Mitochondria regulate the amplitude of simple and complex calcium oscillations.

In a mathematical model for simple calcium oscillations [Biophys. Chem. 71 (1998) 125], it has been shown that mitochondria play an important role in the maintenance of constant amplitudes of cytosolic Ca(2+) oscillations. Simple plausible rate laws for Ca(2+) fluxes across the inner mitochondrial membrane have been used in this model. Here we show that it is possible to use the same rate laws as a plug-in element in other existing mathematical models and obtain the same effect on amplitude regulation. This result appears to be universal, independent of the type of model and the type of Ca(2+) oscillations. We demonstrate this on two models for spiking Ca(2+) oscillations [J. Biol. Chem. 266 (1991) 11068; Cell Calcium 14 (1993) 311] and on two recent models for bursting Ca(2+) oscillations; one of them being a receptor-operated model [Biophys. J. 79 (2000) 1188] and the other one being a store-operated model [BioSystems 57 (2000) 75].     

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.     

Modelling the transition from simple to complex Ca2+oscillations in pancreatic acinar cells

A mathematical model is proposed which systematically investigates complex calcium oscillations in pancreatic acinar cells. This model is based on calcium-induced calcium release via inositol trisphosphate receptors (IPR) and ryanodine receptors (RyR) and includes calcium modulation of inositol (1,4,5) trisphosphate (IP₃) levels through feedback regulation of degradation and production. In our model, the apical and the basal regions are separated by a region containing mitochondria, which is capable of restricting Ca²⁺ responses to the apical region. We were able to reproduce the observed oscillatory patterns, from baseline spikes to sinusoidal oscillations. The model predicts that calcium-dependent production and degradation of IP₃ is a key mechanism for complex calcium oscillations in pancreatic acinar cells. A partial bifurcation analysis is performed which explores the dynamic behaviour of the model in both apical and basal regions.     

Transition from stochastic to deterministic behavior in calcium oscillations

Simulation and modeling is becoming more and more important when studying complex biochemical systems. Most often, ordinary differential equations are employed for this purpose. However, these are only applicable when the numbers of participating molecules in the biochemical systems are large enough to be treated as concentrations. For smaller systems, stochastic simulations on discrete particle basis are more accurate. Unfortunately, there are no general rules for determining which method should be employed for exactly which problem to get the most realistic result. Therefore, we study the transition from stochastic to deterministic behavior in a widely studied system, namely the signal transduction via calcium, especially calcium oscillations. We observe that the transition occurs within a range of particle numbers, which roughly corresponds to the number of receptors and channels in the cell, and depends heavily on the attractive properties of the phase space of the respective systems dynamics. We conclude that the attractive properties of a system, expressed, e.g., by the divergence of the system, are a good measure for determining which simulation algorithm is appropriate in terms of speed and realism.     

Switching Akt: from survival signaling to deadly response

Akt, a protein kinase hyperactivated in many tumors, plays a major role in both cell survival and resistance to tumor therapy. A recent study, 1 along with other evidences, shows interestingly, that Akt is not a single‐function kinase, but may facilitate rather than inhibit cell death under certain conditions. This hitherto undetected function of Akt is accomplished by its ability to increase reactive oxygen species and to suppress antioxidant enzymes. The ability of Akt to down‐regulate antioxidant defenses uncovers a novel Achilles' heel, which could be exploited by oxidant therapies in order to selectively eradicate tumor cells that express high levels of Akt activity.     

Notch signaling and diseases: an evolutionary journey from a simple beginning to complex outcomes

Notch signaling pathway regulates a wide variety of cellular processes during development and it also plays a crucial role in human diseases. This important link is firmly established in cancer, since a rare T-ALL-associated genetic lesion has been initially reported to result in deletion of Notch1 ectodomain and constitutive activation of its intracellular region. Interestingly, the cellular response to Notch signaling can be extremely variable depending on the cell type and activation context. Notch signaling triggers signals implicated in promoting carcinogenesis and autoimmune diseases, whereas it can also sustain responses that are critical to suppress carcinogenesis and to negatively regulate immune response. However, Notch signaling induces all these effects via an apparently simple signal transduction pathway, diversified into a complex network along evolution from Drosophila to mammals. Indeed, an explanation of this paradox comes from a number of evidences accumulated during the last few years, which dissected the intrinsic canonical and non-canonical components of the Notch pathway as well as several modulatory extrinsic signaling events. The identification of these signals has shed light onto the mechanisms whereby Notch and other pathways collaborate to induce a particular cellular phenotype. In this article, we review the role of Notch signaling in cells as diverse as T lymphocytes and epithelial cells of the epidermis, with the main focus on understanding the mechanisms of Notch versatility.     

The calcium sensing receptor: from calcium sensing to signaling

The Ca2+-sensing receptor(the Ca SR),a G-protein-coupled receptor,regulates Ca2+ homeostasis in the body by monitoring extracellular levels of Ca2+([Ca2+]o) and responding to a diverse array of stimuli.Mutations in the Ca2+-sensing receptor result in hypercalcemic or hypocalcemic disorders,such as familial hypocalciuric hypercalcemia,neonatal severe primary hyperparathyroidism,and autosomal dominant hypocalcemic hypercalciuria.Compelling evidence suggests that the Ca SR plays multiple roles extending well beyond not only regulating the level of extracellular Ca2+ in the human body,but also controlling a diverse range of biological processes.In this review,we focus on the structural biology of the Ca SR,the ligand interaction sites as well as their relevance to the disease associated mutations.This systematic summary will provide a comprehensive exploration of how the Ca SR integrates extracellular Ca2+ into intracellular Ca2+ signaling.     

Hypoxia-induced acidosis uncouples the STIM-Orai calcium signaling complex

The endoplasmic reticulum Ca²⁺-sensing STIM proteins mediate Ca²⁺ entry signals by coupling to activate plasma membrane Orai channels. We reveal that STIM-Orai coupling is rapidly blocked by hypoxia and the ensuing decrease in cytosolic pH. In smooth muscle cells or HEK293 cells coexpressing STIM1 and Orai1, acute hypoxic conditions rapidly blocked store-operated Ca²⁺ entry and the Orai1-mediated Ca²⁺ release-activated Ca²⁺ current (I_CRAC). Hypoxia-induced blockade of Ca²⁺ entry and I_CRAC was reversed by NH₄⁺-induced cytosolic alkalinization. Hypoxia and acidification both blocked I_CRAC induced by the short STIM1 Orai-activating region. Although hypoxia induced STIM1 translocation into junctions, it did not dissociate the STIM1-Orai1 complex. However, both hypoxia and cytosolic acidosis rapidly decreased Förster resonance energy transfer (FRET) between STIM1-YFP and Orai1-CFP. Thus, although hypoxia promotes STIM1 junctional accumulation, the ensuing acidification functionally uncouples the STIM1-Orai1 complex providing an important mechanism protecting cells from Ca²⁺ overload under hypoxic stress conditions.     

Mitochondrial respiration links TOR complex 2 signaling to calcium regulation and autophagy

The target of rapamycin (TOR) kinase is a conserved regulator of cell growth and functions within 2 different protein complexes, TORC1 and TORC2, where TORC2 positively controls macroautophagy/autophagy during amino acid starvation. Under these conditions, TORC2 signaling inhibits the activity of the calcium-regulated phosphatase calcineurin and promotes the general amino acid control (GAAC) response and autophagy. Here we demonstrate that TORC2 regulates calcineurin by controlling the respiratory activity of mitochondria. In particular, we find that mitochondrial oxidative stress affects the calcium channel regulatory protein Mid1, which we show is an essential upstream activator of calcineurin. Thus, these findings describe a novel regulation for autophagy that involves TORC2 signaling, mitochondrial respiration, and calcium homeostasis.     

Intercellular signaling in glial cells: calcium waves and oscillations in response to mechanical stimulation and glutamate.

Intercellular Ca2+ signaling in primary cultures of glial cells was investigated with digital fluorescence video imaging. Mechanical stimulation of a single cell induced a wave of increased [Ca2+]i that was communicated to surrounding cells. This was followed by asynchronous Ca2+ oscillations in some cells. Similar communicated Ca2+ responses occurred in the absence of extracellular Ca2+, despite an initial decrease in [Ca2+]i in the stimulated cell. Mechanical stimulation in the presence of glutamate induced a typical communicated Ca2+ wave through cells undergoing asynchronous Ca2+ oscillations in response to glutamate. The coexistence of communicated Ca2+ waves and asynchronous Ca2+ oscillations suggests distinct mechanisms for intra- and intercellular Ca2+ signaling. This intercellular signaling may coordinate cooperative glial function.     

Collective oscillations in developing cells: insights from simple systems

From hormonal secretion to gene expression, multicellular dynamics are rich in oscillatory regulation. When organized in space and time, periodic cell-cell signaling can give rise to long-range coordination of gene expression and cell movement in tissues. Lack of synchrony of the oscillations on the other hand can serve as a source of initial divergence of cell fate in stem cells. How properties of individual cells can account for collective rhythmic behaviors at the tissue level remains elusive in most cases. Recently, studies in chemical reactions, synthetic gene circuits, yeast and social amoeba Dictyostelium have greatly enhanced our view of collective oscillations in cell populations. From these relatively simple systems, a unified view of how excitable and oscillatory regulations could be tuned and coupled to give rise to tissue-level oscillations is emerging. The review focuses on recent progress in cyclic adenosine monophosphate oscillations in Dictyostelium and highlights similarities and differences with other systems. We will see that the autonomy of single-cell level oscillations and different ways in which cells are coupled influence how group-level information can be encoded in collective oscillations.     

Pharmacologically induced calcium oscillations protect neurons from increases in cytosolic calcium after trauma

Increases in cytosolic calcium ([Ca(2+)](i)) following mechanical injury are often considered a major contributing factor to the cellular sequelae in traumatic brain injury (TBI). However, very little is known on how developmental changes may affect the calcium signaling in mechanically injured neurons. One key feature in the developing brain that may directly impact its sensitivity to stretch is the reduced inhibition which results in spontaneous [Ca(2+)](i) oscillations. In this study, we examined the mechanism of stretch-induced [Ca(2+)](i) transients in 18-days in vitro (DIV) neurons exhibiting bicuculline-induced [Ca(2+)](i) oscillations. We used an in vitro model of mechanical trauma to apply a defined uniaxial strain to cultured cortical neurons and used increases in [Ca(2+)](i) as a measure of the neuronal response to the stretch insult. We found that stretch-induced increases in [Ca(2+)](i) in 18-DIV neurons were inhibited by pretreatment with either the NMDA receptor antagonist, APV [D(-)-2-Amino-5-phosphonopentanoic acid], or by depolymerizing the actin cytoskeleton prior to stretch. Blocking synaptic NMDA receptors prior to stretch significantly attenuated most of the [Ca(2+)](i) transient. In comparison, cultures with pharmacologically induced [Ca(2+)](i) oscillations showed a substantially reduced [Ca(2+)](i) peak after stretch. We provide evidence showing that a contributing factor to this mechanical desensitization from induced [Ca(2+)](i) oscillations is the PKC-mediated uncoupling of NMDA receptors (NMDARs) from spectrin, an actin-associated protein, thereby rendering neurons insensitive to stretch. These results provide novel insights into how the [Ca(2+)](i) response to stretch is initiated, and how reduced inhibition - a feature of the developing brain - may affect the sensitivity of the immature brain to trauma.     

Hormone-induced calcium oscillations in liver cells can be explained by a simple one pool model

Hormone-induced oscillations of the free intracellular calcium concentration are thought to be relevant for frequency encoding of hormone signals. In liver cells, such Ca2+ oscillations occur in response to stimulation by hormones acting via phosphoinositide breakdown. This observation may be explained by cooperative, positive feedback of Ca2+ on its own release from one inositol 1,4,5-trisphosphate-sensitive pool, obviating oscillations of inositol 1,4,5-trisphosphate. The kinetic rate laws of the associated model have a mathematical structure reminiscent of the Brusselator, a hypothetical chemical model involving a rather improbable trimolecular reaction step, thus giving a realistic biological interpretation to this hallmark of dissipative structures. We propose that calmodulin is involved in mediating this cooperativity and positive feedback, as suggested by the presented experiments. For one, hormone-induced calcium oscillations can be inhibited by the (nonphenothiazine) calmodulin antagonists calmidazolium or CGS 9343 B. Alternatively, in cells overstimulated by hormone, as characterized by a non-oscillatory elevated Ca2+ concentration, these antagonists could again restore sustained calcium oscillations. The experimental observations, including modulation of the oscillations by extracellular calcium, were in qualitative agreement with the predictions of our mathematical model.     

Scaling up keystone effects from simple to complex ecological networks

Predicting the consequences of species loss requires extending our traditional understanding of simpler dynamic systems of few interacting species to the more complex ecological networks found in natural ecosystems. Especially important is the scaling up of our limited understanding of how and under what conditions loss of ‘keystone’ species causes large declines of many other species. Here we explore how these keystone effects vary among simulations progressively scaled up from simple to more complex systems. Simpler simulations of four to seven interacting species suggest that species up to four links away can strongly alter keystone effects and make the consequences of keystone loss potentially indeterminate in more realistically complex communities. Instead of indeterminacy, we find that more complex networks of up to 32 species generally buffer distant influences such that variation in keystone effects is well predicted by surprisingly local ‘top‐down’, ‘bottom‐up’, and ‘horizontal‘ constraints acting within two links of the keystone subsystem. These results demonstrate that: (1) strong suppression of the competitive dominant by the keystone may only weakly affect subordinate competitors; (2) the community context of the target species determines whether strong keystone effects are realized; (3) simple, measurable, and local attributes of complex communities may explain much of the empirically observed variation in keystone effects; and (4) increasing network complexity per se does not inherently make the prediction of strong keystone effects more complicated.     

ATP autocrine/paracrine signaling induces calcium oscillations and NFAT activation in human mesenchymal stem cells

Human bone marrow-derived mesenchymal stem cells (hMSCs) have the potential to differentiate into several types of cells. Calcium ions (Ca(2+)) play an important role in the differentiation and proliferation of hMSCs. We have demonstrated that spontaneous [Ca(2+)](i) oscillations occur without agonist stimulation in hMSCs. However, the precise mechanism of its generation remains unclear. In this study, we investigated the mechanism and role of spontaneous [Ca(2+)](i) oscillations in hMSCs and found that IP(3)-induced Ca(2+) release is essential for spontaneous [Ca(2+)](i) oscillations. We also found that an ATP autocrine/paracrine signaling pathway is involved in the oscillations. In this pathway, an ATP is secreted via a hemi-gap-junction channel; it stimulates the P(2)Y(1) receptors, resulting in the activation of PLC-beta to produce IP(3). We were able to pharmacologically block this pathway, and thereby to completely halt the [Ca(2+)](i) oscillations. Furthermore, we found that [Ca(2+)](i) oscillations were associated with NFAT translocation into the nucleus in undifferentiated hMSCs. Once the ATP autocrine/paracrine signaling pathway was blocked, it was not possible to detect the nuclear translocation of NFAT, indicating that the activation of NFAT is closely linked to [Ca(2+)](i) oscillations. As the hMSCs differentiated to adipocytes, the [Ca(2+)](i) oscillations disappeared and the translocation of NFAT ceased. These results provide new insight into the molecular and physiological mechanism of [Ca(2+)](i) oscillations in undifferentiated hMSCs.     

Amplitude-encoded calcium oscillations in fish cells

The reaction of intracellular Ca(2+) to different agonist stimuli in primary hepatocytes from rainbow trout (Oncorhynchus mykiss) as well as the permanent fish cell line RTL-W1 was investigated systematically. In addition to "classical" agonists such as phenylephrine and ATP, model environmental toxicants like 4-nitrophenol and 3,4-dichloroaniline were used to elucidate possible interactions between toxic effects and Ca(2+) signaling. We report Ca(2+) oscillations in response to several stimuli in RTL-W1 cells and to a lesser extent in primary hepatocytes. Moreover, these Ca(2+) oscillations are amplitude-encoded in contrast to their mammalian counterpart. Bioinformatics and computational analysis were employed to identify key players of Ca(2+) signaling in fish and to determine likely causes for the experimentally observed differences between the Ca(2+) dynamics in fish cells compared to those in mammalian liver cells.     

Sustained oscillations in glycolysis: an experimental and theoretical study of chaotic and complex periodic behavior and of quenching of simple oscillations

We report sustained oscillations in glycolysis conducted in an open system (a continuous-flow, stirred tank reactor; CSTR) with inflow of yeast extract as well as glucose. Depending on the operating conditions, we observe simple or complex periodic oscillations or chaos. We report the response of the system to instantaneous additions of small amounts of several substrates as functions of the amount added and the phase of the addition. We simulate oscillations and perturbations by a kinetic model based on the mechanism of glycolysis in a CSTR. We find that the response to particular perturbations forms an efficient tool for elucidating the mechanism of biochemical oscillations.     

Cell signaling microdomain with Na,K-ATPase and inositol 1,4,5-trisphosphate receptor generates calcium oscillations

Recent studies indicate novel roles for the ubiquitous ion pump, Na,K-ATPase, in addition to its function as a key regulator of intracellular sodium and potassium concentration. We have previously demonstrated that ouabain, the endogenous ligand of Na,K-ATPase, can trigger intracellular Ca2+ oscillations, a versatile intracellular signal controlling a diverse range of cellular processes. Here we report that Na,K-ATPase and inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) form a cell signaling microdomain that, in the presence of ouabain, generates slow Ca2+ oscillations in renal cells. Using fluorescent resonance energy transfer (FRET) measurements, we detected a close spatial proximity between Na,K-ATPase and InsP3R. Ouabain significantly enhanced FRET between Na,K-ATPase and InsP3R. The FRET effect and ouabain-induced Ca2+ oscillations were not observed following disruption of the actin cytoskeleton. Partial truncation of the NH2 terminus of Na,K-ATPase catalytic alpha1-subunit abolished Ca2+ oscillations and downstream activation of NF-kappaB. Ouabain-induced Ca2+ oscillations occurred in cells expressing an InsP3 sponge and were hence independent of InsP3 generation. Thus, we present a novel principle for a cell signaling microdomain where an ion pump serves as a receptor.