Cell type-specific differences in the coupling of recombinant mGlu1alpha receptors to endogenous G protein sub-populations

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

Pharmacological characterization of mGlu1 receptors in cerebellar granule cells reveals biased agonism

The majority of existing research on the function of metabotropic glutamate (mGlu) receptor 1 focuses on G protein-mediated outcomes. However, similar to other G protein-coupled receptors (GPCR), it is becoming apparent that mGlu1 receptor signaling is multi-dimensional and does not always involve G protein activation. Previously, in transfected CHO cells, we showed that mGlu1 receptors activate a G protein-independent, β-arrestin-dependent signal transduction mechanism and that some mGlu1 receptor ligands were incapable of stimulating this response. Here we set out to investigate the physiological relevance of these findings in a native system using primary cultures of cerebellar granule cells. We tested the ability of a panel of compounds to stimulate two mGlu1 receptor-mediated outcomes: (1) protection from decreased cell viability after withdrawal of trophic support and (2) G protein-mediated phosphoinositide (PI) hydrolysis. We report that the commonly used mGlu1 receptor ligands quisqualate, DHPG, and ACPD are completely biased towards PI hydrolysis and do not induce mGlu1 receptor-stimulated neuroprotection. On the other hand, endogenous compounds including glutamate, aspartate, cysteic acid, cysteine sulfinic acid, and homocysteic acid stimulate both responses. These results show that some commonly used mGlu1 receptor ligands are biased agonists, stimulating only a fraction of mGlu1 receptor-mediated responses in neurons. This emphasizes the importance of utilizing multiple agonists and assays when studying GPCR function.

Extracellular calcium modulates actions of orthosteric and allosteric ligands on metabotropic glutamate receptor 1α

Metabotropic glutamate receptor 1α (mGluR1α), a member of the family C G protein-coupled receptors, is emerging as a potential drug target for various disorders, including chronic neuronal degenerative diseases. In addition to being activated by glutamate, mGluR1α is also modulated by extracellular Ca(2+). However, the underlying mechanism is unknown. Moreover, it has long been challenging to develop receptor-specific agonists due to homologies within the mGluR family, and the Ca(2+)-binding site(s) on mGluR1α may provide an opportunity for receptor-selective targeting by therapeutics. In the present study, we show that our previously predicted Ca(2+)-binding site in the hinge region of mGluR1α is adjacent to the site where orthosteric agonists and antagonists bind on the extracellular domain of the receptor. Moreover, we found that extracellular Ca(2+) enhanced mGluR1α-mediated intracellular Ca(2+) responses evoked by the orthosteric agonist l-quisqualate. Conversely, extracellular Ca(2+) diminished the inhibitory effect of the mGluR1α orthosteric antagonist (S)-α-methyl-4-carboxyphenylglycine. In addition, selective positive (Ro 67-4853) and negative (7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester) allosteric modulators of mGluR1α potentiated and inhibited responses to extracellular Ca(2+), respectively, in a manner similar to their effects on the response of mGluR1α to glutamate. Mutations at residues predicted to be involved in Ca(2+) binding, including E325I, had significant effects on the modulation of responses to the orthosteric agonist l-quisqualate and the allosteric modulator Ro 67-4853 by extracellular Ca(2+). These studies reveal that binding of extracellular Ca(2+) to the predicted Ca(2+)-binding site in the extracellular domain of mGluR1α modulates not only glutamate-evoked signaling but also the actions of both orthosteric ligands and allosteric modulators on mGluR1α.

cAMP-induced expression of neuropilin1 promotes retinal axon crossing in the zebrafish optic chiasm

Growing axons navigate a complex environment as they respond to attractive and repellent guidance cues. Axons can modulate their responses to cues through a G-protein-coupled, cAMP-dependent signaling pathway. To examine the role of G-protein signaling in axon guidance in vivo, we used the GAL4/UAS system to drive expression of dominant-negative heterotrimeric G-proteins (DNG) in retinal ganglion cells (RGCs) of embryonic zebrafish. Retinal axons normally cross at the ventral midline and project to the contralateral tectum. Expression of DNGα(S) in RGCs causes retinal axons to misproject to the ipsilateral tectum. These errors resemble misprojections in adcy1, adcy8, nrp1a, sema3D, or sema3E morphant embryos, as well as in sema3D mutant embryos. nrp1a is expressed in RGCs as their axons extend toward and across the midline. sema3D and sema3E are expressed adjacent to the chiasm, suggesting that they facilitate retinal midline crossing. We demonstrate synergistic induction of ipsilateral misprojections between adcy8 knockdown and transgenic DNGα(S) expression, adcy8 and nrp1a morphants, or nrp1a morphants and transgenic DNGα(S) expression. Using qPCR analysis, we show that either transgenic DNGα(S)-expressing embryos or adcy8 morphant embryos have decreased levels of nrp1a and nrp1b mRNA. Ipsilateral misprojections in adcy8 morphants are corrected by the expression of an nrp1a rescue construct expressed in RGCs. These findings are consistent with the idea that elevated cAMP levels promote Neuropilin1a expression in RGCs, increasing the sensitivity of retinal axons to Sema3D, Sema3E, or other neuropilin ligands at the midline, and consequently facilitate retinal axon crossing in the chiasm.

Glutamate, glutamate receptors, and downstream signaling pathways

Glutamate is a nonessential amino acid, a major bioenergetic substrate for proliferating normal and neoplastic cells, and an excitatory neurotransmitter that is actively involved in biosynthetic, bioenergetic, metabolic, and oncogenic signaling pathways. Glutamate signaling activates a family of receptors consisting of metabotropic glutamate receptors (mGluRs) and ionotropic glutamate receptors (iGluRs), both of which have been implicated in chronic disabling brain disorders such as Schizophrenia and neurodegenerative diseases like Alzheimer's, Parkinson's, and multiple sclerosis. In this review, we discuss the structural and functional relationship of mGluRs and iGluRs and their downstream signaling pathways. The three groups of mGluRs, the associated second messenger systems, and subsequent activation of PI3K/Akt, MAPK, NFkB, PLC, and Ca/CaM signaling systems will be discussed in detail. The current state of human mGluR1a as one of the most important isoforms of Group I-mGluRs will be highlighted. The lack of studies on the human orthologues of mGluRs family will be outlined. We conclude that upon further study, human glutamate-initiated signaling pathways may provide novel therapeutic opportunities for a variety of non-malignant and malignant human diseases.

Cell type influences the molecular mechanisms involved in hormonal regulation of ERG K+ channels

While the thyrotropin-releasing hormone (TRH) effect of raising intracellular Ca(2+) levels has been shown to rely on G(q/11) and PLC activation, the molecular mechanisms involved in the regulation of ERG K(+) channels by TRH are still partially unknown. We have analysed the effects of βγ scavengers, Akt/PKB inactivation, and TRH receptor (TRH-R) overexpression on such regulation in native and heterologous expression cell systems. In native rat pituitary GH(3) cells β-ARK/CT, Gα(t), and phosducin significantly reduced TRH inhibition of rERG currents, whereas in HEK-H36/T1 cells permanently expressing TRH-R and hERG, neither of the βγ scavengers affected the TRH-induced shift in V (1/2). Use of specific siRNAs to knock Akt/PKB expression down abolished the TRH effect on HEK-H36/T1 cell hERG, but not on rERG from GH(3) cells. Indeed, wortmannin or long insulin pretreatment also blocked TRH regulation of ERG currents in HEK-H36/T1 but not in GH(3) cells. To determine whether these differences could be related to the amount of TRH-Rs in the cell, we studied the TRH concentration dependence of the Ca(2+) and ERG responses in GH(3) cells overexpressing the receptors. The data indicated that independent of the receptor number additional cellular factor(s) contribute differently to couple the TRH-R to hERG channel modulation in HEK-H36/T1 cells. We conclude that regulation of ERG currents by TRH and its receptor is transduced in GH(3) and HEK-H36/T1 cell systems through common and different elements, and hence that the cell type influences the signalling pathways involved in the TRH-evoked responses.

The intra-molecular activation mechanisms of the dimeric metabotropic glutamate receptor 1 differ depending on the type of G proteins

Metabotropic glutamate receptor 1 (mGlu1) functions as a homodimer and activates not only the Gq but also the Gi/o and Gs pathways. Because of the dimeric configuration, different pathways could be activated either through the glutamate-bound subunit (cis-activation) and/or the other one (trans-activation). We here examined whether the intra-molecular activation mechanisms in the mGlu1 differ depending on the type of coupling G proteins, using various combinations of mGlu1 constructs that lack glutamate binding and/or G-protein coupling. The cis- or trans-activation alone was confirmed to trigger the Gq-coupled intracellular Ca(2+) transient. In contrast, the Gi/o-coupled G protein-dependent inward rectifying potassium (GIRK) channels were not activated either through the cis- or trans-activation alone. When one subunit of dimeric mGlu1 lacked the G-protein coupling, a significant decrease in the glutamate-induced GIRK current density was also observed. As the G protein-coupling-deficient subunit did not decrease the cell surface expression of mGlu1 and the Gq-coupled Ca(2+) transient, it was suggested that the coupling deficiency in one subunit of mGlu1 attenuates the Gi/o but not Gq coupling. In conclusion, multiple G-protein signaling was differentially activated by different intra-molecular activation mechanisms of the dimeric mGlu1.

SDF1-induced antagonism of axonal repulsion requires multiple G-protein coupled signaling components that work in parallel

SDF1 reduces the responsiveness of axonal growth cones to repellent guidance cues in a pertussis-toxin-sensitive, cAMP-dependent manner. Here, we show that SDF1's antirepellent effect can be blocked in embryonic chick dorsal root ganglia (DRGs) by expression of peptides or proteins inhibiting either Gα_i, Gα_q, or Gβγ. SDF1 antirepellent activity is also blocked by pharmacological inhibition of PLC, a common effector protein for Gα_q. We also show that SDF1 antirepellent activity can be mimicked by overexpression of constitutively active Gα_i, Gα_q, or Gα_s. These results suggest a model in which multiple G protein components cooperate to produce the cAMP levels required for SDF1 antirepellent activity.

The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology

The melanocortin-4 receptor (MC4R) was cloned in 1993 by degenerate PCR; however, its function was unknown. Subsequent studies suggest that the MC4R might be involved in regulating energy homeostasis. This hypothesis was confirmed in 1997 by a series of seminal studies in mice. In 1998, human genetic studies demonstrated that mutations in the MC4R gene can cause monogenic obesity. We now know that mutations in the MC4R are the most common monogenic form of obesity, with more than 150 distinct mutations reported thus far. This review will summarize the studies on the MC4R, from its cloning and tissue distribution to its physiological roles in regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain inflammation, and anxiety. I will then review the studies on the pharmacology of the receptor, including ligand binding and receptor activation, signaling pathways, as well as its regulation. Finally, the pathophysiology of the MC4R in obesity pathogenesis will be reviewed. Functional studies of the mutant MC4Rs and the therapeutic implications, including small molecules in correcting binding and signaling defect, and their potential as pharmacological chaperones in rescuing intracellularly retained mutants, will be highlighted.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Pharmacological assessment of m1 muscarinic acetylcholine receptor-gq/11 protein coupling in membranes prepared from postmortem human brain tissue

Using a selective Galpha(q/11) protein antibody capture guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding approach, it has been possible to perform a quantitative pharmacological examination of the functional activity of the M(1) muscarinic acetylcholine receptor (mAChR) in membranes prepared from human postmortem cerebral cortex. Oxotremorine-M caused a > or = 2-fold increase in [35S]GTPgammaS-Galpha(q/11) binding with a pEC(50) of 6.06 +/- 0.16 in Brodmann's areas 23 and 25 that was almost completely inhibited by preincubation of membranes with the M(1) mAChR subtype-selective antagonist muscarinic toxin-7. In addition, the orthosteric and allosteric agonists, xanomeline [3(3-hexyloxy-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine] and AC-42 (4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl]-piperidine hydrogen chloride), increased [35S]-GTPgammaS-Galpha(q/11) binding, but with reduced intrinsic activities, inducing maximal responses that were 42 +/- 1 and 44 +/- 2% of the oxotremorine-M-induced response, respectively. These data indicate that the M(1) receptor is the predominant mAChR subtype coupling to the Galpha(q/11) G protein in these brain regions and that it is possible to quantify the potency and intrinsic activity of full and partial M(1) mAChR receptor agonists in postmortem human brain using a selective Galpha(q/11) protein antibody capture [35S]GTPgammaS binding assay.

Identification of differential melanocortin 4 receptor agonist profiles at natively expressed receptors in rat cortical astrocytes and recombinantly expressed receptors in human embryonic kidney cells

Using cAMP accumulation as a functional readout, we pharmacologically characterized the response of native melanocortin receptors in cultured rat astrocytes, and found this response to be mediated by the melanocortin 4 receptor (MC4R). Melancortin agonists stimulate cAMP in a concentration-dependent manner in both astrocytes and human embryonic kidney cells recombinantly expressing rat MC4R (HEK-rMC4R), however, the relative potency and intrinsic activity of both small molecule and peptide agonists are reduced in the native system. As such, the small molecules THIQ, NBI-702 and MB243 display 43, 30 and 18% of the maximal response elicited by alpha-MSH in astrocytes. Likewise, the peptides MTII and ACTH display 55 and 72% of the maximal response elicited by alpha-MSH in these cells. In contrast, all of these compounds elicit full agonist responses with similar intrinsic activity to alpha-MSH in HEK-rMC4R cells. MC4R mRNA was detected in astrocytes, however radioligand binding experiments failed to detect measurable MC4R in astrocyte membranes, in contrast to membranes from HEK-rMC4R cells that display a binding site density of 18.1+/-1.5 fmol/mg. We propose that the divergent observations in functional activity between the cell types reflect differences in receptor expression and that caution should be exercised when interpreting agonist activity in over-expression systems for the purposes of drug discovery.

Differential regulation of two distinct voltage-dependent sodium currents by group III metabotropic glutamate receptor activation in insect pacemaker neurons

Using whole cell patch-clamp technique and immunocytochemistry on adult dorsal unpaired median (DUM) neurons isolated from the cockroach Periplaneta americana CNS, we reported the characterization of a native mGluR, sharing pharmacological properties with vertebrate metabotropic glutamate receptor III (mGluRIII) that regulated voltage-dependent sodium current (I(Na)). The global I(Na) was dissociated by means of l-glutamate sensitivity, deactivation time constant, voltage dependence of activation and inactivation, recovery from inactivation, and intracellular regulation process. These two currents were respectively designated I(Na1) and I(Na2) for l-glutamate-sensitive and -insensitive sodium currents. l-glutamate selectively reduced I(Na1) by an increase of intracellular cAMP level. Using different activators and/or inhibitors of G proteins and cAMP/PKA cascade, together with St-Ht31 (an inhibitor of PKA binding to AKAP) and AKAP-79 antibodies, we established that mGluRIII was linked to I(Na1) by a Gi/o and a suspected Gs protein. According to the activated signaling pathway, l-glutamate elevated the cAMP level, which thereby activated cytosolic PKA and released PKA bound to AKAP. As expected from both biophysical and pharmacological studies, we showed that, through an inhibition of I(Na1), l-glutamate increased DUM neuron spontaneous electrical activity. These results indicated that such mGluRIII-activated dual processes provided a new physiological control of pacemaker neuronal firing.

Regional differences in the coupling of 5-hydroxytryptamine-1A receptors to G proteins in the rat brain

Numerous data showed that 5-hydroxytryptamine-1A (5-HT1A) receptors couple to Galpha(o)/alpha(i) proteins for signal transduction. However, the alpha subunit isoforms really involved in 5-HT1A receptor coupling in brain remain to be identified. Moreover, regional differences in the functional characteristics of brain 5-HT1A receptors have been evidenced repeatedly. Because such differences could be due to variations in G proteins interacting with the same receptor, relevant approaches were used for identifying alpha subunits physically coupled to 5-HT1A receptors in different regions of the rat brain. Using immunoaffinity chromatography coupled to Western blot detection, 5-HT1A receptors were found to interact equally with Galpha(o) and Galpha(i3) in the cerebral cortex, mainly with Galpha(o) and weakly with Galpha(i3) in the hippocampus and exclusively with Galpha(i3) in the anterior raphe area. In the hypothalamus, 5-HT(1A) receptors seemed to be coupled to the latter two G proteins plus Galpha(i1) and Galpha(z). Complementary experiments based on an antibody capture technique coupled to both classic radioactivity and scintillation proximity assay detections showed that hippocampal 5-HT1A receptor stimulation induced 5'-O-(3-[35S]thio)triphosphate binding to immunoprecipitates with Galpha(i3) and Galpha(o) antisera. In the anterior raphe, such 5-HT1A receptor-mediated effect was obtained with Galpha(i3) antiserum only. These results demonstrated the existence of regional differences in the coupling of 5-HT1A receptors to G proteins in the rat brain. In the anterior raphe, 5-HT1A receptors seem to interact specifically with Galpha(i3), whereas in the hippocampus, they are mainly coupled to Galpha(o) proteins. Such a disparity in G-protein coupling might explain regional differences in adaptive regulations of brain 5-HT1A receptors.

The functional role of cysteines adjacent to the NRY motif of the human MT1 melatonin receptor

All G protein-coupled melatonin receptors have two conserved cysteines in the interphase between transmembrane helix III and the second intracellular loop, in the region assumed important in receptor/G protein coupling. The cysteines are also potential targets of receptor S-nitrosylation. The effects of site-directed mutagenesis of these cysteines in the human MT1 melatonin receptor were investigated. The cysteines were mutated into serines either individually or as a pair and stable Chinese hamster ovary cell lines expressing the wild-type and mutant MT1 receptors were created. Receptor expression level, subcellular localization, ligand binding and G protein activation of the cell lines were analyzed. Serine substitution of C127 (Cys(3.52)) did not affect the ligand binding affinity and agonist potency but had an influence on receptor trafficking and G protein activation capacity. Serine substitution of C130 (Cys(3.55)) resulted in a decrease in the potency of melatonin to activate G proteins. When both cysteines were mutated into serines, normal MT1 receptor binding and activation were abolished. Computer modeling revealed that the mutations did not change the structure of the ligand binding pocket. Cysteine S-nitrosylation had no influence on G protein activation through MT1 receptors. Taken together, these data show that the two conserved cysteines in the end of transmembrane domain III of the MT1 melatonin receptor, especially C130 (Cys(3.55)), are needed for normal G protein activation and receptor trafficking.

Group I metabotropic glutamate receptors, mGlu1a and mGlu5a, couple to cyclic AMP response element binding protein (CREB) through a common Ca2+ - and protein kinase C-dependent pathway

Coupling of the group I metabotropic glutamate receptors, mGlu1a and mGlu5a, to the cAMP response element binding protein (CREB) has been studied in Chinese hamster ovary cell lines where receptor expression is under the control of an inducible promoter. Both receptors stimulate CREB phosphorylation with similar time courses, and agonist potency was also comparable between the two receptors. Stimulation of cells in Ca(2+)-free medium containing EGTA (100 microm), with or without the additional depletion of intracellular stores, caused marked decreases in agonist-mediated responses in both cell lines. Down-regulation of protein kinase C (PKC) activity by phorbol ester treatment, or treatment with the broad spectrum PKC inhibitor Ro 31-8220, partially attenuated both mGlu1a and mGlu5a receptor-mediated responses. Furthermore, stimulation of cells in the absence of extracellular Ca(2+) following prior PKC down-regulation resulted in additive inhibitory effects. The involvement of extracellular signal-regulated kinases (ERK1/2), Ca(2+)/calmodulin or Ca(2+)/calmodulin-dependent protein kinases was assessed using pharmacological inhibitors. Results indicated that coupling of the group I mGlu receptors to CREB phosphorylation occurs independently of these pathways. Thus, although the [Ca(2+)](i) signatures activated by these mGlu receptors differ, they couple to CREB with comparable potency and recruit similar downstream components to execute CREB phosphorylation.

S-nitrosothiols modulate G protein-coupled receptor signaling in a reversible and highly receptor-specific manner

Background

Recent studies indicate that the G protein-coupled receptor (GPCR) signaling machinery can serve as a direct target of reactive oxygen species, including nitric oxide (NO) and S-nitrosothiols (RSNOs). To gain a broader view into the way that receptor-dependent G protein activation – an early step in signal transduction – might be affected by RSNOs, we have studied several receptors coupling to the G_i family of G proteins in their native cellular environment using the powerful functional approach of [³⁵S]GTPγS autoradiography with brain cryostat sections in combination with classical G protein activation assays.

Results

We demonstrate that RSNOs, like S-nitrosoglutathione (GSNO) and S-nitrosocysteine (CysNO), can modulate GPCR signaling via reversible, thiol-sensitive mechanisms probably involving S-nitrosylation. RSNOs are capable of very targeted regulation, as they potentiate the signaling of some receptors (exemplified by the M2/M4 muscarinic cholinergic receptors), inhibit others (P2Y₁₂ purinergic, LPA₁lysophosphatidic acid, and cannabinoid CB₁ receptors), but may only marginally affect signaling of others, such as adenosine A₁, μ-opioid, and opiate related receptors. Amplification of M2/M4 muscarinic responses is explained by an accelerated rate of guanine nucleotide exchange, as well as an increased number of high-affinity [³⁵S]GTPγS binding sites available for the agonist-activated receptor. GSNO amplified human M4 receptor signaling also under heterologous expression in CHO cells, but the effect diminished with increasing constitutive receptor activity. RSNOs markedly inhibited P2Y₁₂ receptor signaling in native tissues (rat brain and human platelets), but failed to affect human P2Y₁₂ receptor signaling under heterologous expression in CHO cells, indicating that the native cellular signaling partners, rather than the P2Y₁₂ receptor protein, act as a molecular target for this action.

Conclusion

These in vitro studies show for the first time in a broader general context that RSNOs are capable of modulating GPCR signaling in a reversible and highly receptor-specific manner. Given that the enzymatic machinery responsible for endogenous NO production is located in close proximity with the GPCR signaling complex, especially with that for several receptors whose signaling is shown here to be modulated by exogenous RSNOs, our data suggest that GPCR signaling in vivo is likely to be subject to substantial, and highly receptor-specific modulation by NO-derived RSNOs.

Activation of mGluR5 modulates Ca2+ currents in retinal amacrine cells from the chick

In the inner plexiform layer, amacrine cells receive glutamatergic input from bipolar cells. Glutamate can depolarize amacrine cells by activation of ionotropic glutamate receptors or mediate potentially more diverse changes via activation of G protein-coupled metabotropic glutamate receptors (mGluR5). Here, we asked whether selective activation of metabotropic glutamate receptor 5 is linked to modulation of the voltage-gated Ca2+ channels expressed by cultured GABAergic amacrine cells. To address this, we performed whole-cell voltage clamp experiments, primarily in the perforated-patch configuration. We found that agonists selective for mGluR5, including (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), enhanced the amplitude of the voltage-dependent Ca2+ current. The voltage-dependent Ca2+ current and CHPG-dependent current enhancement were blocked by nifedipine, indicating that L-type Ca2+ channels, specifically, were being modulated. We have previously shown that activation of mGluR5 produces Ca2+ elevations in cultured amacrine cells (Sosa et al., 2002). Loading the cells with 5 mM BAPTA inhibited the mGluR5-dependent enhancement, suggesting that the cytosolic Ca2+ elevations are required for modulation of the current. Although activation of mGluR5 is typically linked to activation of protein kinase C, we found that direct activation of this kinase leads to inhibition of the Ca2+ current, indicating that stimulation of this enzyme is not responsible for the mGluR5-dependent enhancement. Interestingly, direct stimulation of protein kinase A produced an enhancement of the Ca2+ current similar to that observed with activation of mGluR5. Thus, activation of mGluR5 may modulate the L-type voltage-gated Ca2+ current in these GABAergic amacrine cells via activation of protein kinase A, possibly via direct activation of a Ca2(+)-dependent adenylate cyclase.

Galpha protein selectivity determinant specified by a viral chemokine receptor-conserved region in the C tail of the human herpesvirus 8 g protein-coupled receptor

The viral G-protein coupled receptor (vGPCR) specified by human herpesvirus 8 (HHV-8) open reading frame 74 (ORF74) is a ligand-independent chemokine receptor that has structural and functional homologues among other characterized gammaherpesviruses and related receptors in the betaherpesviruses. Sequence comparisons of the gammaherpesvirus vGPCRs revealed a highly conserved region in the C tail, just distal to the seventh transmembrane domain. Mutagenesis of the corresponding codons of HHV-8 ORF74 was carried out to provide C-tail-altered proteins for functional analyses. By measuring receptor-activated vascular endothelial growth factor promoter induction and NF-kappaB, mitogen-activated protein kinase, and Ca(2+) signaling, we found that while some altered receptors showed general signaling deficiencies, others had distinguishable activation profiles, suggestive of selective Galpha protein coupling. This was supported by the finding that vGPCR and representative functionally altered variants, vGPCR.8 (R322W) and vGPCR.15 (M325S), were affected differently by inhibitors of Galpha(i) (pertussis toxin), protein kinase C (GF109203X), and phosphatidylinositol 3-kinase (wortmannin). Consistent with the signaling data, [(35)S]GTPgammaS incorporation assays revealed preferential coupling of vGPCR.15 to Galpha(q) and an inability of vGPCR.8 to couple functionally to Galpha(q). However, both variants, wild-type vGPCR, and a C-tail deletion version of the receptor were equally able to associate physically with Galpha(q). Combined, our data demonstrate that HHV-8 vGPCR contains discrete sites of Galpha interaction and that receptor residues in the proximal region of the cytoplasmic tail are determinants of Galpha protein coupling specificity.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

Group-I metabotropic glutamate receptors, mGlu1a and mGlu5a, couple to extracellular signal-regulated kinase (ERK) activation via distinct, but overlapping, signalling pathways

The coupling of the group I metabotropic glutamate receptors, mGlu1a and mGlu5a, to the extracellular signal-regulated protein kinase (ERK) pathway has been studied in Chinese hamster ovary cell-lines where receptor expression is under inducible control. Both mGlu receptors stimulated comparable, robust and agonist concentration-dependent ERK activations in the CHO cell-lines. The mGlu1a receptor-mediated ERK response was almost completely attenuated by pertussis toxin (PTx) pretreatment, whereas the mGlu5a-ERK response, and the phosphoinositide response to activation of either receptor, was PTx-insensitive. mGlu1a and mGlu5a receptor coupling to ERK occurred via mechanisms independent of phosphoinositide 3-kinase activity and intracellular and/or extracellular Ca2+ concentration. While acute treatment with a protein kinase C (PKC) inhibitor did not attenuate agonist-stimulated ERK activation, down-regulation of PKCs by phorbol ester treatment for 24 h did attenuate both mGlu1a and mGlu5a receptor-mediated responses. Further, inhibition of Src non-receptor tyrosine kinase activity by PP1 attenuated the ERK response generated by both receptor subtypes, but only mGlu1a receptor-ERK activation was attenuated by PDGF receptor tyrosine kinase inhibitor AG1296. These findings demonstrate that, although expressed in a common cell background, these closely related mGlu receptors utilize different G proteins to cause ERK activation and may recruit different tyrosine kinases to facilitate this response.

Determinants of metabotropic glutamate receptor-5-mediated Ca2+ and inositol 1,4,5-trisphosphate oscillation frequency. Receptor density versus agonist concentration

Diverse patterns of Ca(2+)(i) release differentially regulate Ca(2+)-sensitive enzymes and gene transcription, and generally the extent of agonist activation of phospholipase C-linked G protein-coupled receptors determines the type of Ca(2+) signal. We have studied global Ca(2+) oscillations arising through activation of the metabotropic glutamate receptor mGluR5a expressed in Chinese hamster ovary cells and find that these oscillations are largely insensitive to agonist concentration. Using an inducible receptor expression system and a non-competitive antagonist, in conjunction with the translocation of eGFP-PH(PLCdelta) to monitor inositol 1,4,5-trisphosphate (InsP(3)) oscillations in single cells, we show that mGluR5a density determines the frequency of these oscillations. The predominant underlying mechanism resulted from a negative feedback loop whereby protein kinase C (PKC) inhibited InsP(3) generation. Down-regulation of PKC by prolonged exposure to phorbol ester revealed a second form of Ca(2+)(i) oscillation at low agonist concentrations. These Ca(2+)(i) signals showed features typical of classic repetitive Ca(2+)-induced Ca(2+) release and were sensitive to agonist concentration. Therefore, a single receptor can stimulate two types of InsP(3)-mediated Ca(2+) signal dependent upon feedback inhibition, producing two distinct means of controlling the final pattern of Ca(2+)(i) release. Our results have physiological implications for Ca(2+) signaling in general and emphasize the importance of mGluR5 surface expression for modulating synaptic plasticity.

Measurement of agonist-dependent and -independent signal initiation of alpha(1b)-adrenoceptor mutants by direct analysis of guanine nucleotide exchange on the G protein galpha(11)

Immunoprecipitation of a fusion protein between the alpha(1b)-adrenoceptor and Galpha(11) following a [(35)S]GTPgammaS [guanosine-5'-O-(3-thio)triphosphate] binding assay resulted in incorporation of low levels of nucleotide. The agonist phenylephrine increased incorporation some 30-fold. Agonist-induced binding represented 1.0 mol of [(35)S]GTPgammaS/mol of fusion protein. This was to the G protein linked to the receptor rather than endogenous Galpha(q)/Galpha(11) as a fusion protein containing the alpha(1b)-adrenoceptor and a form of Galpha(11) (G(208)A) unable to exchange guanine nucleotides effectively, bound [(35)S]GTPgammaS very poorly. Fusion proteins between A(293)E, D(142)A, and 3CAM mutants of the alpha(1b)-adrenoceptor and Galpha(11) bound substantially greater levels of [(35)S]GTPgammaS in the absence of agonist than the fusion incorporating the wild-type receptor. Constitutive binding of the nucleotide induced by these mutants was only 20% of the level achieved by phenylephrine. These mutant receptors thus do not provide an accurate mimic of the agonist-occupied state. Phentolamine reduced the binding of [(35)S]GTPgammaS and acted as a partial inverse agonist for each of the constitutively active mutants. [(35)S]GTPgammaS binding to Galpha(11) was elevated by phenylephrine in both wild-type and constitutively active mutant forms of the fusion proteins, but agonist potency and binding affinity were 50 times higher for the fusions containing the mutated receptors. These studies provide the first direct demonstration of the capacity of constitutively active mutants of a receptor to stimulate guanine nucleotide exchange on the alpha subunit of a G(q) family G protein and defines a strategy potentially suitable for any receptor that couples to these G proteins.

Metabotropic glutamate receptor 5 and calcium signaling in retinal amacrine cells

To begin to understand the modulatory role of glutamate in the inner retina, we examined the mechanisms underlying metabotropic glutamate receptor 5 (mGluR5)-dependent Ca(2+) elevations in cultured GABAergic amacrine cells. A partial sequence of chicken retinal mGluR5 encompassing intracellular loops 2 and 3 suggests that it can couple to both G(q) and G(s). Selective activation of mGluR5 stimulated Ca(2+) elevations that varied in waveform from cell to cell. Experiments using high external K(+) revealed that the mGluR5-dependent Ca(2+) elevations are distinctive in amplitude and time course from those engendered by depolarization. Experiments with a Ca(2+) -free external solution demonstrated that the variability in the time course of mGluR5-dependent Ca(2+) elevations is largely due to the influx of extracellular Ca(2+). The sensitivity of the initial phase of the Ca(2+) elevation to thapsigargin indicates that this phase of the response is due to the release of Ca(2+) from the endoplasmic reticulum. Pharmacological evidence indicates that mGluR5-mediated Ca(2+) elevations are dependent upon the activation of phospholipase C. We rule out a role for L-type Ca(2+) channels and cAMP-gated channels as pathways for Ca(2+) entry, but provide evidence of transient receptor potential (TRP) channel-like immunoreactivity, suggesting that Ca(2+) influx may occur through TRP channels. These results indicate that GABAergic amacrine cells express an avian version of mGluR5 that is linked to phospholipase C-dependent Ca(2+) release and Ca(2+) influx, possibly through TRP channels.

Structural, signalling and regulatory properties of the group I metabotropic glutamate receptors: prototypic family C G-protein-coupled receptors

In 1991 a new type of G-protein-coupled receptor (GPCR) was cloned, the type 1a metabotropic glutamate (mGlu) receptor, which, despite possessing the defining seven-transmembrane topology of the GPCR superfamily, bore little resemblance to the growing number of other cloned GPCRs. Subsequent studies have shown that there are eight mammalian mGlu receptors that, together with the calcium-sensing receptor, the GABA(B) receptor (where GABA is gamma-aminobutyric acid) and a subset of pheromone, olfactory and taste receptors, make up GPCR family C. Currently available data suggest that family C GPCRs share a number of structural, biochemical and regulatory characteristics, which differ markedly from those of the other GPCR families, most notably the rhodopsin/family A GPCRs that have been most widely studied to date. This review will focus on the group I mGlu receptors (mGlu1 and mGlu5). This subgroup of receptors is widely and differentially expressed in neuronal and glial cells within the brain, and receptor activation has been implicated in the control of an array of key signalling events, including roles in the adaptative changes needed for long-term depression or potentiation of neuronal synaptic connectivity. In addition to playing critical physiological roles within the brain, the mGlu receptors are also currently the focus of considerable attention because of their potential as drug targets for the treatment of a variety of neurological and psychiatric disorders.

Enhanced inducible mGlu1alpha receptor expression in Chinese hamster ovary cells

Inducible expression of the group-I metabotropic glutamate receptor (mGlu1alpha) in Chinese hamster ovary cells allows for the study of receptor density dependent effects. However, expression levels attainable with this system are lower than those reported for various brain regions and achieved by conventional (constitutive) transfection. Thus, direct comparison of mGlu1alpha receptor-mediated responses in this inducible expression system with those for receptors expressed heterologously or in vivo is compounded. We show here that inducible expression can be selectively augmented by butyrate pretreatment to levels approaching those reported for cerebral tissue. Enhanced mGlu1alpha receptor protein levels, agonist-induced inositol phosphate accumulation, as well as single-cell inositol 1,4,5-trisphosphate production and intracellular Ca(2+) mobilization occurred following co-induction with butyrate. In contrast, endogenous purinoceptor function was unaffected. Importantly, the ability to titrate receptor expression by varying isopropyl beta-thiogalactoside concentration was retained. Sodium butyrate thus offers a simple and convenient method to enhance inducible gene expression to levels found in vivo.