Hyperphosphorylation of RyRs Underlies Triggered Activity in Transgenic Rabbit Model of LQT2 Syndrome
Rationale: Loss-of function mutations in HERG potassium channels underlie long QT syndrome (LQTS) type 2 (LQT2), and are associated with fatal ventricular tachyarrhythmia. Previously, most studies focused on plasmamembrane-related pathways involved in arrhythmogenesis in LQTS, while pro-arrhythmic changes in intracellular Ca2+ handling remained unexplored.
Objective: We investigated the remodeling of Ca2+ homeostasis in ventricular cardiomyocytes derived from transgenic rabbit model of LQT2 in order to determine whether these changes contribute to triggered activity in the form of early afterdepolarizations (EADs).
Methods and Results: Confocal Ca2+ imaging revealed decrease in amplitude of Ca2+ transients and SR Ca2+ content in LQT2 myocytes. Experiments using SR-entrapped Ca2+ indicator demonstrated enhanced RyR-mediated SR Ca2+ leak in LQT2 cells. Western blot analyses showed increased phosphorylation of RyR in LQT2 myocytes vs. controls. Co-immunoprecipitation experiments demonstrated loss of protein phosphatases type 1 and type 2 from the RyR complex. Stimulation of LQT2 cells with β-adrenergic agonist isoproterenol resulted in prolongation of the plateau of action potentials accompanied by aberrant Ca2+ releases and EADs, which were abolished by inhibition of CaMKII. Computer simulations showed that late aberrant Ca2+ releases caused by RyR hyperactivity promote EADs and underlie the enhanced triggered activity through increased forward mode of NCX1.
Conclusions: Hyperactive, hyperphosphorylated RyRs due to reduced local phosphatase activity enhance triggered activity in LQT2 syndrome. EADs are promoted by aberrant RyR-mediated Ca2+ releases that are present despite a reduction of sarcoplasmic reticulum (SR) content. Those releases increase forward mode NCX1, thereby slowing repolarization and enabling L-type Ca2+ current reactivation.
- Ca2+-induced Ca2+ release
- Computational Model
- long QT syndrome
- cardiac arrhythmia
- calcium regulation
- ryanodine receptor
- Received August 26, 2014.
- Revision received September 18, 2014.
- Accepted September 22, 2014.