glycosylation during zebrafish development, revealing a striking accumulation into glycan-rich ridges within the organism's enveloping layer. UDP-Gal analogues represent a new class of glycan metabolic probes for revealing physiological and pathological changes in glycosylation in vivo.