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

mGluR1/TRPC3-mediated Synaptic Transmission and Calcium Signaling in Mammalian Central Neurons

Metabotropic glutamate receptors type 1 (mGluR1s) are required for a normal function of the mammalian brain. They are particularly important for synaptic signaling and plasticity in the cerebellum. Unlike ionotropic glutamate receptors that mediate rapid synaptic transmission, mGluR1s produce in cerebellar Purkinje cells a complex postsynaptic response consisting of two distinct signal components, namely a local dendritic calcium signal and a slow excitatory postsynaptic potential. The basic mechanisms underlying these synaptic responses were clarified in recent years. First, the work of several groups established that the dendritic calcium signal results from IP(3) receptor-mediated calcium release from internal stores. Second, it was recently found that mGluR1-mediated slow excitatory postsynaptic potentials are mediated by the transient receptor potential channel TRPC3. This surprising finding established TRPC3 as a novel postsynaptic channel for glutamatergic synaptic transmission.

Combining voltage and calcium imaging from neuronal dendrites

The ability to monitor membrane potential (V(m)) and calcium (Ca(2+)) transients at multiple locations on the same neuron can facilitate further progress in our understanding of neuronal function. Here we describe a method to combine V(m) and Ca(2+) imaging using styryl voltage sensitive dyes and Fura type UV-excitable Ca(2+) indicators. In all cases V(m) optical signals are linear with membrane potential changes, but the calibration of optical signals on an absolute scale is presently possible only in some neurons. The interpretation of Ca(2+) optical signals depends on the indicator Ca(2+) buffering capacity relative to the cell endogenous buffering capacity. In hippocampal CA1 pyramidal neurons, loaded with JPW-3028 and 300 microM Bis-Fura-2, V(m) optical signals cannot be calibrated and the physiological Ca(2+) dynamics are compromised by the presence of the indicator. Nevertheless, at each individual site, relative changes in V (m) and Ca(2+) fluorescence signals under different conditions can provide meaningful new information on local dendritic integration. In cerebellar Purkinje neurons, loaded with JPW-1114 and 1 mM Fura-FF, V(m) optical signals can be calibrated in terms of mV and Ca(2+) optical signals quantitatively reveal the physiological changes in free Ca(2+). Using these two examples, the method is explained in detail.

Synapse specificity of calcium release probed by chemical two-photon uncaging of inositol 1,4,5-trisphosphate

Biological messengers can be "caged" by adding a single photosensitive group that can be photolyzed by a light flash to achieve spatially and temporally precise biochemical control. Here we report that photolysis of a double-caged form of the second messenger inositol 1,4,5-trisphosphate (IP3) triggers focal calcium release in Purkinje cell somata, dendrites, and spines as measured by two-photon microscopy. In calbindin knock-out Purkinje cells, peak calcium increased with flash energy with higher cooperativity for double-caged IP3 than for conventional single-caged IP3, consistent with a chemical two-photon effect. Spine photolysis of double-caged IP3 led to local calcium release. Uncaging of glycerophosphoryl-myo-inositol 4,5-bisphosphate (gPIP2), a poorly metabolizable IP3 analog, led to less well localized release. Thus, IP3 breakdown is necessary for spine-specificity. IP3- and gPIP2-evoked signals declined from peak with similar, slow time courses, indicating that release lasts hundreds of milliseconds and is terminated not by IP3 degradation but by intrinsic receptor dynamics. Based on measurements of spine-dendrite coupling, IP3-evoked calcium signals are expected to be at least 2.4-fold larger in their spine of origin than in nearby spines, allowing IP3 to act as a synapse-specific second messenger. Unexpectedly, single-caged IP3 led to less release in somata and was ineffective in dendrites and spines. Calcium release using caged gPIP2 was inhibited by the addition of single-caged IP3, suggesting that single-caged IP3 is an antagonist of calcium release. Caging at multiple sites may be an effective general approach to reducing residual receptor interaction.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

Remodelling of calcium signalling during liver regeneration in the rat

BACKGROUND/AIMS

During liver regeneration, a network of cytokines and growth factors interact with hepatocytes, helping to restore the liver mass and functions after partial tissue loss. Agonists that trigger Ca2+ signals in the liver contribute to this process, although little is known about calcium signalling during liver regeneration.

RESULTS

We observed two phases in which the hepatocyte response to calcium-mobilising agonists was greatly reduced versus control cells at 24h and five days after partial hepatectomy. We found that both phases of hepatocyte desensitisation involved the down-regulation of cell surface receptors and the type II InsP3 receptor. Single cell studies with flash photolysis of caged InsP3 revealed that InsP3-mediated Ca2+ release was slower in regenerating hepatocytes at 24, 48 h and 5 days than in control cells. Also, the temporal pattern of vasopressin-elicited intracellular calcium oscillations studied on fura2-loaded cells was altered, with the duration of each Ca2+ peak being longer. Finally, we showed an association between hepatocyte desensitisation and progression through the cell cycle towards the S phase at 24 h after hepatectomy.

CONCLUSIONS

Our study supports the remodelling of hepatocyte calcium signalling during liver regeneration, and that this change is partly linked with cell cycle progression.

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.

Signaling microdomains regulate inositol 1,4,5-trisphosphate-mediated intracellular calcium transients in cultured neurons

Ca2+ signals in neurons use specific temporal and spatial patterns to encode unambiguous information about crucial cellular functions. To understand the molecular basis for initiation and propagation of inositol 1,4,5-trisphosphate (InsP3)-mediated intracellular Ca2+ signals, we correlated the subcellular distribution of components of the InsP3 pathway with measurements of agonist-induced intracellular Ca2+ transients in cultured rat hippocampal neurons and pheochromocytoma cells. We found specialized domains with high levels of phosphatidylinositol-4-phosphate kinase (PIPKI) and chromogranin B (CGB), proteins acting synergistically to increase InsP3 receptor (InsP3R) activity and sensitivity. In contrast, Ca2+ pumps in the plasma membrane (PMCA) and sarco-endoplasmic reticulum as well as buffers that antagonize the rise in intracellular Ca2+ were distributed uniformly. By pharmacologically blocking phosphatidylinositol-4-kinase and PIPKI or disrupting the CGB-InsP3R interaction by transfecting an interfering polypeptide fragment, we produced major changes in the initiation site and kinetics of the Ca2+ signal. This study shows that a limited number of proteins can reassemble to form unique, spatially restricted signaling domains to generate distinctive signals in different regions of the same neuron. The finding that the subcellular location of initiation sites and protein microdomains was cell type specific will help to establish differences in spatiotemporal Ca2+ signaling in different types of neurons.

Kinetic, pharmacological and activity-dependent separation of two Ca2+ signalling pathways mediated by type 1 metabotropic glutamate receptors in rat Purkinje neurones

Type 1 metabotropic glutamate receptors (mGluR1) in Purkinje neurones (PNs) are important for motor learning and coordination. Here, two divergent mGluR1 Ca2+-signalling pathways and the associated membrane conductances were distinguished kinetically and pharmacologically after activation by 1-ms photorelease of L-glutamate or by bursts of parallel fibre (PF) stimulation. A new, mGluR1-mediated transient K+ conductance was seen prior to the slow EPSC (sEPSC). It was seen only in PNs previously allowed to fire spontaneously or held at depolarized potentials for several seconds and was slowly inhibited by agatoxin IVA, which blocks P/Q-type Ca2+ channels. It peaked in 148 ms, had well-defined kinetics and, unlike the sEPSC, was abolished by the phospholipase C (PLC) inhibitor U73122. It was blocked by the BK Ca2+-activated K+ channel blocker iberiotoxin and unaffected by apamin, indicating selective activation of BK channels by PLC-dependent store-released Ca2+. The K+ conductance and underlying transient Ca2+ release showed a highly reproducible delay of 99.5 ms following PF burst stimulation, with a precision of 1-2 ms in repeated responses of the same PN, and a subsequent fast rise and fall of Ca2+ concentration. Analysis of Ca2+ signals showed that activation of the K+ conductance by Ca2+ release occurred in small dendrites and subresolution structures, most probably spines. The results show that PF burst stimulation activates two pathways of mGluR1 signalling in PNs. First, transient, PLC-dependent Ca2+ release from stores with precisely reproducible timing and second, slower Ca2+ influx in the cation-permeable sEPSC channel. The priming by prior Ca2+ influx in P/Q-type Ca2+ channels may determine the path of mGluR1 signalling. The precise timing of PLC-mediated store release may be important for interactions of PF mGluR1 signalling with other inputs to the PN.

Involvement of ryanodine receptors in IP3-mediated calcium signalling in neurons. A modelling approach

Phospholipase C (PLC)-coupled metabotropic receptors trigger the release of intracellular Ca2+ through activation of IP3 receptors (IP3Rs). Increasing evidence suggests that they can also and perhaps more efficiently mobilize Ca2+ through ryanodine receptors (RyRs). We constructed a model allowing a variable PLC stimulation level (via the parameter gamma) as well as a variable involvement of RyRs (via the parameter A). The sole presence of RyRs (A not = 0) affected the basal Ca2+ concentration [Ca2+]i. To keep Ca2+ below 160 nM, we fixed the upper limit of A to 0.2, a value that is compatible with the numerical ratio between RyRs and IP3Rs in cerebellar Purkinje neurons. Metabotropic responses were simulated by abruptly raising the value of gamma to various levels. In the absence of RyRs, the model starts to oscillate with gamma=0.4. For lower levels of PLC stimulation (gamma< or =0.3), the presence of RyR is capable of triggering an oscillatory signal. When A< or =0.18, the frequency of the Ca2+ oscillations augments from 0.1 to 1.3 Hz as a function of gamma. Conversely, as the frequency increases, the amplitude of the oscillations is reduced from 1 microM to 50 nM. With higher values of A, the oscillating pattern is definitively inhibited. It is concluded that RyRs have the potentiality to strikingly affect the temporal pattern of the Ca2+ signalling triggered by IP3-related metabotropic responses.

InsP3-mediated intracellular calcium signalling is altered by expression of synaptojanin-1

Phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] plays an important physiological role as a precursor for the InsP3-mediated intracellular calcium (Ca2+) signalling cascade. It also regulates membrane trafficking, actin function and transmembrane proteins. SJ-1 (synaptojanin-1), a phosphoinositide phosphatase, regulates the turnover of a PtdIns(4,5)P2 pool involved in clathrin and actin dynamics at the cell surface. We tested the interrelationship of this pool with PtdIns(4,5)P2 pools involved in Ca2+ signalling by expressing in Chinese-hamster ovary cells full-length SJ-1 or its 5-Pase (inositol 5-phosphatase) domain. SJ-1 significantly attenuated the generation of Ca2+ oscillations induced by ATP and the 5-Pase domain mimicked this effect. These changes correlated with increased PtdIns(4,5)P2 phosphatase activity of cellular extracts. Overexpression of the endoplasmic reticulum-anchored PtdIns(4)P phosphatase Sac1 did not affect Ca2+ oscillations, although it increased the Ca2+ efflux rate from intracellular stores. The ability of SJ-1 to alter intracellular Ca2+ signalling indicates a close functional interrelationship between plasma membrane PtdIns(4,5)P2 pools that control actin and endocytosis and those involved in the regulation of specific spatio-temporal Ca2+ signals.

InsP3 receptor type 2 and oscillatory and monophasic Ca2+ transients in rat adrenal chromaffin cells

Muscarinic receptor stimulation induced oscillatory and monophasic Ca(2+) transients in rat adrenal chromaffin cells in the absence of external Ca(2+). As this Ca(2+) mobilization may be mediated by InsP(3), we first explored types of InsP(3) receptors and their intracellular distribution in chromaffin cells. The InsP(3) receptor type 1 was not immunodetected in precipitates of adrenal medulla homogenates and in dissociated adrenal chromaffin cells, whereas an anti-type 3 mAb recognized a faint band with about 250 kDa, but no significant immunoreaction was visible in chromaffin cells. The anti-type 2 mAb strongly detected a band with about 220 kDa and the immunoreaction was observed perinuclearly and at the cell periphery. These results indicate that InsP(3) receptor type 2 is predominant in chromaffin cells. The oscillatory and monophasic Ca(2+) transients were reproduced in simulation based on a three-state kinetic model (shut, open, and inactivated states). Ca(2+) ions were found experimentally and theoretically to turn over rapidly between stores and the cytosol during stimulation. The results suggest that InsP(3) receptor type 2 is responsible for both oscillatory and monophasic Ca(2+) transients and that change in mode of Ca(2+) responses may be accounted for by the kinetic property of the type 2 receptor.

Ca2+ ion permeability and single-channel properties of the metabotropic slow EPSC of rat Purkinje neurons

The slow EPSC (sEPSC) of cerebellar parallel fiber --> Purkinje neuron synapses is mediated by metabotropic glutamate receptor type 1 (mGluR1) activation of nonselective cation channels. Here, the channel properties were studied with uniform calibrated photorelease of L-glutamate with ionotropic receptors blocked, allowing isolation of postsynaptic processes, or with parallel fiber stimulation or mGluR1 agonist application. Evoked current and fluorescence from Ca(2+) indicators were recorded. Noise analysis of the mGluR1 current gave a single-channel conductance of 0.6 pS and showed low open probability at maximal mGluR1 activation. Similar small single-channel conductances were obtained with the mGluR1 agonist (S)-dihydroxyphenylglycine, with parallel fiber or climbing fiber stimulation. The mGluR1 current fluctuations were unaffected by potassium channel blockers. Photoreleased L-glutamate triggered a Ca(2+) concentration increase in the distal dendrites with a time course similar to that of the mGluR1 current. The proximal dendritic and somatic Ca(2+) changes were delayed with respect to the current. Ca(2+) channel blockers and the phospholipase Cdelta inhibitor 1-[6-[((17delta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione, which inhibits mGluR1-activated intracellular Ca(2+) release, did not prevent the dendritic Ca(2+) concentration increase. Polyamine naphthylacetyl spermine and cationic adamantanes that block the pore of the channel were used to vary the mGluR1 current over a wide range in each cell but still at maximal mGluR1 activation. The Ca(2+) influx was inhibited in parallel with the current. The results show that the mGluR1-activated current and the sEPSC are attributable to small-conductance, low-open probability Ca(2+)-permeable cation channels that will mediate spine-specific Ca(2+) influx during the parallel fiber sEPSP.

Histamine potentiates IP(3)-mediated Ca(2+) release via thapsigargin-sensitive Ca(2+) pumps

We have studied the histamine-induced potentiation of inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) release in HeLa cells. Intracellular IP(3) levels were increased by IP(3) dialysis with the whole-cell configuration of the patch-clamp technique (cell dialysis of IP(3)). Low concentrations of extracellular histamine (1 microM) accelerated the rate of IP(3)-mediated Ca(2+) release, an effect that required the coincidence of both histamine signalling and the increase in IP(3) levels. Our data suggest that the potentiation effect of histamine cannot be explained simply by agonist-induced increase in IP(3) levels. Disordering microfilaments with cytochalasin D and microtubules with colchicine caused a decrease in the histamine-induced Ca(2+) response. Furthermore, both cytochalasin D and colchicine diminished the rate of IP(3)-mediated Ca(2+) release, while only the former reduced slightly the histamine-induced potentiation effect. Remarkably, rapid inhibition of SERCA pumps with thapsigargin to avoid the depletion of internal Ca(2+) stores diminished the histamine-induced potentiation of IP(3)-mediated Ca(2+) release, without affecting the rate of IP(3)-mediated Ca(2+) release. These data indicate that histamine-induced potentiation of Ca(2+) release in HeLa cells requires active SERCA pumps and suggest that SERCA pumps are an important factor in determining the efficiency of agonist-induced Ca(2+) release.

Spatiotemporal laser inactivation of inositol 1,4,5-trisphosphate receptors using synthetic small-molecule probes

A malachite green-conjugated inositol 1,4,5-trisphosphate (MGIP(3)) induces specific inactivation of IP(3) receptor (IP(3)R) in tissue samples upon laser irradiation. To verify potential usefulness of the method for studies of cellular Ca(2+) signaling, we conducted laser inactivation at the single-cell level and show that IP(3)R was inactivated with extremely high spatiotemporal resolution. In the presence of MGIP(3), the Ca(2+) release function of IP(3)R in single B lymphoma cells decayed exponentially with increasing duration of laser irradiation with a time constant of 3.4 s. Moreover, by confining laser irradiation to a spatially distinct region of differentiated PC12 cells, subcellular inactivation of IP(3)R was attained, as revealed by a loss of local Ca(2+) signal. Such real-time inactivation of IP(3)R only within a subcellular region may provide a powerful method for investigating spatiotemporal dynamics of Ca(2+) signaling.

Carbonic anhydrase-related protein is a novel binding protein for inositol 1,4,5-trisphosphate receptor type 1

The inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) is an intracellular IP(3)-gated Ca(2+) channel that is located on intracellular Ca(2+) stores and modulates Ca(2+) signalling. Using the yeast two-hybrid system, we screened a mouse brain cDNA library with bait constructs for mouse IP(3)R type 1 (IP(3)R1) to identify IP(3)R1-associated proteins. In this way, we found that carbonic anhydrase-related protein (CARP) is a novel IP(3)R1-binding protein. Western blot analysis revealed that CARP is expressed exclusively in Purkinje cells of the cerebellum, in which IP(3)R1 is abundantly expressed. Immunohistochemical analysis showed that the subcellular localization of CARP in Purkinje cells is coincident with that of IP(3)R1. Biochemical analysis also showed that CARP is co-precipitated with IP(3)R1. Using deletion mutagenesis, we established that amino acids 45-291 of CARP are essential for its association with IP(3)R1, and that the CARP-binding site is located within the modulatory domain of IP(3)R1 amino acids 1387-1647. CARP inhibits IP(3) binding to IP(3)R1 by reducing the affinity of the receptor for IP(3). As reported previously, sensitivity to IP(3) for IP(3)-induced Ca(2+) release in Purkinje cells is low compared with that in other tissues. This could be due to co-expression of CARP with IP(3)R in Purkinje cells and its inhibitory effects on IP(3) binding.

Distinct intracellular calcium transients in neurites and somata integrate neuronal signals

Intracellular calcium signals have distinct temporal and spatial patterns in neurons in which signal initiation and repetitive spiking occurs predominantly in the neurite. We investigated the functional implications of the coexpression of different isoforms of ryanodine receptors (RyR) and inositol 1,4,5-trisphosphate receptors (InsP3Rs) using immunocytochemistry, Western blotting, and calcium imaging in neuronally differentiated PC12 cells. InsP3R type III, an isoform that has been shown to be upregulated in neuronal apoptosis, is exclusively expressed in the soma, serving as a gatekeeper for high-magnitude calcium surges. InsP3R type I is expressed throughout the cell and can be related to signal initiation and repetitive spiking in the neurite. RyR types 2 and 3 are distributed throughout the cell. In the soma, they serve as amplifying molecular switches, facilitating recruitment of the InsP3R type III-dependent pool. In the neurite, they decrease the probability of repetitive spiking. Use of a cell-permeant analog of InsP3 suggested that regional specificity in InsP3 production and surface-to-volume effects play minor roles in determining temporal and spatial calcium signaling patterns in neurons. Our findings suggest that additional modulatory processes acting on the intracellular channels are necessary to generate spatially specific calcium signaling.

Fast biphasic regulation of type 3 inositol trisphosphate receptors by cytosolic calcium

In cytosol-like medium (CLM) with a free [Ca(2+)] of 200 nm, a supramaximal concentration of inositol 1,4,5-trisphosphate (IP(3)) (30 microm) evoked (45)Ca(2+) release from type 3 IP(3) receptors only after a latency of 48 +/- 6 ms; this latency could not be reduced by increasing the IP(3) concentration. In CLM containing a low free [Ca(2+)] ( approximately 4 nm), 300 microm IP(3) evoked (45)Ca(2+) release after a latency of 66 +/- 11 ms; this was reduced to 14 +/- 3 ms when the [Ca(2+)] was 1 mm. Preincubation with CLM containing 100 microm Ca(2+) caused a rapid (half-time = 33 +/- 9 ms), complete, and fully reversible inhibition that could not be overcome by a high concentration of IP(3) (300 microm). Hepatic (type 2) IP(3) receptors were not inhibited by Ca(2+) once they had bound IP(3), but 100 microm Ca(2+) rapidly inhibited type 3 IP(3) receptors whether it was delivered before addition of IP(3) or at any stage during a response to IP(3). Ca(2+) increases the affinity of IP(3) for hepatic receptors by slowing IP(3) dissociation, but Ca(2+) had no effect on IP(3) binding to type 3 receptors. The rate of inhibition of type 3 IP(3) receptors by Ca(2+) was faster than the rate of IP(3) dissociation, and occurred at similar rates whether receptors had bound a high (adenophostin) or low affinity (3-deoxy-3-fluoro-IP(3)) agonist. Dissociation of agonist is not therefore required for Ca(2+) to inhibit type 3 IP(3) receptors. We conclude that type 2 and 3 IP(3) receptors are each biphasically regulated by Ca(2+), but by different mechanisms. For both, IP(3) binding causes a stimulatory Ca(2+)-binding site to be exposed allowing Ca(2+) to bind and open the channel. IP(3) binding protects type 2 receptors from Ca(2+) inhibition, but type 3 receptors are inhibited by Ca(2+) whether or not they have IP(3) bound. Increases in cytosolic [Ca(2+)] will immediately inhibit type 3 receptors, but inhibit type 2 receptors only after IP(3) has dissociated.

Cytosolic Ca(2+) and Ca(2+)-activated Cl(-) current dynamics: insights from two functionally distinct mouse exocrine cells

The dynamics of Ca(2+) release and Ca(2+)-activated Cl(-) currents in two related, but functionally distinct exocrine cells, were studied to gain insight into how the molecular specialization of Ca(2+) signalling machinery are utilized to produce different physiological endpoints: in this case, fluid or exocytotic secretion. Digital imaging and patch-clamp methods were used to monitor the temporal and spatial properties of changes in cytosolic Ca(2+) concentration ([Ca(2+)](c)) and Cl(-) currents following the controlled photolytic release of caged-InsP(3) or caged-Ca(2+). In parotid and pancreatic acinar cells, changes in [Ca(2+)](c) and activation of a Ca(2+)-activated Cl(-) current occurred with close temporal coincidence. In parotid, a rapid global Ca(2+) signal was invariably induced, even with low-level photolytic release of threshold amounts of InsP(3). In pancreas, threshold stimulation generated an apically delimited [Ca(2+)](c) signal, while a stronger stimulus induced a global [Ca(2+)](c) signal which exhibited characteristics of a propagating wave. InsP(3) was more effective in parotid, where [Ca(2+)](c) signals initiated with shorter latency and exhibited a faster time-to-peak than in pancreas. The increased potency of InsP(3) in parotid probably results from a four-fold higher number of InsP(3) receptors as measured by radiolabelled InsP(3) binding and western blot analysis. The Ca(2+) sensitivity of the Cl(-) channels in parotid and pancreas was determined from the [Ca(2+)]-current relationship measured during a dynamic 'Ca(2+) ramp' produced by the continuous, low-level photolysis of caged-Ca(2+). In addition to a greater number of InsP(3) receptors, the Cl(-) current density of parotid acinar cells was more than four-fold greater than that of pancreatic cells. Whereas activation of the current was tightly coupled to increases in Ca(2+) in both cell types, local Ca(2+) clearance was found to contribute substantially to the deactivation of the current in parotid. These data reveal specializations of common modules of Ca(2+)-release machinery and subsequent effector activation that are specifically suited to the distinct functional roles of these two related cell types.

A dynamic model of the type-2 inositol trisphosphate receptor

The dynamic properties of the inositol (1,4,5)-trisphosphate (IP(3)) receptor are crucial for the control of intracellular Ca(2+), including the generation of Ca(2+) oscillations and waves. However, many models of this receptor do not agree with recent experimental data on the dynamic responses of the receptor. We construct a model of the IP(3) receptor and fit the model to dynamic and steady-state experimental data from type-2 IP(3) receptors. Our results indicate that, (i) Ca(2+) binds to the receptor using saturating, not mass-action, kinetics; (ii) Ca(2+) decreases the rate of IP(3) binding while simultaneously increasing the steady-state sensitivity of the receptor to IP(3); (iii) the rate of Ca(2+)-induced receptor activation increases with Ca(2+) and is faster than Ca(2+)-induced receptor inactivation; and (iv) IP(3) receptors are sequentially activated and inactivated by Ca(2+) even when IP(3) is bound. Our results emphasize that measurement of steady-state properties alone is insufficient to characterize the functional properties of the receptor.

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.

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.

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.

Elucidation of a PTS-carbohydrate chemotactic signal pathway in Escherichia coli using a time-resolved behavioral assay

Chemotaxis of Escherichia coli toward phosphotransferase systems (PTSs)-carbohydrates requires phosphoenolpyruvate-dependent PTSs as well as the chemotaxis response regulator CheY and its kinase, CheA. Responses initiated by flash photorelease of a PTS substrates D-glucose and its nonmetabolizable analog methyl alpha-D-glucopyranoside were measured with 33-ms time resolution using computer-assisted motion analysis. This, together with chemotactic mutants, has allowed us to map out and characterize the PTS chemotactic signal pathway. The responses were absent in mutants lacking the general PTS enzymes EI or HPr, elevated in PTS transport mutants, retarded in mutants lacking CheZ, a catalyst of CheY autodephosphorylation, and severely reduced in mutants with impaired methyl-accepting chemotaxis protein (MCP) signaling activity. Response kinetics were comparable to those triggered by MCP attractant ligands over most of the response range, the most rapid being 11.7 +/- 3.1 s-1. The response threshold was <10 nM for glucose. Responses to methyl alpha-D-glucopyranoside had a higher threshold, commensurate with a lower PTS affinity, but were otherwise kinetically indistinguishable. These facts provide evidence for a single pathway in which the PTS chemotactic signal is relayed rapidly to MCP-CheW-CheA signaling complexes that effect subsequent amplification and slower CheY dephosphorylation. The high sensitivity indicates that this signal is generated by transport-induced dephosphorylation of the PTS rather than phosphoenolpyruvate consumption.

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.

ATP-activated cation currents in single guinea-pig hepatocytes

1. Responses of single guinea-pig liver cells to the application of external ATP were studied using the whole-cell voltage clamp technique. 2. When the cells were loaded with 5 mM EGTA in the absence of K+ and Cl- in both internal and external solutions, application of ATP (0.03-100 microM) elicited a large cation-selective inward current at negative holding potentials. The current densities at the peak of the response to 100 microM ATP were 4.5 +/- 0.5 pA pF-1 (mean +/- s.e.m., n = 18) in the presence of Na+ and Ca2+ in the external medium and 3.3 +/- 0.7 pA pF-1 (n = 6) with Ca2+ as the major permeant ion. 3. Divalent cations, when added during the response to ATP in the presence of Na+ and Ca2+, exerted different effects: CdSO4 (2 mM) totally and NiSO4 (2 mM) partially blocked the inward current whereas MnSO4 (2 mM) did not block it. The ATP-activated conductance was permeable to all the divalent cations tested in this study, i.e. Ca2+, Cd2+, Ni2+, Mn2+ and Mg2+. No response to ATP was observed in the absence of external cations. 4. The activation of the inward current was not maintained in the continuous presence of ATP. The effect of Ca2+ ions on the desensitization of the response was studied in different external solutions. The decline in the amplitude of the inward current after the peak was fitted with a single exponential with a time constant of about 2 s for pure Ca2+, Cd2+ or Ni2+ currents, 3 s for Mg2+ or Mn2+ and 4 s in the presence of both Na+ and Ca2+. 5. Under more physiological conditions, the entry of Ca2+ evoked after the stimulation of P2X purinoceptors was associated with an increase in fluo-3 fluorescence and a marked reduction in the delay before the mobilization of internal Ca2+ stores following the activation of P2Y purinoceptors.

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.