Kalium channelrhodopsins are natural light-gated potassium channels that mediate optogenetic inhibition

Genetics-Based Targeting Strategies for Precise Neuromodulation

Genetics-based neuromodulation schemes are capable of selectively manipulating the activity of defined cell populations with high temporal-spatial resolution, providing unprecedented opportunities for probing cellular biological mechanisms, resolving neuronal projection pathways, mapping neural profiles, and precisely treating neurological and psychiatric disorders. Multimodal implementation schemes, which involve the use of exogenous stimuli such as light, heat, mechanical force, chemicals, electricity, and magnetic stimulation in combination with specific genetically engineered effectors, greatly expand their application space and scenarios. In particular, advanced wireless stimulation schemes have enabled low-invasive targeted neuromodulation through local delivery of navigable micro- and nanosized stimulators. In this review, the fundamental principles and implementation protocols of genetics-based precision neuromodulation are first introduced.The implementation schemes are systematically summarized, including optical, thermal, force, chemical, electrical, and magnetic stimulation, with an emphasis on those wireless and low-invasive strategies. Representative studies are dissected and analyzed for their advantages and disadvantages. Finally, the significance of genetics-based precision neuromodulation is emphasized and the open challenges and future perspectives are concluded.

Dual-color optical activation and suppression of neurons with high temporal precision

A well-known phenomenon in the optogenetic toolbox is that all light-gated ion channels, including red-shifted channelrhodopsins (ChRs), are activated by blue light, whereas blue-shifted ChRs are minimally responsive to longer wavelengths. Here, we took advantage of this feature to create a system which allows high-frequency activation of neurons with pulses of red light, while permitting the suppression of action potentials (APs) with millisecond precision by blue light. We achieved this by pairing an ultrafast red-shifted ChR with a blue light-sensitive anion channel of appropriately matching kinetics. This required screening several anion-selective ChRs, followed by a model-based mutagenesis strategy to optimize their kinetics and light spectra. Slice electrophysiology in the hippocampus as well as behavioral inspection of vibrissa movement demonstrate a minimal excitation from blue light. Of significant potential value, in contrast to existing tools, the system we introduce here allows high-frequency optogenetic excitation of neurons with red light, while blue light suppression of APs is confined within the duration of the light pulse.

Voltage-gated potassium channels associated with head and neck cancer

Head and neck cancer (HNC) is a common disease in otorhinolaryngology. Its prevalence is higher in men than in women and is mostly related to tobacco, alcohol and viral infections. Despite significant advances in the treatment of HNC in recent years, the mortality rate is still high and most patients are diagnosed at an advanced stage, and the prognosis for these patients is even worse. Earlier metastasis makes the treatment of HNC trickier. Therefore, actively seeking ways to treat HNC more effectively has been the goal of head and neck surgeons. Potassium (K+) channels are the most diverse ion channels found in all areas of life. Voltage-gated potassium (Kv) channels are the most important subfamily of K+ channels. Multiple Kv channels are associated with the development of HNC. This review focuses on several Kv channels associated with HNC.

Light-Boosted Osmotic Energy Conversion and Ion Pumping through a Graphdiyne Oxide-Based Membrane

Two-dimensional (2D) membranes with engineered light-responsive ion transport dynamics have been explored to construct efficient nanofluidic platforms that show great potential in osmotic and solar-osmotic energy conversion. However, the power density is still limited by poor responsivity and the inevitable trade-off effect between ion selectivity and flux. Here, we observed light-pumping ion transport behavior in graphdiyne oxide (GDYO) with a unique carbon hybrid skeleton that provides sensitive photoelectric responsivity and high-speed cation pathways. Molecular dynamics simulations verify that the coexistent interaction effects between cations and the negatively charged sites in GDYO (i.e., oxygen-containing groups and electron-rich acetylenic bonds) could significantly promote cation transmembrane transport via an absorption-acceleration mechanism. Furthermore, the GDYO-based system, possessing a coupled photon-electron-ion transport behavior due to its inherent semiconductor properties, could subtly realize unidirectional ion movement, consuming luminous energy either from low concentration to high concentration or vice versa, flexibly promoting the osmotic power density by ∼195% to 11.91 W m-2.

Foundry-fabricated dual-color nanophotonic neural probes for photostimulation and electrophysiological recording

Significance

Compact tools capable of delivering multicolor optogenetic stimulation to deep tissue targets with sufficient span, spatiotemporal resolution, and optical power remain challenging to realize. Here, we demonstrate foundry-fabricated nanophotonic neural probes for blue and red photostimulation and electrophysiological recording, which use a combination of spatial multiplexing and on-shank wavelength demultiplexing to increase the number of on-shank emitters.

Aim

We demonstrate silicon (Si) photonic neural probes with 26 photonic channels and 26 recording sites, which were fabricated on 200-mm diameter wafers at a commercial Si photonics foundry. Each photonic channel consists of an on-shank demultiplexer and separate grating coupler emitters for blue and red light, for a total of 52 emitters.

Approach

We evaluate neural probe functionality through bench measurements and in vivo experiments by photostimulating through 16 of the available 26 emitter pairs.

Results

We report neural probe electrode impedances, optical transmission, and beam profiles. We validated a packaged neural probe in optogenetic experiments with mice sensitive to blue or red photostimulation.

Conclusions

Our foundry-fabricated nanophotonic neural probe demonstrates dense dual-color emitter integration on a single shank for targeted photostimulation. Given its two emission wavelengths, high emitter density, and long site span, this probe will facilitate experiments involving bidirectional circuit manipulations across both shallow and deep structures simultaneously.

Mechanosensitive and pH-Gated Butterfly-Shaped Artificial Ion Channel for High-Selective K+ Transport and Cancer Cell Apoptosis

To advance the exploration of mechanisms underlying natural multi-gated ion channels, a novel butterfly-shaped biomimetic K+ channel GnC7 (n = 3, 4) is developed with dual mechanical and pH responsiveness, exhibiting unprecedented K+/Na+ selectivity (G3C7: 34.4; G4C7: 41.3). These channels constructed from poly(propylene imine) dendrimer and benzo-21-crown-7-ethers achieve high K+ transport activity (EC50: 0.72 µm for G3C7; 0.9 µm for G4C7) due to their arc-like mechanical rotation. The dynamic mode relies on butterfly-shaped topology derived from the highly symmetrical core and multiple intramolecular hydrogen bonds. GnC7 can sense mechanical stimulus applied to liposomes/cells and then adapt the K+ transport rate accordingly. Furthermore, reversible ON/OFF switching of K+ transport is realized through the pH-controllable host-guest complexation. G4C7-induced ultrafast cellular K+ efflux (70% within only 9 min) efficiently triggers mitochondrial-dependent apoptosis of cancer cells by provoking endoplasmic reticulum stress accompanied by drastic Ca2+ sparks. This work embodies a multi-dimensional regulation of channel functions; it will provide insights into the dynamic behaviors of biological analogs and promote the innovative design of artificial ion channels and therapeutic agents.

Optogenetic engineering for ion channel modulation

Optogenetics, which integrates photonics and genetic engineering to control protein activity and cellular processes, has transformed biomedical research. Its precise spatiotemporal control, minimal invasiveness, and tunable reversibility have spurred its widespread adoption in both basic and clinical research. Optogenetic techniques have been applied to partially restore vision in blind patients and are being actively explored as innovative treatments for neurological, psychiatric, cardiac, and immunological disorders. Microbial channelrhodopsins (ChRs) allow precise manipulation of neuronal and cardiac activities, while vertebrate rhodopsins offer unique opportunities for ion channel modulation through G-protein-coupled receptor (GPCR) pathways. Plant-derived photoswitchable domains can also be engineered into ion channels to confer photosensitivity. This review summarizes the latest progress in engineering genetically encoded light-sensitive ion channel actuators and modulators (GELICAMs) with diverse ion selectivity and spectral sensitivity. We further discuss the potential applications and challenges of these tools in advancing biomedical research and therapeutic interventions.

Theoretical analysis of low-power deep synergistic sono-optogenetic excitation of neurons by co-expressing light-sensitive and mechano-sensitive ion-channels

The present challenge in neuroscience is to non-invasively exercise low-power and high-fidelity control of neurons situated deep inside the brain. Although, two-photon optogenetic excitation can activate neurons to millimeter depth with sub-cellular specificity and millisecond temporal resolution, it can also cause heating of the targeted tissue. On the other hand, sonogenetics can non-invasively modulate the cellular activity of neurons expressed with mechano-sensitive proteins in deeper areas of the brain with less spatial selectivity. We present a theoretical analysis of a synergistic sono-optogenetic method to overcome these limitations by co-expressing a mechano-sensitive (MscL-I92L) ion-channel with a light-sensitive (CoChR/ChroME2s/ChRmine) ion-channel in hippocampal neurons. It is shown that in the presence of low-amplitude subthreshold ultrasound pulses, the two-photon excitation threshold for neural spiking reduces drastically by 73% with MscL-I92L-CoChR (0.021 mW/µm2), 66% with MscL-I92L-ChroME2s (0.029 mW/µm2), and 64% with MscL-I92L-ChRmine (0.013 mW/µm2) at 5 Hz. It allows deeper excitation of up to 1.2 cm with MscL-I92L-ChRmine combination. The method is useful to design new experiments for low-power deep excitation of neurons and multimodal neuroprosthetic devices and circuits.

Blue-shifted ancyromonad channelrhodopsins for multiplex optogenetics

Light-gated ion channels from protists (channelrhodopsins or ChRs) are optogenetic tools widely used for controlling neurons and cardiomyocytes. Multiplex optogenetic applications require spectrally separated molecules that must be found in nature, as they are difficult to engineer without disrupting channel function. Scanning numerous sequence databases, we identified three robust naturally blue-shifted ChRs from ancyromonads. They form a separate branch on the phylogenetic tree and contain residue motifs characteristic of anion ChRs (ACRs). However, only two conduct chloride, whereas the close Nutomonas longa homolog (peak absorption at ~440 nm) generates inward cation currents in mammalian cells under physiological conditions, significantly exceeding those by previously known tools. Measurements of transient absorption changes and pH titration of purified Ancyromonas sigmoides ACR (AnsACR) combined with mutant analysis revealed the roles of the residues in the photoactive site. Both ancyromonad ACRs allowed optogenetic silencing of mouse cortical neurons in brain slices. AnsACR expression in the cholinergic neurons enabled photoinhibition of pharyngeal muscle contraction in live worms. AnsACR could be activated by near-infrared two-photon illumination, which is required to control specific neurons in thick tissue. Our results improved the mechanistic understanding of light-gated channel function and expanded the optogenetic toolkit.

Impact statement

Ancyromonad channelrhodopsins advance our understanding of ionic selectivity and wavelength regulation in light-gated ion channels and also expand the toolkit for all-optical electrophysiology.

Structural insights into light-gating of potassium-selective channelrhodopsin

Structural information on channelrhodopsins' mechanism of light-gated ion conductance is scarce, limiting its engineering as optogenetic tools. Here, we use single-particle cryo-electron microscopy of peptidisc-incorporated protein samples to determine the structures of the slow-cycling mutant C110A of kalium channelrhodopsin 1 from Hyphochytrium catenoides (HcKCR1) in the dark and upon laser flash excitation. Upon photoisomerization of the retinal chromophore, the retinylidene Schiff base NH-bond reorients from the extracellular to the cytoplasmic side. This switch triggers a series of side chain reorientations and merges intramolecular cavities into a transmembrane K+ conduction pathway. Molecular dynamics simulations confirm K+ flux through the illuminated state but not through the resting state. The overall displacement between the closed and the open structure is small, involving mainly side chain rearrangements. Asp105 and Asp116 play a key role in K+ conductance. Structure-guided mutagenesis and patch-clamp analysis reveal the roles of the pathway-forming residues in channel gating and selectivity.

Suppression of epileptic seizures by transcranial activation of K+-selective channelrhodopsin

Optogenetics is a valuable tool for studying the mechanisms of neurological diseases and is now being developed for therapeutic applications. In rodents and macaques, improved channelrhodopsins have been applied to achieve transcranial optogenetic stimulation. While transcranial photoexcitation of neurons has been achieved, noninvasive optogenetic inhibition for treating hyperexcitability-induced neurological disorders has remained elusive. There is a critical need for effective inhibitory optogenetic tools that are highly light-sensitive and capable of suppressing neuronal activity in deep brain tissue. In this study, we developed a highly sensitive moderately K+-selective channelrhodopsin (HcKCR1-hs) by molecular engineering of the recently discovered Hyphochytrium catenoides kalium (potassium) channelrhodopsin 1. Transcranial activation of HcKCR1-hs significantly prolongs the time to the first seizure, increases survival, and decreases seizure activity in several status epilepticus mouse models. Our approach for transcranial optogenetic inhibition of neural hyperactivity may be adapted for cell type-specific neuromodulation in both basic and preclinical settings.

Tools and methods for cell ablation and cell inhibition in Caenorhabditis elegans

To understand the function of cells such as neurons within an organism, it can be instrumental to inhibit cellular function, or to remove the cell (type) from the organism, and thus to observe the consequences on organismic and/or circuit function and animal behavior. A range of approaches and tools were developed and used over the past few decades that act either constitutively or acutely and reversibly, in systemic or local fashion. These approaches make use of either drugs or genetically encoded tools. Also, there are acutely acting inhibitory tools that require an exogenous trigger like light. Here, we give an overview of such methods developed and used in the nematode Caenorhabditis elegans.

Structural Insights Into the Opening Mechanism of C1C2 Channelrhodopsin

Channelrhodopsins, light-gated cation channels, enable precise control of neural cell depolarization or hyperpolarization with light in the field of optogenetics. This study integrates time-resolved serial crystallography and atomistic molecular dynamics (MD) simulations to resolve the structural changes during C1C2 channelrhodopsin activation. Our observations reveal that within the crystal environment, C1C2 predominantly remains in a light-activated state with characteristics of the M390 intermediate. Here, rearrangement of retinal within its binding pocket partially opens the central gate toward the extracellular vestibule. These structural changes initiate channel opening but were insufficient to allow K+ flow. Adjusting protonation states to represent the subsequent N520 intermediate in our MD simulations induced further conformational changes, including rearrangements of transmembrane helices 2 and 7, that opened the inner gate and the putative ion-translocation pathway. This allowed spontaneous cation conduction with low conductance, aligning with experimental findings. Our findings provide critical structural insights into key intermediates of the channel opening mechanism, enhancing our understanding of ion conduction and selectivity in channelrhodopsins at an atomistic level.

HulaCCR1, a pump-like cation channelrhodopsin discovered in a lake microbiome

Channelrhodopsins are light-gated ion channels consisting of seven transmembrane helices and a retinal chromophore, which are used as popular optogenetic tools for modulating neuronal activity. Cation channelrhodopsins (CCRs), first recognized as the photoreceptors in the chlorophyte Chlamydomonas reinhardtii, have since been identified in diverse species of green algae, as well in other unicellular eukaryotes. The CCRs from non-chlorophyte species are commonly referred to as bacteriorhodopsin-like cation channelrhodopsins, or BCCRs, as most of them feature the three characteristic amino acid residues of the "DTD motif" in the third transmembrane helix (TM3 or helix C) matching the canonical DTD motif of the well-studied archaeal light-driven proton pump bacteriorhodopsin. Here, we report characterization of HulaCCR1, a novel BCCR identified through metatranscriptomic analysis of a unicellular eukaryotic community in Lake Hula, Israel. Interestingly, HulaCCR1 has an ETD motif in which the first residue of the canonical motif is substituted for glutamate. Electrophysiological measurements of the wild-type and a mutant with a DTD motif of HulaCCR1 suggest the critical role of the first glutamate in spectral tuning and channel gating. Additionally, HulaCCR1 exhibits long extensions at the N- and C-termini. Photocurrents recorded from a truncated variant without the signal peptide predicted at the N-terminus were diminished, and membrane localization of the truncated variant significantly decreased, indicating that the signal peptide is important for membrane trafficking of HulaCCR1. These characteristics of HulaCCR1 would be related to a new biological significance in the original unidentified species, distinct from those known for other BCCRs.

Multimodal evaluation of network activity and optogenetic interventions in human hippocampal slices

Seizures are made up of the coordinated activity of networks of neurons, suggesting that control of neurons in the pathologic circuits of epilepsy could allow for control of the disease. Optogenetics has been effective at stopping seizure-like activity in non-human disease models by increasing inhibitory tone or decreasing excitation, although this effect has not been shown in human brain tissue. Many of the genetic means for achieving channelrhodopsin expression in non-human models are not possible in humans, and vector-mediated methods are susceptible to species-specific tropism that may affect translational potential. Here we demonstrate adeno-associated virus-mediated, optogenetic reductions in network firing rates of human hippocampal slices recorded on high-density microelectrode arrays under several hyperactivity-provoking conditions. This platform can serve to bridge the gap between human and animal studies by exploring genetic interventions on network activity in human brain tissue.

The Role of Photobiomodulation to Modulate Ion Channels in the Nervous System: A Systematic Review

Photobiomodulation (PBM) is a safe and effective neurotherapy that modulates cellular pathways by altering cell membrane potentials, leading to beneficial biological effects such as anti-inflammatory and neuroregenerative responses. This review compiles studies from PubMed up to March 2024, investigating the impact of light at wavelengths ranging from 620 to 1270 nm on ion channels. Out of 330 articles screened, 19 met the inclusion criteria. Research indicates that PBM can directly affect various ion channels by influencing neurotransmitter synthesis in neighboring cells, impacting receptors like glutamate and acetylcholine, as well as potassium, sodium channels, and transient receptor potential channels. The diversity of studies hampers a comprehensive meta-analysis for evaluating treatment strategies effectively. This systematic review aims to explore the potential role of optoelectronic signal transduction in PBM, studying the neurobiological mechanisms and therapeutic significance of PBM on ion channels. However, the lack of uniformity in current treatment methods underscores the necessity of establishing standardized and reliable therapeutic approaches.

Sodium-Selective Channelrhodopsins

Channelrhodopsins (ChRs) are light-gated ion channels originally discovered in algae and are commonly used in neuroscience for controlling the electrical activity of neurons with high precision. Initially-discovered ChRs were non-selective cation channels, allowing the flow of multiple ions, such as Na+, K+, H+, and Ca2+, leading to membrane depolarization and triggering action potentials in neurons. As the field of optogenetics has evolved, ChRs with more specific ion selectivity were discovered or engineered, offering more precise optogenetic manipulation. This review highlights the natural occurrence and engineered variants of sodium-selective channelrhodopsins (NaChRs), emphasizing their importance in optogenetic applications. These tools offer enhanced specificity in Na+ ion conduction, reducing unwanted effects from other ions, and generating strong depolarizing currents. Some of the NaChRs showed nearly no desensitization upon light illumination. These characteristics make them particularly useful for experiments requiring robust depolarization or direct Na+ ion manipulation. The review further discusses the molecular structure of these channels, recent advances in their development, and potential applications, including a proposed drug delivery system using NaChR-expressing red blood cells that could be triggered to release therapeutic agents upon light activation. This review concludes with a forward-looking perspective on expanding the use of NaChRs in both basic research and clinical settings.

Integrated Bioelectronic and Optogenetic Methods to Study Brain-Body Circuits

The peripheral nervous system, consisting of somatic sensory circuits and autonomic effector circuits, enables communication between the body's organs and the brain. Dysregulation in these circuits is implicated in an array of disorders and represents a potential target for neuromodulation therapies. In this Perspective, we discuss recent advances in the neurobiological understanding of these brain-body pathways and the expansion of neurotechnologies beyond the brain to the viscera. We focus primarily on the development of integrated technologies that leverage bioelectronic devices with optogenetic tools. We highlight the discovery and application of ultrapotent and red-shifted channelrhodopsins for minimally invasive optogenetics and as tools to study brain-body circuits. These innovations enable studies of freely behaving animals and have enhanced our understanding of the role physiological signals play in brain states and behavior.

Unique hydrogen-bonding network in a viral channelrhodopsin

Channelrhodopsins (CRs) are used as key tools in optogenetics, and novel CRs, either found from nature or engineered by mutation, have greatly contributed to the development of optogenetics. Recently CRs were discovered from viruses, and crystal structure of a viral CR, OLPVR1, reported a very similar water-containing hydrogen-bonding network near the retinal Schiff base to that of a light-driven proton-pump bacteriorhodopsin (BR). In both OLPVR1 and BR, nearly planar pentagonal cluster structures are comprised of five oxygen atoms, three oxygens from water molecules and two oxygens from the Schiff base counterions. The planar pentagonal cluster stabilizes a quadrupole, two positive charges at the Schiff base and an arginine, and two negative charges at the counterions, and thus plays important roles in light-gated channel function of OLPVR1 and light-driven proton pump function of BR. Despite similar pentagonal cluster structures, present FTIR analysis revealed different hydrogen-bonding networks between OLPVR1 and BR. The hydrogen bond between the protonated Schiff base and a water is stronger in OLPVR1 than in BR, and internal water molecules donate hydrogen bonds much weaker in OLPVR1 than in BR. In OLPVR1, the bridged water molecule between the Schiff base and counterions forms hydrogen bonds to D76 and D200 equally, while the hydrogen-bonding interaction is much stronger to D85 than to D212 in BR. The present interpretation is supported by the mutation results, where D76 and D200 equally work as the Schiff base counterions in OLPVR1, but D85 is the primary counterion in BR. This work reports highly sensitive hydrogen-bonding network in the Schiff base region, which would be closely related to each function through light-induced alterations of the network.

Robust optogenetic inhibition with red-light-sensitive anion-conducting channelrhodopsins

Channelrhodopsins (ChRs) are light-gated ion channels widely used to optically activate or silence selected electrogenic cells, such as individual brain neurons. Here, we describe identifying and characterizing a set of anion-conducting ChRs (ACRs) from diverse taxa and representing various branches of the ChR phylogenetic tree. The Mantoniella squamata ACR (MsACR1) showed high sensitivity to yellow-green light (λmax at 555 nm) and was further engineered for optogenetic applications. A single amino-acid substitution that mimicked red-light-sensitive rhodopsins like Chrimson shifted the photosensitivity 20 nm toward red light and accelerated photocurrent kinetics. Hence, it was named red and accelerated ACR, raACR. Both wild-type and mutant are capable optical silencers at low light intensities in mouse neurons in vitro and in vivo, while raACR offers a higher temporal resolution.

Dopamine dynamics in nucleus accumbens across reward-based learning of goal-directed whisker-to-lick sensorimotor transformations in mice

The synaptic and neuronal circuit mechanisms underlying reward-based learning remain to be fully determined. In the mammalian brain, dopamine release in nucleus accumbens is thought to contribute importantly to reward signals for learning and promoting synaptic plasticity. Here, through longitudinal fiber photometry of a genetically-encoded fluorescent sensor, we investigated dopamine signals in the nucleus accumbens of thirsty head-restrained mice as they learned to lick a liquid reward spout in response to single deflections of the C2 whisker across varying reward contingencies. Reward delivery triggered by well-timed licking drove fast transient dopamine increases in nucleus accumbens. On the other hand, unrewarded licking was overall associated with reduced dopamine sensor fluorescence, especially in trials where reward was unexpectedly omitted. The dopamine reward signal upon liquid delivery decreased within individual sessions as mice became sated. As mice learned to lick the reward spout in response to whisker deflection, a fast transient sensory-evoked dopamine signal developed, correlating with the learning of the whisker detection task across consecutive training days, as well as within single learning sessions. The well-defined behavioral paradigm involving a unitary stimulus of a single whisker as a reward-predicting cue along with precisely quantified licking allows untangling of sensory, motor and reward-related dopamine signals and how they evolve across the learning of whisker-dependent goal-directed sensorimotor transformations. Our longitudinal measurements of dopamine dynamics across reward-based learning are overall consistent with the hypothesis that dopamine could play an important role as a reward signal for reinforcement learning.

The high-light-sensitivity mechanism and optogenetic properties of the bacteriorhodopsin-like channelrhodopsin GtCCR4

Channelrhodopsins are microbial light-gated ion channels that can control the firing of neurons in response to light. Among several cation channelrhodopsins identified in Guillardia theta (GtCCRs), GtCCR4 has higher light sensitivity than typical channelrhodopsins. Furthermore, GtCCR4 shows superior properties as an optogenetic tool, such as minimal desensitization. Our structural analyses of GtCCR2 and GtCCR4 revealed that GtCCR4 has an outwardly bent transmembrane helix, resembling the conformation of activated G-protein-coupled receptors. Spectroscopic and electrophysiological comparisons suggested that this helix bend in GtCCR4 omits channel recovery time and contributes to high light sensitivity. An electrophysiological comparison of GtCCR4 and the well-characterized optogenetic tool ChRmine demonstrated that GtCCR4 has superior current continuity and action-potential spike generation with less invasiveness in neurons. We also identified highly active mutants of GtCCR4. These results shed light on the diverse structures and dynamics of microbial rhodopsins and demonstrate the strong optogenetic potential of GtCCR4.

Characterization of the tail current of Channelrhodopsin-2 variants

Our study focused on specific ChR2 variants, particularly those with the Step function Opsins (SFO) mutation at the D156-C128 gate. These are widely used in optogenetics due to their heightened sensitivity to light and bi-stable prolonged activation. However, in some ChR2 variants, specifically D156 mutants, a tail current occurs when continuous light exposure is stopped. We specifically examined the D156H-T159S ChR2 variant, which demonstrated a tail current that was somewhat responsive to light and voltage, with a single-channel current of around 9fA, similar to wt-ChR2 as determined by stationary noise analysis. To further investigate, we used nonstationary noise analysis in cell-attached patching mode, which revealed that the tail current's single-channel current falls within the same range as the peak current, albeit with mild contamination from adaptation and desensitization. This finding strongly supports the notion that a portion of the ChR2 molecules open or re-open at the end of illumination, leading to further membrane depolarization.

Channelrhodopsins with distinct chromophores and binding patterns

Channelrhodopsins are popular optogenetic tools in neuroscience, but remain poorly understood mechanistically. Here we report the cryo-EM structures of channelrhodopsin-2 (ChR2) from Chlamydomonas reinhardtii and H. catenoides kalium channelrhodopsin (KCR1). We show that ChR2 recruits an endogenous N-retinylidene-PE-like molecule to a previously unidentified lateral retinal binding pocket, exhibiting a reduced light response in HEK293 cells. In contrast, H. catenoides kalium channelrhodopsin (KCR1) binds an endogenous retinal in its canonical retinal binding pocket under identical condition. However, exogenous ATR reduces the photocurrent magnitude of wild type KCR1 and also inhibits its leaky mutant C110T. Our results uncover diverse retinal chromophores with distinct binding patterns for channelrhodopsins in mammalian cells, which may further inspire next generation optogenetics for complex tasks such as cell fate control.

Calcium Binding Mechanism in TAT Rhodopsin

TAT rhodopsin binds Ca2+ near the Schiff base region, which accompanies deprotonation of the Schiff base. This paper reports the Ca2+-free and Ca2+-bound structures of TAT rhodopsin by molecular dynamics (MD) simulation launched from AlphaFold structures. In the Ca2+-bound TAT rhodopsin, Ca2+ is directly coordinated by eight oxygen atoms, four oxygens of the side chains of E54 and D227, and four oxygens of water molecules. E54 is not involved in the hydrogen-bonding network of the Ca2+-free TAT rhodopsin, while flipping motion of E54 allows Ca2+ binding to TAT rhodopsin with deformation of helices observed by FTIR spectroscopy. The hydrogen-bonding network plays a crucial role in maintaining the Ca2+ binding, as mutations of E54, Y55, R79, Y200, E220, and D227 abolished the binding. Only T82V exhibited the Ca2+ binding like the wild type among the mutants in this study. The molecular mechanism of Ca2+ binding is discussed based on the present computational and experimental analysis.

A bistable inhibitory optoGPCR for multiplexed optogenetic control of neural circuits

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

Calcium-Sensitive Microbial Rhodopsin VirChR1: A Femtosecond to Second Photocycle Study

Viral rhodopsins are light-gated cation channels representing a novel class of microbial rhodopsins. For viral rhodopsin 1 subfamily members VirChR1 and OLPVR1, channel activity is abolished above a certain calcium concentration. Here we present a calcium-dependent spectroscopic analysis of VirChR1 on the femtosecond to second time scale. Unlike channelrhodopsin-2, VirChR1 possesses two intermediate states P1 and P2 on the ultrafast time scale, similar to J and K in ion-pumping rhodopsins. Subsequently, we observe multifaceted photocycle kinetics with up to seven intermediate states. Calcium predominantly affects the last photocycle steps, including the appearance of additional intermediates P6Ca and P7 representing the blocked channel. Furthermore, the photocycle of the counterion variant D80N is drastically altered, yielding intermediates with different spectra and kinetics compared to those of the wt. These findings demonstrate the central role of the counterion within the defined reaction sequence of microbial rhodopsins that ultimately defines the protein function.

Engineering a human P2X2 receptor with altered ligand selectivity in yeast

P2X receptors are a family of ligand gated ion channels found in a range of eukaryotic species including humans but are not naturally present in the yeast Saccharomyces cerevisiae. We demonstrate the first recombinant expression and functional gating of the P2X2 receptor in baker's yeast. We leverage the yeast host for facile genetic screens of mutant P2X2 by performing site saturation mutagenesis at residues of interest, including SNPs implicated in deafness and at residues involved in native binding. Deep mutational analysis and rounds of genetic engineering yield mutant P2X2 F303Y A304W, which has altered ligand selectivity toward the ATP analog AMP-PNP. The F303Y A304W variant shows over 100-fold increased intracellular calcium amplitudes with AMP-PNP compared to the WT receptor and has a much lower desensitization rate. Since AMP-PNP does not naturally activate P2X receptors, the F303Y A304W P2X2 may be a starting point for downstream applications in chemogenetic cellular control. Interestingly, the A304W mutation selectively destabilizes the desensitized state, which may provide a mechanistic basis for receptor opening with suboptimal agonists. The yeast system represents an inexpensive, scalable platform for ion channel characterization and engineering by circumventing the more expensive and time-consuming methodologies involving mammalian hosts.

Kalium channelrhodopsins effectively inhibit neurons

The analysis of neural circuits has been revolutionized by optogenetic methods. Light-gated chloride-conducting anion channelrhodopsins (ACRs)-recently emerged as powerful neuron inhibitors. For cells or sub-neuronal compartments with high intracellular chloride concentrations, however, a chloride conductance can have instead an activating effect. The recently discovered light-gated, potassium-conducting, kalium channelrhodopsins (KCRs) might serve as an alternative in these situations, with potentially broad application. As yet, KCRs have not been shown to confer potent inhibitory effects in small genetically tractable animals. Here, we evaluated the utility of KCRs to suppress behavior and inhibit neural activity in Drosophila, Caenorhabditis elegans, and zebrafish. In direct comparisons with ACR1, a KCR1 variant with enhanced plasma-membrane trafficking displayed comparable potency, but with improved properties that include reduced toxicity and superior efficacy in putative high-chloride cells. This comparative analysis of behavioral inhibition between chloride- and potassium-selective silencing tools establishes KCRs as next-generation optogenetic inhibitors for in vivo circuit analysis in behaving animals.

Addressing Challenges in Ion-Selectivity Characterization in Nanopores

Ion selectivity is the basis for designing smart nanopore/channel-based devices, e.g., ion separators and biosensors. Quantitative characterization of ion selectivities in nanopores often employs the Nernst or Goldman-Hodgkin-Katz (GHK) equation to interpret transmembrane potentials. However, the direction of the measured transmembrane potential drop is not specified in these equations, and selectivity values calculated using absolute values of transmembrane potentials do not directly reveal the ion for which the membrane is selective. Moreover, researchers arbitrarily choose whether to use the Nernst or GHK equation and overlook the significant differences between them, leading to ineffective quantitative comparisons between studies. This work addresses these challenges through (a) specifying the transmembrane potential (sign) and salt concentrations in terms of working and reference electrodes and the solutions in which they reside when using the Nernst and GHK equations, (b) reporting of both Nernst-selectivity and GHK-selectivity along with solution compositions and transmembrane potentials when comparing different nanopores/channels, and (c) performing simulations to define an ideal selectivity for nanochannels. Experimental and modeling studies provide significant insight into these fundamental equations and guidelines for the development of nanopore/channel-based devices.

Light Control in Microbial Systems

Light is a key environmental component influencing many biological processes, particularly in prokaryotes such as archaea and bacteria. Light control techniques have revolutionized precise manipulation at molecular and cellular levels in recent years. Bacteria, with adaptability and genetic tractability, are promising candidates for light control studies. This review investigates the mechanisms underlying light activation in bacteria and discusses recent advancements focusing on light control methods and techniques for controlling bacteria. We delve into the mechanisms by which bacteria sense and transduce light signals, including engineered photoreceptors and light-sensitive actuators, and various strategies employed to modulate gene expression, protein function, and bacterial motility. Furthermore, we highlight recent developments in light-integrated methods of controlling microbial responses, such as upconversion nanoparticles and optical tweezers, which can enhance the spatial and temporal control of bacteria and open new horizons for biomedical applications.

Intracellular microbial rhodopsin-based optogenetics to control metabolism and cell signaling

Microbial rhodopsin (MRs) ion channels and pumps have become invaluable optogenetic tools for neuroscience as well as biomedical applications. Recently, MR-optogenetics expanded towards subcellular organelles opening principally new opportunities in optogenetic control of intracellular metabolism and signaling via precise manipulations of organelle ion gradients using light. This new optogenetic field expands the opportunities for basic and medical studies of cancer, cardiovascular, and metabolic disorders, providing more detailed and accurate control of cell physiology. This review summarizes recent advances in studies of the cellular metabolic processes and signaling mediated by optogenetic tools targeting mitochondria, endoplasmic reticulum (ER), lysosomes, and synaptic vesicles. Finally, we discuss perspectives of such an optogenetic approach in both fundamental and applied research.

Tuning neural circuits and behaviors by microglia in the adult brain

Microglia are the primary immune cells of the CNS, contributing to both inflammatory damage and tissue repair in neurological disorder. In addition, emerging evidence highlights the role of homeostatic microglia in regulating neuronal activity, interacting with synapses, tuning neural circuits, and modulating behaviors. Herein, we review how microglia sense and regulate neuronal activity through synaptic interactions, thereby directly engaging with neural networks and behaviors. We discuss current studies utilizing microglial optogenetic and chemogenetic approaches to modulate adult neural circuits. These manipulations of microglia across different CNS regions lead to diverse behavioral consequences. We propose that spatial heterogeneity of microglia-neuron interaction lays the groundwork for understanding diverse functions of microglia in neural circuits and behaviors.

Channel Gating in Kalium Channelrhodopsin Slow Mutants

Kalium channelrhodopsin 1 from Hyphochytrium catenoides (HcKCR1) is the first discovered natural light-gated ion channel that shows higher selectivity to K+ than to Na+ and therefore is used to silence neurons with light (optogenetics). Replacement of the conserved cysteine residue in the transmembrane helix 3 (Cys110) with alanine or threonine results in a >1,000-fold decrease in the channel closing rate. The phenotype of the corresponding mutants in channelrhodopsin 2 is attributed to breaking of a specific interhelical hydrogen bond (the "DC gate"). Unlike CrChR2 and other ChRs with long distance "DC gates", the HcKCR1 structure does not reveal any hydrogen bonding partners to Cys110, indicating that the mutant phenotype is likely caused by disruption of direct interaction between this residue and the chromophore. In HcKCR1_C110A, fast photochemical conversions corresponding to channel gating were followed by dramatically slower absorption changes. Full recovery of the unphotolyzed state in HcKCR1_C110A was extremely slow with two time constants 5.2 and 70 min. Analysis of the light-minus-dark difference spectra during these slow processes revealed accumulation of at least four spectrally distinct blue light-absorbing photocycle intermediates, L, M1 and M2, and a UV light-absorbing form, typical of bacteriorhodopsin-like channelrhodopsins from cryptophytes. Our results contribute to better understanding of the mechanistic links between the chromophore photochemistry and channel conductance, and provide the basis for using HcKCR1_C110A as an optogenetic tool.

Local Electric Potential-Driven Nanofluidic Ion Transport for Ultrasensitive Biochemical Sensing

Nanofluidic biosensors have been widely used for detection of analytes based on the change of system resistance before and after target-probe interactions. However, their sensitivity is limited when system resistance barely changes toward low-concentration targets. Here, we proposed a strategy to address this issue by means of target-induced change of local membrane potential under relatively unchanged system resistance. The local membrane potential originated from the directional diffusion of photogenerated carriers across nanofluidic biosensors and gated photoinduced ionic current signal before and after target-probe interactions. The sensitivity of such biosensors for the detection of biomolecules such as circulating tumor DNA (ctDNA) and lysozyme exceeds that of applying a traditional strategy by more than 3 orders of magnitude under unchanged system resistance. Such biosensors can specifically detect the small molecule biomarker in the blood sample between prostate cancer patients and healthy humans. The key advantages of such nanofluidic biosensors are therefore complementary to traditional nanofluidic biosensors, with potential applications in a point-of-care analytical tool.

Ion-Selective Micropipette Sensor for In Vivo Monitoring of Sodium Ion with Crown Ether-Encapsulated Metal-Organic Framework Subnanopores

In vivo sensing of the dynamics of ions with high selectivity is essential for gaining molecular insights into numerous physiological and pathological processes. In this work, we report an ion-selective micropipette sensor (ISMS) through the integration of functional crown ether-encapsulated metal-organic frameworks (MOFs) synthesized in situ within the micropipette tip. The ISMS features distinctive sodium ion (Na+) conduction and high selectivity toward Na+ sensing. The selectivity is attributed to the synergistic effects of subnanoconfined space and the specific coordination of 18-crown-6 toward potassium ions (K+), which largely increase the steric hindrance and transport resistance for K+ to pass through the ISMS. Furthermore, the ISMS exhibits high stability and sensitivity, facilitating real-time monitoring of Na+ dynamics in the living rat brain during spreading of the depression events process. In light of the diversity of crown ethers and MOFs, we believe this study paves the way for a nanofluidic platform for in vivo sensing and neuromorphic electrochemical sensing.

Optical mapping and optogenetics in cardiac electrophysiology research and therapy: a state-of-the-art review

State-of-the-art innovations in optical cardiac electrophysiology are significantly enhancing cardiac research. A potential leap into patient care is now on the horizon. Optical mapping, using fluorescent probes and high-speed cameras, offers detailed insights into cardiac activity and arrhythmias by analysing electrical signals, calcium dynamics, and metabolism. Optogenetics utilizes light-sensitive ion channels and pumps to realize contactless, cell-selective cardiac actuation for modelling arrhythmia, restoring sinus rhythm, and probing complex cell-cell interactions. The merging of optogenetics and optical mapping techniques for 'all-optical' electrophysiology marks a significant step forward. This combination allows for the contactless actuation and sensing of cardiac electrophysiology, offering unprecedented spatial-temporal resolution and control. Recent studies have performed all-optical imaging ex vivo and achieved reliable optogenetic pacing in vivo, narrowing the gap for clinical use. Progress in optical electrophysiology continues at pace. Advances in motion tracking methods are removing the necessity of motion uncoupling, a key limitation of optical mapping. Innovations in optoelectronics, including miniaturized, biocompatible illumination and circuitry, are enabling the creation of implantable cardiac pacemakers and defibrillators with optoelectrical closed-loop systems. Computational modelling and machine learning are emerging as pivotal tools in enhancing optical techniques, offering new avenues for analysing complex data and optimizing therapeutic strategies. However, key challenges remain including opsin delivery, real-time data processing, longevity, and chronic effects of optoelectronic devices. This review provides a comprehensive overview of recent advances in optical mapping and optogenetics and outlines the promising future of optics in reshaping cardiac electrophysiology and therapeutic strategies.

Interareal Synaptic Inputs Underlying Whisking-Related Activity in the Primary Somatosensory Barrel Cortex

Body movements influence brain-wide neuronal activities. In the sensory cortex, thalamocortical bottom-up inputs and motor-sensory top-down inputs are thought to affect the dynamics of membrane potentials (Vm ) of neurons and change their processing of sensory information during movements. However, direct perturbation of the axons projecting to the sensory cortex from other remote areas during movements has remained unassessed, and therefore the interareal circuits generating motor-related signals in sensory cortices remain unclear. Using a Gi/o -coupled opsin, eOPN3, we here inhibited interareal signals incoming to the whisker primary somatosensory barrel cortex (wS1) of awake male mice and tested their effects on whisking-related changes in neuronal activities in wS1. Spontaneous whisking in air induced the changes in spike rates of a subset of wS1 neurons, which were accompanied by depolarization and substantial reduction of slow-wave oscillatory fluctuations of Vm Despite an extensive innervation, inhibition of inputs from the whisker primary motor cortex (wM1) to wS1 did not alter the spike rates and Vm dynamics of wS1 neurons during whisking. In contrast, inhibition of axons from the whisker-related thalamus (wTLM) and the whisker secondary somatosensory cortex (wS2) to wS1 largely attenuated the whisking-related supra- and sub-threshold Vm dynamics of wS1 neurons. Notably, silencing inputs from wTLM markedly decreased the modulation depth of whisking phase-tuned neurons in wS1, while inhibiting wS2 inputs did not impact the whisking variable tuning of wS1 neurons. Thus, sensorimotor integration in wS1 during spontaneous whisking is predominantly facilitated by direct synaptic inputs from wTLM and wS2 rather than from wM1.

Opto-electronic feedback control of membrane potential for real-time control of action potentials

To unlock new research possibilities by acquiring control of action potential (AP) morphologies in excitable cells, we developed an opto-electronic feedback loop-based system integrating cellular electrophysiology, real-time computing, and optogenetic approaches and applied it to monolayers of heart muscle cells. This allowed accurate restoration and preservation of cardiac AP morphologies in the presence of electrical perturbations of different origin in an unsupervised, self-regulatory manner, without any prior knowledge of the disturbance. Moreover, arbitrary AP waveforms could be enforced onto these cells. Collectively, these results set the stage for the refinement and application of opto-electronic control systems to enable in-depth investigation into the regulatory role of membrane potential in health and disease.

Light-gated channelrhodopsin sparks proton-induced calcium release in guard cells

Although there has been long-standing recognition that stimuli-induced cytosolic pH alterations coincide with changes in calcium ion (Ca2+) levels, the interdependence between protons (H+) and Ca2+ remains poorly understood. We addressed this topic using the light-gated channelrhodopsin HcKCR2 from the pseudofungus Hyphochytrium catenoides, which operates as a H+ conductive, Ca2+ impermeable ion channel on the plasma membrane of plant cells. Light activation of HcKCR2 in Arabidopsis guard cells evokes a transient cytoplasmic acidification that sparks Ca2+ release from the endoplasmic reticulum. A H+-induced cytosolic Ca2+ signal results in membrane depolarization through the activation of Ca2+-dependent SLAC1/SLAH3 anion channels, which enabled us to remotely control stomatal movement. Our study suggests a H+-induced Ca2+ release mechanism in plant cells and establishes HcKCR2 as a tool to dissect the molecular basis of plant intracellular pH and Ca2+ signaling.

Cardiac optogenetics: shining light on signaling pathways

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

Combining different ion-selective channelrhodopsins to control water flux by light

Water transport through water channels, aquaporins (AQPs), is vital for many physiological processes including epithelial fluid secretion, cell migration and adipocyte metabolism. Water flux through AQPs is driven by the osmotic gradient that results from concentration differences of solutes including ions. Here, we developed a novel optogenetic toolkit that combines the light-gated anion channel GtACR1 either with the light-gated K+ channel HcKCR1 or the new Na+ channelrhodopsin HcNCR1 with high Na+ permeability, to manipulate water transport in Xenopus oocytes non-invasively. Water efflux through AQP was achieved by light-activating K+ and Cl- efflux through HcKCR1 and GtACR1. Contrarily, when GtACR1 was co-expressed with HcNCR1, inward movement of Na+ and Cl- was light-triggered, and the resulting osmotic gradient led to water influx through AQP1. In sum, we demonstrate a novel optogenetic strategy to manipulate water movement into or out of Xenopus oocytes non-invasively. This approach provides a new avenue to interfere with water homeostasis as a means to study related biological phenomena across cell types and organisms.

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

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

En route to sound coding strategies for optical cochlear implants

Hearing loss is the most common human sensory deficit. Severe-to-complete sensorineural hearing loss is often treated by electrical cochlear implants (eCIs) bypassing dysfunctional or lost hair cells by direct stimulation of the auditory nerve. The wide current spread from each intracochlear electrode array contact activates large sets of tonotopically organized neurons limiting spectral selectivity of sound coding. Despite many efforts, an increase in the number of independent eCI stimulation channels seems impossible to achieve. Light, which can be better confined in space than electric current may help optical cochlear implants (oCIs) to overcome eCI shortcomings. In this review, we present the current state of the optogenetic sound encoding. We highlight optical sound coding strategy development capitalizing on the optical stimulation that requires fine-grained, fast, and power-efficient real-time sound processing controlling dozens of microscale optical emitters as an emerging research area.

Isospectral intermediates in the photochemical reaction cycle of anion channelrhodopsin GtACR1

The most effective tested optogenetic tools available for neuronal silencing are the light-gated anion channel proteins found in the cryptophyte alga Guillardia theta (GtACRs). Molecular mechanisms of GtACRs, including the photointermediates responsible for the open channel state, are of great interest for understanding their exceptional conductance. In this study, the photoreactions of GtACR1 and its D234N, A75E, and S97E mutants were investigated using multichannel time-resolved absorption spectroscopy. For each of the proteins, the analysis showed two early microsecond transitions between K-like and L-like forms and two late millisecond recovery steps. Spectral forms associated with potential molecular intermediates of the proteins were derived and their evolutions in time were analyzed. The results indicate the presence of isospectral intermediates in the photocycles and expand the range of potential intermediates responsible for the open channel state.

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.

Illuminating the Brain: Advances and Perspectives in Optoelectronics for Neural Activity Monitoring and Modulation

Optogenetic modulation of brain neural activity that combines optical and electrical modes in a unitary neural system has recently gained robust momentum. Controlling illumination spatial coverage, designing light-activated modulators, and developing wireless light delivery and data transmission are crucial for maximizing the use of optical neuromodulation. To this end, biocompatible electrodes with enhanced optoelectrical performance, device integration for multiplexed addressing, wireless transmission, and multimodal operation in soft systems have been developed. This review provides an outlook for uniformly illuminating large brain areas while spatiotemporally imaging the neural responses upon optoelectrical stimulation with little artifacts. Representative concepts and important breakthroughs, such as head-mounted illumination, multiple implanted optical fibers, and micro-light-delivery devices, are discussed. Examples of techniques that incorporate electrophysiological monitoring and optoelectrical stimulation are presented. Challenges and perspectives are posed for further research efforts toward high-density optoelectrical neural interface modulation, with the potential for nonpharmacological neurological disease treatments and wireless optoelectrical stimulation.

Channelrhodopsins: From Phototaxis to Optogenetics

Channelrhodopsins stand out among other retinal proteins because of their capacity to generate passive ionic currents following photoactivation. Owing to that, channelrhodopsins are widely used in neuroscience and cardiology as instruments for optogenetic manipulation of the activity of excitable cells. Photocurrents generated by channelrhodopsins were first discovered in the cells of green algae in the 1970s. In this review we describe this discovery and discuss the current state of research in the field.

Structural basis for ion selectivity in potassium-selective channelrhodopsins

KCR channelrhodopsins (K+-selective light-gated ion channels) have received attention as potential inhibitory optogenetic tools but more broadly pose a fundamental mystery regarding how their K+ selectivity is achieved. Here, we present 2.5-2.7 Å cryo-electron microscopy structures of HcKCR1 and HcKCR2 and of a structure-guided mutant with enhanced K+ selectivity. Structural, electrophysiological, computational, spectroscopic, and biochemical analyses reveal a distinctive mechanism for K+ selectivity; rather than forming the symmetrical filter of canonical K+ channels achieving both selectivity and dehydration, instead, three extracellular-vestibule residues within each monomer form a flexible asymmetric selectivity gate, while a distinct dehydration pathway extends intracellularly. Structural comparisons reveal a retinal-binding pocket that induces retinal rotation (accounting for HcKCR1/HcKCR2 spectral differences), and design of corresponding KCR variants with increased K+ selectivity (KALI-1/KALI-2) provides key advantages for optogenetic inhibition in vitro and in vivo. Thus, discovery of a mechanism for ion-channel K+ selectivity also provides a framework for next-generation optogenetics.