The STIM-Orai Pathway: Orai, the Pore-Forming Subunit of the CRAC Channel

ELD607 specifically traffics Orai1 to the lysosome leading to inhibition of store operated calcium entry

Orai1 is a plasma membrane Ca2+ channel involved in store operated calcium entry (SOCE). SOCE can regulate cell growth, exocytosis, gene expression and inflammation. We previously found that short palate lung and nasal epithelial clone 1's (SPLUNC1) sixth α-helix (α6) bound Orai1 to inhibit SOCE. SPLUNC1 was not proteolytically stable, so we developed ELD607, an 11 amino acid peptide based on SPLUNC1's α6 region which was more stable and more potent than SPLUNC1/α6. Here, we studied ELD607's mechanism of action. We overexpressed either Orai1-HA or Orai1-YFP in HEK293T cells to probe ELD607-Orai1 interactions by confocal microscopy. We also measured changes in Fluo-4 fluorescence in a multiplate reader as a marker of cytoplasmic Ca2+ levels. ELD607 internalized Orai1 independently of STIM1. Both 15 min and 3 h exposure to ELD607 similarly depleted Orai1 in the plasma membrane. However, 3 h exposure to ELD607 yielded greater inhibition of SOCE. ELD607 continued to colocalize with Orai1 after internalization and this process was dependent on the presence of the ubiquitin ligase NEDD4.2. Similarly, ELD607 increased the colocalization between Orai1 and ubiquitin. ELD607 also increased the colocalization between Orai1 and Rab5 and 7, but not Rab11, suggesting that Orai1 trafficked through early and late but not recycling endosomes. Finally, ELD607 caused Orai1, but not Orai2, Orai3, or STIM1 to traffic to lysosomes. We conclude that ELD607 rapidly binds to Orai1 and works in an identical fashion as full length SPLUNC1 by internalizing Orai1 and sending it to lysosomes, leading to a decrease in SOCE.

Molecular regulation of calcium-sensing receptor (CaSR)-mediated signaling

Calcium-sensing receptor (CaSR), a family C G-protein-coupled receptor, plays a crucial role in regulating calcium homeostasis by sensing small concentration changes of extracellular Ca2+, Mg2+, amino acids (e.g., L-Trp and L-Phe), small peptides, anions (e.g., HCO3 - and PO4 3-), and pH. CaSR-mediated intracellular Ca2+ signaling regulates a diverse set of cellular processes including gene transcription, cell proliferation, differentiation, apoptosis, muscle contraction, and neuronal transmission. Dysfunction of CaSR with mutations results in diseases such as autosomal dominant hypocalcemia, familial hypocalciuric hypercalcemia, and neonatal severe hyperparathyroidism. CaSR also influences calciotropic disorders, such as osteoporosis, and noncalciotropic disorders, such as cancer, Alzheimer's disease, and pulmonary arterial hypertension. This study first reviews recent advances in biochemical and structural determination of the framework of CaSR and its interaction sites with natural ligands, as well as exogenous positive allosteric modulators and negative allosteric modulators. The establishment of the first CaSR protein-protein interactome network revealed 94 novel players involved in protein processing in endoplasmic reticulum, trafficking, cell surface expression, endocytosis, degradation, and signaling pathways. The roles of these proteins in Ca2+-dependent cellular physiological processes and in CaSR-dependent cellular signaling provide new insights into the molecular basis of diseases caused by CaSR mutations and dysregulated CaSR activity caused by its protein interactors and facilitate the design of therapeutic agents that target CaSR and other family C G-protein-coupled receptors.

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

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

The alkalinizing, lysosomotropic agent ML-9 induces a pH-dependent depletion of ER Ca2+ stores in cellulo

ML-9 elicits a broad spectrum of effects in cells, including inhibition of myosin light chain kinase, inhibition of store-operated Ca²⁺ entry and lysosomotropic actions that result in prostate cancer cell death. Moreover, the compound also affects endoplasmic reticulum (ER) Ca²⁺ homeostasis, although the underlying mechanisms remain unclear. We found that ML-9 provokes a rapid mobilization of Ca²⁺ from ER independently of IP₃Rs or TMBIM6/Bax Inhibitor-1, two ER Ca²⁺-leak channels. Moreover, in unidirectional ⁴⁵Ca²⁺ fluxes in permeabilized cells, ML-9 was able to reduce ER Ca²⁺-store content. Although the ER Ca²⁺ store content was decreased, ML-9 did not directly inhibit SERCA's ATPase activity in vitro using microsomal preparations. Consistent with its chemical properties as a cell-permeable weak alkalinizing agent (calculated pKa of 8.04), ML-9 provoked a rapid increase in cytosolic pH preceding the Ca²⁺ efflux from the ER. Pre-treatment with the weak acid 3NPA blunted the ML-9-evoked increase in intracellular pH and subsequent ML-9-induced Ca²⁺ mobilization from the ER. This experiment underpins a causal link between ML-9's impact on the pH and Ca²⁺ dynamics. Overall, our work indicates that the lysosomotropic drug ML-9 may not only impact lysosomal compartments but also have severe impacts on ER Ca²⁺ handling in cellulo.

X-ray irradiation triggers immune response in human T-lymphocytes via store-operated Ca2+ entry and NFAT activation

Radiation therapy efficiently eliminates cancer cells and reduces tumor growth. To understand collateral agonistic and antagonistic effects of this treatment on the immune system, we examined the impact of x-ray irradiation on human T cells. We find that, in a major population of leukemic Jurkat T cells and peripheral blood mononuclear cells, clinically relevant radiation doses trigger delayed oscillations of the cytosolic Ca²⁺ concentration. They are generated by store-operated Ca²⁺ entry (SOCE) following x-ray–induced clustering of Orai1 and STIM1 and formation of a Ca²⁺ release–activated Ca²⁺ (CRAC) channel. A consequence of the x-ray–triggered Ca²⁺ signaling cascade is translocation of the transcription factor nuclear factor of activated T cells (NFAT) from the cytosol into the nucleus, where it elicits the expression of genes required for immune activation. The data imply activation of blood immune cells by ionizing irradiation, with consequences for toxicity and therapeutic effects of radiation therapy.

Advances in Intracellular Calcium Signaling Reveal Untapped Targets for Cancer Therapy

Intracellular Ca2+ distribution is a tightly regulated process. Numerous Ca2+ chelating, storage, and transport mechanisms are required to maintain normal cellular physiology. Ca2+-binding proteins, mainly calmodulin and calbindins, sequester free intracellular Ca2+ ions and apportion or transport them to signaling hubs needing the cations. Ca2+ channels, ATP-driven pumps, and exchangers assist the binding proteins in transferring the ions to and from appropriate cellular compartments. Some, such as the endoplasmic reticulum, mitochondria, and lysosomes, act as Ca2+ repositories. Cellular Ca2+ homeostasis is inefficient without the active contribution of these organelles. Moreover, certain key cellular processes also rely on inter-organellar Ca2+ signaling. This review attempts to encapsulate the structure, function, and regulation of major intracellular Ca2+ buffers, sensors, channels, and signaling molecules before highlighting how cancer cells manipulate them to survive and thrive. The spotlight is then shifted to the slow pace of translating such research findings into anticancer therapeutics. We use the PubMed database to highlight current clinical studies that target intracellular Ca2+ signaling. Drug repurposing and improving the delivery of small molecule therapeutics are further discussed as promising strategies for speeding therapeutic development in this area.

Cross-Talk between Mechanosensitive Ion Channels and Calcium Regulatory Proteins in Cardiovascular Health and Disease

Mechanosensitive ion channels are widely expressed in the cardiovascular system. They translate mechanical forces including shear stress and stretch into biological signals. The most prominent biological signal through which the cardiovascular physiological activity is initiated or maintained are intracellular calcium ions (Ca²⁺). Growing evidence show that the Ca²⁺ entry mediated by mechanosensitive ion channels is also precisely regulated by a variety of key proteins which are distributed in the cell membrane or endoplasmic reticulum. Recent studies have revealed that mechanosensitive ion channels can even physically interact with Ca²⁺ regulatory proteins and these interactions have wide implications for physiology and pathophysiology. Therefore, this paper reviews the cross-talk between mechanosensitive ion channels and some key Ca²⁺ regulatory proteins in the maintenance of calcium homeostasis and its relevance to cardiovascular health and disease.

A549 in-silico 1.0: A first computational model to simulate cell cycle dependent ion current modulation in the human lung adenocarcinoma

Lung cancer is still a leading cause of death worldwide. In recent years, knowledge has been obtained of the mechanisms modulating ion channel kinetics and thus of cell bioelectric properties, which is promising for oncological biomarkers and targets. The complex interplay of channel expression and its consequences on malignant processes, however, is still insufficiently understood. We here introduce the first approach of an in-silico whole-cell ion current model of a cancer cell, in particular of the A549 human lung adenocarcinoma, including the main functionally expressed ion channels in the plasma membrane as so far known. This hidden Markov-based model represents the electrophysiology behind proliferation of the A549 cell, describing its rhythmic oscillation of the membrane potential able to trigger the transition between cell cycle phases, and it predicts membrane potential changes over the cell cycle provoked by targeted ion channel modulation. This first A549 in-silico cell model opens up a deeper insight and understanding of possible ion channel interactions in tumor development and progression, and is a valuable tool for simulating altered ion channel function in lung cancer electrophysiology.

Orai channels: key players in Ca2+ homeostasis

Maintaining a precise calcium (Ca²⁺) balance is vital for cellular survival. The most prominent pathway to shuttle Ca²⁺ into cells is the Ca²⁺ release activated Ca²⁺ (CRAC) channel. Orai proteins are indispensable players in this central mechanism of Ca²⁺ entry. This short review traces the latest articles published in the field of CRAC channel signalling with a focus on the structure of the pore-forming Orai proteins, the propagation of the binding signal from STIM1 through the channel to the central pore and their role in human health and disease.

Folium Sennae and emodin reverse airway smooth muscle contraction

The objective of this project was to find a bronchodilatory compound from herbs and clarify the mechanism. We found that the ethanol extract of Folium Sennae (EEFS) can relax airway smooth muscle (ASM). EEFS inhibited ASM contraction, induced by acetylcholine, in mouse tracheal rings and lung slices. High-performance liquid chromatography assay showed that EEFS contained emodin. Emodin had a similar reversal action. Acetylcholine-evoked contraction was also partially reduced by nifedipine (a selective inhibitor of L-type voltage-dependent Ca²⁺ channels, LVDCCs), YM-58483 (a selective inhibitor of store-operated Ca²⁺ entry, SOCE), as well as Y-27632 (an inhibitor of Rho-associated protein kinase). In addition, LVDCC- and SOCE-mediated currents and cytosolic Ca²⁺ elevations were inhibited by emodin. Emodin reversed acetylcholine-caused increases in phosphorylation of myosin phosphatase target subunit 1. Furthermore, emodin, in vivo, inhibited acetylcholine-induced respiratory system resistance in mice. These results indicate that EEFS-induced relaxation results from emodin inhibiting LVDCC, SOCE, and Ca²⁺ sensitization. These findings suggest that Folium Sennae and emodin may be new sources of bronchodilators.

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.

NS8593 inhibits Ca2+ permeant channels reversing mouse airway smooth muscle contraction

AIMS

This study focused on investigating whether NS8593 reverses airway smooth muscle (ASM) contraction and the underlying mechanism.

MAIN METHODS

ASM contraction in mouse tracheal rings and lung slices was measured. Currents mediated by voltage dependent Ca²⁺ channels (VDCCs) and ACH-activated channels were measured using the whole-cell patch-clamp technique in single tracheal smooth muscle cells (TSMCs). Intracellular Ca²⁺ level and cell length were measured using an LSM 700 laser confocal microscope and a Zen 2010 software. Mouse respiratory system resistance (Rrs) was assessed using a FlexiVent FX system.

KEY FINDINGS

High K⁺ (80 mM K⁺) and ACH induced ASM contraction in mouse tracheal rings and lung slices, which was partially relaxed by nifedipine (blocker of L-type VDCCs, LVDCCs), YM-58483 (blocker of store-operated Ca²⁺ entry (SOCE), transient receptor potential C3 (TRPC3) and TRPC5 channels), respectively. However, the contraction was completely reversed by NS8593, whereas, slightly relaxed by formoterol. ACH activated inward currents, which displayed linear and reversed around 0 mV, indicating the currents were mediated by non-selective cation channels (NSCCs). Moreover, these currents were blocked by YM-58483. In addition, such currents were abolished by NS8593, implicating that NS8593 inhibits the same channels. Besides, NS8593 inhibited increases of intracellular Ca²⁺ and the associated cell shortening. Finally, NS8593 inhibited ACH-induced increases of mouse respirator system resistance (Rrs).

SIGNIFICANCE

Our results indicate that NS8593 inhibits LVDCCs and NSCCs, resulting in decreases of intracellular Ca²⁺ and then leading to ASM relaxation. These data suggest that NS8593 might be a new bronchodilator.

ER Ca2+ release and store-operated Ca2+ entry - partners in crime or independent actors in oncogenic transformation?

Ca²⁺ is a pleiotropic messenger that controls life and death decisions from fertilisation until death. Cellular Ca²⁺ handling mechanisms show plasticity and are remodelled throughout life to meet the changing needs of the cell. In turn, as the demands on a cell alter, for example through a change in its niche environment or its functional requirements, Ca²⁺ handling systems may be targeted to sustain the remodelled cellular state. Nowhere is this more apparent than in cancer. Oncogenic transformation is a multi-stage process during which normal cells become progressively differentiated towards a cancerous state that is principally associated with enhanced proliferation and avoidance of death. Ca²⁺ signalling is intimately involved in almost all aspects of the life of a transformed cell and alterations in Ca²⁺ handling have been observed in cancer. Moreover, this remodelling of Ca²⁺ signalling pathways is also required in some cases to sustain the transformed phenotype. As such, Ca²⁺ handling is hijacked by oncogenic processes to deliver and maintain the transformed phenotype. Central to generation of intracellular Ca²⁺ signals is the release of Ca²⁺ from the endoplasmic reticulum intracellular (ER) Ca²⁺ store via inositol 1,4,5-trisphosphate receptors (InsP₃Rs). Upon depletion of ER Ca²⁺, store-operated Ca²⁺ entry (SOCE) across the plasma membrane occurs via STIM-gated Orai channels. SOCE serves to both replenish stores but also sustain Ca²⁺ signalling events. Here, we will discuss the role and regulation of these two signalling pathways and their interplay in oncogenic transformation.

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

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

FOXO1 transcription factor plays a key role in T cell-HIV-1 interaction

HIV-1 is dependent on the host cell for providing the metabolic resources for completion of its viral replication cycle. Thus, HIV-1 replicates efficiently only in activated CD4⁺ T cells. Barriers preventing HIV-1 replication in resting CD4⁺ T cells include a block that limits reverse transcription and also the lack of activity of several inducible transcription factors, such as NF-κB and NFAT. Because FOXO1 is a master regulator of T cell functions, we studied the effect of its inhibition on T cell/HIV-1 interactions. By using AS1842856, a FOXO1 pharmacologic inhibitor, we observe that FOXO1 inhibition induces a metabolic activation of T cells with a G0/G1 transition in the absence of any stimulatory signal. One parallel outcome of this change is the inhibition of the activity of the HIV restriction factor SAMHD1 and the activation of the NFAT pathway. FOXO1 inhibition by AS1842856 makes resting T cells permissive to HIV-1 infection. In addition, we found that FOXO1 inhibition by either AS1842856 treatment or upon FOXO1 knockdown induces the reactivation of HIV-1 latent proviruses in T cells. We conclude that FOXO1 has a central role in the HIV-1/T cell interaction and that inhibiting FOXO1 with drugs such as AS1842856 may be a new therapeutic shock-and-kill strategy to eliminate the HIV-1 reservoir in human T cells.

HIV-1 is controlled by host restriction factors that interfere with its life cycle. However, the virus has equipped itself to counter these strategies. We report a new interplay between HIV-1 and human T lymphocytes through the FOXO1 transcription factor. By using AS1842856, a drug targeting FOXO1, we found that FOXO1 inhibition triggers metabolic activation and G₀/G₁ transition of resting T cells and also by the inactivation of the SAMHD1 viral restriction factor. FOXO1 inhibition makes resting CD4⁺ T cells permissive to HIV-1 infection. We finally found that pharmacologic (AS1842856 treatment) or genetic (shRNA) silencing of FOXO1 reactivate HIV-1 latent proviruses. Thus FOXO1 appears as an important player of the HIV-1/T-cell relationship and a new potential therapeutic target for intervention during HIV-1 infection.

Store-operated calcium entry in disease: Beyond STIM/Orai expression levels

Precise intracellular calcium signaling is crucial to numerous cellular functions. In non-excitable cells, store-operated calcium entry (SOCE) is a key step in the generation of intracellular calcium signals. Tight regulation of SOCE is important, and dysregulation is involved in several pathophysiological cellular malfunctions. The current underlying SOCE, calcium release-activated calcium current (I_CRAC), was first discovered almost three decades ago. Since its discovery, the molecular components of I_CRAC, Orai1 and stromal interaction molecule 1 (STIM1), have been extensively investigated. Several regulatory mechanisms and proteins contribute to alterations in SOCE and cellular malfunctions in cancer, immune and neurodegenerative diseases, inflammation, and neuronal disorders. This review summarizes these regulatory mechanisms, including glycosylation, pH sensing, and the regulatory proteins golli, α-SNAP, SARAF, ORMDL3, CRACR2A, and TRPM4 channels.