Gated regulation of CRAC channel ion selectivity by STIM1

Structural aspects of calcium-release activated calcium channel function

Store-operated calcium (Ca(2+)) entry is the process by which molecules located on the endo/sarcoplasmic reticulum (ER/SR) respond to decreased luminal Ca(2+) levels by signaling Ca(2+) release activated Ca(2+) channels (CRAC) channels to open on the plasma membrane (PM). This activation of PM CRAC channels provides a sustained cytosolic Ca(2+) elevation associated with myriad physiological processes. The identities of the molecules which mediate SOCE include stromal interaction molecules (STIMs), functioning as the ER/SR luminal Ca(2+) sensors, and Orai proteins, forming the PM CRAC channels. This review examines the current available high-resolution structural information on these CRAC molecular components with particular focus on the solution structures of the luminal STIM Ca(2+) sensing domains, the crystal structures of cytosolic STIM fragments, a closed Orai hexameric crystal structure and a structure of an Orai1 N-terminal fragment in complex with calmodulin. The accessible structural data are discussed in terms of potential mechanisms of action and cohesiveness with functional observations.

How many Orai's does it take to make a CRAC channel?

CRAC (Calcium Release-Activated Calcium) channels represent the primary pathway for so-called “store-operated calcium entry” – the cellular entry of calcium induced by depletion of intracellular calcium stores. These channels play a key role in diverse cellular activities, most noticeably in the differentiation and activation of Tcells, and in the response of mast cells to inflammatory signals. CRAC channels are formed by members of the recently discovered Orai protein family, with previous studies indicating that the functional channel is formed by a tetramer of Orai subunits. However, a recent report has shown that crystals obtained from the purified Drosophila Orai protein display a hexameric channel structure. Here, by comparing the biophysical properties of concatenated hexameric and tetrameric human Orai1 channels expressed in HEK293 cells, we show that the tetrameric channel displays the highly calcium-selective conductance properties consistent with endogenous CRAC channels, whilst the hexameric construct forms an essentially non-selective cation channel.

Pore waters regulate ion permeation in a calcium release-activated calcium channel

The recent crystal structure of Orai, the pore unit of a calcium release-activated calcium (CRAC) channel, is used as the starting point for molecular dynamics and free-energy calculations designed to probe this channel's conduction properties. In free molecular dynamics simulations, cations localize preferentially at the extracellular channel entrance near the ring of Glu residues identified in the crystal structure, whereas anions localize in the basic intracellular half of the pore. To begin to understand ion permeation, the potential of mean force (PMF) was calculated for displacing a single Na(+) ion along the pore of the CRAC channel. The computed PMF indicates that the central hydrophobic region provides the major hindrance for ion diffusion along the permeation pathway, thereby illustrating the nonconducting nature of the crystal structure conformation. Strikingly, further PMF calculations demonstrate that the mutation V174A decreases the free energy barrier for conduction, rendering the channel effectively open. This seemingly dramatic effect of mutating a nonpolar residue for a smaller nonpolar residue in the pore hydrophobic region suggests an important role for the latter in conduction. Indeed, our computations show that even without significant channel-gating motions, a subtle change in the number of pore waters is sufficient to reshape the local electrostatic field and modulate the energetics of conduction, a result that rationalizes recent experimental findings. The present work suggests the activation mechanism for the wild-type CRAC channel is likely regulated by the number of pore waters and hence pore hydration governs the conductance.

The extended transmembrane Orai1 N-terminal (ETON) region combines binding interface and gate for Orai1 activation by STIM1

STIM1 and Orai1 represent the two molecular key components of the Ca(2+) release-activated Ca(2+) channels. Their activation involves STIM1 C terminus coupling to both the N terminus and the C terminus of Orai. Here we focused on the extended transmembrane Orai1 N-terminal (ETON, aa73-90) region, conserved among the Orai family forming an elongated helix of TM1 as recently shown by x-ray crystallography. To identify "hot spot" residues in the ETON binding interface for STIM1 interaction, numerous Orai1 constructs with N-terminal truncations or point mutations within the ETON region were generated. N-terminal truncations of the first four residues of the ETON region or beyond completely abolished STIM1-dependent Orai1 function. Loss of Orai1 function resulted from neither an impairment of plasma membrane targeting nor pore damage, but from a disruption of STIM1 interaction. In a complementary approach, we monitored STIM1-Orai interaction via Orai1 V102A by determining restored Ca(2+) selectivity as a consequence of STIM1 coupling. Orai1 N-terminal truncations that led to a loss of function consistently failed to restore Ca(2+) selectivity of Orai1 V102A in the presence of STIM1, demonstrating impairment of STIM1 binding. Hence, the major portion of the ETON region (aa76-90) is essential for STIM1 binding and Orai1 activation. Mutagenesis within the ETON region revealed several hydrophobic and basic hot spot residues that appear to control STIM1 coupling to Orai1 in a concerted manner. Moreover, we identified two basic residues, which protrude into the elongated pore to redound to Orai1 gating. We suggest that several hot spot residues in the ETON region contribute in aggregate to the binding of STIM1, which in turn is coupled to a conformational reorientation of the gate.

The STIM1/Orai signaling machinery

Ca(2+) influx via store-operated Ca(2+) release activated Ca(2+) (CRAC) channels represents a main signaling pathway for T-cell activation as well as mast-cell degranulation. The ER-located Ca(2+)-sensor, STIM1 and the Ca(2+)-selective ion pore, Orai1 in the membrane are sufficient to fully reconstitute CRAC currents. Their identification, but even more the recent structural resolution of both proteins by X-ray crystallography has substantially advanced the understanding of the activation mechanism of CRAC channels. In this review, we provide a detailed description of the STIM1/Orai1 signaling pathway thereby focusing on the critical domains mediating both, intra- as well as intermolecular interactions and on the ion permeation pathway. Based on the results of functional studies as well as the recently published crystal structures, we portray a mechanistic view of the steps in the CRAC channel signaling cascade ranging from STIM1 oligomerization over STIM1-Orai1 coupling to the ultimate Orai1 channel activation and permeation.

A novel homology model of TRPC3 reveals allosteric coupling between gate and selectivity filter

Utilizing a novel molecular model of TRPC3, based on the voltage-gated sodium channel from Arcobacter butzleri (Na(V)AB) as template, we performed structure-guided mutagenesis experiments to identify amino acid residues involved in divalent permeation and gating. Substituted cysteine accessibility screening within the predicted selectivity filter uncovered amino acids 629-631 as the narrowest part of the permeation pathway with an estimated pore diameter of < 5.8Å. E630 was found to govern not only divalent permeability but also sensitivity of the channel to block by ruthenium red. Mutations in a hydrophobic cluster at the cytosolic termini of transmembrane segment 6, corresponding to the S6 bundle crossing structure in Na(V)AB, distorted channel gating. Removal of a large hydrophobic residue (I667A or I667E) generated channels with approximately 60% constitutive activity, suggesting I667 as part of the dynamic structure occluding the permeation path. Destabilization of the gate was associated with reduced Ca2+ permeability, altered cysteine cross-linking in the selectivity filter and promoted channel block by ruthenium red. Collectively, we present a structural model of the TRPC3 permeation pathway and localize the channel's selectivity filter and the occluding gate. Moreover, we provide evidence for allosteric coupling between the gate and the selectivity filter in TRPC3.

Molecular pharmacology of store-operated CRAC channels

Calcium influx through store-operated Ca(2+) release-activated Ca(2+) channels (CRAC channels) is a well-defined mechanism of generating cellular Ca(2+) elevations that regulates many functions including gene expression, exocytosis and cell proliferation. The identifications of the ER Ca(2+) sensing proteins, STIM1-2 and the CRAC channel proteins, Orai1-3, have led to improved understanding of the physiological roles and the activation mechanism of CRAC channels. Defects in CRAC channel function are associated with serious human diseases such as immunodeficiency and auto-immunity. In this review, we discuss several pharmacological modulators of CRAC channels, focusing specifically on the molecular mechanism of drug action and their utility in illuminating the mechanism of CRAC channel operation and their physiological roles in different cells.

STIM1/Orai1-mediated SOCE: current perspectives and potential roles in cardiac function and pathology

Store-operated Ca²⁺ entry (SOCE) is critical for Ca²⁺ signaling in nonexcitable cells; however, its role in the regulation of cardiomyocyte Ca²⁺ homeostasis has only recently been investigated. The increased understanding of the role of stromal interaction molecule 1 (STIM1) in regulating SOCE combined with recent studies demonstrating the presence of STIM1 in cardiomyocytes provides support that this pathway co-exists in the heart with the more widely recognized Ca²⁺ handling pathways associated with excitation-contraction coupling. There is now substantial evidence that STIM1-mediated SOCE plays a key role in mediating cardiomyocyte hypertrophy, both in vitro and in vivo, and there is growing support for the contribution of SOCE to Ca²⁺ overload associated with ischemia/reperfusion injury. Here, we provide an overview of our current understanding of the molecular regulation of SOCE and discuss the evidence supporting the role of STIM1/Orai1-mediated SOCE in regulating cardiomyocyte function.

An essential and NSF independent role for α-SNAP in store-operated calcium entry

Store-operated calcium entry (SOCE) by calcium release activated calcium (CRAC) channels constitutes a primary route of calcium entry in most cells. Orai1 forms the pore subunit of CRAC channels and Stim1 is the endoplasmic reticulum (ER) resident Ca(2+) sensor. Upon store-depletion, Stim1 translocates to domains of ER adjacent to the plasma membrane where it interacts with and clusters Orai1 hexamers to form the CRAC channel complex. Molecular steps enabling activation of SOCE via CRAC channel clusters remain incompletely defined. Here we identify an essential role of α-SNAP in mediating functional coupling of Stim1 and Orai1 molecules to activate SOCE. This role for α-SNAP is direct and independent of its known activity in NSF dependent SNARE complex disassembly. Importantly, Stim1-Orai1 clustering still occurs in the absence of α-SNAP but its inability to support SOCE reveals that a previously unsuspected molecular re-arrangement within CRAC channel clusters is necessary for SOCE. DOI:http://dx.doi.org/10.7554/eLife.00802.001.

A TRPC1 protein-dependent pathway regulates osteoclast formation and function

Ca(2+) signaling is essential for bone homeostasis and skeletal development. Here, we show that the transient receptor potential canonical 1 (TRPC1) channel and the inhibitor of MyoD family, I-mfa, function antagonistically in the regulation of osteoclastogenesis. I-mfa null mice have an osteopenic phenotype characterized by increased osteoclast numbers and surface, which are normalized in mice lacking both Trpc1 and I-mfa. In vitro differentiation of pre-osteoclasts derived from I-mfa-deficient mice leads to an increased number of mature osteoclasts and higher bone resorption per osteoclast. These parameters return to normal levels in osteoclasts derived from double mutant mice. Consistently, whole cell currents activated in response to the depletion of intracellular Ca(2+) stores are larger in pre-osteoclasts derived from I-mfa knock-out mice compared with currents in wild type mice and normalized in cells derived from double mutant mice, suggesting a cell-autonomous effect of I-mfa on TRPC1 in these cells. A new splice variant of TRPC1 (TRPC1ε) was identified in early pre-osteoclasts. Heterologous expression of TRPC1ε in HEK293 cells revealed that it is unique among all known TRPC1 isoforms in its ability to amplify the activity of the Ca(2+) release-activated Ca(2+) (CRAC) channel, mediating store-operated currents. TRPC1ε physically interacts with Orai1, the pore-forming subunit of the CRAC channel, and I-mfa is recruited to the TRPC1ε-Orai1 complex through TRPC1ε suppressing CRAC channel activity. We propose that the positive and negative modulation of the CRAC channel by TRPC1ε and I-mfa, respectively, fine-tunes the dynamic range of the CRAC channel regulating osteoclastogenesis.

The C- and N-terminal STIM1 binding sites on Orai1 are required for both trapping and gating CRAC channels

Ca(2+) release-activated Ca(2+) (CRAC) channels are activated through a mechanism wherein depletion of intracellular calcium stores results in the aggregation of stromal interaction molecule 1 (STIM1), the endoplasmic reticulum (ER) Ca(2+) sensor, and Orai1, the CRAC channel protein, at overlapping sites in the ER and plasma membranes (PMs). The redistribution of CRAC channels is driven through direct STIM1-Orai1 binding, an important event that not only controls gating, but also regulates Orai1 ion selectivity. Orai1 harbours two STIM1 binding sites, one each on the intracellular C- and N-termini. Previous studies have proposed modular functions for these sites, with the C-terminal site thought to regulate STIM1-Orai1 binding and trapping of Orai1 at the ER-PM junctions, and the N-terminal site mediating gating. However, here we find that a variety of mutations in the N-terminal site impair the binding of Orai1 to STIM1 and to the soluble CRAC activation domain (CAD). Gating could be restored in several N- and C-terminal point mutants by directly tethering the minimal STIM1 activation domain (S) to Orai1 (Orai1-SS channels), indicating that loss of gating in these mutants by full-length STIM1 results from insufficient ligand binding. By contrast, gating could not be restored in mutant Orai1-SS channels carrying more drastic deletions that removed the STIM1 binding sites (1-85, 73-85, or 272-279 Orai1), suggesting that STIM1 binding to both sites is essential for channel activation. Moreover, analysis of ion selectivity indicated that the molecular requirements for gating and modulation of ion selectivity are similar, yet substantively different from those for Orai1 puncta formation, suggesting that ion selectivity and gating are mechanistically coupled in CRAC channels. Our results indicate that the C- and N-terminal STIM1 binding sites are both essential for multiple aspects of Orai1 function including STIM1-Orai1 association, Orai1 trapping, and channel activation.

ER-PM connections: sites of information transfer and inter-organelle communication

Eukaryotic cells are divided into distinct membrane-bound organelles with unique identities and specialized metabolic functions. Communication between organelles must take place to regulate the size, shape, and composition of individual organelles, as well as to coordinate transport between organelles. The endoplasmic reticulum (ER) forms an expansive membrane network that contacts and participates in crosstalk with several other organelles in the cell, most notably the plasma membrane (PM). ER-PM junctions have well-established functions in the movement of small molecules, such as lipids and ions, between the ER and PM. Recent discoveries have revealed additional exciting roles for ER-PM junctions in the regulation of cell signaling, ER shape and architecture, and PM domain organization.

Orai channel pore properties and gating by STIM: implications from the Orai crystal structure

The Orai channels are unusual, yet prominent, calcium (Ca2+) signal mediators in most cell types. Orai proteins are structurally unique, having little sequence homology with other ion channels. They are also functionally unique with exceedingly high selectivity for Ca2+, mediating both short-term Ca2+ homeostasis and long-term Ca2+ signals important for transcriptional control. Operating in the plasma membrane (PM), Orai channel regulation is unprecedented among ion channels; channel gating occurs through an elaborate intermembrane coupling with stromal interaction molecule (STIM) proteins in the endoplasmic reticulum (ER). STIM proteins function as sensors of Ca2+ stored in the ER lumen and translocate into ER-PM junctions to tether and activate Orai channels when ER Ca2+ concentration decreases. Crystallization studies reveal an unexpected hexameric structure for the Orai channel and provide important insights into the pore architecture, the structural basis of its unusual cation selectivity, and how channel gating occurs through its coupling with STIM proteins.

Regulation of CRAC channels by protein interactions and post-translational modification

Store-operated Ca(2+) entry (SOCE) is a widespread mechanism to elevate the intracellular Ca(2+) concentrations and stimulate downstream signaling pathways affecting proliferation, secretion, differentiation and death in different cell types. In immune cells, immune receptor stimulation induces intracellular Ca(2+) store depletion that subsequently activates Ca(2+)-release-activated-Ca(2+) (CRAC) channels, a prototype of store-operated Ca(2+) (SOC) channels. CRAC channel opening leads to activation of diverse downstream signaling pathways affecting proliferation, differentiation, cytokine production and cell death. Recent identification of STIM1 as the endoplasmic reticulum Ca(2+) sensor and Orai1 as the pore subunit of CRAC channels has provided the much-needed molecular tools to dissect the mechanism of activation and regulation of CRAC channels. In this review, we discuss the recent advances in understanding the associating partners and posttranslational modifications of Orai1 and STIM1 proteins that regulate diverse aspects of CRAC channel function.

Orai1-NFAT signalling pathway triggered by T cell receptor stimulation

T cell receptor (TCR) stimulation plays a crucial role in development, homeostasis, proliferation, cell death, cytokine production, and differentiation of T cells. Thus, in depth understanding of TCR signalling is crucial for development of therapy targeting inflammatory diseases, improvement of vaccination efficiency, and cancer therapy utilizing T cell-based strategies. TCR activation turns on various signalling pathways, one of the important one being the Ca(2+)-calcineurin-nuclear factor of activated T cells (NFAT) signalling pathway. Stimulation of TCRs triggers depletion of intracellular Ca(2+) store and in turn, initiates store-operated Ca(2+) entry (SOCE), one of the major mechanisms to raise the intracellular Ca(2+) concentrations in T cells. Ca(2+)-release-activated-Ca(2+) (CRAC) channels are a prototype of store-operated Ca(2+) (SOC) channels in immune cells that are very well characterized. Recent identification of STIM1 as the endoplasmic reticulum (ER) Ca(2+) sensor and Orai1 as the pore subunit has dramatically advanced the understanding of CRAC channels and provides a molecular tool to investigate the physiological outcomes of Ca(2+) signalling during immune responses. In this review, we focus on our current understanding of CRAC channel activation, regulation, and downstream calcineurin-NFAT signaling pathway.

Differential roles of the C and N termini of Orai1 protein in interacting with stromal interaction molecule 1 (STIM1) for Ca2+ release-activated Ca2+ (CRAC) channel activation

The entry of extracellular Ca(2+), which is mediated by Ca(2+) release-activated Ca(2+) (CRAC) channels, is essential for T cell activation and the normal functioning of other immune cells. Although the molecular components of CRAC channels, the Orai1 pore-forming subunit and the STIM1-activating subunit have been recently identified, the gating mechanism by which Orai1 channels conduct Ca(2+) entry upon Orai1-STIM1 interaction following Ca(2+) store release remains elusive. Herein, we show that C-terminal truncations or point mutations prevented Orai1 from binding to STIM1 and subsequent channel opening. In contrast, an Orai1 mutant with an N-terminal truncation interacted with but failed to be activated by STIM1. Moreover, Orai1 channels with C-terminal disruption, but not N-terminal truncation, could be gated by fused functional domains of STIM1. Interestingly, the channel activities of Orai1 mutants carrying either an N-terminal or a C-terminal truncation were restored by a methionine mutation at the putative gating hinge, the conserved Gly-98 site in the first transmembrane segment (TM1) of Orai1. Collectively, these results support a stepwise gating mechanism of STIM1-operated Orai1 channels; the initial binding between STIM1 and the C terminus of Orai1 docks STIM1 onto the N terminus of Orai1 to initiate conformational changes of the pore-lining TM1 helix of Orai1, leading to the opening of the channel.

Store-independent Orai1/3 channels activated by intracrine leukotriene C4: role in neointimal hyperplasia

RATIONALE

Through largely unknown mechanisms, Ca(2+) signaling plays important roles in vascular smooth muscle cell (VSMC) remodeling. Orai1-encoded store-operated Ca(2+) entry has recently emerged as an important player in VSMC remodeling. However, the role of the exclusively mammalian Orai3 protein in native VSMC Ca(2+) entry pathways, its upregulation during VSMC remodeling, and its contribution to neointima formation remain unknown.

OBJECTIVE

The goal of this study was to determine the agonist-evoked Ca(2+) entry pathway contributed by Orai3; Orai3 potential upregulation and role during neointima formation after balloon injury of rat carotid arteries.

METHODS AND RESULTS

Ca(2+) imaging and patch-clamp recordings showed that although the platelet-derived growth factor activates the canonical Ca(2+) release-activated Ca(2+) channels via store depletion in VSMC, the pathophysiological agonist thrombin activates a distinct Ca(2+)-selective channel contributed by Orai1, Orai3, and stromal interacting molecule1 in the same cells. Unexpectedly, Ca(2+) store depletion is not required for activation of Orai1/3 channel by thrombin. Rather, the signal for Orai1/3 channel activation is cytosolic leukotrieneC4 produced downstream thrombin receptor stimulation through the catalytic activity of leukotrieneC4 synthase. Importantly, Orai3 is upregulated in an animal model of VSMC neointimal remodeling, and in vivo Orai3 knockdown inhibits neointima formation.

CONCLUSIONS

These results demonstrate that distinct native Ca(2+)-selective Orai channels are activated by different agonists/pathways and uncover a mechanism whereby leukotrieneC4 acts through hitherto unknown intracrine mode to elicit store-independent Ca(2+) signaling that promotes vascular occlusive disease. Orai3 and Orai3-containing channels provide novel targets for control of VSMC remodeling during vascular injury or disease.

Structure and function of endoplasmic reticulum STIM calcium sensors

Store-operated calcium (Ca(2+)) entry (SOCE) is a vital Ca(2+) signaling pathway in nonexcitable as well as electrically excitable cells, regulating countless physiological and pathophysiological pathways. Stromal interaction molecules (STIMs) are the principal regulating molecules of SOCE, sensing changes in sarco-/endoplasmic reticulum (S/ER) luminal Ca(2+) levels and directly interacting with the Orai channel subunits to orchestrate the opening of Ca(2+) release-activated Ca(2+) (CRAC) channels. Recent atomic resolution structures on human STIM1 and STIM2 have illuminated critical mechanisms of STIM function in SOCE; further, the first high-resolution structure of the Drosophila melanogaster Orai channel has revealed vital data on the atomic composition of the CRAC channel pore and the assembly of individual Orai subunits. This chapter focuses on the mechanistic information garnered from these high-resolution structures and the supporting biophysical, biochemical, and live cell work that has enhanced our understanding of the relationship between STIM and Orai structural features and CRAC channel function.

Emerging roles for native Orai Ca2+ channels in cardiovascular disease

Orai proteins form highly calcium (Ca(2+))-selective channels located in the plasma membrane of both nonexcitable and excitable cells, where they make important contributions to many cellular processes. The well-characterized Ca(2+) release-activated Ca(2+) current is mediated by Orai1 multimers and is activated, upon depletion of inositol 1,4,5-trisphosphate-sensitive stores, by direct interaction of Orai1 with the endoplasmic reticulum Ca(2+) sensor, stromal interaction molecule 1 (STIM1). This pathway is known as capacitative Ca(2+) entry or store-operated Ca(2+) entry. While most investigations have focused on STIM1 and Orai1 in their store-dependent mode, emerging evidence suggests that Orai1 and Orai3 heteromultimeric channels can form store-independent Ca(2+)-selective channels. The role of store-dependent and store-independent channels in excitation-transcription coupling and the pathological remodeling of the cardiovascular system are beginning to come forth. Recent evidence suggests that STIM/Orai-generated Ca(2+) signaling couples to gene transcription and subsequent phenotypic changes associated with the processes of cardiac and vascular remodeling. This short review will explore the contributions of native Orai channels to heart and vessel physiology and their role in cardiovascular diseases.

STIM-ORAI interactions that control the CRAC channel

The Ca(2+) sensor STIM1 and the Ca(2+) channel ORAI1 are the fundamental working machinery of the CRAC channel, a classical pathway for store-operated Ca(2+) entry. This chapter focuses on the protein-protein interactions of STIM and ORAI proteins that control the channel.

Store-operated Orai channels: structure and function

In many animal cells, store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels function as an essential route for Ca(2+) entry. CRAC channels control many fundamental cellular functions including gene expression, motility, and cell proliferation, are involved in the etiology of several disease processes including a severe combined immunodeficiency syndrome, and have emerged as major targets for drug development. Although little was known of the molecular mechanisms of CRAC channel operation for several decades, the discovery of Orai1 as a prototypic CRAC channel protein and STIM1 as the endoplasmic reticulum (ER) Ca(2+) sensor has led to rapid progress in our understanding of the mechanisms and functions of CRAC channels. It is now known that activation of CRAC channels following ER Ca(2+) store depletion is governed by several events, which include the redistributions and accumulations of STIM1 and Orai1 into overlapping puncta at peripheral cellular sites, resulting in direct protein-protein interactions between the two proteins. In this chapter, I review the molecular features of the STIM and Orai proteins that regulate the gating and ion conduction mechanisms of CRAC channels.

Crystal structure of the calcium release-activated calcium channel Orai

The plasma membrane protein Orai forms the pore of the calcium release-activated calcium (CRAC) channel and generates sustained cytosolic calcium signals when triggered by depletion of calcium from the endoplasmic reticulum. The crystal structure of Orai from Drosophila melanogaster, determined at 3.35 angstrom resolution, reveals that the calcium channel is composed of a hexameric assembly of Orai subunits arranged around a central ion pore. The pore traverses the membrane and extends into the cytosol. A ring of glutamate residues on its extracellular side forms the selectivity filter. A basic region near the intracellular side can bind anions that may stabilize the closed state. The architecture of the channel differs markedly from other ion channels and gives insight into the principles of selective calcium permeation and gating.

STIM proteins: dynamic calcium signal transducers

Stromal interaction molecule (STIM) proteins function in cells as dynamic coordinators of cellular calcium (Ca(2+)) signals. Spanning the endoplasmic reticulum (ER) membrane, they sense tiny changes in the levels of Ca(2+) stored within the ER lumen. As ER Ca(2+) is released to generate primary Ca(2+) signals, STIM proteins undergo an intricate activation reaction and rapidly translocate into junctions formed between the ER and the plasma membrane. There, STIM proteins tether and activate the highly Ca(2+)-selective Orai channels to mediate finely controlled Ca(2+) signals and to homeostatically balance cellular Ca(2+). Details are emerging on the remarkable organization within these STIM-induced junctional microdomains and the identification of new regulators and alternative target proteins for STIM.

Molecular regulation of CRAC channels and their role in lymphocyte function

Calcium (Ca(2+)) influx is required for the activation and function of all cells in the immune system. It is mediated mainly by store-operated Ca(2+) entry (SOCE) through Ca(2+) release-activated Ca(2+) (CRAC) channels located in the plasma membrane. CRAC channels are composed of ORAI proteins that form the channel pore and are activated by stromal interaction molecules (STIM) 1 and 2. Located in the membrane of the endoplasmic reticulum, STIM1 and STIM2 have the dual function of sensing the intraluminal Ca(2+) concentration in the ER and to activate CRAC channels. A decrease in the ER's Ca(2+) concentration induces STIM multimerization and translocation into puncta close to the plasma membrane where they bind to and activate ORAI channels. Since the identification of ORAI and STIM genes as the principal mediators of CRAC channel function, substantial advances have been achieved in understanding the molecular regulation and physiological role of CRAC channels in cells of the immune system and other organs. In this review, we discuss the mechanisms that regulate CRAC channel function and SOCE, the role of recently identified proteins and mechanisms that modulate the activation of ORAI/STIM proteins and the consequences of CRAC channel dysregulation for lymphocyte function and immunity.

Modification of STIM1 by O-linked N-acetylglucosamine (O-GlcNAc) attenuates store-operated calcium entry in neonatal cardiomyocytes

Store-operated calcium entry (SOCE) is a major Ca(2+) signaling pathway responsible for regulating numerous transcriptional events. In cardiomyocytes SOCE has been shown to play an important role in regulating hypertrophic signaling pathways, including nuclear translocation of NFAT. Acute activation of pathways leading to O-GlcNAc synthesis have been shown to impair SOCE-mediated transcription and in diabetes, where O-GlcNAc levels are chronically elevated, cardiac hypertrophic signaling is also impaired. Therefore the goal of this study was to determine whether changes in cardiomyocyte O-GlcNAc levels impaired the function of STIM1, a widely recognized mediator of SOCE. We demonstrated that acute activation of SOCE in neonatal cardiomyocytes resulted in STIM1 puncta formation, which was inhibited in a dose-dependent manner by increasing O-GlcNAc synthesis with glucosamine or inhibiting O-GlcNAcase with thiamet-G. Glucosamine and thiamet-G also inhibited SOCE and were associated with increased O-GlcNAc modification of STIM1. These results suggest that activation of cardiomyocyte O-GlcNAcylation attenuates SOCE via STIM1 O-GlcNAcylation and that this may represent a new mechanism by which increased O-GlcNAc levels regulate Ca(2+)-mediated events in cardiomyocytes. Further, since SOCE is a fundamental mechanism underlying Ca(2+) signaling in most cells and tissues, it is possible that STIM1 represents a nexus linking protein O-GlcNAcylation with Ca(2+)-mediated transcription.

Ca(2+) release-activated Ca(2+) (CRAC) current, structure, and function

Store-operated Ca(2+) entry describes the phenomenon that connects a depletion of internal Ca(2+) stores to an activation of plasma membrane-located Ca(2+) selective ion channels. Tremendous progress towards the underlying molecular mechanism came with the discovery of the two respective limiting components, STIM and Orai. STIM1 represents the ER-located Ca(2+) sensor and transmits the signal of store depletion to the plasma membrane. Here it couples to and activates Orai, the highly Ca(2+)-selective pore-forming subunit of Ca(2+) release-activated Ca(2+) channels. In this review, we focus on the molecular steps that these two proteins undergo from store-depletion to their coupling, the activation, and regulation of Ca(2+) currents.

Temperature and RyR1 regulate the activation rate of store-operated Ca²+ entry current in myotubes

Store-operated calcium entry (SOCE) is an important Ca(2+) entry pathway in skeletal muscle. However, direct electrophysiological recording and full characterization of the underlying SOCE current in skeletal muscle cells (I(SkCRAC)) has not been reported. Here, we characterized the biophysical properties, pharmacological profile, and molecular identity of I(SkCRAC) in skeletal myotubes, as well as the regulation of its rate of activation by temperature and the type I ryanodine receptor (RyR1). I(SkCRAC) exhibited many hallmarks of Ca(2+) release activated Ca(2+) currents (I(CRAC)): store dependence, strong inward rectification, positive reversal potential, limited cesium permeability, and sensitivity to SOCE channel blockers. I(SkCRAC) was reduced by siRNA knockdown of stromal interaction molecule 1 and expression of dominant negative Orai1. Average I(SkCRAC) current density at -80mV was 1.00 ± 0.05 pA/pF. In the presence of 20 mM intracellular EGTA, I(SkCRAC) activation occurred over tens of seconds during repetitive depolarization at 0.5Hz and was inhibited by treatment with 100 μM ryanodine. The rate of SOCE activation was reduced threefold in myotubes from RyR1-null mice and increased 4.6-fold at physiological temperatures (35-37°C). These results show that I(SkCRAC) exhibits similar biophysical, pharmacological, and molecular properties as I(CRAC) in nonexcitable cells and its rate of activation during repetitive depolarization is strongly regulated by temperature and RyR1 activity.

Permeation, selectivity and gating in store-operated CRAC channels

Store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels are a widespread mechanism for generating cellular Ca(2+) signals and regulate many Ca(2+)-dependent functions, including transcription, motility and proliferation. The opening of CRAC channels in response to depletion of intracellular Ca(2+) stores involves a cascade of cellular events that culminate in direct interactions between STIM1, the endoplasmic reticulum Ca(2+) sensor, and the channels composed of Orai proteins. Evidence gathered over the last two decades indicates that CRAC channels display a unique functional pore fingerprint characterized by exquisite Ca(2+) selectivity, low unitary conductance, and low permeability to large cations. Here, we review the key pore properties of CRAC channels and discuss recent progress in addressing the molecular foundations of these properties. Structure-function and cysteine-scanning studies have revealed the identity and organization of pore-lining residues, including those that form the selectivity filter, providing a structural framework for understanding CRAC channel pore properties. Recent studies in pore mutants that produce STIM1-independent constitutive channel activation indicate that exquisite Ca(2+) selectivity in CRAC channels is not hardwired into Orai proteins, but is instead manifested only following the binding of STIM1 to the intrinsically poorly Ca(2+)-selective Orai channels. These findings reveal new functional aspects of CRAC channels and suggest that the selectivity filter of the CRAC channel is a dynamic structure whose conformation and functional properties are powerfully regulated by the channel activation stimulus.