Glutamate induces a calcineurin-mediated dephosphorylation of Na+,K(+)-ATPase that results in its activation in cerebellar neurons in culture

Abstract

In primary cultures of cerebellar neurons glutamate neurotoxicity is mainly mediated by activation of the NMDA receptor, which allows the entry of Ca2+ and Na+ into the neuron. To maintain Na+ homeostasis, the excess Na+ entering through the ion channel should be removed by Na+,K(+)-ATPase. It is shown that incubation of primary cultured cerebellar neurons with glutamate resulted in activation of the Na+,K(+)-ATPase. The effect was rapid, peaking between 5 and 15 min (85% activation), and was maintained for at least 2 h. Glutamate-induced activation of Na+,K(+)-ATPase was dose dependent: It was appreciable (37%) at 0.1 microM and peaked (85%) at 100 microM. The increase in Na+,K(+)-ATPase activity by glutamate was prevented by MK-801, indicating that it is mediated by activation of the NMDA receptor. Activation of the ATPase was reversed by phorbol 12-myristate 13-acetate, an activator of protein kinase C, indicating that activation of Na+,K(+)-ATPase is due to decreased phosphorylation by protein kinase C. W-7 or cyclosporin, both inhibitors of calcineurin, prevented the activation of Na+,K(+)-ATPase by glutamate. These results suggest that activation of NMDA receptors leads to activation of calcineurin, which dephosphorylates an amino acid residue of the Na+,K(+)-ATPase that was previously phosphorylated by protein kinase C. This dephosphorylation leads to activation of Na+,K(+)-ATPase.