Omnitemporal choreographies of all five STIM/Orai and IP3Rs underlie the complexity of mammalian Ca2+ signaling

The MCU and MCUb amino-terminal domains tightly interact: mechanisms for low conductance assembly of the mitochondrial calcium uniporter complex

The mitochondrial calcium (Ca2+) uniporter (MCU) complex is regulated via integration of the MCU dominant negative beta subunit (MCUb), a low conductance paralog of the main MCU pore forming protein. The MCU amino (N)-terminal domain (NTD) also modulates channel function through cation binding to the MCU regulating acidic patch (MRAP). MCU and MCUb have high sequence similarities, yet the structural and functional roles of MCUb-NTD remain unknown. Here, we report that MCUb-NTD exhibits α-helix/β-sheet structure with a high thermal stability, dependent on protein concentration. Remarkably, MCU- and MCUb-NTDs heteromerically interact with ∼nM affinity, increasing secondary structure and stability and structurally perturbing MRAP. Further, we demonstrate MCU and MCUb co-localization is suppressed upon NTD deletion concomitant with increased mitochondrial Ca2+ uptake. Collectively, our data show that MCU:MCUb NTD tight interactions are promoted by enhanced regular structure and stability, augmenting MCU:MCUb co-localization, lowering mitochondrial Ca2+ uptake and implicating an MRAP-sensing mechanism.

Postbiotics of Lacticaseibacillus paracasei CECT 9610 and Lactiplantibacillus plantarum CECT 9608 attenuates store-operated calcium entry and FAK phosphorylation in colorectal cancer cells

Store-operated Ca2+ entry (SOCE) is a major mechanism for Ca2+ influx in colorectal cancer (CRC) cells. This mechanism, regulated by the filling state of the intracellular Ca2+ stores, is mediated by the endoplasmic reticulum Ca2+ sensors of the stromal interaction molecules (STIM) family [stromal interaction molecule 1 (STIM1) and STIM2] and the Ca2+-release-activated Ca2+ channels constituted by Orai family members, with predominance of calcium release-activated calcium channel protein 1 (Orai1). CRC cells exhibit enhanced SOCE due to remodeling of the expression of the key SOCE molecular components. The enhanced SOCE supports a variety of cancer hallmarks. Here, we show that treatment of the colorectal adenocarcinoma cell lines HT-29 and Caco-2 with inanimate Lacticaseibacillus paracasei (CECT9610) and Lactiplantibacillus plantarum (CECT9608) attenuates SOCE, although no detectable effect is seen on SOCE in normal colon mucosa cells. The effect of Lacticaseibacillus paracasei and Lactiplantibacillus plantarum postbiotics was mediated by downregulation of Orai1 and STIM1, while the expression levels of Orai3 and STIM2 remained unaltered. Treatment of HT-29 and Caco-2 cells with inanimate Lacticaseibacillus paracasei and Lactiplantibacillus plantarum impairs in vitro migration by a mechanism likely involving attenuation of focal adhesion kinase (FAK) tyrosine phosphorylation. Cell treatment with the Orai1 inhibitor synta-66 attenuates SOCE and prevents any further effect of Lacticaseibacillus paracasei and Lactiplantibacillus plantarum postbiotics. Together, our results indicate for the first time that Lacticaseibacillus paracasei and Lactiplantibacillus plantarum postbiotics selectively exert negative effects on Ca2+ influx through SOCE in colorectal adenocarcinoma cell lines, providing evidence for an attractive strategy against CRC.

Orai1/STIMs modulators in pulmonary vascular diseases

Calcium (Ca2+) is a secondary messenger that regulates various cellular processes. However, Ca2+ mishandling could lead to pathological conditions. Orai1 is a Ca2+channel contributing to the store-operated calcium entry (SOCE) and plays a critical role in Ca2+ homeostasis in several cell types. Dysregulation of Orai1 contributed to severe combined immune deficiency syndrome, some cancers, pulmonary arterial hypertension (PAH), and other cardiorespiratory diseases. During its activation process, Orai1 is mainly regulated by stromal interacting molecule (STIM) proteins, especially STIM1; however, many other regulatory partners have also been recently described. Increasing knowledge about these regulatory partners provides a better view of the downstream signalling pathways of SOCE and offers an excellent opportunity to decipher Orai1 dysregulation in these diseases. These proteins participate in other cellular functions, making them attractive therapeutic targets. This review mainly focuses on Orai1 regulatory partners in the physiological and pathological conditions of the pulmonary circulation and inflammation.

Emergence of broad cytosolic Ca2+ oscillations in the absence of CRAC channels: A model for CRAC-mediated negative feedback on PLC and Ca2+ oscillations through PKC

The role of Ca2+ release-activated Ca2+ (CRAC) channels mediated by ORAI isoforms in calcium signalling has been extensively investigated. It has been shown that the presence or absence of different isoforms has a significant effect on store-operated calcium entry (SOCE). Yoast et al. (2020) showed that, in addition to the reported narrow-spike oscillations (whereby cytosolic calcium decreases quickly after a sharp increase), ORAI1 knockout HEK293 cells were able to oscillate with broad-spike oscillations (whereby cytosolic calcium decreases in a prolonged manner after a sharp increase) when stimulated with a muscarinic agonist. This suggests that Ca2+ influx through ORAI-mediated CRAC channels negatively regulates the duration of Ca2+ oscillations. We hypothesise that, through the activation of protein kinase C (PKC), ORAI1 negatively regulates phospholipase C (PLC) activity to decrease inositol 1,4,5-trisphosphate (IP3) production and limit the duration of agonist-evoked Ca2+ oscillations. Based on this hypothesis, we construct a new mathematical model, which shows that the formation of broad-spike oscillations is highly dependent on the absence of ORAI1. Predictions of this model are consistent with the experimental results.

Specific Inhibition of Orai1-mediated Calcium Signalling Resolves Inflammation and Clears Bacteria in an Acute Respiratory Distress Syndrome Model

Rationale: Acute respiratory distress syndrome (ARDS) has an unacceptably high mortality rate (35%) and is without effective therapy. Orai1 is a Ca2+ channel involved in store-operated Ca2+ entry (SOCE), a process that exquisitely regulates inflammation. Orai1 is considered a druggable target, but no Orai1-specific inhibitors exist to date. Objectives: To evaluate whether ELD607, a first-in-class Orai1 antagonist, can treat ARDS caused by bacterial pneumonia in preclinical models. Methods: ELD607 pharmacology was evaluated in HEK293T cells and freshly isolated immune cells from patients with ARDS. A murine acute lung injury model caused by bacterial pneumonia was then used: mice were infected with Pseudomonas aeruginosa, Staphylococcus aureus, methicillin-resistant S. aureus, or multidrug-resistant P. aeruginosa and then treated with ELD607 intranasally. Measurements and Main Results: ELD607 specifically inhibited SOCE in HEK293T cells with a half-maximal inhibitory concentration of 9 nM. ELD607 was stable in ARDS airway secretions and inhibited SOCE in ARDS immune cells. In vivo, inhaled ELD607 significantly reduced neutrophilia and improved survival. Surprisingly, Orai1 inhibition by ELD607 caused a significant reduction in lung bacteria, including methicillin-resistant S. aureus. ELD607 worked as an immunomodulator that reduced cytokine levels, reduced neutrophilia, and promoted macrophage-mediated resolution of inflammation and clearance of bacteria. Indeed, when alveolar macrophages were depleted with inhaled clodronate, ELD607 was no longer able to resolve inflammation or clear bacteria. Conclusions: These data indicate that specific Orai1 inhibition by ELD607 may be a novel approach to reduce multiorgan inflammation and treat antibiotic-resistant bacteria.

A dual-clock-driven model of lymphatic muscle cell pacemaking to emulate knock-out of Ano1 or IP3R

Lymphatic system defects are involved in a wide range of diseases, including obesity, cardiovascular disease, and neurological disorders, such as Alzheimer's disease. Fluid return through the lymphatic vascular system is primarily provided by contractions of muscle cells in the walls of lymphatic vessels, which are in turn driven by electrochemical oscillations that cause rhythmic action potentials and associated surges in intracellular calcium ion concentration. There is an incomplete understanding of the mechanisms involved in these repeated events, restricting the development of pharmacological treatments for dysfunction. Previously, we proposed a model where autonomous oscillations in the membrane potential (M-clock) drove passive oscillations in the calcium concentration (C-clock). In this paper, to model more accurately what is known about the underlying physiology, we extend this model to the case where the M-clock and the C-clock oscillators are both active but coupled together, thus both driving the action potentials. This extension results from modifications to the model's description of the IP3 receptor, a key C-clock mechanism. The synchronised dual-driving clock behaviour enables the model to match IP3 receptor knock-out data, thus resolving an issue with previous models. We also use phase-plane analysis to explain the mechanisms of coupling of the dual clocks. The model has the potential to help determine mechanisms and find targets for pharmacological treatment of some causes of lymphoedema.

A multiple-oscillator mechanism underlies antigen-induced Ca2+ oscillations in Jurkat T-cells

T-cell receptor stimulation triggers cytosolic Ca2+ signaling by inositol-1,4,5-trisphosphate (IP3)-mediated Ca2+ release from the endoplasmic reticulum (ER) and Ca2+ entry through Ca2+ release-activated Ca2+ (CRAC) channels gated by ER-located stromal-interacting molecules (STIM1/2). Physiologically, cytosolic Ca2+ signaling manifests as regenerative Ca2+ oscillations, which are critical for nuclear factor of activated T-cells-mediated transcription. In most cells, Ca2+ oscillations are thought to originate from IP3 receptor-mediated Ca2+ release, with CRAC channels indirectly sustaining them through ER refilling. Here, experimental and computational evidence support a multiple-oscillator mechanism in Jurkat T-cells whereby both IP3 receptor and CRAC channel activities oscillate and directly fuel antigen-evoked Ca2+ oscillations, with the CRAC channel being the major contributor. KO of either STIM1 or STIM2 significantly reduces CRAC channel activity. As such, STIM1 and STIM2 synergize for optimal Ca2+ oscillations and activation of nuclear factor of activated T-cells 1 and are essential for ER refilling. The loss of both STIM proteins abrogates CRAC channel activity, drastically reduces ER Ca2+ content, severely hampers cell proliferation and enhances cell death. These results clarify the mechanism and the contribution of STIM proteins to Ca2+ oscillations in T-cells.

Pivotal role of the ORAI3-STIM2 complex in the control of mitotic death and prostate cancer cell cycle progression

Prostate cancer (PCa) represents one of the most frequent diagnosed cancer in males worldwide. Due to routine screening tests and the efficiency of available treatments, PCa-related deaths have significantly decreased over the past decades. However, PCa remains a critical threat if detected at a late stage in which, cancer cells would have already detached from the primary tumor to spread and invade other parts of the body. Calcium (Ca2+) channels and their protein regulators are now considered as hallmarks of cancer and some of them have been well examined in PCa. Among these Ca2+ channels, isoform 3 of the ORAI channel family has been shown to regulate the proliferation of PCa cells via the Arachidonic Acid-mediated Ca2+ entry, requiring the involvement of STIM1 (Stromal Interaction Molecule 1). Still, no study has yet demonstrated a role of the "neglected" STIM2 isoform in PCa or if it may interact with ORAI3 to promote an oncogenic behavior. In this study, we demonstrate that ORAI3 and STIM2 are upregulated in human PCa tissues. In old KIMAP (Knock-In Mouse Prostate Adenocarcinoma) mice, ORAI3 and STIM2 mRNA levels were significantly higher than ORAI1 and STIM1. In vitro, we show that ORAI3-STIM2 interact under basal conditions in PC-3 cells. ORAI3 silencing increased Store Operated Ca2+ Entry (SOCE) and induced a significant increase of the cell population in G2/M phase of the cell cycle, consistent with the role of ORAI3 as a negative regulator of SOCE. Higher expression levels of CDK1-Y15/Cyclin B1 were detected and mitotic arrest-related death occurred after ORAI3 silencing, which resulted in activating Bax/Bcl-2-mediated apoptotic pathway and caspase-8 activation and cleavage. STIM2 and ORAI3 expression increased in M phase while STIM1 expression and SOCE amplitude significantly decreased. Taken together, ORAI3 -STIM2 complex allows a successful progression through mitosis of PCa cells by evading mitotic catastrophe.

Loss of STIM2 in colorectal cancer drives growth and metastasis through metabolic reprogramming and PERK-ATF4 endoplasmic reticulum stress pathway

The endoplasmic reticulum (ER) stores large amounts of calcium (Ca2+), and the controlled release of ER Ca2+ regulates a myriad of cellular functions. Although altered ER Ca2+ homeostasis is known to induce ER stress, the mechanisms by which ER Ca2+ imbalance activate ER stress pathways are poorly understood. Stromal-interacting molecules STIM1 and STIM2 are two structurally homologous ER-resident Ca2+ sensors that synergistically regulate Ca2+ influx into the cytosol through Orai Ca2+ channels for subsequent signaling to transcription and ER Ca2+ refilling. Here, we demonstrate that reduced STIM2, but not STIM1, in colorectal cancer (CRC) is associated with poor patient prognosis. Loss of STIM2 causes SERCA2-dependent increase in ER Ca2+, increased protein translation and transcriptional and metabolic rewiring supporting increased tumor size, invasion, and metastasis. Mechanistically, STIM2 loss activates cMyc and the PERK/ATF4 branch of ER stress in an Orai-independent manner. Therefore, STIM2 and PERK/ATF4 could be exploited for prognosis or in targeted therapies to inhibit CRC tumor growth and metastasis.

Loss of INPP5K attenuates IP3-induced Ca2+ responses in the glioblastoma cell line U-251 MG cells

INPP5K (inositol polyphosphate 5-phosphatase K) is an endoplasmic reticulum (ER)-resident enzyme that acts as a phosphoinositide (PI) 5-phosphatase, capable of dephosphorylating various PIs including PI 4,5-bisphosphate (PI(4,5)P2), a key phosphoinositide found in the plasma membrane. Given its ER localization and substrate specificity, INPP5K may play a role in ER-plasma membrane contact sites. Furthermore, PI(4,5)P2 serves as a substrate for phospholipase C, an enzyme activated downstream of extracellular agonists acting on Gq-coupled receptors or tyrosine-kinase receptors, leading to IP3 production and subsequent release of Ca2+ from the ER, the primary intracellular Ca2+ storage organelle. In this study, we investigated the impact of INPP5K on ER Ca2+ dynamics using a previously established INPP5K-knockdown U-251 MG glioblastoma cell model. We here describe that loss of INPP5K impairs agonist-induced, IP3 receptor (IP3R)-mediated Ca2+ mobilization in intact cells, while the ER Ca2+ content and store-operated Ca2+ influx remain unaffected. To further elucidate the underlying mechanisms, we examined Ca2+ release in permeabilized cells stimulated with exogenous IP3. Interestingly, the absence of INPP5K also disrupted IP3-induced Ca2+ release events. These results suggest that INPP5K may directly influence IP3R activity through mechanisms yet to be resolved. The findings from this study point towards role of INPP5K in modulating ER calcium dynamics, particularly in relation to IP3-mediated signaling pathways. However, further work is needed to establish the general nature of our findings and to unravel the exact molecular mechanisms underlying the interplay between INNP5K function and Ca2+ signaling.

Functional differences in agonist-induced plasma membrane expression of Orai1α and Orai1β

Orai1 is the pore-forming subunit of the store-operated Ca2+ release-activated Ca2+ (CRAC) channels involved in a variety of cellular functions. Two Orai1 variants have been identified, the long form, Orai1α, containing 301 amino acids, and the short form, Orai1β, which arises from alternative translation initiation from methionines 64 or 71, in Orai1α. Orai1 is mostly expressed in the plasma membrane, but a subset of Orai1 is located in intracellular compartments. Here we show that Ca2+ store depletion leads to trafficking and insertion of compartmentalized Orai1α in the plasma membrane via a mechanism that is independent on changes in cytosolic free-Ca2+ concentration, as demonstrated by cell loading with the fast intracellular Ca2+ chelator dimethyl BAPTA in the absence of extracellular Ca2+ . Interestingly, thapsigargin (TG) was found to be unable to induce translocation of Orai1β to the plasma membrane when expressed individually; by contrast, when Orai1β is co-expressed with Orai1α, cell treatment with TG induced rapid trafficking and insertion of compartmentalized Orai1β in the plasma membrane. Translocation of Orai1 forms to the plasma membrane was found to require the integrity of the actin cytoskeleton. Finally, expression of a dominant negative mutant of the small GTPase ARF6, and ARF6-T27N, abolished the translocation of compartmentalized Orai1 variants to the plasma membrane upon store depletion. These findings provide new insights into the mechanism that regulate the plasma membrane abundance of Orai1 variants after Ca2+ store depletion.

Store-Operated Ca2+ Entry as a Putative Target of Flecainide for the Treatment of Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is a genetic disorder that may lead patients to sudden cell death through the occurrence of ventricular arrhythmias. ACM is characterised by the progressive substitution of cardiomyocytes with fibrofatty scar tissue that predisposes the heart to life-threatening arrhythmic events. Cardiac mesenchymal stromal cells (C-MSCs) contribute to the ACM by differentiating into fibroblasts and adipocytes, thereby supporting aberrant remodelling of the cardiac structure. Flecainide is an Ic antiarrhythmic drug that can be administered in combination with β-adrenergic blockers to treat ACM due to its ability to target both Nav1.5 and type 2 ryanodine receptors (RyR2). However, a recent study showed that flecainide may also prevent fibro-adipogenic differentiation by inhibiting store-operated Ca2+ entry (SOCE) and thereby suppressing spontaneous Ca2+ oscillations in C-MSCs isolated from human ACM patients (ACM C-hMSCs). Herein, we briefly survey ACM pathogenesis and therapies and then recapitulate the main molecular mechanisms targeted by flecainide to mitigate arrhythmic events, including Nav1.5 and RyR2. Subsequently, we describe the role of spontaneous Ca2+ oscillations in determining MSC fate. Next, we discuss recent work showing that spontaneous Ca2+ oscillations in ACM C-hMSCs are accelerated to stimulate their fibro-adipogenic differentiation. Finally, we describe the evidence that flecainide suppresses spontaneous Ca2+ oscillations and fibro-adipogenic differentiation in ACM C-hMSCs by inhibiting constitutive SOCE.

Too much of a good thing: The case of SOCE in cellular apoptosis

Intracellular calcium (Ca2+) is an essential second messenger in eukaryotic cells regulating numerous cellular functions such as contraction, secretion, immunity, growth, and metabolism. Ca2+ signaling is also a key signal transducer in the intrinsic apoptosis pathway. The store-operated Ca2+ entry pathway (SOCE) is ubiquitously expressed in eukaryotic cells, and is the primary Ca2+ influx pathway in non-excitable cells. SOCE is mediated by the endoplasmic reticulum Ca2+ sensing STIM proteins, and the plasma membrane Ca2+-selective Orai channels. A growing number of studies have implicated SOCE in regulating cell death primarily via the intrinsic apoptotic pathway in a variety of tissues and in response to physiological stressors such as traumatic brain injury, ischemia reperfusion injury, sepsis, and alcohol toxicity. Notably, the literature points to excessive cytosolic Ca2+ influx through SOCE in vulnerable cells as a key factor tipping the balance towards cellular apoptosis. While the literature primarily addresses the functions of STIM1 and Orai1, STIM2, Orai2 and Orai3 are also emerging as potential regulators of cell death. Here, we review the functions of STIM and Orai proteins in regulating cell death and the implications of this regulation to human pathologies.

Orai1 is an Entotic Ca2+ Channel for Non-Apoptotic Cell Death, Entosis in Cancer Development

Entosis is a non-apoptotic cell death process that forms characteristic cell-in-cell structures in cancers, killing invading cells. Intracellular Ca²⁺ dynamics are essential for cellular processes, including actomyosin contractility, migration, and autophagy. However, the significance of Ca²⁺ and Ca²⁺ channels participating in entosis is unclear. Here, it is shown that intracellular Ca²⁺ signaling regulates entosis via SEPTIN-Orai1-Ca²⁺ /CaM-MLCK-actomyosin axis. Intracellular Ca²⁺ oscillations in entotic cells show spatiotemporal variations during engulfment, mediated by Orai1 Ca²⁺ channels in plasma membranes. SEPTIN controlled polarized distribution of Orai1 for local MLCK activation, resulting in MLC phosphorylation and actomyosin contraction, leads to internalization of invasive cells. Ca²⁺ chelators and SEPTIN, Orai1, and MLCK inhibitors suppress entosis. This study identifies potential targets for treating entosis-associated tumors, showing that Orai1 is an entotic Ca²⁺ channel that provides essential Ca²⁺ signaling and sheds light on the molecular mechanism underlying entosis that involves SEPTIN filaments, Orai1, and MLCK.

Neuronal Store-Operated Calcium Channels

The endoplasmic reticulum (ER) is the major intracellular calcium (Ca²⁺) storage compartment in eukaryotic cells. In most instances, the mobilization of Ca²⁺ from this store is followed by a delayed and sustained uptake of Ca²⁺ through Ca²⁺-permeable channels of the cell surface named store-operated Ca²⁺ channels (SOCCs). This gives rise to a store-operated Ca²⁺ entry (SOCE) that has been thoroughly investigated in electrically non-excitable cells where it is the principal regulated Ca²⁺ entry pathway. The existence of this Ca²⁺ route in neurons has long been a matter of debate. However, a growing body of experimental evidence indicates that the recruitment of Ca²⁺ from neuronal ER Ca²⁺ stores generates a SOCE. The present review summarizes the main studies supporting the presence of a depletion-dependent Ca²⁺ entry in neurons. It also addresses the question of the molecular composition of neuronal SOCCs, their expression, pharmacological properties, as well as their physiological relevance.

Optimal inference of molecular interaction dynamics in FRET microscopy

Intensity-based time-lapse fluorescence resonance energy transfer (FRET) microscopy has been a major tool for investigating cellular processes, converting otherwise unobservable molecular interactions into fluorescence time series. However, inferring the molecular interaction dynamics from the observables remains a challenging inverse problem, particularly when measurement noise and photobleaching are nonnegligible-a common situation in single-cell analysis. The conventional approach is to process the time-series data algebraically, but such methods inevitably accumulate the measurement noise and reduce the signal-to-noise ratio (SNR), limiting the scope of FRET microscopy. Here, we introduce an alternative probabilistic approach, B-FRET, generally applicable to standard 3-cube FRET-imaging data. Based on Bayesian filtering theory, B-FRET implements a statistically optimal way to infer molecular interactions and thus drastically improves the SNR. We validate B-FRET using simulated data and then apply it to real data, including the notoriously noisy in vivo FRET time series from individual bacterial cells to reveal signaling dynamics otherwise hidden in the noise.

Role of Orai-family channels in the activation and regulation of transcriptional activity

Store operated Ca²⁺ entry (SOCE) is a cornerstone for the maintenance of intracellular Ca²⁺ homeostasis and the regulation of a variety of cellular functions. SOCE is mediated by STIM and Orai proteins following the activation of inositol 1,4,5-trisphosphate receptors. Then, a reduction of the endoplasmic reticulum intraluminal Ca²⁺ concentration is sensed by STIM proteins, which undergo a conformational change and activate plasma membrane Ca²⁺ channels comprised by Orai proteins. STIM1/Orai-mediated Ca²⁺ signals are finely regulated and modulate the activity of different transcription factors, including certain isoforms of the nuclear factor of activated T-cells, the cAMP-response element binding protein, the nuclear factor κ-light chain-enhancer of activated B cells, c-fos, and c-myc. These transcription factors associate SOCE with a plethora of signaling events and cellular functions. Here we provide an overview of the current knowledge about the role of Orai channels in the regulation of transcription factors through Ca²⁺ -dependent signaling pathways.

Orai3 and Orai1 mediate CRAC channel function and metabolic reprogramming in B cells

The essential role of store-operated Ca2+ entry (SOCE) through Ca2+ release-activated Ca2+ (CRAC) channels in T cells is well established. In contrast, the contribution of individual Orai isoforms to SOCE and their downstream signaling functions in B cells are poorly understood. Here, we demonstrate changes in the expression of Orai isoforms in response to B cell activation. We show that both Orai3 and Orai1 mediate native CRAC channels in B cells. The combined loss of Orai1 and Orai3, but not Orai3 alone, impairs SOCE, proliferation and survival, nuclear factor of activated T cells (NFAT) activation, mitochondrial respiration, glycolysis, and the metabolic reprogramming of primary B cells in response to antigenic stimulation. Nevertheless, the combined deletion of Orai1 and Orai3 in B cells did not compromise humoral immunity to influenza A virus infection in mice, suggesting that other in vivo co-stimulatory signals can overcome the requirement of BCR-mediated CRAC channel function in B cells. Our results shed important new light on the physiological roles of Orai1 and Orai3 proteins in SOCE and the effector functions of B lymphocytes.

The Molecular Heterogeneity of Store-Operated Ca2+ Entry in Vascular Endothelial Cells: The Different roles of Orai1 and TRPC1/TRPC4 Channels in the Transition from Ca2+-Selective to Non-Selective Cation Currents

Store-operated Ca²⁺ entry (SOCE) is activated in response to the inositol-1,4,5-trisphosphate (InsP₃)-dependent depletion of the endoplasmic reticulum (ER) Ca²⁺ store and represents a ubiquitous mode of Ca²⁺ influx. In vascular endothelial cells, SOCE regulates a plethora of functions that maintain cardiovascular homeostasis, such as angiogenesis, vascular tone, vascular permeability, platelet aggregation, and monocyte adhesion. The molecular mechanisms responsible for SOCE activation in vascular endothelial cells have engendered a long-lasting controversy. Traditionally, it has been assumed that the endothelial SOCE is mediated by two distinct ion channel signalplexes, i.e., STIM1/Orai1 and STIM1/Transient Receptor Potential Canonical 1(TRPC1)/TRPC4. However, recent evidence has shown that Orai1 can assemble with TRPC1 and TRPC4 to form a non-selective cation channel with intermediate electrophysiological features. Herein, we aim at bringing order to the distinct mechanisms that mediate endothelial SOCE in the vascular tree from multiple species (e.g., human, mouse, rat, and bovine). We propose that three distinct currents can mediate SOCE in vascular endothelial cells: (1) the Ca²⁺-selective Ca²⁺-release activated Ca²⁺ current (I_CRAC), which is mediated by STIM1 and Orai1; (2) the store-operated non-selective current (I_SOC), which is mediated by STIM1, TRPC1, and TRPC4; and (3) the moderately Ca²⁺-selective, I_CRAC-like current, which is mediated by STIM1, TRPC1, TRPC4, and Orai1.

The store-operated Ca2+ channel Orai1α is required for agonist-evoked NF-κB activation by a mechanism dependent on PKCβ2

Store-operated Ca²⁺ entry is a ubiquitous mechanism for Ca²⁺ influx in mammalian cells that regulates a variety of physiological processes. The identification of two forms of Orai1, the predominant store-operated channel, Orai1α and Orai1β, raises the question whether they differentially regulate cell function. Orai1α is the full-length Orai1, containing 301 amino acids, whereas Orai1β lacks the N-terminal 63 amino acids. Here, using a combination of biochemistry and imaging combined with the use of human embryonic kidney 293 KO cells, missing the native Orai1, transfected with plasmids encoding for either Orai1α or Orai1β, we show that Orai1α plays a relevant role in agonist-induced NF-κB transcriptional activity. In contrast, functional Orai1β is not required for the activation of these transcription factors. The role of Orai1α in the activation of NF-κB is entirely dependent on Ca²⁺ influx and involves PKCβ activation. Our results indicate that Orai1α interacts with PKCβ2 by a mechanism involving the Orai1α exclusive AKAP79 association region, which strongly suggests a role for AKAP79 in this process. These findings provide evidence of the role of Orai1α in agonist-induced NF-κB transcriptional activity and reveal functional differences between Orai1 variants.

Missense mutations in inositol 1,4,5-trisphosphate receptor type 3 result in leaky Ca2+ channels and activation of store-operated Ca2+ entry

Mutations in all subtypes of the inositol 1,4,5-trisphosphate receptor Ca2+ release channel are associated with human diseases. In this report, we investigated the functionality of three neuropathy-associated missense mutations in IP3R3 (V615M, T1424M, and R2524C). The mutants only exhibited function when highly over-expressed compared to endogenous hIP3R3. All variants resulted in elevated basal cytosolic Ca2+ levels, decreased endoplasmic reticulum Ca2+ store content, and constitutive store-operated Ca2+ entry in the absence of any stimuli, consistent with a leaky IP3R channel pore. These variants differed in channel function; when stably over-expressed the R2524C mutant was essentially dead, V615M was poorly functional, and T1424M exhibited activity greater than that of the corresponding wild-type following threshold stimulation. These results demonstrate that a common feature of these mutations is decreased IP3R3 function. In addition, these mutations exhibit a novel phenotype manifested as a constitutively open channel, which inappropriately gates SOCE in the absence of stimulation.

Similarities and Differences between the Orai1 Variants: Orai1α and Orai1β

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.

Histamine activates an intracellular Ca2+ signal in normal human lung fibroblast WI-38 cells

Histamine is an inflammatory mediator that can be released from mast cells to induce airway remodeling and cause persistent airflow limitation in asthma. In addition to stimulating airway smooth muscle cell constriction and hyperplasia, histamine promotes pulmonary remodeling by inducing fibroblast proliferation, contraction, and migration. It has long been known that histamine receptor 1 (H1R) mediates the effects of histamine on human pulmonary fibroblasts through an increase in intracellular Ca2+ concentration ([Ca2+]i), but the underlying signaling mechanisms are still unknown. Herein, we exploited single-cell Ca2+ imaging to assess the signal transduction pathways whereby histamine generates intracellular Ca2+ signals in the human fetal lung fibroblast cell line, WI-38. WI-38 fibroblasts were loaded with the Ca2+-sensitive fluorophore, FURA-2/AM, and challenged with histamine in the absence and presence of specific pharmacological inhibitors to dissect the Ca2+ release/entry pathways responsible for the onset of the Ca2+ response. Histamine elicited complex intracellular Ca2+ signatures in WI-38 fibroblasts throughout a concentration range spanning between 1 µM and 1 mM. In accord, the Ca2+ response to histamine adopted four main temporal patterns, which were, respectively, termed peak, peak-oscillations, peak-plateau-oscillations, and peak-plateau. Histamine-evoked intracellular Ca2+ signals were abolished by pyrilamine, which selectively blocks H1R, and significantly reduced by ranitidine, which selectively inhibits H2R. Conversely, the pharmacological blockade of H3R and H4R did not affect the complex increase in [Ca2+]i evoked by histamine in WI-38 fibroblasts. In agreement with these findings, histamine-induced intracellular Ca2+ signals were initiated by intracellular Ca2+ release from the endoplasmic reticulum through inositol-1,4,5-trisphosphate (InsP3) receptors (InsP3R) and sustained by store-operated Ca2+ channels (SOCs). Conversely, L-type voltage-operated Ca2+ channels did not support histamine-induced extracellular Ca2+ entry. A preliminary transcriptomic analysis confirmed that WI-38 human lung fibroblasts express all the three InsP3R isoforms as well as STIM2 and Orai3, which represent the molecular components of SOCs. The pharmacological blockade of InsP3 and SOC, therefore, could represent an alternative strategy to prevent the pernicious effects of histamine on lung fibroblasts in asthmatic patients.

STIM Proteins and Regulation of SOCE in ER-PM Junctions

ER-PM junctions are membrane contact sites formed by the endoplasmic reticulum (ER) and plasma membrane (PM) in close apposition together. The formation and stability of these junctions are dependent on constitutive and dynamic enrichment of proteins, which either contribute to junctional stability or modulate the lipid levels of both ER and plasma membranes. The ER-PM junctions have come under much scrutiny recently as they serve as hubs for assembling the Ca²⁺ signaling complexes. This review summarizes: (1) key findings that underlie the abilities of STIM proteins to accumulate in ER-PM junctions; (2) the modulation of Orai/STIM complexes by other components found within the same junction; and (3) how Orai1 channel activation is coordinated and coupled with downstream signaling pathways.

Dynamic S-acylation of the ER-resident protein stromal interaction molecule 1 (STIM1) is required for store-operated Ca2+ entry

Many cell surface stimuli cause calcium release from endoplasmic reticulum (ER) stores to regulate cellular physiology. Upon ER calcium store depletion, the ER-resident protein stromal interaction molecule 1 (STIM1) physically interacts with plasma membrane protein Orai1 to induce calcium release–activated calcium (CRAC) currents that conduct calcium influx from the extracellular milieu. Although the physiological relevance of this process is well established, the mechanism supporting the assembly of these proteins is incompletely understood. Earlier we demonstrated a previously unknown post-translational modification of Orai1 with long-chain fatty acids, known as S-acylation. We found that S-acylation of Orai1 is dynamically regulated in a stimulus-dependent manner and essential for its function as a calcium channel. Here using the acyl resin–assisted capture assay, we show that STIM1 is also rapidly S-acylated at cysteine 437 upon ER calcium store depletion. Using a combination of live cell imaging and electrophysiology approaches with a mutant STIM1 protein, which could not be S-acylated, we determined that the S-acylation of STIM1 is required for the assembly of STIM1 into puncta with Orai1 and full CRAC channel function. Together with the S-acylation of Orai1, our data suggest that stimulus-dependent S-acylation of CRAC channel components Orai1 and STIM1 is a critical mechanism facilitating the CRAC channel assembly and function.

PKC-Mediated Orai1 Channel Phosphorylation Modulates Ca2+ Signaling in HeLa Cells

The overexpression of the Orai1 channel inhibits SOCE when using the Ca²⁺ readdition protocol. However, we found that HeLa cells overexpressing the Orai1 channel displayed enhanced Ca²⁺ entry and a limited ER depletion in response to the combination of ATP and thapsigargin (TG) in the presence of external Ca²⁺. As these effects require the combination of an agonist and TG, we decided to study whether the phosphorylation of Orai1 S27/S30 residues had any role using two different mutants: Orai1-S27/30A (O1-AA, phosphorylation-resistant) and Orai1-S27/30D (O1-DD, phosphomimetic). Both O1-wt and O1-AA supported enhanced Ca²⁺ entry, but this was not the case with O1-E106A (dead-pore mutant), O1-DD, and O1-AA-E106A, while O1-wt, O1-E106A, and O1-DD inhibited the ATP and TG-induced reduction of ER [Ca²⁺], suggesting that the phosphorylation of O1 S27/30 interferes with the IP₃R activity. O1-wt and O1-DD displayed an increased interaction with IP₃R in response to ATP and TG; however, the O1-AA channel decreased this interaction. The expression of mCherry-O1-AA increased the frequency of ATP-induced sinusoidal [Ca²⁺]_i oscillations, while mCherry-O1-wt and mCherry-O1-DD decreased this frequency. These data suggest that the combination of ATP and TG stimulates Ca²⁺ entry, and the phosphorylation of Orai1 S27/30 residues by PKC reduces IP₃R-mediated Ca²⁺ release.

Dysregulated Ca2+ Homeostasis as a Central Theme in Neurodegeneration: Lessons from Alzheimer's Disease and Wolfram Syndrome

Calcium ions (Ca²⁺) operate as important messengers in the cell, indispensable for signaling the underlying numerous cellular processes in all of the cell types in the human body. In neurons, Ca²⁺ signaling is crucial for regulating synaptic transmission and for the processes of learning and memory formation. Hence, the dysregulation of intracellular Ca²⁺ homeostasis results in a broad range of disorders, including cancer and neurodegeneration. A major source for intracellular Ca²⁺ is the endoplasmic reticulum (ER), which has close contacts with other organelles, including mitochondria. In this review, we focus on the emerging role of Ca²⁺ signaling at the ER–mitochondrial interface in two different neurodegenerative diseases, namely Alzheimer’s disease and Wolfram syndrome. Both of these diseases share some common hallmarks in the early stages, including alterations in the ER and mitochondrial Ca²⁺ handling, mitochondrial dysfunction and increased Reactive oxygen species (ROS) production. This indicates that similar mechanisms may underly these two disease pathologies and suggests that both research topics might benefit from complementary research.

Jaw1/LRMP increases Ca2+ influx upon GPCR stimulation with heterogeneous effect on the activity of each ITPR subtype

Ca²⁺ influx upon G protein-coupled receptor (GPCR) stimulation is observed as a cytosolic Ca²⁺ concentration oscillation crucial to initiating downstream responses including cell proliferation, differentiation, and cell–cell communication. Although Jaw1 is known to interact with inositol 1,4,5-triphosphate receptor (ITPRs), Ca²⁺ channels on the endoplasmic reticulum, the function of Jaw1 in the Ca²⁺ dynamics with physiological stimulation remains unclear. In this study, using inducible Jaw1-expressing HEK293 cells, we showed that Jaw1 increases Ca²⁺ influx by GPCR stimulation via changing the Ca²⁺ influx oscillation pattern. Furthermore, we showed that Jaw1 increases the Ca²⁺ release activity of all ITPR subtypes in a subtly different manner. It is well known that the Ca²⁺ influx oscillation pattern varies from cell type to cell type, therefore these findings provide an insight into the relationship between the heterogeneous Ca²⁺ dynamics and the specific ITPR and Jaw1 expression patterns.

STIM and Orai Mediated Regulation of Calcium Signaling in Age-Related Diseases

Tight spatiotemporal regulation of intracellular Ca²⁺ plays a critical role in regulating diverse cellular functions including cell survival, metabolism, and transcription. As a result, eukaryotic cells have developed a wide variety of mechanisms for controlling Ca²⁺ influx and efflux across the plasma membrane as well as Ca²⁺ release and uptake from intracellular stores. The STIM and Orai protein families comprising of STIM1, STIM2, Orai1, Orai2, and Orai3, are evolutionarily highly conserved proteins that are core components of all mammalian Ca²⁺ signaling systems. STIM1 and Orai1 are considered key players in the regulation of Store Operated Calcium Entry (SOCE), where release of Ca²⁺ from intracellular stores such as the Endoplasmic/Sarcoplasmic reticulum (ER/SR) triggers Ca²⁺ influx across the plasma membrane. SOCE, which has been widely characterized in non-excitable cells, plays a central role in Ca²⁺-dependent transcriptional regulation. In addition to their role in Ca²⁺ signaling, STIM1 and Orai1 have been shown to contribute to the regulation of metabolism and mitochondrial function. STIM and Orai proteins are also subject to redox modifications, which influence their activities. Considering their ubiquitous expression, there has been increasing interest in the roles of STIM and Orai proteins in excitable cells such as neurons and myocytes. While controversy remains as to the importance of SOCE in excitable cells, STIM1 and Orai1 are essential for cellular homeostasis and their disruption is linked to various diseases associated with aging such as cardiovascular disease and neurodegeneration. The recent identification of splice variants for most STIM and Orai isoforms while complicating our understanding of their function, may also provide insight into some of the current contradictions on their roles. Therefore, the goal of this review is to describe our current understanding of the molecular regulation of STIM and Orai proteins and their roles in normal physiology and diseases of aging, with a particular focus on heart disease and neurodegeneration.

Lymphatic Collecting Vessel: New Perspectives on Mechanisms of Contractile Regulation and Potential Lymphatic Contractile Pathways to Target in Obesity and Metabolic Diseases

Obesity and metabolic syndrome pose a significant risk for developing cardiovascular disease and remain a critical healthcare challenge. Given the lymphatic system's role as a nexus for lipid absorption, immune cell trafficking, interstitial fluid and macromolecule homeostasis maintenance, the impact of obesity and metabolic disease on lymphatic function is a burgeoning field in lymphatic research. Work over the past decade has progressed from the association of an obese phenotype with Prox1 haploinsufficiency and the identification of obesity as a risk factor for lymphedema to consistent findings of lymphatic collecting vessel dysfunction across multiple metabolic disease models and organisms and characterization of obesity-induced lymphedema in the morbidly obese. Critically, recent findings have suggested that restoration of lymphatic function can also ameliorate obesity and insulin resistance, positing lymphatic targeted therapies as relevant pharmacological interventions. There remain, however, significant gaps in our understanding of lymphatic collecting vessel function, particularly the mechanisms that regulate the spontaneous contractile activity required for active lymph propulsion and lymph return in humans. In this article, we will review the current findings on lymphatic architecture and collecting vessel function, including recent advances in the ionic basis of lymphatic muscle contractile activity. We will then discuss lymphatic dysfunction observed with metabolic disruption and potential pathways to target with pharmacological approaches to improve lymphatic collecting vessel function.

Physiological Functions of CRAC Channels

Store-operated Ca2+ entry (SOCE) is a ubiquitous Ca2+ signaling pathway that is evolutionarily conserved across eukaryotes. SOCE is triggered physiologically when the endoplasmic reticulum (ER) Ca2+ stores are emptied through activation of inositol 1,4,5-trisphosphate receptors. SOCE is mediated by the Ca2+ release-activated Ca2+ (CRAC) channels, which are highly Ca2+ selective. Upon store depletion, the ER Ca2+-sensing STIM proteins aggregate and gain extended conformations spanning the ER-plasma membrane junctional space to bind and activate Orai, the pore-forming proteins of hexameric CRAC channels. In recent years, studies on STIM and Orai tissue-specific knockout mice and gain- and loss-of-function mutations in humans have shed light on the physiological functions of SOCE in various tissues. Here, we describe recent findings on the composition of native CRAC channels and their physiological functions in immune, muscle, secretory, and neuronal systems to draw lessons from transgenic mice and human diseases caused by altered CRAC channel activity.

P2X4 and P2X7 are essential players in basal T cell activity and Ca2+ signaling milliseconds after T cell activation

Initial T cell activation is triggered by the formation of highly dynamic, spatiotemporally restricted Ca²⁺ microdomains. Purinergic signaling is known to be involved in Ca²⁺ influx in T cells at later stages compared to the initial microdomain formation. Using a high-resolution Ca²⁺ live-cell imaging system, we show that the two purinergic cation channels P2X4 and P2X7 not only are involved in the global Ca²⁺ signals but also promote initial Ca²⁺ microdomains tens of milliseconds after T cell stimulation. These Ca²⁺ microdomains were significantly decreased in T cells from P2rx4^-/- and P2rx7^-/- mice or by pharmacological inhibition or blocking. Furthermore, we show a pannexin-1-dependent activation of P2X4 in the absence of T cell receptor/CD3 stimulation. Subsequently, upon T cell receptor/CD3 stimulation, ATP release is increased and autocrine activation of both P2X4 and P2X7 then amplifies initial Ca²⁺ microdomains already in the first second of T cell activation.

Highlighting the Multifaceted Role of Orai1 N-Terminal- and Loop Regions for Proper CRAC Channel Functions

Orai1, the Ca2+-selective pore in the plasma membrane, is one of the key components of the Ca2+release-activated Ca2+ (CRAC) channel complex. Activated by the Ca2+ sensor in the endoplasmic reticulum (ER) membrane, stromal interaction molecule 1 (STIM1), via direct interaction when ER luminal Ca2+ levels recede, Orai1 helps to maintain Ca2+ homeostasis within a cell. It has already been proven that the C-terminus of Orai1 is indispensable for channel activation. However, there is strong evidence that for CRAC channels to function properly and maintain all typical hallmarks, such as selectivity and reversal potential, additional parts of Orai1 are needed. In this review, we focus on these sites apart from the C-terminus; namely, the second loop and N-terminus of Orai1 and on their multifaceted role in the functioning of CRAC channels.

Functional communication between IP3R and STIM2 at subthreshold stimuli is a critical checkpoint for initiation of SOCE

Stromal interaction molecules, STIM1 and STIM2, sense decreases in the endoplasmic reticulum (ER) [Ca2+] ([Ca2+]ER) and cluster in ER-plasma membrane (ER-PM) junctions where they recruit and activate Orai1. While STIM1 responds when [Ca2+]ER is relatively low, STIM2 displays constitutive clustering in the junctions and is suggested to regulate basal Ca2+ entry. The cellular cues that determine STIM2 clustering under basal conditions is not known. By using gene editing to fluorescently tag endogenous STIM2, we report that endogenous STIM2 is constitutively localized in mobile and immobile clusters. The latter associate with ER-PM junctions and recruit Orai1 under basal conditions. Agonist stimulation increases immobile STIM2 clusters, which coordinate recruitment of Orai1 and STIM1 to the junctions. Extended synaptotagmin (E-Syt)2/3 are required for forming the ER-PM junctions, but are not sufficient for STIM2 clustering. Importantly, inositol 1,4,5-triphosphate receptor (IP3R) function and local [Ca2+]ER are the main drivers of immobile STIM2 clusters. Enhancing, or decreasing, IP3R function at ambient [IP3] causes corresponding increase, or attenuation, of immobile STIM2 clusters. We show that immobile STIM2 clusters denote decreases in local [Ca2+]ER mediated by IP3R that is sensed by the STIM2 N terminus. Finally, under basal conditions, ambient PIP2-PLC activity of the cell determines IP3R function, immobilization of STIM2, and basal Ca2+ entry while agonist stimulation augments these processes. Together, our findings reveal that immobilization of STIM2 clusters within ER-PM junctions, a first response to ER-Ca2+ store depletion, is facilitated by the juxtaposition of IP3R and marks a checkpoint for initiation of Ca2+ entry.

Orai1α, but not Orai1β, co-localizes with TRPC1 and is required for its plasma membrane location and activation in HeLa cells

The identification of two variants of the canonical pore-forming subunit of the Ca²⁺ release-activated Ca²⁺ (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 Ca²⁺-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 Ca²⁺ oscillations, while TRPC1 plays a role in agonist-induced Ca²⁺ influx but is not essential for Ca²⁺ oscillations. Using APEX2 proximity labeling, co-immunoprecipitation and the fluorescence of G-GECO1.2 fused to Orai1α our results indicate that agonist stimulation and Ca²⁺ store depletion enhance Orai1α–TRPC1 interaction. Orai1α is essential for TRPC1 plasma membrane location and activation. Thus, TRPC1 function in HeLa cells depends on Ca²⁺ influx through Orai1α exclusively.

STIM1 is a core trigger of airway smooth muscle remodeling and hyperresponsiveness in asthma

Stromal-interacting molecule 1 (STIM1) proteins are essential for the function of store-operated Ca²⁺ entry (SOCE). Using transcriptomics, metabolomics, imaging, and inducible smooth muscle–specific STIM1 knockout mice expressing genetically encoded Ca²⁺ sensors, we reveal a crucial function of STIM1 in airway remodeling and airway hyperresponsiveness in asthma. STIM1-mediated Ca²⁺ oscillations in airway smooth muscle (ASM) cells are critical for ASM remodeling through metabolic and transcriptional reprogramming and cytokine secretion, including IL-6. These effects are driven by Ca²⁺-dependent activation of the transcription factor isoform NFAT4 specifically in ASM. Our data provide evidence that ASM STIM1 and SOCE are central triggers of asthma manifestations and advocate for the future use of STIM1 as a molecular target in asthma therapy.

Airway remodeling and airway hyperresponsiveness are central drivers of asthma severity. Airway remodeling is a structural change involving the dedifferentiation of airway smooth muscle (ASM) cells from a quiescent to a proliferative and secretory phenotype. Here, we show up-regulation of the endoplasmic reticulum Ca²⁺ sensor stromal-interacting molecule 1 (STIM1) in ASM of asthmatic mice. STIM1 is required for metabolic and transcriptional reprogramming that supports airway remodeling, including ASM proliferation, migration, secretion of cytokines and extracellular matrix, enhanced mitochondrial mass, and increased oxidative phosphorylation and glycolytic flux. Mechanistically, STIM1-mediated Ca²⁺ influx is critical for the activation of nuclear factor of activated T cells 4 and subsequent interleukin-6 secretion and transcription of pro-remodeling transcription factors, growth factors, surface receptors, and asthma-associated proteins. STIM1 drives airway hyperresponsiveness in asthmatic mice through enhanced frequency and amplitude of ASM cytosolic Ca²⁺ oscillations. Our data advocates for ASM STIM1 as a target for asthma therapy.

Control of STIM and Orai function by post-translational modifications

Store-operated calcium entry (SOCE) is mediated by the endoplasmic reticulum (ER) Ca²⁺ sensors stromal interaction molecules (STIM1 and STIM2) and the plasma membrane Orai (Orai1, Orai2, Orai3) Ca²⁺ channels. Although primarily regulated by ER Ca²⁺ content, there have been numerous studies over the last 15 years demonstrating that all 5 proteins are also regulated through post-translational modification (PTM). Focusing primarily on phosphorylation, glycosylation and redox modification, this review focuses on how PTMs modulate the key events in SOCE; Ca²⁺ sensing, STIM translocation, Orai interaction and/or Orai1 activation.

Extracellular cysteines C226 and C232 mediate hydrogen sulfide-dependent inhibition of Orai3-mediated store-operated calcium entry

The gaseous signaling molecule hydrogen sulfide (H₂S) physiologically regulates store-operated Ca²⁺ entry (SOCE). The SOCE machinery consists of the plasma membrane-localized Orai channels (Orai1-3) and endoplasmic reticulum-localized stromal interaction molecule (STIM)1 and STIM2 proteins. H₂S inhibits Orai3- but not Orai1- or Orai2-mediated SOCE. The current objective was to define the mechanism by which H₂S selectively modifies Orai3. We measured SOCE and STIM1/Orai3 dynamics and interactions in HEK293 cells exogenously expressing fluorescently tagged human STIM1 and Orai3 in the presence and absence of the H₂S donor GYY4137. Two cysteines (C226 and C232) are present in Orai3 that are absent in the Orai1 and Orai2. When we mutated either of these cysteines to serine, alone or in combination, SOCE inhibition by H₂S was abolished. We also established that inhibition was dependent on an interaction with STIM1. To further define the effects of H₂S on STIM1/Orai3 interaction, we performed a series of fluorescence recovery after photobleaching (FRAP), colocalization, and fluorescence resonance energy transfer (FRET) experiments. Treatment with H₂S did not affect the mobility of Orai3 in the membrane, nor did it influence STIM1/Orai3 puncta formation or STIM1-Orai3 protein-protein interactions. These data support a model in which H₂S modification of Orai3 at cysteines 226 and 232 limits SOCE evoked upon store depletion and STIM1 engagement, by a mechanism independent of the interaction between Orai3 and STIM1.

Orai2 Modulates Store-Operated Ca2+ Entry and Cell Cycle Progression in Breast Cancer Cells

Breast cancer is a heterogeneous disease from the histological and molecular expression point of view, and this heterogeneity determines cancer aggressiveness. Store-operated Ca²⁺ entry (SOCE), a major mechanism for Ca²⁺ entry in non-excitable cells, is significantly remodeled in cancer cells and plays an important role in the development and support of different cancer hallmarks. The store-operated CRAC (Ca²⁺ release-activated Ca²⁺) channels are predominantly comprised of Orai1 but the participation of Orai2 and Orai3 subunits has been reported to modulate the magnitude of Ca²⁺ responses. Here we provide evidence for a heterogeneous expression of Orai2 among different breast cancer cell lines. In the HER2 and triple negative breast cancer cell lines SKBR3 and BT20, respectively, where the expression of Orai2 was greater, Orai2 modulates the magnitude of SOCE and sustain Ca²⁺ oscillations in response to carbachol. Interestingly, in these cells Orai2 modulates the activation of NFAT1 and NFAT4 in response to high and low agonist concentrations. Finally, we have found that, in cells with high Orai2 expression, Orai2 knockdown leads to cell cycle arrest at the G0-G1 phase and decreases apoptosis resistance upon cisplatin treatment. Altogether, these findings indicate that, in breast cancer cells with a high Orai2 expression, Orai2 plays a relevant functional role in agonist-evoked Ca²⁺ signals, cell proliferation and apoptosis resistance.

Spatiotemporal regulation of store-operated calcium entry in cancer metastasis

The store-operated calcium (Ca2+) entry (SOCE) is the Ca2+ entry mechanism used by cells to replenish depleted Ca2+ store. The dysregulation of SOCE has been reported in metastatic cancer. It is believed that SOCE promotes migration and invasion by remodeling the actin cytoskeleton and cell adhesion dynamics. There is recent evidence supporting that SOCE is critical for the spatial and the temporal coding of Ca2+ signals in the cell. In this review, we critically examined the spatiotemporal control of SOCE signaling and its implication in the specificity and robustness of signaling events downstream of SOCE, with a focus on the spatiotemporal SOCE signaling during cancer cell migration, invasion and metastasis. We further discuss the limitation of our current understanding of SOCE in cancer metastasis and potential approaches to overcome such limitation.

Orai3 Regulates Pancreatic Cancer Metastasis by Encoding a Functional Store Operated Calcium Entry Channel

Store operated Ca²⁺ entry (SOCE) mediated by Orai1/2/3 channels is a highly regulated and ubiquitous Ca²⁺ influx pathway. Although the role of Orai1 channels is well studied, the significance of Orai2/3 channels is still emerging in nature. In this study, we performed extensive bioinformatic analysis of publicly available datasets and observed that Orai3 expression is inversely associated with the mean survival time of PC patients. Orai3 expression analysis in a battery of PC cell lines corroborated its differential expression profile. We then carried out thorough Ca²⁺ imaging experiments in six PC cell lines and found that Orai3 forms a functional SOCE channel in PC cells. Our in vitro functional assays show that Orai3 regulates PC cell cycle progression, apoptosis and migration. Most importantly, our in vivo xenograft studies demonstrate a critical role of Orai3 in PC tumor growth and secondary metastasis. Mechanistically, Orai3 controls G₁ phase progression, matrix metalloproteinase expression and epithelial-mesenchymal transition in PC cells. Taken together, this study for the first-time reports that Orai3 drives aggressive phenotypes of PC cells, i.e., migration in vitro and metastasis in vivo. Considering that Orai3 overexpression leads to poor prognosis in PC patients, it appears to be a highly attractive therapeutic target.

Termination of Ca2+ puffs during IP3-evoked global Ca2+ signals

We previously described that cell-wide cytosolic Ca²⁺ transients evoked by inositol trisphosphate (IP₃) are generated by two modes of Ca²⁺ liberation from the ER; 'punctate' release via an initial flurry of transient Ca²⁺ puffs from local clusters of IP₃ receptors, succeeded by a spatially and temporally 'diffuse' Ca²⁺ liberation. Those findings were derived using statistical fluctuation analysis to monitor puff activity which is otherwise masked as global Ca²⁺ levels rise. Here, we devised imaging approaches to resolve individual puffs during global Ca²⁺ elevations to better investigate the mechanisms terminating the puff flurry. We find that puffs contribute about 40% (∼90 attomoles) of the total Ca²⁺ liberation, largely while the global Ca²⁺ signal rises halfway to its peak. The major factor terminating punctate Ca²⁺ release is an abrupt decline in puff frequency. Although the amplitudes of large puffs fall during the flurry, the amplitudes of more numerous small puffs remain steady, so overall puff amplitudes decline only modestly (∼30%). The Ca²⁺ flux through individual IP₃ receptor/channels does not measurably decline during the flurry, or when puff activity is depressed by pharmacological lowering of Ca²⁺ levels in the ER lumen, indicating that the termination of punctate release is not a simple consequence of reduced driving force for Ca²⁺ liberation. We propose instead that the gating of IP₃ receptors at puff sites is modulated such that their openings become suppressed as the bulk [Ca²⁺] in the ER lumen falls during global Ca²⁺ signals.

Role of Orai3 in the Pathophysiology of Cancer

The mammalian exclusive Orai3 channel participates in the generation and/or modulation of two independent Ca²⁺ currents, the store-operated current, I_crac, involving functional interactions between the stromal interaction molecules (STIM), STIM1/STIM2, and Orai1/Orai2/Orai3, as well as the store-independent arachidonic acid (AA) (or leukotriene C4)-regulated current I_arc, which involves Orai1, Orai3 and STIM1. Overexpression of functional Orai3 has been described in different neoplastic cells and cancer tissue samples as compared to non-tumor cells or normal adjacent tissue. In these cells, Orai3 exhibits a cell-specific relevance in Ca²⁺ influx. In estrogen receptor-positive breast cancer cells and non-small cell lung cancer (NSCLC) cells store-operated Ca²⁺ entry (SOCE) is strongly dependent on Orai3 expression while in colorectal cancer and pancreatic adenocarcinoma cells Orai3 predominantly modulates SOCE. On the other hand, in prostate cancer cells Orai3 expression has been associated with the formation of Orai1/Orai3 heteromeric channels regulated by AA and reduction in SOCE, thus leading to enhanced proliferation. Orai3 overexpression is associated with supporting several cancer hallmarks, including cell cycle progression, proliferation, migration, and apoptosis resistance. This review summarizes the current knowledge concerning the functional role of Orai3 in the pathogenesis of cancer.

CREB regulates the expression of type 1 inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) play a central role in regulating intracellular Ca2+ signals in response to a variety of internal and external cues. Dysregulation of IP3R signaling is the underlying cause for numerous pathological conditions. It is well established that the activities of IP3Rs are governed by several post-translational modifications, including phosphorylation by protein kinase A (PKA). However, the long-term effects of PKA activation on expression of IP3R subtypes remains largely unexplored. In this report, we investigate the effects of chronic stimulation and tonic activity of PKA on the expression of IP3R subtypes. We demonstrate that expression of the type 1 IP3R (IP3R1) is augmented upon prolonged activation of PKA or upon ectopic overexpression of cyclic AMP-response element-binding protein (CREB) without altering IP3R2 and IP3R3 abundance. By contrast, inhibition of PKA or blocking CREB diminished IP3R1 expression. We also demonstrate that agonist-induced Ca2+-release mediated by IP3R1 is significantly attenuated upon blocking of CREB. Moreover, CREB - by regulating the expression of KRAS-induced actin-interacting protein (KRAP) - ensures correct localization and licensing of IP3R1. Overall, we report a crucial role for CREB in governing both the expression and correct localization of IP3R1. This article has an associated First Person interview with the first author of the paper.

A protocol for detecting elemental calcium signals (Ca2+ puffs) in mammalian cells using total internal reflection fluorescence microscopy

This protocol outlines steps to visualize and detect Ca2+ puffs following photo-liberation of caged inositol-1,4,5-trisphosphate (IP3) from HEK-293 cells expressing only the native IP3R type 1 receptor using total internal reflection fluorescence (TIRF) microscopy. TIRF microscopy offers high axial resolution and allows imaging at high speed, with a higher signal-to-background ratio. Additionally, we shed light on commonly encountered pitfalls, which should be considered while recording Ca2+ puffs using TIRF microscopy. For complete details on the use and execution of this protocol, please refer to Emrich et al. (2021) and Lock et al. (2015a).

Reactive Oxygen Species and Endothelial Ca2+ Signaling: Brothers in Arms or Partners in Crime?

An increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) controls virtually all endothelial cell functions and is, therefore, crucial to maintain cardiovascular homeostasis. An aberrant elevation in endothelial can indeed lead to severe cardiovascular disorders. Likewise, moderate amounts of reactive oxygen species (ROS) induce intracellular Ca²⁺ signals to regulate vascular functions, while excessive ROS production may exploit dysregulated Ca²⁺ dynamics to induce endothelial injury. Herein, we survey how ROS induce endothelial Ca²⁺ signals to regulate vascular functions and, vice versa, how aberrant ROS generation may exploit the Ca²⁺ handling machinery to promote endothelial dysfunction. ROS elicit endothelial Ca²⁺ signals by regulating inositol-1,4,5-trisphosphate receptors, sarco-endoplasmic reticulum Ca²⁺-ATPase 2B, two-pore channels, store-operated Ca²⁺ entry (SOCE), and multiple isoforms of transient receptor potential (TRP) channels. ROS-induced endothelial Ca²⁺ signals regulate endothelial permeability, angiogenesis, and generation of vasorelaxing mediators and can be exploited to induce therapeutic angiogenesis, rescue neurovascular coupling, and induce cancer regression. However, an increase in endothelial [Ca²⁺]_i induced by aberrant ROS formation may result in endothelial dysfunction, inflammatory diseases, metabolic disorders, and pulmonary artery hypertension. This information could pave the way to design alternative treatments to interfere with the life-threatening interconnection between endothelial ROS and Ca²⁺ signaling under multiple pathological conditions.

The Mitochondrial Ca2+ uniporter is a central regulator of interorganellar Ca2+ transfer and NFAT activation

Mitochondrial Ca²⁺ uptake tailors the strength of stimulation of plasma membrane phospholipase C–coupled receptors to that of cellular bioenergetics. However, how Ca²⁺ uptake by the mitochondrial Ca²⁺ uniporter (MCU) shapes receptor-evoked interorganellar Ca²⁺ signaling is unknown. Here, we used CRISPR/Cas9 gene knockout, subcellular Ca²⁺ imaging, and mathematical modeling to show that MCU is a universal regulator of intracellular Ca²⁺ signaling across mammalian cell types. MCU activity sustains cytosolic Ca²⁺ signaling by preventing Ca²⁺-dependent inactivation of store-operated Ca²⁺ release–activated Ca²⁺ channels and by inhibiting Ca²⁺ extrusion. Paradoxically, MCU knockout (MCU-KO) enhanced cytosolic Ca²⁺ responses to store depletion. Physiological agonist stimulation in MCU-KO cells led to enhanced frequency of cytosolic Ca²⁺ oscillations, endoplasmic reticulum Ca²⁺ refilling, nuclear translocation of nuclear factor for activated T cells transcription factors, and cell proliferation, without altering inositol-1,4,5-trisphosphate receptor activity. Our data show that MCU has dual counterbalancing functions at the cytosol–mitochondria interface, whereby the cell-specific MCU-dependent cytosolic Ca²⁺ clearance and buffering capacity of mitochondria reciprocally regulate interorganellar Ca²⁺ transfer and nuclear factor for activated T cells nuclear translocation during receptor-evoked signaling. These findings highlight the critical dual function of the MCU not only in the acute Ca²⁺ buffering by mitochondria but also in shaping endoplasmic reticulum and cytosolic Ca²⁺ signals that regulate cellular transcription and function.

The role of STIM1/ORAI1 channel in the analgesic effect of grain-sized moxibustion on inflammatory pain mice model

The therapeutic effect of grain-sized moxibustion (GS-Moxi) on inflammatory pain has been well recognized clinically, but the mechanism remains unclear. STIM1/ORAI1 is a sensible temperature channel, therefore; this study aimed to investigate the analgesic effect of GS-Moxi and the association with STIM1/ORAI1 expression. CFA-induced inflammatory pain model was established and was treated with GS-Moxi after 3 days of CFA injection. The behavioral test was measured after the GS-Moxi; then, serum was prepared for IL-1β, IL-6, and TNF-α, and the stimulated skin was used for measuring STIM1 and ORAI1 expression. The results indicated GS-Moxi had an analgesic effect on inflammatory pain and the heat variation was significant for the analgesia. GS-Moxi decreased the expression of IL-1β, IL-6, and TNF-α. Immunofluorescence and western blot analysis illustrated that heat change was associated with the stimulation of STIM1 and ORAI1. Suggesting that heat variation created by GS-Moxi could be crucial in this therapy and STIM1 and ORAI1 were potential enhancers in regulating analgesia of GS-Moxi.

Three-Dimensional Model of Sub-Plasmalemmal Ca2+ Microdomains Evoked by the Interplay Between ORAI1 and InsP3 Receptors

Ca²⁺ signaling plays an essential role in T cell activation, which is a key step to start an adaptive immune response. During the transition from a quiescent to a fully activated state, Ca²⁺ microdomains characterized by reduced spatial and temporal extents are observed in the junctions between the plasma membrane (PM) and the endoplasmic reticulum (ER). Such Ca²⁺ responses can also occur in response to T cell adhesion to other cells or extracellular matrix proteins in otherwise unstimulated T cells. These non-TCR/CD3-dependent Ca²⁺ microdomains rely on d-myo-inositol 1,4,5-trisphosphate (IP₃) signaling and subsequent store operated Ca²⁺ entry (SOCE) via the ORAI/STIM system. The detailed molecular mechanism of adhesion-dependent Ca²⁺ microdomain formation remains to be fully elucidated. We used mathematical modeling to investigate the spatiotemporal characteristics of T cell Ca²⁺ microdomains and their molecular regulators. We developed a reaction-diffusion model using COMSOL Multiphysics to describe the evolution of cytosolic and ER Ca²⁺ concentrations in a three-dimensional ER-PM junction. Equations are based on a previously proposed realistic description of the junction, which is extended to take into account IP₃ receptors (IP₃R) that are located next to the junction. The first model only considered the ORAI channels and the SERCA pumps. Taking into account the existence of preformed clusters of ORAI1 and STIM2, ORAI1 slightly opens in conditions of a full ER. These simulated Ca²⁺ microdomains are too small as compared to those observed in unstimulated T cells. When considering the opening of the IP₃Rs located near the junction, the local depletion of ER Ca²⁺ allows for larger Ca²⁺ fluxes through the ORAI1 channels and hence larger local Ca²⁺ concentrations. Computational results moreover show that Ca²⁺ diffusion in the ER has a major impact on the Ca²⁺ changes in the junction, by affecting the local Ca²⁺ gradients in the sub-PM ER. Besides pointing out the likely involvement of the spontaneous openings of IP₃Rs in the activation of SOCE in conditions of T cell adhesion prior to full activation, the model provides a tool to investigate how Ca²⁺ microdomains extent and interact in response to T cell receptor activation.