Hormone signalling via G-protein: regulation of phosphatidylinositol 4,5-bisphosphate hydrolysis by G q

Enhanced membrane binding of oncogenic G protein αqQ209L confers resistance to inhibitor YM-254890

Heterotrimeric G proteins couple activated G protein–coupled receptors (GPCRs) to intracellular signaling pathways. They can also function independently of GPCR activation upon acquiring mutations that prevent GTPase activity and result in constitutive signaling, as occurs with the αqQ209L mutation in uveal melanoma. YM-254890 (YM) can inhibit signaling by both GPCR-activated WT αq and GPCR-independent αqQ209L. Although YM inhibits WT αq by binding to αq-GDP and preventing GDP/GTP exchange, the mechanism of YM inhibition of cellular αqQ209L remains to be fully understood. Here, we show that YM promotes a subcellular redistribution of αqQ209L from the plasma membrane (PM) to the cytoplasm. To test if this loss of PM localization could contribute to the mechanism of inhibition of αqQ209L by YM, we developed and examined N-terminal mutants of αqQ209L, termed PM-restricted αqQ209L, in which the addition of membrane-binding motifs enhanced PM localization and prevented YM-promoted redistribution. Treatment of cells with YM failed to inhibit signaling by these PM-restricted αqQ209L. Additionally, pull-down experiments demonstrated that YM promotes similar conformational changes in both αqQ209L and PM-restricted αqQ209L, resulting in increased binding to βγ and decreased binding to regulator RGS2, and effectors p63RhoGEF-DH/PH and phospholipase C-β. GPCR-dependent signaling by PM-restricted WT αq is strongly inhibited by YM, demonstrating that resistance to YM inhibition by membrane-binding mutants is specific to constitutively active αqQ209L. Together, these results indicate that changes in membrane binding impact the ability of YM to inhibit αqQ209L and suggest that YM contributes to inhibition of αqQ209L by promoting its relocalization.

Cannabinoids in the descending pain modulatory circuit: Role in inflammation

The legalization of cannabis in some states has intensified interest in the potential for cannabis and its constituents to lead to novel therapeutics for pain. Our understanding of the cellular mechanisms underlying cannabinoid actions in the brain have lagged behind opioids; however, the current opioid epidemic has also increased attention on the use of cannabinoids as alternatives to opioids for pain, especially chronic pain that requires long-term use. Endogenous cannabinoids are lipid signaling molecules that have complex roles in modulating neuronal function throughout the brain. In this review, we discuss cannabinoid functions in the descending pain modulatory pathway, a brain circuit that integrates cognitive and emotional processing of pain to modulate incoming sensory inputs. In addition, we highlight areas where further studies are necessary to understand cannabinoid regulation of descending pain modulation.

DGKι regulates presynaptic release during mGluR-dependent LTD

Diacylglycerol (DAG) is an important lipid second messenger. DAG signalling is terminated by conversion of DAG to phosphatidic acid (PA) by diacylglycerol kinases (DGKs). The neuronal synapse is a major site of DAG production and action; however, how DGKs are targeted to subcellular sites of DAG generation is largely unknown. We report here that postsynaptic density (PSD)-95 family proteins interact with and promote synaptic localization of DGKι. In addition, we establish that DGKι acts presynaptically, a function that contrasts with the known postsynaptic function of DGKζ, a close relative of DGKι. Deficiency of DGKι in mice does not affect dendritic spines, but leads to a small increase in presynaptic release probability. In addition, DGKι-/- synapses show a reduction in metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) at neonatal (∼2 weeks) stages that involve suppression of a decrease in presynaptic release probability. Inhibition of protein kinase C normalizes presynaptic release probability and mGluR-LTD at DGKι-/- synapses. These results suggest that DGKι requires PSD-95 family proteins for synaptic localization and regulates presynaptic DAG signalling and neurotransmitter release during mGluR-LTD.

Regulation of hippocampal long-term potentiation and long-term depression by diacylglycerol kinase ζ

Diacylglycerol (DAG) is an important signaling molecule at neuronal synapses. Generation of synaptic DAG is triggered by the activation of diverse surface receptors including N-methyl-D-aspartate (NMDA) receptors and metabotropic glutamate receptors. The action of DAG is terminated by enzymatic conversion of DAG to phosphatidic acid (PA) by DAG kinases (DGKs). DGKζ, one of many mammalian DGKs, is localized to synapses through direct interaction with the postsynaptic scaffolding protein PSD-95, and regulates dendritic spine maintenance by promoting DAG-to-PA conversion. However, a role for DGKζ in the regulation of synaptic plasticity has not been explored. We report here that Schaffer collateral-CA1 pyramidal synapses in the hippocampus of DGKζ-knockout (DGKζ(-/-) ) mice show enhanced long-term potentiation (LTP) and attenuated long-term depression (LTD). The attenuated LTD at DGKζ(-/-) synapses involves both NMDA receptors and metabotropic glutamate receptors. These changes in LTP and LTD were reversed by phospholipase C inhibition, which blocks DAG production. Similar reversals in both LTP and LTD were also induced by inhibition of protein kinase C, which acts downstream of DAG. These results suggest that DGKζ regulates hippocampal LTP and LTD by promoting DAG-to-PA conversion, and establish that phospholipase C and protein kinase C lie upstream and downstream, respectively, of DGKζ-dependent regulation of hippocampal LTP and LTD.

Synaptic removal of diacylglycerol by DGKzeta and PSD-95 regulates dendritic spine maintenance

Diacylglycerol (DAG) is an important lipid signalling molecule that exerts an effect on various effector proteins including protein kinase C. A main mechanism for DAG removal is to convert it to phosphatidic acid (PA) by DAG kinases (DGKs). However, it is not well understood how DGKs are targeted to specific subcellular sites and tightly regulates DAG levels. The neuronal synapse is a prominent site of DAG production. Here, we show that DGKzeta is targeted to excitatory synapses through its direct interaction with the postsynaptic PDZ scaffold PSD-95. Overexpression of DGKzeta in cultured neurons increases the number of dendritic spines, which receive the majority of excitatory synaptic inputs, in a manner requiring its catalytic activity and PSD-95 binding. Conversely, DGKzeta knockdown reduces spine density. Mice deficient in DGKzeta expression show reduced spine density and excitatory synaptic transmission. Time-lapse imaging indicates that DGKzeta is required for spine maintenance but not formation. We propose that PSD-95 targets DGKzeta to synaptic DAG-producing receptors to tightly couple synaptic DAG production to its conversion to PA for the maintenance of spine density.

Effect of hydrogen peroxide on VIP-induced relaxation of the cat lower esophageal sphincter

We investigated the effects of hydrogen peroxide (H2O2) on relaxation of the cat lower esophageal sphincter (LES). Vasoactive intestinal peptide (VIP) caused dose-dependent relaxation of LES, and H2O2 reduced VIP-induced relaxation. Relaxation was also attenuated by pertussis toxin (PTX), indicating a Gi/o component. VIP treatment increased [35S]GTPgammaS binding to Gs and Gi3 protein, but not to Go, Gq, Gil or Gi2. This increase in Gs or Gi3 binding was reduced by H2O2. However, the relaxation induced by sodium nitroprusside (SNP), 3-morpholino sydnomine (SIN-1), 8-br cGMP (cGMP analog), forskolin (adenylate cyclase activator), and dibutyryl-cAMP (a stable cAMP analog) was not reduced by H2O2. These data suggest that H202 inhibits VIP-induced relaxation via a Gi-dependent pathway, perhaps by inhibiting the activation of G(i3) or Gs downstream of the VIP receptor and independent of cAMP or NO-cGMP signaling.

Presynaptic modulation by endocannabinoids

Modulation of neurotransmitter release by G-protein-coupled receptors (GPCRs) is a prominent presynaptic mechanism for regulation of synaptic transmission. Activation of GPCRs located at the presynaptic terminal can decrease the probability of neurotransmitter release. This presynaptic depression involves activation of Gi/o-type G-proteins that mediate different inhibitory mechanisms, including inhibition of voltage-gated calcium channels, activation of potassium channels, and direct inhibition of the vesicle fusion process. A variety of neurotransmitters and modulatory agents can activate GPCRs that produce presynaptic depression. Among these are lipid metabolites that serve as agonists for GPCRs. The discovery of endocannabinoids and their cognate receptors, including the CB1 receptor, has stimulated intense investigation into the neurophysiological roles of these lipid metabolites. It is now clear that presynaptic depression is the major physiological role for the CB1 receptor. Endocannabinoids activate this receptor mainly via a retrograde signaling process in which these compounds are synthesized in and released from postsynaptic neuronal elements, and travel back to the presynaptic terminal to act on the CB1 receptor. This retrograde endocannabinoid modulation has been implicated in short-term synaptic depression, including suppression of excitatory or inhibitory transmission induced by postsynaptic depolarization and transient synaptic depression induced by activation of postsynaptic GPCRs during agonist treatment or synaptic activation. Endocannabinoids and the CB1 receptor also play a key role in one form of long-term synaptic depression (LTD) that involves a longlasting decrease in neurotransmitter release.

G proteins in signal transduction: the regulation of phospholipase C

The hydrolysis of phosphatidylinositol 4,5-bisphosphate by specific phospholipase C (PLC) enzymes produces two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) of the Gq subfamily activate the PLC beta 1 isoform of PLC. We have purified three isozymes of PLC beta: PLC beta 1 and PLC beta 3 from rat brain and PLC beta 2 from HL-60 cells. Whereas the beta 1 and beta 2 isozymes appear restricted to a few cell types, beta 3 is broadly distributed. Gq alpha (the alpha subunit of the Gq subfamily) can activate all three isoforms but PLC beta 2 is much less sensitive. Thus all three enzymes are potential effectors for pertussis toxin-insensitive regulation by hormones. The three beta isozymes can also be activated by purified beta gamma subunits. The PLC beta 3 isoform gives the greatest activation with beta gamma; PLC beta 1 is least responsive. The results indicate that all the known isoforms of mammalian PLC beta can be regulated at unique sites by both Gq alpha and beta gamma subunits. The effect of beta gamma subunits may provide a pathway for the regulation of PLC beta isozymes by pertussis toxin-sensitive G proteins or may indicate that the alpha subunit of Gq and its associated beta gamma both participate in regulation of the same phospholipase molecule.

Targeting of diacylglycerol degradation to M1 muscarinic receptors by beta-arrestins

Seven-transmembrane receptor (7TMR) signaling is transduced by second messengers such as diacylglycerol (DAG) generated in response to the heterotrimeric guanine nucleotide-binding protein Gq and is terminated by receptor desensitization and degradation of the second messengers. We show that beta-arrestins coordinate both processes for the Gq-coupled M1 muscarinic receptor. beta-Arrestins physically interact with diacylglycerol kinases (DGKs), enzymes that degrade DAG. Moreover, beta-arrestins are essential for conversion of DAG to phosphatidic acid after agonist stimulation, and this activity requires recruitment of the beta-arrestin-DGK complex to activated 7TMRs. The dual function of beta-arrestins, limiting production of diacylglycerol (by receptor desensitization) while enhancing its rate of degradation, is analogous to their ability to recruit adenosine 3',5'-monophosphate phosphodiesterases to Gs-coupled beta2-adrenergic receptors. Thus, beta-arrestins can serve similar regulatory functions for disparate classes of 7TMRs through structurally dissimilar enzymes that degrade chemically distinct second messengers.

Low mobility of phosphatidylinositol 4,5-bisphosphate underlies receptor specificity of Gq-mediated ion channel regulation in atrial myocytes

We have shown previously that cardiac G protein-gated inwardly rectifying K+ (GIRK) channels are inhibited by Gq protein-coupled receptors (GqPCRs) via phosphatidylinositol 4,5-bisphosphate (PIP2) depletion in a receptor-specific manner. To investigate the mechanism of receptor specificity, we examined whether the activation of GqPCRs induces localized PIP2 depletion. When we applied endothelin-1 to the bath, GIRK channel activities recorded in cell-attached patches were not changed, implying that PIP2 signal is not diffusible but is a localized signal. To test this possibility, we directly measured lateral diffusion by introducing fluorescence-labeled phosphoinositides to a small area of the membrane with patch pipettes. After pipettes were attached, phosphatidylinositol 4-monophosphate or phosphatidylinositol diffused rapidly to the entire membrane, whereas PIP2 was confined to the membrane patch inside the pipette. The confinement of PIP2 was disrupted after cytochalasin D treatment, suggesting that the cytoskeleton is responsible for the low mobility of PIP2. The diffusion coefficient (D) of PIP2 in the plasma membrane measured with the fluorescence recovery after photobleaching technique was 0.00039 microm2/s (n = 6), which is markedly lower than D of phosphatidylinositol (5.8 microm2/s, n = 5). Simulation of PIP2 concentration profiles by the diffusion model confirms that when D is small, the kinetics of PIP2 depletion at different distances from phospholipase C becomes similar to the characteristic kinetics of GIRK inhibition by different agonists. These results imply that PIP2 depletion is localized adjacent to GqPCRs because of its low mobility, and that spatial proximity of GqPCR and the target protein underlies the receptor specificity of PIP2-mediated signaling.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

Regulation of phospholipase C-delta by GTP-binding proteins-rhoA as an inhibitory modulator

The regulation of Phospholipase C (PLC)delta activity remains obscure. These studies show that PLCdelta1 activity is significantly enhanced by both guanosine thiotriphosphate (GTPgammaS) and Clostridium botulinum exoenzyme C3 (C3) but not by aluminium fluoride. C3 ADP ribosylated a 21-kDa protein in the PLCdelta1 preparation and Western blotting identified rhoA in these samples. RhoA acts as an inhibitory modulator of PLCdelta activity.

Constitutively active Gq-alpha stimulates prolactin promoter activity via a pathway involving Raf activity

We have investigated the ability of a constitutively active Gq-alpha mutant, Q209L-alpha q, to regulate target gene expression. Transient expression in GH3 pituitary cells of a rat proximal prolactin promoter-chloramphenicol acetyltransferase construct (-187)PRL-CAT, was stimulated by co-expression of Q209L alpha q, but not by wild-type alpha q. Q209L-alpha q stimulated expression of constructs driven by promoters for either rat prolactin or growth hormone, but not of a control construct driven by the thymidine kinase promoter. Thus, transcriptional effects of alpha q are specific both for the activated state of this G-alpha subunit and the promoter examined. Since both the prolactin and growth hormone promoters are activated by the pituitary cell-specific transcription factor Pit-1, we examined whether a Pit-1 binding site could direct a response to Q209L-alpha q. Two copies of prolactin promoter Pit-1 binding site 1P conferred upon a heterologous metallothionein promoter a response to Q209L-alpha q, implying an involvement of this site in the transcriptional action of Q209L-alpha q on the prolactin promoter. The phorbol ester activator of protein kinase C, 12-O-tetradecanoylphorbol-13-acetate, stimulated (-187)PRL-CAT activity, but opposed the action of Q209L-alpha q on activity of this PRL-CAT construct. Q209L-alpha q stimulation of (-187)PRL-CAT activity was inhibited by co-expression of a dominant negative Raf mutant, Raf-C4, but not by a point mutant of Raf-C4 with reduced inhibitory properties. These results imply that activated alpha q subunits can stimulate prolactin promoter activity via a pathway that involves a Pit-1 DNA binding site(s), is opposed by protein kinase C, and is mediated by a pathway in which Raf-1 kinase plays a role.

Regulation of phospholipase C by G proteins

Specific phospholipase C enzymes can hydrolyse phosphatidylinositol 4,5-bisphosphate into two products: inositol 1,4,5-trisphosphate, which regulates the release of intracellular calcium stores, and diacylglycerol, which can stimulate protein kinase C. A new group of G proteins, the Gq subfamily, have recently been shown to mediate the regulation of this activity by a variety of hormones. How do different members of this family modulate unique phospholipase C isozymes? What is the mechanism of this regulation? How might the Gq subfamily act to modulate other important second messenger pathways? The tools to answer these questions are being rapidly developed.