Agonist regulation of cellular G protein levels and distribution: mechanisms and functional implications

Attenuation of relaxing response induced by pituitary adenylate cyclase-activating polypeptide in bronchial smooth muscle of experimental asthma

Bronchomotor tone is regulated by contraction and relaxation of airway smooth muscle (ASM). A weakened ASM relaxation might be a cause of airway hyperresponsiveness (AHR), a characteristic feature of bronchial asthma. Pituitary adenylyl cyclase-activating polypeptide (PACAP) is known as a mediator that causes ASM relaxation. To date, whether or not the PACAP responsiveness is changed in asthmatic ASM is unknown. The current study examined the hypothesis that relaxation induced by PACAP is reduced in bronchial smooth muscle (BSM) of allergic asthma. The ovalbumin (OA)-sensitized mice were repeatedly challenged with aerosolized OA to induce asthmatic reaction. Twenty-four hours after the last antigen challenge, the main bronchial smooth muscle (BSM) tissues were isolated. Tension study showed a BSM hyperresponsiveness to acetylcholine in the OA-challenged mice. Both quantitative RT-PCR and immunoblot analyses revealed a significant decrease in PAC₁ receptor expression in BSMs of the diseased mice. Accordingly, in the antigen-challenged group, the PACAP-induced PAC₁ receptor-mediated BSM relaxation was significantly attenuated, whereas the relaxation induced by vasoactive intestinal polypeptide was not changed. These findings suggest that the relaxation induced by PACAP is impaired in BSMs of experimental asthma due to a downregulation of its binding partner PAC₁ receptor. Impaired BSM responsiveness to PACAP might contribute to the AHR in asthma.

Chronic intermittent ethanol exposure selectively alters the expression of Gα subunit isoforms and RGS subtypes in rat prefrontal cortex

Chronic alcohol exposure induces pronounced changes in GPCR-mediated G-protein signaling. Recent microarray and RNA-seq analyses suggest associations between alcohol abuse and the expression of genes involved in G-protein signaling. The activity of G-proteins (e.g. Gαi/o and Gαq) is negatively modulated by regulator of G-protein signaling (RGS) proteins which are implicated in drugs of abuse including alcohol. The present study used 7days of chronic intermittent ethanol exposure followed by 24h withdrawal (CIE) to investigate changes in mRNA and protein levels of G-protein subunit isoforms and RGS protein subtypes in rat prefrontal cortex, a region associated with cognitive deficit attributed to excessive alcohol drinking. We found that this ethanol paradigm induced differential expression of Gα subunits and RGS subtypes. For example, there were increased mRNA and protein levels of Gαi1/3 subunits and no changes in the expression of Gαs and Gαq subunits in ethanol-treated animals. Moreover, CIE increased the mRNA but not the protein levels of Gαo. Additionally, a modest increase in Gαi2 mRNA level by CIE was accompanied by a pronounced increase in its protein level. Interestingly, we found that CIE increased mRNA and protein levels of RGS2, RGS4, RGS7 and RGS19 but had no effect on the expression of RGS5, RGS6, RGS8, RGS12 or RGS17. Changes in the expression of Gα subunits and RGS subtypes could contribute to the functional alterations of certain GPCRs following chronic ethanol exposure. The present study suggests that RGS proteins may be potential new targets for intervention of alcohol abuse via modification of Gα-mediated GPCR function.

Neuropeptide Y-stimulated [(35) S]GTPγs functional binding is reduced in the hippocampus after kainate-induced seizures in mice

Kainate-induced seizures constitute a model of temporal lobe epilepsy where prominent changes are observed in the hippocampal neuropeptide Y (NPY) system. However, little is known about the functional state and signal transduction of the NPY receptor population resulting from kainate exposure. Thus, in this study, we explored functional NPY receptor activity in the mouse hippocampus and neocortex after kainate-induced seizures using NPY-stimulated [(35) S]GTPγS binding. Moreover, we also studied levels of [(125) I]-peptide YY (PYY) binding and NPY, Y1, Y2, and Y5 receptor mRNA in these kainate-treated mice. Functional NPY binding was unchanged up to 12 h post-kainate, but decreased significantly in all hippocampal regions after 24 h and 1 week. Similarly, a decrease in [(125) I]-PYY binding was found in the dentate gyrus (DG) 1 week post-kainate. However, at 2 h, 6 h, and 12 h, [(125) I]-PYY binding was increased in all regions, and in the CA1 also at 24 h post-kainate. NPY mRNA levels were prominently increased in hippocampal regions, reaching maximum at 12 and 24 h. Y1 and Y5 mRNA levels were lowered in the DG at 24 and 2 h, respectively, while Y2 mRNA levels were elevated at 24 h in the DG and CA3. This study confirms rat kainate studies by showing pronounced adaptive changes in the mouse hippocampus both with regard to NPY synthesis and NPY receptor synthesis and binding, which may contribute to regulating neuronal seizure susceptibility after kainate. However, the potential seizure-suppressant effects of increased NPY gene expression at late time points post-kainate could be attenuated by the novel finding of reduced NPY-receptor G-protein activation.

Modification of heterotrimeric G-proteins in Swiss 3T3 cells stimulated with Pasteurella multocida toxin

Many bacterial toxins covalently modify components of eukaryotic signalling pathways in a highly specific manner, and can be used as powerful tools to decipher the function of their molecular target(s). The Pasteurella multocida toxin (PMT) mediates its cellular effects through the activation of members of three of the four heterotrimeric G-protein families, G(q), G(12) and G(i). PMT has been shown by others to lead to the deamidation of recombinant Gα(i) at Gln-205 to inhibit its intrinsic GTPase activity. We have investigated modification of native Gα subunits mediated by PMT in Swiss 3T3 cells using 2-D gel electrophoresis and antibody detection. An acidic change in the isoelectric point was observed for the Gα subunit of the G(q) and G(i) families following PMT treatment of Swiss 3T3 cells, which is consistent with the deamidation of these Gα subunits. Surprisingly, PMT also induced a similar modification of Gα(11), a member of the G(q) family of G-proteins that is not activated by PMT. Furthermore, an alkaline change in the isoelectric point of Gα(13) was observed following PMT treatment of cells, suggesting differential modification of this Gα subunit by PMT. G(s) was not affected by PMT treatment. Prolonged treatment with PMT led to a reduction in membrane-associated Gα(i), but not Gα(q). We also show that PMT inhibits the GTPase activity of G(q).

Identification of a specific assembly of the g protein golf as a critical and regulated module of dopamine and adenosine-activated cAMP pathways in the striatum

In the principal neurons of striatum (medium spiny neurons, MSNs), cAMP pathway is primarily activated through the stimulation of dopamine D1 and adenosine A_2A receptors, these receptors being mainly expressed in striatonigral and striatopallidal MSNs, respectively. Since cAMP signaling pathway could be altered in various physiological and pathological circumstances, including drug addiction and Parkinson’s disease, it is of crucial importance to identify the molecular components involved in the activation of this pathway. In MSNs, cAMP pathway activation is not dependent on the classical Gs GTP-binding protein but requires a specific G protein subunit heterotrimer containing Gαolf/β2/γ7 in particular association with adenylyl cyclase type 5. This assembly forms an authentic functional signaling unit since loss of one of its members leads to defects of cAMP pathway activation in response to D1 or A_2A receptor stimulation, inducing dramatic impairments of behavioral responses dependent on these receptors. Interestingly, D1 receptor (D1R)-dependent cAMP signaling is modulated by the neuronal levels of Gαolf, indicating that Gαolf represents the rate-limiting step in this signaling cascade and could constitute a critical element for regulation of D1R responses. In both Parkinsonian patients and several animal models of Parkinson’s disease, the lesion of dopamine neurons produces a prolonged elevation of Gαolf levels. This observation gives an explanation for the cAMP pathway hypersensitivity to D1R stimulation, occurring despite an unaltered D1R density. In conclusion, alterations in the highly specialized assembly of Gαolf/β2/γ7 subunits can happen in pathological conditions, such as Parkinson’s disease, and it could have important functional consequences in relation to changes in D1R signaling in the striatum.

Reconstitution of a chemical defense signaling pathway in a heterologous system

Chemical signaling plays an important role in ecological interactions, such as communication and predator-prey dynamics. Since sessile species cannot physically escape predators, many contain compounds that deter predation; however, it is largely unknown how predators physiologically detect deterrent chemicals. Few studies have investigated ecologically relevant aversive taste responses in any predator. Our objective was to determine if a signaling pathway for detecting marine sponge-derived deterrent compounds could be reconstituted in a heterologous expression system to ultimately facilitate investigation of the molecular mechanism of such an aversive behavioral response. Zebrafish (Danio rerio) rejected artificial diets laced with sponge chemical defense compounds that were previously shown to deter a generalist marine predator, Thalassoma bifasciatum, suggesting that zebrafish can recognize deterrent compounds relevant to coral reef systems. Transcripts made from a zebrafish cDNA library were expressed in a heterologous system, Xenopus laevis oocytes, and tested for chemoreceptor activation via electrophysiology, using the cystic fibrosis transmembrane conductance regulator (CFTR) as a reporter. Oocytes expressing gene sequences from the library and CFTR exhibited a CFTR-like electrophysiological response to formoside and ectyoplasides A and B, sponge defense compounds. Therefore, the chemical defense-activated signaling pathway can be reconstituted in Xenopus oocytes. Kinetics of the responses suggested that the responses to formoside and ectyoplasides A and B were receptor-mediated and capable of using the G(alphas) signaling pathway in this system. This bioassay has the potential to lead to the identification of genes that encode receptors capable of interacting with deterrent chemicals, which would enable understanding of predator detection of chemical defenses.

Inverse agonist exposure enhances ligand binding and G protein activation of the human MT1 melatonin receptor, but leads to receptor down-regulation

Melatonin binds and activates G protein-coupled melatonin receptors. The density and affinity of the endogenous melatonin receptors change throughout the 24-hr day, and the exposure of recombinant melatonin receptors to melatonin often results in desensitization of the receptors. Receptor density, G protein activation and expression level were analyzed in CHO cell lines stably expressing the human MT1 receptors after 1 or 72 hr of exposure to melatonin (agonist, 10 nm) and luzindole (antagonist/inverse agonist, 10 microm). The 72-hr exposure to luzindole significantly increased the apparent receptor density in cell lines with both high and low MT1 receptor expression levels (MT1(high) and MT1(low) cells, respectively). In the constitutively active MT1(high) cells, luzindole pretreatment also stimulated the functional response to melatonin in [(35)S]GTPgammaS binding assays, whereas melatonin pretreatment attenuated the functional response at both time points. Receptor ELISA was used to analyze the cell membrane and total expression level of the MT1 receptor in intact and permeabilized cells, respectively. Luzindole pretreatment decreased the total cellular level of MT1 receptor in the MT1(high) cells at both time points but increased the cell surface expression of MT1 receptor at 72 hr. Melatonin significantly decreased MT1 receptor cell surface expression only in MT1(high) cells after a 1-hr treatment. These results indicate that melatonin treatment desensitizes MT1 receptors, whereas luzindole increases ligand binding and G-protein activation. Luzindole also stimulates downregulation of the MT1 receptor protein, interfering with the synthesis and/or degradation of the receptor.

Diversity-Oriented Synthetic Approach to Naturally Abundant S-Amino Acid Based Benzannulated Enantiomerically Pure Medium Ring Heterocyclic Scaffolds Employing Inter- and Intramolecular Mitsunobu Reactions

The diversity-oriented organic synthesis of enantiomerically pure benzannulated oxazepine, diazepine, thiazepine, oxazocine, and diazocine scaffolds is described from easily accessible naturally occurring S-amino acids as chiral synthons. Inter- and intramolecular Mitsunobu reactions for the formation of carbon-nitrogen, carbon-oxygen, and carbon-sulfur bonds have been used as key transformations to construct these biologically important privileged heterocycles. This is the first example where the Mitsunobu approach has been utilized for the construction of S-amino acid based seven- and eight-membered ring systems.

Effect of chronic gestational treatment with caffeine or theophylline on Group I metabotropic glutamate receptors in maternal and fetal brain

Pregnant rats were treated throughout the gestational period with either caffeine or theophylline, and its effect on the metabotropic glutamate receptor (mGluRs) signal transduction pathway was studied in both maternal and fetal brain. In maternal brain, radioligand binding assays showed that chronic treatment with methylxanthines caused a significant decrease in the total number of mGluRs. This decrease was accompanied by an increase in receptor affinity. Immunodetection showed that mGluR1a and phospholipase C beta1 (PLCbeta1) were significantly decreased in response to chronic methylxanthine treatment, whereas alphaG(q/11) was not affected. A loss was also detected of PLC stimulation mediated by (S)-3,5-dihydroxyphenylglycine (DHPG), a selective Group I mGluR agonist, suggesting desensitization of the mGluR/PLC pathway. In fetal brain, a loss in total mGluRs was observed in fetuses from mothers treated with caffeine or theophylline, without variation in receptor affinity. A decrease in mGluR1a, alphaG(q/11) and PLCbeta1 levels was also observed in response to treatment. However, changes detected in this immature tissue were not associated with variations in PLC activity. These results suggest that chronic caffeine or theophylline treatment down-regulates several mGluR/PLC transduction pathway components in both maternal and fetal brain, causing a loss of receptor responsiveness only in maternal brain.

Chronic caffeine or theophylline intake during pregnancy inhibits A1 receptor function in the rat brain

The aim of this work was to study whether caffeine or theophylline chronically consumed during pregnancy affect inhibitory adenylyl cyclase pathway mediated by adenosine, in rat brain of both mothers and full-term fetuses. Immunoblotting analysis revealed a significant decrease in alphaGi(1,2) subunit level (27-29% in mothers, 15-18% in fetuses), associated with a significant increase in the mRNA level coding alphaGi(1) in both maternal and fetal rat brain (12-22%) after methylxanthine intake. No significant differences in alphaGi(3) level were detected in any case. On the other hand, forskolin- and forskolin plus guanosine-5'-O(3-thiotriphosphate) tetralithium salt-stimulated adenylyl cyclase activity was significantly decreased (30-36%) in maternal brain. Moreover, adenylyl cyclase inhibition elicited by N(6)-cyclohexyladenosine, specific adenosine A(1) receptor agonist, was also significantly decreased in caffeine- (40.5%) and theophylline- (55.0%) treated mothers, suggesting a desensitization of adenosine A(1) receptor/adenylyl cyclase pathway in maternal brain. However, no significant differences were detected in fetal brain between control and treated animals. Therefore, caffeine or theophylline chronic intake during pregnancy differently modulates inhibitory adenylyl cyclase pathway mediated by adenosine in maternal and fetal brain causing a loss of the system responsiveness only in maternal brain but down-regulating Gi(1) protein in both mother and fetus brain.

Identification of a novel site within G protein alpha subunits important for specificity of receptor-G protein interaction

Several domains of G protein alpha subunits are implicated in the control of receptor-G protein coupling specificity. Among these are the extreme N-and C-termini, the alpha4/beta6-loops, and the loop linking the N-terminal alpha-helix to the beta1-strand of the ras-like domain. In this study, we illustrate that single-point mutations of a highly conserved glycine residue within the linker I region of the Galpha(q) subunit confers upon the mutant Galpha(q) the ability to be activated by Galpha(i)- and Galpha(s) -coupled receptors, as evidenced by guanosine 5'-O-(3-[(35)S]thio)triphosphate binding and inositol phosphate turnover assays. The mutations did not affect expression of Galpha(q) proteins nor their ability to stimulate phospholipase Cbeta. It is noteworthy that both mutant and wild-type Galpha(q) proteins are indistinguishable in their ability to reconstitute a functional Gq-PLCbeta-calcium signaling pathway when cotransfected with the Galpha(q)-coupled neurokinin 1 or muscarinic M3 receptor into mouse embryonic fibroblasts derived from Galpha(q/11) knockout mice. On a three-dimensional model of the receptor-G protein complex, the highly conserved linker I region connecting the helical and the GTPase domain of the Galpha protein is inaccessible to the intracellular surface of the receptors. Our data indicate that receptor-G protein coupling specificity is not exclusively governed by direct receptor-G protein interaction and that it even bypasses the requirement of the extreme C terminus of Galpha, a well accepted receptor recognition domain, suggesting a novel allosteric mechanism for G protein-coupled receptor-G protein selectivity.

Persistent increase in olfactory type G-protein alpha subunit levels may underlie D1 receptor functional hypersensitivity in Parkinson disease

Although L-dopa remains the most effective treatment of Parkinson disease, its long-term administration is hampered by the appearance of dyskinesia. Hypersensitivity of dopamine D1 receptors in the striatum has been suggested to contribute to the genesis of these delayed adverse effects. However, D1 receptor amounts are unchanged in Parkinson disease, suggesting alterations of downstream effectors. In rodents, striatal D1 receptors activate adenylyl cyclase through olfactory type G-protein alpha subunit (Galphaolf) and G-protein gamma 7 subunit (Ggamma7). We found that Galphaolf was enriched in human basal ganglia and was markedly diminished in the putamen of patients with Huntington disease, in relation with the degeneration of medium spiny neurons. In contrast, in the putamen of patients with Parkinson disease, Galphaolf and Ggamma7 levels were both significantly increased. In the rat, the degeneration of dopamine neurons augmented Galphaolf levels in the striatal neurons, specifically at the plasma membrane, an effect accounting for the increase of D1 response on cAMP production in dopamine-depleted striatum. In lesioned rats, Galphaolf levels were normalized by a 3 week treatment with l-dopa or a D1 agonist but not with aD2-D3 agonist, supporting a Galphaolf regulation by D1 receptor usage. In contrast, the increases of Galphaolf levels in patients were not affected by the duration of l-dopa treatment but correlated with duration of disease. In conclusion, our results revealed in the parkinsonian putamen a prolonged elevation of Galphaolf levels that may lead to a persistent D1 receptor hypersensitivity and contribute to the genesis of long-term complications of L-dopa.

Long-term agonist stimulation of IP prostanoid receptor depletes the cognate G(s)alpha protein in membrane domains but does not change the receptor level

Iloprost (IP) stimulation (1 microM, 2 h) of Flag-epitope-tagged human IP prostanoid receptor (FhIPR) expressed in HEK293 cells resulted in specific decrease of endogenous G(s)alpha protein in detergent-insensitive, caveolin-enriched, membrane domains (DIMs). Receptor protein FhIPR, caveolin, G(i)alpha and GPI-linked, domain markers CD55 and CD59 were unchanged. The same result was obtained in HEK293 cells expressing FhIPR-G(s)alpha fusion protein. The endogenous G(s)alpha decreased, but the level of Flag-hIPR-G(s)alpha protein did not change. The specific depletion of domain-bound pool of G(s)alpha as consequence of iloprost stimulation was also demonstrated in membrane domains prepared according to alkaline treatment plus sonication protocol (detergent-free procedure of Song et al.). Our data further indicated that in control, quiescent cells only a very small amount of IP prostanoid receptor was present in DIMs together with large amount of its cognate G(s)alpha protein. Expressed in quantitative terms, DIMs contained 30-40% of the total cellular amount of G proteins whereas the content of IP prostanoid receptors was 1-3%. The dominant portion (>95%) of FhIPR as well as FhIPR-G(s)alpha was localised in high-density area of the gradient containing detergent-solubilised proteins. FhIPR and FhIPR-G(s)alpha distribution was similar to that of transmembrane plasma membrane (PM) markers (CD147, MHCI, CD29, Tapa1, the alpha subunit of Na,K-ATPase, transmembrane form of CD58 and CD44). All these proteins are known to be fully solubilised by detergent and thus unable to float in density gradient. Our data indicate that (i) long-term agonist stimulation of IP prostanoid receptor is associated with preferential decrease of its cognate G protein G(s)alpha from membrane domains; receptor level is unchanged. (ii) Very small fraction (1-3%) of total cellular amount of receptors is recovered in DIMs together with roughly 40% of G proteins. These data suggest a "supra-stoichiometric" arrangement of G proteins and corresponding receptors in DIMs.

Chronic morphine-induced changes in mu-opioid receptors and G proteins of different subcellular loci in rat brain

Prolonged exposure to opioid agonists can induce adaptive changes resulting in tolerance and dependence. Here, rats were rendered tolerant by subcutaneous injections of increasing doses of morphine from 10 to 60 mg/kg for 3, 5, or 10 consecutive days. Binding parameters of the mu-opioid receptor in subcellular fractions were measured with [(3)H]DAMGO ([D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin). Although the density of surface mu-sites did not change after the 5-day morphine treatment, up-regulation of synaptic plasma membrane binding was detected after the 10-day drug administration. In contrast, the number of mu-binding sites in a light vesicle or microsomal fraction (MI) was elevated by 68 and 30% after 5 and 10 days of morphine exposure, respectively. The up-regulated MI mu-sites displayed enhanced coupling to G proteins compared with those detected in saline-treated controls. Pertussis toxin catalyzed ADP ribosylation, and Western blotting with specific antisera was used to quantitate chronic morphine-induced changes in levels of various G protein alpha-subunits. Morphine treatment of 5 days and longer induced significant increases in levels of Galpha(o), Galpha(i1), and Galpha(i2) in MI fractions that are part of an adaptation process. Up-regulation of intracellular mu-sites may be the result of post-translational changes and in part de novo synthesis. The results provide the first evidence that distinct regulation of intracellular mu-opioid receptor G protein coupling and G protein levels may accompany the development of morphine tolerance.

A novel phenylaminotetralin radioligand reveals a subpopulation of histamine H(1) receptors

Previously, (-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalene ([-]-trans-H(2)-PAT) was shown to activate stereospecifically histamine H(1) receptors coupled to modulation of tyrosine hydroxylase activity in guinea pig and rat forebrain in vitro and in vivo. Furthermore, the novel radioligand [(3)H](-)-trans-H(2)-PAT was shown to label selectively H(1) receptors in guinea pig and rat brain with high affinity (K(D), ~0.1 and 0.5 nM, respectively) and a B(max) about 50 and 15%, respectively, of that observed for the H(1) antagonist radioligand [(3)H]mepyramine. In the current study, [(3)H](-)-trans-H(2)-PAT-labeled cloned guinea pig and human H(1) receptors in Chinese hamster ovary (CHO) cell membranes with high affinity (K(D), ~0.08 and 0.23 nM, respectively) and a B(max) about 15% of that observed for [(3)H]mepyramine. The binding of H(2)-PAT to H(1) receptors in both CHO-H(1) cell lines was stereoselective with the (-)-trans-isomer having affinity (K(i), ~1.5 nM) about 4-, 20-, and 50-times higher than the (-)-cis-, (+)-trans-, and (+)-cis-isomers, respectively; the affinity of (-)-trans-H(2)-PAT was unaffected by excess GTP. In functional assays, (-)-trans-H(2)-PAT was a full antagonist of histamine H(1)-mediated stimulation of phospholipase C (PLC) and [(3)H]inositol phosphates (IP) formation in CHO-H(1) cells, a full inverse agonist of constitutively active H(1) receptors in COS-7-H(1) cells, and a full competitive antagonist (pA(2) = 9.2) of histamine H(1)-mediated contraction of guinea pig ileum. It is concluded that (-)-trans-H(2)-PAT is an antagonist at H(1) receptors coupled to PLC/IP formation and smooth muscle contraction. Meanwhile, the observation that [(3)H](-)-trans-H(2)-PAT labels only a subpopulation of H(1) receptors and that (-)-trans-H(2)-PAT activates H(1) receptors coupled to modulation of tyrosine hydroxylase suggests that there may be post-translational H(1) receptor heterogeneity.

G protein-coupled receptors: dominant players in cell-cell communication

The G protein-coupled receptors (GPCRs) are the most numerous and the most diverse type of receptors (1-5% of the complete invertebrate and vertebrate genomes). They transduce messages as different as odorants, nucleotides, nucleosides, peptides, lipids, and proteins. There are at least eight families of GPCRs that show no sequence similarities and that use different domains to bind ligands and activate a similar set of G proteins. Homo- and heterodimerization of GPCRs seem to be the rule, and in some cases an absolute requirement, for activation. There are about 100 orphan GPCRs in the human genome which will be used to find new message molecules. Mutations of GPCRs are responsible for a wide range of genetic diseases. The importance of GPCRs in physiological processes is illustrated by the fact that they are the target of the majority of therapeutical drugs and drugs of abuse.

Syntheses of fused heterocyclic compounds and their inhibitory activities for squalene synthase

A variety of fused heterocyclic compounds (2-11) were synthesized as a modification of the lead compound 1a and evaluated for their inhibition of squalene synthase. 4,1-Benzothiazepine derivative 2, 1,4-benzodiazepine derivative 6, 1,3-benzodiazepine derivative 7, 1-benzazepine derivative 9, and 4,1-benzoxazocine derivative 10 potently inhibited squalene synthase activity, whereas the 4,1-benzoxazepine derivatives 1 was the most potent inhibitor. 4,1-Benzothiazepine S-oxide derivative 4, 1,4-benzodiazepine derivative 5, 1,3,4-benzotriazepine derivative 8, and 1,2,3,4-tetrahydroquinoline derivative 11 were found to be weakly active. Comparison of the X-ray structures of these compounds (1a, 2, 4, 5, 7 and 10) suggests that orientation of the 5- (or 6)-phenyl group is important for activity.

The human serotonin 5-HT4 receptor regulates secretion of non-amyloidogenic precursor protein

The serotonin 5-HT(4) receptor has recently gained a lot of attention for its functional roles in central processes such as memory and cognition. In this study, we show that activation of the human 5-HT(4) (h5-HT(4)) receptor stimulates the secretion of the non-amyloidogenic soluble form of the amyloid precursor protein (sAPPalpha). 5-HT enhanced the level of secreted sAPPalpha in a time- and dose-dependent manner in Chinese hamster ovary cells stably expressing the h5-HT(4(e)) receptor isoform. The increase was inhibited by the selective 5-HT(4) receptor antagonist, GR113808. The 5-HT(4) selective agonists, prucalopride and renzapride, also increased secreted sAPPalpha in IMR32 human neuroblastoma cells. The stimulatory effect of 5-HT was mimicked by forskolin, a direct activator of adenylyl cyclase, and 8-bromo-cAMP, a membrane-permeant cAMP analogue. On the contrary, inhibition of protein kinase A (PKA) by H89 potentiated the 5-HT-induced increase in both secreted and cellular sAPPalpha. This phenomenon involves a novel PKA-independent stimulatory process that overcomes a PKA-dependent inhibitory one. Finally, activation of the h5-HT(4(e)) receptor did not modify extracellular amyloid beta-protein in Chinese hamster ovary cells transfected with the human APP695. Given the neuroprotective and enhancing memory effects of sAPPalpha, our results may open a new avenue for the treatment of Alzheimer's disease.

Galpha(olf) levels are regulated by receptor usage and control dopamine and adenosine action in the striatum

In the striatum, dopamine D(1) and adenosine A(2A) receptors stimulate the production of cAMP, which is involved in neuromodulation and long-lasting changes in gene expression and synaptic function. Positive coupling of receptors to adenylyl cyclase can be mediated through the ubiquitous GTP-binding protein Galpha(S) subunit or through the olfactory isoform, Galpha(olf), which predominates in the striatum. In this study, using double in situ hybridization, we show that virtually all striatal efferent neurons, identified by the expression of preproenkephalin A, substance P, or D(1) receptor mRNA, contained high amounts of Galpha(olf) mRNA and undetectable levels of Galpha(s) mRNA. In contrast, the large cholinergic interneurons contained both Galpha(olf) and Galpha(s) transcripts. To assess the functional relationship between dopamine or adenosine receptors and G-proteins, we examined G-protein levels in the striatum of D(1) and A(2A) receptor knock-out mice. A selective increase in Galpha(olf) protein was observed in these animals, without change in mRNA levels. Conversely, Galpha(olf) levels were decreased in animals lacking a functional dopamine transporter. These results indicate that Galpha(olf) protein levels are regulated through D(1) and A(2A) receptor usage. To determine the functional consequences of changes in Galpha(olf) levels, we used heterozygous Galpha(olf) knock-out mice, which possess half of the normal Galpha(olf) levels. In these animals, the locomotor effects of amphetamine and caffeine, two psychostimulant drugs that affect dopamine and adenosine signaling, respectively, were markedly reduced. Together, these results identify Galpha(olf) as a critical and regulated component of both dopamine and adenosine signaling.

Specific desensitization of sulfonylurea- but not imidazoline-induced insulin release after prolonged tolbutamide exposure

Functional effects of prolonged exposure to the sulfonylurea, tolbutamide, were examined in the clonal electrofusion-derived BRIN-BD11 cell line. In acute 20-min incubations, 50-400 microM tolbutamide stimulated a dose-dependent increase (P < 0.01) in insulin release at both non-stimulatory (1.1 mM) and stimulatory (8.4 mM) glucose. Culture with 100 microM tolbutamide (18 hr) caused a marked (67%) decrease in subsequent insulin-secretory responsiveness to acute challenge with 200 microM tolbutamide, though notably, tolbutamide culture exerted no influence on 200 microM efaroxan-induced insulin secretion. Duration of exposure (3-18 hr) to 100 microM tolbutamide in culture also time-dependently influenced subsequent responsiveness to acute tolbutamide challenge, with progressive 47-58% decreases from 6-18 hr (P < 0.001). Similarly, 6- to 18-hr culture with 100 microM efaroxan specifically desensitized efaroxan-induced insulin release. Tolbutamide- and efaroxan-induced desensitization exhibited a time-dependent reversibility, with a sustained return to full insulin-secretory responsiveness by 12 hr. Notably, 18-hr culture with tolbutamide or efaroxan did not significantly affect insulinotropic responses to 16.7 mM glucose, 10 mM 2-ketoisocaproic acid, 10 mM alanine, 10 mM arginine, or 30 mM KCl. Diverse inhibitory actions of tolbutamide or efaroxan culture on late events in stimulus-secretion coupling reveal that drug desensitization is both a specific and important phenomenon. As such, the model system described could prove an important tool in determining the complex modes of action of established and novel clinically useful insulinotropic compounds.

Chronic systemic administration of salmeterol to rats promotes pulmonary beta(2)-adrenoceptor desensitization and down-regulation of G(s alpha)

1. The aim of the present study was to examine the effects of chronic infusion of the long-acting agonist salmeterol on pulmonary beta(2)-adrenoceptor function in Sprague-Dawley rats in vivo and to elucidate the molecular basis of any altered state. 2. Systemic administration of rats with salmeterol for 7 days compromised the ability of salmeterol and prostaglandin E(2) (PGE(2)), given acutely by the intravenous route, to protect against ACh-induced bronchoconstriction when compared to rats treated identically with vehicle. 3. beta(1)- and beta(2)-adrenoceptor density was significantly reduced in lung membranes harvested from salmeterol-treated animals, which was associated with impaired salmeterol- and PGE(2)-induced cyclic AMP accumulation ex vivo. 4. Three variants of G(s alpha) that migrated as 42, 44 and 52 kDa peptides on SDS polyacrylamide gels were detected in lung membranes prepared from both groups of rats but the intensity of each isoform was markedly reduced in rats that received salmeterol. 5. The activity of cytosolic, but not membrane-associated, G-protein receptor-coupled kinase was elevated in the lung of salmeterol-treated rats when compared to vehicle-treated animals. 6. The ability of salmeterol, administered systemically, to protect the airways of untreated rats against ACh-induced bronchoconstriction was short-acting (t(off) approximately 45 min), which contrasts with its long-acting nature when given to asthmatic subjects by inhalation. 7. These results indicate that chronic treatment of rats with salmeterol results in heterologous desensitization of pulmonary G(s)-coupled receptors. In light of previous data obtained in rats treated chronically with salbutamol, we propose that a primary mechanism responsible for this effect is a reduction in membrane-associated G(s alpha). The short-acting nature of salmeterol, when administered systemically, and the reduction in beta-adrenoceptor number may be due to metabolism to a biologically-active, short-acting and non-selective beta-adrenoceptor agonist.

The regulator of G protein signaling family

Regulator of G protein signaling (RGS) proteins are responsible for the rapid turnoff of G protein-coupled receptor signaling pathways. The major mechanism whereby RGS proteins negatively regulate G proteins is via the GTPase activating protein activity of their RGS domain. Structural and mutational analyses have characterized the RGS/G alpha interaction in detail, explaining the molecular mechanisms of the GTPase activating protein activity of RGS proteins. More than 20 RGS proteins have been isolated, and there are indications that specific RGS proteins regulate specific G protein-coupled receptor pathways. This specificity is probably created by a combination of cell type-specific expression, tissue distribution, intracellular localization, posttranslational modifications, and domains other than the RGS domain that link them to other signaling pathways. In this review we discuss what has been learned so far about the role of RGS proteins in regulating G protein-coupled receptor signaling and point out areas that may be fruitful for future research.

Albuterol-induced downregulation of Gsalpha accounts for pulmonary beta(2)-adrenoceptor desensitization in vivo

The aim of the present study was to develop a chronic in vivo model of pulmonary beta(2)-adrenoceptor desensitization and to elucidate the nature and molecular basis of this state. Subcutaneous infusion of rats with albuterol for 7 days compromised the ability of albuterol, given acutely, to protect against acetylcholine-induced bronchoconstriction. The bronchoprotective effect of prostaglandin E(2), but not forskolin, was also impaired, indicating that the desensitization was heterologous and that the primary defect in signaling was upstream of adenylyl cyclase. beta(2)-Adrenoceptor density was reduced in lung membranes harvested from albuterol-treated animals, and this was associated with impaired albuterol-induced cyclic adenosine monophosphate (cAMP) accumulation and activation of cAMP-dependent protein kinase ex vivo. Gsalpha expression was reduced in the lung and tracheae of albuterol-treated rats, and cholera toxin-induced cAMP accumulation was blunted. Chronic treatment of rats with albuterol also increased cAMP phosphodiesterase activity and G protein-coupled receptor kinase-2, but the extent to which these events contributed to beta(2)-adrenoceptor desensitization was unclear given that forskolin was active in both groups of animals and that desensitization was heterologous. Collectively, these results indicate that albuterol effects heterologous desensitization of pulmonary Gs-coupled receptors in this model, with downregulation of Gsalpha representing a primary molecular etiology.

The human nucleus accumbens is highly susceptible to G protein down-regulation by methamphetamine and heroin

Although the nucleus accumbens is assumed to be a critical brain "pleasure center," its function in humans is unknown. As animal data suggest that a unique feature of this small brain area is its high sensitivity to down-regulation of an inhibitory G protein by drugs of abuse, we compared G protein levels in postmortem nucleus accumbens with those in seven other brain regions of chronic users of cocaine, methamphetamine, and heroin, and of matched controls. Biochemical changes were restricted to the nucleus accumbens in which concentrations of G(alpha)1 and/or G(alpha)2 were reduced by 32-49% in the methamphetamine and heroin users. This selective responsiveness to these abused drugs implies a special role for the human nucleus accumbens in mechanisms of drug reinforcement and suggests that some features of the drug-dependent state (e.g., tolerance) might be related to inhibition of G(alpha)1-linked receptor activity.

Properties of the ventricular adrenergic signal transduction system during ontogeny of spontaneous hypertension in rats

The purpose of this study was to characterize adrenergic receptors and associated G proteins in ventricles of spontaneously hypertensive rats (SHRs) at different stages of development. The beta- and alpha1-adrenoceptor densities and subtype distribution, and beta-adrenoceptor-G protein coupling were studied by radioligand binding, and levels of G(Salpha), G(ialpha), and G(q/11alpha) protein species were determined by Western blotting in SHRs and Wistar-Kyoto (WKY) control rats aged 3.5 weeks, 3 months, and 8 months. In 3.5-week-old SHRs, both the beta-adrenoceptor density and the percentage of agonist high-affinity binding sites were higher than in age-matched WKY rats. The beta1/beta2-subtype distribution, the alpha1-adrenoceptor density, and the alpha1B/alpha1A-subtype distribution were similar in rats of both strains at all ages. Although essentially no differences in G(salpha) levels between SHRs and WKY rats were detected, higher G(ialpha) and lower Gq/1alpha concentrations were found in 3.5-week-old SHRs. In 3-month-old SHRs, increased levels of Gq/11alpha proteins were observed. In 8-month-old SHRs, none of the parameters was different from those of controls. We conclude that the differences in properties of the adrenergic signal transduction system between SHRs and WKY rats are exclusively observable before and at the onset of the overt hypertension. Moreover, the hypertensive genotype apparently affects G proteins more readily than adrenoceptors.

Functional significance of stimulatory GTP-binding protein in hippocampus is associated with kindling-elicited epileptogenesis

In order to evaluate the involvement of the stimulatory G-protein (Gs)-related transduction system in the basic mechanisms of epilepsy, we examine the expression levels of Gsalpha mRNA and specific GTP-binding ability in the hippocampus of amygdaloid-kindled rats at various seizure stages. Northern blot analysis showed a significant increase in the Gsalpha mRNA expression level in the bilateral hippocampus at 24h after the last generalized seizure. The [3H]-GTP-binding assay with isoproterenol (IPN), a beta-receptor agonist, revealed a remarkable increase of Bmax values in the sham-operated control and partially kindled groups. However, the IPN-induced increase of Bmax values was abolished on both sides of the hippocampus at 24 h after and at 4 weeks after the last generalized seizure in fully kindled rats. These data suggest that alteration in the Gs function and beta-adrenergic receptor-Gs coupling might be implicated in the neurobiological basis of the induction mechanisms of the generalization of seizures and the mechanisms of the maintenance of enduring epileptogenesis. Conversely, the Gs-related transduction system might have a lesser impact on the acquisition process of epileptogenesis.

Low phosphoinositide-specific phospholipase C activity and expression of phospholipase C beta1 protein in the prefrontal cortex of teenage suicide subjects

OBJECTIVE

The enzyme phosphoinositide-specific phospholipase C (PI-PLC) is a component of the phosphoinositide signal transduction system. Other components of this system have been found to be abnormal in adults and adolescents who have committed suicide, and so the authors examined whether PI-PLC activity and protein expression of PLC isozymes are abnormal in postmortem brains of teenage suicide subjects.

METHOD

PI-PLC activity and protein expression of the PLC beta1, delta1, and gamma1 isozymes were examined in Brodmann's areas 8 and 9 of postmortem brains obtained from 18 teenage suicide subjects and 18 matched comparison subjects. PI-PLC activity was determined by enzymatic assay, and protein expression of the PLC isozymes was determined by the Western blot technique.

RESULTS

Compared with the normal subjects, the teenage suicide subjects had significantly lower PI-PLC activity and immunolabeling of the specific PLC beta1 isozyme in both membrane and cytosol fractions of Brodmann's areas 8 and 9 combined (prefrontal cortex). There was also a significant correlation between PI-PLC activity and protein levels of the PLC beta1 isozyme in the brains of the teenage suicide subjects. There was no significant difference in PI-PLC activity or level of PLC beta1 protein between the suicide subjects with a history of mental disorders and those with no history of mental disorders; however, both groups had significantly lower PI-PLC activity and expression of PLC beta1 protein than the normal subjects.

CONCLUSIONS

Low PI-PLC activity and expressed levels of the PLC beta1 isozyme in postmortem brains of suicide subjects may have clinical relevance in the pathophysiology of suicidal behavior.

Activation of the beta(2)-adrenergic receptor-Galpha(s) complex leads to rapid depalmitoylation and inhibition of repalmitoylation of both the receptor and Galpha(s)

Palmitoylation is unique among lipid modifications in that it is reversible. In recent years, dynamic palmitoylation of G protein alpha subunits and of their cognate receptors has attracted considerable attention. However, very little is known concerning the acylation/deacylation cycle of the proteins in relation to their activity status. In particular, the relative contribution of the activation and desensitization of the signaling unit to the regulation of the receptors and G proteins palmitoylation state is unknown. To address this issue, we took advantage of the fact that a fusion protein composed of the stimulatory alpha subunit of trimeric G protein (Galpha(s)) covalently attached to the beta(2)-adrenergic receptor (beta(2)AR) as a carboxyl-terminal extension (beta(2)AR-Galpha(s)) can be stimulated by agonists but does not undergo rapid inactivation, desensitization, or internalization. When expressed in Sf9 cells, both the receptor and the Galpha(s) moieties of the fusion protein were found to be palmitoylated via thioester linkage. Stimulation with the beta-adrenergic agonist isoproterenol led to a rapid depalmitoylation of both the beta(2)AR and Galpha(s) and inhibited repalmitoylation. The extent of depalmitoylation induced by a series of agonists was correlated (0.99) with their intrinsic efficacy to stimulate the adenylyl cyclase activity. However, forskolin-stimulated cAMP production did not affect the palmitoylation state of beta(2)AR-Galpha(s), indicating that the agonist-promoted depalmitoylation is linked to conformational changes and not to second messenger generation. Given that, upon activation, the fusion protein mimics the activated receptor-G protein complex but cannot undergo desensitization, the data demonstrate that early steps in the activation process lead to the depalmitoylation of both receptor and G protein and that repalmitoylation requires later events that cannot be accommodated by the activated fusion protein.

Stimulation of cAMP synthesis by Gi-coupled receptors upon ablation of distinct Galphai protein expression. Gi subtype specificity of the 5-HT1A receptor

The three Galphai subunits were independently depleted from rat pituitary GH4C1 cells by stable transfection of each Galphai antisense rat cDNA construct. Depletion of any Galphai subunit eliminated receptor-induced inhibition of basal cAMP production, indicating that all Galphai subunits are required for this response. By contrast, receptor-mediated inhibition of vasoactive intestinal peptide (VIP)-stimulated cAMP production was blocked by selective depletions for responses induced by the transfected serotonin 1A (5-HT1A) (Galphai2 or Galphai3) or endogenous muscarinic-M4 (Galphai1 or Galphai2) receptors. Strikingly, receptor activation in Galphai1-depleted clones (for the 5-HT1A receptor) or Galphai3-depleted clones (for the muscarinic receptor) induced a pertussis toxin-sensitive increase in basal cAMP production, whereas the inhibitory action on VIP-stimulated cAMP synthesis remained. Finally, in Galphai2-depleted clones, activation of 5-HT1A receptors increased VIP-stimulated cAMP synthesis. Thus, 5-HT1A and muscarinic M4 receptor may couple dominantly to Galphai1 and Galphai3, respectively, to inhibit cAMP production. Upon removal of these Galphai subunits to reduce inhibitory coupling, stimulatory receptor coupling is revealed that may involve Gbetagamma-induced activation of adenylyl cyclase II, a Gi-stimulated cyclase that is predominantly expressed in GH4C1 cells. Thus Gi-coupled receptor activation involves integration of both inhibitory and stimulatory outputs that can be modulated by specific changes in alphai subunit expression level.

Distinct mechanisms for the endocytosis of muscarinic receptors and Gq/11

To determine whether the agonist-mediated endocytosis of muscarinic receptors and Gq/11 are mechanistically related events, the internalization of Gq/11 was monitored under conditions established to prevent muscarinic receptor endocytosis. Incubation of SH-SY5Y neuroblastoma cells with oxotremorine-M resulted in the translocation of both muscarinic receptors and Gq/11 into a 'light vesicle' membrane fraction. Although muscarinic receptor translocation was prevented by either the depletion of phosphoinositides or by disruption of clathrin assembly, the endocytosis of Gq/11 was unaffected. These results demonstrate that the agonist-induced internalization of muscarinic receptors and Gq/11 proceed via distinct mechanisms.

Lithium and haloperidol treatments differently affect the mononuclear leukocyte Galphas protein levels in bipolar affective disorder

Despite numerous suggestions of the involvement of GTP-binding proteins in the mechanisms of action of psychoactive drugs in bipolar affective disorder, few studies have been conducted during the drug treatment of patients. The aim of the present study was to investigate the effects of a mood stabilizer and an antipsychotic drug on Galphas proteins. Patients with bipolar affective disorder under lithium treatment with or without haloperidol were assessed with respect to their mononuclear leukocyte (MNL) Galphas subunit protein. Galphas-45 protein subunit levels were analyzed by the Western immunoblot method. The subjects consisted of a group of 20 patients, all diagnosed as euthymic bipolars, and a comparison group of 15 drug-free healthy subjects. Results showed that Galphas levels were significantly decreased in the bipolar patients (BP) compared to drug-free healthy subjects (Mann-Whitney U test, p < 0.002). The drug effect was evaluated by a factorial analysis of variance and showed significant differences between groups (Kruskal-Wallis H test, p < 0.02). Lithium-treated patients displayed the most decreased Galphas levels (normalized mean values 53.2 +/- 31 vs. 122 +/- 45% for BP and controls, respectively, p < 0.001), while no change was observed in Galphas levels of haloperidol-treated patients compared to controls (mean values: 124.9 +/- 37%; NS). The data indicate that lithium and haloperidol affect the mechanism of Galphas protein signal transduction differently, consistent with previous animal studies.

Agonist-induced release of splice variants of the alpha subunit of the stimulatory G-protein from rat cardiac membranes

It was the aim of the present study to evaluate whether, in physiological cardiac tissue, long and short splice variants of G(s)alpha (the alpha subunit of the stimulatory G-protein) differ in their susceptibility to guanine nucleotide-mediated activation. As a measure of G(s)alpha activation, we determined the proportion of G(s)alpha subunits which were released from the plasma membrane upon stimulation. Membrane preparations from heart ventricles of Wistar rats were incubated with increasing concentrations of the non-hydrolyzable GTP analogue guanylyl-imidodiphosphate (GppNHp, 0-100 micromol/L) in the absence or presence of the beta-adrenoceptor agonist isoprenaline (1 micromol/L). The 45 and 52 kDa forms of G(s)alpha (G(s)alpha-S and -L, respectively) were measured in the supernatant of the incubation mixture by immunoblotting and densitometry. The increase in cyclic AMP induced by GppNHp was measured in the same supernatant. In the absence of isoprenaline, GppNHp increased cyclic AMP formation and the concentration-dependent release of G(s)alpha-L (Friedman test, P < 0.05), whereas the amount of soluble G(s)alpha-S was not affected. After addition of isoprenaline, the redistribution of G(s)alpha-S into the soluble fraction could be stimulated by GppNHp in a concentration-dependent manner (P < 0.05). Kinetic experiments revealed that activation of G(s)alpha-S by GppNHp was rather slow, but could be markedly enhanced by isoprenaline. Thus, it is likely that the different susceptibilities of G(s)alpha-S and -L towards GppNHp reflects differences in the rate of spontaneous GDP release.

Carbachol-induced desensitization of PLC-beta pathway in rat myometrium: downregulation of Gqalpha/G11alpha

In the estrogen-treated rat myometrium, carbachol increased the generation of inositol phosphates by stimulating the muscarinic receptor-Gq/G11-phospholipase C-beta3 (PLC-beta3) cascade. Exposure to carbachol resulted in a rapid and specific (homologous) attenuation of the subsequent muscarinic responses in terms of inositol phosphate production, PLC-beta3 translocation to membrane, and contraction. Refractoriness was accompanied by a reduction of membrane muscarinic binding sites and an uncoupled state of residual receptors. Protein kinase C (PKC) altered the functionality of muscarinic receptors and contributed to the initial period of desensitization. A delayed phase of the muscarinic refractoriness was PKC independent and was associated with a downregulation of Gqalpha/G11alpha. Atropine failed to induce desensitization as well as Gqalpha/G11alpha downregulation, indicating that both events involve active occupancy of the receptor. Prolonged exposure to AlF-4 reduced subsequent AlF-4 as well as carbachol-mediated inositol phosphate responses and similarly induced downregulation of Gqalpha/G11alpha. Data suggest that a decrease in the level of Gqalpha/G11alpha is subsequent to its activation and may account for heterologous desensitization.

Agonist-induced internalization of the G protein G11alpha and thyrotropin-releasing hormone receptors proceed on different time scales

Using a combination of confocal immunofluorescence microscopy and subcellular fractionation, we demonstrate for the first time active internalization, trafficking, and down-regulation of a G protein alpha subunit subsequent to agonist occupation of a receptor. This proceeds on a much slower time scale than internalization of the corresponding receptor. In intact E2M11 HEK293 cells that express high levels of murine G11alpha and the rat thyrotropin-releasing hormone (TRH) receptor, the immunofluorescence signal of G11alpha was restricted almost exclusively to the plasma membrane. Exposure to TRH (10 microM) resulted first in partial relocation of G11alpha to discrete, segregated patches within the plasma membrane (10-60 min). Further exposure to TRH caused internalization of G11alpha to discrete, punctate, intracellular bodies (2-4 h) and subsequently to a virtually complete loss of G11alpha from plasma membranes and the cells (8-16 h). Short-term treatment with TRH followed by wash-out of the ligand allowed G11alpha immunofluorescence to be restored to the plasma membrane within 12 h. In subcellular membrane fractions, G11alpha was centered on plasma membranes, and this was not altered by up to 1-2 h of incubation with TRH. Further exposure to TRH (2-4 h) resulted in transfer of a significant portion of G11alpha to light-vesicular and cytosol fractions. At longer time intervals (4-16 h), an overall decrease in G11alpha content was observed.

Galpha14 and Galphaq mediate the response to trypsin in Xenopus oocytes

Xenopus oocytes respond to trypsin with a characteristic chloride current, virtually indistinguishable from responses mediated by a large number of native and expressed G protein-coupled receptors. We studied the involvement of G proteins of the Galphaq family as possible mediators of this and other G protein-coupled receptor-mediated responses in Xenopus oocytes. We have cloned the third member of the Galphaq family, Xenopus Galpha14, in addition to the previously cloned Xenopus Galphaq and Galpha11 (Shapira, H., Way, J., Lipinsky, D., Oron, Y., and Battey, J. F. (1994) FEBS Lett. 348, 89-92). Amphibian Galpha14 is 354 amino acids long and is 93% identical to its mammalian counterpart. Based on the Galpha14 cDNA sequence, we designed a specific antisense DNA oligonucleotide (antiGalpha14) that, together with antiGalphaq and antiGalpha11, was used in antisense depletion experiments. 24 h after injection into oocytes, either antiGalphaq or antiGalpha14 reduced the response to 1 microg/ml trypsin by 70%, whereas antiGalpha11 had no effect. A mixture of antiGalphaq and antiGalpha14 virtually abolished the response. These data strongly suggest that Galphaq and Galpha14 are the exclusive mediators of the trypsin-evoked response in Xenopus oocytes. Similar experiments with the expressed gastrin-releasing peptide receptor and muscarinic m1 receptor revealed the coupling of Galphaq and Galpha11, but not Galpha14, to these receptors in oocytes. These results confirm the hypothesis that endogenous members of the Galphaq family discriminate among different native receptors in vivo.

Sensitivity of norepinephrine-evoked vasoconstriction to pertussis toxin in the old rat

In male Wistar rats, the in vitro vasoconstrictor response of the perfused tail artery elicited by norepinephrine or serotonin decreased with age (24 mo old vs. 3 mo old), whereas the fluorescent signal (fura 2) produced by intracellular calcium (Ca2+i) mobilization increased. Both vasoconstriction and the increase in intracellular calcium concentration elicited by a high-K+, depolarizing solution were unaffected by aging. Pertussis toxin, a G protein inhibitor, had no effect on vasoconstriction induced by high K+ but diminished vasoconstrictor responses to norepinephrine in 3- and 12-mo-old animals but not in 24-mo-old animals. Pertussis toxin had no effect on Ca2+i mobilization. The sensitivity of receptor activation to pertussis toxin in tail arteries from 24-mo-old animals was restored by pretreatment with the alpha-adrenoceptor antagonist nicergoline. Nicergoline had no effect on vasoconstriction induced by high K+. Plasma norepinephrine concentration rose with age; nicergoline had no effect on this rise. We suggest that aging leads to a decrease in the intracellular G protein-modulated amplification of vasoconstriction produced by receptor activation and that this could be linked to the hyperadrenergic state. Ca2+ sensitivity can be restored by chronic treatment with an alpha-adrenoceptor antagonist.

Endothelin-1 modulation of cAMP in rat pulmonary arteries: effect of chronic hypoxia

The effect of hypoxic shock on the ability of endothelin (ET) receptors to interact with the cAMP second messenger system was assessed in rat pulmonary arteries. Whole pieces of tissue were dissected from the pulmonary arterial system of control and hypoxic (10% O2, 14 days) rats, incubated, where appropriate, with ET-1 (0.1 microM), and the levels of intracellular cAMP measured. Maintenance of rats under hypoxic conditions significantly reduced the basal cAMP levels in all of the arterial branches with the exception of the pulmonary resistance vessels, in which no change was observed. Incubation of the main and first branch extralobar pulmonary arteries from control rats with ET-1 resulted in a consistent decrease in the levels of intracellular cAMP. The ETA receptor antagonist FR139317 partially blocked this ET-1-mediated inhibition of cAMP accumulation in the main extralobar artery. In contrast, ET-1 caused a threefold increase in the levels of this cyclic nucleotide in the pulmonary resistance vessels from the normoxic rat. No ET-1-mediated reduction in intracellular cAMP levels was observed in any of the vessels isolated from hypoxic animals. All vessels showed ligand-activated increases in cAMP production. These results suggest differential modulation of cAMP in the different pulmonary arteries, either by direct activation through Gi and Gs or indirectly via a uncharacterized cross-talk mechanism.

Down-regulation of phospholipase C-beta1 following chronic muscarinic receptor activation

To determine whether prolonged activation of a phospholipase C-coupled receptor can lead to a down-regulation of its effector enzyme, SH-SY5Y neuroblastoma cells were incubated for 24 h with the muscarinic receptor agonist, oxotremorine-M. Under these conditions, significant reductions (46-53%) in muscarinic cholinergic receptor density, G(alphaq/11) and phospholipase C-beta1 (but not the beta3-or gamma1 isoforms) were observed. These results suggest that a selective down-regulation of phospholipase C-beta1 may play a role in adaptation to chronic muscarinic receptor activation.

Altered G protein activity in a desensitization-resistant mutant of the Y1 adrenocortical tumor cell line

Mutant isolates [designated desensitization resistant (DR)] from the Y1 mouse adrenocortical tumor cell line resist agonist-induced desensitization of adenylyl cyclase by preventing the uncoupling of receptors from their guanyl nucleotide-binding regulatory G proteins. In this study, we tested the hypothesis that an underlying G protein defect is associated with the DR phenotype. We found that the G protein reagent guanyl-5'-yl imidodiphosphate [Gpp(NH)p] shifted beta2-adrenergic receptors from a high affinity state to a low affinity state 4-fold more effectively in mutant DR cells than in parent Y1 cells. In the DR mutant, Gpp(NH)p was able to shift receptors to a low affinity state in the absence of NaCl, whereas the effect of Gpp(NH)p in parent Y1 cells was dependent upon the presence of NaCl. Moreover, these differences in sensitivity to Gpp(NH)p and NaCl were transferred to Gs alpha-deficient S49(CYC-) lymphoma cell membranes in G protein reconstitution assays. These observations suggested that the DR mutation was associated with altered activity of the stimulatory G protein, Gs. Cloning and sequence analysis demonstrated that Gs alpha transcripts in the DR mutant were normal, suggesting that another factor involved in guanyl nucleotide exchange is responsible for the altered G protein activity in DR mutant cells.

Role of carboxyl terminus of mu-and delta-opioid receptor in agonist-induced down-regulation

Chronic exposure of mu-and delta-opioid receptors to their agonists leads to different rates in receptor down-regulation. In order to analyze the role of the carboxyl terminus of mu-and delta-opioid receptors in the difference in the rate of down-regulation, two chimeras of these receptors were generated by swapping the carboxyl termini; MORTAGDT and DORTAGMT. These chimeras were tagged at the N-terminus with hemagglutinin (HA) epitope (YPYDVPDYA), which can be recognized by the monoclonal antibody 12CA5, and then stably expressed in Neuro 2A (N2A) cells. The swapping of the carboxyl termini did not alter the ligand selectivity of these receptor chimeras. However, they did exhibit a reduction in agonist potency to inhibit forskolin-stimulated adenylyl cyclase activity for all agonists tested except etorphine which had a potency comparable to that of wild type receptors. Treatment of the N2A cells expressing MORTAGDT with 50 nM etorphine produced a faster rate of receptor down-regulation when compared to the wild type mu-opioid receptor. Immunofluorescence microscopy of the MORTAGDT chimera using a monoclonal antibody against HA confirmed internalization of the receptors after treatment with etorphine for 1 and 6h. There was a reduction in the HA-immunoreactivity at the cell surface of the MORTAGDT chimera concurrent with more noticeable HA-immunoreactivity inside the cell compared to the wild type receptor. On the other hand, the rate of down-regulation of DORTAGMT receptors was seen to be the same as the wild type delta-opioid receptor after etorphine treatment. Immunofluorescence studies showed more reduction in cell surface staining of the DORTAGMT chimera compared to the wild type receptor. These data suggest the involvement of the carboxyl terminus in agonist-induced down-regulation and internalization of the nu-opioid receptor. However, different mechanisms that are unrelated to the carboxyl terminus may operate in the down-regulation of delta-opioid receptor.

Inverse agonism and the regulation of receptor number

Inverse agonists are ligands that preferentially stabilize inactive conformations of G protein-coupled receptors. In a range of systems, sustained treatment with inverse agonists can produce substantially greater upregulation of receptor levels than antagonists. The use of constitutively active mutant receptors can exaggerate this effect but may also allow agonists and antagonists to mimic the effect by preventing denaturation of the mutant receptor polypeptide. In this review Graeme Milligan and Richard Bond consider the basis for these effects and their therapeutic implications.

Subsensitivity of adenylyl cyclase-coupled receptors on mononuclear leukocytes from drug-free inpatients with a major depressive episode

Previous studies have demonstrated blunted beta-adrenergic responsivity in leukocytes from depressed patients. We sought to determine if this blunted cyclic adenosine monophosphate (AMP) response is specific for beta-adrenergic receptors (homologous), or whether other adenylyl cyclase-coupled receptors are also involved (heterologous), in order to localize this effect at the level of the receptor versus the coupling protein or the transducer, adenylyl cyclase. We studied adenylyl cyclase-mediated responses in peripheral blood mononuclear cells from 95 drug-free patients with a major depressive episode and 69 healthy controls. We found a similar degree of decrease in the peak cyclic AMP response to activation of the beta-adrenergic receptor (28%) and the prostaglandin receptor (34%) in the depressed patients, which indicated heterologous desensitization. Forskolin cyclic AMP responses were not blunted. Blunting of cyclic AMP responses to isoproterenol did not appear to correlate with levels of plasma norepinephrine and epinephrine or hypothalamic-pituitary-adrenocortical function. The absence of a decrease in the peak forskolin-generated cyclic AMP response, which involves direct activation of adenylyl cyclase, suggests an abnormality at the level of the coupling protein in these adenylyl-coupled receptors in depressed patients. Future studies need to determine whether this leukocyte signal transduction defect in depression also involves brain adenylyl cyclase-coupled receptors.

Is signal transduction modulated by an interaction between heterotrimeric G-proteins and tubulin?

Although it is generally accepted that tubulin plays an important role in G-protein-mediated signal transduction in a variety of systems, the mechanism of this phenomenon is not completely understood. G-protein-tubulin interaction at the cell membrane and the cytosol, and the influence of such an interaction on cellular signaling are discussed in this review article. Because the diameter of a microtubule is 25 nm and the plasma membrane is 9-11 nm thick, it is not possible for membrane-associated tubulin to assemble into a complete microtubule in the membrane environment. However, tubulin heterodimers may be able to function in the membrane environment as individual heterodimers or as polymers arranged into short protofilaments. At the cell membrane, membrane-associated tubulin may influence hormone-receptor interaction, receptor-G-protein coupling, and G-protein-effector coupling. Structural proteins, such as tubulin, can participate in cellular signaling by communicating through physical forces. By virtue of its interaction with the submembranous network of cytoskeletal proteins, tubulin, when perturbed in one locus, can transmit large changes in conformations to other points. Thus, GTP binding to membrane-associated tubulin might lead to a conformational change in either receptors or G proteins. This may, in turn, influence the binding of an agonist to its receptor. On the other hand, in the cell cytosol, subsequent to agonist-induced translocation of G-proteins from the membrane compartment to the cytosol, G-proteins may affect microtubule formation. In GH3 and AtT-20 cells (stably expressing TRH receptor), transiently transfected with Gq alpha cDNA, soluble tubulin levels decreased in Gq alpha-transfected GH3 and AtT-20 cells, by 33% and 52%, respectively. These results suggest that G-proteins may have a direct effect on the microtubule function in vivo. Because tubulin and G-protein families are ubiquitous and highly conserved, an interaction between these two protein families may occur in vivo, and this, in turn, can have an impact on signal transduction. However, the physiological significance of this interaction remains to be demonstrated.

Structure-function analysis of inhibitory adenosine receptor regulation

Pharmacological and molecular cloning studies have revealed the presence of four adenosine receptor (AR) subtypes, termed A1, A2A, A2B and A3. Given that the A1 and A3ARs can both bind adenosine and couple productively to inhibitory G-proteins, the significance of the existence of multiple inhibitory AR subtypes remains obscure, although one possibility is that these receptors are regulated in a subtype-specific manner. In this review, we summarize our investigations into the mechanisms underlying the agonist-induced desensitization of inhibitory AR function. The results of this work demonstrate that while the A1AR desensitizes slowly over a time course of several hours, the A3AR desensitizes within minutes of agonist exposure. Molecular biological studies have begun to delineate the structural requirements responsible for these differences, and will provide a basis for future experiments designed to determine whether the ability of an inhibitory AR receptor subtype to 'turn-off' at a specific rate has implications for the physiological role of that receptor.