CONICET | Buscador de Institutos y Recursos Humanos

Gangliosides, complex glycosphingolipids containing sialicacids, are synthesized in the endoplasmic reticulum and inthe Golgi complex. These neobiosynthesized gangliosidesmove via vesicular transport to the plasma membrane, becomingcomponents of the external leaflet. Gangliosidescan undergo endocytosis followed by recycling to the cellsurface or sorting to the Golgi complex or lysosomes forremodeling and catabolism. Recently, glycosphingolipid catabolicenzymes (glycohydrolases) have been found to be associatedwith the plasma membrane, where they display activityon the membrane components. In this work, we demonstratedthat ecto-ganglioside glycosyltransferases may catalyzeganglioside synthesis outside the Golgi compartment,particularly at the cell surface. Specifically, we report the firstdirect evidence of expression and activity of CMP-NeuAc:GM3 sialyltransferase (Sial-T2) at the cell surface of epithelialand melanoma cells, with membrane-integrated ecto-Sial-T2 being able to sialylate endogenously synthesizedGM3 ganglioside as well as exogenously incorporated substrate.Interestingly, we also showed that ecto-Sial-T2 wasable to synthesize GD3 ganglioside at the cell surface usingthe endogenously synthesized cytidine monophospho-N-acetylneuraminicacid (CMP-NeuAc) available at the extracellularmilieu. In addition, the expression of UDP-GalNAc:LacCer/GM3/GD3 N-acetylgalactosaminyltransferase (GalNAc-T) was alsodetected at the cell surface of epithelial cells, whose catalyticactivity was only observed after feeding the cells with exogenousGM3 substrate. Thus, the relative interplay between theplasma membrane-associated glycosyltransferase and glycohydrolaseactivities, even when acting on a common substrate,emerges as a potential level of regulation of the localglycosphingolipid composition in response to different externaland internal stimuli.