Proarrhythmic defects in Timothy syndrome require calmodulin kinase II

Area of Science:

• Cardiovascular Physiology
• Molecular Cardiology
• Cardiac Electrophysiology

Background:

• Timothy syndrome (TS) is characterized by excessive cellular calcium (Ca2+) entry and life-threatening arrhythmias.
• The condition arises from a mutation in the cardiac L-type Ca2+ channel (CaV1.2), leading to a loss of normal voltage-dependent inactivation.
• Calmodulin-dependent protein kinase II (CaMKII) activation during cellular Ca2+ overload is implicated in TS arrhythmias.

Purpose of the Study:

• To investigate the role of CaMKII in the proarrhythmic mechanisms of Timothy syndrome.
• To develop and utilize a rat ventricular myocyte model of TS to study CaV1.2 channel function and CaMKII activity.
• To elucidate the upstream events leading to arrhythmia phenotypes in TS.

Main Methods:

• Developed an adult rat ventricular myocyte model of TS (G406R) using lentivirus-mediated transfer of mutated CaV1.2.
• Utilized a dihydropyridine-resistant CaV1.2 mutation (T1066Y) to selectively study exogenous channel function.
• Employed Ca2+ buffering conditions and physiological Ca2+ solutions to assess CaMKII activation and proarrhythmic events.
• Administered CaMKII inhibitory peptide to evaluate its effects on cellular electrophysiology.
• Developed a revised mathematical model incorporating CaMKII-dependent and independent effects.

Main Results:

• TS CaV1.2-expressing myocytes showed loss of voltage-dependent inactivation and increased CaMKII activity in physiological Ca2+.
• Proarrhythmic phenotypes observed included action potential prolongation, increased ICa facilitation, and afterdepolarizations.
• Inhibition of CaMKII reversed ICa facilitation, normalized action potentials, and prevented afterdepolarizations.
• A revised mathematical model was developed to account for CaMKII's role.

Conclusions:

• Loss of voltage-dependent inactivation in TS is an initiating event for arrhythmias.
• CaMKII activation is a critical downstream mediator of TS-associated proarrhythmic phenotypes.
• Understanding this pathway is crucial for developing therapeutic strategies for Timothy syndrome.