Occurrence of glycosylation and deglycosylation of exogenously administered ganglioside GM1 in mouse liver

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.

Uptake of exogenous gangliosides by rat brain synaptosomes

Synaptosomes incorporated mixed brain gangliosides at a rapid initial rate followed by a slower phase of net movement from the protein-associated fraction into the membrane core. The pattern of incorporated gangliosides reflected the pattern available for incorporation. Intact synaptosomes incorporated approximately 100 pmol GM1/mg protein. Synaptosomes preincubated with proteolytic enzymes (trypsin, chymotrypsin, and papain) at different pH values (6.2, 7.4, 7.8) incorporated more exogenous gangliosides than synaptosomes preincubated in buffer alone. This effect was maximal at pH 7.8, though analysis of variance revealed that the proteolytic treatment and pH effects were probably independent processes. Overall uptake of exogenous gangliosides correlated significantly with amount of membrane protein loss, indicating that initial access of exogenous gangliosides to synaptosomal membranes is retarded by cell-surface proteins. These results suggest synaptosomes as a useful alternative to cultured cells for investigating the interaction of gangliosides with other cell surface constituents.

Presence in Human Erythrocyte Membranes of a Novel Form of Sialidase Acting Optimally at Neutral pH

The feature of intact human erythrocytes and erythrocyte white ghosts is a unique sialidase activity with acidic optimal pH (acidic sialidase). The treatment of white ghosts with mildly alkaline isotonic solutions at 37°C, like that used to produce resealed ghosts, is accompanied by the expression, together with the acidic sialidase, of a novel sialidase with a pH optimum of 7.2 (neutral sialidase) that remained masked in the inside-out vesicles prepared from white ghosts. Exhaustive treatment of resealed ghosts with Bacillus Thuringiensis phosphatidylinositol-specific phospholipase C causes an almost complete release of the acidic sialidase, with the neutral enzyme remaining totally unaffected. The treatment of resealed ghosts with 1.2% Triton X-100 resulted in the solubilization of only the neutral sialidase, whereas 3.6% octylglucoside also solubilized the acidic sialidase. The neutral enzyme affected not only the artificial substrate but also any sialoderivatives of a ganglioside, glycoprotein, and oligosaccharide nature; the acidic enzyme did not affect sialoglycoproteins. Erythrocyte endogenous gangliosides were hydrolyzed by both sialidases, whereas the endogenous sialoglycoproteins responded to only the neutral enzyme. It was definitely proved that the acidic sialidase is located on the outer erythrocyte membrane surface, so presumably the neutral enzyme has the same location. It could be that the newly discovered neutral sialidase has a physiologic role in the releasing of sialic acid from erythrocytes during the erythrocyte aging process, leading to eventual phagocytosis by macrophages.

Metabolic processing of gangliosides by normal and Salla human fibroblasts in culture. A study performed by administering radioactive GM3 ganglioside

Cultured fibroblasts from normal subjects and from subjects affected by Salla disease, characterized by the lack or misfunction of the membrane carrier responsible for the egress of sialic acid from lysosomes, were fed with ganglioside GM3 labeled at the sialic acid acetyl group, [Neu5Ac-3H]GM3, or at C-3 of sphingosine (Sph), [Sph-3H]GM3, or at C-1 of stearoyl chain, [stearoyl-14C]GM3. After a 15-h pulse the total amount of cell-bound GM3 corresponded to about 2% of the endogenous ganglioside content. Cells were then subjected to a 72-h chase, and the radioactive products from both ganglioside catabolism and salvage processes of catabolic fragments were measured. These data indicated that about 50% of the cell-bound ganglioside underwent metabolic processing, suggesting a ganglioside half-life of 2-3 days. [Neu5Ac-3H] formed from [Neu5Ac-3H]GM3 degradation was mostly re-cycled for the biosynthesis of gangliosides and sialoglycoproteins, only a minor part being degraded to [3H]water, which constituted only 1.6% of total metabolite linked radioactivity. [Sph-3H] from the [Sph-3H]GM3 degradation was partly re-cycled for the biosynthesis of gangliosides, neutral glycosphingolipids and sphingomyelin, and partly (about 20% of the total metabolite linked radioactivity) degraded to [3H]water. In Salla fibroblasts metabolic processing of [Neu5Ac-3H]GM3 produced large amounts of free [3H]Neu5Ac, and a reduced incorporation of radioactivity into glycoconjugates (as compared to normal cells). However, the accumulation of free Neu5Ac was not accompanied by an increase of tritiated water. LacCer and Cer from [stearoyl-14C]GM3 catabolism were found to accumulate in Salla fibroblasts, an indication that the enzymes of glycosphingolipid metabolism were affected by the impairment of Neu5Ac egress from lysosomes. Particularly relevant was the accumulation of ceramide which was hardly detectable in control cells.

Placental transfer of (3H)-GM1 and its distribution to maternal and fetal tissues of the rat

The demonstration that ganglioside GM1 pretreatment reduced the ethanol induced neurobehavioral effects in adult pups exposed to ethanol in utero, prompted study to examine whether GM1 crosses the placenta and penetrates fetal tissues. The present results indicate that 3H-galactose labeled GM1 not only passes the placenta but also served as a substrate for the synthesis of polysialogangliosides, and remained in various tissues up to 48 h after maternal (3H)-GM1 administration.

Specific ganglioside-cell protein interactions: a study performed with GM1 ganglioside derivative containing photoactivable azide and rat cerebellar granule cells in culture

The incubation of cultured rat cerebellar granule cells with a photoreactive derivative of radiolabeled GM1 ganglioside, [3H]GM1(N3), followed by illumination, led to the specific association of ganglioside to cell proteins. After 30 min of incubation only a few out of the cell proteins became radiolabeled. Two of these, at apparent molecular weights of 95 and 112 kDa, are interacting with the portion of associated ganglioside that is released by trypsin treatment; others, in the region between 31 and 44 kDa, are probably bound to molecules of ganglioside inserted into the outer membrane layer, thus showing that the ganglioside association to the cell surface is a selective phenomenon, involving specific proteins. Increasing the incubation time up to 24 h resulted in a larger number of radiolabeled proteins, probably as a consequence of the internalization and metabolic processing of administered [3H]GM1(N3). In fact, photoreactive and radioactive metabolic derivatives of [3H]GM1(N3) can also interact with a number of proteins. After 24 h incubation, some radioactivity was also associated to cytosolic proteins. Again in this case the interaction with proteins seems to be a specific process involving only a few out of the total cytosolic proteins.

Rat cerebellar granule cells in culture associate and metabolize differently exogenous GM1 ganglioside molecular species containing a C18 or C20 long chain base

A study has been made of the association properties of the two GM1 ganglioside molecular species GM1-C18 and GM1-C20 (containing C18 and C20 long chain bases, respectively) to rat cerebellar granule cells in culture. Both gangliosides recognized, to the same extent, and associated with them to give a form of association, the trypsin-labile form. This form was removed by treatment with trypsin enzyme. Both gangliosides associated stably with the cells to become components of the cell membranes. Although similar amounts of the two gangliosides entered the cells, being then metabolized, the time course of the association was different for the two gangliosides: after 15 h of ganglioside-cell incubation the amount of GM1-C18 inserted into the cell membrane was 2.43 times higher than that of GM1-C20.

Content, pattern and metabolic processing of rat-liver gangliosides during liver regeneration

During rat liver regeneration, the ganglioside content and distribution undergo significant changes after partial hepatectomy; total liver gangliosides increase remarkably till the 4th day after surgery, thereafter progressively decreasing to reach the values of sham-operated controls at the 12th day. The qualitative pattern is characterized by the 95% relative increase of GD1a at the 4th day and the 40% relative decrease of GD1b. In order to investigate the processes of ganglioside penetration into cells, degradation and biosynthesis, radiolabelled GM1 ([Sph-3H] GM1) was administered. One day after hepatectomy the liver uptake and metabolism of exogenous ganglioside were significantly reduced. Three days post-surgery these parameters were restored to control values; however an increased radioactivity incorporation was found in GD1a, thus suggesting an enhancement of its biosynthesis around the 4th day. The data reported here suggest that in the first two days after partial hepatectomy, the ganglioside degradation is reduced with a consequent increase of ganglioside content; later on the catabolic routes normalize and some biosynthetic processes leading to GD1a are enhanced. GD1a seems to be a marker of a peculiar transition phase of liver regeneration.

Recycling of glucosylceramide and sphingosine for the biosynthesis of gangliosides and sphingomyelin in rat liver

It was previously shown that sphingomyelin and gangliosides can be biosynthesized starting from sphingosine or sphingosine-containing fragments which originated in the course of GM1 ganglioside catabolism. In the present paper we investigated which fragments were specifically re-used for sphingomyelin and ganglioside biosynthesis in rat liver. At 30 h after intravenous injection of GM1 labelled at the level of the fatty acid ([stearoyl-14C]GM1) or of the sphingosine ([Sph-3H]) moiety, it was observed that radioactive sphingomyelin was formed almost exclusively after the sphingosine-labelled-GM1 administration. This permitted the recognition of sphingosine as the metabolite re-used for sphingomyelin biosynthesis. Conversely, gangliosides more complex than GM1 were similarly radiolabelled after the two treatments, thus ruling out sphingosine re-utilization for ganglioside biosynthesis. For the identification of the lipid fragment re-used for ganglioside biosynthesis, we administered to rats neutral glycosphingolipids (galactosylceramide, glucosylceramide and lactosylceramide) each radiolabelled in the sphingosine moiety or in the terminal sugar residue. Thereafter we compared the formation of radiolabelled gangliosides in the liver with respect to the species administered and the label location. After galactosylceramide was injected, no radiolabelled gangliosides were formed. After the administration of differently labelled glucosylceramide, radiolabelled gangliosides were formed, regardless of the position of the label. After lactosylceramide administration, the ganglioside fraction became more radioactive when the long-chain-base-labelled precursors were used. These results suggest that glucosylceramide, derived from glycosphingolipid and ganglioside catabolism, is recycled for ganglioside biosynthesis.

Subcellular biosynthesis and transport of gangliosides formed from exogenous lactosylceramide in rat liver

In order to clarify the mechanisms of ganglioside biosynthesis and transport we intravenously administered a liposomal dispersion of radiolabelled lactosylceramide (LacCer) to rats and then followed the time course of the individual gangliosides which became radioactive in the Golgi-apparatus and plasma-membrane fractions prepared from the liver. After administration of radiolabelled LacCer the liver retained a substantial amount of radioactivity, which was distributed among an organic phase (mainly residual LacCer), a fraction containing low-Mr substances (mainly 3H2O) and a ganglioside fraction. The hepatocytes were found to provide the bulk of gangliosides biosynthesized from exogenous LacCer. After subcellular fractionation, the total radioactive gangliosides increased in the Golgi apparatus up to 8 h, to then decrease and practically disappear at 24 h; in the plasma membranes they were progressively concentrated, accounting for high absolute values. Ganglioside patterns were greatly modified with time in both the Golgi apparatus and plasma membrane, but without significant differences between them. Biosynthesis in the Golgi apparatus and accumulation in the plasma membrane of each individual ganglioside followed a precursor-product relationship. The obtained results indicated that once a ganglioside is biosynthesized in the Golgi apparatus, it is in part made available for translocation to the plasma membrane, which rapidly occurs, and is in part retained in the Golgi apparatus, where it acts as a precursor for the biosynthesis of more glycosylated gangliosides.

Lactonization of GD1b ganglioside under acidic conditions

Gangliosides that contain the disialosyl residue alpha-Neu5Ac-(2--8)-alpha-Neu5Ac-(2--3)- can lactonize in the presence of traces of acid and this reaction has been studied in detail on GD1b [beta-Gal-(1--3)-beta-GalNAc-(1 --4)-[alpha-Neu5Ac-(2--8)-alpha-Neu5Ac-(2 --3)]-beta-Gal-(1--4)-beta-Glc-1--1)-Cer]. Lactonization occurs rapidly at a proton-ganglioside molar ratio of less than 1. At equilibrium, the ratio of GD1b to its lactone is 3:7. The data suggest the possibility that a proton-driven lactonization of gangliosides may occur in vivo.

Formation of ganglioside GD1b-lactone in rat brain from intracisternally administered GD1b

The presence of ganglioside GD1b, in lactone form GD1b-L, was ascertained in rat brain. The possible formation of GD1b-L from GD1b in brain was explored by the intracisternal injection of GD1b, 3H-labelled at the level of the terminal galactose. This was followed by recognition of the radioactive gangliosides formed at different times (1, 3, and 7 days) after injection. Whereas at 0 time after injection the only radioactive ganglioside was GD1b, after 1, 3, and 7 days other radioactive gangliosides were also found, thus indicating GD1b penetration into the brain tissue, followed by metabolic processing. Besides GD1b, the following radioactive gangliosides were recognized: GM1 and GM2, derived from GD1b degradation; GT1b, formed by the direct sialylation of GD1b; and GD1b-L, produced by metabolic lactonization. The radioactivity carried by GD1b-L was maximal 3 days after injection; its time course was different from that of the other gangliosides, suggesting that the process of lactonization is separate from that of both degradation and glycosylation. Under the same experimental conditions, some radioactive gangliosides also appeared in the liver, although in much smaller amounts than in brain. Radioactive GD1b-L could not be detected in liver, thus indicating that metabolic lactonization is a tissue- or organ-specific process.

Different metabolic recycling of the lipid components of exogenous sulphatide in human fibroblasts

Cultured human fibroblasts were fed with two differently labelled sulphatide molecules [one labelled on C-3 of the sphingosine (Sph) moiety [( Sph-3H]sulphatide), the second on C-1 of stearic acid [( stearoyl-14C]sulphatide)], and the intracellular metabolic fate of radioactivity was monitored. Incorporated radioactivity was almost all recovered in the total lipid extract, regardless of the labelling position of the added sulphatide; however, large differences in the level of incorporation occurred among labelled glycosphingolipids. For example, sphingomyelin was present as the major radiolabelled lipid after [Sph-3H]-sulphatide incubation, but was detectable only in trace amounts after [stearoyl-14C]sulphatide administration; in the latter case the radioactivity was located predominantly in glycerophospholipids. From this finding it can be inferred that the free long-chain base (sphingosine) that originates from lysosomal catabolism of sulphatide is mainly, and quite specifically, utilized for sphingomyelin biosynthesis, whereas the ceramide moiety is not; conversely the fatty acid released from ceramide is non-specifically re-utilized for phospholipid biosynthesis.

New trends in ganglioside chemistry

New methods have been developed for the preparation of highly purified gangliosides, homogeneous in the saccharide, long chain base, and fatty acid moieties and gangliosides carrying different kinds of labelled probes. Gangliosides, homogeneous in the oligosaccharide portion, were prepared by preparative normal phase HPLC on a Lichrosorb-NH-2 column, using a gradient of acetonitrile-phosphate buffer, pH 5.6, as solvent system. Each class of ganglioside (from monosialo- to tetrasialogangliosides) was then submitted to reversed phase HPLC on a preparative RP-8 column, using acetonitrile-5 mM phosphate buffer, pH 7, as solvent system, to obtain gangliosides homogeneous in the long chain base moiety. Gangliosides containing C18 and C20 sphinganine were prepared by catalytic hydrogenation of the corresponding unsaturated gangliosides. GM1 with homogeneous acyl chain was prepared by alkaline hydrolysis in the presence of tetramethylammonium hydroxide (which forms a GM1 deacetylated at the level of sialic acid, and a GM1 deacetylated at the level of sialic acid and deacylated at the level ceramide), followed by re-N-acylation, carried out in the presence of dimethylaminopropyl, ethylcarbodiimide and natural fatty acids, or of mixed anhydride of ethylchloroformate and 14C-stearic acid, and re-N-acetylation performed with acetic anhydride or labelled acetic anhydride. The GM1 derivative, de-acetylated at the level of sialic acid, also produced by alkaline treatment of GM1, was submitted to re-N-acetylation with 14C-acetic anhydride to produce specifically 14C-labelled GM1. Re-N-acylation was carried out a) in the presence of dimethylaminopropyl, ethylcarbodiimide and natural fatty acids, b) with mixed anhydride of ethylchloroformate and 14C-stearic acid. After re-N-acylations, re-N-acetylation was performed with acetic anhydride or labelled acetic anhydride. Gangliosides tritium labelled in the oligosaccharide moiety were prepared by the galactose oxidase/3H NaBH4 method, and gangliosides tritium labelled at carbon-3 of unsaturated long chain bases by the dicyano-dichlorobenzoquinone (DDQ)/3H NaBH4 method.

The sialic acid residue of exogenous GM1 ganglioside is recycled for biosynthesis of sialoglycoconjugates in rat liver

In order to assess metabolic recycling of sialic acid, GM1 ganglioside [nomenclature of Svennerholm (1964) J. Lipid. Res. 5, 145-155; IUPAC-IUB Recommendations (1977) Lipids 12, 455-468], 14C-radiolabelled at the acetyl group of sialic acid, was intravenously injected into Wistar rats, and the presence of radioactive sialic acid in liver sialoglycolipids (gangliosides) and sialoglycoproteins was ascertained. A time-course study (20 min-72 h) showed that the radioactivity present in the liver distributed in the following fractions, with reciprocal proportion varying with time: the protein (glycoprotein) fraction, the ganglioside fraction and the diffusible fraction, which contained low-Mr compounds, including sialic acid. Ganglioside-linked radioactivity gradually decreased with time; protein-linked radioactivity appeared soon after injection (20 min), reached a maximum around 20 h, then slowly diminished; diffusible radioactivity provided a sharp peak at 4 h, then rapidly decreased till disappearing after 40 h. The behaviour of bound radioactivity in the individual liver gangliosides was as follows: (a) rapid diminution with time in GM1, although with a lower rate at the longer times after injection; (b) early appearance (20 min) with a peak at 1 h, followed by continuous diminution, in GM2; (c) early appearance (20 min), peak at 1 h, diminution till 4 h, followed by a plateau, in GM3; (d) appearance at 60 min, maximum around 40 h and slow diminution thereafter, in GD1a, GD1b and GT1b. A detailed study, accomplished at 40 h after injection, demonstrated that almost all radioactivity present in the protein fraction was released by mild acid treatment and recovered in purified sialic acid; most of radioactive glycoprotein-bound sialic acid was releasable by sialidase action. In addition, the radioactivity present in the different gangliosides was exclusively carried by sialic acid and present in both sialidase-resistant and sialidase-labile residues. Only in the case of GD1a was the specific radioactivity of sialidase-resistant sialic acid superior to that of sialidase-releasable sialic acid. The results obtained lead to the following conclusions: (a) radioactive GM3 and GM2 were produced by degradation of GM1 taken up; GM3 originated partly by a process of neosynthesis; (b) radioactive GM1 consisted in part of residual exogenous GM1 and in part of a neosynthetized product; (c) radioactive GD1a originated in part by direct sialylation of GM1 taken up and in part by a neosynthetic process; (d) radioactive GD1b and GT1b resulted only from neosynthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

New chemical trends in ganglioside research

A report is given of recent progress in the methodology for isolation of gangliosides from natural sources, for the preparation of molecular species of gangliosides homogeneous in both the oligosaccharide and ceramide portions of the molecule, for chemical manipulation and derivatization of gangliosides, and for the preparation of gangliosides radiolabelled in different parts of the molecule. Particular emphasis has been given to: high performance liquid chromatographic procedures capable to separate gangliosides on the basis of their oligosaccharide or ceramide moieties and yielding completely homogeneous compounds, that is gangliosides with a single oligosaccharide, a single long chain base and a single fatty acid; two-dimensional thin-layer chromatographic procedures, provided with a fully computerized quantification system, particularly suitable to identifying gangliosides containing alkali-labile linkages, including ganglioside lactones; chemical procedures of high yield for reducing gangliosides at the double bond of long chain base, for selective removal of the fatty acyl moiety and replacement with a novel fatty acid, and for the synthesis of ganglioside lactones; chemical procedures for inserting fluorescent, paramagnetic or photoreactive probes at the fatty acyl part of the ganglioside molecule; procedures for chemical isotopic radiolabelling of gangliosides at the level of sialic acid acetyl group and at the fatty acid moiety. Examples are provided evidencing the significance and potential use of a variety of ganglioside derivatives in the study of ganglioside metabolism and functional implications.

Mouse liver gangliosides

The major gangliosides from mouse liver were purified and characterized by t.l.c., g.l.c., sialidase treatment, and a methylation study. GM3(NeuAc), GM3(NeuGc), GM2(NeuGc), GM1(NeuGc), and GDla(NeuGc, NeuGc) were identified. The structural identification of three of the gangliosides, GM2(NeuGc), GM1(NeuGc), and GDla(NeuGc, NeuGc), was supported by the results of 1H-n.m.r. analysis, and the structures of GM3(NeuGc), GM2(NeuGc), and GM1(NeuGc) were further confirmed by negative-ion fast-atom bombardment mass spectrometry. Ganglioside mapping showed that there was polymorphic variation of gangliosides in the liver of inbred strains of mice and that the major gangliosides were GM3(NeuGc) in WHT/Ht, GM2(NeuGc) in BALB/c and C3H/He, and GM2(NeuGc), GM1(NeuGc), and GDla(NeuGc, NeuGc) in ICR mice. Gangliosides containing N-acetylneuraminic acid, except for GM3(NeuAc), were not detected as major gangliosides in the strains of mice we analyzed.

Incorporation and metabolism of exogenous GM1 ganglioside in rat liver

The pathways of metabolic processing of exogenously administered GM1 ganglioside in rat liver was investigated at the subcellular level. The GM1 used was 3H-labelled at the level of long-chain base ([Sph(sphingosine)-3H]GM1) or of terminal galactose ([Gal-3H]GM1). The following radioactive compounds, derived from exogenous GM1, were isolated and chemically characterized: gangliosides GM2, GM3, GD1a and GD1b (nomenclature of Svennerholm [(1964) J. Lipid Res. 5, 145-155] and IUPAC-IUB Recommendations [(1977) Lipids 12, 455-468]); lactosylceramide, glucosylceramide and ceramide; sphingomyelin. GM2, GM3, lactosylceramide, glucosylceramide and ceramide, relatively more abundant shortly after GM1 administration, were mainly present in the lysosomal fraction and reflected the occurrence of a degradation process. 3H2O was also produced in relevant amounts, indicating complete degradation of GM1, although no free long-chain bases could be detected. GD1a and GD1b, relatively more abundant later on after administration, were preponderant in the Golgi-apparatus fraction and originated from a biosynthetic process. More GD1a was produced starting from [Sph-3H]GM1 than from [Gal-3H]GM1, and radioactive GD1b was present only after [Sph-3H]GM1 injection. This indicates the use of two biosynthetic routes, one starting from a by-product of GM1 degradation, the other implicating direct sialylation of GM1. Both routes were used to produce GD1a, but only the first one for producing GD1b. Sphingomyelin was the major product of GM1 processing, especially at the longer times after injection, and arose from a by-product of GM1 degradation, most likely ceramide.

Incorporation and metabolism of ganglioside GM2 in skin fibroblasts from normal and GM2 gangliosidosis subjects

Ganglioside GM2, 3H-labeled in the sphingoid base, was added to the culture medium of normal and GM2 gangliosidosis fibroblasts. Ganglioside was found to adsorb rapidly to the cell surface, most of it could however be removed by trypsination. The trypsin-resistant incorporation was about 10 nmol/mg cell protein, after 48 h. The rates of adsorption and incorporation depended strongly on the concentration of fetal calf serum in the medium, higher serum concentrations being inhibitory. After various incubation times, the lipids were extracted, separated by thin-layer chromatography and visualized by fluorography. In normal cells a variety of degradation products as well as sphingomyelin was found whereas in GM2 gangliosidosis cells, only trace amounts of such products (mainly GA2) were found. In contrast, the higher gangliosides GM1 and GD1a were formed in comparable amounts (2.2-3.6% of total radioactivity after 92 h) in normal and pathologic cell lines. Supplementation of cells from GM2 gangliosidosis, variant AB, with purified GM2-activator protein restored ganglioside GM2 degradation to almost normal rates but had no effect on its glycosylation to gangliosides GM1 and GD1a. From these results we conclude that the synthesis of higher gangliosides from incorporated GM2 can occur by direct glycosylation and not only via lysosomal degradation and resynthesis from [3H]sphinganine-containing degradation products. Preliminary studies with subcellular fractionation after various times of [3H]ganglioside incorporation indicated biphasic kinetics for the net transport of membrane-inserted ganglioside to lysosomes, compatible with the notion that a portion of the glycolipids can also escape from secondary lysosomes and migrate to Golgi compartment or cell surface.

Approaches in the study of ganglioside metabolism

Ganglioside GM1, 3H-labeled in the sphingosine or terminal galactose moiety was injected into mice and its metabolic fate in the liver was followed. After administration of sphingosine-labeled GM1 all major liver gangliosides (GM3, GM2, GM1, GD1a-NeuAc, NeuG1) became radioactive, the radioactivity residing in all cases on the sphingosine moiety. The specific radioactivity was highest on GM1, followed by GM2, GM3 and GD1a-NeuAc, NeuG1. Several neutral glycosphingolipids and sphingomyelin were also formed. After administration of galactose-labelled GM1 the only radioactive gangliosides present in the liver were GM1 and GD1a-NeuAc, NeuG1, both carrying the radioactivity on the terminal galactose residue, with no formation of labelled neutral glycosphingolipids. Subcellular studies gave clear evidence that GM1, after being taken up by the liver, was mainly degraded to GM2, GM3 and neutral glycosphingolipids at the level of lysosomes. A part of it was sialylated to more complex gangliosides and some of its metabolic by-products were used for the biosynthesis of other sphingolipid species, likely at the level of the Golgi apparatus. All this suggests that exogenous GM1 is introduced in the metabolic routes of endogenous gangliosides and of other sphingolipids, which are operating in the liver.