Functional Analysis of Orai1 Concatemers Supports a Hexameric Stoichiometry for the CRAC Channel

Activation mechanisms and structural dynamics of STIM proteins

The family of stromal interaction molecules (STIM) includes two widely expressed single-pass endoplasmic reticulum (ER) transmembrane proteins and additional splice variants that act as precise ER-luminal Ca2+ sensors. STIM proteins mainly function as one of the two essential components of the so-called Ca2+ release-activated Ca2+ (CRAC) channel. The second CRAC channel component is constituted by pore-forming Orai proteins in the plasma membrane. STIM and Orai physically interact with each other to enable CRAC channel opening, which is a critical prerequisite for various downstream signalling pathways such as gene transcription or proliferation. Their activation commonly requires the emptying of the intracellular ER Ca2+ store. Using their Ca2+ sensing capabilities, STIM proteins confer this Ca2+ content-dependent signal to Orai, thereby linking Ca2+ store depletion to CRAC channel opening. Here we review the conformational dynamics occurring along the entire STIM protein upon store depletion, involving the transition from the quiescent, compactly folded structure into an active, extended state, modulation by a variety of accessory components in the cell as well as the impairment of individual steps of the STIM activation cascade associated with disease.

ORAI Ca2+ Channels in Cancers and Therapeutic Interventions

The ORAI proteins serve as crucial pore-forming subunits of calcium-release-activated calcium (CRAC) channels, pivotal in regulating downstream calcium-related signaling pathways. Dysregulated calcium homeostasis arising from mutations and post-translational modifications in ORAI can lead to immune disorders, myopathy, cardiovascular diseases, and even cancers. Small molecules targeting ORAI present an approach for calcium signaling modulation. Moreover, emerging techniques like optogenetics and optochemistry aim to offer more precise regulation of ORAI. This review focuses on the role of ORAI in cancers, providing a concise overview of their significance in the initiation and progression of cancers. Additionally, it highlights state-of-the-art techniques for ORAI channel modulation, including advanced optical tools, potent pharmacological inhibitors, and antibodies. These novel strategies offer promising avenues for the functional regulation of ORAI in research and may inspire innovative approaches to cancer therapy targeting ORAI.

Reconstitution of Calcium Channel Protein Orai3 into Liposomes for Functional Studies

Store-operated calcium entry (SOCE) is the main mechanism for the Ca2+ influx in non-excitable cells. The two major components of SOCE are stromal interaction molecule 1 (STIM1) in the endoplasmic reticulum and Ca2+ release-activated Ca2+ channel (CRAC) Orai on the plasma membrane. SOCE requires interaction between STIM1 and Orai. Mammals have three Orai homologs: Orai1, Orai2, and Orai3. Although Orai1 has been widely studied and proven to essential for numerous cellular processes, Orai3 has also attracted a significant attention recently. The gating and activation mechanisms of Orai3 have yet to be fully elucidated. Here, we expressed, purified, and reconstituted Orai3 protein into liposomes and investigated its orientation and oligomeric state in the resulting proteoliposomes. STIM1 interacted with the Orai3-containing proteoliposomes and mediated calcium release from the them, suggesting that the Orai3 channel was functional and that recombinant STIM1 could directly open the Orai3 channel in vitro. The developed in vitro calcium release system could be used to study the structure, function, and pharmacology of Orai3 channel.

Photocrosslinking-induced CRAC channel-like Orai1 activation independent of STIM1

Ca2+ release-activated Ca2+ (CRAC) channels, indispensable for the immune system and various other human body functions, consist of two transmembrane (TM) proteins, the Ca2+-sensor STIM1 in the ER membrane and the Ca2+ ion channel Orai1 in the plasma membrane. Here we employ genetic code expansion in mammalian cell lines to incorporate the photocrosslinking unnatural amino acids (UAA), p-benzoyl-L-phenylalanine (Bpa) and p-azido-L-phenylalanine (Azi), into the Orai1 TM domains at different sites. Characterization of the respective UAA-containing Orai1 mutants using Ca2+ imaging and electrophysiology reveal that exposure to UV light triggers a range of effects depending on the UAA and its site of incorporation. In particular, photoactivation at A137 using Bpa in Orai1 activates Ca2+ currents that best match the biophysical properties of CRAC channels and are capable of triggering downstream signaling pathways such as nuclear factor of activated T-cells (NFAT) translocation into the nucleus without the need for the physiological activator STIM1.

ORAI Calcium Channels: Regulation, Function, Pharmacology, and Therapeutic Targets

The changes in intracellular free calcium (Ca2+) levels are one of the most widely regulators of cell function; therefore, calcium as a universal intracellular mediator is involved in very important human diseases and disorders. In many cells, Ca2+ inflow is mediated by store-operated calcium channels, and it is recognized that the store-operated calcium entry (SOCE) is mediated by the two partners: the pore-forming proteins Orai (Orai1-3) and the calcium store sensor, stromal interaction molecule (STIM1-2). Importantly, the Orai/STIM channels are involved in crucial cell signalling processes such as growth factors, neurotransmitters, and cytokines via interaction with protein tyrosine kinase coupled receptors and G protein-coupled receptors. Therefore, in recent years, the issue of Orai/STIM channels as a drug target in human diseases has received considerable attention. This review summarizes and highlights our current knowledge of the Orai/STIM channels in human diseases and disorders, including immunodeficiency, myopathy, tubular aggregate, Stormorken syndrome, York platelet syndrome, cardiovascular and metabolic disorders, and cancers, as well as suggesting these channels as drug targets for pharmacological therapeutic intervention. Moreover, this work will also focus on the pharmacological modulators of Orai/STIM channel complexes. Together, our thoughtful of the biology and physiology of the Orai/STIM channels have grown remarkably during the past three decades, and the next important milestone in the field of store-operated calcium entry will be to identify potent and selective small molecules as a therapeutic agent with the purpose to target human diseases and disorders for patient benefit.

CRAC and SK Channels: Their Molecular Mechanisms Associated with Cancer Cell Development

Cancer represents a major health burden worldwide. Several molecular targets have been discovered alongside treatments with positive clinical outcomes. However, the reoccurrence of cancer due to therapy resistance remains the primary cause of mortality. Endeavors in pinpointing new markers as molecular targets in cancer therapy are highly desired. The significance of the co-regulation of Ca²⁺-permeating and Ca²⁺-regulated ion channels in cancer cell development, proliferation, and migration make them promising molecular targets in cancer therapy. In particular, the co-regulation of the Orai1 and SK3 channels has been well-studied in breast and colon cancer cells, where it finally leads to an invasion-metastasis cascade. Nevertheless, many questions remain unanswered, such as which key molecular components determine and regulate their interplay. To provide a solid foundation for a better understanding of this ion channel co-regulation in cancer, we first shed light on the physiological role of Ca²⁺ and how this ion is linked to carcinogenesis. Then, we highlight the structure/function relationship of Orai1 and SK3, both individually and in concert, their role in the development of different types of cancer, and aspects that are not yet known in this context.

Validation of the binding stoichiometry between HCN channels and their neuronal regulator TRIP8b by single molecule measurements

Tetratricopeptide repeat-containing Rab8b-interacting (TRIP8b) protein is a brain-specific subunit of Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channels, a class of voltage-gated channels modulated by cyclic nucleotides. While the interaction between TRIP8b and the cytosolic C terminus of the channel has been structurally described, the HCN:TRIP8b stoichiometry is less characterized. We employed single molecule mass photometry (MP) to image HCN4 particles purified in complex with TRIP8b. Our data show that four TRIP8b subunits are bound to the tetrameric HCN4 particle, confirming a 1:1 stoichiometry.

SPLUNC1 is a negative regulator of the Orai1 Ca2+ channel

Orai1 is a ubiquitously-expressed plasma membrane Ca2+ channel that is involved in store-operated Ca2+ entry (SOCE): a fundamental biological process that regulates gene expression, the onset of inflammation, secretion, and the contraction of airway smooth muscle (ASM). During SOCE, Ca2+ leaves the endoplasmic reticulum, which then stimulates a second, amplifying wave of Ca2+ influx through Orai1 into the cytoplasm. Short Palate LUng and Nasal epithelial Clone 1 (SPLUNC1; gene name BPIFA1) is a multi-functional, innate defense protein that is highly abundant in the lung. We have previously reported that SPLUNC1 was secreted from epithelia, where it bound to and inhibited Orai1, leading to reduced SOCE and ASM relaxation. However, the underlying mechanism of action is unknown. Here, we probed the SPLUNC1-Orai1 interactions in ASM and HEK293T cells using biochemical and imaging techniques. We observed that SPLUNC1 caused a conformational change in Orai1, as measured using Forster resonance energy transfer (FRET). SPLUNC1 binding also led to Nedd4-2 dependent ubiquitination of Orai1. Moreover, SPLUNC1 internalized Orai1 to lysosomes, leading to Orai1 degradation. Thus, we conclude that SPLUNC1 is an allosteric regulator of Orai1. Our data indicate that SPLUNC1-mediated Orai1 inhibition could be utilized as a therapeutic strategy to reduce SOCE.

The roles of transmembrane family proteins in the regulation of store-operated Ca2+ entry

Store-operated Ca2+ entry (SOCE) is a major pathway for calcium signaling, which regulates almost every biological process, involving cell proliferation, differentiation, movement and death. Stromal interaction molecule (STIM) and ORAI calcium release-activated calcium modulator (ORAI) are the two major proteins involved in SOCE. With the deepening of studies, more and more proteins are found to be able to regulate SOCE, among which the transmembrane (TMEM) family proteins are worth paying more attention. In addition, the ORAI proteins belong to the TMEM family themselves. As the name suggests, TMEM family is a type of proteins that spans biological membranes including plasma membrane and membrane of organelles. TMEM proteins are in a large family with more than 300 proteins that have been already identified, while the functional knowledge about the proteins is preliminary. In this review, we mainly summarized the TMEM proteins that are involved in SOCE, to better describe a picture of the interaction between STIM and ORAI proteins during SOCE and its downstream signaling pathways, as well as to provide an idea for the study of the TMEM family proteins.

Highlighting the Multifaceted Role of Orai1 N-Terminal- and Loop Regions for Proper CRAC Channel Functions

Orai1, the Ca2+-selective pore in the plasma membrane, is one of the key components of the Ca2+release-activated Ca2+ (CRAC) channel complex. Activated by the Ca2+ sensor in the endoplasmic reticulum (ER) membrane, stromal interaction molecule 1 (STIM1), via direct interaction when ER luminal Ca2+ levels recede, Orai1 helps to maintain Ca2+ homeostasis within a cell. It has already been proven that the C-terminus of Orai1 is indispensable for channel activation. However, there is strong evidence that for CRAC channels to function properly and maintain all typical hallmarks, such as selectivity and reversal potential, additional parts of Orai1 are needed. In this review, we focus on these sites apart from the C-terminus; namely, the second loop and N-terminus of Orai1 and on their multifaceted role in the functioning of CRAC channels.

Structural Insights into Ca2+ Permeation through Orai Channels

Orai channels belong to the calcium release-activated calcium (CRAC) channel family. Orai channels are responsible for the influx of extracellular Ca²⁺ that is triggered by Ca²⁺ depletion from the endoplasmic reticulum (ER); this function is essential for many types of non-excitable cells. Extensive structural and functional studies have advanced the knowledge of the molecular mechanism by which Orai channels are activated. However, the gating mechanism that allows Ca²⁺ permeation through Orai channels is less well explained. Here, we reviewed and summarized the existing structural studies of Orai channels. We detailed the structural features of Orai channels, described structural comparisons of their closed and open states, and finally proposed a "push-pull" model of Ca²⁺ permeation.

Immune Synapse Residency of Orai1 Alters Ca2+ Response of T Cells

CRAC, which plays important role in Ca²⁺-dependent T-lymphocyte activation, is composed of the ER-resident STIM1 and the plasma membrane Orai1 pore-forming subunit. Both accumulate at the immunological synapse (IS) between a T cell and an antigen-presenting cell (APC). We hypothesized that adapter/interacting proteins regulate Orai1 residence in the IS. We could show that mGFP-tagged Orai1-Full channels expressed in Jurkat cells had a biphasic IS-accumulation kinetics peaked at 15 min. To understand the background of Orai1 IS-redistribution we knocked down STIM1 and SAP97 (adaptor protein with a short IS-residency (15 min) and ability to bind Orai1 N-terminus): the mGFP-Orai1-Full channels kept on accumulating in the IS up to the 60th minute in the STIM1- and SAP97-lacking Jurkat cells. Deletion of Orai1 N terminus (mGFP-Orai1-Δ72) resulted in the same time course as described for STIM1/SAP97 knock-down cells. Ca²⁺-imaging of IS-engaged T-cells revealed that of Orai1 residency modifies the Ca²⁺-response: cells expressing mGFP-Orai1-Δ72 construct or mGFP-Orai1-Full in SAP-97 knock-down cells showed higher number of Ca²⁺-oscillation up to the 90th minute after IS formation. Overall, these data suggest that SAP97 may contribute to the short-lived IS-residency of Orai1 and binding of STIM1 to Orai1 N-terminus is necessary for SAP97-Orai1 interaction.

Cell-wide mapping of Orai1 channel activity reveals functional heterogeneity in STIM1-Orai1 puncta

Upon Ca²⁺ store depletion, Orai1 channels cluster and open at endoplasmic reticulum–plasma membrane (ER–PM) junctions in signaling complexes called puncta. Little is known about whether and how Orai1 channel activity may vary between individual puncta. Previously, we developed and validated optical recording of Orai channel activity, using genetically encoded Ca²⁺ indicators fused to Orai1 or Orai3 N or C termini. We have now combined total internal reflection fluorescence microscopy with whole-cell recording to map functional properties of channels at individual puncta. After Ca²⁺ store depletion in HEK cells cotransfected with mCherry-STIM1 and Orai1-GCaMP6f, Orai1-GCaMP6f fluorescence increased progressively with increasingly negative test potentials and robust responses could be recorded from individual puncta. Cell-wide fluorescence half-rise and -fall times during steps to −100 mV test potential indicated probe response times of <50 ms. The in situ Orai1-GCaMP6f affinity for Ca²⁺ was 620 nM, assessed by monitoring fluorescence using buffered Ca²⁺ solutions in “unroofed” cells. Channel activity and temporal activation profile were tracked in individual puncta using image maps and automated puncta identification and recording. Simultaneous measurement of mCherry-STIM1 fluorescence uncovered an unexpected gradient in STIM1/Orai1 ratio that extends across the cell surface. Orai1-GCaMP6f channel activity was found to vary across the cell, with inactive channels occurring in the corners of cells where the STIM1/Orai1 ratio was lowest; low-activity channels typically at edges displayed a slow activation phase lasting hundreds of milliseconds. Puncta with high STIM1/Orai1 ratios exhibited a range of channel activity that appeared unrelated to the stoichiometric requirements for gating. These findings demonstrate functional heterogeneity of Orai1 channel activity between individual puncta and establish a new experimental platform that facilitates systematic comparisons between puncta composition and activity.

Isoform-Specific Properties of Orai Homologues in Activation, Downstream Signaling, Physiology and Pathophysiology

Ca2+ ion channels are critical in a variety of physiological events, including cell growth, differentiation, gene transcription and apoptosis. One such essential entry pathway for calcium into the cell is the Ca2+ release-activated Ca2+ (CRAC) channel. It consists of the Ca2+ sensing protein, stromal interaction molecule 1 (STIM1) located in the endoplasmic reticulum (ER) and a Ca2+ ion channel Orai in the plasma membrane. The Orai channel family includes three homologues Orai1, Orai2 and Orai3. While Orai1 is the "classical" Ca2+ ion channel within the CRAC channel complex and plays a universal role in the human body, there is increasing evidence that Orai2 and Orai3 are important in specific physiological and pathophysiological processes. This makes them an attractive target in drug discovery, but requires a detailed understanding of the three Orai channels and, in particular, their differences. Orai channel activation is initiated via Ca2+ store depletion, which is sensed by STIM1 proteins, and induces their conformational change and oligomerization. Upon STIM1 coupling, Orai channels activate to allow Ca2+ permeation into the cell. While this activation mechanism is comparable among the isoforms, they differ by a number of functional and structural properties due to non-conserved regions in their sequences. In this review, we summarize the knowledge as well as open questions in our current understanding of the three isoforms in terms of their structure/function relationship, downstream signaling and physiology as well as pathophysiology.

Orai channel C-terminal peptides are key modulators of STIM-Orai coupling and calcium signal generation

Junctional coupling between endoplasmic reticulum (ER) Ca2+-sensor STIM proteins and plasma membrane (PM) Orai channels mediates Ca2+ signals in most cells. We reveal that PM-tethered, fluorescently tagged C-terminal M4x (fourth transmembrane helix contains a cytoplasmic C-terminal extension) peptides from Orai channels undergo a Leu-specific signature of direct interaction with the STIM1 Orai-activating region (SOAR), exactly mimicking STIM1 binding to gate Orai channels. The 20-amino-acid Orai3-M4x peptide associates avidly with STIM1 within ER-PM junctions, functions to competitively block native Ca2+ signals, and mediates a key modification of STIM-Orai coupling induced by 2-aminoethoxydiphenyl borate. By blocking STIM-Orai coupling, the Orai3-M4x peptide reveals the critical role of Orai channels in driving Ca2+ oscillatory signals and transcriptional control through NFAT. The M4x peptides interact independently with SOAR dimers consistent with unimolecular coupling between Orai subunits and STIM1 dimers. We reveal the critical role of M4x helices in defining the coupling interface between STIM and Orai proteins to mediate store-operated Ca2+ signals.

Endoplasmic Reticulum-Plasma Membrane Contact Sites as an Organizing Principle for Compartmentalized Calcium and cAMP Signaling

In eukaryotic cells, ultimate specificity in activation and action-for example, by means of second messengers-of the myriad of signaling cascades is primordial. In fact, versatile and ubiquitous second messengers, such as calcium (Ca²⁺) and cyclic adenosine monophosphate (cAMP), regulate multiple-sometimes opposite-cellular functions in a specific spatiotemporal manner. Cells achieve this through segregation of the initiators and modulators to specific plasma membrane (PM) subdomains, such as lipid rafts and caveolae, as well as by dynamic close contacts between the endoplasmic reticulum (ER) membrane and other intracellular organelles, including the PM. Especially, these membrane contact sites (MCSs) are currently receiving a lot of attention as their large influence on cell signaling regulation and cell physiology is increasingly appreciated. Depletion of ER Ca²⁺ stores activates ER membrane STIM proteins, which activate PM-residing Orai and TRPC Ca²⁺ channels at ER-PM contact sites. Within the MCS, Ca²⁺ fluxes relay to cAMP signaling through highly interconnected networks. However, the precise mechanisms of MCS formation and the influence of their dynamic lipid environment on their functional maintenance are not completely understood. The current review aims to provide an overview of our current understanding and to identify open questions of the field.

ORAI1 Ca2+ Channel as a Therapeutic Target in Pathological Vascular Remodelling

In the adult, vascular smooth muscle cells (VSMC) are normally physiologically quiescent, arranged circumferentially in one or more layers within blood vessel walls. Remodelling of native VSMC to a proliferative state for vascular development, adaptation or repair is driven by platelet-derived growth factor (PDGF). A key effector downstream of PDGF receptors is store-operated calcium entry (SOCE) mediated through the plasma membrane calcium ion channel, ORAI1, which is activated by the endoplasmic reticulum (ER) calcium store sensor, stromal interaction molecule-1 (STIM1). This SOCE was shown to play fundamental roles in the pathological remodelling of VSMC. Exciting transgenic lineage-tracing studies have revealed that the contribution of the phenotypically-modulated VSMC in atherosclerotic plaque formation is more significant than previously appreciated, and growing evidence supports the relevance of ORAI1 signalling in this pathologic remodelling. ORAI1 has also emerged as an attractive potential therapeutic target as it is accessible to extracellular compound inhibition. This is further supported by the progression of several ORAI1 inhibitors into clinical trials. Here we discuss the current knowledge of ORAI1-mediated signalling in pathologic vascular remodelling, particularly in the settings of atherosclerotic cardiovascular diseases (CVDs) and neointimal hyperplasia, and the recent developments in our understanding of the mechanisms by which ORAI1 coordinates VSMC phenotypic remodelling, through the activation of key transcription factor, nuclear factor of activated T-cell (NFAT). In addition, we discuss advances in therapeutic strategies aimed at the ORAI1 target.

Gating and regulation of the calcium release-activated calcium channel: Recent progress from experiments and molecular modeling

Calcium release-activated calcium (CRAC) channels are highly calcium ion (Ca²⁺)-selective channels in the plasma membrane. The transient drop of endoplasmic reticulum Ca²⁺ level activates its calcium sensor stromal interaction molecule (STIM) and then triggers the gating of the CRAC channel pore unit Orai. This process involves a variety of activities of the immune system. Therefore, understanding how the activation and regulation of the CRAC channel can be accomplished is essential. Here we briefly summarize the recent progress on Orai gating and its regulation by 2-aminoethoxydiphenylborate (2-APB) obtained from structural biology studies, biochemical and electrophysiological measurements, as well as molecular modeling. Indeed, integration between experiments and computations has further deepened our understanding of the channel gating and regulation.

Orai channels: key players in Ca2+ homeostasis

Maintaining a precise calcium (Ca²⁺) balance is vital for cellular survival. The most prominent pathway to shuttle Ca²⁺ into cells is the Ca²⁺ release activated Ca²⁺ (CRAC) channel. Orai proteins are indispensable players in this central mechanism of Ca²⁺ entry. This short review traces the latest articles published in the field of CRAC channel signalling with a focus on the structure of the pore-forming Orai proteins, the propagation of the binding signal from STIM1 through the channel to the central pore and their role in human health and disease.

The Orai Pore Opening Mechanism

Cell survival and normal cell function require a highly coordinated and precise regulation of basal cytosolic Ca²⁺ concentrations. The primary source of Ca²⁺ entry into the cell is mediated by the Ca²⁺ release-activated Ca²⁺ (CRAC) channel. Its action is stimulated in response to internal Ca²⁺ store depletion. The fundamental constituents of CRAC channels are the Ca²⁺ sensor, stromal interaction molecule 1 (STIM1) anchored in the endoplasmic reticulum, and a highly Ca²⁺-selective pore-forming subunit Orai1 in the plasma membrane. The precise nature of the Orai1 pore opening is currently a topic of intensive research. This review describes how Orai1 gating checkpoints in the middle and cytosolic extended transmembrane regions act together in a concerted manner to ensure an opening-permissive Orai1 channel conformation. In this context, we highlight the effects of the currently known multitude of Orai1 mutations, which led to the identification of a series of gating checkpoints and the determination of their role in diverse steps of the Orai1 activation cascade. The synergistic action of these gating checkpoints maintains an intact pore geometry, settles STIM1 coupling, and governs pore opening. We describe the current knowledge on Orai1 channel gating mechanisms and summarize still open questions of the STIM1-Orai1 machinery.

Proteins Interacting with STIM1 and Store-Operated Ca2+ Entry

The endoplasmic reticulum (ER) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) interacts with ORAI Ca²⁺ channels at the plasma membrane to regulate immune and muscle cell function. The conformational changes underlying STIM1 activation, translocation, and ORAI1 trapping and gating, are stringently regulated by post-translational modifications and accessory proteins. Here, we review the recent progress in the identification and characterization of ER and cytosolic proteins interacting with STIM1 to control its activation and deactivation during store-operated Ca²⁺ entry (SOCE).

Physiological and Pathological Relevance of Selective and Nonselective Ca2+ Channels in Skeletal and Cardiac Muscle

Contraction of the striated muscle is fundamental for human existence. The action of voluntary skeletal muscle enables activities such as breathing, establishing body posture, and diverse body movements. Additionally, highly precise motion empowers communication, artistic expression, and other activities that define everyday human life. The involuntary contraction of striated muscle is the core function of the heart and is essential for blood flow. Several ion channels are important in the transduction of action potentials to cytosolic Ca2+ signals that enable muscle contraction; however, other ion channels are involved in the progression of muscle pathologies that can impair normal life or threaten it. This chapter describes types of selective and nonselective Ca2+ permeable ion channels expressed in the striated muscle, their participation in different aspects of muscle excitation and contraction, and their relevance to the progression of some pathological states.

Dysregulation of Neuronal Calcium Signaling via Store-Operated Channels in Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder that is characterized by motor, cognitive, and psychiatric problems. It is caused by a polyglutamine expansion in the huntingtin protein that leads to striatal degeneration via the transcriptional dysregulation of several genes, including genes that are involved in the calcium (Ca²⁺) signalosome. Recent research has shown that one of the major Ca²⁺ signaling pathways, store-operated Ca²⁺ entry (SOCE), is significantly elevated in HD. SOCE refers to Ca²⁺ flow into cells in response to the depletion of endoplasmic reticulum Ca²⁺ stores. The dysregulation of Ca²⁺ homeostasis is postulated to be a cause of HD progression because the SOCE pathway is indirectly and abnormally activated by mutant huntingtin (HTT) in γ-aminobutyric acid (GABA)ergic medium spiny neurons (MSNs) from the striatum in HD models before the first symptoms of the disease appear. The present review summarizes recent studies that revealed a relationship between HD pathology and elevations of SOCE in different models of HD, including YAC128 mice (a transgenic model of HD), cellular HD models, and induced pluripotent stem cell (iPSC)-based GABAergic medium spiny neurons (MSNs) that are obtained from adult HD patient fibroblasts. SOCE in MSNs was shown to be mediated by currents through at least two different channel groups, Ca²⁺ release-activated Ca²⁺ current (I_CRAC) and store-operated Ca²⁺ current (I_SOC), which are composed of stromal interaction molecule (STIM) proteins and Orai or transient receptor potential channel (TRPC) channels. Their role under physiological and pathological conditions in HD are discussed. The role of Huntingtin-associated protein 1 isoform A in elevations of SOCE in HD MSNs and potential compounds that may stabilize elevations of SOCE in HD are also summarized. Evidence is presented that shows that the dysregulation of molecular components of SOCE or pathways upstream of SOCE in HD MSN neurons is a hallmark of HD, and these changes could lead to HD pathology, making them potential therapeutic targets.

Synthesis and Characterization of Store-Operated Calcium Entry Inhibitors Active in the Submicromolar Range

The store-operated calcium entry, better known as SOCE, forms the main Ca²⁺ influx pathway in non-excitable cells, especially in leukocytes, where it is required for cell activation and the immune response. During the past decades, several inhibitors were developed, but they lack specificity or efficacy. From the non-specific SOCE inhibitor 2-aminoethyl diphenylborinate (2-APB), we synthetized 16 new analogues by replacing/modifying the phenyl groups. Among them, our compound P11 showed the best inhibitory capacity with a K_i ≈ 75 nM. Furthermore, below 1 µM, P11 was devoid of any inhibitory activity on the two other main cellular targets of 2-APB, the IP₃ receptors, and the SERCA pumps. Interestingly, Jurkat T cells secrete interleukin-2 under phytohemagglutinin stimulation but undergo cell death and stop IL-2 synthesis when stimulated in the presence of increasing P11 concentrations. Thus, P11 could represent the first member of a new and potent family of immunosuppressors.

Cryo-EM structure of the calcium release-activated calcium channel Orai in an open conformation

The calcium release-activated calcium channel Orai regulates Ca²⁺ entry into non-excitable cells and is required for proper immune function. While the channel typically opens following Ca²⁺ release from the endoplasmic reticulum, certain pathologic mutations render the channel constitutively open. Previously, using one such mutation (H206A), we obtained low (6.7 Å) resolution X-ray structural information on Drosophila melanogaster Orai in an open conformation (Hou et al., 2018). Here we present a structure of this open conformation at 3.3 Å resolution using fiducial-assisted cryo-electron microscopy. The improved structure reveals the conformations of amino acids in the open pore, which dilates by outward movements of subunits. A ring of phenylalanine residues repositions to expose previously shielded glycine residues to the pore without significant rotational movement of the associated helices. Together with other hydrophobic amino acids, the phenylalanines act as the channel's gate. Structured M1-M2 turrets, not evident previously, form the channel's extracellular entrance.

A sulfur-aromatic gate latch is essential for opening of the Orai1 channel pore

Sulfur-aromatic interactions occur in the majority of protein structures, yet little is known about their functional roles in ion channels. Here, we describe a novel molecular motif, the M101 gate latch, which is essential for gating of human Orai1 channels via its sulfur-aromatic interactions with the F99 hydrophobic gate. Molecular dynamics simulations of different Orai variants reveal that the gate latch is mostly engaged in open but not closed channels. In experimental studies, we use metal-ion bridges to show that promoting an M101-F99 bond directly activates Orai1, whereas disrupting this interaction triggers channel closure. Mutational analysis demonstrates that the methionine residue at this position has a unique combination of length, flexibility, and chemistry to act as an effective latch for the phenylalanine gate. Because sulfur-aromatic interactions provide additional stabilization compared to purely hydrophobic interactions, we infer that the six M101-F99 pairs in the hexameric channel provide a substantial energetic contribution to Orai1 activation.

The anatomy of native CRAC channel(s)

The ubiquitous store-operated Ca²⁺ entry pathway mediated by plasma membrane Ca²⁺ release-activated Ca²⁺ (CRAC) channels regulates a wide variety of physiological functions. While it is clearly established that the ORAI1 protein is essential for native mammalian CRAC channels, the contribution of ORAI2 and ORAI3 have remained nebulous. The crystal structure of the sole Orai isoform in drosophila (dOrai) revealed a hexameric assembly, suggesting that mammalian CRAC channels are hexamers of ORAI. Nevertheless, the relative contribution of each isoform of the mammalian ORAI trio to the stoichiometry of native CRAC channels remains elusive. The recent generation of ORAI isoform single, double and triple knockout cell lines and tissue-specific knockout mice has shed light on how native ORAI isoform heteromerization fine tunes CRAC-mediated Ca²⁺ signaling.

Distinct pharmacological profiles of ORAI1, ORAI2, and ORAI3 channels

The ubiquitous Ca²⁺ release-activated Ca²⁺ (CRAC) channel is crucial to many physiological functions. Both gain and loss of CRAC function is linked to disease. While ORAI1 is a crucial subunit of CRAC channels, recent evidence suggests that ORAI2 and ORAI3 heteromerize with ORAI1 to form native CRAC channels. Furthermore, ORAI2 and ORAI3 can form CRAC channels independently of ORAI1, suggesting diverse native CRAC stoichiometries. Yet, most available CRAC modifiers are presumed to target ORAI1 with little knowledge of their effects on ORAI2/3 or heteromers of ORAIs. Here, we used ORAI1/2/3 triple-null cells to express individual ORAI1, ORAI2, ORAI3 or ORAI1/2/3 concatemers. We reveal that GSK-7975A and BTP2 essentially abrogate ORAI1 and ORAI2 activity while causing only a partial inhibition of ORAI3. Interestingly, Synta66 abrogated ORAI1 channel function, while potentiating ORAI2 with no effect on ORAI3. CRAC channel activities mediated by concatenated ORAI1-1, ORAI1-2 and ORAI1-3 dimers were inhibited by Synta66, while ORAI2-3 dimers were unaffected. The CRAC enhancer IA65 significantly potentiated ORAI1 and ORAI1-1 activity with marginal effects on other ORAIs. Further, we characterized the profiles of individual ORAI isoforms in the presence of Gd³⁺ (5μM), 2-APB (5 μM and 50 μM), as well as changes in intracellular and extracellular pH. Our data reveal unique pharmacological features of ORAI isoforms expressed in an ORAI-null background and provide new insights into ORAI isoform selectivity of widely used CRAC pharmacological compounds.

The Intricate Coupling Between STIM Proteins and Orai Channels

Store-operated Ca²⁺ entry signals are critical for cellular regulation. This intricate signaling pathway involves coupling of proteins in two different membranes: the ER-resident Ca²⁺-sensing STIM proteins and the highly Ca²⁺-selective PM Orai channels. The molecular nature of the STIM-Orai coupling interface in ER-PM junctions and consequent Orai channel gating, are processes under intense study. We describe recent developments in determining the mechanism of Orai activation through the key STIM-Orai Activating Region (SOAR) of STIM1. We describe the unexpected unimolecular coupling of STIM with Orai and explain the observed variable stoichiometry of STIM-Orai interactions. We also define the discrete C-terminal regions in Orai channels that initially latch onto STIM proteins and mediate allosteric activation of the channel. A critical "nexus" region closely associated with the STIM-activated C-terminus of Orai1, propagates the STIM-binding signal through the four tightly-associated transmembrane helices of Orai1, finally to modify the pore-forming helices and effect channel opening.

Store-Operated Calcium Channels: From Function to Structure and Back Again

Store-operated calcium (Ca²⁺) entry (SOCE) occurs through a widely distributed family of ion channels activated by the loss of Ca²⁺ from the endoplasmic reticulum (ER). The best understood of these is the Ca²⁺ release-activated Ca²⁺ (CRAC) channel, which is notable for its unique activation mechanism as well as its many essential physiological functions and the diverse pathologies that result from dysregulation. In response to ER Ca²⁺ depletion, CRAC channels are formed through a diffusion trap mechanism at ER-plasma membrane (PM) junctions, where the ER Ca²⁺-sensing stromal interaction molecule (STIM) proteins bind and activate hexamers of Orai pore-forming proteins to trigger Ca²⁺ entry. Cell biological studies are clarifying the architecture of ER-PM junctions, their roles in Ca²⁺ and lipid transport, and functional interactions with cytoskeletal proteins. Molecular structures of STIM and Orai have inspired a multitude of mutagenesis and electrophysiological studies that reveal potential mechanisms for how STIM is toggled between inactive and active states, how it binds and activates Orai, and the importance of STIM-binding stoichiometry for opening the channel and establishing its signature characteristics of extremely high Ca²⁺ selectivity and low Ca²⁺ conductance.

The Number and Position of Orai3 Units within Heteromeric Store-Operated Ca2+ Channels Alter the Pharmacology of ICRAC

Store-operated heteromeric Orai1/Orai3 channels have been discussed in the context of aging, cancer, and immune cell differentiation. In contrast to homomeric Orai1 channels, they exhibit a different pharmacology upon application of reactive oxygen species (ROS) or 2-aminoethoxydiphenyl borate (2-APB) in various cell types. In endogenous cells, subunit composition and arrangement may vary and cannot be defined precisely. In this study, we used patch-clamp electrophysiology to investigate the 2-APB profile of store-operated and store-independent homomeric Orai1 and heteromeric Orai1/Orai3 concatenated channels with defined subunit compositions. As has been shown previous, one or more Orai3 subunit(s) within the channel result(s) in decreased Ca²⁺ release activated Ca²⁺ current (I_CRAC). Upon application of 50 µM 2-APB, channels with two or more Orai3 subunits exhibit large outward currents and can be activated by 2-APB independent from storedepletion and/or the presence of STIM1. The number and position of Orai3 subunits within the heteromeric store-operated channel change ion conductivity of 2-APB-activated outward current. Compared to homomeric Orai1 channels, one Orai3 subunit within the channel does not alter 2-APB pharmacology. None of the concatenated channel constructs were able to exactly simulate the complex 2-APB pharmacology observed in prostate cancer cells. However, 2-APB profiles of prostate cancer cells are similar to those of concatenated channels with Orai3 subunit(s). Considering the presented and previous results, this indicates that distinct subtypes of heteromeric SOCE channels may be selectively activated or blocked. In the future, targeting distinct heteromeric SOCE channel subtypes may be the key to tailored SOCE-based therapies.

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

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

Detecting single ORAI1 proteins within the plasma membrane reveals higher-order channel complexes

ORAI1 proteins form highly selective Ca²⁺ channels in the plasma membrane. Crystallographic data point towards a hexameric stoichiometry of ORAI1 channels, whereas optical methods postulated ORAI1 channels to reside as dimers at rest, and other data suggests that they have a tetrameric configuration. Here, liquid-phase scanning transmission electron microscopy (STEM) and quantum dot (QD) labeling was utilized to study the conformation of ORAI1 proteins at rest. To address the question of whether ORAI1 was present as a dimer, experiments were designed using single ORAI1 monomers and covalently linked ORAI1 dimers with either one or two label-binding positions. The microscopic data was statistically analyzed via the pair correlation function. Label pairs were found in all cases, even for concatenated dimers with one label-binding position, which is only possible if a significant fraction of ORAI1 was assembled in larger order oligomers than dimers, binding at least two QDs. This interpretation of the data was consistent with Blue Native PAGE analysis showing that ORAI1 is mainly present as a complex of an apparent molecular mass larger than that calculated for a dimer.

Review: Structure and Activation Mechanisms of CRAC Channels

Ca²⁺ release activated Ca²⁺ (CRAC) channels represent a primary pathway for Ca²⁺ to enter non-excitable cells. The two key players in this process are the stromal interaction molecule (STIM), a Ca²⁺ sensor embedded in the membrane of the endoplasmic reticulum, and Orai, a highly Ca²⁺ selective ion channel located in the plasma membrane. Upon depletion of the internal Ca²⁺ stores, STIM is activated, oligomerizes, couples to and activates Orai. This review provides an overview of novel findings about the CRAC channel activation mechanisms, structure and gating. In addition, it highlights, among diverse STIM and Orai mutants, also the disease-related mutants and their implications.

Changes in Calcium Homeostasis and Gene Expression Implicated in Epilepsy in Hippocampi of Mice Overexpressing ORAI1

Previously, we showed that the overexpression of ORAI1 calcium channel in neurons of murine brain led to spontaneous occurrence of seizure-like events in aged animals of transgenic line FVB/NJ-Tg(ORAI1)Ibd (Nencki Institute of Experimental Biology). We aimed to identify the mechanism that is responsible for this phenomenon. Using a modified Ca²⁺-addback assay in the CA1 region of acute hippocampal slices and FURA-2 acetomethyl ester (AM) Ca²⁺ indicator, we found that overexpression of ORAI1 in neurons led to altered Ca²⁺ response. Next, by RNA sequencing (RNAseq) we identified a set of genes, whose expression was changed in our transgenic animals. These data were validated using customized real-time PCR assays and digital droplet PCR (ddPCR) ddPCR. Using real-time PCR, up-regulation of hairy and enhancer of split-5 (Hes-5) gene and down-regulation of aristaless related homeobox (Arx), doublecortin-like kinase 1 (Dclk1), and cyclin-dependent kinase-like 5 (Cdkl5, also known as serine/threonine kinase 9 (Stk9)) genes were found. Digital droplet PCR (ddPCR) analysis revealed down-regulation of Arx. In humans, ARX, DCLK1, and CDLK5 were shown to be mutated in some rare epilepsy-associated disorders. We conclude that the occurrence of seizure-like events in aged mice overexpressing ORAI1 might be due to the down-regulation of Arx, and possibly of Cdkl5 and Dclk1 genes.

Molecular basis of allosteric Orai1 channel activation by STIM1

Store-operated Ca²⁺ entry through Orai1 channels is a primary mechanism for Ca²⁺ entry in many cells and mediates numerous cellular effector functions ranging from gene transcription to exocytosis. Orai1 channels are amongst the most Ca²⁺ -selective channels known and are activated by direct physical interactions with the endoplasmic reticulum Ca²⁺ sensor stromal interaction molecule 1 (STIM1) in response to store depletion triggered by stimulation of a variety of cell surface G-protein coupled and tyrosine kinase receptors. Work in the last decade has revealed that the Orai1 gating process is highly cooperative and strongly allosteric, likely driven by a wave of interdependent conformational changes throughout the protein originating in the peripheral C-terminal ligand binding site and culminating in pore opening. In this review, we survey the structural and molecular features in Orai1 that contribute to channel gating and consider how they give rise to the unique biophysical fingerprint of Orai1 currents.

Molecular understanding of calcium permeation through the open Orai channel

The Orai channel is characterized by voltage independence, low conductance, and high Ca²⁺ selectivity and plays an important role in Ca²⁺ influx through the plasma membrane (PM). How the channel is activated and promotes Ca²⁺ permeation is not well understood. Here, we report the crystal structure and cryo-electron microscopy (cryo-EM) reconstruction of a Drosophila melanogaster Orai (dOrai) mutant (P288L) channel that is constitutively active according to electrophysiology. The open state of the Orai channel showed a hexameric assembly in which 6 transmembrane 1 (TM1) helices in the center form the ion-conducting pore, and 6 TM4 helices in the periphery form extended long helices. Orai channel activation requires conformational transduction from TM4 to TM1 and eventually causes the basic section of TM1 to twist outward. The wider pore on the cytosolic side aggregates anions to increase the potential gradient across the membrane and thus facilitate Ca²⁺ permeation. The open-state structure of the Orai channel offers insights into channel assembly, channel activation, and Ca²⁺ permeation.

The structure of a constitutively active mutant of the fruit fly Orai calcium influx channel reveals a conformational transduction pathway upon channel activation and suggests an anion-assisted mechanism for calcium permeation.

Detecting In-Situ oligomerization of engineered STIM1 proteins by diffraction-limited optical imaging

Several signaling proteins require self-association of individual monomer units to be activated for triggering downstream signaling cascades in cells. Methods that allow visualizing their underlying molecular mechanisms will immensely benefit cell biology. Using enhanced Green Fluorescent Protein (eGFP) complementation, here I present a functional imaging approach for visualizing the protein-protein interaction in cells. Activation mechanism of an ER (endoplasmic reticulum) resident Ca²⁺ sensor, STIM1 (Stromal Interaction Molecule 1) that regulates store-operated Ca²⁺ entry in cells is considered as a model system. Co-expression of engineered full-length human STIM1 (ehSTIM1) with N-terminal complementary split eGFP pairs in mammalian cells fluoresces to form ‘puncta’ upon a drop in ER lumen Ca²⁺ concentration. Quantization of discrete fluorescent intensities of ehSTIM1 molecules at a diffraction-limited resolution revealed a diverse set of intensity levels not exceeding six-fold. Detailed screening of the ehSTIM1 molecular entities characterized by one to six fluorescent emitters across various in-plane sections shows a greater probability of occurrence for entities with six emitters in the vicinity of the plasma membrane (PM) than at the interior sections. However, the number density of entities with six emitters was lesser than that of others localized close to the PM. This finding led to hypothesize that activated ehSTIM1 dimers perhaps oligomerize in bundles ranging from 1–6 with an increased propensity for the occurrence of hexamers of ehSTIM1 dimer units close to PM even when its partner protein, ORAI1 (PM resident Ca²⁺ channel) is not sufficiently over-expressed in cells. The experimental data presented here provide direct evidence for luminal domain association of ehSTIM1 monomer units to trigger activation and allow enumerating various oligomers of ehSTIM1 in cells.

Reaction-diffusion model for STIM-ORAI interaction: The role of ROS and mutations

Release of Ca²⁺ from endoplasmatic retriculum (ER) Ca²⁺ stores causes stromal interaction molecules (STIM) in the ER membrane and ORAI proteins in the plasma membrane (PM) to interact and form the Ca²⁺ release activated Ca²⁺ (CRAC) channels, which represent a major Ca²⁺ entry route in non-excitable cells and thus control various cell functions. It is experimentally possible to mutate ORAI1 proteins and therefore modify, especially block, the Ca²⁺ influx into the cell. On the basis of the model of Hoover and Lewis (2011), we formulate a reaction-diffusion model to quantify the STIM1-ORAI1 interaction during CRAC channel formation and analyze different ORAI1 channel stoichiometries and different ratios of STIM1 and ORAI1 in comparison with experimental data. We incorporate the inhibition of ORAI1 channels by ROS into our model and calculate its contribution to the CRAC channel amplitude. We observe a large decrease of the CRAC channel amplitude evoked by mutations of ORAI1 proteins.

Numbers count: How STIM and Orai stoichiometry affect store-operated calcium entry

Substantial progress has been made in the past several years in establishing the stoichiometries of STIM and Orai proteins and understanding their influence on store-operated calcium entry. Depletion of ER Ca²⁺ triggers STIM1 to accumulate at ER-plasma membrane junctions where it binds and opens Ca²⁺ release-activated Ca²⁺ (CRAC) channels. STIM1 is a dimer, and release of Ca²⁺ from its two luminal domains is reported to promote their association as well as drive formation of higher-order STIM1 oligomers. The CRAC channel, originally thought to be tetrameric, is now considered to be a hexamer of Orai1 subunits based on crystallographic and electrophysiological studies. STIM1 binding activates CRAC channels in a highly nonlinear way, such that all six Orai1 binding sites must be occupied to account for the activation and signature properties of native channels. The structural basis of STIM1 engagement with the channel is currently unclear, with evidence suggesting that STIM1 dimers bind to individual or pairs of Orai1 subunits. This review examines evidence that has led to points of consensus and debate about STIM1 and Orai1 stoichiometries, and explains the importance of STIM-Orai complex stoichiometry for the regulation of store-operated calcium entry.

ORAI channels in cellular remodeling of cardiorespiratory disease

Cardiorespiratory disease, which includes systemic arterial hypertension, restenosis, atherosclerosis, pulmonary arterial hypertension, asthma, and chronic obstructive pulmonary disease (COPD) are highly prevalent and devastating diseases with limited therapeutic modalities. A common pathophysiological theme to these diseases is cellular remodeling, which is contributed by changes in expression and activation of ion channels critical for either excitability or growth. Calcium (Ca²⁺) signaling and specifically ORAI Ca²⁺ channels have emerged as significant regulators of smooth muscle, endothelial, epithelial, platelet, and immune cell remodeling. This review details the dysregulation of ORAI in cardiorespiratory diseases, and how this dysregulation of ORAI contributes to cellular remodeling.

Structural features of STIM and Orai underlying store-operated calcium entry

Store-operated calcium entry (SOCE) through Orai channels is triggered by receptor-stimulated depletion of Ca²⁺ from the ER. Orai1 is unique in terms of its activation mechanism, biophysical properties, and structure, and its precise regulation is essential for human health. Recent studies have begun to reveal the structural basis of the major steps in the SOCE pathway and how the system is reliably suppressed in resting cells but able to respond robustly to ER Ca²⁺ depletion. In this review, we discuss current models describing the activation of ER Ca²⁺ sensor STIM1, its binding to Orai1, propagation of the binding signal from the channel periphery to the central pore, and the resulting conformational changes underlying opening of the highly Ca²⁺ selective Orai1 channel.

STIM1 activation of Orai1

A primary calcium (Ca2+) entry pathway into non-excitable cells is through the store-operated Ca2+ release activated Ca2+ (CRAC) channel. Ca2+ entry into cells is responsible for the initiation of diverse signalling cascades that affect essential cellular processes like gene regulation, cell growth and death, secretion and gene transcription. Upon depletion of intracellular Ca2+ stores within the endoplasmic reticulum (ER), the CRAC channel opens to refill depleted stores. The two key limiting molecular players of the CRAC channel are the stromal interaction molecule (STIM1) embedded in the ER-membrane and Orai1, residing in the plasma membrane (PM), respectively. Together, they form a highly Ca2+ selective ion channel complex. STIM1 senses the Ca2+ content of the ER and confers Ca2+ store-depletion into the opening of Orai1 channels in the PM for triggering Ca2+-dependent gene transcription, T-cell activation or mast cell degranulation. The interplay of Orai and STIM proteins in the CRAC channel signalling cascade has been the main focus of research for more than twelve years. This chapter focuses on current knowledge and main experimental advances in the understanding of Orai1 activation by STIM1, thereby portraying key mechanistic steps in the CRAC channel signalling cascade.

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

KEY POINTS

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

ABSTRACT

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

IP3 receptors and Ca2+ entry

Inositol 1,4,5-trisphosphate receptors (IP₃R) are the most widely expressed intracellular Ca²⁺ release channels. Their activation by IP₃ and Ca²⁺ allows Ca²⁺ to pass rapidly from the ER lumen to the cytosol. The resulting increase in cytosolic [Ca²⁺] may directly regulate cytosolic effectors or fuel Ca²⁺ uptake by other organelles, while the decrease in ER luminal [Ca²⁺] stimulates store-operated Ca²⁺ entry (SOCE). We are close to understanding the structural basis of both IP₃R activation, and the interactions between the ER Ca²⁺-sensor, STIM, and the plasma membrane Ca²⁺ channel, Orai, that lead to SOCE. IP₃Rs are the usual means through which extracellular stimuli, through ER Ca²⁺ release, stimulate SOCE. Here, we review evidence that the IP₃Rs most likely to respond to IP₃ are optimally placed to allow regulation of SOCE. We also consider evidence that IP₃Rs may regulate SOCE downstream of their ability to deplete ER Ca²⁺ stores. Finally, we review evidence that IP₃Rs in the plasma membrane can also directly mediate Ca²⁺ entry in some cells.

The exquisitely cooperative nature of Orai1 channel activation

Yeung and Prakriya highlight new research showing that STIM1 must bind to all six Orai1 subunits to effectively activate the channel.

Structures reveal opening of the store-operated calcium channel Orai

The store-operated calcium (Ca²⁺) channel Orai governs Ca²⁺ influx through the plasma membrane of many non-excitable cells in metazoans. The channel opens in response to the depletion of Ca²⁺ stored in the endoplasmic reticulum (ER). Loss- and gain-of-function mutants of Orai cause disease. Our previous work revealed the structure of Orai with a closed pore. Here, using a gain-of-function mutation that constitutively activates the channel, we present an X-ray structure of Drosophila melanogaster Orai in an open conformation. Well-defined electron density maps reveal that the pore is dramatically dilated on its cytosolic side in comparison to the slender closed pore. Cations and anions bind in different regions of the open pore, informing mechanisms for ion permeation and Ca²⁺ selectivity. Opening of the pore requires the release of cytosolic latches. Together with additional X-ray structures of an unlatched-but-closed conformation, we propose a sequence for store-operated activation.

Physiological CRAC channel activation and pore properties require STIM1 binding to all six Orai1 subunits

The quantitative relation between STIM1 binding to Orai1 and store-operated CRAC channel activation is not well understood. Yen and Lewis find that STIM1 must bind to all six Orai1 subunits to effectively open the channel and generate the high ion selectivity and low conductance seen in native cells.

The binding of STIM1 to Orai1 controls the opening of store-operated CRAC channels as well as their extremely high Ca²⁺ selectivity. Although STIM1 dimers are known to bind directly to the cytosolic C termini of the six Orai1 subunits (SUs) that form the channel hexamer, the dependence of channel activation and selectivity on the number of occupied binding sites is not well understood. Here we address these questions using dimeric and hexameric Orai1 concatemers in which L273D mutations were introduced to inhibit STIM1 binding to specific Orai1 SUs. By measuring FRET between fluorescently labeled STIM1 and Orai1, we find that homomeric L273D mutant channels fail to bind STIM1 appreciably; however, the L273D SU does bind STIM1 and contribute to channel activation when located adjacent to a WT SU. These results suggest that STIM1 dimers can interact with pairs of neighboring Orai1 SUs. Surprisingly, a single L273D mutation within the Orai1 hexamer reduces channel open probability by ∼90%, triples the size of the single-channel current, weakens the Ca²⁺ binding affinity of the selectivity filter, and lowers the selectivity for Na⁺ over Cs⁺ in the absence of divalent cations. These findings reveal a surprisingly strong functional coupling between STIM1 binding and CRAC channel gating and pore properties. We conclude that under physiological conditions, all six Orai1 SUs of the native CRAC channel bind STIM1 to effectively open the pore and generate the signature properties of extremely low conductance and high ion selectivity.

Pore properties of Orai1 calcium channel dimers and their activation by the STIM1 ER calcium sensor

Store-operated Ca2+ entry signals are mediated by plasma membrane Orai channels activated through intermembrane coupling with Ca2+-sensing STIM proteins in the endoplasmic reticulum (ER). The nature of this elaborate Orai-gating mechanism has remained enigmatic. Based on the Drosophila Orai structure, mammalian Orai1 channels are hexamers comprising three dimeric subunit pairs. We utilized concatenated Orai1 dimers to probe the function of key domains within the channel pore and gating regions. The Orai1-E106Q selectivity-filter mutant, widely considered a dominant pore blocker, was surprisingly nondominant within concatenated heterodimers with Orai1-WT. The Orai1-E106Q/WT heterodimer formed STIM1-activated nonselective cation channels with significantly enlarged apparent pore diameter. Other Glu-106 substitutions entirely blocked the function of heterodimers with Orai1-WT. The hydrophobic pore-lining mutation V102C, which constitutively opens channels, was suppressed by Orai1-WT in the heterodimer. In contrast, the naturally occurring R91W pore-lining mutation associated with human immunodeficiency was completely dominant-negative over Orai-WT in heterodimers. Heterodimers containing the inhibitory K85E mutation extending outward from the pore helix gave an interesting partial effect on both channel activation and STIM1 binding, indicating an important allosteric link between the cytosolic Orai1 domains. The Orai1 C-terminal STIM1-binding domain mutation L273D powerfully blocked STIM1-induced channel activation. The Orai1-L273D/WT heterodimer had drastically impaired STIM1-induced channel gating but, unexpectedly, retained full STIM1 binding. This reveals the critical role of Leu-273 in transducing the STIM1-binding signal into the allosteric conformational change that initiates channel gating. Overall, our results provide important new insights into the role of key functional domains that mediate STIM1-induced gating of the Orai1 channel.