Regulation of store-operated calcium entry during cell division

Mitosis, Focus on Calcium

The transformation of a single fertilised egg into an adult human consisting of tens of trillions of highly diverse cell types is a marvel of biology. The expansion is largely achieved by cell duplication through the process of mitosis. Mitosis is essential for normal growth, development, and tissue repair and is one of the most tightly regulated biological processes studied. This regulation is designed to ensure accurate segregation of chromosomes into each new daughter cell since errors in this process can lead to genetic imbalances, aneuploidy, that can lead to diseases including cancer. Understanding how mitosis operates and the molecular mechanisms that ensure its fidelity are therefore not only of significant intellectual value but provide unique insights into disease pathology. The purpose of this review is to revisit historical evidence that mitosis can be influenced by the ubiquitous second messenger calcium and to discuss this in the context of new findings revealing exciting new information about its role in cell division.

Ca2+ Release by IP3 Receptors Is Required to Orient the Mitotic Spindle

The mitotic spindle distributes chromosomes evenly to daughter cells during mitosis. The orientation of the spindle, guided by internal and external cues, determines the axis of cell division and thereby contributes to tissue morphogenesis. Progression through mitosis requires local Ca²⁺ signals at critical steps, and because store-operated Ca²⁺ entry is inhibited during mitosis, those signals probably require Ca²⁺ release through inositol 1,4,5-trisphosphate receptors (IP₃Rs). In cells without IP₃Rs, astral microtubules around the daughter centrosome are shorter than those at the mother centrosome, and the mitotic spindle fails to align with the substratum during metaphase. The misalignment is due to the spindle ineffectively detecting internal cues rather than a failure of cells to recognize the substratum. Expression of type 3 IP₃R is sufficient to rescue spindle alignment, but only if the IP₃R has a functional pore. We conclude that Ca²⁺ signals evoked by IP₃Rs are required to orient the mitotic spindle.

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IP₃ receptors are required for mitotic spindle orientation

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Only IP₃ receptors with a functional channel restore spindle orientation

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Ca²⁺ release through IP₃ receptors is required for spindle orientation

Kv10.1 Regulates Microtubule Dynamics during Mitosis

Kv10.1 (potassium voltage-gated channel subfamily H member 1, known as EAG1 or Ether-à-go-go 1), is a voltage-gated potassium channel, prevailingly expressed in the central nervous system. The aberrant expression of Kv10.1 is detected in over 70% of all human tumor tissues and correlates with poorer prognosis. In peripheral tissues, Kv10.1 is expressed almost exclusively during the G2/M phase of the cell cycle and regulates its progression-downregulation of Kv10.1 extends the duration of the G2/M phase both in cancer and healthy cells. Here, using biochemical and imaging techniques, such as live-cell measurements of microtubule growth and of cytosolic calcium, we elucidate the mechanisms of Kv10.1-mediated regulation at the G2/M phase. We show that Kv10.1 has a dual effect on mitotic microtubule dynamics. Through the functional interaction with ORAI1 (calcium release-activated calcium channel protein 1), it modulates cytosolic calcium oscillations, thereby changing microtubule behavior. The inhibition of either Kv10.1 or ORAI1 stabilizes the microtubules. In contrast, the knockdown of Kv10.1 increases the dynamicity of mitotic microtubules, resulting in a stronger spindle assembly checkpoint, greater mitotic spindle angle, and a decrease in lagging chromosomes. Understanding of Kv10.1-mediated modulation of the microtubule architecture will help to comprehend how cancer tissue benefits from the presence of Kv10.1, and thereby increase the efficacy and safety of Kv10.1-directed therapeutic strategies.

Modulators of calcium signalling at fertilization

Calcium (Ca2+) signals initiate egg activation across the animal kingdom and in at least some plants. These signals are crucial for the success of development and, in the case of mammals, health of the offspring. The mechanisms associated with fertilization that trigger these signals and the molecules that regulate their characteristic patterns vary widely. With few exceptions, a major contributor to fertilization-induced elevation in cytoplasmic Ca2+ is release from endoplasmic reticulum stores through the IP3 receptor. In some cases, Ca2+ influx from the extracellular space and/or release from alternative intracellular stores contribute to the rise in cytoplasmic Ca2+. Following the Ca2+ rise, the reuptake of Ca2+ into intracellular stores or efflux of Ca2+ out of the egg drive the return of cytoplasmic Ca2+ back to baseline levels. The molecular mediators of these Ca2+ fluxes in different organisms include Ca2+ release channels, uptake channels, exchangers and pumps. The functions of these mediators are regulated by their particular activating mechanisms but also by alterations in their expression and spatial organization. We discuss here the molecular basis for modulation of Ca2+ signalling at fertilization, highlighting differences across several animal phyla, and we mention key areas where questions remain.

A centrosome-localized calcium signal is essential for mammalian cell mitosis

Mitosis defects can lead to premature ageing and cancer. Understanding mitosis regulation therefore has important implications for human disease. Early data suggested that calcium (Ca2+) signals could influence mitosis, but these have hitherto not been observed in mammalian cells. Here, we reveal a prolonged yet spatially restricted Ca2+ signal at the centrosomes of actively dividing cells. Local buffering of the centrosomal Ca2+ signals, by flash photolysis of the caged Ca2+ chelator diazo-2-acetoxymethyl ester, arrests mitosis. We also provide evidence that this Ca2+ signal emanates from the endoplasmic reticulum. In summary, we characterize a unique centrosomal Ca2+ signal as a functionally essential input into mitosis.-Helassa, N., Nugues, C., Rajamanoharan, D., Burgoyne, R. D., Haynes, L. P. A centrosome-localized calcium signal is essential for mammalian cell mitosis.

Sequential activation of STIM1 links Ca2+ with luminal domain unfolding

The stromal interaction molecule 1 (STIM1) has two important functions, Ca2+ sensing within the endoplasmic reticulum and activation of the store-operated Ca2+ channel Orai1, enabling plasma-membrane Ca2+ influx. We combined molecular dynamics (MD) simulations with live-cell recordings and determined the sequential Ca2+-dependent conformations of the luminal STIM1 domain upon activation. Furthermore, we identified the residues within the canonical and noncanonical EF-hand domains that can bind to multiple Ca2+ ions. In MD simulations, a single Ca2+ ion was sufficient to stabilize the luminal STIM1 complex. Ca2+ store depletion destabilized the two EF hands, triggering disassembly of the hydrophobic cleft that they form together with the stable SAM domain. Point mutations associated with tubular aggregate myopathy or cancer that targeted the canonical EF hand, and the hydrophobic cleft yielded constitutively clustered STIM1, which was associated with activation of Ca2+ entry through Orai1 channels. On the basis of our results, we present a model of STIM1 Ca2+ binding and refine the currently known initial steps of STIM1 activation on a molecular level.

Transient receptor potential channel 6 knockdown prevents apoptosis of renal tubular epithelial cells upon oxidative stress via autophagy activation

Reactive oxygen species (ROS) are generated under various pathological conditions such as renal ischemia/reperfusion (I/R) injury and provoke damage to multiple cellular organelles and processes. Overproduction of ROS causes oxidative stress and contributes to damages of renal proximal tubular cells (PTC), which are the main cause of the pathogenesis of renal I/R injury. Autophagy is a dynamic process that removes long-lived proteins and damaged organelles via lysosome-mediated degradation, which has an antioxidant effect that relieves oxidative stress. The canonical transient receptor potential channel 6 (TRPC6), a nonselective cation channel that allows passage of Ca²⁺, plays an important role in renal disease. Yet, the relationship between TRPC6 and autophagy, as well as their functions in renal oxidative stress injury, remains unclear. In this study, we found that oxidative stress triggered TRPC6-dependent Ca²⁺ influx in PTC to inhibit autophagy, thereby rendering cells more susceptible to death. We also demonstrated that TRPC6 knockout (TRPC6^-/-) or inhibition by SAR7334, a TRPC6-selective inhibitor, increased autophagic flux and mitigated oxidative stress-induced apoptosis of PTC. The protective effects of TRPC6 ablation were prevented by autophagy inhibitors Chloroquine and Bafilomycin A1. Moreover, this study also shows that TRPC6 blockage promotes autophagic flux via inhibiting the PI3K/Akt/mTOR and ERK1/2 signaling pathways. This is the first evidence showing that TRPC6-mediated Ca²⁺ influx plays a novel role in suppressing cytoprotective autophagy triggered by oxidative stress in PTC, and it may become a novel therapeutic target for the treatment of renal oxidative stress injury in the future.

A Calpain-Like Protein Is Involved in the Execution Phase of Programmed Cell Death of Entamoeba histolytica

Oxygen or nitrogen oxidative species and chemical stress induce the programmed cell death (PCD) of Entamoeba histolytica trophozoites. PCD caused by the aminoglycoside G418 is reduced by incubation with the cysteine protease inhibitor E-64; however, no typical caspases or metacaspases have been detected in this parasite. Calpain, a cysteine protease activated by calcium, has been suggested to be part of a specific PCD pathway in this parasite because the specific calpain inhibitor Z-Leu-Leu-Leu-al diminishes the PCD of trophozoites. Here, we predicted the hypothetical 3D structure of a calpain-like protein of E. histolytica and produced specific antibodies against it. We detected the protein in the cytoplasm and near the nucleus. Its expression gradually increased during incubation with G418, with the highest level after 9 h of treatment. In addition, a specific calpain-like siRNA sequence reduced the cell death rate by 65%. All these results support the hypothesis that the calpain-like protein is one of the proteases involved in the execution phase of PCD in E. histolytica. The hypothetical interactome of the calpain-like protein suggests that it may activate or regulate other proteins that probably participate in PCD, including those with EF-hand domains or other calcium-binding sites.

Evidence for the interaction of peroxiredoxin-4 with the store-operated calcium channel activator STIM1 in liver cells

Ca²⁺ entry through store-operated Ca²⁺ channels (SOCs) in the plasma membrane (PM) of hepatocytes plays a central role in the hormonal regulation of liver metabolism. SOCs are composed of Orai1, the channel pore protein, and STIM1, the activator protein, and are regulated by hormones and reactive oxygen species (ROS). In addition to Orai1, STIM1 also interacts with several other intracellular proteins. Most previous studies of the cellular functions of Orai1 and STIM1 have employed immortalised cells in culture expressing ectopic proteins tagged with a fluorescent polypeptide such as GFP. Little is known about the intracellular distributions of endogenous Orai1 and STIM1. The aims are to determine the intracellular distribution of endogenous Orai1 and STIM1 in hepatocytes and to identify novel STIM1 binding proteins. Subcellular fractions of rat liver were prepared by homogenisation and differential centrifugation. Orai1 and STIM1 were identified and quantified by western blot. Orai1 was found in the PM (0.03%), heavy (44%) and light (27%) microsomal fractions, and STIM1 in the PM (0.09%), and heavy (85%) and light (13%) microsomal fractions. Immunoprecipitation of STIM1 followed by LC/MS or western blot identified peroxiredoxin-4 (Prx-4) as a potential STIM1 binding protein. Prx-4 was found principally in the heavy microsomal fraction. Knockdown of Prx-4 using siRNA, or inhibition of Prx-4 using conoidin A, did not affect Ca²⁺ entry through SOCs but rendered SOCs susceptible to inhibition by H₂O₂. It is concluded that, in hepatocytes, a considerable proportion of endogenous Orai1 and STIM1 is located in the rough ER. In the rough ER, STIM1 interacts with Prx-4, and this interaction may contribute to the regulation by ROS of STIM1 and SOCs.

TRPC1-STIM1 activation modulates transforming growth factor β-induced epithelial-to-mesenchymal transition

Activation of Epithelial-to-Mesenchymal Transition (EMT) is important for tumor metastasis. Although growth factors such as TGFβ and EGF have been shown to induce EMT in breast epithelial cells, the mechanism resulting in migration is not well understood. Herein, we provide evidence that Ca2+ entry into the cell, especially upon store-depletion, plays an important role in TGFβ-induced EMT by promoting cellular migration and potentially leading to metastasis. The increased migration by TGFβ in non-cancerous cells was due to the loss of E-cadherin along with a subsequent increase in N-cadherin levels. Importantly, TGFβ-treatment increases store-mediated Ca2+ entry, which was essential for the activation of calpain leading to the loss of E-cadherin and MMP activation. Inhibition of Ca2+ entry by using Ca2+ channel blocker SKF-96365, significantly decreased Ca2+ entry, decreased TGFβ-induced calpain activation, and suppressed the loss of E-cadherin along with inhibiting cell migration. Furthermore, TRPC1 function as an endogenous Ca2+ entry channel and silencing of either TRPC1 or its activator, STIM1, significantly decreased TGFβ induced Ca2+ entry, inhibited TGFβ-mediated calpain activation and cell migration. In contrast, overexpression of TRPC1 showed increased Ca2+ entry and promoted TGFβ-mediated cell migration. Moreover, increased TRPC1 expression was observed in ductal carcinoma cells. Together these results suggest that disrupting Ca2+ influx via TRPC1/STIM1 mechanism reduces calpain activity, which could restore intercellular junction proteins thereby inhibiting EMT induced motility.

SOCE proteins, STIM1 and Orai1, are localized to the cleavage furrow during cytokinesis of the first and second cell division cycles in zebrafish embryos

In zebrafish embryos, distinct Ca2+ transients are localized to the early cleavage furrows during the first few cell division cycles. These transients are generated mainly by release via IP3Rs in the endoplasmic reticulum, and they are necessary for furrow positioning, propagation, deepening and apposition. We previously showed, via the use of inhibitors, that store-operated Ca2+ entry (SOCE) also appears to be essential for maintaining the IP3R-mediated elevated levels of [Ca2+]i for the extended periods required for the completion of successful furrow deepening and daughter cell apposition in these large embryonic cells. Here, newly fertilized, dechorionated embryos were fixed at various times during the first and second cell division cycles and immunolabelled with antibodies to STIM1 and/or Orai1 (key components of SOCE). We show that both of these proteins have a dynamic pattern of localization during cytokinesis of the first two cell division cycles. These new data help to support our previous inhibitor results, and provide additional evidence that SOCE contributes to the maintenance of the sustained elevated Ca2+ that is required for the successful completion of cytokinesis in the large cells of embryonic zebrafish.

TRPC1-mediated Ca²⁺ entry is essential for the regulation of hypoxia and nutrient depletion-dependent autophagy

Autophagy is a cellular catabolic process needed for the degradation and recycling of protein aggregates and damaged organelles. Although Ca²⁺ is suggested to have an important role in cell survival, the ion channel(s) involved in autophagy have not been identified. Here we demonstrate that increase in intracellular Ca²⁺ via transient receptor potential canonical channel-1 (TRPC1) regulates autophagy, thereby preventing cell death in two morphologically distinct cells lines. The addition of DMOG or DFO, a cell permeable hypoxia-mimetic agents, or serum starvation, induces autophagy in both epithelial and neuronal cells. The induction of autophagy increases Ca²⁺ entry via the TRPC1 channel, which was inhibited by the addition of 2APB and SKF96365. Importantly, TRPC1-mediated Ca²⁺ entry resulted in increased expression of autophagic markers that prevented cell death. Furthermore, hypoxia-mediated autophagy also increased TRPC1, but not STIM1 or Orai1, expression. Silencing of TRPC1 or inhibition of autophagy by 3-methyladenine, but not TRPC3, attenuated hypoxia-induced increase in intracellular Ca²⁺ influx, decreased autophagy, and increased cell death. Furthermore, the primary salivary gland cells isolated from mice exposed to hypoxic conditions also showed increased expression of TRPC1 as well as increase in Ca²⁺ entry along with increased expression of autophagic markers. Altogether, we provide evidence for the involvement of Ca²⁺ influx via TRPC1 in regulating autophagy to protect against cell death.

Fructus Corni extract-induced neuritogenesis in PC12 cells is associated with the suppression of stromal interaction molecule 1 expression and inhibition of Ca2+ influx

Fructus Corni (Cornus officinalis Sieb. et Zucc.) is commonly prescribed as a traditional Chinese herbal medicine that possesses pharmacological actions against inflammation, diabetic nephropathy, tumors, oxidation and aging. However, its function and mode of action within the nervous system remain largely unclear. In this study, the effects of Fructus Corni extract (FCE) on neuronal differentiation were investigated. It was found that FCE significantly increased the percentage of PC12 cells bearing neurites (P<0.001). Following the generation of neurite outgrowth, FCE treatment decreased the mRNA expression of stromal interaction molecule 1 (STIM1; P<0.05) and suppressed the expression of STIM1 protein (P<0.001). In addition, extracellular calcium (Ca²⁺) influx was inhibited resulting in a reduction in the intracellular Ca²⁺ level, suggesting that the inhibition of Ca²⁺ influx may be involved in the FCE-promoted neurite outgrowth of PC12 cells. These results demonstrate that FCE induces neurite outgrowth in PC12 cells and that this is associated with the suppression of STIM1 expression and the inhibition of Ca²⁺ influx, which may partially explain the FCE-induced neuritogenesis.

Endoplasmic reticulum stress in insulin resistance and diabetes

The endoplasmic reticulum is the main intracellular Ca(2+) store for Ca(2+) release during cell signaling. There are different strategies to avoid ER Ca(2+) depletion. Release channels utilize first Ca(2+)-bound to proteins and this minimizes the reduction of the free luminal [Ca(2+)]. However, if release channels stay open after exhaustion of Ca(2+)-bound to proteins, then the reduction of the free luminal ER [Ca(2+)] (via STIM proteins) activates Ca(2+) entry at the plasma membrane to restore the ER Ca(2+) load, which will work provided that SERCA pump is active. Nevertheless, there are several noxious conditions that result in decreased activity of the SERCA pump such as oxidative stress, inflammatory cytokines, and saturated fatty acids, among others. These conditions result in a deficient restoration of the ER [Ca(2+)] and lead to the ER stress response that should facilitate recovery of the ER. However, if the stressful condition persists then ER stress ends up triggering cell death and the ensuing degenerative process leads to diverse pathologies; particularly insulin resistance, diabetes and several of the complications associated with diabetes. This scenario suggests that limiting ER stress should decrease the incidence of diabetes and the mobility and mortality associated with this illness.

Remodeling of channel-forming ORAI proteins determines an oncogenic switch in prostate cancer

ORAI family channels have emerged as important players in malignant transformation, yet the way in which they reprogram cancer cells remains elusive. Here we show that the relative expression levels of ORAI proteins in prostate cancer are different from that in noncancerous tissue. By mimicking ORAI protein remodeling observed in primary tumors, we demonstrate in in vitro models that enhanced ORAI3 expression favors heteromerization with ORAI1 to form a novel channel. These channels support store-independent Ca(2+) entry, thereby promoting cell proliferation and a smaller number of functional homomeric ORAI1-based store-operated channels, which are important in supporting susceptibility to apoptosis. Thus, our findings highlight disrupted dynamic equilibrium of channel-forming proteins as an oncogenic mechanism.

The KSR2-calcineurin complex regulates STIM1-ORAI1 dynamics and store-operated calcium entry (SOCE)

Store-operated calcium entry (SOCE) is the predominant Ca(2+) entry mechanism in nonexcitable cells and controls a variety of physiological and pathological processes. Although significant progress has been made in identifying the components required for SOCE, the molecular mechanisms underlying it are elusive. The present study provides evidence for a direct involvement of kinase suppressor of Ras 2 (KSR2) in SOCE. Using lymphocytes and fibroblasts from ksr2(-/-) mice and shKSR2-depleted cells, we find that KSR2 is critical for the elevation of cytosolic Ca(2+) concentration. Specifically, our results show that although it is dispensable for Ca(2+)-store depletion, KSR2 is required for optimal calcium entry. We observe that KSR2 deficiency affects stromal interaction molecule 1 (STIM1)/ORAI1 puncta formation, which is correlated with cytoskeleton disorganization. Of interest, we find that KSR2-associated calcineurin is crucial for SOCE. Blocking calcineurin activity impairs STIM1/ORAI1 puncta-like formation and cytoskeleton organization. In addition, we observe that calcineurin activity and its role in SOCE are both KSR2 dependent.

A model for the regulation of T-type Ca(2+) channels in proliferation: roles in stem cells and cancer

Ca(2+) influx at critical points in the cell cycle is required for proliferation. This requirement is so ubiquitous that its occurrence is often treated as background noise. Yet without it, cells stop dividing, suggesting an obvious and potentially effective way to treat cancer. To control proliferation by controlling Ca(2+) influx requires that the mechanism be elucidated, but this field of study has been filled with controversy and devoid of therapeutic utility. In this study, the authors present a model for the regulation of Ca(2+) influx at the G1/S restriction point in cancer and stem cells that is simple, cohesive and, we believe, reasonably complete. The model illustrates the essential role of T-type Ca(2+) channels in mediating influx and points clearly to the therapeutic strategies that have recently entered clinical trials.

Phosphoregulation of STIM1 leads to exclusion of the endoplasmic reticulum from the mitotic spindle

The endoplasmic reticulum (ER) undergoes significant reorganization between interphase and mitosis, but the underlying mechanisms are unknown. Stromal interaction molecule 1 (STIM1) is an ER Ca(2+) sensor that activates store-operated Ca(2+) entry (SOCE) and also functions in ER morphogenesis through its interaction with the microtubule +TIP protein end binding 1 (EB1). We previously demonstrated that phosphorylation of STIM1 during mitosis suppresses SOCE. We now show that STIM1 phosphorylation is a major regulatory mechanism that excludes ER from the mitotic spindle. In mitotic HeLa cells, the ER forms concentric sheets largely excluded from the mitotic spindle. We show that STIM1 dissociates from EB1 in mitosis and localizes to the concentric ER sheets. However, a nonphosphorylatable STIM1 mutant (STIM1(10A)) colocalized extensively with EB1 and drove ER mislocalization by pulling ER tubules into the spindle. This effect was rescued by mutating the EB1 interaction site of STIM1(10A), demonstrating that aberrant association of STIM1(10A) with EB1 is responsible for the ER mislocalization. A STIM1 phosphomimetic exhibited significantly impaired +TIP tracking in interphase but was ineffective at inhibiting SOCE, suggesting different mechanisms of regulation of these two STIM1 functions by phosphorylation. Thus, ER spindle exclusion and ER-dependent Ca(2+) signaling during mitosis require multimodal STIM1 regulation by phosphorylation.

Regulation of Orai1/STIM1 by the kinases SGK1 and AMPK

STIM and Orai isoforms orchestrate store operated Ca2+ entry (SOCE) and thus cytosolic Ca2+ fluctuations following stimulation by hormones, growth factors and further mediators. Orai1 is a target of Nedd4-2, an ubiquitin ligase preparing several plasma membrane proteins for degradation. Phosphorylation of Nedd4-2 by the serum and glucocorticoid inducible kinase SGK1 leads to the binding of Nedd4-2 to the protein 14-3-3 thus preventing its interaction with Orai1. Nedd4-2 is activated by the energy sensing AMP activated kinase AMPK. Thus, SGK1 disrupts and AMPK fosters degradation of Orai1. New synthesis of both, Orai1 and STIM1, is stimulated by the transcription factor NF-κB (nuclear factor kappa B), which binds to the respective promoter regions of the genes encoding STIM1 and Orai1. SGK1 upregulates and AMPK presumably downregulates NF-κB and thus de novo synthesis of Orai1 and STIM1 proteins. The regulation by SGK1 links SOCE to the signaling of a wide variety of hormones and growth factors, the AMPK dependent regulation of Orai1 and STIM1 may serve to limit inadequate activation of SOCE following energy depletion, which is otherwise expected to activate SOCE by depletion of intracellular Ca2+ stores due to impairment of the ATP consuming sarco/endoplasmatic reticulum Ca2+ ATPase SERCA.