Store-operated Ca²+ signaling in dendritic cells occurs independently of STIM1

New insights into the stromal interaction molecule 2 function and its impact on the immunomodulation of tumor microenvironment

Immune cells-enhanced immunotherapy exhibits unprecedented overall survival-prolongation even curable in some cancer patients. Although so, most of the patients show no response. Tumor microenvironment (TME) where immune cells settle down has multi-faceted influences, but usually creates an immunosuppressive niche that facilitating tumor cells escape from immune attack. The metabolites and malnutrition of TME exert enormous effects on the resident immune cells, but the underlying mechanism is largely unknown. The stromal interaction molecules 2 (STIM2) is an endoplasmic reticulum (ER) calcium (Ca2+) sensor to maintain Ca2+ homeostasis. Notably, the cytosol STIM2 C-terminus is long with various domains that are available for the combination or/and molecular modification. This distinct structure endows STIM2 with a high susceptibility to numerous permeable physico-chemical molecules or protein interactions. STIM2 and its variants are extensively expressed in various immune cells, especially in T immune cells. STIM2 was reported closely correlated with the function of immune cells via regulating Ca2+ signaling, energy metabolism and cell fitness. Herein, we sum the latest findings on the STIM2 structure, focusing on its distinct characteristics and profound effect on the regulation of Ca2+ homeostasis and multi-talented functionality. We also outline the advancements on the underlying mechanism how STIM2 anomalies influence the function of immune cells and on the turbulent expression or/and amenably modification of STIM2 within the tumor niches. Then we discuss the translation of these researches into antitumor approaches, emphasizing the potential of STIM2 as a therapeutic target for direct inhibition of tumor cells or more activation towards immune cells driving to flare TME. This review is an update on STIM2, aiming to rationalize the potential of STIM2 as a therapeutic target for immunomodulation, engaging immune cells to exert the utmost anti-tumor effect.

Sex-specific Stone-forming Phenotype in Mice During Hypercalciuria/Urine Alkalinization

Sex differences in kidney stone formation are well known. Females generally have slightly acidic blood and higher urine pH when compared with males, which makes them more vulnerable to calcium stone formation, yet the mechanism is still unclear. We aimed to examine the role of sex in stone formation during hypercalciuria and urine alkalinization through acetazolamide and calcium gluconate supplementation, respectively, for 4 weeks in wild-type (WT) and moderately hypercalciuric [TRPC3 knockout [KO](-/-)] male and female mice. Our goal was to develop calcium phosphate (CaP) and CaP+ calcium oxalate mixed stones in our animal model to understand the underlying sex-based mechanism of calcium nephrolithiasis. Our results from the analyses of mice urine, serum, and kidney tissues show that female mice (WT and KO) produce more urinary CaP crystals, higher [Ca2+], and pH in urine compared to their male counterparts. We identified a sex-based relationship of stone-forming phenotypes (types of stones) in our mice model following urine alkalization/calcium supplementation, and our findings suggest that female mice are more susceptible to CaP stones under those conditions. Calcification and fibrotic and inflammatory markers were elevated in treated female mice compared with their male counterparts, and more so in TRPC3 KO mice compared with their WT counterparts. Together these findings contribute to a mechanistic understanding of sex-influenced CaP and mixed stone formation that can be used as a basis for determining the factors in sex-related clinical studies.

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.

Anti-inflammatory role of extracellular l-arginine through calcium sensing receptor in human renal proximal tubular epithelial (HK-2) cells

Renal tubular epithelial cells are capable of synthesizing interleukins (IL) in response to a variety of proinflammatory cytokines. Moreover, elevated urinary levels of IL have been shown in patients with various forms of nephritic diseases. However, the underlying intracellular signaling mechanism is unclear. Here we show the immunological signaling role of l-Arginine (l-Arg) through Ca2+-sensing receptor (CaSR) in human kidney 2 (HK-2) renal proximal tubular epithelial cells, using Ca2+ imaging and patch clamp techniques and its mechanistic link to the downstream cellular function. Both pharmacological and siRNA inhibitors support the activation CaSR by extracellular l-Arg to induced Ca2+ entry via a Transient receptor potential canonical (TRPC) channel in HK-2 cells mainly through the receptor operated Ca2+ entry (ROCE). Activation of CaSR by l-Arg led to the rise in p-p38/p38 expression suggesting [Ca2+]i as a regulator for p38-signaling pathways. Notably, l-Arg activated CaSR-induced Ca2+ signaling reduced the expressions of key fibrotic, inflammatory, and apoptotic genes, suggesting its nephroprotective role via Ca2+ signaling through CaSR in HK-2 cells. Since we found that the IL-6 expressions were inversely proportional to the increasing concentrations of l-Arg in HK-2 cells, we measured the release of IL-6, which steadily decreased as the concentrations of l-Arg were elevated. Taken together, extracellular l-Arg is a negative regulator for IL-6-induced inflammatory process, through the activation of CaSR and TRPC channel by ROCE pathway and can have a potential to alleviate inflammatory renal diseases.

Hypercalciuria switches Ca2+ signaling in proximal tubular cells, induces oxidative damage to promote calcium nephrolithiasis

Proximal tubule (PT) transports most of the renal Ca2+, which was usually described as paracellular (passive). We found a regulated Ca2+ entry pathway in PT cells via the apical transient receptor potential canonical 3 (TRPC3) channel, which initiates transcellular Ca2+ transport. Although TRPC3 knockout (-/-) mice were mildly hypercalciuric and displayed luminal calcium phosphate (CaP) crystals at Loop of Henle (LOH), no CaP + calcium oxalate (CaOx) mixed urine crystals were spotted, which are mostly found in calcium nephrolithiasis (CaNL). Thus, we used oral calcium gluconate (CaG; 2%) to raise the PT luminal [Ca2+]o further in TRPC3 -/- mice for developing such mixed stones to understand the mechanistic role of PT-Ca2+ signaling in CaNL. Expectedly, CaG-treated mice urine samples presented with numerous mixed crystals with remains of PT cells, which were pronounced in TRPC3 -/- mice, indicating PT cell damage. Notably, PT cells from CaG-treated groups switched their mode of Ca2+ entry from receptor-operated to store-operated pathway with a sustained rise in intracellular [Ca2+] ([Ca2+]i), indicating the stagnation in PT Ca2+ transport. Moreover, those PT cells from CaG-treated groups demonstrated an upregulation of calcification, inflammation, fibrotic, oxidative stress, and apoptotic genes; effects of which were more robust in TRPC3 ablated condition. Furthermore, kidneys from CaG-treated groups exhibited fibrosis, tubular injury and calcifications with significant reactive oxygen species generation in the urine, thus, indicating in vivo CaNL. Taken together, excess PT luminal Ca2+ due to escalation of hypercalciuria in TRPC3 ablated mice induced surplus CaP crystal formation and caused stagnation of PT [Ca2+]i, invoking PT cell injury, hence mixed stone formation.

Small Molecule Calcium Channel Activator Potentiates Adjuvant Activity

There remains an unmet need for reliable fully synthetic adjuvants that increase lasting protective immune responses from vaccines. We previously reported a high-throughput screening for small molecules that extended nuclear factor kappa-light-chain enhancer of activated B cells (NF-κB) activation after a Toll-like receptor 4 (TLR4) ligand, lipopolysaccharide (LPS), stimulation using a human myeloid reporter cell line. We identified compounds with a conserved aminothiazole scaffold including 2D216 [N-(4-(2,5-dimethylphenyl)thiazol-2-yl)-4-(piperidin-1-ylsulfonyl)benzamide], which increased murine antigen-specific antibody responses when used as a co-adjuvant with LPS. Here, we examined the mechanism of action in human cells. Although 2D216 activated the major mitogen-activated protein kinases, it did not interact with common kinases and phosphatases and did not stimulate many of the pattern recognition receptors (PRRs). Instead, the mechanism of action was linked to intracellular Ca2+ elevation via Ca2+ channel(s) at the plasma membrane and nuclear translocation of the nuclear factor of activated T-cells (NFAT) as supported by RNA-seq data, analysis by reporter cells, Ca2+ flux assays, and immunoblots. Interestingly, 2D216 had minimal, if any, activity on Jurkat T cells but induced cytokine production and surface expression of costimulatory molecules on cells with antigen-presenting functions. A small series of analogs of 2D216 were tested for the ability to enhance a TLR4 ligand-stimulated autologous mixed lymphocyte reaction (MLR). In the MLR, 2E151, N-(4-(2,5-dimethylphenyl)thiazol-2-yl)-4-((4-propylpiperidin-1-yl)sulfonyl)benzamide, was more potent than 2D216. These results indicate that a small molecule that is not a direct PRR agonist can act as a co-adjuvant to an approved adjuvant to enhance human immune responses via a complementary mechanism of action.

Tannic acid attenuates vascular calcification-induced proximal tubular cells damage through paracrine signaling

Vascular calcification is common in chronic kidney disease; however, the extent to which such condition can affect the renal microvasculature and the neighboring cell types is unclear. Our induced-calcification model in renal proximal tubular (PT) cells exhibited endoplasmic reticulum (ER) stress and oxidative damage, leading to apoptosis. Here, we utilized such calcification in mouse vascular smooth muscle (MOVAS-1) cells as a vascular calcification model, because it exhibited reactive oxygen species (ROS) generation, ER and oxidative stress, inflammatory, and apoptotic gene expressions. To demonstrate whether the vascular calcification condition can dictate the function of the adjacent PT cell layer, we utilized a Transwell multilayer culture system by combining those MOVAS-1 cells in the bottom chamber and polarized PT cells in the upper chamber to show the dimensional cross-signaling effect. Interestingly, calcification of MOVAS-1 cells, in this co-culture, induced H2O2 and lactate dehydrogenase (LDH) release leading to store-operated Ca2+ entry, ROS generation, and activation of oxidative, inflammatory, and apoptotic gene expressions in PT cells through paracrine signaling. Interestingly, application of tannic acid (TA) to either calcified MOVAS-1 or uncalcified PT cells diminished such detrimental pathway activation. Furthermore, the TA-mediated protection was much higher in the PT cells when applied on the calcified MOVAS-1 cells, and the delayed the pathological effects in neighboring PT cells can well be via paracrine signaling. Together, these results provide evidence of vascular calcification-induced PT cell damage, and the protective role of TA in preventing such pathological consequences, which can potentially be used as a nephroprotective remedy.

Modulation of Tubular pH by Acetazolamide in a Ca2+ Transport Deficient Mice Facilitates Calcium Nephrolithiasis

Proximal tubular (PT) acidosis, which alkalinizes the urinary filtrate, together with Ca²⁺ supersaturation in PT can induce luminal calcium phosphate (CaP) crystal formation. While such CaP crystals are known to act as a nidus for CaP/calcium oxalate (CaOx) mixed stone formation, the regulation of PT luminal Ca²⁺ concentration ([Ca²⁺]) under elevated pH and/or high [Ca²⁺] conditions are unknown. Since we found that transient receptor potential canonical 3 (TRPC3) knockout (KO; -/-) mice could produce mild hypercalciuria with CaP urine crystals, we alkalinized the tubular pH in TRPC3-/- mice by oral acetazolamide (0.08%) to develop mixed urinary crystals akin to clinical signs of calcium nephrolithiasis (CaNL). Our ratiometric (λ340/380) intracellular [Ca²⁺] measurements reveal that such alkalization not only upsurges Ca²⁺ influx into PT cells, but the mode of Ca²⁺ entry switches from receptor-operated to store-operated pathway. Electrophysiological experiments show enhanced bicarbonate related current activity in treated PT cells which may determine the stone-forming phenotypes (CaP or CaP/CaOx). Moreover, such alkalization promotes reactive oxygen species generation, and upregulation of calcification, inflammation, fibrosis, and apoptosis in PT cells, which were exacerbated in absence of TRPC3. Altogether, the pH-induced alteration of the Ca²⁺ signaling signature in PT cells from TRPC3 ablated mice exacerbated the pathophysiology of mixed urinary stone formation, which may aid in uncovering the downstream mechanism of CaNL.

Targeting Calcium Release-activated Calcium Channel Is Not Sufficient to Prevent Rejection in Nonhuman Primate Kidney Transplantation

BACKGROUND

Calcineurin inhibitors successfully control rejection of transplanted organs but also cause nephrotoxicity. This study, using a rhesus monkey renal transplantation model, sought to determine the applicability of a new immunomodulatory drug inhibiting the store-operated calcium release-activated calcium channel of lymphocytes to control transplant rejection without nephrotoxicity.

METHODS

Animals underwent kidney transplantation and were treated with tacrolimus alone (n = 3), a CRACM1 inhibitor (PRCL-02) (n = 6) alone, or with initial tacrolimus monotherapy followed by gradual conversion at 3 weeks to PRCL-02 alone (n = 3). PRCL-02 was administered via a surgically inserted gastrostomy tube BID.

RESULTS

Dose-related drug exposure in monkeys was established and renal transplants were then performed using PRCL-02 monotherapy. Oral dosing of PRCL-02 was well tolerated and resulted in suppressed T-cell proliferation in in vitro MLR comparable to animals in the tacrolimus control arm. Animals receiving tacrolimus monotherapy were e on day 100 without rejection. PRCL-02 monotherapy only marginally prolonged graft survival (MST = 13.16 d; group 2) compared with untreated controls. Animals treated initially with tacrolimus and converted to PRCL-02 monotherapy had a mean graft survival of 35.3 days which was prolonged compared with PRCL-02 monotherapy but not compared with the tacrolimus-treated group. Pharmacokinetic studies showed inconsistent drug exposures despite attempts to adjust dose and exposure which may have contributed to the rejections.

CONCLUSIONS

We conclude that, in this nonhuman primate model of kidney transplantation, PRCL-02 demonstrated evidence of in vivo immunosuppressive activity but was inferior to tacrolimus treatment with respect to suppressing immune transplant rejection.

l-ornithine activates Ca2+ signaling to exert its protective function on human proximal tubular cells

Oxidative stress and reactive oxygen species (ROS) generation can be influenced by G-protein coupled receptor (GPCR)-mediated regulation of intracellular Ca2+ ([Ca2+]i) signaling. ROS production are much higher in proximal tubular (PT) cells; in addition, the lack of antioxidants enhances the vulnerability to oxidative damage. Despite such predispositions, PT cells show resiliency, and therefore must possess some inherent mechanism to protect from oxidative damage. While the mechanism in unknown, we tested the effect of l-ornithine, since it is abundantly present in PT luminal fluid and can activate Ca2+-sensing receptor (CaSR), a GPCR, expressed in the PT luminal membrane. We used human kidney 2 (HK2) cells, a PT cell line, and performed Ca2+ imaging and electrophysiological experiments to show that l-ornithine has a concentration-dependent effect on CaSR activation. We further demonstrate that the operation of CaSR activated Ca2+ signaling in HK-2 cells mediated by the transient receptor potential canonical (TRPC) dependent receptor-operated Ca2+ entry (ROCE) using pharmacological and siRNA inhibitors. Since PT cells are vulnerable to ROS, we simulated such deleterious effects using genetically encoded peroxide-induced ROS production (HyperRed indicator) to show that the l-ornithine-induced ROCE mediated [Ca2+]i signaling protects from ROS production. Furthermore, we performed cell viability, necrosis and apoptosis assays, and mitochondrial oxidative gene expression to establish that presence of l-ornithine rescued the ROS-induced damage in HK-2 cells. Moreover, l-ornithine-activation of CaSR can reverse ROS production and apoptosis via mitogen-activated protein kinase p38 activation. Such nephroprotective role of l-ornithine can be useful as the translational option for reversing kidney diseases involving PT cell damage due to oxidative stress or crystal nephropathies.

2-Aminoethyldiphenyl Borinate: A Multitarget Compound with Potential as a Drug Precursor

BACKGROUND

Boron is considered a trace element that induces various effects in systems of the human body. However, each boron-containing compound exerts different effects.

OBJECTIVE

To review the effects of 2-Aminoethyldiphenyl borinate (2-APB), an organoboron compound, on the human body, but also, its effects in animal models of human disease.

METHODS

In this review, the information to showcase the expansion of these reported effects through interactions with several ion channels and other receptors has been reported. These effects are relevant in the biomedical and chemical fields due to the application of the reported data in developing therapeutic tools to modulate the functions of the immune, cardiovascular, gastrointestinal and nervous systems.

RESULTS

Accordingly, 2-APB acts as a modulator of adaptive and innate immunity, including the production of cytokines and the migration of leukocytes. Additionally, reports show that 2-APB exerts effects on neurons, smooth muscle cells and cardiomyocytes, and it provides a cytoprotective effect by the modulation and attenuation of reactive oxygen species.

CONCLUSION

The molecular pharmacology of 2-APB supports both its potential to act as a drug and the desirable inclusion of its moieties in new drug development. Research evaluating its efficacy in treating pain and specific maladies, such as immune, cardiovascular, gastrointestinal and neurodegenerative disorders, is scarce but interesting.

Evidence for a regulated Ca2+ entry in proximal tubular cells and its implication in calcium stone formation

Calcium phosphate (CaP) crystals, which begin to form in the early segments of the loop of Henle (LOH), are known to act as precursors for calcium stone formation. The proximal tubule (PT), which is just upstream of the LOH and is a major site for Ca²⁺ reabsorption, could be a regulator of such CaP crystal formation. However, PT Ca²⁺ reabsorption is mostly described as being paracellular. Here, we show the existence of a regulated transcellular Ca²⁺ entry pathway in luminal membrane PT cells induced by Ca²⁺-sensing receptor (CSR, also known as CASR)-mediated activation of transient receptor potential canonical 3 (TRPC3) channels. In support of this idea, we found that both CSR and TRPC3 are physically and functionally coupled at the luminal membrane of PT cells. More importantly, TRPC3-deficient mice presented with a deficiency in PT Ca²⁺ entry/transport, elevated urinary [Ca²⁺], microcalcifications in LOH and urine microcrystals formations. Taken together, these data suggest that a signaling complex comprising CSR and TRPC3 exists in the PT and can mediate transcellular Ca²⁺ transport, which could be critical in maintaining the PT luminal [Ca²⁺] to mitigate formation of the CaP crystals in LOH and subsequent formation of calcium stones.

CRAC channel regulation of innate immune cells in health and disease

Calcium is a major intracellular signaling messenger in innate immune cells. Similar to other immune cell subsets, the majority of calcium entry into innate immune cells is induced by cell surface receptors that stimulate store-operated calcium entry through calcium-release activated calcium (CRAC) channels. Since the molecular description of the STIM family of calcium sensors and the ORAI family of CRAC channel proteins, the majority of studies support a dominant role for these proteins in calcium signaling in innate cells. In reviewing the literature on CRAC channel function in innate cells, several general themes emerge. All innate cells express multiple members of the STIM and ORAI family members, however the ratio and relative contribution of individual isoforms changes depending on the cell type and activation state of the cell. It is evident that study of functional roles for STIM molecules is clearly ahead of studies of specific ORAI family members in all innate cell types, and that studies of CRAC channels in innate cells are not nearly as advanced as studies in lymphocytes. However, taken together, evidence from both STIM calcium sensors and ORAI channels in innate cells indicates that deficiency of STIM and ORAI proteins tends not to affect the development of any innate cell lineage, but certainly affects their function, in particular activation of the neutrophil oxidase and mast cell activation via IgE receptors. Furthermore, there are clearly hints that therapeutic targeting of CRAC channels in innate cells offers a new approach to various inflammatory and allergic diseases.

Melamine promotes calcium crystal formation in three-dimensional microfluidic device

Melamine, which induces proximal tubular (PT) cell damage has a greater nephrotoxic effect when combined with cyanuric and uric acids; however, it is unknown whether such effect can stimulate calcium phosphate (CaP)/calcium oxalate (CaOx) stone formation. Here, we show that melamine acts as an inducer of CaP, CaOx and CaP + CaOx (mixed) crystal formations in a time and concentration-dependent manner by stabilizing those crystals and further co-aggregating with melamine. To explore the physiological relevance of such melamine-augmented calcium crystal formation, we used 2-dimensional (2D) and 3D microfluidic (MF) device, embedded with PT cells, which also resembled the effect of melamine-stimulated CaP, CaOx and mixed crystal formation. Significantly, addition of preformed CaP and/or CaOx crystal in the presence of melamine, further potentiated those crystal formations in 3D MFs, which helped the growth and aggregation of mixed crystals. Our data show that the mechanism of such predisposition of stone formation could be largely due to co-crystallization between melamine and CaP/CaOx and pronounced effect on induction of stone-forming pathway activation in 3D MF. Taken together, melamine-induced CaP and/or CaOx crystal formation ex-vivo will help us in understanding the larger role of melamine as an environmental toxicant in producing the pathology in similar cellular microenvironments.

Interplay between ER Ca2+ Binding Proteins, STIM1 and STIM2, Is Required for Store-Operated Ca2+ Entry

Store-operated calcium entry (SOCE), a fundamentally important homeostatic and Ca²⁺ signaling pathway in many types of cells, is activated by the direct interaction of stromal interaction molecule 1 (STIM1), an endoplasmic reticulum (ER) Ca²⁺-binding protein, with Ca²⁺-selective Orai1 channels localized in the plasma membrane. While much is known about the regulation of SOCE by STIM1, the role of stromal interaction molecule 2 (STIM2) in SOCE remains incompletely understood. Here, using clustered regularly interspaced short palindromic repeats -CRISPR associated protein 9 (CRISPR-Cas9) genomic editing and molecular imaging, we investigated the function of STIM2 in NIH 3T3 fibroblast and αT3 cell SOCE. We found that deletion of Stim2 expression reduced SOCE by more than 90% in NIH 3T3 cells. STIM1 expression levels were unaffected in the Stim2 null cells. However, quantitative confocal fluorescence imaging demonstrated that in the absence of Stim2 expression, STIM1 did not translocate or form punctae in plasma membrane-associated ER membrane (PAM) junctions following ER Ca²⁺ store depletion. Fluorescence resonance energy transfer (FRET) imaging of intact, living cells revealed that the formation of STIM1 and Orai1 complexes in PAM nanodomains was significantly reduced in the Stim2 knockout cells. Our findings indicate that STIM2 plays an essential role in regulating SOCE in NIH 3T3 and αT3 cells and suggests that dynamic interplay between STIM1 and STIM2 induced by ER Ca²⁺ store discharge is necessary for STIM1 translocation, its interaction with Orai1, and activation of SOCE.

Regulation of human dendritic cell immune functions by ion channels

Dendritic cells (DCs) are highly specialized antigen-presenting cells (APCs) able to induce both specific immunity and immune tolerance. Using information gathered from the tissue where they reside, DCs adjust their functional activity to ensure that protective immunity is favoured while unwanted or exaggerated immune responses are prevented. The remarkable ability of these cells to induce, enhance and orient the immune response, while at the same time maintaining self-tolerance, makes them key players in the immune system. Despite the fact that the role of Ca²⁺ has been clearly established in human DC functions, the link between ion homeostasis, mainly Ca²⁺, and DC functions is not fully understood. After all, a growing number of works clearly show the role of SOCE and associated channels in the maturation step, and those of K⁺ channels in migration. This review highlights the key papers published over the past few years and summarizes prospects for the near future.

Molecular physiology and pathophysiology of stromal interaction molecules

Ca²⁺ release from the endoplasmic reticulum is an important component of Ca²⁺ signal transduction that controls numerous physiological processes in eukaryotic cells. Release of Ca²⁺ from the endoplasmic reticulum is coupled to the activation of store-operated Ca²⁺ entry into cells. Store-operated Ca²⁺ entry provides Ca²⁺ for replenishing depleted endoplasmic reticulum Ca²⁺ stores and a Ca²⁺ signal that regulates Ca²⁺-dependent intracellular biochemical events. Central to connecting discharge of endoplasmic reticulum Ca²⁺ stores following G protein-coupled receptor activation with the induction of store-operated Ca²⁺ entry are stromal interaction molecules (STIM1 and STIM2). These highly homologous endoplasmic reticulum transmembrane proteins function as sensors of the Ca²⁺ concentration within the endoplasmic reticulum lumen and activators of Ca²⁺ release-activated Ca²⁺ channels. Emerging evidence indicates that in addition to their role in Ca²⁺ release-activated Ca²⁺ channel gating and store-operated Ca²⁺ entry, STIM1 and STIM2 regulate other cellular signaling events. Recent studies have shown that disruption of STIM expression and function is associated with the pathogenesis of several diseases including autoimmune disorders, cancer, cardiovascular disease, and myopathies. Here, we provide an overview of the latest developments in the molecular physiology and pathophysiology of STIM1 and STIM2. Impact statement Intracellular Ca²⁺ signaling is a fundamentally important regulator of cell physiology. Recent studies have revealed that Ca²⁺-binding stromal interaction molecules (Stim1 and Stim2) expressed in the membrane of the endoplasmic reticulum (ER) are essential components of eukaryote Ca²⁺ signal transduction that control the activity of ion channels and other signaling effectors present in the plasma membrane. This review summarizes the most recent information on the molecular physiology and pathophysiology of stromal interaction molecules. We anticipate that the work presented in our review will provide new insights into molecular interactions that participate in interorganelle signaling crosstalk, cell function, and the pathogenesis of human diseases.

STIM1 promotes migration, phagosomal maturation and antigen cross-presentation in dendritic cells

Antigen cross-presentation by dendritic cells (DC) stimulates cytotoxic T cell activation to promote immunity to intracellular pathogens, viruses and cancer. Phagocytosed antigens generate potent T cell responses, but the signalling and trafficking pathways regulating their cross-presentation are unclear. Here, we show that ablation of the store-operated-Ca²⁺-entry regulator STIM1 in mouse myeloid cells impairs cross-presentation and DC migration in vivo and in vitro. Stim1 ablation reduces Ca²⁺ signals, cross-presentation, and chemotaxis in mouse bone-marrow-derived DCs without altering cell differentiation, maturation or phagocytic capacity. Phagosomal pH homoeostasis and ROS production are unaffected by STIM1 deficiency, but phagosomal proteolysis and leucyl aminopeptidase activity, IRAP recruitment, as well as fusion of phagosomes with endosomes and lysosomes are all impaired. These data suggest that STIM1-dependent Ca²⁺ signalling promotes the delivery of endolysosomal enzymes to phagosomes to enable efficient cross-presentation.

STIM proteins sense Ca²⁺ depletion in the ER and activate store-operated Ca²⁺-entry (SOCE) in response, a process associated with dendritic cell functions. Here the authors show STIM1 is the major isoform controlling SOCE in mouse dendritic cells and provide a mechanism for its requirement in antigen cross-presentation.

Role of SOCE architects STIM and Orai proteins in Cell Death

Calcium (Ca²⁺) signaling plays a critical role in regulating plethora of cellular functions including cell survival, proliferation and migration. The perturbations in cellular Ca²⁺ homeostasis can lead to cell death either by activating autophagic pathways or through induction of apoptosis. Endoplasmic reticulum (ER) is the major storehouse of Ca²⁺ within cells and a number of physiological agonists mediate ER Ca²⁺ release by activating IP₃ receptors (IP₃R). This decrease in ER Ca²⁺ levels is sensed by STIM, which physically interacts and activates plasma membrane Ca²⁺ selective Orai channels. Emerging literature implicates a key role for STIM1, STIM2, Orai1 and Orai3 in regulating both cell survival and death pathways. In this review, we will retrospect the work highlighting the role of STIM and Orai homologs in regulating cell death signaling. We will further discuss the rationales that could explain the dual role of STIM and Orai proteins in regulating cell fate decisions.

Melamine induces Ca2+-sensing receptor activation and elicits apoptosis in proximal tubular cells

Melamine causes renal tubular cell injury through inflammation, fibrosis, and apoptosis. Although melamine affects the rise in intracellular Ca²⁺ concentration ([Ca²⁺]_i)_, reactive oxygen species (ROS) production, and proapoptotic pathway activation, the mechanism of upstream Ca²⁺ signaling is unknown. Because melamine has some structural similarities with l-amino acids, which endogenously activate Ca²⁺-sensing receptors (CSR), we examined the effect of melamine on CSR-induced Ca²⁺ signaling and apoptotic cell death. We show here that melamine activates CSR, causing a sustained Ca²⁺ entry in the renal epithelial cell line, LLC-PK1. Moreover, such CSR stimulation resulted in a rise in [Ca²⁺]_i, leading to enhanced ROS production. Furthermore, melamine-induced elevated [Ca²⁺]_i and ROS production caused a dose-dependent increase in apoptotic (by DAPI staining, DNA laddering, and annexin V assay) and necrotic (propidium iodide staining) cell death. Upon examining the downstream mechanism, we found that transforming growth factor β1 (TGF-β1), which increases extracellular matrix genes and proapoptotic signaling, was also upregulated at lower doses of melamine, which could be due to an early event inducing apoptosis. Additionally, cells exposed to melamine displayed a rise in pERK activation and lactate dehydrogenase release resulting in cytotoxicity. These results offer a novel insight into the molecular mechanisms by which melamine exerts its effect on CSR, causing a sustained elevation of [Ca²⁺]_i, leading to ROS generation, fibronectin production, proapoptotic pathway activation, and renal cell damage. Together, these results thus suggest that melamine-induced apoptosis and/or necrosis may subsequently result in acute kidney injury and promote kidney stone formation.

The TRPM7 channel kinase regulates store-operated calcium entry

KEY POINTS

Pharmacological and molecular inhibition of transient receptor potential melastatin 7 (TRPM7) reduces store-operated calcium entry (SOCE). Overexpression of TRPM7 in TRPM7^-/- cells restores SOCE. TRPM7 is not a store-operated calcium channel. TRPM7 kinase rather than channel modulates SOCE. TRPM7 channel activity contributes to the maintenance of store Ca²⁺ levels at rest.

ABSTRACT

The transient receptor potential melastatin 7 (TRPM7) is a protein that combines an ion channel with an intrinsic kinase domain, enabling it to modulate cellular functions either by conducting ions through the pore or by phosphorylating downstream proteins via its kinase domain. In the present study, we report store-operated calcium entry (SOCE) as a novel target of TRPM7 kinase activity. TRPM7-deficient chicken DT40 B lymphocytes exhibit a strongly impaired SOCE compared to wild-type cells as a result of reduced calcium release activated calcium currents, and independently of potassium channel regulation, membrane potential changes or changes in cell-cycle distribution. Pharmacological blockade of TRPM7 with NS8593 or waixenicin A in wild-type B lymphocytes results in a significant decrease in SOCE, confirming that TRPM7 activity is acutely linked to SOCE, without TRPM7 representing a store-operated channel itself. Using kinase-deficient mutants, we find that TRPM7 regulates SOCE through its kinase domain. Furthermore, Ca²⁺ influx through TRPM7 is essential for the maintenance of endoplasmic reticulum Ca²⁺ concentration in resting cells, and for the refilling of Ca²⁺ stores after a Ca²⁺ signalling event. We conclude that the channel kinase TRPM7 and SOCE are synergistic mechanisms regulating intracellular Ca²⁺ homeostasis.

Role of STIM2 in cell function and physiopathology

An endoplasmic reticulum (ER)-resident protein that regulates cytosolic and ER free-Ca²⁺ concentration by induction of store-operated calcium entry: that is the original definition of STIM2 and its function. While its activity strongly depends on the amount of calcium stored in the ER, its function goes further, to intracellular signalling and gene expression. Initially under-studied owing to the prominent function of STIM1, STIM2 came to be regarded as vital in mice, gradually emerging as an important player in the nervous system, and cooperating with STIM1 in the immune system. STIM2 has also been proposed as a relevant player in pathological conditions related to ageing, Alzheimer's and Huntington's diseases, autoimmune disorders and cancer. The discovery of additional functions, together with new splicing forms with opposite roles, has clarified existing controversies about STIM2 function in SOCE. With STIM2 being essential for life, but apparently not for development, newly available data demonstrate a complex and still intriguing behaviour that this review summarizes, updating current knowledge of STIM2 function.

Cardiovascular and Hemostatic Disorders: Role of STIM and Orai Proteins in Vascular Disorders

Store-operated Ca²⁺ entry (SOCE) mediated by STIM and Orai proteins is a highly regulated and ubiquitous signaling pathway that plays an important role in various cellular and physiological functions. Endoplasmic reticulum (ER) serves as the major site for intracellular Ca²⁺ storage. Stromal Interaction Molecule 1/2 (STIM1/2) sense decrease in ER Ca²⁺ levels and transmits the message to plasma membrane Ca²⁺ channels constituted by Orai family members (Orai1/2/3) resulting in Ca²⁺ influx into the cells. This increase in cytosolic Ca²⁺ in turn activates a variety of signaling cascades to regulate a plethora of cellular functions. Evidence from the literature suggests that SOCE dysregulation is associated with several pathophysiologies, including vascular disorders. Interestingly, recent studies have suggested that STIM proteins may also regulate vascular functions independent of their contribution to SOCE. In this updated book chapter, we will focus on the physiological role of STIM and Orai proteins in the vasculature (endothelial cells and vascular smooth muscle cells). We will further retrospect the literature implicating a critical role for these proteins in vascular disease.

The role of STIM and ORAI proteins in phagocytic immune cells

Phagocytic cells, such as neutrophils, macrophages, and dendritic cells, migrate to sites of infection or damage and are integral to innate immunity through two main mechanisms. The first is to directly neutralize foreign agents and damaged or infected cells by secreting toxic substances or ingesting them through phagocytosis. The second is to alert the adaptive immune system through the secretion of cytokines and the presentation of the ingested materials as antigens, inducing T cell maturation into helper, cytotoxic, or regulatory phenotypes. While calcium signaling has been implicated in numerous phagocyte functions, including differentiation, maturation, migration, secretion, and phagocytosis, the molecular components that mediate these Ca(2+) signals have been elusive. The discovery of the STIM and ORAI proteins has allowed researchers to begin clarifying the mechanisms and physiological impact of store-operated Ca(2+) entry, the major pathway for generating calcium signals in innate immune cells. Here, we review evidence from cell lines and mouse models linking STIM and ORAI proteins to the control of specific innate immune functions of neutrophils, macrophages, and dendritic cells.

STIM and Orai1 Variants in Store-Operated Calcium Entry

Store-operated Ca²⁺ entry (SOCE) is an ubiquitous mechanism for Ca²⁺ entry in eukaryotic cells. This route for Ca²⁺ influx is regulated by the filling state of the intracellular Ca²⁺ stores communicated to the plasma membrane channels by the proteins of the Stromal Interaction Molecule (STIM) family, STIM1, and STIM2. Store-dependent, STIM1-modulated, channels include the Ca²⁺ release-activated Ca²⁺ channels, comprised of subunits of Orai proteins, as well as the store-operated Ca²⁺ (SOC) channels, involving Orai1, and members of the canonical transient receptor potential family of proteins. Recent studies have revealed the expression of splice variants of STIM1, STIM2, and Orai1 in different cell types. While certain variants are ubiquitously expressed, others, such as STIM1L, show a more restricted expression. The splice variants for STIM and Orai1 proteins exhibit significant functional differences and reveal that alternative splicing enhance the functional diversity of STIM1, STIM2, and Orai1 genes to modulate the dynamics of Ca²⁺ signals.

Novel Protein Kinase C-Mediated Control of Orai1 Function in Invasive Melanoma

The incidence of malignant melanoma, a cancer of the melanocyte cell lineage, has nearly doubled in the past 20 years. Wnt5A, akey driver of melanoma invasiveness, induces Ca2 signals. To understand how store-operated calcium entry (SOCE) contributes to Wnt5A-induced malignancy in melanoma models, we examined the expression and function of STIM1 and Orai1 in patient-derived malignant melanoma cells, previously characterized as either highly invasive (metastatic) or noninvasive. Using both fluorescence microscopy and electrophysiological approaches, we show that SOCE is greatly diminished in invasive melanoma compared to its level in noninvasive cell types. However, no loss of expression of any members of the STIM and Orai families was observed in invasive melanoma cells. Moreover, overexpressed wild-type STIM1 and Orai1 failed to restore SOCE in invasive melanoma cells, and we observed no defects in their localization before or after store depletion in any of the invasive celllines. Importantly, however, we determined that SOCE was restored by inhibition of protein kinase C, a known downstream target of Wnt5A. Furthermore, coexpression of STIM1 with an Orai1 mutant insensitive to protein kinase C-mediated phosphorylation fully restored SOCE in invasive melanoma. These findings reveal a level of control for STIM/Orai function in invasive melanoma not previously reported.

Ca2+ Signaling but Not Store-Operated Ca2+ Entry Is Required for the Function of Macrophages and Dendritic Cells

Store-operated Ca(2+) entry (SOCE) through Ca(2+) release-activated Ca(2+) (CRAC) channels is essential for immunity to infection. CRAC channels are formed by ORAI1 proteins in the plasma membrane and activated by stromal interaction molecule (STIM)1 and STIM2 in the endoplasmic reticulum. Mutations in ORAI1 and STIM1 genes that abolish SOCE cause severe immunodeficiency with recurrent infections due to impaired T cell function. SOCE has also been observed in cells of the innate immune system such as macrophages and dendritic cells (DCs) and may provide Ca(2+) signals required for their function. The specific role of SOCE in macrophage and DC function, as well as its contribution to innate immunity, however, is not well defined. We found that nonselective inhibition of Ca(2+) signaling strongly impairs many effector functions of bone marrow-derived macrophages and bone marrow-derived DCs, including phagocytosis, inflammasome activation, and priming of T cells. Surprisingly, however, macrophages and DCs from mice with conditional deletion of Stim1 and Stim2 genes, and therefore complete inhibition of SOCE, showed no major functional defects. Their differentiation, FcR-dependent and -independent phagocytosis, phagolysosome fusion, cytokine production, NLRP3 inflammasome activation, and their ability to present Ags to activate T cells were preserved. Our findings demonstrate that STIM1, STIM2, and SOCE are dispensable for many critical effector functions of macrophages and DCs, which has important implications for CRAC channel inhibition as a therapeutic strategy to suppress pathogenic T cells while not interfering with myeloid cell functions required for innate immunity.

Stim and Orai proteins in neuronal Ca(2+) signaling and excitability

Stim1 and Orai1 are ubiquitous proteins that have long been known to mediate Ca²⁺ release-activated Ca²⁺ (CRAC) current (I_CRAC) and store-operated Ca²⁺ entry (SOCE) only in non-excitable cells. SOCE is activated following the depletion of the endogenous Ca²⁺ stores, which are mainly located within the endoplasmic reticulum (ER), to replete the intracellular Ca²⁺ reservoir and engage specific Ca²⁺-dependent processes, such as proliferation, migration, cytoskeletal remodeling, and gene expression. Their paralogs, Stim2, Orai2 and Orai3, support SOCE in heterologous expression systems, but their physiological role is still obscure. Ca²⁺ inflow in neurons has long been exclusively ascribed to voltage-operated and receptor-operated channels. Nevertheless, recent work has unveiled that Stim1–2 and Orai1-2, but not Orai3, proteins are also expressed and mediate SOCE in neurons. Herein, we survey current knowledge about the neuronal distribution of Stim and Orai proteins in rodent and human brains; we further discuss that Orai2 is the main pore-forming subunit of CRAC channels in central neurons, in which it may be activated by either Stim1 or Stim2 depending on species, brain region and physiological stimuli. We examine the functions regulated by SOCE in neurons, where this pathway is activated under resting conditions to refill the ER, control spinogenesis and regulate gene transcription. Besides, we highlighted the possibility that SOCE also controls neuronal excitation and regulate synaptic plasticity. Finally, we evaluate the involvement of Stim and Orai proteins in severe neurodegenerative and neurological disorders, such as Alzheimer’s disease and epilepsy.

Potentiation of the store-operated calcium entry (SOCE) induces phytohemagglutinin-activated Jurkat T cell apoptosis

Store-operated Ca(2+) entry (SOCE) is the main Ca(2+) entry pathway of non-excitable cells. In the past decade, the activation of this entry has been unveiled, with STIM1, a protein of the endoplasmic reticulum able to sense the intraluminal Ca(2+) content, and Orai1, the pore-forming unit of the Ca(2+) release activated Ca(2+) (CRAC) channels. When Ca(2+) ions are released from the endoplasmic reticulum, STIM1 proteins oligomerize and directly interact with Orai1 proteins, allowing the opening of the CRAC channels and a massive Ca(2+) ion influx known as SOCE. As Ca(2+) is involved in various cellular processes, the discovery of new drugs acting on the SOCE should be of interest to control the cell activity. By testing analogs of 2-aminoethyl diphenylborinate (2-APB), a well known, though not so selective effector of the SOCE, we identified methoxy diethylborinate (MDEB), a molecule able to potentiate the SOCE in three leukocyte and two breast cancer cell lines by increasing the Ca(2+) influx amplitude. Unlike 2-APB, MDEB does not affect the Ca(2+) pumps or the Ca(2+) release from the endoplasmic reticulum. MDEB could therefore represent the first member of a new group of molecules, specifically able to potentiate SOCE. Although not toxic for non-activated Jurkat T cells, it could induce the apoptosis of phytohemagglutinin-stimulated cells.

Store-operated calcium signaling in neutrophils

Calcium signals in neutrophils are initiated by a variety of cell-surface receptors, including formyl peptide and other GPCRs, FcRs, and integrins. The predominant pathway by which calcium enters immune cells is termed SOCE, whereby plasma membrane CRAC channels allow influx of extracellular calcium into the cytoplasm when intracellular ER stores are depleted. The identification of 2 key families of SOCE regulators, STIM calcium "sensors" and ORAI calcium channels, has allowed for genetic manipulation of SOCE pathways and provided valuable insight into the molecular mechanism of calcium signaling in immune cells, including neutrophils. This review focuses on our current knowledge of the molecules involved in neutrophil SOCE and how study of these molecules has further informed our understanding of the role of calcium signaling in neutrophil activation.

Store-operated calcium entry: unveiling the calcium handling signalplex

Store-operated Ca(2+) entry (SOCE) is an important mechanism for Ca(2+) influx in non-excitable cells, also present in excitable cells. The activation of store-operated channels (SOCs) is finely regulated by the filling state of the intracellular agonist-sensitive Ca(2+) compartments, and both, the mechanism of sensing the Ca(2+) stores and the nature and functional properties of the SOCs, have been a matter of intense investigation and debate. The identification of STIM1 as the endoplasmic reticulum Ca(2+) sensor and both Orai1, as the pore-forming subunit of the channels mediating the Ca(2+)-selective store-operated current, and the members of the TRPC subfamily of proteins, as the channels mediating the cation-permeable SOCs, has shed new light on the underlying events. This review summarizes the initial hypothesis and the current advances on the mechanism of activation of SOCE.

STIM1 and STIM2 proteins differently regulate endogenous store-operated channels in HEK293 cells

The endoplasmic reticulum calcium sensors stromal interaction molecules 1 and 2 (STIM1 and STIM2) are key modulators of store-operated calcium entry. Both these sensors play a major role in physiological functions in normal tissue and in pathology, but available data on native STIM2-regulated plasma membrane channels are scarce. Only a few studies have recorded STIM2-induced CRAC (calcium release-activated calcium) currents. On the other hand, many cell types display store-operated currents different from CRAC. The STIM1 protein regulates not only CRAC but also transient receptor potential canonical (TRPC) channels, but it has remained unclear whether STIM2 is capable of regulating store-operated non-CRAC channels. Here we present for the first time experimental evidence for the existence of endogenous non-CRAC STIM2-regulated channels. As shown in single-channel patch clamp experiments on HEK293 cells, selective activation of native STIM2 proteins or STIM2 overexpression results in store-operated activation of Imin channels, whereas STIM1 activation blocks this process. Changes in the ratio between active STIM2 and STIM1 proteins can switch the regulation of Imin channels between store-operated and store-independent modes. We have previously characterized electrophysiological properties of different Ca(2+) influx channels coexisting in HEK293 cells. The results of this study show that STIM1 and STIM2 differ in the ability to activate these store-operated channels; Imin channels are regulated by STIM2, TRPC3-containing INS channels are induced by STIM1, and TRPC1-composed Imax channels are activated by both STIM1 and STIM2. These new data about cross-talk between STIM1 and STIM2 and their different roles in store-operated channel activation are indicative of an additional level in the regulation of store-operated calcium entry pathways.

Differential role of STIM1 and STIM2 during transient inward (T in) current generation and the maturation process in the Xenopus oocyte

BACKGROUND

The Xenopus oocyte is a useful cell model to study Ca2+ homeostasis and cell cycle regulation, two highly interrelated processes. Here, we used antisense oligonucleotides to investigate the role in the oocyte of stromal interaction molecule (STIM) proteins that are fundamental elements of the store-operated calcium-entry (SOCE) phenomenon, as they are both sensors for Ca2+ concentration in the intracellular reservoirs as well as activators of the membrane channels that allow Ca2+ influx.

RESULTS

Endogenous STIM1 and STIM2 expression was demonstrated, and their synthesis was knocked down 48-72 h after injecting oocytes with specific antisense sequences. Selective elimination of their mRNA and protein expression was confirmed by PCR and Western blot analysis, and we then evaluated the effect of their absence on two endogenous responses: the opening of SOC channels elicited by G protein-coupled receptor (GPCR)-activated Ca2+ release, and the process of maturation stimulated by progesterone. Activation of SOC channels was monitored electrically by measuring the T in response, a Ca2+-influx-dependent Cl- current, while maturation was assessed by germinal vesicle breakdown (GVBD) scoring and electrophysiology.

CONCLUSIONS

It was found that STIM2, but not STIM1, was essential in both responses, and T in currents and GVBD were strongly reduced or eliminated in cells devoid of STIM2; STIM1 knockdown had no effect on the maturation process, but it reduced the T in response by 15 to 70%. Thus, the endogenous SOCE response in Xenopus oocytes depended mainly on STIM2, and its expression was necessary for entry into meiosis induced by progesterone.

STIM1-mediated bidirectional regulation of Ca(2+) entry through voltage-gated calcium channels (VGCC) and calcium-release activated channels (CRAC)

The spatial and temporal regulation of cellular calcium signals is modulated via two main Ca(2+) entry routes. Voltage-gated Ca(2+) channels (VGCC) and Ca(2+)-release activated channels (CRAC) enable Ca(2+) flow into electrically excitable and non-excitable cells, respectively. VGCC are well characterized transducers of electrical activity that allow Ca(2+) signaling into the cell in response to action potentials or subthreshold depolarizing stimuli. The identification of STromal Interaction Molecule (STIM) and Orai proteins has provided significant insights into the understanding of CRAC function and regulation. This review will summarize the current state of knowledge of STIM-Orai interaction and their contribution to cellular Ca(2+) handling mechanisms. We will then discuss the bidirectional actions of STIM1 on VGCC and CRAC. In contrast to the stimulatory role of STIM1 on Orai channel activity that facilitates Ca(2+) entry, recent reports indicated the ability of STIM1 to suppress VGCC activity. This new concept changes our traditional understanding of Ca(2+) handling mechanisms and highlights the existence of dynamically regulated signaling complexes of surface expressed ion channels and intracellular store membrane-embedded Ca(2+) sensors. Overall, STIM1 is emerging as a new class of regulatory proteins that fine-tunes Ca(2+) entry in response to endoplasmic/sarcoplasmic reticulum stress.

The neglected CRAC proteins: Orai2, Orai3, and STIM2

Plasma-membrane-localized Orai1 ion channel subunits interacting with ER-localized STIM1 molecules comprise the major subunit composition responsible for calcium release-activated calcium channels. STIM1 "translates" the Ca(2+) store content into Orai1 activity, making it a store-operated channel. Surprisingly, in addition to being the physical activator, STIM1 also modulates Orai1 properties, including its inactivation and permeation (see Chapter 1). STIM1 is thus more than a pure Orai1 activator. Within the past 7 years following the discovery of STIM and Orai proteins, the molecular mechanisms of STIM1/Orai1 activity and their functional importance have been studied in great detail. Much less is currently known about the other isoforms STIM2, Orai2, and Orai3. In this chapter, we summarize the current knowledge about STIM2, Orai2, and Orai3 properties and function. Are these homologues mainly modulators of predominantly STIM1/Orai1-mediated complexes or do store-dependent or -independent functions such as regulation of basal Ca(2+) concentration and activation of Orai3-containing complexes by arachidonic acid or by estrogen receptors point toward their "true" physiological function? Is Orai2 the Orai1 of neurons? A major focus of the review is on the functional relevance of STIM2, Orai2, and Orai3, some of which still remains speculative.

Decreased store operated Ca2+ entry in dendritic cells isolated from mice expressing PKB/SGK-resistant GSK3

Dendritic cells (DCs), key players of immunity, are regulated by glycogen synthase kinase GSK3. GSK3 activity is suppressed by PKB/Akt and SGK isoforms, which are in turn stimulated by the PI3K pathway. Exposure to bacterial lipopolysaccharides increases cytosolic Ca²⁺-concentration ([Ca²⁺]_i), an effect augmented in DCs isolated from mutant mice expressing PKB/SGK-resistant GSK3α,β (gsk3^KI). Factors affecting [Ca²⁺]_i include Ca²⁺-release from intracellular stores (CRIS), store-operated Ca²⁺-entry (SOCE) through STIM1/STIM2-regulated Orai1, K⁺-dependent Na⁺/Ca²⁺-exchangers (NCKX), K⁺-independent Na⁺/Ca²⁺-exchangers (NCX) and calbindin-D28k. The present study explored whether PKB/SGK-dependent GSK3α, β-activity impacts on CRIS, SOCE, NCKX, NCX or calbindin. DCs were isolated from gsk3^KI mice and respective wild-type mice (gsk3^WT), [Ca²⁺]_i estimated from Fura2 fluorescence, Orai1, STIM1, STIM2 as well as calbindin-D28k protein abundance determined by Western blotting and mRNA levels quantified by real time PCR. As a result, thapsigargin-induced CRIS and SOCE were significantly blunted by GSK3-inhibitors SB216763 (1–10 µM, 30 min) or GSK-XIII (10 µM, 30 min) but were significantly lower in gsk3^WT than in gsk3^KIDCs. Orai1, STIM1 and STIM2 protein abundance was significantly lower and calbindin-D28k abundance significantly higher in gsk3^KI than in gsk3^WTDCs. Activity of NCKX and NCX was significantly higher in gsk3^KI than in gsk3^WTDCs and was significantly increased by SB216763 (1 µM, 30 min) or GSK-XIII (10 µM, 30 min). Treatment of gsk3^WT DCs with SB216763 (1 µM, 4–24 h) or GSK-XIII (10 µM, 4–24 h) did not significantly modify the protein abundance of Orai1, STIM1 and STIM2. The present observations point to a dual role of GSK3 in the regulation of Ca²⁺ in DCs. Acute inhibition of GSK3 blunted the increase of [Ca²⁺]_i following CRIS and SOCE and stimulated NCKX/NCX activity. However, expression of PKB/SGK-resistant GSK3α, β downregulated the increase of [Ca²⁺]_i following CRIS and SOCE, an effect at least partially due to downregulation of Orai1, STIM1 and STIM2 expression as well as upregulation of Na⁺/Ca²⁺-exchanger activity and calbindin D28k expression.

Targeting ion channels for the treatment of autoimmune neuroinflammation

Pharmacological targeting of ion channels has long been recognized as an attractive strategy for the treatment of various diseases. Multiple sclerosis (MS) is an autoimmune disorder of the central nervous system with a prominent neurodegenerative component. A multitude of different cell types are involved in the complex pathophysiology of this disorder, including cells of the immune system (e.g. T and B lymphocytes and microglia), the neurovascular unit (e.g. endothelial cells and astrocytes) and the central nervous system (e.g. astrocytes and neurons). The pleiotropic expression and function of ion channels gives rise to the attractive opportunity of targeting different players and pathophysiological aspects of MS by the modulation of ion channel function in a cell-type and context-specific manner. We discuss the emerging knowledge about ion channels in the context of autoimmune neuroinflammation. While some pharmacological targets are at the edge of clinical translation, others have only recently been discovered and are still under investigation. Special focus is given to those candidates that could be attractive novel targets for future therapeutic approaches in neuroimmune autoinflammation.

Native STIM2 and ORAI1 proteins form a calcium-sensitive and thapsigargin-insensitive complex in cortical neurons

In non-excitatory cells, stromal interaction molecule 1 (STIM1) and STIM2 mediate store-operated calcium entry via an interaction with ORAI1 calcium channels. However, in neurons, STIM2 over-expression appears to play a role in calcium homeostasis that is different from STIM1 over-expression. The aim of this study was to establish the role and localization of native STIM2 in the neuronal cell. Co-immunoprecipitation experiments revealed that the interaction between endogenous STIM2 and ORAI1 was greater in a low-calcium medium than in a high-calcium medium. Using a Proximity Ligation Assay (PLA), the number of apparent complexes of endogenous STIM2 with ORAI1 was quantified. No change in the number of PLA signals was observed in the presence of thapsigargin, which depletes calcium from the endoplasmic reticulum (ER). However, the number of apparent STIM2-ORAI1 complexes increased when intracellular and subsequently ER calcium concentrations were decreased by BAPTA-AM or a low-calcium medium. Both Fura-2 acetoxymethyl ester calcium imaging and PLA in the same neuronal cell indicated that the calcium responses correlated strongly with the number of endogenous STIM2-ORAI1 complexes. The small drop in calcium levels in the ER caused by decreased intracellular calcium levels appeared to initiate the calcium-sensitive and thapsigargin-insensitive interaction between STIM2 and ORAI1. We show in neuronal somata the formation of endogenous complexes of stromal interaction molecule 2 (STIM2) with ORAI1 calcium channels. Their number increased when intracellular Ca²⁺ concentrations were decreased by the Ca²⁺ chelator BAPTA-AM or a low-calcium medium (EGTA), but did not in the presence of thapsigargin (TG). We conclude that the small drop of Ca²⁺ level in endoplasmic reticulum, due to the decreased level of intracellular Ca²⁺, is sufficient to trigger STIM2-ORAI1 complex formation in a thapsigargin-insensitive manner.

The Orai-1 and STIM-1 complex controls human dendritic cell maturation

Ca(2+) signaling plays an important role in the function of dendritic cells (DC), the professional antigen presenting cells. Here, we described the role of Calcium released activated (CRAC) channels in the maturation and cytokine secretion of human DC. Recent works identified STIM1 and Orai1 in human T lymphocytes as essential for CRAC channel activation. We investigated Ca(2+) signaling in human DC maturation by imaging intracellular calcium signaling and pharmalogical inhibitors. The DC response to inflammatory mediators or PAMPs (Pathogen-associated molecular patterns) is due to a depletion of intracellular Ca(2+) stores that results in a store-operated Ca(2+) entry (SOCE). This Ca(2+) influx was inhibited by 2-APB and exhibited a Ca(2+)permeability similar to the CRAC (Calcium-Released Activated Calcium), found in T lymphocytes. Depending on the PAMPs used, SOCE profiles and amplitudes appeared different, suggesting the involvement of different CRAC channels. Using siRNAi, we identified the STIM1 and Orai1 protein complex as one of the main pathways for Ca(2+) entry for LPS- and TNF-α-induced maturation in DC. Cytokine secretions also seemed to be SOCE-dependent with profile differences depending on the maturating agents since IL-12 and IL10 secretions appeared highly sensitive to 2-APB whereas IFN-γ was less affected. Altogether, these results clearly demonstrate that human DC maturation and cytokine secretions depend on SOCE signaling involving STIM1 and Orai1 proteins.

Multifaceted roles of STIM proteins

Stromal interaction molecules (STIM1 and STIM2) are critical components of store-operated calcium entry. Sensing depletion of endoplasmic reticulum (ER) Ca(2+) stores, STIM couples with plasma membrane Orai channels, resulting in the influx of Ca(2+) across the PM into the cytosol. Although best recognized for their primary role as ER Ca(2+) sensors, increasing evidence suggests that STIM proteins have a broader variety of sensory capabilities than first envisaged, reacting to cell stressors such as oxidative stress, temperature, and hypoxia. Further, the array of partners for STIM proteins is now understood to range far beyond the Orai channel family. Here we discuss the implications of STIM's expanding role, both as a stress sensor and a general modulator of multiple physiological processes in the cell.

STIM2 drives Ca2+ oscillations through store-operated Ca2+ entry caused by mild store depletion

Abstract  Agonist-induced Ca(2+) oscillations in many cell types are triggered by Ca(2+) release from intracellular stores and driven by store-operated Ca(2+) entry. Stromal cell-interaction molecule (STIM) 1 and STIM2 serve as endoplasmic reticulum Ca(2+) sensors that, upon store depletion, activate Ca(2+) release-activated Ca(2+) channels (Orai1-3, CRACM1-3) in the plasma membrane. However, their relative roles in agonist-mediated Ca(2+) oscillations remain ambiguous. Here we report that while both STIM1 and STIM2 contribute to store-refilling during Ca(2+) oscillations in mast cells (RBL), T cells (Jurkat) and human embryonic kidney (HEK293) cells, they do so dependent on the level of store depletion. Molecular silencing of STIM2 by siRNA or inhibition by G418 suppresses store-operated Ca(2+) entry and agonist-mediated Ca(2+) oscillations at low levels of store depletion, without interfering with STIM1-mediated signals induced by full store depletion. Thus, STIM2 is preferentially activated by low-level physiological agonist concentrations that cause mild reductions in endoplasmic reticulum Ca(2+) levels. We conclude that with increasing agonist concentrations, store-operated Ca(2+) entry is mediated initially by endogenous STIM2 and incrementally by STIM1, enabling differential modulation of Ca(2+) entry over a range of agonist concentrations and levels of store depletion.

SGK3 regulates Ca(2+) entry and migration of dendritic cells

BACKGROUND/AIMS

Dendritic cells (DCs) are antigen-presenting cells linking innate and adaptive immunity. DC maturation and migration are governed by alterations of cytosolic Ca(2+) concentrations ([Ca(2+)](i)). Ca(2+) entry is in part accomplished by store-operated Ca(2+) (SOC) channels consisting of the membrane pore-forming subunit Orai and the ER Ca(2+) sensing subunit STIM. Moreover, DC functions are under powerful regulation of the phosphatidylinositol-3-kinase (PI3K) pathway, which suppresses proinflammatory cytokine production but supports DC migration. Downstream targets of PI3K include serum- and glucocorticoid-inducible kinase isoform SGK3. The present study explored, whether SGK3 participates in the regulation of [Ca(2+)](i) and Ca(2+)-dependent functions of DCs, such as maturation and migration.

METHODS/RESULTS

Experiments were performed with bone marrow derived DCs from gene targeted mice lacking SGK3 (sgk3(-/-)) and DCs from their wild type littermates (sgk3(+/+)). Maturation, phagocytosis and cytokine production were similar in sgk3(-/-) and sgk3(+/+) DCs. However, SOC entry triggered by intracellular Ca(2+) store depletion with the endosomal Ca(2+) ATPase inhibitor thapsigargin (1 µM) was significantly reduced in sgk3(-/-) compared to sgk3(+/+) DCs. Similarly, bacterial lipopolysaccharide (LPS, 1 µg/ml)- and chemokine CXCL12 (300 ng/ml)- induced increase in [Ca(2+)](i) was impaired in sgk3(-/-) DCs. Moreover, currents through SOC channels were reduced in sgk3(-/-) DCs. STIM2 transcript levels and protein abundance were significantly lower in sgk3(-/-) DCs than in sgk3(+/+) DCs, whereas Orai1, Orai2, STIM1 and TRPC1 transcript levels and/or protein abundance were similar in sgk3-/- and sgk3(+/+) DCs. Migration of both, immature DCs towards CXCL12 and LPS-matured DCs towards CCL21 was reduced in sgk3(-/-) as compared to sgk3(+/+) DCs. Migration of sgk3(+/+) DCs was further sensitive to SOC channel inhibitor 2-APB (50 µM) and to STIM1/STIM2 knock-down.

CONCLUSION

SGK3 contributes to the regulation of store-operated Ca(2+) entry into and migration of dendritic cells, effects at least partially mediated through SGK3-dependent upregulation of STIM2 expression.

STIM proteins: dynamic calcium signal transducers

Stromal interaction molecule (STIM) proteins function in cells as dynamic coordinators of cellular calcium (Ca(2+)) signals. Spanning the endoplasmic reticulum (ER) membrane, they sense tiny changes in the levels of Ca(2+) stored within the ER lumen. As ER Ca(2+) is released to generate primary Ca(2+) signals, STIM proteins undergo an intricate activation reaction and rapidly translocate into junctions formed between the ER and the plasma membrane. There, STIM proteins tether and activate the highly Ca(2+)-selective Orai channels to mediate finely controlled Ca(2+) signals and to homeostatically balance cellular Ca(2+). Details are emerging on the remarkable organization within these STIM-induced junctional microdomains and the identification of new regulators and alternative target proteins for STIM.

Extracellular Ca(2+) sensing in salivary ductal cells

Ca(2+) is secreted from the salivary acinar cells as an ionic constituent of primary saliva. Ions such as Na(+) and Cl(-) get reabsorbed whereas primary saliva flows through the salivary ductal system. Although earlier studies have shown that salivary [Ca(2+)] decreases as it flows down the ductal tree into the oral cavity, ductal reabsorption of Ca(2+) remains enigmatic. Here we report a potential role for the G protein-coupled receptor, calcium-sensing receptor (CSR), in the regulation of Ca(2+) reabsorption by salivary gland ducts. Our data show that CSR is present in the apical region of ductal cells where it is co-localized with transient receptor potential canonical 3 (TRPC3). CSR is activated in isolated salivary gland ducts as well as a ductal cell line (SMIE) by altering extracellular [Ca(2+)] or by aromatic amino acid, L-phenylalanine (L-Phe, endogenous component of saliva), as well as neomycin. CSR activation leads to Ca(2+) influx that, in polarized cells grown on a filter support, is initiated in the luminal region. We show that TRPC3 contributes to Ca(2+) entry triggered by CSR activation. Further, stimulation of CSR in SMIE cells enhances the CSR-TRPC3 association as well as surface expression of TRPC3. Together our findings suggest that CSR could serve as a Ca(2+) sensor in the luminal membrane of salivary gland ducts and regulate reabsorption of [Ca(2+)] from the saliva via TRPC3, thus contributing to maintenance of salivary [Ca(2+)]. CSR could therefore be a potentially important protective mechanism against formation of salivary gland stones (sialolithiasis) and infection (sialoadenitis).

Extracellular Ca(2+) sensing in salivary ductal cells

Ca(2+) is secreted from the salivary acinar cells as an ionic constituent of primary saliva. Ions such as Na(+) and Cl(-) get reabsorbed whereas primary saliva flows through the salivary ductal system. Although earlier studies have shown that salivary [Ca(2+)] decreases as it flows down the ductal tree into the oral cavity, ductal reabsorption of Ca(2+) remains enigmatic. Here we report a potential role for the G protein-coupled receptor, calcium-sensing receptor (CSR), in the regulation of Ca(2+) reabsorption by salivary gland ducts. Our data show that CSR is present in the apical region of ductal cells where it is co-localized with transient receptor potential canonical 3 (TRPC3). CSR is activated in isolated salivary gland ducts as well as a ductal cell line (SMIE) by altering extracellular [Ca(2+)] or by aromatic amino acid, L-phenylalanine (L-Phe, endogenous component of saliva), as well as neomycin. CSR activation leads to Ca(2+) influx that, in polarized cells grown on a filter support, is initiated in the luminal region. We show that TRPC3 contributes to Ca(2+) entry triggered by CSR activation. Further, stimulation of CSR in SMIE cells enhances the CSR-TRPC3 association as well as surface expression of TRPC3. Together our findings suggest that CSR could serve as a Ca(2+) sensor in the luminal membrane of salivary gland ducts and regulate reabsorption of [Ca(2+)] from the saliva via TRPC3, thus contributing to maintenance of salivary [Ca(2+)]. CSR could therefore be a potentially important protective mechanism against formation of salivary gland stones (sialolithiasis) and infection (sialoadenitis).

Orais and STIMs: physiological mechanisms and disease

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

Regulation of Orai1/STIM1 by the kinases SGK1 and AMPK

STIM and Orai isoforms orchestrate store operated Ca2+ entry (SOCE) and thus cytosolic Ca2+ fluctuations following stimulation by hormones, growth factors and further mediators. Orai1 is a target of Nedd4-2, an ubiquitin ligase preparing several plasma membrane proteins for degradation. Phosphorylation of Nedd4-2 by the serum and glucocorticoid inducible kinase SGK1 leads to the binding of Nedd4-2 to the protein 14-3-3 thus preventing its interaction with Orai1. Nedd4-2 is activated by the energy sensing AMP activated kinase AMPK. Thus, SGK1 disrupts and AMPK fosters degradation of Orai1. New synthesis of both, Orai1 and STIM1, is stimulated by the transcription factor NF-κB (nuclear factor kappa B), which binds to the respective promoter regions of the genes encoding STIM1 and Orai1. SGK1 upregulates and AMPK presumably downregulates NF-κB and thus de novo synthesis of Orai1 and STIM1 proteins. The regulation by SGK1 links SOCE to the signaling of a wide variety of hormones and growth factors, the AMPK dependent regulation of Orai1 and STIM1 may serve to limit inadequate activation of SOCE following energy depletion, which is otherwise expected to activate SOCE by depletion of intracellular Ca2+ stores due to impairment of the ATP consuming sarco/endoplasmatic reticulum Ca2+ ATPase SERCA.

Unraveling STIM2 function

The discovery of molecular players in capacitative calcium (Ca(2+)) entry, also referred to as store-operated Ca(2+) entry (SOCE), supposed a great advance in the knowledge of cellular mechanisms of Ca(2+) entry, which are essential for a broad range of cellular functions. The identification of STIM1 and STIM2 proteins as the sensors of Ca(2+) stored in the endoplasmic reticulum unraveled the mechanism by which depletion of intracellular Ca(2+) stores is communicated to store-operated Ca(2+) channels located in the plasma membrane, triggering the activation of SOCE and intracellular Ca(2+)-dependent signaling cascades. Initial studies suggested a dominant function of STIM1 in SOCE and SOCE-dependent cellular functions compared to STIM2, especially those that participate in immune responses. Consequently, most of the subsequent studies focused on STIM1. However, during the last years, STIM2 has been demonstrated to play a more relevant and complex function than initially reported, being even important to sustain normal life in mice. These studies have led to reconsider the role of STIM2 in SOCE and its relevance in cellular physiology. This review is intended to summarize and provide an overview of the current data available about this exciting isoform, STIM2, and its actual position together with STIM1 in the mechanism of SOCE.

Enhanced Ca²⁺ entry and Na+/Ca²⁺ exchanger activity in dendritic cells from AMP-activated protein kinase-deficient mice

In dendritic cells (DCs), chemotactic chemokines, such as CXCL12, rapidly increase cytosolic Ca(2+)concentrations ([Ca(2+)](i)) by triggering Ca(2+) release from intracellular stores followed by store-operated Ca(2+) (SOC) entry. Increase of [Ca(2+)](i) is blunted and terminated by Ca(2+) extrusion, accomplished by K(+)-independent Na(+)/Ca(2+) exchangers (NCXs) and K(+)-dependent Na(+)/Ca(2+) exchangers (NCKXs). Increased [Ca(2+)](i) activates energy-sensing AMP-activated protein kinase (AMPK), which suppresses proinflammatory responses of DCs and macrophages. The present study explored whether AMPK participates in the regulation of DC [Ca(2+)](i) and migration. DCs were isolated from AMPKα1-deficient (ampk(-/-)) mice and, as control, from their wild-type (ampk(+/+)) littermates. AMPKα1, Orai1-2, STIM1-2, and mitochondrial calcium uniporter protein expression was determined by Western blotting, [Ca(2+)](i) by Fura-2 fluorescence, SOC entry by inhibition of endosomal Ca(2+) ATPase with thapsigargin (1 μM), Na(+)/Ca(2+) exchanger activity from increase of [Ca(2+)](i), and respective whole-cell current in patch clamp following removal of extracellular Na(+). Migration was quantified utilizing transwell chambers. AMPKα1 protein is expressed in ampk(+/+) DCs but not in ampk(-/-) DCs. CXCL12 (300 ng/ml)-induced increase of [Ca(2+)](i), SOC entry, Orai 1 protein abundance, NCX, and NCKX were all significantly higher in ampk(-/-) DCs than in ampk(+/+) DCs. NCX and NCKX currents were similarly increased in ampk(-/-) DCs. Moreover, CXCL12 (50 ng/ml)-induced DC migration was enhanced in ampk(-/-) DCs. AMPK thus inhibits SOC entry, Na(+)/Ca(2+) exchangers, and migration of DCs.