Selective ganglioside desialylation in the plasma membrane of human neuroblastoma cells

'To whom correspondence should be addressed •Dedicated to Professor Glinter Quadbeck on the occasion of his 80th birthday Gangliosides of the plasma membrane are important mod-ulators of cellular functions. Previous work from our labo-ratory had suggested that a plasma membrane sialidase was involved in growth control and differentiation in cultured human neuroblastoma cells (SK-N-MC), but its substrates had remained obscure. We now performed sialidase speci-ficity studies in subcellular fractions and found ganglioside GM3 desialylating activity in presence of Triton X-100 to be associated with the plasma membrane, but absent in lysosomes. This Triton-activated plasma membrane en-zyme desialylated also gangliosides GDla, GDlb, and GTlb, thereby forming GM1; cleavage of GM1 and GM2, however, was not observed. Sialidase activity towards the glycoprotein fetuin with modified C-7 sialic acids and to-wards 4-methylumbeUiferyl neuraminate was solely found in lysosomal, but not in plasma membrane fractions. The role of the plasma membrane sialidase in ganglioside desialylation of living cells was examined by following the fate of [JHlgalactose-labelled individual gangliosides in pulse-chase experiments in absence and presence of the extracellular sialidase inhibitor 2-deoxy-2,3-dehydro-N-acetylneuraminic acid. When the plasma membrane siali-dase was inhibited, radioactivity of all gangliosides chased at the same rate. In the absence of inhibitor, GM3, GDla, GDlb, GD2, GD3 and GTlb were degraded at a consider-ably faster rate in confluent cultures, whereas the GM1-pool seemed to be filled by the desialylation of higher gangliosides. The results thus suggest that the plasma mem-brane sialidase causes selective ganglioside desialylation, and that such surface glycolipid modification triggers growth control and differentiation in human neuro-blastoma cells. Key words: ganglioside/neuroblastoma cells/plasma mem-brane/sialidas