Human IgM anti-GM1 autoantibodies modulate intracellular calcium homeostasis in neuroblastoma cells

Increased titers of IgM anti-GM1 antibodies are present in some patients wi th Lower Motor Neuron Disease (LMND) or Motor Neuropathy (MN), but their pa thogenic role and the mechanism of action are unclear. Previous studies hav e shown that the B subunit of Cholera Toxin (CT), which binds and crosslink s ganglioside GM1, modulate intracellular calcium in murine neuroblastoma c ells via the activation of L-type voltage-dependent calcium channels (VGCC) . Therefore, using a fluorimetric approach, we have examined the hypothesis that the pentameric IgM anti-GM1 antibodies, could similarly alter calcium concentration in N18 neuroblastoma cells. Sera with human IgM anti-GM1 ant ibodies were obtained from 5 patients with LMND and 2 patients with MN. Hum an IgG anti-GM1, IgM anti-Myelin Associated Glycoprotein (MAG), IgM anti-su lfatide antibodies and lectin peanut agglutinin (PNA), that recognizes spec ifically the Gal(beta1-S)GalNAc epitope, were used as control sera. Direct application of either human IgM anti-GM1 antibodies or the B subunit of CT to N18 neuroblastoma cells induced a sustained influx of manganese ions, as indicated by a quench of the intracellular fura-2 fluorescence. Furthermor e, the dihydropyridine L-type channel antagonists completely inhibited the manganese influx, suggesting that it is due to activation of an L-type VGCC . The magnitude of the influx was correlated with antibody titers. None of human IgG anti-GM1, IgM anti-MAG, IgM anti-sulfatide antibodies or PNA indu ce an ion influx, pointing to the selective participation of the pentameric IgM isotype of anti-GM1 in the modulation of L-type calcium channels openi ng. Given that L-type calcium channels are present on motor neurons, the mo dulation of L-type calcium channels by IgM GM1 antisera may have important implications in diseases such as LMND and MN. (C) 2001 Elsevier Science B.V . All rights reserved.