The Na+-dependent Inactivation of NCX1.1 is Physiologically Relevant to Cardiac Function

Abstract

In cardiac cells, the Na+/Ca2+ exchanger (NCX) regulates both Ca2+ homeostasis and the action potential waveform. NCX activity is allosterically modulated by cytosolic protons, Ca2+ and Na+, although their physiological relevance is not well-understood. To start unraveling the physiological role of NCX modulation by intracellular Na+ in cardiac function, we generated a mouse model in which NCX1.1 lacks Na+ regulation. Lys229 was replaced with Glu using the CRISPR/Cas9 technique. This mutation (K299Q) removes the inhibitory effects that millimolar [Na+] exerts on NCX activity (Matsouka et al, JGP 109:273-86, 1997). K229Q mice live to adulthood with no cardiac hypertrophy. Cardiac function was assessed in anesthetized mice by echocardiography in B-mode and M-mode. The results indicate a modest but significant decrease in the left ventricular ejection fraction in K229Q mice at 12 weeks of age (Wild-type Male: 62.7±0.7%, n=6; Female: 65.8±1.0%, n=9) (Heterozygous Male: 58.9±0.8%, n=16, P=0.002; Female: 57.6±1.3%, n=12, P<0.001) (Homozygous Male: 57.7±1.0%, n=9, P=0.001; Female: 59.9±1.3%, n=9, P=0.002). The left ventricular fractional shortening was also reduced by ∼12% in K229Q mice (Wild-type Male: 33.4±0.5%, n=6; Female: 35.4±0.7%, n=9) (Heterozygous Male: 30.8±0.5%, n=16, P=0.003; Female: 29.7±0.8%, n=12, P<0.001) (Homozygous Male: 29.8±0.7%, n=9, P=0.001; Female: 30.7±1.3%, n=9, P=0.006). EKG recordings, captured using paw leads and corrected using the Bazett formula, indicate a prolonged QT interval in K229Q mice when compared to wild-type (Wild-type Male: 44.2±2.2 ms, n=6; Female: 42.4±0.4 ms, n=9) (Heterozygous Male: 48.1±0.8 ms, n=14, P=0.03; Female: 49.6±0.8 ms, n=11, P<0.001) (Homozygous Male: 48.5±0.7 ms, n=7, P=0.04; Female: 47.7±0.4 ms, n=9, P<0.001). These results suggest that the Na+-dependent inactivation of the exchanger plays a physiological role in cardiac function.