Regulation of TRP channels via lipid second messengers

Rolling blackout is required for synaptic vesicle exocytosis

Rolling blackout (RBO) is a putative transmembrane lipase required for phospholipase C-dependent phosphatidylinositol 4,5-bisphosphate-diacylglycerol signaling in Drosophila neurons. Conditional temperature-sensitive (TS) rbo mutants display complete, reversible paralysis within minutes, demonstrating that RBO is acutely required for movement. RBO protein is localized predominantly in presynaptic boutons at neuromuscular junction (NMJ) synapses and throughout central synaptic neuropil, and rbo TS mutants display a complete, reversible block of both central and peripheral synaptic transmission within minutes. This phenotype appears limited to adults, because larval NMJs do not manifest the acute blockade. Electron microscopy of adult rbo TS mutant boutons reveals an increase in total synaptic vesicle (SV) content, with a concomitant shrinkage of presynaptic bouton size and an accumulation of docked SVs at presynaptic active zones within minutes. Genetic tests reveal a synergistic interaction between rbo and syntaxin1A TS mutants, suggesting that RBO is required in the mechanism of N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated SV exocytosis, or in a parallel pathway necessary for SV fusion. The rbo TS mutation does not detectably alter SNARE complex assembly, suggesting a downstream requirement in SV fusion. We conclude that RBO plays an essential role in neurotransmitter release, downstream of SV docking, likely mediating SV fusion.

Neural cell adhesion molecule induces intracellular signaling via multiple mechanisms of Ca2+ homeostasis

The neural cell adhesion molecule (NCAM) plays a pivotal role in the development of the nervous system, promoting neuronal differentiation via homophilic (NCAM-NCAM) as well as heterophilic (NCAM-fibroblast growth factor receptor [FGFR]) interactions. NCAM-induced intracellular signaling has been shown to affect and be dependent on the cytoplasmic Ca2+ concentration ([Ca2+]i). However, the molecular basis of this remains unclear. In this study, we determined [Ca2+]i regulating mechanisms involved in intracellular signaling induced by NCAM. To mimic the effect of homophilic NCAM interaction on [Ca2+]i in vitro, we used a peptide derived from a homophilic binding site of NCAM, termed P2, which triggers signaling cascades similar to those activated by NCAM-NCAM interaction. We found that P2 increased [Ca2+]i in primary hippocampal neurons. This effect depended on two signaling pathways. The first pathway was associated with activation of FGFR, phospholipase Cgamma, and production of diacylglycerol, and the second pathway involved Src-family kinases. Moreover, NCAM-mediated Ca2+ entry required activation of nonselective cation and T-type voltage-gated Ca2+ channels. These channels, together with the Src-family kinases, were also involved in neuritogenesis induced by physiological, homophilic NCAM interactions. Thus, unanticipated mechanisms of Ca2+ homeostasis are shown to be activated by NCAM and to contribute to neuronal differentiation.

Charge screening by internal pH and polyvalent cations as a mechanism for activation, inhibition, and rundown of TRPM7/MIC channels

The Mg²⁺-inhibited cation (MIC) current, believed to represent activity of TRPM7 channels, is found in lymphocytes and mast cells, cardiac and smooth muscle, and several other eukaryotic cell types. MIC current is activated during whole-cell dialysis with divalent-free internal solutions. Millimolar concentrations of intracellular Mg²⁺ (or other divalent metal cations) inhibit the channels in a voltage-independent manner. The nature of divalent inhibition and the mechanism of channel activation in an intact cell remain unknown. We show that the polyamines (spermine, spermidine, and putrescine) inhibit the MIC current, also in a voltage-independent manner, with a potency that parallels the number of charges. Neomycin and poly-lysine also potently inhibited MIC current in the absence of Mg²⁺. These same positively charged ions inhibited IRK1 current in parallel with MIC current, suggesting that they probably act by screening the head group phosphates on PIP₂ and other membrane phospholipids. In agreement with this hypothesis, internal protons also inhibited MIC current. By contrast, tetramethylammonium, tetraethylammonium, and hexamethonium produced voltage-dependent block but no inhibition. We show that inhibition by internal polyvalent cations can be relieved by alkalinizing the cytosol using externally applied ammonium or by increasing pH in inside-out patches. Furthermore, in perforated-patch and cell-attached recordings, when intracellular Mg²⁺ is not depleted, endogenous MIC or recombinant TRPM7 currents are activated by cytosolic alkalinization and inhibited by acidification; and they can be reactivated by PIP₂ following rundown in inside-out patches. We propose that MIC (TRPM7) channels are regulated by a charge screening mechanism and may function as sensors of intracellular pH.

The transient receptor potential vanilloid 1: role in airway inflammation and disease

The transient receptor potential vanilloid 1 (TRPV1) is an excitatory cation channel, rather selectively expressed in a subpopulation of nociceptive, primary sensory neurons that promote neurogenic inflammation via neuropeptide release. TRPV1 is activated by noxious temperature, low extracellular pH and diverse lipid derivatives, and is uniquely sensitive to vanilloid molecules, including capsaicin. TRPV1 expression and sensitivity is highly regulated by diverse G protein-coupled and tyrosine kinase receptors. Other exogenous or endogenous chemical agents, including reactive oxygen species, ethanol and hydrogen sulphide sensitize/activate TRPV1. In the airways, TRPV1 agonists cause cough, bronchoconstriction, microvascular leakage, hyperreactivity and hypersecretion. Patients with asthma and chronic obstructive pulmonary disease are more sensitive to the tussive effect of TRPV1 agonists and TRPV1 activation may contribute to respiratory symptoms caused by acidic media present in the airways during asthma exacerbation, gastroesophageal reflux induced asthma or in other conditions. TRPV1 antagonists may be useful in the treatment of these diseases.

Expression of canonical transient receptor potential (TRPC) proteins in human glomerular mesangial cells

Mesangial cells are located within glomerular capillary loops and contribute to the physiological regulation of glomerular hemodynamics. The function of mesangial cells is controlled by a variety of ion channels in the plasma membrane, including nonselective cation channels, receptor-operated Ca2+ channels, and recently identified store-operated Ca2+ channels. Although the significance of these channels has been widely acknowledged, their molecular identities are still unknown. Recently, the members of the canonical transient receptor potential (TRPC) protein family have been demonstrated to behave as cation channels. The present study was performed to identify the isoforms of endogenous TRPC proteins in human mesangial cells (HMCs) and their interactions. Western blotting showed that TRPC1, 3, 4, and 6 were expressed in cultured HMCs. Consistently, immunofluorescent confocal microscopy revealed specific stainings for TRPC1, 3, 4, and 6 with predominant intracellular localization. However, TRPC5 and 7 were not detectable at protein level by either Western blotting or immunofluorescent staining. The expression of TRPC1, 3, 4, and 6 was also observed in rat and human glomeruli using fluorescent immunohistochemistry. Furthermore, coimmunoprecipitation experiments and immunofluorescent double staining displayed that TRPC1 had physical interaction with TRPC4 and 6, while no interactions were detected among other isoforms of TRPCs. Ca2+ fluorescent ratiometry measurement showed that store-operated Ca2+ entry in HMCs was significantly reduced by knocking down TRPC1, but enhanced by overexpressing TRPC1. These results suggest that HMCs specifically express isoforms of TRPC1, 3, 4, and 6 proteins. These isoforms of TRPCs might selectively assemble to form functional complexes.

Insights on TRP channels from in vivo studies in Drosophila

Transient receptor potential (TRP) channels mediate responses in a large variety of signaling mechanisms. Most studies on mammalian TRP channels rely on heterologous expression, but their relevance to in vivo tissues is not entirely clear. In contrast, Drosophila TRP and TRP-like (TRPL) channels allow direct analyses of in vivo function. In Drosophila photoreceptors, activation of TRP and TRPL is mediated via the phosphoinositide cascade, with both Ca2+ and diacylglycerol (DAG) essential for generating the light response. In tissue culture cells, TRPL channels are constitutively active, and lipid second messengers greatly facilitate this activity. Inhibition of phospholipase C (PLC) completely blocks lipid activation of TRPL, suggesting that lipid activation is mediated via PLC. In vivo studies in mutant Drosophila also reveal an acute requirement for lipid-producing enzyme, which may regulate PLC activity. Thus, PLC and its downstream second messengers, Ca2+ and DAG, constitute critical mediators of TRP/TRPL gating in vivo.

Lymphocyte calcium signaling from membrane to nucleus

Ca(2+) signals control a variety of lymphocyte responses, ranging from short-term cytoskeletal modifications to long-term changes in gene expression. The identification of molecules and channels that modulate Ca(2+) entry into T and B lymphocytes has both provided details of the molecular events leading to immune responses and raised controversy. Here we review studies of the pathways that allow Ca(2+) entry, the function of Ca(2+) in the regulation of cell polarity and motility and the principles by which Ca(2+)-dependent transcription regulates lymphocyte function.

Ca2+ signaling microdomains:platforms for the assembly and regulation of TRPC channels

The transient receptor potential canonical family (TRPC1-TRPC7) of ion channel proteins, which are activated in response to agonist-stimulated phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] hydrolysis, are proposed components of the elusive store-operated Ca(2+) (SOC) channel. TRPC channels display distinct properties and interact to form homomeric or heteromeric channels that differ in their function and regulation. Although the exact function of TRPC channels and how they are regulated has not been established, increasing data suggest that they are localized and regulated within Ca(2+) signaling microdomains. TRPC channels contribute to store-operated and store-independent Ca(2+) entry mechanisms, both of which are activated by agonist-stimulated PtdIns(4,5)P(2) hydrolysis. Elucidation of how cells achieve specificity and precise temporal and spatial coordination of channel activation is crucial for understanding the molecular basis of agonist-mediated stimulation of Ca(2+) entry and identifying downstream physiological functions. This review will address the assembly and localization of TRPC channels and how these processes impact their function.

Calcium Signals Activated by Arachidonic Acid in Embryonic Chick Ciliary Ganglion Neurons

Arachidonic acid (AA, 20:4) has been reported to modulate a variety of calcium-permeable ionic channels, both in the plasma membrane and in the endoplasmic reticulum. We have studied the effects of AA on calcium signaling in a well-characterized model of developing peripheral neurons, embryonic chick ciliary ganglion neurons in culture. When given at low non-micellar concentrations (5 microM), in the majority of cells AA directly activated a delayed and long-lasting increase in [Ca2+]i, involving both the cytoplasm and the nucleoplasm, that was completely reversed by abolition of extracellular calcium. Other fatty acids (FAs), either saturated like arachidic acid (20:0), or unsaturated like linoleic (18:2) and docosahexaenoic acid (22:6), shared its ability to activate calcium influx. This entry was not suppressed by voltage-dependent calcium channel inhibitors omega-conotoxin and nifedipine, by the voltage-independent calcium channel antagonist LOE-908, by pre-treatment with blockers of AA metabolic pathways or with pertussis toxin. The arachidonate-activated calcium pathway was permeable to Mn2+ and blocked by La3+, Gd3+ and Ni2+. In a neuronal subpopulation, AA at the same concentration was also able to elicit calcium release from thapsigargin-sensitive intracellular stores; we provide evidence that cytochrome P450 epoxygenase is involved in this process.

Phototransduction in primate cones and blowfly photoreceptors: different mechanisms, different algorithms, similar response

Phototransduction in primate cones is compared with phototransduction in blowfly photoreceptor cells. Phototransduction in the two cell types utilizes not only different molecular mechanisms, but also different signal processing steps, producing range compression, contrast constancy, and an intensity-dependent integration time. The dominant processing step in the primate cone is a strongly compressive nonlinearity due to cGMP hydrolysis by phosphodiesterase. In the blowfly photoreceptor a considerable part of the range compression is performed by the nonlinear membrane of the cell. Despite these differences, both photoreceptor cell types are similarly effective in compressing the wide range of naturally occurring intensities, and in converting intensity variations into contrast variations. A direct comparison of the responses to a natural time series of intensities, simulated in the cone and measured in the blowfly photoreceptor, shows that the responses are quite similar.

Oscillatory Ca2+ signaling in the isolated Caenorhabditis elegans intestine: role of the inositol-1,4,5-trisphosphate receptor and phospholipases C beta and gamma

Defecation in the nematode Caenorhabditis elegans is a readily observable ultradian behavioral rhythm that occurs once every 45-50 s and is mediated in part by posterior body wall muscle contraction (pBoc). pBoc is not regulated by neural input but instead is likely controlled by rhythmic Ca(2+) oscillations in the intestinal epithelium. We developed an isolated nematode intestine preparation that allows combined physiological, genetic, and molecular characterization of oscillatory Ca(2+) signaling. Isolated intestines loaded with fluo-4 AM exhibit spontaneous rhythmic Ca(2+) oscillations with a period of approximately 50 s. Oscillations were only detected in the apical cell pole of the intestinal epithelium and occur as a posterior-to-anterior moving intercellular Ca(2+) wave. Loss-of-function mutations in the inositol-1,4,5-trisphosphate (IP(3)) receptor ITR-1 reduce pBoc and Ca(2+) oscillation frequency and intercellular Ca(2+) wave velocity. In contrast, gain-of-function mutations in the IP(3) binding and regulatory domains of ITR-1 have no effect on pBoc or Ca(2+) oscillation frequency but dramatically increase the speed of the intercellular Ca(2+) wave. Systemic RNA interference (RNAi) screening of the six C. elegans phospholipase C (PLC)-encoding genes demonstrated that pBoc and Ca(2+) oscillations require the combined function of PLC-gamma and PLC-beta homologues. Disruption of PLC-gamma and PLC-beta activity by mutation or RNAi induced arrhythmia in pBoc and intestinal Ca(2+) oscillations. The function of the two enzymes is additive. Epistasis analysis suggests that PLC-gamma functions primarily to generate IP(3) that controls ITR-1 activity. In contrast, IP(3) generated by PLC-beta appears to play little or no direct role in ITR-1 regulation. PLC-beta may function instead to control PIP(2) levels and/or G protein signaling events. Our findings provide new insights into intestinal cell Ca(2+) signaling mechanisms and establish C. elegans as a powerful model system for defining the gene networks and molecular mechanisms that underlie the generation and regulation of Ca(2+) oscillations and intercellular Ca(2+) waves in nonexcitable cells.

Putative ClC-2 Chloride Channel Mediates Inward Rectification in Drosophila Retinal Photoreceptors

We report that Drosophila retinal photoreceptors express inwardly rectifying chloride channels that seem to be orthologous to mammalian ClC-2 inward rectifier channels. We measured inwardly rectifying Cl(-) currents in photoreceptor plasma membranes: Hyperpolarization under whole-cell tight-seal voltage clamp induced inward Cl(-) currents; and hyperpolarization of voltage-clamped inside-out patches excised from plasma membrane induced Cl(-) currents that have a unitary channel conductance of approximately 3.7 pS. The channel was inhibited by 1 mM: Zn(2+) and by 1 mM: 9-anthracene, but was insensitive to DIDS. Its anion permeability sequence is Cl(-) = SCN(-)> Br(-)>> I(-), characteristic of ClC-2 channels. Exogenous polyunsaturated fatty acid, linolenic acid, enhanced or activated the inward rectifier Cl(-) currents in both whole-cell and excised patch-clamp recordings. Using RT-PCR, we found expression in Drosophila retina of a ClC-2 gene orthologous to mammalian ClC-2 channels. Antibodies to rat ClC-2 channels labeled Drosophila photoreceptor plasma membranes and synaptic regions. Our results provide evidence that the inward rectification in Drosophila retinal photoreceptors is mediated by ClC-2-like channels in the non-transducing (extra-rhabdomeral) plasma membrane, and that this inward rectification can be modulated by polyunsaturated fatty acid.

Mechanisms of light adaptation in Drosophila photoreceptors

Phototransduction in Drosophila is mediated by a phospholipase C (PLC) cascade culminating in activation of transient receptor potential (TRP) channels. Ca(2+) influx via these channels is required for light adaptation, but although several molecular targets of Ca(2+)-dependent feedback have been identified, their contribution to adaptation is unclear. By manipulating cytosolic Ca(2+) via the Na(+)/Ca(2+) exchange equilibrium, we found that Ca(2+) inhibited the light-induced current (LIC) over a range corresponding to steady-state light-adapted Ca(2+) levels (0.1-10 microM Ca(2+)) and accurately mimicked light adaptation. However, PLC activity monitored with genetically targeted PIP(2)-sensitive ion channels (Kir2.1) was first inhibited by much higher (>/= approximately 50 microM) Ca(2+) levels, which occur only transiently in vivo. Ca(2+)-dependent inhibition of PLC, but not the LIC, was impaired in mutants (inaC) of protein kinase C (PKC). The results indicate that light adaptation is primarily mediated downstream of PLC and independently of PKC by Ca(2+)-dependent inhibition of TRP channels. This is interpreted as a strategy to prevent inhibition of PLC by global steady-state light-adapted Ca(2+) levels, whereas rapid inhibition of PLC by local Ca(2+) transients is required to terminate the response and ensures that PIP(2) reserves are not depleted during stimulation.

Inhibition of phospholipase C activity in Drosophila photoreceptors by 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA) and di-bromo BAPTA

In vivo light-induced and basal hydrolysis of phosphatidyl inositol 4,5-bisphosphate (PIP2) by phospholipase C (PLC) were monitored in Drosophila photoreceptors using genetically targeted PIP2-sensitive ion channels (Kir2.1) as electrophysiological biosensors for PIP2. In cells loaded via patch pipettes with varying concentrations of Ca2+ buffered by 4 mM free BAPTA, light-induced PLC activity, showed an apparent bell-shaped dependence on free Ca2+ (maximum at "100 nM", approximately 10-fold inhibition at <10nM or approximately 1 microM). However, experiments where the total BAPTA concentration was varied whilst free [Ca2+] was maintained constant indicated that inhibition of PLC at higher (>100 nM) nominal Ca2+ concentrations was independent of Ca2+ and due to inhibition by BAPTA itself (IC50 approximately 8 mM). Di-bromo BAPTA (DBB) was yet more potent at inhibiting PLC activity (IC50 approximately 1mM). Both BAPTA and DBB also appeared to induce a modest, but less severe inhibition of basal PLC activity. By contrast, EGTA, failed to inhibit PLC activity when pre-loaded with Ca2+, but like BAPTA, inhibited both basal and light-induced PLC activity when introduced without Ca2+. The results indicate that both BAPTA and DBB inhibit PLC activity independently of their role as Ca2+ chelators, whilst non-physiologically low (<100 nM) levels of Ca2+ suppress both basal and light-induced PLC activity.

Physiological mechanisms of TRPC activation

TRPC (canonical transient receptor potential) channels are vertebrate homologs of the Drosophila photoreceptor channel, TRP. Considerable research has been brought to bear on the seven members of this family, especially with regard to their possible role in calcium entry. Unfortunately, the current literature presents a confusing picture, with different laboratories producing widely differing results and interpretations. It appears that ectopically expressed TRPC channels can be activated by phospholipase C products (generally, diacylglycerols), by stimulation of trafficking to the plasma membrane, or by depletion of intracellular Ca(2+) stores. Here, I discuss the possibility that these diverse experimental findings arise because TRPC channels can, under both experimental as well as physiological conditions, be activated in three distinct ways, possibly depending on their subunit composition and/or signaling complex environment. The TRPCs may be unique among ion-channel subunit families in being able to participate in the assembly and function of multiple types of physiologically important ion channels.

Determining the functional role of TRPC channels in primary cells

Although the TRPC members of the mammalian transient receptor potential TRP cation channel family were the first to be described in 1995, the depth of knowledge of TRPC channels has fallen behind that of their counterparts in the TRPV and TRPM subfamilies in the intervening years. The complexities and controversies of TRPC channel composition and regulation have hindered their progress as therapeutic targets in the drug discovery environment to date, however embracing these challenges as opportunities may bring TRPC channels to the forefront of the discovery of novel therapies for many diseases. These challenges and opportunities of exploring TRPC channels as therapeutic targets are highlighted and discussed in this review with respect to respiratory diseases.

Functional role of TRPC proteins in native systems: implications from knockout and knock-down studies

Available data on transient receptor potential channel (TRPC) protein functions indicate that these proteins represent essential constituents of agonist-activated and phospholipase C-dependent cation entry pathways in primary cells which contribute to the elevation of cytosolic Ca2+. In addition, a striking number of biological functions have already been assigned to the various TRPC proteins, including mechanosensing activity (TRPC1), chemotropic axon guidance (TRPC1 and TRPC3), pheromone sensing and the regulation of sexual and social behaviour (TRPC2), endothelial-dependent regulation of vascular tone, endothelial permeability and neurotransmitter release (TRPC4), axonal growth (TRPC5), modulation of smooth muscle tone in blood vessels and lung and regulation of podocyte structure and function in the kidney (TRPC6). The lack of compounds which specifically block or activate TRPC proteins impairs the analysis of TRPC function in primary cells. We therefore concentrate in this contribution on (i) studies of TRPC-deficient mouse lines, (ii) data obtained by gene-silencing approaches using antisense oligonucleotides or RNA interference, (iii) expression experiments employing dominant negative TRPC constructs, and (iv) recent data correlating mutations of TRPC genes associated with human disease.

Mutations in the Arabidopsis phosphoinositide phosphatase gene SAC9 lead to overaccumulation of PtdIns(4,5)P2 and constitutive expression of the stress-response pathway

Phosphoinositides (PIs) are signaling molecules that regulate cellular events including vesicle targeting and interactions between membrane and cytoskeleton. Phosphatidylinositol (PtdIns)(4,5)P(2) is one of the best characterized PIs; studies in which PtdIns(4,5)P(2) localization or concentration is altered lead to defects in the actin cytoskeleton and exocytosis. PtdIns(4,5)P(2) and its derivative Ins(1,4,5)P(3) accumulate in salt, cold, and osmotically stressed plants. PtdIns(4,5)P(2) signaling is terminated through the action of inositol polyphosphate phosphatases and PI phosphatases including supressor of actin mutation (SAC) domain phosphatases. In some cases, these phosphatases also act on Ins(1,4,5)P(3). We have characterized the Arabidopsis (Arabidopsis thaliana) sac9 mutants. The SAC9 protein is different from other SAC domain proteins in several ways including the presence of a WW protein interaction domain within the SAC domain. The rice (Oryza sativa) and Arabidopsis SAC9 protein sequences are similar, but no apparent homologs are found in nonplant genomes. High-performance liquid chromatography studies show that unstressed sac9 mutants accumulate elevated levels of PtdIns(4,5)P(2) and Ins(1,4,5)P(3) as compared to wild-type plants. The sac9 mutants have characteristics of a constitutive stress response, including dwarfism, closed stomata, and anthocyanin accumulation, and they overexpress stress-induced genes and overaccumulate reactive-oxygen species. These results suggest that the SAC9 phosphatase is involved in modulating phosphoinsitide signals during the stress response.

KCNQ-like Potassium Channels in Caenorhabditis elegans

The human KCNQ gene family encodes potassium channels linked to several genetic syndromes including neonatal epilepsy, cardiac arrhythmia, and progressive deafness. KCNQ channels form M-type potassium channels, which are critical regulators of neuronal excitability that mediate autonomic responses, pain, and higher brain function. Fundamental mechanisms of the normal and abnormal cellular roles for these channels may be gained from their study in simple model organisms. Here we report that a multigene family of KCNQ-like channels is present in the nematode, Caenorhabditis elegans. We show that many aspects of the functional properties, tissue expression pattern, and modulation of these C. elegans channels are conserved, including suppression by the M1 muscarinic receptor. We also describe a conserved mechanism of modulation by diacylglycerol for a subset of C. elegans and vertebrate KCNQ/KQT channels, which is dependent upon the carboxyl-terminal domains of channel subunits and activated protein kinase C.

Role of phospholipase D and diacylglycerol in activating constitutive TRPC-like cation channels in rabbit ear artery myocytes

Previously we have described a constitutively active Ca2+-permeable non-selective cation channel in freshly dispersed rabbit ear artery myocytes that has similar properties to canonical transient receptor potential (TRPC) channel proteins. In the present study we have investigated the transduction pathways responsible for stimulating constitutive channel activity in these myocytes. Application of the pharmacological inhibitors of phosphatidylcholine-phospholipase D (PC-PLD), butan-1-ol and C2 ceramide, produced marked inhibition of constitutive channel activity in cell-attached patches and also butan-1-ol produced pronounced suppression of resting membrane conductance measured with whole-cell recording whereas the inactive isomer butan-2-ol had no effect on constitutive whole-cell or channel activity. In addition butan-1-ol had no effect on channel activity evoked by the diacylglycerol (DAG) analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG). Inhibitors of PC-phospholipase C (PC-PLC) and phospholipase A2 (PLA2) had no effect on constitutive channel activity. Application of a purified PC-PLD enzyme and its metabolite phosphatidic acid to inside-out patches markedly increased channel activity. The phosphatidic acid phosphohydrolase (PAP) inhibitor dl-propranolol also inhibited constitutive and phosphatidic acid-induced increases in channel activity but had no effect on OAG-evoked responses. The DAG lipase and DAG kinase inhibitors, RHC80267 and R59949 respectively, which inhibit DAG metabolism, produced transient increases in channel activity which were mimicked by relatively high concentrations (40 microm) of OAG. The protein kinase C (PKC) inhibitor chelerythrine did not prevent channel activation by OAG but blocked the secondary inhibitory response of OAG. It is proposed that endogenous DAG is involved in the activation of channel activity and that its effects on channel activity are concentration-dependent with higher concentrations of DAG also inhibiting channel activity through activation of PKC. This study indicates that constitutive cation channel activity in ear artery myocytes is mediated by DAG which is generated by PC-PLD via phosphatidic acid which represents a novel activation pathway of cation channels in vascular myocytes.

Co-contribution of IP3R and Ca2+ influx pathways to pacemaker Ca2+ activity in stomach ICC

Intracellular Ca2+ oscillations in interstitial cells of Cajal (ICCs) are thought to be the primary pacemaker activity in the gut. In the present study, the authors prepared small tissues of 100-to 300-microm diameter (cell cluster preparation) from the stomach smooth muscle (including the myenteric plexus) of mice by enzymatic and mechanical treatments. After 2 to 4 days of culture, the intracellular Ca2+ concentration ([Ca2+]i) was measured. In the presence of nifedipine, a dihydropyridine Ca2+ channel antagonist, spontaneous [Ca2+]i oscillations were observed within limited regions showing positive c-Kitimmunoreactivity, a maker for ICCs. In the majority of cell cluster preparations with multiple regions of [Ca2+]i oscillations, [Ca2+]i oscillated synchronously in the same phase. A small number of cell clusters (8 of 53) showed multiple regions of [Ca2+]i oscillations synchronized but with a considerable phase shift. Neither tetrodotoxin (250 nM) nor atropine (10 microM) significantly affected [Ca2+]i oscillations in the presence of nifedipine. Low concentrations (40 microM) of Ni2+ had little effect on the spontaneous [Ca2+]i oscillation, but SK&F96365 (40 microM) and Cd2+ (120 microM) terminated it. Applications of either 2-aminoethoxydiphenyl borate (10 microM) or xestosponginC(10 microM) completely and rather rapidly (approximately 2 min) abolished the spontaneous [Ca2+]i oscillations. The results suggest that pacemaker [Ca2+]i oscillations in ICCs are produced by close interaction of intracellular Ca2+ release channels, especially inositol 1,4,5-trisphosphate receptor (InsP3R) and Ca2+ influx pathways, presumably corresponding to store-operated type channels. Reverse transcription polymerase chain reaction examinations revealed expression of TRPC2, 4, and 6, as well as InsP3R1 and 2 in ICCs.

The mechanosensitive nature of TRPV channels

Transient receptor potential vanilloid (TRPV) channels are widely expressed in both sensory and nonsensory cells. Whereas the channels display a broad diversity to activation by chemical and physical stimuli, activation by mechanical stimuli is common to many members of this group in both lower and higher organisms. Genetic screening in Caenorhabditis elegans has demonstrated an essential role for two TRPV channels in sensory neurons. OSM-9 and OCR-2, for example, are essential for both osmosensory and mechanosensory (nose-touch) behaviors. Likewise, two Drosophila TRPV channels, NAN and IAV, have been shown to be critical for hearing by the mechanosensitive chordotonal organs located in the fly's antennae. The mechanosensitive nature of the channels appears to be conserved in higher organisms for some TRPV channels. Two vertebrate channels, TRPV2 and TRPV4, are sensitive to hypotonic cell swelling, shear stress/fluid flow (TRPV4), and membrane stretch (TRPV2). In the osmosensing neurons of the hypothalamus (circumventricular organs), TRPV4 appears to function as an osmoreceptor, or part of an osmoreceptor complex, in control of vasopressin release, whereas in inner ear hair cells and vascular baroreceptors a mechanosensory role is suggestive, but not demonstrated. Finally, in many nonsensory cells expressing TRPV4, such as vascular endothelial cells and renal tubular epithelial cells, the channel exhibits well-developed local mechanosensory transduction processes where both cell swelling and shear stress/fluid flow lead to channel activation. Hence, many TRPV channels, or combinations of TRPV channels, display a mechanosensitive nature that underlies multiple mechanosensitive processes from worms to mammals.

PI(4,5)P2 regulates the activation and desensitization of TRPM8 channels through the TRP domain

The subjective feeling of cold is mediated by the activation of TRPM8 channels in thermoreceptive neurons by cold or by cooling agents such as menthol. Here, we demonstrate a central role for phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) in the activation of recombinant TRPM8 channels by both cold and menthol. Moreover, we show that Ca(2+) influx through these channels activates a Ca(2+)-sensitive phospholipase C and that the subsequent depletion of PI(4,5)P(2) limits channel activity, serving as a unique mechanism for desensitization of TRPM8 channels. Finally, we find that mutation of conserved positive residues in the highly conserved proximal C-terminal TRP domain of TRPM8 and two other family members, TRPM5 and TRPV5, reduces the sensitivity of the channels for PI(4,5)P(2) and increases inhibition by PI(4,5)P(2) depletion. These data suggest that the TRP domain of these channels may serve as a PI(4,5)P(2)-interacting site and that regulation by PI(4,5)P(2) is a common feature of members of the TRP channel family.

Functional INAD complexes are required to mediate degeneration in photoreceptors of the Drosophila rdgA mutant

The TRP family of ion channels mediates a wide range of calcium-influx phenomena in eukaryotic cells. Many members of this family are activated downstream of phosphoinositide hydrolysis but the subsequent steps that lead to TRP channel activation in vivo remain unclear. Recently, the lipid products of phosphoinositide hydrolysis (such as diacylglycerol and its metabolites) have been implicated in activating TRP channels in both Drosophila and mammals. In Drosophila photoreceptors, lack of diacylglycerol kinase (DGK) activity (encoded by rdgA) leads to both constitutive TRP-channel activity and retinal degeneration. In this study, using a novel forward-genetic screen, we identified InaD, a multivalent PDZ domain protein as a suppresser of retinal degeneration in rdgA mutants. We show that InaD suppresses rdgA and that the rescue is correlated with reduced levels of phospholipase Cbeta (PLCbeta), a key enzyme for TRP channel activation. Furthermore, we show that light, Gq and PLCbeta all modulate retinal degeneration in rdgA. The results demonstrate a previously unknown requirement for a balance of PLCbeta and DGK activity for retinal degeneration in rdgA. They also suggest a key role for the lipid products of phosphoinositide hydrolysis in the activation of TRP channels in vivo.

Impaired KCNQ1-KCNE1 and phosphatidylinositol-4,5-bisphosphate interaction underlies the long QT syndrome

Nearly a hundred different KCNQ1 mutations have been reported as leading to the cardiac long QT syndrome, characterized by prolonged QT interval, syncopes, and sudden death. We have previously shown that phosphatidylinositol-4,5-bisphosphate (PIP2) regulates the KCNQ1-KCNE1 complex. In the present study, we show that PIP2 affinity is reduced in three KCNQ1 mutant channels (R243H, R539W, and R555C) associated with the long QT syndrome. In giant excised patches, direct application of PIP2 on the cytoplasmic face of the three mutant channels counterbalances the loss of function. Reintroduction of a positive charge by application of methanethiosulfonate ethylammonium on the cytoplasmic face of R555C mutant channels also restores channel activity. The channel affinity for a soluble analog of PIP2 is decreased in the three mutant channels. By using a model that describes the KCNQ1-KCNE1 channel behavior and by fitting the relationship between the kinetics of deactivation and the current amplitude obtained in whole-cell experiments, we estimated the PIP2 binding and dissociation rates on wild-type and mutant channels. The dissociation rate of the three mutants was higher than for the wild-type channel, suggesting a decreased affinity for PIP2. PIP2 binding was magnesium-dependent, and the PIP2-dependent equilibrium constant in the absence of magnesium was higher with the wild-type than with the mutant channels. Altogether, our data suggest that a reduced PIP2 affinity of KCNQ1 mutants can lead to the long QT syndrome.

Signal transduction pathway for the substance P-induced inhibition of rat Kir3 (GIRK) channel

Certain transmitters inhibit Kir3 (GIRK) channels, resulting in neuronal excitation. We analysed signalling mechanisms for substance P (SP)-induced Kir3 inhibition in relation to the role of phosphatidylinositol 4,5-bisphosphate (PIP(2)). SP rapidly - with a half-time of approximately 10 s with intracellular GTPgammaS and approximately 14 s with intracellular GTP - inhibits a robustly activated Kir3.1/Kir3.2 current. A mutant Kir3 channel, Kir3.1(M223L)/Kir3.2(I234L), which has a stronger binding to PIP(2) than does the wild type Kir3.1/Kir3.2, is inhibited by SP as rapidly as the wild type Kir3.1/Kir3.2. This result contradicts the idea that Kir3 inhibition originates from the depletion of PIP(2). A Kir2.1 (IRK1) mutant, Kir2.1(R218Q), despite having a weaker binding to PIP(2) than wild type Kir3.1/Kir3.2, shows a SP-induced inhibition slower than the wild type Kir3.1/Kir3.2 channel, again conflicting with the PIP(2) theory of channel inhibition. Co-immunoprecipitation reveals that Galpha(q) binds with Kir3.2, but not with Kir2.2 or Kir2.1. These functional results and co-immunoprecipitation data suggest that G(q) activation rapidly inhibits Kir3 (but not Kir2), possibly by direct binding of Galpha(q) to the channel.

Effects of unsaturated fatty acids on calcium-activated potassium current in gastric myocytes of guinea pigs

AIM: To investigate the effects of exogenous unsaturated fatty acids on calcium-activated potassium current [I_K(Ca)] in gastric antral circular myocytes of guinea pigs.

METHODS: Gastric myocytes were isolated by collagenase from the antral circular layer of guinea pig stomach. The whole-cell patch clamp technique was used to record I_K(Ca) in the isolated single smooth muscle cells with or without different concentrations of arachidonic acid (AA), linoleic acid (LA), and oleic acid (OA).

RESULTS: AA at concentrations of 2,5 and 10 μmol/L markedly increased I_K(Ca) in a dose-dependent manner. LA at concentrations of 5, 10 and 20 μmol/L also enhanced I_K(Ca) in a dose-dependent manner. The increasing potency of AA, LA, and oleic acid (OA) on I_K(Ca) at the same concentration (10 μmol/L) was in the order of AA>LA>OA. AA (10 μmol/L)-induced increase of I_K(Ca) was not blocked by H-7 (10 μmol/L), an inhibitor of protein kinase C (PKC), or indomethacin (10 μmol/L), an inhibitor of the cyclooxygenase pathway, and 17-octadecynoic acid (10 μmol/L), an inhibitor of the cytochrome P450 pathway, but weakened by nordihydroguaiaretic acid (10 μmol/L), an inhibitor of the lipoxygenase pathway.

CONCLUSION: Unsaturated fatty acids markedly increase I_K(Ca), and the enhancing potencies are related to the number of double bonds in the fatty acid chain. The lipoxygenase pathway of unsaturated fatty acid metabolism is involved in the unsaturated fatty acid-induced increase of I_K(Ca) in gastric antral circular myocytes of guinea pigs.

Lipid regulation of the synaptic vesicle cycle

Membrane vesicle cycling is orchestrated through the combined actions of proteins and lipids. At neuronal synapses, this orchestration must meet the stringent demands of speed, fidelity and sustainability of the synaptic vesicle cycle that mediates neurotransmission. Historically, the lion's share of the attention has been focused on the proteins that are involved in this cycle; but, in recent years, it has become clear that the previously unheralded plasma membrane and vesicle lipids are also key regulators of this cycle. This article reviews recent insights into the roles of lipid-modifying enzymes and lipids in the acute modulation of neurotransmission.

Hypoxia sensing and pathways of cytosolic Ca2+ increases

Oxygen-sensing and reactivity to changes in the concentration of oxygen is a fundamental property of cellular physiology. This central role is determined, mainly, by, to the fact that oxygen represents the final acceptor of electrons, derived from the normal cellular metabolism, at the end of the mitochondrial respiratory chain. Despite significant advances in molecular characterization of various oxygen-sensitive processes, the nature of the oxygen-sensor molecules and the mechanisms that link sensors to effects remains unclear. One such controversy is about the role and nature of reactive oxygen species (ROS) changes during hypoxia. Irrespective of the mechanisms of oxygen sensing, one of the constant early responses to hypoxia in almost all cell types is an increase in intracellular Ca2+ ([Ca2+]i). In many instances, this increase is mediated by the activation of various plasma membrane Ca2+ conductances. Some of these channels have specific Ca2+ permeability (e.g. voltage-operated Ca2+ channels), whereas others have non-specific cation conductances and are activated by a variety of ligands (ligand-operated channels). In the last decade, a large superfamily of channels with significant Ca2+ permeability has been progressively identified and characterised: the TRP channels. Through their properties, some groups of the TRP channels provide a link to the other hypoxia-activated mechanism of [Ca2+]i increase: the release of Ca2+ from intracellular Ca2+ stores. Since the [Ca2+]i signals, depending on their localization and intensity, are important regulators of the subsequent cellular responses to hypoxia, a deeper understanding of the mechanisms through which hypoxia regulate the activity of these pathways that increase intracellular Ca2+ could point the way towards the development of new therapeutic approaches to reduce or suppress the pathological effects of cellular hypoxia, such as those seen in stroke or myocardial ischemia.

Induction of photosensitivity by heterologous expression of melanopsin

Melanopsin has been proposed to be the photopigment of the intrinsically photosensitive retinal ganglion cells (ipRGCs); these photoreceptors of the mammalian eye drive circadian and pupillary adjustments through direct projections to the brain. Their action spectrum (lambda(max) approximately 480 nm) implicates an opsin and melanopsin is the only opsin known to exist in these cells. Melanopsin is required for ipRGC photosensitivity and for behavioural photoresponses that survive disrupted rod and cone function. Heterologously expressed melanopsin apparently binds retinaldehyde and mediates photic activation of G proteins. However, its amino-acid sequence differs from vertebrate photosensory opsins and some have suggested that melanopsin may be a photoisomerase, providing retinoid chromophore to an unidentified opsin. To determine whether melanopsin is a functional sensory photopigment, here we transiently expressed it in HEK293 cells that stably expressed TRPC3 channels. Light triggered a membrane depolarization in these cells and increased intracellular calcium. The light response resembled that of ipRGCs, with almost identical spectral sensitivity (lambda(max) approximately 479 nm). The phototransduction pathway included Gq or a related G protein, phospholipase C and TRPC3 channels. We conclude that mammalian melanopsin is a functional sensory photopigment, that it is the photopigment of ganglion-cell photoreceptors, and that these photoreceptors may use an invertebrate-like phototransduction cascade.

Intracellular calcium signals and control of cell proliferation: how many mechanisms?

The progression through the cell cycle in non-transformed cells is under the strict control of extracellular signals called mitogens, that act by eliciting complex cascades of intracellular messengers. Among them, increases in cytosolic free calcium concentration have been long realized to play a crucial role; however, the mechanisms coupling membrane receptor activation to calcium signals are still only partially understood, as are the pathways of calcium entry in the cytosol. This article centers on the role of calcium influx from the extracellular medium in the control of proliferative processes, and reviews the current understanding of the pathways responsible for this influx and of the second messengers involved in their activation.

Ca(2+)- and volume-sensitive chloride currents are differentially regulated by agonists and store-operated Ca2+ entry

Using patch-clamp and calcium imaging techniques, we characterized the effects of ATP and histamine on human keratinocytes. In the HaCaT cell line, both receptor agonists induced a transient elevation of [Ca²⁺]_i in a Ca²⁺-free medium followed by a secondary [Ca²⁺]_i rise upon Ca²⁺ readmission due to store-operated calcium entry (SOCE). In voltage-clamped cells, agonists activated two kinetically distinct currents, which showed differing voltage dependences and were identified as Ca²⁺-activated (I_Cl(Ca)) and volume-regulated (I_{Cl, swell}) chloride currents. NPPB and DIDS more efficiently inhibited I_Cl(Ca) and I_{Cl, swell}, respectively. Cell swelling caused by hypotonic solution invariably activated I_{Cl, swell} while regulatory volume decrease occurred in intact cells, as was found in flow cytometry experiments. The PLC inhibitor U-73122 blocked both agonist- and cell swelling–induced I_{Cl, swell}, while its inactive analogue U-73343 had no effect. I_Cl(Ca) could be activated by cytoplasmic calcium increase due to thapsigargin (TG)-induced SOCE as well as by buffering [Ca²⁺]_i in the pipette solution at 500 nM. In contrast, I_{Cl, swell} could be directly activated by 1-oleoyl-2-acetyl-sn-glycerol (OAG), a cell-permeable DAG analogue, but neither by InsP₃ infusion nor by the cytoplasmic calcium increase. PKC also had no role in its regulation. Agonists, OAG, and cell swelling induced I_{Cl, swell} in a nonadditive manner, suggesting their convergence on a common pathway. I_{Cl, swell} and I_Cl(Ca) showed only a limited overlap (i.e., simultaneous activation), although various maneuvers were able to induce these currents sequentially in the same cell. TG-induced SOCE strongly potentiated I_Cl(Ca), but abolished I_{Cl, swell}, thereby providing a clue for this paradox. Thus, we have established for the first time using a keratinocyte model that I_{Cl, swell} can be physiologically activated under isotonic conditions by receptors coupled to the phosphoinositide pathway. These results also suggest a novel function for SOCE, which can operate as a “selection” switch between closely localized channels.

Possible role of phosphatidylinositol 4,5, bisphosphate in luteinizing hormone releasing hormone-mediated M-current inhibition in bullfrog sympathetic neurons

Luteinizing hormone releasing hormone (LHRH) is a physiological modulator of neuronal excitability in bullfrog sympathetic ganglia (BFSG). Actions of LHRH involve suppression of the noninactivating, voltage-dependent M-type K+ channel conductance (gM). We found, using whole-cell recordings from these neurons, that LHRH-induced suppression of gM was attenuated by the phospholipase C (PLC) inhibitor U73122 (10 microM) but not by the inactive isomer U73343 (10 microM). Buffering internal Ca2+ to 117 nM with intracellular 20 mM BAPTA + 8 mM Ca2+ or to < 10 nM with intracellular 20 mM BAPTA + 0.4 mM Ca2+ did not attenuate LHRH-induced gM suppression. Suppression of gM by LHRH was not antagonized by the inositol 1,4,5 trisphosphate (InsP3) receptor antagonist heparin (approximately 300 microM). Preventing phosphatidylinositol-4,5-bisphosphate (PIP2) synthesis by blocking phosphatidylinositol-4-kinase with wortmannin (10 microM) or with the nonhydrolysable ATP analogue AMP-PNP (3 mM) prolonged recovery of LHRH-induced gM suppression. This effect was not produced by blocking phosphatidyl inositol-3-kinase with LY294002 (10 microM). Rundown of gM was attenuated when cells were dialysed with 240 microM di-octanoyl PIP2 or 240 microM di-octanoyl phosphatidylinositol-3,4,5-trisphosphate (PIP3) but not with 240 microM di-octanoyl phosphatidylcholine. LHRH-induced gM suppression was competitively antagonized by dialysis with 240 microM di-octanoyl PIP2, but not with di-octanoyl phosphatidylcholine. These results would be expected if LHRH-induced gM suppression reflects a PLC-mediated decrease in plasma membrane PIP2 levels.

Protein kinase-independent activation of CFTR by phosphatidylinositol phosphates

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that is expressed in many epithelia and in the heart. Phosphorylation of CFTR by protein kinases is thought to be an absolute prerequisite for the opening of CFTR channels. In addition, nucleoside triphosphates were shown to regulate the opening of phosphorylated CFTR. Here, we report that phosphatidylinositol 4,5-bisphosphate (PIP(2)) activates human CFTR, resulting in ATP responsiveness of PIP(2)-treated CFTR. PIP(2) alone is not sufficient to open CFTR, but ATP opens nonphosphorylated CFTR after application of PIP(2). The effect of PIP(2) is independent of protein kinases, as PIP(2) activates CFTR in the complete absence of Mg. Phosphatidylinositol and phosphatidylinositol monophosphate activate CFTR less efficiently than PIP(2). PIP(2) application to phosphorylated CFTR may inhibit the CFTR chloride current. We suggest that regulation of CFTR by PIP(2) is a previously unrecognized, alternative mechanism to control chloride conductance.

Effects of muscarinic acetylcholine receptor activation on membrane currents and intracellular messengers in medium spiny neurones of the rat striatum

Acetylcholine, acting through muscarinic receptors, modulates the excitability of striatal medium spiny neurones. However, the underlying membrane conductances and intracellular signalling pathways have not been fully determined. Our aim was to characterize excitatory effects mediated by M1 muscarinic acetylcholine receptors in these neurones using whole-cell patch-clamp recordings in brain slices of postnatal rats. Under voltage-clamp, muscarine evoked an inward current associated with an increase in cell membrane resistance. The current, which reversed at -85 mV, was sensitive to the M1 receptor antagonist pirenzepine. Blocking the potassium conductance attenuated the response and the residual current was further reduced by ruthenium red (50 microm) and reversed at +15 mV. Simultaneous recordings from cholinergic interneurones and medium spiny neurones in conjunction with spike-triggered averaging revealed small unitary excitatory postsynaptic currents in four of 39 cell pairs tested. The muscarine-induced inward current was attenuated by a phospholipase C (PLC) inhibitor, U73122, but not by a protein kinase C inhibitor, chelerythrine, or by the intracellular calcium chelator 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid, suggesting that the current was associated with PLC in a protein kinase C- and Ca2+ -independent manner. The phosphatidylinositol 4-kinase inhibitor wortmannin (10 microm) reduced the recovery of the inward current, indicating that the recovery process was dependent on the removal of diacylglycerol and/or inositol 1,4,5 triphosphate or resynthesis of phospholipid phosphatidylinositol 4,5-bisphophate. Ratiometric measurement of intracellular calcium after cell loading with fura-2 demonstrated a muscarine-induced increase in calcium signal that originated mainly from intracellular stores. Thus, the cholinergic excitatory effect in striatal medium spiny neurones, which is important in motor disorders associated with altered cholinergic transmission in the striatum such as Parkinson's disease, is mediated through M1 receptors and the PLC-dependent pathway.

Negative regulation of TRPC3 channels by protein kinase C-mediated phosphorylation of serine 712

TRPC3 is a nonselective cation channel member of the "canonical" transient receptor potential (TRPC) family whose members are activated by phospholipase C-coupled receptors. TRPC3 can be activated by the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol (OAG) in a protein kinase C-independent manner. On the other hand, phorbol 12-myristate 13-acetate (PMA) inhibits OAG-mediated TRPC3 channel activation, suggesting that phosphorylation of TRPC3 by protein kinase C is a mechanism of receptor-mediated negative feedback. Here, we show PMA-induced phosphorylation of TRPC3 channels in vivo. We demonstrate by site-directed mutagenesis that a single site containing Ser(712) and conserved among all members of the TRPC family is essential for protein kinase C-mediated negative regulation of TRPC3. In human embryonic kidney 293 cells expressing a TRPC3 mutant in which Ser(712) was replaced by alanine (S712A), PMA failed to block channel activation, whereas wild-type TRPC3 activity was completely inhibited. Phosphorylation of the S712A TRPC3 mutant was not stimulated in response to PMA treatment. Furthermore, S712A TRPC3 mutant-mediated Ca(2+) entry after methacholine activation was significantly greater than that of wild-type TRPC3. These findings demonstrate a dual role for phospholipase C-generated diacylglycerol, which serves as a signal for TRPC3 activation as well as a signal for negative feedback via protein kinase C-mediated phosphorylation.

The super-cooling agent icilin reveals a mechanism of coincidence detection by a temperature-sensitive TRP channel

TRPM8, a member of the transient receptor potential family of ion channels, depolarizes somatosensory neurons in response to cold. TRPM8 is also activated by the cooling agents menthol and icilin. When exposed to menthol or cold, TRPM8 behaves like many ligand-gated channels, exhibiting rapid activation followed by moderate Ca(2+)-dependent adaptation. In contrast, icilin activates TRPM8 with extremely variable latency followed by extensive desensitization, provided that calcium is present. Here, we show that, to achieve full efficacy, icilin requires simultaneous elevation of cytosolic Ca2+, either via permeation through TRPM8 channels or by release from intracellular stores. Thus, two stimuli must be paired to elicit full channel activation, illustrating the potential for coincidence detection by TRP channels. Determinants of icilin sensitivity map to a region of TRPM8 that corresponds to the capsaicin binding site on the noxious heat receptor TRPV1, suggesting a conserved molecular logic for gating of these thermosensitive channels by chemical agonists.

In Vivo Light-induced and Basal Phospholipase C Activity in Drosophila Photoreceptors Measured with Genetically Targeted Phosphatidylinositol 4,5-Bisphosphate-sensitive Ion Channels (Kir2.1)

The phosphatidylinositol 4,5-bisphosphate (PIP(2))-sensitive inward rectifier channel Kir2.1 was expressed in Drosophila photoreceptors and used to monitor in vivo PIP(2) levels. Since the wild-type (WT) Kir2.1 channel appeared to be saturated by the prevailing PIP(2) concentration, we made a single amino acid substitution (R228Q), which reduced the effective affinity for PIP(2) and yielded channels generating currents proportional to the PIP(2) levels relevant for phototransduction. To isolate Kir2.1 currents, recordings were made from mutants lacking both classes of light-sensitive transient receptor potential channels (TRP and TRPL). Light resulted in the effective depletion of PIP(2) by phospholipase C (PLC) in approximately three or four microvilli per absorbed photon at rates exceeding approximately 150% of total microvillar phosphoinositides per second. PIP(2) was resynthesized with a half-time of approximately 50 s. When PIP(2) resynthesis was prevented by depriving the cell of ATP, the Kir current spontaneously decayed at maximal rates representing a loss of approximately 40% loss of total PIP(2) per minute. This loss was attributed primarily to basal PLC activity, because it was greatly decreased in norpA mutants lacking PLC. We tried to confirm this by using the PLC inhibitor U73122; however, this was found to act as a novel inhibitor of the Kir2.1 channel. PIP(2) levels were reduced approximately 5-fold in the diacylglycerol kinase mutant (rdgA), but basal PLC activity was still pronounced, consistent with the suggestion that raised diacylglycerol levels are responsible for the constitutive TRP channel activity characteristic of this mutant.

Rolling blackout, a newly identified PIP2-DAG pathway lipase required for Drosophila phototransduction

The rolling blackout (rbo) gene encodes an integral plasma membrane lipase required for Drosophila phototransduction. Photoreceptors are enriched for the RBO protein, and temperature-sensitive rbo mutants show reversible elimination of phototransduction within minutes, demonstrating an acute requirement for the protein. The block is activity dependent, indicating that the action of RBO is use dependent. Conditional rbo mutants show activity-dependent depletion of diacylglycerol and concomitant accumulation of phosphatidylinositol phosphate and phosphatidylinositol 4,5-bisphosphate within minutes of induction, suggesting rapid downregulation of phospholipase C (PLC) activity. The RBO requirement identifies an essential regulatory step in G-protein-coupled, PLC-dependent inositol lipid signaling mediating activation of TRP and TRPL channels during phototransduction.

P2Y-mediated Ca2+ response is spatiotemporally graded and synchronized in sensory neurons: a two-photon photolysis study

ATP is thought to be an initiator and modulator of noxious pain sensation. We employed two-photon photolysis to apply ATP locally and transiently, thus mimicking ATP release upon cell damage or exocytosis. Using this technique, an increase in intracellular Ca2+ concentration ([Ca2+]i) was induced via P2Y receptors in individual sensory neurons, or in a neurite region. The ATP-induced [Ca2+]i rise was attenuated by applications of either a phospholipase C inhibitor, or inhibitors for IP3 or ryanodine receptors. These results indicate that intracellular Ca2+ stores play a major role in contributing to the increase in [Ca2+]i. Spatiotemporal analysis revealed that local and transient applications of ATP increased [Ca2+]i by release from intracellular stores, but in a unique, graded, and synchronized manner. 1) As the duration of local ATP application was prolonged, the latency decreased and the magnitude of the [Ca2+]i rise increased; 2) The time course of the rising phase of the [Ca2+]i response to ATP was essentially the same over the cell body, once [Ca2+]i had started to rise. It is anticipated that sensory responses may be modulated variably, depending on the spatiotemporal characteristics of the ATP-related [Ca2+]i profile.

Receptor-operated cation entry--more than esoteric terminology?

Many hormones and neurotransmitters elicit an increase in the intracellular calcium concentration by binding to phospholipase C-linked G protein-coupled receptors. Activated receptors signal to calcium-permeable cation channels in the plasma membrane, which are distinct from those engaged by emptying of intracellular stores of calcium. The TRPC family of the mammalian homologs of the Drosophila transient receptor potential (TRP) cation channel represents likely molecular correlates underlying receptor-operated cation entry. While all TRPC family members are gated in a phospholipase C-dependent manner, the exact activation mechanism still remains elusive, although lipids such as diacylglycerol and polyunsaturated fatty acids are potential diffusible messengers. Functional TRPC channel complexes in the plasma membrane are thought to be composed of four distinct subunits whose stoichiometry and composition under physiological conditions are still largely unknown. However, recent progress in defining the combinatorial rules of TRPC channel assembly may lead to the identification of TRPC-dependent ion fluxes in living cells. Because of the large number of TRP proteins and their frequently overlapping functional characteristics, the central question is whether TRP proteins are functionally interchangeable or whether unique physiological roles can be ascribed to them. Receptor-operated cation entry is critically involved in the control of airway and vascular smooth muscle tone; hence, TRPC proteins are promising new drug targets.

Regulation of calcium signaling by polycystin-2

Autosomal dominant PKD (ADPKD) is a common lethal genetic disorder characterized by progressive development of fluid-filled cysts in the kidney and other target organs. ADPKD is caused by mutations in the PKD1 and PKD2 genes, encoding the transmembrane proteins polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Although the function and putative interacting ligands of PC1 are largely unknown, recent evidence indicates that PC2 behaves as a TRP-type Ca2+-permeable nonselective cation channel. The PC2 channel is implicated in the transient increase in cytosolic Ca2+in renal epithelial cells and may be linked to the activation of subsequent signaling pathways. Recent studies also indicate that PC1 functionally interacts with PC2 such that the PC1-PC2 channel complex is an obligatory novel signaling pathway implicated in the transduction of environmental signals into cellular events. The present review purposely avoids issues of regulation of PC2 expression and trafficking and focuses instead on the evidence for the TRP-type cation channel function of PC2. How its role as a cation channel may unmask mechanisms that trigger Ca2+transport and regulation is the focus of attention. PC2 channel function may be essential in renal cell function and kidney development. Nonrenal-targeted expression of PC2 and related proteins, including the cardiovascular system, also suggests previously unforeseeable roles in signal transduction.

Activation and sensitisation of the vanilloid receptor: role in gastrointestinal inflammation and function

The exquisite specific excitatory and desensitising actions of capsaicin on a subpopulation of primary sensory neurons have been instrumental in identifying the roles of these neurons in nociception, reflex responses and neurogenic inflammation. Structure activity studies with capsaicin-like molecules have suggested that a "receptor" should mediate the effects of capsaicin on sensory neurons. The cloning of the vanilloid receptor-1 (VR1) has confirmed this hypothesis. VR1 (TRPV1) belongs to the transient receptor potential (TRP) family of channels, and its activation by various xenobiotics, noxious temperature, extracellular low pH and high concentration of certain lipid derivatives results in cation influx and sensory nerve terminal excitation. TRPV1 may dimerise or form tetramers or heteromers with PLC-gamma and TrkA or even with other TRPs. TRPV1 is markedly upregulated and/or "sensitised" under inflammatory conditions via protein kinase C-epsilon-, cAMP-dependent PK- and PLC-gamma-dependent pathways or by exposure to dietary agents as ethanol. TRPV1 is expressed on sensory neurons distributed in all the regions of the gastrointestinal tract in myenteric ganglia, muscle layer and mucosa. There is evidence of TRPV1 expression also in epithelial cells of the gastrointestinal tract. High expression of TRPV1 has been detected in several inflammatory diseases of the colon and ileum, whereas neuropeptides released upon sensory nerve stimulation triggered by TRPV1 activation seem to play a role in intestinal motility disorders. TRPV1 antagonists, which will soon be available for clinical testing, may undergo scrutiny for the treatment of inflammatory diseases of the gut.

Activation of human TRPC6 channels by receptor stimulation

The human TRPC6 channel was expressed in human embryonic kidney (HEK) cells, and activity was monitored using the giga-seal technique. Whole cell membrane currents with distinctive inward and outward rectification were activated by carbachol (CCh) in TRPC6-expressing cells, but not in lacZ-transfected controls. The effect of CCh was steeply dose-dependent with a K(0.5) of approximately 10 microm and a Hill coefficient of 3-4. A steep concentration-response relationship was also observed when TRPC6 activity was measured using a fluorescence-based imaging plate reader (FLIPR) assay for membrane depolarization. Ionomycin, thapsigargin, and dialysis of the cell with inositol 1,4,5-trisphosphate via the patch pipette had no effect on TRPC6 currents, but exogenous application of 1-oleoyl acetyl-sn-glycerol (OAG, 30-300 microm) produced a slow increase in channel activity. The PKC activator, phorbol 12-myristate 13-acetate (PMA, 0.5 microm) had no significant acute effect on TRPC6, or on the subsequent response to OAG. In contrast, the response to CCh was blocked >90% by PMA pretreatment. To further explore the role of DAG in receptor stimulation, TRPC6 currents were monitored following the sequential addition of CCh and OAG. Surprisingly, concentrations of CCh that produced little or no response in the absence of OAG, produced increases in TRPC6 currents in the presence of OAG that were larger than the sum of either agent alone. Likewise, the response to OAG was superadditive following prior stimulation of the cells with near threshold concentrations of CCh. Overall, these results suggest that generation of DAG alone may not fully account for activation of TRPC6, and that other receptor-mediated events act synergistically with DAG to stimulate channel activity. This synergy may explain, at least in part, the steep dose-response relationship observed for CCh-induced TRPC6 currents expressed in HEK cells.