Essential role for the CRAC activation domain in store-dependent oligomerization of STIM1

Essential role of N-terminal SAM regions in STIM1 multimerization and function

The single-pass transmembrane protein Stromal Interaction Molecule 1 (STIM1), located in the endoplasmic reticulum (ER) membrane, possesses two main functions: It senses the ER-Ca2+ concentration and directly binds to the store-operated Ca2+ channel Orai1 for its activation when Ca2+ recedes. At high resting ER-Ca2+ concentration, the ER-luminal STIM1 domain is kept monomeric but undergoes di/multimerization once stores are depleted. Luminal STIM1 multimerization is essential to unleash the STIM C-terminal binding site for Orai1 channels. However, structural basis of the luminal association sites has so far been elusive. Here, we employed molecular dynamics (MD) simulations and identified two essential di/multimerization segments, the α7 and the adjacent region near the α9-helix in the sterile alpha motif (SAM) domain. Based on MD results, we targeted the two STIM1 SAM domains by engineering point mutations. These mutations interfered with higher-order multimerization of ER-luminal fragments in biochemical assays and puncta formation in live-cell experiments upon Ca2+ store depletion. The STIM1 multimerization impeded mutants significantly reduced Ca2+ entry via Orai1, decreasing the Ca2+ oscillation frequency as well as store-operated Ca2+ entry. Combination of the ER-luminal STIM1 multimerization mutations with gain of function mutations and coexpression of Orai1 partially ameliorated functional defects. Our data point to a hydrophobicity-driven binding within the ER-luminal STIM1 multimer that needs to switch between resting monomeric and activated multimeric state. Altogether, these data reveal that interactions between SAM domains of STIM1 monomers are critical for multimerization and activation of the protein.

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.

Regulatory mechanisms controlling store-operated calcium entry

Calcium influx through plasma membrane ion channels is crucial for many events in cellular physiology. Cell surface stimuli lead to the production of inositol 1,4,5-trisphosphate (IP3), which binds to IP3 receptors (IP3R) in the endoplasmic reticulum (ER) to release calcium pools from the ER lumen. This leads to the depletion of ER calcium pools, which has been termed store depletion. Store depletion leads to the dissociation of calcium ions from the EF-hand motif of the ER calcium sensor Stromal Interaction Molecule 1 (STIM1). This leads to a conformational change in STIM1, which helps it to interact with the plasma membrane (PM) at ER:PM junctions. At these ER:PM junctions, STIM1 binds to and activates a calcium channel known as Orai1 to form calcium release-activated calcium (CRAC) channels. Activation of Orai1 leads to calcium influx, known as store-operated calcium entry (SOCE). In addition to Orai1 and STIM1, the homologs of Orai1 and STIM1, such as Orai2/3 and STIM2, also play a crucial role in calcium homeostasis. The influx of calcium through the Orai channel activates a calcium current that has been termed the CRAC current. CRAC channels form multimers and cluster together in large macromolecular assemblies termed "puncta". How CRAC channels form puncta has been contentious since their discovery. In this review, we will outline the history of SOCE, the molecular players involved in this process, as well as the models that have been proposed to explain this critical mechanism in cellular physiology.

Regulatory mechanisms controlling store-operated calcium entry

Calcium influx through plasma membrane ion channels is crucial for many events in cellular physiology. Cell surface stimuli lead to the production of inositol 1,4,5-trisphosphate (IP3), which binds to IP3 receptors (IP3R) in the endoplasmic reticulum (ER) to release calcium pools from the ER lumen. This leads to the depletion of ER calcium pools, which has been termed store depletion. Store depletion leads to the dissociation of calcium ions from the EF-hand motif of the ER calcium sensor Stromal Interaction Molecule 1 (STIM1). This leads to a conformational change in STIM1, which helps it to interact with the plasma membrane (PM) at ER:PM junctions. At these ER:PM junctions, STIM1 binds to and activates a calcium channel known as Orai1 to form calcium-release activated calcium (CRAC) channels. Activation of Orai1 leads to calcium influx, known as store-operated calcium entry (SOCE). In addition to Orai1 and STIM1, the homologs of Orai1 and STIM1, such as Orai2/3 and STIM2, also play a crucial role in calcium homeostasis. The influx of calcium through the Orai channel activates a calcium current that has been termed the CRAC current. CRAC channels form multimers and cluster together in large macromolecular assemblies termed "puncta". How CRAC channels form puncta has been contentious since their discovery. In this review, we will outline the history of SOCE, the molecular players involved in this process, as well as the models that have been proposed to explain this critical mechanism in cellular physiology.

Proteomic Analyses of the G Protein-Coupled Estrogen Receptor GPER1 Reveal Constitutive Links to Endoplasmic Reticulum, Glycosylation, Trafficking, and Calcium Signaling

The G protein-coupled estrogen receptor 1 (GPER1) has been proposed to mediate rapid responses to the steroid hormone estrogen. However, despite a strong interest in its potential role in cancer, whether it is indeed activated by estrogen and how this works remain controversial. To provide new tools to address these questions, we set out to determine the interactome of exogenously expressed GPER1. The combination of two orthogonal methods, namely APEX2-mediated proximity labeling and immunoprecipitation followed by mass spectrometry, gave us high-confidence results for 73 novel potential GPER1 interactors. We found that this GPER1 interactome is not affected by estrogen, a result that mirrors the constitutive activity of GPER1 in a functional assay with a Rac1 sensor. We specifically validated several hits highlighted by a gene ontology analysis. We demonstrate that CLPTM1 interacts with GPER1 and that PRKCSH and GANAB, the regulatory and catalytic subunits of α-glucosidase II, respectively, associate with CLPTM1 and potentially indirectly with GPER1. An imbalance in CLPTM1 levels induces nuclear association of GPER1, as does the overexpression of PRKCSH. Moreover, we show that the Ca2+ sensor STIM1 interacts with GPER1 and that upon STIM1 overexpression and depletion of Ca2+ stores, GPER1 becomes more nuclear. Thus, these new GPER1 interactors establish interesting connections with membrane protein maturation, trafficking, and calcium signaling.

Molecular dynamics simulations reveal the hidden EF-hand of EF-SAM as a possible key thermal sensor for STIM1 activation by temperature

Intracellular calcium signaling is essential for many cellular processes, including store-operated Ca2+ entry (SOCE), which is initiated by stromal interaction molecule 1 (STIM1) detecting endoplasmic reticulum (ER) Ca2+ depletion. STIM1 is also activated by temperature independent of ER Ca2+ depletion. Here we provide evidence, from advanced molecular dynamics simulations, that EF-SAM may act as a true temperature sensor for STIM1, with the prompt and extended unfolding of the hidden EF-hand subdomain (hEF) even at slightly elevated temperatures, exposing a highly conserved hydrophobic Phe108. Our study also suggests an interplay between Ca2+ and temperature sensing, as both, the canonical EF-hand subdomain (cEF) and the hidden EF-hand subdomain (hEF), exhibit much higher thermal stability in the Ca2+-loaded form compared to the Ca2+-free form. The SAM domain, surprisingly, displays high thermal stability compared to the EF-hands and may act as a stabilizer for the latter. We propose a modular architecture for the EF-hand-SAM domain of STIM1 composed of a thermal sensor (hEF), a Ca2+ sensor (cEF), and a stabilizing domain (SAM). Our findings provide important insights into the mechanism of temperature-dependent regulation of STIM1, which has broad implications for understanding the role of temperature in cellular physiology.

Phosphatidylinositol 4,5-bisphosphate and calcium at ER-PM junctions - Complex interplay of simple messengers

Endoplasmic reticulum-plasma membrane contact sites (ER-PM MCS) are a specialised domain involved in the control of Ca²⁺ dynamics and various Ca²⁺-dependent cellular processes. Intracellular Ca²⁺ signals are broadly supported by Ca²⁺ release from intracellular Ca²⁺ channels such as inositol 1,4,5-trisphosphate receptors (IP₃Rs) and subsequent store-operated Ca²⁺ entry (SOCE) across the PM to replenish store content. IP₃Rs sit in close proximity to the PM where they can easily access newly synthesised IP₃, interact with binding partners such as actin, and localise adjacent to ER-PM MCS populated by the SOCE machinery, STIM1-2 and Orai1-3, to possibly form a locally regulated unit of Ca²⁺ influx. PtdIns(4,5)P₂ is a multiplex regulator of Ca²⁺ signalling at the ER-PM MCS interacting with multiple proteins at these junctions such as actin and STIM1, whilst also being consumed as a substrate for phospholipase C to produce IP₃ in response to extracellular stimuli. In this review, we consider the mechanisms regulating the synthesis and turnover of PtdIns(4,5)P₂ via the phosphoinositide cycle and its significance for sustained signalling at the ER-PM MCS. Furthermore, we highlight recent insights into the role of PtdIns(4,5)P₂ in the spatiotemporal organization of signalling at ER-PM junctions and raise outstanding questions on how this multi-faceted regulation occurs.

The SOAR of STIM1 interacts with plasma membrane lipids to form ER-PM contact sites

Depletion of Ca²⁺ from the endoplasmic reticulum (ER) causes the ER Ca²⁺ sensor STIM1 to form membrane contact sites (MCSs) with the plasma membrane (PM). At the ER-PM MCS, STIM1 binds to Orai channels to induce cellular Ca²⁺ entry. The prevailing view of this sequential process is that STIM1 interacts with the PM and with Orai1 using two separate modules: a C-terminal polybasic domain (PBD) for the interaction with PM phosphoinositides and the STIM-Orai activation region (SOAR) for the interaction with Orai channels. Here, using electron and fluorescence microscopy and protein-lipid interaction assays, we show that oligomerization of the SOAR promotes direct interaction with PM phosphoinositides to trap STIM1 at ER-PM MCSs. The interaction depends on a cluster of conserved lysine residues within the SOAR and is co-regulated by the STIM1 coil-coiled 1 and inactivation domains. Collectively, our findings uncover a molecular mechanism for formation and regulation of ER-PM MCSs by STIM1.

Progress on role of ion channels of cardiac fibroblasts in fibrosis

Cardiac fibrosis is defined as excessive deposition of extracellular matrix (ECM) in pathological conditions. Cardiac fibroblasts (CFs) activated by injury or inflammation differentiate into myofibroblasts (MFs) with secretory and contractile functions. In the fibrotic heart, MFs produce ECM which is composed mainly of collagen and is initially involved in maintaining tissue integrity. However, persistent fibrosis disrupts the coordination of excitatory contractile coupling, leading to systolic and diastolic dysfunction, and ultimately heart failure. Numerous studies have demonstrated that both voltage- and non-voltage-gated ion channels alter intracellular ion levels and cellular activity, contributing to myofibroblast proliferation, contraction, and secretory function. However, an effective treatment strategy for myocardial fibrosis has not been established. Therefore, this review describes the progress made in research related to transient receptor potential (TRP) channels, Piezo1, Ca²⁺ release-activated Ca²⁺ (CRAC) channels, voltage-gated Ca²⁺ channels (VGCCs), sodium channels, and potassium channels in myocardial fibroblasts with the aim of providing new ideas for treating myocardial fibrosis.

Nuanced Interactions between AKAP79 and STIM1 with Orai1 Ca2+ Channels at Endoplasmic Reticulum-Plasma Membrane Junctions Sustain NFAT Activation

A-kinase anchoring protein 79 (AKAP79) is a human scaffolding protein that organizes Ca²⁺/calmodulin-dependent protein phosphatase calcineurin, calmodulin, cAMP-dependent protein kinase, protein kinase C, and the transcription factor nuclear factor of activated T cells (NFAT1) into a signalosome at the plasma membrane. Upon Ca²⁺ store depletion, AKAP79 interacts with the N-terminus of STIM1-gated Orai1 Ca²⁺ channels, enabling Ca²⁺ nanodomains to stimulate calcineurin. Calcineurin then dephosphorylates and activates NFAT1, which then translocates to the nucleus. A fundamental question is how signalosomes maintain long-term signaling when key effectors are released and therefore removed beyond the reach of the activating signal. Here, we show that the AKAP79-Orai1 interaction is considerably more transient than that of STIM1-Orai1. Free AKAP79, with calcineurin and NFAT1 in tow, is able to replace rapidly AKAP79 devoid of NFAT1 on Orai1, in the presence of continuous Ca²⁺ entry. We also show that Ca²⁺ nanodomains near Orai1 channels activate almost the entire cytosolic pool of NFAT1. Recycling of inactive NFAT1 from the cytoplasm to AKAP79 in the plasma membrane, coupled with the relatively weak interaction between AKAP79 and Orai1, maintain excitation-transcription coupling. By measuring rates for AKAP79-NFAT interaction, we formulate a mathematical model that simulates NFAT dynamics at the plasma membrane.

LIPID transfer proteins regulate store-operated calcium entry via control of plasma membrane phosphoinositides

The ER-resident proteins STIM1 together with the plasma membrane (PM)-localized Orai1 channels constitute the molecular components of the store-operated Ca2+ entry (SOCE) pathway. Prepositioning of STIM1 to the peripheral ER close to the PM ensures its efficient interaction with Orai1 upon a decrease in the ER luminal Ca2+ concentration. The C-terminal polybasic domain of STIM1 has been identified as mediating the interaction with PM phosphoinositides and hence positions the molecule to ER-PM contact sites. Here we show that STIM1 requires PM phosphatidylinositol 4-phosphate (PI4P) for efficient PM interaction. Accordingly, oxysterol binding protein related proteins (ORPs) that work at ER-PM junctions and consume PI4P gradients exert important control over the Ca2+ entry process. These studies reveal an important connection between non-vesicular lipid transport at ER-PM contact sites and regulation of ER Ca2+store refilling.

Proteolytically Activated CRAC Effectors through Designed Intramolecular Inhibition

Highly regulated intracellular calcium entry affects numerous cellular physiological events. External regulation of intracellular calcium signaling presents a great opportunity for the artificial regulation of cellular activity. Calcium entry can be mediated by STIM proteins interacting with Orai calcium channels; therefore, the STIM1–Orai1 pair has become a tool for artificially modulating calcium entry. We report on an innovative genetically engineered protease-activated Orai activator called PACE. CAD self-dimerization and activation were inhibited with a coiled-coil forming peptide pair linked to CAD via a protease cleavage site. PACE generated sustained calcium entry after its activation with a reconstituted split protease. We also generated PACE, whose transcriptional activation of NFAT was triggered by PPV or TEV protease. Using PACE, we successfully activated the native NFAT signaling pathway and the production of cytokines in a T-cell line. PACE represents a useful tool for generating sustained calcium entry to initiate calcium-dependent protein translation. PACE provides a promising template for the construction of links between various protease activation pathways and calcium signaling.

Identification of a STIM1 Splicing Variant that Promotes Glioblastoma Growth

Deregulated store-operated calcium entry (SOCE) mediated by aberrant STIM1-ORAI1 signaling is closely implicated in cancer initiation and progression. Here the authors report the identification of an alternatively spliced variant of STIM1, designated STIM1β, that harbors an extra exon to encode 31 additional amino acids in the cytoplasmic domain. STIM1β, highly conserved in mammals, is aberrantly upregulated in glioma tissues to perturb Ca²⁺ signaling. At the molecular level, the 31-residue insertion destabilizes STIM1β by perturbing its cytosolic inhibitory domain and accelerating its activation kinetics to efficiently engage and gate ORAI calcium channels. Functionally, STIM1β depletion affects SOCE in glioblastoma cells, suppresses tumor cell proliferation and growth both in vitro and in vivo. Collectively, their study establishes a splicing variant-specific tumor-promoting role of STIM1β that can be potentially targeted for glioblastoma intervention.

Transcriptomes of electrophysiologically recorded Dbx1-derived respiratory neurons of the preBötzinger complex in neonatal mice

Breathing depends on interneurons in the preBötzinger complex (preBötC) derived from Dbx1-expressing precursors. Here we investigate whether rhythm- and pattern-generating functions reside in discrete classes of Dbx1 preBötC neurons. In a slice model of breathing with ~ 5 s cycle period, putatively rhythmogenic Type-1 Dbx1 preBötC neurons activate 100-300 ms prior to Type-2 neurons, putatively specialized for output pattern, and 300-500 ms prior to the inspiratory motor output. We sequenced Type-1 and Type-2 transcriptomes and identified differential expression of 123 genes including ionotropic receptors (Gria3, Gabra1) that may explain their preinspiratory activation profiles and Ca2+ signaling (Cracr2a, Sgk1) involved in inspiratory and sigh bursts. Surprisingly, neuropeptide receptors that influence breathing (e.g., µ-opioid and bombesin-like peptide receptors) were only sparsely expressed, which suggests that cognate peptides and opioid drugs exert their profound effects on a small fraction of the preBötC core. These data in the public domain help explain the neural origins of breathing.

Control of STIM and Orai function by post-translational modifications

Store-operated calcium entry (SOCE) is mediated by the endoplasmic reticulum (ER) Ca²⁺ sensors stromal interaction molecules (STIM1 and STIM2) and the plasma membrane Orai (Orai1, Orai2, Orai3) Ca²⁺ channels. Although primarily regulated by ER Ca²⁺ content, there have been numerous studies over the last 15 years demonstrating that all 5 proteins are also regulated through post-translational modification (PTM). Focusing primarily on phosphorylation, glycosylation and redox modification, this review focuses on how PTMs modulate the key events in SOCE; Ca²⁺ sensing, STIM translocation, Orai interaction and/or Orai1 activation.

Altered Expression of ORAI and STIM Isoforms in Activated Human Cardiac Fibroblasts

Cardiac fibrotization is a well-known process characteristic of many cardiac pathological conditions. The key element is excessive activation of cardiac fibroblasts, their transdifferentiation into myofibroblasts, increased production, and accumulation of extracellular matrix proteins, resulting in cardiac stiffness. The exact cellular mechanisms and molecular components involved in the process are not fully elucidated, but the SOCE mechanism could play an important role. Its key molecules are the molecular sensor of calcium in ER/SR - STIM and the highly selective calcium channels Orai located in the plasma membrane. This study aims to evaluate selected SOCE-associated genes in the activation of HCF cell culture by several known substances (phenylephrine, isoprenaline) that represent cardiovascular overload. After cell cultivation, cell medium was collected to measure the soluble collagen content. From the harvested cells, qRT-PCR was performed to determine the mRNA levels of the corresponding genes. The activation of cells was based on changes in the relative expression of collagen genes as well as the collagen content in the medium of the cell culture. We detected an increase in the expression of the Orai2 isoform, a change in the Orai1/Orai3 ratio and also an increase in the expression of the STIM2 isoform. These results suggest an increased activation of the SOCE mechanism under stress conditions of fibroblasts, which supports the hypothesis of fibroblast activation in pathological processes by altering calcium homeostasis through the SOCE mechanism.

Conformational dynamics of auto-inhibition in the ER calcium sensor STIM1

The dimeric ER Ca²⁺ sensor STIM1 controls store-operated Ca²⁺ entry (SOCE) through the regulated binding of its CRAC activation domain (CAD) to Orai channels in the plasma membrane. In resting cells, the STIM1 CC1 domain interacts with CAD to suppress SOCE, but the structural basis of this interaction is unclear. Using single-molecule Förster resonance energy transfer (smFRET) and protein crosslinking approaches, we show that CC1 interacts dynamically with CAD in a domain-swapped configuration with an orientation predicted to sequester its Orai-binding region adjacent to the ER membrane. Following ER Ca²⁺ depletion and release from CAD, cysteine crosslinking indicates that the two CC1 domains become closely paired along their entire length in the active Orai-bound state. These findings provide a structural basis for the dual roles of CC1: sequestering CAD to suppress SOCE in resting cells and propelling it toward the plasma membrane to activate Orai and SOCE after store depletion.

Alteration of STIM1/Orai1-Mediated SOCE in Skeletal Muscle: Impact in Genetic Muscle Diseases and Beyond

Intracellular Ca²⁺ ions represent a signaling mediator that plays a critical role in regulating different muscular cellular processes. Ca²⁺ homeostasis preservation is essential for maintaining skeletal muscle structure and function. Store-operated Ca²⁺ entry (SOCE), a Ca²⁺-entry process activated by depletion of intracellular stores contributing to the regulation of various function in many cell types, is pivotal to ensure a proper Ca²⁺ homeostasis in muscle fibers. It is coordinated by STIM1, the main Ca²⁺ sensor located in the sarcoplasmic reticulum, and ORAI1 protein, a Ca²⁺-permeable channel located on transverse tubules. It is commonly accepted that Ca²⁺ entry via SOCE has the crucial role in short- and long-term muscle function, regulating and adapting many cellular processes including muscle contractility, postnatal development, myofiber phenotype and plasticity. Lack or mutations of STIM1 and/or Orai1 and the consequent SOCE alteration have been associated with serious consequences for muscle function. Importantly, evidence suggests that SOCE alteration can trigger a change of intracellular Ca²⁺ signaling in skeletal muscle, participating in the pathogenesis of different progressive muscle diseases such as tubular aggregate myopathy, muscular dystrophy, cachexia, and sarcopenia. This review provides a brief overview of the molecular mechanisms underlying STIM1/Orai1-dependent SOCE in skeletal muscle, focusing on how SOCE alteration could contribute to skeletal muscle wasting disorders and on how SOCE components could represent pharmacological targets with high therapeutic potential.

Defects in the STIM1 SOARα2 domain affect multiple steps in the CRAC channel activation cascade

The calcium release-activated calcium (CRAC) channel consists of STIM1, a Ca2+ sensor in the endoplasmic reticulum (ER), and Orai1, the Ca2+ ion channel in the plasma membrane. Ca2+ store depletion triggers conformational changes and oligomerization of STIM1 proteins and their direct interaction with Orai1. Structural alterations include the transition of STIM1 C-terminus from a folded to an extended conformation thereby exposing CAD (CRAC activation domain)/SOAR (STIM1-Orai1 activation region) for coupling to Orai1. In this study, we discovered that different point mutations of F394 in the small alpha helical segment (STIM1 α2) within the CAD/SOAR apex entail a rich plethora of effects on diverse STIM1 activation steps. An alanine substitution (STIM1 F394A) destabilized the STIM1 quiescent state, as evident from its constitutive activity. Single point mutation to hydrophilic, charged amino acids (STIM1 F394D, STIM1 F394K) impaired STIM1 homomerization and subsequent Orai1 activation. MD simulations suggest that their loss of homomerization may arise from altered formation of the CC1α1-SOAR/CAD interface and potential electrostatic interactions with lipid headgroups in the ER membrane. Consistent with these findings, we provide experimental evidence that the perturbing effects of F394D depend on the distance of the apex from the ER membrane. Taken together, our results suggest that the CAD/SOAR apex is in the immediate vicinity of the ER membrane in the STIM1 quiescent state and that different mutations therein can impact the STIM1/Orai1 activation cascade in various manners. Legend: Upon intracellular Ca2+ store depletion of the endoplasmic reticulum (ER), Ca2+ dissociates from STIM1. As a result, STIM1 adopts an elongated conformation and elicits Ca2+ influx from the extracellular matrix (EM) into the cell due to binding to and activation of Ca2+-selective Orai1 channels (left). The effects of three point mutations within the SOARα2 domain highlight the manifold roles of this region in the STIM1/Orai1 activation cascade: STIM1 F394A is active irrespective of the intracellular ER Ca2+ store level, but activates Orai1 channels to a reduced extent (middle). On the other hand, STIM1 F394D/K cannot adopt an elongated conformation upon Ca2+ store-depletion due to altered formation of the CC1α1-SOAR/CAD interface and/or electrostatic interaction of the respective side-chain charge with corresponding opposite charges on lipid headgroups in the ER membrane (right).

The interplay between mitochondria and store-operated Ca2+ entry: Emerging insights into cardiac diseases

Store‐operated Ca²⁺ entry (SOCE) machinery, including Orai channels, TRPCs, and STIM1, is key to cellular calcium homeostasis. The following characteristics of mitochondria are involved in the physiological and pathological regulation of cells: mitochondria mediate calcium uptake through calcium uniporters; mitochondria are regulated by mitochondrial dynamic related proteins (OPA1, MFN1/2, and DRP1) and form mitochondrial networks through continuous fission and fusion; mitochondria supply NADH to the electron transport chain through the Krebs cycle to produce ATP; under stress, mitochondria will produce excessive reactive oxygen species to regulate mitochondria‐endoplasmic reticulum interactions and the related signalling pathways. Both SOCE and mitochondria play critical roles in mediating cardiac hypertrophy, diabetic cardiomyopathy, and cardiac ischaemia‐reperfusion injury. All the mitochondrial characteristics mentioned above are determinants of SOCE activity, and vice versa. Ca²⁺ signalling dictates the reciprocal regulation between mitochondria and SOCE under the specific pathological conditions of cardiomyocytes. The coupling of mitochondria and SOCE is essential for various pathophysiological processes in the heart. Herein, we review the research focussing on the reciprocal regulation between mitochondria and SOCE and provide potential interplay patterns in cardiac diseases.

Store-operated Ca2+ entry as a key oncogenic Ca2+ signaling driving tumor invasion-metastasis cascade and its translational potential

Tumor metastasis is the primary cause of treatment failure and cancer-related deaths. Store-operated Ca²⁺ entry (SOCE), which is mediated by stromal interaction molecules (STIM) and ORAI proteins, has been implicated in the tumor invasion-metastasis cascade. Epithelial-mesenchymal transition (EMT) is a cellular program that enables tumor cells to acquire the capacities needed for migration and invasion and the formation of distal metastases. Tumor-associated angiogenesis contributes to metastasis because aberrantly developed vessels offer a path for tumor cell dissemination as well as supply sufficient nutrients for the metastatic colony to develop into metastasis. Recently, increasing evidence has indicated that SOCE alterations actively participate in the multi-step process of tumor metastasis. In addition, the dysregulated expression of STIM/ORAI has been reported to be a predictor of poor prognosis. Herein, we review the latest advances about the critical role of SOCE in the tumor metastasis cascade and the underlying regulatory mechanisms. We emphasize the contributions of SOCE to the EMT program, tumor cell migration and invasion, and angiogenesis. We further discuss the possibility of modulating SOCE or intervening in the downstream signaling pathways as a feasible targeting therapy for cancer treatment.

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.

The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells

The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca²⁺ signals and Ca²⁺ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca²⁺ signal on arterial contractility depends on the type of Ca²⁺ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca²⁺ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca²⁺ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca²⁺ signals have already been confirmed, while further investigation is needed for other Ca²⁺ signals. This article focuses on endothelial and smooth muscle Ca²⁺ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca²⁺ entry pathways at the plasma membrane, Ca²⁺ release signals from the intracellular stores, the functional and physiological relevance of Ca²⁺ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca²⁺ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.

STIM Proteins: An Ever-Expanding Family

Stromal interaction molecules (STIM) are a distinct class of ubiquitously expressed single-pass transmembrane proteins in the endoplasmic reticulum (ER) membrane. Together with Orai ion channels in the plasma membrane (PM), they form the molecular basis of the calcium release-activated calcium (CRAC) channel. An intracellular signaling pathway known as store-operated calcium entry (SOCE) is critically dependent on the CRAC channel. The SOCE pathway is activated by the ligand-induced depletion of the ER calcium store. STIM proteins, acting as calcium sensors, subsequently sense this depletion and activate Orai ion channels via direct physical interaction to allow the influx of calcium ions for store refilling and downstream signaling processes. This review article is dedicated to the latest advances in the field of STIM proteins. New results of ongoing investigations based on the recently published functional data as well as structural data from nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations are reported and complemented with a discussion of the latest developments in the research of STIM protein isoforms and their differential functions in regulating SOCE.

Molecular Choreography and Structure of Ca2+ Release-Activated Ca2+ (CRAC) and KCa2+ Channels and Their Relevance in Disease with Special Focus on Cancer

Ca2+ ions play a variety of roles in the human body as well as within a single cell. Cellular Ca2+ signal transduction processes are governed by Ca2+ sensing and Ca2+ transporting proteins. In this review, we discuss the Ca2+ and the Ca2+-sensing ion channels with particular focus on the structure-function relationship of the Ca2+ release-activated Ca2+ (CRAC) ion channel, the Ca2+-activated K+ (KCa2+) ion channels, and their modulation via other cellular components. Moreover, we highlight their roles in healthy signaling processes as well as in disease with a special focus on cancer. As KCa2+ channels are activated via elevations of intracellular Ca2+ levels, we summarize the current knowledge on the action mechanisms of the interplay of CRAC and KCa2+ ion channels and their role in cancer cell development.

Orai1 Channels Are Essential for Amplification of Glutamate-Evoked Ca2+ Signals in Dendritic Spines to Regulate Working and Associative Memory

Store-operated Orai1 calcium channels function as highly Ca²⁺-selective ion channels and are broadly expressed in many tissues including the central nervous system, but their contributions to cognitive processing are largely unknown. Here, we report that many measures of synaptic, cellular, and behavioral models of learning are markedly attenuated in mice lacking Orai1 in forebrain excitatory neurons. Results with focal glutamate uncaging in hippocampal neurons support an essential role of Orai1 channels in amplifying NMDA-receptor-induced dendritic Ca²⁺ transients that drive activity-dependent spine morphogenesis and long-term potentiation at Schaffer collateral-CA1 synapses. Consistent with these signaling roles, mice lacking Orai1 in pyramidal neurons (but not interneurons) exhibit striking deficits in working and associative memory tasks. These findings identify Orai1 channels as essential regulators of dendritic spine Ca²⁺ signaling, synaptic plasticity, and cognition.

Mechanism of STIM activation

Store-operated calcium entry (SOCE) through Orai ion channels is an intracellular signaling pathway that is initiated by ligand-induced depletion of calcium from the endoplasmic reticulum (ER) store. The molecular link between SOCE and ER store depletion is thereby provided by a distinct class of single pass ER transmembrane proteins known as stromal interaction molecules (STIM). STIM proteins are equipped with a precise N-terminal calcium sensing domain that enables them to react to changes of the ER luminal calcium concentration. Additionally, a C-terminal coiled-coil domain permits relaying of signals to Orai ion channels via direct physical interaction. In this review, we provide a brief introduction to STIM proteins with a focus on structure and function and give an overview of recent developments in the field of STIM research.

Store-Operated Ca2+ Channels: Mechanism, Function, Pharmacology, and Therapeutic Targets

Calcium (Ca²⁺) release-activated Ca²⁺ (CRAC) channels are a major route for Ca²⁺ entry in eukaryotic cells. These channels are store operated, opening when the endoplasmic reticulum (ER) is depleted of Ca²⁺, and are composed of the ER Ca²⁺ sensor protein STIM and the pore-forming plasma membrane subunit Orai. Recent years have heralded major strides in our understanding of the structure, gating, and function of the channels. Loss-of-function and gain-of-function mutants combined with RNAi knockdown strategies have revealed important roles for the channel in numerous human diseases, making the channel a clinically relevant target. Drugs targeting the channels generally lack specificity or exhibit poor efficacy in animal models. However, the landscape is changing, and CRAC channel blockers are now entering clinical trials. Here, we describe the key molecular and biological features of CRAC channels, consider various diseases associated with aberrant channel activity, and discuss targeting of the channels from a therapeutic perspective.

Structural Mechanisms of Store-Operated and Mitochondrial Calcium Regulation: Initiation Points for Drug Discovery

Calcium (Ca2+) is a universal signaling ion that is essential for the life and death processes of all eukaryotes. In humans, numerous cell stimulation pathways lead to the mobilization of sarco/endoplasmic reticulum (S/ER) stored Ca2+, resulting in the propagation of Ca2+ signals through the activation of processes, such as store-operated Ca2+ entry (SOCE). SOCE provides a sustained Ca2+ entry into the cytosol; moreover, the uptake of SOCE-mediated Ca2+ by mitochondria can shape cytosolic Ca2+ signals, function as a feedback signal for the SOCE molecular machinery, and drive numerous mitochondrial processes, including adenosine triphosphate (ATP) production and distinct cell death pathways. In recent years, tremendous progress has been made in identifying the proteins mediating these signaling pathways and elucidating molecular structures, invaluable for understanding the underlying mechanisms of function. Nevertheless, there remains a disconnect between using this accumulating protein structural knowledge and the design of new research tools and therapies. In this review, we provide an overview of the Ca2+ signaling pathways that are involved in mediating S/ER stored Ca2+ release, SOCE, and mitochondrial Ca2+ uptake, as well as pinpoint multiple levels of crosstalk between these pathways. Further, we highlight the significant protein structures elucidated in recent years controlling these Ca2+ signaling pathways. Finally, we describe a simple strategy that aimed at applying the protein structural data to initiating drug design.

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.

STIM1 interacts with termini of Orai channels in a sequential manner

Store-operated Ca2+ entry (SOCE) is critical for numerous Ca2+-related processes. The activation of SOCE requires engagement between stromal interaction molecule 1 (STIM1) molecules on the endoplasmic reticulum and Ca2+ release-activated channel (CRAC) Orai on the plasma membrane. However, the molecular details of their interactions remain elusive. Here, we analyzed STIM1-Orai interactions using synthetic peptides derived from the N- and C-termini of Orai channels (Orai-NT and Orai-CT, respectively) and purified fragments of STIM1. The binding of STIM1 to Orai-NT is hydrophilic based, whereas binding to the Orai-CT is mostly hydrophobic. STIM1 decreases its affinity for Orai-CT when Orai-NT is present, supporting a stepwise interaction. Orai3-CT exhibits stronger binding to STIM1 than Orai1-CT, largely due to the shortness of one helical turn. The role of newly identified residues was confirmed by co-immunoprecipitation and Ca2+ imaging using full-length molecules. Our results provide important insight into CRAC gating by STIM1.

Ion channels in capillary endothelium

Vascular beds are anatomically and functionally compartmentalized into arteries, capillaries, and veins. The bulk of the vasculature consists of the dense, anastomosing capillary network, composed of capillary endothelial cells (cECs) that are intimately associated with the parenchyma. Despite their abundance, the ion channel expression and function and Ca²⁺ signaling behaviors of capillaries have only recently begun to be explored in detail. Here, we discuss the established and emerging roles of ion channels and Ca²⁺ signaling in cECs. By mining a publicly available RNA-seq dataset, we outline the wide variety of ion channel genes that are expressed in these cells, which potentially imbue capillaries with a broad range of sensing and signal transduction capabilities. We also underscore subtle but critical differences between cEC and arteriolar EC ion channel expression that likely underlie key functional differences in ECs at these different levels of the vascular tree. We focus our discussion on the cerebral vasculature, but the findings and principles being elucidated in this area likely generalize to other vascular beds.

Calcium Signaling in Glioma Cells: The Role of Nucleotide Receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y₁ and P2Y₁₂ nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

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.

Phosphoproteomics reveals conserved exercise-stimulated signaling and AMPK regulation of store-operated calcium entry

Exercise stimulates cellular and physiological adaptations that are associated with widespread health benefits. To uncover conserved protein phosphorylation events underlying this adaptive response, we performed mass spectrometry-based phosphoproteomic analyses of skeletal muscle from two widely used rodent models: treadmill running in mice and in situ muscle contraction in rats. We overlaid these phosphoproteomic signatures with cycling in humans to identify common cross-species phosphosite responses, as well as unique model-specific regulation. We identified > 22,000 phosphosites, revealing orthologous protein phosphorylation and overlapping signaling pathways regulated by exercise. This included two conserved phosphosites on stromal interaction molecule 1 (STIM1), which we validate as AMPK substrates. Furthermore, we demonstrate that AMPK-mediated phosphorylation of STIM1 negatively regulates store-operated calcium entry, and this is beneficial for exercise in Drosophila. This integrated cross-species resource of exercise-regulated signaling in human, mouse, and rat skeletal muscle has uncovered conserved networks and unraveled crosstalk between AMPK and intracellular calcium flux.

Changes in STIM Isoforms Expression and Gender-Specific Alterations in Orai Expression in Human Heart Failure

Store-operated calcium entry (SOCE) is one of regulatory mechanisms which regulates Ca2+ cycling in the heart. SOCE alterations in pathological conditions contribute to progression of heart failure and cardiac hypertrophy by multiple signaling pathways such as Cn/NFAT and CaMKII/MEF2. Several components mediating SOCE have been identified, such as STIM and Orai. Different isoforms of both Orai and STIM have been detected in animal studies, exhibiting distinct functional properties. This study is focused on the analysis of STIM and Orai isoforms expression in the end-stage human failing myocardium. Left ventricle samples isolated from 43 explanted hearts from patients undergoing heart transplant and from 5 healthy donor hearts were used to determine the mRNA levels of Orai1, Orai2 and Orai3, STIM1, STIM2 and STIM2.1 by qRT-PCR. The expression was further analyzed for connection with gender, related co-morbidities, pathoetiology, clinical data and biochemical parameters. We show that Orai1 expression is decreased by 30 % in failing myocardium, even though we detected no significant changes in expression of Orai2 or Orai3. Interestingly, this decrease in Orai1 was gender-specific and was present only in men, with no change in women. The ratio Orai1/Orai3 was significantly lower in males as well. The novel STIM2.1 isoform was detected both in healthy and failing human myocardium. In the end-stage heart failure, the expression of STIM2.1 was significantly decreased. The lower ratio of STIM2.1/STIM2 in failing hearts indicates a switch from SOCE-inhibiting STIM2.1 isoform to stimulatory STIM2.2. STIM1 mRNA levels were not significantly changed. These observed alterations in Orai and STIM expression were independent of functional heart parameters, clinical or biochemical patient characteristics. These results provide detailed insight into the alterations of SOCE regulation in human failing myocardium. Gender-specific change in Orai1 expression might represent a possible mechanism of cardioprotective effects of estrogens. The switch from STIM2.1 to STIM2.2 indicates an amplification of SOCE and could contribute to the hypertrophy development in the filing heart.

Septins organize endoplasmic reticulum-plasma membrane junctions for STIM1-ORAI1 calcium signalling

ORAI1 Ca2+ channels in the plasma membrane (PM) are gated by STIM1 at endoplasmic reticulum (ER)-PM junctions to effect store-dependent Ca2+ entry into cells, but little is known about how local STIM-ORAI signalling at junctions is coordinated with overall cellular architecture. Filamentous septins can specify cytoskeletal rearrangements and have been found recently to modulate STIM-ORAI signalling. Here we show by super-resolution imaging of ORAI1, STIM1, and septin 4 in living cells that septins facilitate Ca2+ signalling indirectly. Septin 4 does not colocalize preferentially with ORAI1 in resting or stimulated cells, assemble stably at ER-PM junctions, or specify a boundary that directs or confines ORAI1 to junctions. Rather, ORAI1 is recruited to junctions solely through interaction with STIM proteins, while septins regulate the number of ER-PM junctions and enhance STIM1-ORAI1 interactions within junctions. Thus septins communicate with STIM1 and ORAI1 through protein or lipid intermediaries, and are favorably positioned to coordinate Ca2+ signalling with rearrangements in cellular architecture.

STIM calcium sensing and conformational change

The control of calcium influx at the plasma membrane by endoplasmic reticulum (ER) calcium stores, a process common to invertebrates and vertebrates, is central to physiological calcium signalling and cellular calcium balance. Stromal interaction molecule 1 (STIM1) is a calcium sensor and regulatory protein localized to the ER. ORAI1 is a calcium channel in the plasma membrane (PM). In outline, STIM1 senses an ER-luminal calcium decrease, relocalizes to ER-PM junctions, and recruits and gates ORAI1 channels. Recent work, reviewed here, has offered detailed insight into the process of sensing and communicating ER calcium-store depletion, and particularly into the STIM1 conformational change that is the basis for communication between the ER and the PM.

STIM1-dependent membrane insertion of heteromeric TRPC1-TRPC4 channels in response to muscarinic receptor stimulation

Muscarinic receptor stimulation results in activation of nonselective cation (NSC) channels in guinea pig adrenal medullary (AM) cells. The biophysical and pharmacological properties of the NSC channel suggest the involvement of heteromeric channels of TRPC1 with TRPC4 or TRPC5. This possibility was explored in PC12 cells and guinea pig AM cells. Proximity ligation assay (PLA) revealed that when exogenously expressed in PC12 cells, TRPC1 forms a heteromeric channel with TRPC4, but not with TRPC5, in a STIM1-dependent manner. The heteromeric TRPC1-TRPC4 channel was also observed in AM cells and trafficked to the cell periphery in response to muscarine stimulation. To explore whether heteromeric channels are inserted into the cell membrane, tags were attached to the extracellular domains of TRPC1 and TRPC4. PLA products developed between the tags in cells stimulated by muscarine, but not in resting cells, indicating that muscarinic stimulation results in the membrane insertion of channels. This membrane insertion required expression of full-length STIM1. We conclude that muscarinic receptor stimulation results in the insertion of heteromeric TRPC1-TRPC4 channels into the cell membrane in PC12 cells and guinea pig AM cells.

Tmem178 negatively regulates store-operated calcium entry in myeloid cells via association with STIM1

Store-operated calcium entry (SOCE) modulates cytosolic calcium in multiple cells. Endoplasmic reticulum (ER)-localized STIM1 and plasma membrane (PM)-localized ORAI1 are two main components of SOCE. STIM1:ORAI1 association requires STIM1 oligomerization, its re-distribution to ER-PM junctions, and puncta formation. However, little is known about the negative regulation of these steps to prevent calcium overload. Here, we identified Tmem178 as a negative modulator of STIM1 puncta formation in myeloid cells. Using site-directed mutagenesis, co-immunoprecipitation assays and FRET imaging, we determined that Tmem178:STIM1 association occurs via their transmembrane motifs. Mutants that increase Tmem178:STIM1 association reduce STIM1 puncta formation, SOCE activation, impair inflammatory cytokine production in macrophages and osteoclastogenesis. Mutants that reduce Tmem178:STIM1 association reverse these effects. Furthermore, exposure to plasma from arthritic patients decreases Tmem178 expression, enhances SOCE activation and cytoplasmic calcium. In conclusion, Tmem178 modulates the rate-limiting step of STIM1 puncta formation and therefore controls SOCE in inflammatory conditions.

Coiled-Coil Formation Conveys a STIM1 Signal from ER Lumen to Cytoplasm

STIM1 and STIM2 are endoplasmic reticulum (ER) membrane proteins that sense decreases in ER-luminal free Ca²⁺ and, through a conformational change in the STIM cytoplasmic domain, control gating of the plasma membrane Ca²⁺ channel ORAI1. To determine how STIM1 conveys a signal from the ER lumen to the cytoplasm, we studied the Ca²⁺-dependent conformational change of engineered STIM1 proteins in isolated ER membranes and, in parallel, physiological activation of these proteins in cells. We find that conserved "sentinel" features of the CC1 region help to prevent activation while Ca²⁺ is bound to STIM ER-luminal domains. Reduced ER-luminal Ca²⁺ drives a concerted conformational change, in which STIM luminal domains rearrange and the STIM transmembrane helices and initial parts of the CC1 regions pair in an extended coiled coil. This intradimer rearrangement overcomes the relatively weak CC1-SOAR/CAD interactions that hold STIM in an inactive conformation, releasing the SOAR/CAD domain to activate ORAI channels.

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.

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.

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.

Calcium sensing by the STIM1 ER-luminal domain

Stromal interaction molecule 1 (STIM1) monitors ER-luminal Ca2+ levels to maintain cellular Ca2+ balance and to support Ca2+ signalling. The prevailing view has been that STIM1 senses reduced ER Ca2+ through dissociation of bound Ca2+ from a single EF-hand site, which triggers a dramatic loss of secondary structure and dimerization of the STIM1 luminal domain. Here we find that the STIM1 luminal domain has 5-6 Ca2+-binding sites, that binding at these sites is energetically coupled to binding at the EF-hand site, and that Ca2+ dissociation controls a switch to a second structured conformation of the luminal domain rather than protein unfolding. Importantly, the other luminal-domain Ca2+-binding sites interact with the EF-hand site to control physiological activation of STIM1 in cells. These findings fundamentally revise our understanding of physiological Ca2+ sensing by STIM1, and highlight molecular mechanisms that govern the Ca2+ threshold for activation and the steep Ca2+ concentration dependence.

The 2β Splice Variation Alters the Structure and Function of the Stromal Interaction Molecule Coiled-Coil Domains

Stromal interaction molecule (STIM)-1 and -2 regulate agonist-induced and basal cytosolic calcium (Ca²⁺) levels after oligomerization and translocation to endoplasmic reticulum (ER)-plasma membrane (PM) junctions. At these junctions, the STIM cytosolic coiled-coil (CC) domains couple to PM Orai1 proteins and gate these Ca²⁺ release-activated Ca²⁺ (CRAC) channels, which facilitate store-operated Ca²⁺ entry (SOCE). Unlike STIM1 and STIM2, which are SOCE activators, the STIM2β splice variant contains an 8-residue insert located within the conserved CCs which inhibits SOCE. It remains unclear if the 2β insert further depotentiates weak STIM2 coupling to Orai1 or independently causes structural perturbations which prevent SOCE. Here, we use far-UV circular dichroism, light scattering, exposed hydrophobicity analysis, solution small angle X-ray scattering, and a chimeric STIM1/STIM2β functional assessment to provide insights into the molecular mechanism by which the 2β insert precludes SOCE activation. We find that the 2β insert reduces the overall α-helicity and enhances the exposed hydrophobicity of the STIM2 CC domains in the absence of a global conformational change. Remarkably, incorporation of the 2β insert into the STIM1 context not only affects the secondary structure and hydrophobicity as observed for STIM2, but also eliminates the more robust SOCE response mediated by STIM1. Collectively, our data show that the 2β insert directly precludes Orai1 channel activation by inducing structural perturbations in the STIM CC region.

With the greatest care, stromal interaction molecule (STIM) proteins verify what skeletal muscle is doing

Skeletal muscle contracts or relaxes to maintain the body position and locomotion. For the contraction and relaxation of skeletal muscle, Ca²⁺ in the cytosol of skeletal muscle fibers acts as a switch to turn on and off a series of contractile proteins. The cytosolic Ca²⁺ level in skeletal muscle fibers is governed mainly by movements of Ca²⁺ between the cytosol and the sarcoplasmic reticulum (SR). Store-operated Ca²⁺ entry (SOCE), a Ca²⁺ entryway from the extracellular space to the cytosol, has gained a significant amount of attention from muscle physiologists. Orai1 and stromal interaction molecule 1 (STIM1) are the main protein identities of SOCE. This mini-review focuses on the roles of STIM proteins and SOCE in the physiological and pathophysiological functions of skeletal muscle and in their correlations with recently identified proteins, as well as historical proteins that are known to mediate skeletal muscle function.

A charge-sensing region in the stromal interaction molecule 1 luminal domain confers stabilization-mediated inhibition of SOCE in response to S-nitrosylation

Store-operated Ca2+ entry (SOCE) is a major Ca2+ signaling pathway facilitating extracellular Ca2+ influx in response to the initial release of intracellular endo/sarcoplasmic reticulum (ER/SR) Ca2+ stores. Stromal interaction molecule 1 (STIM1) is the Ca2+ sensor that activates SOCE following ER/SR Ca2+ depletion. The EF-hand and the adjacent sterile α-motif (EFSAM) domains of STIM1 are essential for detecting changes in luminal Ca2+ concentrations. Low ER Ca2+ levels trigger STIM1 destabilization and oligomerization, culminating in the opening of Orai1-composed Ca2+ channels on the plasma membrane. NO-mediated S-nitrosylation of cysteine thiols regulates myriad protein functions, but its effects on the structural mechanisms that regulate SOCE are unclear. Here, we demonstrate that S-nitrosylation of Cys49 and Cys56 in STIM1 enhances the thermodynamic stability of its luminal domain, resulting in suppressed hydrophobic exposure and diminished Ca2+ depletion-dependent oligomerization. Using solution NMR spectroscopy, we pinpointed a structural mechanism for STIM1 stabilization driven by complementary charge interactions between an electropositive patch on the core EFSAM domain and the S-nitrosylated nonconserved region of STIM1. Finally, using live cells, we found that the enhanced luminal domain stability conferred by either Cys49 and Cys56S-nitrosylation or incorporation of negatively charged residues into the EFSAM electropositive patch in the full-length STIM1 context significantly suppresses SOCE. Collectively, our results suggest that S-nitrosylation of STIM1 inhibits SOCE by interacting with an electropositive patch on the EFSAM core, which modulates the thermodynamic stability of the STIM1 luminal domain.