Normal and abnormal biosynthesis of gangliosides

Polypeptide N-acetylgalactosamine transferase 3: a post-translational writer on human health

Polypeptide N-acetylgalactosamine transferase 3 (ppGalNAc-T3) is an enzyme involved in the initiation of O-GalNAc glycan biosynthesis. Acting as a writer of frequent post-translational modification (PTM) on human proteins, ppGalNAc-T3 has key functions in the homeostasis of human cells and tissues. We review the relevant roles of this molecule in the biosynthesis of O-GalNAc glycans, as well as in biological functions related to human physiological and pathological conditions. With main emphasis in ppGalNAc-T3, we draw attention to the different ways involved in the modulation of ppGalNAc-Ts enzymatic activity. In addition, we take notice on recent reports of ppGalNAc-T3 having different subcellular localizations, highlight critical intrinsic and extrinsic functions in cellular physiology that are exerted by ppGalNAc-T3-synthesized PTMs, and provide an update on several human pathologies associated with dysfunctional ppGalNAc-T3. Finally, we propose biotechnological tools as new therapeutic options for the treatment of pathologies related to altered ppGalNAc-T3. KEY MESSAGES: ppGalNAc-T3 is a key enzyme in the human O-GalNAc glycans biosynthesis. enzyme activity is regulated by PTMs, lectin domain and protein-protein interactions. ppGalNAc-T3 is located in human Golgi apparatus and cell nucleus. ppGalNAc-T3 has a central role in cell physiology as well as in several pathologies. Biotechnological tools for pathological management are proposed.

Glycosyltransferase complexes in eukaryotes: long-known, prevalent but still unrecognized

Glycosylation is the most common and complex cellular modification of proteins and lipids. It is critical for multicellular life and its abrogation often leads to a devastating disease. Yet, the underlying mechanistic details of glycosylation in both health and disease remain unclear. Partly, this is due to the complexity and dynamicity of glycan modifications, and the fact that not all the players are taken into account. Since late 1960s, a vast number of studies have demonstrated that glycosyltransferases typically form homomeric and heteromeric complexes with each other in yeast, plant and animal cells. To propagate their acceptance, we will summarize here accumulated data for their prevalence and potential functional importance for glycosylation focusing mainly on their mutual interactions, the protein domains mediating these interactions, and enzymatic activity changes that occur upon complex formation. Finally, we will highlight the few existing 3D structures of these enzyme complexes to pinpoint their individual nature and to emphasize that their lack is the main obstacle for more detailed understanding of how these enzyme complexes interact and function in a eukaryotic cell.

Anti-ganglioside anti-idiotypic vaccination: more than molecular mimicry

Surgery, chemotherapy, and radiation therapy are standard modalities for cancer treatment, but the effectiveness of these treatments has reached a plateau. Thus, other strategies are being explored to combine with the current treatment paradigms in order to reach better clinical results. One of these approaches is the active immunotherapy based on the induction of anti-tumor responses by anti-idiotypic vaccination. This approach arose from Jerne’s idiotypic network theory, which postulates that B lymphocytes forms a functional network, with a role in the establishment of the immune repertoires, in the regulation of natural antibody production and even in the establishment of natural tolerance. Due to the large potential diversity of the immunoglobulin variable regions, the idiotypes repertoire can mimic the universe of self and foreign epitopes, even those of non-protein nature, like gangliosides. Gangliosides are sialic acid-containing glycolipids that have been considered attractive targets for cancer immunotherapy, based on the qualitative and quantitative changes they suffer during malignant transformation and due to their importance for tumor biology. Although any idiotype could be able to mimic any antigen, only those related to antigens involved in functions relevant for organism homeostasis, and that in consequence has been fixed by evolution, would be able not only to mimic, but also to activate the idiotypic cascades related with the nominal antigen. The present review updates the results, failures and hopes, obtained with ganglioside mimicking anti-idiotypic antibodies and presents evidences of the existence of a natural response against gangliosides, suggesting that these glycolipids could be idiotypically relevant antigens.

Excitotoxicity induces changes in rat brain gangliosides

The patterns of major gangliosides in the rat hippocampi and olfactory bulbs was examined in vivo after microinjections of Ibotenic acid and L-BOAA (NMDA and AMPA receptor agonists, respectively) which were given under free-movement conditions. The excitotoxicity induced by injections of Ibotenic acid promoted transient ganglioside changes in olfactory bulbs and permanent changes in hippocampus. Four days after injections, the amount of gangliosides in the hippocampus increased significantly for GQ1b, GT1b and GD1b and decreased in the olfactory bulb for GQ1b, GT1b, GD1b, GD1a and GM1 compared to normal ganglioside levels. The alterations of gangliosides were minimal 1 day after injections. After 5 weeks, the amounts of GQ1b, GT1b and GD1b dramatically decreased in the hippocampus while in the olfactory bulbs gangliosides recovered to normal levels. The results obtained with L-BOAA 4 days after injections strengthen the results observed in the experiments using Ibotenic acid and corroborate our suggestion that gangliosides have an active role in the compensatory mechanism to maintain the number of glutamate receptors during the excitotoxicity.

GD3 and GM2 synthase activities in rat testes during the period of sexual development

Activities of two key enzymes of gangliosides biosynthesis were determined in rat testes during development. GD3 synthase activity was low and showed small variations with age. GM2 synthase activity increased 10-fold in testes from 10- to 30-d-old animals, showing a maximum activity at 30 d, followed by a small decrease until 45 d and then a constant activity up to adulthood. These developmental changes in the activity of both glycosyltransferases were related to the increasing complexity in the ganglioside pattern observed in rats testes during the period of sexual development.

Incorporation of 14C-galactose into gangliosides of rabbit lens

Rabbit lenses were incubated in organ culture with 14C-galactose for 6, 12 and 20 hours. Gangliosides were extracted using the Folch-Suzuki method, purified by dialysis and analyzed by thin-layer chromatography. Six radioactive bands, including the origin, were observed. Tentative identification of these bands as N-acetylneuraminylgalactosylglucosylceramide (GM3), N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (GM2), galactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)- galactosylglucosylceramide (GM1), N-acetylneuraminylgalactosyl-N- acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (GD1a), N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-(N- acetylneuraminyl-N-acetylneuraminyl)-galactosylglucosylceramide (GT) was made by comparison with authentic standard gangliosides. Galactose incorporation into GM3 and GM2 increased during the first 12 hours but decreased during the period from 12 to 20 hours. GD1a and GT incorporated the greatest amount of label during the period from 12 to 20 hours. Incorporation of labeled galactose into GM1 was nearly constant during this time period. Specific activities for GM1, GM3 and GT were nearly the same at 6 hours and were about half those of GM2 and GD1a for the same time period.

Lateral distribution of gangliosides in bilayer membranes: lipid and ionic effects

Native Ricinus communis agglutinin (RCA 60) has been used as a visual marker to localize the ganglioside GM1 under a variety of conditions in model membranes. By employing the technique of freeze-etch electron microscopy, it was possible to resolve membrane structural details down to some 3 nm. The most striking feature was frequent existence of the marker lectin in clusters--presumably reflecting the underlying presence of clustered receptor. This feature persisted at glycolipid concentrations from 0.5 to 7 mol %. It was visible in membranes of single pure phospholipids above and below their phase transition temperatures, in membranes of mixed phospholipids, and in membranes containing cholesterol. The presence of cations, including Ca2+, was not seen to alter the pattern of lectin binding at a resolution of 3 nm. In pure dipalmitoyl phosphatidylcholine, a lining-up of glycolipid in the "troughs" between ripples in rigid lipid was apparent, in agreement with a similar phenomenon reported by Tillack et al for a neutral glycosphingolipid in pure dimyristoyl phosphatidylcholine [1982: Biochim Biophys Acta 691:261-273]; but this feature was not evident in host matrices composed of several different lipids.

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)

Evidence of a distribution difference between two gangliosides in bilayer membranes

Freeze-etch electron microscopy, a platinum shadowing technique, has been used to compare the lateral distribution of several gangliosides in bilayer model membranes by directly visualizing bound lectin molecules. In particular, GM1 and GD1a, major components of brain ganglioside, were studied in phase-separated mixtures of dipalmitoyl- and dielaidoylphosphatidylcholines exposed to Ricinus communis agglutinin and wheat germ agglutinin. The distribution of glycolipid showed evidence of microheterogeneity in that bound lectin tended to occur in clusters of several or more molecules. With GD1a as receptor such clusters were small and very uniformly distributed over the membrane surface. Somewhat larger, irregularly spaced clusters of up to a dozen lectin particles were more typical of membranes bearing GM1 and, in addition, there were occasional extensive patches of bound lectin coexisting with areas apparently devoid of glycolipid receptor in phase-separated mixtures of dipalmitoyl- and dielaidoylphosphatidylcholine. Gangliosides in the latter mixtures were not obviously influenced in their lateral distribution by the presence of coexisting fluid and rigid domains. These basic observations seem to extend to bilayer membranes containing mixtures of two gangliosides. The patterns of lectin binding were not grossly affected by incubation time or history of warming and cooling. This study was extended to bilayers of pure dipalmitoylphosphatidylcholine in expectation that the distinctive features characteristic of the P beta' phase of this lipid might accentuate any behavioural differences between GM1 and GD1a. GM1 was found to exist preferentially in the 'trough' regions between P beta' ripples, while GD1a showed no apparent preferential arrangement. Given that bound lectins adequately reflect glycolipid distribution in membranes, it would appear that structurally different glycolipids from the same host membrane can assume different distributions on the basis of interactions with defined lipid host matrices.

Translocation of newly synthesized gangliosides to the cell surface

A new method was developed to follow the translocation of gangliosides from their site of synthesis within the cell to the plasma membrane. Cultured mouse neuroblastoma N18 and rat glioma C6 cells were labeled for increasing times with D- [1-3H]galactose and then subjected to mild oxidation with NaIO4. Under the conditions chosen, oxidation was essentially restricted to cell-surface sialic acid residues, which were converted to derivatives with an aldehyde function. The labeled gangliosides were isolated from the cells and reacted with dinitrophenylhydrazine to form dinitrophenyl (DNP) derivatives of the oxidized gangliosides. The DNP-gangliosides then were separated from their unmodified counterparts by thin-layer chromatography. Thus, the rate of labeling of surface gangliosides was distinguished from the rate of labeling of total gangliosides. Our results indicated that the transfer of gangliosides from the site of synthesis to the cell surface required approximately 20 min and that newly synthesized gangliosides appeared to be transported to the plasma membrane at a constant rate. No essential differences were found in the rates of translocation of different ganglioside species by N18 cells or between gangliosides of N18 and C6 cells.

Biosynthesis in vitro of mono- and di-sialosylgangliosides from gangliotetraosylceramide by cultured cell lines and young rat brain. Structure of the products, and activity and specificity of sialosyltransferase

Incubations in vitro of GA1, labeled with 3H in the terminal D-galactopyranosyl group, with nonradioactive CMP-NeuNAc in the presence of homogenates of C21 rat brain glial cells, NIE mouse neuroblastoma cells, 3T3 mouse fibroblasts, SV 40-transformed 3T3 cells, chick embryo fibroblasts, Rous sarcoma virus-transformed chick embryo fibroblasts, and 9-day old rat brain resulted in all cases in the formation in high yield of GM1b, in which the neuraminidase-labile NeuNAc group is linked at O-3 of the terminal D-galactosyl residue, as shown by permethylation studies. No trace of the naturally occurring neuraminidase-stable GM1a was detected in any case. In addition, with NIE cells, and normal and RSV-transformed chick embryo fibroblasts, a disialosylganglioside (GD1) differing from GD1a and GD1b, and bearing only one substituent at O-3 of the terminal D-galactopyranosyl residue was formed. It was also biosynthesized from GM1b and CMP-NeuNAc by NIE and chick embryo cells but not by C21 cells, or rat brain. However, C21 cells and rat brain were capable of synthesizing GD1a from GM1a. Periodate oxidation degraded both NeuNAc groups in GD1 to a 7-carbon fragm:nt, indicating lack of substitution at O-8. GM1b could not be detected as a natural product in rat brain.

Genetic variability for regional brain gangliosides in five strains of young mice

The quantitative and qualitative distributions of gangliosides were determined in the cerebrum, cerebellum, and brain stem of five inbred strains (C57BL/6J, DBA/2J, LG/J, C3H/HeJ, BALB/cJ) of mice at 21 days of age. Genetic differences were found among the strains for wet weight, absolute amount of gangliosides per region, and concentration of ganglioside (expressed on bolth a wet and a dry weight basis) in all three regions of the brain. The water content of the various brain regions showed the least amount of genetic variability. Coefficients of genetic determination were used to estimate the magnitude of genetic influence on these traits in each brain region. Significant differences were also found among the five strains for the distribution of certain gangliosides. The DBA strain, which is susceptible to audiogenic seizure at this age, had the highest level of the myelin-enriched ganglioside GM1 in all brain regions. Most of the genetic variation that influences the content and distribution of gangliosides among neurologically normal mice can be considered polygenic. Several possible sources of this genetic variation that may contribute to the differences observed among the strains are discussed.

Tissue distribution of EDTA encapsulated within liposomes containing glycolipids or brain phospholipids

Multilameller liposomes were prepared with various asialoglycolipids, gangliosides, sialic acid, or brain phospholipids in the liposome membrane and with ethylenediaminetetraacetic acid (EDTA) encapsulated in the aqueous compartments. The liposomes containing glycolipids or sialic acid were prepared from a mixture of phosphatidylcholine, cholesterol, and one of the following test substances: galactocerebroside, glucocerebroside, galactocerebroside sulfate, mixed gangliosides, monosialoganglioside GM1, monosialoganglioside GM2, monosialoganglioside GM3, disialoganglioside GD1a, or sialic acid. The liposomes containing brain phospholipids were mixtures of either sphingomyelin and cholesterol or a brain total phospholipid extract and cholesterol. Distributions of 14C-labeled EDTA were determined in mouse tissues from 15 min to 6 h or 12 h after a single injection of liposome preparation. Liver uptake up encapsulated EDTA was lowest from all liposome preparations containing sialic acid or sialogangliosides, regardless of the amount of sialic acid moiety present or the identity of the particular ganglioside; highest uptake of encapsulated EDTA by liver was from liposomes containing galactocerebroside or brain phospholipids. Lungs and brain took up the largest amounts of EDTA from liposomes containing sphingomyelin and lesser amounts from liposomes containing GD1a. Use of mouse brain phospholipid extract to prepare liposomes did not increase uptake of encapsulated EDTA by the brain. EDTA in liposomes containing monosialogangliosides, brain phospholipids, galactocerebroside, or sialic acid was taken up well by spleen and marrow. Highest thymus uptake of encapsulated EDTA was from liposomes containing GD1a. These results demonstrate that inclusion of sialogangliosides in liposome membranes decreases uptake of liposomes by liver, thus making direction of encapsulated drugs to other organs more feasible. Liposomes containing glycolipids also have potential uses as probes of cell surface receptors.

The biosynthesis of brain gangliosides. Separation of membranes with different ratios of ganglioside sialylating activity to gangliosides

Brain subcellular fractions were analysed for ganglioside-sialylating activity by measuring the incorporation of N-[3H]acetylneuraminic acid from CMP-N-[3H]acetylneuraminic acid into endogenous ganglioside acceptors (endogenous incorporation) and into exogenous lactosyceramide (haematoside synthetase activity). The ratios of endogenous incorporation to gangliosides and of haematoside synthetase to gangliosides for the synaptosomal and mitochondrial fractions from a washed crude mitochondrial fraction were lower than those obtained for other membrane fractions. The differences appear to reflect intrinsic characteristics of each membrane fraction. The results of labelling in vitro and the time course of labelling of gangliosides of the different subcellular fractions in vivo after injection of N-[3H]acetylmannosamine are consistent with the possibility of a subcellular site for synthesis of gangliosides different from that of ganglioside deposition.

Biosynthesis and function of gangliosides

Gangliosides are unique acidic glycolipids that are selectively concentrated in the plasma membrane of cells. Surface labeling studies have demonstrated that at least a portion of the oligosaccharde chain of gangliosides extends beyond the hydrophe) is imbedded in the membrane bilayer. It is becoming increasingly apparent that gangliosides participate in the internalization of environmental signals elicited by cholera toxin and glycoprotein hormones such as thyrotropic hormone and chorionic gonadotropin as well as other substances such as interferon and possibly serotonin. The mechanism by which cholera toxin binds to a specific ganglioside receptor on the celraction of trophic agents with gangliosides. We would predict that analyogous phenomena involving gangliosides will be discovered in brain. The biosynthesis of gangliosides proceeds by the ordered sequential addition of sugars to the lipid moiety. These reactions are catalyzed by a cluster of membrane-bound glycosyltransferases. Any alteration in the activity or specificity of one of these enzymes will result in a dramatic change in the ganglioside pattern of an afflicted cell or organ. The drastic consequences that accompany abnormalities of ganglioside synthesis have been documented in a heritable metabolic disorder in vivo and in tumorigenic transformation of cells in vitro. In this article, we have attempted to unify these observations and to provide a reasonable interpretation of the role of gangliosides in mediating cell surface phenomena.

Isolation of a Golgi-apparatus-enriched fraction from leukaemic cells

1. A Golgi-apparatus-enriched fraction was isolated from acute leukaemic lymphoblasts of AKR mice by using an homogenate stabilized with 1 mM-glutaraldehyde. 2. The isolated fraction, which was shown morphologically to be enriched in dictyosomes, possessed between 44- and 76-fold increase in specific activity, compared with the tumour homogenate, of UDP-galactose-glycoprotein galactosyltransferase and between 3- and10.5-fold increase in relative specific activity of UDP-N-acetygalactosamine-polypeptide N-acetylgalactosaminyltransferase. 3. Plasma membranes isolated from the leukaemic lymphoblasts also possessed glycoprotein galactosyltransferase activity, though in contrast with Golgi-apparatus-enriched material had no detectable polypeptide N-acetygalactosaminyltransferase. 4. The difficulties associated with maintaining the morphological integrity of the Golgi apparatus in subcellular fractionation are discussed.

Gangliosides of hepatoma 27, normal and regenerating rat liver

The highly malignant rat hepatoma 27 was found to have increased amounts of lipid-bound sialic acid as compared with normal liver whereas in regenerating liver the lipid-bound sialic acid level was reduced. In contrast to the liver the hepatoma contained higher amounts of disialogangliosides and no trisialogangliosides, which are abundant in the liver. The main disialoganglioside of the hepatoma had no analogue among the liver gangliosides and was identified as Gal-GalNAc(AcNeu-AcNeu)-Glc-Cer (GD1b), which in other tissues is known to be a precursor of trisialogangliosides. These findings may be explained by a reduced activity of glycosyltransferases in the hepatoma and apparently do not simply reflect differences in growth rate since the ganglioside pattern of regenerating rat liver was not altered significantly in comparison with the liver. Liver and hepatoma microsomes were found to be enriched in gangliosides as compared with whole cells, liver mitochondria were slightly poorer, while the ganglioside level of hepatoma mitochondria was much higher than that of the hepatoma cells. It thus appears that the existing image of the plasma membranes as the only sites of high ganglioside concentration may not hold true for weakly differentiated hepatomas of high malignancy.

Scanning electron microscopy of in vitro chemically transformed mouse embryo cells

A cloned nontumorigenic control cell line of C3H mouse embryo cells (C3H/1OT1/2CL8) and two cell lines derived from it by treatment in vitro with 7,12-dimethylbenz(a)anthracene (DMBA) or 3-methylcholanthrene (MCA) were studied by scanning electron microscopy. Confluent control cells were polygonal in shape and extensively flattened with smooth surfaces. Both in vitro transformants were pleomorphic to fusiform in shape, thicker than the control cells, and lacked contact inhibition. Microvilli of variable length and small marginal ruffles were characteristic surface alterations of the MCA-transformed cells, while blebs and numerous cytoplasmic strands extending between cells were typical of the DMBA transformant. Inoculation of the DMBA-transformed cells into C3H mice and re-establishment of cells from one of the subsequent fibrosarcomas in culture revealed an increased number of microvilli on the surface of the cells and an alteration in growth pattern. Other surface characteristics remained the same. A possible relationship between surface topography and outer membrane glycolipids is discussed.