Modulation of plasma membrane calcium-ATPase activity by local calcium microdomains near CRAC channels in human T cells

Mitochondrial Calcium Overload Plays a Causal Role in Oxidative Stress in the Failing Heart

Heart failure is a serious global health challenge, affecting more than 6.2 million people in the United States and is projected to reach over 8 million by 2030. Independent of etiology, failing hearts share common features, including defective calcium (Ca2+) handling, mitochondrial Ca2+ overload, and oxidative stress. In cardiomyocytes, Ca2+ not only regulates excitation-contraction coupling, but also mitochondrial metabolism and oxidative stress signaling, thereby controlling the function and actual destiny of the cell. Understanding the mechanisms of mitochondrial Ca2+ uptake and the molecular pathways involved in the regulation of increased mitochondrial Ca2+ influx is an ongoing challenge in order to identify novel therapeutic targets to alleviate the burden of heart failure. In this review, we discuss the mechanisms underlying altered mitochondrial Ca2+ handling in heart failure and the potential therapeutic strategies.

PMCA Ca2+ clearance in dental enamel cells depends on the magnitude of cytosolic Ca2

Enamel formation (amelogenesis) is a two-step process whereby crystals partially grow during the secretory stage followed by a significant growth expansion during the maturation stage concurrent with an increase in vectorial Ca2+ transport. This requires tight regulation of cytosolic Ca2+ (c Ca2+ ) concentration in the enamel forming ameloblasts by controlling Ca2+ influx (entry) and Ca2+ extrusion (clearance). Gene and protein expression studies suggest that the plasma membrane Ca2+ -ATPases (PMCA1-4) are likely involved in c Ca2+ extrusion in ameloblasts, yet no functional analysis of these pumps has been reported nor whether their activity changes across amelogenesis. PMCAs have high Ca2+ affinity and low Ca2+ clearance which may be a limiting factor in their contribution to enamel formation as maturation stage ameloblasts handle high Ca2+ loads. We analyzed PMCA function in rat secretory and maturation ameloblasts by blocking or potentiating these pumps. Low/moderate elevations in c Ca2+ measured using the Ca2+ probe Fura-2-AM show that secretory ameloblasts clear Ca2+ faster than maturation stage cells through PMCAs. This process was completely inhibited by an external alkaline (pH 9.0) solution or was significantly delayed by the PMCA blockers vanadate and caloxin 1b1. Eliciting higher c Ca2+ transients via the activation of the ORAI1 Ca2+ channel showed that the PMCAs of maturation ameloblasts were more efficient. Inhibiting PMCAs decreased the rate of Ca2+ influx via ORAI1 but potentiation with forskolin had no effect. Our findings suggest that PMCAs are functional Ca2+ pumps during amelogenesis regulating c Ca2+ upon low and/or moderate Ca2+ stimulus in secretory stage, thus participating in amelogenesis.

Nuanced Interactions between AKAP79 and STIM1 with Orai1 Ca2+ Channels at Endoplasmic Reticulum-Plasma Membrane Junctions Sustain NFAT Activation

A-kinase anchoring protein 79 (AKAP79) is a human scaffolding protein that organizes Ca²⁺/calmodulin-dependent protein phosphatase calcineurin, calmodulin, cAMP-dependent protein kinase, protein kinase C, and the transcription factor nuclear factor of activated T cells (NFAT1) into a signalosome at the plasma membrane. Upon Ca²⁺ store depletion, AKAP79 interacts with the N-terminus of STIM1-gated Orai1 Ca²⁺ channels, enabling Ca²⁺ nanodomains to stimulate calcineurin. Calcineurin then dephosphorylates and activates NFAT1, which then translocates to the nucleus. A fundamental question is how signalosomes maintain long-term signaling when key effectors are released and therefore removed beyond the reach of the activating signal. Here, we show that the AKAP79-Orai1 interaction is considerably more transient than that of STIM1-Orai1. Free AKAP79, with calcineurin and NFAT1 in tow, is able to replace rapidly AKAP79 devoid of NFAT1 on Orai1, in the presence of continuous Ca²⁺ entry. We also show that Ca²⁺ nanodomains near Orai1 channels activate almost the entire cytosolic pool of NFAT1. Recycling of inactive NFAT1 from the cytoplasm to AKAP79 in the plasma membrane, coupled with the relatively weak interaction between AKAP79 and Orai1, maintain excitation-transcription coupling. By measuring rates for AKAP79-NFAT interaction, we formulate a mathematical model that simulates NFAT dynamics at the plasma membrane.

Plasma Membrane Ca2+ ATPase Activity Enables Sustained Store-operated Ca2+ Entry in the Absence of a Bulk Cytosolic Ca2+ Rise

In many cell types, the rise in cytosolic Ca2+ due to opening of Ca2+ release-activated Ca2+ (CRAC) channels drives a plethora of responses, including secretion, motility, energy production, and gene expression. The amplitude and time course of the cytosolic Ca2+ rise is shaped by the rates of Ca2+ entry into and removal from the cytosol. However, an extended bulk Ca2+ rise is toxic to cells. Here, we show that the plasma membrane Ca2+ ATPase (PMCA) pump plays a major role in preventing a prolonged cytosolic Ca2+ signal following CRAC channel activation. Ca2+ entry through CRAC channels leads to a sustained sub-plasmalemmal Ca2+ rise but bulk Ca2+ is kept low by the activity of PMCA4b. Despite the low cytosolic Ca2+, membrane permeability to Ca2+ is still elevated and Ca2+ continues to enter through CRAC channels. Ca2+-dependent NFAT activation, driven by Ca2+ nanodomains near the open channels, is maintained despite the return of bulk Ca2+ to near pre-stimulation levels. Our data reveal a central role for PMCA4b in determining the pattern of a functional Ca2+ signal and in sharpening local Ca2+ gradients near CRAC channels, whilst protecting cells from a toxic Ca2+ overload.

STIM1-dependent peripheral coupling governs the contractility of vascular smooth muscle cells

Peripheral coupling between the sarcoplasmic reticulum (SR) and plasma membrane (PM) forms signaling complexes that regulate the membrane potential and contractility of vascular smooth muscle cells (VSMCs). The mechanisms responsible for these membrane interactions are poorly understood. In many cells, STIM1 (stromal interaction molecule 1), a single-transmembrane-domain protein that resides in the endoplasmic reticulum (ER), transiently moves to ER-PM junctions in response to depletion of ER Ca²⁺ stores and initiates store-operated Ca²⁺ entry (SOCE). Fully differentiated VSMCs express STIM1 but exhibit only marginal SOCE activity. We hypothesized that STIM1 is constitutively active in contractile VSMCs and maintains peripheral coupling. In support of this concept, we found that the number and size of SR-PM interacting sites were decreased, and SR-dependent Ca²⁺-signaling processes were disrupted in freshly isolated cerebral artery SMCs from tamoxifen-inducible, SMC-specific STIM1-knockout (Stim1-smKO) mice. VSMCs from Stim1-smKO mice also exhibited a reduction in nanoscale colocalization between Ca²⁺-release sites on the SR and Ca²⁺-activated ion channels on the PM, accompanied by diminished channel activity. Stim1-smKO mice were hypotensive, and resistance arteries isolated from them displayed blunted contractility. These data suggest that STIM1 – independent of SR Ca²⁺ store depletion – is critically important for stable peripheral coupling in contractile VSMCs.

Calcium signals regulate the functional differentiation of thymic iNKT cells

How natural or innate-like lymphocytes generate the capacity to produce IL-4 and other cytokines characteristic of type 2 immunity remains unknown. Invariant natural killer T (iNKT) cells differentiate in the thymus into NKT1, NKT2, and NKT17 subsets, similar to mature, peripheral CD4+ T helper cells. The mechanism for this differentiation was not fully understood. Here, we show that NKT2 cells required higher and prolonged calcium (Ca2+ ) signals and continuing activity of the calcium release-activated calcium (CRAC) channel, than their NKT1 counterparts. The sustained Ca2+ entry via CRAC pathway in NKT2 cells was apparently mediated by ORAI and controlled in part by the large mitochondrial Ca2+ uptake. Unique properties of mitochondria in NKT2 cells, including high activity of oxidative phosphorylation, may regulate mitochondrial Ca2+ buffering in NKT2 cells. In addition, the low Ca2+ extrusion rate may also contribute to the higher Ca2+ level in NKT2 cells. Altogether, we identified ORAI-dependent Ca2+ signaling connected with mitochondria and cellular metabolism, as a central regulatory pathway for the differentiation of NKT2 cells.

Investigating the landscape of intracellular [Ca2+] in live cells by rapid photoactivated cross-linking of calmodulin-protein interactions

The Ca²⁺ sensor protein calmodulin interacts in a Ca²⁺-dependent manner with a large number of proteins that among them encompass a diverse assortment of functions and subcellular localizations. A method for monitoring calmodulin-protein interactions as they occur throughout a living cell would thus uniquely enable investigations of the intracellular landscape of [Ca²⁺] and its relationship to cell function. We have developed such a method based on capture of calmodulin-protein interactions by rapid photoactivated cross-linking (t_1/2 ∼7s) in cells stably expressing a tandem affinity tagged calmodulin that have been metabolically labeled with a photoreactive methionine analog. Tagged adducts are stringently enriched, and captured calmodulin interactors are then identified and quantified based on tandem mass spectrometry data for their tryptic peptides. In this paper we show that the capture behaviors of interactors in cells are consistent with the presence of basal microdomains of elevated [Ca²⁺]. Ca²⁺ sensitivities for capture were determined, and these suggest that [Ca²⁺] levels are above ∼1 μM in these regions. Although the microdomains appear to affect capture of most proteins, capture of some is at an apparent Ca²⁺-dependent maximum, suggesting they are targeted to the domains. Removal of extracellular Ca²⁺ has both immediate (5 min) and delayed (30 min) effects on capture, implying that the microdomains are supported by a combination of Ca²⁺ influx across the cell membrane and Ca²⁺ derived from internal stores. The known properties of the presumptive microdomain targeted proteins suggestroles in a variety of Ca²⁺-dependent basal metabolism and in formation and maintenance of the domains.

Plasma Membrane Calcium ATPase Regulates Stoichiometry of CD4+ T-Cell Compartments

Immune responses involve mobilization of T cells within naïve and memory compartments. Tightly regulated Ca²⁺ levels are essential for balanced immune outcomes. How Ca²⁺ contributes to regulating compartment stoichiometry is unknown. Here, we show that plasma membrane Ca²⁺ ATPase 4 (PMCA4) is differentially expressed in human CD4⁺ T compartments yielding distinct store operated Ca²⁺ entry (SOCE) profiles. Modulation of PMCA4 yielded a more prominent increase of SOCE in memory than in naïve CD4⁺ T cell. Interestingly, downregulation of PMCA4 reduced the effector compartment fraction and led to accumulation of cells in the naïve compartment. In silico analysis and chromatin immunoprecipitation point towards Ying Yang 1 (YY1) as a transcription factor regulating PMCA4 expression. Analyses of PMCA and YY1 expression patterns following activation and of PMCA promoter activity following downregulation of YY1 highlight repressive role of YY1 on PMCA expression. Our findings show that PMCA4 adapts Ca²⁺ levels to cellular requirements during effector and quiescent phases and thereby represent a potential target to intervene with the outcome of the immune response.

TRPM5 Negatively Regulates Calcium-Dependent Responses in Lipopolysaccharide-Stimulated B Lymphocytes

B cells produce high amounts of cytokines and immunoglobulins in response to lipopolysaccharide (LPS) stimulation. Calcium signaling cascades are critically involved in cytokine production of T cells, and the cytosolic calcium concentration is regulated by calcium-activated monovalent cation channels (CAMs). Calcium signaling is also implicated in B cell activation; however, its involvement in the cytokine production of LPS-stimulated B cells remains less well characterized. Here, we show that the transient receptor potential melastatin 5 channel (TRPM5), which is one of the CAMs, negatively modulates calcium signaling, thereby regulating LPS-induced proliferative and inflammatory responses by B cells. LPS-stimulated B cells of Trpm5-deficient mice exhibit an increased cytosolic calcium concentration, leading to enhanced proliferation and the production of the inflammatory cytokines interleukin-6 and CXCL10. Furthermore, Trpm5-deficient mice show an exacerbation of endotoxic shock with high mortality. Our findings demonstrate the importance of TRPM5-dependent regulatory mechanisms in LPS-induced calcium signaling of splenic B cells.

Store-Operated Ca2+ Channels: Mechanism, Function, Pharmacology, and Therapeutic Targets

Calcium (Ca²⁺) release-activated Ca²⁺ (CRAC) channels are a major route for Ca²⁺ entry in eukaryotic cells. These channels are store operated, opening when the endoplasmic reticulum (ER) is depleted of Ca²⁺, and are composed of the ER Ca²⁺ sensor protein STIM and the pore-forming plasma membrane subunit Orai. Recent years have heralded major strides in our understanding of the structure, gating, and function of the channels. Loss-of-function and gain-of-function mutants combined with RNAi knockdown strategies have revealed important roles for the channel in numerous human diseases, making the channel a clinically relevant target. Drugs targeting the channels generally lack specificity or exhibit poor efficacy in animal models. However, the landscape is changing, and CRAC channel blockers are now entering clinical trials. Here, we describe the key molecular and biological features of CRAC channels, consider various diseases associated with aberrant channel activity, and discuss targeting of the channels from a therapeutic perspective.

Signaling through Ca2+ Microdomains from Store-Operated CRAC Channels

Calcium (Ca²⁺) ion microdomains are subcellular regions of high Ca²⁺ concentration that develop rapidly near open Ca²⁺ channels in the plasma membrane or internal stores and generate local regions of high Ca²⁺ concentration. These microdomains are remarkably versatile in that they activate a range of responses that differ enormously in both their temporal and spatial profile. In this review, we describe how Ca²⁺ microdomains generated by store-operated calcium channels, a widespread and conserved Ca²⁺ entry pathway, stimulate different signaling pathways, and how the spatial extent of a Ca²⁺ microdomain can be influenced by Ca²⁺ ATPase pumps.

Orai, STIM, and PMCA contribute to reduced calcium signal generation in CD8+ T cells of elderly mice

Ca²⁺ is a crucial second messenger for proper T cell function. Considering the relevance of Ca²⁺ signals for T cell functionality it is surprising that no mechanistic insights into T cell Ca²⁺ signals from elderly individuals are reported. The main Ca²⁺ entry mechanism in T cells are STIM-activated Orai channels. Their role during lymphocyte aging is completely unknown. Here, we report not only reduced Ca²⁺ signals in untouched and stimulated, but also in central and effector memory CD8⁺ T cells from elderly (18-24 months) compared to adult (3-6 months) mice. Two mechanisms contribute to the overall reduction in Ca²⁺ signals of CD8⁺ T cells of elderly mice: 1) Reduced Ca²⁺ currents through Orai channels due to decreased expressions of STIMs and Orais. 2) A faster extrusion of Ca²⁺ owing to an increased expression of PMCA4. The reduced Ca²⁺ signals correlated with a resistance of the cytotoxic efficiency of CD8⁺ T cells to varying free [Ca²⁺]_ext with age. In summary, reduced STIM/Orai expression and increased Ca²⁺ clearing rates following enhanced PMCA4 expression contribute to reduced Ca²⁺ signals in CD8⁺ T cells of elderly mice. These changes are apparently relevant to immune function as they reduce the Ca²⁺ dependency of CTL cytotoxicity.

A muscular hypotonia-associated STIM1 mutant at R429 induces abnormalities in intracellular Ca2+ movement and extracellular Ca2+ entry in skeletal muscle

Stromal interaction molecule 1 (STIM1) mediates extracellular Ca²⁺ entry into the cytosol through a store-operated Ca²⁺ entry (SOCE) mechanism, which is involved in the physiological functions of various tissues, including skeletal muscle. STIM1 is also associated with skeletal muscle diseases, but its pathological mechanisms have not been well addressed. The present study focused on examining the pathological mechanism(s) of a mutant STIM1 (R429C) that causes human muscular hypotonia. R429C was expressed in mouse primary skeletal myotubes, and the properties of the skeletal myotubes were examined using single-cell Ca²⁺ imaging of myotubes and transmission electron microscopy (TEM) along with biochemical approaches. R429C did not interfere with the terminal differentiation of myoblasts to myotubes. Unlike wild-type STIM1, there was no further increase of SOCE by R429C. R429C bound to endogenous STIM1 and slowed down the initial rate of SOCE that were mediated by endogenous STIM1. Moreover, R429C increased intracellular Ca²⁺ movement in response to membrane depolarization by eliminating the attenuation on dihydropyridine receptor-ryanodine receptor (DHPR-RyR1) coupling by endogenous STIM1. The cytosolic Ca²⁺ level was also increased due to the reduction in SR Ca²⁺ level. In addition, R429C-expressing myotubes showed abnormalities in mitochondrial shape, a significant decrease in ATP levels, and the higher expression levels of mitochondrial fission-mediating proteins. Therefore, serial defects in SOCE, intracellular Ca²⁺ movement, and cytosolic Ca²⁺ level along with mitochondrial abnormalities in shape and ATP level could be a pathological mechanism of R429C for human skeletal muscular hypotonia. This study also suggests a novel clue that STIM1 in skeletal muscle could be related to mitochondria via regulating intra and extracellular Ca²⁺ movements.

The Ca2+ export pump PMCA clears near-membrane Ca2+ to facilitate store-operated Ca2+ entry and NFAT activation

Ca²⁺ signals, which facilitate pluripotent changes in cell fate, reflect the balance between cation entry and export. We found that overexpression of either isoform of the Ca²⁺-extruding plasma membrane calcium ATPase 4 (PMCA4) pump in Jurkat T cells unexpectedly increased activation of the Ca²⁺-dependent transcription factor nuclear factor of activated T cells (NFAT). Coexpression of the endoplasmic reticulum-resident Ca²⁺ sensor stromal interaction molecule 1 (STIM1) with the PMCA4b splice variant further enhanced NFAT activity; however, coexpression with PMCA4a depressed NFAT. No PMCA4 splice variant dependence in STIM1 association was observed, whereas partner of STIM1 (POST) preferentially associated with PMCA4b over PMCA4a, which enhanced, rather than inhibited, PMCA4 function. A comparison of global and near-membrane cytosolic Ca²⁺ abundances during store-operated Ca²⁺ entry revealed that PMCA4 markedly depressed near-membrane Ca²⁺ concentrations, particularly when PMCA4b was coexpressed with STIM1. PMCA4b closely associated with both POST and the store-operated Ca²⁺ channel Orai1. Furthermore, POST knockdown increased the near-membrane Ca²⁺ concentration, inhibiting the global cytosolic Ca²⁺ increase. These observations reveal an unexpected role for POST in coupling PMCA4 to Orai1 to promote Ca²⁺ entry during T cell activation through Ca²⁺ disinhibition.

Selective recruitment of different Ca2+-dependent transcription factors by STIM1-Orai1 channel clusters

Store-operated Ca2+ entry, involving endoplasmic reticulum Ca2+ sensing STIM proteins and plasma membrane Orai1 channels, is a widespread and evolutionary conserved Ca2+ influx pathway. This form of Ca2+ influx occurs at discrete loci where peripheral endoplasmic reticulum juxtaposes the plasma membrane. Stimulation evokes numerous STIM1-Orai1 clusters but whether distinct signal transduction pathways require different cluster numbers is unknown. Here, we show that two Ca2+-dependent transcription factors, NFAT1 and c-fos, have different requirements for the number of STIM1-Orai1 clusters and on the Ca2+ flux through them. NFAT activation requires fewer clusters and is more robustly activated than c-fos by low concentrations of agonist. For similar cluster numbers, transcription factor recruitment occurs sequentially, arising from intrinsic differences in Ca2+ sensitivities. Variations in the number of STIM1-Orai1 clusters and Ca2+ flux through them regulate the robustness of signalling to the nucleus whilst imparting a mechanism for selective recruitment of different Ca2+-dependent transcription factors.

Tuning store-operated calcium entry to modulate Ca2+-dependent physiological processes

The intracellular calcium signaling processes are tightly regulated to ensure the generation of calcium signals with the specific spatiotemporal characteristics required for regulating various cell functions. Compartmentalization of the molecular components involved in the generation of these signals at discrete intracellular sites ensures the signaling specificity and transduction fidelity of the signal for regulating downstream effector processes. Store-operated calcium entry (SOCE) is ubiquitously present in cells and is critical for essential cell functions in a variety of tissues. SOCE is mediated via plasma membrane Ca²⁺ channels that are activated when luminal [Ca²⁺] of the endoplasmic reticulum ([Ca²⁺]_ER) is decreased. The ER-resident stromal interaction molecules, STIM1 and STIM2, respond to decreases in [Ca²⁺]_ER by undergoing conformational changes that cause them to aggregate at the cell periphery in ER-plasma membrane (ER-PM) junctions. At these sites, STIM proteins recruit Orai1 channels and trigger their activation. Importantly, the two STIM proteins concertedly modulate Orai1 function as well as the sensitivity of SOCE to ER-Ca²⁺ store depletion. Another family of plasma membrane Ca²⁺ channels, known as the Transient Receptor Potential Canonical (TRPC) channels (TRPC1-7) also contribute to sustained [Ca²⁺]_i elevation. Although Ca²⁺ signals generated by these channels overlap with those of Orai1, they regulate distinct functions in the cells. Importantly, STIM1 is also required for plasma membrane localization and activation of some TRPCs. In this review, we will discuss various molecular components and factors that govern the activation, regulation and modulation of the Ca²⁺ signal generated by Ca²⁺ entry pathways in response to depletion of ER-Ca²⁺ stores. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Profiling calcium signals of in vitro polarized human effector CD4+ T cells

Differentiation of naïve CD4⁺ T cells into effector subtypes with distinct cytokine profiles and physiological roles is a tightly regulated process, the imbalance of which can lead to an inadequate immune response or autoimmune disease. The crucial role of Ca²⁺ signals, mainly mediated by the store operated Ca²⁺ entry (SOCE) in shaping the immune response is well described. However, it is unclear if human effector CD4⁺ T cell subsets show differential Ca²⁺ signatures in response to different stimulation methods. Herein, we provide optimized in vitro culture conditions for polarization of human CD4⁺ effector T cells and characterize their SOCE following both pharmacological store depletion and direct T-cell receptor (TCR) activation. Moreover, we measured whole cell Ca²⁺ release activated Ca²⁺ currents (I_CRAC) and investigated whether the observed differences correlate to the expression of CRAC genes. Our results show that Ca²⁺ profiles of helper CD4⁺ Th1, Th2 and Th17 are distinct and in part shaped by the intensity of stimulation. Regulatory T cells (Treg) are unique being the subtype with the most prominent SOCE response. Analysis of in vivo differentiated Treg unraveled the role of differential expression of ORAI2 in fine-tuning signals in Treg vs. conventional CD4⁺ T cells.

Ca2+ Microdomains in T-Lymphocytes

Early Ca²⁺ signaling is characterized by occurrence of Ca²⁺ microdomains formed by opening of single or clusters of Ca²⁺ channels, thereby initiating first signaling and subsequently activating global Ca²⁺ signaling mechanisms. However, only few data are available focusing on the first seconds and minutes of Ca²⁺ microdomain formation and related signaling pathways in activated T-lymphocytes. In this review, we condense current knowledge on Ca²⁺ microdomain formation in T-lymphocytes and early Ca²⁺ signaling, function of Ca²⁺ microdomains, and microdomain organization. Interestingly, considering the first seconds of T cell activation, a triphasic Ca²⁺ signal is becoming apparent: (i) initial Ca²⁺ microdomains occurring in the first second of T cell activation, (ii) amplification of Ca²⁺ microdomains by recruitment of further channels in the next 5-10 s, and (iii) a transition to global Ca²⁺ increase. Apparently, the second messenger nicotinic acid adenine dinucleotide phosphate is the first second messenger involved in initiation of Ca²⁺ microdomains. Ryanodine receptors type 1 act as initial Ca²⁺ release channels in CD4⁺ T-lymphocytes. Regarding the temporal correlation of Ca²⁺ microdomains with other molecular events of T cell activation, T cell receptor-dependent microdomain organization of signaling molecules Grb2 and Src homology [SH2] domain-containing leukocyte protein of 65 kDa was observed within the first 20 s. In addition, fast cytoskeletal changes are initiated. Furthermore, the involvement of additional Ca²⁺ channels and organelles, such as the Ca²⁺ buffering mitochondria, is discussed. Future research developments will comprise analysis of the causal relation between these temporally coordinated signaling events. Taken together, high-resolution Ca²⁺ imaging techniques applied to T cell activation in the past years paved the way to detailed molecular understanding of initial Ca²⁺ signaling mechanisms in non-excitable cells.

Subcellular ROS imaging methods: Relevance for the study of calcium signaling

Recent advances in genetically encoded fluorescent probes have dramatically increased the toolkit available for imaging the intracellular environment. Perhaps the biggest improvements have been made in sensing specific reactive oxygen species (ROS) and redox changes under physiological conditions. The new generation of probes may be targeted to a wide range of subcellular environments. By targeting such probes to compartments and organelle surfaces they may be exposed to environments, which support local signal transduction and regulation. The close apposition of the endoplasmic reticulum (ER) with mitochondria and other organelles forms such a local environment where Ca(2+) dynamics are greatly enhanced compared to the bulk cytosol. We describe here how newly developed genetically encoded redox indicators (GERIs) might be used to monitor ROS and probe their interaction with Ca(2+) at both global and local level.

The role of Orai-STIM calcium channels in melanocytes and melanoma

Calcium signalling within normal and cancer cells regulates many important cellular functions such as migration, proliferation, differentiation and cytokine secretion. Store operated Ca(2+) entry (SOCE) via the Ca(2+) release activated Ca(2+) (CRAC) channels, which are composed of the plasma membrane based Orai channels and the endoplasmic reticulum stromal interaction molecules (STIMs), is a major Ca(2+) entry route in many cell types. Orai and STIM have been implicated in the growth and metastasis of multiple cancers; however, while their involvement in cancer is presently indisputable, how Orai-STIM-controlled Ca(2+) signals affect malignant transformation, tumour growth and invasion is not fully understood. Here, we review recent studies linking Orai-STIM Ca(2+) channels with cancer, with a particular focus on melanoma. We highlight and examine key molecular players and the signalling pathways regulated by Orai and STIM in normal and malignant cells, we expose discrepancies, and we reflect on the potential of Orai-STIMs as anticancer drug targets. Finally, we discuss the functional implications of future discoveries in the field of Ca(2+) signalling.

Regulation of neurogenesis by calcium signaling

Calcium (Ca(2+)) signaling has essential roles in the development of the nervous system from neural induction to the proliferation, migration, and differentiation of neural cells. Ca(2+) signaling pathways are shaped by interactions among metabotropic signaling cascades, intracellular Ca(2+) stores, ion channels, and a multitude of downstream effector proteins that activate specific genetic programs. The temporal and spatial dynamics of Ca(2+) signals are widely presumed to control the highly diverse yet specific genetic programs that establish the complex structures of the adult nervous system. Progress in the last two decades has led to significant advances in our understanding of the functional architecture of Ca(2+) signaling networks involved in neurogenesis. In this review, we assess the literature on the molecular and functional organization of Ca(2+) signaling networks in the developing nervous system and its impact on neural induction, gene expression, proliferation, migration, and differentiation. Particular emphasis is placed on the growing evidence for the involvement of store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels in these processes.

Modeling Ca(2+) currents and buffered diffusion of Ca(2+) in human β-cells during voltage clamp experiments

Macroscopic Ca(2+) currents of the human β-cells were characterized using the Hodgkin-Huxley formalism. Expressions describing the Ca(2+)-dependent inactivation process of the L-type Ca(2+) channels in terms of the concentration of Ca(2+) were obtained. By coupling the modeled Ca(2+) currents to a three-dimensional model of buffered diffusion of Ca(2+), we simulated the Ca(2+) transients formed in the immediate vicinity of the cell membrane during voltage clamp experiments performed in high buffering conditions. Our modeling approach allowed us to consider the distribution of the Ca(2+) sources over the cell membrane. The effect of exogenous (EGTA) and endogenous Ca(2+) buffers on the temporal course of the Ca(2+) transients was evaluated. We show that despite the high Ca(2+) buffering capacity, nanodomains are formed in the submembrane space, where a peak Ca(2+) concentration between ∼76 and 143 µM was estimated from our simulations. In addition, the contribution of each Ca(2+) current to the formation of the Ca(2+) nanodomains was also addressed. Here we provide a general framework to incorporate the spatial aspects to the models of the pancreatic β-cell, such as a more detailed and realistic description of Ca(2+) dynamics in response to electrical activity in physiological conditions can be provided by future models.

Store-Operated Calcium Channels

Store-operated calcium channels (SOCs) are a major pathway for calcium signaling in virtually all metozoan cells and serve a wide variety of functions ranging from gene expression, motility, and secretion to tissue and organ development and the immune response. SOCs are activated by the depletion of Ca(2+) from the endoplasmic reticulum (ER), triggered physiologically through stimulation of a diverse set of surface receptors. Over 15 years after the first characterization of SOCs through electrophysiology, the identification of the STIM proteins as ER Ca(2+) sensors and the Orai proteins as store-operated channels has enabled rapid progress in understanding the unique mechanism of store-operate calcium entry (SOCE). Depletion of Ca(2+) from the ER causes STIM to accumulate at ER-plasma membrane (PM) junctions where it traps and activates Orai channels diffusing in the closely apposed PM. Mutagenesis studies combined with recent structural insights about STIM and Orai proteins are now beginning to reveal the molecular underpinnings of these choreographic events. This review describes the major experimental advances underlying our current understanding of how ER Ca(2+) depletion is coupled to the activation of SOCs. Particular emphasis is placed on the molecular mechanisms of STIM and Orai activation, Orai channel properties, modulation of STIM and Orai function, pharmacological inhibitors of SOCE, and the functions of STIM and Orai in physiology and disease.

Synthesis of new indole-based bisphosphonates and evaluation of their chelating ability in PE/CA-PJ15 cells

Bisphosphonates are the most important class of antiresorptive agents used against osteoclast-mediated bone loss, and, more recently, in oncology. These compounds have high affinity for calcium ions (Ca(2+)) and therefore target bone mineral, where they appear to be internalized selectively by bone-resorbing osteoclasts and inhibit osteoclast function. They are extensively used in healthcare, however they are affected by severe side effects; pharmacological properties of bisphosphonates depend on their molecular structure, which is frequently the cause of poor intestinal adsorption and low distribution. In this work we synthesized six novel bisphosphonate compounds having a variably substituted indole moiety to evaluate their extra- and intracellular calcium chelating ability in PE/CA-PJ15 cells. Preliminary in silico and in vitro ADME studies were also performed and the results suggested that the indole moiety plays an important role in cell permeability and metabolism properties.

The STIM1-ORAI1 microdomain

The regulatory protein STIM1 controls gating of the Ca(2+) channel ORAI1 by a direct protein-protein interaction. Because STIM1 is anchored in the ER membrane and ORAI1 is in the plasma membrane, the STIM-ORAI pathway can support Ca(2+) influx only where the two membranes come into close apposition, effectively demarcating a microdomain for Ca(2+) signalling. This review begins with a brief summary of the STIM-ORAI pathway of store-operated Ca(2+) influx, then turns to the special geometry of the STIM-ORAI microdomain and the expected characteristics of the microdomain Ca(2+) signal. A final section of the review seeks to place the STIM-ORAI microdomain into a broader context of cellular Ca(2+) signalling.

The Impact of Vitamin D3 Supplementation on Mechanisms of Cell Calcium Signaling in Chronic Kidney Disease

Intracellular calcium concentration in peripheral blood mononuclear cells (PBMCs) of patients with chronic kidney disease (CKD) is significantly increased, and the regulatory mechanisms maintaining cellular calcium homeostasis are impaired. The purpose of this study was to examine the effect of vitamin D3 on predominant regulatory mechanisms of cell calcium homeostasis. The study involved 16 CKD stages 2-3 patients with vitamin D deficiency treated with cholecalciferol 7000-14000 IU/week for 6 months. The regulatory mechanisms of calcium signaling were studied in PBMCs and red blood cells. After vitamin D3 supplementation, serum concentration of 25(OH)D3 increased (P < 0.001) and [Ca(2+)]i decreased (P < 0.001). The differences in [Ca(2+)]i were inversely related to differences in 25(OH)D3 concentration (P < 0.01). Vitamin D3 supplementation decreased the calcium entry through calcium release activated calcium (CRAC) channels and purinergic P2X7 channels. The function of P2X7 receptors was changed in comparison with their baseline status, and the expression of these receptors was reduced. There was no effect of vitamin D3 on P2X7 pores and activity of plasma membrane Ca(2+)-ATPases. Vitamin D3 supplementation had a beneficial effect on [Ca(2+)]i decreasing calcium entry via CRAC and P2X7 channels and reducing P2X7 receptors expression.

Molecular regulation of the pore component of CRAC channels, Orai1

Store-operated Ca(2+) entry (SOCE) is a fundamental mechanism ubiquitously employed by cells to elevate intracellular Ca(2+) concentrations ([Ca(2+)]i). Increased intracellular Ca(2+) ions act as a second messenger that can stimulate a variety of downstream signaling pathways affecting proliferation, secretion, differentiation, and death of cells. In immune cells, immune receptor stimulation induces endoplasmic reticulum Ca(2+) store depletion that subsequently activates Ca(2+)-release-activated-Ca(2+) (CRAC) channels, a prototype of store-operated Ca(2+) (SOC) channels. Identification of Orai1 as the pore subunit of CRAC channels has provided the much-needed molecular tool to dissect the mechanism of activation and regulation of these channels. In this review, we discuss the recent advances in understanding the regulatory mechanisms and posttranslational modifications that regulate diverse aspects of CRAC channel function.

Apart from its known function, the plasma membrane Ca²⁺ATPase can regulate Ca²⁺ signaling by controlling phosphatidylinositol 4,5-bisphosphate levels

Plasma membrane Ca(2+) ATPases (PMCAs, also known as ATP2B1-ATP2B4) are known targets of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂], but if and how they control the PtdIns(4,5)P₂ pool has not been considered. We demonstrate here that PMCAs protect PtdIns(4,5)P₂ in the plasma membrane from hydrolysis by phospholipase C (PLC). Comparison of active and inactive PMCAs indicates that the protection operates by two mechanisms; one requiring active PMCAs, the other not. It appears that the mechanism requiring activity is the removal of the Ca(2+) required for sustained PLC activity, whereas the mechanism not requiring activity is PtdIns(4,5)P₂ binding. We show that in PMCA overexpressing cells, PtdIns(4,5)P₂ binding can lead to less inositol 1,4,5-triphosphate (InsP₃) and diminished Ca(2+) release from intracellular Ca(2+) pools. Inspection of a homology model of PMCA suggests that PMCAs have a conserved cluster of basic residues forming a 'blue collar' at the interface between the membrane core and the cytoplasmic domains. By molecular dynamics simulation, we found that the blue collar forms four binding pockets for the phosphorylated inositol head group of PtdIns(4,5)P₂; these pockets bind PtdIns(4,5)P₂ strongly and frequently. Our studies suggest that by having the ability to bind PtdIns(4,5)P₂, PMCAs can control the accessibility of PtdIns(4,5)P₂ for PLC and other PtdIns(4,5)P₂-mediated processes.

A mathematical model of T lymphocyte calcium dynamics derived from single transmembrane protein properties

Fate decision processes of T lymphocytes are crucial for health and disease. Whether a T lymphocyte is activated, divides, gets anergic, or initiates apoptosis depends on extracellular triggers and intracellular signaling. Free cytosolic calcium dynamics plays an important role in this context. The relative contributions of store-derived calcium entry and calcium entry from extracellular space to T lymphocyte activation are still a matter of debate. Here we develop a quantitative mathematical model of T lymphocyte calcium dynamics in order to establish a tool which allows to disentangle cause-effect relationships between ion fluxes and observed calcium time courses. The model is based on single transmembrane protein characteristics which have been determined in independent experiments. This reduces the number of unknown parameters in the model to a minimum and ensures the predictive power of the model. Simulation results are subsequently used for an analysis of whole cell calcium dynamics measured under various experimental conditions. The model accounts for a variety of these conditions, which supports the suitability of the modeling approach. The simulation results suggest a model in which calcium dynamics dominantly relies on the opening of channels in calcium stores while calcium entry through calcium-release activated channels (CRAC) is more associated with the maintenance of the T lymphocyte calcium levels and prevents the cell from calcium depletion. Our findings indicate that CRAC guarantees a long-term stable calcium level which is required for cell survival and sustained calcium enhancement.

The calcium feedback loop and T cell activation: how cytoskeleton networks control intracellular calcium flux

During T cell activation, the engagement of a T cell with an antigen-presenting cell (APC) results in rapid cytoskeletal rearrangements and a dramatic increase of intracellular calcium (Ca(2+)) concentration, downstream to T cell antigen receptor (TCR) ligation. These events facilitate the organization of an immunological synapse (IS), which supports the redistribution of receptors, signaling molecules and organelles towards the T cell-APC interface to induce downstream signaling events, ultimately supporting T cell effector functions. Thus, Ca(2+) signaling and cytoskeleton rearrangements are essential for T cell activation and T cell-dependent immune response. Rapid release of Ca(2+) from intracellular stores, e.g. the endoplasmic reticulum (ER), triggers the opening of Ca(2+) release-activated Ca(2+) (CRAC) channels, residing in the plasma membrane. These channels facilitate a sustained influx of extracellular Ca(2+) across the plasma membrane in a process termed store-operated Ca(2+) entry (SOCE). Because CRAC channels are themselves inhibited by Ca(2+) ions, additional factors are suggested to enable the sustained Ca(2+) influx required for T cell function. Among these factors, we focus here on the contribution of the actin and microtubule cytoskeleton. The TCR-mediated increase in intracellular Ca(2+) evokes a rapid cytoskeleton-dependent polarization, which involves actin cytoskeleton rearrangements and microtubule-organizing center (MTOC) reorientation. Here, we review the molecular mechanisms of Ca(2+) flux and cytoskeletal rearrangements, and further describe the way by which the cytoskeletal networks feedback to Ca(2+) signaling by controlling the spatial and temporal distribution of Ca(2+) sources and sinks, modulating TCR-dependent Ca(2+) signals, which are required for an appropriate T cell response. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Contribution and regulation of TRPC channels in store-operated Ca2+ entry

Store-operated calcium entry (SOCE) is activated in response to depletion of the endoplasmic reticulum-Ca(2+) stores following stimulation of plasma membrane receptors that couple to PIP2 hydrolysis and IP3 generation. Search for the molecular components of SOCE channels led to the identification of mammalian transient receptor potential canonical (TRPC) family of calcium-permeable channels (TRPC1-TRPC7), which are all activated in response to stimuli that result in PIP2 hydrolysis. While several TRPCs, including TRPC1, TRPC3, and TRPC4, have been implicated in SOCE, the data are most consistent for TRPC1. Extensive studies in cell lines and knockout mouse models have established the contribution of TRPC1 to SOCE. Furthermore, there is a critical functional interaction between TRPC1 and the key components of SOCE, STIM1, and Orai1, which determines the activation of TRPC1. Orai1-mediated Ca(2+) entry is required for recruitment of TRPC1 and its insertion into surface membranes while STIM1 gates the channel. Notably, TRPC1 and Orai1 generate distinct patterns of Ca(2+) signals in cells that are decoded for the regulation of specific cellular functions. Thus, SOCE appears to be a complex process that depends on temporal and spatial coordination of several distinct steps mediated by proteins in different cellular compartments. Emerging data suggest that, in many cell types, the net Ca(2+) entry measured in response to store depletion is the result of the coordinated regulation of different calcium-permeable ion channels. Orai1 and STIM1 are central players in this process, and by mediating recruitment or activation of other Ca(2+) channels, Orai1-CRAC function can elicit rapid changes in global and local [Ca(2+)]i signals in cells. It is most likely that the type of channels and the [Ca(2+)]i signature that are generated by this process reflect the physiological function of the cell that is regulated by Ca(2+).

STIM proteins: dynamic calcium signal transducers

Stromal interaction molecule (STIM) proteins function in cells as dynamic coordinators of cellular calcium (Ca(2+)) signals. Spanning the endoplasmic reticulum (ER) membrane, they sense tiny changes in the levels of Ca(2+) stored within the ER lumen. As ER Ca(2+) is released to generate primary Ca(2+) signals, STIM proteins undergo an intricate activation reaction and rapidly translocate into junctions formed between the ER and the plasma membrane. There, STIM proteins tether and activate the highly Ca(2+)-selective Orai channels to mediate finely controlled Ca(2+) signals and to homeostatically balance cellular Ca(2+). Details are emerging on the remarkable organization within these STIM-induced junctional microdomains and the identification of new regulators and alternative target proteins for STIM.

Orai1, STIM1, and their associating partners

Store-operated Ca(2+) (SOC) entry is one of the major mechanisms to raise intracellular Ca(2+) concentration in non-excitable cells. Ca(2+)-release-activated Ca(2+) (CRAC) channels are a subtype of SOC channels that are extensively characterized in immune cells. Identification of STIM1 as an endoplasmic reticulum Ca(2+) sensor and Orai1 as the pore subunit has dramatically advanced the molecular understanding of CRAC channels. Recent efforts have focused on understanding the physiological aspects of CRAC channels at an organism level using transgenic animal models and at a molecular level using electrophysiological and biochemical tools. In this review, we summarize our current understanding of the interacting partners of Orai and STIM proteins in the regulation of CRAC channel activity and other non-CRAC channel-related functions.

Privileged coupling between Ca(2+) entry through plasma membrane store-operated Ca(2+) channels and the endoplasmic reticulum Ca(2+) pump

The sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) is the third element of capacitative calcium entry. It colocalizes with STIM1 and Orai1 at puncta, where couples plasma membrane store-operated Ca(2+) channels (SOC) to Ca(2+) pumping into the ER. The efficiency of this calcium entry-calcium refilling (CECR) coupling is comparable to the classic excitation-response transduction mechanisms. This allows efficient filling of the endoplasmic reticulum (ER) with the Ca(2+) entering through SOC channels with little progression of the Ca(2+) wave towards the cell core. CECR coupling is very sensitive to changes in stoichiometry among STIM, Orai and SERCA, with excess Orai antagonizing ER refilling. ER takes up most of the calcium load that enters through SOC, whereas mitochondria take up a very small fraction. This difference is due to the spatial positioning with regard to SOC, the amplitude of the high Ca(2+) microdomains, and the differences in the Ca(2+) affinity of the uptake mechanisms.

Nanodomain Ca²⁺ of Ca²⁺ channels detected by a tethered genetically encoded Ca²⁺ sensor

Coupling of excitation to secretion, contraction and transcription often relies on Ca(2+) computations within the nanodomain-a conceptual region extending tens of nanometers from the cytoplasmic mouth of Ca(2+) channels. Theory predicts that nanodomain Ca(2+) signals differ vastly from the slow submicromolar signals routinely observed in bulk cytoplasm. However, direct visualization of nanodomain Ca(2+) far exceeds optical resolution of spatially distributed Ca(2+) indicators. Here we couple an optical, genetically encoded Ca(2+) indicator (TN-XL) to the carboxy tail of Ca(V)2.2 Ca(2+) channels, enabling near-field imaging of the nanodomain. Under total internal reflection fluorescence microscopy, we detect Ca(2+) responses indicative of large-amplitude pulses. Single-channel electrophysiology reveals a corresponding Ca(2+) influx of only 0.085 pA, and fluorescence resonance energy transfer measurements estimate TN-XL distance to the cytoplasmic mouth at ~55 Å. Altogether, these findings raise the possibility that Ca(2+) exits the channel through the analogue of molecular portals, mirroring the crystallographic images of side windows in voltage-gated K channels.

Mitochondrial dynamics and their impact on T cell function

Energy supply is the most prominent function of mitochondria, but in addition, mitochondria are indispensable for a multitude of other important cellular functions including calcium (Ca(2+)) signaling and buffering, the supply of metabolites and the sequestration of apoptotic factors. The efficiency of those functions highly depends on the proper positioning of mitochondria within the cytosol. In lymphocytes, mitochondria preferentially localize into the vicinity (∼200nm) of the immune synapse (IS). This localization is regulated by motor-based cytoskeleton-mediated transport, the fusion/fission dynamics of mitochondria, and probably also through tethering with the ER. IS formation also induces the accumulation of CRAC/ORAI1 Ca(2+) channels, the CRAC/ORAI channel activator STIM1, K(+) channels and plasma membrane Ca(2+) ATPase (PMCA) within the IS. Such a large agglomeration of Ca(2+) binding organelles and proteins highlights the IS as a critical cellular compartment for Ca(2+) dependent lymphocyte activation. At the IS, Ca(2+) microdomains generated beneath open CRAC/ORAI channels provide a rapid, robust and reliable mechanism for driving cellular responses in mast cells and T cells. Here, we discuss the relevance of motor-based mitochondrial transport, fusion, fission and tethering for mitochondrial localization in T cells and the importance of subplasmalemmal mitochondria to control local CRAC/ORAI1-dependent Ca(2+) microdomains at the IS for efficient T lymphocyte activation.

Modulation of T cell activation by localized K⁺ accumulation at the immunological synapse--a mathematical model

The response of T cells to antigens (T cell activation) is marked by an increase in intracellular Ca²⁺ levels. Voltage-gated and Ca²⁺-dependent K⁺ channels control the membrane potential of human T cells and regulate Ca²⁺ influx. This regulation is dependent on proper accumulation of K⁺ channels at the immunological synapse (IS) a signaling zone that forms between a T cell and antigen presenting cell. It is believed that the IS provides a site for regulation of the activation response and that K⁺ channel inhibition occurs at the IS, but the underlying mechanisms are unknown. A mathematical model was developed to test whether K⁺ efflux through K⁺ channels leads to an accumulation of K⁺ in the IS cleft, ultimately reducing K⁺ channel function and intracellular Ca²⁺ concentration ([Ca²⁺](i)). Simulations were conducted in models of resting and activated T cell subsets, which express different levels of K⁺ channels, by varying the K⁺ diffusion constant and the spatial localization of K⁺ channels at the IS. K⁺ accumulation in the IS cleft was calculated to increase K⁺ concentration ([K⁺]) from its normal value of 5.0 mM to 5.2-10.0 mM. Including K⁺ accumulation in the model of the IS reduced calculated K⁺ current by 1-12% and consequently, reduced calculated [Ca²⁺](i) by 1-28%. Significant reductions in K⁺ current and [Ca²⁺](i) only occurred in activated T cell simulations when most K⁺ channels were centrally clustered at the IS. The results presented show that the localization of K⁺ channels at the IS can produce a rise in [K⁺] in the IS cleft and lead to a substantial decrease in K⁺ currents and [Ca²⁺](i) in activated T cells thus providing a feedback inhibitory mechanism during T cell activation.

STIM1 is required for attenuation of PMCA-mediated Ca2+ clearance during T-cell activation

T-cell activation involves a complex signalling cascade uniquely dependent on elevated cytosolic Ca(2+) levels. Further, the spatiotemporal characteristics of this Ca(2+) signal play a critical role in this process via selective activation of transcription factors. In T cells, store-operated Ca(2+) entry (SOCe) is the primary Ca(2+) influx pathway; however, cytosolic Ca(2+) concentration depends upon the balance between Ca(2+) influx and extrusion. The plasma membrane Ca(2+) ATPase (PMCA) has previously been identified as a critical player in Ca(2+) clearance in T cells. Here, we provide data revealing both functional and physical links between the activation of stromal interacting molecule 1 (STIM1) and PMCA-mediated Ca(2+) clearance. Due to the ubiquitous expression of both STIM1 and PMCA, these findings have wide-ranging implications for Ca(2+) signalling in multiple cell types.

Store-operated calcium channels: new perspectives on mechanism and function

Store-operated calcium channels (SOCs) are a nearly ubiquitous Ca(2+) entry pathway stimulated by numerous cell surface receptors via the reduction of Ca(2+) concentration in the ER. The discovery of STIM proteins as ER Ca(2+) sensors and Orai proteins as structural components of the Ca(2+) release-activated Ca(2+) (CRAC) channel, a prototypic SOC, opened the floodgates for exploring the molecular mechanism of this pathway and its functions. This review focuses on recent advances made possible by the use of STIM and Orai as molecular tools. I will describe our current understanding of the store-operated Ca(2+) entry mechanism and its emerging roles in physiology and disease, areas of uncertainty in which further progress is needed, and recent findings that are opening new directions for research in this rapidly growing field.

Orai channel-dependent activation of phospholipase C-δ: a novel mechanism for the effects of calcium entry on calcium oscillations

The frequency of oscillatory Ca(2+) signals is a major determinant in the selective activation of discrete downstream responses in non-excitable cells. An important modulator of this oscillation frequency is known to be the rate of agonist-activated Ca(2+) entry. However precisely how this is achieved and the respective roles of store-operated versus store-independent Ca(2+) entry pathways in achieving this are unclear. Here, we examine the possibility that a direct stimulation of a phospholipase C (PLC) by the entering Ca(2+) can induce a modulation of Ca(2+) oscillation frequency, and examine the roles of the endogenous store-operated and store-independent Orai channels (CRAC and ARC channels, respectively) in such a mechanism. Using the decline in the magnitude of currents through expressed PIP(2)-dependent Kir2.1 channels as a sensitive assay for PLC activity, we show that simple global increases in Ca(2+) concentrations over the physiological range do not significantly affect PLC activity. Similarly, maximal activation of endogenous CRAC channels also fails to affect PLC activity. In contrast, equivalent activation of endogenous ARC channels resulted in a 10-fold increase in the measured rate of PIP(2) depletion. Further experiments show that this effect is strictly dependent on the Ca(2+) entering via these channels, rather than the gating of the channels or the arachidonic acid used to activate them, and that it reflects the activation of a PLCδ by local Ca(2+) concentrations immediately adjacent to the active channels. Finally, based on the effects of expression of either a dominant-negative mutant Orai3 that is an essential component of the ARC channel, or a catalytically compromised mutant PLCδ, it was shown that this specific action of the store-independent ARC channel-mediated Ca(2+) entry on PLCδ has a significant impact on the oscillation frequency of the Ca(2+) signals activated by low concentrations of agonist.

Purinergic P2X7 receptors participate in disturbed intracellular calcium homeostasis in peripheral blood mononuclear cells of patients with chronic kidney disease

BACKGROUND

P2X(7) receptors intervene with lymphocyte activation and are responsible for multiple processes, including calcium influx. Here, we studied the participation of P2X(7) receptors in disturbed intracellular calcium homeostasis regulation in early-stage chronic kidney disease (CKD).

METHODS

The study involved 20 healthy volunteers and 20 CKD stage 2-3 patients. The free cytosolic calcium concentration ([Ca(2+)](i)) was measured using fluorimetry. The P2X(7) pore function was evaluated by the fluorescent dye ethidium bromide.

RESULTS

In peripheral blood mononuclear cells (PBMCs) of patients, [Ca(2+)](i), intracellular calcium stores and the capacitative calcium entry were increased when compared with healthy subjects. The agonist of P2X(7) receptor BzATP caused a sustained increase in [Ca(2+)](i) in both groups, but the effect was smaller in patients. The antagonist at the P2X(7) receptor KN-62 reduced [Ca(2+)](i) in patients, but had no effect in healthy subjects. In patients, the permeability of ethidium bromide through P2X(7) pores, as well as through BzATP-activated and KN-62-inhibited pores, was distinct from permeability in healthy volunteers.

CONCLUSIONS

These results demonstrate that the calcium signaling pathway in PBMCs of CKD patients is defective already in CKD stage 2-3, and the pore-forming P2X(7) receptors are involved in these pathophysiological processes.

Calcium microdomains at the immunological synapse: how ORAI channels, mitochondria and calcium pumps generate local calcium signals for efficient T-cell activation

Cell polarization enables restriction of signalling into microdomains. Polarization of lymphocytes following formation of a mature immunological synapse (IS) is essential for calcium-dependent T-cell activation. Here, we analyse calcium microdomains at the IS with total internal reflection fluorescence microscopy. We find that the subplasmalemmal calcium signal following IS formation is sufficiently low to prevent calcium-dependent inactivation of ORAI channels. This is achieved by localizing mitochondria close to ORAI channels. Furthermore, we find that plasma membrane calcium ATPases (PMCAs) are re-distributed into areas beneath mitochondria, which prevented PMCA up-modulation and decreased calcium export locally. This nano-scale distribution-only induced following IS formation-maximizes the efficiency of calcium influx through ORAI channels while it decreases calcium clearance by PMCA, resulting in a more sustained NFAT activity and subsequent activation of T cells.

Leucine zipper EF hand-containing transmembrane protein 1 (Letm1) and uncoupling proteins 2 and 3 (UCP2/3) contribute to two distinct mitochondrial Ca2+ uptake pathways

Cytosolic Ca²⁺ signals are transferred into mitochondria over a huge concentration range. In our recent work we described uncoupling proteins 2 and 3 (UCP2/3) to be fundamental for mitochondrial uptake of high Ca²⁺ domains in mitochondria-ER junctions. On the other hand, the leucine zipper EF hand-containing transmembrane protein 1 (Letm1) was identified as a mitochondrial Ca²⁺/H⁺ antiporter that achieved mitochondrial Ca²⁺ sequestration at small Ca²⁺ increases. Thus, the contributions of Letm1 and UCP2/3 to mitochondrial Ca²⁺ uptake were compared in endothelial cells. Knock-down of Letm1 did not affect the UCP2/3-dependent mitochondrial uptake of intracellularly released Ca²⁺ but strongly diminished the transfer of entering Ca²⁺ into mitochondria, subsequently, resulting in a reduction of store-operated Ca²⁺ entry (SOCE). Knock-down of Letm1 and UCP2/3 did neither impact on cellular ATP levels nor the membrane potential. The enhanced mitochondrial Ca²⁺ signals in cells overexpressing UCP2/3 rescued SOCE upon Letm1 knock-down. In digitonin-permeabilized cells, Letm1 exclusively contributed to mitochondrial Ca²⁺ uptake at low Ca²⁺ conditions. Neither the Letm1- nor the UCP2/3-dependent mitochondrial Ca²⁺ uptake was affected by a knock-down of mRNA levels of mitochondrial calcium uptake 1 (MICU1), a protein that triggers mitochondrial Ca²⁺ uptake in HeLa cells. Our data indicate that Letm1 and UCP2/3 independently contribute to two distinct, mitochondrial Ca²⁺ uptake pathways in intact endothelial cells.

Selective activation of the transcription factor NFAT1 by calcium microdomains near Ca2+ release-activated Ca2+ (CRAC) channels

NFATs are a family of Ca(2+)-dependent transcription factors that play a central role in the morphogenesis, development, and physiological activities of numerous distinct cell types and organ systems. Here, we visualize NFAT1 movement in and out of the nucleus in response to transient activation of store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels in nonexcitable cells. We show that NFAT migration is exquisitely sensitive to Ca(2+) microdomains near open CRAC channels. Another Ca(2+)-permeable ion channel (TRPC3) was ineffective in driving NFAT1 to the nucleus. NFAT1 movement is temporally dissociated from the time course of the Ca(2+) signal and remains within the nucleus for 10 times longer than the duration of the trigger Ca(2+) signal. Kinetic analyses of each step linking CRAC channel activation to NFAT1 nuclear residency reveals that the rate-limiting step is transcription factor exit from the nucleus. The slow deactivation of NFAT provides a mechanism whereby Ca(2+)-dependent responses can be sustained despite the termination of the initial Ca(2+) signal and helps explain how gene expression in nonexcitable cells can continue after the primary stimulus has been removed.

Effect of rare earth elements on beef cattle growth performance, blood clinical chemical parameters and mitogen stimulated proliferation of bovine peripheral blood mononuclear cells in vitro and ex vivo

Rare earth elements (REE) are possible performance enhancers in animal production, but little is known about their effects on ruminants. Therefore a feeding trial was conducted with 40 fattening bulls who received 0, 100, 200 or 300mg REE-citrate/kg dry matter (DM), containing 34.30% La, 58.09% Ce and 7.61% other REE. DM intake was measured daily and live weight weekly. Ex vivo ConcanavalinA (ConA)-stimulated cell proliferation of peripheral blood mononuclear cells (PBMC) was tested by MTT and alamar blue (AB) assay. Serum was analysed for clinical chemical parameters, ion (Mg, Ca and P) and REE concentrations. The effects of LaCl(3), CeCl(3), NdCl(3) and YCl(3) on ConA-stimulated proliferation of PBMC were tested in vitro, using MTT and AB assay. REE-citrate supplementation did affect DM intake, but not live weight gain, clinical chemical parameters, and ion concentrations significantly. In REE-300 group ex vivo proliferation of PBMC was significantly increased. In vitro ConA-stimulated proliferation decreased with rising REE-chloride concentrations. At least at the highest tested concentration (approximately 290μM) the inhibition reached significance. Proliferation of non-stimulated PBMC was not affected dose-dependently. REE affect the proliferation of PBMC, thus an effect on the bovine immune system is possible. However, the great differences in effective doses in vitro and ex vivo (serum REE concentrations) might explain the different results from the experiments.

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.

Calcium-activated K(+) channel (K(Ca)3.1) activity during Ca(2+) store depletion and store-operated Ca(2+) entry in human macrophages

STIM1 'senses' decreases in endoplasmic reticular (ER) luminal Ca(2+) and induces store-operated Ca(2+) (SOC) entry through plasma membrane Orai channels. The Ca(2+)/calmodulin-activated K(+) channel K(Ca)3.1 (previously known as SK4) has been implicated as an 'amplifier' of the Ca(2+)-release activated Ca(2+) (CRAC) current, especially in T lymphocytes. We have previously shown that human macrophages express K(Ca)3.1, and here we used the whole-cell patch-clamp technique to investigate the activity of these channels during Ca(2+) store depletion and store-operated Ca(2+) influx. Using RT-PCR, we found that macrophages express the elementary CRAC channel components Orai1 and STIM1, as well as Orai2, Orai3 and STIM2, but not the putatively STIM1-activated channels TRPC1, TRPC3-7 or TRPV6. In whole-cell configuration, a robust Ca(2+)-induced outwardly rectifying K(+) current inhibited by clotrimazole and augmented by DC-EBIO could be detected, consistent with K(Ca)3.1 channel current (also known as intermediate-conductance IK1). Introduction of extracellular Ca(2+) following Ca(2+) store depletion via P2Y(2) receptors induced a robust charybdotoxin (CTX)- and 2-APB-sensitive outward K(+) current and hyperpolarization. We also found that SOC entry induced by thapsigargin treatment induced CTX-sensitive K(+) current in HEK293 cells transiently expressing K(Ca)3.1. Our data suggest that SOC and K(Ca)3.1 channels are tightly coupled, such that a small Ca(2+) influx current induces a much large K(Ca)3.1 channel current and hyperpolarization, providing the necessary electrochemical driving force for prolonged Ca(2+) signaling and store repletion.

Coupling of mitochondria to store-operated Ca(2+)-signaling sustains constitutive activation of protein kinase B/Akt and augments survival of malignant melanoma cells

Mitochondria are emerging as a major hub for cellular Ca(2+)-signaling, though their contribution to Ca(2+)-driven growth- and survival-promoting events in cancer is poorly understood. Here employing flow cytometry to monitor mitochondrial and cytosolic Ca(2+), we assessed trans-mitochondrial Ca(2+)-transport and store-operated Ca(2+)-influx (store-operated channels (SOC)) in malignant vs. non-malignant B16BL6 melanoma clones. Remarkably, mitochondrial Ca(2+)-fluxes measured in whole cells or in isolated mitochondria were accelerated in the malignant clones compared to their non-malignant counterpart clones. This coincided with enhanced SOC-mediated Ca(2+)-influx and high levels of constitutively active protein kinase B/Akt (PKB). Interruption of trans-mitochondrial Ca(2+)-transport in the malignant cells with an antagonist of the mitochondrial Na(+)/Ca(2+) exchanger, CGP-37157, abolsihed SOC-mediated Ca(2+)-influx, inactivated PKB, retarded cell growth and increased vulnerability to apoptosis. Similarly, direct SOC blockade by silencing Stim1 inhibited PKB, indicating that the crosstalk between SOC and mitochondria is essential to preserve PKB in constitutively active state. Finally, the retraction of mitochondria from sub-plasmalemmal micro-domains triggered by Fis1 over-expression inhibited SOC-coupled trans-mitochondrial Ca(2+)-flux, Ca(2+)-entry via SOC and PKB activity. Taken together, our data show that in the malignant melanoma cells, the functional and spatial relationship of up-regulated mitochondrial Ca(2+)-transport to the SOC sustains the robust Ca(2+)-responses and down-stream signaling critical for apoptosis-resistance and proliferation.

Store-operated CRAC channels: function in health and disease

Elevation of cytosolic Ca(2+) levels through the activation of store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels is involved in mediating a disparate array of cellular responses. These include secretion, metabolism and gene expression, as well as cell growth and proliferation. Moreover, emerging evidence points to the involvement of aberrant CRAC channel activity in human diseases, such as certain types of immunodeficiency and autoimmunity disorders, allergy, and inflammatory bowel disease. This article summarizes recent advances in understanding the gating and function of CRAC channels, their links to human disease and key issues for the development of channel blockers.