Intracellular signalling. Receptor-specific messenger oscillations

A molecular systems perspective on calcium oscillations beyond ion fluxes

Calcium (Ca2+) oscillations, marked by periodic fluctuations in cytosolic Ca2+ levels, are a universal feature of both excitable and non-excitable cells, regulating key functions like immune responses, neuronal activity and oocyte activation. Despite significant progress over the past few decades in identifying the molecular toolkits involved in Ca2+ mobilization, fundamental questions remain unresolved: How do Ca2+oscillations arise? In dynamical systems, oscillations arise as closed-loop trajectories in phase space, known as limit cycles. In this framework, [Ca2+] is the variable that oscillates along the limit cycle. Is [Ca2+] also the control parameter that defines the system's stability? Understanding how oscillations arise and how instability is controlled are essential for determining what these oscillations encode. This review revisits classic categorizations of Ca2+ oscillation models, focusing on the minimal mathematical models, their assumptions and gaps linking models with experimental data. We examine historical arguments in light of recent discoveries of plasma membrane lipid oscillations in non-excitable cells. While growing evidence support the pivotal role of lipid signaling in regulating Ca2+ dynamics, they mostly focused on the upstream role of signaling in Ca2+ mobilization, rather than viewing membrane-dependent signal transduction as the core control loop that is responsible for oscillatory Ca2+ dynamics. Here we summarize recent molecular studies of phosphoinositide signaling in modulating Ca2+ dynamics, by considering a broader chemical perspective as essential for understanding Ca2+ oscillations beyond ion fluxes.

ER stress as a sentinel mechanism for ER Ca2+ homeostasis

Endoplasmic reticulum (ER) stress is triggered upon the interference with oxidative protein folding that aims to produce fully folded, disulfide-bonded and glycosylated proteins, which are then competent to exit the ER. Many of the enzymes catalyzing this process require the binding of Ca2+ ions, including the chaperones BiP/GRP78, calnexin and calreticulin. The induction of ER stress with a variety of drugs interferes with chaperone Ca2+ binding, increases cytosolic Ca2+through the opening of ER Ca2+ channels, and activates store-operated Ca2+ entry (SOCE). Posttranslational modifications (PTMs) of the ER Ca2+ handling proteins through ER stress-dependent phosphorylation or oxidation control these mechanisms, as demonstrated in the case of the sarco/endoplasmic reticulum ATPase (SERCA), inositol 1,4,5 trisphosphate receptors (IP3Rs) or stromal interaction molecule 1 (STIM1). Their aim is to restore ER Ca2+ homeostasis but also to increase Ca2+ transfer from the ER to mitochondria during ER stress. This latter function boosts ER bioenergetics, but also triggers apoptosis if ER Ca2+ signaling persists. ER Ca2+ toolkit oxidative modifications upon ER stress can occur within the ER lumen or in the adjacent cytosol. Enzymes involved in this redox control include ER oxidoreductin 1 (ERO1) or the thioredoxin-family protein disulfide isomerases (PDI) and ERp57. A tight, but adaptive connection between ER Ca2+ content, ER stress and mitochondrial readouts allows for the proper functioning of many tissues, including skeletal muscle, the liver, and the pancreas, where ER stress either maintains or compromises their function, depending on its extent and context. Upon mutation of key regulators of ER Ca2+ signaling, diseases such as muscular defects (e.g., from mutated selenoprotein N, SEPN1/SELENON), or diabetes (e.g., from mutated PERK) are the result.

PAR1-mediated Non-periodical Synchronized Calcium Oscillations in Human Mesangial Cells

Mesangial cells offer structural support to the glomerular tuft and regulate glomerular capillary flow through their contractile capabilities. These cells undergo phenotypic changes, such as proliferation and mesangial expansion, resulting in abnormal glomerular tuft formation and reduced capillary loops. Such adaptation to the changing environment is commonly associated with various glomerular diseases, including diabetic nephropathy and glomerulonephritis. Thrombin-induced mesangial remodeling was found in diabetic patients, and expression of the corresponding protease-activated receptors (PARs) in the renal mesangium was reported. However, the functional PAR-mediated signaling in mesangial cells was not examined. This study investigated protease-activated mechanisms regulating mesangial cell calcium waves that may play an essential role in the mesangial proliferation or constriction of the arteriolar cells. Our results indicate that coagulation proteases such as thrombin induce synchronized oscillations in cytoplasmic Ca2+ concentration of mesangial cells. The oscillations required PAR1 G-protein coupled receptors-related activation, but not a PAR4, and were further mediated presumably through store-operated calcium entry and transient receptor potential canonical 3 (TRPC3) channel activity. Understanding thrombin signaling pathways and their relation to mesangial cells, contractile or synthetic (proliferative) phenotype may play a role in the development of chronic kidney disease and requires further investigation.

Breaking reflection symmetry: evolving long dynamical cycles in Boolean systems

In interacting dynamical systems, specific local interaction rules for system components give rise to diverse and complex global dynamics. Long dynamical cycles are a key feature of many natural interacting systems, especially in biology. Examples of dynamical cycles range from circadian rhythms regulating sleep to cell cycles regulating reproductive behavior. Despite the crucial role of cycles in nature, the properties of network structure that give rise to cycles still need to be better understood. Here, we use a Boolean interaction network model to study the relationships between network structure and cyclic dynamics. We identify particular structural motifs that support cycles, and other motifs that suppress them. More generally, we show that the presence of dynamical reflection symmetry in the interaction network enhances cyclic behavior. In simulating an artificial evolutionary process, we find that motifs that break reflection symmetry are discarded. We further show that dynamical reflection symmetries are over-represented in Boolean models of natural biological systems. Altogether, our results demonstrate a link between symmetry and functionality for interacting dynamical systems, and they provide evidence for symmetry's causal role in evolving dynamical functionality.

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.

The role of Zn2+ in shaping intracellular Ca2+ dynamics in the heart

Increasing evidence suggests that Zn2+ acts as a second messenger capable of transducing extracellular stimuli into intracellular signaling events. The importance of Zn2+ as a signaling molecule in cardiovascular functioning is gaining traction. In the heart, Zn2+ plays important roles in excitation-contraction (EC) coupling, excitation-transcription coupling, and cardiac ventricular morphogenesis. Zn2+ homeostasis in cardiac tissue is tightly regulated through the action of a combination of transporters, buffers, and sensors. Zn2+ mishandling is a common feature of various cardiovascular diseases. However, the precise mechanisms controlling the intracellular distribution of Zn2+ and its variations during normal cardiac function and during pathological conditions are not fully understood. In this review, we consider the major pathways by which the concentration of intracellular Zn2+ is regulated in the heart, the role of Zn2+ in EC coupling, and discuss how Zn2+ dyshomeostasis resulting from altered expression levels and efficacy of Zn2+ regulatory proteins are key drivers in the progression of cardiac dysfunction.

Mapping of structural arrangement of cells and collective calcium transients: an integrated framework combining live cell imaging using confocal microscopy and UMAP-assisted HDBSCAN-based approach

Live cell calcium (Ca2+) imaging is one of the important tools to record cellular activity during in vitro and in vivo preclinical studies. Specially, high-resolution microscopy can provide valuable dynamic information at the single cell level. One of the major challenges in the implementation of such imaging schemes is to extract quantitative information in the presence of significant heterogeneity in Ca2+ responses attained due to variation in structural arrangement and drug distribution. To fill this gap, we propose time-lapse imaging using spinning disk confocal microscopy and machine learning-enabled framework for automated grouping of Ca2+ spiking patterns. Time series analysis is performed to correlate the drug induced cellular responses to self-assembly pattern present in multicellular systems. The framework is designed to reduce the large-scale dynamic responses using uniform manifold approximation and projection (UMAP). In particular, we propose the suitability of hierarchical DBSCAN (HDBSCAN) in view of reduced number of hyperparameters. We find UMAP-assisted HDBSCAN outperforms existing approaches in terms of clustering accuracy in segregation of Ca2+ spiking patterns. One of the novelties includes the application of non-linear dimension reduction in segregation of the Ca2+ transients with statistical similarity. The proposed pipeline for automation was also proved to be a reproducible and fast method with minimal user input. The algorithm was used to quantify the effect of cellular arrangement and stimulus level on collective Ca2+ responses induced by GPCR targeting drug. The analysis revealed a significant increase in subpopulation containing sustained oscillation corresponding to higher packing density. In contrast to traditional measurement of rise time and decay ratio from Ca2+ transients, the proposed pipeline was used to classify the complex patterns with longer duration and cluster-wise model fitting. The two-step process has a potential implication in deciphering biophysical mechanisms underlying the Ca2+ oscillations in context of structural arrangement between cells.

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

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

GPCR mediated control of calcium dynamics: A systems perspective

G-protein coupled receptor (GPCR) mediated calcium (Ca²⁺)-signaling transduction remains crucial in designing drugs for various complex diseases including neurodegeneration, chronic heart failure as well as respiratory diseases. Although there are several reviews detailing various aspects of Ca²⁺-signaling such as the role of IP₃ receptors and Ca²⁺-induced-Ca²⁺-release, none of them provide an integrated view of the mathematical descriptions of GPCR signal transduction and investigations on dose-response curves. This article is the first study in reviewing the network structures underlying GPCR signal transduction that control downstream [Ca_c²⁺]-oscillations. The central theme of this paper is to present the biochemical pathways, as well as molecular mechanisms underlying the GPCR-mediated Ca²⁺-dynamics in order to facilitate a better understanding of how agonist concentration is encoded in Ca²⁺-signals for G_αq, G_αs, and G_αi/o signaling pathways. Moreover, we present the GPCR targeting drugs that are relevant for treating cardiac, respiratory, and neuro-diseases. The current paper presents the ODE formulation for various models along with the detailed schematics of signaling networks. To provide a systems perspective, we present the network motifs that can provide readers an insight into the complex and intriguing science of agonist-mediated Ca²⁺-dynamics. One of the features of this review is to pinpoint the interplay between positive and negative feedback loops that are involved in controlling intracellular [Ca_c²⁺]-oscillations. Furthermore, we review several examples of dose-response curves obtained from [Ca_c²⁺]-spiking for various GPCR pathways. This paper is expected to be useful for pharmacologists and computational biologists for designing clinical applications of GPCR targeting drugs through modulation of Ca²⁺-dynamics.

IP3-Dependent Ca2+ Oscillations Switch into a Dual Oscillator Mechanism in the Presence of PLC-Linked Hormones

Ca2+ oscillations that depend on inositol-1,4,5-trisphosphate (IP3) have been ascribed to biphasic Ca2+ regulation of the IP3 receptor (IP3R) or feedback mechanisms controlling IP3 levels in different cell types. IP3 uncaging in hepatocytes elicits Ca2+ transients that are often localized at the subcellular level and increase in magnitude with stimulus strength. However, this does not reproduce the broad baseline-separated global Ca2+ oscillations elicited by vasopressin. Addition of hormone to cells activated by IP3 uncaging initiates a qualitative transition from high-frequency spatially disorganized Ca2+ transients, to low-frequency, oscillatory Ca2+ waves that propagate throughout the cell. A mathematical model with dual coupled oscillators that integrates Ca2+-induced Ca2+ release at the IP3R and mutual feedback mechanisms of cross-coupling between Ca2+ and IP3 reproduces this behavior. Thus, multiple Ca2+ oscillation modes can coexist in the same cell, and hormonal stimulation can switch from the simpler to the more complex to yield robust signaling.

The Endoplasmic Reticulum-Plasma Membrane Junction: A Hub for Agonist Regulation of Ca2+ Entry

Stimulation of cell-surface receptors induces cytosolic Ca²⁺ ([Ca²⁺]_i) increases that are detected and transduced by effector proteins for regulation of cell function. Intracellular Ca²⁺ release, via endoplasmic reticulum (ER) proteins inositol 1,4,5-trisphosphate receptors (IP₃R) and ryanodine receptors (RyR), and Ca²⁺ influx, via store-operated Ca²⁺ entry (SOCE), contribute to the increase in [Ca²⁺]_i The amplitude, frequency, and spatial characteristics of the [Ca²⁺]_i increases are controlled by the compartmentalization of proteins into signaling complexes such as receptor-signaling complexes and SOCE complexes. Both complexes include protein and lipid components, located in the plasma membrane (PM) and ER. Receptor signaling initiates in the PM via phospholipase C (PLC)-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂), and culminates with the activation of IP₃R in the ER. Conversely, SOCE is initiated in the ER by Ca²⁺-sensing stromal interaction molecule (STIM) proteins, which then interact with PM channels Orai1 and TRPC1 to activate Ca²⁺ entry. This review will address how ER-PM junctions serve a central role in agonist regulation of SOCE.

The Arabidopsis heterotrimeric G-protein β subunit, AGB1, is required for guard cell calcium sensing and calcium-induced calcium release

Cytosolic calcium concentration ([Ca²⁺ ]_cyt ) and heterotrimeric G-proteins are universal eukaryotic signaling elements. In plant guard cells, extracellular calcium (Ca_o ) is as strong a stimulus for stomatal closure as the phytohormone abscisic acid (ABA), but underlying mechanisms remain elusive. Here, we report that the sole Arabidopsis heterotrimeric Gβ subunit, AGB1, is required for four guard cell Ca_o responses: induction of stomatal closure; inhibition of stomatal opening; [Ca²⁺ ]_cyt oscillation; and inositol 1,4,5-trisphosphate (InsP3) production. Stomata in wild-type Arabidopsis (Col) and in mutants of the canonical Gα subunit, GPA1, showed inhibition of stomatal opening and promotion of stomatal closure by Ca_o . By contrast, stomatal movements of agb1 mutants and agb1/gpa1 double-mutants, as well as those of the agg1agg2 Gγ double-mutant, were insensitive to Ca_o . These behaviors contrast with ABA-regulated stomatal movements, which involve GPA1 and AGB1/AGG3 dimers, illustrating differential partitioning of G-protein subunits among stimuli with similar ultimate impacts, which may facilitate stimulus-specific encoding. AGB1 knockouts retained reactive oxygen species and NO production, but lost YC3.6-detected [Ca²⁺ ]_cyt oscillations in response to Ca_o , initiating only a single [Ca²⁺ ]_cyt spike. Experimentally imposed [Ca²⁺ ]_cyt oscillations restored stomatal closure in agb1. Yeast two-hybrid and bimolecular complementation fluorescence experiments revealed that AGB1 interacts with phospholipase Cs (PLCs), and Ca_o induced InsP3 production in Col but not in agb1. In sum, G-protein signaling via AGB1/AGG1/AGG2 is essential for Ca_o -regulation of stomatal apertures, and stomatal movements in response to Ca_o apparently require Ca²⁺ -induced Ca²⁺ release that is likely dependent on Gβγ interaction with PLCs leading to InsP3 production.

Lysophosphatidylinositol-acyltransferase-1 is involved in cytosolic Ca2+ oscillations in macrophages

Lysophosphatidylinositol-acyltransferase-1 (LPIAT1) specifically catalyzes the transfer of arachidonoyl-CoA to lysophosphoinositides. LPIAT^-/- mice have been shown to have severe defects in the brain and liver; however, the exact molecular mechanisms behind these conditions are not well understood. As immune cells have been implicated in liver inflammation based on disfunction of LPIAT1, we generated Raw264.7 macrophages deficient in LPIAT1, using shRNA and CRISPR/Cas9. The amount of C38:4 species in phosphoinositides, especially in PtdInsP₂ , was remarkably decreased in these cells. Unlike in wild-type cells, LPIAT1-deficient cells showed prolonged oscillations of intracellular Ca²⁺ upon UDP stimulation, which is known to activate phospholipase Cβ through the Gq-coupled P2Y6 receptor, even in the absence of extracellular Ca²⁺ . It is speculated that the prolonged Ca²⁺ response may be relevant to the increased risk of liver inflammation induced by LPIAT1 disfunction.

Coding and decoding of oscillatory Ca2+ signals

About 30 years after their first observation, Ca²⁺ oscillations are now recognised as a universal mechanism of signal transduction. These oscillations are driven by periodic cycles of release and uptake of Ca²⁺ between the cytoplasm and the endoplasmic reticulum. Their frequency often increases with the level of stimulation, which can be decoded by some molecules. However, it is becoming increasingly evident that the widespread core oscillatory mechanism is modulated in many ways, depending on the cell type and on the physiological conditions. Interplay with inositol 1,4,5-trisphosphate metabolism and with other Ca²⁺ stores as the extracellular medium or mitochondria can much affect the properties of these oscillations. In many cases, these finely tuned characteristics of Ca²⁺ oscillations impact the physiological response that is triggered by the signal. Moreover, oscillations are intrinsically irregular. This randomness can also be exploited by the cell. In this review, we discuss evidences of these additional manifestations of the versatility of Ca²⁺ signalling.

Robustness of frequency vs. amplitude coding of calcium oscillations during changing temperatures

Intracellular calcium oscillations have been widely studied. It is assumed that information is conveyed in the frequency, amplitude and shape of these oscillations. In particular, calcium signalling in mammalian liver cells has repeatedly been reported to display frequency coding so that an increasing amount of stimulus is translated into an increasing frequency of the oscillations. However, recently, we have shown that calcium oscillations in fish liver cells rather exhibit amplitude coding with increasing stimuli being translated into increasing amplitudes. Practical consequences of this difference are unknown so far. Here we investigated advantages and disadvantages of frequency vs. amplitude coding, in particular in environments with substantially changing temperatures (e.g. 10-20 degrees). For this purpose, we use computational modelling and a new approach to generate a calcium model exactly displaying a specific frequency and/or amplitude. We conclude that despite the advantages in flexibility that frequencies might offer for the transmission of information in the cell, amplitude coding is obviously more robust with respect to changes in environmental temperatures. This potentially explains the observed differences between two classes of organisms, one operating at constant temperatures whereas the other is not.

Biased agonism and allosteric modulation of metabotropic glutamate receptor 5

Metabotropic glutamate receptors belong to class C G-protein-coupled receptors and consist of eight subtypes that are ubiquitously expressed throughout the central nervous system. In recent years, the metabotropic glutamate receptor subtype 5 (mGlu₅) has emerged as a promising target for a broad range of psychiatric and neurological disorders. Drug discovery programs targetting mGlu₅ are primarily focused on development of allosteric modulators that interact with sites distinct from the endogenous agonist glutamate. Significant efforts have seen mGlu₅ allosteric modulators progress into clinical trials; however, recent failures due to lack of efficacy or adverse effects indicate a need for a better understanding of the functional consequences of mGlu₅ allosteric modulation. Biased agonism is an interrelated phenomenon to allosterism, describing how different ligands acting through the same receptor can differentially influence signaling to distinct transducers and pathways. Emerging evidence demonstrates that allosteric modulators can induce biased pharmacology at the level of intrinsic agonism as well as through differential modulation of orthosteric agonist-signaling pathways. Here, we present key considerations in the discovery and development of mGlu₅ allosteric modulators and the opportunities and pitfalls offered by biased agonism and modulation.

[Advances in methods for analyzing IP3 signaling and understanding of coupled Ca2+ and IP3 oscillations]

Inositol 1,4,5-trisphosphate (IP₃) is an important intracellular messenger produced by phospholipase C via the activation of G-protein-coupled receptor- or receptor-tyrosine-kinase-mediated pathways, and is involved in numerous responses to hormones, neurotransmitters, and growth factors through the releases of Ca²⁺ from intracellular stores via IP₃ receptors. IP₃-mediated Ca²⁺ signals often exhibit complex spatial and temporal organizations, such as Ca²⁺ oscillations. Recently, new methods have become available to measure IP₃ concentration ([IP₃]) using AlphaScreen technology, fluorescence polarization, and competitive ligand binding assay (CFLA). These methods are useful for the high throughput screening in drug discovery. Calcium ions generate versatile intracellular signals such as Ca²⁺ oscillations and waves. Fluorescent sensors molecules to monitor changes in [IP₃] in single living cells are crucial to study the mechanism for the spatially and temporally regulated Ca²⁺ signals. In particular, FRET-based IP₃ sensors are useful for the quantitative monitoring intracellular [IP₃], and allowed to uncovered the oscillatory IP₃ dynamics in association with Ca²⁺ oscillations. A mathematical model of coupled Ca²⁺ and IP₃ oscillations predicts that Ca²⁺ oscillations are the result of modulation of the IP₃ receptor by intracellular Ca²⁺, and that the period is modulated by the accompanying IP₃ oscillations. These model predictions have also been confirmed experimentally. At present, however, usefulness of FRET-based IP₃ sensors are limited by their relatively small change in fluorescence. Development of novel IP₃ sensors with improve dynamic range would be important for understanding the regulatory mechanism of Ca²⁺ signaling and for in vivo IP₃ imaging.

Multiple calcium patterns of rat osteoblasts under fluidic shear stress

The intracellular calcium ([Ca²⁺ ]_i ) response induced by external forces can be diverse and complex. Using primary osteoblasts from Wistar rats, we found multiple patterns of [Ca²⁺ ]_i responses induced by fluidic shear stress (Fss), including homogeneous non-oscillation and heterogeneous oscillations. These multiple-patterned [Ca²⁺ ]_i responses could be influenced by Fss intensity, cell density, and cell differentiation. Our real-time measurements with free calcium, ATP, ATP without calcium, suramin, apyrase, and thapsigargin confirmed homogeneous [Ca²⁺ ]_i patterns and/or heterogeneous [Ca²⁺ ]_i oscillations with respect to the combined degree of external Ca²⁺ influx, and intracellular Ca²⁺ release. Our theoretical model supported diverse Fss-induced calcium activities as well. We concluded that a singular factor of Ca²⁺ influx or release dominated to produce smooth homogeneous patterns, but combined factors produced oscillatory heterogeneous patterns. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2039-2051, 2018.

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.

Electrical control of calcium oscillations in mesenchymal stem cells using microsecond pulsed electric fields

BACKGROUND

Human mesenchymal stem cells are promising tools for regenerative medicine due to their ability to differentiate into many cellular types such as osteocytes, chondrocytes and adipocytes amongst many other cell types. These cells present spontaneous calcium oscillations implicating calcium channels and pumps of the plasma membrane and the endoplasmic reticulum. These oscillations regulate many basic functions in the cell such as proliferation and differentiation. Therefore, the possibility to mimic or regulate these oscillations might be useful to regulate mesenchymal stem cells biological functions.

METHODS

One or several electric pulses of 100 μs were used to induce Ca2+ spikes caused by the penetration of Ca2+ from the extracellular medium, through the transiently electropermeabilized plasma membrane, in human adipose mesenchymal stem cells from several donors. Attached cells were preloaded with Fluo-4 AM and exposed to the electric pulse(s) under the fluorescence microscope. Viability was also checked.

RESULTS

According to the pulse(s) electric field amplitude, it is possible to generate a supplementary calcium spike with properties close to those of calcium spontaneous oscillations, or, on the contrary, to inhibit the spontaneous calcium oscillations for a very long time compared to the pulse duration. Through that inhibition of the oscillations, Ca2+ oscillations of desired amplitude and frequency could then be imposed on the cells using subsequent electric pulses. None of the pulses used here, even those with the highest amplitude, caused a loss of cell viability.

CONCLUSIONS

An easy way to control Ca2+ oscillations in mesenchymal stem cells, through their cancellation or the addition of supplementary Ca2+ spikes, is reported here. Indeed, the direct link between the microsecond electric pulse(s) delivery and the occurrence/cancellation of cytosolic Ca2+ spikes allowed us to mimic and regulate the Ca2+ oscillations in these cells. Since microsecond electric pulse delivery constitutes a simple technology available in many laboratories, this new tool might be useful to further investigate the role of Ca2+ in human mesenchymal stem cells biological processes such as proliferation and differentiation.

Chronic alcohol feeding potentiates hormone-induced calcium signalling in hepatocytes

KEY POINTS

Chronic alcohol consumption causes a spectrum of liver diseases, but the pathogenic mechanisms driving the onset and progression of disease are not clearly defined. We show that chronic alcohol feeding sensitizes rat hepatocytes to Ca²⁺ -mobilizing hormones resulting in a leftward shift in the concentration-response relationship and the transition from oscillatory to more sustained and prolonged Ca²⁺ increases. Our data demonstrate that alcohol-dependent adaptation in the Ca²⁺ signalling pathway occurs at the level of hormone-induced inositol 1,4,5 trisphosphate (IP₃ ) production and does not involve changes in the sensitivity of the IP₃ receptor or size of internal Ca²⁺ stores. We suggest that prolonged and aberrant hormone-evoked Ca²⁺ increases may stimulate the production of mitochondrial reactive oxygen species and contribute to alcohol-induced hepatocyte injury. ABSTRACT: 'Adaptive' responses of the liver to chronic alcohol consumption may underlie the development of cell and tissue injury. Alcohol administration can perturb multiple signalling pathways including phosphoinositide-dependent cytosolic calcium ([Ca²⁺ ]_i ) increases, which can adversely affect mitochondrial Ca²⁺ levels, reactive oxygen species production and energy metabolism. Our data indicate that chronic alcohol feeding induces a leftward shift in the dose-response for Ca²⁺ -mobilizing hormones resulting in more sustained and prolonged [Ca²⁺ ]_i increases in both cultured hepatocytes and hepatocytes within the intact perfused liver. Ca²⁺ increases were initiated at lower hormone concentrations, and intercellular calcium wave propagation rates were faster in alcoholics compared to controls. Acute alcohol treatment (25 mm) completely inhibited hormone-induced calcium increases in control livers, but not after chronic alcohol-feeding, suggesting desensitization to the inhibitory actions of ethanol. Hormone-induced inositol 1,4,5 trisphosphate (IP₃ ) accumulation and phospholipase C (PLC) activity were significantly potentiated in hepatocytes from alcohol-fed rats compared to controls. Removal of extracellular calcium, or chelation of intracellular calcium did not normalize the differences in hormone-stimulated PLC activity, indicating calcium-dependent PLCs are not upregulated by alcohol. We propose that the liver 'adapts' to chronic alcohol exposure by increasing hormone-dependent IP₃ formation, leading to aberrant calcium increases, which may contribute to hepatocyte injury.

Aβ and Inflammatory Stimulus Activate Diverse Signaling Pathways in Monocytic Cells: Implications in Retaining Phagocytosis in Aβ-Laden Environment

Background: Accumulation of amyloid β (Aβ) is one of the main hallmarks of Alzheimer’s disease (AD). The enhancement of Aβ clearance may provide therapeutic means to restrict AD pathology. The cellular responses to different forms of Aβ in monocytic cells are poorly known. We aimed to study whether different forms of Aβ induce inflammatory responses in monocytic phagocytes and how Aβ may affect monocytic cell survival and function to retain phagocytosis in Aβ-laden environment.

Methods: Monocytic cells were differentiated from bone marrow hematopoietic stem cells (HSC) in the presence of macrophage-colony stimulating factor. Monocytic cells were stimulated with synthetic Aβ42 and intracellular calcium responses were recorded with calcium imaging. The formation of reactive oxygen species (ROS), secretion of cytokines and cell viability were also assessed. Finally, monocytic cells were introduced to native Aβ deposits ex vivo and the cellular responses in terms of cell viability, pro-inflammatory activation and phagocytosis were determined. The ability of monocytic cells to phagocytose Aβ plaques was determined after intrahippocampal transplantation in vivo.

Results: Freshly solubilized Aβ induced calcium oscillations, which persisted after removal of the stimulus. After few hours of aggregation, Aβ was not able to induce oscillations in monocytic cells. Instead, lipopolysaccharide (LPS) induced calcium responses divergent from Aβ-induced response. Furthermore, while LPS induced massive production of pro-inflammatory cytokines, neither synthetic Aβ species nor native Aβ deposits were able to induce pro-inflammatory activation of monocytic cells, contrary to primary microglia. Finally, monocytic cells retained their viability in the presence of Aβ and exhibited phagocytic activity towards native fibrillar Aβ deposits and congophilic Aβ plaques.

Conclusion: Monocytic cells carry diverse cellular responses to Aβ and inflammatory stimulus LPS. Even though Aβ species cause specific responses in calcium signaling, they completely lack the ability to induce pro-inflammatory phenotype of monocytic cells. Monocytes retain their viability and function in Aβ-laden brain.

Store-Operated Ca2+ Entry in Oocytes Modulate the Dynamics of IP3 -Dependent Ca2+ Release From Oscillatory to Tonic

Ca²⁺ signaling is ubiquitous and mediates various cellular functions encoded in its spatial, temporal, and amplitude features. Here, we investigate the role of store-operated Ca²⁺ entry (SOCE) in regulating the temporal dynamics of Ca²⁺ signals in Xenopus oocytes, which can be either oscillatory or tonic. Oscillatory Ca²⁺ release from intracellular stores is typically observed at physiological agonist concentration. When Ca²⁺ release leads to Ca²⁺ store depletion, this triggers the activation of SOCE that translates into a low-amplitude tonic Ca²⁺ signal. SOCE has also been implicated in fueling Ca²⁺ oscillations when activated at low levels. Here, we show that sustained SOCE activation in the presence of IP₃ to gate IP₃ receptors (IP₃ R) results in a pump-leak steady state across the endoplasmic reticulum (ER) membrane that inhibits Ca²⁺ oscillations and produces a tonic Ca²⁺ signal. Tonic signaling downstream of SOCE activation relies on focal Ca²⁺ entry through SOCE ER-plasma membrane (PM) junctions, Ca²⁺ uptake into the ER, followed by release through open IP₃ Rs at distant sites, a process we refer to as "Ca²⁺ teleporting." Therefore, sustained SOCE activation in the presence of an IP₃ -dependent "leak" pathway at the ER membrane results in a switch from oscillatory to tonic Ca²⁺ signaling. J. Cell. Physiol. 232: 1095-1103, 2017. © 2016 Wiley Periodicals, Inc.

A G-protein subunit translocation embedded network motif underlies GPCR regulation of calcium oscillations

G-protein βγ subunits translocate reversibly from the plasma membrane to internal membranes on receptor activation. Translocation rates differ depending on the γ subunit type. There is limited understanding of the role of the differential rates of Gβγ translocation in modulating signaling dynamics in a cell. Bifurcation analysis of the calcium oscillatory network structure predicts that the translocation rate of a signaling protein can regulate the damping of system oscillation. Here, we examined whether the Gβγ translocation rate regulates calcium oscillations induced by G-protein-coupled receptor activation. Oscillations in HeLa cells expressing γ subunit types with different translocation rates were imaged and quantitated. The results show that differential Gβγ translocation rates can underlie the diversity in damping characteristics of calcium oscillations among cells. Mathematical modeling shows that a translocation embedded motif regulates damping of G-protein-mediated calcium oscillations consistent with experimental data. The current study indicates that such a motif may act as a tuning mechanism to design oscillations with varying damping patterns by using intracellular translocation of a signaling component.

Differential Regulation of Multiple Steps in Inositol 1,4,5-Trisphosphate Signaling by Protein Kinase C Shapes Hormone-stimulated Ca2+ Oscillations

How Ca(2+) oscillations are generated and fine-tuned to yield versatile downstream responses remains to be elucidated. In hepatocytes, G protein-coupled receptor-linked Ca(2+) oscillations report signal strength via frequency, whereas Ca(2+) spike amplitude and wave velocity remain constant. IP3 uncaging also triggers oscillatory Ca(2+) release, but, in contrast to hormones, Ca(2+) spike amplitude, width, and wave velocity were dependent on [IP3] and were not perturbed by phospholipase C (PLC) inhibition. These data indicate that oscillations elicited by IP3 uncaging are driven by the biphasic regulation of the IP3 receptor by Ca(2+), and, unlike hormone-dependent responses, do not require PLC. Removal of extracellular Ca(2+) did not perturb Ca(2+) oscillations elicited by IP3 uncaging, indicating that reloading of endoplasmic reticulum stores via plasma membrane Ca(2+) influx does not entrain the signal. Activation and inhibition of PKC attenuated hormone-induced Ca(2+) oscillations but had no effect on Ca(2+) increases induced by uncaging IP3. Importantly, PKC activation and inhibition differentially affected Ca(2+) spike frequencies and kinetics. PKC activation amplifies negative feedback loops at the level of G protein-coupled receptor PLC activity and/or IP3 metabolism to attenuate IP3 levels and suppress the generation of Ca(2+) oscillations. Inhibition of PKC relieves negative feedback regulation of IP3 accumulation and, thereby, shifts Ca(2+) oscillations toward sustained responses or dramatically prolonged spikes. PKC down-regulation attenuates phenylephrine-induced Ca(2+) wave velocity, whereas responses to IP3 uncaging are enhanced. The ability to assess Ca(2+) responses in the absence of PLC activity indicates that IP3 receptor modulation by PKC regulates Ca(2+) release and wave velocity.

Cardiac resynchronization therapy restores sympathovagal balance in the failing heart by differential remodeling of cholinergic signaling

RATIONALE

Cardiac resynchronization therapy (CRT) is the only heart failure (HF) therapy documented to improve left ventricular function and reduce mortality. The underlying mechanisms are incompletely understood. Although β-adrenergic signaling has been studied extensively, the effect of CRT on cholinergic signaling is unexplored.

OBJECTIVE

We hypothesized that remodeling of cholinergic signaling plays an important role in the aberrant calcium signaling and depressed contractile and β-adrenergic responsiveness in dyssynchronous HF that are restored by CRT.

METHODS AND RESULTS

Canine tachypaced dyssynchronous HF and CRT models were generated to interrogate responses specific to dyssynchronous versus resynchronized ventricular contraction during hemodynamic decompensation. Echocardiographic, electrocardiographic, and invasive hemodynamic data were collected from normal controls, dyssynchronous HF and CRT models. Left ventricular tissue was used for biochemical analyses and functional measurements (calcium transient, sarcomere shortening) from isolated myocytes (n=42-104 myocytes per model; 6-9 hearts per model). Human left ventricular myocardium was obtained for biochemical analyses from explanted failing (n=18) and nonfailing (n=7) hearts. The M2 subtype of muscarinic acetylcholine receptors was upregulated in human and canine HF compared with nonfailing controls. CRT attenuated the increased M2 subtype of muscarinic acetylcholine receptor expression and Gαi coupling and enhanced M3 subtype of muscarinic acetylcholine receptor expression in association with enhanced calcium cycling, sarcomere shortening, and β-adrenergic responsiveness. Despite model-dependent remodeling, cholinergic stimulation completely abolished isoproterenol-induced triggered activity in both dyssynchronous HF and CRT myocytes.

CONCLUSIONS

Remodeling of cholinergic signaling is a critical pathological component of human and canine HF. Differential remodeling of cholinergic signaling represents a novel mechanism for enhancing sympathovagal balance with CRT and may identify new targets for treatment of systolic HF.

Modeling the intracellular organization of calcium signaling

Calcium (Ca²⁺) is a key signaling ion that plays a fundamental role in many cellular processes in most types of tissues and organisms. The versatility of this signaling pathway is remarkable. Depending on the cell type and the stimulus, intracellular Ca²⁺ increases can last over different periods, as short spikes or more sustained signals. From a spatial point of view, they can be localized or invade the whole cell. Such a richness of behaviors is possible thanks to numerous exchange processes with the external medium or internal Ca²⁺ pools, mainly the endoplasmic or sarcoplasmic reticulum and mitochondria. These fluxes are also highly regulated. In order to get an accurate description of the spatiotemporal organization of Ca²⁺ signaling, it is useful to resort to modeling. Thus, each flux can be described by an appropriate kinetic expression. Ca²⁺ dynamics in a given cell type can then be simulated by a modular approach, consisting of the assembly of computational descriptions of the appropriate fluxes and regulations. Modeling can also be used to get insight into the mechanisms of decoding of the Ca²⁺ signals responsible for cellular responses. Cells can use frequency or amplitude coding, as well as take profit of Ca²⁺ oscillations to increase their sensitivity to small average Ca²⁺ increases.

Calcium-dependent regulation of glucose homeostasis in the liver

A major role of the liver is to integrate multiple signals to maintain normal blood glucose levels. The balance between glucose storage and mobilization is primarily regulated by the counteracting effects of insulin and glucagon. However, numerous signals converge in the liver to ensure energy demand matches the physiological status of the organism. Many circulating hormones regulate glycogenolysis, gluconeogenesis and mitochondrial metabolism by calcium-dependent signaling mechanisms that manifest as cytosolic Ca(2+) oscillations. Stimulus-strength is encoded in the Ca(2+) oscillation frequency, and also by the range of intercellular Ca(2+) wave propagation in the intact liver. In this article, we describe how Ca(2+) oscillations and waves can regulate glucose output and oxidative metabolism in the intact liver; how multiple stimuli are decoded though Ca(2+) signaling at the organ level, and the implications of Ca(2+) signal dysregulation in diseases such as metabolic syndrome and non-alcoholic fatty liver disease.

Endothelial calcium dynamics, connexin channels and blood-brain barrier function

Situated between the circulation and the brain, the blood-brain barrier (BBB) protects the brain from circulating toxins while securing a specialized environment for neuro-glial signaling. BBB capillary endothelial cells exhibit low transcytotic activity and a tight, junctional network that, aided by the cytoskeleton, restricts paracellular permeability. The latter is subject of extensive research as it relates to neuropathology, edema and inflammation. A key determinant in regulating paracellular permeability is the endothelial cytoplasmic Ca(2+) concentration ([Ca(2+)]i) that affects junctional and cytoskeletal proteins. Ca(2+) signals are not one-time events restricted to a single cell but often appear as oscillatory [Ca(2+)]i changes that may propagate between cells as intercellular Ca(2+) waves. The effect of Ca(2+) oscillations/waves on BBB function is largely unknown and we here review current evidence on how [Ca(2+)]i dynamics influence BBB permeability.

Distinct roles of M1 and M3 muscarinic acetylcholine receptors controlling oscillatory and non-oscillatory [Ca2+]i increase

We examined ACh-induced [Ca2+]i dynamics in pancreatic acinar cells prepared from mAChR subtype-specific knockout (KO) mice. ACh did not induce any [Ca2+]i increase in the cells isolated from M1/M3 double KO mice. In the cells from M3KO mice, ACh (0.3-3 μM) caused a monotonic [Ca2+]i increase. However, we found characteristic oscillatory [Ca2+]i increases in cells from M1KO mice in lower concentrations of ACh (0.03-0.3 μM). We investigated the receptor specific pattern of [Ca2+]i increase in COS-7 cells transfected with M1 or M3 receptors. ACh induced the oscillatory [Ca2+]i increase in M3 expressing cells, but not in cells expressing M1, which exhibited monotonic [Ca2+]i increases. IP3 production detected in fluorescent indicator co-transfected cells was higher in M1 than in M3 expressing cells. From the examination of four types of M1/M3 chimera receptors we found that the carboxyl-terminal region of M3 was responsible for the generation of Ca2+ oscillations. The present results suggest that the oscillatory Ca2+ increase in response to M3 stimulation is dependent upon a moderate IP3 increase, which is suitable for causing Ca(2+)-dependent IP3-induced Ca2+ release. The C-terminal domain of M3 may contribute as a regulator of the efficiency of Gq and PLC cooperation.

Acetylcholine promotes Ca2+ and NO-oscillations in adipocytes implicating Ca2+→NO→cGMP→cADP-ribose→Ca2+ positive feedback loop--modulatory effects of norepinephrine and atrial natriuretic peptide

PURPOSE

This study investigated possible mechanisms of autoregulation of Ca(2+) signalling pathways in adipocytes responsible for Ca(2+) and NO oscillations and switching phenomena promoted by acetylcholine (ACh), norepinephrine (NE) and atrial natriuretic peptide (ANP).

METHODS

Fluorescent microscopy was used to detect changes in Ca(2+) and NO in cultures of rodent white adipocytes. Agonists and inhibitors were applied to characterize the involvement of various enzymes and Ca(2+)-channels in Ca(2+) signalling pathways.

RESULTS

ACh activating M3-muscarinic receptors and Gβγ protein dependent phosphatidylinositol 3 kinase induces Ca(2+) and NO oscillations in adipocytes. At low concentrations of ACh which are insufficient to induce oscillations, NE or α1, α2-adrenergic agonists act by amplifying the effect of ACh to promote Ca(2+) oscillations or switching phenomena. SNAP, 8-Br-cAMP, NAD and ANP may also produce similar set of dynamic regimes. These regimes arise from activation of the ryanodine receptor (RyR) with the implication of a long positive feedback loop (PFL): Ca(2+)→NO→cGMP→cADPR→Ca(2+), which determines periodic or steady operation of a short PFL based on Ca(2+)-induced Ca(2+) release via RyR by generating cADPR, a coagonist of Ca(2+) at the RyR. Interplay between these two loops may be responsible for the observed effects. Several other PFLs, based on activation of endothelial nitric oxide synthase or of protein kinase B by Ca(2+)-dependent kinases, may reinforce functioning of main PFL and enhance reliability. All observed regimes are independent of operation of the phospholipase C/Ca(2+)-signalling axis, which may be switched off due to negative feedback arising from phosphorylation of the inositol-3-phosphate receptor by protein kinase G.

CONCLUSIONS

This study presents a kinetic model of Ca(2+)-signalling system operating in adipocytes and integrating signals from various agonists, which describes it as multivariable multi feedback network with a family of nested positive feedback.

Molecular mechanisms of muscarinic acetylcholine receptor-stimulated increase in cytosolic free Ca(2+) concentration and ERK1/2 activation in the MIN6 pancreatic β-cell line

Muscarinic acetylcholine receptor (mAChR) activation of pancreatic β-cells elevates intracellular Ca(2+) and potentiates glucose-stimulated insulin secretion. In addition, it activates a number of signaling molecules, including ERK1/2, whose activation has been shown to play an important role in regulating pancreatic β-cell function and mass. The aim of this work was to determine how mAChR activation elevates intracellular Ca(2+) concentration ([Ca(2+)]( i )) and activates ERK1/2 in the pancreatic β-cell line MIN6. We demonstrate that agonist-stimulated ERK1/2 activation is dependent on the activation of phospholipase C and an elevation in [Ca(2+)]( i ), but is independent of the activation of diacylglycerol-dependent protein kinase C isoenzymes. Using a pharmacological approach, we provide evidence that agonist-induced increases in [Ca(2+)]( i ) and ERK activity require (1) IP(3) receptor-mediated mobilization of Ca(2+) from the endoplasmic reticulum, (2) influx of extracellular Ca(2+) through store-operated channels, (3) closure of K(ATP) channels, and (4) Ca(2+) entry via L-type voltage-operated Ca(2+) channels. Moreover, this Ca(2+)-dependent activation of ERK is mediated via both Ras-dependent and Ras-independent mechanisms. In summary, this study provides important insights into the multifactorial signaling mechanisms linking mAChR activation to increases in [Ca(2+)]( i ) and ERK activity.

Connexin 43 hemichannels contribute to cytoplasmic Ca2+ oscillations by providing a bimodal Ca2+-dependent Ca2+ entry pathway

Many cellular functions are driven by changes in the intracellular Ca(2+) concentration ([Ca(2+)](i)) that are highly organized in time and space. Ca(2+) oscillations are particularly important in this respect and are based on positive and negative [Ca(2+)](i) feedback on inositol 1,4,5-trisphosphate receptors (InsP(3)Rs). Connexin hemichannels are Ca(2+)-permeable plasma membrane channels that are also controlled by [Ca(2+)](i). We aimed to investigate how hemichannels may contribute to Ca(2+) oscillations. Madin-Darby canine kidney cells expressing connexin-32 (Cx32) and Cx43 were exposed to bradykinin (BK) or ATP to induce Ca(2+) oscillations. BK-induced oscillations were rapidly (minutes) and reversibly inhibited by the connexin-mimetic peptides (32)Gap27/(43)Gap26, whereas ATP-induced oscillations were unaffected. Furthermore, these peptides inhibited the BK-triggered release of calcein, a hemichannel-permeable dye. BK-induced oscillations, but not those induced by ATP, were dependent on extracellular Ca(2+). Alleviating the negative feedback of [Ca(2+)](i) on InsP(3)Rs using cytochrome c inhibited BK- and ATP-induced oscillations. Cx32 and Cx43 hemichannels are activated by <500 nm [Ca(2+)](i) but inhibited by higher concentrations and CT9 peptide (last 9 amino acids of the Cx43 C terminus) removes this high [Ca(2+)](i) inhibition. Unlike interfering with the bell-shaped dependence of InsP(3)Rs to [Ca(2+)](i), CT9 peptide prevented BK-induced oscillations but not those triggered by ATP. Collectively, these data indicate that connexin hemichannels contribute to BK-induced oscillations by allowing Ca(2+) entry during the rising phase of the Ca(2+) spikes and by providing an OFF mechanism during the falling phase of the spikes. Hemichannels were not sufficient to ignite oscillations by themselves; however, their contribution was crucial as hemichannel inhibition stopped the oscillations.

New concepts in calcium-sensing receptor pharmacology and signalling

The calcium-sensing receptor (CaR) is the key controller of extracellular calcium (Ca(2+)(o)) homeostasis via its regulation of parathyroid hormone (PTH) secretion and renal Ca(2+) reabsorption. The CaR-selective calcimimetic drug Cinacalcet stimulates the CaR to suppress PTH secretion in chronic kidney disease and represents the world's first clinically available receptor positive allosteric modulator (PAM). Negative CaR allosteric modulators (NAMs), known as calcilytics, can increase PTH secretion and are being investigated as possible bone anabolic treatments against age-related osteoporosis. Here we address the current state of development and clinical use of a series of positive and negative CaR modulators. In addition, clinical CaR mutations and transgenic mice carrying tissue-specific CaR deletions have provided a novel understanding of the relative functional importance of CaR in both calciotropic tissues and those elsewhere in the body. The development of CaR-selective modulators and signalling reagents have provided us with a more detailed appreciation of how the CaR signals in vivo. Thus, both of these areas of CaR research will be reviewed.

Analysis of IP3 receptors in and out of cells

BACKGROUND

Inositol 1,4,5-trisphosphate receptors (IP3R) are expressed in almost all animal cells. Three mammalian genes encode closely related IP3R subunits, which assemble into homo- or hetero-tetramers to form intracellular Ca2+ channels.

SCOPE OF THE REVIEW

In this brief review, we first consider a variety of complementary methods that allow the links between IP3 binding and channel gating to be defined. How does IP3 binding to the IP3-binding core in each IP3R subunit cause opening of a cation-selective pore formed by residues towards the C-terminal? We then describe methods that allow IP3, Ca2+ signals and IP3R mobility to be examined in intact cells. A final section briefly considers genetic analyses of IP3R signalling.

MAJOR CONCLUSIONS

All IP3R are regulated by both IP3 and Ca2+. This allows them to initiate and regeneratively propagate intracellular Ca2+ signals. The elementary Ca2+ release events evoked by IP3 in intact cells are mediated by very small numbers of active IP3R and the Ca2+-mediated interactions between them. The spatial organization of these Ca2+ signals and their stochastic dependence on so few IP3Rs highlight the need for methods that allow the spatial organization of IP3R signalling to be addressed with single-molecule resolution.

GENERAL SIGNIFICANCE

A variety of complementary methods provide insight into the structural basis of IP3R activation and the contributions of IP3-evoked Ca2+ signals to cellular physiology. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Connexin channels provide a target to manipulate brain endothelial calcium dynamics and blood-brain barrier permeability

The cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) is an important factor determining the functional state of blood-brain barrier (BBB) endothelial cells but little is known on the effect of dynamic [Ca(2+)](i) changes on BBB function. We applied different agonists that trigger [Ca(2+)](i) oscillations and determined the involvement of connexin channels and subsequent effects on endothelial permeability in immortalized and primary brain endothelial cells. The inflammatory peptide bradykinin (BK) triggered [Ca(2+)](i) oscillations and increased endothelial permeability. The latter was prevented by buffering [Ca(2+)](i) with BAPTA, indicating that [Ca(2+)](i) oscillations are crucial in the permeability changes. Bradykinin-triggered [Ca(2+)](i) oscillations were inhibited by interfering with connexin channels, making use of carbenoxolone, Gap27, a peptide blocker of connexin channels, and Cx37/43 knockdown. Gap27 inhibition of the oscillations was rapid (within minutes) and work with connexin hemichannel-permeable dyes indicated hemichannel opening and purinergic signaling in response to stimulation with BK. Moreover, Gap27 inhibited the BK-triggered endothelial permeability increase in in vitro and in vivo experiments. By contrast, [Ca(2+)](i) oscillations provoked by exposure to adenosine 5' triphosphate (ATP) were not affected by carbenoxolone or Gap27 and ATP did not disturb endothelial permeability. We conclude that interfering with endothelial connexin hemichannels is a novel approach to limiting BBB-permeability alterations.

Quantitative analysis reveals multiple mechanisms of allosteric modulation of the mGlu5 receptor in rat astroglia

Positive and negative allosteric modulators (PAMs and NAMs, respectively) of the type 5 metabotropic glutamate (mGlu5) receptor have demonstrable therapeutic potential in an array of neurological and psychiatric disorders. Here, we have used rat cortical astrocytes to investigate how PAMs and NAMs mediate their activity and reveal marked differences between PAMs with respect to their modulation of orthosteric agonist affinity and efficacy. Affinity cooperativity factors (α) were assessed using [(3)H]2-methyl-6-(phenylethynyl)-pyridine (MPEP)-PAM competition binding in the absence and presence of orthosteric agonist, whereas efficacy cooperativity factors (β) were calculated from net affinity/efficacy cooperativity parameters (αβ) obtained from analyses of the abilities of PAMs to potentiate [(3)H]inositol phosphate accumulation in astrocytes stimulated with a submaximal (EC(20)) concentration of orthosteric agonist. We report that whereas 3,3'-difluorobenzaldazine (DFB) and 3-cyano-N-(1,3-diphenyl-1H-prazol-5-yl)benzamide (CDPPB) primarily exert their allosteric modulatory effects through modifying the apparent orthosteric agonist affinity at the astrocyte mGlu5 receptor, the effects of S-(4-fluoro-phenyl)-{3-[3-(4-fluoro-phenyl)-[1,2,4]oxadiazol-5-yl]-piperidinl-1-yl}-methanone (ADX47273) are mediated primarily via efficacy-driven modulation. In [(3)H]MPEP-NAM competition binding assays, both MPEP and 2-(2-(3-methoxyphenyl)ethynyl)-5-methylpyridine (M-5MPEP) defined similar specific binding components, with affinities that were unaltered in the presence of orthosteric agonist, indicating wholly negative efficacy-driven modulations. It is noteworthy that whereas M-5MPEP only partially inhibited orthosteric agonist-stimulated [(3)H]inositol phosphate accumulation in astrocytes, it could completely suppress Ca(2+) oscillations stimulated by quisqualate or (S)-3,5-dihydroxyphenylglycine. In contrast, MPEP was fully inhibitory with respect to both functional responses. The finding that M-5MPEP has different functional effects depending on the endpoint measured is discussed as a possible example of permissive allosteric antagonism.

Inositol 1,4,5-trisphosphate receptor subtype-specific regulation of calcium oscillations

Oscillatory fluctuations in the cytosolic concentration of free calcium ions (Ca²⁺) are considered a ubiquitous mechanism for controlling multiple cellular processes. Inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃R) are intracellular Ca²⁺ release channels that mediate Ca²⁺ release from endoplasmic reticulum (ER) Ca²⁺ stores. The three IP₃R subtypes described so far exhibit differential structural, biophysical, and biochemical properties. Subtype specific regulation of IP₃R by the endogenous modulators IP₃, Ca²⁺, protein kinases and associated proteins have been thoroughly examined. In this article we will review the contribution of each IP₃R subtype in shaping cytosolic Ca²⁺ oscillations.

Decoding cytosolic Ca2+ oscillations

A rise in cytosolic Ca(2+) concentration is used as a universal signalling mechanism to control biological processes as diverse as exocytosis, contraction, cell growth and cell death. Ca(2+) signals are often presented to cells in the form of Ca(2+) oscillations, with signalling information encoded in both amplitude and frequency of the Ca(2+) spikes. Recent studies have revealed that the sub-cellular spatial profile of the Ca(2+) oscillation is also important in activating cellular responses, thereby suggesting a new mechanism for extracting information from the ubiquitous Ca(2+) oscillation.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Nonlinear gap junctions enable long-distance propagation of pulsating calcium waves in astrocyte networks

A new paradigm has recently emerged in brain science whereby communications between glial cells and neuron-glia interactions should be considered together with neurons and their networks to understand higher brain functions. In particular, astrocytes, the main type of glial cells in the cortex, have been shown to communicate with neurons and with each other. They are thought to form a gap-junction-coupled syncytium supporting cell-cell communication via propagating Ca²⁺ waves. An identified mode of propagation is based on cytoplasm-to-cytoplasm transport of inositol trisphosphate (IP₃) through gap junctions that locally trigger Ca²⁺ pulses via IP₃-dependent Ca²⁺-induced Ca²⁺ release. It is, however, currently unknown whether this intracellular route is able to support the propagation of long-distance regenerative Ca²⁺ waves or is restricted to short-distance signaling. Furthermore, the influence of the intracellular signaling dynamics on intercellular propagation remains to be understood. In this work, we propose a model of the gap-junctional route for intercellular Ca²⁺ wave propagation in astrocytes. Our model yields two major predictions. First, we show that long-distance regenerative signaling requires nonlinear coupling in the gap junctions. Second, we show that even with nonlinear gap junctions, long-distance regenerative signaling is favored when the internal Ca²⁺ dynamics implements frequency modulation-encoding oscillations with pulsating dynamics, while amplitude modulation-encoding dynamics tends to restrict the propagation range. As a result, spatially heterogeneous molecular properties and/or weak couplings are shown to give rise to rich spatiotemporal dynamics that support complex propagation behaviors. These results shed new light on the mechanisms implicated in the propagation of Ca²⁺ waves across astrocytes and the precise conditions under which glial cells may participate in information processing in the brain.

In recent years, the focus of Cellular Neuroscience has progressively stopped only being on neurons but started to include glial cells as well. Indeed, astrocytes, the main type of glial cells in the cortex, dynamically modulate neuron excitability and control the flow of information across synapses. Moreover, astrocytes have been shown to communicate with each other over long distances using calcium waves. These waves spread from cell to cell via molecular gates called gap junctions, which connect neighboring astrocytes. In this work, we used a computer model to question what biophysical mechanisms could support long-distance propagation of Ca²⁺ wave signaling. The model shows that the coupling function of the gap junction must be non-linear and include a threshold. This prediction is largely unexpected, as gap junctions are classically considered to implement linear functions. Recent experimental observations, however, suggest their operation could actually be more complex, in agreement with our prediction. The model also shows that the distance traveled by waves depends on characteristics of the internal astrocyte dynamics. In particular, long-distance propagation is facilitated when internal calcium oscillations are in their frequency-modulation encoding mode and are pulsating. Hence, this work provides testable experimental predictions to decipher long-distance communication between astrocytes.

Inositol trisphosphate 3-kinases: focus on immune and neuronal signaling

The localized control of second messenger levels sculpts dynamic and persistent changes in cell physiology and structure. Inositol trisphosphate [Ins(1,4,5)P(3)] 3-kinases (ITPKs) phosphorylate the intracellular second messenger Ins(1,4,5)P(3). These enzymes terminate the signal to release Ca(2+) from the endoplasmic reticulum and produce the messenger inositol tetrakisphosphate [Ins(1,3,4,5)P(4)]. Independent of their enzymatic activity, ITPKs regulate the microstructure of the actin cytoskeleton. The immune phenotypes of ITPK knockout mice raise new questions about how ITPKs control inositol phosphate lifetimes within spatial and temporal domains during lymphocyte maturation. The intense concentration of ITPK on actin inside the dendritic spines of pyramidal neurons suggests a role in signal integration and structural plasticity in the dendrite, and mice lacking neuronal ITPK exhibit memory deficits. Thus, the molecular and anatomical features of ITPKs allow them to regulate the spatiotemporal properties of intracellular signals, leading to the formation of persistent molecular memories.

Increased receptor stimulation elicits differential calcium-sensing receptor(T888) dephosphorylation

The calcium-sensing receptor (CaR) elicits oscillatory Ca(2+)(i) mobilization associated with dynamic, inhibitory protein kinase C-mediated phosphorylation of CaR(T888). While modest CaR stimulation elicits Ca(2+)(i) oscillations, greater stimulation either increases oscillation frequency or elicits sustained responses by an unknown mechanism. Here, moderate CaR stimulation (2.5 mm Ca(2+)(o), 10 min) increased CaR(T888) phosphorylation (160-kDa mature receptor) 5-fold in CaR stably transfected HEK-293 cells, whereas 3-5 mm Ca(2+)(o) treatments were without apparent effect. Treatment with 2 mm Ca(2+)(o) caused sustained CaR(T888) phosphorylation (> or = 20 min) and oscillatory Ca(2+)(i) mobilization. However, 5 mm Ca(2+)(o) increased CaR(T888) phosphorylation only briefly while eliciting sustained Ca(2+)(i) mobilization, suggesting that greater CaR activation induces rapid CaR(T888) dephosphorylation, thus permitting sustained Ca(2+)(i) responses. Indeed, 5 mm Ca(2+)(o) stimulated protein phosphatase 2A activity and induced CaR(T888) dephosphorylation following acute phorbol ester pretreatment, the latter effect being mimicked by CaR-positive allosteric modulators (NPS-R467 and l-Phe). Finally, the phosphatase inhibitor calyculin-A reversed CaR-induced inhibition of parathyroid hormone secretion from bovine parathyroid slices and normal human parathyroid cells, demonstrating the physiological importance of phosphorylation status on parathyroid function. Therefore, high Ca(2+)(o)-stimulated protein kinase C acts in concert with high Ca(2+)(o)-induced phosphatase activity to generate and maintain CaR-induced Ca(2+)(i) oscillations via the dynamic phosphorylation and dephosphorylation of CaR(T888).

Spatiotemporal dynamics of Ca2+ signaling and its physiological roles

Changes in the intracellular Ca(2+) concentration regulate numerous cell functions and display diverse spatiotemporal dynamics, which underlie the versatility of Ca(2+) in cell signaling. In many cell types, an increase in the intracellular Ca(2+) concentration starts locally, propagates within the cell (Ca(2+) wave) and makes oscillatory changes (Ca(2+) oscillation). Studies of the intracellular Ca(2+) release mechanism from the endoplasmic reticulum (ER) showed that the Ca(2+) release mechanism has inherent regenerative properties, which is essential for the generation of Ca(2+) waves and oscillations. Ca(2+) may shuttle between the ER and mitochondria, and this appears to be important for pacemaking of Ca(2+) oscillations. Importantly, Ca(2+) oscillations are an efficient mechanism in regulating cell functions, having effects supra-proportional to the sum of duration of Ca(2+) increase. Furthermore, Ca(2+) signaling mechanism studies have led to the development of a method for specific inhibition of Ca(2+) signaling, which has been used to identify hitherto unrecognized functions of Ca(2+) signals.

Effects of positive allosteric modulators on single-cell oscillatory Ca2+ signaling initiated by the type 5 metabotropic glutamate receptor

Agonist stimulation of the type 5 metabotropic glutamate (mGlu5) receptor initiates robust oscillatory changes in cytosolic Ca2+ concentration ([Ca2+]i) in single cells by rapid, repeated cycles of phosphorylation/dephosphorylation of the mGlu5 receptor, involving protein kinase C and as-yet-unspecified protein phosphatase activities. An emergent property of this type of Ca2+ oscillation-generating mechanism (termed "dynamic uncoupling") is that once a threshold concentration has been reached to initiate the Ca2+ oscillation, its frequency is largely insensitive to further increases in orthosteric agonist concentration. Here, we report the effects of positive allosteric modulators (PAMs) on the patterns of single-cell Ca2+ signaling in recombinant and native mGlu5 receptor-expressing systems. In a Chinese hamster ovary cell-line (CHO-lac-mGlu5a), none of the mGlu5 receptor PAMs studied [3,3'-difluorobenzaldazine (DFB), N-{4-chloro-2-[(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl) methyl]phenyl}-2-hydroxy-benzamide (CPPHA), 3-cyano-N-(1, 3-diphenyl-1H-prazol-5-yl)benzamide (CDPPB), S-(4-fluoro-phenyl)-{3-[3-(4-fluoro-phenyl)-[1,2,4]oxadiazol-5-yl]-piperidinl-1-yl}-methanone (ADX47273)], stimulated a Ca2+ response when applied alone, but each PAM concentration-dependently increased the frequency, without affecting the amplitude, of Ca2+ oscillations induced by glutamate or quisqualate. Therefore, PAMs can cause graded increases (and negative allosteric modulator-graded decreases) in the Ca2+ oscillation frequency stimulated by orthosteric agonist. Initial data in rat cerebrocortical astrocytes demonstrated that similar effects of PAMs could be observed in a native cell background, although at high orthosteric agonist concentrations, PAM addition could much more often be seen to drive rapid Ca2+ oscillations into peak-plateau responses. These data demonstrate that allosteric modulators can "tune" the Ca2+ oscillation frequency initiated by mGlu5 receptor activation, and this might allow pharmacological modification of the downstream processes (e.g., transcriptional regulation) that is unachievable through orthosteric ligand interactions.

Full and partial agonists of muscarinic M3 receptors reveal single and oscillatory Ca2+ responses by beta 2-adrenoceptors

Under physiological circumstances, cellular responses often reflect integration of signaling by two or more different receptors activated coincidentally or sequentially. In addition to heterologous desensitization, there are examples in which receptor activation either reveals or potentiates signaling by a different receptor type, although this is perhaps less well explored. Here, we characterize one such interaction between endogenous receptors in human embryonic kidney 293 cells in which Galpha(q/11)-coupled muscarinic M(3) receptors facilitate Ca(2+) signaling by Galpha(s)-coupled beta(2)-adrenoceptors. Measurement of changes in intracellular [Ca(2+)] demonstrated that noradrenaline released Ca(2+) from thapsigargin-sensitive intracellular stores only during activation of muscarinic receptors. Agonists with low efficacy for muscarinic receptor-mediated Ca(2+) responses facilitated cross-talk more effectively than full agonists. The cross-talk required Galpha(s) and was dependent upon intracellular Ca(2+) release channels, particularly inositol (1,4,5)-trisphosphate receptors. However, beta(2)-adrenoceptor-mediated Ca(2+) release was independent of measurable increases in phospholipase C activity and resistant to inhibitors of protein kinases A and C. Interestingly, single-cell imaging demonstrated that particularly lower concentrations of muscarinic receptor agonists facilitated marked oscillatory Ca(2+) signaling to noradrenaline. Thus, activation of muscarinic M(3) receptors profoundly influences the magnitude and oscillatory behavior of intracellular Ca(2+) signaling by beta(2)-adrenoceptors. Although these receptor subtypes are often coexpressed and mediate contrasting acute physiological effects, altered oscillatory Ca(2+) signaling suggests that cross-talk could influence longer term events through, for example, regulating gene transcription.