Calcineurin involvement in the regulation of high-threshold Ca2+ channels in NG108-15 (rodent neuroblastoma x glioma hybrid) cells

1. We examined the relationship between calcineurin (protein phosphatase 2B (PP2B)) and voltage-operated Ca2+ channels (VOCCs) in NG108-15 cells. PP2B expression in NG108-15 cells was altered by transfection with plasmid constructs containing a full length cDNA of human PP2B beta(3) in sense (CN-15) and antisense (CN-21) orientation. 2. Confocal immunocytochemical localization showed that in wild-type cells, PP2B immunoreactivity is uniformly distributed in undifferentiated cells and located at the inner surface of soma membrane and neurites in differentiated cells. 3. To test the Ca2+ dependence of the VOCC, we used high-frequency stimulation (HFS). The L- and N-type VOCCs decreased by 37 and 52%, respectively, whereas the T-type current was only marginally sensitive to this procedure. FK-506 (2 mu M), a specific blocker of PP2B, reduced the inhibition of L- and N-type VOCCs induced by HFS by 30 and 33%, respectively. 4. In CN-15-transfected cells overxpressing PP2B, total high-voltage-activated (HVA) VOCCs were suppressed by about 60% at a test potential of +20 mV. Intracellular addition of EGTA or FK-506 into CN-15-transfected cells induced an up to 5-fold increase of HVA VOCCs. 5. These findings indicate that PP2B activity does not influence the expression of HVA C2+ channels, but modulates their function by C2+-dependent dephosphorylation. Thus HVA VOCCs, in a phosphorylated state under control conditions, are downregulated by PP2B upon stimulation, with the major effect being on N-type VOCCs.