SELECTIVE GANGLIOSIDE DESIALYLATION IN THE PLASMA-MEMBRANE OF HUMAN NEUROBLASTOMA-CELLS

Gangliosides of the plasma membrane are important modulators of cellul ar functions, Previous work from our laboratory had suggested that a p lasma membrane sialidase was involved in growth control and differenti ation in cultured human neuroblastoma cells (SK-N-MC), but its substra tes had remained obscure, We now performed sialidase specificity studi es in subcellular fractions and found ganglioside GM3 desialylating ac tivity in presence of Triton X-100 to be associated with the plasma me mbrane, but absent in lysosomes, This Triton-activated plasma membrane enzyme desialylated also gangliosides GD1a, GD1b, and GT1b, thereby f orming GM1; cleavage of GM1 and GM2, however, was not observed, Sialid ase activity towards the glycoprotein fetuin with modified C-7 sialic acids and towards 4-methylumbelliferyl neuraminate was solely found in lysosomal, but not in plasma membrane fractions, The role of the plas ma membrane sialidase in ganglioside desialylation of living cells was examined by following the fate of [H-3]galactose-labelled individual gangliosides in pulse-chase experiments in absence and presence of the extracellular sialidase inhibitor 2-deoxy-2,3-dehydro-N-acetylneurami nic acid, When the plasma membrane sialidase was inhibited, radioactiv ity of all gangliosides chased at the same rate, In the absence of inh ibitor, GM3, GD1a, GD1b, GD2, GD3 and GT1b 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 th us suggest that the plasma membrane sialidase causes selective ganglio side desialylation, and that such surface glycolipid modification trig gers growth control and differentiation in human neuroblastoma cells.