Disruption of Rho signaling results in progressive atrioventricular conduction defects while ventricular function remains preserved

Abstract

Recent studies suggest that RhoA and Rac1 mediate hypertrophic signals in cardiac myocyte hypertrophy. However, effects on cardiac function caused by inhibition of their activity in the heart have yet to be evaluated. Cardiac-specific inhibition of Rho family protein activities was achieved by expressing Rho GDIalpha, an endogenous specific GDP dissociation inhibitor for Rho family proteins, using the alpha-myosin heavy-chain promoter. Increased expression of Rho GDIalpha led to atrial arrhythmias and mild ventricular hypertrophy in adult mice (4-7 months). However, left ventricular systolic and diastolic function was largely preserved before and after the development of cardiac hypertrophy, indicating that Rho GTPases are not required to maintain ventricular contractile function under basal physiological condition. Electrocardiography and intracardiac electrophysiological studies revealed first-degree atrioventricular (AV) block in the transgenic heart at 1 week of age, which further progressed into second-degree AV block at 4 weeks of age before the development of cardiac hypertrophy. Expression of connexin 40 dramatically decreased from 1 week to 4 weeks of age in the transgenic heart, which may contribute in part to the conduction defects in the transgenic mice. This study provides novel evidence for an important role of Rho GTPases in regulating AV conduction.

RhoA GTPase regulates L-type Ca2+currents in cardiac myocytes

“…SR Ca 2ϩ content was also assessed by caffeine-induced Ca 2ϩ transients upon termination of a steady train of stimulation. Peak Ca 2ϩ transients were not significantly different between NTG (0.47 Ϯ 0.1, n ϭ 20) and TG (0.46 Ϯ 1.2, n ϭ 3; P ϭ NS) myocytes, which is consistent with the previous observation (30) that the mRNA abundances of sarco(endo)plasmic reticulum Ca 2ϩ -ATPase and phospholamban in TG ventricles were unchanged at 4 wk of age. The data suggest that a submaximal I Ca can trigger a maximal SR Ca 2ϩ release, and that the reduction in peak I Ca may not result in serious contractile alterations in TG hearts assuming that the SR Ca 2ϩ -loading function is normal (14).…”

“…Myocyte capacitance values, which are a measure of cell size, were not different between TG and NTG myocytes at 4 wk of age (TG: 124 Ϯ 6 pF, n ϭ 29; NTG: 122 Ϯ 3 pF, n ϭ 76) but became significantly increased at 4 mo of age in TG myocytes (TG: 153 Ϯ 4 pF, n ϭ 119; NTG: 122 Ϯ 6 pF, n ϭ 75; P Ͻ 0.01), which is consistent with mild hypertrophy observed at this age (ϳ15% increase in ventricular weight in TG compared with NTG mice; Ref. 30).…”

“…Pharmacological agents that either enhance or reduce I Ca density also cause changes in myocardial and myocyte contractility. However, both contractile and relaxation functions were largely preserved in Rho GDI-␣ transgenic hearts (30), which suggests functional compensation in this animal model under basal physiological conditions. Consistent with in vivo observations, there were no alterations in myocyte contractility and Ca 2ϩ transients in TG ventricular myocytes, which suggests that a submaximal I Ca can trigger a maximal SR Ca 2ϩ release, and that this reduction in peak I Ca may not result in serious contractile alterations in TG ventricular myocytes assuming that SR Ca 2ϩ loading function is normal.…”

“…We previously described that Rho GDI-␣ TG mice (M2 line), in which the transgene level is approximately sixfold higher than endogenous Rho GDI-␣ ( Fig. 1A), had no early-lethal embryonic phenotype, and that the activity of Rho family proteins was inhibited in the TG hearts (29,30). Whole cell patch-clamp studies were performed on ventricular myocytes of M2 line mice.…”

“…We observed that firstgeneration TG mice that expressed the highest levels of the transgene died around E10.5, and that heart tube looping and ventricular maturation were disrupted in these TG embryos (29). Heterozygotes of middle-copy lines had no early-lethal embryonic phenotype but did display progressive atrioventricular conduction defects (30), which suggests that Rho GTPases are involved in the regulation of cardiac electrical activity. Because L-type Ca 2ϩ channels are crucial for cardiac excitation-contraction coupling and are regulated by intracellular signals such as heterotrimeric G proteins, protein kinases, and calmodulins (4,7,12,18,25), we examined the effects of Rho GTPase in ventricular myocytes isolated from these TG mice.…”