The endoplasmic reticulum luminal Ca²⁺ regulates cardiac Ca²⁺ pump function

Area of Science:

• Biochemistry
• Cardiology
• Molecular Biology

Background:

• The type 2a sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA2a) is crucial for cardiomyocyte Ca2+ signaling.
• SERCA2a activity regulates diastolic relaxation and systolic contraction strength by controlling SR Ca2+ load.

Purpose of the Study:

• To investigate how dynamic changes in sarcoplasmic reticulum Ca2+ concentration ([Ca2+]SR) affect SERCA2a function.
• To identify the regulatory mechanisms of SERCA2a by luminal Ca2+.

Main Methods:

• Measured endoplasmic reticulum Ca2+ ([Ca2+]ER) using the Ca2+ sensor R-CEPIA1er.
• Characterized recombinant human and native mouse SERCA2a function.
• Utilized site-directed mutagenesis (E877L/D878L/E883L) and molecular dynamics simulations.

Main Results:

• SERCA2a-mediated Ca2+ transport is slower at low [Ca2+]ER than at intermediate [Ca2+]ER, indicating a requirement for optimal [Ca2+]ER.
• Mutations in the M7-M8 loop (E877L/D878L/E883L) reduced SERCA2a transport, particularly at intermediate [Ca2+]ER.
• Molecular dynamics simulations revealed that mutations stabilize an E2 state, slowing Ca2+ release into the ER.

Conclusions:

• SERCA2a Ca2+ transport is regulated by luminal Ca2+ through interactions with the M7-M8 loop.
• This finding provides novel insights into the intricate regulation of cardiac Ca2+ handling.