Characterization of the binding epitope of the monoclonal antibody DMAb-1 to ganglioside GM2

Altered distribution of the gangliosides GM1 and GM2 in Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder where β-amyloid tends to aggregate and form plaques. Lipid raft-associated ganglioside GM1 has been suggested to facilitate β-amyloid aggregation; furthermore, GM1 and GM2 are increased in lipid rafts isolated from cerebral cortex of AD cases.

AIM/METHOD

The distribution of GM1 and GM2 was studied by immunohistochemistry in the frontal and temporal cortex of AD cases. Frontotemporal dementia (FTD) was included as a contrast group.

RESULTS

The distribution of GM1 and GM2 changes during the process of AD (n = 5) and FTD (n = 3) compared to controls (n = 5). Altered location of the GM1-positive small circular structures seems to be associated with myelin degradation. In the grey matter, the staining of GM1-positive plasma membranes might reflect neuronal loss in the AD/FTD tissue. The GM1-positive compact bundles were only visible in cells located in the AD frontal grey matter, possibly reflecting raft formation of GM1 and thus a pathological connection. Furthermore, our results suggest GM2 to be enriched within vesicles of pyramidal neurons of the AD/FTD brain.

CONCLUSION

Our study supports the biochemical finding of ganglioside accumulation in cellular membranes of AD patients and shows a redistribution of these molecules.

Structural membrane alterations in Alzheimer brains found to be associated with regional disease development; increased density of gangliosides GM1 and GM2 and loss of cholesterol in detergent-resistant membrane domains

The formation of neurotoxic beta-amyloid fibrils in Alzheimer's disease (AD) is suggested to involve membrane rafts and to be promoted, in vitro, by enriched concentrations of gangliosides, particularly GM1, and the cholesterol therein. In our study, the presence of rafts and their content of the major membrane lipids and gangliosides in the temporal cortex, reflecting late stages of AD pathology, and the frontal cortex, presenting earlier stages, has been investigated. Whole tissue and isolated detergent-resistant membrane fractions (DRMs) were analysed from 10 AD and 10 age-matched control autopsy brains. DRMs from the frontal cortex of AD brains contained a significantly higher concentration (micromol/micromol glycerophospholipids), of ganglioside GM1 (22.3 +/- 4.6 compared to 10.3 +/- 6.4, p <0.001) and GM2 (2.5 +/- 1.0 compared to 0.55 +/- 0.3, p <0.001). Similar increases of these gangliosides were also seen in DRMs from the temporal cortex of AD brains, which, in addition, comprised significantly lower proportions of DRMs. Moreover, these remaining rafts were depleted in cholesterol (from 1.5 +/- 0.2 to 0.6 +/- 0.3 micromol/micromol glycerophospholipids, p <0.001). In summary, we found an increased proportion of GM1 and GM2 in DRMs, and accelerating plaque formation at an early stage, which may gradually lead to membrane raft disruptions and thereby affect cellular functions associated with the presence of such membrane domains.

Lipids and glycosphingolipids in caveolae and surrounding plasma membrane of primary rat adipocytes

We have made a comprehensive and quantitative analysis of the lipid composition of caveolae from primary rat fat cells and compared the composition of plasma membrane inside and outside caveolae. We isolated caveolae from purified plasma membranes using ultrasonication in carbonate buffer to disrupt the membrane, or extraction with nonionic detergent, followed by density gradient ultracentrifugation. The carbonate-isolated caveolae fraction was further immunopurified using caveolin antibodies. Carbonate-isolated caveolae were enriched in cholesterol and sphingomyelin, and the concentration was three- and twofold higher, respectively, in caveolae compared to the surrounding plasma membrane. The concentration of glycerophospholipids was similar suggesting that glycerophospholipids constitute a constant core throughout the plasma membrane. The composition of detergent-insoluble fractions of the plasma membrane was very variable between preparations, but strongly enriched in sphingomyelin and depleted of glycerophospholipids compared to carbonate-isolated caveolae; indicating that detergent extraction is not a suitable technique for caveolae preparation. An average adipocyte caveola contained about 22 x 10(3) molecules of cholesterol, 7.5 x 10(3) of sphingomyelin and 23 x 10(3) of glycerophospholipid. The glycosphingolipid GD3 was highly enriched in caveolae, whereas GM3, GM1 and GD1a were present inside as well as outside the caveolae membrane. GD1b, GT1b, GM2, GQ1b, sulfatide and lactosylceramide sulfate were not detected in caveolae.

Gangliosides as therapeutic targets for cancer

Gangliosides, sialic acid-containing glycosphingolipids, have engendered great interest for more than 20 years in the search for target molecules of relevance for tumour growth and formation of metastases and as potential targets for immunotherapy. These molecules show large quantitative and structural variability, which is related to cell type and developmental stage. Their potential role in the formation of tumour metastases was suggested from data supporting that they are involved in cell growth regulation and in cell-cell and cell-matrix adhesion. Moreover, gangliosides are expressed on the cell surface and thereby are accessible for antibodies or other ganglioside-binding molecules to induce cell death, inhibit cell growth and/or inhibit formation of tumour metastasis. All tumours exhibit aberrant ganglioside expression. This includes overexpression of normal ganglioside constituents, which appears to be common among various tumours, and expression of gangliosides not found in normal adult tissue but often found during fetal development. The ganglioside composition of melanoma cells has been found to correlate with their metastatic potential and also to be selectively expressed in cells of a tumour mass and invading tumour cells. Passive immunotherapy using murine or murine/human chimeric monoclonal antiganglioside antibodies in their native form or combined with various effector molecules has been investigated. However, the vaccination strategy using native or structurally modified tumour-associated gangliosides in combination with adjuvants is currently the dominant method in clinical trials. The outcomes reported so far vary between type of tumour and treatment strategies. However, we believe that targeting gangliosides is as promising as any other immune therapeutic strategy, and basic research as well as clinical trials utilising new aspects is encouraged.

Selective lack of the C16:0 fatty acid isoform of sulfatide in pancreas of type II diabetic animal models

Sulfatide (3'-sulfogalactosyl-ceramide) is a glycosphingolipid mainly located in the nervous system, but has also been found in the islets of Langerhans. Previous studies have suggested that sulfatide is involved in insulin processing and secretion. In this study, sulfatide expression and metabolism in pancreas and isolated islets of the type II diabetes models, ob/ob- and db/db mouse, was investigated using TLC-ELISA, metabolic labelling and electron microscopy. As in non-diabetic Lewis rat and human pancreas, sulfatide was located in secretory granules of the beta cells. However, the type II diabetic animal models and their background strains had an altered sulfatide expression, involving the lack of the C16:0 sulfatide fatty acid isoform, compared to non-diabetic Lewis rat, BALB/c mouse and human pancreatic tissue, in which the two dominating pancreatic sulfatide isoforms C16:0 and C24:0 are expressed. Correspondingly, in isolated ob/ob islets, sulfatide synthesis excluded the production of C16:0 sulfatide. Insulin administration to ob/ob mouse, which lowers beta cell activity, resulted in significantly increased sulfatide expression in pancreas (p=0.0003), but still no expression of the C16:0 sulfatide isoform. In vitro, the C16:0 sulfatide was shown to be the isomer involved in the preservation of insulin crystals. Thus, it is hypothesized that the selection of sulfatide isomers in pancreas might be a genetic factor contributing to disease development in type II diabetic animal models.

Anti-ganglioside antibodies bind with enhanced affinity to gangliosides containing very long chain fatty acids

Gangliosides function in both physiological and pathological molecular recognition. Although much research has focused on the role of ganglioside glycans in recognition, fewer studies have addressed the role of the ceramide moiety. Ceramides of major brain gangliosides are composed predominantly of monounsaturated 18-carbon and 20-carbon long chain bases with a saturated 18-carbon fatty acid amide. In contrast, gangliosides of X-linked adrenoleukodystrophy patients are characterized by abnormal very long chain fatty acids that are proposed to be associated with autoimmune inflammation. In the current study we synthesized and characterized derivatives of the major brain ganglioside GD1a bearing defined very long chain fatty acid amides (C24:0, C24:1, and C26:0). When tested in a solid phase binding assay in the presence of auxiliary membrane lipids, GD1a species with long chain fatty acids were up to 8-fold more potent than normal brain GD1a in binding four different anti-GD1a monoclonal antibodies. These data support the hypothesis that gangliosides bearing very long chain fatty acids are differentially displayed on membranes, which may lead to altered antigenicity.

Dynamic association of human insulin receptor with lipid rafts in cells lacking caveolae

Cholesterol-sphingolipid rich plasma membrane domains, known as rafts, have emerged as important regulators of signal transduction. The adipocyte insulin receptor (IR) is localized to and signals via caveolae that are formed by polymerization of caveolins. Caveolin binds to IR and stimulates signalling. We report that, in liver-derived cells lacking caveolae, autophosphorylation of the endogenous IR is dependent on raft lipids, being compromised by acute cyclodextrin-mediated cholesterol depletion or by antibody clustering of glycosphingolipids. Moreover, we provide evidence that IR becomes recruited to detergent-resistant domains upon ligand binding and that clustering of GM2 ganglioside inhibits IR signalling apparently by excluding the ligand-bound IR from these domains. Our results indicate that, in cells derived from liver, an important insulin target tissue, caveolae are not required for insulin signalling. Rather, the dynamic recruitment of the ligand-bound IR into rafts may serve to regulate interactions in the initiation of the IR signalling cascade.

Monoclonal anti-GD3 antibodies selectively inhibit the proliferation of human malignant glioma cells in vitro

The frequently occurring alteration of ganglioside expression in tumor cells has been implicated to play a role in the uncontrolled growth of these cells; antibodies to such gangliosides might affect tumor cell growth. We have studied the effect of IgM monoclonal antibodies to two glioma-associated gangliosides, GD3 and GM2, on cell proliferation of four human glioma cell lines and one renal tumor cell line. Of the two anti-ganglioside antibodies tested, only the anti-GD3 antibody resulted in a significant (p<0.005) inhibition of cell proliferation as measured by thymidine incorporation and Brd-U labeling, after 24h incubation. The effect was not dependent on any serum factor and no increased cell death was observed. All cell lines contained higher or similar amounts of GM2 than GD3, and both antigens were shown to be expressed on the cell surface and accessible to antibodies. The selective effect of anti-GD3 antibodies as contrasted to the inactivity of anti-GM2 antibodies suggests a possible role for ganglioside GD3 in tumor cell proliferation.

Shedding of gangliosides by human medulloblastoma cells

Shedding of immunosuppressive gangliosides is an important characteristic of both experimental and human tumors. Using a medulloblastoma cell line, Daoy, with a very high ganglioside expression (141 +/- 13 nmol/10(8) cells) and a well-characterized ganglioside complement, we have now studied ganglioside shedding by human brain tumor cells. Shedding of gangliosides, quantified by metabolic radiolabeling, was significant (169 pmol/10(8) cells/h) and was generalized with respect to the major ganglioside carbohydrate structures (G(M2), G(M3), and G(D1a)). For each ganglioside, however, shedding was selective for ceramide structures containing shorter fatty acyl chains. Rapid and ceramide-selective shedding was confirmed in two additional human medulloblastoma cell lines, D341 Med and D283 Med (112 and 59 pmol/10(8) cells/h). Significant ganglioside shedding is therefore a common characteristic of human medulloblastoma cells and may influence the biological behavior of this tumor, in view of immunosuppressive and other biological properties of shed gangliosides.

Encephaloneuropathy with lysosomal zebra bodies and GM2 ganglioside storage

An 11-year-old girl died of a neuronal storage disorder that clinically was characterized by failure to thrive and muscular hypotonia from birth, with the subsequent evolution of motor neuron disease, epilepsy, and dementia. A wide range of metabolic disorders, including all forms of GM2 gangliosidosis, could be excluded. Electron microscopy demonstrated neuronal zebra body inclusions, and immunohistochemistry demonstrated that GM2 ganglioside was a major constituent of the storage material. We suggest that the patient died of a lysosomal storage disease that is clinically and biochemically different from Tay-Sachs disease, Sandhoff disease, and other GM2 gangliosidoses described previously. This case also further demonstrates that significant accumulation of GM2 ganglioside, which is crucial for dendritic formation, may occur in neuronal storage diseases lacking known defects in ganglioside catabolism.

T cell-independent B cell response to self-monosialogangliosides: primary response monoclonal antibodies

Purified GM1 and GM2 gangliosides incorporated into liposomes were injected subcutaneously in BALB/c mice every 3-4 days after pretreatment of the animals with low-dose cyclophosphamide. Serum samples were collected at different intervals and tests by ELISA for the presence of anti-ganglioside antibodies. Four doses (50 micrograms each) were sufficient to raise a measurable primary type of response to GM1, while nine doses were required to obtain measurable IgM antibody titers to GM2. Three monoclonal antibodies (MAbs) wer generated by fusing splenocytes with mouse myeloma cells. The specificity of MAbs was determined by ELISA and HPTLC-immunostaining using a panel of purified glycolipids. The MAb designated E1 showed a high degree of specificity because it reacted only with N-acetyl GM2. Monoclonal antibody A3 reacted predominantly with GM2 and GM1, but also reacted moderately with the GM3 ganglioside. The epitope recognized by this MAb is suggested to be the trisaccharide sequence GalNAc beta 1-4(NeuAc alpha 2-3)Gal. The third MAb (F6) reacted strongly with GM1 but a weak reactivity was also observed with GD1b as well as with asialo-GM1, indicating that the terminal tetrasaccharide Gal beta 1-3GalNAc beta 1-4(NeuAc alpha 2-3)Gal- structure is probably involved in antigenic recognition. Formalin-fixed and paraffin-embedded tissue sections were stained with the E1 and A3 MAbs, using the avidin-biotin complex (ABC) technique. Strong immunoreactivity for E1 appeared in the tumor cells of five primary lung carcinomas and in five malignant melanomas. No immunoreactivity was demonstrated in the parenchyma of a lung without malignancy, or in a metastasis from a colon carcinoma.

Ceramide structure predicts tumor ganglioside immunosuppressive activity

Molecular determinants of biological activity of gangliosides are generally believed to be carbohydrate in nature. However, our studies of immunomodulation by highly purified naturally occurring tumor gangliosides provide another perspective: while the immunosuppressive activity of gangliosides requires the intact molecule (both carbohydrate and ceramide moieties), ceramide structure strikingly influences ganglioside immunosuppressive activity. Molecular species of human neuroblastoma GD2 ganglioside in which the ceramide contains a shorter fatty acyl chain (C16:0, C18:0) were 6- to 10-fold more active than those with a longer fatty acyl chain (C22:0/C24:1, C24:0). These findings were confirmed in studies of ceramide species of human leukemia sialosylparagloboside and murine lymphoma GalNAcGM1b. Gangliosides that contain shorter-chain fatty acids (and are most immunosuppressive) are known to be preferentially shed by tumor cells. Therefore, the results suggest that the tumor cell is optimized to protect itself from host immune destruction by selective shedding of highly active ceramide species of gangliosides.

Antibody responses to ganglio-series gangliosides in different strains of inbred mice

We studied antibody responses after immunization with ganglio-series gangliosides against 10 strains of inbred mice, including Balb/c, C57BL/6, A/J, C3H/HeN, C3H/HeJ, CBA/N, AKR/N, NZB/N, DBA/2 and nu/nu Balb/c. Twelve gangliosides having NeuAc as their sialic acid moiety (GM4, GM3, GM2, GM1, GD3, O-Ac-GD3, GD2, GD1a, GD1b, GT1a, GT1b and GQ1b), four gangliosides having NeuGc (GM3, GM2, GM1 and GD3) and four asialo-gangliosides (GA4, GA3, GA2 and GA1) were injected intravenously adsorbed to Salmonella minnesota. The antibody titers of the mice sera were determined by an enzyme-linked immunosorbent assay and an immune adherence assay. Antibody responses were found to depend not only on the ganglioside used as an immunogen but also on the mouse strain. Gangliosides having a trisaccharide sequence (NeuAc alpha 2----8NeuAc alpha 2----3Gal-) such as GD3, GD2, GD1b, GT1a and GQ1b, in particular O-Ac-GD3, induced high-titer antibody responses, whereas those having a disaccharide sequence (NeuAc alpha 2----3Gal-) such as GM4, GM3, GM2, GM1, GD1a and GT1b induced low-titer antibody responses. On the other hand, gangliosides with NeuGc developed minimum titers. In contrast, asialogangliosides induced much higher responses than the corresponding gangliosides. The differences in ceramide portions of these gangliosides did not appear to be involved in inducing antibody responses. Mice could be divided into three groups according to the magnitude of their antibody responses: Group 1, those that produce the highest antibody responses (C3H/HeN and A/J); Group 2, those that demonstrate moderate antibody titers (Balb/c, C57BL/6, DBA/2 and nu/nu Balb/c); and Group 3, those that make minimum responses (AKR/N, C3H/HeJ, CBA/N and NZB/N). The pattern of reactivity to the various gangliosides was similar in all the strains tested.

Determination of gangliosides and sulfatide in human cerebrospinal fluid with a microimmunoaffinity technique

An immunostaining procedure has been developed for the assay of the gangliotetraose gangliosides and sulfatide in cerebrospinal fluid. Gangliosides of the gangliotetraose series were individually determined with cholera toxin B-subunit (CT-B) and an anti CT-B monoclonal antibody after chromatography and sialidase hydrolysis to GM1 on high performance thin-layer plates. Sulfatide was determined by thin-layer chromatography using an anti-sulfatide antibody. The method was applied to normal cerebrospinal fluid from 20 adults and 30 children. The concentration of the gangliotetraose series gangliosides in adults varied from 100-300 nmol/l with a mean value of 230 +/- 56 nmol/l. Corresponding values for sulfatide were 30-225 nmol/l and 140 +/- 46 nmol/l. The values for gangliosides and sulfatide in children increased during development. The major gangliosides in cerebrospinal fluid of adults were GT1b and GD1b and in children GD1a and GT1b.