Store-operated Ca2+ entry: A STIMulating stOrai

Calcium and Neural Stem Cell Proliferation

Intracellular calcium plays a pivotal role in central nervous system (CNS) development by regulating various processes such as cell proliferation, migration, differentiation, and maturation. However, understanding the involvement of calcium (Ca2+) in these processes during CNS development is challenging due to the dynamic nature of this cation and the evolving cell populations during development. While Ca2+ transient patterns have been observed in specific cell processes and molecules responsible for Ca2+ homeostasis have been identified in excitable and non-excitable cells, further research into Ca2+ dynamics and the underlying mechanisms in neural stem cells (NSCs) is required. This review focuses on molecules involved in Ca2+ entrance expressed in NSCs in vivo and in vitro, which are crucial for Ca2+ dynamics and signaling. It also discusses how these molecules might play a key role in balancing cell proliferation for self-renewal or promoting differentiation. These processes are finely regulated in a time-dependent manner throughout brain development, influenced by extrinsic and intrinsic factors that directly or indirectly modulate Ca2+ dynamics. Furthermore, this review addresses the potential implications of understanding Ca2+ dynamics in NSCs for treating neurological disorders. Despite significant progress in this field, unraveling the elements contributing to Ca2+ intracellular dynamics in cell proliferation remains a challenging puzzle that requires further investigation.

Zinc-sensing receptor activation induces endothelium-dependent hyperpolarization-mediated vasorelaxation of arterioles

BACKGROUND

The micronutrient zinc (Zn2+) is critical for cell function as intracellular signaling and endogenous ligand for Zn2+ sensing receptor (ZnR). Although cytosolic Zn2+ (cyt) signaling in the vascular system was studied previously, role of the ZnR has not been explored in vascular physiology.

METHODS

ZnR-mediated relaxation response of human submucosal arterioles and the mesenteric arterioles from wide-type (WT), ZnR-/- and TRPV4-/- mice were determined by a Mulvany-style wire myograph. The perfused vessel density (PVD) of mouse mesenteric arterioles was also measured in in vivo study. The expression of ZnR in arterioles and vascular endothelial cells (VEC) were examined by immunofluorescence staining, and its function was characterized in VEC by Ca2+ imaging and patch clamp study.

RESULTS

ZnR expression was detected on human submucosal arterioles, murine mesenteric arterioles and VEC but not in ZnR-/- mice. ZnR activation predominately induced endothelium-dependent hyperpolarization (EDH)-mediated vasorelaxation of arterioles in vitro and in vivo via Ca2+ signaling, which is totally different from endothelium-dependent vasorelaxation via Zn2+ (cyt) signaling reported previously. Furthermore, ZnR-induced vasorelaxation via EDH was significantly impaired in ZnR-/- and TRPV4-/- mice. Mechanistically, ZnR induced endothelium-dependent vasorelaxation predominately via PLC/IP3/IP3R and TRPV4/SOCE. The role of ZnR in regulating Ca2+ signaling and ion channels on VEC was verified by Ca2+ imaging and patch clamp techniques.

CONCLUSION

ZnR activation induces endothelium-dependent vasorelaxation of resistance vessels predominately via TRPV4/Ca2+/EDH pathway. We therefore not only provide new insights into physiological role of ZnR in vascular system but also may pave a potential pathway for developing Zn2+-based treatments for vascular disease.

STIM and Orai Mediated Regulation of Calcium Signaling in Age-Related Diseases

Tight spatiotemporal regulation of intracellular Ca²⁺ plays a critical role in regulating diverse cellular functions including cell survival, metabolism, and transcription. As a result, eukaryotic cells have developed a wide variety of mechanisms for controlling Ca²⁺ influx and efflux across the plasma membrane as well as Ca²⁺ release and uptake from intracellular stores. The STIM and Orai protein families comprising of STIM1, STIM2, Orai1, Orai2, and Orai3, are evolutionarily highly conserved proteins that are core components of all mammalian Ca²⁺ signaling systems. STIM1 and Orai1 are considered key players in the regulation of Store Operated Calcium Entry (SOCE), where release of Ca²⁺ from intracellular stores such as the Endoplasmic/Sarcoplasmic reticulum (ER/SR) triggers Ca²⁺ influx across the plasma membrane. SOCE, which has been widely characterized in non-excitable cells, plays a central role in Ca²⁺-dependent transcriptional regulation. In addition to their role in Ca²⁺ signaling, STIM1 and Orai1 have been shown to contribute to the regulation of metabolism and mitochondrial function. STIM and Orai proteins are also subject to redox modifications, which influence their activities. Considering their ubiquitous expression, there has been increasing interest in the roles of STIM and Orai proteins in excitable cells such as neurons and myocytes. While controversy remains as to the importance of SOCE in excitable cells, STIM1 and Orai1 are essential for cellular homeostasis and their disruption is linked to various diseases associated with aging such as cardiovascular disease and neurodegeneration. The recent identification of splice variants for most STIM and Orai isoforms while complicating our understanding of their function, may also provide insight into some of the current contradictions on their roles. Therefore, the goal of this review is to describe our current understanding of the molecular regulation of STIM and Orai proteins and their roles in normal physiology and diseases of aging, with a particular focus on heart disease and neurodegeneration.

Ablation of Collagen VI leads to the release of platelets with altered function

Megakaryocytes express collagen VI that regulates the release of functional platelets.

Collagen VI–null megakaryocytes and platelets display increased mTOR signaling and store-operated calcium entry.

Hemostatic abnormalities and impaired platelet function have been described in patients affected by connective tissue disorders. We observed a moderate bleeding tendency in patients affected by collagen VI–related disorders and investigated the defects in platelet functionality, whose mechanisms are unknown. We demonstrated that megakaryocytes express collagen VI that is involved in the regulation of functional platelet production. By exploiting a collagen VI–null mouse model (Col6a1^−/−), we found that collagen VI–null platelets display significantly increased susceptibility to activation and intracellular calcium signaling. Col6a1^−/− megakaryocytes and platelets showed increased expression of stromal interaction molecule 1 (STIM1) and ORAI1, the components of store-operated calcium entry (SOCE), and activation of the mammalian target of rapamycin (mTOR) signaling pathway. In vivo mTOR inhibition by rapamycin reduced STIM1 and ORAI1 expression and calcium flows, resulting in a normalization of platelet susceptibility to activation. These defects were cell autonomous, because transplantation of lineage-negative bone marrow cells from Col6a1^−/− mice into lethally irradiated wild-type animals showed the same alteration in SOCE and platelet activation seen in Col6a1^−/− mice. Peripheral blood platelets of patients affected by collagen VI–related diseases, Bethlem myopathy and Ullrich congenital muscular dystrophy, displayed increased expression of STIM1 and ORAI1 and were more prone to activation. Altogether, these data demonstrate the importance of collagen VI in the production of functional platelets by megakaryocytes in mouse models and in collagen VI–related diseases.

Cyclopiazonic Acid-Induced Ca2+ Store Depletion Initiates Endothelium-Dependent Hyperpolarization-Mediated Vasorelaxation of Mesenteric Arteries in Healthy and Colitis Mice

Purposes: Since the role of store-operated calcium entry (SOCE) in endothelium-dependent hyperpolarization (EDH)-mediated vasorelaxation of mesenteric arteries in health and colitis is not fully understood, cyclopiazonic acid (CPA), a specific inhibitor of the sarco(endo) plasmic reticulum calcium-ATPases (SERCA), was used as a SOCE activator to investigate its role in normal mice and its alteration in colitis mice.

Methods: The changes in Ca²⁺ signaling in vascular endothelial cells (VEC) were examined by single cell Ca²⁺ imaging and tension of mesenteric arteries in response to CPA were examined using Danish DMT520A microvascular measuring system.

Results: CPA activated the SOCE through depletion of the endoplasmic reticulum (ER) Ca²⁺ in endothelial cells. CPA had a concentration-dependent vasorelaxing effect in endothelium-intact mesenteric arteries, which was lost after endothelial removal. Both nitric oxide (NO) and prostacyclin (PGI₂) inhibitors did not affect CPA-induced vasorelaxation; however, after both NO and PGI₂ were inhibited, K_Ca channel blocker [10 mM tetraethylammonium chloride (TEA)] inhibited CPA-induced vasorelaxation while K_Ca channel activator (0.3 μM SKA-31) promoted it. Two SOCE blockers [30 μM SKF96365 and 100 μM flufenamic acid (FFA)], and an Orai channel blocker (30 μM GSK-7975A) inhibited this vasorelaxation. The inhibition of both Na⁺/K⁺-ATPase (NKA) and Na⁺/Ca²⁺-exchange (NCX) also inhibited CPA-induced vasorelaxation. Finally, the CPA involved in EDH-induced vasorelaxation by the depletion of ER Ca²⁺ of mesenteric arteries was impaired in colitis mice.

Conclusion: Depletion of ER Ca²⁺ by CPA induces a vasorelaxation of mesenteric arteries that is mediated through EDH mechanism and invokes the activation of SOCE. The CPA-induced endothelium-dependent dilation is impaired in colitis which may limit blood perfusion to the intestinal mucosa.

Priority Strategy of Intracellular Ca2+ Homeostasis in Skeletal Muscle Fibers During the Multiple Stresses of Hibernation

: Intracellular calcium (Ca²⁺) homeostasis plays a vital role in the preservation of skeletal muscle. In view of the well-maintained skeletal muscle found in Daurian ground squirrels (Spermophilus dauricus) during hibernation, we hypothesized that hibernators possess unique strategies of intracellular Ca²⁺ homeostasis. Here, cytoplasmic, sarcoplasmic reticulum (SR), and mitochondrial Ca²⁺ levels, as well as the potential Ca²⁺ regulatory mechanisms, were investigated in skeletal muscle fibers of Daurian ground squirrels at different stages of hibernation. The results showed that cytoplasmic Ca²⁺ levels increased in the skeletal muscle fibers during late torpor (LT) and inter-bout arousal (IBA), and partially recovered when the animals re-entered torpor (early torpor, ET). Furthermore, compared with levels in the summer active or pre-hibernation state, the activity and protein expression levels of six major Ca²⁺ channels/proteins were up-regulated during hibernation, including the store-operated Ca²⁺ entry (SOCE), ryanodine receptor 1 (RyR1), leucine zipper-EF-hand containing transmembrane protein 1 (LETM1), SR Ca²⁺ ATPase 1 (SERCA1), mitochondrial calcium uniporter complex (MCU complex), and calmodulin (CALM). Among these, the increased extracellular Ca²⁺ influx mediated by SOCE, SR Ca²⁺ release mediated by RyR1, and mitochondrial Ca²⁺ extrusion mediated by LETM1 may be triggers for the periodic elevation in cytoplasmic Ca²⁺ levels observed during hibernation. Furthermore, the increased SR Ca²⁺ uptake through SERCA1, mitochondrial Ca²⁺ uptake induced by MCU, and elevated free Ca²⁺ binding capacity mediated by CALM may be vital strategies in hibernating ground squirrels to attenuate cytoplasmic Ca²⁺ levels and restore Ca²⁺ homeostasis during hibernation. Compared with that in LT or IBA, the decreased extracellular Ca²⁺ influx mediated by SOCE and elevated mitochondrial Ca²⁺ uptake induced by MCU may be important mechanisms for the partial cytoplasmic Ca²⁺ recovery in ET. Overall, under extreme conditions, hibernating ground squirrels still possess the ability to maintain intracellular Ca²⁺ homeostasis.

An SAR study of hydroxy-trifluoromethylpyrazolines as inhibitors of Orai1-mediated store operated Ca2+ entry in MDA-MB-231 breast cancer cells using a convenient Fluorescence Imaging Plate Reader assay

The proteins Orai1 and STIM1 control store-operated Ca²⁺ entry (SOCE) into cells. SOCE is important for migration, invasion and metastasis of MDA-MB-231 human triple negative breast cancer (TNBC) cells and has been proposed as a target for cancer drug discovery. Two hit compounds from a medium throughput screen, displayed encouraging inhibition of SOCE in MDA-MB-231 cells, as measured by a Fluorescence Imaging Plate Reader (FLIPR) Ca²⁺ assay. Following NMR spectroscopic analysis of these hits and reassignment of their structures as 5-hydroxy-5-trifluoromethylpyrazolines, a series of analogues was prepared via thermal condensation reactions between substituted acylhydrazones and trifluoromethyl 1,3-dicarbonyl arenes. Structure-activity relationship (SAR) studies showed that small lipophilic substituents at the 2- and 3-positions of the RHS and 2-, 3- and 4-postions of the LHS terminal benzene rings improved activity, resulting in a novel class of potent and selective inhibitors of SOCE.

ADP-Ribosylation and Cross-Linking of Actin by Bacterial Protein Toxins

Actin and the actin cytoskeleton play fundamental roles in host-pathogen interactions. Proper function of the actin cytoskeleton is crucial for innate and acquired immune defense. Bacterial toxins attack the actin cytoskeleton by targeting regulators of actin. Moreover, actin is directly modified by various bacterial protein toxins and effectors, which cause ADP-ribosylation or cross-linking of actin. Modification of actin can result in inhibition or stimulation of actin polymerization. Toxins, acting directly on actin, are reviewed.

When under pressure, get closer: PERKing up membrane contact sites during ER stress

The endoplasmic reticulum (ER) is the main hub of cellular Ca(2+)signalling and protein synthesis and folding. The ER moreover is the central player in the formation of contact sites with other organelles and structures, including mitochondria, plasma membrane (PM) and endosomes. The most studied of these, the ER-mitochondria contact sites, are crucial regulators of cellular Ca(2+)homoeostasis, metabolism and cell death signalling. Protein kinase RNA-like ER kinase (PERK), an ER stress kinase and crucial signalling protein in the unfolded protein response (UPR), was found to be able to orchestrate contact sites between the ER and mitochondria and to be indispensable for the pre-apoptotic trafficking of calreticulin (CRT) at the PM during immunogenic cell death (ICD). Furthermore, PERK has recently been linked with ER and PM contact sites through the mechanism of store-operated Ca(2+)entry (SOCE). Here we discuss emerging findings disclosing novel roles of the ER stress sensor PERK in orchestrating inter-organellar communication in the context of ER stress.

Evaluation of known and novel inhibitors of Orai1-mediated store operated Ca2+ entry in MDA-MB-231 breast cancer cells using a Fluorescence Imaging Plate Reader assay

The Orai1 Ca²⁺ permeable ion channel is an important component of store operated Ca²⁺ entry (SOCE) in cells. It's over-expression in basal molecular subtype breast cancers has been linked with poor prognosis, making it a potential target for drug development. We pharmacologically characterised a number of reported inhibitors of SOCE in MDA-MB-231 breast cancer cells using a convenient Fluorescence Imaging Plate Reader (FLIPR) assay, and show that the rank order of their potencies in this assay is the same as those reported in a wide range of published assays. The assay was also used in a screening project seeking novel inhibitors. Following a broad literature survey of classes of calcium channel inhibitors we used simplified ligand structures to query the ZINC on-line database, and following two iterations of refinement selected a novel Orai1-selective dichlorophenyltriazole hit compound. Analogues of this were synthesized and evaluated in the FLIPR assay to develop structure-activity relationships (SAR) for the three domains of the hit; triazole (head), dichlorophenyl (body) and substituted phenyl (tail). For this series, the results suggested the need for a lipophilic tail domain and an out-of-plane twist between the body and tail domains.

The effect of bradykinin on the electrical activity of rat myenteric neurons

Bradykinin is a mediator involved in inflammatory processes in the gut. Here we investigated the effect of bradykinin on the electrical activity of rat myenteric neurons, the key players for regulation of gastrointestinal motility. Bradykinin (2 × 10(-8)mol/l) induced a biphasic increase in frequency of action potentials measured with microelectrode arrays. This increase was mirrored by a biphasic increase in cytosolic Ca(2+) concentration ([Ca(2+)]i), which was observed in about 40% of the myenteric neurons. The bradykinin B1 receptor agonist des-arg(9)-bradykinin as well as the bradykinin B2 receptor agonist hyp(3)-bradykinin induced a similar effect on [Ca(2+)]i. Immunocytochemical stainings confirmed the expression of both receptor types by myenteric ganglionic cells. Real time PCR showed that the inducible B1 receptor was upregulated during cell culture. The inhibition of cyclooxygenases with piroxicam reduced the effect of bradykinin on the electrical activity of myenteric neurons. The suppression of the glial growth on microelectrode arrays did not affect the bradykinin-induced change in frequency of action potentials. This suggests that prostaglandins, which probably mediate the effect of bradykinin, are not exclusively released from glial cells. The bradykinin-induced increase in [Ca(2+)]i was dependent on the presence of extracellular Ca(2+) and was inhibited by Co(2+), Cd(2+), and Ni(2+), blockers of voltage-dependent Ca(2+) channels, indicating a stimulation of the influx of extracellular Ca(2+) by the kinin. Consequently, bradykinin induces a Ca(2+) influx in myenteric neurons via Ca(2+) channels in the plasma membrane.

STIM1-regulated Ca2+ influx across the apical and the basolateral membrane in colonic epithelium

In nonexcitable cells, store-operated Ca(2+) entry is the most important pathway for influx of extracellular Ca(2+) serving as a second messenger in the cytoplasm. The present study investigated the expression, localization and polar distribution of two key components of store-operated Ca(2+) entry identified, e.g., in lymphocytes or epithelial cell lines-STIM1 (stromal interacting molecule 1), working as a Ca(2+) sensor in the endoplasmic reticulum, and Orai1, working as the (or part of the) store-operated Ca(2+) channel in the plasma membrane-in a native intestinal epithelium, i.e., rat colon. Immunohistochemical investigations revealed expression of STIM1 and Orai1 in the rat colonic epithelium. Ca(2+) store depletion led to a translocation of STIM1 both to the basolateral as well as to the apical cell pole as observed by confocal microscopy. A Ca(2+) depletion/repletion protocol was used in Ussing chamber experiments to investigate the contribution of basolateral and apical store-operated Ca(2+) entry to the induction of anion secretion. These experiments revealed that Ca(2+)-dependent anion secretion was induced not only by basolateral Ca(2+) repletion but also, to a lesser extent, by apical Ca(2+) repletion. Both responses were suppressed by La(3+). The effect of basolateral Ca(2+) repletion was significantly inhibited by brefeldin A, a blocker of vesicular transport from the endoplasmic reticulum to the Golgi apparatus. In a final series of experiments, fura-2-loaded HT29/B6 cells were used. A carbachol-induced increase in the cytosolic Ca(2+) concentration was significantly reduced when cells were pretreated with siRNA against STIM1. In conclusion, these results demonstrate that STIM1 as a key component of intracellular Ca(2+) signaling is expressed by rat colonic epithelium and is involved in the regulation not only of basolateral but also of apical Ca(2+) influx.

Caveolin-1 regulation of store-operated Ca(2+) influx in human airway smooth muscle

Caveolae, plasma membrane invaginations with constitutive caveolin proteins, harbour proteins involved in intracellular calcium ([Ca(2+)](i)) regulation. In human airway smooth muscle (ASM), store-operated Ca(2+) entry (SOCE) is a key component of [Ca(2+)](i) regulation, and contributes to increased [Ca(2+)](i) in inflammation. SOCE involves proteins Orai1 and stromal interaction molecule (STIM)1. We investigated the link between caveolae, SOCE and inflammation in ASM. [Ca(2+)](i) was measured in human ASM cells using fura-2. Small interference RNA (siRNA) or overexpression vectors were used to alter expression of caveolin-1 (Cav-1), Orai1 or STIM1. Tumour necrosis factor (TNF)-α was used as a representative pro-inflammatory cytokine. TNF-α increased SOCE following sarcoplasmic reticulum Ca(2+) depletion, and increased whole-cell and caveolar Orai1 (but only intracellular STIM1). Cav-1 siRNA decreased caveolar and whole-cell Orai1 (but not STIM1) expression, and blunted SOCE, even in the presence of TNF-α. STIM1 overexpression substantially enhanced SOCE: an effect only partially reversed by Cav-1 siRNA. In contrast, Orai1 siRNA substantially blunted SOCE even in the presence of TNF-α. Cav-1 overexpression significantly increased Orai1 expression and SOCE, especially in the presence of TNF-α. These results demonstrate that caveolar expression and regulation of proteins such as Orai1 are important for [Ca(2+)](i) regulation in human ASM cells and its modulation during inflammation.

Calcium influx, a new potential therapeutic target in the control of neutrophil-dependent inflammatory diseases in bovines

Neutrophils are the first line of defense against pathogens in bovines; however, they are also one of the most aggressive cells during the inflammatory process, causing injury in surrounding tissues. At present, anti-inflammatory drugs are limited in acute diseases, such as pneumonia, mastitis and endometritis, because neutrophils are mostly insensitive. One of the earliest events during neutrophil activation is the increase in intracellular calcium concentration. The calcium movement is attributed to the release from intracellular stores and influx through the calcium channels in the plasma membrane, a process called store operated calcium entry (SOCE). Recently, several calcium influx blockers have been shown to have strong effects on bovine neutrophils, and this suggests that the manipulation of this pathway can be useful in the control of neutrophil functions during acute inflammatory processes. In this paper, we will review the role of calcium influx as a potential anti-inflammatory target and summarize the most recent evidences for this in bovine neutrophils.

Extracellular nucleotides elicit cytosolic free calcium oscillations in Arabidopsis

Extracellular ATP induces a rise in the level of cytosolic free calcium ([Ca(2+)](cyt)) in plant cells. To expand our knowledge about the function of extracellular nucleotides in plants, the effects of several nucleotide analogs and pharmacological agents on [Ca(2+)](cyt) changes were studied using transgenic Arabidopsis (Arabidopsis thaliana) expressing aequorin or the fluorescence resonance energy transfer-based Ca(2+) sensor Yellow Cameleon 3.6. Exogenously applied CTP caused elevations in [Ca(2+)](cyt) that displayed distinct time- and dose-dependent kinetics compared with the purine nucleotides ATP and GTP. The inhibitory effects of antagonists of mammalian P2 receptors and calcium influx inhibitors on nucleotide-induced [Ca(2+)](cyt) elevations were distinct between CTP and purine nucleotides. These results suggest that distinct recognition systems may exist for the respective types of nucleotides. Interestingly, a mutant lacking the heterotrimeric G protein Gβ-subunit exhibited a remarkably higher [Ca(2+)](cyt) elevation in response to all tested nucleotides in comparison with the wild type. These data suggest a role for Gβ in negatively regulating extracellular nucleotide signaling and point to an important role for heterotrimeric G proteins in modulating the cellular effects of extracellular nucleotides. The addition of extracellular nucleotides induced multiple temporal [Ca(2+)](cyt) oscillations, which could be localized to specific root cells. The oscillations were attenuated by a vesicle-trafficking inhibitor, indicating that the oscillations likely require ATP release via exocytotic secretion. The results reveal new molecular details concerning extracellular nucleotide signaling in plants and the importance of fine control of extracellular nucleotide levels to mediate specific plant cell responses.

Intracellular Ca2+ signaling and store-operated Ca2+ entry are required in Drosophila neurons for flight

Neuronal Ca(2+) signals can affect excitability and neural circuit formation. Ca(2+) signals are modified by Ca(2+) flux from intracellular stores as well as the extracellular milieu. However, the contribution of intracellular Ca(2+) stores and their release to neuronal processes is poorly understood. Here, we show by neuron-specific siRNA depletion that activity of the recently identified store-operated channel encoded by dOrai and the endoplasmic reticulum Ca(2+) store sensor encoded by dSTIM are necessary for normal flight and associated patterns of rhythmic firing of the flight motoneurons of Drosophila melanogaster. Also, dOrai overexpression in flightless mutants for the Drosophila inositol 1,4,5-trisphosphate receptor (InsP(3)R) can partially compensate for their loss of flight. Ca(2+) measurements show that Orai gain-of-function contributes to the quanta of Ca(2+)-release through mutant InsP(3)Rs and elevates store-operated Ca(2+) entry in Drosophila neurons. Our data show that replenishment of intracellular store Ca(2+) in neurons is required for Drosophila flight.

Molecular determinants of the coupling between STIM1 and Orai channels: differential activation of Orai1-3 channels by a STIM1 coiled-coil mutant

STIM1 and Orai1 have been reported to interact upon store depletion culminating in Ca(2+) release-activated Ca(2+) current activation. Recently, the essential region has been identified within the STIM1 C terminus that includes the second coiled-coil domain C-terminally extended by approximately 50 amino acids and exhibits a strong binding to the Orai1 C terminus. Based on the homology within the Orai family, an analogous scenario might be assumed for Orai2 as well as Orai3 channels as both are activated in a similar STIM1-dependent manner. A combined approach of electrophysiology and Foerster resonance energy transfer microscopy uncovered a general mechanism in the communication of STIM1 with Orai proteins that involved the conserved putative coiled-coil domains in the respective Orai C terminus and the second coiled-coil motif in the STIM1 C terminus. A coiled-coil single mutation in the Orai1 C terminus abrogated communication with the STIM1 C terminus, whereas an analogous mutation in Orai2 and Orai3 still allowed for their moderate activation. However, increasing coiled-coil probability by a gain of function deletion in Orai1 or by generating an Orai1-Orai3 chimera containing the Orai3 C terminus recovered stimulation to a similar extent as with Orai2/3. At the level of STIM1, decreasing probability of the second coiled-coil domain by a single mutation within the STIM1 C terminus abolished activation of Orai1 but still enabled partial stimulation of Orai2/3 channels. A double mutation within the second coiled-coil motif of the STIM1 C terminus fully disrupted communication with all three Orai channels. In aggregate, the impairment in the overall communication between STIM1 and Orai channels upon decreasing probabilities of either one of the putative coiled-coil domains in the C termini might be compatible with the concept of their functional, heteromeric interaction.

Properties of Orai1 mediated store-operated current depend on the expression levels of STIM1 and Orai1 proteins

Two cellular proteins, stromal interaction molecule 1 (STIM1) and Orai1, are recently discovered essential components of the Ca2+ release activated Ca2+ (CRAC) channel. Orai1 polypeptides form the pore of the CRAC channel, while STIM1 plays the role of the endoplasmic reticulum Ca2+ sensor required for activation of CRAC current (I(CRAC)) by store depletion. It is not known, however, if the role of STIM1 is limited exclusively to Ca2+ sensing, or whether interaction between Orai1 and STIM1, either direct or indirect, also defines the properties of I(CRAC). In this study we investigated how the relative expression levels of ectopic Orai1 and STIM1 affect the properties of I(CRAC). The results show that cells expressing low Orai1 : STIM1 ratios produce I(CRAC) with strong fast Ca2+-dependent inactivation, while cells expressing high Orai1 : STIM1 ratios produce I(CRAC) with strong activation at negative potentials. Moreover, the expression ratio of Orai1 and STIM1 affects Ca2+, Ba2+ and Sr2+ conductance, but has no effect on the current in the absence of divalent cations. The results suggest that several key properties of Ca2+ channels formed by Orai1 depend on its interaction with STIM1, and that the stoichiometry of this interaction may vary depending on the relative expression levels of these proteins.

Identification of alternative splicing markers for breast cancer

Breast cancer is the most common cause of cancer death among women under age 50 years, so it is imperative to identify molecular markers to improve diagnosis and prognosis of this disease. Here, we present a new approach for the identification of breast cancer markers that does not measure gene expression but instead uses the ratio of alternatively spliced mRNAs as its indicator. Using a high-throughput reverse transcription-PCR-based system for splicing annotation, we monitored the alternative splicing profiles of 600 cancer-associated genes in a panel of 21 normal and 26 cancerous breast tissues. We validated 41 alternative splicing events that significantly differed in breast tumors relative to normal breast tissues. Most cancer-specific changes in splicing that disrupt known protein domains support an increase in cell proliferation or survival consistent with a functional role for alternative splicing in cancer. In a blind screen, a classifier based on the 12 best cancer-associated splicing events correctly identified cancer tissues with 96% accuracy. Moreover, a subset of these alternative splicing events could order tissues according to histopathologic grade, and 5 markers were validated in a further blind set of 19 grade 1 and 19 grade 3 tumor samples. These results provide a simple alternative for the classification of normal and cancerous breast tumor tissues and underscore the putative role of alternative splicing in the biology of cancer.

Crosstalk between calcium and redox signaling: from molecular mechanisms to health implications

Studies done many years ago established unequivocally the key role of calcium as a universal second messenger. In contrast, the second messenger roles of reactive oxygen and nitrogen species have emerged only recently. Therefore, their contributions to physiological cell signaling pathways have not yet become universally accepted, and many biological researchers still regard them only as cellular noxious agents. Furthermore, it is becoming increasingly apparent that there are significant interactions between calcium and redox species, and that these interactions modify a variety of proteins that participate in signaling transduction pathways and in other fundamental cellular functions that determine cell life or death. This review article addresses first the central aspects of calcium and redox signaling pathways in animal cells, and continues with the molecular mechanisms that underlie crosstalk between calcium and redox signals under a number of physiological or pathological conditions. To conclude, the review focuses on conditions that, by promoting cellular oxidative stress, lead to the generation of abnormal calcium signals, and how this calcium imbalance may cause a variety of human diseases including, in particular, degenerative diseases of the central nervous system and cardiac pathologies.

Dynamic simulation of the effect of calcium-release activated calcium channel on cytoplasmic Ca2+ oscillation

A mathematical model is proposed to illustrate the activation of STIM1 (stromal interaction molecule 1) protein, the assembly and activation of calcium-release activated calcium (CRAC) channels in T cells. In combination with De Young-Keizer-Li-Rinzel model, we successfully reproduce a sustained Ca(2+) oscillation in cytoplasm. Our results reveal that Ca(2+) oscillation dynamics in cytoplasm can be significantly affected by the way how the Orai1 CRAC channel are assembled and activated. A low sustained Ca(2+) influx is observed through the CRAC channels across the plasma membrane. In particular, our model shows that a tetrameric channel complex can effectively regulate the total quantity of the channels and the ratio of the active channels to the total channels, and a period of Ca(2+) oscillation about 29 s is in agreement with published experimental data. The bifurcation analyses illustrate the different dynamic properties between our mixed Ca(2+) feedback model and the single positive or negative feedback models.

Reversible translocation of EYFP-tagged STIM1 is coupled to calcium influx in insulin secreting beta-cells

Calcium (Ca(2+)) signaling regulates insulin secretion in pancreatic beta-cells. STIM1 has been proposed to function as an endoplasmic reticulum (ER) Ca(2+) sensor regulating store-operated Ca(2+) entry (SOCE). Here we studied the translocation of EYFP-STIM1 in response to ER calcium depletion in mouse insulinoma MIN6 cells by fluorescent microscopy. While in resting cells EYFP-STIM1 is co-localized with an ER marker, in thapsigargin (Tg)-stimulated cells it occupied highly defined areas of the peri-PM space in punctae adjacent to, but not entirely coincident with the ER. Co-staining with fluorescent phalloidin revealed that EYFP-STIM1 punctae was located in actin-poor areas. Use of the SOCE blocker in MIN6 cells, 2-aminoethoxy diphenylborate (2-APB), prevented store depletion-dependent translocation of EYFP-STIM1 to the PM in a concentration-dependent (3.75-100muM) and reversible manner. TIRF microscopy revealed that 2-APB treatment led to the reversible disappearance of peri-PM EYFP-STIM1 punctae, while the ER structure in this compartment remained grossly unaffected. We conclude from this data that in these cells EYFP-STIM1 is delivered to a peri-PM location from the ER upon store depletion and this trafficking is reversibly blocked by 2-APB.

Store-operated Ca(2+) channels and Stromal Interaction Molecule 1 (STIM1) are targets for the actions of bile acids on liver cells

Cholestasis is a significant contributor to liver pathology and can lead to primary sclerosis and liver failure. Cholestatic bile acids induce apoptosis and necrosis in hepatocytes but these effects can be partially alleviated by the pharmacological application of choleretic bile acids. These actions of bile acids on hepatocytes require changes in the release of Ca(2+) from intracellular stores and in Ca(2+) entry. However, the nature of the Ca(2+) entry pathway affected is not known. We show here using whole cell patch clamp experiments with H4-IIE liver cells that taurodeoxycholic acid (TDCA) and other choleretic bile acids reversibly activate an inwardly-rectifying current with characteristics similar to those of store-operated Ca(2+) channels (SOCs), while lithocholic acid (LCA) and other cholestatic bile acids inhibit SOCs. The activation of Ca(2+) entry was observed upon direct addition of the bile acid to the incubation medium, whereas the inhibition of SOCs required a 12 h pre-incubation. In cells loaded with fura-2, choleretic bile acids activated a Gd(3+)-inhibitable Ca(2+) entry, while cholestatic bile acids inhibited the release of Ca(2+) from intracellular stores and Ca(2+) entry induced by 2,5-di-(tert-butyl)-1,4-benzohydro-quinone (DBHQ). TDCA and LCA each caused a reversible redistribution of stromal interaction molecule 1 (STIM1, the endoplasmic reticulum Ca(2+) sensor required for the activation of Ca(2+) release-activated Ca(2+) channels and some other SOCs) to puncta, similar to that induced by thapsigargin. Knockdown of Stim1 using siRNA caused substantial inhibition of Ca(2+)-entry activated by choleretic bile acids. It is concluded that choleretic and cholestatic bile acids activate and inhibit, respectively, the previously well-characterised Ca(2+)-selective hepatocyte SOCs through mechanisms which involve the bile acid-induced redistribution of STIM1.

Ca(2+) -permeable channels in the hepatocyte plasma membrane and their roles in hepatocyte physiology

Hepatocytes are highly differentiated and spatially polarised cells which conduct a wide range of functions, including intermediary metabolism, protein synthesis and secretion, and the synthesis, transport and secretion of bile acids. Changes in the concentrations of Ca(2+) in the cytoplasmic space, endoplasmic reticulum (ER), mitochondria, and other intracellular organelles make an essential contribution to the regulation of these hepatocyte functions. While not yet fully understood, the spatial and temporal parameters of the cytoplasmic Ca(2+) signals and the entry of Ca(2+) through Ca(2+)-permeable channels in the plasma membrane are critical to the regulation by Ca(2+) of hepatocyte function. Ca(2+) entry across the hepatocyte plasma membrane has been studied in hepatocytes in situ, in isolated hepatocytes and in liver cell lines. The types of Ca(2+)-permeable channels identified are store-operated, ligand-gated, receptor-activated and stretch-activated channels, and these may vary depending on the animal species studied. Rat liver cell store-operated Ca(2+) channels (SOCs) have a high selectivity for Ca(2+) and characteristics similar to those of the Ca(2+) release activated Ca(2+) channels in lymphocytes and mast cells. Liver cell SOCs are activated by a decrease in Ca(2+) in a sub-region of the ER enriched in type1 IP(3) receptors. Activation requires stromal interaction molecule type 1 (STIM1), and G(i2alpha,) F-actin and PLCgamma1 as facilitatory proteins. P(2x) purinergic channels are the only ligand-gated Ca(2+)-permeable channels in the liver cell membrane identified so far. Several types of receptor-activated Ca(2+) channels have been identified, and some partially characterised. It is likely that TRP (transient receptor potential) polypeptides, which can form Ca(2+)- and Na(+)-permeable channels, comprise many hepatocyte receptor-activated Ca(2+)-permeable channels. A number of TRP proteins have been detected in hepatocytes and in liver cell lines. Further experiments are required to characterise the receptor-activated Ca(2+) permeable channels more fully, and to determine the molecular nature, mechanisms of activation, and precise physiological functions of each of the different hepatocyte plasma membrane Ca(2+) permeable channels.

Store-operated calcium entry mediates intracellular alkalinization, ERK1/2, and Akt/PKB phosphorylation in bovine neutrophils

Neutrophil's responses to G protein-coupled chemoattractants are highly dependent on store-operated calcium (Ca(2+)) entry (SOCE). Platelet-activating factor (PAF), a primary chemoattractant, simultaneously increases cytosolic-free Ca(2+), intracellular pH (pH(i)), ERK1/2, and Akt/protein kinase B (PKB) phosphorylation. In this study, we looked at the efficacy of several putative SOCE inhibitors and whether SOCE mediates intracellular alkalinization, ERK1/2, and Akt/PKB phosphorylation in bovine neutrophils. We demonstrated that the absence of external Ca(2+) and the presence of EGTA reduced the intracellular alkalinization and ERK1/2 phosphorylation induced by PAF, apparently via SOCE influx inhibition. Next, we tested the efficacy of several putative SOCE inhibitors such as 2-aminoethoxydiphenyl borate (2-APB), capsaicin, flufenamic acid, 1-{beta-[3-(4-methoxy-phenyl)propoxy]-4-methoxyphenethyl}-1H-imidazole hydrochloride (SK&F 96365), and N-(4-[3,5-bis(trifluoromethyl)-1H-pyrazol-1-yl]phenyl)-4-methyl-1,2,3-thiadiazole-5-carboxamide (BTP2) on Ca(2+) entry induced by PAF or thapsigargin. 2-APB was the most potent SOCE inhibitor, followed by capsaicin and flufenamic acid. Conversely, SK&F 96365 reduced an intracellular calcium ([Ca(2+)](i)) peak but SOCE partially. BTP2 did not show an inhibitory effect on [Ca(2+)](i) following PAF stimuli. 2-APB strongly reduced the pH(i) recovery, whereas the effect of flufenamic acid and SK&F 96365 was partial. Capsaicin and BTP2 did not affect the pH(i) changes induced by PAF. Finally, we observed that 2-APB reduced the ERK1/2 and Akt phosphorylation completely, whereas the inhibition with flufenamic acid was partial. The results suggest that 2-APB is the most potent SOCE inhibitor and support a key role of SOCE in pH alkalinization and PI-3K-ERK1/2 pathway control. Finally, 2-APB could be an important tool to characterize Ca(2+) signaling in neutrophils.

Molecular evolution and functional divergence of the Ca(2+) sensor protein in store-operated Ca(2+) entry: stromal interaction molecule

Receptor-mediated Ca²⁺ signaling in many non-excitable cells initially induces Ca²⁺ release from intracellular Ca²⁺ stores, followed by Ca²⁺ influx across the plasma membrane. Recent findings have suggested that stromal interaction molecules (STIMs) function as the Ca²⁺ sensor to detect changes of Ca²⁺ content in the intracellular Ca²⁺ stores. Human STIMs and invertebrate STIM share several functionally important protein domains, but diverge significantly in the C-terminus. To better understand the evolutionary significance of STIM activity, phylogenetic analysis of the STIM protein family was conducted after extensive database searching. Results from phylogeny and sequence analysis revealed early adaptation of the C-terminal divergent domains in Urochordata, before the expansion of STIMs in Vertebrata. STIMs were subsequently subjected to one round of gene duplication as early as in the Euteleostomi lineage in vertebrates, with a second round of fish-specific gene duplication. After duplication, STIM-1 and STIM-2 molecules appeared to have undergone purifying selection indicating strong evolutionary constraints within each group. Furthermore, sequence analysis of the EF-hand Ca²⁺ binding domain and the SAM domain, together with functional divergence studies, identified critical regions/residues likely underlying functional changes, and provided evidence for the hypothesis that STIM-1 and STIM-2 might have developed distinct functional properties after duplication.

The TRP channel superfamily: insights into how structure, protein-lipid interactions and localization influence function

TRP (transient receptor potential) cationic channels are key molecules that are involved in a variety of diverse biological processes ranging from fertility to osmosensation and nociception. Increasing our knowledge of these channels will help us to understand a range of physiological and pathogenic processes, as well as highlighting potential therapeutic drug targets. The founding members of the TRP family, Drosophila TRP and TRPL (TRP-like) proteins, were identified within the last two decades and there has been a subsequent explosion in the number and type of TRP channel described. Although information is accumulating as to the function of some of the TRP channels, the activation and inactivation mechanisms, structure, and interacting proteins of many, if not most, are awaiting elucidation. The Cell and Molecular Biology of TRP Channels Meeting held at the University of Bath included speakers working on a number of the different subfamilies of TRP channels and provided a basis for highlighting both similarities and differences between these groups. As the TRP channels mediate diverse functions, this meeting also brought together an audience with wide-ranging research interests, including biochemistry, cell biology, physiology and neuroscience, and inspired lively discussion on the issues reviewed herein.

Dissecting ICRAC, a store-operated calcium current

The use of Ca(2+) for intracellular signalling necessitates tight local and global control of cytoplasmic Ca(2+) concentration, and mechanisms for maintaining the net Ca(2+) balance. It has long been recognized that intracellular Ca(2+) stores exert control over Ca(2+) influx at the plasma membrane through a process of store-operated Ca(2+) entry (SOCE). The Ca(2+) current I(CRAC) is the best characterized instance of SOCE, but the elements of the pathway leading to I(CRAC) have eluded biochemical definition for more than a decade. However, the recent identification of key proteins underlying I(CRAC)--STIM1 and Orai1--has led to several insights into this ER-to-plasma membrane signalling system and to the recognition that it is an ancient and conserved mechanism in multicellular organisms.

Biochemical properties and cellular localisation of STIM proteins

Human and murine STIM1 were originally discovered as candidate growth regulators in tumours and in the bone marrow stroma, and the structurally related vertebrate family members, STIM2 and the Drosophila homologue D-Stim, were subsequently identified. STIM proteins are ubiquitously expressed type I single-pass transmembrane proteins which have a unique combination of structural motifs within their polypeptide sequences. The extracellular regions contain an N-terminal unpaired EF-hand Ca(2+) binding motif adjacent to an unconventional glycosylated SAM domain, while the cytoplasmic regions contain alpha-helical coiled-coil domains within a region having homology to ERM domains adjacent to the transmembrane region, and phosphorylated proline-rich domains near the C-terminus. STIM1, STIM2 and D-Stim diverge significantly only in their structure C-terminal to the coiled-coil/ERM domains. The STIM structural domains were predicted to function in Ca(2+) binding as well as in mediating interactions between STIM proteins and other proteins, and homotypic STIM1-STIM1 and heterotypic STIM1-STIM2 interactions were demonstrated biochemically. However, the functional significance of the cellular localisation of STIM1 and its domain structure only became evident after recent breakthrough research identified STIM1 as a key regulator of store-operated calcium (SOC) entry into cells. It is now clear that STIM1 is both a sensor of Ca(2+) depletion in the endoplasmic reticulum (ER) lumen and an activator of Orai1-containing SOC channels in the plasma membrane. On the basis of recent functional studies a model can be proposed to explain how the biochemical properties of STIM1 contribute to its precise membrane localisation and its function in regulating SOC entry.

Molecular evolution and structural analysis of the Ca(2+) release-activated Ca(2+) channel subunit, Orai

Depletion of intracellular Ca(2+) stores evokes Ca(2+) entry across the plasma membrane by inducing Ca(2+) release-activated Ca(2+) (CRAC) currents in many cell types. Recently, Orai and STIM proteins were identified as the molecular identities of the CRAC channel subunit and the endoplasmic reticulum Ca(2+) sensor, respectively. Here, extensive database searching and phylogenetic analysis revealed several lineage-specific duplication events in the Orai protein family, which may account for the evolutionary origins of distinct functional properties among mammalian Orai proteins. Based on similarity to key structural domains and essential residues for channel functions in Orai proteins, database searching also identifies a putative primordial Orai sequence in hyperthermophilic archaeons. Furthermore, modern Orai appears to acquire new structural domains as early as Urochodata, before divergence into vertebrates. The evolutionary patterns of structural domains might be related to distinct functional properties of Drosophila and mammalian CRAC currents. Interestingly, Orai proteins display two conserved internal repeats located at transmembrane segments 1 and 3, both of which contain key amino acids essential for channel function. These findings demonstrate biochemical and physiological relevance of Orai proteins in light of different evolutionary origins and will provide novel insights into future structural and functional studies of Orai proteins.