Reduced expression of Na+/Ca2+ exchangers is associated with cognitive deficits seen in Alzheimer's disease model mice

Abstract

Na+/Ca2+ exchangers (NCXs) are expressed primarily in the plasma membrane of most cell types, where they mediate electrogenic exchange of one Ca2+ for three Na+ ions, depending on Ca2+ and Na+ electrochemical gradients across the membrane. Three mammalian NCX isoforms (NCX1, NCX2, and NCX3) are each encoded by a distinct gene. Here, we report that NCX2 and NCX3 protein and mRNA levels are relatively reduced in hippocampal CA1 of APP23 and APP-KI mice. Likewise, NCX2+/− or NCX3+/− mice exhibited impaired hippocampal LTP and memory-related behaviors. Moreover, relative to controls, calcium/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation significantly decreased in NCX2+/− mouse hippocampus but increased in hippocampus of NCX3+/− mice. NCX2 or NCX3 heterozygotes displayed impaired maintenance of hippocampal LTP, a phenotype that in NCX2+/− mice was correlated with elevated calcineurin activity and rescued by treatment with the calcineurin (CaN) inhibitor FK506. Likewise, FK506 treatment significantly restored impaired hippocampal LTP in APP-KI mice. Moreover, Ca2+ clearance after depolarization following high frequency stimulation was slightly delayed in hippocampal CA1 regions of NCX2+/− mice. Electron microscopy revealed relatively decreased synaptic density in CA1 of NCX2+/− mice, while the number of spines with perforated synapses in CA1 significantly increased in NCX3+/− mice. We conclude that memory impairment seen in NCX2+/− and NCX3+/− mice reflect dysregulated hippocampal CaMKII activity, which alters dendritic spine morphology, findings with implications for memory deficits seen in Alzheimer's disease model mice.