Activation Of Gq-coupled Receptors Research Articles

Activation of Gq-Coupled Receptors in Astrocytes Restores Cognitive Function in Alzheimer's Disease Mice Model.

Alzheimer's disease (AD) is one of the most widespread neurodegenerative diseases. Most of the current AD therapeutic developments are directed towards improving neuronal cell function or facilitating Aβ amyloid clearance from the brain. However, some recent evidence suggests that astrocytes may play a significant role in the pathogenesis of AD. In this paper, we evaluated the effects of the optogenetic activation of Gq-coupled exogenous receptors expressed in astrocytes as a possible way of restoring brain function in the AD mouse model. We evaluated the effects of the optogenetic activation of astrocytes on long-term potentiation, spinal morphology and behavioral readouts in 5xFAD mouse model of AD. We determined that in vivo chronic activation of astrocytes resulted in the preservation of spine density, increased mushroom spine survival, and improved performance in cognitive behavioral tests. Furthermore, chronic optogenetic stimulation of astrocytes resulted in the elevation of EAAT-2 glutamate uptake transporter expression, which could be a possible explanation for the observed in vivo neuroprotective effects. The obtained results suggest that the persistent activation of astrocytes may be considered a potential therapeutic approach for the treatment of AD and possibly other neurodegenerative disorders.

Histamine/H1 receptor signaling in the parafacial region increases activity of chemosensitive neurons and respiratory activity in rats.

Histaminergic neurons of the tuberomammillary nucleus (TMN) are pH sensitive and contribute to CO2/H+-dependent behaviors including arousal and respiratory activity. TMN neurons project to several respiratory centers including the ventral parafacial region (pF), where the chemosensitive retrotrapezoid (RTN) neurons are located, and since RTN neurons are an important source of CO2/H+-dependent respiratory drive, we wondered whether histamine contributes to RTN chemoreception. To test this, we characterized effects of histamine on mean arterial pressure (MAP) and diaphragm muscle activity (DIAEMG) in urethane-anesthetized, vagotomized, and artificially ventilated male Wistar rats. Unilateral injection of histamine in the pF (25 mM) increased DIAEMG amplitude without changing DIAEMG frequency and MAP. Bilateral injections of the H1 receptor antagonist diphenhydramine hydrochloride (DPH; 0.5 mM) into the pF decreased baseline DIAEMG amplitude and frequency and MAP. Despite the strong inhibitory effect of DPH on baseline breathing, the hypercapnic ventilatory response was preserved under these experimental conditions. At the cellular level, chemosensitive RTN neurons showed a dose-dependent excitatory response to histamine that was blunted by DPH and mimicked by H1 receptor agonist 2-pyridylethylamine dihydrochloride (2PYEA) both under control conditions and when fast neurotransmitter receptors were blocked. We also tested effects of 2PYEA in the presence of serotonin, another wake-on neurotransmitter that activates RTN chemoreceptors partly by activation of Gq-coupled receptors. We found that the response to 2PYEA was diminished in serotonin, suggesting that RTN neurons have a limited capacity to respond to multiple Gq-coupled modulators. These results suggest that histamine can modulate breathing at the pF level by a mechanism involving H1 receptors.NEW & NOTEWORTHY Histamine/H1 receptor signaling activates retrotrapezoid (RTN) neurons under control conditions and to a lesser extent in the presence of serotonin. These results suggest that RTN neurons have a limited capacity to respond to simultaneous activation of multiple Gq-coupled receptors.

Gq neuromodulation of BLA parvalbumin interneurons induces burst firing and mediates fear-associated network and behavioral state transition in mice

Patterned coordination of network activity in the basolateral amygdala (BLA) is important for fear expression. Neuromodulatory systems play an essential role in regulating changes between behavioral states, however the mechanisms underlying this neuromodulatory control of transitions between brain and behavioral states remain largely unknown. We show that chemogenetic Gq activation and α1 adrenoreceptor activation in mouse BLA parvalbumin (PV) interneurons induces a previously undescribed, stereotyped phasic bursting in PV neurons and time-locked synchronized bursts of inhibitory postsynaptic currents and phasic firing in BLA principal neurons. This Gq-coupled receptor activation in PV neurons suppresses gamma oscillations in vivo and in an ex vivo slice model, and facilitates fear memory recall, which is consistent with BLA gamma suppression during conditioned fear expression. Thus, here we identify a neuromodulatory mechanism in PV inhibitory interneurons of the BLA which regulates BLA network oscillations and fear memory recall.

BRET-based effector membrane translocation assay monitors GPCR-promoted and endocytosis-mediated Gq activation at early endosomes

G protein-coupled receptors (GPCRs) are gatekeepers of cellular homeostasis and the targets of a large proportion of drugs. In addition to their signaling activity at the plasma membrane, it has been proposed that their actions may result from translocation and activation of G proteins at endomembranes-namely endosomes. This could have a significant impact on our understanding of how signals from GPCR-targeting drugs are propagated within the cell. However, little is known about the mechanisms that drive G protein movement and activation in subcellular compartments. Using bioluminescence resonance energy transfer (BRET)-based effector membrane translocation assays, we dissected the mechanisms underlying endosomal Gq trafficking and activity following activation of Gq-coupled receptors, including the angiotensin II type 1, bradykinin B2, oxytocin, thromboxane A2 alpha isoform, and muscarinic acetylcholine M3 receptors. Our data reveal that GPCR-promoted activation of Gq at the plasma membrane induces its translocation to endosomes independently of β-arrestin engagement and receptor endocytosis. In contrast, Gq activity at endosomes was found to rely on both receptor endocytosis-dependent and -independent mechanisms. In addition to shedding light on the molecular processes controlling subcellular Gq signaling, our study provides a set of tools that will be generally applicable to the study of G protein translocation and activation at endosomes and other subcellular organelles, as well as the contribution of signal propagation to drug action.

Dissociation of the G protein βγ from the Gq–PLCβ complex partially attenuates PIP2 hydrolysis

Phospholipase C β (PLCβ), which is activated by the Gq family of heterotrimeric G proteins, hydrolyzes the inner membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2), generating diacylglycerol and inositol 1,4,5-triphosphate (IP3). Because Gq and PLCβ regulate many crucial cellular processes and have been identified as major disease drivers, activation and termination of PLCβ signaling by the Gαq subunit have been extensively studied. Gq-coupled receptor activation induces intense and transient PIP2 hydrolysis, which subsequently recovers to a low-intensity steady-state equilibrium. However, the molecular underpinnings of this equilibrium remain unclear. Here, we explored the influence of signaling crosstalk between Gq and Gi/o pathways on PIP2 metabolism in living cells using single-cell and optogenetic approaches to spatially and temporally constrain signaling. Our data suggest that the Gβγ complex is a component of the highly efficient lipase GαqGTP–PLCβ–Gβγ. We found that over time, Gβγ dissociates from this lipase complex, leaving the less-efficient GαqGTP–PLCβ lipase complex and allowing the significant partial recovery of PIP2 levels. Our findings also indicate that the subtype of the Gγ subunit in Gβγ fine-tunes the lipase activity of Gq–PLCβ, in which cells expressing Gγ with higher plasma membrane interaction show lower PIP2 recovery. Given that Gγ shows cell- and tissue-specific subtype expression, our findings suggest the existence of tissue-specific distinct Gq–PLCβ signaling paradigms. Furthermore, these results also outline a molecular process that likely safeguards cells from excessive Gq signaling.

Abstract P277: Ceramide Modulates GPCR And TK Receptor Signaling To Impact Endothelial Functions And Blood Pressure

Ceramides are sphingolipids that modulate a variety of cellular processes, via two major modes of actions: by functioning as second messengers and by regulating the formation of lipid raft, important signaling platform. Receptor-mediated signaling orchestrates multiple pathophysiological processes, hence more than 50% of drugs clinically prescribed target mainly GPCRs. Alterations of ceramide levels are implicated in endothelial dysfunction, a key event in many cardiovascular diseases, including hypertension and atherosclerosis. However, specific molecular mechanisms of how ceramides modulate the structure and the function of GPCRs and TK receptors to impact endothelial functions remains unknown and unexplored. Thus, we generated mice lacking the endothelial sphingolipid de novo biosynthesis, by deleting the serine palmitoyltransferase long subunit 2, of the first enzyme of the pathway, named ECKO-Sptlc2. Systolic blood pressure was markedly increased in ECKO-Sptlc2 vs. control mice (122.1± 1.9 vs. 103.6±0.7 mmHg, n=9). Vasodilation of mesenteric arteries to acetylcholine and histamine (Gq-coupled receptors) and the NO-signaling downstream was preserved suggesting that altered membrane sphingolipid levels did not affect Gq-coupled receptor activation. On the contrary, vasodilation induced by the activation of tyrosine kinase receptors, i.e. VEGFR2 (Emax 32.8±3.5 vs. 55.7±3.6 % vasodilation, n=5), Gi-coupled GPCRs, i.e. S1PR1 (Emax 9.0±11.0 vs. 58.7 ±3.5 % vasodilation, n=5) and flow were markedly reduced. C16-, C24- and C24:1-ceramide are the most abundant ceramides in the endothelium and were markedly reduced by the loss of Sptlc2. Interestingly, the treatment of the mice and endothelial cells in culture with C16-, C24- and C24:1-ceramides, showed that C16-ceramide play a specific role in VEGFR2, IR, S1PR1 and flow mediated vasodilation. Mechanistically, C16-ceramide modulates VEGFR2 phosphorylation and downstream signaling in a concentration-dependent manner. The finding that C16-ceramide affects specific the function of Gi-coupled, TK but not Gq-coupled receptor is of great significance considering that alterations in C16-ceramide strongly correlate with CV conditions, such as CAD and heart failure.

Fatty-acyl chain profiles of cellular phosphoinositides

Phosphoinositides are rapidly turning-over phospholipids that play key roles in intracellular signaling and modulation of membrane effectors. Through technical refinements we have improved sensitivity in the analysis of the phosphoinositide PI, PIP, and PIP2 pools from living cells using mass spectrometry. This has permitted further resolution in phosphoinositide lipidomics from cell cultures and small samples of tissue. The technique includes butanol extraction, derivatization of the lipids, post-column infusion of sodium to stabilize formation of sodiated adducts, and electrospray ionization mass spectrometry in multiple reaction monitoring mode, achieving a detection limit of 20pg. We describe the spectrum of fatty-acyl chains in the cellular phosphoinositides. Consistent with previous work in other mammalian primary cells, the 38:4 fatty-acyl chains dominate in the phosphoinositides of the pineal gland and of superior cervical ganglia, and many additional fatty acid combinations are found at low abundance. However, Chinese hamster ovary cells and human embryonic kidney cells (tsA201) in culture have different fatty-acyl chain profiles that change with growth state. Their 38:4 lipids lose their dominance as cultures approach confluence. The method has good time resolution and follows well the depletion in <20s of both PIP2 and PIP that results from strong activation of Gq-coupled receptors. The receptor-activated phospholipase C exhibits no substrate selectivity among the various fatty-acyl chain combinations.

Molecular Aspects of Modulation of L-type Calcium Channels by Protein Kinase C.

Ca(2+) influx via L-type Ca(2+) channel (L-VDCC; CaV1.2) is required for cardiac and smooth muscle contraction. These channels are located in the plasma membrane and along the T-tubules (in cardiomyocytes), along with various scaffold and trafficking proteins. CaV1.2 is modulated by different hormones and transmitters and was implicated in a variety of cardiovascular pathologies, many of which also involve protein kinase C (PKC). One of the prominent pathways of PKC activation in cardiac and smooth muscle cells is via activation of Gq-coupled receptors and subsequent activation of protein lipase C (PLC). CaV1.2 was shown to be modulated, phosphorylated by, and associated with PKC both in vitro and in vivo. Despite the well documented enhancing effect of vasoconstrictors operating via Gq on CaV1.2 channels, the molecular mechanism by which PKC affects the channel has not yet been resolved. Furthermore, the nature of PKC modulation of CaV1.2 appears to be species-, age- and tissue-dependent. Results from experiments in heterologous expression systems are often contradicting and are difficult to coalesce. The choice of both the heterologous expression system and the CaV1.2 isoform expressed are at the core of this conundrum. Complete reconstitution of the enhancing effect of PKC was successful only in Xenopus oocytes and only when the long N-terminus (NT) isoform of the channel was expressed. This review summarizes past and new findings regarding the mechanism by which activated PKC modulates CaV1.2 channels in native tissues and heterologous expression systems, and suggests perspectives for future research.

M-current preservation contributes to anticonvulsant effects of valproic acid.

Valproic acid (VPA) has been widely used for decades to treat epilepsy; however, its mechanism of action remains poorly understood. Here, we report that the anticonvulsant effects of nonacute VPA treatment involve preservation of the M-current, a low-threshold noninactivating potassium current, during seizures. In a wide variety of neurons, activation of Gq-coupled receptors, such as the m1 muscarinic acetylcholine receptor, suppresses the M-current and induces hyperexcitability. We demonstrated that VPA treatment disrupts muscarinic suppression of the M-current and prevents resultant agonist-induced neuronal hyperexcitability. We also determined that VPA treatment interferes with M-channel signaling by inhibiting palmitoylation of a signaling scaffold protein, AKAP79/150, in cultured neurons. In a kainate-induced murine seizure model, administration of a dose of an M-channel inhibitor that did not affect kainate-induced seizure transiently eliminated the anticonvulsant effects of VPA. Retigabine, an M-channel opener that does not open receptor-suppressed M-channels, provided anticonvulsant effects only when administered prior to seizure induction in control animals. In contrast, treatment of VPA-treated mice with retigabine induced anticonvulsant effects even when administered after seizure induction. Together, these results suggest that receptor-induced M-current suppression plays a role in the pathophysiology of seizures and that preservation of the M-current during seizures has potential as an effective therapeutic strategy.

The initiation of synaptic 2-AG mobilization requires both an increased supply of diacylglycerol precursor and increased postsynaptic calcium

On-demand postsynaptic synthesis and release of endocannabinoid lipids and subsequent binding to presynaptic CB1 receptors (CB1Rs) mediates short and long-term depression (LTD) of excitatory transmission in many brain regions. However, mechanisms involved in the synthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG) by diacylglycerol lipase α (DGLα) are poorly understood. Since Gq-coupled receptor activation can stimulate production of a major DGL substrate 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG) by PLCβ, we sought to determine if 2-AG biosynthesis was limited only by a lack of substrate availability, or if other pathways, such as Ca2+ signaling, also need to be simultaneously engaged. To address this question, we loaded medium spiny neurons of the dorsolateral striatum with SAG while monitoring excitatory synaptic inputs. SAG-loading had no significant effect on evoked excitatory synaptic currents when cells were voltage-clamped at −80 mV. However, depolarization of MSNs to −50 mV revealed a SAG-loading dependent decrease in the amplitude of excitatory currents that was accompanied by an increase in paired pulse ratio, consistent with decreased glutamate release. Both effects of loading SAG at −50 mV were blocked by chelation of postsynaptic Ca2+ using BAPTA or by bath application of tetrahydrolipstatin (THL), a DGL inhibitor. Loading of SAG into glutamatergic pyramidal neurons of the amygdala similarly inhibited excitatory synaptic inputs and increased the PPR. SAG-induced depression was absent in both regions from mice lacking CB1Rs. These data show that increasing substrate availability alone is insufficient to drive 2-AG mobilization and that DGL-dependent synaptic depression via CB1R activation requires postsynaptic Ca2+ signals.

Differential inhibition of the TRPM8 ion channel by Gαq and Gα11

Cold temperature is encoded by the cold-sensitive ion channel TRPM8 in somatosensory neurons. It has been unclear how TRPM8 is modulated so that it can mediate distinct type of cold signaling. We have recently reported that activated Gαq directly inhibits TRPM8 after activation of Gq-coupled receptors. Here, we further show that activation of the muscarinic receptor M1R, which is known to inhibit M currents through PLCβ-mediated hydrolysis of PtdIns(4,5)P2, similarly inhibited TRPM8 potently, but inhibition was not prevented by the PLC inhibitor U73122. Interestingly, although Gαq and Gα11 are indistinguishable in activating PLCβ and hydrolysing PtdIns(4,5)P2, activated Gα11 inhibited TRPM8 to a lesser extent than activated Gαq. The differential TRPM8 inhibition is determined by a specific residue E197 on Gα11, because mutating this residue to the corresponding residue on Gαq restored TRPM8 inhibition to a similar degree as mediated by Gαq. These results reinforce the idea that activated Gαq directly inhibits TRPM8 independently from PtdIns(4,5)P2 hydrolysis-mediated inhibition of TRPM8.

CREB mediates Gq-coupled receptor activation of Vmp1, a switch for autophagy in pancreas biology and diseases

CREB mediates Gq-coupled receptor activation of Vmp1, a switch for autophagy in pancreas biology and diseases

Type 2 inositol 1,4,5-trisphosphate receptor is predominantly involved in agonist-induced Ca2+ signaling in Bergmann glia

Ca2+ release via inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) plays a crucial role in astrocyte functions such as modulation of neuronal activity and regulation of local blood flow in the cerebral cortex and hippocampus. Bergmann glia are unipolar cerebellar astrocytes that release Ca2+ through IP3Rs in response to the activation of Gq-coupled receptors. The composition of the three subtypes of IP3R is a factor that determines the spatiotemporal pattern of Ca2+ release. However, the functional expression of IP3R subtypes and their contribution to Ca2+ release in Bergmann glia remain controversial. In this study, we first characterized the Ca2+ response in Bergmann glia to noradrenaline and histamine stimulation in organotypic cultures of the mouse cerebellum using a Ca2+ indicator, Inverse-Pericam, and found that Bergmann glial processes exhibit a higher agonist-induced Ca2+ indicator response than the soma. Furthermore, we performed Ca2+ imaging using mutant mice lacking each IP3R subtype. This revealed that Bergmann glia lacking type 2 IP3R exhibited reduced responses to noradrenaline or histamine compared with wild-type Bergmann glia and Bergmann glia with other genotypes, suggesting that type 2 IP3R is the major functional IP3R subtype involved in agonist-induced Ca2+ release in Bergmann glia, although types 1 and 3 IP3R could also contribute to rapid agonist-induced [Ca2+]i elevation in the processes.

Phosphoinositide Signaling Regulates the Surface Localization of the δ2 Ionotropic Glutamate Receptor

The δ2 glutamate receptor (GluD2) is a member of the ionotropic glutamate receptor family. Since GluD2 remains an orphan receptor, it has not been shown to conduct ions. It is highly expressed in the parallel fiber-Purkinje cell (PF-PC) synapse and its role in cerebellar physiology is increasingly appreciated. GluD2 has been linked to the induction of cerebellar long-term depression (LTD) and presynaptic terminal differentiation. A single point mutation in the second transmembrane domain (A654T), named Lurcher (GluD2Lc), confers constitutive activity to the receptor.We have used the Lurcher mutant as a model to examine whether phosphoinositides regulate GluD2. We show that decreasing PIP2 levels in the membrane, through Gq-coupled receptor activation or pretreatment with a PI4K inhibitor, potentiates GluD2Lc currents. Conversely, increasing PIP2 levels by co-expressing PIP5K leads to decreased GluD2Lc currents. On the other hand, utilizing PI3K inhibitors wortmannin or LY294002, and presumably decreasing PIP3 levels, reduces GluD2Lc currents, while co-expression of PI3K leads to potentiation of GluD2Lc currents.A chemiluminescence-based assay that quantifies surface localization of the GluD2 and GluD2Lc receptors, showed that manipulations of the membrane phosphoinositide levels evoke changes in the cell surface localization of both wild-type and mutant receptors. These changes in surface localization of the receptor correspond to the effects we have observed by monitoring GluD2Lc currents, suggesting that current measurements from this mutant receptor serve as a good reporter for the localization of the wild-type δ2 receptor. These results are consistent with the interpretation that increased PIP2 levels decrease, while increased PIP3 levels increase localization of the receptor at the cell surface. Signals that affect the levels of these phosphoinositides simultaneously are likely to regulate the surface localization of GluD2 based on the net change of the two opposing effects.

Membrane Depolarization Increases Membrane PtdIns(4,5)P2 Levels through Mechanisms Involving PKC βII and PI4 Kinase

In a previous study, we showed that membrane depolarization induced elevation of membrane phosphatidylinositol 4,5-bisphosphates (PtdIns(4,5)P(2), also known as PIP(2)) and subsequently increased the KCNQ2/Q3 currents expressed in Xenopus oocytes through increased PI4 kinase activity. In this study, the underlying mechanism for this depolarization-induced enhancement of PIP(2) synthesis was further investigated. Our results indicate that activation of protein kinase C (PKC) isozyme βII was responsible for the enhanced PIP(2) synthesis. We found that phorbol-12-myristate, 13-acetate (PMA), an activator of PKC, mimicked the effects of the membrane depolarization by increasing KCNQ2/Q3 activity, elevating membrane PIP(2) levels and increasing activity of PI4 kinase β. Furthermore, membrane depolarization enhanced PKC activity. The effects of both depolarization and PMA were blocked by a PKC inhibitor or PI4 kinase β RNA interference. Further results demonstrate that the depolarization selectively activated the PKC βII isoform and enhanced its interaction with PI4 kinase β. These results reveal that the depolarization-induced elevation of membrane PIP(2) is through activation of PKC and the subsequent increased activity of PI4 kinase β.

G PROTEIN‐COUPLED RECEPTORS

The invention provides human G-protein coupled receptors (GCREC) and polynucleotides which identify and encode GCREC. The invention also provides expression vectors, host cells, antibodies, agonists, and antagonists. The invention also provides methods for diagnosing, treating, or preventing disorders associated with aberrant expression of GCREC.

Monitoring Gq-coupled receptor response through inositol phosphate quantification with the IP-One assay

Introduction:G-protein-coupled receptors (GPCRs) are transmembrane proteins that play a key role in the signal transduction of extracellular stimuli. GPCRs associate to a complex assembly of intracellular proteins regulating a large variety of signaling pathways. In particular, the production of inositol 1,4,5 triphosphate (IP3) signs the activation of Gq-coupled receptors. However, its very short half-life makes its assessment too challenging for drug screening operations and the monitoring of calcium release, triggered by IP3, has been extensively used as a downstream readout of this signaling pathway. Recently, a new homogeneous time-resolved fluorescence (HTRF) assay, detecting a downstream metabolite of IP3, inositol monophosphate (IP1), has overcome the drawbacks of the IP3 quantification, allowing its use in primary or secondary screening.Areas covered:This review provides an overview of the use of the IP-One assay in screening processes, providing comparisons with results obtained with other existing techniques traditionally used to investigate Gq-coupled receptors. Moreover, the review highlights two key features of the IP-One assay, the discrimination of slow acting compounds and the characterization of inverse agonists, which are impossible to achieve using calcium release.Expert opinion:The IP-One assay is well established to perform screening in the pharmaceutical industry. A number of criteria can be taken into account, including the impact of the sensitivity improvement of the assay, to position the IP-One assay in the different stages of the drug screening process. Moreover, the IP-One assay can be used as a valuable solution to investigate new research concepts such as ligand-biased signaling or receptor heteromerization.

Probing the regulation of TASK potassium channels by PI(4,5)P2 with switchable phosphoinositide phosphatases

TASK channels are background K+ channels that contribute to the resting conductance in many neurons. A key feature of TASK channels is the reversible inhibition by Gq-coupled receptors, thereby mediating the dynamic regulation of neuronal activity by modulatory transmitters. The mechanism that mediates channel inhibition is not fully understood. While it is clear that activation of Gαq is required, the immediate signal for channel closure remains controversial. Experimental evidence pointed to either phospholipase C (PLC)-mediated depletion of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) as the cause for channel closure or to a direct inhibitory interaction of active Gαq with the channel. Here, we address the role of PI(4,5)P2 for G-protein-coupled receptor (GPCR)-mediated TASK inhibition by using recently developed genetically encoded tools to alter phosphoinositide (PI) concentrations in the living cell.When expressed in CHO cells, TASK-1- and TASK-3-mediated currents were not affected by depletion of plasma membrane PI(4,5)P2 either via the voltage-activated phosphatase Ci-VSP or via chemically triggered recruitment of a PI(4,5)P2-5'-phosphatase. Depletion of both PI(4,5)P2 and PI(4)P via membrane recruitment of a novel engineered dual-specificity phosphatase also did not inhibit TASK currents. In contrast, each of these methods produced robust inhibition of the bona fide PI(4,5)P2-dependent channel KCNQ4. Efficient depletion of PI(4,5)P2 and PI(4)P was further confirmed with a fluorescent phosphoinositide sensor. Moreover, TASK channels recovered normally from inhibition by co-expressed muscarinic M1 receptors when resynthesis of PI(4,5)P2 was prevented by depletion of cellular ATP. These results demonstrate that TASK channel activity is independent of phosphoinositide concentrations within the physiological range. Consequently, Gq-mediated inhibition of TASK channels is not mediated by depletion of PI(4,5)P2.

The Lurcher Mutant of δ2 Ionotropic Glutamate Receptor is Regulated by Phosphoinositides

The δ2 glutamate receptor (GluD2) is an orphan glutamate-like receptor, since no specific ligand capable of activating it has been identified. Nevertheless, it is increasingly appreciated that GluD2 is an important part of the physiology of the cerebellum. It is highly expressed in the parallel fiber-Purkinje cell (PF-PC synapse) and it has been linked to presynaptic terminal differentiation and the induction of long-term depression (LTD). Although, the wild-type receptor is non-conductive, a mutation in the third transmembrane domain (A654T), named lurcher (GluD2Lc), exhibits constitutive activity. Motivated by our previous work showing that NMDA receptor activity was dependent on phosphoinositides (PIPs) indirectly through the PIP2-sensitive and NMDA receptor-associated protein a-actinin, we asked whether GluD2 behavior was also dependent on PIPs and used GluD2Lc as our model to address this question.Our results in X. laevis oocytes have shown that GluD2Lc currents were potentiated upon activation of Gq-coupled receptors (M1R and mGluR1). Pretreatment with the PI4K inhibitor phenylarsine oxide (PAO) led to potentiated GluD2Lc currents as well. Moreover, pretreatment with the PI3K inhibitors wortmannin (at low μM concentrations) and LY294002 inhibited the GluD2Lc current. GluD2Lc co-expression with PIP5K or PI3K yielded consistent results with PIP5K inhibiting, while PI3K stimulating activity. These results are consistent with a dependence of GluD2Lc activity on PIPs.In order to distinguish between direct versus indirect phosphoinositide effects on GluD2 we are pursuing two strategies: 1) we are testing effects of PIPs on the activity of GluD2Lc in inside-out macropatches, and 2) we utilize site-directed mutagenesis to probe for critical residues in the proximal C-terminus that alter GluD2Lc dependence on PIPs. The proximal C-terminus could be responsible for direct interactions with phosphoinositides but could also couple to PIPs indirectly through PIP-sensitive proteins that interact with GluD2.

Diacylglycerol Signaling Underlies Astrocytic ATP Release

Astrocytes have the ability to modulate neuronal excitability and synaptic transmission by the release of gliotransmitters. The importance of ATP released downstream of the activation of Gq-coupled receptors has been well established, but the mechanisms by which this release is regulated are unclear. The current work reveals that the elevation of diacylglycerol (DAG) in astrocytes induces vesicular ATP release. Unexpectedly, DAG-induced ATP release was found to be independent of PKC activation, but dependent upon activation of a C1 domain-containing protein. Astrocytes express the C1 domain-containing protein Munc13-1, which has been implicated in neuronal transmitter release, and RNAi-targeted downregulation of Munc13-1 inhibits astrocytic ATP release. These studies demonstrate that elevations of DAG induce the exocytotic release of ATP in astrocytes, likely via a Munc13-1-dependent mechanism.