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

Human Milk Oligosaccharides (HMOS): Structure, Function, and Enzyme-Catalyzed Synthesis

The important roles played by human milk oligosaccharides (HMOS), the third major component of human milk, in the health of breast-fed infants have been increasingly recognized, as the structures of more than 100 different HMOS have now been elucidated. Despite the recognition of the various functions of HMOS as prebiotics, antiadhesive antimicrobials, and immunomodulators, the roles and the applications of individual HMOS species are less clear. This is mainly due to the limited accessibility to large amounts of individual HMOS in their pure forms. Current advances in the development of enzymatic, chemoenzymatic, whole-cell, and living-cell systems allow for the production of a growing number of HMOS in increasing amounts. This effort will greatly facilitate the elucidation of the important roles of HMOS and allow exploration into the applications of HMOS both as individual compounds and as mixtures of defined structures with desired functions. The structures, functions, and enzyme-catalyzed synthesis of HMOS are briefly surveyed to provide a general picture about the current progress on these aspects. Future efforts should be devoted to elucidating the structures of more complex HMOS, synthesizing more complex HMOS including those with branched structures, and developing HMOS-based or HMOS-inspired prebiotics, additives, and therapeutics.

Fuc-TIX: a versatile alpha1,3-fucosyltransferase with a distinct acceptor- and site-specificity profile

alpha1,3-Fucosyltransferases (Fuc-Ts) convert N-acetyllactosamine (LN, Galbeta1-4GlcNAc) to Galbeta1-4(Fucalpha1-3)GlcNAc, the Lewis x (CD15, SSEA-1) epitope, which is involved in various recognition phenomena. We describe details of the acceptor specificity of alpha1,3-fucosyltransferase IX (Fuc-TIX). The unconjugated N- and O-glycan analogs LNbeta1-2Man, LNbeta1-6Manalpha1-OMe, LNbeta1-2Manalpha1-3(LNbeta1-2Manalpha1-6)Manbeta1-4GlcNAc, and Galbeta1-3(LNbeta1-6)GalNAc reacted well in vitro with Fuc-TIX present in lysates of appropriately transfected Namalwa cells. Fuc-TIX reacted well with the reducing end LN of GlcNAcbeta1-3'LN (underscored site reacted) and GlcNAcbeta1-3'LNbeta1-3'LN (both LNs reacted), but very poorly with the reducing end LN of LNbeta1-3'LN. However, Fuc-TIX reacted significantly better with the non-reducing end LN as compared to the other LN units in the glycans LNbeta1-3'LN and LNbeta1-3'LNbeta1-3'LNbeta1-3'LN, confirming our previous data on LNbeta1-3'LNbeta1-OR. In contrast, the sialylated glycan Neu5Acalpha2-3'LNbeta1-3'LNbeta1-3'LNbeta1-3'LN was fucosylated preferentially at the two most reducing end LN units. We conclude that Fuc-TIX is a versatile alpha1,3-Fuc-T, that (1) generates distal Lewis x epitopes from many different acceptors, (2) possesses inherent ability for the biosynthesis of internal Lewis x epitopes on growing polylactosamine backbones, and (3) fucosylates the remote internal LN units of alpha2,3-sialylated i-type polylactosamines.

The acceptor and site specificity of alpha 3-fucosyltransferase V. High reactivity of the proximal and low of the distal galbeta 1-4GlcNAc unit in i-type polylactosamines

We report here on in vitro acceptor and site specificity of recombinant alpha3-fucosyltransferase V (Fuc-TV) with 40 oligosaccharide acceptors. Galbeta1-4GlcNAc (LN) and GalNAcbeta1-4GlcNAc (LDN) reacted rapidly; Galbeta1-3GlcNAc (LNB) reacted moderately, and GlcNAcbeta1-4GlcNAc (N, N'-diacetyl-chitobiose) reacted slowly yet distinctly. In neutral and terminally alpha3-sialylated polylactosamines of i-type, the reducing end LN unit reacted rapidly and the distal (sialyl)LN group very slowly; the midchain LNs revealed intermediate reactivities. The data suggest that a distal LN neighbor enhances but a proximal LN neighbor reduces the reactivity of the midchain LNs. This implies that Fuc-TV may bind preferably the tetrasaccharide sequence Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAc for transfer at the underlined monosaccharide. Terminal alpha3-sialylation of i-type polylactosamines almost doubled the reactivities of the LN units at all positions of the chains. We conclude that, in comparison with human Fuc-TIV and Fuc-TIX, Fuc-TV reacted with a highly distinct site specificity with i-type polylactosamines. The Fuc-TV reactivity of free LNB resembled that of LNBbeta1-3'R of a polylactosamine, contrasting strongly with the dissimilarity of the reactivities of the analogous pair of LN and LNbeta1-3'R. This observation supports the notion that LN and LNB may be functionally bound at distinct sites on Fuc-TV surface. Our data show that Fuc-TV worked well with a very wide range of LN-glycans, showing weak reactivity only with distal (sialyl)LN units of i-type polylactosamines, biantennary N-glycans, and I branches of polylactosamines.

Complementary acceptor and site specificities of Fuc-TIV and Fuc-TVII allow effective biosynthesis of sialyl-TriLex and related polylactosamines present on glycoprotein counterreceptors of selectins

The P-selectin counterreceptor PSGL-1 is covalently modified by mono alpha2,3-sialylated, multiply alpha1,3-fucosylated polylactosamines. These glycans are required for the adhesive interactions that allow this adhesion receptor-counterreceptor pair to facilitate leukocyte extravasation. To begin to understand the biosynthesis of these glycans, we have characterized the acceptor and site specificities of the two granulocyte alpha1,3-fucosyltransferases, Fuc-TIV and Fuc-TVII, using recombinant forms of these two enzymes and a panel of synthetic polylactosamine-based acceptors. We find that Fuc-TIV can transfer fucose effectively to all N-acetyllactosamine (LN) units in neutral polylactosamines, and to the "inner" LN units of alpha2,3-sialylated acceptors but is ineffective in transfer to the distal alpha2,3-sialylated LN unit in alpha2,3-sialylated acceptors. Fuc-TVII, by contrast, effectively fucosylates only the distal alpha2,3-sialylated LN unit in alpha2,3-sialylated acceptors and thus exhibits an acceptor site-specificity that is complementary to Fuc-TIV. Furthermore, the consecutive action of Fuc-TIV and Fuc-TVII, in vitro, can convert the long chain sialoglycan SAalpha2-3'LNbeta1-3'LNbeta1-3'LN (where SA is sialic acid) into the trifucosylated molecule SAalpha2-3'Lexbeta1-3'Lexbeta1-3'Lex (where Lex is the trisaccharide Galbeta1-4(Fucalpha1-3)GlcNAc) known to decorate PSGL-1. The complementary in vitro acceptor site-specificities of Fuc-TIV and Fuc-TVII imply that these enzymes cooperate in vivo in the biosynthesis of monosialylated, multifucosylated polylactosamine components of selectin counterreceptors on human leukocytes.

Enzymatic synthesis of site-specifically (alpha 1-3)-fucosylated polylactosamines containing either a sialyl Lewis (x), a VIM-2, or a sialylated and internally difucosylated sequence

By using two different reaction pathways, we generated enzymatically three sialylated and site-specifically alpha 1-3-fucosylated polylactosamines. Two of these are isomeric hexasaccharides Neu5Ac(alpha 2-3)Gal(beta 1-4)GlcNAc(beta 1-3)Gal(beta 1-4)[Fuc(alpha 1-3)] GlcNAc and Neu5Ac(alpha 2-3)Gal(beta 1-4)[Fuc(alpha 1-3)]GlcNAc(beta 1-3)Gal(beta 1-4) GlcNAc, containing epitopes that correspond to VIM-2 and sialyl Lewis (x), respectively. The third one, nonasaccharide Neu5Ac(alpha 2-3)Gal(beta 1-4)GlcNAc(beta 1-3)Gal(beta 1-4)[Fuc(alpha 1-3)] GlcNAc(beta 1-3)Gal(beta 1-4)[Fuc(alpha 1-3)]GlcNAc, is a sialylated and internally difucosylated derivative of a trimeric N-acetyllactosamine. All three oligosaccharides have one fucose-free N-acetyllactosaminyl unit and can be used as acceptors for recombinant alpha 1-3-fucosyltransferases in determining the biosynthesis pathways leading to polyfucosylated selectin ligands.

Enzymatic midchain branching of polylactosamine backbones is restricted in a site-specific manner in alpha 1,3-fucosylated chains

Branched polylactosamines on animal cell surfaces are believed to contribute to multivalent interactions in cell adhesion and cell signalling. Their biosynthesis proceeds via linear precursors that become branched by beta1,6-GlcNAc transferases (IGnT6, GlcNAc to Gal). Previous work has identified the tetrasaccharide Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAc (1) and the hexasaccharide Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1- 3Galbeta1-4GlcNAc (4) as acceptors for a rat serum enzyme activity (cIGnT6), which transfers GlcNAcbeta1-6 units to the midchain galactose residues. Thereby, 1 is converted to the branched pentasaccharide Galbeta1-4GlcNAcbeta1-3(GlcNAcbeta1-6)Galbeta1-4 GlcNAc and 4 to the doubly branched octasaccharide Galbeta1-4GlcNAcbeta1-3(GlcNAcbeta1-6)Galbeta1-+ ++4GlcNAcbeta1-3(GlcNAcb eta1-6)Galbeta1-4GlcNAc [Leppänen, A., Salminen, H., Zhu, Y., Maaheimo, H., Helin, J., Costello, C. E., & Renkonen, O. (1997) Biochemistry 36, 7026-7036]. Here we report that neither the alpha1, 3-fucose-containing derivatives of 1 [Galbeta1-4GlcNAcbeta1-3Galbeta1-4(Fucalpha1-3)G lcNAc and Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Gl cNAc] nor a similar derivative of 4 [Galbeta1-4GlcNAcbeta1-3Galbeta1-4(Fucalpha1-3)+ ++GlcNAcbeta1-3Galbeta1- 4GlcNAc] were acceptors for the rat serum cIGnT6 activity. Hence, the enzyme's branch-forming action was completely prevented at sites in the immediate neighborhood of the fucosylated loci of the polylactosamines. In Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1- 3Galbeta1-4(Fucalpha1-3) GlcNAc, the inhibition of the branch-forming reaction was restricted to the fucose-carrying LacNAc unit; at the middle LacNAc, the branching proceeded normally. However, in the isomeric Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1- 4GlcNAcbeta1-3Galbeta1-4 GlcNAc, the fucose residue prevented branching completely at the middle LacNAc and almost completely at the reducing end LacNAc. In summary, alpha1,3-fucose residues in polylactosamine chains inhibited the cIGnT6 reaction in a site-specific manner, at the fucosylated LacNAc unit itself and also at sites one and two LacNAc units upstream, but not at the LacNAc units downstream from the fucosylated locus. These data imply that site-directed branching in polylactosamines is possible in vitro with the aid of specifically positioned alpha1,3-fucosyl units, that can be removed afterward without harming the branched backbones.

In vitro experimental studies of sialyl Lewis x and sialyl Lewis a on endothelial and carcinoma cells: crucial glycans on selectin ligands

Extravasation from the blood of malignant tumour cells that form metastasis and leukocytes that go into tissues require contact between selectins and their sialyl Lewis x and sialyl Lewis a (sLe(x) and sLe(a) respectively) decorated ligands. Endothelial cells have been shown to express sLe(x) epitopes in lymph nodes and at sites of inflammation, and this is crucial for the selectin-dependent leukocyte traffic. Besides the ability to synthesize sLe(x) on sialylated N-acetyllactosamine via the action of alpha(1,3)fucosyltransferase(s), endothelial cells can also degrade sLe(x) to Lewis x through the action of alpha(2,3)sialidase(s). In addition, several epithelial tumors possess the machinery to synthesize sLe(x), which facilitates their adhesion to endothelial E- and P-selectin.

Expression and function of alpha 2,3-sialyl- and alpha 1,3/1,4-fucosyltransferases in colon adenocarcinoma cell lines: role in synthesis of E-selectin counter-receptors

We show here that colon-carcinoma cell lines adhere to E-selectin via sialyl Lewis x and sialyl Lewis a (s(Lex) and s(Lea)) oligosaccharides and that this adhesion can be enhanced by TNF stimulation. To study in greater detail this endothelial binding, we analysed the mRNA expression and function of the enzymes participating in the generation of s(Lex) and s(Lea on cancer cells. These oligosaccharides are synthesized by sequential action of alpha 2,3 sialyl (alpha 2,3-ST) and alpha 1,3/1/4 fucosyltransferases (alpha 1,3/1,4-FT) on existing (poly)N-acetyllactosamine chains. We report here that mRNAs of 2 recently cloned alpha 2,3-STs and 4 alpha 1,3/1,4-FTs are expressed in adenocarcinoma cells. In functional assays alpha 2,3-ST and alpha 1,3- or 1,4-FT activities were observed in adenocarcinoma cell lysates to exogenous N-acetyllactosamine and lacto-N-biose acceptors and to their sialylated derivatives, leading to the synthesis of the sialyl-N-acetyllactosamine and s(Lex) or the sialyllacto-N-biose and s(Lea), respectively. Furthermore, the inflammatory cytokine TNF could enhance some alpha 2,3-ST and alpha 1,3/1,4-FT activities capable of generating E-selectin counter-receptors. Taken together, these data show that COLO 205 and HT-29 adenocarcinoma cell lines adhere to E-selectin in a TNF-inducible manner via their cell-surface s(Lex) and s(Lea). These cells also express mRNA as well as inducible enzyme activities of several alpha 2,3-STs and alpha 1,3/1,4-FTs responsible for the final steps in the synthesis of s(Lex) and s(Lea).(ABSTRACT TRUNCATED AT 250 WORDS)