Dynamic imaging of proteins in live cells is routinely performed by using genetically encoded reporters, an approach that cannot be extended to other classes of biomolecules such as glycans and lipids. Here, we report a Cu-free variant of click chemistry that can label these biomolecules rapidly and selectively in living systems, overcoming the intrinsic toxicity of the canonical Cu-catalyzed reaction. The critical reagent, a substituted cyclooctyne, possesses ring strain and electron-withdrawing fluorine substituents that together promote the [3 ؉ 2] dipolar cycloaddition with azides installed metabolically into biomolecules. This Cu-free click reaction possesses comparable kinetics to the Cu-catalyzed reaction and proceeds within minutes on live cells with no apparent toxicity. With this technique, we studied the dynamics of glycan trafficking and identified a population of sialoglycoconjugates with unexpectedly rapid internalization kinetics.azide ͉ bioorthogonal reaction ͉ cyclooctyne glycan trafficking ͉ molecular imaging

Glycans are attractive targets for molecular imaging but have been inaccessible because of their incompatibility with genetically encoded reporters. We demonstrated the noninvasive imaging of glycans in live developing zebrafish, using a chemical reporter strategy. Zebrafish embryos were treated with an unnatural sugar to metabolically label their cell-surface glycans with azides. Subsequently, the embryos were reacted with fluorophore conjugates by means of copper-free click chemistry, enabling the visualization of glycans in vivo at subcellular resolution during development. At 60 hours after fertilization, we observed an increase in de novo glycan biosynthesis in the jaw region, pectoral fins, and olfactory organs. Using a multicolor detection strategy, we performed a spatiotemporal analysis of glycan expression and trafficking and identified patterns that would be undetectable with conventional molecular imaging approaches.The cell-surface glycome is a rich source of information that reports on the cell's physiological state. For example, changes in glycan structures serve as markers of altered gene expression during development (1) and disease progression (2). The dynamics of glycans at the plasma membrane reflect the activity of the cell's secretory machinery (3), and their relative abundances report on flux in metabolic pathways inside the cell (4). Glycans are therefore attractive targets for in vivo imaging but have been inaccessible because of their incompatibility with genetically encoded reporters (5).To image glycans in vivo, we employed a strategy in which an azide is introduced into target biomolecules, priming them for selective covalent reaction with fluorescent probes (5). The azide is small, stable in biological systems, and selectively reactive with phosphines or activated alkynes. Previously, the Staudinger ligation (6,7) or copper-catalyzed click chemistry (8,9) have been used to detect azide-labeled biomolecules on cells ex vivo. However, in vivo †To whom correspondence should be addressed. E-mail: E-mail: crb@berkeley.edu. * These authors contributed equally to this work. imaging of dynamic biological processes using these chemistries could be complicated by slow reaction kinetics or reagent toxicity. The copper-free click reaction of azides with difluorinated cyclooctyne (DIFO) reagents (10) overcomes these limitations, suggesting its potential application to in vivo imaging.We chose zebrafish as a model organism because of their well-defined developmental program (11), emerging disease models (12), and amenability to optical imaging. The metabolic substrate peracetylated N-azidoacetylgalactosamine (Ac 4 GalNAz) was selected on the basis of its known incorporation into mucin-type O-linked glycoproteins in mammalian cells and mice via the N-acetylgalactosamine (GalNAc) salvage pathway (13,14) ( fig. S1). We envisioned an imaging experiment ( Before performing imaging experiments, we confirmed that the zebrafish glycan biosynthetic enzymes are permissive of the unnatural sugar. The...

Detection of metabolites and post-translational modifications can be achieved using the azide as a bioorthogonal chemical reporter. Once introduced into target biomolecules, either metabolically or through chemical modification, the azide can be tagged with probes using one of three highly selective reactions: the Staudinger ligation, the Cu(I)-catalyzed azide-alkyne cycloaddition, or the strain-promoted [3 + 2] cycloaddition. Here, we compared these chemistries in the context of various biological applications, including labeling of biomolecules in complex lysates and on live cell surfaces. The Cu(I)-catalyzed reaction was found to be most efficient for detecting azides in protein samples but was not compatible with live cells due to the toxicity of the reagents. Both the Staudinger ligation and the strain-promoted [3 + 2] cycloaddition using optimized cyclooctynes were effective for tagging azides on live cells. The best reagent for this application was dependent upon the specific structure of the azide. These results provide a guide for biologists in choosing a suitable ligation chemistry.

Chemical reactions that enable selective biomolecule labeling in living organisms offer a means to probe biological processes in vivo. Very few reactions possess the requisite bioorthogonality, and, among these, only the Staudinger ligation between azides and triarylphosphines has been employed for direct covalent modification of biomolecules with probes in the mouse, an important model organism for studies of human disease. Here we explore an alternative bioorthogonal reaction, the 1,3-dipolar cycloaddition of azides and cyclooctynes, also known as “Cu-free click chemistry,” for labeling biomolecules in live mice. Mice were administered peracetylated N-azidoacetylmannosamine (Ac4ManNAz) to metabolically label cell-surface sialic acids with azides. After subsequent injection with cyclooctyne reagents, glycoconjugate labeling was observed on isolated splenocytes and in a variety of tissues including the intestines, heart, and liver, with no apparent toxicity. The cyclooctynes tested displayed various labeling efficiencies that likely reflect the combined influence of intrinsic reactivity and bioavailability. These studies establish Cu-free click chemistry as a bioorthogonal reaction that can be executed in the physiologically relevant context of a mouse.