A molecular mechanism of the cardiac alternans, one of the causes of severe cardiac arrhythmia, has been identified

An international project with participation of IIBB-CSIC has identified one of the molecular mechanisms that causes the so-called “cardiac alternans”, an alteration of the cardiac rhythm that can cause ventricular fibrillation and potentially lethal cardiac arrhythmia. This finding opens a new path to the development of pharmacological treatment.

Cardiac Rhythm and Contraction group at IIBB-CSIC.Cardiovascular diseases are one of the main causes of mortality worldwide, being responsible for around 18 million deaths each year. In Spain, they represent almost a third of all deaths. These diseases also cause a significant decrease in the quality of life.

An international team of multidisciplinary researchers led by Professor SRW Chen from the University of Calgary, the Cardiac Rhythm and Contraction group at the Institute of Biomedical Research of Barcelona IIBB-CSIC and IIB Sant Pau have identified one of the molecular mechanisms underlying cardiac alternans, an alteration of the heart rhythm that facilitates the induction of ventricular fibrillation1, and that causes a potentially fatal arrhythmia. Furthermore, they show that people with loss of ryanodine receptor (RyR) function, responsible for the emission of calcium ions, are prone to induction of ventricular fibrillation and sudden death caused by cardiac alternans2.

The research group of Computational Biology and Complex Systems BIOCOM-SC and the ANCORA group of biomedical images analysis at the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) also participate in the project.

Cardiac alternans have been known since the late nineteenth century, when an alteration in the pulse (hence the name pulsus alternans) was described, alternating a strong heart beat with a weak one. Almost a century later, this alteration was related to the propensity to suffer episodes of ventricular fibrillation, where heart cells suddenly stop beating in a synchronized way, in many cases with fatal results in just minutes. Since then, work has been done to understand the molecular mechanisms responsible for this arrhythmia, which would open up the possibility to the development of possible pharmacological treatments.

Background of the study of alternans at IIBB-CSIC and IIB Sant Pau

The Cardiac Rhythm and Contraction group at IIBB-CSIC and IIB Sant Pau, led by Dr. Leif Hove-Madsen, has been studying cellular electrophysiological alterations responsible for the induction of cardiac arrhythmias for two decades. The molecular mechanism responsible for cardiac alternans has been the subject of debate over the past decade where different origins for RyR malfunction have been proposed.

As Dr. Leif Hove-Madsen points out, “the RyR is a protein found in a network of membranes within the cardiac cell where large amounts of calcium are stored. The RyR behaves like a gate that, when opened, releases the stored calcium and activates the contraction of the heart. However, if these calcium deposits are overloaded, the opening of the door becomes uncontrolled, leading to irregularities in the calcium released and cardiac arrhythmias. This study has managed to identify the key molecular mechanism regulating the opening of this gate.”

The project sees the union of RyR to the protein calmodulin (CaM), in charge of regulating RyR’s function, as a possible explanation for its alteration.

It was Professor SRW Chen from the University of Calgary, and one of the leading experts on the RyR, who proposed that this alteration in the function of the receptor could be due to its binding to another protein CaM, that regulates its function. To confirm this, Professor Chen designed an experiment, in collaboration with Dr. Leif Hove-Madsen. He changed the function of CaM by introducing modified adenoviruses in mouses to produce either the protein in its normal state or mutations in the protein that increased or decreased its function, all the while observing whether this decreased or increased the propensity to develop alternans.

Data analysis

The analysis of these data has been carried out by the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) through statistical techniques. These techniques consist on the analysis of image sequences with thousands or millions of pixels per image, as it is more useful than to average out the sign of each image. The group is lead by Raul Benítez, who indicates that “the result of the analysis of these experiments is clear. A decrease in calcium binding to CaM was correlated with a decrease in the occurrence of alternans.” However, experimentally it is not possible to observe the physical interaction between CaM and the RyR, so there is always the possibility that CaM is affecting another regulatory mechanism, and that there is an alternative explanation for this effect.

To confirm whether the effect of CaM on RyR is responsible for this change, the members of BIOCOM-SC used a computational model that describes in detail the interaction between CaM and RyR and its effect on cardiac dynamics.

Thanks to multidisciplinary collaborations, the BIOCOM-SC group has managed to develop computational techniques that allow the study of different physiological scenarios that explain the origin of cardiac alternans. These techniques are based on a mathematical model of a ventricular myocyte. As indicated by Blas Echebarria, principal researcher of the computational part of the study, “it is known that, in most cases, cardiac alternans is due to irregularities in the regulation of the calcium level within the cell. Mathematical models predicted that the alternans could be caused by alterations in the protein that regulates the release of calcium in the cell, the RyR.” The results of the mathematical model agree perfectly with the experimental observations.

Blas Echebarria and Leif Hove-Madsen concur that, "the agreement of the mathematical model with the experimental data is so good that we can be quite sure that the regulation of RyR opening by CaM is the underlying mechanism."

Researchers do not rule out that there may be other mechanisms depending on the electrical functioning of the heart, which varies among species and cardiac tissues. Interdisciplinary work is fundamental in this research, in which the identification of a specific molecular mechanism causing cardiac alternans should help with the development of pharmacological treatments for arrhythmias triggered by cardiac alternans.

The contribution of the research groups from Barcelona in presenting discoveries on ventricular arrhythmia is thanks to multidisciplinary research projects on atrial fibrillation and the development of atrial models, funded by the Spanish Ministry of Science and Innovation, the Generalitat de Catalunya and the Fundació Marató TV3. "Thanks to the knowledge generated in these projects, we had the opportunity to develop the ventricular model in collaboration with Professor Chen", the researchers from Barcelona conclude.

ARTICLES

1 Ca2+-CaM Dependent Inactivation of RyR2 Underlies Ca2+ Alternans in Intact Heart. Wei J, Yao J, Belke D, Guo W, Zhong X, Sun B, Wang R, Estillore JP, Alexander V, Benitez R, Hove-Madsen L, Alvarez-Lacalle E, Echebarria B, Chen SW. Circ Res. 2020 Dec 30. doi:10.1161/CIRCRESAHA.120.318429. Online ahead of print. PMID: 33375811

2 Cardiac ryanodine receptor calcium release deficiency syndrome. Sun B, Yao J, Ni M, Wei J, Zhong X, Guo W, Zhang L, Wang R, Belke D, Chen YX, Lieve KVV, Broendberg AK, Roston TM, Blankoff I, Kammeraad JA, von Alvensleben JC, Lazarte J, Vallmitjana A, Bohne LJ, Rose RA, Benitez R, Hove-Madsen L, Napolitano C, Hegele RA, Fill M, Sanatani S, Wilde AAM, Roberts JD, Priori SG, Jensen HK, Chen SRW. Sci Transl Med. 2021 Feb 3;13(579):eaba7287. doi:10.1126/scitranslmed.aba7287. PMID:  33536282