Optogenetic manipulation of neural and non-neural functions

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.

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.

Near-Infrared-Light Remote-Controlled Activation of Cancer Immunotherapy Using Photothermal Conjugated Polymer Nanoparticles

Remote control of the therapeutic process is an ideal strategy for maximizing efficacy and avoiding side effects, especially for cancer immunotherapy. Herein, a conjugated polymer nanoparticles (CPNs)-mediated optogenetic system for in situ activation of immunotherapy under near-infrared laser irradiation is reported. This system is composed of photothermal CPNs and interferon-gamma (IFN-γ) plasmid driven by heat shock promoter HSP70. The photothermally responsive CPNs serve as a photo-heat nanotransducer to trigger the gene transcription of IFN-γ cytokine. The secreted IFN-γ from cancer cells can sufficiently elicit surrounding tumor-associated macrophages activation through IFN-γ-JAK-STAT1 transcription-factor signaling pathway and finally induce cancer cell killing by immunotherapy. Therefore, this synergetic optogenetic system provides a promising approach to remotely control the process of cancer immunotherapy.

Dose-independent threshold illumination for non-invasive time-lapse fluorescence imaging of live cells

Fluorescent microscopy employs monochromatic light for excitation, which can adversely affect the cells being observed. We reported earlier that fibroblasts relax their contractile force in response to green light of typical intensity. Here we show that such effects are independent of extracellular matrix and cell lines. In addition, we establish a threshold intensity that elicits minimal or no adverse effect on cell contractility even for long-time exposure. This threshold intensity is wavelength dependent. We cultured fibroblasts on soft 2D elastic hydrogels embedded with fluorescent beads to trace substrate deformation and cell forces. The beads move towards cell center when cells contract, but they move away when cells relax. We use relaxation/contraction ratio (λ _r), in addition to traction force, as measures of cell response to red (wavelength, λ=635-650 nm), green (λ=545-580 nm) and blue (λ=455-490 nm) lights with varying intensities. Our results suggest that intensities below 57, 31 and 3.5 W/m² for red, green and blue lights, respectively, do not perturb force homeostasis. To our knowledge, these intensities are the lowest reported safe thresholds, implying that cell traction is a highly sensitive readout of the effect of light on cells. Most importantly, we find these threshold intensities to be dose-independent; i.e., safe regardless of the energy dosage or time of exposure. Conversely, higher intensities result in widespread force-relaxation in cells with λ _r > 1. Furthermore, we present a photo-reaction based model that simulates photo-toxicity and predicts threshold intensity for different wavelengths within the visible spectra. In conclusion, we recommend employing illumination intensities below aforementioned wavelength-specific thresholds for time-lapse imaging of cells and tissues in order to avoid light-induced artifacts in experimental observations.

Nonlinear response characteristics of neural networks and single neurons undergoing optogenetic excitation

Optogenetic stimulation has become the method of choice for investigating neural computation in populations of neurons. Optogenetic experiments often aim to elicit a network response by stimulating specific groups of neurons. However, this is complicated by the fact that optogenetic stimulation is nonlinear, more light does not always equal to more spikes, and neurons that are not directly but indirectly stimulated could have a major impact on how networks respond to optogenetic stimulation. To clarify how optogenetic excitation of some neurons alters the network dynamics, we studied the temporal and spatial response of individual neurons and recurrent neural networks. In individual neurons, we find that neurons show a monotonic, saturating rate response to increasing light intensity and a nonmonotonic rate response to increasing pulse frequency. At the network level, we find that Gaussian light beams elicit spatial firing rate responses that are substantially broader than the stimulus profile. In summary, our analysis and our network simulation code allow us to predict the outcome of an optogenetic experiment and to assess whether the observed effects can be attributed to direct or indirect stimulation of neurons.

Optogenetic circuit manipulation has become a popular tool to manipulate the activity of neurons. During optogenetic stimulation, the firing rate of a neuron can rise because of direct light excitation or indirect activation via other neurons. To disentangle these influences and predict the effects of optogenetic excitation, we set up a spiking network model with controlled connectivity and studied its response to light stimulation. We find that the optogenetically evoked activity in a network can spread far beyond the light-stimulated area. We further found a nonmonotonic rate response of single neurons to increasing light intensities and frequencies. Our results help to interpret optogenetic experiments in vivo, and we provide computer code that can be customized to simulate 2D connectivity scenarios and explore their consequences.

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

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

Application of Optogenetics for Muscle Cells and Stem Cells

This chapter describes the current progress of basic research, and potential therapeutic applications primarily focused on the optical manipulation of muscle cells and neural stem cells using microbial rhodopsin as a light-sensitive molecule. Since the contractions of skeletal, cardiac, and smooth muscle cells are mainly regulated through their membrane potential, several studies have been demonstrated to up- or downregulate the muscle contraction directly or indirectly using optogenetic actuators or silencers with defined stimulation patterns and intensities. Light-dependent oscillation of membrane potential also facilitates the maturation of myocytes with the development of T tubules and sarcomere structures, tandem arrays of minimum contractile units consists of contractile proteins and cytoskeletal proteins. Optogenetics has been applied to various stem cells and multipotent/pluripotent cells such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) to generate light-sensitive neurons and to facilitate neuroscience. The chronic optical stimulation of the channelrhodopsin-expressing neural stem cells facilitates their neural differentiation. There are potential therapeutic applications of optogenetics in cardiac pacemaking, muscle regeneration/maintenance, locomotion recovery for the treatment of muscle paralysis due to motor neuron diseases such as amyotrophic lateral sclerosis (ALS). Optogenetics would also facilitate maturation, network integration of grafted neurons, and improve the microenvironment around them when applied to stem cells.

New Pioneers of Optogenetics in Neuroscience

Optogenetics have recently increased in popularity as tools to study behavior in response to the brain and how these trends relate back to a neuronal circuit. Additionally, the high demand for human cerebral tissue in research has led to the generation of a new model to investigate human brain development and disease. Human Pluripotent Stem Cells (hPSCs) have been previously used to recapitulate the development of several tissues such as intestine, stomach and liver and to model disease in a human context, recently new improvements have been made in the field of hPSC-derived brain organoids to better understand overall brain development but more specifically, to mimic inter-neuronal communication. This review aims to highlight the recent advances in these two separate approaches of brain research and to emphasize the need for overlap. These two novel approaches would combine the study of behavior along with the specific circuits required to produce the signals causing such behavior. This review is focused on the current state of the field, as well as the development of novel optogenetic technologies and their potential for current scientific study and potential therapeutic use.

Gate-keeper of ion transport-a highly conserved helix-3 tryptophan in a channelrhodopsin chimera, C1C2/ChRWR

Microbial rhodopsin is a large family of membrane proteins having seven transmembrane helices (TM1-7) with an all-trans retinal (ATR) chromophore that is covalently bound to Lys in the TM7. The Trp residue in the middle of TM3, which is homologous to W86 of bacteriorhodopsin (BR), is highly conserved among microbial rhodopsins with various light-driven functions. However, the significance of this Trp for the ion transport function of microbial rhodopsins has long remained unknown. Here, we replaced the W163 (BR W86 counterpart) of a channelrhodopsin (ChR), C1C2/ChRWR, which is a chimera between ChR1 and 2, with a smaller aromatic residue, Phe to verify its role in the ion transport. Under whole-cell patch clamp recordings from the ND7/23 cells that were transfected with the DNA plasmid coding human codon optimized C1C2/ChRWR (hWR) or its W163F mutant (hWR-W163F), the photocurrents were evoked by a pulsatile light at 475 nm. The ion-transporting activity of hWR was strongly altered by the W163F mutation in 3 points: (1) the H⁺ leak at positive membrane potential (V_m) and its light-adaptation, (2) the attenuation of cation channel activity and (3) the manifestation of outward H⁺ pump activity. All of these results strongly suggest that W163 has a role in stabilizing the structure involved in the gating-on and -off of the cation channel, the role of “gate keeper”. We can attribute the attenuation of cation channel activity to the incomplete gating-on and the H⁺ leak to the incomplete gating-off.

Neurogenic differentiation of human dental pulp stem cells by optogenetics stimulation

INTRODUCTION

Human dental pulp stem cells (hDPSCs), a promising source for autologous transplantation in regenerative medicine, have been shown to be able to differentiate into neural precursors. Optogenetics is considered as an advanced biological technique in neuroscience which is able to control the activity of genetically modified stem cells by light. The purpose of this study is to investigate the neurogenic differentiation of hDPSCs following optogenetic stimulation.

METHODS

The hDPSCs were isolated by mechanical enzymatic digestion from an impacted third molar and cultured in DMEM/F12. The cells were infected with lentiviruses carrying CaMKIIa-hChR2 (H134R). Opsin-expressing hDPSCs were plated at the density of 5 × 10⁴ cells/well in 6-well plates and optical stimulation was conducted with blue light (470 nm) pulsing at 15 Hz, 90 % Duty Cycle and 10 mW power for 10 s every 90 minutes, 6 times a day for 5 days. Two control groups including non-opsin-expressing hDPSCs and opsin-expressing hDPSCs with no optical stimulation were also included in the study. A day after last light stimulation, the viability of cells was analyzed by the MTT assay and the morphological changes were examined by phase contrast microscopy. The expression of Nestin, Microtubule-Associated protein 2 (MAP2) and Doublecortin (DCX) were examined by immunocytochemistry.

RESULTS

Human DPSCs expressed the reporter gene, mCherry, 72 hours after lentiviral infection. The result of MTT assay revealed a significant more viability in optical stimulated opsin-expressing hDPSCs as compared with two control groups. Moreover, optical stimulation increased the expression of Nestin, Doublecortin and MAP2 along with morphological changes from spindle shape to neuron-like shape.

CONCLUSION

Optogenetics stimulation through depolarizing the hDPSCs can increase the cells viability and/or proliferation and also promote the differentiation toward neuron-like cells.

Single-Cell Optogenetic Control of Calcium Signaling with a High-Density Micro-LED Array

Precise optogenetic control, ideally down to single cells in dense cell populations, is essential in understanding the heterogeneity of cell networks. Devices with such capability, if built in a chip scale, will advance optogenetic studies at cellular levels in a variety of experimental settings. Here we demonstrate optogenetic control of intracellular Ca2+ dynamics at the single cell level using a 16-μm pitched micro-light emitting diode (LED) array that features high brightness, small spot size, fast response, and low voltage operation. Individual LED pixels are able to reliably trigger intracellular Ca2+ transients, confirmed by fluorescence microscopy and control experiments and cross-checked by two genetically coded Ca2+ indicators. Importantly, our array can optogenetically address individual cells that are sub-10 μm apart in densely packed cell populations. These results suggest the possible use of the micro-LED array toward a lab-on-a-chip for single-cell optogenetics, which may allow for pharmaceutical screening and fundamental studies on a variety of cell networks.

Driving Neurogenesis in Neural Stem Cells with High Sensitivity Optogenetics

Optogenetic stimulation of neural stem cells (NSCs) enables their activity-dependent photo-modulation. This provides a spatio-temporal tool for studying activity-dependent neurogenesis and for regulating the differentiation of the transplanted NSCs. Currently, this is mainly driven by viral transfection of channelrhodopsin-2 (ChR2) gene, which requires high irradiance and complex in vivo/vitro stimulation systems. Additionally, despite the extensive application of optogenetics in neuroscience, the transcriptome-level changes induced by optogenetic stimulation of NSCs have not been elucidated yet. Here, we made transformed NSCs (SFO-NSCs) stably expressing one of the step-function opsin (SFO)-variants of chimeric channelrhodopsins, ChRFR(C167A), which is more sensitive to blue light than native ChR2, via a non-viral transfection system using piggyBac transposon. We set up a simple low-irradiance optical stimulation (OS)-incubation system that induced c-fos mRNA expression, which is activity-dependent, in differentiating SFO-NSCs. More neuron-like SFO-NCSs, which had more elongated axons, were differentiated with daily OS than control cells without OS. This was accompanied by positive/negative changes in the transcriptome involved in axonal remodeling, synaptic plasticity, and microenvironment modulation with the up-regulation of several genes involved in the Ca²⁺-related functions. Our approach could be applied for stem cell transplantation studies in tissue with two strengths: lower carcinogenicity and less irradiance needed for tissue penetration.

Expanding the Toolbox of Upconversion Nanoparticles for In Vivo Optogenetics and Neuromodulation

Optogenetics is an optical technique that exploits visible light for selective neuromodulation with spatio-temporal precision. Despite enormous effort, the effective stimulation of targeted neurons, which are located in deeper structures of the nervous system, by visible light, remains a technical challenge. Compared to visible light, near-infrared illumination offers a higher depth of tissue penetration owing to a lower degree of light attenuation. Herein, an overview of advances in developing new modalities for neural circuitry modulation utilizing upconversion-nanoparticle-mediated optogenetics is presented. These developments have led to minimally invasive optical stimulation and inhibition of neurons with substantially improved selectivity, sensitivity, and spatial resolution. The focus is to provide a comprehensive review of the mechanistic basis for evaluating upconversion parameters, which will be useful in designing, executing, and reporting optogenetic experiments.

Optogenetic study of the response interaction among multi-afferent inputs in the barrel cortex of rats

We investigated the relationship between whisker mechanoreceptive inputs and the neural responses to optical stimulation in layer 2/upper 3 (L2/U3) of the barrel cortex using optogenetics since, ideally, we should investigate interactions among inputs with spatiotemporal acuity. Sixteen whisker points of a transgenic rat (W-TChR2V4), that expresses channelrhodopsin 2 (ChR2)-Venus conjugate (ChR2V) in the peripheral nerve endings surrounding the whisker follicles, were respectively connected one-by-one with 16 LED-coupled optical fibres, which illuminated the targets according to a certain pattern in order to evaluate interactions among the inputs in L2/U3. We found that the individual L2/U3 neurons frequently received excitatory inputs from multiple whiskers that were arrayed in a row. Although the interactions among major afferent inputs (MAIs) were negligible, negative interactions with the surrounding inputs suggest that the afferent inputs were integrated in the cortical networks to enhance the contrast of an array to its surroundings. With its simplicity, reproducibility and spatiotemporal acuity, the optogenetic approach would provide an alternative way to understand the principles of afferent integration in the cortex and should complement knowledge obtained by experiments using more natural stimulations.

Holographic two-photon activation for synthetic optogenetics

Optogenetic tools provide users the ability to photocontrol the activity of cells. Commonly, activation is achieved by expression of proteins from photosynthetic organisms, for example, microbial opsins (e.g., ChR2). Alternatively, a sister approach, synthetic optogenetics, enables photocontrol over proteins of mammalian origin by use of photoswitches, visible light (typically), and genetic modification. Thus, synthetic optogenetics facilitates interrogation of native neuronal signaling mechanisms. However, the poor tissue penetration of visible wavelengths impedes the use of the technique in tissue, as two-photon excitation (2PE) is typically required to access the near-infrared window. Here, we describe an alternative technique that uses 2PE-compatible photoswitches (section 1) for photoactivation of genetically modified glutamate receptors (section 2). Furthermore, for fast, multi-region photoactivation, we describe the use of 2P-digital holography (2P-DH) (section 3). We detail how to combine 2P-DH and synthetic optogenetics with electrophysiology, or with red fluorescence Ca²⁺ recordings, for all-optical neural interrogation. The time required to complete the methods, aside from obtaining the necessary reagents and illumination equipment, is ~3 weeks.

Functional emergence of a column-like architecture in layer 5 of mouse somatosensory cortex in vivo

To investigate how the functional architecture is organized in layer 5 (L5) of the somatosensory cortex of a mouse in vivo, the input-output relationship was investigated using an all-optical approach. The neural activity in L5 was optically recorded using a Ca2+ sensor, R-CaMP2, through a microprism inserted in the cortex under two-photon microscopy, while the L5 was regionally excited using optogenetics. The excitability was spread around the blue-light irradiated region, but the horizontal propagation was limited to within a certain distance (λ < 130 μm from the center of the illumination spot). When two regions were photostimulated with a short interval, the excitability of each cluster was reduced. Therefore, a column-like architecture had functionally emerged with reciprocal inhibition through a minimal number of synaptic relays. This could generate a synchronous output from a region of L5 with simultaneous enhancement of the signal-to-noise ratio by silencing of the neighboring regions.

Locally induced neuronal synchrony precisely propagates to specific cortical areas without rhythm distortion

Propagation of oscillatory spike firing activity at specific frequencies plays an important role in distributed cortical networks. However, there is limited evidence for how such frequency-specific signals are induced or how the signal spectra of the propagating signals are modulated during across-layer (radial) and inter-areal (tangential) neuronal interactions. To directly evaluate the direction specificity of spectral changes in a spiking cortical network, we selectively photostimulated infragranular excitatory neurons in the rat primary visual cortex (V1) at a supra-threshold level with various frequencies, and recorded local field potentials (LFPs) at the infragranular stimulation site, the cortical surface site immediately above the stimulation site in V1, and cortical surface sites outside V1. We found a significant reduction of LFP powers during radial propagation, especially at high-frequency stimulation conditions. Moreover, low-gamma-band dominant rhythms were transiently induced during radial propagation. Contrastingly, inter-areal LFP propagation, directed to specific cortical sites, accompanied no significant signal reduction nor gamma-band power induction. We propose an anisotropic mechanism for signal processing in the spiking cortical network, in which the neuronal rhythms are locally induced/modulated along the radial direction, and then propagate without distortion via intrinsic horizontal connections for spatiotemporally precise, inter-areal communication.

Targeted expression of step-function opsins in transgenic rats for optogenetic studies

Rats are excellent animal models for experimental neuroscience. However, the application of optogenetics in rats has been hindered because of the limited number of established transgenic rat strains. To accomplish cell-type specific targeting of an optimized optogenetic molecular tool, we generated ROSA26/CAG-floxed STOP-ChRFR(C167A)-Venus BAC rats that conditionally express the step-function mutant channelrhodopsin ChRFR(C167A) under the control of extrinsic Cre recombinase. In primary cultured cortical neurons derived from this reporter rat, only Cre-positive cells expressing ChRFR(C167A) became bi-stable, that is, their excitability was enhanced by blue light and returned to the baseline by yellow~red light. In bigenic pups carrying the Phox2B-Cre driver, ChRFR(C167A) was specifically expressed in the rostral parafacial respiratory group (pFRG) in the medulla, where endogenous Phox2b immunoreactivity was detected. These neurons were sensitive to blue light with an increase in the firing frequency. Thus, this transgenic rat actuator/reporter system should facilitate optogenetic studies involving the effective in vivo manipulation of the activities of specific cell fractions using light of minimal intensity.

Kinetic profiles of photocurrents in cells expressing two types of channelrhodopsin genes

Channelrhodopsin-2 (ChR2), a light-activated cation-selective ion channel, has been widely used as a tool in optogenetic research. ChR2 is specifically sensitive to wavelengths less than 550 nm. One of the methods to expand the sensitivity of a channelrhodopsin to a wider range of wavelengths is to express another channelrhodopsin in the cells by the transduction of an additional gene. Here, we report the characteristic features of cells expressing two types of channelrhodopsins, each having different wavelength sensitivities. In HEK293 cells stably expressing ChR2, photocurrents were elicited at stimuli of 400-550 nm, and the wavelength sensitivity range was expanded by the additional transduction of the modified Volvox channelrhodopsin-1 (mVChR1) gene, which has broad wavelength sensitivities, ranging from 400 to 600 nm. However, the photocurrent at 550 nm was lower than that of the mVChR1-expressing cell; moreover, the turning-on and turning-off constants were delayed, and the deactivation rates were decreased. Meanwhile, the response to lower light intensity was improved by the additional gene. Thus, the transduction of an additional gene is a useful method to improve the light and wavelength sensitivities, as well as photocurrent kinetic profiles, of channelrhodopsins.

Photothermal-Responsive Conjugated Polymer Nanoparticles for Remote Control of Gene Expression in Living Cells

Remote control and noninvasive manipulation of cellular bioprocess has received intensive attention as a powerful technology to control cell functions. Here, a strategy is developed to remotely control intracellular gene expression with high spatial and temporal resolutions by using photothermal-responsive conjugated polymer nanoparticles (CPNs) as the transducer under near-infrared light irradiation. After being modified with positive charged peptide, the CPNs with superior photothermal conversion capacity could effectively coat on the surface of living cells and generate localized heat to trigger target gene expression. The heat-inducible heat shock protein-70 promoter starts transcription of downstream EGFP gene in response to heat shock, thus producing green fluorescent protein in the living cells. The combination of heat-inducible gene promoter and photothermal-responsive CPNs provides a method for the development of thermogenetics.

Optogenetic and chemogenetic techniques for neurogastroenterology

Optogenetics and chemogenetics comprise a wide variety of applications in which genetically encoded actuators and indicators are used to modulate and monitor activity with high cellular specificity. Over the past 10 years, development of these genetically encoded tools has contributed tremendously to our understanding of integrated physiology. In concert with the continued refinement of probes, strategies to target transgene expression to specific cell types have also made much progress in the past 20 years. In addition, the successful implementation of optogenetic and chemogenetic techniques thrives thanks to ongoing advances in live imaging microscopy and optical technology. Although innovation of optogenetic and chemogenetic methods has been primarily driven by researchers studying the central nervous system, these techniques also hold great promise to boost research in neurogastroenterology. In this Review, we describe the different classes of tools that are currently available and give an overview of the strategies to target them to specific cell types in the gut wall. We discuss the possibilities and limitations of optogenetic and chemogenetic technology in the gut and provide an overview of their current use, with a focus on the enteric nervous system. Furthermore, we suggest some experiments that can advance our understanding of how the intrinsic and extrinsic neural networks of the gut control gastrointestinal function.

Optogenetic manipulation of ENS - The brain in the gut

Optogenetics has emerged as an important tool in neuroscience, especially in central nervous system research. It allows for the study of the brain's highly complex network with high temporal and spatial resolution. The enteric nervous system (ENS), the brain in the gut, plays critical roles for life. Although advanced progress has been made, the neural circuits of the ENS remain only partly understood because the appropriate research tools are lacking. In this review, I highlight the potential application of optogenetics in ENS research. Firstly, I describe the development of optogenetics with focusing on its three main components. I discuss the applications in vitro and in vivo, and summarize current findings in the ENS research field obtained by optogenetics. Finally, the challenges for the application of optogenetics to the ENS research will be discussed.

Myogenic Maturation by Optical-Training in Cultured Skeletal Muscle Cells

Optogenetic techniques are powerful tools for manipulating biological processes in identified cells using light under high temporal and spatial resolutions. Here, we describe an optogenetic training strategy to promote morphological maturation and functional development of skeletal muscle cells in vitro. Optical stimulation with a rhythmical frequency facilitates specific structural alignment of sarcomeric proteins. Optical stimulation also depolarizes the membrane potential, and induces contractile responses in synchrony with the given pattern of light pulses. These results suggest that optogenetic techniques can be employed to manipulate activity-dependent processes during myogenic development and control contraction of photosensitive skeletal muscle cells with high temporal and special precision.

Electrophysiological Properties and Viability of Neonatal Rat Ventricular Myocyte Cultures with Inducible ChR2 Expression

Channelrhodopsin-2 (ChR2)-based optogenetic technique has been increasingly applied to cardiovascular research. However, the potential effects of ChR2 protein overexpression on cardiomyocytes are not completely understood. The present work aimed to examine how the doxycycline-inducible lentiviral-mediated ChR2 expression may affect cell viability and electrophysiological property of neonatal rat ventricular myocyte (NRVM) cultures. Primary NVRMs were infected with lentivirus containing ChR2 or YFP gene and subjected to cytotoxicity analysis. ChR2-expressing cultures were then paced electrically or optically with a blue light-emitting diode, with activation spread recorded simultaneously using optical mapping. Results showed that ChR2 could be readily transduced to NRVMs by the doxycycline-inducible lentiviral system; however, high-level ChR2 (but not YFP) expression was associated with substantial cytotoxicity, which hindered optical pacing. Application of bromodeoxyuridine significantly reduced cell damage, allowing stimulation with light. Simultaneous optical V_m mapping showed that conduction velocity, action potential duration, and dV_m/dt_max were similar in ChR2-expressing and control cultures. Finally, the ChR2-expressing cultures could be optically paced at multiple sites, with significantly reduced overall activation time. In summary, we demonstrated that inducible lentiviral-mediated ChR2 overexpression might cause cytotoxicity in NRVM cultures, which could be alleviated without impairing electrophysiological function, allowing simultaneous optical pacing and V_m mapping.

Surpassing light-induced cell damage in vitro with novel cell culture media

Light is extensively used to study cells in real time (live cell imaging), separate cells using fluorescence activated cell sorting (FACS) and control cellular functions with light sensitive proteins (Optogenetics). However, photo-sensitive molecules inside cells and in standard cell culture media generate toxic by-products that interfere with cellular functions and cell viability when exposed to light. Here we show that primary cells from the rat central nervous system respond differently to photo-toxicity, in that astrocytes and microglia undergo morphological changes, while in developing neurons and oligodendrocyte progenitor cells (OPCs) it induces cellular death. To prevent photo-toxicity and to allow for long-term photo-stimulation without causing cellular damage, we formulated new photo-inert media called MEMO and NEUMO, and an antioxidant rich and serum free supplement called SOS. These new media reduced the detrimental effects caused by light and allowed cells to endure up to twenty times more light exposure without adverse effects, thus bypassing the optical constraints previously limiting experiments.

Kinetic characteristics of chimeric channelrhodopsins implicate the molecular identity involved in desensitization

Channelrhodopsin (ChR)-1 and ChR2 were the first-identified members of ChRs which are a growing subfamily of microbial-type rhodopsins. Light absorption drives the generation of a photocurrent in cell membranes expressing ChR2. However, the photocurrent amplitude attenuates and becomes steady-state during prolonged irradiation. This process, called desensitization or inactivation, has been attributed to the accumulation of intermediates less conductive to cations. Here we provided evidence that the dark-adapted (DA) photocurrent before desensitization is kinetically different from the light-adapted (LA) one after desensitization, that is, the deceleration of both basal-to-conductive and conductive-to-basal transitions. When the kinetics were compared between the DA and LA photocurrents for the ChR1/2 chimeras, the transmembrane helices, TM1 and TM2, were the determinants of both basal-to-conductive and conductive-to-basal transitions, whereas TM4 may contribute to the basal-to-conductive transitions and TM5 may contribute to the conductive-to-basal transitions, respectively. The fact that the desensitization-dependent decrease of the basal-to-conductive and conductive-to-basal transitions was facilitated by the TM1 exchange from ChR2 to ChR1 and reversed by the further TM2 exchange suggests that the conformation change for the channel gating is predominantly regulated by the interaction between TM1 and TM2. Although the exchange of TM1 from ChR2 to ChR1 showed no obvious influence on the spectral sensitivity, this exchange significantly induced the desensitization-dependent blue shift. Therefore, the TM1 and 2 are the main structures involved in two features of the desensitization, the stabilization of protein conformation and the charge distribution around the retinal-Schiff base (RSB⁺).

Role of Neuromodulation and Optogenetic Manipulation in Pain Treatment

Neuromodulation, including invasive and non-invasive stimulation, has been used to treat intractable chronic pain. However, the mechanisms by which neuromodulation produces antinociceptive effect still remain uncertain. Optogenetic manipulation, a recently developed novel approach, has already proven its value to clinicians by providing new insights into mechanisms of current clinical neuromodulation methods as well as pathophysiology of nervous system diseases at the circuit level. Here, we discuss the principles of two neuromodulation methods (deep brain stimulation and motor cortex stimulation) and their applications in pain treatment. More important, we summarize the new information from recent studies regarding optogenetic manipulation in neuroscience research and its potential utility in pain study.

A Chimera Na+-Pump Rhodopsin as an Effective Optogenetic Silencer

With the progress of optogenetics, the activities of genetically identified neurons can be optically silenced to determine whether the neurons in question are necessary for the network performance of the behavioral expression. This logical induction is expected to be improved by the application of the Na⁺ pump rhodopsins (NaRs), which hyperpolarize the membrane potential with negligible influence on the ionic/pH balance. Here, we made several chimeric NaRs between two NaRs, KR2 and IaNaR from Krokinobacter eikastus and Indibacter alkaliphilus, respectively. We found that one of these chimeras, named I₁K₆NaR, exhibited some improvements in the membrane targeting and photocurrent properties over native NaRs. The I₁K₆NaR-expressing cortical neurons were stably silenced by green light irradiation for a certain long duration. With its rapid kinetics and voltage dependency, the photoactivation of I₁K₆NaR would specifically counteract the generation of action potentials with less hyperpolarization of the neuronal membrane potential than KR2.

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

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

Near-infrared (NIR) up-conversion optogenetics

Non-invasive remote control technologies designed to manipulate neural functions have been long-awaited for the comprehensive and quantitative understanding of neuronal network in the brain as well as for the therapy of neurological disorders. Recently, it has become possible for the neuronal activity to be optically manipulated using biological photo-reactive molecules such as channelrhodopsin (ChR)-2. However, ChR2 and its relatives are mostly reactive to visible light, which does not effectively penetrate through biological tissues. In contrast, near-infrared (NIR) light (650-1450 nm) penetrates deep into the tissues because biological systems are almost transparent to light within this so-called 'imaging window'. Here we used lanthanide nanoparticles (LNPs), composed of rare-earth elements, as luminous bodies to activate ChRs since they absorb low-energy NIR light to emit high-energy visible light (up-conversion). Here, we created a new type of optogenetic system which consists of the donor LNPs and the acceptor ChRs. The NIR laser irradiation emitted visible light from LNPs, then induced the photo-reactive responses in the near-by cells that expressed ChRs. However, there remains room for large improvements in the energy efficiency of the LNP-ChR system.

Optogenetic control of nerve growth

Due to the limited regenerative ability of neural tissue, a diverse set of biochemical and biophysical cues for increasing nerve growth has been investigated, including neurotrophic factors, topography, and electrical stimulation. In this report, we explore optogenetic control of neurite growth as a cell-specific alternative to electrical stimulation. By investigating a broad range of optical stimulation parameters on dorsal root ganglia (DRGs) expressing channelrhodopsin 2 (ChR2), we identified conditions that enhance neurite outgrowth by three-fold as compared to unstimulated or wild-type (WT) controls. Furthermore, optogenetic stimulation of ChR2 expressing DRGs induces directional outgrowth in WT DRGs co-cultured within a 10 mm vicinity of the optically sensitive ganglia. This observed enhancement and polarization of neurite growth was accompanied by an increased expression of neural growth and brain derived neurotrophic factors (NGF, BDNF). This work highlights the potential for implementing optogenetics to drive nerve growth in specific cell populations.

Optogenetic induction of contractile ability in immature C2C12 myotubes

Myoblasts can be differentiated into multinucleated myotubes, which provide a well-established and reproducible muscle cell model for skeletal myogenesis in vitro. However, under conventional differentiation conditions, each myotube rarely exhibits robust contraction as well as sarcomere arrangement. Here, we applied trains of optical stimulation (OS) to C2C12 myotubes, which were genetically engineered to express a channelrhodopsin variant, channelrhodopsin-green receiver (ChRGR), to investigate whether membrane depolarization facilitates the maturation of myotubes. We found that light pulses induced membrane depolarization and evoked action potentials in ChRGR-expressing myotubes. Regular alignments of sarcomeric proteins were patterned periodically after OS training. In contrast, untrained control myotubes rarely exhibited the striated patterns. OS-trained and untrained myotubes also differed in terms of their resting potential. OS training significantly increased the number of contractile myotubes. Treatment with nifedipine during OS training significantly decreased the fraction of contractile myotubes, whereas tetrodotoxin was less effective. These results suggest that oscillations of membrane potential and intracellular Ca²⁺ accompanied by OS promoted sarcomere assembly and the development of contractility during the myogenic process. These results also suggest that optogenetic techniques could be used to manipulate the activity-dependent process during myogenic development.

Role of a helix B lysine residue in the photoactive site in channelrhodopsins

In most studied microbial rhodopsins two conserved carboxylic acid residues (the homologs of Asp-85 and Asp-212 in bacteriorhodopsin) and an arginine residue (the homolog of Arg-82) form a complex counterion to the protonated retinylidene Schiff base, and neutralization of the negatively charged carboxylates causes red shifts of the absorption maximum. In contrast, the corresponding neutralizing mutations in some relatively low-efficiency channelrhodopsins (ChRs) result in blue shifts. These ChRs do not contain a lysine residue in the second helix, conserved in higher efficiency ChRs (Lys-132 in the crystallized ChR chimera). By action spectroscopy of photoinduced channel currents in HEK293 cells and absorption spectroscopy of detergent-purified pigments, we found that in tested ChRs the Lys-132 homolog controls the direction of spectral shifts in the mutants of the photoactive site carboxylic acid residues. Analysis of double mutants shows that red spectral shifts occur when this Lys is present, whether naturally or by mutagenesis, and blue shifts occur when it is replaced with a neutral residue. A neutralizing mutation of the Lys-132 homolog alone caused a red spectral shift in high-efficiency ChRs, whereas its introduction into low-efficiency ChR1 from Chlamydomonas augustae (CaChR1) caused a blue shift. Taking into account that the effective charge of the carboxylic acid residues is a key factor in microbial rhodopsin spectral tuning, these findings suggest that the Lys-132 homolog modulates their pKa values. On the other hand, mutation of the Arg-82 homolog that fulfills this role in bacteriorhodopsin caused minimal spectral changes in the tested ChRs. Titration revealed that the pKa of the Asp-85 homolog in CaChR1 lies in the alkaline region unlike in most studied microbial rhodopsins, but is substantially decreased by introduction of a Lys-132 homolog or neutralizing mutation of the Asp-212 homolog. In the three ChRs tested the Lys-132 homolog also alters channel current kinetics.

Programmable wireless light-emitting diode stimulator for chronic stimulation of optogenetic molecules in freely moving mice

We produced a miniaturized, multicode, multiband, and programmable light-emitting diode (LED) stimulator for wireless control of optogenetic experiments. The LED stimulator is capable of driving three independent LEDs upon reception of an infrared (IR) signal generated by a custom-made IR transmitter. Individual LED photopulse patterns are assigned to different codes of the IR signals (up to 256 codes). The photopulse patterns can be programmed in the on-board microcontroller by specifying the parameters of duration ([Formula: see text]), frequency ([Formula: see text]), and pulse width ([Formula: see text]). The IR signals were modulated at multiple carrier frequencies to establish multiband IR transmission. Using these devices, we could remotely control the moving direction of a Thy1-ChR2-YFP transgenic mouse by transcranially illuminating the corresponding hemisphere of the primary motor cortex. IR transmitter and LED stimulator will be particularly useful in experiments where free movement or patterned concurrent stimulation is desired, such as testing social communication of rodents.

Optobiology: optical control of biological processes via protein engineering

Enabling optical control over biological processes is a defining goal of the new field of optogenetics. Control of membrane voltage by natural rhodopsin family ion channels has found widespread acceptance in neuroscience, due to the fact that these natural proteins control membrane voltage without further engineering. In contrast, optical control of intracellular biological processes has been a fragmented effort, with various laboratories engineering light-responsive properties into proteins in different manners. In the present article, we review the various systems that have been developed for controlling protein functions with light based on vertebrate rhodopsins, plant photoregulatory proteins and, most recently, the photoswitchable fluorescent protein Dronpa. By allowing biology to be controlled with spatiotemporal specificity and tunable dynamics, light-controllable proteins will find applications in the understanding of cellular and organismal biology and in synthetic biology.

Characterization of a highly efficient blue-shifted channelrhodopsin from the marine alga Platymonas subcordiformis

Rhodopsin photosensors of phototactic algae act as light-gated cation channels when expressed in animal cells. These proteins (channelrhodopsins) are extensively used for millisecond scale photocontrol of cellular functions (optogenetics). We report characterization of PsChR, one of the phototaxis receptors in the alga Platymonas (Tetraselmis) subcordiformis. PsChR exhibited ∼3-fold higher unitary conductance and greater relative permeability for Na(+) ions, as compared with the most frequently used channelrhodopsin-2 from Chlamydomonas reinhardtii (CrChR2). Photocurrents generated by PsChR in HEK293 cells showed lesser inactivation and faster peak recovery than those by CrChR2. Their maximal spectral sensitivity was at 445 nm, making PsChR the most blue-shifted channelrhodopsin so far identified. The λmax of detergent-purified PsChR was 437 nm at neutral pH and exhibited red shifts (pKa values at 6.6 and 3.8) upon acidification. The purified pigment undergoes a photocycle with a prominent red-shifted intermediate whose formation and decay kinetics match the kinetics of channel opening and closing. The rise and decay of an M-like intermediate prior to formation of this putative conductive state were faster than in CrChR2. PsChR mediated sufficient light-induced membrane depolarization in cultured hippocampal neurons to trigger reliable repetitive spiking at the upper threshold frequency of the neurons. At low frequencies spiking probability decreases less with PsChR than with CrChR2 because of the faster recovery of the former. Its blue-shifted absorption enables optogenetics at wavelengths even below 400 nm. A combination of characteristics makes PsChR important for further research on structure-function relationships in ChRs and potentially useful for optogenetics, especially for combinatorial applications when short wavelength excitation is required.

Mechanism divergence in microbial rhodopsins

A fundamental design principle of microbial rhodopsins is that they share the same basic light-induced conversion between two conformers. Alternate access of the Schiff base to the outside and to the cytoplasm in the outwardly open "E" conformer and cytoplasmically open "C" conformer, respectively, combined with appropriate timing of pKa changes controlling Schiff base proton release and uptake make the proton path through the pumps vectorial. Phototaxis receptors in prokaryotes, sensory rhodopsins I and II, have evolved new chemical processes not found in their proton pump ancestors, to alter the consequences of the conformational change or modify the change itself. Like proton pumps, sensory rhodopsin II undergoes a photoinduced E→C transition, with the C conformer a transient intermediate in the photocycle. In contrast, one light-sensor (sensory rhodopsin I bound to its transducer HtrI) exists in the dark as the C conformer and undergoes a light-induced C→E transition, with the E conformer a transient photocycle intermediate. Current results indicate that algal phototaxis receptors channelrhodopsins undergo redirected Schiff base proton transfers and a modified E→C transition which, contrary to the proton pumps and other sensory rhodopsins, is not accompanied by the closure of the external half-channel. The article will review our current understanding of how the shared basic structure and chemistry of microbial rhodopsins have been modified during evolution to create diverse molecular functions: light-driven ion transport and photosensory signaling by protein-protein interaction and light-gated ion channel activity. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.