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

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Ca2+ signalling system initiated by endoplasmic reticulum stress stimulates PERK activation

The accumulation of unfolded proteins within the Endoplasmic Reticulum (ER) activates a signal transduction pathway termed the unfolded protein response (UPR), which attempts to restore ER homoeostasis. If this cannot be done, UPR signalling ultimately induces apoptosis. Ca²⁺ depletion in the ER is a potent inducer of ER stress. Despite the ubiquity of Ca²⁺ as an intracellular messenger, the precise mechanism(s) by which Ca²⁺ release affects the UPR remains unknown. Tethering a genetically encoded Ca²⁺ indicator (GCamP6) to the ER membrane revealed novel Ca²⁺ signalling events initiated by Ca²⁺ microdomains in human astrocytes under ER stress, induced by tunicamycin (Tm), an N-glycosylation inhibitor, as well as in a cell model deficient in all three inositol triphosphate receptor isoforms. Pharmacological and molecular studies indicate that these local events are mediated by translocons and that the Ca²⁺ microdomains impact (PKR)-like-ER kinase (PERK), an UPR sensor, activation. These findings reveal the existence of a Ca²⁺ signal mechanism by which stressor-mediated Ca²⁺ release regulates ER stress.

Quantal Ca2+ release mediated by very few IP3 receptors that rapidly inactivate allows graded responses to IP3

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are intracellular Ca²⁺ channels that link extracellular stimuli to Ca²⁺ signals. Ca²⁺ release from intracellular stores is "quantal": low IP₃ concentrations rapidly release a fraction of the stores. Ca²⁺ release then slows or terminates without compromising responses to further IP₃ additions. The mechanisms are unresolved. Here, we synthesize a high-affinity partial agonist of IP₃Rs and use it to demonstrate that quantal responses do not require heterogenous Ca²⁺ stores. IP₃Rs respond incrementally to IP₃ and close after the initial response to low IP₃ concentrations. Comparing functional responses with IP₃ binding shows that only a tiny fraction of a cell's IP₃Rs mediate incremental Ca²⁺ release; inactivation does not therefore affect most IP₃Rs. We conclude, and test by simulations, that Ca²⁺ signals evoked by IP₃ pulses arise from rapid activation and then inactivation of very few IP₃Rs. This allows IP₃Rs to behave as increment detectors mediating graded Ca²⁺ release.

Calcium cytotoxicity sensitizes prostate cancer cells to standard-of-care treatments for locally advanced tumors

Therapy resistance is a major roadblock in oncology. Exacerbation of molecular dysfunctions typical of cancer cells have proven effective in twisting oncogenic mechanisms to lethal conditions, thus offering new therapeutic avenues for cancer treatment. Here, we demonstrate that selective agonists of Transient Receptor Potential cation channel subfamily M member 8 (TRPM8), a cation channel characteristic of the prostate epithelium frequently overexpressed in advanced stage III/IV prostate cancers (PCa), sensitize therapy refractory models of PCa to radio, chemo or hormonal treatment. Overall, our study demonstrates that pharmacological-induced Ca²⁺ cytotoxicity is an actionable strategy to sensitize cancer cells to standard therapies.

Spatial-temporal patterning of Ca2+ signals by the subcellular distribution of IP3 and IP3 receptors

The patterning of cytosolic Ca²⁺ signals in space and time underlies their ubiquitous ability to specifically regulate numerous cellular processes. Signals mediated by liberation of Ca²⁺ sequestered in the endoplasmic reticulum (ER) through inositol trisphosphate receptor (IP₃R) channels constitute a hierarchy of events; ranging from openings of individual IP₃ channels, through the concerted openings of several clustered IP₃Rs to generate local Ca²⁺ puffs, to global Ca²⁺ waves and oscillations that engulf the entire cell. Here, we review recent progress in elucidating how this hierarchy is shaped by an interplay between the functional gating properties of IP₃Rs and their spatial distribution within the cell. We focus in particular on the subset of IP₃Rs that are organized in stationary clusters and are endowed with the ability to preferentially liberate Ca²⁺.

Using two dyes to observe the competition of Ca2+ trapping mechanisms and their effect on intracellular Ca2+ signals

The specificity and universality of intracellular [Formula: see text] signals rely on the variety of spatio-temporal patterns that the [Formula: see text] concentration can display. [Formula: see text] liberation through inositol 1,4,5-trisphosphate receptors ([Formula: see text]) is key for this variety. In this paper, we study how the competition between buffers of different kinetics affects [Formula: see text] signals that involve [Formula: see text] release through [Formula: see text]. The study also provides insight into the underlying spatial distribution of the channels that participate in the signals. Previous works on the effects of [Formula: see text] buffers have drawn conclusions 'indirectly' by observing the [Formula: see text]-bound dye distributions in the presence of varying concentrations of exogenous buffers and using simulations to interpret the results. In this paper, we make visible the invisible by observing the signals simultaneously with two dyes, [Formula: see text] and [Formula: see text], each of which plays the role of a slow or fast [Formula: see text] buffer, respectively. Our observations obtained for different concentrations of [Formula: see text] highlight the dual role that fast buffers exert on the dynamics, either reducing the intracluster channel coupling or preventing channel inhibition and allowing the occurrence of relatively long cycles of [Formula: see text] release. Our experiments also show that signals with relatively high [Formula: see text] release rates remain localized in the presence of large [Formula: see text] concentrations, while the mean speed of the elicited waves increases. We interpret this as a consequence of the more effective uncoupling between [Formula: see text] clusters as the slow dye concentration increases. Combining the analysis of the experiments with numerical simulations, we also conclude that [Formula: see text] release not only occurs within the close vicinity of the centers of the clearly identifiable release sites ([Formula: see text] clusters) but there are also functional [Formula: see text] in between them.

Comparison of Ca2+ puffs evoked by extracellular agonists and photoreleased IP3

The inositol trisphosphate (IP3) signaling pathway evokes local Ca2+ signals (Ca2+ puffs) that arise from the concerted openings of clustered IP3 receptor/channels in the ER membrane. Physiological activation is triggered by binding of agonists to G-protein-coupled receptors (GPCRs) on the cell surface, leading to cleavage of phosphatidyl inositol bisphosphate and release of IP3 into the cytosol. Photorelease of IP3 from a caged precursor provides a convenient and widely employed means to study the final stage of IP3-mediated Ca2+ liberation, bypassing upstream signaling events to enable more precise control of the timing and relative concentration of cytosolic IP3. Here, we address whether Ca2+ puffs evoked by photoreleased IP3 fully replicate those arising from physiological agonist stimulation. We imaged puffs in individual SH-SY5Y neuroblastoma cells that were sequentially stimulated by picospritzing extracellular agonist (carbachol, CCH or bradykinin, BK) followed by photorelease of a poorly-metabolized IP3 analog, i-IP3. The centroid localizations of fluorescence signals during puffs evoked in the same cells by agonists and photorelease substantially overlapped (within ∼1μm), suggesting that IP3 from both sources accesses the same, or closely co-localized clusters of IP3Rs. Moreover, the time course and spatial spread of puffs evoked by agonists and photorelease matched closely. Because photolysis generates IP3 uniformly throughout the cytoplasm, our results imply that IP3 generated in SH-SY5Y cells by activation of receptors to CCH and BK also exerts broadly distributed actions, rather than specifically activating a subpopulation of IP3Rs that are scaffolded in close proximity to cell surface receptors to form a signaling nanodomain.

Functional Role of Intracellular Calcium Receptor Inositol 1,4,5-Trisphosphate Type 1 in Rat Hippocampus after Neonatal Anoxia

Anoxia is one of the most prevalent causes of neonatal morbidity and mortality, especially in preterm neonates, constituting an important public health problem due to permanent neurological sequelae observed in patients. Oxygen deprivation triggers a series of simultaneous cascades, culminating in cell death mainly located in more vulnerable metabolic brain regions, such as the hippocampus. In the process of cell death by oxygen deprivation, cytosolic calcium plays crucial roles. Intracellular inositol 1,4,5-trisphosphate receptors (IP3Rs) are important regulators of cytosolic calcium levels, although the role of these receptors in neonatal anoxia is completely unknown. This study focused on the functional role of inositol 1,4,5-trisphosphate receptor type 1 (IP3R1) in rat hippocampus after neonatal anoxia. Quantitative real-time PCR revealed a decrease of IP3R1 gene expression 24 hours after neonatal anoxia. We detected that IP3R1 accumulates specially in CA1, and this spatial pattern did not change after neonatal anoxia. Interestingly, we observed that anoxia triggers translocation of IP3R1 to nucleus in hippocampal cells. We were able to observe that anoxia changes distribution of IP3R1 immunofluorescence signals, as revealed by cluster size analysis. We next examined the role of IP3R1 in the neuronal cell loss triggered by neonatal anoxia. Intrahippocampal injection of non-specific IP3R1 blocker 2-APB clearly reduced the number of Fluoro-Jade C and Tunel positive cells, revealing that activation of IP3R1 increases cell death after neonatal anoxia. Finally, we aimed to disclose mechanistics of IP3R1 in cell death. We were able to determine that blockade of IP3R1 did not reduced the distribution and pixel density of activated caspase 3-positive cells, indicating that the participation of IP3R1 in neuronal cell loss is not related to classical caspase-mediated apoptosis. In summary, this study may contribute to new perspectives in the investigation of neurodegenerative mechanisms triggered by oxygen deprivation.

Mode switching of Inositol 1,4,5-trisphosphate receptor channel shapes the Spatiotemporal scales of Ca2+ signals

The inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) channel is crucial for the generation and modulation of highly specific intracellular Ca2+ signals performing numerous functions in animal cells. However, the single channel behavior during Ca2+ signals of different spatiotemporal scales is not well understood. To elucidate the correlation between the gating dynamics of single InsP3Rs and spatiotemporal Ca2+ patterns, we simulate a cluster of InsP3Rs under varying ligand concentrations and extract comprehensive gating statistics of all channels during events of different sizes and durations. Our results show that channels gating predominantly in the low activity mode with negligible occupancy of intermediate and high modes leads to single channel Ca2+ release event blips. Increasing occupancies of intermediate and high modes results in events with increasing size. When the channel has more than 50% probability of gating in the intermediate and high modes, the cluster generates very large puffs that would most likely result in global Ca2+ signals. The size, duration and frequency of Ca2+ signals all increase linearly with the total probability of channel gating in the intermediate and high modes. To our knowledge, this is the first study that quantitatively relates the modal characteristics of InsP3R to the shaping of different spatiotemporal scales of Ca2+ signals.

Luminal Ca(2+) dynamics during IP3R mediated signals

The role of cytosolic Ca(2+) on the kinetics of Inositol 1,4,5-triphosphate receptors (IP3Rs) and on the dynamics of IP3R-mediated Ca(2+) signals has been studied at large both experimentally and by modeling. The role of luminal Ca(2+) has not been investigated with that much detail although it has been found that it is relevant for signal termination in the case of Ca(2+) release through ryanodine receptors. In this work we present the results of observing the dynamics of luminal and cytosolic Ca(2+) simultaneously in Xenopus laevis oocytes. Combining observations and modeling we conclude that there is a rapid mechanism that guarantees the availability of free Ca(2+) in the lumen even when a relatively large Ca(2+) release is evoked. Comparing the dynamics of cytosolic and luminal Ca(2+) during a release, we estimate that they are consistent with a 80% of luminal Ca(2+) being buffered. The rapid availability of free luminal Ca(2+) correlates with the observation that the lumen occupies a considerable volume in several regions across the images.

The role of Ca(2+) influx in spontaneous Ca(2+) wave propagation in interstitial cells of Cajal from the rabbit urethra

KEY POINTS

Tonic contractions of rabbit urethra are associated with spontaneous electrical slow waves that are thought to originate in pacemaker cells termed interstitial cells of Cajal (ICC). ICC pacemaker activity results from their ability to generate propagating Ca(2+) waves, although the exact mechanisms of propagation are not understood. In this study, we have identified spontaneous localised Ca(2+) events for the first time in urethral ICC; these were due to Ca(2+) release from the endoplasmic reticulum (ER) via ryanodine receptors (RyRs) and, while they often remained localised, they sometimes initiated propagating Ca(2+) waves. We show that propagation of Ca(2+) waves in urethral ICC is critically dependent upon Ca(2+) influx via reverse mode NCX. Our data provide a clearer understanding of the intracellular mechanisms involved in the generation of ICC pacemaker activity. Interstitial cells of Cajal (ICC) are putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous propagating Ca(2+) waves that are modulated by [Ca(2+)]o and whose propagation is inhibited by inositol tri-phosphate receptor (IP3 R) blockers. The purpose of this study was to further examine the role of Ca(2+) influx and Ca(2+) release in the propagation of Ca(2+) waves. Intracellular Ca(2+) was measured in Fluo-4-loaded ICC using a Nipkow spinning disc confocal microscope at fast acquisition rates (50 fps). We identified previously undetected localised Ca(2+) events originating from ryanodine receptors (RyRs). Inhibiting Ca(2+) influx by removing [Ca(2+)]o or blocking reverse mode sodium-calcium exchange (NCX) with KB-R 7943 or SEA-0400 abolished Ca(2+) waves, while localised Ca(2+) events persisted. Stimulating RyRs with 1 mm caffeine restored propagation. Propagation was also inhibited when Ca(2+) release sites were uncoupled by buffering intracellular Ca(2+) with EGTA-AM. This was reversed when Ca(2+) influx via NCX was increased by reducing [Na(+)]o to 13 mm. Low [Na(+)]o also increased the frequency of Ca(2+) waves and this effect was blocked by tetracaine and ryanodine but not 2-aminoethoxydiphenyl borate (2-APB). RT-PCR revealed that isolated ICC expressed both RyR2 and RyR3 subtypes. We conclude: (i) RyRs are required for the initiation of Ca(2+) waves, but wave propagation normally depends on activation of IP3 Rs; (ii) under resting conditions, propagation by IP3 Rs requires sensitisation by influx of Ca(2+) via reverse mode NCX; (iii) propagation can be maintained by RyRs if they have been sensitised to Ca(2+).

Modulation of elementary calcium release mediates a transition from puffs to waves in an IP3R cluster model

The oscillating concentration of intracellular calcium is one of the most important examples for collective dynamics in cell biology. Localized releases of calcium through clusters of inositol 1,4,5-trisphosphate receptor channels constitute elementary signals called calcium puffs. Coupling by diffusing calcium leads to global releases and waves, but the exact mechanism of inter-cluster coupling and triggering of waves is unknown. To elucidate the relation of puffs and waves, we here model a cluster of IP3R channels using a gating scheme with variable non-equilibrium IP3 binding. Hybrid stochastic and deterministic simulations show that puffs are not stereotyped events of constant duration but are sensitive to stimulation strength and residual calcium. For increasing IP3 concentration, the release events become modulated at a timescale of minutes, with repetitive wave-like releases interspersed with several puffs. This modulation is consistent with experimental observations we present, including refractoriness and increase of puff frequency during the inter-wave interval. Our results suggest that waves are established by a random but time-modulated appearance of sustained release events, which have a high potential to trigger and synchronize activity throughout the cell.

Termination of calcium puffs and coupled closings of inositol trisphosphate receptor channels

Calcium puffs are localized Ca(2+) signals mediated by Ca(2+) release from the endoplasmic reticulum (ER) through clusters of inositol trisphosphate receptor (IP3R) channels. The recruitment of IP3R channels during puffs depends on Ca(2+)-induced Ca(2+) release, a regenerative process that must be terminated to maintain control of cell signaling and prevent Ca(2+) cytotoxicity. Here, we studied puff termination using total internal reflection microscopy to resolve the gating of individual IP3R channels during puffs in intact SH-SY5Y neuroblastoma cells. We find that the kinetics of IP3R channel closing differ from that expected for independent, stochastic gating, in that multiple channels tend to remain open together longer than predicted from their individual open lifetimes and then close in near-synchrony. This behavior cannot readily be explained by previously proposed termination mechanisms, including Ca(2+)-inhibition of IP3Rs and local depletion of Ca(2+) in the ER lumen. Instead, we postulate that the gating of closely adjacent IP3Rs is coupled, possibly via allosteric interactions, suggesting an important mechanism to ensure robust puff termination in addition to Ca(2+)-inactivation.

Multi-scale data-driven modeling and observation of calcium puffs

The spatiotemporal dynamics of elementary Ca(2+) release events, such as "blips" and "puffs" shapes the hierarchal Ca(2+) signaling in many cell types. Despite being the building blocks of Ca(2+) patterning, the mechanism responsible for the observed properties of puffs, especially their termination is incompletely understood. In this paper, we employ a data-driven approach to gain insights into the complex dynamics of blips and puffs. We use a model of inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) derived directly from single channel patch clamp data taken at 10 μM concentration of IP(3) to simulate calcium puffs. We first reproduce recent observations regarding puffs and blips and then investigate the mechanism of puff termination. Our model suggests that during a puff, IP(3)R s proceed around a loop through kinetic states from "rest" to "open" to "inhibited" and back to "rest". A puff terminates because of self-inhibition. Based on our simulations, we rule out the endoplasmic reticulum (ER) Ca(2+) depletion as a possible cause for puff termination. The data-driven approach also enables us to estimate the current through a single IP(3)R and the peak Ca(2+) concentration near the channel pore.

Cooperative and stochastic calcium releases from multiple calcium puff sites generate calcium microdomains in intact Hela cells

Ca(2+) microdomains or locally restricted Ca(2+) increases in the cell have recently been reported to regulate many essential physiological events. Ca(2+) increases through the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)/Ca(2+) release channels contribute to the formation of a class of such Ca(2+) microdomains, which were often observed and referred to as Ca(2+) puffs in their isolated states. In this report, we visualized IP(3)-evoked Ca(2+) microdomains in histamine-stimulated intact HeLa cells using a total internal reflection fluorescence microscope, and quantitatively characterized the spatial profile by fitting recorded images to a two-dimensional Gaussian distribution. Ca(2+) concentration profiles were marginally spatially anisotropic, with the size increasing linearly even after the amplitude began to decline. We found the event centroid drifted with an apparent diffusion coefficient of 4.20 ± 0.50 μm(2)/s, which is significantly larger than those estimated for IP(3)Rs. The sites of maximal Ca(2+) increase, rather than initiation or termination sites, were detected repeatedly at the same location. These results indicate that Ca(2+) microdomains in intact HeLa cell are generated from spatially distributed multiple IP(3)R clusters or Ca(2+) puff sites, rather than a single IP(3)R cluster reported in cells loaded with Ca(2+) buffers.

Custom-made modification of a commercial confocal microscope to photolyze caged compounds using the conventional illumination module and its application to the observation of Inositol 1,4,5-trisphosphate-mediated calcium signals

The flash photolysis of "caged" compounds is a powerful experimental technique for producing rapid changes in concentrations of bioactive signaling molecules. These caged compounds are inactive and become active when illuminated with ultraviolet light. This paper describes an inexpensive adaptation of an Olympus confocal microscope that uses as source of ultraviolet light the mercury lamp that comes with the microscope for conventional fluorescence microscopy. The ultraviolet illumination from the lamp (350 - 400 nm) enters through an optical fiber that is coupled to a nonconventional port of the microscope. The modification allows to perform the photolysis of caged compounds over wide areas (∼ 200 μm) and obtain confocal fluorescence images simultaneously. By controlling the ultraviolet illumination exposure time and intensity it is possible to regulate the amount of photolyzed compounds. In the paper we characterize the properties of the system and show its capabilities with experiments done in aqueous solution and in Xenopus Laevis oocytes. The latter demonstrate its applicability for the study of Inositol 1,4,5-trisphosphate-mediated intracellular calcium signals.

Recording single-channel activity of inositol trisphosphate receptors in intact cells with a microscope, not a patch clamp

Optical single-channel recording is a novel tool for the study of individual Ca2+-permeable channels within intact cells under minimally perturbed physiological conditions. As applied to the functioning and spatial organization of IP3Rs, this approach complements our existing knowledge, which derives largely from reduced systems - such as reconstitution into lipid bilayers and patch clamping of IP3Rs on the membrane of excised nuclei - where the spatial arrangement and interactions among IP3Rs via CICR are disrupted. The ability to image the activity of single IP3R channels with millisecond resolution together with localization of their positions with a precision of a few tens of nanometers both raises several intriguing questions and holds promise of answers. In particular, what mechanism underlies the anchoring of puffs and blips to static locations; why do these Ca2+ release events appear to involve only a very small fraction of the IP3Rs within a cell; and how can we reconcile the relative immotility of functional IP3Rs with numerous studies reporting free diffusion of IP3R protein in the ER membrane?

Puff-wave transition in an inhomogeneous model for calcium signals

In many cell types, calcium ion channels on the endoplasmic reticulum membrane occur in a clustered distribution. The channels generate either localized puffs, each comprising channels of only one cluster, or global calcium waves. In this work we model the calcium system as a two-dimensional lattice of active elements distributed regularly in an otherwise passive space. We address an important feature of the puff-wave transition, which is the difference in lifetime of puffs at a few hundred milliseconds and long-lived global waves with periods of several seconds. We show that such a lifetime difference between puffs and waves can be understood with strongly reduced ordinary differential equations modified by a time-scale factor that takes into account the coupling strength of active and passive regions determined by the Ca2+ diffusion coefficient. Furthermore, we show that the point model can also describe very well the dependence of Ca2+ oscillation characteristics on the cluster-cluster distance in the case of large diffusivity.

Mitochondrial Ca2+ uptake increases Ca2+ release from inositol 1,4,5-trisphosphate receptor clusters in smooth muscle cells

Smooth muscle activities are regulated by inositol 1,4,5-trisphosphate (InsP(3))-mediated increases in cytosolic Ca2+ concentration ([Ca2+](c)). Local Ca2+ release from an InsP(3) receptor (InsP(3)R) cluster present on the sarcoplasmic reticulum is termed a Ca2+ puff. Ca2+ released via InsP(3)R may diffuse to adjacent clusters to trigger further release and generate a cell-wide (global) Ca2+ rise. In smooth muscle, mitochondrial Ca2+ uptake maintains global InsP(3)-mediated Ca2+ release by preventing a negative feedback effect of high [Ca2+] on InsP(3)R. Mitochondria may regulate InsP(3)-mediated Ca2+ signals by operating between or within InsP(3)R clusters. In the former mitochondria could regulate only global Ca2+ signals, whereas in the latter both local and global signals would be affected. Here whether mitochondria maintain InsP(3)-mediated Ca2+ release by operating within (local) or between (global) InsP(3)R clusters has been addressed. Ca2+ puffs evoked by localized photolysis of InsP(3) in single voltage-clamped colonic smooth muscle cells had amplitudes of 0.5-4.0 F/F(0), durations of approximately 112 ms at half-maximum amplitude, and were abolished by the InsP(3)R inhibitor 2-aminoethoxydiphenyl borate. The protonophore carbonyl cyanide 3-chloropheylhydrazone and complex I inhibitor rotenone each depolarized DeltaPsi(M) to prevent mitochondrial Ca2+ uptake and attenuated Ca2+ puffs by approximately 66 or approximately 60%, respectively. The mitochondrial uniporter inhibitor, RU360, attenuated Ca2+ puffs by approximately 62%. The "fast" Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acted like mitochondria to prolong InsP(3)-mediated Ca2+ release suggesting that mitochondrial influence is via their Ca2+ uptake facility. These results indicate Ca2+ uptake occurs quickly enough to influence InsP(3)R communication at the intra-cluster level and that mitochondria regulate both local and global InsP(3)-mediated Ca2+ signals.

Imaging the quantal substructure of single IP3R channel activity during Ca2+ puffs in intact mammalian cells

The spatiotemporal patterning of Ca(2+) signals regulates numerous cellular functions, and is determined by the functional properties and spatial clustering of inositol trisphosphate receptor (IP(3)R) Ca(2+) release channels in the endoplasmic reticulum membrane. However, studies at the single-channel level have been hampered because IP(3)Rs are inaccessible to patch-clamp recording in intact cells, and because excised organelle and bilayer reconstitution systems disrupt the Ca(2+)-induced Ca(2+) release (CICR) process that mediates channel-channel coordination. We introduce here the use of total internal reflection fluorescence microscopy to image single-channel Ca(2+) flux through individual and clustered IP(3)Rs in intact mammalian cells. This enables a quantal dissection of the local calcium puffs that constitute building blocks of cellular Ca(2+) signals, revealing stochastic recruitment of, on average, approximately 6 active IP(3)Rs clustered within <500 nm. Channel openings are rapidly ( approximately 10 ms) recruited by opening of an initial trigger channel, and a similarly rapid inhibitory process terminates puffs despite local [Ca(2+)] elevation that would otherwise sustain Ca(2+)-induced Ca(2+) release indefinitely. Minimally invasive, nano-scale Ca(2+) imaging provides a powerful tool for the functional study of intracellular Ca(2+) release channels while maintaining the native architecture and dynamic interactions essential for discrete and selective cell signaling.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Modeling Ca2+ feedback on a single inositol 1,4,5-trisphosphate receptor and its modulation by Ca2+ buffers

The inositol 1,4,5-trisphosphate receptor/channel (IP(3)R) is a major regulator of intracellular Ca(2+) signaling, and liberates Ca(2+) ions from the endoplasmic reticulum in response to binding at cytosolic sites for both IP(3) and Ca(2+). Although the steady-state gating properties of the IP(3)R have been extensively studied and modeled under conditions of fixed [IP(3)] and [Ca(2+)], little is known about how Ca(2+) flux through a channel may modulate the gating of that same channel by feedback onto activating and inhibitory Ca(2+) binding sites. We thus simulated the dynamics of Ca(2+) self-feedback on monomeric and tetrameric IP(3)R models. A major conclusion is that self-activation depends crucially on stationary cytosolic Ca(2+) buffers that slow the collapse of the local [Ca(2+)] microdomain after closure. This promotes burst-like reopenings by the rebinding of Ca(2+) to the activating site; whereas inhibitory actions are substantially independent of stationary buffers but are strongly dependent on the location of the inhibitory Ca(2+) binding site on the IP(3)R in relation to the channel pore.

Localization of puff sites adjacent to the plasma membrane: functional and spatial characterization of Ca2+ signaling in SH-SY5Y cells utilizing membrane-permeant caged IP3

The Xenopus oocyte has been a favored model system in which to study spatio-temporal mechanisms of intracellular Ca2+ dynamics, in large part because this giant cell facilitates intracellular injections of Ca2+ indicator dyes, buffers and caged compounds. However, the recent commercial availability of membrane-permeant ester forms of caged IP3 (ci-IP3) and EGTA, now allows for facile loading of these compounds into smaller mammalian cells, permitting control of [IP3]i and cytosolic Ca2+ buffering. Here, we establish the human neuroblastoma SH-SY5Y cell line as an advantageous experimental system for imaging Ca2+ signaling, and characterize IP3-mediated Ca2+ signaling mechanisms in these cells. Flash photo-release of increasing amounts of i-IP3 evokes Ca2+ puffs that transition to waves, but intracellular loading of EGTA decouples release sites, allowing discrete puffs to be studied over a wide range of [IP3]. Puff activity persists for minutes following a single photo-release, pointing to a slow rate of i-IP3 turnover in these cells and suggesting that repetitive Ca2+ spikes with periods of 20-30s are not driven by oscillations in [IP3]. Puff amplitudes are independent of [IP3], whereas their frequencies increase with increasing photo-release. Puff sites in SH-SY5Y cells are not preferentially localized near the nucleus, but instead are concentrated close to the plasma membrane where they can be visualized by total internal reflection microscopy, offering the potential for unprecedented spatio-temporal resolution of Ca2+ puff kinetics.

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

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

Characterization of local calcium signals in tubular networks of endoplasmic reticulum

To explain the large time and space scales of elementary calcium events in Xenopus oocytes it is assumed that the Ca2+ source is located on tubules of the endoplasmic reticulum, which provide local barriers for diffusion. The event duration, width and signal mass dependence on the total quantity of released Ca2+ is determined at different orientations of the scan line and different ionic currents. Excellent agreement with published data is obtained with on- and off-rate constants of the fluorescent indicator of 15 microM(-1) s(-1) and 2.55 s(-1), respectively. It is found that one signal mass unit, calculated with the classical method that assumes spherical symmetry of the cytosolic space surrounding the release site, corresponds to 0.189 fC of released Ca2+ in the presence of a tubular network. It is estimated that release Ca2+ currents and amounts are randomly distributed, with averages of 0.165 pA and 3.66 fC per event and average release duration of 22.2 ms. The total quantity of liberated Ca2+ and the release current amplitude in the presence of endoplasmic reticulum tubules is predicted to be about one order of magnitude lower than estimated within the isotropic diffusion formalism. This could have implications in muscle cell Ca2+ imaging as well.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

F-actin-based Ca signaling-a critical comparison with the current concept of Ca signaling

A short comparative survey on the current idea of Ca signaling and the alternative concept of F-actin-based Ca signaling is given. The two hypotheses differ in one central aspect, the mechanism of Ca storage. The current theory rests on the assumption of Ca-accumulating endoplasmic/sarcoplasmic reticulum-derived vesicles equipped with an ATP-dependent Ca pump and IP3- or ryanodine-sensitive channel-receptors for Ca-release. The alternative hypothesis proceeds from the idea of Ca storage at the high-affinity binding sites of actin filaments. Cellular sites of F-actin-based Ca storage are microvilli and the submembrane cytoskeleton. Several specific features of Ca signaling such as store-channel coupling, quantal Ca release, spiking and oscillations, biphasic and "phasic" uptake kinetics, and Ca-induced Ca release (CICR), which are not adequately described by the current concept, are inherent properties of the F-actin system and its dynamic state of treadmilling.

'Trigger' events precede calcium puffs in Xenopus oocytes

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

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

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

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.

Models of the inositol trisphosphate receptor

The inositol (1,4,5)-trisphosphate receptor (IPR) plays a crucial role in calcium dynamics in a wide range of cell types, and is often a central feature in quantitative models of calcium oscillations and waves. We review deterministic and stochastic mathematical models of the IPR, from the earliest ones of the 1970s and 1980s, to the most recent. The effects of IPR stochasticity on Ca2+ dynamics are briefly discussed.

Modeling stochastic Ca2+ release from a cluster of IP3-sensitive receptors

We focused our attention on Ca(2+) release from the endoplasmic reticulum through a cluster of inositol(1,4,5)-trisphosphate (IP(3)) receptor channels. The random opening and closing of these receptors introduce stochastic effects that have been observed experimentally. Here, we present a stochastic version of Othmer-Tang model (OTM) for IP(3) receptor clusters. We address the average behavior of the channels in response to IP(3) stimuli. In our stochastic simulation we found that the fraction of open channels versus [IP(3)] follows a Hill curve, whose associate Hill coefficient increases when intracellular Ca(2+) level increase. This finding suggests that feedback from cytosolic Ca(2+) plays a key role in the channel response to IP(3). We also study several aspects of the stochastic properties of Ca(2+) release and we compare with experimental observations.

Gating mechanisms of the type-1 inositol trisphosphate receptor

A large amount of data and observations on inositol 1,4,5-trisphosphate (IP(3)) binding to the IP(3) receptor/Ca(2+) channel, the steady-state activity of the channel, and its inactivation by IP(3) can be explained by assuming one activation and one inhibition module, both allosterically operated by Ca(2+), IP(3), and ATP, and one adaptation element, driven by IP(3), Ca(2+), and the interconversion between two possible conformations of the receptor. The adaptation module becomes completely insensitive to a second IP(3) pulse within 80 s. Observed kinetic responses are well reproduced if, in addition, two module open states are rendered inactive by the current charge carrier Mn(2+). The inactivation time constants are 59 s in the activation, and 0.75 s in the adaptation module. The in vivo open probability of the channel is predicted to be almost in coincidence with the behavior in lipid bilayers for IP(3) levels of 0.2 and 2 microM and one-order-higher at 0.02 microM IP(3), whereas at 180 microM IP(3) the maximal in vivo activity may be 2.5-orders higher than in bilayers and restricted to a narrower Ca(2+) domain (approximately 10 microM-wide versus approximately 100 microM-wide). IP(3) is likely to inhibit channel activity at < or =120 nM Ca(2+) in vivo.

Molecular Cloning of Mouse Type 2 and Type 3 Inositol 1,4,5-Trisphosphate Receptors and Identification of a Novel Type 2 Receptor Splice Variant

We isolated cDNAs encoding type 2 and type 3 inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)R2 and IP(3)R3, respectively) from mouse lung and found a novel alternative splicing segment, SI(m2), at 176-208 of IP(3)R2. The long form (IP(3)R2 SI(m2)(+)) was dominant, but the short form (IP(3)R2 SI(m2)(-)) was detected in all tissues examined. IP(3)R2 SI(m2)(-) has neither IP(3) binding activity nor Ca(2+) releasing activity. In addition to its reticular distribution, IP(3)R2 SI(m2)(+) is present in the form of clusters in the endoplasmic reticulum of resting COS-7 cells, and after ATP or Ca(2+) ionophore stimulation, most of the IP(3)R2 SI(m2)(+) is in clusters. IP(3)R3 is localized uniformly on the endoplasmic reticulum of resting cells and forms clusters after ATP or Ca(2+) ionophore stimulation. IP(3)R2 SI(m2)(-) does not form clusters in either resting or stimulated cells. IP(3) binding-deficient site-directed mutants of IP(3)R2 SI(m2)(+) and IP(3)R3 fail to form clusters, indicating that IP(3) binding is involved in the cluster formation by these isoforms. Coexpression of IP(3)R2 SI(m2)(-) prevents stimulus-induced IP(3)R clustering, suggesting that IP(3)R2 SI(m2)(-) functions as a negative coordinator of stimulus-induced IP(3)R clustering. Expression of IP(3)R2 SI(m2)(-) in CHO-K1 cells significantly reduced ATP-induced Ca(2+) entry, but not Ca(2+) release, suggesting that the novel splice variant of IP(3)R2 specifically influences the dynamics of the sustained phase of Ca(2+) signals.

A model of IP3 receptor with a luminal calcium binding site: stochastic simulations and analysis

We have constructed a stochastic model of the inositol 1,4,5-trisphosphate receptor-Ca2+ channel that is based on quantitative measurements of the channel's properties. It displays the observed dependence of the open probability of the channel with cytosolic [Ca2+] and [IP3] and gives values for the dwell times that agree with the observations. The model includes an explicit dependence of channel gating with luminal calcium. This not only explains several observations reported in the literature, but also provides a possible explanation of why the open probabilities and shapes of the bell-shaped curves reported in [Nature 351 (1991) 751] and in [Proc. Natl. Acad. Sci. U.S.A. 269 (1998) 7238] are so different.

Saltatory and continuous calcium waves and the rapid buffering approximation

Calcium waves propagate inside cells due to a regenerative mechanism known as calcium-induced calcium release. Buffer-mediated calcium diffusion in the cytosol plays a crucial role in the process. However, most models of calcium waves either treat buffers phenomenologically or assume that they are in equilibrium with calcium (the rapid buffering approximation). In this article we address the issue of whether this approximation provides a good description of wave propagation. We first compare the timescales present in the problem, and determine the situations in which the equilibrium hypothesis fails. We then present a series of numerical studies based on the simple fire-diffuse-fire model of wave propagation. We find that the differences between the full and reduced descriptions may lead to errors that are above experimental resolution even for relatively fast buffers in the case of saltatory waves. Conversely, in the case of continuous waves, the approximation may give accurate results even for relatively slow buffers.

Role of complexing agents in the appearance of Turing patterns

In this paper we study a four-species reaction-diffusion system where Turing patterns are stabilized by the presence of fast reversible reactions between the morphogens and two different mobile complexing agents (CAs) that are not necessarily in excess. We provide a quantitative explanation of how the interaction with the CA changes the size of the Turing space making it possible to observe patterns even in a region where the free diffusion coefficients of the relevant species are equal, as is usually the case in real systems. Our analytical treatment gives a series of mathematical relations that can be helpful for those designing experiments where Turing patterns are expected to appear. We also show how the mobility of CAs affect the characteristic size of the pattern. Finally, we provide an example of biological interest in order to illustrate the main procedures and results.

Buffer kinetics shape the spatiotemporal patterns of IP3-evoked Ca2+ signals

Ca2+ liberation through inositol 1,4,5-trisphosphate receptors (IP3Rs) plays a universal role in cell regulation, and specificity of cell signalling is achieved through the spatiotemporal patterning of Ca2+ signals. IP3Rs display Ca2+-induced Ca2+ release (CICR), but are grouped in clusters so that regenerative Ca2+ signals may remain localized to individual clusters, or propagate globally between clusters by successive cycles of Ca2+ diffusion and CICR. We used confocal microscopy and photoreleased IP3 in Xenopus oocytes to study how these properties are modulated by mobile cytosolic Ca2+ buffers. EGTA (a buffer with slow 'on-rate') speeded Ca2+ signals and 'balkanized' Ca2+ waves by dissociating them into local signals. In contrast, BAPTA (a fast buffer with similar affinity) slowed Ca2+ responses and promoted 'globalization' of spatially uniform Ca2+ signals. These actions are likely to arise through differential effects on Ca2+ feedback within and between IP3R clusters, because Ca2+ signals evoked by influx through voltage-gated channels were little affected. We propose that cell-specific expression of Ca2+-binding proteins with distinct kinetics may shape the time course and spatial distribution of IP3-evoked Ca2+ signals for specific physiological roles.

Selection of intracellular calcium patterns in a model with clustered Ca2+ release channels

A two-dimensional model is proposed for intracellular Ca2+ waves, which incorporates both the discrete nature of Ca2+ release sites in the endoplasmic reticulum membrane and the stochastic dynamics of the clustered inositol 1,4,5-triphosphate (IP3) receptors. Depending on the Ca2+ diffusion coefficient and concentration of IP3, various spontaneous Ca2+ patterns, such as calcium puffs, local waves, abortive waves, global oscillation, and tide waves, can be observed. We further investigate the speed of the global waves as a function of the IP3 concentration and the Ca2+ diffusion coefficient and under what conditions the spatially averaged Ca2+ response can be described by a simple set of ordinary differential equations.

Buffers and oscillations in intracellular Ca2+ dynamics

I model the behavior of intracellular Ca(2+) release with high buffer concentrations. The model uses a spatially discrete array of channel clusters. The channel subunit dynamics is a stochastic representation of the DeYoung-Keizer model. The calculations show that the concentration profile of fast buffer around an open channel is more localized than that of slow buffers. Slow buffers allow for release of larger amounts of Ca(2+) from the endoplasmic reticulum and hence bind more Ca(2+) than fast buffers with the same dissociation constant and concentration. I find oscillation-like behavior for high slow buffer concentration and low Ca(2+) content of the endoplasmic reticulum. High concentration of slow buffer leads to oscillation-like behavior by repetitive wave nucleation for high Ca(2+) content of the endoplasmic reticulum. Localization of Ca(2+) release by slow buffer, as used in experiments, can be reproduced by the modeling approach.

On the role of stochastic channel behavior in intracellular Ca2+ dynamics

I present a stochastic model for intracellular Ca(2+) oscillations. The model starts from stochastic binding and dissociation of Ca(2+) to binding sites on a single subunit of the IP(3)-receptor channel but is capable of simulating large numbers of clusters for many oscillation periods too. I find oscillations with variable periods ranging from 17 s to 120 s and a standard deviation well in the experimentally observed range. Long period oscillations can be perceived as nucleation phenomenon and can be observed for a large variety of single channel dynamics. Their period depends on the geometric characteristics of the cluster array. Short periods are in the range of the time scale of channel dynamics. Both long and short period oscillations occur for parameters with a nonoscillatory deterministic regime.

Stochastic properties of Ca(2+) release of inositol 1,4,5-trisphosphate receptor clusters

Intracellular Ca(2+) release is controlled by inositol 1,4,5-trisphosphate (IP(3)) receptors or ryanodine receptors. These receptors are typically distributed in clusters with several or tens of channels. The random opening and closing of these channels introduces stochasticity into the elementary calcium release mechanism. Stochastic release events have been experimentally observed in a variety of cell types and have been termed sparks and puffs. We put forward a stochastic version of the Li-Rinzel model (the deactivation binding process is described by a Markovian scheme) and a computationally more efficient Langevin approach to model the stochastic Ca(2+) oscillation of single clusters. Statistical properties such as Ca(2+) puff amplitudes, lifetimes, and interpuff intervals are studied with both models and compared with experimental observations. For clusters with tens of channels, a simply decaying amplitude distribution is typically observed at low IP(3) concentration, while a single peak distribution appears at high IP(3) concentration.

Polarized expression of G protein-coupled receptors and an all-or-none discharge of Ca2+ pools at initiation sites of [Ca2+]i waves in polarized exocrine cells

In the present work we examined localization and behavior of G protein-coupled receptors (GPCR) in polarized exocrine cells to address the questions of how luminal to basal Ca(2+) waves can be generated in a receptor-specific manner and whether quantal Ca(2+) release reflects partial release from a continuous pool or an all-or-none release from a compartmentalized pool. Immunolocalization revealed that expression of GPCRs in polarized cells is not uniform, with high levels of GPCR expression at or near the tight junctions. Measurement of phospholipase Cbeta activity and receptor-dependent recruitment and trapping of the box domain of RGS4 in GPCRs complexes indicated autonomous functioning of G(q)-coupled receptors in acinar cells. These findings explain the generation of receptor-specific Ca(2+) waves and why the waves are always initiated at the apical pole. The initiation site of Ca(2+) wave at the apical pole and the pattern of wave propagation were independent of inositol 1,4,5-trisphosphate concentration. Furthermore, a second Ca(2+) wave with the same initiation site and pattern was launched by inhibition of sarco/endoplasmic reticulum Ca(2+)-ATPase pumps of cells continuously stimulated with sub-maximal agonist concentration. By contrast, rapid sequential application of sub-maximal and maximal agonist concentrations to the same cell triggered Ca(2+) waves with different initiation sites. These findings indicate that signaling specificity in pancreatic acinar cells is aided by polarized expression and autonomous functioning of GPCRs and that quantal Ca(2+) release is not due to a partial Ca(2+) release from a continuous pool, but rather, it is due to an all-or-none Ca(2+) release from a compartmentalized Ca(2+) pool.

Ragged spiking of free calcium in ADP-stimulated human platelets: regulation of puff-like calcium signals in vitro and ex vivo

1. Human platelets respond to agonists of G protein (G(q))-coupled receptors by generating an irregular pattern of spiking changes in cytosolic Ca2+ ([Ca2+]i). We have investigated the ADP-induced Ca2+ responses of single, Fluo-3-loaded platelets in the presence or absence of autologous plasma or whole blood under flow conditions. 2. In plasma-free platelets, incubated in buffer medium, baseline separated [Ca2+]i peaks always consisted of a rapid rising phase (median time 0.8 s) which was abruptly followed by a slower, mono-exponential decay phase. The decay constant differed from platelet to platelet, ranging from 0.23 +/- 0.02 to 0.63 +/- 0.03 s(-1) (mean +/- S.E.M., n = 3-5), and was used to identify individual Ca2+ release events and to determine the Ca2+ fluxes of the events. 3. Confocal, high-frequency measurements of adherent, spread platelets (diameter 3-5 microm) indicated that different optical regions had simultaneous patterns of both low- and high-amplitude Ca2+ release events. 4. With or without plasma or flowing blood, the ADP-induced Ca2+ signals in platelets had the characteristics of irregular Ca2+ puffs as well as more regular Ca2+ oscillations. Individual [Ca2+]i peaks varied in amplitude and peak-to-peak interval, as observed for separated Ca2+ puffs within larger cells. On the other hand, the peaks appeared to group into periods of ragged, shorter-interval Ca2+ release events with little integration, which were alternated with longer-interval events. 5. We conclude that the spiking Ca2+ signal generated in these small cells has the characteristics of a 'poor' oscillator with an irregular frequency being reactivated from period to period. This platelet signal appears to be similar in an environment of non-physiological buffer medium and in flowing, whole blood.

Single-cell imaging of graded Ins(1,4,5)P3 production following G-protein-coupled-receptor activation

The pleckstrin homology domain of phospholipase Cdelta1 (PH(PLCdelta)) binds Ins(1,4,5)P(3) and PtdIns(4,5)P(2) specifically, and can be used to detect changes in Ins(1,4,5)P(3) in single cells. A fusion construct of PH(PLCdelta) and enhanced green fluorescent protein (EGFP-PH(PLCdelta)) associates with the plasma membrane due to its association with PtdIns(4,5)P(2). However, PH(PLCdelta) has greater affinity for Ins(1,4,5)P(3) than PtdIns(4,5)P(2), and translocates to the cytosol as Ins(1,4,5)P(3) levels rise. Prolonged activation of group I metabotropic glutamate receptor 1alpha expressed in Chinese-hamster ovary cells or endogenous M(3) muscarinic receptors in SH-SY5Y neuroblastoma cells gave an initial transient peak in translocation, followed by a sustained plateau phase. This closely followed changes in cell population Ins(1,4,5)P(3) mass, but not PtdIns(4,5)P(2) levels, which decreased monophasically, as determined by radioreceptor assay. Translocation thus provides a real-time method to follow increases in Ins(1,4,5)P(3). Graded changes in Ins(1,4,5)P(3) in Chinese-hamster ovary-lac-mGlu1alpha cells could be detected with increasing glutamate concentrations, and dual loading with fura 2 and EGFP-PH(PLCdelta) showed that changes in intracellular Ca(2+) concentration closely paralleled Ins(1,4,5)P(3) production. Moreover, Ins(1,4,5)P(3) accumulation and intracellular Ca(2+) mobilization within single cells is graded in nature and dependent on both agonist concentration and receptor density.

The endoplasmic reticulum: one continuous or several separate Ca(2+) stores?

The Ca2+ store and sink in the endoplasmic reticulum (ER) is important for Ca2+ signal integration and for conveyance of information in spatial and temporal domains. Textbooks regard the ER as one continuous network, but biochemical and biophysical studies revealed apparently discrete ER Ca2+ stores. Recent direct studies of ER lumenal Ca2+ movements show that this organelle system is one continuous Ca2+ store, which can function as a Ca2+ tunnel. The concept of a fully connected ER network is entirely compatible with evidence indicating that the distribution of Ca2+ -release channels in the ER membrane is discontinuous with clustering in certain localities.