Endogenous GM1 ganglioside of the plasma membrane promotes neuritogenesis by two mechanisms

The influence of GM1 on the neuritogenic phase of neuronal differentiation has been highlighted in recent reports showing upregulation of this ganglio side in the plasma and nuclear membranes concomitant with axonogenesis, The se changes are accompanied by alterations in Ca2+ flux which constitute an essential component of the signaling mechanism for axon outgrowth. This stu dy examines 2 distinct mechanisms of induced neurite outgrowth involving pl asma membrane GM1, as expressed in 3 neuroblastoma cell lines. Growth of Ne uro-2a and NG108-15 cells in the presence of neuraminidase (N'ase), an enzy me that increases the cell surface content of GM1, caused prolific outgrowt h of neurites which, in the case of Neuro-2a, could be blocked by the B sub unit of cholera toxin (Ctx B) which binds specifically to GM1; however, the latter agent applied to NG108-15 cells proved neuritogenic and potentiated the effect of N'ase. With N18 cells, the combination was also neuritogenic as was Ctx B alone, whereas N'ase by itself had no effect. Neurite outgrow th correlated with influx of extracellular Ca2+, determined with fura-2, Tr eatment of NG108-15 and N18 cells with Ctx B alone caused modest but persis tent elevation of intracellular Ca2+ while a more pronounced increase occur red with the combination Ctx B + N'ase. Treatment with N'ase alone also cau sed modest but prolonged elevation of intracellular Ca2+ in NG108-15 and Ne uro-2a hut not N18; in the case of Neuro-2a this effect was blocked by Ctx B. Neuro-2a and N18 thus possess 2 distinctly different mechanisms for neur itogenesis based on Ca2+ modulation by plasma membrane GM1, while NG108-15 cells show both capabilities. The neurites stimulated by N'ase + Ctx B trea tment of N18 cells were shown to have axonal character, as previously demon strated fur NG108-15 cells stimulated in this manner and for Neuro-2a cells stimulated by N'ase alone.