Ganglioside-basic protein interaction: protection of gangliosides against neuraminidase action

Ganglioside GM3 suppresses lipopolysaccharide-induced inflammatory responses in rAW 264.7 macrophage cells through NF-κB, AP-1, and MAPKs signaling

Gangliosides are known to specifically inhibit vascular leukocyte recruitment and consequent interaction with the injured endothelium, the basic inflammatory process. In this study, we have found that the production of nitric oxide (NO), a main regulator of inflammation, is suppressed by GM3 on murine macrophage RAW 264.7 cells, when induced by LPS. In addition, GM3 attenuated the increase in cyclooxyenase-2 (COX-2) protein and mRNA levels in lipopolysaccharide (LPS)-activated RAW 264.7 cells in a dose-dependent manner. Moreover, GM3 inhibited the expression and release of pro-inflammatory cytokines of tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and interleukin-1β (IL-1β) in RAW 264.7 macrophages. At the intracellular level, GM3 inhibited LPS-induced nuclear translocation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and activator protein (AP)-1 in RAW 264.7 macrophages. We, therefore, investigated whether GM3 affects mitogen-activated protein kinase (MAPK) phosphorylation, a process known as the upstream signaling regulator. GM3 dramatically reduced the expression levels of the phosphorylated forms of ERK, JNK, and p38 in LPS-activated RAW 264.7 cells. These results indicate that GM3 is a promising suppressor of the vascular inflammatory responses and ganglioside GM3 suppresses the LPS-induced inflammatory response in RAW 264.7 macrophages by suppression of NF-κB, AP-1, and MAPKs signaling. Accordingly, GM3 is suggested as a beneficial agent for the treatment of diseases that are associated with inflammation.

Isolation and structure elucidation of GM4-type gangliosides from the Okinawan starfish Protoreaster nodosus

Three new ganglioside molecular species, termed PNG-1, PNG-2A, and PNG-2B were isolated from pyloric caeca of the starfish Protoreaster nodosus. Their structures were elucidated using a combination of spectroscopic and chemical methods, and characterized as 1-O-[8-O-methyl-N-acetyl-α-neuraminosyl-(2→3)-β-galactopyranosyl]-ceramide for PNG-1, 1-O-[β-galactofuranosyl-(1→3)-α-galactopyranosyl-(1→4)-8-O-methyl-N-acetyl-α-neuraminosyl-(2→3)-β-galactopyranosyl]-ceramide for PNG-2A, and 1-O-[β-galacto furanosyl-(1→3)-α-galactopyranosyl-(1→9)-N-acetyl-α-neuraminosyl-(2→3)-β-galactopyr anosyl]-ceramide for PNG-2B. PNG-2A and PNG-2B represent the first GM4 elongation products in nature.

Imaging of sialidase activity in rat brain sections by a highly sensitive fluorescent histochemical method

Sialidase (EC 3.2.1.18) removes sialic acid from sialoglycoconjugates. Since sialidase extracellularly applied to the rat hippocampus influences many neural functions, including synaptic plasticity and innervations of glutamatergic neurons, endogenous sialidase activities on the extracellular membrane surface could also affect neural functions. However, the distribution of sialidase activity in the brain remains unknown. To visualize extracellular sialidase activity on the membrane surface in the rat brain, acute brain slices were incubated with 5-bromo-4-chloroindol-3-yl-α-d-N-acetylneuraminic acid (X-Neu5Ac) and Fast Red Violet LB (FRV LB) at pH 7.3. After 1h, myelin-abundant regions showed intense fluorescence in the rat brain. Although the hippocampus showed weak fluorescence in the brain, mossy fiber terminals in the hippocampus showed relatively intense fluorescence. These fluorescence intensities were attenuated with a sialidase-specific inhibitor, 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (DANA, 1mM). Additionally, the fluorescence intensities caused by X-Neu5Ac and FRV LB were correlated with the sialidase activity measured with 4-methylumbelliferyl-α-d-N-acetylneuraminic acid (4MU-Neu5Ac), a classical substrate for quantitative measurement of sialidase activity, in each brain region. Therefore, staining with X-Neu5Ac and FRV LB is specific for sialidase and useful for quantitative analysis of sialidase activities. The results suggest that white matter of the rat brain has intense sialidase activity.

Ganglioside/glycosphingolipid turnover: new concepts

In this review focus is given to the metabolic turnover of gangliosides/glycosphingolipids. The metabolism and accompanying intracellular trafficking of gangliosides/glycosphingolipids is illustrated with particular attention to the following events: (a) the de novo biosynthesis in the endoplasmic reticulum and Golgi apparatus, followed by vesicular sorting to the plasma membrane; (b) the enzyme-assisted chemical modifications occurring at the plasma membrane level; (c) the internalization via endocytosis and recycling to the plasma membrane; (d) the direct glycosylations taking place after sorting from endosomes to the Golgi apparatus; (e) the degradation at the late endosomal/lysosomal level with formation of fragments of sugar (glucose, galactose, hexosamine, sialic acid) and lipid (ceramide, sphingosine, fatty acid) nature; (f) the metabolic recycling of these fragments for biosynthetic purposes (salvage pathways); and (g) further degradation of fragments to waste products. Noteworthy, the correct course of ganglioside/glycosphingolipid metabolism requires the presence of the vimentin intracellular filament net work, likely to assist intracellular transport of sphingoid molecules. ut of the above events those that can be quantitatively evaluated with acceptable reliability are the processes of de novo biosynthesis, metabolic salvage and direct glycosylation. Depending on the cultured cells employed, the percentage of distribution of de novo biosynthesis, salvage pathways, and direct glycosylation, over total metabolism were reported to be: 35% (range: 10-90%) for de novo biosynthesis, 7% (range: 5-10%) for direct glycosylation, and 58% (range: 10-90%) for salvage pathways. The attempts made to calculate the half-life of overall ganglioside turnover provided data of unsure reliability, especially because in many studies salvage pathways were not taken into consideration. The values of half-life range from 2 to 6.5 h to 3 days depending on the cells used. Available evidence for changes of ganglioside/glycosphingolipid turnover, due to extracellular stimuli, is also considered and discussed.

Salvage pathways in glycosphingolipid metabolism

In this review, the focus is on the role of salvage pathways in glycosphingolipid, particularly, ganglioside metabolism. Ganglioside de novo biosynthesis, that begins with the formation of ceramide and continues with the sequential glycosylation steps producing the oligosaccharide moieties, is briefly outlined in its enzymological and cell-topological aspects. Neo-synthesized gangliosides are delivered to the plasma membrane, where their oligosaccharide chains protrude toward the cell exterior. The metabolic fate of gangliosides after internalization via endocytosis is then described, illustrating: (a) the direct recycling of gangliosides to the plasma membrane through vesicles gemmated from sorting endosomes; (b) the sorting through endosomal vesicles to the Golgi apparatus where additional glycosylations may take place; and (c) the channelling to the endosomal/lysosomal system, where complete degradation occurs with formation of the individual sugar (glucose, galactose, hexosamine, sialic acid) and lipid (ceramide, sphingosine, fatty acid) components of gangliosides. The in vivo and in vitro evidence concerning the metabolic recycling of these components is examined in detail. The notion arises that these salvage pathways, leading to the formation of gangliosides and other glycosphingolipids, sphingomyelin, glycoproteins and glycosaminoglycans, represent an important saving of energy in the cell economy and constitute a relevant event in overall ganglioside (or glycosphingolipid, in general) turnover, covering from 50% to 90% of it, depending on the cell line and stage of cell life. Sialic acid is the moiety most actively recycled for metabolic purposes, followed by sphingosine, hexosamine, galactose and fatty acid. Finally, the importance of salvage processes in controlling the active concentrations of ceramide and sphingosine, known to carry peculiar bioregulatory/signalling properties, is discussed.

Association of GM4 ganglioside with the membrane surrounding lipid droplets in shark liver

By TLC, GM4 was found to be the major ganglioside in the liver of six shark species examined: Odontaspis taurus, Negaprion brevirostris, Sphyrna lewini, Mustelus griseus, Mustelus manazo, and Prionace glauca. A detailed analysis of the glycosphingolipids (GSLs) in the liver of O. taurus (sand tiger shark) showed that it contained approximately 110 nmol of lipid-bound sialic acid per gram of wet tissue, of which 80% was GM4. By extracting the liver of O. taurus with chloroform/methanol, followed by chromatographic separation of GSLs using DEAE-Sephadex A-25 and Iatrobeads columns, we have isolated GM4 in pure form with a yield of approximately 5 mg per 100 g of wet tissue. The structures of both the sugar chain and the ceramide moiety of this GM4 were analyzed by chemical analysis, mass spectrometry, and NMR spectroscopy. Similar to GM4 isolated from other sources, 92% of fatty acids in the ceramide of this GM4 were 2-hydroxylated. However, unlike the long-chain bases found in other GSLs, the total long-chain bases in this GM4 were found to contain 43% octadecasphingenine and 50% nonadecasphingenine. Immunohistochemical analysis using a monoclonal antibody against GM4 revealed that the hepatocytes of both M. griseus (spotless smooth hound) and M. manazo (smooth hound) were filled with lipid droplets and GM4 was primarily associated with the membrane structure surrounding lipid droplets.

Concerted modulation by myelin basic protein and sulfatide of the activity of phospholipase A2 against phospholipid monolayers

The effect of myelin basic protein (MBP) on the activity of phospholipase A2 (PLA2, EC 3.1.1.4) against monolayers of dilauroylphosphatidylcholine (dlPC) or dilauroylphosphatidic acid (dlPA) containing different proportions of sulfatide (Sulf) and galactocerebroside (GalCer) was investigated. MBP was introduced into the interface by direct spreading as an initial constitutive component of the lipid-protein film or by adsorption and penetration from the subphase into the preformed lipid monolayers. The effect of MBP on PLA2 activity depends on the type of phospholipid and on the proportion of MBP at the interface. At a low mole fraction of MBP, homogeneously mixed lipid-protein monolayers are formed, and the PLA2 activity against dlPC is only slightly modified while the degradation of dlPA is markedly inhibited. This is probably due to favorable charge-charge interactions between dlPA and MBP that interfere with the enzyme action. The PLA2 activity against either phospholipid is increased when the mole fraction of MBP exceeds the proportion at which immiscible surface domains are formed. GalCer has little effect on the modulation by MBP of the phospholipase activity. The effect of Sulf depends on its proportions in relation to MBP. The individual effects of both components balance each other, and a finely tuned modulation is regulated by the interactions of MBP with Sulf or with the phospholipid.

Myelin basic protein component C1 in increasing concentrations can elicit fusion, aggregation, and fragmentation of myelin-like membranes

Myelin basic protein (MBP) is considered to have a primary role in the formation and maintenance of the myelin sheath. Many studies using artificial vesicle systems of simple lipid composition, and generally small size, have shown that MBP can elicit vesicle fusion, aggregation, or even fragmentation under different conditions. Here, we have studied the effects of increasing concentrations of bovine MBP charge isomer C1 (MBP/C1) on large unilamellar vesicles (LUVs) composed of phosphatidylcholine and phosphatidylserine (92:8 molar ratio), or with a lipid composition similar to that of the myelin membrane in vivo (Cyt-LUVs). Using absorbance spectrophotometry, fluorescence resonance energy transfer, dynamic light scattering and transmission electron microscopy, we have shown that vesicle aggregation and some vesicle fusion occurred upon addition of MBP/C1, and as the molar protein-lipid ratio increased. Fragmentation of Cyt-LUVs was observed at very high protein concentrations. These results showed that the phenomena of vesicle fusion, aggregation, and fragmentation can all be observed in one in vitro system, but were dependent on lipid composition and on the relative proportions of protein and lipid.

Two mitogenic regions of myelin basic protein interact with different receptors to induce Schwann cell proliferation in a cAMP dependent process

Previous studies have shown that myelin basic protein (MBP) is mitogenic for Schwann cells (SCs) in the presence of elevated intracellular cAMP. Two mitogenic regions of MBP have been identified: one mitogenic region within the first 44 residues of the aminoterminus (1-44) and the other mitogenic region within the terminal 15 residues of the carboxyl end of the molecule (152-167). Unlike the mitogenic effect of a myelin enriched fraction (MEF), the mitogenic effect of MBP was not reduced by the addition of the lysosomal inhibitor, ammonium chloride. These data indicate that MBP causes SC proliferation by direct interaction of MBP with a surface receptor. Using Scatchard analysis of the binding of MBP to SCs, we report that treatment with forskolin does not cause the upregulation of receptors for MBP. Moreover, MBP blocks the cross-linking of 125I-bFGF with two fibroblast growth factor (FGF) receptors having apparent molecular weights of 140 kDa and 120 kDa, respectively. Since neither TGF-beta nor PDGF-BB displaced cell surface bound 125I-MBP, we conclude that MBP binds to the FGF receptor rather than other growth factor receptors. Furthermore, only MBP interacted with ganglioside GM1, whereas MBP did not interact with this ganglioside. These results are consistent with the view that ganglioside GM1 mediates the mitogenic effects of MBP, while the FGF receptor mediates the mitogenic effect of MBP. Intracellular cAMP of SCs was transiently increased after the addition of macrophage conditioned medium, suggesting that macrophages may produce factors in vivo which can transiently elevate intracellular cAMP levels, allowing a wave of SC proliferation in response to MBP-related mitogens.

Exogenous myelin basic protein promotes oligodendrocyte death via increased calcium influx

Treatment of cultured oligodendrocytes (OLGs) with micromolar quantities of myelin basic protein (MBP) caused a rapid, MBP-dose-dependent cell death. In contrast, a 72-hr incubation of OLGs with MBP peptides (1-44, 47-87, 88-151, or 152-167) at comparable concentrations had no effect on cell viability. MBP and MBP peptides (1-44 and 88-151) have been shown to interact with ganglioside GM1 (Tzeng et al.: J Neurochem Res: 42:758-767, 1995). This interaction has been reported to increase calcium influx. Therefore, using the fluorescent dye Indo-1 and an ACAS laser cytometer, we examined the level of intracellular calcium in OLGs after MBP treatment. MBP was shown to provoke a rapid, dramatic, and sustained rise of intracellular calcium in most OLGs. The levels of elevated intracellular calcium were sustained and did not return to baseline even after 10 min. This increase of intracellular calcium was suppressed in the presence of EGTA, indicating that the [Ca2+]i rise was due to the entry of extracellular calcium. Incubation of cultured OLGs with MBP peptides (1-44 or 88-151) caused a modest and transitory elevation of intracellular calcium ions in a lower percentage of OLGs. The potent OLG cytotoxicity of intact MBP and the loss of potency after proteolysis raise the possibility that MBP proteolysis during demyelination protects OLGs from death.

Sialyllactose-mediated cell interaction during granulosa cell differentiation. Identification of its binding proteins

The present study was designed to prove the carbohydrate-binding proteins interacting with cell surface sialyllactosylceramide (GM3, NeuAc alpha 2-->3Gal beta 1-->4Glc beta 1-->1'Cer), which is highly expressed during differentiation of rat ovarian granulosa cells. As a specific ligand for the sialyllactose (SL)-binding proteins on granulosa cells, we used a radioiodinated multivalent SL-linked albumin (Alb-(SL)17). The specific association of the ligand to the putative proteins on the intact cells was competitively inhibited by GM3 more effectively than other gangliosides, sialyllactotetraosylceramide, sialylneolactotetraosylceramide, and several glycoproteins with N-linked oligosaccharides. However, the proteins had no specificity for the side chain (N-acetyl or N-glycolyl forms) of sialic acid in GM3. Scatchard analysis of Alb-(SL)17 binding showed high (Kd = 6.4 x 10(-10)M) and low (Kd = 3.1 x 10(-8)M) affinity population of binding sites. By direct binding of 125I-Alb-(SL)17 to SL-binding proteins on Western blots, the putative proteins with molecular masses of 35, 18, and 14 kDa were detected. The interaction of the multivalent derivative with these binding proteins was differently modulated by Ca2+ and Mn2+. The SL-binding proteins occurred in immature granulosa cells and progressively decreased during differentiation, whereas their endogenous ligand GM3 increased. These results indicate that relatively low molecular weight SL-binding proteins exist on the surface of immature granulosa cells and that they may serve as receptor sites for newly synthesized GM3 during differentiation.

Characterization of sialidase activity in mouse synaptic plasma membranes and its age-related changes

Sialidase activity in synaptic plasma membranes (SPM) isolated from C57BL/6 mouse brain was examined using exogenous ganglioside substrates. The enzyme activity directed toward GM3 showed sharp pH dependency with optimal pH of 4.0, and was greatly enhanced by Triton CF-54, Nonidet P-40 or CHAPS. The apparent Km and Vmax values for enzyme activity in SPM were 11 microM and 164 pmol/mg protein/min, respectively. Examination of sialidase activities in subcellular fractions of brain tissues showed the enrichment of enzyme activity in SPM prepared from either young adult or senescent mice. Substrate specificity of SPM sialidase was compared with that of myelin sialidase using delipidated, solubilized enzyme preparations. The SPM sialidase hydrolyzed GD1a more effectively as compared with the myelin enzyme. While SPM sialidase could hydrolyze GM1, the hydrolytic rate by the SPM enzyme was significantly lower than that by the myelin enzyme. The sialidase activity in SPM decreased with increasing age; activity was highest between the ages of 4-7 months, decreased to a relatively constant level between 13-25 months, and reached its lowest level at 31 months. These results demonstrate that SPM contain a distinct sialidase activity which is regulated in an age-dependent manner.

Possible role of myelin-associated neuraminidase in membrane adhesion

Previous studies from our laboratory have demonstrated the presence of a specific interaction between myelin-associated neuraminidase and GM1 (Saito and Yu, J Neurochem 47:632-641, 1986). In the present study, we further characterized this neuraminidase-GM1 interaction and examined its role in the adhesion of rat oligodendroglial cells to GM1. Hydrolysis of N-acetylneuramin-lactitol by the enzyme was inhibited by GM1 in a competitive manner; GM1 itself was not hydrolyzed, suggesting that GM1 may serve as a competitive inhibitor of the enzyme. Asialo-GM1 had no inhibitory effect. When a soluble enzyme preparation was applied to a GM1-linked affinity column, the enzyme activity was retained on the column and was recovered from the column only by elution with a buffer containing 5 mM 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (Neu2en5Ac), a competitive inhibitor of neuraminidase. A binding study with 51Cr-labeled rat oligodendroglial cells showed that oligodendroglial cells bound preferentially to GM1 developed on a thin-layer plate, but not to other gangliosides such as GM3, GD1a, GD1b, and GT1b. The binding reaction to GM1 was inhibited by Neu2en5Ac (5 mM). These results suggest that myelin-associated neuraminidase specifically interacts with GM1 and may be involved in adhesion of oligodendroglial cells to GM1. This neuraminidase-GM1 interaction may play an important role in the formation and stabilization of the multilamellar structure of the myelin sheath.

Isolation of complex gangliosides from human brain

A simple procedure that enables the isolation of ganglioside GQ1b and other complex gangliosides from the human brain is described. The tissue was extracted with a mixture of chloroform, methanol, and water, and the extract purified twice by means of silica gel column chromatography and preparative HPTLC. Phase partition and ion exchange chromatography were omitted. The silica gel chromatography was based on a two step developing system, which provided an efficient separation of oligosialogangliosides. The yields of chromatographically homogenous fractions of ganglioside GQ1b isolated from the whole cerebrum, cerebellar cortex and occipital grey matter of a 60-year-old woman were 62, 138 and 110 nmol SA per g of fresh tissue. The problem of co-extraction of protein-positive material with gangliosides into the organic solvents is discussed. Chromatographic search of gangliosides in different regions of the human brain revealed the presence of small quantities of more complex gangliosides than GQ1b.

Neuraminidase activities in oligodendroglial cells of the rat brain

Neuraminidase activities in oligodendroglial cells were characterized using rats of different ages. Rat oligodendroglial cells had intrinsic neuraminidase activities directed toward GM3 and N-acetylneuramin(2-3)lactitol (NL). Developmental profiles of the neuraminidase activities toward the two substrates in oligodendroglial cells were different from each other. The neuraminidase activity toward GM3 increased rapidly with the onset of active myelination and, after 26 days of development, reached the adult level which was about 18 times higher than that in myelin. At the adult age, oligodendroglial cells had the highest neuraminidase activity toward GM3 among the individual brain cell types examined. The activity of NL-neuraminidase showed a less remarkable developmental profile, with a peak value at 26 days. The UDP-galactose:ceramide galactosyltransferase activity in oligodendroglial cells increased during the period of active myelination and, afterward, returned to the basal level. The enrichment and unique developmental profile in oligodendroglial cells of the neuraminidase activity toward GM3 suggest that this enzyme may play an important role in the formation and maintenance of the myelin sheath.

Role of myelin-associated neuraminidase in the ganglioside metabolism of rat brain myelin

The role of myelin-associated neuraminidase in ganglioside metabolism was examined using rats of ages ranging from 17 to 97 days. The neuraminidase activity directed toward the ganglioside GM3 in the total myelin fraction was high during the period of active myelination and, thereafter, decreased rapidly to the adult level. The ganglioside composition became simpler during development with an increasing amount of GM1 and decreasing percentages of di- and polysialogangliosides. The decrease in the proportion of GD1a was most prominent, whereas relative amounts of GD1b and GT1b increased transiently before reducing to the adult levels. The heavy myelin subfraction contained higher percentages of di- and polysialo-species compared to the light myelin fraction at young and adult ages. The in vitro incubation of myelin of young rats under an optimal condition for neuraminidase action produced a profile of ganglioside changes similar to that observed in in vivo development. These results strongly suggest that myelin-associated neuraminidase may play a pivotal role in the developmental changes in the ganglioside composition of rat brain myelin.

Ganglioside and lipid composition of bulk-isolated rat and bovine oligodendroglia

We have examined the ganglioside composition of 30-day and 60-day postnatal rat oligodendroglia, adult bovine oligodendroglia, gray matter, white matter, and myelin and also the total lipid composition of the oligodendroglial preparations. The ganglioside patterns of rat and bovine oligodendroglia, as previously found for human oligodendroglia, were more complex than those of myelin. These data indicate that oligodendroglial perikarya can synthesize many brain type gangliosides, not all of which are incorporated into the compact myelin. Alternatively, the ganglioside composition of myelin may be altered in situ by the myelin-associated neuraminidase. In these two species, as in human, GM4 appears specific to oligodendroglia and myelin, while GD3 and GM3 are enriched in oligodendroglia but not myelin. In bovine oligodendrocytes GD3 is the major ganglioside. The total lipid concentration, as well as the percentage of cholesterol, sphingomyelin, phosphatidylinositol, and phosphatidylserine, differ for 30- and 60-day-old rat oligodendroglia and may be developmentally correlated with changes in myelin composition during myelinogenesis. There are also marked differences in the lipid composition of bovine oligodendroglia compared to rat oligodendroglia, with the former having more galactolipid and less ethanolamine phosphoglycerides.

Soluble bovine galactose-binding lectin. cDNA cloning reveals the complete amino acid sequence and an antigenic relationship with the major encephalitogenic domain of myelin basic protein

A full-length cDNA clone for the 13-14 kDa soluble beta-galactoside-binding lectin was isolated from a bovine fibroblast cDNA library. The derived amino acid sequence shows eight differences from a preliminary partial amino acid sequence given previously for the bovine heart lectin. This observation led to a re-examination of the data and correction of the heart lectin protein sequence. Except for a possible polymorphism of the heart lectin at position 57, the fibroblast and heart lectin sequences are considered identical. The epitope recognized by two monoclonal anti-(bovine lectin) antibodies, 36/8 and 9/5, was identified as the tetrapeptide sequence W-G-A/S-E/D by the isolation of several different cDNA clones from a human intestine cDNA library. A similar tetrapeptide is present in all of the soluble beta-galactoside-binding animal lectins sequenced thus far. It is also found in myelin basic protein, which we show is antigenically cross-reactive with the lectin. In myelin basic protein the tetrapeptide is a part of the major domain previously shown to be responsible for the induction of experimental allergic encephalomyelitis.

Induction of experimental autoimmune encephalomyelitis in guinea pigs using myelin basic protein and myelin glycolipids

Strain 13 guinea pigs were immunized with galactocerebroside, asialo-GM1 (GA1) or GM4 ganglioside in association with myelin basic protein (MBP) in complete Freund's adjuvant (CFA) to produce experimental autoimmune encephalomyelitis (EAE). The clinical and pathological features, serum antibodies, and lipid compositions of affected brains and spinal cords were compared with those of guinea pigs immunized with MBP, in CFA, alone. Perivascular demyelination was seen in brains from all guinea pigs immunized with GA1/MBP. The incidence and degree of demyelination in this group were significantly higher than in the group immunized with only MBP. The onset of EAE was slightly, but significantly, retarded in groups of animals immunized with GM4/MBP and there was no detectable demyelination. Otherwise, no significant differences were detected between groups. Augmentation of EAE by myelin glycolipids may provide some important clues in understanding the mechanism of demyelinating diseases.

A novel fragmentation of human myelin basic protein: identification of phosphorylated domains

Human myelin basic protein (MBP) was fragmented into three major polypeptides comprised of a NH2-terminal domain (residues 1-83), a middle domain (residues 84-119) which contains an experimental allergic encephalitogenic determinant and a highly conserved triproline sequence, and a COOH-terminal domain (residues 120-170) by Staphylococcus aureus V8 protease at pH 4.0. These three polypeptides could be identified and purified by reversed-phase high-performance liquid chromatography. Analysis of the sites of phosphorylation of the component 1 of human MBP, the most cationic species, catalyzed by a purified Ca2+-activated and phospholipid-dependent protein kinase and cAMP-dependent protein kinase revealed that although these protein kinases could incorporate approximately 6 and 4 mol 32P, respectively, into MBP, none of the potential sites were located within the middle domain.

Molecular interactions and thermotropic behavior of glycosphingolipids in model membrane systems

The oligosaccharide chain of glycosphingolipids (GSLs) has a marked influence on their thermotropic behavior, intermolecular packing and surface electrical potential. The transition temperature and enthalpy of GSLs decrease proportionally to the complexity of the polar head group and show a linear dependence with the intermolecular spacings. Interactions occurring among GSLs and phospholipids induce changes of the molecular area and surface potential that depend on the type of GSLs. Increasing proportions of phospholipids perturb the thermodynamic properties of the GSLs up to a point where phase separated phospholipid domains separate out but no phase separation of pure GSLs occurs. Heterogeneous equilibria among different structures occur for some systems. Large changes of the molecular free energy, eccentricity, asymmetry ratio and phase state of the GSLs-containing structure can be triggered by small changes of the molecular parameters, lipid composition and lateral surface pressure. The thermotropic behavior of GSLs is considerably perturbed by myelin basic protein. Phase separation occurs depending on the amount of protein and type of GSLs. The protein induces a decrease of the lipid molecular area, the more so the more complex the oligosaccharide chain in the GSLs. These membrane systems can not be described only on the basis of the individual properties of the molecules involved in a simple causal manner. Still scarcely explored long range thermodynamic, geometric and field effects that belong simultaneously to the intervening molecules, to the morphological properties of the structure involved and to the aqueous environment, are important determinants of their behavior.

Sulfatide-binding proteins

Sulfatides (galactosyl ceramide-I3-sulfate) and other sulfated glycolipids are found in many tissues. The cell adhesion proteins laminin, thrombospondin, and von Willebrand factor bind specifically to sulfated glycolipids. Methods for characterizing the specificity of these interactions using surface-adsorbed glycolipids are reviewed. The three proteins do not bind to other anionic lipids, including gangliosides, phospholipids, or cholesterol 3-sulfate. Binding to sulfatides is saturable and of relatively high affinity. Relative binding avidity depends on the oligosaccharide structure of the glycolipids. Binding to sulfatides in erythrocyte membranes can account for the hemagglutinating activities of the three proteins and may play a role in the interactions of these proteins with other cell types.

Further characterization of a myelin-associated neuraminidase: properties and substrate specificity

A neuraminidase activity in myelin isolated from adult rat brains was examined. The enzyme activity in myelin was first compared with that in microsomes using N-acetylneuramin(alpha 2----3)lactitol (NL) as a substrate. In contrast to the microsomal neuraminidase which exhibited a sharp pH dependency for its activity, the myelin enzyme gave a very shallow pH activity curve over a range between 3.6 and 5.9. The myelin enzyme was more stable to heat denaturation (65 degrees C) than the microsomal enzyme. Inhibition studies with a competitive inhibitor, 2,3-dehydro-2-deoxy-N-acetylneuraminic acid, showed the Ki value for the myelin neuraminidase to be about one-fifth of that for the microsomal enzyme (1.3 X 10(-6) M versus 6.3 X 10(-6) M). The apparent Km values for the myelin and the microsomal enzyme were 1.3 X 10(-4) M and 4.3 X 10(-4) M, respectively. An enzyme preparation that was practically devoid of myelin lipids was then prepared and its substrate specificity examined. The "delipidated enzyme" could hydrolyze fetuin, NL, and ganglioside substrates, including GM1 and GM2. When the delipidated enzyme was exposed to high temperature (55 degrees C) or low pH (pH 2.54), the neuraminidase activities toward NL and GM3 decreased at nearly the same rate. Both fetuin and 2,3-dehydro-2-deoxy-N-acetylneuraminic acid inhibited NL and GM3 hydrolysis. With 2,3-dehydro-2-deoxy-N-acetylneuraminic acid, inhibition of NL was greater than that of GM3; however, the Ki values for each substrate were almost identical. GM3 and GM1 also competitively inhibited the hydrolysis of NL and NL similarly inhibited GM3 hydrolysis by the enzyme. These results indicate that rat brain myelin has intrinsic neuraminidase activities toward nonganglioside as well as ganglioside substrates, and that these two enzyme activities are likely catalyzed by a single enzyme entity.

Anti-glycolipid antibodies and their immune complexes in multiple sclerosis

Antibody titers against myelin constituents in sera and CSF of patients with multiple sclerosis (MS) were examined by a solid-phase radioimmunoassay. Anti-GM4 and anti-galactocerebroside antibody titers were significantly elevated in the CSF of MS patients, but not anti-GM1 and anti-myelin basic protein antibodies. In sera of MS patients, the titers of antibodies against these myelin constituents were not elevated. Total IgG level was also significantly elevated in the CSF, but not in the sera of MS patients. Immune complexes from the CSF of MS patients were dissociated by acid-ultrafiltration and assayed for antibodies to GM4, GM1, and galactocerebroside. Anti-GM4 and antigalactocerebroside antibody titers were significantly enhanced after acid dissociation and ultrafiltration. These data suggest that antibodies of the IgG class against GM4 and galactocerebroside are present in CSF of MS patients, and a significant number of them exist as immune complexes with their corresponding glycolipid antigens.

Prevention of experimental allergic encephalomyelitis by ganglioside GM4. A follow-up study

The myelin-specific ganglioside GM4, when incubated with myelin basic protein prior to inoculation, prevents experimental allergic encephalomyelitis in the guinea pig. The monosialoganglioside GM1, the predominant ganglioside of mammalian myelin, also prevents experimental allergic encephalomyelitis, whereas a mixture of neural gangliosides rich in polysialogangliosides causes enhanced disease. Guinea pigs inoculated with both myelin basic protein and GM4 develop serum IgG antibodies against basic protein, but not against GM4.

Further evidence for an intrinsic neuraminidase in CNS myelin

An intrinsic neuraminidase activity in rat brain CNS myelin has been demonstrated and compared with the neuraminidase activity in rat brain microsomes. With use of ganglioside GM3 as a substrate, the myelin-associated neuraminidase exhibited a shallow pH curve with an optimum at pH 4.8 whereas the microsomal activity had a marked optimum at pH 4-4.3. Neuraminidase activity in both fractions was optimized in 0.3% Triton CF-54 but activation was much greater in the microsomes. When the neuraminidase activities were examined at 60 degrees C, the myelin neuraminidase activity was more than sevenfold of that observed at 37 degrees C and was linear for at least 2 h; the microsomal activity increased only fivefold initially and exhibited a continual loss in activity. Addition of excess microsomes to the total homogenate prior to myelin isolation resulted in no change in myelin neuraminidase activity. When the two membrane fractions were examined at equivalent protein concentrations in the presence of additional cations or EDTA (1 mM), similar but not identical effects on neuraminidase activity were seen. The microsomal neuraminidase was considerably more susceptible to inhibition by divalent copper ion. Activity in both fractions was markedly inhibited by Hg2+ and Ag+ whereas EDTA had no effect on either activity. The myelin-associated neuraminidase activity was the highest in cerebral hemispheres, followed by brainstem, cerebellum, and spinal cord and was extremely low in sciatic nerve. In fact, the myelin neuraminidase activity was higher than the microsomal enzyme activity in the cerebral hemispheres.(ABSTRACT TRUNCATED AT 250 WORDS)

Acidic lipids enhance cathepsin D cleavage of the myelin basic protein

Some acidic lipids including sulfatide and phosphatidylinositol were found to increase greatly the rate of cathepsin D cleavage of the myelin basic protein. Since a similar effect was seen when the substrate was changed to cytochrome C, but not when the enzyme was changed to pepsin, these acidic lipids seem to be acting on cathepsin D rather than on myelin basic protein itself. Even so, chemical modification studies suggest that this phenomenon is only seen when the myelin basic protein is in its native conformation. Succinylation of MBP increases its rate of cleavage by cathepsin D by at least tenfold and, in addition, with this modified and presumably denatured MBP as substrate, activation of cathepsin D is no longer seen with acidic lipids. These findings suggest that the native conformation of MBP is both an important determinant of its rate of cleavage by cathepsin D and is also essential for observing activation of this reaction by acidic lipids. The acidic lipids seem to alter the "extended active site" of cathepsin D in such a way as to enable this enzyme better to utilize the native myelin basic protein as a substrate. Cathepsin D has previously been implicated as the protease responsible for the release into cerebrospinal fluid in multiple sclerosis patients of an encephalitogenic fragment derived from myelin basic protein. It is possible that the elevated levels of cathepsin D and sulfatide that have previously been found associated with multiple sclerosis plaques in vivo act in concert to bring about the rapid cleavage and subsequent loss of the myelin basic protein from these localized regions in the myelin sheath.

Gangliosides inhibit phospholipid-sensitive Ca2+-dependent kinase phosphorylation of rat myelin basic proteins

Gangliosides inhibit the phosphorylation of both small and large rat myelin basic proteins (SMBP, LMBP) by an endogenous phospholipid-sensitive Ca2+-dependent protein kinase (C-kinase). Using a rat brain myelin preparation in an in vitro phosphorylation assay system, we determined the inhibition constants (IC50's) of the gangliosides GM1, GD1a, GD1b, and GT1b to be approximately 160 microM, 65 microM, 65 microM, and 40 microM, respectively. Asialoganglioside GA1, ceramide, and N-acetylneuraminic acid (NANA, sialic acid) failed to produce similar inhibition, suggesting that both the lipid and the sialic acid moieties are necessary, but neither alone is sufficient to produce inhibition. The results indicate that gangliosides may regulate protein kinase C activities in the nervous system.

Myelin basic protein has lectin-like properties

Myelin basic proteins (MBP) obtained from bovine, guinea pig and human nervous tissues exhibited hemagglutinating activity for erythrocytes of several animal species. The activity to each erythrocyte was actually equal among the 3 MBPs. With bovine MBP and chicken erythrocytes, the hemagglutinating activity did not require divalent cations and it was resistant to denaturing conditions such as urea treatment or heating of the protein. The activity was specifically inhibited by antisera to MBP, allowing hemagglutination inhibition assay to be a useful method for estimating antibody titers for MBP. Various sugars and glycoproteins were tested for their ability to inhibit MBP-mediated hemagglutination. Among mono- and disaccharides, only D-galactose and D-galactosamine exhibited an inhibitory effect at high concentration. Beta-Galactopyranoside was found to be 8-fold effective compared to alpha-anomer. Among glycoproteins, glycophorin and kappa-casein were potent inhibitors, while fetuin, ovalbumin and ovomucoid were ineffective. Either of the former two glycoproteins was observed to form a clear precipitin band with MBP in Ouchterlony double diffusion. Possible interaction of MBP with certain saccharide receptor was briefly discussed.

Prevention of experimental allergic encephalomyelitis by ganglioside GM4

Guinea pigs inoculated with a mixture of myelin basic protein and the myelin-specific ganglioside, sialosylgalactosylceramide (GM4), do not develop the signs or neuropathology of experimental allergic encephalomyelitis. The results indicate that GM4 cloaks the basic protein molecule so that it is no longer immunopathogenic in these animals. The interaction of basic protein with GM4, previously shown in vitro, appears to be relevant to the pathogenesis of experimental allergic encephalomyelitis in guinea pigs.

Interaction of ganglioside GM1 and myelin basic protein studied by carbon-13 and proton nuclear magnetic resonance spectroscopy

The interaction of the myelin basic protein (MBP) and the major endogenous ganglioside GM1 in myelin of the central nervous system has been investigated using both 500-MHz 1H and 67.89 MHz 13C NMR. Titration of MBP by GM1 resulted in 13C NMR signal shifts for the I1e and His residues of MBP at a GM1/MBP mole ratio of one or less. The carbohydrate head group of GM1 was also found to be perturbed. 1H NMR results obtained in a similar manner demonstrated the perturbation of His and Phe residues. At a GM1/MBP mole ratio of 0.5, small perturbation of Trp #116 was observed, and at mole ratios of two and beyond significant involvement of Phe residues and methylated Arg #107 was found. Met #167 was more perturbed than Met #20; hence, more extensive interaction of the lipid is occurring with the C-terminus of the protein than with the N-terminus. No resonances from GM1 bound to MBP at mole ratios of up to one appeared in the spectra. However, as the GM1/MBP mole ratio was increased to eight or greater a major conformational change of MBP was detected. An upfield shift of the GM1 midchain methylene resonance was observed for the GM1/MBP complex. This observation provides strong evidence that the state of GM1 interacting with MBP is different from that of GM1 micelles. The number of saturable GM1 binding sites on MBP is estimated to be four. The data also favor a rapid exchange between bound GM1 and GM1 micelles. Interaction of MBP with the oligosaccharide derived from GM1 was found to be weaker than with GM1. Based on our data, a model for the interaction can be proposed: the first GM1 molecule is bound to the protein molecule through its head group and hydrocarbon chains, followed by the formation of a GM1/MBP complex with a concomitant conformational change of MBP as more GM1 is added.