Ca signaling and STIM1

An increase in the intracellular calcium ion concentration ([Ca²⁺]) impacts a diverse range of cell functions, including adhesion, motility, gene expression and proliferation. Elevation of intracellular calcium ion (Ca²⁺) regulates various cellular events after the stimulation of cells. Initial increase in Ca²⁺ comes from the endoplasmic reticulum (ER), intracellular storage space. However, the continuous influx of extracellular Ca²⁺ is required to maintain the increased level of Ca²⁺ inside cells. Store-operated Ca²⁺ entry (SOCE) manages this process, and STIM1, a newly discovered molecule, has a unique and essential role in SOCE. STIM1 can sense the exhaustion of Ca²⁺ in the ER, and activate the SOC channel in the plasma membrane, leading to the continuous influx of extracellular Ca²⁺. STIM1 senses the status of the intracellular Ca²⁺ stores via a luminal N-terminal Ca²⁺-binding EF-hand domain. Dissociation of Ca²⁺ from this domain induces the clustering of STIM1 to regions of the ER that lie close to the plasma membrane, where it regulates the activity of the store-operated Ca²⁺ channels/entry (calcium-release-activated calcium channels/entry). In this review, we summarize the mechanism by which STIM1 regulates SOCE, and also its role in the control of mast cell functions and allergic responses.     

Oligomerization and Ca2+/calmodulin control binding of the ER Ca2+-sensors STIM1 and STIM2 to plasma membrane lipids

Ca²⁺ (calcium) homoeostasis and signalling rely on physical contacts between Ca²⁺ sensors in the ER (endoplasmic reticulum) and Ca²⁺ channels in the PM (plasma membrane). STIM1 (stromal interaction molecule 1) and STIM2 Ca²⁺ sensors oligomerize upon Ca²⁺ depletion in the ER lumen, contact phosphoinositides at the PM via their cytosolic lysine (K)-rich domains, and activate Ca²⁺ channels. Differential sensitivities of STIM1 and STIM2 towards ER luminal Ca²⁺ have been studied but responses towards elevated cytosolic Ca²⁺ concentration and the mechanism of lipid binding remain unclear. We found that tetramerization of the STIM1 K-rich domain is necessary for efficient binding to PI(4,5)P₂-containing PM-like liposomes consistent with an oligomerization-driven STIM1 activation. In contrast, dimerization of STIM2 K-rich domain was sufficient for lipid binding. Furthermore, the K-rich domain of STIM2, but not of STIM1, forms an amphipathic α-helix. These distinct features of the STIM2 K-rich domain cause an increased affinity for PI(4,5)P₂, consistent with the lower activation threshold of STIM2 and a function as regulator of basal Ca²⁺ levels. Concomitant with higher affinity for PM lipids, binding of CaM (calmodulin) inhibited the interaction of the STIM2 K-rich domain with liposomes in a Ca²⁺ and PI(4,5)P₂ concentration-dependent manner. Therefore we suggest that elevated cytosolic Ca²⁺ concentration down-regulates STIM2-mediated ER–PM contacts via CaM binding.     

The role of STIM1 and SOCE in smooth muscle contractility

Contraction is a central feature for skeletal, cardiac and smooth muscle; this unique feature is largely dependent on calcium (Ca²⁺) signaling and therefore maintenance of internal Ca²⁺ stores. Stromal interaction molecule 1 (STIM1) is a single-pass transmembrane protein that functions as a Ca²⁺ sensor for the activation store-operated calcium channels (SOCCs) on the plasma membrane in response to depleted internal sarco(endo)plasmic (S/ER) reticulum Ca²⁺ stores. STIM1 was initially characterized in non-excitable cells; however, evidence from both animal models and human mutations suggests a role for STIM1 in modulating Ca²⁺ homeostasis in excitable tissues as well. STIM1-dependent SOCE is particularly important in tissues undergoing sustained contraction, leading us to believe STIM1 may play a role in smooth muscle contraction. To date, the role of STIM1 in smooth muscle is unknown. In this review, we provide a brief overview of the role of STIM1-dependent SOCE in striated muscle and build off that knowledge to investigate whether STIM1 contributes to smooth muscle contractility. We conclude by discussing the translational implications of targeting STIM1 in the treatment of smooth muscle disorders.     

Polarized but Differential Localization and Recruitment of STIM1, Orai1 and TRPC Channels in Secretory Cells

Polarized Ca²⁺ signals in secretory epithelial cells are determined by compartmentalized localization of Ca²⁺ signaling proteins at the apical pole. Recently the ER Ca²⁺ sensor STIM1 (stromal interaction molecule 1) and the Orai channels were shown to play a critical role in store‐dependent Ca²⁺ influx. STIM1 also gates the transient receptor potential‐canonical (TRPC) channels. Here, we asked how cell stimulation affects the localization, recruitment and function of the native proteins in polarized cells. Inhibition of Orai1, STIM1, or deletion of TRPC1 reduces Ca²⁺ influx and frequency of Ca²⁺ oscillations. Orai1 localization is restricted to the apical pole of the lateral membrane. Surprisingly, cell stimulation does not lead to robust clustering of native Orai1, as is observed with expressed Orai1. Unexpectedly, cell stimulation causes polarized recruitment of native STIM1 to both the apical and lateral regions, thus to regions with and without Orai1. Accordingly, STIM1 and Orai1 show only 40% colocalization. Consequently, STIM1 shows higher colocalization with the basolateral membrane marker E‐cadherin than does Orai1, while Orai1 showed higher colocalization with the tight junction protein ZO1. TRPC1 is expressed in both apical and basolateral regions of the plasma membrane. Co‐IP of STIM1/Orai1/IP₃ receptors (IP₃Rs)/TRPCs is enhanced by cell stimulation and disrupted by 2‐aminoethoxydiphenyl borate (2APB). The polarized localization and recruitment of these proteins results in preferred Ca²⁺ entry that is initiated at the apical pole. These findings reveal that in addition to Orai1, STIM1 likely regulates other Ca²⁺ permeable channels, such as the TRPCs. Both channels contribute to the frequency of [Ca²⁺] oscillations and thus impact critical cellular functions.     

Increased Confinement and Polydispersity of STIM1 and Orai1 after Ca2+ Store Depletion

STIM1 (a Ca²⁺ sensor in the endoplasmic reticulum (ER) membrane) and Orai1 (a pore-forming subunit of the Ca²⁺-release-activated calcium channel in the plasma membrane) diffuse in the ER membrane and plasma membrane, respectively. Upon depletion of Ca²⁺ stores in the ER, STIM1 translocates to the ER-plasma membrane junction and binds Orai1 to trigger store-operated Ca²⁺ entry. However, the motion of STIM1 and Orai1 during this process and its roles to Ca²⁺ entry is poorly understood. Here, we report real-time tracking of single STIM1 and Orai1 particles in the ER membrane and plasma membrane in living cells before and after Ca²⁺ store depletion. We found that the motion of single STIM1 and Orai1 particles exhibits anomalous diffusion both before and after store depletion, and their mobility—measured by the radius of gyration of the trajectories, mean-square displacement, and generalized diffusion coefficient—decreases drastically after store depletion. We also found that the measured displacement distribution is non-Gaussian, and the non-Gaussian parameter drastically increases after store depletion. Detailed analyses and simulations revealed that single STIM1 and Orai1 particles are confined in the compartmentalized membrane both before and after store depletion, and the changes in the motion after store depletion are explained by increased confinement and polydispersity of STIM1-Orai1 complexes formed at the ER-plasma membrane junctions. Further simulations showed that this increase in the confinement and polydispersity after store depletion localizes a rapid increase of Ca²⁺ influx, which can facilitate the rapid activation of local Ca²⁺ signaling pathways and the efficient replenishing of Ca²⁺ store in the ER in store-operated Ca²⁺ entry.     

内质网上的蛋白质RCN2与STIM1-Orai1复合体相互作用

膜蛋白质Orail组成了一类被称为钙释放激活钙通道（CRAC）的离子通道,并且由相互作用的蛋白质STIM1作为其在内质网上的钙感受器．但是这类通道的调节机制还未研究透彻．通过串连亲和纯化STIM1-Orai1复合体,发现与之相互作用的内质网蛋白质RCN2．共聚焦显微术显示RCN2与STIM1在钙库排空前后完全共定位．对RCN2的EFhands结构突变体所作单细胞测钙,结果显示其对钙库操控通道电流特性有微弱影响．全内反射荧光显微术显示,RCN2以花环状围绕包围STIM1聚集堆,这提示RCN2在STIM1聚集中起到一种结构约束作用．     

Role of SOCE architects STIM and Orai proteins in Cell Death

Calcium (Ca²⁺) signaling plays a critical role in regulating plethora of cellular functions including cell survival, proliferation and migration. The perturbations in cellular Ca²⁺ homeostasis can lead to cell death either by activating autophagic pathways or through induction of apoptosis. Endoplasmic reticulum (ER) is the major storehouse of Ca²⁺ within cells and a number of physiological agonists mediate ER Ca²⁺ release by activating IP₃ receptors (IP₃R). This decrease in ER Ca²⁺ levels is sensed by STIM, which physically interacts and activates plasma membrane Ca²⁺ selective Orai channels. Emerging literature implicates a key role for STIM1, STIM2, Orai1 and Orai3 in regulating both cell survival and death pathways. In this review, we will retrospect the work highlighting the role of STIM and Orai homologs in regulating cell death signaling. We will further discuss the rationales that could explain the dual role of STIM and Orai proteins in regulating cell fate decisions.     

STIM1 Positively Regulates the Ca2+ Release Activity of the Inositol 1,4,5-Trisphosphate Receptor in Bovine Aortic Endothelial Cells

The endothelium is actively involved in many functions of the cardiovascular system, such as the modulation of arterial pressure and the maintenance of blood flow. These functions require a great versatility of the intracellular Ca²⁺ signaling that resides in the fact that different signals can be encoded by varying the frequency and the amplitude of the Ca²⁺ response. Cells use both extracellular and intracellular Ca²⁺ pools to modulate the intracellular Ca²⁺ concentration. In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP₃R), located on the endoplasmic reticulum (ER), is responsible for the release of Ca²⁺ from the intracellular store. The proteins STIM1 and STIM2 are also located on the ER and they are involved in the activation of a store-operated Ca²⁺ entry (SOCE). Due to their Ca²⁺ sensor property and their close proximity with IP₃Rs on the ER, STIMs could modulate the activity of IP₃R. In this study, we showed that STIM1 and STIM2 are expressed in bovine aortic endothelial cells and they both interact with IP₃R. While STIM2 appears to play a minor role, STIM1 plays an important role in the regulation of agonist-induced Ca²⁺ mobilization in BAECs by a positive effect on both the SOCE and the IP₃R-dependent Ca²⁺ release.     

Constitutive Activation of the Calcium Sensor STIM1 Causes Tubular-Aggregate Myopathy

Tubular aggregates are regular arrays of membrane tubules accumulating in muscle with age. They are found as secondary features in several muscle disorders, including alcohol- and drug-induced myopathies, exercise-induced cramps, and inherited myasthenia, but also exist as a pure genetic form characterized by slowly progressive muscle weakness. We identified dominant STIM1 mutations as a genetic cause of tubular-aggregate myopathy (TAM). Stromal interaction molecule 1 (STIM1) is the main Ca²⁺ sensor in the endoplasmic reticulum, and all mutations were found in the highly conserved intraluminal Ca²⁺-binding EF hands. Ca²⁺ stores are refilled through a process called store-operated Ca²⁺ entry (SOCE). Upon Ca²⁺-store depletion, wild-type STIM1 oligomerizes and thereby triggers extracellular Ca²⁺ entry. In contrast, the missense mutations found in our four TAM-affected families induced constitutive STIM1 clustering, indicating that Ca²⁺ sensing was impaired. By monitoring the calcium response of TAM myoblasts to SOCE, we found a significantly higher basal Ca²⁺ level in TAM cells and a dysregulation of intracellular Ca²⁺ homeostasis. Because recessive STIM1 loss-of-function mutations were associated with immunodeficiency, we conclude that the tissue-specific impact of STIM1 loss or constitutive activation is different and that a tight regulation of STIM1-dependent SOCE is fundamental for normal skeletal-muscle structure and function.     

Knockdown of STIM1 inhibits 6-hydroxydopamine-induced oxidative stress through attenuating calcium-dependent ER stress and mitochondrial dysfunction in undifferentiated PC12 cells

Abstract

Stromal interaction molecule (STIM) proteins are parts of elaborate eukaryotic Ca²⁺ signaling systems and are considered to be important players in regulating neuronal Ca²⁺ homeostasis under normal ageing and pathological conditions. Here, we investigated the potential role of STIM1 in 6-hydroxydopamine (6-OHDA)-induced toxicity in undifferentiated PC12 cell lines. Cells exposed to 6-OHDA demonstrated alterations in the generation of reactive oxygen species (ROS) in a Ca²⁺-dependent manner. Downregulation of STIM1 expression by specific small interfering RNA (siRNA) attenuated apoptotic cell death, reduced intracellular ROS production, and partially prevented the impaired endogenous antioxidant enzyme activities after 6-OHDA treatment. Furthermore, STIM1 knockdown significantly attenuated 6-OHDA-induced intracellular Ca²⁺ overload by inhibiting endogenous store-operated calcium entry (SOCE). The effect of STIM1 siNRA on SOCE was related to orai1 and L-type Ca²⁺ channels, but not to transient receptor potential canonical type 1 (TRPC1) channel. In addition, silencing of STIM1 increased the Ca²⁺ buffering capacity of the endoplasmic reticulum (ER) in 6-OHDA-injured cells. ER vacuoles formed from the destruction of ER structural integrity and activation of ER-related apoptotic factors (CHOP and Caspase-12) were partially prevented by STIM1 knockdown. Moreover, STIM1 knockdown attenuated 6-OHDA-induced mitochondrial Ca²⁺ uptake and mitochondrial dysfunction, including the collapse of mitochondrial membrane potential (MMP) and the decrease of ATP generation. Taken together, our data provide the first evidence that inhibition of STIM1-meditated intracellular Ca²⁺ dyshomeostasis protects undifferentiated PC12 cells against 6-OHDA toxicity and indicate that STIM1 may be responsible for neuronal oxidative stress induced by ER stress and mitochondrial dysfunction in PD.     

STIM and Orai: Dynamic Intermembrane Coupling to Control Cellular Calcium Signals

Ca²⁺ signals controlling a vast array of cell functions involve both Ca²⁺ store release and external Ca²⁺ entry. These two events are coordinated through a dynamic intermembrane coupling between two distinct membrane proteins, STIM and Orai. STIM proteins are endoplasmic reticulum (ER) luminal Ca²⁺ sensors that undergo a profound redistribution into discrete junctional ER domains closely juxtaposed with the plasma membrane (PM). Orai proteins are PM Ca²⁺ channels that migrate and become tethered by STIM within the ER-PM junctions, where they mediate exceedingly selective Ca²⁺ entry. We describe a new understanding of the nature of the proteins and how they function to mediate this remarkable intermembrane signaling process controlling Ca²⁺ signals.     

The distinct role of STIM1 and STIM2 in the regulation of store-operated Ca2+ entry and cellular function

Stromal interaction molecules STIM1 and STIM2 are endoplasmic reticulum (ER) Ca²⁺ sensors that initiate store-operated Ca ²⁺ entry (SOCE). The roles of STIM1-mediated SOCE in cancer biology have been highlighted in different types of cancer, but that of STIM2 is unknown. By the model of cervical cancer, here we focus on the cooperative regulation of SOCE by STIM proteins and their distinct roles in cellular function. Immunofluorescent stainings of surgical specimens of cervical cancer show that STIM1 and STIM2 are abundant in tumor tissues, but STIM1 is the major ER Ca ²⁺ sensor identified in the invasive front of cancer tissues. STIM1 or STIM2 overexpression in cervical cancer SiHa cells induces an upregulated SOCE. Regarding cellular function, STIM1 and STIM2 are necessary for cell proliferation, whereas STIM1 is the dominant ER Ca ²⁺ sensor involved in cell migration. During SOCE, STIM1 is aggregated and translocated towards the Orai1-containing plasma membrane in association with the microtubule plus-end binding protein EB1. In contrast, STIM2 is constitutively aggregated without significant trafficking or association with microtubules. These results show the distinct role of STIM1 and STIM2 in SOCE and cellular function of cervical cancer cells.     

STIM1 Is a Novel Component of ER-Chlamydia trachomatis Inclusion Membrane Contact Sites

Productive developmental cycle of the obligate intracellular bacterial pathogen Chlamydia trachomatis depends on the interaction of the replicative vacuole, named the inclusion, with cellular organelles. We have recently reported the formation of ER-Inclusion membrane contact sites (MCSs), where the endoplasmic reticulum (ER) is in apposition to the inclusion membrane. These platforms contain the C. trachomatis inclusion membrane protein IncD, the mammalian ceramide transfer protein CERT and the ER resident proteins VAPA/B and were proposed to play a role in the non-vesicular trafficking of lipids to the inclusion. Here, we identify STIM1 as a novel component of ER-Inclusion MCSs. STIM1, an ER calcium (Ca²⁺) sensor that relocate to ER-Plasma Membrane (PM) MCSs upon Ca²⁺ store depletion, associated with C. trachomatis inclusion. STIM1, but not the general ER markers Rtn3C and Sec61ß, was enriched at the inclusion membrane. Ultra-structural studies demonstrated that STIM1 localized to ER-Inclusion MCSs. Time-course experiments showed that STIM1, CERT and VAPB co-localized throughout the developmental cycle. By contrast, Orai1, the PM Ca²⁺ channel that interacts with STIM1 at ER-PM MCSs, did not associate with C. trachomatis inclusion. Upon ER Ca²⁺ store depletion, a pool of STIM1 relocated to ER-PM MCSs, while the existing ER-Inclusion MCSs remained enriched in STIM1. Finally, we have identified the CAD domain, which mediates STIM1-Orai1 interaction, as the minimal domain required for STIM1 enrichment at ER-Inclusion MCSs. Altogether this study identifies STIM1 as a novel component of ER-C. trachomatis inclusion MCSs. We discuss the potential role(s) of STIM1 during the infection process.     

Role of the STIM1 C-terminal Domain in STIM1 Clustering

Store-operated Ca²⁺ entry (SOCE) represents a ubiquitous Ca²⁺ influx pathway activated by the filling state of intracellular Ca²⁺ stores. SOCE is mediated by coupling of STIM1, the endoplasmic reticulum Ca²⁺ sensor, to the Orai1 channel. SOCE inactivates during meiosis, partly because of the inability of STIM1 to cluster in response to store depletion. STIM1 has several functional domains, including the Orai1 interaction domain (STIM1 Orai Activating Region (SOAR) or CRAC Activation Domain (CAD)) and STIM1 homomerization domain. When Ca²⁺ stores are full, these domains are inactive to prevent constitutive Ca²⁺ entry. Here we show, using the Xenopus oocyte as an expression system, that the C-terminal 200 residues of STIM1 are important to maintain STIM1 in an inactive state when Ca²⁺ stores are full, through predicted intramolecular shielding of the active STIM1 domains (SOAR/CAD and STIM1 homomerization domain). Interestingly, our data argue that the C-terminal 200 residues accomplish this through a steric hindrance mechanism because they can be substituted by GFP or mCherry while maintaining all aspects of STIM1 function. We further show that STIM1 clustering inhibition during meiosis is independent of the C-terminal 200 residues.     

A Coiled-coil Clamp Controls Both Conformation and Clustering of Stromal Interaction Molecule 1 (STIM1)

Store-operated Ca²⁺ entry, essential for the adaptive immunity, is initiated by the endoplasmic reticulum (ER) Ca²⁺ sensor STIM1. Ca²⁺ entry occurs through the plasma membrane resident Ca²⁺ channel Orai1 that directly interacts with the C-terminal STIM1 domain, named SOAR/CAD. Depletion of the ER Ca²⁺ store controls this STIM1/Orai1 interaction via transition to an extended STIM1 C-terminal conformation, exposure of the SOAR/CAD domain, and STIM1/Orai1 co-clustering. Here we developed a novel approach termed FRET-derived Interaction in a Restricted Environment (FIRE) in an attempt to dissect the interplay of coiled-coil (CC) interactions in controlling STIM1 quiescent as well as active conformation and cluster formation. We present evidence of a sequential activation mechanism in the STIM1 cytosolic domains where the interaction between CC1 and CC3 segment regulates both SOAR/CAD exposure and CC3-mediated higher-order oligomerization as well as cluster formation. These dual levels of STIM1 auto-inhibition provide efficient control over the coupling to and activation of Orai1 channels.     

Reticulon 4 Is Necessary for Endoplasmic Reticulum Tubulation,STIM1-Orai1 Coupling,and Store-operated Calcium Entry

Despite recent advances in understanding store-operated calcium entry (SOCE) regulation, the fundamental question of how ER morphology affects this process remains unanswered. Here we show that the loss of RTN4, is sufficient to alter ER morphology and severely compromise SOCE. Mechanistically, we show this to be the result of defective STIM1-Orai1 coupling because of loss of ER tubulation and redistribution of STIM1 to ER sheets. As a functional consequence, RTN4-depleted cells fail to sustain elevated cytoplasmic Ca²⁺ levels via SOCE and therefor are less susceptible to Ca²⁺ overload induced apoptosis. Thus, for the first time, our results show a direct correlation between ER morphology and SOCE and highlight the importance of RTN4 in cellular Ca²⁺ homeostasis.     

STIM1 activation of adenylyl cyclase 6 connects Ca2+ and cAMP signaling during melanogenesis

Endoplasmic reticulum (ER)–plasma membrane (PM) junctions form functionally active microdomains that connect intracellular and extracellular environments. While the key role of these interfaces in maintenance of intracellular Ca²⁺ levels has been uncovered in recent years, the functional significance of ER‐PM junctions in non‐excitable cells has remained unclear. Here, we show that the ER calcium sensor protein STIM1 (stromal interaction molecule 1) interacts with the plasma membrane‐localized adenylyl cyclase 6 (ADCY6) to govern melanogenesis. The physiological stimulus α‐melanocyte‐stimulating hormone (αMSH) depletes ER Ca²⁺ stores, thus recruiting STIM1 to ER‐PM junctions, which in turn activates ADCY6. Using zebrafish as a model system, we further established STIM1's significance in regulating pigmentation in vivo. STIM1 domain deletion studies reveal the importance of Ser/Pro‐rich C‐terminal region in this interaction. This mechanism of cAMP generation creates a positive feedback loop, controlling the output of the classical αMSH‐cAMP‐MITF axis in melanocytes. Our study thus delineates a signaling module that couples two fundamental secondary messengers to drive pigmentation. Given the central role of calcium and cAMP signaling pathways, this module may be operative during various other physiological processes and pathological conditions.     

3,3'-Diindolylmethane induces gastric cancer cells death via STIM1 mediated store-operated calcium entry

3,3''-Diindolylmethane (DIM), a natural phytochemicals isolated from cruciferous vegetables, has been reported to inhibit human gastric cancer cells proliferation and induce cells apoptosis as well as autophagy, but its mechanisms are still unclear. Store-operated calcium entry (SOCE) is a main Ca²⁺ influx pathway in various of cancers, which is activated by the depletion of endoplasmic reticulum (ER) Ca²⁺ store. Stromal interaction molecular 1 (STIM1) is the necessary component of SOCE. In this study, we focus on to examine the regulatory mechanism of SOCE on DIM-induced death in gastric cancer. After treating the human BGC-823 and SGC-7901 gastric cancer cells with DIM, cellular proliferation was determined by MTT, apoptosis and autophagy were detected by flow cytometry or Hoechst 33342 staining. The expression levels of related proteins were evaluated by Western blotting. Free cytosolilc Ca²⁺ level was assessed by fluorescence monitoring under a laser scanning confocal microscope. The data have shown that DIM could significantly inhibit proliferation and induce apoptosis as well as autophagy in two gastric cancer cell lines. After DIM treatment, the STIM1-mediated SOCE was activated by upregulating STIM1 and decreasing ER Ca²⁺ level. Knockdown STIM1 with siRNA or pharmacological inhibition of SOCE attenuated DIM induced apoptosis and autophagy by inhibiting p-AMPK mediated ER stress pathway. Our data highlighted that the potential of SOCE as a promising target for treating cancers. Developing effective and selective activators targeting STIM1-mediated SOCE pathway will facilitate better therapeutic sensitivity of phytochemicals acting on SOCE in gastric cancer. Moreover, more research should be performed to validate the efficacy of combination chemotherapy of anti-cancer drugs targeting SOCE for clinical application.