Distribution of serotonin receptor 5-HT6 mRNA in rat neuronal subpopulations: A double in situ hybridization study

The 5-HT6 receptor (5-HT6R) is almost exclusively expressed in the brain and has emerged as a promising target for cognitive disorders, including Alzheimer's disease. In the present study, we have determined the cell types on which the 5-HT6R is expressed by colocalizing 5-HT6R mRNA with that of a range of neuronal and interneuronal markers in the rat brain. Here, we show that 5-HT6R mRNA was expressed at high levels in medium spiny neurons in caudate putamen and in nucleus accumbens, as well as in the olfactory tubercle. Striatal 5-HT6R mRNA was colocalized with both dopamine D1 and D2 receptor mRNA. 5-HT6R mRNA was moderately expressed in the hippocampus and throughout cortical regions in glutamatergic neurons coexpressing vGluT1. A subset of GAD67-positive GABAergic interneurons (approximately 15%) expressed 5-HT6R mRNA in the cortex and hippocampus, the majority of which belonged to the 5-HT3a receptor (5-HT3aR)-expressing subpopulation. In contrast, 5-HT6R mRNA was only expressed to a minor extent in the parvalbumin and somatostatin subpopulations. A subset of calbindin- and calretinin-positive GABAergic interneurons expressed 5-HT6R mRNA while only a very minor fraction of VIP or NPY interneurons in forebrain structures expressed 5-HT6R mRNA. Serotonergic, dopaminergic or cholinergic neurons did not express 5-HT6R mRNA. These data indicate that the 5-HT6R is located on GABAergic and glutamatergic principal neurons, and on a subset of interneurons mainly belonging to the 5-HT3aR subgroup suggesting that the 5-HT6R is positioned to regulate the balance between excitatory and inhibitory signaling in the brain. These data provide new insights into the mechanisms of 5-HT6R signaling.

Selectivity of (3)H-MADAM binding to 5-hydroxytryptamine transporters in vitro and in vivo in mice; correlation with behavioural effects

1. Binding of the novel radioligand (3)H-2-(2-dimethylaminomethyl-phenylsulphanyl)-5-methyl-phenylamine ((3)H-MADAM) to the serotonin transporter (SERT) was used to characterise a range of selective serotonin re-uptake inhibitors (SSRIs) in vitro and in vivo. 2. (3)H-MADAM bound with high affinity in a saturable manner to both human SERT expressed in CHO cells (K(d)=0.20 nm (pK(d)=9.74+/-0.12), B(max)=35+/-4 fmol mg(-1) protein) and mouse cerebral cortex membranes (K(d)=0.21 nm (pK(d)=9.66+/-0.10), B(max)=50+/-24 fmol mg(-1) protein). 3. Binding of (3)H-MADAM was highly selective for SERT in vitro as demonstrated by the in vitro profile of MADAM tested at 75 different receptors, ion channels and transporters. This was further substantiated by the pharmacological profile of the binding. Hence, the binding of (3)H-MADAM was potently inhibited by SSRIs but not by selective inhibitors of noradrenaline transport and dopamine transport. Likewise, a 5-HT(2A/2C) receptor antagonist did not inhibit (3)H-MADAM binding. 4. (3)H-MADAM binding in vivo was inhibited only by compounds which also inhibited the binding of (3)H-MADAM in vitro (the SSRIs, mixed SERT/noradrenaline transport inhibitors and clomipramine), confirming the selectivity of (3)H-MADAM for SERT also in vivo. Moreover, compounds effective in inhibiting (3)H-MADAM binding were the only ones found to be active in the mouse 5-HTP potentiation test confirming the model as a behavioural correlate to in vivo 5-HT uptake. 5. Finally, it was found that a SERT occupancy of 85-95% was necessary to produce 50% of the maximum behavioural response (ED(50)).

GluR2 ligand-binding core complexes: importance of the isoxazolol moiety and 5-substituent for the binding mode of AMPA-type agonists

X-ray structures of the GluR2 ligand-binding core in complex with (S)-Des-Me-AMPA and in the presence and absence of zinc ions have been determined. (S)-Des-Me-AMPA, which is devoid of a substituent in the 5-position of the isoxazolol ring, only has limited interactions with the partly hydrophobic pocket of the ligand-binding site, and adopts an AMPA-like binding mode. The structures, in comparison with other agonist complex structures, disclose the relative importance of the isoxazolol ring and of the substituent in the 5-position for the mode of binding. A relationship appears to exist between the extent of interaction of the ligand with the hydrophobic pocket and the affinity of the ligand.

Structural basis for AMPA receptor activation and ligand selectivity: crystal structures of five agonist complexes with the GluR2 ligand-binding core

Glutamate is the principal excitatory neurotransmitter within the mammalian CNS, playing an important role in many different functions in the brain such as learning and memory. In this study, a combination of molecular biology, X-ray structure determinations, as well as electrophysiology and binding experiments, has been used to increase our knowledge concerning the ionotropic glutamate receptor GluR2 at the molecular level. Five high-resolution X-ray structures of the ligand-binding domain of GluR2 (S1S2J) complexed with the three agonists (S)-2-amino-3-[3-hydroxy-5-(2-methyl-2H-tetrazol-5-yl)isoxazol-4-yl]propionic acid (2-Me-Tet-AMPA), (S)-2-amino-3-(3-carboxy-5-methylisoxazol-4-yl)propionic acid (ACPA), and (S)-2-amino-3-(4-bromo-3-hydroxy-isoxazol-5-yl)propionic acid (Br-HIBO), as well as of a mutant thereof (S1S2J-Y702F) in complex with ACPA and Br-HIBO, have been determined. The structures reveal that AMPA agonists with an isoxazole moiety adopt different binding modes in the receptor, dependent on the substituents of the isoxazole. Br-HIBO displays selectivity among different AMPA receptor subunits, and the design and structure determination of the S1S2J-Y702F mutant in complex with Br-HIBO and ACPA have allowed us to explain the molecular mechanism behind this selectivity and to identify key residues for ligand recognition. The agonists induce the same degree of domain closure as AMPA, except for Br-HIBO, which shows a slightly lower degree of domain closure. An excellent correlation between domain closure and efficacy has been obtained from electrophysiology experiments undertaken on non-desensitising GluR2i(Q)-L483Y receptors expressed in oocytes, providing strong evidence that receptor activation occurs as a result of domain closure. The structural results, combined with the functional studies on the full-length receptor, form a powerful platform for the design of new selective agonists.

CRM1 mediates the export of ADAR1 through a nuclear export signal within the Z-DNA binding domain

RNA editing of specific residues by adenosine deamination is a nuclear process catalyzed by adenosine deaminases acting on RNA (ADAR). Different promoters in the ADAR1 gene give rise to two forms of the protein: a constitutive promoter expresses a transcript encoding (c)ADAR1, and an interferon-induced promoter expresses a transcript encoding an N-terminally extended form, (i)ADAR1. Here we show that (c)ADAR1 is primarily nuclear whereas (i)ADAR1 encompasses a functional nuclear export signal in the N-terminal part and is a nucleocytoplasmic shuttle protein. Mutation of the nuclear export signal or treatment with the CRM1-specific drug leptomycin B induces nuclear accumulation of (i)ADAR1 fused to the green fluorescent protein and increases the nuclear editing activity. In concurrence, CRM1 and RanGTP interact specifically with the (i)ADAR1 nuclear export signal to form a tripartite export complex in vitro. Furthermore, our data imply that nuclear import of (i)ADAR1 is mediated by at least two nuclear localization sequences. These results suggest that the nuclear editing activity of (i)ADAR1 is modulated by nuclear export.

Stereochemistry and molecular pharmacology of (S)-thio-ATPA, a new potent and selective GluR5 agonist

(RS)-2-Amino-3-(5-tert-butyl-3-hydroxy-4-isothiazolyl)propionic acid (thio-ATPA), a 3-isothiazolol analogue of (RS)-2-amino-3-(5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid (ATPA), has previously been shown to be a relatively weak AMPA receptor agonist at native (S)-glutamic acid ((S)-Glu) receptors (EC(50)=14 microM), comparable in potency with ATPA (EC(50)=34 microM). Recent findings, that (S)-ATPA is a potent (EC(50)=0.48 microM) and selective agonist at homomerically expressed ionotropic GluR5, prompted us to resolve thio-ATPA using chiral chromatography and pharmacologically characterize the two enantiomers at native as well as cloned ionotropic glutamate receptors. The enantiomers, (S)- and (R)-thio-ATPA, were obtained in high enantiomeric excess, and their absolute stereochemistry established by an X-ray crystallographic analysis. Electrophysiologically, the two enantiomers were evaluated in the rat cortical wedge preparation, and the S-enantiomer was found to be an AMPA receptor agonist (EC(50)=8.7 microM) twice as potent as the racemate, whereas the R-enantiomer was devoid of activity. In accordance with this, (S)-thio-ATPA proved to be an agonist at homomerically expressed recombinant AMPA receptors (GluR1o, GluR3o, and GluR4o) with EC(50) values of 5, 32 and 20 microM, respectively, producing maximal steady state currents of 78--168% of those maximally evoked by kainic acid, and 120-1600% of those maximally evoked by (S)-ATPA. At homomerically expressed GluR5, (S)-thio-ATPA was found to be a potent agonist (EC(50)=0.10 microM), thus being approximately five times more potent than (S)-ATPA. (R)-Thio-ATPA induced saturating currents with an estimated EC(50) value of 10 microM, most likely due to a contamination with (S)-thio-ATPA. At heteromerically expressed GluR6+KA2 receptors, (S)-thio-ATPA showed relatively weak agonistic properties (EC(50)=4.9 microM). Thus, (S)-thio-ATPA has been shown to be a very potent agonist at GluR5, and may be a valuable tool for the investigation of desensitization properties of AMPA receptors.

Larger intercellular variation in (Q/R) editing of GluR6 than GluR5 revealed by single cell RT-PCR

RNA editing of the pre-mRNA encoding the kainate receptor subtypes determines the Ca2+ permeability and the rectifying properties of the receptors in which these are assembled. GluR6 pre-mRNA contains three characterized editing sites: Q/R, IN and the Y/C, whereas GluR5 pre-mRNA contains only the (Q/R) site. Single cell RT-PCR was used on cultured cortical neurons to determine the relative expression and editing levels of the kainate receptor subunits encoding mRNA. The analysis showed a large intercellular variation in editing efficiency. The overall lower level of GluR5 editing, in the culture, compared to GluR6 editing is a result of an approximately 60% lower editing efficiency of GluR5 pre-mRNA, within single cells, compared with GluR6.

Resolution, configurational assignment, and enantiopharmacology of 2-amino-3-[3-hydroxy-5-(2-methyl-2H- tetrazol-5-yl)isoxazol-4-yl]propionic acid, a potent GluR3- and GluR4-preferring AMPA receptor agonist

We have previously shown that (RS)-2-amino-3-[3-hydroxy-5-(2-methyl-2H-tetrazol-5-yl)isoxazol -4-yl] propionic acid (2-Me-Tet-AMPA) is a selective agonist at (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptors, markedly more potent than AMPA itself, whereas the isomeric compound 1-Me-Tet-AMPA is essentially inactive. We here report the enantiopharmacology of 2-Me-Tet-AMPA in radioligand binding and cortical wedge electrophysiological assay systems, and using cloned AMPA (GluR1-4) and kainic acid (KA) (GluR5, 6, and KA2) receptor subtypes expressed in Xenopus oocytes. 2-Me-Tet-AMPA was resolved using preparative chiral HPLC. Zwitterion (-)-2-Me-Tet-AMPA was assigned the (R)-configuration based on an X-ray crystallographic analysis supported by the elution order of (-)- and (+)-2-Me-Tet-AMPA using four different chiral HPLC columns and by circular dichroism spectra. None of the compounds tested showed detectable affinity for N-methyl-D-aspartic acid (NMDA) receptor sites, and (R)-2-Me-Tet-AMPA was essentially inactive in all of the test systems used. Whereas (S)-2-Me-Tet-AMPA showed low affinity (IC(50) = 11 microM) in the [(3)H]KA binding assay, it was significantly more potent (IC(50) = 0.009 microM) than AMPA (IC(50) = 0.039 microM) in the [(3)H]AMPA binding assay, and in agreement with these findings, (S)-2-Me-Tet-AMPA (EC(50) = 0.11 microM) was markedly more potent than AMPA (EC(50) = 3.5 microM) in the electrophysiological cortical wedge model. In contrast to AMPA, which showed comparable potencies (EC(50) = 1.3-3.5 microM) at receptors formed by the AMPA receptor subunits (GluR1-4) in Xenopus oocytes, more potent effects and a substantially higher degree of subunit selectivity were observed for (S)-2-Me-Tet-AMPA: GluR1o (EC(50) = 0.16 microM), GluR1o/GluR2i (EC(50) = 0.12 microM), GluR3o (EC(50) = 0.014 microM) and GluR4o (EC(50) = 0.009 microM). At the KA-preferring receptors GluR5 and GluR6/KA2, (S)-2-Me-Tet-AMPA showed much weaker agonist effects (EC(50) = 8.7 and 15.3 microM, respectively). It is concluded that (S)-2-Me-Tet-AMPA is a subunit-selective and highly potent AMPA receptor agonist and a potentially useful tool for studies of physiological AMPA receptor subtypes.

Effects of topiramate on kainate- and domoate-activated [14C]guanidinium ion flux through GluR6 channels in transfected BHK cells using Cytostar-T scintillating microplates

PURPOSE

This study was undertaken to test the hypothesis that topiramate (TPM) exerts a negative modulatory effect on some types of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA)/kainate receptors by binding to the site at which protein kinase A (PKA) phosphorylates the receptor-channel complex.

METHODS

The effect of TPM on kainate- or domoate-induced [14C]guanidinium ion flux through iGluR6 channels expressed in baby hamster kidney (BHK) cells was evaluated. Because the hypothesis predicts that TPM will bind only in the dephosphorylated state, a variety of experimental conditions were used to either promote or impede the phosphorylation of the receptor-channel complex. These included the use of dibutyryl cyclic adenosine monophosphate (cAMP) and forskolin to activate PKA, H-9 and H-89 to inhibit PKA, and okadaic acid to inhibit protein phosphatases.

RESULTS

Kainate (1 microM) induced a gradual accumulation of [14C]guanidinium into the cells that plateaued approximately 30 min after initiation of the reaction, whereas domoate (0.1 microM) caused a rapid accumulation into the cells that peaked within 5 min; thereafter, the amount of [14C]guanidinium in the cells declined gradually. Topiramate, at 0.1 and 100 microM, did not significantly affect the [14C]guanidinium accumulation under any of the experimental conditions used.

CONCLUSIONS

The results of this study are not consistent with the hypothesis tested. However, the results must be interpreted cautiously because iGluR6 receptors expressed in the BHK cells and the functional state of proteins that regulate AMPA/receptors (e.g., PSD-95) may not be sufficiently similar to the receptors and functional state in neurons to serve as a true test of the hypothesis.

Resolution, absolute stereochemistry, and enantiopharmacology of the GluR1-4 and GluR5 antagonist 2-amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl]propionic acid

The phosphono amino acid, (RS)-2-amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl+ ++]propio nic acid (ATPO), is a structural hybrid between the NMDA antagonist (RS)-2-amino-7-phosphonoheptanoic acid (AP7) and the AMPA and GluR5 agonist, (RS)-2-amino-3-(5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid (ATPA). ATPO has been resolved into (S)-ATPO and (R)-ATPO using chiral HPLC, and the absolute stereochemistry of the two enantiomers was established by an X-ray crystallographic analysis of (R)-ATPO. (S)-ATPO and (R)-ATPO were characterized pharmacologically using rat brain membrane binding and electrophysiologically using the cortical wedge preparation as well as homo- or heteromeric GluR1-4, GluR5-6, and KA2 receptors expressed in Xenopus oocytes. (R)-ATPO was essentially inactive as an agonist or antagonist in all test systems. (S)-ATPO was an inhibitor of the binding of [(3)H]AMPA (IC(50) = 16 +/- 1 microM) and of [(3)H]-6-cyano-7-nitroquinoxaline-2,3-dione ([(3)H]CNQX) (IC(50) = 1.8 +/- 0.2 microM), but was inactive in the [(3)H]kainic acid and the [(3)H]-(RS)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid ([(3)H]CPP) binding assays. (S)-ATPO did not show detectable agonist effects at any of the receptors under study, but antagonized AMPA-induced depolarization in the cortical wedge preparation (IC(50) = 15 +/- 1 microM). (S)-ATPO also blocked kainic acid agonist effects at GluR1 (K(i) = 2.0 microM), GluR1+2 (K(i) = 3.6 microM), GluR3 (K(i) = 3.6 microM), GluR4 (K(i) = 6.7 microM), and GluR5 (K(i) = 23 microM), but was inactive at GluR6 and GluR6+KA2. Thus, although ATPO is a structural analog of AP7 neither (S)-ATPO nor (R)-ATPO are recognized by NMDA receptor sites.

Resolution, absolute stereochemistry and molecular pharmacology of the enantiomers of ATPA

(RS)-2-Amino-3-(5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid (ATPA), an analogue of (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA). has previously been shown to be a relatively weak AMPA receptor agonist and a very potent agonist at the GluR5 subtype of kainic acid-preferring (S)-glutamic acid ((S)-Glu) receptors. We report here the separation of (+)- and (-)-ATPA, obtained at high enantiomeric purity (enantiomeric excess values of 99.8% and > 99.8%, respectively) using chiral chromatography, and the unequivocal assignment of the stereochemistry of (S)-(+)-ATPA and (R)-(-)-ATPA. (S)- and (R)-ATPA were characterized in receptor binding studies using rat brain membranes, and electrophysiologically using the rat cortical wedge preparation and cloned AMPA-preferring (GluR1, GluR3, and GluR4) and kainic acid-preferring (GluR5, GluR6, and GluR6 + KA2) receptors expressed in Xenopus oocytes. In the cortical wedge, (S)-ATPA showed AMPA receptor agonist effects (EC50 = 23 microM) approximately twice as potent as those of ATPA. (R)-ATPA antagonized depolarizations induced by AMPA (Ki = 253 microM) and by (S)-ATPA (Ki = 376 microM), and (R)-ATPA antagonized the biphasic depolarizing effects induced by kainic acid (Ki = 301 microM and 1115 microM). At cloned AMPA receptors, (S)-ATPA showed agonist effects at GluR3 and GluR4 with EC50 values of approximately 8 microM and at GluR1 (EC50 = 22 microM), producing maximal steady state currents only 5.4-33% of those evoked by kainic acid. (R)-ATPA antagonized currents evoked by kainic acid at cloned AMPA receptor subtypes with Ki values of 33-75 microM. (S)-ATPA produced potent agonist effects at GluR5 (EC50 = 0.48 microM). Due to desensitization of GluR5 receptors, which could not be fully prevented by treatment with concanavalin A, (S)-ATPA-induced agonist effects were normalized to those of kainic acid. Under these circumstances, maximal currents produced by (S)-ATPA and kainic acid were not significantly different. (R)-ATPA did not attenuate currents produced by kainic acid at GluR5, and neither (S)- nor (R)-ATPA showed significant effects at GluR6. (S)-ATPA as well as AMPA showed weak agonist effects at heteromeric GluR6 + KA2 receptors, whereas (R)-ATPA was inactive. Thus, (S)- and (R)-ATPA may be useful tools for mechanistic studies of ionotropic non-NMDA (S)-Glu receptors, and lead structures for the design of new subtype-selective ligands for such receptors.

Characterization of a new AMPA receptor radioligand, [3H]2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid

(RS)-2-Amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA), which is a potent and selective agonist at (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptors, has previously been shown to desensitize AMPA receptors to a much lower degree than AMPA itself. We now report the synthesis of [3H]ACPA (32.5 Ci/mmol), the neurochemical and pharmacological characterization of [3H]ACPA binding, and a comparison of the distribution of [3H]ACPA, [3H]AMPA, and [3H](S)-5-fluorowillardiine binding sites in rat brain. Under equilibrium conditions, [3H]ACPA was shown to bind to a single population of receptor sites on rat brain membranes. [3H]ACPA was shown to bind with single and similar affinities (15-45 nM) to cloned AMPA receptor subunits (GluR1-4), expressed in insect cells, whereas a K(D) value of 330 nM was determined for the binding of [3H]ACPA to cloned kainic acid preferring GluR5 subunits. Whereas Bmax and K(D) values for [3H]ACPA binding, determined using filtration techniques, were different from such obtained in centrifugation assays, Bmax and K(D) values as well as association and dissociation constants were not significantly affected by the addition of the chaotropic agent KSCN. K(D) values, determined under equilibrium conditions, were, however, markedly different from K(D) values derived from kinetic data. Furthermore, the results of analyses of these kinetic data were consistent with the existence of two different populations of [3H]ACPA binding sites. The pharmacology of [3H]ACPA binding sites was characterized using a series of AMPA receptor agonists and antagonists. Whereas addition of KSCN had little effect on the affinities of AMPA receptor agonists for [3H]ACPA binding, this chaotropic agent reduced the affinities of AMPA receptor antagonists structurally related to AMPA. Based on these and previously reported data, the AMPA receptor agonists, ACPA, AMPA and (S)-5-fluorowillardiine, seem to bind to and activate AMPA receptors in a nonidentical fashion, and these three agonists together may be useful tools for studies of AMPA receptor mechanisms.

A six-domain structural model for Escherichia coli translation initiation factor IF2. Characterisation of twelve surface epitopes

The Escherichia coli translation initiation factor IF2 is a 97 kDa protein which interacts with the initiator fMet-tRNAfMet, GTP and the ribosomal subunits during initiation of protein biosynthesis. For structural and functional investigations of the factor, we have raised and characterised monoclonal antibodies against E. coli IF2. Twelve epitopes have been localised at the surface of the protein molecule by three different methods: Interactions of the monoclonal antibodies with nested deletion mutants of IF2, comparison of the relative location of the epitopes in a competition immunoassay and cross-reactivity analyses of the monoclonal antibodies towards IF2 from Salmonella typhimurium, Klebsiella oxytoca, Enterobacter cloacae, Proteus vulgaris, and Bacillus stearothermophilus. These data are combined with predicted secondary structure and discussed in relation to a six-domain structural model for IF2. The model describes IF2 as a slightly elongated molecule with a structurally compact C-terminal domain, a well-conserved central GTP-binding domain, and a highly charged, solvent exposed N-terminal with protruding alpha-helical structures.

Assembly of proteins and 5 S rRNA to transcripts of the major structural domains of 23 S rRNA

The six major structural domains of 23 S rRNA from Escherichia coli, and all combinations thereof, were synthesized as separate T7 transcripts and reconstituted with total 50 S subunit proteins. Analysis by one and two-dimensional gel electrophoresis demonstrated the presence of at least one primary binding protein associated with each RNA domain and additional proteins assembled to domains I, II, V and VI. For all the combinations of two to five domains, enhanced assembly yields and/or new proteins were observed primarily to those transcripts containing either domains I+II or domains V+VI. This indicates that there are two major protein assembly centres located at the ends of the 23 S rRNA, which is consistent with an earlier view that in vitro protein assembly nucleates around proteins L24 and L3. Although similar protein assembly patterns were observed over a range of temperature and magnesium concentrations, protein L2 assembled strongly with domains II and IV at 4-8 mM Mg2+ (the first step of the two-step reconstitution procedure) and with domain IV alone at higher Mg2+ concentrations (the second step). It is proposed that this change in protein-RNA binding provides a basis for the two-step reconstitution in vitro. A chemical footprinting approach was employed on the reconstituted protein-domain complexes to localize a putative L4 binding region within domain I to a region that is partially co-structural with the site on the L4-mRNA where L4 binds and inhibits its own translation. A similar approach was used to map the putative binding regions on domain V of protein L9 and the 5 S RNA-L5-L18 complex.

Antagonist properties of a phosphono isoxazole amino acid at glutamate R1-4 (R,S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid receptor subtypes

The activity of the (R, S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor antagonist, (R,S) -2-amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl] propionic acid (ATPO), at recombinant ionotropic glutamate receptors (GluRs) was evaluated using electrophysiological techniques. Responses at homo- or heterooligomeric AMPA-preferring GluRs expressed in human embryonic kidney (HEK) 293 cells (GluR1-flip) or Xenopus laevis oocytes (GluR1-4-flop or GluR1-flop + GluR2) were potently inhibited by ATPO with apparent dissociation constants (Kb values) ranging from 3.9 to 26 microM. A Schild analysis for kainate (KA)-activated GluR1 receptors showed ATPO to have a KB of 8.2 microM and a slope of unity, indicating competitive inhibition. The antagonism by ATPO at GluR1 was of similar magnitude at holding potentials between -100 mV and +20 mV. In contrast, ATPO (<300 microM), does not inhibit responses to kainate at homomeric GluR6 or heterooligomeric GluR6/KA2 expressed in HEK 293 cells but activated GluR5 and GluR5/KA2 expressed in X. laevis oocytes. ATPO produced <15% inhibition at the maximal concentration (300 microM) of current responses through NR1A + NR2B receptors expressed in X. laevis oocytes. Thus, ATPO shows a unique pharmacological profile, being an antagonist at GluR1-4 and a weak partial agonist at GluR5 and GluR5/KA2.

Modern methods for probing RNA structure

Molecular biologists have been remarkably successful in dividing large RNAs into small functional modules manageable for NMR and X-ray studies. At the same time biophysical, biochemical and genetic tools in RNA structure determination have reached a level of sophistication, at which we start to see a glimpse of molecular dynamics and the mechanism of RNA mediated catalysis.

Immunochemical determination of the cellular content of polypeptide chain release factor RF3 in Escherichia coli

Polypeptide chain termination in Escherichia coli is known to require two codon specific release factors, RF1 and RF2. A third factor, RF3, has been described to stimulate the termination. Earlier investigations have estimated the cellular content of factors RF1 and RF2. Two different immunological techniques for measuring the amount of RF3 per cell in crude E coli cell extracts are reported here, using a sensitive immunoblotting method and a sandwich assay by ELISA. Monoclonal murine antibodies and polyclonal rabbit antibodies were raised against extensively purified recombinant E coli RF3. The immunoblotting involves a specific monoclonal antibody (mAb), biotinylated second antibody and finally radioactive iodinated streptavidin. In the sandwich assay polyclonal antibodies are immobilised on a polystyrene surface before addition of crude cell extract; a specific mAb serves as primary antibody and an HRP-labelled anti-mouse Ig as secondary antibody. Both methods are accurate and rapid to perform. The number of RF3 molecules per cell in exponentially growing E coli cells was found to vary considerably according to the K12 strain examined and depended on the culture medium (from 20 to 500 molecules per cell), faster growth being positively correlated with the number of RF3 molecules per cell.

1-(3-(9H-carbazol-9-yl)-1-propyl)-4-(2-methoxyphenyl)-4-piperidinol, a novel subtype selective inhibitor of the mouse type II GABA-transporter

The selectivity of new derivatives of the gamma-aminobutyric acid (GABA)-uptake inhibitor, tiagabine was characterized at the four cloned mouse GABA transporters (mGAT1 through mGAT4) by measuring [3H]-GABA uptake into stably transfected baby hamster kidney cells. While tiagabine is a highly selective inhibitor of mGAT1 (Ki = 0.11 +/- 0.02 microM), these derivatives exhibited low potencies at mGAT1 but differential activities at mGAT2, mGAT3 and mGAT4. In particular, 1-(3-(9H-carbazol-9-yl)-1-propyl)-4-(2-methoxyphenyl)-4-piperidino l (NNC 05-2090) was a potent inhibitor of mGAT2 (Ki = 1.4 +/- 0.3 microM) showing at least 10 fold selectivity over mGAT1, mGAT3 and mGAT4. NNC 05-2090 is the first subtype selective inhibitor of mGAT2 and may represent a novel useful tool for investigating the physiological roles of GAT2 in the brain and periphery.

Intron sequence directs RNA editing of the glutamate receptor subunit GluR2 coding sequence

The Ca2+ permeability and the rectifying properties of the glutamate receptors assembled from the subunits GluR1-GluR4 depend upon a critical Arg in the GluR2 subunit located in a domain that has been proposed to span the membrane. The GluR2 subunit gene encodes a Gln (CAG) at this position, whereas the mRNA is edited so that it encodes an Arg (CGG) at this position [Sommer, B., Kohler, M., Sprengel, R. & Seeburg, P. H. (1991) Cell 67, 11-20]. The editing process is specific since only the GluR2 subunit RNA is edited even though the GluR1, GluR3, and GluR4 RNAs have a similar sequence. We show that this selective RNA editing depends upon a critical intron sequence in the GluR2 gene. This critical intron sequence is sufficient to cause editing of the GluR3 subunit exon in a chimera minigene constructed so that the GluR3 exon is placed upstream to the GluR2 intron sequence. Transfections of a neuronal cell line, N2a, with minigene constructs encoding different fragments of the GluR2 gene demonstrate that the 5' part of the 3' intron is essential for editing. Part of the exon and this critical intron sequence contains an inverted repeat that can fold into a structure consisting of three helical elements. Similar conclusions were reached by Higuchi, M., Single, F. n., Köhler, M., Sommer, B., Sprengel, R. & Seeburg, P. H. [(1993) Cell 75, 1361-1370]. These experiments demonstrate that the low Ca2+ permeability of the ionotropic non-N-methyl-D-aspartate glutamate receptors depends upon RNA editing, which requires a sequence in an intron 3' to the exon.

Selective RNA editing and subunit assembly of native glutamate receptors

RNA editing and subunit assembly of ionotropic glutamate receptors (GluRs) were examined in an oligodendrocyte progenitor cell line, CG4, which expresses GluR2-GluR4, GluR6, GluR7, KA1, and KA2. AMPA-evoked currents rapidly desensitize, whereas kainate-evoked currents contain a steady-state component with a nearly linear current-voltage relation and a fast desensitizing component that is inwardly rectifying. The Q/R site is edited > 95% to the arginine codon in GluR2(Q607) mRNA, and < 5% in GluR6(Q621) mRNA. Immunoprecipitation experiments demonstrate that GluR6 and/or GluR7 subunits assemble with KA2, but not with GluR2-GluR4. These results indicate that oligodendrocyte progenitor cells selectively edit and assemble glutamate receptors into at least two functionally and structurally distinct heteromeric channels.

Ca2+ permeability of unedited and edited versions of the kainate selective glutamate receptor GluR6

The Ca2+ permeability of the kainate selective glutamate receptor GluR6 depends on the editing of the RNA (or DNA). The unedited version of GluR6, GluR6Q, encodes a glutamine at position 621 (Q/R site) and exhibits a Ca2+/monovalent ion permeability ratio of 1.2, while the edited version of GluR6, GluR6R, encodes an arginine at position 621 and exhibits a permeability ratio of 0.47. Kainate activation of the GluR6 receptor results in currents that are modulated by extracellular calcium ions. Permeability ratios of other divalent ions indicate that the Q/R site is not the only determinant for divalent ion permeability. The level of editing of the receptor will determine the Ca2+ influx through the GluR6 receptor channels and, consequently, may modulate the synaptic activity.

Cloning of a putative glutamate receptor: A low affinity kainate-binding subunit

Kainate, a glutamate receptor agonist, is a potent neuroexcitatory agent that produces epileptiform activity and selective neuronal degeneration. Binding studies using neuronal membrane homogenates or brain sections have identified sites having either high or low affinity for [3H]kainate. Here we report the cloning of a gene, GluR7, with approximately 75% sequence identity with the previously cloned GluR5 and GluR6 subunit genes. Transcripts of the GluR7 gene are evident in brain areas that bind [3H]kainate and are susceptible to kainate-induced neurotoxicity. We have performed ligand binding studies with membranes of transfected HeLa cells expressing GluR6 or GluR7 subunits. Our data show that the GluR6 and GluR7 subunits have a rank order of agonist affinity (domoate greater than kainate much greater than L-glutamate, quisqualate much greater than AMPA, NMDA) and a dissociation constant for kainate (95 and 77 nM, respectively) characteristic of the low affinity kainate-binding sites described in the brain.

Attachment sites of primary binding proteins L1, L2 and L23 on 23 S ribosomal RNA of Escherichia coli

The attachment sites of the primary binding proteins L1, L2 and L23 on 23 S ribosomal RNA of Escherichia coli were examined by a chemical and ribonuclease footprinting method using several probes with different specificities. The results show that the sites are confined to localized RNA regions within the large ribonuclease-protected ribonucleoprotein fragments that were characterized earlier. They are as follows: (1) L1 recognizes a tertiary structural motif in domain V centred on two interacting internal loops; the main protein interaction sites occur at the internal loop/helix junctions. (2) The L2 site constitutes a single irregular stem/loop structure in the centre of domain IV where non-Watson-Crick pairing is likely to occur. (3) L23 recognizes a tertiary structural motif involving a single terminal loop structure and part of an adjacent internal loop at the centre of domain III. Each of the three primary binding proteins, whose presence is essential for ribosomal assembly, has been associated with important ribosomal functions: L1 lies in the E-site for deacylated tRNA binding while L2 and L23 have been implicated in the P and A substrate sites, respectively, of the peptidyl transferase centre. Moreover, each of the protein sites, but particularly those of L2 and L23, lies at the centre of RNA domains where they can maximally influence both the assembly of secondary binding proteins and the function of the RNA region.

Binding sites of the antibiotics pactamycin and celesticetin on ribosomal RNAs

The binding sites of the antibiotics pactamycin and celesticetin on the rRNAs of Escherichia coli ribosomes were investigated by a chemical footprinting procedure. Pactamycin protected residues G-693 and C-795 in 16S RNA which are located in an important functional region of the 30S subunit participating in initiation complex formation and ribosomal subunit interaction. Celesticetin altered the reactivities of 5 residues A-2058, A-2059, A-2062, A-2451 and G-2505 within the central loop of domain V of 23S RNA which has been implicated in peptidyltransferase activity. Inferences are drawn concerning the mode of action of the antibiotics.

Cloning of a cDNA for a glutamate receptor subunit activated by kainate but not AMPA

Fast excitatory transmission in the vertebrate central nervous system is mediated mainly by L-glutamate. On the basis of pharmacological, physiological and agonist binding properties, the ionotropic glutamate receptors are classified into NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate) and kainate subtypes. Sequence homology between complementary DNA clones encoding non-NMDA glutamate receptor subunits reveals at least two subunit classes: the GluR1 to GluR4 class and the GluR5 class. Here we report the cloning and expression of a functional rat glutamate receptor subunit cDNA, GluR6, which has a very different pharmacology from that of the GluR1-GluR4 class. Receptors generated from the GluR1-GluR4 class have a higher apparent affinity for AMPA than for kainate. When expressed in Xenopus oocytes the homomeric GluR6 receptor is activated by kainate, quisqualate and L-glutamate but not by AMPA, and the apparent affinity for kainate is higher than for receptors from the GluR1-GluR4 class. Desensitization of the receptor was observed with continuous application of agonist. The homomeric GluR6 glutamate receptor exhibits an outwardly rectifying current-voltage relationship. In situ hybridizations reveal a pattern of GluR6 gene expression reminiscent of the binding pattern obtained with [3H]kainate.

Characterization of the binding sites of protein L11 and the L10.(L12)4 pentameric complex in the GTPase domain of 23 S ribosomal RNA from Escherichia coli

Ribonuclease and chemical probes were used to investigate the binding sites of ribosomal protein L11 and the pentameric complex L10.(L12)4 on Escherichia coli 23 S RNA. Protein complexes were formed with an RNA fragment constituting most of domains I and II or with 23 S RNA and they were investigated by an end-labelling method and a reverse transcriptase procedure, respectively. The results demonstrate that the two protein moieties bind at adjacent sites within a small RNA region. The L11 binding region overlaps with those of the modified peptide antibiotics thiostrepton and micrococcin and is constrained structurally by a three-helix junction while the L10.(L12)4 site is centred on an adjacent internal loop. The secondary structure of the whole region was determined in detail by the phylogenetic sequence comparison method, and the results for the L11 binding region, together with the experimental data, were used in a computer graphics approach to build a partial RNA tertiary structural model. The model provides insight into the topography of the L11 binding site. It also provides a structural rationale for the mutually co-operative binding of protein L11 with the antibiotics thiostrepton and micrococcin, and with the L10.(L12)4 protein complex.

Protein L18 binds primarily at the junctions of helix II and internal loops A and B in Escherichia coli 5 S RNA. Implications for 5 S RNA structure

Ribonuclease and chemical probes were used to investigate the binding sites of ribosomal protein L18 on Escherichia coli 5 S RNA using both end-labelling and reverse transcriptase procedures. The results, together with earlier data, were superimposed on a cylindrical projection of RNA double helices and most of the protection effects were found to cluster in the major groove at two sites located on one side of the RNA at the junctions of helix II with the adjoining internal loops A and B. The loop A/helix II junction was investigated using 5 S RNA mutants, produced by site-directed mutagenesis, that exhibited altered binding properties to L18. These results, together with those from a circular dichroism study of L18 complexed with the wild-type and different mutant RNAs, enabled us to assign an L18-induced conformational change to loop A. We infer that this change contributes to the co-operative binding of L5 to helix I, which may be reinforced by the binding of the very basic N-terminal peptide of L18 within the minor groove of helix I. A psoralen derivative formed a mono-addition product with U25 within loop B in the free RNA but not in the L18 complex. Moreover, the modified molecules were selected against in L18 binding experiments. Protection effects that occurred within the adjoining helix III and loop C were compatible with a tertiary interaction between loop C and loop B/helix III that could be stabilized by the L18 binding to the junction of helix II and loop B. Further support for a bipartite binding site derived from the finding that ethidium bromide molecules that are displaced from E. coli 5 S RNA by L18 intercalate both at the loop A/helix II junction and in loop B at the binding site of the psoralen derivative.

Antibiotic interactions at the GTPase-associated centre within Escherichia coli 23S rRNA

A comprehensive range of chemical reagents and ribonucleases was employed to investigate the interaction of the antibiotics thiostrepton and micrococcin with the ribosomal protein L11-23S RNA complex and with the 50S subunit. Both antibiotics block processes associated with the ribosomal A-site but differ in their effects on GTP hydrolysis, which is inhibited by thiostrepton and stimulated by micrococcin. The interaction sites of both drugs were shown to occur within the nucleotide sequences A1067-A1098 within the protein L11 binding site on 23S RNA. This region of the ribosome structure is involved in elongation factor-G-dependent GTP hydrolysis and in the stringent response. No effects of drug binding were detected elsewhere in the 23S RNA. In general, the two drugs afforded 23S RNA similar protection from the chemical and nuclease probes in accord with their similar modes of action. One important exception, however, occurred at nucleotide A1067 within a terminal loop where thiostrepton protected the N-1 position while micrococcin rendered it more reactive. This difference correlates with the opposite effects of the two antibiotics on GTPase activity.

Domain VI of Escherichia coli 23 S ribosomal RNA. Structure, assembly and function

Domain VI at the 3' end of the 23 S ribosomal RNA from Escherichia coli was prepared using the in vitro T7 RNA polymerase system. Its structure was examined by probing with ribonucleases and chemical reagents, including a psoralen derivative, of various nucleotide specificities, using a reverse transcriptase procedure for analysis. The data provided support for the most recent secondary structure derived from phylogenetic sequence comparisons and for additional structuring that was inferred from earlier experimental data. Moreover, the structure was essentially the same in the free domain, in renatured 23 S RNA and in 50 S subunits. Protein L3 bound to the isolated domain and its binding site was located at a long-range double helix containing a large internal loop. This structure is unusual for a protein-RNA binding site and it may characterize a new (third) class of site. Protein L3 has been implicated, together with L24, in initiating assembly of the 50 S subunit and it shares the exceptional property with L24 that it binds adjacent to the junction of two RNA domains from where it can maximally influence RNA folding. Protein L6 also assembled to domain VI and, in a control experiment, protein L2 bound to isolated domain IV. Domain VI was largely inaccessible in the 50 S subunit and the few accessible RNA sites occurred mainly within conserved sequence regions that constitute potential functional sites. alpha-Sarcin inactivates ribosomes by cutting at one of these sites in 50 S subunits; it also recognized the same site in the free 23 S RNA and in the free domain. Both the EF-Tu ternary complex, and the EF-G ternary complex stabilized by fusidic acid or by a non-hydrolyzable GTP derivative, inhibited alpha-sarcin attack while non-enzymatically bound tRNA did not, thus providing evidence, more direct than before, for the involvement of the RNA region in a common elongation factor binding site.

Studies of the isolation and viability of human islets of Langerhans

Pancreas obtained from 34 adult human cadaver organ donors was divided into proximal and distal segments, and the duct to each segment was cannulated. Collagenase was injected into the proximal duct of 7 glands and into the distal duct of 7 others; the duct of the opposite segment was perfused with collagenase. The pancreas was then dispersed by teasing, trituration, and passage through filters. Perfused proximal and distal segments released 1461 +/- 287 and 2728 +/- 797 islets/g (+/- SEM) versus 710 +/- 149 (P less than 0.05) and 1950 +/- 636 after injection. Twenty other pancreases were perfused with collagenase warmed rapidly to 39 degrees C (n = 4) or warmed slowly to 37 degrees C (n = 6) or 39 degrees C (n = 10): the yield was 1625 +/- 632, 1320 +/- 116, and 2009 +/- 277 islets/g respectively. Total yields from the latter were 76 X 10(3) large (greater than 100 microns) and 85 X 10(3) small (less than 100 microns) islets with recoveries of 61% and 42%, respectively, after Ficoll density gradient purification. Histology showed highly purified islets. Perifusion with glucose elicited a biphasic release of insulin with the mean response (microU/islet/min) rising to a first peak of 0.5 and constant second phase secretion of 0.25, followed by a return to baseline. Reduced response was observed for islets from pancreas stored greater than 6 hr and tissue obtained from multiple centers. Less insulin was produced by freshly isolated islets, islets less than 100 microns, and after Ficoll separation. Secretion was similar for islets derived from proximal or distal segments. Perfusion of collagenase via the ducts of human pancreas improves islet isolation and Ficoll gradient separation yields highly purified islets. Important factors influencing insulin secretion are the source of donor tissue, cold storage of pancreas, Ficoll purification, islet size, and tissue culture.

Structure and accessibility of domain I of Escherichia coli 23 S RNA in free RNA, in the L24-RNA complex and in 50 S subunits. Implications for ribosomal assembly

Domain I of 23 S RNA of Escherichia coli was probed in renatured RNA, in the protein L24-RNA complex and in 50 S subunits with ribonucleases specific for single- and double-stranded regions and with chemical reagents specific for guanosines (N-1 and N-2), adenosines (N-1, N-7 and N-6), cytidines (N-3) and uridines (N-3). Reactive sites were detected by a reverse transcriptase procedure. The results support most new features of the latest version of the Santa Cruz/Urbana model of the secondary structure, which is based on evidence from sequence comparison. Most double-helical segments were reactive to cobra venom ribonuclease to some degree; the exceptions were the five "long-range" helices that are probably compactly folded within the structure. The data provide evidence for the occurrence of A(syn) X G(anti) pairings in internal loops and at the ends of some helices; they also support the existence of extensive higher-order structuring, especially within the interhelical regions, and are compatible with two of three tertiary interactions in the free RNA that were predicted from comparative sequence studies. Protein L24 is the only primary binding protein that associates with domain I and it strongly protects two sites against ribonuclease and chemical activity. Site A has the properties of a classic protein binding site and we conclude from four lines of evidence that it is the primary attachment site. Site B is rich in highly conserved, unpaired adenosine residues and lies in a potentially critical region of the structure adjoining a group of long-range helices; we infer that L24 binding here is related to the important role of L24 in initiating ribosomal assembly; the existence of both sites is supported, independently, by genetic experiments. L24-induced enhanced reactivities were detected throughout the domain and are consistent with a general "tuning" of the RNA structure. The RNA domain in the 50 S subunits is almost completely resistant to ribonucleases and only a few sites, mainly interhelical, are accessible to chemical reagents. The appearance of several newly reactive nucleotides in the subunit RNA and the enhancement of some others suggest that some minor conformational changes occur on assembly. Nevertheless, the minimal secondary structure of the renatured RNA appears to be retained. We draw the general conclusion that domain I is a highly structured domain that is important for initiating assembly and for the subsequent organization of the ribosome.