Essential Role of the Mitochondrial Na <sup>+</sup>/Ca <sup>2+</sup> Exchanger - NCLX in Mediating the PDE2 Dependent Neuronal Survival and Learning

Abstract

Consumption of coffee and caffeine enhances learning efficiency and reduces the risk of stroke and neurodegenerative syndromes by a molecular pathway that is poorly understood. Here we fluorescently monitored mitochondrial Ca2+ transients in hippocampal neurons evoked by caffeine. Surprisingly, caffeine also had a long-lasting potentiation effect on mitochondrial Ca2+ ([Ca2+]m) efflux evoked by depolarization, in WT but not in neurons deficient in the mitochondrial Na+/Ca2+ exchanger NCLX. We reasoned that the caffeine effect is linked to phosphodiesterase inhibition, and focused on PDE2 that has the highest affinity to caffeine and is targeted to the mitochondria. Pretreatment of neurons with the selective PDE2 inhibitor Bay 60-7550 mimicked caffeine and rescued [Ca2+]m efflux triggered by neuronal depolarization. PDE2 inhibition also enhanced [Ca2+]m efflux triggered by neuromodulators. Inhibition of PDE2 acted by increasing mitochondrial cAMP thereby promoting NCLX phosphorylation. We found that protection of neurons against excitotoxic insults, conferred by PDE2 inhibition was diminished in NCLX KO neurons, thus is NCLX dependent. Finally, administration of Bay 60-7550 enhanced new object recognition learning in WT but not in NCLX KO mice. Our results identify a pathway linking caffeine and PDE to [Ca2+]m signaling, thereby providing new therapeutic targets for treating cognitive impairment and excitotoxicity.