The N terminus of Orai1 couples to the AKAP79 signaling complex to drive NFAT1 activation by local Ca2+ entry

CRAC channel activity pulsates during cytosolic Ca2+ oscillations

Intracellular Ca2+ ions are used as second messengers throughout the phylogenetic tree. They are indispensable for diverse biological processes ranging from fertilization to cell death. In Metazoans, signaling information is conveyed via the amplitude, frequency and spatial profile of cytosolic Ca2+ oscillations. In non-excitable cells, these oscillations generally arise from regenerative release of Ca2+ from inositol 1,4,5-trisphosphate (InsP3)-sensitive intracellular stores, which are refilled by entry of Ca2+ through Ca2+ release-activated Ca2+ (CRAC) channels in the plasma membrane. However, the precise contribution of these store-operated CRAC channels to Ca2+ oscillations has remained controversial for decades. One view proposes that CRAC channels remain open throughout stimulation, functioning as the pacemaker in setting Ca2+ oscillation frequency. An alternative hypothesis is that channel activity oscillates in parallel with InsP3-driven regenerative Ca2+ release. Here, by tethering a genetically encoded Ca2+ indicator to the pore- forming subunit of the CRAC channel, Orai1, we distinguish between these hypotheses and demonstrate that CRAC channel activity fluctuates in phase with cytosolic Ca2+ oscillations during physiological levels of stimulation. We also find that spatially distinct CRAC channel clusters fire in a coordinated manner, revealing that CRAC channels are not independent units but might function in a synchronized manner to provide pulses of Ca2+ signal at the same time.

Nanodomain cAMP signaling in cardiac pathophysiology: potential for developing targeted therapeutic interventions

The 3',5'-cyclic adenosine monophosphate (cAMP) mediates the effects of sympathetic stimulation on the rate and strength of cardiac contraction. Beyond this pivotal role, in cardiac myocytes cAMP also orchestrates a diverse array of reactions to various stimuli. To ensure specificity of response, the cAMP signaling pathway is intricately organized into multiple, spatially confined, subcellular domains, each governing a distinct cellular function. In this review, we describe the molecular components of the cAMP signaling pathway with a specific focus on adenylyl cyclases, A-kinase anchoring proteins, and phosphodiesterases. We discuss how they are organized inside the intracellular space and how they achieve exquisite regulation of signaling within nanometer-size domains. We delineate the key experimental findings that lead to the current model of compartmentalized cAMP signaling, and we offer an overview of our present understanding of how cAMP nanodomains are structured and regulated within cardiac myocytes. Furthermore, we discuss how compartmentalized cAMP signaling is affected in cardiac disease and consider the potential therapeutic opportunities arising from understanding such organization. By exploiting the nuances of compartmentalized cAMP signaling, novel and more effective therapeutic strategies for managing cardiac conditions may emerge. Finally, we highlight the unresolved questions and hurdles that must be addressed to translate these insights into interventions that may benefit patients.

Filamin A C-terminal fragment modulates Orai1 expression by inhibition of protein degradation

Filamin A (FLNA) is an actin-binding protein that has been reported to interact with STIM1 modulating the activation of Orai1 channels. Cleaving of FLNA by calpain leads to a C-terminal fragment that is involved in a variety of functional and pathological events, including pro-oncogenic activity in different types of cancer. Here, we show that full-length FLNA is downregulated in samples from patients with colon cancer as well as in the adenocarcinoma cell line HT-29. This is consistent with an increased calpain-dependent FLNA cleaving with enhanced expression of the C-terminal FLNA fragment accompanied by enhanced expression of Orai1 and STIM1, as well as store-operated Ca2+ entry (SOCE). To further explore the mechanism underlying the enhancement of SOCE by the C-terminal FLNA fragment, we expressed in HEK-293 cells the C-terminal FLNA region encompassing repeats 16-24 (FLNA16-24 fragment), which enhanced both Orai1 and STIM1 as well as SOCE. Transfection of the FLNA16-24 fragment attenuates Orai1 and STIM1 protein degradation, and, specifically, abrogates Orai1α lysosomal degradation and retains this channel in the plasma membrane. However, the C-terminal FLNA fragment did not induce a detectable modification in Orai1β degradation. Due to the relevance of SOCE in cell physiology, our results provide evidence of a novel mechanism for the regulation of Ca2+ influx with relevant pathophysiological implications.NOTE & NOTEWORTHY FLNA cleaving by calpain has been observed in a variety of tumoral, including prostate and colorectal cancer cells, as well as in nontumoral cells, leading to a C-terminal fragment encompassing repeats 16-24. Expression of the FLNA16-24 fragment in HEK-293 cells enhances Orai1 and STIM1 expression, as well as SOCE, a mechanism mediated by attenuation of Orai1α and STIM1 degradation, providing evidence for a novel mechanism for the regulation of SOCE in normal and malignant cells.

Ca2+ tunneling architecture and function are important for secretion

Ca2+ tunneling requires both store-operated Ca2+ entry (SOCE) and Ca2+ release from the endoplasmic reticulum (ER). Tunneling expands the SOCE microdomain through Ca2+ uptake by SERCA into the ER lumen where it diffuses and is released via IP3 receptors. In this study, using high-resolution imaging, we outline the spatial remodeling of the tunneling machinery (IP3R1; SERCA; PMCA; and Ano1 as an effector) relative to STIM1 in response to store depletion. We show that these modulators redistribute to distinct subdomains laterally at the plasma membrane (PM) and axially within the cortical ER. To functionally define the role of Ca2+ tunneling, we engineered a Ca2+ tunneling attenuator (CaTAr) that blocks tunneling without affecting Ca2+ release or SOCE. CaTAr inhibits Cl- secretion in sweat gland cells and reduces sweating in vivo in mice, showing that Ca2+ tunneling is important physiologically. Collectively our findings argue that Ca2+ tunneling is a fundamental Ca2+ signaling modality.

Feedback modulation of Orai1α and Orai1β protein content mediated by STIM proteins

Store-operated Ca2+ entry is a mechanism controlled by the filling state of the intracellular Ca2+ stores, predominantly the endoplasmic reticulum (ER), where ER-resident proteins STIM1 and STIM2 orchestrate the activation of Orai channels in the plasma membrane, and Orai1 playing a predominant role. Two forms of Orai1, Orai1α and Orai1β, have been identified, which arises the question whether they are equally regulated by STIM proteins. We demonstrate that STIM1 preferentially activates Orai1α over STIM2, yet both STIM proteins similarly activate Orai1β. Under resting conditions, there is a pronounced association between STIM2 and Orai1α. STIM1 and STIM2 are also shown to influence the protein levels of the Orai1 variants, independently of Ca2+ influx, via lysosomal degradation. Interestingly, Orai1α and Orai1β appear to selectively regulate the protein level of STIM1, but not STIM2. These observations offer crucial insights into the regulatory dynamics between STIM proteins and Orai1 variants, enhancing our understanding of the intricate processes that fine-tune intracellular Ca2+ signaling.

Visualization and Quantification of NFAT1 Translocation in T Cells

NFAT1 translocation to the nucleus and consequent IL-2 production in T cells can be used as a downstream read-out for initial T cell activation via the T cell receptor complex. Here, we describe a protocol for the detection of NFAT1 translocation using anti-CD3/anti-CD28 coated beads as a stimulus of primary murine CD4+ T cells by immunohistochemistry. Further, an open-source Python code is described to determine NFAT1 translocation.

House dust mite allergens, store-operated Ca2+ channels and asthma

The house dust mite is the principal source of aero-allergen worldwide. Exposure to mite-derived allergens is associated with the development of asthma in susceptible individuals, and the majority of asthmatics are allergic to the mite. Mite-derived allergens are functionally diverse and activate multiple cell types within the lung that result in chronic inflammation. Allergens activate store-operated Ca2+ release-activated Ca2+ (CRAC) channels, which are widely expressed in multiple cell types within the lung that are associated with the pathogenesis of asthma. Opening of CRAC channels stimulates Ca2+-dependent transcription factors, including nuclear factor of activated T cells and nuclear factor-κB, which drive expression of a plethora of pro-inflammatory cytokines and chemokines that help to sustain chronic inflammation. Here, I describe drivers of asthma, properties of mite-derived allergens, how the allergens are recognized by cells, the signalling pathways used by the receptors and how these are transduced into functional effects, with a focus on CRAC channels. In vivo experiments that demonstrate the effectiveness of targeting CRAC channels as a potential new therapy for treating mite-induced asthma are also discussed, in tandem with other possible approaches.

Multifaceted control of T cell differentiation by STIM1

In T cells, stromal interaction molecule (STIM) and Orai are dispensable for conventional T cell development, but critical for activation and differentiation. This review focuses on novel STIM-dependent mechanisms for control of Ca2+ signals during T cell activation and its impact on mitochondrial function and transcriptional activation for control of T cell differentiation and function. We highlight areas that require further work including the roles of plasma membrane Ca2+ ATPase (PMCA) and partner of STIM1 (POST) in controlling Orai function. A major knowledge gap also exists regarding the independence of T cell development from STIM and Orai, despite compelling evidence that it requires Ca2+ signals. Resolving these and other outstanding questions ensures that the field will remain active for many years to come.

Adhesion to laminin-1 and collagen IV induces the formation of Ca2+ microdomains that sensitize mouse T cells for activation

During an immune response, T cells migrate from blood vessel walls into inflamed tissues by migrating across the endothelium and through extracellular matrix (ECM). Integrins facilitate T cell binding to endothelial cells and ECM proteins. Here, we report that Ca²⁺ microdomains observed in the absence of T cell receptor (TCR)/CD3 stimulation are initial signaling events triggered by adhesion to ECM proteins that increase the sensitivity of primary murine T cells to activation. Adhesion to the ECM proteins collagen IV and laminin-1 increased the number of Ca²⁺ microdomains in a manner dependent on the kinase FAK, phospholipase C (PLC), and all three inositol 1,4,5-trisphosphate receptor (IP₃R) subtypes and promoted the nuclear translocation of the transcription factor NFAT-1. Mathematical modeling predicted that the formation of adhesion-dependent Ca²⁺ microdomains required the concerted activity of two to six IP₃Rs and ORAI1 channels to achieve the increase in the Ca²⁺ concentration in the ER-plasma membrane junction that was observed experimentally and that required SOCE. Further, adhesion-dependent Ca²⁺ microdomains were important for the magnitude of the TCR-induced activation of T cells on collagen IV as assessed by the global Ca²⁺ response and NFAT-1 nuclear translocation. Thus, adhesion to collagen IV and laminin-1 sensitizes T cells through a mechanism involving the formation of Ca²⁺ microdomains, and blocking this low-level sensitization decreases T cell activation upon TCR engagement.

Role of Orai-family channels in the activation and regulation of transcriptional activity

Store operated Ca²⁺ entry (SOCE) is a cornerstone for the maintenance of intracellular Ca²⁺ homeostasis and the regulation of a variety of cellular functions. SOCE is mediated by STIM and Orai proteins following the activation of inositol 1,4,5-trisphosphate receptors. Then, a reduction of the endoplasmic reticulum intraluminal Ca²⁺ concentration is sensed by STIM proteins, which undergo a conformational change and activate plasma membrane Ca²⁺ channels comprised by Orai proteins. STIM1/Orai-mediated Ca²⁺ signals are finely regulated and modulate the activity of different transcription factors, including certain isoforms of the nuclear factor of activated T-cells, the cAMP-response element binding protein, the nuclear factor κ-light chain-enhancer of activated B cells, c-fos, and c-myc. These transcription factors associate SOCE with a plethora of signaling events and cellular functions. Here we provide an overview of the current knowledge about the role of Orai channels in the regulation of transcription factors through Ca²⁺ -dependent signaling pathways.

Aerobic exercise reverses aging-induced depth-dependent decline in cerebral microcirculation

Aging is a major risk factor for cognitive impairment. Aerobic exercise benefits brain function and may promote cognitive health in older adults. However, underlying biological mechanisms across cerebral gray and white matter are poorly understood. Selective vulnerability of the white matter to small vessel disease and a link between white matter health and cognitive function suggests a potential role for responses in deep cerebral microcirculation. Here, we tested whether aerobic exercise modulates cerebral microcirculatory changes induced by aging. To this end, we carried out a comprehensive quantitative examination of changes in cerebral microvascular physiology in cortical gray and subcortical white matter in mice (3-6 vs. 19-21 months old), and asked whether and how exercise may rescue age-induced deficits. In the sedentary group, aging caused a more severe decline in cerebral microvascular perfusion and oxygenation in deep (infragranular) cortical layers and subcortical white matter compared with superficial (supragranular) cortical layers. Five months of voluntary aerobic exercise partly renormalized microvascular perfusion and oxygenation in aged mice in a depth-dependent manner, and brought these spatial distributions closer to those of young adult sedentary mice. These microcirculatory effects were accompanied by an improvement in cognitive function. Our work demonstrates the selective vulnerability of the deep cortex and subcortical white matter to aging-induced decline in microcirculation, as well as the responsiveness of these regions to aerobic exercise.

Celastrol inhibits store operated calcium entry and suppresses psoriasis

Introduction: Psoriasis is an inflammatory autoimmune skin disease that is hard to cure and prone to relapse. Currently available global immunosuppressive agents for psoriasis may cause severe side effects, thus it is crucial to identify new therapeutic reagents and druggable signaling pathways for psoriasis. Methods: To check the effects of SOCE inhibitors on psoriasis, we used animal models, biochemical approaches, together with various imaging techniques, including calcium, confocal and FRET imaging. Results and discussion: Store operated calcium (Ca²⁺) entry (SOCE), mediated by STIM1 and Orai1, is crucial for the function of keratinocytes and immune cells, the two major players in psoriasis. Here we showed that a natural compound celastrol is a novel SOCE inhibitor, and it ameliorated the skin lesion and reduced PASI scores in imiquimod-induced psoriasis-like mice. Celastrol dose- and time-dependently inhibited SOCE in HEK cells and HaCaT cells, a keratinocyte cell line. Mechanistically, celastrol inhibited SOCE via its actions both on STIM1 and Orai1. It inhibited Ca²⁺ entry through constitutively-active Orai1 mutants independent of STIM1. Rather than blocking the conformational switch and oligomerization of STIM1 during SOCE activation, celastrol diminished the transition from oligomerized STIM1 into aggregates, thus locking STIM1 in a partially active state. As a result, it abolished the functional coupling between STIM1 and Orai1, diminishing SOCE signals. Overall, our findings identified a new SOCE inhibitor celastrol that suppresses psoriasis, suggesting that SOCE pathway may serve as a new druggable target for treating psoriasis.

The store-operated Ca2+ channel Orai1α is required for agonist-evoked NF-κB activation by a mechanism dependent on PKCβ2

Store-operated Ca²⁺ entry is a ubiquitous mechanism for Ca²⁺ influx in mammalian cells that regulates a variety of physiological processes. The identification of two forms of Orai1, the predominant store-operated channel, Orai1α and Orai1β, raises the question whether they differentially regulate cell function. Orai1α is the full-length Orai1, containing 301 amino acids, whereas Orai1β lacks the N-terminal 63 amino acids. Here, using a combination of biochemistry and imaging combined with the use of human embryonic kidney 293 KO cells, missing the native Orai1, transfected with plasmids encoding for either Orai1α or Orai1β, we show that Orai1α plays a relevant role in agonist-induced NF-κB transcriptional activity. In contrast, functional Orai1β is not required for the activation of these transcription factors. The role of Orai1α in the activation of NF-κB is entirely dependent on Ca²⁺ influx and involves PKCβ activation. Our results indicate that Orai1α interacts with PKCβ2 by a mechanism involving the Orai1α exclusive AKAP79 association region, which strongly suggests a role for AKAP79 in this process. These findings provide evidence of the role of Orai1α in agonist-induced NF-κB transcriptional activity and reveal functional differences between Orai1 variants.

A cellular atlas of calcineurin signaling

Intracellular Ca2+ signals are temporally controlled and spatially restricted. Signaling occurs adjacent to sites of Ca2+ entry and/or release, where Ca2+-dependent effectors and their substrates co-localize to form signaling microdomains. Here we review signaling by calcineurin, the Ca2+/calmodulin regulated protein phosphatase and target of immunosuppressant drugs, Cyclosporin A and FK506. Although well known for its activation of the adaptive immune response via NFAT dephosphorylation, systematic mapping of human calcineurin substrates and regulators reveals unexpected roles for this versatile phosphatase throughout the cell. We discuss calcineurin function, with an emphasis on where signaling occurs and mechanisms that target calcineurin and its substrates to signaling microdomains, especially binding of cognate short linear peptide motifs (SLiMs). Calcineurin is ubiquitously expressed and regulates events at the plasma membrane, other intracellular membranes, mitochondria, the nuclear pore complex and centrosomes/cilia. Based on our expanding knowledge of localized CN actions, we describe a cellular atlas of Ca2+/calcineurin signaling.

Similarities and Differences between the Orai1 Variants: Orai1α and Orai1β

Orai1, the first identified member of the Orai protein family, is ubiquitously expressed in the animal kingdom. Orai1 was initially characterized as the channel responsible for the store-operated calcium entry (SOCE), a major mechanism that allows cytosolic calcium concentration increments upon receptor-mediated IP₃ generation, which results in intracellular Ca²⁺ store depletion. Furthermore, current evidence supports that abnormal Orai1 expression or function underlies several disorders. Orai1 is, together with STIM1, the key element of SOCE, conducting the Ca²⁺ release-activated Ca²⁺ (CRAC) current and, in association with TRPC1, the store-operated Ca²⁺ (SOC) current. Additionally, Orai1 is involved in non-capacitative pathways, as the arachidonate-regulated or LTC4-regulated Ca²⁺ channel (ARC/LRC), store-independent Ca²⁺ influx activated by the secretory pathway Ca²⁺-ATPase (SPCA2) and the small conductance Ca²⁺-activated K⁺ channel 3 (SK3). Furthermore, Orai1 possesses two variants, Orai1α and Orai1β, the latter lacking 63 amino acids in the N-terminus as compared to the full-length Orai1α form, which confers distinct features to each variant. Here, we review the current knowledge about the differences between Orai1α and Orai1β, the implications of the Ca²⁺ signals triggered by each variant, and their downstream modulatory effect within the cell.

Nuanced Interactions between AKAP79 and STIM1 with Orai1 Ca2+ Channels at Endoplasmic Reticulum-Plasma Membrane Junctions Sustain NFAT Activation

A-kinase anchoring protein 79 (AKAP79) is a human scaffolding protein that organizes Ca²⁺/calmodulin-dependent protein phosphatase calcineurin, calmodulin, cAMP-dependent protein kinase, protein kinase C, and the transcription factor nuclear factor of activated T cells (NFAT1) into a signalosome at the plasma membrane. Upon Ca²⁺ store depletion, AKAP79 interacts with the N-terminus of STIM1-gated Orai1 Ca²⁺ channels, enabling Ca²⁺ nanodomains to stimulate calcineurin. Calcineurin then dephosphorylates and activates NFAT1, which then translocates to the nucleus. A fundamental question is how signalosomes maintain long-term signaling when key effectors are released and therefore removed beyond the reach of the activating signal. Here, we show that the AKAP79-Orai1 interaction is considerably more transient than that of STIM1-Orai1. Free AKAP79, with calcineurin and NFAT1 in tow, is able to replace rapidly AKAP79 devoid of NFAT1 on Orai1, in the presence of continuous Ca²⁺ entry. We also show that Ca²⁺ nanodomains near Orai1 channels activate almost the entire cytosolic pool of NFAT1. Recycling of inactive NFAT1 from the cytoplasm to AKAP79 in the plasma membrane, coupled with the relatively weak interaction between AKAP79 and Orai1, maintain excitation-transcription coupling. By measuring rates for AKAP79-NFAT interaction, we formulate a mathematical model that simulates NFAT dynamics at the plasma membrane.

Orai1 Inhibitors as Potential Treatments for Pulmonary Arterial Hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is characterized by progressive distal pulmonary artery (PA) obstruction, leading to right ventricular hypertrophy and failure. Exacerbated intracellular calcium (Ca²⁺) signaling contributes to abnormalities in PA smooth muscle cells (PASMCs), including aberrant proliferation, apoptosis resistance, exacerbated migration, and arterial contractility. Store-operated Ca²⁺ entry is involved in Ca²⁺ homeostasis in PASMCs, but its properties in PAH are unclear.

METHODS

Using a combination of Ca²⁺ imaging, molecular biology, in vitro, ex vivo, and in vivo approaches, we investigated the roles of the Orai1 SOC channel in PA remodeling in PAH and determined the consequences of pharmacological Orai1 inhibition in vivo using experimental models of pulmonary hypertension (PH).

RESULTS

Store-operated Ca²⁺ entry and Orai1 mRNA and protein were increased in human PASMCs (hPASMCs) from patients with PAH (PAH-hPASMCs). We found that MEK1/2 (mitogen-activated protein kinase kinase 1/2), NFAT (nuclear factor of activated T cells), and NFκB (nuclear factor-kappa B) contribute to the upregulation of Orai1 expression in PAH-hPASMCs. Using small interfering RNA (siRNA) and Orai1 inhibitors, we found that Orai1 inhibition reduced store-operated Ca²⁺ entry, mitochondrial Ca²⁺ uptake, aberrant proliferation, apoptosis resistance, migration, and excessive calcineurin activity in PAH-hPASMCs. Orai1 inhibitors reduced agonist-evoked constriction in human PAs. In experimental rat models of PH evoked by chronic hypoxia, monocrotaline, or Sugen/hypoxia, administration of Orai1 inhibitors (N-{4-[3,5-bis(Trifluoromethyl)-1H-pyrazol-1-yl]phenyl}-4-methyl-1,2,3-thiadiazole-5-carboxamide [BTP2], 4-(2,5-dimethoxyphenyl)-N-[(pyridin-4-yl)methyl]aniline [JPIII], or 5J4) protected against PH.

CONCLUSIONS

In human PAH and experimental PH, Orai1 expression and activity are increased. Orai1 inhibition normalizes the PAH-hPASMCs phenotype and attenuates PH in rat models. These results suggest that Orai1 should be considered as a relevant therapeutic target for PAH.

Plasma Membrane Ca2+ ATPase Activity Enables Sustained Store-operated Ca2+ Entry in the Absence of a Bulk Cytosolic Ca2+ Rise

In many cell types, the rise in cytosolic Ca2+ due to opening of Ca2+ release-activated Ca2+ (CRAC) channels drives a plethora of responses, including secretion, motility, energy production, and gene expression. The amplitude and time course of the cytosolic Ca2+ rise is shaped by the rates of Ca2+ entry into and removal from the cytosol. However, an extended bulk Ca2+ rise is toxic to cells. Here, we show that the plasma membrane Ca2+ ATPase (PMCA) pump plays a major role in preventing a prolonged cytosolic Ca2+ signal following CRAC channel activation. Ca2+ entry through CRAC channels leads to a sustained sub-plasmalemmal Ca2+ rise but bulk Ca2+ is kept low by the activity of PMCA4b. Despite the low cytosolic Ca2+, membrane permeability to Ca2+ is still elevated and Ca2+ continues to enter through CRAC channels. Ca2+-dependent NFAT activation, driven by Ca2+ nanodomains near the open channels, is maintained despite the return of bulk Ca2+ to near pre-stimulation levels. Our data reveal a central role for PMCA4b in determining the pattern of a functional Ca2+ signal and in sharpening local Ca2+ gradients near CRAC channels, whilst protecting cells from a toxic Ca2+ overload.

STIM Proteins and Regulation of SOCE in ER-PM Junctions

ER-PM junctions are membrane contact sites formed by the endoplasmic reticulum (ER) and plasma membrane (PM) in close apposition together. The formation and stability of these junctions are dependent on constitutive and dynamic enrichment of proteins, which either contribute to junctional stability or modulate the lipid levels of both ER and plasma membranes. The ER-PM junctions have come under much scrutiny recently as they serve as hubs for assembling the Ca²⁺ signaling complexes. This review summarizes: (1) key findings that underlie the abilities of STIM proteins to accumulate in ER-PM junctions; (2) the modulation of Orai/STIM complexes by other components found within the same junction; and (3) how Orai1 channel activation is coordinated and coupled with downstream signaling pathways.

Discovery of novel gating checkpoints in the Orai1 calcium channel by systematic analysis of constitutively active mutants of its paralogs and orthologs

In humans, there are three paralogs of the Orai Ca2+ channel that form the core of the store-operated calcium entry (SOCE) machinery. While the STIM-mediated gating mechanism of Orai channels is still under active investigation, several artificial and natural variants are known to cause constitutive activity of the human Orai1 channel. Surprisingly, little is known about the conservation of the gating checkpoints among the different human Orai paralogs and orthologs in other species. In our work, we show that the mutation corresponding to the activating mutation H134A in transmembrane helix 2 (TM2) of human Orai1 also activates Orai2 and Orai3, likely via a similar mechanism. However, this cross-paralog conservation does not apply to the "ANSGA" nexus mutations in TM4 of human Orai1, which is reported to mimic the STIM1-activated state of the channel. In investigating the mechanistic background of these differences, we identified two positions, H171 and F246 in human Orai1, that are not conserved among paralogs and that seem to be crucial for the channel activation triggered by the "ANSGA" mutations in Orai1. However, mutations of the same residues still allow gating of Orai1 by STIM1, suggesting that the ANSGA mutant of Orai1 may not be a surrogate for the STIM1-activated state of the Orai1 channel. Our results shed new light on these important gating checkpoints and show that the gating mechanism of Orai channels is affected by multiple factors that are not necessarily conserved among orai homologs, such as the TM4-TM3 coupling.

The Role of Lipids in CRAC Channel Function

The composition and dynamics of the lipid membrane define the physical properties of the bilayer and consequently affect the function of the incorporated membrane transporters, which also applies for the prominent Ca²⁺ release-activated Ca²⁺ ion channel (CRAC). This channel is activated by receptor-induced Ca²⁺ store depletion of the endoplasmic reticulum (ER) and consists of two transmembrane proteins, STIM1 and Orai1. STIM1 is anchored in the ER membrane and senses changes in the ER luminal Ca²⁺ concentration. Orai1 is the Ca²⁺-selective, pore-forming CRAC channel component located in the plasma membrane (PM). Ca²⁺ store-depletion of the ER triggers activation of STIM1 proteins, which subsequently leads to a conformational change and oligomerization of STIM1 and its coupling to as well as activation of Orai1 channels at the ER-PM contact sites. Although STIM1 and Orai1 are sufficient for CRAC channel activation, their efficient activation and deactivation is fine-tuned by a variety of lipids and lipid- and/or ER-PM junction-dependent accessory proteins. The underlying mechanisms for lipid-mediated CRAC channel modulation as well as the still open questions, are presented in this review.

Calcium Signalling in Breast Cancer Associated Bone Pain

Calcium (Ca²⁺) is involved as a signalling mediator in a broad variety of physiological processes. Some of the fastest responses in human body like neuronal action potential firing, to the slowest gene transcriptional regulation processes are controlled by pathways involving calcium signalling. Under pathological conditions these mechanisms are also involved in tumoral cells reprogramming, resulting in the altered expression of genes associated with cell proliferation, metastatisation and homing to the secondary metastatic site. On the other hand, calcium exerts a central function in nociception, from cues sensing in distal neurons, to signal modulation and interpretation in the central nervous system leading, in pathological conditions, to hyperalgesia, allodynia and pain chronicization. It is well known the relationship between cancer and pain when tumoral metastatic cells settle in the bones, especially in late breast cancer stage, where they alter the bone micro-environment leading to bone lesions and resulting in pain refractory to the conventional analgesic therapies. The purpose of this review is to address the Ca²⁺ signalling mechanisms involved in cancer cell metastatisation as well as the function of the same signalling tools in pain regulation and transmission. Finally, the possible interactions between these two cells types cohabiting the same Ca²⁺ rich environment will be further explored attempting to highlight new possible therapeutical targets.

A novel FBW7/NFAT1 axis regulates cancer immunity in sunitinib-resistant renal cancer by inducing PD-L1 expression

Background

Tyrosine kinase inhibitors (TKIs) alone and in combination with immune checkpoint inhibitors (ICIs) have been shown to be beneficial for the survival of metastatic renal cell carcinoma (mRCC) patients, but resistance to targeted therapy and ICIs is common in the clinic. Understanding the underlying mechanism is critical for further prolonging the survival of renal cancer patients. Nuclear factor of activated T cell 1 (NFAT1) is expressed in immune and nonimmune cells, and the dysregulation of NFAT1 contributes to the progression of various type of malignant tumors. However, the specific role of NFAT1 in RCC is elusive. As a regulator of the immune response, we would like to systemically study the role of NFAT1 in RCC.

Methods

TCGA-KIRC dataset analysis, Western blot analysis and RT-qPCR analysis was used to determine the clinic-pathological characteristic of NFAT1 in RCC. CCK-8 assays, colony formation assays and xenograft assays were performed to examine the biological role of NFAT1 in renal cancer cells. RNA-seq analysis was used to examine the pathways changed after NFAT1 silencing. ChIP-qPCR, coimmunoprecipitation analysis, Western blot analysis and RT-qPCR analysis were applied to explore the mechanism by NAFT1 was regulated in the renal cancer cells.

Results

In our study, we found that NFAT1 was abnormally overexpressed in RCC and that NFAT1 overexpression was associated with an unfavorable prognosis. Then, we showed that NFAT1 enhanced tumor growth and regulated the immune response by increasing PD-L1 expression in RCC. In addition, we demonstrated that NFAT1 was stabilized in sunitinib-resistant RCC via hyperactivation of the PI3K/AKT/GSK-3β signaling pathway. Furthermore, our study indicated that downregulation of the expression of FBW7, which promotes NFAT1 degradation, was induced by FOXA1 and SETD2 in sunitinib-resistant RCC. Finally, FBW7 was found to contribute to modulating the immune response in RCC.

Conclusions

Our data reveal a novel role for the FBW7/NFAT1 axis in the RCC response to TKIs and ICIs. NFAT1 and its associated signaling pathway might be therapeutic targets for RCC treatment, especially when combined with ICIs and/or TKIs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-022-02253-0.

Highlighting the Multifaceted Role of Orai1 N-Terminal- and Loop Regions for Proper CRAC Channel Functions

Orai1, the Ca2+-selective pore in the plasma membrane, is one of the key components of the Ca2+release-activated Ca2+ (CRAC) channel complex. Activated by the Ca2+ sensor in the endoplasmic reticulum (ER) membrane, stromal interaction molecule 1 (STIM1), via direct interaction when ER luminal Ca2+ levels recede, Orai1 helps to maintain Ca2+ homeostasis within a cell. It has already been proven that the C-terminus of Orai1 is indispensable for channel activation. However, there is strong evidence that for CRAC channels to function properly and maintain all typical hallmarks, such as selectivity and reversal potential, additional parts of Orai1 are needed. In this review, we focus on these sites apart from the C-terminus; namely, the second loop and N-terminus of Orai1 and on their multifaceted role in the functioning of CRAC channels.

Orai1α, but not Orai1β, co-localizes with TRPC1 and is required for its plasma membrane location and activation in HeLa cells

The identification of two variants of the canonical pore-forming subunit of the Ca²⁺ release-activated Ca²⁺ (CRAC) channel Orai1, Orai1α and Orai1β, in mammalian cells arises the question whether they exhibit different functional characteristics. Orai1α and Orai1β differ in the N-terminal 63 amino acids, exclusive of Orai1α, and show different sensitivities to Ca²⁺-dependent inactivation, as well as distinct ability to form arachidonate-regulated channels. We have evaluated the role of both Orai1 variants in the activation of TRPC1 in HeLa cells. We found that Orai1α and Orai1β are required for the maintenance of regenerative Ca²⁺ oscillations, while TRPC1 plays a role in agonist-induced Ca²⁺ influx but is not essential for Ca²⁺ oscillations. Using APEX2 proximity labeling, co-immunoprecipitation and the fluorescence of G-GECO1.2 fused to Orai1α our results indicate that agonist stimulation and Ca²⁺ store depletion enhance Orai1α–TRPC1 interaction. Orai1α is essential for TRPC1 plasma membrane location and activation. Thus, TRPC1 function in HeLa cells depends on Ca²⁺ influx through Orai1α exclusively.

Orai2 Modulates Store-Operated Ca2+ Entry and Cell Cycle Progression in Breast Cancer Cells

Breast cancer is a heterogeneous disease from the histological and molecular expression point of view, and this heterogeneity determines cancer aggressiveness. Store-operated Ca²⁺ entry (SOCE), a major mechanism for Ca²⁺ entry in non-excitable cells, is significantly remodeled in cancer cells and plays an important role in the development and support of different cancer hallmarks. The store-operated CRAC (Ca²⁺ release-activated Ca²⁺) channels are predominantly comprised of Orai1 but the participation of Orai2 and Orai3 subunits has been reported to modulate the magnitude of Ca²⁺ responses. Here we provide evidence for a heterogeneous expression of Orai2 among different breast cancer cell lines. In the HER2 and triple negative breast cancer cell lines SKBR3 and BT20, respectively, where the expression of Orai2 was greater, Orai2 modulates the magnitude of SOCE and sustain Ca²⁺ oscillations in response to carbachol. Interestingly, in these cells Orai2 modulates the activation of NFAT1 and NFAT4 in response to high and low agonist concentrations. Finally, we have found that, in cells with high Orai2 expression, Orai2 knockdown leads to cell cycle arrest at the G0-G1 phase and decreases apoptosis resistance upon cisplatin treatment. Altogether, these findings indicate that, in breast cancer cells with a high Orai2 expression, Orai2 plays a relevant functional role in agonist-evoked Ca²⁺ signals, cell proliferation and apoptosis resistance.

A longer isoform of Stim1 is a negative SOCE regulator but increases cAMP-modulated NFAT signaling

Alternative splicing is a potent modifier of protein function. Stromal interaction molecule 1 (Stim1) is the essential activator of store‐operated Ca²⁺ entry (SOCE) triggering activation of transcription factors. Here, we characterize Stim1A, a splice variant with an additional 31 amino acid domain inserted in frame within its cytosolic domain. Prominent expression of exon A is found in astrocytes, heart, kidney, and testes. Full‐length Stim1A functions as a dominant‐negative regulator of SOCE and I_CRAC, facilitating sequence‐specific fast calcium‐dependent inactivation and destabilizing gating of Orai channels. Downregulation or absence of native Stim1A results in increased SOCE. Despite reducing SOCE, Stim1A leads to increased NFAT translocation. Differential proteomics revealed an interference of Stim1A with the cAMP‐SOCE crosstalk by altered modulation of phosphodiesterase 8 (PDE8), resulting in reduced cAMP degradation and increased PIP5K activity, facilitating NFAT activation. Our study uncovers a hitherto unknown mechanism regulating NFAT activation and indicates that cell‐type‐specific splicing of Stim1 is a potent means to regulate the NFAT signalosome and cAMP‐SOCE crosstalk.

Spatiotemporal regulation of store-operated calcium entry in cancer metastasis

The store-operated calcium (Ca2+) entry (SOCE) is the Ca2+ entry mechanism used by cells to replenish depleted Ca2+ store. The dysregulation of SOCE has been reported in metastatic cancer. It is believed that SOCE promotes migration and invasion by remodeling the actin cytoskeleton and cell adhesion dynamics. There is recent evidence supporting that SOCE is critical for the spatial and the temporal coding of Ca2+ signals in the cell. In this review, we critically examined the spatiotemporal control of SOCE signaling and its implication in the specificity and robustness of signaling events downstream of SOCE, with a focus on the spatiotemporal SOCE signaling during cancer cell migration, invasion and metastasis. We further discuss the limitation of our current understanding of SOCE in cancer metastasis and potential approaches to overcome such limitation.

Orai1 Boosts SK3 Channel Activation

The interplay of SK3, a Ca2+ sensitive K+ ion channel, with Orai1, a Ca2+ ion channel, has been reported to increase cytosolic Ca2+ levels, thereby triggering proliferation of breast and colon cancer cells, although a molecular mechanism has remained elusive to date. We show in the current study, via heterologous protein expression, that Orai1 can enhance SK3 K+ currents, in addition to constitutively bound calmodulin (CaM). At low cytosolic Ca2+ levels that decrease SK3 K+ permeation, co-expressed Orai1 potentiates SK3 currents. This positive feedback mechanism of SK3 and Orai1 is enabled by their close co-localization. Remarkably, we discovered that loss of SK3 channel activity due to overexpressed CaM mutants could be restored by Orai1, likely via its interplay with the SK3-CaM binding site. Mapping for interaction sites within Orai1, we identified that the cytosolic strands and pore residues are critical for a functional communication with SK3. Moreover, STIM1 has a bimodal role in SK3-Orai1 regulation. Under physiological ionic conditions, STIM1 is able to impede SK3-Orai1 interplay by significantly decreasing their co-localization. Forced STIM1-Orai1 activity and associated Ca2+ influx promote SK3 K+ currents. The dynamic regulation of Orai1 to boost endogenous SK3 channels was also determined in the human prostate cancer cell line LNCaP.

Impact of SOCE Abolition by ORAI1 Knockout on the Proliferation, Adhesion, and Migration of HEK-293 Cells

Store-operated calcium entry (SOCE) provided through channels formed by ORAI proteins is a major regulator of several cellular processes. In immune cells, it controls fundamental processes such as proliferation, cell adhesion, and migration, while in cancer, SOCE and ORAI1 gene expression are dysregulated and lead to abnormal migration and/or cell proliferation. In the present study, we used the CRISPR/Cas9 technique to delete the ORAI1 gene and to identify its role in proliferative and migrative properties of the model cell line HEK-293. We showed that ORAI1 deletion greatly reduced SOCE. Thereby, we found that this decrease and the absence of ORAI1 protein did not affect HEK-293 proliferation. In addition, we determined that ORAI1 suppression did not affect adhesive properties but had a limited impact on HEK-293 migration. Overall, we showed that ORAI1 and SOCE are largely dispensable for cellular proliferation, migration, and cellular adhesion of HEK-293 cells. Thus, despite its importance in providing Ca2+ entry in non-excitable cells, our results indicate that the lack of SOCE does not deeply impact HEK-293 cells. This finding suggests the existence of compensatory mechanism enabling the maintenance of their physiological function.

Immune Synapse Residency of Orai1 Alters Ca2+ Response of T Cells

CRAC, which plays important role in Ca²⁺-dependent T-lymphocyte activation, is composed of the ER-resident STIM1 and the plasma membrane Orai1 pore-forming subunit. Both accumulate at the immunological synapse (IS) between a T cell and an antigen-presenting cell (APC). We hypothesized that adapter/interacting proteins regulate Orai1 residence in the IS. We could show that mGFP-tagged Orai1-Full channels expressed in Jurkat cells had a biphasic IS-accumulation kinetics peaked at 15 min. To understand the background of Orai1 IS-redistribution we knocked down STIM1 and SAP97 (adaptor protein with a short IS-residency (15 min) and ability to bind Orai1 N-terminus): the mGFP-Orai1-Full channels kept on accumulating in the IS up to the 60th minute in the STIM1- and SAP97-lacking Jurkat cells. Deletion of Orai1 N terminus (mGFP-Orai1-Δ72) resulted in the same time course as described for STIM1/SAP97 knock-down cells. Ca²⁺-imaging of IS-engaged T-cells revealed that of Orai1 residency modifies the Ca²⁺-response: cells expressing mGFP-Orai1-Δ72 construct or mGFP-Orai1-Full in SAP-97 knock-down cells showed higher number of Ca²⁺-oscillation up to the 90th minute after IS formation. Overall, these data suggest that SAP97 may contribute to the short-lived IS-residency of Orai1 and binding of STIM1 to Orai1 N-terminus is necessary for SAP97-Orai1 interaction.

AKAP79 Orchestrates a Cyclic AMP Signalosome Adjacent to Orai1 Ca2+ Channels

To ensure specificity of response, eukaryotic cells often restrict signalling molecules to sub-cellular regions. The Ca²⁺ nanodomain is a spatially confined signal that arises near open Ca²⁺ channels. Ca²⁺ nanodomains near store-operated Orai1 channels stimulate the protein phosphatase calcineurin, which activates the transcription factor NFAT1, and both enzyme and target are initially attached to the plasma membrane through the scaffolding protein AKAP79. Here, we show that a cAMP signalling nexus also forms adjacent to Orai1. Protein kinase A and phosphodiesterase 4, an enzyme that rapidly breaks down cAMP, both associate with AKAP79 and realign close to Orai1 after stimulation. PCR and mass spectrometry failed to show expression of Ca²⁺-activated adenylyl cyclase 8 in HEK293 cells, whereas the enzyme was observed in neuronal cell lines. FRET and biochemical measurements of bulk cAMP and protein kinase A activity consistently failed to show an increase in adenylyl cyclase activity following even a large rise in cytosolic Ca²⁺. Furthermore, expression of AKAP79-CUTie, a cAMP FRET sensor tethered to AKAP79, did not report a rise in cAMP after stimulation, despite AKAP79 association with Orai1. Hence, HEK293 cells do not express functional active Ca²⁺-activated adenylyl cyclases including adenylyl cyclase 8. Our results show that two ancient second messengers are independently generated in nanodomains close to Orai1 Ca²⁺ channels.

Isoform-Specific Properties of Orai Homologues in Activation, Downstream Signaling, Physiology and Pathophysiology

Ca2+ ion channels are critical in a variety of physiological events, including cell growth, differentiation, gene transcription and apoptosis. One such essential entry pathway for calcium into the cell is the Ca2+ release-activated Ca2+ (CRAC) channel. It consists of the Ca2+ sensing protein, stromal interaction molecule 1 (STIM1) located in the endoplasmic reticulum (ER) and a Ca2+ ion channel Orai in the plasma membrane. The Orai channel family includes three homologues Orai1, Orai2 and Orai3. While Orai1 is the "classical" Ca2+ ion channel within the CRAC channel complex and plays a universal role in the human body, there is increasing evidence that Orai2 and Orai3 are important in specific physiological and pathophysiological processes. This makes them an attractive target in drug discovery, but requires a detailed understanding of the three Orai channels and, in particular, their differences. Orai channel activation is initiated via Ca2+ store depletion, which is sensed by STIM1 proteins, and induces their conformational change and oligomerization. Upon STIM1 coupling, Orai channels activate to allow Ca2+ permeation into the cell. While this activation mechanism is comparable among the isoforms, they differ by a number of functional and structural properties due to non-conserved regions in their sequences. In this review, we summarize the knowledge as well as open questions in our current understanding of the three isoforms in terms of their structure/function relationship, downstream signaling and physiology as well as pathophysiology.

The Orai1-AC8 Interplay: How Breast Cancer Cells Escape from Orai1 Channel Inactivation

The interplay between the Ca2+-sensitive adenylyl cyclase 8 (AC8) and Orai1 channels plays an important role both in the activation of the cAMP/PKA signaling and the modulation of Orai1-dependent Ca2+ signals. AC8 interacts with a N-terminal region that is exclusive to the Orai1 long variant, Orai1α. The interaction between both proteins allows the Ca2+ that enters the cell through Orai1α to activate the generation of cAMP by AC8. Subsequent PKA activation results in Orai1α inactivation by phosphorylation at serine-34, thus shaping Orai1-mediated cellular functions. In breast cancer cells, AC8 plays a relevant role supporting a variety of cancer hallmarks, including proliferation and migration. Breast cancer cells overexpress AC8, which shifts the AC8-Orai1 stoichiometry in favor of the former and leads to the impairment of PKA-dependent Orai1α inactivation. This mechanism contributes to the enhanced SOCE observed in triple-negative breast cancer cells. This review summarizes the functional interaction between AC8 and Orai1α in normal and breast cancer cells and its relevance for different cancer features.