The ryanodine receptor type 2 (RyR2) is an intracellular calcium channel in the heart that contributes to the translation of electrical signals into mechanical force, a process known as excitation-contraction coupling (ECC). While deletion of RyR2 is embryonically lethal, constitutive, and conditional knockout models with ∼50% reduction in expression show preserved heart function and calcium homeostasis. Hence, we hypothesized that ECC possesses plasticity and adapts to a significant loss of RyR2 expression without inducing overt alterations to calcium homeostasis. To test this, we used a previously reported tamoxifen-inducible conditional RyR2 knockout (RyR2cKO) mouse model. Mice survived for up to 6 weeks after tamoxifen injection. At three weeks post-injection, RyR2cKO hearts showed a ∼90% depletion in RyR2 expression compared to controls. Immunostaining of RyR2cKO heart sections uncovered a heterogeneous RyR2 expression pattern in which most cardiomyocytes have a nearly complete loss of RyR2 while few cells retain low levels of expression. Calcium transient (CaT) amplitude in isolated myocytes was not different compared to controls, while the sarcoplasmic reticulum (SR) calcium load was increased in RyR2cKO myocytes. We observed an increase in time to peak and a decrease in rise velocity of the CaT in RyR2cKO myocytes, suggesting that RyR2 expression regulates the rate of calcium release. ECC gain was preserved in RyR2cKO myocytes as evidenced by no changes in I CaL and CaT amplitude compared to control myocytes. Single-cell RNA-sequencing showed limited transcriptomic remodeling in RyR2cKO myocytes, while western blots uncovered only a decrease in SERCA2a. Our data show that ECC is highly plastic and functionally adapts to a significant decrease in RyR2 expression without overt changes to ECC protein expression or Ca 2+ handling dynamics. Additional components of the compensation apparatus that functionally regulates ECC require further examination.
Perez et al. (Sun,) studied this question.