Elsevier

Developmental Biology

Volume 373, Issue 1, 1 January 2013, Pages 149-162
Developmental Biology

Spatiotemporal regulation of an Hcn4 enhancer defines a role for Mef2c and HDACs in cardiac electrical patterning

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Abstract

Regional differences in cardiomyocyte automaticity permit the sinoatrial node (SAN) to function as the leading cardiac pacemaker and the atrioventricular (AV) junction as a subsidiary pacemaker. The regulatory mechanisms controlling the distribution of automaticity within the heart are not understood. To understand regional variation in cardiac automaticity, we carried out an in vivo analysis of cis-regulatory elements that control expression of the hyperpolarization-activated cyclic-nucleotide gated ion channel 4 (Hcn4). Using transgenic mice, we found that spatial and temporal patterning of Hcn4 expression in the AV conduction system required cis-regulatory elements with multiple conserved fragments. One highly conserved region, which contained a myocyte enhancer factor 2C (Mef2C) binding site previously described in vitro, induced reporter expression specifically in the embryonic non-chamber myocardium and the postnatal AV bundle in a Mef2c-dependent manner in vivo. Inhibition of histone deacetylase (HDAC) activity in cultured transgenic embryos showed expansion of reporter activity to working myocardium. In adult animals, hypertrophy induced by transverse aortic constriction, which causes translocation of HDACs out of the nucleus, resulted in ectopic activation of the Hcn4 enhancer in working myocardium, recapitulating pathological electrical remodeling. These findings reveal mechanisms that control the distribution of automaticity among cardiomyocytes during development and in response to stress.

Highlights

In-vivo transgenic enhancer assay reveals an enhancer for murine Hcn4 in intron 1. ► Hcn4 enhancer has two short conserved domains that are required in combination. ► Hcn4 enhancer is active in non-chamber myocardium and AV bundle but not SA node. ► The Mef2C–HDAC system regulates Hcn4 enhancer and AV bundle Hcn4 expression. ► The Hcn4 enhancer is ectopically activated in pathological cardiac hypertrophy.

Abbreviations

SA
Sinoatrial
AV
Atrioventricular
AVB/LNC
AV bundle and lower nodal cells
R
Genomic region
CRE
Cis-regulatory element
TFBS
Transcription factor binding site
EMSA
Electromobility shift assay

Keywords

Hcn4
Cardiac automaticity
Mef2C
Cis-regulatory element
Atrioventricular bundle
Electrical remodeling

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