Development of a Cre-recombination-based color-switching reporter system for cell fusion detection

Cell fusion plays a key role in the development and formation of tissues and organs in several organisms. Skeletal myogenesis is assessed in vitro by cell shape and gene and protein expression using immunofluorescence and immunoblotting assays. However, these conventional methods are complex and do not allow for easy time-course observation in living cells. Therefore, this study aimed to develop a Cre recombination-based fluorescent reporter system to monitor cell-cell fusion. We combined green and red fluorescent proteins with a Cre-loxP system to detect syncytium formation using a fluorescent binary switch. This allowed us to visualize mononucleated cells with green fluorescence before fusion and multinucleated syncytia with red fluorescence by conditional expression after cell fusion. The formation of multinuclear myotubes during myogenic differentiation was detected by the change in fluorescence from green to red after Cre-mediated recombination. The distribution of the fluorescence signal correlated with the expression of myogenic differentiation markers. Moreover, red reporter fluorescence intensity was correlated with the number of nuclei contained in the red fluorescent-positive myotubes. We also successfully demonstrated that our fusion monitoring system is applicable to the formation of skeletal muscle myotube and placental syncytiotrophoblast. These results suggest that the color-switching fluorescent reporter system, using Cre-mediated recombination, could be a robust tool used to facilitate the study of cell-to-cell fusion.

Enzymatic vitamin A2 production enables red-shifted optogenetics

Optogenetics is a technology using light-sensitive proteins to control signaling pathways and physiological processes in cells and organs and has been applied in neuroscience, cardiovascular sciences, and many other research fields. Most commonly used optogenetic actuators are sensitive to blue and green light, but red-light activation would allow better tissue penetration and less phototoxicity. Cyp27c1 is a recently deorphanized cytochrome P450 enzyme that converts vitamin A1 to vitamin A2, thereby red-shifting the spectral sensitivity of visual pigments and enabling near-infrared vision in some aquatic species.Here, we investigated the ability of Cyp27c1-generated vitamin A2 to induce a shift in spectral sensitivity of the light-gated ion channel Channelrhodopsin-2 (ChR2) and its red-shifted homolog ReaChR. We used patch clamp to measure photocurrents at specific wavelengths in HEK 293 cells expressing ChR2 or ReaChR. Vitamin A2 incubation red-shifted the wavelength for half-maximal currents (λ50%) by 6.8 nm for ChR2 and 12.4 nm for ReaChR. Overexpression of Cyp27c1 in HEK 293 cells showed mitochondrial localization, and HPLC analysis showed conversion of vitamin A1 to vitamin A2. Notably, the λ50% of ChR2 photocurrents was red-shifted by 10.5 nm, and normalized photocurrents at 550 nm were about twofold larger with Cyp27c1 expression. Similarly, Cyp27c1 shifted the λ50% of ReaChR photocurrents by 14.3 nm and increased normalized photocurrents at 650 nm almost threefold.Since vitamin A2 incubation is not a realistic option for in vivo applications and expression of Cyp27c1 leads to a greater red-shift in spectral sensitivity, we propose co-expression of this enzyme as a novel strategy for red-shifted optogenetics.

β-adrenergic signaling triggers enteric glial reactivity and acute enteric gliosis during surgery

BACKGROUND

Enteric glia contribute to the pathophysiology of various intestinal immune-driven diseases, such as postoperative ileus (POI), a motility disorder and common complication after abdominal surgery. Enteric gliosis of the intestinal muscularis externa (ME) has been identified as part of POI development. However, the glia-restricted responses and activation mechanisms are poorly understood. The sympathetic nervous system becomes rapidly activated by abdominal surgery. It modulates intestinal immunity, innervates all intestinal layers, and directly interfaces with enteric glia. We hypothesized that sympathetic innervation controls enteric glia reactivity in response to surgical trauma.

METHODS

Sox10iCreERT2/Rpl22HA/+ mice were subjected to a mouse model of laparotomy or intestinal manipulation to induce POI. Histological, protein, and transcriptomic analyses were performed to analyze glia-specific responses. Interactions between the sympathetic nervous system and enteric glia were studied in mice chemically depleted of TH+ sympathetic neurons and glial-restricted Sox10iCreERT2/JellyOPfl/+/Rpl22HA/+ mice, allowing optogenetic stimulation of β-adrenergic downstream signaling and glial-specific transcriptome analyses. A laparotomy model was used to study the effect of sympathetic signaling on enteric glia in the absence of intestinal manipulation. Mechanistic studies included adrenergic receptor expression profiling in vivo and in vitro and adrenergic agonism treatments of primary enteric glial cell cultures to elucidate the role of sympathetic signaling in acute enteric gliosis and POI.

RESULTS

With ~ 4000 differentially expressed genes, the most substantial enteric glia response occurs early after intestinal manipulation. During POI, enteric glia switch into a reactive state and continuously shape their microenvironment by releasing inflammatory and migratory factors. Sympathetic denervation reduced the inflammatory response of enteric glia in the early postoperative phase. Optogenetic and pharmacological stimulation of β-adrenergic downstream signaling triggered enteric glial reactivity. Finally, distinct adrenergic agonists revealed β-1/2 adrenoceptors as the molecular targets of sympathetic-driven enteric glial reactivity.

CONCLUSIONS

Enteric glia act as early responders during post-traumatic intestinal injury and inflammation. Intact sympathetic innervation and active β-adrenergic receptor signaling in enteric glia is a trigger of the immediate glial postoperative inflammatory response. With immune-activating cues originating from the sympathetic nervous system as early as the initial surgical incision, adrenergic signaling in enteric glia presents a promising target for preventing POI development.

A bistable inhibitory OptoGPCR for multiplexed optogenetic control of neural circuits

Information is transmitted between brain regions through the release of neurotransmitters from long-range projecting axons. Understanding how the activity of such long-range connections contributes to behavior requires efficient methods for reversibly manipulating their function. Chemogenetic and optogenetic tools, acting through endogenous G-protein coupled receptor (GPCRs) pathways, can be used to modulate synaptic transmission, but existing tools are limited in sensitivity, spatiotemporal precision, or spectral multiplexing capabilities. Here we systematically evaluated multiple bistable opsins for optogenetic applications and found that the Platynereis dumerilii ciliary opsin (PdCO) is an efficient, versatile, light-activated bistable GPCR that can suppress synaptic transmission in mammalian neurons with high temporal precision in-vivo. PdCO has superior biophysical properties that enable spectral multiplexing with other optogenetic actuators and reporters. We demonstrate that PdCO can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, thereby enabling detailed synapse-specific functional circuit mapping.

How Carvedilol activates β2-adrenoceptors

Carvedilol is among the most effective β-blockers for improving survival after myocardial infarction. Yet the mechanisms by which carvedilol achieves this superior clinical profile are still unclear. Beyond blockade of β1-adrenoceptors, arrestin-biased signalling via β2-adrenoceptors is a molecular mechanism proposed to explain the survival benefits. Here, we offer an alternative mechanism to rationalize carvedilol's cellular signalling. Using primary and immortalized cells genome-edited by CRISPR/Cas9 to lack either G proteins or arrestins; and combining biological, biochemical, and signalling assays with molecular dynamics simulations, we demonstrate that G proteins drive all detectable carvedilol signalling through β2ARs. Because a clear understanding of how drugs act is imperative to data interpretation in basic and clinical research, to the stratification of clinical trials or to the monitoring of drug effects on the target pathway, the mechanistic insight gained here provides a foundation for the rational development of signalling prototypes that target the β-adrenoceptor system.

TLR4 pathway induces proarrhythmogenic slow conduction by impaired depolarisation

Background

Cardiac inflammation driven by the Toll-Like-Receptor 4 (TLR4) is correlated to increased risk of arrhythmia. Cardiac arrhythmogenesis and the formation of re-entry tachycardia is highly dependent on conduction velocity (CV) and action potential (AP) duration (APD). As TLR4 induced APD shortening has been shown, we analyze in this study for the first time the TLR4 effect on conductance disturbances in a LPS induced septic mouse model.

Methods

Systemic activation of TLR4 in mice was achieved by intraperitoneal LPS injection 3.5 hours prior to experiments. In vivo electrophysiological investigation (EPI) was performed using an octapolar transvenous catheter placed to the right heart. Ex vivo experiments were performed on Langendorff-perfused hearts. AP propagation was measured by optical voltage mapping (OVM) with voltage sensitive dye Di-4-ANEPPS. For AP analysis including RMP, intracellular electrical recordings were performed using sharp microelectrodes. Wildtype mice after LPS injection were compared to wildtype mice after vehicle (NaCl) injection or ubiquitous TLR4 knockout (TLR4−/−) mice with LPS or vehicle application.

Results

In vivo EPI showed a tendency to more atrial fibrillation after LPS injection (+LPS 5/6, +NaCl 2/6, p=0.2). Ventricular stimulation evoked ventricular tachycardia in every LPS treated WT mouse but less in controls (+LPS 6/6, +NaCl 1/6, p=0.01). OVM measured decreased CV in both atria and ventricle after LPS treatment (atria: +LPS: 43.1±3.1cm/s, n=5; +NaCl: 72.6±9.8cm/s, n=10, p=0.04; ventricle: +LPS: 50.2±2.2cm/s, n=6; +NaCl: 67.7±5.0cm/s, n=10, p=0.02). In analysis of AP in atria upstroke velocity was slightly decreased (max.dV/dt: +LPS: 123.1±4.8V/s, n=22; +NaCl: 158.5±5.3V/s, n=39, p=0.04) but highly reduced in ventricle (max.dV/dt: +LPS: 91.8±3.6V/s, n=27; +NaCl: 140.7±6.3V/s, n=35, p<0.0001). RMP in atria was depolarised after LPS injection (+LPS: −70.1±1.9mV, n=22; +NaCl: −81.1±1.2mV, n=39, p=0.004) explaining decreased upstroke velocity and CV slowing in atria by voltage-dependent Na+ channel inactivation. Ventricular RMP was unaffected by LPS injection (+LPS: −75.1±1.1mV, n=44; +NaCl: −76.3±0.9mV, n=55, p=0.83). Therefore the Na+ currents were measured in isolated ventricular cardiomyocytes using whole cell patch clamp revealing the maximum Na+ current density lowered after LPS treatment (+LPS: −20.6±1.7 pA/pF, n=16, +NaCl 27.1±2.6 pA/pF, n=10, p=0.03). LPS did not affect EPI, CV, upstroke velocity or current density in TLR4−/− mice.

Conclusion

Herein we report for the first time impaired cardiac depolarisation and conduction after short term activation of TLR4 in vivo. Pro arrhythmogenic mechanisms differ in atria and ventricle: Increased atrial RMP inactivates Na+ current leading to reduction of CV. Ventricular slow CV is caused by reduced current density of Na+ channels. This different TLR4 effect might be important for novel antiarrhythmic and antiinflammatory applications.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): GEROK-grant, University Bonn

Combined fibre atrophy and decreased muscle regeneration capacity driven by mitochondrial DNA alterations underlie the development of sarcopenia

BACKGROUND

Mitochondrial dysfunction caused by mitochondrial (mtDNA) deletions have been associated with skeletal muscle atrophy and myofibre loss. However, whether such defects occurring in myofibres cause sarcopenia is unclear. Also, the contribution of mtDNA alterations in muscle stem cells (MuSCs) to sarcopenia remains to be investigated.

METHODS

We expressed a dominant-negative variant of the mitochondrial helicase, which induces mtDNA alterations, specifically in differentiated myofibres (K320E^skm mice) and MuSCs (K320E^msc mice), respectively, and investigated their impact on muscle structure and function by immunohistochemistry, analysis of mtDNA and respiratory chain content, muscle transcriptome and functional tests.

RESULTS

K320E^skm mice at 24 months of age had higher levels of mtDNA deletions compared with controls in soleus (SOL, 0.07673% vs. 0.00015%, P = 0.0167), extensor digitorum longus (EDL, 0.0649 vs. 0.000925, P = 0.0015) and gastrocnemius (GAS, 0.09353 vs. 0.000425, P = 0.0004). K320E^skm mice revealed a progressive increase in the proportion of cytochrome c oxidase deficient (COX^- ) fibres in skeletal muscle cross sections, reaching a maximum of 3.03%, 4.36%, 13.58%, and 17.08% in EDL, SOL, tibialis anterior (TA) and GAS, respectively. However, mice did not show accelerated loss of muscle mass, muscle strength or physical performance. Histological analyses revealed ragged red fibres but also stimulated regeneration, indicating activation of MuSCs. RNAseq demonstrated enhanced expression of genes associated with protein synthesis, but also degradation, as well as muscle fibre differentiation and cell proliferation. In contrast, 7 days after destruction by cardiotoxin, regenerating TA of K320E^msc mice showed 30% of COX^- fibres. Notably, regenerated muscle showed dystrophic changes, increased fibrosis (2.5% vs. 1.6%, P = 0.0003), increased abundance of fat cells (2.76% vs. 0.23%, P = 0.0144) and reduced muscle mass (regenerated TA: 40.0 mg vs. 60.2 mg, P = 0.0171). In contrast to muscles from K320E^skm mice, freshly isolated MuSCs from aged K320E^msc mice were completely devoid of mtDNA alterations. However, after passaging, mtDNA copy number as well as respiratory chain subunits and p62 levels gradually decreased.

CONCLUSIONS

Taken together, accumulation of large-scale mtDNA alterations in myofibres alone is not sufficient to cause sarcopenia. Expression of K320E-Twinkle is tolerated in quiescent MuSCs, but progressively leads to mtDNA and respiratory chain depletion upon activation, in vivo and in vitro, possibly caused by an increased mitochondrial removal. Altogether, our results suggest that the accumulation of mtDNA alterations in myofibres activates regeneration during aging, which leads to sarcopenia if such alterations have expanded in MuSCs as well.

Holographic optogenetic stimulation with calcium imaging as an all optical tool for cardiac electrophysiology

All optical approaches to control and read out the electrical activity in a cardiac syncytium can improve our understanding of cardiac electrophysiology. Here, we demonstrate optogenetic stimulation of cardiomyocytes with high spatial precision using light foci generated with a ferroelectric spatial light modulator. Computer generated holograms binarized by bidirectional error diffusion create multiple foci with more even intensity distribution compared with thresholding approach. We evoke the electrical activity of cardiac HL1 cells expressing the channelrhodopsin-2 variant, ChR2(H134R) using single and multiple light foci and at the same time visualize the action potential using a calcium sensitive indicator called Cal-630. We show that localized regions in the cardiac monolayer can be stimulated enabling us to initiate signal propagation from a precise location. Furthermore, we demonstrate that probing the cardiac cells with multiple light foci enhances the excitability of the cardiac network. This approach opens new applications in manipulating and visualizing the electrical activity in a cardiac syncytium.

Selective optogenetic control of Gq signaling using human Neuropsin

G_q proteins are universally important for signal transduction in mammalian cells. The underlying kinetics and transformation from extracellular stimuli into intracellular signaling, however could not be investigated in detail so far. Here we present the human Neuropsin (hOPN5) for specific and repetitive manipulation of G_q signaling in vitro and in vivo with high spatio-temporal resolution. Properties and G protein specificity of hOPN5 are characterized by UV light induced IP₃ generation, Ca²⁺ transients and inhibition of G_IRK channel activity in HEK cells. In adult hearts from a transgenic animal model, light increases the spontaneous beating rate. In addition, we demonstrate light induced contractions in the small intestine, which are not detectable after pharmacological G_q protein block. All-optical high-throughput screening for TRPC6 inhibitors is more specific and sensitive than conventional pharmacological screening. Thus, we demonstrate specific G_q signaling of hOPN5 and unveil its potential for optogenetic applications.

Optogenetic Stimulation of Gi Signaling Enables Instantaneous Modulation of Cardiomyocyte Pacemaking

G-protein signaling pathways are central in the regulation of cardiac function in physiological and pathophysiological conditions. Their functional analysis through optogenetic techniques with selective expression of opsin proteins and activation by specific wavelengths allows high spatial and temporal precision. Here, we present the application of long wavelength-sensitive cone opsin (LWO) in cardiomyocytes for activation of the G_i signaling pathway by red light. Murine embryonic stem (ES) cells expressing LWO were generated and differentiated into beating cardiomyocytes in embryoid bodies (EBs). Illumination with red light (625 nm) led to an instantaneous decrease up to complete inhibition (84–99% effectivity) of spontaneous beating, but had no effect on control EBs. By using increasing light intensities with 10 s pulses, we determined a half maximal effective light intensity of 2.4 μW/mm² and a maximum effect at 100 μW/mm². Pre-incubation of LWO EBs with pertussis toxin completely inhibited the light effect proving the specificity for G_i signaling. Frequency reduction was mainly due to the activation of GIRK channels because the specific channel blocker tertiapin reduced the light effect by ~80%. Compared with pharmacological stimulation of M₂ receptors with carbachol with slow kinetics (>30 s), illumination of LWO had an identical efficacy, but much faster kinetics (<1 s) in the activation and deactivation demonstrating the temporal advantage of optogenetic stimulation. Thus, LWO is an effective optogenetic tool for selective stimulation of the G_i signaling cascade in cardiomyocytes with red light, providing high temporal precision.

Immobilization of Recombinant Fluorescent Biosensors Permits Imaging of Extracellular Ion Signals

Given the importance of ion gradients and fluxes in biology, monitoring ions locally at the exterior of the plasma membrane of intact cells in a noninvasive manner is highly desirable but challenging. Classical targeting of genetically encoded biosensors at the exterior of cell surfaces would be a suitable approach; however, it often leads to intracellular accumulation of the tools in vesicular structures and adverse modifications, possibly impairing sensor functionality. To tackle these issues, we generated recombinant fluorescent ion biosensors fused to traptavidin (TAv) specifically coupled to a biotinylated AviTag expressed on the outer cell surface of cells. We show that purified chimeras of TAv and pH-Lemon or GEPII 1.0, Förster resonance energy transfer-based pH and K⁺ biosensors, can be immobilized directly and specifically on biotinylated surfaces including glass platelets and intact cells, thereby remaining fully functional for imaging of ion dynamics. The immobilization of recombinant TAv–GEPII 1.0 on the extracellular cell surface of primary cortical rat neurons allowed imaging of excitotoxic glutamate-induced K⁺ efflux in vitro. We also performed micropatterning of purified TAv biosensors using a microperfusion system to generate spatially separated TAv–pH-Lemon and TAv–GEPII 1.0 spots for simultaneous pH and K⁺ measurements on cell surfaces. Our results suggest that the approach can be greatly expanded by immobilizing various biosensors on extracellular surfaces to quantitatively visualize microenvironmental transport and signaling processes in different cell culture models and other experimental settings.

Novel Optics-Based Approaches for Cardiac Electrophysiology: A Review

Optical techniques for recording and manipulating cellular electrophysiology have advanced rapidly in just a few decades. These developments allow for the analysis of cardiac cellular dynamics at multiple scales while largely overcoming the drawbacks associated with the use of electrodes. The recent advent of optogenetics opens up new possibilities for regional and tissue-level electrophysiological control and hold promise for future novel clinical applications. This article, which emerged from the international NOTICE workshop in 2018, reviews the state-of-the-art optical techniques used for cardiac electrophysiological research and the underlying biophysics. The design and performance of optical reporters and optogenetic actuators are reviewed along with limitations of current probes. The physics of light interaction with cardiac tissue is detailed and associated challenges with the use of optical sensors and actuators are presented. Case studies include the use of fluorescence recovery after photobleaching and super-resolution microscopy to explore the micro-structure of cardiac cells and a review of two photon and light sheet technologies applied to cardiac tissue. The emergence of cardiac optogenetics is reviewed and the current work exploring the potential clinical use of optogenetics is also described. Approaches which combine optogenetic manipulation and optical voltage measurement are discussed, in terms of platforms that allow real-time manipulation of whole heart electrophysiology in open and closed-loop systems to study optimal ways to terminate spiral arrhythmias. The design and operation of optics-based approaches that allow high-throughput cardiac electrophysiological assays is presented. Finally, emerging techniques of photo-acoustic imaging and stress sensors are described along with strategies for future development and establishment of these techniques in mainstream electrophysiological research.

Selective optogenetic stimulation of fibroblasts enables quantification of hetero-cellular coupling to cardiomyocytes in a three-dimensional model of heart tissue

AIMS

Besides providing mechanical stability, fibroblasts in the heart could modulate the electrical properties of cardiomyocytes. Here, we aim to develop a three-dimensional hetero-cellular model to analyse the electric interaction between fibroblasts and human cardiomyocytes in vitro using selective optogenetic de- or hyperpolarization of fibroblasts.

METHODS AND RESULTS

NIH3T3 cell lines expressing the light-sensitive ion channel Channelrhodopsin2 or the light-induced proton pump Archaerhodopsin were generated for optogenetic depolarization or hyperpolarization, respectively, and characterized by patch clamp. Cardiac bodies consisting of 50% fibroblasts and 50% human pluripotent stem cell-derived cardiomyocytes were analysed by video microscopy and membrane potential was measured with sharp electrodes. Myofibroblast activation in cardiac bodies was enhanced by transforming growth factor-β1 (TGF-β1)-stimulation. Connexin-43 expression was analysed by qPCR and fluorescence recovery after photobleaching. Illumination of Channelrhodopsin2 or Archaerhodopsin expressing fibroblasts induced inward currents and depolarization or outward currents and hyperpolarization. Transforming growth factor-β1-stimulation elevated connexin-43 expression and increased cell-cell coupling between fibroblasts as well as increased basal beating frequency and cardiomyocyte resting membrane potential in cardiac bodies. Illumination of cardiac bodies generated with Channelrhodopsin2 fibroblasts accelerated spontaneous beating, especially after TGF-β1-stimulation. Illumination of cardiac bodies prepared with Archaerhodopsin expressing fibroblasts led to hyperpolarization of cardiomyocytes and complete block of spontaneous beating after TGF-β1-stimulation. Effects of light were significantly smaller without TGF-β1-stimulation.

CONCLUSION

Transforming growth factor-β1-stimulation leads to increased hetero-cellular coupling and optogenetic hyperpolarization of fibroblasts reduces TGF-β1 induced effects on cardiomyocyte spontaneous activity. Optogenetic membrane potential manipulation selectively in fibroblasts in a new hetero-cellular cardiac body model allows direct quantification of fibroblast-cardiomyocyte coupling in vitro.

OPN5 a new optogenetic receptor to study Gq signalling in the heart with light

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Deutsche Forschungsgemeinschaft (DFG) Deutsche Zentrum für Herz-Kreislauf-Forschung (DZHK)

Background

Gq signalling plays an imperative role in cardiac physiology as well as pathophysiologies such as arrhythmias, hypertrophy, and heart failure. However, the underlying kinetics and the transition from physiological to pathological signalling is yet to be investigated. Optogenetics provides an unprecedented spatial and temporal precision as well as cell specificity, but up to now, selective control of Gq signalling in the intact heart with light has not been achieved.

Aim

To prove the potential of human Neuropsin (OPN5) to selectively control Gq signalling in the adult heart.

Methods

The biophysical behaviour of OPN5 in HEK293 cells was characterized by measuring IP1 accumulation, Ca2+ imaging, and patch-clamp analysis of GIRK current activity. We generated a new transgenic mouse model expressing OPN5 in fusion with eYFP under the control of the chicken-β-actin promotor and analysed the expression pattern in the whole heart after tissue clearing with light sheet microscopy and the expression rate after single cell dissociation. Adult isolated cardiomyocytes were examined to measure the effect of OPN5 activation on contractility with a novel custom-made software for on-line contraction analysis by pixel tracking. In Langendorff-perfused hearts, illumination of the sinus node region with UV light (385 nm, 10 s, 1 mW/mm2) was used to measure the influence on the heart rate in electrogram recordings. The light-induced effects and basal ECG parameters as well as the heart-weight-to-femur-length ratio of OPN5 hearts were compared to hearts from non-expressing siblings as well as from CD1 wild-type.

Results

We prove the selective activation of the Gq proteins by UV light-induced increase in IP3 production and induction of Ca2+ transients in HEK293 cells because both effects were abolished after applying the Gq protein specific blocker FR900359. Promiscuous activation of Gi proteins was excluded by light-induced inhibition of GIRK channels instead of activation. Within two different mouse lines, we found an OPN5/eYFP expression rate of ∼70% within the cardiomyocytes targeted to the plasmamembrane. UV light induced a significant increase in contraction velocity in ventricular cardiomyocytes, and this effect could be abolished by blocking Gq proteins. In the right atrium HCN4 positive sinus nodal cells expressed OPN5/ eYFP and illumination of this region resulted in OPN5 expressing hearts in an average increase of 5.9 ± 0.97% (n= 20, in founder line 1) in the heart rate, which was significant lager than control hearts (all groups &lt;2.4% increase, n≥17). Importantly we could not find any differences in basal ECG parameters or the heart weight-to-femur-length between OPN5 expressing and control. 

Conclusion

We herein prove the specificity of OPN5 for Gq protein activation and demonstrate its use in cardiomyocytes as well as the whole heart. Thus OPN5 is the first optogenetic tool allowing to control Gq signalling in the whole heart precisely.

Carnosine synthase deficiency is compatible with normal skeletal muscle and olfactory function but causes reduced olfactory sensitivity in aging mice

Carnosine (β-alanyl-l-histidine) and anserine (β-alanyl-3-methyl-l-histidine) are abundant peptides in the nervous system and skeletal muscle of many vertebrates. Many in vitro and in vivo studies demonstrated that exogenously added carnosine can improve muscle contraction, has antioxidant activity, and can quench various reactive aldehydes. Some of these functions likely contribute to the proposed anti-aging activity of carnosine. However, the physiological role of carnosine and related histidine-containing dipeptides (HCDs) is not clear. In this study, we generated a mouse line deficient in carnosine synthase (Carns1). HCDs were undetectable in the primary olfactory system and skeletal muscle of Carns1-deficient mice. Skeletal muscle contraction in these mice, however, was unaltered, and there was no evidence for reduced pH-buffering capacity in the skeletal muscle. Olfactory tests did not reveal any deterioration in 8-month-old mice lacking carnosine. In contrast, aging (18-24-month-old) Carns1-deficient mice exhibited olfactory sensitivity impairments that correlated with an age-dependent reduction in the number of olfactory receptor neurons. Whereas we found no evidence for elevated levels of lipoxidation and glycation end products in the primary olfactory system, protein carbonylation was increased in the olfactory bulb of aged Carns1-deficient mice. Taken together, these results suggest that carnosine in the olfactory system is not essential for information processing in the olfactory signaling pathway but does have a role in the long-term protection of olfactory receptor neurons, possibly through its antioxidant activity.

Light-cell interactions in depth-resolved optogenetics

Light as a tool in medical therapy and biological research has been studied extensively and its application is subject to continuous improvement. However, safe and efficient application of light-based methods in photomedicine or optogenetics requires knowledge about the optical properties of the target tissue as well as the response characteristics of the stimulated cells. Here, we used tissue phantoms and a heart-like light-sensitive cell line to investigate optogenetic stimulation through tissue layers. The input power necessary for successful stimulation could be described as a function of phantom thickness. A model of light transmission through the tissue phantoms gives insights into the expected stimulation efficiency. Cell-type specific effects are identified that result in deviations of the stimulation threshold from the modelled predictions. This study provides insights into the complex interplay between light, tissue and cells during deep-tissue optogenetics. It can serve as an orientation for safe implementation of light-based methods in vivo.

Optogenetic Termination of Cardiac Arrhythmia: Mechanistic Enlightenment and Therapeutic Application?

Optogenetic methods enable selective de- and hyperpolarization of cardiomyocytes expressing light-sensitive proteins within the myocardium. By using light, this technology provides very high spatial and temporal precision, which is in clear contrast to electrical stimulation. In addition, cardiomyocyte-specific expression would allow pain-free stimulation. In light of these intrinsic technical advantages, optogenetic methods provide an intriguing opportunity to understand and improve current strategies to terminate cardiac arrhythmia as well as for possible pain-free arrhythmia termination in patients in the future. In this review, we give a concise introduction to optogenetic stimulation of cardiomyocytes and the whole heart and summarize the recent progress on optogenetic defibrillation and cardioversion to terminate cardiac arrhythmia. Toward this aim, we specifically focus on the different mechanisms of optogenetic arrhythmia termination and how these might influence the prerequisites for success. Furthermore, we critically discuss the clinical perspectives and potential patient populations, which might benefit from optogenetic defibrillation devices.

Optogenetic Hyperpolarization of Cardiomyocytes Terminates Ventricular Arrhythmia

Cardiac defibrillation to terminate lethal ventricular arrhythmia (VA) is currently performed by applying high energy electrical shocks. In cardiac tissue, electrical shocks induce simultaneously de- and hyperpolarized areas and only depolarized areas are considered to be responsible for VA termination. Because electrical shocks do not allow proper control over spatial extent and level of membrane potential changes, the effects of hyperpolarization have not been explored in the intact heart. In contrast, optogenetic methods allow cell type-selective induction of de- and hyperpolarization with unprecedented temporal and spatial control. To investigate effects of cardiomyocyte hyperpolarization on VA termination, we generated a mouse line with cardiomyocyte-specific expression of the light-driven proton pump ArchT. Isolated cardiomyocytes showed light-induced outward currents and hyperpolarization. Free-running VA were evoked by electrical stimulation of explanted hearts perfused with low K⁺ and the K_ATP channel opener Pinacidil. Optogenetic hyperpolarization was induced by epicardial illumination, which terminated VA with an average efficacy of ∼55%. This value was significantly higher compared to control hearts without illumination or ArchT expression (p = 0.0007). Intracellular recordings with sharp electrodes within the intact heart revealed hyperpolarization and faster action potential upstroke upon illumination, which should fasten conduction. However, conduction speed was lower during illumination suggesting enhanced electrical sink by hyperpolarization underlying VA termination. Thus, selective hyperpolarization in cardiomyocytes is able to terminate VA with a completely new mechanism of increased electrical sink. These novel insights could improve our mechanistic understanding and treatment strategies of VA termination.

Optogenetic stimulation of Gs-signaling in the heart with high spatio-temporal precision

The standard technique for investigating adrenergic effects on heart function is perfusion with pharmaceutical agonists, which does not provide high temporal or spatial precision. Herein we demonstrate that the light sensitive G_s-protein coupled receptor JellyOp enables optogenetic stimulation of G_s-signaling in cardiomyocytes and the whole heart. Illumination of transgenic embryonic stem cell-derived cardiomyocytes or of the right atrium of mice expressing JellyOp elevates cAMP levels and instantaneously accelerates spontaneous beating rates similar to pharmacological β-adrenergic stimulation. Light application to the dorsal left atrium instead leads to supraventricular extrabeats, indicating adverse effects of localized G_s-signaling. In isolated ventricular cardiomyocytes from JellyOp mice, we find increased Ca²⁺ currents, fractional cell shortening and relaxation rates after illumination enabling the analysis of differential G_s-signaling with high temporal precision. Thus, JellyOp expression allows localized and time-restricted G_s stimulation and will provide mechanistic insights into different effects of site-specific, long-lasting and pulsatile G_s activation.

Studying adrenergic signalling in the heart requires perfusion with receptor agonists, which lacks cell specificity and spatiotemporal control. Here the authors use the light sensitive G-coupled receptor JellyOp to optogenetically control Gs-signaling in cardiomyocytes and intact hearts with high spatiotemporal precision.

Optical control of L-type Ca2+ channels using a diltiazem photoswitch

L-type Ca²⁺ channels (LTCCs) play a crucial role in excitation-contraction coupling and release of hormones from secretory cells. They are targets of antihypertensive and antiarrhythmic drugs such as diltiazem. Here, we present a photoswitchable diltiazem, FHU-779, which can be used to reversibly block endogenous LTCCs by light. FHU-779 is as potent as diltiazem and can be used to place pancreatic β-cell function and cardiac activity under optical control.

Synchronization of excitable cardiac cultures of different origin

In the present work, we investigated the synchronization of electrical activity in cultured cardiac cells of different origin put in direct contact. In the first set of experiments synchronization was studied in the primary culture cells of neonatal rats taken at different developmental ages, and in the second - in the neonatal rat cardiomyocytes and HL-1 cells. The electrical excitation of cells was recorded using the calcium transient marker Fluor-4. In the confluent cell layers created with the aid of a specially devised mask, the excitation waves and their propagation between areas occupied by cells of different origin were observed. On the level of individual cells, their contact and synchronization was monitored with the aid of scanning fluorescence microscopy. It was found that populations of cultured cells of different origin are able to synchronize, suggesting the formation of electrical coupling between them. The results obtained may be considered as a proof of concept that implanted alien grafted cells are able to create electrical coupling with the host cardiac tissue.

Optogenetic termination of atrial fibrillation in mice

Aims

The primary goal in the treatment of symptomatic atrial fibrillation/flutter (AF) is to restore sinus rhythm by cardioversion. Electrical shocks are highly effective, but have to be applied under analgo-sedation and can further harm the heart. In order to develop a novel pain-free and less harmful approach, we explored herein the optogenetic cardioversion by light-induced depolarization.

Methods and results

Hearts from mice expressing Channelrhodopsin-2 (ChR2) and the AF-promoting loss-of-function Connexin 40 Ala96Ser mutation were explanted and perfused with low K+ Tyrode's solution and an atrial KATP-channel activator. This new protocol shortened atrial refractoriness as well as slowed atrial conduction and thereby enabled the induction of sustained AF. AF episodes could be terminated by epicardial illumination of the atria with focussed blue light (470 nm, 0.4 mW/mm2) with an efficacy of ∼97% (n = 17 hearts). In > 80% of cases, light directly terminated the AF episode with onset of illumination. Because similar illumination intensity was able to locally inhibit atrial activity, we propose that a light-induced block of electrical activity is responsible for reliable AF termination. The success rate was strongly depending on the illuminated area, applied light intensity and duration of illumination. Importantly, we were also able to demonstrate optogenetic termination of AF in vivo, using epicardial illumination through the open chest (n = 3 hearts). To point towards a translational potential, we systemically injected an adeno-associated virus to express ChR2 in wild type hearts. After 6-8 months, we found robust ChR2 expression in the atria, enabling light-mediated AF termination in six of seven mice tested.

Conclusion

We provide the first evidence for optogenetic termination of atrial tachyarrhythmia in intact hearts from transgenic as well as wild type mice ex and in vivo. Thus, this report could lay the foundation for the development of implantable devices for pain-free termination of AF.

Frequency-Dependent Multi-Well Cardiotoxicity Screening Enabled by Optogenetic Stimulation

Side effects on cardiac ion channels causing lethal arrhythmias are one major reason for drug withdrawals from the market. Field potential (FP) recording from cardiomyocytes, is a well-suited tool to assess such cardiotoxic effects of drug candidates in preclinical drug development, but it is currently limited to the spontaneous beating of the cardiomyocytes and manual analysis. Herein, we present a novel optogenetic cardiotoxicity screening system suited for the parallel automated frequency-dependent analysis of drug effects on FP recorded from human-induced pluripotent stem cell-derived cardiomyocytes. For the expression of the light-sensitive cation channel Channelrhodopsin-2, we optimised protocols using virus transduction or transient mRNA transfection. Optical stimulation was performed with a new light-emitting diode lid for a 96-well FP recording system. This enabled reliable pacing at physiologically relevant heart rates and robust recording of FP. Thereby we detected rate-dependent effects of drugs on Na⁺, Ca2+ and K⁺ channel function indicated by FP prolongation, FP shortening and the slowing of the FP downstroke component, as well as generation of afterdepolarisations. Taken together, we present a scalable approach for preclinical frequency-dependent screening of drug effects on cardiac electrophysiology. Importantly, we show that the recording and analysis can be fully automated and the technology is readily available using commercial products.

Improved heart repair upon myocardial infarction: Combination of magnetic nanoparticles and tailored magnets strongly increases engraftment of myocytes

Cell replacement in the heart is considered a promising strategy for the treatment of post-infarct heart failure. Direct intramyocardial injection of cells proved to be the most effective application route, however, engraftment rates are very low (<5%) strongly hampering its efficacy. Herein we combine magnetic nanoparticle (MNP) loading of EGFP labeled embryonic cardiomyocytes (eCM) and embryonic stem cell-derived cardiomyocytes (ES-CM) with application of custom designed magnets to enhance their short and long-term engraftment. To optimize cellular MNP uptake and magnetic force within the infarct area, first numerical simulations and experiments were performed in vitro. All tested cell types could be loaded efficiently with SOMag5-MNP (200 pg/cell) without toxic side effects. Application of a 1.3 T magnet at 5 mm distance from the heart for 10 min enhanced engraftment of both eCM and ES-CM by approximately 7 fold at 2 weeks and 3.4 fold (eCM) at 8 weeks after treatment respectively and also strongly improved left ventricular function at all time points. As underlying mechanisms we found that application of the magnetic field prevented the initial dramatic loss of cells via the injection channel. In addition, grafted eCM displayed higher proliferation and lower apoptosis rates. Electron microscopy revealed better differentiation of engrafted eCM, formation of cell to cell contacts and more physiological matrix formation in magnet-treated grafts. These results were corroborated by gene expression data. Thus, combination of MNP-loaded cells and magnet-application strongly increases long-term engraftment of cells addressing a major shortcoming of cardiomyoplasty.

Spot light on skeletal muscles: optogenetic stimulation to understand and restore skeletal muscle function

Damage of peripheral nerves results in paralysis of skeletal muscle. Currently, the only treatment option to restore proper function is electrical stimulation of the innervating nerve or of the skeletal muscles directly. However this approach has low spatial and temporal precision leading to co-activation of antagonistic muscles and lacks cell-type selectivity resulting in pain or discomfort by stimulation of sensible nerves. In contrast to electrical stimulation, optogenetic methods enable spatially confined and cell-type selective stimulation of cells expressing the light sensitive channel Channelrhodopsin-2 with precise temporal control over the membrane potential. Herein we summarize the current knowledge about the use of this technology to control skeletal muscle function with the focus on the direct, non-neuronal stimulation of muscle fibers. The high temporal flexibility of using light pulses allows new stimulation patterns to investigate skeletal muscle physiology. Furthermore, the high spatial precision of focused illumination was shown to be beneficial for selective stimulation of distinct nearby muscle groups. Finally, the cell-type specific expression of the light-sensitive effector proteins in muscle fibers will allow pain-free stimulation and open new options for clinical treatments. Therefore, we believe that direct optogenetic stimulation of skeletal muscles is a very potent method for basic scientists that also harbors several distinct advantages over electrical stimulation to be considered for clinical use in the future.

EBIO Does Not Induce Cardiomyogenesis in Human Pluripotent Stem Cells but Modulates Cardiac Subtype Enrichment by Lineage-Selective Survival

Subtype-specific human cardiomyocytes (CMs) are valuable for basic and applied research. Induction of cardiomyogenesis and enrichment of nodal-like CMs was described for mouse pluripotent stem cells (mPSCs) in response to 1-ethyl-2-benzimidazolinone (EBIO), a chemical modulator of small-/intermediate-conductance Ca²⁺-activated potassium channels (SKs 1-4). Investigating EBIO in human pluripotent stem cells (PSCs), we have applied three independent differentiation protocols of low to high cardiomyogenic efficiency. Equivalent to mPSCs, timed EBIO supplementation during hPSC differentiation resulted in dose-dependent enrichment of up to 80% CMs, including an increase in nodal- and atrial-like phenotypes. However, our study revealed extensive EBIO-triggered cell loss favoring cardiac progenitor preservation and, subsequently, CMs with shortened action potentials. Proliferative cells were generally more sensitive to EBIO, presumably via an SK-independent mechanism. Together, EBIO did not promote cardiogenic differentiation of PSCs, opposing previous findings, but triggered lineage-selective survival at a cardiac progenitor stage, which we propose as a pharmacological strategy to modulate CM subtype composition.

Engineered Context-Sensitive Agonism: Tissue-Selective Drug Signaling through a G Protein-Coupled Receptor

Drug discovery strives for selective ligands to achieve targeted modulation of tissue function. Here we introduce engineered context-sensitive agonism as a postreceptor mechanism for tissue-selective drug action through a G protein-coupled receptor. Acetylcholine M₂-receptor activation is known to mediate, among other actions, potentially dangerous slowing of the heart rate. This unwanted side effect is one of the main reasons that limit clinical application of muscarinic agonists. Herein we show that dualsteric (orthosteric/allosteric) agonists induce less cardiac depression ex vivo and in vivo than conventional full agonists. Exploration of the underlying mechanism in living cells employing cellular dynamic mass redistribution identified context-sensitive agonism of these dualsteric agonists. They translate elevation of intracellular cAMP into a switch from full to partial agonism. Designed context-sensitive agonism opens an avenue toward postreceptor pharmacologic selectivity, which even works in target tissues operated by the same subtype of pharmacologic receptor.

Optogenetic defibrillation terminates ventricular arrhythmia in mouse hearts and human simulations

Ventricular arrhythmias are among the most severe complications of heart disease and can result in sudden cardiac death. Patients at risk currently receive implantable defibrillators that deliver electrical shocks to terminate arrhythmias on demand. However, strong electrical shocks can damage the heart and cause severe pain. Therefore, we have tested optogenetic defibrillation using expression of the light-sensitive channel channelrhodopsin-2 (ChR2) in cardiac tissue. Epicardial illumination effectively terminated ventricular arrhythmias in hearts from transgenic mice and from WT mice after adeno-associated virus-based gene transfer of ChR2. We also explored optogenetic defibrillation for human hearts, taking advantage of a recently developed, clinically validated in silico approach for simulating infarct-related ventricular tachycardia (VT). Our analysis revealed that illumination with red light effectively terminates VT in diseased, ChR2-expressing human hearts. Mechanistically, we determined that the observed VT termination is due to ChR2-mediated transmural depolarization of the myocardium, which causes a block of voltage-dependent Na+ channels throughout the myocardial wall and interrupts wavefront propagation into illuminated tissue. Thus, our results demonstrate that optogenetic defibrillation is highly effective in the mouse heart and could potentially be translated into humans to achieve nondamaging and pain-free termination of ventricular arrhythmia.

Robust Generation of Cardiomyocytes from Human iPS Cells Requires Precise Modulation of BMP and WNT Signaling

Various strategies have been published enabling cardiomyocyte differentiation of human induced pluripotent stem (iPS) cells. However the complex nature of signaling pathways involved as well as line-to-line variability compromises the application of a particular protocol to robustly obtain cardiomyocytes from multiple iPS lines. Hence it is necessary to identify optimized protocols with alternative combinations of specific growth factors and small molecules to enhance the robustness of cardiac differentiation. Here we focus on systematic modulation of BMP and WNT signaling to enhance cardiac differentiation. Moreover, we improve the efficacy of cardiac differentiation by enrichment via lactate. Using our protocol we show efficient derivation of cardiomyocytes from multiple human iPS lines. In particular we demonstrate cardiomyocyte differentiation within 15 days with an efficiency of up to 95 % as judged by flow cytometry staining against cardiac troponin T. Cardiomyocytes derived were functionally validated by alpha-actinin staining, transmission electron microscopy as well as electrophysiological analysis. We expect our protocol to provide a robust basis for scale-up production of functional iPS cell-derived cardiomyocytes that can be used for cell replacement therapy and disease modeling.

Universal cardiac induction of human pluripotent stem cells in two and three-dimensional formats: implications for in vitro maturation

Directed cardiac differentiation of human pluripotent stem cells (hPSCs) enables disease modeling, investigation of human cardiogenesis, as well as large-scale production of cardiomyocytes (CMs) for translational purposes. Multiple CM differentiation protocols have been developed to individually address specific requirements of these diverse applications, such as enhanced purity at a small scale or mass production at a larger scale. However, there is no universal high-efficiency procedure for generating CMs both in two-dimensional (2D) and three-dimensional (3D) culture formats, and undefined or complex media additives compromise functional analysis or cost-efficient upscaling. Using systematic combinatorial optimization, we have narrowed down the key requirements for efficient cardiac induction of hPSCs. This implied differentiation in simple serum and serum albumin-free basal media, mediated by a minimal set of signaling pathway manipulations at moderate factor concentrations. The method was applicable both to 2D and 3D culture formats as well as to independent hPSC lines. Global time-course gene expression analyses over extended time periods and in comparison with human heart tissue were used to monitor culture-induced maturation of the resulting CMs. This suggested that hPSC-CMs obtained with our procedure reach a rather stable transcriptomic state after approximately 4 weeks of culture. The underlying gene expression changes correlated well with a decline of immature characteristics as well as with a gain of structural and physiological maturation features within this time frame. These data link gene expression patterns of hPSC-CMs to functional readouts and thus define the cornerstones of culture-induced maturation.

Optogenetic control of contractile function in skeletal muscle

Optogenetic stimulation allows activation of cells with high spatial and temporal precision. Here we show direct optogenetic stimulation of skeletal muscle from transgenic mice expressing the light-sensitive channel Channelrhodopsin-2 (ChR2). Largest tetanic contractions are observed with 5-ms light pulses at 30 Hz, resulting in 84% of the maximal force induced by electrical stimulation. We demonstrate the utility of this approach by selectively stimulating with a light guide individual intralaryngeal muscles in explanted larynges from ChR2-transgenic mice, which enables selective opening and closing of the vocal cords. Furthermore, systemic injection of adeno-associated virus into wild-type mice provides sufficient ChR2 expression for optogenetic opening of the vocal cords. Thus, direct optogenetic stimulation of skeletal muscle generates large force and provides the distinct advantage of localized and cell-type-specific activation. This technology could be useful for therapeutic purposes, such as restoring the mobility of the vocal cords in patients suffering from laryngeal paralysis.

Human Connexin43E42K Mutation From a Sudden Infant Death Victim Leads to Impaired Ventricular Activation and Neonatal Death in Mice

Background—

Sudden infant death syndrome (SIDS) describes the sudden, unexplained death of a baby during its first year of age and is the third leading cause of infant mortality. It is assumed that ≤20% of all SIDS cases are because of cardiac arrhythmias resulting from mutations in ion channel proteins. Besides ion channels also cardiac gap junction channels are important for proper conduction of cardiac electric activation. In the mammalian heart Connexin43 (Cx43) is the major gap junction protein expressed in ventricular cardiomyocytes. Recently, a novel Connexin43 loss-of-function mutation (Cx43E42K) was identified in a 2-month-old SIDS victim.

Methods and Results—

We have generated Cx43E42K-expressing mice as a model for SIDS. Heterozygous cardiac-restricted Cx43E42K-mutated mice die neonatally without major cardiac morphological defects. Electrocardiographic recordings of embryonic Cx43+/E42K mice reveal severely disturbed ventricular activation, whereas immunohistochemical analyses show normal localization and expression patterns of gap junctional Connexin43 protein in the Cx43E42K-mutated newborn mouse heart.

Conclusions—

Because we did not find heterogeneous gap junction loss in Cx43E42K mouse hearts, we conclude that the Cx43E42K gap junction channel creates an arrhythmogenic substrate leading to lethal ventricular arrhythmias. The strong cardiac phenotype of Cx43E42K expressing mice supports the association between the human Cx43E42K mutation and SIDS and indicates that Connexin43 mutations should be considered in future studies when SIDS cases are to be molecularly explained.

Systemic gene transfer enables optogenetic pacing of mouse hearts

AIMS

Optogenetic pacing of the heart has been demonstrated in transgenic animals expressing channelrhodopsin-2 (ChR2). However, for the clinical use of optogenetics to treat cardiac arrhythmias, gene transfer to non-transgenic hearts is required. The aim of this study was to describe a reliable method for gene transfer of ChR2 into a sufficient percentage of cardiomyocytes to overcome the electrical sink of all the coupled non-expressing cardiomyocytes during optical pacing of the whole heart in vivo.

METHODS AND RESULTS

Adeno-associated virus (AAV) with cardiac tropism for expression of ChR2 in fusion with mCherry was systemically injected into wild-type mouse hearts. Bright mCherry fluorescence was detected in the whole heart 4-10 weeks later. Single-cell dissociation revealed that on average 58% cardiomyocytes were mCherry-positive. These showed light-induced inward currents, action potentials, and contractions. Pulsed illumination of the left ventricle induced ventricular pacing in vivo in 74% of mice, and higher light intensities were required for reduced pulse duration or size of illumination. Non-responding hearts showed low AAV expression, and the threshold for optical pacing was estimated to be 35-40% ChR2-expressing cardiomyocytes. Optical pacing in vivo was stable over extended periods without negative effects on normal sinus rhythm and ECG parameters after termination of stimulation indicating sufficient cardiac output during pacing.

CONCLUSIONS

Gene transfer generates sufficient ChR2 photocurrent for reliable optogenetic pacing in vivo and lays out the basis for future optogenetic pacemaker and pain-free defibrillation therapies.

MiRNA-1/133a clusters regulate adrenergic control of cardiac repolarization

The electrical properties of the heart are primarily determined by the activity of ion channels and the activity of these molecules is permanently modulated and adjusted to the physiological needs by adrenergic signaling. miRNAs are known to control the expression of many proteins and to fulfill distinct functions in the mammalian heart, though the in vivo effects of miRNAs on the electrical activity of the heart are poorly characterized. The miRNAs miR-1 and miR-133a are the most abundant miRNAs of the heart and are expressed from two miR-1/133a genomic clusters. Genetic modulation of miR-1/133a cluster expression without concomitant severe disturbance of general cardiomyocyte physiology revealed that these miRNA clusters govern cardiac muscle repolarization. Reduction of miR-1/133a dosage induced a longQT phenotype in mice especially at low heart rates. Longer action potentials in cardiomyocytes are caused by modulation of the impact of β-adrenergic signaling on the activity of the depolarizing L-type calcium channel. Pharmacological intervention to attenuate β-adrenergic signaling or L-type calcium channel activity in vivo abrogated the longQT phenotype that is caused by modulation of miR-1/133a activity. Thus, we identify the miR-1/133a miRNA clusters to be important to prevent a longQT-phenotype in the mammalian heart.

Optogenetic activation of Gq signalling modulates pacemaker activity of cardiomyocytes

AIMS

Investigation of Gq signalling with pharmacological agonists of Gq-coupled receptors lacks spatio-temporal precision. The aim of this study was to establish melanopsin, a light-sensitive Gq-coupled receptor, as a new tool for the investigation of spatial and temporal effects of Gq stimulation on pacemaking in cardiomyocytes at an early developmental stage.

METHODS AND RESULTS

A vector for ubiquitous expression of melanopsin was tested in HEK293FT cells, which showed light-induced production of inositol-1,4,5-trisphosphate and elevation of intracellular Ca(2+) concentration. Mouse embryonic stem cells were stably transfected with this plasmid and differentiated into spontaneously beating embryoid bodies (EBs). Cardiomyocytes within EBs showed melanopsin expression and illumination (60 s, 308.5 nW/mm(2), 470 nm) of EBs increased beating rate within 10.2 ± 1.7 s to 317.1 ± 16.3% of baseline frequency. Illumination as short as 5 s was sufficient for generating the maximal frequency response. After termination of illumination, baseline frequency was reached with a decay constant of 27.1 ± 2.5 s. The light-induced acceleration of beating frequency showed a sigmoid dependence on light intensity with a half maximal effective light intensity of 41.7 nW/mm(2). Interestingly, EBs showed a high rate of irregular contractions after termination of high-intensity illumination. Local Gq activation by illumination of a small region in a functional syncytium of cardiomyocytes led to pacemaker activity within the illuminated area.

CONCLUSIONS

Light-induced Gq activation in melanopsin-expressing cardiomyocytes increases beating rate and generates local pacemaker activity. We propose that melanopsin is a powerful optogenetic tool for the investigation of spatial and temporal aspects of Gq signalling in cardiovascular research.