Pharmacological analysis of intracellular Ca2+ signalling: problems and pitfalls

End binding-3 inhibitor activates regenerative program in age-related macular degeneration

Wet age-related macular degeneration (AMD), characterized by leaky neovessels emanating from the choroid, is a main cause of blindness. As current treatments for wet AMD require regular intravitreal injections of anti-vascular endothelial growth factor (VEGF) biologics, there is a need for the development of less invasive treatments. Here, we designed an allosteric inhibitor of end binding-3 (EB3) protein, termed EBIN, which reduces the effects of environmental stresses on endothelial cells by limiting pathological calcium signaling. Delivery of EBIN via eye drops in mouse and non-human primate (NHP) models of wet AMD prevents both neovascular leakage and choroidal neovascularization. EBIN reverses the epigenetic changes induced by environmental stresses, allowing an activation of a regenerative program within metabolic-active endothelial cells comprising choroidal neovascularization (CNV) lesions. These results suggest the therapeutic potential of EBIN in preventing the degenerative processes underlying wet AMD.

Differential use of BTK and PLC in FcεRI- and KIT-mediated mast cell activation: A marginal role of BTK upon KIT activation

In mast cells (MCs), the TEC family kinase (TFK) BTK constitutes a central regulator of antigen (Ag)-triggered, FcεRI-mediated PLCγ phosphorylation, Ca²⁺ mobilization, degranulation, and pro-inflammatory cytokine production. Less is known about the function of BTK in the context of stem cell factor (SCF)-induced KIT signaling. In bone marrow-derived MCs (BMMCs), Ag stimulation caused intense phosphorylation of BTK at Y551 in its active center and at Y223 in its SH3-domain, whereas in response to SCF only Y223 was significantly phosphorylated. Further data using the TFK inhibitor Ibrutinib indicated that BTK Y223 is phosphorylated by a non-BTK TFK upon SCF stimulation. In line, SCF-induced PLCγ1 phosphorylation was stronger attenuated by Ibrutinib than by BTK deficiency. Subsequent pharmacological analysis of PLCγ function revealed a total block of SCF-induced Ca²⁺ mobilization by PLC inhibition, whereas only the sustained phase of Ca²⁺ flux was curtailed in Ag-stimulated BMMCs. Despite this severe stimulus-dependent difference in inducing Ca²⁺ mobilization, PLCγ inhibition suppressed Ag- and SCF-induced degranulation and pro-inflammatory cytokine production to comparable extents, suggesting involvement of additional TFK(s) or PLCγ-dependent signaling components. In addition to PLCγ, the MAPKs p38 and JNK were activated by Ag in a BTK-dependent manner; this was not observed upon SCF stimulation. Hence, FcεRI and KIT employ different mechanisms for activating PLCγ, p38, and JNK, which might strengthen their cooperation regarding pro-inflammatory MC effector functions. Importantly, our data clearly demonstrate that analyzing BTK Y223 phosphorylation is not sufficient to prove BTK activation.

Evidence for metabotropic function of epithelial nicotinic cholinergic receptors in rat colon

BACKGROUND AND PURPOSE

ACh exerts its actions via nicotinic (nAChR) and muscarinic receptors. In the peripheral nervous system, ionotropic nAChR mediate responses in excitable cells. However, recent studies demonstrate the expression of nAChR in the colonic epithelium, which are coupled to an induction of Cl^- secretion via activation of the Na⁺ -K⁺ -pump.

EXPERIMENTAL APPROACH

In order to find out whether these epithelial nAChR function as ionotropic receptors, intracellular microelectrode and imaging experiments were performed in isolated crypts from rat colon. Apically permeabilized epithelia were used to measure pump current across the basolateral membrane.

KEY RESULTS

Imaging experiments with the Na⁺ -sensitive dye SBFI revealed that nicotine induced a decrease in the cytosolic Na⁺ concentration concomitant with a fall in the cytosolic Ca²⁺ concentration in about 50% of the cells. as shown in fura-2 experiments. Nicotine hyperpolarized the membrane by 6.4 ± 2.1 mV. These observations contradict the assumption that epithelial nAChR function as ligand-gated non-selective cation channels. The decrease in the cytosolic Na⁺ concentration was strongly delayed, when the Na⁺ -K⁺ -pump was inhibited by scilliroside. Ussing chamber experiments revealed a strong dependence of the nicotine-induced pump current on the presence of Ca²⁺ , and chelation of cytosolic Ca²⁺ with BAPTA prevented the fall in the cytosolic Na⁺ concentration in SBFI-loaded crypts. Inhibition of PKC with GF 109203X or Goe 6983 significantly reduced the nicotine-induced pump current.

CONCLUSIONS AND IMPLICATIONS

These results suggest that epithelial nAChR activate the Na⁺ -K⁺ -pump via a PKC dependent on a sufficient cytosolic Ca²⁺ concentration.

Synaptopodin is regulated by aromatase activity

Locally synthesized estradiol plays an important role in synaptic plasticity in the hippocampus. We have previously shown that in hippocampal neurons, activity of the enzyme aromatase, which converts testosterone into estradiol, is reduced via Ca²⁺ -dependent phosphorylation. Synaptopodin is a highly estrogen responsive protein, and it has been shown that it is an important regulator of synaptic plasticity, mediated by its close association with internal calcium stores. In this study, we show that the expression of synaptopodin is stronger in the hippocampus of female animals than in that of male animals. Phosphorylation of aromatase, using letrozole, however, down-regulates synaptopodin immunohistochemistry in the hippocampus of both male and females. Similarly, in aromatase knock-out mice synaptopodin expression in the hippocampus is reduced sex independently. Using primary-dissociated hippocampal neurons, we found that evoked release of Ca²⁺ from internal stores down-regulates aromatase activity, which is paralleled by reduced expression of synaptopodin. Opposite effects were achieved after inhibition of the release. Calcium-dependent regulation of synaptopodin expression was abolished when the control of aromatase activity by the Ca²⁺ transients was disrupted. Our data suggest that the regulation of aromatase activity by Ca²⁺ transients in neurons contributes to synaptic plasticity in the hippocampus of male and female animals as an on-site regulatory mechanism.

Hypocretin/Orexin Peptides Alter Spike Encoding by Serotonergic Dorsal Raphe Neurons through Two Distinct Mechanisms That Increase the Late Afterhyperpolarization

Orexins (hypocretins) are neuropeptides that regulate multiple homeostatic processes, including reward and arousal, in part by exciting serotonergic dorsal raphe neurons, the major source of forebrain serotonin. Here, using mouse brain slices, we found that, instead of simply depolarizing these neurons, orexin-A altered the spike encoding process by increasing the postspike afterhyperpolarization (AHP) via two distinct mechanisms. This orexin-enhanced AHP (oeAHP) was mediated by both OX1 and OX2 receptors, required Ca(2+) influx, reversed near EK, and decayed with two components, the faster of which resulted from enhanced SK channel activation, whereas the slower component decayed like a slow AHP (sAHP), but was not blocked by UCL2077, an antagonist of sAHPs in some neurons. Intracellular phospholipase C inhibition (U73122) blocked the entire oeAHP, but neither component was sensitive to PKC inhibition or altered PKA signaling, unlike classical sAHPs. The enhanced SK current did not depend on IP3-mediated Ca(2+) release but resulted from A-current inhibition and the resultant spike broadening, which increased Ca(2+) influx and Ca(2+)-induced-Ca(2+) release, whereas the slower component was insensitive to these factors. Functionally, the oeAHP slowed and stabilized orexin-induced firing compared with firing produced by a virtual orexin conductance lacking the oeAHP. The oeAHP also reduced steady-state firing rate and firing fidelity in response to stimulation, without affecting the initial rate or fidelity. Collectively, these findings reveal a new orexin action in serotonergic raphe neurons and suggest that, when orexin is released during arousal and reward, it enhances the spike encoding of phasic over tonic inputs, such as those related to sensory, motor, and reward events.

SIGNIFICANCE STATEMENT

Orexin peptides are known to excite neurons via slow postsynaptic depolarizations. Here we elucidate a significant new orexin action that increases and prolongs the postspike afterhyperpolarization (AHP) in 5-HT dorsal raphe neurons and other arousal-system neurons. Our mechanistic studies establish involvement of two distinct Ca(2+)-dependent AHP currents dependent on phospholipase C signaling but independent of IP3 or PKC. Our functional studies establish that this action preserves responsiveness to phasic inputs while attenuating responsiveness to tonic inputs. Thus, our findings bring new insight into the actions of an important neuropeptide and indicate that, in addition to producing excitation, orexins can tune postsynaptic excitability to better encode the phasic sensory, motor, and reward signals expected during aroused states.

OX2 orexin/hypocretin receptor signal transduction in recombinant Chinese hamster ovary cells

There are two subtypes of orexin receptors, OX1 and OX2. Signalling pathways have been mapped in much higher detail for OX1 receptors than OX2 receptors. Almost all the detailed studies have been performed in Chinese hamster ovary cells, and we thus chose the same cell background for the studies on human OX2 receptors to allow comparison to human OX1 receptors. Adenylyl cyclase, phospholipase A2, C and D and diacylglycerol lipase activities were assessed by precursor radiolabelling and chromatographic separation (ion exchange, affinity or thin layer), calcium by a fluorescent method, and receptor binding with [(125)I]-orexin-A. Upon activation with orexin-A, OX2 receptors stimulated phospholipase A2, C and D, diacylglycerol lipase and calcium elevation, and both inhibited and stimulated adenylyl cyclase; i.e., the responses to OX2 activation by orexin-A were principally like those of OX1, in contrast to some previous suggestions. The responses occurred mostly in the same concentration range as those for OX1 activation and via the same signal cascades. However, some responses were weaker, suggesting a partially differential coupling to some cascades. In summary, OX2 receptor signalling is principally similar to OX1 receptor signalling suggesting also a physiologically similar coupling, though this needs to be verified in physiological contexts. Some (relatively weak) differences between the receptors may be investigated in further studies.

Involvement of plasma membrane Ca2+ channels, IP3 receptors, and ryanodine receptors in the generation of spontaneous rhythmic contractions of the cricket lateral oviduct

In the present study, the isolated cricket (Gryllus bimaculatus) lateral oviduct exhibited spontaneous rhythmic contractions (SRCs) with a frequency of 0.29±0.009 Hz (n=43) and an amplitude of 14.6±1.25 mg (n=29). SRCs completely disappeared following removal of extracellular Ca2+ using a solution containing 5mM EGTA. Application of the non-specific Ca2+ channel blockers Co2+, Ni2+, and Cd2+ also decreased both the frequency and amplitude of SRCs in dose-dependent manners, suggesting that Ca2+ entry through plasma membrane Ca2+ channels is essential for the generation of SRCs. Application of ryanodine (30 μM), which depletes intracellular Ca2+ by locking ryanodine receptor (RyR)-Ca2+ channels in an open state, gradually reduced the frequency and amplitude of SRCs. A RyR antagonist, tetracaine, reduced both the frequency and amplitude of SRCs, whereas a RyR activator, caffeine, increased the frequency of SRCs with a subsequent increase in basal tonus, indicating that RyRs are essential for generating SRCs. To further investigate the involvement of phospholipase C (PLC) and inositol 1,4,5-trisphosphate receptors (IP3Rs) in SRCs, we examined the effect of a PLC inhibitor, U73122, and an IP3R antagonist, 2-aminoethoxydiphenyl borate (2-APB), on SRCs. Separately, U73122 (10 μM) and 2-APB (30-50 μM) both significantly reduced the amplitude of SRCs with little effect on their frequency, further indicating that the PLC/IP3R signaling pathway is fundamental to the modulation of the amplitude of SRCs. A hypotonic-induced increase in the frequency and amplitude of SRCs and a hypertonic-induced decrease in the frequency and amplitude of SRCs indicated that mechanical stretch of the lateral oviduct is involved in the generation of SRCs. The sarcoplasmic reticulum Ca2+-pump ATPase inhibitors thapsigargin and cyclopiazonic acid impaired or suppressed the relaxation phase of SRCs. Taken together, the present results indicate that Ca2+ influx through plasma membrane Ca2+ channels and Ca2+ release from RyRs play an essential role in pacing SRCs and that Ca2+ release from IP3Rs may play a role in modulating the amplitude of SRCs, probably via activation of PLC.

Scaffold hopping with virtual screening from IP3 to a drug-like partial agonist of the inositol trisphosphate receptor

Inositol 1,4,5-trisphosphate (IP3 ) is a universal signalling molecule that releases calcium from stores within cells by activating the IP3 receptor. Although chemical tools that modulate the IP3 receptor exist, none is ideal due to trade offs between potency, selectivity and cell permeability, and their chemical properties make them challenging starting points for optimisation. Therefore, to find new leads, we used virtual screening to scaffold hop from IP3 by using the program ROCS to perform a 3D ligand-based screen of the ZINC database of purchasable compounds. We then used the program FRED to dock the top-ranking hits into the IP3 binding pocket of the receptor. We tested the 12 highest-scoring hits in a calcium-release bioassay and identified SI-9 as a partial agonist. SI-9 competed with [(3) H]IP3 binding, and reduced histamine-induced calcium signalling in HeLa cells. SI-9 has a novel 2D scaffold that represents a tractable lead for designing improved IP3 receptor modulators.

Sphingosine derivatives inhibit cell signaling by electrostatically neutralizing polyphosphoinositides at the plasma membrane

Mast cell stimulation via IgE receptors causes activation of multiple processes, including Ca(2+) mobilization, granule exocytosis, and outward trafficking of recycling endosomes to the plasma membrane. We used fluorescein-conjugated cholera toxin B (FITC-CTxB) to label GM(1) in recycling endsomes and to monitor antigen-stimulated trafficking to the plasma membrane in both fluorimeter and imaging-based assays. We find that the sphingosine derivatives D-sphingosine and N,N'-dimethylsphingosine effectively inhibit this outward trafficking response, whereas a quarternary ammonium derivative, N,N',N″-trimethylsphingosine, does not inhibit. This pattern of inhibition is also found for Ca(2+) mobilization and secretory lysosomal exocytosis, indicating a general effect on Ca(2+)-dependent signaling processes. This inhibition correlates with the capacity of sphingosine derivatives to flip to the inner leaflet of the plasma membrane that is manifested as changes in plasma membrane-associated FITC-CTxB fluorescence and cytoplasmic pH. Using a fluorescently labeled MARCKS effector domain to monitor plasma membrane-associated polyphosphoinositides, we find that these sphingosine derivatives displace the electrostatic binding of this MARCKS effector domain to the plasma membrane in parallel with their capacity to inhibit Ca(2+)-dependent signaling. Our results support roles for plasma membrane polyphosphoinositides in Ca(2+) signaling and stimulated exocytosis, and they illuminate a mechanism by which D-sphingosine regulates signaling responses in mammalian cells.

Orexin/hypocretin receptor signalling cascades

Orexin (hypocretin) peptides and their two known G-protein-coupled receptors play essential roles in sleep-wake control and powerfully influence other systems regulating appetite/metabolism, stress and reward. Consequently, drugs that influence signalling by these receptors may provide novel therapeutic opportunities for treating sleep disorders, obesity and addiction. It is therefore critical to understand how these receptors operate, the nature of the signalling cascades they engage and their physiological targets. In this review, we evaluate what is currently known about orexin receptor signalling cascades, while a sister review (Leonard & Kukkonen, this issue) focuses on tissue-specific responses. The evidence suggests that orexin receptor signalling is multifaceted and is substantially more diverse than originally thought. Indeed, orexin receptors are able to couple to members of at least three G-protein families and possibly other proteins, through which they regulate non-selective cation channels, phospholipases, adenylyl cyclase, and protein and lipid kinases. In the central nervous system, orexin receptors produce neuroexcitation by postsynaptic depolarization via activation of non-selective cation channels, inhibition of K⁺ channels and activation of Na⁺/Ca²⁺ exchange, but they also can stimulate the release of neurotransmitters by presynaptic actions and modulate synaptic plasticity. Ca²⁺ signalling is also prominently influenced by these receptors, both via the classical phospholipase C-Ca²⁺ release pathway and via Ca²⁺ influx, mediated by several pathways. Upon longer-lasting stimulation, plastic effects are observed in some cell types, while others, especially cancer cells, are stimulated to die. Thus, orexin receptor signals appear highly tunable, depending on the milieu in which they are operating.

Alzheimer's disease-associated peptide Aβ42 mobilizes ER Ca(2+) via InsP3R-dependent and -independent mechanisms

Dysregulation of Ca²⁺ homeostasis is considered to contribute to the toxic action of the Alzheimer's disease (AD)-associated amyloid-β-peptide (Aβ). Ca²⁺ fluxes across the plasma membrane and release from intracellular stores have both been reported to underlie the Ca²⁺ fluxes induced by Aβ₄₂. Here, we investigated the contribution of Ca²⁺ release from the endoplasmic reticulum (ER) to the effects of Aβ₄₂ upon Ca²⁺ homeostasis and the mechanism by which Aβ₄₂ elicited these effects. Consistent with previous reports, application of soluble oligomeric forms of Aβ₄₂ induced an elevation in intracellular Ca²⁺. The Aβ₄₂-stimulated Ca²⁺ signals persisted in the absence of extracellular Ca²⁺ indicating a significant contribution of Ca²⁺ release from the ER Ca²⁺ store to the generation of these signals. Moreover, inositol 1,4,5-trisphosphate (InsP₃) signaling contributed to Aβ₄₂-stimulated Ca²⁺ release. The Ca²⁺ mobilizing effect of Aβ₄₂ was also observed when applied to permeabilized cells deficient in InsP₃ receptors, revealing an additional direct effect of Aβ₄₂ upon the ER, and a mechanism for induction of toxicity by intracellular Aβ₄₂.

Mechanisms underlying odorant-induced and spontaneous calcium signals in olfactory receptor neurons of spiny lobsters, Panulirus argus

We determined if a newly developed antennule slice preparation allows studying chemosensory properties of spiny lobster olfactory receptor neurons under in situ conditions with Ca(2+) imaging. We show that chemical stimuli reach the dendrites of olfactory receptor neurons but not their somata, and that odorant-induced Ca(2+) signals in the somata are sufficiently stable over time to allow stimulation with a substantial number of odorants. Pharmacological manipulations served to elucidate the source of odorant-induced Ca(2+) transients and spontaneous Ca(2+) oscillations in the somata of olfactory receptor neurons. Both Ca(2+) signals are primarily mediated by an influx of extracellular Ca(2+) through voltage-activated Ca(2+) channels that can be blocked by CoCl2 and the L-type Ca(2+) channel blocker verapamil. Intracellular Ca(2+) stores contribute little to odorant-induced Ca(2+) transients and spontaneous Ca(2+) oscillations. The odorant-induced Ca(2+) transients as well as the spontaneous Ca(2+) oscillations depend on action potentials mediated by Na(+) channels that are largely TTX-insensitive but blocked by the local anesthetics tetracaine and lidocaine. Collectively, these results corroborate the conclusion that odorant-induced Ca(2+) transients and spontaneous Ca(2+) oscillations in the somata of olfactory receptor neurons closely reflect action potential activity associated with odorant-induced phasic-tonic responses and spontaneous bursting, respectively. Therefore, both types of Ca(2+) signals represent experimentally accessible proxies of spiking.

Probing endoplasmic reticulum dynamics using fluorescence imaging and photobleaching techniques

This unit describes approaches and tools for studying the dynamics and organization of endoplasmic reticulum (ER) membranes and proteins in living cells using fluorescence microscopy. The ER plays a key role in secretory protein biogenesis, calcium regulation, and lipid synthesis. However, study of these processes has often been restricted to biochemical assays that average millions of lysed cells or imaging of static fixed cells. With new fluorescent protein (FP) reporter tools, sensitive commercial microscopes, and photobleaching techniques, investigators can interrogate the behaviors of ER proteins, membranes, and stress pathways in single live cells. Solutions are described for imaging challenges relevant to the ER, including the mobility of ER membranes, a range of ER structures, and the influence of post-translational modifications on FP reporters. Considerations for performing photobleaching assays for ER proteins are discussed. Finally, reporters and drugs for studying misfolded secretory protein stress and the unfolded protein response are described.

Cytotoxicity of intracellular aβ42 amyloid oligomers involves Ca2+ release from the endoplasmic reticulum by stimulated production of inositol trisphosphate

Oligomeric forms of β-amyloid (Aβ(42)) peptides associated with Alzheimer's disease (AD) disrupt cellular Ca(2+) regulation by liberating Ca(2+) into the cytosol from both extracellular and intracellular sources. We elucidated the actions of intracellular Aβ by imaging Ca(2+) responses to injections of Aβ oligomers into Xenopus oocytes. Two types of signal were observed: (1) local, "channel-like" transients dependent on extracellular Ca(2+) influx, which resembled signals from amlyoid pores formed by extracellular application of oligomers; and (2) local transients and global Ca(2+) waves, resembling Ca(2+) puffs and waves mediated by inositol trisphosphate (IP(3)). The latter responses were suppressed by antagonists of the IP(3) receptor (caffeine and heparin), pretreatment with the G(i)(o)-protein inhibitor pertussis toxin, and pretreatment with lithium to deplete membrane inositol lipids. We show that G-protein-mediated stimulation of IP(3) production and consequent liberation of Ca(2+) from the endoplasmic reticulum by intracellular Aβ oligomers is cytotoxic, potentially representing a novel pathological mechanism in AD which may be further exacerbated by AD-linked mutations in presenilins to promote opening of IP(3) receptor/channels.

P2Y receptor subtypes evoke different Ca2+ signals in cultured aortic smooth muscle cells

Adenine and uridine nucleotides evoke Ca(2+) signals via four subtypes of P2Y receptor in cultured aortic smooth muscle cells, but the mechanisms underlying the different patterns of these Ca(2+) signals are unresolved. Cytosolic Ca(2+) signals were recorded from single cells and populations of cultured rat aortic smooth muscle cells, loaded with a fluorescent Ca(2+) indicator and stimulated with agonists that allow subtype-selective activation of P2Y1, P2Y2, P2Y4, or P2Y6 receptors. Activation of P2Y1, P2Y2, and P2Y6 receptors caused homologous desensitisation, while activation of P2Y2 receptors also caused heterologous desensitisation of the other subtypes. The Ca(2+) signals evoked by each P2Y receptor subtype required activation of phospholipase C and release of Ca(2+) from intracellular stores via inositol 1,4,5-trisphosphate (IP(3)) receptors, but they were unaffected by inhibition of ryanodine or nicotinic acid adenine dinucleotide phosphate (NAADP) receptors. Sustained Ca(2+) signals were independent of the Na(+)/Ca(2+) exchanger and were probably mediated by store-operated Ca(2+) entry. Analyses of single cells established that most cells express P2Y2 receptors and at least two other P2Y receptor subtypes. We conclude that four P2Y receptor subtypes evoke Ca(2+) signals in cultured aortic smooth muscle cells using the same intracellular (IP(3) receptors) and Ca(2+) entry pathways (store-operated Ca(2+) entry). Different rates of homologous desensitisation and different levels of receptor expression account for the different patterns of Ca(2+) signal evoked by each P2Y receptor subtype.

Physiology of the orexinergic/hypocretinergic system: a revisit in 2012

The neuropeptides orexins and their G protein-coupled receptors, OX(1) and OX(2), were discovered in 1998, and since then, their role has been investigated in many functions mediated by the central nervous system, including sleep and wakefulness, appetite/metabolism, stress response, reward/addiction, and analgesia. Orexins also have peripheral actions of less clear physiological significance still. Cellular responses to the orexin receptor activity are highly diverse. The receptors couple to at least three families of heterotrimeric G proteins and other proteins that ultimately regulate entities such as phospholipases and kinases, which impact on neuronal excitation, synaptic plasticity, and cell death. This article is a 10-year update of my previous review on the physiology of the orexinergic/hypocretinergic system. I seek to provide a comprehensive update of orexin physiology that spans from the molecular players in orexin receptor signaling to the systemic responses yet emphasizing the cellular physiological aspects of this system.

OX1 orexin/hypocretin receptor activation of phospholipase D

BACKGROUND AND PURPOSE

Orexin receptors potently signal to lipid messenger systems, and our previous studies have suggested that PLD would be one of these. We thus wanted to verify this by direct measurements and clarify the molecular mechanism of the coupling.

EXPERIMENTAL APPROACH

Orexin receptor-mediated PLD activation was investigated in CHO cells stably expressing human OX(1) orexin receptors using [(14) C]-oleic acid-prelabelling and the transphosphatidylation assay.

KEY RESULTS

Orexin stimulation strongly increased PLD activity - even more so than the phorbol ester TPA (12-O-tetradecanoyl-phorbol-13-acetate), a highly potent activator of PLD. Both orexin and TPA responses were mediated by PLD1. Orexin-A and -B showed approximately 10-fold difference in potency, and the concentration-response curves were biphasic. Using pharmacological inhibitors and activators, both orexin and TPA were shown to signal to PLD1 via the novel PKC isoform, PKCδ. In contrast, pharmacological or molecular biological inhibitors of Rho family proteins RhoA/B/C, cdc42 and Rac did not inhibit the orexin (or the TPA) response, nor did the molecular biological inhibitors of PKD. In addition, neither cAMP elevation, Gα(i/o) nor Gβγ seemed to play an important role in the orexin response.

CONCLUSIONS AND IMPLICATIONS

Stimulation of OX(1) receptors potently activates PLD (probably PLD1) in CHO cells and this is mediated by PKCδ but not other PKC isoforms, PKDs or Rho family G-proteins. At present, the physiological significance of orexin-induced PLD activation is unknown, but this is not the first time we have identified PKCδ in orexin signalling, and thus some specific signalling cascade may exist between orexin receptors and PKCδ.

OX1 orexin/hypocretin receptor signaling through arachidonic acid and endocannabinoid release

We showed previously that OX(1) orexin receptor stimulation produced a strong (3)H overflow response from [(3)H]arachidonic acid (AA)-labeled cells. Here we addressed this issue with a novel set of tools and methods, to distinguish the enzyme pathways responsible for this response. CHO-K1 cells heterologously expressing human OX(1) receptors were used as a model system. By using selective pharmacological inhibitors, we showed that, in orexin-A-stimulated cells, the AA-derived radioactivity was released as two distinct components, i.e., free AA and the endocannabinoid 2-arachidonoyl glycerol (2-AG). Two orexin-activated enzymatic cascades are responsible for this response: cytosolic phospholipase A(2) (cPLA(2)) and diacylglycerol lipase; the former cascade is responsible for part of the AA release, whereas the latter is responsible for all of the 2-AG release and part of the AA release. Essentially only diacylglycerol released by phospholipase C but not by phospholipase D was implicated as a substrate for 2-AG production, although both phospholipases were strongly activated. The 2-AG released acted as a potent paracrine messenger through cannabinoid CB(1) receptors in an artificial cell-cell communication assay that was developed. The cPLA(2) cascade, in contrast, was involved in the activation of orexin receptor-operated Ca(2+) influx. 2-AG was also released upon OX(1) receptor stimulation in recombinant HEK-293 and neuro-2a cells. The results directly show, for the first time, that orexin receptors are able to generate potent endocannabinoid signals in addition to arachidonic acid signals, which may explain the proposed orexin-cannabinoid interactions (e.g., in neurons).

Calcium-release channels in paramecium. Genomic expansion, differential positioning and partial transcriptional elimination

The release of Ca²⁺ from internal stores is a major source of signal Ca²⁺ in almost all cell types. The internal Ca²⁺ pools are activated via two main families of intracellular Ca²⁺-release channels, the ryanodine and the inositol 1,4,5-trisphosphate (InsP₃) receptors. Among multicellular organisms these channel types are ubiquitous, whereas in most unicellular eukaryotes the identification of orthologs is impaired probably due to evolutionary sequence divergence. However, the ciliated protozoan Paramecium allowed us to prognosticate six groups, with a total of 34 genes, encoding proteins with characteristics typical of InsP₃ and ryanodine receptors by BLAST search of the Paramecium database. We here report that these Ca²⁺-release channels may display all or only some of the characteristics of canonical InsP₃ and ryanodine receptors. In all cases, prediction methods indicate the presence of six trans-membrane regions in the C-terminal domains, thus corresponding to canonical InsP₃ receptors, while a sequence homologous to the InsP₃-binding domain is present only in some types. Only two types have been analyzed in detail previously. We now show, by using antibodies and eventually by green fluorescent protein labeling, that the members of all six groups localize to distinct organelles known to participate in vesicle trafficking and, thus, may provide Ca²⁺ for local membrane-membrane interactions. Whole genome duplication can explain radiation within the six groups. Comparative and evolutionary evaluation suggests derivation from a common ancestor of canonical InsP₃ and ryanodine receptors. With one group we could ascertain, to our knowledge for the first time, aberrant splicing in one thoroughly analyzed Paramecium gene. This yields truncated forms and, thus, may indicate a way to pseudogene formation. No comparable analysis is available for any other, free-living or parasitic/pathogenic protozoan.

The Na(+)/Ca(2+)-exchanger: an essential component in the mechanism governing cardiac steroid-induced slow Ca(2+) oscillations

Plasma membrane (PM) Na(+), K(+)-ATPase, plays crucial roles in numerous physiological processes. Cardiac steroids (CS), such as ouabain and bufalin, specifically bind to the Na(+), K(+)-ATPase and affect ionic homeostasis, signal transduction, and endocytosed membrane traffic. CS-like compounds, synthesized in and released from the adrenal gland, are considered a new family of steroid hormones. Previous studies showed that ouabain induces slow Ca(2+) oscillations in COS-7 cells by enhancing the interactions between Na(+), K(+)-ATPase, inositol 1,4,5-trisphosphate receptor (IP(3)R) and Ankyrin B (Ank-B) to form a Ca(2+) signaling micro-domain. The activation of this micro-domain, however, is independent of InsP3 generation. Thus, the mechanism underlying the induction of these slow Ca(2+) oscillations remained largely unclear. We now show that other CS, such as bufalin, can also induce Ca(2+) oscillations. These oscillations depend on extracellular Ca(2+) concentrations [Ca(2+)](out) and are inhibited by Ni(2+). Furthermore, we found that these slow oscillations are Na(+)(out) dependent, abolished by Na(+)/Ca(2+) exchanger1 (NCX1)-specific inhibitors and markedly attenuated by NCX1 siRNA knockdown. Based on these results, a model is presented for the CS-induced slow Ca(2+) oscillations in COS-7 cells.

Ryanodine receptor-2 upregulation and nicotine-mediated plasticity

Nicotine, the major psychoactive component of cigarette smoke, modulates neuronal activity to produce Ca2+-dependent changes in gene transcription. However, the downstream targets that underlie the long-term effects of nicotine on neuronal function, and hence behaviour, remain to be elucidated. Here, we demonstrate that nicotine administration to mice upregulates levels of the type 2 ryanodine receptor (RyR2), a Ca2+-release channel present on the endoplasmic reticulum, in a number of brain areas associated with cognition and addiction, notably the cortex and ventral midbrain. Nicotine-mediated RyR2 upregulation was driven by CREB, and caused a long-lasting reinforcement of Ca2+ signalling via the process of Ca2+-induced Ca2+ release. RyR2 upregulation was itself required for long-term phosphorylation of CREB in a positive-feedback signalling loop. We further demonstrate that inhibition of RyR-activation in vivo abolishes sensitization to nicotine-induced habituated locomotion, a well-characterised model for onset of drug dependence. Our findings, therefore, indicate that gene-dependent reprogramming of Ca2+ signalling is involved in nicotine-induced behavioural changes.

P2Y1 and P2Y13 purinergic receptors mediate Ca2+ signaling and proliferative responses in pulmonary artery vasa vasorum endothelial cells

Extracellular ATP and ADP have been shown to exhibit potent angiogenic effects on pulmonary artery adventitial vasa vasorum endothelial cells (VVEC). However, the molecular signaling mechanisms of extracellular nucleotide-mediated angiogenesis remain not fully elucidated. Since elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) is required for cell proliferation and occurs in response to extracellular nucleotides, this study was undertaken to delineate the purinergic receptor subtypes involved in Ca(2+) signaling and extracellular nucleotide-mediated mitogenic responses in VVEC. Our data indicate that stimulation of VVEC with extracellular ATP resulted in the elevation of [Ca(2+)](i) via Ca(2+) influx through plasma membrane channels as well as Ca(2+) mobilization from intracellular stores. Moreover, extracellular ATP induced simultaneous Ca(2+) responses in both cytosolic and nuclear compartments. An increase in [Ca(2+)](i) was observed in response to a wide range of purinergic receptor agonists, including ATP, ADP, ATPγS, ADPβS, UTP, UDP, 2-methylthio-ATP (MeSATP), 2-methylthio-ADP (MeSADP), and BzATP, but not adenosine, AMP, diadenosine tetraphosphate, αβMeATP, and βγMeATP. Using RT-PCR, we identified mRNA for the P2Y1, P2Y2, P2Y4, P2Y13, P2Y14, P2X2, P2X5, P2X7, A1, A2b, and A3 purinergic receptors in VVEC. Preincubation of VVEC with the P2Y1 selective antagonist MRS2179 and the P2Y13 selective antagonist MRS2211, as well as with pertussis toxin, attenuated at varying degrees agonist-induced intracellular Ca(2+) responses and activation of ERK1/2, Akt, and S6 ribosomal protein, indicating that P2Y1 and P2Y13 receptors play a major role in VVEC growth responses. Considering the broad physiological implications of purinergic signaling in the regulation of angiogenesis and vascular homeostasis, our findings suggest that P2Y1 and P2Y13 receptors may represent novel and specific targets for treatment of pathological vascular remodeling involving vasa vasorum expansion.

Thapsigargin induces expression of activating transcription factor 3 in human keratinocytes involving Ca2+ ions and c-Jun N-terminal protein kinase

Thapsigargin is a specific inhibitor of the sarco/endoplasmic reticulum Ca(2+) ATPase of the endoplasmic reticulum. Here, we show that stimulation of human HaCaT keratinocytes with nanomolar concentrations of thapsigargin triggers expression of activating transcription factor (ATF) 3, a basic-region leucin zipper transcription factor. ATF3 expression was also up-regulated in thapsigargin-stimulated glioma cells, hepatoma cells, retinal pigment epithelial cells, and airway epithelial cells. Thapsigargin-induced up-regulation of ATF3 expression in keratinocytes was attenuated by BAPTA-acetoxymethyl ester or by expression of the Ca(2+)-binding protein parvalbumin in the cytosol of HaCaT cells but not by a panel of pharmacological agents that chelate extracellular Ca(2+) (EGTA) or inhibit either ryanodine receptors (dantrolene) or voltage-gated Ca(2+) channels (nifedipine). Hence, elevated levels of intracellular Ca(2+), released from intracellular stores, are essential for the effect of thapsigargin on the biosynthesis of ATF3. The thapsigargin-induced signaling pathway was blocked by expression of either mitogen-activated protein kinase phosphatase-1 or -5. Experiments involving pharmacological and genetic tools revealed the importance of c-Jun N-terminal protein kinase (JNK) within the signaling cascade, whereas inhibition of extracellular signal-regulated protein kinase or p38 protein kinase did not attenuate thapsigargin-induced expression of ATF3. Functional studies showed that treatment of HaCaT keratinocytes with thapsigargin led to a 2-fold induction of caspase-3/7 activity. The up-regulation of caspase-3/7 activity in thapsigargin-stimulated HaCaT cells was attenuated by inhibition of JNK. Together, these data show that stimulation of HaCaT cells with thapsigargin induces a specific signaling pathway in keratinocytes involving activation of JNK, biosynthesis of ATF3, and up-regulation of caspase-3/7 activity.

Dantrolene mitigates caerulein-induced pancreatitis in vivo in mice

Acute pancreatitis is a painful, inflammatory disorder for which adequate treatments are lacking. An early, critical step in its development is the aberrant signaling of Ca(2+) within the pancreatic acinar cell. This Ca(2+) release is modulated by the intracellular Ca(2+) channel the ryanodine receptor (RYR). We have previously shown that RYR inhibition reduces pathological intra-acinar protease activation, an early marker of pancreatitis. In this study, we examined whether pretreatment with the RYR inhibitor dantrolene attenuates the severity of caerulein-induced pancreatitis in mice. Immunofluorescent labeling for RYR from mouse pancreatic sections showed localization to the basolateral region of the acinar cell. After 1 h of caerulein hyperstimulation in vivo, dantrolene 1) reduced pancreatic trypsin activity by 59% (P < 0.05) and 2) mitigated early ultrastructural derangements within the acinar cell. Eight hours after pancreatitis induction, dantrolene reduced pancreatic trypsin activity and serum amylase by 61 and 32%, respectively (P < 0.05). At this later time point, overall histological severity of pancreatitis was reduced by 63% with dantrolene pretreatment (P < 0.05). TUNEL-positive cells were reduced by 58% (P < 0.05). These data suggest that the RYR plays an important role in mediating early acinar cell events during in vivo pancreatitis and contributes to disease severity. Blockade of Ca(2+) signals and particularly RYR-Ca(2+) may be useful as prophylactic treatment for this disease in high-risk settings for pancreatitis.

Virus-induced Ca2+ influx extends survival of west nile virus-infected cells

West Nile virus (WNV) infection leads to rapid and sustained Ca(2+) influx. This influx was observed with different strains of WNV and in different types of cells. Entry during virion endocytosis as well as through calcium channels contributed to the Ca(2+) influx observed in WNV-infected cells. Ca(2+) influx was not detected after infection with vesicular stomatitis virus (VSV) and occurred only through endocytosis in Sindbis virus-infected cells. Caspase 3 cleavage and activation of several kinases, including focal adhesion kinase (FAK), mitogen-activated extracellular signal-regulated protein kinase (ERK1/2), and protein-serine kinase B alpha (Akt), at early times after WNV infection were shown to be dependent on Ca(2+) influx. Although the activation of these kinases was sustained in virus-infected cells throughout infection, UV-inactivated WNV induced only a transient activation of FAK and ERK1/2 at early times after infection. The Ca(2+)-dependent FAK activation observed in WNV-infected cells was not mediated by alphavbeta3 integrins. Reduction of Ca(2+) influx at early times of infection by various treatments decreased the viral yield and delayed both the early transient caspase 3 cleavage and the activation of FAK, Akt, and ERK signaling. The results indicate that Ca(2+) influx is required for early infection events needed for efficient viral replication, possibly for virus-induced rearrangement of the endoplasmic reticulum (ER) membrane. Increased caspase 3 cleavage at both early (transient) and late times of infection correlated with decreased activation of the FAK and ERK1/2 pathways, indicating a role for these kinases in extending the survival of flavivirus-infected cells.

Multiple Ca2+ signaling pathways regulate intracellular Ca2+ activity in human cardiac fibroblasts

Ca(2+) signaling pathways are well studied in cardiac myocytes, but not in cardiac fibroblasts. The aim of the present study is to characterize Ca(2+) signaling pathways in cultured human cardiac fibroblasts using confocal scanning microscope and RT-PCR techniques. It was found that spontaneous intracellular Ca(2+) (Ca(i) (2+)) oscillations were present in about 29% of human cardiac fibroblasts, and the number of cells with Ca(i) (2+) oscillations was increased to 57.3% by application of 3% fetal bovine serum. Ca(i) (2+) oscillations were dependent on Ca(2+) entry. Ca(i) (2+) oscillations were abolished by the store-operated Ca(2+) (SOC) entry channel blocker La(3+), the phospholipase C inhibitor U-73122, and the inositol trisphosphate receptors (IP3Rs) inhibitor 2-aminoethoxydiphenyl borate, but not by ryanodine. The IP3R agonist thimerosal enhanced Ca(i) (2+) oscillations. Inhibition of plasma membrane Ca(2+) pump (PMCA) and Na(+)-Ca(2+) exchanger (NCX) also suppressed Ca(i) (2+) oscillations. In addition, the frequency of Ca(i) (2+) oscillations was reduced by nifedipine, and increased by Bay K8644 in cells with spontaneous Ca(2+) oscillations. RT-PCR revealed that mRNAs for IP3R1-3, SERCA1-3, Ca(V)1.2, NCX3, PMCA1,3,4, TRPC1,3,4,6, STIM1, and Orai1-3, were readily detectable, but not RyRs. Our results demonstrate for the first time that spontaneous Ca(i) (2+) oscillations are present in cultured human cardiac fibroblasts and are regulated by multiple Ca(2+) pathways, which are not identical to those of the well-studied contractile cardiomyocytes. This study provides a base for future investigations into how Ca(2+) signals regulate biological activity in human cardiac fibroblasts and cardiac remodeling under pathological conditions.

Ca2+ waves initiate antigen-stimulated Ca2+ responses in mast cells

Ca(2+) mobilization is central to many cellular processes, including stimulated exocytosis and cytokine production in mast cells. Using single cell stimulation by IgE-specific Ag and high-speed imaging of conventional or genetically encoded Ca(2+) sensors in rat basophilic leukemia and bone marrow-derived rat mast cells, we observe Ca(2+) waves that originate most frequently from the tips of extended cell protrusions, as well as Ca(2+) oscillations throughout the cell that usually follow the initiating Ca(2+) wave. In contrast, Ag conjugated to the tip of a micropipette stimulates local, repetitive Ca(2+) puffs at the region of cell contact. Initiating Ca(2+) waves are observed in most rat basophilic leukemia cells stimulated with soluble Ag and are sensitive to inhibitors of Ca(2+) release from endoplasmic reticulum stores and to extracellular Ca(2+), but they do not depend on store-operated Ca(2+) entry. Knockdown of transient receptor potential channel (TRPC)1 and TRPC3 channel proteins by short hairpin RNA reduces the sensitivity of these cells to Ag and shifts the wave initiation site from protrusions to the cell body. Our results reveal spatially encoded Ca(2+) signaling in response to immunoreceptor activation that utilizes TRPC channels to specify the initiation site of the Ca(2+) response.

Ca2+ concentrations are key determinants of ischemia-reperfusion-induced apoptosis: significance for the molecular mechanism of Bcl-2 action

The mechanism of action of the anti-apoptotic oncogene Bcl-2 and Ca2+ regulation in ischemia-reperfusion injury is still obscure. In this present study, we investigated mitochondrial Ca2+ overloads and mechanism of action of Bcl-2. Eighteen Wistar rats were divided into sham-operated control group (I) (n = 6), ischemia and reperfusion group (II) (n = 6), and amlodipine-treated group (1 mg kg(-1) body weight/daily by oral route for 7 days before inducing ischemia-reperfusion maneuver) (III) (n = 6). Rats were subjected to 1 h of hepatic ischemia followed by 3-h reperfusion. Mitochondrial Ca2+ content was determined and damage was confirmed by transmission electron microscopy. Decrease of mitochondrial Ca2+ level is related to reduction of apoptosis and cellular changes, viz. increased Bcl-2 expression followed by reduction in secondary endoplasmic reticulum, whereas ischemia/reperfusion group shows overloading Ca2+ ions and decrease in Bcl-2 expression as compared to sham-operated rats. Thus, Bcl-2-dependent reduction of Ca2+ is an important component of the anti-apoptotic program in ischemia-reperfusion injury.

The contribution of inositol 1,4,5-trisphosphate and ryanodine receptors to agonist-induced Ca(2+) signaling of airway smooth muscle cells

The relative contribution of inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) and ryanodine receptors (RyRs) to agonist-induced Ca(2+) signaling in mouse airway smooth muscle cells (SMCs) was investigated in lung slices with phase-contrast or laser scanning microscopy. At room temperature (RT), methacholine (MCh) or 5-hydroxytryptamine (5-HT) induced Ca(2+) oscillations and an associated contraction in small airway SMCs. The subsequent exposure to an IP(3)R antagonist, 2-aminoethoxydiphenyl borate (2-APB), inhibited the Ca(2+) oscillations and induced airway relaxation in a concentration-dependent manner. 2-APB also inhibited Ca(2+) waves generated by the photolytic release of IP(3). However, the RyR antagonist ryanodine had no significant effect, at any concentration, on airway contraction or agonist- or IP(3)-induced Ca(2+) oscillations or Ca(2+) wave propagation. By contrast, a second RyR antagonist, tetracaine, relaxed agonist-contracted airways and inhibited agonist-induced Ca(2+) oscillations in a concentration-dependent manner. However, tetracaine did not affect IP(3)-induced Ca(2+) release or wave propagation nor the Ca(2+) content of SMC Ca(2+) stores as evaluated by Ca(2+)-release induced by caffeine. Conversely, both ryanodine and tetracaine completely blocked agonist-independent slow Ca(2+) oscillations induced by KCl. The inhibitory effects of 2-APB and absence of an effect of ryanodine on MCh-induced airway contraction or Ca(2+) oscillations of SMCs were also observed at 37 degrees C. In Ca(2+)-permeable SMCs, tetracaine inhibited agonist-induced contraction without affecting intracellular Ca(2+) levels indicating that relaxation also resulted from a reduction in Ca(2+) sensitivity. These results indicate that agonist-induced Ca(2+) oscillations in mouse small airway SMCs are primary mediated via IP(3)Rs and that tetracaine induces relaxation by both decreasing Ca(2+) sensitivity and inhibiting agonist-induced Ca(2+) oscillations via an IP(3)-dependent mechanism.

Bidirectional Hebbian plasticity at hippocampal mossy fiber synapses on CA3 interneurons

Hippocampal area CA3 is critically involved in the formation of nonoverlapping neuronal subpopulations ("pattern separation") to store memory representations as distinct events. Efficient pattern separation relies on the strong and sparse excitatory input from the mossy fibers (MFs) to pyramidal cells and feedforward inhibitory interneurons. However, MF synapses on CA3 pyramidal cells undergo long-term potentiation (LTP), which, if unopposed, will degrade pattern separation because MF activation will now recruit additional CA3 pyramidal cells. Here, we demonstrate MF LTP in stratum lacunosum-moleculare (L-M) interneurons induced by the same stimulation protocol that induces MF LTP in pyramidal cells. This LTP was NMDA receptor (NMDAR) independent and occurred at MF Ca(2+)-impermeable AMPA receptor synapses. LTP was prevented by with voltage clamping the postsynaptic cell soma during high-frequency stimulation (HFS), intracellular injections of the Ca(2+) chelator BAPTA (20 mm), or bath applications of the L-type Ca(2+) channel blocker nimodipine (10 microm). We propose that MF LTP in L-M interneurons preserves the sparsity of pyramidal cell activation, thus allowing CA3 to maintain its role in pattern separation. In the presence of the mGluR1alpha antagonist LY367385 [(S)-(+)-a-amino-4-carboxy-2-methylbenzeneacetic acid] (100 microm), the same HFS that induces MF LTP in naive slices triggered NMDAR-independent MF LTD. This LTD, like LTP, required activation of the L-type Ca(2+) channel and also was induced after blockade of IP(3) receptors with heparin (4 mg/ml) or the selective depletion of receptor-gated Ca(2+) stores with ryanodine (10 or 100 microm). We conclude that L-M interneurons are endowed with Ca(2+) signaling cascades suitable for controlling the polarity of MF long-term plasticity induced by joint presynaptic and postsynaptic activities.

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

Inositol 1,4,5-trisphosphate (IP(3)) is a ubiquitous intracellular messenger regulating diverse functions in almost all mammalian cell types. It is generated by membrane receptors that couple to phospholipase C (PLC), an enzyme which liberates IP(3) from phosphatidylinositol 4,5-bisphosphate (PIP(2)). The major action of IP(3), which is hydrophilic and thus translocates from the membrane into the cytoplasm, is to induce Ca(2+) release from endogenous stores through IP(3) receptors (IP(3)Rs). Cardiac excitation-contraction coupling relies largely on ryanodine receptor (RyR)-induced Ca(2+) release from the sarcoplasmic reticulum. Myocytes express a significantly larger number of RyRs compared to IP(3)Rs (~100:1), and furthermore they experience substantial fluxes of Ca(2+) with each heartbeat. Therefore, the role of IP(3) and IP(3)-mediated Ca(2+) signaling in cardiac myocytes has long been enigmatic. Recent evidence, however, indicates that despite their paucity cardiac IP(3)Rs may play crucial roles in regulating diverse cardiac functions. Strategic localization of IP(3)Rs in cytoplasmic compartments and the nucleus enables them to participate in subsarcolemmal, bulk cytoplasmic and nuclear Ca(2+) signaling in embryonic stem cell-derived and neonatal cardiomyocytes, and in adult cardiac myocytes from the atria and ventricles. Intriguingly, expression of both IP(3)Rs and membrane receptors that couple to PLC/IP(3) signaling is altered in cardiac disease such as atrial fibrillation or heart failure, suggesting the involvement of IP(3) signaling in the pathology of these diseases. Thus, IP(3) exerts important physiological and pathological functions in the heart, ranging from the regulation of pacemaking, excitation-contraction and excitation-transcription coupling to the initiation and/or progression of arrhythmias, hypertrophy and heart failure.

Calcium release via activation of presynaptic IP3 receptors contributes to kainate-induced IPSC facilitation in rat neocortex

We examined the mechanisms of kainate (KA) induced modulation of GABA release in rat prefrontal cortex. Pharmacologically isolated IPSCs were recorded from visually identified layer II/III pyramidal cells using whole-cell patch clamp techniques. KA produced an increase in evoked IPSC amplitude at low nanomolar concentrations (100-500 nM). The frequency but not the amplitude of miniature (m) IPSCs was also increased. The GluR5 subunit selective agonist (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA) caused an increase in mIPSC frequency whereas (3S,4aR,6S,8aR)-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid (LY382884), a selective GluR5 subunit antagonist, inhibited this facilitation. Philanthotoxin-433 (PhTx) blocked the effect of KA, indicating involvement of Ca(2+)-permeable GluR5 receptors. No IPSC facilitation was seen when Ca(2+) was omitted from the bathing solution. Facilitation was observed when slices were preincubated in ruthenium red or high concentrations of ryanodine, but was inhibited with application of thapsigargin. The IP3 receptor (IP3R) antagonists diphenylboric acid 2-amino-ethyl ester (2-APB) (15 microM) and Xestospongin C (XeC) blocked IPSC facilitation. These results show that activation of KA receptors (KARs) on GABAergic nerve terminals results is linked to intracellular Ca(2+) release via activation of IP3, but not ryanodine, receptors. This represents a new mechanism of presynaptic modulation whereby Ca(2+) entry through Ca(2+)-permeable GluR5 subunit containing KARs activates IP3Rs receptors leading to an increase in GABA release.

Alpha1-adrenoceptors trigger the snake venom production cycle in secretory cells by activating phosphatidylinositol 4,5-bisphosphate hydrolysis and ERK signaling pathway

Loss of venom from the venom gland after biting or manual extraction leads to morphological changes in venom secreting cells and the start of a cycle of production of new venom. We have previously shown that stimulation of both alpha- and beta-adrenoceptors in the secretory cells of the venom gland is essential for the onset of the venom production cycle in Bothrops jararaca. We investigated the signaling pathway by which the alpha-adrenoceptor initiates the venom production cycle. Our results show that the alpha(1)-adrenoceptor subtype is present in venom gland of the snake. In quiescent cells, stimulation of alpha(1)-adrenoceptor with phenylephrine increased the total inositol phosphate concentration, and this effect was blocked by the phospholipase C inhibitor U73122. Phenylephrine mobilized Ca(2+) from thapsigargin-sensitive stores and increased protein kinase C activity. In addition, alpha(1)-adrenoceptor stimulation increased the activity of ERK 1/2, partially via protein kinase C. Using RT-PCR approach we obtained a partial sequence of a snake alpha(1)-adrenoceptor (260 bp) with higher identity with alpha(1D) and alpha(1B)-adrenoceptors from different species. These results suggest that alpha(1)-adrenoceptors in the venom secreting cells are probably coupled to a G(q) protein and trigger the venom production cycle by activating the phosphatidylinositol 4,5-bisphosphate and ERK signaling pathway.

The metabolism of serotonin in neuronal cells in culture and platelets

The aim of this study is to find a relationship between serotonin (5-HT) and its metabolite 5-hydroxy indol acetic acid (5-HIAA) in hippocampus, frontal neocortex and platelets. Serotonin and 5-HIAA were measured in cultured neurons and compared with those produced by human platelets. The cortical neuronal 5-HIAA/serotonin ratio was 4.7 and for hippocampal neurons it was 3.2. In human platelets, this ratio was 1.35 suggesting that the highest serotonin metabolism occurs in the frontal neocortex followed by the hippocampus and platelets. In the presence of 0.3 microM of p-chlorophenylalanine both cultured neurons and platelets exhibited an approximately 50% decrease in serotonin and 5-HIAA concentration suggesting similarities in the metabolic profile in both preparations. In addition, we found that serotonin by itself does not play any role in platelet aggregation but potentiates this phenomenon in the presence of calcium ionophore A23187. This synergistic interaction between serotonin (2-5 microM) and A23187 (0.5-2 microM) was inhibited by serotonin receptor blockers [methysergide (IC50 = 18 microM) and cyproheptadine (IC50, 20 microM)] and calcium channel blockers (verapamil and diltiazem, IC50 = 20 and 40 microM, respectively) that indicate both mechanisms are receptor mediated. Similarly, U73122, an inhibitor of phospholipase C (PLC), blocked the synergistic effect of serotonin and ionophore at an IC50 value of 9.2 microM. Wortmannin, a phosphoinositide 3-kinase (PI 3-K) inhibitor, also blocked the response (IC50 = 2.6 microM) by inhibiting respiratory burst. However, neither genistein, a tyrosine-specific protein kinase inhibitor, nor chelerythrine, a protein kinase C (PKC) inhibitor, affected aggregation. Our results are strongly suggestive of a synergistic interaction between serotonin type-2 and Ca-ionophore via a PLC/Ca signalling pathway.

Ca2+-induced Cl- efflux at rat distal colonic epithelium

With the aid of the halide-sensitive dye 6-methoxy-N-ethylquinolinium iodide (MEQ), changes in intracellular Cl(-) concentration were measured to characterize the role of Ca(2+)-dependent Cl(-) channels at the rat distal colon. In order to avoid indirect effects of secretagogues mediated by changes in the driving force for Cl(-) exit (i.e., mediated by opening of Ca(2+)-dependent K(+) channels), all experiments were performed under depolarized conditions, i.e., in the presence of high extracellular K(+) concentrations. The Ca(2+)-dependent secretagogue carbachol induced a stilbene-sensitive Cl(-) efflux, which was mimicked by the Ca(2+) ionophore ionomycin. Surprisingly, the activation of Ca(2+)-dependent Cl(-) efflux was resistant against blockers of classical Ca(2+) signaling pathways such as phospholipase C, protein kinase C and calmodulin. Hence, alternative pathways must be involved in the signaling cascade. One possible signaling molecule seems to be nitric oxide (NO) as the NO donor sodium nitroprusside could induce Cl(- )efflux. Vice versa, the NO synthase inhibitor N-omega-monomethyl-arginine (L: -NMMA) reduced the carbachol-induced Cl(- )efflux. This indicates that NO may be involved in part of the signaling cascade. In order to test the ability of the epithelium to produce NO, the expression of different isoforms of NO synthase was verified by immunohistochemistry. In addition, the cytoskeleton seems to play a role in the activation of Ca(2+)-dependent Cl(-) channels. Inhibitors of microtubule association such as nocodazole and colchicine as well as jasplakinolide, a drug that enhances actin polymerization, inhibited the carbachol-induced Cl(-) efflux. Consequently, the activation of apical Cl(-) channels by muscarinic receptor stimulation differs in signal transduction from the classical phospholipase C/protein kinase C way.

Signaling mechanism of thrombin-induced gingival fibroblast-populated collagen gel contraction

1.--Thrombin is activated during gingival tissue injury and inflammation. Thrombin (platelet)-rich plasma has been used for periodontal regeneration with success. Thrombin and other bacterial proteases also affect the functions of adjacent periodontal cells via stimulation of protease-activated receptors (PARs). 2.--We noted that thrombin (0.1-2 U ml(-1)), human, and frog PAR-1 agonist peptide (20-240 microM) induced the gingival fibroblast (GF)-populated collagen gel contraction within 2 h of exposure. However, PAR-2, PAR-3, and PAR-4 agonist peptide (20-240 microM) showed little effect on collagen gel contraction. U73122 (phospholipase C inhibitor) and 2-APB (IP3 antagonist) were effective in inhibition of GF contraction. 3.--Thrombin-induced GF contraction was inhibited by 5 mM EGTA (an extracellular calcium chelator) and verapamil (an L-type calcium channel blocker). In addition, W7 (10 and 25 microM, a calcium/calmodulin (CaM) inhibitor), ML-7 (50 microM, myosin light chain kinase (MLCK) inhibitor), and HA1077 (100 microM, Rho kinase inhibitor) completely inhibited the thrombin-induced collagen gel contraction. Thrombin also induced the phosphorylation of ERK1/ERK2 and elevated the Rho-GTP levels in GF. 4.--However, U0126 only partially inhibited the thrombin-induced GF contraction. Similarly, wortmannin (100 nM), LY294002 (20 microM) (two PI3K inhibitor) and genistein also showed partial inhibition. Moreover, NAC was not able to suppress the GF contraction, as supported by the slight decrease in reactive oxygen species production in GF by thrombin. 5.--Thrombin also stimulated metalloproteinase-2 (MMP-2) and MMP-3 production in GF. But addition of GM6001 or 1,10-phenanthroline, two MMP inhibitors, could not inhibit the thrombin-induced GF contraction. 6.--These results indicate that thrombin is crucial in the periodontal inflammation and wound healing by promoting GF contraction. This event is mainly mediated via PAR-1 activation, PLC activation, extracellular calcium influx via L-type calcium channel, and the calcium/CaM-MLCK and Rho kinase activation pathway.

Effect of the stable thromboxane derivative, carbocyclic thromboxane A2, on membrane potential of rat myenteric neurones in culture

The effects of carbocyclic thromboxane A(2) (cTXA(2); 10(-6) mol L(-1)) on membrane potential and cytosolic Ca(2+) concentration were measured with the whole-cell patch-clamp or the fura-2 method, respectively, at rat myenteric ganglia. cTXA(2) caused a hyperpolarization of myenteric neurones from -19.3 +/- 2.5 to -29.3 +/- 2.3 mV. In addition, the eicosanoid potentiated the carbachol-induced depolarization from 4.2 +/- 1.0 mV under control conditions to 11.1 +/- 1.1 mV in the presence of the cTXA(2) (n = 9). The hyperpolarization was abolished by internal application of CsCl (140 mmol L(-1)), a non-selective blocker of K(+) channels, or EGTA (11 mmol L(-1)in the pipette solution), a chelator of intracellular Ca(2+). A similar inhibition was observed in the presence of charybdotoxin (10(-7) mol L(-1)). Fura-2 imaging experiments revealed a cTXA(2)-evoked increase in the intracellular Ca(2+) concentration as indicated by a rise in the fura-2 ratio signal. This response was mediated by a release of Ca(2+) from intracellular stores as sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase blockade with cyclopiazonic acid (5 x 10(-5) mol L(-1)) completely abolished the response to cTXA(2). A similar inhibition was observed after blockade of phospholipase C with U-73122 (10(-5) mol L(-1)). These results suggest an activation of Ca(2+)-activated K(+) channels by cTXA(2) after stimulation of phospholipase C.

Ca2+ release-dependent hyperpolarizations modulate the firing pattern of juvenile GABA neurons in mouse substantia nigra pars reticulata in vitro

A phasic activation of small-conductance Ca(2+)-dependent K(+) channels (SK channels) underlies spike-afterhyperpolarizations and spike-independent, transient hyperpolarizations in juvenile substantia nigra neurons. Outward current pulses that cause the spike-independent hyperpolarizations result from ryanodine receptor-mediated Ca(2+) release from intracellular stores. To study the modulation of excitability by the outward current pulses, we recorded from GABAergic pars reticulata neurons of mice at postnatal days 12-16. We induced a prolongation of SK channel open states by 1-ethyl-2-benzimidazolinone (1-EBIO). In addition to a prolongation of spike-afterhyperpolarizations, 1-EBIO (200 microm) potentiated outward current pulses by increasing their duration. Neurons were manipulated by current injection to display continuous or discontinuous discharge. Despite the prolongation of the outward current pulses by 1-EBIO, continuous action potential discharge became more regular, although its frequency declined. Durations of silent periods (periods of >2x average interspike interval) increased. Caffeine (1 mm) further increased the duration of such silent periods. Caffeine, however, had no effect at short interspike intervals (<600 ms). Cyclopiazonic acid (10 microm) silenced discharge in 1-EBIO, but discharge reappeared with the depletion of Ca(2+) stores. We conclude that the modulation of excitability by an activation of SK channels through ryanodine receptor-mediated release of Ca(2+) critically depends on the frequency of discharge. Outward current pulses occur only if interspike intervals exceed the duration of spike-afterhyperpolarizations. In this instance, the phasic, spike-independent activation of SK channels supports pauses to interrupt autonomous discharge in juvenile GABAergic pars reticulata neurons.

Bitter stimuli induce Ca2+ signaling and CCK release in enteroendocrine STC-1 cells: role of L-type voltage-sensitive Ca2+ channels

We previously demonstrated the expression of bitter taste receptors of the type 2 family (T2R) and the alpha-subunits of the G protein gustducin (Galpha(gust)) in the rodent gastrointestinal (GI) tract and in GI endocrine cells. In this study, we characterized mechanisms of Ca(2+) fluxes induced by two distinct T2R ligands: denatonium benzoate (DB) and phenylthiocarbamide (PTC), in mouse enteroendocrine cell line STC-1. Both DB and PTC induced a marked increase in intracellular [Ca(2+)] ([Ca(2+)](i)) in a dose- and time-dependent manner. Chelating extracellular Ca(2+) with EGTA blocked the increase in [Ca(2+)](i) induced by either DB or PTC but, in contrast, did not prevent the effect induced by bombesin. Thapsigargin blocked the transient increase in [Ca(2+)](i) induced by bombesin, but did not attenuate the [Ca(2+)](i) increase elicited by DB or PTC. These results indicate that Ca(2+) influx mediates the increase in [Ca(2+)](i) induced by DB and PTC in STC-1 cells. Preincubation with the L-type voltage-sensitive Ca(2+) channel (L-type VSCC) blockers nitrendipine or diltiazem for 30 min inhibited the increase in [Ca(2+)](i) elicited by DB or PTC. Furthermore, exposure to the L-type VSCCs opener BAY K 8644 potentiated the increase in [Ca(2+)](i) induced by DB and PTC. Stimulation with DB also induced a marked increase in the release of cholecystokinin from STC-1 cells, an effect also abrogated by prior exposure to EGTA or L-type VSCC blockers. Collectively, our results demonstrate that bitter tastants increase [Ca(2+)](i) and cholecystokinin release through Ca(2+) influx mediated by the opening of L-type VSCCs in enteroendocrine STC-1 cells.

Characterisation of serum-induced intracellular Ca2+ oscillations in primary bone marrow stromal cells

Intracellular Ca2+ signalling is pivotal to cell function and [Ca2+]i oscillations permit precise and prolonged modulation of an array of Ca2+-sensitive processes without the need for extended, global elevations in [Ca2+]i. We have studied [Ca2+]i signalling in primary rat marrow stromal cells exposed to foetal calf serum (FCS) constituents at concentrations up to those required to promote growth and differentiation in culture. Spontaneous [Ca2+]i signalling was not observed, but exposure to 1% FCS induced regular, sustained Ca2+ oscillations in 41 +/- 3% of cells. Incidence of FCS-induced oscillations was dose-dependent, saturating at 0.5%. These oscillations were arrested by disruption of Ca2+ stores with 100 nM-1 microM thapsigargin or discharge of mitochondrial membrane potential and were sensitive to blockade of IP3-receptors by 50 microM 2-amino-ethoxydiphenyl borate (2-APB) and inhibition of phospholipase C with 5 microM U73122. The oscillations decreased in frequency and amplitude following inhibition of Ca2+ influx with EGTA or La3+ but were poorly sensitive to nifedipine (1-10 microM) and Bay K 8644 (300 nM). The factor(s) responsible for inducing [Ca2+]i oscillations are heat stable, insensitive to disulphide bond reduction with 20 mM dithioerythritol and retained by a 30 kDa molecular weight filter. Serum is routinely present in culture medium at 10%-15% [v/v] and marrow stromal cells maintained under culture conditions exhibited sustained oscillations. This is the first demonstration of agonist-induced complex Ca2+ signals in marrow stromal cells. We conclude that Ca2+ oscillations occur constantly in these cells in culture and are potentially important regulators of cell proliferation and differentiation.

Multiple receptors coupled to phospholipase C gate long-term depression in visual cortex

Long-term depression (LTD) in sensory cortices depends on the activation of NMDA receptors. Here, we report that in visual cortical slices, the induction of LTD (but not long-term potentiation) also requires the activation of receptors coupled to the phospholipase C (PLC) pathway. Using immunolesions in combination with agonists and antagonists, we selectively manipulated the activation of alpha1 adrenergic, M1 muscarinic, and mGluR5 glutamatergic receptors. Inactivation of these PLC-coupled receptors prevents the induction of LTD, but only when the three receptors were inactivated together. LTD is fully restored by activating any one of them or by supplying intracellular D-myo-inositol-1,4,5-triphosphate (IP3). LTD was also impaired by intracellular application of PLC or IP3 receptor blockers, and it was absent in mice lacking PLCbeta1, the predominant PLC isoform in the forebrain. We propose that visual cortical LTD requires a minimum of PLC activity that can be supplied independently by at least three neurotransmitter systems. This essential requirement places PLC-linked receptors in a unique position to control the induction of LTD and provides a mechanism for gating visual cortical plasticity via extra-retinal inputs in the intact organism.

Ryanodine receptor/Ca(2+) release mechanisms in rhythmically active respiratory neurons of cats in vivo

The cytosolic Ca(2+) released from internal stores is important for distinctive cell functions. To assess the role of ryanodine/Ca(2+) releasing mechanisms in the rhythmic activity of respiratory neurons, effects of intracellular injection of ryanodine on the membrane potential trajectory of postinspiratory and augmenting inspiratory neurons were investigated in unanesthetized, decerebrate, paralyzed and artificially ventilated cats. Ryanodine injection hyperpolarized the membrane and decreased input resistance throughout the respiratory cycle in both types of respiratory neurons. Specifically, membrane repolarization during postinspiration was accelerated in postinspiratory neurons, and the large hyperpolarization at the onset of postinspiration was increased in augmenting inspiratory neurons. Spike-afterhyperpolarization consisting of a fast, early component and slow, late component increased in size after ryanodine, resulting in prolongation of inter-spike intervals and decrease of burst discharge. Intracellular injection of caffeine produced similar effects on these respiratory neurons, and Ruthenium Red, an antagonist of ryanodine receptors, had opposite effects. Immunoreactivity for ryanodine receptors was detected in all respiratory neurons labeled intracellularly with neurobiotin. These results demonstrate that ryanodine-sensitive Ca(2+) stores modulate the periodic membrane potential fluctuations and spike activity in respiratory neurons.

Synaptic facilitation and behavioral dishabituation in Aplysia: dependence on release of Ca2+ from postsynaptic intracellular stores, postsynaptic exocytosis, and modulation of postsynaptic AMPA receptor efficacy

Sensitization and dishabituation of the defensive withdrawal reflex in Aplysia have been ascribed to presynaptic mechanisms, particularly presynaptic facilitation of transmission at sensorimotor synapses in the CNS of Aplysia. Here, we show that facilitation of sensorimotor synapses in cell culture during and after serotonin (5-HT) exposure depends on a rise in postsynaptic intracellular Ca(2+) and release of Ca(2+) from postsynaptic stores. We also provide support for the idea that postsynaptic AMPA receptor insertion mediates a component of synaptic facilitation by showing that facilitation after 5-HT offset is blocked by injecting botulinum toxin, an exocytotic inhibitor, into motor neurons before application of 5-HT. Using a reduced preparation, we extend our results to synaptic facilitation in the abdominal ganglion. We show that tail nerve shock-induced facilitation of siphon sensorimotor synapses also depends on elevated postsynaptic Ca(2+) and release of Ca(2+) from postsynaptic stores and recruits a late phase of facilitation that involves selective enhancement of the AMPA receptor-mediated synaptic response. To examine the potential role of postsynaptic exocytosis of AMPA receptors in learning in Aplysia, we test the effect of injecting botulinum toxin into siphon motor neurons on dishabituation of the siphon-withdrawal reflex. We find that postsynaptic injections of the toxin block dishabituation resulting from tail shock. Our results indicate that postsynaptic mechanisms, particularly Ca(2+)-dependent modulation of AMPA receptor trafficking, play a critical role in synaptic facilitation as well as in dishabituation and sensitization in Aplysia.

Involvement of Ca(2+)-induced Ca2+ releasing system in interleukin-1beta-associated adenosine release

Interleukin-1beta (IL-1beta) plays an important role in neuroprotective and neurodegenerative events in the central nervous system. To clarify the mechanism of controversial actions of IL-1beta, we determined the effect of IL-1beta, as well as the interaction between IL-1beta and Ca(2+)-induced Ca2+ releasing system (CICR), on adenosine releases in mice hippocampus using mini-slices method. Basal and K(+)-stimulated adenosine releases were regulated by two types of CICRs, including inositol-1,4,5-trisphosphate (IP3) receptor and ryanodine receptor. Lower concentration of IL-1beta increased both adenosine releases, whereas higher concentration did not affect their releases. The stimulatory effect of IL-1beta on basal adenosine release was reduced by removal of extracellular Ca2+ and IP3 receptor inhibitor, while the stimulatory effect of IL-1beta on K(+)-stimulated adenosine release was reduced by ryanodine receptor inhibitor. These results suggest that the potent effect of IL-1beta upon adenosine release might contribute to the neuroprotective action of IL-1beta, whereas IL-1beta-induced neurodegeneration might be due to the overload response of Ca2+ mobilization and the inactivation of adenosine exocytosis.