An improved chloride-conducting channelrhodopsin for light-induced inhibition of neuronal activity in vivo

Optogenetic control of neural activity: The biophysics of microbial rhodopsins in neuroscience

Optogenetics, the use of microbial rhodopsins to make the electrical activity of targeted neurons controllable by light, has swept through neuroscience, enabling thousands of scientists to study how specific neuron types contribute to behaviors and pathologies, and how they might serve as novel therapeutic targets. By activating a set of neurons, one can probe what functions they can initiate or sustain, and by silencing a set of neurons, one can probe the functions they are necessary for. We here review the biophysics of these molecules, asking why they became so useful in neuroscience for the study of brain circuitry. We review the history of the field, including early thinking, early experiments, applications of optogenetics, pre-optogenetics targeted neural control tools, and the history of discovering and characterizing microbial rhodopsins. We then review the biophysical attributes of rhodopsins that make them so useful to neuroscience - their classes and structure, their photocycles, their photocurrent magnitudes and kinetics, their action spectra, and their ion selectivity. Our hope is to convey to the reader how specific biophysical properties of these molecules made them especially useful to neuroscientists for a difficult problem - the control of high-speed electrical activity, with great precision and ease, in the brain.

Two Ascending Thermosensory Pathways from the Lateral Parabrachial Nucleus That Mediate Behavioral and Autonomous Thermoregulation

Thermoregulatory behavior in homeothermic animals is an innate behavior to defend body core temperature from environmental thermal challenges in coordination with autonomous thermoregulatory responses. In contrast to the progress in understanding the central mechanisms of autonomous thermoregulation, those of behavioral thermoregulation remain poorly understood. We have previously shown that the lateral parabrachial nucleus (LPB) mediates cutaneous thermosensory afferent signaling for thermoregulation. To understand the thermosensory neural network for behavioral thermoregulation, in the present study, we investigated the roles of ascending thermosensory pathways from the LPB in avoidance behavior from innocuous heat and cold in male rats. Neuronal tracing revealed two segregated groups of LPB neurons projecting to the median preoptic nucleus (MnPO), a thermoregulatory center (LPB→MnPO neurons), and those projecting to the central amygdaloid nucleus (CeA), a limbic emotion center (LPB→CeA neurons). While LPB→MnPO neurons include separate subgroups activated by heat or cold exposure of rats, LPB→CeA neurons were only activated by cold exposure. By selectively inhibiting LPB→MnPO or LPB→CeA neurons using tetanus toxin light chain or chemogenetic or optogenetic techniques, we found that LPB→MnPO transmission mediates heat avoidance, whereas LPB→CeA transmission contributes to cold avoidance. In vivo electrophysiological experiments showed that skin cooling-evoked thermogenesis in brown adipose tissue requires not only LPB→MnPO neurons but also LPB→CeA neurons, providing a novel insight into the central mechanism of autonomous thermoregulation. Our findings reveal an important framework of central thermosensory afferent pathways to coordinate behavioral and autonomous thermoregulation and to generate the emotions of thermal comfort and discomfort that drive thermoregulatory behavior.SIGNIFICANCE STATEMENT Coordination of behavioral and autonomous thermoregulation is important for maintaining thermal homeostasis in homeothermic animals. However, the central mechanism of thermoregulatory behaviors remains poorly understood. We have previously shown that the lateral parabrachial nucleus (LPB) mediates ascending thermosensory signaling that drives thermoregulatory behavior. In this study, we found that one pathway from the LPB to the median preoptic nucleus mediates heat avoidance, whereas the other pathway from the LPB to the central amygdaloid nucleus is required for cold avoidance. Surprisingly, both pathways are required for skin cooling-evoked thermogenesis in brown adipose tissue, an autonomous thermoregulatory response. This study provides a central thermosensory network that coordinates behavioral and autonomous thermoregulation and generates thermal comfort and discomfort that drive thermoregulatory behavior.

Pump-like channelrhodopsins: Not just bridging the gap between ion pumps and ion channels

Channelrhodopsins are microbial rhodopsins that work as light-gated ion channels. Their importance has become increasingly recognized due to their ability to control the membrane potential of specific cells in a light-dependent manner. This technology, termed optogenetics, has revolutionized neuroscience, and numerous channelrhodopsin variants have been isolated or engineered to expand the utility of optogenetics. Pump-like channelrhodopsins (PLCRs), one of the recently discovered channelrhodopsin subfamilies, have attracted broad attention due to their high sequence similarity to ion-pumping rhodopsins and their distinct properties, such as high light sensitivity and ion selectivity. In this review, we summarize the current understanding of the structure-function relationships of PLCRs and discuss the challenges and opportunities of channelrhodopsin research.

Potassium-selective channelrhodopsins

Since their discovery 21 years ago, channelrhodopsins have come of age and have become indispensable tools for optogenetic control of excitable cells such as neurons and myocytes. Potential therapeutic utility of channelrhodopsins has been proven by partial vision restoration in a human patient. Previously known channelrhodopsins are either proton channels, non-selective cation channels almost equally permeable to Na+ and K+ besides protons, or anion channels. Two years ago, we discovered a group of channelrhodopsins that exhibit over an order of magnitude higher selectivity for K+ than for Na+. These proteins, known as "kalium channelrhodopsins" or KCRs, lack the canonical tetrameric selectivity filter found in voltage- and ligand-gated K+ channels, and use a unique selectivity mechanism intrinsic to their individual protomers. Mutant analysis has revealed that the key residues responsible for K+ selectivity in KCRs are located at both ends of the putative cation conduction pathway, and their role has been confirmed by high-resolution KCR structures. Expression of KCRs in mouse neurons and human cardiomyocytes enabled optical inhibition of these cells' electrical activity. In this minireview we briefly discuss major results of KCR research obtained during the last two years and suggest some directions of future research.

Tailoring baker's yeast Saccharomyces cerevisiae for functional testing of channelrhodopsin

Channelrhodopsin 2 (ChR2) and its variants are the most frequent tools for remote manipulation of electrical properties in cells via light. Ongoing attempts try to enlarge their functional spectrum with respect to ion selectivity, light sensitivity and protein trafficking by mutations, protein engineering and environmental mining of ChR2 variants. A shortcoming in the required functional testing of large numbers of ChR2 variants is the lack of an easy screening system. Baker's yeast, which was successfully employed for testing ion channels from eukaryotes has not yet been used for screening of ChR2s, because they neither produce the retinal chromophore nor its precursor carotenoids. We found that addition of retinal to the external medium was not sufficient for detecting robust ChR activity in yeast in simple growth assays. This obstacle was overcome by metabolic engineering of a yeast strain, which constitutively produces retinal. In proof of concept experiments we functionally express different ChR variants in these cells and monitor their blue light induced activity in simple growth assays. We find that light activation of ChR augments an influx of Na+ with a consequent inhibition of cell growth. In a K+ uptake deficient yeast strain, growth can be rescued in selective medium by the blue light induced K+ conductance of ChR. This yeast strain can now be used as chassis for screening of new functional ChR variants and mutant libraries in simple yeast growth assays under defined selective conditions.

Parabrachial-to-parasubthalamic nucleus pathway mediates fear-induced suppression of feeding in male mice

Feeding behavior is adaptively regulated by external and internal environment, such that feeding is suppressed when animals experience pain, sickness, or fear. While the lateral parabrachial nucleus (lPB) plays key roles in nociception and stress, neuronal pathways involved in feeding suppression induced by fear are not fully explored. Here, we investigate the parasubthalamic nucleus (PSTN), located in the lateral hypothalamus and critically involved in feeding behaviors, as a target of lPB projection neurons. Optogenetic activation of lPB-PSTN terminals in male mice promote avoidance behaviors, aversive learning, and suppressed feeding. Inactivation of the PSTN and lPB-PSTN pathway reduces fear-induced feeding suppression. Activation of PSTN neurons expressing pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide enriched in the PSTN, is sufficient for inducing avoidance behaviors and feeding suppression. Blockade of PACAP receptors impaires aversive learning induced by lPB-PSTN photomanipulation. These findings indicate that lPB-PSTN pathway plays a pivotal role in fear-induced feeding suppression.

ΔFosB accumulation in hippocampal granule cells drives cFos pattern separation during spatial learning

Mice display signs of fear when neurons that express cFos during fear conditioning are artificially reactivated. This finding gave rise to the notion that cFos marks neurons that encode specific memories. Here we show that cFos expression patterns in the mouse dentate gyrus (DG) change dramatically from day to day in a water maze spatial learning paradigm, regardless of training level. Optogenetic inhibition of neurons that expressed cFos on the first training day affected performance days later, suggesting that these neurons continue to be important for spatial memory recall. The mechanism preventing repeated cFos expression in DG granule cells involves accumulation of ΔFosB, a long-lived splice variant of FosB. CA1 neurons, in contrast, repeatedly expressed cFos. Thus, cFos-expressing granule cells may encode new features being added to the internal representation during the last training session. This form of timestamping is thought to be required for the formation of episodic memories.

Contractile Force of Transplanted Cardiomyocytes Actively Supports Heart Function After Injury

BACKGROUND

Transplantation of pluripotent stem cell-derived cardiomyocytes represents a promising therapeutic strategy for cardiac regeneration, and the first clinical studies in patients with heart failure have commenced. Yet, little is known about the mechanism of action underlying graft-induced benefits. Here, we explored whether transplanted cardiomyocytes actively contribute to heart function.

METHODS

We injected cardiomyocytes with an optogenetic off-on switch in a guinea pig cardiac injury model.

RESULTS

Light-induced inhibition of engrafted cardiomyocyte contractility resulted in a rapid decrease of left ventricular function in ≈50% (7/13) animals that was fully reversible with the offset of photostimulation.

CONCLUSIONS

Our optogenetic approach demonstrates that transplanted cardiomyocytes can actively participate in heart function, supporting the hypothesis that the delivery of new force-generating myocardium can serve as a regenerative therapeutic strategy.

Aion is a bistable anion-conducting channelrhodopsin that provides temporally extended and reversible neuronal silencing

Optogenetic silencing allows to reveal the necessity of selected neuronal populations for various neurophysiological functions. These range from synaptic transmission and coordinated neuronal network activity to control of specific behaviors. An ideal single-component optogenetic silencing tool should be switchable between active and inactive states with precise timing while preserving its activity in the absence of light until switched to an inactive state. Although bistable anion-conducting channelrhodopsins (ACRs) were previously engineered to reach this goal, their conducting state lifetime was limited to only a few minutes and some ACRs were not fully switchable. Here we report Aion, a bistable ACR displaying a long-lasting open state with a spontaneous closing time constant close to 15 min. Moreover, Aion can be switched between the open and closed state with millisecond precision using blue and orange light, respectively. The long conducting state enables overnight silencing of neurons with minimal light exposure. We further generated trafficking-optimized versions of Aion, which show enhanced membrane localization and allow precisely timed, long-lasting all-optical control of nociceptive responses in larvae of Drosophila melanogaster. Thus, Aion is an optogenetic silencing tool for inhibition of neuronal activity over many hours which can be switched between an active and inactive state with millisecond precision.

Aion is an anion-conducting, bistable channelrhodopsin that enables long-term silencing of neuronal networks, as demonstrated in organotypic hippocampal cultures and Drosophila melanogaster larvae.

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

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.

Advancements in the Quest to Map, Monitor, and Manipulate Neural Circuitry

Neural circuits and the cells that comprise them represent the functional units of the brain. Circuits relay and process sensory information, maintain homeostasis, drive behaviors, and facilitate cognitive functions such as learning and memory. Creating a functionally-precise map of the mammalian brain requires anatomically tracing neural circuits, monitoring their activity patterns, and manipulating their activity to infer function. Advancements in cell-type-specific genetic tools allow interrogation of neural circuits with increased precision. This review provides a broad overview of recombination-based and activity-driven genetic targeting approaches, contemporary viral tracing strategies, electrophysiological recording methods, newly developed calcium, and voltage indicators, and neurotransmitter/neuropeptide biosensors currently being used to investigate circuit architecture and function. Finally, it discusses methods for acute or chronic manipulation of neural activity, including genetically-targeted cellular ablation, optogenetics, chemogenetics, and over-expression of ion channels. With this ever-evolving genetic toolbox, scientists are continuing to probe neural circuits with increasing resolution, elucidating the structure and function of the incredibly complex mammalian brain.

In Utero Electroporation for Manipulation of Specific Neuronal Populations

The complexity of brain functions is supported by the heterogeneity of brain tissue and millisecond-scale information processing. Understanding how complex neural circuits control animal behavior requires the precise manipulation of specific neuronal subtypes at high spatiotemporal resolution. In utero electroporation, when combined with optogenetics, is a powerful method for precisely controlling the activity of specific neurons. Optogenetics allows for the control of cellular membrane potentials through light-sensitive ion channels artificially expressed in the plasma membrane of neurons. Here, we first review the basic mechanisms and characteristics of in utero electroporation. Then, we discuss recent applications of in utero electroporation combined with optogenetics to investigate the functions and characteristics of specific regions, layers, and cell types. These techniques will pave the way for further advances in understanding the complex neuronal and circuit mechanisms that underlie behavioral outputs.

Emerging Diversity of Channelrhodopsins and Their Structure-Function Relationships

Cation and anion channelrhodopsins (CCRs and ACRs, respectively) from phototactic algae have become widely used as genetically encoded molecular tools to control cell membrane potential with light. Recent advances in polynucleotide sequencing, especially in environmental samples, have led to identification of hundreds of channelrhodopsin homologs in many phylogenetic lineages, including non-photosynthetic protists. Only a few CCRs and ACRs have been characterized in detail, but there are indications that ion channel function has evolved within the rhodopsin superfamily by convergent routes. The diversity of channelrhodopsins provides an exceptional platform for the study of structure-function evolution in membrane proteins. Here we review the current state of channelrhodopsin research and outline perspectives for its further development.

Single-Cell Resolution Optogenetics Via Expression of Soma-Targeted Rhodopsins

Optogenetics allows control of neural activity in genetically targeted neuron populations by light. Optogenetic control of individual neurons in neural circuits would enable powerful, causal investigations of neural connectivity and function at single-cell level and provide insights into how neural circuits operate. Such single-cell resolution optogenetics in neuron populations requires precise sculpting of light and subcellular targeting of optogenetic molecules. Here we describe a group of methods for single-cell resolution optogenetics in neuron cultures, in mouse brain slices, and in mouse cortex in-vivo, via patterned light and soma-targeted optogenetic molecules.

Cation and Anion Channelrhodopsins: Sequence Motifs and Taxonomic Distribution

Cation and anion channelrhodopsins (CCRs and ACRs, respectively) primarily from two algal species, Chlamydomonas reinhardtii and Guillardia theta, have become widely used as optogenetic tools to control cell membrane potential with light. We mined algal and other protist polynucleotide sequencing projects and metagenomic samples to identify 75 channelrhodopsin homologs from four channelrhodopsin families, including one revealed in dinoflagellates in this study. We carried out electrophysiological analysis of 33 natural channelrhodopsin variants from different phylogenetic lineages and 10 metagenomic homologs in search of sequence determinants of ion selectivity, photocurrent desensitization, and spectral tuning in channelrhodopsins. Our results show that association of a reduced number of glutamates near the conductance path with anion selectivity depends on a wider protein context, because prasinophyte homologs with a glutamate pattern identical to that in cryptophyte ACRs are cation selective. Desensitization is also broadly context dependent, as in one branch of stramenopile ACRs and their metagenomic homologs, its extent roughly correlates with phylogenetic relationship of their sequences. Regarding spectral tuning, we identified two prasinophyte CCRs with red-shifted spectra to 585 nm. They exhibit a third residue pattern in their retinal-binding pockets distinctly different from those of the only two types of red-shifted channelrhodopsins known (i.e., the CCR Chrimson and RubyACRs). In cryptophyte ACRs we identified three specific residue positions in the retinal-binding pocket that define the wavelength of their spectral maxima. Lastly, we found that dinoflagellate rhodopsins with a TCP motif in the third transmembrane helix and a metagenomic homolog exhibit channel activity.

Targeted Neurostimulation in Mouse Brains with Non-invasive Ultrasound

Recently developed brain stimulation techniques have significantly advanced our ability to manipulate the brain's function. However, stimulating specific neurons in a desired region without significant surgical invasion remains a challenge. Here, we demonstrate a neuron-specific and region-targeted neural excitation strategy using non-invasive ultrasound through activation of heterologously expressed mechanosensitive ion channels (MscL-G22S). Low-intensity ultrasound is significantly better at inducing Ca²⁺ influx and neuron activation in vitro and at evoking electromyogram (EMG) responses in vivo in targeted cells expressing MscL-G22S. Neurons in the cerebral cortex or dorsomedial striatum of mice are made to express MscL-G22S and stimulated ultrasonically. We find significant upregulation of c-Fos in neuron nuclei only in the regions expressing MscL-G22S compared with the non-MscL controls, as well as in various other regions in the same brain. Thus, we detail an effective approach for activating specific regions and cell types in intact mouse brains by sensitizing them to ultrasound using a mechanosensitive ion channel.

Attenuation of Hippocampal Evoked Potentials in vivo by Activation of GtACR2, an Optogenetic Chloride Channel

AIM

GtACR2, a light-activated chloride channel, is an attractive tool for neural inhibition as it can shunt membrane depolarizations. In this study, we assessed the effect of activating GtACR2 on in vivo hippocampal CA1 activity evoked by Schaffer collateral (SC) stimulation.

METHODS

Adult male Wistar rats were unilaterally injected with 0.5 μL of adeno associated viral vector for induction of GtACR2-mCherry (n = 10, GtACR2 group) or mCherry (n = 4, Sham group) expression in CA1 pyramidal neurons of the hippocampus. Three weeks later, evoked potentials (EPs) were recorded from the CA1 subfield placing an optrode (bipolar recording electrode attached to an optic fiber) at the injection site and a stimulation electrode targeting SCs. Effects of illumination parameters required to activate GtACR2 such as light power densities (LPDs), illumination delays, and light-pulse durations were tested on CA1 EP parameters [population spike (PS) amplitude and field excitatory postsynaptic potential (fEPSP) slope].

RESULTS

In the GtACR2 group, delivery of a 10 ms light-pulse induced a negative deflection in the local field potential which increased with increasing LPD. When combined with electrical stimulation of the SCs, light-induced activation of GtACR2 had potent inhibitory effects on CA1 EPs. An LPD of 160 mW/mm2 was sufficient to obtain maximal inhibition CA1 EPs. To quantify the duration of the inhibitory effect, a 10 ms light-pulse of 160 mW/mm2 was delivered at increasing delays before the CA1 EPs. Inhibition of EPs was found to last up to 9 ms after the cessation of the light-pulse. Increasing light-pulse durations beyond 10 ms did not result in larger inhibitory effects.

CONCLUSION

Precisely timed activation of GtACR2 potently blocks evoked activity of CA1 neurons. The strength of inhibition depends on LPD, lasts up to 9 ms after a light-pulse of 10 ms, and is independent of the duration of the light-pulse given.

Freeze-Frame Imaging of Dendritic Calcium Signals With TubuTag

The extensive dendritic arbor of neurons is thought to be actively involved in the processing of information. Dendrites contain a rich diversity of ligand- and voltage-activated ion channels as well as metabotropic receptors. In addition, they are capable of releasing calcium from intracellular stores. Under specific conditions, large neurons produce calcium spikes that are locally restricted to a dendritic section. To investigate calcium signaling in dendrites, we introduce TubuTag, a genetically encoded ratiometric calcium sensor anchored to the cytoskeleton. TubuTag integrates cytoplasmic calcium signals by irreversible photoconversion from green to red fluorescence when illuminated with violet light. We used a custom two-photon microscope with a large field of view to image pyramidal neurons in CA1 at subcellular resolution. Photoconversion was strongest in the most distal parts of the apical dendrite, suggesting a gradient in the amplitude of dendritic calcium signals. As the read-out of fluorescence can be performed several hours after photoconversion, TubuTag will help investigating dendritic signal integration and calcium homeostasis in large populations of neurons.

Structure-Function Relationship of Channelrhodopsins

Ion-translocating rhodopsins, especially channelrhodopsins (ChRs), have attracted broad attention as a powerful tool to modulate the membrane potential of cells with light (optogenetics). Because of recent biophysical, spectroscopic, and computational studies, including the structural determination of cation and anion ChRs, our understanding of the molecular mechanism underlying light-gated ion conduction has been greatly advanced. In this chapter, I first describe the background of rhodopsin family proteins including ChR, and how the optogenetics technology has been established from the discovery of first ChR in 2002. I later introduce the recent findings of the structure-function relationship of ChR by comparing the crystal structures of cation and anion ChRs. I further discuss the future goal in the fields of ChR research and optogenetic tool development.

Bioluminescence-Optogenetics

In this chapter, we introduce a relatively new, emerging method for molecular neuromodulation-bioluminescence-optogenetics. Bioluminescence-optogenetics is mediated by luminopsin fusion proteins-light-sensing opsins fused to light-emitting luciferases. We describe their structures and working mechanisms and discuss their unique benefits over conventional optogenetics and chemogenetics. We also summarize applications of bioluminescence-optogenetics in various neurological disease models in rodents.

Optogenetic Modulation of Ion Channels by Photoreceptive Proteins

In these 15 years, researches to control cellular responses by light have flourished dramatically to establish "optogenetics" as a research field. In particular, light-dependent excitation/inhibition of neural cells using channelrhodopsins or other microbial rhodopsins is the most powerful and the most widely used optogenetic technique. New channelrhodopsin-based optogenetic tools having favorable characteristics have been identified from a wide variety of organisms or created through mutagenesis. Despite the great efforts, some neuronal activities are still hard to be manipulated by the channelrhodopsin-based tools, indicating that complementary approaches are needed to make optogenetics more comprehensive. One of the feasible and complementary approaches is optical control of ion channels using photoreceptive proteins other than channelrhodopsins. In particular, animal opsins can modulate various ion channels via light-dependent G protein activation. In this chapter, we summarize how such alternative optogenetic tools work and they will be improved.

Functional interrogation of neural circuits with virally transmitted optogenetic tools

The vertebrate brain comprises a plethora of cell types connected by intertwined pathways. Optogenetics enriches the neuroscientific tool set for disentangling these neuronal circuits in a manner which exceeds the spatio-temporal precision of previously existing techniques. Technically, optogenetics can be divided in three types of optical and genetic combinations: (1) it is primarily understood as the manipulation of the activity of genetically modified cells (typically neurons) with light, i.e. optical actuators. (2) A second combination refers to visualizing the activity of genetically modified cells (again typically neurons), i.e. optical sensors. (3) A completely different interpretation of optogenetics refers to the light activated expression of a genetically induced construct. Here, we focus on the first two types of optogenetics, i.e. the optical actuators and sensors in an attempt to give an overview into the topic. We first cover methods to express opsins into neurons and introduce strategies of targeting specific neuronal populations in different animal species. We then summarize combinations of optogenetics with behavioral read out and neuronal imaging. Finally, we give an overview of the current state-of-the-art and an outlook on future perspectives.

Challenges for Therapeutic Applications of Opsin-Based Optogenetic Tools in Humans

As the technological hurdles are overcome and optogenetic techniques advance to have more control over neurons, therapies based on these approaches will begin to emerge in the clinic. Here, we consider the technical challenges surrounding the transition of this breakthrough technology from an investigative tool to a true therapeutic avenue. The emerging strategies and remaining tasks surrounding genetically encoded molecules which respond to light as well as the vehicles required to deliver them are discussed.The use of optogenetics in humans would represent a completely new paradigm in medicine and would be associated with unprecedented technical considerations. To be applied for stimulation of neurons in humans, an ideal optogenetic tool would need to be non-immunogenic, highly sensitive, and activatable with red light or near-infrared light (to maximize light penetration while minimizing photodamage). To enable sophisticated levels of neuronal control, the combined use of optogenetic actuators and indicators could enable closed-loop all-optical neuromodulation. Such systems would introduce additional challenges related to spectral orthogonality between actuator and indicator, the need for decision making computational algorithms and requirements for large gene cassettes. As in any gene therapy, the therapeutic efficiency of optogenetics will rely on vector delivery and expression in the appropriate cell type. Although viral vectors such as those based on AAVs are showing great potential in human trials, barriers to their general use remain, including immune responses, delivery/transport, and liver clearance. Limitations associated with the gene cassette size which can be packaged in currently approved vectors also need to be addressed.

Light Up the Brain: The Application of Optogenetics in Cell-Type Specific Dissection of Mouse Brain Circuits

The exquisite intricacies of neural circuits are fundamental to an animal’s diverse and complex repertoire of sensory and motor functions. The ability to precisely map neural circuits and to selectively manipulate neural activity is critical to understanding brain function and has, therefore been a long-standing goal for neuroscientists. The recent development of optogenetic tools, combined with transgenic mouse lines, has endowed us with unprecedented spatiotemporal precision in circuit analysis. These advances greatly expand the scope of tractable experimental investigations. Here, in the first half of the review, we will present applications of optogenetics in identifying connectivity between different local neuronal cell types and of long-range projections with both in vitro and in vivo methods. We will then discuss how these tools can be used to reveal the functional roles of these cell-type specific connections in governing sensory information processing, and learning and memory in the visual cortex, somatosensory cortex, and motor cortex. Finally, we will discuss the prospect of new optogenetic tools and how their application can further advance modern neuroscience. In summary, this review serves as a primer to exemplify how optogenetics can be used in sophisticated modern circuit analyses at the levels of synapses, cells, network connectivity and behaviors.

A central master driver of psychosocial stress responses in the rat

The mechanism by which psychological stress elicits various physiological responses is unknown. We discovered a central master neural pathway in rats that drives autonomic and behavioral stress responses by connecting the corticolimbic stress circuits to the hypothalamus. Psychosocial stress signals from emotion-related forebrain regions activated a VGLUT1-positive glutamatergic pathway from the dorsal peduncular cortex and dorsal tenia tecta (DP/DTT), an unexplored prefrontal cortical area, to the dorsomedial hypothalamus (DMH), a hypothalamic autonomic center. Genetic ablation and optogenetics revealed that the DP/DTT→DMH pathway drives thermogenic, hyperthermic, and cardiovascular sympathetic responses to psychosocial stress without contributing to basal homeostasis. This pathway also mediates avoidance behavior from psychosocial stressors. Given the variety of stress responses driven by the DP/DTT→DMH pathway, the DP/DTT can be a potential target for treating psychosomatic disorders.

Molecular Dynamics Simulation of Transmembrane Transport of Chloride Ions in Mutants of Channelrhodopsin

Channelrhodopsins (ChRs) are light-gated transmembrane cation channels which are widely used for optogenetic technology. Replacing glutamate located at the central gate of the ion channel with positively charged amino acid residues will reverse ion selectivity and allow anion conduction. The structures and properties of the ion channel, the transport of chloride, and potential of mean force (PMF) of the chimera protein (C1C2) and its mutants, EK-TC, ER-TC and iChloC, were investigated by molecular dynamics simulation. The results show that the five-fold mutation in E122Q-E129R-E140S-D195N-T198C (iChloC) increases the flexibility of the transmembrane channel protein better than the double mutations in EK-TC and ER-TC, and results in an expanded ion channel pore size and decreased steric resistance. The iChloC mutant was also found to have a higher affinity for chloride ions and, based on surface electrostatic potential analysis, provides a favorable electrostatic environment for anion conduction. The PMF free energy curves revealed that high affinity Cl^- binding sites are generated near the central gate of the three mutant proteins. The energy barriers for the EK-TC and ER-TC were found to be much higher than that of iChloC. The results suggest that the transmembrane ion channel of iChloC protein is better at facilitating the capture and transport of chloride ions.

Optimized photo-stimulation of halorhodopsin for long-term neuronal inhibition

Background

Optogenetic silencing techniques have expanded the causal understanding of the functions of diverse neuronal cell types in both the healthy and diseased brain. A widely used inhibitory optogenetic actuator is eNpHR3.0, an improved version of the light-driven chloride pump halorhodopsin derived from Natronomonas pharaonis. A major drawback of eNpHR3.0 is related to its pronounced inactivation on a time-scale of seconds, which renders it unsuited for applications that require long-lasting silencing.

Results

Using transgenic mice and Xenopus laevis oocytes expressing an eNpHR3.0-EYFP fusion protein, we here report optimized photo-stimulation techniques that profoundly increase the stability of eNpHR3.0-mediated currents during long-term photo-stimulation. We demonstrate that optimized photo-stimulation enables prolonged hyperpolarization and suppression of action potential discharge on a time-scale of minutes.

Conclusions

Collectively, our findings extend the utility of eNpHR3.0 to the long-lasting inhibition of excitable cells, thus facilitating the optogenetic dissection of neural circuits.

Sub-millisecond Control of Neuronal Firing by Organic Light-Emitting Diodes

Optogenetics combines optics and genetics to enable minimally invasive cell-type-specific stimulation in living tissue. For the purposes of bio-implantation, there is a need to develop soft, flexible, transparent and highly biocompatible light sources. Organic semiconducting materials have key advantages over their inorganic counterparts, including low Young's moduli, high strain resistances, and wide color tunability. However, until now it has been unclear whether organic light emitting diodes (OLEDs) are capable of providing sufficient optical power for successful neuronal stimulation, while still remaining within a biologically acceptable temperature range. Here we investigate the use of blue polyfluorene- and orange poly(p-phenylenevinylene)-based OLEDs as stimuli for blue-light-activated Sustained Step Function Opsin (SFFO) and red-light-activated ChrimsonR opsin, respectively. We show that, when biased using high frequency (multi-kHz) drive schemes, the OLEDs permit safe and controlled photostimulation of opsin-expressing neurons and were able to control neuronal firing with high temporal-resolution at operating temperatures lower than previously demonstrated.

MerMAIDs: a family of metagenomically discovered marine anion-conducting and intensely desensitizing channelrhodopsins

Channelrhodopsins (ChRs) are algal light-gated ion channels widely used as optogenetic tools for manipulating neuronal activity. ChRs desensitize under continuous bright-light illumination, resulting in a significant decline of photocurrents. Here we describe a metagenomically identified family of phylogenetically distinct anion-conducting ChRs (designated MerMAIDs). MerMAIDs almost completely desensitize during continuous illumination due to accumulation of a late non-conducting photointermediate that disrupts the ion permeation pathway. MerMAID desensitization can be fully explained by a single photocycle in which a long-lived desensitized state follows the short-lived conducting state. A conserved cysteine is the critical factor in desensitization, as its mutation results in recovery of large stationary photocurrents. The rapid desensitization of MerMAIDs enables their use as optogenetic silencers for transient suppression of individual action potentials without affecting subsequent spiking during continuous illumination. Our results could facilitate the development of optogenetic tools from metagenomic databases and enhance general understanding of ChR function.

Monosynaptic Hippocampal-Prefrontal Projections Contribute to Spatial Memory Consolidation in Mice

Time locking between neocortical sleep slow oscillations, thalamo-cortical spindles, and hippocampal sharp-wave ripples has convincingly been shown to be a key element of systems consolidation. Here we investigate the role of monosynaptic projections from ventral/intermediate hippocampus to medial prefrontal cortex (mPFC) in sleep-dependent memory consolidation in male mice. Following acquisition learning in the Barnes maze, we optogenetically silenced the axonal terminals of hippocampal projections within mPFC during slow-wave sleep. This silencing during SWS selectively impaired recent but not remote memory in the absence of effects on error rate and escape latencies. Furthermore, it prevented the development of the most efficient search strategy and sleep spindle time-locking to slow oscillation. An increase in post-learning sleep sharp-wave ripple (SPWR) density and reduced time locking of learning-associated SPWR activity to sleep spindles may be a less specific response. Our results demonstrate that monosynaptic projections from hippocampus to mPFC contribute to sleep-dependent memory consolidation, potentially by affecting the temporal coupling of sleep-associated electrophysiological events.SIGNIFICANCE STATEMENT Convincing evidence supports the role of slow-wave sleep (SWS), and the relevance of close temporal coupling of neuronal activity between brain regions for systems consolidation. Less attention has been paid so far to the specific neuronal pathways underlying these processes. Here, we optogenetically silenced the direct monosynaptic projection from ventral/intermediate hippocampus (HC) to medial prefrontal cortex (mPFC) during SWS in male mice following repeated learning trials in a weakly aversive spatial task. Our results confirm the concept that the monosynaptic projection between HC and mPFC contributes to memory consolidation and support an important functional role of this pathway in shaping the temporal precision among sleep-associated electrophysiological events.

Precise cell behaviors manipulation through light-responsive nano-regulators: recent advance and perspective

Nanotechnology-assisted spatiotemporal manipulation of biological events holds great promise in advancing the practice of precision medicine in healthcare systems. The progress in internal and/or external stimuli-responsive nanoplatforms for highly specific cellular regulations and theranostic controls offer potential clinical translations of the revolutionized nanomedicine. To successfully implement this new paradigm, the emerging light-responsive nanoregulators with unparalleled precise cell functions manipulation have gained intensive attention, providing UV-Vis light-triggered photocleavage or photoisomerization studies, as well as near-infrared (NIR) light-mediated deep-tissue applications for stimulating cellular signal cascades and treatment of mortal diseases. This review discusses current developments of light-activatable nanoplatforms for modulations of various cellular events including neuromodulations, stem cell monitoring, immunomanipulation, cancer therapy, and other biological target intervention. In summary, the propagation of light-controlled nanomedicine would place a bright prospect for future medicine.

Three-dimensional drivable optrode array for high-resolution neural stimulations and recordings in multiple brain regions

The brain-computer interface (BCI) devices are of prime important for study of nervous system as well as diagnosis and treatment of neurological disorders. To meet the needs of the BCI devices in high-density integration and multi-functionalization, 3-dimensional (3D) drivable optrode array with laser diodes (LDs) coupled waveguides was developed. The unique device realizes the 3D integration of the optrodes and avoids fiber tangle and tissue heating by adopting LD coupled waveguide structure. Besides, the postoperative position adjustment of the optrode array was achieved by integrating with a 3D printed micro-drive. Most importantly, high-resolution neural stimulations and recordings were achieved for study of working memory related neural circuits in four brain regions of mice including prelimbic cortex (PrL), mediodorsal thalamic nucleus (MD), dorsal medial caudate nucleus (dmCP) and posterior motor cortex 2 (pM2). The results indicate that this novel device is promising for the research of complex neural networks.

Ion channel engineering for modulation and de novo generation of electrical excitability

Ion channels play essential roles in regulating electrical properties of excitable tissues. By leveraging various ion channel gating mechanisms, scientists have developed a versatile set of genetically encoded tools to modulate intrinsic tissue excitability under different experimental settings. In this article, we will review how ion channels activated by voltage, light, small chemicals, stretch, and temperature have been customized to enable control of tissue excitability both in vitro and in vivo. Advantages and limitations of each of these ion channel-engineering platforms will be discussed and notable applications will be highlighted. Furthermore, we will describe recent progress on de novo generation of excitable tissues via expression of appropriate sets of engineered voltage-gated ion channels and discuss potential therapeutic implications.

Crystal structure of a natural light-gated anion channelrhodopsin

The anion channelrhodopsin GtACR1 from the alga Guillardia theta is a potent neuron-inhibiting optogenetics tool. Presented here, its X-ray structure at 2.9 Å reveals a tunnel traversing the protein from its extracellular surface to a large cytoplasmic cavity. The tunnel is lined primarily by small polar and aliphatic residues essential for anion conductance. A disulfide-immobilized extracellular cap facilitates channel closing and the ion path is blocked mid-membrane by its photoactive retinylidene chromophore and further by a cytoplasmic side constriction. The structure also reveals a novel photoactive site configuration that maintains the retinylidene Schiff base protonated when the channel is open. These findings suggest a new channelrhodopsin mechanism, in which the Schiff base not only controls gating, but also serves as a direct mediator for anion flux.

Cardiac Electrophysiological Effects of Light-Activated Chloride Channels

During the last decade, optogenetics has emerged as a paradigm-shifting technique to monitor and steer the behavior of specific cell types in excitable tissues, including the heart. Activation of cation-conducting channelrhodopsins (ChR) leads to membrane depolarization, allowing one to effectively trigger action potentials (AP) in cardiomyocytes. In contrast, the quest for optogenetic tools for hyperpolarization-induced inhibition of AP generation has remained challenging. The green-light activated ChR from Guillardia theta (GtACR1) mediates Cl^--driven photocurrents that have been shown to silence AP generation in different types of neurons. It has been suggested, therefore, to be a suitable tool for inhibition of cardiomyocyte activity. Using single-cell electrophysiological recordings and contraction tracking, as well as intracellular microelectrode recordings and in vivo optical recordings of whole hearts, we find that GtACR1 activation by prolonged illumination arrests cardiac cells in a depolarized state, thus inhibiting re-excitation. In line with this, GtACR1 activation by transient light pulses elicits AP in rabbit isolated cardiomyocytes and in spontaneously beating intact hearts of zebrafish. Our results show that GtACR1 inhibition of AP generation is caused by cell depolarization. While this does not address the need for optogenetic silencing through physiological means (i.e., hyperpolarization), GtACR1 is a potentially attractive tool for activating cardiomyocytes by transient light-induced depolarization.

A light-gated potassium channel for sustained neuronal inhibition

Currently available inhibitory optogenetic tools provide short and transient silencing of neurons, but they cannot provide long-lasting inhibition because of the requirement for high light intensities. Here we present an optimized blue-light-sensitive synthetic potassium channel, BLINK2, which showed good expression in neurons in three species. The channel is activated by illumination with low doses of blue light, and in our experiments it remained active over (tens of) minutes in the dark after the illumination was stopped. This activation caused long periods of inhibition of neuronal firing in ex vivo recordings of mouse neurons and impaired motor neuron response in zebrafish in vivo. As a proof-of-concept application, we demonstrated that in a freely moving rat model of neuropathic pain, the activation of a small number of BLINK2 channels caused a long-lasting (>30 min) reduction in pain sensation.

High-efficiency optogenetic silencing with soma-targeted anion-conducting channelrhodopsins

Optogenetic silencing allows time-resolved functional interrogation of defined neuronal populations. However, the limitations of inhibitory optogenetic tools impose stringent constraints on experimental paradigms. The high light power requirement of light-driven ion pumps and their effects on intracellular ion homeostasis pose unique challenges, particularly in experiments that demand inhibition of a widespread neuronal population in vivo. Guillardia theta anion-conducting channelrhodopsins (GtACRs) are promising in this regard, due to their high single-channel conductance and favorable photon-ion stoichiometry. However, GtACRs show poor membrane targeting in mammalian cells, and the activity of such channels can cause transient excitation in the axon due to an excitatory chloride reversal potential in this compartment. Here, we address these problems by enhancing membrane targeting and subcellular compartmentalization of GtACRs. The resulting soma-targeted GtACRs show improved photocurrents, reduced axonal excitation and high light sensitivity, allowing highly efficient inhibition of neuronal activity in the mammalian brain.

Current optogenetic inhibition methods like light-controlled ion pumps require high-intensity light and disrupt physiological ion gradients. Here, the authors somatically target the anion-conducting opsin GtACR to eliminate spiking in distal axons and improve photocurrents, thus enhancing its utility.

Crystal structure of the red light-activated channelrhodopsin Chrimson

Channelrhodopsins are light-activated ion channels that mediate cation permeation across cell membranes upon light absorption. Red-light-activated channelrhodopsins are of particular interest, because red light penetrates deeper into biological tissues and also enables dual-color experiments in combination with blue-light-activated optogenetic tools. Here we report the crystal structure of the most red-shifted channelrhodopsin from the algae Chlamydomonas noctigama, Chrimson, at 2.6 Å resolution. Chrimson resembles prokaryotic proton pumps in the retinal binding pocket, while sharing similarity with other channelrhodopsins in the ion-conducting pore. Concomitant mutation analysis identified the structural features that are responsible for Chrimson’s red light sensitivity; namely, the protonation of the counterion for the retinal Schiff base, and the polar residue distribution and rigidity of the retinal binding pocket. Based on these mechanistic insights, we engineered ChrimsonSA, a mutant with a maximum activation wavelength red-shifted beyond 605 nm and accelerated closing kinetics.

Channelrhodopsins are light-activated ion channels that mediate cation permeation across cell membranes upon light absorption. Here, the authors report the crystal structure of the most red-shifted channelrhodopsin from the algae Chlamydomonas noctigama at 2.6 Å resolution.

Crystal structure of the natural anion-conducting channelrhodopsin GtACR1

The naturally occurring channelrhodopsin variant anion channelrhodopsin-1 (ACR1), discovered in the cryptophyte algae Guillardia theta, exhibits large light-gated anion conductance and high anion selectivity when expressed in heterologous settings, properties that support its use as an optogenetic tool to inhibit neuronal firing with light. However, molecular insight into ACR1 is lacking owing to the absence of structural information underlying light-gated anion conductance. Here we present the crystal structure of G. theta ACR1 at 2.9 Å resolution. The structure reveals unusual architectural features that span the extracellular domain, retinal-binding pocket, Schiff-base region, and anion-conduction pathway. Together with electrophysiological and spectroscopic analyses, these findings reveal the fundamental molecular basis of naturally occurring light-gated anion conductance, and provide a framework for designing the next generation of optogenetic tools.

Structural mechanisms of selectivity and gating in anion channelrhodopsins

Both designed and natural anion-conducting channelrhodopsins (dACRs and nACRs, respectively) have been widely applied in optogenetics (enabling selective inhibition of target-cell activity during animal behaviour studies), but each class exhibits performance limitations, underscoring trade-offs in channel structure-function relationships. Therefore, molecular and structural insights into dACRs and nACRs will be critical not only for understanding the fundamental mechanisms of these light-gated anion channels, but also to create next-generation optogenetic tools. Here we report crystal structures of the dACR iC++, along with spectroscopic, electrophysiological and computational analyses that provide unexpected insights into pH dependence, substrate recognition, channel gating and ion selectivity of both dACRs and nACRs. These results enabled us to create an anion-conducting channelrhodopsin integrating the key features of large photocurrent and fast kinetics alongside exclusive anion selectivity.

Transient Magnetothermal Neuronal Silencing Using the Chloride Channel Anoctamin 1 (TMEM16A)

Determining the role and necessity of specific neurons in a network calls for precisely timed, reversible removal of these neurons from the circuit via remotely triggered transient silencing. Previously, we have shown that alternating magnetic field mediated heating of magnetic nanoparticles, bound to neurons, expressing temperature-sensitive cation channels TRPV1 remotely activates these neurons, evoking behavioral responses in mice. Here, we demonstrate how to apply magnetic nanoparticle heating to silence target neurons. Rat hippocampal neuronal cultures were transfected to express the temperature gated chloride channel, anoctamin 1 (TMEM16A). Spontaneous firing was suppressed within seconds of alternating magnetic field application to anoctamin 1 (TMEM16A) channel expressing, magnetic nanoparticle decorated neurons. Five seconds of magnetic field application leads to 12 s of silencing, with a latency of 2 s and an average suppression ratio of more than 80%. Immediately following the silencing period spontaneous activity resumed. The method provides a promising avenue for tether free, remote, transient neuronal silencing in vivo for both scientific and therapeutic applications.

Targeting light-gated chloride channels to neuronal somatodendritic domain reduces their excitatory effect in the axon

Light-gated chloride channels are emerging as promising optogenetic tools for inhibition of neural activity. However, their effects depend on the transmembrane chloride electrochemical gradient and may be complex due to the heterogeneity of this gradient in different developmental stages, neuronal types, and subcellular compartments. Here we characterized a light-gated chloride channel, GtACR2, in mouse cortical neurons. We found that GtACR2 activation inhibited the soma, but unexpectedly depolarized the presynaptic terminals resulting in neurotransmitter release. Other light-gated chloride channels had similar effects. Reducing the chloride concentrations in the axon and presynaptic terminals diminished the GtACR2-induced neurotransmitter release, indicating an excitatory effect of chloride channels in these compartments. A novel hybrid somatodendritic targeting motif reduced the GtACR2-induced neurotransmitter release while enhancing the somatic photocurrents. Our results highlight the necessity of precisely determining the effects of light-gated chloride channels under specific experimental conditions and provide a much-improved light-gated chloride channel for optogenetic inhibition.

Dual color optogenetic control of neural populations using low-noise, multishank optoelectrodes

Optogenetics allows for optical manipulation of neuronal activity and has been increasingly combined with intra- and extra-cellular electrophysiological recordings. Genetically-identified classes of neurons are optically manipulated, though the versatility of optogenetics would be increased if independent control of distinct neural populations could be achieved on a sufficient spatial and temporal resolution. We report a scalable multi-site optoelectrode design that allows simultaneous optogenetic control of two spatially intermingled neuronal populations in vivo. We describe the design, fabrication, and assembly of low-noise, multi-site/multi-color optoelectrodes. Each shank of the four-shank assembly is monolithically integrated with 8 recording sites and a dual-color waveguide mixer with a 7 × 30 μm cross-section, coupled to 405 nm and 635 nm injection laser diodes (ILDs) via gradient-index (GRIN) lenses to meet optical and thermal design requirements. To better understand noise on the recording channels generated during diode-based activation, we developed a lumped-circuit modeling approach for EMI coupling mechanisms and used it to limit artifacts to amplitudes under 100 μV upto an optical output power of 450 μW. We implanted the packaged devices into the CA1 pyramidal layer of awake mice, expressing Channelrhodopsin-2 in pyramidal cells and ChrimsonR in paravalbumin-expressing interneurons, and achieved optical excitation of each cell type using sub-mW illumination. We highlight the potential use of this technology for functional dissection of neural circuits.

A synaptic threshold mechanism for computing escape decisions

Escaping from imminent danger is an instinctive behaviour that is fundamental for survival, and requires the classification of sensory stimuli as harmless or threatening. The absence of threat enables animals to forage for essential resources, but as the level of threat and potential for harm increases, they have to decide whether or not to seek safety 1 . Despite previous work on instinctive defensive behaviours in rodents2-11, little is known about how the brain computes the threat level for initiating  escape. Here we show that the probability and vigour of escape in mice scale with the saliency of innate threats, and are well described by a model that computes the distance between the threat level and an escape threshold. Calcium imaging and optogenetics in the midbrain of freely behaving mice show that the activity of excitatory neurons in the deep layers of the medial superior colliculus (mSC) represents the saliency of the threat stimulus and is predictive of escape, whereas glutamatergic neurons of the dorsal periaqueductal grey (dPAG) encode exclusively the choice to escape and control escape vigour. We demonstrate a feed-forward monosynaptic excitatory connection from mSC to dPAG neurons, which is weak and unreliable-yet required for escape behaviour-and provides a synaptic threshold for dPAG activation and the initiation of escape. This threshold can be overcome by high mSC network activity because of short-term synaptic facilitation and recurrent excitation within the mSC, which amplifies and sustains synaptic drive to the dPAG. Therefore, dPAG glutamatergic neurons compute escape decisions and escape vigour using a synaptic mechanism to  threshold threat information received from the mSC, and provide a biophysical model of how the brain performs a critical behavioural computation.

Defining the ionic mechanisms of optogenetic control of vascular tone by channelrhodopsin-2

BACKGROUND AND PURPOSE

Optogenetic control of electromechanical coupling in vascular smooth muscle cells (VSMCs) is emerging as a powerful research tool with potential applications in drug discovery and therapeutics. However, the precise ionic mechanisms involved in this control remain unclear.

EXPERIMENTAL APPROACH

Cell imaging, patch-clamp electrophysiology and muscle tension recordings were used to define these mechanisms over a wide range of light stimulations.

KEY RESULTS

Transgenic mice expressing a channelrhodopsin-2 variant [ChR2(H134R)] selectively in VSMCs were generated. Isolated VSMCs obtained from these mice demonstrated blue light-induced depolarizing whole-cell currents. Fine control of artery tone was attained by varying the intensity of the light stimulus. This arterial response was sufficient to overcome the endogenous, melanopsin-mediated, light-evoked, arterial relaxation observed in the presence of contractile agonists. Ca2+ entry through voltage-gated Ca2+ channels, and opening of plasmalemmal depolarizing channels (TMEM16A and TRPM) and intracellular IP3 receptors were involved in the ChR2(H134R)-dependent arterial response to blue light at intensities lower than ~0.1 mW·mm-2 . Light stimuli of greater intensity evoked a significant Ca2+ influx directly through ChR2(H134R) and produced marked intracellular alkalinization of VSMCs.

CONCLUSIONS AND IMPLICATIONS

We identified the range of light intensity allowing optical control of arterial tone, primarily by means of endogenous channels and without substantial alteration to intracellular pH. Within this range, mice expressing ChR2(H134R) in VSMCs are a powerful experimental model for achieving accurate and tuneable optical voltage-clamp of VSMCs and finely graded control of arterial tone, offering new approaches to the discovery of vasorelaxant drugs.

Extending the Time Domain of Neuronal Silencing with Cryptophyte Anion Channelrhodopsins

Optogenetic inhibition of specific neuronal types in the brain enables analysis of neural circuitry and is promising for the treatment of a number of neurological disorders. Anion channelrhodopsins (ACRs) from the cryptophyte alga Guillardia theta generate larger photocurrents than other available inhibitory optogenetic tools, but more rapid channels are needed for temporally precise inhibition, such as single-spike suppression, of high-frequency firing neurons. Faster ACRs have been reported, but their potential advantages for time-resolved inhibitory optogenetics have not so far been verified in neurons. We report RapACR, nicknamed so for "rapid," an ACR from Rhodomonas salina, that exhibits channel half-closing times below 10 ms and achieves equivalent inhibition at 50-fold lower light intensity in lentivirally transduced cultured mouse hippocampal neurons as the second-generation engineered Cl--conducting channelrhodopsin iC++. The upper limit of the time resolution of neuronal silencing with RapACR determined by measuring the dependence of spiking recovery after photoinhibition on the light intensity was calculated to be 100 Hz, whereas that with the faster of the two G. theta ACRs was 13 Hz. Further acceleration of RapACR channel kinetics was achieved by site-directed mutagenesis of a single residue in transmembrane helix 3 (Thr111 to Cys). We also show that mutation of another ACR (Cys to Ala at the same position) with a greatly extended lifetime of the channel open state acts as a bistable photochromic tool in mammalian neurons. These molecules extend the time domain of optogenetic neuronal silencing while retaining the high light sensitivity of Guillardia ACRs.

The form and function of channelrhodopsin

Channelrhodopsins are light-gated ion channels that, via regulation of flagellar function, enable single-celled motile algae to seek ambient light conditions suitable for photosynthesis and survival. These plant behavioral responses were initially investigated more than 150 years ago. Recently, major principles of function for light-gated ion channels have been elucidated by creating channelrhodopsins with kinetics that are accelerated or slowed over orders of magnitude, by discovering and designing channelrhodopsins with altered spectral properties, by solving the high-resolution channelrhodopsin crystal structure, and by structural model-guided redesign of channelrhodopsins for altered ion selectivity. Each of these discoveries not only revealed basic principles governing the operation of light-gated ion channels, but also enabled the creation of new proteins for illuminating, via optogenetics, the fundamentals of brain function.