THE FIRST TECHNIQUE for imaging carbohydrates as they are produced on cell surfaces of live animals has been developed by glycobiology specialist Carolyn R. Bertozzi and coworkers at the University of California, Berkeley. Specific glycan (oligosaccharide) structures in cells are important markers and mediators of many physiological processes, but up to now changes in these glycans couldn't be visualized in living organisms.
The new technique, based on cell-friendly fluorescent-labeling chemistry devised by the same group, will help scientists more closely probe the functional roles of carbohydrates during development and in both health and disease.
Chemistry professor M. G. Finn of Scripps Research Institute says the Bertozzi group's technique "provides a 'global positioning system' for tracking carbohydrates through organisms. This is chemical biology at its finest: refined and reliable chemical tools illuminating—literally—molecular interactions in living biological systems."
In recent years, Bertozzi and coworkers have developed techniques for imaging glycans in live cells. First, they feed azide-derivatized sugars to live organisms, which incorporate the modified sugars into their glycans. After cells are removed from the organisms, the labeled glycans can be imaged by using the Staudinger ligation, a reaction developed by Bertozzi's group in 2000, to mark the glycans with fluorescent probes. But that reaction and similar ones are too slow or too toxic for use in live animals.
Last year, Bertozzi's group developed copper-free click chemistry with difluorinated cyclooctyne (DIFO) reagents (Proc. Natl. Acad. Sci. USA 2007, 104, 16793; C&EN, Oct. 22, 2007, page 15). This ligation reaction is faster than the Staudinger ligation and is nontoxic, unlike the copper-catalyzed click chemistry reactions on which it's based. In the new study the researchers introduce azide-derivatized N-acetylgalactosamine (GalNAc) metabolically into developing zebrafish embryos and treat the embryos with DIFO reagent to fluorescently label GalNAc-containing glycans. They then use microscopy to image the labeled glycans in the developing animals (Science 2008, 320, 664).
The technique makes it possible for Bertozzi and coworkers to detect increases in glycan biosynthesis in specific locations during zebrafish development and to visualize patterns of glycan expression that previously would have been undetectable. The researchers believe the technique can be extended to other sugars, such as sialic acid.
The new study represents "a significant advance in visualizing the distribution of sugars on cell surfaces, where they are believed to mediate cell adhesion, migration, and recognition processes," says glycan-imaging specialist Mary L. Kraft of the University of Illinois, Urbana-Champaign. "Changes in cell-surface glycans also occur during embryo development and are characteristic of several diseases, such as cancer. However, how they mediate these processes is largely unknown" because no methods have existed to visualize them in live animals. The ability of the new approach "to determine how glycans are distributed spatially within a tissue and in time can shed light on their functional roles and help identify cell-surface markers indicative of the physiological status of cells."
Chemistry Nobel Prize winner K. Barry Sharpless of Scripps, who helped develop the concept of click chemistry, comments that Bertozzi's "in vivo light-up of the zebrafish is a stunning first."