Abstract 13563: Mef2 Regulates Junctional and Stress-Responsive Automaticity Through Cooperative Cis-Regulatory Elements at the Hcn4 Locus
Rationale: Regional differences in cardiomyocyte automaticity permit the sinoatrial node to function as the primary cardiac pacemaker with the atrioventricular (AV) junction as a subsidiary pacemaker. Under pathological conditions, sinoatrial rhythmicity is impaired and areas of ectopic automaticity can arise. The regulatory networks that direct the expression of genes required for normal and ectopic automaticity are unknown.
Hypothesis: Gene regulatory networks controlling cardiac automaticity can be identified through cis-regulatory analysis of the spatiotemporal and stress-responsive expression of a molecular marker of cellular automaticity, the hyperpolarization-activated, cyclic nucleotide-gated ion channel 4 (Hcn4).
Methods and Results: Putative cis-regulatory elements (CREs) from the Hcn4 locus were identified by interspecies conservation and cloned upstream of a LacZ reporter. CRE-reporter constructs were injected into the pronuclei of fertilized ova to generate transgenic mice. Patterns of reporter activity were assessed with X-gal staining and compared to the expression pattern of Hcn4. We found that spatial and temporal patterning of Hcn4 expression requires combinations of discrete, non-contiguous CREs, as opposed to individual, autonomously functioning CREs. A combination of two individually silent CREs from distinct areas of the Hcn4 locus direct reporter expression specifically to the developing AV bundle up to its interface with the compact AV node. This CRE system requires binding of myocyte enhancer factor 2C (Mef2C) and is exquisitely sensitive to Mef2C dosage. After induction of pathological cardiac hypertrophy with transverse aortic constriction, the AV junction CRE system becomes ectopically activated in working myocardium, mimicking expression changes observed in pathological electrical remodeling.
Conclusions: (1) Regional control of Hcn4 expression requires combinations of CREs; (2) Mef2C regulates regional patterning of cardiac automaticity; (3) A conduction system-specific and Mef2-dependent system of CREs is ectopically activated in a model of heart disease, providing a plausible mechanistic link between normal and abnormal automaticity.
- © 2011 by American Heart Association, Inc.