Determination of in situ dissociation constant for Fura-2 and quantitation of background fluorescence in astrocyte cell line U373-MG

An Inorganic Fluorescent Chemosensor: Rational Design and Selective Mg2+ Detection

A Zn²⁺ based complex, 3, displays greatly increased fluorescence emission in the presence of Mg²⁺. Fluorescent and computational studies suggest that 3 selectively interacts with Mg²⁺ due to optimal cavity size formation between two uncoordinated pyrazole side arms. This work thus represents a new approach to the development of fluorescent chemosensors.

Intracellular Calcium Recording After Purinoceptor Activation Using a Video-Microscopy Equipment

Calcium is one of the most important intracellular messengers, triggering a wide range of cellular responses. Changes in intracellular free calcium concentration can be measured using calcium sensitive fluorescent dyes, which are either EGTA- or BAPTA-based organic molecules that change their spectral properties in response to Ca²⁺ binding. One of the most common calcium indicators is the ratiometric dye Fura-2. The main advantage of using ratiometric dyes is that the ratio signal is independent of the illumination intensity, dye concentration, photobleaching, and focus changes among others, allowing for the concentration of intracellular calcium to be determined independently of these artifacts. In this protocol, we describe the use of Fura-2 to measure intracellular calcium elevations in single cultured cells after purinoceptor activation using a video-microscopy equipment. This method, usually known as calcium imaging, allows for real-time quantification of intracellular calcium dynamics and can be adapted to measure agonist mediated intracellular calcium responses due to the activation of different purinergic receptors in several cellular models using the appropriate growth conditions.

Transient Receptor Potential V Channels Are Essential for Glucose Sensing by Aldolase and AMPK

Fructose-1,6-bisphosphate (FBP) aldolase links sensing of declining glucose availability to AMPK activation via the lysosomal pathway. However, how aldolase transmits lack of occupancy by FBP to AMPK activation remains unclear. Here, we show that FBP-unoccupied aldolase interacts with and inhibits endoplasmic reticulum (ER)-localized transient receptor potential channel subfamily V, inhibiting calcium release in low glucose. The decrease of calcium at contact sites between ER and lysosome renders the inhibited TRPV accessible to bind the lysosomal v-ATPase that then recruits AXIN:LKB1 to activate AMPK independently of AMP. Genetic depletion of TRPVs blocks glucose starvation-induced AMPK activation in cells and liver of mice, and in nematodes, indicative of physical requirement of TRPVs. Pharmacological inhibition of TRPVs activates AMPK and elevates NAD+ levels in aged muscles, rejuvenating the animals' running capacity. Our study elucidates that TRPVs relay the FBP-free status of aldolase to the reconfiguration of v-ATPase, leading to AMPK activation in low glucose.

Biosensors for spatiotemporal detection of reactive oxygen species in cells and tissues

Redox biology has become a major issue in numerous areas of physiology. Reactive oxygen species (ROS) have a broad range of roles from signal transduction to growth control and cell death. To understand the nature of these roles, accurate measurement of the reactive compounds is required. An increasing number of tools for ROS detection is available; however, the specificity and sensitivity of these tools are often insufficient. Furthermore, their specificity has been rarely evaluated in complex physiological conditions. Many ROS probes are sensitive to environmental conditions in particular pH, which may interfere with ROS detection and cause misleading results. Accurate detection of ROS in physiology and pathophysiology faces additional challenges concerning the precise localization of the ROS and the timing of their production and disappearance. Certain ROS are membrane permeable, and certain ROS probes move across cells and organelles. Targetable ROS probes such as fluorescent protein-based biosensors are required for accurate localization. Here we analyze these challenges in more detail, provide indications on the strength and weakness of current tools for ROS detection, and point out developments that will provide improved ROS detection methods in the future. There is no universal method that fits all situations in physiology and cell biology. A detailed knowledge of the ROS probes is required to choose the appropriate method for a given biological problem. The knowledge of the shortcomings of these probes should also guide the development of new sensors.

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

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

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.

A polarized Ca2+, diacylglycerol and STIM1 signalling system regulates directed cell migration

Ca(2+) signals control cell migration by regulating forward movement and cell adhesion. However, it is not well understood how Ca(2+)-regulatory proteins and second messengers are spatially organized in migrating cells. Here we show that receptor tyrosine kinase and phospholipase C signalling are restricted to the front of migrating endothelial leader cells, triggering local Ca(2+) pulses, local depletion of Ca(2+) in the endoplasmic reticulum and local activation of STIM1, supporting pulsatile front retraction and adhesion. At the same time, the mediator of store-operated Ca(2+) influx, STIM1, is transported by microtubule plus ends to the front. Furthermore, higher Ca(2+) pump rates in the front relative to the back of the plasma membrane enable effective local Ca(2+) signalling by locally decreasing basal Ca(2+). Finally, polarized phospholipase C signalling generates a diacylglycerol gradient towards the front that promotes persistent forward migration. Thus, cells employ an integrated Ca(2+) control system with polarized Ca(2+) signalling proteins and second messengers to synergistically promote directed cell migration.

Endoplasmic reticulum Ca2+ dysregulation and endoplasmic reticulum stress following in vitro neuronal ischemia: role of Na+-K+-Cl- cotransporter

We investigated the role of Na(+)-K(+)-Cl(-) cotransporter (NKCC1) in conjunction with Na(+)/Ca(2+) exchanger (NCX) in disruption of endoplasmic reticulum (ER) Ca(2+) homeostasis and ER stress development in primary cortical neurons following in vitro ischemia. Oxygen-glucose deprivation (OGD) and reoxygenation (REOX) caused a rise in [Na(+)](cyt) which was accompanied by an elevation in [Ca(2+)](cyt). Inhibition of NKCC1 with its potent inhibitor bumetanide abolished the OGD/REOX-induced rise in [Na(+)](cyt) and [Ca(2+)](cyt). Moreover, OGD significantly increased Ca(2+)(ER) accumulation. Following REOX, a biphasic change in Ca(2+)(ER) occurred with an initial release of Ca(2+)(ER) which was sensitive to inositol 1,4,5-trisphosphate receptor (IP(3)R) inhibition and a subsequent refilling of Ca(2+)(ER) stores. Inhibition of NKCC1 activity with its inhibitor or genetic ablation prevented the release of Ca(2+)(ER). A similar result was obtained with inhibition of reversed mode operation of NCX (NCX(rev)). OGD/REOX also triggered a transient increase of glucose regulated protein 78 (GRP78), phospho-form of the alpha subunit of eukaryotic initiation factor 2 (p-eIF2alpha), and cleaved caspase 12 proteins. Pre-treatment of neurons with NKCC1 inhibitor bumetanide inhibited upregulation of GRP78 and attenuated the level of cleaved caspase 12 and p-eIF2alpha. Inhibition of NKCC1 reduced cytochrome C release and neuronal death. Taken together, these results suggest that NKCC1 and NCX(rev) may be involved in ischemic cell damage in part via disrupting ER Ca(2+) homeostasis and ER function.

Dual probe fluorescence monitoring of intracellular free calcium during ischemia in mouse heart by using continuous compensation for pH dependence of the dissociation constant of Fura-2, and the interference of myoglobin

Mitochondrial damage is the main source of cellular injury upon ischemia-reperfusion, and calcium loading has been implicated in this phenomenon. The use of optical probes for calcium monitoring of the intact heart is hampered by internal filter effects of intracellular hemoproteins, endogenous fluorescence, and their sensitivity to pH. We describe here a method for measurement of intracellular free calcium in isolated myoglobin-deficient perfused mouse hearts under conditions of large intracellular pH fluctuations by simultaneous fluorescence monitoring of the calcium-probe Fura-2 and the pH probe BCECF through dual wavelength excitation of both probes. In myoglobin-containing mouse heart endogenous chromophores interfere with Fura-2 fluorometry. It is shown that a paradoxical decrease in Fura-2 fluorescence occurs during ischemia in isolated mouse hearts. Simultaneous recording of BCECF fluorescence (calibrated against pH measurement with phosphorus NMR) and data reduction based on continual recalculation of the apparent dissociation constant of the calcium-probe complex revealed that a marked increase in intracellular free calcium occurs, and that the Fura-2 fluorescence decrease was caused by an increase in dissociation constant due to intracellular acidification. Intracellular free calcium rose almost linearly during a 20-min period of ischemia and returned to basal values rapidly upon the commencement of perfusion.

Impaired host defense in mice lacking ONZIN

ONZIN is a small, cysteine-rich peptide of unique structure that is conserved in all vertebrates examined to date. We show that ONZIN is expressed at high levels in epithelial cells of the intestinal tract, the lung, and in cells of the immune system including macrophages and granulocytes. Because this pattern of expression is suggestive of a role in innate immune function, we have generated mice lacking this protein and examined their ability to respond to challenge with infectious agents. Onzin(-/-) mice show a heightened innate immune response after induction of acute peritonitis with Klebsiella pneumoniae. This increased response is consistent with an increased bacterial burden in the Onzin(-/-) mice. Ex vivo studies show that, whereas phagocytosis is not altered in Onzin(-/-) neutrophils, phagocytes lacking this protein kill bacteria less effectively. This result identifies ONZIN as a novel class of intracellular protein required for optimal function of the neutrophils after uptake of bacteria.

Enhanced mast cell activation in mice deficient in the A2b adenosine receptor

Antigen-mediated cross-linking of IgE bound to mast cells via the high affinity receptor for IgE triggers a signaling cascade that results in the release of intracellular calcium stores, followed by an influx of extracellular calcium. The collective increase in intracellular calcium is critical to the release of the granular contents of the mast cell, which include the mediators of acute anaphylaxis. We show that the sensitivity of the mast cell to antigen-mediated degranulation through this pathway can be dramatically influenced by the A2b adenosine receptor. Loss of this G_s-coupled receptor on mouse bone marrow–derived mast cells results in decreased basal levels of cyclic AMP and an excessive influx of extracellular calcium through store-operated calcium channels following antigen activation. Mice lacking the A2b receptor display increased sensitivity to IgE-mediated anaphylaxis. Collectively, these findings show that the A2b adenosine receptor functions as a critical regulator of signaling pathways within the mast cell, which act in concert to limit the magnitude of mast cell responsiveness when antigen is encountered.

Vacuolar-type H+-ATPases at the plasma membrane regulate pH and cell migration in microvascular endothelial cells

Microvascular endothelial cells involved in angiogenesis are exposed to an acidic environment that is not conducive for growth and survival. These cells must exhibit a dynamic intracellular (cytosolic) pH (pHcyt) regulatory mechanism to cope with acidosis, in addition to the ubiquitous Na+/H+exchanger and HCO3−-based H+-transporting systems. We hypothesize that the presence of plasmalemmal vacuolar-type proton ATPases (pmV-ATPases) allows microvascular endothelial cells to better cope with this acidic environment and that pmV-ATPases are required for cell migration. This study indicates that microvascular endothelial cells, which are more migratory than macrovascular endothelial cells, express pmV-ATPases. Spectral imaging microscopy indicates a more alkaline pHcytat the leading than at the lagging edge of microvascular endothelial cells. Treatment of microvascular endothelial cells with V-ATPase inhibitors decreases the proton fluxes via pmV-ATPases and cell migration. These data suggest that pmV-ATPases are essential for pHcytregulation and cell migration in microvascular endothelial cells.

Cholesterol deficiency in a mouse model of Smith-Lemli-Opitz syndrome reveals increased mast cell responsiveness

Mutation of the 3β-hydroxysterol Δ⁷-reductase gene (Dhcr7^−/−) results in Smith-Lemli-Opitz syndrome (SLOS). Patients, and genetically altered mice, are unable to produce cholesterol and accumulate 7-dehydrocholesterol (DHC) in serum and tissue. This causes multiple growth and developmental abnormalities as well as immune system anomalies including allergy. Because cholesterol is a key component of liquid-ordered membranes (lipid rafts) and these domains have been implicated in regulating mast cell activation, we examined whether mast cell responsiveness is altered in this model. Mast cells derived from Dhcr7^−/− mice (DHCR KO) showed constitutive cytokine production and hyper-degranulation after stimulation of the high affinity IgE receptor (FcɛRI). DHCR KO mast cells, but not wild-type mast cells, accumulated DHC in lipid rafts. DHC partially disrupted lipid raft stability and displaced Lyn kinase protein and activity from lipid rafts. This led to down-regulation of some Lyn-dependent signaling events but increased Fyn kinase activity and Akt phosphorylation. The Lyn-dependent phosphorylation of Csk-binding protein, which negatively regulates Fyn activity, was decreased. This phenotype reproduces some of the characteristics of Lyn-null mast cells, which also demonstrate hyper-degranulation. These findings provide the first evidence of lipid raft dysfunction in SLOS and may explain the observed association of allergy with SLOS.

Spectral imaging microscopy demonstrates cytoplasmic pH oscillations in glial cells

Glial cells exhibit distinct cellular domains, somata, and filopodia. Thus the cytoplasmic pH (pH(cyt)) and/or the behavior of the fluorescent ion indicator might be different in these cellular domains because of distinct microenvironments. To address these issues, we loaded C6 glial cells with carboxyseminaphthorhodafluor (SNARF)-1 and evaluated pH(cyt) using spectral imaging microscopy. This approach allowed us to study pH(cyt) in discrete cellular domains with high temporal, spatial, and spectral resolution. Because there are differences in the cell microenvironment that may affect the behavior of SNARF-1, we performed in situ titrations in discrete cellular regions of single cells encompassing the somata and filopodia. The in situ titration parameters apparent acid-base dissociation constant (pK'(a)), maximum ratio (R(max)), and minimum ratio (R(min)) had a mean coefficient of variation approximately six times greater than those measured in vitro. Therefore, the individual in situ titration parameters obtained from specific cellular domains were used to estimate the pH(cyt) of each region. These studies indicated that glial cells exhibit pH(cyt) heterogeneities and pH(cyt) oscillations in both the absence and presence of physiological HCO(3)(-). The amplitude and frequency of the pH(cyt) oscillations were affected by alkalosis, by acidosis, and by inhibitors of the ubiquitous Na(+)/H(+) exchanger- and HCO(3)(-)-based H(+)-transporting mechanisms. Optical imaging approaches used in conjunction with BCECF as a pH probe corroborated the existence of pH(cyt) oscillations in glial cells.

Augmentation increases vesicular release probability in the presence of masking depression at the frog neuromuscular junction

Synaptic augmentation is a short-term component of synaptic plasticity that increases transmitter release during repetitive stimulation and decays thereafter with a time constant of approximately 7 sec. Augmentation has typically been observed under conditions where there is little or no depression because of depletion of synaptic vesicles from the readily releasable pool (RRP) of transmitter. We now study augmentation under conditions of pronounced depression at the frog neuromuscular junction to gain additional insight into mechanism. If augmentation reflects an increase in the size of the RRP of transmitter, then augmentation should not be present with depression. Our findings using four different experimental approaches suggested that augmentation was still present in the presence of pronounced depression: mathematical extraction of augmentation from the changes in transmitter release after repetitive stimulation, identification of augmentation with Ba2+, correction of the data for the measured depletion of the RRP, and identification of an augmentation component of residual Ca2+. We conclude that the augmentation machinery still acts to increase transmitter release when depression reduces the RRP sufficiently to mask obvious augmentation. The masked augmentation was found to increase transmitter release by increasing the probability of releasing individual vesicles from the depressed RRP, countering the effects of depression. Because augmentation and depression have similar time courses, either process can mask the other, depending on their relative magnitudes. Consequently, the apparent absence of one of the processes does not exclude that it is still contributing to short-term synaptic plasticity.

Monitoring Cytosolic Calcium in the Dinoflagellate Crypthecodinium cohnii with Calcium Orange-AM

Calcium plays several important roles in the signal transduction pathways of dinoflagellates. We describe here the development of calcium orange-AM as an intracellular calcium reporter for the heterotrophic dinoflagellate Crypthecodinium cohnii. We demonstrated with confocal microscopy that by restricting the incubation period to 30-45 min, no compartmentalization of the dye occurs in the mitochondria or endoplasmic reticulum. The dye fluorescence responded well to the effects of calcium ionophores and calcium chelators. By calibrating the dye with known calcium concentrations, we determined the intracellular calcium concentration of C. cohnii to be 158 +/- 56 nM, which rose to about 550 nM upon mechanical stimulation.

Na-K-Cl cotransporter-mediated intracellular Na+ accumulation affects Ca2+ signaling in astrocytes in an in vitro ischemic model

Na-K-Cl cotransporter isoform 1 (NKCC1) plays an important role in maintenance of intracellular Na+, K+, and Cl- levels in astrocytes. We propose that NKCC1 may contribute to perturbations of ionic homeostasis in astrocytes under ischemic conditions. After 3-8 hr of oxygen and glucose deprivation (OGD), NKCC1-mediated 86Rb influx was significantly increased in astrocytes from NKCC1 wild-type (NKCC1+/+) and heterozygous mutant (NKCC1+/-) mice. Phosphorylated NKCC1 protein was increased in NKCC1+/+ astrocytes at 2 hr of OGD. Two hours of OGD and 1 hr of reoxygenation (OGD/REOX) triggered an 3.6-fold increase in intracellular Na+ concentration ([Na+]i) in NKCC1+/+ astrocytes. Inhibition of NKCC1 activity by bumetanide or ablation of the NKCC1 gene significantly attenuated the rise in [Na+]i. Moreover, NKCC1+/+ astrocytes swelled by 10-30% during 20-60 min of OGD. Either genetic ablation of NKCC1 or inhibition of NKCC1 by bumetanide-attenuated OGD-mediated swelling. An NKCC1-mediated increase in [Na+]i may subsequently affect Ca2+ signaling through the Na+/Ca2+ exchanger (NCX). A rise in [Ca2+]i was detected after OGD/REOX in the presence of a sarcoplasmic-endoplasmic reticulum (ER) Ca2+-ATPase inhibitor thapsigargin. Moreover, OGD/REOX led to a significant increase in Ca2+ release from ER Ca2+ stores. Furthermore, KB-R7943 (2-[2-[4(4-nitrobenzyloxy)phenyl]ethyl]isothiourea mesylate), an inhibitor of reverse-mode operation of NCX, abolished the OGD/REOX-induced enhancement in filling of ER Ca2+ stores. OGD/REOX-mediated Ca2+ accumulation in ER Ca2+ stores was absent when NKCC1 activity was ablated or pharmacologically inhibited. These findings imply that stimulation of NKCC1 activity leads to Na+ accumulation after OGD/REOX and that subsequent reverse-mode operation of NCX contributes to increased Ca2+ accumulation by intracellular Ca2+ stores.

Theoretical analysis on ratiometric fluorescent indicators caused biased estimates of intracellular free calcium concentrations

Ratiometric fluorescent calcium indicator dyes have been widely used for the study of the role of Ca2+ in cell physiopathology. Although these ratiometric dyes offer several advantages over others, they suffer some drawbacks which cause serious errors in measurement of intracellular Ca2+ concentration, [Ca2+]i. The present study systematically analyzes theoretical reasons and technical sources of discrepancies occurring in the measurement of the characteristics of the agonists-induced cells [Ca2+]i. In order to avoid the errors and achieve the accurate determination of [Ca2+]i, this study proposes solutions and suggests some critical measures in both theoretical and technical aspects. Therefore, this analysis can be a valuable tool in clarifying proper usages of fluorescent dyes for [Ca2+]i measurements.

Dexamethasone Suppresses Antigen-Induced Activation of Phosphatidylinositol 3-Kinase and Downstream Responses in Mast Cells

Dexamethasone and other glucocorticoids suppress FcepsilonRI-mediated release of inflammatory mediators from mast cells. Suppression of cytokine production is attributed to repression of cytokine gene transcription but no mechanism has been described for the suppression of degranulation. We show that therapeutic concentrations of dexamethasone inhibit intermediate signaling events, in particular the activation of phosphatidylinositol (PI)3-kinase and downstream signaling events that lead to degranulation in rat basophilic leukemia 2H3 cells. This inhibitory action is mediated via the glucocorticoid receptor and is not apparent when cells are stimulated via Kit in a mouse bone marrow-derived mast cell line. The primary perturbation appears to be the failure of the regulatory p85 subunit of PI3-kinase to engage with the adaptor protein Grb2-associated binder 2 leading to suppression of phosphorylation of phospholipase Cgamma2, the calcium signal, and degranulation. Suppression of PI3-kinase activation by dexamethasone may also contribute to reduced cytokine production because the PI3-kinase inhibitor LY294002, like dexamethasone, inhibits Ag-induced transcription of cytokine genes as well as degranulation.

Calcium and Its Role in the Nuclear Translocation and Activation of Cytosolic Phospholipase A2in Cells Rendered Sensitive to TNF-Induced Apoptosis by Cycloheximide

In these experiments, we investigated the role of calcium as a second messenger in the apoptotic activation of cytosolic phospholipase A(2) (cPLA(2)). As our model, we used a murine fibroblast cell line (C3HA) that was induced to undergo apoptosis by a combination of TNF and cycloheximide. Using fura 2 Ca(2+) imaging, we found strong evidence for an intracellular calcium response after 1 h of treatment, which correlated with the onset of phosphatidylserine externalization, but preceded effector procaspase processing by several hours. The response was strongest in the perinuclear region, where mean levels rose 83% (144 +/- 14 nM in untreated cells vs 264 +/- 39 nM in treated), while cells displaying morphological evidence of apoptosis had the highest levels of calcium (250-1000 nM). Verapamil blocked this response, indicating an extracellular source for the calcium. Fluorescence microscopy revealed a pattern of nuclear translocation of cPLA(2) during apoptosis, which was also blocked by verapamil, indicating an important role for calcium in this process. In addition, we found that verapamil prevented the release of [(3)H]arachidonic acid from C3HA cells induced to undergo apoptosis by the chemotherapeutic agents vinblastine, melphalan, and cis-platinum. Together, these data suggest that calcium is important for cPLA(2) activation by diverse apoptotic stimuli.

P/Q Ca2+ channel blockade stops spreading depression and related pyramidal neuronal Ca2+ rise in hippocampal organ culture

Ca2+ channels and pyramidal cell Ca2+ are involved in hippocampal spreading depression (SD), but their roles remain elusive. Accordingly, we characterized Ca2+ changes during SD in CA3 pyramidal neurons and determined whether Ca2+ channel antagonists could prevent SD. SD was induced in hippocampal organotypic cultures (HOTCs), in which experimental conditions can be rigorously controlled. SD was triggered by transient exposure to sodium acetate (NaAc)-based Ringer's coupled to an electrical pulse in the dentate gyrus and its occurrence confirmed with interstitial DC recordings. Pyramidal cell Ca2+ was measured with fura-2 filled cells and was quantified at the soma, proximal and more distal apical dendrites. Regional Ca2+ changes began simultaneously with the triggering pulse of SD and reached three distinct peaks before returning to baseline concomitant with the interstitial DC potential of SD. The first peak occurred within 5 s of the triggering pulse, was smallest, and heralded the onset of SD. The second Ca2+ change was the greatest and reached a peak 6 s later, during the early phase of SD. The third was intermediate in size and occurred 18 s later, as SD reached its maximum interstitial DC change. SD was prevented by nonselective Ca2+ blockade (Ni2+ and Cd2+) but not by either L-Ca2+ channel (nifedipine) or N-Ca2+ channel inhibition (omega-conotoxin GVIA). Importantly, SD was blocked by P/Q Ca2+ channel antagonism (omega-agatoxin-IVA), which also prompted a significant reduction in pyramidal cell Ca2+ change and hyperexcitability. These results show that the spatiotemporal pattern of pyramidal cell Ca2+ change with SD is multiphasic; they provide further evidence that these changes begin before electrophysiologic evidence of SD. Furthermore, they show that P/Q Ca2+ channel antagonism can prevent SD in HOTCs and it appears to do so by preventing the NaAc-induced increased pyramidal cell excitability from NaAc exposure, which may involve altered GABAergic transmission.

Do we know the absolute values of intracellular free calcium concentration?

More than 20 years ago, it was shown that the addition of EGTA increases the affinity of the plasma membrane Ca2+ pump for Ca2+ by an order of magnitude. The left-hand shift of Ca2+-dependencies in the presence of EGTA has been also documented in studies of the sarcoplasmic reticulum Ca2+ pump, mitochondrial Ca2+-transporter as well as Ca2+-binding by calmodulin and troponin C. These data allow us to hypothesise that this effect is caused by an admixture of di- and trivalent cations possessing high affinity for EGTA and interacting with Ca2+-transporting and binding proteins. Here, we propose that polyvalent cations affect the estimation of absolute values of free intracellular Ca2+ concentration. Indeed, EGTA sharply increases the apparent affinity of the fluorescent Ca2+ indicators quin-2 and fluo-3 for Ca2+. The impact of polyvalent cations on Ca2+ measurement was further confirmed by the study showing the high sensitivity of Ca2+-induced fluo-3 fluorescence to Mn2+, Fe2+, Cu2+, and Co2+ seen in the absence of EGTA.

Fast calcium signals in Drosophila motor neuron terminals

Drosophila is a powerful model for neuroscientists, but physiological techniques have not kept pace with advances in molecular genetics. We introduce a reliable assay for intracellular calcium dynamics in Drosophila larval motor neuron terminals, and a new physiological solution that improves the longevity of the larval preparation. By loading calcium indicators into motor neuron terminals through cut axons, we obtained a high signal-to-noise ratio with confocal microscopy, and good temporal resolution of calcium-dependent fluorescence changes. We provide an estimate for the resting intracellular calcium concentration, the first description of calcium kinetics for a single action potential (AP), and improved resolution of calcium kinetics during AP trains. The very rapid decay of the calcium signal following a single AP (tau ~60 ms) indicates a previously unreported fast calcium extrusion mechanism in Drosophila motor neuron terminals well suited for sustaining physiological processes during the high rates of impulse activity which drive locomotor activity.

Calcium channel antagonist effect on in vitro meningioma signal transduction pathways after growth factor stimulation

OBJECTIVE

We have previously demonstrated that calcium channel antagonists inhibit the growth of human meningiomas in culture after stimulation with growth factors. This study examined the effects of these drugs on signaling transduction pathways in an attempt to elucidate potential mechanisms by which this growth inhibition is mediated.

METHODS

Primary cell cultures from patients with intracranial meningiomas were established. Cell growth studies were performed with inhibitors and stimulators of tyrosine kinase signal transduction. Intracellular calcium changes and inositol phosphate production were measured after growth factor exposure, with or without pretreatment by calcium channel antagonists.

RESULTS

The growth of meningiomas in culture can be inhibited by tyrosine kinase receptor inhibitors. Inhibitors and stimulators of phospholipase C can stimulate or inhibit the growth of in vitro meningiomas, respectively. Calcium channel antagonists inhibit intracellular calcium changes induced by serum and epidermal growth factor. Inositol phosphate production is increased after growth factor stimulation, and calcium channel antagonists potentiate this effect.

CONCLUSION

Calcium channel antagonists interfere with intracellular signaling pathways of cultured meningioma cells. This inhibition is unrelated to voltage-sensitive calcium channels. The findings of this project may aid in the understanding of the signal transduction mechanisms involved in growth factor-mediated meningioma proliferation and may lead to clinically relevant strategies for growth inhibition.

PLA2 activity regulates Ca2+ storage-dependent cellular proliferation

The objective of this study is to determine the role of arachidonic acid (AA) in cell proliferation by inhibiting AA synthetic enzyme phospholipase A2 (PLA2) and to determine its involvement in the role of the second messenger intracellular calcium (Ca2+). Methods used to determine the effects on proliferation of cell cultures of primary meningioma and astrocytoma U373-MG included treatment with micromolar concentrations of PLA2 inhibitors 4-bromophenacylbromide and quinacrine. Effects of these drugs on proliferation were further investigated by the application of concentrations that inhibit growth by 50% while antagonizing these agents with AA replacement. Free cytosolic Ca2+ was measured with the use of fluorescent dye Fura-2 during PLA2 agonist/antagonist studies. These Ca2+ measurements were performed in the absence of extracellular Ca2+ to identify the contribution of intracellular Ca2+ sources. PLA2 inhibition resulted in decreased growth of cultured astrocytoma and meningioma cells in a dose-dependent manner in the micromolar range. This inhibitory effect was antagonized by the addition of AA. PLA2 inhibition caused an elevation of basal-cytosolic-free [Ca2+] while depleting internal Ca2+ stores. These Ca2+ changes were also antagonized by the addition of AA. In conclusion, these results demonstrate that AA, a PLA2 enzyme product, is involved in regulating the growth rate of these cell types. The PLA2 pathway also regulates the maintenance of the internal Ca2+ stores. Ca2+ is known to be a growth-related intracellular second messenger. These results suggest that the growth regulatory functions of AA are mediated by Ca2+-dependent mechanisms.

Measuring [Ca2+] with fluorescent indicators: theoretical approach to the ratio method

In this work we present a theoretical analysis of the ratio method, a widely used technique for measuring intracellular calcium concentration, [Ca2+]i, in isolated cells. From the ratio of fluorescence measured at two different excitation or emission wavelengths, [Ca2+]i may be estimated from the equation: [Ca2+]i = Kd.beta.(R-Rmin)/(Rmax-R). From this equation we determined the method sensitivity showing that its maximum is located at [Ca2+] = Kd.beta.(Rmin/Rmax)1/2, i.e. for [Ca2+] < Kd.beta. We also analyzed the error propagation due to inaccuracies in the calibration parameters. The fluorescence phenomenon was described, aiming at providing a basis for the microscopic interpretation of the method and giving physical meaning to the calibration parameters. In this sense beta, is shown to depend not only on the set-up, but also on the spectrum of the indicator for the particular sample studied. A new approach to estimate beta with higher accuracy is also proposed. Experimentally obtained beta values using this approach were not statistically different from those determined as Fmin2/Fmax2. A graphical interpretation of the method is presented to provide users of fluorescence systems with a simple technique to help understand equipment performance and design.