Dual functions for the endoplasmic reticulum calcium sensors STIM1 and STIM2 in T cell activation and tolerance

The Role of Calcium-Calcineurin-NFAT Signaling Pathway in Health and Autoimmune Diseases

Calcium (Ca²⁺) is an essential signaling molecule that controls a wide range of biological functions. In the immune system, calcium signals play a central role in a variety of cellular functions such as proliferation, differentiation, apoptosis, and numerous gene transcriptions. During an immune response, the engagement of T-cell and B-cell antigen receptors induces a decrease in the intracellular Ca²⁺ store and then activates store-operated Ca²⁺ entry (SOCE) to raise the intracellular Ca²⁺ concentration, which is mediated by the Ca²⁺ release-activated Ca²⁺ (CRAC) channels. Recently, identification of the two critical regulators of the CRAC channel, stromal interaction molecule (STIM) and Orai1, has broadened our understanding of the regulatory mechanisms of Ca²⁺ signaling in lymphocytes. Repetitive or prolonged increase in intracellular Ca²⁺ is required for the calcineurin-mediated dephosphorylation of the nuclear factor of an activated T cell (NFAT). Recent data indicate that Ca²⁺-calcineurin-NFAT1 to 4 pathways are dysregulated in autoimmune diseases. Therefore, calcineurin inhibitors, cyclosporine and tacrolimus, have been used for the treatment of such autoimmune diseases as systemic lupus erythematosus and rheumatoid arthritis. Here, we review the role of the Ca²⁺-calcineurin–NFAT signaling pathway in health and diseases, focusing on the STIM and Orai1, and discuss the deregulated calcium-mediated calcineurin-NFAT pathway in autoimmune diseases.

Stim1 Polymorphism Disrupts Immune Signaling and Creates Renal Injury in Hypertension

Background

Spontaneously hypertensive rats of the stroke‐prone line (SHR‐A3) develop hypertensive renal disease as a result of naturally occurring genetic variation. Our prior work identified a single‐nucleotide polymorphism unique to SHR‐A3 that results in truncation of the carboxy terminus of STIM1. The SHR‐B2 line, which is also hypertensive but resists hypertensive renal injury, expresses the wild‐type STIM1. STIM1 plays a central role in lymphocyte calcium signaling that directs immune effector responses. Here we show that major defects in lymphocyte function affecting calcium signaling, nuclear factor of activated T cells activation, cytokine production, proliferation, apoptosis, and regulatory T‐cell development are present in SHR‐A3 and attributable to STIM1.

Methods and Results

To assess the role of Stim1 variation in susceptibility to hypertensive renal injury, we created a Stim1 congenic line, SHR‐A3(Stim1‐B2), and STIM1 function was rescued in SHR‐A3. We found that Stim1 gene rescue restores disturbed lymphocyte function in SHR‐A3. Hypertensive renal injury was compared in SHR‐A3 and the SHR‐A3(Stim1‐B2) congenic line. Histologically assessed renal injury was markedly reduced in SHR‐A3(Stim1‐B2), as were renal injury biomarker levels measured in urine. Stim1 deficiency has been linked to the emergence of antibody‐mediated autoimmunity. Renal glomerular immunoglobulin deposition was greater in SHR‐A3 than SHR‐B2 and was reduced by Stim1 congenic substitution. Serum anti–double‐stranded DNA antibody titers in SHR‐A3 were elevated compared with SHR‐B2 and were reduced in SHR‐A3(Stim1‐B2).

Conclusions

Stim1 deficiency in lymphocyte function originating from Stim1 truncation in SHR‐A3 combines with hypertension to create end organ disease and may do so as a result of antibody formation.

T Cell Co-stimulation and Functional Modulation by Innate Signals

Pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), NOD-like receptors (NLRs), and RIG-I-like receptors (RLRs), play a pivotal role in the initiation of innate immune responses. Certain PRRs are also expressed by CD4⁺ and CD8⁺ T cells, where they function to provide co-stimulatory signals for their activation and differentiation. Recently, stimulator of interferon genes (STING) was found to be highly expressed in CD4⁺ and CD8⁺ T cells and to modulate T cell function. STING signaling inhibits cell growth and stimulates type I interferon (IFN-I) responses in T cells through reciprocal regulation between T cell receptor (TCR) and STING signals. Here, we propose a model whereby innate signals by TLRs and STING regulate TCR signals and T cell functions.

The Endoplasmic Reticulum-Plasma Membrane Junction: A Hub for Agonist Regulation of Ca2+ Entry

Stimulation of cell-surface receptors induces cytosolic Ca²⁺ ([Ca²⁺]_i) increases that are detected and transduced by effector proteins for regulation of cell function. Intracellular Ca²⁺ release, via endoplasmic reticulum (ER) proteins inositol 1,4,5-trisphosphate receptors (IP₃R) and ryanodine receptors (RyR), and Ca²⁺ influx, via store-operated Ca²⁺ entry (SOCE), contribute to the increase in [Ca²⁺]_i The amplitude, frequency, and spatial characteristics of the [Ca²⁺]_i increases are controlled by the compartmentalization of proteins into signaling complexes such as receptor-signaling complexes and SOCE complexes. Both complexes include protein and lipid components, located in the plasma membrane (PM) and ER. Receptor signaling initiates in the PM via phospholipase C (PLC)-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂), and culminates with the activation of IP₃R in the ER. Conversely, SOCE is initiated in the ER by Ca²⁺-sensing stromal interaction molecule (STIM) proteins, which then interact with PM channels Orai1 and TRPC1 to activate Ca²⁺ entry. This review will address how ER-PM junctions serve a central role in agonist regulation of SOCE.

STIM1-Ca2+ signaling in coronary sinus cardiomyocytes contributes to interatrial conduction

Pacemaker action potentials emerge from the sinoatrial node (SAN) and rapidly propagate through the atria to the AV node via preferential conduction pathways, including one associated with the coronary sinus. However, few distinguishing features of these tracts are known. Identifying specific molecular markers to distinguish among these conduction pathways will have important implications for understanding atrial conduction and atrial arrhythmogenesis. Using a Stim1 reporter mouse, we discovered stromal interaction molecule 1 (STIM1)-expressing coronary sinus cardiomyocytes (CSC)s in a tract from the SAN to the coronary sinus. Our studies here establish that STIM1 is a molecular marker of CSCs and we propose a role for STIM1-CSCs in interatrial conduction. Deletion of Stim1 from the CSCs slowed interatrial conduction and increased susceptibility to atrial arrhythmias. Store-operated Ca²⁺ currents (I_soc) in response to Ca²⁺ store depletion were markedly reduced in CSCs and their action potentials showed electrical remodeling. Our studies identify STIM1 as a molecular marker for a coronary sinus interatrial conduction pathway. We propose a role for SOCE in Ca²⁺ signaling of CSCs and implicate STIM1 in atrial arrhythmogenesis.

Molecular Basis and Regulation of Store-Operated Calcium Entry

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous mechanism for Ca²⁺ influx in mammalian cells with important physiological implications. Since the discovery of SOCE more than three decades ago, the mechanism that communicates the information about the amount of Ca²⁺ accumulated in the intracellular Ca²⁺ stores to the plasma membrane channels and the nature of these channels have been matters of intense investigation and debate. The stromal interaction molecule-1 (STIM1) has been identified as the Ca²⁺ sensor of the intracellular Ca²⁺ compartments that activates the store-operated channels. STIM1 regulates two types of store-dependent channels: the Ca²⁺ release-activated Ca²⁺ (CRAC) channels, formed by Orai1 subunits, that conduct the highly Ca²⁺ selective current I _CRAC and the cation permeable store-operated Ca²⁺ (SOC) channels, which consist of Orai1 and TRPC1 proteins and conduct the non-selective current I _SOC. While the crystal structure of Drosophila CRAC channel has already been solved, the architecture of the SOC channels still remains unclear. The dynamic interaction of STIM1 with the store-operated channels is modulated by a number of proteins that either support the formation of the functional STIM1-channel complex or protect the cell against Ca²⁺ overload.

Calcium signalling in T cells

Calcium (Ca²⁺) signalling is of paramount importance to immunity. Regulated increases in cytosolic and organellar Ca²⁺ concentrations in lymphocytes control complex and crucial effector functions such as metabolism, proliferation, differentiation, antibody and cytokine secretion and cytotoxicity. Altered Ca²⁺ regulation in lymphocytes leads to various autoimmune, inflammatory and immunodeficiency syndromes. Several types of plasma membrane and organellar Ca²⁺-permeable channels are functional in T cells. They contribute highly localized spatial and temporal Ca²⁺ microdomains that are required for achieving functional specificity. While the mechanistic details of these Ca²⁺ microdomains are only beginning to emerge, it is evident that through crosstalk, synergy and feedback mechanisms, they fine-tune T cell signalling to match complex immune responses. In this article, we review the expression and function of various Ca²⁺-permeable channels in the plasma membrane, endoplasmic reticulum, mitochondria and endolysosomes of T cells and their role in shaping immunity and the pathogenesis of immune-mediated diseases.

Metalloimmunology: The metal ion-controlled immunity

Metals are essential components in all forms of life required for the function of nearly half of all enzymes and are critically involved in virtually all fundamental biological processes. Especially, the transition metals iron (Fe), zinc (Zn), manganese (Mn), nickel (Ni), copper (Cu) and cobalt (Co) are crucial micronutrients known to play vital roles in metabolism as well due to their unique redox properties. Metals carry out three major functions within metalloproteins: to provide structural support, to serve as enzymatic cofactors, and to mediate electron transportation. Metal ions are also involved in the immune system from metal allergies to nutritional immunity. Within the past decade, much attention has been drawn to the roles of metal ions in the immune system, since increasing evidence has mounted to suggest that metals are critically implicated in regulating both the innate immune sensing of and the host defense against invading pathogens. The importance of ions in immunity is also evidenced by the identification of various immunodeficiencies in patients with mutations in ion channels and transporters. In addition, cancer immunotherapy has recently been conclusively demonstrated to be effective and important for future tumor treatment, although only a small percentage of cancer patients respond to immunotherapy because of inadequate immune activation. Importantly, metal ion-activated immunotherapy is becoming an effective and potential way in tumor therapy for better clinical application. Nevertheless, we are still in a primary stage of discovering the diverse immunological functions of ions and mechanistically understanding the roles of these ions in immune regulation. This review summarizes recent advances in the understanding of metal-controlled immunity. Particular emphasis is put on the mechanisms of innate immune stimulation and T cell activation by the essential metal ions like calcium (Ca²⁺), zinc (Zn²⁺), manganese (Mn²⁺), iron (Fe²⁺/Fe³⁺), and potassium (K⁺), followed by a few unessential metals, in order to draw a general diagram of metalloimmunology.

Sequential activation of STIM1 links Ca2+ with luminal domain unfolding

The stromal interaction molecule 1 (STIM1) has two important functions, Ca2+ sensing within the endoplasmic reticulum and activation of the store-operated Ca2+ channel Orai1, enabling plasma-membrane Ca2+ influx. We combined molecular dynamics (MD) simulations with live-cell recordings and determined the sequential Ca2+-dependent conformations of the luminal STIM1 domain upon activation. Furthermore, we identified the residues within the canonical and noncanonical EF-hand domains that can bind to multiple Ca2+ ions. In MD simulations, a single Ca2+ ion was sufficient to stabilize the luminal STIM1 complex. Ca2+ store depletion destabilized the two EF hands, triggering disassembly of the hydrophobic cleft that they form together with the stable SAM domain. Point mutations associated with tubular aggregate myopathy or cancer that targeted the canonical EF hand, and the hydrophobic cleft yielded constitutively clustered STIM1, which was associated with activation of Ca2+ entry through Orai1 channels. On the basis of our results, we present a model of STIM1 Ca2+ binding and refine the currently known initial steps of STIM1 activation on a molecular level.

Cardiomyocyte-Specific STIM1 (Stromal Interaction Molecule 1) Depletion in the Adult Heart Promotes the Development of Arrhythmogenic Discordant Alternans

BACKGROUND

STIM1 (stromal interaction molecule 1) is a calcium (Ca²⁺) sensor that regulates cardiac hypertrophy by triggering store-operated Ca²⁺ entry. Because STIM1 binding to phospholamban increases sarcoplasmic reticulum Ca²⁺ load independent of store-operated Ca²⁺ entry, we hypothesized that it controls electrophysiological function and arrhythmias in the adult heart.

METHODS

Inducible myocyte-restricted STIM1-KD (STIM1 knockdown) was achieved in adult mice using an αMHC (α-myosin heavy chain)-MerCreMer system. Mechanical and electrophysiological properties were examined using echocardiography in vivo and optical action potential (AP) mapping ex vivo in tamoxifen-induced STIM1^flox/flox-Cre^tg/- (STIM1-KD) and littermate controls for STIM1^flox/flox (referred to as STIM1-Ctl) and for Cre^tg/- without STIM deletion (referred to as Cre-Ctl).

RESULTS

STIM1-KD mice (N=23) exhibited poor survival compared with STIM1-Ctl (N=22) and Cre-Ctl (N=11) with >50% mortality after only 8-days of cardiomyocyte-restricted STIM1-KD. STIM1-KD but not STIM1-Ctl or Cre-Ctl hearts exhibited a proclivity for arrhythmic behavior, ranging from frequent ectopy to pacing-induced ventricular tachycardia/ventricular fibrillation (VT/VF). Examination of the electrophysiological substrate revealed decreased conduction velocity and increased AP duration (APD) heterogeneity in STIM1-KD. These features, however, were comparable in VT/VF(+) and VT/VF(-) hearts. We also uncovered a marked increase in the magnitude of APD alternans during rapid pacing, and the emergence of a spatially discordant alternans profile in STIM1-KD hearts. Unlike conduction velocity slowing and APD heterogeneity, the magnitude of APD alternans was greater (by 80%, P<0.05) in VT/VF(+) versus VT/VF(-) STIM1-KD hearts. Detailed phase mapping during the initial beats of VT/VF identified one or more rotors that were localized along the nodal line separating out-of-phase alternans regions.

CONCLUSIONS

In an adult murine model with inducible and myocyte-specific STIM1 depletion, we demonstrate for the first time the regulation of spatially discordant alternans by STIM1. Early mortality in STIM1-KD mice is likely related to enhanced susceptibility to VT/VF secondary to discordant APD alternans.

Loss of Orai2-Mediated Capacitative Ca 2+ Entry Is Neuroprotective in Acute Ischemic Stroke

Background and Purpose—

Ischemic stroke is one of the leading causes of disability and death. The principal goal of acute stroke treatment is the recanalization of the occluded cerebral arteries, which is, however, only effective in a very narrow time window. Therefore, neuroprotective treatments that can be combined with recanalization strategies are needed. Calcium overload is one of the major triggers of neuronal cell death. We have previously shown that capacitative Ca 2+ entry, which is triggered by the depletion of intracellular calcium stores, contributes to ischemia-induced calcium influx in neurons, but the responsible Ca 2+ channel is not known.

Methods—

Here, we have generated mice lacking the calcium channel subunit Orai2 and analyzed them in experimental stroke.

Results—

Orai2-deficient mice were protected from ischemic neuronal death both during acute ischemia under vessel occlusion and during ischemia/reperfusion upon successful recanalization. Calcium signals induced by calcium store depletion or oxygen/glucose deprivation were significantly diminished in Orai2-deficient neurons demonstrating that Orai2 is a central mediator of neuronal capacitative Ca 2+ entry and is involved in calcium overload during ischemia.

Conclusions—

Our experimental data identify Orai2 as an attractive target for pharmaceutical intervention in acute stroke.

GPR40 activation initiates store-operated Ca2+ entry and potentiates insulin secretion via the IP3R1/STIM1/Orai1 pathway in pancreatic β-cells

The long-chain fatty acid receptor GPR40 plays an important role in potentiation of glucose-induced insulin secretion (GIIS) from pancreatic β-cells. Previous studies demonstrated that GPR40 activation enhances Ca²⁺ release from the endoplasmic reticulum (ER) by activating inositol 1,4,5-triphosphate (IP3) receptors. However, it remains unknown how ER Ca²⁺ release via the IP3 receptor is linked to GIIS potentiation. Recently, stromal interaction molecule (STIM) 1 was identified as a key regulator of store-operated Ca²⁺ entry (SOCE), but little is known about its contribution in GPR40 signaling. We show that GPR40-mediated potentiation of GIIS is abolished by knockdown of IP3 receptor 1 (IP3R1), STIM1 or Ca²⁺-channel Orai1 in insulin-secreting MIN6 cells. STIM1 and Orai1 knockdown significantly impaired SOCE and the increase of intracellular Ca²⁺ by the GPR40 agonist, fasiglifam. Furthermore, β-cell-specific STIM1 knockout mice showed impaired fasiglifam-mediated GIIS potentiation not only in isolated islets but also in vivo. These results indicate that the IP3R1/STIM1/Orai1 pathway plays an important role in GPR40-mediated SOCE initiation and GIIS potentiation in pancreatic β-cells.

What's Bred in the Bone: Calcium Channels in Lymphocytes

Calcium (Ca²⁺) is an important second messenger in lymphocytes and is essential in regulating various intracellular pathways that control critical cell functions. Ca²⁺ channels are located in the plasma membrane and intracellular membranes, facilitating Ca²⁺ entry into the cytoplasm. Upon Ag receptor stimulation, Ca²⁺ can enter the lymphocyte via the Ca²⁺ release-activated Ca²⁺ channel found in the plasma membrane. The increase of cytosolic Ca²⁺ modulates signaling pathways, resulting in the transcription of target genes implicated in differentiation, activation, proliferation, survival, and apoptosis of lymphocytes. Along with Ca²⁺ release-activated Ca²⁺ channels, several other channels have been found in the membranes of T and B lymphocytes contributing to key cellular events. Among them are the transient receptor potential channels, the P2X receptors, voltage-dependent Ca²⁺ channels, and the inositol 1,4,5-trisphosphate receptor as well as the N-methyl-d-aspartate receptors. In this article, we review the contributions of these channels to mediating Ca²⁺ currents that drive specific lymphocyte functions.

Calcium-sensing stromal interaction molecule 2 upregulates nuclear factor of activated T cells 1 and transforming growth factor-β signaling to promote breast cancer metastasis

Background

Stromal interaction molecule (STIM) 2 is a key calcium-sensing molecule that regulates the stabilization of calcium ions (Ca²⁺) and therefore regulates downstream Ca²⁺-associated signaling and cellular events. We hypothesized that STIM2 regulates epithelial-mesenchymal transition (EMT) to promote breast cancer metastasis.

Methods

We determined the effects of gain, loss, and rescue of STIM2 on cellular motility, levels of EMT-related proteins, and secretion of transforming growth factor-β (TGF-β). We also conducted bioinformatics analyses and in vivo assessments of breast cancer growth and metastasis using xenograft models.

Results

We found a significant association between STIM2 overexpression and metastatic breast cancer. STIM2 overexpression activated the nuclear factor of activated T cells 1 (NFAT1) and TGF-β signaling. Knockdown of STIM2 inhibited the motility of breast cancer cells by inhibiting EMT via specific suppression of NFAT1 and inhibited mammary tumor metastasis in mice. In contrast, STIM2 overexpression promoted metastasis. These findings were validated in human tissue arrays of 340 breast cancer samples for STIM2.

Conclusion

Taken together, our results demonstrated that STIM2 specifically regulates NFAT1, which in turn regulates the expression and secretion of TGF-β1 to promote EMT in vitro and in vivo, leading to metastasis of breast cancer.

Electronic supplementary material

The online version of this article (10.1186/s13058-019-1185-1) contains supplementary material, which is available to authorized users.

Acylglycerol Kinase Maintains Metabolic State and Immune Responses of CD8+ T Cells

CD8⁺ T cell expansions and functions rely on glycolysis, but the mechanisms underlying CD8⁺ T cell glycolytic metabolism remain elusive. Here, we show that acylglycerol kinase (AGK) is required for the establishment and maintenance of CD8⁺ T cell metabolic and functional fitness. AGK deficiency dampens CD8⁺ T cell antitumor functions in vivo and perturbs CD8⁺ T cell proliferation in vitro. Activation of phosphatidylinositol-3-OH kinase (PI3K)-mammalian target of rapamycin (mTOR) signaling, which mediates elevated CD8⁺ T cell glycolysis, is tightly dependent on AGK kinase activity. Mechanistically, T cell antigen receptor (TCR)- and CD28-stimulated recruitment of PTEN to the plasma membrane facilitates AGK-PTEN interaction and AGK-triggered PTEN phosphorylation, thereby restricting PTEN phosphatase activity in CD8⁺ T cells. Collectively, these results demonstrate that AGK maintains CD8⁺ T cell metabolic and functional state by restraining PTEN activity and highlight a critical role for AGK in CD8⁺ T cell metabolic programming and effector function.

Gut Bacterial Metabolite Urolithin A (UA) Mitigates Ca2+ Entry in T Cells by Regulating miR-10a-5p

The gut microbiota influences several biological functions including immune responses. Inflammatory bowel disease is favorably influenced by consumption of several dietary natural plant products such as pomegranate, walnuts, and berries containing polyphenolic compounds such as ellagitannins and ellagic acid. The gut microbiota metabolizes ellagic acid resulting in the formation of bioactive urolithins A, B, C, and D. Urolithin A (UA) is the most active and effective gut metabolite and acts as a potent anti-inflammatory and anti-oxidant agent. However, whether gut metabolite UA affects the function of immune cells remains incompletely understood. T cell proliferation is stimulated by store operated Ca2+ entry (SOCE) resulting from stimulation of Orai1 by STIM1/STIM2. We show here that treatment of murine CD4+ T cells with UA (10 μM, 3 days) significantly blunted SOCE in CD4+ T cells, an effect paralleled by significant downregulation of Orai1 and STIM1/2 transcript levels and protein abundance. UA treatment further increased miR-10a-5p abundance in CD4+ T cells in a dose dependent fashion. Overexpression of miR-10a-5p significantly decreased STIM1/2 and Orai1 mRNA and protein levels as well as SOCE in CD4+ T cells. UA further decreased CD4+ T cell proliferation. Thus, the gut bacterial metabolite UA increases miR-10a-5p levels thereby downregulating Orai1/STIM1/STIM2 expression, store operated Ca2+ entry, and proliferation of murine CD4+ T cells.

Elevated plasma catecholamines functionally compensate for the reduced myogenic tone in smooth muscle STIM1 knockout mice but with deleterious cardiac effects

Aims

Stromal interaction molecule 1 (STIM1) has emerged as an important player in the regulation of growth and proliferation of smooth muscle cells. Therefore, we hypothesized that STIM1 plays a crucial role in the maintenance of vascular integrity. The objective of this study was to evaluate whether reduced expression of STIM1 could modify the structure and function of the vasculature, leading to changes in blood pressure (BP).

Methods and results

Smooth muscle-specific STIM1 knockout (sm-STIM1 KO) in mice resulted in arteries with ∼80% reduced STIM1 protein expression as compared with control mice. Mesenteric vessels exposed to increasing transmural pressure revealed attenuated myogenic reactivity and reduced vasoconstrictor response to phenylephrine in sm-STIM1 KO arteries. BP monitored via telemetry in sm-STIM1 KO and matched controls did not reveal differences. However, heart rate was significantly increased in sm-STIM1 KO mice. Consistent with these findings, plasma catecholamine levels were higher in sm-STIM1 KO than in control mice. Increased sympathetic activity in sm-STIM1 KO mice was unmasked by apha1-adrenergic receptor inhibitor (prazosin) and by treatment with the ganglion-blocking agent, hexamethonium. Both treatments resulted in a greater reduction of BP in sm-STIM1 KO mice. Cytoskeleton of cultured smooth muscle cells was studied by immunocytochemistry using specific antibodies. Staining for actin and vinculin revealed significant alterations in the cytoskeletal architecture of cells isolated from sm-STIM1 KO arteries. Finally, although sm-STIM1 KO mice were protected from Ang II-induced hypertension, such treatment resulted in significant fibrosis and a rapid deterioration of cardiac function.

Conclusions

STIM1 deletion in smooth muscle results in attenuated myogenic tone and cytoskeletal defects with detrimental effects on the mechanical properties of arterial tissue. Although BP is maintained by elevated circulating catecholamine, this compensatory stimulation has a deleterious long-term effect on the myocardium.

Enhancement of Cardiac Store Operated Calcium Entry (SOCE) within Novel Intercalated Disk Microdomains in Arrhythmic Disease

Store-operated Ca²⁺ entry (SOCE), a major Ca²⁺ signaling mechanism in non-myocyte cells, has recently emerged as a component of Ca²⁺ signaling in cardiac myocytes. Though it has been reported to play a role in cardiac arrhythmias and to be upregulated in cardiac disease, little is known about the fundamental properties of cardiac SOCE, its structural underpinnings or effector targets. An even greater question is how SOCE interacts with canonical excitation-contraction coupling (ECC). We undertook a multiscale structural and functional investigation of SOCE in cardiac myocytes from healthy mice (wild type; WT) and from a genetic murine model of arrhythmic disease (catecholaminergic ventricular tachycardia; CPVT). Here we provide the first demonstration of local, transient Ca²⁺ entry (LoCE) events, which comprise cardiac SOCE. Although infrequent in WT myocytes, LoCEs occurred with greater frequency and amplitude in CPVT myocytes. CPVT myocytes also evidenced characteristic arrhythmogenic spontaneous Ca²⁺ waves under cholinergic stress, which were effectively prevented by SOCE inhibition. In a surprising finding, we report that both LoCEs and their underlying protein machinery are concentrated at the intercalated disk (ID). Therefore, localization of cardiac SOCE in the ID compartment has important implications for SOCE-mediated signaling, arrhythmogenesis and intercellular mechanical and electrical coupling in health and disease.

Store-Operated Ca2+ Entry in Tumor Progression: From Molecular Mechanisms to Clinical Implications

The remodeling of Ca²⁺ homeostasis has been implicated as a critical event in driving malignant phenotypes, such as tumor cell proliferation, motility, and metastasis. Store-operated Ca²⁺ entry (SOCE) that is elicited by the depletion of the endoplasmic reticulum (ER) Ca²⁺ stores constitutes the major Ca²⁺ influx pathways in most nonexcitable cells. Functional coupling between the plasma membrane Orai channels and ER Ca²⁺-sensing STIM proteins regulates SOCE activation. Previous studies in the human breast, cervical, and other cancer types have shown the functional significance of STIM/Orai-dependent Ca²⁺ signals in cancer development and progression. This article reviews the information on the regulatory mechanisms of STIM- and Orai-dependent SOCE pathways in the malignant characteristics of cancer, such as proliferation, resistance, migration, invasion, and metastasis. The recent investigations focusing on the emerging importance of SOCE in the cells of the tumor microenvironment, such as tumor angiogenesis and antitumor immunity, are also reviewed. The clinical implications as cancer therapeutics are discussed.

Signaling and functional competency of neutrophils derived from bone-marrow cells expressing the ER-HOXB8 oncoprotein

Neutrophils play a central role in immunity and inflammation via their intrinsic ability to migrate into inflamed tissue, to phagocytose pathogens, and to kill bacterial and fungi by releasing large quantities of superoxide anions and lytic enzymes. The molecular pathways controlling neutrophil microbicidal functions are still unclear, because neutrophils have a short half-life and are resistant to genetic manipulation. Neutrophil-like cells (NLC) can be generated from myeloid progenitors conditionally immortalized with the ER-HoxB8 oncoprotein, but whether these cells can replace neutrophils in high-throughput functional assays is unclear. Here, we assess the ability of NLC derived from ER-HoxB8 progenitors to produce ROS and to perform chemotaxis and phagocytosis. We compare the Ca²⁺ responses and effector functions of NLC to primary murine neutrophils and document the molecular basis of their functional differences by mRNA profiling. Pro-inflammatory cytokines enhanced the expression by NLC of neutrophil surface markers and transcription factors. Ca²⁺ elevations evoked in NLC by agonists, adhesion receptors, and store depletion resembled the physiological responses recorded in primary neutrophils, but NLC expressed reduced amounts of Ca²⁺ signaling proteins and of chemotactic receptors. Unlike their myeloid progenitors, NLC produced H₂ O₂ when adhered to fibronectin, migrated toward chemotactic peptides, phagocytosed opsonized particles, and generated intracellular ROS. NLC phagocytosed as efficiently as primary neutrophils but produced 50 times less ROS and migrated less efficiently toward chemoattractant. Our data indicate that NLC can replace neutrophils to study Ca²⁺ signaling and phagocytosis, but that their incomplete granulocytic differentiation limits their use for chemotaxis and ROS production assays.

STIM1-dependent membrane insertion of heteromeric TRPC1-TRPC4 channels in response to muscarinic receptor stimulation

Muscarinic receptor stimulation results in activation of nonselective cation (NSC) channels in guinea pig adrenal medullary (AM) cells. The biophysical and pharmacological properties of the NSC channel suggest the involvement of heteromeric channels of TRPC1 with TRPC4 or TRPC5. This possibility was explored in PC12 cells and guinea pig AM cells. Proximity ligation assay (PLA) revealed that when exogenously expressed in PC12 cells, TRPC1 forms a heteromeric channel with TRPC4, but not with TRPC5, in a STIM1-dependent manner. The heteromeric TRPC1-TRPC4 channel was also observed in AM cells and trafficked to the cell periphery in response to muscarine stimulation. To explore whether heteromeric channels are inserted into the cell membrane, tags were attached to the extracellular domains of TRPC1 and TRPC4. PLA products developed between the tags in cells stimulated by muscarine, but not in resting cells, indicating that muscarinic stimulation results in the membrane insertion of channels. This membrane insertion required expression of full-length STIM1. We conclude that muscarinic receptor stimulation results in the insertion of heteromeric TRPC1-TRPC4 channels into the cell membrane in PC12 cells and guinea pig AM cells.

Calcium Signaling Controls Pathogenic Th17 Cell-Mediated Inflammation by Regulating Mitochondrial Function

Pathogenic Th17 cells play important roles in many autoimmune and inflammatory diseases. Their function depends on T cell receptor (TCR) signaling and cytokines that activate signal transducer and activator of transcription 3 (STAT3). TCR engagement activates stromal interaction molecule 1 (STIM1) and calcium (Ca²⁺) influx through Ca²⁺-release-activated Ca²⁺ (CRAC) channels. Here, we show that abolishing STIM1 and Ca²⁺ influx in T cells expressing a hyperactive form of STAT3 (STAT3C) attenuates pathogenic Th17 cell function and inflammation associated with STAT3C expression. Deletion of STIM1 in pathogenic Th17 cells reduces the expression of genes required for mitochondrial function and oxidative phosphorylation (OXPHOS) but enhances reactive oxygen species (ROS) production. STIM1 deletion or inhibition of OXPHOS is associated with a non-pathogenic Th17 gene expression signature and impaired pathogenic Th17 cell function. Our findings establish Ca²⁺ influx as a critical regulator of mitochondrial function and oxidative stress in pathogenic Th17 cell-mediated multiorgan inflammation.

Remodeling of ER-plasma membrane contact sites but not STIM1 phosphorylation inhibits Ca2+ influx in mitosis

The mechanisms blocking Ca²⁺ influx in mitosis are complex and involve a decrease in stable endoplasmic reticulum (ER)–plasma membrane (PM) contact sites and degradation of the ER Ca²⁺ sensor stromal interaction molecule 1 (STIM1) but not its phosphorylation. This challenges the current view that STIM1 phosphorylation is essential for mitotic store-operated Ca²⁺ entry inhibition and sheds light on the dynamics of ER–PM contact sites and of Ca²⁺ influx in mitosis.

Store-operated Ca²⁺ entry (SOCE), mediated by the endoplasmic reticulum (ER) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) and the plasma membrane (PM) channel Orai1, is inhibited during mitosis. STIM1 phosphorylation has been suggested to mediate this inhibition, but it is unclear whether additional pathways are involved. Here, we demonstrate using various approaches, including a nonphosphorylatable STIM1 knock-in mouse, that STIM1 phosphorylation is not required for SOCE inhibition in mitosis. Rather, multiple pathways converge to inhibit Ca²⁺ influx in mitosis. STIM1 interacts with the cochaperone BAG3 and localizes to autophagosomes in mitosis, and STIM1 protein levels are reduced. The density of ER–PM contact sites (CSs) is also dramatically reduced in mitosis, thus physically preventing STIM1 and Orai1 from interacting to activate SOCE. Our findings provide insights into ER–PM CS remodeling during mitosis and a mechanistic explanation of the inhibition of Ca²⁺ influx that is required for cell cycle progression.

Targeting the NFAT:AP-1 transcriptional complex on DNA with a small-molecule inhibitor

The transcription factor nuclear factor of activated T cells (NFAT) has a key role in both T cell activation and tolerance and has emerged as an important target of immune modulation. NFAT directs the effector arm of the immune response in the presence of activator protein-1 (AP-1), and T cell anergy/exhaustion in the absence of AP-1. Envisioning a strategy for selective modulation of the immune response, we designed a FRET-based high-throughput screen to identify compounds that disrupt the NFAT:AP-1:DNA complex. We screened ∼202,000 small organic compounds and identified 337 candidate inhibitors. We focus here on one compound, N-(3-acetamidophenyl)-2-[5-(1H-benzimidazol-2-yl)pyridin-2-yl]sulfanylacetamide (Compound 10), which disrupts the NFAT:AP-1 interaction at the composite antigen-receptor response element-2 site without affecting the binding of NFAT or AP-1 alone to DNA. Compound 10 binds to DNA in a sequence-selective manner and inhibits the transcription of the Il2 gene and several other cyclosporin A-sensitive cytokine genes important for the effector immune response. This study provides proof-of-concept that small molecules can inhibit the assembly of specific DNA-protein complexes, and opens a potential new approach to treat human diseases where known transcription factors are deregulated.

Pivotal role of STIM2, but not STIM1, in IL-4 production by IL-3-stimulated murine basophils

Basophils have nonredundant roles in various immune responses that require Ca²⁺ influx. Here, we examined the role of two Ca²⁺ sensors, stromal interaction molecule 1 and 2 (STIM1 and STIM2), in basophil activation. We found that loss of STIM1, but not STIM2, impaired basophil IL-4 production after stimulation with immunoglobulin E (IgE)-containing immune complexes. In contrast, when basophils were stimulated with IL-3, loss of STIM2, but not STIM1, reduced basophil IL-4 production. This difference in STIM proteins was associated with distinct time courses of Ca²⁺ influx and transcription of the Il4 gene that were elicited by each stimulus. Similarly, basophil-specific STIM1 expression was required for IgE-driven chronic allergic inflammation in vivo, whereas STIM2 was required for IL-4 production after combined IL-3 and IL-33 treatment in mice. These data indicate that STIM1 and STIM2 have differential roles in the production of IL-4, which are stimulus dependent. Furthermore, these results illustrate the vital role of STIM2 in basophils, which is often considered to be less important than STIM1.

Stromal Interaction Molecule Deficiency in T Cells Promotes Spontaneous Follicular Helper T Cell Development and Causes Type 2 Immune Disorders

Appropriate T cell responses are controlled by strict balance between activatory and inhibitory pathways downstream of TCR. Although mice or humans with impaired TCR signaling develop autoimmunity, the precise molecular mechanisms linking reduced TCR signaling to autoimmunity are not fully understood. Engagement of TCR activates Ca²⁺ signaling mainly through store-operated Ca²⁺ entry activated by stromal interaction molecule (Stim) 1 and Stim2. Despite defective T cell activation, mice deficient in both Stim1 and Stim2 in T cells (conditional double knockout [cDKO]) developed lymphoproliferative disorders and skin inflammation with a concomitant increase in serum IgG1 and IgE levels. In cDKO mice, follicular helper T (Tfh) cells were dramatically increased in number, and they produced IL-4 spontaneously. These inflammatory symptoms were abolished by the deletion of IL-4 in cDKO mice. Tfh development and inflammatory symptoms in cDKO mice were abrogated by further deletion of NFAT2 in T cells. These findings suggest that Tfh cells spontaneously developed in the absence of Ca²⁺ signaling and caused unregulated type 2 responses.

Phosphatase inhibitor PPP1R11 modulates resistance of human T cells toward Treg-mediated suppression of cytokine expression

Regulatory T cells (Tregs) act as indispensable unit for maintaining peripheral immune tolerance mainly by regulating effector T cells. T cells resistant to suppression by Tregs pose therapeutic challenges in the treatment of autoimmune diseases, while augmenting susceptibility to suppression may be desirable for cancer therapy. To understand the cell intrinsic signals in T cells during suppression by Tregs, we have previously performed a global phosphoproteomic characterization. We revealed altered phosphorylation of protein phosphatase 1 regulatory subunit 11 (PPP1R11; Inhibitor‐3) in conventional T cells upon suppression by Tregs. Here, we show that silencing of PPP1R11 renders T cells resistant toward Treg‐mediated suppression of TCR‐induced cytokine expression. Furthermore, whole‐transcriptome sequencing revealed that PPP1R11 differentially regulates not only the expression of specific T cell stimulation‐induced cytokines but also other molecules and pathways in T cells. We further confirmed the target of PPP1R11, PP1, to augment TCR‐induced cytokine expression. In conclusion, we present PPP1R11 as a novel negative regulator of T cell activation‐induced cytokine expression. Targeting PPP1R11 may have therapeutic potential to regulate the T cell activation status including modulating the susceptibility of T cells toward Treg‐mediated suppression, specifically altering the stimulation‐induced T cell cytokine milieu.

Tissue resident and follicular Treg cell differentiation is regulated by CRAC channels

T regulatory (Treg) cells maintain immunological tolerance and organ homeostasis. Activated Treg cells differentiate into effector Treg subsets that acquire tissue-specific functions. Ca²⁺ influx via Ca²⁺ release-activated Ca²⁺ (CRAC) channels formed by STIM and ORAI proteins is required for the thymic development of Treg cells, but its function in mature Treg cells remains unclear. Here we show that deletion of Stim1 and Stim2 genes in mature Treg cells abolishes Ca²⁺ signaling and prevents their differentiation into follicular Treg and tissue-resident Treg cells. Transcriptional profiling of STIM1/STIM2-deficient Treg cells reveals that Ca²⁺ signaling regulates transcription factors and signaling pathways that control the identity and effector differentiation of Treg cells. In the absence of STIM1/STIM2 in Treg cells, mice develop a broad spectrum of autoantibodies and fatal multiorgan inflammation. Our findings establish a critical role of CRAC channels in controlling lineage identity and effector functions of Treg cells.

Regulatory T (Treg) cells are important for maintaining immune homeostasis. Here the authors show that STIM1 and STIM2, which activate the Ca²⁺ channel ORAI1, are essential for the differentiation of peripheral Treg cells into tissue-resident and follicular Treg cells and their ability to limit autoimmunity in mice.

ORAI Channels as Potential Therapeutic Targets in Pulmonary Hypertension

Pulmonary hypertension is a complex and fatal disease that lacks treatments. Its pathophysiology involves pulmonary artery hyperreactivity, endothelial dysfunction, wall remodelling, inflammation, and thrombosis, which could all depend on ORAI Ca²⁺ channels. We review the knowledge about ORAI channels in pulmonary artery and discuss the interest to target them in the treatment of pulmonary hypertension.

STIM1 thermosensitivity defines the optimal preference temperature for warm sensation in mice

Mammals possess a remarkable ability to sense subtle temperature deviations from the thermoneutral skin temperature of ~33 °C, which ensures precise warm sensation. However, the underlying mechanisms remain unclear. Here we show that STIM1, an endoplasmic reticulum (ER) resident transmembrane protein that responds to both ER Ca²⁺ depletion and heat, mediates temperature-induced Ca²⁺ influx in skin keratinocytes via coupling to Orai Ca²⁺ channels in plasma membrane. Behaviorally, the keratinocyte-specific knockout of STIM1 shifts the optimal preference temperature (OPT) of mice from ~32 °C to ~34 °C, resulting in a strikingly reversed preference between 32 °C and 34 °C. Importantly, the thermally inactive STIM1-ΔK knock-in mice show altered OPT and warm preference behaviors as well, demonstrating the requirement of STIM1 thermosensitivity for warm sensation. Furthermore, the wild-type and mutant mice prefer temperatures closer to their respective OPTs, but poorly distinguish temperatures that are equally but oppositely deviated from their OPTs. Mechanistically, keratinocyte STIM1 affects the in vivo warm responses of sensory neurons by likely involving TRPA1 as a downstream transduction channel. Collectively, our data suggest that STIM1 serves as a novel in vivo thermosensor in keratinocytes to define the OPT, which might be utilized as a peripheral reference temperature for precise warm sensation.

ORAI1, STIM1/2, and RYR1 shape subsecond Ca2+ microdomains upon T cell activation

The earliest intracellular signals that occur after T cell activation are local, subsecond Ca²⁺ microdomains. Here, we identified a Ca²⁺ entry component involved in Ca²⁺ microdomain formation in both unstimulated and stimulated T cells. In unstimulated T cells, spontaneously generated small Ca²⁺ microdomains required ORAI1, STIM1, and STIM2. Super-resolution microscopy of unstimulated T cells identified a circular subplasmalemmal region with a diameter of about 300 nm with preformed patches of colocalized ORAI1, ryanodine receptors (RYRs), and STIM1. Preformed complexes of STIM1 and ORAI1 in unstimulated cells were confirmed by coimmunoprecipitation and Förster resonance energy transfer studies. Furthermore, within the first second after T cell receptor (TCR) stimulation, the number of Ca²⁺ microdomains increased in the subplasmalemmal space, an effect that required ORAI1, STIM2, RYR1, and the Ca²⁺ mobilizing second messenger NAADP (nicotinic acid adenine dinucleotide phosphate). These results indicate that preformed clusters of STIM and ORAI1 enable local Ca²⁺ entry events in unstimulated cells. Upon TCR activation, NAADP-evoked Ca²⁺ release through RYR1, in coordination with Ca²⁺ entry through ORAI1 and STIM, rapidly increases the number of Ca²⁺ microdomains, thereby initiating spread of Ca²⁺ signals deeper into the cytoplasm to promote full T cell activation.

Tuning store-operated calcium entry to modulate Ca2+-dependent physiological processes

The intracellular calcium signaling processes are tightly regulated to ensure the generation of calcium signals with the specific spatiotemporal characteristics required for regulating various cell functions. Compartmentalization of the molecular components involved in the generation of these signals at discrete intracellular sites ensures the signaling specificity and transduction fidelity of the signal for regulating downstream effector processes. Store-operated calcium entry (SOCE) is ubiquitously present in cells and is critical for essential cell functions in a variety of tissues. SOCE is mediated via plasma membrane Ca²⁺ channels that are activated when luminal [Ca²⁺] of the endoplasmic reticulum ([Ca²⁺]_ER) is decreased. The ER-resident stromal interaction molecules, STIM1 and STIM2, respond to decreases in [Ca²⁺]_ER by undergoing conformational changes that cause them to aggregate at the cell periphery in ER-plasma membrane (ER-PM) junctions. At these sites, STIM proteins recruit Orai1 channels and trigger their activation. Importantly, the two STIM proteins concertedly modulate Orai1 function as well as the sensitivity of SOCE to ER-Ca²⁺ store depletion. Another family of plasma membrane Ca²⁺ channels, known as the Transient Receptor Potential Canonical (TRPC) channels (TRPC1-7) also contribute to sustained [Ca²⁺]_i elevation. Although Ca²⁺ signals generated by these channels overlap with those of Orai1, they regulate distinct functions in the cells. Importantly, STIM1 is also required for plasma membrane localization and activation of some TRPCs. In this review, we will discuss various molecular components and factors that govern the activation, regulation and modulation of the Ca²⁺ signal generated by Ca²⁺ entry pathways in response to depletion of ER-Ca²⁺ stores. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Store-operated calcium entry in thrombosis and thrombo-inflammation

Cytosolic free calcium (Ca²⁺) is a second messenger regulating a wide variety of functions in blood cells, including adhesion, activation, proliferation and migration. Store-operated Ca²⁺ entry (SOCE), triggered by depletion of Ca²⁺ from the endoplasmic reticulum, provides a main mechanism of regulated Ca²⁺ influx in blood cells. SOCE is mediated and regulated by isoforms of the ion channel proteins ORAI and TRP, and the transmembrane Ca²⁺ sensors stromal interaction molecules (STIMs), respectively. This report provides an overview of the (patho)physiological importance of SOCE in blood cells implicated in thrombosis and thrombo-inflammation, i.e. platelets and immune cells. We also discuss the physiological consequences of dysregulated SOCE in platelets and immune cells and the potential of SOCE inhibition as a therapeutic option to prevent or treat arterial thrombosis as well as thrombo-inflammatory disease states such as ischemic stroke.

Identification of molecular determinants that govern distinct STIM2 activation dynamics

The endoplasmic reticulum (ER) Ca²⁺ sensors stromal interaction molecule 1 (STIM1) and STIM2, which connect ER Ca²⁺ depletion with extracellular Ca²⁺ influx, are crucial for the maintenance of Ca²⁺ homeostasis in mammalian cells. Despite the recent progress in unraveling the role of STIM2 in Ca²⁺ signaling, the mechanistic underpinnings of its activation remain underexplored. We use an engineering approach to direct ER-resident STIMs to the plasma membrane (PM) while maintaining their correct membrane topology, as well as Förster resonance energy transfer (FRET) sensors that enabled in cellulo real-time monitoring of STIM activities. This allowed us to determine the calcium affinities of STIM1 and STIM2 both in cellulo and in situ, explaining the current discrepancies in the literature. We also identified the key structural determinants, especially the corresponding G residue in STIM1, which define the distinct activation dynamics of STIM2. The chimeric E470G mutation could switch STIM2 from a slow and weak Orai channel activator into a fast and potent one like STIM1 and vice versa. The systemic dissection of STIM2 activation by protein engineering sets the stage for the elucidation of the regulation and function of STIM2-mediated signaling in mammals.

Calcium ions play a major regulatory role in the physiology and biochemistry of the cell, and thus their levels and activities should be tightly regulated. The stromal interaction molecules (STIMs) are sensors of the calcium levels within the endoplasmic reticulum (ER)—which serves as a major intracellular calcium store—to mediate communication between the ER and the plasma membrane and are regarded as ubiquitous central players of calcium signaling in mammalian cells. STIM2 acts as a slow and weak activator of Orai1 calcium channels on the plasma membrane by direct binding; however, the affinity of STIMs for calcium or how Orai1 channels are activated remain unclear. In this study, we systematically analyzed the molecular determinants that govern the activation of STIM proteins. Adopting protein engineering approaches that enable the relocation of ER-resident STIM proteins at the plasma membrane, we determined the calcium affinities of STIMs under physiological conditions in mammalian cells. We identified a critical position within STIMs, which defines their distinct resting states and activation kinetics, as well as the efficacy to activate Orai1 channels. These findings shed new light on how STIM2 can efficiently respond to small changes within the ER lumen to regulate calcium homeostasis and signaling in mammalian cells.

STIM1 and Orai1 regulate Ca2+ microdomains for activation of transcription

Since calcium (Ca²⁺) regulates a large variety of cellular signaling processes in a cell's life, precise control of Ca²⁺ concentrations within the cell is essential. This enables the transduction of information via Ca²⁺ changes in a time-dependent and spatially defined manner. Here, we review molecular and functional aspects of how the store-operated Ca²⁺ channel Orai1 creates spatiotemporal Ca²⁺ microdomains. The architecture of this channel is unique, with a long helical pore and a six-fold symmetry. Energetic barriers within the Ca²⁺ channel pathway limit permeation to allow an extensive local Ca²⁺ increase in close proximity to the channel. The precise timing of the Orai1 channel function is controlled by direct binding to STIM proteins upon Ca²⁺ depletion in the endoplasmic reticulum. These induced Ca²⁺ microdomains are tailored to, and sufficient for, triggering long-term activation processes, such as transcription factor activation and subsequent gene regulation. We describe the principles of spatiotemporal activation of the transcription factor NFAT and compare its signaling characteristics to those of the autophagy regulating transcription factors, MITF and TFEB.

CRAC channels in dental enamel cells

Enamel mineralization relies on Ca2+ availability provided by Ca2+ release activated Ca2+ (CRAC) channels. CRAC channels are modulated by the endoplasmic reticulum Ca2+ sensor STIM1 which gates the pore subunit of the channel known as ORAI1, found the in plasma membrane, to enable sustained Ca2+ influx. Mutations in the STIM1 and ORAI1 genes result in CRAC channelopathy, an ensemble of diseases including immunodeficiency, muscular hypotonia, ectodermal dysplasia with defects in sweat gland function and abnormal enamel mineralization similar to amelogenesis imperfecta (AI). In some reports, the chief medical complain has been the patient's dental health, highlighting the direct and important link between CRAC channels and enamel. The reported enamel defects are apparent in both the deciduous and in permanent teeth and often require extensive dental treatment to provide the patient with a functional dentition. Among the dental phenotypes observed in the patients, discoloration, increased wear, hypoplasias (thinning of enamel) and chipping has been reported. These findings are not universal in all patients. Here we review the mutations in STIM1 and ORAI1 causing AI-like phenotype, and evaluate the enamel defects in CRAC channel deficient mice. We also provide a brief overview of the role of CRAC channels in other mineralizing systems such as dentine and bone.

Impaired Store-Operated Calcium Entry and STIM1 Loss Lead to Reduced Insulin Secretion and Increased Endoplasmic Reticulum Stress in the Diabetic β-Cell

Store-operated Ca2+ entry (SOCE) is a dynamic process that leads to refilling of endoplasmic reticulum (ER) Ca2+ stores through reversible gating of plasma membrane Ca2+ channels by the ER Ca2+ sensor Stromal Interaction Molecule 1 (STIM1). Pathogenic reductions in β-cell ER Ca2+ have been observed in diabetes. However, a role for impaired SOCE in this phenotype has not been tested. We measured the expression of SOCE molecular components in human and rodent models of diabetes and found a specific reduction in STIM1 mRNA and protein levels in human islets from donors with type 2 diabetes (T2D), islets from hyperglycemic streptozotocin-treated mice, and INS-1 cells (rat insulinoma cells) treated with proinflammatory cytokines and palmitate. Pharmacologic SOCE inhibitors led to impaired islet Ca2+ oscillations and insulin secretion, and these effects were phenocopied by β-cell STIM1 deletion. STIM1 deletion also led to reduced ER Ca2+ storage and increased ER stress, whereas STIM1 gain of function rescued β-cell survival under proinflammatory conditions and improved insulin secretion in human islets from donors with T2D. Taken together, these data suggest that the loss of STIM1 and impaired SOCE contribute to ER Ca2+ dyshomeostasis under diabetic conditions, whereas efforts to restore SOCE-mediated Ca2+ transients may have the potential to improve β-cell health and function.

Pyrtriazoles, a Novel Class of Store-Operated Calcium Entry Modulators: Discovery, Biological Profiling, and in Vivo Proof-of-Concept Efficacy in Acute Pancreatitis

In recent years, channels that mediate store-operated calcium entry (SOCE, i.e., the ability of cells to sense a decrease in endoplasmic reticulum luminal calcium and induce calcium entry across the plasma membrane) have been associated with a number of disorders, spanning from immune disorders to acute pancreatitis and have been suggested to be druggable targets. In the present contribution, we exploited the click chemistry approach to synthesize a class of SOCE modulators where the arylamide substructure that characterizes most inhibitors so far described is substituted by a 1,4-disubstituted 1,2,3-triazole ring. Within this series, inhibitors of SOCE were identified and the best compound proved effective in an animal model of acute pancreatitis, a disease characterized by a hyperactivation of SOCE. Strikingly, two enhancers of the process were discovered, affording invaluable research tools to further explore the (patho)physiological role of capacitative calcium entry.

Unfolded Protein Response-Dependent Communication and Contact among Endoplasmic Reticulum, Mitochondria, and Plasma Membrane

The function of the endoplasmic reticulum (ER) can be impaired by changes to the extra- and intracellular environment, such as disruption of calcium homeostasis, expression of mutated proteins, and oxidative stress. In response to disruptions to ER homeostasis, eukaryotic cells activate canonical branches of signal transduction cascades, collectively termed the unfolded protein response (UPR). The UPR functions to remove or recover the activity of misfolded proteins that accumulated in the ER and to avoid irreversible cellular damage. Additionally, the UPR plays unique physiological roles in the regulation of diverse cellular events, including cell differentiation and development and lipid biosynthesis. Recent studies have shown that these important cellular events are also regulated by contact and communication among organelles. These reports suggest strong involvement among the UPR, organelle communication, and regulation of cellular homeostasis. However, the precise mechanisms for the formation of contact sites and the regulation of ER dynamics by the UPR remain unresolved. In this review, we summarize the current understanding of how the UPR regulates morphological changes to the ER and the formation of contact sites between the ER and other organelles. We also review how UPR-dependent connections between the ER and other organelles affect cellular and physiological functions.

Modulation of intracellular calcium signaling by microRNA-34a-5p

Adjusting intracellular calcium signaling is an important feature in the regulation of immune cell function and survival. Here we show that miR-34a-5p, a small non-coding RNA that is deregulated in many common diseases, is a regulator of store-operated Ca²⁺ entry (SOCE) and calcineurin signaling. Upon miR-34a-5p overexpression, we observed both a decreased depletion of ER calcium content and a decreased Ca²⁺ influx through Ca²⁺ release-activated Ca²⁺ channels. Based on an in silico target prediction we identified multiple miR-34a-5p target genes within both pathways that are implicated in the balance between T-cell activation and apoptosis including ITPR2, CAMLG, STIM1, ORAI3, RCAN1, PPP3R1, and NFATC4. Functional analysis revealed a decrease in Ca²⁺ activated calcineurin pathway activity measured by a reduced IL-2 secretion due to miR-34a-5p overexpression. Impacting SOCE and/or downstream calcineurin/NFAT signaling by miR-34a-5p offers a possible future approach to manipulate immune cells for clinical interventions.

The Pyrazole Derivative BTP2 Attenuates IgG Immune Complex-induced Inflammation

Store-operated calcium entry (SOCE) is the most common mode of calcium influx in non-excitable cells, including immune cells. The two STIM isoforms mediate SOCE as well as Fc receptor (FcR)-downstream activation of macrophages and mast cells-which appears to be relevant in vivo, in models of antibody-dependent tissue injury and allergy. Hence, the pathway of SOCE may be a therapeutic target for treatment of immune complex (IC)-mediated autoimmunity and allergic asthma. The pyrazole derivative, BTP2 is an efficient inhibitor of SOCE, which has already been shown to attenuate allergic inflammation. However, its effect on Fc gamma receptor (FcγR) signaling and IC-induced tissue injury had not yet been studied. Here, we show that BTP2 is a potent inhibitor of SOCE in primary macrophages, blocking FcγR-mediated responses. To investigate the effect of inhibition of SOCE in IC-mediated tissue injury, we induced reverse passive Arthus reaction to IgG immune complexes in the skin and lungs of BTP2- or control-treated mice. Treatment with BTP2 resulted in markedly attenuated inflammation in both the skin and the lungs. Our findings indicate the involvement of SOCE in FcγR-mediated responses in vitro and in vivo and suggest that BTP2-mediated inhibition of SOCE may have a therapeutic potential on IC-mediated autoimmunity.

M1 Macrophage Polarization Is Dependent on TRPC1-Mediated Calcium Entry

Macrophage plasticity is essential for innate immunity, but in-depth signaling mechanism(s) regulating their functional phenotypes are ill-defined. Here we report that interferon (IFN) γ priming of naive macrophages induces store-mediated Ca²⁺ entry and inhibition of Ca²⁺ entry impairs polarization to M1 inflammatory phenotype. In vitro and in vivo functional analyses revealed ORAI1 to be a primary contributor to basal Ca²⁺ influx in macrophages, whereas IFNγ-induced Ca²⁺ influx was mediated by TRPC1. Deficiency of TRPC1 displayed abrogated IFNγ-induced M1 inflammatory mediators in macrophages. In a preclinical model of peritonitis by Klebsiella pneumoniae infection, macrophages showed increased Ca²⁺ influx, which was TRPC1 dependent. Macrophages from infected TRPC1^−/− mice showed inhibited expression of M1-associated signature molecules. Furthermore, in human patients with systemic inflammatory response syndrome, the level of TRPC1 expression in circulating macrophages directly correlated with M1 inflammatory mediators. Overall, TRPC1-mediated Ca²⁺ influx is essential for the induction/shaping of macrophage polarization to M1 inflammatory phenotype.

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TRPC1 mediates sterile or infection-induced Ca²⁺ influx and M1 phenotype in macrophages

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ORAI1 mediates the basal Ca²⁺ influx in macrophages

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In patients with SIRS, the TRPC1 level correlates with M1 inflammatory mediators in macrophages

T cell activation triggers reversible inosine-5'-monophosphate dehydrogenase assembly

T cell-mediated adaptive immunity requires naïve, unstimulated T cells to transition from a quiescent metabolic state into a highly proliferative state upon T cell receptor engagement. This complex process depends on transcriptional changes mediated by Ca²⁺-dependent NFAT signaling, mTOR-mediated signaling and increased activity of the guanine nucleotide biosynthetic inosine-5'-monophosphate (IMP) dehydrogenase 1 and 2 enzymes (IMPDH1 and IMPDH2, hereafter IMPDH). Inhibitors of these pathways serve as potent immunosuppressants. Unexpectedly, we discovered that all three pathways converge to promote the assembly of IMPDH protein into micron-scale macromolecular filamentous structures in response to T cell activation. Assembly is post-transcriptionally controlled by mTOR and the Ca²⁺ influx regulator STIM1. Furthermore, IMPDH assembly and catalytic activity were negatively regulated by guanine nucleotide levels, suggesting a negative feedback loop that limits biosynthesis of guanine nucleotides. Filamentous IMPDH may be more resistant to this inhibition, facilitating accumulation of the higher GTP levels required for T cell proliferation.

SEPT7-mediated regulation of Ca2+ entry through Orai channels requires other septin subunits

Orai channels are plasma membrane resident Ca²⁺ channels that allow extracellular Ca²⁺ uptake after depletion of ER-Ca²⁺ stores by a process called store-operated Ca²⁺ entry (SOCE). Septins of the SEPT2 subgroup act as positive regulators of SOCE in human nonexcitable cells. SEPT2 subgroup septins form the central core of hetero-hexameric or hetero-octameric complexes with SEPT6, SEPT7 and SEPT9 subgroup septins. The presence of fewer septin encoding genes coupled with ease of genetic manipulation allows for better understanding of septin subgroup function in Drosophila. Our earlier findings show that although dSEPT7 reduction does not alter Orai-mediated Ca²⁺ entry during SOCE, it results in constitutive activation of Orai channels in resting neurons. Here, we have investigated the role of other septin subgroup members in regulating Orai channel activation in Drosophila neurons by both cellular and functional assays. We show that dSEPT1, a SEPT2 subgroup septin can exist in a complex with dSEPT2 and dSEPT7 in the central nervous system (CNS) of Drosophila. Our findings suggest that the nature of septin filaments and heteromers obtained after reducing septins of different subgroups alters their ability to regulate Orai channel opening. The molecular mechanisms underlying this complex regulation of Orai function by septins require further cellular investigations.

Calcium release-activated calcium channels and pain

Calcium release-activated calcium (CRAC) channels are unique among ion channels that are activated in response to depletion of intracellular calcium stores and are highly permeable to Ca²⁺ compared to other cations. CRAC channels mediate an important calcium signal for a wide variety of cell types and are well studied in the immune system. They have been implicated in a number of disorders such as immunodeficiency, musculosketal disorders and cancer. There is growing evidence showing that CRAC channels are expressed in the nervous system and are involved in pathological conditions including pain. This review summarizes the expression, distribution, and function of the CRAC channel family in the dorsal root ganglion, spinal cord and some brain regions, and discusses their functional significance in neurons and glial cells and involvement in nociception and chronic pain. Although further studies are needed to understand how these channels are activated under physiological conditions, the recent findings indicate that the CRAC channel Orai1 is an important player in pain modulation and could represent a new target for pathological pain.

Stim and Orai mediate constitutive Ca2+ entry and control endoplasmic reticulum Ca2+ refilling in primary cultures of colorectal carcinoma cells

Store-operated Ca²⁺ entry (SOCE) provides a major Ca²⁺ entry route in cancer cells. SOCE is mediated by the assembly of Stim and Orai proteins at endoplasmic reticulum (ER)-plasma membrane junctions upon depletion of the ER Ca²⁺ store. Additionally, Stim and Orai proteins underpin constitutive Ca²⁺ entry in a growing number of cancer cell types due to the partial depletion of their ER Ca²⁺ reservoir. Herein, we investigated for the first time the structure and function of SOCE in primary cultures of colorectal carcinoma (CRC) established from primary tumor (pCRC) and metastatic lesions (mCRC) of human subjects. Stim1-2 and Orai1-3 transcripts were equally expressed in pCRC and mCRC cells, although Stim1 and Orai3 proteins were up-regulated in mCRC cells. The Mn²⁺-quenching technique revealed that constitutive Ca2⁺ entry was significantly enhanced in pCRC cells and was inhibited by the pharmacological and genetic blockade of Stim1, Stim2, Orai1 and Orai3. The larger resting Ca²⁺ influx in pCRC was associated to their lower ER Ca²⁺ content as compared to mCRC cells. Pharmacological and genetic blockade of Stim1, Stim2, Orai1 and Orai3 prevented ER-dependent Ca²⁺ release, thereby suggesting that constitutive SOCE maintains ER Ca²⁺ levels. Nevertheless, pharmacological and genetic blockade of Stim1, Stim2, Orai1 and Orai3 did not affect CRC cell proliferation and migration. These data provide the first evidence that Stim and Orai proteins mediate constitutive Ca²⁺ entry and replenish ER with Ca²⁺ in primary cultures of CRC cells. However, SOCE is not a promising target to design alternative therapies for CRC.

CRAC channels as targets for drug discovery and development

Calcium release-activated calcium (CRAC) channels have been the target of drug discovery for many years. The identification of STIM and Orai proteins as key components of CRAC channels greatly facilitated this process because their co-expression in cell lines produced electrophysiological currents (I_CRAC) much larger than those in native cells, making it easier to confirm and characterize the effects of modulatory compounds. A driving force in the quest for CRAC channel drugs has been the immunocompromised phenotype displayed by humans and mice with null or loss-of-function mutations in STIM1 or Orai1, suggesting that CRAC channel inhibitors could be useful therapeutics for autoimmune or inflammatory conditions. Emerging data also suggests that other therapeutic conditions may benefit from CRAC channel inhibition. However, only recently have CRAC channel inhibitors reached clinical trials. This review discusses the challenges associated with drug discovery and development on CRAC channels and the approaches employed to date, as well as the results, starting from initial high-throughput screens for CRAC channel modulators and progressing through target selection and justification, descriptions of pharmacological, safety and toxicological profiles of compounds, and finally the entry of CRAC channel inhibitors into clinical trials.