K007 Functional impact of mineralocorticoid receptor overexpression in embryonic stem cell-derived cardiomyocytes

Mineralocoticoid receptor (MR or NR3C2) is expressed in a wide range of tissues including in the heart, where its role is not fully understood. Recent clinical trials had underscored the importance of aldosterone, the main mineralocorticoid hormone, in the cardiovascular system based on the beneficial effect of mineralocorticoid antagonists to improve the prognosis of heart failure patients. We generated transgenic mice models overexpressing the human MR (hMR) under the control of its own P1 proximal (P1.hMR) or P2 distal (P2.hMR) promoters. Both transgenes were expressed in the heart among other mineralocorticoid target tissues. P1.hMR mice present with a dilated cardiomyopathy associated with tachycardia and arrhythmia. To further investigate the role of MR and aldosterone in the heart using an in vitro cellular model, we established several embryonic stem (ES) cell lines from wild type (WT), P1.hMR and P2.hMR blastocysts. Cardiac differentiation was initiated by culturing ES cells in hanging drops to derive embryoid bodies (EB). Patches of spontaneously beating cardiomyocytes appeared from day 7 and were detected in 70 % of EB at day 16. Recombinant and endogenous MR mRNA were expressed concomitantly with cardiac specific markers such as Nkx2.5, alphaMHC and Troponin T both during differentiation and in excised beating patches. hMR overexpression was confirmed by immunocytochemistry. In WT ES cells, aldosterone sharply enhanced cardiomyocyte differentiation efficiency since up to 100 % of EB exhibited beating areas. The most striking phenotype was the 2 fold-increase of basal cardiomyocyte beating rate induced by hMR overexpression, using several independent cell lines (WT: 1.09±0.2Hz, P1.hMR: 1.8±0.3Hz, P2.hMR: 1.7±0.5Hz, p