Abstract 5272: Perinatal Loss of Nkx2–5 Results in Rapid Conduction and Contraction Defects
Homeobox transcription factor Nkx2–5, highly expressed in heart, is known to be a critical factor during early embryonic cardiac development. Having previously demonstrated the persistent expression of Nkx2–5 in cardiac myocytes from the precardiac mesoderm through to the adult heart, we hypothesized that Nkx2–5 actively regulates a critical set of genes in postnatal cardiomyocytes to maintain proper cardiac function. We generated tamoxifen-inducible Nkx2–5 knockout mice that carry homozygous floxed-Nkx2–5 alleles and a heterozygous Cre-ERTM transgene under the control of CMV enhancer and chicken-beta-globin promoter. A single injection of tamoxifen into pregnant mice on gestation day 19 almost completely deleted Nkx2–5 genes, as demonstrated by Southern and Northern blotting. These mice demonstrated rapid conduction and contraction defects within 4 days of tamoxifen-injection. Conduction defect was accompanied by reduction of the cardiac voltage-gated Na+ channel pore-forming a-subunit (Nav1.5-alpha) in ventricles, the largest ion channel in the heart responsive for rapid depolarization of the action potential. In addition, Ca2+ release from the sarcoplasmic reticulum was diminished accompanied by substantial reduction in ryanodine receptor 2. Because excitation and contraction is tightly coupled in hearts, reduced handling of Na+ and Ca2+ demonstrated in Nkx2–5 knockout hearts may mutually lead to rapid conduction and contraction defects. Survival studies demonstrated that approximately 50% of Nkx2–5 knockout mice died before 3 weeks of age, and all the knockout mice that survived demonstrated conduction defects and enlarged hearts at 4 weeks of age. These defects appeared sooner and progressed more rapidly than those seen in mice with embryonic ventricular-restricted deletion of Nkx2–5, likely due to nearly complete deletion of Nkx2–5 in the entire hearts, without compensatory mechanisms during embryonic stages after loss of Nkx2–5. Our results demonstrate that Nkx2–5 is not limited to embryonic heart development; it is critical for both conduction and contraction in the perinatal heart by regulating expression of several important gene products in conduction and contraction.
This research has received full or partial funding support from the American Heart Association, AHA National Center.