Pado, a fluorescent protein with proton channel activity can optically monitor membrane potential, intracellular pH, and map gap junctions

Hydrophobic residues in S1 modulate enzymatic function and voltage sensing in voltage-sensing phosphatase

The voltage-sensing domain (VSD) is a four-helix modular protein domain that converts electrical signals into conformational changes, leading to open pores and active enzymes. In most voltage-sensing proteins, the VSDs do not interact with one another, and the S1-S3 helices are considered mainly scaffolding, except in the voltage-sensing phosphatase (VSP) and the proton channel (Hv). To investigate its contribution to VSP function, we mutated four hydrophobic amino acids in S1 to alanine (F127, I131, I134, and L137), individually or in combination. Most of these mutations shifted the voltage dependence of activity to higher voltages; however, not all substrate reactions were the same. The kinetics of enzymatic activity were also altered, with some mutations significantly slowing down dephosphorylation. The voltage dependence of VSD motions was consistently shifted to lower voltages and indicated a second voltage-dependent motion. Additionally, none of the mutations broke the VSP dimer, indicating that the S1 impact could stem from intra- and/or intersubunit interactions. Lastly, when the same mutations were introduced into a genetically encoded voltage indicator, they dramatically altered the optical readings, making some of the kinetics faster and shifting the voltage dependence. These results indicate that the S1 helix in VSP plays a critical role in tuning the enzyme's conformational response to membrane potential transients and influencing the function of the VSD.

Imaging different cell populations in the mouse olfactory bulb using the genetically encoded voltage indicator ArcLight

Genetically encoded voltage indicators (GEVIs) are protein-based optical sensors that allow for measurements from genetically defined populations of neurons. Although in vivo imaging in the mammalian brain with early generation GEVIs was difficult due to poor membrane expression and low signal-to-noise ratio, newer and more sensitive GEVIs have begun to make them useful for answering fundamental questions in neuroscience. We discuss principles of imaging using GEVIs and genetically encoded calcium indicators, both useful tools for in vivo imaging of neuronal activity, and review some of the recent mechanistic advances that have led to GEVI improvements. We provide an overview of the mouse olfactory bulb (OB) and discuss recent studies using the GEVI ArcLight to study different cell types within the bulb using both widefield and two-photon microscopy. Specific emphasis is placed on using GEVIs to begin to study the principles of concentration coding in the OB, how to interpret the optical signals from population measurements in the in vivo brain, and future developments that will push the field forward.

S1 hydrophobic residues modulate voltage sensing phosphatase enzymatic function and voltage sensing

The voltage sensing domain (VSD) is a four-helix modular protein domain that converts electrical signals into conformational changes, leading to open pores and active enzymes. In most voltage sensing proteins, the VSDs do not interact with one another and the S1-S3 helices are considered mainly as scaffolding. The two exceptions are the voltage sensing phosphatase (VSP) and the proton channel (Hv). VSP is a voltage-regulated enzyme and Hvs are channels that only have VSDs. To investigate the S1 contribution to VSP function, we individually mutated four hydrophobic amino acids in S1 to alanine (F127, I131, I134 and L137). We also combined these mutations to generate quadruple mutation designated S1-Q. Most of these mutations shifted the voltage dependence of activity to higher voltages though interestingly, not all substrate reactions were the same. The kinetics of enzymatic activity were also altered with some mutations significantly slowing down dephosphorylation. The voltage dependence of VSD motions were consistently shifted to lower voltages and indicated a second voltage dependent motion. Co-immunoprecipitation demonstrated that none of the mutations broke the VSP dimer indicating that the S1 impact could stem from intrasubunit and/or intersubunit interactions. Lastly, when the same alanine mutations were introduced into a genetically encoded voltage indicator, they dramatically altered the optical readings, making some of the kinetics faster and shifting the voltage dependence. These results indicate that the S1 helix in VSP plays a critical role in tuning the enzymes conformational response to membrane potential transients and influencing the function of the VSD.

Transcendent Aspects of Proton Channels

A handful of biological proton-selective ion channels exist. Some open at positive or negative membrane potentials, others open at low or high pH, and some are light activated. This review focuses on common features that result from the unique properties of protons. Proton conduction through water or proteins differs qualitatively from that of all other ions. Extraordinary proton selectivity is needed to ensure that protons permeate and other ions do not. Proton selectivity arises from a proton pathway comprising a hydrogen-bonded chain that typically includes at least one titratable amino acid side chain. The enormously diverse functions of proton channels in disparate regions of the phylogenetic tree can be summarized by considering the chemical and electrical consequences of proton flux across membranes. This review discusses examples of cells in which proton efflux serves to increase pHi, decrease pHo, control the membrane potential, generate action potentials, or compensate transmembrane movement of electrical charge.

A new player in circadian networks: Role of electrical synapses in regulating functions of the circadian clock

Several studies have indicated that coherent circadian rhythms in behaviour can be manifested only when the underlying circadian oscillators function as a well-coupled network. The current literature suggests that circadian pacemaker neuronal networks rely heavily on communication mediated by chemical synapses comprising neuropeptides and neurotransmitters to regulate several behaviours and physiological processes. It has become increasingly clear that chemical synapses closely interact with electrical synapses and function together in the neuronal networks of most organisms. However, there are only a few studies which have examined the role of electrical synapses in circadian networks and here, we review our current understanding of gap junction proteins in circadian networks of various model systems. We describe the general mechanisms by which electrical synapses function in neural networks, their interactions with chemical neuromodulators and their contributions to the regulation of circadian rhythms. We also discuss the various methods available to characterize functional electrical synapses in these networks and the potential directions that remain to be explored to understand the roles of this relatively understudied mechanism of communication in modulating circadian behaviour.

Conserved Amino Acids Residing Outside the Voltage Field Can Shift the Voltage Sensitivity and Increase the Signal Speed and Size of Ciona Based GEVIs

To identify potential regions of the voltage-sensing domain that could shift the voltage sensitivity of Ciona intestinalis based Genetically Encoded Voltage Indicators (GEVIs), we aligned the amino acid sequences of voltage-gated sodium channels from different organisms. Conserved polar residues were identified at multiple transmembrane/loop junctions in the voltage sensing domain. Similar conservation of polar amino acids was found in the voltage-sensing domain of the voltage-sensing phosphatase gene family. These conserved residues were mutated to nonpolar or oppositely charged amino acids in a GEVI that utilizes the voltage sensing domain of the voltage sensing phosphatase from Ciona fused to the fluorescent protein, super ecliptic pHluorin (A227D). Different mutations shifted the voltage sensitivity to more positive or more negative membrane potentials. Double mutants were then created by selecting constructs that shifted the optical signal to a more physiologically relevant voltage range. Introduction of these mutations into previously developed GEVIs resulted in Plos6-v2 which improved the dynamic range to 40% ΔF/F/100 mV, a 25% increase over the parent, ArcLight. The onset time constant of Plos6-v2 is also 50% faster than ArcLight. Thus, Plos6-v2 appears to be the GEVI of choice.

Improving the flexibility of genetically encoded voltage indicators via intermolecular FRET

A new family of genetically encoded voltage indicators (GEVIs) has been developed based on intermolecular Förster resonance energy transfer (FRET). To test the hypothesis that the GEVI ArcLight functions via interactions between the fluorescent protein (FP) domains of neighboring probes, the FP of ArcLight was replaced with either a FRET donor or acceptor FP. We discovered relatively large FRET signals only when cells were cotransfected with both the FRET donor and acceptor GEVIs. Using a cyan fluorescent protein donor and an RFP acceptor, we were able to observe a voltage-dependent signal with an emission peak separated by over 200 nm from the excitation wavelength. The intermolecular FRET strategy also works for rhodopsin-based probes, potentially improving their flexibility as well. Separating the FRET pair into two distinct proteins has important advantages over intramolecular FRET constructs. The signals are larger because the voltage-induced conformational change moves two FPs independently. The expression of the FRET donor and acceptor can also be restricted independently, enabling greater cell type specificity as well as refined subcellular voltage reporting.

Mechanism of ArcLight derived GEVIs involves electrostatic interactions that can affect proton wires

The genetically encoded voltage indicators ArcLight and its derivatives mediate voltage-dependent optical signals by intermolecular, electrostatic interactions between neighboring fluorescent proteins (FPs). A random mutagenesis event placed a negative charge on the exterior of the FP, resulting in a greater than 10-fold improvement of the voltage-dependent optical signal. Repositioning this negative charge on the exterior of the FP reversed the polarity of voltage-dependent optical signals, suggesting the presence of “hot spots” capable of interacting with the negative charge on a neighboring FP, thereby changing the fluorescent output. To explore the potential effect on the chromophore state, voltage-clamp fluorometry was performed with alternating excitation at 390 nm followed by excitation at 470 nm, resulting in several mutants exhibiting voltage-dependent, ratiometric optical signals of opposing polarities. However, the kinetics, voltage ranges, and optimal FP fusion sites were different depending on the wavelength of excitation. These results suggest that the FP has external, electrostatic pathways capable of quenching fluorescence that are wavelength specific. One mutation to the FP (E222H) showed a voltage-dependent increase in fluorescence when excited at 390 nm, indicating the ability to affect the proton wire from the protonated chromophore to the H222 position. ArcLight-derived sensors may therefore offer a novel way to map how conditions external to the β-can structure can affect the fluorescence of the chromophore and transiently affect those pathways via conformational changes mediated by manipulating membrane potential.

Biophysical Parameters of GEVIs: Considerations for Imaging Voltage

Genetically encoded voltage indicators (GEVIs) continue to evolve, resulting in many different probes with varying strengths and weaknesses. Developers of new GEVIs tend to highlight their positive features. A recent article from an independent laboratory has compared the signal/noise ratios of a number of GEVIs. Such a comparison can be helpful to investigators eager to try to image the voltage of excitable cells. In this perspective, we will present examples of how the biophysical features of GEVIs affect the imaging of excitable cells in an effort to assist researchers when considering probes for their specific needs.

Engineering Photoactivatability in Genetically Encoded Voltage and pH Indicators

Genetically-encoded indicators of neuronal activity enable the labeling of a genetically defined population of neurons to optically monitor their activities. However, researchers often find difficulties in identifying relevant signals from excessive background fluorescence. A photoactivatable version of a genetically encoded calcium indicator, sPA-GCaMP6f is a good example of circumventing such an obstacle by limiting the fluorescence to a region of interest defined by the user. Here, we apply this strategy to genetically encoded voltage (GEVI) and pH (GEPI) indicators. Three photoactivatable GEVI candidates were considered. The first one used a circularly-permuted fluorescent protein, the second design involved a Förster resonance energy transfer (FRET) pair, and the third approach employed a pH-sensitive variant of GFP, ecliptic pHluorin. The candidate with a variant of ecliptic pHluorin exhibited photoactivation and a voltage-dependent fluorescence change. This effort also yielded a pH-sensitive photoactivatable GFP that varies its brightness in response to intracellular pH changes.

Using Genetically Encoded Voltage Indicators (GEVIs) to Study the Input-Output Transformation of the Mammalian Olfactory Bulb

Genetically encoded voltage indicators (GEVIs) are fluorescent protein reporters of membrane potential. These tools can, in principle, be used to monitor the neural activity of genetically distinct cell types in the brain. Although introduced in 1997, they have been a challenge to use to study intact neural circuits due to a combination of small signal-to-noise ratio, slow kinetics, and poor membrane expression. New strategies have yielded novel GEVIs such as ArcLight, which have improved properties. Here, we compare the in vivo properties of ArcLight with Genetically Encoded Calcium Indicators (GECIs) in the mouse olfactory bulb. We show how voltage imaging can be combined with organic calcium sensitive dyes to measure the input-output transformation of the olfactory bulb. Finally, we demonstrate that ArcLight can be targeted to olfactory bulb interneurons. The olfactory bulb contributes substantially to the perception of the concentration invariance of odor recognition.

Comparative genomic analysis suggests that the sperm-specific sodium/proton exchanger and soluble adenylyl cyclase are key regulators of CatSper among the Metazoa

BACKGROUND

CatSper is a sperm-specific calcium ion (Ca2+) channel, which regulates sperm flagellar beating by tuning cytoplasmic Ca2+ concentrations. Although this Ca2+ channel is essential for mammalian fertilization, recent bioinformatics analyses have revealed that genes encoding CatSper are heterogeneously distributed throughout the eukaryotes, including vertebrates. As this channel is activated by cytoplasmic alkalization in mammals and sea urchins, it has been proposed that the sperm-specific Na+/H+ exchanger (sNHE, a product of the SLC9C gene family) positively regulates its activity. In mouse, sNHE is functionally coupled to soluble adenylyl cyclase (sAC). CatSper, sNHE, and sAC have thus been considered functionally interconnected in the control of sperm motility, at least in mouse and sea urchin.

RESULTS

We carried out a comparative genomic analysis to explore phylogenetic relationships among CatSper, sNHE and sAC in eukaryotes. We found that sNHE occurs only in Metazoa, although sAC occurs widely across eukaryotes. In animals, we found correlated and restricted distribution patterns of the three proteins, suggesting coevolution among them in the Metazoa. Namely, nearly all species in which CatSper is conserved also preserve sNHE and sAC. In contrast, in species without sAC, neither CatSper nor sNHE is conserved. On the other hand, the distribution of another testis-specific NHE (NHA, a product of the SLC9B gene family) does not show any apparent association with that of CatSper.

CONCLUSIONS

Our results suggest that CatSper, sNHE and sAC form prototypical machinery that functions in regulating sperm flagellar beating in Metazoa. In non-metazoan species, CatSper may be regulated by other H+ transporters, or its activity might be independent of cytoplasmic pH.

Imaging and analysis of genetically encoded calcium indicators linking neural circuits and behaviors

Confirming the direct link between neural circuit activity and animal behavior has been a principal aim of neuroscience. The genetically encoded calcium indicator (GECI), which binds to calcium ions and emits fluorescence visualizing intracellular calcium concentration, enables detection of in vivo neuronal firing activity. Various GECIs have been developed and can be chosen for diverse purposes. These GECI-based signals can be acquired by several tools including two-photon microscopy and microendoscopy for precise or wide imaging at cellular to synaptic levels. In addition, the images from GECI signals can be analyzed with open source codes including constrained non-negative matrix factorization for endoscopy data (CNMF_E) and miniscope 1-photon-based calcium imaging signal extraction pipeline (MIN1PIPE), and considering parameters of the imaged brain regions (e.g., diameter or shape of soma or the resolution of recorded images), the real-time activity of each cell can be acquired and linked with animal behaviors. As a result, GECI signal analysis can be a powerful tool for revealing the functions of neuronal circuits related to specific behaviors.

Optimizing Strategies for Developing Genetically Encoded Voltage Indicators

Genetically encoded optical indicators of neuronal activity enable unambiguous recordings of input-output activity patterns from identified cells in intact circuits. Among them, genetically encoded voltage indicators (GEVIs) offer additional advantages over calcium indicators as they are direct sensors of membrane potential and can adeptly report subthreshold events and hyperpolarization. Here, we outline the major GEVI designs and give an account of properties that need to be carefully optimized during indicator engineering. While designing the ideal GEVI, one should keep in mind aspects such as membrane localization, signal size, signal-to-noise ratio, kinetics and voltage dependence of optical responses. Using ArcLight and derivatives as prototypes, we delineate how a probe should be optimized for the former properties and developed along other areas in a need-based manner. Finally, we present an overview of the GEVI engineering process and lend an insight into their discovery, delivery and diagnosis.

PARIS, an optogenetic method for functionally mapping gap junctions

Cell-cell communication via gap junctions regulates a wide range of physiological processes by enabling the direct intercellular electrical and chemical coupling. However, the in vivo distribution and function of gap junctions remain poorly understood, partly due to the lack of non-invasive tools with both cell-type specificity and high spatiotemporal resolution. Here, we developed PARIS (pairing actuators and receivers to optically isolate gap junctions), a new fully genetically encoded tool for measuring the cell-specific gap junctional coupling (GJC). PARIS successfully enabled monitoring of GJC in several cultured cell lines under physiologically relevant conditions and in distinct genetically defined neurons in Drosophila brain, with ~10 s temporal resolution and sub-cellular spatial resolution. These results demonstrate that PARIS is a robust, highly sensitive tool for mapping functional gap junctions and study their regulation in both health and disease.

For the tissues and organs of our bodies to work properly, the cells within them need to communicate with each other. One important part of cellular communication is the movement of signals – usually small molecules or ions – directly from one cell to another. This happens via structures called gap junctions, a type of sealed ‘channel’ that connects two cells.

Gap junctions are found throughout the body, but investigating their precise roles in health and disease has been difficult. This is due to problems with the tools available to detect and monitor gap junctions. Some are simply harmful to cells, while others cannot be restricted to specific cell populations within a tissue. This lack of specificity makes it difficult to study gap junctions in the brain, where it is important to understand the connectivity patterns between distinct types of nerve cells. Wu et al. wanted to develop a new, non-harmful method to track gap junctions in distinct groups of cells within living tissues.

To do this, Wu et al. devised PARIS, a two-part, genetically encoded system. The first part comprises a light-sensitive molecular ‘pump’, which can only be turned on by shining a laser onto the cell of interest. When the pump is active, it transports hydrogen ions out of the cell. The second part of the system is a fluorescent sensor, present inside ‘receiving’ cells, which responds to the outcoming hydrogen ions (small enough to pass through gap junctions). If an illuminated ‘signaling’ cell is connected via gap junctions to cells containing the fluorescent sensor, they will light up within seconds, but other cells not connected through gap junctions will not.

The researchers first tested PARIS in cultured human and rat cells that had been genetically engineered to produce both components of the system. The experiments confirmed that PARIS could both detect networks of gap junctions in healthy cells and reveal when these networks had been disrupted, for instance by drugs or genetic mutations. Experiments using fruit flies demonstrated that PARIS was stable in living tissue and could also map the gap junctions connecting specific groups of nerve cells.

PARIS is a valuable addition to the toolbox available to study cell communication. In the future, it could help increase our understanding of diseases characterized by defective gap junctions, such as seizures, cardiac irregularities, and even some cancers.

Voltage-Sensing Phosphatases: Biophysics, Physiology, and Molecular Engineering

Voltage-sensing phosphatase (VSP) contains a voltage sensor domain (VSD) similar to that in voltage-gated ion channels, and a phosphoinositide phosphatase region similar to phosphatase and tensin homolog deleted on chromosome 10 (PTEN). The VSP gene is conserved from unicellular organisms to higher vertebrates. Membrane depolarization induces electrical driven conformational rearrangement in the VSD, which is translated into catalytic enzyme activity. Biophysical and structural characterization has revealed details of the mechanisms underlying the molecular functions of VSP. Coupling between the VSD and the enzyme is tight, such that enzyme activity is tuned in a graded fashion to the membrane voltage. Upon VSP activation, multiple species of phosphoinositides are simultaneously altered, and the profile of enzyme activity depends on the history of the membrane potential. VSPs have been the obvious candidate link between membrane potential and phosphoinositide regulation. However, patterns of voltage change regulating VSP in native cells remain largely unknown. This review addresses the current understanding of the biophysical biochemical properties of VSP and provides new insight into the proposed functions of VSP.

Optical consequences of a genetically-encoded voltage indicator with a pH sensitive fluorescent protein

Genetically-Encoded Voltage Indicators (GEVIs) are capable of converting changes in membrane potential into an optical signal. Here, we focus on recent insights into the mechanism of ArcLight-type probes and the consequences of utilizing a pH-dependent Fluorescent Protein (FP). A negative charge on the exterior of the β-can of the FP combined with a pH-sensitive FP enables voltage-dependent conformational changes to affect the fluorescence of the probe. This hypothesis implies that interaction/dimerization of the FP creates a microenvironment for the probe that is altered via conformational changes. This mechanism explains why a pH sensitive FP with a negative charge on the outside of the β-can is needed, but also suggests that pH could affect the optical signal as well. To better understand the effects of pH on the voltage-dependent signal of ArcLight, the intracellular pH (pHi) was tested at pH 6.8, 7.2, or 7.8. The resting fluorescence of ArcLight gets brighter as the pHi increases, yet only pH 7.8 significantly affected the ΔF/F. ArcLight could also simultaneously report voltage and pH changes during the acidification of a neuron firing multiple action potentials revealing different buffering capacities of the soma versus the processes of the cell.

A dimeric fluorescent protein yields a bright, red-shifted GEVI capable of population signals in brain slice

A bright, red-shifted Genetically Encoded Voltage Indicator (GEVI) was developed using a modified version of the fluorescent protein, tdTomato. Dimerization of the fluorescent domain for ArcLight-type GEVIs has been shown to affect the signal size of the voltage-dependent optical signal. For red-shifted GEVI development, tdTomato was split fusing a single dTomato chromophore to the voltage sensing domain. Optimization of the amino acid length and charge composition of the linker region between the voltage sensing domain and the fluorescent protein resulted in a probe that is an order of magnitude brighter than FlicR1 at a resting potential of -70 mV and exhibits a ten-fold larger change in fluorescence (ΔF) upon 100 mV depolarization of the plasma membrane in HEK 293 cells. Unlike ArcLight, the introduction of charged residues to the exterior of dTomato did not substantially improve the dynamic range of the optical signal. As a result, this new GEVI, Ilmol, yields a 3-fold improvement in the signal-to-noise ratio compared to FlicR1 despite a smaller fractional change in fluorescence of 4% per 100 mV depolarization of the plasma membrane. Ilmol expresses well in neurons resolving action potentials in neuronal cultures and reporting population signals in mouse hippocampal acute brain slice recordings. Ilmol is the brightest red-shifted GEVI to date enabling imaging with 160-fold less light than Archon1 for primary neuron recordings (50 mW/cm² versus 8 W/cm²) and 600-fold less light than QuasAr2 for mouse brain slice recordings (500 mW/cm² versus 300 W/cm²). This new GEVI uses a distinct mechanism from other approaches, opening an alternate engineering path to improve sensitivity and speed.

Gap Junctions in the Nervous System: Probing Functional Connections Using New Imaging Approaches

Gap junctions are channels that physically connect adjacent cells, mediating the rapid exchange of small molecules, and playing an essential role in a wide range of physiological processes in nearly every system in the body, including the nervous system. Thus, altered function of gap junctions has been linked with a plethora of diseases and pathological conditions. Being able to measure and characterize the distribution, function, and regulation of gap junctions in intact tissue is therefore essential for understanding the physiological and pathophysiological roles that gap junctions play. In recent decades, several robust in vitro and in vivo methods have been developed for detecting and characterizing gap junctions. Here, we review the currently available methods with respect to invasiveness, signal-to-noise ratio, temporal resolution and others, highlighting the recently developed chemical tracers and hybrid imaging systems that use novel chemical compounds and/or genetically encoded enzymes, transporters, channels, and fluorescent proteins in order to map gap junctions. Finally, we discuss possible avenues for further improving existing techniques in order to achieve highly sensitive, cell type-specific, non-invasive measures of in vivo gap junction function with high throughput and high spatiotemporal resolution.

Monitoring voltage fluctuations of intracellular membranes

In eukaryotic cells, the endoplasmic reticulum (ER) is the largest continuous membrane-enclosed network which surrounds a single lumen. Using a new genetically encoded voltage indicator (GEVI), we applied the patch clamp technique to cultured HEK293 cells and neurons and found that there is a very fast electrical interaction between the plasma membrane and internal membrane(s). This discovery suggests a novel mechanism for interaction between the external membrane and internal membranes as well as mechanisms for interactions between the various internal membranes. The ER may transfer electrical signals between the plasma membrane and other internal organelles. The internal membrane optical signal is reversed in polarity but has a time course similar to that of the plasma membrane signal. The optical signal of the GEVI in the plasma membrane is consistent from trial to trial. However, the internal signal decreases in size with repeated trials suggesting that the electrical coupling is degrading and/or the resistance of the internal membrane is decaying.

Voltage and pH sensing by the voltage-gated proton channel, HV1

Voltage-gated proton channels are unique ion channels, membrane proteins that allow protons but no other ions to cross cell membranes. They are found in diverse species, from unicellular marine life to humans. In all cells, their function requires that they open and conduct current only under certain conditions, typically when the electrochemical gradient for protons is outwards. Consequently, these proteins behave like rectifiers, conducting protons out of cells. Their activity has electrical consequences and also changes the pH on both sides of the membrane. Here we summarize what is known about the way these proteins sense the membrane potential and the pH inside and outside the cell. Currently, it is hypothesized that membrane potential is sensed by permanently charged arginines (with very high pKa) within the protein, which results in parts of the protein moving to produce a conduction pathway. The mechanism of pH sensing appears to involve titratable side chains of particular amino acids. For this purpose their pKa needs to be within the operational pH range. We propose a 'counter-charge' model for pH sensing in which electrostatic interactions within the protein are selectively disrupted by protonation of internally or externally accessible groups.

Insights into the structure and function of HV1 from a meta-analysis of mutation studies

The voltage-gated proton channel (HV1) is a widely distributed, proton-specific ion channel with unique properties. Since 2006, when genes for HV1 were identified, a vast array of mutations have been generated and characterized. Accessing this potentially useful resource is hindered, however, by the sheer number of mutations and interspecies differences in amino acid numbering. This review organizes all existing information in a logical manner to allow swift identification of studies that have characterized any particular mutation. Although much can be gained from this meta-analysis, important questions about the inner workings of HV1 await future revelation.

Voltage and Calcium Imaging of Brain Activity

Sensors for imaging brain activity have been under development for almost 50 years. The development of some of these tools is relatively mature, whereas qualitative improvements of others are needed and are actively pursued. In particular, genetically encoded voltage indicators are just now starting to be used to answer neurobiological questions and, at the same time, more than 10 laboratories are working to improve them. In this Biophysical Perspective, we attempt to discuss the present state of the art and indicate areas of active development.

Modulating the Voltage-sensitivity of a Genetically Encoded Voltage Indicator

Saturation mutagenesis was performed on a single position in the voltage-sensing domain (VSD) of a genetically encoded voltage indicator (GEVI). The VSD consists of four transmembrane helixes designated S1-S4. The V220 position located near the plasma membrane/extracellular interface had previously been shown to affect the voltage range of the optical signal. Introduction of polar amino acids at this position reduced the voltage-dependent optical signal of the GEVI. Negatively charged amino acids slightly reduced the optical signal by 33 percent while positively charge amino acids at this position reduced the optical signal by 80%. Surprisingly, the range of V220D was similar to that of V220K with shifted optical responses towards negative potentials. In contrast, the V220E mutant mirrored the responses of the V220R mutation suggesting that the length of the side chain plays in role in determining the voltage range of the GEVI. Charged mutations at the 219 position all behaved similarly slightly shifting the optical response to more negative potentials. Charged mutations to the 221 position behaved erratically suggesting interactions with the plasma membrane and/or other amino acids in the VSD. Introduction of bulky amino acids at the V220 position increased the range of the optical response to include hyperpolarizing signals. Combining The V220W mutant with the R217Q mutation resulted in a probe that reduced the depolarizing signal and enhanced the hyperpolarizing signal which may lead to GEVIs that only report neuronal inhibition.

Improving a genetically encoded voltage indicator by modifying the cytoplasmic charge composition

An improved genetically encoded voltage indicator (GEVI) was achieved by altering the charge composition of the region linking the voltage-sensing domain of the GEVI to a pH-sensitive fluorescent protein. Negatively charged linker segments reduced the voltage-dependent optical signal while positively charged linkers increased the signal size. Arginine scanning mutagenesis of the linker region improved the signal size of the GEVI, Bongwoori, yielding fluorescent signals as high as 20% ΔF/F during the firing of action potentials. The speed of this new sensor was also capable of optically resolving action potentials firing at 65 Hz. This large signal size enabled individual pixels to become surrogate electrodes. Plotting the highest correlated pixels based only on fluorescence changes reproduced the image of the neuron exhibiting activity. Furthermore, the use of a pH-sensitive fluorescent protein facilitated the detection of the acidification of the neuron during the firing of action potentials.

NADPH Oxidases: Insights into Selected Functions and Mechanisms of Action in Cancer and Stem Cells

NADPH oxidases (NOX) are reactive oxygen species- (ROS-) generating enzymes regulating numerous redox-dependent signaling pathways. NOX are important regulators of cell differentiation, growth, and proliferation and of mechanisms, important for a wide range of processes from embryonic development, through tissue regeneration to the development and spread of cancer. In this review, we discuss the roles of NOX and NOX-derived ROS in the functioning of stem cells and cancer stem cells and in selected aspects of cancer cell physiology. Understanding the functions and complex activities of NOX is important for the application of stem cells in tissue engineering, regenerative medicine, and development of new therapies toward invasive forms of cancers.

Directed Evolution of Key Residues in Fluorescent Protein Inverses the Polarity of Voltage Sensitivity in the Genetically Encoded Indicator ArcLight

Genetically encoded calcium indicators (GECIs) produce unprecedentedly large signals that have enabled routine optical recording of single neuron activity in vivo in rodent brain. Genetically encoded voltage indicators (GEVIs) offer a more direct measure of neuronal electrical status, however the signal-to-noise characteristics and signal polarity of the probes developed to date have precluded routine use in vivo. We applied directed evolution to target modulable areas of the fluorescent protein in GEVI ArcLight to create the first GFP-based GEVI (Marina) that exhibits a ΔF/ΔV with a positive slope relationship. We found that only three rounds of site-directed mutagenesis produced a family of “brightening” GEVIs with voltage sensitivities comparable to that seen in the parent probe ArcLight. This shift in signal polarity is an essential first step to producing voltage indicators with signal-to-noise characteristics comparable to GECIs to support widespread use in vivo.

Deciphering the molecular mechanism responsible for GCaMP6m's Ca2+-dependent change in fluorescence

The goal of this work is to determine how GCaMP6m's fluorescence is altered in response to Ca2+-binding. Our detailed spectroscopic study reveals the simplest explanation for how GCaMP6m changes fluorescence in response to Ca2+ is with a four-state model, in which a Ca2+-dependent change of the chromophore protonation state, due to a shift in pKa, is the predominant factor. The pKa shift is quantitatively explained by a change in electrostatic potential around the chromophore due to the conformational changes that occur in the protein when calmodulin binds Ca2+ and interacts with the M13 peptide. The absolute pKa values for the Ca2+-free and Ca2+-saturated states of GCaMP6m are critical to its high signal-to-noise ratio. This mechanism has important implications for further improvements to GCaMP6m and potentially for other similarly designed biosensors.

Genetically Encoded Voltage Indicators: Opportunities and Challenges

A longstanding goal in neuroscience is to understand how spatiotemporal patterns of neuronal electrical activity underlie brain function, from sensory representations to decision making. An emerging technology for monitoring electrical dynamics, voltage imaging using genetically encoded voltage indicators (GEVIs), couples the power of genetics with the advantages of light. Here, we review the properties that determine indicator performance and applicability, discussing both recent progress and technical limitations. We then consider GEVI applications, highlighting studies that have already deployed GEVIs for biological discovery. We also examine which classes of biological questions GEVIs are primed to address and which ones are beyond their current capabilities. As GEVIs are further developed, we anticipate that they will become more broadly used by the neuroscience community to eavesdrop on brain activity with unprecedented spatiotemporal resolution.

SIGNIFICANCE STATEMENT

Genetically encoded voltage indicators are engineered light-emitting protein sensors that typically report neuronal voltage dynamics as changes in brightness. In this review, we systematically discuss the current state of this emerging method, considering both its advantages and limitations for imaging neural activity. We also present recent applications of this technology and discuss what is feasible now and what we anticipate will become possible with future indicator development. This review will inform neuroscientists of recent progress in the field and help potential users critically evaluate the suitability of genetically encoded voltage indicator imaging to answer their specific biological questions.

Optogenetic Monitoring of Synaptic Activity with Genetically Encoded Voltage Indicators

The age of genetically encoded voltage indicators (GEVIs) has matured to the point that changes in membrane potential can now be observed optically in vivo. Improving the signal size and speed of these voltage sensors has been the primary driving forces during this maturation process. As a result, there is a wide range of probes using different voltage detecting mechanisms and fluorescent reporters. As the use of these probes transitions from optically reporting membrane potential in single, cultured cells to imaging populations of cells in slice and/or in vivo, a new challenge emerges—optically resolving the different types of neuronal activity. While improvements in speed and signal size are still needed, optimizing the voltage range and the subcellular expression (i.e., soma only) of the probe are becoming more important. In this review, we will examine the ability of recently developed probes to report synaptic activity in slice and in vivo. The voltage-sensing fluorescent protein (VSFP) family of voltage sensors, ArcLight, ASAP-1, and the rhodopsin family of probes are all good at reporting changes in membrane potential, but all have difficulty distinguishing subthreshold depolarizations from action potentials and detecting neuronal inhibition when imaging populations of cells. Finally, we will offer a few possible ways to improve the optical resolution of the various types of neuronal activities.