Subcellular calcium transients visualized by confocal microscopy in a voltage-clamped vertebrate neuron

Activity-dependent compensation of cell size is vulnerable to targeted deletion of ion channels

In many species, excitable cells preserve their physiological properties despite significant variation in physical size across time and in a population. For example, neurons in crustacean central pattern generators generate similar firing patterns despite several-fold increases in size between juveniles and adults. This presents a biophysical problem because the electrical properties of cells are highly sensitive to membrane area and channel density. It is not known whether specific mechanisms exist to sense membrane area and adjust channel expression to keep a consistent channel density, or whether regulation mechanisms that sense activity alone are capable of compensating cell size. We show that destabilising effects of growth can be specifically compensated by feedback mechanism that senses average calcium influx and jointly regulate multiple conductances. However, we further show that this class of growth-compensating regulation schemes is necessarily sensitive to perturbations that alter the expression of subsets of ion channel types. Targeted perturbations of specific ion channels can trigger a pathological response of the regulation mechanism and a failure of homeostasis. Our findings suggest that physiological regulation mechanisms that confer robustness to growth may be specifically vulnerable to deletions or mutations that affect subsets of ion channels.

Distinct oscillations in cytosolic and nuclear free calcium in single intact living cells demonstrated by confocal microscopy

Many growth factors stimulate both a rapid increase in free calcium (Ca2+i) and protooncogene expression in cells following binding to their cell surface receptors. Increases in Ca2+i and proto-oncogene expression are thought to be important in the transduction of growth factor binding and regulation of other cellular activities (i.e. mitogenesis), yet a direct role for the interaction of these two growth factor-stimulated events has been lacking. We have previously reported the observation of changes in free calcium in the nuclear region of vascular smooth muscle cells (VSMC) which were temporally and spatially distinct form those of the cytoplasm. We hypothesized that these changes in nuclear free calcium (Ca2+n) might activate gene expression necessary for growth factor activity and thus provide a direct link between growth factor-induced increases in Ca2+n and proto-oncogene expression. That growth factor stimulated alterations in Ca2+n may occur is strengthened by the recent description of an ATP-dependent nuclear Cna2+ pump, an inositol 1,4,5-trisphosphatesensitive Ca2+ pool in liver nuclei and subcellular heterogeneity of Ca2+ transients in voltage clamped vertebrate neurons.

Control of cardiac contraction by sodium: Promises, reckonings, and new beginnings

Several generations of cardiac physiologists have verified that basal cardiac contractility depends strongly on the transsarcolemmal Na gradient, and the underlying molecular mechanisms that link cardiac excitation-contraction coupling (ECC) to the Na gradient have been elucidated in good detail for more than 30 years. In brief, small increases of cytoplasmic Na push cardiac (NCX1) Na/Ca exchangers to increase contractility by increasing the myocyte Ca load. Accordingly, basal cardiac contractility is expected to be physiologically regulated by pathways that modify the cardiac Na gradient and the function of Na transporters. Assuming that this expectation is correct, it remains to be elucidated how in detail signaling pathways affecting the cardiac Na gradient are controlled in response to changing cardiac output requirements. Some puzzle pieces that may facilitate progress are outlined in this short review. Key open issues include (1) whether the concept of local Na gradients is viable, (2) how in detail Na channels, Na transporters and Na/K pumps are regulated by lipids and metabolic processes, (3) the physiological roles of Na/K pump inactivation, and (4) the possibility that key diffusible signaling molecules remain to be discovered.

Multiple genetic loci define Ca++ utilization by bloodstream malaria parasites

Background

Bloodstream malaria parasites require Ca⁺⁺ for their development, but the sites and mechanisms of Ca⁺⁺ utilization are not well understood. We hypothesized that there may be differences in Ca⁺⁺ uptake or utilization by genetically distinct lines of P. falciparum. These differences, if identified, may provide insights into molecular mechanisms.

Results

Dose response studies with the Ca⁺⁺ chelator EGTA (ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid) revealed stable differences in Ca⁺⁺ requirement for six geographically divergent parasite lines used in previous genetic crosses, with the largest difference seen between the parents of the HB3 x Dd2 cross. Genetic mapping of Ca⁺⁺ requirement yielded complex inheritance in 34 progeny clones with a single significant locus on chromosome 7 and possible contributions from other loci. Although encoded by a gene in the significant locus and a proposed Ca⁺⁺ target, PfCRT (P. falciparum chloroquine resistance transporter), the primary determinant of clinical resistance to the antimalarial drug chloroquine, does not appear to contribute to this quantitative trait. Stage-specific application of extracellular EGTA also excluded determinants associated with merozoite egress and erythrocyte reinvasion.

Conclusions

We have identified differences in Ca⁺⁺ utilization amongst P. falciparum lines. These differences are under genetic regulation, segregating as a complex trait in genetic cross progeny. Ca⁺⁺ uptake and utilization throughout the bloodstream asexual cycle of malaria parasites represents an unexplored target for therapeutic intervention.

Calcium-induced release of calcium in muscle: 50 years of work and the emerging consensus

Ryanodine-sensitive intracellular Ca2+ channels (RyRs) open upon binding Ca2+ at cytosolic-facing sites. This results in concerted, self-reinforcing opening of RyRs clustered in specialized regions on the membranes of Ca2+ storage organelles (endoplasmic reticulum and sarcoplasmic reticulum), a process that produces Ca2+-induced Ca2+ release (CICR). The process is optimized to achieve large but brief and localized increases in cytosolic Ca2+ concentration, a feature now believed to be critical for encoding the multiplicity of signals conveyed by this ion. In this paper, I trace the path of research that led to a consensus on the physiological significance of CICR in skeletal muscle, beginning with its discovery. I focus on the approaches that were developed to quantify the contribution of CICR to the Ca2+ increase that results in contraction, as opposed to the flux activated directly by membrane depolarization (depolarization-induced Ca2+ release [DICR]). Although the emerging consensus is that CICR plays an important role alongside DICR in most taxa, its contribution in most mammalian muscles appears to be limited to embryogenesis. Finally, I survey the relevance of CICR, confirmed or plausible, to pathogenesis as well as the multiple questions about activation of release channels that remain unanswered after 50 years.

Endothelial Dysfunction in Patients with Severe Acute Pancreatitis: Improved by Continuous Blood Purification Therapy

Objectives.

Endothelium dysfunction is one of the critical pathophysiologic disorders in patients with severe acute pancreatitis (SAP). To investigate the effect of continuous blood purification (CBP) on endothelial function, we conducted a prospective study of 20 patients with SAP, 9 of whom had evidence of sepsis.

Methods.

All patients underwent CVVH for 72 h. Soluble E-selectin (sE-selectin), soluble thrombomodulin, permeability of the endothelial monolayer, and endothelial intracellular calcium ([Ca2+]i) levels were used as the markers for the assessment of endothelial function and the effect of CBP therapy in patients with SAP. Blood samples were taken from the patients at 0, 2, 12, 24, 48, and 72 h during CVVH therapy. sE-selectin and thrombomoduiln were measured by ELISA. The endothelial permeability and activation were evaluated using cultured endothelial monolayer and intracellular Ca2+ concentration.

Results.

The results showed that during CVVH treatment, the hemodynamics and mean arterial pressure (MAP) were stable. The Acute Physiology and Chronic Health Evaluation (APACHE) II score was improved significantly after CVVH. Endothelial dysfunction was evident in patients with SAP as compared to normal controls. Patients with SAP had increased levels of sE-selectin, endothelial permeability and intracellular [Ca2+]i, which was higher in patients with sepsis than in those without sepsis. The level of thrombomodulin showed a tendency to increase; however, these changes were not significant between SAP patients and controls. After CBP treatment, sE-selectin levels substantially decreased in all patients. CBP treatment also significantly diminished the endothelial permeability and decreased the intracellular [Ca2+] concentration.

Conclusions.

These data demonstrate that endothelial dysfunction is present in patients with SAP and the degree of endothelial damage may be correlated with the disease severity. CBP therapy can not only improve the general conditions, as measured by the APACHE II score, but also effectively improve endothelial dysfunction.

The discovery of the sub-threshold currents M and Q/H in central neurons

The history, content and consequences of the highly-cited 1982 Brain Research paper by Halliwell and Adams are summarized. The paper pioneered the use of the single-electrode voltage clamp in mammalian brain slices, described 2 novel sub-threshold voltage-dependent ionic currents, IM and IQ/H, and suggested that cholinergic inputs "enabled" pyramidal cell firing in response to conventional synaptic input, the first example of central neuromodulation. The paper, published in Brain Research to give the first author appropriate importance, heralded an ongoing tidal wave of quantitative electrophysiology in mammalian central neurons.

ORIGINAL ARTICLE ABSTRACT

Voltage-clamp analysis of muscarinic excitation in hippocampal neurons Pyramidal cells in the CA1 field of guinea pig hippocampal slices were voltage-clamped using a single microelectrode, at 23-30°C. Small inwardly relaxing currents triggered by step hyperpolarizations from holding potentials of -80 to -40mV were investigated. Inward relaxations occurring for negative steps between -40mV and -70mV resembled M-currents of sympathetic ganglion cells: they were abolished by addition of carbachol, muscarine or bethanechol, as well as by 1mM barium; the relaxations appeared to invert at around -80mV; they became faster at more negative potentials; and the inversion potential was shifted positively by raising external K(+) concentration. Inward relaxations triggered by steps negative to -80mV, in contrast, appeared to reflect passage of another current species, which has been labeled IQ.Thus IQ did not invert negative to -80mV, it was insensitive to muscarinic agonizts or to barium, and it was blocked by 0.5-3mM cesium (which does not block IM). Turn-on of IQ causes the well known droop in the hyperpolarizing electrotonic potential in these cells. The combined effects of IQ and IM make the steady-state current-voltage relation of CA1 cells slightly sigmoidal around rest potential. It is suggested that activation of cholinergic septal inputs to the hippocampus facilitates repetitive firing off pyramidal cells by turning off the M-conductance, without much change in the resting potential of the cell. © 1982. This article is part of a Special Issue entitled SI:50th Anniversary Issue.

Effect of midazolam on the proliferation of neural stem cells isolated from rat hippocampus

In many recent studies, the inhibitory transmitter gamma-aminobutyric acid has been shown to modulate the proliferation, differentiation and survival of neural stem cells. Most general anesthetics are partial or allosteric gamma-aminobutyric acid A receptor agonists, suggesting that general anesthetics could alter the behavior of neural stem cells. The neuroprotective efficacy of general anesthetics has been recognized for decades, but their effects on the proliferation of neural stem cells have received little attention. This study investigated the potential effect of midazolam, an extensively used general anesthetic and allosteric gamma-aminobutyric acid A receptor agonist, on the proliferation of neural stem cells in vitro and preliminarily explored the underlying mechanism. The proliferation of neural stem cells was tested using both Cell Counting Kit 8 and bromodeoxyuridine incorporation experiments. Cell distribution analysis was performed to describe changes in the cell cycle distribution in response to midazolam. Calcium imaging was employed to explore the molecular signaling pathways activated by midazolam. Midazolam (30–90 μM) decreased the proliferation of neural stem cells in vitro. Pretreatment with the gamma-aminobutyric acid A receptor antagonist bicuculline or Na-K-2Cl cotransport inhibitor furosemide partially rescued this inhibition. In addition, midazolam triggered a calcium influx into neural stem cells. The suppressive effect of midazolam on the proliferation of neural stem cells can be partly attributed to the activation of gamma-aminobutyric acid A receptor. The calcium influx triggered by midazolam may be a trigger factor leading to further downstream events.

Intracellular organelles in the saga of Ca2+ homeostasis: different molecules for different purposes?

An increase in the concentration of cytosolic free Ca(2+) is a key component regulating different cellular processes ranging from egg fertilization, active secretion and movement, to cell differentiation and death. The multitude of phenomena modulated by Ca(2+), however, do not simply rely on increases/decreases in its concentration, but also on specific timing, shape and sub-cellular localization of its signals that, combined together, provide a huge versatility in Ca(2+) signaling. Intracellular organelles and their Ca(2+) handling machineries exert key roles in this complex and precise mechanism, and this review will try to depict a map of Ca(2+) routes inside cells, highlighting the uniqueness of the different Ca(2+) toolkit components and the complexity of the interactions between them.

Voltage clamp methods for the study of membrane currents and SR Ca(2+) release in adult skeletal muscle fibres

Skeletal muscle excitation-contraction (E-C)(1) coupling is a process composed of multiple sequential stages, by which an action potential triggers sarcoplasmic reticulum (SR)(2) Ca(2+) release and subsequent contractile activation. The various steps in the E-C coupling process in skeletal muscle can be studied using different techniques. The simultaneous recordings of sarcolemmal electrical signals and the accompanying elevation in myoplasmic Ca(2+), due to depolarization-initiated SR Ca(2+) release in skeletal muscle fibres, have been useful to obtain a better understanding of muscle function. In studying the origin and mechanism of voltage dependency of E-C coupling a variety of different techniques have been used to control the voltage in adult skeletal fibres. Pioneering work in muscles isolated from amphibians or crustaceans used microelectrodes or 'high resistance gap' techniques to manipulate the voltage in the muscle fibres. The development of the patch clamp technique and its variant, the whole-cell clamp configuration that facilitates the manipulation of the intracellular environment, allowed the use of the voltage clamp techniques in different cell types, including skeletal muscle fibres. The aim of this article is to present an historical perspective of the voltage clamp methods used to study skeletal muscle E-C coupling as well as to describe the current status of using the whole-cell patch clamp technique in studies in which the electrical and Ca(2+) signalling properties of mouse skeletal muscle membranes are being investigated.

Label-free and noninvasive optical detection of the distribution of nanometer-size mitochondria in single cells

A microfluidic flow cytometric technique capable of obtaining information on nanometer-sized organelles in single cells in a label-free, noninvasive optical manner was developed. Experimental two-dimensional (2D) light scattering patterns from malignant lymphoid cells (Jurkat cell line) and normal hematopoietic stem cells (cord blood CD34+ cells) were compared with those obtained from finite-difference time-domain simulations. In the simulations, we assumed that the mitochondria were randomly distributed throughout a Jurkat cell, and aggregated in a CD34+ cell. Comparison of the experimental and simulated light scattering patterns led us to conclude that distinction from these two types of cells may be due to different mitochondrial distributions. This observation was confirmed by conventional confocal fluorescence microscopy. A method for potential cell discrimination was developed based on analysis of the 2D light scattering patterns. Potential clinical applications using mitochondria as intrinsic biological markers in single cells were discussed in terms of normal cells (CD34+ cell and lymphocytes) versus malignant cells (THP-1 and Jurkat cell lines).

Calcium-dependent mitochondrial function and dysfunction in neurons

Calcium is an extraordinarily versatile signaling ion, encoding cellular responses to a wide variety of external stimuli. In neurons, mitochondria can accumulate enormous amounts of calcium, with the consequence that mitochondrial calcium uptake, sequestration and release play pivotal roles in orchestrating calcium-dependent responses as diverse as gene transcription and cell death. In this review, we consider the basic chemistry of calcium as a 'sticky' cation, which leads to extremely high bound/free ratios, and discuss areas of current interest or controversy. Topics addressed include methodologies for measuring local intracellular calcium, mitochondrial calcium buffering and loading capacity, mitochondrially directed spatial calcium gradients, and the role of calcium overload-dependent mitochondrial dysfunction in glutamate-evoked excitotoxic injury and neurodegeneration. Finally, we consider the relationship between delayed calcium de-regulation, the mitochondrial permeability transition and the generation of reactive oxygen species, and propose a unified view of the 'source specificity' and 'calcium overload' models of N-methyl-d-aspartate (NMDA) receptor-dependent excitotoxicity. Non-NMDA receptor mechanisms of excitotoxicity are discussed briefly.

Somatic Ca2+ signaling in cerebellar Purkinje neurons

Activity-driven Ca(2+) signaling plays an important role in a number of neuronal functions, including neuronal growth, differentiation, and plasticity. Both cytosolic and nuclear Ca(2+) has been implicated in these functions. In the current study, we investigated membrane-to-nucleus Ca(2+) signaling in cerebellar Purkinje neurons in culture to gain insight into the pathways and mechanisms that can initiate nuclear Ca(2+) signaling in this neuronal type. Purkinje neurons are known to express an abundance of Ca(2+) signaling molecules such as voltage-gated Ca(2+) channels, ryanodine receptors, and IP3 receptors. Results show that membrane depolarization evoked by brief stimulation with K(+) saline elicits a prominent Ca(2+) signal in the cytosol and nucleus of the Purkinje neurons. Ca(2+) influx through P/Q- and L-type voltage-gated Ca(2+) channels and Ca(2+)-induced Ca(2+) release (CICR) from intracellular stores contributed to the Ca(2+) signal, which spread from the plasma membrane to the nucleus. At strong K(+) stimulations, the amplitude of the nuclear Ca(2+) signal exceeded that of the cytosolic Ca(2+) signal, suggesting the involvement of a nuclear amplification mechanism and/or differences in Ca(2+) buffering in these two cellular compartments. An enhanced nuclear Ca(2+) signal was more prominent for Ca(2+) signals elicited by membrane depolarization than for Ca(2+) signals elicited by activation of the metabotropic glutamate receptor pathway (mGluR1), which is linked to Ca(2+) release from intracellular stores controlled by the IP3 receptor.

Imaging nervous system activity

Optical imaging methods rely upon visualization of three types of signals: (1) intrinsic optical signals, including light scattering and reflectance, birefringence, and spectroscopic changes of intrinsic molecules, such as NADH or oxyhemoglobin; (2) changes in fluorescence or absorbance of voltage-sensitive membrane dyes; and (3) changes in fluorescence or absorbance of calcium-sensitive indicator dyes. Of these, the most widely used approach is fluorescent microscopy of calcium-sensitive dyes. This unit describes protocols for the use of calcium-sensitive dyes and voltage-dependent dyes for studies of neuronal activity in culture, tissue slices, and en-bloc preparations of the central nervous system.

Chapter 5: Imaging in depth: controversies and opportunities

One enduring challenge of biological imaging is achieving depth of penetration-into cells, tissues, and animals. How deeply can we probe and with what resolution and efficacy? These are critical issues as microscopists seek to push ever deeper, while resolving structural details and observing specific molecular events. In this guide to depth-appropriate modalities, standard optical platforms such as confocal and two-photon microscopes are considered along with complementary imaging modalities that range in depth of penetration. After an introduction to basic techniques, the trade-offs and limitations that distinguish competing technologies are considered, with emphasis on the visualization of subcellular structures and dynamic events. Not surprisingly, there are differences of opinion regarding imaging technologies, as highlighted in a section on point-scanning and Nipkow-disk style confocal microscopes. Confocal microscopy is then contrasted with deconvolution and multi-photon imaging modalities. It is also important to consider the detectors used by current instruments (such as PMTs and CCD cameras). Ultimately specimen properties, in conjunction with instrumentation, determine the depth at which subcellular operations and larger-scale biological processes can be visualized. Relative advantages are mentioned in the context of experiment planning and instrument-purchase decisions. Given the rate at which new optical techniques are being invented, this report should be viewed as a snapshot of current capabilities, with the goal of providing a framework for thinking about new developments.

Subcellular organization of calcium signalling in hepatocytes and the intact liver

Hepatocytes respond to inositol 1,4,5-trisphosphate (InsP3)-linked agonists with frequency-modulated oscillations in the intracellular free calcium concentration ([Ca2+]i), that occur as waves propagating from a specific origin within each cell. The subcellular distribution and functional organization of InsP3-sensitive Ca2+ pools has been investigated, in both intact and permeabilized cells, by fluorescence imaging of dyes which can be used to monitor luminal Ca2+ content and InsP3-activated ion permeability in a spatially resolved manner. The Ca2+ stores behave as a luminally continuous system distributed throughout the cytoplasm. The structure of the stores, an important determinant of their function, is controlled by the cytoskeleton and can be modulated in a guanine nucleotide-dependent manner. The nuclear matrix is devoid of Ca2+ stores, but Ca2+ waves in the intact cell propagate through this compartment. The organization of [Ca2+]i signals has also been investigated in the perfused liver. Frequency-modulated [Ca2+]i oscillations are still observed at the single cell level, with similar properties to those in the isolated hepatocyte. The [Ca2+]i oscillations propagate between cells in the intact liver, leading to the synchronization of [Ca2+]i signals across part or all of each hepatic lobule.

Regulation of nuclear calcium concentration

Transient increases in nuclear calcium concentration have been shown to activate gene expression and other nuclear processes. It has been suggested that nuclear calcium signals are controlled by a mechanism that is independent of calcium signalling in the cytosol. This would be possible if calcium diffusion is slow and a separate calcium release mechanism is localized to the nuclear region. Alternatively, the nuclear envelope could act as a diffusion barrier for calcium ions released either inside or outside the nucleus. It has also been proposed that inositol 1,4,5-trisphosphate (InsP3) can be generated inside the nucleus and that there are calcium release channels in the inner membrane of the nuclear envelope. Most of the experimental evidence supporting these hypotheses is based on the calibration of nuclear and cytosolic calcium concentrations. However, recent studies suggest that the local calibration of calcium indicators may not be accurate. We propose that nuclear calcium signals can be investigated by a different approach that does not rely on accurate calibration of indicators. We have developed calcium indicators that minimize facilitated calcium diffusion and are localized to either the nucleus or the cytosol. Using the diffusion coefficient of calcium ions, and measuring the delay between cytosolic and nuclear calcium increases, we show that the nuclear envelope is not a substantial barrier for calcium ions in PC12 (phaeochromocytoma) cells. This suggests that nuclear and cytosolic calcium signals equilibrate rapidly in these cells.

Secret message at the plant surface

In general, stomata open during the day and close at night. This behavior has a crucial importance because it maximizes the update of CO(2) for photosynthesis and minimizes the water loss. Blue light is one of the environmental factors that regulates this process. Certainly, when either entire plants or epidermal strips adapted to the dark are exposed to blue light, the stomata open widely their pores. But, what does happen if we illuminate individual stomata instead of peels or entire plants? In the inaugural issue of PLoS ONE, we have answered this question by irradiating individual stomata with a laser attached to a confocal microscope. Our study not only demonstrates that the stomata function independently from the behavior of their neighbors, and illuminates the implication of the blue light receptors PHOTOTROPIN1 and PHOTOTROPIN2 in such response. It also gives clues about the physiological relevancy of this behavior.

Determining calcium concentration in heterogeneous model systems using multiple indicators

Intracellular free calcium concentrations ([Ca2+]i) are assessed by measuring indicator fluorescence in entire cells or subcellular regions using fluorescence microscopy. [Ca2+]i is calculated using equations which link fluorescence intensities (or intensity ratios) to calcium concentrations [G. Grynkiewicz, M. Poenie, R.Y. Tsien, A new generation of Ca2+ indicators with greatly improved fluorescence properties, J. Biol. Chem. 260 (1985) 3440-3450]. However, if calcium ions are heterogeneously distributed within a region of interest, then the observed average fluorescence intensity may not reflect average [Ca2+]i. We assessed potential calcium determination errors in mathematical and experimental models consisting of 'low' and 'high' calcium compartments, using indicators with different affinity for calcium. [Ca2+] calculated using average fluorescence intensity was lower than the actual mean concentrations. Low affinity indicators reported higher (more accurate) values than their high affinity counterparts. To estimate compartment dimensions and respective [Ca2+], we extended the standard approach by using different indicator responses to the same [Ca2+]. While two indicators were sufficient to provide a partial characterization of two-compartment model systems, the use of three or more indicators offered full description of the model provided compartmental [Ca2+] were within the indicator sensitivity ranges. These results show that uneven calcium distribution causes underestimation of actual [Ca2+], and offers novel approaches to estimating calcium heterogeneity.

Bioluminescence imaging of mitochondrial Ca2+ dynamics in soma and neurites of individual adult mouse sympathetic neurons

Changes in the cytosolic Ca(2+) concentration ([Ca(2+)](c)) are essential for triggering neurotransmitter release from presynaptic nerve terminals. Calcium-induced Ca(2+) release (CICR) from the endoplasmic reticulum (ER) may amplify the [Ca(2+)](c) signals and facilitate neurotransmitter release in sympathetic neurons. In adrenal chromaffin cells, functional triads are formed by voltage-operated Ca(2+) channels (VOCCs), CICR sites and mitochondria. In fact, mitochondria take up most of the Ca(2+) load entering the cells and are essential for shaping [Ca(2+)](c) signals and exocytosis. Here we have investigated the existence of such functional triads in sympathetic neurons. The mitochondrial Ca(2+) concentration ([Ca(2+)](m)) in soma and neurites of individual mouse superior cervical ganglion (SCG) neurons was monitored by bioluminescence imaging of targeted aequorins. In soma, Ca(2+) entry through VOCCs evoked rapid, near millimolar [Ca(2+)](m) increases in a subpopulation of mitochondria containing about 40% of the aequorin. Caffeine evoked a similar [Ca(2+)](m) increase in a mitochondrial pool containing about 30% of the aequorin and overlapping with the VOCC-sensitive pool. These observations suggest the existence of functional triads similar to the ones described in chromaffin cells. In neurites, mitochondria were able to buffer [Ca(2+)](c) increases resulting from activation of VOCCs but not those mediated by caffeine-induced Ca(2+) release from the ER. The weaker Ca(2+) buffering by mitochondria in neurites could contribute to facilitate Ca(2+)-induced exocytosis at the presynaptic sites.

Modulation of calcium wave propagation in the dendrites and to the soma of rat hippocampal pyramidal neurons

Repetitive synaptic stimulation in the stratum radiatum (SR) evokes large amplitude Ca2+ waves in the thick apical dendrites of hippocampal CA1 pyramidal neurons. These waves are initiated by activation of metabotropic glutamate receptors (mGluRs), which mobilize inositol-1,4,5-trisphospate (IP3) and release Ca2+ from intracellular stores. We explored mechanisms that modulate the spatial properties of these waves. Higher stimulus current evoked waves of increasing spatial extent. Most waves did not propagate through the soma; the majority stopped close to the junction of the soma and apical dendrite. Pairing strong stimulation with one electrode and subthreshold stimulation with another (associative activation) extended the waves distally but failed to extend waves into the cell body. Pairing synaptic stimulation with backpropagating action potentials enhanced the likelihood of wave generation but did not extend the waves to the somatic region. Priming the stores with Ca2+ entry through voltage dependent channels modulated wave properties but did not extend them past the dendrites. These results are consistent with propagation failing due to the dilution of synaptically generated IP3 as it diffuses into the large volume of the soma (impedance mismatch). Synaptically activating waves in the presence of low concentrations of carbachol, which probably increased the tonic level of IP3 throughout the cell, enhanced the extent of propagation and generated waves that invaded the soma, as long as low-affinity indicators were used to detect the [Ca2+]i changes. Consistent with this explanation direct injection of IP3 into the soma promoted wave propagation into this region. Ca2+ waves that propagated through the cell body were interesting because they did not fill the volume of the soma, but passed through the centre, often with large amplitude. These waves may be particularly effective in activating gene expression and protein synthesis.

Biphasic Ca2+-dependent switching in a calmodulin-IQ domain complex

The relationship between the free Ca2+ concentration and the apparent dissociation constant for the complex between calmodulin (CaM) and the neuromodulin IQ domain consists of two phases. In the first phase, Ca2+ bound to the C-ter EF hand pair in CaM increases the Kd for the complex from the Ca2+-free value of 2.3 +/- 0.1 microM to a value of 14.4 +/- 1.3 microM. In the second phase, Ca2+ bound to the N-ter EF hand pair reduces the Kd for the complex to a value of 2.5 +/- 0.1 microM, reversing the effect of the first phase. Due to energy coupling effects associated with these phases, the mean dissociation constant for binding of Ca2+ to the C-ter EF hand pair is increased approximately 3-fold, from 1.8 +/- 0.1 to 5.1 +/- 0.7 microM, and the mean dissociation constant for binding of Ca2+ to the N-ter EF hand pair is decreased by the same factor, from 11.2 +/- 1.0 to 3.5 +/- 0.6 microM. These characteristics produce a bell-shaped relationship between the apparent dissociation constant for the complex and the free Ca2+ concentration, with a distance of 5-6 microM between the midpoints of the rising and falling phases. Release of CaM from the neuromodulin IQ domain therefore appears to be promoted over a relatively narrow range of free Ca2+ concentrations. Our results demonstrate that CaM-IQ domain complexes can function as biphasic Ca2+ switches through opposing effects of Ca2+ bound sequentially to the two EF hand pairs in CaM.

High-speed addressable confocal microscopy for functional imaging of cellular activity

Due to cellular complexity, studying fast signaling in neurons is often limited by: 1. the number of sites that can be simultaneously probed with conventional tools, such as patch pipettes, and 2. the recording speed of imaging tools, such as confocal or multiphoton microscopy. To overcome these spatiotemporal limitations, we develop an addressable confocal microscope that permits concurrent optical recordings from multiple user-selected sites of interest at high frame rates. Our system utilizes acousto-optic deflectors (AODs) for rapid positioning of a focused laser beam and a digital micromirror device (DMD) for addressable spatial filtering to achieve confocality. A registration algorithm synchronizes the AODs and DMD such that point illumination and point detection are always colocalized in conjugate image planes. The current system has an adjustable spatial resolution of approximately 0.5 to 1 microm. Furthermore, we show that recordings can be made at an aggregate frame rate of approximately 40 kHz. The system is capable of optical sectioning; this property is used to create 3-D reconstructions of fluorescently labeled test specimens and visualize neurons in brain slices. Additionally, we use the system to record intracellular calcium transients at several sites in hippocampal neurons using the fluorescent calcium indicator Oregon Green BAPTA-1.

Transcriptional repressor DREAM regulates T-lymphocyte proliferation and cytokine gene expression

Downstream Regulatory Element Antagonist Modulator (DREAM) is a Ca2+-dependent transcriptional repressor expressed in the brain, thyroid gland and thymus. Here, we analyzed the function of DREAM and the related protein KChIP-2 in the immune system using transgenic (tg) mice expressing a cross-dominant active mutant (EFmDREAM) for DREAM and KChIPs Ca2+-dependent transcriptional derepression. EFmDREAM tg mice showed reduced T-cell proliferation. Tg T cells exhibited decreased interleukin (IL)-2, -4 and interferon (IFN)gamma production after polyclonal activation and following antigen-specific response. Chromatin immunoprecipitation and transfection assays showed that DREAM binds to and represses transcription from these cytokine promoters. Importantly, specific transient knockdown of DREAM or KChIP-2 induced basal expression of IL-2 and IFNgamma in wild-type splenocytes. These data propose DREAM and KChIP-2 as Ca2+-dependent repressors of the immune response.

Calcium gradients and exocytosis in bovine adrenal chromaffin cells

The relationship between the localized Ca(2+) concentration and depolarization-induced exocytosis was studied in patch-clamped adrenal chromaffin cells using pulsed-laser Ca(2+) imaging and membrane capacitance measurements. Short depolarizing voltage steps induced Ca(2+) gradients and small "synchronous" increases in capacitance during the pulses. Longer pulses increased the capacitance changes, which saturated at 16 fF, suggesting the presence of a small immediately releasable pool of fusion-ready vesicles. A Hill plot of the capacitance changes versus the estimated Ca(2+) concentration in a thin (100 nm) shell beneath the membrane gave n = 2.3 and K(d) = 1.4 microM. Repetitive stimulation elicited a more complex pattern of exocytosis: early pulses induced synchronous capacitance increases, but after five or more pulses there was facilitation of the synchronous responses and gradual increases in capacitance continued between pulses (asynchronous exocytosis) as the steep submembrane Ca(2+) gradients collapsed. Raising the pipette Ca(2+) concentration led to early facilitation of the synchronous response and early appearance of asynchronous exocytosis. We used this data to develop a kinetic model of depolarization-induced exocytosis, where Ca(2+)-dependent fusion of vesicles occurs from a small immediately releasable pool with an affinity of 1-2 microM and vesicles are mobilized to this pool in a Ca(2+)-dependent manner.

Calcium sparks in skeletal muscle fibers

Ca(2+) sparks monitor transient local releases of Ca(2+) from the sarcoplasmic reticulum (SR) into the myoplasm. The release takes place through ryanodine receptors (RYRs), the Ca(2+)-release channels of the SR. In intact fibers from frog skeletal muscle, the temporal and spatial properties of voltage-activated Ca(2+) sparks are well simulated by a model that assumes that the Ca(2+) flux underlying a spark is 2.5 pA (units of Ca(2+) current) for 4.6 ms (18 degrees C). This flux amplitude suggests that 1-5 active RYRs participate in the generation of a typical voltage-activated spark under physiological conditions. A major goal of future experiments is to estimate this number more precisely and, if it is two or more, to investigate the communication mechanism that allows multiple RYRs to be co-activated in a rapid but self-limited fashion.

Calcium-induced calcium release contributes to somatic secretion of serotonin in leech Retzius neurons

We analyzed the contribution of calcium (Ca2+)-induced Ca2+ release to somatic secretion in serotonergic Retzius neurons of the leech. Somatic secretion was studied by the incorporation of fluorescent dye FM1-43 upon electrical stimulation with trains of 10 impulses and by electron microscopy. Quantification of secretion with FM1-43 was made in cultured neurons to improve optical resolution. Stimulation in the presence of FM1-43 produced a frequency-dependent number of fluorescent spots. While a 1-Hz train produced 19.5+/-5.0 spots/soma, a 10-Hz train produced 146.7+/-20.2 spots/soma. Incubation with caffeine (10 mM) to induce Ca2+ release from intracellular stores without electrical stimulation and external Ca2+, produced 168+/-21.7 spots/soma. This staining was reduced by 49% if neurons were preincubated with the Ca2+- ATPase inhibitor thapsigargin (200 nM). Moreover, in neurons stimulated at 10 Hz in the presence of ryanodine (100 microM) to block Ca2+-induced Ca2+ release, FM1-43 staining was reduced by 42%. In electron micrographs of neurons at rest or stimulated at 1 Hz in the ganglion, endoplasmic reticulum lay between clusters of dense core vesicles and the plasma membrane. In contrast, in neurons stimulated at 20 Hz, the vesicle clusters were apposed to the plasma membrane and flanked by the endoplasmic reticulum. These results suggest that Ca2+-induced Ca2+ release produces vesicle mobilization and fusion in the soma of Retzius neurons, and supports the idea that neuronal somatic secretion shares common mechanisms with secretion by excitable endocrine cells.

Spatial distribution of Ca(2+) signals during repetitive depolarizing stimuli in adrenal chromaffin cells

Exocytosis in adrenal chromaffin cells is strongly influenced by the pattern of stimulation. To understand the dynamic and spatial properties of the underlying Ca(2+) signal, we used pulsed laser Ca(2+) imaging to capture Ca(2+) gradients during stimulation by single and repetitive depolarizing stimuli. Short single pulses (10-100 ms) lead to the development of submembrane Ca(2+) gradients, as previously described (F. D. Marengo and J. R. Monck, 2000, Biophysical Journal, 79:1800-1820). Repetitive stimulation with trains of multiple pulses (50 ms each, 2Hz) produce a pattern of intracellular Ca(2+) increase that progressively changes from the typical Ca(2+) gradient seen after a single pulse to a Ca(2+) increase throughout the cell that peaks at values 3-4 times higher than the maximum values obtained at the end of single pulses. After seven or more pulses, the fluorescence increase was typically larger in the interior of the cell than in the submembrane region. The pattern of Ca(2+) gradient was not modified by inhibitors of Ca(2+)-induced Ca(2+) release (ryanodine), inhibitors of IP(3)-induced Ca(2+) release (xestospongin), or treatments designed to deplete intracellular Ca(2+) stores (thapsigargin). However, we found that the large fluorescence increase in the cell interior spatially colocalized with the nucleus. These results can be simulated using mathematical models of Ca(2+) redistribution in which the nucleus takes up Ca(2+) by active or passive transport mechanisms. These results show that chromaffin cells can respond to depolarizing stimuli with different dynamic Ca(2+) signals in the submembrane space, the cytosol, and the nucleus.

Local calcium signaling in neurons

Transient rises in the cytoplasmic concentration of calcium ions serve as second messenger signals that control many neuronal functions. Selective triggering of these functions is achieved through spatial localization of calcium signals. Several qualitatively different forms of local calcium signaling can be distinguished by the location of open calcium channels as well as by the distance between these channels and the calcium binding proteins that serve as the molecular targets of calcium action. Local calcium signaling is especially prominent at presynaptic active zones and postsynaptic densities, structures that are distinguished by highly organized macromolecular arrays that yield precise spatial arrangements of calcium signaling proteins. Similar forms of local calcium signaling may be employed throughout the nervous system, though much remains to be learned about the molecular underpinnings of these events.

Probing neural circuits in the zebrafish: a suite of optical techniques

The ability to image neural activity in populations of neurons inside an intact animal, while obtaining single-cell or subcellular spatial resolution, has led to several advances in our understanding of vertebrate locomotor control. This result, first reported in a 1995 study of motoneurons in larval zebrafish, was the beginning of a series of technical developments that exploited the transparency and simplicity of the larval CNS. Presented here, in chronological fashion, is a suite of imaging techniques that have extended the ability to probe and optically dissect neural control systems. Included are methodological details pertaining to: (1). the in vivo optical recording of neural activity, (2). the optical dissection of complex neural architectures, and (3). additional fluorescence imaging-based techniques for the anatomical and physiological characterization of these systems. These approaches have provided insights into the descending neural control of escape and other locomotive behaviors, such as swimming and prey capture. The methods employed are discussed in relation to complementary and alternative imaging techniques, including, for example, the Nipkow disk confocal. While these methodologies focus on descending motor control in the larval zebrafish, the extension of such approaches to other neural systems is viewed as a promising and necessary step if neurobiologists are to bridge the gap between synaptic and brain region levels of analysis. The efficiency of optical techniques for surveying the cellular elements of intricate neural systems is of particular relevance because a comprehensive description of such elements is deemed necessary for a precise understanding of vertebrate neural architectures.

The role of Ca2+-dependent cationic current in generating gamma frequency rhythmic bursts: modeling study

Fast rhythmic bursting pyramidal neuron or chattering neuron is a promising candidate for the pacemaker of coherent gamma-band (25-70 Hz) cortical oscillation. It, however, still remains to be clarified how the neuron generates such high-frequency bursts. Here, we demonstrate in a single-compartment model neuron that the fast rhythmic bursts (FRBs) can be achieved through Ca2+-activated channels in the entire gamma frequency range. In a previous in vitro study, a subset of rat cortical pyramidal cells displayed a long-lasting depolarizing afterpotential (DAP) following a plateau-type action potential when K+ conductances were suppressed with Cs+, and this DAP was found to be mediated by a Ca2+-dependent cationic current. This current appeared also suitable for producing a hump-like DAP, a characteristic of the chattering neurons, because of its reversal potential being approximately -40 mV. In the present theoretical study, we show that the enhancement of such a DAP leads to generation of doublet/triplet spikes seen during FRBs. The firing pattern during FRBs is primarily determined by a Ca2+-dependent cationic current and a small-conductance Ca2+-dependent potassium current, which are differentially activated by a biphasically decaying Ca2+ transient produced by fast buffering and a slow pump extrusion after each spike. With varying intensities of injected current pulses, the interburst frequencies of the FRBs range over the entire gamma frequency band (25-70 Hz) in our model, while the intraburst frequencies remain higher than 300 Hz. Our model suggests that FRBs are essentially generated in the soma, unlike the model based on a persistent sodium current, and that the alteration of Ca2+ sensitivity of Ca2+-dependent cationic current plays an essential role in controlling the FRB pattern.

Ca(2+)-dependent prodynorphin transcriptional derepression in neuroblastoma cells is exerted through DREAM protein activity in a kinase-independent manner

Prodynorphin transcription has been postulated as an important molecular mechanism involved in adaptation/repair processes. Expression of prodynorphin is modulated according to the levels of the second messengers cAMP and Ca(2+). In the neuroblastoma cell lines, the increase of prodynorphin mRNA levels is coupled to an elevation of intracellular cAMP levels. Promoter analyses have revealed that the DRE site, a silencer element present in the prodynorphin promoter, is involved in PKA-dependent prodynorphin derepression. In this way, DREAM, a calcium-dependent repressor, plays an outstanding role. In this study, Ca(2+) release from internal stores has been found to promote an increase of prodynorphin mRNA levels in NB69 cells. Surprisingly, Ca(2+)-dependent prodynorphin gene transcription was insensitive to the broad-spectrum kinase inhibitors and sensitive to agents that alter internal Ca(2+) accumulation. Moreover, we demonstrate that in NB69 cells, the Ca(2+) signaling pathway uses DREAM as an effector to evoke prodynorphin transcription derepression in a kinase-independent manner.

The endoplasmic reticulum and neuronal calcium signalling

The endoplasmic reticulum (ER) is a multifunctional signalling organelle regulating a wide range of neuronal functional responses. The ER is intimately involved in intracellular Ca(2+) signalling, producing local or global cytosolic calcium fluctuations via Ca(2+)-induced Ca(2+) release (CICR) or inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). The CICR and IICR are controlled by two subsets of Ca(2+) release channels residing in the ER membrane, the Ca(2+)-gated Ca(2+) release channels, generally known as ryanodine receptors (RyRs) and InsP(3)-gated Ca(2+) release channels, referred to as InsP(3)-receptors (InsP(3)Rs). Both types of Ca(2+) release channels are expressed abundantly in nerve cells and their activation triggers cytoplasmic Ca(2+) signals important for synaptic transmission and plasticity. The RyRs and InsP(3)Rs show heterogeneous localisation in distinct cellular sub-compartments, conferring thus specificity in local Ca(2+) signals. At the same time, the ER Ca(2+) store emerges as a single interconnected pool fenced by the endomembrane. The continuity of the ER Ca(2+) store could play an important role in various aspects of neuronal signalling. For example, Ca(2+) ions may diffuse within the ER lumen with comparative ease, endowing this organelle with the capacity for "Ca(2+) tunnelling". Thus, continuous intra-ER Ca(2+) highways may be very important for the rapid replenishment of parts of the pool subjected to excessive stimulation (e.g. in small compartments within dendritic spines), the facilitated removal of localised Ca(2+) loads, and finally in conveying Ca(2+) signals from the site of entry towards the cell interior and nucleus.

Ca2+-dependent block of CREB-CBP transcription by repressor DREAM

The calcium-binding protein DREAM binds specifically to DRE sites in the DNA and represses transcription of target genes. Derepression at DRE sites following PKA activation depends on a specific interaction between alphaCREM and DREAM. Two leucine-charged residue-rich domains (LCD) located in the kinase-inducible domain (KID) and in the leucine zipper of alphaCREM and two LCDs in DREAM participate in a two-site interaction that results in the loss of DREAM binding to DRE sites and derepression. Since the LCD motif located within the KID in CREM is also present in CREB, and maps in a region critical for the recruitment of CBP, we investigated whether DREAM may affect CRE-dependent transcription. Here we show that in the absence of Ca2+ DREAM binds to the LCD in the KID of CREB. As a result, DREAM impairs recruitment of CBP by phospho CREB and blocks CBP-mediated transactivation at CRE sites in a Ca2+-dependent manner. Thus, Ca2+-dependent interactions between DREAM and CREB represent a novel point of cross-talk between cAMP and Ca2+ signalling pathways in the nucleus.

Role of ATP decrease in secretion induced by mitochondrial dysfunction in guinea-pig adrenal chromaffin cells

The mechanism related to mitochondrial dysfunction-induced catecholamine (CA) secretion in dispersed guinea-pig adrenal chromaffin cells was investigated using amperometry and confocal laser microscopy. Application of CCCP, which does not stimulate generation of reactive oxygen species (ROS), reversibly induced CA secretion, whereas application of either cyanide or oligomycin (OL), a stimulator for ROS, enhanced CA secretion to a smaller extent. The CCCP-induced secretion was abolished by removal of external Ca2+ ions and was markedly diminished by D600. The mitochondrial membrane potential, measured using rhodamine 123, was rapidly lost in response to CCCP, but did not change noticeably during a 3 min exposure to OL. Prior exposure to OL markedly facilitated depolarization of the mitochondrial membrane potential in response to cyanide. The mitochondrial inhibitors rapidly produced an increase in Magnesium Green (MgG) fluorescence in the absence of external Ca2+ and Mg2+ ions, an increase that was larger in the cytoplasm than in the nucleus. The rank order of potency in increasing MgG fluorescence among the inhibitors was similar to that in increasing secretion. Thus, mitochondrial inhibition rapidly decreases [ATP] and the mitochondrial dysfunction-induced secretion is not due to ROS generation or to mitochondrial depolarization, but is possibly mediated by a decrease in ATP.

Spike frequency decoding and autonomous activation of Ca2+-calmodulin-dependent protein kinase II in dorsal root ganglion neurons

Autonomous activation of calcium-calmodulin kinase (CaMKII) has been proposed as a molecular mechanism for decoding Ca(2+) spike frequencies resulting from action potential firing, but this has not been investigated in intact neurons. This was studied in mouse DRG neurons in culture using confocal measurements of [Ca(2+)](i) and biochemical measurements of CaMKII autophosphorylation and autonomous activity. Using electrical stimulation at different frequencies, we find that CaMKII autonomous activity reached near maximal levels after approximately 45 impulses, regardless of firing frequency (1-10 Hz), and autonomous activity declined with prolonged stimulation. Frequency-dependent activation of CaMKII was limited to spike frequencies in the range of 0.1-1 Hz, despite marked increases in [Ca(2+)](i) at higher frequencies (1-30 Hz). The high levels of autonomous activity measured before stimulation and the relatively long duration of Ca(2+) spikes induced by action potentials ( approximately 300 msec) are consistent with the lower frequency range of action potential decoding by CaMKII. The high autonomous activity under basal conditions was associated with extracellular [Ca(2+)], independently from changes in [Ca(2+)](i), and unrelated to synaptic or spontaneous impulse activity. CaMKII autonomous activity in response to brief bursts of action potentials correlated better with the frequency of Ca(2+) transients than with the concentration of [Ca(2+)](i). In conclusion, CaMKII may decode frequency-modulated responses between 0.1 and 1 Hz in these neurons, but other mechanisms may be required to decode higher frequencies. Alternatively, CaMKII may mediate high-frequency responses in subcellular microdomains in which the enzyme is maintained at a low level of autonomous activity or the Ca(2+) transients have faster kinetics.

Modeling temporal behavior of postnatal cat retinal ganglion cells

During development, mammalian retinal ganglion cells (RGCs) go through marked ontogenetic changes with respect to their excitable membrane properties. Voltage-clamp studies conducted in our laboratory have shown that the amplitude, voltage-dependence and kinetics of activation and inactivation (where present) of Na(+), K(+) and Ca(2+) conductances all exhibit developmental changes during a time when the firing patterns of mammalian ganglion cells shift from being transient to being predominantly sustained in nature. In order to better understand the contribution of each conductance to the generation of spikes and spiking patterns, we have developed a model based on our experimental data. For simplicity, we have initially used experimental data obtained from postnatal ganglion cells. At this age the ontogenetic changes observed in the characteristics of the various ionic currents are complete. Utilizing the methods adopted by Hodgkin and Huxley for the giant squid axon, we have determined rate equations for the activation and inactivation properties of the I(A), I(K dr), I(Na), I(Ca L), I(Ca N), and I(leak) currents in postnatal cat RGCs. Combining these with a simplified model of the calcium-activated potassium current (I(KCa)), we have solved and analysed the resulting differential equations. While spikes and spiking patterns resembling experimental data could be obtained from a model in which [Ca(2+)i] was averaged across the whole cell, more accurate simulations were obtained when the diffusion of intracellular Ca(2+) was modeled spatially. The resulting spatial calcium gradients were more effective in gating I(KCa), and our simulations more accurately matched the recorded amplitude and shape of individual spikes as well as the frequency of maintained discharges observed in mammalian postnatal RGCs.

Multiple modes of calcium-induced calcium release in sympathetic neurons II: a [Ca2+](i)- and location-dependent transition from endoplasmic reticulum Ca accumulation to net Ca release

CICR from an intracellular store, here directly characterized as the ER, usually refers to net Ca(2)+ release that amplifies evoked elevations in cytosolic free calcium [Ca2+](i). However, the companion paper (Albrecht, M.A., S.L. Colegrove, J. Hongpaisan, N.B. Pivovarova, S.B. Andrews, and D.D. Friel. 2001. J. Gen. Physiol. 118:83-100) shows that in sympathetic neurons, small [Ca2+](i) elevations evoked by weak depolarization stimulate ER Ca accumulation, but at a rate attenuated by activation of a ryanodine-sensitive CICR pathway. Here, we have measured depolarization-evoked changes in total ER Ca concentration ([Ca](ER)) as a function of [Ca2+](i), and found that progressively larger [Ca2+](i) elevations cause a graded transition from ER Ca accumulation to net release, consistent with the expression of multiple modes of CICR. [Ca](ER) is relatively high at rest (12.8 +/- 0.9 mmol/kg dry weight, mean +/- SEM) and is reduced by thapsigargin or ryanodine (5.5 +/- 0.7 and 4.7 +/- 1.1 mmol/kg, respectively). [Ca](ER) rises during weak depolarization (to 17.0 +/- 1.6 mmol/kg over 120s, [Ca2+](i) less than approximately 350 nM), changes little in response to stronger depolarization (12.1 +/- 1.1 mmol/kg, [Ca2+](i) approximately 700 nM), and declines (to 6.5 +/- 1.0 mmol/kg) with larger [Ca2+](i) elevations (>1 microM) evoked by the same depolarization when mitochondrial Ca2+ uptake is inhibited (FCCP). Thus, net ER Ca2+ transport exhibits a biphasic dependence on [Ca2+](i). With mitochondrial Ca2+ uptake enabled, [Ca](ER) rises after repolarization (to 16.6 +/- 1.8 mmol/kg at 15 min) as [Ca2+](i) falls within the permissive range for ER Ca accumulation over a period lengthened by mitochondrial Ca2+ release. Finally, although spatially averaged [Ca](ER) is unchanged during strong depolarization, net ER Ca2+ release still occurs, but only in the outermost approximately 5-microm cytoplasmic shell where [Ca2+](i) should reach its highest levels. Since mitochondrial Ca accumulation occurs preferentially in peripheral cytoplasm, as demonstrated here by electron energy loss Ca maps, the Ca content of ER and mitochondria exhibit reciprocal dependencies on proximity to sites of Ca2+ entry, possibly reflecting indirect mitochondrial regulation of ER Ca(2)+ transport.

How does calcium trigger neurotransmitter release?

Recent work has established that different geometric arrangements of calcium channels are found at different presynaptic terminals, leading to a wide spectrum of calcium signals for triggering neurotransmitter release. These calcium signals are apparently transduced by synaptotagmins - calcium-binding proteins found in synaptic vesicles. New biochemical results indicate that all synaptotagmins undergo calcium-dependent interactions with membrane lipids and a number of other presynaptic proteins, but which of these interactions is responsible for calcium-triggered transmitter release remains unclear.

Modifications of intracellular Ca2+ signalling during nerve growth factor-induced neuronal differentiation of rat adrenal chromaffin cells

Postnatal sympathetic neurons (SNs) and chromaffin cells (CCs) derive from neural crest precursors. CCs can differentiate in vitro into SN-like cells after nerve growth factor (NGF) exposure. This study examines changes of intracellular Ca2+ homeostasis and dynamics of CCs under conditions that promote a neuronal phenotype. Spontaneous Ca2+ fluctuations, a frequent observation in early cultures of CCs, diminished after > 10 days in vitro in control cells and ceased in NGF-treated ones. At the same time, Ca2+ rises resulting from entry upon membrane depolarization, gradually increased both their size and peak d[Ca2+]i/dt, resembling those recorded in SNs. Concomitantly, caffeine-induced Ca2+ rises, resulting from Ca2+ release from intracellular stores, increased their size and their peak d[Ca2+]i/dt by > 1000%, and developed transient and sustained release components, similar to those of SNs. The transient component, linked to regenerative Ca2+ release, appeared after > 10 days of NGF treatment, suggesting a delayed steep enhancement of Ca2+-induced Ca2+ release (CICR). Immunostaining showed that proteins coded by the three known isoforms of ryanodine receptors (RyRs) are present in CCs, but that only RyR2 increased significantly after NGF treatment. Since the transient release component increased more steeply than RyR2 immunostaining, we suggest that the development of robust CICR requires both an increased expression of RyRs and more efficient functional coupling among them. NGF-induced transdifferentiation of chromaffin cells involves the enhancement of both voltage-gated Ca2+ influx and Ca2+ release from intracellular stores. These modifications are likely to complement the extensive morphological and functional reorganization required for the replacement of the endocrine phenotype with the neuronal one.

Origin sites of calcium release and calcium oscillations in frog sympathetic neurons

In many neurons, Ca(2+) signaling depends on efflux of Ca(2+) from intracellular stores into the cytoplasm via caffeine-sensitive ryanodine receptors (RyRs) of the endoplasmic reticulum. We have used high-speed confocal microscopy to image depolarization- and caffeine-evoked increases in cytoplasmic Ca(2+) levels in individual cultured frog sympathetic neurons. Although caffeine-evoked Ca(2+) wave fronts propagated throughout the cell, in most cells the initial Ca(2+) release was from one or more discrete sites that were several micrometers wide and located at the cell edge, even in Ca(2+)-free external solution. During cell-wide cytoplasmic [Ca(2+)] oscillations triggered by continual caffeine application, the initial Ca(2+) release that began each Ca(2+) peak was from the same subcellular site or sites. The Ca(2+) wave fronts propagated with constant amplitude; the spread was mostly via calcium-induced calcium release. Propagation was faster around the cell periphery than radially inward. Local Ca(2+) levels within the cell body could increase or decrease independently of neighboring regions, suggesting independent action of spatially separate Ca(2+) stores. Confocal imaging of fluorescent analogs of ryanodine and thapsigargin, and of MitoTracker, showed potential structural correlates to the patterns of Ca(2+) release and propagation. High densities of RyRs were found in a ring around the cell periphery, mitochondria in a broader ring just inside the RyRs, and sarco-endoplasmic reticulum Ca(2+) ATPase pumps in hot spots at the cell edge. Discrete sites at the cell edge primed to release Ca(2+) from intracellular stores might preferentially convert Ca(2+) influx through a local area of plasma membrane into a cell-wide Ca(2+) increase.

The DREAM-DRE interaction: key nucleotides and dominant negative mutants

Transcriptional repressor DREAM, an EF-hand containing calcium-binding protein, blocks basal expression of target genes through specific interaction with DRE sites in the DNA. The sequence GTCA forms the central core of the DRE site, whereas flanking nucleotides contribute notably to the affinity for DREAM. Release of binding of DREAM from the DRE results in derepression, a process that is regulated by Ca(2+). Change of two amino acids within an EF-hand in DREAM blocks Ca(2+)-induced derepression and results in potent dominant negative mutants of endogenous DREAM.

The nuclear chloride ion channel NCC27 is involved in regulation of the cell cycle

NCC27 is a nuclear chloride ion channel, identified in the PMA-activated U937 human monocyte cell line. NCC27 mRNA is expressed in virtually all cells and tissues and the gene encoding NCC27 is also highly conserved. Because of these factors, we have examined the hypothesis that NCC27 is involved in cell cycle regulation. Electrophysiological studies in Chinese hamster ovary (CHO-K1) cells indicated that NCC27 chloride conductance varied according to the stage of the cell cycle, being expressed only on the plasma membrane of cells in G2/M phase. We also demonstrate that Cl- ion channel blockers known to block NCC27 led to arrest of CHO-K1 cells in the G2/M stage of the cell cycle, the same stage at which this ion channel is selectively expressed on the plasma membrane. These data strongly support the hypothesis that NCC27 is involved, in some as yet undetermined manner, in regulation of the cell cycle.

Functional triads consisting of ryanodine receptors, Ca(2+) channels, and Ca(2+)-activated K(+) channels in bullfrog sympathetic neurons. Plastic modulation of action potential

Fluorescent ryanodine revealed the distribution of ryanodine receptors in the submembrane cytoplasm (less than a few micrometers) of cultured bullfrog sympathetic ganglion cells. Rises in cytosolic Ca(2+) ([Ca(2+)](i)) elicited by single or repetitive action potentials (APs) propagated at a high speed (150 microm/s) in constant amplitude and rate of rise in the cytoplasm bearing ryanodine receptors, and then in the slower, waning manner in the deeper region. Ryanodine (10 microM), a ryanodine receptor blocker (and/or a half opener), or thapsigargin (1-2 microM), a Ca(2+)-pump blocker, or omega-conotoxin GVIA (omega-CgTx, 1 microM), a N-type Ca(2+) channel blocker, blocked the fast propagation, but did not affect the slower spread. Ca(2+) entry thus triggered the regenerative activation of Ca(2+)-induced Ca(2+) release (CICR) in the submembrane region, followed by buffered Ca(2+) diffusion in the deeper cytoplasm. Computer simulation assuming Ca(2+) release in the submembrane region reproduced the Ca(2+) dynamics. Ryanodine or thapsigargin decreased the rate of spike repolarization of an AP to 80%, but not in the presence of iberiotoxin (IbTx, 100 nM), a BK-type Ca(2+)-activated K(+) channel blocker, or omega-CgTx, both of which decreased the rate to 50%. The spike repolarization rate and the amplitude of a single AP-induced rise in [Ca(2+)](i) gradually decreased to a plateau during repetition of APs at 50 Hz, but reduced less in the presence of ryanodine or thapsigargin. The amplitude of each of the [Ca(2+)](i) rise correlated well with the reduction in the IbTx-sensitive component of spike repolarization. The apamin-sensitive SK-type Ca(2+)-activated K(+) current, underlying the afterhyperpolarization of APs, increased during repetitive APs, decayed faster than the accompanying rise in [Ca(2+)](i), and was suppressed by CICR blockers. Thus, ryanodine receptors form a functional triad with N-type Ca(2+) channels and BK channels, and a loose coupling with SK channels in bullfrog sympathetic neurons, plastically modulating AP.