Pore opening mechanism of CRAC channels

Calcium Signaling in Airway Epithelial Cells: Current Understanding and Implications for Inflammatory Airway Disease

Airway epithelial cells play an indispensable role in protecting the lung from inhaled pathogens and allergens by releasing an array of mediators that orchestrate inflammatory and immune responses when confronted with harmful environmental triggers. While this process is undoubtedly important for containing the effects of various harmful insults, dysregulation of the inflammatory response can cause lung diseases including asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis. A key cellular mechanism that underlies the inflammatory responses in the airway is calcium signaling, which stimulates the production and release of chemokines, cytokines, and prostaglandins from the airway epithelium. In this review, we discuss the role of major Ca2+ signaling pathways found in airway epithelial cells and their contributions to airway inflammation, mucociliary clearance, and surfactant production. We highlight the importance of store-operated Ca2+ entry as a major signaling hub in these processes and discuss therapeutic implications of targeting Ca2+ signaling for airway inflammation.

Synthetic Biology Meets Ca2+ Release-Activated Ca2+ Channel-Dependent Immunomodulation

Many essential biological processes are triggered by the proximity of molecules. Meanwhile, diverse approaches in synthetic biology, such as new biological parts or engineered cells, have opened up avenues to precisely control the proximity of molecules and eventually downstream signaling processes. This also applies to a main Ca2+ entry pathway into the cell, the so-called Ca2+ release-activated Ca2+ (CRAC) channel. CRAC channels are among other channels are essential in the immune response and are activated by receptor-ligand binding at the cell membrane. The latter initiates a signaling cascade within the cell, which finally triggers the coupling of the two key molecular components of the CRAC channel, namely the stromal interaction molecule, STIM, in the ER membrane and the plasma membrane Ca2+ ion channel, Orai. Ca2+ entry, established via STIM/Orai coupling, is essential for various immune cell functions, including cytokine release, proliferation, and cytotoxicity. In this review, we summarize the tools of synthetic biology that have been used so far to achieve precise control over the CRAC channel pathway and thus over downstream signaling events related to the immune response.

Effects of Ion-Transporting Proteins on the Digestive System Under Hypoxia

Hypoxia refers to a state of oxygen limitation, which mainly mediates pathological processes in the human body and participates in the regulation of normal physiological processes. In the hypoxic environment, the main regulator of human body homeostasis is the hypoxia-inducible factor family (HIF). HIF can regulate the expression of many hypoxia-induced genes and then participate in various physiological and pathological processes of the human body. Ion-transporting proteins are extremely important types of proteins. Ion-transporting proteins are distributed on cell membranes or organelles and strictly control the inflow or outflow of ions in cells or organelles. Changes in ions in cells are often closely related to extensive physiological and pathological processes in the human body. Numerous studies have confirmed that hypoxia and its regulatory factors can regulate the transcription and expression of ion-transporting protein-related genes. Under hypoxic stress, the regulation and interaction of ion-transporting proteins by hypoxia often leads to diseases of various human systems and even tumors. Using ion-transporting proteins and hypoxia as targets to explore the mechanism of digestive system diseases and targeted therapy is expected to become a new breakthrough point.

TRPC3 channel gating by lipids requires localization at the ER/PM junctions defined by STIM1

TRPC3, a member of the transient receptor potential (TRP) superfamily of cation channels, is a lipid-regulated, Ca²⁺-permeable channel that mediates essential components of the receptor evoked Ca²⁺ signal. The modes and mechanisms by which lipids regulate TRPC3 and other members of the TRPC channel family are not well understood. Here, we report that PI(4,5)P₂ regulates TRPC3 in three independent modes. PLC-dependent hydrolysis generates diacylglycerol (DAG) that interacts with lipid-binding site 2 in the channel pore. PI(4,5)P₂ interacts with lipid site 1 to inhibit TRPC3 opening and regulate access of DAG to the pore lipid site 2. PI(4,5)P₂ is required for regulating pore ionic selectivity by receptor stimulation. Notably, the activation and regulation of TRPC3 by PI(4,5)P₂ require recruitment of TRPC3 to the ER/PM junctions at a PI(4,5)P₂-rich domain. Accordingly, we identified an FFAT site at the TRPC3 N-terminal loop within the linker helices that envelope the C-terminus pole helix. The FFAT site interacts with the ER-resident VAPB to recruit TRPC3 to the ER/PM junctions and control its receptor-mediated activation. The TRPC3’s lipid interacting sites are fully conserved in TRPC6 and TRPC7 and in part in other TRPC channels. These findings inform on multiple modes of regulation of ion channels by lipids that may be relevant to diseases affected by aberrant TRPC channel functions.

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.

Ca2+ signalling is critical for autoantibody-induced blistering of human epidermis in pemphigus

BACKGROUND

Pemphigus is a severe bullous autoimmune skin disease. Pemphigus foliaceus (PF) is characterized by antidesmoglein (Dsg) 1 IgG causing epidermal blistering; mucosal pemphigus vulgaris (mPV) by anti-Dsg3 IgG inducing erosions in the mucosa; and mucocutaneous pemphigus vulgaris (PV) by affecting both, with autoantibodies targeting Dsg1 and Dsg3.

OBJECTIVES

To characterize the Ca²⁺ flux pathway and delineate its importance in pemphigus pathogenesis and clinical phenotypes caused by different antibody profiles.

METHODS

Immunoprecipitation, Ca²⁺ flux analysis, Western blotting, immunofluorescence staining, dissociation assays and a human skin ex vivo model were used.

RESULTS

PV IgG and PF IgG, but neither Dsg3-specific monoclonal antibody (AK23) nor mPV IgG, caused Ca²⁺ influx in primary human keratinocytes. Phosphatidylinositol 4-kinase α interacts with Dsg1 but not with Dsg3. Its downstream target - phospholipase-C-γ1 (PLC) - was activated by PV IgG and PF IgG but not AK23 or mPV IgG. PLC releases inositol 1,4,5-trisphosphate (IP3) causing IP3 receptor (IP3R) activation and Ca²⁺ flux from the endoplasmic reticulum into the cytosol, which stimulates Ca²⁺ release-activated channels (CRAC)-mediated Ca²⁺ influx. Inhibitors against PLC, IP3R and CRAC effectively blocked PV IgG and PF IgG-induced Ca²⁺ influx; ameliorated alterations of Dsg1 and Dsg3 localization, and reorganization of keratin and actin filaments; and inhibited loss of cell adhesion in vitro. Finally, inhibiting PLC or IP3R was protective against PV IgG-induced blister formation and redistribution of Dsg1 and Dsg3 in human skin ex vivo.

CONCLUSIONS

Ca²⁺ -mediated signalling is important for epidermal blistering and dependent on the autoantibody profile, which indicates different roles for signalling complexes organized by Dsg1 and Dsg3. Interfering with PLC and Ca²⁺ signalling may be a promising approach to treat epidermal manifestations of pemphigus.

Interrogating permeation and gating of Orai channels using chemical modification of cysteine residues

Chemical modification of ion channels using the substituted cysteine accessibility method has a rich and successful history in elucidating the structural basis of ion channel function. In this approach, cysteine residues are introduced in regions of interest into the protein and their accessibility to water soluble thiol-reactive reagents is determined by monitoring ion channel activity. Because a wide range of these reagents are available with differing size, charge, and membrane solubility, the physio-chemical environment of the introduced cysteine residue and therefore the protein domain of interest can be probed with great precision. The approach has been widely employed for determining the secondary structure of specific ion channel domains, the location and nature of the channel gate, and the conformational rearrangements in the channel pore that underlie the opening/closing of the pore. In this chapter, we describe the use of these and related approaches to probe the functional architecture and gating of store-operated Orai1 channels.

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.

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.

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.

Inhibition of store-operated calcium channels by N-arachidonoyl glycine (NAGly): no evidence for the involvement of lipid-sensing G protein coupled receptors

N-arachidonoyl glycine (NAGly) is an endogenous lipid deriving from the endocannabinoid anandamide (AEA). Identified as a ligand of several G-protein coupled receptors (GPCRs), it can however exert biological responses independently of GPCRs. NAGly was recently shown to depress store-operated Ca²⁺ entry (SOCE) but its mechanism of action remains elusive. The major aim of this study was to gain a better knowledge on the NAGly-dependent impairment of SOCE in neurons of the central nervous system (CNS) from mice. First, we examined the expression of genes encoding for putative lipid sensing GPCRs using transcriptomic data publicly available. This analysis showed that the most abundant GPCRs transcripts present in the cerebral cortices of embryonic brains were coding for lysophosphatidic acid (LPA) and sphingosine-1 phosphate (S1P) receptors. Next, the presence of functional receptors was assessed with live-cell calcium imaging experiments. In primary cortical cells S1P and LPA mobilize Ca²⁺ from internal stores via a mechanism sensitive to the S1P and LPA receptor antagonists Ex26, H2L5186303, or Ki16425. However, none of these compounds prevented or attenuated the NAGly-dependent impairment of SOCE. We found no evidence for the requirement of lipid sensing GPCRs in this inhibitory process, indicating that NAGly is an endogenous modulator interfering with the core machinery of SOCE. Moreover, these data also raise the intriguing possibility that the depression of SOCE could play a role in the central effects of NAGly.

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.

Structural and Mechanistic Insights of CRAC Channel as a Drug Target in Autoimmune Disorder

BACKGROUND

Calcium (Ca2+) ion is a major intracellular signaling messenger, controlling a diverse array of cellular functions like gene expression, secretion, cell growth, proliferation, and apoptosis. The major mechanism controlling this Ca2+ homeostasis is store-operated Ca2+ release-activated Ca2+ (CRAC) channels. CRAC channels are integral membrane protein majorly constituted via two proteins, the stromal interaction molecule (STIM) and ORAI. Following Ca2+ depletion in the Endoplasmic reticulum (ER) store, STIM1 interacts with ORAI1 and leads to the opening of the CRAC channel gate and consequently allows the influx of Ca2+ ions. A plethora of studies report that aberrant CRAC channel activity due to Loss- or gain-of-function mutations in ORAI1 and STIM1 disturbs this Ca2+ homeostasis and causes several autoimmune disorders. Hence, it clearly indicates that the therapeutic target of CRAC channels provides the space for a new approach to treat autoimmune disorders.

OBJECTIVE

This review aims to provide the key structural and mechanical insights of STIM1, ORAI1 and other molecular modulators involved in CRAC channel regulation.

RESULTS AND CONCLUSION

Understanding the structure and function of the protein is the foremost step towards improving the effective target specificity by limiting their potential side effects. Herein, the review mainly focusses on the structural underpinnings of the CRAC channel gating mechanism along with its biophysical properties that would provide the solid foundation to aid the development of novel targeted drugs for an autoimmune disorder. Finally, the immune deficiencies caused due to mutations in CRAC channel and currently used pharmacological blockers with their limitation are briefly summarized.

Tspan18 is a novel regulator of the Ca2+ channel Orai1 and von Willebrand factor release in endothelial cells

Ca2+ entry via Orai1 store-operated Ca2+ channels in the plasma membrane is critical to cell function, and Orai1 loss causes severe immunodeficiency and developmental defects. The tetraspanins are a superfamily of transmembrane proteins that interact with specific 'partner proteins' and regulate their trafficking and clustering. The aim of this study was to functionally characterize tetraspanin Tspan18. We show that Tspan18 is expressed by endothelial cells at several-fold higher levels than most other cell types analyzed. Tspan18-knockdown primary human umbilical vein endothelial cells have 55-70% decreased Ca2+ mobilization upon stimulation with the inflammatory mediators thrombin or histamine, similar to Orai1-knockdown. Tspan18 interacts with Orai1, and Orai1 cell surface localization is reduced by 70% in Tspan18-knockdown endothelial cells. Tspan18 overexpression in lymphocyte model cell lines induces 20-fold activation of Ca2+ -responsive nuclear factor of activated T cell (NFAT) signaling, in an Orai1-dependent manner. Tspan18-knockout mice are viable. They lose on average 6-fold more blood in a tail-bleed assay. This is due to Tspan18 deficiency in non-hematopoietic cells, as assessed using chimeric mice. Tspan18-knockout mice have 60% reduced thrombus size in a deep vein thrombosis model, and 50% reduced platelet deposition in the microcirculation following myocardial ischemia-reperfusion injury. Histamine- or thrombin-induced von Willebrand factor release from endothelial cells is reduced by 90% following Tspan18-knockdown, and histamine-induced increase of plasma von Willebrand factor is reduced by 45% in Tspan18-knockout mice. These findings identify Tspan18 as a novel regulator of endothelial cell Orai1/Ca2+ signaling and von Willebrand factor release in response to inflammatory stimuli.

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.

Differential regulation of Ca2+ influx by ORAI channels mediates enamel mineralization

Store-operated Ca²⁺ entry (SOCE) channels are highly selective Ca²⁺ channels activated by the endoplasmic reticulum (ER) sensors STIM1 and STIM2. Their direct interaction with the pore-forming plasma membrane ORAI proteins (ORAI1, ORAI2, and ORAI3) leads to sustained Ca²⁺ fluxes that are critical for many cellular functions. Mutations in the human ORAI1 gene result in immunodeficiency, anhidrotic ectodermal dysplasia, and enamel defects. In our investigation of the role of ORAI proteins in enamel, we identified enamel defects in a patient with an ORAI1 null mutation. Targeted deletion of the Orai1 gene in mice showed enamel defects and reduced SOCE in isolated enamel cells. However, Orai2^-/- mice showed normal enamel despite having increased SOCE in the enamel cells. Knockdown experiments in the enamel cell line LS8 suggested that ORAI2 and ORAI3 modulated ORAI1 function, with ORAI1 and ORAI2 being the main contributors to SOCE. ORAI1-deficient LS8 cells showed altered mitochondrial respiration with increased oxygen consumption rate and ATP, which was associated with altered redox status and enhanced ER Ca²⁺ uptake, likely due to S-glutathionylation of SERCA pumps. Our findings demonstrate an important role of ORAI1 in Ca²⁺ influx in enamel cells and establish a link between SOCE, mitochondrial function, and redox homeostasis.

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.

Trace levels of peptidoglycan in serum underlie the NOD-dependent cytokine response to endoplasmic reticulum stress

NOD1 and NOD2 are intracellular sensors of bacterial peptidoglycan that belong to the Nod-like receptor family of innate immune proteins. In addition to their role as direct bacterial sensors, it was proposed that the nucleotide-binding oligomerization domain (NOD) proteins could detect endoplasmic reticulum (ER) stress induced by thapsigargin, an inhibitor of the sarcoplasmic or endoplasmic reticulum calcium ATPase family that pumps Ca²⁺ into the ER, resulting in pro-inflammatory signaling. Here, we confirm that thapsigargin induces NOD-dependent pro-inflammatory signaling in epithelial cells. However, the effect was specific to thapsigargin, as tunicamycin and the subtilase cytotoxin SubAB from Shiga toxigenic Escherichia coli, which induce ER stress by other mechanisms, did not induce cytokine expression. The calcium ionophore A23187 also induced NOD-dependent signaling, and calcium chelators demonstrated a role for both intracellular and extracellular calcium in mediating thapsigargin-induced and NOD-dependent pro-inflammatory signaling, in part through the activation of plasma membrane-associated calcium release-activated channels. Moreover, our results demonstrate that both endocytosis and the addition of serum to the cell culture medium were required for thapsigargin-mediated NOD activation. Finally, we analyzed cell culture grade fetal calf serum as well as serum from laboratory mice using HPLC and MS identified the presence of various peptidoglycan fragments. We propose that cellular perturbations that affect intracellular Ca²⁺ can trigger internalization of peptidoglycan trace contaminants found in culture serum, thereby stimulating pro-inflammatory signaling. The presence of peptidoglycan in animal serum suggests that a homeostatic function of NOD signaling may have been previously overlooked.

Modulation of calcium signaling pathway by hepatitis C virus core protein stimulates NLRP3 inflammasome activation

Hepatitis C virus (HCV) infection remains a major cause of hepatic inflammation and liver disease. HCV triggers NLRP3 inflammasome activation and interleukin-1β (IL-1β) production from hepatic macrophages, or Kupffer cells, to drive the hepatic inflammatory response. Here we examined HCV activation of the NLRP3 inflammasome signaling cascade in primary human monocyte derived macrophages and THP-1 cell models of hepatic macrophages to define the HCV-specific agonist and cellular processes of inflammasome activation. We identified the HCV core protein as a virion-specific factor of inflammasome activation. The core protein was both necessary and sufficient for IL-1β production from macrophages exposed to HCV or soluble core protein alone. NLRP3 inflammasome activation by the HCV core protein required calcium mobilization linked with phospholipase-C activation. Our findings reveal a molecular basis of hepatic inflammasome activation and IL-1β release triggered by HCV core protein.

This study deciphers the molecular mechanism of Hepatitis C virus (HCV)-induced hepatic inflammation. HCV triggers NLRP3 inflammasome activation and IL-1β release from hepatic macrophages, thus driving liver inflammation. Using biochemical, virological, and genetic approaches we identified the HCV core protein as the specific viral stimulus that triggers intracellular calcium signaling linked with phospholipase-C activation to drive NLRP3 inflammasome activation and IL-1β release in macrophages.

STIM1 overexpression in hypoxia microenvironment contributes to pancreatic carcinoma progression

Objective

Stromal interaction molecule 1 (STIM1) overexpression has been reported to play an important role in progression of several cancers. However, the mechanism of STIM1 overexpression and its relationship with hypoxia in pancreatic ductal adenocarcinoma (PDAC) remains unclear.

Methods

STIM1 and HIF-1α expression was tested using immunohistochemistry in tissue microarray (TMA) including pancreatic cancer and matched normal pancreatic tissues, and their relationships with clinicopathological parameters were statistically analyzed. q-PCR, Western blot, ChIP, and luciferase assay were employed to 030 analyze transcriptional regulation between HIF-1α and STIM1 in pancreatic cancer PANC-1 cells.

Results

Both STIM1 and HIF-1α showed higher positive rates and up-regulated expression in cancer tissues compared to that of normal tissues (P < 0.05). The Kaplan–Meier method revealed that higher HIF-1α and STIM1 expression levels were significantly correlated with decreased disease-free survival ( P = 0.025 and P = 0.029, respectively). The expression of HIF-1α showed a significant positive correlation with that of STIM1 in cancer tissues (r_s = 0.3343, P = 0.0011) and pancreatic cancer cell lines. Furthermore, ChIP and luciferase assays confirmed that HIF-1α bound to the STIM1 promoter and regulated its expression in PANC-1 cells.

Conclusions

In hypoxia microenvironment, up-regulated expression of STIM1 mediated by HIF-1α promotes PDAC progression. HIF-1α and STIM1 are potential prognostic markers and/or therapeutic targets for PDAC treatment.

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.

IL-3-producing basophils are required to exacerbate airway hyperresponsiveness in a murine inflammatory model

BACKGROUND

Basophils are commonly associated with allergic responses because of their ability to produce large amounts of pro-Th2 cytokines and histamine. However, the mechanisms through which bone marrow-resident basophils (BMRB) become fully competent cytokine and histamine producers in response to IgE crosslinking are poorly understood. Here, we sought to determine the role of IL-3 in promoting pro-Th2 basophils.

METHODS

BMRB and basophils exposed to IL-3 in vitro and in vivo were evaluated for their production of Th2 cytokines and histamine in response to FcεRI crosslinking on both protein and gene expression levels. In vivo relevance of our findings was assessed in a model of ovalbumin-induced allergic asthma using IL-3-deficient and wild-type mice in a protocol of adoptive basophil transfer.

RESULTS

We show that BMRB and basophils previously exposed to IL-3 differ in their ability to generate cytokines (IL-4, IL-6, IL-13, and GM-CSF) and histamine in response to FcεRI crosslinking, reflecting two stages of maturation. Exposure to IL-3 initiated an autocrine loop of endogenous IL-3 production that enhanced histamine and cytokine production upon FcεRI crosslinking. This increased responsiveness required calcium flux and was dependent on calcineurin and store-operated calcium channels. Our findings are of pathophysiological relevance, as assessed by the failure of IL-3-deficient mice to develop airway hyperreactivity, which could be restored by adoptive transfer of IL-3-derived basophils recovered from wild-type mice.

CONCLUSION

IL-3-dependent basophils promote Th2 allergic AHR, which designates the IL-3/basophil axis as a promising therapeutic target for the treatment of basophil-dependent asthma.

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.

New insights into the role of mitochondrial calcium homeostasis in cell migration

Mitochondria are dynamic organelles involved in numerous physiological functions. Beyond their function in ATP production, mitochondria regulate cell death, reactive oxygen species (ROS) generation, immunity and metabolism. Mitochondria also play a key role in the buffering of cytosolic calcium, and calcium transported into the matrix regulates mitochondrial metabolism. Recently, the identification of the mitochondrial calcium uniporter (MCU) and associated regulators has allowed the characterization of new physiological roles for calcium in both mitochondrial and cellular homeostasis. Indeed, recent work has highlighted the importance of mitochondrial calcium homeostasis in regulating cell migration. Cell migration is a property common to all metazoans and is critical to embryogenesis, cancer progression, wound-healing and immune surveillance. Previous work has established that cytoplasmic calcium is a key regulator of cell migration, as oscillations in cytosolic calcium activate cytoskeletal remodelling, actin contraction and focal adhesion (FA) turnover necessary for cell movement. Recent work using animal models and in cellulo experiments to genetically modulate MCU and partners have shed new light on the role of mitochondrial calcium dynamics in cytoskeletal remodelling through the modulation of ATP and ROS production, as well as intracellular calcium signalling. This review focuses on MCU and its regulators in cell migration during physiological and pathophysiological processes including development and cancer. We also present hypotheses to explain the molecular mechanisms by which MCU may regulate mitochondrial dynamics and motility to drive cell migration.

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

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

Calcium oscillations in fertilized pig oocytes are associated with repetitive interactions between STIM1 and ORAI1

The Ca2+ entry mechanism that sustains the Ca2+ oscillations in fertilized pig oocytes was investigated. Stromal interaction molecule 1 (STIM1) and ORAI1 proteins tagged with various fluorophores were expressed in the oocytes. In some cells, the Ca2+ stores were depleted using cyclopiazonic acid (CPA); others were inseminated. Changes in the oocytes' cytosolic free Ca2+ concentration were monitored, while interaction between the expressed fusion proteins was investigated using fluorescence resonance energy transfer (FRET). Store depletion led to an increase of the FRET signal in oocytes co-expressing mVenus-STIM1 and mTurquoise2-ORAI1, indicating that Ca2+ release was followed by an interaction between these proteins. A similar FRET increase in response to CPA was also detected in oocytes co-expressing mVenus-STIM1 and mTurquoise2-STIM1, which is consistent with STIM1 forming punctae after store depletion. ML-9, an inhibitor that can interfere with STIM1 puncta formation, blocked store-operated Ca2+ entry (SOCE) induced by Ca2+ add-back after a CPA treatment; it also disrupted the Ca2+ oscillations in fertilized oocytes. In addition, oocytes overexpressing mVenus-STIM1 showed high-frequency Ca2+ oscillations when fertilized, arguing for an active role of the protein. High-frequency Ca2+ oscillations were also detected in fertilized oocytes co-expressing mVenus-STIM1 and mTurquoise2-ORAI1, and both of these high-frequency Ca2+ oscillations could be stopped by inhibitors of SOCE. Importantly, in oocytes co-expressing mVenus-STIM1 and mTurquoise2-ORAI1, we were also able to detect cyclic increases of the FRET signal indicating repetitive interactions between STIM1 and ORAI1. The results confirm the notion that in pig oocytes, SOCE is involved in the maintenance of the repetitive Ca2+ transients at fertilization.

Molecular physiology and pathophysiology of stromal interaction molecules

Ca²⁺ release from the endoplasmic reticulum is an important component of Ca²⁺ signal transduction that controls numerous physiological processes in eukaryotic cells. Release of Ca²⁺ from the endoplasmic reticulum is coupled to the activation of store-operated Ca²⁺ entry into cells. Store-operated Ca²⁺ entry provides Ca²⁺ for replenishing depleted endoplasmic reticulum Ca²⁺ stores and a Ca²⁺ signal that regulates Ca²⁺-dependent intracellular biochemical events. Central to connecting discharge of endoplasmic reticulum Ca²⁺ stores following G protein-coupled receptor activation with the induction of store-operated Ca²⁺ entry are stromal interaction molecules (STIM1 and STIM2). These highly homologous endoplasmic reticulum transmembrane proteins function as sensors of the Ca²⁺ concentration within the endoplasmic reticulum lumen and activators of Ca²⁺ release-activated Ca²⁺ channels. Emerging evidence indicates that in addition to their role in Ca²⁺ release-activated Ca²⁺ channel gating and store-operated Ca²⁺ entry, STIM1 and STIM2 regulate other cellular signaling events. Recent studies have shown that disruption of STIM expression and function is associated with the pathogenesis of several diseases including autoimmune disorders, cancer, cardiovascular disease, and myopathies. Here, we provide an overview of the latest developments in the molecular physiology and pathophysiology of STIM1 and STIM2. Impact statement Intracellular Ca²⁺ signaling is a fundamentally important regulator of cell physiology. Recent studies have revealed that Ca²⁺-binding stromal interaction molecules (Stim1 and Stim2) expressed in the membrane of the endoplasmic reticulum (ER) are essential components of eukaryote Ca²⁺ signal transduction that control the activity of ion channels and other signaling effectors present in the plasma membrane. This review summarizes the most recent information on the molecular physiology and pathophysiology of stromal interaction molecules. We anticipate that the work presented in our review will provide new insights into molecular interactions that participate in interorganelle signaling crosstalk, cell function, and the pathogenesis of human diseases.

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

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

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

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

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

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