Year: 2009
Type: article
Abstract: Normal cardiac rhythms are generated by an organized propagation of depolarization and repolarization. The voltage-gated transient potassium current (Ito) is a major determinant of cardiac action potential. The Ito is expressed in a gradient across the left ventricular wall of the hearts, which is essential for the proper repolarization sequence in the left ventricle. Altered expression of Ito is seen in hypertrophied and failing hearts and may contribute to the increased incidence of cardiac sudden death. Thus, elucidating mechanisms underlying the expression of Ito channels will provide basic knowledge essential for the prevention and treatment of cardiac diseases with high public health significance.In the mammalian heart, Ito is produced by assembly of pore-forming Kv4 and accessory KChIP2 subunits. Differential expression of Kv4.2 gene underlies transmural gradient of Ito in the left ventricle in small rodents, whereas the size of Ito is correlated with different levels of KChIP2 in large animals. Recent studies have shown that atypical homeodomain Iroquois proteins are distributed in a gradient in the left ventricle and influence the expression of Kv4.2 and Ito. Therefore, this thesis examines the hypothesis that Irx proteins control Kv4.2 gene transcription in a cell-type specific manner and analyzes the underlying molecular mechanism. Irx3 and Irx5 are differentially expressed in a steep gradient in the left ventricle of rat hearts in an inverse pattern to Kv4.2 expression, whereas Irx4 is equally abundant in the ventricle. Irx5 activates Kv4.2 promoter in several non-myocyte cell lines, whereas the transcription factorinhibits the promoter activity in neonatal ventricular myocytes. Moreover, Irx4 prevents Irx5 to activate the channel promoter. Structure-function studies establish that the C-terminus of Irx5 is required for its regulation of channel promoter, whereas the N-terminus of Irx4 mediates its action. Addition of histone deacetylase inhibitor relieves the inhibitory effect of Irx4. Deletion and mutation analyses demonstrate the presence of a previously unidentified Irx5-responsive element in the Kv4.2 distant promoter region. Collectively, these results indicate that the interplay between Irx4 and Irx5 contributes to the heterogeneous expression of Kv4.2 gene, and hence Ito density, in the left ventricle of rat hearts.
Author: Wen-Jie He
Language: English
Subfield: Cardiology and Cardiovascular Medicine
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W1522797824