Rapid changes in intracellular free calcium concentration. Detection by metallochromic indicator dyes in squid giant axon

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.

Mapping light distribution in the brain via MRI

Liposomal nanoparticles incorporating photosensitive lipids and enclosing paramagnetic molecules enable the mapping, via magnetic resonance imaging, of spatial variations of light intensity in illuminated brain tissue in living animals.

Transcriptional Genetically Encoded Calcium Indicators in Drosophila

Knowing which neurons are active during behavior is a crucial step toward understanding how nervous systems work. Neuronal activation is generally accompanied by an increase in intracellular calcium levels. Therefore, intracellular calcium levels are widely used as a proxy for neuronal activity. Many types of synthetic components and bioluminescent or fluorescent proteins that report transient and long-term changes in intracellular calcium levels have been developed over the past 60 years. Calcium indicators that enable imaging of the dynamic activity of a large ensemble of neurons in behaving animals have revolutionized the field of neuroscience. Among these, transcription-based genetically encoded calcium indicators (transcriptional GECIs) have proven easy to use and do not depend on sophisticated imaging systems, offering unique advantages over other types of calcium indicators. Here, we describe the two currently available fly transcriptional GECIs-calcium-dependent nuclear import of LexA (CaLexA) and transcriptional reporter of intracellular calcium (TRIC)-and review studies that have used them. In the accompanying protocol, we present step-by-step details for generating CaLexA- and TRIC-ready flies and for imaging CaLexA and TRIC signals in dissected brains after experimental manipulations of intact free-moving flies.

Ca2+ entry through NaV channels generates submillisecond axonal Ca2+ signaling

Calcium ions (Ca²⁺) are essential for many cellular signaling mechanisms and enter the cytosol mostly through voltage-gated calcium channels. Here, using high-speed Ca²⁺ imaging up to 20 kHz in the rat layer five pyramidal neuron axon we found that activity-dependent intracellular calcium concentration ([Ca²⁺]_i) in the axonal initial segment was only partially dependent on voltage-gated calcium channels. Instead, [Ca²⁺]_i changes were sensitive to the specific voltage-gated sodium (Na_V) channel blocker tetrodotoxin. Consistent with the conjecture that Ca²⁺ enters through the Na_V channel pore, the optically resolved I_Ca in the axon initial segment overlapped with the activation kinetics of Na_V channels and heterologous expression of Na_V1.2 in HEK-293 cells revealed a tetrodotoxin-sensitive [Ca²⁺]_i rise. Finally, computational simulations predicted that axonal [Ca²⁺]_i transients reflect a 0.4% Ca²⁺ conductivity of Na_V channels. The findings indicate that Ca²⁺ permeation through Na_V channels provides a submillisecond rapid entry route in Na_V-enriched domains of mammalian axons.

Nerve cells communicate using tiny electrical impulses called action potentials. Special proteins termed ion channels produce these electric signals by allowing specific charged particles, or ions, to pass in or out of cells across its membrane. When a nerve cell ‘fires’ an action potential, specific ion channels briefly open to let in a surge of positively charged ions which electrify the cell. Action potentials begin in the same place in each nerve cell, at an area called the axon initial segment. The large number of sodium channels at this site kick-start the influx of positively charged sodium ions ensuring that every action potential starts from the same place.

Previous research has shown that, when action potentials begin, the concentration of calcium ions at the axon initial segment also increases, but it was not clear which ion channels were responsible for this entry of calcium. Channels that are selective for calcium ions are the prime candidates for this process. However, research in squid nerve cells gave rise to an unexpected idea by suggesting that sodium channels may not exclusively let in sodium but also allow some calcium ions to pass through. Hanemaaijer, Popovic et al. therefore wanted to test the routes that calcium ions take and see whether the sodium channels in mammalian nerve cells are also permeable to calcium.

Experiments using fluorescent dyes to track the concentration of calcium in rat and human nerve cells showed that calcium ions accumulated at the axon initial segment when action potentials fired. Most of this increase in calcium could be stopped by treating the neurons with a toxin that prevents sodium channels from opening. Electrical manipulations of the cells revealed that, in this context, the calcium ions were effectively behaving like sodium ions. Human kidney cells were then engineered to produce the sodium channel protein. This confirmed that calcium and sodium ions were indeed both passing through the same channel.

These results shed new light on the relationship between calcium ions and sodium channels within the mammalian nervous system and that this interplay occurs at the axon initial segment of the cell. Genetic mutations that ‘nudge’ sodium channels towards favoring calcium entry are also found in patients with autism spectrum disorders, and so this new finding may contribute to our understanding of these conditions.

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.

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.

The emerging use of bioluminescence in medical research

Bioluminescence is the light produced by a living organism and is commonly emitted by sea life with Ca²⁺-regulated photoproteins being the most responsible for bioluminescence emission. Marine coelenterates provide important functions involved in essential purposes such as defense, feeding, and breeding. In this review, the main characteristics of marine photoproteins including aequorin, clytin, obelin, berovin, pholasin and symplectin from different marine organisms will be discussed. We will focused on the recent use of recombinant photoproteins in different biomedical research fields including the measurement of Ca²⁺ in different intracellular compartments of animal cells, as labels in the design and development of binding assays. This review will also outline how bioluminescent photoproteins have been used in a plethora of analytical methods including ultra-sensitive assays and in vivo imaging of cellular processes. Due to their unique properties including elective intracellular distribution, wide dynamic range, high signal-to-noise ratio and low Ca²⁺-buffering effect, recombinant photoproteins represent a promising future analytical tool in several in vitro and in vivo experiments.

Calcium imaging method to visualize the spatial patterns of neural responses in the pygmy squid, Idiosepius paradoxus, central nervous system

BACKGROUND

Cephalopods exhibit unique behaviors such as camouflage and tactile learning. The brain functions correlated to these behaviors have long been analyzed through behavioral observations of animals subject to surgical manipulation or electrical stimulation of brain lobes. However, physiological methods have rarely been introduced to investigate the functions of each individual lobe, though physiological work on giant axons and slices of the vertical lobe system of the cephalopods have provided deep insights into ion conductance of nerves and long-term synaptic plasticity. The lack of in vivo physiological work is partly due to difficulties in immobilizing the brain which is contained within the soft body and applying calcium indicators to the cephalopod central nervous system.

NEW METHOD

We here present a calcium imaging method to visualize neural responses in the central nervous system of the smallest squid, Idiosepius paradoxus.

RESULTS

We injected calcium indicator Cal-520 into the brachial lobes and revealed a spatiotemporal pattern of neural responses to the electrical stimulations of the axial nerve cord in the first arm.

COMPARISON WITH EXISTING METHODS

We established a method to immobilize the central nervous system which is contained within the soft body and record the calcium responses from the intact central nervous system.

CONCLUSIONS

Our method provides a novel approach to investigate the mechanisms of how the characteristic organization of the cephalopod brain functions to induce their unique behaviors.

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.

Advances in two photon scanning and scanless microscopy technologies for functional neural circuit imaging

Recent years have seen substantial developments in technology for imaging neural circuits, raising the prospect of large scale imaging studies of neural populations involved in information processing, with the potential to lead to step changes in our understanding of brain function and dysfunction. In this article we will review some key recent advances: improved fluorophores for single cell resolution functional neuroimaging using a two photon microscope; improved approaches to the problem of scanning active circuits; and the prospect of scanless microscopes which overcome some of the bandwidth limitations of current imaging techniques. These advances in technology for experimental neuroscience have in themselves led to technical challenges, such as the need for the development of novel signal processing and data analysis tools in order to make the most of the new experimental tools. We review recent work in some active topics, such as region of interest segmentation algorithms capable of demixing overlapping signals, and new highly accurate algorithms for calcium transient detection. These advances motivate the development of new data analysis tools capable of dealing with spatial or spatiotemporal patterns of neural activity, that scale well with pattern size.

A Bright and Fast Red Fluorescent Protein Voltage Indicator That Reports Neuronal Activity in Organotypic Brain Slices

Optical imaging of voltage indicators based on green fluorescent proteins (FPs) or archaerhodopsin has emerged as a powerful approach for detecting the activity of many individual neurons with high spatial and temporal resolution. Relative to green FP-based voltage indicators, a bright red-shifted FP-based voltage indicator has the intrinsic advantages of lower phototoxicity, lower autofluorescent background, and compatibility with blue-light-excitable channelrhodopsins. Here, we report a bright red fluorescent voltage indicator (fluorescent indicator for voltage imaging red; FlicR1) with properties that are comparable to the best available green indicators. To develop FlicR1, we used directed protein evolution and rational engineering to screen libraries of thousands of variants. FlicR1 faithfully reports single action potentials (∼3% ΔF/F) and tracks electrically driven voltage oscillations at 100 Hz in dissociated Sprague Dawley rat hippocampal neurons in single trial recordings. Furthermore, FlicR1 can be easily imaged with wide-field fluorescence microscopy. We demonstrate that FlicR1 can be used in conjunction with a blue-shifted channelrhodopsin for all-optical electrophysiology, although blue light photoactivation of the FlicR1 chromophore presents a challenge for applications that require spatially overlapping yellow and blue excitation.

Genetically Encoded Protein Sensors of Membrane Potential

Organic voltage-sensitive dyes offer very high spatial and temporal resolution for imaging neuronal function. However these dyes suffer from the drawbacks of non-specificity of cell staining and low accessibility of the dye to some cell types. Further progress in imaging activity is expected from the development of genetically encoded fluorescent sensors of membrane potential. Cell type specificity of expression of these fluorescent protein (FP) voltage sensors can be obtained via several different mechanisms. One is cell type specificity of infection by individual virus subtypes. A second mechanism is specificity of promoter expression in individual cell types. A third, depends on the offspring of transgenic animals with cell type specific expression of cre recombinase mated with an animal that has the DNA for the FP voltage sensor in all of its cells but its expression is dependent on the recombinase activity. Challenges remain. First, the response time constants of many of the new FP voltage sensors are slower (2-10 ms) than those of organic dyes. This results in a relatively small fractional fluorescence change, ΔF/F, for action potentials. Second, the largest signal presently available is only ~40% for a 100 mV depolarization and many of the new probes have signals that are substantially smaller. Large signals are especially important when attempting to detect fast events because the shorter measurement interval results in a relatively small number of detected photons and therefore a relatively large shot noise (see Chap. 1). Another kind of challenge has occurred when attempts were made to transition from one species to another or from one cell type to another or from cell culture to in vivo measurements.Several laboratories have recently described a number of novel FP voltage sensors. Here we attempt to critically review the current status of these developments in terms of signal size, time course, and in vivo function.

Historical Overview and General Methods of Membrane Potential Imaging

Voltage imaging was first conceived in the late 1960s and efforts to find better organic voltage sensitive dyes began in the 1970s and continue until today. At the beginning it was difficult to measure an action potential signal from a squid giant axon in a single trial. Now it is possible to measure the action potential in an individual spine. Other chapters will discuss advances in voltage imaging technology and applications in a variety of biological preparations. The development of genetically encoded voltage sensors has started. A genetically encoded sensor could provide cell type specific expression and voltage recording (see Chap. 20). Optimizing the signal-to-noise ratio of an optical recording requires attention to several aspects of the recording apparatus. These include the light source, the optics and the recording device. All three have improved substantially in recent years. Arc lamp, LED, and laser sources are now stable, more powerful, and less expensive. Cameras for recording activity have frames rates above 1 kHz and quantum efficiencies near 1.0 although they remain expensive. The sources of noise in optical recordings are well understood. Both the apparatus and the noise sources are discussed in this chapter.

Monitoring brain activity with protein voltage and calcium sensors

Understanding the roles of different cell types in the behaviors generated by neural circuits requires protein indicators that report neural activity with high spatio-temporal resolution. Genetically encoded fluorescent protein (FP) voltage sensors, which optically report the electrical activity in distinct cell populations, are, in principle, ideal candidates. Here we demonstrate that the FP voltage sensor ArcLight reports odor-evoked electrical activity in the in vivo mammalian olfactory bulb in single trials using both wide-field and 2-photon imaging. ArcLight resolved fast odorant-responses in individual glomeruli, and distributed odorant responses across a population of glomeruli. Comparisons between ArcLight and the protein calcium sensors GCaMP3 and GCaMP6f revealed that ArcLight had faster temporal kinetics that more clearly distinguished activity elicited by individual odorant inspirations. In contrast, the signals from both GCaMPs were a saturating integral of activity that returned relatively slowly to the baseline. ArcLight enables optical electrophysiology of mammalian neuronal population activity in vivo.

Optical recording of calcium currents during impulse conduction in cardiac tissue

We explore the feasibility of obtaining a spatially resolved picture of [Formula: see text] inward currents ([Formula: see text]) in multicellular cardiac tissue by differentiating optically recorded [Formula: see text] transients that accompany propagating action potentials. Patterned growth strands of neonatal rat ventricular cardiomyocytes were stained with the [Formula: see text] indicators Fluo-4 or Fluo-4FF. Preparations were stimulated at 1 Hz, and [Formula: see text] transients were recorded with high spatiotemporal resolution ([Formula: see text], 2 kHz analog bandwidth) with a photodiode array. Signals were differentiated after appropriate digital filtering. Differentiation of [Formula: see text] transients resulted in optically recorded calcium currents (ORCCs) that carried the temporal and pharmacological signatures of L-type [Formula: see text] inward currents: the time to peak amounted to [Formula: see text] (Fluo-4FF) and [Formula: see text] (Fluo-4), full-width at half-maximum was [Formula: see text], and ORCCs were completely suppressed by [Formula: see text][Formula: see text]. Also, and as reported before from patch-clamp studies, caffeine reversibly depressed the amplitude of ORCCs. The results demonstrate that the differentiation of [Formula: see text] transients can be used to obtain a spatially resolved picture of the initial phase of [Formula: see text] in cardiac tissue and to assess relative changes of activation/fast inactivation of [Formula: see text] following pharmacological interventions.

Imaging with organic indicators and high-speed charge-coupled device cameras in neurons: some applications where these classic techniques have advantages

Dynamic calcium and voltage imaging is a major tool in modern cellular neuroscience. Since the beginning of their use over 40 years ago, there have been major improvements in indicators, microscopes, imaging systems, and computers. While cutting edge research has trended toward the use of genetically encoded calcium or voltage indicators, two-photon microscopes, and in vivo preparations, it is worth noting that some questions still may be best approached using more classical methodologies and preparations. In this review, we highlight a few examples in neurons where the combination of charge-coupled device (CCD) imaging and classical organic indicators has revealed information that has so far been more informative than results using the more modern systems. These experiments take advantage of the high frame rates, sensitivity, and spatial integration of the best CCD cameras. These cameras can respond to the faster kinetics of organic voltage and calcium indicators, which closely reflect the fast dynamics of the underlying cellular events.

Utilizing GCaMP transgenic mice to monitor endogenous Gq/11-coupled receptors

The family of GCaMPs are engineered proteins that contain Ca²⁺ binding motifs within a circularly permutated variant of the Aequorea Victoria green fluorescent protein (cp-GFP). The rapidly advancing field of utilizing GCaMP reporter constructs represents a major step forward in our ability to monitor intracellular Ca²⁺ dynamics. With the use of these genetically encoded Ca²⁺ sensors, investigators have studied activation of endogenous G_q types of G protein-coupled receptors (GPCRs) and subsequent rises in intracellular calcium. Escalations in intracellular Ca²⁺ from GPCR activation can be faithfully monitored in space and time as an increase in fluorescent emission from these proteins. Further, transgenic mice are now commercially available that express GCaMPs in a Cre recombinase dependent fashion. These GCaMP reporter mice can be bred to distinct Cre recombinase driver mice to direct expression of this sensor in unique populations of cells. Concerning the central nervous system (CNS), sources of calcium influx, including those arising from G_q activation can be observed in targeted cell types like neurons or astrocytes. This powerful genetic method allows simultaneous monitoring of the activity of dozens of cells upon activation of endogenous G_q-coupled GPCRs. Therefore, in combination with pharmacological tools, this strategy of monitoring GPCR activation is amenable to analysis of orthosteric and allosteric ligands of G_q-coupled receptors in their endogenous environments.

Light sources and cameras for standard in vitro membrane potential and high-speed ion imaging

Membrane potential and fast ion imaging are now standard optical techniques routinely used to record dynamic physiological signals in several preparations in vitro. Although detailed resolution of optical signals can be improved by confocal or two-photon microscopy, high spatial and temporal resolution can be obtained using conventional microscopy and affordable light sources and cameras. Thus, standard wide-field imaging methods are still the most common in research laboratories and can often produce measurements with a signal-to-noise ratio that is superior to other optical approaches. This paper seeks to review the most important instrumentation used in these experiments, with particular reference to recent technological advances. We analyse in detail the optical constraints dictating the type of signals that are obtained with voltage and ion imaging and we discuss how to use this information to choose the optimal apparatus. Then, we discuss the available light sources with specific attention to light emitting diodes and solid state lasers. We then address the current state-of-the-art of available charge coupled device, electron multiplying charge coupled device and complementary metal oxide semiconductor cameras and we analyse the characteristics that need to be taken into account for the choice of optimal detector. Finally, we conclude by discussing prospective future developments that are likely to further improve the quality of the signals expanding the capability of the techniques and opening the gate to novel applications.

Dopaminergic regulation of dendritic calcium: fast multisite calcium imaging

Optimal dopamine tone is required for the normal cortical function; however it is still unclear how cortical-dopamine-release affects information processing in individual cortical neurons. Thousands of glutamatergic inputs impinge onto elaborate dendritic trees of neocortical pyramidal neurons. In the process of ensuing synaptic integration (information processing), a variety of calcium transients are generated in remote dendritic compartments. In order to understand the cellular mechanisms of dopaminergic modulation it is important to know whether and how dopaminergic signals affect dendritic calcium transients. In this chapter, we describe a relatively inexpensive method for monitoring dendritic calcium fluctuations at multiple loci across the pyramidal dendritic tree, at the same moment of time (simultaneously). The experiments have been designed to measure the amplitude, time course and spatial extent of action potential-associated dendritic calcium transients before and after application of dopaminergic drugs. In the examples provided here the dendritic calcium transients were evoked by triggering the somatic action potentials (backpropagation-evoked), and puffs of exogenous dopamine were applied locally onto selected dendritic branches.

Genetically encoded fluorescent voltage sensors using the voltage-sensing domain of Nematostella and Danio phosphatases exhibit fast kinetics

A substantial increase in the speed of the optical response of genetically encoded fluorescent protein voltage sensors (FP voltage sensors) was achieved by using the voltage-sensing phosphatase genes of Nematostella vectensis and Danio rerio. A potential N. vectensis voltage-sensing phosphatase was identified in silico. The voltage-sensing domain (S1-S4) of the N. vectensis homolog was used to create an FP voltage sensor called Nema. By replacing the phosphatase with a cerulean/citrine FRET pair, a new FP voltage sensor was synthesized with fast off kinetics (Tau(off)<5ms). However, the signal was small (ΔF/F=0.4%/200mV). FP voltage sensors using the D. rerio voltage-sensing phosphatase homolog, designated Zahra and Zahra 2, exhibited fast on and off kinetics within 2ms of the time constants observed with the organic voltage-sensitive dye, di4-ANEPPS. Mutagenesis of the S4 region of the Danio FP voltage sensor shifted the voltage dependence to more negative potentials but did not noticeably affect the kinetics of the optical signal.

Random insertion of split-cans of the fluorescent protein venus into Shaker channels yields voltage sensitive probes with improved membrane localization in mammalian cells

FlaSh-YFP, a fluorescent protein (FP) voltage sensor that is a fusion of the Shaker potassium channel with yellow fluorescent protein (YFP), is primarily expressed in the endoplasmic reticulum (ER) of mammalian cells, possibly due to misfolded monomers. In an effort to improve plasma membrane expression, the FP was split into two non-fluorescent halves. Each half was randomly inserted into Shaker monomers via a transposon reaction. Shaker subunits containing the 5' half were co-expressed with Shaker subunits containing the 3' half. Tetramerization of Shaker subunits is required for re-conjugation of the FP. The misfolded monomers trapped in ER are unlikely to tetramerize and reconstitute the beta-can structure, and thus intracellular fluorescence might be reduced. This split-can transposon approach yielded 56 fluorescent probes, 30 (54%) of which were expressed at the plasma membrane and were capable of optically reporting changes in membrane potential. The largest signal from these novel FP-sensors was a -1.4% in ΔF/F for a 100 mV depolarization, with on time constants of about 15 ms and off time constants of about 200 ms. This split-can transposon approach has the potential to improve other multimeric probes.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Genetically encoded fluorescent sensors of membrane potential

Imaging activity of neurons in intact brain tissue was conceived several decades ago and, after many years of development, voltage-sensitive dyes now offer the highest spatial and temporal resolution for imaging neuronal functions in the living brain. Further progress in this field is expected from the emergent development of genetically encoded fluorescent sensors of membrane potential. These fluorescent protein (FP) voltage sensors overcome the drawbacks of organic voltage sensitive dyes such as non-specificity of cell staining and the low accessibility of the dye to some cell types. In a transgenic animal, a genetically encoded sensor could in principle be expressed specifically in any cell type and would have the advantage of staining only the cell population determined by the specificity of the promoter used to drive expression. Here we critically review the current status of these developments.

Three fluorescent protein voltage sensors exhibit low plasma membrane expression in mammalian cells

Three first-generation fluorescent protein voltage sensitive probes (FP-voltage sensors) were characterized in mammalian cells. Flare, a Kv1.4 variant of FlaSh [Siegel MS, Isacoff EY. Neuron 1997;19(October (4)):735-41], SPARC [Ataka K, Pieribone VA. Biophys J 2002;82(January (1 Pt 1)):509-16], and VSFP-1 [Sakai R, Repunte-Canonigo V, Raj CD, Knopfel T. Eur J Neurosci 2001;13(June (12)):2314-18] were expressed, imaged and voltage clamped in HEK 293 cells and in dissociated hippocampal neurons. We were unable to detect a signal in response to changes in membrane potential after averaging16 trials with any of the three constructs. Using the hydrophobic voltage sensitive dye, di8-ANEPPS, as a surface marker, confocal analyses demonstrated poor plasma membrane expression for Flare, SPARC and VSFP-1 in both HEK 293 cells and dissociated hippocampal neurons. Almost all of the expressed FP-voltage sensors reside in internal membranes in both cell types. This internal expression generates a background fluorescence that increases the noise in the optical measurement.

Imaging microdomain Ca2+ in muscle cells

Ca2+ ions passing through a single or a cluster of Ca2+-permeable channels create microscopic, short-lived Ca2+ gradients that constitute the building blocks of cellular Ca2+ signaling. Over the last decade, imaging microdomain Ca2+ in muscle cells has unveiled the exquisite spatial and temporal architecture of intracellular Ca2+ dynamics and has reshaped our understanding of Ca2+ signaling mechanisms. Major advances include the visualization of "Ca2+ sparks" as the elementary events of Ca2+ release from the sarcoplasmic reticulum (SR), "Ca2+ sparklets" produced by openings of single Ca2+-permeable channels, miniature Ca2+ transients in single mitochondria ("marks"), and SR luminal Ca2+ depletion transients ("scraps"). As a model system, a cardiac myocyte contains a 3-dimensional grid of 104 spark ignition sites, stochastic activation of which summates into global Ca2+ transients. Tracking intermolecular coupling between single L-type Ca2+ channels and Ca2+ sparks has provided direct evidence validating the local control theory of Ca2+-induced Ca2+ release in the heart. In vascular smooth muscle myocytes, Ca2+ can paradoxically signal both vessel constriction (by global Ca2+ transients) and relaxation (by subsurface Ca2+ sparks). These findings shed new light on the origin of Ca2+ signaling efficiency, specificity, and versatility. In addition, microdomain Ca2+ imaging offers a novel modality that complements electrophysiological approaches in characterizing Ca2+ channels in intact cells.

20 years of Ins(1,4,5)P3, and 40 years before

This year marks the 20th birthday of the discovery of inositol-1,4,5-trisphosphate as a second messenger. The background to this discovery is a complex story that goes back more than 50 years and involves a large cast of characters, both chemical and human.

Caffeine interaction with fluorescent calcium indicator dyes

We report that caffeine, in millimolar concentrations, interacts strongly with four common calcium indicator dyes: mag-fura-2, magnesium green, fura-2, and fluo-3. Fluorescence intensities are either noticeably enhanced (mag-fura-2, fura-2) or diminished (magnesium green, fluo-3). The caffeine-induced changes in the fluorescence spectra are clearly distinct from those of metal ion binding at the indicator chelation sites. Binding affinities for calcium of either mag-fura-2 or magnesium green increased only slightly in the presence of caffeine. Caffeine also alters the fluorescence intensities of two other fluorescent dyes lacking a chelation site, fluorescein and sulforhodamine 101, implicating the fluorophore itself as the interaction site for caffeine. In the absence of caffeine, variation of solution hydrophobicity by means of water/dioxane mixtures yielded results similar to those for caffeine. These observations suggest that hydrophobic substances, in general, can alter dye fluorescence in a dye-specific manner. For the particular case of caffeine, and perhaps other commonly used pharmacological agents, the dye interactions can seriously distort fluorescence measurements of intracellular ion concentrations with metal indicator dyes.

Direct evidence of Na+/Ca2+ exchange in squid rhabdomeric membranes

Na+/Ca2+ exchange has been investigated in squid (Loligo pealei) rhabdomeric membranes. Ca2+-containing vesicles have been prepared from purified rhabdomeric membranes by extrusion through polycarbonate filters of 1-micrometer pore size. After removal of external Ca2+, up to 90% of the entrapped Ca2+ could be specifically released by the addition of Na+; this finding indicates that most of the vesicles contained Na+/Ca2+ exchanger. The Na+-induced Ca2+ efflux had a half-maximum value (K1/2) of approximately 44 mM and a Hill coefficient of approximately 1.7. The maximal Na+-induced Ca2+ efflux was approximately 0.6 nmol Ca2+. s-1. mg protein-1. Similar Na+-induced Ca2+ effluxes were measured if K+ was replaced with Li+ or Cs+. Vesicles loaded with Ca2+ by Na+/Ca2+ exchange also released this Ca2+ by Na+/Ca2+ exchange, suggesting that Na+/Ca2+ exchange operated in both forward and reverse modes. Limited proteolysis by trypsin resulted in a rate of Ca2+ efflux enhanced by approximately fivefold when efflux was activated with 95 mM NaCl. For vesicles subjected to limited proteolysis by trypsin, Na+/Ca2+ exchange was characterized by a K1/2 of approximately 25 mM and a Hill coefficient of 1.6. For these vesicles, the maximal Na+-induced Ca2+ efflux was about twice as great as in control vesicles. We conclude that Na+/Ca2+ exchange proteins localized in rhabdomeric membranes mediate Ca2+ extrusion in squid photoreceptors.

Fluorescence imaging of intracellular Ca2+

The measurement of intracellular Ca2+ concentrations ([Ca2+]i) is of critical importance, because many cellular functions are tightly regulated by [Ca2+]i. The fluorescent indicator, fura-2, has been used frequently to measure [Ca2+]i because of its sensitivity and specificity, and because it can be loaded into living cells with little disruption of function. Most importantly, the peak excitation wavelength of fura-2 changes when it binds Ca2+. As a consequence, measurements of fluorescence at two excitation wavelengths can be used to obtain an estimate of [Ca2+]i that is independent of dye concentration and cell thickness. Fura-2 acetoxymethyl ester (AM) is a lipid-soluble derivative that is often used because of its ability to pass through cell membranes. There are, however, several problems with the use of fura-2 AM such as intracellular compartmentation and incomplete deesterification. The availability of low-light-level cameras and computer hardware for the digitization of fluorescent images has made quantitative fluorescence microscopy possible. This technique has shown a striking spatial heterogeneity of [Ca2+]i in a variety of cell types, and has revealed substantial new information on dynamic intracellular biochemistry and signal transduction. However, the current imaging technology is not fully developed because of dye and instrumentation limitations. Further development of techniques and new probes are required to improve temporal and spatial resolution.

Intracellular signal transduction in four dimensions: from molecular design to physiology

Designed fluorescent indicators are the basis for a major new technique in cell physiology, the quantitative measurement and dynamic imaging of intracellular concentrations of important ions and messengers such as Ca2+, Na+, H+, and adenosine 3',5'-cyclic monophosphate. Molecular engineering has now produced indicators with quite good selectivity and sensitivity for these analytes. In many cases, these probes can be introduced into large populations of cells by means of membrane-permeant chemical derivatives, so that the plasma membrane need never be disrupted or physically breached at any point. Like many other optical microscopic techniques, fluorescent indicators are readily applied to study living cells and tissues, with an unparalleled combination of spatial and temporal resolution. They offer one of the few methods for continuous nondestructive monitoring of dynamic intracellular biochemistry and signal transduction in single cells or subregions of cells.

The light-induced rise in cytosolic calcium starts later than the receptor current of the Limulus ventral photoreceptor

The intracellular arsenazo signal indicating the transient light-evoked change in cytosolic Ca2+ (or Sr2+) concentration was measured in Limulus ventral photoreceptors simultaneously with the receptor current under voltage clamp conditions at 15 degrees C. The latency of the light-evoked arsenazo response was consistently more than 25 msec longer than the latency of the electrical light response (receptor current or -potential). Replacing calcium by strontium in the superfusate caused, within 30-40 min, reversible changes: an enlargement of the arsenazo response and a considerable prolongation of both latencies, that of the electrical and that of the arsenazo response; the difference between the two latencies, however, stayed essentially constant.

Spatial and temporal analysis of calcium-dependent electrical activity in guinea pig Purkinje cell dendrites

We have used the calcium indicator dye arsenazo III, together with a photodiode array, to record intracellular calcium changes simultaneously from all regions of individual guinea pig cerebellar Purkinje cells in slices. The optical signals, recorded with millisecond time resolution, are good indicators of calcium-dependent electrical events. For many cells the sensitivity of the recordings was high enough to detect signals from each array element without averaging. Consequently, it was possible to use these signals to follow the complex spatial and temporal patterns of plateau and spike potentials. Calcium entry corresponding to action potentials was detected from all parts of the dendritic field including the fine spiny branchlets, demonstrating that calcium action potentials spread over the entire arbor. Usually, the entire dendritic tree fired at once. But sometimes only restricted areas had signals at any one moment with transients detected in different regions at other times. In one cell, six separate zones were distinguished. These results show that calcium action potentials could be regenerative in some dendrites and could fail to propagate into others. Signals from plateau potentials were also detected from extensive areas in the dendritic field but were always smaller than those caused by a burst of action potentials.

Ca2+ imaging in single living cells: theoretical and practical issues

The measurement of intracellular calcium ion concentrations [( Ca2+]i) in single living cells using quantitative fluorescence microscopy draws from a diverse set of disciplines, including cellular biology, optical physics, statistics and computer science. Over the last few years, we have devised and built a number of systems for measuring [Ca2+]i with Fura-2, and have applied them in the exploration of a wide range of biological processes controlled by Ca2+. In this report we discuss these systems and their advantages and limitations. We also describe the theoretical and practical problems associated with using Fura-2 to measure [Ca2+]i, and the solutions that we, and others, have developed to overcome them. The approaches described should provide useful guidance for others interested in imaging [Ca2+] distribution in living cells. The factors that limit current methods are discussed, and areas for future development are highlighted.

Design of an imaging spectrophotometer for use with voltage-sensitive and calcium-indicator dyes

This paper describes the design for an imaging spectrophotometer which is suitable for use with both fluorescence and absorbance probes and which has a bandwidth appropriate for the study of a wide range of biological phenomena.

Calcium changes in isolated peptidergic neurons during activation by a cAMP analog

The bag cell neurons of intact Aplysia control egg-laying by producing an approximately 30 min afterdischarge of action potentials, during which the neuropeptide egg-laying hormone is released. Previous studies have shown that cAMP, protein phosphorylation and Ca2+ are involved in the mechanisms of the afterdischarge. Spontaneous discharge and afterdischarge can be produced in dissociated bag cells by treatment with cAMP analogs. While pharmacological evidence exists for a large calcium component in the action potentials of bag cells, direct evidence by measuring (Ca2+)i has not appeared, nor has Ca-buffering during cAMP activation been examined. Our studies were directed at measuring (Ca2+)i changes in isolated bag cells in attached cell culture using the metallochromic indicator, arsenazo III, simultaneously with voltage recordings across the cell membrane. It was found that a single induced action potential causes a negligible rise in (Ca2+)i while a train of 35 spikes causes a significant rise (about 3 X 10(-4) absorbance units). Increasing external (Ca2+) enhances the action potential and Ca2+ signal while cobaltous ions block both the action potentials and the rise in (Ca2+)i. Lengthening the duration of spikes by TEA (50 mM) or 8-benzylthio cAMP (8-BTcAMP; 0.5 mM) enhances Ca2+ influx and allows the detection of Ca2+ transients due to single spikes. During spontaneous groups of action potentials in either TEA or 8-BTcAMP, (Ca2+)i increases but the bag cell's Ca-buffering systems are able to restore the Ca2+ levels with a half-time of about 20 s.(ABSTRACT TRUNCATED AT 250 WORDS)

Activity-dependent calcium transients in central nervous system myelinated axons revealed by the calcium indicator Fura-2

Optical measurements from rat optic nerve, loaded with the new Ca2+ indicator Fura-2, provide the first evidence for the presence of activity-dependent fast intracellular [Ca2+] transients in mammalian central nervous system (CNS) myelinated axons. The results suggest that voltage-dependent Ca2+ channels are present in some of the myelinated axons. Optical measurements from axons stained with anterogradely transported voltage-sensitive dye suggest the presence of Ca2+-dependent potassium conductances in these axons. This report also demonstrates that Fura-2 can readily detect changes in [Ca2+] inside cells as a result of electrical activity, and establishes its suitability for measurements of intracellular Ca2+ transients in the millisecond time domain.

Mapping calcium transients in the dendrites of Purkinje cells from the guinea-pig cerebellum in vitro

1. A 10 X 10 photodiode array was used to detect stimulation-dependent absorbance changes simultaneously from many positions in the dendrite field of guinea-pig Purkinje cells which had been injected with the calcium indicator Arsenazo III in thin cerebellar slices. Signals from each element of the array were matched to positions on the cells by mapping them onto fluorescence photographs of Lucifer Yellow which had been co-injected into the cells with the Arsenazo III. 2. In response to intrasomatic stimulation the rising phase of the absorbance signals corresponded in time with the calcium spikes recorded with an intracellular electrode. There was no increase in absorbance during bursts of fast sodium spikes. Absorbance signals persisted after the sodium spikes were blocked by tetrodotoxin (TTX). In addition, the signals were largest at 660 nm and small signals of opposite polarity were found at 540 nm. These results indicate that the absorbance signals came from calcium entry into the cell resulting from the turning on of voltage-dependent calcium conductances. 3. In these experiments signals were usually seen all over the dendritic field and were weak or totally absent over the soma. In some cases signals were seen over a more restricted area. With a spatial resolution of 25 microns we were not able to see any evidence for highly localized sites of calcium entry. 4. Sometimes the rising phase of the calcium signals was separated by almost 13 ms in different parts of the dendritic field, too long to be explained by active propagation delay. This suggests that calcium spikes causing these signals can be evoked separately in different regions of the Purkinje cell dendritic field by long-lasting potentials which may reach local threshold at different times. 5. Calcium signals resulting from slow plateau after-potentials and the calcium spikes produced by them were also detected in all locations in the dendritic field. The relative distribution of amplitudes from these plateau signals was different from the distribution of evoked signals during current injection. 6. Climbing fibre synaptic activation produced calcium signals which were distributed over the dendritic arborization, but larger at the main dendritic tree where most of the synaptic contacts are located. 7. Calcium signals were also detected from the dendrites of other neurone types in the in vitro slice preparation. Thus, it is likely that these kind of measurements can be used to analyse the electroresponsiveness of many kinds of neurones in the mammalian brain.

Comparison of arsenazo III optical signals in intact and cut frog twitch fibers

The Ca indicator arsenazo III was introduced into cut frog twitch fibers by diffusion from end-pool segments rendered permeable by saponin. After 2-3 h, the arsenazo III concentration at the optical recording site in the center of a fiber reached two to three times that in the end-pool solutions. Thus, arsenazo III was bound to or taken up by intracellular constituents. The time course of indicator appearance was fitted by equations for diffusion plus linear reversible binding; on average, 0.73 of the indicator was bound and the free diffusion constant was 0.86 x 10(-6) cm2/s at 18 degrees C. When the indicator was removed from the end pools, it failed to diffuse away from the optical site as rapidly as it had diffused in. The wavelength dependence of resting arsenazo III absorbance was the same in cut fibers and injected intact fibers. After action potential stimulation, the active Ca and dichroic signals were similar in the two preparations, which indicates that arsenazo III undergoes the same changes in absorbance and orientation in both cut and intact fibers. Ca transients in freshly prepared cut fibers appeared to be similar to those in intact fibers. As a cut fiber experiment progressed, however, the Ca signal changed. With action potential stimulation, the half-width of the signal gradually increased, regardless of whether the indicator concentration was increasing or decreasing. This increase was usually not accompanied by any change in the amplitude of the Ca signal at a given indicator concentration or by any obvious deterioration in the electrical condition of the fiber. In voltage-clamp experiments near threshold, the relation between peak [Ca] and voltage usually became less steep with time and shifted to more negative potentials. All these changes were also observed in cut fibers containing antipyrylazo III (Maylie, J., M. Irving, N. L. Sizto, and W. K. Chandler. 1987. Journal of General Physiology. 89:83-143). They are considered to represent a progressive change in the physiological state of a cut fiber during the time course of an experiment.

Calcium levels measured in a presynaptic neurone of Aplysia under conditions that modulate transmitter release

We have utilized the Ca2+ indicator dye, Arsenazo III, to examine the role of presynaptic Ca2+ concentration in two types of synaptic plasticity observed at the synapses of cell L10 in Aplysia californica; post-tetanic potentiation (p.t.p. - the increased transmitter release which follows high frequency stimulation), and resting membrane potential modulation of release. Intracellular Ca2+ was monitored in the cell body and main neurites of L10 injected with Arsenazo III. Tetanic stimulation caused an increase in intracellular Ca2+ concentration that decayed, after tetanus, with fast and slow time constants which paralleled the time course of decay of p.t.p. When the voltage-sensitive Ca2+ current was reduced by removing external Ca2+ (0 mM-Ca2+, 4 mM-EGTA) or by blocking Ca2+ channels with divalent cation channel blocker (4 mM-Cd2+), tetanic stimulation did not cause increases in Arsenazo absorbance even when Na+ currents were not blocked. This finding suggests that Ca2+ entering the cell through voltage-dependent Ca2+ channels was the major source of Ca2+ which accumulated during the tetanus. Transmitter release is increased when L10 is maintained at a depolarized membrane potential, and is decreased when L10 is hyperpolarized. We found that the base-line Arsenazo absorbance signal in L10 increased when L10 was depolarized from -60 to -40 mV and decreased when L10 was hyperpolarized. This finding supports the idea that the steady-state Ca2+ concentration contributes to the membrane-potential modulation of transmitter release. These results support the idea that transmitter release can be modulated by the residual or resting Ca2+ concentration of the presynaptic cell.

Arsenazo III transients and calcium current in a normally non-spiking neuronal soma of crayfish

Arsenazo III was used to investigate Ca2+ transients in the normally non-excitable soma of the motor giant neurones of the crayfish Procambarus clarkii. Two kinds of regenerative potentials could be obtained depending on membrane potential conditioning: a fast spike after a pre-hyperpolarization to -90 mV and a slow action potential after a pre-depolarization to -50 mV. Only the second of these was accompanied by an Arsenazo III transient. In voltage-clamped, somata injected, with tetraethylammonium chloride, an absorbance change could be obtained by pulsing the membrane potential above -44 mV. The relationship between absorbance change and potential peaked between 0 and +10 mV then fell off to zero at ca. +150 mV. Changes in light absorbance studied using double-pulse protocols suggested that the inactivation of Ca2+ entry was predominantly mediated by the intracellular free Ca2+ concentration. External application of 1 mM-CdCl2 abolished both the absorbance changes and the (Ca2+) inward current. The voltage dependence of this current was similar to that of the absorbance change. For positive membrane potential the current-voltage relationship showed a voltage-dependent conductance property, the origin of which is discussed.