Gangliosides have been considered to modulate cell signals in the microdomain of the cell membrane, lipid/rafts, or glycolipid-enriched microdomain/rafts (GEM/rafts). In particular, cancer-associated gangliosides were reported to enhance the malignant properties of cancer cells. In fact, GD2-positive (GD2+) cells showed increased proliferation, invasion, and adhesion, compared with GD2-negative (GD2−) cells. However, the precise mechanisms by which gangliosides regulate cell signaling in GEM/rafts are not well understood. In order to analyze the roles of ganglioside GD2 in the malignant properties of melanoma cells, we searched for GD2-associating molecules on the cell membrane using the enzyme-mediated activation of radical sources combined with mass spectrometry, and integrin β1 was identified as a representative GD2-associating molecule. Then, we showed the physical association of GD2 and integrin β1 by immunoprecipitation/immunoblotting. Close localization was also shown by immuno-cytostaining and the proximity ligation assay. During cell adhesion, GD2+ cells showed multiple phospho-tyrosine bands, i.e., the epithelial growth factor receptor and focal adhesion kinase. The knockdown of integrin β1 revealed that the increased malignant phenotypes in GD2+ cells were clearly cancelled. Furthermore, the phosphor-tyrosine bands detected during the adhesion of GD2+ cells almost completely disappeared after the knockdown of integrin β1. Finally, immunoblotting to examine the intracellular distribution of integrins during cell adhesion revealed that large amounts of integrin β1 were localized in GEM/raft fractions in GD2+ cells before and just after cell adhesion, with the majority being localized in the non-raft fractions in GD2− cells. All these results suggest that GD2 and integrin β1 cooperate in GEM/rafts, leading to enhanced malignant phenotypes of melanomas.

Acidic glycosphingolipids, i.e., gangliosides, are predominantly and consistently expressed in nervous tissues of vertebrates at high levels. Therefore, they are considered to be involved in the development and function of nervous systems. Recent studies involving genetic engineering of glycosyltransferase genes have revealed novel aspects of the roles of gangliosides in the regulation of nervous tissues. In this review, novel findings regarding ganglioside functions and their modes of action elucidated mainly by studies of gene knockout mice are summarized. In particular, the roles of gangliosides in the regulation of lipid rafts to maintain the integrity of nervous systems are reported with a focus on the roles in the regulation of neuro-inflammation and neurodegeneration via complement systems. In addition, recent advances in studies of congenital neurological disorders due to genetic mutations of ganglioside synthase genes and also in the techniques for the analysis of ganglioside functions are introduced.

The readthrough of premature termination codon (PTC) by ribosome sometimes produces full-length proteins. We previously reported a readthrough of PTC of glycosyltransferase gene B4GALNT1 with hereditary spastic paraplegia (HSP). Here we featured the readthrough of B4GALNT1 of two mutants, M4 and M2 with PTC by immunoblotting and flow cytometry after transfection of B4GALNT1 cDNAs into cells. Immunoblotting showed a faint band of full-length mutant protein of M4 but not M2 at a similar position with that of wild-type B4GALNT1. AGC sequences at immediately before and after the PTC in M4 were critical for the readthrough. Treatment of cells transfected with mutant M4 cDNA with aminoglycosides resulted in increased readthrough of PTC. Furthermore, treatment of transfectants of mutant M2 cDNA with G418 also resulted in the induction of readthrough of PTC. Both M4 and M2 cDNA transfectants showed increased/induced bands in immunoblotting and GM2 expression in a dose-dependent manner of aminoglycosides. Results of mass spectrometry supported this effect. Here, we showed for the first time the induction and/or enhancement of the readthrough of PTCs of B4GALNT1 by aminoglycoside treatment, suggesting that aminoglycosides are efficient for patients with HSP caused by PTC of B4GALNT1, in which gradual neurological disorders emerged with aging.

Extracellular vesicles (EVs) play important roles in intercellular communication in various biological events. In particular, EVs released from cancer cells have attracted special attention. Although it has been reported that cancer-associated glycosphingolipids play important roles in the enhancement of malignant properties of cancer cells, the presence, behavior, and roles of glycosphingolipids in EVs have not been elucidated. Recently, we reported crucial roles of EVs expressing gangliosides, GD2, and/or GD3 in the enhancement of cancer properties in malignant melanomas and gliomas. However, how EVs containing cancer-associated glycosphingolipids play their roles has not been reported to date. Here, we studied spatio-temporal mechanisms for GD3/GD2-containing EVs released from gliomas in the actions toward target cells. Proteome analyses of EVs with/without GD3/GD2 revealed an equally high concentration of integrin isoforms in both GD3/GD2+ and GD3/GD2- EVs. PKH26-labeled EVs attached, invaded, and distributed to/in the target cells within 1 h. GD3/GD2 formed molecular complexes with integrins on EVs as elucidated by immunoprecipitation/immunoblotting and immunocytostaining. The addition of antibodies reactive with GD3, GD2, or integrins resulted in the suppression of the enhancing effects of EVs in the cell adhesion assay. The addition of GD3/GD2 + EVs to GD3/GD2- cells clearly increased the phosphorylation levels of the PDGF receptor, FAK, and Erk1/2 in immunoblotting, suggesting GD3/GD2+ EVs activate the signaling pathway in the target cells within 15 min after addition. Anti-ganglioside antibodies clearly blocked signaling with EVs. In conclusion, EVs released from GD3/GD2-expressing glioma cells enhance cancer phenotypes and malignant signals via the cluster formation of integrins and GD3/GD2 on EVs, leading to the regulation of the cancer microenvironment.

B4GALNT1 is a glycosyltransferase essential for the synthesis of complex gangliosides and plays important roles in the nervous system. Although N-glycosylation is thought to regulate B4GALNT1, its functional significance has remained unclear. In this study, we generated human B4GALNT1 mutants carrying single or combined substitutions at three N-glycosylation sites (N1–N3) and analyzed their enzymatic activity, intracellular localization, and molecular assembly using transfected B78 mouse melanoma cells. Loss of N-glycosylation reduced GM2 expression and enzymatic activity in a site-dependent manner, with multiple-site mutants showing greater impairment. Golgi localization was also progressively disrupted as the number of mutated N-glycosylation sites increased. Non-reducing immunoblotting demonstrated that loss of the N1 glycosylation site markedly impaired B4GALNT1 dimer formation, with additional defects caused by the combined loss of the N3 and N2 sites. Predicted dimeric B4GALNT1 structures generated by AlphaFold2 revealed the spatial arrangement of N-glycans, suggesting a dominant role for the N1 site and a supportive role for the N3 site in stabilizing intermolecular contacts. These results indicated that N-glycosylation sites in the three-dimensional structure of B4GALNT1 are critical for the molecular integrity and enzymatic activity.