C6 cells express a sodium-calcium Exchanger/GM1 complex in the nuclear envelope but have no exchanger in the plasma membrane: Comparison to astrocytes

Previous work demonstrated the presence of an isoform of Na+/Ca2+ exchanger in the nuclear envelope of neurons and NG108-15 cells that is tightly associated with GM1 ganglioside and potentiated by the latter. This contrasted with the Na+/Ca2+ exchanger(s) in the plasma membrane, which were suggested to associate more loosely with GM1. To study these aspects of Na+/Ca2+ exchanger expression in nonneuronal neural cells, we have examined nuclear and plasma membrane exchanger patterns in astrocytes and C6 cells, a glia-derived line. We find both cell types contain the tightly associated exchanger/GM1 complex in the nuclear envelope but, surprisingly, only astrocytes possess Na+/ Ca2+ exchanger activity in the plasma membrane. This is the first reported example of a cell (C6) with Na+/Ca2+ exchangers in the nuclear envelope but not in the plasma membrane. RT-PCR established the presence of the NCX1 subtype in C6 cells and both NCX1 and NCX2 in astrocytes. Comparison was made with NG108-15 cells, which have Na+/Ca2+ exchangers in both nuclear and plasma membranes, and Jurkat cells, which have no Na+/Ca2+ exchanger in either membrane. Culturing of C6 cells in the presence dibutyryl-cAMP caused upregulation of a high molecular weight isoform of the exchanger together with GM1 in the nuclear envelope, resulting in significant elevation of Na+/Ca2+ exchanger activity in the latter. Application of exogenous GM1 to nuclei from non-treated cells also potentiated exchanger activity, although to a lesser degree. The Na+/Ca2+ exchanger/GM1 complex occurs in the inner membrane of the nuclear envelope, suggesting a functional role in transferring Ca2+ between nucleoplasm and the envelope lumen. (C) 2004 Wiley-Liss, Inc.