Ultra light-sensitive and fast neuronal activation with the Ca²+-permeable channelrhodopsin CatCh

On-demand optogenetic activation of human stem-cell-derived neurons

The widespread application of human stem-cell-derived neurons for functional studies is impeded by complicated differentiation protocols, immaturity, and deficient optogene expression as stem cells frequently lose transgene expression over time. Here we report a simple but precise Cre-loxP-based strategy for generating conditional, and thereby stable, optogenetic human stem-cell lines. These cells can be easily and efficiently differentiated into functional neurons, and optogene expression can be triggered by administering Cre protein to the cultures. This conditional expression system may be applied to stem-cell-derived neurons whenever timed transgene expression could help to overcome silencing at the stem-cell level.

An Embedded Real-Time Processing Platform for Optogenetic Neuroprosthetic Applications

Optogenetics offers a powerful new approach for controlling neural circuits. It has numerous applications in both basic and clinical science. These applications require stimulating devices with small processors that can perform real-time neural signal processing, deliver high-intensity light with high spatial and temporal resolution, and do not consume a lot of power. In this paper, we demonstrate the implementation of neuronal models in a platform consisting of an embedded system module and a portable digital light processing projector. As a replacement for damaged neural circuitry, the embedded module processes neural signals and then directs the projector to optogenetically activate a downstream neural pathway. We present a design in the context of stimulating circuits in the visual system, but the approach is feasible for a broad range of biomedical applications.

Optogenetic manipulation of anatomical re-entry by light-guided generation of a reversible local conduction block

Aims

Anatomical re-entry is an important mechanism of ventricular tachycardia, characterized by circular electrical propagation in a fixed pathway. It's current investigative and therapeutic approaches are non-biological, rather unspecific (drugs), traumatizing (electrical shocks), or irreversible (ablation). Optogenetics is a new biological technique that allows reversible modulation of electrical function with unmatched spatiotemporal precision using light-gated ion channels. We therefore investigated optogenetic manipulation of anatomical re-entry in ventricular cardiac tissue.

Methods and results

Transverse, 150-μm-thick ventricular slices, obtained from neonatal rat hearts, were genetically modified with lentiviral vectors encoding Ca2+-translocating channelrhodopsin (CatCh), a light-gated depolarizing ion channel, or enhanced yellow fluorescent protein (eYFP) as control. Stable anatomical re-entry was induced in both experimental groups. Activation of CatCh was precisely controlled by 470-nm patterned illumination, while the effects on anatomical re-entry were studied by optical voltage mapping. Regional illumination in the pathway of anatomical re-entry resulted in termination of arrhythmic activity only in CatCh-expressing slices by establishing a local and reversible, depolarization-induced conduction block in the illuminated area. Systematic adjustment of the size of the light-exposed area in the re-entrant pathway revealed that re-entry could be terminated by either wave collision or extinction, depending on the depth (transmurality) of illumination. In silico studies implicated source-sink mismatches at the site of subtransmural conduction block as an important factor in re-entry termination.

Conclusions

Anatomical re-entry in ventricular tissue can be manipulated by optogenetic induction of a local and reversible conduction block in the re-entrant pathway, allowing effective re-entry termination. These results provide distinctively new mechanistic insight into re-entry termination and a novel perspective for cardiac arrhythmia management.

A high-conductance chemo-optogenetic system based on the vertebrate channel Trpa1b

Optogenetics is a powerful research approach that allows localized optical modulation of selected cells within an animal via the expression of genetically encoded photo-excitable ion channels. Commonly used optogenetic techniques rely on the expression of microbial opsin variants, which have many excellent features but suffer from various degrees of blue spectral overlap and limited channel conductance. Here, we expand the optogenetics toolbox in the form of a tunable, high-conductance vertebrate cation channel, zTrpa1b, coupled with photo-activated channel ligands, such as optovin and 4g6. Our results demonstrate that zTrpa1b/ligand pairing offers high light sensitivity, millisecond-scale response latency in vivo, as well as adjustable channel off latency. Exogenous in vivo expression of zTrpa1b in sensory neurons allowed subcellular photo-activation, enabling light-dependent motor control. zTrpa1b/ligand was also suitable for cardiomyocyte pacing, as shown in experiments performed on zebrafish hearts in vivo as well as in human stem cell-derived cardiomyocytes in vitro. Therefore, zTrpa1b/optovin represents a novel tool for flexible, high-conductance optogenetics.

Molecular determinants of proton selectivity and gating in the red-light activated channelrhodopsin Chrimson

Channelrhodopsins are light-gated ion channels of green algae used for the precise temporal and spatial control of transmembrane ion fluxes. The channelrhodopsin Chrimson from Chlamydomonas noctigama allows unprecedented deep tissue penetration due to peak absorption at 590 nm. We demonstrate by electrophysiological recordings and imaging techniques that Chrimson is highly proton selective causing intracellular acidification in HEK cells that is responsible for slow photocurrent decline during prolonged illumination. We localized molecular determinants of both high proton selectivity and red light activation to the extracellular pore. Whereas exchange of Glu143 only drops proton conductance and generates an operational Na-channel with 590 nm activation, exchange of Glu139 in addition increased the open state lifetime and shifted the absorption hypsochromic by 70 nm. In conjunction with Glu300 in the center and Glu124 and Glu125 at the intracellular end of the pore, Glu139 contributes to a delocalized activation gate and stabilizes by long-range interaction counterion configuration involving protonation of Glu165 that we identified as a key determinant of the large opsin shift in Chrimson.

Genetically Encoded Photoactuators and Photosensors for Characterization and Manipulation of Pluripotent Stem Cells

Our knowledge of pluripotent stem cell biology has advanced considerably in the past four decades, but it has yet to deliver on the great promise of regenerative medicine. The slow progress can be mainly attributed to our incomplete understanding of the complex biologic processes regulating the dynamic developmental pathways from pluripotency to fully-differentiated states of functional somatic cells. Much of the difficulty arises from our lack of specific tools to query, or manipulate, the molecular scale circuitry on both single-cell and organismal levels. Fortunately, the last two decades of progress in the field of optogenetics have produced a variety of genetically encoded, light-mediated tools that enable visualization and control of the spatiotemporal regulation of cellular function. The merging of optogenetics and pluripotent stem cell biology could thus be an important step toward realization of the clinical potential of pluripotent stem cells. In this review, we have surveyed available genetically encoded photoactuators and photosensors, a rapidly expanding toolbox, with particular attention to those with utility for studying pluripotent stem cells.

Palladium-Mediated Synthesis of a Near-Infrared Fluorescent K+ Sensor

Potassium (K+) exits electrically excitable cells during normal and pathophysiological activity. Currently, K+-sensitive electrodes and electrical measurements are the primary tools to detect K+ fluxes. Here, we describe the synthesis of a near-IR, oxazine fluorescent K+ sensor (KNIR-1) with a dissociation constant suited for detecting changes in intracellular and extracellular K+ concentrations. KNIR-1 treatment of cells expressing voltage-gated K+ channels enabled the visualization of intracellular K+ depletion upon channel opening and restoration of cytoplasmic K+ after channel closing.

Reaction dynamics of the chimeric channelrhodopsin C1C2

Channelrhodopsin (ChR) is a key protein of the optogenetic toolkit. C1C2, a functional chimeric protein of Chlamydomonas reinhardtii ChR1 and ChR2, is the only ChR whose crystal structure has been solved, and thus uniquely suitable for structure-based analysis. We report C1C2 photoreaction dynamics with ultrafast transient absorption and multi-pulse spectroscopy combined with target analysis and structure-based hybrid quantum mechanics/molecular mechanics calculations. Two relaxation pathways exist on the excited (S₁) state through two conical intersections CI₁ and CI₂, that are reached via clockwise and counter-clockwise rotations: (i) the C13=C14 isomerization path with 450 fs via CI₁ and (ii) a relaxation path to the initial ground state with 2.0 ps and 11 ps via CI₂, depending on the hydrogen-bonding network, hence indicating active-site structural heterogeneity. The presence of the additional conical intersection CI₂ rationalizes the relatively low quantum yield of photoisomerization (30 ± 3%), reported here. Furthermore, we show the photoreaction dynamics from picoseconds to seconds, characterizing the complete photocycle of C1C2.

Cardiac neuroanatomy - Imaging nerves to define functional control

The autonomic nervous system regulates normal cardiovascular function and plays a critical role in the pathophysiology of cardiovascular disease. Further understanding of the interplay between the autonomic nervous system and cardiovascular system holds promise for the development of neuroscience-based cardiovascular therapeutics. To this end, techniques to image myocardial innervation will help provide a basis for understanding the fundamental underpinnings of cardiac neural control. In this review, we detail the evolution of gross and microscopic anatomical studies for functional mapping of cardiac neuroanatomy.

Optogenetik als mögliche Therapie bei degenerativen Netzhauterkrankungen

Bei neurodegenerativen Erkrankungen der Netzhaut sind die lichtempfindlichen Zellen, die Photorezeptoren, oft als Erstes betroffen. Die Optogenetik ist ein vielversprechender Ansatz, die Netzhaut wieder lichtempfindlich zu machen und dadurch das Sehvermögen wiederherzustellen. Bei der Optogenetik werden lichtempfindliche Proteine über gentechnische Methoden in die Netzhaut eingebracht; die Aktivität der Zielzellen wird durch diese Behandlung durch Licht beeinflussbar. Dieser Einfluss kann die direkte lichtinduzierte Änderung des Membranpotenzials sein (sowohl hemmend als auch erregend) oder die lichtinduzierte Aktivierung intrazellulärer Signalkaskaden. Dies hat zur Folge, dass das Zielgewebe, die Netzhaut, wieder auf Licht reagiert. Diese Übersicht beschreibt die Prinzipien der Optogenetik und den gegenwärtigen Stand im Hinblick auf ihre Anwendung zur Behandlung von Blindheit.

Optogenetic Approaches to Drug Discovery in Neuroscience and Beyond

Recent advances in optogenetics have opened new routes to drug discovery, particularly in neuroscience. Physiological cellular assays probe functional phenotypes that connect genomic data to patient health. Optogenetic tools, in particular tools for all-optical electrophysiology, now provide a means to probe cellular disease models with unprecedented throughput and information content. These techniques promise to identify functional phenotypes associated with disease states and to identify compounds that improve cellular function regardless of whether the compound acts directly on a target or through a bypass mechanism. This review discusses opportunities and unresolved challenges in applying optogenetic techniques throughout the discovery pipeline - from target identification and validation, to target-based and phenotypic screens, to clinical trials.

Microbial Rhodopsins: Diversity, Mechanisms, and Optogenetic Applications

Microbial rhodopsins are a family of photoactive retinylidene proteins widespread throughout the microbial world. They are notable for their diversity of function, using variations of a shared seven-transmembrane helix design and similar photochemical reactions to carry out distinctly different light-driven energy and sensory transduction processes. Their study has contributed to our understanding of how evolution modifies protein scaffolds to create new protein chemistry, and their use as tools to control membrane potential with light is fundamental to optogenetics for research and clinical applications. We review the currently known functions and present more in-depth assessment of three functionally and structurally distinct types discovered over the past two years: (a) anion channelrhodopsins (ACRs) from cryptophyte algae, which enable efficient optogenetic neural suppression; (b) cryptophyte cation channelrhodopsins (CCRs), structurally distinct from the green algae CCRs used extensively for neural activation and from cryptophyte ACRs; and

Morphological evaluation of retinal ganglion cells expressing the L132C/T159C ChR2 mutant transgene in young adult cynomolgus monkeys

To characterize recombinant AAV2 (rAAV2)-mediated expression of L132C/T159C ChR2 mutant in retinal ganglion cells (RGCs) of young adult cynomolgus monkeys. rAAV2 vectors carrying a fusion construct of the ChR2 mutant and GFP (ChR2-GFP) were delivered to the vitreous chamber by intravitreal injection. Expression patterns of the ChR2 mutant in RGCs were examined by immunohistochemical methods three months after injection. The RNA-binding protein with multiple splicing (RBPMS) was used as an RGC specific marker to differentiate RGCs from other retinal neurons and non-neuronal cells. The numbers of RBPMS+ and GFP+ double-labeled RGCs in the central foveal varied with the eccentricity. The expression peaked within 100 μm from the edge of the foveola and drastically decreased to a single superficial RGC layer approximately 300 μm from the edge. On average, the ratio of the double-labeled RGCs versus RBPMS+ RGCs approached 0.32±0.15 (n=14 fields) at the central foveal region (0.1 to 0.53 mm). We observed that the ratio reached 0.78±0.16 (n=21 fields) at peripheral retinal locations (eccentricity >7 mm). This investigation demonstrates that RBPMS could serve as a valuable RGC specific marker for future investigations in this field.

Whole-cell Patch-clamp Recordings for Electrophysiological Determination of Ion Selectivity in Channelrhodopsins

Over the past decade, channelrhodopsins became indispensable in neuroscientific research where they are used as tools to non-invasively manipulate electrical processes in target cells. In this context, ion selectivity of a channelrhodopsin is of particular importance. This article describes the investigation of chloride selectivity for a recently identified anion-conducting channelrhodopsin of Proteomonas sulcata via electrophysiological patch-clamp recordings on HEK293 cells. The experimental procedure for measuring light-gated photocurrents demands a fast switchable - ideally monochromatic - light source coupled into the microscope of an otherwise conventional patch-clamp setup. Preparative procedures prior to the experiment are outlined involving preparation of buffered solutions, considerations on liquid junction potentials, seeding and transfection of cells, and pulling of patch pipettes. The actual recording of current-voltage relations to determine the reversal potentials for different chloride concentrations takes place 24 h to 48 h after transfection. Finally, electrophysiological data are analyzed with respect to theoretical considerations of chloride conduction.

Targeted calcium influx boosts cytotoxic T lymphocyte function in the tumour microenvironment

Adoptive cell transfer utilizing tumour-targeting cytotoxic T lymphocytes (CTLs) is one of the most effective immunotherapies against haematological malignancies, but significant clinical success has not yet been achieved in solid tumours due in part to the strong immunosuppressive tumour microenvironment. Here, we show that suppression of CTL killing by CD4+CD25+Foxp3+ regulatory T cell (Treg) is in part mediated by TGFβ-induced inhibition of inositol trisphosphate (IP3) production, leading to a decrease in T cell receptor (TCR)-dependent intracellular Ca2+ response. Highly selective optical control of Ca2+ signalling in adoptively transferred CTLs enhances T cell activation and IFN-γ production in vitro, leading to a significant reduction in tumour growth in mice. Altogether, our findings indicate that the targeted optogenetic stimulation of intracellular Ca2+ signal allows for the remote control of cytotoxic effector functions of adoptively transferred T cells with outstanding spatial resolution by boosting T cell immune responses at the tumour sites.

Optogenetics gets to the heart: A guiding light beyond defibrillation

Optogenetics provides a tool for controlling the electrical activity of excitable cells by means of the interaction of light with light-gated ion channels. Despite the fact that optogenetics has been intensively utilized in the neurosciences, it has been more rarely employed as an instrument for studying cardiac pathophysiology. However, the advantages of optical approaches to perturb cardiac electrical activity are numerous, especially when the spatio-temporal qualities of light are utterly exploited. Here, we review the main breakthroughs employing optogenetics to perturb cardiac pathophysiology and attempt a comparison of methods and procedures that have employed optogenetics in the heart. We particularly focus on light-based defibrillation strategies that represent one of the latest achievements in this field. We highlight the important role of advanced optical methods for detecting and stimulating electrical activity for optimizing defibrillation strategies and, more generally, for dissecting novel insights in cardiac physiology. Finally, we discuss the main future perspectives that we envision for optogenetics in the heart, both in terms of translational applications and for addressing fundamental questions of cardiac function.

Adjacent channelrhodopsin-2 residues within transmembranes 2 and 7 regulate cation selectivity and distribution of the two open states

Channelrhodopsin-2 (ChR2) is a light-activated channel that can conduct cations of multiple valencies down the electrochemical gradient. Under continuous light exposure, ChR2 transitions from a high-conducting open state (O1) to a low-conducting open state (O2) with differing ion selectivity. The molecular basis for the O1 → O2 transition and how ChR2 modulates selectivity between states is currently unresolved. To this end, we used steered molecular dynamics, electrophysiology, and kinetic modeling to identify residues that contribute to gating and selectivity in discrete open states. Analysis of steered molecular dynamics experiments identified three transmembrane residues (Val-86, Lys-93, and Asn-258) that form a putative barrier to ion translocation. Kinetic modeling of photocurrents generated from ChR2 proteins with conservative mutations at these positions demonstrated that these residues contribute to cation selectivity (Val-86 and Asn-258), the transition between the two open states (Val-86), open channel stability, and the hydrogen-bonding network (K93I and K93N). These results suggest that this approach can be used to identify residues that contribute to the open-state transitions and the discrete ion selectivity within these states. With the rise of ChR2 use in optogenetics, it will be critical to identify residues that contribute to O1 or O2 selectivity and gating to minimize undesirable effects.

Optogenetic Control of Ca2+ and Voltage-Dependent Large Conductance (BK) Potassium Channels

Ca²⁺ concentration jumps for the activation of Ca²⁺-dependent ion channels or transporters can be obtained either by fast solution exchange or by the use of caged Ca²⁺. Here, we report on an alternate optogenetic method for the activation of Ca²⁺ and voltage-dependent large conductance (BK) potassium channels. This was achieved through the use of the light-gated channelrhodopsin 2 variant, CatCh(Calcium translocating Channelrhodopsin) with enhanced Ca, which produces locally [Ca²⁺] in the μM range on the inner side of the membrane, without significant [Ca²⁺] increase in the cytosol. BK channel subunits α and β1 were expressed together with CatCh in HEK293 cells, and voltage and Ca²⁺ dependence were analyzed. Light activation of endogenous BK channels under native conditions in astrocytes and glioma cells was also investigated. Additionally, BK channels were used as sensors for the calibration of the [Ca²⁺] on the inner surface of the cell membrane.

Functional Maturation of Human Stem Cell-Derived Neurons in Long-Term Cultures

Differentiated neurons can be rapidly acquired, within days, by inducing stem cells to express neurogenic transcription factors. We developed a protocol to maintain long-term cultures of human neurons, called iNGNs, which are obtained by inducing Neurogenin-1 and Neurogenin-2 expression in induced pluripotent stem cells. We followed the functional development of iNGNs over months and they showed many hallmark properties for neuronal maturation, including robust electrical and synaptic activity. Using iNGNs expressing a variant of channelrhodopsin-2, called CatCh, we could control iNGN activity with blue light stimulation. In combination with optogenetic tools, iNGNs offer opportunities for studies that require precise spatial and temporal resolution. iNGNs developed spontaneous network activity, and these networks had excitatory glutamatergic synapses, which we characterized with single-cell synaptic recordings. AMPA glutamatergic receptor activity was especially dominant in postsynaptic recordings, whereas NMDA glutamatergic receptor activity was absent from postsynaptic recordings but present in extrasynaptic recordings. Our results on long-term cultures of iNGNs could help in future studies elucidating mechanisms of human synaptogenesis and neurotransmission, along with the ability to scale-up the size of the cultures.

Exploring poisonous mechanism of honeybee, Apis mellifera ligustica Spinola, caused by pyrethroids

As the important intracellular secondary messengers, calcium channel is the target of many neurotoxic pesticides as calcium homeostasis in the neuroplasm play important role in neuronal functions and behavior in insects. This study investigated the effect of deltamethrin (DM) on calcium channel in the brain nerve cells of adult workers of Apis mellifera ligustica Spinola that were cultured in vitro. The results showed that the intracellular calcium concentration was significantly elevated even with a very low concentration of the DM (3.125×10^-2mg/L). Further testing revealed that T-type voltage-gated calcium channels (VGCCs), except for sodium channels, was one of the target of DM on toxicity of Apis mellifera, while DM has no significant effect on the L-type VGCCs, N-methyl-d-aspartate receptor-gated calcium channels and calcium store. These results suggesting that the DM may act on T-type VGCCs in brain cells of honeybees and result in behavioral abnormalities including swarming, feeding, learning, and acquisition.

Challenges for the application of optical stimulation in the cochlea for the study and treatment of hearing loss

INTRODUCTION

Electrical stimulation has long been the most effective strategy for evoking neural activity from bionic devices and has been used with great success in the cochlear implant to allow deaf people to hear speech and sound. Despite its success, the spread of electrical current stimulates a broad region of neural tissue meaning that contemporary devices have limited precision. Optical stimulation as an alternative has attracted much recent interest for its capacity to provide highly focused stimuli, and therefore, potentially improved sensory perception. Given its specificity of activation, optical stimulation may also provide a useful tool in the study of fundamental neuroanatomy and neurophysiological processes. Areas covered: This review examines the advances in optical stimulation - infrared, nanoparticle-enhanced, and optogenetic-based - and its application in the inner ear for the restoration of auditory function following hearing loss. Expert opinion: Initial outcomes suggest that optogenetic-based approaches hold the greatest potential and viability amongst optical techniques for application in the cochlea. The future success of this approach will be governed by advances in the targeted delivery of opsins to auditory neurons, improvements in channel kinetics, development of optical arrays, and innovation of opsins that activate within the optimal near-infrared therapeutic window.

Optogenetic Engineering of Atrial Cardiomyocytes

Optogenetics is emerging in the cardiology field as a new strategy to explore biological functions through the use of light-sensitive proteins and dedicated light sources. For example, this technology allows modification of the electrophysiological properties of cardiac muscle cells with superb spatiotemporal resolution and quantitative control. In this chapter, the optogenetic modification of atrial cardiomyocytes (aCMCs) from 2-day-old Wistar rats using lentiviral vector (LV) technology and the subsequent activation of the light-sensitive proteins (i.e., ion channels) through light-emitting diodes (LEDs) are described.

Natural Resources for Optogenetic Tools

Photoreceptors are found in all kingdoms of life and mediate crucial responses to environmental challenges. Nature has evolved various types of photoresponsive protein structures with different chromophores and signaling concepts for their given purpose. The abundance of these signaling proteins as found nowadays by (meta-)genomic screens enriched the palette of optogenetic tools significantly. In addition, molecular insights into signal transduction mechanisms and design principles from biophysical studies and from structural and mechanistic comparison of homologous proteins opened seemingly unlimited possibilities for customizing the naturally occurring proteins for a given optogenetic task. Here, a brief overview on the photoreceptor concepts already established as optogenetic tools in natural or engineered form, their photochemistry and their signaling/design principles is given. Finally, so far not regarded photosensitive modules and protein architectures with potential for optogenetic application are described.

Opsin spectral sensitivity determines the effectiveness of optogenetic termination of ventricular fibrillation in the human heart: a simulation study

KEY POINTS

Optogenetics-based defibrillation, a theoretical alternative to electrotherapy, involves expression of light-sensitive ion channels in the heart (via gene or cell therapy) and illumination of the cardiac surfaces (via implanted LED arrays) to elicit light-induced activations. We used a biophysically detailed human ventricular model to determine whether such a therapy could terminate fibrillation (VF) and identify which combinations of light-sensitive ion channel properties and illumination configurations would be effective. Defibrillation was successful when a large proportion (> 16.6%) of ventricular tissue was directly stimulated by light that was bright enough to induce an action potential in an uncoupled cell. While illumination with blue light never successfully terminated VF, illumination of red light-sensitive ion channels with dense arrays of implanted red light sources resulted in successful defibrillation. Our results suggest that cardiac expression of red light-sensitive ion channels is necessary for the development of effective optogenetics-based defibrillation therapy using LED arrays.

ABSTRACT

Optogenetics-based defibrillation has been proposed as a novel and potentially pain-free approach to enable cardiomyocyte-selective defibrillation in humans, but the feasibility of such a therapy remains unknown. This study aimed to (1) assess the feasibility of terminating sustained ventricular fibrillation (VF) via light-induced excitation of opsins expressed throughout the myocardium and (2) identify the ideal (theoretically possible) opsin properties and light source configurations that would maximise therapeutic efficacy. We conducted electrophysiological simulations in an MRI-based human ventricular model with VF induced by rapid pacing; light sensitisation via systemic, cardiac-specific gene transfer of channelrhodopsin-2 (ChR2) was simulated. In addition to the widely used blue light-sensitive ChR2-H134R, we also modelled theoretical ChR2 variants with augmented light sensitivity (ChR2+), red-shifted spectral sensitivity (ChR2-RED) or both (ChR2-RED+). Light sources were modelled as synchronously activating LED arrays (LED radius: 1 mm; optical power: 10 mW mm-2 ; array density: 1.15-4.61 cm-2 ). For each unique optogenetic configuration, defibrillation was attempted with two different optical pulse durations (25 and 500 ms). VF termination was only successful for configurations involving ChR2-RED and ChR2-RED+ (for LED arrays with density ≥ 2.30 cm-2 ), suggesting that opsin spectral sensitivity was the most important determinant of optogenetic defibrillation efficacy. This was due to the deeper penetration of red light in cardiac tissue compared with blue light, which resulted in more widespread light-induced propagating wavefronts. Longer pulse duration and higher LED array density were associated with increased optogenetic defibrillation efficacy. In all cases observed, the defibrillation mechanism was light-induced depolarisation of the excitable gap, which led to block of reentrant wavefronts.

Probes for manipulating and monitoring IP3

Inositol 1,4,5-trisphosphate (IP₃) is an important second messenger produced via G-protein-coupled receptor- or receptor tyrosine kinase-mediated pathways. IP₃ levels induce Ca²⁺ release from the endoplasmic reticulum (ER) via IP₃ receptor (IP₃R) located in the ER membrane. The resultant spatiotemporal pattern of Ca²⁺ signals regulates diverse cellular functions, including fertilization, gene expression, synaptic plasticity, and cell death. Therefore, monitoring and manipulating IP₃ levels is important to elucidate not only the functions of IP₃-mediated pathways but also the encoding mechanism of IP₃R as a converter of intracellular signals from IP₃ to Ca²⁺.

Chronic activation of the D156A point mutant of Channelrhodopsin-2 signals apoptotic cell death: the good and the bad

Channelrhodopsin-2 (ChR2) has become a celebrated research tool and is considered a promising potential therapeutic for neurological disorders. While making its way into the clinic, concerns about the safety of chronic ChR2 activation have emerged; in particular as the high-intensity blue light illumination needed for ChR2 activation may be phototoxic. Here we set out to quantify for the first time the cytotoxic effects of chronic ChR2 activation. We studied the safety of prolonged illumination on ChR2(D156A)-expressing human melanoma cells as cancer cells are notorious for their resistance to killing. Three days of illumination eradicated the entire ChR2(D156A)-expressing cell population through mitochondria-mediated apoptosis, whereas blue light activation of non-expressing control cells did not significantly compromise cell viability. In other words, chronic high-intensity blue light illumination alone is not phototoxic, but prolonged ChR2 activation induces mitochondria-mediated apoptosis. The results are alarming for gain-of-function translational neurological studies but open the possibility to optogenetically manipulate the viability of non-excitable cells, such as cancer cells. In a second set of experiments we therefore evaluated the feasibility to put melanoma cell proliferation and apoptosis under the control of light by transdermally illuminating in vivo melanoma xenografts expressing ChR2(D156A). We show clear proof of principle that light treatment inhibits and even reverses tumor growth, rendering ChR2s potential tools for targeted light-therapy of cancers.

Light-based Approaches to Cardiac Arrhythmia Research: From Basic Science to Translational Applications

Light has long been used to image the heart, but now it can be used to modulate its electrophysiological function. Imaging modalities and techniques have long constituted an indispensable part of arrhythmia research and treatment. Recently, advances in the fields of optogenetics and photodynamic therapy have provided scientists with more effective approaches for probing, studying and potentially devising new treatments for cardiac arrhythmias. This article is a review of research toward the application of these techniques. It contains (a) an overview of advancements in technology and research that have contributed to light-based cardiac applications and (b) a summary of current and potential future applications of light-based control of cardiac cells, including modulation of heart rhythm, manipulation of cardiac action potential morphology, quantitative analysis of arrhythmias, defibrillation and cardiac ablation.

Optochemokine Tandem for Light-Control of Intracellular Ca2

An optochemokine tandem was developed to control the release of calcium from endosomes into the cytosol by light and to analyze the internalization kinetics of G-protein coupled receptors (GPCRs) by electrophysiology. A previously constructed rhodopsin tandem was re-engineered to combine the light-gated Ca²⁺-permeable cation channel Channelrhodopsin-2(L132C), CatCh, with the chemokine receptor CXCR4 in a functional tandem protein tCXCR4/CatCh. The GPCR was used as a shuttle protein to displace CatCh from the plasma membrane into intracellular areas. As shown by patch-clamp measurements and confocal laser scanning microscopy, heterologously expressed tCXCR4/CatCh was internalized via the endocytic SDF1/CXCR4 signaling pathway. The kinetics of internalization could be followed electrophysiologically via the amplitude of the CatCh signal. The light-induced release of Ca²⁺ by tandem endosomes into the cytosol via CatCh was visualized using the Ca²⁺-sensitive dyes rhod2 and rhod2-AM showing an increase of intracellular Ca²⁺ in response to light.

Optogenetic Control of Mouse Outer Hair Cells

Normal hearing in mammals depends on sound amplification by outer hair cells (OHCs) presumably by their somatic motility and force production. However, the role of OHC force production in cochlear amplification and frequency tuning are not yet fully understood. Currently, available OHC manipulation techniques for physiological or clinical studies are limited by their invasive nature, lack of precision, and poor temporal-spatial resolution. To overcome these limitations, we explored an optogenetic approach based on channelrhodopsin 2 (ChR-2), a direct light-activated nonselective cation channel originally discovered in Chlamydomonas reinhardtii. Three approaches were compared: 1) adeno-associated virus-mediated in utero transfer of the ChR-2 gene into the developing murine otocyst, 2) expression of ChR-2(H134R) in an auditory cell line (HEI-OC1), and 3) expression of ChR-2 in the OHCs of a mouse line carrying a ChR-2 conditional allele. Whole cell recording showed that blue light (470 nm) elicited the typical nonselective cation current of ChR-2 with reversal potential around zero in both mouse OHCs and HEI-OC1 cells and generated depolarization in both cell types. In addition, pulsed light stimulation (10 Hz) elicited a 1:1 repetitive depolarization and ChR-2 currents in mouse OHCs and HEI-OC1 cells, respectively. The time constant of depolarization in OHCs, 1.45 ms, is 10 times faster than HEI-OC1 cells, which allowed light stimulation up to rates of 10/s to elicit corresponding membrane potential changes. Our study demonstrates that ChR-2 can successfully be expressed in mouse OHCs and HEI-OC1 cells and that these present a typical light-sensitive current and depolarization. However, the amount of ChR-2 current induced in our in vivo experiments was insufficient to result in measurable cochlear effects.

Optogenetic and Chemogenetic Approaches for Studying Astrocytes and Gliotransmitters

The brain consists of heterogeneous populations of neuronal and non-neuronal cells. The revelation of their connections and interactions is fundamental to understanding normal brain functions as well as abnormal changes in pathological conditions. Optogenetics and chemogenetics have been developed to allow functional manipulations both in vitro and in vivo to examine causal relationships between cellular changes and functional outcomes. These techniques are based on genetically encoded effector molecules that respond exclusively to exogenous stimuli, such as a certain wavelength of light or a synthetic ligand. Activation of effector molecules provokes diverse intracellular changes, such as an influx or efflux of ions, depolarization or hyperpolarization of membranes, and activation of intracellular signaling cascades. Optogenetics and chemogenetics have been applied mainly to the study of neuronal circuits, but their use in studying non-neuronal cells has been gradually increasing. Here we introduce recent studies that have employed optogenetics and chemogenetics to reveal the function of astrocytes and gliotransmitters.

The Physiology of Fear: Reconceptualizing the Role of the Central Amygdala in Fear Learning

The historically understood role of the central amygdala (CeA) in fear learning is to serve as a passive output station for processing and plasticity that occurs elsewhere in the brain. However, recent research has suggested that the CeA may play a more dynamic role in fear learning. In particular, there is growing evidence that the CeA is a site of plasticity and memory formation, and that its activity is subject to tight regulation. The following review examines the evidence for these three main roles of the CeA as they relate to fear learning. The classical role of the CeA as a routing station to fear effector brain structures like the periaqueductal gray, the lateral hypothalamus, and paraventricular nucleus of the hypothalamus will be briefly reviewed, but specific emphasis is placed on recent literature suggesting that the CeA 1) has an important role in the plasticity underlying fear learning, 2) is involved in regulation of other amygdala subnuclei, and 3) is itself regulated by intra- and extra-amygdalar input. Finally, we discuss the parallels of human and mouse CeA involvement in fear disorders and fear conditioning, respectively.

Combining Optogenetics and Electrophysiology to Analyze Projection Neuron Circuits

A set of methods is described for channelrhodopsin-2 (ChR2)-based synaptic circuit analysis that combines photostimulation of virally transfected presynaptic neurons' axons with whole-cell electrophysiological recordings from retrogradely labeled postsynaptic neurons. The approach exploits the preserved photoexcitability of ChR2-expressing axons in brain slices and can be used to assess either local or long-range functional connections. Stereotaxic injections are used both to express ChR2 selectively in presynaptic axons of interest (using rabies virus [RV] or adeno-associated virus [AAV]) and to label two types of postsynaptic projection neurons of interest with fluorescent retrograde tracers. In brain slices, tracer-labeled postsynaptic neurons are targeted for whole-cell electrophysiological recordings, and synaptic connections are assessed by sampling voltage or current responses to light-emitting diode (LED) photostimulation of ChR2-expressing axons. The data are analyzed to estimate the relative amplitude of synaptic input and other connectivity parameters. Pharmacological and electrophysiological manipulations extend the versatility of the basic approach, allowing the dissection of monosynaptic versus disynaptic responses, excitatory versus inhibitory responses, and more. The method enables rapid, quantitative characterization of synaptic connectivity between defined pre- and postsynaptic classes of neurons.

The Road to Optogenetics: Microbial Rhodopsins

Optogenetics technology (using light-sensitive microbial proteins to control animal cell physiology) is becoming increasingly popular in laboratories around the world. Among these proteins, particularly important are rhodopsins that transport ions across the membrane and are used in optogenetics to regulate membrane potential by light, mostly in neurons. Although rhodopsin ion pumps transport only one charge per captured photon, channelrhodopsins are capable of more efficient passive transport. In this review, we follow the history of channelrhodopsin discovery in flagellate algae and discuss the latest addition to the channelrhodopsin family, channels with anion, rather than cation, selectivity.

Optogenetic stimulation of the cochlea-A review of mechanisms, measurements, and first models

This review evaluates the potential of optogenetic methods for the stimulation of the auditory nerve and assesses the feasability of optogenetic cochlear implants (CIs). It provides an overview of all critical steps like opsin targeting strategies, how opsins work, how their function can be modeled and included in neuronal models and the properties of light sources available for optical stimulation. From these foundations, quantitative estimates for the number of independent stimulation channels and the temporal precision of optogenetic stimulation of the auditory nerve are derived and compared with state-of-the-art electrical CIs. We conclude that optogenetic CIs have the potential to increase the number of independent stimulation channels by up to one order of magnitude to about 100, but only if light sources are able to deliver confined illumination patterns independently and parallelly. Already now, opsin variants like ChETA and Chronos enable driving of the auditory nerve up to rates of 200 spikes/s, close to the physiological value of their maximum sustained firing rate. Apart from requiring 10 times more energy than electrical stimulation, optical CIs still face major hurdles concerning the safety of gene transfection and optrode array implantation, for example, before becoming an option to replace electrical CIs.

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.

Biophysical Properties of Optogenetic Tools and Their Application for Vision Restoration Approaches

Optogenetics is the use of genetically encoded light-activated proteins to manipulate cells in a minimally invasive way using light. The most prominent example is channelrhodopsin-2 (ChR2), which allows the activation of electrically excitable cells via light-dependent depolarization. The combination of ChR2 with hyperpolarizing-light-driven ion pumps such as the Cl(-) pump halorhodopsin (NpHR) enables multimodal remote control of neuronal cells in culture, tissue, and living animals. Very soon, it became obvious that this method offers a chance of gene therapy for many diseases affecting vision. Here, we will give a brief introduction to retinal function and retinal diseases; optogenetic vision restoration strategies will be highlighted. We will discuss the functional and structural properties of rhodopsin-based optogenetic tools and analyze the potential for the application of vision restoration.

Maintaining ocular safety with light exposure, focusing on devices for optogenetic stimulation

Optogenetics methods are rapidly being developed as therapeutic tools for treating neurological diseases, in particular, retinal degenerative diseases. A critical component of the development is testing the safety of the light stimulation used to activate the optogenetic proteins. While the stimulation needs to be sufficient to produce neural responses in the targeted retinal cell class, it also needs to be below photochemical and photothermal limits known to cause ocular damage. The maximal permissible exposure is determined by a variety of factors, including wavelength, exposure duration, visual angle, pupil size, pulse width, pulse pattern, and repetition frequency. In this paper, we develop utilities to systematically and efficiently assess the contributions of these parameters in relation to the limits, following directly from the 2014 American National Standards Institute (ANSI). We also provide an array of stimulus protocols that fall within the bounds of both safety and effectiveness. Additional verification of safety is provided with a case study in rats using one of these protocols.

ChR2 transgenic animals in peripheral sensory system: Sensing light as various sensations

Since the introduction of Channelrhodopsin-2 (ChR2) to neuroscience, optogenetics technology was developed, making it possible to activate specific neurons or circuits with spatial and temporal precision. Various ChR2 transgenic animal models have been generated and are playing important roles in revealing the mechanisms of neural activities, mapping neural circuits, controlling the behaviors of animals as well as exploring new strategy for treating the neurological diseases in both central and peripheral nervous system. An animal including humans senses environments through Aristotle's five senses (sight, hearing, smell, taste and touch). Usually, each sense is associated with a kind of sensory organ (eyes, ears, nose, tongue and skin). Is it possible that one could hear light, smell light, taste light and touch light? When ChR2 is targeted to different peripheral sensory neurons by viral vectors or generating ChR2 transgenic animals, the animals can sense the light as various sensations such as hearing, touch, pain, smell and taste. In this review, we focus on ChR2 transgenic animals in the peripheral nervous system. Firstly the working principle of ChR2 as an optogenetic actuator is simply described. Then the current transgenic animal lines where ChR2 was expressed in peripheral sensory neurons are presented and the findings obtained by these animal models are reviewed.

The history and future of neural modeling for cochlear implants

This special issue of Network: Computation in Neural Systems on the topic of "Computational models of the electrically stimulated auditory system" incorporates review articles spanning a wide range of approaches to modeling cochlear implant stimulation of the auditory system. The purpose of this overview paper is to provide a historical context for the different modeling endeavors and to point toward how computational modeling could play a key role in the understanding, evaluation, and improvement of cochlear implants in the future.

Enhancing Channelrhodopsins: An Overview

After the discovery of Channelrhodopsin, a light-gated ion channel, only a few people saw the diverse range of applications for such a protein. Now, more than 10 years later Channelrhodopsins have become widely accepted as the ultimate tool to control the membrane potential of excitable cells via illumination. The demand for more application-specific Channelrhodopsin variants started a race between protein engineers to design improved variants. Even though many engineered variants have undisputable advantages compared to wild-type variants, many users are alienated by the tremendous amount of new variants and their perplexing names. Here, we review new variants whose efficacy has already been proven in neurophysiological experiments, or variants which are likely to extend the optogenetic toolbox. Variants are described based on their mechanistic and operational properties in terms of expression, kinetics, ion selectivity, and wavelength responsivity.

Optogenetics: Basic Concepts and Their Development

The discovery of light-gated ion channels and their application to controlling neural activities have had a transformative impact on the field of neuroscience. In recent years, the concept of using light-activated proteins to control biological processes has greatly diversified into other fields, driven by the natural diversity of photoreceptors and decades of knowledge obtained from their biophysical characterization. In this chapter, we will briefly discuss the origin and development of optogenetics and highlight the basic concepts that make it such a powerful technology. We will review how these enabling concepts have developed over the past decade, and discuss future perspectives.

Selective keratinocyte stimulation is sufficient to evoke nociception in mice

The skin epidermis is densely innervated by peripheral sensory nerve endings. Nociceptive neurons, whose terminals are in close contact with epidermal keratinocytes, can be activated directly by noxious physical and chemical stimuli to trigger pain. However, whether keratinocytes can signal acutely to sensory nerve terminals to initiate pain in vivo remains unclear. Here, using the keratin 5 promoter to selectively express the capsaicin receptor TRPV1 in keratinocytes of TRPV1-knockout mice, we achieved specific stimulation of keratinocytes with capsaicin. Using this approach, we found that keratinocyte stimulation was sufficient to induce strong expression of the neuronal activation marker, c-fos, in laminae I and II of the ipsilateral spinal cord dorsal horn and to evoke acute paw-licking nocifensive behavior and conditioned place aversion. These data provide direct evidence that keratinocyte stimulation is sufficient to evoke acute nociception-related responses.

Optogenetic Approaches to Restoring Vision

Severe loss of photoreceptor cells in inherited or acquired retinal degenerative diseases can result in partial loss of sight or complete blindness. The optogenetic strategy for restoration of vision utilizes optogenetic tools to convert surviving inner retinal neurons into photosensitive cells; thus, light sensitivity is imparted to the retina after the death of photoreceptor cells. Proof-of-concept studies, especially those using microbial rhodopsins, have demonstrated restoration of light responses in surviving retinal neurons and visually guided behaviors in animal models. Significant progress has also been made in improving microbial rhodopsin-based optogenetic tools, developing virus-mediated gene delivery, and targeting specific retinal neurons and subcellular compartments of retinal ganglion cells. In this article, we review the current status of the field and outline further directions and challenges to the advancement of this strategy toward clinical application and improvement in the outcomes of restored vision.

Optogenetic tools for modulating and probing the epileptic network

Epilepsy affects roughly 1% of the population worldwide. Although effective treatments with antiepileptic drugs are available, more than 20% of patients have seizures that are refractory to medical therapy and many patients experience adverse effects. Hence, there is a continued need for novel therapies for those patients. A new technique called "optogenetics" may offer a new hope for these refractory patients. Optogenetics is a technology based on the combination of optics and genetics, which can control or record neural activity with light. Following delivery of light-sensitive opsin genes such as channelrhodopsin-2 (ChR2), halorhodopsin (NpHR), and others into brain, excitation or inhibition of specific neurons in precise brain areas can be controlled by illumination at different wavelengths with very high temporal and spatial resolution. Neuromodulation with the optogenetics toolbox have already been shown to be effective at treating seizures in animal models of epilepsy. This review will outline the most recent advances in epilepsy research with optogenetic techniques and discuss how this technology can contribute to our understanding and treatment of epilepsy in the future.

Pulse trains to percepts: the challenge of creating a perceptually intelligible world with sight recovery technologies

An extraordinary variety of sight recovery therapies are either about to begin clinical trials, have begun clinical trials, or are currently being implanted in patients. However, as yet we have little insight into the perceptual experience likely to be produced by these implants. This review focuses on methodologies, such as optogenetics, small molecule photoswitches and electrical prostheses, which use artificial stimulation of the retina to elicit percepts. For each of these technologies, the interplay between the stimulating technology and the underlying neurophysiology is likely to result in distortions of the perceptual experience. Here, we describe some of these potential distortions and discuss how they might be minimized either through changes in the encoding model or through cortical plasticity.