Characteristics of membrane currents evoked by photoreleased inositol trisphosphate in Xenopus oocytes

Noise analysis of cytosolic calcium image data

Cellular Ca²⁺ signals are often constrained to cytosolic micro- or nano-domains where stochastic openings of Ca²⁺ channels cause large fluctuations in local Ca²⁺ concentration (Ca²⁺ 'noise'). With the advent of TIRF microscopy to image the fluorescence of Ca²⁺-sensitive probes from attoliter volumes it has become possible to directly monitor these signals, which closely track the gating of plasmalemmal and ER Ca²⁺-permeable channels. Nevertheless, it is likely that many physiologically important Ca²⁺ signals are too small to resolve as discrete events in fluorescence recordings. By analogy with noise analysis of electrophysiological data, we explore here the use of statistical approaches to detect and analyze such Ca²⁺ noise in images obtained using Ca²⁺-sensitive indicator dyes. We describe two techniques - power spectrum analysis and spatio-temporal correlation - and demonstrate that both effectively identify discrete, spatially localized calcium release events (Ca²⁺ puffs). Moreover, we show they are able to detect localized noise fluctuations in a case where discrete events cannot directly be resolved.

All three IP3 receptor isoforms generate Ca2+ puffs that display similar characteristics

Inositol 1,4,5-trisphosphate (IP₃) evokes Ca²⁺ release through IP₃ receptors (IP₃Rs) to generate both local Ca²⁺ puffs arising from concerted openings of clustered IP₃Rs and cell-wide Ca²⁺ waves. Imaging Ca²⁺ puffs with single-channel resolution yields information on the localization and properties of native IP₃Rs in intact cells, but interpretation has been complicated because cells express varying proportions of three structurally and functionally distinct isoforms of IP₃Rs. Here, we used TIRF and light-sheet microscopy to image Ca²⁺ puffs in HEK-293 cell lines generated by CRISPR-Cas9 technology to express exclusively IP₃R type 1, 2, or 3. Photorelease of the IP₃ analog i-IP₃ in all three cell lines evoked puffs with largely similar mean amplitudes, temporal characteristics, and spatial extents. Moreover, the single-channel Ca²⁺ flux was similar among isoforms, indicating that clusters of different IP₃R isoforms contain comparable numbers of active channels. Our results show that all three IP₃R isoforms cluster to generate local Ca²⁺ puffs and, contrary to findings of divergent properties from in vitro electrophysiological studies, display similar conductances and gating kinetics in intact cells.

Picomolar sensitivity to inositol trisphosphate in Xenopus oocytes

Ca(2+) liberation from the endoplasmic reticulum mediated by inositol trisphosphate receptor/channels (IP3Rs) in response to production of the second messenger IP3 regulates numerous signaling pathways. However, estimates of resting and physiologically relevant cytosolic concentrations of IP3 vary appreciably. Here we directly address this question, taking advantage of the large size of Xenopus oocytes to image Ca(2+) liberation evoked by bolus intracellular injections of known concentrations of IP3. Our principal finding is that IP3 evokes both global and local Ca(2+) signals in freshly isolated oocytes at concentrations as low as a few pM. A corollary is that basal, resting [IP3] must be even lower, given the absence of detectable Ca(2+) signals before injection. The dose/response curve for IP3-activation of Ca(2+) liberation suggests that freshly isolated oocytes express two distinct functional populations of IP3 receptors with EC50 values around 200 pM and tens of nM, whereas the high-affinity receptors are not apparent in oocytes examined later than about 3 days after isolation from the ovary.

Differential role of STIM1 and STIM2 during transient inward (T in) current generation and the maturation process in the Xenopus oocyte

BACKGROUND

The Xenopus oocyte is a useful cell model to study Ca2+ homeostasis and cell cycle regulation, two highly interrelated processes. Here, we used antisense oligonucleotides to investigate the role in the oocyte of stromal interaction molecule (STIM) proteins that are fundamental elements of the store-operated calcium-entry (SOCE) phenomenon, as they are both sensors for Ca2+ concentration in the intracellular reservoirs as well as activators of the membrane channels that allow Ca2+ influx.

RESULTS

Endogenous STIM1 and STIM2 expression was demonstrated, and their synthesis was knocked down 48-72 h after injecting oocytes with specific antisense sequences. Selective elimination of their mRNA and protein expression was confirmed by PCR and Western blot analysis, and we then evaluated the effect of their absence on two endogenous responses: the opening of SOC channels elicited by G protein-coupled receptor (GPCR)-activated Ca2+ release, and the process of maturation stimulated by progesterone. Activation of SOC channels was monitored electrically by measuring the T in response, a Ca2+-influx-dependent Cl- current, while maturation was assessed by germinal vesicle breakdown (GVBD) scoring and electrophysiology.

CONCLUSIONS

It was found that STIM2, but not STIM1, was essential in both responses, and T in currents and GVBD were strongly reduced or eliminated in cells devoid of STIM2; STIM1 knockdown had no effect on the maturation process, but it reduced the T in response by 15 to 70%. Thus, the endogenous SOCE response in Xenopus oocytes depended mainly on STIM2, and its expression was necessary for entry into meiosis induced by progesterone.

Enhanced ER Ca2+ store filling by overexpression of SERCA2b promotes IP3-evoked puffs

Liberation of Ca(2+) from the endoplasmic reticulum (ER) through inositol trisphosphate receptors (IP(3)R) is modulated by the ER Ca(2+) content, and overexpression of SERCA2b to accelerate Ca(2+) sequestration into the ER has been shown to potentiate the frequency and amplitude of IP(3)-evoked Ca(2+) waves in Xenopus oocytes. Here, we examined the effects of SERCA overexpression on the elementary IP(3)-evoked puffs to elucidate whether ER [Ca(2+)] may modulate IP(3)R function via luminal regulatory sites in addition to simply determining the size of the available store and electrochemical driving force for Ca(2+) release. SERCA2b and Ca(2+) permeable nicotinic plasmalemmal channels were expressed in oocytes, and hyperpolarizing pulses were delivered to induce Ca(2+) influx and thereby load ER stores. Puffs evoked by photoreleased IP(3) were significantly potentiated in terms of numbers of responding sites, frequency and amplitude following transient Ca(2+) influx in SERCA-overexpressing cells, whereas little change was evident with SERCA overexpression alone or following Ca(2+) influx in control cells not overexpressing SERCA. Intriguingly, we observed the appearance of a new population of puffs that arose after long latencies and had prolonged durations supporting the notion of luminal regulation of IP(3)R gating kinetics.

A novel postsynaptic mechanism for heterosynaptic sharing of short-term plasticity

Postsynaptic release of Ca(2+) from intracellular stores is an important means of cellular signaling that mediates numerous forms of synaptic plasticity. Previous studies have identified a postsynaptic intracellular Ca(2+) requirement for a form of short-term plasticity, post-tetanic potentiation (PTP) at sensory neuron (SN)-motor neuron synapses in Aplysia. Here, we show that postsynaptic IP(3)-mediated Ca(2+) release in response to a presynaptic tetanus in an SN that induces PTP can confer transient plasticity onto a neighboring SN synapse receiving subthreshold activation. This heterosynaptic sharing of plasticity represents a dynamic, short-term synaptic enhancement of synaptic inputs onto a common postsynaptic target. Heterosynaptic sharing is blocked by postsynaptic disruption of Ca(2+)- and IP(3)-mediated signaling, and, conversely, it is mimicked by postsynaptic injection of nonhydrolyzable IP(3), and by photolysis of caged IP(3) in the MN. The molecular mechanism for heterosynaptic sharing involves metabotropic glutamate receptors and Homer-dependent interactions, indicating that Homer can facilitate the integration of Ca(2+)-dependent plasticity at neighboring postsynaptic sites and provides a postsynaptic mechanism for the spread of plasticity induced by presynaptic activation. Our results support a model in which postsynaptic summation of IP(3) signals from suprathreshold and subthreshold inputs results in molecular coincidence detection that gives rise to a novel form of heterosynaptic plasticity.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

Use of Fluorescence Resonance Energy Transfer-based Biosensors for the Quantitative Analysis of Inositol 1,4,5-Trisphosphate Dynamics in Calcium Oscillations

Inositol 1,4,5-trisphosphate (IP(3)) is an intracellular messenger that elicits a wide range of spatial and temporal Ca(2+) signals, and this signaling versatility is exploited to regulate diverse cellular responses. In this study, we have developed a series of IP(3) biosensors that exhibit strong pH stability and varying affinities for IP(3), as well as a method for the quantitative measurement of cytosolic concentrations of IP(3) ([IP(3)](i)) in single living cells. We applied this method to elucidate IP(3) dynamics during agonist-induced Ca(2+) oscillations, and we demonstrated cell type-dependent differences in IP(3) dynamics, a nonfluctuating rise in [IP(3)](i) and repetitive IP(3) spikes during Ca(2+) oscillations in COS-7 cells and HSY-EA1 cells, respectively. The size of the IP(3) spikes in HSY-EA1 cells varied from 10 to 100 nm, and the [IP(3)](i) spike peak was preceded by a Ca(2+) spike peak. These results suggest that repetitive IP(3) spikes in HSY-EA1 cells are passive reflections of Ca(2+) oscillations, and are unlikely to be essential for driving Ca(2+) oscillations. In addition, the interspike periods of Ca(2+) oscillations that occurred during the slow rise in [IP(3)](i) were not shortened by the rise in [IP(3)](i), indicating that IP(3)-dependent and -independent mechanisms may regulate the frequency of Ca(2+) oscillations. The novel method described herein as well as the quantitative information obtained by using this method should provide a valuable and sound basis for future studies on the spatial and temporal regulations of IP(3) and Ca(2+).

Calcium puffs in Xenopus oocytes

The second messenger inositol 1,4,5-trisphosphate (InsP3) functions in large part by liberating calcium ions from intracellular stores. This release process is highly non-linear and shows a regenerative characteristic that allows production of all-or-none calcium spikes which propagate as waves. However, at low concentrations of InsP3 an additional mode of calcium liberation is seen in Xenopus oocytes, transient 'puffs' of cytosolic calcium that last for a few hundred milliseconds and are restricted to within a few micrometres. Puffs are generally of similar size and the amount of calcium released (about 3 x 10(-18) mol) suggests that they arise through the concerted opening of several InsP3-gated calcium release channels. Puff sites are present at a density of about one per 30 microns 2 in the animal hemisphere of the oocyte. Each site functions autonomously, producing puffs at largely random intervals. We conclude that calcium puffs represent 'quantal' units of InsP3-evoked calcium liberation, which may result from local regenerative feedback by cytosolic calcium ions at functionally discrete release sites.

Enhanced ryanodine receptor recruitment contributes to Ca2+ disruptions in young, adult, and aged Alzheimer's disease mice

Neuronal Ca2+ signaling through inositol triphosphate receptors (IP3R) and ryanodine receptors (RyRs) must be tightly regulated to maintain cell viability, both acutely and over a lifetime. Exaggerated intracellular Ca2+ levels have been associated with expression of Alzheimer's disease (AD) mutations in young mice, but little is known of Ca2+ dysregulations during normal and pathological aging processes. Here, we used electrophysiological recordings with two-photon imaging to study Ca2+ signaling in nontransgenic (NonTg) and several AD mouse models (PS1KI, 3xTg-AD, and APPSweTauP301L) at young (6 week), adult (6 months), and old (18 months) ages. At all ages, the PS1KI and 3xTg-AD mice displayed exaggerated endoplasmic reticulum (ER) Ca2+ signals relative to NonTg mice. The PS1 mutation was the predominant "calciopathic" factor, because responses in 3xTg-AD mice were similar to PS1KI mice, and APPSweTauP301L mice were not different from controls. In addition, we uncovered powerful signaling interactions and differences between IP3R- and RyR-mediated Ca2+ components in NonTg and AD mice. In NonTg mice, RyR contributed modestly to IP3-evoked Ca2+, whereas the exaggerated signals in 3xTg-AD and PS1KI mice resulted primarily from enhanced RyR-Ca2+ release and were associated with increased RyR expression across all ages. Moreover, IP3-evoked membrane hyperpolarizations in AD mice were even greater than expected from exaggerated Ca2+ signals, suggesting increased coupling efficiency between cytosolic [Ca2+] and K+ channel regulation. We conclude that lifelong ER Ca2+ disruptions in AD are related to a modulation of RyR signaling associated with PS1 mutations and represent a discrete "calciumopathy," not merely an acceleration of normal aging.

Analysis of puff dynamics in oocytes: interdependence of puff amplitude and interpuff interval

Puffs are localized Ca(2+) signals that arise in oocytes in response to inositol 1,4,5-trisphosphate (IP(3)). They are analogous to the sparks of myocytes and are believed to be the result of the liberation of Ca(2+) from the endoplasmic reticulum through the coordinated opening of IP(3) receptor/channels clustered at a functional release site. In this article, we analyze sequences of puffs that occur at the same site to help elucidate the mechanisms underlying puff dynamics. In particular, we show a dependence of the interpuff time on the amplitude of the preceding puff, and of the amplitude of the following puff on the preceding interval. These relationships can be accounted for by an inhibitory role of the Ca(2+) that is liberated during puffs. We construct a stochastic model for a cluster of IP(3) receptor/channels that quantitatively replicates the observed behavior, and we determine that the characteristic time for a channel to escape from the inhibitory state is of the order of seconds.

An inwardly rectifying whole cell current induced by Gq-coupled receptors

Ca(2+) influx across the plasma membrane after stimulation of G protein-coupled receptors is important for many physiological functions. Here we studied the regulation of an inwardly rectifying whole cell current and its putative role in Ca(2+) entry in Xenopus oocytes. Expression of P2Y(1) or M1 receptors in Xenopus oocytes elicited a characteristic inwardly rectifying current without receptor stimulation. This current displayed distinct activation and inactivation kinetics and was highly Ca(2+)-dependent. After stimulation of endogenous G(q)-coupled receptors in water-injected cells similar currents were observed. We therefore speculated that the current could be activated via Ca(2+) store depletion induced by constitutive stimulation of the IP(3) cascade in cells overexpressing G(q)-coupled receptors. Receptor-independent Ca(2+) store depletion also induced the current. In conclusion, this current is activated after store depletion suggesting a role in Ca(2+) entry after stimulation of G(q)-coupled receptors. Finally, our data do not support the proposed ionotropic properties of the P2Y(1) receptor.

Fluorescent biosensor for quantitative real-time measurements of inositol 1,4,5-trisphosphate in single living cells

The second messenger inositol 1,4,5-trisphosphate (IP(3)) plays a central role in the generation of a variety of spatiotemporally complex intracellular Ca(2+) signals involved in the regulation of many essential physiological processes. Here we describe the development of "LIBRA", a novel ratiometric fluorescent IP(3) biosensor that allows for the quantitative monitoring of intracellular IP(3) concentrations in single living cells in real time. LIBRA consists of the IP(3)-binding domain of the rat type 3 IP(3) receptor fused between the fluorescence resonance energy transfer pair cyan fluorescent protein and yellow fluorescent protein and preceded by a membrane-targeting signal. We show that the LIBRA fluorescent signal is highly selective for IP(3) and unaffected by concentrations of Ca(2+) and ATP in the physiological range. In addition, LIBRA can be calibrated in situ. We demonstrate the utility of LIBRA by monitoring the temporal relationship between the responses intracellular IP(3) and Ca(2+) concentrations in SH-SY5Y cells following acetylcholine stimulation.

Dysregulated IP3 signaling in cortical neurons of knock-in mice expressing an Alzheimer's-linked mutation in presenilin1 results in exaggerated Ca2+ signals and altered membrane excitability

Disruptions in intracellular Ca2+ signaling are proposed to underlie the pathophysiology of Alzheimer's disease (AD), and it has recently been shown that AD-linked mutations in the presenilin 1 gene (PS1) enhance inositol triphosphate (IP3)-mediated Ca2+ liberation in nonexcitable cells. However, little is known of these actions in neurons, which are the principal locus of AD pathology. We therefore sought to determine how PS1 mutations affect Ca2+ signals and their subsequent downstream effector functions in cortical neurons. Using whole-cell patch-clamp recording, flash photolysis, and two-photon imaging in brain slices from 4-5-week-old mice, we show that IP3-evoked Ca2+ responses are more than threefold greater in PS1(M146V) knock-in mice relative to age-matched nontransgenic controls. Electrical excitability is thereby reduced via enhanced Ca2+ activation of K+ conductances. Action potential-evoked Ca2+ signals were unchanged, indicating that PS1(M146V) mutations specifically disrupt intracellular Ca2+ liberation rather than reduce cytosolic Ca2+ buffering or clearance. Moreover, IP3 receptor levels are not different in cortical homogenates, further suggesting that the exaggerated cytosolic Ca2+ signals may result from increased store filling and not from increased flux through additional IP3-gated channels. Even in young animals, PS1 mutations have profound effects on neuronal Ca2+ and electrical signaling: cumulatively, these disruptions may contribute to the long-term pathophysiology of AD.

Ca2+ signaling in mouse cortical neurons studied by two-photon imaging and photoreleased inositol triphosphate

IP(3)-mediated Ca(2+) release is a crucial neuronal signaling mechanism that has not been extensively characterized in the mammalian cerebral cortex. We used two-photon, video-rate microscopy to image Ca(2+) signals evoked by photoreleased IP(3) in pyramidal neurons of mouse prefrontal cortex. Ca(2+) responses to photoreleased IP(3) varied greatly between different neurons; however, within IP(3)-responsive neurons, the soma invariably showed highest sensitivity, with signals increasing nonlinearly with [IP(3)]. Responses to paired photorelease displayed inhibition, whereas IP(3)-evoked Ca(2+) liberation was potentiated by Ca(2+) entry during action potentials and vice versa. IP(3)-mediated Ca(2+) signals strongly inhibited spike firing through activation of K(+) membrane conductance. Metabotropic signaling via the phosphoinositide pathway thus serves as a powerful and sustained modulator of excitability in cortical neurons and displays complex reciprocal interactions between electrical and chemical signals.

Phasic characteristic of elementary Ca(2+) release sites underlies quantal responses to IP(3)

Ca(2+) liberation by inositol 1,4,5-trisphosphate (IP(3)) is 'quantal', in that low [IP(3)] causes only partial Ca(2+) release, but further increasing [IP(3)] evokes more release. This characteristic allows cells to generate graded Ca(2+) signals, but is unexpected, given the regenerative nature of Ca(2+)-induced Ca(2+) release through IP(3) receptors. Two models have been proposed to resolve this paradox: (i) all-or-none Ca(2+) release from heterogeneous stores that empty at varying [IP(3)]; and (ii) phasic liberation from homogeneously sensitive stores. To discriminate between these hypotheses, we imaged subcellular Ca(2+) puffs evoked by IP(3) in Xenopus oocytes where release sites were functionally uncoupled using EGTA. Puffs were little changed by 300 microM intracellular EGTA, but sites operated autonomously and did not propagate waves. Photoreleased IP(3) generated flurries of puffs-different to the prolonged Ca(2+) elevation following waves in control cells-and individual sites responded repeatedly to successive increments of [IP(3)]. These data support the second hypothesis while refuting the first, and suggest that local Ca(2+) signals exhibit rapid adaptation, different to the slower inhibition following global Ca(2+) waves.

Ca(2+)-dependent activation of Cl(-) currents in Xenopus oocytes is modulated by voltage

Ca(2+)-activated Cl(-) currents (I(Cl,Ca)) were examined using fluorescence confocal microscopy to monitor intracellular Ca(2+) liberation evoked by flash photolysis of caged inositol 1,4, 5-trisphosphate (InsP(3)) in voltage-clamped Xenopus oocytes. Currents at +40 mV exhibited a steep dependence on InsP(3) concentration ([InsP(3)]), whereas currents at -140 mV exhibited a higher threshold and more graded relationship with [InsP(3)]. Ca(2+) levels required to half-maximally activate I(Cl,Ca) were about 50% larger at -140 mV than at +40 mV, and currents evoked by small Ca(2+) elevations were reduced >25-fold. The half-decay time of Ca(2+) signals shortened at increasingly positive potentials, whereas the decay of I(Cl,Ca) lengthened. The steady-state current-voltage (I-V) relationship for I(Cl,Ca) exhibited outward rectification with weak photolysis flashes but became more linear with stronger stimuli. Instantaneous I-V relationships were linear with both strong and weak stimuli. Current relaxations following voltage steps during activation of I(Cl,Ca) decayed with half-times that shortened from about 100 ms at +10 mV to 20 ms at -160 mV. We conclude that InsP(3)-mediated Ca(2+) liberation activates a single population of Cl(-) channels, which exhibit voltage-dependent Ca(2+) activation and voltage-independent instantaneous conductance.

The epithelial calcium channel, ECaC, is activated by hyperpolarization and regulated by cytosolic calcium

The recently cloned epithelial Ca(2+) channel, ECaC, which is expressed in the apical membrane of 1,25-dihydroxyvitamin D(3)-responsible epithelia, was characterized in Xenopus laevis oocytes by measuring the Ca(2+)-activated Cl(-) current which is a sensitive read-out of the Ca(2+) influx. ECaC-expressing oocytes responded to a voltage ramp with a maximal inward current of -2.1 +/- 0.3 microA at a holding potential of -99 +/- 1 mV. The inward current decreased progressively at less negative potentials and at +50 mV a small Ca(2+)-induced outward current was observed. The Ca(2+) influx-evoked current at a hyperpolarizing pulse to -100 mV displayed a fast activation followed by a rapid but partial inactivation. Loading of the oocytes with the Ca(2+) chelator BAPTA delayed the activation and blocked the inactivation of ECaC. When a series of brief hyperpolarizing pulses were given a significant decline in the peak response and subsequent plateau phase was observed. In conclusion, the distinct electrophysiological features of ECaC are hyperpolarization-dependent activation, Ca(2+)-dependent regulation of channel conductance and desensitization during repetitive stimulation.

Reversible Ca gradients between the subplasmalemma and cytosol differentially activate Ca-dependent Cl currents

Xenopus oocytes express several different Ca-activated Cl currents that have different waveforms and biophysical properties. We compared the stimulation of Ca-activated Cl currents measured by two-microelectrode voltage clamp with the Ca transients measured in the same cell by confocal microscopy and Ca-sensitive fluorophores. The purpose was to determine how the amplitude and/or spatio-temporal features of the Ca signal might explain how these different Cl currents were activated by Ca. Because Ca release from stores was voltage independent, whereas Ca influx depended upon the electrochemical driving force, we were able to separately assess the contribution of Ca from these two sources. We were surprised to find that Ca signals measured with a cytosolic Ca-sensitive dye, dextran-conjugated Ca-green-1, correlated poorly with Cl currents. This suggested that Cl channels located at the plasma membrane and the Ca-sensitive dye located in the bulk cytosol were sensing different [Ca]. This was true despite Ca measurement in a confocal slice very close to the plasma membrane. In contrast, a membrane-targeted Ca-sensitive dye (Ca-green-C18) reported a Ca signal that correlated much more closely with the Cl currents. We hypothesize that very local, transient, reversible Ca gradients develop between the subplasmalemmal space and the bulk cytosol. [Ca] is higher near the plasma membrane when Ca is provided by Ca influx, whereas the gradient is reversed when Ca is released from stores, because Ca efflux across the plasma membrane is faster than diffusion of Ca from the bulk cytosol to the subplasmalemmal space. Because dissipation of the gradients is accelerated by inhibition of Ca sequestration into the endoplasmic reticulum with thapsigargin, we conclude that [Ca] in the bulk cytosol declines slowly partly due to futile recycling of Ca through the endoplasmic reticulum.

Dynamics of calcium regulation of chloride currents in Xenopus oocytes

Ca-activated Cl currents are widely expressed in many cell types and play diverse and important physiological roles. The Xenopus oocyte is a good model system for studying the regulation of these currents. We previously showed that inositol 1,4,5-trisphosphate (IP3) injection into Xenopus oocytes rapidly elicits a noninactivating outward Cl current (ICl1-S) followed several minutes later by the development of slow inward (ICl2) and transient outward (ICl1-T) Cl currents. In this paper, we investigate whether these three currents are mediated by the same or different Cl channels. Outward Cl currents were more sensitive to Ca than inward Cl currents, as shown by injection of different amounts of Ca or by Ca influx through a heterologously expressed ligand-gated Ca channel, the ionotropic glutamate receptor iGluR3. These data could be explained by two channels with different Ca affinities or one channel with a higher Ca affinity at depolarized potentials. To distinguish between these possibilities, we determined the anion selectivity of the three currents. The anion selectivity sequences for the three currents were the same (I > Br > Cl), but ICl1-S had an I-to-Cl permeability ratio more than twofold smaller than the other two currents. The different anion selectivities and instantaneous current-voltage relationships were consistent with at least two different channels mediating these currents. However, after consideration of possible errors, the hypothesis that a single type of Cl channel underlies the complex waveforms of the three different macroscopic Ca-activated Cl currents in Xenopus oocytes remains a viable alternative.

Activation and co-ordination of InsP3-mediated elementary Ca2+ events during global Ca2+ signals in Xenopus oocytes

1. The activation of elementary calcium release events ('puffs') and their co-ordination to generate calcium waves was studied in Xenopus oocytes by confocal linescan imaging together with photorelease of inositol 1,4,5-trisphosphate (InsP3) from a caged precursor. 2. Weak photolysis flashes evoked no responses or isolated calcium puffs, whereas flashes of increasing strength evoked more frequent puffs, often occurring in flurries as abortive waves, and then a near-simultaneous calcium liberation originating at multiple sites. The numbers of sites activated increased initially as about the fourth power of photoreleased [InsP3]. 3. Following repeated, identical photolysis flashes, puffs arose after stochastically varying latencies of a few hundred milliseconds to several seconds. The cumulative number of events initially increased as about the third power of time. No rise in free [Ca2+] was detected preceding the puffs, suggesting that this co-operativity arises through binding of multiple InsP3 molecules, rather than through calcium feedback. 4. The mean latency to onset of calcium liberation shortened as about the square of the flash strength, and the dispersion in latencies between events reduced correspondingly. 5. Weak stimuli often evoked coupled puffs involving adjacent sites, and stronger flashes evoked saltatory calcium waves, propagating with non-constant velocity. During waves, [Ca2+] rose slowly between puff sites, but more abruptly at active sites following an initial diffusive rise in calcium. 6. Initial rates of rise of local [Ca2+] at release sites were similar during puffs and release induced by much (> 10-fold) greater [InsP3]. In contrast, macroscopic calcium measurements averaged over the scan line showed a graded dependence of rate of calcium liberation upon [InsP3], due to recruitment of additional sites and decreasing dispersion in activation latencies. 7. We conclude that the initiation of calcium liberation depends co-operatively upon [InsP3] whereas the subsequent regenerative increase in calcium flux depends upon local calcium feedback and is largely independent of [InsP3]. Wave propagation is consistent with the diffusive spread of calcium evoking regenerative liberation at heterogeneous discrete sites, the sensitivity of which is primed by InsP3.

Expression of the mammalian calcium signaling response to Trypanosoma cruzi in Xenopus laevis oocytes

Infective stages of the protozoan parasite Trypanosoma cruzi contain a soluble factor that induces elevation in the intracellular free Ca2+ concentration ([Ca2+]i) of mammalian cells. The process is pertussis toxin (PTx)-sensitive, and involves phospholipase C (PLC) activation, inositol 1,4,5-trisphosphate (IP3) formation and Ca2+ release from intracellular stores (Tardieux I, et al. J Exp Med 1994;179:1017-1022; Rodriguez A, et al. J Cell Biol 1995;129:1263-1273). We now report that a molecule exposed on the surface of the target cells is required to trigger the signaling cascade, and that a response with identical characteristics can be induced in Xenopus laevis oocytes injected with mRNA from normal rat kidney (NRK) fibroblasts. Xenopus oocytes do not show an endogenous response to the trypomastigote Ca2+ signaling factor, but a vigorous response in the form of a propagating Ca2+ wave is expressed after injection of NRK cell mRNA. As previously demonstrated for mammalian cells, the response is inhibited when injected oocytes are pretreated with PTx, implicating Galphai or Galphao trimeric G-proteins, and with thapsigargin, which depletes intracellular Ca2+ stores. Moreover, the [Ca2+]i transients triggered by the T. cruzi soluble factor in mRNA-injected oocytes are blocked by the same inhibitors of the parasite oligopeptidase B that abolish the [Ca2+]i response in NRK cells (Burleigh B, Andrews NW. J Biol Chem 1995;270:5172-5180; Burleigh BA et al. J Cell Biol 1997;136:609-620). The NRK mRNA fraction that induces expression of the [Ca2+]i response to the T. cruzi signaling factor contains messages from 1.5 to 2.0 kb, a size range consistent with the family of seven-transmembrane G-protein-coupled receptors.

Elementary events of InsP3-induced Ca2+ liberation in Xenopus oocytes: hot spots, puffs and blips

Liberation of sequestered Ca2+ ions in Xenopus oocytes by the second messenger inositol 1,4,5-trisphosphate (InP3) occurs from functionally discrete sites, which are spaced at intervals of several microns and probably represent clusterings of InsP3 receptor/channels (InsP3R) in the endoplasmic reticulum. As well as requiring InsP3, opening of release channels is regulated by dual positive and negative feedback by cytosolic Ca2+, leading to regenerative Ca2+ transients. Because the sensitivity of this process is determined by [InsP3], the ability of Ca2+ ions diffusing from one location to activate increasingly distant InsP3R is enhanced by increasing [InsP3]. Together with the spatial distribution of receptors, this results in generation of a hierarchy of Ca2+ release events, which may involve individual InsP3R (Ca2+ 'blips'), concerted activation of several receptors within a single release site (Ca2+ 'puffs'), and recruitment of successive sites by Ca2+ diffusing over micron distances to produce propagating Ca2+ waves. Thus, Ca2+ signalling in the oocyte is organized as at least two sizes of elemental 'building blocks'; highly localized Ca2+ transients that arise autonomously and stochastically from discrete sites at low [InsP3], but which become coordinated at higher [InsP3] to produce global Ca2+ responses.

Effects of protein phosphorylation on the regulation of capacitative calcium influx in Xenopus oocytes

The regulation of capacitative Ca2+ influx in Xenopus oocytes was investigated using both the two electrode voltage-clamp (where Ca2+ is monitored through the Ca2+-dependent Cl- current) and patch-clamp techniques. Following stimulation of expressed 5-hydroxytryptamine (5-HT) receptors, capacitative Ca2+ influx deactivated in around 15 min. Following injection of [adenosine 5'-O-(3-Thiotriphosphate)] (ATP [gamma-S]), an ATP analogue that is readily used by protein kinases, capacitative Ca2+ influx activated by 5-HT application either did not deactivate or was prolonged around twofold. However, injection of adenylyl 5'-(beta,gamma-methylene)-diphosphonate (AMP-PCP), another ATP analogue that is not utilised by kinases, did not affect the time-course of Ca2+ influx. When capacitative Ca2+ influx was activated by readmission of Ca2+ to oocytes incubated in thapsigargin/0 Ca2+ solution for several hours, Ca2+ influx occurred and a weakly saturating relationship between external Ca2+ and Ca2+ influx was found. Ca2+ influx in thapsigargin-treated cells was unaffected by ATP [gamma-S]. ATP [gamma-s] and several kinases had no effect on the Ca2+-dependent Cl- current when the latter was activated by elevation of Ca2+ independent of capacitative Ca2+ influx. Protein kinase C slowly and partially inhibited the Cl- current. Outside-out patches taken from thapsigargin-treated cells failed to demonstrated any Ca2+ current or Ca2+-dependent Cl- current on reapplying high Ca2+ to the patch, despite the oocyte showing a large capacitative Ca2+ influx. The results suggest that a kinase, activated on receptor stimulation, prolongs the activation time-course of capacitative Ca2+ influx.

Fast kinetics of calcium liberation induced in Xenopus oocytes by photoreleased inositol trisphosphate

Inositol 1,4,5-trisphosphate (InsP3) acts on intracellular receptors to cause liberation of Ca2+ ions into the cytosol as repetitive spikes and propagating waves. We studied the processes underlying this regenerative release of Ca2+ by monitoring with high resolution the kinetics of Ca2+ flux evoked in Xenopus oocytes by flash photolysis of caged InsP3. Confocal microfluorimetry was used to monitor intracellular free [Ca2+] from femtoliter volumes within the cell, and the underlying Ca2+ flux was then derived from the rate of increase of the fluorescence signals. A threshold amount of InsP3 had to be photoreleased to evoke any appreciable Ca2+ signal, and the amount of liberated Ca2+ then increased only approximately fourfold with maximal stimulation, whereas the peak rate of increase of Ca2+ varied over a range of nearly 20-fold, reaching a maximum of approximately 150 microMs-1. Ca2+ flux increased as a first-order function of [InsP3]. Indicating a lack of cooperativity in channel opening, and was half-maximal with stimuli approximately 10 times threshold. After a brief photolysis flash, Ca2+ efflux began after a quiescent latent period that shortened from several hundred milliseconds with near-threshold stimuli to 25 ms with maximal flashes. This delay could not be explained by an initial "foot" of Ca2+ increasing toward a threshold at which regenerative release was triggered, and the onset of release seemed too abrupt to be accounted for by multiple sequential steps involved in channel opening. Ca2+ efflux increased to a maximum after the latent period in a time that reduced from > 100 ms to approximately 8 ms with increasing [InsP3] and subsequently declined along a two-exponential time course: a rapid fall with a time constant shortening from > 100 ms to approximately 25 ms with increasing [InsP3], followed by a much smaller fail persisting for several seconds. The results are discussed in terms of a model in which InsP3 receptors must undergo a slow transition after binding InsP3 before they can be activated by cytosolic Ca2+ acting as a co-agonist. Positive feedback by liberated Ca2+ ions then leads to a rapid increase in efflux to a maximal rate set by the proportion of receptors binding InsP3. Subsequently, Ca2+ efflux terminates because of a slower inhibitory action of cytosolic Ca2+ on gating of InsP3 receptor-channels.

Quantal puffs of intracellular Ca2+ evoked by inositol trisphosphate in Xenopus oocytes

1. Ca2+ liberation induced in Xenopus oocytes by a poorly metabolized derivative of inositol 1,4,5-trisphosphate (3-deoxy-3-fluoro-D-myo-inositol 1,4,5-trisphosphate; 3-F-InsP3) was visualized using a video-rate confocal microscope to image fluorescence signals reported by the indicator dye calcium green-1. 2. Low (10-30 nM) intracellular concentrations of 3-F-InsP3 evoked Ca2+ release as localized transient 'puffs'. Progressively higher concentrations (30-60 nM) gave rise to abortive Ca2+ waves triggered by puffs, and then (> 60 nM) to a sustained elevation of Ca2+ followed by the appearance of propagating Ca2+ waves. At concentrations up to that giving waves, the frequency of puffs increased as about the third power of [InsP3], whereas their amplitudes increased only slightly. 3. The rise of cytosolic Ca2+ during a puff began abruptly, and peaked within about 50 ms. The peak free Ca2+ level was about 180 nM, and the total amount of Ca2+ liberated was several attomoles (10(-18) mol), too much to be accounted for by opening of a single InsP3-gated channel. The subsequent decline of Ca2+ occurred over a few hundred milliseconds, determined largely by diffusion of Ca2+ away from the release site, rather than by resequestration. Lateral spread of Ca2+ was restricted to a few micrometres, consistent with an effective diffusion coefficient for Ca2+ ions of about 27 microns2 s-1. 4. The peak amplitudes of puffs recorded at a given site were distributed in a roughly Gaussian manner, and a small proportion of sites consistently gave puffs much larger than the main population. Intervals between successive puffs at a single site were exponentially distributed, except for a progressive fall-off in puffs seen at intervals shorter than about 10 s. Thus, triggering of puffs appeared to be stochastically determined after recovery from a refractory period. 5. There was little correlation between the occurrence of puffs at sites more than a few micrometres apart, indicating that puff sites can function autonomously, but closely (ca 2 microns) adjacent sites showed highly correlated behaviour. 6. Puffs arose from sites-present at a density of about 1 per 30 microns2 in the animal hemisphere, located within a narrow band about 5-7 microns below the plasma membrane. 7. We conclude that Ca2+ puffs represent a 'quantal' unit of InsP3-evoked Ca2+ liberation, which may arise because local regenerative feedback by cytosolic Ca2+ ions causes the concerted opening of several closely clustered InsP3 receptor channels.

Overexpression of inositol 1,4,5-trisphosphate 3-kinase in Xenopus oocytes inhibits agonist-evoked capacitative calcium entry

Ins(1,4,5)P3 3-kinase is a key enzyme in the regulation of Ins(1,4,5)P3. Overexpression of Ins(1,4,5)P3 3-kinase inhibited agonist-evoked and Ins(1,3,4,5)P4-evoked Ca2+ entry in Xenopus oocytes, but did not inhibit Ca2+ entry evoked by thapsigargin or non-metabolizable Ins(1,4,5)P3 analogues. The data suggest that Ins(1,4,5)P3 alone plays the crucial role in the activation of capacitative Ca2+ entry by emptying intracellular stores.

The initiation of a calcium signal in Xenopus oocytes

Application of acetylcholine to Xenopus oocytes evoked increases in the cytosolic free calcium ion concentration ([Ca2+]i) after latencies of up to several seconds depending on the agonist dose. Higher acetylcholine concentrations evoked responses with larger amplitudes and shorter latencies. The latencies of responses to acetylcholine could be increased by application of caffeine, injection of calcium buffers or depletion of intracellular calcium stores. Acute inhibition of endoplasmic reticulum calcium pumps without substantial reduction of the calcium store content (by application of thapsigargin shortly before agonist stimulation) reduced the latencies of responses to acetylcholine. A schematic and mathematical model are presented to show a possible mechanism by which a calcium signal is initiated following a latent period after the elevation of the inositol trisphosphate concentration. During the latent period, calcium is slowly released from the intracellular stores. The released calcium is rapidly buffered by cytosolic calcium-binding proteins and some is resequestered into the stores by calcium pumps. The [Ca2+]i changes very little until the buffering is locally saturated. The [Ca2+]i then rises above a threshold concentration which evokes an explosive release of calcium due to positive feedback by calcium on the inositol trisphosphate receptor.

A membrane potential model with counterions for cytosolic calcium oscillations

An initial model has been proposed to describe a mechanism for cytosolic calcium oscillations [Jafri MS. Vajda S. Pasik P. Gillo B. (1992) A membrane model for cytosolic calcium oscillations: a study using Xenopus oocytes. Biophys. J., 63, 235-246]. In this paper we extend our original model to include the effects of counterion movement into the ER in response to calcium release. This produces smoother oscillations over a wider parameter range. We have lowered the endoplasmic reticulum (ER) intraluminal free calcium concentration and shown that the oscillations can occur at lower ER membrane potentials, consistent with physiological values. The improved model is then tested with two representative paradigms that are currently under investigation by many researchers. The model predicts that the reduction of the ER calcium pump (Ca-ATPase) rate can cause the termination of cytosolic calcium oscillations in an active cell, and induce oscillations in a resting cell. This result is consistent with experiments with thapsigargin, a Ca-ATPase activity inhibitor. In addition, we simulate the latency period for the response to the application of agonist and offer a plausible explanation for it. Our mathematical model is currently the only model that formulates the contributions of calcium binding proteins, ER membrane potential, ER counterion movements, and distinct calcium pump populations, and describes their effects on cytosolic calcium oscillations.

Role of cytosolic Ca2+ in inhibition of InsP3-evoked Ca2+ release in Xenopus oocytes

1. Calcium liberation induced in Xenopus oocytes by flash photorelease of inositol 1,4,5-trisphosphate (InsP3) from a caged precursor was monitored by confocal microfluorimetry. The object was to determine whether inhibition of Ca2+ release seen with paired flashes arose as a direct consequence of elevated cytosolic free [Ca2+]. 2. Responses evoked by just-suprathreshold test flashes were not inhibited by subthreshold conditioning flashes, but were strongly suppressed when conditioning flashes were raised above threshold. 3. Inhibition at first increased progressively as the inter-flash interval was lengthened to about 2 s and thereafter declined, with a half-recovery at about 4 s. 4. Intracellular injections of Ca2+ caused relatively slight inhibition of InsP3-evoked signals, even when cytosolic free [Ca2+] was elevated to levels similar to those at which strong inhibition was seen in paired-flash experiments. 5. Recovery from inhibition was not appreciably slowed when Ca2+ was injected to raise the free Ca2+ level between paired flashes. 6. We conclude that inhibition of InsP3-evoked Ca2+ liberation is not directly proportional to cytosolic free Ca2+ level and that recovery from inhibition in paired-pulse experiments involves factors other than the decline of cytosolic [Ca2+] following a conditioning response.

Mouse oocyte maturation is affected by lithium via the polyphosphoinositide metabolism and the microtubule network

The incubation of mechanically denuded mouse oocytes in medium containing LiCl delayed both germinal vesicle breakdown (GVBD) and polar body extrusion in a dose-dependent and reversible manner. When myo-inositol alone was added to the culture medium, we observed that it accelerated GVBD and increased the rate of polar body extrusion, whereas, when combined with LiCl, the normal timing of GVBD was recovered. In the same way, when inositol trisphosphate (InsP3) was microinjected into the ooplasma, we observed an important improvement of the rate of GVBD, as compared to control oocytes, and prevention of lithium inhibition. However, neither myo-inositol nor InsP3 were able to rescue totally the oocytes from the negative effect of lithium on polar body extrusion. Moreover, lithium induced some important changes in microtubule and chromosome organizations. Before extrusion of the first polar body, the reduction of the spindle size or the appearance of short individualized chromosomes dispersed around a large aster of microtubules were often observed, whereas, after polar body extrusion, the spindle appeared smaller and chromosomes were often trapped in the midbody. Thus lithium affects mouse oocyte maturation at two different levels: GVBD and polar body extrusion. Whereas the former seems to be affected via polyphosphoinositide turnover, the latter is InsP3-independent and seems to be influenced negatively via underdevelopment of microtubular structures.

A slowly ADP-ribosylated pertussis-toxin-sensitive GTP-binding regulatory protein is required for vasopressin-stimulated Ca2+ inflow in hepatocytes

The roles of heterotrimeric GTP-binding regulatory proteins (G-proteins) and inositol polyphosphates in the mechanism by which vasopressin stimulates Ca2+ inflow in hepatocytes were investigated by using single cells loaded with fura2 by microinjection. Vasopressin-stimulated Ca2+ inflow was mimicked by microinjection of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) or guanosine 5'-[beta gamma-imido]triphosphate to the cells, but not adenosine 5'-[gamma-thio]triphosphate (ATP[S]) or guanosine 5'-[beta-thio]diphosphate (GDP[S]). Extracellular Gd3+ (5 microM) inhibited both vasopressin- and GTP[S]-stimulated Ca2+ inflow. GDP[S], but not GMP, administered to hepatocytes by microinjection, completely inhibited vasopressin-stimulated Ca2+ inflow and partially inhibited vasopressin-induced release of Ca2+ from intracellular stores. The microinjection of pertussis toxin had no effect either on the release of Ca2+ from intracellular stores or on Ca2+ inflow induced by vasopressin, but completely inhibited changes in these processes induced by epidermal growth factor (EGF). Hepatocytes isolated from rats treated with pertussis toxin for 24 h exhibited no vasopressin- or GTP[S]-stimulated Ca2+ inflow, whereas the vasopressin-stimulated release of Ca2+ from intracellular stores was similar to that observed for control cells. Heparin or ATP[S] inhibited, or delayed the onset of, both vasopressin-induced release of Ca2+ from intracellular stores and vasopressin-stimulated Ca2+ inflow. Vasopressin-induced oscillations in intracellular [Ca2+] were observed in some heparin-treated cells. It is concluded that the stimulation by vasopressin of Ca2+ inflow to hepatocytes requires inositol 1,4,5-trisphosphate (InsP3) and, by implication, the pertussis-toxin-insensitive G-protein required for the activation of phospholipase C beta [Taylor, Chae, Rhee and Exton (1991) Nature (London) 350, 516-518], and another G-protein which is slowly ADP-ribosylated by pertussis toxin and acts between InsP3 and the putative plasma-membrane Ca2+ channel. EGF-stimulated Ca2+ inflow involves at least one G-protein which is rapidly ADP-ribosylated and is most likely required for InsP3 formation.

Relation between intracellular Ca2+ signals and Ca(2+)-activated Cl- current in Xenopus oocytes

Activation of inositol 1,4,5-trisphosphate (InsP3) signalling in Xenopus oocytes causes intracellular Ca2+ mobilization and thereby activates a Ca(2+)-dependent Cl- membrane conductance. Measurements of cytosolic Ca2+ levels using fluorescent indicators, however, revealed little correspondence with Cl- currents. Intracellular photorelease of InsP3 from a caged precursor evoked transient currents that peaked while the Ca(2+)-fluorescence signal was rising, and subsequently declined within a few seconds, even though the Ca2+ signal remained elevated much longer. Also, Cl- currents evoked by agonist activation showed transient spikes while a wave of Ca2+ liberation swept across the cell, but then decreased when the Ca2+ signal attained a maximal level. Thus, the Cl- current corresponded better to the rate of rise of intracellular free Ca2+, rather than to its steady state level. Experiments using paired flashes to photolyse caged InsP3 and caged Ca2+ indicated that this relationship did not arise through desensitization or inactivation of the Cl- conductance. Furthermore, fluorescence measurements made at different depths into the cell using a confocal microscope revealed no evidence that a rapid decline of local Ca2+ levels near the plasma membrane was responsible for the decay of Ca(2+)-activated Cl- current. Instead, Cl- channels may show an adaptive or incremental response to Ca2+, which is likely to be important for the encoding and transmission of information by Ca2+ spikes.

Inositol 1,4,5-trisphosphate receptors in Xenopus laevis oocytes: localization and modulation by Ca2+

Inositol 1,4,5-trisphosphate receptors (InsP3R) in Xenopus laevis oocytes were localized and their regulation by Ca2+ was investigated. Antibodies raised against the C-terminal region of the mouse cerebellar InsP3R (cAb) cross-reacted with a 255 kD protein in Western blots of Xenopus microsomal membranes. Immunolocalization of this protein in cryosections of oocytes revealed diffuse staining of the cytoplasm, intense staining of the sub-plasma membrane region of the animal hemisphere, and punctate staining in association with the germinal vesicle. In the presence of 40 microM free Ca2+, isolated oocyte membranes exhibited a high affinity binding site for Ins 1,4,5-P3 (KD = 5nM) and a binding capacity of 450 fmol/mg protein. The specific binding capacity of oocyte membranes for [3H]-Ins 1,4,5-P3 increased as the level of free Ca2+ present in binding assays was raised from < 0.1 nM to 4.0 microM, with an apparent EC50 of 60 nM. Increasing the concentration of free Ba2+ failed to facilitate [3H]-Ins1,4,5-P3 binding. Other inositol phosphates competed for Ins1,4,5-P3 binding sites with approximate IC50 values of: Ins1,3,4,5-P4 = 79 nM, Ins2,4,5-P3 = 455 nM and L-Ins1,4,5-P3 = 20 microM. In addition, 150 micrograms/ml (approximately 12 microM) heparin displaced 50% of bound [3H]-Ins1,4,5-P3, whereas caffeine (10 mM) had little effect. Functional reconstitution of solubilized InsP3Rs into lipid bilayers revealed that Ca2+ was a necessary co-agonist for activation of the InsP3R. When InsP3 (5 microM) and Ca2+ (5 microM) were applied together, conductance steps were observed. InsP3 or Ca2+ alone had little effect. These results suggest that the subcellular organization of InsP3Rs and the facilitation of InsP3 binding and channel opening by Ca2+ contribute to the Ins1,4,5-P3-mediated Ca2+ spikes, waves, and oscillations observed in Xenopus oocytes.

Intracellular calcium waves generated by Ins(1,4,5)P3-dependent mechanisms

Cellular oscillations of cytosolic free Ca2+ ([Ca2+]i) have been observed in many cell types in response to cell surface receptor agonists acting through inositol 1,4,5-trisphosphate (InsP3). In a number of cases where appropriate spatial and temporal resolution have been used to examine these [Ca2+]i oscillations, they have been found to be organized as repetitive waves of Ca2+ increase that propagate through the cytosol of individual cells. In some cases Ca2+ waves also occur as a single pass through stimulated cells. This review discusses the factors underlying the spatial organization of [Ca2+]i signals in the form of Ca2+ waves. In addition, potential mechanisms for the initiation and subsequent propagation of these Ca2+ waves are described.

A single-pool model for intracellular calcium oscillations and waves in the Xenopus laevis oocyte

We construct a minimal model of cytosolic free Ca2+ oscillations based on Ca2+ release via the inositol 1,4,5-trisphosphate (IP3) receptor/Ca2+ channel (IP3R) of a single intracellular Ca2+ pool. The model relies on experimental evidence that the cytosolic free calcium concentration ([Ca2+]c) modulates the IP3R in a biphasic manner, with Ca2+ release inhibited by low and high [Ca2+]c and facilitated by intermediate [Ca2+]c, and that channel inactivation occurs on a slower time scale than activation. The model produces [Ca2+]c oscillations at constant [IP3] and reproduces a number of crucial experiments. The two-dimensional spatial model with IP3 dynamics, cytosolic diffusion of IP3 (Dp = 300 microns 2 s-1), and cytosolic diffusion of Ca2+ (Dc = 20 microns 2 s-1) produces circular, planar, and spiral waves of Ca2+ with speeds of 7-15 microns.s-1, which annihilate upon collision. Increasing extracellular [Ca2+] influx increases wave speed and baseline [Ca2+]c. A [Ca2+]c-dependent Ca2+ diffusion coefficient does not alter the qualitative behavior of the model. An important model prediction is that channel inactivation must occur on a slower time scale than activation in order for waves to propagate. The model serves to capture the essential macroscopic mechanisms that are involved in the production of intracellular Ca2+ oscillations and traveling waves in the Xenopus laevis oocyte.

Inositol trisphosphate-mediated Ca2+ influx into Xenopus oocytes triggers Ca2+ liberation from intracellular stores

1. Inositol 1,4,5-trisphosphate (InsP3) functions as a second messenger by liberating Ca2+ from intracellular stores and by promoting influx of extracellular Ca2+. We examined whether Ca2+ influx modulates intracellular Ca2+ liberation in Xenopus oocytes by fluorescence monitoring of cytosolic free Ca2+ together with voltage clamp recording of Ca(2+)-activated Cl- membrane currents. Sustained activation of membrane Ca2+ permeability was induced by intracellular injections of a non-metabolizable InsP3 analogue, 3-deoxy-3-fluoro-D-myo-inositol 1,4,5-trisphosphate (3-F-InsP3), and Ca2+ influx was controlled by applying step changes in membrane potential to alter the driving force for Ca2+ entry. 2. Negative-going potential steps evoked intracellular Ca2+ signals comprising two components; an initial transient peak followed by a slower rise. The initial transient grew steeply over a narrow (ca 40 mV) voltage range but then increased little with further polarization, whereas the second component showed a nearly linear voltage dependence. 3. The transient Ca2+ signal continued to rise almost unchanged when Ca2+ influx was interrupted by stepping the potential to more positive values after brief hyperpolarization. In contrast, Ca2+ levels declined monotonically when positive-going steps were applied after longer intervals during the second component of the Ca2+ signal. 4. Large Ca(2+)-dependent transient inward (T(in)) membrane currents were evoked during the rising phase of the initial Ca2+ signal, but little current was associated with the second component of the Ca2+ signal. 5. The T(in) currents evoked by hyperpolarization were mimicked at fixed clamp potential by re-admitting Ca2+ to the bathing solution, and by flash photolysis of caged Ca2+ loaded into the oocyte. 6. T(in) currents were strongly inhibited by prior release of Ca2+ from InsP3-sensitive intracellular stores, and vice versa. Experiments with paired hyperpolarizing pulses and paired photorelease of InsP3 showed that responses to both stimuli recovered with similar time courses. 7. We conclude that the transient Ca2+ signal and associated T(in) current evoked by hyperpolarization arise because Ca2+ entering the oocyte triggers regenerative release of Ca2+ from InsP3-sensitive intracellular stores. Since membrane currents evoked by liberated Ca2+ were much greater than those evoked by Ca2+ entry per se, a major function of InsP3-mediated Ca2+ entry may be to modulate the activity of intracellular Ca2+ stores.

Calcium waves

Intracellular Ca2+ oscillations and waves are commonly observed both in excitable cells, including neurons, and in non-excitable cells. Current attempts to describe and explain these complex intracellular signals suggest that the oscillations are the result of a highly regulated mechanism, the details of which vary among different cells. Recently, the Xenopus oocyte has become an important model system in which a single pool of IP3 receptors release Ca2+ to initiate waves. The intrinsic bell-shaped dependence of the IP3 receptor on Ca2+ is sufficient to explain the regenerative wave phenomenon.

Confocal microfluorimetry of Ca2+ signals evoked in Xenopus oocytes by photoreleased inositol trisphosphate

1. The subcellular characteristics of inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ liberation were studied in Xenopus oocytes by the use of confocal microfluorimetry to monitor Ca2+ signals from minutely localized region of the cell in response to photorelease of InsP3 from a caged precursor. 2. Photorelease of increasing amounts of InsP3 by progressively longer light flashes evoked transient Ca2+ responses that appeared abruptly at a certain threshold duration, and then grew steeply over a narrow range of flash durations to reach a maximum. Further lengthening of flash duration gave no increase in size of the Ca2+ signals, but their rate of rise continued to increase and their duration became longer. Simultaneous measurements of Ca(2+)-activated Cl- currents showed a slightly higher threshold than the Ca2+ signal, and a more graded dependence upon flash duration. 3. The threshold flash durations required to evoke Ca2+ and membrane current signals grew by more than 100-fold as the area of the oocyte exposed to photolysis light was reduced from a square of 140 microns to 5 microns. 4. Ca2+ signals evoked by photoreleased InsP3 began following a dose-dependent latency that was as long as several seconds with low intensity light, but shortened to about 50 ms at maximum intensity. The extrapolated minimum latency with infinite photorelease of InsP3 was about 30 ms. 5. InsP3-evoked membrane currents began 30 ms or longer after the corresponding Ca2+ signals, whereas currents evoked by photorelease of Ca2+ from a caged precursor began within 5 ms of the onset of the light flash. 6. No differences in duration of InsP3-evoked Ca2+ signals were apparent when the confocal measuring spot was positioned close to the plasma membrane or about 10 microns more deeply into the oocyte. At both locations the Ca2+ signals were more prolonged than the associated membrane current signals. 7. Ca2+ signals to a test light flash were suppressed for about 2 s following a conditioning suprathreshold flash, but recovered almost completely after 6 s. The associated membrane current signals were facilitated at short intervals, suppressed at intervals between 0.5 and 3 s, and subsequently recovered more slowly than the Ca2+ signals. 8. Photorelease of InsP3 during 30 s exposures of low intensity evoked trains of repetitive Ca2+ spikes. The overall amplitudes of these responses changed little with increasing in frequency, and became smaller and superimposed on a more sustained elevation of Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)