| Myocardial gap junctions allow electrical and metabolic coupling throughout the heart. Closure of these junctions disrupts electrical continuity and may provide foci for arrhythmia. On the other hand, excessive coupling between myocardial cells may extend cellular injury when the heart is subjected to insults such as ischemia. Therefore, functional uncoupling of cardiac gap junctions may serve as a protective role in the heart. Connexin 43 (Cx43) is the main gap junction protein expressed in the adult working myocardium and it has been demonstrated that phosphorylation of Cx43 by the mitogen-activated protein kinase (MAPK) family members, ERK1/2 and p38 results in inhibition of gap junction communication (GJC). We demonstrated by immunoprecipitation, Western blotting and site-directed mutagenesis that BMK1, another member of the MAPK family, associates with and directly phosphorylates Cx43 on serine 255 in vitro and in vivo. This BMK1-mediated Cx43 phosphorylation event leads to a consequent decrease in cell-to-cell communication, measured by the fluorescence recovery after photobleaching (FRAP) assay to assess GJC. In spontaneously-beating neonatal rat ventricular myocytes, disruption of gap junction communication by an adenovirus expressing either dominant negative Cx43 or the constitutively active upstream BMK1 kinase (MEK5α) prevents chemical hypoxia-induced diastolic intracellular calcium ([Ca2+]i) overload. This novel finding was extended to the characterization of a transgenic mouse which expresses cardiac-restricted constitutively active MEK5α. Increased BMK1 activation and hyper-phosphorylation of Cx43 were observed in the ventricles of these transgenic mouse hearts. These transgenic mouse hearts, however, demonstrated basal contractile function and cardiac rhythm that was indistinguishable from wild type mouse hearts. Upon exposure to an ischemia/reperfusion insult, the transgenic mouse hearts exhibited an accelerated recovery of left ventricular developed pressure measured ex vivo in the Langendorff preparation. Reduced infarct size and reduced cardiac enzyme release in the CA-MEK5α transgenic mice were accompanied by a decreased activity of the calcium-activated protease, calpain. We propose a novel role for BMK1 in protecting the heart from ischemia-induced [Ca2+]i overload and subsequent contractile dysfunction, in part, by disrupting gap junction communication. |
