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

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.

Opportunities and challenges for the development of covalent chemical immunomodulators

Compounds that react irreversibly with cysteines have reemerged as potent and selective tools for altering protein function, serving as chemical probes and even clinically approved drugs. The exquisite sensitivity of human immune cell signaling pathways to oxidative stress indicates the likely, yet still underexploited, general utility of covalent probes for selective chemical immunomodulation. Here, we provide an overview of immunomodulatory cysteines, including identification of electrophilic compounds available to label these residues. We focus our discussion on three protein classes essential for cell signaling, which span the 'druggability' spectrum from amenable to chemical probes (kinases), somewhat druggable (proteases), to inaccessible (phosphatases). Using existing inhibitors as a guide, we identify general strategies to guide the development of covalent probes for selected undruggable classes of proteins and propose the application of such compounds to alter immune cell functions.

Myopathology of Congenital Myopathies: Bridging the Old and the New

Congenital myopathies (CM) are a genetically heterogeneous group of neuromuscular disorders most commonly presenting with neonatal/childhood-onset hypotonia and muscle weakness, a relatively static or slowly progressive disease course, and originally classified into subcategories based on characteristic histopathologic findings in muscle biopsies. This enduring concept of disease definition and classification based on the clinicopathologic phenotype was pioneered in the premolecular era. Advances in molecular genetics have brought into focus the increased blurring of the original seemingly "watertight" categories through broadening of the clinical phenotypes in existing genes, and continuous identification of novel genetic backgrounds. This review summarizes the histopathologic landscape of the 4 "classical" subtypes of CM-nemaline myopathies, core myopathies, centronuclear myopathies, and congenital fiber type disproportion and some of the emerging and novel genetic diseases with a CM presentation.

Memantine ameliorates pulmonary inflammation in a mice model of COPD induced by cigarette smoke combined with LPS

An enhanced chronic inflammatory response in the airways has been regarded as a critical characteristic of chronic obstructive pulmonary disease (COPD). Memantine, an N-methyl-d-aspartate (NMDA) receptors antagonist, has been reported to alleviate lung inflammation. In this study, we investigated the effect and mechanism of memantine on the COPD model induced by cigarette smoke (CS) combined with LPS. Mice and RAW264.7 cells were treated with LPS in the presence or absence of CS. We performed H&E staining to analysis the lung histopathological characteristics. Cytokines (IL-6, TNF-α, and IFN-γ) levels in bronchoalveolar lavage fluid (BALF), lung tissue homogenates and RAW264.7 cell culture medium were determined. Glutamate levels in plasma and culture medium of RAW264.7 were determined. The intracellular Ca2+ flux in RAW264.7 cells was measured by fluo-3 AM staining. The protein levels of NR-1, xCT, ERK1/2, and AKT signaling in the lung tissue and cells were investigated. The result showed that CS and LPS stimulation caused inflammation response, a significant increase in the release of cytokines, including TNF-α, IL-6, and IFN-γ, the elevated release of glutamate and protein levels of NR-1 and xCT, increased Ca2+ influx, and the activation of the ERK1/2 pathway in vitro and in vivo. The above effects of CS and LPS stimulation could be significantly attenuated by memantine treatment. In conclusion, memantine can effectively ameliorate pulmonary inflammation in CS + LPS-induced COPD in mice via reducing NR-1 and xCT expression, glutamate release, Ca2+ influx, and the phosphorylation of Erk1/2. We provided a possible mechanism by which memantine ameliorates COPD in mice.

Chemoinformatic Analysis of Selected Cacalolides from Psacalium decompositum (A. Gray) H. Rob. & Brettell and Psacalium peltatum (Kunth) Cass. and Their Effects on FcεRI-Dependent Degranulation in Mast Cells

Cacalolides are a kind of sesquiterpenoids natural compounds synthesized by Psacalium decompositum (A. Gray) H. Rob. & Brettell or Psacalium peltatum (Kunth) Cass. Antioxidant and hypoglycemic effects have been found for cacalolides such as cacalol, cacalone or maturine, however, their effects on inflammatory processes are still largely unclear. The main aim of this study was to investigate the biological activities of secondary metabolites from P. decompositum and P. peltatum through two approaches: (1) chemoinformatic and toxicoinformatic analysis based on ethnopharmacologic background; and (2) the evaluation of their potential anti-inflammatory/anti-allergic effects in bone marrow-derived mast cells by IgE/antigen complexes. The bioinformatics properties of the compounds: cacalol; cacalone; cacalol acetate and maturin acetate were evaluated through Osiris DataWarrior software and Molinspiration and PROTOX server. In vitro studies were performed to test the ability of these four compounds to inhibit antigen-dependent degranulation and intracellular calcium mobilization, as well as the production of reactive oxygen species in bone marrow-derived mast cells. Our findings showed that cacalol displayed better bioinformatics properties, also exhibited a potent inhibitory activity on IgE/antigen-dependent degranulation and significantly reduced the intracellular calcium mobilization on mast cells. These data suggested that cacalol could reduce the negative effects of the mast cell-dependent inflammatory process.

Pharmacological inhibition of store-operated calcium entry in MDA-MB-468 basal A breast cancer cells: consequences on calcium signalling, cell migration and proliferation

Store-operated Ca2+ entry is a pathway that is remodelled in a variety of cancers, and altered expression of the components of store-operated Ca2+ entry is a feature of breast cancer cells of the basal molecular subtype. Studies of store-operated Ca2+ entry in breast cancer cells have used non-specific pharmacological inhibitors, complete depletion of intracellular Ca2+ stores and have mostly focused on MDA-MB-231 cells (a basal B breast cancer cell line). These studies compared the effects of the selective store-operated Ca2+ entry inhibitors Synta66 and YM58483 (also known as BTP2) on global cytosolic free Ca2+ ([Ca2+]CYT) changes induced by physiological stimuli in a different breast cancer basal cell line model, MDA-MB-468. The effects of these agents on proliferation as well as serum and epidermal growth factor (EGF) induced migration were also assessed. Activation with the purinergic receptor activator adenosine triphosphate, produced a sustained increase in [Ca2+]CYT that was entirely dependent on store-operated Ca2+ entry. The protease activated receptor 2 activator, trypsin, and EGF also produced Ca2+ influx that was sensitive to both Synta66 and YM58483. Serum-activated migration of MDA-MB-468 breast cancer cells was sensitive to both store-operated Ca2+ inhibitors. However, proliferation and EGF-activated migration was differentially affected by Synta66 and YM58483. These studies highlight the need to define the exact mechanisms of action of different store-operated calcium entry inhibitors and the impact of such differences in the control of tumour progression pathways.

ORAI1 channel gating and selectivity is differentially altered by natural mutations in the first or third transmembrane domain

KEY POINTS

Gain-of-function mutations in the highly selective Ca²⁺ channel ORAI1 cause tubular aggregate myopathy (TAM) characterized by muscular pain, weakness and cramping. TAM-associated mutations in ORAI1 first and third transmembrane domain facilitate channel opening by STIM1, causing constitutive Ca²⁺ influx and increasing the currents evoked by Ca²⁺ store depletion. Mutation V107M additionally decreases the channel selectivity for Ca²⁺ ions and its inhibition by acidic pH, while mutation T184M does not alter the channel sensitivity to pH or to reactive oxygen species. The ORAI blocker GSK-7975A prevents the constitutive activity of TAM-associated channels and might be used in therapy for patients suffering from TAM.

ABSTRACT

Skeletal muscle differentiation relies on store-operated Ca²⁺ entry (SOCE) mediated by STIM proteins linking the depletion of endoplasmic/sarcoplasmic reticulum Ca²⁺ stores to the activation of membrane Ca²⁺ -permeable ORAI channels. Gain-of-function mutations in STIM1 or ORAI1 isoforms cause tubular aggregate myopathy (TAM), a skeletal muscle disorder with muscular pain, weakness and cramping. Here, we characterize two overactive ORAI1 mutants from patients with TAM: V107M and T184M, located in the first and third transmembrane domain of the channel. When ectopically expressed in HEK-293T cells or human primary myoblasts, the mutated channels increased basal and store-operated Ca²⁺ entry. The constitutive activity of V107M, L138F, T184M and P245L mutants was prevented by low concentrations of GSK-7975A while the G98S mutant was resistant to inhibition. Electrophysiological recordings confirmed ORAI1-V107M constitutive activity and revealed larger STIM1-gated V107M- and T184M-mediated currents with conserved fast and slow Ca²⁺ -dependent inactivation. Mutation V107M altered the channel selectivity for Ca²⁺ ions and conferred resistance to acidic inhibition. Ca²⁺ imaging and molecular dynamics simulations showed a preserved sensitivity of T184M to the negative regulation by reactive oxygen species. Both mutants were able to mediate SOCE in Stim1^-/- /Stim2^-/- mouse embryonic fibroblasts expressing the binding-deficient STIM1-F394H mutant, indicating a higher sensitivity for STIM1-mediated gating, with ORAI1-T184M gain-of-function being strictly dependent on STIM1. These findings provide new insights into the permeation and regulatory properties of ORAI1 mutants that might translate into therapies against diseases with gain-of-function mutations in ORAI1.

SOCE and STIM1 signaling in the heart: Timing and location matter

Store operated Ca²⁺ entry (SOCE) is an ancient and ubiquitous Ca²⁺ signaling pathway discovered decades ago, but the function of SOCE in human physiology is only now being revealed. The relevance of this pathway to striated muscle was solidified with the description of skeletal myopathies that result from mutations in STIM1 and Orai1, the two SOCE components. Here, we consider the evidence for STIM1 and SOCE in cardiac muscle and the sinoatrial node. We highlight recent studies revealing a role for STIM1 in cardiac growth in response to developmental and pathologic cues. We also review the role of STIM1 in the regulation of SOCE and Ca²⁺ store refilling in a non-Orai dependent manner. Finally, we discuss the importance of this pathway in ventricular cardiomyocytes where SOCE contribute to developmental growth and in pacemaker cells where SOCE likely has a fundamental to generating the cardiac rhythm.

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.

Calcium signalling and related ion channels in neutrophil recruitment and function

The recruitment of neutrophils to sites of inflammation, their battle against invading microorganisms through phagocytosis and the release of antimicrobial agents is a highly coordinated and tightly regulated process that involves the interplay of many different receptors, ion channels and signalling pathways. Changes in intracellular calcium levels, caused by cytosolic Ca2+ store depletion and the influx of extracellular Ca2+ via ion channels, play a critical role in synchronizing neutrophil activation and function. In this review, we provide an overview of how Ca2+ signalling is initiated in neutrophils and how changes in intracellular Ca2+ levels modulate neutrophil function.

Role of STIM1/ORAI1-mediated store-operated Ca2+ entry in skeletal muscle physiology and disease

Store-operated Ca²⁺ entry (SOCE) is a Ca²⁺ entry mechanism activated by depletion of intracellular Ca²⁺ stores. In skeletal muscle, SOCE is mediated by an interaction between stromal-interacting molecule-1 (STIM1), the Ca²⁺ sensor of the sarcoplasmic reticulum, and ORAI1, the Ca²⁺-release-activated-Ca²⁺ (CRAC) channel located in the transverse tubule membrane. This review focuses on the molecular mechanisms and physiological role of SOCE in skeletal muscle, as well as how alterations in STIM1/ORAI1-mediated SOCE contribute to muscle disease. Recent evidence indicates that SOCE plays an important role in both muscle development/growth and fatigue. The importance of SOCE in muscle is further underscored by the discovery that loss- and gain-of-function mutations in STIM1 and ORAI1 result in an eclectic array of disorders with clinical myopathy as central defining component. Despite differences in clinical phenotype, all STIM1/ORAI1 gain-of-function mutations-linked myopathies are characterized by the abnormal accumulation of intracellular membranes, known as tubular aggregates. Finally, dysfunctional STIM1/ORAI1-mediated SOCE also contributes to the pathogenesis of muscular dystrophy, malignant hyperthermia, and sarcopenia. The picture to emerge is that tight regulation of STIM1/ORAI1-dependent Ca²⁺ signaling is critical for optimal skeletal muscle development/function such that either aberrant increases or decreases in SOCE activity result in muscle dysfunction.

STIM1 R304W causes muscle degeneration and impaired platelet activation in mice

STIM1 and ORAI1 regulate store-operated Ca²⁺ entry (SOCE) in most cell types, and mutations in these proteins have deleterious and diverse effects. We established a mouse line expressing the STIM1 R304 W gain-of-function mutation causing Stormorken syndrome to explore effects on organ and cell physiology. While STIM1 R304 W was lethal in the homozygous state, surviving mice presented with reduced growth, skeletal muscle degeneration, and reduced exercise endurance. Variable STIM1 expression levels between tissues directly impacted cellular SOCE capacity. In contrast to patients with Stormorken syndrome, STIM1 was downregulated in fibroblasts from Stim1^R304W/R304W mice, which maintained SOCE despite constitutive protein activity. In studies using foetal liver chimeras, STIM1 protein was undetectable in homozygous megakaryocytes and platelets, resulting in impaired platelet activation and absent SOCE. These data indicate that downregulation of STIM1 R304 W effectively opposes the gain-of-function phenotype associated with this mutation, and highlight the importance of STIM1 in skeletal muscle development and integrity.

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.

CRAC channel-based optogenetics

Store-operated Ca²⁺ entry (SOCE) constitutes a major Ca²⁺ influx pathway in mammals to regulate a myriad of physiological processes, including muscle contraction, synaptic transmission, gene expression, and metabolism. In non-excitable cells, the Ca²⁺ release-activated Ca²⁺ (CRAC) channel, composed of ORAI and stromal interaction molecules (STIM), constitutes a prototypical example of SOCE to mediate Ca²⁺ entry at specialized membrane contact sites (MCSs) between the endoplasmic reticulum (ER) and the plasma membrane (PM). The key steps of SOCE activation include the oligomerization of the luminal domain of the ER-resident Ca²⁺ sensor STIM1 upon Ca²⁺ store depletion, subsequent signal propagation toward the cytoplasmic domain to trigger a conformational switch and overcome the intramolecular autoinhibition, and ultimate exposure of the minimal ORAI-activating domain to directly engage and gate ORAI channels in the plasma membrane. This exquisitely coordinated cellular event is also facilitated by the C-terminal polybasic domain of STIM1, which physically associates with negatively charged phosphoinositides embedded in the inner leaflet of the PM to enable efficient translocation of STIM1 into ER-PM MCSs. Here, we present recent progress in recapitulating STIM1-mediated SOCE activation by engineering CRAC channels with optogenetic approaches. These STIM1-based optogenetic tools make it possible to not only mechanistically recapture the key molecular steps of SOCE activation, but also remotely and reversibly control Ca²⁺-dependent cellular processes, inter-organellar tethering at MCSs, and transcriptional reprogramming when combined with CRISPR/Cas9-based genome-editing tools.

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.

Calcium signaling defects underlying salivary gland dysfunction

Salivary glands secrete saliva, a mixture of proteins and fluids, which plays an extremely important role in the maintenance of oral health. Loss of salivary secretion causes a dry mouth condition, xerostomia, which has numerous deleterious consequences including opportunistic infections within the oral cavity, difficulties in eating and swallowing food, and problems with speech. Saliva secretion is regulated by stimulation of specific signaling mechanisms within the acinar cells of the gland. Neurotransmitter-stimulated increase in cytosolic [Ca²⁺] ([Ca²⁺]_i) in acinar cells is the primary trigger for salivary fluid secretion from salivary glands, the loss of which is a critical factor underlying dry mouth conditions in patients. The increase in [Ca²⁺]_i regulates multiple ion channel and transport activities that together generate the osmotic gradient which drives fluid secretion across the apical membrane. Ca²⁺ entry mediated by the Store-Operated Ca²⁺ Entry (SOCE) mechanism provides the essential [Ca²⁺]_i signals to trigger salivary gland fluid secretion. Under physiological conditions depletion of ER-Ca²⁺ stores is caused by activation of IP3R by IP3 and this provides the stimulus for SOCE. Core components of SOCE in salivary gland acinar cells are the plasma membrane Ca²⁺ channels, Orai1 and TRPC1, and STIM1, a Ca²⁺-sensor protein in the ER, which regulates both channels. In addition, STIM2 likely enhances the sensitivity of cells to ER-Ca²⁺ depletion thereby tuning the cellular response to agonist stimulation. Two major, clinically relevant, conditions which cause irreversible salivary gland dysfunction are radiation treatment for head-and-neck cancers and the autoimmune exocrinopathy, Sjögren's syndrome (pSS). However, the exact mechanism(s) that causes the loss of fluid secretion, in either condition, is not clearly understood. A number of recent studies have identified that defects in critical Ca²⁺ signaling mechanisms underlie salivary gland dysfunction caused by radiation treatment or Sjögren's syndrome (pSS). This chapter will discuss these very interesting and important studies.

Evidence That Calcium Entry Into Calcium-Transporting Dental Enamel Cells Is Regulated by Cholecystokinin, Acetylcholine and ATP

Dental enamel is formed by specialized epithelial cells which handle large quantities of Ca²⁺ while producing the most highly mineralized tissue. However, the mechanisms used by enamel cells to handle bulk Ca²⁺ safely remain unclear. Our previous work contradicted the dogma that Ca²⁺ is ferried through the cytosol of Ca²⁺-transporting cells and instead suggested an organelle-based route across enamel cells. This new paradigm involves endoplasmic reticulum (ER)-associated Ca²⁺ stores and their concomitant refilling by store-operated Ca²⁺ entry (SOCE) mediated by Ca²⁺ release activated Ca²⁺ (CRAC) channels. Given that Ca²⁺ handling is maximal during the enamel-mineralization stage (maturation), we anticipated that SOCE would also be elevated then. Confirmation was obtained here using single-cell recordings of cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) in rat ameloblasts. A candidate SOCE agonist, cholecystokinin (CCK), was found to be upregulated during maturation, with Cck transcript abundance reaching 30% of that in brain. CCK-receptor transcripts were also detected and Ca²⁺ imaging showed that CCK stimulation increased [Ca²⁺]_cyt in a dose-responsive manner that was sensitive to CRAC-channel inhibitors. Similar effects were observed with two other SOCE activators, acetylcholine and ATP, whose receptors were also found in enamel cells. These results provide the first evidence of a potential regulatory system for SOCE in enamel cells and so strengthen the Ca²⁺ transcytosis paradigm for ER-based transport of bulk Ca²⁺. Our findings also implicate enamel cells as a new physiological target of CCK and raise the possibility of an auto/paracrine system for regulating Ca²⁺ transport.

Molecular Determinants for STIM1 Activation During Store- Operated Ca2+ Entry

BACKGROUND

STIM/ORAI-mediated store-operated Ca2+ entry (SOCE) mediates a myriad of Ca2+-dependent cellular activities in mammals. Genetic defects in STIM1/ORAI1 lead to devastating severe combined immunodeficiency; whereas gain-offunction mutations in STIM1/ORAI1 are intimately associated with tubular aggregate myopathy. At molecular level, a decrease in the Ca2+ concentrations within the lumen of endoplasmic reticulum (ER) initiates multimerization of the STIM1 luminal domain to switch on the STIM1 cytoplasmic domain to engage and gate ORAI channels, thereby leading to the ultimate Ca2+ influx from the extracellular space into the cytosol. Despite tremendous progress made in dissecting functional STIM1-ORAI1 coupling, the activation mechanism of SOCE remains to be fully characterized.

OBJECTIVE AND METHODS

Building upon a robust fluorescence resonance energy transfer assay designed to monitor STIM1 intramolecular autoinhibition, we aimed to systematically dissect the molecular determinants required for the activation and oligomerization of STIM1.

RESULTS

Here we showed that truncation of the STIM1 luminal domain predisposes STIM1 to adopt a more active conformation. Replacement of the single transmembrane (TM) domain of STIM1 by a more rigid dimerized TM domain of glycophorin A abolished STIM1 activation. But this adverse effect could be partially reversed by disrupting the TM dimerization interface. Moreover, our study revealed regions that are important for the optimal assembly of hetero-oligomers composed of full-length STIM1 with its minimal STIM1-ORAI activating region, SOAR.

CONCLUSIONS

Our study clarifies the roles of major STIM1 functional domains in maintaining a quiescent configuration of STIM1 to prevent preactivation of SOCE.

Mapping the functional anatomy of Orai1 transmembrane domains for CRAC channel gating

Store-operated Orai1 channels are activated through a unique inside-out mechanism involving binding of the endoplasmic reticulum Ca²⁺ sensor STIM1 to cytoplasmic sites on Orai1. Although atomic-level details of Orai structure, including the pore and putative ligand binding domains, are resolved, how the gating signal is communicated to the pore and opens the gate is unknown. To address this issue, we used scanning mutagenesis to identify 15 residues in transmembrane domains (TMs) 1-4 whose perturbation activates Orai1 channels independently of STIM1. Cysteine accessibility analysis and molecular-dynamics simulations indicated that constitutive activation of the most robust variant, H134S, arises from a pore conformational change that opens a hydrophobic gate to augment pore hydration, similar to gating evoked by STIM1. Mutational analysis of this locus suggests that H134 acts as steric brake to stabilize the closed state of the channel. In addition, atomic packing analysis revealed distinct functional contacts between the TM1 pore helix and the surrounding TM2/3 helices, including one set mediated by a cluster of interdigitating hydrophobic residues and another by alternative ridges of polar and hydrophobic residues. Perturbing these contacts via mutagenesis destabilizes STIM1-mediated Orai1 channel gating, indicating that these bridges between TM1 and the surrounding TM2/3 ring are critical for conveying the gating signal to the pore. These findings help develop a framework for understanding the global conformational changes and allosteric interactions between topologically distinct domains that are essential for activation of Orai1 channels.

Short-term high-glucose treatment decreased abundance of Orai1 protein through posttranslational mechanisms in rat mesangial cells

The short-term effect of high-glucose (HG) treatment on store-operated Ca²⁺ entry in mesangial cells (MCs) is not well-known. The aim of the present study was to determine whether and how HG treatment for a short period altered protein abundance of Orai1, the channel mediating store-operated Ca²⁺ entry in MCs. Rat and human MCs were exposed to HG (25 mM) for 2, 4, 8, and 24 h, and the abundance of Orai1 protein was significantly decreased at the time points of 8 and 16 h. Consistently, HG treatment for 8 h significantly reduced store-operated Ca²⁺ entry in rat MCs. However, HG treatment for the same time periods did not alter the levels of Orai1 transcript. Cycloheximide, a protein synthesis inhibitor, did not affect the HG-induced decrease of Orai1 protein, suggesting a posttranslational mechanism was involved. However, the HG effect on Orai1 protein was significantly attenuated by MG132 (a ubiquitin-proteasome inhibitor) and NH₄Cl (a lysosomal pathway inhibitor). Furthermore, HG treatment for 8 h stimulated ubiquitination of Orai1 protein. We further found that polyethylene glycol-catalase, an antioxidant, significantly blunted the HG-induced reduction of Orai1 protein. In support of involvement of reactive oxygen species in the HG effects, hydrogen peroxide (H₂O₂) itself significantly decreased abundance of Orai1 protein and increased the level of ubiquitinated Orai1. Taken together, these results suggest that a short-term HG treatment decreased abundance of Orai1 protein in MCs by promoting the protein degradation through the ubiquitination-proteasome and -lysosome mechanisms. This HG-stimulated posttranslational mechanism was mediated by H₂O₂.

Orai1 downregulation impairs lymphocyte function in type 2 diabetes mellitus

BACKGROUND/AIMS

It has been suggested that diabetes is associated with immune dysfunction, in which Ca²⁺ signaling malfunction in lymphocyte may contributes most. However, the pattern of the Ca²⁺ signal disorder and the mechanism(s) that explains the change are unclear. Here, in this study we aimed to investigate possible changes and mechanism(s) accounting for the internal Ca²⁺ signals in diabetic T lymphocyte upon stimulation.

METHODS AND RESULTS

Using Fura-2-AM, we found a significant decrease in Ca²⁺ influx induced by thapsigargin (TG) and anti-CD3 antibody (OKT3) in T lymphocytes from blood of both diabetes patients and animals. Furthermore, a downregulated Orai1 protein expression, but not mRNA, was also observed in these cells using western blot and qRT-PCR, respectively. In addition, in high-glucose and agonist treated Jurkat T cells, Ca²⁺ entry and the release of interleukin-2 (IL-2) were also decreased. Orai1 expression reduced, while stromal interaction molecule 1 (STIM1) and other downstream proteins remained unchanged.

CONCLUSION

This study demonstrates that the declined Orai1 expression, at least partly, contributes to the downregulated Ca²⁺ entry during lymphocyte excitation, providing an important mechanism for T lymphocyte malfunction in diabetes.

Extension of chemotactic pseudopods by nonadherent human neutrophils does not require or cause calcium bursts

Global bursts in free intracellular calcium (Ca²⁺) are among the most conspicuous signaling events in immune cells. To test the common view that Ca²⁺ bursts mediate rearrangement of the actin cytoskeleton in response to the activation of G protein-coupled receptors, we combined single-cell manipulation with fluorescence imaging and monitored the Ca²⁺ concentration in individual human neutrophils during complement-mediated chemotaxis. By decoupling purely chemotactic pseudopod formation from cell-substrate adhesion, we showed that physiological concentrations of anaphylatoxins, such as C5a, induced nonadherent human neutrophils to form chemotactic pseudopods but did not elicit Ca²⁺ bursts. By contrast, pathological or supraphysiological concentrations of C5a often triggered Ca²⁺ bursts, but pseudopod protrusion stalled or reversed in such cases, effectively halting chemotaxis, similar to sepsis-associated neutrophil paralysis. The maximum increase in cell surface area during pseudopod extension in pure chemotaxis was much smaller-by a factor of 8-than the known capacity of adherent human neutrophils to expand their surface. Because the measured rise in cortical tension was not sufficient to account for this difference, we attribute the limited deformability to a reduced ability of the cytoskeleton to generate protrusive force in the absence of cell adhesion. Thus, we hypothesize that Ca²⁺ bursts in neutrophils control a mechanistic switch between two distinct modes of cytoskeletal organization and dynamics. A key element of this switch appears to be the expedient coordination of adhesion-dependent lock or release events of cytoskeletal membrane anchors.

BTP2, a Store-Operated Calcium Channel Inhibitor, Attenuates Lung Ischemia-Reperfusion Injury in Rats

Lung ischemia-reperfusion (I/R) injury is a critical complication following a lung transplant, cardiopulmonary bypass, pulmonary embolism, and trauma. Immune cells and their effector functions are involved in the lung I/R injury. Store-operated calcium channels (SOCC) are highly Ca²⁺-selective cation channels and have crucial effects on the immune system. It has been indicated that BTP2, a potent SOCC blocker, could inhibit pro-inflammatory cytokine production from immune cells both in vitro and in vivo. Therefore, this study was conducted to investigate the beneficial effects of BTP2 on lung I/R injury in Sprague-Dawley (SD) rats. The left lungs of male SD rats underwent ischemia for 60 min and reperfusion for 2 h. Treated animals received BTP2 4 mg/kg or 10 mg/kg intraperitoneally 30 min before the ischemia. The results revealed that pretreatment with BTP2 markedly attenuated I/R injury-induced pulmonary edema, microvascular protein leakage, neutrophil infiltration, adhesion molecules, cytokine production (e.g., ICAM-1, TNF-α, IL-1β, and IL-2), and the transcription factor nuclear factor of activated T cells c1 nuclear translocation in the lung tissue. These findings indicate that BTP2 can be a potential therapeutic drug for lung I/R injury and suggest that SOCC may play a critical role in lung I/R injury.

Nanoluciferase Reporter Gene System Directed by Tandemly Repeated Pseudo-Palindromic NFAT-Response Elements Facilitates Analysis of Biological Endpoint Effects of Cellular Ca2+ Mobilization

NFAT is a cytoplasm-localized hyper-phosphorylated transcription factor that is activated through dephosphorylation by calcineurin, a Ca²⁺/calmodulin-dependent phosphatase. A non-palindromic NFAT-response element (RE) found in the IL2 promoter region has been commonly used for a Ca²⁺-response reporter gene system, but requirement of concomitant activation of AP-1 (Fos/Jun) often complicates the interpretation of obtained results. A new nanoluciferase (NanoLuc) reporter gene containing nine-tandem repeats of a pseudo-palindromic NFAT-RE located upstream of the IL8 promoter was designed to monitor Ca²⁺-induced transactivation activity of NFAT in human embryonic kidney (HEK) 293 cells by measuring luciferase activities of NanoLuc and co-expressed firefly luciferase for normalization. Ionomycin treatment enhanced the relative luciferase activity (RLA), which was suppressed by calcineurin inhibitors. HEK293 cells that stably express human STIM1 and Orai1, components of the store-operated calcium entry (SOCE) machinery, gave a much higher RLA by stimulation with thapsigargin, an inhibitor of sarcoplasmic/endoplamic reticulum Ca²⁺-ATPase (SERCA). HEK293 cells deficient in a penta-EF-hand Ca²⁺-binding protein ALG-2 showed a higher RLA value than the parental cells by stimulation with an acetylcholine receptor agonist carbachol. The novel reporter gene system is found to be useful for applications to cell signaling research to monitor biological endpoint effects of cellular Ca²⁺ mobilization.

Mast cell degranulation and calcium influx are inhibited by an Echinacea purpurea extract and the alkylamide dodeca-2E,4E-dienoic acid isobutylamide

ETHNOPHARMACOLOGICAL RELEVANCE

Native Americans used plants from the genus Echinacea to treat a variety of different inflammatory conditions including swollen gums, sore throats, skin inflammation, and gastrointestinal disorders. Today, various Echinacea spp. preparations are used primarily to treat upper respiratory infections.

AIM OF THE STUDY

The goal of this study was to evaluate the effects of an ethanolic E. purpurea (L) Moench root extract and the alkylamide dodeca-2E,4E-dienoic acid isobutylamide (A15) on mast cells, which are important mediators of allergic and inflammatory responses. Inhibition of mast cell activation may help explain the traditional use of Echinacea.

MATERIALS AND METHODS

A15 was evaluated for its effects on degranulation, calcium influx, cytokine and lipid mediator production using bone marrow derived mast cells (BMMCs) and the transformed rat basophilic leukemia mast cell line RBL-2H3. Methods included enzymatic assays, fluorimetry, ELISAs, and microscopy. A root extract of E. purpurea, and low and high alkylamide-containing fractions prepared from this extract, were also tested for effects on mast cell function. Finally, we tested A15 for effects on calcium responses in RAW 264.7 macrophage and Jurkat T cell lines.

RESULTS

A15 inhibited ß-hexosaminidase release from BMMCs and RBL-2H3 cells after treatment with the calcium ionophore A23187 by 83.5% and 48.4% at 100µM, respectively. Inhibition also occurred following stimulation with IgE anti-DNP/DNP-HSA. In addition, A15 inhibited 47% of histamine release from A23187-treated RBL-2H3 cells. A15 prevented the rapid rise in intracellular calcium following FcεRI crosslinking and A23187 treatment suggesting it acts on the signals controlling granule release. An E. purpurea root extract and a fraction with high alkylamide content derived from this extract also displayed these activities while fractions with little to no detectable amounts of alkylamide did not. A15 mediated inhibition of calcium influx was not limited to mast cells as A23187-stimulated calcium influx was blocked in both RAW 264.7 and Jurkat cell lines with 60.2% and 43.6% inhibition at 1min post-stimulation, respectively. A15 also inhibited the release of TNF-α, and PGE₂ to a lesser degree, following A23187 stimulation indicating its broad activity on mast cell mediator production.

CONCLUSIONS

These findings suggest that Echinacea extracts and alkylamides may be useful for treating allergic and inflammatory responses mediated by mast cells. More broadly, since calcium is a critical second messenger, the inhibitory effects of alkylamides on calcium uptake would be predicted to dampen a variety of pathological responses, suggesting new uses for this plant and its constituents.

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.

Areca nut extracts mobilize calcium and release pro-inflammatory cytokines from various immune cells

Betel nut consumption has significant implications for the public health globally, as the wide-spread habit of Areca chewing throughout Asia and the Pacific is associated with a high prevalence of oral carcinoma and other diseases. Despite a clear causal association of betel nut chewing and oral mucosal diseases, the biological mechanisms that link Areca nut-contained molecules, inflammation and cancer remain underexplored. In this study we show that the whole Areca nut extract (ANE) is capable of mobilizing Ca²⁺ in various immune cell lines. Interestingly, none of the four major alkaloids or a range of other known constituents of Areca nut were able to induce such Ca²⁺ signals, suggesting that the active components might represent novel or so far unappreciated chemical structures. The separation of ANE into aqueous and organic fractions has further revealed that the calcium-mobilizing molecules are exclusively present in the aqueous extract. In addition, we found that these calcium signals are associated with the activation of several immune cell lines as shown by the release of pro-inflammatory cytokines and increased cell proliferation. These results indicate that calcium-mobilizing molecules present in the aqueous fraction of the Areca nut may critically contribute to the inflammatory disorders affecting betel nut chewers.

STIM1 deficiency protects the liver from ischemia/reperfusion injury in mice

Hepatic ischemia reperfusion (I/R) injury is unavoidable in various clinical conditions. Despite considerable investigation, the underlying molecular mechanism revealing liver I/R injury remains elusive. Stromal interaction molecule 1 (STIM1) plays essential role in regulating the induction of cellular responses to a number of stress conditions, including temperature changes, elevated ROS, and hypoxia. Here, to explore if STIM1 is involved in hepatic injury, wild type (WT) and STIM1-knockout (STIM1^-/-) mice were subjected to I/R. Our results indicated that the WT mice with hepatic I/R injury showed higher STIM1 expressions from gene and protein levels in liver tissue samples. Similar results were observed in hypoxia-exposed cells in vitro. Significantly, STIM1^-/- attenuated hepatic injury compared to the WT mice after I/R, as evidenced by the improved pathological alterations in liver sections. WT mice subjected to liver I/R showed higher serum alanine aminotransferase (ALT) and aminotransferase (AST) levels, as well as pro-inflammatory cytokines, tumor necrosis factor-α (TNF-α), interleukin (IL)-6 and IL-1β, which were significantly reduced by STIM1^-/-. In addition, STIM1^-/- also decreased the liver mRNA levels of pro-inflammatory cytokines in mice after I/R injury. Furthermore, significantly decreased oxidative stress was found in STIM1^-/- mice after I/R injury compared to the WT group of mice, evidenced by the enhanced superoxide dismutase (SOD) activity and the reduced malondialdehyde (MDA) and reactive oxygen species (ROS) levels in liver tissue samples. Moreover, STIM1^-/- mice with hepatic I/R injury displayed the down-regulated nuclear factor of activated T cell (NFAT1), Orai1 and cleaved Caspase-3 levels in liver, contributing to apoptosis suppression. The results above were confirmed in hypoxia-treated cells lacking of STIM1 expression. Together, the findings suggested that STIM1-deletion protects the liver from I/R injury in mice through inhibiting inflammation, oxidative stress and apoptosis. STIM1 could be considered as a potential therapeutic target to ameliorate I/R injury.

Engineered Cross-Linking to Study the Pore Architecture of the CRAC Channel

ORAI1 constitutes the pore-forming subunit of the calcium release-activated calcium (CRAC) channel, a prototypical store-operated channel that is essential for the activation of cells of the immune system. Here we describe a convenient yet powerful cross-linking approach to examine the pore architecture of CRAC channels using ORAI1 proteins engineered to contain one or two cysteine residues. The generalizable cross-linking in situ approach can also be readily extended to study other integral membrane proteins expressed in various types of cells.

STIM2 regulates both intracellular Ca2+ distribution and Ca2+ movement in skeletal myotubes

Stromal interaction molecule 1 (STIM1) along with Orai1 mediates extracellular Ca²⁺ entry into the cytosol through a store-operated Ca²⁺ entry (SOCE) mechanism in various tissues including skeletal muscle. However, the role(s) of STIM2, a homolog of STIM1, in skeletal muscle has not been well addressed. The present study, first, was focused on searching for STIM2-binding proteins from among proteins mediating skeletal muscle functions. This study used a binding assay, quadrupole time-of-flight mass spectrometry, and co-immunoprecipitation assay with bona-fide STIM2- and SERCA1a-expressing rabbit skeletal muscle. The region for amino acids from 453 to 729 of STIM2 binds to sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase 1a (SERCA1a). Next, oxalate-supported ⁴⁵Ca²⁺-uptake experiments and various single-myotube Ca²⁺ imaging experiments using STIM2-knockdown mouse primary skeletal myotubes have suggested that STIM2 attenuates SERCA1a activity during skeletal muscle contraction, which contributes to the intracellular Ca²⁺ distribution between the cytosol and the SR at rest. In addition, STIM2 regulates Ca²⁺ movement through RyR1 during skeletal muscle contraction as well as SOCE. Therefore, via regulation of SERCA1a activity, STIM2 regulates both intracellular Ca²⁺ distribution and Ca²⁺ movement in skeletal muscle, which makes it both similar to, yet different from, STIM1.

A study on blocking store-operated Ca2+ entry in pulmonary arterial smooth muscle cells with xyloketals from marine fungi

In this study, the effect of four xyloketals 1-4 on store-operated calcium entry (SOCE) was investigated in primary distal pulmonary arterial smooth muscle cells (PASMCs) isolated from mice. The results showed that xyloketal A (1), an unusual ketal with C-3 symmetry, exhibited strong SOCE blocking activity. Secretion of interleukin-8 (IL-8) was also inhibited by xyloketal A. The parallel artificial membrane permeability assay (PAMPA) of 1-4 suggested that these xyloketals penetrated easily through the cell membrane. Moreover, the molecular docking study of xyloketal A with activation region of the stromal interaction molecule (STIM) 1 and the calcium release-activated calcium modulator (ORAI) 1 (STIM1-ORAI1) protein complex, the key domain of SOCE, revealed that xyloketal A exhibited a noncovalent interaction with the key residue lysine 363 (LYS363) in the identified cytosolic regions in STIM1-C. These findings provided useful information about xyloketal A as a SOCE inhibitor for further evaluation.

LEFTY2 inhibits endometrial receptivity by downregulating Orai1 expression and store-operated Ca2+ entry

Abstract

Early embryo development and endometrial differentiation are initially independent processes, and synchronization, imposed by a limited window of implantation, is critical for reproductive success. A putative negative regulator of endometrial receptivity is LEFTY2, a member of the transforming growth factor (TGF)-β family. LEFTY2 is highly expressed in decidualizing human endometrial stromal cells (HESCs) during the late luteal phase of the menstrual cycle, coinciding with the closure of the window of implantation. Here, we show that flushing of the uterine lumen in mice with recombinant LEFTY2 inhibits the expression of key receptivity genes, including Cox2, Bmp2, and Wnt4, and blocks embryo implantation. In Ishikawa cells, a human endometrial epithelial cell line, LEFTY2 downregulated the expression of calcium release-activated calcium channel protein 1, encoded by ORAI1, and inhibited store-operated Ca²⁺ entry (SOCE). Furthermore, LEFTY2 and the Orai1 blockers 2-APB, MRS-1845, as well as YM-58483, inhibited, whereas the Ca²⁺ ionophore, ionomycin, strongly upregulated COX2, BMP2 and WNT4 expression in decidualizing HESCs. These findings suggest that LEFTY2 closes the implantation window, at least in part, by downregulating Orai1, which in turn limits SOCE and antagonizes expression of Ca²⁺-sensitive receptivity genes.

Key messages

•Endometrial receptivity is negatively regulated by LEFTY2.

•LEFTY2 inhibits the expression of key murine receptivity genes, including Cox2, Bmp2and Wnt4, and blocks embryo implantation.

•LEFTY2 downregulates the expression of Orai1 and inhibits SOCE.

•LEFTY2 and the Orai1 blockers 2-APB, MRS-1845, and YM-58483 inhibit COX2, BMP2, and WNT4 expression in endometrial cells.

•Targeting LEFTY2 and Orai1 may represent a novel approach for treating unexplained infertility.

Electronic supplementary material

The online version of this article (10.1007/s00109-017-1610-9) contains supplementary material, which is available to authorized users.

Forms and functions of store-operated calcium entry mediators, STIM and Orai

Calcium signals arise by multiple mechanisms, including mechanisms of release of intracellular stored Ca²⁺, and the influx of Ca²⁺ through channels in the plasma membrane. One mechanism that links these two sources of Ca²⁺ is store-operated Ca²⁺ entry, the most commonly encountered version of which involves the extensively studied calcium-release-activated Ca²⁺ (CRAC) channel. The minimal and essential molecular components of the CRAC channel are the STIM proteins that function as Ca²⁺ sensors in the endoplasmic reticulum, and the Orai proteins that comprise the pore forming subunits of the CRAC channel. CRAC channels are known to play significant roles in a wide variety of physiological functions. This review discusses the multiple forms of STIM and Orai proteins encountered in mammalian cells, and discusses some specific examples of how these proteins modulate or mediate important physiological processes.

The steroid hormone 20-hydroxyecdysone upregulates calcium release-activated calcium channel modulator 1 expression to induce apoptosis in the midgut of Helicoverpa armigera

Animal steroid hormones stimulate extracellular Ca²⁺ influx into cells; however, the mechanism remains unclear. In this study, we determined that the Ca²⁺ influx induced by steroid hormone 20-hydroxyecdysone (20E) is mediated by the calcium release-activated calcium channel modulator 1 (CRACM1/Orai1). The Orai1 mRNA is highly expressed during midgut programmed cell death in the lepidopteran insect Helicoverpa armigera. 20E upregulated the expression of Orai1 in H. armigera larvae and in an epidermal cell line (HaEpi). Knockdown of Orai1 in HaEpi cells blocked 20E-induced Ca²⁺ influx, and the inhibitor of inositol 1, 4, 5-trisphosphate receptor (IP₃R) Xestospongin (XeC) blocked 20E-induced Ca²⁺ influx, suggesting that 20E, via Orai1, induces stored-operated Ca²⁺ influx. Orai1 interacts with stromal interaction molecule 1(Stim1) to exert its function in 20E-induced Ca²⁺ influx. 20E promotes Orai1 aggregation through G-protein-coupled receptors, phospholipase C gamma 1, and Stim1. Knockdown of Orai1 in the HaEpi cell line repressed apoptosis and maintained autophagy under 20E regulation. Knockdown of Orai1 in larvae delayed pupation, repressed midgut apoptosis, maintained the midgut in an autophagic state, and repressed 20E-pathway gene expression. These results revealed that steroid hormone 20E, via Orai1, induces Ca²⁺ influx to promote the transition of midgut from autophagy to apoptosis.

The secret life of ion channels: Kv1.3 potassium channels and proliferation

Kv1.3 channels are involved in the switch to proliferation of normally quiescent cells, being implicated in the control of cell cycle in many different cell types and in many different ways. They modulate membrane potential controlling K⁺ fluxes, sense changes in potential, and interact with many signaling molecules through their intracellular domains. From a mechanistic point of view, we can describe the role of Kv1.3 channels in proliferation with at least three different models. In the "membrane potential model," membrane hyperpolarization resulting from Kv1.3 activation provides the driving force for Ca²⁺ influx required to activate Ca²⁺-dependent transcription. This model explains most of the data obtained from several cells from the immune system. In the "voltage sensor model," Kv1.3 channels serve mainly as sensors that transduce electrical signals into biochemical cascades, independently of their effect on membrane potential. Kv1.3-dependent proliferation of vascular smooth muscle cells (VSMCs) could fit this model. Finally, in the "channelosome balance model," the master switch determining proliferation may be related to the control of the Kv1.3 to Kv1.5 ratio, as described in glial cells and also in VSMCs. Since the three mechanisms cannot function independently, these models are obviously not exclusive. Nevertheless, they could be exploited differentially in different cells and tissues. This large functional flexibility of Kv1.3 channels surely gives a new perspective on their functions beyond their elementary role as ion channels, although a conclusive picture of the mechanisms involved in Kv1.3 signaling to proliferation is yet to be reached.

A focus on extracellular Ca2+ entry into skeletal muscle

The main task of skeletal muscle is contraction and relaxation for body movement and posture maintenance. During contraction and relaxation, Ca²⁺ in the cytosol has a critical role in activating and deactivating a series of contractile proteins. In skeletal muscle, the cytosolic Ca²⁺ level is mainly determined by Ca²⁺ movements between the cytosol and the sarcoplasmic reticulum. The importance of Ca²⁺ entry from extracellular spaces to the cytosol has gained significant attention over the past decade. Store-operated Ca²⁺ entry with a low amplitude and relatively slow kinetics is a main extracellular Ca²⁺ entryway into skeletal muscle. Herein, recent studies on extracellular Ca²⁺ entry into skeletal muscle are reviewed along with descriptions of the proteins that are related to extracellular Ca²⁺ entry and their influences on skeletal muscle function and disease.

Cutting Edge: Murine NK Cells Degranulate and Retain Cytotoxic Function without Store-Operated Calcium Entry

Sustained Ca2+ signaling, known as store-operated calcium entry (SOCE), occurs downstream of immunoreceptor engagement and is critical for cytotoxic lymphocyte signaling and effector function. CD8+ T cells require sustained Ca2+ signaling for inflammatory cytokine production and the killing of target cells; however, much less is known about its role in NK cells. In this study, we use mice deficient in stromal interacting molecules 1 and 2, which are required for SOCE, to examine the contribution of sustained Ca2+ signaling to murine NK cell function. Surprisingly, we found that, although SOCE is required for NK cell IFN-γ production in an NFAT-dependent manner, NK cell degranulation/cytotoxicity and tumor rejection in vivo remained intact in the absence of sustained Ca2+ signaling. Our data suggest that mouse NK cells use different signaling mechanisms for cytotoxicity compared with other cytotoxic lymphocytes.

Stim1 Regulates Enamel Mineralization and Ameloblast Modulation

Loss-of-function mutations in the Ca²⁺ release-activated Ca²⁺ channel genes ORAI1 and STIM1 abolish store-operated Ca²⁺ entry (SOCE) and result in ectodermal dysplasia with amelogenesis imperfecta. However, because of the limited availability of patient tissue, analyses of enamel mineralization or possible changes in ameloblast function or morphology have not been possible. Here, we generated mice with ectodermal tissue-specific deletion of Stim1 ( Stim1 cKO [conditional knockout]), Stim2 ( Stim2 cKO), and Stim1 and Stim2 ( Stim1/2 cKO) and analyzed their enamel phenotypes as compared with those of control ( Stim1/2^fl/fl) animals. Ablation of Stim1 and Stim1/2 but not Stim2 expression resulted in chalky enamel and severe attrition at the incisor tips and molar cusps. Stim1 and Stim1/2 cKO, but not Stim2 cKO, demonstrated inferior enamel mineralization with impaired structural integrity, whereas the shape of the teeth and enamel thickness appeared to be normal in all animals. The gene expression levels of the enamel matrix proteins Amelx and Ambn and the enamel matrix proteases Mmp20 and Klk4 were not altered by the abrogation of SOCE in Stim1/2 cKO mice. The morphology of ameloblasts during the secretory and maturation stages was not significantly altered in either the incisors or molars of the cKO animals. However, in Stim1 and Stim1/2 cKO incisors, the alternating modulation of maturation-stage ameloblasts between the smooth- and ruffle-ended cell types continued beyond the regular cycle and extended to the areas corresponding to the zone of postmodulation ameloblasts in the teeth of control animals. These results indicate that SOCE is essential for proper enamel mineralization, in which Stim1 plays a critical role during the maturation process.

Tobacco Smoke Constituents Trigger Cytoplasmic Calcium Release

Cytosolic Ca²⁺ is a universal second messenger that is involved in many processes throughout the body, including the regulation of cell growth/cell division, apoptosis, and the secretion of both ions, and macromolecules. Tobacco smoke exerts multiple effects on airway epithelia and we have previously shown that Kentucky reference cigarette smoke exposure elevated the second messenger Ca²⁺, leading to dysfunctional ion secretion. In this study, we tested whether little cigar and commercial cigarette smoke exposure exerts similar effects on intracellular Ca²⁺ levels. Indeed, Swisher Sweets, Captain Black, and Cheyenne little cigars, as well as Camel, Marlboro, and Newport cigarettes, triggered a comparable increase in intracellular Ca²⁺ as seen with Kentucky reference cigarettes in human bronchial epithelia. We also found that Kentucky reference cigarette smoke exposure caused increases in Ca²⁺ in HEK293T cells and that similar increases in Ca²⁺ were seen with the tobacco smoke metabolites 1-NH₂-naphthalene, formaldehyde, nicotine, and nicotine-derived nitrosamine ketone. Given the large number of physiological processes governed by changes in cytosolic Ca²⁺, our data suggest that Ca²⁺ signaling is a useful and reproducible assay that can be used to probe the propensity of tobacco products and their constituents to cause toxicity.

Ca2+ transport and signalling in enamel cells

Dental enamel is one of the most remarkable examples of matrix-mediated biomineralization. Enamel crystals form de novo in a rich extracellular environment in a stage-dependent manner producing complex microstructural patterns that are visually stunning. This process is orchestrated by specialized epithelial cells known as ameloblasts which themselves undergo striking morphological changes, switching function from a secretory role to a cell primarily engaged in ionic transport. Ameloblasts are supported by a host of cell types which combined represent the enamel organ. Fully mineralized enamel is the hardest tissue found in vertebrates owing its properties partly to the unique mixture of ionic species represented and their highly organized assembly in the crystal lattice. Among the main elements found in enamel, Ca2+ is the most abundant ion, yet how ameloblasts modulate Ca2+ dynamics remains poorly known. This review describes previously proposed models for passive and active Ca2+ transport, the intracellular Ca2+ buffering systems expressed in ameloblasts and provides an up-dated view of current models concerning Ca2+ influx and extrusion mechanisms, where most of the recent advances have been made. We also advance a new model for Ca2+ transport by the enamel organ.

Inhibitory effect of Salvia plebeia leaf extract on ultraviolet-induced photoaging-associated ion channels and enzymes

In traditional Korean/Asian medicine, Salvia plebeia R.Br. (S. plebeia) leaves are used to treat inflammatory diseases, including dermatitis, cough, asthma and toothache. Recently, S. plebeia leaves have been applied in skin care, as they promote skin lightening and elasticity. Therefore, the present study investigated the anti-aging effects of S. plebeia leaf methanolic extract and its fractions (dichloromethane, ethylacetate and n-butanol). The results of a whole-cell patch clamp analysis indicated that the methanolic extract mediated ultraviolet (UV)-induced photoaging-associated ion channels, transient receptor potential vanilloid 1 (TRPV1) and calcium release-activated calcium channel protein 1 (ORAI1) channel activity in HEK293T cells overexpressing TRPV1 or ORAI1 and STIM1. Electrophysiological analysis revealed that the butanol fraction inhibited capsaicin-induced TRPV1 (84±8% at -60 mV/86±1% at 100 mV at 100 µg/ml) and ORAI1 (87±2% at -120 mV at 100 µg/ml) currents. Furthermore, the dichloromethane and hexane fractions inhibited tyrosinase activity by 32.4±0.69 and 22.6±0.96% at 330 µg/ml, respectively. Furthermore, the ethylacetate and butanol fractions inhibited elastase activity by 65.2±1.30 and 31.7±1.23% at 330 µg/ml, respectively. Tyrosinase and elastase, which are UV-induced photoaging-associated enzymes, regulate skin pigmentation and wrinkle formation, respectively. The results of the present study indicated that S. plebeia leaves may be a novel treatment for UV-induced photoaging.

Assembly of ER-PM Junctions: A Critical Determinant in the Regulation of SOCE and TRPC1

Store-operated calcium entry (SOCE), a unique plasma membrane Ca²⁺ entry mechanism, is activated when ER-[Ca²⁺] is decreased. SOCE is mediated via the primary channel, Orai1, as well as others such as TRPC1. STIM1 and STIM2 are ER-Ca²⁺ sensor proteins that regulate Orai1 and TRPC1. SOCE requires assembly of STIM proteins with the plasma membrane channels which occurs within distinct regions in the cell that have been termed as endoplasmic reticulum (ER)-plasma membrane (PM) junctions. The PM and ER are in close proximity to each other within this region, which allows STIM1 in the ER to interact with and activate either Orai1 or TRPC1 in the plasma membrane. Activation and regulation of SOCE involves dynamic assembly of various components that are involved in mediating Ca²⁺ entry as well as those that determine the formation and stabilization of the junctions. These components include proteins in the cytosol, ER and PM, as well as lipids in the PM. Recent studies have also suggested that SOCE and its components are compartmentalized within ER-PM junctions and that this process might require remodeling of the plasma membrane lipids and reorganization of structural and scaffolding proteins. Such compartmentalization leads to the generation of spatially- and temporally-controlled Ca²⁺signals that are critical for regulating many downstream cellular functions.

Mitochondrial Calcium Handling in Physiology and Disease

Calcium (Ca²⁺) accumulation inside mitochondria represents a pleiotropic signal controlling a wide range of cellular functions, including key metabolic pathways and life/death decisions. This phenomenon has been first described in the 1960s, but the identity of the molecules controlling this process remained a mystery until just few years ago, when both mitochondrial Ca²⁺ uptake and release systems were genetically dissected. This finally opened the possibility to develop genetic models to directly test the contribution of mitochondrial Ca²⁺ homeostasis to cellular functions. Here we summarize our current understanding of the molecular machinery that controls mitochondrial Ca²⁺ handling and critically evaluate the physiopathological role of mitochondrial Ca²⁺ signaling, based on recent evidences obtained through in vitro and in vivo models.

The STIM-Orai Pathway: Regulation of STIM and Orai by Thiol Modifications

Cysteines are among the least abundant amino acids found in proteins. Due to their unique nucleophilic thiol group, they are able to undergo a broad range of chemical modifications besides their known role in disulfide formation, such as S-sulfenylation (-SOH), S-sulfinylation (-SO(2)H), S-sufonylation (-SO(3)H), S-glutathionylation (-SSG), and S-sulfhydration (-SSH), among others. These posttranslational modifications can be irreversible and act as transitional modifiers or as reversible on-off switches for the function of proteins. Disturbances of the redox homeostasis, for example, in situations of increased oxidative stress, can contribute to a range of diseases. Because Ca²⁺ signaling mediated by store-operated calcium entry (SOCE) is involved in a plethora of cellular responses, the cross-talk between reactive oxygen species (ROS) and Ca²⁺ is critical for homeostatic control. Identification of calcium regulatory protein targets of thiol redox modifications is needed to understand their role in biology and disease.