The Na(+)/Ca(2+) Exchanger 3 Is Functionally Coupled With the Na(V)1.6 Voltage-Gated Channel and Promotes an Endoplasmic Reticulum Ca(2+) Refilling in a Transgenic Model of Alzheimer’s Disease

The remodelling of neuronal ionic homeostasis by altered channels and transporters is a critical feature of the Alzheimer’s disease (AD) pathogenesis. Different reports converge on the concept that the Na(+)/Ca(2+) exchanger (NCX), as one of the main regulators of Na(+) and Ca(2+) concentrations and signalling, could exert a neuroprotective role in AD. The activity of NCX has been found to be increased in AD brains, where it seemed to correlate with an increased neuronal survival. Moreover, the enhancement of the NCX3 currents (I(NCX)) in primary neurons treated with the neurotoxic amyloid β 1–42 (Aβ(1–42)) oligomers prevented the endoplasmic reticulum (ER) stress and neuronal death. The present study has been designed to investigate any possible modulation of the I(NCX), the functional interaction between NCX and the Na(V)1.6 channel, and their impact on the Ca(2+) homeostasis in a transgenic in vitro model of AD, the primary hippocampal neurons from the Tg2576 mouse, which overproduce the Aβ(1–42) peptide. Electrophysiological studies, carried in the presence of siRNA and the isoform-selective NCX inhibitor KB-R7943, showed that the activity of a specific NCX isoform, NCX3, was upregulated in its reverse, Ca(2+) influx mode of operation in the Tg2576 neurons. The enhanced NCX activity contributed, in turn, to increase the ER Ca(2+) content, without affecting the cytosolic Ca(2+) concentrations of the Tg2576 neurons. Interestingly, our experiments have also uncovered a functional coupling between NCX3 and the voltage-gated Na(V)1.6 channels. In particular, the increased Na(V)1.6 currents appeared to be responsible for the upregulation of the reverse mode of NCX3, since both TTX and the Streptomyces griseolus antibiotic anisomycin, by reducing the Na(V)1.6 currents, counteracted the increase of the I(NCX) in the Tg2576 neurons. In agreement, our immunofluorescence analyses revealed that the NCX3/Na(V)1.6 co-expression was increased in the Tg2576 hippocampal neurons in comparison with the WT neurons. Collectively, these findings indicate that NCX3 might intervene in the Ca(2+) remodelling occurring in the Tg2576 primary neurons thus emerging as a molecular target with a neuroprotective potential, and provide a new outcome of the Na(V)1.6 upregulation related to the modulation of the intracellular Ca(2+) concentrations in AD neurons.