Dynamic movement of the calcium sensor STIM1 and the calcium channel Orai1 in activated T-cells: puncta and distal caps

Ribosome-free terminals of rough ER allow formation of STIM1 puncta and segregation of STIM1 from IP(3) receptors

Store-operated Ca(2+) entry is a ubiquitous mechanism that prevents the depletion of endoplasmic reticulum (ER) calcium. A reduction of ER calcium triggers translocation of STIM proteins, which serve as calcium sensors in the ER, to subplasmalemmal puncta where they interact with and activate Orai channels. In pancreatic acinar cells, inositol 1,4,5-trisphosphate (IP(3)) receptors populate the apical part of the ER. Here, however, we observe that STIM1 translocates exclusively to the lateral and basal regions following ER Ca(2+) loss. This finding is paradoxical because the basal and lateral regions of the acinar cells contain rough ER (RER); the size of the ribosomes that decorate RER is larger than the distance that can be spanned by a STIM-Orai complex, and STIM1 function should therefore not be possible. We resolve this paradox and characterize ribosome-free terminals of the RER that form junctions between the reticulum and the plasma membrane in the basal and lateral regions of the acinar cells. Our findings indicate that different ER compartments specialize in different calcium-handling functions (Ca(2+) release and Ca(2+) reloading) and that any potential interference between Ca(2+) release and Ca(2+) influx is minimized by the spatial separation of the two processes.

Recent progress on STIM1 domains controlling Orai activation

Ca(2+) entry in non-excitable cells is mainly carried by store-operated channels among which the CRAC channel is best characterized. Its two limiting molecular components are represented by the Ca(2+) sensor protein STIM1 located in the endoplasmic reticulum and Orai1 in the plasma membrane. STIM1 senses a decrease of the Ca(2+) content in internal stores and triggers its accumulation into puncta like structures resulting in coupling to as well as activation of Orai1 channels. The STIM1-Orai coupling process is determined by an interaction via their C-termini. This review highlights recent developments on domains particularly within the cytosolic part of STIM1 that govern this interaction.

The functional network of ion channels in T lymphocytes

For more than 25 years, it has been widely appreciated that Ca2+ influx is essential to trigger T-lymphocyte activation. Patch clamp analysis, molecular identification, and functional studies using blockers and genetic manipulation have shown that a unique contingent of ion channels orchestrates the initiation, intensity, and duration of the Ca2+ signal. Five distinct types of ion channels--Kv1.3, KCa3.1, Orai1+ stromal interacting molecule 1 (STIM1) [Ca2+-release activating Ca2+ (CRAC) channel], TRPM7, and Cl(swell)--comprise a network that performs functions vital for ongoing cellular homeostasis and for T-cell activation, offering potential targets for immunomodulation. Most recently, the roles of STIM1 and Orai1 have been revealed in triggering and forming the CRAC channel following T-cell receptor engagement. Kv1.3, KCa3.1, STIM1, and Orai1 have been found to cluster at the immunological synapse following contact with an antigen-presenting cell; we discuss how channels at the synapse might function to modulate local signaling. Immuno-imaging approaches are beginning to shed light on ion channel function in vivo. Importantly, the expression pattern of Ca2+ and K+ channels and hence the functional network can adapt depending upon the state of differentiation and activation, and this allows for different stages of an immune response to be targeted specifically.

Formation of STIM and Orai complexes: puncta and distal caps

In the last few years, great progress has been made in understanding how stromal interacting molecule 1 (STIM1), a protein containing a calcium sensor that is located in the endoplasmic reticulum, and Orai1, a protein that forms a calcium channel in the plasma membrane, interact and give rise to store-operated calcium entry. Pharmacological depletion of calcium stores leads to the formation of clusters containing STIM and Orai that appear to be sites for calcium influx. Similar puncta are also produced in response to physiological stimuli in immune cells. In T cells engaged with antigen-presenting cells, clusters containing STIM and Orai accumulate at the immunological synapse. We recently discovered that in activated T cells, STIM1 and Orai1 also accumulate in cap-like structures opposite the immune synapse at the distal pole of the cell. Both caps and puncta are long-lived stable structures containing STIM1 and Orai1 in close proximity. The function of puncta as sites of calcium influx is clear. We speculate that the caps may provide a secondary site of calcium entry. Alternatively, they may serve as a source of preformed channel complexes that move to new immune synapses as T cells repeatedly engage antigen-presenting cells.

A Ca2(+ )release-activated Ca2(+) (CRAC) modulatory domain (CMD) within STIM1 mediates fast Ca2(+)-dependent inactivation of ORAI1 channels

STIM1 and ORAI1, the two limiting components in the Ca(2+) release-activated Ca(2+) (CRAC) signaling cascade, have been reported to interact upon store depletion, culminating in CRAC current activation. We have recently identified a modulatory domain between amino acids 474 and 485 in the cytosolic part of STIM1 that comprises 7 negatively charged residues. A STIM1 C-terminal fragment lacking this domain exhibits enhanced interaction with ORAI1 and 2-3-fold higher ORAI1/CRAC current densities. Here we focused on the role of this CRAC modulatory domain (CMD) in the fast inactivation of ORAI1/CRAC channels, utilizing the whole-cell patch clamp technique. STIM1 mutants either with C-terminal deletions including CMD or with 7 alanines replacing the negative amino acids within CMD gave rise to ORAI1 currents that displayed significantly reduced or even abolished inactivation when compared with STIM1 mutants with preserved CMD. Consistent results were obtained with cytosolic C-terminal fragments of STIM1, both in ORAI1-expressing HEK 293 cells and in RBL-2H3 mast cells containing endogenous CRAC channels. Inactivation of the latter, however, was much more pronounced than that of ORAI1. The extent of inactivation of ORAI3 channels, which is also considerably more prominent than that of ORAI1, was also substantially reduced by co-expression of STIM1 constructs missing CMD. Regarding the dependence of inactivation on Ca(2+), a decrease in intracellular Ca(2+) chelator concentrations promoted ORAI1 current fast inactivation, whereas Ba(2+) substitution for extracellular Ca(2+) completely abrogated it. In summary, CMD within the STIM1 cytosolic part provides a negative feedback signal to Ca(2+) entry by triggering fast Ca(2+)-dependent inactivation of ORAI/CRAC channels.

STIMulating store-operated Ca(2+) entry

Calcium influx through plasma membrane store-operated Ca(2+) (SOC) channels is triggered when the endoplasmic reticulum (ER) Ca(2+) store is depleted - a homeostatic Ca(2+) signalling mechanism that remained enigmatic for more than two decades. RNA-interference (RNAi) screening and molecular and cellular physiological analysis recently identified STIM1 as the mechanistic 'missing link' between the ER and the plasma membrane. STIM proteins sense the depletion of Ca(2+) from the ER, oligomerize, translocate to junctions adjacent to the plasma membrane, organize Orai or TRPC (transient receptor potential cation) channels into clusters and open these channels to bring about SOC entry.

The calcium/NFAT pathway: role in development and function of regulatory T cells

Calcium signals are essential for diverse cellular functions in the immune system. Sustained Ca(2+) entry is necessary for complete and long-lasting activation of calcineurin/NFAT pathways. A growing number of studies have emphasized that Ca(2+)/calcineurin/NFAT pathway is crucial for both development and function of regulatory T cells.

Lipid rafts/caveolae as microdomains of calcium signaling

Ca(2+) is a major signaling molecule in both excitable and non-excitable cells, where it serves critical functions ranging from cell growth to differentiation to cell death. The physiological functions of these cells are tightly regulated in response to changes in cytosolic Ca(2+) that is achieved by the activation of several plasma membrane (PM) Ca(2+) channels as well as release of Ca(2+) from the internal stores. One such channel is referred to as store-operated Ca(2+) channel that is activated by the release of endoplasmic reticulum (ER) Ca(2+) which initiates store-operated Ca(2+) entry (SOCE). Recent advances in the field suggest that some members of TRPCs and Orai channels function as SOCE channels. However, the molecular mechanisms that regulate channel activity and the exact nature of where these channels are assembled and regulated remain elusive. Research from several laboratories has demonstrated that key proteins involved in Ca(2+) signaling are localized in discrete PM lipid rafts/caveolar microdomains. Lipid rafts are cholesterol and sphingolipid-enriched microdomains that function as unique signal transduction platforms. In addition lipid rafts are dynamic in nature which tends to scaffold certain signaling molecules while excluding others. By such spatial segregation, lipid rafts not only provide a favorable environment for intra-molecular cross-talk but also aid to expedite the signal relay. Importantly, Ca(2+) signaling is shown to initiate from these lipid raft microdomains. Clustering of Ca(2+) channels and their regulators in such microdomains can provide an exquisite spatiotemporal regulation of Ca(2+)-mediated cellular function. Thus in this review we discuss PM lipid rafts and caveolae as Ca(2+)-signaling microdomains and highlight their importance in organizing and regulating SOCE channels.

STIM and Orai: the long-awaited constituents of store-operated calcium entry

Rapid changes in cytosolic Ca(2+) concentrations [Ca(2+)](i) are the most commonly used signals in biology to regulate a whole host of cellular functions including contraction, secretion and gene activation. A widely utilized form of Ca(2+) influx is termed store-operated Ca(2+) entry (SOCE) owing to its control by the Ca(2+) content of the endoplasmic reticulum (ER). The underlying molecular mechanism of SOCE has eluded identification until recently when two groups of proteins, the ER Ca(2+) sensors stromal interaction molecule (STIM)1 and STIM2 and the plasma-membrane channels Orai1, Orai2 and Orai3, have been identified. These landmark discoveries have enabled impressive progress in clarifying how these proteins work in concert and what developmental and cellular processes require their participation most. As we begin to better understand the biology of the STIM and Orai proteins, the attention to the pharmacological tools to influence their functions quickly follow suit. Here, we briefly summarize recent developments in this exciting area of Ca(2+) signaling.

A Cytosolic Homomerization and a Modulatory Domain within STIM1 C Terminus Determine Coupling to ORAI1 Channels

In immune cells, generation of sustained Ca(2+) levels is mediated by the Ca(2+) release-activated Ca(2+) (CRAC) current. Molecular key players in this process comprise the stromal interaction molecule 1 (STIM1) that functions as a Ca(2+) sensor in the endoplasmic reticulum and ORAI1 located in the plasma membrane. Depletion of endoplasmic reticulum Ca(2+) stores leads to STIM1 multimerization into discrete puncta, which co-cluster with ORAI1 to couple to and activate ORAI1 channels. The cytosolic C terminus of STIM1 is sufficient to activate ORAI1 currents independent of store depletion. Here we identified an ORAI1-activating small fragment (OASF, amino acids 233-450/474) within STIM1 C terminus comprising the two coiled-coil domains and additional 50-74 amino acids that exhibited enhanced interaction with ORAI1, resulting in 3-fold increased Ca(2+) currents. This OASF, similar to the complete STIM1 C terminus, displayed the ability to homomerize by a novel assembly domain that occurred subsequent to the coiled-coil domains. A smaller fragment (amino acids 233-420) generated by a further deletion of 30 amino acids substantially reduced the ability to homomerize concomitant to a loss of coupling to as well as activation of ORAI1. Extending OASF by 35 amino acids (233-485) did not alter homomerization but substantially decreased efficiency in coupling to and activation of ORAI1. Expressing OASF in rat basophilic leukemia (RBL) mast cells demonstrated its enhanced plasma membrane targeting associated with 2.5-fold larger CRAC currents in comparison with the complete STIM1 C terminus. In aggregate, we have identified two cytosolic key regions within STIM1 C terminus that control ORAI1/CRAC activation: a homomerization domain indispensable for coupling to ORAI1 and a modulatory domain that controls the extent of coupling to ORAI1.

Calcium signaling in immune cells

Calcium acts as a second messenger in many cell types, including lymphocytes. Resting lymphocytes maintain a low concentration of Ca2+. However, engagement of antigen receptors induces calcium influx from the extracellular space by several routes. A chief mechanism of Ca2+ entry in lymphocytes is through store-operated calcium (SOC) channels. The identification of two important molecular components of SOC channels, CRACM1 (the pore-forming subunit) and STIM1 (the sensor of stored calcium), has allowed genetic and molecular manipulation of the SOC entry pathway. In this review, we highlight advances in the understanding of Ca2+ signaling in lymphocytes with special emphasis on SOC entry. We also discuss outstanding questions and probable future directions of the field.

Molecular clustering of STIM1 with Orai1/CRACM1 at the plasma membrane depends dynamically on depletion of Ca2+ stores and on electrostatic interactions

Activation of store operated Ca(2+) entry involves stromal interaction molecule 1 (STIM1), localized to the endoplasmic reticulum (ER), and calcium channel subunit (Orai1/CRACM1), localized to the plasma membrane. Confocal microscopy shows that thapsigargin-mediated depletion of ER Ca(2+) stores in RBL mast cells causes a redistribution of STIM1, labeled with monomeric red fluorescent protein (mRFP), to micrometer-scale ER-plasma membrane junctions that contain Orai1/CRACM1, labeled with monomeric Aequorea coerulescens green fluorescent protein (AcGFP). Using fluorescence resonance energy transfer (FRET), we determine that this visualized coredistribution is accompanied by nanoscale interaction of STIM1-mRFP and AcGFP-Orai1/CRACM1. We find that antigen stimulation of immunoglobulin E receptors causes much less Orai1/CRACM1 and STIM1 association, but strong interaction is observed under conditions that prevent refilling of ER stores. Stimulated association monitored by FRET is inhibited by sphingosine derivatives in parallel with inhibition of Ca(2+) influx. Similar structural and functional effects are caused by mutation of acidic residues in the cytoplasmic segment of Orai1/CRACM1, suggesting a role for electrostatic interactions via these residues in the coupling of Orai1/CRACM1 to STIM1. Our results reveal dynamic molecular interactions between STIM1 and Orai1/CRACM1 that depend quantitatively on electrostatic interactions and on the extent of store depletion.

Functional stoichiometry of the unitary calcium-release-activated calcium channel

Two proteins, STIM1 in the endoplasmic reticulum and Orai1 in the plasma membrane, are required for the activation of Ca(2+) release-activated Ca(2+) (CRAC) channels at the cell surface. How these proteins interact to assemble functional CRAC channels has remained uncertain. Here, we determine how many Orai1 and STIM1 molecules are required to form a functional CRAC channel. We engineered several genetically expressed fluorescent Orai1 tandem multimers and a fluorescent, constitutively active STIM1 mutant. The tandem multimers assembled into CRAC channels, as seen by rectifying inward currents and by cytoplasmic calcium elevations. CRAC channels were visualized as fluorescent puncta in total internal reflection microscopy. With single-molecule imaging techniques, it was possible to observe photo-bleaching of individual fluorophores and to count the steps of bleaching as a measure of the stoichiometry of each CRAC channel complex. We conclude that the subunit stoichiometry in an active CRAC channel is four Orai1 molecules and two STIM1 molecules. Fluorescence resonance energy transfer experiments also showed that four Orai1 subunits form the assembled channel. From the fluorescence intensity of single fluorophores, we could estimate that our transfected HEK293 cells had almost 400,000 CRAC channels and that, when intracellular Ca(2+) stores were depleted, the channels clustered in aggregates containing approximately 1,300 channels, amplifying the local Ca(2+) entry.