LL-00066471, a novel positive allosteric modulator of α7 nicotinic acetylcholine receptor ameliorates cognitive and sensorimotor gating deficits in animal models: Discovery and preclinical characterization

α7 nicotinic acetylcholine receptor (α7 nAChR) is an extensively validated target for several neurological and psychiatric conditions namely, dementia and schizophrenia, owing to its vital roles in cognition and sensorimotor gating. Positive allosteric modulation (PAM) of α7 nAChR represents an innovative approach to amplify endogenous cholinergic signaling in a temporally restricted manner in learning and memory centers of brain. α7 nAChR PAMs are anticipated to side-step burgeoning issues observed with several clinical-stage orthosteric α7 nAChR agonists, related to selectivity, tolerance/tachyphylaxis, thus providing a novel dimension in therapeutic strategy and pharmacology of α7 nAChR ion-channel. Here we describe a novel α7 nAChR PAM, LL-00066471, which potently amplified agonist-induced Ca2+ fluxes in neuronal IMR-32 neuroblastoma cells in a α-bungarotoxin (α-BTX) sensitive manner. LL-00066471 showed excellent oral bioavailability across species (mouse, rat and dog), low clearance and good brain penetration (B/P ratio > 1). In vivo, LL-00066471 robustly attenuated cognitive deficits in both procognitive and antiamnesic paradigms of short-term episodic and recognition memory in novel object recognition task (NORT) and social recognition task (SRT), respectively. Additionally, LL-00066471 mitigated apomorphine-induced sensorimotor gating deficits in acoustic startle reflex (ASR) and enhanced antipsychotic efficacy of olanzapine in conditioned avoidance response (CAR) task. Further, LL-00066471 corrected redox-imbalances and reduced cortico-striatal infarcts in stroke model. These finding together suggest that LL-00066471 has potential to symptomatically alleviate cognitive deficits associated with dementias, attenuate sensorimotor gating deficits in schizophrenia and correct redox-imbalances in cerebrovascular disorders. Therefore, LL-00066471 presents potential for management of cognitive impairments associated with neurological and psychiatric conditions.

Synthesis and structure-activity relationship of the isoindolinyl benzisoxazolpiperidines as potent, selective, and orally active human dopamine D4 receptor antagonists

A new class of potent dopamine D(4) antagonists was discovered with selectivity over dopamine D(2) and the alpha-1 adrenoceptor. The lead compound was discovered by screening our compound collection. The structure-activity relationships of substituted isoindoline rings and the chirality about the hydroxymethyl side chain were explored. The isoindoline analogues showed modest differences in potency and selectivity. The S enantiomer proved to be the more potent enantiomer at the D(4) receptor. Several analogues with greater than 100-fold selectivity for D(4) over D(2) and the alpha-1 adrenoreceptor were discovered. Several selective analogues were active in vivo upon oral or intraperitoneal administration. A chiral synthesis starting from either D- or L-O-benzylserine is also described.

Design and synthesis of a novel series of 1,2-disubstituted cyclopentanes as small, potent potentiators of 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA) receptors

2-Amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA) potentiators are ligands that act as positive allosteric modulators at the AMPA receptors. We recently disclosed a novel series of 2-arylpropylsulfonamides that were potent potentiators of responses mediated through AMPA receptors. To further define the structural requirements for activity in this series, new ring-constrained analogues were prepared and a new stereocenter was introduced. The potentiating activity was highly dependent on the stereochemistry at the 2-position of the disubstituted cyclopentane and was independent of the relative stereochemistry at the 1-position. Compound (R,R)-10 represents a potent, novel potentiator of iGluR4 flip receptors (EC(50) = 22.6 nM).

4-Alkylidenyl glutamic acids, potent and selective GluR5 agonists

Twenty-four 4-alkylidene glutamic acids were synthesised and tested as potential subtype selective GluR5 and 6 ligands. It was found that a critical size of alkylidene group gave potent and selective GluR5 receptor agonists. LY339624 had Kis of 0.0326 and >100 microM on GluR5 and 6 receptors, respectively.

4-Alkyl- and 4-cinnamylglutamic acid analogues are potent GluR5 kainate receptor agonists

Enantiomerically pure (2S,4R)-4-substituted glutamic acids were prepared and tested for homomeric GluR5 and GluR6 kainate subtype receptor affinity. Some of the 4-cinnamyl analogues showed high selectivity and potency (K(i) < 25 nM) for the GluR5 receptors. The greatest selectivity and potency were achieved with the 3-(2-naphthyl)prop-2-enyl compound. This compound, LY339434, has negligible activity at the AMPA and kainate receptors GluR1, -2, -4 and -6. Although, LY339434 shows agonist activity at NMDA receptors in cultural hippocampal neurons (approximate EC(50) of 2.5 microM), we consider that LY339434 should be a useful pharmacological tool for the investigation of the functional role of GluR5 kainate receptors.

Prototypic G protein-coupled receptor for the intestinotrophic factor glucagon-like peptide 2

Glucagon-like peptide 2 (GLP-2) is a 33-aa proglucagon-derived peptide produced by intestinal enteroendocrine cells. GLP-2 stimulates intestinal growth and up-regulates villus height in the small intestine, concomitant with increased crypt cell proliferation and decreased enterocyte apoptosis. Moreover, GLP-2 prevents intestinal hypoplasia resulting from total parenteral nutrition. However, the mechanism underlying these actions has remained unclear. Here we report the cloning and characterization of cDNAs encoding rat and human GLP-2 receptors (GLP-2R), a G protein-coupled receptor superfamily member expressed in the gut and closely related to the glucagon and GLP-1 receptors. The human GLP-2R gene maps to chromosome 17p13.3. Cells expressing the GLP-2R responded to GLP-2, but not GLP-1 or related peptides, with increased cAMP production (EC50 = 0.58 nM) and displayed saturable high-affinity radioligand binding (Kd = 0.57 nM), which could be displaced by synthetic rat GLP-2 (Ki = 0.06 nM). GLP-2 analogs that activated GLP-2R signal transduction in vitro displayed intestinotrophic activity in vivo. These results strongly suggest that GLP-2, like glucagon and GLP-1, exerts its actions through a distinct and specific novel receptor expressed in its principal target tissue, the gastrointestinal tract.

A hippocampal GluR5 kainate receptor regulating inhibitory synaptic transmission

The principal excitatory neurotransmitter in the vertebrate central nervous system, L-glutamate, acts on three classes of ionotripic glutamate receptors, named after the agonists AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxalole-4-propionic acid), NMDA (N-methyl-D-aspartate) and kainate. The development of selective pharmacological agents has led to a detailed understanding of the physiological and pathological roles of AMPA and NMDA receptors. In contrast, the lack of selective kainate receptor ligands has greatly hindered progress in understanding the roles of kainate receptors. Here we describe the effects of a potent and selective agonist, ATPA ((RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid) and a selective antagonist, LY294486 ((3SR, 4aRS, 6SR, 8aRS)-6-((((1H-tetrazol-5-yl) methyl)oxy)methyl)-1, 2, 3, 4, 4a, 5, 6, 7, 8, 8a-decahydroisoquinoline-3-carboxylic acid), of the GluR5 subtype of kainate receptor. We have used these agents to show that kainate receptors, comprised of or containing GluR5 subunits, regulate synaptic inhibition in the hippocampus, an action that could contribute to the epileptogenic effects of kainate.

Calcium-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors: a molecular determinant of selective vulnerability in amyotrophic lateral sclerosis

The cause of the selective degeneration of motor neurons in amyotrophic lateral sclerosis (ALS) remains unexplained. One potential pathogenetic mechanism is chronic toxicity due to disturbances of the glutamatergic neurotransmitter system, mediated via alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-sensitive glutamate receptors. Functional AMPA receptors consist of various combinations of four subunits (designated GluR1-4). The GluR2 subunit is functionally dominant and renders AMPA receptors impermeable to calcium. Most native AMPA receptors in the mammalian central nervous system (CNS) contain the GluR2 subunit and are calcium impermeable. We have investigated the composition of AMPA receptors expressed on normal human spinal motor neurons by in situ hybridization to determine their likely subunit stoichiometry. Highly significant levels of mRNA were detected for the GluR1, GluR3, and GluR4 subunits. However, GluR2 subunit mRNA was not detectable in this cell group. The absence of detectable GluR2 mRNA in normal human spinal motor neurons predicts that they express calcium-permeable AMPA receptors unlike most neuronal groups in the human CNS. Expression of atypical calcium-permeable AMPA receptors by human motor neurons provides a possible mechanism whereby disturbances of glutamate neurotransmission in ALS may selectively injure this cell group.

Synthesis of a series of aryl kainic acid analogs and evaluation in cells stably expressing the kainate receptor humGluR6

The synthesis and pharmacological characterization of a novel series of 4-aryl-substituted kainic acid analogs are described. Receptor affinities were determined on recombinantly expressed humGluR6 kainate receptors and on [3H]kainate binding to rat forebrain kainate receptors. Functional agonist potencies were assessed using whole cell voltage clamp recordings in cells expressing humGluR6 receptors. Substitution of phenyl for the methyl at the C-4 position of kainic acid produced 11 which has high affinity and agonist potency at the GluR6 receptor. Substitution on phenyl led to a series of compounds with varying affinity for this kainate receptor. Agonist potency correlated with receptor affinity and with no derivative could antagonist activity be identified. Affinities for the humGluR6 kainate receptor were approximately 10-50 less than the observed affinities at rat forebrain kainate receptors. Furthermore, within the series of 4-aryl-substituted kainic acid analogs, there was a high degree of correlation between binding affinities for humGluR6 receptors and competition with kainate binding to rat forebrain kainate receptors.

Pharmacological discrimination of GluR5 and GluR6 kainate receptor subtypes by (3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]decahyd roisdoquinoline-3 carboxylic-acid

The pharmacological tools available for the discrimination of kainate receptor subtypes are limited. We examined the effects of (3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]decahydr oisoquinoline-3-carboxylic acid (LY293558) and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline (NBQX) on inward currents associated with activation of non-N-methyl-D-asparate (NMDA) receptors in acutely isolated rat cerebellar Purkinje neurons, rat dorsal root ganglion neurons, and human embryonic kidney 293 cells transfected with human glutamate receptors (GluR) 5 and 6. LY293558 and NBQX inhibited kainate-induced currents in cerebellar Purkinje cells, DRG neurons, and human GluR5-transfected cells. In contrast, human embryonic kidney 293 cells expressing GluR6 receptors, although blocked by NBQX, were unaffected by LY293558 at concentrations of < / = 100 microM. The selective antagonism by LY293558 of GluR5 receptors should allow the determination of the functional role of GluR5 and GluR6 in more complex systems.

Activity of 2,3-benzodiazepines at native rat and recombinant human glutamate receptors in vitro: stereospecificity and selectivity profiles

The activity and selectivity of the glutamate receptor antagonists belonging to the 2,3-benzodiazepine class of compounds have been examined at recombinant human non-NMDA glutamate receptors expressed in HEK293 cells and on native rat NMDA and non-NMDA receptors in vitro. The racemic 2,3-benzodiazepines GYKI52466, LY293606 (GYKI53405) and LY300168 (GYKI53655) inhibited AMPA (10 microM)-mediated responses in recombinant human GluR1 receptors expressed in HEK293 cells with approximate IC50 values of 18 microM, 24 microM and 6 microM, respectively and AMPA (10 microM) responses in recombinant human GluR4 expressing HEK293 cells with approximate IC50 values of 22 microM, 28 microM and 5 microM, respectively. GYKI 52466, LY293606 and LY300168 were non-competitive antagonists of AMPA receptor-mediated responses in acutely isolated rat cerebellar Purkinje neurons with approximate IC50 values of 10 microM, 8 microM and 1.5 microM, respectively. The activity of racemic compounds LY293606 and LY300168 was established to reside in the (-) isomer of each compound. At a concentration of 100 microM, GYKI52466, LY293606 and LY300168 produced < 30% inhibition of kainate-activated currents evoked in HEK293 cells expressing either human homomeric GluR5 or GluR6 receptors or heteromeric GluR6+KA2 kainate receptors. The activity of the 2,3-benzodiazepines at 100 microM was weak at kainate receptors, but was stereoselective. Similar levels of inhibition were observed for kainate-induced currents in dorsal root ganglion neurons. Intact tissue preparations were also used to examine the stereoselective actions of the 2,3-benzodiazepines. In the cortical wedge preparation, the active isomer of LY300168, LY303070, produced a non-competitive antagonism of AMPA-evoked depolarizations with smaller changes in depolarizations induced by kainate and no effect on NMDA-dependent depolarizations. LY303070 was also effective in preventing 30 microM AMPA-induced depolarizations in isolated spinal cord dorsal roots with an approximate IC50 value of 1 microM. Synaptic transmission in the hemisected spinal cord preparation was stereoselectively antagonized by the active isomers of LY300168 and LY293606. In summary, these results indicate that 2,3-benzodiazepines are potent, selective and stereospecific antagonists of the AMPA subtype of the non-NMDA glutamate receptor.

Distribution of AMPA-selective glutamate receptor subunits in the human hippocampus and cerebellum

The distribution of AMPA-selective subunits, GluR1-4, was determined in the human hippocampus and cerebellum by in situ hybridization and immunocytochemistry. In the hippocampus, in situ hybridization revealed that GluR1 and GluR2 mRNAs were similarly distributed and highly expressed in the dentate gyrus, with lower levels in the CA regions. GluR3 and GluR4 mRNAs were expressed at very low levels. Immunocytochemical studies showed that GluR1- and GluR2/3-immunoreactivity were highest in the dentate molecular and granular layers. In the CA regions, GluR1 and GluR2/3 staining was observed in pyramidal cell bodies and surrounding neuropil and was more intense in CA4/3/2 compared with CA1. GluR4-immunoreactivity was low throughout the hippocampus. In the cerebellum, GluR1 and GluR4 transcripts were expressed in the granular and Purkinje cell/Bergmann glia layers. GluR2 mRNA was highly expressed in the granular layer and individual Purkinje cells, while GluR3 mRNA was not detectable in the cerebellum. GluR1- and GluR4-immunoreactivity were localized to Purkinje cells and putative Bergmann glia, as well as their processes extending into the molecular layer. GluR2/3 staining was intense in Purkinje cells, with moderate staining in the granular layer. Thus, GluR1-4 subunits are differentially distributed in the hippocampus and cerebellum. In addition, the distribution of subunit mRNA and protein correlate well with each other and with the glutamatergic neuroanatomy of the hippocampus and cerebellum.

Expression of human glutamate receptors (GluR) in neuroblastoma cell lines

Neuroblastoma cell lines are useful for the investigation of neuronal receptor regulation since these cells display various neuronal features. Here we report the analysis of human AMPA and kainate receptor expression in four neuroblastoma cell lines (SK-N-MC, IMR-32, CHP 126 and NMB/N7). The reverse transcriptase-polymerase chain reaction (RT-PCR) demonstrated the presence of hGluR1, hGluR4 and EAA4 transcripts in all four cell lines whereas hGluR3 mRNA was undetectable. The pattern of expression of hGluR2, EAA1, EAA2, EAA3 and EAA5 was more complex and differed among the cell lines tested. Immunoblot analysis and electrophysiological recordings failed to demonstrate expression of hGluR1-hGluR4, EAA1/EAA2 proteins and the formation of functional AMPA/kainate receptor channels. These cell lines might provide a valuable model to study the transcriptional and post-transcriptional regulation of AMPA and kainate receptor expression.

Human N-methyl-D-aspartate receptor modulatory subunit hNR3: cloning and sequencing of the cDNA and primary structure of the protein

Several cDNA clones encoding the human N-methyl-D-aspartate receptor modulatory subunit hNR3, were isolated from a human fetal brain library. The hNR3 cDNA demonstrated a 91.3-91.5% nucleotide (nt) identity with the rat NR2B and mouse epsilon 2 cDNAs. The nt sequence of hNR3 would encode a 1484 amino acid (aa) protein that has a 98.4-98.5% identity with the mouse epsilon 2 and rat NR2B subunits.

cDNA cloning and functional properties of human glutamate receptor EAA3 (GluR5) in homomeric and heteromeric configuration

We have isolated a new member of the human glutamate receptor family from a fetal brain cDNA library. This cDNA clone, designated EAA3a, shares a 90% nucleotide identity with the previously reported rat GluR5-2b cDNA splice variant and differed from human GluR5-1d in the amino and carboxy terminal regions. Cell lines stably expressing EAA3a protein formed homomeric ligand-gated ion channels responsive, in order of decreasing affinity to domoate, kainate, L-glutamate and (RS)-alpha-amino-3-hydroxy-5- methylisoxazole-propionate (AMPA). Kainate-evoked currents showed partial desensitization that was reduced on incubation with concanavalin A (conA) but not cyclothiazide and were attenuated by the non-N-methyl-D-aspartate (NMDA) receptor antagonist CNQX (6-cyano-7-nitro-quinoxalinedione). Coexpression of EAA3a and human EAA1 cDNAs in HEK 293 cells formed a heteromeric channel with unique properties. Kainate and AMPA activated the heteromeric channel with significantly higher affinities than observed for EAA3a alone. Ligand binding studies with the recombinant EAA3a receptor expressed in mammalian cells indicated a high affinity kainate binding site (Kd = 120 +/- 15.0 nM). The relative potency of compounds in displacing [3H]-kainate binding to EAA3a receptor was: domoate > kainate > L-glutamate = quisqualate > 6,7-dinitroquinoxaline-2,3-dione (DNQX) = CNQX > AMPA > dihydrokainate > NMDA.

RNA editing of human kainate receptor subunits

RNA editing has been shown to be critical in generating the molecular diversity of rodent kainate receptors. We have examined cDNAs derived from various human brain sources to assess the occurrence and extent of RNA editing in human brain. Comparison of genomic and cDNA sequences revealed extensive editing of the human EAA4 (GluR6) mRNA at the isoleucine/valine, tyrosine/cysteine sites of the transmembrane I region, and the glutamine/arginine site of the transmembrane II region. Of the eight potential molecular variants generated by the nucleotide exchange, five were observed in the tissues examined. The distribution of the various RNA editing combinations were not uniform, and displayed tissue and/or age dependent distribution. Editing of the glutamine/arginine site was also confirmed for EAA3 (GluR5), which displays a significantly higher extent of editing in specific human brain regions compared with rodent whole brain. Hence, it can be concluded that RNA editing is a determinant of the phenotype of human kainate receptor complexes.

Human glutamate receptor hGluR3 flip and flop isoforms: cloning and sequencing of the cDNAs and primary structure of the proteins

Several cDNA clones encoding the human glutamate receptor subunit GluR3 flip and flop isoforms, were isolated from human hippocampus and fetal brain libraries. DNA sequence analysis revealed overlapping clones permitting the reconstruction of full-length GluR3-flip and GluR3-flop cDNAs. The GluR3 cDNAs demonstrated an 94.1-94.7% nucleotide (nt) identity with the corresponding rat cDNAs. The nt sequence of the GluR3 cDNAs would encode 894 amino acid proteins that have a 99.4% identity with the rat GluR3 isoforms. The human GluR3 cDNAs predict an additional 6 amino acid in the N-terminal signal peptide as compared to the rat GluR3.

Cloning and sequence analysis of additional splice variants encoding human N-methyl-D-aspartate receptor (hNR1) subunits

Two cDNA clones representing previously unidentified human N-methyl-D-aspartate receptor (hNR1) subunit polypeptides were isolated and sequenced. Clone hNR1-4 was isolated from a human hippocampus cDNA library and was presumably generated by alternative RNA splicing in the 3' amino acid (aa) coding regions. The hNR1-4 cDNA demonstrated an 85.7% nucleotide (nt) identity to the corresponding rat NR1 (rNR1) cDNA. The nt sequence of hNR1-4 would encode a protein that has a 99.8% identity with the corresponding rNR1 subunit. Clone hNR1N was isolated by polymerase chain reaction (PCR)-mediated amplification of a 0.6-kb DNA fragment from human cerebellum cDNA. The nt sequence of this DNA fragment was identical to previously isolated hNR1 cDNA clones, except for the presence of a 63-bp DNA insertion that would encode an additional 21 aa. This DNA insertion occurs in the 5' aa coding regions of hNR1 and presumably represents an exon that is subject to alternative splicing. The nt and aa sequences of this exon are identical between human and rat.

Differential RNA editing efficiency of AMPA receptor subunit GluR-2 in human brain

RNA editing in rat brain has been found to control a determinant of cation flow in alpha-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA)-gated channels. Here we provide the first evidence that this RNA editing phenomenon occurs in human brain and is differentially regulated. Sequence analysis of human genomic DNA revealed a Q codon (CAG) in the putative channel-forming segment of human GluR-2, whereas in the majority of cDNA clones an R codon (CGG) was found. Examination of editing in various brain tissues revealed differences in the efficiency of this process. The hippocampus, cerebellum and temporal cortex harbour 100% edited GluR-2, whereas only 72% of substantia nigra, 89% of corpus striatum and 96% of fetal cDNAs have been found to be edited. This new discovery of differential efficiency of RNA editing has important implications in AMPA receptor channel-mediated calcium influx. AMPA receptors are thought to mediate the majority of the fast excitatory synaptic neurotransmission; the RNA editing process may therefore play a critical role in normal brain function and development. Dysfunction of this RNA editing process may have neuropathological consequences and could be related to certain neurodegenerative diseases.

Human N-methyl-D-aspartate receptor modulatory subunit hNR2A: cloning and sequencing of the cDNA and primary structure of the protein

Several cDNA clones encoding the human N-methyl-D-aspartate receptor modulatory subunit hNR2A, were isolated from human hippocampus and fetal brain libraries. DNA sequence analysis revealed overlapping clones permitting the reconstruction of full-length hNR2A cDNA. The hNR2A cDNA demonstrated an 88-89% nucleotide (nt) identity with the corresponding rodent cDNAs. The nt sequence of hNR2A would encode a 1464-aa protein that has a 95.2% identity with the rodent NR2A subunits.

Functional expression and pharmacological characterization of the human EAA4 (GluR6) glutamate receptor: a kainate selective channel subunit

A cDNA encoding an ionotropic glutamate receptor subunit protein humEAA4 (GluR6), has been cloned from a human fetal brain library. This cDNA when expressed in COS or HEK-293 cells is associated with high-affinity kainate receptor binding and ion channel formation. We have successfully established cell lines stably expressing humEAA4 in HEK-293 cells This is the first report of the establishment of stable cell lines expressing a glutamate receptor channel. The relative potency of compounds for displacing [3H]-kainate binding to humEAA4 receptors expressed in COS or HEK-293 cells is domoate > kainate > quisqualate > 6-cyano-7-nitroquinoxaline-2,3-dione > L-glutamate = 6,7- dinitroquinoxaline-2,3-dione > dihydrokainate. Applications of kainate, glutamate, and domoate but not AMPA evoked rapidly desensitizing currents in cells expressing homo-oligomeric humEAA4 in a concentration dependent manner. The order of potency was: domoate > kainate > L-glutamate. Although AMPA did not itself activate humEAA4 receptors it did reduce, to a limited extent, kainate-evoked responses. AMPA may therefore be a weak partial agonist for this receptor. To date this effect has not been demonstrated with rat GluR6. It is possible that subtle species differences may exist in the nature of agonist receptor interaction. Kainate evoked currents were attenuated by the quinoxalinediones CNQX and DNQX but not by DAP5. The receptor desensitization was attenuated on application of concanavalin A. Ion-permeability studies indicated that the receptor-linked ion channel is permeable to both Na+ and Ca2+ ions.

Molecular cloning, expression, and pharmacological characterization of humEAA1, a human kainate receptor subunit

Kainate is a potent neuroexcitatory agent; its neurotoxicity is thought to be mediated by an ionotropic receptor with a nanomolar affinity for kainate. In this report, we describe the cloning of a cDNA encoding a human glutamate ionotropic receptor subunit protein from a human hippocampal library. This cDNA, termed humEAA1, is most closely related to rat and human cDNAs for kainate receptor proteins and, when expressed in COS or Chinese hamster ovary cells, is associated with high-affinity kainate receptor binding. We have successfully established cell lines stably expressing humEAA1. This is the first report of establishment of stable cell lines expressing a glutamate receptor subunit. The relative potency of compounds for displacing [3H]kainate binding of humEAA1 receptors expressed in these stable cell lines was kainate > quisqualate > domoate > L-glutamate >> (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid > dihydrokainate > 6,7-dinitroquinoxaline-2,3-dione > 6-cyano-7-nitroquinoxaline-2,3-dione. Homooligomeric expression of humEAA1 does not appear to elicit ligand-gated ion channel activity. Nevertheless, the molecular structure and pharmacological characterization of high-affinity kainate binding of the humEAA1 expressed in the stable cell line (ppEAA1-16) suggest that the humEAA1 is a subunit protein of a human kainate receptor complex.

Molecular characterization of the human EAA5 (GluR7) receptor: a high-affinity kainate receptor with novel potential RNA editing sites

Several cDNA clones encoding EAA5 receptor polypeptides were isolated from a human fetal brain library. The EAA5 cDNAs demonstrated an 88.7-90.1% nucleotide identity with rat GluR7 cDNAs. The nucleotide sequence of EAA5 would encode a 919-amino acid protein, that has a 97.7-98.9% identity with the rat GluR7 receptor. Two variation of the EAA5 cDNA were identified which result in amino acid substitutions in the predicted extracellular amino-terminal region; Ser310-->Ala and Arg352-->Gln. These variations can be attributed to RNA editing involving T-->G and G-->A substitutions. Both the location (with respect to glutamate receptors), and the nucleotides involved, in this putative RNA editing are novel and may therefore involve novel mechanisms. Ligand binding studies with membranes of transfected COS-1 cells expressing EAA5 polypeptides demonstrate a rank order of ligand affinity similar to that observed with the rat GluR7 receptor, and a dissociation constant for kainate (2.72 +/- 0.12 nM (n = 3)) that is approximately 20- to 30-fold higher than that observed for the rat GluR7 receptor. All of the ligands tested had a higher affinity for the human EAA5 receptor as compared to the rat GluR7 receptor. This report provides another example of pharmacological differences for similar receptors across species.

Cloning and sequence analysis of cDNAs encoding human hippocampus N-methyl-D-aspartate receptor subunits: evidence for alternative RNA splicing

Several cDNA clones encoding human N-methyl-D-aspartate receptor (hNR1) subunit polypeptides were isolated from a human hippocampus library. Degenerate oligodeoxyribonucleotide (oligo) primers based on the published rat NR1 (rNR1) amino acid (aa) sequence [K. Moriyoshi et al. Nature 354 (1991) 31-37] amplified a 0.7-kb fragment from a human hippocampus cDNA library, via the polymerase chain reaction (PCR). This fragment was used as a probe for subsequent hybridization screening. DNA sequence analysis of 28 plaque-purified clones indicated three distinct classes, designated hNR1-1, hNR1-2 and hNR1-3, presumably generated by alternative RNA splicing. One of these clones, hNR1-1(5A), was isolated as a full-length cDNA. The hNR1-2 and hNR1-3 cDNAs represented 66.8 and 98.9%, respectively, of the total aa coding information predicted for the polypeptides. The hNR1 cDNAs demonstrated an 84-90.8% nucleotide (nt) identity with the corresponding rodent cDNAs. The nt sequences of hNR1-1, hNR1-2 and hNR1-3 would encode 885-, 901- and 938-aa proteins, respectively, that have 99.1-99.8% identity with the corresponding rodent NR1 (roNR1) subunits. The changes between the predicted aa sequences of hNR1 and the corresponding roNR1 subunits are confined to the extracellular N-terminal regions. We have also identified two possible allelic variations of the hNR1-3 cDNA that result in aa substitutions in the extracellular N- and C-terminal regions. One of these naturally occurring aa variations is situated within a potential glutamate-binding site.

Molecular structure and pharmacological characterization of humEAA2, a novel human kainate receptor subunit

A cDNA encoding a novel human glutamate receptor subunit protein was isolated from a human hippocampal library. This cDNA, termed humEAA2, is most closely related to rat cDNAs for kainate receptor proteins and, when expressed in COS cells, is associated with high affinity kainate receptor binding. The relative potency of compounds in displacing [3H]kainate binding was kainate greater than quisqualate greater than domoate greater than L-glutamate much greater than 6,7-dinitroquinoxaline-2,3-dione greater than dihydrokainate greater than 6-cyano-7-nitroquinoxaline-2,3-dione greater than (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid. Homomeric expression of humEAA2 does not appear to elicit ligand-gated channel activity. Nevertheless, the molecular structure and pharmacology of high affinity kainate binding suggest that humEAA2 is a novel subunit protein of a human kainate receptor complex.

The 80L5C4 epitope overlaps with the homophilic binding site of the cell adhesion molecule gp80 of Dictyostelium

The monoclonal antibody (mAb) 80L5C4 is a potent inhibitor of the cell adhesion molecule gp80 of Dictyostelium discoideum. To map the exact location of the epitope recognized by mAb 80L5C4, overlapping hexapeptides were synthesized on plastic pins and the binding p6 mAb 80L5C4 to these peptides was monitored by enzyme-linked immunosorbent assay. The 80L5C4 epitope is mapped to a single hexapeptide sequence GYKLNV, which shares five amino acid residues with the octapeptide sequence YKLNVNDS involved in gp80 homophilic binding. Analogue studies indicate that the hydrophobic residues within this sequence are crucial for antigen recognition.

Regulation of slug size by the cell adhesion molecule gp80 in Dictyostelium discoideum

We previously provided in vitro evidence that the cell surface glycoprotein of Mr80,000 (gp80) of Dictyostelium discoideum is capable of mediating EDTA-resistant cell-cell binding. Expression of gp80 is specific for the aggregation stage when cells form tight aggregates. To investigate the physiological role of gp80, Dictyostelium cells were transformed with a vector containing gp80 cDNA fused to an actin promoter. gp80 transcripts were detected in transformed cells in their vegetative growth phase. Transformants at this stage also exhibited EDTA-resistant cell cohesion, thus providing direct in vivo evidence that gp80 mediates cell-cell binding via homophilic interaction. While aggregates of the parental strain KAX3 had the tendency to break up to form small slugs, transformants expressing an increased amount of gp80 were able to maintain the integrity of aggregates, giving rise to larger slugs, resulting in the formation of bigger fruiting bodies. To further demonstrate that the increase in slug size could be correlated with the expression of gp80, cells of the parental strain were treated with exogenous cAMP pulses to stimulate an over-expression of gp80. The treated cells also gave rise to larger slugs, consistent with the notion that slug size is influenced by intercellular adhesiveness during development.

Cell-cell adhesion and morphogenesis in Dictyostelium discoideum

During development of Dictyostelium discoideum, cells acquire EDTA-resistant cell-cell adhesion at the aggregation stage. The EDTA-resistant cell binding activity is associated with a cell surface glycoprotein of Mr 80,000 (gp80), which mediates cell-cell binding via homophilic interaction. Analysis of the structure of gp80 deduced from cDNA sequence reveals the presence of three internally homologous segments in the NH2-terminal domain, which also contains regions with homology to the neural cell adhesion molecule. Secondary structure predictions show an abundance of beta-structures and very few alpha-helices. This is confirmed by circular dichroism measurements. It is likely that the homologous segments are organized into globular structures, extended from the cell surface by a Pro-rich stalk domain. The cell binding activity of gp80 resides within the first globular repeat of the NH2-terminal domain and has been mapped to a 51 amino acid region between Val123 and Leu173. Synthetic oligopeptides corresponding to sequences within this region have been prepared and assayed for their ability to bind to cell surface gp80. Results lead to identification of the homophilic binding site to an octapeptide sequence within this region. Synthetic peptides containing this octapeptide sequence and univalent antibodies directed against this site block the formation of organized cell streams during aggregation. Although cell aggregates are eventually formed, most fail to undergo further development to give rise to slugs and fruiting bodies, indicating that cell-cell adhesion involving gp80 is an important step in normal morphogenesis.

Identification of an octapeptide involved in homophilic interaction of the cell adhesion molecule gp80 of dictyostelium discoideum

During development of Dictyostelium discoideum, a surface glycoprotein of Mr 80,000 (gp80) is known to mediate EDTA-resistant cell-cell adhesion via homophilic interaction. Antibodies directed against a 13 amino acid sequence (13-mer) near the NH2 terminus of the protein were found to inhibit cell reassociation. This 13-mer also inhibited gp80-cell interaction and gp80-gp80 interaction. The cell binding site was mapped to the octapeptide sequence YKLNVNDS by using shorter peptide sequences to inhibit gp80 interaction. High salt concentrations inhibited homophilic interactions of both the 13-mer and gp80, suggesting that ionic interactions are involved in the forward binding reaction. Since disruption of homophilic interactions between the bound molecules required the presence of Triton X-100, hydrophobic interactions may occur after the initial ionic binding.

Mapping of the monoclonal antibody 80L5C4 epitope on the cell adhesion molecule gp80 of Dictyostelium discoideum

EDTA-resistant cell-cell binding sites are expressed on Dictyostelium discoideum cells at the aggregation stage of development. A cell surface glycoprotein of Mr 80,000 (gp80) has been found to mediate these binding sites via homophilic interaction. We have previously raised a monospecific monoclonal antibody 80L5C4 against gp80, which blocks the cell binding site of gp80 (Siu, C.-H., Lam, T.Y. and Choi, A.H.C. (1985) J. Biol. Chem. 260, 16030-16036). To map the 80L5C4 epitope, gp80 was digested with protease V8, and the smallest proteolytic fragment that retained immunoreactivity with 80L5C4 was about 27,000 Da, corresponding to the amino-terminal fragment predicted from the cleavage sites. In addition, cDNA fragments containing different gp80 coding regions were used to construct trpE/gp80 gene fusions in the expression vector pATH10. An analysis of these fusion proteins led to the mapping of the 80L5C4 epitope to a 51 amino-acid segment between residues 123 and 173.

Mapping of a cell-binding domain in the cell adhesion molecule gp80 of Dictyostelium discoideum

At the aggregation stage of Dictyostelium discoideum development, a cell surface glycoprotein of Mr 80,000 (gp80) has been found to mediate the EDTA-resistant type of cell-cell adhesion via homophilic interaction (Siu, C.-H., A. Cho, and A. H. C. Choi. 1987. J. Cell Biol. 105:2523-2533). To investigate the structure-function relationships of gp80, we have isolated full length cDNA clones for gp80 and determined the DNA sequence. The deduced structure of gp80 showed three major domains. An amino-terminal globular domain composed of the bulk of the protein is supported by a short stalk region, which is followed by a membrane anchor at the carboxy terminus. Structural analysis suggested that the cell-binding domain of gp80 resides within the globular domain near the amino terminus. To investigate the relationship of the cell-binding activity to this region of the polypeptide, three protein A/gp80 (PA80) gene fusions were constructed using the expression vector pRIT2T. These PA80 fusion proteins were assayed for their ability to bind to aggregation stage cells. Binding of 125I-labeled fusion proteins PA80I (containing the Val123 to Ile514 fragment of gp80) and PA80II (Val123 to Ala258) was dosage dependent and could be inhibited by precoating cells with the cell cohesion-blocking mAb 80L5C4. On the other hand, there was no appreciable binding of PA80III (Ile174 to Ile514) to cells. Reassociation of cells was significantly inhibited in the presence of PA80I or PA80II. In addition, 125I-labeled PA80II exhibited homophilic interaction with immobilized PA80I, PA80II, or gp80. The results of these studies lead to the mapping of a cell-binding domain in the region between Val123 and Leu173 of gp80 and provide direct evidence that the cell-binding activity of gp80 resides in the protein moiety.

Molecular mechanisms of cell-cell interaction in Dictyostelium discoideum

During development of the cellular slime mold Dictyostelium discoideum, cells migrate in response to cAMP to form aggregates, which give rise to fruiting bodies consisting of two major cell types: spores and stalk cells. Multicellularity is achieved by the expression of two types of cell-cell adhesion sites. The EDTA-sensitive binding sites are expressed at the initial stage of development. At the aggregation stage, cells acquire EDTA-resistant binding sites, which are mediated by a cell-surface glycoprotein of Mr80,000 (gp80). gp80 is preferentially associated with cell surface filopodia, which are probably involved in the initiation of contact formation between cells. Covaspheres conjugated with gp80 bind specifically to aggregation-stage cells. The binding can be inhibited by precoating cells with an anti-gp80 monoclonal antibody, thus suggesting that gp80 mediates cell-cell binding via homophilic interaction. The structure of gp80 predicted from its cDNA sequence can be divided into three major domains: a membrane anchor, a hinge, and a globular region. An analysis of fusion proteins containing different gp80 segments shows that the cell-binding activity resides in the globular region. In the postaggregation stages, gp80 is replaced by other surface glycoproteins in maintaining cell-cell adhesion. One of them has a Mr of 150,000 (gp150). Anti-gp150 antibodies have no effect on aggregation-stage cells, but they disrupt cell-cell adhesion at subsequent stages. It becomes evident that the complex phenomena of cell adhesion and tissue organization involve the participation of a number of surface glycoproteins.

Purine Nucleoside Transport in Petunia Pollen Is an Active, Carrier-Mediated System Not Sensitive to Nitrobenzylthioinosine and Not Renewed during Pollen Tube Growth

Adenosine and guanosine are transported into Petunia hybrida pollen by a saturable, carrier-mediated mechanism. The energy poisons carbonylcyanide-m-chlorophenylhydrazone, 2,4-dinitrophenol, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, and N,N'-dicyclohexylcarbodiimide all inhibit uptake, suggesting an energy coupled (active) transport process. Transport takes place against a concentration gradient, strongly favoring an active transport mechanism. The purine nucleoside transport in Petunia pollen differs from that already reported for pyrimidine nucleosides in that it exhibits a significantly higher K(m) for nucleoside and is not so severely inhibited by the polyamine, spermine. Like that for the pyrimidine nucleosides uridine and cytosine, however, the system exhibits a broad pH optimum, is inhibited by sulfydryl-binding reagents, while the potent inhibitors of nucleoside transport in animal cells, nitrobenzylthioinosine and dipyridamole, have no effect. Transport of both purine and pyrimidine nucleosides in germinating pollen decreases steadily with time, a finding consistent with reports that RNA synthesis and DNA repair are early events of pollen germination and tube elongation. However, since these precursors are often used to demonstrate nucleic acid synthesis, it cannot be ruled out that the lack of precursor transport itself leads to scoring nucleic acid synthesis as negative. The results indicate that the newly synthesized pollen tube membranes contain little or no nucleoside transporters.

Self-incompatibility alleles control a low molecular weight, basic protein in pistils of Petunia hybrida

Proteins extracted from the pistils of several clones of Petunia hybrida carrying differing pairs of S alleles were examined by gel electrophoresis. The major protein of pistils, a basic glycoprotein of relatively low molecular weight, showed properties which varied in a simple manner with the S genotype. For each S allele we were able to assign a specific molecular weight (ranging from 27,000 to 33,000) and isoelectric point (in the range 8.3 to 8.7) for this putative S protein. Pistils homozygous at the S locus showed only one major protein on two-dimensional gel electrophoresis, while pistils from plants heterozygous at the S locus showed two. No evidence was obtained for the presence of this putative S protein in pollen extracts.

Pyrimidine nucleoside uptake by petunia pollen: specificity and inhibitor studies on the carrier-mediated transport

Transport of pyrimidine nucleosides into germinating Petunia hybrida pollen is carrier-mediated, and, except for thymidine, is inhibited by the energy poisons N,N'-dicyclohexylcarbodiimide, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, 2,4-dinitrophenol, and carbonylcyanide-m-chlorophenylhydrazone. Kinetic studies with analogs deoxyuridine and 5-bromodeoxyuridine show that they too are taken up faster than thymidine and inhibited by the energy poisons. These and other analogs inhibit uridine and cytidine transport more than thymidine, as do the inhibitors parachloromercuribenzoic acid, N-ethylmaleimide, phenylarsine oxide, o-phenanthroline, ethylene diamenetetraacetate, and ethylene glycol-bis (beta-aminoethyl ether) N,N,N'N'-tetraacetic acid. Citrate, phosphate, succinate, and tartrate inhibited uptake of all pyrimidine nucleosides. The specific inhibitor of nucleoside transport in animal cells, nitrobenzylthioinosine, has little effect on pollen transport. Uridine and deoxyuridine accumulate against a concentration gradient, suggesting active transport. Except for thymidine, however, transported nucleosides were found to be extensively phosphorylated. Until mutant plants are found which do not phosphorylate uridine, it is not possible to decide unequivocally between active and nonactive transport for uridine. However, consistent with a low level of DNA synthesis in germinating Petunia pollen, it is clear that thymidine transport is nonactive and relatively slow. It is apparent from these experiments that a more sensitive way to study DNA repair in this pollen would be to use 5-bromodeoxyuridine or deoxyuridine instead of thymidine to label repaired DNA. The results show that pollen has the transport systems necessary to take up pyrimidine nucleosides from Petunia styles, where it is known that the concentration of free nucleosides increase after pollination.

Divergent transport mechanisms for pyrimidine nucleosides in petunia pollen

Petunia hybrida pollen exhibits divergent transport mechanisms for pyrimidine nucleosides. Uridine and cytidine show all the properties of being actively transported, a nucleoside transport mechanism not hitherto reported in plant cells. Contrasting with this, thymidine transport has the properties of a nonactive, carrier-mediated system. Reasons for these different mechanisms are considered to lie in the high demand for uridine and cytidine, obtained perhaps from stylar tissue, for the biosynthetic reactions of the pollen tube, while thymidine demand is lower due to the absence of DNA replication in germinating Petunia pollen.

Localization of phytic acid in the floral structure of Petunia hybrida and relation to the incompatibility genes

In three clones of Petunia hybrida with different incompatibility genes, phytic acid is detected exclusively in pollen, stigma and style. These are all parts of the floral structure involved in the incompatibility reaction. Phytase activity was detected in these tissues as well as in the ovary. The level of phytic acid and phytase activity varied between clones with different S alleles. This difference was most evident in stigma and style. The pattern of phytic acid breakdown following pollination depends on whether pollen and style form a compatible or incompatible combination. Incompatible pollination results in a higher rate of degradation. Consideration is given to the relationship between breakdown of phytic acid to myo-inositol and cell wall thickening and plug formation, which occurs to a greater extent in the incompatible combination.