Cardiac Optogenetics: A move in the light direction
Computer simulations show how low-intensity illumination can be used to terminate cardiac arrhythmias.
- Institute for Experimental Cardiovascular Medicine, University Heart Center Freiburg – Bad Krozingen, Germany
- Faculty of Medicine, University of Freiburg, Germany
For a human heart to pump blood through our circulatory system, billions of muscle cells, called cardiomyocytes, must contract in a well-orchestrated manner. Ordered contraction is achieved via sequential electrical excitation of cells. Cardiomyocytes are excited when a sufficiently strong electric stimulus causes them to depolarize; they then remain in their activated state for some time; and, lastly, they return to their resting state, ready to be activated again. For normal heartbeats, electrical excitation originates from specialized pacemaker cells in the sinus node of the heart, which depolarize automatically. This rhythmically generated electrical signal then propagates along cardiomyocytes throughout the heart. A short time after depolarization, cardiomyocytes contract. Thus, rhythmical electrical activity leads to a regular heartbeat.
Cardiac arrhythmias can be caused by various factors – such as diseased cells and scar tissue – and they are associated with serious clinical conditions, including myocardial infarction, heart failure, and sudden cardiac death. One specific mechanism responsible for cardiac arrhythmia is so-called re-entrant electrical activity: this happens when the electrical impulse that excites cardiomyocytes does not terminate after all cells have been activated. Instead, due to pathological changes in cell or tissue properties, the excitation wave finds a way to circle back and re-activate cells out-of-turn. Under certain conditions, a self-sustaining ‘rotor’ can form, where electrical activity keeps circling in the heart for extended periods of time. While rotating, it emits spiral waves of electrical excitation that override the normal heart rhythm (Pandit and Jalife, 2013; Figure 1A).
Figure 1
Optogenetic control of spiral waves.
(A) Under certain conditions in the heart, spiral waves of electrical excitation are emitted by a ‘rotor’ of self-sustained electrical excitation. Hussaini et al. have used computer simulations to explore how a rotor in a two-dimensional model of heart tissue responds to various patterns of low-intensity illumination. (B) Upon illumination with a spatial gradient (here, the intensity increases from left to right), the rotor moves in the direction of higher light intensity. (C) Partial illumination of the model tissue with constant sub-threshold intensity light also causes the rotor to move towards the illuminated area.
One elegant way to study mechanisms of cardiac arrhythmias is optogenetics. In cardiac optogenetics, light-sensitive proteins are expressed in heart cells and are used to monitor or steer their electrical properties and function (Schneider-Warme, 2018; Zgierski-Johnston and Schneider-Warme, 2021). For example, blue light can be used to activate an ion-channel protein called channelrhodopsin-2, which results in the depolarization of cells. By activating this protein in cardiomyocytes, researchers have already successfully generated optical pacemakers (Arrenberg et al., 2010; Bruegmann et al., 2010) and conducted optical defibrillation in animal models (Bruegmann et al., 2018; Bruegmann et al., 2016; Crocini et al., 2016; Nyns et al., 2017). Now, in eLife, Stefan Luther and colleagues – including Sayedeh Hussaini as first author – report that low-intensity light may be used to steer rotors (Hussaini et al., 2021). Using a combination of cardiac optogenetics and computational modelling, they describe guiding rotors towards locations where re-entrant electrical activity is no longer possible, thereby terminating cardiac arrhythmias.
Computational modelling has long been used by cardiac researchers to explore how cellular (and sub-cellular) mechanisms act together in healthy or diseased heart tissue (Loewe et al., 2018). Hussaini et al. started with an established mathematical model of cardiomyocyte electrical activity (Petkova-Kirova et al., 2012) and added differential equations that describe light-activated currents mediated by channelrhodopsin-2 (Williams et al., 2013). A reaction-diffusion equation (the so-called monodomain model) was then used to couple multiple model cells in a two-dimensional tissue model.
Utilizing this model, the researchers – who are based at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, the University of Göttingen and other institutions in Germany, France, Italy and Canada – first investigated the effect of illumination with intensities so low that the cardiomyocytes were slightly depolarized from their resting state, but were not excited. They found that increasing the intensity of such ‘sub-threshold’ illumination has two effects: it decreases the velocity at which electrical excitation is relayed from cell to cell, and it decreases the dominant frequency of spiral waves. Both results are in keeping with classically expected single-cell behaviour, but Hussaini et al. confirmed them in real tissue for the first time by performing ex vivo experiments with intact mouse hearts.
Next, Hussaini et al. simulated what could happen when using sub-threshold illumination with linearly graded light intensity. They found that the rotor – which previously had been stable – tended to move towards the part of the tissue where the light intensity was higher (Figure 1B). Moreover, the steeper the gradient, the faster the rotor moved. In another simulation, one region of the tissue was exposed to light of constant, low intensity, while the remaining tissue was not illuminated: here the rotor moved to the illuminated region (Figure 1C). Finally, they predicted that successive partial illuminations of the 2D tissue could be used to steer the rotor to parts of the tissue where the rotor could not continue to self-sustain.
Hussaini and colleagues propose a novel approach for the termination of cardiac arrhythmias using sub-threshold illumination. In fact, the underlying mechanism may have played a role in previous breakthroughs in cardiac optogenetics, such as the demonstration of optical defibrillation in animal models mentioned above. Further work is needed to verify effects of interventions proposed based on computational modelling, as hearts are characterized not only by a complex geometry, but also by tissue heterogeneities that rotors might become anchored to. Looking to the future, the obvious question is whether optogenetic modulation of cardiac electrophysiology, as described here, may ultimately be used to terminate arrhythmias in humans. Answering this question will require further research – it will be necessary, for example, to overcome the challenges associated with the expression of light-gated channels in cardiomyocytes, and with targeted light delivery. The work reported in this paper is undoubtedly an important step in the light direction.
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Author details
Franziska Schneider-Warme
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- Version of Record published: January 27, 2021 (version 1)
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© 2021, Wülfers and Schneider-Warme
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Cardiac Optogenetics: A move in the light direction
eLife 10:e65360.
https://doi.org/10.7554/eLife.65360
Further reading
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- Computational and Systems Biology
Accurate prediction of the structurally diverse complementarity determining region heavy chain 3 (CDR-H3) loop structure remains a primary and long-standing challenge for antibody modeling. Here, we present the H3-OPT toolkit for predicting the 3D structures of monoclonal antibodies and nanobodies. H3-OPT combines the strengths of AlphaFold2 with a pre-trained protein language model and provides a 2.24 Å average RMSDCα between predicted and experimentally determined CDR-H3 loops, thus outperforming other current computational methods in our non-redundant high-quality dataset. The model was validated by experimentally solving three structures of anti-VEGF nanobodies predicted by H3-OPT. We examined the potential applications of H3-OPT through analyzing antibody surface properties and antibody–antigen interactions. This structural prediction tool can be used to optimize antibody–antigen binding and engineer therapeutic antibodies with biophysical properties for specialized drug administration route.
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Background:
Preterm birth is the leading cause of neonatal morbidity and mortality worldwide. Most cases of preterm birth occur spontaneously and result from preterm labor with intact (spontaneous preterm labor [sPTL]) or ruptured (preterm prelabor rupture of membranes [PPROM]) membranes. The prediction of spontaneous preterm birth (sPTB) remains underpowered due to its syndromic nature and the dearth of independent analyses of the vaginal host immune response. Thus, we conducted the largest longitudinal investigation targeting vaginal immune mediators, referred to herein as the immunoproteome, in a population at high risk for sPTB.
Methods:
Vaginal swabs were collected across gestation from pregnant women who ultimately underwent term birth, sPTL, or PPROM. Cytokines, chemokines, growth factors, and antimicrobial peptides in the samples were quantified via specific and sensitive immunoassays. Predictive models were constructed from immune mediator concentrations.
Results:
Throughout uncomplicated gestation, the vaginal immunoproteome harbors a cytokine network with a homeostatic profile. Yet, the vaginal immunoproteome is skewed toward a pro-inflammatory state in pregnant women who ultimately experience sPTL and PPROM. Such an inflammatory profile includes increased monocyte chemoattractants, cytokines indicative of macrophage and T-cell activation, and reduced antimicrobial proteins/peptides. The vaginal immunoproteome has improved predictive value over maternal characteristics alone for identifying women at risk for early (<34 weeks) sPTB.
Conclusions:
The vaginal immunoproteome undergoes homeostatic changes throughout gestation and deviations from this shift are associated with sPTB. Furthermore, the vaginal immunoproteome can be leveraged as a potential biomarker for early sPTB, a subset of sPTB associated with extremely adverse neonatal outcomes.
Funding:
This research was conducted by the Perinatology Research Branch, Division of Obstetrics and Maternal-Fetal Medicine, Division of Intramural Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, U.S. Department of Health and Human Services (NICHD/NIH/DHHS) under contract HHSN275201300006C. ALT, KRT, and NGL were supported by the Wayne State University Perinatal Initiative in Maternal, Perinatal and Child Health.
