RESEARCH HIGHLIGHTS

CHEMICAL BIOLOGY Carbohydrate chemistry in California

By combining an improved oligosaccharide molecule but to make a variety of different that the interactions between proteins and methods synthetic route with protein-binding and molecules where we control the sulfation glycosaminoglycans can be extremely selec- cell-growth assays, Caltech researchers can pattern,” says Hsieh-Wilson. tive,” says Hsieh-Wilson. CS-E also stimulated investigate the biological roles of specific With chondroitin sulfate sequences in the outgrowth of several other neuronal cell glycosaminoglycan sulfation patterns. hand, the researchers immobilized the mol- types and its activity was inhibited by block- .com/nature e To Atkins dieters everywhere, carbohy- ecules on a microarray to investigate their ing the receptors for midkine and brain- drates are the enemy. To a lot of chemi- interaction with the growth factor midkine, derived neurotrophic factor, suggesting that .natur cal biologists, however, carbohydrates are which is involved in the development and this sequence may recruit growth factors and w pretty neat. repair of neural tissue. Midkine bound activate downstream signaling pathways. Carbohydrates in the form of oligosaccha- strongly to one of the sulfation sequences, These results led the researchers to hypoth- ride and polysaccharide sequences are crucial CS-E. Furthermore, they discovered that esize the existence of a precise ‘sulfation code’ http://ww

protein modifications involved in countless brain-derived neurotrophic factor, which on the backbone of chondroitin sulfate. “It’s

oup biological functions, from development to is involved in regulation of nervous system been challenging to probe these molecular r

G immune response to cell-cell communica- development, also had a strong preference interactions using just biological approaches

tion. Studying protein glycosylation is a for CS-E. alone,” remarks Hsieh-Wilson. “Synthetic challenging pursuit because of the complex Next they examined the effects of sulfation organic chemistry can provide unique lishing structures of carbohydrates. Furthermore, patterns on primary hippocampal neuron insights and, when used with a battery of b they are difficult to probe via genetic inter- cell growth. Overwhelmingly, they found that other biological techniques, you can start to Pu vention because glycosylation is a nontem- CS-E was required for neurite outgrowth on address some really interesting questions.” plated post-translational process. Fortunately, coverslips coated with the molecule, whereas Allison Doerr

Nature a variety of tools are rapidly being generated, cell growth did not occur on surfaces coated

6 as researchers have begun to recognize the with any of the other sequences. “Even a close- RESEARCH PAPERS Gama, C.I. et al. Sulfation patterns of

200 biological and medical importance of glyco- ly related analog with the same structure and glycosaminoglycans encode molecular recognition and

© sylation (see Box 1). overall charge had no effect, which suggests activity. Nat. Chem. Biol. 2, 467–473 (2006). California Institute of Technology profes- sor Linda Hsieh-Wilson and her colleagues are interested in one such class of carbohy- BOX 1 CHEMICAL REPORTERS OF FUCOSYLATION drates known as the glycosaminoglycans, or GAGs for short, and their roles in neuronal The chemical labeling of another type tolerate and incorporate azido-modified development and regeneration. GAGs are of polysaccharide post-translational fucose analogs into polysaccharides. Using often sulfated in different patterns, which modification, the fucosylated glycan, is either the Staudinger ligation or ‘click’ have been implicated as encoding impor- the subject of two recent reports led by chemistry, both groups selectively labeled tant functional information for various Chi-Huey Wong of the Scripps Research azidofucose on the cell surface and then biological processes. “The complex chemi- Institute (Sawa et al., 2006) and Carolyn stained the cells with a fluorescent cal structures of GAGs provide instructions Bertozzi of University of California, probe. Additionally, Wong’s group for protein binding and regulation,” explains Berkeley (Rabuka et al., 2006). showed that fucosylated glycans could be Hsieh-Wilson. Fucose is often an important imaged inside cells by directly labeling Until now, however, a molecular-level component of glycans involved in cell- azidofucose with a new fluorogenic understanding of these sulfation sequences cell interactions and cell migration. It is naphthalimide-based molecule bearing an was missing, mainly because the chemical also known to be particularly prevalent alkyne handle. synthesis of GAGs is extremely challenging. in cancer tissues, so there is a great deal of interest in investigating fucosylated The first step Hsieh-Wilson and her col- RESEARCH PAPERS antigens as potential cancer biomarkers. leagues took was to design a streamlined Sawa, M. et al. Glycoproteomic probes for Wong’s and Bertozzi’s laboratories have fluorescent imaging of fucosylated glycans in synthetic route to making tetrasaccharide devised similar labeling strategies by vivo. Proc. Natl. Acad. Sci. USA 103, 12371–12376 chondroitin sulfate glycosaminoglycans (2006). harnessing the fucose ‘salvage’ metabolic with varied sulfation patterns. “I had two or Rabuka, D. et al. A chemical reporter strategy to pathway to incorporate a bio-orthogonal three people on the project for four years, probe glycoprotein fucosylation. J. Am. Chem. chemical handle. Enzymes in this pathway Soc.; published online 23 August 2006. with the goal being not only to make one

NATURE METHODS | VOL.3 NO.10 | OCTOBER 2006 | 769