Mitochondrial control of calcium-channel gating: a mechanism for sustained signaling and transcriptional activation in T lymphocytes

Cytoskeleton rotation relocates mitochondria to the immunological synapse and increases calcium signals

Ca²⁺ microdomains and spatially resolved Ca²⁺ signals are highly relevant for cell function. In T cells, local Ca²⁺ signaling at the immunological synapse (IS) is required for downstream effector functions. We present experimental evidence that the relocation of the MTOC towards the IS during polarization drags mitochondria along with the microtubule network. From time-lapse fluorescence microscopy we conclude that mitochondria rotate together with the cytoskeleton towards the IS. We hypothesize that this movement of mitochondria towards the IS together with their functionality of absorption and spatial redistribution of Ca²⁺ is sufficient to significantly increase the cytosolic Ca²⁺ concentration. To test this hypothesis we developed a whole cell model for Ca²⁺ homoeostasis involving specific geometries for mitochondria and use the model to calculate the spatial distribution of Ca²⁺ concentrations within the cell body as a function of the rotation angle and the distance from the IS. We find that an inhomogeneous distribution of PMCA pumps on the cell membrane, in particular an accumulation of PMCA at the IS, increases the global Ca²⁺ concentration and decreases the local Ca²⁺ concentration at the IS with decreasing distance of the MTOC from the IS. Unexpectedly, a change of CRAC/Orai activity is not required to explain the observed Ca²⁺ changes. We conclude that rotation-driven relocation of the MTOC towards the IS together with an accumulation of PMCA pumps at the IS are sufficient to control the observed Ca²⁺ dynamics in T-cells during polarization.

Store-operated CRAC channel inhibitors: opportunities and challenges

Aberrant Ca(2+) release-activated Ca(2+) (CRAC) channel activity has been implicated in a number of human disorders, including immunodeficiency, autoimmunity, occlusive vascular diseases and cancer, thus placing CRAC channels among the important targets for the treatment of these disorders. We briefly summarize herein the molecular basis and activation mechanism of CRAC channel and focus on discussing several pharmacological inhibitors of CRAC channels with respect to their biological activity, mechanisms of action and selectivity over other types of Ca(2+) channel in different types of cells.

Imaging calcium and redox signals using genetically encoded fluorescent indicators

Calcium and redox signals are presently established as essential regulators of many cellular processes. Nevertheless, we are still far from fully understanding the physiological and pathological importance of these universal second messengers. It is becoming increasingly apparent that many cellular functions are not regulated by global changes in the abundance of Ca(2+) ions and/or reactive oxygen and nitrogen species (ROS and RNS), but by the formation of transient local micro-domains or by signaling limited to a particular cellular compartment. Therefore, it is essential to identify and quantify Ca(2+) and redox signals in single cells with a high spatial and temporal resolution. The best tools for this purpose are the genetically encoded fluorescent indicators (GEFI). These protein sensors can be targeted into different cellular compartments, feature different colors, can be used to establish transgenic animal models, and are relatively inert to the cellular environment. Based on the chemical properties of Ca(2+) and ROS/RNS, currently more sensors exist for the detection of Ca(2+)- than for redox signals. Here, we shortly describe the most popular genetically encoded fluorescent Ca(2+) and redox indicators, discuss advantages and disadvantages based on our experience, show examples of different applications, and thus provide a brief guide that will help scientists choose the right combination of Ca(2+) and redox sensors to answer specific scientific questions.

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.

Modulation of Calcium Entry by Mitochondria

The role of mitochondria in intracellular Ca(2+) signaling relies mainly in its capacity to take up Ca(2+) from the cytosol and thus modulate the cytosolic [Ca(2+)]. Because of the low Ca(2+)-affinity of the mitochondrial Ca(2+)-uptake system, this organelle appears specially adapted to take up Ca(2+) from local high-Ca(2+) microdomains and not from the bulk cytosol. Mitochondria would then act as local Ca(2+) buffers in cellular regions where high-Ca(2+) microdomains form, that is, mainly close to the cytosolic mouth of Ca(2+) channels, both in the plasma membrane and in the endoplasmic reticulum (ER). One of the first targets proposed already in the 1990s to be regulated in this way by mitochondria were the store-operated Ca(2+) channels (SOCE). Mitochondria, by taking up Ca(2+) from the region around the cytosolic mouth of the SOCE channels, would prevent its slow Ca(2+)-dependent inactivation, thus keeping them active for longer. Since then, evidence for this mechanism has accumulated mainly in immunitary cells, where mitochondria actually move towards the immune synapse during T cell activation. However, in many other cell types the available data indicate that the close apposition between plasma and ER membranes occurring during SOCE activation precludes mitochondria from getting close to the Ca(2+)-entry sites. Alternative pathways for mitochondrial modulation of SOCE, both Ca(2+)-dependent and Ca(2+)-independent, have also been proposed, but further work will be required to elucidate the actual mechanisms at work. Hopefully, the recent knowledge of the molecular nature of the mitochondrial Ca(2+) uniporter will allow soon more precise studies on this matter.

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.

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.

An imaging flow cytometry-based approach to measuring the spatiotemporal calcium mobilisation in activated T cells

Calcium ions (Ca(2+)) are a ubiquitous transducer of cellular signals controlling key processes such as proliferation, differentiation, secretion and metabolism. In the context of T cells, stimulation through the T cell receptor has been shown to induce the release of Ca(2+) from intracellular stores. This sudden elevation within the cytoplasm triggers the opening of ion channels in the plasma membrane allowing an influx of extracellular Ca(2+) that in turn activates key molecules such as calcineurin. This cascade ultimately results in gene transcription and changes in the cellular state. Traditional methods for measuring Ca(2+) include spectrophotometry, conventional flow cytometry (CFC) and live cell imaging techniques. While each method has strengths and weaknesses, none can offer a detailed picture of Ca(2+) mobilisation in response to various agonists. Here we report an Imaging Flow Cytometry (IFC)-based method that combines the throughput and statistical rigour of CFC with the spatial information of a microscope. By co-staining cells with Ca(2+) indicators and organelle-specific dyes we can address the spatiotemporal patterns of Ca(2+) flux in Jurkat cells after stimulation with anti-CD3. The multispectral, high-throughput nature of IFC means that the organelle co-staining functions to direct the measurement of Ca(2+) indicator fluorescence to either the endoplasmic reticulum (ER) or the mitochondrial compartments without the need to treat cells with detergents such as digitonin to eliminate cytoplasmic background. We have used this system to look at the cellular localisation of Ca(2+) after stimulating cells with CD3, thapsigargin or ionomycin in the presence or absence of extracellular Ca(2+). Our data suggest that there is a dynamic interplay between the ER and mitochondrial compartments and that mitochondria act as a sink for both intracellular and extracellular derived Ca(2+). Moreover, by generating an NFAT-GFP expressing Jurkat line, we were able to combine mitochondrial Ca(2+) measurements with nuclear translocation. In conclusion, this method enables the high throughput study of spatiotemporal patterns of Ca2(+) signals in T cells responding to different stimuli.

Redox regulation of T-cell receptor signaling

T-cell receptor (TCR) triggering by antigens activates a sophisticated intracellular signaling network leading to transcriptional activation, proliferation and differentiation of T cells. These events ultimately culminate in adaptive immune responses. Over recent years it has become evident that reactive oxygen species (ROS) play an important role in T-cell activation. It is now clear that ROS are involved in the regulation of T-cell mediated physiological and pathological processes. Upon TCR triggering, T cells produce oxidants, which originate from different cellular sources. In addition, within inflamed tissues, T cells are exposed to exocrine ROS produced by activated phagocytes or other ROS-producing cells. Oxidative modifications can have different effects on T-cell function. Indeed, they can stimulate T-cell activation but they can be also detrimental. These opposite effects of oxidation likely depend on different factors such as ROS concentration and source and also on the differentiation status of the T cells. Despite the well-stablished fact that ROS represent important modulators of T-cell activation, the precise molecular mechanisms of their action are far from clear. Here, we summarize the present knowledge on redox regulation of T-cell function with a particular emphasis on the redox regulation of TCR signaling.

Lack of GDAP1 induces neuronal calcium and mitochondrial defects in a knockout mouse model of charcot-marie-tooth neuropathy

Mutations in GDAP1, which encodes protein located in the mitochondrial outer membrane, cause axonal recessive (AR-CMT2), axonal dominant (CMT2K) and demyelinating recessive (CMT4A) forms of Charcot-Marie-Tooth (CMT) neuropathy. Loss of function recessive mutations in GDAP1 are associated with decreased mitochondrial fission activity, while dominant mutations result in impairment of mitochondrial fusion with increased production of reactive oxygen species and susceptibility to apoptotic stimuli. GDAP1 silencing in vitro reduces Ca2+ inflow through store-operated Ca2+ entry (SOCE) upon mobilization of endoplasmic reticulum (ER) Ca2+, likely in association with an abnormal distribution of the mitochondrial network. To investigate the functional consequences of lack of GDAP1 in vivo, we generated a Gdap1 knockout mouse. The affected animals presented abnormal motor behavior starting at the age of 3 months. Electrophysiological and biochemical studies confirmed the axonal nature of the neuropathy whereas histopathological studies over time showed progressive loss of motor neurons (MNs) in the anterior horn of the spinal cord and defects in neuromuscular junctions. Analyses of cultured embryonic MNs and adult dorsal root ganglia neurons from affected animals demonstrated large and defective mitochondria, changes in the ER cisternae, reduced acetylation of cytoskeletal α-tubulin and increased autophagy vesicles. Importantly, MNs showed reduced cytosolic calcium and SOCE response. The development and characterization of the GDAP1 neuropathy mice model thus revealed that some of the pathophysiological changes present in axonal recessive form of the GDAP1-related CMT might be the consequence of changes in the mitochondrial network biology and mitochondria-endoplasmic reticulum interaction leading to abnormalities in calcium homeostasis.

Subcellular localization of calcium in the incompatible and compatible interactions of wheat and Puccinia striiformis f. sp. tritici

Ca(2+) is an ubiquitous intracellular molecule which is used as a second messenger to control many physiological activities in plant cells. In the present work, the relationship between calcium localization and the hypersensitive response (HR)one of the most crucial and indispensable pathway to resist a pathogenwas studied in the wheat-wheat strip rust system using cytochemical technique. Our results show that calcium is involved in the interaction between wheat and wheat stripe rust. In the incompatible interaction associated with necrosis of host mesophyll cells, an influx of Ca(2+) from the intercellular space to the cytoplasm and finally an efflux to the intercellular space again was detected in an incompatible interaction. Calcium precipitates were also observed in mesophyll cells adjacent to necrotic cells. On the contrary, calcium flow was not significantly altered in a compatible interaction. These results suggest that calcium might induce HR as a secondary messenger in the incompatible interaction of wheat and wheat stripe rust.

Redox regulation of store-operated Ca2+ entry

SIGNIFICANCE

Store-operated Ca2+ entry (SOCE) is a ubiquitous Ca2+ signaling mechanism triggered by Ca2+ depletion of the endoplasmic reticulum (ER) and by a variety of cellular stresses. Reactive oxygen species (ROS) are often concomitantly produced in response to these stresses, however, the relationship between redox signaling and SOCE is not completely understood. Various cardiovascular, neurological, and immune diseases are associated with alterations in both Ca2+ signaling and ROS production, and thus understanding this relationship has therapeutic implications.

RECENT ADVANCES

Several reactive cysteine modifications in stromal interaction molecule (STIM) and Orai proteins comprising the core SOCE machinery were recently shown to modulate SOCE in a redox-dependent manner. Moreover, STIM1 and Orai1 expression levels may reciprocally regulate and be affected by responses to oxidative stress. ER proteins involved in oxidative protein folding have gained increased recognition as important sources of ROS, and the recent discovery of their accumulation in contact sites between the ER and mitochondria provides a further link between ROS production and intracellular Ca2+ handling.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Future research should aim to establish the complete set of SOCE controlling molecules, to determine their redox-sensitive residues, and to understand how intracellular Ca2+ stores dynamically respond to different types of stress. Mapping the precise nature and functional consequence of key redox-sensitive components of the pre- and post-translational control of SOCE machinery and of proteins regulating ER calcium content will be pivotal in advancing our understanding of the complex cross-talk between redox and Ca2+ signaling.

Mitochondrial calcium uniporter MCU supports cytoplasmic Ca2+ oscillations, store-operated Ca2+ entry and Ca2+-dependent gene expression in response to receptor stimulation

Ca²⁺ flux into mitochondria is an important regulator of cytoplasmic Ca²⁺ signals, energy production and cell death pathways. Ca²⁺ uptake can occur through the recently discovered mitochondrial uniporter channel (MCU) but whether the MCU is involved in shaping Ca²⁺ signals and downstream responses to physiological levels of receptor stimulation is unknown. Here, we show that modest stimulation of leukotriene receptors with the pro-inflammatory signal LTC₄ evokes a series of cytoplasmic Ca²⁺ oscillations that are rapidly and faithfully propagated into mitochondrial matrix. Knockdown of MCU or mitochondrial depolarisation, to reduce the driving force for Ca²⁺ entry into the matrix, prevents the mitochondrial Ca²⁺ rise and accelerates run down of the oscillations. The loss of cytoplasmic Ca²⁺ oscillations appeared to be a consequence of enhanced Ca²⁺-dependent inactivation of InsP₃ receptors, which arose from the loss of mitochondrial Ca²⁺ buffering. Ca²⁺ dependent gene expression in response to leukotriene receptor activation was suppressed following knockdown of the MCU. In addition to buffering Ca²⁺ release, mitochondria also sequestrated Ca²⁺ entry through store-operated Ca²⁺ channels and this too was prevented following loss of MCU. MCU is therefore an important regulator of physiological pulses of cytoplasmic Ca²⁺.

Patch-clamp measurement of ICRAC and ORAI channel activity

Depletion of internal Ca(2+) stores activates store-operated Ca(2+) channels. The most prominent members of this class of channels are Ca(2+) release-activated Ca(2+) (CRAC) channels, which are present in a variety of cell types including immune cells. CRAC channels are composed of ORAI proteins, which are activated by endoplasmic reticulum-bound STIM proteins on Ca(2+) store depletion. The underlying Ca(2+) current is called ICRAC, which is required for many cellular functions including T-cell activation, mast cell activation, Ca(2+)-dependent gene expression, and refilling of internal Ca(2+) stores. To analyze ICRAC or the Ca(2+) current through heterologously expressed ORAI channels, whole-cell patch clamp is the technique of choice. It allows the direct analysis of ion currents through CRAC/ORAI channels. The patch-clamp technique has been used to determine selectivity, permeability, rectification, inactivation, and several other biophysical and pharmacological properties of the channels, and is the most direct and reliable technique to analyze ICRAC.

The excitation-contraction coupling mechanism in skeletal muscle

First coined by Alexander Sandow in 1952, the term excitation-contraction coupling (ECC) describes the rapid communication between electrical events occurring in the plasma membrane of skeletal muscle fibres and Ca2+ release from the SR, which leads to contraction. The sequence of events in twitch skeletal muscle involves: (1) initiation and propagation of an action potential along the plasma membrane, (2) spread of the potential throughout the transverse tubule system (T-tubule system), (3) dihydropyridine receptors (DHPR)-mediated detection of changes in membrane potential, (4) allosteric interaction between DHPR and sarcoplasmic reticulum (SR) ryanodine receptors (RyR), (5) release of Ca2+ from the SR and transient increase of Ca2+ concentration in the myoplasm, (6) activation of the myoplasmic Ca2+ buffering system and the contractile apparatus, followed by (7) Ca2+ disappearance from the myoplasm mediated mainly by its reuptake by the SR through the SR Ca2+ adenosine triphosphatase (SERCA), and under several conditions movement to the mitochondria and extrusion by the Na+/Ca2+ exchanger (NCX). In this text, we review the basics of ECC in skeletal muscle and the techniques used to study it. Moreover, we highlight some recent advances and point out gaps in knowledge on particular issues related to ECC such as (1) DHPR-RyR molecular interaction, (2) differences regarding fibre types, (3) its alteration during muscle fatigue, (4) the role of mitochondria and store-operated Ca2+ entry in the general ECC sequence, (5) contractile potentiators, and (6) Ca2+ sparks.

How to win ATP and influence Ca(2+) signaling

This brief review discusses recent advances in studies of mitochondrial Ca(2+) signaling and considers how the relationships between mitochondria and Ca(2+) responses are shaped in secretory epithelial cells. Perhaps the more precise title of this review could have been "How to win ATP and influence Ca(2+) signaling in secretory epithelium with emphasis on exocrine secretory cells and specific focus on pancreatic acinar cells". But "brevity is a virtue" and the authors hope that many of the mechanisms discussed are general and applicable to other tissues and cell types. Among these mechanisms are mitochondrial regulation of Ca(2+) entry and the role of mitochondria in the formation of localized Ca(2+) responses. The roles of Ca(2+) signaling in the physiological adjustment of bioenergetics and in mitochondrial damage are also briefly discussed.

Modulation of T Cell Metabolism and Function through Calcium Signaling

As a vital second messenger in the activation of lymphocytes, the divalent cation Ca(2+) plays numerous roles in adaptive immune responses. Importantly, Ca(2+) signaling is essential for T cell activation, tolerance of self-antigens, and homeostasis. Supporting the essential role of Ca(2+) signaling in T cell biology, the Ca(2+) regulated protein phosphatase calcineurin is a key target of pharmacologic inhibition for preventing allograft rejection and for autoimmune therapy. Recent studies have highlighted the unique role of Stim1 and Orai1/2 proteins in the regulation of store-operated/calcium release activated calcium (CRAC) channels in the context of T cells. While Ca(2+) is known to modulate T cell activation via effects on calcineurin and its target, nuclear factor of activated T cells (NFAT), this second messenger also regulates other pathways, including protein kinase C, calmodulin kinases, and cytoskeletal proteins. Ca(2+) also modulates the unique metabolic changes that occur during in distinct T cell stages and subsets. Herein, we discuss the means by which Ca(2+) mobilization modulates cellular metabolism following T cell receptor ligation. Further, we highlight the crosstalk between mitochondrial metabolism, reactive oxygen species (ROS) generation, and CRAC channel activity. As a target of mitochondrial ROS and Ca(2+) regulation, we describe the involvement of the serine/threonine kinase DRAK2 in the context of these processes. Given the important roles for Ca(2+) dependent signaling and cellular metabolism in adaptive immune responses, the crosstalk between these pathways is likely to be important for the regulation of T cell activation, tolerance, and homeostasis.

The mitochondrial calcium uniporter is a multimer that can include a dominant-negative pore-forming subunit

Mitochondrial calcium uniporter (MCU) channel is responsible for Ruthenium Red-sensitive mitochondrial calcium uptake. Here, we demonstrate MCU oligomerization by immunoprecipitation and Förster resonance energy transfer (FRET) and characterize a novel protein (MCUb) with two predicted transmembrane domains, 50% sequence similarity and a different expression profile from MCU. Based on computational modelling, MCUb includes critical amino-acid substitutions in the pore region and indeed MCUb does not form a calcium-permeable channel in planar lipid bilayers. In HeLa cells, MCUb is inserted into the oligomer and exerts a dominant-negative effect, reducing the [Ca(2+)]mt increases evoked by agonist stimulation. Accordingly, in vitro co-expression of MCUb with MCU drastically reduces the probability of observing channel activity in planar lipid bilayer experiments. These data unveil the structural complexity of MCU and demonstrate a novel regulatory mechanism, based on the inclusion of dominant-negative subunits in a multimeric channel, that underlies the fine control of the physiologically and pathologically relevant process of mitochondrial calcium homeostasis.

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.

Mitochondrial calcium uptake

Calcium (Ca(2+)) uptake into the mitochondrial matrix is critically important to cellular function. As a regulator of matrix Ca(2+) levels, this flux influences energy production and can initiate cell death. If large, this flux could potentially alter intracellular Ca(2+) ([Ca(2+)]i) signals. Despite years of study, fundamental disagreements on the extent and speed of mitochondrial Ca(2+) uptake still exist. Here, we review and quantitatively analyze mitochondrial Ca(2+) uptake fluxes from different tissues and interpret the results with respect to the recently proposed mitochondrial Ca(2+) uniporter (MCU) candidate. This quantitative analysis yields four clear results: (i) under physiological conditions, Ca(2+) influx into the mitochondria via the MCU is small relative to other cytosolic Ca(2+) extrusion pathways; (ii) single MCU conductance is ∼6-7 pS (105 mM [Ca(2+)]), and MCU flux appears to be modulated by [Ca(2+)]i, suggesting Ca(2+) regulation of MCU open probability (P(O)); (iii) in the heart, two features are clear: the number of MCU channels per mitochondrion can be calculated, and MCU probability is low under normal conditions; and (iv) in skeletal muscle and liver cells, uptake per mitochondrion varies in magnitude but total uptake per cell still appears to be modest. Based on our analysis of available quantitative data, we conclude that although Ca(2+) critically regulates mitochondrial function, the mitochondria do not act as a significant dynamic buffer of cytosolic Ca(2+) under physiological conditions. Nevertheless, with prolonged (superphysiological) elevations of [Ca(2+)]i, mitochondrial Ca(2+) uptake can increase 10- to 1,000-fold and begin to shape [Ca(2+)]i dynamics.

Mitochondria as oxidative signaling organelles in T-cell activation: physiological role and pathological implications

Early scientific reports limited the cell biological role of reactive oxygen species (ROS) to the cause of pathological damage. However, extensive research performed over the last decade led to a wide recognition of intracellular oxidative/redox signaling as a crucial mechanism of homeostatic regulation. Amongst different cellular processes known to be influenced by redox signaling, T-cell activation is one of the most established. Numerous studies reported an indispensible role for ROS as modulators of T-cell receptor-induced transcription. Nevertheless, mechanistic details regarding signaling pathways triggered by ROS are far from being delineated. The nature and interplay between enzymatic sources involved in the generation of "oxidative signals" are also a matter of ongoing research. In particular, active participation of the mitochondrial respiratory chain as ROS producer constitutes an intriguing issue with various implications for bioenergetics of activated T cells as well as for T-cell-mediated pathologies. The aim of the current review is to address these interesting concepts.

Generation and characterization of fully human monoclonal antibodies against human Orai1 for autoimmune disease

Calcium entry into T cells following antigen stimulation is crucial for nuclear factor of activated T cells (NFAT)-mediated T cell activation. The movement of calcium is mediated by calcium release-activated calcium (CRAC) channels. There are two key components of this channel: Orai1 is the pore-forming subunit located in the plasma membrane, and stromal interaction molecule 1 (STIM1) functions as a Ca(2+) sensor in the endoplasmic reticulum. A subset of human patients carry mutations in either STIM1 or Orai1 that affect protein function or expression, resulting in defective store-operated Ca(2+) influx and CRAC channel function, and impaired T cell activation. These patients suffer from a hereditary form of severe combined immune deficiency syndrome, highlighting the importance of the CRAC channel for T lymphocyte function in humans. Since autoreactive T cells play an important role in the development of autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, and organ transplantation, Orai1 becomes an attractive therapeutic target for ameliorating autoimmune disease. We developed a novel approach to inhibiting CRAC function by generating high-affinity fully human monoclonal antibodies to human Orai1. These antibodies inhibited ICRAC current, store-operated Ca(2+) influx, NFAT transcription, and cytokine release. These fully human antibodies to human Orai1 may represent a novel therapeutic approach for the treatment of autoimmunity.

MICU1 is an essential gatekeeper for MCU-mediated mitochondrial Ca(2+) uptake that regulates cell survival

Mitochondrial Ca(2+) (Ca(2+)(m)) uptake is mediated by an inner membrane Ca(2+) channel called the uniporter. Ca(2+) uptake is driven by the considerable voltage present across the inner membrane (ΔΨ(m)) generated by proton pumping by the respiratory chain. Mitochondrial matrix Ca(2+) concentration is maintained five to six orders of magnitude lower than its equilibrium level, but the molecular mechanisms for how this is achieved are not clear. Here, we demonstrate that the mitochondrial protein MICU1 is required to preserve normal [Ca(2+)](m) under basal conditions. In its absence, mitochondria become constitutively loaded with Ca(2+), triggering excessive reactive oxygen species generation and sensitivity to apoptotic stress. MICU1 interacts with the uniporter pore-forming subunit MCU and sets a Ca(2+) threshold for Ca(2+)(m) uptake without affecting the kinetic properties of MCU-mediated Ca(2+) uptake. Thus, MICU1 is a gatekeeper of MCU-mediated Ca(2+)(m) uptake that is essential to prevent [Ca(2+)](m) overload and associated stress.

ORMDL3 modulates store-operated calcium entry and lymphocyte activation

T lymphocytes rely on a Ca(2+) signal known as store-operated calcium entry (SOCE) for their activation. This Ca(2+) signal is generated by activation of a T-cell receptor, depletion of endoplasmic reticulum (ER) Ca(2+) stores and activation of Ca(2+) release-activated Ca(2+) currents (I(CRAC)). Here, we report that the ER protein orosomucoid like 3 (ORMDL3), the product of the ORMDL3 gene associated with several autoimmune and/or inflammatory diseases, negatively modulates I(CRAC), SOCE, nuclear factor of activated T cells nuclear translocation and interleukin-2 production. ORMDL3 inhibits the Ca(2+) influx mechanism at the outer mitochondrial membrane, resulting in a Ca(2+)-dependent inhibition of I(CRAC) and reduced SOCE. The effect of ORMDL3 could be mimicked by interventions that decreased mitochondrial Ca(2+) influx and reverted by buffering of cytosolic Ca(2+) or activation of mitochondrial Ca(2+) influx. In conclusion, ORMDL3 modifies key steps in the process of T-lymphocyte activation, providing a functional link between the genetic associations of the ORMDL3 gene with autoimmune and/or inflammatory diseases.

Molecular regulation of CRAC channels and their role in lymphocyte function

Calcium (Ca(2+)) influx is required for the activation and function of all cells in the immune system. It is mediated mainly by store-operated Ca(2+) entry (SOCE) through Ca(2+) release-activated Ca(2+) (CRAC) channels located in the plasma membrane. CRAC channels are composed of ORAI proteins that form the channel pore and are activated by stromal interaction molecules (STIM) 1 and 2. Located in the membrane of the endoplasmic reticulum, STIM1 and STIM2 have the dual function of sensing the intraluminal Ca(2+) concentration in the ER and to activate CRAC channels. A decrease in the ER's Ca(2+) concentration induces STIM multimerization and translocation into puncta close to the plasma membrane where they bind to and activate ORAI channels. Since the identification of ORAI and STIM genes as the principal mediators of CRAC channel function, substantial advances have been achieved in understanding the molecular regulation and physiological role of CRAC channels in cells of the immune system and other organs. In this review, we discuss the mechanisms that regulate CRAC channel function and SOCE, the role of recently identified proteins and mechanisms that modulate the activation of ORAI/STIM proteins and the consequences of CRAC channel dysregulation for lymphocyte function and immunity.

Mitochondria as sensors and regulators of calcium signalling

During the past two decades calcium (Ca(2+)) accumulation in energized mitochondria has emerged as a biological process of utmost physiological relevance. Mitochondrial Ca(2+) uptake was shown to control intracellular Ca(2+) signalling, cell metabolism, cell survival and other cell-type specific functions by buffering cytosolic Ca(2+) levels and regulating mitochondrial effectors. Recently, the identity of mitochondrial Ca(2+) transporters has been revealed, opening new perspectives for investigation and molecular intervention.

Endothelial mitochondria--less respiration, more integration

Lining the inner surface of the circulatory system, the vascular endothelium accomplishes a vast variety of specialized functions. Even slight alterations of these functions are implicated in the development of certain cardiovascular diseases that represent major causes of morbidity and mortality in developed countries. Endothelial mitochondria are essential to the functional integrity of the endothelial cell as they integrate a wide range of cellular processes including Ca²⁺ handling, redox signaling and apoptosis, all of which are closely interrelated. Growing evidence supports the notion that impairment of mitochondrial signaling in the endothelium is an early event and a causative factor in the development of diseases such as atherosclerosis or diabetic complications. In this review, we want to outline the significance of mitochondria in both physiology and pathology of the vascular endothelium.

Inflammation alters regional mitochondrial Ca²+ in human airway smooth muscle cells

Regulation of cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in airway smooth muscle (ASM) is a key aspect of airway contractility and can be modulated by inflammation. Mitochondria have tremendous potential for buffering [Ca(2+)](cyt), helping prevent Ca(2+) overload, and modulating other intracellular events. Here, compartmentalization of mitochondria to different cellular regions may subserve different roles. In the present study, we examined the role of Ca(2+) buffering by mitochondria and mitochondrial Ca(2+) transport mechanisms in the regulation of [Ca(2+)](cyt) in enzymatically dissociated human ASM cells upon exposure to the proinflammatory cytokines TNF-α and IL-13. Cells were loaded simultaneously with fluo-3 AM and rhod-2 AM, and [Ca(2+)](cyt) and mitochondrial Ca(2+) concentration ([Ca(2+)](mito)) were measured, respectively, using real-time two-color fluorescence microscopy in both the perinuclear and distal, perimembranous regions of cells. Histamine induced a rapid increase in both [Ca(2+)](cyt) and [Ca(2+)](mito), with a significant delay in the mitochondrial response. Inhibition of the mitochondrial Na(+)/Ca(2+) exchanger (1 μM CGP-37157) increased [Ca(2+)](mito) responses in perinuclear mitochondria but not distal mitochondria. Inhibition of the mitochondrial uniporter (1 μM Ru360) decreased [Ca(2+)](mito) responses in perinuclear and distal mitochondria. CGP-37157 and Ru360 significantly enhanced histamine-induced [Ca(2+)](cyt). TNF-α and IL-13 both increased [Ca(2+)](cyt), which was associated with decreased [Ca(2+)](mito) in the case of TNF-α but not IL-13. The effects of TNF-α on both [Ca(2+)](cyt) and [Ca(2+)](mito) were affected by CGP-37157 but not by Ru360. Overall, these data demonstrate that in human ASM cells, mitochondria buffer [Ca(2+)](cyt) after agonist stimulation and its enhancement by inflammation. The differential regulation of [Ca(2+)](mito) in different parts of ASM cells may serve to locally regulate Ca(2+) fluxes from intracellular sources versus the plasma membrane as well as respond to differential energy demands at these sites. We propose that such differential mitochondrial regulation, and its disruption, may play a role in airway hyperreactivity in diseases such as asthma, where [Ca(2+)](cyt) is increased.

Mitochondrial organization and Ca2+ uptake

Mitochondria may function as multiple separate organelles or as a single electrically coupled continuum to modulate changes in [Ca2+]c (cytoplasmic Ca2+ concentration) in various cell types. Mitochondria may also be tethered to the internal Ca2+ store or plasma membrane in particular parts of cells to facilitate the organelles modulation of local and global [Ca2+]c increases. Differences in the organization and positioning contributes significantly to the at times apparently contradictory reports on the way mitochondria modulate [Ca2+]c signals. In the present paper, we review the organization of mitochondria and the organelles role in Ca2+ signalling.

Neurotoxin-induced ER stress in mouse dopaminergic neurons involves downregulation of TRPC1 and inhibition of AKT/mTOR signaling

Individuals with Parkinson's disease (PD) experience a progressive decline in motor function as a result of selective loss of dopaminergic (DA) neurons in the substantia nigra. The mechanism(s) underlying the loss of DA neurons is not known. Here, we show that a neurotoxin that causes a disease that mimics PD upon administration to mice, because it induces the selective loss of DA neurons in the substantia nigra, alters Ca²⁺ homeostasis and induces ER stress. In a human neuroblastoma cell line, we found that endogenous store-operated Ca²⁺ entry (SOCE), which is critical for maintaining ER Ca²⁺ levels, is dependent on transient receptor potential channel 1 (TRPC1) activity. Neurotoxin treatment decreased TRPC1 expression, TRPC1 interaction with the SOCE modulator stromal interaction molecule 1 (STIM1), and Ca²⁺ entry into the cells. Overexpression of functional TRPC1 protected against neurotoxin-induced loss of SOCE, the associated decrease in ER Ca²⁺ levels, and the resultant unfolded protein response (UPR). In contrast, silencing of TRPC1 or STIM1 increased the UPR. Furthermore, Ca²⁺ entry via TRPC1 activated the AKT pathway, which has a known role in neuroprotection. Consistent with these in vitro data, Trpc1⁻/⁻ mice had an increased UPR and a reduced number of DA neurons. Brain lysates of patients with PD also showed an increased UPR and decreased TRPC1 levels. Importantly, overexpression of TRPC1 in mice restored AKT/mTOR signaling and increased DA neuron survival following neurotoxin administration. Overall, these results suggest that TRPC1 is involved in regulating Ca²⁺ homeostasis and inhibiting the UPR and thus contributes to neuronal survival.

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.

Endoplasmic reticulum-mitochondria coupling: local Ca²⁺ signalling with functional consequences

Plasma membrane store-operated Ca²⁺ release-activated Ca²⁺ (CRAC) channels are a widespread and conserved Ca²⁺ influx pathway, driving activation of a range of spatially and temporally distinct cellular responses. Although CRAC channels are activated by the loss of Ca²⁺ from the endoplasmic reticulum, their gating is regulated by mitochondria. Through their ability to buffer cytoplasmic Ca²⁺, mitochondria take up Ca²⁺ released from the endoplasmic reticulum by InsP₃ receptors, leading to more extensive store depletion and stronger activation of CRAC channels. Mitochondria also buffer Ca²⁺ that enters through CRAC channels, reducing Ca²⁺-dependent slow inactivation of the channels. In addition, depolarised mitochondria impair movement of the CRAC channel activating protein STIM1 across the endoplasmic reticulum membrane. Because they regulate CRAC channel activity, particularly Ca²⁺-dependent slow inactivation, mitochondria influence CRAC channel-driven enzyme activation, secretion and gene expression. Mitochondrial regulation of CRAC channels therefore provides an important control element to the regulation of intracellular Ca²⁺ signalling.

Effect of mtDNA point mutations on cellular bioenergetics

This overview discusses the results of research on the effects of most frequent mtDNA point mutations on cellular bioenergetics. Thirteen proteins coded by mtDNA are crucial for oxidative phosphorylation, 11 of them constitute key components of the respiratory chain complexes I, III and IV and 2 of mitochondrial ATP synthase. Moreover, pathogenic point mutations in mitochondrial tRNAs and rRNAs generate abnormal synthesis of the mtDNA coded proteins. Thus, pathogenic point mutations in mtDNA usually disturb the level of key parameter of the oxidative phosphorylation, i.e. the electric potential on the inner mitochondrial membrane (Δψ), and in a consequence calcium signalling and mitochondrial dynamics in the cell. Mitochondrial generation of reactive oxygen species is also modified in the mutated cells. The results obtained with cultured cells and describing biochemical consequences of mtDNA point mutations are full of contradictions. Still they help elucidate the biochemical basis of pathologies and provide a valuable tool for finding remedies in the future. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

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.

Mitochondrial Ca(2+) as a key regulator of mitochondrial activities

Mitochondria play a central role in cell biology, not only as producers of ATP but also as regulators of the Ca(2+) signal. The translocation by respiratory chain protein complexes of H(+) across the ion-impermeable inner membrane generates a very large H(+) electrochemical gradient that can be employed not only by the H(+) ATPase to run the endoergonic reaction of ADP phosphorylation, but also to accumulate cations into the matrix. Mitochondria can rapidly take up Ca(2+) through an electrogenic pathway, the uniporter, that acts to equilibrate Ca(2+) with its electrochemical gradient, and thus accumulates the cation into the matrix, and they can release it through two exchangers (with H(+) and Na(+), mostly expressed in non-excitable and excitable cells, respectively), that utilize the electrochemical gradient of the monovalent cations to prevent the attainment of electrical equilibrium.The uniporter, due to its low Ca(2+) affinity, demands high local Ca(2+) concentrations to work. In different cell systems these high Ca(2+) concentration microdomains are generated, upon cell stimulation, in proximity of the plasma membrane and the sarco/endoplasmic reticulum Ca(2+) channels.Recent work has revealed the central role of mitochondria in signal transduction pathways: evidence is accumulating that, by taking up Ca(2+), they not only modulate mitochondrial activities but also tune the cytosolic Ca(2+) signals and their related functions. This review analyses recent developments in the area of mitochondrial Ca(2+) signalling and attempts to summarize cell physiology aspects of the mitochondrial Ca(2+) transport machinery.

The hepatitis B virus X protein elevates cytosolic calcium signals by modulating mitochondrial calcium uptake

Chronic hepatitis B virus (HBV) infections are associated with the development of hepatocellular carcinoma (HCC). The HBV X protein (HBx) is thought to play an important role in the development of HBV-associated HCC. One fundamental HBx function is elevation of cytosolic calcium signals; this HBx activity has been linked to HBx stimulation of cell proliferation and transcription pathways, as well as HBV replication. Exactly how HBx elevates cytosolic calcium signals is not clear. The studies described here show that HBx stimulates calcium entry into cells, resulting in an increased plateau level of inositol 1,4,5-triphosphate (IP3)-linked calcium signals. This increased calcium plateau can be inhibited by blocking mitochondrial calcium uptake and store-operated calcium entry (SOCE). Blocking SOCE also reduced HBV replication. Finally, these studies also demonstrate that there is increased mitochondrial calcium uptake in HBx-expressing cells. Cumulatively, these studies suggest that HBx can increase mitochondrial calcium uptake and promote increased SOCE to sustain higher cytosolic calcium and stimulate HBV replication.

ORAI1 deficiency impairs activated T cell death and enhances T cell survival

ORAI1 is a pore subunit of Ca(2+) release-activated Ca(2+) channels that mediate TCR stimulation-induced Ca(2+) entry. A point mutation in ORAI1 (ORAI1(R91W)) causes SCID in human patients that is recapitulated in Orai1(-/-) mice, emphasizing its important role in the immune cells. In this study, we have characterized a novel function of ORAI1 in T cell death. CD4(+) T cells from Orai1(-/-) mice showed robust proliferation with repetitive stimulations and strong resistance to stimulation-induced cell death due to reduced mitochondrial Ca(2+) uptake and altered gene expression of proapoptotic and antiapoptotic molecules (e.g., Fas ligand, Noxa, and Mcl-1). Nuclear accumulation of NFAT was severely reduced in ORAI1-deficient T cells, and expression of ORAI1 and a constitutively active mutant of NFAT recovered cell death. These results indicate NFAT-mediated cell death pathway as one of the major downstream targets of ORAI1-induced Ca(2+) entry. By expressing various mutants of ORAI1 in wild-type and Orai1(-/-) T cells to generate different levels of intracellular Ca(2+), we have shown that activation-induced cell death is directly proportional to the intracellular Ca(2+) concentration levels. Consistent with the in vitro results, Orai1(-/-) mice showed strong resistance to T cell depletion induced by injection of anti-CD3 Ab. Furthermore, ORAI1-deficient T cells showed enhanced survival after adoptive transfer into immunocompromised hosts. Thus, our results demonstrate a crucial role of the ORAI1-NFAT pathway in T cell death and highlight the important role of ORAI1 as a major route of Ca(2+) entry during activated T cell death.

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.

Regulators of Ca(2+) signaling in mast cells: potential targets for treatment of mast cell-related diseases?

A calcium signal is essential for degranulation, generation of eicosanoids and optimal production of cytokines in mast cells in response to antigen and other stimulants. The signal is initiated by phospholipase C-mediated production of inositol1,4,5-trisphosphate resulting in release of stored Ca(2+) from the endoplasmic reticulum (ER) and Golgi. Depletion of these stores activates influx of extracellular Ca(2+), usually referred to as store-operated calcium entry (SOCE), through the interaction of the Ca(2+)-sensor, stromal interacting molecule-1 (STIM1 ), in ER with Orai1(CRACM1) and transient receptor potential canonical (TRPC) channel proteins in the plasma membrane (PM). This interaction is enabled by microtubular-directed reorganization of ER to form ER/PM contact points or "punctae" in which STIM1 and channel proteins colocalize. The ensuing influx of Ca(2+) replenishes Ca(2+) stores and sustains elevated levels of cytosolic Ca(2+) ions-the obligatory signal for mast-cell activation. In addition, the signal can acquire spatial and dynamic characteristics (e.g., calcium puffs, waves, oscillations) that encode signals for specific functional outputs. This is achieved by coordinated regulation of Ca(2+) fluxes through ATP-dependent Ca(2+)-pumps and ion exchangers in mitochondria, ER and PM. As discussed in this chapter, studies in mast cells revealed much about the mechanisms described above but little about allergic and autoimmune diseases although studies in other types of cells have exposed genetic defects that lead to aberrant calcium signaling in immune diseases. Pharmacologic agents that inhibit or activate the regulatory components of calcium signaling in mast cells are also discussed along with the prospects for development of novel SOCE inhibitors that may prove beneficial in the treatment inflammatory mast-cell related diseases.

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.

Nonsteroidal anti-inflammatory drugs inhibit vascular smooth muscle cell proliferation by enabling the Ca2+-dependent inactivation of calcium release-activated calcium/orai channels normally prevented by mitochondria

Abnormal vascular smooth muscle cell (VSMC) proliferation contributes to occlusive and proliferative disorders of the vessel wall. Salicylate and other nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit VSMC proliferation by an unknown mechanism unrelated to anti-inflammatory activity. In search for this mechanism, we have studied the effects of salicylate and other NSAIDs on subcellular Ca(2+) homeostasis and Ca(2+)-dependent cell proliferation in rat aortic A10 cells, a model of neointimal VSMCs. We found that A10 cells displayed both store-operated Ca(2+) entry (SOCE) and voltage-operated Ca(2+) entry (VOCE), the former being more important quantitatively than the latter. Inhibition of SOCE by specific Ca(2+) released-activated Ca(2+) (CRAC/Orai) channels antagonists prevented A10 cell proliferation. Salicylate and other NSAIDs, including ibuprofen, indomethacin, and sulindac, inhibited SOCE and thereby Ca(2+)-dependent, A10 cell proliferation. SOCE, but not VOCE, induced mitochondrial Ca(2+) uptake in A10 cells, and mitochondrial depolarization prevented SOCE, thus suggesting that mitochondrial Ca(2+) uptake controls SOCE (but not VOCE) in A10 cells. NSAIDs depolarized mitochondria and prevented mitochondrial Ca(2+) uptake, suggesting that they favor the Ca(2+)-dependent inactivation of CRAC/Orai channels. NSAIDs also inhibited SOCE in rat basophilic leukemia cells where mitochondrial control of CRAC/Orai is well established. NSAIDs accelerate slow inactivation of CRAC currents in rat basophilic leukemia cells under weak Ca(2+) buffering conditions but not in strong Ca(2+) buffer, thus excluding that NSAIDs inhibit SOCE directly. Taken together, our results indicate that NSAIDs inhibit VSMC proliferation by facilitating the Ca(2+)-dependent inactivation of CRAC/Orai channels which normally is prevented by mitochondria clearing of entering Ca(2+).

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.

Mitofusin 2 regulates STIM1 migration from the Ca2+ store to the plasma membrane in cells with depolarized mitochondria

Store-operated Ca2+ channels in the plasma membrane (PM) are activated by the depletion of Ca2+ from the endoplasmic reticulum (ER) and constitute a widespread and highly conserved Ca2+ influx pathway. After store emptying, the ER Ca2+ sensor STIM1 forms multimers, which then migrate to ER-PM junctions where they activate the Ca2+ release-activated Ca2+ channel Orai1. Movement of an intracellular protein to such specialized sites where it gates an ion channel is without precedence, but the fundamental question of how STIM1 migrates remains unresolved. Here, we show that trafficking of STIM1 to ER-PM junctions and subsequent Ca2+ release-activated Ca2+ channel activity is impaired following mitochondrial depolarization. We identify the dynamin-related mitochondrial protein mitofusin 2, mutations of which causes the inherited neurodegenerative disease Charcot-Marie-Tooth IIa in humans, as an important component of this mechanism. Our results reveal a molecular mechanism whereby a mitochondrial fusion protein regulates protein trafficking across the endoplasmic reticulum and reveals a homeostatic mechanism whereby mitochondrial depolarization can inhibit store-operated Ca2+ entry, thereby reducing cellular Ca2+ overload.

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.

Multiple Ca2+-dependent mechanisms regulate L-type Ca2+ current in retinal amacrine cells

Understanding the regulation of L-type voltage-gated Ca(2+) current is an important component of elucidating the signaling capabilities of retinal amacrine cells. Here we ask how the cytosolic Ca(2+) environment and the balance of Ca(2+)-dependent effectors shape native L-type Ca(2+) channel function in these cells. To achieve this, whole cell voltage clamp recordings were made from cultured amacrine cells under conditions that address the contribution of mitochondrial Ca(2+) uptake (MCU), Ca(2+)/calmodulin (CaM)-dependent channel inactivation (CDI), protein kinase A (PKA), and Ca(2+)-induced Ca(2+) release (CICR). Under control conditions, repeated activation of the L-type channels produces a progressive enhancement of the current. Inhibition of MCU causes a reduction in the Ca(2+) current amplitude that is dependent on Ca(2+) influx as well as cytosolic Ca(2+) buffering, consistent with CDI. Including the Ca(2+) buffer bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) internally can shift the balance between enhancement and inhibition such that inhibition of MCU can enhance the current. Inhibition of PKA can remove the enhancing effect of BAPTA suggesting that cyclic AMP-dependent phosphorylation is involved. Inhibition of CaM suppresses CDI but spares the enhancement, consistent with the substantially higher sensitivity of the Ca(2+)-sensitive adenylate cyclase 1 (AC1) to Ca(2+)/CaM. Inhibition of the ryanodine receptor reduces the current amplitude, suggesting that CICR might normally amplify the activation of AC1 and stimulation of PKA activity. These experiments reveal that the amplitude of L-type Ca(2+) currents in retinal amacrine cells are both positively and negatively regulated by Ca(2+)-dependent mechanisms.

Mitochondrial Ca2+ uptake and not mitochondrial motility is required for STIM1-Orai1-dependent store-operated Ca2+ entry

Store-operated Ca2+ entry (SOCE) is established by formation of subplasmalemmal clusters of the endoplasmic reticulum (ER) protein, stromal interacting molecule 1 (STIM1) upon ER Ca2+ depletion. Thereby, STIM1 couples to plasma membrane channels such as Orai1. Thus, a close proximity of ER domains to the plasma membrane is a prerequisite for SOCE activation, challenging the concept of local Ca2+ buffering by mitochondria as being essential for SOCE. This study assesses the impact of mitochondrial Ca2+ handling and motility on STIM1–Orai1-dependent SOCE. High-resolution microscopy showed only 10% of subplasmalemmal STIM1 clusters to be colocalized with mitochondria. Impairments of mitochondrial Ca2+ handling by inhibition of mitochondrial Na+-Ca2+ exchanger (NCXmito) or depolarization only partially suppressed Ca2+ entry in cells overexpressing STIM1-Orai1. However, SOCE was completely abolished when both NCXmito was inhibited and the inner mitochondrial membrane was depolarized, in STIM1- and Orai1-overexpressing cells. Immobilization of mitochondria by expression of mAKAP-RFP-CAAX, a construct that physically links mitochondria to the plasma membrane, affected the Ca2+ handling of the organelles but not the activity of SOCE. Our observations indicate that mitochondrial Ca2+ uptake, including reversal of NCXmito, is fundamental for STIM1–Orai1-dependent SOCE, whereas the proximity of mitochondria to STIM1-Orai1 SOCE units and their motility is not required.