The Orai1 severe combined immune deficiency mutation and calcium release-activated Ca2+ channel function in the heterozygous condition

Store-operated calcium entry controls innate and adaptive immune cell function in inflammatory bowel disease

Inflammatory bowel disease (IBD) is characterized by dysregulated intestinal immune responses. Using mass cytometry (CyTOF) to analyze the immune cell composition in the lamina propria (LP) of patients with ulcerative colitis (UC) and Crohn's disease (CD), we observed an enrichment of CD4⁺ effector T cells producing IL‐17A and TNF, CD8⁺ T cells producing IFNγ, T regulatory (Treg) cells, and innate lymphoid cells (ILC). The function of these immune cells is regulated by store‐operated Ca²⁺ entry (SOCE), which results from the opening of Ca²⁺ release‐activated Ca²⁺ (CRAC) channels formed by ORAI and STIM proteins. We observed that the pharmacologic inhibition of SOCE attenuated the production of proinflammatory cytokines including IL‐2, IL‐4, IL‐6, IL‐17A, TNF, and IFNγ by human colonic T cells and ILCs, reduced the production of IL‐6 by B cells and the production of IFNγ by myeloid cells, but had no effect on the viability, differentiation, and function of intestinal epithelial cells. T cell‐specific deletion of CRAC channel genes in mice showed that Orai1, Stim1, and Stim2‐deficient T cells have quantitatively distinct defects in SOCE, which correlate with gradually more pronounced impairment of cytokine production by Th1 and Th17 cells and the severity of IBD. Moreover, the pharmacologic inhibition of SOCE with a selective CRAC channel inhibitor attenuated IBD severity and colitogenic T cell function in mice. Our data indicate that SOCE inhibition may be a suitable new approach for the treatment of IBD.

The role of genetic factors in familial case of acne

Acne is one of the most common dermatoses. A prominent genetic component for this disease has been reported and the manifestation in first-line relatives is considered an important risk factor. Here we present a clinical case illustrating the relevance of particular genetic polymorphisms mapped to NCF1, CD3E, ORAI1, IGHM and TAZ in patients with severe forms and burdened family history of the disease. Genetic examination identified the same allelic variants in five candidate target genes (NCF1, CD3E, ORAI1, IGHM and TAZ) in two closely related patients (father and son) with severe acne. The identified genetic configuration may interfere with the oxidase activity and promote defects in mitochondrial function along with reduced T cell proliferation and imbalanced immunoglobulin production. The findings may provide an important reference point for further clinical investigation and treatment of severe torpid dermatoses.

Isoform-Specific Properties of Orai Homologues in Activation, Downstream Signaling, Physiology and Pathophysiology

Ca2+ 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 Ca2+ release-activated Ca2+ (CRAC) channel. It consists of the Ca2+ sensing protein, stromal interaction molecule 1 (STIM1) located in the endoplasmic reticulum (ER) and a Ca2+ ion channel Orai in the plasma membrane. The Orai channel family includes three homologues Orai1, Orai2 and Orai3. While Orai1 is the "classical" Ca2+ 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 Ca2+ store depletion, which is sensed by STIM1 proteins, and induces their conformational change and oligomerization. Upon STIM1 coupling, Orai channels activate to allow Ca2+ 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 Orai Pore Opening Mechanism

Cell survival and normal cell function require a highly coordinated and precise regulation of basal cytosolic Ca²⁺ concentrations. The primary source of Ca²⁺ entry into the cell is mediated by the Ca²⁺ release-activated Ca²⁺ (CRAC) channel. Its action is stimulated in response to internal Ca²⁺ store depletion. The fundamental constituents of CRAC channels are the Ca²⁺ sensor, stromal interaction molecule 1 (STIM1) anchored in the endoplasmic reticulum, and a highly Ca²⁺-selective pore-forming subunit Orai1 in the plasma membrane. The precise nature of the Orai1 pore opening is currently a topic of intensive research. This review describes how Orai1 gating checkpoints in the middle and cytosolic extended transmembrane regions act together in a concerted manner to ensure an opening-permissive Orai1 channel conformation. In this context, we highlight the effects of the currently known multitude of Orai1 mutations, which led to the identification of a series of gating checkpoints and the determination of their role in diverse steps of the Orai1 activation cascade. The synergistic action of these gating checkpoints maintains an intact pore geometry, settles STIM1 coupling, and governs pore opening. We describe the current knowledge on Orai1 channel gating mechanisms and summarize still open questions of the STIM1-Orai1 machinery.

Review: Structure and Activation Mechanisms of CRAC Channels

Ca²⁺ release activated Ca²⁺ (CRAC) channels represent a primary pathway for Ca²⁺ to enter non-excitable cells. The two key players in this process are the stromal interaction molecule (STIM), a Ca²⁺ sensor embedded in the membrane of the endoplasmic reticulum, and Orai, a highly Ca²⁺ selective ion channel located in the plasma membrane. Upon depletion of the internal Ca²⁺ 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.

Critical parameters maintaining authentic CRAC channel hallmarks

Ca2+ ions represent versatile second messengers that regulate a huge diversity of processes throughout the cell's life. One prominent Ca2+ entry pathway into the cell is the Ca2+ release-activated Ca2+ (CRAC) ion channel. It is fully reconstituted by the two molecular key players: the stromal interaction molecule (STIM1) and Orai. STIM1 is a Ca2+ sensor located in the membrane of the endoplasmic reticulum, and Orai, a highly Ca2+ selective ion channel embedded in the plasma membrane. Ca2+ store-depletion leads initially to the activation of STIM1 which subsequently activates Orai channels via direct binding. Authentic CRAC channel hallmarks and biophysical characteristics include high Ca2+ selectivity with a reversal potential in the range of + 50 mV, small unitary conductance, fast Ca2+-dependent inactivation and enhancements in currents upon the switch from a Na+-containing divalent-free to a Ca2+-containing solution. This review provides an overview on the critical determinants and structures within the STIM1 and Orai proteins that establish these prominent CRAC channel characteristics.

Pore properties of Orai1 calcium channel dimers and their activation by the STIM1 ER calcium sensor

Store-operated Ca2+ entry signals are mediated by plasma membrane Orai channels activated through intermembrane coupling with Ca2+-sensing STIM proteins in the endoplasmic reticulum (ER). The nature of this elaborate Orai-gating mechanism has remained enigmatic. Based on the Drosophila Orai structure, mammalian Orai1 channels are hexamers comprising three dimeric subunit pairs. We utilized concatenated Orai1 dimers to probe the function of key domains within the channel pore and gating regions. The Orai1-E106Q selectivity-filter mutant, widely considered a dominant pore blocker, was surprisingly nondominant within concatenated heterodimers with Orai1-WT. The Orai1-E106Q/WT heterodimer formed STIM1-activated nonselective cation channels with significantly enlarged apparent pore diameter. Other Glu-106 substitutions entirely blocked the function of heterodimers with Orai1-WT. The hydrophobic pore-lining mutation V102C, which constitutively opens channels, was suppressed by Orai1-WT in the heterodimer. In contrast, the naturally occurring R91W pore-lining mutation associated with human immunodeficiency was completely dominant-negative over Orai-WT in heterodimers. Heterodimers containing the inhibitory K85E mutation extending outward from the pore helix gave an interesting partial effect on both channel activation and STIM1 binding, indicating an important allosteric link between the cytosolic Orai1 domains. The Orai1 C-terminal STIM1-binding domain mutation L273D powerfully blocked STIM1-induced channel activation. The Orai1-L273D/WT heterodimer had drastically impaired STIM1-induced channel gating but, unexpectedly, retained full STIM1 binding. This reveals the critical role of Leu-273 in transducing the STIM1-binding signal into the allosteric conformational change that initiates channel gating. Overall, our results provide important new insights into the role of key functional domains that mediate STIM1-induced gating of the Orai1 channel.

Functional Analysis of Orai1 Concatemers Supports a Hexameric Stoichiometry for the CRAC Channel

Store-operated Ca²⁺ entry occurs through the binding of the endoplasmic reticulum (ER) Ca²⁺ sensor STIM1 to Orai1, the pore-forming subunit of the Ca²⁺ release-activated Ca²⁺ (CRAC) channel. Although the essential steps leading to channel opening have been described, fundamental questions remain, including the functional stoichiometry of the CRAC channel. The crystal structure of Drosophila Orai indicates a hexameric stoichiometry, while studies of linked Orai1 concatemers and single-molecule photobleaching suggest that channels assemble as tetramers. We assessed CRAC channel stoichiometry by expressing hexameric concatemers of human Orai1 and comparing in detail their ionic currents to those of native CRAC channels and channels generated from monomeric Orai1 constructs. Cell surface biotinylation results indicated that Orai1 channels in the plasma membrane were assembled from intact hexameric polypeptides and not from truncated protein products. In addition, the L273D mutation depressed channel activity equally regardless of which Orai1 subunit in the concatemer carried the mutation. Thus, functional channels were generated from intact Orai1 hexamers in which all subunits contributed equally. These hexameric Orai1 channels displayed the biophysical fingerprint of native CRAC channels, including the distinguishing characteristics of gating (store-dependent activation, Ca²⁺-dependent inactivation, open probability), permeation (ion selectivity, affinity for Ca²⁺ block, La³⁺ sensitivity, unitary current magnitude), and pharmacology (enhancement and inhibition by 2-aminoethoxydiphenyl borate). Because permeation characteristics depend strongly on pore geometry, it is unlikely that hexameric and tetrameric pores would display identical Ca²⁺ affinity, ion selectivity, and unitary current magnitude. Thus, based on the highly similar pore properties of the hexameric Orai1 concatemer and native CRAC channels, we conclude that the CRAC channel functions as a hexamer of Orai1 subunits.

Increased expression of STIM1/Orai1 in platelets of stroke patients predictive of poor outcomes

BACKGROUND AND PURPOSE

The platelet activation that is mediated by store-operated Ca²⁺ entry via stromal interaction molecule (STIM1) and Orai1 Ca²⁺ influx channels has been shown to play a key role in ischaemic stroke (IS). This study aimed to evaluate the impact of platelet STIM1/Orai1 protein expression on outcomes of IS.

METHODS

A total of 160 patients with acute non-cardioembolic IS, among whom 45 patients had small-vessel diseases and 115 patients had large-vessel diseases, were evaluated. Patients were divided into two groups according to their baseline platelet STIM1/Orai1 protein expression: high-expression group (HG) (n = 80) and low-expression group (LG) (n = 80). Univariate and multivariate regression models were used to assess the correlation between STIM1/Orai1 expression and clinical outcomes, which included stroke severity that was measured based on the National Institutes of Health Stroke Scale and Stroke Impact Scale (SIS) at baseline and during the 3-month follow-up.

RESULTS

There were no significant differences in age, sex and cardiovascular risk factors between patients in HG and LG. However, HG had very high levels of biomarkers such as glycosylated hemoglobin, C-reactive protein, homocysteine and high mobility group box-1 protein (all P < 0.05). Although the baseline stroke severity (National Institutes of Health Stroke Scale score) was not obviously higher in HG than in LG, patients showed a better recovery score (SIS score) in LG than in HG (90.75 ± 13.65 vs. 80.68 ± 7.09; P = 0.022). STIM1/Orai1 expression was an independent predictor of the 3-month stroke recovery (hazard ratio, 4.543; 95% confidence interval, 1.941-29.145; P = 0.029).

CONCLUSIONS

A high expression level of platelet Orai1/STIMI1 was associated with poor clinical outcome (mortality and recurrence) and functional recovery (SIS scores) during the 3-month follow-up. Thus, we propose that these proteins are strongly predictive of life quality in patients with IS.

Cardiovascular and Hemostatic Disorders: SOCE and Ca2+ Handling in Platelet Dysfunction

Among the Ca²⁺ entry mechanisms in platelets, store-operated Ca²⁺ entry (SOCE) plays a prominent role as it is necessary to achieve full activation of platelet functions and replenish intracellular Ca²⁺ stores. In platelets, as in other non-excitable cells, SOCE has been reported to involve the activation of plasma membrane channels by the ER Ca²⁺ sensor STIM1. Despite electrophysiological studies are not possible in human platelets, indirect analyses have revealed that the Ca²⁺-permeable channels involve Orai1 and, most likely, TRPC1 subunits. A relevant role for the latter has not been found in mouse platelets. There is a body of evidence revealing a number of abnormalities in SOCE or in its molecular regulators that result in qualitative platelet disorders and, as a consequence, altered platelet responsiveness upon stimulation with multiple physiological agonists. Platelet SOCE abnormalities include STIM1 and Orai1 mutations. This chapter summarizes the current knowledge in this field, as well as the disorders associated to platelet SOCE dysfunction.

The Orai1 Store-operated Calcium Channel Functions as a Hexamer

Orai channels mediate store-operated Ca²⁺ signals crucial in regulating transcription in many cell types, and implicated in numerous immunological and inflammatory disorders. Despite their central importance, controversy surrounds the basic subunit structure of Orai channels, with several biochemical and biophysical studies suggesting a tetrameric structure yet crystallographic evidence indicating a hexamer. We systematically investigated the subunit configuration of the functional Orai1 channel, generating a series of tdTomato-tagged concatenated Orai1 channel constructs (dimers to hexamers) expressed in CRISPR-derived ORAI1 knock-out HEK cells, stably expressing STIM1-YFP. Surface biotinylation demonstrated that the full-length concatemers were surface membrane-expressed. Unexpectedly, Orai1 dimers, trimers, tetramers, pentamers, and hexamers all mediated similar and substantial store-operated Ca²⁺ entry. Moreover, each Orai1 concatemer mediated Ca²⁺ currents with inward rectification and reversal potentials almost identical to those observed with expressed Orai1 monomer. In Orai1 tetramers, subunit-specific replacement with Orai1 E106A "pore-inactive" subunits revealed that functional channels utilize only the N-terminal dimer from the tetramer. In contrast, Orai1 E106A replacement in Orai1 hexamers established that all the subunits can contribute to channel formation, indicating a hexameric channel configuration. The critical Ca²⁺ selectivity filter-forming Glu-106 residue may mediate Orai1 channel assembly around a central Ca²⁺ ion within the pore. Thus, multiple E106A substitutions in the Orai1 hexamer may promote an alternative "trimer-of-dimers" channel configuration in which the C-terminal E106A subunits are excluded from the hexameric core. Our results argue strongly against a tetrameric configuration for Orai1 channels and indicate that the Orai1 channel functions as a hexamer.

Pharmacodynamic evaluation of RP3128, a novel and potent CRAC channel inhibitor in guinea pig models of allergic asthma

The increase in intracellular Ca(2+) levels through the activation of Ca(2+) release-activated Ca(2+) (CRAC) channels is essential for mediating a wide scale of immune cell responses. Emerging evidence indicates an involvement of abnormal CRAC channel activity in human diseases such as certain types of immunodeficiency, autoimmunity and allergic disorders. This objective of this study was to evaluate the therapeutic potency of a novel CRAC channel inhibitor, RP3128, in experimental models of allergic asthma using guinea pigs. Ovalbumin-induced allergic airway inflammation was determined upon acute and long-term (14 days) oral administration of RP3128. In vivo changes in specific airways resistance (sRaw) and amplitude of isometric contraction (mN) of ASM (in vitro) were estimated to evaluate bronchodilatory effect upon acute and long-term administration of RP3128 or salbutamol. Exhaled nitric oxide (eNO), immunohistochemical and histological analysis of cellular infiltration in airways tissue, and levels of cytokines in plasma as well as bronchoalveolar lavage fluid (BALF), were determined using Bio-Plex® 200 System (BIO-RAD, USA). Ciliary beat frequency (CBF, in Hz) was estimated using a high-speed video camera and LabVIEW™ Software. Additionally, the impact of RP3128 and budesonide on mucociliary clearance was determined. Acute and long-term administration of RP3128 resulted in significant bronchodilation. Long-term administration of RP3128 exceeded the bronchodilatory effect of salbutamol and significantly decreased eNO and cytokine levels in plasma and BALF, which together with histological and immunohistochemical analysis validated its anti-inflammatory effect compared to budesonide. Data demonstrate the therapeutic potential of RP3128 in respiratory diseases causally associated with allergic inflammation.

Orai1 expression pattern in tooth and craniofacial ectodermal tissues and potential functions during ameloblast differentiation

BACKGROUND

Orai1 is a plasma membrane protein that forms the pore of the calcium release activated calcium channel. Humans with mutated Orai1 present with hereditary combined immunodeficiency, congenital myopathy and anhidrotic ectodermal dysplasia. Consistent with the ectodermal dysplasia phenotype, enamel formation and mineralization is also abnormal in Orai1 deficient patients. The expression pattern and potential functions of Orai1 in enamel formation remains unclear. To contribute toward understanding the role of Orai1 in amelogenesis we characterized ORAI1 protein developmental pattern in comparison with other ectodermal organs. We also examined the effects of Orai1 down-regulation in ameloblast cell proliferation and differentiation.

RESULTS

Our data show strong expression of ORAI1 protein during the ameloblast secretory stage, which weans at the end of the maturation stage. In salivary glands, ORAI1 is expressed mainly in acini cells. ORAI1 expression is also found in hair follicle and oral epithelium. Knockdown of Orai1 expression decreases cell proliferation and results in RNA expression levels changes of key ameloblast genes regulating enamel thickness and mineralization.

CONCLUSIONS

This study provides insights in the anhidrotic ectodermal dysplasia phenotype due to Orai1 mutation and highlights the importance of calcium signaling in controlling ameloblast differentiation and maturation during tooth development.

How many Orai's does it take to make a CRAC channel?

CRAC (Calcium Release-Activated Calcium) channels represent the primary pathway for so-called “store-operated calcium entry” – the cellular entry of calcium induced by depletion of intracellular calcium stores. These channels play a key role in diverse cellular activities, most noticeably in the differentiation and activation of Tcells, and in the response of mast cells to inflammatory signals. CRAC channels are formed by members of the recently discovered Orai protein family, with previous studies indicating that the functional channel is formed by a tetramer of Orai subunits. However, a recent report has shown that crystals obtained from the purified Drosophila Orai protein display a hexameric channel structure. Here, by comparing the biophysical properties of concatenated hexameric and tetrameric human Orai1 channels expressed in HEK293 cells, we show that the tetrameric channel displays the highly calcium-selective conductance properties consistent with endogenous CRAC channels, whilst the hexameric construct forms an essentially non-selective cation channel.

The STIM1/Orai signaling machinery

Ca(2+) influx via store-operated Ca(2+) release activated Ca(2+) (CRAC) channels represents a main signaling pathway for T-cell activation as well as mast-cell degranulation. The ER-located Ca(2+)-sensor, STIM1 and the Ca(2+)-selective ion pore, Orai1 in the membrane are sufficient to fully reconstitute CRAC currents. Their identification, but even more the recent structural resolution of both proteins by X-ray crystallography has substantially advanced the understanding of the activation mechanism of CRAC channels. In this review, we provide a detailed description of the STIM1/Orai1 signaling pathway thereby focusing on the critical domains mediating both, intra- as well as intermolecular interactions and on the ion permeation pathway. Based on the results of functional studies as well as the recently published crystal structures, we portray a mechanistic view of the steps in the CRAC channel signaling cascade ranging from STIM1 oligomerization over STIM1-Orai1 coupling to the ultimate Orai1 channel activation and permeation.

Functional T cell immunodeficiencies (with T cells present)

Severe combined immunodeficiency (SCID) comprises a group of disorders that are fatal owing to genetic defects that abrogate T cell development. Numerous related defects have recently been identified that allow T cell development but that compromise T cell function by affecting proximal or distal steps in intracellular signaling. These functional T cell immunodeficiencies are characterized by immune dysregulation and increased risk of malignancies, in addition to infections. The study of patients with these rare conditions, and of corresponding animal models, illustrates the importance of intracellular signaling to maintain T cell homeostasis.

Structure and function of endoplasmic reticulum STIM calcium sensors

Store-operated calcium (Ca(2+)) entry (SOCE) is a vital Ca(2+) signaling pathway in nonexcitable as well as electrically excitable cells, regulating countless physiological and pathophysiological pathways. Stromal interaction molecules (STIMs) are the principal regulating molecules of SOCE, sensing changes in sarco-/endoplasmic reticulum (S/ER) luminal Ca(2+) levels and directly interacting with the Orai channel subunits to orchestrate the opening of Ca(2+) release-activated Ca(2+) (CRAC) channels. Recent atomic resolution structures on human STIM1 and STIM2 have illuminated critical mechanisms of STIM function in SOCE; further, the first high-resolution structure of the Drosophila melanogaster Orai channel has revealed vital data on the atomic composition of the CRAC channel pore and the assembly of individual Orai subunits. This chapter focuses on the mechanistic information garnered from these high-resolution structures and the supporting biophysical, biochemical, and live cell work that has enhanced our understanding of the relationship between STIM and Orai structural features and CRAC channel function.

Orais and STIMs: physiological mechanisms and disease

The stromal interaction molecules STIM1 and STIM2 are Ca²⁺ sensors, mostly located in the endoplasmic reticulum, that detect changes in the intraluminal Ca²⁺ concentration and communicate this information to plasma membrane store-operated channels, including members of the Orai family, thus mediating store-operated Ca²⁺ entry (SOCE). Orai and STIM proteins are almost ubiquitously expressed in human cells, where SOCE has been reported to play a relevant functional role. The phenotype of patients bearing mutations in STIM and Orai proteins, together with models of STIM or Orai deficiency in mice, as well as other organisms such as Drosophila melanogaster, have provided compelling evidence on the relevant role of these proteins in cellular physiology and pathology. Orai1-deficient patients suffer from severe immunodeficiency, congenital myopathy, chronic pulmonary disease, anhydrotic ectodermal dysplasia and defective dental enamel calcification. STIM1-deficient patients showed similar abnormalities, as well as autoimmune disorders. This review summarizes the current evidence that identifies and explains diseases induced by disturbances in SOCE due to deficiencies or mutations in Orai and STIM proteins.

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.

Structure, regulation and biophysics of I(CRAC), STIM/Orai1

Ca(2+) release activated Ca(2+) (CRAC) channels mediate robust Ca(2+) influx when the endoplasmic reticulum Ca(2+) stores are depleted. This essential process for T-cell activation as well as degranulation of mast cells involves the Ca(2+) sensor STIM1, located in the endoplasmic reticulum and the Ca(2+) selective Orai1 channel in the plasma membrane. Our review describes the CRAC signaling pathway, the activation of which is initiated by a drop in the endoplasmic Ca(2+) level sensed by STIM1. This in term induces multimerisation and puncta-formation of STIM1 proteins is followed by their coupling to and activation of Orai channels. Consequently Ca(2+) entry is triggered through the Orai pore into the cytosol with subsequent closure of the channel by Ca(2+)-dependent inactivation. We will portray a mechanistic view of the events coupling STIM1 to Orai activation based on their structure and biophysics.

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.

Mutations in Orai1 transmembrane segment 1 cause STIM1-independent activation of Orai1 channels at glycine 98 and channel closure at arginine 91

Stim and Orai proteins comprise the molecular machinery of Ca(2+) release-activated Ca(2+) (CRAC) channels. As an approach toward understanding the gating of Orai1 channels, we investigated effects of selected mutations at two conserved sites in the first transmembrane segment (TM1): arginine 91 located near the cytosolic end of TM1 and glycine 98 near the middle of TM1. Orai1 R91C, when coexpressed with STIM1, was activated normally by Ca(2+)-store depletion. Treatment with diamide, a thiol-oxidizing agent, induced formation of disulfide bonds between R91C residues in adjacent Orai1 subunits and rapidly blocked STIM1-operated Ca(2+) current. Diamide-induced blocking was reversed by disulfide bond-reducing agents. These results indicate that R91 forms a very narrow part of the conducting pore at the cytosolic side. Alanine replacement at G98 prevented STIM1-induced channel activity. Interestingly, mutation to aspartate (G98D) or proline (G98P) caused constitutive channel activation in a STIM1-independent manner. Both Orai1 G98 mutants formed a nonselective Ca(2+)-permeable conductance that was relatively resistant to block by Gd(3+). The double mutant R91W/G98D was also constitutively active, overcoming the normal inhibition of channel activity by tryptophan at the 91 position found in some patients with severe combined immunodeficiency (SCID), and the double mutant R91C/G98D was resistant to diamide block. These data suggest that the channel pore is widened and ion selectivity is altered by mutations at the G98 site that may perturb α-helical structure. We propose distinct functional roles for G98 as a gating hinge and R91 as part of the physical gate at the narrow inner mouth of the channel.

Subunit stoichiometry of human Orai1 and Orai3 channels in closed and open states

We applied single-molecule photobleaching to investigate the stoichiometry of human Orai1 and Orai3 channels tagged with eGFP and expressed in mammalian cells. Orai1 was detected predominantly as dimers under resting conditions and as tetramers when coexpressed with C-STIM1 to activate Ca(2+) influx. Orai1 was also found to be tetrameric when coexpressed with STIM1 and evaluated following fixation. We show that fixation rapidly causes release of Ca(2+), redistribution of STIM1 to the plasma membrane, and STIM1/Orai1 puncta formation, and may cause the channel to be in the activated state. Consistent with this possibility, Orai1 was found predominantly as a dimer when coexpressed with STIM1 in living cells under resting conditions. We further show that Orai3, like Orai1, is dimeric under resting conditions and is predominantly tetrameric when activated by C-STIM1. Interestingly, a dimeric Orai3 stoichiometry was found both before and during application of 2-aminoethyldiphenyl borate (2-APB) to activate a nonselective cation conductance in its STIM1-independent mode. We conclude that the human Orai1 and Orai3 channels undergo a dimer-to-tetramer transition to form a Ca(2+)-selective pore during store-operated activation and that Orai3 forms a dimeric nonselective cation pore upon activation by 2-APB.

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

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

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.

The N-terminal domain of Orai3 determines selectivity for activation of the store-independent ARC channel by arachidonic acid

Although highly selective Ca²(+) entry pathways play a critical role in agonist-activated Ca²(+) signals in non-excitable cells, only with the recent discovery of the Orai proteins have the first insights into the molecular nature of these pathways been possible. To date, just two such highly Ca²(+)-selective "Orai channels" have been identified in native cells - the store-operated CRAC channels and the store-independent, arachidonic acid-activated ARC channels. Studies have shown that the functional CRAC channel pore is formed by a tetrameric arrangement of Orai1 subunits, whilst a heteropentamer of three Orai1 subunits and two Orai3 subunits forms the functional ARC channel pore. Importantly, this inclusion of Orai3 subunits in the ARC channel structure has been shown to play a specific role in determining the selectivity of these channels for activation by arachidonic acid. Using an approach based on the expression of various concatenated constructs, we examined the basis for this Orai3-dependent effect on selectivity for arachidonic acid. We show that, whilst heteropentamers containing only one Orai3 subunit are sensitive to arachidonic acid, specific selectivity for activation by this fatty acid is only achieved on inclusion of the second Orai3 subunit in the pentamer. Further studies identified the cytosolic N-terminal domain of Orai3 as the region specifically responsible for this switch in selectivity. Substitution of just this domain into an otherwise complete single Orai1 subunit within a concatenated 31111 pentamer is sufficient to change the resulting channel from one that is predominantly store-operated, to one that is exclusively activated by arachidonic acid.

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.

Pore architecture of the ORAI1 store-operated calcium channel

ORAI1 is the pore-forming subunit of the calcium release-activated calcium (CRAC) channel, a store-operated channel that is central to Ca(2+) signaling in mammalian cells. Electrophysiological data have shown that the acidic residues E106 in transmembrane helix 1 (TM1) and E190 in TM3 contribute to the high selectivity of ORAI1 channels for Ca(2+). We have examined the pore architecture of the ORAI1 channel using ORAI1 proteins engineered to contain either one or two cysteine residues. Disulfide cross-linking shows that ORAI1 assembles as a tetramer or a higher oligomer with TM1 centrally located. Cysteine side chains projecting from TM1 at position 88, 95, 102, or 106 cross-link efficiently to the corresponding side chain in a second ORAI1 monomer. Cysteine residues at position 190 or at surrounding positions in TM3 do not cross-link. We conclude that E106 residues in wild-type ORAI1 are positioned to form a Ca(2+) binding site in the channel pore and that E190 interacts less directly with ions traversing the pore. The cross-linking data further identify a relatively rigid segment of TM1 adjacent to E106 that is likely to contribute to the selectivity filter.

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

Store-operated Ca2+ entry (SOCE) is a mechanism used by many cells types including lymphocytes and other immune cells to increase intracellular Ca2+ concentrations to initiate signal transduction. Activation of immunoreceptors such as the T-cell receptor, B-cell receptor, or Fc receptors results in the release of Ca2+ ions from endoplasmic reticulum (ER) Ca2+ stores and subsequent activation of plasma membrane Ca2+ channels such as the well-characterized Ca2+ release-activated Ca2+ (CRAC) channel. Two genes have been identified that are essential for SOCE: ORAI1 as the pore-forming subunit of the CRAC channel in the plasma membrane and stromal interaction molecule-1 (STIM1) sensing the ER Ca2+ concentration and activating ORAI1-CRAC channels. Intense efforts in the past several years have focused on understanding the molecular mechanism of SOCE and the role it plays for cell functions in vitro and in vivo. A number of transgenic mouse models have been generated to investigate the role of ORAI1 and STIM1 in immunity. In addition, mutations in ORAI1 and STIM1 identified in immunodeficient patients provide valuable insight into the role of both genes and SOCE. This review focuses on the role of ORAI1 and STIM1 in vivo, discussing the phenotypes of ORAI1- and STIM1-deficient human patients and mice.

Increased hydrophobicity at the N terminus/membrane interface impairs gating of the severe combined immunodeficiency-related ORAI1 mutant

Patients with severe combined immune deficiency (SCID) suffer from defective T-cell Ca2+ signaling. A loss of Ca2+ entry has been linked at the molecular level to single missense mutation R91W in the store-operated Ca2+ channel ORAI1. However, the mechanistic impact of this mutation on ORAI1 function remains unclear. Confocal Förster resonance energy transfer microscopy revealed that dynamic store-operated coupling of STIM1 to ORAI1 R91W was largely sustained similar to wild-type ORAI1. Characterization of various point mutants at position 91 by whole cell patch clamp recordings displayed that neutral or even negatively charged amino acids did not abolish ORAI1 function. However, substitution by hydrophobic leucine, valine, or phenylalanine resulted in non-functional ORAI1 channels, despite preserved STIM1 coupling. Besides conformational constraints at the N terminus/membrane interface predicted for the hydrophobic mutants, additional key factor(s) were suggested to determine ORAI1 functionality. Calculation of the probability for the 1st transmembrane domain and its hydrophobicity revealed a substantial increase for all hydrophobic substitutions that lead to non-functional ORAI1 R91X mutants in contrast to those with hydrophilic residues. Hence, increased hydrophobicity might lead to disrupted permeation/gating, as an ORAI1 channel with increased pore size and R91W mutation failed to recover activity. In conclusion, the increase in hydrophobicity at the N terminus/membrane interface represents the major cause for yielding non-functional ORAI1 channels.

Signal transduction--receptors, mediators, and genes

The 2008 annual meeting of the Signal Transduction Society covered a broad spectrum of topics, with signaling in immune cells as the special focus of the meeting. Many of the immune signaling talks concerned B and T lymphocytes in particular; the role of inflammatory cytokines in cancer progression was also addressed. Neoplastic development was also discussed with regard to aspects of cell cycle control, aging, and transformation. Topics extended to signaling pathways induced by bacteria, viruses, and environmental toxins, as well as those involved in differentiation, morphogenesis, and cell death. This international and interdisciplinary scientific gathering induced lively discussions and close interactions between participants.