The first engineering of cell surfaces in living animals

ave harmful effects -- indeed they are useful in some drugs: the A in AZT, azidothymidine, is for azide -- but they rapidly react with each other under physiological conditions. Suitably modified by Bertozzi's methods, the ligation product formed from these compounds is stable under physiological conditions.

The trick is to get the azide half of the reaction to show up on cell surfaces. In cell cultures, this is done by diffusing into the cells a precursor, containing the azide group, of a commonly expressed form of oligosaccharide, or sugar, that includes sialic acid. The cells manufacture these azide-bearing sugars and display them on their surfaces.

With mice, Bertozzi's group injected the azide-containing precursor directly into the abdomen, where cells in the spleen and some other organs took it up. Later examination of spleen cells, which are rich in sialic-acid sugars, showed an abundance of cell-surface azides.

Because rodent blood has high levels of esterase, enzymes that might interfere with the conversion of the modified precursor, the first experiments were done with esterase-deficient knockout mice. Later the investigators found that wild-type mice displayed just as many cell-surface azides, so esterase was apparently no hindrance to the uptake of the azide-containing precursor.

The other half of the Staudinger ligation, the phosphine, may be attached to a great variety of markers. To confirm the success of the Staudinger ligation in mice, Bertozzi and her collaborators used a peptide that could be identified by a fluorescing antibody.

"The future holds a range of opportunities for using the Staudinger ligation for noninvasive imaging," says Bertozzi. "Phosphine markers might include radiochemicals for detection by positron emission tomography, or magnetically active molecules for detection by magnetic resonance imaging, or nanocrystals with unique optical properties, or many other methods. These markers cou

Contact: Paul Preuss
DOE/Lawrence Berkeley National Laboratory

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