Research Highlights

Development: The colors of sweetness

Azido analogs of sugars are readily metabolized by cells and converted to cell surface azido sugars. Click here for a full-size image.

Evidence from model organisms shows that cell surface glycan expression modulates and determines embryonic development and cell fate determination. Concurrent with the refinement of analytical methods such as mass spectrometry and transcription profiling, visualization of glycan expression on the cell surface has progressed in recent years. However, research has been lacking that shows how ontogenic processes coincide with the expression of specific glycans. Research by Bertozzi and colleagues published in Science now reports major progress in glycosylation tracking during embryogenesis in Danio rerio (zebrafish).

In previous studies, Bertozzi and colleagues showed that living cells or organisms can incorporate N-azido sugars as a replacement for N-acetyl sugars without disturbance of mitosis or metabolism. The N-azido group, in turn, can be readily connected in vivo to a chromophore through difluorinated cyclooctynes (DIFOs) as a physiologically viable linker. Laughlin et al. incubated zebrafish embryos with N-azidogalactosamine (GalNAz). They found it was readily incorporated into mucin-type O-glycans as incubation of embryos with PNGase F (cleaving most of the N-glycans) or chondroitinase (cutting many glycosaminoglycans) did not alter the reactivity of the cell surface glycans towards the labeling reaction. Next, they performed in situ labeling with DIFOs substituted by various chromophores.

Bertozzi and colleagues then performed the labeling reaction on zebrafish embryos over five days every 12 hours post fertilization (hpf). GalNAz-labeled glycans were detected from 24 hpf onwards. They selected the jaw regions, pectoral fins and olfactory organs for further exploration as fluorescence intensity was concentrated in these regions after 60 and 72 hpf. Using differently labeled DIFOs, they distinguished 'old' glycans appearing after 60 hpf and 'newer' ones expressed after 72 hpf in regions such as the mouth invagination. Additionally, labeling by a third DIFO-linked fluorescent chromophore showed that another group of mucin-type O-glycans appeared after 60 hpf and disappeared again by 72 hpf.

The findings of Bertozzi and colleagues beautifully reveal the fine orchestration of O-glycosylation during embryogenesis. This study is complemented by research that identifies signaling pathways guiding development as targets of specific glycosylation types.

Author: Mirko von Elstermann