An EF-hand in the sodium channel couples intracellular calcium to cardiac excitability

Abstract

Sodium channels initiate the electrical cascade responsible for cardiac rhythm, and certain life-threatening arrhythmias arise from Na(+) channel dysfunction. We propose a novel mechanism for modulation of Na(+) channel function whereby calcium ions bind directly to the human cardiac Na(+) channel (hH1) via an EF-hand motif in the C-terminal domain. A functional role for Ca(2+) binding was identified electrophysiologically, by measuring Ca(2+)-induced modulation of hH1. A small hH1 fragment containing the EF-hand motif was shown to form a structured domain and to bind Ca(2+) with affinity characteristic of calcium sensor proteins. Mutations in this domain reduce Ca(2+) affinity in vitro and the inactivation gating effects of Ca(2+) in electrophysiology experiments. These studies reveal the molecular basis for certain forms of long QT syndrome and other arrhythmia-producing syndromes, and suggest a potential pharmacological target for antiarrhythmic drug design.

Functional Interactions between Distinct Sodium Channel Cytoplasmic Domains through the Action of Calmodulin

“…Fig. 1A demonstrates that the likelihood of voltage-dependent inactivation of Na V 1.5 is reduced at any given voltage by increasing [Ca 2ϩ ] i , as such, the availability of channels to open is enhanced (14,15). Hence, in elevated (1 M free) [Ca 2ϩ ] i , I Na is increased when elicited from a voltage near the cardiac cell resting potential (compare Ϫ80 mV small traces, Fig.…”

“…CaM was expressed in BL21(DE3) host cells at 37°C in LB media or M9 media supplemented with 15 Isothermal Titration Calorimetry-ITC measurements were carried out with a VP-ITC MicroCalorimeter (MicroCal, Inc., Northampton, MA). Titration experiments were performed in 50 mM Tris (pH 7.5) containing 50 mM NaCl and 1 mM CaCl 2 .…”

“…To test for direct physical interaction, NMR chemical shift perturbation analysis was performed using 15 N-enriched CaM. Fig.…”

“…This domain exhibits multiple CaM binding modes, which are influenced by [Ca 2ϩ ] i (14), as well as an interaction with a C-terminal EF-hand domain that directly binds the IQ domain (15). In conditions where [Ca 2ϩ ] i is elevated, interaction between the EF-hand and the IQ domain "destabilizes" channel inactivation, i.e.…”

CaMKII modulates sodium current in neurons from epileptic Scn2a mutant mice

“…Previous work has shown that the cardiac sodium channel Na V 1.5 can be modulated by intracellular Ca 2+ and by CaMKII-mediated phosphorylation, which have profound effects on channel inactivation and persistent sodium current (8,10,19). Elevated intracellular Ca 2+ has been demonstrated to evoke a depolarized shift in the voltage dependence of inactivation and suppression of persistent sodium current for cardiac Na V 1.5 channels (8,20). Similarly, CaMKIImediated phosphorylation of Na V 1.5 causes a shift in the voltage dependence of inactivation, as well as increased persistent sodium current (10,19,21,22).…”