Acute Genetic Ablation of Cardiac Sodium/Calcium Exchange in Adult Mice: Implications for Cardiomyocyte Calcium Regulation, Cardioprotection, and Arrhythmia

Abstract

Background Sodium‐calcium (Ca 2+ ) exchanger isoform 1 (NCX1) is the dominant Ca 2+ efflux mechanism in cardiomyocytes and is critical to maintaining Ca 2+ homeostasis during excitation‐contraction coupling. NCX1 activity has been implicated in the pathogenesis of cardiovascular diseases, but a lack of specific NCX1 blockers complicates experimental interpretation. Our aim was to develop a tamoxifen‐inducible NCX1 knockout (KO) mouse to investigate compensatory adaptations of acute ablation of NCX1 on excitation‐contraction coupling and intracellular Ca 2+ regulation, and to examine whether acute KO of NCX1 confers resistance to triggered arrhythmia and ischemia/reperfusion injury. Methods and Results We used the α‐myosin heavy chain promoter (Myh6)‐MerCreMer promoter to create a tamoxifen‐inducible cardiac‐specific NCX1 KO mouse. Within 1 week of tamoxifen injection, NCX1 protein expression and current were dramatically reduced. Diastolic Ca 2+ increased despite adaptive reductions in Ca 2+ current and action potential duration and compensatory increases in excitation‐contraction coupling gain, sarcoplasmic reticulum Ca 2+ ATPase 2 and plasma membrane Ca2+ ATPase. As these adaptations progressed over 4 weeks, diastolic Ca 2+ normalized and SR Ca 2+ load increased. Left ventricular function remained normal, but mild fibrosis and hypertrophy developed. Transcriptomics revealed modification of cardiovascular‐related gene networks including cell growth and fibrosis. NCX1 KO reduced spontaneous action potentials triggered by delayed afterdepolarizations and reduced scar size in response to ischemia/reperfusion. Conclusions Tamoxifen‐inducible NCX1 KO mice adapt to acute genetic ablation of NCX1 by reducing Ca 2+ influx, increasing alternative Ca 2+ efflux pathways, and increasing excitation‐contraction coupling gain to maintain contractility at the cost of mild Ca 2+ ‐activated hypertrophy and fibrosis and decreased survival. Nevertheless, KO myocytes are protected against spontaneous action potentials and ischemia/reperfusion injury.

Mechanisms of spontaneous Ca2+ release‐mediated arrhythmia in a novel 3D human atrial myocyte model: II. Ca2+‐handling protein variation

“…In the healthy heart, NCX inhibition prevents Ca 2+ extrusion, leading to Ca 2+ overload (Bers, 2002) and thus enhancement of SCRs (Lotteau et al., 2021). This is seen in our simulations, whereby a decrease in NCX is associated with a reduction in the pacing threshold for SCRs.…”

Spatial multi-omic map of human myocardial infarction

“…These three molecular niches were enriched mainly in cardiomyocytes (Fig. 3c ), but with a distinct molecular profile: among the top 5 upregulated genes of niche 2 was XIRP1 , which encodes an intercalated-disc ion-channel-interacting protein and RRAD , which encodes a GTPase known to regulate L-type Ca 2+ channels and contractile functions of the heart 19 ; molecular niche 4 was enriched for SLC8A1 (also known as NCX1 ), which encodes the Na + /Ca 2+ exchanger that is the major regulator of the Ca 2+ efflux in cardiomyocytes and is critical to maintain Ca 2+ homeostasis during excitation–contraction coupling 20 , and MPC1 , which encodes mitochondrial pyruvate carrier, a known mitochondrial metabolic regulator of heart function 21 (Extended Data Fig. 5h ).…”

Dual loss of regulator of G protein signaling 2 and 5 exacerbates ventricular myocyte arrhythmias and disrupts the fine-tuning of Gi/o signaling

“…In addition, the expression of cardiomyopathy markers, particularly titin and troponin T, is reduced in Rgs2/5 dbKO hearts of both sexes, and individual cardiomyocyte contractions are markedly decreased despite increased mobilization of intracellular Ca 2+ . The augmented Ca 2+ mobilization, however, appears to contribute to arrhythmogenesis as Ca 2+ recycling is impaired, leading to prolonged elevation of cytoplasmic Ca 2+ concentration, which is known to be arrhythmogenic [30,[52][53][54]. Taken together with the decrease in the expression of contractile genes, our data suggest that the cardiomyopathy in Rgs2/5 dbKO is accompanied by unraveling of the mechanisms that couple cardiac electrical excitation and contractility, thereby contributing to the marked decrease in baseline ejection fraction and fractional shortening.…”