Novel approach for quantification of endoplasmic reticulum Ca²⁺ transport

The type 2a sarco-/endoplasmic reticulum Ca²⁺-ATPase (SERCA2a) plays a key role in Ca²⁺ regulation in the heart. However, available techniques to study SERCA function are either cell destructive or lack sensitivity. The goal of this study was to develop an approach to selectively measure SERCA2a function in the cellular environment. The genetically encoded Ca²⁺ sensor R-CEPIA1er was used to measure the concentration of Ca²⁺ in the lumen of the endoplasmic reticulum (ER) ([Ca²⁺]_ER) in HEK293 cells expressing human SERCA2a. Coexpression of the ER Ca²⁺ release channel ryanodine receptor (RyR2) created a Ca²⁺ release/reuptake system that mimicked aspects of cardiac myocyte Ca²⁺ handling. SERCA2a function was quantified from the rate of [Ca²⁺]_ER refilling after ER Ca²⁺ depletion; then, ER Ca²⁺ leak was measured after SERCA inhibition. ER Ca²⁺ uptake and leak were analyzed as a function of [Ca²⁺]_ER to determine maximum ER Ca²⁺ uptake rate and maximum ER Ca²⁺ load. The sensitivity of this assay was validated by analyzing effects of SERCA inhibitors, [ATP]/[ADP], oxidative stress, phospholamban, and a loss-of-function SERCA2a mutation. In addition, the feasibility of using R-CEPIA1er to study SERCA2a in a native system was evaluated by using in vivo gene delivery to express R-CEPIA1er in mouse hearts. After ventricular myocyte isolation, the same methodology used in HEK293 cells was applied to study endogenous SERCA2a. In conclusion, this new approach can be used as a sensitive screening tool to study the effect of different drugs, posttranslational modifications, and mutations on SERCA function. NEW & NOTEWORTHY The aim of this study was to develop a sensitive approach to selectively measure sarco-/endoplasmic reticulum Ca²⁺-ATPase (SERCA) function in the cellular environment. The newly developed Ca²⁺ sensor R-CEPIA1er was used to successfully analyze Ca²⁺ uptake mediated by recombinant and native cardiac SERCA. These results demonstrate that this new approach can be used as a powerful tool to study new mechanisms of Ca²⁺ pump regulation.