Fine-tuning SERCA activity to treat distinct heart failure syndromes

The understanding of excitation-contraction (E-C) coupling in cardiac cells consolidated as an enormous foundational knowledge over the last decades. The phenomenon of calcium (Ca²⁺)-induced Ca²⁺ release is a fundamental event for the initiation of E-C coupling, while the dynamics of Ca²⁺ events occuring during E-C coupling are temporally and spatially arranged by Ca²⁺ handling machinery. Although multiple factors regulate and fine-tune the initiation and termination of intracellular Ca²⁺ handling, more attention has been given to the function of ryanodine receptors (RyR), sarcoplasmic reticulum (SR), sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA), sodium/calcium exchanger, and phospholamban (PLB). The synchronized function of these components is notoriously important for maintaining the Ca²⁺ homeostasis, while individual or collective disturbance(s) of these components are linked to Ca²⁺ mishandling, a hallmark feature of cardiac diseases. For instance, increased SR Ca²⁺ leak is an arrhythmogenic substrate that may result in a higher occurrence of self-propagating intracellular Ca²⁺ waves.

In a recent article in The Journal of Physiology, Sato et al. (2021) explored the effects of SERCA activity on the propensity of Ca²⁺ wave propagation. The authors started this study by fine-tuning a preexisting computational model of a rabbit ventricular myocyte. Since the development of LabHEART by Bers’ group in 2001, multiple increments have been made to further approximate this model to the cellular physiology. In the latest upgrade, Sato et al. (2021) inserted Ca²⁺ diffusion in the submembrane space, which helped to make the simulations of Ca²⁺ spark and wave propagation more realistic. The accuracy of the findings obtained from this model was then experimentally verified.

Taking advantage of this physiologically detailed computational model, they demonstrated that the high dependency of luminal Ca²⁺ to recruit its neighboring clusters of RyRs (Ca²⁺ Release Units - CRUs) to initiate a new Ca²⁺ spark is transversally facilitated by the geographic location of CRUs. While progressive elevations of [Ca²⁺]_SR and [Ca²⁺]_i triggered Ca²⁺ waves, these events are also spatially more frequent in a transversal direction. By interrogating the role of SR Ca²⁺ re-uptake on the wave formation and propagation, the authors demonstrated that enhanced SERCA activity can act as a regulator preventing the formation of Ca²⁺ waves. Interestingly, the suppressive Ca²⁺ wave formation was more effective in the longitudinal propagation. Considering that enhanced SERCA activity may cause SR Ca²⁺ overload favoring Ca²⁺ waves formation and despite the high occurrence of spontaneous Ca²⁺ release under high SERCA activity, the majority of these events were spatially restricted. To experimentally validate the computational simulations, PLB knockout mice were used. The predicted biphasic response of graded SERCA activity on Ca²⁺ waves was confirmed by the pharmacological blockage of SERCA activity by cyclopiazonic acid in isolated cardiomyocytes. Thus, the authors are to be commended for implementing a physiologically detailed biophysical model in combination with experimental validations.

These findings add granular details for a better understanding of Ca²⁺ homeostasis in heart failure (HF) settings, while the model may serve as an excellent platform for further interrogations. For instance, a recent publication from Frisk et al. (2021) showed fundamental Ca²⁺ handling and t-tubule density changes in different HF etiologies, including HF with reduced ejection fraction (HFrEF) and preserved (HFpEF). While transversely-oriented t-tubule density was reduced in human HFrEF biopsies, increased transverse tubule density is observed in patients with HFpEF. By using diabetic or hypertensive HFpEF animal models, Frisk et al. (2021) demonstrated that the magnitude and release kinetics remain unchanged in both HFpEF models, but were impaired in HFrEF. Although SERCA abundance was reduced in HFrEF and HFpEF, this is particularly compensated in hypertensive HFpEF by increased PLB phosphorylation, but not in HFrEF and diabetic HFpEF, resulting in slowed Ca²⁺ reuptake. Therefore, the diversity of HF etiologies is reflected in an ununiform Ca²⁺ homeostasis disturbance. Considering that ventricular arrhythmias are frequently observed regardless of the HF etiology, the observed biphasic effect of SERCA activity on the propensity of arrhythmogenic Ca²⁺ waves may motivate future investigations across the HFpEF etiologies.

The divergent structural remodeling of t-tubules in different forms of HF (Frisk et al., 2021) and its association with the dyssynchronous rise of [Ca²⁺]_i during systole is another hallmark of HF. The loss of t-tubules in HFrEF and fragmentation of RyR clusters into smaller and less dense CRUs groupings were accompanied by increased Ca²⁺ sparks but also favor the appearance of “silent” CRUs. The impact of CRUs geometry changes on Ca²⁺ sparks was also replicated in a mathematical model (Kolstad et al., 2018). Thus, the model proposed by Sato et al. (2021) may be useful to investigate the propagative properties of Ca²⁺ waves under fragmented RyR cluster conditions and graded SERCA activity. Moreover, considering the preservation of t-tubules in HFpEF but with Ca²⁺ reuptake abnormalities (Frisk et al., 2021), the computational model described by Sato et al. (2021) might provide mechanistic hints of the arrhythmic phenotype in HFpEF. Furthermore, although the intraluminal components of the SR are often overlooked, luminal Ca²⁺ sensors/buffers also contribute to the open probability of the RyR directly affecting the SR Ca²⁺ load, Ca²⁺ leak, and spontaneous Ca²⁺ waves.

Several preclinical studies have reported promising results in treating HF using gene therapy to increase the expression or activity of SERCA2a. Sato’s findings also reinforce the possibility of using strategies that can fine-tune SERCA activity and expression with the aim of treating cardiac diseases related to Ca²⁺ dysfunction. The large body of evidence obtained in multiple proof-of-concept studies advanced towards mechanistic-based clinical trials in HF patients, including the Calcium Upregulation by Percutaneous Administration of Gene Therapy in Cardiac Disease – CUPID trial. Although the efficacy, safety and feasibility of a recombinant adeno-associated viral vector type 1 (AAV1/SERCA2a – CUPID) gene delivery via percutaneous intracoronary infusion demonstrated promising outcomes in phase 1/2a CUPID (Jessup et al., 2011), subsequent phase 2b CUPID reported neutral clinical outcomes in HFrEF patients (Greenberg et al., 2016). Shortly after the announcement of CUPID 2 results, similar trials (AGENT-HF and SERCA-LVAD) were prematurely terminated. A detailed description of potential reasons that might justify the poor translatability of proof-of-concept studies into clinically-verified therapies is beyond the scope of this journal club. However, the lessons learned from the first AAV-based gene therapy to enter a clinical trial for HF span over multiple areas of the scientific community. As a result, the genetic engineering field has evolved at a fast pace, opening up new avenues for the development of alternative technologies for improving the gene therapy approaches in humans.

In summary, the study showed SERCA activity has a dual effect on the probability of Ca²⁺ release, affecting Ca²⁺ spark formation and Ca²⁺ waves propagation. These findings open avenues to a better understanding of the involvement of SERCA activity in cardiac diseases, such as HF. Additionally, the implemented computational model makes it possible to motivate future investigations using this model.