Large store-operated calcium selective currents due to co-expression of Orai1 or Orai2 with the intracellular calcium sensor, Stim1

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.

Orai1-Orai2 complex is involved in store-operated calcium entry in chondrocyte cell lines

Ca(2+) influx via store-operated Ca(2+) entry (SOCE) plays critical roles in many essential cellular functions. The Ca(2+) release-activated Ca(2+) (CRAC) channel complex, consisting of Orai and STIM, is one of the major components of store-operated Ca(2+) (SOC) channels. Our previous study demonstrated that histamine can cause sustained Ca(2+) entry through SOC channels in OUMS-27 cells derived from human chondrosarcoma. This SOCE was increased by low- and decreased by high-concentrations of 2-aminoethoxydiphenyl borate. Quantitative reverse transcription PCR and Western blot analyses revealed abundant expressions of Orai1, Orai2 and STIM1. Introduction of dominant negative mutant of Orai1, or siOrai1 knockdown significantly attenuated SOCE. Following histamine application, single molecule imaging using total internal reflection fluorescence (TIRF) microscopy demonstrated punctate Orai1-STIM1 complex formation in plasma membrane. In contrast, knockdown or over-expression of Orai2 resulted in an increase or a decrease in SOCE, respectively. Finally, TIRF imaging revealed direct coupling between Orai1 and Orai2, and suggested that Orai2 reduces Orai1 function by formation of a hetero-tetramer. These results provide substantial evidence that Orai1, Orai2 and STIM1 form functional CRAC channels in OUMS-27 cells and that these complexes are responsible for sustained Ca(2+) entry in response to agonist stimulation.

Role of Calcium Signaling in B Cell Activation and Biology

Increase in intracellular levels of calcium ions (Ca2+) is one of the key triggering signals for the development of B cell response to the antigen. The diverse Ca2+ signals finely controlled by multiple factors participate in the regulation of gene expression, B cell development, and effector functions. B cell receptor (BCR)-initiated Ca2+ mobilization is sourced from two pathways: one is the release of Ca2+ from the intracellular stores, endoplasmic reticulum (ER), and other is the prolonged influx of extracellular Ca2+ induced by depleting the stores via store-operated calcium entry (SOCE) and calcium release-activated calcium (CRAC) channels. The identification of stromal interaction molecule 1(STIM1), the ER Ca2+ sensor, and Orai1, a key subunit of the CRAC channel pore, has now provided the tools to understand the mode of Ca2+ influx regulation and physiological relevance. Herein, we discuss our current understanding of the molecular mechanisms underlying BCR-triggered Ca2+ signaling as well as its contribution to the B cell biological processes and diseases.

Open Sesame: treasure in store-operated calcium entry pathway for cancer therapy

Store-operated Ca²⁺ entry (SOCE) controls intracellular Ca²⁺ homeostasis and regulates a wide range of cellular events including proliferation, migration and invasion. The discovery of STIM proteins as Ca²⁺ sensors and Orai proteins as Ca²⁺ channel pore forming units provided molecular tools to understand the physiological function of SOCE. Many studies have revealed the pathophysiological roles of Orai and STIM in tumor cells. This review focuses on recent advances in SOCE and its contribution to tumorigenesis. Altered Orai and/or STIM functions may serve as biomarkers for cancer prognosis, and targeting the SOCE pathway may provide a novel means for cancer treatment.

STIM1, STIM2, and Orai1 regulate store-operated calcium entry and purinergic activation of microglia

Activation of microglia is the first and main immune response to brain injury. Release of the nucleotides ATP, ADP, and UDP from damaged cells regulate microglial migration and phagocytosis via purinergic P2Y receptors. We hypothesized that store-operated Ca(2+) entry (SOCE), the prevalent Ca(2+) influx mechanism in non-excitable cells, is a potent mediator of microglial responses to extracellular nucleotides. Expression analyses of STIM Ca(2+) sensors and Orai Ca(2+) channel subunits, that comprise the molecular machinery of SOCE, showed relevant levels of STIM1, STIM2, and Orai1 in cultured mouse microglia. STIM1 expression and SOCE were down-regulated by treatment of microglia with lipopolysaccharide, suggesting that inflammation limits SOCE by lower STIM1 abundance. Ca(2+) entry induced by cyclopiazonic acid, ATP, the P2Y6 receptor agonist UDP, or the P2Y12 receptor agonist 2-methylthio-ADP (2-MeSADP) was clearly affected in microglia from Stim1(-/-) , Stim2(-/-) , and Orai1(-/-) mice. SOCE blockers or ablation of STIM1, STIM2, or Orai1 severely impaired nucleotide-induced migration and phagocytosis in microglia. Thus, this study assigns SOCE, regulated by STIM1, STIM2, and Orai1 an essential role in purinergic signaling and activation of microglia.

Complex role of STIM1 in the activation of store-independent Orai1/3 channels

Orai proteins contribute to Ca(2+) entry into cells through both store-dependent, Ca(2+) release-activated Ca(2+) (CRAC) channels (Orai1) and store-independent, arachidonic acid (AA)-regulated Ca(2+) (ARC) and leukotriene C4 (LTC4)-regulated Ca(2+) (LRC) channels (Orai1/3 heteromultimers). Although activated by fundamentally different mechanisms, CRAC channels, like ARC and LRC channels, require stromal interacting molecule 1 (STIM1). The role of endoplasmic reticulum-resident STIM1 (ER-STIM1) in CRAC channel activation is widely accepted. Although ER-STIM1 is necessary and sufficient for LRC channel activation in vascular smooth muscle cells (VSMCs), the minor pool of STIM1 located at the plasma membrane (PM-STIM1) is necessary for ARC channel activation in HEK293 cells. To determine whether ARC and LRC conductances are mediated by the same or different populations of STIM1, Orai1, and Orai3 proteins, we used whole-cell and perforated patch-clamp recording to compare AA- and LTC4-activated currents in VSMCs and HEK293 cells. We found that both cell types show indistinguishable nonadditive LTC4- and AA-activated currents that require both Orai1 and Orai3, suggesting that both conductances are mediated by the same channel. Experiments using a nonmetabolizable form of AA or an inhibitor of 5-lipooxygenase suggested that ARC and LRC currents in both cell types could be activated by either LTC4 or AA, with LTC4 being more potent. Although PM-STIM1 was required for current activation by LTC4 and AA under whole-cell patch-clamp recordings in both cell types, ER-STIM1 was sufficient with perforated patch recordings. These results demonstrate that ARC and LRC currents are mediated by the same cellular populations of STIM1, Orai1, and Orai3, and suggest a complex role for both ER-STIM1 and PM-STIM1 in regulating these store-independent Orai1/3 channels.

Clinical, histological and genetic characterisation of patients with tubular aggregate myopathy caused by mutations in STIM1

BACKGROUND

Tubular aggregate myopathies (TAMs) are muscle disorders characterised by abnormal accumulations of densely packed single-walled or double-walled membrane tubules in muscle fibres. Recently, STIM1, encoding a major calcium sensor of the endoplasmic reticulum, was identified as a TAM gene.

METHODS

The present study aims to define the clinical, histological and ultrastructural phenotype of tubular aggregate myopathy and to assess the STIM1 mutation spectrum.

RESULTS

We describe six new TAM families harbouring one known and four novel STIM1 mutations. All identified mutations are heterozygous missense mutations affecting highly conserved amino acids in the calcium-binding EF-hand domains, demonstrating the presence of a mutation hot spot for TAM. We show that the mutations induce constitutive STIM1 clustering, strongly suggesting that calcium sensing and consequently calcium homoeostasis is impaired. Histological and ultrastructural analyses define a common picture with tubular aggregates labelled with Gomori trichrome and Nicotinamide adenine dinucleotide (NADH) tetrazolium reductase, substantiating their endoplasmic reticulum origin. The aggregates were observed in both fibre types and were often accompanied by nuclear internalisation and fibre size variability. The phenotypical spectrum ranged from childhood onset progressive muscle weakness and elevated creatine kinase levels to adult-onset myalgia without muscle weakness and normal CK levels.

CONCLUSIONS

The present study expands the phenotypical spectrum of STIM1-related tubular aggregate myopathy. STIM1 should therefore be considered for patients with tubular aggregate myopathies involving either muscle weakness or myalgia as the first and predominant clinical sign.

Stromal interaction molecule 1 (STIM1) and Orai1 mediate histamine-evoked calcium entry and nuclear factor of activated T-cells (NFAT) signaling in human umbilical vein endothelial cells

Histamine is an important immunomodulator involved in allergic reactions and inflammatory responses. In endothelial cells, histamine induces Ca(2+) mobilization by releasing Ca(2+) from the endoplasmic reticulum and eliciting Ca(2+) entry across the plasma membrane. Herein, we show that histamine-evoked Ca(2+) entry in human umbilical vein endothelial cells (HUVECs) is sensitive to blockers of Ca(2+) release-activated Ca(2+) (CRAC) channels. RNA interference against STIM1 or Orai1, the activating subunit and the pore-forming subunit of CRAC channels, respectively, abolishes this histamine-evoked Ca(2+) entry. Furthermore, overexpression of dominant-negative CRAC channel subunits inhibits while co-expression of both STIM1 and Orai1 enhances histamine-induced Ca(2+) influx. Interestingly, gene silencing of STIM1 or Orai1 also interrupts the activation of calcineurin/nuclear factor of activated T-cells (NFAT) pathway and the production of interleukin 8 triggered by histamine in HUVECs. Collectively, these results suggest a central role of STIM1 and Orai1 in mediating Ca(2+) mobilization linked to inflammatory signaling of endothelial cells upon histamine stimulation.

Regulation of endogenous and heterologous Ca²⁺ release-activated Ca²⁺ currents by pH

Deviations from physiological pH (∼pH 7.2) as well as altered Ca(2+) signaling play important roles in immune disease and cancer. One of the most ubiquitous pathways for cellular Ca(2+) influx is the store-operated Ca(2+) entry (SOCE) or Ca(2+) release-activated Ca(2+) current (ICRAC), which is activated upon depletion of intracellular Ca(2+) stores. We here show that extracellular and intracellular changes in pH regulate both endogenous ICRAC in Jurkat T lymphocytes and RBL2H3 cells, and heterologous ICRAC in HEK293 cells expressing the molecular components STIM1/2 and Orai1/2/3 (CRACM1/2/3). We find that external acidification suppresses, and alkalization facilitates IP3-induced ICRAC. In the absence of IP3, external alkalization did not elicit endogenous ICRAC but was able to activate heterologous ICRAC in HEK293 cells expressing Orai1/2/3 and STIM1 or STIM2. Similarly, internal acidification reduced IP3-induced activation of endogenous and heterologous ICRAC, while alkalization accelerated its activation kinetics without affecting overall current amplitudes. Mutation of two aspartate residues to uncharged alanine amino acids (D110/112A) in the first extracellular loop of Orai1 significantly attenuated both the inhibition of ICRAC by external acidic pH as well as its facilitation by alkaline conditions. We conclude that intra- and extracellular pH differentially regulates ICRAC. While intracellular pH might affect aggregation and/or binding of STIM to Orai, external pH seems to modulate ICRAC through its channel pore, which in Orai1 is partially mediated by residues D110 and D112.

Key components of store-operated Ca2+ entry in non-excitable cells

Store-operated Ca(2+) entry (SOCE) is a ubiquitous Ca(2+) entry pathway in non-excitable cells. It is activated by the depletion of Ca(2+) from intracellular Ca(2+) stores, notably the endoplasmic reticulum (ER). In the past 9 years, it has been established that two key proteins, stromal interacting molecule 1 (STIM1) and Orai1, play critical roles in SOCE. STIM1 is a single-pass transmembrane protein located predominantly in the ER that serves as a Ca(2+) sensor within the ER, while Orai1 is a tetraspanning plasma membrane (PM) protein that functions as the pore-forming subunit of store-operated Ca(2+) channels. A decrease in the ER Ca(2+) concentration induces translocation of STIM1 into puncta close to the PM. STIM1 oligomers directly interact with Orai1 channels and activates them. This review summarizes the molecular basis of the interaction between STIM1 and Orai1 in SOCE. Further, we describe current findings on additional regulatory proteins, such as Ca(2+) release-activated Ca(2+) regulator 2A and septin, novel roles of STIM1, and modulation of SOCE by protein phosphorylation.

Orai-2 is localized on secretory granules and regulates antigen-evoked Ca²⁺ mobilization and exocytosis in mast cells

The increase in intracellular Ca(2+) through the Ca(2+) channel is an indispensable step for the secretion of inflammatory mediators by mast cells. It was recently reported that Orai-1 is responsible for the Ca(2+) influx that is activated by depletion of stored Ca(2+). There are three isoforms of Orai: Orai-1, Orai-2, and Orai-3; however, isoforms other than Orai-1 are poorly understood. We found that Orai-2 is expressed and localized on secretory granules in RBL-2H3. Ca(2+) release from Ca(2+) store, induced by antigen stimulation, was significantly attenuated by knockdown of Orai-2, while that induced by thapsigargin was not affected. Furthermore, exocytotic release induced by antigen stimulation was inhibited in knockdown cells. This observation suggests a new role of Orai isoforms in secretory cells.

Remodeling of channel-forming ORAI proteins determines an oncogenic switch in prostate cancer

ORAI family channels have emerged as important players in malignant transformation, yet the way in which they reprogram cancer cells remains elusive. Here we show that the relative expression levels of ORAI proteins in prostate cancer are different from that in noncancerous tissue. By mimicking ORAI protein remodeling observed in primary tumors, we demonstrate in in vitro models that enhanced ORAI3 expression favors heteromerization with ORAI1 to form a novel channel. These channels support store-independent Ca(2+) entry, thereby promoting cell proliferation and a smaller number of functional homomeric ORAI1-based store-operated channels, which are important in supporting susceptibility to apoptosis. Thus, our findings highlight disrupted dynamic equilibrium of channel-forming proteins as an oncogenic mechanism.

STIM1 controls neuronal Ca²⁺ signaling, mGluR1-dependent synaptic transmission, and cerebellar motor behavior

In central mammalian neurons, activation of metabotropic glutamate receptor type1 (mGluR1) evokes a complex synaptic response consisting of IP3 receptor-dependent Ca(2+) release from internal Ca(2+) stores and a slow depolarizing potential involving TRPC3 channels. It is largely unclear how mGluR1 is linked to its downstream effectors. Here, we explored the role of stromal interaction molecule 1 (STIM1) in regulating neuronal Ca(2+) signaling and mGluR1-dependent synaptic transmission. By analyzing mouse cerebellar Purkinje neurons, we demonstrate that STIM1 is an essential regulator of the Ca(2+) level in neuronal endoplasmic reticulum Ca(2+) stores. Both mGluR1-dependent synaptic potentials and IP3 receptor-dependent Ca(2+) signals are strongly attenuated in the absence of STIM1. Furthermore, the Purkinje neuron-specific deletion of Stim1 causes impairments in cerebellar motor behavior. Together, our results demonstrate that in the mammalian nervous system STIM1 is a key regulator of intracellular Ca(2+) signaling, metabotropic glutamate receptor-dependent synaptic transmission, and motor coordination.

Amelioration of Muc5b mucin hypersecretion is enhanced by IL-33 after 2-APB administration in a murine model of allergic rhinitis

We attempted to clarify whether hypersecretion of Muc5b mucin from mouse nasal submucosal glands that is enhanced by interleukin (IL)-33 under allergic conditions can be ameliorated by administration of 2-APB. Immunohistochemistry was used to examine both the distribution of T cells in the nasal mucosa of an allergic rhinitis mouse model and expressions of IL-33 receptor ST2 and Muc5b protein in mouse submucosal gland cells. The amounts of protein and mRNA of Orai1, Muc5b, IL-4, IL-5, IL-13 and IL-33 in mouse nasal lavage fluid (NLF) and nasal mucosa were determined using enzyme-linked immunosorbent assay and real-time reverse transcription-polymerase chain reaction. Expressions of Orai1, Muc5b, IL-4, IL-5, IL-13 and IL-33 were up-regulated in the allergic state and IL-33 increased the levels of Muc5b, IL-4, IL-5 and IL-13, but did not influence proliferation of T cells; however, ST2 was diminished in nasal submucosal gland cells. 2-APB reduced proliferation of T cells and the Orai1 level in the nasal mucosa. It also reduced the concentrations of IL-4, IL-5 and IL-13 in NLF and nasal mucosa, and hypersecretion of Muc5b from glandular cells that was enhanced by IL-33, but did not affect IL-33 production. 2-APB decreased Muc5b mucin hypersecretion from submucosal gland that was enhanced by IL-33 in allergic mice by limiting Ca(2+) release-activated Ca(2+) channel activity in which Orai1 plays a crucial role in the gland cells and/or by controlling channel activation in T cells and proliferation of these cells.

How to win ATP and influence Ca(2+) signaling

This brief review discusses recent advances in studies of mitochondrial Ca(2+) signaling and considers how the relationships between mitochondria and Ca(2+) responses are shaped in secretory epithelial cells. Perhaps the more precise title of this review could have been "How to win ATP and influence Ca(2+) signaling in secretory epithelium with emphasis on exocrine secretory cells and specific focus on pancreatic acinar cells". But "brevity is a virtue" and the authors hope that many of the mechanisms discussed are general and applicable to other tissues and cell types. Among these mechanisms are mitochondrial regulation of Ca(2+) entry and the role of mitochondria in the formation of localized Ca(2+) responses. The roles of Ca(2+) signaling in the physiological adjustment of bioenergetics and in mitochondrial damage are also briefly discussed.

Molecular regulation of the pore component of CRAC channels, Orai1

Store-operated Ca(2+) entry (SOCE) is a fundamental mechanism ubiquitously employed by cells to elevate intracellular Ca(2+) concentrations ([Ca(2+)]i). Increased intracellular Ca(2+) ions act as a second messenger that can stimulate a variety of downstream signaling pathways affecting proliferation, secretion, differentiation, and death of cells. In immune cells, immune receptor stimulation induces endoplasmic reticulum Ca(2+) store depletion that subsequently activates Ca(2+)-release-activated-Ca(2+) (CRAC) channels, a prototype of store-operated Ca(2+) (SOC) channels. Identification of Orai1 as the pore subunit of CRAC channels has provided the much-needed molecular tool to dissect the mechanism of activation and regulation of these channels. In this review, we discuss the recent advances in understanding the regulatory mechanisms and posttranslational modifications that regulate diverse aspects of CRAC channel function.

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.

Structural and stoichiometric determinants of Ca2+ release-activated Ca2+ (CRAC) channel Ca2+-dependent inactivation

Depletion of intracellular Ca(2+) stores in mammalian cells results in Ca(2+) entry across the plasma membrane mediated primarily by Ca(2+) release-activated Ca(2+) (CRAC) channels. Ca(2+) influx through these channels is required for the maintenance of homeostasis and Ca(2+) signaling in most cell types. One of the main features of native CRAC channels is fast Ca(2+)-dependent inactivation (FCDI), where Ca(2+) entering through the channel binds to a site near its intracellular mouth and causes a conformational change, closing the channel and limiting further Ca(2+) entry. Early studies suggested that FCDI of CRAC channels was mediated by calmodulin. However, since the discovery of STIM1 and Orai1 proteins as the basic molecular components of the CRAC channel, it has become apparent that FCDI is a more complex phenomenon. Data obtained using heterologous overexpression of STIM1 and Orai1 suggest that, in addition to calmodulin, several cytoplasmic domains of STIM1 and Orai1 and the selectivity filter within the channel pore are required for FCDI. The stoichiometry of STIM1 binding to Orai1 also has emerged as an important determinant of FCDI. Consequently, STIM1 protein expression levels have the potential to be an endogenous regulator of CRAC channel Ca(2+) influx. This review discusses the current understanding of the molecular mechanisms governing the FCDI of CRAC channels, including an evaluation of further experiments that may delineate whether STIM1 and/or Orai1 protein expression is endogenously regulated to modulate CRAC channel function, or may be dysregulated in some pathophysiological states.

Astrocyte glycogenolysis is triggered by store-operated calcium entry and provides metabolic energy for cellular calcium homeostasis

Astrocytic glycogen, the only storage form of glucose in the brain, has been shown to play a fundamental role in supporting learning and memory, an effect achieved by providing metabolic support for neurons. We have examined the interplay between glycogenolysis and the bioenergetics of astrocytic Ca(2+) homeostasis, by analyzing interdependency of glycogen and store-operated Ca(2+) entry (SOCE), a mechanism in cellular signaling that maintains high endoplasmatic reticulum (ER) Ca(2+) concentration and thus provides the basis for store-dependent Ca(2+) signaling. We stimulated SOCE in primary cultures of murine cerebellar and cortical astrocytes, and determined glycogen content to investigate the effects of SOCE on glycogen metabolism. By blocking glycogenolysis, we tested energetic dependency of SOCE-related Ca(2+) dynamics on glycogenolytic ATP. Our results show that SOCE triggers astrocytic glycogenolysis. Upon inhibition of adenylate cyclase with 2',5'-dideoxyadenosine, glycogen content was no longer significantly different from that in unstimulated control cells, indicating that SOCE triggers astrocytic glycogenolysis in a cAMP-dependent manner. When glycogenolysis was inhibited in cortical astrocytes by 1,4-dideoxy-1,4-imino-D-arabinitol, the amount of Ca(2+) loaded into ER via sarco/endoplasmic reticulum Ca(2)-ATPase (SERCA) was reduced, which suggests that SERCA pumps preferentially metabolize glycogenolytic ATP. Our study demonstrates SOCE as a novel pathway in stimulating astrocytic glycogenolysis. We also provide first evidence for a new functional role of brain glycogen, in providing local ATP to SERCA, thus establishing the bioenergetic basis for astrocytic Ca(2+) signaling. This mechanism could offer a novel explanation for the impact of glycogen on learning and memory.

Store-operated Ca²⁺ entry mediated regulation of polarization in differentiated human neutrophil-like HL-60 cells under hypoxia

The regulation of neutrophil polarization by calcium entry is critical for maintaining an effective host response. Hypoxia has a major effect on the apoptosis of neutrophils, however the role of store-operated Ca2+ entry (SOCE) in neutrophil polarization under hypoxia remains to be elucidated. In the present study, we examined the polarization of differentiated human neutrophil-like HL-60 (dHL-60) cells exposed to hypoxia (3% O2) and the results demonstrated that the percentage of polarized cells following exposure to an N-formyl-Met-Leu-Phe (fMLP) gradient in the Zigmond chamber was increased. We examined stromal interaction molecule 1 (STIM1) and Orai1 expression in dHL-60 cells during hypoxia, and it was observed that the expression of STIM1 and Orai1 was significantly reduced at day 2. However, no apparent change was observed on the first day, indicating that this effect is dependent on stimulation time. Fluo-4/acetoxymethyl (AM) ester imaging also demonstrated that SOCE was decreased in dHL-60 cells. The plasmid overexpression assay demonstrated that the response of polarization was returned to the control level. We demonstrated the inhibitory role of SOCE on the polarization of dHL-60 cells under hypoxic conditions, which may be the mechanism for the adaptation of neutrophils to hypoxia. SOCE is also suggested to be a key modulator of immune deficiency under hypoxic conditions and is potentially a therapeutic target.

The TRPC family of TRP channels: roles inferred (mostly) from knockout mice and relationship to ORAI proteins

Aside from entering into cells through voltage gated Ca channels and Na/Ca exchangers in those cells that express these proteins, for all cells be they excitable or non-excitable, Ca(2+) enters through channels that are activated downstream of phosphoinositide mobilization (activation of phospholipase C, PLC) and through channels that are activated secondary to depletion of internal stores. Depletion of internal stores activates plasma membrane channels known as ORAIs. Activation of PLCs activates the canonical class of transient receptor potential channels (TRPCs), and, because this activation also causes depletion of Ca(2+) stores, also ORAI based channels. Whereas the activation of ORAI is a well-accepted phenomenon, it appears that TRPC channels also participate in Ca(2+) entry triggered by store depletion with or without participation of ORAI molecules. Regardless of molecular makeup of TRPC containing channels, a plethora of studies have shown TRPCs to be important both in physiologic systems as well as in pathophysiologic phenomena. Particularly important in defining roles of TRPCs, have been studies with mice with targeted disruption of their genes, i.e., with TRPC KO mice. In this chapter we first focus on TRPCs as regulators of body functions in health and disease, and then focus on the possible make-up of the channels of which they participate. A hypothesis is set forth, whereby ORAI dimers are proposed to be regulatory subunits of tetrameric TRPC channels and serve as structural units that form ORAI channels either as dimers of dimers or trimers of dimers.

Antibody-mediated targeting of the Orai1 calcium channel inhibits T cell function

Despite the attractiveness of ion channels as therapeutic targets, there are no examples of monoclonal antibodies directed against ion channels in clinical development. Antibody-mediated inhibition of ion channels could offer a directed, specific therapeutic approach. To investigate the potential of inhibiting ion channel function with an antibody, we focused on Orai1, the pore subunit of the calcium channel responsible for store-operated calcium entry (SOCE) in T cells. Effector T cells are key drivers of autoimmune disease pathogenesis and calcium signaling is essential for T cell activation, proliferation, and cytokine production. We show here the generation of a specific anti-human Orai1 monoclonal antibody (mAb) against an extracellular loop of the plasma membrane-spanning protein. The anti-Orai1 mAb binds native Orai1 on lymphocytes and leads to cellular internalization of the channel. As a result, T cell proliferation, and cytokine production is inhibited in vitro. In vivo, anti-Orai1 mAb is efficacious in a human T cell-mediated graft-versus host disease (GvHD) mouse model. This study demonstrates the feasibility of antibody-mediated inhibition of Orai1 function and, more broadly, reveals the possibility of targeting ion channels with biologics for the treatment of autoimmunity and other diseases.

How many Orai's does it take to make a CRAC channel?

CRAC (Calcium Release-Activated Calcium) channels represent the primary pathway for so-called “store-operated calcium entry” – the cellular entry of calcium induced by depletion of intracellular calcium stores. These channels play a key role in diverse cellular activities, most noticeably in the differentiation and activation of Tcells, and in the response of mast cells to inflammatory signals. CRAC channels are formed by members of the recently discovered Orai protein family, with previous studies indicating that the functional channel is formed by a tetramer of Orai subunits. However, a recent report has shown that crystals obtained from the purified Drosophila Orai protein display a hexameric channel structure. Here, by comparing the biophysical properties of concatenated hexameric and tetrameric human Orai1 channels expressed in HEK293 cells, we show that the tetrameric channel displays the highly calcium-selective conductance properties consistent with endogenous CRAC channels, whilst the hexameric construct forms an essentially non-selective cation channel.

Molecular pharmacology of store-operated CRAC channels

Calcium influx through store-operated Ca(2+) release-activated Ca(2+) channels (CRAC channels) is a well-defined mechanism of generating cellular Ca(2+) elevations that regulates many functions including gene expression, exocytosis and cell proliferation. The identifications of the ER Ca(2+) sensing proteins, STIM1-2 and the CRAC channel proteins, Orai1-3, have led to improved understanding of the physiological roles and the activation mechanism of CRAC channels. Defects in CRAC channel function are associated with serious human diseases such as immunodeficiency and auto-immunity. In this review, we discuss several pharmacological modulators of CRAC channels, focusing specifically on the molecular mechanism of drug action and their utility in illuminating the mechanism of CRAC channel operation and their physiological roles in different cells.

Ca2+ influx through store-operated Ca2+ channels reduces Alzheimer disease β-amyloid peptide secretion

Alzheimer disease (AD), the leading cause of dementia, is characterized by the accumulation of β-amyloid peptides (Aβ) in senile plaques in the brains of affected patients. Many cellular mechanisms are thought to play important roles in the development and progression of AD. Several lines of evidence point to the dysregulation of Ca(2+) homeostasis as underlying aspects of AD pathogenesis. Moreover, direct roles in the regulation of Ca(2+) homeostasis have been demonstrated for proteins encoded by familial AD-linked genes such as PSEN1, PSEN2, and APP, as well as Aβ peptides. Whereas these studies support the hypothesis that disruption of Ca(2+) homeostasis contributes to AD, it is difficult to disentangle the effects of familial AD-linked genes on Aβ production from their effects on Ca(2+) homeostasis. Here, we developed a system in which cellular Ca(2+) homeostasis could be directly manipulated to study the effects on amyloid precursor protein metabolism and Aβ production. We overexpressed stromal interaction molecule 1 (STIM1) and Orai1, the components of the store-operated Ca(2+) entry pathway, to generate cells with constitutive and store depletion-induced Ca(2+) entry. We found striking effects of Ca(2+) entry induced by overexpression of the constitutively active STIM1(D76A) mutant on amyloid precursor protein metabolism. Specifically, constitutive activation of Ca(2+) entry by expression of STIM1(D76A) significantly reduced Aβ secretion. Our results suggest that disruptions in Ca(2+) homeostasis may influence AD pathogenesis directly through the modulation of Aβ production.

Mechanisms of STIM1 activation of store-independent leukotriene C4-regulated Ca2+ channels

We recently showed, in primary vascular smooth muscle cells (VSMCs), that the platelet-derived growth factor activates canonical store-operated Ca(2+) entry and Ca(2+) release-activated Ca(2+) currents encoded by Orai1 and STIM1 genes. However, thrombin activates store-independent Ca(2+) selective channels contributed by both Orai3 and Orai1. These store-independent Orai3/Orai1 channels are gated by cytosolic leukotriene C4 (LTC4) and require STIM1 downstream LTC4 action. However, the source of LTC4 and the signaling mechanisms of STIM1 in the activation of this LTC4-regulated Ca(2+) (LRC) channel are unknown. Here, we show that upon thrombin stimulation, LTC4 is produced through the sequential activities of phospholipase C, diacylglycerol lipase, 5-lipo-oxygenease, and leukotriene C4 synthase. We show that the endoplasmic reticulum-resident STIM1 is necessary and sufficient for LRC channel activation by thrombin. STIM1 does not form sustained puncta and does not colocalize with Orai1 either under basal conditions or in response to thrombin. However, STIM1 is precoupled to Orai3 and Orai3/Orai1 channels under basal conditions as shown using Forster resonance energy transfer (FRET) imaging. The second coiled-coil domain of STIM1 is required for coupling to either Orai3 or Orai3/Orai1 channels and for LRC channel activation. We conclude that STIM1 employs distinct mechanisms in the activation of store-dependent and store-independent Ca(2+) entry pathways.

The calcium feedback loop and T cell activation: how cytoskeleton networks control intracellular calcium flux

During T cell activation, the engagement of a T cell with an antigen-presenting cell (APC) results in rapid cytoskeletal rearrangements and a dramatic increase of intracellular calcium (Ca(2+)) concentration, downstream to T cell antigen receptor (TCR) ligation. These events facilitate the organization of an immunological synapse (IS), which supports the redistribution of receptors, signaling molecules and organelles towards the T cell-APC interface to induce downstream signaling events, ultimately supporting T cell effector functions. Thus, Ca(2+) signaling and cytoskeleton rearrangements are essential for T cell activation and T cell-dependent immune response. Rapid release of Ca(2+) from intracellular stores, e.g. the endoplasmic reticulum (ER), triggers the opening of Ca(2+) release-activated Ca(2+) (CRAC) channels, residing in the plasma membrane. These channels facilitate a sustained influx of extracellular Ca(2+) across the plasma membrane in a process termed store-operated Ca(2+) entry (SOCE). Because CRAC channels are themselves inhibited by Ca(2+) ions, additional factors are suggested to enable the sustained Ca(2+) influx required for T cell function. Among these factors, we focus here on the contribution of the actin and microtubule cytoskeleton. The TCR-mediated increase in intracellular Ca(2+) evokes a rapid cytoskeleton-dependent polarization, which involves actin cytoskeleton rearrangements and microtubule-organizing center (MTOC) reorientation. Here, we review the molecular mechanisms of Ca(2+) flux and cytoskeletal rearrangements, and further describe the way by which the cytoskeletal networks feedback to Ca(2+) signaling by controlling the spatial and temporal distribution of Ca(2+) sources and sinks, modulating TCR-dependent Ca(2+) signals, which are required for an appropriate T cell response. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

A TRPC1 protein-dependent pathway regulates osteoclast formation and function

Ca(2+) signaling is essential for bone homeostasis and skeletal development. Here, we show that the transient receptor potential canonical 1 (TRPC1) channel and the inhibitor of MyoD family, I-mfa, function antagonistically in the regulation of osteoclastogenesis. I-mfa null mice have an osteopenic phenotype characterized by increased osteoclast numbers and surface, which are normalized in mice lacking both Trpc1 and I-mfa. In vitro differentiation of pre-osteoclasts derived from I-mfa-deficient mice leads to an increased number of mature osteoclasts and higher bone resorption per osteoclast. These parameters return to normal levels in osteoclasts derived from double mutant mice. Consistently, whole cell currents activated in response to the depletion of intracellular Ca(2+) stores are larger in pre-osteoclasts derived from I-mfa knock-out mice compared with currents in wild type mice and normalized in cells derived from double mutant mice, suggesting a cell-autonomous effect of I-mfa on TRPC1 in these cells. A new splice variant of TRPC1 (TRPC1ε) was identified in early pre-osteoclasts. Heterologous expression of TRPC1ε in HEK293 cells revealed that it is unique among all known TRPC1 isoforms in its ability to amplify the activity of the Ca(2+) release-activated Ca(2+) (CRAC) channel, mediating store-operated currents. TRPC1ε physically interacts with Orai1, the pore-forming subunit of the CRAC channel, and I-mfa is recruited to the TRPC1ε-Orai1 complex through TRPC1ε suppressing CRAC channel activity. We propose that the positive and negative modulation of the CRAC channel by TRPC1ε and I-mfa, respectively, fine-tunes the dynamic range of the CRAC channel regulating osteoclastogenesis.

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.

Emerging roles of Orai3 in pathophysiology

Calcium (Ca(2+)) is a ubiquitous second messenger that regulates a plethora of physiological functions. Deregulation of calcium homeostasis has been reported in a wide variety of pathological conditions including cardiovascular disorders, cancer and neurodegenerative diseases. One of the most ubiquitous pathways involved in regulated Ca(2+) influx into cells is the store-operated Ca(2+) entry (SOCE) pathway. In 2006, Orai1 was identified as the channel protein that mediates SOCE in immune cells. Orai1 has two mammalian homologs, Orai2 and Orai3. Although Orai1 has been the most widely studied Orai isoform, Orai3 has recently received significant attention. Under native conditions, Orai3 was demonstrated to be an important component of store-independent arachidonate-regulated Ca(2+) (ARC) entry in HEK293 cells, and more recently of a store-independent leukotrieneC4-regulated Ca(2+) (LRC) entry pathway in vascular smooth muscle cells. Recent studies have shown upregulation of Orai3 in estrogen receptor-expressing breast cancers and a critical role for Orai3 in breast cancer development in immune-compromised mice. Orai3 upregulation was also shown to contribute to vascular smooth muscle remodeling and neointimal hyperplasia caused by vascular injury. Furthermore, Orai3 has been shown to contribute to proliferation of effector T-lymphocytes under oxidative stress. In this review, we will discuss the role of Orai3 in reported pathophysiological conditions and will contribute ideas on the potential role of Orai3 in native Ca(2+) signaling pathways and human disease.

Store-operated calcium entry in neuroglia

Neuroglial cells are homeostatic neural cells. Generally, they are electrically non-excitable and their activation is associated with the generation of complex intracellular Ca(2+) signals that define the "Ca(2+) excitability" of glia. In mammalian glial cells the major source of Ca(2+) for this excitability is the lumen of the endoplasmic reticulum (ER), which is ultimately (re)filled from the extracellular space. This occurs via store-operated Ca(2+) entry (SOCE) which is supported by a specific signaling system connecting the ER with plasmalemmal Ca(2+) entry. Here, emptying of the ER Ca(2+) store is necessary and sufficient for the activation of SOCE, and without Ca(2+) influx via SOCE the ER store cannot be refilled. The molecular arrangements underlying SOCE are relatively complex and include plasmalemmal channels, ER Ca(2+) sensors, such as stromal interaction molecule, and possibly ER Ca(2+) pumps (of the SERCA type). There are at least two sets of plasmalemmal channels mediating SOCE, the Ca(2+)-release activated channels, Orai, and transient receptor potential (TRP) channels. The molecular identity of neuroglial SOCE has not been yet identified unequivocally. However, it seems that Orai is predominantly expressed in microglia, whereas astrocytes and oligodendrocytes rely more on TRP channels to produce SOCE. In physiological conditions the SOCE pathway is instrumental for the sustained phase of the Ca(2+) signal observed following stimulation of metabotropic receptors on glial cells.

Staurosporine maintains the activation of store-operated Ca²⁺ entry even after the refilling of Ca²⁺ stores

Store-operated Ca²⁺ entry (SOCE) from the extracellular space plays a critical role in agonist-mediated Ca²⁺ signaling in non-excitable cells. Here we show that SOCE is enhanced in COS-7 cells treated with staurosporine (ST), a protein kinase inhibitor. In COS-7 cells, stimulation with ATP induced Ca²⁺ release from intracellular Ca²⁺ stores and Ca²⁺ entry from the extracellular space. Ca²⁺ release was not affected by treatment with ST, but Ca²⁺ entry continued in the ST-treated cells even after the removal of ATP. ST did not inhibit Ca²⁺ sequestration into Ca²⁺ stores. The Ca²⁺ entry induced by cyclopiazonic acid (CPA), a reversible ER Ca²⁺ pump inhibitor, was maintained in ST-treated cells even after the removal of CPA, but was not maintained in the control cells. The sustained Ca²⁺ entry in ST-treated cells was completely attenuated by the SOCE inhibitors, La³⁺ and 2-APB. The large increase in Ca²⁺ entry produced in the cells co-expressing Venus-Orai1 and STIM1-mKO1 was stabilized with ST treatment, and confocal imaging of these cells suggested that the complex between Orai1 and STIM1 did not completely dissociate following the refilling of Ca²⁺ stores. These results show that SOCE remains activated even after the refilling of Ca²⁺ stores in ST-treated cells and that the effect of ST on SOCE may result from a stabilization of the Orai1-STIM1 interaction.

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.

CRAC inhibitors: identification and potential

BACKGROUND

Ca(2+) release-activated Ca(2+) (CRAC) channels, a subfamily of store-operated channels, play an essential role in various diseases such as immune disorders and allergic responses.

OBJECTIVE

The successful treatment of these diseases requires the identification of specific inhibitors. So far, a variety of chemical compounds blocking CRAC have been identified; however, they have all turned out to be less specific. Recently two proteins, STIM1 and ORAI1, have been identified as the essential components that fully reconstitute CRAC currents with a similar biophysical fingerprint.

METHOD

These two proteins and their activation process represent direct targets for the application of specific CRAC inhibitors.

RESULTS/CONCLUSION

For drug development, fluorescence microscopy adaptable for high-throughput screening will provide a powerful assay to mechanistically identify potential CRAC inhibitors that act on various stages within the STIM1/ORAI1 activation pathway visualized by fluorescent-tagged proteins.

STIM1 controls endothelial barrier function independently of Orai1 and Ca2+ entry

Endothelial barrier function is critical for tissue fluid homeostasis, and its disruption contributes to various pathologies, including inflammation and sepsis. Thrombin is an endogenous agonist that impairs endothelial barrier function. We showed that the thrombin-induced decrease in transendothelial electric resistance of cultured human endothelial cells required the endoplasmic reticulum-localized, calcium-sensing protein stromal interacting molecule 1 (STIM1), but was independent of Ca2+ entry across the plasma membrane and the Ca2+ release-activated Ca2+ channel protein Orai1, which is the target of STIM1 in the store-operated calcium entry pathway. We found that STIM1 coupled the thrombin receptor to activation of the guanosine triphosphatase RhoA, stimulation of myosin light chain phosphorylation, formation of actin stress fibers, and loss of cell-cell adhesion. Thus, STIM1 functions in pathways that are dependent on and independent of Ca2+ entry.

The polybasic lysine-rich domain of plasma membrane-resident STIM1 is essential for the modulation of store-operated divalent cation entry by extracellular calcium

STIM1 acts as an endoplasmic reticulum Ca(2+) sensor that communicates the filling state of the intracellular stores to the store-operated channels. In addition, STIM1 is expressed in the plasma membrane, with the Ca(2+) binding EF-hand motif facing the extracellular medium; however, its role sensing extracellular Ca(2+) concentrations in store-operated Ca(2+) entry (SOCE), as well as the underlying mechanism remains unclear. Here we report that divalent cation entry stimulated by thapsigargin (TG) is attenuated by extracellular Ca(2+) in a concentration-dependent manner. Expression of the Ca(2+)-binding defective STIM1(D76A) mutant did not alter the surface expression of STIM1 but abolishes the regulation of divalent cation entry by extracellular Ca(2+). Orai1 and TRPC1 have been shown to play a major role in SOCE. Expression of the STIM1(D76A) mutant did not alter Orai1 phosphoserine content. TRPC1 silencing significantly attenuated TG-induced Mn(2+) entry. Expression of the STIM1(K684,685E) mutant impaired the association of plasma membrane STIM1 with TRPC1, as well as the regulation of TG-induced divalent cation entry by extracellular Ca(2+), which suggests that TRPC1 might be involved in the regulation of divalent cation entry by extracellular Ca(2+) mediated by plasma membrane-resident STIM1. Expression of the STIM1(D76A) or STIM1(K684,685E) mutants reduced store-operated divalent cation entry and resulted in loss of dependence on the extracellular Ca(2+) concentration, providing evidence for a functional role of plasma membrane-resident STIM1 in the regulation of store-operated divalent cation entry, which at least involves the EF-hand motif and the C-terminal polybasic lysine-rich domain.

Constitutive activation of the calcium sensor STIM1 causes tubular-aggregate myopathy

Tubular aggregates are regular arrays of membrane tubules accumulating in muscle with age. They are found as secondary features in several muscle disorders, including alcohol- and drug-induced myopathies, exercise-induced cramps, and inherited myasthenia, but also exist as a pure genetic form characterized by slowly progressive muscle weakness. We identified dominant STIM1 mutations as a genetic cause of tubular-aggregate myopathy (TAM). Stromal interaction molecule 1 (STIM1) is the main Ca(2+) sensor in the endoplasmic reticulum, and all mutations were found in the highly conserved intraluminal Ca(2+)-binding EF hands. Ca(2+) stores are refilled through a process called store-operated Ca(2+) entry (SOCE). Upon Ca(2+)-store depletion, wild-type STIM1 oligomerizes and thereby triggers extracellular Ca(2+) entry. In contrast, the missense mutations found in our four TAM-affected families induced constitutive STIM1 clustering, indicating that Ca(2+) sensing was impaired. By monitoring the calcium response of TAM myoblasts to SOCE, we found a significantly higher basal Ca(2+) level in TAM cells and a dysregulation of intracellular Ca(2+) homeostasis. Because recessive STIM1 loss-of-function mutations were associated with immunodeficiency, we conclude that the tissue-specific impact of STIM1 loss or constitutive activation is different and that a tight regulation of STIM1-dependent SOCE is fundamental for normal skeletal-muscle structure and function.

Organization and function of membrane contact sites

Membrane-bound organelles are a wonderful evolutionary acquisition of the eukaryotic cell, allowing the segregation of sometimes incompatible biochemical reactions into specific compartments with tailored microenvironments. On the flip side, these isolating membranes that crowd the interior of the cell, constitute a hindrance to the diffusion of metabolites and information to all corners of the cell. To ensure coordination of cellular activities, cells use a network of contact sites between the membranes of different organelles. These membrane contact sites (MCSs) are domains where two membranes come to close proximity, typically less than 30nm. Such contacts create microdomains that favor exchange between two organelles. MCSs are established and maintained in durable or transient states by tethering structures, which keep the two membranes in proximity, but fusion between the membranes does not take place. Since the endoplasmic reticulum (ER) is the most extensive cellular membrane network, it is thus not surprising to find the ER involved in most MCSs within the cell. The ER contacts diverse compartments such as mitochondria, lysosomes, lipid droplets, the Golgi apparatus, endosomes and the plasma membrane. In this review, we will focus on the common organizing principles underlying the many MCSs found between the ER and virtually all compartments of the cell, and on how the ER establishes a network of MCSs for the trafficking of vital metabolites and information. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

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.

Structure and function of endoplasmic reticulum STIM calcium sensors

Store-operated calcium (Ca(2+)) entry (SOCE) is a vital Ca(2+) signaling pathway in nonexcitable as well as electrically excitable cells, regulating countless physiological and pathophysiological pathways. Stromal interaction molecules (STIMs) are the principal regulating molecules of SOCE, sensing changes in sarco-/endoplasmic reticulum (S/ER) luminal Ca(2+) levels and directly interacting with the Orai channel subunits to orchestrate the opening of Ca(2+) release-activated Ca(2+) (CRAC) channels. Recent atomic resolution structures on human STIM1 and STIM2 have illuminated critical mechanisms of STIM function in SOCE; further, the first high-resolution structure of the Drosophila melanogaster Orai channel has revealed vital data on the atomic composition of the CRAC channel pore and the assembly of individual Orai subunits. This chapter focuses on the mechanistic information garnered from these high-resolution structures and the supporting biophysical, biochemical, and live cell work that has enhanced our understanding of the relationship between STIM and Orai structural features and CRAC channel function.

Emerging roles for native Orai Ca2+ channels in cardiovascular disease

Orai proteins form highly calcium (Ca(2+))-selective channels located in the plasma membrane of both nonexcitable and excitable cells, where they make important contributions to many cellular processes. The well-characterized Ca(2+) release-activated Ca(2+) current is mediated by Orai1 multimers and is activated, upon depletion of inositol 1,4,5-trisphosphate-sensitive stores, by direct interaction of Orai1 with the endoplasmic reticulum Ca(2+) sensor, stromal interaction molecule 1 (STIM1). This pathway is known as capacitative Ca(2+) entry or store-operated Ca(2+) entry. While most investigations have focused on STIM1 and Orai1 in their store-dependent mode, emerging evidence suggests that Orai1 and Orai3 heteromultimeric channels can form store-independent Ca(2+)-selective channels. The role of store-dependent and store-independent channels in excitation-transcription coupling and the pathological remodeling of the cardiovascular system are beginning to come forth. Recent evidence suggests that STIM/Orai-generated Ca(2+) signaling couples to gene transcription and subsequent phenotypic changes associated with the processes of cardiac and vascular remodeling. This short review will explore the contributions of native Orai channels to heart and vessel physiology and their role in cardiovascular diseases.

STIM-ORAI interactions that control the CRAC channel

The Ca(2+) sensor STIM1 and the Ca(2+) channel ORAI1 are the fundamental working machinery of the CRAC channel, a classical pathway for store-operated Ca(2+) entry. This chapter focuses on the protein-protein interactions of STIM and ORAI proteins that control the channel.

Store-operated Orai channels: structure and function

In many animal cells, store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels function as an essential route for Ca(2+) entry. CRAC channels control many fundamental cellular functions including gene expression, motility, and cell proliferation, are involved in the etiology of several disease processes including a severe combined immunodeficiency syndrome, and have emerged as major targets for drug development. Although little was known of the molecular mechanisms of CRAC channel operation for several decades, the discovery of Orai1 as a prototypic CRAC channel protein and STIM1 as the endoplasmic reticulum (ER) Ca(2+) sensor has led to rapid progress in our understanding of the mechanisms and functions of CRAC channels. It is now known that activation of CRAC channels following ER Ca(2+) store depletion is governed by several events, which include the redistributions and accumulations of STIM1 and Orai1 into overlapping puncta at peripheral cellular sites, resulting in direct protein-protein interactions between the two proteins. In this chapter, I review the molecular features of the STIM and Orai proteins that regulate the gating and ion conduction mechanisms of CRAC channels.

Measurement of intracellular Ca2+ concentration in single cells using ratiometric calcium dyes

Measurement of intracellular Ca(2+) concentration ([Ca(2+)](i)) is useful to study the upstream and downstream events of Ca(2+) signaling. Ca(2+)-binding proteins including EF-hand-containing proteins are important downstream effector molecules after an increase of [Ca(2+)](i). Conversely, these proteins can also act as key modulators for regulation of [Ca(2+)](i) by sensing the Ca(2+) levels in the intracellular organelles and cytoplasm. Here we describe a single-cell Ca(2+) imaging technique that was used to measure the intracellular Ca(2+) levels to examine the function of Ca(2+)-binding proteins, STIM1 and Calcium release-activated Calcium channel regulator 2A (CRACR2A), using ratiometric Ca(2+) dye Fura-2 in adherent and non-adherent cells.

How to make a good egg!: The need for remodeling of oocyte Ca(2+) signaling to mediate the egg-to-embryo transition

The egg-to-embryo transition marks the initiation of multicellular organismal development and is mediated by a specialized Ca(2+) transient at fertilization. This explosive Ca(2+) signal has captured the interest and imagination of scientists for many decades, given its cataclysmic nature and necessity for the egg-to-embryo transition. Learning how the egg acquires the competency to generate this Ca(2+) transient at fertilization is essential to our understanding of the mechanisms controlling egg and the transition to embryogenesis. In this review we discuss our current knowledge of how Ca(2+) signaling pathways remodel during oocyte maturation in preparation for fertilization with a special emphasis on the frog oocyte as additional reviews in this issue will touch on this in other species.