Principles of Optogenetic Methods and Their Application to Cardiac Experimental Systems

The proarrhythmogenic role of autonomics and emerging neuromodulation approaches to prevent sudden death in cardiac ion channelopathies

Ventricular arrhythmias in cardiac channelopathies are linked to autonomic triggers, which are sub-optimally targeted in current management strategies. Improved molecular understanding of cardiac channelopathies and cellular autonomic signalling could refine autonomic therapies to target the specific signalling pathways relevant to the specific aetiologies as well as the central nervous system centres involved in the cardiac autonomic regulation. This review summarizes key anatomical and physiological aspects of the cardiac autonomic nervous system and its impact on ventricular arrhythmias in primary inherited arrhythmia syndromes. Proarrhythmogenic autonomic effects and potential therapeutic targets in defined conditions including the Brugada syndrome, early repolarization syndrome, long QT syndrome, and catecholaminergic polymorphic ventricular tachycardia will be examined. Pharmacological and interventional neuromodulation options for these cardiac channelopathies are discussed. Promising new targets for cardiac neuromodulation include inhibitory and excitatory G-protein coupled receptors, neuropeptides, chemorepellents/attractants as well as the vagal and sympathetic nuclei in the central nervous system. Novel therapeutic strategies utilizing invasive and non-invasive deep brain/brain stem stimulation as well as the rapidly growing field of chemo-, opto-, or sonogenetics allowing cell-specific targeting to reduce ventricular arrhythmias are presented.

Cardiac optogenetics: shining light on signaling pathways

In the early 2000s, the field of neuroscience experienced a groundbreaking transformation with the advent of optogenetics. This innovative technique harnesses the properties of naturally occurring and genetically engineered rhodopsins to confer light sensitivity upon target cells. The remarkable spatiotemporal precision offered by optogenetics has provided researchers with unprecedented opportunities to dissect cellular physiology, leading to an entirely new level of investigation. Initially revolutionizing neuroscience, optogenetics quickly piqued the interest of the wider scientific community, and optogenetic applications were expanded to cardiovascular research. Over the past decade, researchers have employed various optical tools to observe, regulate, and steer the membrane potential of excitable cells in the heart. Despite these advancements, achieving control over specific signaling pathways within the heart has remained an elusive goal. Here, we review the optogenetic tools suitable to control cardiac signaling pathways with a focus on GPCR signaling, and delineate potential applications for studying these pathways, both in healthy and diseased hearts. By shedding light on these exciting developments, we hope to contribute to the ongoing progress in basic cardiac research to facilitate the discovery of novel therapeutic possibilities for treating cardiovascular pathologies.

Optogenetically mediated large volume suppression and synchronized excitation of human ventricular cardiomyocytes

A major challenge in cardiac optogenetics is to have minimally invasive large volume excitation and suppression for effective cardioversion and treatment of tachycardia. It is important to study the effect of light attenuation on the electrical activity of cells in in vivo cardiac optogenetic experiments. In this computational study, we present a detailed analysis of the effect of light attenuation in different channelrhodopsins (ChRs)-expressing human ventricular cardiomyocytes. The study shows that sustained illumination from the myocardium surface used for suppression, simultaneously results in spurious excitation in deeper tissue regions. Tissue depths of suppressed and excited regions have been determined for different opsin expression levels. It is shown that increasing the expression level by 5-fold enhances the depth of suppressed tissue from 2.24 to 3.73 mm with ChR2(H134R) (ChR2 with a single point mutation at position H134), 3.78 to 5.12 mm with GtACR1 (anion-conducting ChR from cryptophyte algae Guillardia theta) and 6.63 to 9.31 mm with ChRmine (a marine opsin gene from Tiarina fusus). Light attenuation also results in desynchrony in action potentials in different tissue regions under pulsed illumination. It is further shown that gradient-opsin expression not only enables suppression up to the same level of tissue depth but also enables synchronized excitation under pulsed illumination. The study is important for the effective treatment of tachycardia and cardiac pacing and for extending the scale of cardiac optogenetics.

Timing is Everything: Stochastic Optogenetic Stimulation Reduces Adaptation in Retinal Ganglion Cells

Optogenetics gives us unprecedented power to investigate brain connectivity. The ability to activate neural circuits with single cell resolution and its ease of application has provided a wealth of knowledge in brain function. More recently, optogenetics has shown tremendous utility in prosthetics applications, including vision restoration for patients with retinitis pigmentosa. One of the disadvantages of optogenetics, however, is its poor temporal bandwidth, i.e. the cell's inability to fire at a rate that matches the optical stimulation rate at high frequencies (>30 Hz). This research proposes a new strategy to overcome the temporal limits of optogenetic stimulation. Using whole-cell current clamp recordings in mouse retinal ganglion cells expressing channelrhodopsin-2 (H134R variant), we observed that randomizing inter-pulse intervals can significantly increase a retinal ganglion cell's temporal response to high frequency stimulation.Clinical Relevance- A significant disadvantage of optogenetic stimulation is its poor temporal dynamics which prohibit its widespread use in retinal prosthetics. We have shown that randomizing the interval between stimulation pulses reduces adaptation in retinal ganglion cells. This stimulation strategy may contribute to new levels of functional restoration in therapeutics which incorporate optogenetics.

Biosensors integrated 3D organoid/organ-on-a-chip system: A real-time biomechanical, biophysical, and biochemical monitoring and characterization

As a full-fidelity simulation of human cells, tissues, organs, and even systems at the microscopic scale, Organ-on-a-Chip (OOC) has significant ethical advantages and development potential compared to animal experiments. The need for the design of new drug high-throughput screening platforms and the mechanistic study of human tissues/organs under pathological conditions, the evolving advances in 3D cell biology and engineering, etc., have promoted the updating of technologies in this field, such as the iteration of chip materials and 3D printing, which in turn facilitate the connection of complex multi-organs-on-chips for simulation and the further development of technology-composite new drug high-throughput screening platforms. As the most critical part of organ-on-a-chip design and practical application, verifying the success of organ model modeling, i.e., evaluating various biochemical and physical parameters in OOC devices, is crucial. Therefore, this paper provides a logical and comprehensive review and discussion of the advances in organ-on-a-chip detection and evaluation technologies from a broad perspective, covering the directions of tissue engineering scaffolds, microenvironment, single/multi-organ function, and stimulus-based evaluation, and provides a more comprehensive review of the progress in the significant organ-on-a-chip research areas in the physiological state.

Optical modulation of excitation-contraction coupling in human-induced pluripotent stem cell-derived cardiomyocytes

Non-genetic photostimulation is a novel and rapidly growing multidisciplinary field that aims to induce light-sensitivity in living systems by exploiting exogeneous phototransducers. Here, we propose an intramembrane photoswitch, based on an azobenzene derivative (Ziapin2), for optical pacing of human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). The light-mediated stimulation process has been studied by applying several techniques to detect the effect on the cell properties. In particular, we recorded changes in membrane capacitance, in membrane potential (V_m), and modulation of intracellular Ca²⁺ dynamics. Finally, cell contractility was analyzed using a custom MATLAB algorithm. Photostimulation of intramembrane Ziapin2 causes a transient V_m hyperpolarization followed by a delayed depolarization and action potential firing. The observed initial electrical modulation nicely correlates with changes in Ca²⁺ dynamics and contraction rate. This work represents the proof of principle that Ziapin2 can modulate electrical activity and contractility in hiPSC-CMs, opening up a future development in cardiac physiology.

In Vivo and 3D Imaging Technique(s) for Spatiotemporal Mapping of Pathological Events in Experimental Model(s) of Spinal Cord Injury

Endothelial damage, astrogliosis, microgliosis, and neuronal degeneration are the most common events after spinal cord injury (SCI). Studies highlighted that studying the spatiotemporal profile of these events might provide a deeper understanding of the pathophysiology of SCI. For imaging of these events, available conventional techniques such as 2-dimensional histology and immunohistochemistry (IHC) are well established and frequently used to visualize and detect the altered expression of the protein of interest involved in these events. However, the technique requires the physical sectioning of the tissue, and results are also open to misinterpretation. Currently, researchers are focusing more attention toward the advanced tools for imaging the spinal cord's various physiological and pathological parameters. The tools include two-photon imaging, light sheet fluorescence microscopy, in vivo imaging system with fluorescent probes, and in vivo chemical and fluorescent protein-expressing viral-tracers. These techniques outperform the limitations associated with conventional techniques in various aspects, such as optical sectioning of tissue, 3D reconstructed imaging, and imaging of particular planes of interest. In addition to this, these techniques are minimally invasive and less time-consuming. In this review, we will discuss the various advanced imaging methodologies that will evolve in the future to explore the fundamental mechanisms after SCI.

A simple flash and freeze system for cryogenic time-resolved electron microscopy

As the resolution revolution in CryoEM expands to encompass all manner of macromolecular complexes, an important new frontier is the implementation of cryogenic time resolved EM (cryoTREM). Biological macromolecular complexes are dynamic systems that undergo conformational changes on timescales from microseconds to minutes. Understanding the dynamic nature of biological changes is critical to understanding function. To realize the full potential of CryoEM, time resolved methods will be integral in coupling static structures to dynamic functions. Here, we present an LED-based photo-flash system as a core part of the sample preparation phase in CryoTREM. The plug-and-play system has a wide range of operational parameters, is low cost and ensures uniform irradiation and minimal heating of the sample prior to plunge freezing. The complete design including electronics and optics, manufacturing, control strategies and operating procedures are discussed for the Thermo Scientific™ Vitrobot and Leica™ EM GP2 plunge freezers. Possible adverse heating effects on the biological sample are also addressed through theoretical as well as experimental studies.

Ultra-low power deep sustained optogenetic excitation of human ventricular cardiomyocytes with red-shifted opsins: A computational study

KEY POINTS

Formulation of accurate theoretical models of optogenetic control of HVCMs expressed with newly-discovered opsins (ChRmine, bReaChES, and CsChrimson). Under continuous illumination, action potentials in each opsin-expressing HVCMs can only be evoked in a certain range of irradiances. Action potentials in ChRmine-expressing HVCMs can be triggered at ultra-low power (6 μW/mm² at 10 ms pulse or 0.7 μW/mm² at 100 ms pulse at 585 nm), which is 2-3 orders of magnitude lower than reported results. Ongoing APs in ChRmine-expressing HVCMs can be suppressed by continuous illumination of 585 nm light at 2 μW/mm² . ChRmine enables sustained excitation due to its faster recovery from the desensitized state. Optogenetic excitation of deeply situated cardiac cells is possible upto ∼ 7.46 mm and 10.2 mm with ChRmine on illuminating the outer surface of pericardium at safe irradiance at 585 nm and 650 nm, respectively. The study opens up prospects for designing energy-efficient light-induced pacemakers, resynchronization, and termination of ventricular tachycardia.

ABSTRACT

The main challenge in cardiac optogenetics is to have low-power, high-fidelity, and deep excitation of cells with minimal invasiveness and heating. We present a detailed computational study of optogenetic excitation of human ventricular cardiomyocytes (HVCMs) with new ChRmine, bReaChES and CsChrimson red-shifted opsins to overcome the challenge. Action potentials (APs) in ChRmine expressing HVCMs can be triggered at 6 μW/mm² (10 ms pulse) and 0.7 μW/mm2 (100 ms pulse) at 585 nm which are two orders of magnitude lower than ChR2(H134R). This enables safe sustained excitation of deeply situated cardiac cells with ChRmine (7.46 mm) and with bReaChES (6.21 mm) with the light source at the pericardium surface. Deeper excitation upto 10.2 mm can be achieved with ChRmine by illuminating at 650 nm. Photostimulation conditions for minimum charge transfer during AP have been determined, which are important for tissue health under sustained excitation. The action potential duration for all the opsins is constant upto 100 ms pulse-width but increases thereafter. Interestingly, the AP frequency increases with irradiance under continuous illumination, which gets suppressed at higher irradiances. Optimal range of irradiance for each opsin to excite HVCMs has been determined. Under optimal photostimulation conditions, each opsin can precisely excite APs up to 2.5 Hz, while latency and power of light pulse for each AP in a sequence remain most stable and an order lower respectively, in ChRmine-expressing HVCMs. The study highlights the importance of ChRmine and bReaChES for resynchronization, termination of ventricular tachycardia, and designing optogenetic cardiac pacemakers with enhanced battery life. Abstract figure legend Deep optogenetic excitation of opsin-expressing cardiomyocytes by placing the light source (maximum output 5.5 mW/mm² ) at the outer surface of the pericardium. Excitation of cardiomyocytes upto 10.2 mm (at 650 nm) and 7.46 mm (at 585 nm) is possible with ChRmine. This article is protected by copyright. All rights reserved.

The Integration of Optical Stimulation in a Mechanically Dynamic Cell Culture Substrate

A cell culture well with integrated mechanical and optical stimulation is presented. This is achieved by combining dielectric elastomer soft actuators, also known as artificial muscles, and a varifocal micro-electromechanical mirror that couples light from an optical fiber and focuses it onto the transparent cell substrate. The device enables unprecedented control of in vitro cell cultures by allowing the experimenter to tune and synchronize mechanical and optical stimuli, thereby enabling new experimental assays in optogenetics, fluorescent microscopy, or laser stimulation that include dynamic mechanical strain as a controlled input parameter.

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.

Implantable Micro-Light-Emitting Diode (µLED)-based optogenetic interfaces toward human applications

Optogenetics has received wide attention in biomedical fields because of itsadvantages in temporal precision and spatial resolution. Beyond contributions to important advances in fundamental research, optogenetics is inspiring a shift towards new methods of improving human well-being and treating diseases. Soft, flexible and biocompatible systems using µLEDs as a light source have been introduced to realize brain-compatible optogenetic implants, but there are still many technical challenges to overcome before their human applications. In this review, we address progress in the development of implantable µLED probes and recent achievements in (i) device engineering design, (ii) driving power, (iii) multifunctionality and (iv) closed-loop systems. (v) Expanded optogenetic applications based on remarkable advances in µLED implants will also be discussed.

Optogenetic Methods to Investigate Brain Alterations in Preclinical Models

Investigating the neuronal dynamics supporting brain functions and understanding how the alterations in these mechanisms result in pathological conditions represents a fundamental challenge. Preclinical research on model organisms allows for a multiscale and multiparametric analysis in vivo of the neuronal mechanisms and holds the potential for better linking the symptoms of a neurological disorder to the underlying cellular and circuit alterations, eventually leading to the identification of therapeutic/rescue strategies. In recent years, brain research in model organisms has taken advantage, along with other techniques, of the development and continuous refinement of methods that use light and optical approaches to reconstruct the activity of brain circuits at the cellular and system levels, and to probe the impact of the different neuronal components in the observed dynamics. These tools, combining low-invasiveness of optical approaches with the power of genetic engineering, are currently revolutionizing the way, the scale and the perspective of investigating brain diseases. The aim of this review is to describe how brain functions can be investigated with optical approaches currently available and to illustrate how these techniques have been adopted to study pathological alterations of brain physiology.

Photonic Nanojet-Mediated Optogenetics

Optogenetics has become a widely used technique in neuroscience research, capable of controlling neuronal activity with high spatiotemporal precision and cell-type specificity. Expressing exogenous opsins in the selected cells can induce neuronal activation upon light irradiation, and the activation depends on the power of incident light. However, high optical power can also lead to off-target neuronal activation or even cell damage. Limiting the incident power, but enhancing power distribution to the targeted neurons, can improve optogenetic efficiency and reduce off-target effects. Here, the use of optical lenses made of polystyrene microspheres is demonstrated to achieve effective focusing of the incident light of relatively low power to neighboring neurons via photonic jets. The presence of microspheres significantly localizes and enhances the power density to the target neurons both in vitro and ex vivo, resulting in increased inward current and evoked action potentials. In vivo results show optogenetic stimulation with microspheres that can evoke significantly more motor behavior and neuronal activation at lowered power density. In all, a proof-of-concept of a strategy is demonstrated to increase the efficacy of optogenetic neuromodulation using pulses of reduced optical power.

Seeing the Light: The Use of Zebrafish for Optogenetic Studies of the Heart

Optogenetics, involving the optical measurement and manipulation of cellular activity with genetically encoded light-sensitive proteins ("reporters" and "actuators"), is a powerful experimental technique for probing (patho-)physiological function. Originally developed as a tool for neuroscience, it has now been utilized in cardiac research for over a decade, providing novel insight into the electrophysiology of the healthy and diseased heart. Among the pioneering cardiac applications of optogenetic actuators were studies in zebrafish, which first demonstrated their use for precise spatiotemporal control of cardiac activity. Zebrafish were also adopted early as an experimental model for the use of optogenetic reporters, including genetically encoded voltage- and calcium-sensitive indicators. Beyond optogenetic studies, zebrafish are becoming an increasingly important tool for cardiac research, as they combine many of the advantages of integrative and reduced experimental models. The zebrafish has striking genetic and functional cardiac similarities to that of mammals, its genome is fully sequenced and can be modified using standard techniques, it has been used to recapitulate a variety of cardiac diseases, and it allows for high-throughput investigations. For optogenetic studies, zebrafish provide additional advantages, as the whole zebrafish heart can be visualized and interrogated in vivo in the transparent, externally developing embryo, and the relatively small adult heart allows for in situ cell-specific observation and control not possible in mammals. With the advent of increasingly sophisticated fluorescence imaging approaches and methods for spatially-resolved light stimulation in the heart, the zebrafish represents an experimental model with unrealized potential for cardiac optogenetic studies. In this review we summarize the use of zebrafish for optogenetic investigations in the heart, highlighting their specific advantages and limitations, and their potential for future cardiac research.

Advances in Implantable Optogenetic Technology for Cardiovascular Research and Medicine

Optogenetic technology provides researchers with spatiotemporally precise tools for stimulation, sensing, and analysis of function in cells, tissues, and organs. These tools can offer low-energy and localized approaches due to the use of the transgenically expressed light gated cation channel Channelrhodopsin-2 (ChR2). While the field began with many neurobiological accomplishments it has also evolved exceptionally well in animal cardiac research, both in vitro and in vivo. Implantable optical devices are being extensively developed to study particular electrophysiological phenomena with the precise control that optogenetics provides. In this review, we highlight recent advances in novel implantable optogenetic devices and their feasibility in cardiac research. Furthermore, we also emphasize the difficulties in translating this technology toward clinical applications and discuss potential solutions for successful clinical translation.

Advances and prospects of rhodopsin-based optogenetics in plant research

Microbial rhodopsins have advanced optogenetics since the discovery of channelrhodopsins almost two decades ago. During this time an abundance of microbial rhodopsins has been discovered, engineered, and improved for studies in neuroscience and other animal research fields. Optogenetic applications in plant research, however, lagged largely behind. Starting with light-regulated gene expression, optogenetics has slowly expanded into plant research. The recently established all-trans retinal production in plants now enables the use of many microbial opsins, bringing extra opportunities to plant research. In this review, we summarize the recent advances of rhodopsin-based plant optogenetics and provide a perspective for future use, combined with fluorescent sensors to monitor physiological parameters.

Ventricular voltage-gated ion channels: Detection, characteristics, mechanisms, and drug safety evaluation

Cardiac voltage-gated ion channels (VGICs) play critical roles in mediating cardiac electrophysiological signals, such as action potentials, to maintain normal heart excitability and contraction. Inherited or acquired alterations in the structure, expression, or function of VGICs, as well as VGIC-related side effects of pharmaceutical drug delivery can result in abnormal cellular electrophysiological processes that induce life-threatening cardiac arrhythmias or even sudden cardiac death. Hence, to reduce possible heart-related risks, VGICs must be acknowledged as important targets in drug discovery and safety studies related to cardiac disease. In this review, we first summarize the development and application of electrophysiological techniques that are employed in cardiac VGIC studies alone or in combination with other techniques such as cryoelectron microscopy, optical imaging and optogenetics. Subsequently, we describe the characteristics, structure, mechanisms, and functions of various well-studied VGICs in ventricular myocytes and analyze their roles in and contributions to both physiological cardiac excitability and inherited cardiac diseases. Finally, we address the implications of the structure and function of ventricular VGICs for drug safety evaluation. In summary, multidisciplinary studies on VGICs help researchers discover potential targets of VGICs and novel VGICs in heart, enrich their knowledge of the properties and functions, determine the operation mechanisms of pathological VGICs, and introduce groundbreaking trends in drug therapy strategies, and drug safety evaluation.

The Effects of Repetitive Use and Pathological Remodeling on Channelrhodopsin Function in Cardiomyocytes

Aim: Channelrhodopsins (ChRs) are a large family of light-gated ion channels with distinct properties, which is of great importance in the selection of a ChR variant for a given application. However, data to guide such selection for cardiac optogenetic applications are lacking. Therefore, we investigated the functioning of different ChR variants in normal and pathological hypertrophic cardiomyocytes subjected to various illumination protocols.

Methods and Results: Isolated neonatal rat ventricular cardiomyocytes (NRVMs) were transduced with lentiviral vectors to express one of the following ChR variants: H134R, CatCh, ReaChR, or GtACR1. NRVMs were treated with phenylephrine (PE) to induce pathological hypertrophy (PE group) or left untreated [control (CTL) group]. In these groups, ChR currents displayed unique and significantly different properties for each ChR variant on activation by a single 1-s light pulse (1 mW/mm²: 470, 565, or 617 nm). The concomitant membrane potential (V_m) responses also showed a ChR variant-specific profile, with GtACR1 causing a slight increase in average V_m during illumination (V_plateau: −38 mV) as compared with a V_plateau > −20 mV for the other ChR variants. On repetitive activation at increasing frequencies (10-ms pulses at 1–10 Hz for 30 s), peak currents, which are important for cardiac pacing, decreased with increasing activation frequencies by 17–78% (p < 0.05), while plateau currents, which are critical for arrhythmia termination, decreased by 10–75% (p < 0.05), both in a variant-specific manner. In contrast, the corresponding V_plateau remained largely stable. Importantly, current properties and V_m responses were not statistically different between the PE and CTL groups, irrespective of the variant used (p > 0.05).

Conclusion: Our data show that ChR variants function equally well in cell culture models of healthy and pathologically hypertrophic myocardium but show strong, variant-specific use-dependence. This use-dependent nature of ChR function should be taken into account during the design of cardiac optogenetic studies and the interpretation of the experimental findings thereof.

Theoretical analysis of optogenetic spiking with ChRmine, bReaChES and CsChrimson-expressing neurons for retinal prostheses

Objective.Optogenetics has emerged as a promising technique for neural prosthetics, especially retinal prostheses, with unprecedented spatiotemporal resolution. Newly discovered opsins with high light sensitivity and fast temporal kinetics can provide sufficient temporal resolution at safe light powers and overcome the limitations of presently used opsins. It is also important to formulate accurate mathematical models for optogenetic retinal prostheses, which can facilitate optimization of photostimulation factors to improve the performance.Approach.A detailed theoretical analysis of optogenetic excitation of model retinal ganglion neurons (RGNs) and hippocampal neurons expressed with already tested opsins for retinal prostheses, namely, ChR2, ReaChR and ChrimsonR, and also with recently discovered potent opsins CsChrimson, bReaChES and ChRmine, was carried out.Main results.Under continuous illumination, ChRmine-expressing RGNs begin to respond at very low irradiances ∼10^-4mW mm^-2, and evoke firing upto ∼280 Hz, highest among other opsin-expressing RGNs, at 10^-2mW mm^-2. Under pulsed illumination at randomized photon fluxes, ChRmine-expressing RGNs respond to changes in pulse to pulse irradiances upto four logs, although very bright pulses >10¹⁴photons mm^-2s^-1block firing in these neurons. The minimum irradiance threshold for ChRmine-expressing RGNs is lower by two orders of magnitude, whereas, the first spike latency in ChRmine-expressing RGNs is shorter by an order of magnitude, alongwith stable latency of subsequest spikes compared to others. Further, a good set of photostimulation parameters were determined to achieve high-frequency control with single spike resolution at minimal power. Although ChrimsonR enables spiking upto 100 Hz in RGNs, it requires very high irradiances. ChRmine provides control at light powers that are two orders of magnitude smaller than that required with experimentally studied opsins, while maintaining single spike temporal resolution upto 40 Hz.Significance.The present study highlights the importance of ChRmine as a potential opsin for optogenetic retinal prostheses.

Optogenetic approaches for understanding homeostatic and degenerative processes in Drosophila

Many organs and tissues have an intrinsic ability to regenerate from a dedicated, tissue-specific stem cell pool. As organisms age, the process of self-regulation or homeostasis begins to slow down with fewer stem cells available for tissue repair. Tissues become more fragile and organs less efficient. This slowdown of homeostatic processes leads to the development of cellular and neurodegenerative diseases. In this review, we highlight the recent use and future potential of optogenetic approaches to study homeostasis. Optogenetics uses photosensitive molecules and genetic engineering to modulate cellular activity in vivo, allowing precise experiments with spatiotemporal control. We look at applications of this technology for understanding the mechanisms governing homeostasis and degeneration as applied to widely used model organisms, such as Drosophila melanogaster, where other common tools are less effective or unavailable.

Benefits of pharmacological and electrical cholinergic stimulation in hypertension and heart failure

Systemic arterial hypertension and heart failure are cardiovascular diseases that affect millions of individuals worldwide. They are characterized by a change in the autonomic nervous system balance, highlighted by an increase in sympathetic activity associated with a decrease in parasympathetic activity. Most therapeutic approaches seek to treat these diseases by medications that attenuate sympathetic activity. However, there is a growing number of studies demonstrating that the improvement of parasympathetic function, by means of pharmacological or electrical stimulation, can be an effective tool for the treatment of these cardiovascular diseases. Therefore, this review aims to describe the advances reported by experimental and clinical studies that addressed the potential of cholinergic stimulation to prevent autonomic and cardiovascular imbalance in hypertension and heart failure. Overall, the published data reviewed demonstrate that the use of central or peripheral acetylcholinesterase inhibitors is efficient to improve the autonomic imbalance and hemodynamic changes observed in heart failure and hypertension. Of note, the baroreflex and the vagus nerve activation have been shown to be safe and effective approaches to be used as an alternative treatment for these cardiovascular diseases. In conclusion, pharmacological and electrical stimulation of the parasympathetic nervous system has the potential to be used as a therapeutic tool for the treatment of hypertension and heart failure, deserving to be more explored in the clinical setting.

Nanotransducers for Wireless Neuromodulation

Understanding the signal transmission and processing within the central nervous system (CNS) is a grand challenge in neuroscience. The past decade has witnessed significant advances in the development of new tools to address this challenge. Development of these new tools draws diverse expertise from genetics, materials science, electrical engineering, photonics and other disciplines. Among these tools, nanomaterials have emerged as a unique class of neural interfaces due to their small size, remote coupling and conversion of different energy modalities, various delivery methods, and mitigated chronic immune responses. In this review, we will discuss recent advances in nanotransducers to modulate and interface with the neural system without physical wires. Nanotransducers work collectively to modulate brain activity through optogenetic, mechanical, thermal, electrical and chemical modalities. We will compare important parameters among these techniques including the invasiveness, spatiotemporal precision, cell-type specificity, brain penetration, and translation to large animals and humans. Important areas for future research include a better understanding of the nanomaterials-brain interface, integration of sensing capability for bidirectional closed-loop neuromodulation, and genetically engineered functional materials for cell-type specific neuromodulation.

IPG-based field potential measurement of cultured cardiomyocytes for optogenetic applications

BACKGROUND

Electrophysiological sensing of cardiomyocytes (CMs) in optogenetic preparations applies various techniques, such as patch-clamp, microelectrode array, and optical mapping. However, challenges remain in decreasing the cost, system dimensions, and operating skills required for these technologies.

OBJECTIVE

This study developed a low-cost, portable impedance plethysmography (IPG)-based electrophysiological measurement of cultured CMs for optogenetic applications.

METHODS

To validate the efficacy of the proposed sensor, optogenetic stimulation with different pacing cycle lengths (PCL) was performed to evaluate whether the channelrhodopsin-2 (ChR2)-expressing CM beating rhythm measured by the IPG sensor was consistent with biological responses.

RESULTS

The experimental results show that the CM field potential was synchronized with external optical pacing with PCLs ranging from 250 ms to 1000 ms. Moreover, irregular fibrillating waveforms induced by CM arrhythmia were detected after overdrive optical pacing. Through the combined evidence of the theoretical model and experimental results, this study confirmed the feasibility of long-term electrophysiological sensing for optogenetic CMs.

CONCLUSION

This study proposes an IPG-based sensor that is low-cost, portable, and requires low-operating skills to perform real-time CM field potential measurement in response to optogenetic stimulation.

SIGNIFICANCE

This study demonstrates a new methodology for convenient electrophysiological sensing of CMs in optogenetic applications.

Using a bistable animal opsin for switchable and scalable optogenetic inhibition of neurons

There is no consensus on the best inhibitory optogenetic tool. Since Gi/o signalling is a native mechanism of neuronal inhibition, we asked whether Lamprey Parapinopsin ("Lamplight"), a Gi/o-coupled bistable animal opsin, could be used for optogenetic silencing. We show that short (405 nm) and long (525 nm) wavelength pulses repeatedly switch Lamplight between stable signalling active and inactive states, respectively, and that combining these wavelengths can be used to achieve intermediate levels of activity. These properties can be applied to produce switchable neuronal hyperpolarisation and suppression of spontaneous spike firing in the mouse hypothalamic suprachiasmatic nucleus. Expressing Lamplight in (predominantly) ON bipolar cells can photosensitise retinas following advanced photoreceptor degeneration, with 405 and 525 nm stimuli producing responses of opposite sign in the output neurons of the retina. We conclude that bistable animal opsins can co-opt endogenous signalling mechanisms to allow optogenetic inhibition that is scalable, sustained and reversible.

Structures of the archaerhodopsin-3 transporter reveal that disordering of internal water networks underpins receptor sensitization

Many transmembrane receptors have a desensitized state, in which they are unable to respond to external stimuli. The family of microbial rhodopsin proteins includes one such group of receptors, whose inactive or dark-adapted (DA) state is established in the prolonged absence of light. Here, we present high-resolution crystal structures of the ground (light-adapted) and DA states of Archaerhodopsin-3 (AR3), solved to 1.1 Å and 1.3 Å resolution respectively. We observe significant differences between the two states in the dynamics of water molecules that are coupled via H-bonds to the retinal Schiff Base. Supporting QM/MM calculations reveal how the DA state permits a thermodynamic equilibrium between retinal isomers to be established, and how this same change is prevented in the ground state in the absence of light. We suggest that the different arrangement of internal water networks in AR3 is responsible for the faster photocycle kinetics compared to homologs.

Dynamic real-time subtraction of stray-light and background for multiphoton imaging

We introduce a new approach to reduce uncorrelated background signals from fluorescence imaging data, using real-time subtraction of background light. This approach takes advantage of the short fluorescence lifetime of most popular fluorescent activity reporters, and the low duty-cycle of ultrafast lasers. By synchronizing excitation and recording, laser-induced multiphoton fluorescence can be discriminated from background light levels with each laser pulse. We demonstrate the ability of our method to - in real-time - remove image artifacts that in a conventional imaging setup lead to clipping of the signal. In other words, our method enables imaging under conditions that in a conventional setup would yield corrupted data from which no accurate information can be extracted. This is advantageous in experimental setups requiring additional light sources for applications such as optogenetic stimulation.

Application of Targeting-Optimized Chronos for Stimulation of the Auditory Pathway

In the last 15 years, optogenetics has revolutionized the life sciences and enabled studies of complex biological systems such as the brain. Applying optogenetics also has great potential for restorative medicine, such as hearing restoration, by stimulating genetically modified spiral ganglion neurons of the cochlea with light. To this end, opsins with short closing kinetics are required, given the high firing rates and utmost temporal precision of spiking in these neurons. Chronos is the fastest native blue channelrhodopsin (ChR) reported so far with a closing kinetics bellow 1 ms at body temperature and an interesting candidate for the development of the future optogenetic cochlear implants. This book chapter explains in more details the development and application of Chronos with optimized membrane targeting for temporally precise optical stimulation of spiral ganglion neurons. In addition, the generation of adeno-associated virus (AAV) and AAV delivery to the cochlea of postnatal mice and the procedure to record optically evoked auditory brainstem responses are described.

Optogenetic activation of muscle contraction in vivo

Purpose: Optogenetics is an emerging alternative to traditional electrical stimulation to initiate action potentials in activatable cells both ex vivo and in vivo. Optogenetics has been commonly used in mammalian neurons and more recently, it has been adapted for activation of cardiomyocytes and skeletal muscle. Therefore, the aim of this study was to evaluate the stimulation feasibility and sustain isometric muscle contraction and limit decay for an extended period of time (1s), using non-invasive transdermal light activation of skeletal muscle (triceps surae) in vivo.

MATERIALS AND METHODS

We used inducible Cre recombination to target expression of Channelrhodopsin-2 (ChR2(H134R)-EYFP) in skeletal muscle (Acta1-Cre) in mice. Fluorescent imaging confirmed that ChR2 expression is localized in skeletal muscle and does not have specific expression in sciatic nerve branch, therefore, allowing for non-nerve mediated optical stimulation of skeletal muscle. We induced muscle contraction using transdermal exposure to blue light and selected 10 Hz stimulation after controlled optimization experiments to sustain prolonged muscle contraction.

RESULTS

Increasing the stimulation frequency from 10 Hz to 40 Hz increased the muscle contraction decay during prolonged 1s stimulation, highlighting frequency dependency and importance of membrane repolarization for effective light activation. Finally, we showed that optimized pulsed optogenetic stimulation of 10 Hz resulted in comparable ankle torque and contractile functionality to that of electrical stimulation.

CONCLUSIONS

Our results demonstrate the feasibility and repeatability of non-invasive optogenetic stimulation of muscle in vivo and highlight optogenetic stimulation as a powerful tool for non-invasive in vivo direct activation of skeletal muscle.

Cardiac optogenetics: a decade of enlightenment

The electromechanical function of the heart involves complex, coordinated activity over time and space. Life-threatening cardiac arrhythmias arise from asynchrony in these space-time events; therefore, therapies for prevention and treatment require fundamental understanding and the ability to visualize, perturb and control cardiac activity. Optogenetics combines optical and molecular biology (genetic) approaches for light-enabled sensing and actuation of electrical activity with unprecedented spatiotemporal resolution and parallelism. The year 2020 marks a decade of developments in cardiac optogenetics since this technology was adopted from neuroscience and applied to the heart. In this Review, we appraise a decade of advances that define near-term (immediate) translation based on all-optical electrophysiology, including high-throughput screening, cardiotoxicity testing and personalized medicine assays, and long-term (aspirational) prospects for clinical translation of cardiac optogenetics, including new optical therapies for rhythm control. The main translational opportunities and challenges for optogenetics to be fully embraced in cardiology are also discussed.

A guide to the design of synthetic gene networks in mammalian cells

Synthetic biology aims to harness natural and synthetic biological parts and engineering them in new combinations and systems, producing novel therapies, diagnostics, bioproduction systems, and providing information on the mechanism of function of biological systems. Engineering cell function requires the rewiring or de novo construction of cell information processing networks. Using natural and synthetic signal processing elements, researchers have demonstrated a wide array of signal sensing, processing and propagation modules, using transcription, translation, or post-translational modification to program new function. The toolbox for synthetic network design is ever-advancing and has still ample room to grow. Here, we review the diversity of synthetic gene networks, types of building modules, techniques of regulation, and their applications.

Organoids, Assembloids, and Novel Biotechnology: Steps Forward in Developmental and Disease-Related Neuroscience

In neuroscience research, the efforts to find the model through which we can mimic the in vivo microenvironment of a developing or defective brain have been everlasting. While model organisms are used for over a hundred years, many more methods have been introduced with immortalized or primary cell lines and later induced pluripotent stem cells and organoids to be some of these. As the use of organoids becomes more and more common by many laboratories in biology and neuroscience in particular, it is crucial to deeper understand the challenges and possible pitfalls of their application in research, many of which can be surpassed with the support of state-of-the art bioengineering solutions. In this review, after a brief chronicle of the path to the discovery of organoids, we focus on the latest approaches to study neuroscience related topics with organoids, such as the use of assembloids, CRISPR technology, patch-clamp and optogenetics techniques and discuss how modern 3-dimensional biomaterials, miniaturized bioreactors and microfluidic chips can help to overcome the disadvantages of their use.

Optical capture and defibrillation in rats with monocrotaline-induced myocardial fibrosis 1 year after a single intravenous injection of adeno-associated virus channelrhodopsin-2

BACKGROUND

Optogenetics uses light to regulate cardiac rhythms and terminate malignant arrhythmias.

OBJECTIVE

The purpose of this study was to investigate the long-term validity of optical capture properties based on virus-transfected channelrhodopsin-2 (ChR2) and evaluate the effects of optogenetic-based defibrillation in an in vivo rat model of myocardial fibrosis enhanced by monocrotaline (MCT).

METHODS

Fifteen infant rats received jugular vein injection of adeno-associated virus (AAV). After 8 weeks, 5 rats were randomly selected to verify the effectiveness ChR2 transfection. The remaining rats were administered MCT at 11 months. Four weeks after MCT, the availability of 473-nm blue light to capture heart rhythm in these rats was verified again. Ventricular tachycardia (VT) and ventricular fibrillation (VF) were induced by burst stimulation on the basis of enhanced myocardial fibrosis, and the termination effects of the optical manipulation were tested.

RESULTS

Eight weeks after AAV injection, there was ChR2 expression throughout the ventricular myocardium as reflected by both fluorescence imaging and optical pacing. Four weeks after MCT, significant myocardial fibrosis was achieved. Light could still trigger the corresponding ectopic heart rhythm, and the pulse width and illumination area could affect the light capture rate. VT/VF was induced successfully in 1-year-observation rats, and the rate of termination of VT/VF under light was much higher than that of spontaneous termination.

CONCLUSION

Viral ChR2 transfection can play a long-term role in the rat heart, and light can successfully regulate heart rhythm and defibrillate after cardiac fibrosis.

Multifaceted roles of pericytes in central nervous system homeostasis and disease

Pericytes, the mural cells surrounding microcirculation, are gaining increasing attention for their roles in health and disease of the central nervous system (CNS). As an essential part of the neurovascular unit (NVU), pericytes are actively engaged in interactions with neighboring cells and work in synergy with them to maintain homeostasis of the CNS, such as maintaining the blood-brain barrier (BBB), regulating cerebral blood flow (CBF) and the glymphatic system as well as mediating immune responses. However, the dysfunction of pericytes may contribute to the progression of various pathologies. In this review, we discuss: (1) origin of pericytes and different pericyte markers; (2) interactions of pericytes with endothelial cells (ECs), astrocytes, microglia, oligodendrocytes, and neurons; (3) physiological roles of pericytes in the CNS; (4) effects of pericytes in different CNS diseases; (5) relationship of pericytes with extracellular vesicles (EVs) and microRNAs (miRs); (6) recent advances in pericytes studies and future perspective.

Optogenetics for neural transplant manipulation and functional analysis

Transplantation of neural stem cells (NSCs) or NSC-derived neurons into the brain is a promising therapeutic approach to restore neuronal function. Rapid progress in the NSCs research field, particularly due to the exploitation of induced pluripotent stem cells (iPSCs), offers great potential and an unlimited source of stem cell-derived neural grafts. Studying the functional integration of these grafts into host brain tissues and their effects on each other have been boosted by the implementation of optogenetic technologies. Optogenetics provides high spatiotemporal functional manipulations of grafted or host neurons in parallel. This review aims to highlight the impact of optogenetics in neural stem cell transplantations.

Comparison of low-power, high-frequency and temporally precise optogenetic inhibition of spiking in NpHR, eNpHR3.0 and Jaws-expressing neurons

A detailed theoretical analysis of low-power, high-frequency and temporally precise optogenetic inhibition of neuronal spiking, with red-shifted opsins namely, NpHR, eNpHR3.0 and Jaws, has been presented. An accurate model for inhibition of spiking in these opsins expressed hippocampal neurons that includes the important rebound activity of chloride ions across the membrane has been formulated. The effect of various parameters including irradiance, pulse width, frequency, opsin-expression density and chloride concentration has been studied in detail. Theoretical simulations are in very good agreement with reported experimental results. The chloride concentration gradient directly affects the photocurrent and inhibition capacity in all three variants. eNpHR3.0 shows smallest inhibitory post-synaptic potential plateau at higher frequencies. The time delay between light stimulus and target spike is crucial to minimize irradiance and expression density thresholds for suppressing individual spike. Good practical values of photostimulation parameters have been obtained empirically for peak photocurrent, time delay and 100% spiking inhibition, at continuous and pulsed illumination. Under continuous illumination, complete inhibition of neural activity in Jaws-expressing neurons takes place at minimum irradiance of 0.2 mW mm^-2 and expression density of 0.2 mS cm^-2, whereas for pulsed stimulation, it is at minimum irradiance of 0.6 mW mm^-2 and 5 ms pulse width, at 10 Hz. It is shown that Jaws and eNpHR3.0 are able to invoke single spike precise inhibition up to 160 and 200 Hz, respectively. The study is useful in designing new experiments, understanding temporal spike coding and bidirectional control, and curing neurological disorders.

Towards the clinical translation of optogenetic skeletal muscle stimulation

Paralysis is a frequent phenomenon in many diseases, and to date, only functional electrical stimulation (FES) mediated via the innervating nerve can be employed to restore skeletal muscle function in patients. Despite recent progress, FES has several technical limitations and significant side effects. Optogenetic stimulation has been proposed as an alternative, as it may circumvent some of the disadvantages of FES enabling cell type–specific, spatially and temporally precise stimulation of cells expressing light-gated ion channels, commonly Channelrhodopsin2. Two distinct approaches for the restoration of skeletal muscle function with optogenetics have been demonstrated: indirect optogenetic stimulation through the innervating nerve similar to FES and direct optogenetic stimulation of the skeletal muscle. Although both approaches show great promise, both have their limitations and there are several general hurdles that need to be overcome for their translation into clinics. These include successful gene transfer, sustained optogenetic protein expression, and the creation of optically active implantable devices. Herein, a comprehensive summary of the underlying mechanisms of electrical and optogenetic approaches is provided. With this knowledge in mind, we substantiate a detailed discussion of the advantages and limitations of each method. Furthermore, the obstacles in the way of clinical translation of optogenetic stimulation are discussed, and suggestions on how they could be overcome are provided. Finally, four specific examples of pathologies demanding novel therapeutic measures are discussed with a focus on the likelihood of direct versus indirect optogenetic stimulation.

Optogenetics: Background, Methodological Advances and Potential Applications for Cardiovascular Research and Medicine

Optogenetics is an elegant approach of precisely controlling and monitoring the biological functions of a cell, group of cells, tissues, or organs with high temporal and spatial resolution by using optical system and genetic engineering technologies. The field evolved with the need to precisely control neurons and decipher neural circuity and has made great accomplishments in neuroscience. It also evolved in cardiovascular research almost a decade ago and has made considerable progress in both in vitro and in vivo animal studies. Thus, this review is written with an objective to provide information on the evolution, background, methodical advances, and potential scope of the field for cardiovascular research and medicine. We begin with a review of literatures on optogenetic proteins related to their origin, structure, types, mechanism of action, methods to improve their performance, and the delivery vehicles and methods to express such proteins on target cells and tissues for cardiovascular research. Next, we reviewed historical and recent literatures to demonstrate the scope of optogenetics for cardiovascular research and regenerative medicine and examined that cardiac optogenetics is vital in mimicking heart diseases, understanding the mechanisms of disease progression and also in introducing novel therapies to treat cardiac abnormalities, such as arrhythmias. We also reviewed optogenetics as promising tools in providing high-throughput data for cardiotoxicity screening in drug development and also in deciphering dynamic roles of signaling moieties in cell signaling. Finally, we put forth considerations on the need of scaling up of the optogenetic system, clinically relevant in vivo and in silico models, light attenuation issues, and concerns over the level, immune reactions, toxicity, and ectopic expression with opsin expression. Detailed investigations on such considerations would accelerate the translation of cardiac optogenetics from present in vitro and in vivo animal studies to clinical therapies.

The Emergence of Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes (hiPSC-CMs) as a Platform to Model Arrhythmogenic Diseases

There is a need for improved in vitro models of inherited cardiac diseases to better understand basic cellular and molecular mechanisms and advance drug development. Most of these diseases are associated with arrhythmias, as a result of mutations in ion channel or ion channel-modulatory proteins. Thus far, the electrophysiological phenotype of these mutations has been typically studied using transgenic animal models and heterologous expression systems. Although they have played a major role in advancing the understanding of the pathophysiology of arrhythmogenesis, more physiological and predictive preclinical models are necessary to optimize the treatment strategy for individual patients. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have generated much interest as an alternative tool to model arrhythmogenic diseases. They provide a unique opportunity to recapitulate the native-like environment required for mutated proteins to reproduce the human cellular disease phenotype. However, it is also important to recognize the limitations of this technology, specifically their fetal electrophysiological phenotype, which differentiates them from adult human myocytes. In this review, we provide an overview of the major inherited arrhythmogenic cardiac diseases modeled using hiPSC-CMs and for which the cellular disease phenotype has been somewhat characterized.