Distinct spatial Ca2+ signatures selectively activate different NFAT transcription factor isoforms

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

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

Differential engagement of ORAI1 and TRPC1 in the induction of vimentin expression by different stimuli

The Ca²⁺ signal is essential in both hypoxia- and epidermal growth factor (EGF)-mediated epithelial to mesenchymal transition (EMT) in MDA-MB-468 breast cancer cells. This finding suggests that Ca²⁺-permeable ion channels participate in the induction of expression of some mesenchymal markers such as vimentin. However, the ion channels involved in vimentin expression induction have not been fully characterized. This work sought to define how differential modulation of the calcium signal effects the induction of vimentin and the Ca²⁺ influx pathways involved. We identified that the intracellular Ca²⁺ chelator EGTA-AM, cytochalasin D (a modulator of cytoskeletal dynamics and cell morphology), and the sarco/endoplasmic reticulum ATPase inhibitor thapsigargin are all inducers of vimentin in MDA-MB-468 breast cancer cells. EGTA-AM- and thapsigargin-mediated induction of vimentin expression in MDA-MB-468 cells involves store-operated Ca²⁺ entry, as evidenced by sensitivity to silencing of the molecular components of this pathway, STIM1 and ORAI1. In stark contrast, cytochalasin D-mediated vimentin induction was insensitive to silencing of ORAI1, despite sensitivity to silencing of its canonical activator the endoplasmic reticulum Ca²⁺ sensor STIM1. Cytochalasin D-mediated vimentin induction was, however, sensitive to silencing of another reported STIM1 target, TRPC1. Subsequent studies identified that EGTA-AM-induced vimentin expression also partially involved a TRPC1-dependent pathway. These studies define a complex interplay between vimentin expression in this model and the specific Ca²⁺-permeable ion channels involved. The complexity in the engagement of different Ca²⁺ influx pathways that regulate vimentin induction are opportunities but also potential challenges in targeting Ca²⁺ signaling to block EMT in cancer cells. Our findings further highlight the need to identify potential indispensable ion channels that can regulate induction of specific mesenchymal markers via different stimuli.

NO1, a New Sigma 2 Receptor/TMEM97 Fluorescent Ligand, Downregulates SOCE and Promotes Apoptosis in the Triple Negative Breast Cancer Cell Lines

(1) Background: The structure of the Sigma 2 receptor/TMEM97 (σ2RTMEM97) has recently been reported. (2, 3) Methods and results: We used genetic and biochemical approaches to identify the molecular mechanism downstream of σ2R/TMEM97. The novel σ2R/TMEM97 fluorescent ligand, NO1, reduced the proliferation and survival of the triple negative breast cancer cell lines (TNBC: MDA-MB-231 and MDA-MB-468 cell lines), due to NO1-induced apoptosis. Greater bioaccumulation and faster uptake of NO1 in MDA-MB-231 cells compared to MCF10A or MCF7 cell lines were also shown. Accordingly, elevated σ2R/TMEM97 expression was confirmed by Western blotting. In contrast to NO1, other σ2R/TMEM97 ligands, such as SM21 and PB28, enhanced MDA-MB-231 cell proliferation and migration. Store-operated calcium entry (SOCE) is crucial for different cancer hallmarks. Here, we show that NO1, but not other σ2R/TMEM97 ligands, reduced SOCE in MDA-MB-231 cells. Similarly, TMEM97 silencing in MDA-MB-231 cells also impaired SOCE. NO1 administration downregulated STIM1-Orai1 interaction, probably by impairing the positive regulatory effect of σ2R/TMEM97 on STIM1, as we were unable to detect interaction with Orai1. (4) Conclusion: σ2R/TMEM97 is a key protein for the survival of triple negative breast cancer cells by promoting SOCE; therefore, NO1 may become a good pharmacological tool to avoid their proliferation.

A new selective pharmacological enhancer of the Orai1 Ca2+ channel reveals roles for Orai1 in smooth and skeletal muscle functions

Store operated calcium (Ca²⁺) entry is an important homeostatic mechanism in cells, whereby the release of Ca²⁺ from intracellular endoplasmic reticulum stores triggers the activation of a Ca²⁺ influx pathway. Mediated by Orai1, this Ca²⁺ influx has specific and essential roles in biological processes as diverse as lactation to immunity. Although pharmacological inhibitors of this Ca²⁺ influx mechanism have helped to define the role of store operated Ca²⁺ entry in many cellular events, the lack of isoform specific modulators and activators of Orai1 has limited our full understanding of these processes. Here we report the identification and synthesis of an Orai1 activity enhancer that concurrently potentiated Orai1 Ca²⁺ -dependent inactivation (CDI). This unique enhancer of Orai1 had only a modest effect on Orai3 with weak inhibitory effects at high concentrations in intact MCF-7 breast cancer cells. The Orai1 enhancer heightened vascular smooth muscle cell migration induced by platelet-derived growth factor and the unique store operated Ca²⁺ entry pathway present in skeletal muscle cells. These studies show that IA65 is an exemplar for the translation and development of Orai isoform selective agents. The ability of IA65 to activate CDI demonstrates that agents can be developed that can enhance Orai1-mediated Ca²⁺ influx but avoid the cytotoxicity associated with sustained Orai1 activation. IA65 and/or future analogues with similar Orai1 and CDI activating properties could be fine tuners of physiological processes important in specific disease states, such as cellular migration and immune cell function.

Transient receptor potential vanilloid-3 (TRPV3) channel induces dermal fibrosis via the TRPV3/TSLP/Smad2/3 pathways in dermal fibroblasts

BACKGROUND

Excessive wound healing can lead to hypertrophic scars, which are not only a cosmetic issue but could also be itchy or painful. Previously, we reported that, in comparison with normal tissue, thymic stromal lymphopoietin (TSLP) expression was increased in skin burn scars when the activation of transient receptor potential vanilloid-3 (TRPV3) of keratinocytes was increased. However, the functional role of TRPV3 in dermal fibrosis remains unclear.

OBJECTIVE

We aimed to determine whether TRPV3 affects the collagen production of human primary dermal fibroblasts (HPDFs) and to investigate the mechanism involved.

METHOD

Human primary dermal fibroblasts were cultured and transformed into myofibroblasts using TSLP and carvacrol. Expression levels of α-SMA, fibronectin, and COL1A1 were determined using qPCR, western blotting, and immunofluorescence staining. Ca²⁺ influx was measured using a calcium-sensitive fluorescent dye, Fura3-AM. Nuclear factor of activated T-cells (NFAT) and phosphorylated-Smad2/3 were determined by western blotting. Silencing of TRPV3 with TRPV3-specific small interference RNA was evaluated using qPCR and western blotting.

RESULTS

The expression levels of α-SMA, fibronectin, COL1A1, and TSLP were significantly increased in carvacrol-treated HPDFs. The expression levels of α-SMA, fibronectin, and COL1A1 were significantly increased by TSLP. The expression levels of TSLP and COL1A1 were significantly blocked by TRPV3 silencing in HPDFs. Regulation of Ca²⁺ influx and the expression levels of NFAT and p-Smad2/3 were significantly increased in carvacrol-treated HPDFs. ECM productions induced via the TRPV3/TSLP/Smad2/3 pathways.

CONCLUSIONS

The activation of the TRPV3 channels regulates dermal fibrosis by reducing extracellular matrix production via the TRPV3/TSLP/Smad2/3 pathways in dermal fibroblasts.

Review: Structure and Activation Mechanisms of CRAC Channels

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

The Calcium-Signaling Toolkit in Cancer: Remodeling and Targeting

Processes that are important in cancer progression, such as sustained cell growth, invasion to other organs, and resistance to cell death inducers, have a clear overlap with pathways regulated by Ca²⁺ signaling. It is therefore not surprising that proteins important in Ca²⁺ signaling, sometimes referred to as the "Ca²⁺ signaling toolkit," can contribute to cancer cell proliferation and invasiveness, and the ability of agents to induce cancer cell death. Ca²⁺ signaling is also critical in other aspects of cancer progression, including events in the tumor microenvironment and processes involved in the acquisition of resistance to anticancer therapies. This review will consider the role of Ca²⁺ signaling in tumor progression and highlight areas in which a better understanding of the interplay between the Ca²⁺-signaling toolkit and tumorigenesis is still required.

Redox signals at the ER-mitochondria interface control melanoma progression

Reactive oxygen species (ROS) are emerging as important regulators of cancer growth and metastatic spread. However, how cells integrate redox signals to affect cancer progression is not fully understood. Mitochondria are cellular redox hubs, which are highly regulated by interactions with neighboring organelles. Here, we investigated how ROS at the endoplasmic reticulum (ER)-mitochondria interface are generated and translated to affect melanoma outcome. We show that TMX1 and TMX3 oxidoreductases, which promote ER-mitochondria communication, are upregulated in melanoma cells and patient samples. TMX knockdown altered mitochondrial organization, enhanced bioenergetics, and elevated mitochondrial- and NOX4-derived ROS. The TMX-knockdown-induced oxidative stress suppressed melanoma proliferation, migration, and xenograft tumor growth by inhibiting NFAT1. Furthermore, we identified NFAT1-positive and NFAT1-negative melanoma subgroups, wherein NFAT1 expression correlates with melanoma stage and metastatic potential. Integrative bioinformatics revealed that genes coding for mitochondrial- and redox-related proteins are under NFAT1 control and indicated that TMX1, TMX3, and NFAT1 are associated with poor disease outcome. Our study unravels a novel redox-controlled ER-mitochondria-NFAT1 signaling loop that regulates melanoma pathobiology and provides biomarkers indicative of aggressive disease.

Selective recruitment of different Ca2+-dependent transcription factors by STIM1-Orai1 channel clusters

Store-operated Ca2+ entry, involving endoplasmic reticulum Ca2+ sensing STIM proteins and plasma membrane Orai1 channels, is a widespread and evolutionary conserved Ca2+ influx pathway. This form of Ca2+ influx occurs at discrete loci where peripheral endoplasmic reticulum juxtaposes the plasma membrane. Stimulation evokes numerous STIM1-Orai1 clusters but whether distinct signal transduction pathways require different cluster numbers is unknown. Here, we show that two Ca2+-dependent transcription factors, NFAT1 and c-fos, have different requirements for the number of STIM1-Orai1 clusters and on the Ca2+ flux through them. NFAT activation requires fewer clusters and is more robustly activated than c-fos by low concentrations of agonist. For similar cluster numbers, transcription factor recruitment occurs sequentially, arising from intrinsic differences in Ca2+ sensitivities. Variations in the number of STIM1-Orai1 clusters and Ca2+ flux through them regulate the robustness of signalling to the nucleus whilst imparting a mechanism for selective recruitment of different Ca2+-dependent transcription factors.

Photopharmacology and opto-chemogenetics of TRPC channels-some therapeutic visions

Non-selective cation conductances formed by transient receptor potential canonical (TRPC) proteins govern the function and fate of a wide range of human cell types. In the past decade, evidence has accumulated for a pivotal role of these channels in human diseases, raising substantial interest in their therapeutic targeting. As yet, an appreciable number of small molecules for block and modulation of recombinant TRPC conductances have been identified. However, groundbreaking progress in TRPC pharmacology towards therapeutic applications is lagging behind due to incomplete understanding of their molecular pharmacology and their exact role in disease. A major breakthrough that is expected to overcome these hurdles is the recent success in obtaining high-resolution structure information on TRPC channel complexes and the advent of TRP photopharmacology and optogenetics. Here, we summarize current concepts of enhancing the precision of therapeutic interference with TRPC signaling and TRPC-mediated pathological processes.

TRPC-mediated Ca2+ signaling and control of cellular functions

Canonical members of the TRP superfamily of ion channels have long been recognized as key elements of Ca²⁺ handling in a plethora of cell types. The emerging role of TRPC channels in human physiopathology has generated considerable interest in their pharmacological targeting, which requires detailed understanding of their molecular function. Although consent has been reached that receptor-phospholipase C (PLC) pathways and generation of lipid mediators constitute the prominent upstream signaling process that governs channel activity, multimodal sensing features of TRPC complexes have been demonstrated repeatedly. Downstream signaling by TRPC channels is similarly complex and involves the generation of local and global cellular Ca²⁺ rises, which are well-defined in space and time to govern specific cellular functions. These TRPC-mediated Ca²⁺ signals rely in part on Ca²⁺ permeation through the channels, but are essentially complemented by secondary mechanisms such as Ca²⁺ mobilization from storage sites and Na⁺/Ca²⁺ exchange, which involve coordinated interaction with signaling partners. Consequently, the control of cell functions by TRPC molecules is critically determined by dynamic assembly and subcellular targeting of the TRPC complexes. The very recent availability of high-resolution structure information on TRPC channel complexes has paved the way towards a comprehensive understanding of signal transduction by TRPC channels. Here, we summarize current concepts of cation permeation in TRPC complexes, TRPC-mediated shaping of cellular Ca²⁺ signals and the associated control of specific cell functions.

Calcium signal dynamics in T lymphocytes: Comparing in vivo and in vitro measurements

Amplitude and kinetics of intracellular Ca²⁺ signals ([Ca²⁺]_int) determine many immune cell functions. To mimic in vivo changes of [Ca²⁺]_int in human immune cells, two approaches may be best suited: 1) Analyze primary human immune cells taken from blood under conditions resembling best physiological or pathophysiological conditions. 2.) Analyze the immune system in vivo or ex vivo in explanted tissue from small vertebrate animals, such as mice. With the help of genetically encoded Ca²⁺ indicators and intravital microscopy, [Ca²⁺]_int have been investigated in murine T lymphocytes (T cells) in vivo during the last five years and in explanted lymph node (LN) during the last 10 years. There are several important reasons to compare [Ca²⁺]_int measured in primary murine T lymphocytes in vivo and in vitro with [Ca²⁺]_int measured in primary human T lymphocytes in vitro. First, how do human and murine data compare? Second, how do in vivo and in vitro data compare? Third, can in vitro data predict in vivo data? The last point is particularly important considering the many technical challenges that limit in vivo measurements and to reduce the number of animals sacrificed. This review summarizes and compares the results of the available publications on in vivo and in vitro [Ca²⁺]_int measurements in T lymphocytes stimulated focally by antigen-presenting cells (APC) after forming an immunological synapse.

Expression of calcium release-activated and voltage-gated calcium channels genes in peripheral blood mononuclear cells is altered in pregnancy and in type 1 diabetes

Calcium (Ca2+) is an important ion in physiology and is found both outside and inside cells. The intracellular concentration of Ca2+ is tightly regulated as it is an intracellular signal molecule and can affect a variety of cellular processes. In immune cells Ca2+ has been shown to regulate e.g. gene transcription, cytokine secretion, proliferation and migration. Ca2+ can enter the cytoplasm either from intracellular stores or from outside the cells when Ca2+ permeable ion channels in the plasma membrane open. The Ca2+ release-activated (CRAC) channel is the most prominent Ca2+ ion channel in the plasma membrane. It is formed by ORAI1-3 and the channel is opened by the endoplasmic reticulum Ca2+ sensor proteins stromal interaction molecules (STIM) 1 and 2. Another group of Ca2+ channels in the plasma membrane are the voltage-gated Ca2+ (CaV) channels. We examined if a change in immunological tolerance is accompanied by altered ORAI, STIM and CaV gene expression in peripheral blood mononuclear cells (PBMCs) in pregnant women and in type 1 diabetic individuals. Our results show that in pregnancy and type 1 diabetes ORAI1-3 are up-regulated whereas STIM1 and 2 are down-regulated in pregnancy but only STIM2 in type 1 diabetes. Expression of L-, P/Q-, R- and T-type voltage-gated Ca2+ channels was detected in the PBMCs where the CaV2.3 gene was up-regulated in pregnancy and type 1 diabetes whereas the CaV 2.1 and CaV3.2 genes were up-regulated only in pregnancy and the CaV1.3 gene in type 1 diabetes. The results are consistent with that expression of ORAI, STIM and CaV genes correlate with a shift in immunological status of the individual in health, as during pregnancy, and in the autoimmune disease type 1 diabetes. Whether the changes are in general protective or in type 1 diabetes include some pathogenic components remains to be clarified.

Stimulation of TRPM3 channels increases the transcriptional activation potential of Elk-1 involving cytosolic Ca2+, extracellular signal-regulated protein kinase, and calcineurin

Stimulation of transient receptor potential M3 (TRPM3) channels with the steroid pregnenolone sulfate increases the transcriptional activation potential of Elk-1, a transcription factor that regulates serum response element-mediated transcription. Here, we show that an influx of Ca²⁺ ions into the cells is essential for the activation of Elk-1 following stimulation of TRPM3. Using genetically encoded Ca²⁺ buffers, we show that a rise in cytoplasmic Ca²⁺ is required for the upregulation of the transcriptional activation potential of Elk-1, while buffering of Ca²⁺ in the nucleus had no inhibitory effect on the transcriptional activity of Elk-1. Pharmacological and genetic experiments showed that extracellular signal-regulated protein kinase (ERK1/2) functions as signal transducer connecting TRPM3 channels with the Elk-1 transcription factor. Accordingly, dephosphorylation of ERK1/2 in the nucleus by MAP kinase phosphatase attenuated TRPM3-mediated Elk-1 activation. Moreover, we show that the Ca²⁺/calmodulin-dependent protein phosphatase calcineurin is part of a shut-off-device for the signaling cascade connecting TRPM3 channels with the activation of Elk-1. The fact that TRPM3 channel stimulation activates Elk-1 connects TRPM3 with the biological functions of Elk-1, including the regulation of proliferation, differentiation, survival, transcription, and cell migration.

Pharmacological inhibition of store-operated calcium entry in MDA-MB-468 basal A breast cancer cells: consequences on calcium signalling, cell migration and proliferation

Store-operated Ca2+ entry is a pathway that is remodelled in a variety of cancers, and altered expression of the components of store-operated Ca2+ entry is a feature of breast cancer cells of the basal molecular subtype. Studies of store-operated Ca2+ entry in breast cancer cells have used non-specific pharmacological inhibitors, complete depletion of intracellular Ca2+ stores and have mostly focused on MDA-MB-231 cells (a basal B breast cancer cell line). These studies compared the effects of the selective store-operated Ca2+ entry inhibitors Synta66 and YM58483 (also known as BTP2) on global cytosolic free Ca2+ ([Ca2+]CYT) changes induced by physiological stimuli in a different breast cancer basal cell line model, MDA-MB-468. The effects of these agents on proliferation as well as serum and epidermal growth factor (EGF) induced migration were also assessed. Activation with the purinergic receptor activator adenosine triphosphate, produced a sustained increase in [Ca2+]CYT that was entirely dependent on store-operated Ca2+ entry. The protease activated receptor 2 activator, trypsin, and EGF also produced Ca2+ influx that was sensitive to both Synta66 and YM58483. Serum-activated migration of MDA-MB-468 breast cancer cells was sensitive to both store-operated Ca2+ inhibitors. However, proliferation and EGF-activated migration was differentially affected by Synta66 and YM58483. These studies highlight the need to define the exact mechanisms of action of different store-operated calcium entry inhibitors and the impact of such differences in the control of tumour progression pathways.

STIM1 and Orai1 regulate Ca2+ microdomains for activation of transcription

Since calcium (Ca²⁺) regulates a large variety of cellular signaling processes in a cell's life, precise control of Ca²⁺ concentrations within the cell is essential. This enables the transduction of information via Ca²⁺ changes in a time-dependent and spatially defined manner. Here, we review molecular and functional aspects of how the store-operated Ca²⁺ channel Orai1 creates spatiotemporal Ca²⁺ microdomains. The architecture of this channel is unique, with a long helical pore and a six-fold symmetry. Energetic barriers within the Ca²⁺ channel pathway limit permeation to allow an extensive local Ca²⁺ increase in close proximity to the channel. The precise timing of the Orai1 channel function is controlled by direct binding to STIM proteins upon Ca²⁺ depletion in the endoplasmic reticulum. These induced Ca²⁺ microdomains are tailored to, and sufficient for, triggering long-term activation processes, such as transcription factor activation and subsequent gene regulation. We describe the principles of spatiotemporal activation of the transcription factor NFAT and compare its signaling characteristics to those of the autophagy regulating transcription factors, MITF and TFEB.

Loss of barrier integrity in alveolar epithelial cells downregulates ENaC expression and activity via Ca2+ and TRPV4 activation

The epithelial Na channel (ENaC) plays an essential role in lung physiology by modulating the amount of liquid lining the respiratory epithelium. Here, we tested the effect of breaking alveolar epithelial cell barrier integrity on ENaC expression and function. We found that either mechanical wounding by scratching the monolayer or disruption of tight junction with EDTA induced a ~ 50% decrease of α,β and γENaC mRNA expression and an 80% reduction of ENaC short-circuit current (I_sc) at 6 h. Scratching the cell monolayer generated a Ca²⁺ wave that spread from the margin of the scratch to distant cells. Pretreatment with BAPTA-AM, an intracellular Ca²⁺ chelator, abolished the effect of mechanical wounding and EDTA on αENaC mRNA expression, suggesting that [Ca²⁺]_i is important for this modulation. We tested the hypothesis that a mechanosensitive channel such as TRPV4, a cationic channel known to increase [Ca²⁺]_i, could mediate this effect. Activation of the channel with the TRPV4 specific agonist GSK-1016790A (GSK) decreased αENAC mRNA expression and almost completely abolished ENaC I_sc. Pretreatment of alveolar epithelial cells with HC-067047 (HC0), a specific TRPV4 antagonist, reduced the extent of αENAC mRNA downregulation by mechanical wounding and EDTA. Altogether, our results suggest that mechanical stress induced by wounding or TRPV4-mediated loss of tight junction increases [Ca²⁺]_i and elicits a Ca²⁺ wave that affects ENaC expression and function away from the site of injury. These data are important to better understand how Ca²⁺ signaling affects lung liquid clearance in injured lungs.

Spatially restricted subcellular Ca2+ signaling downstream of store-operated calcium entry encoded by a cortical tunneling mechanism

Agonist-dependent Ca²⁺ mobilization results in Ca²⁺ store depletion and Store-Operated Calcium Entry (SOCE), which is spatially restricted to microdomains defined by cortical ER – plasma membrane contact sites (MCS). However, some Ca²⁺-dependent effectors that localize away from SOCE microdomains, are activated downstream of SOCE by mechanisms that remain obscure. One mechanism proposed initially in acinar cells and termed Ca²⁺ tunneling, mediates the uptake of Ca²⁺ flowing through SOCE into the ER followed by release at distal sites through IP₃ receptors. Here we show that Ca²⁺ tunneling encodes exquisite specificity downstream of SOCE signal by dissecting the sensitivity and dependence of multiple effectors in HeLa cells. While mitochondria readily perceive Ca²⁺ release when stores are full, SOCE shows little effect in raising mitochondrial Ca²⁺, and Ca²⁺-tunneling is completely inefficient. In contrast, gK_Ca displays a similar sensitivity to Ca²⁺ release and tunneling, while the activation of NFAT1 is selectively responsive to SOCE and not to Ca²⁺ release. These results show that in contrast to the previously described long-range Ca²⁺ tunneling, in non-specialized HeLa cells this mechanism mediates spatially restricted Ca²⁺ rise within the cortical region of the cell to activate a specific subset of effectors.

ORAI Calcium Channels

In this review article, we discuss the different gene products and translational variants of ORAI proteins and their contribution to the makeup of different native calcium-conducting channels with distinct compositions and modes of activation. We also review the different modes of regulation of these distinct calcium channels and their impact on downstream cellular signaling controlling important physiological functions.

Ion channelopathies of the immune system

Ion channels and transporters move ions across membrane barriers and are essential for a host of cell functions in many organs. They conduct K+, Na+ and Cl-, which are essential for regulating the membrane potential, H+ to control intracellular and extracellular pH and divalent cations such as Ca2+, Mg2+ and Zn2+, which function as second messengers and cofactors for many proteins. Inherited channelopathies due to mutations in ion channels or their accessory proteins cause a variety of diseases in the nervous, cardiovascular and other tissues, but channelopathies that affect immune function are not as well studied. Mutations in ORAI1 and STIM1 genes that encode the Ca2+ release-activated Ca2+ (CRAC) channel in immune cells, the Mg2+ transporter MAGT1 and the Cl- channel LRRC8A all cause immunodeficiency with increased susceptibility to infection. Mutations in the Zn2+ transporters SLC39A4 (ZIP4) and SLC30A2 (ZnT2) result in nutritional Zn2+ deficiency and immune dysfunction. These channels, however, only represent a fraction of ion channels that regulate immunity as demonstrated by immune dysregulation in channel knockout mice. The immune system itself can cause acquired channelopathies that are associated with a variety of diseases of nervous, cardiovascular and endocrine systems resulting from autoantibodies binding to ion channels. These autoantibodies highlight the therapeutic potential of functional anti-ion channel antibodies that are being developed for the treatment of autoimmune, inflammatory and other diseases.

Calcium signaling and the therapeutic targeting of cancer cells

The calcium signal is implicated in a variety of processes important in tumor progression (e.g. proliferation and invasiveness). The calcium signal has also been shown to be important in other processes important in cancer progression including the development of resistance to current cancer therapies. In this review, we discuss how Ca²⁺ channels, pumps and exchangers may be drug targets in some cancer types. We consider what factors should be taken into account when considering an optimal Ca²⁺ channel, pump or exchanger as a candidate for further assessment as a novel drug target in cancer. We also present and summarize how some therapies for the treatment of cancer intersect with Ca²⁺ signaling and how pharmacological manipulation of the machinery of Ca²⁺ signaling could promote the effectiveness of some therapies. We also review new therapeutic opportunities for Ca²⁺ signal modulators in the context of the tumor microenvironment.

DYRK1A Kinase Positively Regulates Angiogenic Responses in Endothelial Cells

Angiogenesis is a highly regulated process essential for organ development and maintenance, and its deregulation contributes to inflammation, cardiac disorders, and cancer. The Ca2+/nuclear factor of activated T cells (NFAT) signaling pathway is central to endothelial cell angiogenic responses, and it is activated by stimuli like vascular endothelial growth factor (VEGF) A. NFAT phosphorylation by dual-specificity tyrosine phosphorylation-regulated kinases (DYRKs) is thought to be an inactivating event. Contrary to expectations, we show that the DYRK family member DYRK1A positively regulates VEGF-dependent NFAT transcriptional responses in primary endothelial cells. DYRK1A silencing reduces intracellular Ca2+ influx in response to VEGF, which dampens NFAT activation. The effect is exerted at the level of VEGFR2 accumulation leading to impairment in PLCγ1 activation. Notably, Dyrk1a heterozygous mice show defects in developmental retinal vascularization. Our data establish a regulatory circuit, DYRK1A/ Ca2+/NFAT, to fine-tune endothelial cell proliferation and angiogenesis.

Cross-linking of Orai1 channels by STIM proteins

The transmembrane docking of endoplasmic reticulum (ER) Ca²⁺-sensing STIM proteins with plasma membrane (PM) Orai Ca²⁺ channels is a critical but poorly understood step in Ca²⁺ signal generation. STIM1 protein dimers unfold to expose a discrete STIM-Orai activating region (SOAR1) that tethers and activates Orai1 channels within discrete ER-PM junctions. We reveal that each monomer within the SOAR dimer interacts independently with single Orai1 subunits to mediate cross-linking between Orai1 channels. Superresolution imaging and mobility measured by fluorescence recovery after photobleaching reveal that SOAR dimer cross-linking leads to substantial Orai1 channel clustering, resulting in increased efficacy and cooperativity of Orai1 channel function. A concatenated SOAR1 heterodimer containing one monomer point mutated at its critical Orai1 binding residue (F394H), although fully activating Orai channels, is completely defective in cross-linking Orai1 channels. Importantly, the naturally occurring STIM2 variant, STIM2.1, has an eight-amino acid insert in its SOAR unit that renders it functionally identical to the F394H mutant in SOAR1. Contrary to earlier predictions, the SOAR1-SOAR2.1 heterodimer fully activates Orai1 channels but prevents cross-linking and clustering of channels. Interestingly, combined expression of full-length STIM1 with STIM2.1 in a 5:1 ratio causes suppression of sustained agonist-induced Ca²⁺ oscillations and protects cells from Ca²⁺ overload, resulting from high agonist-induced Ca²⁺ release. Thus, STIM2.1 exerts a powerful regulatory effect on signal generation likely through preventing Orai1 channel cross-linking. Overall, STIM-mediated cross-linking of Orai1 channels is a hitherto unrecognized functional paradigm that likely provides an organizational microenvironment within ER-PM junctions with important functional impact on Ca²⁺ signal generation.

A calcium optimum for cytotoxic T lymphocyte and natural killer cell cytotoxicity

KEY POINTS

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells are required to eliminate cancer cells. We analysed the Ca²⁺ dependence of CTL and NK cell cytotoxicity and found that in particular CTLs have a very low optimum of [Ca²⁺ ]_i (between 122 and 334 nm) and [Ca²⁺ ]_o (between 23 and 625 μm) for efficient cancer cell elimination, well below blood plasma Ca²⁺ levels. As predicted from these results, partial down-regulation of the Ca²⁺ channel Orai1 in CTLs paradoxically increases perforin-dependent cancer cell killing. Lytic granule release at the immune synapse between CTLs and cancer cells has a Ca²⁺ optimum compatible with this low Ca²⁺ optimum for efficient cancer cell killing, whereas the Ca²⁺ optimum for CTL migration is slightly higher and proliferation increases monotonously with increasing [Ca²⁺ ]_o . We propose that a partial inhibition of Ca²⁺ signals by specific Orai1 blockers at submaximal concentrations could contribute to tumour elimination.

ABSTRACT

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells are required to protect the human body against cancer. Ca²⁺ is a key metabolic factor for lymphocyte function and cancer homeostasis. We analysed the Ca²⁺ dependence of CTL and NK cell cytotoxicity against cancer cells and found that CTLs have a bell-shaped Ca²⁺ dependence with an optimum for cancer cell elimination at rather low [Ca²⁺ ]_o (23-625 μm) and [Ca²⁺ ]_i (122-334 nm). This finding predicts that a partial inhibition of Orai1 should increase (rather than decrease) cytotoxicity of CTLs at [Ca²⁺ ]_o higher than 625 μm. We tested this hypothesis in CTLs and indeed found that partial down-regulation of Orai1 by siRNA increases the efficiency of cancer cell killing. We found two mechanisms that may account for the Ca²⁺ optimum of cancer cell killing: (1) migration velocity and persistence have a moderate optimum between 500 and 1000 μm [Ca²⁺ ]_o in CTLs, and (2) lytic granule release at the immune synapse between CTLs and cancer cells is increased at 146 μm compared to 3 or 800 μm, compatible with the Ca²⁺ optimum for cancer cell killing. It has been demonstrated in many cancer cell types that Orai1-dependent Ca²⁺ signals enhance proliferation. We propose that a decrease of [Ca²⁺ ]_o or partial inhibition of Orai1 activity by selective blockers in the tumour microenvironment could efficiently reduce cancer growth by simultaneously increasing CTL and NK cell cytotoxicity and decreasing cancer cell proliferation.

NFAT control of immune function: New Frontiers for an Abiding Trooper

Nuclear factor of activated T cells (NFAT) was first described almost three decades ago as a Ca ²⁺/calcineurin-regulated transcription factor in T cells. Since then, a large body of research uncovered the regulation and physiological function of different NFAT homologues in the immune system and many other tissues. In this review, we will discuss novel roles of NFAT in T cells, focusing mainly on its function in humoral immune responses, immunological tolerance, and the regulation of immune metabolism.

The Calcium-Induced Regulation in the Molecular and Transcriptional Circuitry of Human Inflammatory Response and Autoimmunity

Rheumatoid arthritis synovial fibroblasts (RASFs) are fundamental effector cells in RA driving the joint inflammation and deformities. Celastrol is a natural compound that exhibits a potent anti-arthritic effect promoting endoplasmic reticulum (ER) stress mediated by intracellular calcium (Ca²⁺) mobilization. Ca²⁺ is a second messenger regulating a variety of cellular processes. We hypothesized that the compound, celastrol, affecting cytosolic Ca²⁺ mobilization could serve as a novel strategy to combat RA. To address this issue, celastrol was used as a molecular tool to assay the inflammatory gene expression profile regulated by Ca²⁺. We confirmed that celastrol treatment mobilized cytosolic Ca²⁺ in patient-derived RASFs. It was found that 23 genes out of 370 were manipulated by Ca²⁺ mobilization using an inflammatory and autoimmunity PCR array following independent quantitative PCR validation. Most of the identified genes were downregulated and categorized into five groups corresponding to their cellular responses participating in RA pathogenesis. Accordingly, a signaling network map demonstrating the possible molecular circuitry connecting the functions of the products of these genes was generated based on literature review. In addition, a bioinformatics analysis revealed that celastrol-induced Ca²⁺ mobilization gene expression profile showed a novel mode of action compared with three FDA-approved rheumatic drugs (methotrexate, rituximab and tocilizumab). To the best of our knowledge, this is a pioneer work charting the Ca²⁺ signaling network on the regulation of RA-associated inflammatory gene expression.

Orai1 is critical for Notch-driven aggressiveness under hypoxic conditions in triple-negative breast cancers

It is believed that hypoxia stimulates triple-negative breast cancers (TNBCs) metastasis, which is associated with a poor prognosis. However, the underlying mechanism remains unclear. Here, we demonstrated that hypoxia up-regulates both the levels of Orai1 and Notch1, and the increase in Orai1 is mediated by Notch1 signaling in TNBCs. Functionally, Orai1 caused a sustained elevation of intracellular Ca²⁺ via Store-operated Ca²⁺ entry (SOCE), then activated the calcineurin-nuclear factor of activated T-cell 4 (NFAT4, also named NFATc3) in hypoxic TNBCs. Furthermore, pharmacologic inhibition or gene-silencing studies showed that the aggressiveness mediated by Orai1 during hypoxia is dependent on the Notch1/Orai1/SOCE/NFAT4 signaling. Moreover, Orai1 signaling also mediated hypoxia-induced angiogenesis in TNBCs. Thus, our results revealed a novel role of Orai1 as an inducer of aggression and angiogenesis under hypoxic conditions, and this suggests a novel mechanism of hypoxia-induced invasion. It may be worthwhile to further explore the potential of using Orai1 signaling as new target for anti-tumor therapy in TNBCs.

Transmembrane helix connectivity in Orai1 controls two gates for calcium-dependent transcription

The channel Orai1 requires Ca2+ store depletion in the endoplasmic reticulum and an interaction with the Ca2+ sensor STIM1 to mediate Ca2+ signaling. Alterations in Orai1-mediated Ca2+ influx have been linked to several pathological conditions including immunodeficiency, tubular myopathy, and cancer. We screened large-scale cancer genomics data sets for dysfunctional Orai1 mutants. Five of the identified Orai1 mutations resulted in constitutively active gating and transcriptional activation. Our analysis showed that certain Orai1 mutations were clustered in the transmembrane 2 helix surrounding the pore, which is a trigger site for Orai1 channel gating. Analysis of the constitutively open Orai1 mutant channels revealed two fundamental gates that enabled Ca2+ influx: Arginine side chains were displaced so they no longer blocked the pore, and a chain of water molecules formed in the hydrophobic pore region. Together, these results enabled us to identify a cluster of Orai1 mutations that trigger Ca2+ permeation associated with gene transcription and provide a gating mechanism for Orai1.

Acetylcholine induces intracellular Ca2+ oscillations and nitric oxide release in mouse brain endothelial cells

Basal forebrain neurons increase cortical blood flow by releasing acetylcholine (Ach), which stimulates endothelial cells (ECs) to produce the vasodilating gasotransmitter, nitric oxide (NO). Surprisingly, the mechanism whereby Ach induces NO synthesis in brain microvascular ECs is unknown. An increase in intracellular Ca²⁺ concentration recruits a multitude of endothelial Ca²⁺-dependent pathways, such as Ca²⁺/calmodulin endothelial NO synthase (eNOS). The present investigation sought to investigate the role of intracellular Ca²⁺ signaling in Ach-induced NO production in bEND5 cells, an established model of mouse brain microvascular ECs, by conventional imaging of cells loaded with the Ca²⁺-sensitive dye, Fura-2/AM, and the NO-sensitive fluorophore, DAF-DM diacetate. Ach induced dose-dependent Ca²⁺ oscillations in bEND5 cells, 300 μM being the most effective dose to generate a prolonged Ca²⁺ burst. Pharmacological manipulation revealed that Ach-evoked Ca²⁺ oscillations required metabotropic muscarinic receptor (mAchR) activation and were patterned by a complex interplay between repetitive ER Ca²⁺ release via inositol-1,4,5-trisphosphate receptors (InsP₃Rs) and store-operated Ca²⁺ entry (SOCE). A comprehensive real time-polymerase chain reaction analysis demonstrated the expression of the transcripts encoding for M3-mAChRs, InsP₃R1 and InsP₃R3, Stim1-2 and Orai2. Next, we found that Ach-induced NO production was hindered by L-NAME, a selective NOS inhibitor, and BAPTA, a membrane permeable intracellular Ca²⁺ buffer. Moreover, Ach-elicited NO synthesis was blocked by the pharmacological abrogation of the accompanying Ca²⁺ spikes. Overall, these data shed novel light on the molecular mechanisms whereby neuronally-released Ach controls neurovascular coupling in blood microvessels.

The calcium-cancer signalling nexus

The calcium signal is a powerful and multifaceted tool by which cells can achieve specific outcomes. Cellular machinery important in tumour progression is often driven or influenced by changes in calcium ions; in some cases this regulation occurs within spatially defined regions. Over the past decade there has been a deeper understanding of how calcium signalling is remodelled in some cancers and the consequences of calcium signalling on key events such as proliferation, invasion and sensitivity to cell death. Specific calcium signalling pathways have also now been identified as playing important roles in the establishment and maintenance of multidrug resistance and the tumour microenvironment.

The STIM-Orai coupling interface and gating of the Orai1 channel

In virtually all cells, store-operated Ca2+ entry signals are vital in controlling a spectrum of functions. The signals are mediated by STIM proteins in the ER and Orai channels in the PM which undergo a dynamic coupling process within discrete ER-PM junctional regions. This coupling is initiated by depletion of ER stored Ca2+ triggering STIM proteins to undergo an intricate activation process. Thereafter, STIM proteins become trapped in the ER-PM junctions where they tether and gate PM Orai Ca2+ channels. STIM1 exists as a dimer, with a single STIM-Orai activating region (SOAR) buried in the resting protein that becomes exposed upon activation. An exposed region on SOAR including the Phe-394 residue forms a critical Orai1 interacting site. Using dimeric SOAR concatemers, we reveal only one of the two sites in the SOAR dimer is needed for Orai1 activation. This unimolecular interaction of SOAR with Orai1 suggests STIM1 can cross-link Orai channels with important significance for Ca2+ signaling. A critical "nexus" region in Orai1 close to the STIM1-binding site can be mutated to constitutively activate the channel mimicking the gating action of STIM1. This indicates STIM1 remotely controls Orai1 channel gating through an allosteric switch triggered by STIM1 binding only to the exposed C-terminal tail of the Orai1 channel.

Calcineurin signaling in the heart: The importance of time and place

The calcium-activated protein phosphatase, calcineurin, lies at the intersection of protein phosphorylation and calcium signaling cascades, where it provides an essential nodal point for coordination between these two fundamental modes of intracellular communication. In excitatory cells, such as neurons and cardiomyocytes, that experience rapid and frequent changes in cytoplasmic calcium, calcineurin protein levels are exceptionally high, suggesting that these cells require high levels of calcineurin activity. Yet, it is widely recognized that excessive activation of calcineurin in the heart contributes to pathological hypertrophic remodeling and the progression to failure. How does a calcium activated enzyme function in the calcium-rich environment of the continuously contracting heart without pathological consequences? This review will discuss the wide range of calcineurin substrates relevant to cardiovascular health and the mechanisms calcineurin uses to find and act on appropriate substrates in the appropriate location while potentially avoiding others. Fundamental differences in calcineurin signaling in neonatal verses adult cardiomyocytes will be addressed as well as the importance of maintaining heterogeneity in calcineurin activity across the myocardium. Finally, we will discuss how circadian oscillations in calcineurin activity may facilitate integration with other essential but conflicting processes, allowing a healthy heart to reap the benefits of calcineurin signaling while avoiding the detrimental consequences of sustained calcineurin activity that can culminate in heart failure.

New insights into mammalian signaling pathways using microfluidic pulsatile inputs and mathematical modeling

Temporally modulated input mimics physiology. This chemical communication strategy filters the biochemical noise through entrainment and phase-locking. Under laboratory conditions, it also expands the observability space for downstream responses. A combined approach involving microfluidic pulsatile stimulation and mathematical modeling has led to deciphering of hidden/unknown temporal motifs in several mammalian signaling pathways and has provided mechanistic insights, including how these motifs combine to form distinct band-pass filters and govern fate regulation under dynamic microenvironment. This approach can be utilized to understand signaling circuit architectures and to gain mechanistic insights for several other signaling systems. Potential applications include synthetic biology and biotechnology, in developing pharmaceutical interventions, and in developing lab-on-chip models.

The STIM-Orai Pathway: Conformational Coupling Between STIM and Orai in the Activation of Store-Operated Ca2+ Entry

Store-operated Ca²⁺ entry fulfills a crucial role in controlling Ca²⁺ signals in almost all cells. The Ca²⁺-sensing stromal interaction molecule (STIM) proteins in the endoplasmic reticulum (ER) undergo complex conformational changes in response to depleted ER luminal Ca²⁺, allowing them to unfold and become trapped in ER-plasma membrane (PM) junctions. Dimers of STIM proteins trap and gate the plasma membrane Orai Ca²⁺ channels within these junctions to generate discrete zones of high Ca²⁺ and regulate sensitive Ca²⁺-dependent intracellular signaling pathways. The STIM-Orai activating region (SOAR) of STIM1 becomes exposed upon store depletion and promotes trapping of Orai1 at the PM. Residue Phe-394 within SOAR forms an integral part of the high-affinity Orai1-interacting site. Our results demonstrate that only a single active site within the dimeric SOAR domain of STIM1 is required for the activation of Orai1 channel activity. This unimolecular model is strongly supported by evidence of variable STIM1:Orai1 stoichiometry reported in many studies. We hypothesize that unimolecular coupling promotes cross-linking of channels, localizing Ca²⁺ signals, and regulating channel activity. We have also identified a key "nexus" region in Orai1 near the C-terminal STIM1-binding site that can be mutated to constitutively activate Ca²⁺ entry, mimicking STIM1 activated channels. This suggests that STIM1 mediates gating of Orai1 in an allosteric manner via interaction with the Orai1 C-terminus alone. This model suggests the dual role of STIM1 in regulating both localization and gating of Orai1 channels and has important implications for the regulation of SOCE-mediated downstream signaling and the kinetics of channel activation.

Tissue Specificity: Store-Operated Ca2+ Entry in Cardiac Myocytes

Calcium (Ca²⁺) is a key regulator of cardiomyocyte contraction. The Ca²⁺ channels, pumps, and exchangers responsible for the cyclical cytosolic Ca²⁺ signals that underlie contraction are well known. In addition to those Ca²⁺ signaling components responsible for contraction, it has been proposed that cardiomyocytes express channels that promote the influx of Ca²⁺ from the extracellular milieu to the cytosol in response to depletion of intracellular Ca²⁺ stores. With non-excitable cells, this store-operated Ca²⁺ entry (SOCE) is usually easily demonstrated and is essential for prolonging cellular Ca²⁺ signaling and for refilling depleted Ca²⁺ stores. The role of SOCE in cardiomyocytes, however, is rather more elusive. While there is published evidence for increased Ca²⁺ influx into cardiomyocytes following Ca²⁺ store depletion, it has not been universally observed. Moreover, SOCE appears to be prominent in embryonic cardiomyocytes but declines with postnatal development. In contrast, there is overwhelming evidence that the molecular components of SOCE (e.g., STIM, Orai, and TRPC proteins) are expressed in cardiomyocytes from embryo to adult. Moreover, these proteins have been shown to contribute to disease conditions such as pathological hypertrophy, and reducing their expression can attenuate hypertrophic growth. It is plausible that SOCE might underlie Ca²⁺ influx into cardiomyocytes and may have important signaling functions perhaps by activating local Ca²⁺-sensitive processes. However, the STIM, Orai, and TRPC proteins appear to cooperate with multiple protein partners in signaling complexes. It is therefore possible that some of their signaling activities are not mediated by Ca²⁺ influx signals, but by protein-protein interactions.

Spatial Ca2+ profiling: decrypting the universal cytosolic Ca2+ oscillation

Stimulation of cell-surface receptors that couple to phospholipase C to generate the second messenger inositol trisphosphate often evokes repetitive oscillations in cytosolic Ca²⁺ . Signalling information is encoded in both the amplitude and frequency of the Ca²⁺ spikes. Recent studies have revealed that the spatial profile of the oscillation also imparts signalling power; Ca²⁺ microdomains near store-operated CRAC channels in the plasma membrane and inositol trisphosphate-gated channels in the endoplasmic reticulum both signal to distinct downstream targets. Spatial profiling therefore increases the transduction power of the universal oscillatory cytosolic Ca²⁺ signal.

Control of NFAT Isoform Activation and NFAT-Dependent Gene Expression through Two Coincident and Spatially Segregated Intracellular Ca2+ Signals

Excitation-transcription coupling, linking stimulation at the cell surface to changes in nuclear gene expression, is conserved throughout eukaryotes. How closely related coexpressed transcription factors are differentially activated remains unclear. Here, we show that two Ca2+-dependent transcription factor isoforms, NFAT1 and NFAT4, require distinct sub-cellular InsP3 and Ca2+ signals for physiologically sustained activation. NFAT1 is stimulated by sub-plasmalemmal Ca2+ microdomains, whereas NFAT4 additionally requires Ca2+ mobilization from the inner nuclear envelope by nuclear InsP3 receptors. NFAT1 is rephosphorylated (deactivated) more slowly than NFAT4 in both cytoplasm and nucleus, enabling a more prolonged activation phase. Oscillations in cytoplasmic Ca2+, long considered the physiological form of Ca2+ signaling, play no role in activating either NFAT protein. Instead, effective sustained physiological activation of NFAT4 is tightly linked to oscillations in nuclear Ca2+. Our results show how gene expression can be controlled by coincident yet geographically distinct Ca2+ signals, generated by a freely diffusible InsP3 message.

Leishmania donovani-Induced Prostaglandin E2 Generation Is Critically Dependent on Host Toll-Like Receptor 2-Cytosolic Phospholipase A2 Signaling

Visceral leishmaniasis (VL) is the second-largest parasitic killer disease after malaria. During VL, the protozoan Leishmania donovani induces prostaglandin E2 (PGE2) generation within host macrophages to aid parasite survival. PGE2 significantly influences leishmanial pathogenesis, as L. donovani proliferation is known to be attenuated in PGE2-inhibited macrophages. Here, we report for the first time that signaling via macrophage Toll-like receptor 2 (TLR2) plays an instrumental role in inducing PGE2 release from L. donovani-infected macrophages. This signaling cascade, mediated via the TLR2-phosphatidylinositol 3-kinase (PI3K)-phospholipase C (PLC) signaling pathway, was found to be indispensable for activation of two major enzymes required for PGE2 generation: cytosolic phospholipase A2 (cPLA2) and cyclooxygenase 2 (Cox2). Inhibition of cPLA2, but not secreted phospholipase A2 (sPLA2) or calcium-independent phospholipase A2 (iPLA2), arrested L. donovani infection. During infection, cPLA2 activity increased >7-fold in a calcium-dependent and extracellular signal-regulated kinase (ERK)-dependent manner, indicating that elevation of intracellular calcium and ERK-mediated phosphorylation was necessary for L. donovani-induced cPLA2 activation. For transcriptional upregulation of cyclooxygenase 2, activation of the calcium-calcineurin-nuclear factor of activated T cells (NFAT) signaling was required in addition to the TLR2-PI3K-PLC pathway. Detailed studies by site-directed mutagenesis of potential NFAT binding sites and chromatin immunoprecipitation (ChIP) analysis revealed that the binding of macrophage NFATc2, at the -73/-77 site on the cox2 promoter, induced L. donovani-driven cox2 transcriptional activation. Collectively, these findings highlight the contribution of TLR2 downstream signaling toward activation of cPLA2 and Cox2 and illustrate how the TLR2-PI3K-PLC pathway acts in a concerted manner with calcium-calcineurin-NFATc2 signaling to modulate PGE2 release from L. donovani-infected macrophages.

A cholesterol-binding domain in STIM1 modulates STIM1-Orai1 physical and functional interactions

STIM1 and Orai1 are the main components of a widely conserved Calcium influx pathway known as store-operated calcium entry (SOCE). STIM1 is a calcium sensor, which oligomerizes and activates Orai channels when calcium levels drop inside the endoplasmic reticulum (ER). The series of molecular rearrangements that STIM1 undergoes until final activation of Orai1 require the direct exposure of the STIM1 domain known as SOAR (Stim Orai Activating Region). In addition to these complex molecular rearrangements, other constituents like lipids at the plasma membrane, play critical roles orchestrating SOCE. PI(4,5)P₂ and enriched cholesterol microdomains have been shown as important signaling platforms that recruit the SOCE machinery in steps previous to Orai1 activation. However, little is known about the molecular role of cholesterol once SOCE is activated. In this study we provide clear evidence that STIM1 has a cholesterol-binding domain located inside the SOAR region and modulates Orai1 channels. We demonstrate a functional association of STIM1 and SOAR to cholesterol, indicating a close proximity of SOAR to the inner layer of the plasma membrane. In contrast, the depletion of cholesterol induces the SOAR detachment from the plasma membrane and enhances its association to Orai1. These results are recapitulated with full length STIM1.

The coming of age of the mitochondria-ER contact: a matter of thickness

The sites of near-contact between the mitochondrion and the endoplasmic reticulum (ER) have earned a lot of attention due to their key role in the maintenance of lipid and calcium (Ca(2+)) homeostasis, in the initiation of autophagy and mitochondrial division, and in sensing metabolic shifts. At these sites, typically called MAMs (mitochondria-associated ER membranes) or MERCs (mitochondria-ER contacts), the organelles juxtapose at a distance that can range from ~10 to ~50 nm. The multifunctional role of this subcellular compartment is puzzling; further, recent studies have shown that mitochondria-ER contacts are highly plastic structures that remodel upon metabolic transitions and that their activity in controlling lipid homeostasis could be involved in Alzheimer's disease pathogenesis. This review aims at integrating the functions of this subcellular compartment to its most characterizing and unexplored structural parameter, their 'thickness': that is, the width of the cleft that separates the cytosolic face of the outer mitochondrial membrane from that of the ER. We describe and discuss the reasons why the thickness of a MERC should be considered a regulated structural parameter of the cell that defines and controls its function. Further, we propose a MERC classification that will help organize the expanding field of MERCs biology and of their role in cell physiology and human disease.

ASIC1-mediated calcium entry stimulates NFATc3 nuclear translocation via PICK1 coupling in pulmonary arterial smooth muscle cells

The development of chronic hypoxia (CH)-induced pulmonary hypertension is associated with increased pulmonary arterial smooth muscle cell (PASMC) Ca(2+) influx through acid-sensing ion channel-1 (ASIC1) and activation of the Ca(2+)/calcineurin-dependent transcription factor known as nuclear factor of activated T-cells isoform c3 (NFATc3). Whether Ca(2+) influx through ASIC1 contributes to NFATc3 activation in the pulmonary vasculature is unknown. Furthermore, both ASIC1 and calcineurin have been shown to interact with the scaffolding protein known as protein interacting with C kinase-1 (PICK1). In the present study, we tested the hypothesis that ASIC1 contributes to NFATc3 nuclear translocation in PASMC in a PICK1-dependent manner. Using both ASIC1 knockout (ASIC1(-/-)) mice and pharmacological inhibition of ASIC1, we demonstrate that ASIC1 contributes to CH-induced (1 wk at 380 mmHg) and endothelin-1 (ET-1)-induced (10(-7) M) Ca(2+) responses and NFATc3 nuclear import in PASMC. The interaction between ASIC1/PICK1/calcineurin was shown using a Duolink in situ Proximity Ligation Assay. Inhibition of PICK1 by using FSC231 abolished ET-1-induced and ionomycin-induced NFATc3 nuclear import, but it did not alter ET-1-mediated Ca(2+) responses, suggesting that PICK1 acts downstream of Ca(2+) influx. The key findings of the present work are that 1) Ca(2+) influx through ASIC1 mediates CH- and ET-1-induced NFATc3 nuclear import and 2) the scaffolding protein PICK1 is necessary for NFATc3 nuclear import. Together, these data provide an essential link between CH-induced ASIC1-mediated Ca(2+) influx and activation of the NFATc3 transcription factor. Identification of this ASIC1/PICK1/NFATc3 signaling complex increases our understanding of the mechanisms contributing to the vascular remodeling and increased vascular contractility that are associated with CH-induced pulmonary hypertension.

Endoplasmic reticulum-plasma membrane junctions: structure, function and dynamics

Endoplasmic reticulum (ER)-plasma membrane (PM) junctions are contact sites between the ER and the PM; the distance between the two organelles in the junctions is below 40 nm and the membranes are connected by protein tethers. A number of molecular tools and technical approaches have been recently developed to visualise, modify and characterise properties of ER-PM junctions. The junctions serve as the platforms for lipid exchange between the organelles and for cell signalling, notably Ca(2+) and cAMP signalling. Vice versa, signalling events regulate the development and properties of the junctions. Two Ca(2+) -dependent mechanisms of de novo formation of ER-PM junctions have been recently described and characterised. The junction-forming proteins and lipids are currently the focus of vigorous investigation. Junctions can be relatively short-lived and simple structures, forming and dissolving on the time scale of a few minutes. However, complex, sophisticated and multifunctional ER-PM junctions, capable of attracting numerous protein residents and other cellular organelles, have been described in some cell types. The road from simplicity to complexity, i.e. the transformation from simple 'nascent' ER-PM junctions to advanced stable multiorganellar complexes, is likely to become an attractive research avenue for current and future junctologists. Another area of considerable research interest is the downstream cellular processes that can be activated by specific local signalling events in the ER-PM junctions. Studies of the cell physiology and indeed pathophysiology of ER-PM junctions have already produced some surprising discoveries, likely to expand with advances in our understanding of these remarkable organellar contact sites.

Optogenetic Control of Calcium Oscillation Waveform Defines NFAT as an Integrator of Calcium Load

It is known that the calcium-dependent transcription factor NFAT initiates transcription in response to pulsatile loads of calcium signal. However, the relative contributions of calcium oscillation frequency, amplitude, and duty cycle to transcriptional activity remain unclear. Here, we engineer HeLa cells to permit optogenetic control of intracellular calcium concentration using programmable LED arrays. This approach allows us to generate calcium oscillations of constant peak amplitude, in which frequency is varied while holding duty cycle constant, or vice versa. Using this setup and mathematical modeling, we show that NFAT transcriptional activity depends more on duty cycle, defined as the proportion of the integrated calcium concentration over the oscillation period, than on frequency alone. This demonstrates that NFAT acts primarily as a signal integrator of cumulative load rather than a frequency-selective decoder. This approach resolves a fundamental question in calcium encoding and demonstrates the value of optogenetics for isolating individual dynamical components of larger signaling behaviors.