Mechanisms of altered Ca²⁺ handling in heart failure

Abstract

Ca²⁺ plays a crucial role in connecting membrane excitability with contraction in myocardium. The hallmark features of heart failure are mechanical dysfunction and arrhythmias; defective intracellular Ca²⁺ homeostasis is a central cause of contractile dysfunction and arrhythmias in failing myocardium. Defective Ca²⁺ homeostasis in heart failure can result from pathological alteration in the expression and activity of an increasingly understood collection of Ca²⁺ homeostatic and structural proteins, ion channels, and enzymes. This review focuses on the molecular mechanisms of defective Ca²⁺ cycling in heart failure and considers how fundamental understanding of these pathways may translate into novel and innovative therapies.

Keywords: CaMKII; calcium; excitation-contraction coupling; heart failure; mitochondria.

Fig 1. Ca²⁺ homeostasis and Excitation Coupling (ECC)

The ECC process is initiated when an action potential (AP) excites the myocyte cell membrane (sarcolemma) along its transverse tubules. This depolarization rapidly opens voltage-gated Na⁺ channels (mostly Na_V1.5) that further depolarize the cell membrane, allowing opening of voltage-gated Ca²⁺ channels (mostly Ca_V1.2). Inward Ca²⁺ current triggers opening of ryanodine receptor (RyR2) channels by a Ca²⁺-induced Ca²⁺ release process, resulting in coordinated release of sarcoplasmic reticulum (SR) Ca²⁺ that contributes the major portion of the myofilament-activating increase in [Ca²⁺]_i. The Ca²⁺ released from the SR binds to troponin C of the troponin-tropomyosin complex on the actin filaments in sarcomeres, facilitating formation of cross bridges between actin and myosin and myocardial contraction. Voltage-gated K⁺ channels open to allow an outward current that favors action potential repolarization, establishing conditions required for relaxation. Relaxation occurs when Ca²⁺ is taken back up into the SR through the action of the SR Ca²⁺ adenosine triphosphatase SERCA2a and is extruded from the cell by the sarcolemmal Na⁺ and Ca²⁺ exchanger (NCX). SERCA2a is constrained by phospholamban (PLN) under resting conditions.

Figure 2. Regulation of [Ca²⁺]_i homeostasis by Ca²⁺ binding proteins and kinases

Regulation of Ca²⁺ homeostasis involves a multitude of Ca²⁺ binding proteins and enzymes, including CaMKII, PKC, PKA and S100A1: (1). CaMKII catalyzes phosphorylation of voltage-gated Ca²⁺ channels (mostly Ca_V1.2 in ventricle) to increase Ca²⁺ entry, RyR2 to increase Ca²⁺ release, voltage-gated Na⁺ channels (mostly Na_V1.5 in ventricle) to increase subsarcolemmal [Na+]_i,, which decreases the driving force for Ca²⁺ extrusion by the Na⁺/Ca²⁺ exchanger (NCX), and PLN to reduce the inhibitory activity of PLN on SERCA2a. In general, the increased phosphorylation of these proteins by CaMKII increases Ca²⁺ influx, and storage by the SR, which leads to increased systolic [Ca²⁺]_i and increased rate and magnitude of force (pressure) generation and improved lusitropy. (2) PKA is activated by β–AR agonists and catalyzes phosphorylation of the same Ca²⁺ regulatory proteins modified by CaMKII, but at different amino acids. (3) Classical PKC isoforms are activated downstream to a variety of G protein coupled receptors and are activated by increased [Ca²⁺]_i, leading to decreased activity SERCA2 by phosphorylating inhibitor 1 (I-1) resulting in PLN dephosphorylation, reducing SR Ca²⁺ load and Ca²⁺ release, causing reduced contractility. (4) S100A1 interacts with the SERCA2a/PLN complex in a Ca²⁺-dependent manner to augment SR Ca²⁺ uptake and increase SR Ca²⁺ content. S100A1 also directly regulates RyR2 function, stimulates ATP synthase activity and promotes the adenosine nucleotide translocator (ANT) function to increase ATP synthesis and mitochondrial ATP efflux in cardiomyocytes.

Figure 3. A scenario for mitochondrial Ca²⁺ overload, impaired metabolism and cell death in heart failure

The mitochondrial Ca²⁺ uniporter is a Ca²⁺ selective channel residing in the inner mitochondrial membrane. MCU is a phosphorylation substrate for CaMKII. Mitochondrial CaMKII inhibition reduces MCU current, increases mitochondrial Ca²⁺ retention capacity and is protective against myocardial death in response to ischemia-reperfusion injury, myocardial infarction and toxic doses of isoproterenol. Excessive mitochondrial Ca²⁺ and ROS trigger mitochondrial permeability transition pore (mPTP) opening, leading to cell death. Mitochondria Ca²⁺ overload also promotes ROS generation, which could oxidize CaMKII (ox-CaMKII) and cause sustained activation of CaMKII. ox-CaMKII could enhance MCU activity and further increase mitochondrial Ca²⁺ overload, promoting mPTP opening and impairing energy metabolism in heart failure. At the same time, myocardial energy deficiency could adversely affect [Ca²⁺]_i homeostasis.

Figure 4. Structure and activation of CaMKII

CaMKII consists of stacked hexamers and each monomer consists of an N-terminus catalytic domain and a C-terminus association domain that flank a core regulatory domain. CaMKII is activated when [Ca²⁺]_i binds to calmodulin causing CaMKII to assume an active, extended conformation. Sustained binding to calcified calmodulin (Ca²⁺/CaM) leads to threonine 287 autophosphorylation and sustained CaMKII activation. Oxidation of paired regulatory domain methionines (281/282) also causes sustained activation of CaMKII as oxidized CaMKII resets its Ca²⁺ sensitivity so that lower levels of intracellular Ca²⁺ are required for initial activation. Thus, both threonine 287 autophosphoryation and methionine 281/282 oxidation can convert CaMKII into a constitutively active enzyme to drive myocardial disease phenotypes.

Fig 5. CaMKII and Mechanisms of arrhythmia

Sustained activation of CaMKII by oxidative stress and elevated [Ca²⁺]_i contributes to arrhythmia in heart failure by several mechanisms: 1) CaMKII phosphorylates L-type Ca channels (Ca_V1.2) to increase its open probability, causing early afterdepolarizations (EADs). Increased I_Ca also contributes to action potential prolongation, augmented [Ca²⁺]_i and DADs. 2) CaMKII phosphorylates Na⁺ channels (Na_V1.5) and enhances the long-lasting late I_Na (gain of function) promoting EADs and increasing subsarcolemmal [Na⁺]_i to favor delayed afterdepolarizations (DADs). 3) CaMKII favors phosphorylation of RyR2 to increase SR Ca²⁺ leak, which shifts Na⁺/Ca²⁺ exchanger (NCX) to a forward mode, causing DADs. CaMKII contributes to arrhythmogenic structural features of injured myocardium by promoting myocyte death and collagen deposition.