Functional expression of bacteriorhodopsin in oocytes allows direct measurement of voltage dependence of light induced H+ pumping

Plastid-localized xanthorhodopsin increases diatom biomass and ecosystem productivity in iron-limited surface oceans

Microbial rhodopsins are photoreceptor proteins that convert light into biological signals or energy. Proteins of the xanthorhodopsin family are common in eukaryotic photosynthetic plankton including diatoms. However, their biological role in these organisms remains elusive. Here we report on a xanthorhodopsin variant (FcR1) isolated from the polar diatom Fragilariopsis cylindrus. Applying a combination of biophysical, biochemical and reverse genetics approaches, we demonstrate that FcR1 is a plastid-localized proton pump which binds the chromophore retinal and is activated by green light. Enhanced growth of a Thalassiora pseudonana gain-of-function mutant expressing FcR1 under iron limitation shows that the xanthorhodopsin proton pump supports growth when chlorophyll-based photosynthesis is iron-limited. The abundance of xanthorhodopsin transcripts in natural diatom communities of the surface oceans is anticorrelated with the availability of dissolved iron. Thus, we propose that these proton pumps convey a fitness advantage in regions where phytoplankton growth is limited by the availability of dissolved iron.

Single cell in vivo optogenetic stimulation by two-photon excitation fluorescence transfer

Optogenetic manipulation with single-cell resolution can be achieved by two-photon excitation. However, this frequently requires relatively high laser powers. Here, we developed a novel strategy that can improve the efficiency of current two-photon stimulation technologies by positioning fluorescent proteins or small fluorescent molecules with high two-photon cross-sections in the vicinity of opsins. This generates a highly localized source of endogenous single-photon illumination that can be tailored to match the optimal opsin absorbance. Through neuronal and vascular stimulation in the live mouse brain, we demonstrate the utility of this technique to achieve efficient opsin stimulation, without loss of cellular resolution. We also provide a theoretical framework for understanding the potential advantages and constrains of this methodology, with directions for future improvements. Altogether, this fluorescence transfer illumination method opens new possibilities for experiments difficult to implement in the live brain such as all-optical neural interrogation and control of regional cerebral blood flow.

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.

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.

Quantitative analysis of the optogenetic excitability of CA1 neurons

Introduction

Optogenetics has emerged as a promising technique for modulating neuronal activity and holds potential for the treatment of neurological disorders such as temporal lobe epilepsy (TLE). However, clinical translation still faces many challenges. This in-silico study aims to enhance the understanding of optogenetic excitability in CA1 cells and to identify strategies for improving stimulation protocols.

Methods

Employing state-of-the-art computational models coupled with Monte Carlo simulated light propagation, the optogenetic excitability of four CA1 cells, two pyramidal and two interneurons, expressing ChR2(H134R) is investigated.

Results and discussion

The results demonstrate that confining the opsin to specific neuronal membrane compartments significantly improves excitability. An improvement is also achieved by focusing the light beam on the most excitable cell region. Moreover, the perpendicular orientation of the optical fiber relative to the somato-dendritic axis yields superior results. Inter-cell variability is observed, highlighting the importance of considering neuron degeneracy when designing optogenetic tools. Opsin confinement to the basal dendrites of the pyramidal cells renders the neuron the most excitable. A global sensitivity analysis identified opsin location and expression level as having the greatest impact on simulation outcomes. The error reduction of simulation outcome due to coupling of neuron modeling with light propagation is shown. The results promote spatial confinement and increased opsin expression levels as important improvement strategies. On the other hand, uncertainties in these parameters limit precise determination of the irradiance thresholds. This study provides valuable insights on optogenetic excitability of CA1 cells useful for the development of improved optogenetic stimulation protocols for, for instance, TLE treatment.

Optogenetic Methods in Plant Biology

Optogenetics is a technique employing natural or genetically engineered photoreceptors in transgene organisms to manipulate biological activities with light. Light can be turned on or off, and adjusting its intensity and duration allows optogenetic fine-tuning of cellular processes in a noninvasive and spatiotemporally resolved manner. Since the introduction of Channelrhodopsin-2 and phytochrome-based switches nearly 20 years ago, optogenetic tools have been applied in a variety of model organisms with enormous success, but rarely in plants. For a long time, the dependence of plant growth on light and the absence of retinal, the rhodopsin chromophore, prevented the establishment of plant optogenetics until recent progress overcame these difficulties. We summarize the recent results of work in the field to control plant growth and cellular motion via green light-gated ion channels and present successful applications to light-control gene expression with single or combined photoswitches in plants. Furthermore, we highlight the technical requirements and options for future plant optogenetic research.

Plant optogenetics: Applications and perspectives

To understand cell biological processes, like signalling pathways, protein movements, or metabolic processes, precise tools for manipulation are desired. Optogenetics allows to control cellular processes by light and can be applied at a high temporal and spatial resolution. In the last three decades, various optogenetic applications have been developed for animal, fungal, and prokaryotic cells. However, using optogenetics in plants has been difficult due to biological and technical issues, like missing cofactors, the presence of endogenous photoreceptors, or the necessity of light for photosynthesis, which potentially activates optogenetic tools constitutively. Recently developed tools overcome these limitations, making the application of optogenetics feasible also in plants. Here, we highlight the most useful recent applications in plants and give a perspective for future optogenetic approaches in plants science.

Characterization and Modification of Light-Sensitive Phosphodiesterases from Choanoflagellates

Enzyme rhodopsins, including cyclase opsins (Cyclops) and rhodopsin phosphodiesterases (RhoPDEs), were recently discovered in fungi, algae and protists. In contrast to the well-developed light-gated guanylyl/adenylyl cyclases as optogenetic tools, ideal light-regulated phosphodiesterases are still in demand. Here, we investigated and engineered the RhoPDEs from Salpingoeca rosetta, Choanoeca flexa and three other protists. All the RhoPDEs (fused with a cytosolic N-terminal YFP tag) can be expressed in Xenopus oocytes, except the AsRhoPDE that lacks the retinal-binding lysine residue in the last (8th) transmembrane helix. An N296K mutation of YFP::AsRhoPDE enabled its expression in oocytes, but this mutant still has no cGMP hydrolysis activity. Among the RhoPDEs tested, SrRhoPDE, CfRhoPDE1, 4 and MrRhoPDE exhibited light-enhanced cGMP hydrolysis activity. Engineering SrRhoPDE, we obtained two single point mutants, L623F and E657Q, in the C-terminal catalytic domain, which showed ~40 times decreased cGMP hydrolysis activity without affecting the light activation ratio. The molecular characterization and modification will aid in developing ideal light-regulated phosphodiesterase tools in the future.

Molecular Biology of Microbial Rhodopsins

Research on type 1 rhodopsins spans now a history of 50 years. Originally, just archaeal ion pumps and sensors have been discovered. However, with modern genetic techniques and gene sequencing tools, more and more proteins were identified in all kingdoms of life. Spectroscopic and other biophysical studies revealed quite diverse functions. Ion pumps, sensors, and channels are imprinted in the same seven-helix transmembrane protein scaffold carrying a retinal prosthetic group. In this review, molecular biology methods are described, which enabled the elucidation of their function and structure leading to optogenetic applications.

In Vivo and In Vitro Characterization of Cyclase and Phosphodiesterase Rhodopsins

Rhodopsins with enzymatic activity were found in microbes, in 2004 hypothetically from sequence data and since 2014 by experimental proof. So far three different types are known: light-activated guanylyl cyclase opsins (Cyclop) in fungi, light-inhibited two-component guanylyl cyclase opsins (2c-Cyclop) in green algae, and rhodopsin phosphodiesterases (RhoPDE) in choanoflagellates. They are integral membrane proteins with eight transmembrane helices (TM), different to the other microbial (type I) rhodopsins with 7 TM. Therefore, we propose a classification as type Ib rhodopsins for opsins with 8 TM and type Ia for the ones with 7 TM. To characterize those rhodopsins or their mutants, the expression in Xenopus laevis oocytes proved to be an efficient strategy. Functional analysis was initially performed "in oocyte" (in vivo), but more detailed characterization can be obtained with an in vitro assay. In this chapter, we describe procedures how to extract membranes from oocytes after cRNA microinjection and heterologous protein expression. Enzymatic activity of these membranes is then analyzed under different illumination conditions. In addition, fluorescent labeling of the rhodopsins is employed to quantify the expression level and the absolute activity of designed mutants. We discuss strengths and pitfalls, associated with this expression system, and strategies for selecting potentially useful optogenetic tools.

Rhodopsin-Based Optogenetics: Basics and Applications

Optogenetics has revolutionized not only neuroscience but also had an impact on muscle physiology and cell biology. Rhodopsin-based optogenetics started with the discovery of the light-gated cation channels, called channelrhodopsins. Together with the light-driven ion pumps, these channels allow light-mediated control of electrically excitable cells in culture tissue and living animals. They can be activated (depolarized) or silenced (hyperpolarized) by light with incomparably high spatiotemporal resolution. Optogenetics allows the light manipulation of cells under electrode-free conditions in a minimally invasive manner. Through modern genetic techniques, virus-induced transduction can be performed with extremely high cell specificity in tissue and living animals, allowing completely new approaches for analyzing neural networks, behavior studies, and investigations of neurodegenerative diseases. First clinical trials for the optogenetic recovery of vision are underway.This chapter provides a comprehensive description of the structure and function of the different light-gated channels and some new light-activated ion pumps. Some of them already play an essential role in optogenetics while others are supposed to become important tools for more specialized applications in the future.At the moment, a large number of publications are available concerning intrinsic mechanisms of microbial rhodopsins. Mostly they describe CrChR2 and its variants, as CrChR2 is still the most prominent optogenetic tool. Therefore, many biophysically and biochemically oriented groups contributed to the overwhelming mass of information on this unique ion channel's molecular mechanism. In this context, the function of new optogenetic tools is discussed, which is essential for rational optimization of the optogenetic approach for an eventual biomedical application. The comparison of the effectivity of ion pumps versus ion channels is discussed as well.Applications of rhodopsins-based optogenetic tools are also discussed in the chapter. Because of the enormous number of these applications in neuroscience, only exemplary studies on cell culture neural tissue, muscle physiology, and remote control of animal behavior are presented.

Structural Factors Determining the Absorption Spectrum of Channelrhodopsins: A Case Study of the Chimera C1C2

Channelrhodopsins are photosensitive proteins that trigger flagella motion in single-cell algae and have been successfully utilized in optogenetic applications. In optogenetics, light is used to activate neural cells in living organisms, which can be achieved by exploiting the ion channel signaling of channelrhodopsins. Tailoring channelrhodopsins for such applications includes the tuning of the absorption maximum. In order to establish rational design and to obtain a desired spectral shift, a basic understanding of the absorption spectrum is required. We have studied the chimera C1C2 as a representative of this protein family and the first member with an available crystal structure. For this purpose, we sampled the conformations of C1C2 using quantum mechanical/molecular mechanical molecular dynamics and subjected the resulting snapshots of the trajectory to excitation energy calculations using ADC(2) and simplified time-dependent density functional theory. In contrast to previous reports, we found that different hydrogen-bonding networks-involving the retinal protonated Schiff base, the putative counterions E162 and D292, and water molecules-had only a small impact on the absorption spectrum. However, in the case of deprotonated E162, increasing the distance to the Schiff base hydrogen-bonding partner led to a systematic blue shift. The β-ionone ring rotation was identified as another important contributor. Yet the most important factors were found to be the bond length alternation and bond order alternation that were linearly correlated to the absorption maximum by up to 62 and 82%, respectively. We ascribe this novel insight into the structural basis of the absorption spectrum to our enhanced protein setup that includes membrane embedding as well as long and extensive sampling.

Molecular Tools for Targeted Control of Nerve Cell Electrical Activity. Part I

In modern life sciences, the issue of a specific, exogenously directed manipulation of a cell's biochemistry is a highly topical one. In the case of electrically excitable cells, the aim of the manipulation is to control the cells' electrical activity, with the result being either excitation with subsequent generation of an action potential or inhibition and suppression of the excitatory currents. The techniques of electrical activity stimulation are of particular significance in tackling the most challenging basic problem: figuring out how the nervous system of higher multicellular organisms functions. At this juncture, when neuroscience is gradually abandoning the reductionist approach in favor of the direct investigation of complex neuronal systems, minimally invasive methods for brain tissue stimulation are becoming the basic element in the toolbox of those involved in the field. In this review, we describe three approaches that are based on the delivery of exogenous, genetically encoded molecules sensitive to external stimuli into the nervous tissue. These approaches include optogenetics (Part I) as well as chemogenetics and thermogenetics (Part II), which are significantly different not only in the nature of the stimuli and structure of the appropriate effector proteins, but also in the details of experimental applications. The latter circumstance is an indication that these are rather complementary than competing techniques.

In-vitro Recordings of Neural Magnetic Activity From the Auditory Brainstem Using Color Centers in Diamond: A Simulation Study

Magnetometry based on nitrogen-vacancy (NV) centers in diamond is a novel technique capable of measuring magnetic fields with high sensitivity and high spatial resolution. With the further advancements of these sensors, they may open up novel approaches for the 2D imaging of neural signals in vitro. In the present study, we investigate the feasibility of NV-based imaging by numerically simulating the magnetic signal from the auditory pathway of a rodent brainstem slice (ventral cochlear nucleus, VCN, to the medial trapezoid body, MNTB) as stimulated by both electric and optic stimulation. The resulting signal from these two stimulation methods are evaluated and compared. A realistic pathway model was created based on published data of the neural morphologies and channel dynamics of the globular bushy cells in the VCN and their axonal projections to the principal cells in the MNTB. The pathway dynamics in response to optic and electric stimulation and the emitted magnetic fields were estimated using the cable equation. For simulating the optic stimulation, the light distribution in brain tissue was numerically estimated and used to model the optogenetic neural excitation based on a four state channelrhodopsin-2 (ChR2) model. The corresponding heating was also estimated, using the bio-heat equation and was found to be low (<2°C) even at excessively strong optic signals. A peak magnetic field strength of ∼0.5 and ∼0.1 nT was calculated from the auditory brainstem pathway after electrical and optical stimulation, respectively. By increasing the stimulating light intensity four-fold (far exceeding commonly used intensities) the peak magnetic signal strength only increased to 0.2 nT. Thus, while optogenetic stimulation would be favorable to avoid artefacts in the recordings, electric stimulation achieves higher peak fields. The present simulation study predicts that high-resolution magnetic imaging of the action potentials traveling along the auditory brainstem pathway will only be possible for next generation NV sensors. However, the existing sensors already have sufficient sensitivity to support the magnetic sensing of cumulated neural signals sampled from larger parts of the pathway, which might be a promising intermediate step toward further maturing this novel technology.

Charge Transport by Light-Activated Rhodopsins Determined by Electrophysiological Recordings

Electrophysiological experiments are required to determine the ion transport properties of light-activated currents from microbial rhodopsin expressing cells. The recordings set the quantitative basis for correlation with spectroscopic data and for understanding of channel gating, ion transport vectoriality, or ion selectivity. This chapter focuses on voltage-clamp recordings of channelrhodopsin-2-expressing cells, and it will describe different illumination protocols that reveal the kinetic properties of gating. While the opening and closing reaction is determined from a single turnover upon a short laser flash, desensitization of the light-gated currents is studied under continuous illumination. Recovery from the desensitized state is probed after prolonged illumination with a subsequent light activation upon different dark intervals. Compiling the experimental data will define a minimum number of states in kinetic schemes used to describe the light-gated currents in channelrhodopsins, and emphasis will be given on how to correlate the results with the different time-resolved spectroscopic experiments.

Stationary photocurrent generation from bacteriorhodopsin-loaded lipo-polymersomes in polyelectrolyte multilayer assembly on polyethersulfone membrane

Vesicles constructed of either synthetic polymers alone (polymersomes) or a combination of polymers and lipids (lipo-polymersomes) demonstrate excellent long-term stability and ability to integrate membrane proteins. Applications using lipo-polymersomes with integrated membrane proteins require suitable supports to maintain protein functionality. Using lipo-polymersomes loaded with the light-driven proton pump bacteriorhodopsin (BR), we demonstrate here how the photocurrent is influenced by a chosen support. In our study, we deposited BR-loaded lipo-polymersomes in a cross-linked polyelectrolyte multilayer assembly either directly physisorbed on gold electrode microchips or cross-linked on an intermediary polyethersulfone (PES) membrane covalently grafted using a hydrogel cushion. In both cases, electrochemical impedance spectroscopic characterization demonstrated successful polyelectrolyte assembly with BR-loaded lipo-polymersomes. Light-induced proton pumping by BR-loaded lipo-polymersomes in the different support constructs was characterized by amperometric recording of the generated photocurrent. Application of the hydrogel/PES membrane support together with the polyelectrolyte assembly decreased the transient current response upon light activation of BR, while enhancing the generated stationary current to over 700 nA/cm². On the other hand, the current response from BR-loaded lipo-polymersomes in a polyelectrolyte assembly without the hydrogel/PES membrane support was primarily a transient peak combined with a low-nanoampere-level stationary photocurrent. Hence, the obtained results demonstrated that by using a hydrogel/PES support it was feasible to monitor continuously light-induced proton flux in biomimetic applications of lipo-polymersomes. Graphical abstract.

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

Background

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

Results

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

Conclusions

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

Design of a light-gated proton channel based on the crystal structure of Coccomyxa rhodopsin

The light-driven proton pump Coccomyxa subellipsoidea rhodopsin (CsR) provides-because of its high expression in heterologous host cells-an opportunity to study active proton transport under controlled electrochemical conditions. In this study, solving crystal structure of CsR at 2.0-Å resolution enabled us to identify distinct features of the membrane protein that determine ion transport directivity and voltage sensitivity. A specific hydrogen bond between the highly conserved Arg⁸³ and the nearby nonconserved tyrosine (Tyr¹⁴) guided our structure-based transformation of CsR into an operational light-gated proton channel (CySeR) that could potentially be used in optogenetic assays. Time-resolved electrophysiological and spectroscopic measurements distinguished pump currents from channel currents in a single protein and emphasized the necessity of Arg⁸³ mobility in CsR as a dynamic extracellular barrier to prevent passive conductance. Our findings reveal that molecular constraints that distinguish pump from channel currents are structurally more confined than was generally expected. This knowledge might enable the structure-based design of novel optogenetic tools, which derive from microbial pumps and are therefore ion specific.

Light-Driven Biocatalysis in Liposomes and Polymersomes: Where Are We Now?

The utilization of light energy to power organic-chemical transformations is a fundamental strategy of the terrestrial energy cycle. Inspired by the elegance of natural photosynthesis, much interdisciplinary research effort has been devoted to the construction of simplified cell mimics based on artificial vesicles to provide a novel tool for biocatalytic cascade reactions with energy-demanding steps. By inserting natural or even artificial photosynthetic systems into liposomes or polymersomes, the light-driven proton translocation and the resulting formation of electrochemical gradients have become possible. This is the basis for the conversion of photonic into chemical energy in form of energy-rich molecules such as adenosine triphosphate (ATP), which can be further utilized by energy-dependent biocatalytic reactions, e.g. carbon fixation. This review compares liposomes and polymersomes as artificial compartments and summarizes the types of light-driven proton pumps that have been employed in artificial photosynthesis so far. We give an overview over the methods affecting the orientation of the photosystems within the membranes to ensure a unidirectional transport of molecules and highlight recent examples of light-driven biocatalysis in artificial vesicles. Finally, we summarize the current achievements and discuss the next steps needed for the transition of this technology from the proof-of-concept status to preparative applications.

Optogenetic stimulation of complex spatio-temporal activity patterns by acousto-optic light steering probes cerebellar granular layer integrative properties

Optogenetics provides tools to control afferent activity in brain microcircuits. However, this requires optical methods that can evoke asynchronous and coordinated activity within neuronal ensembles in a spatio-temporally precise way. Here we describe a light patterning method, which combines MHz acousto-optic beam steering and adjustable low numerical aperture Gaussian beams, to achieve fast 2D targeting in scattering tissue. Using mossy fiber afferents to the cerebellar cortex as a testbed, we demonstrate single fiber optogenetic stimulation with micron-scale lateral resolution, >100 µm depth-penetration and 0.1 ms spiking precision. Protracted spatio-temporal patterns of light delivered by our illumination system evoked sustained asynchronous mossy fiber activity with excellent repeatability. Combining optical and electrical stimulations, we show that the cerebellar granular layer performs nonlinear integration, whereby sustained mossy fiber activity provides a permissive context for the transmission of salient inputs, enriching combinatorial views on mossy fiber pattern separation.

High frequency neural spiking and auditory signaling by ultrafast red-shifted optogenetics

Optogenetics revolutionizes basic research in neuroscience and cell biology and bears potential for medical applications. We develop mutants leading to a unifying concept for the construction of various channelrhodopsins with fast closing kinetics. Due to different absorption maxima these channelrhodopsins allow fast neural photoactivation over the whole range of the visible spectrum. We focus our functional analysis on the fast-switching, red light-activated Chrimson variants, because red light has lower light scattering and marginal phototoxicity in tissues. We show paradigmatically for neurons of the cerebral cortex and the auditory nerve that the fast Chrimson mutants enable neural stimulation with firing frequencies of several hundred Hz. They drive spiking at high rates and temporal fidelity with low thresholds for stimulus intensity and duration. Optical cochlear implants restore auditory nerve activity in deaf mice. This demonstrates that the mutants facilitate neuroscience research and future medical applications such as hearing restoration.

Optogenetic applications would benefit from channelrhodopsins (ChRs) with faster photostimulation, increased tissue transparency and lower phototoxicity. Here, the authors develop fast red-shifted ChR variants and show the abilities for temporal precise spiking of cerebral interneurons and restoring auditory activity in deaf mice.

A novel rhodopsin phosphodiesterase from Salpingoeca rosetta shows light-enhanced substrate affinity

It is since many years textbook knowledge that the concentration of the second messenger cGMP is regulated in animal rod and cone cells by type II rhodopsins via a G-protein signaling cascade. Microbial rhodopsins with enzymatic activity for regulation of cGMP concentration were only recently discovered: in 2014 light-activated guanylyl-cyclase opsins in fungi and in 2017 a novel rhodopsin phosphodiesterase (RhoPDE) in the protist Salpingoeca rosetta (SrRhoPDE). The light regulation of SrRhoPDE, however, seemed very weak or absent. Here, we present strong evidence for light regulation by studying SrRhoPDE, expressed in Xenopus laevis oocytes, at different substrate concentrations. Hydrolysis of cGMP shows an ∼100-fold higher turnover than that of cAMP. Light causes a strong decrease in the K_m value for cGMP from 80 to 13 µM but increases the maximum turnover only by ∼30%. The PDE activity for cAMP is similarly enhanced by light at low substrate concentrations. Illumination does not affect the cGMP degradation of Lys296 mutants that are not able to form a covalent bond of Schiff base type to the chromophore retinal. We demonstrate that SrRhoPDE shows cytosolic N- and C-termini, most likely via an eight-transmembrane helix structure. SrRhoPDE is a new optogenetic tool for light-regulated cGMP manipulation which might be further improved by genetic engineering.

Bacteriorhodopsin-like channelrhodopsins: Alternative mechanism for control of cation conductance

The recently discovered cation-conducting channelrhodopsins in cryptophyte algae are far more homologous to haloarchaeal rhodopsins, in particular the proton pump bacteriorhodopsin (BR), than to earlier known channelrhodopsins. They uniquely retain the two carboxylate residues that define the vectorial proton path in BR in which Asp-85 and Asp-96 serve as acceptor and donor, respectively, of the photoactive site Schiff base (SB) proton. Here we analyze laser flash-induced photocurrents and photochemical conversions in Guillardia theta cation channelrhodopsin 2 (GtCCR2) and its mutants. Our results reveal a model in which the GtCCR2 retinylidene SB chromophore rapidly deprotonates to the Asp-85 homolog, as in BR. Opening of the cytoplasmic channel to cations in GtCCR2 requires the Asp-96 homolog to be unprotonated, as has been proposed for the BR cytoplasmic channel for protons. However, reprotonation of the GtCCR2 SB occurs not from the Asp-96 homolog, but by proton return from the earlier protonated acceptor, preventing vectorial proton translocation across the membrane. In GtCCR2, deprotonation of the Asp-96 homolog is required for cation channel opening and occurs >10-fold faster than reprotonation of the SB, which temporally correlates with channel closing. Hence in GtCCR2, cation channel gating is tightly coupled to intramolecular proton transfers involving the same residues that define the vectorial proton path in BR.

The Road to Optogenetics: Microbial Rhodopsins

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

Optogenetic Approaches to Restoring Vision

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

Conversion of a light-driven proton pump into a light-gated ion channel

Interest in microbial rhodopsins with ion pumping activity has been revitalized in the context of optogenetics, where light-driven ion pumps are used for cell hyperpolarization and voltage sensing. We identified an opsin-encoding gene (CsR) in the genome of the arctic alga Coccomyxa subellipsoidea C-169 that can produce large photocurrents in Xenopus oocytes. We used this property to analyze the function of individual residues in proton pumping. Modification of the highly conserved proton shuttling residue R83 or its interaction partner Y57 strongly reduced pumping power. Moreover, this mutation converted CsR at moderate electrochemical load into an operational proton channel with inward or outward rectification depending on the amino acid substitution. Together with molecular dynamics simulations, these data demonstrate that CsR-R83 and its interacting partner Y57 in conjunction with water molecules forms a proton shuttle that blocks passive proton flux during the dark-state but promotes proton movement uphill upon illumination.

The CarO rhodopsin of the fungus Fusarium fujikuroi is a light-driven proton pump that retards spore germination

Rhodopsins are membrane-embedded photoreceptors found in all major taxonomic kingdoms using retinal as their chromophore. They play well-known functions in different biological systems, but their roles in fungi remain unknown. The filamentous fungus Fusarium fujikuroi contains two putative rhodopsins, CarO and OpsA. The gene carO is light-regulated, and the predicted polypeptide contains all conserved residues required for proton pumping. We aimed to elucidate the expression and cellular location of the fungal rhodopsin CarO, its presumed proton-pumping activity and the possible effect of such function on F. fujikuroi growth. In electrophysiology experiments we confirmed that CarO is a green-light driven proton pump. Visualization of fluorescent CarO-YFP expressed in F. fujikuroi under control of its native promoter revealed higher accumulation in spores (conidia) produced by light-exposed mycelia. Germination analyses of conidia from carO(-) mutant and carO(+) control strains showed a faster development of light-exposed carO(-) germlings. In conclusion, CarO is an active proton pump, abundant in light-formed conidia, whose activity slows down early hyphal development under light. Interestingly, CarO-related rhodopsins are typically found in plant-associated fungi, where green light dominates the phyllosphere. Our data provide the first reliable clue on a possible biological role of a fungal rhodopsin.

Stable closure of the cytoplasmic half-channel is required for efficient proton transport at physiological membrane potentials in the bacteriorhodopsin catalytic cycle

The bacteriorhodopsin (BR) Asp96Gly/Phe171Cys/Phe219Leu triple mutant has been shown to translocate protons 66% as efficiently as the wild-type protein. Light-dependent ATP synthesis in haloarchaeal cells expressing the triple mutant is 85% that of the wild-type BR expressing cells. Therefore, the functional activity of BR seems to be largely preserved in the triple mutant despite the observations that its ground-state structure resembles that of the wild-type M state (i.e., the so-called cytoplasmically open state) and that the mutant shows no significant structural changes during its photocycle, in sharp contrast to what occurs in the wild-type protein in which a large structural opening and closing occurs on the cytoplasmic side. To resolve the contradiction between the apparent functional robustness of the triple mutant and the presumed importance of the opening and closing that occurs in the wild-type protein, we conducted additional experiments to compare the behavior of wild-type and mutant proteins under different operational loads. Specifically, we characterized the ability of the two proteins to generate light-driven proton currents against a range of membrane potentials. The wild-type protein showed maximal conductance between −150 and −50 mV, whereas the mutant showed maximal conductance at membrane potentials >+50 mV. Molecular dynamics (MD) simulations of the triple mutant were also conducted to characterize structural changes in the protein and in solvent accessibility that might help to functionally contextualize the current–voltage data. These simulations revealed that the cytoplasmic half-channel of the triple mutant is constitutively open and dynamically exchanges water with the bulk. Collectively, the data and simulations help to explain why this mutant BR does not mediate photosynthetic growth of haloarchaeal cells, and they suggest that the structural closing observed in the wild-type protein likely plays a key role in minimizing substrate back flow in the face of electrochemical driving forces present at physiological membrane potentials.

Gloeobacter rhodopsin, limitation of proton pumping at high electrochemical load

We studied the photocurrents of a cyanobacterial rhodopsin Gloeobacter violaceus (GR) in Xenopus laevis oocytes and HEK-293 cells. This protein is a light-driven proton pump with striking similarities to marine proteorhodopsins, including the D121-H87 cluster of the retinal Schiff base counterion and a glutamate at position 132 that acts as a proton donor for chromophore reprotonation during the photocycle. Interestingly, at low extracellular pH(o) and negative voltage, the proton flux inverted and directed inward. Using electrophysiological measurements of wild-type and mutant GR, we demonstrate that the electrochemical gradient limits outward-directed proton pumping and converts it into a purely passive proton influx. This conclusion contradicts the contemporary paradigm that at low pH, proteorhodopsins actively transport H(+) into cells. We identified E132 and S77 as key residues that allow inward directed diffusion. Substitution of E132 with aspartate or S77 with either alanine or cysteine abolished the inward-directed current almost completely. The proton influx is likely caused by the pK(a) of E132 in GR, which is lower than that of other microbial ion pumping rhodopsins. The advantage of such a low pK(a) is an acceleration of the photocycle and high pump turnover at high light intensities.

Proteorhodopsin

Proteorhodopsins are the most abundant retinal based photoreceptors and their phototrophic function might be relevant in marine ecosystems. Here, we describe their remarkable molecular properties with a special focus on the green absorbing variant. Its distinct features include a high pKa value of the primary proton acceptor stabilized through an interaction with a conserved histidine, a long-range interaction between the cytoplasmic EF loop and the chromophore entailing a particular mode of color tuning and a variable proton pumping vectoriality with complex voltage-dependence. The proteorhodopsin family represents a profound example for structure-function relationships. Especially the development of a biophysical understanding of green proteorhodopsin is an excellent example for the unique opportunities offered by a combined approach of advanced spectroscopic and electrophysiological methods. This article is part of a Special Issue entitled: Retinal Proteins-You can teach an old dog new tricks.

Voltage dependence of proton pumping by bacteriorhodopsin mutants with altered lifetime of the M intermediate

The light-driven proton pump bacteriorhodopsin (BR) from Halobacterium salinarum is tightly regulated by the [H(+)] gradient and transmembrane potential. BR exhibits optoelectric properties, since spectral changes during the photocycle are kinetically controlled by voltage, which predestines BR for optical storage or processing devices. BR mutants with prolonged lifetime of the blue-shifted M intermediate would be advantageous, but the optoelectric properties of such mutants are still elusive. Using expression in Xenopus oocytes and two-electrode voltage-clamping, we analyzed photocurrents of BR mutants with kinetically destabilized (F171C, F219L) or stabilized (D96N, D96G) M intermediate in response to green light (to probe H(+) pumping) and blue laser flashes (to probe accumulation/decay of M). These mutants have divergent M lifetimes. As for BR-WT, this strictly correlates with the voltage dependence of H(+) pumping. BR-F171C and BR-F219L showed photocurrents similar to BR-WT. Yet, BR-F171C showed a weaker voltage dependence of proton pumping. For both mutants, blue laser flashes applied during and after green-light illumination showed reduced M accumulation and shorter M lifetime. In contrast, BR-D96G and BR-D96N exhibited small photocurrents, with nonlinear current-voltage curves, which increased strongly in the presence of azide. Blue laser flashes showed heavy M accumulation and prolonged M lifetime, which accounts for the strongly reduced H(+) pumping rate. Hyperpolarizing potentials augmented these effects. The combination of M-stabilizing and -destabilizing mutations in BR-D96G/F171C/F219L (BR-tri) shows that disruption of the primary proton donor Asp-96 is fatal for BR as a proton pump. Mechanistically, M destabilizing mutations cannot compensate for the disruption of Asp-96. Accordingly, BR-tri and BR-D96G photocurrents were similar. However, BR-tri showed negative blue laser flash-induced currents even without actinic green light, indicating that Schiff base deprotonation in BR-tri exists in the dark, in line with previous spectroscopic investigations. Thus, M-stabilizing mutations, including the triple mutation, drastically interfere with electrochemical H(+) gradient generation.

Of ion pumps, sensors and channels - perspectives on microbial rhodopsins between science and history

We present a historical overview of research on microbial rhodopsins ranging from the 1960s to the present date. Bacteriorhodopsin (BR), the first identified microbial rhodopsin, was discovered in the context of cell and membrane biology and shown to be an outward directed proton transporter. In the 1970s, BR had a big impact on membrane structural research and bioenergetics, that made it to a model for membrane proteins and established it as a probe for the introduction of various biophysical techniques that are widely used today. Halorhodopsin (HR), which supports BR physiologically by transporting negatively charged Cl⁻ into the cell, is researched within the microbial rhodopsin community since the late 1970s. A few years earlier, the observation of phototactic responses in halobacteria initiated research on what are known today as sensory rhodopsins (SR). The discovery of the light-driven ion channel, channelrhodopsin (ChR), serving as photoreceptors for behavioral responses in green alga has complemented inquiries into this photoreceptor family. Comparing the discovery stories, we show that these followed quite different patterns, albeit the objects of research being very similar. The stories of microbial rhodopsins present a comprehensive perspective on what can nowadays be considered one of nature's paradigms for interactions between organisms and light. Moreover, they illustrate the unfolding of this paradigm within the broader conceptual and instrumental framework of the molecular life sciences. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.

Intramolecular proton transfer in channelrhodopsins

Channelrhodopsins serve as photoreceptors that control the motility behavior of green flagellate algae and act as light-gated ion channels when heterologously expressed in animal cells. Here, we report direct measurements of proton transfer from the retinylidene Schiff base in several channelrhodopsin variants expressed in HEK293 cells. A fast outward-directed current precedes the passive channel current that has the opposite direction at physiological holding potentials. This rapid charge movement occurs on the timescale of the M intermediate formation in microbial rhodopsins, including that for channelrhodopsin from Chlamydomonas augustae and its mutants, reported in this study. Mutant analysis showed that the glutamate residue corresponding to Asp(85) in bacteriorhodopsin acts as the primary acceptor of the Schiff-base proton in low-efficiency channelrhodopsins. Another photoactive-site residue corresponding to Asp(212) in bacteriorhodopsin serves as an alternative proton acceptor and plays a more important role in channel opening than the primary acceptor. In more efficient channelrhodopsins from Chlamydomonas reinhardtii, Mesostigma viride, and Platymonas (Tetraselmis) subcordiformis, the fast current was apparently absent. The inverse correlation of the outward proton transfer and channel activity is consistent with channel function evolving in channelrhodopsins at the expense of their capacity for active proton transport.

Theoretical principles underlying optical stimulation of myelinated axons expressing channelrhodopsin-2

Numerous clinical conditions can be treated by neuromodulation of the peripheral nervous system (PNS). Typical electrical PNS therapies activate large diameter axons at lower electrical stimulus thresholds than small diameter axons. However, recent animal experiments with peripheral optogenetic neural stimulation (PONS) of myelinated axons expressing channelrhodopsin-2 (ChR2) have shown that this technique activates small diameter axons at lower irradiances than large diameter axons. We hypothesized that the small-to-large diameter recruitment order primarily arises from the internodal spacing relationship of myelinated axons. Small diameter axons have shorter distances between their nodes of Ranvier, which increases the number of nodes of Ranvier directly illuminated relative to larger diameter axons. We constructed "light-axon" PONS models that included multi-compartment, double cable, myelinated axon models embedded with ChR2 membrane dynamics, coupled with a model of blue light dynamics in the tissue medium from a range of different light sources. The light-axon models enabled direct calculation of threshold irradiance for different diameter axons. Our simulations demonstrate that illumination of multiple nodal sections reduces the threshold irradiance and enhances the small-to-large diameter recruitment order. In addition to addressing biophysical questions, our light-axon model system could also be useful in guiding the engineering design of optical stimulation technology that could maximize the efficiency and selectivity of PONS.

Experimental and modeling analysis of Synechocystis sp. PCC 6803 growth

BACKGROUND/AIMS

The influence of different parameters such as temperature, irradiance, nitrate concentration, pH, and an external carbon source on Synechocystis PCC 6803 growth was evaluated.

METHODS

4.5-ml cuvettes containing 2 ml of culture, a high-throughput system equivalent to batch cultures, were used with gas exchange ensured by the use of a Parafilm™ cover. The effect of the different variables on maximum growth was assessed by a multi-way statistical analysis.

RESULTS

Temperature and pH were identified as the key factors. It was observed that Synechocystis cells have a strong influence on the external pH. The optimal growth temperature was 33°C while light-saturating conditions were reached at 40 µE·m⁻²·s⁻¹.

CONCLUSION

It was demonstrated that Synechocystis exhibits a marked difference in behavior between autotrophic and glucose-based mixotrophic conditions, and that nitrate concentrations did not have a significant influence, probably due to endogenous nitrogen reserves. Furthermore, a dynamic metabolic model of Synechocystis photosynthesis was developed to gain insights on the underlying mechanism enabling this cyanobacterium to control the levels of external pH. The model showed a coupled effect between the increase of the pH and ATP production which in turn allows a higher carbon fixation rate.

Theoretical principles underlying optical stimulation of a channelrhodopsin-2 positive pyramidal neuron

Optogenetics is an emerging field of neuromodulation that permits scaled, millisecond temporal control of the membrane dynamics of genetically targeted cells using light. Optogenetic technology has revolutionized neuroscience research; however, numerous biophysical questions remain on the optical and neuronal factors impacting the modulation of neural activity with photon-sensitive ion channels. To begin to address such questions, we developed a computational tool to explore the underlying principles of optogenetic neural stimulation. This "light-neuron" model consists of theoretical representations of the light dynamics generated by a fiber optic in brain tissue, coupled to a multicompartment cable model of a cortical pyramidal neuron embedded with channelrhodopsin-2 (ChR2) membrane dynamics. Simulations revealed that the large energies required to generate an action potential are primarily due to the limited conductivity of ChR2, and that the major determinants of stimulation threshold are the surface area of illuminated cell membrane and proximity to the light source. Our results predict that the activation threshold is sensitive to many of the properties of ChR2 (density, conductivity, and kinetics), tissue medium (scattering and absorbance), and the fiber-optic light source (diameter and numerical aperture). We also illustrate the impact of redistributing the ChR2 expression density (uniform vs. nonuniform) on the activation threshold. The model system developed in this study represents a scientific instrument to characterize the effects of optogenetic neuromodulation, as well as an engineering design tool to help guide future development of optogenetic technology.

A history of optogenetics: the development of tools for controlling brain circuits with light

Understanding how different kinds of neuron in the brain work together to implement sensations, feelings, thoughts, and movements, and how deficits in specific kinds of neuron result in brain diseases, has long been a priority in basic and clinical neuroscience. “Optogenetic” tools are genetically encoded molecules that, when targeted to specific neurons in the brain, enable their activity to be driven or silenced by light. These molecules are microbial opsins, seven-transmembrane proteins adapted from organisms found throughout the world, which react to light by transporting ions across the lipid membranes of cells in which they are genetically expressed. These tools are enabling the causal assessment of the roles that different sets of neurons play within neural circuits, and are accordingly being used to reveal how different sets of neurons contribute to the emergent computational and behavioral functions of the brain. These tools are also being explored as components of prototype neural control prosthetics capable of correcting neural circuit computations that have gone awry in brain disorders. This review gives an account of the birth of optogenetics and discusses the technology and its applications.

A history of optogenetics: the development of tools for controlling brain circuits with light

Understanding how different kinds of neuron in the brain work together to implement sensations, feelings, thoughts, and movements, and how deficits in specific kinds of neuron result in brain diseases, has long been a priority in basic and clinical neuroscience. "Optogenetic" tools are genetically encoded molecules that, when targeted to specific neurons in the brain, enable their activity to be driven or silenced by light. These molecules are microbial opsins, seven-transmembrane proteins adapted from organisms found throughout the world, which react to light by transporting ions across the lipid membranes of cells in which they are genetically expressed. These tools are enabling the causal assessment of the roles that different sets of neurons play within neural circuits, and are accordingly being used to reveal how different sets of neurons contribute to the emergent computational and behavioral functions of the brain. These tools are also being explored as components of prototype neural control prosthetics capable of correcting neural circuit computations that have gone awry in brain disorders. This review gives an account of the birth of optogenetics and discusses the technology and its applications.

Enhancement of survival and electricity production in an engineered bacterium by light-driven proton pumping

Microorganisms can use complex photosystems or light-dependent proton pumps to generate membrane potential and/or reduce electron carriers to support growth. The discovery that proteorhodopsin is a light-dependent proton pump that can be expressed readily in recombinant bacteria enables development of new strategies to probe microbial physiology and to engineer microbes with new light-driven properties. Here, we describe functional expression of proteorhodopsin and light-induced changes in membrane potential in the bacterium Shewanella oneidensis strain MR-1. We report that there were significant increases in electrical current generation during illumination of electrochemical chambers containing S. oneidensis expressing proteorhodopsin. We present evidence that an engineered strain is able to consume lactate at an increased rate when it is illuminated, which is consistent with the hypothesis that proteorhodopsin activity enhances lactate uptake by increasing the proton motive force. Our results demonstrate that there is coupling of a light-driven process to electricity generation in a nonphotosynthetic engineered bacterium. Expression of proteorhodopsin also preserved the viability of the bacterium under nutrient-limited conditions, providing evidence that fulfillment of basic energy needs of organisms may explain the widespread distribution of proteorhodopsin in marine environments.

Modeling the membrane potential generation of bacteriorhodopsin

The archaeon Halobacterium salinarum can grow phototrophically with only light as its energy source. It uses the retinal containing and light-driven proton pump bacteriorhodopsin to enhance the membrane potential which drives the ATP synthase. Therefore, a model of the membrane potential generation of bacteriorhodopsin is of central importance to the development of a mathematical model of the bioenergetics of H. salinarum. To measure the current produced by bacteriorhodopsin at different light intensities and clamped voltages, we expressed the gene in Xenopus laevis oocytes. We present current-voltage measurements and a mathematical model of the current-voltage relationship of bacteriorhodopsin and its generation of the membrane potential. The model consists of three intermediate states, the BR, L, and M states, and comparisons between model predictions and experimental data show that the L to M reaction must be inhibited by the membrane potential. The model is not able to fit the current-voltage measurements when only the M to BR phase is membrane potential dependent, while it is able to do so when either only the L to M reaction or both reactions (L to M and M to BR) are membrane potential dependent. We also show that a decay term is necessary for modeling the rate of change of the membrane potential.

Mechanistic insights in light-induced cAMP production by photoactivated adenylyl cyclase alpha (PACalpha)

The flagellate Euglena gracilis contains as photoreceptor complex a heterotetrameric light-sensitive adenylyl cyclase (AC), consisting of the flavoproteins PACalpha and PACbeta. Previously, we demonstrated the functional expression of PACalpha and PACbeta in oocytes from Xenopus laevis and of PACalpha in different animal cell types. Both yielded a blue light-induced increase of cellular [cAMP]. Here, we report that the action spectrum of PACalpha is flavoprotein-typical, with maxima at approximately 380 and approximately 470 nm. Mutational analysis of PACalpha yields a model for its structure and function. PACalpha shows a basal AC activity in the dark which is unaffected by mutating the conserved tyrosines in the two flavin-binding domains (F1, F2), Y60 in F1 and Y472 in F2. Y60 in F1 is, however, essential for photoactivation as light-stimulation of cyclase activity is completely lost in the F1 mutant Y60F. This effect does not occur in the respective mutation in F2 (Y472F). Mutating the two cyclase domains (C1, C2) indicated that C1 and C2 form a heterodimeric catalytic center as in mammalian class III cyclases. Interaction of C1 with C2 in the same molecule could be excluded as coexpression of non-functional C1 and C2 mutants restored light-induced cyclase activity. Our results strongly suggest an intermolecular dimerization of C1 and C2 domains on PACalpha for a functional enzyme.

Light-energy conversion in engineered microorganisms

Increasing interest in renewable resources by the energy and chemical industries has spurred new technologies both to capture solar energy and to develop biologically derived chemical feedstocks and fuels. Advances in molecular biology and metabolic engineering have provided new insights and techniques for increasing biomass and biohydrogen production, and recent efforts in synthetic biology have demonstrated that complex regulatory and metabolic networks can be designed and engineered in microorganisms. Here, we explore how light-driven processes may be incorporated into nonphotosynthetic microbes to boost metabolic capacity for the production of industrial and fine chemicals. Progress towards the introduction of light-driven proton pumping or anoxygenic photosynthesis into Escherichia coli to increase the efficiency of metabolically-engineered biosynthetic pathways is highlighted.

In-depth activation of channelrhodopsin 2-sensitized excitable cells with high spatial resolution using two-photon excitation with a near-infrared laser microbeam

We used two-photon excitation with a near-infrared (NIR) laser microbeam to investigate activation of channelrhodopsin 2 (ChR2) in excitable cells for the first time to our knowledge. By measuring the fluorescence intensity of the calcium (Ca) indicator dye, Ca orange, at different wavelengths as a function of power of the two-photon excitation microbeam, we determined the activation potential of the NIR microbeam as a function of wavelength. The two-photon activation spectrum is found to match measurements carried out with single-photon activation. However, two-photon activation is found to increase in a nonlinear manner with the power density of the two-photon laser microbeam. This approach allowed us to activate different regions of ChR2-sensitized excitable cells with high spatial resolution. Further, in-depth activation of ChR2 in a spheroid cellular model as well as in mouse brain slices was demonstrated by the use of the two-photon NIR microbeam, which was not possible using single-photon activation. This all-optical method of identification, activation, and detection of ChR2-induced cellular activation in genetically targeted cells with high spatial and temporal resolution will provide a new method of performing minimally invasive in-depth activation of specific target areas of tissues or organisms that have been rendered photosensitive by genetic targeting of ChR2 or similar photo-excitable molecules.

Dynamics of voltage profile in enzymatic ion transporters, demonstrated in electrokinetics of proton pumping rhodopsin

H(+)-pumping rhodopsins mediate a primordial conversion of light to metabolic energy. Bacteriorhodopsin from Halobacterium salinarium is the first identified and (biochemically) best-studied H(+)-pumping rhodopsin. The electrical properties of H(+)-pumping rhodopsins, however, are known in more detail for the homolog Acetabularia rhodopsin, isolated from the eukaryotic green alga Acetabularia acetabulum. Based on data from Acetabularia rhodopsin we present a general reaction kinetic model of H(+)-pumping rhodopsins with only seven independent parameters, which fits the kinetic properties of photocurrents as functions of light, transmembrane voltage, internal and external pH, and time. The model describes fast photoisomerization of retinal with simultaneous H(+) transfer to an H(+) acceptor, reprotonation of retinal from the intracellular face via an H(+) donor, and proton release to the extracellular space via an H(+) release complex. The voltage sensitivities of the individual reaction steps and their temporal changes are treated here by a novel approach, whereby--as in an Ohmic voltage divider--the effective portions of the total transmembrane voltage decrease with the relative velocities of the individual reaction steps. This analysis quantitatively infers dynamic changes of the voltage profile and of the pK values of the H(+)-binding sites involved.