In vitro biosynthesis of the tetrasialoganglioside GQ1b

Regulation of ganglioside biosynthesis in the nervous system

Ganglioside biosynthesis is strictly regulated by the activities of glycosyltransferases and is necessarily controlled at the levels of gene transcription and posttranslational modification. Cells can switch between expressing simple and complex gangliosides or between different series within these two groups during brain development. The sequential biosynthesis of gangliosides in parallel enzymatic pathways, however, requires fine-tuned subcellular sequestration and orchestration of glycosyltransferases. A popular model predicts that this regulation is achieved by the vectorial organization of ganglioside biosynthesis: sequential biosynthetic steps occur with the traffic of ganglioside intermediates through subsequent subcellular compartments. Here, we review current models for the subcellular distribution of glycosyltransferases and discuss results that suggest a critical role of N-glycosylation for the processing, transport, and complex formation of these enzymes. In this context, we attempt to illustrate the regulation of ganglioside biosynthesis as well as the biological significance of N-glycosylation as a posttranslational regulatory mechanism. We also review the results of analyses of the 5' regulatory sequences of several glycosyltransferases in ganglioside biosynthesis and provide insights into how their synthesis can be regulated at the level of transcription.

Ganglioside GD3 biosynthesis in normal and mutant mouse embryos

CMP-sialic acid:GM3 sialyltransferase (GD3 synthase; EC 2.4.99.8) was characterized in a membrane-enriched preparation (P2 pellet) from mouse embryos at embryonic day 12 (E-12). Gangliosides GD3 and GM3 were the major radiolabeled products of the reaction. Optimum GD3 synthase activity was obtained at pH 6.0 using 0.1% detergent Triton CF-54. The Km values for GM3 and CMP-sialic acid were 55 and 80 microM, respectively. The Vmax value was calculated as 622 pmol/mg protein/hr. Ganglioside GD3, as end product, induced a two-step reduction of enzyme activity in the range of concentrations from 0 to 34 microM (40%) and from 150 to 300 microM (65%). The rate of GD3 formation was similar in whole embryos and in embryo head and body regions. GD3 synthase activity in tw1/tw1 mutant mouse embryos, which express defects in neuronal differentiation, was only 40% of that in normal wild-type (+/+) embryos. Enzyme activity in heterozygous (+/twl) embryos was similar to that in +/+ embryos. These findings suggest that the reduced GD3 synthase activity in the mutants might arise as a consequence of failed nervous system development and might reflect a secondary rather than a primary effect of the mutation.

GD3 expression by cultured human tumor cells of neuroectodermal origin

Seven monoclonal antibodies (mAbs) reactive with ganglioside II3(NeuAc)2-LacCer (GD3) were generated; four of these mAbs (DMAb-21, DMAb-22, DMAb-23, and DMAb-24) by immunizing mice with GD3 adsorbed to Salmonella minnesota and the remaining three (DMAb-7, DMAb-8, and DMAb-17) with melanoma line SK-MEL 28, which contains 1.4 nmol sialic acid of GD3 per mg protein. The specificities of the mAbs were defined by high-performance thin-layer chromatography (HPTLC) immunostain and solid-phase radioimmunoassay (SP-RIA) with a panel of purified gangliosides. DMAb-7 and DMAb-8 reacted with GD3, IV3(NeuAc)2nLcOse4Cer(3',8'-LD1), and very weakly with IV3(NeuAc)2II3NeuAcGgOse4Cer (GT1a), but not with II3NeuAc-LacCer (GM3), II3NeuAcGgOse3Cer(GM2), II3NeuAcGgOse4Cer (GM1), II3NeuAc, IV3NeuAcGgOse4Cer (GD1a), II3(NeuAc)2GgOse3(GD2), II3(NeuAc)2GgOse4Cer (GD1b), IV3NeuAcII3(NeuAc)2, GgOse4Cer(GT1b), suggesting the binding epitope to be a terminal tetrasaccharide NeuAc alpha 2-8NeuAc alpha 2-3Gal beta 1-4(Glc or GlcNAc). DMAb-7 and DMAb-8 were used to investigate the expression of GD3 on cultured human tumor cells of neuroectodermal origin. Thirteen of 19 gliomas, 3 of 5 medulloblastomas, 5 of 5 neuroblastomas, 2 of 2 melanomas, and 1 of 3 teratomas were shown to react with DMAb-8 and/or DMAb-7 by cell surface-RIA (CS-RIA) and immunofluorescence (IF) assays. HPTLC and densitometric analysis confirmed these results, as positive immunostains in the GD3 region were obtained with oligoganglioside fractions from 9 glioma, 1 medulloblastoma, 2 neuroblastoma, 1 melanoma, and 1 teratoma cell line. Glioma cell line U-105 MG and medulloblastoma cell line Daoy contain GD3 as shown by HPTLC immunostain analysis of extracts, although GD3 was undetectable on the cell surface as determined by CS-RIA and IF.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

The monoclonal antibody A2B5 is specific to ganglioside GQ1c

The neural specific monoclonal antibody A2B5 was found to interact with GQ1c but not with Gq1b, nor did it interact with other glycolipids such as GM1, GD1a, GD1b, GT1a, GT1b and GA1. Since GQ1c is enriched in embryonic chicken brains but not in adult chicken brains, this antibody should be useful as a tool in assessing the role of GQ1c in neuronal maturation.

Cellular localization of gangliosides in the developing mouse cerebellum: analysis using the weaver mutant

The distribution of gangliosides was studied in the weaver (wv/wv) mutant mouse, where the vast majority of postmitotic granule cell neurons die prior to their differentiation. The wv mutation also shows a dosage effect, as granule cell migration is slowed or retarded in the +/wv heterozygotes. By correlating changes in ganglioside composition with the well-documented histological events that occur during cerebellar development in the normal (+/+), heterozygous (+/wv), and weaver (wv/wv) mutant mice, information was obtained on the cellular localization and function of gangliosides. Ganglioside GM1 may be enriched in granule cell growth cones and play an important role in neurite outgrowth. A striking accumulation of GM1, which may result from altered metabolism, occurred in the adult wv/wv mice. GD3 was heavily concentrated in undifferentiated granule cells, but was rapidly displaced by the more complex gangliosides during differentiation. GD1a became enriched in granule cells during formation of synaptic and dendritic membranes, whereas GT1a appeared enriched in Purkinje cell synaptic spines. A possible fucose-containing ganglioside was quantitated only in the wv/wv mice. Ganglioside GT1b became enriched in granule cells during synaptogenesis, whereas GQ1b became enriched in these cells after synaptogenesis. The concentrations of GT1b and especially GQ1b increased continuously with age. Our results provide further evidence for a differential cellular enrichment of gangliosides in the mouse cerebellum and also suggest that certain gangliosides may be differentially distributed within the membranes of these cells at various stages of development.