AMPA receptor phosphorylation is selectively regulated by constitutive phospholipase A₂ and 5-lipoxygenase activities

The present investigation provides the first indication that constitutive, calcium-independent phospholipase A₂ activity (iPLA₂) modulates phosphorylation of the α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) subtype of glutamate receptors. Preincubation of frozen-thawed brain sections with two iPLA₂ inhibitors, bromoenol lactone (BEL) or palmitoyl trifluoromethyl ketone (PACO), produced a dose-dependent enhancement in phosphorylation at both Ser831 and Ser845 sites on the GluR1 subunit of AMPA receptors. This effect was not associated with changes in phosphorylation at the Ser sites of either the GluR2/3 subunits of AMPA receptors or the NR1 subunits of N-methyl-D-aspartate (NMDA) receptors, nor was it reproduced by inhibition of the calcium-dependent form of PLA₂ activity. These results suggest that the effects of these inhibitors are selective to GluR1 subunits and that they are dependent on iPLA₂ activity. The ability of iPLA₂ inhibitors to increase GluR1 phosphorylation was mimicked by the 5-lipoxygenase (5-LO) inhibitor MK-886, but not by blockers of 12-lipoxygenase (12-LO) or cyclooxygenase. Additional experiments indicated that calcium-mediated truncation of GluR1 subunits was reduced by iPLA₂ inhibitors, an effect that was not correlated with overall changes in the distribution of AMPA receptors between intracellular and membrane compartments prepared from whole brain sections. However, quantitative autoradiographic analysis indicated enhanced ³H-AMPA binding to the CA1 stratum radiatum of the hippocampus in BEL-treated sections. Saturation kinetics experiments demonstrated that this binding augmentation was due to an increase in the maximal number of AMPA binding sites. Altogether, our results point to the conclusion that basal iPLA₂ activity, through the generation of 5-LO metabolites, regulates AMPA receptor phosphorylation of GluR1 subunits, an effect that might selectively influence the number of membrane receptors in area CA1 of the hippocampus. © 2005 Wiley-Liss, Inc.