A radioactive assay for sialyltransferase activity using 96-well multiscreen filtration plates

Dietary sialic acid supplementation improves learning and memory in piglets

BACKGROUND

Sialic acid, a key component of both human milk oligosaccharides and neural tissues, may be a conditional nutrient during periods of rapid brain growth.

OBJECTIVE

We tested the hypothesis that variations in the sialic acid content of a formula milk would influence early learning behavior and gene expression of enzymes involved in sialic acid metabolism in piglets.

DESIGN

Piglets (n = 54) were allocated to 1 of 4 groups fed sow milk replacer supplemented with increasing amounts of sialic acid as casein glycomacropeptide for 35 d. Learning performance and memory were assessed with the use of easy and difficult visual cues in an 8-arm radial maze. Brain ganglioside and sialoprotein concentrations and mRNA expression of 2 learning-associated genes (ST8SIA4 and GNE) were measured.

RESULTS

In both tests, the supplemented groups learned in significantly fewer trials than did the control group, with a dose-response relation for the difficult task (P = 0.018) but not the easy task. In the hippocampus, significant dose-response relations were observed between amount of sialic acid supplementation and mRNA levels of ST8SIA4 (P = 0.002) and GNE (P = 0.004), corresponding with proportionate increases in protein-bound sialic acid concentrations in the frontal cortex.

CONCLUSIONS

Feeding a protein-bound source of sialic acid during early development enhanced learning and increased expression of 2 genes associated with learning in developing piglets. Sialic acid in mammalian milks could play a role in cognitive development.

Development of a S. cerevisiae whole cell biocatalyst for in vitro sialylation of oligosaccharides

Absence of sialylation on recombinant glycoproteins compromises their efficacy as therapeutic agents, as it results in rapid clearance from the human bloodstream. To circumvent this, several strategies are followed, including the implementation of a post-secretion glycosylation step. In this paper we describe the engineering of yeast cells expressing active surface exposed Trypanosoma cruzi trans-sialidase (TS) fused to the yeast Aga2 protein, and the use of this yeast in the sialylation of synthetic oligosaccharides. In an attempt to improve overall protein accessibility on the yeast surface, we abolished hyperglycosylation on the yeast cell wall proteins. This was achieved by disrupting the OCH1 gene of the TS surface expressing strain, which resulted in increased enzymatic activity. Using a fluorescence-based activity assay and DSA-FACE structural analysis, we obtained almost complete conversion to a fully sialylated acceptor, whereas in the wild type situation this conversion was only partial. Increasing protein accessibility on the yeast surface by modifying the glycosylation content thus proved to be a valuable approach in increasing the cell wall associated activity of an immobilised enzyme, hence resulting in a more effective biocatalyst system.

A rapid, semi-automated method for detection of Galbeta1-4GlcNAc alpha2,6-sialyltransferase (EC 2.4.99.1) activity using the lectin Sambucus nigra agglutinin

Sialyltransferase activity has traditionally been studied by determining the rate at which the enzyme transfers a labeled donor sugar to an acceptor substrate. These types of assays can be difficult to quantitate, and the separation of untransfered donor sugar from the sialylated acceptor is time-consuming. The biosensor-based method described here is both rapid and semi-automated. The NeuAc-alpha2-6Gal-R-specific lectin Sambucus nigra agglutinin (SNA) immobilized to the carboxymethyl dextran surface of a BIAcore sensor chip was used to detect and measure the formation of the NeuAc-alpha2-6Gal-R moieties. The sialyltransferase assays were carried out using modified protocols based on the method described in Rearick, J.I., Sadler, J.E., Paulson, J.C., and Hill, R.L. (1979) Enzymatic characterization of betaD-galactoside alpha2-3 sialyltransferase from porcine submaxillary gland. J. Biol. Chem., 254, 4444-4451. The complete assay mixture was simply diluted before injection into the instrument. All injections were performed automatically using the robotics of the BIAcore instrument. Using this technique it is possible to detect product from 0.4 microU of commercial Galbeta1-4GlcNAc alpha2,6-sialyltransferase (EC 2.4.99.1) (ST6Gal I). One unit of sialyltransferase is defined as the quantity that will transfer 1 micromol of N-acetylneuraminic acid from cytidine monophosphate (CMP)-N-acetylneuraminic acid to asialofetuin per min at pH 6.5 and 37 degrees C. The method described here requires as little as 10 microl total assay volume, thus reducing the consumption of reagents. In addition, the sample is completely recoverable from the sensor chip surface, which allows for downstream analysis of the reaction product if desired. This method eliminates the need for labeled donor and acceptor molecules and does not require the separation of the substrates from the product before analysis. Although some kinetic properties of the enzyme can be estimated using this method, further development and validation is required. The method is most useful in determining qualitative estimates of ST6Gal I activity in tissue extracts and in characterizing the production of enzymes in cultured cell systems. The use of a microtiter plate assay format enables the rapid screening of multiple fractions for sialyltransferase activity.

Characterization of sialyltransferase mutants using surface plasmon resonance

Sialyltransferases are enzymes responsible for the important sialylation of glycoconjugates. Since crystal structures are not available, other tools are needed to study enzymatic mechanisms. As a model, we used human alpha2,6-sialyltransferase. A putative acceptor-binding domain containing the small and the very small sialyl motifs was randomly mutated. This resulted in enzymes with altered enzymatic activity. Affinity chromatography demonstrated that their binding to donor substrate was maintained. To illustrate the role of the mutated domain in acceptor binding, a method based on surface plasmon resonance was set up. Only at low salt and high acceptor concentration was association of wild-type ST6GalI with asialofetuin demonstrated. As expected, this interaction was affected by cytidine 5'-monophospho-N-acetylneuraminic acid, the donor substrate, which proves the specificity of the interaction. Different types of mutants were found. For some, the drop in activity could be explained by loss in affinity for the acceptor. For others, the catalytic center, but not the acceptor-binding site, was affected. Neither acceptor binding nor catalytic activity were limited to the sialyl motifs. To our knowledge, this is the first example in which surface plasmon resonance is successfully used to demonstrate the binding of a glycosyltransferase to its natural acceptor.

One-step separation of endocytic organelles, Golgi/trans-Golgi network and plasma membrane by density gradient electrophoresis

Many different methods for the fractionation of subcellular organelles have been reported. However, no protocol for rapid separation of plasma membrane, Golgi/trans-Golgi network (TGN) and endosomes is available to date. Such a method is a prerequisite for a quantitative biochemical analysis of vesicular transport from the Golgi/TGN compartment to plasma membrane and endosomes. Here a density gradient electrophoresis protocol is described that allows the fractionation of these organelles in one step. This protocol requires only low-cost instrumentation available in most biochemical laboratories.

Thin-layer chromatography and polyacrylamide gel electrophoresis-based assays for sialyltransferases using tetramethylrhodamine-labeled acceptors

Two novel assay systems for the determination of sialyltransferase activity using a tetramethylrhodamine-labeled disaccharide Galbeta1-4GlcNAc (2) as the acceptor are described. The TMR-labeled disaccharide 2 was synthesized by directly coupling Galbeta1-4GlcNAc-O-(CH(2))(6)NH(2) (1) with 5-tetramethylrhodamine N-hydroxysuccinimide ester. The K(m) value for compound 2 obtained with alpha-2,6-sialyltransferase from rat liver (EC 2.4.99.1) was 160 +/- 20 microM. After incubation of compound 2 with sialyltransferase the product and the unreacted acceptor substrate were separated either by thin-layer chromatography (TLC) on C-18 silica gel plates or by polyacrylamide gel electrophoresis (PAGE). The density of the spots on the TLC plates and the fluorescence of the bands on the gel were quantified. The assay conditions were optimized using crude bovine colostrum extract and also alpha-2, 6-sialyltransferase from rat liver. The detection limits for the TLC and PAGE assays were 1 and 0.4 microU of the rat liver enzyme, respectively. Either assay allows the parallel investigation of several samples at a time and is useful for the testing of fractions during enzyme purification.

In vitro conversion of the carbohydrate moiety of fungal glycoproteins to mammalian-type oligosaccharides--evidence for N-acetylglucosaminyltransferase-I-accepting glycans from Trichoderma reesei

To investigate the potential of filamentous fungi to synthesize N-glycans that are convertible to a mammalian type, in vitro glycosylation assays were performed. Recombinant human N-acetylglucosaminyltransferase I, human beta-1,4-galactosyltransferase and rat alpha-2,6-sialyltransferase were successively used to mimic part of the mammalian glycosylation synthesis pathway. High-mannose carbohydrates on Trichoderma reesei cellobiohydrolase I were converted to a hybrid mammalian-type structure. Successful modification varied markedly with the strain of T. reesei used to produce cellobiohydrolase I. In vitro pretreatment of fungal glycoproteins with Aspergillus saitoi alpha-1,2-mannosidase improved subsequent hybrid formation. It was, however, not possible to trim all fungal oligosaccharides to an acceptor substrate for mammalian glycosyltransferases. With T. reesei RUTC 30, capping glucose residues and phosphate groups were shown to be responsible for this lack of trimming. N-glycan processing in T. reesei apparently involves different steps, including alpha-1,2-mannosidase trimmings, and thus resembles the first mammalian glycosylation processes. The alpha-1,2-mannosidase trimming steps can be exploited for further in vitro and/or in vivo synthesis of complex oligosaccharides on (heterologous) glycoproteins from filamentous fungi.