The cardiac pacemaker-specific channel Hcn4 is a direct transcriptional target of MEF2

Hcn4, which encodes the hyperpolarization-activated, cyclic nucleotide-sensitive channel (I-h), is a well-established marker of the cardiac sino-atrial node. We aimed to identify cis-elements in the genomic locus of the Hcn4 gene that regulate the transcription of Hcn4. We screened evolutionarily conserved non-coding sequences (CNSs) that are often involved in the regulation of gene expression. The VISTA Enhancer Browser identified 16 regions, termed CNS 1-16, within the Hcn4 locus. Using the luciferase reporter assay in primary neonatal rat cardiomyocytes, we found that CNS13 conferred a prominent enhancer activity (more than 30-fold) on the Hcn4 promoter. Subsequent mutation analysis revealed that the Hcn4 enhancer function was dependent on myocyte enhancer factor-2 (MEF2) and activator protein-1 (AP1) binding sequences located in CNS13. Electrophoretic mobility shift assay and chromatin immunoprecipitation confirmed that MEF2 and AP1 proteins bound CNS13. Furthermore, overexpression of a dominant negative MEF2 mutant inhibited the enhancer activity of CNS13, decreased Hcn4 mRNA expression and also decreased the amplitude of I-h current in myocytes isolated from the inflow tract of embryonic heart. These results suggest that the novel enhancer CNS13 and MEF2 may play a critical role in the transcription of Hcn4 in the heart.