Deletion of Trpm4 Alters the Function of the Nav1.5 Channel in Murine Cardiac Myocytes

Abstract

Transient receptor potential melastatin member 4 (TRPM4) encodes a Ca2+-activated, non-selective cation channel that is functionally expressed in several tissues, including the heart. Pathogenic mutants in TRPM4 have been reported in patients with inherited cardiac diseases, including conduction blockage and Brugada syndrome. Heterologous expression of mutant channels in cell lines indicates that these mutations can lead to an increase or decrease in TRPM4 expression and function at the cell surface. While the expression and clinical variant studies further stress the importance of TRPM4 in cardiac function, the cardiac electrophysiological phenotypes in Trpm4 knockdown mouse models remain incompletely characterized. To study the functional consequences of Trpm4 deletion on cardiac electrical activity in mice, we performed perforated-patch clamp and immunoblotting studies on isolated atrial and ventricular cardiac myocytes and surfaces, as well as on pseudo- and intracardiac ECGs, either in vivo or in Langendorff-perfused explanted mouse hearts. We observed that TRPM4 is expressed in atrial and ventricular cardiac myocytes and that deletion of Trpm4 unexpectedly reduces the peak Na+ currents in myocytes. Hearts from Trpm4−/− mice presented increased sensitivity towards mexiletine, a Na+ channel blocker, and slower intraventricular conduction, consistent with the reduction of the peak Na+ current observed in the isolated cardiac myocytes. This study suggests that TRPM4 expression impacts the Na+ current in murine cardiac myocytes and points towards a novel function of TRPM4 regulating the Nav1.5 function in murine cardiac myocytes.

Electrophysiological Effects of the Transient Receptor Potential Melastatin 4 Channel Inhibitor (4-Chloro-2-(2-chlorophenoxy)acetamido) Benzoic Acid (CBA) in Canine Left Ventricular Cardiomyocytes

“…There was no significant difference in TRPM4 expression between isolated cells of the left ventricle and whole left ventricular wall tissue, suggesting that the source of the TRPM4 signal is mostly from cardiomyocytes. Our results are in agreement with a recent study where equal TRPM4 expression was shown in mice atrial and ventricular samples, and in isolated ventricular cardiomyocytes [14].…”

“…The CBA-induced V + max reduction was about 15%, not reversible, and not observed in murine ventricular myocardium [16]. A recent study, however, described the smaller value of V + max in ventricular myocytes of TRPM4 knockout mice compared to that in wild-type animals [14]. The smaller V + max value was due to the reduction of peak Na + current density as it was approximately 30% smaller in TRPM4 knockout mice [14].…”

“…A recent study, however, described the smaller value of V + max in ventricular myocytes of TRPM4 knockout mice compared to that in wild-type animals [14]. The smaller V + max value was due to the reduction of peak Na + current density as it was approximately 30% smaller in TRPM4 knockout mice [14]. It might be that the CBA-induced reduction of V + max in our study reflects the reduction peak Na + current, as TRPM4 and Nav1.5 were shown to interact with each other [14].…”

“…In healthy rat ventricle, TRPM4 expression was localized in the plasma membrane [13], and its expression was stronger in atrial than in the ventricular muscle of bovine hearts [9]. In mice, TRPM4 expression was detected at both protein and mRNA levels in atrial and ventricular samples, and also in isolated ventricular cells [14]. In human cardiac tissue, TRPM4 protein expression was described in the conductive system [10], in the atrial muscle [43], and in cardiac samples of 5-month-old Fallot tetralogy patients [40].…”

“…The expression of TRPM4 is weaker in atrial muscle [12] but it is also expressed to a small extent in ventricular cells [9]. Recent studies reported definitive TRPM4 expression in ventricular muscle and cardiomyocytes [13,14]. TRPM4 current contributes to pacemaking in several species including rat, mouse, and rabbit by preventing bradycardia [15], prolongs action potential duration (APD) in atrial muscle [16], and is involved in the positive inotropic effect of β-adrenergic stimulation [17].…”