Mitogen-activated protein kinase kinase 4 deficiency in cardiomyocytes causes connexin 43 reduction and couples hypertrophic signals to ventricular arrhythmogenesis.

Connexin 43 (Cx43) is the predominant isoform of gap junction proteins in the working myocardium. In the heart, mitogenactivated protein (MAP) kinases are implicated in regulating Cx43 remodeling; however, their precise roles remain obscure. Mitogen-activated protein kinase kinase 4 (MKK4) is a critical component of the stressactivated MAP kinase signaling pathway. We have previously demonstrated that MKK4 antagonizes cardiomyocyte hypertrophy. Herein, we investigate the role of MKK4 in regulating Cx43 expression in cardiomyocytes. We found that knockdown of MKK4 expression or inhibition of its kinase activity in neonatal rat cardiomyocytes (NRCMs) significantly reduced phenylephrine-induced Cx43 expression. Furthermore, two activator protein-1 (AP-1) binding elements in the Cx43 promoter region were identified as being responsible for the MKK4-regulated Cx43 expression. Consistently, we also detected heterogeneously reduced Cx43 expression and attenuated zonula occludens-1 (ZO-1) content in the hearts of MKK4 cardiomyocyte-specific knockout mice (MKK4) following pressure overload. To test whether heterogeneously reduced Cx43 expression contributes to ventricular arrhythmic vulnerability, MKK4 and control mice were subjected to pressure overload followed by programmed electrical stimulation (PES). 6 of 13 MKK4 mice, but none of the controls, developed ventricular tachycardia. Epicardial activation mapping recorded from the MKK4 hypertrophied heart showed ventricular activation delay. Mathematical models have simulated that the spatially heterogeneous decrease in Cx43 http://www.jbc.org/cgi/doi/10.1074/jbc.M111.228791 The latest version is at JBC Papers in Press. Published on March 28, 2011 as Manuscript M111.228791