Nogo-C causes post-MI arrhythmia through increasing calcium leakage from sarcoplasmic reticulum

Abstract

Disturbance of calcium homeostasis in cardiomyocytes plays a pivotal role in the pathogenesis of ventricular arrhythmias after myocardial infarction(MI). Nogo-C is a sarcoplasmic reticulum (SR) protein ubiquitously expressed in tissues including heart. We previously reported that Nogo-C played a critical role in the pathogenesis of MI through inducing cardiomyocyte apoptosis. However, the role and mechanisms of Nogo-C in the regulation of calcium homeostasis and in the pathogenesis of arrhythmia post-MI remain unclear. In the present study, wildtype and Nogo-C knockout mice were subjected with MI operation followed by intraperitoneal injection of caffeine and isoproterenol to induce arrhythmia. We found that Nogo-C knockout mice displayed lower incidence of arrhythmia compared with the high incidence of arrhythmia of wildtype mice. Moreover, Nogo-C overexpression by adenovirus in cardiomyocytes caused decreased SR calcium content, reduced contractile ability, and increased diastolic cytoplasma calcium concentration, as compared with control cells. Oxalate-facilitated 45Ca uptake assay showed no difference of SR calcium reuptake ability between control and Nogo-C overexpressing cardiomyocytes, indicating that the decreased SR calcium content was not through disturbing SR calcium reuptake in Nogo-C overexpressing cells. In contrast, the rate of SR calcium leak, spontaneous calcium wave, and calcium spark markedly increased in Nogo-C overexpressing cardiomyocytes, suggesting that the increased SR calcium leak causes the decreased SR calcium content. Furthermore, We found that Nogo-C overexpression markedly increased, while Nogo-C knockdown decreased Sec61 protein, a newly found calcium leak channel, in cardiomyocytes. Nogo-C bound with Sec61 through its transmembrane domains loop-6 and loop-8, thus inhibited the ubiquitination of Sec61, leading to the increased calcium leak from SR. Together, our data show that Nogo-C is increased in MI hearts, and the increased Nogo-C inhibits the degradation of Sec61 by ubiquitination, leading to increased SR calcium leak, thus promoting arrhythmia after MI.