Calcineurin controls the expression of isoform 4CII of the plasma membrane Ca²⁺ pump in neurons

The expression of the CII splice variant of the plasma membrane Ca²⁺ ATPase 4 (PMCA4) was down-regulated in granule neurons when they were cultured under conditions of partial membrane depolarization (25 mM KCl), which are required for long term in vitro survival of the neurons. These conditions, which cause a chronic increase of the resting free Ca²⁺ concentration in the neurons, have recently been shown to promote up- regulation of the PMCA2, 3, and 1CII isoforms. Whereas the chronic, i.e. >3 days, Ca²⁺ increase was necessary for the up-regulation of the PMCA1CII, 2, and 3, the downregulation of the PMCA4CII mRNA was already evident 1-2 h after the start of culturing in 25 mM KCl. The immunosuppressant calcineurin inhibitor FK506 inhibited the down-regulation of the PMCA4CII at both the protein and the mRNA level but did not affect the changes of the other PMCA pumps. Direct evidence for the involvement of calcineurin in the down- regulation of the PMCA4CII was obtained by overexpressing a truncated, constitutively active, and Ca²⁺-independent form of calcineurin; under these conditions, depolarization was not required for the down-regulation of the PMCA4CII pump. De novo synthesis of (transcription) factors was required for the down-regulation of the PMCA4CII mRNA. Calcineurin, therefore, controls the neuronal transcription of PMCA4CII, a splice variant of the pump isoforms that is found almost exclusively in brain.