Store-operated calcium channels: new perspectives on mechanism and 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.

STIM1 participates in the contractile rhythmicity of HL-1 cells by moderating T-type Ca(2+) channel activity

STIM1 plays a crucial role in Ca(2+) homeostasis, particularly in replenishing the intracellular Ca(2+) store following its depletion. In cardiomyocytes, the Ca(2+) content of the sarcoplasmic reticulum must be tightly controlled to sustain contractile activity. The presence of STIM1 in cardiomyocytes suggests that it may play a role in regulating the contraction of cardiomyocytes. The aim of the present study was to determine how STIM1 participates in the regulation of cardiac contractility. Atomic force microscopy revealed that knocking down STIM1 disrupts the contractility of cardiomyocyte-derived HL-1 cells. Ca(2+) imaging also revealed that knocking down STIM1 causes irregular spontaneous Ca(2+) oscillations in HL-1 cells. Action potential recordings further showed that knocking down STIM1 induces early and delayed afterdepolarizations. Knocking down STIM1 increased the peak amplitude and current density of T-type voltage-dependent Ca(2+) channels (T-VDCC) and shifted the activation curve toward more negative membrane potentials in HL-1 cells. Biotinylation assays revealed that knocking down STIM1 increased T-VDCC surface expression and co-immunoprecipitation assays suggested that STIM1 directly regulates T-VDCC activity. Thus, STIM1 is a negative regulator of T-VDCC activity and maintains a constant cardiac rhythm by preventing a Ca(2+) overload that elicits arrhythmogenic events.

Quantitative interactions between the A-type K+ current and inositol trisphosphate receptors regulate intraneuronal Ca2+ waves and synaptic plasticity

The A-type potassium current has been implicated in the regulation of several physiological processes. Here, we explore a role for the A-type potassium current in regulating the release of calcium through inositol trisphosphate receptors (InsP3R) that reside on the endoplasmic reticulum (ER) of hippocampal pyramidal neurons. To do this, we constructed morphologically realistic, conductance-based models equipped with kinetic schemes that govern several calcium signalling modules and pathways, and constrained the distributions and properties of constitutive components by experimental measurements from these neurons. Employing these models, we establish a bell-shaped dependence of calcium release through InsP3Rs on the density of A-type potassium channels, during the propagation of an intraneuronal calcium wave initiated through established protocols. Exploring the sensitivities of calcium wave initiation and propagation to several underlying parameters, we found that ER calcium release critically depends on dendritic diameter and that wave initiation occurred at branch points as a consequence of a high surface area to volume ratio of oblique dendrites. Furthermore, analogous to the role of A-type potassium channels in regulating spike latency, we found that an increase in the density of A-type potassium channels led to increases in the latency and the temporal spread of a propagating calcium wave. Next, we incorporated kinetic models for the metabotropic glutamate receptor (mGluR) signalling components and a calcium-controlled plasticity rule into our model and demonstrate that the presence of mGluRs induced a leftward shift in a Bienenstock-Cooper-Munro-like synaptic plasticity profile. Finally, we show that the A-type potassium current could regulate the relative contribution of ER calcium to synaptic plasticity induced either through 900 pulses of various stimulus frequencies or through theta burst stimulation. Our results establish a novel form of interaction between active dendrites and the ER membrane, uncovering a powerful mechanism that could regulate biophysical/biochemical signal integration and steer the spatiotemporal spread of signalling microdomains through changes in dendritic excitability.

The ARC channel--an endogenous store-independent Orai channel

Although Orai channels and their regulator stromal interacting molecule 1 (STIM1) were originally identified and described as the key components of the store-operated highly calcium-selective CRAC channels, it is now clear that these proteins are equally essential components of the agonist-activated, store-independent calcium entry pathway mediated by the arachidonic acid-regulated calcium-selective (ARC) channel. Correspondingly, ARC channels display biophysical properties that closely resemble those of CRAC channels but, whereas the latter is formed exclusively by Orai1 subunits, the ARC channel is formed by a combination of Orai1 and Orai3 subunits. Moreover, while STIM1 in the membrane of the endoplasmic reticulum is the critical sensor of intracellular calcium store depletion that results in the activation of the CRAC channels, it is the pool of STIM1 resident in the plasma membrane that regulates the activity of the store-independent ARC channels. Here, we describe the unique features of the ARC channels and their activation and discuss recent evidence indicating how these two coexisting, and biophysically very similar, Orai channels act to play entirely distinct roles in the regulation of various important cellular activities.

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.

Measurement of intracellular Ca2+ concentration in single cells using ratiometric calcium dyes

Measurement of intracellular Ca(2+) concentration ([Ca(2+)](i)) is useful to study the upstream and downstream events of Ca(2+) signaling. Ca(2+)-binding proteins including EF-hand-containing proteins are important downstream effector molecules after an increase of [Ca(2+)](i). Conversely, these proteins can also act as key modulators for regulation of [Ca(2+)](i) by sensing the Ca(2+) levels in the intracellular organelles and cytoplasm. Here we describe a single-cell Ca(2+) imaging technique that was used to measure the intracellular Ca(2+) levels to examine the function of Ca(2+)-binding proteins, STIM1 and Calcium release-activated Calcium channel regulator 2A (CRACR2A), using ratiometric Ca(2+) dye Fura-2 in adherent and non-adherent cells.

Targeted STIM deletion impairs calcium homeostasis, NFAT activation, and growth of smooth muscle

The Ca(2+)-sensing stromal interaction molecule (STIM) proteins are crucial Ca(2+) signal coordinators. Cre-lox technology was used to generate smooth muscle (sm)-targeted STIM1-, STIM2-, and double STIM1/STIM2-knockout (KO) mouse models, which reveal the essential role of STIM proteins in Ca(2+) homeostasis and their crucial role in controlling function, growth, and development of smooth muscle cells (SMCs). Compared to Cre(+/-) littermates, sm-STIM1-KO mice showed high mortality (50% by 30 d) and reduced bodyweight. While sm-STIM2-KO was without detectable phenotype, the STIM1/STIM double-KO was perinatally lethal, revealing an essential role of STIM1 partially rescued by STIM2. Vascular and intestinal smooth muscle tissues from sm-STIM1-KO mice developed abnormally with distended, thinned morphology. While depolarization-induced aortic contraction was unchanged in sm-STIM1-KO mice, α1-adrenergic-mediated contraction was 26% reduced, and store-dependent contraction almost eliminated. Neointimal formation induced by carotid artery ligation was suppressed by 54%, and in vitro PDGF-induced proliferation was greatly reduced (79%) in sm-STIM1-KO. Notably, the Ca(2+) store-refilling rate in STIM1-KO SMCs was substantially reduced, and sustained PDGF-induced Ca(2+) entry was abolished. This defective Ca(2+) homeostasis prevents PDGF-induced NFAT activation in both contractile and proliferating SMCs. We conclude that STIM1-regulated Ca(2+) homeostasis is crucial for NFAT-mediated transcriptional control required for induction of SMC proliferation, development, and growth responses to injury.-Mancarella, S., Potireddy, S., Wang, Y., Gao, H., Gandhirajan, K., Autieri, M., Scalia, R., Cheng, Z., Wang, H., Madesh, M., Houser, S. R., Gill, D. L. Targeted STIM deletion impairs calcium homeostasis, NFAT activation, and growth of smooth muscle.

Role of molecular determinants of store-operated Ca(2+) entry (Orai1, phospholipase A2 group 6, and STIM1) in focal adhesion formation and cell migration

BACKGROUND

Store-operated Ca(2+) entry is important for cell migration.

RESULTS

This study presents characterization of localization and roles of Orai1, STIM1, and PLA2g6 in adhesion dynamics during cell migration.

CONCLUSION

Orai1 and PLA2g6 are involved in adhesion formation at the front, whereas STIM1 participates in both adhesion formation and disassembly.

SIGNIFICANCE

Results uncovered new parameters of Orai1, STIM1, and PLA2g6 involvement in cell migration. Store-operated Ca(2+) entry and its major determinants are known to be important for cell migration, but the mechanism of their involvement in this complex process is unknown. This study presents a detailed characterization of distinct roles of Orai1, STIM1, and PLA2g6 in focal adhesion (FA) formation and migration. Using HEK293 cells, we discovered that although molecular knockdown of Orai1, STIM1, or PLA2g6 resulted in a similar reduction in migration velocity, there were profound differences in their effects on number, localization, and lifetime of FAs. Knockdown of STIM1 caused an increase in lifetime and number of FAs, their redistribution toward lamellae region, and an increase in cell tail length. In contrast, the number of FAs in Orai1- or PLA2g6-deficient cells was significantly reduced, and FAs accumulated closer to the leading edge. Assembly rate and Vinculin phosphorylation of FAs was similarly reduced in Orai1, PLA2g6, or STIM1-deficient cells. Although Orai1 and PLA2g6 accumulated and co-localized at the leading edge, STIM1 distribution was more complex. We found STIM1 protrusions in lamellipodia, which co-localized with FAs, whereas major accumulation could be seen in central and retracting parts of the cell. Interestingly, knockdown of Orai1 and PLA2g6 produced similar and non-additive effect on migration, whereas knockdown of STIM1 simultaneously with either Orai1 or PLA2g6 produced additional inhibition. Together these data suggest that although Orai1, PLA2g6, and STIM1 play major roles in formation of new FAs at the leading edge, STIM1 may also be involved in Orai1- and PLA2g6-independent disassembly of FAs in the back of cells.

Roles of cADPR and NAADP in pancreatic cells

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are Ca(2+)-mobilizing nucleotides that were discovered in the late 1980s. Two decades of investigations have built up a considerable understanding about these two molecules that are related because both are derived from pyridine nucleotides and known to be generated by CD38/ADP-ribosyl cyclases. cADPR has been shown to target the ryanodine receptors in the endoplasmic reticulum whereas NAADP stimulates the two-pore channels in the endo-lysosomes. Accumulating results indicate that cADPR and NAADP are second messenger molecules mediating Ca(2+) signaling activated by a wide range of agonists. This article reviews what is known about these two molecules, especially regarding their signaling roles in the pancreatic cells.

Orai1, STIM1, and their associating partners

Store-operated Ca(2+) (SOC) entry is one of the major mechanisms to raise intracellular Ca(2+) concentration in non-excitable cells. Ca(2+)-release-activated Ca(2+) (CRAC) channels are a subtype of SOC channels that are extensively characterized in immune cells. Identification of STIM1 as an endoplasmic reticulum Ca(2+) sensor and Orai1 as the pore subunit has dramatically advanced the molecular understanding of CRAC channels. Recent efforts have focused on understanding the physiological aspects of CRAC channels at an organism level using transgenic animal models and at a molecular level using electrophysiological and biochemical tools. In this review, we summarize our current understanding of the interacting partners of Orai and STIM proteins in the regulation of CRAC channel activity and other non-CRAC channel-related functions.

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.

Ca2+ signaling during mammalian fertilization: requirements, players, and adaptations

Changes in the intracellular concentration of calcium ([Ca(2+)](i)) represent a vital signaling mechanism enabling communication among cells and between cells and the environment. The initiation of embryo development depends on a [Ca(2+)](i) increase(s) in the egg, which is generally induced during fertilization. The [Ca(2+)](i) increase signals egg activation, which is the first stage in embryo development, and that consist of biochemical and structural changes that transform eggs into zygotes. The spatiotemporal patterns of [Ca(2+)](i) at fertilization show variability, most likely reflecting adaptations to fertilizing conditions and to the duration of embryonic cell cycles. In mammals, the focus of this review, the fertilization [Ca(2+)](i) signal displays unique properties in that it is initiated after gamete fusion by release of a sperm-derived factor and by periodic and extended [Ca(2+)](i) responses. Here, we will discuss the events of egg activation regulated by increases in [Ca(2+)](i), the possible downstream targets that effect these egg activation events, and the property and identity of molecules both in sperm and eggs that underpin the initiation and persistence of the [Ca(2+)](i) responses in these species.