Reversible desensitization of inositol trisphosphate-induced calcium release provides a mechanism for repetitive calcium spikes

Endothelial TRPV4 channels modulate vascular tone by Ca2+ -induced Ca2+ release at inositol 1,4,5-trisphosphate receptors

BACKGROUND AND PURPOSE

The TRPV4 ion channels are Ca²⁺ permeable, non-selective cation channels that mediate large, but highly localized, Ca²⁺ signals in the endothelium. The mechanisms that permit highly localized Ca²⁺ changes to evoke cell-wide activity are incompletely understood. Here, we tested the hypothesis that TRPV4-mediated Ca²⁺ influx activates Ca²⁺ release from internal Ca²⁺ stores to generate widespread effects.

EXPERIMENTAL APPROACH

Ca²⁺ signals in large numbers (~100) of endothelial cells in intact arteries were imaged and analysed separately.

KEY RESULTS

Responses to the TRPV4 channel agonist GSK1016790A were heterogeneous across the endothelium. In activated cells, Ca²⁺ responses comprised localized Ca²⁺ changes leading to slow, persistent, global increases in Ca²⁺ followed by large propagating Ca²⁺ waves that moved within and between cells. To examine the mechanisms underlying each component, we developed methods to separate slow persistent Ca²⁺ rise from the propagating Ca²⁺ waves in each cell. TRPV4-mediated Ca²⁺ entry was required for the slow persistent global rise and propagating Ca²⁺ signals. The propagating waves were inhibited by depleting internal Ca²⁺ stores, inhibiting PLC or blocking IP₃ receptors. Ca²⁺ release from stores was tightly controlled by TRPV4-mediated Ca²⁺ influx and ceased when influx was terminated. Furthermore, Ca²⁺ release from internal stores was essential for TRPV4-mediated control of vascular tone.

CONCLUSIONS AND IMPLICATIONS

Ca²⁺ influx via TRPV4 channels is amplified by Ca²⁺ -induced Ca²⁺ release acting at IP₃ receptors to generate propagating Ca²⁺ waves and provide a large-scale endothelial communication system. TRPV4-mediated control of vascular tone requires Ca²⁺ release from the internal store.

Mitochondrial ATP production provides long-range control of endothelial inositol trisphosphate-evoked calcium signaling

Endothelial cells are reported to be glycolytic and to minimally rely on mitochondria for ATP generation. Rather than providing energy, mitochondria in endothelial cells may act as signaling organelles that control cytosolic Ca2+ signaling or modify reactive oxygen species (ROS). To control Ca2+ signaling, these organelles are often observed close to influx and release sites and may be tethered near Ca2+ transporters. In this study, we used high-resolution, wide-field fluorescence imaging to investigate the regulation of Ca2+ signaling by mitochondria in large numbers of endothelial cells (∼50 per field) in intact arteries from rats. We observed that mitochondria were mostly spherical or short-rod structures and were distributed widely throughout the cytoplasm. The density of these organelles did not increase near contact sites with smooth muscle cells. However, local inositol trisphosphate (IP3)-mediated Ca2+ signaling predominated near these contact sites and required polarized mitochondria. Of note, mitochondrial control of Ca2+ signals occurred even when mitochondria were far from Ca2+ release sites. Indeed, the endothelial mitochondria were mobile and moved throughout the cytoplasm. Mitochondrial control of Ca2+ signaling was mediated by ATP production, which, when reduced by mitochondrial depolarization or ATP synthase inhibition, eliminated local IP3-mediated Ca2+ release events. ROS buffering did not significantly alter local Ca2+ release events. These results highlight the importance of mitochondrial ATP production in providing long-range control of endothelial signaling via IP3-evoked local Ca2+ release in intact endothelium.

Examining the role of mitochondria in Ca²⁺ signaling in native vascular smooth muscle

Mitochondrial Ca²⁺ uptake contributes important feedback controls to limit the time course of Ca²⁺signals. Mitochondria regulate cytosolic [Ca²⁺] over an exceptional breath of concentrations (∼200 nM to >10 μM) to provide a wide dynamic range in the control of Ca²⁺ signals. Ca²⁺ uptake is achieved by passing the ion down the electrochemical gradient, across the inner mitochondria membrane, which itself arises from the export of protons. The proton export process is efficient and on average there are less than three protons free within the mitochondrial matrix. To study mitochondrial function, the most common approaches are to alter the proton gradient and to measure the electrochemical gradient. However, drugs which alter the mitochondrial proton gradient may have substantial off target effects that necessitate careful consideration when interpreting their effect on Ca²⁺ signals. Measurement of the mitochondrial electrochemical gradient is most often performed using membrane potential sensitive fluorophores. However, the signals arising from these fluorophores have a complex relationship with the electrochemical gradient and are altered by changes in plasma membrane potential. Care is again needed in interpreting results. This review provides a brief description of some of the methods commonly used to alter and measure mitochondrial contribution to Ca²⁺ signaling in native smooth muscle.

Mitochondrial regulation of cytosolic Ca²⁺ signals in smooth muscle

The cytosolic Ca²⁺ concentration ([Ca²⁺]c) controls virtually every activity of smooth muscle, including contraction, migration, transcription, division and apoptosis. These processes may be activated by large (>10 μM) amplitude [Ca²⁺]c increases, which occur in small restricted regions of the cell or by smaller (<1 μM) amplitude changes throughout the bulk cytoplasm. Mitochondria contribute to the regulation of these signals by taking up Ca²⁺. However, mitochondria's reported low affinity for Ca²⁺ is thought to require the organelle to be positioned close to ion channels and within a microdomain of high [Ca²⁺]. In cultured smooth muscle, mitochondria are highly dynamic structures but in native smooth muscle mitochondria are immobile, apparently strategically positioned organelles that regulate the upstroke and amplitude of IP₃-evoked Ca²⁺ signals and IP₃ receptor (IP₃R) cluster activity. These observations suggest mitochondria are positioned within the high [Ca²⁺] microdomain arising from an IP₃R cluster to exert significant local control of channel activity. On the other hand, neither the upstroke nor amplitude of voltage-dependent Ca²⁺ entry is modulated by mitochondria; rather, it is the declining phase of the transient that is regulated by the organelle. Control of the declining phase of the transient requires a high mitochondrial affinity for Ca²⁺ to enable uptake to occur over the normal physiological Ca²⁺ range (<1 μM). Thus, in smooth muscle, mitochondria regulate Ca²⁺ signals exerting effects over a large range of [Ca²⁺] (∼200 nM to at least tens of micromolar) to provide a wide dynamic range in the control of Ca²⁺ signals.

IP(3) receptors: toward understanding their activation

Inositol 1,4,5-trisphosphate receptors (IP(3)R) and their relatives, ryanodine receptors, are the channels that most often mediate Ca(2+) release from intracellular stores. Their regulation by Ca(2+) allows them also to propagate cytosolic Ca(2+) signals regeneratively. This brief review addresses the structural basis of IP(3)R activation by IP(3) and Ca(2+). IP(3) initiates IP(3)R activation by promoting Ca(2+) binding to a stimulatory Ca(2+)-binding site, the identity of which is unresolved. We suggest that interactions of critical phosphate groups in IP(3) with opposite sides of the clam-like IP(3)-binding core cause it to close and propagate a conformational change toward the pore via the adjacent N-terminal suppressor domain. The pore, assembled from the last pair of transmembrane domains and the intervening pore loop from each of the four IP(3)R subunits, forms a structure in which a luminal selectivity filter and a gate at the cytosolic end of the pore control cation fluxes through the IP(3)R.

Agonist-evoked Ca(2+) wave progression requires Ca(2+) and IP(3)

Smooth muscle responds to IP(3)-generating agonists by producing Ca(2+) waves. Here, the mechanism of wave progression has been investigated in voltage-clamped single smooth muscle cells using localized photolysis of caged IP(3) and the caged Ca(2+) buffer diazo-2. Waves, evoked by the IP(3)-generating agonist carbachol (CCh), initiated as a uniform rise in cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) over a single though substantial length (approximately 30 microm) of the cell. During regenerative propagation, the wave-front was about 1/3 the length (approximately 9 microm) of the initiation site. The wave-front progressed at a relatively constant velocity although amplitude varied through the cell; differences in sensitivity to IP(3) may explain the amplitude changes. Ca(2+) was required for IP(3)-mediated wave progression to occur. Increasing the Ca(2+) buffer capacity in a small (2 microm) region immediately in front of a CCh-evoked Ca(2+) wave halted progression at the site. However, the wave front does not progress by Ca(2+)-dependent positive feedback alone. In support, colliding [Ca(2+)](c) increases from locally released IP(3) did not annihilate but approximately doubled in amplitude. This result suggests that local IP(3)-evoked [Ca(2+)](c) increases diffused passively. Failure of local increases in IP(3) to evoke waves appears to arise from the restricted nature of the IP(3) increase. When IP(3) was elevated throughout the cell, a localized increase in Ca(2+) now propagated as a wave. Together, these results suggest that waves initiate over a surprisingly large length of the cell and that both IP(3) and Ca(2+) are required for active propagation of the wave front to occur.

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.

`Quantal' Ca2+ release at the cytoplasmic aspect of the Ins(1,4,5)P3R channel in smooth muscle

Smooth muscle responds to activation of the inositol (1,4,5)-trisphosphate receptor [Ins(1,4,5)P3R] with a graded concentration-dependent (`quantal') Ca2+ release from the sarcoplasmic reticulum (SR) store. Graded release seems incompatible both with the finite capacity of the store and the Ca2+-induced Ca2+ release (CICR)-like facility, at Ins(1,4,5)P3Rs, that, once activated, should release the entire content of SR Ca2+. The structural organization of the SR and the regulation of Ins(1,4,5)P3R activity by inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] and Ca2+ have each been proposed to explain `quantal' Ca2+ release. Here, we propose that regulation of Ins(1,4,5)P3R activity by lumenal Ca2+ acting at the cytoplasmic aspect of the receptor might explain `quantal' Ca2+ release in smooth muscle. The entire SR store was found to be lumenally continuous and Ca2+ could diffuse freely throughout: peculiarities of SR structure are unlikely to account for `quantal' release. While Ca2+ release was regulated by [Ca2+] within the SR, the velocity of release increased (accelerated) during the release process. The extent of acceleration of release determined the peak cytoplasmic [Ca2+] and was attenuated by a reduction in SR [Ca2+] or an increase in cytoplasmic Ca2+ buffering. Positive feedback by released Ca2+ acting at the cytoplasmic aspect of Ins(1,4,5)P3Rs (i.e. CICR-like) might (a) account for the acceleration, (b) provide the regulation of release by SR [Ca2+] and (c) explain the `quantal' release process itself. During Ca2+ release, SR [Ca2+] and thus unitary Ins(1,4,5)P3R currents decline, CICR reduces and stops. With increasing [Ins(1,4,5)P3], coincidental activation of several neighbouring Ins(1,4,5)P3Rs offsets the reduced Ins(1,4,5)P3R current to renew CICR and Ca2+ release.

Graded recruitment and inactivation of single InsP3 receptor Ca2+-release channels: implications for quantal [corrected] Ca2+release

Modulation of cytoplasmic free Ca2+ concentration ([Ca2+]i) by receptor-mediated generation of inositol 1,4,5-trisphosphate (InsP3) and activation of its receptor (InsP3R), a Ca2+-release channel in the endoplasmic reticulum, is a ubiquitous signalling mechanism. A fundamental aspect of InsP3-mediated signalling is the graded release of Ca2+ in response to incremental levels of stimuli. Ca2+ release has a transient fast phase, whose rate is proportional to [InsP3], followed by a much slower one even in constant [InsP3]. Many schemes have been proposed to account for quantal Ca2+ release, including the presence of heterogeneous channels and Ca2+ stores with various mechanisms of release termination. Here, we demonstrate that mechanisms intrinsic to the single InsP3R channel can account for quantal Ca2+ release. Patch-clamp electrophysiology of isolated insect Sf9 cell nuclei revealed a consistent and high probability of detecting functional endogenous InsP3R channels, enabling InsP3-induced channel inactivation to be identified as an inevitable consequence of activation, and allowing the average number of activated channels in the membrane patch (N(A)) to be accurately quantified. InsP3-activated channels invariably inactivated, with average duration of channel activity reduced by high [Ca2+]i and suboptimal [InsP3]. Unexpectedly, N(A) was found to be a graded function of both [Ca2+]i and [InsP3]. A qualitative model involving Ca2+-induced InsP3R sequestration and inactivation can account for these observations. These results suggest that apparent heterogeneous ligand sensitivity can be generated in a homogeneous population of InsP3R channels, providing a mechanism for graded Ca2+ release that is intrinsic to the InsP3R Ca2+ release channel itself.

Modelling of calcium handling in airway myocytes

Airway myocytes are the primary effectors of airway reactivity which modulates airway resistance and hence ventilation. Stimulation of airway myocytes results in an increase in the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and the subsequent activation of the contractile apparatus. Many contractile agonists, including acetylcholine, induce [Ca(2+)](i) increase via Ca(2+) release from the sarcoplasmic reticulum through InsP(3) receptors. Several models have been developed to explain the characteristics of InsP(3)-induced [Ca(2+)](i) responses, in particular Ca(2+) oscillations. The article reviews the modelling of the major structures implicated in intracellular Ca(2+) handling, i.e., InsP(3) receptors, SERCAs, mitochondria and Ca(2+)-binding cytosolic proteins. We developed theoretical models specifically dedicated to the airway myocyte which include the major mechanisms responsible for intracellular Ca(2+) handling identified in these cells. These biocomputations pointed out the importance of the relative proportion of InsP(3) receptor isoforms and the respective role of the different mechanisms responsible for cytosolic Ca(2+) clearance in the pattern of [Ca(2+)](i) variations. We have developed a theoretical model of membrane conductances that predicts the variations in membrane potential and extracellular Ca(2+) influx. Stimulation of this model by simulated increase in [Ca(2+)](i) predicts membrane depolarisation, but not great enough to trigger a significant opening of voltage-dependant Ca(2+) channels. This may explain why airway contraction induced by cholinergic stimulation does not greatly depend on extracellular calcium. The development of such models of airway myocytes is important for the understanding of the cellular mechanisms of airway reactivity and their possible modulation by pharmacological agents.

The sarcoplasmic reticulum, Ca2+ trapping, and wave mechanisms in smooth muscle

The sarcoplasmic reticulum (SR) and apposed regions of the sarcolemma passively trap Ca2+ entering the cell to limit the rise in cytoplasmic Ca2+ concentration without SR pump involvement. When "leaky," the SR facilitates Ca2+ entry to the cytoplasm. SR Ca2+ release via inositol 1,4,5-trisphosphate receptors (IP(3)Rs) propagates as calcium waves; IP(3)Rs alone account for wave propagation.

Origin and Mechanisms of Ca2+ Waves in Smooth Muscle as Revealed by Localized Photolysis of Caged Inositol 1,4,5-Trisphosphate

The cytosolic Ca(2+) concentration ([Ca(2+)](c)) controls diverse cellular events via various Ca(2+) signaling patterns; the latter are influenced by the method of cell activation. Here, in single-voltage clamped smooth muscle cells, sarcolemma depolarization generated uniform increases in [Ca(2+)](c) throughout the cell entirely by Ca(2+) influx. On the other hand, the Ca(2+) signal produced by InsP(3)-generating agonists was a propagated wave. Using localized uncaged InsP(3), the forward movement of the Ca(2+) wave arose from Ca(2+)-induced Ca(2+) release at the InsP(3) receptor (InsP(3)R) without ryanodine receptor involvement. The decline in [Ca(2+)](c) (the back of the wave) occurred from a functional compartmentalization of the store, which rendered the site of InsP(3)-mediated Ca(2+) release, and only this site, refractory to the phosphoinositide. The functional compartmentalization arose by a localized feedback deactivation of InsP(3) receptors produced by an increased [Ca(2+)](c) rather than a reduced luminal [Ca(2+)] or an increased cytoplasmic [InsP(3)]. The deactivation of the InsP(3) receptor was delayed in onset, compared with the time of the rise in [Ca(2+)](c), persisted (>30 s) even when [Ca(2+)](c) had regained resting levels, and was not prevented by kinase or phosphatase inhibitors. Thus different forms of cell activation generate distinct Ca(2+) signaling patterns in smooth muscle. Sarcolemma Ca(2+) entry increases [Ca(2+)](c) uniformly; agonists activate InsP(3)R and produce Ca(2+) waves. Waves progress by Ca(2+)-induced Ca(2+) release at InsP(3)R, and persistent Ca(2+)-dependent inhibition of InsP(3)R accounts for the decline in [Ca(2+)](c) at the back of the wave.

Integrated luminal and cytosolic aspects of the calcium release control

We propose here a unitary approach to the luminal and cytosolic control of calcium release. A minimal number of model elements that realistically describe different data sets are combined and adapted to correctly respond to various physiological constraints. We couple the kinetic properties of the inositol 1,4,5 trisphosphate receptor/calcium channel with the dynamics of Ca(2+) and K(+) in both the lumen and cytosol, and by using a detailed simulation approach, we propose that local (on a radial distance approximately 2 micro m) calcium oscillations in permeabilized cells are driven by the slow inactivation of channels organized in discrete clusters composed of between six and 15 channels. Moreover, the character of these oscillations is found to be extremely sensitive to K(+), so that the cytosolic and luminal calcium variations are in or out of phase if the store at equilibrium has tens or hundreds micro M Ca(2+), respectively, depending on the K(+) gradient across the reticulum membrane. Different patterns of calcium signals can be reproduced through variation of only a few parameters.

IP3 receptors and their regulation by calmodulin and cytosolic Ca2+

Inositol 1,4,5-trisphosphate (IP(3)) receptors are tetrameric intracellular Ca(2+) channels, the opening of which is regulated by both IP(3) and Ca(2+). We suggest that all IP(3) receptors are biphasically regulated by cytosolic Ca(2+), which binds to two distinct sites. IP(3) promotes channel opening by controlling whether Ca(2+) binds to the stimulatory or inhibitory sites. The stimulatory site is probably an integral part of the receptor lying just upstream of the pore region. Inhibition of IP(3) receptors by Ca(2+) probably requires an accessory protein, which has not yet been unequivocally identified, but calmodulin is a prime candidate. We speculate that one lobe of calmodulin tethers it to the IP(3) receptor, while the other lobe can bind Ca(2+) and then interact with a second site on the receptor to cause inhibition.

Modelling of simple and complex calcium oscillations. From single-cell responses to intercellular signalling

This review provides a comparative overview of recent developments in the modelling of cellular calcium oscillations. A large variety of mathematical models have been developed for this wide-spread phenomenon in intra- and intercellular signalling. From these, a general model is extracted that involves six types of concentration variables: inositol 1,4,5-trisphosphate (IP3), cytoplasmic, endoplasmic reticulum and mitochondrial calcium, the occupied binding sites of calcium buffers, and the fraction of active IP3 receptor calcium release channels. Using this framework, the models of calcium oscillations can be classified into 'minimal' models containing two variables and 'extended' models of three and more variables. Three types of minimal models are identified that are all based on calcium-induced calcium release (CICR), but differ with respect to the mechanisms limiting CICR. Extended models include IP3--calcium cross-coupling, calcium sequestration by mitochondria, the detailed gating kinetics of the IP3 receptor, and the dynamics of G-protein activation. In addition to generating regular oscillations, such models can describe bursting and chaotic calcium dynamics. The earlier hypothesis that information in calcium oscillations is encoded mainly by their frequency is nowadays modified in that some effect is attributed to amplitude encoding or temporal encoding. This point is discussed with reference to the analysis of the local and global bifurcations by which calcium oscillations can arise. Moreover, the question of how calcium binding proteins can sense and transform oscillatory signals is addressed. Recently, potential mechanisms leading to the coordination of oscillations in coupled cells have been investigated by mathematical modelling. For this, the general modelling framework is extended to include cytoplasmic and gap-junctional diffusion of IP3 and calcium, and specific models are compared. Various suggestions concerning the physiological significance of oscillatory behaviour in intra- and intercellular signalling are discussed. The article is concluded with a discussion of obstacles and prospects.

Adenophostin A and ribophostin, but not inositol 1,4,5-trisphosphate or manno-adenophostin, activate the Ca2+ release-activated Ca2+ current, I(CRAC), in weak intracellular Ca2+ buffer

Under physiological conditions of weak intracellular Ca(2+) buffering (0.1 mM EGTA), the second messenger Ins(1,4,5)P(3) often fails to activate any detectable store-operated Ca(2+) current. However, it has been reported that the fungal metabolite adenophostin A [which has a severalfold higher affinity than Ins(1,4,5)P(3) for Ins(1,4,5)P(3) receptors] consistently activates the current under similar conditions. Here, whole-cell patch clamp experiments have been performed to examine how adenophostin A can activate the store-operated Ca(2+) current (I(CRAC)) in RBL-1 (rat basophilic leukaemia) cells. In a strong intracellular Ca(2+) buffer, saturating concentrations of adenophostin A activated I(CRAC) maximally and the current amplitude and kinetics were indistinguishable from those obtained with high concentrations of Ins(1,4,5)P(3). In a weak Ca(2+) buffer, adenophostin A consistently activated I(CRAC), but the current was submaximal. High concentrations of Ins(1,4,5)P(3) or the non-metabolizable analogue Ins(2,4,5)P(3) were largely ineffective under these conditions. The size of I(CRAC) to adenophostin A in weak Ca(2+) buffer could be significantly increased by either inhibiting sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase ('SERCA') pumps with thapsi-gargin or enhancing mitochondrial Ca(2+) uptake, although blocking the mitochondrial Ca(2+) uniporter with Ruthenium Red did not suppress the activation of the current. Changing the levels of free ATP in the recording pipette did not enhance the size of I(CRAC) evoked by adenophostin A. We also examined two structurally distinct analogues of adenophostin A (manno-adenophostin and ribophostin), for which the affinities for the Ins(1,4,5)P(3) receptor are similar to that of Ins(1,4,5)P(3) in equilibrium binding experiments. Although these analogues were able to activate I(CRAC) to its maximal extent in strong buffer, ribophostin, but not manno-adenophostin, consistently activated the current in weak buffer. We conclude that adenophostin A and ribophostin are able to activate I(CRAC) in weak buffer through a mechanism that is quite distinct from that employed by Ins(1,4,5)P(3) and manno-adenophostin and is not related to equilibrium affinities.

Characterization of transepithelial potential oscillations in theDrosophilaMalpighian tubule

The Malpighian tubule of Drosophila melanogaster is a useful model system for studying the regulation of epithelial ion transport. In acutely isolated tubules, the transepithelial potential (TEP) undergoes large oscillations in amplitude with a period of approximately 30s. The TEP oscillations are diminished by reductions in the peritubular chloride concentration in a manner consistent with their being caused by fluctuations in chloride conductance. The oscillations are eliminated by pretreating tubules with the calcium chelator BAPTA-AM, although removal of peritubular calcium has no effect, suggesting that the oscillations are a result of either the release of calcium from intracellular stores or the entry of calcium from the tubule lumen. Transcripts encoding two calcium-release channels, the ryanodine receptor and the inositol trisphosphate receptor, are detectable in the tubule by reverse transcription–polymerase chain reaction. To identify the cell type responsible for the oscillations, tubules were treated with diuretic hormones known to alter calcium levels in each of the two cell types. Leucokinin-IV, which increases calcium levels in the stellate cells, suppressed the oscillations, whereas cardioacceleratory peptide 2b (CAP2b), which increases calcium levels in the principal cells, had no effect. These data are consistent with a model in which rhythmic changes in transepithelial chloride conductance, regulated by intracellular calcium levels in the stellate cells, cause the TEP oscillations.

Sarcoplasmic/endoplasmic-reticulum-Ca2+-ATPase-mediated Ca2+ reuptake, and not Ins(1,4,5)P3 receptor inactivation, prevents the activation of macroscopic Ca2+ release-activated Ca2+ current in the presence of physiological Ca2+ buffer in rat basophilic leukaemia-1 cells

Whole-cell patch-clamp experiments were performed to examine the mechanism underlying the inability of intracellular Ins(1,4,5)P(3) to activate the Ca(2+) release-activated Ca(2+) current (I(CRAC)) in rat basophilic leukaemia (RBL)-1 cells under conditions of weak cytoplasmic Ca(2+) buffering. Dialysis with Ins(1,4,5)P(3) in weak Ca(2+) buffer did not activate any macroscopic I(CRAC) even after precautions had been taken to minimize the extent of Ca(2+) entry during the experiment. Following intracellular dialysis with Ins(1,4,5)P(3) for >150 s in weak buffer, external application of the sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase (SERCA) pump blocker thapsigargin activated I(CRAC), and the current developed much more quickly than when thapsigargin was applied in the absence of Ins(1,4,5)P(3). This indicates that the Ins(1,4,5)P(3) receptors had not inactivated much over this timecourse. When external Ca(2+) was replaced by Ba(2+), Ins(1,4,5)P(3) still failed to generate any detectable I(CRAC) even though Ba(2+) permeates CRAC channels and is not taken up into the intracellular Ca(2+) stores. In strong Ca(2+) buffer, I(CRAC) could be activated by muscarinic-receptor stimulation, provided protein kinase C (PKC) was blocked. In weak buffer, however, as with Ins(1,4,5)P(3), stimulation of these receptors with carbachol did not activate I(CRAC) even after inhibition of PKC. The inability of Ins(1,4,5)P(3) to activate macroscopic I(CRAC) in weak Ca(2+) buffer was not altered by inhibition of Ca(2+)-dependent phosphorylation/dephosphorylation reactions. Our results suggest that the inability of Ins(1,4,5)P(3) to activate I(CRAC) under conditions of weak intracellular Ca(2+) buffering is not due to strong inactivation of the Ins(1,4,5)P(3) receptors. Instead, a futile Ca(2+) cycle across the stores seems to be occurring and SERCA pumps resequester sufficient Ca(2+) to ensure that the threshold for activation of macroscopic I(CRAC) has not been exceeded.

The machinery of local Ca2+ signalling between sarco-endoplasmic reticulum and mitochondria

Growing evidence suggests that propagation of cytosolic [Ca2+] ([Ca2+]c) spikes and oscillations to the mitochondria is important for the control of fundamental cellular functions. Delivery of [Ca2+]c spikes to the mitochondria may utilize activation of the mitochondrial Ca2+ uptake sites by the large local [Ca2+]c rise occurring in the vicinity of activated sarco-endoplasmic reticulum (SR/ER) Ca2+ release channels. Although direct measurement of the local [Ca2+]c sensed by the mitochondria has been difficult, recent studies shed some light onto the molecular mechanism of local Ca2+ communication between SR/ER and mitochondria. Subdomains of the SR/ER are in close contact with mitochondria and display a concentration of Ca2+ release sites, providing the conditions for an effective delivery of released Ca2+ to the mitochondrial targets. Furthermore, many functional properties of the signalling between SR/ER Ca2+ release sites and mitochondrial Ca2+ uptake sites, including transient microdomains of high [Ca2+], saturation of mitochondrial Ca2+ uptake sites by released Ca2+, connection of multiple release sites to each uptake site and quantal transmission, are analogous to the features of the coupling between neurotransmitter release sites and postsynaptic receptors in synaptic transmission. As such, Ca2+ signal transmission between SR/ER and mitochondria may utilize discrete communication sites and a closely related functional architecture to that used for synaptic signal propagation between cells.

Control of InsP3-induced Ca2+ oscillations in permeabilized blowfly salivary gland cells: contribution of mitochondria

Many agonists linked to the generation of inositol 1,4, 5-trisphosphate (InsP3) and release of Ca2+ from intracellular stores induce repetitive transients in cytosolic Ca2+ whose frequency increases over a certain range of agonist concentrations. In order to investigate the mechanisms underlying this frequency modulation, the fluorescent Ca2+ sensor mag-fura-2 was loaded into intracellular calcium stores and used to monitor InsP3-induced dynamics of the intraluminal calcium concentration ([Ca2+]L) in secretory cells of permeabilized blowfly Calliphora vicina salivary glands. In this preparation, increasing concentrations of InsP3 induced graded decreases in [Ca2+]L that were often superimposed with repetitive [Ca2+]L transients produced by sequential Ca2+ release and re-uptake. These [Ca2+]L oscillations developed at frequencies of 3-11 min-1 unrelated to the concentration of InsP3 present. In contrast, incremental concentrations of InsP3 applied in the presence of the oxidizable mitochondrial substrates citrate, succinate, or pyruvate-malate induced repetitive [Ca2+]L transients whose frequency increased with the concentration of InsP3. This InsP3 concentration-dependent modulation of oscillation frequency was abolished after dissipating the mitochondrial membrane potential (Delta psi m) by combined treatment with carbonyl cyanide p-trifluoromethoxyphenyl hydrazone + oligomycin or after application of Ruthenium Red, an inhibitor of mitochondrial Ca2+ uptake. Taken together, the data indicate that energized mitochondria exert negative control over the frequency of InsP3-induced Ca2+ oscillations. It is concluded that mitochondria play a crucial role in determining the duration of the interspike period and, therefore, for the encoding of amplitude-modulated, InsP3-liberating stimuli into the frequency of cytosolic Ca2+ oscillations.

Dual regulation of calcium mobilization by inositol 1,4, 5-trisphosphate in a living cell

Changes in cytosolic free calcium ([Ca(2+)](i)) often take the form of a sustained response or repetitive oscillations. The frequency and amplitude of [Ca(2+)](i) oscillations are essential for the selective stimulation of gene expression and for enzyme activation. However, the mechanism that determines whether [Ca(2+)](i) oscillates at a particular frequency or becomes a sustained response is poorly understood. We find that [Ca(2+)](i) oscillations in rat megakaryocytes, as in other cells, results from a Ca(2+)-dependent inhibition of inositol 1,4,5-trisphosphate (IP(3))-induced Ca(2+) release. Moreover, we find that this inhibition becomes progressively less effective with higher IP(3) concentrations. We suggest that disinhibition, by increasing IP(3) concentration, of Ca(2+)-dependent inhibition is a common mechanism for the regulation of [Ca(2+)](i) oscillations in cells containing IP(3)-sensitive Ca(2+) stores.

Differential codes for free Ca(2+)-calmodulin signals in nucleus and cytosol

BACKGROUND

Many targets of calcium signaling pathways are activated or inhibited by binding the Ca(2+)-liganded form of calmodulin (Ca(2+)-CaM). Here, we test the hypothesis that local Ca(2+)-CaM-regulated signaling processes can be selectively activated by local intracellular differences in free Ca(2+)-CaM concentration.

RESULTS

Energy-transfer confocal microscopy of a fluorescent biosensor was used to measure the difference in the concentration of free Ca(2+)-CaM between nucleus and cytoplasm. Strikingly, short receptor-induced calcium spikes produced transient increases in free Ca(2+)-CaM concentration that were of markedly higher amplitude in the cytosol than in the nucleus. In contrast, prolonged increases in calcium led to equalization of the nuclear and cytosolic free Ca(2+)-CaM concentrations over a period of minutes. Photobleaching recovery and translocation measurements with fluorescently labeled CaM showed that equalization is likely to be the result of a diffusion-mediated net translocation of CaM into the nucleus. The driving force for equalization is a higher Ca(2+)-CaM-buffering capacity in the nucleus compared with the cytosol, as the direction of the free Ca(2+)-CaM concentration gradient and of CaM translocation could be reversed by expressing a Ca(2+)-CaM-binding protein at high concentration in the cytosol.

CONCLUSIONS

Subcellular differences in the distribution of Ca(2+)-CaM-binding proteins can produce gradients of free Ca(2+)-CaM concentration that result in a net translocation of CaM. This provides a mechanism for dynamically regulating local free Ca(2+)-CaM concentrations, and thus the local activity of Ca(2+)-CaM targets. Free Ca(2+)-CaM signals in the nucleus remain low during brief or low-frequency calcium spikes, whereas high-frequency spikes or persistent increases in calcium cause translocation of CaM from the cytoplasm to the nucleus, resulting in similar concentrations of nuclear and cytosolic free Ca(2+)-CaM.

Morphological control of inositol-1,4,5-trisphosphate-dependent signals

Inositol-1,4,5-trisphosphate (InsP(3))-mediated calcium signals represent an important mechanism for transmitting external stimuli to the cell. However, information about intracellular spatial patterns of InsP(3) itself is not generally available. In particular, it has not been determined how the interplay of InsP(3) generation, diffusion, and degradation within complex cellular geometries can control the patterns of InsP(3) signaling. Here, we explore the spatial and temporal characteristics of [InsP(3)](cyt) during a bradykinin-induced calcium wave in a neuroblastoma cell. This is achieved by using a unique image-based computer modeling system, Virtual Cell, to integrate experimental data on the rates and spatial distributions of the key molecular components of the process. We conclude that the characteristic calcium dynamics requires rapid, high-amplitude production of [InsP(3)](cyt) in the neurite. This requisite InsP(3) spatiotemporal profile is provided, in turn, as an intrinsic consequence of the cell's morphology, demonstrating how geometry can locally and dramatically intensify cytosolic signals that originate at the plasma membrane. In addition, the model predicts, and experiments confirm, that stimulation of just the neurite, but not the soma or growth cone, is sufficient to generate a calcium response throughout the cell.

The role of Ca2+ feedback in shaping InsP3-evoked Ca2+ signals in mouse pancreatic acinar cells

1. Cytosolic Ca2+ has been proposed to act as both a positive and a negative feedback signal on the inositol trisphosphate (InsP3) receptor. However, it is unclear how this might affect the Ca2+ response in vivo. 2. Mouse pancreatic acinar cells were whole-cell patch clamped to record the Ca2+-dependent chloride (Cl(Ca)) current spikes and imaged to record the cytosolic Ca2+ spikes elicited by the injection of Ins(2,4,5)P3. Increasing concentrations of Ca2+ buffer (up to 200 microM EGTA or BAPTA) were associated with the appearance of steps in the current activation phase and a prevalence of smaller-amplitude Cl(Ca) spikes. Imaging experiments showed that with increased buffer the secretory pole cytosolic Ca2+ signal became fragmented and spatially discrete Ca2+ release events were observed. 3. At higher buffer concentrations (200-500 microM), increasing concentrations of EGTA increased spike frequency and reduced spike amplitude. In contrast, BAPTA decreased spike frequency and maintained large spike amplitudes. 4. We conclude that, during InsP3-evoked spiking, long-range Ca2+ feedback ( approximately 2-4 microm) shapes the rising phase of the Ca2+ signal by acting to co-ordinate discrete Ca2+ release events and short-range ( approximately 40 nm) Ca2+ feedback acts to inhibit further Ca2+ release.

The inositol trisphosphate receptor regulates a 50-second behavioral rhythm in C. elegans

The C. elegans defecation cycle is characterized by the contraction of three distinct sets of muscles every 50 s. Our data indicate that this cycle is regulated by periodic calcium release mediated by the inositol trisphosphate receptor (IP3 receptor). Mutations in the IP3 receptor slow down or eliminate the cycle, while overexpression speeds up the cycle. The IP3 receptor controls these periodic muscle contractions nonautonomously from the intestine. In the intestinal cells, calcium levels oscillate with the same period as the defecation cycle and peak calcium levels immediately precede the first muscle contraction. Mutations in the IP3 receptor slow or eliminate these calcium oscillations. Thus, the IP3 receptor is an essential component of the timekeeper for this cycle and represents a novel mechanism for the control of behavioral rhythms.

Regulation of cerebellar Ins(1,4,5)P3 receptor by interaction between Ins(1,4,5)P3 and Ca2+

We have characterized in detail the Ca(2+)-dependent inhibition of [(3)H]Ins(1,4,5)P(3) ([(3)H]InsP(3)) binding to sheep cerebellar microsomes, over a short duration (3 s), with the use of a perfusion protocol. This procedure prevented artifacts previously identified in studies of this Ca(2+) effect. In a cytosol-like medium at pH 7.1 and 20 degrees C, a maximal inhibition of approx. 50% was measured. Both inhibition and its reversal were complete within 3 s. Ca(2+) decreased the affinity of the receptor for InsP(3) by approx. 50% (K(d) 146+/-24 nM at pCa 9 and 321+/-56 nM at pCa 5.3), without changing the total number of binding sites. Conversely, increasing the [(3)H]InsP(3) concentration from 30 to 400 nM tripled the IC(50) for Ca(2+) and decreased the maximal inhibition by 63%. This is similar to a partial competitive inhibition between InsP(3) binding and inhibitory Ca(2+) binding and is consistent with InsP(3) and Ca(2+) converting InsP(3) receptor into two different states with different affinities for these ligands. Mn(2+) and Sr(2+) also inhibited [(3)H]InsP(3) binding but were respectively only 1/10 and 1/200 as effective as Ca(2+). No inhibition was observed with Ba(2+). This selectivity is the same as that previously reported for the inhibitory Ca(2+) site of InsP(3)-induced Ca(2+) flux, suggesting that the same site is used by Ca(2+) to convert cerebellar InsP(3) receptor to a low-affinity state and to inhibit its channel activity. Our results also suggest a mechanism by which InsP(3) counteracts this Ca(2+)-dependent inhibition.

Determination of time-dependent inositol-1,4,5-trisphosphate concentrations during calcium release in a smooth muscle cell

The level of [InsP3]cyt required for calcium release in A7r5 cells, a smooth muscle cell line, was determined by a new set of procedures using quantitative confocal microscopy to measure release of InsP3 from cells microinjected with caged InsP3. From these experiments, the [InsP3]cyt required to evoke a half-maximal calcium response is 100 nM. Experiments with caged glycerophosphoryl-myo-inositol 4, 5-bisphosphate (GPIP2), a slowly metabolized analogue of InsP3, gave a much slower recovery and a half-maximal response of an order of magnitude greater than InsP3. Experimental data and highly constrained variables were used to construct a mathematical model of the InsP3-dependent [Ca2+]cyt changes; the resulting simulations show high fidelity to experiment. Among the elements considered in constructing this model were the mechanism of the InsP3-receptor, InsP3 degradation, calcium buffering in the cytosol, and refilling of the ER stores via sarcoplasmic endoplasmic reticulum ATPase (SERCA) pumps. The model predicts a time constant of 0.8 s for InsP3 degradation and 13 s for GPIP2. InsP3 degradation was found to be a prerequisite for [Ca2+]cyt recovery to baseline levels and is therefore critical to the pattern of the overall [Ca2+]cyt signal. Analysis of the features of this model provides insights into the individual factors controlling the amplitude and shape of the InsP3-mediated calcium signal.

Regulatory and spatial aspects of inositol trisphosphate-mediated calcium signals

Hormones that act to release Ca2+ from intracellular stores initiate a signaling cascade that culminates in the production of inositol 1,4,5-trisphosphate (InsP3). The Ca2+ response mediated by InsP3 is not a sustained increase in the cytosolic Ca2+ concentration, but rather a series of periodic spikes that manifest as waves in larger cells. In vitro studies have determined that the key positive feedback parameter driving spikes and waves is a highly localized direct Ca(2+)-activation of InsP3-gated Ca2+ channels. Advances in fluorescent Ca2+ imaging have facilitated the resolution of individual positive feedback units. These studies have revealed that there are several modes of channel coupling underlying global Ca2+ signals; single channel openings or Ca2+ "blips," synchronized clusters of channels or Ca2+ "puffs," and cell wide calcium waves. It appears that the channel clusters that produce Ca2+ puffs are synchronized by the highly localized positive feedback that was predicted by the in vitro studies of channel regulation. Localization of InsP3-induced Ca2+ signals has been shown to be important for activation of several cellular processes including uni-directional salt flow and mitochondrial activation.

Agonist-dependent phosphorylation of the inositol 1,4,5-trisphosphate receptor: A possible mechanism for agonist-specific calcium oscillations in pancreatic acinar cells

The properties of inositol 1,4,5-trisphosphate (IP3)-dependent intracellular calcium oscillations in pancreatic acinar cells depend crucially on the agonist used to stimulate them. Acetylcholine or carbachol (CCh) cause high-frequency (10-12-s period) calcium oscillations that are superimposed on a raised baseline, while cholecystokinin (CCK) causes long-period (>100-s period) baseline spiking. We show that physiological concentrations of CCK induce rapid phosphorylation of the IP3 receptor, which is not true of physiological concentrations of CCh. Based on this and other experimental data, we construct a mathematical model of agonist-specific intracellular calcium oscillations in pancreatic acinar cells. Model simulations agree with previous experimental work on the rates of activation and inactivation of the IP3 receptor by calcium (DuFour, J.-F., I.M. Arias, and T.J. Turner. 1997. J. Biol. Chem. 272:2675-2681), and reproduce both short-period, raised baseline oscillations, and long-period baseline spiking. The steady state open probability curve of the model IP3 receptor is an increasing function of calcium concentration, as found for type-III IP3 receptors by Hagar et al. (Hagar, R.E., A.D. Burgstahler, M.H. Nathanson, and B.E. Ehrlich. 1998. Nature. 396:81-84). We use the model to predict the effect of the removal of external calcium, and this prediction is confirmed experimentally. We also predict that, for type-III IP3 receptors, the steady state open probability curve will shift to lower calcium concentrations as the background IP3 concentration increases. We conclude that the differences between CCh- and CCK-induced calcium oscillations in pancreatic acinar cells can be explained by two principal mechanisms: (a) CCK causes more phosphorylation of the IP3 receptor than does CCh, and the phosphorylated receptor cannot pass calcium current; and (b) the rate of calcium ATPase pumping and the rate of calcium influx from the outside the cell are greater in the presence of CCh than in the presence of CCK.

Quasi-synaptic calcium signal transmission between endoplasmic reticulum and mitochondria

Transmission of cytosolic [Ca2+] ([Ca2+]c) oscillations into the mitochondrial matrix is thought to be supported by local calcium control between IP3 receptor Ca2+ channels (IP3R) and mitochondria, but study of the coupling mechanisms has been difficult. We established a permeabilized cell model in which the Ca2+ coupling between endoplasmic reticulum (ER) and mitochondria is retained, and mitochondrial [Ca2+] ([Ca2+]m) can be monitored by fluorescence imaging. We demonstrate that maximal activation of mitochondrial Ca2+ uptake is evoked by IP3-induced perimitochondrial [Ca2+] elevations, which appear to reach values >20-fold higher than the global increases of [Ca2+]c. Incremental doses of IP3 elicited [Ca2+]m elevations that followed the quantal pattern of Ca2+ mobilization, even at the level of individual mitochondria. In contrast, gradual increases of IP3 evoked relatively small [Ca2+]m responses despite eliciting similar [Ca2+]c increases. We conclude that each mitochondrial Ca2+ uptake site faces multiple IP3R, a concurrent activation of which is required for optimal activation of mitochondrial Ca2+ uptake. This architecture explains why calcium oscillations evoked by synchronized periodic activation of IP3R are particularly effective in establishing dynamic control over mitochondrial metabolism. Furthermore, our data reveal fundamental functional similarities between ER-mitochondrial Ca2+ coupling and synaptic transmission.

Inositol 1,4,5-trisphosphate [correction of tris-phosphate] activation of inositol trisphosphate [correction of tris-phosphate] receptor Ca2+ channel by ligand tuning of Ca2+ inhibition

Inositol 1,4,5-trisphosphate (IP3) [corrected] binding to its receptors (IP3R) in the endoplasmic reticulum (ER) activates Ca2+ release from the ER lumen to the cytoplasm, generating complex cytoplasmic Ca2+ concentration signals including temporal oscillations and propagating waves. IP3-mediated Ca2+ release is also controlled by cytoplasmic Ca2+ concentration with both positive and negative feedback. Single-channel properties of the IP3R in its native ER membrane were investigated by patch clamp electrophysiology of isolated Xenopus oocyte nuclei to determine the dependencies of IP3R on cytoplasmic Ca2+ and IP3 concentrations under rigorously defined conditions. Instead of the expected narrow bell-shaped cytoplasmic free Ca2+ concentration ([Ca2+]i) response centered at approximately 300 nM-1 microM, the open probability remained elevated (approximately 0.8) in the presence of saturating levels (10 microM) of IP3, even as [Ca2+]i was raised to high concentrations, displaying two distinct types of functional Ca2+ binding sites: activating sites with half-maximal activating [Ca2+]i (Kact) of 210 nM and Hill coefficient (Hact) approximately 2; and inhibitory sites with half-maximal inhibitory [Ca2+]i (Kinh) of 54 microM and Hill coefficient (Hinh) approximately 4. Lowering IP3 concentration was without effect on Ca2+ activation parameters or Hinh, but decreased Kinh with a functional half-maximal activating IP3 concentration (KIP3) of 50 nM and Hill coefficient (HIP3) of 4 for IP3. These results demonstrate that Ca2+ is a true receptor agonist, whereas the sole function of IP3 is to relieve Ca2+ inhibition of IP3R. Allosteric tuning of Ca2+ inhibition by IP3 enables the individual IP3R Ca2+ channel to respond in a graded fashion, which has implications for localized and global cytoplasmic Ca2+ concentration signaling and quantal Ca2+ release.

Inositol trisphosphate receptors: Ca2+-modulated intracellular Ca2+ channels

The three subtypes of inositol trisphosphate (InsP3) receptor expressed in mammalian cells are each capable of forming intracellular Ca2+ channels that are regulated by both InsP3 and cytosolic Ca2+. The InsP3 receptors of many, though perhaps not all, tissues are biphasically regulated by cytosolic Ca2+: a rapid stimulation of the receptors by modest increases in Ca2+ concentration is followed by a slower inhibition at higher Ca2+ concentrations. Despite the widespread occurrence of this form of regulation and the belief that it is an important element of the mechanisms responsible for the complex Ca2+ signals evoked by physiological stimuli, the underlying mechanisms are not understood. Both accessory proteins and Ca2+-binding sites on InsP3 receptors themselves have been proposed to mediate the effects of cytosolic Ca2+ on InsP3 receptor function, but the evidence is equivocal. The effects of cytosolic Ca2+ on InsP3 binding and channel opening, and the possible means whereby the effects are mediated are discussed in this review.

Cell-permeant caged InsP3 ester shows that Ca2+ spike frequency can optimize gene expression

Inositol 1,4,5-trisphosphate (InsP3) releases calcium from intracellular stores and triggers complex waves and oscillations in levels of cytosolic free calcium. To determine which longer-term responses are controlled by oscillations in InsP3 and cytosolic free calcium, it would be useful to deliver exogenous InsP3, under spatial and temporal control, into populations of unpermeabilized cells. Here we report the 15-step synthesis of a membrane-permeant, caged InsP3 derivative from myo-inositol This derivative diffused into intact cells and was hydrolysed to produce a caged, metabolically stable InsP3 derivative. This latter derivative accumulated in the cytosol at concentrations of hundreds of micromolar, without activating the InsP3 receptor. Ultraviolet illumination uncaged an InsP3 analogue nearly as potent as real InsP3, and generated spikes of cytosolic free calcium, and stimulated gene expression via the nuclear factor of activated T cells. The same total amount of InsP3 analogue elicited much more gene expression when released by repetitive flashes at 1-minute intervals than when released at 0.5- or > or = 2-minute intervals, as a single pulse, or as a slow sustained plateau. Thus, oscillations in cytosolic free calcium levels at roughly physiological rates maximize gene expression for a given amount of InsP3.

Perturbation of myo-inositol-1,4,5-trisphosphate levels during agonist-induced Ca2+ oscillations

Agonist-induced Ca2+ oscillations in rat hepatocytes involve the production of myo-inositol-1,4,5-trisphosphate (IP3), which stimulates the release of Ca2+ from intracellular stores. The oscillatory frequency is conditioned by the agonist concentration. This study investigated the role of IP3 concentration in the modulation of oscillatory frequency by using microinjected photolabile IP3 analogs. Photorelease of IP3 during hormone-induced oscillations evoked a Ca2+ spike, after which oscillations resumed with a delay corresponding to the period set by the agonists. IP3 photorelease had no influence on the frequency of oscillations. After photorelease of 1-(alpha-glycerophosphoryl)-D-myo-inositol-4,5-diphosphate (GPIP2), a slowly metabolized IP3 analog, the frequency of oscillations initially increased by 34% and declined to its original level within approximately 6 min. Both IP3 and GPIP2 effects can be explained by their rate of degradation: the half-life of IP3, which is a few seconds, can account for the lack of influence of IP3 photorelease on the frequency, whereas the slower metabolism of GPIP2 allowed a transient acceleration of the oscillations. The phase shift introduced by IP3 is likely the result of the brief elevation of Ca2+ during spiking that resets the IP3 receptor to a state of maximum inactivation. A mathematical model of Ca2+ oscillations is in satisfactory agreement with the observed results.

Agonist-specific behaviour of the intracellular Ca2+ response in rat hepatocytes

A variety of agonists stimulate in hepatocytes a response that takes the shape of repetitive cytosolic free Ca2+ transients called Ca2+ oscillations. The shape of spikes and the pattern of oscillations in a given cell differ depending on the agonist of the phosphoinositide pathway that is applied. In this study, the response of individual rat hepatocytes to maximal stimulation by arginine vasopressin (AVP), phenylephrine and ADP was investigated by fluorescence microscopy and flash photolysis. Hepatocytes loaded with Ca2+-sensitive probes were stimulated with a first agonist to evoke a maximal response, and then a second agonist was added. When phenylephrine or ADP was used as the first agonist, AVP applied subsequently could elicit an additional response, which did not happen when AVP was first applied and phenylephrine or ADP was applied later. Cells microinjected with caged myo-inositol 1,4,5-trisphosphate (IP3) were challenged with the different agonists and, when a maximal response was obtained, photorelease of IP3 was triggered. Cells maximally stimulated with AVP did not respond to IP3 photorelease, whereas those stimulated with phenylephrine or ADP responded with a fast Ca2+ spike above the elevated steady-state level, which was followed by an undershoot. In contrast, with all three agonists, IP3 photorelease triggered at the top of an oscillatory Ca2+ transient was able to mobilize additional Ca2+. These experiments indicate that the differential response of cells to agonists is found not only during Ca2+ oscillations but also during maximal agonist stimulation and that potency and efficacy differences exist among agonists.

Molecular and functional evidence for multiple Ca2+-binding domains in the type 1 inositol 1,4,5-trisphosphate receptor

Structural and functional analyses were used to investigate the regulation of the inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) by Ca2+. To define the structural determinants for Ca2+ binding, cDNAs encoding GST fusion proteins that covered the complete linear cytosolic sequence of the InsP3R-1 were expressed in bacteria. The fusion proteins were screened for Ca2+ and ruthenium red binding through the use of 45Ca2+ and ruthenium red overlay procedures. Six new cytosolic Ca2+-binding regions were detected on the InsP3R in addition to the one described earlier (Sienaert, I., De Smedt, H., Parys, J. B., Missiaen, L., Vanlingen, S., Sipma, H., and Casteels, R. (1996) J. Biol. Chem. 271, 27005-27012). Strong 45Ca2+ and ruthenium red binding domains were localized in the N-terminal region of the InsP3R as follows: two Ca2+-binding domains were located within the InsP3-binding domain, and three Ca2+ binding stretches were localized in a 500-amino acid region just downstream of the InsP3-binding domain. A sixth Ca2+-binding stretch was detected in the proximity of the calmodulin-binding domain. Evidence for the involvement of multiple Ca2+-binding sites in the regulation of the InsP3R was obtained from functional studies on permeabilized A7r5 cells, in which we characterized the effects of Ca2+ and Sr2+ on the EC50 and cooperativity of the InsP3-induced Ca2+ release. The activation by cytosolic Ca2+ was due to a shift in EC50 toward lower InsP3 concentrations, and this effect was mimicked by Sr2+. The inhibition by cytosolic Ca2+ was caused by a decrease in cooperativity and by a shift in EC50 toward higher InsP3 concentrations. The effect on the cooperativity occurred at lower Ca2+ concentrations than the inhibitory effect on the EC50. In addition, Sr2+ mimicked the effect of Ca2+ on the cooperativity but not the inhibitory effect on the EC50. The different [Ca2+] and [Sr2+] dependencies suggest that three different cytosolic interaction sites were involved. Luminal Ca2+ stimulated the release without affecting the Hill coefficient or the EC50, excluding the involvement of one of the cytosolic Ca2+-binding sites. We conclude that multiple Ca2+-binding sites are localized on the InsP3R-1 and that at least four different Ca2+-interaction sites may be involved in the complex feedback regulation of the release by Ca2+.

Electroporation-induced formation of individual calcium entry sites in the cell body and processes of adherent cells

Electroporation is a widely used method for introducing macromolecules into cells. We developed an electroporation device that requires only 1 microl of sample to load adherent cells in a 10-mm2 surface area while retaining greater than 90% cell survivability. To better understand this device, field-induced permeabilization of adherent rat basophilic leukemia and neocortical neuroblastoma cells was investigated by using fluorescent calcium and voltage indicators. Rectangular field pulses led to the formation of only a few calcium entry sites, preferentially in the hyperpolarized parts of the cell body and processes. Individual entry sites were formed at the same locations when field pulses were repeated. Before calcium entry, a partial breakdown of the membrane potential was observed in both polar regions. Based on our results, a model is proposed for the formation and closure of macromolecule entry sites in adherent cells. First, the rapid formation of a large number of small pores leads to a partial membrane potential breakdown in both polar regions of the cell. Second, over tens of milliseconds, a few entry sites for macromolecules are formed, preferentially in the hyperpolarized part of cell body and processes, at locations defined by the local membrane structure. These entry sites reseal on a time scale of 50 ms to several seconds, with residual small pores remaining open for several minutes.

Photodynamic triggering of calcium oscillation in the isolated rat pancreatic acini

1. Photodynamic agents, due to their photon-dependent selective activation, can selectively activate a number of physiological processes and may directly modulate signal transduction in a number of cells including pancreatic acinar cells. 2. Activation of the photodynamic agent sulphonated aluminium phthalocyanine (SALPC) triggered recurrent cytosolic calcium ([Ca2+]i) spiking in pancreatic acinar cells. 3. The photodynamically triggered calcium spiking could be blocked by phosphatidylinositol-specific phospholipase C (PI-PLC) inhibitor U73122, but not by phosphatidylcholine-specific phospholipase C inhibitor D609. 4. Removal of extracellular Ca2+ abolished spiking, as did 2-aminoethoxydiphenylborate (2-APB), an inhibitory modulator of IP3-mediated Ca2+ release from intracellular stores. 5. These data suggest that SALPC photodynamic action may permanently fix PI-PLC in an active conformation, and this produced recurrent [Ca2+]i spiking.

Kinetics of Ca2+ release by InsP3 in pig single aortic endothelial cells: evidence for an inhibitory role of cytosolic Ca2+ in regulating hormonally evoked Ca2+ spikes

1. The role of the InsP3 receptor and its interaction with Ca2+ in shaping endothelial Ca2+ spikes was investigated by comparing InsP3-evoked intracellular Ca2+ release with hormonally evoked Ca2+ spikes in single endothelial cells. 2. InsP3 was generated by flash photolysis of intracellular caged InsP3. InsP3 at 0.2 microM or higher released Ca2+ from stores with a time course comprising a well-defined delay, a fast rise of free [Ca2+] to a peak where net flux into the cystosol is zero, and a slow decline to preflash levels. InsP3-evoked Ca2+ flux into unit cytosolic volume was measured as the rate of change of free [Ca2+]i during the fast rise, d[Ca2+]i/dt (mol s-1 l-1). 3. The mean delay decreased from 433 ms at 0.2 microM to 30 ms at 5 microM. At very high InsP3 concentrations, 78 microM, the delay was shorter, < 10 ms. At low InsP3 concentration the delay was reduced by approximately 30% by prior elevation of free [Ca2+]i, supporting a co-operative action of free [Ca2+] and InsP3 in activation. 4. Both Ca2+ flux and peak free [Ca2+]i increased with InsP3 concentration within each cell. Maximal activation was at > 5 microM, 50% maximum Ca2+ flux was at 1.6 microM InsP3 and the Hill coefficient was between 3.6 and 4.3. A large variation of Ca2+ flux and peak [Ca2+]i was found from cell to cell at the same InsP3 concentration. 5. Strong inhibition of InsP3-evoked flux was produced by an immediately preceding response, with complete inhibition at peak free [Ca2+]i due to the first pulse. InsP3 sensitivity returned over 1-2 min, with 50% recovery at approximately 25 s. The recovery of InsP3 sensitivity may determine the minimum interval between hormonally evoked spikes. 6. Ca2+ flux due to a pulse of InsP3 terminated rapidly, in the continued presence of InsP3, producing a well-defined peak [Ca2+]. A reciprocal relation was found between the duration and the rate of Ca2+ flux, such that high Ca2+ flux was of brief duration. The rate of termination of flux measured as the reciprocal of the 10-90% rise time of free [Ca2+]i showed a linear correlation with Ca2+ flux over a large range in all cells. A systematic deviation from linearity at low InsP3 concentration showed a greater rate of termination at low InsP3 concentration than at high for the same flux. 7. Elevating cytosolic free [Ca2+] by 0.1-2.5 microM strongly inhibited Ca2+ release by InsP3, and buffering free [Ca2+] to low levels greatly prolonged Ca2+ release. Both results support the idea that Ca2+ flux quickly produces locally high free [Ca2+] which inhibits the receptor and terminates Ca2+ release. 8. Hormonally evoked Ca2+ spikes showed a similar reciprocal relation between rise time and Ca2+ flux, seen in the initial Ca2+ spike evoked by extracellular ATP in porcine aortic endothelial cells and by acetylcholine in rat aortic endothelial cells in situ, supporting the idea that the same mechanism of cytosolic Ca2+ inhibition determines the duration of hormonally and InsP3-evoked Ca2+ spikes.

Minimal requirements for calcium oscillations driven by the IP3 receptor

Hormones and neurotransmitters that act through inositol 1,4,5-trisphosphate (IP3) can induce oscillations of cytosolic Ca2+ ([Ca2+]c), which render dynamic regulation of intracellular targets. Imaging of fluorescent Ca2+ indicators located within intracellular Ca2+ stores was used to monitor IP3 receptor channel (IP3R) function and to demonstrate that IP3-dependent oscillations of Ca2+ release and re-uptake can be reproduced in single permeabilized hepatocytes. This system was used to define the minimum essential components of the oscillation mechanism. With IP3 clamped at a submaximal concentration, coordinated cycles of IP3R activation and subsequent inactivation were observed in each cell. Cycling between these states was dependent on feedback effects of released Ca2+ and the ensuing [Ca2+]c increase, but did not require Ca2+ re-accumulation. [Ca2+]c can act at distinct stimulatory and inhibitory sites on the IP3R, but whereas the Ca2+ release phase was driven by a Ca2+-induced increase in IP3 sensitivity, Ca2+ release could be terminated by intrinsic inactivation after IP3 bound to the Ca2+-sensitized IP3R without occupation of the inhibitory Ca2+-binding site. These findings were confirmed using Sr2+, which only interacts with the stimulatory site. Moreover, vasopressin induced Sr2+ oscillations in intact cells in which intracellular Ca2+ was completely replaced with Sr2+. Thus, [Ca2+]c oscillations can be driven by a coupled process of Ca2+-induced activation and obligatory intrinsic inactivation of the Ca2+-sensitized state of the IP3R, without a requirement for occupation of the inhibitory Ca2+-binding site.

Elementary calcium-release units induced by inositol trisphosphate

The extent to which inositol 1,4,5-trisphosphate (InsP3)-induced calcium signals are localized is a critical parameter for understanding the mechanism of effector activation. The spatial characteristics of InsP3-mediated calcium signals were determined by targeting a dextran-based calcium indicator to intracellular membranes through the in situ addition of a geranylgeranyl lipid group. Elementary calcium-release events observed with this indicator typically lasted less than 33 milliseconds, had diameters less than 2 micrometers, and were uncoupled from each other by the calcium buffer EGTA. Cellwide calcium transients are likely to result from synchronized triggering of such local release events, suggesting that calcium-dependent effector proteins could be selectively activated by localization near sites of local calcium release.

Regulation of Ca2+ release by InsP3 in single guinea pig hepatocytes and rat Purkinje neurons

The repetitive spiking of free cytosolic [Ca2+] ([Ca2+]i) during hormonal activation of hepatocytes depends on the activation and subsequent inactivation of InsP3-evoked Ca2+ release. The kinetics of both processes were studied with flash photolytic release of InsP3 and time resolved measurements of [Ca2+]i in single cells. InsP3 evoked Ca2+ flux into the cytosol was measured as d[Ca2+]i/dt, and the kinetics of Ca2+ release compared between hepatocytes and cerebellar Purkinje neurons. In hepatocytes release occurs at InsP3 concentrations greater than 0.1-0.2 microM. A comparison with photolytic release of metabolically stable 5-thio-InsP3 suggests that metabolism of InsP3 is important in determining the minimal concentration needed to produce Ca2+ release. A distinct latency or delay of several hundred milliseconds after release of low InsP3 concentrations decreased to a minimum of 20-30 ms at high concentrations and is reduced to zero by prior increase of [Ca2+]i, suggesting a cooperative action of Ca2+ in InsP3 receptor activation. InsP3-evoked flux and peak [Ca2+]i increased with InsP3 concentration up to 5-10 microM, with large variation from cell to cell at each InsP3 concentration. The duration of InsP3-evoked flux, measured as 10-90% risetime, showed a good reciprocal correlation with d[Ca2+]i/dt and much less cell to cell variation than the dependence of flux on InsP3 concentration, suggesting that the rate of termination of the Ca2+ flux depends on the free Ca2+ flux itself. Comparing this data between hepatocytes and Purkinje neurons shows a similar reciprocal correlation for both, in hepatocytes in the range of low Ca2+ flux, up to 50 microM. s-1 and in Purkinje neurons at high flux up to 1,400 microM. s-1. Experiments in which [Ca2+]i was controlled at resting or elevated levels support a mechanism in which InsP3-evoked Ca2+ flux is inhibited by Ca2+ inactivation of closed receptor/channels due to Ca2+ accumulation local to the release sites. Hepatocytes have a much smaller, more prolonged InsP3-evoked Ca2+ flux than Purkinje neurons. Evidence suggests that these differences in kinetics can be explained by the much lower InsP3 receptor density in hepatocytes than Purkinje neurons, rather than differences in receptor isoform, and, more generally, that high InsP3 receptor density promotes fast rising, rapidly inactivating InsP3-evoked [Ca2+]i transients.

Serotonin-induced intercellular calcium waves in salivary glands of the blowfly Calliphora erythrocephala

1. Blowfly salivary glands have been used extensively as a model system for the analysis of inositol phosphate-dependent signal transduction. To detect and characterize changes in intracellular free calcium ([Ca2+]i) that might be expected to be triggered by stimulation with serotonin (5-HT), we have carried out digital calcium-imaging experiments on intact glands using the Ca2+-sensitive dye fura-2. 2. 5-HT (1-10 nM) induced repetitive transient increases in [Ca2+]i, i.e. Ca2+ spikes whose frequency was a function of agonist concentration (EC50 = 2.8 nM). 3. Pre-incubation in EGTA decreased the frequency but did not inhibit spiking. Thapsigargin abolished periodic spike activity indicating that the [Ca2+]i rise results from Ca2+ release. Neither caffeine (10 mM) nor ryanodine (10 and 50 microM) induced increases in [Ca2+]i. 4. Oscillatory activity in individual cells was synchronized by regenerative intercellular Ca2+ waves that propagated over distances greater than 400 microm. Colliding waves annihilated each other. 5. Desynchronization of the oscillation pattern by 100 microM 1-octanol suggests the involvement of gap junctions and an intracellular messenger in wave propagation. 6. Local stimulation of glands elicited [Ca2+]i elevations in the stimulated area, but not in adjacent cells, indicating that local increases in [Ca2+]i are not sufficient to trigger Ca2+ waves. However, local stimulation was capable of evoking propagating Ca2+ waves when combined with low-dose 5-HT stimulation of the whole gland. 7. The data are consistent with the hypothesis that: (1) Ca2+ acts as the intercellular messenger and modulates its own release via positive and negative feedback on the inosital 1,4,5-trisphosphate (InsP3) receptor, and (2) sensitization of the InsP3 receptor to Ca2+ by InsP3 is required for the propagation of intercellular Ca2+ waves, as proposed for intracellular Ca2+ waves in Xenopus oocytes.