Abstract 20060: Irx5 Regulates Ito-Dependent Contractility and Calcineurin-Nfat-Dependent Hypertrophic Response in the Heart
The homeodomain transcription factor, Irx5, negatively regulates KV4.2 gene encoding the transient outward K+ current (Ito) in the heart. Ito levels are invariably reduced in heart diseases, and Ito changes can modulate both cardiac contractility via effects on action potential profile and cardiac hypertrophy. We therefore examined contractility and hypertrophic responses in hearts from Irx5 knockout (Irx5 KO) mice. As expected, Irx5 KO hearts had reduced contractility (in Langendorff and in vivo hearts), in association with elevated Ito and abbreviated action potential duration, as well as decreased [Ca2+]i transients and cell shortening in endocardial, but not epicardial myocytes. Since Irx5 KO mice showed no evidence of heart disease despite lower cardiac function, we subjected the mice to 2 weeks of transverse aortic constriction (TAC). Irx5 KO mice with TAC developed less cardiac hypertrophy than wild-type (WT) mice. Reduced hypertrophy was also observed in cultured neonatal mouse ventricular myocytes (NMVM) from Irx5 KO hearts when exposed to norepinephrine (NE) while Irx5 over-expression in Irx5 KO NMVM restored hypertrophic responses to NE. To investigate if these Irx5 KO phenotypes are Ito-dependent, we crossed Irx5 KO mice with KV4.2 KO mice in which Ito is virtually eliminated. Cardiac contractility of Irx5-KV4.2 double KO hearts was greater than either WT (WT) or Irx5 KO, and was the same as the cardiac contractility of KV4.2 KO mice (elevated above WT). Nevertheless, Irx5-KV4.2 double KO hearts still showed reduced hypertrophy following pressure-overload. Thus, the effects of Irx5 on contractility are Ito-mediated while its effects on hypertrophy are Ito-independent. Biochemical assessments of transcriptional mechanisms revealed that Irx5 immunoprecipates with Nfatc3 and Gata4, suggesting Irx5 is associated with Calcineurin-Nfat signalling. Indeed, calcineurin (Cn) activity measured by Rcan1 (Mcip) expression was reduced in NE-treated NMVMs and pressure-overload hearts from Irx5 KO mice. We conclude that Irx5 regulates of cardiac contractility in an Ito-dependent and hypertrophy in a Cn-Nfat-dependent but Ito-independent manner.
- © 2010 by American Heart Association, Inc.