Optical single-channel recording: imaging Ca2+ flux through individual N-type voltage-gated channels expressed in Xenopus oocytes

Experimentally monitored calcium dynamics at synaptic active zones during neurotransmitter release in neuron-muscle cell cultures

Ca2+-dependent K+ (BK) channels at varicosities in Xenopus nerve-muscle cell cultures were used to quantify experimentally the instantaneous active zone [Ca2+]AZ resulting from different rates and durations of Ca2+ entry in the absence of extrinsic buffers and correlate this with neurotransmitter release. Ca2+ tail currents produce mean peak [Ca2+]AZ ~ 30 μM; with continued influx, [Ca2+]AZ reaches ~45-60 μM at different rates depending on Ca2+ driving force and duration of influx. Both IBK and release are dependent on Ca2+ microdomains composed of both N- and L-type Ca channels. Domains collapse with a time constant of ~0.6 ms. We have constructed an active zone (AZ) model that approximately fits this data, and depends on incorporation of the high-capacity, low-affinity fixed buffer represented by phospholipid charges in the plasma membrane. Our observations suggest that in this preparation, (1) some BK channels, but few if any of the Ca2+ sensors that trigger release, are located within Ca2+ nanodomains while a large fraction of both are located far enough from Ca channels to be blockable by EGTA, (2) the IBK is more sensitive than the excitatory postsynaptic current (EPSC) to [Ca2+]AZ (K1/2-26 μM vs. ~36 μM [Ca2+]AZ); (3) with increasing [Ca2+]AZ, the IBK grows with a Hill coefficient of 2.5, the EPSC with a coefficient of 3.9; (4) release is dependent on the highest [Ca2+] achieved, independent of the time to reach it; (5) the varicosity synapses differ from mature frog nmjs in significant ways; and (6) BK channels are useful reporters of local [Ca2+]AZ.

Xenopus Oocytes as a Powerful Cellular Model to Study Foreign Fully-Processed Membrane Proteins

The use of Xenopus oocytes in electrophysiological and biophysical research constitutes a long and successful story, providing major advances to the knowledge of the function and modulation of membrane proteins, mostly receptors, ion channels, and transporters. Earlier reports showed that these cells are capable of correctly expressing heterologous proteins after injecting the corresponding mRNA or cDNA. More recently, the Xenopus oocyte has become an outstanding host-cell model to carry out detailed studies on the function of fully-processed foreign membrane proteins after their microtransplantation to the oocyte. This review focused on the latter overall process of transplanting foreign membrane proteins to the oocyte after injecting plasma membranes or purified and reconstituted proteins. This experimental approach allows for the study of both the function of mature proteins, with their native stoichiometry and post-translational modifications, and their putative modulation by surrounding lipids, mostly when the protein is purified and reconstituted in lipid matrices of defined composition. Remarkably, this methodology enables functional microtransplantation to the oocyte of membrane receptors, ion channels, and transporters from different sources including human post-mortem tissue banks. Despite the large progress achieved over the last decades on the structure, function, and modulation of neuroreceptors and ion channels in healthy and pathological tissues, many unanswered questions remain and, most likely, Xenopus oocytes will continue to help provide valuable responses.

The application of single molecule nanopore sensing for quantitative analysis

Nanopore-based sensors typically work by monitoring transient pulses in conductance via current-time traces as molecules translocate through the nanopore. The unique property of being able to monitor single molecules gives nanopore sensors the potential as quantitative sensors based on the counting of single molecules. This review provides an overview of the concepts and fabrication of nanopore sensors as well as nanopore sensing with a view toward using nanopore sensors for quantitative analysis. We first introduce the classification of nanopores and highlight their applications in molecular identification with some pioneering studies. The review then shifts focus to recent strategies to extend nanopore sensors to devices that can rapidly and accurately quantify the amount of an analyte of interest. Finally, future prospects are provided and briefly discussed. The aim of this review is to aid in understanding recent advances, challenges, and prospects for nanopore sensors for quantitative analysis.

CellSpecks: A Software for Automated Detection and Analysis of Calcium Channels in Live Cells

To couple the fidelity of patch-clamp recording with a more high-throughput screening capability, we pioneered a, to our knowledge, novel approach to single-channel recording that we named "optical patch clamp." By using highly sensitive fluorescent Ca²⁺ indicator dyes in conjunction with total internal fluorescence microscopy techniques, we monitor Ca²⁺ flux through individual Ca²⁺-permeable channels. This approach provides information about channel gating analogous to patch-clamp recording at a time resolution of ∼2 ms with the additional advantage of being massively parallel, providing simultaneous and independent recording from thousands of channels in the native environment. However, manual analysis of the data generated by this technique presents severe challenges because a video recording can include many thousands of frames. To overcome this bottleneck, we developed an image processing and analysis framework called CellSpecks capable of detecting and fully analyzing the kinetics of ion channels within a video sequence. By using randomly generated synthetic data, we tested the ability of CellSpecks to rapidly and efficiently detect and analyze the activity of thousands of ion channels, including openings for a few milliseconds. Here, we report the use of CellSpecks for the analysis of experimental data acquired by imaging muscle nicotinic acetylcholine receptors and the Alzheimer's disease-associated amyloid β pores with multiconductance levels in the plasma membrane of Xenopus laevis oocytes. We show that CellSpecks can accurately and efficiently generate location maps and create raw and processed fluorescence time traces; histograms of mean open times, mean close times, open probabilities, durations, and maximal amplitudes; and a "channel chip" showing the activity of all channels as a function of time. Although we specifically illustrate the application of CellSpecks for analyzing data from Ca²⁺ channels, it can be easily customized to analyze other spatially and temporally localized signals.

Single-Vesicle Assays Using Liposomes and Cell-Derived Vesicles: From Modeling Complex Membrane Processes to Synthetic Biology and Biomedical Applications

The plasma membrane is of central importance for defining the closed volume of cells in contradistinction to the extracellular environment. The plasma membrane not only serves as a boundary, but it also mediates the exchange of physical and chemical information between the cell and its environment in order to maintain intra- and intercellular functions. Artificial lipid- and cell-derived membrane vesicles have been used as closed-volume containers, representing the simplest cell model systems to study transmembrane processes and intracellular biochemistry. Classical examples are studies of membrane translocation processes in plasma membrane vesicles and proteoliposomes mediated by transport proteins and ion channels. Liposomes and native membrane vesicles are widely used as model membranes for investigating the binding and bilayer insertion of proteins, the structure and function of membrane proteins, the intramembrane composition and distribution of lipids and proteins, and the intermembrane interactions during exo- and endocytosis. In addition, natural cell-released microvesicles have gained importance for early detection of diseases and for their use as nanoreactors and minimal protocells. Yet, in most studies, ensembles of vesicles have been employed. More recently, new micro- and nanotechnological tools as well as novel developments in both optical and electron microscopy have allowed the isolation and investigation of individual (sub)micrometer-sized vesicles. Such single-vesicle experiments have revealed large heterogeneities in the structure and function of membrane components of single vesicles, which were hidden in ensemble studies. These results have opened enormous possibilities for bioanalysis and biotechnological applications involving unprecedented miniaturization at the nanometer and attoliter range. This review will cover important developments toward single-vesicle analysis and the central discoveries made in this exciting field of research.

Multi-scale and multi-physics model of the uterine smooth muscle with mechanotransduction

Preterm labor is an important public health problem. However, the efficiency of the uterine muscle during labor is complex and still poorly understood. This work is a first step towards a model of the uterine muscle, including its electrical and mechanical components, to reach a better understanding of the uterus synchronization. This model is proposed to investigate, by simulation, the possible role of mechanotransduction for the global synchronization of the uterus. The electrical diffusion indeed explains the local propagation of contractile activity, while the tissue stretching may play a role in the synchronization of distant parts of the uterine muscle. This work proposes a multi-physics (electrical, mechanical) and multi-scales (cell, tissue, whole uterus) model, which is applied to a realistic uterus 3D mesh. This model includes electrical components at different scales: generation of action potentials at the cell level, electrical diffusion at the tissue level. It then links these electrical events to the mechanical behavior, at the cellular level (via the intracellular calcium concentration), by simulating the force generated by each active cell. It thus computes an estimation of the intra uterine pressure (IUP) by integrating the forces generated by each active cell at the whole uterine level, as well as the stretching of the tissue (by using a viscoelastic law for the behavior of the tissue). It finally includes at the cellular level stretch activated channels (SACs) that permit to create a loop between the mechanical and the electrical behavior (mechanotransduction). The simulation of different activated regions of the uterus, which in this first "proof of concept" case are electrically isolated, permits the activation of inactive regions through the stretching (induced by the electrically active regions) computed at the whole organ scale. This permits us to evidence the role of the mechanotransduction in the global synchronization of the uterus. The results also permit us to evidence the effect on IUP of this enhanced synchronization induced by the presence of SACs. This proposed simplified model will be further improved in order to permit a better understanding of the global uterine synchronization occurring during efficient labor contractions.

Analyzing and Modeling the Kinetics of Amyloid Beta Pores Associated with Alzheimer's Disease Pathology

Amyloid beta (Aβ) oligomers associated with Alzheimer’s disease (AD) form Ca²⁺-permeable plasma membrane pores, leading to a disruption of the otherwise well-controlled intracellular calcium (Ca²⁺) homeostasis. The resultant up-regulation of intracellular Ca²⁺ concentration has detrimental implications for memory formation and cell survival. The gating kinetics and Ca²⁺ permeability of Aβ pores are not well understood. We have used computational modeling in conjunction with the ability of optical patch-clamping for massively parallel imaging of Ca²⁺ flux through thousands of pores in the cell membrane of Xenopus oocytes to elucidate the kinetic properties of Aβ pores. The fluorescence time-series data from individual pores were idealized and used to develop data-driven Markov chain models for the kinetics of the Aβ pore at different stages of its evolution. Our study provides the first demonstration of developing Markov chain models for ion channel gating that are driven by optical-patch clamp data with the advantage of experiments being performed under close to physiological conditions. Towards the end, we demonstrate the up-regulation of gating of various Ca²⁺ release channels due to Aβ pores and show that the extent and spatial range of such up-regulation increases as Aβ pores with low open probability and Ca²⁺ permeability transition into those with high open probability and Ca²⁺ permeability.

Imaging potassium-flux through individual electropores in droplet interface bilayers

Using total internal reflection fluorescence microscopy of droplet interface bilayers containing the potassium-sensitive fluorophore APG-4, we imaged the ionic flux through individual electropores. We are able to monitor up to 30 individual pores in parallel and show voltage dependent responses in fluorescence that corresponds to the measured ionic current. These experiments help quantify the scope and current limitations of optical single channel recordings of potassium flux. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

An integrated system for optical and electrical detection of single molecules/particles inside a solid-state nanopore

Nanopore techniques have proven to be useful tools for single-molecule detection. The combination of optical detection and ionic current measurements enables a new possibility for the parallel readout of multiple nanopores without complex nanofluidics and embedded electrodes. In this study, we developed a new integrated system for the label-free optical and electrical detection of single molecules based on a metal-coated nanopore. The entire system, containing a dark-field microscopy system and an ultralow current detection system with high temporal resolution, was designed and fabricated. An Au-coated nanopore was used to generate the optical signal. Light scattering from a single Au-coated nanopore was measured under a dark-field microscope. A lab-built ultralow current detection system was designed for the correlated optical and electrical readout. This integrated system might provide more direct and detailed information on single analytes inside the nanopore compared with classical ionic current measurements.

Label-free optical detection of biomolecular translocation through nanopore arrays

In recent years, nanopores have emerged as exceptionally promising single-molecule sensors due to their ability to detect biomolecules at subfemtomole levels in a label-free manner. Development of a high-throughput nanopore-based biosensor requires multiplexing of nanopore measurements. Electrical detection, however, poses a challenge, as each nanopore circuit must be electrically independent, which requires complex nanofluidics and embedded electrodes. Here, we present an optical method for simultaneous measurements of the ionic current across an array of solid-state nanopores, requiring no additional fabrication steps. Proof-of-principle experiments are conducted that show simultaneous optical detection and characterization of ssDNA and dsDNA using an array of pores. Through a comparison with electrical measurements, we show that optical measurements are capable of accessing equivalent transmembrane current information.

TRP channel Ca(2+) sparklets: fundamental signals underlying endothelium-dependent hyperpolarization

Important functions of the vascular endothelium, including permeability, production of antithrombotic factors, and control of vascular tone, are regulated by changes in intracellular Ca(2+). The molecular identities and regulation of Ca(2+) influx channels in the endothelium are incompletely understood, in part because of experimental difficulties associated with application of patch-clamp electrophysiology to native endothelial cells. However, advances in confocal and total internal reflection fluorescence microscopy and the development of fast, high-affinity Ca(2+)-binding fluorophores have recently allowed for direct visualization and characterization of single-channel transient receptor potential (TRP) channel Ca(2+) influx events in endothelial cells. These events, called "TRP channel Ca(2+) sparklets," have been optically recorded from primary endothelial cells and the intact endothelium, and the biophysical properties and fundamental significance of these Ca(2+) signals in vasomotor regulation have been characterized. This review will first briefly discuss the role of endothelial cell TRP channel Ca(2+) influx in endothelium-dependent vasodilation, describe improved methods for recording unitary TRP channel activity using optical methods, and highlight discoveries regarding the regulation and physiological significance of TRPV4 Ca(2+) sparklets in the vascular endothelium enabled by this new technology. Perspectives on the potential use of these techniques to evaluate changes in TRP channel Ca(2+) influx activity associated with endothelial dysfunction are offered.

Capturing single L-type Ca(2+) channel function with optics

Advances in imaging technology have allowed optical analysis of Ca(2+)-permeable ion channel activity. Here, we briefly review novel developments in optical recording of L-type voltage-dependent Ca(2+) channel (LTCC) function with high spatial and temporal resolution. Underlying principles supporting the use of total internal reflection fluorescence (TIRF) microscopy for optical measurement of channel activity and new functional characteristics of LTCCs revealed by application of this approach are discussed. Visualization of Ca(2+) influx through single LTCCs ("LTCC sparklets") has demonstrated that channel activity is regionally heterogeneous and that clustered channels are capable of operating in a cooperative, or "coupled" manner. In light of these findings, we describe a current molecular model for the local control of LTCC activity and coupled gating in physiological and pathological contexts. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Regulation of L-type calcium channel sparklet activity by c-Src and PKC-α

The activity of persistent Ca²⁺ sparklets, which are characterized by longer and more frequent channel open events than low-activity sparklets, contributes substantially to steady-state Ca²⁺ entry under physiological conditions. Here, we addressed two questions related to the regulation of Ca²⁺ sparklets by PKC-α and c-Src, both of which increase whole cell Cav1.2 current: 1) Does c-Src activation enhance persistent Ca²⁺ sparklet activity? 2) Does PKC-α activate c-Src to produce persistent Ca²⁺ sparklets? With the use of total internal reflection fluorescence microscopy, Ca²⁺ sparklets were recorded from voltage-clamped tsA-201 cells coexpressing wild-type (WT) or mutant Cav1.2c (the neuronal isoform of Cav1.2) constructs ± active or inactive PKC-α/c-Src. Cells expressing Cav1.2c exhibited both low-activity and persistent Ca²⁺ sparklets. Persistent Ca²⁺ sparklet activity was significantly reduced by acute application of the c-Src inhibitor PP2 or coexpression of kinase-dead c-Src. Cav1.2c constructs mutated at one of two COOH-terminal residues (Y²¹²²F and Y²¹³⁹F) were used to test the effect of blocking putative phosphorylation sites for c-Src. Expression of Y²¹²²F but not Y²¹³⁹F Cav1.2c abrogated the potentiating effect of c-Src on Ca²⁺ sparklet activity. We could not detect a significant change in persistent Ca²⁺ sparklet activity or density in cells coexpressing Cav1.2c + PKC-α, regardless of whether WT or Y²¹²²F Cav1.2c was used, or after PP2 application, suggesting that PKC-α does not act upstream of c-Src to produce persistent Ca²⁺ sparklets. However, our results indicate that persistent Ca²⁺ sparklet activity is promoted by the action of c-Src on residue Y²¹²² of the Cav1.2c COOH terminus.

Measuring calcium signals and exocytosis in tissues

BACKGROUND

Since the 1960s it has been clear that calcium is a key regulator of exocytosis. Early experiments directly showed that the secretory output was calcium dependent. But it has taken improvements in technology and clever experimentation to determine the relationships between the calcium signal and exocytosis. Today controversies still remain because of limitations in our ability to record both the calcium responses within the local domains that control secretion and in the methods used to record exocytosis.

SCOPE OF REVIEW

Here the techniques used to measure calcium and exocytosis are reviewed with a distinction being drawn between measurements in excitable cells versus measurements in non-excitable cells. The review has a focus on techniques that are relevant to in vitro studies of native tissues and recent in vivo recordings.

MAJOR CONCLUSIONS

There are a range of methods used to study the stimulus-secretion pathway. Each presents their own advantages and drawbacks. These are discussed with reference to the latest work determining the factors controlling exocytosis in tissues.

GENERAL SIGNIFICANCE

Stimulus-secretion coupling is the fundamental step in the control of neurotransmitter release, hormone secretion and protein secretion. Understanding secretory control is therefore important in understanding the physiological regulation of processes ranging from learning and memory to pancreatic secretion. Recent technological advances are now enabling us to study stimulus-secretion coupling within native tissues. This is helping us to understand the physiological complexities of secretory control. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Single-channel Ca(2+) imaging implicates Aβ1-42 amyloid pores in Alzheimer's disease pathology

High-resolution imaging of calcium influx reveals that the Aβ peptides implicated in Alzheimer’s disease form highly toxic Ca²⁺-permeable pores.

Oligomeric forms of Aβ peptides are implicated in Alzheimer’s disease (AD) and disrupt membrane integrity, leading to cytosolic calcium (Ca²⁺) elevation. Proposed mechanisms by which Aβ mediates its effects include lipid destabilization, activation of native membrane channels, and aggregation of Aβ into Ca²⁺-permeable pores. We distinguished between these using total internal reflection fluorescence (TIRF) microscopy to image Ca²⁺ influx in Xenopus laevis oocytes. Aβ1–42 oligomers evoked single-channel Ca²⁺ fluorescence transients (SCCaFTs), which resembled those from classical ion channels but which were not attributable to endogenous oocyte channels. SCCaFTs displayed widely variable open probabilities (P_o) and stepwise transitions among multiple amplitude levels reminiscent of subconductance levels of ion channels. The proportion of high P_o, large amplitude SCCaFTs grew with time, suggesting that continued oligomer aggregation results in the formation of highly toxic pores. We conclude that formation of intrinsic Ca²⁺-permeable membrane pores is a major pathological mechanism in AD and introduce TIRF imaging for massively parallel single-channel studies of the incorporation, assembly, and properties of amyloidogenic oligomers.

Superresolution localization of single functional IP3R channels utilizing Ca2+ flux as a readout

The subcellular localization of membrane Ca2+ channels is crucial for their functioning, but is difficult to study because channels may be distributed more closely than the resolution of conventional microscopy is able to detect. We describe a technique, stochastic channel Ca2+ nanoscale resolution (SCCaNR), employing Ca2+-sensitive fluorescent dyes to localize stochastic openings and closings of single Ca2+-permeable channels within <50 nm, and apply it to examine the clustered arrangement of inositol trisphosphate receptor (IP3R) channels underlying local Ca2+ puffs. Fluorescence signals (blips) arising from single functional IP3Rs are almost immotile (diffusion coefficient<0.003 microm2 s(-1)), as are puff sites over prolonged periods, suggesting that the architecture of this signaling system is stable and not subject to rapid, dynamic rearrangement. However, rapid stepwise changes in centroid position of fluorescence are evident within the durations of individual puffs. These apparent movements likely result from asynchronous gating of IP3Rs distributed within clusters that have an overall diameter of approximately 400 nm, indicating that the nanoscale architecture of IP3R clusters is important in shaping local Ca2+ signals. We anticipate that SCCaNR will complement superresolution techniques such as PALM and STORM for studies of Ca2+ channels as it obviates the need for photoswitchable labels and provides functional as well as spatial information.

Ion trapping with fast-response ion-selective microelectrodes enhances detection of extracellular ion channel gradients

Previously, functional mapping of channels has been achieved by measuring the passage of net charge and of specific ions with electrophysiological and intracellular fluorescence imaging techniques. However, functional mapping of ion channels using extracellular ion-selective microelectrodes has distinct advantages over the former methods. We have developed this method through measurement of extracellular K+ gradients caused by efflux through Ca2+-activated K+ channels expressed in Chinese hamster ovary cells. We report that electrodes constructed with short columns of a mechanically stable K+-selective liquid membrane respond quickly and measure changes in local [K+] consistent with a diffusion model. When used in close proximity to the plasma membrane (<4 microm), the ISMs pose a barrier to simple diffusion, creating an ion trap. The ion trap amplifies the local change in [K+] without dramatically changing the rise or fall time of the [K+] profile. Measurement of extracellular K+ gradients from activated rSlo channels shows that rapid events, 10-55 ms, can be characterized. This method provides a noninvasive means for functional mapping of channel location and density as well as for characterizing the properties of ion channels in the plasma membrane.

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.

Multi-dimensional resolution of elementary Ca2+ signals by simultaneous multi-focal imaging

Elementary events such as puffs and sparks are cytosolic microdomains of Ca2+ from which cellular Ca2+ signals are constructed. Because of the tight localization and fast kinetics of elementary events, imaging studies have been hindered by instrumental limitations of confocal and deconvolution fluorescence microscopy which necessitate compromises between spatial and temporal resolution. Here, we describe a novel, yet simple 'multi-focal' fluorescence microscopy system that employs three high-speed cameras focused at different axial depths to enable 4-dimensional imaging with millisecond resolution. We demonstrate the utility of this system for studies of puffs in Xenopus oocytes by mapping the axial distribution of puff sites, by obtaining measurements of puff amplitudes undistorted by focus error, and by deriving deblurred images that reveal novel sub-micron jumps of Ca2+ release sites.

Imaging single-channel calcium microdomains

The Ca(2+) microdomains generated around the mouth of open ion channels represent the basic building blocks from which cytosolic Ca(2+) signals are constructed. Recent improvements in optical imaging techniques now allow these microdomains to be visualized as single channel calcium fluorescence transients (SCCaFTs), providing information about channel properties that was previously accessible only by electrophysiological patch-clamp recordings. We review recent advances in single channel Ca(2+) imaging methodologies, with emphasis on total internal reflection fluorescence microscopy (TIRFM) as the technique of choice for recording SCCaFTs from voltage- and ligand-gated plasmalemmal ion channels. This technique of 'optical patch-clamp recording' is massively parallel, permitting simultaneous imaging of hundreds of channels; provides millisecond resolution of gating kinetics together with sub-micron spatial resolution of channel locations; and is applicable to diverse families of membrane channels that display partial permeability to Ca(2+) ions.

'Trigger' events precede calcium puffs in Xenopus oocytes

The liberation of calcium ions sequestered in the endoplasmic reticulum through inositol 1,4,5-trisphosphate receptors/channels (IP(3)Rs) results in a spatiotemporal hierarchy of calcium signaling events that range from single-channel openings to local Ca(2+) puffs believed to arise from several to tens of clustered IP(3)Rs to global calcium waves. Using high-resolution confocal linescan imaging and a sensitive Ca(2+) indicator dye (fluo-4-dextran), we show that puffs are often preceded by small, transient Ca(2+) elevations that we christen "trigger events". The magnitude of triggers is consistent with their arising from the opening of a single IP(3) receptor/channel, and we propose that they initiate puffs by recruiting neighboring IP(3)Rs within the cluster by a regenerative process of Ca(2+)-induced Ca(2+) release. Puff amplitudes (fluorescence ratio change) are on average approximately 6 times greater than that of the triggers, suggesting that at least six IP(3)Rs may simultaneously be open during a puff. Trigger events have average durations of approximately 12 ms, as compared to 19 ms for the mean rise time of puffs, and their spatial extent is approximately 3 times smaller than puffs (respective widths at half peak amplitude 0.6 and 1.6 micro m). All these parameters were relatively independent of IP(3) concentration, although the proportion of puffs showing resolved triggers was greatest (approximately 80%) at low [IP(3)]. Because Ca(2+) puffs constitute the building blocks from which cellular IP(3)-mediated Ca(2+) signals are constructed, the events that initiate them are likely to be of fundamental importance for cell signaling. Moreover, the trigger events provide a useful yardstick by which to derive information regarding the number and spatial arrangement of IP(3)Rs within clusters.

The number and spatial distribution of IP3 receptors underlying calcium puffs in Xenopus oocytes

Calcium puffs are local Ca(2+) release events that arise from a cluster of inositol 1,4,5-trisphosphate receptor channels (IP(3)Rs) and serve as a basic "building block" from which global Ca(2+) waves are generated. Important questions remain as to the number of IP(3)Rs that open during a puff, their spatial distribution within a cluster, and how much Ca(2+) current flows through each channel. The recent discovery of "trigger" events-small Ca(2+) signals that immediately precede puffs and are interpreted to arise through opening of single IP(3)R channels-now provides a useful yardstick by which to calibrate the Ca(2+) flux underlying puffs. Here, we describe a deterministic numerical model to simulate puffs and trigger events. Based on confocal linescan imaging in Xenopus oocytes, we simulated Ca(2+) release in two sequential stages; representing the trigger by the opening of a single IP(3)R in the center of a cluster for 12 ms, followed by the concerted opening of some number of IP(3)Rs for 19 ms, representing the rising phase of the puff. The diffusion of Ca(2+) and Ca(2+)-bound indicator dye were modeled in a three-dimensional cytosolic volume in the presence of immobile and mobile Ca(2+) buffers, and were used to predict the observed fluorescence signal after blurring by the microscope point-spread function. Optimal correspondence with experimental measurements of puff spatial width and puff/trigger amplitude ratio was obtained assuming that puffs arise from the synchronous opening of 25-35 IP(3)Rs, each carrying a Ca(2+) current of approximately 0.4 pA, with the channels distributed through a cluster 300-800 nm in diameter.

Simple data-driven models of intracellular calcium dynamics with predictive power

Biology is complex. However, it is not clear how much of this complexity must necessarily translate into complicated mathematical models of biological processes. Simple models can be appealing to physicists but are usually deceiving for biologists. Complicated models, on the other hand, depend on too many parameters whose values are frequently unknown. Therefore, complicated models, although in principle more realistic, can lead to erroneous results if they are sensitive to these unknown parameter values. Intracellular calcium signals provide an example of utmost biological importance in which the issue of "simple vs complex" can be explored. In this paper we show that simple models describing the dynamics of intracellular calcium can be directly inferred from experimental data, without no a priori information on unknown parameters. A similar approach can be followed to study other reaction-diffusion systems. In spite of their simplicity, these models can provide quantitative information on some of the processes that shape calcium signals, such as the calcium current that underlies an experimental observation. This shows that simple models of biological systems are not limited to qualitative descriptions.

Simultaneous flux and current measurement from single plant protoplasts reveals a strong link between K+ fluxes and current, but no link between Ca2+ fluxes and current

We present a thorough calibration and verification of a combined non-invasive self-referencing microelectrode-based ion-flux measurement and whole-cell patch clamp system as a novel and powerful tool for the study of ion transport. The system is shown to be capable of revealing the movement of multiple ions across the plasma membrane of a single protoplast at multiple voltages and in complex physiologically relevant solutions. Wheat root protoplasts are patch clamped in the whole-cell configuration and current-voltage relations obtained whilst monitoring net K+ and Ca2+ flux adjacent to the membrane with ion-selective electrodes. At each voltage, net ion flux (nmol m(-2) sec(-1)) is converted to an equivalent current density (mA m(-2)) taking into account geometry and electrode efficiency, and compared with the net current density measured with the patch clamp system. Using this technique, it is demonstrated that the K+-permeable outwardly rectifying conductance (KORC) is responsible for net outward K+ movement across the plasma membrane [1:1 flux-to-current ratio (1.21 +/- 0.14 SEM, n = 15)]. Variation in the K+ flux-to-current ratio among single protoplasts suggests a heterogeneous distribution of KORC channels on the membrane surface. As a demonstration of the power of the technique we show that despite a significant Ca2+ permeability being associated with KORC (analysis of tail current reversal potentials), there is no correlation between Ca2+ flux and KORC activity. A very significant observation is that large Ca2+ fluxes are electrically silent and probably tightly coupled to compensatory charge movements. This analysis demonstrates that it is mandatory to measure flux and currents simultaneously to investigate properly Ca2+ transport mechanisms and selectivity of ion channels in general.

"Optical patch-clamping": single-channel recording by imaging Ca2+ flux through individual muscle acetylcholine receptor channels

We describe an optical technique using total internal reflection fluorescence (TIRF) microscopy to obtain simultaneous and independent recordings from numerous ion channels via imaging of single-channel Ca²⁺ flux. Muscle nicotinic acetylcholine (ACh) receptors made up of αβγδ subunits were expressed in Xenopus oocytes, and single channel Ca²⁺ fluorescence transients (SCCaFTs) were imaged using a fast (500 fps) electron-multiplied c.c.d. camera with fluo-4 as the indicator. Consistent with their arising through openings of individual nicotinic channels, SCCaFTs were seen only when a nicotinic agonist was present in the bathing solution, were blocked by curare, and increased in frequency as roughly the second power of [ACh]. Their fluorescence amplitudes varied linearly with membrane potential and extrapolated to zero at about +60 mV. The rise and fall times of fluorescence were as fast as 2 ms, providing a kinetic resolution adequate to characterize channel gating kinetics; which showed mean open times of 7.9 and 15.8 ms when activated, respectively, by ACh or suberyldicholine. Simultaneous records were obtained from >400 channels in the imaging field, and we devised a novel “channel chip” representation to depict the resultant large dataset as a single image. The positions of SCCaFTs remained fixed (<100 nm displacement) over tens of seconds, indicating that the nicotinic receptor/channels are anchored in the oocyte membrane; and the spatial distribution of channels appeared random without evidence of clustering. Our results extend single-channel TIRFM imaging to ligand-gated channels that display only partial permeability to Ca²⁺, and demonstrate an order-of-magnitude improvement in kinetic resolution. We believe that functional single-channel imaging opens a new approach to ion channel study, having particular advantages over patch-clamp recording in that it is massively parallel, and provides high-resolution spatial information that is inaccessible by electrophysiological techniques.

Optical single-channel recording by imaging Ca2+ flux through individual ion channels: theoretical considerations and limits to resolution

Recent developments in microscopy and fluorescent indicators now make it possible to monitor the activity and localization of membrane ion channels by imaging Ca(2+) flux through individual channels. Such optical approaches have advantages over electrophysiological single-channel techniques in that they are less invasive, provide spatial information and can simultaneously and independently monitor hundreds of channels. However, their kinetic resolution does not yet approach that of patch-clamp recordings. To help understand the processes that determine the temporal resolution and noise level of single-channel Ca(2+) fluorescence signals (SCCaFTs), we simulated the microdomains of Ca(2+) ions and Ca(2+)-bound indicator dye that exist around the mouth of an open channel. Further, as an aid to development of improved optical techniques, we modeled the dependence of the amplitude and kinetics of SCCaFTs on parameters such as the imaging volume, the indicator concentration, affinity and mobility, and the presence of endogenous and exogenous Ca(2+) buffers. The results indicate that under optimal conditions, including the use of confocal or total-internal reflection microscopy to image from sub-femtolitre volumes, SCCaFTs should resolve channel openings as brief as 1ms with a signal-to-noise ratio >10.

A model-independent algorithm to derive Ca2+ fluxes underlying local cytosolic Ca2+ transients

Local intracellular Ca(2+) signals result from Ca(2+) flux into the cytosol through individual channels or clusters of channels. To gain a mechanistic understanding of these events we need to know the magnitude and spatial distribution of the underlying Ca(2+) flux. However, this is difficult to infer from fluorescence Ca(2+) images because the distribution of Ca(2+)-bound dye is affected by poorly characterized processes including diffusion of Ca(2+) ions, their binding to mobile and immobile buffers, and sequestration by Ca(2+) pumps. Several methods have previously been proposed to derive Ca(2+) flux from fluorescence images, but all require explicit knowledge or assumptions regarding these processes. We now present a novel algorithm that requires few assumptions and is largely model-independent. By testing the algorithm with both numerically generated image data and experimental images of sparklets resulting from Ca(2+) flux through individual voltage-gated channels, we show that it satisfactorily reconstructs the magnitude and time course of the underlying Ca(2+) currents.

Optical single-channel recording: imaging Ca2+ flux through individual ion channels with high temporal and spatial resolution

Developments in imaging technology now enable visualization of the functioning of individual ion channels in living cells: something previously possible only by the electrophysiological patch-clamp technique. We review techniques that track channel gating via changes in intracellular [Ca2+] resulting from openings of Ca(2+)-permeable channels. Spatial and temporal resolution are optimized by monitoring Ca2+ close to the channel mouth, and we describe the use of two imaging modalities: confocal laser scan microscopy (linescan CLSM) and total internal reflection fluorescence microscopy (TIRFM). Both currently achieve a kinetic resolution of <10 ms, provide a simultaneous and independent readout from many channels, and enable their locations to be mapped with submicrometer resolution. TIRFM provides 2-D images from a very thin (approximately 100 nm) optical section, but it is restricted to channels in the plasma membrane of cells adhering close to a cover glass. In contrast, CLSM can image channels in intracellular membranes but, to achieve good temporal resolution, has been utilized only in a linescan mode with limited spatial information. We anticipate that imaging techniques will develop as a useful adjunct to patch-clamping for single-channel studies, with capabilities including simultaneous readout from multiple channels, high-resolution mapping of channel location, and mobility that is inaccessible by electrophysiological means. Optical single-channel recording is applicable to diverse voltage- and ligand-gated Ca(2+)-permeable channels and has potential for high-throughput functional analysis.

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.

Imaging calcium entering the cytosol through a single opening of plasma membrane ion channels: SCCaFTs—fundamental calcium events

Recently, it has become possible to record the localized fluorescence transient associated with the opening of a single plasma membrane Ca(2+) permeable ion channel using Ca(2+) indicators like fluo-3. These Single Channel Ca(2+) Fluorescence Transients (SCCaFTs) share some of the characteristics of such elementary events as Ca(2+) sparks and Ca(2+) puffs caused by Ca(2+) release from intracellular stores (due to the opening of ryanodine receptors and IP(3) receptors, respectively). In contrast to intracellular Ca(2+) release events, SCCaFTs can be observed while simultaneously recording the unitary channel currents using patch-clamp techniques to verify the channel openings. Imaging SCCaFTs provides a way to examine localized Ca(2+) handling in the vicinity of a channel with a known Ca(2+) influx, to obtain the Ca(2+) current passing through plasma membrane cation channels in near physiological solutions, to localize Ca(2+) permeable ion channels on the plasma membrane, and to estimate the Ca(2+) currents underlying those elementary events where the Ca(2+) currents cannot be recorded. Here we review studies of these fluorescence transients associated with caffeine-activated channels, L-type Ca(2+) channels, and stretch-activated channels. For the L-type Ca(2+) channel, SCCaFTs have been termed sparklets. In addition, we discuss how SCCaFTs have been used to estimate Ca(2+) currents using the rate of rise of the fluorescence transient as well as the signal mass associated with the total fluorescence increase.

Imaging the activity and localization of single voltage-gated Ca(2+) channels by total internal reflection fluorescence microscopy

The patch-clamp technique has enabled functional studies of single ion channels, but suffers limitations including lack of spatial information and inability to independently monitor currents from more than one channel. Here, we describe the use of total internal reflection fluorescence microscopy as an alternative, noninvasive approach to optically monitor the activity and localization of multiple Ca(2+)-permeable channels in the plasma membrane. Images of near-membrane Ca(2+) signals were obtained from >100 N-type channels expressed within restricted areas (80 x 80 micro m) of Xenopus oocytes, thereby permitting simultaneous resolution of their gating kinetics, voltage dependence, and localization. Moreover, this technique provided information inaccessible by electrophysiological means, demonstrating that N-type channels are immobile in the membrane, show a patchy distribution, and display diverse gating kinetics even among closely adjacent channels. Total internal reflection fluorescence microscopy holds great promise for single-channel recording of diverse voltage- and ligand-gated Ca(2+)-permeable channels in the membrane of neurons and other isolated or cultured cells, and has potential for high-throughput functional analysis of single channels.