Anabolic sialosylation of gangliosides in situ in rat brain cortical slices

Gangliosides in Membrane Organization

Since the structure of GM1 was elucidated 55years ago, researchers have been attracted by the sialylated glycans of gangliosides. Gangliosides head groups, protruding toward the extracellular space, significantly contribute to the cell glycocalyx; and in certain cells, such as neurons, are major determinants of the features of the cell surface. Expression of glycosyltransferases involved in the de novo biosynthesis of gangliosides is tightly regulated along cell differentiation and activation, and is regarded as the main metabolic mechanism responsible for the acquisition of cell-specific ganglioside patterns. The resulting sialooligosaccharides are characterized by a high degree of geometrical complexity and by highly dynamic properties, which seem to be functional for complex interactions with other molecules sitting on the same cellular membrane (cis-interactions) or soluble molecules present in the extracellular environment, or molecules associated with the surface of other cells (trans-interactions). There is no doubt that the multifaceted biological functions of gangliosides are largely dependent on oligosaccharide-mediated molecular interactions. However, gangliosides are amphipathic membrane lipids, and their chemicophysical, aggregational, and, consequently, biological properties are dictated by the properties of the monomers as a whole, which are not merely dependent on the structures of their polar head groups. In this chapter, we would like to focus on the peculiar chemicophysical features of gangliosides (in particular, those of the nervous system), that represent an important driving force determining the organization and properties of cellular membranes, and to emphasize the causal connections between altered ganglioside-dependent membrane organization and relevant pathological conditions.

Remodeling of sphingolipids by plasma membrane associated enzymes

The sphingolipid plasma membrane content and pattern is the result of several processes, among which the main, in term of quantity, are: neo-biosynthesis in endoplasmic reticulum and Golgi apparatus, membrane turnover with final catabolism in lysosomes and membrane shedding. In addition to this, past and recent data suggest that the head group of sphingolipids can be opportunely modified at the plasma membrane level, probably inside specific membrane lipid domains, by the action of enzymes involved in the sphingolipids metabolism, working directly at the cell surface. The number of membrane enzymes, hydrolases and transferases, acting on membrane sphingolipids is growing very rapidly. In this report we describe some properties of these enzymes.

Deregulated sphingolipid metabolism and membrane organization in neurodegenerative disorders

Sphingolipids are polar membrane lipids present as minor components in eukaryotic cell membranes. Sphingolipids are highly enriched in nervous cells, where they exert important biological functions. They deeply affect the structural and geometrical properties and the lateral order of cellular membranes, modulate the function of several membrane-associated proteins, and give rise to important intra- and extracellular lipid mediators. Sphingolipid metabolism is regulated along the differentiation and development of the nervous system, and the expression of a peculiar spatially and temporarily regulated sphingolipid pattern is essential for the maintenance of the functional integrity of the nervous system: sphingolipids in the nervous system participate to several signaling pathways controlling neuronal survival, migration, and differentiation, responsiveness to trophic factors, synaptic stability and synaptic transmission, and neuron-glia interactions, including the formation and stability of central and peripheral myelin. In several neurodegenerative diseases, sphingolipid metabolism is deeply deregulated, leading to the expression of abnormal sphingolipid patterns and altered membrane organization that participate to several events related to the pathogenesis of these diseases. The most impressive consequence of this deregulation is represented by anomalous sphingolipid-protein interactions that are at least, in part, responsible for the misfolding events that cause the fibrillogenic and amyloidogenic processing of disease-specific protein isoforms, such as amyloid beta peptide in Alzheimer's disease, huntingtin in Huntington's disease, alpha-synuclein in Parkinson's disease, and prions in transmissible encephalopathies. Targeting sphingolipid metabolism represents today an underexploited but realistic opportunity to design novel therapeutic strategies for the intervention in these diseases.

Gangliosides as regulators of cell membrane organization and functions

Gangliosides, characteristic complex lipids present in the external layer of plasma membranes, deeply influence the organization of the membrane as a whole and the function of specific membrane associated proteins due to lipid-lipid and lipid-protein lateral interaction. Here we discuss the basis for the membrane-organizing potential of gangliosides, examples of ganglioside-regulated membrane protein complexes and the mechanisms for the regulation of ganglioside membrane composition.

Purification and characterization of alpha (2-6)-sialyltransferase from human liver

A Gal beta 1-4GlcNAc alpha (2-6)-sialyltransferase from human liver was purified 34,340-fold with 18% yield by dye chromatography on Cibacron Blue F3GA and cation exchange FPLC. The enzyme preparation was free of other sialyltransferases. It did not contain CMP-NeuAc hydrolase, protease, or sialidase activity, and was stable at -20 degrees C for at least eight months. The donor substrate specificity was examined with CMP-NeuAc analogues modified at C-5 or C-9 of the N-acetylneuraminic acid moiety. Affinity of the human enzyme for parent CMP-NeuAc and each CMP-NeuAc analogue was substantially higher than the corresponding Gal beta 1-4GlcNAc alpha (2-6)-sialyltransferase from rat liver.

A highly sensitive fluorometric assay for sialyltransferase activity using CMP-9-fluoresceinyl-NeuAc as donor

This paper presents a very sensitive fluorometric assay for sialyltransferase activity based on the transfer of 5-acetamido-9-deoxy-9-fluoresceinylthioure-idoneuraminic acid onto distinct glycoproteins, thus allowing determination of acceptor specificities. Acceptor protein-bound fluorescence was quantified after gel filtration which separated fluorescent sialoglycoprotein from the fluorescence-labeled CMP-glycoside donor. Kinetic constants obtained for five different purified sialyltransferases indicated that CMP-9-fluoresceinyl-NeuAc was a suitable donor substrate for each enzyme, affording low Km values and Vmax values comparable in magnitude (15-100%) to that obtained with the parent CMP-NeuAc. Sensitivity was enhanced 200- to 1000-fold compared to the radiometric sialyltransferase assay as it is used routinely. The method was applied to determination of the kinetic properties of purified rat liver alpha 2,6-sialyltransferase with four separate glycoprotein acceptors differing in glycan structure, employing very small amounts of donor, acceptor, and enzyme, and to the study of sialyltransferase activity of the human promyelocytic cell line HL-60 toward three different acceptors.