The Role of NADPH Oxidase in the Modulation of PP2A

Transient, but severe global ischemia results in AMPA receptor (AMPAR) mediated delayed neuronal death (DND). AMPARs, a major glutamatergic receptor in the CNS, are heteromeric complexes composed of GluA1 - GluA4 subunits. Most AMPARs in the hippocampus are calcium-impermeable due to the presence of the edited form of the GluA2 subunit. Ischemia results in a down-regulation of GluA2 mRNA and protein expression, resulting in the expression of GluA2- lacking, calcium/zinc-permeable AMPARs. It has been indicated that these GluA2-lacking AMPARs play a key role in promoting DND following ischemic injury. Recent studies report that an oxidative stress signaling pathway is responsible for the ischemia/reperfusion-induced changes in AMPAR subunit composition. Studies suggest that NADPH oxidase, a superoxide generator, is the source that initiates the oxidative stress-signaling cascade during post-ischemic reperfusion. We observed that inhibition of reactive oxygen species (ROS) generated by mitochondria and xanthine oxidase failed to diminish the oxygen-glucose deprivation/ reperfusion (OGD/R)-induced degradation of GluA2. However, inhibition of NADPH oxidase did diminish the OGD/R-induced degradation of GluA2, supporting a role for NADPH oxidase in the oxidative stress-signaling cascade. We also demonstrated that the treatment of acute adult rat hippocampal slices to OGD/R results in the sustained activation of PKCα and sustained Ser880 phosphorylation of GluA2, priming the subunit for internalization. Inhibition of NADPH oxidase resulted in a decrease in activated PKCα and Ser880 phosphorylation of GluA2. The objective of this study was to investigate the role of NADPH oxidase in modulating PKCα activity. PKCα activity is positively regulated by increases in calcium, therefore any enhancement in calcium concentrations will increase PKCα activity. Oxidative stress and NADPH oxidase activity have been linked to effecting calcium levels, therefore, inhibition of NADPH oxidase activity during OGD/R could potentially dampen PKCα activity through the attenuation of oxidative stress-enhanced rises in calcium entry and intracellular calcium release necessary for PKCα activation. A second possibility is that PKCα may be redox sensitive and its activity could be increased with oxidative stress. A third possibility is that the phosphatases responsible for regulating PKCα activity may be attenuated by a NADPH oxidase-mediated signaling cascade. Here, we show that protein phosphatase 2A (PP2A), a phosphatase responsible for the dephosphorylation and inactivation of PKC, undergoes a NADPH oxidase-mediated increase in phosphorylation, which has been reported to inactivate the phosphatase. Collectively, these results identify a mechanism that may underlie the post-ischemic-induced degradation of the GluA2 subunit