Gonadotropin-releasing hormone receptor-coupled gene network organization

An early gene cDNA microarray was developed to study genes that are regulated immediately following gonadotropin-releasing hormone (GnRH) receptor activation. 956 selected candidate genes were printed in triplicate, a t statistic-based regulation algorithm was used for data analysis, and the response to GnRH in a time course from 1 to 6 h was determined. Measurements were highly reproducible within arrays, between arrays, and between experiments. Accuracy and algorithm reliability were established by real-time polymerase chain reaction assays of 60 genes. Gene changes ranging from 1.3- to 31-fold on the microarray were confirmed by real-time polymerase chain reaction. Many of the genes were found to be highly regulated. The regulated genes identified were all elevated at 1 h of treatment and returned nearly or completely to baseline levels of expression by 3 h of treatment. This broad, robust, and transient transcriptional response to constant GnRH exposure includes modulators of signal transduction (e.g. Rgs2 and IkappaB), cytoskeletal proteins (e.g. gamma-actin), and transcription factors (e.g. c-Fos, Egr1, and LRG21). The interplay of the activators, repressors, and feedback inhibitors identified embodies a combinatorial code to direct the activity of specific downstream secondary genes.

Insertional mutagenesis of the arginine cage domain of the gonadotropin-releasing hormone receptor

The pattern of side-chain conservation at the cytoplasmic side of the third transmembrane domain of rhodopsin family G protein-coupled receptors, Asp/Glu-Arg-Tyr/X-X-X-Ile/Val, defines a structural "arginine cage" domain. Previous computational and mutagenesis studies of the GnRH receptor indicated an important contribution of local interactions to the function of this domain. We have investigated the functional importance of the intrahelical position and orientation of the arginine cage using insertional mutagenesis. Introduction of a single Ala proximal to the conserved Asp-Arg of this domain caused loss of detectable ligand binding. Inserting a second Ala, however, restored high-affinity agonist binding. Further insertion of three or four Ala residues at this site generated receptors that bound agonist with an affinity 3- to 10-fold higher than that of the wild-type receptor. Loss of detectable coupling to inositol phosphate turnover in all these mutant receptors confirms that the structure required in this region for efficient signaling is highly constrained. In contrast, the recovery of agonist binding with the progressive insertion of two to four Ala residues indicates that specific orientations of this segment can stabilize high-affinity receptor conformations that are uncoupled from signal transduction.

Conserved helix 7 tyrosine functions as an activation relay in the serotonin 5HT(2C) receptor

The function of the helix VII Tyr in the conserved Asn-Pro-X-X-Tyr segment of rhodopsin-like G protein coupled receptors has been investigated in many receptors. Various effects of site-directed mutation of this locus have been found, including altered coupling, sequestration and agonist affinity. We report the first constitutively active mutations of this Tyr. In the serotonin 5HT(2C) receptor, substituting Ala or Cys for Tyr resulted in a marked increase in the basal level of inositol phosphate accumulation in transfected COS-1 cells. This constitutive signaling was abolished by the inverse agonist SB206553. Introducing Phe at this locus eliminated both basal and agonist-stimulated signaling. All three mutant receptors showed an increase in binding affinity for the structurally dissimilar agonists 5-hydroxytryptamine (5HT), (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), and quipazine, suggesting that both the activating and inactivating mutations stabilize a high affinity state. These results implicate the conserved Tyr in the conformational rearrangements that occur during agonist complexing and receptor activation.

Evaluation of the role of the D2 dopamine receptor in myoclonus dystonia

A novel Val154-->Ile mutation in the D2 dopamine receptor (DRD2) on chromosome 11q23 has recently been shown to be associated with myoclonus dystonia (M-D) in one large family. Sequence analysis of the DRD2 gene in 5 M-D patients from different families did not reveal any mutations, nor was there evidence of linkage to the 11q23 region in the DRD2 gene in four other families. Receptor binding and signal transduction assays of the DRD2 mutant and wild-type receptors revealed identical agonist and antagonist affinities and functional responses. These studies suggest that M-D is genetically heterogeneous. The molecular mechanisms through which the Val-->Ile mutation may contribute to M-D remain to be determined.

Contribution of a helix 5 locus to selectivity of hallucinogenic and nonhallucinogenic ligands for the human 5-hydroxytryptamine2A and 5-hydroxytryptamine2C receptors: direct and indirect effects on ligand affinity mediated by the same locus

An important determinant of the neurobehavioral responses induced by a drug is its relative receptor selectivity. The molecular basis of ligand selectivity of hallucinogenic and nonhallucinogenic compounds of varying structural classes for the human 5-hydroxytryptamine (5-HT)2A and 5-HT2C receptors was investigated with the use of reciprocal site-directed mutagenesis. Because these two closely related receptor subtypes differ in the amino acid present at position 5.46 (residues 242 and 222 in the sequences, respectively), the effects of corresponding substitutions in the 5-HT2A[S5.46(242)-->A] and 5-HT2C[A5.46(222)-->S] receptors were studied in tandem. By studying both receptors, the direct and indirect effects of mutations on affinity and selectivity can be distinguished. The ergolines studied, mesulergine (selective for the 5-HT2C receptor) and d-lysergic acid diethylamide (selective for the 5-HT2A receptor), reversed their relative affinity with mutations in each receptor, supporting a direct role of this locus in the selectivity of these ligands. However, interchange mutations in either receptor led to decreased or unchanged affinity for (+/-)-1-)(2,5-dimethoxy-4-iodophenyl)-2-aminopropane and ketanserin, which have higher affinity for the 5-HT2A receptor, consistent with little contribution of this locus to the selectivity of these ligands. The indoleamines studied were affected differently by mutations in each receptor, suggesting that they bind differently to the two receptor subtypes. Mutation of this locus in the 5-HT2A receptor decreased the affinity of all indoleamines, whereas the interchange mutation of the 5-HT2C receptor did not affect indoleamine affinity. These results are consistent with a direct interaction between this side chain and indoleamines for the 5-HT2A receptor but not for the 5-HT2C receptor. Furthermore, this analysis shows that the higher affinity of 5-HT and tryptamine for the 5-HT2C receptor than for the 5-HT2A receptors is not due to the difference at this locus. The hallucinogens studied [d-lysergic acid diethylamide, psilocin, bufotenin, and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane] fell into different classes in this analysis. For the classes of ligand studied, the side-chain difference at this position directly determines relative ligand selectivity only for ergolines and may contribute to the specific effects of hallucinogens in this class.

Mapping the binding site pocket of the serotonin 5-Hydroxytryptamine2A receptor. Ser3.36(159) provides a second interaction site for the protonated amine of serotonin but not of lysergic acid diethylamide or bufotenin

Like other amine neurotransmitters that activate G-protein-coupled receptors, 5-hydroxytryptamine (5-HT) binds to the 5-HT2A receptor through the interaction of its cationic primary amino group with the conserved Asp3.32(155) in transmembrane helix 3. Computational experiments with a 5-HT2A receptor model suggest that the same functional group of 5-hydroxytryptamine also forms a hydrogen bond with the side chain of Ser3.36(159), which is adjacent in space to Asp3.32(155). However, other 5-HT2A receptor ligands like lysergic acid diethylamide (LSD), in which the amine nitrogen is embedded in a heterocycle, or N,N-dimethyl 5-HT, in which the side chain is a tertiary amine, are found in the computational simulations to interact with the aspartate but not with the serine, due mainly to steric hindrance. The predicted difference in the interaction of various ligands in the same receptor binding pocket was tested with site-directed mutagenesis of Ser3.36(159) --> Ala and Ser3.36(159) --> Cys. The alanine substitution led to an 18-fold reduction in 5-HT affinity and the cysteine substitution to an intermediate 5-fold decrease. LSD affinity, in contrast, was unaffected by either mutation. N,N-Dimethyl 5-HT affinity was unaffected by the cysteine mutation and had a comparatively small 3-fold decrease in affinity for the alanine mutant. These findings identify a mode of ligand-receptor complexation that involves two receptor side chains interacting with the same functional group of specific serotonergic ligands. This interaction serves to orient the ligands in the binding pocket and may influence the degree of receptor activation.

Related contribution of specific helix 2 and 7 residues to conformational activation of the serotonin 5-HT2A receptor

A conserved helix 2 Asp is required for the proper function of many G-protein-coupled receptors. To reveal the structural basis for the role of this residue, the additive effects of mutations at this locus and at a conserved helix 7 locus were investigated in the 5-HT2A receptor. All mutant receptors studied retained high affinity agonist and antagonist binding. Whereas an Asp-->Asn mutation in helix 2 eliminated coupling, interchanging the residues at the two positions by a second mutation of Asn-->Asp in helix 7 restored receptor function. These data suggest that these residues are adjacent in space and interact. The loss of function observed with Ala at either position is consistent with each side chain forming hydrogen bonds. Molecular dynamics simulations were performed on three-dimensional computational models of agonist-receptor complexes of both the wild-type receptor and the Asp-->Asn mutant receptor. Consonant with the lack of coupling observed for the mutant construct, introducing the mutation into the computational model produced a conformational change in a direction opposite to that seen from computational simulations of activation of the wild-type receptor model. These results implicate both loci in a common hydrogen-bonding network underlying receptor activation by agonist.

5-Hydroxytryptamine type 2A receptors regulate cyclic AMP accumulation in a neuronal cell line by protein kinase C-dependent and calcium/calmodulin-dependent mechanisms

The effects of 5-hydroxytryptamine (5-HT)2A receptor activation on cAMP formation were studied in a cell line derived from embryonic rat cortex (A1A1). 5-HT (EC50 = 0.87 microM) amplified the amount of cAMP formed in response to 5'-N-ethylcarboxamidoadenosine (an adenosine A2 receptor agonist), cholera toxin, and forskolin after 15 min of coincubation in the presence of the phosphodiesterase inhibitor rolipram. This effect of 5-HT was blocked by 10 nM ketanserin as well as by 10 nM spiperone, indicating a response mediated by the 5-HT2A receptor subtype. Similarly, cAMP accumulation was enhanced by coincubation with the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and the calcium ionophore A23187. After exposure to PMA for 24 hr (PKC-depleted cells), 5-HT and A23187 still enhanced cAMP formed in response to forskolin and 5'-N-ethylcarboxamidoadenosine, whereas the amplifying effects of PMA were abolished. Analysis by Western blots and PKC activity measurements revealed that, of three PKC isoforms detected in A1A1 cells (alpha, delta, and epsilon), only the calcium-independent isoform PKC-epsilon remained in membrane fractions after long term PMA treatment. In PKC-depleted cells, 5-HT-mediated amplification was greatly reduced after treatment with the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethyl)-ester or the calmodulin antagonists calmidazolium and N-(6-aminohexyl)-5-chloro-1-napthalenesulfonamide hydrochloride. In addition, 5-HT-mediated amplification of cAMP accumulation was reduced by the PKC inhibitor staurosporine in normal cells but was unaffected in PKC-depleted cells. In conclusion, these data suggest that 5-HT2A receptor activation can amplify cAMP formation in A1A1 cells by two distinct pathways coupled to the hydrolysis of inositol phosphates, i.e., PKC and calcium/calmodulin.

5-Hydroxytryptamine1-like receptors linked to increases in intracellular calcium concentration and inhibition of cyclic AMP accumulation in cultured vascular smooth muscle cells derived from bovine basilar artery

Vascular smooth muscle cells derived from bovine basilar artery by the explant method were grown in culture. In the presence of 1 microM forskolin and the phosphodiesterase inhibitor rolipram, 5-hydroxytryptamine (5-HT) agonists inhibited by 90 to 100% the accumulation of intracellular cyclic AMP (cAMP) with a rank order of potency 5-carboxamidotryptamine (5-CT) > or = 5-HT > 5-benzyloxytryptamine = sumatriptan > RU24969 [5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H indole succinate] > (+/-)-8-hydroxydipropylaminotetralin. In suspensions of cells loaded with the calcium-sensitive probe fura-2, 5-CT and 5-HT caused a biphasic increase in the concentration of intracellular free calcium ([Ca++]i) that consisted of both transient and sustained phases. The transient phase was reduced and the sustained phase abolished in the absence of extracellular calcium. The EC50 for 5-CT-induced increase in [Ca++]i (6 nM) was similar to that for inhibition of cAMP accumulation (1.3 nM). Both the inhibition of cAMP accumulation and increase in [Ca++]i were inhibited by the antagonist methiothepin (pA2 = 8.9), but not by the antagonists ketanserin, spiperone and pindolol. Both the inhibition of cAMP accumulation and increase in [Ca++]i were attenuated by greater than 85% in cells that were pretreated with pertussis toxin. PI turnover was not stimulated by 5-CT. The rank order of agonist potency, as well as the antagonist sensitivity, indicates responses mediated by one or more 5-HT1-like-type receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of ascorbate as an endogenous substance that irreversibly inhibits binding of dihydropyridine calcium channel blockers

Endogenous material present in heat-denatured extracts of rat brain that inhibited the binding of [3H]-isopropyl-4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-5-metho xyca rbonyl-2,6-dimethyl-3-pyridinecarboxylate ([3H]-PN200-110) to calcium channels in brain membranes was purified. Spectrophotometric analysis of material purified by strong anion-exchange and reverse-phase chromatography showed an absorption maximum at 266 nm at pH 7.0 that shifted to 245 nm at pH 2.0. This pH-dependent spectral shift was indistinguishable from that of ascorbic acid. Samples of the purified extract contained ascorbic acid; however, the inhibition of binding by purified material was always greater than the inhibition seen with equivalent concentrations of ascorbate, implying the presence of additional inhibitory factors. Attempts to detect and identify such inhibitory substances by chromatography showed that inhibition activity was coincident with the presence of ascorbate, and the inhibitory activity of purified material was abolished after treatment with ascorbic acid oxidase. Iron enhanced the inhibition produced by ascorbate, and chemical analysis of purified preparations revealed the presence of iron. Studies comparing the potency of the purified material with that of a mixture of ascorbate plus iron showed that the content of ascorbate and iron in the purified brain extract is sufficient to explain the observed inhibition of binding of [3H]PN200-110.

Inhibition of binding of [3H]PN200-110 to membranes from rat brain and heart by ascorbate is mediated by lipid peroxidation

Ascorbate inhibited the binding of the calcium channel blocker [3H]PN200-110 to membranes prepared from rat brain and heart. The inhibition was increased in the presence of Fe++ and Fe in a dose-dependent manner, with Fe++ being more potent than Fe . Concentration-response curves were biphasic in the absence and presence of 1 microM Fe++, with little inhibition being observed at concentrations of Fe++ below 0.03 mM or above 3 mM. Exposure of membranes to 0.1 mM ascorbate resulted in a 35 to 40% decrease in the density of binding sites for [3H] PN200-110. A further decrease in the density of binding sites of 75 to 80% was observed in the presence of 1 microM Fe++. In the presence of ascorbate a small increase in the Kd for binding of [3H]PN200-110 was also observed. A further increase was observed in the presence of 1 microM Fe++. The concentration-response curves for ascorbate- and ascorbate/iron-induced inhibition of binding were coincident with those for production of malondialdehyde, an index of lipid peroxidation. The effects were eliminated by iron chelators and antioxidants. In both tissues, the time course for ascorbate-induced inhibition of binding of [3H]PN200-110 was coincident with that for formation of malondialdehyde, with a significant lag occurring in experiments with heart membranes but not with brain membranes. Addition of 1 microM FeSO4 increased the rates of inhibition of binding and formation of malondialdehyde, but did not alter the lag time. These results suggest that the properties of voltage-dependent calcium channels are modified by conditions that promote lipid peroxidation.

Endogenous modulators of binding of [3H]nitrendipine in extracts of rat brain

Gel-filtration chromatography of neutral extract of rat brain revealed the presence of both large (LF; MW greater than 66,000) and small (SF; MW congruent to 1300) MW factors that inhibit the specific binding of [3H]nitrendipine ([3H]NT) to membranes prepared from rat brain. Neither factor inhibited the specific binding of [3H]NT to membranes prepared from rat heart. LF was heat-sensitive, was destroyed by treatment with trypsin and was not converted to SF by boiling. SF was heat-stable but was destroyed on incubation with Pronase. SF was partially purified by boiling, acid treatment and ion-exchange chromatography on Q-Sepharose. Incubation of membranes with either factor resulted in a decrease in the density of binding sites for [3H]NT with no change in the affinity of the binding sites for [3H]NT. The inhibitory effect of LF was fully reversible and was not affected by increasing the concentration of Ca++ in the binding assay. In contrast, inhibition by SF was not reversible but could be prevented by increasing the concentration of Ca++ or other divalent cations in the assay. The presence of LF during preincubation of membranes with SF attenuated the irreversible inhibition of binding of [3H]NT caused by SF. These results suggest that there are at least two distinct factors in rat brain that are capable of modulating the interaction of [3H]NT with binding sites on voltage-dependent calcium channels in rat brain.

Differences in d-[3H]lysergic acid diethylamide binding in mouse cortex and hippocampus in vivo and in vitro revealed by radioautography and rapid filtration studies

The localization of d-[3H]lysergic acid diethylamide ([3H]LSD) binding sites in mouse brain was compared in vivo and in vitro. Radioautography of brain sections incubated with 6 nM [3H]LSD in vitro revealed substantial specific binding in cortex (CTX), especially in layers III to IV and anterior cingulate gyrus, and in areas CA1 and dentate gyrus of hippocampus (HIP). In sections of brains from mice that received 100 nmol of [3H]LSD per kg and were killed 10, 15 or 30 min later, specific [3H]LSD binding in CTX had a pattern of distribution similar to that observed in vitro. In contrast, the pattern of specific [3H]LSD binding in HIP in vivo differed from the results obtained in vitro, in that it was sparse and lacked differential subregional distribution. The low specific [3H]LSD binding in vivo in HIP but not in CTX was confirmed by homogenate filtration studies of brain areas from mice that received 100 nmol of [3H]LSD per kg. The levels of free [3H]LSD, obtained after correction for time-dependent metabolism of [3H]LSD, did not vary among regions, but [3H]LSD specifically bound in HIP was 30 to 50% of that in CTX. In contrast, steady-state binding studies in vitro in membrane preparations from CTX and HIP demonstrated a similar density and affinity of [3H]LSD binding sites in the two regions. Comparison of [3H]LSD binding characteristics in vivo and in vitro suggests possible mechanisms causing the lower specific binding in HIP in vivo, including modulation of the binding sites that differ in CTX and HIP.

Histamine H2-receptors on guinea-pig ileum myenteric plexus neurons mediate the release of contractile agents

Dimaprit, a highly selective H2-agonist, caused a multiphasic contraction of guinea-pig ileal segments and ileal myenteric plexus-longitudinal muscle preparations. The initial phase was characterized by a twitch which reached a maximum in 15 to 30 sec and was followed by a partial relaxation. The later phase was variable and consisted of a series of twitch responses or of a slowly developing contracture which sometimes was accompanied by oscillatory changes in tension. dose-response curves were generated for the initial response; for isolated ileal segments the EC50 was 5.1 +/- 1.8 micrometers (mean +/- S.D., N = 7) and the Hill coefficient was 1.1 +/- 0.2 and for longitudinal muscle strips the EC50 was 5.8 +/- 1.2 micrometer and the Hill coefficient was 1.2 +/- 0.1 (N = 7). Both the initial and secondary components of the contractile responses to dimaprit were prevented by 0.2 micron tetrodotoxin or 10 microns mefenamic acid and by the production of tachphylaxis to either substance P or serotonin. Scopolamine, 0.001 to 0.1 micron, insurmountably antagonized only the initial component of the response. Mepyramine (1.0 micrometer), hexamethonium (100 microns), bromolysergic acid (0.25 microns) and p-(imidazol-1-yl)phenyl (10 microns) were without effect on the response to dimaprit. The histamine H2-receptor antagonist, tiotidine, produced parallel dextral shifts in the dose-response curve for dimaprit. The apparent pA2 value for tiotidine was 7.65. The results suggest that dimaprit acts on H2-receptors located on myenteric plexus neurons to cause the release of contractile substances. The mediators of the contractile response are tentatively identified as acetylcholine, substance P, serotonin and a product(s) of the arachadonic acid cascade.

A comparison of 2-hydroxyestradiol and U-0521 (3'4'-dihydroxy-2-methylpropiophenone, Upjohn) as in situ and in vitro inhibitors of tyrosine hydroxylase

Feedback inhibition of tyrosine hydroxylase by catechols was evaluated using in situ and in vitro enzyme assays. The three catechol compounds used were norepinephrine, 2-hydroxyestradiol, and 3'4'-dihydroxy-2-methylpropiophenone (U-0521, Upjohn); representing endogenous catecholamines, catechol estrogens, and a synthetic catechol, respectively. The in situ experiments were performed with dissociated retinal cells from rats and with stationary phase adrenergic-like neuroblastoma cells (N1E-115). The catechol estrogen, 2-hydroxyestradiol, resembled the endogenous catecholamine in its potency to inhibit in vitro and in situ tyrosine hydroxylations with IC50 values of 10 microM in vitro and 100 microM in situ. The drug U-0521, which has been used as an inhibitor of catechol-O-methyltransferase (COMT), was also found to be an inhibitor of tyrosine hydroxylase. Further, it was shown to be more potent than the natural catechols, both in vitro and in situ, with IC50 values of 30--600 nM.