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Using genetically engineered mice and cell lines derived from genetically engineered mice we show that depletion of ER delimited Ca 2+ stores activates heteromeric Ca 2+ entry (SOCE) channels formed obligatorily, but not exclusively by Orai1 molecules. Comparison of Orai-dependent Ca 2+ entries revealed Orai1 to be dominant when compared to Orai2 and Orai3. Unexpectedly, we found that store-depletion-activated Ca 2+ entry does not depend obligatorily on functionally intact TRPC molecules, as SOCE monitored with the Fura2 Ca 2+ reporter dye is unaffected in cells in which all seven TRPC coding genes have been structurally and functionally inactivated. Unexpectedly as well, we found that TRPC-independent Gq-coupled receptor-operated Ca 2+ entry (ROCE) also depends on Orai1. Biophysical measurements of Ca 2+ release activated Ca 2+ currents (Icrac) are likewise unaffected by ablation of all seven TRPC genes. We refer to mice and cells carrying the seven-fold disruption of TRPC genes as TRPC heptaKO mice and cells. TRPC heptaKO mice are fertile allowing the creation of a new homozygous inbred strain., Competing Interests: Competing interests statement:The authors declare no competing interest.

Store-operated calcium entry (SOCE) plays a crucial role in maintaining cellular calcium homeostasis. This mechanism involves proteins, such as stromal interaction molecule 1 (STIM1) and ORAI1. Mutations in the genes encoding these proteins, especially STIM1, can lead to various diseases, including CRAC channelopathies associated with severe combined immunodeficiency. Herein, we describe a novel homozygous mutation, NM_003156 c.792-3C > G, in STIM1 in a patient with a clinical profile of CRAC channelopathy, including immune system deficiencies and muscle weakness. Functional analyses revealed three distinct spliced forms in the patient cells: wild-type, exon 7 skipping, and intronic retention. Calcium influx analysis revealed impaired SOCE in the patient cells, indicating a loss of STIM1 function. We developed an antisense oligonucleotide treatment that improves STIM1 splicing and highlighted its potential as a therapeutic approach. Our findings provide insights into the complex effects of STIM1 mutations and shed light on the multifaceted clinical presentation of the patient., Competing Interests: Declaration of competing interest None., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

T-cell receptor stimulation triggers cytosolic Ca 2+ signaling by inositol-1,4,5-trisphosphate (IP 3 )-mediated Ca 2+ release from the endoplasmic reticulum (ER) and Ca 2+ entry through Ca 2+ release-activated Ca 2+ (CRAC) channels gated by ER-located stromal-interacting molecules (STIM1/2). Physiologically, cytosolic Ca 2+ signaling manifests as regenerative Ca 2+ oscillations, which are critical for nuclear factor of activated T-cells-mediated transcription. In most cells, Ca 2+ oscillations are thought to originate from IP 3 receptor-mediated Ca 2+ release, with CRAC channels indirectly sustaining them through ER refilling. Here, experimental and computational evidence support a multiple-oscillator mechanism in Jurkat T-cells whereby both IP 3 receptor and CRAC channel activities oscillate and directly fuel antigen-evoked Ca 2+ oscillations, with the CRAC channel being the major contributor. KO of either STIM1 or STIM2 significantly reduces CRAC channel activity. As such, STIM1 and STIM2 synergize for optimal Ca 2+ oscillations and activation of nuclear factor of activated T-cells 1 and are essential for ER refilling. The loss of both STIM proteins abrogates CRAC channel activity, drastically reduces ER Ca 2+ content, severely hampers cell proliferation and enhances cell death. These results clarify the mechanism and the contribution of STIM proteins to Ca 2+ oscillations in T-cells., Competing Interests: Conflict of interest Mohamed Trebak is a consultant for Seeker Biologics Inc, and is a member of the editorial board of the J. Biol. Chem. The remaining authors declare no competing interests., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

Ca 2+ release-activated Ca 2+ (CRAC) channels are activated by direct physical interactions between Orai1, the channel protein, and STIM1, the endoplasmic reticulum Ca 2+ sensor. A hallmark of CRAC channels is fast Ca 2+ -dependent inactivation (CDI) which provides negative feedback to limit Ca 2+ entry through CRAC channels. Although STIM1 is thought to be essential for CDI, its molecular mechanism remains largely unknown. Here, we examined a poorly understood gain-of-function (GOF) human Orai1 disease mutation, L138F, that causes tubular aggregate myopathy. Through pairwise mutational analysis, we determine that large amino acid substitutions at either L138 or the neighboring T92 locus located on the pore helix evoke highly Ca 2+ -selective currents in the absence of STIM1. We find that the GOF phenotype of the L138 pathogenic mutation arises due to steric clash between L138 and T92. Surprisingly, strongly activating L138 and T92 mutations showed CDI in the absence of STIM1, contradicting prevailing views that STIM1 is required for CDI. CDI of constitutively open T92W and L138F mutants showed enhanced intracellular Ca 2+ sensitivity, which was normalized by re-adding STIM1 to the cells. Truncation of the Orai1 C-terminus reduced T92W CDI, indicating a key role for the Orai1 C-terminus for CDI. Overall, these results identify the molecular basis of a disease phenotype with broad implications for activation and inactivation of Orai1 channels., Competing Interests: PY, MY No competing interests declared, MP Reviewing editor, eLife, (© 2023, Yeung, Yamashita et al.)

Stormorken syndrome is a multiorgan hereditary disease caused by dysfunction of the endoplasmic reticulum (ER) Ca 2+ sensor protein STIM1, which forms the Ca 2+ release-activated Ca 2+ (CRAC) channel together with the plasma membrane channel Orai1. ER Ca 2+ store depletion activates STIM1 by releasing the intramolecular "clamp" formed between the coiled coil 1 (CC1) and CC3 domains of the protein, enabling the C terminus to extend and interact with Orai1. The most frequently occurring mutation in patients with Stormorken syndrome is R304W, which destabilizes and extends the STIM1 C terminus independently of ER Ca 2+ store depletion, causing constitutive binding to Orai1 and CRAC channel activation. We found that in cis deletion of one amino acid residue, Glu 296 (which we called E296del) reversed the pathological effects of R304W. Homozygous Stim1 E296del+R304W mice were viable and phenotypically indistinguishable from wild-type mice. NMR spectroscopy, molecular dynamics simulations, and cellular experiments revealed that although the R304W mutation prevented CC1 from interacting with CC3, the additional deletion of Glu 296 opposed this effect by enabling CC1-CC3 binding and restoring the CC domain interactions within STIM1 that are critical for proper CRAC channel function. Our results provide insight into the activation mechanism of STIM1 by clarifying the molecular basis of mutation-elicited protein dysfunction and pathophysiology.

SignificanceCalcium release-activated calcium (CRAC) channels play key roles in the regulation of cellular signaling, transcription, and migration. Here, we describe the design, chemical synthesis, and characterization of photoswitchable channel inhibitors that can be switched on and off depending on the wavelength of light used. We use the compounds to induce light-dependent modulation of channel activity and downstream gene expression in human immune cells. We further expand the usage of the compounds to control seeding of cancer cells in target tissue and regulation of response to noxious stimuli in vivo in mice.

Mast cells (MCs) are tissue resident immune cells that play important roles in the pathogenesis of allergic disorders. These responses are mediated via the cross-linking of cell surface high affinity IgE receptor (FcϵRI) by antigen resulting in calcium (Ca 2+ ) mobilization, followed by degranulation and release of proinflammatory mediators. In addition to FcϵRI, cutaneous MCs express Mas-related G protein-coupled receptor X2 (MRGPRX2; mouse ortholog MrgprB2). Activation of MRGPRX2/B2 by the neuropeptide substance P (SP) is implicated in neurogenic inflammation, chronic urticaria, mastocytosis and atopic dermatitis. Although Ca 2+ entry is required for MRGPRX2/B2-mediated MC responses, the possibility that calcium release-activated calcium (CRAC/Orai) channels participate in these responses has not been tested. Lentiviral shRNA-mediated silencing of Orai1, Orai2 or Orai3 in a human MC line (LAD2 cells) resulted in partial inhibition of SP-induced Ca 2+ mobilization, degranulation and cytokine/chemokine generation (TNF-α, IL-8, and CCL-3). Synta66, which blocks homo and hetero-dimerization of Orai channels, caused a more robust inhibition of SP-induced responses than knockdown of individual Orai channels. Synta66 also blocked SP-induced extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt phosphorylation and abrogated cytokine/chemokine production. It also inhibited SP-induced Ca 2+ mobilization and degranulation in primary human skin MCs and mouse peritoneal MCs. Furthermore, Synta66 attenuated both SP-induced cutaneous vascular permeability and leukocyte recruitment in mouse peritoneum. These findings demonstrate that Orai channels contribute to MRGPRX2/B2-mediated MC activation and suggest that their inhibition could provide a novel approach for the modulation of SP-induced MC/MRGPRX2-mediated disorders., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Chaki, Alkanfari, Roy, Amponnawarat, Hui, Oskeritzian and Ali.)

CRAC channel regulator 2 A (CRACR2A) is a large Rab GTPase that is expressed abundantly in T cells and acts as a signal transmitter between T cell receptor stimulation and activation of the Ca 2+ -NFAT and JNK-AP1 pathways. CRACR2A has been linked to human diseases in numerous genome-wide association studies, however, to date no patient with damaging variants in CRACR2A has been identified. In this study, we describe a patient harboring biallelic variants in CRACR2A [paternal allele c.834 gaG> gaT (p.E278D) and maternal alelle c.430 Aga > Gga (p.R144G) c.898 Gag> Tag (p.E300*)], the gene encoding CRACR2A. The 33-year-old patient of East-Asian origin exhibited late onset combined immunodeficiency characterised by recurrent chest infections, panhypogammaglobulinemia and CD4+ T cell lymphopenia. In vitro exposure of patient B cells to a T-dependent stimulus resulted in normal generation of antibody-secreting cells, however the patient's T cells showed pronounced reduction in CRACR2A protein levels and reduced proximal TCR signaling, including dampened SOCE and reduced JNK phosphorylation, that contributed to a defect in proliferation and cytokine production. Expression of individual allelic mutants in CRACR2A-deleted T cells showed that the CRACR2A E278D mutant did not affect JNK phosphorylation, but impaired SOCE which resulted in reduced cytokine production. The truncated double mutant CRACR2A R144G/E300* showed a pronounced defect in JNK phosphorylation as well as SOCE and strong impairment in cytokine production. Thus, we have identified variants in CRACR2A that led to late-stage combined immunodeficiency characterized by loss of function in T cells., Competing Interests: BW, LR, JS, DL, KW, CC, LM, RA, GD, SS, YG No competing interests declared, SS Received research grant funding and payments from CSL Behring for advisory board participation, (© 2021, Wu et al.)

The calcium release-activated calcium (CRAC) channel consists of STIM1, a Ca 2+ sensor in the endoplasmic reticulum (ER), and Orai1, the Ca 2+ ion channel in the plasma membrane. Ca 2+ store depletion triggers conformational changes and oligomerization of STIM1 proteins and their direct interaction with Orai1. Structural alterations include the transition of STIM1 C-terminus from a folded to an extended conformation thereby exposing CAD (CRAC activation domain)/SOAR (STIM1-Orai1 activation region) for coupling to Orai1. In this study, we discovered that different point mutations of F394 in the small alpha helical segment (STIM1 α2) within the CAD/SOAR apex entail a rich plethora of effects on diverse STIM1 activation steps. An alanine substitution (STIM1 F394A) destabilized the STIM1 quiescent state, as evident from its constitutive activity. Single point mutation to hydrophilic, charged amino acids (STIM1 F394D, STIM1 F394K) impaired STIM1 homomerization and subsequent Orai1 activation. MD simulations suggest that their loss of homomerization may arise from altered formation of the CC1α1-SOAR/CAD interface and potential electrostatic interactions with lipid headgroups in the ER membrane. Consistent with these findings, we provide experimental evidence that the perturbing effects of F394D depend on the distance of the apex from the ER membrane. Taken together, our results suggest that the CAD/SOAR apex is in the immediate vicinity of the ER membrane in the STIM1 quiescent state and that different mutations therein can impact the STIM1/Orai1 activation cascade in various manners. Legend: Upon intracellular Ca 2+ store depletion of the endoplasmic reticulum (ER), Ca 2+ dissociates from STIM1. As a result, STIM1 adopts an elongated conformation and elicits Ca 2+ influx from the extracellular matrix (EM) into the cell due to binding to and activation of Ca 2+ -selective Orai1 channels (left). The effects of three point mutations within the SOARα2 domain highlight the manifold roles of this region in the STIM1/Orai1 activation cascade: STIM1 F394A is active irrespective of the intracellular ER Ca 2+ store level, but activates Orai1 channels to a reduced extent (middle). On the other hand, STIM1 F394D/K cannot adopt an elongated conformation upon Ca 2+ store-depletion due to altered formation of the CC1α1-SOAR/CAD interface and/or electrostatic interaction of the respective side-chain charge with corresponding opposite charges on lipid headgroups in the ER membrane (right)., (© 2021. The Author(s).)

Ca 2+ ion channels are critical in a variety of physiological events, including cell growth, differentiation, gene transcription and apoptosis. One such essential entry pathway for calcium into the cell is the Ca 2+ release-activated Ca 2+ (CRAC) channel. It consists of the Ca 2+ sensing protein, stromal interaction molecule 1 (STIM1) located in the endoplasmic reticulum (ER) and a Ca 2+ ion channel Orai in the plasma membrane. The Orai channel family includes three homologues Orai1, Orai2 and Orai3. While Orai1 is the "classical" Ca 2+ ion channel within the CRAC channel complex and plays a universal role in the human body, there is increasing evidence that Orai2 and Orai3 are important in specific physiological and pathophysiological processes. This makes them an attractive target in drug discovery, but requires a detailed understanding of the three Orai channels and, in particular, their differences. Orai channel activation is initiated via Ca 2+ store depletion, which is sensed by STIM1 proteins, and induces their conformational change and oligomerization. Upon STIM1 coupling, Orai channels activate to allow Ca 2+ permeation into the cell. While this activation mechanism is comparable among the isoforms, they differ by a number of functional and structural properties due to non-conserved regions in their sequences. In this review, we summarize the knowledge as well as open questions in our current understanding of the three isoforms in terms of their structure/function relationship, downstream signaling and physiology as well as pathophysiology.

The population of regulatory T cells (Tregs) is critical for immunological self-tolerance and homeostasis. Proper ion regulation contributes to Treg lineage identity, regulation, and effector function. Identified ion channels include Ca 2+ release-activated Ca 2+ , transient receptor potential, P2X, volume-regulated anion and K + channels Kv1.3 and KCa3.1. Ion channel modulation represents a promising therapeutic approach for the treatment of autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. This review summarizes studies with gene-targeted mice and pharmacological modulators affecting Treg number and function. Furthermore, participation of ion channels is illustrated and the power of future research possibilities is discussed., Competing Interests: The authors have no conflicts of interest to declare., (© Copyright by the Author(s). Published by Cell Physiol Biochem Press.)

Calcium (Ca 2+ ) signals play fundamental roles in immune cell function. The main sources of Ca 2+ influx in mammalian lymphocytes following antigen receptor stimulation are Ca 2+ release-activated Ca 2+ (CRAC) channels. These are formed by ORAI proteins in the plasma membrane and are activated by stromal interaction molecules (STIM) located in the endoplasmic reticulum (ER). Human loss-of-function (LOF) mutations in ORAI1 and STIM1 that abolish Ca 2+ influx cause a unique disease syndrome called CRAC channelopathy that is characterized by immunodeficiency autoimmunity and non-immunological symptoms. Studies in mice lacking Stim and Orai genes have illuminated many cellular and molecular mechanisms by which these molecules control lymphocyte function. CRAC channels are required for the differentiation and function of several T lymphocyte subsets that provide immunity to infection, mediate inflammation and prevent autoimmunity. This review examines new insights into how CRAC channels control T cell-mediated immunity., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Calcium (Ca 2+ ) signals initiate egg activation across the animal kingdom and in at least some plants. These signals are crucial for the success of development and, in the case of mammals, health of the offspring. The mechanisms associated with fertilization that trigger these signals and the molecules that regulate their characteristic patterns vary widely. With few exceptions, a major contributor to fertilization-induced elevation in cytoplasmic Ca 2+ is release from endoplasmic reticulum stores through the IP3 receptor. In some cases, Ca 2+ influx from the extracellular space and/or release from alternative intracellular stores contribute to the rise in cytoplasmic Ca 2+ . Following the Ca 2+ rise, the reuptake of Ca 2+ into intracellular stores or efflux of Ca 2+ out of the egg drive the return of cytoplasmic Ca 2+ back to baseline levels. The molecular mediators of these Ca 2+ fluxes in different organisms include Ca 2+ release channels, uptake channels, exchangers and pumps. The functions of these mediators are regulated by their particular activating mechanisms but also by alterations in their expression and spatial organization. We discuss here the molecular basis for modulation of Ca 2+ signalling at fertilization, highlighting differences across several animal phyla, and we mention key areas where questions remain.

Approximately one-fourth of patients with essential thrombocythemia or primary myelofibrosis carry a somatic mutation of the calreticulin gene (CALR), the gene encoding for calreticulin. A 52-bp deletion (type I mutation) and a 5-bp insertion (type II mutation) are the most frequent genetic lesions. The mechanism(s) by which a CALR mutation leads to a myeloproliferative phenotype has been clarified only in part. We studied the interaction between calreticulin and store-operated calcium (Ca2+) entry (SOCE) machinery in megakaryocytes (Mks) from healthy individuals and from patients with CALR-mutated myeloproliferative neoplasms (MPNs). In Mks from healthy subjects, binding of recombinant human thrombopoietin to c-Mpl induced the activation of signal transducer and activator of transcription 5, AKT, and extracellular signal-regulated kinase 1/2, determining inositol triphosphate-dependent Ca2+ release from the endoplasmic reticulum (ER). This resulted in the dissociation of the ER protein 57 (ERp57)-mediated complex between calreticulin and stromal interaction molecule 1 (STIM1), a protein of the SOCE machinery that leads to Ca2+ mobilization. In Mks from patients with CALR-mutated MPNs, defective interactions between mutant calreticulin, ERp57, and STIM1 activated SOCE and generated spontaneous cytosolic Ca2+ flows. In turn, this resulted in abnormal Mk proliferation that was reverted using a specific SOCE inhibitor. In summary, the abnormal SOCE regulation of Ca2+ flows in Mks contributes to the pathophysiology of CALR-mutated MPNs. In perspective, SOCE may represent a new therapeutic target to counteract Mk proliferation and its clinical consequences in MPNs., (© 2020 by The American Society of Hematology.)

Background: Calcium (Ca2+) ion is a major intracellular signaling messenger, controlling a diverse array of cellular functions like gene expression, secretion, cell growth, proliferation, and apoptosis. The major mechanism controlling this Ca2+ homeostasis is store-operated Ca2+ release-activated Ca2+ (CRAC) channels. CRAC channels are integral membrane protein majorly constituted via two proteins, the stromal interaction molecule (STIM) and ORAI. Following Ca2+ depletion in the Endoplasmic reticulum (ER) store, STIM1 interacts with ORAI1 and leads to the opening of the CRAC channel gate and consequently allows the influx of Ca2+ ions. A plethora of studies report that aberrant CRAC channel activity due to Loss- or gain-of-function mutations in ORAI1 and STIM1 disturbs this Ca2+ homeostasis and causes several autoimmune disorders. Hence, it clearly indicates that the therapeutic target of CRAC channels provides the space for a new approach to treat autoimmune disorders., Objective: This review aims to provide the key structural and mechanical insights of STIM1, ORAI1 and other molecular modulators involved in CRAC channel regulation., Results and Conclusion: Understanding the structure and function of the protein is the foremost step towards improving the effective target specificity by limiting their potential side effects. Herein, the review mainly focusses on the structural underpinnings of the CRAC channel gating mechanism along with its biophysical properties that would provide the solid foundation to aid the development of novel targeted drugs for an autoimmune disorder. Finally, the immune deficiencies caused due to mutations in CRAC channel and currently used pharmacological blockers with their limitation are briefly summarized., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.)

Ca 2+ release activated Ca 2+ (CRAC) channels represent a primary pathway for Ca 2+ to enter non-excitable cells. The two key players in this process are the stromal interaction molecule (STIM), a Ca 2+ sensor embedded in the membrane of the endoplasmic reticulum, and Orai, a highly Ca 2+ selective ion channel located in the plasma membrane. Upon depletion of the internal Ca 2+ stores, STIM is activated, oligomerizes, couples to and activates Orai. This review provides an overview of novel findings about the CRAC channel activation mechanisms, structure and gating. In addition, it highlights, among diverse STIM and Orai mutants, also the disease-related mutants and their implications.

The ORAI family of ion channel-forming proteins in mammals includes three members, ORAI1, ORAI2 and ORAI3, encoded by homologous genes. Of these proteins the ORAI1 one received major attention as plasma membrane constituent of store-operated calcium entry (SOCE) in non-excitable cells. The functional significance of two other proteins, ORAI2 and ORAI3, is much less defined, although both of them participate to various extends in cell-specific modulation of SOCE as well as in supporting some of the store-independent calcium entry mechanisms. Calcium signaling becomes remodeled in cancer to promote cancer hallmarks - enhanced proliferation, resistance to apoptosis, motility and metastasizing. Although such remodeling commonly involves rearrangements of the whole molecular Ca 2+ -handling toolkit of the cell (Ca 2+ pumps and transporters, Ca 2+ -binding and storage proteins, Ca 2+ entry and release channels, Ca 2+ -dependent effectors), Ca 2+ entry through Orai-based channels is especially important, as its dysregulation may contribute to several cancer hallmarks. The latter depend on the type of Ca 2+ -permeable channel formed by ORAI-proteins, spatiotemporal characteristics of Ca 2+ signal that this channel contributes to, and the type Ca 2+ -dependent effector(s) targeted by this signal, all of which may be cancer-specific. By participating in global Ca 2+ entry, ORAI-based SOCE may also contribute to cytosolic Ca 2+ overload of cancer cells thereby playing pro-apoptotic, antineoplastic roles which can potentially be exploited for cancer treatment. This mini review examines various aspects of ORAI proteins in malignant transformation.

Inositol 1,4,5-trisphosphate receptors (IP 3 R) are the most widely expressed intracellular Ca 2+ release channels. Their activation by IP 3 and Ca 2+ allows Ca 2+ to pass rapidly from the ER lumen to the cytosol. The resulting increase in cytosolic [Ca 2+ ] may directly regulate cytosolic effectors or fuel Ca 2+ uptake by other organelles, while the decrease in ER luminal [Ca 2+ ] stimulates store-operated Ca 2+ entry (SOCE). We are close to understanding the structural basis of both IP 3 R activation, and the interactions between the ER Ca 2+ -sensor, STIM, and the plasma membrane Ca 2+ channel, Orai, that lead to SOCE. IP 3 Rs are the usual means through which extracellular stimuli, through ER Ca 2+ release, stimulate SOCE. Here, we review evidence that the IP 3 Rs most likely to respond to IP 3 are optimally placed to allow regulation of SOCE. We also consider evidence that IP 3 Rs may regulate SOCE downstream of their ability to deplete ER Ca 2+ stores. Finally, we review evidence that IP 3 Rs in the plasma membrane can also directly mediate Ca 2+ entry in some cells., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Astrocytes are the major glial subtype in the brain and mediate numerous functions ranging from metabolic support to gliotransmitter release through signaling mechanisms controlled by Ca 2+ Despite intense interest, the Ca 2+ influx pathways in astrocytes remain obscure, hindering mechanistic insights into how Ca 2+ signaling is coupled to downstream astrocyte-mediated effector functions. Here, we identified store-operated Ca 2+ release-activated Ca 2+ (CRAC) channels encoded by Orai1 and STIM1 as a major route of Ca 2+ entry for driving sustained and oscillatory Ca 2+ signals in astrocytes after stimulation of metabotropic purinergic and protease-activated receptors. Using synaptopHluorin as an optical reporter, we showed that the opening of astrocyte CRAC channels stimulated vesicular exocytosis to mediate the release of gliotransmitters, including ATP. Furthermore, slice electrophysiological recordings showed that activation of astrocytes by protease-activated receptors stimulated interneurons in the CA1 hippocampus to increase inhibitory postsynaptic currents on CA1 pyramidal cells. These results reveal a central role for CRAC channels as regulators of astrocyte Ca 2+ signaling, gliotransmitter release, and astrocyte-mediated tonic inhibition of CA1 pyramidal neurons., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Basal forebrain neurons control cerebral blood flow (CBF) by releasing acetylcholine (Ach), which binds to endothelial muscarinic receptors to induce nitric (NO) release and vasodilation in intraparenchymal arterioles. Nevertheless, the mechanism whereby Ach stimulates human brain microvascular endothelial cells to produce NO is still unknown. Herein, we sought to assess whether Ach stimulates NO production in a Ca 2+ -dependent manner in hCMEC/D3 cells, a widespread model of human brain microvascular endothelial cells. Ach induced a dose-dependent increase in intracellular Ca 2+ concentration ([Ca 2+ ] i ) that was prevented by the genetic blockade of M5 muscarinic receptors (M5-mAchRs), which was the only mAchR isoform coupled to phospholipase Cβ (PLCβ) present in hCMEC/D3 cells. A comprehensive real-time polymerase chain reaction analysis revealed the expression of the transcripts encoding for type 3 inositol-1,4,5-trisphosphate receptors (InsP 3 R3), two-pore channels 1 and 2 (TPC1-2), Stim2, Orai1-3. Pharmacological manipulation showed that the Ca 2+ response to Ach was mediated by InsP 3 R3, TPC1-2, and store-operated Ca 2+ entry (SOCE). Ach-induced NO release, in turn, was inhibited in cells deficient of M5-mAchRs. Likewise, Ach failed to increase NO levels in the presence of l-NAME, a selective NOS inhibitor, or BAPTA, a membrane-permeant intracellular Ca 2+ buffer. Moreover, the pharmacological blockade of the Ca 2+ response to Ach also inhibited the accompanying NO production. These data demonstrate for the first time that synaptically released Ach may trigger NO release in human brain microvascular endothelial cells by stimulating a Ca 2+ signal via M5-mAchRs., (© 2018 Wiley Periodicals, Inc.)

Calcium (Ca2+) is an important ion in physiology and is found both outside and inside cells. The intracellular concentration of Ca2+ is tightly regulated as it is an intracellular signal molecule and can affect a variety of cellular processes. In immune cells Ca2+ has been shown to regulate e.g. gene transcription, cytokine secretion, proliferation and migration. Ca2+ can enter the cytoplasm either from intracellular stores or from outside the cells when Ca2+ permeable ion channels in the plasma membrane open. The Ca2+ release-activated (CRAC) channel is the most prominent Ca2+ ion channel in the plasma membrane. It is formed by ORAI1-3 and the channel is opened by the endoplasmic reticulum Ca2+ sensor proteins stromal interaction molecules (STIM) 1 and 2. Another group of Ca2+ channels in the plasma membrane are the voltage-gated Ca2+ (CaV) channels. We examined if a change in immunological tolerance is accompanied by altered ORAI, STIM and CaV gene expression in peripheral blood mononuclear cells (PBMCs) in pregnant women and in type 1 diabetic individuals. Our results show that in pregnancy and type 1 diabetes ORAI1-3 are up-regulated whereas STIM1 and 2 are down-regulated in pregnancy but only STIM2 in type 1 diabetes. Expression of L-, P/Q-, R- and T-type voltage-gated Ca2+ channels was detected in the PBMCs where the CaV2.3 gene was up-regulated in pregnancy and type 1 diabetes whereas the CaV 2.1 and CaV3.2 genes were up-regulated only in pregnancy and the CaV1.3 gene in type 1 diabetes. The results are consistent with that expression of ORAI, STIM and CaV genes correlate with a shift in immunological status of the individual in health, as during pregnancy, and in the autoimmune disease type 1 diabetes. Whether the changes are in general protective or in type 1 diabetes include some pathogenic components remains to be clarified., Competing Interests: The authors have declared that no competing interests exist.

The binding of STIM1 to Orai1 controls the opening of store-operated CRAC channels as well as their extremely high Ca 2+ selectivity. Although STIM1 dimers are known to bind directly to the cytosolic C termini of the six Orai1 subunits (SUs) that form the channel hexamer, the dependence of channel activation and selectivity on the number of occupied binding sites is not well understood. Here we address these questions using dimeric and hexameric Orai1 concatemers in which L273D mutations were introduced to inhibit STIM1 binding to specific Orai1 SUs. By measuring FRET between fluorescently labeled STIM1 and Orai1, we find that homomeric L273D mutant channels fail to bind STIM1 appreciably; however, the L273D SU does bind STIM1 and contribute to channel activation when located adjacent to a WT SU. These results suggest that STIM1 dimers can interact with pairs of neighboring Orai1 SUs. Surprisingly, a single L273D mutation within the Orai1 hexamer reduces channel open probability by ∼90%, triples the size of the single-channel current, weakens the Ca 2+ binding affinity of the selectivity filter, and lowers the selectivity for Na + over Cs + in the absence of divalent cations. These findings reveal a surprisingly strong functional coupling between STIM1 binding and CRAC channel gating and pore properties. We conclude that under physiological conditions, all six Orai1 SUs of the native CRAC channel bind STIM1 to effectively open the pore and generate the signature properties of extremely low conductance and high ion selectivity., (© 2018 Yen and Lewis.)

Dendritic cells (DCs) are highly specialized antigen-presenting cells (APCs) able to induce both specific immunity and immune tolerance. Using information gathered from the tissue where they reside, DCs adjust their functional activity to ensure that protective immunity is favoured while unwanted or exaggerated immune responses are prevented. The remarkable ability of these cells to induce, enhance and orient the immune response, while at the same time maintaining self-tolerance, makes them key players in the immune system. Despite the fact that the role of Ca 2+ has been clearly established in human DC functions, the link between ion homeostasis, mainly Ca 2+ , and DC functions is not fully understood. After all, a growing number of works clearly show the role of SOCE and associated channels in the maturation step, and those of K + channels in migration. This review highlights the key papers published over the past few years and summarizes prospects for the near future., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Ion channels and transporters move ions across membrane barriers and are essential for a host of cell functions in many organs. They conduct K + , Na + and Cl - , which are essential for regulating the membrane potential, H + to control intracellular and extracellular pH and divalent cations such as Ca 2+ , Mg 2+ and Zn 2+ , which function as second messengers and cofactors for many proteins. Inherited channelopathies due to mutations in ion channels or their accessory proteins cause a variety of diseases in the nervous, cardiovascular and other tissues, but channelopathies that affect immune function are not as well studied. Mutations in ORAI1 and STIM1 genes that encode the Ca 2+ release-activated Ca 2+ (CRAC) channel in immune cells, the Mg 2+ transporter MAGT1 and the Cl - channel LRRC8A all cause immunodeficiency with increased susceptibility to infection. Mutations in the Zn 2+ transporters SLC39A4 (ZIP4) and SLC30A2 (ZnT2) result in nutritional Zn 2+ deficiency and immune dysfunction. These channels, however, only represent a fraction of ion channels that regulate immunity as demonstrated by immune dysregulation in channel knockout mice. The immune system itself can cause acquired channelopathies that are associated with a variety of diseases of nervous, cardiovascular and endocrine systems resulting from autoantibodies binding to ion channels. These autoantibodies highlight the therapeutic potential of functional anti-ion channel antibodies that are being developed for the treatment of autoimmune, inflammatory and other diseases., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Calcium release-activated calcium (CRAC) channels open upon depletion of Ca 2+ from the endoplasmic reticulum, and when open, they are permeable to a selective flux of calcium ions. The atomic structure of Orai, the pore domain of CRAC channels, from Drosophila melanogaster has revealed many details about conduction and selectivity in this family of ion channels. However, it is still unclear how residues on the third transmembrane helix can affect the conduction properties of the channel. Here, molecular dynamics and Brownian dynamics simulations were employed to analyze how a conserved glutamate residue on the third transmembrane helix (E262) contributes to selectivity. The comparison between the wild-type and mutated channels revealed a severe impact of the mutation on the hydration pattern of the pore domain and on the dynamics of residues K270, and Brownian dynamics simulations proved that the altered configuration of residues K270 in the mutated channel impairs selectivity to Ca 2+ over Na + . The crevices of water molecules, revealed by molecular dynamics simulations, are perfectly located to contribute to the dynamics of the hydrophobic gate and the basic gate, suggesting a possible role in channel opening and in selectivity function.

Calcium (Ca 2+ ) is an essential second messenger required for diverse signaling processes in immune cells. Ca 2+ release-activated Ca 2+ (CRAC) channels represent one main Ca 2+ entry pathway into the cell. They are fully reconstituted via two proteins, the stromal interaction molecule 1 (STIM1), a Ca 2+ sensor in the endoplasmic reticulum, and the Ca 2+ ion channel Orai in the plasma membrane. After Ca 2+ store depletion, STIM1 and Orai couple to each other, allowing Ca 2+ influx. CRAC-/STIM1-mediated Orai channel currents display characteristic hallmarks such as high Ca 2+ selectivity, an increase in current density when switching from a Ca 2+ -containing solution to a divalent-free Na + one, and fast Ca 2+ -dependent inactivation. Here, we discovered several constitutively active Orai1 and Orai3 mutants, containing substitutions in the TM3 and/or TM4 regions, all of which displayed a loss of the typical CRAC channel hallmarks. Restoring authentic CRAC channel activity required both the presence of STIM1 and the conserved Orai N-terminal portion. Similarly, these structural requisites were found in store-operated Orai channels. Key molecular determinants within the Orai N terminus that together with STIM1 maintained the typical CRAC channel hallmarks were distinct from those that controlled store-dependent Orai activation. In conclusion, the conserved portion of the Orai N terminus is essential for STIM1, as it fine-tunes the open Orai channel gating, thereby establishing authentic CRAC channel activity., (© 2018 by The American Society for Biochemistry and Molecular Biology, Inc.)

Since last decade real-time qPCR has become a routine and robust approach for measuring the expression of genes of interest. Indeed, using qPCR, expression profile analyses are now possible and participate to the understanding of physiological or pathological role of channels such as calcium release-activated channels (CRAC). Initially discovered in lymphocyte T and immunity perturbations, recent studies have highlighted the role of CRAC channels in other pathologies such as cancer. Here we describe a protocol to quantify CRAC components expression, in tissue sample and cell lines, to validate knockdown strategies or identify their roles in physiological and pathological conditions (Hoth and Penner, J Physiol 465:359-386, 1993; Hoth and Penner, Nature 355:353-356, 1992; and Zweifach and Lewis, Proc Natl Acad Sci U S A 90:6295-6299, 1993).

Intracellular calcium signaling cascades are integral to early and late allergic responses involving mast cell degranulation and type 2 helper T cell activation, respectively. Both the responses are accompanied by the movement of calcium through the calcium release-activated calcium (CRAC) channel, encoded by the ORAI1 gene. Spirodela polyrhiza (L.) Schleid (SP) has anti-inflammatory and anti-allergic effects, but its effect on calcium signaling has not been reported. This study investigated whether a 30% ethanolic SP extract (SP EtOH ) and its constituents can reduce CRAC currents ([Formula: see text]), and thus inhibit mast cell degranulation and T cell activation. In Jurkat T lymphocytes, we found that 3[Formula: see text]mg/mL SP EtOH inhibited the [Formula: see text] by [Formula: see text]%, whereas one of its constituents vitexin (100[Formula: see text][Formula: see text]M) inhibited the [Formula: see text] by [Formula: see text]%. Furthermore, in the RBL-2H3 mast cell, the [Formula: see text] was inhibited by 3[Formula: see text]mg/mL SP EtOH ([Formula: see text]%) and 100[Formula: see text][Formula: see text]M vitexin ([Formula: see text]%). Investigation of human primary T cell proliferation induced by co-stimulation with antibodies to cluster of differentiation 3 and 28, and of RBL-2H3 mast cell degranulation following IgE-antigen complex stimulation revealed that 100[Formula: see text][Formula: see text]M vitexin inhibited both T-cell proliferation (by [Formula: see text]%) and mast cell degranulation (by [Formula: see text]%). These effects were concentration-dependent, and no cytotoxicity was observed. Our findings suggest that vitexin is a promising candidate compound for the development of therapeutic agents to prevent and treat allergic diseases.

Fast Ca 2+ -dependent inactivation (FCDI) is a safety mechanism limiting Ca 2+ entry through some types of Ca 2+ channels, including Ca 2+ release-activated Ca 2+ (CRAC) channels. This type of inactivation is caused by Ca 2+ , which passes through Ca 2+ channel and binds to a specific site within a short distance from the inner mouth of the pore, causing channel to shut.The main technique that is used to investigate FCDI is whole-cell patch clamping. Since the cloning of the molecular components of the CRAC channel, STIM1 and Orai1, FCDI of CRAC channel has been studied using HEK293T heterologous expression system. In this paper we describe a method of quantifying CRAC channel FCDI by using instantaneous tail currents.

The Ca 2+ release-activated Ca 2+ (CRAC) current is a major signaling event in non-excitable cells whereby Ca 2+ store depletion activates Ca 2+ entry across the plasma membrane from the extracellular space. Stromal interaction molecule 1 (STIM1) and Orai1 proteins are the key molecular players of the CRAC channel. Previous studies have linked activity of this channel to many physiological functions, and dysregulation of the CRAC channel has been associated with various diseases. In the absence of inducible tissue-specific knockout mice, in vivo knockdown studies examining the endogenous function of CRAC channel proteins, STIM1 and Orai1, are a challenge. In this chapter, we describe a lentiviral delivery system of shRNA-mediated gene silencing that has proven successful in studying the endogenous CRAC channel in vivo.

We describe two methods to study CRAC channel function in human lung mast cells. Both methods involve suppression of endogenous channel function. In the first we use Orai-targeting shRNAs to knock down Orai channel mRNA transcripts. In the second we overexpress dominant-negative mutants of the three members of the Orai channel family. To overcome the poor transfection efficiency of mast cells, we employ an adenoviral delivery system for cell transduction. Knockdown of CRAC channel transcripts is assessed initially using quantitative RT-PCR. We describe an assay for β-hexosaminidase release as a measure of mast cell degranulation to assess the effect of overexpression of dominant-negative mutants.

A wide variety of cellular function depends on the dynamics of intracellular Ca(2+) signals. Especially for relatively slow and lasting processes such as gene expression, cell proliferation, and often migration, cells rely on the store-operated Ca(2+) entry (SOCE) pathway, which is particularly prominent in immune cells. SOCE is initiated by the sensor proteins (STIM1, STIM2) located within the endoplasmic reticulum (ER) registering the Ca(2+) concentration within the ER, and upon its depletion, cluster and trap Orai (Orai1-3) proteins located in the plasma membrane (PM) into ER-PM junctions. These regions become sites of highly selective Ca(2+) entry predominantly through Orai1-assembled channels, which, among other effector functions, is necessary for triggering NFAT translocation into the nucleus. What is less clear is how the spatial and temporal spread of intracellular Ca(2+) is shaped and regulated by differential expression of the individual SOCE genes and their splice variants, their heteromeric combinations and pre- and posttranslational modifications. This review focuses on principle mechanisms regulating expression, splicing, and targeting of Ca(2+) release-activated Ca(2+) (CRAC) channels., (Copyright © 2016 the American Physiological Society.)

In phagocytes, pathogen recognition is followed by Ca(2+) mobilization and NADPH oxidase 2 (NOX2)-mediated "oxidative burst," which involves the rapid production of large amounts of reactive oxygen species (ROS). We showed that ORAI Ca(2+) channels control store-operated Ca(2+) entry, ROS production, and bacterial killing in primary human monocytes. ROS inactivate ORAI channels that lack an ORAI3 subunit. Staphylococcal infection of mice reduced the expression of the gene encoding the redox-sensitive Orai1 and increased the expression of the gene encoding the redox-insensitive Orai3 in the lungs or in bronchoalveolar lavages. A similar switch from ORAI1 to ORAI3 occurred in primary human monocytes exposed to bacterial peptides in culture. These alterations in ORAI1 and ORAI3 abundance shifted the channel assembly toward a more redox-insensitive configuration. Accordingly, silencing ORAI3 increased the redox sensitivity of the channel and enhanced oxidation-induced inhibition of NOX2. We generated a mathematical model that predicted additional features of the Ca(2+)-redox interplay. Our results identified the ORAI-NOX2 feedback loop as a determinant of monocyte immune responses., (Copyright © 2016, American Association for the Advancement of Science.)