Elongation of both branches of biantennary backbones of oligo-(N-acetyllactosamino)glycans by human serum (1----3)-N-acetyl-beta-D- glucosaminyltransferase

Biosynthesis and function of beta 1,6 branched mucin-type glycans

The contribution of carbohydrate structure to biomolecular, cellular, and organismal function is well-established, but has not yet received the attention it deserves, perhaps due to the complexity of the structures involved and to a lack of simple experimental methods for relating structure and function. In particular, beta1,6 GlcNAc branching plays a key functional role in processes ranging from inflammation and immune system function to tumor cell metastasis. For instance, synthesis of the core 2 beta1,6 branched structure in the mucin glycan chain by C2GnT enables the expression of functional structures at the termini of polylactosamine chains, such as blood group antigens and sialyl Lewis x. Also, IGnT can create multiple branches on the polylactosamine chain, which may serve as a mechanism for amplifying the functional potency of cell surface glycoproteins and glycolipids. The family of enzymes which creates beta1,6 branched structure in mucin glycans is proving to be quite complex, since multiple isoforms appear to exist for these enzymes, and some of the enzymes are adept at forming more than one type of beta1,6 branched structure, as in the case of C2GnT-M. Furthermore, the enzymes do not appear to be restricted to acting on mucin-type acceptor structures, but are able to act on glycolipid structures as well. Much remains to be learned regarding the specific biological niche filled by each of these enzymes and how their activities complement one another, as well as the manner in which the activities of these enzymes are regulated in the cell.

Enzymatic synthesis of alpha3'sialylated and multiply alpha3fucosylated biantennary polylactosamines. A bivalent [sialyl diLex]-saccharide inhibited lymphocyte-endothelium adhesion organ-selectively

Multifucosylated sialo-polylactosamines are known to be high affinity ligands for E-selectin. PSGL-1, the physiological ligand of P-selectin, is decorated in HL-60 cells by a sialylated and triply fucosylated polylactosamine that is believed to be of functional importance. Mimicking some of these saccharide structures, we have synthesized enzymatically a bivalent [sialyl diLex]-glycan, Neu5Acalpha2-3'Lexbeta1-3'Lexbeta1-3'(Neu5Acalpha2-3'Lexbeta1-3Lexbe ta1-6')LN [where Neu5Ac is N-acetylneuraminic acid, Lex is the trisaccharide Galbeta1-4(Fucalpha1-3)GlcNAc and LN is the disaccharide Galbeta1-4GlcNAc]. Several structurally related, novel polylactosamine glycans were also constructed. The inhibitory effects of these glycans on two L-selectin-dependent, lymphocyte-to-endothelium adhesion processes of rats were analysed in ex-vivo Stamper-Woodruff binding assays. The IC50 value of the bivalent [sialyl diLex]-glycan at lymph node high endothelium was 50 nm, but at the capillaries of rejecting cardiac allografts it was only 5 nm. At both adhesion sites, the inhibition was completely dependent on the presence of fucose units on the sialylated LN units of the inhibitor saccharide. These data show that the bivalent [sialyl diLex]-glycan is a high affinity ligand for L-selectin, and may reduce extravasation of lymphocytes at sites of inflammation in vivo without severely endangering the normal recirculation of lymphocytes via lymph nodes.

Biosynthesis of branched polylactosaminoglycans. Embryonal carcinoma cells express midchain beta1,6-N-acetylglucosaminyltransferase activity that generates branches to preformed linear backbones

Two types of beta1,6-GlcNAc transferases (IGnT6) are involved in in vitro branching of polylactosamines: dIGnT6 (distally acting), transferring to the penultimate galactose residue in acceptors like GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1-R, and cIGnT6 (centrally acting), transferring to the midchain galactoses in acceptors of the type (GlcNAcbeta1-3)Galbeta1-4GlcNAcbeta1-3Galbeta1-+ ++4GlcNAcbeta1-R. The roles of the two transferases in the biosynthesis of branched polylactosamine backbones have not been clearly elucidated. We report here that cIGnT6 activity is expressed in human (PA1) and murine (PC13) embryonal carcinoma (EC) cells, both of which contain branched polylactosamines in large amounts. In the presence of exogenous UDP-GlcNAc, lysates from both EC cells catalyzed the formation of the branched pentasaccharide Galbeta1-4GlcNAcbeta1-3(GlcNAcbeta1-6)Galbeta1-4 GlcNAc from the linear tetrasaccharide Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAc. The PA1 cell lysates were shown to also catalyze the formation of the branched heptasaccharides Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1-3(+ ++GlcNAcbeta1-6)Galbeta1 -4GlcNAc and Galbeta1-4GlcNAcbeta1-3(GlcNAcbeta1-6)Galbeta1-+ ++4GlcNAcbeta1-3Galbeta1 -4GlcNAc from the linear hexasaccharide Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1- 3Galbeta1-4GlcNAc in reactions characteristic to cIGnT6. By contrast, dIGnT6 activity was not detected in the lysates of the two EC cells that were incubated with UDP-GlcNAc and the acceptor trisaccharide GlcNAcbeta1-3Galbeta1-4GlcNAc. Hence, it appears likely that cIGnT6, rather than dIGnT6 is responsible for the synthesis of the branched polylactosamine chains in these cells.

De novo expression of endothelial sialyl Lewis(a) and sialyl Lewis(x) during cardiac transplant rejection: superior capacity of a tetravalent sialyl Lewis(x) oligosaccharide in inhibiting L-selectin-dependent lymphocyte adhesion

Acute organ transplant rejection is characterized by a heavy lymphocyte infiltration. We have previously shown that alterations in the graft endothelium lead to increased lymphocyte traffic into the graft. Here, we demonstrate that lymphocytes adhere to the endothelium of rejecting cardiac transplants, but not to the endothelium of syngeneic grafts or normal hearts analyzed with the in vitro Stamper-Woodruff binding assay. Concomitant with the enhanced lymphocyte adhesion, the cardiac endothelium begins to de novo express sialyl Lewis(a) and sialyl Lewis(x) (sLea and sLex) epitopes, which have been shown to be sequences of L-selectin counterreceptors. The endothelium of allografts, but not that of syngeneic grafts or normal controls, also reacted with the L-selectin-immunoglobulin G fusion protein, giving further proof of inducible L-selectin counterreceptors. The lymphocyte adhesion to endothelium could be significantly decreased either by treating the lymphocytes with anti-L-selectin antibody HRL-1, or by treating the tissue sections with sialidase or anti-sLea or anti-sLex monoclonal antibodies. Finally, we synthetized enzymatically several members of the sLex family oligosaccharides and analyzed their ability to block lymphocyte adhesion to cardiac endothelium. The monovalent sLex (a tetramer), divalent sLex (a decamer), and tetravalent sLex (a 22-mer) could all significantly reduce lymphocyte binding, but the inhibition by the tetravalent sLex-construct was clearly superior to other members of the sLex family. The crucial control oligosaccharides, sialyl lactosamines lacking fucose but being otherwise similar to the members of sLex family, had no effect on lymphocyte binding.

Enzyme-assisted synthesis of a bivalent high-affinity dodecasaccharide inhibitor of mouse gamete adhesion. The length of the chains carrying distal alpha 1,3-bonded galactose residues is critical

Proposing to study the molecular mechanisms of mouse gamete adhesion with the aid of high affinity adhesion inhibitors of saccharide nature, we report here the enzymatic synthesis of a bivalent oligosaccharide Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-3(Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-6)Gal beta 1-4GlcNAc (4), consisting of two long arms that link together two distal alpha 1,3-galactose residues. Binding data reported elsewhere (E. Litscher et al., Biochemistry, 1995, 34, 4662-4669) show that 4 is a high affinity inhibitor of mouse gamete adhesion in vitro (IC50 = 9 microM), while a related octasaccharide Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3(Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-6)Gal beta 1-4GlcNAc, consisting of two short arms is of very low inhibitory activity. The data highlight the importance of the two alpha-galactose residues of 4, and the length of the sugar chains joining them.

alpha 1,3-Fucosylation of branched blood group I-type oligo-(N-acetyllactosamino)glycans by human milk transferases is restricted to distal N-acetyllactosamine units: the resulting isomers are separated by WGA-agarose chromatography

A partially purified preparation of alpha 1,3-fucosyltransferase(s) from human milk was used to [14C]fucosylate oligosaccharides containing Gal beta 1-4GlcNAc units. Substitution of N-acetyllactosamine at position 3' with a beta-linked N-acetyl-glucosamine enhanced the reactivity of the acceptor, whereas similar substitution at position 6' was inhibitory. Thus, the trisaccharide GlcNAc beta 1-6Gal beta 1-4GlcNAc (5), the branched tetrasaccharide GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4GlcNAc (11) and the triply branched decasaccharide GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4GlcNAc beta 1-3[GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4GlcNAc beta 1-6]Gal beta 1-4GlcNAc (26) gave remarkably poor yields of alpha 1,3-fucosylated products in comparison to GlcNAc beta 1-3Gal beta 1-4GlcNAc (3). beta 1,4-Galactosyl derivatives of 5 and 11, however, gave good yields of alpha 1,3-fucosylated products, but the fucosylation was restricted to the distal N-acetyllactosamine units of Gal beta 1-4GlcNAc beta 1-6Gal beta 1-4GlcNAc (16), Gal beta 1-4GlcNAc beta 1-3(Gal beta 1-4GlcNAc beta 1-6)Gal beta 1-4GlcNAc (18) and also in Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3(Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-6)Gal beta 1-4GlcNAc (22). Immobilized wheat germ agglutinin (WGA), possessing high affinity for 16 [1], revealed no affinity for the fucosylated derivative Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-6Gal beta 1-4GlcNAc (17). The isomeric heptasaccharides Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-3(Gal beta 1-4GlcNAc beta 1-6)Gal beta 1-4GlcNAc (19) and Gal beta 1-4GlcNAc beta 1-3[Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-6]Gal beta 1-4GlcNAc (20) were readily separated from each other on WGA agarose, and so were the isomeric nonasaccharides Gal alpha 1-3Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-3(Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-6)Gal beta 1-4GlcNAc (23) and Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3[Gal alpha 1-3Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-6]Gal beta 1-4GlcNAc (24).

Bi-antennary oligo-(N-acetyllactosamino)glycans of I-type are galactosylated preferentially at the GlcNAc beta 1-6Gal linked arms by alpha 1,3-galactosyltransferase of bovine thymus

alpha 1,3-Galactosylation of radiolabelled bi-antennary acceptors Gal beta 1-4GlcNAc beta 1-3(Gal beta 1-4GlcNAc beta 1-6)Gal-R (R = 1-OH, beta 1-4GlcNAc or beta 1-4Glc) with bovine thymus alpha 1,3-galactosyltransferase was studied. At all stages of the reactions the three acceptors reacted faster at the 1-->6 linked arm than at the 1-->3 linked branch. Hence, in addition to the doubly alpha 1,3-galactosylated products, practically pure Gal beta 1-4GlcNAc beta 1-3(Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-6)Gal-R could be obtained from the three acceptors in reactions that had proceeded to near completion. The isomeric mono-alpha 1,3-galactosylated products were identified by using exoglycosidases to remove the branches unprotected by alpha 1,3-galactoses and by subsequently identifying the resulting linear glycans chromatographically.