Immunodeficiency due to defects in store-operated calcium entry

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.

The ER stress sensor IRE1 interacts with STIM1 to promote store-operated calcium entry, T cell activation, and muscular differentiation

Store-operated Ca2+ entry (SOCE) mediated by stromal interacting molecule (STIM)-gated ORAI channels at endoplasmic reticulum (ER) and plasma membrane (PM) contact sites maintains adequate levels of Ca2+ within the ER lumen during Ca2+ signaling. Disruption of ER Ca2+ homeostasis activates the unfolded protein response (UPR) to restore proteostasis. Here, we report that the UPR transducer inositol-requiring enzyme 1 (IRE1) interacts with STIM1, promotes ER-PM contact sites, and enhances SOCE. IRE1 deficiency reduces T cell activation and human myoblast differentiation. In turn, STIM1 deficiency reduces IRE1 signaling after store depletion. Using a CaMPARI2-based Ca2+ genome-wide screen, we identify CAMKG2 and slc105a as SOCE enhancers during ER stress. Our findings unveil a direct crosstalk between SOCE and UPR via IRE1, acting as key regulator of ER Ca2+ and proteostasis in T cells and muscles. Under ER stress, this IRE1-STIM1 axis boosts SOCE to preserve immune cell functions, a pathway that could be targeted for cancer immunotherapy.

A bibliometric analysis: Ca2+ fluxes and inflammatory phenotyping by flow cytometry in peripheral blood mononuclear cells

Background

The immune system, composed of organs, tissues, cells, and proteins, is the key to protecting the body from external biological attacks and inflammation. The latter occurs in several pathologies, such as cancers, type 1 diabetes, and human immunodeficiency virus infection. Immunophenotyping by flow cytometry is the method of choice for diagnosing these pathologies. Under inflammatory conditions, the peripheral blood mononuclear cells (PBMCs) are partially activated and generate intracellular pathways involving Ca2+-dependent signaling cascades leading to transcription factor expression. Ca2+ signaling is typically studied by microscopy in cell lines but can present some limitations to explore human PBMCs, where flow cytometry can be a good alternative.

Objective

In this review, we dived into the research field of inflammation and Ca2+ signaling in PBMCs. We aimed to investigate the structure and evolution of this field in a physio-pathological context, and then we focused our review on flow cytometry analysis of Ca2+ fluxes in PBMCs.

Methods

From 1984 to 2022, 3865 articles on inflammation and Ca2+ signaling in PBMCs were published, according to The Clarivate Web of Science (WOS) database used in this review. A bibliometric study was designed for this collection and consisted of a co-citation and bibliographic coupling analysis.

Results

The co-citation analysis was performed on 133 articles: 4 clusters highlighted the global context of Ca2+ homeostasis, including chemical probe development, identification of the leading players in Ca2+ signaling, and the link with chemokine production in immune cell function. Next, the bibliographic coupling analysis combined 998 articles in 8 clusters. This analysis outlined the mechanisms of PBMC activation, from signal integration to cellular response. Further explorations of the bibliographic coupling network, focusing on flow cytometry, revealed 21 articles measuring cytosolic Ca2+ in PBMCs, with only 5 since 2016. This final query showed that Ca2+ signaling analysis in human PBMCs using flow cytometry is still underdeveloped and investigates mainly the cytosolic Ca2+ compartment.

Conclusion

Our review uncovers remaining knowledge gaps of intracellular players involved in Ca2+ signaling in PBMCs, such as reticulum and mitochondria, and presents flow cytometry as a solid option to supplement gold-standard microscopy studies.

Methods to Quantify the Dynamic Recycling of Plasma Membrane Channels

Store-operated Ca2+ entry (SOCE) is a ubiquitous Ca2+ signaling modality mediated by Orai Ca2+ channels at the plasma membrane (PM) and the endoplasmic reticulum (ER) Ca2+ sensors STIM1/2. At steady state, Orai1 constitutively cycles between an intracellular compartment and the PM. Orai1 PM residency is modulated by its endocytosis and exocytosis rates. Therefore, Orai1 trafficking represents an important regulatory mechanism to define the levels of Ca2+ influx. Here, we present a protocol using the dually tagged YFP-HA-Orai1 with a cytosolic YFP and extracellular hemagglutinin (HA) tag to quantify Orai1 cycling rates. For measuring Orai1 endocytosis, cells expressing YFP-HA-Orai1 are incubated with mouse anti-HA antibody for various periods of time before being fixed and stained for surface Orai1 with Cy5-labeled anti-mouse IgG. The cells are fixed again, permeabilized, and stained with Cy3-labeled anti-mouse IgG to reveal anti-HA that has been internalized. To quantify Orai1 exocytosis rate, cells are incubated with anti-HA antibody for various incubation periods before being fixed, permeabilized, and then stained with Cy5-labeled anti-mouse IgG. The Cy5/YFP ratio is plotted over time and fitted with a mono-exponential growth curve to determine exocytosis rate. Although the described assays were developed to measure Orai1 trafficking, they are readily adaptable to other PM channels. Key features Detailed protocols to quantify endocytosis and exocytosis rates of Orai1 at the plasma membrane that can be used in various cell lines. The endocytosis and exocytosis assays are readily adaptable to study the trafficking of other plasma membrane channels.

Case Report: Novel STIM1 Gain-of-Function Mutation in a Patient With TAM/STRMK and Immunological Involvement

Gain-of-function (GOF) mutations in STIM1 are responsible for tubular aggregate myopathy and Stormorken syndrome (TAM/STRMK), a clinically overlapping multisystemic disease characterised by muscle weakness, miosis, thrombocytopaenia, hyposplenism, ichthyosis, dyslexia, and short stature. Several mutations have been reported as responsible for the disease. Herein, we describe a patient with TAM/STRMK due to a novel L303P STIM1 mutation, who not only presented clinical manifestations characteristic of TAM/STRMK but also manifested immunological involvement with respiratory infections since childhood, with chronic cough and chronic bronchiectasis. Despite the seemingly normal main immunological parameters, immune cells revealed GOF in calcium signalling compared with healthy donors. The calcium flux dysregulation in the immune cells could be responsible for our patient’s immune involvement. The patient’s mother carried the mutation but did not exhibit TAM/STRMK, manifesting an incomplete penetrance of the mutation. More cases and evidence are necessary to clarify the dual role of STIM1 in immune system dysregulation and myopathy.

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.

Structural Insights into Ca2+ Permeation through Orai Channels

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

Spontaneous calcium responses of SF2 rat dental epithelial cells stably expressing the calcium sensor G-GECO

Genetically-encoded calcium indicators such as G-GECO are useful for studying Ca²⁺ responses during long-term processes. In this study, we employed a lentiviral vector and established a rat dental epithelial cell line that stably expressed G-GECO (SF2-G-GECO). Ca²⁺ imaging analysis under cell culture conditions revealed that SF2-G-GECO cells exhibited spontaneous Ca²⁺ responses, which could be classified into the following three major patterns depending on the cell density: localized Ca²⁺ responses at cell protrusions at a low density, a cell-wide spread of Ca²⁺ responses at a medium density, and Ca²⁺ responses in clusters of 3-20 cells at a high density. The P2Y receptor inhibitor suramin (10 μM), the ATP-degrading enzyme apyrase (5 units/mL), and the fibroblast growth factor (FGF) receptor inhibitor FIIN-2 (1 μM) decreased the frequency of spontaneous Ca²⁺ responses. These results indicate that ATP and FGF are involved in the spontaneous Ca²⁺ responses. SF2 cells differentiate into ameloblasts via interactions with mesenchymal cells. Therefore, SF2-G-GECO cells are expected to be a useful tool for studying the functions of Ca²⁺ responses in regulating gene expression during tooth development.

Stormorken Syndrome Caused by a Novel STIM1 Mutation: A Case Report

Objective: To identify the gene mutation of Stormorken syndrome and review the published Stromal Interaction Molecule 1 (STIM1) mutation phenotype.

Methods: We described the clinical and molecular aspects of a Chinese female with Stormorken syndrome by laboratory tests, muscle biopsies, and genetic analysis. We used this information to summarize all the mutation sites reported in the literature. We also reviewed the clinical features of published cases with a gain of function mutations of STIM1.

Results: A 12-year-old Chinese female presented with skin purpura in the lower limbs and stroke-like episodes. Muscle biopsy and microscopic examination revealed atrophy in her skeletal muscle. Genetic analysis identified a novel heterozygous missense mutation, a c.1095G>C transition (NM_003156.3), which caused a p.K365N amino acid substitution in the protein and affected a STIM1-orai1-activation region (SOAR).

Conclusions: The novel variant c.1095G>C transition (NM_003156.3) was located in the SOAR, which expands the phenotypic spectrum of STIM1 variants in human disorders and may define the molecular basis of Stormorken syndrome.

Store-Operated Calcium Channels as Drug Target in Gastroesophageal Cancers

Gastroesophageal cancers, including tumors occurring in esophagus and stomach, usually have poor prognosis and lack effective chemotherapeutic drugs for treatment. The association between dysregulated store-operated calcium entry (SOCE), a key intracellular Ca²⁺ signaling pathway and gastroesophageal cancers are emerging. This review summarizes the recent advances in understanding the contribution of SOCE-mediated intracellular Ca²⁺ signaling to gastroesophageal cancers. It assesses the pathophysiological role of each component in SOCE machinery, such as Orais and STIMs in the cancer cell proliferation, migration, and invasion as well as stemness maintenance. Lastly, it discusses efforts towards development of more specific and potent SOCE inhibitors, which may be a new set of chemotherapeutic drugs appearing at the horizon, to provide either targeted therapy or adjuvant treatment to overcome drug resistance for gastroesophageal cancers.

Novel ORAI1 Mutation Disrupts Channel Trafficking Resulting in Combined Immunodeficiency

Store-operated Ca²⁺ entry (SOCE) represents a predominant Ca²⁺ influx pathway in non-excitable cells. SOCE is required for immune cell activation and is mediated by the plasma membrane (PM) channel ORAI1 and the endoplasmic reticulum (ER) Ca²⁺ sensor STIM1. Mutations in the Orai1 or STIM1 genes abolish SOCE leading to combined immunodeficiency (CID), muscular hypotonia, and anhidrotic ectodermal dysplasia. Here, we identify a novel autosomal recessive mutation in ORAI1 in a child with CID. The patient is homozygous for p.C126R mutation in the second transmembrane domain (TM2) of ORAI1, a region with no previous loss-of-function mutations. SOCE is suppressed in the patient’s lymphocytes, which is associated with impaired T cell proliferation and cytokine production. Functional analyses demonstrate that the p.C126R mutation does not alter protein expression but disrupts ORAI1 trafficking. Orai1-C126R does not insert properly into the bilayer resulting in ER retention. Insertion of an Arg on the opposite face of TM2 (L135R) also results in defective folding and trafficking. We conclude that positive side chains within ORAI1 TM2 are not tolerated and result in misfolding, defective bilayer insertion, and channel trafficking thus abolishing SOCE and resulting in CID.

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

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

Evaluation for the Genetic Association between Store-Operated Calcium Influx Pathway (STIM1 and ORAI1) and Human Hepatocellular Carcinoma in Patients with Chronic Hepatitis B Infection

Hepatocellular carcinoma (HCC) often develops from chronic hepatitis B (CHB) through replication of hepatitis B virus (HBV) infection. Calcium (Ca²⁺) signaling plays an essential role in HBV replication. Store-operated calcium (SOC) channels are a major pathway of Ca²⁺ entry into non-excitable cells such as immune cells and cancer cells. The basic components of SOC signaling include the STIM1 and ORAI1 genes. However, the roles of STIM1 and ORAI1 in HBV-mediated HCC are still unclear. Thus, long-term follow-up of HBV cohort was carried out in this study. This study recruited 3631 patients with chronic hepatitis (345 patients with HCC, 3286 patients without HCC) in a Taiwanese population. Genetic variants of the STIM1 and ORAI1 genes were detected using an Axiom CHB1 genome-wide array. Clinical associations of 40 polymorphisms were analyzed. Three of the STIM1 single-nucleotide polymorphisms (SNPs) (rs6578418, rs7116520, and rs11030472) and one SNP of ORAI1 (rs6486795) showed a trend of being associated with HCC disease (p < 0.05). However, after correction for multiple testing, none of the SNPs reached a significant level (q > 0.05); in contrast, neither STIM1 nor ORAI1 showed a significant association with HCC progression in CHB patients. Functional studies by both total internal reflection fluorescence images and transwell migration assay indicated the critical roles of SOC-mediated signaling in HCC migration. In conclusion, we reported a weak correlation between STIM1/ORAI1 polymorphisms and the risk of HCC progression in CHB patients.

A Mechanism Underlying Sex-Associated Differences in Ankylosing Spondylitis: Troponin C2, Fast Skeletal Type (TNNC2) and Calcium Signaling Pathway

BACKGROUND Ankylosing spondylitis (AS) is a disease that causes pathological changes in the spine and sacroiliac joints. Numerous studies have shown that the characteristics of AS differ between males and females. The purpose of this study was to discover the key molecules that contribute to sex-associated differences in AS, which may provide a new molecular target for personalized treatment. MATERIAL AND METHODS The gene expression profile of GSE39340 was downloaded from the Gene Expression Comprehensive database, and 2 groups (AS vs. No-AS groups and male AS vs. female AS groups) of differentially expressed genes (EDGs) were obtained by GEO2R. The DAVID database was used for DEGs function and enrichment analysis. Based on data in the STRING online database, a protein-protein interaction (PPI) network was constructed in Cytoscape. Hub genes were selected from CytoHubba. With the intersection of the top 30 hub genes of 2 sets of EDGs, genes coexisting with the KEGG-related pathway were found. RESULTS We screened 560 genes between the AS and No-AS groups, and screened 710 genes that were differentially expressed between the male and female AS groups. GO analysis showed that DEGs were mainly co-enriched in molecular functions, including structural constituent of muscle. The KEGG pathway mainly included the structural constituent of muscle. Seven hub genes were obtained. Troponin C2 and fast skeletal type (TNNC2) were the key genes participating in the calcium signaling pathway. CONCLUSIONS This study contributes to understanding the molecular biological mechanism underlying sex-associated differences in AS. TNNC2 and calcium signaling pathway may be new targets for the individualized treatment of AS.

The polyphenol ellagic acid exerts anti-inflammatory actions via disruption of store-operated calcium entry (SOCE) pathway activators and coupling mediators

Ellagic acid, a naturally occurring phenol found in a variety of fruits and nuts has been shown to possess anti-inflammatory properties. However, the mechanism of action behind its anti-inflammatory action is unclear. Using human Jurkat T cells, our study examined the effects of ellagic acid (EA) on Ca²⁺ handling, in particular, store-operated Ca²⁺ entry (SOCE), a process critical to proper T cell function. We observed that the acute addition of EA-induced Ca²⁺ release with an EC₅₀ of 63 μM. The Ca²⁺ release was significantly attenuated by Xestospongin C, a known inhibitor of the Inositol 1,4,5-trisphosphate receptor (IP₃R) channel and was unaffected by the phospholipase C (PLC) inhibitor, U73122. Furthermore, chronic incubation of Jurkat T cells with EA not only decreased the ATP-induced Ca²⁺ release but also diminished the SOCE-mediated Ca²⁺ influx in a dose-dependent manner. This inhibition was confirmed by reduced Mn²⁺ entry rates in the EA-treated cells. The ATP-induced Ca²⁺ entry was also attenuated in EA-treated HEK293 cells transiently transfected with SOCE channel Orai1-myc and ER-sensor stromal interaction molecule (STIM1) (HEK^STIM/Orai). Moreover, EA treatment interfered with the Orai1 and STIM1 coupling by disrupting STIM1 puncta formation in the HEK^STIM/Orai cells. We observed that EA treatment reduced cytokine secretion and nuclear factor of activated T-cell transcriptional activity in stimulated T cells. Hence, by inhibiting SOCE mediated Ca²⁺ influx, EA decreased downstream activation of pro-inflammatory mediators. These results suggest a novel target for EA-mediated effects and provide insight into the mechanisms underlying EA-mediated anti-inflammatory effects.

Molecular Basis and Regulation of Store-Operated Calcium Entry

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous mechanism for Ca²⁺ influx in mammalian cells with important physiological implications. Since the discovery of SOCE more than three decades ago, the mechanism that communicates the information about the amount of Ca²⁺ accumulated in the intracellular Ca²⁺ stores to the plasma membrane channels and the nature of these channels have been matters of intense investigation and debate. The stromal interaction molecule-1 (STIM1) has been identified as the Ca²⁺ sensor of the intracellular Ca²⁺ compartments that activates the store-operated channels. STIM1 regulates two types of store-dependent channels: the Ca²⁺ release-activated Ca²⁺ (CRAC) channels, formed by Orai1 subunits, that conduct the highly Ca²⁺ selective current I _CRAC and the cation permeable store-operated Ca²⁺ (SOC) channels, which consist of Orai1 and TRPC1 proteins and conduct the non-selective current I _SOC. While the crystal structure of Drosophila CRAC channel has already been solved, the architecture of the SOC channels still remains unclear. The dynamic interaction of STIM1 with the store-operated channels is modulated by a number of proteins that either support the formation of the functional STIM1-channel complex or protect the cell against Ca²⁺ overload.

A muscular hypotonia-associated STIM1 mutant at R429 induces abnormalities in intracellular Ca2+ movement and extracellular Ca2+ entry in skeletal muscle

Stromal interaction molecule 1 (STIM1) mediates extracellular Ca²⁺ entry into the cytosol through a store-operated Ca²⁺ entry (SOCE) mechanism, which is involved in the physiological functions of various tissues, including skeletal muscle. STIM1 is also associated with skeletal muscle diseases, but its pathological mechanisms have not been well addressed. The present study focused on examining the pathological mechanism(s) of a mutant STIM1 (R429C) that causes human muscular hypotonia. R429C was expressed in mouse primary skeletal myotubes, and the properties of the skeletal myotubes were examined using single-cell Ca²⁺ imaging of myotubes and transmission electron microscopy (TEM) along with biochemical approaches. R429C did not interfere with the terminal differentiation of myoblasts to myotubes. Unlike wild-type STIM1, there was no further increase of SOCE by R429C. R429C bound to endogenous STIM1 and slowed down the initial rate of SOCE that were mediated by endogenous STIM1. Moreover, R429C increased intracellular Ca²⁺ movement in response to membrane depolarization by eliminating the attenuation on dihydropyridine receptor-ryanodine receptor (DHPR-RyR1) coupling by endogenous STIM1. The cytosolic Ca²⁺ level was also increased due to the reduction in SR Ca²⁺ level. In addition, R429C-expressing myotubes showed abnormalities in mitochondrial shape, a significant decrease in ATP levels, and the higher expression levels of mitochondrial fission-mediating proteins. Therefore, serial defects in SOCE, intracellular Ca²⁺ movement, and cytosolic Ca²⁺ level along with mitochondrial abnormalities in shape and ATP level could be a pathological mechanism of R429C for human skeletal muscular hypotonia. This study also suggests a novel clue that STIM1 in skeletal muscle could be related to mitochondria via regulating intra and extracellular Ca²⁺ movements.

7-Azaindole Analogues as Bioactive Agents and Recent Results

Azaindoles have been accepted as important structures having various biological activities in medicinal chemistry in novel drug discovery. Various azaindole derivatives have been used commercially and newer analogues are synthesized continuously. As in literature, azaindole is a very potent moiety, its derivatives displayed a number of biological activities such as kinase inhibitors, cytotoxic agents, anti-angiogenic activity, CRTh2 receptor antagonists, melanin agonists, nicotine agonists, effectiveness in alzheimer disease, cytokinin analogs, Orai inhibitors in asthma and chemokine receptor- 2 (CCR2) antagonists. This review consists of biological activities of various azaindole analogs, reported so far, and their structure activity relations, along with future perspectives in this field.

Leucine-rich repeat kinase 2 regulates mouse dendritic cell migration by ORAI2

The leucine-rich repeat kinase 2 (LRRK2) is expressed in various immune cells and involved in regulating inflammatory processes. LRRK2 facilitates calcium extrusion exchanger and sodium-calcium exchanger activity and hence influences intracellular Ca²⁺ concentration in dendritic cells (DCs). DC maturation and migration are governed by the intracellular Ca²⁺ concentration, but the related mechanisms whereby LRRK2 regulates DC function and involved Ca²⁺ channels are still under investigation. In the previous study, we found that LRRK2^-/- DCs exhibited higher store-operated Ca²⁺ entry (SOCE) activity than LRRK2^+/+ DCs. Herein, we ascertained the exact SOCE components by using genetic, pharmacological, and fluorescent approaches. Ca²⁺ imaging showed that LRRK2 kinase activity negatively modulated SOCE activity. Moreover, LRRK2 deficiency resulted in an enhanced migration capacity of DCs but had little effect on the maturation process. SOCE is widely known to regulate DC functions; we wanted to dissect the reason why LRRK2 specifically influenced DC migration and therefore silenced ORAI1, ORAI2, and ORAI3, respectively. Transwell assays showed that both ORAI1 and ORAI2 silencing markedly decreased the migration of DCs, but only ORAI1 deficiency influenced the expression of maturation markers CD11c, CD86, and major histocompatibility complex class II. Of note, LRRK2 deficiency increased ORAI2 expression but not that of ORAI1 and ORAI3. Thus, we suggest that LRRK2 modulates DC migration by interfering with ORAI2.-Yan, J., Zhao, W., Gao, C., Liu, X., Zhao, X., Wei, T., Gao, Z. Leucine-rich repeat kinase 2 regulates mouse dendritic cell migration by ORAI2.

Calcium release-activated calcium modulator 1 as a therapeutic target in allergic skin diseases

Allergic skin disease is the most common skin condition, and considerably affects patients' life quality because of its recurrence and pruritus. Numbers of studies point out that immune cells, including mast cells and T cells, play pathogenic roles in allergic skin diseases, and share similarities in the activation and secretion of cytokines. Calcium Release-Activated Calcium Modulator 1(CRACM1/ORAI1) is a subtype of Ca²⁺ membrane channel, causing Ca²⁺ influx into the cells. As a second messenger, Ca²⁺ is an essential element that regulates immune responses, especially in the development and function of T and B cells. Thus, ORAI1 is considered to participate in allergic diseases. However, the specific mechanism of ORAI1 in skin disorders is still unclear. In order to investigate the roles of ORAI1 in allergic skin disorders, we reviewed the related articles and concluded that ORAI1 could be a potential therapeutic target for allergic skin diseases.

Pivotal role of STIM2, but not STIM1, in IL-4 production by IL-3-stimulated murine basophils

Basophils have nonredundant roles in various immune responses that require Ca²⁺ influx. Here, we examined the role of two Ca²⁺ sensors, stromal interaction molecule 1 and 2 (STIM1 and STIM2), in basophil activation. We found that loss of STIM1, but not STIM2, impaired basophil IL-4 production after stimulation with immunoglobulin E (IgE)-containing immune complexes. In contrast, when basophils were stimulated with IL-3, loss of STIM2, but not STIM1, reduced basophil IL-4 production. This difference in STIM proteins was associated with distinct time courses of Ca²⁺ influx and transcription of the Il4 gene that were elicited by each stimulus. Similarly, basophil-specific STIM1 expression was required for IgE-driven chronic allergic inflammation in vivo, whereas STIM2 was required for IL-4 production after combined IL-3 and IL-33 treatment in mice. These data indicate that STIM1 and STIM2 have differential roles in the production of IL-4, which are stimulus dependent. Furthermore, these results illustrate the vital role of STIM2 in basophils, which is often considered to be less important than STIM1.

Early-onset inflammatory bowel disease as a model disease to identify key regulators of immune homeostasis mechanisms

Rare, monogenetic diseases present unique models to dissect gene functions and biological pathways, concomitantly enhancing our understanding of the etiology of complex (and often more common) traits. Although inflammatory bowel disease (IBD) is a generally prototypic complex disease, it can also manifest in an early-onset, monogenic fashion, often following Mendelian modes of inheritance. Recent advances in genomic technologies have spurred the identification of genetic defects underlying rare, very early-onset IBD (VEO-IBD) as a disease subgroup driven by strong genetic influence, pinpointing key players in the delicate homeostasis of the immune system in the gut and illustrating the intimate relationships between bowel inflammation, systemic immune dysregulation, and primary immunodeficiency with increased susceptibility to infections. As for other human diseases, it is likely that adult-onset diseases may represent complex diseases integrating the effects of host genetic susceptibility and environmental triggers. Comparison of adult-onset IBD and VEO-IBD thus provides beautiful models to investigate the relationship between monogenic and multifactorial/polygenic diseases. This review discusses the present and novel findings regarding monogenic IBD as well as key questions and future directions of IBD research.

Functional Interrogation of Primary Human T Cells via CRISPR Genetic Editing

Developing precise and efficient gene editing approaches using CRISPR in primary human T cell subsets would provide an effective tool in decoding their functions. Toward this goal, we used lentiviral CRISPR/Cas9 systems to transduce primary human T cells to stably express the Cas9 gene and guide RNAs that targeted either coding or noncoding regions of genes of interest. We showed that multiple genes (CD4, CD45, CD95) could be simultaneously and stably deleted in naive, memory, effector, or regulatory T cell (Treg) subsets at very high efficiency. Additionally, nuclease-deficient Cas9, associated with a transcriptional activator or repressor, can downregulate or increase expression of genes in T cells. For example, expression of glycoprotein A repetitions predominant (GARP), a gene that is normally and exclusively expressed on activated Tregs, could be induced on non-Treg effector T cells by nuclease-deficient Cas9 fused to transcriptional activators. Further analysis determined that this approach could be used in mapping promoter sequences involved in gene transcription. Through this CRISPR/Cas9-mediated genetic editing we also demonstrated the feasibility of human T cell functional analysis in several examples: 1) CD95 deletion inhibited T cell apoptosis upon reactivation; 2) deletion of ORAI1, a Ca²⁺ release-activated channel, abolished Ca²⁺ influx and cytokine secretion, mimicking natural genetic mutations in immune-deficient patients; and 3) transcriptional activation of CD25 or CD127 expression enhanced cytokine signaling by IL-2 or IL-7, respectively. Taken together, application of the CRISPR toolbox to human T cell subsets has important implications for decoding the mechanisms of their functional outputs.

STIM- and Orai-mediated calcium entry controls NF-κB activity and function in lymphocytes

The central role of Ca²⁺ signaling in the development of functional immunity and tolerance is well established. These signals are initiated by antigen binding to cognate receptors on lymphocytes that trigger store operated Ca²⁺ entry (SOCE). The underlying mechanism of SOCE in lymphocytes involves TCR and BCR mediated activation of Stromal Interaction Molecule 1 and 2 (STIM1/2) molecules embedded in the ER membrane leading to their activation of Orai channels in the plasma membrane. STIM/Orai dependent Ca²⁺ signals guide key antigen induced lymphocyte development and function principally through direct regulation of Ca²⁺ dependent transcription factors. The role of Ca²⁺ signaling in NFAT activation and signaling is well known and has been studied extensively, but a wide appreciation and mechanistic understanding of how Ca²⁺ signals also shape the activation and specificity of NF-κB dependent gene expression has lagged. Here we discuss and interpret what is known about Ca²⁺ dependent mechanisms of NF-kB activation, including what is known and the gaps in our understanding of how these signals control lymphocyte development and function.

ATP triggers a robust intracellular [Ca2+ ]-mediated signalling pathway in human synovial fibroblasts

NEW FINDINGS

What is the central question of this study? What are the main [Ca²⁺ ]_i signalling pathways activated by ATP in human synovial fibroblasts? What is the main finding and its importance? In human synovial fibroblasts ATP acts through a linked G-protein (G_q ) and phospholipase C signalling mechanism to produce IP₃ , which then markedly enhances release of Ca²⁺ from the endoplasmic reticulum. These results provide new information for the detection of early pathophysiology of arthritis.

ABSTRACT

In human articular joints, synovial fibroblasts (HSFs) have essential physiological functions that include synthesis and secretion of components of the extracellular matrix and essential articular joint lubricants, as well as release of paracrine substances such as ATP. Although the molecular and cellular processes that lead to a rheumatoid arthritis (RA) phenotype are not fully understood, HSF cells exhibit significant changes during this disease progression. The effects of ATP on HSFs were studied by monitoring changes in intracellular Ca²⁺ ([Ca²⁺ ]_i ), and measuring electrophysiological properties. ATP application to HSF cell populations that had been enzymatically released from 2-D cell culture revealed that ATP (10-100 μm), or its analogues UTP or ADP, consistently produced a large transient increase in [Ca²⁺ ]_i . These changes (i) were initiated by activation of the P₂ Y purinergic receptor family, (ii) required G_q -mediated signal transduction, (iii) did not involve a transmembrane Ca²⁺ influx, but instead (iv) arose almost entirely from activation of endoplasmic reticulum (ER)-localized inositol 1,4,5-trisphosphate (IP₃ ) receptors that triggered Ca²⁺ release from the ER. Corresponding single cell electrophysiological studies revealed that these ATP effects (i) were insensitive to [Ca²⁺ ]_o removal, (ii) involved an IP₃ -mediated intracellular Ca²⁺ release process, and (iii) strongly turned on Ca²⁺ -activated K⁺ current(s) that significantly hyperpolarized these cells. Application of histamine produced very similar effects in these HSF cells. Since ATP is a known paracrine agonist and histamine is released early in the inflammatory response, these findings may contribute to identification of early steps/defects in the initiation and progression of RA.

On the Role of Store-Operated Calcium Entry in Acute and Chronic Neurodegenerative Diseases

In both excitable and non-excitable cells, calcium (Ca²⁺) signals are maintained by a highly integrated process involving store-operated Ca²⁺ entry (SOCE), namely the opening of plasma membrane (PM) Ca²⁺ channels following the release of Ca²⁺ from intracellular stores. Upon depletion of Ca²⁺ store, the stromal interaction molecule (STIM) senses Ca²⁺ level reduction and migrates from endoplasmic reticulum (ER)-like sites to the PM where it activates the channel proteins Orai and/or the transient receptor potential channels (TRPC) prompting Ca²⁺ refilling. Accumulating evidence suggests that SOCE dysregulation may trigger perturbation of intracellular Ca²⁺ signaling in neurons, glia or hematopoietic cells, thus participating to the pathogenesis of diverse neurodegenerative diseases. Under acute conditions, such as ischemic stroke, neuronal SOCE can either re-establish Ca²⁺ homeostasis or mediate Ca²⁺ overload, thus providing a non-excitotoxic mechanism of ischemic neuronal death. The dualistic role of SOCE in brain ischemia is further underscored by the evidence that it also participates to endothelial restoration and to the stabilization of intravascular thrombi. In Parkinson's disease (PD) models, loss of SOCE triggers ER stress and dysfunction/degeneration of dopaminergic neurons. Disruption of neuronal SOCE also underlies Alzheimer's disease (AD) pathogenesis, since both in genetic mouse models and in human sporadic AD brain samples, reduced SOCE contributes to synaptic loss and cognitive decline. Unlike the AD setting, in the striatum from Huntington's disease (HD) transgenic mice, an increased STIM2 expression causes elevated synaptic SOCE that was suggested to underlie synaptic loss in medium spiny neurons. Thus, pharmacological inhibition of SOCE is beneficial to synapse maintenance in HD models, whereas the same approach may be anticipated to be detrimental to cortical and hippocampal pyramidal neurons. On the other hand, up-regulation of SOCE may be beneficial during AD. These intriguing findings highlight the importance of further mechanistic studies to dissect the molecular pathways, and their corresponding targets, involved in synaptic dysfunction and neuronal loss during aging and neurodegenerative diseases.

EB1 binding restricts STIM1 translocation to ER-PM junctions and regulates store-operated Ca2+ entry

STIM1 activates store-operated Ca²⁺ entry (SOCE) by translocating to endoplasmic reticulum–plasma membrane junctions. Chang et al. reveal that STIM1 localization and SOCE are regulated by a dynamic trapping mechanism mediated by STIM1 binding to EB1 at growing microtubule ends.

The endoplasmic reticulum (ER) Ca²⁺ sensor STIM1 forms oligomers and translocates to ER–plasma membrane (PM) junctions to activate store-operated Ca²⁺ entry (SOCE) after ER Ca²⁺ depletion. STIM1 also interacts with EB1 and dynamically tracks microtubule (MT) plus ends. Nevertheless, the role of STIM1–EB1 interaction in regulating SOCE remains unresolved. Using live-cell imaging combined with a synthetic construct approach, we found that EB1 binding constitutes a trapping mechanism restricting STIM1 targeting to ER–PM junctions. We further showed that STIM1 oligomers retain EB1 binding ability in ER Ca²⁺-depleted cells. By trapping STIM1 molecules at dynamic contacts between the ER and MT plus ends, EB1 binding delayed STIM1 translocation to ER–PM junctions during ER Ca²⁺ depletion and prevented excess SOCE and ER Ca²⁺ overload. Our study suggests that STIM1–EB1 interaction shapes the kinetics and amplitude of local SOCE in cellular regions with growing MTs and contributes to spatiotemporal regulation of Ca²⁺ signaling crucial for cellular functions and homeostasis.

Preliminary Structure-Activity Relationship (SAR) of a Novel Series of Pyrazole SKF-96365 Analogues as Potential Store-Operated Calcium Entry (SOCE) Inhibitors

From a series of (1R, 1S)-1[β-(phenylalkoxy)-(phenetyl)]-1H-pyrazolium hydrochloride as new analogues of SKF-96365, one has an interesting effect for endoplasmic reticulum (ER) Ca²⁺ release and store-operated Ca²⁺ entry (SOCE) (IC₅₀ 25 μM) on the PLP-B lymphocyte cell line. A successful resolution of (±) 1-phenyl-2-(1H-pyrazol-1-yl)ethan-1-ol has been developed by using the method of “half-concentration” in the presence of (+)-(1S)- or (−)-(1R)-CSA.

Contribution of TMEM16F to pyroptotic cell death

Pyroptosis is a highly inflammatory form of programmed cell death that is caused by infection with intracellular pathogens and activation of canonical or noncanonical inflammasomes. The purinergic receptor P2X₇ is activated by the noncanonical inflammasome and contributes essentially to pyroptotic cell death. The Ca²⁺ activated phospholipid scramblase and ion channel TMEM16F has been shown earlier to control cellular effects downstream of purinergic P2X₇ receptors that ultimately lead to cell death. As pyroptotic cell death is accompanied by an increases in intracellular Ca²⁺, we asked whether TMEM16F is activated during pyroptosis. The N-terminal cleavage product of gasdermin D (GD-N) is an executioner of pyroptosis by forming large plasma membrane pores. Expression of GD-N enhanced basal Ca²⁺ levels and induced cell death. We observed that GD-N induced cell death in HEK293 and HAP1 cells, which was depending on expression of endogenous TMEM16F. GD-N activated large whole cell currents that were suppressed by knockdown or inhibition of TMEM16F. The results suggest that whole cell currents induced by the pore forming domain of gasdermin-D, are at least in part due to activation of TMEM16F. Knockdown of other TMEM16 paralogues expressed in HAP1 cells suggest TMEM16F as a crucial element during pyroptosis and excluded a role of other TMEM16 proteins. Thus TMEM16F supports pyroptosis and other forms of inflammatory cell death such as ferroptosis. Its potent inhibition by tannic acid may be part of the anti-inflammatory effects of flavonoids.

ORAI1 mutations abolishing store-operated Ca2+ entry cause anhidrotic ectodermal dysplasia with immunodeficiency

BACKGROUND

Store-operated Ca²⁺ entry (SOCE) through Ca²⁺ release-activated Ca²⁺ channels is an essential signaling pathway in many cell types. Ca²⁺ release-activated Ca²⁺ channels are formed by ORAI1, ORAI2, and ORAI3 proteins and activated by stromal interaction molecule (STIM) 1 and STIM2. Mutations in the ORAI1 and STIM1 genes that abolish SOCE cause a combined immunodeficiency (CID) syndrome that is accompanied by autoimmunity and nonimmunologic symptoms.

OBJECTIVE

We performed molecular and immunologic analysis of patients with CID, anhidrosis, and ectodermal dysplasia of unknown etiology.

METHODS

We performed DNA sequencing of the ORAI1 gene, modeling of mutations on ORAI1 crystal structure, analysis of ORAI1 mRNA and protein expression, SOCE measurements, immunologic analysis of peripheral blood lymphocyte populations by using flow cytometry, and histologic and ultrastructural analysis of patient tissues.

RESULTS

We identified 3 novel autosomal recessive mutations in ORAI1 in unrelated kindreds with CID, autoimmunity, ectodermal dysplasia with anhidrosis, and muscular dysplasia. The patients were homozygous for p.V181SfsX8, p.L194P, and p.G98R mutations in the ORAI1 gene that suppressed ORAI1 protein expression and SOCE in the patients' lymphocytes and fibroblasts. In addition to impaired T-cell cytokine production, ORAI1 mutations were associated with strongly reduced numbers of invariant natural killer T and regulatory T (Treg) cells and altered composition of γδ T-cell and natural killer cell subsets.

CONCLUSION

ORAI1 null mutations are associated with reduced numbers of invariant natural killer T and Treg cells that likely contribute to the patients' immunodeficiency and autoimmunity. ORAI1-deficient patients have dental enamel defects and anhidrosis, representing a new form of anhidrotic ectodermal dysplasia with immunodeficiency that is distinct from previously reported patients with anhidrotic ectodermal dysplasia with immunodeficiency caused by mutations in the nuclear factor κB signaling pathway (IKBKG and NFKBIA).

Knockdown of amyloid precursor protein increases calcium levels in the endoplasmic reticulum

Familial Alzheimer's disease (AD) is caused by mutations in the genes that encode amyloid precursor protein (APP) and presenilins. Disturbances in calcium homeostasis have been observed in various cellular and animal models of AD and are proposed to underlie the pathogenesis of the disease. Furthermore, wildtype presenilins were shown to regulate endoplasmic reticulum (ER) calcium homeostasis, although their precise mechanism of action remains controversial. To investigate whether APP also affects ER calcium levels, we used RNA interference to target the APP gene in cultured T84 cells in combination with two types of ER calcium sensors. Using a genetically encoded calcium indicator, GEM-CEPIA1er, we found that APP-deficient cells exhibited elevated resting calcium levels in the ER and prolonged emptying of ER calcium stores upon the cyclopiazonic acid-induced inhibition of sarco-endoplasmic reticulum calcium-ATPase. These effects could be ascribed to lower ER calcium leakage rates. Consistent with these results, translocation of the endogenous ER calcium sensor STIM1 to its target channel Orai1 was delayed following ER calcium store depletion. Our data suggest a physiological function of APP in the regulation of ER calcium levels.

A focus on extracellular Ca2+ entry into skeletal muscle

The main task of skeletal muscle is contraction and relaxation for body movement and posture maintenance. During contraction and relaxation, Ca²⁺ in the cytosol has a critical role in activating and deactivating a series of contractile proteins. In skeletal muscle, the cytosolic Ca²⁺ level is mainly determined by Ca²⁺ movements between the cytosol and the sarcoplasmic reticulum. The importance of Ca²⁺ entry from extracellular spaces to the cytosol has gained significant attention over the past decade. Store-operated Ca²⁺ entry with a low amplitude and relatively slow kinetics is a main extracellular Ca²⁺ entryway into skeletal muscle. Herein, recent studies on extracellular Ca²⁺ entry into skeletal muscle are reviewed along with descriptions of the proteins that are related to extracellular Ca²⁺ entry and their influences on skeletal muscle function and disease.

Ca2+ transport and signalling in enamel cells

Dental enamel is one of the most remarkable examples of matrix-mediated biomineralization. Enamel crystals form de novo in a rich extracellular environment in a stage-dependent manner producing complex microstructural patterns that are visually stunning. This process is orchestrated by specialized epithelial cells known as ameloblasts which themselves undergo striking morphological changes, switching function from a secretory role to a cell primarily engaged in ionic transport. Ameloblasts are supported by a host of cell types which combined represent the enamel organ. Fully mineralized enamel is the hardest tissue found in vertebrates owing its properties partly to the unique mixture of ionic species represented and their highly organized assembly in the crystal lattice. Among the main elements found in enamel, Ca2+ is the most abundant ion, yet how ameloblasts modulate Ca2+ dynamics remains poorly known. This review describes previously proposed models for passive and active Ca2+ transport, the intracellular Ca2+ buffering systems expressed in ameloblasts and provides an up-dated view of current models concerning Ca2+ influx and extrusion mechanisms, where most of the recent advances have been made. We also advance a new model for Ca2+ transport by the enamel organ.

ORAI2 modulates store-operated calcium entry and T cell-mediated immunity

Store-operated Ca2+ entry (SOCE) through Ca2+ release-activated Ca2+ (CRAC) channels is critical for lymphocyte function and immune responses. CRAC channels are hexamers of ORAI proteins that form the channel pore, but the contributions of individual ORAI homologues to CRAC channel function are not well understood. Here we show that deletion of Orai1 reduces, whereas deletion of Orai2 increases, SOCE in mouse T cells. These distinct effects are due to the ability of ORAI2 to form heteromeric channels with ORAI1 and to attenuate CRAC channel function. The combined deletion of Orai1 and Orai2 abolishes SOCE and strongly impairs T cell function. In vivo, Orai1/Orai2 double-deficient mice have impaired T cell-dependent antiviral immune responses, and are protected from T cell-mediated autoimmunity and alloimmunity in models of colitis and graft-versus-host disease. Our study demonstrates that ORAI1 and ORAI2 form heteromeric CRAC channels, in which ORAI2 fine-tunes the magnitude of SOCE to modulate immune responses.

From Stores to Sinks: Structural Mechanisms of Cytosolic Calcium Regulation

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

Cardiovascular and Hemostatic Disorders: SOCE in Cardiovascular Cells: Emerging Targets for Therapeutic Intervention

The discovery of the store-operated Ca²⁺ entry (SOCE) phenomenon is tightly associated with its recognition as a pathway of high (patho)physiological significance in the cardiovascular system. Early on, SOCE has been investigated primarily in non-excitable cell types, and the vascular endothelium received particular attention, while a role of SOCE in excitable cells, specifically cardiac myocytes and pacemakers, was initially ignored and remains largely enigmatic even to date. With the recent gain in knowledge on the molecular components of SOCE as well as their cellular organization within nanodomains, potential tissue/cell type-dependent heterogeneity of the SOCE machinery along with high specificity of linkage to downstream signaling pathways emerged for cardiovascular cells. The basis of precise decoding of cellular Ca²⁺ signals was recently uncovered to involve correct spatiotemporal organization of signaling components, and even minor disturbances in these assemblies trigger cardiovascular pathologies. With this chapter, we wish to provide an overview on current concepts of cellular organization of SOCE signaling complexes in cardiovascular cells with particular focus on the spatiotemporal aspects of coupling to downstream signaling and the potential disturbance of these mechanisms by pathogenic factors. The significance of these mechanistic concepts for the development of novel therapeutic strategies will be discussed.

Pathophysiological Significance of Store-Operated Calcium Entry in Megakaryocyte Function: Opening New Paths for Understanding the Role of Calcium in Thrombopoiesis

Store-Operated Calcium Entry (SOCE) is a universal calcium (Ca²⁺) influx mechanism expressed by several different cell types. It is now known that Stromal Interaction Molecule (STIM), the Ca²⁺ sensor of the intracellular compartments, together with Orai and Transient Receptor Potential Canonical (TRPC), the subunits of Ca²⁺ permeable channels on the plasma membrane, cooperate in regulating multiple cellular functions as diverse as proliferation, differentiation, migration, gene expression, and many others, depending on the cell type. In particular, a growing body of evidences suggests that a tight control of SOCE expression and function is achieved by megakaryocytes along their route from hematopoietic stem cells to platelet production. This review attempts to provide an overview about the SOCE dynamics in megakaryocyte development, with a focus on most recent findings related to its involvement in physiological and pathological thrombopoiesis.

Calcium Dyshomeostasis in Tubular Aggregate Myopathy

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

Micronutrients and Leptospirosis: A Review of the Current Evidence

BACKGROUND

Leptospirosis is one of the most widespread zoonoses and represents a major threat to human health. Due to the high burden of disease, limitations in diagnostics, and limited coverage and availability of effective human and veterinary vaccines, leptospirosis remains an important neglected zoonotic disease. Improved surveillance and identification of modifiable risk factors for leptospirosis are urgently needed to inform preventive interventions and reduce the risk and severity of Leptospira infection.

METHODOLOGY/PRINCIPAL FINDINGS

This review was conducted to examine the evidence that links micronutrient status and Leptospira infection. A total of 56 studies were included in this review: 28 in vitro, 17 animal, and 11 observational human studies. Findings indicated that Leptospira infection is associated with higher iron and calcium concentrations and hypomagnesemia.

CONCLUSIONS/SIGNIFICANCE

Few prospective studies and no randomized trials have been conducted to date to examine the potential role of micronutrients in Leptospira infection. The limited literature in this area constrains our ability to make specific recommendations; however, the roles of iron, calcium, and magnesium in leptospirosis represent important areas for future research. The role of micronutrients in leptospirosis risk and severity needs to be elucidated in larger prospective human studies to inform public health interventions.

Disturbed canonical nuclear factor of κ light chain signaling in B cells of patients with common variable immunodeficiency

BACKGROUND

Most patients with common variable immunodeficiency (CVID) present with severely reduced switched memory B-cell counts, and some display an increase of CD21^low B-cell counts (CVID 21low), whereas others do not (CVID 21norm). Altered B-cell receptor (BCR) signaling might contribute to the defective memory formation observed in patients with CVID.

OBJECTIVE

We sought to investigate canonical nuclear factor of κ light chain (NF-κB) signaling in B cells from patients with CVID as a central pathway in B-cell differentiation.

METHODS

Degradation of inhibitor of κBα (IκBα) and p65 phosphorylation, nuclear translocation of p65, and regulation of target genes and cell function were investigated after different modes of B-cell stimulation.

RESULTS

BCR-mediated canonical NF-κB signaling was impaired in all mature naive CVID-derived B cells. This impairment was more profound in naive B cells from CVID 21low patients than CVID 21norm patients and most pronounced in CD21^low B cells. The signaling defect translated into reduced induction of Bcl-xL and IκBα, 2 bona fide target genes of the canonical NF-κB pathway. CD40 ligand- and Toll-like receptor 9-mediated signaling were less strongly altered. Signaling in CD21^low B cells but not CD21⁺ B cells of patients with HIV was similarly affected.

CONCLUSION

Combined with the previous description of disturbed Ca²⁺ signaling, the discovery of NF-κB signaling defects, especially in CVID 21low patients, suggests a broad underlying signaling defect affecting especially BCR-derived signals. Given the immune phenotype of monogenic defects affecting Ca²⁺ and NF-κB signaling, the latter is more likely to contribute to the humoral deficiency. The strongly disturbed BCR signaling of CD21^low B cells is characteristic for this cell type and independent of the underlying disease.

Store-Operated Ca(2+) Entry in Follicular T Cells Controls Humoral Immune Responses and Autoimmunity

T follicular helper (Tfh) cells promote affinity maturation of B cells in germinal centers (GCs), whereas T follicular regulatory (Tfr) cells limit the GC reaction. Store-operated Ca(2+) entry (SOCE) through Ca(2+) release-activated Ca(2+) (CRAC) channels mediated by STIM and ORAI proteins is a fundamental signaling pathway in T lymphocytes. Conditional deletion of Stim1 and Stim2 genes in T cells abolished SOCE and strongly reduced antibody-mediated immune responses following viral infection caused by impaired differentiation and function of Tfh cells. Conversely, aging Stim1Stim2-deficient mice developed humoral autoimmunity with spontaneous autoantibody production due to abolished Tfr cell differentiation in the presence of residual Tfh cells. Mechanistically, SOCE controlled Tfr and Tfh cell differentiation through NFAT-mediated IRF4, BATF, and Bcl-6 transcription-factor expression. SOCE had a dual role in controlling the GC reaction by regulating both Tfh and Tfr cell differentiation, thus enabling protective B cell responses and preventing humoral autoimmunity.

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

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

Diseases caused by mutations in ORAI1 and STIM1

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

Selenoprotein K and protein palmitoylation

SIGNIFICANCE

Selenoprotein K (SelK) is an endoplasmic reticulum (ER) membrane protein, and its expression is sensitive to dietary selenium levels. A recently described role for SelK as a cofactor in catalyzing protein palmitoylation reactions provides an important link between low dietary selenium intake and suboptimal cellular functions that depend on this selenoprotein for palmitoylation.

RECENT ADVANCES

A recent breakthrough provided insight into the contribution of SelK to calcium (Ca(2+)) flux in immune cells. In particular, SelK is required for palmitoylation of the Ca(2+) channel protein, inositol-1,4,5-triphosphate receptor (IP3R) in the ER membrane. Without this post-translational modification, expression and function of the IP3R is impaired. SelK is required for palmitoylation of another transmembrane protein, CD36, and very likely other proteins. SelK serves as a cofactor during protein palmitoylation by binding to the protein acyltransferase, DHHC6, thereby facilitating addition of the palmitate via a thioester bond to the sulfhydryl group of cysteine residues of target proteins.

CRITICAL ISSUES

The association of DHHC6 and SelK is clearly important for immune cell functions and possibly other cell types. The step in the DHHC6 catalyzed S-acylation reaction on which SelK acts remains unclear and possible mechanisms of how the kinetics of the reaction are impacted by SelK binding to DHHC6 are presented here.

FUTURE DIRECTIONS

Uncovering the specific role of SelK in promoting DHHC6 catalyzed protein palmitoylation may open a new line of inquiry into other selenoproteins playing similar roles as cofactors for different enzymatic processes.

Discovery and structural optimization of 1-phenyl-3-(1-phenylethyl)urea derivatives as novel inhibitors of CRAC channel

AIM

Ca(2+)-release-activated Ca(2+) (CRAC) channel, a subfamily of store-operated channels, is formed by calcium release-activated calcium modulator 1 (ORAI1), and gated by stromal interaction molecule 1 (STIM1). CRAC channel may be a novel target for the treatment of immune disorders and allergy. The aim of this study was to identify novel small molecule CRAC channel inhibitors.

METHODS

HEK293 cells stably co-expressing both ORAI1 and STIM1 were used for high-throughput screening. A hit, 1-phenyl-3-(1-phenylethyl)urea, was identified that inhibited CRAC channels by targeting ORAI1. Five series of its derivatives were designed and synthesized, and their primary structure-activity relationships (SARs) were analyzed. All derivatives were assessed for their effects on Ca(2+) influx through CRAC channels on HEK293 cells, cytotoxicity in Jurkat cells, and IL-2 production in Jurkat cells expressing ORAI1-SS-eGFP.

RESULTS

A total of 19 hits were discovered in libraries containing 32 000 compounds using the high-throughput screening. 1-Phenyl-3-(1-phenylethyl)urea inhibited Ca(2+) influx with IC50 of 3.25±0.17 μmol/L. SAR study on its derivatives showed that the alkyl substituent on the α-position of the left-side benzylic amine (R1) was essential for Ca(2+) influx inhibition and that the S-configuration was better than the R-configuration. The derivatives in which the right-side R3 was substituted by an electron-donating group showed more potent inhibitory activity than those that were substituted by electron-withdrawing groups. Furthermore, the free N-H of urea was not necessary to maintain the high potency of Ca(2+) influx inhibition. The N,N'-disubstituted or N'-substituted derivatives showed relatively low cytotoxicity but maintained the ability to inhibit IL-2 production. Among them, compound 5b showed an improved inhibition of IL-2 production and low cytotoxicity.

CONCLUSION

1-Phenyl-3-(1-phenylethyl)urea is a novel CRAC channel inhibitor that specifically targets ORAI1. This study provides a new chemical scaffold for design and development of CRAC channel inhibitors with improved Ca(2+) influx inhibition, immune inhibition and low cytotoxicity.