Abstract 1874: The Iroquois Homeobox 3 Transcription Factor Controls Impulse Propagation in the Ventricular Conduction System by Regulating Sodium Channel Function in Purkinje Cells
The fast-conducting cells of the His-Purkinje network are located on the ventricular endomyocardium and are a key component of the cardiac conduction system, which coordinates ventricular contraction. Histological studies of mice expressing lacZ at the locus for the Iroquois homeobox 3 (Irx3) gene revealed that cardiac expression of Irx3 is restricted to the His-Purkinje system. Thus, we hypothesized that Irx3 is involved in regulating electrical properties in these specialized conducting tissues. Functionally, Irx3 knockout (Irx3−/−) mice showed an increase (p < 0.05) in QRS duration measured using telemetry ECG (14.8 ± 0.9 ms for Irx3−/− vs. 12.8 ± 1.7 ms for Irx3+/+), suggesting slowed ventricular conduction. Also, 6 lead ECGs measured in anesthetized mice confirmed that Irx3−/− mice have prolonged QRS durations and notched R waves consistent with bundle branch block. To determine the basis for this conduction deficit patch-clamp studies were performed on isolated Purkinje cells, which were identified using a Cx40eGFP fluorescent reporter that selectively expresses GFP in the ventricular conduction system. Current-clamp recordings of GFP+ Purkinje cells demonstrate that action potential (AP) upstroke velocity was reduced (p < 0.05) in Irx3−/− myocytes (166.1 ± 9.7 V/s; n = 20) versus in Irx3+/+ myocytes (214 ± 11.7 V/s; n = 19). In addition, AP overshoot was smaller (p < 0.05) in Irx3−/− myocytes (29.8 ± 3.2 mV) compared to Irx3+/+ (35.7 ± 2.6 mV) while AP duration at 50% repolarization time was prolonged in the Irx3−/−myocytes (31.7 ± 5.7 vs. 15.9 ± 2.7 ms). Voltage-clamp experiments showed that peak sodium current (INa) was reduced (p < 0.05) in Irx3−/− Purkinje cells (−68.1 ± 11.1 pA/pF; n = 14) compared to Irx3+/+ Purkinje cells (−93.4 ± 7.7 pA/pF; n = 12) without measurable differences in repolarizing K+currents. In contrast, AP parameters in ventricular septal myocytes (which do not express GFP) were not different between genotypes, demonstrating that the effects of Irx3 are unique to the ventricular conduction system. Together, these data suggest that Irx3 is a unique marker of the His-Purkinje network and that it regulates the electrical properties of the ventricular conduction system through a selective modulation of INa in Purkinje cells.