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

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Rapid changes in cytosolic Ca2+ concentrations [Ca2+]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 Ca2+ influx is termed store-operated Ca2+ entry (SOCE) owing to its control by the Ca2+ content of the endoplasmic reticulum (ER). The underlying molecular mechanism of SOCE has eluded identification until recently when two groups of proteins, the ER Ca2+ 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 Ca2+ signaling.

Introduction

A select group of stimuli initiate cellular responses by acting on cell-surface receptors to increase the cytosolic Ca2+ concentration. By the mid 1980 s it was understood that stimulation of these receptors induces the hydrolysis of membrane phosphoinositides by phospholipase C (PLC) enzymes yielding the diffusible Ca2+-mobilizing messenger inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], to release Ca2+ from non-mitochondrial Ca2+ stores [1]. However, Ca2+ release is also followed by a stimulated Ca2+ entry in such cells and in 1986 James Putney proposed that the ultimate reason for the Ca2+ influx is a decrease in the Ca2+ content of the endoplasmic reticulum (ER) introducing the concept of capacitative or store-operated Ca2+ entry (SOCE) [2]. The mechanism by which the ER Ca2+ stores communicate with the plasma membrane (PM) and the molecules participating in the Ca2+-influx process remained elusive until 3 years ago when suddenly a series of key discoveries identified the stromal interaction molecule (STIM) and Orai (also known as calcium-release-activated calcium-modulator [CRACM]) proteins that serve as ER calcium sensors and calcium channels, respectively (see Box 1 for a brief historical overview). Several excellent reviews have summarized the exciting early developments on this research area in more detail 3, 4, 5, 6. Here, we describe the main features and the cell biology of these two classes of molecules in addition to their biological importance. We also highlight some of the pharmacological means by which to manipulate the functions of these proteins.

Section snippets

STIM1 is the ER Ca2+ sensor

STIM1 and STIM2 had been described and characterized before their roles in Ca2+ signaling were recognized. STIM was first identified as a cell-surface molecule that mediates cell-stromal interactions acting as a recessive tumor-suppressor gene [7]. STIM1 and its homologue, STIM2, are type-I membrane proteins with a luminal helix-turn-helix EF-hand Ca2+-sensing module followed by a sterile α motif (SAM) before the single transmembrane segment (see Figure 1 for the structural features of STIM

The Orai proteins form the channel pore

The predicted architecture of the integral membrane protein, Orai1 (also known as CRACM1) (Figure 2), already raised the possibility that it might be the channel component of SOCE 24, 25, 26. This was unequivocally proven by mutational analysis that showed that the properties of the ICRAC (Ca2+-release-activated calcium; CRAC; see Glossary) currents are determined by the Orai1 protein 27, 28, 29. The three Orai proteins display notable differences in their features despite a high degree of

Interaction between STIM and Orai proteins

The Orai1 protein located in the PM shows a uniform PM distribution in resting cells but rapidly clusters after store depletion and co-localizes with STIM1 puncta indicating an interaction between the two proteins 38, 39, 40. The parsimonious assumption is that STIM1 proteins in the ER oligomerize once ER Ca2+ levels drop and they cluster the Orai1 proteins in the PM. This interaction then stabilizes the junctional sites between the ER and the PM (Figure 3). Yet, direct interaction between

Life without STIM and Orai proteins

Analysis of severe combined immunodeficiency (SCID) patients carrying the Arg91Trp mutation has already indicated that Orai1 is not an essential protein, although it is crucially important for proper immune-cell functions [67]. During T-cell activation, the cytoplasmic Ca2+ increase triggers the dephosphorylation and nuclear translocation of the NFAT1 transcription factor (nuclear receptor for activated T-cells) by the Ca2+-dependent phosphatase calcineurin [68]. Apparently, this process relies

Inhibitors of the STIM1 and Orai1 protein-mediated Ca2+-influx pathways

Divalent cations and trivalent lanthanides have long been known to inhibit Ca2+-entry pathways. Among these, La3+ (IC50 = 10–100 μM) and Gd3+ (IC50 < 1.0 μM) have been most widely used. However, these ions inhibit a wide range of Ca2+-entry channels and their apparent selectivity displayed in a narrow concentration range could depend not only on the channel itself but also on the activation mechanism of Ca2+ entry [77]. Therefore, several chemical inhibitors of SOCE have been introduced over the

Outstanding questions and future directions

Several questions remain to be clarified; the most immediate ones are related to the molecular determinants mediating the interactions between the STIM and Orai proteins. Additionally, soluble active STIM1 components capable of activating the Orai1 channels could enable a more direct manipulation and analysis of the electrical properties of these channels in excised membranes. More complex questions include whether specific areas of the ER are preferentially coupled to SOCE with special

Acknowledgements

P.V. is a Bolyai Fellow of the Hungarian Academy of Science (www.mta.hu) and was supported by the Hungarian Scientific Research Fund (OTKA NF-68563; www.otka.hu) and the Medical Research Council (ETT 440/2006; www.ett.hu). The research of T.B. was supported by the Intramural Research Program of the National Institute of Child Health and Human Development of the National Institutes of Health (www.nichd.nih.gov).

Glossary

CIF
Ca2+ influx factor; a putative diffusible molecule that is liberated when the ER Ca2+ store becomes depleted and activates a Ca2+-influx pathway.
Cortical actin
the fraction of polymerized actin formed beneath the PM.
ICRAC
Ca2+-release-activated calcium current; the electrophysiological correlate of STIM-Orai-mediated Ca2+ entry.
EB1
a microtubule plus-end-tracking protein. They accumulate at the distal end of growing microtubules.
Gene trapping
a method by which a cassette containing a selectable

References (113)

  • L. Zheng

    Biophysical characterization of the EF-hand and SAM domain containing Ca2+ sensory region of STIM1 and STIM2

    Biochem. Biophys. Res. Commun.

    (2008)
  • M. Vig

    CRACM1 multimers form the ion-selective pore of the CRAC channel

    Curr. Biol.

    (2006)
  • Y. Gwack

    Biochemical and functional characterization of Orai proteins

    J. Biol. Chem.

    (2007)
  • A. Lis

    CRACM1, CRACM2, and CRACM3 are store-operated Ca2+ channels with distinct functional properties

    Curr. Biol.

    (2007)
  • W.I. DeHaven

    Calcium inhibition and calcium potentiation of Orai1, Orai2, and Orai3 calcium release-activated calcium channels

    J. Biol. Chem.

    (2007)
  • J. Soboloff

    Orai1 and STIM reconstitute store-operated calcium channel function

    J. Biol. Chem.

    (2006)
  • P. Xu

    Aggregation of STIM1 underneath the plasma membrane induces clustering of Orai1

    Biochem. Biophys. Res. Commun.

    (2006)
  • P. Varnai

    Visualization and manipulation of plasma membrane-endoplasmic reticulum contact sites indicates the presence of additional molecular components within the STIM1-Orai1 Complex

    J. Biol. Chem.

    (2007)
  • H.L. Ong

    Relocalization of STIM1 for activation of store-operated Ca2+ entry is determined by the depletion of subplasma membrane endoplasmic reticulum Ca2+ store

    J. Biol. Chem.

    (2007)
  • H. Jousset

    STIM1 knockdown reveals that store-operated Ca2+ channels located close to sarco/endoplasmic Ca2+ ATPases (SERCA) pumps silently refill the endoplasmic reticulum

    J. Biol. Chem.

    (2007)
  • Z. Li

    Mapping the interacting domains of STIM1 and Orai1 in Ca2+ release-activated Ca2+ channel activation

    J. Biol. Chem.

    (2007)
  • M.A. Spassova

    Voltage gating at the selectivity filter of the Ca2+ release-activated Ca2+ channel induced by mutation of the Orai1 protein

    J. Biol. Chem.

    (2008)
  • M. Muik

    Dynamic coupling of the putative coiled-coil domain of ORAI1 with STIM1 mediates ORAI1 channel activation

    J. Biol. Chem.

    (2008)
  • S.L. Zhang

    Store-dependent and -independent modes regulating Ca2+ release-activated Ca2+ channel activity of human Orai1 and Orai3

    J. Biol. Chem.

    (2008)
  • W. Zeng

    STIM1 gates TRPC channels, but not Orai1, by electrostatic interaction

    Mol. Cell

    (2008)
  • I. Grigoriev

    STIM1 is a MT-plus-end-tracking protein involved in remodeling of the ER

    Curr. Biol.

    (2008)
  • R.L. Patterson

    Store-operated Ca2+ entry: evidence for a secretion-like coupling model

    Cell

    (1999)
  • P. Mueller

    Disruption of the cortical actin cytoskeleton does not affect store operated Ca2+ channels in human T-cells

    FEBS Lett.

    (2007)
  • J.J. Lopez

    Interaction of STIM1 with endogenously expressed human canonical TRP1 upon depletion of intracellular Ca2+ stores

    J. Biol. Chem.

    (2006)
  • Y. Takahashi

    Functional role of stromal interaction molecule 1 (STIM1) in vascular smooth muscle cells

    Biochem. Biophys. Res. Commun.

    (2007)
  • K.T. Cheng

    Functional requirement for Orai1 in store-operated TRPC1-STIM1 channels

    J. Biol. Chem.

    (2008)
  • B. Pani

    Lipid rafts determine clustering of STIM1 in endoplasmic reticulum-plasma membrane junctions and regulation of store-operated Ca2+ entry (SOCE)

    J. Biol. Chem.

    (2008)
  • T.J. Shuttleworth

    STIM1 and the noncapacitative ARC channels

    Cell Calcium

    (2007)
  • M. Trebak

    Comparison of human TRPC3 channels in receptor-activated and store-operated modes. Differential sensitivity to channel blockers suggests fundamental differences in channel composition

    J. Biol. Chem.

    (2002)
  • W.I. Dehaven

    Complex actions of 2-aminoethyldiphenyl borate on store-operated calcium entry

    J. Biol. Chem.

    (2008)
  • R. Schindl

    2-aminoethoxydiphenyl borate alters selectivity of orai3 channels by increasing their pore size

    J. Biol. Chem.

    (2008)
  • H. Zhou

    2-Aminoethyl diphenylborinate analogues: selective inhibition for store-operated Ca2+ entry

    Biochem. Biophys. Res. Commun.

    (2007)
  • G. Boulay

    Cloning and expression of a novel mammalian homolog of Drosophila transient receptor potential (Trp) involved in calcium entry secondary to activation of receptors coupled by the Gq class of G protein

    J. Biol. Chem.

    (1997)
  • G. Schwarz

    Multiple effects of SK&F 96365 on ionic currents and intracellular calcium in human endothelial cells

    Cell Calcium

    (1994)
  • H. Watanabe

    Inhibition of agonist-induced Ca2+ entry in endothelial cells by myosin light-chain kinase inhibitor

    Biochem. Biophys. Res. Commun.

    (1996)
  • C. Zitt

    Potent inhibition of Ca2+ release-activated Ca2+ channels and T-lymphocyte activation by the pyrazole derivative BTP2

    J. Biol. Chem.

    (2004)
  • L.P. He

    A functional link between store-operated and TRPC channels revealed by the 3,5-bis(trifluoromethyl)pyrazole derivative, BTP2

    J. Biol. Chem.

    (2005)
  • M.J. Berridge et al.

    Inositol trisphosphate, a novel second messenger in cellular signal transduction

    Nature

    (1984)
  • R.S. Lewis

    The molecular choreography of a store-operated calcium channel

    Nature

    (2007)
  • R.T. Williams

    Identification and characterization of the STIM (stromal interaction molecule) gene family: coding for a novel class of transmembrane proteins

    Biochem. J.

    (2001)
  • S.L. Zhang

    STIM1 is a Ca2+ sensor that activates CRAC channels and migrates from the Ca2+ store to the plasma membrane

    Nature

    (2005)
  • M.M. Wu

    Ca2+ store depletion causes STIM1 to accumulate in ER regions closely associated with the plasma membrane

    J. Cell Biol.

    (2006)
  • M.A. Spassova

    STIM1 has a plasma membrane role in the activation of store-operated Ca2+ channels

    Proc. Natl. Acad. Sci. U. S. A.

    (2006)
  • O. Mignen

    STIM1 regulates Ca2+ entry via arachidonate-regulated Ca2+-selective (ARC) channels without store depletion or translocation to the plasma membrane

    J. Physiol.

    (2007)
  • Y. Baba

    Coupling of STIM1 to store-operated Ca2+ entry through its constitutive and inducible movement in the endoplasmic reticulum

    Proc. Natl. Acad. Sci. U. S. A.

    (2006)
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