Revisiting intracellular calcium signaling semantics

Cells use intracellular free calcium concentration changes for signaling. Signal encoding occurs through both spatial and temporal modulation of the free calcium concentration. The encoded message is detected by an ensemble of intracellular sensors forming the family of calcium-binding proteins (CaBPs) which must faithfully translate the message using a new syntax that is recognized by the cell. The cell is home to a significant although limited number of genes coding for proteins involved in the signal encoding and decoding processes. In a cell, only a subset of this ensemble of genes is expressed, leading to a genetic regulation of the calcium signal pathways. Calmodulin (CaM), the most ubiquitous expressed intracellular calcium-binding protein, plays a major role in calcium signal translation. Similar to a hub, it is central to a large and finely tuned network, receiving information, integrating it and dispatching the cognate response. In this review, we examine the different steps starting with an external stimulus up to a cellular response, with special emphasis on CaM and the mechanism by which it decodes calcium signals and translates it into exquisitely coordinated cellular events. By this means, we will revisit the calcium signaling semantics, hoping that we will ease communication between scientists dealing with calcium signals in different biological systems and different domains.     

胞内钙信号作用的神经模型

细胞溶质内的游离钙离子在许多细胞活动中发挥着重要的作用.对于神经元,细胞膜上的神经电信号和胞内钙离子化学信号之间有着复杂的相互作用,每个神经元都可看作为一个含有细胞膜和内质网膜的双膜系统,而神经细胞的内质网则可视为神经元内的神经元.本综述探讨了神经元膜上神经电信号与内质网钙通道释放的胞内钙信号相耦合的动力学模型.我们认...     

Criticality in intracellular calcium signaling in cardiac myocytes

Calcium (Ca) is a ubiquitous second messenger that regulates many biological functions. The elementary events of local Ca signaling are Ca sparks, which occur randomly in time and space, and integrate to produce global signaling events such as intra- and intercellular Ca waves and whole-cell Ca oscillations. Despite extensive experimental characterization in many systems, the transition from local random to global synchronous events is still poorly understood. Here we show that criticality, a ubiquitous dynamical phenomenon in nature, is responsible for the transition from local to global Ca signaling. We demonstrate this first in a computational model of Ca signaling in a cardiac myocyte and then experimentally in mouse ventricular myocytes, complemented by a theoretical agent-based model to delineate the underlying dynamics. We show that the interaction between the Ca release units via Ca-induced Ca release causes self-organization of Ca spark clusters. When the coupling between Ca release units is weak, the cluster-size distribution is exponential. As the interactions become strong, the cluster-size distribution changes to a power-law distribution, which is characteristic of criticality in thermodynamic and complex nonlinear systems, and facilitates the formation and propagation of Ca waves and whole-cell Ca oscillations. Our findings illustrate how criticality is harnessed by a biological cell to regulate Ca signaling via self-organization of random subcellular events into cellular-scale oscillations, and provide a general theoretical framework for the transition from local Ca signaling to global Ca signaling in biological cells.     

Cell adhesion and intracellular calcium signaling in neurons

Cell adhesion molecules (CAMs) play indispensable roles in the developing and mature brain by regulating neuronal migration and differentiation, neurite outgrowth, axonal fasciculation, synapse formation and synaptic plasticity. CAM-mediated changes in neuronal behavior depend on a number of intracellular signaling cascades including changes in various second messengers, among which CAM-dependent changes in intracellular Ca²⁺ levels play a prominent role. Ca²⁺ is an essential secondary intracellular signaling molecule that regulates fundamental cellular functions in various cell types, including neurons. We present a systematic review of the studies reporting changes in intracellular Ca²⁺ levels in response to activation of the immunoglobulin superfamily CAMs, cadherins and integrins in neurons. We also analyze current experimental evidence on the Ca²⁺ sources and channels involved in intracellular Ca²⁺ increases mediated by CAMs of these families, and systematically review the role of the voltage-dependent Ca²⁺ channels (VDCCs) in neurite outgrowth induced by activation of these CAMs. Molecular mechanisms linking CAMs to VDCCs and intracellular Ca²⁺ stores in neurons are discussed.     

Integrin-mediated calcium signaling and regulation of cell adhesion by intracellular calcium

Integrins are ubiquitous trans-membrane adhesion molecules that mediate the interaction of cells with the extracellular matrix (ECM). Integrins link cells to the ECM by interacting with the cell cytoskeleton. In cases such as leukocyte binding, integrins mediate cell-cell interactions and cell-ECM interactions. Recent research indicates that integrins also function as signal transduction receptors, triggering a number of intracellular signaling pathways that regulate cell behavior and development. A number of integrins are known to stimulate changes in intracellular calcium levels, resulting in integrin activation. Although changes in intracellular calcium regulate a vast number of cellular functions, this review will discuss the stimulation of calcium signaling by integrins and the role of intracellular calcium in the regulation of integrin-mediated adhesion.     

System-size resonance for intracellular and intercellular calcium signaling

Dynamical behaviors of unidirectionally, linearly coupled as well as isolated calcium subsystems are investigated by taking into account the internal noise resulting from finite system size and thus small numbers of interacting molecules. For an isolated calcium system, the internal noise can induce stochastic oscillations for a steady state close to the Hopf-bifurcation point, and the regularity of those stochastic oscillations depends resonantly on the system size, exhibiting system-size resonance. For the coupled system consisting of two subsystems, the system-size resonance effect observed in the subsystem subject to coupling is significantly amplified due to the nontrivial effects of coupling.     

HVEM signaling in monocytes is mediated by intracellular calcium mobilization

Herpes virus entry mediator (HVEM) is a member of the TNF receptor (TNFR) superfamily and is expressed on many immune cells, including T and B cells, NK cells, monocytes, and neutrophils. Interaction of HVEM with its ligand, LIGHT, costimulates T cells and increases the bactericidal activity of monocytes and neutrophils. The interaction recruits cytoplasmic TNFR-associated factor adaptor proteins to the intracellular domain of HVEM. This leads to NFkappaB activation as a result of IkappaBalpha degradation and/or JNK/AP-1 activation, and ultimately results in the expression of genes required for cell survival, cytokine production, or cell proliferation. In this study, we show that treatment of human monocytes with recombinant human LIGHT (rhLIGHT) induces rapid elevation of intracellular calcium concentration ([Ca(2+)](i)) in a HVEM-specific manner in parallel with TNF-alpha production, and enhances the bactericidal activities of monocytes. Immunoprecipitation and Western blotting analyses revealed phosphorylation of phospholipase Cgamma1 (PLCgamma1) but not PLCgamma2. rhLIGHT-induced Ca(2+)response was completely abolished by silencing PLCgamma1, or preincubating monocytes with PLC inhibitors, antagonists of the inositol-1,4,5-triphosphate receptor, or [Ca(2+)](i) chelators. Furthermore, these PLC/Ca(2+) inhibitors also blocked rhLIGHT-mediated IkappaBalpha degradation, generation of reactive oxygen species, TNF-alpha production and the bactericidal activities of monocytes. Our results indicate that Ca(2+)is a downstream mediator of the LIGHT/HVEM interaction in monocytes.     

C-terminal domain of chromogranin B regulates intracellular calcium signaling

The versatility of intracellular calcium as a second messenger is seen in its ability to mediate opposing events such as neuronal cell growth and apoptosis. A leading hypothesis used to explain how calcium regulates such divergent signaling pathways is that molecules responsible for maintaining calcium homeostasis have multiple roles. For example, chromogranin B (CGB), a calcium binding protein found in secretory granules and in the lumen of the endoplasmic reticulum, buffers calcium and also binds to and amplifies the activity of the inositol 1,4,5-trisphosphate receptor (InsP(3)R). Previous studies have identified two conserved domains of CGB, an N-terminal domain (N-CGB) and a C-terminal domain (C-CGB). N-CGB binds to the third intraluminal loop of the InsP(3)R and inhibits binding of full-length CGB. This displacement of CGB decreases InsP(3)R-dependent calcium release and alters normal signaling patterns. In the present study, we further characterized the role of N-CGB and identified roles for C-CGB. The effect of N-CGB on calcium release depended upon endogenous levels of cellular CGB, whereas the regulatory effect of C-CGB was apparent regardless of endogenous levels of CGB. When either full-length CGB or C-CGB was expressed in cells, calcium transients were increased. Additionally, the calcium signal initiation site was altered upon C-CGB expression in neuronally differentiated PC12 and SHSY5Y cells. These results show that CGB has numerous regulatory roles and that CGB is a critical component in modulating InsP(3)R-dependent calcium signaling.     

Cytoprotection and intracellular calcium.

It seems that prostacyclin has an increasing effect on gastric mucosal (antral and fundic) calmodulin level in rats. Using either the calcium channel blocker verapamil or anti-calmodulin drugs (diazepam, trifluoperazine,) the cytoprotective effect of prostacyclin can be inhibited. Therefore, it is probable that calcium ions and calcium-activated calmodulin play a role in the effect of prostacyclin.     

Stress-induced dissociations between intracellular calcium signaling and insulin secretion in pancreatic islets

In healthy pancreatic islets, glucose-stimulated changes in intracellular calcium ([Ca²⁺]_i) provide a reasonable reflection of the patterns and relative amounts of insulin secretion. We report that [Ca²⁺]_i in islets under stress, however, dissociates with insulin release in different ways for different stressors. Islets were exposed for 48 h to a variety of stressors: cytokines (low-grade inflammation), 28 mM glucose (28G, glucotoxicity), free fatty acids (FFAs, lipotoxicity), thapsigargin (ER stress), or rotenone (mitochondrial stress). We then measured [Ca²⁺]_i and insulin release in parallel studies. Islets exposed to all stressors except rotenone displayed significantly elevated [Ca²⁺]_i in low glucose, however, increased insulin secretion was only observed for 28G due to increased nifedipine-sensitive calcium-channel flux. Following 3–11 mM glucose stimulation, all stressors substantially reduced the peak glucose-stimulated [Ca²⁺]_i response (first phase). Thapsigargin and cytokines also substantially impacted aspects of calcium influx and ER calcium handling. Stressors did not significantly impact insulin secretion in 11 mM glucose for any stressor, although FFAs showed a borderline reduction, which contributed to a significant decrease in the stimulation index (11:3 mM glucose) observed for FFAs and also for 28G. We also clamped [Ca²⁺]_i using 30 mM KCl + 250 μM diazoxide to test the amplifying pathway. Only rotenone-treated islets showed a robust increase in 3–11 mM glucose-stimulated insulin secretion under clamped conditions, suggesting that low-level mitochondrial stress might activate the metabolic amplifying pathway. We conclude that different stressors dissociate [Ca²⁺]_i from insulin secretion differently: ER stressors (thapsigargin, cytokines) primarily affect [Ca²⁺]_i but not conventional insulin secretion and ‘metabolic’ stressors (FFAs, 28G, rotenone) impacted insulin secretion.     

Modelling receptor-controlled intracellular calcium oscillators.

This paper presents mathematical models for the hepatocyte calcium oscillator which follow the concepts in a class of informal models developed to account for the striking dependence on the receptor type of several features of the calcium oscillations, in particular the shape and duration of the free calcium transients. The essence of these models is that the transients should be timed by a build-up of activated GTP-binding proteins, which, combined with positive feedback processes and perhaps with cooperative effects, leads to a sudden activation of phospholipase C (PLC), followed by negative feedback processes which switch off the calcium rise and lead to a fall in free calcium back to resting levels. These models predict pulsatile oscillations in inositol (1,4,5)P3 as well as in free calcium. We show that receptor-controlled intracellular calcium oscillators involving an unknown positive feedback pathway onto PLC and negative feedback from protein kinase C (PKC) onto G-proteins and receptors, or negative feedback by stimulation of GTPase activity can simulate many of the features of observed intracellular calcium oscillations. These oscillators exhibit a dependence of frequency on agonist concentration and a dependence of transient duration on receptor and G-protein type. We also show that a PLC-dependent GTPase activating factor (GAF) could provide explanations for some otherwise puzzling features of intracellular calcium oscillations.     

Is there crosstalk between extracellular and intracellular calcium mobilization in jasmonic acid signaling

The changes in cytosolic Ca²⁺ levels play an important role in the jasmonic acid (JA) signal transduction pathway. We demonstrate that an increase in cytosolic free Ca²⁺ concentration ([Ca²⁺]_cyt) of Arabidopsis leaf cells was affected by pretreatment with heparin and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8). With pretreatment of heparin, an antagonist of inositol 1,4,5-trisphosphate (IP₃) sensitive channels, the basal and JA induced fluorescence of [Ca²⁺]_cyt were both decreased. Furthermore, heparin and TMB-8, another antagonist of IP₃ sensitive channels, enhanced the JA-induced gene expression of JR1. These data suggest that there may be a fine tune control system between extracellular and intracellular Ca²⁺ mobilization in JA signaling pathway.     

Mechanism of intracellular calcium transients.

Calcium ions mediate extracellular signals on intracellular processes. The signalling system based on transient rises or oscillations of the cytoplasmic calcium concentration has potential advantages. The relevant mechanisms of intracellular concentration changes include calcium-induced calcium release and calcium dependent inactivation of calcium release. A model has been devised based on these processes to generate repetitive transients of the cytoplasmic calcium concentration.     

2-APB inhibits volume-regulated anion channels independently from intracellular calcium signaling modulation

It has previously been suggested that volume-regulated anion channels (VRACs) and store-operated channels (SOCs) interact with each other according to their expected colocalization in the plasma membrane of LNCaP cells. In order to study interactions between these two channels, we used 2-aminoethoxydiphenyl borate (2-APB) as a regular SOC inhibitor. Surprisingly 2-APB reduced VRAC activity in a dose-dependent manner (IC(50)=122.8 microM), but not 2,2-diphenyltetrahydrofuran (a structural analog of 2-APB). This effect was also present in keratinocytes. We conclude that 2-APB is an inhibitor of the VRAC family, and is also a potent tool to study the SOC-VRAC interaction in LNCaP cells.     

Effect of arachidonic and other fatty acids on the intracellular calcium concentration and calcium signaling in peritoneal macrophages

Effects of arachidonic and other fatty acids on the intracellular Ca2+ concentration ([Ca2+]i) in rat peritoneal macrophages was investigated. It has been shown that cis-polyunsaturated arachidonic and linoleic induce a significant and dose-dependent increase in [Ca2+]i, which is due to depletion of thapsigargin-sensitive Ca2+ store and to stimulation of Ca2+ entry from the extracellular medium. Pharmacological characteristics of Ca2+ entry induced by arachidonic acid appeared to be similar to those of store-dependent Ca2+ entry activated by thapsigargin or cyclopiazonic acid; Ca2+ entry is attenuated by the same Ca2+ channel inhibitors, by tyrosine kinase inhibitor genistein and epoxygenase inhibitor proadifen. Cis-monounsaturated oleic and saturated myristic acids appeared to be less effective and induced only a slight increase in [Ca2+]i at much higher concentrations. Arachidonic and other fatty acids can also stimulate Ca(2+)-ATPase in the macrophage plasma membrane. The data are compatible with the important role played by arachidonic and other free fatty acids in the regulation of [Ca2+]i in peritoneal macrophages.     

Estradiol-induced nongenomic calcium signaling regulates genotropic signaling in macrophages.

Estradiol (E(2)) exerts not only genotropic but also nongenomic actions through nuclear estrogen receptors (ER). Here, we provide a novel paradigm for nongenomic E(2) signaling independent of nuclear ER. E(2) induces a rapid rise in the intracellular free Ca(2+) concentration ([Ca(2+)](i)) through membrane estrogen receptors in murine RAW 264.7 macrophages. This E(2)-induced Ca(2+) signaling is not prevented by different ER blockers and cannot directly activate stably transfected c-fos promoter or the mitogen-activated protein kinases p38, ERK1/2, and SAPK/JNK, or NO production. However, the E(2)-induced rise in [Ca(2+)](i) specifically down-regulates the serum-stimulated activation of c-fos promoter and ERK1/2, and conversely, it specifically up-regulates lipopolysaccharide-stimulated activation of c-fos promoter, p38, and NO production. The E(2)-changed activation of c-fos promoter can be prevented by an intracellular Ca(2+) chelator. Our data indicate that E(2)-induced nongenomic Ca(2+) signaling through membrane ER is able to specifically modulate genotropic signaling pathways with impact on macrophage activation.     

Cholecystokinin-8 induces intracellular calcium signaling in cultured myenteric neurons from neonatal guinea pigs

The responsiveness of cultured myenteric neurons to cholecystokinin (CCK-8) was examined using fura-2-based digital microfluorimetric measurement of intracellular calcium ([Ca(2+)](i)). CCK-8 (10(-10)-10(-6)M) evoked concentration-dependent increases in percentage of neurons responding (8-52%) and delta[Ca(2+)](i) (76-169 nM). Gastrin (1 microM) also induced an increase in [Ca(2+)](i) in 29+/-6% of neurons (delta[Ca(2+)](i): 71+/-3 nM). L-364,718, an antagonist for the CCK-A receptor, blocked [Ca(2+)](i) response to CCK-8. Removal of extracellular calcium eliminated CCK-induced [Ca(2+)](i) increments, as did the addition of the calcium channel inhibitors nickel (1mM) and lanthanum (5mM). Nifedipine (1-50 microM) dose-dependently attenuated CCK-caused [Ca(2+)](i) responses. CCK evokes [Ca(2+)](i) signaling in myenteric neurons by the influx of extracellular calcium, likely through L-type calcium channels.     

Role of mitochondria in intracellular calcium signaling in primary and secondary sensory neurones of rats

The participation of different calcium-regulated mechanisms in the generation of cytosolic Ca(2+) transients during neuronal excitation has been compared in isolated large and small primary (dorsal root ganglia (DRG)) and secondary (spinal dorsal horn (DH)) rat sensory neurones. As it was shown before in murine primary sensory neurones the application of mitochondrial protonophore CCCP by itself induced only small elevation of [Ca(2+)](i). However, its preceding application substantially increased the peak amplitude of depolarization-induced transients. Application of CCCP immediately after termination of the depolarizing pulse induced in both types of primary neurones a massive release of Ca(2+) from mitochondria into the cytosol. In secondary neurones application of CCCP by itself induced a substantial release of Ca(2+) from the mitochondria, but its preceding application resulted in only an insignificant increase in the peak amplitude of depolarization-triggered calcium transients. Application of CCCP immediately after termination of depolarization elicited a small release of Ca(2+), which became more pronounced when the application was delayed. Preceding application of CCCP increased the amplitude of the transients induced by caffeine-triggered Ca(2+) release from the endoplasmic reticulum in secondary neurones and did not affect those in large primary neurones. These findings may be explained by substantial differences in the density and distribution of mitochondria in the cytosol of primary and secondary sensory neurones. This suggestion was confirmed electronmicroscopically, showing a much lower density of mitochondria near plasmalemma in secondary sensory neurones and predominant clustered location of mitochondria beneath the plasmalemma in the primary cells. The possible functional importance of these differences is discussed.