Orai1 internalization and STIM1 clustering inhibition modulate SOCE inactivation during meiosis

Phosphorylation of STIM1 at ERK1/2 target sites regulates interaction with the microtubule plus-end binding protein EB1

STIM1 (stromal interaction molecule 1) is a key regulator of store-operated calcium entry (SOCE). Upon depletion of Ca(2+) concentration within the endoplasmic reticulum (ER), STIM1 relocalizes at ER-plasma membrane junctions, activating store-operated calcium channels (SOCs). Although the molecular details for STIM1-SOC binding is known, the regulation of SOCE remains largely unknown. A detailed list of phosphorylated residues within the STIM1 sequence has been reported. However, the molecular pathways controlling this phosphorylation and its function are still under study. Using phosphospecific antibodies, we demonstrate that ERK1/2 mediates STIM1 phosphorylation at Ser575, Ser608 and Ser621 during Ca(2+) store depletion, and that Ca(2+) entry and store refilling restore phosphorylation to basal levels. This phosphorylation occurs in parallel to the dissociation from end-binding protein 1 (EB1), a regulator of growing microtubule ends. Although Ser to Ala mutation of residues 575, 608 and 621 showed a constitutive binding to EB1 even after Ca(2+) store depletion, Ser to Glu mutation of these residues (to mimic the phosphorylation profile attained after store depletion) triggered full dissociation from EB1. Given that wild-type STIM1 and STIM1(S575E/S608E/S621E) activate SOCE similarly, a model is proposed to explain how ERK1/2-mediated phosphorylation of STIM1 regulates SOCE. This regulation is based on the phosphorylation of STIM1 to trigger dissociation from EB1 during Ca(2+) store depletion, an event that is fully reversed by Ca(2+) entry and store refilling.

Multifaceted roles of STIM proteins

Stromal interaction molecules (STIM1 and STIM2) are critical components of store-operated calcium entry. Sensing depletion of endoplasmic reticulum (ER) Ca(2+) stores, STIM couples with plasma membrane Orai channels, resulting in the influx of Ca(2+) across the PM into the cytosol. Although best recognized for their primary role as ER Ca(2+) sensors, increasing evidence suggests that STIM proteins have a broader variety of sensory capabilities than first envisaged, reacting to cell stressors such as oxidative stress, temperature, and hypoxia. Further, the array of partners for STIM proteins is now understood to range far beyond the Orai channel family. Here we discuss the implications of STIM's expanding role, both as a stress sensor and a general modulator of multiple physiological processes in the cell.

Ca2+ influx and the store-operated Ca2+ entry pathway undergo regulation during mouse oocyte maturation

Changes in Ca²⁺ homeostasis render oocytes competent to undergo [Ca²⁺]_i oscillations and activation. During mouse oocyte maturation Ca²⁺ influx and SOCE are down-regulated, whereas [Ca²⁺]_ER content increases. Bypassing the down-regulation of Ca²⁺ influx disturbs oocyte maturation.

In preparation for fertilization, mammalian oocytes undergo optimization of the mechanisms that regulate calcium homeostasis. Among these changes is the increase in the content of the Ca²⁺ stores ([Ca²⁺]_ER), a process that requires Ca²⁺ influx. Nevertheless, the mechanism(s) that mediates this influx remains obscure, although is known that [Ca²⁺]_ER can regulate Ca²⁺ influx via store-operated Ca²⁺ entry (SOCE). We find that during maturation, as [Ca²⁺]_ER increases, Ca²⁺ influx decreases. We demonstrate that mouse oocytes/eggs express the two molecular components of SOCE—stromal interaction molecule 1 (Stim1) and Orai1—and expression of human (h) Stim1 increases Ca²⁺ influx in a manner that recapitulates endogenous SOCE. We observe that the cellular distribution of hStim1 and hOrai1 during maturation undergoes sweeping changes that curtail their colocalization during the later stages of maturation. Coexpression of hStim1 and hOrai1 enhances influx throughout maturation but increases basal Ca²⁺ levels only in GV oocytes. Further, expression of a constitutive active form of hStim1 plus Orai1, which increases basal Ca²⁺ throughout maturation, disturbs resumption of meiosis. Taken together, our results demonstrate that Ca²⁺ influx and SOCE are regulated during maturation and that alteration of Ca²⁺ homeostasis undermines maturation in mouse oocytes.

The Xenopus oocyte: a single-cell model for studying Ca2+ signaling

In the four decades since the Xenopus oocyte was first demonstrated to have the capacity to translate exogenous mRNAs, this system has been exploited for many different experimental purposes. Typically, the oocyte is used either as a "biological test tube" for heterologous expression of proteins without any particular cell biological insight or, alternatively, it is used for applications where cell biology is paramount, such as investigations of the cellular adaptations that power early development. In this article, we discuss the utility of the Xenopus oocyte for studying Ca(2+) signaling in both these contexts.

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.

Downregulation of store-operated Ca2+ entry during mammalian meiosis is required for the egg-to-embryo transition

A specialized Ca(2+) transient at fertilization represents the universal driver for the egg-to-embryo transition. Ca(2+) signaling remodels during oocyte maturation to endow the egg with the capacity to produce the specialized Ca(2+) transient at fertilization, which takes the form of a single (e.g. Xenopus) or multiple (e.g. mouse) Ca(2+) spikes depending on the species. Store-operated Ca(2+) entry (SOCE) is the predominant Ca(2+) influx pathway in vertebrate oocytes, and in Xenopus SOCE completely inactivates during meiosis. Here, we show that SOCE is downregulated during mouse meiosis, but remains active in mature metaphase II eggs. SOCE inhibition is due to a decreased ability of the Ca(2+) sensor STIM1 to translocate to the cortical endoplasmic reticulum domain and due to internalization of Orai1. Reversing SOCE downregulation by overexpression of STIM1 and Orai1 prolongs the Ca(2+) oscillations at egg activation and disrupts the egg-to-embryo transition. Thus, SOCE downregulation during mammalian oocyte maturation is a crucial determinant of the fertilization-specific Ca(2+) transient, egg activation and early embryonic development.

Ion currents modulating oocyte maturation in animals

Growing oocytes are arrested at the first prophase of meiosis which is morphologically identified by the presence of a large and vesicular nucleus, called the germinal vesicle. The dissolution of the germinal vesicle marks the resumption of meiosis during which the oocyte undergoes massive modifications up to the second meiotic block, which is removed at fertilization. The interval between the first and the second meiotic block is defined as maturation and the events occurring during this period are crucial for ovulation, fertilization, and embryo development. Oocytes are excitable cells that react to stimuli by modifying their electrical properties as a consequence of ion currents flowing through ion channels on the plasma membrane. These electrical changes have been largely described at fertilization whereas little information is available during oocyte maturation. The aim of this review is to give an overview on the involvement of ion channels and ion currents during oocyte maturation in species from invertebrates to mammals. The results summarized here point to the possible functional role of ion channels underlying oocyte growth and maturation.

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.

How to make a good egg!: The need for remodeling of oocyte Ca(2+) signaling to mediate the egg-to-embryo transition

The egg-to-embryo transition marks the initiation of multicellular organismal development and is mediated by a specialized Ca(2+) transient at fertilization. This explosive Ca(2+) signal has captured the interest and imagination of scientists for many decades, given its cataclysmic nature and necessity for the egg-to-embryo transition. Learning how the egg acquires the competency to generate this Ca(2+) transient at fertilization is essential to our understanding of the mechanisms controlling egg and the transition to embryogenesis. In this review we discuss our current knowledge of how Ca(2+) signaling pathways remodel during oocyte maturation in preparation for fertilization with a special emphasis on the frog oocyte as additional reviews in this issue will touch on this in other species.

Ca(2+) homeostasis and regulation of ER Ca(2+) in mammalian oocytes/eggs

The activation of the developmental program in mammalian eggs relies on the initiation at the time of fertilization of repeated rises in the intracellular concentration of free calcium ([Ca(2+)](i)), also known as [Ca(2+)](i) oscillations. The ability to mount the full complement of oscillations is only achieved at the end of oocyte maturation, at the metaphase stage of meiosis II (MII). Over the last decades research has focused on addressing the mechanisms by which the sperm initiates the oscillations and identification of the channels that mediate intracellular Ca(2+) release. This review will describe the up-to-date knowledge of other aspects of Ca(2+) homeostasis in mouse oocytes, such as the mechanisms that transport Ca(2+) out of the cytosol into the endoplasmic reticulum (ER), the Ca(2+) store of the oocyte/egg, into other organelles and also those that extrude Ca(2+). Evidence pointing to channels in the plasma membrane that mediate Ca(2+) entry from the extracellular milieu, which is required for the persistence of the oscillations, is also discussed, along with the modifications that these mechanisms undergo during maturation. Lastly, we highlight areas where additional research is needed to obtain a better understating of the molecules and mechanisms that regulate Ca(2+) homeostasis in this unique Ca(2+) signaling system.

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.

Calcium signaling in mammalian egg activation and embryo development: the influence of subcellular localization

Calcium (Ca(2+) ) signals drive the fundamental events surrounding fertilization and the activation of development in all species examined to date. Initial studies of Ca(2+) signaling at fertilization in marine animals were tightly linked to new discoveries of bioluminescent proteins and their use as fluorescent Ca(2+) sensors. Since that time, there has been rapid progress in our understanding of the key functions for Ca(2+) in many cell types and of the impact of cellular localization on Ca(2+) signaling pathways. In this review, which focuses on mammalian egg activation, we consider how Ca(2+) is regulated and stored at different stages of oocyte development and examine the functions of molecules that serve as both regulators of Ca(2+) release and effectors of Ca(2+) signals. We then summarize studies exploring how Ca(2+) directs downstream effectors mediating both egg activation and later signaling events required for successful preimplantation embryo development. Throughout this review, we focus attention on how localization of Ca(2+) signals influences downstream signaling events, and attempt to highlight gaps in our knowledge that are ripe for future research.

Canonical transient receptor potential (TRPC) 1 acts as a negative regulator for vanilloid TRPV6-mediated Ca2+ influx

TRP proteins mostly assemble to homomeric channels but can also heteromerize, preferentially within their subfamilies. The TRPC1 protein is the most versatile member and forms various TRPC channel combinations but also unique channels with the distantly related TRPP2 and TRPV4. We show here a novel cross-family interaction between TRPC1 and TRPV6, a Ca²⁺ selective member of the vanilloid TRP subfamily. TRPV6 exhibited substantial co-localization and in vivo interaction with TRPC1 in HEK293 cells, however, no interaction was observed with TRPC3, TRPC4, or TRPC5. Ca²⁺ and Na⁺ currents of TRPV6-overexpressing HEK293 cells are significantly reduced by co-expression of TRPC1, correlating with a dramatically suppressed plasma membrane targeting of TRPV6. In line with their intracellular retention, remaining currents of TRPC1 and TRPV6 co-expression resemble in current-voltage relationship that of TRPV6. Studying the N-terminal ankyrin like repeat domain, structurally similar in the two proteins, we have found that these cytosolic segments were sufficient to mediate a direct heteromeric interaction. Moreover, the inhibitory role of TRPC1 on TRPV6 influx was also maintained by expression of only its N-terminal ankyrin-like repeat domain. Our experiments provide evidence for a functional interaction of TRPC1 with TRPV6 that negatively regulates Ca²⁺ influx in HEK293 cells.

Alternative translation initiation gives rise to two isoforms of Orai1 with distinct plasma membrane mobilities

Store-operated calcium entry is an almost ubiquitous signaling pathway in eukaryotic cells. The plasma membrane store-operated channels are comprised of subunits of the recently discovered Orai proteins, the major one being Orai1.We have discovered that native Orai1, as well as expressed Orai1, exists in two forms in similar quantities: a longer form (Orai1α) of approximately 33 kDa, and a shorter form (Orai1β) of approximately 23 kDa. The Orai1β form arises from alternative translation initiation from a methionine at position 64, and possibly also 71, in the longer Orai1α form. In the sequence upstream of the initiation site of Orai1β, there is a poly-arginine sequence previously suggested to be involved in interaction of Orai1 with plasma membrane phosphatidylinositol-4,5-bisphosphate. The loss of this phospholipid binding domain would be expected to influence the mobility of Orai1 protein in the plasma membrane. Indeed, experiments utilizing fluorescence recovery after photobleaching (FRAP) revealed that the recovery half-time for Orai1β was significantly faster than for Orai1α. Since Orai1 must diffuse to sites of interaction with the Ca(2+) sensor, STIM1, these two mobilities might provide for efficient recruitment of Orai1 subunits to sites of store-operated Ca(2+) entry during agonist-induced Ca(2+) signaling.

STIM1 is required for Ca2+ signaling during mammalian fertilization

During fertilization in mammals, a series of oscillations in the oocyte's intracellular free Ca(2+) concentration is responsible for oocyte activation and stimulation of embryonic development. The oscillations are associated with influx of Ca(2+) across the plasma membrane that is probably triggered by the depletion of the intracellular stores, a mechanism known as store-operated Ca(2+) entry. Recently, STIM1 has been identified in oocytes as a key component of the machinery that generates the Ca(2+) influx after store depletion. In this study, the involvement of STIM1 in the sperm-induced Ca(2+) oscillations and its significance in supporting subsequent embryo development were investigated. Downregulation of STIM1 levels in pig oocytes by siRNA completely inhibited the repetitive Ca(2+) signal triggered by the fertilizing sperm. In addition, a significantly lower percentage of oocytes cleaved or formed blastocysts when STIM1 was downregulated prior to fertilization compared to the control groups. Restoring STIM1 levels after fertilization in such oocytes by means of mRNA injection could not rescue embryonic development that in most cases was arrested at the 2-cell stage. On the other hand, STIM1 overexpression prior to fertilization did not alter the pattern of sperm-induced Ca(2+) oscillations and development of these fertilized oocytes up to the blastocyst stage was also similar to that registered in the control group. Finally, downregulation of STIM1 had no effect on oocyte activation when activation was stimulated artificially by inducing a single large elevation in the oocyte's intracellular free Ca(2+) concentration. These findings suggest that STIM1 is essential for normal fertilization as it is involved in the maintenance of the long-lasting repetitive Ca(2+) signal.

Regulation of store-operated calcium entry during cell division

Store-operate Ca2+ channels gate Ca2+ entry into the cytoplasm in response to the depletion of Ca2+ from endoplasmic reticulum Ca2+ stores. The major molecular components of store-operated Ca2+ entry are STIM (stromal-interacting molecule) 1 (and in some instances STIM2) that serves as the endoplasmic reticulum Ca2+ sensor, and Orai (Orai1, Orai2 and Orai3) which function as pore-forming subunits of the store-operated channel. It has been known for some time that store-operated Ca2+ entry is shut down during cell division. Recent work has revealed complex mechanisms regulating the functions and locations of both STIM1 and Orai1 in dividing cells.

The closing and opening of TRPC channels by Homer1 and STIM1

Influx of Ca(2+) is a central component of the receptor-evoked Ca(2+) signal. A ubiquitous form of Ca(2+) influx comes from Ca(2+) channels that are activated in response to depletion of the endoplasmic reticulum Ca(2+) stores and are thus named the store-operated Ca(2+) -influx channels (SOCs). One form of SOC is the transient receptor potential canonical (TRPC) channels. A major question in the field of Ca(2+) signalling is the molecular mechanism that regulates the opening and closing of these channels. All TRPC channels have a Homer-binding ligand and two conserved negative charges that interact with two terminal lysines of the stromal interacting molecule 1 (STIM1). The Homer and STIM1 sites are separated by only four amino acid residues. Based on available results, we propose a molecular mechanism by which Homer couples TRPC channels to IP(3) receptors (IP(3) Rs) to keep these channels in the closed state. Dissociation of the TRPCs-Homer-IP(3) Rs complex allows STIM1 access to the TRPC channels negative charges to gate open these channels.

Regulation of inositol 1,4,5-trisphosphate receptor function during mouse oocyte maturation

At the time of fertilization, an increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) underlies egg activation and initiation of development in all species studied to date. The inositol 1,4,5-trisphosphate receptor (IP(3)R1), which is mostly located in the endoplasmic reticulum (ER) mediates the majority of this Ca(2+) release. The sensitivity of IP(3)R1, that is, its Ca(2+) releasing capability, is increased during oocyte maturation so that the optimum [Ca(2+)](i) response concurs with fertilization, which in mammals occurs at metaphase of second meiosis. Multiple IP(3)R1 modifications affect its sensitivity, including phosphorylation, sub-cellular localization, and ER Ca(2+) concentration ([Ca(2+)](ER)). Here, we evaluated using mouse oocytes how each of these factors affected IP(3)R1 sensitivity. The capacity for IP(3)-induced Ca(2+) release markedly increased at the germinal vesicle breakdown stage, although oocytes only acquire the ability to initiate fertilization-like oscillations at later stages of maturation. The increase in IP(3)R1 sensitivity was underpinned by an increase in [Ca(2+)](ER) and receptor phosphorylation(s) but not by changes in IP(3)R1 cellular distribution, as inhibition of the former factors reduced Ca(2+) release, whereas inhibition of the latter had no impact. Therefore, the results suggest that the regulation of [Ca(2+)](ER) and IP(3)R1 phosphorylation during maturation enhance IP(3)R1 sensitivity rendering oocytes competent to initiate oscillations at the expected time of fertilization. The temporal discrepancy between the initiation of changes in IP(3)R1 sensitivity and acquisition of mature oscillatory capacity suggest that other mechanisms that regulate Ca(2+) homeostasis also shape the pattern of oscillations in mammalian eggs.

Structure, regulation and biophysics of I(CRAC), STIM/Orai1

Ca(2+) release activated Ca(2+) (CRAC) channels mediate robust Ca(2+) influx when the endoplasmic reticulum Ca(2+) stores are depleted. This essential process for T-cell activation as well as degranulation of mast cells involves the Ca(2+) sensor STIM1, located in the endoplasmic reticulum and the Ca(2+) selective Orai1 channel in the plasma membrane. Our review describes the CRAC signaling pathway, the activation of which is initiated by a drop in the endoplasmic Ca(2+) level sensed by STIM1. This in term induces multimerisation and puncta-formation of STIM1 proteins is followed by their coupling to and activation of Orai channels. Consequently Ca(2+) entry is triggered through the Orai pore into the cytosol with subsequent closure of the channel by Ca(2+)-dependent inactivation. We will portray a mechanistic view of the events coupling STIM1 to Orai activation based on their structure and biophysics.

The TR (i)P to Ca²⁺ signaling just got STIMy: an update on STIM1 activated TRPC channels

Calcium is a ubiquitous signaling molecule, indispensable for cellular metabolism of organisms from unicellular life forms to higher eukaryotes. The biological function of most eukaryotic cells is uniquely regulated by changes in cytosolic calcium, which is largely achieved by the universal phenomenon of store-operated calcium entry (SOCE). The canonical TRPs and Orai channels have been described as the molecular components of the store-operated calcium channels (SOCC). Importantly, the ER calcium-sensor STIM1 has been shown to initiate SOCE via gating of SOCC. Since the discovery of STIM1, as the critical regulator of SOCE, there has been a flurry of observations suggesting its obligatory role in regulating TRPC and Orai channel function. Considerable effort has been made to identify the molecular details as how STIM1 activates SOCC. In this context, findings as of yet has substantially enriched our understanding on, the modus operandi of SOCE, the distinct cellular locales that organize STIM1-SOCC complexes, and the physiological outcomes entailing STIM1-activated SOCE. In this review we discuss TRPC channels and provide an update on their functional regulation by STIM1.

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

Store-operated calcium channels (SOCs) are a nearly ubiquitous Ca(2+) entry pathway stimulated by numerous cell surface receptors via the reduction of Ca(2+) concentration in the ER. The discovery of STIM proteins as ER Ca(2+) sensors and Orai proteins as structural components of the Ca(2+) release-activated Ca(2+) (CRAC) channel, a prototypic SOC, opened the floodgates for exploring the molecular mechanism of this pathway and its functions. This review focuses on recent advances made possible by the use of STIM and Orai as molecular tools. I will describe our current understanding of the store-operated Ca(2+) entry mechanism and its emerging roles in physiology and disease, areas of uncertainty in which further progress is needed, and recent findings that are opening new directions for research in this rapidly growing field.

Endoplasmic reticulum remodeling tunes IP₃-dependent Ca²+ release sensitivity

The activation of vertebrate development at fertilization relies on IP₃-dependent Ca²⁺ release, a pathway that is sensitized during oocyte maturation. This sensitization has been shown to correlate with the remodeling of the endoplasmic reticulum into large ER patches, however the mechanisms involved are not clear. Here we show that IP₃ receptors within ER patches have a higher sensitivity to IP₃ than those in the neighboring reticular ER. The lateral diffusion rate of IP₃ receptors in both ER domains is similar, and ER patches dynamically fuse with reticular ER, arguing that IP₃ receptors exchange freely between the two ER compartments. These results suggest that increasing the density of IP₃ receptors through ER remodeling is sufficient to sensitize IP₃-dependent Ca²⁺ release. Mathematical modeling supports this concept of ‘geometric sensitization’ of IP₃ receptors as a population, and argues that it depends on enhanced Ca²⁺-dependent cooperativity at sub-threshold IP₃ concentrations. This represents a novel mechanism of tuning the sensitivity of IP₃ receptors through ER remodeling during meiosis.

Calcium signaling in mouse oocyte maturation: the roles of STIM1, ORAI1 and SOCE

Calcium handling is critical for the oocyte function, since the first steps of fertilization are dependent on the appropriate Ca(2+) mobilization to originate transient spikes of the cytosolic Ca(2+) concentration. It is well known that the Ca(2+) influx from the extracellular milieu is required to maintain this signaling in mammalian oocytes. However, the regulation of the Ca(2+) channels involved in this process is still unknown in oocytes. STIM1, a key regulator of store-operated Ca(2+) entry (SOCE), relocates in the mouse oocyte shortly after sperm stimulation, suggesting that SOCE is involved in the maintenance of cytosolic Ca(2+)-spiking in the fertilized oocyte. Here, we show that there is an up-regulation of the expression of STIM1 at the germinal vesicle breakdown stage, and this expression remains steady during following maturation stages. We found that oocytes express ORAI1, a store-operated Ca(2+) channel, and that ORAI1 expression level was stable during oocyte maturation. Immature oocytes showed no Ca(2+) entry and no increase in STIM1-ORAI1 colocalization in response to the store depletion induced by thapsigargin. On the contrary, in mature oocytes, STIM1-ORAI1 colocalization is enhanced 3-fold by depletion of Ca(2+) stores, enabling the activation of store-operated calcium channels and therefore Ca(2+) entry. Finally, the correlation between SOCE activation during the maturation of oocytes and STIM1-ORAI1 colocalization strongly suggests that ORAI1 is involved in the Ca(2+) entry pathway in the mature oocyte. SOCE up-regulation in the final stage of maturation is further evidence of a major role for SOCE in fully mature oocytes, and therefore in Ca(2+) signaling at fertilization.

Remodelling of the endoplasmic reticulum during store-operated calcium entry

SOCE (store-operated calcium entry) is a ubiquitous cellular mechanism linking the calcium depletion of the ER (endoplasmic reticulum) to the activation of PM (plasma membrane) Ca2+-permeable channels. The activation of SOCE channels favours the entry of extracellular Ca2+ into the cytosol, thereby promoting the refilling of the depleted ER Ca2+ stores as well as the generation of long-lasting calcium signals. The molecules that govern SOCE activation comprise ER Ca2+ sensors [STIM1 (stromal interaction molecule 1) and STIM2], PM Ca2+-permeable channels {Orai and TRPC [TRP (transient receptor potential) canonical]} and regulatory Ca2+-sensitive cytosolic proteins {CRACR2 [CRAC (Ca2+ release-activated Ca2+ current) regulator 2]}. Upon Ca2+ depletion of the ER, STIM molecules move towards the PM to bind and activate Orai or TRPC channels, initiating calcium entry and store refilling. This molecular rearrangement is accompanied by the formation of specialized compartments derived from the ER, the pre-cER (cortical ER) and cER. The pre-cER appears on the electron microscope as thin ER tubules enriched in STIM1 that extend along microtubules and that are devoid of contacts with the PM. The cER is located in immediate proximity to the PM and comprises thinner sections enriched in STIM1 and devoid of chaperones that might be dedicated to calcium signalling. Here, we review the molecular interactions and the morphological changes in ER structure that occur during the SOCE process.

Cell proliferation, calcium influx and calcium channels

Both increases in the basal cytosolic calcium concentration ([Ca(2+)](cyt)) and [Ca(2+)](cyt) transients play major roles in cell cycle progression, cell proliferation and division. Calcium transients are observed at various stages of cell cycle and more specifically during late G(1) phase, before and during mitosis. These calcium transients are mainly due to calcium release and reuptake by the endoplasmic reticulum (ER) and are observed over periods of hours in oocytes and mammalian cells. Calcium entry sustains the ER Ca(2+) load and thereby helps to maintain these calcium transients for such a long period. Calcium influx also controls cell growth and proliferation in several cell types. Various calcium channels are involved in this process and the tight relation between the expression and activity of cyclins and calcium channels also suggests that calcium entry may be needed only at particular stages of the cell cycle. Consistent with this idea, the expression of l-type and T-type calcium channels and SOCE amplitude fluctuate along the cell cycle. But, as calcium influx regulates several other transduction pathways, the presence of a specific connection to trigger activation of proliferation and cell division in mammalian cells will be discussed in this review.

Ca2+ signaling during maturation of cumulus-oocyte complex in mammals

Under the influence of gonadotropins or growth factors, a close cooperation develops between cumulus cells and the oocyte that is implicated in transmitting signals involved in maintaining or releasing the meiotic arrest in the oocyte. While cyclic adenosine 5'-monophosphate (cAMP) is a key molecule in maintaining the meiotic arrest, calcium (Ca(2+)) may play a role in controlling either spontaneous or gonadotropin-induced oocyte maturation, possibly by modulating intracytoplasmic cAMP concentrations via Ca(2+)-sensitive adenylate cyclases. This review focuses on the mechanisms related to the origin of the Ca(2+) wave that travels from the cumulus cells to the oocyte, and discusses the source of variations affecting the dynamics of this wave.

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.

Activation and regulation of store-operated calcium entry

The process of store-operated Ca²⁺ entry (SOCE), whereby Ca²⁺ influx across the plasma membrane is activated in response to depletion of intracellular Ca²⁺ stores in the endoplasmic reticulum (ER), has been under investigation for greater than 25 years; however, only in the past 5 years have we come to understand this mechanism at the molecular level. A surge of recent experimentation indicates that STIM molecules function as Ca²⁺ sensors within the ER that, upon Ca²⁺ store depletion, rearrange to sites very near to the plasma membrane. At these plasma membrane-ER junctions, STIM interacts with and activates SOCE channels of the Orai family. The molecular and biophysical data that have led to these findings are discussed in this review, as are several controversies within this rapidly expanding field.

Role of the STIM1 C-terminal domain in STIM1 clustering

Store-operated Ca(2+) entry (SOCE) represents a ubiquitous Ca(2+) influx pathway activated by the filling state of intracellular Ca(2+) stores. SOCE is mediated by coupling of STIM1, the endoplasmic reticulum Ca(2+) sensor, to the Orai1 channel. SOCE inactivates during meiosis, partly because of the inability of STIM1 to cluster in response to store depletion. STIM1 has several functional domains, including the Orai1 interaction domain (STIM1 Orai Activating Region (SOAR) or CRAC Activation Domain (CAD)) and STIM1 homomerization domain. When Ca(2+) stores are full, these domains are inactive to prevent constitutive Ca(2+) entry. Here we show, using the Xenopus oocyte as an expression system, that the C-terminal 200 residues of STIM1 are important to maintain STIM1 in an inactive state when Ca(2+) stores are full, through predicted intramolecular shielding of the active STIM1 domains (SOAR/CAD and STIM1 homomerization domain). Interestingly, our data argue that the C-terminal 200 residues accomplish this through a steric hindrance mechanism because they can be substituted by GFP or mCherry while maintaining all aspects of STIM1 function. We further show that STIM1 clustering inhibition during meiosis is independent of the C-terminal 200 residues.

The role of store-operated calcium influx in skeletal muscle signaling

In cardiac and skeletal muscle Ca(2+) release from intracellular stores triggers actomyosin cross-bridge formation and the generation of contractile force. In the face of large fluctuations of intracellular calcium ([Ca(2+)](i)) that occur with contractile activity, myocytes are able to sense and respond to changes in workload and patterns of activation through calcium signaling pathways which modulate gene expression and cellular metabolism. Store-operated calcium influx has emerged as a mechanism by which calcium signaling pathways are activated in order to respond to the changing demands of the myocyte. Abnormalities of store-operated calcium influx may contribute to maladaptive muscle remodeling in multiple disease states. The importance of store-operated calcium influx in muscle is confirmed in mice lacking STIM1 which die perinatally and in patients with mutations on STIM1 or Orai1 who exhibit a myopathy exhibited by hypotonia. In this review, we consider the role of store-operated Ca(2+) entry into skeletal muscle as a critical mediator of Ca(2+) dependent gene expression and how alterations in Ca(2+) influx may influence muscle development and disease.

Constitutive recycling of the store-operated Ca2+ channel Orai1 and its internalization during meiosis

The egg's competency to activate at fertilization and transition to embryogenesis is dependent on its ability to generate a fertilization-specific Ca(2+) transient. To endow the egg with this capacity, Ca(2+) signals remodel during oocyte maturation, including inactivation of the primary Ca(2+) influx pathway store-operated Ca(2+) entry (SOCE). SOCE inactivation is coupled to internalization of the SOCE channel, Orai1. In this study, we show that Orai1 internalizes during meiosis through a caveolin (Cav)- and dynamin-dependent endocytic pathway. Cav binds to Orai1, and we map a Cav consensus-binding site in the Orai1 N terminus, which is required for Orai1 internalization. Furthermore, at rest, Orai1 actively recycles between an endosomal compartment and the cell membrane through a Rho-dependent endocytic pathway. A significant percentage of total Orai1 is intracellular at steady state. Store depletion completely shifts endosomal Orai1 to the cell membrane. These results define vesicular trafficking mechanisms in the oocyte that control Orai1 subcellular localization at steady state, during meiosis, and after store depletion.

Ca(2+) signaling, genes and the cell cycle

Changes in the concentration and spatial distribution of Ca(2+) ions in the cytoplasm constitute a ubiquitous intracellular signaling module in cellular physiology. With the advent of Ca(2+) dyes that allow direct visualization of Ca(2+) transients, combined with powerful experimental tools such as electrophysiological recordings, intracellular Ca(2+) transients have been implicated in practically every aspect of cellular physiology, including cellular proliferation. Ca(2+) signals are associated with different phases of the cell cycle and interfering with Ca(2+) signaling or downstream pathways often disrupts progression of the cell cycle. Although there exists a dependence between Ca(2+) signals and the cell cycle the mechanisms involved are not well defined and given the cross-talk between Ca(2+) and other signaling modules, it is difficult to assess the exact role of Ca(2+) signals in cell cycle progression. Two exceptions however, include fertilization and T-cell activation, where well-defined roles for Ca(2+) signals in mediating progression through specific stages of the cell cycle have been clearly established. In the case of T-cell activation Ca(2+) regulates entry into the cell cycle through the induction of gene transcription.

Biphasic effects of nitric oxide on calcium influx in human platelets

In this study the effects of nitric oxide (NO) donors on intracellular free calcium ([Ca(2+)](i)) in human platelets was examined. Inhibition of guanylyl cyclase (GC) with either methylene blue or ODQ slightly inhibited the ability of submaximal concentrations of thrombin to increase [Ca(2+)](i) which suggests that a small portion of the thrombin mediated increase in [Ca(2+)](i) was due to an increase in NO and subsequent increase in cGMP and activation of cGMP dependent protein kinase (cGPK). Thrombin predominantly increases [Ca(2+)](i) by stimulating store-operated Ca(2+) entry (SOCE). The NO donor GEA3162 was previously shown to stimulate SOCE in some cells. In platelets GEA3162 had no effect to increase [Ca(2+)](i) however it inhibited the ability of thrombin to increase [Ca(2+)](i) and this effect was reversed by ODQ. The addition of low concentrations (2.0 - 20 nM) of the NO donor sodium nitroprusside (SNP) slightly potentiated the ability of thrombin to increase [Ca(2+)](i) whereas higher concentrations (>200 nM) of SNP inhibited thrombin induced increases in [Ca(2+)](i). Both of these effects of SNP were reversed by ODQ which implies that they were both mediated by cGPK. Ba(2+) influx was stimulated by low concentrations (2.0 nM) of SNP and inhibited by high concentrations (>200 nM) of SNP and both effects were inhibited by ODQ. Previous studies showed that Ba(2+) influx was blocked by the SOCE inhibitors 2-aminoethoxydipheny borate and diethylstilbestrol. It was concluded that low levels of SNP can stimulate SOCE in platelets and this effect may account for the increased aggregation and secretion previously observed with low concentrations of NO donors. Of the proteins known to be involved in SOCE (e.g. stromal interaction molecule 1 (Stim1), Stim2 and Orai1) only Stim2 has cGPK phosphorylation sites. The possibility that Stim2 phosphorylation regulates SOCE in platelets is discussed.

Regulation of store-operated Ca2+ entry during the cell cycle

Cytoplasmic Ca(2+) signals are central to numerous cell physiological processes, including cellular proliferation. Historically, much of the research effort in this area has focused on the role of Ca(2+) signals in cell-cycle progression. It is becoming clear, however, that the relationship between Ca(2+) signaling and the cell cycle is a 'two-way street'. Specifically, Ca(2+)-signaling pathways are remodeled during M phase, leading to altered Ca(2+) dynamics. Such remodeling probably better serves the large variety of functions that cells must perform during cell division compared with during interphase. This is clearly the case during oocyte meiosis, because remodeling of Ca(2+) signals partially defines the competence of the egg to activate at fertilization. Store-operated Ca(2+) entry (SOCE) is a ubiquitous Ca(2+)-signaling pathway that is regulated during M phase. In this Commentary, we discuss the latest advances in our understanding of how SOCE is regulated during cell division.

Phosphorylation of STIM1 at ERK1/2 target sites modulates store-operated calcium entry

Store-operated calcium entry (SOCE) is an important Ca2+ entry pathway that regulates many cell functions. Upon store depletion, STIM1, a transmembrane protein located in the endoplasmic reticulum (ER), aggregates and relocates close to the plasma membrane (PM) where it activates store-operated calcium channels (SOCs). Although STIM1 was early defined as a phosphoprotein, the contribution of the phosphorylation has been elusive. In the present work, STIM1 was found to be a target of extracellular-signal-regulated kinases 1 and 2 (ERK1/2) in vitro, and we have defined the ERK1/2-phosphorylated sites on the STIM1 sequence. Using HEK293 cells stably transfected for the expression of tagged STIM1, we found that alanine substitution mutants of ERK1/2 target sites reduced SOCE significantly, suggesting that phosphorylation of these residues are required to fully accomplish SOCE. Indeed, the ERK1/2 inhibitors PD184352 and PD0325901 decreased SOCE in transfected cells. Conversely, 12-O-tetradecanoylphorbol-13-acetate, which activates ERK1/2, enhanced SOCE in cells expressing wild-type tagged STIM1, but did not potentiate Ca2+ influx in cells expressing serine to alanine mutations in ERK1/2 target sites of STIM1. Alanine substitution mutations decreased Ca2+ influx without disturbing the aggregation of STIM1 upon store depletion and without affecting the relocalization in ER-PM punctae. However, our results suggest that STIM1 phosphorylation at ERK1/2 target sites can modulate SOCE by altering STIM1 binding to SOCs, because a significant decrease in FRET efficiency was observed between alanine substitution mutants of STIM1-GFP and ORAI1-CFP.

ORAI-mediated calcium entry: mechanism and roles, diseases and pharmacology

ORAI1 is a protein located on the plasma membrane that acts as a calcium channel. Calcium enters via ORAI1 as a mechanism to refill the sarcoplasmic/endoplasmic reticulum calcium stores, the depletion of which can be detected by the sensor protein STIM1. Isoforms of these proteins ORAI2, ORAI3 and STIM2 also have roles in cellular calcium homeostasis but are less well characterized. This pathway of filling the calcium stores is termed store-operated calcium entry and while the pathway itself was proposed in 1986, the identity of the key molecular components was only discovered in 2005 and 2006. The characterization of the ORAI and STIM proteins has provided clearer information on some calcium-regulated pathways that are important in processes from gene transcription to immune cell function. Recent studies have also suggested the importance of the components of ORAI-mediated calcium entry in some diseases or processes significant in disease including the migration of breast cancer cells and thrombus formation. This review will provide a brief overview of ORAI-mediated calcium entry, its role in physiological and pathophysiological processes, as well as current and potential pharmacological modulators of the components of this important cellular calcium entry pathway.

Effect of cell swelling on ER/PM junctional interactions and channel assembly involved in SOCE

Store-operated calcium entry (SOCE) regulates critical cellular functions and is determined by precise ER/plasma membrane (PM) junctional interactions. Here we have assessed the effect of hypotonic cell volume increase on SOCE in a salivary gland epithelial cell line (HSG). Thapsigargin (Tg) activated a 2APB- and 1microM Gd(3+)-sensitive, inwardly rectifying, cation current, I(SOC), while hypotonic solution (150mOsm) induced cell swelling and activated an outwardly rectifying cation current that was blocked by 100microM Gd(3+) but not by 2APB. HTS addition before or after Tg attenuated the sensitivity of Ca(2+) influx to 2APB and 1microM Gd(3+). After HTS-induced volume increase, while stimulation of cells with Tg resulted in intracellular Ca(2+) release without Ca(2+) influx, stimulation with CCh caused neither internal Ca(2+) release nor Ca(2+) influx. Importantly, HTS caused the ER to recede from the plasma membrane which prevented Tg-stimulated clustering of STIM1 in the ER/PM region and association of STIM1 with TRPC1 and Orai1. Disruption of SOCE was dependent on the level of hypotonic stress as 225mOsm HTS induced relatively less cell swelling or disruption of SOCE. These results demonstrate that epithelial cells can tolerate small increases (up to 5%) in cell volume while larger increases lead to disruption of ER-PM interactions that are critical for activation of SOCE. We suggest that loss of SOCE could impact cell function and contribute to the deleterious effects of severe hypotonic stress.

A novel native store-operated calcium channel encoded by Orai3: selective requirement of Orai3 versus Orai1 in estrogen receptor-positive versus estrogen receptor-negative breast cancer cells

Store-operated calcium (Ca(2+)) entry (SOCE) mediated by STIM/Orai proteins is a ubiquitous pathway that controls many important cell functions including proliferation and migration. STIM proteins are Ca(2+) sensors in the endoplasmic reticulum and Orai proteins are channels expressed at the plasma membrane. The fall in endoplasmic reticulum Ca(2+) causes translocation of STIM1 to subplasmalemmal puncta where they activate Orai1 channels that mediate the highly Ca(2+)-selective Ca(2+) release-activated Ca(2+) current (I(CRAC)). Whereas Orai1 has been clearly shown to encode SOCE channels in many cell types, the role of Orai2 and Orai3 in native SOCE pathways remains elusive. Here we analyzed SOCE in ten breast cell lines picked in an unbiased way. We used a combination of Ca(2+) imaging, pharmacology, patch clamp electrophysiology, and molecular knockdown to show that native SOCE and I(CRAC) in estrogen receptor-positive (ER(+)) breast cancer cell lines are mediated by STIM1/2 and Orai3 while estrogen receptor-negative (ER(-)) breast cancer cells use the canonical STIM1/Orai1 pathway. The ER(+) breast cancer cells represent the first example where the native SOCE pathway and I(CRAC) are mediated by Orai3. Future studies implicating Orai3 in ER(+) breast cancer progression might establish Orai3 as a selective target in therapy of ER(+) breast tumors.

An endoplasmic reticulum/plasma membrane junction: STIM1/Orai1/TRPCs

Ca(2+) entering cells through store-operated channels (SOCs) affects most cell functions, and excess SOC is associated with pathologies. The molecular makeup of SOCs and their mechanisms of gating were clarified with the discovery of the Orais and STIM1. Another form of SOCs are the TRPCs. STIM1 gates both Orai and TRPC channels but does so by different mechanisms. Although the STIM1 SOAR domain mediates the binding of STIM1 to both channel types, SOAR is sufficient to open the Orais but the STIM1 polylysine domain mediates opening of the TRPC channels. This short review discusses recent findings on how STIM1 gates and regulates the Orais and TRPCs, and how the STIM1/Orai1/TRPCs complexes may function in vivo to mediate SOC activity.