Fig. 6: Mechanistic role of SERCA2 phosphorylation at S663 in ischemic and healthy/protected hearts.

In healthy heart, during excitation-contraction coupling (ECC) induced after depolarization of the sarcolemma, a small amount of Ca²⁺ enters to the sarcoplasm through the voltage sensitive L-type calcium dihydropyridine channels (LTCC) of the T-tubule, targeting a rapid and large amount of Ca²⁺ release (systolic Ca²⁺) from inside the sarcoplasmic reticulum (SR/ER) through the Ca²⁺ ryanodine channels (RyR), which subsequently activates myofilaments (MF) for muscle contraction. Relaxation occurs when SR Ca²⁺ ATPase (SERCA2) reuptakes Ca²⁺, which is regulated by the balance of the phosphorylation level of SERCA2 at serine 663 (S663) via the GSK3β, lowering cytosolic Ca²⁺ concentration in combination with Ca²⁺ extrusion via Na⁺–Ca²⁺ exchanger (NCX) working in reverse mode. Mitochondria (Mito) also participate taking and extruding Ca²⁺ from the cytosol during Ca²⁺ cycle. During ischemic heart disease, the level of SERCA2 phosphorylation at S663 is increased, reducing the reuptake of Ca²⁺ into SR/ER during the relaxation phase, which contributes to the increase of both intracellular (Ca²⁺ _i) and mitochondrial Ca²⁺ (Ca²⁺_m) overload, driving cells towards death. Conversely, preventing SERCA2 phosphorylation at reperfusion, enhances the Ca²⁺ reuptake into SR/ER, increasing the SR/ER Ca²⁺ content, which subsequently (1) reduces the intracellular and mitochondrial Ca²⁺ content, and (2) improves excitation-contraction coupling of cardiomyocytes, contributing to the protection and recovery of ischemic heart.