Rapid Na+ accumulation by a sustained action potential impairs mitochondria function and induces apoptosis in HEK293 cells expressing non-inactivating Na+ channels.

The mechanisms underlying neuronal cell death induced by the rise of intracellular Na+ concentration ([Na+]i) following abnormal hyperexcitation are not fully understood. Previously, we have established a recombinant cell line derived from HEK293 cells, in which the occurrence of a sustained action potential (AP) induces cell death. Mutated voltage-gated Nav1.5 channel (IFM/QQQ) lacking inactivation, and inward rectifying K+ channel (Kir2.1) were co-expressed in HEK293 cells (IFM/QQQ + Kir2.1 cells). In this cell line, the rise of [Na+]i due to a sustained AP reached maximum within 15 min without concomitant [Ca2+]i rise, and then elicited significant externalization of phosphatidylserine and enhancement of caspase activity. Marked decreases in mitochondrial transmembrane potential and ATP concentration were also detected. The significant cell death occurred at 3 h from the AP onset and reached a steady state at around 12 h. The significant release of lactate dehydrogenase was not detected even after 12 h. These results provide novel findings that Na+ accumulation or/and possibly concomitant K+ loss elicits apoptosis presumably due to the mitochondrial dysfunction, which is attributable to neither the membrane depolarization nor [Ca2+]i change. This apoptotic mechanism may be involved, at least in part, in neuronal cell death under pathophysiological settings with abnormal hyperexcitability.