Calmodulin modulates the termination threshold for cardiac ryanodine receptor-mediated Ca2+ release

Area of Science:

• Cardiology
• Molecular Biology
• Cell Physiology

Background:

• Cardiac ryanodine receptor 2 (RyR2) mediates Ca2+ release in cardiomyocytes, crucial for heart function.
• RyR2-mediated Ca2+ release terminates when sarcoplasmic reticulum Ca2+ content reaches a specific threshold.
• The regulatory mechanisms of this termination threshold remain largely unknown.

Purpose of the Study:

• To investigate the role of calmodulin (CaM) in regulating the termination threshold of RyR2-mediated Ca2+ release.
• To determine how Ca2+ binding to different lobes of CaM affects RyR2 activity and termination.
• To elucidate the function of the CaM-binding domain of RyR2 in Ca2+ release dynamics.

Main Methods:

• Monitoring endoplasmic reticulum Ca2+ dynamics in RyR2-expressing HEK293 cells.
• Utilizing wild-type CaM and various CaM mutants with altered Ca2+ binding affinities.
• Employing RyR2 mutants affecting the CaM-binding domain to assess CaM interaction.

Main Results:

• Wild-type CaM and CaM mutants abolishing N-lobe Ca2+ binding increased the termination threshold, facilitating release termination.
• CaM mutations affecting C-lobe Ca2+ binding decreased the termination threshold, delaying termination.
• RyR2 mutations in the CaM-binding domain generally reduced the termination threshold, with one mutant affecting both activation and termination thresholds.

Conclusions:

• Calmodulin (CaM) facilitates RyR2-mediated Ca2+ release termination by increasing the termination threshold.
• CaM's effect on the termination threshold is dependent on Ca2+ binding to its C-lobe, but not its N-lobe.
• The RyR2 CaM-binding domain is a key determinant of both Ca2+ release activation and termination thresholds.