Calcium Signaling in Cardiomyocyte Function

Abstract

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

Figure 1.

Excitation–contraction coupling (ECC). (1) An action potential depolarizes the cardiomyocyte and induces Na⁺ influx through voltage-gated Na⁺ channels (Na_v). (2) This further depolarizes the cell membrane and induces Ca²⁺ influx through voltage-gated L-type Ca²⁺ channels (LTCCs). (3) This Ca²⁺ entry stimulates Ca²⁺ release via dyadic RyR2 on the SR (4), which in turn triggers cell contraction through activating myofilament crossbridges. (5) LTCC inactivate and RyR close. (6) Cytosolic Ca²⁺ is then moved out of the cell by the Na⁺/Ca²⁺ exchanger (NCX) and pumped back into the SR by SERCA2a, thereby decreasing cytosolic Ca²⁺ concentration and bringing about relaxation. (7) A family of K⁺ channels participate in cell repolarization with K⁺ efflux as a last step for returning membrane potential to its resting value before a new cycle starts. (8) Tuning ECC to meet cardiovascular demands involves β-adrenergic pathways that induce the activation of CaMKII and of cAMP/PKA, which phosphorylates voltage-gated LTCCs and RyRs to enhance their activity and phospholamban (PLB) to remove its inhibition of SERCA activity.

Figure 2.

Difference in the TATS in isolated cardiomyocytes from mouse (A), pig (B), and human (C). The TATS is stained in green (Caveolin-3 [Cav3] or NCX) and RyRs are stained in red. Scale bar, 10 µm. Images were acquired with a Nikon A1R confocal microscope using a 60× oil immersion objective. A4× zoom of the white square is shown (unpubl.).

Figure 3.

Noncanonical Ca²⁺ channels. (1) G-protein-coupled receptors activated by endothelin-1 (ET-1) or angiotensin-II (Ang-II), produce InsP₃ that activates Ca²⁺ release via InsP₃ receptors (IP₃RII), which cross talks with RyR2 to modify Ca²⁺-induced Ca²⁺ release (CICR). (2) Ca²⁺ release from InsP₃R2 present at the perinuclear SR and the nuclear envelope stimulates nuclear Ca²⁺ signaling pathways. (3) β-Adrenergic receptor (β-AR) activation leads to the activation of protein kinase A (PKA), which in turn enhances the activity of the CD38 enzyme. (4) CD38 produces nicotinic acid adenine dinucleotide phosphate (NAADP) that promotes Ca²⁺ leak via endolysosomal two-pore channels (TPCs). (5) Activation of CD38 also generates cyclic ADP-ribose (cADPR), which acts on the RyR2 to enhance its activity. (6) To refill the SR as well as affect cytosolic Ca²⁺ levels, the STIM/ORAI/TRP system may be engaged.

Figure 4.

Mitochondrial Ca²⁺. Mitochondria are densely packed in the cardiomyocyte being aligned along the myofilaments as bricks. Their Ca²⁺ uptake sites are closely localized to the junctional sarcoplasmic reticulum (SR) and a microdomain of elevated Ca²⁺ generated by SR Ca²⁺ release via RyRs. Ca²⁺ enters the mitochondria through the voltage-gated anion channel (VDAC) of the outer mitochondrial membrane and the mitochondrial Ca²⁺ uniporter (MCU) of the inner mitochondrial membrane. Ca²⁺ acts at multiple sites in the electron transport chain and tricarboxylic acid (TCA) cycle to modulate mitochondrial metabolism and ATP production. Ca²⁺ is extruded from the mitochondria through a Na⁺/Li⁺/Ca²⁺ exchanger (NCXL) and/or a Ca²⁺/H⁺ exchanger (mHCX).

Figure 5.

Remodeling of excitation–contraction coupling (ECC) in disease. (1) The coupling between Ca²⁺ influx and Ca²⁺ release is compromised because of a loss of tight connections between the transverse and axial tubule system (TATS) and the sarcoplasmic reticulum (SR) with an atrophy and remodeling of the tubular network. (2) RyRs thus become uncoupled and are then activated with a delay by Ca²⁺ diffusion across the cell from coupled RyRs. (3) Consequently, activation of myofilaments is less homogeneous and contraction impaired. (4) SERCA expression is reduced leading to an elevation in diastolic Ca²⁺ and reduced SR Ca²⁺, which in turn leads to a decrease in Ca²⁺ release via RyRs during each ECC cycle, thereby reducing contraction amplitude. (5) RyRs display spontaneous releases of Ca²⁺, (6) generating Ca²⁺ waves. (7) This cytosolic Ca²⁺ overload leads to NCX activation, triggering after-depolarizations and potentially a spontaneous AP.

Figure 6.

Excitation–transcription coupling. (1) InsP₃R-mediated cytosolic Ca²⁺ increase induces activation of different signaling pathways: (2) Ca²⁺ activates cytosolic and nuclear CaMKII that phosphorylates the inhibitory transcription factor histone deacetylase (HDAC) and (3) induces its export out of the nucleus. (4) MEF2 is activated and this triggers hypertrophic genes expression. (5) Ca²⁺ increases, including via InsP₃Rs, activates calcineurin (CnA), leading to dephosphorylation of nuclear factor of activated T-cells (NFAT) and its translocation to the nucleus. (6) NFAT then activates the transcription of hypertrophic genes including fetal genes (InsP₃R, NPPA, NPPB, RCAN1) and can also repress miR-133. (7) Nuclear Ca²⁺ signals arising from InsP₃R can contribute to activation of nuclear CnA and trigger pathways for hypertrophy (9). (7) miR-133 suppresses expression of NFAT, CnA, and InsP₃R, which will inhibit the hypertrophic responses previously described (8).