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Background Ca 2+ release-activated Ca 2+ (CRAC) channels are highly Ca 2+ selective plasma membrane channels formed by the hexameric assembly of Orai subunits, with a predominant role for Orai1. Two Orai1 variants have been identified, Orai1α, which comprises 301 amino acids, and a short variant, Orai1β, lacking the first N-terminal 63 or 71 amino acids; however, little is known about their possible heteromerization to form CRAC channels. Here we show that Orai1α and Orai1β exhibit different lipid raft distributions in resting cells when expressed individually, likely due to the presence of a caveolin-binding domain exclusively in Orai1α. However, when both variants are co-expressed, they show a similar distribution predominantly in the lipid raft domains, indicating potential interaction between the two Orai1 forms. Methods A lipid raft isolation protocol in combination with Western blotting assay was conducted to detect the expression of each Orai1 variants in the isolated membrane fractions. Ca 2+ mobilization was determined using fura-2 and G-GECO1.2 fused to Orai1α fluorescence. Evidence of physical interaction between both Orai1 variants was provided using co-immunoprecipitation, APEX2 peroxidase-catalyzed proximity labeling, Förster resonance energy transfer (FRET) and super-resolution microscopy. Results Our results indicate that Orai1α and Orai1β exhibit different lipid raft partitioning in resting cells when expressed individually, likely attributed to the presence of a caveolin-binding domain in Orai1α. However, when both variants are co-expressed, they show a similar distribution predominantly in the lipid raft domains, indicating potential interaction between the two Orai1 forms. Expression of a dominant-negative Orai1β mutant has been found to interfere with Orai1α-mediated Ca 2+ entry. Using co-immunoprecipitation, APEX2 peroxidase-catalyzed proximity labeling, Förster resonance energy transfer (FRET) and super-resolution microscopy our results indicate that there is certain interaction between Orai1α and Orai1β although both variants form mostly independent channels. Conclusions Our results indicate that while Orai1α and Orai1β mostly form separate CRAC channels, a small subset of both Orai1 variants combine to form heteromeric channels. These findings provide new insights on the nature of CRAC channels.

The pore-forming Orai1 protein is an essential component of store-operated calcium entry (SOCE), a process vital to diverse cellular and physiological functions. Mutations in human Orai1 cause severe immunodeficiencies and myopathies, yet structural insights have remained largely elusive. To address this, we studied the structure of detergent-solubilized human Orai1 (hOrai1) by cryo-electron microscopy. While the overall resolution is moderate, the reconstructed map confirms a conserved hexameric architecture and enables assignment of transmembrane helices. We observed profound structural heterogeneity, with particles adopting both C6- and C2-symmetric conformations, indicative of dynamic rearrangements. This study establishes a framework for future structural and mechanistic studies of hOrai1.

Calcium signals are initiated in immune cells by the process of store-operated calcium entry (SOCE), where receptor activation triggers transient calcium release from the endoplasmic reticulum, followed by opening of plasma-membrane calcium-release activated calcium (CRAC) channels. ORAI1, ORAI2, and ORAI3 are known to comprise the CRAC channel; however, the contributions of individual isoforms to neutrophil function are not well understood. Here, we show that loss of ORAI1 partially decreases calcium influx, while loss of both ORAI1 and ORAI2 completely abolishes SOCE. In other immune-cell types, loss of ORAI2 enhances SOCE. In contrast, we find that ORAI2-deficient neutrophils display decreased calcium influx, which is correlated with measurable differences in the regulation of neutrophil membrane potential via KCa3.1. Decreased SOCE in ORAI1-, ORAI2-, and ORAI1/2-deficient neutrophils impairs multiple neutrophil functions, including phagocytosis, degranulation, leukotriene, and reactive oxygen species (ROS) production, rendering ORAI1/2-deficient mice highly susceptible to staphylococcal infection. This study demonstrates that ORAI1 and ORAI2 are the primary components of the neutrophil CRAC channel and identifies subpopulations of neutrophils where cell-membrane potential functions as a rheostat to modulate the SOCE response. These findings have implications for mechanisms that modulate neutrophil function during infection, acute and chronic inflammatory conditions, and cancer.

Acute myeloid leukemia (AML) is a hematological malignancy with a high risk of relapse. This issue is associated with the development of mechanisms leading to drug resistance that are not yet fully understood. In this context, we previously showed the clinical significance of the ATP binding cassette subfamily B-member 1 (ABCB1) in AML patients, namely its association with stemness markers and an overall worth prognosis. Calcium signaling dysregulations affect numerous cellular functions and are associated with the development of the hallmarks of cancer. However, in AML, calcium-dependent signaling pathways remain poorly investigated. With this study, we show the involvement of the ORAI1 calcium channel in store-operated calcium entry (SOCE), the main calcium entry pathway in non-excitable cells, in two representative human AML cell lines (KG1 and U937) and in primary cells isolated from patients. Moreover, our data suggest that in these models, SOCE varies according to the differentiation status, ABCB1 activity level and leukemic stem cell (LSC) proportion. Finally, we present evidence that ORAI1 expression and SOCE amplitude are modulated during the establishment of an apoptosis resistance phenotype elicited by the chemotherapeutic drug Ara-C. Our results therefore suggest ORAI1/SOCE as potential markers of AML progression and drug resistance apparition.

The transmembrane docking of endoplasmic reticulum (ER) Ca2+-sensing STIM proteins with plasma membrane (PM) Orai Ca2+ channels is a critical but poorly understood step in Ca2+ signal generation. STIM1 protein dimers unfold to expose a discrete STIM–Orai activating region (SOAR1) that tethers and activates Orai1 channels within discrete ER–PM junctions. We reveal that each monomer within the SOAR dimer interacts independently with single Orai1 subunits to mediate cross-linking between Orai1 channels. Superresolution imaging and mobility measured by fluorescence recovery after photobleaching reveal that SOAR dimer cross-linking leads to substantial Orai1 channel clustering, resulting in increased efficacy and cooperativity of Orai1 channel function. A concatenated SOAR1 heterodimer containing one monomer point mutated at its critical Orai1 binding residue (F394H), although fully activating Orai channels, is completely defective in cross-linking Orai1 channels. Importantly, the naturally occurring STIM2 variant, STIM2.1, has an eight-amino acid insert in its SOAR unit that renders it functionally identical to the F394H mutant in SOAR1. Contrary to earlier predictions, the SOAR1–SOAR2.1 heterodimer fully activates Orai1 channels but prevents cross-linking and clustering of channels. Interestingly, combined expression of full-length STIM1 with STIM2.1 in a 5:1 ratio causes suppression of sustained agonist-induced Ca2+ oscillations and protects cells from Ca2+ overload, resulting from high agonist-induced Ca2+ release. Thus, STIM2.1 exerts a powerful regulatory effect on signal generation likely through preventing Orai1 channel cross-linking. Overall, STIM-mediated cross-linking of Orai1 channels is a hitherto unrecognized functional paradigm that likely provides an organizational microenvironment within ER–PM junctions with important functional impact on Ca2+ signal generation.

The identification of two variants of the canonical pore-forming subunit of the Ca 2+ release-activated Ca 2+ (CRAC) channel Orai1, Orai1α and Orai1β, in mammalian cells arises the question whether they exhibit different functional characteristics. Orai1α and Orai1β differ in the N-terminal 63 amino acids, exclusive of Orai1α, and show different sensitivities to Ca 2+ -dependent inactivation, as well as distinct ability to form arachidonate-regulated channels. We have evaluated the role of both Orai1 variants in the activation of TRPC1 in HeLa cells. We found that Orai1α and Orai1β are required for the maintenance of regenerative Ca 2+ oscillations, while TRPC1 plays a role in agonist-induced Ca 2+ influx but is not essential for Ca 2+ oscillations. Using APEX2 proximity labeling, co-immunoprecipitation and the fluorescence of G-GECO1.2 fused to Orai1α our results indicate that agonist stimulation and Ca 2+ store depletion enhance Orai1α–TRPC1 interaction. Orai1α is essential for TRPC1 plasma membrane location and activation. Thus, TRPC1 function in HeLa cells depends on Ca 2+ influx through Orai1α exclusively.

Orai1, the first identified member of the Orai protein family, is ubiquitously expressed in the animal kingdom. Orai1 was initially characterized as the channel responsible for the store-operated calcium entry (SOCE), a major mechanism that allows cytosolic calcium concentration increments upon receptor-mediated IP3 generation, which results in intracellular Ca2+ store depletion. Furthermore, current evidence supports that abnormal Orai1 expression or function underlies several disorders. Orai1 is, together with STIM1, the key element of SOCE, conducting the Ca2+ release-activated Ca2+ (CRAC) current and, in association with TRPC1, the store-operated Ca2+ (SOC) current. Additionally, Orai1 is involved in non-capacitative pathways, as the arachidonate-regulated or LTC4-regulated Ca2+ channel (ARC/LRC), store-independent Ca2+ influx activated by the secretory pathway Ca2+-ATPase (SPCA2) and the small conductance Ca2+-activated K+ channel 3 (SK3). Furthermore, Orai1 possesses two variants, Orai1α and Orai1β, the latter lacking 63 amino acids in the N-terminus as compared to the full-length Orai1α form, which confers distinct features to each variant. Here, we review the current knowledge about the differences between Orai1α and Orai1β, the implications of the Ca2+ signals triggered by each variant, and their downstream modulatory effect within the cell.

Store-operated Ca2+ entry (SOCE) represents a predominant Ca2+ influx pathway in non-excitable cells. SOCE is required for immune cell activation and is mediated by the plasma membrane (PM) channel ORAI1 and the endoplasmic reticulum (ER) Ca2+ sensor STIM1. Mutations in the Orai1 or STIM1 genes abolish SOCE leading to combined immunodeficiency (CID), muscular hypotonia, and anhidrotic ectodermal dysplasia. Here, we identify a novel autosomal recessive mutation in ORAI1 in a child with CID. The patient is homozygous for p.C126R mutation in the second transmembrane domain (TM2) of ORAI1, a region with no previous loss-of-function mutations. SOCE is suppressed in the patient’s lymphocytes, which is associated with impaired T cell proliferation and cytokine production. Functional analyses demonstrate that the p.C126R mutation does not alter protein expression but disrupts ORAI1 trafficking. Orai1-C126R does not insert properly into the bilayer resulting in ER retention. Insertion of an Arg on the opposite face of TM2 (L135R) also results in defective folding and trafficking. We conclude that positive side chains within ORAI1 TM2 are not tolerated and result in misfolding, defective bilayer insertion, and channel trafficking thus abolishing SOCE and resulting in CID.

ORAI1 constitutes the store-operated Ca 2+ release-activated Ca 2+ (CRAC) channel crucial for life. Whereas ORAI1 activation by Ca 2+ -sensing STIM proteins is known, still obscure is how ORAI1 is turned off through Ca 2+ -dependent inactivation (CDI), protecting against Ca 2+ toxicity. Here we identify a spatially-restricted Ca 2+ /cAMP signaling crosstalk critical for mediating CDI. Binding of Ca 2+ -activated adenylyl cyclase 8 (AC8) to the N-terminus of ORAI1 positions AC8 near the mouth of ORAI1 for sensing Ca 2+ . Ca 2+ permeating ORAI1 activates AC8 to generate cAMP and activate PKA. PKA, positioned by AKAP79 near ORAI1, phosphorylates serine-34 in ORAI1 pore extension to induce CDI whereas recruitment of the phosphatase calcineurin antagonizes the effect of PKA. Notably, CDI shapes ORAI1 cytosolic Ca 2+ signature to determine the isoform and degree of NFAT activation. Thus, we uncover a mechanism of ORAI1 inactivation, and reveal a hitherto unappreciated role for inactivation in shaping cellular Ca 2+ signals and NFAT activation. ORAI1 constitutes the store-operated Ca 2+ release-activated Ca 2+ (CRAC) channel, but how this channel is turned off through Ca 2+ -dependent inactivation (CDI) remained unclear. Here the authors identify a spatially-restricted Ca 2+ /cAMP signaling crosstalk critical for mediating CDI which in turn regulates cellular Ca 2+ signals and NFAT activation.

Pulmonary arterial hypertension (PAH) is a rare and incurable disease characterized by progressive narrowing of pulmonary arteries (PA), resulting in right ventricular (RV) hypertrophy, RV failure, and eventually death. Orai1 inhibition has emerged as promising therapeutic approach to mitigate PAH. In this study, we investigated the efficacy of a clinically applicable selective Orai1 inhibitor, CM5480, and its effects when combined with standard PAH therapies in a preclinical PAH model. In male and female monocrotaline PAH-rats, CM5480 monotherapy improved hemodynamics, PA, and RV remodeling, as confirmed by RV catheterization, echocardiography, histology, and unbiased RNA-Seq. Standard PAH therapies, ambrisentan or sildenafil, achieved modest improvements in experimental PAH. In contrast, combination therapies with CM5480 yielded significantly greater benefits in reducing PA remodeling and improving cardiac function compared with monotherapies. Furthermore, in vitro experiments showed that Orai1 knockdown reduced pulmonary endothelial cell dysfunction in PAH and that the Orai1 pathway is independent of standard PAH-targeted pathways in PA smooth muscle cells (PASMCs). Finally, we found enhanced Orai1 expression/function in PASMCs and pulmonary vein SMCs from patients with pulmonary veno-occlusive disease. These findings suggest that Orai1 inhibition represents a potentially novel and complementary therapeutic strategy for PAH by acting at pulmonary vascular and RV levels.