The inositol trisphosphate receptor antagonist 2-aminoethoxydiphenylborate (2-APB) blocks Ca2+ entry channels in human platelets: cautions for its use in studying Ca2+ influx

Glucocorticoids paradoxically promote steroid resistance in B cell acute lymphoblastic leukemia through CXCR4/PLC signaling

Glucocorticoid (GC) resistance in childhood relapsed B-cell acute lymphoblastic leukemia (B-ALL) represents an important challenge. Despite decades of clinical use, the mechanisms underlying resistance remain poorly understood. Here, we report that in B-ALL, GC paradoxically induce their own resistance by activating a phospholipase C (PLC)-mediated cell survival pathway through the chemokine receptor, CXCR4. We identify PLC as aberrantly activated in GC-resistant B-ALL and its inhibition is able to induce cell death by compromising several transcriptional programs. Mechanistically, dexamethasone (Dex) provokes CXCR4 signaling, resulting in the activation of PLC-dependent Ca2+ and protein kinase C signaling pathways, which curtail anticancer activity. Treatment with a CXCR4 antagonist or a PLC inhibitor improves survival of Dex-treated NSG mice in vivo. CXCR4/PLC axis inhibition significantly reverses Dex resistance in B-ALL cell lines (in vitro and in vivo) and cells from Dex resistant ALL patients. Our study identifies how activation of the PLC signalosome in B-ALL by Dex limits the upfront efficacy of this chemotherapeutic agent.

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.

2,6-Difluorobenzamide derivatives as store-operated calcium channel (SOC) inhibitors

The Ca²⁺ entry from store-operated Ca²⁺ channel (SOC) is involved in regulating colorectal cancer progression, such as cell migration. SOC activation is due to STIM1 translocation and interaction with Orai1 upon Ca²⁺ depletion in the ER. Numerous SOC inhibitors, like 2-APB, have been developed and demonstrated their inhibition effects in the preclinical stage. However, most currently used SOC inhibitors have higher cytotoxicity or opposite effects at different doses, and the drugs to target SOC in the clinic are lacking. In this study, a total of 13 difluorobenzamide compounds had been synthesized and examined the inhibitory effects on SOC with Ca²⁺ imaging and wound-healing migration assay. Among them, 2,6-Difluoro-N-(5-(4-fluorophenyl)pyridine-2-yl)benzamide (MPT0M004, 8a) demonstrated a prominent inhibitory ability on SOC. Furthermore, the cell proliferation assay results showed that MPT0M004 (8a) had lower cytotoxicity than 2-APB, the reference compound. In the pharmacokinetic study, MPT0M004 (8a) has a long half-life (T_1/2 = 24 h) and lower daily dose administered intravenously with an oral bioavailability (F = 34%). Therefore, MPT0M004 (8a) has the potential to be a lead compound as a SOC inhibitor and further develop into a potential drug to treat colorectal cancer.

Role of Orai3 in the Pathophysiology of Cancer

The mammalian exclusive Orai3 channel participates in the generation and/or modulation of two independent Ca²⁺ currents, the store-operated current, I_crac, involving functional interactions between the stromal interaction molecules (STIM), STIM1/STIM2, and Orai1/Orai2/Orai3, as well as the store-independent arachidonic acid (AA) (or leukotriene C4)-regulated current I_arc, which involves Orai1, Orai3 and STIM1. Overexpression of functional Orai3 has been described in different neoplastic cells and cancer tissue samples as compared to non-tumor cells or normal adjacent tissue. In these cells, Orai3 exhibits a cell-specific relevance in Ca²⁺ influx. In estrogen receptor-positive breast cancer cells and non-small cell lung cancer (NSCLC) cells store-operated Ca²⁺ entry (SOCE) is strongly dependent on Orai3 expression while in colorectal cancer and pancreatic adenocarcinoma cells Orai3 predominantly modulates SOCE. On the other hand, in prostate cancer cells Orai3 expression has been associated with the formation of Orai1/Orai3 heteromeric channels regulated by AA and reduction in SOCE, thus leading to enhanced proliferation. Orai3 overexpression is associated with supporting several cancer hallmarks, including cell cycle progression, proliferation, migration, and apoptosis resistance. This review summarizes the current knowledge concerning the functional role of Orai3 in the pathogenesis of cancer.

Canonical transient receptor potential channels and their modulators: biology, pharmacology and therapeutic potentials

Canonical transient receptor potential channels (TRPCs) are nonselective, high calcium permeability cationic channels. The TRPCs family includes TRPC1, TRPC2, TRPC3, TRPC4, TRPC5, TRPC6, and TRPC7. These channels are widely expressed in the cardiovascular and nervous systems and exist in many other human tissues and cell types, playing several crucial roles in the human physiological and pathological processes. Hence, the emergence of TRPCs modulators can help investigate these channels’ applications in health and disease. It is worth noting that the TRPCs subfamilies have structural and functional similarities, which presents a significant difficulty in screening and discovering of TRPCs modulators. In the past few years, only a limited number of selective modulators of TRPCs were detected; thus, additional research on more potent and more selective TRPCs modulators is needed. The present review focuses on the striking desired therapeutic effects of TRPCs modulators, which provides intel on the structural modification of TRPCs modulators and further pharmacological research. Importantly, TRPCs modulators can significantly facilitate future studies of TRPCs and TRPCs related diseases.

Ca2+ Signaling as the Untact Mode during Signaling in Metastatic Breast Cancer

Metastatic features of breast cancer in the brain are considered a common pathology in female patients with late-stage breast cancer. Ca2+ signaling and the overexpression pattern of Ca2+ channels have been regarded as oncogenic markers of breast cancer. In other words, breast tumor development can be mediated by inhibiting Ca2+ channels. Although the therapeutic potential of inhibiting Ca2+ channels against breast cancer has been demonstrated, the relationship between breast cancer metastasis and Ca2+ channels is not yet understood. Thus, we focused on the metastatic features of breast cancer and summarized the basic mechanisms of Ca2+-related proteins and channels during the stages of metastatic breast cancer by evaluating Ca2+ signaling. In particular, we highlighted the metastasis of breast tumors to the brain. Thus, modulating Ca2+ channels with Ca2+ channel inhibitors and combined applications will advance treatment strategies for breast cancer metastasis to the brain.

The herbicide dinitramine affects the proliferation of murine testicular cells via endoplasmic reticulum stress-induced calcium dysregulation

The hazardous effects of herbicides are well known; however, their effects on the reproductive system remain unclear. In this study, we demonstrated the anti-proliferative effects of dinitramine (DN) on immature murine testicular cell lines (Leydig and Sertoli cells) mediated via endoplasmic reticulum (ER) stress-induced calcium dysregulation in the cytosol and mitochondria. The results demonstrated that the viability and proliferation of DN-treated TM3 and TM4 cells decreased significantly, even in the spheroid state. DN induced the apoptosis of TM3 and TM4 cells and decreased the expression of genes related to cell cycle progression. Treatment with DN increased the cytosolic and intramitochondrial levels of calcium by activating ER stress signals. DN activated the Erk/P38/Jnk Mapk pathway and inactivated the Pi3k/Akt pathway in murine testicular cells. Co-treatment with 2-aminoethoxydiphenyl borate (2-APB) mitigated DN-induced calcium upregulation in both testicular cell lines. Although 2-APB did not antagonize the anti-proliferative effect of DN in TM3 cells, treatment with 2-APB and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid restored the proliferation of DN-treated TM4 cells.

Role of IP3 Receptors in Shaping the Carotid Chemoreceptor Response to Hypoxia But Not to Hypercapnia in the Rat Carotid Body: An Evidence Review

This article addresses the disparity in the transduction pathways for hypoxic and hypercapnic stimuli in carotid body glomus cells. We investigated and reviewed the experimental evidence showing that the response to hypoxia, but not to hypercapnia, is mediated by 1,4,5-inositol triphosphate receptors (IP₃R/s) regulating the intracellular calcium content [Ca²⁺]_c in glomus cells. The rationale was based on the past observations that inhibition of oxidative phosphorylation leads to the explicit inhibition of the hypoxic chemoreflex. [Ca²⁺]_c changes were measured using cellular Ca²⁺-sensitive fluorescent probes, and carotid sinus nerve (CSN) sensory discharge was recorded with bipolar electrodes in in vitro perfused-superfused rat carotid body preparations. The cell-permeant, 2-amino-ethoxy-diphenyl-borate (2-APB; 100 μM) and curcumin (50 μM) were used as the inhibitors of IP₃R/s. These agents suppressed the [Ca²⁺]_c, and CSN discharge increases in hypoxia but not in hypercapnia, leading to the conclusion that only the hypoxic effects were mediated via modulation of IP₃R/s. The ATP-induced Ca²⁺ release from intracellular stores in a Ca²⁺-free medium was blocked with 2-APB, supporting this conclusion.

Focused Ultrasound Stimulates ER Localized Mechanosensitive PANNEXIN-1 to Mediate Intracellular Calcium Release in Invasive Cancer Cells

Focused ultrasound (FUS) is a rapidly developing stimulus technology with the potential to uncover novel mechanosensory dependent cellular processes. Since it is non-invasive, it holds great promise for future therapeutic applications in patients used either alone or as a complement to boost existing treatments. For example, FUS stimulation causes invasive but not non-invasive cancer cell lines to exhibit marked activation of calcium signaling pathways. Here, we identify the membrane channel PANNEXIN1 (PANX1) as a mediator for activation of calcium signaling in invasive cancer cells. Knockdown of PANX1 decreases calcium signaling in invasive cells, while PANX1 overexpression enhances calcium elevations in non-invasive cancer cells. We demonstrate that FUS may directly stimulate mechanosensory PANX1 localized in endoplasmic reticulum to evoke calcium release from internal stores. This process does not depend on mechanosensory stimulus transduction through an intact cytoskeleton and does not depend on plasma membrane localized PANX1. Plasma membrane localized PANX1, however, plays a different role in mediating the spread of intercellular calcium waves via ATP release. Additionally, we show that FUS stimulation evokes cytokine/chemokine release from invasive cancer cells, suggesting that FUS could be an important new adjuvant treatment to improve cancer immunotherapy.

2-Aminoethoxydiphenylborate (2-APB) inhibits release of phosphatidylserine-exposing extracellular vesicles from platelets

Activated, procoagulant platelets shed phosphatidylserine (PS)-exposing extracellular vesicles (EVs) from their surface in a Ca2+- and calpain-dependent manner. These PS-exposing EVs are prothrombotic and proinflammatory and are found at elevated levels in many cardiovascular and metabolic diseases. How PS-exposing EVs are shed is not fully understood. A clearer understanding of this process may aid the development of drugs to selectively block their release. In this study we report that 2-aminoethoxydiphenylborate (2-APB) significantly inhibits the release of PS-exposing EVs from platelets stimulated with the Ca2+ ionophore, A23187, or the pore-forming toxin, streptolysin-O. Two analogues of 2-APB, diphenylboronic anhydride (DPBA) and 3-(diphenylphosphino)-1-propylamine (DP3A), inhibited PS-exposing EV release with similar potency. Although 2-APB and DPBA weakly inhibited platelet PS exposure and calpain activity, this was not seen with DP3A despite inhibiting PS-exposing EV release. These data suggest that there is a further target of 2-APB, independent of cytosolic Ca2+ signalling, PS exposure and calpain activity, that is required for PS-exposing EV release. DP3A is likely to inhibit the same target, without these other effects. Identifying the target of 2-APB, DPBA and DP3A may provide a new way to inhibit PS-exposing EV release from activated platelets and inhibit their contribution to thrombosis and inflammation.

The Role of TRPC1 in Modulating Cancer Progression

Calcium ions (Ca²⁺) play an important role as second messengers in regulating a plethora of physiological and pathological processes, including the progression of cancer. Several selective and non-selective Ca²⁺-permeable ion channels are implicated in mediating Ca²⁺ signaling in cancer cells. In this review, we are focusing on TRPC1, a member of the TRP protein superfamily and a potential modulator of store-operated Ca²⁺ entry (SOCE) pathways. While TRPC1 is ubiquitously expressed in most tissues, its dysregulated activity may contribute to the hallmarks of various types of cancers, including breast cancer, pancreatic cancer, glioblastoma multiforme, lung cancer, hepatic cancer, multiple myeloma, and thyroid cancer. A range of pharmacological and genetic tools have been developed to address the functional role of TRPC1 in cancer. Interestingly, the unique role of TRPC1 has elevated this channel as a promising target for modulation both in terms of pharmacological inhibition leading to suppression of tumor growth and metastasis, as well as for agonistic strategies eliciting Ca²⁺ overload and cell death in aggressive metastatic tumor cells.

Fucoidan Derived from Fucus vesiculosus Inhibits the Development of Human Ovarian Cancer via the Disturbance of Calcium Homeostasis, Endoplasmic Reticulum Stress, and Angiogenesis

Marine organisms are sources of several natural compounds with potential clinical use. However, only a few marine-based pharmaceuticals have been approved for use due to limited knowledge on their biological activities. Here, we identified the functional role of fucoidan extracted from Fucus vesiculosus on ovarian cancer. Fucoidan increased the death of ES-2 and OV-90 cells, through a reduction in proliferation, cell cycle arrest, releases of cytochrome c, reactive oxygen species (ROS) generation, and endoplasmic reticulum (ER) stress. Additionally, fucoidan increased the concentration of cytosolic and mitochondrial calcium in both cells. The decrease of cell proliferation was controlled by the inactivation of PI3K and MAPK signaling cascades in ES-2 and OV-90 cells. In a toxicity assay with normal zebrafish larvae, fucoidan did not induce toxicity, cardiotoxicity, development, kinesis, and apoptosis at different concentrations. However, it disrupted tumor formation and vascular development in a zebrafish xenograft model and angiogenesis transgenic (Tg, fli1-eGFP) model, respectively. Collectively, the results indicate that fucoidan may be a novel pharmaceutical for the management of human ovarian cancer.

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.

Long-term facilitation of catecholamine secretion from adrenal chromaffin cells of neonatal rats by chronic intermittent hypoxia

In neonates, catecholamine (CA) secretion from adrenal medullary chromaffin cells (AMC) is an important mechanism for maintaining homeostasis during hypoxia. Nearly 90% of premature infants experience chronic intermittent hypoxia (IH) because of high incidence of apnea of prematurity, which is characterized by periodic stoppage of breathing. The present study examined the effects of repetitive hypoxia, designed to mimic apnea of prematurity, on CA release from AMC of neonatal rats. Neonatal rats were exposed to either control conditions or chronic intermittent hypoxia (IH) from ages postnatal days 0-5 (P0-P5), and CA release from adrenal medullary slices was measured after challenge with repetitive hypoxia (5 episodes of 30-s hypoxia, Po₂ ~35 mmHg). In response to repetitive hypoxia, chronic IH-treated AMC exhibited sustained CA release, and this phenotype was not seen in control AMC. The sustained CA release was associated with long-lasting elevation of intracellular Ca²⁺ concentration ([Ca²⁺]_i), which was due to store-operated Ca²⁺ entry (SOCE). 2-Aminoethoxydiphenyl borate, an inhibitor of SOCE, prevented the long-lasting [Ca²⁺]_i elevation and CA release. Repetitive hypoxia increased H₂O₂ abundance, and polyethylene glycol (PEG)-catalase, a scavenger of H₂O₂ blocked this effect. PEG-catalase also prevented repetitive hypoxia-induced SOCE activation, sustained [Ca²⁺]_i elevation, and CA release. These results demonstrate that repetitive hypoxia induces long-term facilitation of CA release in chronic IH-treated neonatal rat AMC through sustained Ca²⁺ influx mediated by SOCE.NEW & NOTEWORTHY Apnea of prematurity and the resulting chronic intermittent hypoxia are major clinical problems in neonates born preterm. Catecholamine release from adrenal medullary chromaffin cells maintains homeostasis during hypoxia in neonates. Our results demonstrate that chronic intermittent hypoxia induces a hitherto uncharacterized long-term facilitation of catecholamine secretion from neonatal rat chromaffin cells in response to repetitive hypoxia, simulating hypoxic episodes encountered during apnea of prematurity. The sustained catecholamine secretion might contribute to cardiovascular morbidities in infants with apnea of prematurity.

Maintenance of homeostatic plasticity at the Drosophila neuromuscular synapse requires continuous IP3-directed signaling

Synapses and circuits rely on neuroplasticity to adjust output and meet physiological needs. Forms of homeostatic synaptic plasticity impart stability at synapses by countering destabilizing perturbations. The Drosophila melanogaster larval neuromuscular junction (NMJ) is a model synapse with robust expression of homeostatic plasticity. At the NMJ, a homeostatic system detects impaired postsynaptic sensitivity to neurotransmitter and activates a retrograde signal that restores synaptic function by adjusting neurotransmitter release. This process has been separated into temporally distinct phases, induction and maintenance. One prevailing hypothesis is that a shared mechanism governs both phases. Here, we show the two phases are separable. Combining genetics, pharmacology, and electrophysiology, we find that a signaling system consisting of PLCβ, inositol triphosphate (IP₃), IP₃ receptors, and Ryanodine receptors is required only for the maintenance of homeostatic plasticity. We also find that the NMJ is capable of inducing homeostatic signaling even when its sustained maintenance process is absent.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Pyrtriazoles, a Novel Class of Store-Operated Calcium Entry Modulators: Discovery, Biological Profiling, and in Vivo Proof-of-Concept Efficacy in Acute Pancreatitis

In recent years, channels that mediate store-operated calcium entry (SOCE, i.e., the ability of cells to sense a decrease in endoplasmic reticulum luminal calcium and induce calcium entry across the plasma membrane) have been associated with a number of disorders, spanning from immune disorders to acute pancreatitis and have been suggested to be druggable targets. In the present contribution, we exploited the click chemistry approach to synthesize a class of SOCE modulators where the arylamide substructure that characterizes most inhibitors so far described is substituted by a 1,4-disubstituted 1,2,3-triazole ring. Within this series, inhibitors of SOCE were identified and the best compound proved effective in an animal model of acute pancreatitis, a disease characterized by a hyperactivation of SOCE. Strikingly, two enhancers of the process were discovered, affording invaluable research tools to further explore the (patho)physiological role of capacitative calcium entry.

Gold nanoparticle-mediated laser stimulation induces a complex stress response in neuronal cells

Stimulation of neuronal cells generally resorts to electric signals. Recent advances in laser-based stimulation methods could present an alternative with superior spatiotemporal resolution. The avoidance of electronic crosstalk makes these methods attractive for in vivo therapeutic application. In particular, nano-mediators, such as gold nanoparticles, can be used to transfer the energy from a laser pulse to the cell membrane and subsequently activate excitable cells. Although the underlying mechanisms of neuronal activation have been widely unraveled, the overall effect on the targeted cell is not understood. Little is known about the physiological and pathophysiological impact of a laser pulse targeted onto nanoabsorbers on the cell membrane. Here, we analyzed the reaction of the neuronal murine cell line Neuro-2A and murine primary cortical neurons to gold nanoparticle mediated laser stimulation. Our study reveals a severe, complex and cell-type independent stress response after laser irradiation, emphasizing the need for a thorough assessment of this approach’s efficacy and safety.

TRPV3 Channel in Keratinocytes in Scars with Post-Burn Pruritus

Post-burn pruritus is a common and distressing sequela of burn scars. Empirical antipruritic treatments usually fail to have a satisfactory outcome because of their limited selectivity and possible side effects. Therefore, novel drug targets need to be identified. Here, we aimed to investigate the possible role of protease-activated receptor 2 (PAR2) and transient receptor potential vanniloid 3 (TRPV3), along with the relation of TRPV3 to thymic stromal lymphopoietin (TSLP). Specimens from normal (unscarred) or burn-scarred (with or without pruritus) tissue were obtained from burn patients for this study. In each sample, the keratinocytes were isolated and cultured, and the intracellular Ca²⁺ level at the time of stimulation of each factor was quantified and the interaction was screened. PAR2 function was reduced by antagonism of TRPV3. Inhibiting protein kinase A (PKA) and protein kinase C (PKC) reduced TRPV3 function. TSLP mRNA and protein, and TSLPR protein expressions, increased in scars with post-burn pruritus, compared to scars without it or to normal tissues. In addition, TRPV1 or TRPV3 activation induced increased TSLP expression. Conclusively, TRPV3 may contribute to pruritus in burn scars through TSLP, and can be considered a potential therapeutic target for post-burn pruritus.

RCN1 suppresses ER stress-induced apoptosis via calcium homeostasis and PERK-CHOP signaling

Endoplasmic reticulum (ER) stress is caused by the disturbance of ER homeostasis and leads to the activation of the unfolded protein response (UPR), which alleviates stress at an early stage and triggers apoptosis if homeostasis fails over a prolonged timeframe. Here, we report that reticulocalbin 1 (RCN1), a member of the CREC family, is transactivated by nuclear factor kappa B (NF-κB) during ER stress and inhibits ER stress-induced apoptosis. The depletion of RCN1 increases the UPR during drug-induced ER stress by activating PRKR-like ER kinase–CCAAT/enhancer-binding protein-homologous protein (PERK–CHOP) signaling, thus inducing apoptosis. Furthermore, we found that the first two EF-hand calcium-binding motifs of RCN1 specifically interact with inositol 1,4,5-trisphosphate (IP₃) receptor type 1 (IP₃R1) on loop 3 of its ER luminal domain and inhibit ER calcium release and apoptosis. Together, these data indicate that RCN1, a target of NF-κB, suppresses ER calcium release by binding to IP₃R1 and decreases the UPR, thereby inhibiting ER stress-induced apoptosis.

TRP channels: potential drug target for neuropathic pain

Neuropathic pain is a debilitating disease which affects central as well as peripheral nervous system. Transient receptor potential (TRP) channels are ligand-gated ion channels that detect physical and chemical stimuli and promote painful sensations via nociceptor activation. TRP channels have physiological role in the mechanisms controlling several physiological responses like temperature and mechanical sensations, response to painful stimuli, taste, and pheromones. TRP channel family involves six different TRPs (TRPV1, TRPV2, TRPV3, TRPV4, TRPM8, and TRPA1) which are expressed in pain sensing neurons and primary afferent nociceptors. They function as transducers for mechanical, chemical, and thermal stimuli into inward currents, an essential first step for provoking pain sensations. TRP ion channels activated by temperature (thermo TRPs) are important molecular players in acute, inflammatory, and chronic pain states. Different degree of heat activates four TRP channels (TRPV1-4), while cold temperature ranging from affable to painful activate two indistinctly related thermo TRP channels (TRPM8 and TRPA1). Targeting primary afferent nociceptive neurons containing TRP channels that play pivotal role in revealing physical stimuli may be an effective target for the development of successful pharmacotherapeutics for clinical pain syndromes. In this review, we highlighted the potential role of various TRP channels in different types of neuropathic pain. We also discussed the pharmacological activity of naturally and synthetically originated TRP channel modulators for pharmacotherapeutics of nociception and neuropathic pain.

Role of forward-motility-stimulating factor as an extracellular activator of soluble adenylyl cyclase

Forward-motility-stimulating factor (FMSF) is a protein, originally purified from bubaline serum, that promotes progressive motility of mature spermatozoa. FMSF binds to sperm surface receptors and activates transmembrane adenylyl cyclase (tmAC), causing a rise in intracellular cyclic AMP level ([cAMP]i) and subsequent activation of a protein kinase A/tyrosine kinase-mediated pathway that enhances forward motility. This article further evaluates how FMSF works in the caprine system, particularly identifying the stimulatory effect of this glycoprotein on soluble adenylyl cyclase (sAC). Elevated [cAMP]i, initially resulting from FMSF-dependent activation of tmAC, was associated with the release of Ca(2+) from an intracellular calcium store in the sperm head, likely via an inositol triphosphate-sensitive calcium ion channel. This peak Ca(2+) concentration of ∼125-175 nM was capable of stimulating sAC in vitro in a calmodulin-independent manner, thereby triggering more cAMP production. Our model proposes that a positive-feedback loop mediated by cAMP and Ca(2+) is established in FMSF-stimulated sperm, with cAMP playing the role of a chemical messenger at multiple steps, resulting in the observed progressive motility. Thus, FSMF stimulates a novel signaling cascade that synergistically activate both tmAC and sAC to achieve forward sperm motility.

Ca(2+) is a Regulator of the WNK/OSR1/NKCC Pathway in a Human Salivary Gland Cell Line

Wnk kinase maintains cell volume, regulating various transporters such as sodium-chloride cotransporter, potassium-chloride cotransporter, and sodium-potassium-chloride cotransporter 1 (NKCC1) through the phosphorylation of oxidative stress responsive kinase 1 (OSR1) and STE20/SPS1-related proline/alanine-rich kinase (SPAK). However, the activating mechanism of Wnk kinase in specific tissues and specific conditions is broadly unclear. In the present study, we used a human salivary gland (HSG) cell line as a model and showed that Ca²⁺ may have a role in regulating Wnk kinase in the HSG cell line. Through this study, we found that the HSG cell line expressed molecules participating in the WNK-OSR1-NKCC pathway, such as Wnk1, Wnk4, OSR1, SPAK, and NKCC1. The HSG cell line showed an intracellular Ca²⁺ concentration ([Ca²⁺]_i) increase in response to hypotonic stimulation, and the response was synchronized with the phosphorylation of OSR1. Interestingly, when we inhibited the hypotonically induced [Ca²⁺]_i increase with nonspecific Ca²⁺ channel blockers such as 2-aminoethoxydiphenyl borate, gadolinium, and lanthanum, the phosphorylated OSR1 level was also diminished. Moreover, a cyclopiazonic acid-induced passive [Ca²⁺]_i elevation was evoked by the phosphorylation of OSR1, and the amount of phosphorylated OSR1 decreased when the cells were treated with BAPTA, a Ca²⁺ chelator. Finally, through that process, NKCC1 activity also decreased to maintain the cell volume in the HSG cell line. These results indicate that Ca²⁺ may regulate the WNK-OSR1 pathway and NKCC1 activity in the HSG cell line. This is the first demonstration that indicates upstream Ca²⁺ regulation of the WNK-OSR1 pathway in intact cells.

Mepivacaine-induced intracellular calcium increase appears to be mediated primarily by calcium influx in rat aorta without endothelium

Background

Mepivacaine induces contraction or decreased blood flow both in vivo and in vitro. Vasoconstriction is associated with an increase in the intracellular calcium concentration ([Ca²⁺]_i). However, the mechanism responsible for the mepivacaine-evoked [Ca²⁺]_i increase remains to be determined. Therefore, the objective of this in vitro study was to examine the mechanism responsible for the mepivacaine-evoked [Ca²⁺]_i increment in isolated rat aorta.

Methods

Isometric tension was measured in isolated rat aorta without endothelium. In addition, fura-2 loaded aortic muscle strips were illuminated alternately (48 Hz) at two excitation wavelengths (340 and 380 nm). The ratio of F340 to F380 (F340/F380) was regarded as an amount of [Ca²⁺]_i. We investigated the effects of nifedipine, 2-aminoethoxydiphenylborate (2-APB), gadolinium chloride hexahydrate (Gd³⁺), low calcium level and Krebs solution without calcium on the mepivacaine-evoked contraction in isolated rat aorta and on the mepivacaine-evoked [Ca²⁺]_i increment in fura-2 loaded aortic strips. We assessed the effect of verapamil on the mepivacaine-evoked [Ca²⁺]_i increment.

Results

Mepivacaine produced vasoconstriction and increased [Ca²⁺]_i. Nifedipine, 2-APB and low calcium attenuated vasoconstriction and the [Ca²⁺]_i increase evoked by mepivacaine. Verapamil attenuated the mepivacaine-induced [Ca²⁺]_i increment. Calcium-free solution almost abolished mepivacaine-induced contraction and strongly attenuated the mepivacaineinduced [Ca²⁺]_i increase. Gd³⁺ had no effect on either vasoconstriction or the [Ca²⁺]_i increment evoked by mepivacaine.

Conclusions

The mepivacaine-evoked [Ca²⁺]_i increment, which contributes to mepivacaine-evoked contraction, appears to be mediated mainly by calcium influx and partially by calcium released from the sarcoplasmic reticulum.

Altered platelet calsequestrin abundance, Na⁺/Ca²⁺ exchange and Ca²⁺ signaling responses with the progression of diabetes mellitus

INTRODUCTION

Downregulation of calsequestrin (CSQ), a major Ca(2+) storage protein, may contribute significantly to the hyperactivity of internal Ca(2+) ([Ca(2+)]i) in diabetic platelets. Here, we investigated changes in CSQ-1 abundance, Ca(2+) signaling and aggregation responses to stimulation with the progression of diabetes, especially the mechanism(s) underlying the exaggerated Ca(2+) influx in diabetic platelets.

MATERIALS AND METHODS

Type 1 diabetes was induced by streptozotocin in rats. Platelet [Ca(2+)]i and aggregation responses upon ADP stimulation were assessed by fluorescence spectrophotometry and aggregometry, respectively. CSQ-1 expression was evaluated using western blotting.

RESULTS

During the 12-week course of diabetes, the abundance of CSQ-1, basal [Ca(2+)]i and ADP-induced Ca(2+) release were progressively altered in diabetic platelets, while the elevated Ca(2+) influx and platelet aggregation were not correlated with diabetes development. 2-Aminoethoxydiphenyl borate, the store-operated Ca(2+) channel blocker, almost completely abolished ADP-induced Ca(2+) influx in normal and diabetic platelets, whereas nifedipine, an inhibitor of the nicotinic acid adenine dinucleotide phosphate receptor, showed no effect. Additionally, inhibition of Na(+)/Ca(2+) exchange induced much slower Ca(2+) extrusion and more Ca(2+) influx in normal platelets than in diabetic platelets. Furthermore, under the condition of Ca(2+)-ATPase inhibition, ionomycin caused greater Ca(2+) mobilization and Ca(2+) influx in diabetic platelets than in normal platelets.

CONCLUSIONS

These data demonstrate that platelet hyperactivity in diabetes is caused by several integrated factors. Besides the downregulation of CSQ-1 that mainly disrupts basal Ca(2+) homeostasis, insufficient Na(+)/Ca(2+) exchange also contributes, at least in part, to the hyperactive Ca(2+) response to stimulation in diabetic platelets.

Activation of ERK1/2 by store-operated calcium entry in rat parotid acinar cells

The regulation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) plays a critical role in a variety of cellular processes, including transcription, protein activation, vesicle trafficking, and ion movement across epithelial cells. In many cells, the activation of phospholipase C-coupled receptors hydrolyzes membrane phosphoinositides and produces the depletion of endoplasmic reticulum Ca²⁺ stores, followed by the sustained elevation of [Ca²⁺]_i from Ca²⁺ entry across the plasma membrane via store-operated Ca²⁺ entry (SOCE). Ca²⁺ entry is also increased in a store-independent manner by arachidonate-regulated Ca²⁺ (ARC) channels. Using rat parotid salivary gland cells, we examined multiple pathways of Ca²⁺ entry/elevation to determine if they activated cell signaling proteins and whether this occurred in a pathway-dependent manner. We observed that SOCE activates extracellular signal-related kinases 1 and 2 (ERK1/2) to ∼3-times basal levels via a receptor-independent mechanism when SOCE was initiated by depleting Ca²⁺ stores using the endoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin (TG). TG-initiated ERK1/2 phosphorylation increased as rapidly as that initiated by the muscarinic receptor agonist carbachol, which promoted an increase to ∼5-times basal levels. Notably, ERK1/2 phosphorylation was not increased by the global elevation of [Ca²⁺]_i by Ca²⁺ ionophore or by Ca²⁺ entry via ARC channels in native cells, although ERK1/2 phosphorylation was increased by Ca²⁺ ionophore in Par-C10 and HSY salivary cell lines. Agents and conditions that blocked SOCE in native cells, including 2-aminoethyldiphenyl borate (2-APB), SKF96363, and removal of extracellular Ca²⁺, also reduced TG- and carbachol-stimulated ERK1/2 phosphorylation. TG-promoted ERK1/2 phosphorylation was blocked when SRC and Protein Kinases C (PKC) were inhibited, and it was blocked in cells pretreated with β-adrenergic agonist isoproterenol. These observations demonstrate that ERK1/2 is activated by a selective mechanism of Ca²⁺ entry (SOCE) in these cells, and suggest that ERK1/2 may contribute to events downstream of SOCE.

A soft coral natural product, 11-episinulariolide acetate, inhibits gene expression of cyclooxygenase-2 and interleukin-8 through attenuation of calcium signaling

Epidermal growth factor receptor (EGFR) is overexpressed in many types of cancer cells. EGFR-mediated signaling involves inflammatory gene expression including cyclooxygenase (COX)-2 and interleukin (IL)-8, and is associated with cancer pathogenesis. In a search of phytochemicals with anti-inflammatory activity, the COX-2 and IL-8 inhibitory activities of some marine compounds were examined. After screening these compounds 11-episinulariolide acetate (1) from soft coral exhibited the most potent activity. Reverse-transcription PCR; western blotting; ELISA and luciferase assays were used to test the effect of compound 1 on EGF-stimulated expressions of COX-2 and IL-8 in A431 human epidermoid carcinoma cells. After exposure to 10 μM of compound 1, expression levels of COX-2 and IL-8 were reduced. In addition; intracellular Ca²⁺ increase and Ca²⁺-dependent transcription factor activation were blocked by compound 1. Thus, compound 1 can potentially serve as a lead compound for targeting Ca²⁺ signaling-dependent inflammatory diseases.

CRAC inhibitors: identification and potential

BACKGROUND

Ca(2+) release-activated Ca(2+) (CRAC) channels, a subfamily of store-operated channels, play an essential role in various diseases such as immune disorders and allergic responses.

OBJECTIVE

The successful treatment of these diseases requires the identification of specific inhibitors. So far, a variety of chemical compounds blocking CRAC have been identified; however, they have all turned out to be less specific. Recently two proteins, STIM1 and ORAI1, have been identified as the essential components that fully reconstitute CRAC currents with a similar biophysical fingerprint.

METHOD

These two proteins and their activation process represent direct targets for the application of specific CRAC inhibitors.

RESULTS/CONCLUSION

For drug development, fluorescence microscopy adaptable for high-throughput screening will provide a powerful assay to mechanistically identify potential CRAC inhibitors that act on various stages within the STIM1/ORAI1 activation pathway visualized by fluorescent-tagged proteins.

STIM1-regulated Ca2+ influx across the apical and the basolateral membrane in colonic epithelium

In nonexcitable cells, store-operated Ca(2+) entry is the most important pathway for influx of extracellular Ca(2+) serving as a second messenger in the cytoplasm. The present study investigated the expression, localization and polar distribution of two key components of store-operated Ca(2+) entry identified, e.g., in lymphocytes or epithelial cell lines-STIM1 (stromal interacting molecule 1), working as a Ca(2+) sensor in the endoplasmic reticulum, and Orai1, working as the (or part of the) store-operated Ca(2+) channel in the plasma membrane-in a native intestinal epithelium, i.e., rat colon. Immunohistochemical investigations revealed expression of STIM1 and Orai1 in the rat colonic epithelium. Ca(2+) store depletion led to a translocation of STIM1 both to the basolateral as well as to the apical cell pole as observed by confocal microscopy. A Ca(2+) depletion/repletion protocol was used in Ussing chamber experiments to investigate the contribution of basolateral and apical store-operated Ca(2+) entry to the induction of anion secretion. These experiments revealed that Ca(2+)-dependent anion secretion was induced not only by basolateral Ca(2+) repletion but also, to a lesser extent, by apical Ca(2+) repletion. Both responses were suppressed by La(3+). The effect of basolateral Ca(2+) repletion was significantly inhibited by brefeldin A, a blocker of vesicular transport from the endoplasmic reticulum to the Golgi apparatus. In a final series of experiments, fura-2-loaded HT29/B6 cells were used. A carbachol-induced increase in the cytosolic Ca(2+) concentration was significantly reduced when cells were pretreated with siRNA against STIM1. In conclusion, these results demonstrate that STIM1 as a key component of intracellular Ca(2+) signaling is expressed by rat colonic epithelium and is involved in the regulation not only of basolateral but also of apical Ca(2+) influx.

The action of selective CRAC channel blockers is affected by the Orai pore geometry

As the molecular composition of calcium-release activated calcium (CRAC) channels has been unknown for two decades, elucidation of selective inhibitors has been considerably hampered. By the identification of the two key components of CRAC channels, STIM1 and Orai1 have emerged as promising targets for CRAC blockers. The aim of this study was to thoroughly characterize the effects of two selective CRAC channel blockers on currents derived from STIM1/Orai heterologoulsy expressed in HEK293 cells. The novel compounds GSK-7975A and GSK-5503A were tested for effects on STIM1 mediated Orai1 or Orai3 currents by whole-cell patch-clamp recordings and for the effects on STIM1 oligomerisation or STIM1/Orai coupling by FRET microscopy. To investigate their site of action, inhibitory effects of these molecules were explored using Orai pore mutants. The GSK blockers inhibited Orai1 and Orai3 currents with an IC₅₀ of approximately 4 μM and exhibited a substantially slower rate of onset than the typical pore blocker La³⁺, together with almost no current recovery upon wash-out over 4 min. For the less Ca²⁺-selective Orai1 E106D pore mutant, I_CRAC inhibition was significantly reduced. FRET experiments indicated that neither STIM1–STIM1 oligomerization nor STIM1–Orai1 coupling was affected by these compounds.

These CRAC channel blockers are acting downstream of STIM1 oligomerization and STIM1/Orai1 interaction, potentially via an allosteric effect on the selectivity filter of Orai. The elucidation of these CRAC current blockers represents a significant step toward the identification of CRAC channel-selective drug compounds.

Thrombin-stimulated discharge of calcium stores in human platelets: analysis of experimental data

The purpose of this research was to analyze experimental data concerning thrombin-stimulated discharge of calcium stores in human platelets contained in calcium-free medium in view of better understanding the mechanisms involved in calcium fluxes. The model curves are reasonably close to experimental data; the parameters of the models are related to the properties of the entities responsible for control or maintenance of cytosolic calcium concentration. It has been shown that: (a) time-course of calcium concentration in cytosol of human platelets can be acceptably modeled on the basis of reasonable assumptions concerning agonist stimulated calcium redistribution in cellular compartments; (b) those assumptions are of fundamental importance for the model (c) some parameters of the model (taken arbitrarily) cannot be estimated independently of others from fitting the model to experimental data available; (d) special experiments are necessary to determine the unknown parameters; (e) agonist-stimulated change of the permeability of endomembrane of calcium stores can be regarded as a pulse of the permeability; it can be modeled as a sequence of transitions of the system from inactive to active and to inactive state again.

Antithrombotic potential of blockers of store-operated calcium channels in platelets

OBJECTIVE

Platelet Orai1 channels mediate store-operated Ca(2+) entry (SOCE), which is required for procoagulant activity and arterial thrombus formation. Pharmacological blockage of these channels may provide a novel way of antithrombotic therapy. Therefore, the thromboprotective effect of SOCE blockers directed against platelet Orai1 is determined.

METHODS AND RESULTS

Candidate inhibitors were screened for their effects on SOCE in washed human platelets. Tested antagonists included the known compounds, SKF96365, 2-aminoethyl diphenylborate, and MRS1845 and the novel compounds, Synta66 and GSK-7975A. The potency of SOCE inhibition was in the order of Synta66>2-aminoethyl diphenylborate>GSK-7975A>SKF96365>MRS1845. The specificity of the first 3 compounds was verified with platelets from Orai1-deficient mice. Inhibitory activity on procoagulant activity and high-shear thrombus formation was assessed in plasma and whole blood. In the presence of plasma, all 3 compounds suppressed platelet responses and restrained thrombus formation under flow. Using a murine stroke model, arterial thrombus formation was provoked in vivo by transient middle cerebral artery occlusion. Postoperative administration of 2-aminoethyl diphenylborate markedly diminished brain infarct size.

CONCLUSIONS

Plasma-soluble SOCE blockers such as 2-aminoethyl diphenylborate suppress platelet-dependent coagulation and thrombus formation. The platelet Orai1 channel is a novel target for preventing thrombotic events causing brain infarction.

Ion channels in the regulation of platelet migration

Platelets have been shown to migrate and thus to invade the vascular wall. Platelet migration is stimulated by SDF-1. In other cell types, migration is dependent on Ca(2+) entry via Ca(2+) channels. Ca(2+) influx is sensitive to cell membrane potential which is maintained by K(+) channel activity and/or Cl(-) channel activity. The present study explored the role of ion channels in the regulation of SDF-1 induced migration. Platelets were isolated from human volunteers as well as from gene targeted mice lacking the Ca(2+) activated K(+) channel SK4 (sk4(-/-)) and their wild type littermates (sk4(+/+)). According to confocal microscopy human platelets expressed the Ca(2+) channel Orai1 and the Ca(2+)-activated K(+) channel K(Ca)3.1 (SK4). SDF-1 (100 ng/ml) stimulated migration in human platelets, an effect blunted by Orai1 inhibitors 2-aminoethoxydiphenyl borate 2-APB (10 μM) and SKF-96365 (10 μM), by unspecific K(+) channel inhibitor TEA (30 mM), by SK4 specific K(+) channel blocker clotrimazole (10 μM), but not by Cl(-) channel inhibitor 5-nitro-2-(3-phenylpropylamino) benzoic acid NPPB (100 μM). Significant stimulation of migration by SDF-1 was further observed in sk4(+/+) platelets but was virtually absent in sk4(-/-) platelets. In conclusion, platelet migration requires activity of the Ca(2+) channel Orai1 and of the Ca(2+) activated K(+) channel SK4, but not of NPPB-sensitive Cl(-) channels.

Levobupivacaine-induced contraction of isolated rat aorta is calcium dependent

Levobupivacaine is a long-acting local anesthetic that intrinsically produces vasoconstriction in isolated vessels. The goals of this study were to investigate the calcium-dependent mechanism underlying levobupivacaine-induced contraction of isolated rat aorta in vitro and to elucidate the pathway responsible for the endothelium-dependent attenuation of levobupivacaine-induced contraction. Isolated rat aortic rings were suspended to record isometric tension. Cumulative levobupivacaine concentration-response curves were generated in either the presence or absence of the antagonists verapamil, nifedipine, SKF-96365, 2-aminoethoxydiphenylborate, Gd(3+), N(W)-nitro-l-arginine methyl ester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), and methylene blue, either alone or in combination. Verapamil, nifedipine, SKF-96365, 2-aminoethoxydiphenylborate, low calcium concentrations, and calcium-free Krebs solution attenuated levobupivacaine-induced contraction. Gd(3+) had no effect on levobupivacaine-induced contraction. Levobupivacaine increased intracellular calcium levels in vascular smooth muscle cells. L-NAME, ODQ, and methylene blue increased levobupivacaine-induced contraction in endothelium-intact aorta. SKF-96365 attenuated calcium-induced contraction in a previously calcium-free isotonic depolarizing solution containing 100 mmol/L KCl. Levobupivacaine-induced contraction of rat aortic smooth muscle is mediated primarily by calcium influx from the extracellular space mainly via voltage-operated calcium channels and, in part, by inositol 1,4,5-trisphosphate receptor-mediated release of calcium from the sarcoplasmic reticulum. The nitric oxide - cyclic guanosine monophosphate pathway is involved in the endothelium-dependent attenuation of levobupivacaine-induced contraction.

Monitoring the intracellular store Ca2+ concentration in agonist-stimulated, intact human platelets by using Fluo-5N

BACKGROUND

Most Ca(2+) signaling research in platelets has relied solely on monitoring the cytosolic Ca(2+) concentration ([Ca(2+)](cyt)). Changes in [Ca(2+)](cyt) constitute the net effect of Ca(2+) fluxes into the cytosol across the plasma membrane (PM) and from intracellular stores, and Ca(2+) sequestration into the stores and Ca(2+) removal across the PM. This makes interpretation of the effects of pharmacologic or genetic interventions on Ca(2+) signaling difficult and subject to error.

OBJECTIVES

To validate the use of the low-affinity Ca(2+) indicator Fluo-5N to monitor the concentration of Ca(2+) in the intracellular stores ([Ca(2+)](st)) of human platelets as a first step in developing assays for a systems-level analysis of platelet Ca(2+) signaling.

METHODS

Fluo-5N-loaded and Fura-2-loaded human platelets were used to observe the effects of agonist stimulation and other manipulations on [Ca(2+)](cyt) and [Ca(2+)](st).

RESULTS

Fluo-5N fluorescence changed appropriately in response to compounds that induce passive depletion of intracellular Ca(2+) stores and to physiologic agonists. Ca(2+) reuptake inhibitors and blockers of Ca(2+) release channels had the expected effects on Fura-2 and Fluo-5N fluorescence. Agonist-evoked Ca(2+) release was reversed by Ca(2+) addition to the medium, and required intact Ca(2+) reuptake mechanisms. Store refilling was observed in the presence of sarcoplasmic/endoplasmic reticulum Ca(2+) -ATPase (SERCA) inhibitors and ionomycin, suggesting the presence of a non-SERCA Ca(2+) reuptake mechanism. Evidence for a role for Ca(2+) -induced Ca(2+) release in agonist-evoked responses was obtained.

CONCLUSIONS

Our data provide a validation of the use of Fluo-5N as a method for monitoring changes in [Ca(2+)](st) in human platelets.

The effects of 2-APB on the time-frequency distributions of gamma oscillations in rat hippocampal slices

We investigated the influence of 2-APB (2-aminoethoxy-diphenylborate) acute exposure on hippocampal oscillations using time-frequency analysis methods including continuous wavelet transform and short-time Fourier transform. We hypothesized that acute exposure to 2-APB drastically reduced the hippocampal gamma oscillations. We estimated the hippocampal oscillations' time-frequency representations from 24 hippocampal slices in five rats. Our results indicated that it took at least 100 ms to see any hippocampal activities in response to the 100 Hz stimulus. The hippocampal oscillations' spectral energies dominated in the 31-60 Hz and 61-90 Hz frequency bands in the early time (100-200 ms) segment post-stimulus and in the 31-60 Hz and 61-90 Hz frequency bands after 200 ms until 400 ms post-stimulus. They were noticeably reduced in the late time segment (above 400 ms). The hippocampal oscillations' spectral energies in the 31-60 and 61-90 Hz frequency bands still dominated the early time segment after the acute 2-APB exposure. The 2-APB exposure never changed the energy content in all three frequency bands in the early time segment (p > 0.01). The exposure significantly reduced the energy content in both the mid-time segment and in the 31-60 Hz frequency band (p < 0.001) and in both the second time segment and in the 61-90 Hz frequency band (p < 0.01). Additionally, in the late time segment, the energy content in all three frequency bands was notably reduced post-drug exposure (p < 0.001).

Carbon nanotubes activate blood platelets by inducing extracellular Ca2+ influx sensitive to calcium entry inhibitors

To elucidate a mechanism of prothrombotic effects of carbon nanotubes (CNTs), we report here that multiwalled CNTs activate blood platelets by inducing extracellular Ca(2+) influx that could be inhibited by calcium channel blockers SKF 96365 and 2-APB. We also demonstrate platelet aggregating activity of different single-walled and multiwalled CNTs. In addition, we show that CNT-induced platelet activation is associated with a marked release of platelet membrane microparticles positive for the granular secretion markers CD62P and CD63.

Hypoglycaemia predisposes platelets to death by affecting calcium homeostasis and mitochondrial integrity

Factors affecting platelet survival are poorly understood. To explore the hypothesis that platelet lifespan correlates with the lifespan of a key housekeeping process we subjected human platelets to in vitro incubation at 37 degrees C for 24 h to several days under hypoglycaemic conditions. Viability was assessed both by microscopy and flow cytometry using calcein-AM and/or FM4-64. In keeping with previous data we found that, under control conditions platelets died at a linear rate during 120 h of incubation. Hypoglycaemia did not affect the death rate but did lead to an increase in the frequency of platelets unable to accumulate the mitochondrial potentiometric dye 10-Nonyl Acridine Orange (NAO) and promoted platelet death in response to the pro-apoptotic molecule BH3I-2'. Hypoglycaemia led to an increase in intraplatelet calcium that could be prevented 2-aminoethoxydiphenylborate (2-APB), a store operated calcium channel (SOCC) blocker. However, this agent was unable to rescue the platelets' ability to accumulate NAO. These data suggest that extracellular glucose is utilised by platelets for calcium homeostasis and maintenance of mitochondrial integrity and that hypoglycaemia primes platelets for death.

Spontaneous transient depolarizations in lymphatic vessels of the guinea pig mesentery: pharmacology and implication for spontaneous contractility

Guinea pig mesenteric lymphatic vessels exhibit rhythmic constrictions induced by action potential (AP)-like spikes and initiated by entrainment of spontaneous transient depolarizations (STDs). To characterize STDs and the signaling mechanisms responsible for their occurrence, we used intracellular microelectrodes, Ca2+ imaging, and pharmacological agents. In our investigation of the role of intracellular Ca2+ released from Ca2+ stores, we observed that intracellular Ca2+ transients accompanied some STDs, although there were many exceptions where Ca2+ transients occurred without accompanying STDs. STD frequency and amplitude were markedly affected by activators/inhibitors of inositol 1,4,5-trisphosphate receptors (IP3Rs) but not by treatments known to alter Ca2+ release via ryanodine receptors. A role for Ca2+-activated Cl(-) (Cl(Ca)) channels was indicated, as STDs were dependent on the Cl(-) but not Na+ concentration of the superfusing solution and were inhibited by the Cl(Ca) channel blockers niflumic acid (NFA), anthracene 9-carboxylic acid, and 5-nitro-2-(3-phenylpropylamino)benzoic acid but not by the volume-regulated Cl(-) blocker DIDS. Increases in STD frequency and amplitude induced by agonist stimulation were also inhibited by NFA. Nifedipine, the hyperpolarization-activated inward current blocker ZD-7288, and the nonselective cation/store-operated channel blockers SKF-96365, Gd3+, and Ni2+ had no or marginal effects on STD activity. However, nifedipine, 2-aminoethoxydiphenyl borate, NFA, SKF-96365, Gd3+, and Ni2+ altered the occurrence of spontaneous APs. Our findings support a role for Ca2+ release through IP3Rs and a resultant opening of Cl(Ca) channels in STD generation and confirm the importance of these events in the initiation of lymphatic spontaneous APs and subsequent contractions. The abolition of spontaneous APs by blockers of other excitatory ion channels suggests a contribution of these conductances to lymphatic pacemaking.

2-Aminoethoxydiphenyl borate as a common activator of TRPV1, TRPV2, and TRPV3 channels

2-Aminoethoxydiphenyl borate (2APB) had been depicted as a universal blocker of transient receptor potential (TRP) channels. While evidence has accumulated showing that some TRP channels are indeed inhibited by 2APB, especially in heterologous expression systems, there are other TRP channels that are unaffected or affected very little by this compound. More interestingly, the thermosensitive TRPV1, TRPV2, and TRPV3 channels are activated by 2APB. This has been demonstrated both in heterologous systems and in native tissues that express these channels. A number of 2APB analogs have been examined for their effects on native store-operated channels and heterologously expressed TRPV3. These studies revealed a complex mechanism of action for 2APB and its analogs on ion channels. In this review, we have summarized the current results on 2APB-induced activation of TRPV1-3 and discussed the potential mechanisms by which 2APB may regulate TRP channels.

TRP channels in platelet function

Ca2+ entry forms an essential component of platelet activation; however, the mechanisms associated with this process are not understood. Ca2+ entry upon receptor activation occurs as a consequence of intracellular store depletion (referred to as store-operated Ca2+ entry or SOCE), a direct action of second messengers on cation entry channels or the direct occupancy of a ligand-gated P2(Xi) receptor. The molecular identity of the SOCE channel has yet to be established. Transient receptor potential (TRP) proteins are candidate cation entry channels and are classified into a number of closely related subfamilies including TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin) and TRPML (mucolipins). From the TRPC family, platelets have been shown to express TRPC6 and TRPC1, and are likely to express other TRPC and other TRP members. TRPC6 is suggested to be involved with receptor-activated, diacyl-glycerol-mediated cation entry. TRPC1 has been suggested to be involved with SOCE, though many of the suggested mechanisms remain controversial. As no single TRP channel has the properties described for SOCE in platelets, it is likely that it is composed of a heteromeric association of TRP and related subunits, some of which may be present in intracellular compartments in the resting cell.

Altered calcium signalling in platelets from bile-duct-ligated rats

In the present study, we have analysed the mechanisms of Ca2+ entry and release in platelets obtained from BDL (bile-duct-ligated) rats, 11–13 days and 4 weeks after surgery. Platelets were washed and loaded with fura-2, and [Ca2+]i (cytosolic Ca2+ concentration) was determined in cell suspensions by means of fluorescence spectroscopy. Basal [Ca2+]i was similar in platelets from BDL rats compared with those from their respective controls, both in the absence and presence of extracellular Ca2+. Platelet stimulation with thrombin in the absence and presence of extracellular Ca2+ induced a rapid rise in [Ca2+]i that was of greater magnitude in platelets from BDL rats than in controls. Ca2+ storage was significantly elevated in platelets from BDL rats, as well as the activity of SERCA (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase). Capacitative Ca2+ entry, as evaluated by inhibition of SERCA with thapsigargin, was also altered in platelets from BDL rats, having lower rates of Ca2+ entry. In conclusion, chronic BDL alters intracellular Ca2+ homoeostasis in platelets, such that an enhanced Ca2+ release is evoked by thrombin, which may be due to an increased amount of Ca2+ stored in the intracellular organelles and secondary to an enhanced activity of SERCA. These alterations are already evident before cirrhosis has completely developed and occurs during the cholestasis phase.

Calcium signalling in the endothelium

Elevations in cytosolic Ca2+ concentration are the usual initial response of endothelial cells to hormonal and chemical transmitters and to changes in physical parameters, and many endothelial functions are dependent upon changes in Ca2+ signals produced. Endothelial cell Ca2+ signalling shares similar features with other electrically non-excitable cell types, but has features unique to endothelial cells. This chapter discusses the major components of endothelial cell Ca2+ signalling.

PAR-1-stimulated factor IXa binding to a small platelet subpopulation requires a pronounced and sustained increase of cytoplasmic calcium

We previously reported that only a subpopulation of PAR-1-stimulated platelets binds coagulation factor IXa, since confirmed by other laboratories. Since calcium changes have been implicated in exposure of procoagulant aminophospholipids, we have now examined calcium fluxes in this subpopulation by measuring fluorescence changes in Fura Red/AM-loaded platelets following PAR-1 stimulation. While fluorescence changes in all platelets indicated calcium release from internal stores and influx of external calcium, a subpopulation of platelets displayed a pronounced increase in calcium transients by 15 s and positive factor IXa binding by 2 min, with calcium transients sustained for 45 min. Pretreatment of platelets with Xestospongin C to inhibit IP3-mediated dense tubule calcium release, and the presence of impermeable calcium channel blockers nifedipine, SKF96365, or LaCl3, inhibited PAR-1-induced development of a subpopulation with pronounced calcium transients, factor IXa binding, and platelet support of FXa generation, suggesting the importance of both release of calcium from internal stores and influx of extracellular calcium. When platelets were stimulated in EDTA for 5-20 min before addition of calcium, factor IXa binding sites developed on a smaller subpopulation but with unchanged rate, indicating sustained opening of calcium channels and continued availability of signaling elements required for binding site exposure. While pretreatment of platelets with 100 microM BAPTA/AM (Kd 160 nM) had minimal effects, 100 microM 5,5'-dimethylBAPTA/AM (Kd 40 nM) completely inhibited the appearance and function of the platelet subpopulation, indicating the importance of minor increases of cytoplasmic calcium. We conclude that PAR-1-stimulated development of factor IXa binding sites in a subpopulation of platelets is dependent upon release of calcium from internal stores leading to sustained and pronounced calcium transients.

TRPC7 is a receptor-operated DAG-activated channel in human keratinocytes

Muscarinic and purinergic receptors expressed in keratinocytes are an important part of a functional system for cell growth. While several aspects of this process are clearly dependent on Ca(2+) homeostasis, less is known about the mechanisms controlling Ca(2+) entry during epidermal receptor stimulation. We used patch-clamp technique to study responses to carbachol (CCh) and adenosine triphosphate (ATP) in HaCaT human keratinocytes. Both agonists induced large currents mediated by cation-selective channels about three times more permeable to Ca(2+) than Na(+), suggesting that they play an important role in receptor-operated Ca(2+) entry. CCh- and ATP-induced currents were inhibited by 1-[6-([(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino)hexyl]-1H-pyrrole-2,5-dione, a phospholipase C (PLC) blocker. Investigation of the pathways downstream of PLC activation revealed that InsP(3) did not affect the agonist responses. In contrast, 1-oleoyl-2-acetyl-sn-glycerol (OAG), a membrane-permeable analog of 1,2-diacylglycerol (DAG), evoked a similar cation current. This action appears to be direct, since the effects of activators or inhibitors of protein kinase C were comparatively small. Finally, transient receptor potential canonical 7 (TRPC7) specific knockdown by antisense oligonucleotides led to a decrease in ATP- and CCh-induced calcium entry, as well as OAG-evoked current. We concluded that activation of both muscarinic and purinergic receptors via a common DAG-dependent link opens Ca(2+)-permeable TRPC7 channels.