Characterization of cholera toxin B subunit-induced Ca2+ influx in neuroblastoma cells:: Evidence for a voltage-independent GM1 ganglioside-associated Ca2+ channel

The role of endogenous GM1 ganglioside in neurite outgrowth has been studied in N18 and NG108-15 neuroblastoma cells with the GM1-specific ligand cholera toxin B subunit (Ctx B), which stimulates Ca2+ influx together with neuritogenesis. Our primary goal has been to identify the nature of the calcium channel that is modulated by GM1. An L-type voltage-operated Ca2+ channel (VOCC) was previously proposed as the mediator of this phenomenon. This investigation, employing fura-2 fluorescent measurements and specific channel blockers and other agents, revealed that GM1 modulates a hitherto unidentified Ca2+ channel not of the L type. It was opened by Ctx B; was permeable to Ca2+ and Ba2+ but not Mn2+; and was blocked by Ni2+, Cd2+, and La3+. Although most dihydropyridines inhibited Ctx B-induced Ca2+ influx as well as neurite outgrowth at higher concentrations, they and other VOCC blockers at normally employed concentrations failed to do so, suggesting un-involvement of VOCC. In addition, Ca2+ influx induced by Ctx B was not mediated by cGMP-dependent or G-protein-coupled nonselective cation channels, as demonstrated by the cGMP antagonist Rp-cGMPS or the G-protein/receptor uncoupling agent suramin, respectively. Finally, Ca2+ influx was unlikely to be due to inhibition or reversal of Na+-Ca2+ exchanger via Ctx B induction of Na+ uptake, insofar as no effect was seen on blocking Na+ channels, inhibiting Na+-K+-ATPase, or eliminating extracellular Na+. The results suggest that this novel channel is gated by interaction with GM1, which, when associated with the channel and bound by appropriate ligand, promotes Ca2+ influx. This in turn induces signaling for the onset of neuritogenesis. (C) 2002 Wiley-Liss, Inc.