ORAI1 and STIM1 deficiency in human and mice: roles of store-operated Ca2+ entry in the immune system and beyond

Orai1 calcium channels in the vasculature

Orai1 was discovered in T cells as a calcium-selective channel that is activated by store depletion. Recent studies suggest that it is expressed and functionally important also in blood vessels, not only because haematopoietic cells can incorporate in the vascular wall but also because Orai1 is expressed and functional in vascular smooth muscle cells and endothelial cells. This article summarises the arising observations in this new area of vascular research and debates underlying issues and challenges for future investigations. The primary focus is on vascular smooth muscle cells and endothelial cells. Specific topics include Orai1 expression; Orai1 roles in store-operated calcium entry and ionic currents of store-depleted cells; blockade of Orai1-related signals by Synta 66 and other pharmacology; activation or regulation of Orai1-related signals by physiological substances and compartments; stromal interaction molecules and the relationship of Orai1 to other ion channels, transporters and pumps; transient receptor potential canonical channels and their contribution to store-operated calcium entry; roles of Orai1 in vascular tone, remodelling, thrombus formation and inflammation; and Orai2 and Orai3. Overall, the observations suggest the existence of an additional, previously unrecognised, calcium channel of the vascular wall that is functionally important particularly in remodelling but probably also in certain vasoconstrictor contexts.

Fluorescence-based measurement of store-operated calcium entry in live cells: from cultured cancer cell to skeletal muscle fiber

Store operated Ca²⁺ entry (SOCE), earlier termed capacitative Ca²⁺ entry, is a tightly regulated mechanism for influx of extracellular Ca²⁺ into cells to replenish depleted endoplasmic reticulum (ER) or sarcoplasmic reticulum (SR) Ca²⁺ stores^1,2. Since Ca²⁺ is a ubiquitous second messenger, it is not surprising to see that SOCE plays important roles in a variety of cellular processes, including proliferation, apoptosis, gene transcription and motility. Due to its wide occurrence in nearly all cell types, including epithelial cells and skeletal muscles, this pathway has received great interest^3,4. However, the heterogeneity of SOCE characteristics in different cell types and the physiological function are still not clear^5-7.

The functional channel properties of SOCE can be revealed by patch-clamp studies, whereas a large body of knowledge about this pathway has been gained by fluorescence-based intracellular Ca²⁺ measurements because of its convenience and feasibility for high-throughput screening. The objective of this report is to summarize a few fluorescence-based methods to measure the activation of SOCE in monolayer cells, suspended cells and muscle fibers^5,8-10. The most commonly used of these fluorescence methods is to directly monitor the dynamics of intracellular Ca²⁺ using the ratio of F_340nm and F_380nm (510 nm for emission wavelength) of the ratiometric Ca²⁺ indicator Fura-2. To isolate the activity of unidirectional SOCE from intracellular Ca²⁺ release and Ca²⁺ extrusion, a Mn²⁺ quenching assay is frequently used. Mn²⁺ is known to be able to permeate into cells via SOCE while it is impervious to the surface membrane extrusion processes or to ER uptake by Ca²⁺ pumps due to its very high affinity with Fura-2. As a result, the quenching of Fura-2 fluorescence induced by the entry of extracellular Mn²⁺ into the cells represents a measurement of activity of SOCE⁹. Ratiometric measurement and the Mn⁺² quenching assays can be performed on a cuvette-based spectrofluorometer in a cell population mode or in a microscope-based system to visualize single cells. The advantage of single cell measurements is that individual cells subjected to gene manipulations can be selected using GFP or RFP reporters, allowing studies in genetically modified or mutated cells. The spatiotemporal characteristics of SOCE in structurally specialized skeletal muscle can be achieved in skinned muscle fibers by simultaneously monitoring the fluorescence of two low affinity Ca²⁺ indicators targeted to specific compartments of the muscle fiber, such as Fluo-5N in the SR and Rhod-5N in the transverse tubules^9,11,12.

Influence of Orai1 intervention on mouse airway epithelium reactions in vivo and in vitro

BACKGROUND

Orai1 is crucial for store-operated Ca2(+) entry and Ca2(+) release-activated Ca2(+) channel activities. However, little is known about its function in allergic diseases.

OBJECTIVE

To assess the influence of Orai1 intervention on mouse airway epithelium reactions in vivo and in vitro.

METHODS

We used immunohistochemical staining, enzyme-linked immunosorbent assay, and real-time reverse transcription-polymerase chain reaction to evaluate Orai1 expression in nasal and tracheal mucosa epithelium of nonsensitized, control, and 2-aminoethoxydiphenyl borate (2-APB)-treated groups in vivo and in vitro. In addition, we analyzed concentrations of interleukin 1β, interleukin 6, macrophage inflammatory protein 2, and tumor necrosis factor α in nasal lavage fluid, bronchoalveolar lavage fluid, and culture supernatant and their messenger RNAs in nasal and tracheal mucosa and cultured nasal and tracheal epithelium.

RESULTS

Administration of 2-APB into the nostrils suppressed Orai1 expression in nasal and tracheal mucosa of treated mice compared with that in control mice and restrained the mediators in nasal lavage fluid, bronchoalveolar lavage fluid, and airway mucosa of treated groups compared with those in control groups. Similarly, the 2-APB intervention also alleviated Orai1 and the production of the mediators in culture supernatant and cultured airway epithelium under allergic conditions.

CONCLUSIONS

Our results indicate that 2-APB could effectively ameliorate reactions of upper and lower airway epithelial cells in mice in allergic states in vivo and in vitro.

Immunodeficiency due to defects in store-operated calcium entry

Mutations in genes encoding the calcium-release activated calcium (CRAC) channel abolish calcium influx in cells of the immune system and cause severe congenital immunodeficiency. Patients with autosomal recessive mutations in the CRAC channel gene ORAI1, its activator stromal interaction molecule 1 (STIM1), and mice with targeted deletion of Orai1, Stim1, and Stim2 genes reveal important roles for CRAC channels in adaptive and innate immune responses to infection and in autoimmunity. Because CRAC channels have important functions outside the immune system, deficiency of either ORAI1 or STIM1 is associated with a unique clinical phenotype. This review will give an overview of CRAC channel function in the immune system, examine the consequences of CRAC channel deficiency for immunity in human patients and mice, and discuss genetic defects in immunoreceptor-associated signaling molecules that compromise calcium influx and cause immunodeficiency.

Pharmacology of ORAI channels as a tool to understand their physiological functions

Store-operated Ca(2+) entry is a major Ca(2+) entry mechanism that is present in most cell types. In immune cells, store-operated Ca(2+) entry is almost exclusively mediated by Ca(2+) release-activated Ca(2+) (CRAC) channels. Ca(2+) entry through these channels and the corresponding cytosolic Ca(2+) signals are required for many immune cell functions, including all aspects of T-cell activation. ORAI proteins are the molecular correlates for the CRAC channels. The three human members, ORAI1, ORAI2 and ORAI3, are activated through the stromal interaction molecules (STIM)1 and 2 following depletion of endoplasmic reticulum Ca(2+) stores. Different combinations of STIM and ORAI can form different CRAC channels with distinct biophysical properties. In this article, we review and discuss mechanistic and functional implications of two important CRAC/ORAI inhibitors, 2-APB and BTP2, and the antibiotic G418 that has also been reported to interfere with ORAI channel function. The use of pharmacological tools should help to assign distinct physiological and pathophysiological functions to different STIM-ORAI protein complexes.

Physiological and pathophysiological functions of SOCE in the immune system

Calcium signals play a critical role in many cell-type specific effector functions during innate and adaptive immune responses. The predominant mechanism to raise intracellular (Ca²⁺) used by most immune cells is store-operated Ca²⁺ entry (SOCE), whereby the depletion of endoplasmic reticulum (ER) Ca²⁺ stores triggers the influx of extracellular Ca²⁺. SOCE in immune cells is mediated by the highly Ca²⁺ selective Ca²⁺-release-activated Ca²⁺ (CRAC) channel, encoded by ORAI1, ORAI2 and ORAI3 genes. ORAI proteins are activated by stromal interaction molecules (STIM) 1 and 2, which act as sensors of ER Ca²⁺ store depletion. The importance of SOCE mediated by STIM and ORAI proteins for immune function is evident from the immunodeficiency and autoimmunity in patients with mutations in STIM1 and ORAI1 genes. These patients and studies in gene-targeted mice have revealed an essential role for ORAI/STIM proteins in the function of several immune cells. This review focuses on recent advances made towards understanding the role of SOCE in immune cells with an emphasis on the immune dysregulation that results from defects in SOCE in human patients and transgenic mice.

Physiological and pathophysiological functions of SOCE in the immune system

Calcium signals play a critical role in many cell-type specific effector functions during innate and adaptive immune responses. The predominant mechanism to raise intracellular (Ca²⁺) used by most immune cells is store-operated Ca²⁺ entry (SOCE), whereby the depletion of endoplasmic reticulum (ER) Ca²⁺ stores triggers the influx of extracellular Ca²⁺. SOCE in immune cells is mediated by the highly Ca²⁺ selective Ca²⁺-release-activated Ca²⁺ (CRAC) channel, encoded by ORAI1, ORAI2 and ORAI3 genes. ORAI proteins are activated by stromal interaction molecules (STIM) 1 and 2, which act as sensors of ER Ca²⁺ store depletion. The importance of SOCE mediated by STIM and ORAI proteins for immune function is evident from the immunodeficiency and autoimmunity in patients with mutations in STIM1 and ORAI1 genes. These patients and studies in gene-targeted mice have revealed an essential role for ORAI/STIM proteins in the function of several immune cells. This review focuses on recent advances made towards understanding the role of SOCE in immune cells with an emphasis on the immune dysregulation that results from defects in SOCE in human patients and transgenic mice.

Antiviral and regulatory T cell immunity in a patient with stromal interaction molecule 1 deficiency

Stromal interaction molecule 1 (STIM1) deficiency is a rare genetic disorder of store-operated calcium entry, associated with a complex syndrome including immunodeficiency and immune dysregulation. The link from the molecular defect to these clinical manifestations is incompletely understood. We report two patients with a homozygous R429C point mutation in STIM1 completely abolishing store-operated calcium entry in T cells. Immunological analysis of one patient revealed that despite the expected defect of T cell proliferation and cytokine production in vitro, significant antiviral T cell populations were generated in vivo. These T cells proliferated in response to viral Ags and showed normal antiviral cytotoxicity. However, antiviral immunity was insufficient to prevent chronic CMV and EBV infections with a possible contribution of impaired NK cell function and a lack of NKT cells. Furthermore, autoimmune cytopenia, eczema, and intermittent diarrhea suggested impaired immune regulation. FOXP3-positive regulatory T (Treg) cells were present but showed an abnormal phenotype. The suppressive function of STIM1-deficient Treg cells in vitro, however, was normal. Given these partial defects in cytotoxic and Treg cell function, impairment of other immune cell populations probably contributes more to the pathogenesis of immunodeficiency and autoimmunity in STIM1 deficiency than previously appreciated.

MST1 mutations in autosomal recessive primary immunodeficiency characterized by defective naive T-cell survival

The molecular mechanisms that underlie T-cell quiescence are poorly understood. In the present study, we report a primary immunodeficiency phenotype associated with MST1 deficiency and primarily characterized by a progressive loss of naive T cells. The in vivo consequences include recurrent bacterial and viral infections and autoimmune manifestations. MST1-deficient T cells poorly expressed the transcription factor FOXO1, the IL-7 receptor, and BCL2. Conversely, FAS expression and the FAS-mediating apoptotic pathway were up-regulated. These abnormalities suggest that increased cell death of naive and proliferating T cells is the main mechanism underlying this novel immunodeficiency. Our results characterize a new mechanism in primary T-cell immunodeficiencies and highlight a role of the MST1/FOXO1 pathway in controlling the death of human naive T cells.

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

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

Tripartite motif containing protein 27 negatively regulates CD4 T cells by ubiquitinating and inhibiting the class II PI3K-C2β

The K(+) channel KCa3.1 is required for Ca(2+) influx and the subsequent activation of CD4 T cells. The class II phosphatidylinositol 3 kinase C2β (PI3KC2β) is activated by the T-cell receptor (TCR) and is critical for KCa3.1 channel activation. Tripartite motif containing protein 27 (TRIM27) is a member of a large family of proteins that function as Really Interesting New Gene (RING) E3 ubiquitin ligases. We now show that TRIM27 functions as an E3 ligase and mediates lysine 48 polyubiquitination of PI3KC2β, leading to a decrease in PI3K enzyme activity. By inhibiting PI3KC2β, TRIM27 also functions to negatively regulate CD4 T cells by inhibiting KCa3.1 channel activity and TCR-stimulated Ca(2+) influx and cytokine production in Jurkat, primary human CD4 T cells, and Th0, Th1, and Th2 CD4 T cells generated from TRIM27(-/-) mice. These findings provide a unique mechanism for regulating class II PI3Ks, and identify TRIM27 as a previously undescribed negative regulator of CD4 T cells.

Orai1 and Ca2+-independent phospholipase A2 are required for store-operated Icat-SOC current, Ca2+ entry, and proliferation of primary vascular smooth muscle cells

Store-operated Ca(2+) entry (SOCE) is important for multiple functions of vascular smooth muscle cells (SMC), which, depending of their phenotype, can resemble excitable and nonexcitable cells. Similar to nonexcitable cells, Orai1 was found to mediate Ca(2+)-selective (CRAC-like) current and SOCE in dedifferentiated cultured SMC and smooth muscle-derived cell lines. However, the role of Orai1 in cation-selective store-operated channels (cat-SOC), which are responsible for SOCE in primary SMC, remains unclear. Here we focus on primary SMC, and assess the role of Orai1 and Ca(2+)-independent phospholipase A(2) (iPLA(2)β, or PLA2G6) in activation of cat-SOC current (I(cat-SOC)), SOCE, and SMC proliferation. Using molecular, electrophysiological, imaging, and functional approaches, we demonstrate that molecular knockdown of either Orai1 or iPLA(2)β leads to similar inhibition of the whole cell cat-SOC current and SOCE in primary aortic SMC and results in significant reduction in DNA synthesis and impairment of SMC proliferation. This is the first demonstration that Orai1 and iPLA(2)β are equally important for cat-SOC, SOCE, and proliferation of primary aortic SMC.

Insights into NK cell biology from human genetics and disease associations

Rare human primary immunodeficiency disorders with extreme susceptibility to infections in infancy have provided important insights into immune function. Increasingly, however, primary immunodeficiencies are also recognized as a cause of other more common, often discrete, infectious susceptibilities. In a wider context, loss-of-function mutations in immune genes may also cause disorders of immune regulation and predispose to cancer. Here, we review the associations between human diseases and mutations in genetic elements affecting natural killer (NK) cell development and function. Although many such genetic aberrations significantly reduce NK cell numbers or severely impair NK cell responses, inferences regarding the role of NK cells in disease are confounded by the fact that most mutations also affect the development or function of other cell types. Still, data suggest an important role for NK cells in diseases ranging from classical immunodeficiency syndromes with susceptibility to viruses and other intracellular pathogens to cancer, autoimmunity, and hypersensitivity reactions.

Orai3--the 'exceptional' Orai?

The field of agonist-activated Ca(2+) entry in non-excitable cells underwent a revolution some 5 years ago with the discovery of the Orai proteins as the essential pore-forming components of the low-conductance, highly Ca(2+)-selective CRAC channels whose activation is dependent on depletion of intracellular stores. Mammals possess three distinct Orai proteins (Orai1, 2 and 3) of which Orai3 is unique to this class, apparently evolving from Orai1. However, the sequence of Orai3 shows marked differences from that of Orai1, particularly in those regions of the protein outside of the essential pore-forming domains. Correspondingly, studies from several different groups have indicated that the inclusion of Orai3 is associated with the appearance of conductances that display unique features in their gating, selectivity, regulation and mode of activation. In this Topical Review, these features are discussed with the purpose of proposing that the evolutionary appearance of Orai3 in mammals, and the consequent development of conductances displaying novel properties - whether formed by Orai3 alone or in conjunction with the other Orai proteins - is associated with the specific role of this member of the Orai family in a unique range of distinct cellular activities.

Remodelling of the endoplasmic reticulum during store-operated calcium entry

SOCE (store-operated calcium entry) is a ubiquitous cellular mechanism linking the calcium depletion of the ER (endoplasmic reticulum) to the activation of PM (plasma membrane) Ca2+-permeable channels. The activation of SOCE channels favours the entry of extracellular Ca2+ into the cytosol, thereby promoting the refilling of the depleted ER Ca2+ stores as well as the generation of long-lasting calcium signals. The molecules that govern SOCE activation comprise ER Ca2+ sensors [STIM1 (stromal interaction molecule 1) and STIM2], PM Ca2+-permeable channels {Orai and TRPC [TRP (transient receptor potential) canonical]} and regulatory Ca2+-sensitive cytosolic proteins {CRACR2 [CRAC (Ca2+ release-activated Ca2+ current) regulator 2]}. Upon Ca2+ depletion of the ER, STIM molecules move towards the PM to bind and activate Orai or TRPC channels, initiating calcium entry and store refilling. This molecular rearrangement is accompanied by the formation of specialized compartments derived from the ER, the pre-cER (cortical ER) and cER. The pre-cER appears on the electron microscope as thin ER tubules enriched in STIM1 that extend along microtubules and that are devoid of contacts with the PM. The cER is located in immediate proximity to the PM and comprises thinner sections enriched in STIM1 and devoid of chaperones that might be dedicated to calcium signalling. Here, we review the molecular interactions and the morphological changes in ER structure that occur during the SOCE process.

STIM1L is a new actin-binding splice variant involved in fast repetitive Ca2+ release

A newly identified splice variant of STIM1 called STIM1L forms constitutive clusters that interact with actin and Orai1 and allows fast repetitive Ca²⁺ release.

Cytosolic Ca²⁺ signals encoded by repetitive Ca²⁺ releases rely on two processes to refill Ca²⁺ stores: Ca²⁺ reuptake from the cytosol and activation of a Ca²⁺ influx via store-operated Ca²⁺ entry (SOCE). However, SOCE activation is a slow process. It is delayed by >30 s after store depletion because stromal interaction molecule 1 (STIM1), the Ca²⁺ sensor of the intracellular stores, must form clusters and migrate to the membrane before being able to open Orai1, the plasma membrane Ca²⁺ channel. In this paper, we identify a new protein, STIM1L, that colocalizes with Orai1 Ca²⁺ channels and interacts with actin to form permanent clusters. This property allowed the immediate activation of SOCE, a characteristic required for generating repetitive Ca²⁺ signals with frequencies within seconds such as those frequently observed in excitable cells. STIM1L was expressed in several mammalian tissues, suggesting that many cell types rely on this Ca²⁺ sensor for their Ca²⁺ homeostasis and intracellular signaling.

Local cytosolic Ca2+ elevations are required for stromal interaction molecule 1 (STIM1) de-oligomerization and termination of store-operated Ca2+ entry

The Ca²⁺ depletion of the endoplasmic reticulum (ER) activates the ubiquitous store-operated Ca²⁺ entry (SOCE) pathway that sustains long-term Ca²⁺ signals critical for cellular functions. ER Ca²⁺ depletion initiates the oligomerization of stromal interaction molecules (STIM) that control SOCE activation, but whether ER Ca²⁺ refilling controls STIM de-oligomerization and SOCE termination is not known. Here, we correlate the changes in free luminal ER Ca²⁺ concentrations ([Ca²⁺]_ER) and in STIM1 oligomerization, using fluorescence resonance energy transfer (FRET) between CFP-STIM1 and YFP-STIM1. We observed that STIM1 de-oligomerized at much lower [Ca²⁺]_ER levels during store refilling than it oligomerized during store depletion. We then refilled ER stores without adding exogenous Ca²⁺ using a membrane-permeable Ca²⁺ chelator to provide a large reservoir of buffered Ca²⁺. This procedure rapidly restored pre-stimulatory [Ca²⁺]_ER levels but did not trigger STIM1 de-oligomerization, the FRET signals remaining elevated as long as the external [Ca²⁺] remained low. STIM1 dissociation evoked by Ca²⁺ readmission was prevented by SOC channel inhibition and was associated with cytosolic Ca²⁺ elevations restricted to STIM1 puncta, indicating that Ca²⁺ acts on a cytosolic target close to STIM1 clusters. These data indicate that the refilling of ER Ca²⁺ stores is not sufficient to induce STIM1 de-oligomerization and that localized Ca²⁺ elevations in the vicinity of assembled SOCE complexes are required for the termination of SOCE.

Calcium store sensor stromal-interaction molecule 1-dependent signaling plays an important role in cervical cancer growth, migration, and angiogenesis

Store-operated Ca(2+) entry (SOCE) is the principal Ca(2+) entry mechanism in nonexcitable cells. Stromal-interaction molecule 1 (STIM1) is an endoplasmic reticulum Ca(2+) sensor that triggers SOCE activation. However, the role of STIM1 in regulating cancer progression remains controversial and its clinical relevance is unclear. Here we show that STIM1-dependent signaling is important for cervical cancer cell proliferation, migration, and angiogenesis. STIM1 overexpression in tumor tissue is noted in 71% cases of early-stage cervical cancer. In tumor tissues, the level of STIM1 expression is significantly associated with the risk of metastasis and survival. EGF-stimulated cancer cell migration requires STIM1 expression and EGF increases the interaction between STIM1 and Orai1 in juxta-membrane areas, and thus induces Ca(2+) influx. STIM1 involves the activation of Ca(2+)-regulated protease calpain, as well as Ca(2+)-regulated cytoplasmic kinase Pyk2, which regulate the focal-adhesion dynamics of migratory cervical cancer cells. Because of an increase of p21 protein levels and a decrease of Cdc25C protein levels, STIM1-silencing in cervical cancer cells significantly inhibits cell proliferation by arresting the cell cycle at the S and G2/M phases. STIM1 also regulates the production of VEGF in cervical cancer cells. Interference with STIM1 expression or blockade of SOCE activity inhibits tumor angiogenesis and growth in animal models, confirming the crucial role of STIM1-mediated Ca(2+) influx in aggravating tumor development in vivo. These results make STIM1-dependent signaling an attractive target for therapeutic intervention.

Ion channels in asthma

Ion channels are specialized transmembrane proteins that permit the passive flow of ions following their electrochemical gradients. In the airways, ion channels participate in the production of epithelium-based hydroelectrolytic secretions and in the control of intracellular Ca(2+) levels that will ultimately activate almost all lung cells, either resident or circulating. Thus, ion channels have been the center of many studies aiming to understand asthma pathophysiological mechanisms or to identify therapeutic targets for better control of the disease. In this minireview, we focus on molecular, genetic, and animal model studies associating ion channels with asthma.

β-Thalassemia trait association with autoimmune diseases: β-globin locus proximity to the immunity genes or role of hemorphins?

Thalassemia major continues to be a significant health problem for Mediterranean, Afro-Arabic countries, India and South Easth Asia. It was generally assumed that the β-thalassemia heterozygotes do not bear significant medical risks except a mild microcytic anemia. Nonetheless, increasing number of reports associate β-thalassemia trait with autoimmune conditions, nephritis, diabetes, arthritis, fibromyalgia and asthma. Available sparse data indicate reduced incidence of systemic lupus erythematosus (SLE) in β-thalassemia heterozygotes; yet, if two conditions coexist, the SLE manifestations occur much severer. These associations make sense when considering that the hemoglobin β-chain locus at 11p15.5 resides in close proximity to eight genes with profound roles in immune regulation: STIM1, CD151, TC21/RRAS2, SIGIRR/TOLL/IL1R8, pp52/LSP1 (lymphocyte specific protein), TRIM21, toll interacting protein (TOLLIP) and SLEN3. β-Thalassemia trait accompaniment to autoimmune disease may be the result of haplotypal associations between the close proximity genes. An alternative explanation to thalassemia heterozygosity: autoimmune disease association may be the changed concentrations of hemorphins. Hemorphins are endogenous opioid peptides derived via proteolytical cleavage of hemoglobin. They are shown to bind diverse opioid receptors and act anti-inflammatory. Their reduced expression in thalassemia heterozygosity may explain a proinflammatory stage and autoimmunity vulnerability.

Migrational guidance of neutrophils is mechanotransduced via high-affinity LFA-1 and calcium flux

Acute inflammation triggers the innate immune response of neutrophils that efficiently traffic from the bloodstream to concentrate at high numbers at the site of tissue infection or wounding. A gatekeeper in this process is activation of β(2) integrins, which form bond clusters with ICAM-1 on the endothelial surface. These bond clusters serve dual functions of providing adhesive strength to anchor neutrophils under the shear forces of blood flow and directional guidance for cell polarization and subsequent transmigration on inflamed endothelium. We hypothesized that shear forces transmitted through high-affinity LFA-1 facilitates the cooperation with the calcium release-activated channel Orai1 in directing localized cytoskeletal activation and directed migration. By using vascular mimetic microfluidic channels, we observed neutrophil arrest on a substrate of either ICAM-1 or allosteric Abs that stabilize a high- or low-affinity conformation of LFA-1. Neutrophils captured via low-affinity LFA-1 did not exhibit intracellular calcium flux, F-actin polymerization, cell polarization, or directional migration under shear flow. In contrast, high-affinity LFA-1 provided orientation along a uropod-pseudopod axis that required calcium flux through Orai1. We demonstrate how the shear stress of blood flow can transduce distinct outside-in signals at focal sites of high-affinity LFA-1 that provide contact-mediated guidance for neutrophil emigration.

STIM1 and STIM2 are located in the acidic Ca2+ stores and associates with Orai1 upon depletion of the acidic stores in human platelets

Mammalian cells accumulate Ca2+ into agonist-sensitive acidic organelles, vesicles that possess a vacuolar proton-ATPase. Acidic Ca2+ stores include secretory granules and lysosome-related organelles. Current evidence clearly indicates that acidic Ca2+ stores participate in cell signaling and function, including the activation of store-operated Ca2+ entry in human platelets upon depletion of the acidic stores, although the mechanism underlying the activation of store-operated Ca2+ entry controlled by the acidic stores remains unclear. STIM1 has been presented as the endoplasmic reticulum Ca2+ sensor, but its role sensing intraluminal Ca2+ concentration in the acidic stores has not been investigated. Here we report that STIM1 and STIM2 are expressed in the lysosome-related organelles and dense granules in human platelets isolated by immunomagnetic sorting. Depletion of the acidic Ca2+ stores using the specific vacuolar proton-ATPase inhibitor, bafilomycin A1, enhanced the association between STIM1 and STIM2 as well as between these proteins and the plasma membrane channel Orai1. Depletion of the acidic Ca2+ stores also induces time-dependent co-immunoprecipitation of STIM1 with the TRPC proteins hTRPC1 and hTRPC6, as well as between Orai1 and both TRPC proteins. In addition, bafilomycin A1 enhanced the association between STIM2 and SERCA3. These findings demonstrate the location of STIM1 and STIM2 in the acidic Ca2+ stores and their association with Ca2+ channels and ATPases upon acidic stores discharge.

STIM1 as a key regulator for Ca2+ homeostasis in skeletal-muscle development and function

Stromal interaction molecules (STIM) were identified as the endoplasmic-reticulum (ER) Ca2+ sensor controlling store-operated Ca2+ entry (SOCE) and Ca2+-release-activated Ca2+ (CRAC) channels in non-excitable cells. STIM proteins target Orai1-3, tetrameric Ca2+-permeable channels in the plasma membrane. Structure-function analysis revealed the molecular determinants and the key steps in the activation process of Orai by STIM. Recently, STIM1 was found to be expressed at high levels in skeletal muscle controlling muscle function and properties. Novel STIM targets besides Orai channels are emerging.Here, we will focus on the role of STIM1 in skeletal-muscle structure, development and function. The molecular mechanism underpinning skeletal-muscle physiology points toward an essential role for STIM1-controlled SOCE to drive Ca2+/calcineurin/nuclear factor of activated T cells (NFAT)-dependent morphogenetic remodeling programs and to support adequate sarcoplasmic-reticulum (SR) Ca2+-store filling. Also in our hands, STIM1 is transiently up-regulated during the initial phase of in vitro myogenesis of C2C12 cells. The molecular targets of STIM1 in these cells likely involve Orai channels and canonical transient receptor potential (TRPC) channels TRPC1 and TRPC3. The fast kinetics of SOCE activation in skeletal muscle seem to depend on the triad-junction formation, favoring a pre-localization and/or pre-formation of STIM1-protein complexes with the plasma-membrane Ca2+-influx channels. Moreover, Orai1-mediated Ca2+ influx seems to be essential for controlling the resting Ca2+ concentration and for proper SR Ca2+ filling. Hence, Ca2+ influx through STIM1-dependent activation of SOCE from the T-tubule system may recycle extracellular Ca2+ losses during muscle stimulation, thereby maintaining proper filling of the SR Ca2+ stores and muscle function. Importantly, mouse models for dystrophic pathologies, like Duchenne muscular dystrophy, point towards an enhanced Ca2+ influx through Orai1 and/or TRPC channels, leading to Ca2+-dependent apoptosis and muscle degeneration. In addition, human myopathies have been associated with dysfunctional SOCE. Immunodeficient patients harboring loss-of-function Orai1 mutations develop myopathies, while patients suffering from Duchenne muscular dystrophy display alterations in their Ca2+-handling proteins, including STIM proteins. In any case, the molecular determinants responsible for SOCE in human skeletal muscle and for dysregulated SOCE in patients of muscular dystrophy require further examination.

Orai1 and CRAC channel dependence of VEGF-activated Ca2+ entry and endothelial tube formation

RATIONALE

Orai1 and the associated calcium release-activated calcium (CRAC) channel were discovered in the immune system. Existence also in endothelial cells has been suggested, but the relevance to endothelial biology is mostly unknown.

OBJECTIVE

The aim of this study was to investigate the relevance of Orai1 and CRAC channels to vascular endothelial growth factor (VEGF) and endothelial tube formation.

METHODS AND RESULTS

In human umbilical vein endothelial cells, Orai1 disruption by short-interfering RNA or dominant-negative mutant Orai1 inhibited calcium release-activated (store-operated) calcium entry, VEGF-evoked calcium entry, cell migration, and in vitro tube formation. Expression of exogenous wild-type Orai1 rescued the tube formation. VEGF receptor-2 and Orai1 partially colocalized. Orai1 disruption also inhibited calcium entry and tube formation in endothelial progenitor cells from human blood. A known blocker of the immune cell CRAC channel (3-fluoropyridine-4-carboxylic acid (2',5'-dimethoxybiphenyl-4-yl)amide) was a strong blocker of store-operated calcium entry in endothelial cells and inhibited calcium entry evoked by VEGF in 3 types of human endothelial cell. The compound lacked effect on VEGF-evoked calcium-release, STIM1 clustering, and 2 types of transient receptor potential channels, TRPC6 and TRPV4. Without effect on cell viability, the compound inhibited human endothelial cell migration and tube formation in vitro and suppressed angiogenesis in vivo in the chick chorioallantoic membrane. The compound showed 100-fold greater potency for endothelial compared with immune cell calcium entry.

CONCLUSIONS

The data suggest positive roles for Orai1 and CRAC channels in VEGF-evoked calcium entry and new opportunity for chemical modulation of angiogenesis.

The role of calcium release activated calcium channels in osteoclast differentiation

Osteoclasts are specialized macrophage derivatives that secrete acid and proteinases to mobilize bone for mineral homeostasis, growth, and replacement or repair. Osteoclast differentiation generally requires the monocyte growth factor m-CSF and the TNF-family cytokine RANKL, although differentiation is regulated by many other cytokines and by intracellular signals, including Ca(2+). Studies of osteoclast differentiation in vitro were performed using human monocytic precursors stimulated with m-CSF and RANKL, revealing significant loss in both the expression and function of the required components of store-operated Ca(2+) entry over the course of osteoclast differentiation. However, inhibition of CRAC using either the pharmacological agent 3,4-dichloropropioanilide (DCPA) or by knockdown of Orai1 severely inhibited formation of multinucleated osteoclasts. In contrast, no effect of CRAC channel inhibition was observed on expression of the osteoclast protein tartrate resistant acid phosphatase (TRAP). Our findings suggest that despite the fact that they are down-regulated during osteoclast differentiation, CRAC channels are required for cell fusion, a late event in osteoclast differentiation. Since osteoclasts cannot function properly without multinucleation, selective CRAC inhibitors may have utility in management of hyperresorptive states.

ORAI1-mediated calcium influx is required for human cytotoxic lymphocyte degranulation and target cell lysis

Lymphocytes mediate cytotoxicity by polarized release of the contents of cytotoxic granules toward their target cells. Here, we have studied the role of the calcium release-activated calcium channel ORAI1 in human lymphocyte cytotoxicity. Natural killer (NK) cells obtained from an ORAI1-deficient patient displayed defective store-operated Ca(2+) entry (SOCE) and severely defective cytotoxic granule exocytosis leading to impaired target cell lysis. Similar findings were obtained using NK cells from a stromal interaction molecule 1-deficient patient. The defect occurred at a late stage of the signaling process, because activation of leukocyte functional antigen (LFA)-1 and cytotoxic granule polarization were not impaired. Moreover, pharmacological inhibition of SOCE interfered with degranulation and target cell lysis by freshly isolated NK cells and CD8(+) effector T cells from healthy donors. In addition to effects on lymphocyte cytotoxicity, synthesis of the chemokine macrophage inflammatory protein-1β and the cytokines TNF-α and IFN-γ on target cell recognition was impaired in ORAI1-deficient NK cells, as previously described for T cells. By contrast, NK cell cytokine production induced by combinations of IL-12, IL-15, and IL-18 was not impaired by ORAI1 deficiency. Taken together, these results identify a critical role for ORAI1-mediated Ca(2+) influx in granule exocytosis for lymphocyte cytotoxicity as well as for cytokine production induced by target cell recognition.

The TRPC ion channels: association with Orai1 and STIM1 proteins and participation in capacitative and non-capacitative calcium entry

Transient receptor potential (TRP) proteins are involved in a large number of non-selective cation channels that are permeable to both monovalent and divalent cations. Two general classes of receptor-mediated Ca(2+) entry has been proposed: one of then is conduced by receptor-operated Ca(2+) channels (ROC), the second is mediated by channels activated by the emptying of intracellular Ca(2+) stores (store-operated channels or SOC). TRP channels have been presented as subunits of both ROC and SOC, although the precise mechanism that regulates the participation of TRP proteins in these Ca(2+) entry mechanisms remains unclear. Recently, TRPC proteins have been shown to associate with Orai1 and STIM1 in a dynamic ternary complex regulated by the occupation of membrane receptors in several cell models, which might play an important role in the function of TRPC proteins. The present review summarizes the current knowledge concerning the association of TRP proteins with Orai and STIM proteins and how this affects the participation of TRP proteins in store-operated or receptor-operated Ca(2+) entry.

Competitive modulation of Ca2+ release-activated Ca2+ channel gating by STIM1 and 2-aminoethyldiphenyl borate

Activation of Ca(2+) release-activated Ca(2+) channels by depletion of intracellular Ca(2+) stores involves physical interactions between the endoplasmic reticulum Ca(2+) sensor, STIM1, and the channels composed of Orai subunits. Recent studies indicate that the Orai3 subtype, in addition to being store-operated, is also activated in a store-independent manner by 2-aminoethyldiphenyl borate (2-APB), a small molecule with complex pharmacology. However, it is unknown whether the store-dependent and -independent activation modes of Orai3 channels operate independently or whether there is cross-talk between these activation states. Here we report that in addition to causing direct activation, 2-APB also regulates store-operated gating of Orai3 channels, causing potentiation at low doses and inhibition at high doses. Inhibition of store-operated gating by 2-APB was accompanied by the suppression of several modes of Orai3 channel regulation that depend on STIM1, suggesting that high doses of 2-APB interrupt STIM1-Orai3 coupling. Conversely, STIM1-bound Orai3 (and Orai1) channels resisted direct gating by high doses of 2-APB. The rate of direct 2-APB activation of Orai3 channels increased linearly with the degree of STIM1-Orai3 uncoupling, suggesting that 2-APB has to first disengage STIM1 before it can directly gate Orai3 channels. Collectively, our results indicate that the store-dependent and -independent modes of Ca(2+) release-activated Ca(2+) channel activation are mutually exclusive: channels bound to STIM1 resist 2-APB gating, whereas 2-APB antagonizes STIM1 gating.

Nuclear Ca(2+) signalling

Ca(2+) signalling is important for controlling gene transcription. Changes of the cytosolic Ca(2+) ([Ca(2+)](C)) may promote migration of transcription factors or transcriptional regulators to the nucleus. Changes of the nucleoplasmic Ca(2+) ([Ca(2+)](N)) can also regulate directly gene expression. [Ca(2+)](N) may change by propagation of [Ca(2+)](C) changes through the nuclear envelope or by direct release of Ca(2+) inside the nucleus. In the last case nuclear and cytosolic signalling can be dissociated. Phosphatidylinositol bisphosphate, phospholipase C and cyclic ADP-ribosyl cyclase are present inside the nucleus. Inositol trisphosphate receptors (IP(3)R) and ryanodine receptors (RyR) have also been found in the nucleus and can be activated by agonists. Furthermore, nuclear location of the synthesizing enzymes and receptors may be atypical, not associated to the nuclear envelope or other membranes. The possible role of nuclear subdomains such as speckles, nucleoplasmic reticulum, multi-macromolecular complexes and nuclear nanovesicles is discussed.

Ca(2+) signaling, genes and the cell cycle

Changes in the concentration and spatial distribution of Ca(2+) ions in the cytoplasm constitute a ubiquitous intracellular signaling module in cellular physiology. With the advent of Ca(2+) dyes that allow direct visualization of Ca(2+) transients, combined with powerful experimental tools such as electrophysiological recordings, intracellular Ca(2+) transients have been implicated in practically every aspect of cellular physiology, including cellular proliferation. Ca(2+) signals are associated with different phases of the cell cycle and interfering with Ca(2+) signaling or downstream pathways often disrupts progression of the cell cycle. Although there exists a dependence between Ca(2+) signals and the cell cycle the mechanisms involved are not well defined and given the cross-talk between Ca(2+) and other signaling modules, it is difficult to assess the exact role of Ca(2+) signals in cell cycle progression. Two exceptions however, include fertilization and T-cell activation, where well-defined roles for Ca(2+) signals in mediating progression through specific stages of the cell cycle have been clearly established. In the case of T-cell activation Ca(2+) regulates entry into the cell cycle through the induction of gene transcription.

Selected tetrapeptides lead to a GLP-1 release from the human enteroendocrine cell line NCI-H716

Enteroendocrine cells in the intestine sense the luminal contents and have been shown to respond to not only fatty acids, proteins, and monosaccharides but also artificial sweeteners and bitter compounds. Secretion of hormones such as CCK and GLP-1 from these cells is often associated with a rise in intracellular calcium concentration [Ca²+](i). The human NCI-H716 enteroendocrine cell line has been described as a proper model to study GLP-1 secretion in response to amino acids and protein hydrolysates. Here, we describe that NCI-H716 cells specifically respond to selective tetrapeptides such as tetra-glycine, tetra-alanine and Gly-Trp-Gly-Gly with a dose-dependent [Ca²+](i) response and a GLP-1 secretion, whereas selected free amino acids, dipeptides, tripeptides and pentapeptides failed to elicit such a response. Hormone secretion was not associated with changes in cAMP levels in the cells. The calcium-dependence of hormone secretion appears to involve store-operated calcium channels (SOCCs), since the SOCC blocker 2-APB abolished both the [Ca²+](i) response and GLP-1 release upon tetra-glycine stimulation. The nature of the sensor currently remains elusive, and no obvious common structural pattern in tetrapeptides eliciting GLP-1 secretion was identified. This tetrapeptide sensing in NCI-H716 cells may be underlying the effective stimulation of hormone secretion shown for various protein hydrolysates, and could involve a novel G-protein-coupled receptor (GPCR).

T-cell-specific deletion of STIM1 and STIM2 protects mice from EAE by impairing the effector functions of Th1 and Th17 cells

T-cell function is dependent on store-operated Ca(2+) influx that is activated by the stromal interaction molecules (STIM) 1 and 2. We show that mice with T-cell-specific deletion of STIM1 or STIM2 are protected from EAE, a mouse model of multiple sclerosis (MS). While STIM1- and STIM2-deficient T cells could be successfully primed by autoantigen, they failed to produce the proinflammatory cytokines IL-17 and IFN-γ. STIM1-deficient T cells showed reduced expression of IL-23R, required for Th17 cell homeostasis, and had impaired chemokine-dependent T-cell migration caused by a lack of chemokine-induced Ca(2+) influx. Autoantigen-specific STIM1- or STIM2-deficient T cells failed to expand and accumulate in the CNS and lymph nodes following adoptive transfer to passively induce EAE, suggesting that autoantigen-specific restimulation or homeostasis of STIM1- and STIM2-deficient T cells are impaired. Combined deletion of both STIM1 and STIM2, previously shown to impair Treg development and function, completely protected mice from EAE. This indicates that, in the absence of Ca(2+) influx, autoreactive T cells are severely dysfunctional rendering Treg dispensable for the prevention of CNS inflammation. Our findings demonstrate that both STIM1 and STIM2 are critical for T-cell function and autoimmunity in vivo.

Store-operated Ca2+ entry through ORAI1 is critical for T cell-mediated autoimmunity and allograft rejection

ORAI1 is the pore-forming subunit of the Ca(2+) release-activated Ca(2+) (CRAC) channel, which is responsible for store-operated Ca(2+) entry in lymphocytes. A role for ORAI1 in T cell function in vivo has been inferred from in vitro studies of T cells from human immunodeficient patients with mutations in ORAI1 and Orai1(-/-) mice, but a detailed analysis of T cell-mediated immune responses in vivo in mice lacking functional ORAI1 has been missing. We therefore generated Orai1 knock-in mice (Orai1(KI/KI)) expressing a nonfunctional ORAI1-R93W protein. Homozygosity for the equivalent ORAI1-R91W mutation abolishes CRAC channel function in human T cells resulting in severe immunodeficiency. Homozygous Orai1(KI/KI) mice die neonatally, but Orai1(KI/KI) fetal liver chimeric mice are viable and show normal lymphocyte development. T and B cells from Orai1(KI/KI) mice display severely impaired store-operated Ca(2+) entry and CRAC channel function resulting in a strongly reduced expression of several key cytokines including IL-2, IL-4, IL-17, IFN-γ, and TNF-α in CD4(+) and CD8(+) T cells. Cell-mediated immune responses in vivo that depend on Th1, Th2, and Th17 cell function were severely attenuated in ORAI1-deficient mice. Orai1(KI/KI) mice lacked detectable contact hypersensitivity responses and tolerated skin allografts significantly longer than wild-type mice. In addition, T cells from Orai1(KI/KI) mice failed to induce colitis in an adoptive transfer model of inflammatory bowel disease. These findings reaffirm the critical role of ORAI1 for T cell function and provide important insights into the in vivo functions of CRAC channels for T cell-mediated immunity.

Whole-exome sequencing-based discovery of STIM1 deficiency in a child with fatal classic Kaposi sarcoma

Whole-exome sequencing reveals a homozygous splice-site mutation in the gene encoding STIM1 in a child with classic Kaposi sarcoma.

Classic Kaposi sarcoma (KS) is exceedingly rare in children from the Mediterranean Basin, despite the high prevalence of human herpesvirus-8 (HHV-8) infection in this region. We hypothesized that rare single-gene inborn errors of immunity to HHV-8 may underlie classic KS in childhood. We investigated a child with no other unusually severe infectious or tumoral phenotype who died from disseminated KS at two years of age. Whole-exome sequencing in the patient revealed a homozygous splice-site mutation in STIM1, the gene encoding stromal interaction molecule 1, which regulates store-operated Ca²⁺ entry. STIM1 mRNA splicing, protein production, and Ca²⁺ influx were completely abolished in EBV-transformed B cell lines from the patient, but were rescued by the expression of wild-type STIM1. Based on the previous discovery of STIM1 deficiency in a single family with a severe T cell immunodeficiency and the much higher risk of KS in individuals with acquired T cell deficiencies, we conclude that STIM1 T cell deficiency precipitated the development of lethal KS in this child upon infection with HHV-8. Our report provides the first evidence that isolated classic KS in childhood may result from single-gene defects and provides proof-of-principle that whole-exome sequencing in single patients can decipher the genetic basis of rare inborn errors.

Designing dynamical output feedback controllers for store-operated Ca²+ entry

Store-operated calcium entry (SOCE) has been proposed as the main process controlling Ca²+ entry in non-excitable cells. Although recent breakthroughs in experimental studies of SOCE have been made, its mathematical modeling has not been developed. In the present work, SOCE is viewed as a feedback control system subject to an extracellular agonist disturbance and an extracellular calcium input. We then design a dynamic output feedback controller to reject the disturbance and track Ca²+ resting levels in the cytosol and the endoplasmic reticulum (ER). The constructed feedback control system is validated by published experimental data and its global asymptotic stability is proved by using the LaSalle's invariance principle. We then simulate the dynamic responses of STIM1 and Orai1, two major components in the operation of the store-operated channels, to the depletion of Ca²+ in the ER with thapsigargin, which show that: (1) Upon the depletion of Ca²+ in the ER, the concentrations of activated STIM1 and STIM1-Orai1 cluster are elevated gradually, indicating that STIM1 is accumulating in the ER-PM junctions and that the cytosolic portion of the active STIM1 is binding to Orai1 and driving the opening of CRAC channels for Ca²+ entry; (2) after the extracellular Ca²+ addition, the concentrations of both STIM1 and STIM1-Orai1 cluster decrease but still much higher than the original levels. We also simulate the system responses to the agonist disturbance, which show that, when a sequence of periodic agonist pulses is applied, the system returns to its equilibrium after each pulse. This indicates that the designed feedback controller can reject the disturbance and track the equilibrium.

STIM proteins: integrators of signalling pathways in development, differentiation and disease

The stromal interaction molecules STIM1 and STIM2 are endoplasmic reticulum Ca²⁺ sensors, serving to detect changes in receptor-mediated ER Ca²⁺ store depletion and to relay this information to plasma membrane localized proteins, including the store-operated Ca²⁺ channels of the ORAI family. The resulting Ca²⁺ influx sustains the high cytosolic Ca²⁺ levels required for activation of many intracellular signal transducers such as the NFAT family of transcription factors. Models of STIM protein deficiency in mice, Drosophila melanogaster and Caenorhabditis elegans, in addition to the phenotype of patients bearing mutations in STIM1 have provided great insight into the role of these proteins in cell physiology and pathology. It is now becoming clear that STIM1 and STIM2 are critical for the development and functioning of many cell types, including lymphocytes, skeletal and smooth muscle myoblasts, adipocytes and neurons, and can interact with a variety of signalling proteins and pathways in a cell- and tissue-type specific manner. This review focuses on the role of STIM proteins in development, differentiation and disease, in particular highlighting the functional differences between STIM1 and STIM2.

ORAI-mediated calcium entry: mechanism and roles, diseases and pharmacology

ORAI1 is a protein located on the plasma membrane that acts as a calcium channel. Calcium enters via ORAI1 as a mechanism to refill the sarcoplasmic/endoplasmic reticulum calcium stores, the depletion of which can be detected by the sensor protein STIM1. Isoforms of these proteins ORAI2, ORAI3 and STIM2 also have roles in cellular calcium homeostasis but are less well characterized. This pathway of filling the calcium stores is termed store-operated calcium entry and while the pathway itself was proposed in 1986, the identity of the key molecular components was only discovered in 2005 and 2006. The characterization of the ORAI and STIM proteins has provided clearer information on some calcium-regulated pathways that are important in processes from gene transcription to immune cell function. Recent studies have also suggested the importance of the components of ORAI-mediated calcium entry in some diseases or processes significant in disease including the migration of breast cancer cells and thrombus formation. This review will provide a brief overview of ORAI-mediated calcium entry, its role in physiological and pathophysiological processes, as well as current and potential pharmacological modulators of the components of this important cellular calcium entry pathway.

Molecular basis of calcium signaling in lymphocytes: STIM and ORAI

Ca(2+) entry into cells of the peripheral immune system occurs through highly Ca(2+)-selective channels known as CRAC (calcium release-activated calcium) channels. CRAC channels are a very well-characterized example of store-operated Ca(2+) channels, so designated because they open when the endoplasmic reticulum (ER) Ca(2+) store becomes depleted. Physiologically, Ca(2+) is released from the ER lumen into the cytoplasm when activated receptors couple to phospholipase C and trigger production of the second messenger inositol 1,4,5-trisphosphate (IP(3)). IP(3) binds to IP(3) receptors in the ER membrane and activates Ca(2+) release. The proteins STIM and ORAI were discovered through limited and genome-wide RNAi screens, respectively, performed in Drosophila cells and focused on identifying modulators of store-operated Ca(2+) entry. STIM1 and STIM2 sense the depletion of ER Ca(2+) stores, whereas ORAI1 is a pore subunit of the CRAC channel. In this review, we discuss selected aspects of Ca(2+) signaling in cells of the immune system, focusing on the roles of STIM and ORAI proteins in store-operated Ca(2+) entry.

Human muscle economy myoblast differentiation and excitation-contraction coupling use the same molecular partners, STIM1 and STIM2

Our recent work identified store-operated Ca(2+) entry (SOCE) as the critical Ca(2+) source required for the induction of human myoblast differentiation (Darbellay, B., Arnaudeau, S., König, S., Jousset, H., Bader, C., Demaurex, N., and Bernheim, L. (2009) J. Biol. Chem. 284, 5370-5380). The present work indicates that STIM2 silencing, similar to STIM1 silencing, reduces myoblast SOCE amplitude and differentiation. Because myoblasts in culture can be induced to differentiate into myotubes, which spontaneously contract in culture, we used the same molecular tools to explore whether the Ca(2+) mechanism of excitation-contraction coupling also relies on STIM1 and STIM2. Live cell imaging of early differentiating myoblasts revealed a characteristic clustering of activated STIM1 and STIM2 during the first few hours of differentiation. Thapsigargin-induced depletion of endoplasmic reticulum Ca(2+) content caused STIM1 and STIM2 redistribution into clusters, and co-localization of both STIM proteins. Interaction of STIM1 and STIM2 was revealed by a rapid increase in fluorescence resonance energy transfer between CFP-STIM1 and YFP-STIM2 after SOCE activation and confirmed by co-immunoprecipitation of endogenous STIM1 and STIM2. Although both STIM proteins clearly contribute to SOCE and are required during the differentiation process, STIM1 and STIM2 are functionally largely redundant as overexpression of either STIM1 or STIM2 corrected most of the impact of STIM2 or STIM1 silencing on SOCE and differentiation. With respect to excitation-contraction, we observed that human myotubes rely also on STIM1 and STIM2 to refill their endoplasmic reticulum Ca(2+)-content during repeated KCl-induced Ca(2+) releases. This indicates that STIM2 is a necessary partner of STIM1 for excitation-contraction coupling. Thus, both STIM proteins are required and interact to control SOCE during human myoblast differentiation and human myotube excitation-contraction coupling.

Store-operated CRAC channels: function in health and disease

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

Essential role for the CRAC activation domain in store-dependent oligomerization of STIM1

Oligomerization of the ER Ca²⁺ sensor, STIM1, is the key triggering step in the activation of store-operated Ca channels. We show that the cytosolic CRAC activation domain, previously known only for its role in binding to and activating Orai1 channels, is also required for stable STIM1 oligomerization.

Oligomerization of the ER Ca²⁺ sensor STIM1 is an essential step in store-operated Ca²⁺ entry. The lumenal EF-hand and SAM domains of STIM1 are believed to initiate oligomerization after Ca²⁺ store depletion, but the contributions of STIM1 cytosolic domains (coiled-coil 1, CC1; coiled-coil 2, CC2; CRAC activation domain, CAD) to this process are not well understood. By applying coimmunoprecipitation and fluorescence photobleaching and energy transfer techniques to truncated and mutant STIM1 proteins, we find that STIM1 cytosolic domains play distinct roles in forming both “resting” oligomers in cells with replete Ca²⁺ stores and higher-order oligomers in store-depleted cells. CC1 supports the formation of resting STIM1 oligomers and appears to interact with cytosolic components to slow STIM1 diffusion. On store depletion, STIM1 lacking all cytosolic domains (STIM1-ΔC) oligomerizes through EF-SAM interactions alone, but these oligomers are unstable. Addition of CC1 + CAD, but not CC1 alone, enables the formation of stable store-dependent oligomers. Within the CAD, both CC2 and C-terminal residues contribute to oligomer formation. Our results reveal a new function for the CAD: in addition to binding and activating Orai1, it is directly involved in STIM1 oligomerization, the initial event triggering store-operated Ca²⁺ entry.

Regulation of immune cell development through soluble inositol-1,3,4,5-tetrakisphosphate

The membrane lipid phosphatidylinositol-3,4,5-trisphosphate (PtdInsP(3)) regulates membrane receptor signalling in many cells, including immunoreceptor signalling. Here, we review recent data that have indicated essential roles for the soluble PtdInsP(3) analogue inositol-1,3,4,5-tetrakisphosphate (InsP(4)) in T cell, B cell and neutrophil development and function. Decreased InsP(4) production in leukocytes causes immunodeficiency in mice and might contribute to inflammatory vasculitis in Kawasaki disease in humans. InsP(4)-producing kinases could therefore provide attractive drug targets for inflammatory and infectious diseases.

Immunodeficiency due to mutations in ORAI1 and STIM1

Lymphocyte activation requires Ca(2+) influx through specialized Ca(2+) channels in the plasma membrane. In T cells the predominant Ca(2+) channel is the Ca(2+) release activated Ca(2+) (CRAC) channel encoded by the gene ORAI1. ORAI1 is activated by stromal interaction molecule (STIM) 1 that is localized in the ER where it senses the concentration of stored Ca(2+). Following antigen binding to immunoreceptors such as the TCR, ER Ca(2+) stores are depleted, STIM1 is activated and ORAI1-CRAC channels open resulting in what is referred to as store-operated Ca(2+) entry (SOCE). Mutations in ORAI1 and STIM1 genes in human patients that lead to expression of non-functional ORAI1 or complete lack of ORAI1 or STIM1 protein are associated with a unique clinical phenotype that is characterized by immunodeficiency, muscular hypotonia and anhydrotic ectodermal dysplasia, as well as, in the case of STIM1 deficiency, autoimmunity and lymphoproliferative disease. The immunodeficiency in these patients is due to a severe defect in T cell activation but not in lymphocyte development. This review describes the immunological and non-immunological phenotypes of patients with defects in SOCE and CRAC channel function and discusses them in the context of similar immunodeficiency diseases and animal models of ORAI1 and STIM1 function.

CRAC channelopathies

Store-operated Ca2+ entry (SOCE) is an important Ca2+ influx pathway in many non-excitable and some excitable cells. It is regulated by the filling state of intracellular Ca2+ stores, notably the endoplasmic reticulum (ER). Reduction in [Ca2+]ER results in activation of plasma membrane Ca2+ channels that mediate sustained Ca2+ influx which is required for many cell functions as well as refilling of Ca2+ stores. The Ca2+ release activated Ca2+ (CRAC) channel is the best characterized SOC channel with well-defined electrophysiological properties. In recent years, the molecular components of the CRAC channel, long mysterious, have been defined. ORAI1 (or CRACM1) acts as the pore-forming subunit of the CRAC channel in the plasma membrane. Stromal interaction molecule (STIM) 1 is localized in the ER, senses [Ca2+]ER, and activates the CRAC channel upon store depletion by binding to ORAI1. Both proteins are widely expressed in many tissues in both human and mouse consistent with the widespread prevalence of SOCE and CRAC channel currents in many cells types. CRAC channelopathies in human patients with mutations in STIM1 and ORAI1 are characterized by abolished CRAC channel currents, lack of SOCE and-clinically-immunodeficiency, congenital myopathy, and anhydrotic ectodermal dysplasia. This article reviews the role of ORAI and STIM proteins for SOCE and CRAC channel function in a variety of cell types and tissues and compares the phenotypes of ORAI1 and STIM1-deficient human patients and mice with targeted deletion of Orai and Stim genes.

ORAI1 deficiency and lack of store-operated Ca2+ entry cause immunodeficiency, myopathy, and ectodermal dysplasia

BACKGROUND

Defects in the development or activation of T cells result in immunodeficiency associated with severe infections early in life. T-cell activation requires Ca2+ influx through Ca2+-release activated Ca2+ (CRAC) channels encoded by the gene ORAI1.

OBJECTIVE

Investigation of the genetic causes and the clinical phenotype of immunodeficiency in patients with impaired Ca2+ influx and CRAC channel function.

METHODS

DNA sequence analysis for mutations in the genes ORAI1, ORAI2, ORAI3, and stromal interaction molecule (STIM) 1 and 2, as well as mRNA and protein expression analysis of ORAI1 in immunodeficient patients. Immunohistochemical analysis of ORAI1 tissue distribution in healthy human donors.

RESULTS

We identified mutations in ORAI1 in patients from 2 unrelated families. One patient is homozygous for a frameshift nonsense mutation in ORAI1 (ORAI1-A88SfsX25), and a second patient is compound heterozygous for 2 missense mutations in ORAI1 (ORAI1-A103E/L194P). All 3 mutations abolish ORAI1 expression and impair Ca2+ influx and CRAC channel function. The clinical syndrome associated with ORAI1 deficiency is characterized by immunodeficiency with a defect in the function but not in the development of lymphocytes, congenital myopathy, and anhydrotic ectodermal dysplasia with a defect in dental enamel calcification. In contrast with the limited clinical phenotype, we found ORAI1 protein expression in a wide variety of cell types and organs.

CONCLUSION

Ca2+ influx through ORAI1 is crucial for lymphocyte function in vivo. Despite almost ubiquitous ORAI1 expression, the channel has a nonredundant role in only a few cell types judging from the limited clinical phenotype in ORAI1-deficient patients.

Structural determinants of ion permeation in CRAC channels

CRAC channels generate Ca(2+) signals critical for the activation of immune cells and exhibit an intriguing pore profile distinguished by extremely high Ca(2+) selectivity, low Cs(+) permeability, and small unitary conductance. To identify the ion conduction pathway and gain insight into the structural bases of these permeation characteristics, we introduced cysteine residues in the CRAC channel pore subunit, Orai1, and probed their accessibility to various thiol-reactive reagents. Our results indicate that the architecture of the ion conduction pathway is characterized by a flexible outer vestibule formed by the TM1-TM2 loop, which leads to a narrow pore flanked by residues of a helical TM1 segment. Residues in TM3, and specifically, E190, a residue considered important for ion selectivity, are not close to the pore. Moreover, the outer vestibule does not significantly contribute to ion selectivity, implying that Ca(2+) selectivity is conferred mainly by E106. The ion conduction pathway is sufficiently narrow along much of its length to permit stable coordination of Cd(2+) by several TM1 residues, which likely explains the slow flux of ions within the restrained geometry of the pore. These results provide a structural framework to understand the unique permeation properties of CRAC channels.

Orai1 regulates intracellular calcium, arrest, and shape polarization during neutrophil recruitment in shear flow

Orai1 was reported to function as a calcium channel subunit that facilitates store operated calcium entry (SOCE) in T cells and is necessary for formation of the immune synapse. We reasoned that SOCE via Orai1 might regulate PMNs activation during recruitment to inflamed endothelium. Orai1 function was assessed by real-time imaging of calcium transients as PMNs were stimulated to roll, arrest, and migrate on E-selectin and ICAM-1 in shear flow. Calcium entry was significantly reduced when Orai1 function was impaired by heterozygous knockout in a mouse model or by siRNA knockdown in HL-60 cells. Reduced Orai-1 expression correlated with the delayed onset of arrest and reduced ability to transition to a polarized migratory phenotype. Inhibition of SOCE by treatment with 2-APB, or blocking phospholipase C (PLC) mediated calcium store release with U73122, abrogated formyl peptide induced calcium elevation, and delayed subsequent cell arrest and polarization. These results suggest that calcium entry via Orai1 is the predominant SOCE that cooperates with cytoplasmic calcium store release in coordinating integrin-dependent PMN arrest and migration in the acute response to inflammation.