Dynamics of calcium fluxes in human platelets assessed in calcium-free medium

NO1, a New Sigma 2 Receptor/TMEM97 Fluorescent Ligand, Downregulates SOCE and Promotes Apoptosis in the Triple Negative Breast Cancer Cell Lines

(1) Background: The structure of the Sigma 2 receptor/TMEM97 (σ2RTMEM97) has recently been reported. (2, 3) Methods and results: We used genetic and biochemical approaches to identify the molecular mechanism downstream of σ2R/TMEM97. The novel σ2R/TMEM97 fluorescent ligand, NO1, reduced the proliferation and survival of the triple negative breast cancer cell lines (TNBC: MDA-MB-231 and MDA-MB-468 cell lines), due to NO1-induced apoptosis. Greater bioaccumulation and faster uptake of NO1 in MDA-MB-231 cells compared to MCF10A or MCF7 cell lines were also shown. Accordingly, elevated σ2R/TMEM97 expression was confirmed by Western blotting. In contrast to NO1, other σ2R/TMEM97 ligands, such as SM21 and PB28, enhanced MDA-MB-231 cell proliferation and migration. Store-operated calcium entry (SOCE) is crucial for different cancer hallmarks. Here, we show that NO1, but not other σ2R/TMEM97 ligands, reduced SOCE in MDA-MB-231 cells. Similarly, TMEM97 silencing in MDA-MB-231 cells also impaired SOCE. NO1 administration downregulated STIM1-Orai1 interaction, probably by impairing the positive regulatory effect of σ2R/TMEM97 on STIM1, as we were unable to detect interaction with Orai1. (4) Conclusion: σ2R/TMEM97 is a key protein for the survival of triple negative breast cancer cells by promoting SOCE; therefore, NO1 may become a good pharmacological tool to avoid their proliferation.

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.

The membrane potential modulates thrombin-stimulated Ca²⁺ mobilization and platelet aggregation

G protein-coupled receptors can be directly modulated by changes in transmembrane voltage in a variety of cell types. Here we show that, while changes in the membrane voltage itself do not induce detectable modifications in the cytosolic Ca(2+) concentration, platelet stimulation with thrombin or the PAR-1 and PAR-4 agonist peptides SFLLRN and AYPGKF, respectively, results in Ca(2+) release from intracellular stores that is sensitive to the membrane depolarisation. Direct activation of G proteins or phospholipase C by AlF4(-) and m-3M3FBS, respectively, leads to Ca(2+) release that is insensitive to changes in the membrane potential. Thapsigargin-, as well as OAG-induced Ca(2+) entry are affected by the membrane voltage, probably as a result of the modification in the driving force for Ca(2+) influx; however, hyperpolarisation does not enhance thrombin- or OAG-evoked Ca(2+) entry probably revealing the presence of a voltage-sensitive regulatory mechanism. Transmembrane voltage also modulates the activity of the plasma membrane Ca(2+)-ATPase (PMCA) most likely due to a decrease in the phosphotyrosine content of the pump. Thrombin-stimulated platelet aggregation is modulated by membrane depolarisation by a mechanism that is, at least partially, independent of Ca(2+). These observations indicate that PAR-1 and PAR-4 receptors are modulated by the membrane voltage in human platelets.

Endoplasmic reticulum calcium pumps and cancer cell differentiation

The endoplasmic reticulum (ER) is a major intracellular calcium storage pool and a multifunctional organelle that accomplishes several calcium-dependent functions involved in many homeostatic and signaling mechanisms. Calcium is accumulated in the ER by Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA)-type calcium pumps. SERCA activity can determine ER calcium content available for intra-ER functions and for calcium release into the cytosol, and can shape the spatiotemporal characteristics of calcium signals. SERCA function therefore constitutes an important nodal point in the regulation of cellular calcium homeostasis and signaling, and can exert important effects on cell growth, differentiation and survival. In several cell types such as cells of hematopoietic origin, mammary, gastric and colonic epithelium, SERCA2 and SERCA3-type calcium pumps are simultaneously expressed, and SERCA3 expression levels undergo significant changes during cell differentiation, activation or immortalization. In addition, SERCA3 expression is decreased or lost in several tumor types when compared to the corresponding normal tissue. These observations indicate that ER calcium homeostasis is remodeled during cell differentiation, and may present defects due to decreased SERCA3 expression in tumors. Modulation of the state of differentiation of the ER reflected by SERCA3 expression constitutes an interesting new aspect of cell differentiation and tumor biology.

Calcium fluxes into and out of cytosol in human platelets: analysis of experimental data

The purpose of this research was to analyse experimental data concerning cytosolic calcium concentration in view of the mechanisms involved in calcium fluxes in human platelets. The parameters of model curves are related to the properties of the entities responsible for control or maintenance of cytosolic calcium concentration. It has been shown that: (a) biphasicity of increase in cytosolic calcium concentration caused by inhibition of SERCAs either by TBHQ and TG or by TG alone is related to fast and slow discharge of acidic calcium stores and DTS; (b) biphasicity of decline in cytosolic calcium concentration after its rise caused by stimulation of platelets by the agonists is related to non-synchronous extrusion of calcium by PMCA and NCX; (c) NCX is active only in calcium containing medium: calcium ion(s) are necessary to be bound to the site(s) located on the medium-facing side of the (macro)molecule; (d) PMCA is likely to be activated either by binding calcium ion(s) to the site(s) located on its cytosol-facing side or by unbinding identical ion(s) from the site(s) on its medium-facing side.

Chemical Composition of Fatty Acid and Unsaponifiable Fractions of Leaves, Stems and Roots of Arbutus unedo and in vitro Antimicrobial Activity of Unsaponifiable Extracts

The chemical composition of the fatty acid and unsaponifiable fractions of the leaves, stems and roots of Arbutus unedo L. were determined using gas chromatography and gas chromatography-mass spectrometry. The fatty acid fractions of the leaves, stems and roots contained 38.5%, 31.3% and 14.1% palmitic acid, respectively, along with other long-chain fatty acids (up to C22). The chemical composition of the unsaponifiable fractions differed: the leaf and stem fractions contained high levels of aliphatic (32.1% and 62.6%, respectively) and terpenic compounds (49.6% and 25.7%, respectively), and the root fraction mainly contained esters, of which the most abundant was benzyl cinnamate (36.6%). The antimicrobial activities of the unsaponifiable fractions against nine species of microorganisms were assessed. The unsaponifiable leaf and stem extracts inhibited the growth of Klebsiella pneumoniae, Enterococcus faecalis and Candida albicans.

Identification and investigation of Drosophila postsynaptic density homologs

AMPA receptors are responsible for fast excitatory transmission in the CNS and the trafficking of these receptors has been implicated in LTP and learning and memory. These receptors reside in the postsynaptic density, a network of proteins that links the receptors to downstream signaling components and to the neuronal cytoskeleton. To determine whether the fruit fly, Drosophila melanogaster, possesses a similar array of proteins as are found at the mammalian PSD, we identified Drosophila homologs of 95.8% of mammalian PSD proteins. We investigated, for the first time, the role of one of these PSD proteins, Pod1 in GluR cluster formation at the Drosophila neuromuscular junction and found that mutations in pod1 resulted in a specific loss of A-type receptors at the synapse.

Comparative analysis of the locations of the NR1 and NR2 NMDA receptor subunits in honeybee (Apis mellifera) and fruit fly (Drosophila melanogaster, Canton-S wild-type) cerebral ganglia

The locations of the NR1 and NR2 subunits of the GABA receptor were studied in brain structures in insects--honeybees and fruit flies--using an immunohistochemical method. The specificities of the antibodies to the NR1 and NR2 subunits were confirmed by the antisense knockdown method for the NR1 subunit and western blotting. The data obtained here lead to the conclusion that the distributions of the NR1 and NR2 subunits of the NMDA receptor complex in the cerebral ganglia of the honeybee and fruit fly are similar; areas with the highest concentrations of NR1 and NR2 subunits were identified, and these were found to be different in the different insects. This is associated with the behavioral characteristics of these two insect species.

Orai1 mediates the interaction between STIM1 and hTRPC1 and regulates the mode of activation of hTRPC1-forming Ca2+ channels

Orai1 and hTRPC1 have been presented as essential components of store-operated channels mediating highly Ca(2+) selective I(CRAC) and relatively Ca(2+) selective I(SOC), respectively. STIM1 has been proposed to communicate the Ca(2+) content of the intracellular Ca(2+) stores to the plasma membrane store-operated Ca(2+) channels. Here we present evidence for the dynamic interaction between endogenously expressed Orai1 and both STIM1 and hTRPC1 regulated by depletion of the intracellular Ca(2+) stores, using the pharmacological tools thapsigargin plus ionomycin, or by the physiological agonist thrombin, independently of extracellular Ca(2+). In addition we report that Orai1 mediates the communication between STIM1 and hTRPC1, which is essential for the mode of activation of hTRPC1-forming Ca(2+) permeable channels. Electrotransjection of cells with anti-Orai1 antibody, directed toward the C-terminal region that mediates the interaction with STIM1, and stabilization of an actin cortical barrier with jasplakinolide prevented the interaction between STIM1 and hTRPC1. Under these conditions hTRPC1 was no longer involved in store-operated calcium entry but in diacylglycerol-activated non-capacitative Ca(2+) entry. These findings support the functional role of the STIM1-Orai1-hTRPC1 complex in the activation of store-operated Ca(2+) entry.

STIM1 regulates acidic Ca2+ store refilling by interaction with SERCA3 in human platelets

Ca(2+) mobilization regulates a wide variety of cellular functions. Platelets possess agonist-releasable Ca(2+) stores in acidic organelles where sarcoendoplasmic reticulum Ca(2+)-ATPase-3 (SERCA) pump is involved in store refilling. Stromal interaction molecule 1 (STIM1), which has been presented as a central regulator of platelet function, is a Ca(2+) sensor of the intracellular Ca(2+) stores. Here we present that STIM1 is required for acidic store refilling. Electrotransjection of cells with anti-STIM1 (Y(231)-K(243)) antibody, directed towards a cytoplasmic sequence of STIM1, significantly reduced acidic store refilling, which was tested by remobilizing Ca(2+) from the acidic stores using 2,5-di-(t-butyl)-1,4-hydroquinone (TBHQ) after a brief refilling period that followed thrombin stimulation. Platelet treatment with thrombin or thapsigargin in combination with ionomycin, to induce extensive Ca(2+) store depletion, resulted in a transient increase in the interaction between STIM1 and SERCA3, reaching a maximum 30 s after stimulation. The coupling between STIM1 and SERCA3 was abolished by electrotransjection with anti-STIM1 antibody. The interaction between STIM1 and SERCA3 induced by thrombin or by treatment with thapsigargin plus ionomycin is reduced in platelets from type 2 diabetic patients, as well as Ca(2+) reuptake into the acidic Ca(2+) stores. These findings provide evidence for a role of STIM1 in acidic store refilling in platelets probably acting as a Ca(2+) sensor and regulating the activity of SERCA3. This action is impaired in platelets from type 2 diabetics, which might lead to the enhanced cytosolic Ca(2+) concentration observed and, therefore, in platelet hyperactivity.

Neurotoxicity of a novel local anesthetic agent, ropivacaine: the possible roles of bursts of potential and cytoplasmic second messenger

BACKGROUND/PURPOSE

Ropivacaine has been shown to induce convulsion following overdose or accidental intravenous injection, but the mechanisms are poorly understood. Using an identifiable central neuron from giant African snail, the authors studied the mechanism of ropivacaine-elicited bursts of potential and explored the possible mechanisms of ropivacaine-induced neurotoxicity.

METHODS

Ropivacaine action on a central neuron (RP4) of the giant African snail (Achatina fulica Ferussac) was recorded by conventional electrophysiologic technique. Interactions between ropivacaine and prazosin, propranolol, atropine, d-tubocurarine, calcium-free solution, H89, U73,122, neomycin, high-magnesium solution, and chelerythrine were also observed.

RESULTS

The RP4 neuron showed spontaneous firing of action potentials. Extracellular application of ropivacaine (900 microM) reversibly elicited bursts of potential in the RP4 neuron. The bursts of potential elicited by ropivacaine were not blocked after administration of: (1) prazosin, propranolol, atropine, d-tubocurarine; (2) calcium-free solution; and (3) pretreatment with H89 or chelerythrine. The bursts of potential elicited by ropivacaine were blocked by pretreatment with U73122 (30 microM) or by adding neomycin (3.5 mM) or high-magnesium solution (30 mM).

CONCLUSION

Ropivacaine reversibly elicited bursts of potential in the central snail neuron. The ropivacaine-elicited bursts of potential were associated with phospholipase C activity in the RP4 snail neuron. Our results suggest that ropivacaine-induced neurotoxicity is highly associated with phospholipase C activity and phospholipase C inhibitor may offer a novel therapeutic approach for managing local anesthetic-induced convulsion or other transient neurologic toxicity.

Cinnamtannin B-1 from bay wood reduces abnormal intracellular Ca2+ homeostasis and platelet hyperaggregability in type 2 diabetes mellitus patients

Type 2 diabetes mellitus induces a number of cardiovascular disorders, including platelet hyperactivity and hyperaggregability, which is associated to an increased oxidant production and abnormal cytosolic Ca2+ mobilization. In the present study, we have investigated the effect of cinnamtannin B-1 obtained from bay wood on oxidants production, Ca2+ mobilization and aggregation in platelets from type 2 diabetic donors. Pretreatment of platelets with cinnamtannin B-1 reversed the enhanced oxidants production and Ca2+ mobilization, including Ca2+ entry, evoked by thapsigargin plus ionomycin or thrombin, observed in platelets from diabetic subjects, so that in the presence of cinnamtannin B-1 Ca2+ entry was similar in platelets from healthy and diabetic subjects. In addition, cinnamtannin B-1 reduced thrombin-induced aggregation in platelets from type 2 diabetic subjects. We conclude that cinnamtannin B-1 exerts an effective antioxidant action in platelets from patients with type 2 diabetes mellitus and reverses the enhanced Ca2+ mobilization and hyperaggregability.

Dynamics of Calcium Fluxes in Nonexcitable Cells: Mathematical Modeling

Mathematical models simulating the dynamics of calcium redistribution (elicited by experimental interference with the pathways of calcium fluxes) in cellular compartments have been developed, based on a minimal scheme of the pathways of calcium fluxes in nonexcitable cells suspended in calcium-free medium. The models are consistent with available experimental data. All parameters are quantitatively related to the intrinsic properties of calcium adenosine triphosphatases (ATPases) and cellular membranes; there is no interdependence between the parameters. The models can be used as the basis for quantitative analysis and interpretation of experimental data. The activities of plasma membrane and sarcoendoplasmic reticulum calcium ATPases (PMCA and SERCAs) are governed by different mechanisms. PMCA is likely to undergo transitions from inactive to active to "dormant" (not identical to the initial) and back to inactive states, the mean duration of the cycle lasting for minutes or longer. The sequence of the transitions is initiated, presumably, by an increase in cytosolic calcium concentration. The transition of PMCA from inactive to active (at least at low rates of increase in cytosolic calcium concentration) is likely to be slower than that from active to dormant. SERCA, presumably, transits from inactive to active state in response to increases in calcium leakage from calcium stores. Whereas PMCA extrudes excess calcium (a definite quantity of it) in a short pulse, SERCA retakes calcium back into the stores permanently at a high rate. The models presented here may be the best means for the moment to quantitatively relate the dynamics of calcium fluxes in nonexcitable cells with known or putative properties of the mechanisms underlying activation of calcium ATPases.

Molecular characterization of NMDA-like receptors in Aplysia and Lymnaea: relevance to memory mechanisms

The N-methyl-D-aspartate (NMDA) receptor belongs to the group of ionotropic glutamate receptors and has been implicated in synaptic plasticity, memory acquisition, and learning in both vertebrates and invertebrates, including molluscs. However, the molecular identity of NMDA-type receptors in molluscs remains unknown. Here, we cloned two NMDA-type receptors from the sea slug Aplysia californica, AcNR1-1 and AcNR1-2, as well as their homologs from the freshwater pulmonate snail Lymnaea stagnalis, LsNR1-1 and LsNR1-2. The cloned receptors contain a signal peptide, two extracellular segments with predicted binding sites for glycine and glutamate, three recognized transmembrane regions, and a fourth hydrophobic domain that makes a hairpin turn to form a pore-like structure. Phylogenetic analysis suggests that both the AcNR1s and LsNR1s belong to the NR1 subgroup of ionotrophic glutamate receptors. Our in situ hybridization data indicate highly abundant, but predominantly neuron-specific expression of molluscan NR1-type receptors in all central ganglia, including identified motor neurons in the buccal and abdominal ganglia as well as groups of mechanosensory cells. AcNR1 transcripts were detected extrasynaptically in the neurites of metacerebral cells of Aplysia. The widespread distribution of AcNR1 and LsNR1 transcripts also implies diverse functions, including their involvement in the organization of feeding, locomotory, and defensive behaviors.

Collaborative effect of SERCA and PMCA in cytosolic calcium homeostasis in human platelets

Intracellular free Ca2+ concentration ([Ca2+]c) is finely regulated by several mechanisms that either increase or reduce [Ca2+]c. Two different Ca2+ pumps have been described so far as the main mechanisms for Ca2+ removal from the cytosol, either by its sequestration into the stores, mediated by the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) or by Ca2+ extrusion to the extracellular medium, by the plasma membrane Ca2+-ATPase (PMCA). We have used inhibitors of these pumps to analyze their Ca2+ clearance efficacy in human platelets stimulated by the physiological agonist thrombin. Results demonstrate that, after platelet stimulation with thrombin, activation of SERCA precedes that of PMCA, although the ability of PMCA to remove Ca2+ from the cytosol last longer than that of SERCA. The efficacy of SERCA and PMCA removing Ca2+ from the cytosol is reduced when the concentration of thrombin increases. This phenomenon correlates with the greater increase in [Ca2+]c induced by higher concentrations of thrombin, which further confirms that SERCA and PMCA activities are regulated by [Ca2+]c.