Increases in intracellular calcium via activation of an endogenous P2-purinoceptor in cultured CHO-K1 cells

Versicotide G suppresses osteoclastogenesis and prevents osteolysis

Excessive formation and function of osteoclasts cause various osteolytic bone diseases. Natural products are a potential source for the discovery of new therapeutic candidates to treat bone destruction diseases. In this study, chemical informatics and bioassay guided examination of the marine-derived Aspergillus versicolor F77 fungus chemically resulted in the isolation of seven cyclopeptides, of which versicotides G-J (1-4) are new cyclohexapeptides. Their structures were identified by spectroscopic data in association with Marfey method and single crystal X-ray diffraction data for configurational assignments. Bioassay revealed that versicotide G (1, VG) is the most active among the analogs to suppress the receptor activator of nuclear factor-KB ligand (RANKL)-induced osteoclastogenesis in bone marrow derived monocytes (BMMs) without affecting BMMs viability. VG also suppressed RANKL-induced actin-ring formation and resorbing function of osteoclast dose-dependently. Mechanistically, VG attenuated RANKL-induced intracellular calcium elevation by inhibiting PLCγ1 phosphorylation and blocking the activation of downstream phosphatase calcineurin. In addition, VG abrogated the expression and translocation of nuclear factor of activated T cells cytoplasmic-1 (NFATc1), leading to the downregulation of the expression of osteoclast-specific genes and the abolishment of the osteoclast formation. In the in vivo test, VG suppressed osteoclast formation and bone loss in Ti-induced calvarial osteolytic mouse model.These findings imply that VG is a promising candidate for the remedy of bone destruction-related diseases.

Complex FFA1 receptor (in)dependent modulation of calcium signaling by free fatty acids

The expression of free fatty acid 1 receptors (FFA1R), activated by long chain fatty acids in human pancreatic β-cells and enhancing glucose-stimulated insulin secretion are an attractive target to treat type 2 diabetes. Yet several clinical studies with synthetic FFA1R agonists had to be discontinued due to cytotoxicity and/or so-called "liver concerns". It is not clear whether these obstructions are FFA1R dependent. In this context we used CHO-AEQ cells expressing the bioluminescent calcium-sensitive protein aequorin to investigate calcium signaling elicited by FFA1 receptor ligands α-linolenic acid (ALA), oleic acid (OLA) and myristic acid (MYA). This study revealed complex modulation of intracellular calcium signaling by these fatty acids. First these compounds elicited a typical transient increase of intracellular calcium via binding to FFA1 receptors. Secondly slightly higher concentrations of ALA substantially reduced ATP mediated calcium responses in CHO-AEQ cells and Angiotensin II responses in CHO-AEQ cells expressing human AT₁ receptors. This effect was less pronounced with MYA and OLA and was not linked to FFA1 receptor activation nor to acute cytotoxicity as a result of plasma membrane perturbation. Yet it can be hypothesized that, in line with previous studies, unsaturated long chain fatty acids such as ALA and OLA are capable of inactivating the G-proteins involved in purinergic and Angiotensin AT₁ receptor calcium signaling. Alternatively the ability of fatty acids to deplete intracellular calcium stores might underly the observed cross-inhibition of these receptor responses in the same cells.

A novel bioassay to detect Nociceptin/Orphanin FQ release from single human polymorphonuclear cells

Nociceptin/Orphanin FQ (N/OFQ) is the endogenous opioid agonist for the N/OFQ receptor or NOP. This receptor system is involved in pain processing but also has a role in immune regulation. Indeed, polymorphonuclear cells (PMNs) express mRNA for N/OFQ precursor and are a potential source for circulating N/OFQ. Current measurements are based on ELISA and RIA techniques. In this study we have designed a bioassay to measure N/OFQ release from single PMNs. Chinese Hamster Ovary (CHO) cells transfected with the human (h) NOP receptor and Gαiq5 chimera force receptor coupling in biosensor cells to increase intracellular Ca2+; this can be measured with FLUO-4 dye. If isolated PMNs from healthy human volunteers are layered next to CHOhNOPGαiq5 biosensor cells then stimulated with the chemoattractant N-formyl-methionyl-leucyl-phenylalanine (fMLP) we hypothesise that released N/OFQ will activate the biosensor. PMNs also release ATP and CHO cells express purinergic receptors coupled to elevated Ca2+. In a system where these receptors (P2Y1, P2Y2 and P2X7) are blocked with high concentrations of PPADS and oATP, PMN stimulation with fMLP increases Ca2+ in PMNs then shortly afterwards the biosensor cells. Our data therfore reports detection of single cell N/OFQ release from immune cells. This was absent when cells were preincubated with the selective NOP antagonist; SB-612111. Collectively this is the first description of single cell N/OFQ release. We will deploy this assay with further purified individual cell types and use this to further study the role of the N/OFQ-NOP system in disease; in particular sepsis where there is strong evidence for increased levels of N/OFQ worsening outcome.

A Set of Experimentally Validated Decoys for the Human CC Chemokine Receptor 7 (CCR7) Obtained by Virtual Screening

We present a state-of-the-art virtual screening workflow aiming at the identification of novel CC chemokine receptor 7 (CCR7) antagonists. Although CCR7 is associated with a variety of human diseases, such as immunological disorders, inflammatory diseases, and cancer, this target is underexplored in drug discovery and there are no potent and selective CCR7 small molecule antagonists available today. Therefore, computer-aided ligand-based, structure-based, and joint virtual screening campaigns were performed. Hits from these virtual screenings were tested in a CCL19-induced calcium signaling assay. After careful evaluation, none of the in silico hits were confirmed to have an antagonistic effect on CCR7. Hence, we report here a valuable set of 287 inactive compounds that can be used as experimentally validated decoys.

Identification of positive modulators of TRPM5 channel from a high-throughput screen using a fluorescent membrane potential assay

Transient Receptor Potential Melastatin 5 (TRPM5) is an intracellular calcium-activated cation-selective ion channel expressed in a variety of cell types. Dysfunction of this channel has recently been implied in a range of disease states including diabetes, enteric infections, inflammatory responses, parasitic infection and other pathologies. However, to date, agonists and positive modulators of this channel with sufficient selectivity to enable target validation studies have not been described, limiting the evaluation of TRPM5 biology and its potential as a drug target. We developed a high-throughput assay using a fluorescent membrane potential dye and a medium- and high-throughput electrophysiology assay using QPatch HTX and SyncroPatch 384PE. By employing these assays, we conducted a primary screening campaign and identified hit compounds as TRPM5 channel positive modulators. An initial selectivity profile confirmed hit selectivity to TRPM5 and is presented here. These small molecule TRPM5 compounds have a high potential both as early tool compounds to enable pharmacological studies of TRPM5 and as starting points for the development of potent, selective TRPM5 openers or positive modulators as novel drugs targeting several pathological states.

Pseudo-Membrane Jackets: Two-Dimensional Coordination Polymers Achieving Visible Phase Separation in Cell Membrane

Cell membranes contain lateral systems that consist of various lipid compositions and actin cytoskeleton, providing two-dimensional (2D) platforms for chemical reactions. However, such complex 2D environments have not yet been used as a synthetic platform for artificial 2D nanomaterials. Herein, we demonstrate the direct synthesis of 2D coordination polymers (CPs) at the liquid-cell interface of the plasma membrane of living cells. The coordination-driven self-assembly of networking metal complex lipids produces cyanide-bridged CP layers with metal ions, enabling "pseudo-membrane jackets" that produce long-lived micro-domains with a size of 1-5 μm. The resultant artificial and visible phase separation systems remain stable even in the absence of actin skeletons in cells. Moreover, we show the cell application of the jackets by demonstrating the enhancement of cellular calcium response to ATP.

Receptor Characterization and Functional Activity of Pyrokinins on the Hindgut in the Adult Mosquito, Aedes aegypti

Pyrokinins are structurally related insect neuropeptides, characterized by their myotropic, pheromonotropic and melanotropic roles in some insects, but their function is unclear in blood-feeding arthropods. In the present study, we functionally characterized the pyrokinin-1 and pyrokinin-2 receptors (PK1-R and PK2-R, respectively), in the yellow fever mosquito, Aedes aegypti, using a heterologous cell system to characterize their selective and dose-responsive activation by members of two distinct pyrokinin subfamilies. We also assessed transcript-level expression of these receptors in adult organs and found the highest level of PK1-R transcript in the posterior hindgut (rectum) while PK2-R expression was enriched in the anterior hindgut (ileum) as well as in reproductive organs, suggesting these to be prominent target sites for their peptidergic ligands. In support of this, PRXa-like immunoreactivity (where X = V or L) was localized to innervation along the hindgut. Indeed, we identified a myoinhibitory role for a PK2 on the ileum where PK2-R transcript was enriched. However, although we found that PK1 did not influence myoactivity or Na⁺ transport in isolated recta, the PRXa-like immunolocalization terminating in close association to the rectal pads and the significant enrichment of PK1-R transcript in the rectum suggests this organ could be a target of PK1 signaling and may regulate the excretory system in this important disease vector species.

Molecular identification, transcript expression, and functional deorphanization of the adipokinetic hormone/corazonin-related peptide receptor in the disease vector, Aedes aegypti

The recently discovered adipokinetic hormone/corazonin-related peptide (ACP) is an insect neuropeptide structurally intermediate between corazonin (CRZ) and adipokinetic (AKH) hormones, which all demonstrate homology to the vertebrate gonadotropin-releasing hormone (GnRH). To date, the function of the ACP signaling system remains unclear. In the present study, we molecularly identified the complete open reading frame encoding the Aedes aegypti ACP receptor (ACPR), which spans nine exons and undergoes alternative splicing giving rise to three transcript variants. Only a single variant, AedaeACPR-I, yielding a deduced 577 residue protein, contains all seven transmembrane domains characteristic of rhodopsin-like G protein-coupled receptors. Functional deorphanization of AedaeACPR-I using a heterologous cell culture-based system revealed highly-selective and dose-dependent receptor activation by AedaeACP (EC₅₀ = 10.25 nM). Analysis of the AedaeACPR-I and AedaeACP transcript levels in all post-embryonic developmental stages using quantitative RT-PCR identified enrichment of both transcripts after adult eclosion. Tissue-specific expression profiling in adult mosquitoes reveals expression of the AedaeACPR-I receptor transcript in the central nervous system, including significant enrichment within the abdominal ganglia. Further, the AedaeACP transcript is prominently detected within the brain and thoracic ganglia. Collectively, these results indicate a neuromodulator or neurotransmitter role for ACP and suggest this neuropeptide may function in regulation of post-ecdysis activities.

A new family of insect muscarinic acetylcholine receptors

Most currently used insecticides are neurotoxic chemicals that target a limited number of sites and insect cholinergic neurotransmission is the major target. A potential target for insecticide development is the muscarinic acetylcholine receptor (mAChR), which is a metabotropic G-protein-coupled receptor. Insects have A- and B-type mAChRs and the five mammalian mAChRs are close to the A-type. We isolated a cDNA (CG12796) from the fruit fly, Drosophila melanogaster. After heterologous expression in Chinese hamster ovary K1 cells, CG12796 could be activated by acetylcholine [EC50 (half maximal effective concentration), 73 nM] and the mAChR agonist oxotremorine M (EC50 , 48.2 nM) to increase intracellular Ca(2+) levels. Thus, the new mAChR is coupled to Gq/11 but not Gs and Gi/o . The classical mAChR antagonists atropine and scopolamine N-butylbromide at 100 μM completely blocked the acetylcholine-induced responses. The orthologues of CG12796 can also be found in the genomes of other insects, but not in the genomes of the honeybee or parasitoid wasps. Knockdown of CG12796 in the central nervous system had no effect on male courtship behaviours. We suggest that CG12796 represents the first recognized member of a novel mAChR class.

G-protein inhibition profile of the reported Gq/11 inhibitor UBO-QIC

UBO-QIC (FR900359) is the only currently available Gq/11 protein inhibitor. However, its characterization has not been published, and we thus set out to do this. Gi, Gs and Gq protein-mediated responses were assessed utilizing endogenous or heterologously expressed receptors in Chinese hamster ovary cells. UBO-QIC, at 1 μM, was an effective inhibitor of the Gq-mediated responses, but was inactive at Gi- and Gs-mediated responses. Gq/11 and G16 responses were additionally compared in HEL92.1.7 cells, showing inhibition of Gq/11 responses. However, UBO-QIC also appeared to inhibit G16. Further studies are required to establish its profile with respect to the different Gq-family proteins.

Close association of B2 bradykinin receptors with P2Y2 ATP receptors

Two G-protein-coupled receptors (GPCRs) that couple with Gαq/11, B2 bradykinin (BK) receptor (B2R) and ATP/UTP receptor P2Y2 (P2Y2R), are ubiquitously expressed and responsible for vascular tone, inflammation, and pain. We analysed the cellular signalling of P2Y2Rs in cells that express B2Rs. B2R desensitization induced by BK or B2R internalization-inducing glycans cross-desensitized the P2Y2R response to ATP/UTP. Fluorescence resonance energy transfer from P2Y2R-AcGFP to B2R-DsRed was detected in the cells and on the cell surfaces, showing the close association of these GPCRs. BK- and ATP-induced cross-internalization of P2Y2R and B2R, respectively, was shown in a β-galactosidase complementation assay using P2Y2R or B2R fused to the H31R substituted α donor peptide of a β-galactosidase reporter enzyme (P2Y2R-α or B2R-α) with coexpression of the FYVE domain of endofin, an early endosome protein, fused to the M15 acceptor deletion mutant of β-galactosidase (the ω peptide, FYVE-ω). Arrestin recruitment to the GPCRs by cross-activation was also shown with the similar way. Coimmunoprecipitation showed that B2R and P2Y2R were closely associated in the cotransfected cells. These results indicate that B2R couples with P2Y2R and that these GPCRs act together to fine-tune cellular responsiveness. The collaboration between these receptors may permit rapid onset and turning off of biological events.

A membrane permeable fluorescent Ca2+ probe based on bis-BODIPY with branched PEG

The cellular uptake of MPFCP-2 is improved by the PEG encapsulation method, and then MPFCP-2 could pass through the cell membrane by itself, and monitor the changes of the intracellular Ca²⁺ signal.

Purinergic signalling in the reproductive system in health and disease

There are multiple roles for purinergic signalling in both male and female reproductive organs. ATP, released as a cotransmitter with noradrenaline from sympathetic nerves, contracts smooth muscle via P2X1 receptors in vas deferens, seminal vesicles, prostate and uterus, as well as in blood vessels. Male infertility occurs in P2X1 receptor knockout mice. Both short- and long-term trophic purinergic signalling occurs in reproductive organs. Purinergic signalling is involved in hormone secretion, penile erection, sperm motility and capacitation, and mucous production. Changes in purinoceptor expression occur in pathophysiological conditions, including pre-eclampsia, cancer and pain.

Differential effects of the Gβ5-RGS7 complex on muscarinic M3 receptor-induced Ca2+ influx and release

The G protein β subunit Gβ5 uniquely forms heterodimers with R7 family regulators of G protein signaling (RGS) proteins (RGS6, RGS7, RGS9, and RGS11) instead of Gγ. Although the Gβ5-RGS7 complex attenuates Ca(2+) signaling mediated by the muscarinic M3 receptor (M3R), the route of Ca(2+) entry (i.e., release from intracellular stores and/or influx across the plasma membrane) is unknown. Here, we show that, in addition to suppressing carbachol-stimulated Ca(2+) release, Gβ5-RGS7 enhanced Ca(2+) influx. This novel effect of Gβ5-RGS7 was blocked by nifedipine and 2-aminoethoxydiphenyl borate. Experiments with pertussis toxin, an RGS domain-deficient mutant of RGS7, and UBO-QIC {L-threonine,(3R)-N-acetyl-3-hydroxy-L-leucyl-(aR)-a-hydroxybenzenepropanoyl-2,3-idehydro-N-methylalanyl-L-alanyl-N-methyl-L-alanyl-(3R)-3-[[(2S,3R)-3-hydroxy-4- methyl-1-oxo-2-[(1-oxopropyl)amino]pentyl]oxy]-L-leucyl-N,O-dimethyl-,(7→1)-lactone (9CI)}, a novel inhibitor of Gq, showed that Gβ5-RGS7 modulated a Gq-mediated pathway. These studies indicate that Gβ5-RGS7, independent of RGS7 GTPase-accelerating protein activity, couples M3R to a nifedipine-sensitive Ca(2+) channel. We also compared the action of Gβ5-RGS7 on M3R-induced Ca(2+) influx and release elicited by different muscarinic agonists. Responses to Oxo-M [oxotremorine methiodide N,N,N,-trimethyl-4-(2-oxo-1-pyrrolidinyl)-2-butyn-1-ammonium iodide] were insensitive to Gβ5-RGS7. Pilocarpine responses consisted of a large release and modest influx components, of which the former was strongly inhibited whereas the latter was insensitive to Gβ5-RGS7. McN-A-343 [(4-hydroxy-2-butynyl)-1-trimethylammonium-3-chlorocarbanilate chloride] was the only compound whose total Ca(2+) response was enhanced by Gβ5-RGS7, attributed to, in part, by the relatively small Ca(2+) release this partial agonist stimulated. Together, these results show that distinct agonists not only have differential M3R functional selectivity, but also confer specific sensitivity to the Gβ5-RGS7 complex.

Impact of polymorphic variants on the molecular pharmacology of the two-agonist conformations of the human β1-adrenoceptor

β-blockers are widely used to improve symptoms and prolong life in heart disease primarily by inhibiting the actions of endogenous catecholamines at the β1-adrenoceptor. There are two common naturally occurring polymorphisms within the human β1-adrenoceptor sequence: Ser or Gly at position 49 in the N-terminus and Gly or Arg at position 389 in the C-terminus and some clinical studies have suggested that expression of certain variants may be associated with disease and affect response to treatment with β-blockers. The β1-adrenoceptor also exists in two agonist conformations - a high affinity catecholamine conformation and a low affinity secondary agonist conformation. Receptor-effector coupling and intracellular signalling from the different conformations may be affected by the polymorphic variants. Here, we examine in detail the molecular pharmacology of the β1-adrenoceptor polymorphic variants with respect to ligand affinity, efficacy, activation of the different agonist conformations and signal transduction and determine whether the polymorphic variants do indeed affect this secondary conformation. Stable cell lines expressing the wildtype and polymorphic variants were constructed and receptor pharmacology examined using whole cell binding and intracellular secondary messenger techniques. There was no difference in affinity for agonists and antagonists at the human wildtype β1-adrenoceptor (Ser49/Gly389) and the polymorphic variants Gly49/Gly389 and Ser49/Arg389. Furthermore, the polymorphic variant receptors both have two active agonist conformations with pharmacological properties similar to the wildtype receptor. Although the polymorphism at position 389 is thought to occur in an intracellular domain important for Gs-coupling, the two agonist conformations of the polymorphic variants stimulate intracellular signalling pathways, including Gs-cAMP intracellular signalling, in a manner very similar to that of the wildtype receptor.

Measurement of [Ca²⁺]i in whole cell suspensions using Fura-2

Use of Fura-2 in whole cell suspensions to measure changes in intracellular Ca(2+) is probably one of the simplest, yet most widely used protocols described in this volume. Whole cell suspensions are loaded with Fura-2 and then placed into a cuvette-based fluorimetric system (measuring 510 nm emission at alternating 340/340 nm excitation). Cells can be stimulated with agonists and antagonists to enable temporal response profiling and concentration-response curves to be constructed. The protocol can be used for a wide range of cells including those transfected with Ca(2+)-signaling proteins, e.g., receptors and channels. Loading characteristics and the need for agents to retain loaded dye (e.g., probenecid) need to be determined empirically. Calibration of whole cell suspensions to convert the fluorescent signal into Ca(2+) is simply performed using Triton-X lysis (to determine R (max)) and EGTA chelation (to determine R (min)).

Comparative analysis of calcium spikes upon activation of serotonin(1A) and purinergic receptors

Calcium signaling represents one of the most important signaling cascades in cells and regulates diverse processes such as exocytosis, muscle contraction and relaxation, gene expression and cell growth. G protein-coupled receptors (GPCRs) are the most important family of receptors that activate calcium signaling. Since calcium signaling regulates a large number of physiological responses, it is intriguing that how changes in cytosolic calcium levels by a wide range of stimuli lead to signal-specific physiological responses in the cellular interior. In order to address this issue, we have analyzed temporal calcium profiles induced by two GPCRs, the serotonin(1A) and purinergic receptors. In this work, we have described a set of parameters for the analysis of calcium transients that could provide novel insight into mechanisms responsible for maintaining signal specificity by shaping calcium transients. An interesting feature of calcium signaling that has emerged from our analysis is that the profile of individual transients in a calcium response could play an important role in maintaining downstream signal specificity. In summary, our analysis offers a novel approach to identify differences in calcium response patterns induced by various stimuli.

Investigation of the potential involvement of eicosanoid metabolites in anti-diuretic hormone signaling in Rhodnius prolixus

The use of naturally occurring plant-derived compounds for controlling insect pests remains an attractive alternative to potentially dangerous synthetic chemical compounds. One prospective plant-based compound, isoforms of the so-called jack bean urease (JBU) from the jack bean, Canavalia ensiformis, as well a derived peptide, Jaburetox-2Ec, have insecticidal effects on an array of insect species. In the Chagas' disease vector, Rhodnius prolixus, some of the physiological effects attributed to these urease isoforms include inhibition of serotonin (5-HT)-stimulated fluid secretion by the Malpighian tubules (MTs). Here, we investigated whether the effects of these exogenous urease isoforms were targeting the neuroendocrine network involved in the anti-diuretic hormone (RhoprCAPA-2) signaling cascade. We show that pharmacological agents known to interfere with eicosanoid metabolite biosynthesis do not affect RhoprCAPA-2 inhibition of 5-HT-stimulated fluid secretion by MTs. In addition, we demonstrate that RhoprCAPA-2 inhibition of MTs is independent of extracellular or intracellular calcium. Using a heterologous system for analysis of receptor activation, we show that neither JBU nor Jaburetox-2Ec are agonists of the anti-diuretic hormone receptor, RhoprCAPAr1. Finally, activation of the receptor using sub-maximal doses of the natural ligand, RhoprCAPA-2, was not influenced by the presence of either JBU or Jaburetox-2Ec indicating that the urease isoforms do not compete with RhoprCAPA-2 for binding and activation of RhoprCAPAr1. Taken together, these results suggest that at least two distinct mechanisms leading to inhibition of fluid secretion by MTs exist in R. prolixus and, unlike the urease-related effects, the eicosanoid metabolite pathway is not involved in RhoprCAPA-2 mediated anti-diuresis.

Divergent effect of mammalian PLCζ in generating Ca²⁺ oscillations in somatic cells compared with eggs

Sperm PLCζ (phospholipase Cζ) is a distinct phosphoinositide-specific PLC isoform that is proposed to be the physiological trigger of egg activation and embryo development at mammalian fertilization. Recombinant PLCζ has the ability to trigger Ca²⁺ oscillations when expressed in eggs, but it is not known how PLCζ activity is regulated in sperm or eggs. In the present study, we have transfected CHO (Chinese-hamster ovary) cells with PLCζ fused with either YFP (yellow fluorescent protein) or luciferase and found that PLCζ-transfected cells did not display cytoplasmic Ca²⁺ oscillations any differently from control cells. PLCζ expression was not associated with changes in CHO cell resting Ca²⁺ levels, nor with a significantly changed Ca²⁺ response to extracellular ATP compared with control cells transfected with either YFP alone, a catalytically inactive PLCζ or luciferase alone. Sperm extracts containing PLCζ also failed to cause Ca²⁺ oscillations in CHO cells. Despite these findings, PLCζ-transfected CHO cell extracts exhibited high recombinant protein expression and PLC activity. Furthermore, either PLCζ-transfected CHO cells or derived cell extracts could specifically cause cytoplasmic Ca²⁺ oscillations when microinjected into mouse eggs. These data suggest that PLCζ-mediated Ca²⁺ oscillations may require specific factors that are only present within the egg cytoplasm or be inhibited by factors present only in somatic cell lines.

Acid stimulation (sour taste) elicits GABA and serotonin release from mouse taste cells

Several transmitter candidates including serotonin (5-HT), ATP, and norepinephrine (NE) have been identified in taste buds. Recently, γ-aminobutyric acid (GABA) as well as the associated synthetic enzymes and receptors have also been identified in taste cells. GABA reduces taste-evoked ATP secretion from Receptor cells and is considered to be an inhibitory transmitter in taste buds. However, to date, the identity of GABAergic taste cells and the specific stimulus for GABA release are not well understood. In the present study, we used genetically-engineered Chinese hamster ovary (CHO) cells stably co-expressing GABA(B) receptors and Gαqo5 proteins to measure GABA release from isolated taste buds. We recorded robust responses from GABA biosensors when they were positioned against taste buds isolated from mouse circumvallate papillae and the buds were depolarized with KCl or a stimulated with an acid (sour) taste. In contrast, a mixture of sweet and bitter taste stimuli did not trigger GABA release. KCl- or acid-evoked GABA secretion from taste buds was Ca(2+)-dependent; removing Ca(2+) from the bathing medium eliminated GABA secretion. Finally, we isolated individual taste cells to identify the origin of GABA secretion. GABA was released only from Presynaptic (Type III) cells and not from Receptor (Type II) cells. Previously, we reported that 5-HT released from Presynaptic cells inhibits taste-evoked ATP secretion. Combined with the recent findings that GABA depresses taste-evoked ATP secretion, the present results indicate that GABA and 5-HT are inhibitory transmitters in mouse taste buds and both likely play an important role in modulating taste responses.

Purinergic signalling mobilizes mitochondrial Ca²⁺ in mouse Sertoli cells

Intimate bidirectional communication between Sertoli cells and developing germ cells ensures the integrity and efficiency of spermatogenesis. Yet, a conceptual mechanistic understanding of the physiological principles that underlie Sertoli cell autocrine and paracrine signalling is lacking. Here, we characterize a purinergic Ca(2+) signalling network in immature mouse Sertoli cells that consists of both P2X2 and P2Y2 purinoceptor subtypes, the endoplasmic reticulum and, notably, mitochondria. By combining a transgenic mouse model with a dedicated bioluminescence imaging device, we describe a novel method to monitor mitochondrial Ca(2+) mobilization in Sertoli cells at subcellular spatial and millisecond temporal resolution. Our data identify mitochondria as essential components of the Sertoli cell signalling 'toolkit' that control the shape of purinergic Ca(2+) responses, and probably several other paracrine Ca(2+)-dependent signals.

Triphenylphosphine oxide is a potent and selective inhibitor of the transient receptor potential melastatin-5 ion channel

Transient receptor potential melastatin-5 (TRPM5) is a calcium-gated monovalent cation channel expressed in highly specialized cells of the taste bud and gastrointestinal tract, as well as in pancreatic β-cells. Well established as a critical signaling protein for G protein-coupled receptor-mediated taste pathways, TRPM5 also has recently been implicated as a regulator of incretin and insulin secretion. To date, no inhibitors of practical use have been described that could facilitate investigation of TRPM5 functions in taste or secretion of metabolic hormones. Using recombinant TRPM5-expressing cells in a fluorescence imaging plate reader-based membrane potential assay, we identified triphenylphosphine oxide (TPPO) as a selective and potent inhibitor of TRPM5. TPPO inhibited both human (IC₅₀ = 12 μM) and murine TRPM5 (IC₅₀ = 30 μM) heterologously expressed in HEK293 cells, but had no effect (up to 100 μM) on the membrane potential responses of TRPA1, TRPV1, or TRPM4b. TPPO also inhibited a calcium-gated TRPM5-dependent conductance in taste cells isolated from the tongues of transgenic TRPM5(+/)⁻ mice. In contrast, TPP had no effect on TRPM5 responses, indicating a strict requirement of the oxygen atom for activity. Sixteen additional TPPO derivatives also inhibited TRPM5 but none more potently than TPPO. Structure-activity relationship of tested compounds was used for molecular modeling-based analysis to clarify the positive and negative structural contributions to the potency of TPPO and its derivatives. TPPO is the most potent TRPM5 inhibitor described to date and is the first demonstrated to exhibit selectivity over other channels.

Structure based design of a Ca2+-sensitive RhoA protein that controls cell morphology

The Rho proteins are important regulators of cell morphology, and the prototypical protein RhoA is known to regulate contraction, blebbing and bleb retraction. We have identified and experimentally confirmed that RhoA has a binding site for calmodulin, a ubiquitous transducer of the Ca(2+) second messenger. Using structural modeling, a fusion protein was designed wherein RhoA activity was controlled by Ca(2+) via calmodulin. Living cells transfected with this synthetic protein underwent Ca(2+) sensitive and calmodulin-dependent bleb retraction within minutes. Further, the modularity of Ca(2+) signaling was exploited to induce bleb retraction in response to blue light (using channelrhodopsin-2) or exogenous chemicals (with acetylcholine receptor), showing input signal versatility. The widespread use of Ca(2+) signaling in nature suggests that fully exploring its signaling potential may allow powerful applications to other synthetic biological systems.

Heat shock enhances CMV-IE promoter-driven metabotropic glutamate receptor expression and toxicity in transfected cells

In CHO-K1 cells, heat shock strongly activated reporter-gene expression driven by the cytomegalovirus immediate-early (CMV-IE) promoter from adenoviral and plasmid vectors. Heat shock treatment (2h at 42.5 °C) significantly enhanced the promoter DNA-binding activity in nuclear extracts. In CHO cells expressing mGluR1a and mGluR5a receptors under the control of the CMV promoter, heat shock increased receptor protein expression, mRNA levels and receptor function estimated by measurement of PI hydrolysis, intracellular Ca²+ and cAMP. Hyperthermia increased average amplitudes of Ca²+ responses, the number of responding cells, and revealed the toxic properties of mGluR1a receptor. Heat shock also effectively increased the expression of EGFP. Hence, heat shock effects on mGluR expression and function in CHO cells may be attributed to the activation of the CMV promoter. Moreover, this effect was not limited to CHO cells as heat shock also increased EGFP expression in PC-12 and HEK293 cells. Heat shock treatment may be a useful tool to study the function of proteins expressed in heterologous systems under control of the CMV promoter. It may be especially valuable for increasing protein expression in transient transfections, for enhancing receptor expression in drug screening applications and to control the expression of proteins endowed with toxic properties. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Amphioxus expresses both vertebrate-type and invertebrate-type dopamine D(1) receptors

The cephalochordate amphioxus (Branchiostoma floridae) has recently been placed as the most basal of all the chordates, which makes it an ideal organism for studying the molecular basis of the evolutionary transition from invertebrates to vertebrates. The biogenic amine, dopamine regulates many aspects of motor control in both vertebrates and invertebrates, and in both cases, its receptors can be divided into two main groups (D1 and D2) based on sequence similarity, ligand affinity and effector coupling. A bioinformatic study shows that amphioxus has at least three dopamine D1-like receptor sequences. We have recently characterized one of these receptors, AmphiD1/β, which was found to have high levels of sequence similarity to both vertebrate D1 receptors and to β-adrenergic receptors, but functionally appeared to be a vertebrate-type dopamine D(1) receptor. Here, we report on the cloning of two further dopamine D(1) receptors (AmphiAmR1 and AmphiAmR2) from adult amphioxus cDNA libraries and their pharmacological characterisation subsequent to their expression in cell lines. AmphiAmR1 shows closer structural similarities to vertebrate D(1)-like receptors but shows some pharmacological similarities to invertebrate "DOP1" dopamine D(1)-like receptors. In contrast, AmphiAmR2 shows closer structural and pharmacological similarities to invertebrate "INDR"-like dopamine D(1)-like receptors.

Mitochondrial reactive oxygen species mediate GPCR-induced TACE/ADAM17-dependent transforming growth factor-alpha shedding

Epidermal growth factor receptor (EGFR) activation by GPCRs regulates many important biological processes. ADAM metalloprotease activity has been implicated as a key step in transactivation, yet the regulatory mechanisms are not fully understood. Here, we investigate the regulation of transforming growth factor-alpha (TGF-alpha) shedding by reactive oxygen species (ROS) through the ATP-dependent activation of the P2Y family of GPCRs. We report that ATP stimulates TGF-alpha proteolysis with concomitant EGFR activation and that this process requires TACE/ADAM17 activity in both murine fibroblasts and CHO cells. ATP-induced TGF-alpha shedding required calcium and was independent of Src family kinases and PKC and MAPK signaling. Moreover, ATP-induced TGF-alpha shedding was completely inhibited by scavengers of ROS, whereas calcium-stimulated shedding was partially inhibited by ROS scavenging. Hydrogen peroxide restored TGF-alpha shedding after calcium chelation. Importantly, we also found that ATP-induced shedding was independent of the cytoplasmic NADPH oxidase complex. Instead, mitochondrial ROS production increased in response to ATP and mitochondrial oxidative complex activity was required to activate TACE-dependent shedding. These results reveal an essential role for mitochondrial ROS in regulating GPCR-induced growth factor shedding.

Identification and characterization of a novel amphioxus dopamine D-like receptor

Dopamine receptors function to control many aspects of motor control and other forms of behaviour in both vertebrates and invertebrates. They can be divided into two main groups (D(1) and D(2)) based on sequence similarity, ligand affinity and effector coupling. However, little is known about the pharmacology and functionality of dopamine receptors in the deuterostomian invertebrates, such as the cephalochordate amphioxus (Branchiostoma floridae) which has recently been placed as the most basal of all the chordates. A bioinformatic study shows that amphioxus has at least three dopamine D(1)-like receptor sequences. One of these receptors, AmphiD(1)/beta, was found to have high levels of sequence similarity to both vertebrate D(1) receptors and to beta-adrenergic receptors. Here, we report on the cloning of AmphiD(1)/beta from an adult amphioxus cDNA library, and its pharmacological characterization subsequent to its expression in both mammalian cell lines and Xenopus oocytes. It was found that AmphiD(1)/beta has a similar pharmacology to vertebrate D(1) receptors, including responding to benzodiazepine ligands. The pharmacology of the receptor exhibits 'agonist-specific coupling' depending upon the second messenger pathway to which it is linked. Moreover, no pharmacological characteristics were observed to suggest that AmphiD(1)/beta may be an amphioxus orthologue of vertebrate beta-adrenergic receptors.

Improvements in live cell analysis of G protein coupled receptors using second generation BD calcium assay kits

BD™ Calcium Assay Kits are designed for cell-based calcium mobilization high-throughput screening assays. The kits use a proprietary formulation including a non-fluorescent calcium indicator that becomes activated inside the cell and shows increased fluorescence upon calcium binding. The formulation includes a signal-enhancing reagent to maximize the signal over background in a homogeneous, no-wash assay format, based on a technology developed at BD. We have compared the next generation BD calcium assay kit product family to previous versions of the formulation, and to other commercially available homogeneous calcium assay kits. The improvements have enabled better performance on the cell lines and receptors that we have tested in all plate formats including 1536.

Immunogold electron microscopic demonstration of distinct submembranous localization of the activated gammaPKC depending on the stimulation

We examined the precise intracellular translocation of gamma subtype of protein kinase C (gammaPKC) after various extracellular stimuli using confocal laser-scanning fluorescent microscopy (CLSM) and immunogold electron microscopy. By CLSM, treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in a slow and irreversible accumulation of green fluorescent protein (GFP)-tagged gammaPKC (gammaPKC-GFP) on the plasma membrane. In contrast, treatment with Ca(2+) ionophore and activation of purinergic or NMDA receptors induced a rapid and transient membrane translocation of gammaPKC-GFP. Although each stimulus resulted in PKC localization at the plasma membrane, electron microscopy revealed that gammaPKC showed a subtle but significantly different localization depending on stimulation. Whereas TPA and UTP induced a sustained localization of gammaPKC-GFP on the plasma membrane, Ca(2+) ionophore and NMDA rapidly translocated gammaPKC-GFP to the plasma membrane and then restricted gammaPKC-GFP in submembranous area (<500 nm from the plasma membrane). These results suggest that Ca(2+) influx alone induced the association of gammaPKC with the plasma membrane for only a moment and then located this enzyme at a proper distance in a touch-and-go manner, whereas diacylglycerol or TPA tightly anchored this enzyme on the plasma membrane. The distinct subcellular targeting of gammaPKC in response to various stimuli suggests a novel mechanism for PKC activation.

A microfluidic-based system for analysis of single cells based on Ca2+ flux

A microfluidic format-based system has been developed for in situ monitoring of the calcium flux response to agonists using Chinese hamster ovary (CHO) cells. The assay is based on measuring the fluorescent intensity of the calcium-sensitive indicator, Fluo-4 AM, and was performed in a modified glass chip channel, whose surface was functionalised using a silanisation method with 3-aminopropyltriethoxysilane (APTS) (enabling the cells to be immobilised on the channel surface). CHO cells calcium flux response was measured for different agonists over a range of concentrations. Cells and reagents were introduced into the chip in a continuous flow as a series of plugs in a given sequence.

Dynamic regulation of mammalian numb by G protein-coupled receptors and protein kinase C activation: Structural determinants of numb association with the cortical membrane

The cell fate determinant Numb is a membrane-associated adaptor protein involved in both development and intracellular vesicular trafficking. It has a phosphotyrosine-binding (PTB) domain and COOH-terminal endocytic-binding motifs for alpha-adaptin and Eps15 homology domain-containing proteins. Four isoforms of Numb are expressed in vertebrates, two of which selectively associate with the cortical membrane. In this study, we have characterized a cortical pool of Numb that colocalizes with AP2 and Eps15 at substratum plasma membrane punctae and cortical membrane-associated vesicles. Green fluorescent protein (GFP)-tagged mutants of Numb were used to identify the structural determinants required for localization. In addition to the previously described association of the PTB domain with the plasma membrane, we show that the AP2-binding motifs facilitate the association of Numb with cortical membrane punctae and vesicles. We also show that agonist stimulation of G protein-coupled receptors (GPCRs) that are linked to phospholipase Cbeta and protein kinase C (PKC) activation causes redistribution of Numb from the cortical membrane to the cytosol. This effect is correlated with Numb phosphorylation and an increase in its Triton X-100 solubility. Live-imaging analysis of mutants identified two regions within Numb that are independently responsive to GPCR-mediated lipid hydrolysis and PKC activation: the PTB domain and a region encompassing at least three putative PKC phosphorylation sites. Our data indicate that membrane localization of Numb is dynamically regulated by GPCR-activated phospholipid hydrolysis and PKC-dependent phosphorylation events.

Galpha13 mediates activation of the cytosolic phospholipase A2alpha through fine regulation of ERK phosphorylation

Heterotrimeric GTP-binding (G) proteins transduce hormone-induced signals to their effector enzymes, which include several phospholipases. In particular, the G(o)/G(i) and G(q) protein families have been shown to couple signaling to phospholipase A(2) (PLA(2)), phospholipase C, and phospholipase D, while the G(12)/G(13) family has been linked to the activation of small GTPases of the Rho family, and hence, to phospholipase D activation. Here, we demonstrate that in CHO cells, the G(12)/G(13) family is also able to activate cPLA(2)alpha, through the activation of RhoA and, subsequently, ERK1/2. Hormone-induced arachidonic acid release increased as a consequence of Galpha(13) overexpression, and was inhibited through inhibition of Galpha(13) signaling. The Galpha(13)-mediated cPLA(2)alpha activation was inhibited by pharmacological blockade of ERK1/2 with either U0126 or PD98059, and by RhoA inactivation with C3 toxin or a dominant-negative RhoA (N19RhoA), and was stimulated by the serine-threonine phosphatase inhibitor calyculin A. Our data thus identify a pathway of cPLA(2)alpha regulation that is initiated by thrombin and purinergic receptor activation, and that signals through Galpha(13), RhoA and ERK1/2, with the involvement of a calyculin-sensitive phosphatase.

Characteristics of acid extrusion from Chinese hamster ovary cells expressing different prostaglandin EP receptors

Acid extrusion responses to prostaglandin E2 were investigated in Chinese hamster ovary (CHO) cells heterologously expressing human EP1, EP2, and EP3I receptors (hEP1, hEP2 and hEP3I) by using a microphysiometer that detected small pH changes in the extracellular microenvironment. In the cells expressing hEP1, which is known to increase intracellular Ca2+, prostaglandin E2 (1 and 10 nM) slowly accelerated acid extrusion, but at higher concentrations an initial transient phase (approximately 5 times greater than the basal acidification) overlapped the slowly developing phase. In contrast, the cells expressing hEP2, which evokes cAMP production, showed dual responses to prostaglandin E2: an initial reduction followed by an acceleration of acid extrusion. In the cells expressing hEP3I, which is known to produce both a decrease in cAMP and a modest increase in intracellular Ca2+, acid extrusion was gradually accelerated by prostaglandin E2 and reached a plateau at around 2 min. Elimination of extracellular Ca2+ diminished the responses to prostaglandin E2 in hEP1 cells, but had little effect on the responses in hEP2 and hEP3I cells. Forskolin mimicked the dual effects of prostaglandin E2 observed in the hEP2 cells. Pretreatment with pertussis toxin inhibited the response to prostaglandin E2 in hEP3I cells, but the responses in hEP1 and hEP2 cells were not affected. Na+/H+ exchanger (NHE) inhibitors (EIPA and HOE642) suppressed all the responses induced by prostaglandin E2 in hEP1, hEP2, and hEP3I cells. These results suggest that EP receptor subtypes regulate acid extrusion mainly via NHE-1 through distinct signal transduction pathways in CHO cells.

Identification and characterization of a novel family of Drosophila beta-adrenergic-like octopamine G-protein coupled receptors

Insect octopamine receptors carry out many functional roles traditionally associated with vertebrate adrenergic receptors. These include control of carbohydrate metabolism, modulation of muscular tension, modulation of sensory inputs and modulation of memory and learning. The activation of octopamine receptors mediating many of these actions leads to increases in the levels of cyclic AMP. However, to date none of the insect octopamine receptors that have been cloned have been convincingly shown to be capable of directly mediating selective and significant increases in cyclic AMP levels. Here we report on the identification and characterization of a novel, neuronally expressed family of three Drosophila G-protein coupled receptors that are selectively coupled to increases in intracellular cyclic AMP levels by octopamine. This group of receptors, DmOct beta1R (CG6919), DmOct beta2R (CG6989) and DmOct beta3R (CG7078) shows homology to vertebrate beta-adrenergic receptors. When expressed in Chinese hamster ovary cells all three receptors show a strong preference for octopamine over tyramine for the accumulation of cyclic AMP but show unique pharmacological profiles when tested with a range of synthetic agonists and antagonists. Thus, the pharmacological profile of individual insect tissue responses to octopamine might vary with the combination and the degree of expression of the individual octopamine receptors present.

TG1019/OXE, a Gαi/o-protein-coupled receptor, mediates 5-oxo-eicosatetraenoic acid-induced chemotaxis

We have previously identified a Galpha(i/o)-protein-coupled receptor (TG1019/OXE) using 5-oxo-6E,8Z,11Z,14Z-eicosatetraenoic acid (5-oxo-ETE) as its ligand. We investigated signal transduction from TG1019 following stimulation with 5-oxo-ETE and role of TG1019 in 5-oxo-ETE-induced chemotaxis, using Chinese hamster ovary cells expressing TG1019 (CHO/TG1019 cells). 5-Oxo-ETE induced intracellular calcium mobilization and rapid activation of MEK/ERK and PI3K/Akt pathways in CHO/TG1019 cells. CHO/TG1019 cells stimulated with 5-oxo-ETE and other eicosanoids exhibited chemotaxis with efficacies related to agonistic activity of each eicosanoid for TG1019. Pretreatment of the cells with pertussis toxin, a phospholipase C (PLC) inhibitor (U73122) or a PI3K inhibitor (LY294002), markedly suppressed 5-oxo-ETE-induced chemotaxis, whereas pretreatment with a MEK inhibitor (PD98059) had no significant effect on the chemotaxis. Our results show that TG1019 mediates 5-oxo-ETE-induced chemotaxis and that signals from TG1019 are transduced via Galpha(i/o) protein to PLC/calcium mobilization, MEK/ERK, and PI3K/Akt, among which PLC and PI3K would play important roles in the chemotaxis.

Mutant protein kinase Cgamma found in spinocerebellar ataxia type 14 is susceptible to aggregation and causes cell death

Spinocerebellar ataxia type 14 (SCA14) is an autosomal dominant neurodegenerative disease characterized by various symptoms including cerebellar ataxia. Recently, several missense mutations in the protein kinase Cgamma (gammaPKC) gene have been found in different SCA14 families. To elucidate how the mutant gammaPKC causes SCA14, we examined the molecular properties of seven mutant (H101Y, G118D, S119P, S119F, Q127R, G128D, and F643L) gammaPKCs fused with green fluorescent protein (gammaPKC-GFP). Wild-type gammaPKC-GFP was expressed ubiquitously in the cytoplasm of CHO cells, whereas mutant gammaPKC-GFP tended to aggregate in the cytoplasm. The insolubility of mutant gammaPKC-GFP to Triton X-100 was increased and correlated with the extent of aggregation. gammaPKC-GFP in the Triton-insoluble fraction was rarely phosphorylated at Thr(514), whereas gammaPKC-GFP in the Triton-soluble fraction was phosphorylated. Furthermore, the stimulation of the P2Y receptor triggered the rapid aggregation of mutant gammaPKC-GFP within 10 min after transient translocation to the plasma membrane. Overexpression of the mutant gammaPKC-GFP caused cell death that was more prominent than wild type. The cytotoxicity was exacerbated in parallel with the expression level of the mutant. These results indicate that SCA14 mutations make gammaPKC form cytoplasmic aggregates, suggesting the involvement of this property in the etiology of SCA14.

Neurokinin-1 receptor resensitization precedes receptor recycling

Following agonist binding, neurokinin-1 receptors undergo rapid desensitization followed by internalization and recycling. Desensitization requires receptor phosphorylation but does not require internalization, whereas resensitization is thought to require internalization and recycling. Our previous data, however, have suggested that, following activation and desensitization, the return of responsiveness to the neurokinin-1 agonist substance P (termed "resensitization") occurs hours before internalized receptors are recycled back to the plasma membrane. To further investigate this novel mechanism of neurokinin-1 receptor resensitization, we have studied the time courses of neurokinin-1 receptor responsiveness, recycling, and dephosphorylation by measuring cellular Ca(2+) responses, ligand-receptor binding, and receptor phosphorylation, respectively. Concentration-response curves and competition binding curves were obtained at various times following desensitization. The effects of the nonhydrolyzable GTP analog Gpp(NH)p on substance P binding were also studied to assess receptor-G protein coupling. After receptor activation and desensitization, Ca(2+) signaling in response to substance P occurred within 90 min, whereas the return of receptor binding required 240 min. Receptor dephosphorylation was greater than 90% complete 20 min after agonist washout. In addition, the return of substance P responsiveness coincided with a return in sensitivity of substance P binding to Gpp(NH)p, indicating a return in receptor-G protein coupling. These data show that the resensitization of responsiveness to substance P precedes receptor recycling. This may result from a conversion of nonfunctional neurokinin-1 receptors to functional receptors at the plasma membrane.

Phosphorylation of PKC activation loop plays an important role in receptor-mediated translocation of PKC

Protein kinase C (PKC) is translocated to various cellular regions in a subtype and stimulation-dependent manner. Thereafter, the activated PKC phosphorylates its substrate and causes subsequent cellular responses (PKC targeting). The 3-phosphoinositide-dependent protein kinase-1 (PDK1) has an essential role in the maturation of PKC by phosphorylating a threonine residue in the PKC activation loop. To elucidate the role of PDK1 in PKC targeting, we expressed mutant gamma- or delta-PKC fused with GFP (gamma- or delta-PKC-ALM (activation loop mutant)-GFP), whose threonine residue in the activation loop was replaced with alanine, and compared their P2Y receptor-mediated translocation with wild-type PKC-GFP in CHO cells. ATP (1 mm) induced the transient translocation of wild-type gamma- or delta-PKC-GFP from cytoplasm to plasma membrane and following retranslocation from membrane to the cytoplasm. gamma- or delta-PKC-ALM-GFP was also translocated to plasma membrane, which was, however, retained at the membrane for a longer period than wild type. Similar results were observed in kinase-negative PKC mutants, indicating that the phosphorylation by PDK1 affects the retranslocation step of PKC by regulating the kinase activity. The simultaneous monitoring of [Ca2+]i and diacylglycerol (DG) levels with the translocation of PKC demonstrated that PKC-ALM induced the prolonged accumulation of DG, resulting in the prolonged retention of PKC-ALM at the plasma membrane. It is possible that PKC-ALM with decreased kinase activity could delay the conversion of DG at the plasma membrane. Our present study suggests that the activation loop phosphorylation plays an important role in receptor-mediated PKC targeting.

InsP3-mediated intracellular calcium signalling is altered by expression of synaptojanin-1

Phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] plays an important physiological role as a precursor for the InsP3-mediated intracellular calcium (Ca2+) signalling cascade. It also regulates membrane trafficking, actin function and transmembrane proteins. SJ-1 (synaptojanin-1), a phosphoinositide phosphatase, regulates the turnover of a PtdIns(4,5)P2 pool involved in clathrin and actin dynamics at the cell surface. We tested the interrelationship of this pool with PtdIns(4,5)P2 pools involved in Ca2+ signalling by expressing in Chinese-hamster ovary cells full-length SJ-1 or its 5-Pase (inositol 5-phosphatase) domain. SJ-1 significantly attenuated the generation of Ca2+ oscillations induced by ATP and the 5-Pase domain mimicked this effect. These changes correlated with increased PtdIns(4,5)P2 phosphatase activity of cellular extracts. Overexpression of the endoplasmic reticulum-anchored PtdIns(4)P phosphatase Sac1 did not affect Ca2+ oscillations, although it increased the Ca2+ efflux rate from intracellular stores. The ability of SJ-1 to alter intracellular Ca2+ signalling indicates a close functional interrelationship between plasma membrane PtdIns(4,5)P2 pools that control actin and endocytosis and those involved in the regulation of specific spatio-temporal Ca2+ signals.

Interchange of aequorin and obelin bioluminescence color is determined by substitution of one active site residue of each photoprotein

The bioluminescence spectra from the Ca2+-regulated photoproteins aequorin (lambdamax=469 nm) and obelin (lambdamax=482 nm) differ because aequorin has an H-bond from its Tyr82 to the bound coelenteramide, not present in obelin at the corresponding Phe88. Substitutions of this Phe88 by Tyr, Trp, or His shifted the obelin bioluminescence to shorter wavelength with F88Y having lambdamax=453 nm. Removal of the H-bond by the substitution of Y82F in aequorin shifted its bioluminescence to lambdamax=501 nm. All mutants were stable with good activity and were expressible in mammalian cells, thereby demonstrating potential for monitoring multiple events in cells using multi-color detection.

Regulation of the human organic cation transporter hOCT1

The human organic cation transporter type 1 (hOCT1) is an important transport system for small organic cations in the liver. Organic cation transporters are regulated by different signaling pathways, but the regulation of hOCT1 has not yet been studied. In this work, we have for the first time investigated the regulation of hOCT1. hOCT1 was expressed in Chinese hamster ovary cells (CHO-hOCT1) and in human embryonic kidney cells (HEK293-hOCT1). Its activity was monitored using microfluorimetry with the fluorescent organic cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP(+)) as substrate. hOCT1 expressed in CHO-cells was inhibited by protein kinase A (PKA) activation (1 microM forskolin, -58 +/- 6%, n = 12), calmodulin inhibition (0.1 microM calmidazolium, -68 +/- 3%, n = 6; 10 microM ophiobolin A, -48 +/- 10%, n = 7), calmodulin-dependent kinase II inhibition (1 microM KN62, -78 +/- 4%, n = 12), and inhibition of p56(lck) tyrosine kinase (10 microM aminogenistein, -35 +/- 7%, n = 12). The apparent affinities for TEA(+) were lower in CHO-hOCT1 than in HEK293-hOCT1, while those for TPA(+) and quinine were almost identical; the rank order of EC(50) values (TPA(+) > quinine > TEA(+)) was independent of the expression system. EC(50) values for TEA(+) in CHO-hOCT1 or HEK293-hOCT1 were increased under calmidazolium incubation (6.3 and 1.4 mM, respectively). hOCT1 was inhibited by PKA and endogenously activated by calmodulin, calmodulin-dependent kinase II, and p56(lck) tyrosine kinase. Regulation pathways were the same in the two expression systems. Since apparent substrate affinities depend on activity of regulatory pathways, the expression system plays a role in determining the substrate affinities.

Phospholipase A2 products retain a neuron specific gamma isoform of PKC on the plasma membrane through the C1 domain--a molecular mechanism for sustained enzyme activity

To clarify molecular mechanism for sustained activation of gamma protein kinase C (gammaPKC), a neuron-specific subtype, we investigated the involvement of phospholipase A2 (PLA2) products in the membrane association of gammaPKC upon activation of G protein coupled purinoceptors in CHO-K1 and NG 108-15 cells. In addition, the functional domain responsible for PLA2-product mediated retention of gammaPKC on the plasma membrane was determined by simultaneously monitoring two different fluorescence-tagged gammaPKCs and mutants in the same living CHO-K1 cells. Purinoceptor activation by UTP induced a transient translocation of gammaPKC from the cytoplasm to the plasma membrane. Interestingly, PLA2 inhibitors, bromoenol lactone (BEL) and arachidonyl-trifluoromethyl ketone (AACOF3), shortened the retention time of gammaPKC on the plasma membrane in cells treated with UTP, while a DAG kinase inhibitor did not affect it. The C1 domain deficient mutant (DeltaC1-gammaPKC) also showed short membrane association compared with wild type gammaPKC, when cells are treated with UTP or arachidonic acid (AA) plus a Ca(2+) ionophore. However, deletion of C1A or C1B subdomains (DeltaC1A-gammaPKC or DeltaC1B-gammaPKC) did not alter the retention time on the plasma membrane, whereas PLA2 inhibitor shortened the retention times of both mutants. These results indicate that PLA2 products prolong the retention of gammaPKC on the plasma membrane through the C1A and/or C1B subdomain in purinoceptor-stimulated CHO-K1 cells. The importance of PLA2 product and C1 domain for the retention of gammaPKC on the membrane was also confirmed using neuronal cell line, suggesting that these are part of molecular machinery for sustaining enzyme activity in neurons.

Mepyramine, a histamine H1 receptor inverse agonist, binds preferentially to a G protein-coupled form of the receptor and sequesters G protein

Accurate characterization of the molecular mechanisms of the action of ligands is an extremely important issue for their appropriate research, pharmacological, and therapeutic uses. In view of this fact, the aim of the present work was to investigate the mechanisms involved in the actions of mepyramine at the guinea pig H(1) receptor stably expressed in Chinese hamster ovary cells. We found that mepyramine is able to decrease the basal constitutive activity of the guinea pig H(1) receptor, to bind with high affinity to a G(q/11) protein-coupled form of the receptor and to promote a G protein-coupled inactive state of the H(1) receptor that interferes with the G(q/11)-mediated signaling of the endogenously expressed ATP receptor, as predicted by the Cubic Ternary Complex Model of receptor occupancy. The effect of mepyramine on ATP-induced signaling was specifically neutralized by Galpha(11) overexpression, indicating that mepyramine is able to reduce G protein availability for other non-related receptors associated with the same signaling pathway. Finally, we found a loss of mepyramine efficacy in decreasing basal levels of intracellular calcium at high Galpha(11) expression levels, which can be theoretically explained in terms of high H(1) receptor constitutive activity. The whole of the present work sheds new light on H(1) receptor pharmacology and the mechanisms H(1) receptor inverse agonists could use to exert their observed negative efficacy.

Co-expression of MG29 and ryanodine receptor leads to apoptotic cell death: effect mediated by intracellular Ca2+ release

Perturbation of intracellular Ca2+ homeostasis has been shown to regulate the process of cell proliferation and apoptosis. Our previous studies show that mitsugumin 29 (MG29), a synaptophysin-related protein localized in the triad junction of skeletal muscle, serves an essential role in muscle Ca2+ signaling by regulating the process of store-operated Ca2+ entry. Here we report a functional interaction between MG29 and the ryanodine receptor (RyR)/Ca2+ release channel. The purified MG29 protein enhances activity of the RyR/Ca2+ release channel incorporated into the lipid bilayer membrane. Co-expression of MG29 and RyR in Chinese hamster ovary cells leads to apoptotic cell death resulting from depletion of intracellular Ca2+ stores, despite neither protein expression alone exhibits any significant effect on cell viability. In transient expression studies, the presence of RyR in the endoplasmic reticulum leads to retention of MG29 from the plasma membrane into the intracellular organelles. This functional interaction between MG29 and RyR could have important implications in the Ca2+ signaling processes of muscle cells. Our data also show that perturbation of intracellular Ca2+ homeostasis can serve as a key signal in the initiation of apoptosis.

Negative regulation of CFTR activity by extracellular ATP involves P2Y2 receptors in CFTR-expressing CHO cells

Extracellular nucleotides exert autocrine/ paracrine effects on ion transport by activating P2 receptors. We studied the effects of extracellular ATP and UTP on the cystic fibrosis transmembrane conductance regulator (CFTR) channel stably expressed in Chinese Hamster Ovary cells (CHO-BQI cells). CFTR activity was measured using the (125I) iodide efflux technique and whole-cell patch-clamp recording in response to either forskolin or xanthine derivatives. Using RT-PCR and intracellular calcium concentration ([Ca2+]i) measurement, we showed that CHO-BQI cells express P2Y2 but not P2Y4 receptors. While ATP and UTP induced similar increases in [Ca2+]i, pre-addition by one of these two agonists desensitized the response for the other, suggesting that ATP- and UTP-induced [Ca2+]i increases were mediated by a common receptor, which was identified as the P2Y2 subtype. CFTR activity was reduced by ATP and UTP but not by ADP or adenosine applications. This inhibitory effect of ATP on CFTR activity was not due to a change in cAMP level. Furthermore, CFTR activation by forskolin or IBMX failed to promote [Ca2+]i increase, suggesting that CFTR activation did not generate an ATP release large enough to stimulate P2Y2 receptors. Taken together, our results show that endogenous P2Y2 receptor activation downregulates CFTR activity in a cAMP-independent manner in CHO cells.