Mitochondrial calcium in cardiac ischemia/reperfusion injury and cardioprotection

Abstract

Mitochondrial calcium (Ca²⁺) signals play a central role in cardiac homeostasis and disease. In the healthy heart, mitochondrial Ca²⁺ levels modulate the rate of oxidative metabolism to match the rate of adenosine triphosphate consumption in the cytosol. During ischemia/reperfusion (I/R) injury, pathologically high levels of Ca²⁺ in the mitochondrial matrix trigger the opening of the mitochondrial permeability transition pore, which releases solutes and small proteins from the matrix, causing mitochondrial swelling and ultimately leading to cell death. Pharmacological and genetic approaches to tune mitochondrial Ca²⁺ handling by regulating the activity of the main Ca²⁺ influx and efflux pathways, i.e., the mitochondrial Ca²⁺ uniporter and sodium/Ca²⁺ exchanger, represent promising therapeutic strategies to protect the heart from I/R injury.

Keywords: Calcium handling; Cardiac myocytes; Ischemia/reperfusion injury; Mitochondria; Myocardial infarction; Reactive oxygen species.

Fig. 1

Cardiac mechano-energetic coupling. In the healthy heart, oxidative phosphorylation produces 95% of the adenosine triphosphate (ATP) required to fuel excitation–contraction coupling. ATP phosphorylation is catalyzed by the F₁/F_o-ATP synthase, which harnesses the electrochemical gradient produced by translocation of protons (H⁺) across the inner mitochondrial membrane (IMM) by the electron transport chain complexes. In turn, the electron transport chain derives reducing equivalents from the reduced form of nicotinamide adenine dinucleotide (NADH) produced by the tricarboxylic acid (TCA) cycle. Superoxide (^.O₂⁻) is a physiological by-product of the respiratory chain activity, and is rapidly dismutated to hydrogen peroxide (H₂O₂) by the manganese-dependent superoxide dismutase (Mn-SOD). During elevations in cardiac workload, the increased ATP turnover in the cytosol partially dissipates the mitochondrial membrane potential (ΔΨ_m) and oxidizes the NADH pool. At the same time, calcium (Ca²⁺) accumulation in the mitochondrial matrix stimulates the TCA cycle dehydrogenases to regenerate NADH and also the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH), which sustain the respiratory chain activity and hydrogen peroxide (H₂O₂) elimination, respectively. Also under physiological conditions, transient opening of a low-conductance mitochondrial permeability transition pore (mPTP) in the IMM may operate as an alternative Ca²⁺ efflux pathway in addition to the mitochondrial Na⁺/Ca²⁺ exchanger. Figure created with BioRender.com

Fig. 2

Mechanisms of cardiac ischemia/reperfusion injury. Panel A. Ischemia. During ischemia, the lack of oxygen (O₂) abruptly halts oxidative phosphorylation and induces a metabolic switch to anaerobic glycolysis, which increases lactate production and induces intracellular acidosis. The reduced availability of ATP stops cardiac myocyte contraction and relaxation and alters intracellular ion concentrations. Reduced activity of the sodium (Na⁺)/potassium (K⁺) ATPase, together with the increased activity of the Na⁺/H⁺ exchanger due to intracellular acidosis, induce Na⁺ and Ca²⁺ accumulation in the cytosol. The increase in calcium (Ca²⁺) levels is transmitted to the mitochondrial matrix via the mitochondrial Ca²⁺ uniporter. Mitochondrial Ca²⁺ accumulation progressively dissipates the mitochondrial membrane potential (ΔΨ_m), which can ultimately trigger mitochondrial permeability transition and necrotic cell death. Panel B. Reperfusion. When blood flow is restored, the activity of the respiratory chain resumes, but succinate accumulated during ischemia fuels superoxide (O₂⁻) production at complex I via reverse electron transport. The massive production of mitochondrial reactive oxygen species (ROS), together with mitochondrial Ca²⁺ accumulation and partial restoration of intracellular pH, triggers irreversible opening of the large-conductance mitochondrial permeability transition pore (mPTP), which allows release of ions, solutes and ROS from the mitochondrial matrix. ROS release from the mPTP and other redox-sensitive channels, such as the inner mitochondrial anion channel (IMAC), can induce ROS production from neighboring mitochondria, a process coined ROS-induced ROS release. Figure created with BioRender.com

Fig. 3

Redox-optimized ROS balance. The graph illustrates the relationship between the redox environment of cardiac myocytes and the emission of reactive oxygen species (ROS). ROS emission occurs when the intracellular and/or intramitochondrial environments are either highly reduced (right side of the figure) or highly oxidized (left). Under physiological conditions, the cell and mitochondria operate at intermediate redox state (gray-shaded area), at which ROS production is controlled by the antioxidant systems. Reproduced with permission from [5]. Copyright Elsevier

Fig. 4

Role of mitochondria in ischemic conditioning. The main signaling pathways activated during ischemic conditioning include the reperfusion injury salvage kinase (RISK) pathway, the survival activating factor enhancement (SAFE) pathway, and a pathway involving activation of endothelial nitric oxide synthase (eNOS), protein kinase G (PKG) and protein kinase C (PKC), which mainly converge on mitochondria as the end effector of cardioprotection. Opening of the mitochondrial permeability transition pore (mPTP), which is triggered by mitochondrial Ca²⁺ overload and reactive oxygen species (ROS) production, is a major cause of cardiac myocyte loss during ischemia/reperfusion injury. Modulation of mitochondrial permeability transition is considered a central mediator of cardioprotection induced by ischemic conditioning. Intracellular signaling pathways activated by intermittent cycles of ischemia and reperfusion reduce susceptibility to mPTP opening via multiple mechanisms, including direct mPTP inhibition, mitochondrial translocation of connexin 43 (Cx43), activation of mitochondrial potassium (K⁺) uptake via the mitochondrial ATP-dependent K⁺ channel (mK_ATP), and reduced formation of reactive oxygen species (ROS). In addition, modulation of Ca²⁺ exchange between the sarcoplasmic reticulum (SR) and mitochondria contributes to the cardioprotection afforded by ischemic conditioning. Namely, decreased Ca²⁺ release via ryanodine receptor type 2 (RyR2) and delayed phospholamban (PLN) phosphorylation might prevent exaggerate SR Ca²⁺ release and consequent mitochondrial Ca²⁺ overload during reperfusion. Cyt c cytochrome c, GPCR G protein-coupled receptor, GSK-3β glycogen synthase kinase-3β, JAK Janus kinase, MCU mitochondrial calcium uniporter, MFN1/2 mitofusin ½, PKA protein kinase A, SERCA sarcoplasmic reticulum Ca²⁺ ATPase, SR, sarcoplasmic reticulum, STAT3 signal transducer and activator of transcription 3; TNFR tumor necrosis factor receptor. Figure created with BioRender.com