STIM1 and STIM2 are located in the acidic Ca2+ stores and associates with Orai1 upon depletion of the acidic stores in human platelets

Activation mechanisms and structural dynamics of STIM proteins

The family of stromal interaction molecules (STIM) includes two widely expressed single-pass endoplasmic reticulum (ER) transmembrane proteins and additional splice variants that act as precise ER-luminal Ca2+ sensors. STIM proteins mainly function as one of the two essential components of the so-called Ca2+ release-activated Ca2+ (CRAC) channel. The second CRAC channel component is constituted by pore-forming Orai proteins in the plasma membrane. STIM and Orai physically interact with each other to enable CRAC channel opening, which is a critical prerequisite for various downstream signalling pathways such as gene transcription or proliferation. Their activation commonly requires the emptying of the intracellular ER Ca2+ store. Using their Ca2+ sensing capabilities, STIM proteins confer this Ca2+ content-dependent signal to Orai, thereby linking Ca2+ store depletion to CRAC channel opening. Here we review the conformational dynamics occurring along the entire STIM protein upon store depletion, involving the transition from the quiescent, compactly folded structure into an active, extended state, modulation by a variety of accessory components in the cell as well as the impairment of individual steps of the STIM activation cascade associated with disease.

SARAF overexpression impairs thrombin-induced Ca2+ homeostasis in neonatal platelets

Neonatal platelets present a reduced response to the platelet agonist, thrombin (Thr), thus resulting in a deficient Thr-induced aggregation. These alterations are more pronounced in premature newborns. Here, our aim was to uncover the causes underneath the impaired Ca2+ homeostasis described in neonatal platelets. Both Ca2+ mobilization and Ca2+ influx in response to Thr are decreased in neonatal platelets compared to maternal and control woman platelets. In neonatal platelets, we observed impaired Ca2+ mobilization in response to the PAR-1 agonist (SFLLRN) or by blocking SERCA3 function with tert-butylhydroquinone. Regarding SOCE, the STIM1 regulatory protein, SARAF, was found overexpressed in neonatal platelets, promoting an increase in STIM1/SARAF interaction even under resting conditions. Additionally, higher interaction between SARAF and PDCD61/ALG2 was also observed, reducing SARAF ubiquitination and prolonging its half-life. These results were reproduced by overexpressing SARAF in MEG01 and DAMI cells. Finally, we also observed that pannexin 1 permeability is enhanced in response to Thr in control woman and maternal platelets, but not in neonatal platelets, hence, leading to the deregulation of the Ca2+ entry found in neonatal platelets. Summarizing, we show that in neonatal platelets both Ca2+ accumulation in the intracellular stores and Thr-evoked Ca2+ entry through either capacitative channels or non-selective channels are altered in neonatal platelets, contributing to deregulated Ca2+ homeostasis in neonatal platelets and leading to the altered aggregation observed in these subjects.

Platelet-derived microvesicles induce intracellular calcium mobilization in human platelets

Platelet-derived microvesicles (PMVs) represent a significant proportion of microvesicles in circulation and have been linked to various pathophysiological complications. Recent research suggests that PMVs carry significant amounts of cargo that can affect cellular functions by influencing calcium oscillations in target cells. As calcium is involved in multiple cellular processes, including hemostasis and thrombosis, this study aimed to investigate the impact of PMVs on platelet calcium mobilization. The study found that PMVs increase platelet intracellular calcium levels via both intracellular storage and extracellular space in a dose-dependent manner. The study highlighted the critical role of the dense tubular system, acidic vacuoles, mitochondrial stores, and store-operated calcium entry (SOCE) in PMV-mediated calcium release in human platelets. Moreover, the study revealed that PMV-induced calcium rise in platelets does not occur via sarcoendoplasmic reticulum calcium ATPase, and extracellular calcium addition further increases the calcium level in platelets, demonstrating the involvement of SOCE. These findings provide insights into the platelet stimulation signaling mechanisms and contributes to our understanding of platelet and cell behavior when exposed to PMV-rich environments.

The Ca2+ Sensor STIM in Human Diseases

The STIM family of proteins plays a crucial role in a plethora of cellular functions through the regulation of store-operated Ca2+ entry (SOCE) and, thus, intracellular calcium homeostasis. The two members of the mammalian STIM family, STIM1 and STIM2, are transmembrane proteins that act as Ca2+ sensors in the endoplasmic reticulum (ER) and, upon Ca2+ store discharge, interact with and activate the Orai/CRACs in the plasma membrane. Dysregulation of Ca2+ signaling leads to the pathogenesis of a variety of human diseases, including neurodegenerative disorders, cardiovascular diseases, cancer, and immune disorders. Therefore, understanding the mechanisms underlying Ca2+ signaling pathways is crucial for developing therapeutic strategies targeting these diseases. This review focuses on several rare conditions associated with STIM1 mutations that lead to either gain- or loss-of-function, characterized by myopathy, hematological and immunological disorders, among others, and due to abnormal activation of CRACs. In addition, we summarize the current evidence concerning STIM2 allele duplication and deletion associated with language, intellectual, and developmental delay, recurrent pulmonary infections, microcephaly, facial dimorphism, limb anomalies, hypogonadism, and congenital heart defects.

Reduction in SOCE and Associated Aggregation in Platelets from Mice with Platelet-Specific Deletion of Orai1

Calcium signalling in platelets through store operated Ca²⁺ entry (SOCE) or receptor-operated Ca²⁺ entry (ROCE) mechanisms is crucial for platelet activation and function. Orai1 proteins have been implicated in platelet’s SOCE. In this study we evaluated the contribution of Orai1 proteins to these processes using washed platelets from adult mice from both genders with platelet-specific deletion of the Orai1 gene (Orai1^flox/flox; Pf4-Cre termed as Orai1^Plt-KO) since mice with ubiquitous Orai1 deficiency show early lethality. Platelet aggregation as well as Ca²⁺ entry and release were measured in vitro following stimulation with collagen, collagen related peptide (CRP), thromboxane A2 analogue U46619, thrombin, ADP and the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) inhibitor thapsigargin, respectively. SOCE and aggregation induced by Thapsigargin up to a concentration of 0.3 µM was abrogated in Orai1-deficient platelets. Receptor-operated Ca²⁺-entry and/or platelet aggregation induced by CRP, U46619 or thrombin were partially affected by Orai1 deletion depending on the gender. In contrast, ADP-, collagen- and CRP-induced aggregation was comparable in Orai1^Plt-KO platelets and control cells over the entire concentration range. Our results reinforce the indispensability of Orai1 proteins for SOCE in murine platelets, contribute to understand its role in agonist-dependent signalling and emphasize the importance to analyse platelets from both genders.

CRACking the Molecular Regulatory Mechanism of SOCE during Platelet Activation in Thrombo-Occlusive Diseases

Thrombo-occlusive diseases such as myocardial infarction, ischemic stroke and deep vein thrombosis with subsequent pulmonary embolism still represent a major health burden worldwide. Besides the cells of the vasculature or other hematopoietic cells, platelets are primarily responsible for the development and progression of an occluding thrombus. The activation and function of platelets crucially depend on free cytosolic calcium (Ca2+) as second messenger, which modulates platelet secretion, aggregation and thrombus formation. Ca2+ is elevated upon platelet activation by release of Ca2+ from intracellular stores thus triggering of the subsequent store-operated Ca2+ entry (SOCE), which is facilitated by Ca2+ release-activated channels (CRACs). In general, CRACs are assembled by the pore-forming unit Orai in the plasma membrane and the Ca2+-sensing stromal interaction molecule (STIM) in the endoplasmic reticulum after the depletion of internal Ca2+ stores. In the last few years, there is a growing body of the literature demonstrating the importance of STIM and Orai-mediated mechanism in thrombo-occlusive disorders. Thus, this review provides an overview of the recent understanding of STIM and Orai signaling in platelet function and its implication in the development and progression of ischemic thrombo-occlusive disorders. Moreover, potential pharmacological implications of STIM and Orai signaling in platelets are anticipated and discussed in the end.

Orai2 Modulates Store-Operated Ca2+ Entry and Cell Cycle Progression in Breast Cancer Cells

Breast cancer is a heterogeneous disease from the histological and molecular expression point of view, and this heterogeneity determines cancer aggressiveness. Store-operated Ca²⁺ entry (SOCE), a major mechanism for Ca²⁺ entry in non-excitable cells, is significantly remodeled in cancer cells and plays an important role in the development and support of different cancer hallmarks. The store-operated CRAC (Ca²⁺ release-activated Ca²⁺) channels are predominantly comprised of Orai1 but the participation of Orai2 and Orai3 subunits has been reported to modulate the magnitude of Ca²⁺ responses. Here we provide evidence for a heterogeneous expression of Orai2 among different breast cancer cell lines. In the HER2 and triple negative breast cancer cell lines SKBR3 and BT20, respectively, where the expression of Orai2 was greater, Orai2 modulates the magnitude of SOCE and sustain Ca²⁺ oscillations in response to carbachol. Interestingly, in these cells Orai2 modulates the activation of NFAT1 and NFAT4 in response to high and low agonist concentrations. Finally, we have found that, in cells with high Orai2 expression, Orai2 knockdown leads to cell cycle arrest at the G0-G1 phase and decreases apoptosis resistance upon cisplatin treatment. Altogether, these findings indicate that, in breast cancer cells with a high Orai2 expression, Orai2 plays a relevant functional role in agonist-evoked Ca²⁺ signals, cell proliferation and apoptosis resistance.

Furin Prodomain ppFurin Enhances Ca2+ Entry Through Orai and TRPC6 Channels' Activation in Breast Cancer Cells

Simple Summary

Furin, a proprotein convertase that belongs to a family of Ca²⁺-dependent serine peptidases, is involved in the maturation of a variety of proproteins, including growth factors, receptors and differentiation factors, adhesion molecules and proteases. Furin have been associated with tumorigenesis and tumor progression and metastasis; therefore, it has been hypothesized that Furin may constitute a new potential target for cancer therapy. In triple negative breast cancer cells, inhibition of Furin by the prodomain ppFurin results in enhancement of Ca²⁺ influx, which involves both the increase of store-operated calcium entry (SOCE) and the activation of constitutive Ca²⁺ entry. The latter involves the activation of Orai and TRPC6 channels, while the increase of SOCE observed in ppFurin-expressing cells is entirely dependent on Orai channels. As a result, ppFurin expression reduces triple negative breast cancer cell viability and ability to migrate and enhances their sensitization to hydrogen peroxide-induced apoptosis.

Abstract

The intracellular calcium concentration ([Ca²⁺]_i) modulation plays a key role in the regulation of cellular growth and survival in normal cells and failure of [Ca²⁺]_i homeostasis is involved in tumor initiation and progression. Here we showed that inhibition of Furin by its naturally occurring inhibitor the prodomain ppFurin in the MDA-MB-231 breast cancer cells resulted in enhanced store-operated calcium entry (SOCE) and reduced the cell malignant phenotype. Expression of ppFurin in a stable manner in MDA-MB-231 and the melanoma MDA-MB-435 cell lines inhibits Furin activity as assessed by in vitro digestion assays. Accordingly, cell transfection experiments revealed that the ppFurin-expressing cells are unable to adequately process the proprotein convertase (PC) substrates vascular endothelial growth factor C (proVEGF-C) and insulin-like growth factor-1 receptor (proIGF-1R). Compared to MDA-MB-435 cells, expression of ppFurin in MDA-MB-231 and BT20 cells significantly enhanced SOCE and induced constitutive Ca²⁺ entry. The enhanced SOCE is impaired by inhibition of Orai channels while the constitutive Ca²⁺ entry is attenuated by silencing or inhibition of TRPC6 or inhibition of Orai channels. Analysis of TRPC6 activation revealed its upregulated tyrosine phosphorylation in ppFurin-expressing MDA-MB-231 cells. In addition, while ppFurin had no effect on MDA-MB-435 cell viability, in MDA-MB-231 cells ppFurin expression reduced their viability and ability to migrate and enhanced their sensitization to the apoptosis inducer hydrogen peroxide and similar results were observed in BT20 cells. These findings suggest that Furin inhibition by ppFurin may be a useful strategy to interfere with Ca²⁺ mobilization, leading to breast cancer cells’ malignant phenotype repression and reduction of their resistance to treatments.

Functional role of TRPC6 and STIM2 in cytosolic and endoplasmic reticulum Ca2+ content in resting estrogen receptor-positive breast cancer cells

TRPC6 forms non-selective cation channels activated by a variety of stimuli that are involved in a wide number of cellular functions. In estrogen receptor-positive (ER+) breast cancer cells, the store-operated Ca2+ entry has been reported to be dependent on STIM1, STIM2 and Orai3, with TRPC6 playing a key role in the activation of store-operated Ca2+ entry as well as in proliferation, migration and viability of breast cancer cells. We have used a combination of biotinylation, Ca2+ imaging as well as protein knockdown and overexpression of a dominant-negative TRPC6 mutant (TRPC6dn) to show that TRPC6 and STIM2 are required for the maintenance of cytosolic and endoplasmic reticulum Ca2+ content under resting conditions in ER+ breast cancer MCF7 cells. These cells exhibit a greater plasma membrane expression of TRPC6 under resting conditions than non-tumoral breast epithelial cells. Attenuation of STIM2, TRPC6 and Orai3, alone or in combination, results in impairment of resting cytosolic and endoplasmic reticulum Ca2+ homeostasis. Similar results were observed when cells were transfected with expression plasmid for TRPC6dn. TRPC6 co-immunoprecipitates with STIM2 in resting MCF7 cells, a process that is impaired by rises in cytosolic Ca2+ concentration. Impairment of TRPC6 function leads to abnormal Ca2+ homeostasis and endoplasmic reticulum stress, thus, suggesting that TRPC6 might be a potential target for the development of anti-tumoral therapies.

Melatonin downregulates TRPC6, impairing store-operated calcium entry in triple-negative breast cancer cells

Melatonin has been reported to induce effective reduction in growth and development in a variety of tumors, including breast cancer. In triple-negative breast cancer (TNBC) cells, melatonin attenuates a variety of cancer features, such as tumor growth and apoptosis resistance, through a number of still poorly characterized mechanisms. One biological process that is important for TNBC cells is store-operated Ca2+ entry (SOCE), which is modulated by TRPC6 expression and function. We wondered whether melatonin might intersect with this pathway as part of its anticancer activity. We show that melatonin, in the nanomolar range, significantly attenuates TNBC MDA-MB-231 cell viability, proliferation, and migration in a time- and concentration-dependent manner, without having any effect on nontumoral breast epithelial MCF10A cells. Pretreatment with different concentrations of melatonin significantly reduced SOCE in MDA-MB-231 cells without altering Ca2+ release from the intracellular stores. By contrast, SOCE in MCF10A cells was unaffected by melatonin. In the TNBC MDA-MB-468 cell line, melatonin not only attenuated viability, migration, and SOCE, but also reduced TRPC6 expression in a time- and concentration-dependent manner, without altering expression or function of the Ca2+ channel Orai1. The expression of exogenous TRPC6 overcame the effect of melatonin on SOCE and cell proliferation, and silencing or inhibition of TRPC6 impaired the inhibitory effect of melatonin on SOCE. These findings indicate that TRPC6 downregulation might be involved in melatonin's inhibitory effects on Ca2+ influx and the maintenance of cancer hallmarks and point toward a novel antitumoral mechanism of melatonin in TNBC cells.

STIM Proteins: An Ever-Expanding Family

Stromal interaction molecules (STIM) are a distinct class of ubiquitously expressed single-pass transmembrane proteins in the endoplasmic reticulum (ER) membrane. Together with Orai ion channels in the plasma membrane (PM), they form the molecular basis of the calcium release-activated calcium (CRAC) channel. An intracellular signaling pathway known as store-operated calcium entry (SOCE) is critically dependent on the CRAC channel. The SOCE pathway is activated by the ligand-induced depletion of the ER calcium store. STIM proteins, acting as calcium sensors, subsequently sense this depletion and activate Orai ion channels via direct physical interaction to allow the influx of calcium ions for store refilling and downstream signaling processes. This review article is dedicated to the latest advances in the field of STIM proteins. New results of ongoing investigations based on the recently published functional data as well as structural data from nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations are reported and complemented with a discussion of the latest developments in the research of STIM protein isoforms and their differential functions in regulating SOCE.

Applying Antibodies Inside Cells: Principles and Recent Advances in Neurobiology, Virology and Oncology

To interfere with cell function, many scientists rely on methods that target DNA or RNA due to the ease with which they can be applied. Proteins are usually the final executors of function but are targeted only indirectly by these methods. Recent advances in targeted degradation of proteins based on proteolysis-targeting chimaeras (PROTACs), ubiquibodies, deGradFP (degrade Green Fluorescent Protein) and other approaches have demonstrated the potential of interfering directly at the protein level for research and therapy. Proteins can be targeted directly and very specifically by antibodies, but using antibodies inside cells has so far been considered to be challenging. However, it is possible to deliver antibodies or other proteins into the cytosol using standard laboratory equipment. Physical methods such as electroporation have been demonstrated to be efficient and validated thoroughly over time. The expression of intracellular antibodies (intrabodies) inside cells is another way to interfere with intracellular targets at the protein level. Methodological strategies to target the inside of cells with antibodies, including delivered antibodies and expressed antibodies, as well as applications in the research areas of neurobiology, viral infections and oncology, are reviewed here. Antibodies have already been used to interfere with a wide range of intracellular targets. Disease-related targets included proteins associated with neurodegenerative diseases such as Parkinson's disease (α-synuclein), Alzheimer's disease (amyloid-β) or Huntington's disease (mutant huntingtin [mHtt]). The applications of intrabodies in the context of viral infections include targeting proteins associated with HIV (e.g. HIV1-TAT, Rev, Vif, gp41, gp120, gp160) and different oncoviruses such as human papillomavirus (HPV), hepatitis B virus (HBV), hepatitis C virus (HCV) and Epstein-Barr virus, and they have been used to interfere with various targets related to different processes in cancer, including oncogenic pathways, proliferation, cell cycle, apoptosis, metastasis, angiogenesis or neo-antigens (e.g. p53, human epidermal growth factor receptor-2 [HER2], signal transducer and activator of transcription 3 [STAT3], RAS-related RHO-GTPase B (RHOB), cortactin, vascular endothelial growth factor receptor 2 [VEGFR2], Ras, Bcr-Abl). Interfering at the protein level allows questions to be addressed that may remain unanswered using alternative methods. This review addresses why direct targeting of proteins allows unique insights, what is currently feasible in vitro, and how this relates to potential therapeutic applications.

Readily Releasable Stores of Calcium in Neuronal Endolysosomes: Physiological and Pathophysiological Relevance

Neurons are long-lived post-mitotic cells that possess an elaborate system of endosomes and lysosomes (endolysosomes) for protein quality control. Relatively recently, endolysosomes were recognized to contain high concentrations (400-600 μM) of readily releasable calcium. The release of calcium from this acidic organelle store contributes to calcium-dependent processes of fundamental physiological importance to neurons including neurotransmitter release, membrane excitability, neurite outgrowth, synaptic remodeling, and cell viability. Pathologically, disturbances of endolysosome structure and/or function have been noted in a variety of neurodegenerative disorders including Alzheimer's disease (AD) and HIV-1 associated neurocognitive disorder (HAND). And, dysregulation of intracellular calcium has been implicated in the neuropathogenesis of these same neurological disorders. Thus, it is important to better understand mechanisms by which calcium is released from endolysosomes as well as the consequences of such release to inter-organellar signaling, physiological functions of neurons, and possible pathological consequences. In doing so, a path forward towards new therapeutic modalities might be facilitated.

Review: Structure and Activation Mechanisms of CRAC Channels

Ca²⁺ release activated Ca²⁺ (CRAC) channels represent a primary pathway for Ca²⁺ to enter non-excitable cells. The two key players in this process are the stromal interaction molecule (STIM), a Ca²⁺ sensor embedded in the membrane of the endoplasmic reticulum, and Orai, a highly Ca²⁺ selective ion channel located in the plasma membrane. Upon depletion of the internal Ca²⁺ stores, STIM is activated, oligomerizes, couples to and activates Orai. This review provides an overview of novel findings about the CRAC channel activation mechanisms, structure and gating. In addition, it highlights, among diverse STIM and Orai mutants, also the disease-related mutants and their implications.

Molecular Basis and Regulation of Store-Operated Calcium Entry

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous mechanism for Ca²⁺ influx in mammalian cells with important physiological implications. Since the discovery of SOCE more than three decades ago, the mechanism that communicates the information about the amount of Ca²⁺ accumulated in the intracellular Ca²⁺ stores to the plasma membrane channels and the nature of these channels have been matters of intense investigation and debate. The stromal interaction molecule-1 (STIM1) has been identified as the Ca²⁺ sensor of the intracellular Ca²⁺ compartments that activates the store-operated channels. STIM1 regulates two types of store-dependent channels: the Ca²⁺ release-activated Ca²⁺ (CRAC) channels, formed by Orai1 subunits, that conduct the highly Ca²⁺ selective current I _CRAC and the cation permeable store-operated Ca²⁺ (SOC) channels, which consist of Orai1 and TRPC1 proteins and conduct the non-selective current I _SOC. While the crystal structure of Drosophila CRAC channel has already been solved, the architecture of the SOC channels still remains unclear. The dynamic interaction of STIM1 with the store-operated channels is modulated by a number of proteins that either support the formation of the functional STIM1-channel complex or protect the cell against Ca²⁺ overload.

Adenylyl Cyclase Type 8 Overexpression Impairs Phosphorylation-Dependent Orai1 Inactivation and Promotes Migration in MDA-MB-231 Breast Cancer Cells

Orai1 plays a major role in store-operated Ca²⁺ entry (SOCE) in triple-negative breast cancer (TNBC) cells. This channel is inactivated via different mechanisms, including protein kinase C (PKC) and protein kinase A (PKA)-dependent phosphorylation at Ser-27 and Ser-30 or Ser-34, respectively, which shapes the Ca²⁺ responses to agonists. The Ca²⁺ calmodulin-activated adenylyl cyclase type 8 (AC8) was reported to interact directly with Orai1, thus mediating a dynamic interplay between the Ca²⁺- and cyclic adenosine monophosphate (cAMP)-dependent signaling pathways. Here, we show that the breast cancer cell lines MCF7 and MDA-MB-231 exhibit enhanced expression of Orai1 and AC8 as compared to the non-tumoral breast epithelial MCF10A cell line. In these cells, AC8 interacts with the Orai1α variant in a manner that is not regulated by Orai1 phosphorylation. AC8 knockdown in MDA-MB-231 cells, using two different small interfering RNAs (siRNAs), attenuates thapsigargin (TG)-induced Ca²⁺ entry and also Ca²⁺ influx mediated by co-expression of Orai1 and the Orai1-activating small fragment (OASF) of STIM1 (stromal interaction molecule-1). Conversely, AC8 overexpression enhances SOCE, as well as Ca²⁺ entry, in cells co-expressing Orai1 and OASF. In MDA-MB-231 cells, we found that AC8 overexpression reduces the Orai1 phosphoserine content, thus suggesting that AC8 interferes with Orai1 serine phosphorylation, which takes place at residues located in the AC8-binding site. Consistent with this, the subset of Orai1 associated with AC8 in naïve MDA-MB-231 cells is not phosphorylated in serine residues in contrast to the AC8-independent Orai1 subset. AC8 expression knockdown attenuates migration of MCF7 and MDA-MB-231 cells, while this maneuver has no effect in the MCF10A cell line, which is likely attributed to the low expression of AC8 in these cells. We found that AC8 is required for FAK (focal adhesion kinase) phosphorylation in MDA-MB-231 cells, which might explain its role in cell migration. Finally, we found that AC8 is required for TNBC cell proliferation. These findings indicate that overexpression of AC8 in breast cancer MDA-MB-231 cells impairs the phosphorylation-dependent Orai1 inactivation, a mechanism that might support the enhanced ability of these cells to migrate.

Involvement of organelles and inter-organellar signaling in the pathogenesis of HIV-1 associated neurocognitive disorder and Alzheimer's disease

Endolysosomes, mitochondria, peroxisomes, endoplasmic reticulum, and plasma membranes are now known to physically and functionally interact with each other. Such findings of inter-organellar signaling and communication has led to a resurgent interest in cell biology and an increased appreciation for the physiological actions and pathological consequences of the dynamic physical and chemical communications occurring between intracellular organelles. Others and we have shown that HIV-1 proteins implicated in the pathogenesis of neuroHIV and that Alzheimer's disease both affects the structure and function of intracellular organelles. Intracellular organelles are highly mobile, and their intracellular distribution almost certainly affects their ability to interact with other organelles and to regulate such important physiological functions as endolysosome acidification, cell motility, and nutrient homeostasis. Indeed, compounds that acidify endolysosomes cause endolysosomes to exhibit a mainly perinuclear pattern while compounds that de-acidify endolysosomes cause these organelles to exhibit a larger profile as well as movement towards plasma membranes. Endolysosome pH might be an early event in the pathogenesis of neuroHIV and Alzheimer's disease and in terms of organellar biology endolysosome changes might be upstream of HIV-1 protein-induced changes to other organelles. Thus, inter-organellar signaling mechanisms might be involved in the pathogenesis of neuroHIV and other neurological disorders, and a better understanding of inter-organellar signaling might lead to improved therapeutic strategies.

Store-operated calcium entry in disease: Beyond STIM/Orai expression levels

Precise intracellular calcium signaling is crucial to numerous cellular functions. In non-excitable cells, store-operated calcium entry (SOCE) is a key step in the generation of intracellular calcium signals. Tight regulation of SOCE is important, and dysregulation is involved in several pathophysiological cellular malfunctions. The current underlying SOCE, calcium release-activated calcium current (I_CRAC), was first discovered almost three decades ago. Since its discovery, the molecular components of I_CRAC, Orai1 and stromal interaction molecule 1 (STIM1), have been extensively investigated. Several regulatory mechanisms and proteins contribute to alterations in SOCE and cellular malfunctions in cancer, immune and neurodegenerative diseases, inflammation, and neuronal disorders. This review summarizes these regulatory mechanisms, including glycosylation, pH sensing, and the regulatory proteins golli, α-SNAP, SARAF, ORMDL3, CRACR2A, and TRPM4 channels.

Store-Operated Ca2+ Entry in Breast Cancer Cells: Remodeling and Functional Role

Breast cancer is the most common type of cancer in women. It is a heterogeneous disease that ranges from the less undifferentiated luminal A to the more aggressive basal or triple negative breast cancer molecular subtype. Ca²⁺ influx from the extracellular medium, but more specifically store-operated Ca²⁺ entry (SOCE), has been reported to play an important role in tumorigenesis and the maintenance of a variety of cancer hallmarks, including cell migration, proliferation, invasion or epithelial to mesenchymal transition. Breast cancer cells remodel the expression and functional role of the molecular components of SOCE. This review focuses on the functional role and remodeling of SOCE in breast cancer cells. The current studies suggest the need to deepen our understanding of SOCE in the biology of the different breast cancer subtypes in order to develop new and specific therapeutic strategies.

(-)‑Oleocanthal inhibits proliferation and migration by modulating Ca2+ entry through TRPC6 in breast cancer cells

Triple negative breast cancer is an aggressive type of cancer that does not respond to hormonal therapy and current therapeutic strategies are accompanied by side effects due to cytotoxic actions on normal tissues. Therefore, there is a need for the identification of anti-cancer compounds with negligible effects on non-tumoral cells. Here we show that (-)‑oleocanthal (OLCT), a phenolic compound isolated from olive oil, selectively impairs MDA-MB-231 cell proliferation and viability without affecting the ability of non-tumoral MCF10A cells to proliferate or their viability. Similarly, OLCT selectively impairs the ability of MDA-MB-231 cells to migrate while the ability of MCF10A to migrate was unaffected. The effect of OLCT was not exclusive for triple negative breast cancer cells as we found that OLCT also attenuate cell viability and proliferation of MCF7 cells. Our results indicate that OLCT is unable to induce Ca²⁺ mobilization in non-tumoral cells. By contrast, OLCT induces Ca²⁺ entry in MCF7 and MDA-MB-231 cells, which is impaired by TRPC6 expression silencing. We have found that MDA-MB-231 and MCF7 cells overexpress the channel TRPC6 as compared to non-tumoral MCF10A and treatment with OLCT for 24-72 h downregulates TRPC6 expression in MDA-MB-231 cells. These findings indicate that OLCT impairs the ability of breast cancer cells to proliferate and migrate via downregulation of TRPC6 channel expression while having no effect on the biology of non-tumoral breast cells.

TRPC6 Channels Are Required for Proliferation, Migration and Invasion of Breast Cancer Cell Lines by Modulation of Orai1 and Orai3 Surface Exposure

Transient receptor potential channels convey signaling information from a number of stimuli to a wide variety of cellular functions, mainly by inducing changes in cytosolic Ca²⁺ concentration. Different members of the TRPC, TRPM and TRPV subfamilies have been reported to play a role in tumorigenesis. Here we show that the estrogen receptor positive and triple negative breast cancer cell lines, MCF7 and MDA-MB-231, respectively, exhibit enhanced expression of the TRPC6 channel as compared to the non-tumoral MCF10A cell line. In vitro TRPC6 knockdown using shRNA impaired MCF7 and MDA-MB-231 cell proliferation, migration and invasion detected by BrdU incorporation, wound healing and Boyden chamber assays, respectively. Using RNAi-mediated TRPC6 silencing as well as overexpression of the pore-dead dominant-negative TRPC6 mutant we have found that TRPC6 plays a relevant role in the activation of store-operated Ca²⁺ entry in the breast cancer cell lines but not in non-tumoral breast cells. Finally, we have found that TRPC6 interacts with Orai1 and Orai3 in MCF7 and MDA-MB-231 cells and is required for the translocation of Orai1 and Orai3 to the plasma membrane in MDA-MB-231 and MCF7 cells, respectively, upon Ca²⁺ store depletion. These findings introduce a novel mechanism for the modulation of Ca²⁺ influx and the development of different cancer hallmarks in breast cancer cells.

Interplay between ER Ca2+ Binding Proteins, STIM1 and STIM2, Is Required for Store-Operated Ca2+ Entry

Store-operated calcium entry (SOCE), a fundamentally important homeostatic and Ca²⁺ signaling pathway in many types of cells, is activated by the direct interaction of stromal interaction molecule 1 (STIM1), an endoplasmic reticulum (ER) Ca²⁺-binding protein, with Ca²⁺-selective Orai1 channels localized in the plasma membrane. While much is known about the regulation of SOCE by STIM1, the role of stromal interaction molecule 2 (STIM2) in SOCE remains incompletely understood. Here, using clustered regularly interspaced short palindromic repeats -CRISPR associated protein 9 (CRISPR-Cas9) genomic editing and molecular imaging, we investigated the function of STIM2 in NIH 3T3 fibroblast and αT3 cell SOCE. We found that deletion of Stim2 expression reduced SOCE by more than 90% in NIH 3T3 cells. STIM1 expression levels were unaffected in the Stim2 null cells. However, quantitative confocal fluorescence imaging demonstrated that in the absence of Stim2 expression, STIM1 did not translocate or form punctae in plasma membrane-associated ER membrane (PAM) junctions following ER Ca²⁺ store depletion. Fluorescence resonance energy transfer (FRET) imaging of intact, living cells revealed that the formation of STIM1 and Orai1 complexes in PAM nanodomains was significantly reduced in the Stim2 knockout cells. Our findings indicate that STIM2 plays an essential role in regulating SOCE in NIH 3T3 and αT3 cells and suggests that dynamic interplay between STIM1 and STIM2 induced by ER Ca²⁺ store discharge is necessary for STIM1 translocation, its interaction with Orai1, and activation of SOCE.

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.

Molecular physiology and pathophysiology of stromal interaction molecules

Ca²⁺ release from the endoplasmic reticulum is an important component of Ca²⁺ signal transduction that controls numerous physiological processes in eukaryotic cells. Release of Ca²⁺ from the endoplasmic reticulum is coupled to the activation of store-operated Ca²⁺ entry into cells. Store-operated Ca²⁺ entry provides Ca²⁺ for replenishing depleted endoplasmic reticulum Ca²⁺ stores and a Ca²⁺ signal that regulates Ca²⁺-dependent intracellular biochemical events. Central to connecting discharge of endoplasmic reticulum Ca²⁺ stores following G protein-coupled receptor activation with the induction of store-operated Ca²⁺ entry are stromal interaction molecules (STIM1 and STIM2). These highly homologous endoplasmic reticulum transmembrane proteins function as sensors of the Ca²⁺ concentration within the endoplasmic reticulum lumen and activators of Ca²⁺ release-activated Ca²⁺ channels. Emerging evidence indicates that in addition to their role in Ca²⁺ release-activated Ca²⁺ channel gating and store-operated Ca²⁺ entry, STIM1 and STIM2 regulate other cellular signaling events. Recent studies have shown that disruption of STIM expression and function is associated with the pathogenesis of several diseases including autoimmune disorders, cancer, cardiovascular disease, and myopathies. Here, we provide an overview of the latest developments in the molecular physiology and pathophysiology of STIM1 and STIM2. Impact statement Intracellular Ca²⁺ signaling is a fundamentally important regulator of cell physiology. Recent studies have revealed that Ca²⁺-binding stromal interaction molecules (Stim1 and Stim2) expressed in the membrane of the endoplasmic reticulum (ER) are essential components of eukaryote Ca²⁺ signal transduction that control the activity of ion channels and other signaling effectors present in the plasma membrane. This review summarizes the most recent information on the molecular physiology and pathophysiology of stromal interaction molecules. We anticipate that the work presented in our review will provide new insights into molecular interactions that participate in interorganelle signaling crosstalk, cell function, and the pathogenesis of human diseases.

Cigarette smoke activates CFTR through ROS-stimulated cAMP signaling in human bronchial epithelial cells

Air pollution stimulates airway epithelial secretion through a cholinergic reflex that is unaffected in cystic fibrosis (CF), yet a strong correlation is observed between passive smoke exposure in the home and impaired lung function in CF children. Our aim was to study the effects of low smoke concentrations on cystic fibrosis transmembrane conductance regulator (CFTR) function in vitro. Cigarette smoke extract stimulated robust anion secretion that was transient, mediated by CFTR, and dependent on cAMP-dependent protein kinase activation. Secretion was initiated by reactive oxygen species (ROS) and mediated by at least two distinct pathways: autocrine activation of EP4 prostanoid receptors and stimulation of Ca2+ store-operated cAMP signaling. The response was absent in cells expressing the most common disease-causing mutant F508del-CFTR. In addition to the initial secretion, prolonged exposure of non-CF bronchial epithelial cells to low levels of smoke also caused a gradual decline in CFTR functional expression. F508del-CFTR channels that had been rescued by the CF drug combination VX-809 (lumacaftor) + VX-770 (ivacaftor) were more sensitive to this downregulation than wild-type CFTR. The results suggest that CFTR-mediated secretion during acute cigarette smoke exposure initially protects the airway epithelium while prolonged exposure reduces CFTR functional expression and reduces the efficacy of CF drugs.

TRPs in Pain Sensation

According to the International Association for the Study of Pain (IASP) pain is characterized as an "unpleasant sensory and emotional experience associated with actual or potential tissue damage". The TRP super-family, compressing up to 28 isoforms in mammals, mediates a myriad of physiological and pathophysiological processes, pain among them. TRP channel might be constituted by similar or different TRP subunits, which will result in the formation of homomeric or heteromeric channels with distinct properties and functions. In this review we will discuss about the function of TRPs in pain, focusing on TRP channles that participate in the transduction of noxious sensation, especially TRPV1 and TRPA1, their expression in nociceptors and their sensitivity to a large number of physical and chemical stimuli.

GTPase of the Immune-Associated Nucleotide Protein 5 Regulates the Lysosomal Calcium Compartment in T Lymphocytes

T lymphocytes from Gimap5^lyp/lyp rats carrying a recessive mutation in the GTPase of immune-associated protein 5 (Gimap5) gene undergo spontaneous apoptosis. Molecular mechanisms underlying this survival defect are not yet clear. We have shown that Gimap5^lyp/lyp T lymphocytes display reduced calcium influx following T cell antigen receptor (TCR) stimulation that was associated with impaired buffering of calcium by mitochondria. Here, we investigated the subcellular localization of GIMAP5 and its influence on Ca²⁺ response in HEK293T cells and T lymphocytes. The more abundantly expressed GIMAP5v2 localizes to the lysosome and certain endosomal vesicles. Gimap5^lyp/lyp T lymphocytes showed increased accumulation of calcium in the lysosomes as evidenced by Gly-Phe β-naphthylamide (GPN) triggered Ca²⁺ release. As a corollary, GPN-induced Ca²⁺ flux was decreased in HEK293T cells expressing GIMAP5v2. Strikingly, TCR stimulation of rat, mouse, and human T lymphocytes increased lysosomal calcium content. Overall, our findings show that lysosomes modulate cellular Ca²⁺ response during T cell activation and that GIMAP5 regulates the lysosomal Ca²⁺ compartment in T lymphocytes.

Cardiovascular and Hemostatic Disorders: SOCE and Ca2+ Handling in Platelet Dysfunction

Among the Ca²⁺ entry mechanisms in platelets, store-operated Ca²⁺ entry (SOCE) plays a prominent role as it is necessary to achieve full activation of platelet functions and replenish intracellular Ca²⁺ stores. In platelets, as in other non-excitable cells, SOCE has been reported to involve the activation of plasma membrane channels by the ER Ca²⁺ sensor STIM1. Despite electrophysiological studies are not possible in human platelets, indirect analyses have revealed that the Ca²⁺-permeable channels involve Orai1 and, most likely, TRPC1 subunits. A relevant role for the latter has not been found in mouse platelets. There is a body of evidence revealing a number of abnormalities in SOCE or in its molecular regulators that result in qualitative platelet disorders and, as a consequence, altered platelet responsiveness upon stimulation with multiple physiological agonists. Platelet SOCE abnormalities include STIM1 and Orai1 mutations. This chapter summarizes the current knowledge in this field, as well as the disorders associated to platelet SOCE dysfunction.

Modulation of Calcium Entry by the Endo-lysosomal System

Endo-lysosomes are acidic organelles that besides the role in macromolecules degradation, act as intracellular Ca(2+) stores. Nicotinic acid adenine dinucleotide phosphate (NAADP), the most potent Ca(2+)-mobilizing second messenger, produced in response to agonist stimulation, activates Ca(2+)-releasing channels on endo-lysosomes and modulates a variety of cellular functions. NAADP-evoked signals are amplified by Ca(2+) release from endoplasmic reticulum, via the recruitment of inositol 1,4,5-trisphosphate and/or ryanodine receptors through a Ca(2+)-induced Ca(2+)- release (CICR) mechanism. The endo-lysosomal Ca(2+) channels activated by NAADP were recently identified as the two-pore channels (TPCs). In addition to TPCs, endo-lysosomes express another distinct family of Ca(2+)- permeable channels, namely the transient receptor potential mucolipin (TRPML) channels, functionally distinct from TPCs. TPCs belong to the voltage-gated channels, resembling voltage-gated Na(+) and Ca(2+) channels. TPCs have important roles in vesicular fusion and trafficking, in triggering a global Ca(2+) signal and in modulation of the membrane excitability. Depletion of acidic Ca(2+) stores has been shown to activate store-operated Ca(2+) entry in human platelets and mouse pancreatic β-cells. In human platelets, Ca(2+) influx in response to acidic stores depletion is facilitated by the tubulin-cytoskeleton and occurs through non-selective cation channels and transient receptor potential canonical (TRPC) channels. Emerging evidence indicates that activation of intracellular receptors, situated on endo-lysosomes, elicits canonical and non-canonical signaling mechanisms that involve CICR and activation of non-selective cation channels in plasma membrane. The ability of endo-lysosomal Ca(2+) stores to modulate the Ca(2+) release from other organelles and the Ca(2+) entry increases the diversity and complexity of cellular signaling mechanisms.

Familial Alzheimer's disease-associated presenilin 1 mutants promote γ-secretase cleavage of STIM1 to impair store-operated Ca2+ entry

Some forms of familial Alzheimer's disease (FAD) are caused by mutations in presenilins (PSs), catalytic components of a γ-secretase complex that cleaves target proteins, including amyloid precursor protein (APP). Calcium (Ca(2+)) dysregulation in cells with these FAD-causing PS mutants has been attributed to attenuated store-operated Ca(2+) entry [SOCE; also called capacitative Ca(2+) entry (CCE)]. CCE occurs when STIM1 detects decreases in Ca(2+) in the endoplasmic reticulum (ER) and activates ORAI channels to replenish Ca(2+) stores in the ER. We showed that CCE was attenuated by PS1-associated γ-secretase activity. Endogenous PS1 and STIM1 interacted in human neuroblastoma SH-SY5Y cells, patient fibroblasts, and mouse primary cortical neurons. Forms of PS1 with FAD-associated mutations enhanced γ-secretase cleavage of the STIM1 transmembrane domain at a sequence that was similar to the γ-secretase cleavage sequence of APP. Cultured hippocampal neurons expressing mutant PS1 had attenuated CCE that was associated with destabilized dendritic spines, which were rescued by either γ-secretase inhibition or overexpression of STIM1. Our results indicate that γ-secretase activity may physiologically regulate CCE by targeting STIM1 and that restoring STIM1 may be a therapeutic approach in AD.

Full activation of mouse platelets requires ADP secretion regulated by SERCA3 ATPase-dependent calcium stores

The role of the sarco-endoplasmic reticulum calcium (Ca(2+)) adenosine triphosphatase (ATPase) 3 (SERCA3) in platelet physiology remains poorly understood. Here, we show that SERCA3 knockout (SERCA3(-/-)) mice exhibit prolonged tail bleeding time and rebleeding. Thrombus formation was delayed both in arteries and venules in an in vivo ferric chloride-induced thrombosis model. Defective platelet adhesion and thrombus growth over collagen was confirmed in vitro. Adenosine 5'-diphosphate (ADP) removal by apyrase diminished adhesion and thrombus growth of control platelets to the level of SERCA3(-/-) platelets. Aggregation, dense granule secretion, and Ca(2+) mobilization of SERCA3(-/-) platelets induced by low collagen or low thrombin concentration were weaker than controls. Accordingly, SERCA3(-/-) platelets exhibited a partial defect in total stored Ca(2+) and in Ca(2+) store reuptake following thrombin stimulation. Importantly ADP, but not serotonin, rescued aggregation, secretion, and Ca(2+) mobilization in SERCA3(-/-) platelets, suggesting specificity. Dense granules appeared normal upon electron microscopy, mepacrine staining, and total serotonin content, ruling out a dense granule defect. ADP induced normal platelet aggregation, excluding a defect in ADP activation pathways. The SERCA3-specific inhibitor 2,5-di-(tert-butyl)-1,4-benzohydroquinone diminished both Ca(2+) mobilization and secretion of control platelets, as opposed to the SERCA2b inhibitor thapsigargin. This confirmed the specific role of catalytically active SERCA3 in ADP secretion. Accordingly, SERCA3-dependent Ca(2+) stores appeared depleted in SERCA3(-/-) platelets. Finally, αIIbβ3 integrin blockade did not affect SERCA3-dependent secretion, therefore proving independent of αIIbβ3 engagement. Altogether, these results show that SERCA3-dependent Ca(2+) stores control a specific ADP secretion pathway required for full platelet secretion induced by agonists at low concentration and independent of αIIbβ3.

Molecular modulators of store-operated calcium entry

Three decades ago, store-operated Ca(2+) entry (SOCE) was identified as a unique mechanism for Ca(2+) entry through plasma membrane (PM) Ca(2+)-permeable channels modulated by the intracellular Ca(2+) stores, mainly the endoplasmic reticulum (ER). Extensive analysis of the communication between the ER and the PM leads to the identification of the protein STIM1 as the ER-Ca(2+) sensor that gates the Ca(2+) channels in the PM. Further analysis on the biophysical, electrophysiological and biochemical properties of STIM1-dependent Ca(2+) channels has revealed the presence of a highly Ca(2+)-selective channel termed Ca(2+) release-activated Ca(2+) channel (CRAC), consisting of Orai1 subunits, and non-selective cation channels named store-operated channels (SOC), including both Orai1 and TRPC channel subunits. Since the identification of the key elements of CRAC and SOC channels a number of intracellular modulators have been reported to play essential roles in the stabilization of STIM-Orai interactions, collaboration with STIM1 conformational changes or mediating slow Ca(2+)-dependent inactivation. Here, we review our current understanding of some of the key modulators of STIM1-Orai1 interaction, including the proteins CRACR2A, STIMATE, SARAF, septins, golli and ORMDL3.

Intracellular Trafficking, Localization, and Mobilization of Platelet-Borne Thiol Isomerases

OBJECTIVE

Thiol isomerases facilitate protein folding in the endoplasmic reticulum, and several of these enzymes, including protein disulfide isomerase and ERp57, are mobilized to the surface of activated platelets, where they influence platelet aggregation, blood coagulation, and thrombus formation. In this study, we examined the synthesis and trafficking of thiol isomerases in megakaryocytes, determined their subcellular localization in platelets, and identified the cellular events responsible for their movement to the platelet surface on activation.

APPROACH AND RESULTS

Immunofluorescence microscopy imaging was used to localize protein disulfide isomerase and ERp57 in murine and human megakaryocytes at various developmental stages. Immunofluorescence microscopy and subcellular fractionation analysis were used to localize these proteins in platelets to a compartment distinct from known secretory vesicles that overlaps with an inner cell-surface membrane region defined by the endoplasmic/sarcoplasmic reticulum proteins calnexin and sarco/endoplasmic reticulum calcium ATPase 3. Immunofluorescence microscopy and flow cytometry were used to monitor thiol isomerase mobilization in activated platelets in the presence and absence of actin polymerization (inhibited by latrunculin) and in the presence or absence of membrane fusion mediated by Munc13-4 (absent in platelets from Unc13d(Jinx) mice).

CONCLUSIONS

Platelet-borne thiol isomerases are trafficked independently of secretory granule contents in megakaryocytes and become concentrated in a subcellular compartment near the inner surface of the platelet outer membrane corresponding to the sarco/endoplasmic reticulum of these cells. Thiol isomerases are mobilized to the surface of activated platelets via a process that requires actin polymerization but not soluble N-ethylmaleimide-sensitive fusion protein attachment receptor/Munc13-4-dependent vesicular-plasma membrane fusion.

STIM and Orai1 Variants in Store-Operated Calcium Entry

Store-operated Ca²⁺ entry (SOCE) is an ubiquitous mechanism for Ca²⁺ entry in eukaryotic cells. This route for Ca²⁺ influx is regulated by the filling state of the intracellular Ca²⁺ stores communicated to the plasma membrane channels by the proteins of the Stromal Interaction Molecule (STIM) family, STIM1, and STIM2. Store-dependent, STIM1-modulated, channels include the Ca²⁺ release-activated Ca²⁺ channels, comprised of subunits of Orai proteins, as well as the store-operated Ca²⁺ (SOC) channels, involving Orai1, and members of the canonical transient receptor potential family of proteins. Recent studies have revealed the expression of splice variants of STIM1, STIM2, and Orai1 in different cell types. While certain variants are ubiquitously expressed, others, such as STIM1L, show a more restricted expression. The splice variants for STIM and Orai1 proteins exhibit significant functional differences and reveal that alternative splicing enhance the functional diversity of STIM1, STIM2, and Orai1 genes to modulate the dynamics of Ca²⁺ signals.

Second Messenger-Operated Calcium Entry Through TRPC6

Canonical transient receptor potential 6 (TRPC6) proteins assemble into heteromultimeric structures forming non-selective cation channels. In addition, many TRPC6-interacting proteins have been identified like some enzymes, channels, pumps, cytoskeleton-associated proteins, immunophilins, or cholesterol-binding proteins, indicating that TRPC6 are engaged into macromolecular complexes. Depending on the cell type and the experimental conditions used, TRPC6 activity has been reported to be controlled by diverse modalities. For instance, the second messenger diacylglycerol, store-depletion, the plant extract hyperforin or H2O2 have all been shown to trigger the opening of TRPC6 channels. A well-characterized consequence of TRPC6 activation is the elevation of the cytosolic concentration of Ca(2+). This latter response can reflect the entry of Ca(2+) through open TRPC6 channels but it can also be due to the Na(+)/Ca(2+) exchanger (operating in its reverse mode) or voltage-gated Ca(2+) channels (recruited in response to a TRPC6-mediated depolarization). Although TRPC6 controls a diverse array of biological functions in many tissues and cell types, its pathophysiological functions are far from being fully understood. This chapter covers some key features of TRPC6, with a special emphasis on their biological significance in kidney and blood cells.

Regulation of Platelet Function by Orai, STIM and TRP

Agonist-induced changes in cytosolic Ca(2+) concentration ([Ca(2+)]c) are central events in platelet physiology. A major mechanism supporting agonist-induced Ca(2+) signals is store-operated Ca(2+) entry (SOCE), where the Ca(2+) sensor STIM1 and the channels of the Orai family, as well as TRPC members are the key elements. STIM1-dependent SOCE plays a major role in collagen-stimulated Ca(2+) signaling, phosphatidylserine exposure and thrombin generation. Furthermore, studies involving Orai1 gain-of-function mutants and platelets from Orai1-deficient mice have revealed the importance of this channel in thrombosis and hemostasis to those found in STIM1-deficient mice indicating that SOCE might play a prominent role in thrombus formation. Moreover, increase in TRPC6 expression might lead to thrombosis in humans. The role of STIM1, Orai1 and TRPCs, and thus SOCE, in thrombus formation, suggests that therapies directed against SOCE and targeting these molecules during cardiovascular and cerebrovascular events could significantly improve traditional anti-thrombotic treatments.

Homer proteins mediate the interaction between STIM1 and Cav1.2 channels

STIM1 is a ubiquitous Ca2+ sensor of the intracellular, agonist-sensitive, Ca2+ stores that communicates the filling state of the Ca2+ compartments to plasma membrane store-operated Ca2+ (SOC) channels. STIM1 has been presented as a point of convergence between store-operated and voltage-operated Ca2+ influx, both inducing activation of SOC channels while suppressing Cav1.2 channels. Here we report that Homer proteins play a relevant role in the communication between STIM1 and Cav1.2 channels. HEK-293 cells transiently expressing Cav1.2 channel subunits α1, β2 and α2δ-1 exhibited a significant Ca2+ entry upon treatment with a high concentration of KCl. In Cav1.2-expressing cells, treatment with thapsigargin (TG), to induce passive discharge of the intracellular Ca2+ stores, resulted in Ca2+ influx that was significantly greater than in cells not expressing Cav1.2 channels, a difference that was abolished by nifedipine and diltiazem. Treatment with TG induces co-immunoprecipitation of Homer1 with STIM1 and the Cav1.2 α1 subunit. Impairment of Homer function by introduction of the synthetic PPKKFR peptide into cells, which emulates the proline-rich sequences of the PPXXF motif, or using siRNA Homer1, reduced the association of STIM1 and the Cav1.2 α1 subunit. These findings indicate that Homer is important for the association between both proteins. Finally, treatment with siRNA Homer1 or the PPKKFR peptide enhanced the nifedipine-sensitive component of TG response in Cav1.2-expressing cells. Altogether, these findings provide evidence for a new role of Homer1 supporting the regulation of Cav1.2 channels by STIM1.

STIM1 regulates TRPC6 heteromultimerization and subcellular location

STIM1 regulates TRPC6 heteromultimerization with other TRPC channels and internalization to the endoplasmic reticulum, thus reducing Ca2+ entry mediated by TRPC6.

The neglected CRAC proteins: Orai2, Orai3, and STIM2

Plasma-membrane-localized Orai1 ion channel subunits interacting with ER-localized STIM1 molecules comprise the major subunit composition responsible for calcium release-activated calcium channels. STIM1 "translates" the Ca(2+) store content into Orai1 activity, making it a store-operated channel. Surprisingly, in addition to being the physical activator, STIM1 also modulates Orai1 properties, including its inactivation and permeation (see Chapter 1). STIM1 is thus more than a pure Orai1 activator. Within the past 7 years following the discovery of STIM and Orai proteins, the molecular mechanisms of STIM1/Orai1 activity and their functional importance have been studied in great detail. Much less is currently known about the other isoforms STIM2, Orai2, and Orai3. In this chapter, we summarize the current knowledge about STIM2, Orai2, and Orai3 properties and function. Are these homologues mainly modulators of predominantly STIM1/Orai1-mediated complexes or do store-dependent or -independent functions such as regulation of basal Ca(2+) concentration and activation of Orai3-containing complexes by arachidonic acid or by estrogen receptors point toward their "true" physiological function? Is Orai2 the Orai1 of neurons? A major focus of the review is on the functional relevance of STIM2, Orai2, and Orai3, some of which still remains speculative.

Myeloperoxidase modulates human platelet aggregation via actin cytoskeleton reorganization and store-operated calcium entry

Myeloperoxidase (MPO) is a heme-containing enzyme released from activated leukocytes into the extracellular space during inflammation. Its main function is the production of hypohalous acids that are potent oxidants. MPO can also modulate cell signaling and inflammatory responses independently of its enzymatic activity. Because MPO is regarded as an important risk factor for cardiovascular diseases associated with increased platelet activity, we studied the effects of MPO on human platelet functional properties. Laser scanning confocal microscopy was used to reveal carbohydrate-independent MPO binding to human platelet membrane. Adding MPO to platelets did not activate their aggregation under basal conditions (without agonist). In contrast, MPO augmented agonist-induced platelet aggregation, which was not prevented by MPO enzymatic activity inhibitors. It was found that exposure of platelets to MPO leads to actin cytoskeleton reorganization and an increase in their elasticity. Furthermore, MPO evoked a rise in cytosolic Ca²⁺ through enhancement of store-operated Ca²⁺ entry (SOCE). Together, these findings indicate that MPO is not a direct agonist but rather a mediator that binds to human platelets, induces actin cytoskeleton reorganization and affects the mechanical stiffness of human platelets, resulting in potentiating SOCE and agonist-induced human platelet aggregation. Therefore, an increased activity of platelets in vascular disease can, at least partly, be provided by MPO elevated concentrations.

Regulation of STIM1/Orai1-dependent Ca2+ signalling in platelets

Platelet secretion and aggregation as well as thrombus formation of blood platelets critically depend on increase of cytosolic Ca2+ concentration ([Ca2+]i) mainly resulting from intracellular Ca2+ release followed by store operated Ca2+ entry (SOCE) through Ca2+ release activated channels (CRAC). SOCE is in part accomplished by the pore forming unit Orai and its regulator stromal interaction molecule (STIM). Orai1 and STIM1 transcription is stimulated by NF-κB (nuclear factor kappa B). Serum- and glucocorticoid-inducible kinase 1 (SGK1) up-regulates NF-κB-activity in megakaryocytes and thus Orai1-expression and SOCE in platelets. SGK1 is thus a powerful regulator of platelet Ca2+-signalling and thrombus formation and presumably participates in the regulation of platelet activation by a variety of hormones as well as clinical conditions (e.g. type 2 diabetes or metabolic syndrome) associated with platelet hyperaggregability and increased risk of thromboocclusive events. SOCE in platelets is further regulated by scaffolding protein Homer and chaperone protein cyclophilin A (CyPA). Additional potential regulators of Orai1/STIM1 and thus SOCE in platelets include AMP activated kinase (AMPK), protein kinase A (PKA), reactive oxygen species, lipid rafts, pH and mitochondrial Ca2+ buffering. Future studies are required defining the significance of those mechanisms for platelet Orai1 abundance and function, for SOCE into platelets and for platelet function in cardiovascular diseases.

Native STIM2 and ORAI1 proteins form a calcium-sensitive and thapsigargin-insensitive complex in cortical neurons

In non-excitatory cells, stromal interaction molecule 1 (STIM1) and STIM2 mediate store-operated calcium entry via an interaction with ORAI1 calcium channels. However, in neurons, STIM2 over-expression appears to play a role in calcium homeostasis that is different from STIM1 over-expression. The aim of this study was to establish the role and localization of native STIM2 in the neuronal cell. Co-immunoprecipitation experiments revealed that the interaction between endogenous STIM2 and ORAI1 was greater in a low-calcium medium than in a high-calcium medium. Using a Proximity Ligation Assay (PLA), the number of apparent complexes of endogenous STIM2 with ORAI1 was quantified. No change in the number of PLA signals was observed in the presence of thapsigargin, which depletes calcium from the endoplasmic reticulum (ER). However, the number of apparent STIM2-ORAI1 complexes increased when intracellular and subsequently ER calcium concentrations were decreased by BAPTA-AM or a low-calcium medium. Both Fura-2 acetoxymethyl ester calcium imaging and PLA in the same neuronal cell indicated that the calcium responses correlated strongly with the number of endogenous STIM2-ORAI1 complexes. The small drop in calcium levels in the ER caused by decreased intracellular calcium levels appeared to initiate the calcium-sensitive and thapsigargin-insensitive interaction between STIM2 and ORAI1. We show in neuronal somata the formation of endogenous complexes of stromal interaction molecule 2 (STIM2) with ORAI1 calcium channels. Their number increased when intracellular Ca²⁺ concentrations were decreased by the Ca²⁺ chelator BAPTA-AM or a low-calcium medium (EGTA), but did not in the presence of thapsigargin (TG). We conclude that the small drop of Ca²⁺ level in endoplasmic reticulum, due to the decreased level of intracellular Ca²⁺, is sufficient to trigger STIM2-ORAI1 complex formation in a thapsigargin-insensitive manner.

Orai-STIM-mediated Ca2+ release from secretory granules revealed by a targeted Ca2+ and pH probe

Secretory granules (SGs) sequester significant calcium. Understanding roles for this calcium and potential mechanisms of release is hampered by the difficulty of measuring SG calcium directly in living cells. We adapted the Förster resonance energy transfer-based D1-endoplasmic reticulum (ER) probe to develop a unique probe (D1-SG) to measure calcium and pH in secretory granules. It significantly localizes to SGs and reports resting free Ca(2+) of 69 ± 15 μM and a pH of 5.8. Application of extracellular ATP to activate P2Y receptors resulted in a slow monotonic decrease in SG Ca(2+) temporally correlated with the occurrence of store-operated calcium entry (SOCE). Further investigation revealed a unique receptor-mediated mechanism of calcium release from SGs that involves SG store-operated Orai channels activated by their regulator stromal interaction molecule 1 (STIM1) on the ER. SG Ca(2+) release is completely antagonized by a SOCE antagonist, by switching to Ca(2+)-free medium, and by overexpression of a dominant-negative Orai1(E106A). Overexpression of the CRAC activation domain (CAD) of STIM1 resulted in a decrease of resting SG Ca(2+) by ∼75% and completely abolished the ATP-mediated release of Ca(2+) from SGs. Overexpression of a dominant-negative CAD construct(CAD-A376K) induced no significant changes in SG Ca(2+). Colocalization analysis suggests that, like the plasma membrane, SG membranes also possess Orai1 channels and that during SG Ca(2+) release, colocalization between SGs and STIM1 increases. We propose Orai channel opening on SG membranes as a potential mode of calcium release from SGs that may serve to raise local cytoplasmic calcium concentrations and aid in refilling intracellular calcium stores of the ER and exocytosis.