Elevated intracellular cyclic AMP exerts different modulatory effects on cytosolic free Ca2+ oscillations induced by ADP and ATP in single rat hepatocytes

Modeling IP3-induced Ca2+ signaling based on its interspike interval statistics

Inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ signaling is a second messenger system used by almost all eukaryotic cells. Recent research demonstrated randomness of Ca2+ signaling on all structural levels. We compile eight general properties of Ca2+ spiking common to all cell types investigated and suggest a theory of Ca2+ spiking starting from the random behavior of IP3 receptor channel clusters mediating the release of Ca2+ from the endoplasmic reticulum capturing all general properties and pathway-specific behavior. Spike generation begins after the absolute refractory period of the previous spike. According to its hierarchical spreading from initiating channel openings to cell level, we describe it as a first passage process from none to all clusters open while the cell recovers from the inhibition which terminated the previous spike. Our theory reproduces the exponential stimulation response relation of the average interspike interval Tav and its robustness properties, random spike timing with a linear moment relation between Tav and the interspike interval SD and its robustness properties, sensitive dependency of Tav on diffusion properties, and nonoscillatory local dynamics. We explain large cell variability of Tav observed in experiments by variability of channel cluster coupling by Ca2+-induced Ca2+ release, the number of clusters, and IP3 pathway component expression levels. We predict the relation between puff probability and agonist concentration and [IP3] and agonist concentration. Differences of spike behavior between cell types and stimulating agonists are explained by the different types of negative feedback terminating spikes. In summary, the hierarchical random character of spike generation explains all of the identified general properties.

Receptor-specific Ca2+ oscillation patterns mediated by differential regulation of P2Y purinergic receptors in rat hepatocytes

Extracellular agonists linked to inositol-1,4,5-trisphosphate (IP₃) formation elicit cytosolic Ca²⁺ oscillations in many cell types, but despite a common signaling pathway, distinct agonist-specific Ca²⁺ spike patterns are observed. Using qPCR, we show that rat hepatocytes express multiple purinergic P2Y and P2X receptors (R). ADP acting through P2Y1R elicits narrow Ca²⁺ oscillations, whereas UTP acting through P2Y2R elicits broad Ca²⁺ oscillations, with composite patterns observed for ATP. P2XRs do not play a role at physiological agonist levels. The discrete Ca²⁺ signatures reflect differential effects of protein kinase C (PKC), which selectively modifies the falling phase of the Ca²⁺ spikes. Negative feedback by PKC limits the duration of P2Y1R-induced Ca²⁺ spikes in a manner that requires extracellular Ca²⁺. By contrast, P2Y2R is resistant to PKC negative feedback. Thus, the PKC leg of the bifurcated IP₃ signaling pathway shapes unique Ca²⁺ oscillation patterns that allows for distinct cellular responses to different agonists.

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Distinct stereotypic Ca²⁺ oscillations are elicited by P2Y1 and P2Y2 receptors

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P2X receptors do not contribute to the generation of Ca²⁺ oscillations

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Agonist-specific Ca²⁺ spike shapes reflect discrete modes of PKC negative feedback

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Bifurcation of IP₃/PKC signaling yields unique Ca²⁺ oscillation signatures

On the phase space structure of IP3 induced Ca2+ signalling and concepts for predictive modeling

The correspondence between mathematical structures and experimental systems is the basis of the generalizability of results found with specific systems and is the basis of the predictive power of theoretical physics. While physicists have confidence in this correspondence, it is less recognized in cellular biophysics. On the one hand, the complex organization of cellular dynamics involving a plethora of interacting molecules and the basic observation of cell variability seem to question its possibility. The practical difficulties of deriving the equations describing cellular behaviour from first principles support these doubts. On the other hand, ignoring such a correspondence would severely limit the possibility of predictive quantitative theory in biophysics. Additionally, the existence of functional modules (like pathways) across cell types suggests also the existence of mathematical structures with comparable universality. Only a few cellular systems have been sufficiently investigated in a variety of cell types to follow up these basic questions. IP₃ induced Ca²⁺signalling is one of them, and the mathematical structure corresponding to it is subject of ongoing discussion. We review the system's general properties observed in a variety of cell types. They are captured by a reaction diffusion system. We discuss the phase space structure of its local dynamics. The spiking regime corresponds to noisy excitability. Models focussing on different aspects can be derived starting from this phase space structure. We discuss how the initial assumptions on the set of stochastic variables and phase space structure shape the predictions of parameter dependencies of the mathematical models resulting from the derivation.

Cross-talk between signaling pathways can generate robust oscillations in calcium and cAMP

Background

To control and manipulate cellular signaling, we need to understand cellular strategies for information transfer, integration, and decision-making. A key feature of signal transduction is the generation of only a few intracellular messengers by many extracellular stimuli.

Methodology/Principal Findings

Here we model molecular cross-talk between two classic second messengers, cyclic AMP (cAMP) and calcium, and show that the dynamical complexity of the response of both messengers increases substantially through their interaction. In our model of a non-excitable cell, both cAMP and calcium concentrations can oscillate. If mutually inhibitory, cross-talk between the two second messengers can increase the range of agonist concentrations for which oscillations occur. If mutually activating, cross-talk decreases the oscillation range, but can generate ‘bursting’ oscillations of calcium and may enable better filtering of noise.

Conclusion

We postulate that this increased dynamical complexity allows the cell to encode more information, particularly if both second messengers encode signals. In their native environments, it is unlikely that cells are exposed to one stimulus at a time, and cross-talk may help generate sufficiently complex responses to allow the cell to discriminate between different combinations and concentrations of extracellular agonists.

Complex intracellular calcium oscillations. A theoretical exploration of possible mechanisms

Intracellular Ca(2+) oscillations are commonly observed in a large number of cell types in response to stimulation by an extracellular agonist. In most cell types the mechanism of regular spiking is well understood and models based on Ca(2+)-induced Ca(2+) release (CICR) can account for many experimental observations. However, cells do not always exhibit simple Ca(2+) oscillations. In response to given agonists, some cells show more complex behaviour in the form of bursting, i.e. trains of Ca(2+) spikes separated by silent phases. Here we develop several theoretical models, based on physiologically plausible assumptions, that could account for complex intracellular Ca(2+) oscillations. The models are all based on one- or two-pool models based on CICR. We extend these models by (i) considering the inhibition of the Ca(2+)-release channel on a unique intracellular store at high cytosolic Ca(2+) concentrations, (ii) taking into account the Ca(2+)-activated degradation of inositol 1,4,5-trisphosphate (IP(3)), or (iii) considering explicity the evolution of the Ca(2+) concentration in two different pools, one sensitive and the other one insensitive to IP(3). Besides simple periodic oscillations, these three models can all account for more complex oscillatory behaviour in the form of bursting. Moreover, the model that takes the kinetics of IP(3) into account shows chaotic behaviour.

ADP stimulation of inositol phosphates in hepatocytes: role of conversion to ATP and stimulation of P2Y2 receptors

1 Accumulation of inositol (poly)phosphates (InsP(x)) has been studied in rat hepatocytes labelled with [(3)H]inositol. Stimulation with ADP resulted in a significant increase in total [(3)H]InsP(x), whereas 2-MeSADP had only a small effect and ADPbetaS was ineffective. UTP and ITP also stimulated substantial increases in [(3)H]InsP(x). 2 The dose-response curve to ADP was largely unaltered by the presence of the P2Y(1) antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P). Similarly, inclusion of MRS 2179, a more selective P2Y(1) antagonist, had no effect on the dose-response curve to ADP. 3 The inclusion of hexokinase in the assay reduced, but did not abolish, the response to ADP. 4 HPLC analysis revealed that ADP in the medium was rapidly converted to AMP and ATP. The inclusion of hexokinase removed ATP, but exacerbated the decline in ADP concentration, leading to increased levels of AMP. 2-MeSADP was stable in the medium and ATP was largely unaffected. 5 The addition of the adenylate kinase inhibitor, diadenosine pentaphosphate (Ap(5)A) significantly reduced the ADP response. HPLC analysis conducted in parallel demonstrated that this treatment inhibited conversion of ADP to ATP and AMP. 6 Inclusion of the P1 antagonist CGS 15943 had no effect on the dose-response curve to ADP. 7 These observations indicate that hepatocytes respond to ADP with an increase in inositol (poly)phosphates following conversion to ATP. P2Y(1) activation in hepatocytes does not appear to be coupled to inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) production.

Atrial natriuretic peptide attenuates Ca2+ oscillations and modulates plasma membrane Ca2+ fluxes in rat hepatocytes

BACKGROUND & AIMS

Oscillations in cytosolic free Ca2+ concentration are a fundamental mechanism of intracellular signaling in hepatocytes. The aim of this study was to examine the effects of atrial natriuretic peptide (ANP) on cytosolic Ca2+ oscillations in rat hepatocytes.

METHODS

Cyclic guanosine monophosphate (cGMP) was measured by enzyme immunoassay. Cytosolic Ca2+ oscillations were recorded from single aequorin-injected hepatocytes. Ca2+ efflux from hepatocyte populations was measured by using extracellular fura-2. Ca2+ influx was estimated by Mn2+ quench of fluorescence of fura-2 dextran injected into single hepatocytes.

RESULTS

ANP attenuated cytosolic Ca2+ oscillations through a decrease in their frequency. In addition, ANP dramatically stimulated plasma membrane Ca2+ efflux and modestly inhibited basal Ca2+ influx. All of the observed effects of ANP were mimicked by the cGMP analogue 8-bromo-cGMP (8-Br-cGMP), and were prevented by inhibition of protein kinase G. In contrast, activation of cytosolic guanylyl cyclase by sodium nitroprusside had no effect on Ca2+ efflux, Ca2+ influx, or Ca2+ oscillations.

CONCLUSIONS

ANP decreases the frequency of Ca2+ oscillations and modulates plasma membrane Ca2+ fluxes in rat hepatocytes. Attenuation of oscillatory Ca2+ signaling in hepatocytes may represent a key role for ANP in vivo.

Evidence that 2-methylthioATP and 2-methylthioADP are both agonists at the rat hepatocyte P2Y(1) receptor

In the absence of selective antagonists, pharmacological characterization of P2Y receptor subtypes has relied heavily upon their distinct agonist profiles. 2-methylthioADP (2-MeSADP) is a selective agonist for the P2Y(1) receptor. The agonist action of 2-MeSATP at the P2Y(1) receptor has recently been questioned. The effects of both 2-MeSADP and 2-MeSATP have been studied on rat hepatocytes injected with the bioluminescent Ca(2+) indicator, aequorin. Single hepatocytes generate series of repetitive transients in cytosolic free calcium concentration ([Ca(2+)](i)) when stimulated with agonists acting through the phosphoinositide signalling pathway. The transients induced by 2-MeSADP and 2-MeSATP in the same cell were indistinguishable, indicating that they act at a common receptor. In contrast the transients evoked by ATP and UTP had very different profiles. Treatment of 2-MeSATP with an ATP-regenerating system to remove contaminating 2-MeSADP did not abolish its agonist activity. Application of the P2Y(1) antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P) inhibited the transients induced by both 2-MeSADP and 2-MeSATP. In contrast the transients induced by ATP and UTP were enhanced by the addition of A3P5P. These results indicate that both 2-MeSADP and 2-MeSATP are agonists at the rat hepatocyte P2Y(1) receptor.

Evidence that rat hepatocytes co-express functional P2Y1 and P2Y2 receptors

Previous studies have indicated the expression of multiple P2Y receptors by rat hepatocytes although they have not been identified. Here we show by reverse transcriptase-polymerase chain reaction (RT - PCR) that rat hepatocytes express mRNA encoding all of the four cloned rat P2Y receptors (P2Y(1), P2Y(2), P2Y(4) and P2Y(6)). The effects of UTP have been examined on single aequorin-injected rat hepatocytes. The [Ca(2+)](i) transients induced by UTP were indistinguishable from those induced by ATP in the same cell. The modulatory effects of elevated intracellular cyclic AMP concentration were the same on both UTP- and ATP-induced [Ca(2+)](i) transients. UDP, an agonist at the P2Y(6) receptor, failed to induce transients in hepatocytes, indicating that functional P2Y(6) receptors coupled to increased [Ca(2+)](i) are not expressed. The transients evoked by ADP were more sensitive to inhibition by suramin than those induced by either ATP or UTP. Within an individual cell, the transients induced by ATP and UTP were inhibited by the same concentration of suramin. This sensitivity of ATP and UTP responses to suramin suggests action through P2Y(2) rather than P2Y(4) receptors. Co-application of 30 microM pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) caused a decrease in frequency and amplitude of transients induced by ADP. ATP- and UTP-induced transients also displayed a decrease in amplitude in response to addition of PPADS, but this was accompanied by an increase in frequency of transients. In conclusion the data presented here are consistent with the co-expression of P2Y(1) and P2Y(2) receptors by rat hepatocytes.

Diadenosine polyphosphates and the control of cyclic AMP concentrations in isolated rat liver cells

Extracellular diadenosine polyphosphates (Ap(n)A), through their interactions with appropriate P(2) receptors, influence a diverse range of intracellular activities. In particular, Ap(4)A stimulates alterations in intracellular calcium homeostasis and subsequent activation of glycogen breakdown in isolated liver cells. Here we show that, like ATP, Ap(4)A and other naturally occurring diadenosine polyphosphates attenuate glucagon-stimulated accumulation of cyclic AMP in isolated rat liver cells. The characteristics of Ap(4)A- and ATP-dependent modulation of glucagon-stimulated cyclic AMP accumulation are similar. These results are discussed in the context of the repertoire of intracellular signalling processes modulated by extracellular nucleotides.

Neuropeptide Y modulates ATP-induced increases in internal calcium via the adenylate cyclase/protein kinase A system in a human neuroblastoma cell line

The modulatory effects of neuropeptide Y (NPY) on ATP-induced increases in cytosolic free-calcium concentration ([Ca2+]i) were investigated in the CHP-234 human neuroblastoma cell line. Pretreatment of cells with 100 nM NPY potentiated the increase in [Ca2+]i evoked subsequently by 20 microM ATP, compared with initial application of ATP in a control experiment, whereas a similar pretreatment with 1 microM NPY attenuated the subsequent response to ATP. Both actions of NPY were completely blocked by H-89 [N-[2-((3-(4-bromo-phenyl)-2-propenyl)-amino)-ethyl]-5 isoquinoline sulphonamide dihydrochloride], a selective antagonist of protein kinase A. The effects of 100 nM NPY were mimicked by H-89, while forskolin and 8-Br-cAMP mimicked the effects of 1 microM NPY. Both basal and forskolin-stimulated cAMP levels were inhibited by 100 nM NPY and by 100 nM NPY(13-36), a selective agonist of the NPY Y2-receptor subtype. In contrast, at 1 microM such inhibition was not observed for either NPY or NPY(13-36). It is concluded that NPY has a biphasic modulatory effect on increases in [Ca2+]i produced by ATP, which probably involves the cAMP/protein kinase A cascade.

Partial characterization of a new nucleotide binding glycoprotein of hepatocyte plasma membrane

Hepatocyte plasma membranes contain a glycosylated 230-kDa Ca(2+) -dependent, Mg(2+)-stimulated ATPase (pgp230), which consists of two subunits, one of 120 kDa and the other of 110 kDa. pgp230 can be enriched by the use of affinity chromatography on Concanavalin A-Sepharose, wheat germ lectin-Sepharose, and 5'-AMP-Sepharose. It has a high-affinity Ca2+ binding site. In the presence of Ca2+, it forms a phosphorylated intermediate by autocatalytic transfer of the terminal phosphate residue from ATP. Maximal Ca(2+)-dependent autophosphorylation is observed at pH 5-6. Photoaffinity labeling using 8-azido-[alpha-32P]ATP or [y-32P]ATP confirms the presence of ATP binding sites. Incubation with [alpha-32P]ATP leads to a rapid but transient labeling of pgp230. Various nucleotides, nucleotide receptor agonists, or antagonists inhibit Ca(2+)-dependent phosphorylation by [y-32P]ATP. The concentrations of half-maximal inhibition range from 10(-7) M to 10(-3) M. The rank order of inhibitory potency is: ATP > alpha,beta-methylene-ATP > CTP = TTP > y-4-amino-phenyl-ATP = 2-methyl-thio-ATP > UTP = GTP > GDP = ADP = beta,y-methylene-ATP = beta, y-methylene-TTP = beta,y-methylene-GTP = adenosine-5'-O-2-thiodiphosphate = CMP = AMP > adenosine > cytidine > guanosine = suramin > Reactive blue 2 > iso-butyl-methyl-xanthine > thymidine > uridine. These data suggest a nucleotide binding capacity of this new hepatocyte membrane glycoprotein. Further investigations should be carried out to reveal its biological function.

Characterization of the binding of diadenosine 5',5'''-P1,P4-tetraphosphate (Ap4A) to rat liver cell membranes

Diadenosine polyphosphates present in the extracellular environment can, through interaction with appropriate purinoceptors, influence a range of cellular activities. Here we have investigated the nature of the ligand:receptor interactions involved in diadenosine 5',5'''-P1, P4-tetraphosphate (Ap4A)-mediated stimulation of glycogen breakdown in isolated rat liver cells. [2-3H]Ap4A showed specific binding to both intact isolated liver cells and plasma membrane fractions prepared from isolated liver cells. HPLC analysis confirmed that binding was mediated by intact Ap4A and not by potential breakdown products (e.g. ATP, adenosine, etc.). Binding of [2-3H]Ap4A, to isolated liver cell plasma membrane preparations, was successfully displaced by a range of both naturally occurring and synthetic diadenosine polyphospates with the rank order potency Ap4A > or = Ap5A > Ap6A > Ap3A > Ap2A. [2-3H]Ap4A binding was not displaced by P1 effectors but was successfully displaced by a range of P2 effectors with the rank order potency 2-methylthio-ATP > ATP > ATP > or = adenosine 5'-[alpha beta-methylene]triphosphate > adenosine 5'-[beta gamma-methylene]triphospate. These findings are consistent with the interaction of Ap4A with a P2y-like subclass of purinoceptor and are discussed in relation to (1) the known purinoceptor populations in liver cell plasma membranes and (2) observations concerning the binding of diadenosine polyphosphates to purinoceptors in other tissues.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

A hypothesis for the hormonal regulation of gluconeogenesis, in which increases in cytosolic free-Ca2+ levels ([Ca2+]i) play a major role, is presented. This hypothesis is based on the observation that gluconeogenic hormones evoke a common pattern of Ca2+ redistribution, resulting in increases in [Ca2+]i. Current concepts of hormonally evoked Ca2+ fluxes are presented and discussed. It is suggested that the increase in [Ca2+]i is functionally linked to stimulation of gluconeogenesis. The stimulation of gluconeogenesis is accomplished in two ways: (1) by increasing the activities of the Krebs cycle and the electron-transfer chain, thereby supplying adenosine triphosphates (ATP) and reducing equivalents to the process; and (2) by stimulating the activities of key gluconeogenic enzymes, such as pyruvate carboxylase. The hypothesis presents a conceptual framework that ties together two interrelated manifestations of hormone action: signal transduction and metabolism.

Actions of ADP, but not ATP, on cytosolic free Ca2+ in single rat hepatocytes mimicked by 2-methylthioATP

1. Aequorin-injected, single rat hepatocytes generate series of repetitive transients in cytosolic free calcium concentration ([Ca2+]i) when stimulated with agonists acting through the phosphoinositide signalling pathway, including ADP and ATP. We have previously described differences in the [Ca2+]i responses of aequorin-injected hepatocytes to ADP and ATP. 2. The effects of the phosphorothioate analogue of ATP, 2-methylthioATP (2-meSATP), have been examined on single rat hepatocytes. This analogue is belived to be the most potent agonist at the P2Y1 subclass of purinoceptor. 3. The [Ca2+]i transients induced by 2-meSATP were indistinguishable from those induced by ADP, and in contrast to those induced by ATP. 4. At hig concentrations, 2-meSATP and ADP both induced transients at high frequency. In contrast, hepatocytes responded to high concentrations of ATP with an initial rapid rise in [Ca2+]i, followed by a slowly decaying fall. 5. The modulatory effects of elevated intracellular cyclic AMP concentration were the same on both 2-meSATP- and ADP-induced [Ca2+]i transients; the peak height and frequency of transients were enhanced. ATP-induced transients, however, underwent either an increase in duration or conversion into a sustained rise in [Ca2+]i. 6. ATP-induced transients were specifically potentiated by the co-addition of alpha, beta-methyleneATP, whereas 2-meSATP- and ADP-induced transients were unaffected by this treatment. 7. We conclude that 2-meSATP acts at the same receptor as ADP on rat hepatocytes, and that this is distinct from teh receptor(s) mediating the effects of ATP.

Cytosolic free Ca2+ oscillations induced by diadenosine 5',5"'-P1,P3-triphosphate and diadenosine 5',5"'-P1,P4-tetraphosphate in single rat hepatocytes are indistinguishable from those induced by ADP and ATP respectively

Diadenosine 5',5"'-P1,P3-triphosphate (Ap3A) and diadenosine 5',5"'-P1,P4-tetraphosphate (Ap4A) induce distinctive patterns of [Ca2+]i oscillations in single rat hepatocytes. We show here that [Ca2+]i oscillations induced by Ap3A and ADP are indistinguishable and that [Ca2+]i oscillations induced by Ap4A closely resemble those induced by ATP. These similarities embrace the following: (1) ADP and Ap3A invariably induce [Ca2+]i transients of short duration (approx. 9 s). Ap4A, like ATP, can induce, depending upon the individual cell, either transients of short duration (approx. 9 s), transients of much longer duration or a mixture of short and long transients within a single response. We show here that the pattern of oscillations induced by Ap4A is similar to that induced by ATP in the same hepatocyte. (2) Elevated intracellular cyclic AMP concentration modulates Ap3A-induced transients, like ADP-induced transients, through an increase in both the peak [Ca2+]i and the frequency of the transients. In contrast, Ap4A-induced transients, like ATP-induced transients, develop an increased duration or a sustained rise in [Ca2+]i, with no rise in peak [Ca2+]i. (3) Ap3A-induced transients, like ADP-induced transients, are abolished by low concentrations of the phorbol ester 4 beta-phorbol 12,13-dibutyrate (PDB; 5-10 nM), whereas long Ap4A-induced transients, like long ATP-induced transients, are refractory to high concentrations of PDB (100 nM). We propose that the [Ca2+]i oscillations induced in rat hepatocytes by Ap3A are mediated by the same purinoceptor that mediates the effects of ADP, whereas the oscillations induced by Ap4A are mediated by the same purinoceptor(s) that mediate the effects of ATP.

Oscillations in cytosolic free Ca2+ induced by ADP and ATP in single rat hepatocytes display differential sensitivity to application of phorbol ester

We have previously described differences in the oscillatory responses of cytosolic free Ca2+ concentration ([Ca2+]i) in hepatocytes to ADP and ATP, which we have interpreted as evidence that these two nucleotides are acting at distinct receptors. We show here that ADP- and ATP-induced oscillations are differentially sensitive to application of the phorbol ester 4 beta-phorbol 12,13-dibutyrate (PDB). ADP-induced [Ca2+]i oscillations are abolished by low concentrations of PDB (5-10 nM), whereas ATP-induced oscillations of long duration are refractory to PDB, even at greatly elevated concentrations (100 nM). The data illustrate a further difference in the actions of ADP and ATP, strengthening the argument that these agonists are not acting at the same receptor on rat hepatocytes.