Targeting cardiomyocyte Ca2+ homeostasis in heart failure

Abstract

Improved treatments for heart failure patients will require the development of novel therapeutic strategies that target basal disease mechanisms. Disrupted cardiomyocyte Ca(2+) homeostasis is recognized as a major contributor to the heart failure phenotype, as it plays a key role in systolic and diastolic dysfunction, arrhythmogenesis, and hypertrophy and apoptosis signaling. In this review, we outline existing knowledge of the involvement of Ca(2+) homeostasis in these deficits, and identify four promising targets for therapeutic intervention: the sarcoplasmic reticulum Ca(2+) ATPase, the Na(+)-Ca(2+) exchanger, the ryanodine receptor, and t-tubule structure. We discuss experimental data indicating the applicability of these targets that has led to recent and ongoing clinical trials, and suggest future therapeutic approaches.

Fig. (1)

Excitation-contraction (EC) contraction coupling in the normal and failing heart. In healthy cardiomyocytes (A), the action potential propagates into the t-tubules, opening voltage-gated L-type Ca²⁺ channels (LTCCs). The resulting Ca²⁺ influx triggers the opening of Ca²⁺ release channels (ryanodine receptors, RyRs) in the membrane of the sarcoplasmic reticulum (SR). Released Ca²⁺ initiates contraction as it binds to the contractile apparatus, and relaxation occurs as Ca²⁺ is recycled into the SR by the SR Ca²⁺ ATPase 2a (SERCA2a), and removed from the cell via the Na⁺-Ca²⁺ exchanger (NCX). During heart failure (B), Ca²⁺ release is impaired, leading to slower and smaller contractions. Reduced SR Ca²⁺ content results from decreased SERCA2a activity, increased RyR leak and, in some cases, increased NCX activity. Systolic dysfunction also results from disruption of T-tubule structure, which functionally “orphans” some RyRs from LTCCs. Slowed and incomplete Ca²⁺ removal from the cytosol impairs cardiomyocyte relaxation, and promotes hypertrophy and apoptosis signaling. Triggered cardiac arryhythmia has been linked to RyR leak, and removal of released Ca²⁺ by NCX, causing DADs. EADs may result from inappropriate re-opening of Ca²⁺ channels. Deficient Ca²⁺ cycling is also linked to altered Na+ homeostasis, following downregulation of the Na⁺-K⁺ ATPase (NKA) and increased late Na+ current.

Fig. (2)

Disrupted Ca²⁺ homeostasis in failing cardiomyocytes and therapeutic targeting of SERCA2a. A: Representative confocal line-scan images (left) show that Ca²⁺ release is de-synchronized across failing cells, resulting in smaller and slower Ca²⁺ transients (centre panel, magnified in inset at right). Unpublished data are from a rat model of heart failure following myocardial infarction and sham-operated controls (fluo-4 AM loading). Smaller transients additionally result from declining SR content due to reduced SERCA2a activity. B: SERCA2a activity may be therapeutically increased in heart failure by gene therapy-mediated overexpression or pharmacological stimulation. Phospholamban (PLB)-dependent inhibition of SERCA could be relieved by increasing PLB phosphorylation by: 1) elevating cAMP levels (stimulating β-adrenergic signaling or preventing cAMP breakdown by phosphodiesterases (PDEs)), or 2) preventing PLB dephosphorylation by inhibiting protein phosphatase 1 (PP1), or increasing activity of inhibitor 1 (I-1). Alternatively, competitive inhibition of PLB could be employed.

Fig. (3)

Altered NCX activity as a therapeutic target in heart failure. A: Experimental Ca²⁺ transients from SERCA2 knockout mice are dramatically reduced (left panel). Modeling data predict that simultaneous NCX ablation increases Ca²⁺ transient magnitude (right). This indicates that NCX competes with SERCA2a for the same pool of Ca²⁺ and reduces SR Ca²⁺ content and release. Data are adapted from [227], with permission. B: NCX activity could be therapeutically modulated by direct targeting or altering electrochemical gradients. NCX inhibitors attenuate cellular Ca²⁺ extrusion and thereby increase cellular Ca²⁺ load and ultimately contractility. Inhibition of NKA similarly inhibits NCX-mediated Ca²⁺ extrusion by increasing cellular Na⁺ levels. However, prevention of Ca²⁺ overload is desirable in patients at risk for arrhythmia, and this may be attained by inhibition of Na+ influx pathways, which augments Ca²⁺ extrusion.

Fig. (4)

Altered Ca²⁺ sparks in failing cardiomyocytes and therapeutic targets of RyR activity. A: Line scan confocal imaging of failing cardiomyocytes (post-infarction mouse) reveals more frequent and slower Ca²⁺ sparks (temporal profiles of indicated sparks shown at right; reproduced from [6], with permission). Thus, disrupted RyR function in heart failure promotes SR Ca²⁺ leak and dyssynchronous Ca²⁺ release. B: RyR function is regulated by a large protein complex. Phosphorylation (by PKA and CaMKII) and dephosphorylation (by protein phosphatase 1 or 2a) are important regulatory pathways. Strategies to inhibit RyR phosphorylation, such as CaMKII inhibitors, are demonstrated to reduce SR Ca²⁺ leak. RyRs “blockers” such as rycals are an alternative approach to reducing leak.

Fig. (5)

T-tubule disruption in heart failure and pathways to restore t-tubule structure. A: Confocal images post-infarction murine cardiomyocytes stained with di-8-ANEPPS show disrupted t-tubule structure in heart failure (left panel, magnified in insets). Accordingly, SR Ca²⁺ release was desynchronized in comparison with sham-operated controls. Mathematical modeling quantitatively reproduced the dyssynchronous pattern of Ca²⁺ release when changes in t-tubule organization, RyR threshold, and SR Ca²⁺ content were accounted for (right panels). Data are adapted from [193], with permission. B: Ttubule integrity and function is dependent on several proteins. JP2, along with caveolin 3, anchors t-tubules to the SR membrane. Increased calcineurin-NFAT signaling resulting from mechanical stress downregulates JP2 and disrupts t-tubules. JP2 expression may be restored by mechanical unloading, blockade of calcineurin-NFAT signaling, or by overexpressing the stretch-sensitive protein Tcap. BIN1 is involved in t-tubule growth and is downregulated in heart failure. Overexpression of BIN1 may therefore attenuate t-tubule loss.