A comparison of fluorescent Ca2+ indicator properties and their use in measuring elementary and global Ca2+ signals

Functionalizable red emitting calcium sensor bearing a 1,4-triazole chelating moiety

Herein we developed a functionalizable OFF–ON red emitting fluorescent calcium probe based on a new chelating system formed by CuAAC click chemistry (Huisgen cycloaddition).

Two-dimensional imaging of fast intracellular Ca2+ release

Asynchronous release of calcium (Ca(2+))-for example, the generation of Ca(2+) alternans in cardiac myocytes-is a phenomenon important in the development of cardiac arrhythmogenesis. The development of a failure to release Ca(2+) at individual release sites can be regarded as a major contributor to cardiac pathologies such as hypertrophy. Although confocal linescans provide sufficient temporal resolution to investigate the physiological and pathological cardiac excitation-contraction (EC) coupling, linescans can only image ∼1.5% of the cross section of myocytes, which raises doubts about how representative such recordings are, especially in light of nonhomogeneous uncoupling of Ca(2+) channels and ryanodine receptors. Nowadays, the speed of confocal microscopes has been greatly improved, enabling two-dimensional (2D) imaging at sufficient image rates (>100 frames/sec). To understand better the physiological and pathophysiological EC coupling of cardiomyocytes, we describe here a protocol to monitor fast intracellular Ca(2+) signals using fast 2D confocal scanning.

Upregulation of ABC transporters contributes to chemoresistance of sphingosine 1-phosphate lyase-deficient fibroblasts

Sphingosine 1-phosphate (S1P) is an extra- and intracellular mediator that regulates cell growth, survival, migration, and adhesion in many cell types. S1P lyase is the enzyme that irreversibly cleaves S1P and thereby constitutes the ultimate step in sphingolipid catabolism. It has been reported previously that embryonic fibroblasts from S1P lyase-deficient mice (Sgpl1(-/-)-MEFs) are resistant to chemotherapy-induced apoptosis through upregulation of B cell lymphoma 2 (Bcl-2) and Bcl-2-like 1 (Bcl-xL). Here, we demonstrate that the transporter proteins Abcc1/MRP1, Abcb1/MDR1, Abca1, and spinster-2 are upregulated in Sgpl1(-/-)-MEFs. Furthermore, the cells efficiently sequestered the substrates of Abcc1 and Abcb1, fluo-4 and doxorubicin, in subcellular compartments. In line with this, Abcb1 was localized mainly at intracellular vesicular structures. After 16 h of incubation, wild-type MEFs had small apoptotic nuclei containing doxorubicin, whereas the nuclei of Sgpl1(-/-)-MEFs appeared unchanged and free of doxorubicin. A combined treatment with the inhibitors of Abcb1 and Abcc1, zosuquidar and MK571, respectively, reversed the compartmentalization of doxorubicin and rendered the cells sensitive to doxorubicin-induced apoptosis. It is concluded that upregulation of multidrug resistance transporters contributes to the chemoresistance of S1P lyase-deficient MEFs.

In situ Ca2+ titration in the fluorometric study of intracellular Ca2+ binding

Imaging with Ca(2+)-sensitive fluorescent dye has provided a wealth of insight into the dynamics of cellular Ca(2+) signaling. The spatiotemporal evolution of intracellular free Ca(2+) observed in imaging experiments is shaped by binding and unbinding to cytoplasmic Ca(2+) buffers, as well as the fluorescent indicator used for imaging. These factors must be taken into account in the interpretation of Ca(2+) imaging data, and can be exploited to investigate endogenous Ca(2+) buffer properties. Here we extended the use of Ca(2+) fluorometry in the characterization of Ca(2+) binding molecules within cells, building on a method of titration of intracellular Ca(2+) binding sites in situ with measured amounts of Ca(2+) entering through voltage-gated Ca(2+) channels. We developed a systematic procedure for fitting fluorescence data acquired during a series of voltage steps to models with multiple Ca(2+) binding sites. The method was tested on simulated data, and then applied to 2-photon fluorescence imaging data from rat posterior pituitary nerve terminals patch clamp-loaded with the Ca(2+) indicator fluo-8. Focusing on data sets well described by a single endogenous Ca(2+) buffer and dye, this method yielded estimates of the endogenous buffer concentration and Kd, the dye Kd, and the fraction of Ca(2+) inaccessible cellular volume. The in situ Kd of fluo-8 thus obtained was indistinguishable from that measured in vitro. This method of calibrating Ca(2+)-sensitive fluorescent dyes in situ has significant advantages over previous methods. Our analysis of Ca(2+) titration fluorometric data makes more effective use of the experimental data, and provides a rigorous treatment of multivariate errors and multiple Ca(2+) binding species. This method offers a versatile approach to the study of endogenous Ca(2+) binding molecules in their physiological milieu.

New red-fluorescent calcium indicators for optogenetics, photoactivation and multi-color imaging

Most chemical and, with only a few exceptions, all genetically encoded fluorimetric calcium (Ca(2+)) indicators (GECIs) emit green fluorescence. Many of these probes are compatible with red-emitting cell- or organelle markers. But the bulk of available fluorescent-protein constructs and transgenic animals incorporate green or yellow fluorescent protein (GFP and YFP respectively). This is, in part, not only heritage from the tendency to aggregate of early-generation red-emitting FPs, and due to their complicated photochemistry, but also resulting from the compatibility of green-fluorescent probes with standard instrumentation readily available in most laboratories and core imaging facilities. Photochemical constraints like limited water solubility and low quantum yield have contributed to the relative paucity of red-emitting Ca(2+) probes compared to their green counterparts, too. The increasing use of GFP and GFP-based functional reporters, together with recent developments in optogenetics, photostimulation and super-resolution microscopies, has intensified the quest for red-emitting Ca(2+) probes. In response to this demand more red-emitting chemical and FP-based Ca(2+)-sensitive indicators have been developed since 2009 than in the thirty years before. In this topical review, we survey the physicochemical properties of these red-emitting Ca(2+) probes and discuss their utility for biological Ca(2+) imaging. Using the spectral separability index Xijk (Oheim M., 2010. Methods in Molecular Biology 591: 3-16) we evaluate their performance for multi-color excitation/emission experiments, involving the identification of morphological landmarks with GFP/YFP and detecting Ca(2+)-dependent fluorescence in the red spectral band. We also establish a catalog of criteria for evaluating Ca(2+) indicators that ideally should be made available for each probe. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Laser exposure of gold nanorods can induce intracellular calcium transients

Uncoated and poly(styrene sulphonate) (PSS)-coated gold nanorods were taken up by NG108-15 neuronal cells. Exposure to 780 nm laser light at the plasmon resonance wavelength of the gold nanorods was found to induce intracellular Ca(2+) transients. The higher Ca(2+) peaks were observed at lower laser doses, with the highest levels obtained at a radiant exposure of 0.33 J/cm(2) . In contrast, the cells without nanoparticles showed a consistently small response, independent of the laser dose. These initial results open up new opportunities for peripheral nerve regeneration treatments and for more efficient optical stimulation techniques.

Imaging of mitochondrial Ca2+ dynamics in astrocytes using cell-specific mitochondria-targeted GCaMP5G/6s: mitochondrial Ca2+ uptake and cytosolic Ca2+ availability via the endoplasmic reticulum store

Mitochondrial Ca(2+) plays a critical physiological role in cellular energy metabolism and signaling, and its overload contributes to various pathological conditions including neuronal apoptotic death in neurological diseases. Live cell mitochondrial Ca(2+) imaging is an important approach to understand mitochondrial Ca(2+) dynamics. Recently developed GCaMP genetically-encoded Ca(2+) indicators provide unique opportunity for high sensitivity/resolution and cell type-specific mitochondrial Ca(2+) imaging. In the current study, we implemented cell-specific mitochondrial targeting of GCaMP5G/6s (mito-GCaMP5G/6s) and used two-photon microscopy to image astrocytic and neuronal mitochondrial Ca(2+) dynamics in culture, revealing Ca(2+) uptake mechanism by these organelles in response to cell stimulation. Using these mitochondrial Ca(2+) indicators, our results show that mitochondrial Ca(2+) uptake in individual mitochondria in cultured astrocytes and neurons can be seen after stimulations by ATP and glutamate, respectively. We further studied the dependence of mitochondrial Ca(2+) dynamics on cytosolic Ca(2+) changes following ATP stimulation in cultured astrocytes by simultaneously imaging mitochondrial and cytosolic Ca(2+) increase using mito-GCaMP5G and a synthetic organic Ca(2+) indicator, x-Rhod-1, respectively. Combined with molecular intervention in Ca(2+) signaling pathway, our results demonstrated that the mitochondrial Ca(2+) uptake is tightly coupled with inositol 1,4,5-trisphosphate receptor-mediated Ca(2+) release from the endoplasmic reticulum and the activation of G protein-coupled receptors. The current study provides a novel approach to image mitochondrial Ca(2+) dynamics as well as Ca(2+) interplay between the endoplasmic reticulum and mitochondria, which is relevant for neuronal and astrocytic functions in health and disease.

Physiological epidermal growth factor concentrations activate high affinity receptors to elicit calcium oscillations

Signaling mediated by the epidermal growth factor (EGF) is crucial in tissue development, homeostasis and tumorigenesis. EGF is mitogenic at picomolar concentrations and is known to bind its receptor on high affinity binding sites depending of the oligomerization state of the receptor (monomer or dimer). In spite of these observations, the cellular response induced by EGF has been mainly characterized for nanomolar concentrations of the growth factor, and a clear definition of the cellular response to circulating (picomolar) concentrations is still lacking. We investigated Ca2+ signaling, an early event in EGF responses, in response to picomolar doses in COS-7 cells where the monomer/dimer equilibrium is unaltered by the synthesis of exogenous EGFR. Using the fluo5F Ca2+ indicator, we found that picomolar concentrations of EGF induced in 50% of the cells a robust oscillatory Ca2+ signal quantitatively similar to the Ca2+ signal induced by nanomolar concentrations. However, responses to nanomolar and picomolar concentrations differed in their underlying mechanisms as the picomolar EGF response involved essentially plasma membrane Ca2+ channels that are not activated by internal Ca2+ store depletion, while the nanomolar EGF response involved internal Ca2+ release. Moreover, while the picomolar EGF response was modulated by charybdotoxin-sensitive K+ channels, the nanomolar response was insensitive to the blockade of these ion channels.

Alterations of calcium homoeostasis in cultured rat astrocytes evoked by bioactive sphingolipids

AIM

In the brain, alterations in sphingolipid metabolism contribute to several neurological disorders; however, their effect on astrocytes is largely unknown. Here, we identified bioactive sphingolipids that affect intracellular free calcium concentration ([Ca(2+)]i), mobility of peptidergic secretory vesicles, signalling pathways involved in alterations of calcium homoeostasis and explored the relationship between the stimulus-evoked increase in [Ca(2+)]i and attenuation of vesicle mobility.

METHODS

Confocal time-lapse images were acquired to explore [Ca(2+)]i signals, the mobility of fluorescently tagged peptidergic vesicles and the structural integrity of the microtubules and actin filaments before and after the addition of exogenous sphingolipids to astrocytes.

RESULTS

Fingolimod (FTY720), a recently introduced therapeutic for multiple sclerosis, and sphingosine, a releasable constituent of membrane sphingolipids, evoked long-lasting increases in [Ca(2+)]i in the presence and absence of extracellular Ca(2+); the evoked responses were diminished in the absence of extracellular Ca(2+). Activation of phospholipase C and inositol-1,4,5-triphosphate receptors was necessary and sufficient to evoke increases in [Ca(2+)]i as revealed by the pharmacologic inhibitors; Ca(2+) flux from the extracellular space intensified these responses several fold. The lipid-evoked increases in [Ca(2+)]i coincided with the attenuated vesicle mobility. High and positive correlation between increase in [Ca(2+)]i and decrease in peptidergic vesicle mobility was confirmed independently in astrocytes exposed to evoked, transient Ca(2+) signalling triggered by purinergic and glutamatergic stimulation.

CONCLUSION

Exogenously added cell-permeable sphingosine-like lipids exert complex, Ca(2+)-dependent effects on astrocytes and likely alter their homeostatic function in vivo.

Dantrolene suppresses spontaneous Ca2+ release without altering excitation-contraction coupling in cardiomyocytes of aged mice

Cardiac dysfunction in the aged heart reflects abnormalities in cardiomyocyte Ca(2+) homeostasis including altered Ca(2+) cycling through the sarcoplasmic reticulum (SR). The ryanodine receptor antagonist dantrolene exerts antiarrhythmic effects by preventing spontaneous diastolic Ca(2+) release from the SR. We tested the hypothesis that dantrolene prevents spontaneous Ca(2+) release without altering excitation-contraction coupling in aged myocardium. Left ventricular cardiomyocytes isolated from young (3 to 4 mo) and aged (24-26 mo) C57BL/6 mice were loaded with the Ca(2+) indicator fluo-4. Amplitudes of action potential-induced Ca(2+) transients at 1-Hz pacing were similar between young and aged mice, yet cell shortening was impaired in aged mice. Isoproterenol (1 μM) increased Ca(2+) transient amplitude and cell shortening to identical levels in young and aged; dantrolene (1 μM) had no effect on Ca(2+) transients or cell shortening during pacing. Under Ca(2+) overload conditions induced with 10 mM extracellular Ca(2+) concentration, spontaneous Ca(2+) waves were of diminished amplitude and associated with lower SR Ca(2+) content in aged versus young mice. Despite no effect in young mice, dantrolene increased SR Ca(2+) content and Ca(2+) wave amplitude in aged mice. In the presence of isoproterenol following rest from 1-Hz pacing, Ca(2+) spark frequency was elevated in aged mice, yet the time to spontaneous Ca(2+) wave was similar between young and aged mice; dantrolene decreased Ca(2+) spark frequency and prolonged the time to Ca(2+) wave onset in aged mice with no effect in young mice. Thus dantrolene attenuates diastolic Ca(2+) release in the aged murine heart that may prove useful in preventing cardiac dysfunction.

Genetically encoded Ca2+ indicators in cardiac myocytes

Genetically encoded Ca(2+) indicators constitute a powerful set of tools to investigate functional aspects of Ca(2+) signaling in isolated cardiomyocytes, cardiac tissue, and whole hearts. Here, we provide an overview of the concepts, experiences, state of the art, and ongoing developments in the use of genetically encoded Ca(2+) indicators for cardiac cells and heart tissue. This review is supplemented with in vivo viral gene transfer experiments and comparisons of available genetically encoded Ca(2+) indicators with each other and with the small molecule dye Fura-2. In the context of cardiac myocytes, we provide guidelines for selecting a genetically encoded Ca(2+) indicator. For future developments, we discuss improvements of a broad range of properties, including photophysical properties such as spectral spread and biocompatibility, as well as cellular and in vivo applications.

Fluorescence fluctuations and equivalence classes of Ca²⁺ imaging experiments

release into the cytosol through inositol 1,4,5-trisphosphate receptors (IP₃Rs) plays a relevant role in numerous physiological processes. IP₃R-mediated signals involve -induced -release (CICR) whereby release through one open IP₃R induces the opening of other channels. IP₃Rs are apparently organized in clusters. The signals can remain localized (i.e., puffs) if CICR is limited to one cluster or become waves that propagate between clusters. puffs are the building blocks of waves. Thus, there is great interest in determining puff properties, especially in view of the current controversy on the spatial distribution of activatable IP₃Rs. puffs have been observed in intact cells with optical techniques proving that they are intrinsically stochastic. Obtaining a correct picture of their dynamics then entails being able to detect the whole range of puff sizes. puffs are observed using visible single-wavelength dyes, slow exogenous buffers (e.g., EGTA) to disrupt inter-cluster CICR and UV-photolyzable caged IP₃. Single-wavelength dyes increase their fluorescence upon calcium binding producing images that are strongly dependent on their kinetic, transport and photophysical properties. Determining the artifacts that the imaging setting introduces is particularly relevant when trying to analyze the smallest signals. In this paper we introduce a method to estimate the expected signal-to-noise ratio of imaging experiments that use single-wavelength dyes. The method is based on the Number and Brightness technique. It involves the performance of a series of experiments and their subsequent analysis in terms of a fluorescence fluctuation model with which the model parameters are quantified. Using the model, the expected signal-to-noise ratio is then computed. Equivalence classes between different experimental conditions that produce images with similar signal-to-noise ratios can then be established. The method may also be used to estimate the smallest signals that can reliably be observed with each setting.

Measuring baseline Ca(2+) levels in subcellular compartments using genetically engineered fluorescent indicators

Intracellular Ca(2+) signaling is involved in a series of physiological and pathological processes. In particular, an intimate crosstalk between bioenergetic metabolism and Ca(2+) homeostasis has been shown to determine cell fate in resting conditions as well as in response to stress. The endoplasmic reticulum and mitochondria represent key hubs of cellular metabolism and Ca(2+) signaling. However, it has been challenging to specifically detect highly localized Ca(2+) fluxes such as those bridging these two organelles. To circumvent this issue, various recombinant Ca(2+) indicators that can be targeted to specific subcellular compartments have been developed over the past two decades. While the use of these probes for measuring agonist-induced Ca(2+) signals in various organelles has been extensively described, the assessment of basal Ca(2+) concentrations within specific organelles is often disregarded, in spite of the fact that this parameter is vital for several metabolic functions, including the enzymatic activity of mitochondrial dehydrogenases of the Krebs cycle and protein folding in the endoplasmic reticulum. Here, we provide an overview on genetically engineered, organelle-targeted fluorescent Ca(2+) probes and outline their evolution. Moreover, we describe recently developed protocols to quantify baseline Ca(2+) concentrations in specific subcellular compartments. Among several applications, this method is suitable for assessing how changes in basal Ca(2+) levels affect the metabolic profile of cancer cells.

Temperature-induced labelling of Fluo-3 AM selectively yields brighter nucleus in adherent cells

Fluo-3 is widely used to study cell calcium. Two traditional approaches: (1) direct injection and (2) Fluo-3 acetoxymethyl ester (AM) loading, often bring conflicting results in cytoplasmic calcium ([Ca(2+)]c) and nuclear calcium ([Ca(2+)]n) imaging. AM loading usually yields a darker nucleus than in cytoplasm, while direct injection always induces a brighter nucleus which is more responsive to [Ca(2+)]n detection. In this work, we detailedly investigated the effects of loading and de-esterification temperatures on the fluorescence intensity of Fluo-3 in response to [Ca(2+)]n and [Ca(2+)]c in adherent cells, including osteoblast, HeLa and BV2 cells. Interestingly, it showed that fluorescence intensity of nucleus in osteoblast cells was about two times larger than that of cytoplasm when cells were loaded with Fluo-3 AM at 4 °C and allowed a subsequent step for de-esterification at 20 °C. Brighter nuclei were also acquired in HeLa and BV2 cells using the same experimental condition. Furthermore, loading time and adhesion quality of cells had effect on fluorescence intensity. Taken together, cold loading and room temperature de-esterification treatment of Fluo-3 AM selectively yielded brighter nucleus in adherent cells.

Parvalbumin is overexpressed in the late phase of pharmacological preconditioning in skeletal muscle

Pharmacological preconditioning (PPC) with mitochondrial ATP-sensitive K(+) channel openers such as diazoxide, provides protection against ischemia in cardiac muscle, skeletal muscle, and other tissues. Effects on Ca(2+) homeostasis during the late phase of PPC have been described in cardiomyocytes, but no information is available regarding intracellular Ca(2+) changes in skeletal muscle fibers during late PPC. Intracellular Ca(2+) signals were measured in single fibers of adult mouse skeletal muscle, with fluorescent probes, 48 h after the administration of diazoxide. Parvalbumin levels in the myofibers were quantitated by Western blot. Diazoxide induction of late PPC was confirmed by partial protection of muscles from peroxide-induced damage. Late PPC was associated with a significant decrease in the duration of Ca(2+) signals during single twitches and tetanus with no changes in peak values. This effect was prevented by the reactive oxygen species (ROS) scavenger tiron. Late PPC was accompanied by a 30% increase in parvalbumin levels, and this effect was also blocked by tiron. Our data show, for the first time, a role of parvalbumin in late PPC in skeletal muscle.

Barcoding T cell calcium response diversity with methods for automated and accurate analysis of cell signals (MAAACS)

We introduce a series of experimental procedures enabling sensitive calcium monitoring in T cell populations by confocal video-microscopy. Tracking and post-acquisition analysis was performed using Methods for Automated and Accurate Analysis of Cell Signals (MAAACS), a fully customized program that associates a high throughput tracking algorithm, an intuitive reconnection routine and a statistical platform to provide, at a glance, the calcium barcode of a population of individual T-cells. Combined with a sensitive calcium probe, this method allowed us to unravel the heterogeneity in shape and intensity of the calcium response in T cell populations and especially in naive T cells, which display intracellular calcium oscillations upon stimulation by antigen presenting cells.

The adaptive immune response to pathogen invasion requires the stimulation of lymphocytes by antigen-presenting cells. We hypothesized that investigating the dynamics of the T lymphocyte activation by monitoring intracellular calcium fluctuations might help explain the high specificity and selectivity of this phenomenon. However, the quantitative and exhaustive analysis of calcium fluctuations by video microscopy in the context of cell-to-cell contact is a tough challenge. To tackle this, we developed a complete solution named MAAACS (Methods for Automated and Accurate Analysis of Cell Signals), in order to automate the detection, cell tracking, raw data ordering and analysis of calcium signals. Our algorithm revealed that, when in contact with antigen-presenting cells, T lymphocytes generate oscillating calcium signals and not a massive and sustained calcium response as was originally thought. We anticipate our approach providing many more new insights into the molecular mechanisms triggering adaptive immunity.

Regulators of calcium homeostasis identified by inference of kinetic model parameters from live single cells perturbed by siRNA

Assigning molecular functions and revealing dynamic connections between large numbers of partially characterized proteins in regulatory networks are challenges in systems biology. We showed that functions of signaling proteins can be discovered with a differential equations model of the underlying signaling process to extract specific molecular parameter values from single-cell, time-course measurements. By analyzing the effects of 250 small interfering RNAs on Ca(2+) signals in single cells over time, we identified parameters that were specifically altered in the Ca(2+) regulatory system. Analysis of the screen confirmed known functions of the Ca(2+) sensors STIM1 (stromal interaction molecule 1) and calmodulin and of Ca(2+) channels and pumps localized in the endoplasmic reticulum (ER) or plasma membrane. Furthermore, we showed that the Alzheimer's disease-linked protein presenilin-2 and the channel protein ORAI2 prevented overload of ER Ca(2+) and that feedback from Ca(2+) to phosphatidylinositol 4-kinase and PLCδ (phospholipase Cδ) may regulate the abundance of the plasma membrane lipid PI(4,5)P2 (phosphatidylinositol 4,5-bisphosphate) to control Ca(2+) extrusion. Thus, functions of signaling proteins and dynamic regulatory connections can be identified by extracting molecular parameter values from single-cell, time-course data.

Intracrine endothelin signaling evokes IP3-dependent increases in nucleoplasmic Ca²⁺ in adult cardiac myocytes

Endothelin receptors are present on the nuclear membranes in adult cardiac ventricular myocytes. The objectives of the present study were to determine 1) which endothelin receptor subtype is in cardiac nuclear membranes, 2) if the receptor and ligand traffic from the cell surface to the nucleus, and 3) the effect of increased intracellular ET-1 on nuclear Ca(2+) signaling. Confocal microscopy using fluorescently-labeled endothelin analogs confirmed the presence of ETB at the nuclear membrane of rat cardiomyocytes in skinned-cells and isolated nuclei. Furthermore, in both cardiac myocytes and aortic endothelial cells, endocytosed ET:ETB complexes translocated to lysosomes and not the nuclear envelope. Although ETA and ETB can form heterodimers, the presence or absence of ETA did not alter ETB trafficking. Treatment of isolated nuclei with peptide: N-glycosidase F did not alter the electrophoretic mobility of ETB. The absence of N-glycosylation further indicates that these receptors did not originate at the cell surface. Intracellular photolysis of a caged ET-1 analog ([Trp-ODMNB(21)]ET-1) evoked an increase in nucleoplasmic Ca(2+) ([Ca(2+)]n) that was attenuated by inositol 1,4,5-trisphosphate receptor inhibitor 2-aminoethoxydiphenyl borate and prevented by pre-treatment with ryanodine. A caged cell-permeable analog of the ETB-selective antagonist IRL-2500 blocked the ability of intracellular cET-1 to increase [Ca(2+)]n whereas extracellular application of ETA and ETB receptor antagonists did not. These data suggest that 1) the endothelin receptor in the cardiac nuclear membranes is ETB, 2) ETB traffics directly to the nuclear membrane after biosynthesis, 3) exogenous endothelins are not ligands for ETB on nuclear membranes, and 4) ETB associated with the nuclear membranes regulates nuclear Ca(2+) signaling.

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.

Calcium in red blood cells-a perilous balance

Ca2+ is a universal signalling molecule involved in regulating cell cycle and fate, metabolism and structural integrity, motility and volume. Like other cells, red blood cells (RBCs) rely on Ca2+ dependent signalling during differentiation from precursor cells. Intracellular Ca2+ levels in the circulating human RBCs take part not only in controlling biophysical properties such as membrane composition, volume and rheological properties, but also physiological parameters such as metabolic activity, redox state and cell clearance. Extremely low basal permeability of the human RBC membrane to Ca2+ and a powerful Ca2+ pump maintains intracellular free Ca2+ levels between 30 and 60 nM, whereas blood plasma Ca2+ is approximately 1.8 mM. Thus, activation of Ca2+ uptake has an impressive impact on multiple processes in the cells rendering Ca2+ a master regulator in RBCs. Malfunction of Ca2+ transporters in human RBCs leads to excessive accumulation of Ca2+ within the cells. This is associated with a number of pathological states including sickle cell disease, thalassemia, phosphofructokinase deficiency and other forms of hereditary anaemia. Continuous progress in unravelling the molecular nature of Ca2+ transport pathways allows harnessing Ca2+ uptake, avoiding premature RBC clearance and thrombotic complications. This review summarizes our current knowledge of Ca2+ signalling in RBCs emphasizing the importance of this inorganic cation in RBC function and survival.

Functional stem cell integration assessed by organotypic slice cultures

Re-formation or preservation of functional, electrically active neural networks has been proffered as one of the goals of stem cell-mediated neural therapeutics. A primary issue for a cell therapy approach is the formation of functional contacts between the implanted cells and the host tissue. Therefore, it is of fundamental interest to establish protocols that allow us to delineate a detailed time course of grafted stem cell survival, migration, differentiation, integration, and functional interaction with the host. One option for in vitro studies is to examine the integration of exogenous stem cells into an existing active neuronal network in ex vivo organotypic cultures. Organotypic cultures leave the structural integrity essentially intact while still allowing the microenvironment to be carefully controlled. This allows detailed studies over time of cellular responses and cell-cell interactions, which are not readily performed in vivo. This unit describes procedures for using organotypic slice cultures as ex vivo model systems for studying neural stem cell and embryonic stem cell engraftment and communication with CNS host tissue.

Immunoglobulins G from patients with sporadic amyotrophic lateral sclerosis affects cytosolic Ca2+ homeostasis in cultured rat astrocytes

Astrocytes are considered essential in the etiopathogenesis of amyotrophic lateral sclerosis (ALS). We have demonstrated previously that immunoglobulins G (IgG) isolated from patients with ALS enhance the mobility of acidic vesicles in cultured astrocytes in a Ca(2+)-dependent manner. Here we directly examined the impact of purified sporadic ALS IgG on cytosolic [Ca(2+)] ([Ca(2+)]i) in astrocytes. Confocal time-lapse images were acquired and fluorescence of a non-ratiometric Ca(2+) indicator was recorded before and after the application of IgG. ALS IgG (0.1 mg/ml) from 7 patients evoked transient increases in [Ca(2+)]i in ~50% of tested astrocytes. The probability of observing a response was independent of extracellular Ca(2+). The peak increase in [Ca(2+)]i developed ~3 times faster and the time integral of evoked transients was ~2-fold larger; the peak amplitude itself was not affected by extracellular Ca(2+). Application of pharmacological inhibitors revealed that activation of inositol-1,4,5-triphosphate receptors is necessary and sufficient to initiate transients in [Ca(2+)]i; the Ca(2+) influx through store-operated calcium entry prolongs the transient increase in [Ca(2+)]i. Thus, ALS IgG acutely affect [Ca(2+)]i by mobilizing both, intra- and extracellular Ca(2+) into the cytosol of cultured astrocytes.

Ca2+-sensitive fluorescent dyes and intracellular Ca2+ imaging

Imaging Ca(2+)-sensitive fluorescent indicators provides a common approach for studying Ca(2+) signals in many contexts. Fluorescent indicators are particularly useful for measuring acute Ca(2+) changes in a relatively noninvasive manner. The availability of indicators that can be targeted to specific cellular domains, coupled with variations in affinity, brightness or spectral characteristics, provides tools for exploring spatially and temporally diverse Ca(2+) signals, and moreover, multiplexing the readout of Ca(2+) with other cellular functions. This article aims to give the novice experimentalist some insight into the considerations and potential pitfalls that impinge on the use of fluorescent Ca(2+) indicators.

Converting fluorescence data into Ca2+ concentration

In many situations, fluorescent Ca(2+) reporters are used to simply indicate that a change of Ca(2+) concentration has occurred. Monitoring the emission from a Ca(2+)-sensitive indicator can be sufficient to tell whether a signal has arisen, and what its kinetic/spatial parameters were. The emission from an indicator does not have a linear relationship to the Ca(2+) concentration within a cell; rather, the relationship between fluorescence emission and Ca(2+) concentration is described by a logistic function. Simply recording fluorescence emission, therefore, provides a relative indication of the magnitude of a Ca(2+) signal that should not be used for generating mean amplitude data. However, with a little consideration and effort, the fluorescence output can be calibrated to yield actual Ca(2+) concentration.

Characterization of calcium signals in human embryonic stem cells and in their differentiated offspring by a stably integrated calcium indicator protein

Intracellular calcium signaling pathways play a major role in cellular responses such as proliferation, differentiation and apoptosis. Human embryonic stem cells (hESC) provide new possibilities to explore the development and differentiation of various cell types of the human body. Intracellular calcium responses to various ligands and the calcium signaling pathways, however, have not been thoroughly studied in embryonic stem cells and in their differentiated progenies. In our previous work we demonstrated that the use of the fluorescent calcium indicator Fluo-4 with confocal microscopy allows sensitive and reliable measurements of calcium modulation in human embryonic stem cells and stem-cell derived cardiomyocytes. Here we developed a human embryonic stem cell line stably expressing a genetically encoded Ca(2+) indicator (GCaMP2) using a transposon-based gene delivery system. We found that the differentiation properties were fully preserved in the GCaMP2-expressing hESC lines and Ca imaging could be performed without the need of toxic dye-loading of the cells. In undifferentiated hES cells the calcium signals induced by various ligands, ATP, LPA, trypsin or angiotensin II were comparable to those in Fluo-4 loaded cells. In accordance with previous findings, no calcium signal was evoked by thrombin, histamine or GABA. Cardiomyocyte colonies differentiated from hES-GCaMP2 cells could be recognized by spontaneous contractions and Ca(2+) oscillations. GCaMP2-expressing neural cells were identified based on their morphological and immuno-staining properties and Ca signals were characterized on those cells. Characteristics of both the spontaneous and ligand-induced Ca(2+) signals, as well as their pharmacological modification could be successfully examined in these model cells by fluorescence imaging.

Flow cytometry-based methods for measurement of cytosolic calcium and surface protein expression in Plasmodium falciparum merozoites

An increased level of cytosolic free calcium (Ca(2+)) is an essential second messenger for apical organelle discharge in Plasmodium falciparum merozoites. Here, we describe a method for isolation of viable and invasive P. falciparum merozoites. We also describe methods to measure cytosolic Ca(2+) levels in merozoites using fluorescent intracellular calcium indicators such as Fluo-4AM by flow cytometry. Further, we also describe a flow cytometry-based method to detect translocation of apical organelle proteins to the surface of merozoites. Using these methods, we have advanced our understanding of signaling pathways involved in apical organelle discharge during erythrocyte invasion by P. falciparum merozoites.

Studying the activation of epithelial ion channels using global whole-field photolysis

The production of saliva by parotid acinar cells is stimulated by Ca(2+) activation of Cl(-) and K(+) channels located in the apical plasma membrane of these polarized cells. Here we provide a detailed description of a flash photolysis experiment designed to give a global and relatively uniform photorelease of inositol 1,4,5-trisphosphate (InsP(3)) or Ca(2+) from caged precursors (NPE-InsP(3) or NP-EGTA) combined with the simultaneous measurement of whole-cell Ca(2+)-activated currents. The photolysis light source can be either an ultraviolet (UV) flash lamp or alternatively the output from a 375-nm diode laser, which is defocused to illuminate the entire field.

Functional upregulation of Ca(2+)-activated K(+) channels in the development of substantia nigra dopamine neurons

Many connections in the basal ganglia are made around birth when animals are exposed to a host of new affective, cognitive, and sensori-motor stimuli. It is thought that dopamine modulates cortico-striatal synapses that result in the strengthening of those connections that lead to desired outcomes. We propose that there must be a time before which stimuli cannot be processed into functional connections, otherwise it would imply an effective link between stimulus, response, and reward in uterus. Consistent with these ideas, we present evidence that early in development dopamine neurons are electrically immature and do not produce high-frequency firing in response to salient stimuli. We ask first, what makes dopamine neurons immature? and second, what are the implications of this immaturity for the basal ganglia? As an answer to the first question, we find that at birth the outward current is small (3nS-V), insensitive to Ca(2+), TEA, BK, and SK blockers. Rapidly after birth, the outward current increases to 15nS-V and becomes sensitive to Ca(2+), TEA, BK, and SK blockers. We make a detailed analysis of the kinetics of the components of the outward currents and produce a model for BK and SK channels that we use to reproduce the outward current, and to infer the geometrical arrangement of BK and Ca(2+) channels in clusters. In the first cluster, T-type Ca(2+) and BK channels are coupled within distances of ~20 nm (200 Å). The second cluster consists of L-type Ca(2+) and BK channels that are spread over distances of at least 60 nm. As for the second question, we propose that early in development, the mechanism of action selection is in a "locked-in" state that would prevent dopamine neurons from reinforcing cortico-striatal synapses that do not have a functional experiential-based value.

Intra-axonal calcium changes after axotomy in wild-type and slow Wallerian degeneration axons

Calcium accumulation induces the breakdown of cytoskeleton and axonal fragmentation in the late stages of Wallerian degeneration. In the early stages there is no evidence for any long-lasting, extensive increase in intra-axonal calcium but there does appear to be some redistribution. We hypothesized that changes in calcium distribution could have an early regulatory role in axonal degeneration in addition to the late executionary role of calcium. Schmidt-Lanterman clefts (SLCs), which allow exchange of metabolites and ions between the periaxonal and extracellular space, are likely to have an increased role when axon segments are separated from the cell body, so we used the oxalate-pyroantimonate method to study calcium at SLCs in distal stumps of transected wild-type and slow Wallerian degeneration (Wld(S)) mutant sciatic nerves, in which Wallerian degeneration is greatly delayed. In wild-type nerves most SLCs show a step gradient of calcium distribution, which is lost at around 20% of SLCs within 3mm of the lesion site by 4-24h after nerve transection. To investigate further the association with Wallerian degeneration, we studied nerves from Wld(S) rats. The step gradient of calcium distribution in Wld(S) is absent in around 20% of the intact nerves beneath SLCs but 4-24h following injury, calcium distribution in transected axons remained similar to that in uninjured nerves. We then used calcium indicators to study influx and buffering of calcium in injured neurites in primary culture. Calcium penetration and the early calcium increase in this system were indistinguishable between Wld(S) and wild-type axons. However, a significant difference was observed during the following hours, when calcium increased in wild-type neurites but not in Wld(S) neurites. We conclude that there is little relationship between calcium distribution and the early stages of Wallerian degeneration at the time points studied in vivo or in vitro but that Wld(S) neurites fail to show a later calcium rise that could be a cause or consequence of the later stages of Wallerian degeneration.

Calcium rubies: a family of red-emitting functionalizable indicators suitable for two-photon Ca2+ imaging

We designed Calcium Rubies, a family of functionalizable BAPTA-based red-fluorescent calcium (Ca(2+)) indicators as new tools for biological Ca(2+) imaging. The specificity of this Ca(2+)-indicator family is its side arm, attached on the ethylene glycol bridge that allows coupling the indicator to various groups while leaving open the possibility of aromatic substitutions on the BAPTA core for tuning the Ca(2+)-binding affinity. Using this possibility we now synthesize and characterize three different CaRubies with affinities between 3 and 22 μM. Their long excitation and emission wavelengths (peaks at 586/604 nm) allow their use in otherwise challenging multicolor experiments, e.g., when combining Ca(2+) uncaging or optogenetic stimulation with Ca(2+) imaging in cells expressing fluorescent proteins. We illustrate this capacity by the detection of Ca(2+) transients evoked by blue light in cultured astrocytes expressing CatCh, a light-sensitive Ca(2+)-translocating channelrhodopsin linked to yellow fluorescent protein. Using time-correlated single-photon counting, we measured fluorescence lifetimes for all CaRubies and demonstrate a 10-fold increase in the average lifetime upon Ca(2+) chelation. Since only the fluorescence quantum yield but not the absorbance of the CaRubies is Ca(2+)-dependent, calibrated two-photon fluorescence excitation measurements of absolute Ca(2+) concentrations are feasible.

A comparative assessment of fluo Ca2+ indicators in rat ventricular myocytes

The fluo family of indicators is frequently used in studying Ca(2+) physiology; however, choosing which fluo indicator to use is not obvious. Indicator properties are typically determined in well-defined aqueous solutions. Inside cells, however, the properties can change markedly. We have characterized each of three fluo variants (fluo-2MA, fluo-3 and fluo-4) in two forms-the acetoxymethyl (AM) ester and the K(+) salt. We loaded indicators into rat ventricular myocytes and used confocal microscopy to monitor depolarization-induced fluorescence changes and fractional shortening. Myocytes loaded with the indicator AM esters showed significantly different Ca(2+) transients and fractional shortening kinetics. Loading the K(+) salts via whole-cell patch-pipette eliminated differences between fluo-3 and fluo-4, but not fluo-2MA. Cells loaded with different indicator AM esters showed different staining patterns-suggesting differential loading into organelles. Ca(2+) dissociation constants (K(d,Ca)), measured in protein-rich buffers mimicking the cytosol were significantly higher than values determined in simple buffers. This increase in K(d,Ca) (decrease in Ca(2+) affinity) was greatest for fluo-3 and fluo-4, and least for fluo-2MA. We conclude that the structurally-similar fluo variants differ with respect to cellular loading, subcellular compartmentalization, and intracellular Ca(2+) affinity. Therefore, judicious choice of fluo indicator and loading procedure is advisable when designing experiments.

Ryanodine receptors are involved in nuclear calcium oscillation in primary pancreatic β-cells

Ryanodine receptors (RyRs) are mainly located on the endoplasmic reticulum (ER) and play an important role in regulating glucose-induced cytosolic Ca(2+) oscillation in pancreatic β-cells. However, subcellular locations and functions of RyRs on other cell organelles such as nuclear envelope are not well understood. In order to investigate the role of RyRs in nuclear Ca(2+) oscillation we designed and conducted experiments in intact primary pancreatic β-cells. Immunocytochemistry was used to examine the expression of RYRs on the nuclear envelope. Confocal microscopy was used to evaluate the function of RYRs on the nuclear envelope. We found that RyRs are expressed on the nuclear envelope in single primary pancreatic β-cells and isolated nuclei. Laser scanning confocal microscopy studies indicated that application of glucose to the cells co-incubated with Ca(2+) indicator Fluo-4 AM and cell-permeable nuclear indicator Hoechst 33342 resulted in nuclear Ca(2+) oscillation. The pattern of glucose-induced Ca(2+) oscillation in the nucleus and cytosol was similar. The reduction of Ca(2+) oscillation amplitude by ryanodine was much greater in the nucleus though both the cytosol and the nucleus Ca(2+) amplitude decreased by ryanodine. Our results suggest that functional ryanodine receptors not only exist in endoplasmic reticulum but are also expressed in nuclear envelope of pancreatic β-cells.

Calcium signaling in pluripotent stem cells

Pluripotent stem cells represent a new source of biological material allowing the exploration of signaling phenomena during normal cell development and differentiation. Still, the calcium signaling pathways and intracellular calcium responses to various ligands or stress conditions have not been sufficiently explored as yet in embryonic or induced pluripotent stem cells and in their differentiated offspring. This is partly due to the special culturing conditions of these cell types, the rapid morphological and functional changes in heterogeneous cell populations during early differentiation, and methodological problems in cellular calcium measurements. In this paper, we review the currently available data in the literature on calcium signaling in pluripotent stem cells and discuss the potential shortcomings of these studies. Various assay methods are surveyed for obtaining reliable data both in undifferentiated embryonic stem cells and in specific, stem cell-derived human tissues. In this paper, we present the modulation of calcium signaling in human embryonic stem cells (hESC) and in their derivates; mesenchymal stem cell like (MSCl) cells and cardiac tissues using the fluorescent calcium indicator Fluo-4 and confocal microscopy. LPA, trypsin and angiotensin II were effective in inducing calcium signals both in HUES9 and MSCl cells. Histamine and thrombin induced calcium signal exclusively in the MSCl cells, while ATP was effective only in HUES9 cells. There was no calcium signal evoked by GABA, even at relatively high concentrations. In stem cell-derived cardiomyocytes a rapid increase in the beating rate and an increase of the calcium signal peaks could be observed after the addition of adrenaline, while verapamil led to a strong decrease in cellular calcium and stopped spontaneous contractions in a relaxed state.

Highly electroconductive multiwalled carbon nanotubes as potentially useful tools for modulating calcium balancing in biological environments

Aiming to explore the mechanisms modulating cell-carbon nanotube interactions, we investigated whether Ca(2+) ion balancing between intra- and extracellular environments could be affected by multiwalled carbon nanotubes (MWCNTs). We analyzed the effects induced by two different kinds of MWCNTs (as prepared and annealed at 2400°C) on the intracellular Ca(2+) ion levels in rat electrically sensitive cells and on the intercellular junction integrity of rat adenocarcinoma colon cells and platelet aggregation ability, which depend on the Ca(2+) concentration in the medium. MWCNTs, purified by annealing and more electroconductive as compared to nonannealed MWCNTs, affected Ca(2+) ion balancing between extra- and intracellular environments and induced changes on Ca(2+) ion-dependent cellular junctions and platelet aggregation, behaving as the calcium chelator ethylene glycol tetraacetic acid. This could be due to the sorption of cationic Ca(2+) ions on CNTs surface because of the excess of negatively charged electrons on the aromatic units formed on MWCNTs after annealing. From the ClinicAL Editor: The authors investigated whether Ca(2+) ion balance between intra- and extracellular space can be modulated by multiwalled carbon nanotubes (MWCNTs). Annealed nanotubes induced changes on Ca(2+) dependent cellular junctions and platelet aggregation, behaving similary to ethylene glycol tetraacetic acid, an established calcium chelator.

Changes in chromatin compaction during the cell cycle revealed by micrometer-scale measurement of molecular flow in the nucleus

We present a quantitative fluctuation-based assay to measure the degree of local chromatin compaction and investigate how chromatin density regulates the diffusive path adopted by an inert protein in dividing cells. The assay uses CHO-K1 cells coexpressing untagged enhanced green fluorescent protein (EGFP) and histone H2B tagged mCherry. We measure at the single-cell level the EGFP localization and molecular flow patterns characteristic of each stage of chromatin compaction from mitosis through interphase by means of pair-correlation analysis. We find that the naturally occurring changes in chromatin organization impart a regulation on the spatial distribution and temporal dynamics of EGFP within the nucleus. Combined with the analysis of Ca(2+) intracellular homeostasis during cell division, EGFP flow regulation can be interpreted as the result of controlled changes in chromatin compaction. For the first time, to our knowledge, we were able to probe chromatin compaction on the micrometer scale, where the regulation of molecular diffusion may become relevant for many cellular processes.

Intracellular magnesium detection by fluorescent indicators

Magnesium is essential for a wide variety of biochemical reactions and physiological functions, but its regulatory mechanisms (both at the cellular and at the systemic level) are still poorly characterized. Not least among the reasons for this gap are the technical difficulties in sensing minor changes occurring over a high background concentration. Specific fluorescent indicators are highly sensitive tools for dynamic evaluation of intracellular magnesium concentration. We herein discuss the main criteria to consider when choosing a magnesium-specific fluorescent indicator and provide examples among commercial as well as developmental sensors. We focus on spectrofluorimetric approaches to quantify Mg(2+) concentration in cell or mitochondria suspensions, and on imaging techniques to detect intracellular magnesium distribution and fluxes by live microscopy, reporting a detailed description of standard protocols for each method. The general guidelines we provide should be applicable to specific issues by any researcher in the field.

Development and optimization of FLIPR high throughput calcium assays for ion channels and GPCRs

Ca(2+) permeable ion channels and GPCRs linked to Ca(2+) release are important drug targets, with modulation of Ca(2+) signaling increasingly recognized as a valid therapeutic strategy in a range of diseases. The FLIPR is a high throughput imaging plate reader that has contributed substantially to drug discovery efforts and pharmacological characterization of receptors and ion channels coupled to Ca(2+). Now in its fourth generation, the FLIPR(TETRA) is an industry standard for high throughput Ca(2+) assays. With an increasing number of excitation LED banks and emission filter sets available; FLIPR Ca(2+) assays are becoming more versatile. This chapter describes general methods for establishing robust FLIPR Ca(2+) assays, incorporating practical aspects as well as suggestions for assay optimization, to guide the reader in the development and optimization of high throughput FLIPR assays for ion channels and GPCRs.