Abstract 066: MEF2-induced Loss of Sarcomeres is Mediated by an Alternative Splice Variant of DMPK

Abstract

The pathology of heart failure is characterized by poorly contracting and dilated ventricles. Although this is associated with lengthening of individual cardiomyocytes and loss of sarcomeres, the mechanism underlying these changes in cardiomyocyte structure remains to be elucidated. We have previously identified the transcription factor myocyte enhancer factor-2 (MEF2) as important trigger for adverse cardiomyocyte remodeling. Here, we use microarray analysis and gain- and loss- of function approaches to identify MEF2 target genes involved in structural remodeling of the cardiomyocyte. Isolated neonatal rat cardiomyocytes infected with adenoviruses expressing MEF2 underwent cellular elongation associated with loss of sarcomeric structure. Microarray analysis revealed myotonic dystrophy protein kinase (DMPK) as MEF2 target gene, which we verified by chromatin immunoprecipitation experiments. siRNA mediated knockdown of DMPK prevented MEF2-induced cardiomyocyte elongation and loss of sarcomeres. Interestingly, RT-PCR analysis of known DMPK splice variants demonstrated a relative increase of the DMPK E isoform in failing mouse hearts. To test the role of this specific splice isoform, we generated adenoviruses expressing DMPK E or a kinase dead mutant DMPK E. Overexpression of wildtype DMPK E, but not of the kinase dead mutant, in cardiomyocytes resulted in severe loss of sarcomeric structure. Moreover, quantitative PCR analysis showed a decrease in mRNA levels for several sarcomeric genes after overexpression of DMPK E. These genes are known targets of the transcription factor serum response factor (SRF) and DMPK is known to phosphorylate SRF. Therefore, we tested the effect of DMPK on SRF activity in luciferase experiments, which demonstrated that DMPK E is an inhibitor of SRF transcriptional activity. Our data indicate that MEF2 induces loss of sarcomeres, which is mediated by at least one specific splice variant of DMPK. Moreover, increased expression of this DMPK splice variant results in a decrease in sarcomeric gene expression, which possibly involves inhibition of SRF transcriptional activity. Together, these results assign a novel function to MEF2 and DMPK in adverse cardiomyocyte remodeling during heart failure development.