Trends in Pharmacological Sciences
ReviewSTIM and Orai: the long-awaited constituents of store-operated calcium entry
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
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