Initial activation of STIM1, the regulator of store-operated calcium entry

Ca2+ and lipid signals hold hands at endoplasmic reticulum-plasma membrane contact sites

Discovery of the STIM1 and Orai proteins as the principal components of store-operated Ca²⁺ entry has drawn attention to contact sites between the endoplasmic reticulum (ER) and the plasma membrane (PM). Such contacts between adjacent membranes of different cellular organelles, primarily between the mitochondria and the ER, had already been known as the sites where Ca²⁺ released from the ER can be efficiently channelled to the mitochondria and also where phosphatidylserine synthesis and transfer takes place. Recent studies have identified contact sites between virtually every organelle and the ER and the functional importance of these small specialized membrane domains is increasingly recognized. Most recent developments have highlighted the role of phosphatidylinositol 4-phosphate gradients as critical determinants of the non-vesicular transport of various lipids from the ER to other organelles such as the Golgi or PM. As we learn more about membrane contact sites it becomes apparent that Ca²⁺ is not only transported at these sites but also controls both the dynamics and the lipid transfer efficiency of these processes. Conversely, lipids are critical for regulating the Ca²⁺ entry process. This review will summarize some of the most exciting recent developments in this rapidly expanding research field.

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.

The STIM-Orai Pathway: STIM-Orai Structures: Isolated and in Complex

Considerable progress has been made elucidating the molecular mechanisms of calcium (Ca²⁺) sensing by stromal interaction molecules (STIMs) and the basis for Orai channel activity. This chapter focuses on the available high-resolution structural details of STIM and Orai proteins with respect to the regulation of store-operated Ca²⁺ entry (SOCE). Solution structures of the Ca²⁺-sensing domains of STIM1 and STIM2 are reviewed in detail, crystal structures of cytosolic coiled-coil STIM fragments are discussed, and an overview of the closed Drosophila melanogaster Orai hexameric structure is provided. Additionally, we highlight structures of human Orai1 N-terminal and C-terminal domains in complex with calmodulin and human STIM1, respectively. Ultimately, the accessible structural data are discussed in terms of potential mechanisms of action and cohesiveness with functional observations.

Long-term administration of pyridostigmine attenuates pressure overload-induced cardiac hypertrophy by inhibiting calcineurin signalling

Cardiac hypertrophy is associated with autonomic imbalance, characterized by enhanced sympathetic activity and withdrawal of parasympathetic control. Increased parasympathetic function improves ventricular performance. However, whether pyridostigmine, a reversible acetylcholinesterase inhibitor, can offset cardiac hypertrophy induced by pressure overload remains unclear. Hence, this study aimed to determine whether pyridostigmine can ameliorate pressure overload-induced cardiac hypertrophy and identify the underlying mechanisms. Rats were subjected to either sham or constriction of abdominal aorta surgery and treated with or without pyridostigmine for 8 weeks. Vagal activity and cardiac function were determined using PowerLab. Cardiac hypertrophy was evaluated using various histological stains. Protein markers for cardiac hypertrophy were quantitated by Western blot and immunoprecipitation. Pressure overload resulted in a marked reduction in vagal discharge and a profound increase in cardiac hypertrophy index and cardiac dysfunction. Pyridostigmine increased the acetylcholine levels by inhibiting acetylcholinesterase in rats with pressure overload. Pyridostigmine significantly attenuated cardiac hypertrophy based on reduction in left ventricular weight/body weight, suppression of the levels of atrial natriuretic peptide, brain natriuretic peptide and β-myosin heavy chain, and a reduction in cardiac fibrosis. These effects were accompanied by marked improvement of cardiac function. Additionally, pyridostigmine inhibited the CaN/NFAT3/GATA4 pathway and suppressed Orai1/STIM1 complex formation. In conclusion, pressure overload resulted in cardiac hypertrophy, cardiac dysfunction and a significant reduction in vagal discharge. Pyridostigmine attenuated cardiac hypertrophy and improved cardiac function, which was related to improved cholinergic transmission efficiency (decreased acetylcholinesterase and increased acetylcholine), inhibition of the CaN/NFAT3/GATA4 pathway and suppression of the interaction of Orai1/STIM1.

Optical control of membrane tethering and interorganellar communication at nanoscales

Endoplasmic reticulum (ER) forms an extensive intracellular membranous network in eukaryotes that dynamically connects and communicates with diverse subcellular compartments such as plasma membrane (PM) through membrane contact sites (MCSs), with the inter-membrane gaps separated by a distance of 10-40 nm. Phosphoinositides (PI) constitute an important class of cell membrane phospholipids shared by many MCSs to regulate a myriad of cellular events, including membrane trafficking, calcium homeostasis and lipid metabolism. By installing photosensitivity into a series of engineered PI-binding domains with minimal sizes, we have created an optogenetic toolkit (designated as 'OptoPB') to enable rapid and reversible control of protein translocation and inter-membrane tethering at MCSs. These genetically-encoded, single-component tools can be used as scaffolds for grafting lipid-binding domains to dissect molecular determinants that govern protein-lipid interactions in living cells. Furthermore, we have demonstrated the use of OptoPB as a versatile fusion tag to photomanipulate protein translocation toward PM for reprogramming of PI metabolism. When tethered to the ER membrane with the insertion of flexible spacers, OptoPB can be applied to reversibly photo-tune the gap distances at nanometer scales between the two organellar membranes at MCSs, and to gauge the distance requirement for the free diffusion of protein complexes into MCSs. Our modular optical tools will find broad applications in non-invasive and remote control of protein subcellular localization and interorganellar contact sites that are critical for cell signaling.

Molecular anatomy of the early events in STIM1 activation - oligomerization or conformational change?

Decreased luminal endoplasmic reticulum (ER) Ca²⁺ concentration triggers oligomerization and clustering of the ER Ca²⁺ sensor STIM1 to promote its association with plasma membrane Orai1 Ca²⁺ channels leading to increased Ca²⁺ influx. A key step in STIM1 activation is the release of its SOAR domain from an intramolecular clamp formed with the STIM1 first coiled-coil (CC1) region. Using a truncated STIM1(1-343) molecule that captures or releases the isolated SOAR domain depending on luminal ER Ca²⁺ concentrations, we analyzed the early molecular events that control the intramolecular clamp formed between the CC1 and SOAR domains. We found that STIM1 forms constitutive dimers, and its CC1 domain can bind the SOAR domain of another STIM1 molecule in trans. Artificial oligomerization failed to liberate the SOAR domain or activate STIM1 unless the luminal Ca²⁺-sensing domains were removed. We propose that the release of SOAR from its CC1 interaction is controlled by changes in the orientation of the two CC1 domains in STIM1 dimers. Ca²⁺ unbinding in the STIM1 luminal domains initiates the conformational change allowing SOAR domain liberation and clustering, leading to Orai1 channel activation.

RASSF4 controls SOCE and ER-PM junctions through regulation of PI(4,5)P2

RAS association domain family 4 (RASSF4) is involved in tumorigenesis. Chen et al. show that RASSF4 regulates store-operated Ca²⁺ entry and ER–PM junctions by affecting PI(4,5)P₂ levels. RASSF4 interacts with and regulates the activity of ARF6, an upstream regulator of PIP5K and PI(4,5)P₂.

RAS association domain family 4 (RASSF4) is involved in tumorigenesis and regulation of the Hippo pathway. In this study, we identify new functional roles of RASSF4. First, we discovered that RASSF4 regulates store-operated Ca²⁺ entry (SOCE), a fundamental Ca²⁺ signaling mechanism, by affecting the translocation of the endoplasmic reticulum (ER) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) to ER–plasma membrane (PM) junctions. It was further revealed that RASSF4 regulates the formation of ER–PM junctions and the ER–PM tethering function of extended synaptotagmins E-Syt2 and E-Syt3. Moreover, steady-state PM phosphatidylinositol 4,5-bisphosphate (PI[4,5]P₂) levels, important for localization of STIM1 and E-Syts at ER–PM junctions, were reduced in RASSF4-knockdown cells. Furthermore, we demonstrated that RASSF4 interacts with and regulates the activity of adenosine diphosphate ribosylation factor 6 (ARF6), a small G protein and upstream regulator of type I phosphatidylinositol phosphate kinases (PIP5Ks) and PM PI(4,5)P₂ levels. Overall, our study suggests that RASSF4 controls SOCE and ER–PM junctions through ARF6-dependent regulation of PM PI(4,5)P₂ levels, pivotal for a variety of physiological processes.

Optogenetic toolkit for precise control of calcium signaling

Calcium acts as a second messenger to regulate a myriad of cell functions, ranging from short-term muscle contraction and cell motility to long-term changes in gene expression and metabolism. To study the impact of Ca2+-modulated 'ON' and 'OFF' reactions in mammalian cells, pharmacological tools and 'caged' compounds are commonly used under various experimental conditions. The use of these reagents for precise control of Ca2+ signals, nonetheless, is impeded by lack of reversibility and specificity. The recently developed optogenetic tools, particularly those built upon engineered Ca2+ release-activated Ca2+ (CRAC) channels, provide exciting opportunities to remotely and non-invasively modulate Ca2+ signaling due to their superior spatiotemporal resolution and rapid reversibility. In this review, we briefly summarize the latest advances in the development of optogenetic tools (collectively termed as 'genetically encoded Ca2+ actuators', or GECAs) that are tailored for the interrogation of Ca2+ signaling, as well as their applications in remote neuromodulation and optogenetic immunomodulation. Our goal is to provide a general guide to choosing appropriate GECAs for optical control of Ca2+ signaling in cellulo, and in parallel, to stimulate further thoughts on evolving non-opsin-based optogenetics into a fully fledged technology for the study of Ca2+-dependent activities in vivo.

ER-plasma membrane junctions: Why and how do we study them?

Endoplasmic reticulum (ER)-plasma membrane (PM) junctions are membrane microdomains important for communication between the ER and the PM. ER-PM junctions were first reported in muscle cells in 1957, but mostly ignored in non-excitable cells due to their scarcity and lack of functional significance. In 2005, the discovery of stromal interaction molecule 1 (STIM1) mediating a universal Ca²⁺ feedback mechanism at ER-PM junctions in mammalian cells led to a resurgence of research interests toward ER-PM junctions. In the past decade, several major advancements have been made in this emerging topic in cell biology, including the generation of tools for labeling ER-PM junctions and the unraveling of mechanisms underlying regulation and functions of ER-PM junctions. This review summarizes early studies, recently developed tools, and current advances in the characterization and understanding of ER-PM junctions. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

The STIM-Orai coupling interface and gating of the Orai1 channel

In virtually all cells, store-operated Ca2+ entry signals are vital in controlling a spectrum of functions. The signals are mediated by STIM proteins in the ER and Orai channels in the PM which undergo a dynamic coupling process within discrete ER-PM junctional regions. This coupling is initiated by depletion of ER stored Ca2+ triggering STIM proteins to undergo an intricate activation process. Thereafter, STIM proteins become trapped in the ER-PM junctions where they tether and gate PM Orai Ca2+ channels. STIM1 exists as a dimer, with a single STIM-Orai activating region (SOAR) buried in the resting protein that becomes exposed upon activation. An exposed region on SOAR including the Phe-394 residue forms a critical Orai1 interacting site. Using dimeric SOAR concatemers, we reveal only one of the two sites in the SOAR dimer is needed for Orai1 activation. This unimolecular interaction of SOAR with Orai1 suggests STIM1 can cross-link Orai channels with important significance for Ca2+ signaling. A critical "nexus" region in Orai1 close to the STIM1-binding site can be mutated to constitutively activate the channel mimicking the gating action of STIM1. This indicates STIM1 remotely controls Orai1 channel gating through an allosteric switch triggered by STIM1 binding only to the exposed C-terminal tail of the Orai1 channel.

From Stores to Sinks: Structural Mechanisms of Cytosolic Calcium Regulation

All eukaryotic cells have adapted the use of the calcium ion (Ca²⁺) as a universal signaling element through the evolution of a toolkit of Ca²⁺ sensor, buffer and effector proteins. Among these toolkit components, integral and peripheral proteins decorate biomembranes and coordinate the movement of Ca²⁺ between compartments, sense these concentration changes and elicit physiological signals. These changes in compartmentalized Ca²⁺ levels are not mutually exclusive as signals propagate between compartments. For example, agonist induced surface receptor stimulation can lead to transient increases in cytosolic Ca²⁺ sourced from endoplasmic reticulum (ER) stores; the decrease in ER luminal Ca²⁺ can subsequently signal the opening surface channels which permit the movement of Ca²⁺ from the extracellular space to the cytosol. Remarkably, the minuscule compartments of mitochondria can function as significant cytosolic Ca²⁺ sinks by taking up Ca²⁺ in a coordinated manner. In non-excitable cells, inositol 1,4,5 trisphosphate receptors (IP₃Rs) on the ER respond to surface receptor stimulation; stromal interaction molecules (STIMs) sense the ER luminal Ca²⁺ depletion and activate surface Orai1 channels; surface Orai1 channels selectively permit the movement of Ca²⁺ from the extracellular space to the cytosol; uptake of Ca²⁺ into the matrix through the mitochondrial Ca²⁺ uniporter (MCU) further shapes the cytosolic Ca²⁺ levels. Recent structural elucidations of these key Ca²⁺ toolkit components have improved our understanding of how they function to orchestrate precise cytosolic Ca²⁺ levels for specific physiological responses. This chapter reviews the atomic-resolution structures of IP₃R, STIM1, Orai1 and MCU elucidated by X-ray crystallography, electron microscopy and NMR and discusses the mechanisms underlying their biological functions in their respective compartments within the cell.

The STIM-Orai Pathway: The Interactions Between STIM and Orai

A primary Ca²⁺ entry pathway in non-excitable cells is established by the Ca²⁺ release-activated Ca²⁺ channels. Their two limiting molecular components include the Ca²⁺-sensor protein STIM1 located in the endoplasmic reticulum and the Orai channel in the plasma membrane. STIM1 senses the luminal Ca²⁺ content, and store depletion induces its oligomerization into puncta-like structures, thereby triggering coupling to as well as activation of Orai channels. A C-terminal STIM1 domain is assumed to couple to both C- and N-terminal, cytosolic strands of Orai, accomplishing gating of the channel. Here we highlight the inter- and intramolecular steps of the STIM1-Orai signaling cascade together with critical sites of the pore structure that accomplishes Ca²⁺ permeation.

The STIM-Orai Pathway: Light-Operated Ca2+ Entry Through Engineered CRAC Channels

Ca²⁺ signals regulate a plethora of cellular functions that include muscle contraction, heart beating, hormone secretion, lymphocyte activation, gene expression, and metabolism. To study the impact of Ca²⁺ signals on biological processes, pharmacological tools and caged compounds have been commonly applied to induce fluctuations of intracellular Ca²⁺ concentrations. These conventional approaches, nonetheless, lack rapid reversibility and high spatiotemporal resolution. To overcome these disadvantages, we and others have devised a series of photoactivatable genetically encoded Ca²⁺ actuators (GECAs) by installing light sensitivities into a bona fide highly selective Ca²⁺ channel, the Ca²⁺ release-activated Ca²⁺ (CRAC) channel. Store-operated CRAC channel serves as a major route for Ca²⁺ entry in many cell types. These GECAs enable remote and precise manipulation of Ca²⁺ signaling in both excitable and non-excitable cells. When combined with nanotechnology, it becomes feasible to wirelessly photo-modulate Ca²⁺-dependent activities in vivo. In this chapter, we briefly review most recent advances in engineering CRAC channels to achieve optical control over Ca²⁺ signaling, outline their design principles and kinetic features, and present exemplary applications of GECAs engineered from CRAC channels.

The STIM-Orai Pathway: Conformational Coupling Between STIM and Orai in the Activation of Store-Operated Ca2+ Entry

Store-operated Ca²⁺ entry fulfills a crucial role in controlling Ca²⁺ signals in almost all cells. The Ca²⁺-sensing stromal interaction molecule (STIM) proteins in the endoplasmic reticulum (ER) undergo complex conformational changes in response to depleted ER luminal Ca²⁺, allowing them to unfold and become trapped in ER-plasma membrane (PM) junctions. Dimers of STIM proteins trap and gate the plasma membrane Orai Ca²⁺ channels within these junctions to generate discrete zones of high Ca²⁺ and regulate sensitive Ca²⁺-dependent intracellular signaling pathways. The STIM-Orai activating region (SOAR) of STIM1 becomes exposed upon store depletion and promotes trapping of Orai1 at the PM. Residue Phe-394 within SOAR forms an integral part of the high-affinity Orai1-interacting site. Our results demonstrate that only a single active site within the dimeric SOAR domain of STIM1 is required for the activation of Orai1 channel activity. This unimolecular model is strongly supported by evidence of variable STIM1:Orai1 stoichiometry reported in many studies. We hypothesize that unimolecular coupling promotes cross-linking of channels, localizing Ca²⁺ signals, and regulating channel activity. We have also identified a key "nexus" region in Orai1 near the C-terminal STIM1-binding site that can be mutated to constitutively activate Ca²⁺ entry, mimicking STIM1 activated channels. This suggests that STIM1 mediates gating of Orai1 in an allosteric manner via interaction with the Orai1 C-terminus alone. This model suggests the dual role of STIM1 in regulating both localization and gating of Orai1 channels and has important implications for the regulation of SOCE-mediated downstream signaling and the kinetics of channel activation.

Calcium Dyshomeostasis in Tubular Aggregate Myopathy

Calcium is a crucial mediator of cell signaling in skeletal muscles for basic cellular functions and specific functions, including contraction, fiber-type differentiation and energy production. The sarcoplasmic reticulum (SR) is an organelle that provides a large supply of intracellular Ca²⁺ in myofibers. Upon excitation, it releases Ca²⁺ into the cytosol, inducing contraction of myofibrils. During relaxation, it takes up cytosolic Ca²⁺ to terminate the contraction. During exercise, Ca²⁺ is cycled between the cytosol and the SR through a system by which the Ca²⁺ pool in the SR is restored by uptake of extracellular Ca²⁺ via a specific channel on the plasma membrane. This channel is called the store-operated Ca²⁺ channel or the Ca²⁺ release-activated Ca²⁺ channel. It is activated by depletion of the Ca²⁺ store in the SR by coordination of two main molecules: stromal interaction molecule 1 (STIM1) and calcium release-activated calcium channel protein 1 (ORAI1). Recently, myopathies with a dominant mutation in these genes have been reported and the pathogenic mechanism of such diseases have been proposed. This review overviews the calcium signaling in skeletal muscles and role of store-operated Ca²⁺ entry in calcium homeostasis. Finally, we discuss the phenotypes and the pathomechanism of myopathies caused by mutations in the STIM1 and ORAI1 genes.

Structural perturbations induced by Asn131 and Asn171 glycosylation converge within the EFSAM core and enhance stromal interaction molecule-1 mediated store operated calcium entry

A major intracellular calcium (Ca²⁺) uptake pathway in both excitable and non-excitable eukaryotic cells is store-operated Ca²⁺ entry (SOCE). SOCE is the process by which endoplasmic reticulum (ER)-stored Ca²⁺ depletion leads to activation of plasma membrane Ca²⁺ channels to provide a sustained increase in cytosolic Ca²⁺ levels that mediate a plethora of physiological processes ranging from the immune response to platelet aggregation. Stromal interaction molecule-1 (STIM1) is the principal regulator of SOCE and responds to changes in ER stored Ca²⁺ through luminal sensing machinery composed of EF-hand and SAM domains (EFSAM). The EFSAM domain can undergo N-glycosylation at Asn131 and Asn171 sites; however, the precise role of EFSAM N-glycosylation in the Ca²⁺ sensing mechanism of STIM1 is unclear. By establishing a site-specific chemical approach to covalently linking glucose to EFSAM and examining α-helicity, thermal stability, three dimensional atomic-resolution structure, Ca²⁺ binding affinity and oligomerization, we show that N-glycosylation of the EFSAM domain enhances the properties that promote STIM1 activation. This augmentation occurs through changes in structure localized near the Asn131 and Asn171 sites that together permeate through the protein core and lead to decreased Ca²⁺ binding affinity, reduced stability and enhanced oligomerization. Congruently, Ca²⁺ influx via SOCE in HEK293 cells co-expressing Orai1 and STIM1 was diminished when N-glycosylation was blocked by introducing Asn131Gln and Asn171Gln mutations. Collectively, our data suggests that N-glycosylation enhances the EFSAM destabilization-coupled oligomerization in response to ER Ca²⁺ depletion thereby augmenting the role of STIM1 as a robust ON/OFF regulator of SOCE. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Calcium signalling in salivary gland physiology and dysfunction

Studies over the past four decades have established that Ca(2+) is a critical factor in control of salivary gland function and have led to identification of the critical components of this process. The major ion transport mechanisms and ion channels that are involved in fluid secretion have also been established. The key event in activation of fluid secretion is an increase in [Ca(2+) ]i triggered by inositol 1,4,5-trisphosphate (IP3 )-induced release of Ca(2+) from ER via the IP3 receptor (IP3 R). IP3 Rs determine the site of initiation and the pattern of the [Ca(2+) ]i signal in the cell. However, Ca(2+) entry into the cell is required to sustain the elevation of [Ca(2+) ]i and fluid secretion and is mediated by the store-operated Ca(2+) entry (SOCE) mechanism. Orai1, TRPC1, TRPC3 and STIM1 have been identified as critical components of SOCE in these cells. Cells finely tune the generation and amplification of [Ca(2+) ]i signals for regulation of cell function. An important emerging area is the concept that unregulated [Ca(2+) ]i signals in cells can directly cause cell damage, dysfunction and disease. Alternatively, aberrant [Ca(2+) ]i signals can also amplify and increase the rates of cell damage. Such defects in Ca(2+) signalling have been described in salivary glands in conjunction with radiation-induced loss of salivary gland function as well as in the salivary defects associated with the autoimmune exocrinopathy Sjögren's syndrome. Such defects have been associated with altered function or expression of key Ca(2+) signalling components, such as STIM proteins and TRP channels. These studies offer new avenues for examining the mechanisms underlying the disease and development of novel clinical targets and therapeutic strategies.

Store-operated CRAC channel inhibitors: opportunities and challenges

Aberrant Ca(2+) release-activated Ca(2+) (CRAC) channel activity has been implicated in a number of human disorders, including immunodeficiency, autoimmunity, occlusive vascular diseases and cancer, thus placing CRAC channels among the important targets for the treatment of these disorders. We briefly summarize herein the molecular basis and activation mechanism of CRAC channel and focus on discussing several pharmacological inhibitors of CRAC channels with respect to their biological activity, mechanisms of action and selectivity over other types of Ca(2+) channel in different types of cells.

Roles for Ca2+ mobilization and its regulation in mast cell functions: recent progress

Ca(2+)mobilization in response to cross-linking of IgE bound to its high affinity receptor, FcεRI, on mast cells is central to immune allergic responses. Stimulated tyrosine phosphorylation caused by this cross-linking activates store-operated Ca(2+)entry that results in sustained Ca(2+)oscillations dependent on Rho family GTPases and phosphoinositide synthesis. Coupling of the endoplasmic reticulum (ER) Ca(2+)sensor, stromal interaction molecule 1 (STIM1), to the Ca(2+)-selective channel, Orai1, is regulated by these elements and depends on membrane organization, both at the plasma membrane and at the ER. Mitochondria also contribute to the regulation of Ca(2+)mobilization, and we describe recent evidence that the ER membrane protein vesicle-associated membrane protein-associated protein (VAP) plays a significant role in the coupling between ER and mitochondria in this process. In addition to granule exocytosis, Ca(2+)mobilization in these cells also contributes to stimulated outward trafficking of recycling endosomes and to antigen-stimulated chemotaxis, and it is pathologically regulated by protozoan parasitic invasion.

STIM1 activation is regulated by a 14 amino acid sequence adjacent to the CRAC activation domain

Oligomerization of the Ca²⁺ sensor, STIM1, in the endoplasmic reticulum (ER) membrane, caused by depletion of ER Ca²⁺ stores, results in STIM1 coupling to the plasma membrane Ca²⁺ channel protein, Orai1, to activate Ca²⁺ influx in a process known as store-operated Ca²⁺ entry. We use fluorimetry-based fluorescence resonance energy transfer (FRET) to monitor changes in STIM1 oligomerization in COS7 cells transfected with STIM1 constructs containing selected truncations, deletions, and point mutations, and labeled with donor and acceptor fluorescent proteins at either the luminal (N-terminal) or the cytoplasmic (C-terminal) ends. Our results with sequential truncations of STIM1 from the C-terminus support previous evidence that the CRAC activation domain (CAD/SOAR, human sequence 342–448) is an oligomer-promoting segment of STIM1, and they show that truncation just after CAD/SOAR (1–448) causes significantly elevated basal cytoplasmic Ca²⁺ and spontaneous STIM1 clustering. We find that a 14 amino acid sequence just C-terminal of CAD/SOAR (449–462) prevents spontaneous clustering and activation of STIM1 in COS7 cells. In response to store depletion, C-terminally labeled STIM1 without CAD/SOAR clusters together with CAD/SOAR-containing STIM1 constructs. However, these donor-acceptor pairs do not undergo a stimulated increase in FRET, exhibiting instead a decrease in FRET consistent with a stimulated conformational extension in full length STIM1. We find that the 14 amino acid sequence plays a regulatory role in this process. Overall, our FRET results provide evidence in live cells that Ca²⁺ store depletion stimulates a conformational extension in the cytoplasmic segment of STIM1 that accompanies its oligomerization.

Homeostatic regulation of the PI(4,5)P2-Ca(2+) signaling system at ER-PM junctions

The phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2)-Ca(2+) signaling system is important for cell activation in response to various extracellular stimuli. This signaling system is initiated by receptor-induced hydrolysis of PI(4,5)P2 in the plasma membrane (PM) to generate the soluble second messenger inositol 1,4,5-trisphosphate (IP3). IP3 subsequently triggers the release of Ca(2+) from the endoplasmic reticulum (ER) store to the cytosol to activate Ca(2+)-mediated responses, such as secretion and proliferation. The consumed PM PI(4,5)P2 and ER Ca(2+) must be quickly restored to sustain signaling responses, and to maintain the homeostasis of PI(4,5)P2 and Ca(2+). Since phosphatidylinositol (PI), the precursor lipid for PM PI(4,5)P2, is synthesized in the ER membrane, and a Ca(2+) influx across the PM is required to refill the ER Ca(2+) store, efficient communications between the ER and the PM are critical for the homeostatic regulation of the PI(4,5)P2-Ca(2+) signaling system. This review describes the major findings that established the framework of the PI(4,5)P2-Ca(2+) signaling system, and recent discoveries on feedback control mechanisms at ER-PM junctions that sustain the PI(4,5)P2-Ca(2+) signaling system. Particular emphasis is placed on the characterization of ER-PM junctions where efficient communications between the ER and the PM occur, and the activation mechanisms of proteins that dynamically localize to ER-PM junctions to provide the feedback control during PI(4,5)P2-Ca(2+) signaling, including the ER Ca(2+) sensor STIM1, the extended synaptotagmin E-Syt1, and the PI transfer protein Nir2. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Glu106 targeted inhibitors of ORAI1 as potential Ca2+ release-activated Ca2+ (CRAC) channel blockers - molecular modeling and docking studies

Calcium release-activated calcium modulator 1(ORAI1) is an integral component of the calcium release-activated calcium channel (CRAC) channel complex and plays a central role in regulating Ca^2 +concentrations in T-lymphocytes. It is critical for many physiological processes, including cell-proliferation, cytokine production and activation of the immune system. Loss of ORAI1 function is linked with rheumatoid arthritis (RA) and hence pharmacological blockers of ORAI1 could be potential therapeutic agents for the treatment of RA. In this study, we have used a high-throughput screening approach to inhibit the binding of Ca²⁺toward ORAI1 and the interactions are verified through induced fit docking. The results hint that these compounds act by possibly binding with, and thereby blocking Ca²⁺-binding with ORAI1 (E106). The molecular dynamics (MD) simulations shows strong support toward the hit compounds by showing the ligand potency throughout the simulation timescale of 30 ns. We have thus identified a novel class of highly stable, potential lead compounds that directly bind with the selectivity filter region E106 and block Ca²⁺ binding on ORAI1. This resulting alteration in the pore geometry of ORAI1 due to the strong blocking mechanism of lead compounds will greatly diminish its function and the downstream activities that result from the same including decreased production of cytokines in autoimmune disorders. This study may lay the foundation for finding novel lead compounds for clinical trials that could positively modulate the course of autoimmune disorders with ORAI1 as its specific target.

Molecular mechanisms of STIM/Orai communication

Ca²⁺ entry into the cell via store-operated Ca²⁺ release-activated Ca²⁺ (CRAC) channels triggers diverse signaling cascades that affect cellular processes like cell growth, gene regulation, secretion, and cell death. These store-operated Ca²⁺ channels open after depletion of intracellular Ca²⁺ stores, and their main features are fully reconstituted by the two molecular key players: the stromal interaction molecule (STIM) and Orai. STIM represents an endoplasmic reticulum-located Ca²⁺ sensor, while Orai forms a highly Ca²⁺-selective ion channel in the plasma membrane. Functional as well as mutagenesis studies together with structural insights about STIM and Orai proteins provide a molecular picture of the interplay of these two key players in the CRAC signaling cascade. This review focuses on the main experimental advances in the understanding of the STIM1-Orai choreography, thereby establishing a portrait of key mechanistic steps in the CRAC channel signaling cascade. The focus is on the activation of the STIM proteins, the subsequent coupling of STIM1 to Orai1, and the consequent structural rearrangements that gate the Orai channels into the open state to allow Ca²⁺ permeation into the cell.

The STIM1: Orai Interaction

Ca(2+) influx via store-operated Ca(2+) release activated Ca(2+) (CRAC) channels represents a main signalling pathway for a variety of cell functions, including T-cell activation as well as mast-cell degranulation. Depletion of [Ca(2+)]ER results in activation of Ca(2+) channels within the plasmamembrane that mediate sustained Ca(2+) influx which is required for refilling Ca(2+) stores and down-stream Ca(2+) signalling. The CRAC channel is the best characterized store-operated channel (SOC) with well-defined electrophysiological properties. In recent years, the molecular components of the CRAC channel have been defined. The ER - located Ca(2+)-sensor, STIM1 and the Ca(2+)-selective ion pore, Orai1 in the membrane are sufficient to fully reconstitute CRAC currents. Stromal interaction molecule (STIM) 1 is localized in the ER, senses [Ca(2+)]ER and activates the CRAC channel upon store depletion by direct binding to Orai1 in the plasmamembrane. The identification of STIM1 and Orai1 and recently the structural resolution of both proteins by X-ray crystallography and nuclear magnetic resonance substantiated many findings from structure-function studies which has substantially improved the understanding of CRAC channel activation. Within this review, we summarize the functional and structural mechanisms of CRAC channel regulation, present a detailed overview of the STIM1/Orai1 signalling pathway where we focus on the critical domains mediating interactions and on the ion permeation pathway. We portray a mechanistic view of the steps in the dynamics of CRAC channel signalling ranging from STIM1 oligomerization over STIM1-Orai1 coupling to CRAC channel activation and permeation.

Microdomains Associated to Lipid Rafts

Store Operated Ca(2+) Entry (SOCE), the main Ca(2+) influx mechanism in non-excitable cells, is implicated in the immune response and has been reported to be affected in several pathologies including cancer. The basic molecular constituents of SOCE are Orai, the pore forming unit, and STIM, a multidomain protein with at least two principal functions: one is to sense the Ca(2+) content inside the lumen of the endoplasmic reticulum(ER) and the second is to activate Orai channels upon depletion of the ER. The link between Ca(2+) depletion inside the ER and Ca(2+) influx from extracellular media is through a direct association of STIM and Orai, but for this to occur, both molecules have to interact and form clusters where ER and plasma membrane (PM) are intimately apposed. In recent years a great number of components have been identified as participants in SOCE regulation, including regions of plasma membrane enriched in cholesterol and sphingolipids, the so called lipid rafts, which recruit a complex platform of specialized microdomains, which cells use to regulate spatiotemporal Ca(2+) signals.

Near-infrared photoactivatable control of Ca(2+) signaling and optogenetic immunomodulation

The application of current channelrhodopsin-based optogenetic tools is limited by the lack of strict ion selectivity and the inability to extend the spectra sensitivity into the near-infrared (NIR) tissue transmissible range. Here we present an NIR-stimulable optogenetic platform (termed 'Opto-CRAC') that selectively and remotely controls Ca²⁺ oscillations and Ca²⁺-responsive gene expression to regulate the function of non-excitable cells, including T lymphocytes, macrophages and dendritic cells. When coupled to upconversion nanoparticles, the optogenetic operation window is shifted from the visible range to NIR wavelengths to enable wireless photoactivation of Ca²⁺-dependent signaling and optogenetic modulation of immunoinflammatory responses. In a mouse model of melanoma by using ovalbumin as surrogate tumor antigen, Opto-CRAC has been shown to act as a genetically-encoded 'photoactivatable adjuvant' to improve antigen-specific immune responses to specifically destruct tumor cells. Our study represents a solid step forward towards the goal of achieving remote and wireless control of Ca²⁺-modulated activities with tailored function.

DOI:http://dx.doi.org/10.7554/eLife.10024.001

Optogenetics is a technique that has been used to study nerve cells for several years. It involves genetically engineering these cells to produce proteins from light-sensitive bacteria, and results in nerve cells that will either send, or stop sending, nerve impulses when they are exposed to a particular color of light. Neuroscientists have learned a lot about brain circuits using the technique, and now researchers in many other fields are giving it a try.

There are, however, several challenges to using optogenetics in other types of cells. Nerve cells create a tiny electrical impulses when they are activated, which helps them quickly transmit messages. But other types of cells use more diverse means to communicate and transmit signals. This means that optogenetics techniques must be adapted. Additionally, many cells are located deep in the body and so getting the light to them can be difficult.

He, Zhang et al. have now developed an optogenetic system (termed “Opto-CRAC”) that can control immune cells buried deep in tissue. The action of immune cells can be tuned by controlling the flow of calcium ions through gate-like proteins in their membranes. He, Zhang et al. genetically engineered immune cells so that a calcium gate-controlling protein became light sensitive. When the cells were exposed to a blue light the calcium ion gates opened. When the light was turned off, the gates closed. More intense light caused more calcium to enter into the cells. Further experiments then revealed that exposing these engineered immune cells to blue light in the laboratory could trigger an immune response.

The next obstacle was getting light to immune cells in a live animal. So, He, Zhang et al. used specific nanoparticles that have been shown to help transmit light deep within tissue. In these experiments, mice were injected with the light-sensitive immune cells and the nanoparticles. Then, a near-infrared laser beam that can transmit into the tissues was pointed at the mice. This caused calcium channels to open in the engineered cells deep in the mice. Finally, further experiments were used to show that this light-based stimulation could boost an immune response to aid the killing of cancer cells. Other scientists will likely use the technique to help them study immune, heart, and other types of cells that use calcium to communicate.

DOI:http://dx.doi.org/10.7554/eLife.10024.002

Cooperative Binding of Stromal Interaction Molecule 1 (STIM1) to the N and C Termini of Calcium Release-activated Calcium Modulator 1 (Orai1)

Calcium flux through store-operated calcium entry is a central regulator of intracellular calcium signaling. The two key components of the store-operated calcium release-activated calcium channel are the Ca(2+)-sensing protein stromal interaction molecule 1 (STIM1) and the channel pore-forming protein Orai1. During store-operated calcium entry activation, calcium depletion from the endoplasmic reticulum triggers a series of conformational changes in STIM1 that unmask a minimal Orai1-activating domain (CRAC activation region (CAD)). To gate Orai1 channels, the exposed STIM1-activating domain binds to two sites in Orai1, one in the N terminus and one in the C terminus. Whether the two sites operate as distinct binding domains or cooperate in CAD binding is unknown. In this study, we show that the N and C-terminal domains of Orai1 synergistically contribute to the interaction with STIM1 and couple STIM1 binding with channel gating and modulation of ion selectivity.

Diseases caused by mutations in ORAI1 and STIM1

Ca(2+) release-activated Ca(2+) (CRAC) channels mediate a specific form of Ca(2+) influx called store-operated Ca(2+) entry (SOCE) that contributes to the function of many cell types. CRAC channels are composed of ORAI1 proteins located in the plasma membrane, which form its ion-conducting pore. ORAI1 channels are activated by stromal interaction molecule (STIM) 1 and STIM2 located in the endoplasmic reticulum. Loss- and gain-of-function gene mutations in ORAI1 and STIM1 in human patients cause distinct disease syndromes. CRAC channelopathy is caused by loss-of-function mutations in ORAI1 and STIM1 that abolish CRAC channel function and SOCE; it is characterized by severe combined immunodeficiency (SCID)-like disease, autoimmunity, muscular hypotonia, and ectodermal dysplasia, with defects in sweat gland function and dental enamel formation. The latter defect emphasizes an important role of CRAC channels in tooth development. By contrast, autosomal dominant gain-of-function mutations in ORAI1 and STIM1 result in constitutive CRAC channel activation, SOCE, and increased intracellular Ca(2+) levels that are associated with an overlapping spectrum of diseases, including nonsyndromic tubular aggregate myopathy (TAM) and York platelet and Stormorken syndromes. The latter two syndromes are defined, besides myopathy, by thrombocytopenia, thrombopathy, and bleeding diathesis. The fact that myopathy results from both loss- and gain-of-function mutations in ORAI1 and STIM1 highlights the importance of CRAC channels for Ca(2+) homeostasis in skeletal muscle function. The cellular dysfunction and clinical disease spectrum observed in mutant patients provide important information about the molecular regulation of ORAI1 and STIM1 proteins and the role of CRAC channels in human physiology.

Store-Operated Calcium Channels

Store-operated calcium channels (SOCs) are a major pathway for calcium signaling in virtually all metozoan cells and serve a wide variety of functions ranging from gene expression, motility, and secretion to tissue and organ development and the immune response. SOCs are activated by the depletion of Ca(2+) from the endoplasmic reticulum (ER), triggered physiologically through stimulation of a diverse set of surface receptors. Over 15 years after the first characterization of SOCs through electrophysiology, the identification of the STIM proteins as ER Ca(2+) sensors and the Orai proteins as store-operated channels has enabled rapid progress in understanding the unique mechanism of store-operate calcium entry (SOCE). Depletion of Ca(2+) from the ER causes STIM to accumulate at ER-plasma membrane (PM) junctions where it traps and activates Orai channels diffusing in the closely apposed PM. Mutagenesis studies combined with recent structural insights about STIM and Orai proteins are now beginning to reveal the molecular underpinnings of these choreographic events. This review describes the major experimental advances underlying our current understanding of how ER Ca(2+) depletion is coupled to the activation of SOCs. Particular emphasis is placed on the molecular mechanisms of STIM and Orai activation, Orai channel properties, modulation of STIM and Orai function, pharmacological inhibitors of SOCE, and the functions of STIM and Orai in physiology and disease.

Proteomic mapping of ER-PM junctions identifies STIMATE as a regulator of Ca²⁺ influx

Specialized junctional sites that connect the plasma membrane (PM) and endoplasmic reticulum (ER) play critical roles in controlling lipid metabolism and Ca(2+) signalling. Store-operated Ca(2+) entry mediated by dynamic STIM1-ORAI1 coupling represents a classical molecular event occurring at ER-PM junctions, but the protein composition and how previously unrecognized protein regulators facilitate this process remain ill-defined. Using a combination of spatially restricted biotin labelling in situ coupled with mass spectrometry and a secondary screen based on bimolecular fluorescence complementation, we mapped the proteome of intact ER-PM junctions in living cells without disrupting their architectural integrity. Our approaches led to the discovery of an ER-resident multi-transmembrane protein that we call STIMATE (STIM-activating enhancer, encoded by TMEM110) as a positive regulator of Ca(2+) influx in vertebrates. STIMATE physically interacts with STIM1 to promote STIM1 conformational switch. Genetic depletion of STIMATE substantially reduces STIM1 puncta formation at ER-PM junctions and suppresses the Ca(2+)-NFAT signalling. Our findings enable further genetic studies to elucidate the function of STIMATE in normal physiology and disease, and set the stage to uncover more uncharted functions of hitherto underexplored ER-PM junctions.

SOCE and cancer: Recent progress and new perspectives

Ca²⁺ acts as a universal and versatile second messenger in the regulation of a myriad of biological processes, including cell proliferation, differentiation, migration and apoptosis. Store‐operated Ca²⁺ entry (SOCE) mediated by ORAI and the stromal interaction molecule (STIM) constitutes one of the major routes of calcium entry in nonexcitable cells, in which the depletion of intracellular Ca²⁺ stores triggers activation of the endoplasmic reticulum (ER)‐resident Ca²⁺ sensor protein STIM to gate and open the ORAI Ca²⁺ channels in the plasma membrane (PM). Accumulating evidence indicates that SOCE plays critical roles in cancer cell proliferation, metastasis and tumor neovascularization, as well as in antitumor immunity. We summarize herein the recent advances in our understanding of the function of SOCE in various types of tumor cells, vascular endothelial cells and cells of the immune system. Finally, the therapeutic potential of SOCE inhibitors in the treatment of cancer is also discussed.

STIM1 dimers undergo unimolecular coupling to activate Orai1 channels

The endoplasmic reticulum (ER) Ca(2+) sensor, STIM1, becomes activated when ER-stored Ca(2+) is depleted and translocates into ER-plasma membrane junctions where it tethers and activates Orai1 Ca(2+) entry channels. The dimeric STIM1 protein contains a small STIM-Orai-activating region (SOAR)--the minimal sequence sufficient to activate Orai1 channels. Since SOAR itself is a dimer, we constructed SOAR concatemer-dimers and introduced mutations at F394, which is critical for Orai1 coupling and activation. The F394H mutation in both SOAR monomers completely blocks dimer function, but F394H introduced in only one of the dimeric SOAR monomers has no effect on Orai1 binding or activation. This reveals an unexpected unimolecular coupling between STIM1 and Orai1 and argues against recent evidence suggesting dimeric interaction between STIM1 and two adjacent Orai1 channel subunits. The model predicts that STIM1 dimers may be involved in crosslinking between Orai1 channels with implications for the kinetics and localization of Orai1 channel opening.

Proteomic mapping of ER-PM junctions identifies STIMATE as a regulator of Ca²⁺ influx

Specialized junctional sites that connect the plasma membrane (PM) and endoplasmic reticulum (ER) play critical roles in controlling lipid metabolism and Ca²⁺ signaling^1–4. Store operated Ca²⁺ entry mediated by dynamic STIM1-ORAI1 coupling represents a classical molecular event occurring at ER-PM junctions, but the protein composition and how previously-unrecognized protein regulators facilitate this process remain ill-defined. Using a combination of spatially-restricted biotin-labelling in situ coupled with mass spectrometry^{5, 6} and a secondary screen based on bimolecular fluorescence complementation⁷, we mapped the proteome of intact ER-PM junctions in living cells without disrupting their architectural integrity. Our approaches lead to the discovery of an ER-resident multi-transmembrane protein that we call STIMATE (STIM-activating enhancer, encoded by TMEM110) as a positive regulator of Ca²⁺ influx in vertebrates. STIMATE physically interacts with STIM1 to promote STIM1 conformational switch. Genetic depletion of STIMATE substantially reduces STIM1 puncta formation at ER-PM junctions and suppresses the Ca²⁺-NFAT signaling. Our findings enable further genetic studies to elucidate the function of STIMATE in normal physiology and disease, and set the stage to uncover more uncharted functions of hitherto underexplored ER-PM junctions.

Calmodulin and STIM proteins: Two major calcium sensors in the cytoplasm and endoplasmic reticulum

The calcium (Ca(2+)) ion is a universal signalling messenger which plays vital physiological roles in all eukaryotes. To decode highly regulated intracellular Ca(2+) signals, cells have evolved a number of sensor proteins that are ideally adapted to respond to a specific range of Ca(2+) levels. Among many such proteins, calmodulin (CaM) is a multi-functional cytoplasmic Ca(2+) sensor with a remarkable ability to interact with and regulate a plethora of structurally diverse target proteins. CaM achieves this 'multi-talented' functionality through two EF-hand domains, each with an independent capacity to bind targets, and an adaptable flexible linker. By contrast, stromal interaction molecule-1 and -2 (STIMs) have evolved for a specific role in endoplasmic reticulum (ER) Ca(2+) sensing using EF-hand machinery analogous to CaM; however, whereas CaM structurally adjusts to dissimilar binding partners, STIMs use the EF-hand machinery to self-regulate the stability of the Ca(2+) sensing domain. The molecular mechanisms underlying the Ca(2+)-dependent signal transduction by CaM and STIMs have revealed a remarkable repertoire of actions and underscore the flexibility of nature in molecular evolution and adaption to discrete Ca(2+) levels. Recent genomic sequencing efforts have uncovered a number of disease-associated mutations in both CaM and STIM1. This article aims to highlight the most recent key structural and functional findings in the CaM and STIM fields, and discusses how these two Ca(2+) sensor proteins execute their biological functions.

Store-operated calcium entry: Mechanisms and modulation

Store-operated calcium entry is a central mechanism in cellular calcium signalling and in maintaining cellular calcium balance. This review traces the history of research on store-operated calcium entry, the discovery of STIM and ORAI as central players in calcium entry, and the role of STIM and ORAI in biology and human disease. It describes current knowledge of the basic mechanism of STIM-ORAI signalling and of the varied mechanisms by which STIM-ORAI signalling can be modulated.

Inside-out Ca(2+) signalling prompted by STIM1 conformational switch

Store-operated Ca(2+) entry mediated by STIM1 and ORAI1 constitutes one of the major Ca(2+) entry routes in mammalian cells. The molecular choreography of STIM1-ORAI1 coupling is initiated by endoplasmic reticulum (ER) Ca(2+) store depletion with subsequent oligomerization of the STIM1 ER-luminal domain, followed by its redistribution towards the plasma membrane to gate ORAI1 channels. The mechanistic underpinnings of this inside-out Ca(2+) signalling were largely undefined. By taking advantage of a unique gain-of-function mutation within the STIM1 transmembrane domain (STIM1-TM), here we show that local rearrangement, rather than alteration in the oligomeric state of STIM1-TM, prompts conformational changes in the cytosolic juxtamembrane coiled-coil region. Importantly, we further identify critical residues within the cytoplasmic domain of STIM1 (STIM1-CT) that entail autoinhibition. On the basis of these findings, we propose a model in which STIM1-TM reorganization switches STIM1-CT into an extended conformation, thereby projecting the ORAI-activating domain to gate ORAI1 channels.

The STIM1-ORAI1 microdomain

The regulatory protein STIM1 controls gating of the Ca(2+) channel ORAI1 by a direct protein-protein interaction. Because STIM1 is anchored in the ER membrane and ORAI1 is in the plasma membrane, the STIM-ORAI pathway can support Ca(2+) influx only where the two membranes come into close apposition, effectively demarcating a microdomain for Ca(2+) signalling. This review begins with a brief summary of the STIM-ORAI pathway of store-operated Ca(2+) influx, then turns to the special geometry of the STIM-ORAI microdomain and the expected characteristics of the microdomain Ca(2+) signal. A final section of the review seeks to place the STIM-ORAI microdomain into a broader context of cellular Ca(2+) signalling.

Critical role for Orai1 C-terminal domain and TM4 in CRAC channel gating

Calcium flux through store-operated calcium entry is a major regulator of intracellular calcium homeostasis and various calcium signaling pathways. Two key components of the store-operated calcium release-activated calcium channel are the Ca(2+)-sensing protein stromal interaction molecule 1 (STIM1) and the channel pore-forming protein Orai1. Following calcium depletion from the endoplasmic reticulum, STIM1 undergoes conformational changes that unmask an Orai1-activating domain called CAD. CAD binds to two sites in Orai1, one in the N terminal and one in the C terminal. Most previous studies suggested that gating is initiated by STIM1 binding at the Orai1 N-terminal site, just proximal to the TM1 pore-lining segment, and that binding at the C terminal simply anchors STIM1 within reach of the N terminal. However, a recent study had challenged this view and suggested that the Orai1 C-terminal region is more than a simple STIM1-anchoring site. In this study, we establish that the Orai1 C-terminal domain plays a direct role in gating. We identify a linker region between TM4 and the C-terminal STIM1-binding segment of Orai1 as a key determinant that couples STIM1 binding to gating. We further find that Proline 245 in TM4 of Orai1 is essential for stabilizing the closed state of the channel. Taken together with previous studies, our results suggest a dual-trigger mechanism of Orai1 activation in which binding of STIM1 at the N- and C-terminal domains of Orai1 induces rearrangements in proximal membrane segments to open the channel.

Gain-of-Function Mutation in STIM1 (P.R304W) Is Associated with Stormorken Syndrome

Stormorken syndrome is a rare autosomal dominant disorder characterized by a phenotype that includes miosis, thrombocytopenia/thrombocytopathy with bleeding time diathesis, intellectual disability, mild hypocalcemia, muscle fatigue, asplenia, and ichthyosis. Using targeted sequencing and whole-exome sequencing, we identified the c.910C > T transition in a STIM1 allele (p.R304W) only in patients and not in their unaffected family members. STIM1 encodes stromal interaction molecule 1 protein (STIM1), which is a finely tuned endoplasmic reticulum Ca(2+) sensor. The effect of the mutation on the structure of STIM1 was investigated by molecular modeling, and its effect on function was explored by calcium imaging experiments. Results obtained from calcium imaging experiments using transfected cells together with fibroblasts from one patient are in agreement with impairment of calcium homeostasis. We show that the STIM1 p.R304W variant may affect the conformation of the inhibitory helix and unlock the inhibitory state of STIM1. The p.R304W mutation causes a gain of function effect associated with an increase in both resting Ca(2+) levels and store-operated calcium entry. Our study provides evidence that Stormorken syndrome may result from a single-gene defect, which is consistent with Mendelian-dominant inheritance.

Missense mutation in immunodeficient patients shows the multifunctional roles of coiled-coil domain 3 (CC3) in STIM1 activation

Store-operated Ca(2+) entry (SOCE) is a universal Ca(2+) influx pathway that is important for the function of many cell types. SOCE occurs upon depletion of endoplasmic reticulum (ER) Ca(2+) stores and relies on a complex molecular interplay between the plasma membrane (PM) Ca(2+) channel ORAI1 and the ER Ca(2+) sensor stromal interaction molecule (STIM) 1. Patients with null mutations in ORAI1 or STIM1 genes present with severe combined immunodeficiency (SCID)-like disease. Here, we describe the molecular mechanisms by which a loss-of-function STIM1 mutation (R429C) in human patients abolishes SOCE. R429 is located in the third coiled-coil (CC3) domain of the cytoplasmic C terminus of STIM1. Mutation of R429 destabilizes the CC3 structure and alters the conformation of the STIM1 C terminus, thereby releasing a polybasic domain that promotes STIM1 recruitment to ER-PM junctions. However, the mutation also impairs cytoplasmic STIM1 oligomerization and abolishes STIM1-ORAI1 interactions. Thus, despite its constitutive localization at ER-PM junctions, mutant STIM1 fails to activate SOCE. Our results demonstrate multifunctional roles of the CC3 domain in regulating intra- and intermolecular STIM1 interactions that control (i) transition of STIM1 from a quiescent to an active conformational state, (ii) cytoplasmic STIM1 oligomerization, and (iii) STIM1-ORAI1 binding required for ORAI1 activation.

Store-operated Ca2+ entry plays a role in HMGB1-induced vascular endothelial cell hyperpermeability

Aims

Endothelial dysfunction, including increased endothelial permeability, is considered an early marker for atherosclerosis. High-mobility group box 1 protein (HMGB1) and extracellular Ca2+ entry, primarily mediated through store-operated Ca2+ entry (SOCE), are known to be involved in increasing endothelial permeability. The aim of this study was to clarify how HMGB1 could lead to endothelia hyperpermeability.

Methods and Results

We have shown that human vascular endothelial cell permeability is increased, while transendothelial electrical resistance and VE-cadherin expression were reduced by HMGB1 treatment. Two SOCE inhibitors and knockdown of stromal interaction molecule 1 (STIM1), a Ca²⁺ sensor mediating SOCE, inhibited the HMGB1-induced influx of Ca²⁺ and Src activation followed by significant suppression of endothelial permeability. Moreover, knockdown of Orai1, an essential pore-subunit of SOCE channels, decreased HMGB1-induced endothelial hyperpermeability.

Conclusions

These data suggest that SOCE, acting via STIM1, might be the predominant mechanism of Ca²⁺ entry in the modulation of endothelial cell permeability. STIM1 may thus represent a possible new therapeutic target against atherosclerosis.

Is hydrogen ion (H(+)) the real second messenger in calcium signalling?

Most second messengers have the acknowledged ability to mobilize the segregated Ca(2+) from intracellular stores, although the mechanisms of mobilization are unclear. To study this problem, the fact that inositol 1,4,5-trisphosphate, and six other known endogenous Ca(2+) mobilizers are acids, or acid-generating compounds, is highlighted. In physiological conditions, a newly generated acid releases H(+). The transient rise of H(+) in the cytosol may induce the lowering of pH, mobilization of bound Ca(2+), protein conformational rearrangement, store depletion, and Ca(2+) influx. Accordingly, a new description of the basic mechanism for signal transduction in non-excitable cells and the related consequences is put forward.

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.

Nanodomains in early and later phases of FcɛRI signalling

Our long-term efforts to elucidate receptor-mediated signalling in immune cells, particularly transmembrane signalling initiated by FcɛRI, the receptor for IgE in mast cells, led us unavoidably to contemplate the role of the heterogeneous plasma membrane. Our early investigations with fluorescence microscopy revealed co-redistribution of certain lipids and signalling components with antigen-cross-linked IgE-FcɛRI and pointed to participation of ordered membrane domains in the signalling process. With a focus on this function, we have worked along with others to develop diverse and increasingly sophisticated tools to analyse the complexity of membrane structure that facilitates regulation and targeting of signalling events. The present chapter describes how initial membrane interactions of clustered IgE-FcɛRI lead to downstream cellular responses and how biochemical information integrated with nanoscale resolution spectroscopy and imaging is providing mechanistic insights at the level of molecular complexes.

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

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