Different characteristics of AMPA receptor agonists acting at AMPA receptors expressed in Xenopus oocytes

Relating ligand binding to activation gating in P2X2 receptors using a novel fluorescent ATP derivative

Ionotropic purinergic receptors (P2X receptors) are non-specific cation channels that are activated by the binding of ATP at their extracellular side. P2X receptors contribute to multiple functions, including the generation of pain, inflammation, or synaptic transmission. The channels are trimers and structural information on several of their isoforms is available. In contrast, the cooperation of the subunits in the activation process is poorly understood. We synthesized a novel fluorescent ATP derivative, 2-[DY-547P1]-AET-ATP (fATP) to unravel the complex activation process in P2X2 and mutated P2X2 H319K channels with enhanced apparent affinity by characterizing the relation between ligand binding and activation gating. fATP is a full agonist with respect to ATP that reports the degree of binding by bright fluorescence. For quantifying the binding, a fast automated algorithm was employed on human embryonic kidney cell culture images. The concentrations of half maximum occupancy and activation as well as the respective Hill coefficients were determined. All Hill coefficients exceeded unity, even at an occupancy <10%, suggesting cooperativity of the binding even for the first and second binding step. fATP shows promise for continuative functional studies on other purinergic receptors and, beyond, any other ATP-binding proteins.

Substituted 4-hydroxy-1,2,3-triazoles: synthesis, characterization and first drug design applications through bioisosteric modulation and scaffold hopping approaches

The hydroxytriazole system is here analysed and used to modulate acidic moieties present in lead compounds.

Complete resolution of the microscopic protonation equilibria of N-methyl-d-aspartic acid and related compounds

Protonation equilibria of N-methyl-D-aspartate (NMDA, a specific glutamate receptor agonist) and its derivatives are characterized at the macroscopic and microscopic levels. (1)H NMR-pH and pH-potentiometric titrations were carried out to determine the macroconstants. Microconstants were obtained by appropriate combination of acidity and NMR parameters of the parent compound and its three synthetic derivatives. These derivatives were close models of the NMDA minor microspecies, allowing the calculation of all the 12 microconstants, the 8 microspecies concentrations and 3 site interactivity parameters. Reliability of the microconstants was assessed by three independent test methods. It was found that protonation of the secondary amino site decreases the beta- and alpha-carboxylate basicities almost exactly by one and two orders of magnitude, respectively, whereas protonation of one of the carboxylates lessens the basicity of the other one by a factor of 3. NMR-pH profiles, macro- and microscopic protonation schemes and species-specific distribution diagrams are presented.

Estradiol modulation of kainic acid-induced calcium elevation in neonatal hippocampal neurons

The developing hippocampus of both males and females is exposed to high levels of the gonadal steroid estradiol. The impact of this estradiol exposure on developing hippocampal neurons is essentially unknown. In the rat, the newborn hippocampus is relatively insensitive to excitotoxic brain injury, which in adults is associated with the release of amino acids, in particular glutamate, resulting in a significant increase in intracellular calcium and eventual cell death. We have shown previously in the rat that administration of the glutamate agonist, kainic acid (KA), on the day of birth results in limited hippocampal damage, which is ameliorated by treatment with the gonadal steroid, estradiol. We now show that KA induces an increase in intracellular calcium through L-type voltage-sensitive calcium channels early in development and, later in development, through polyamine-sensitive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors with a modest increase through N-methyl-D-aspartate receptors. Pretreatment with the gonadal steroid, estradiol, decreases the percentage of neurons responding to KA and decreases the peak amplitude of the calcium transient early in development but has no effect later in development. Taken together, these data suggest that there is a developmental shift in the route of KA-induced intracellular calcium and estradiol modulates KA-induced intracellular calcium to a time restricted to early development, but whether this is the basis of the neuroprotective effect of estradiol remains to be determined.

Microtransplantation of membranes from cultured cells to Xenopus oocytes: a method to study neurotransmitter receptors embedded in native lipids

The Xenopus oocyte is used as a convenient cell expression system to study the structure and function of heterogenic transmitter receptors and ion channels. Recently, we introduced a method to microtransplant already assembled neurotransmitter receptors from the human brain to the plasma membrane of Xenopus oocytes. The same approach was used here to transplant neurotransmitter receptors expressed from cultured cells to the oocytes. Membrane vesicles prepared from a human embryonic kidney cell line (HEK293) stably expressing the rat glutamate receptor 1 were injected into oocytes, and, within a few hours, the oocyte plasma membrane acquired alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors, which had the same properties as those expressed in the original HEK cells. Analogously, oocytes injected with membranes prepared from rat pituitary GH(4)C1 cells, stably expressing homomeric human neuronal alpha 7 nicotinic acetylcholine receptors (alpha 7-AcChoRs), incorporated in their plasma membrane AcChoRs that behaved as those expressed in GH(4)C1 cells. Similar results were obtained with HEK cells stably expressing heteromeric human neuronal alpha 4 beta 2-AcChoRs. All this makes the Xenopus oocyte a powerful tool for detailed investigations of receptors and other proteins expressed in the membrane of cultured cells.

Stereostructure-activity studies on agonists at the AMPA and kainate subtypes of ionotropic glutamate receptors

(S)-Glutamic acid (Glu), the major excitatory neurotransmitter in the central nervous system, operates through ionotropic as well as metabotropic receptors and is considered to be involved in certain neurological disorders and degenerative brain diseases that are currently without any satisfactory therapeutic treatment. Until recently, development of selective Glu receptor agonists had mainly been based on lead compounds, which were frequently naturally occurring excitants structurally related to Glu. These Glu receptor agonists generally contain heterocyclic acidic moieties, which has stimulated the use of bioisosteric replacement approaches for the design of subtype-selective agonists. Furthermore, most of these leads are conformationally restricted and stereochemically well-defined Glu analogs. Crystallization of the agonist binding domain of the GluR2 subunit of the (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor subtype of ionotropic Glu receptors in the presence or absence of an agonist has provided important information about ligand-receptor interaction mechanisms. The availability of these binding domain crystal structures has formed the basis for rational design of ligands, especially for the AMPA and kainate subtypes of ionotropic Glu receptors. This mini-review will focus on structure-activity relationships on AMPA and kainate receptor agonists with special emphasis on stereochemical and three-dimensional aspects.

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.

Resolution, configurational assignment, and enantiopharmacology at glutamate receptors of 2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA) and demethyl-ACPA

We have previously described (RS)-2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA) as a potent agonist at the (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor subtype of (S)-glutamic acid (Glu) receptors. We now report the chromatographic resolution of ACPA and (RS)-2-amino-3-(3-carboxy-4-isoxazolyl)propionic acid (demethyl-ACPA) using a Sumichiral OA-5000 column. The configuration of the enantiomers of both compounds have been assigned based on X-ray crystallographic analyses, supported by circular dichroism spectra and elution orders on chiral HPLC columns. Furthermore, the enantiopharmacology of ACPA and demethyl-ACPA was investigated using radioligand binding and cortical wedge electrophysiological assay systems and cloned metabotropic Glu receptors. (S)-ACPA showed high affinity in AMPA binding (IC(50) = 0.025 microM), low affinity in kainic acid binding (IC(50) = 3.6 microM), and potent AMPA receptor agonist activity on cortical neurons (EC(50) = 0.25 microM), whereas (R)-ACPA was essentially inactive. Like (S)-ACPA, (S)-demethyl-ACPA displayed high AMPA receptor affinity (IC(50) = 0.039 microM), but was found to be a relatively weak AMPA receptor agonist (EC(50) = 12 microM). The stereoselectivity observed for demethyl-ACPA was high when based on AMPA receptor affinity (eudismic ratio = 250), but low when based on electrophysiological activity (eudismic ratio = 10). (R)-Demethyl-ACPA also possessed a weak NMDA receptor antagonist activity (IC(50) = 220 microM). Among the enantiomers tested, only (S)-demethyl-ACPA showed activity at metabotropic receptors, being a weak antagonist at the mGlu(2) receptor subtype (K(B) = 148 microM).

Characteristics of AMPA receptor-mediated responses of cultured cortical and spinal cord neurones and their correlation to the expression of glutamate receptor subunits, GluR1-4

Electrophysiological recordings have been used to characterize responses mediated by AMPA receptors expressed by cultured rat cortical and spinal cord neurones. The EC(50) values for AMPA were 17 and 11 microM, respectively. Responses of cortical neurones to AMPA were inhibited competitively by NBQX (pK(i)=6.6). Lower concentrations of NBQX (< or =1 microM) also potentiated the plateau responses of spinal cord neurones to AMPA, which could be attributed to a depression of desensitization to AMPA. GYKI 52466 inhibited responses of spinal cord neurones to AMPA to about twice the extent of responses of cortical neurones. Blockade of AMPA receptor desensitization by cyclothiazide (CTZ) potentiated responses of spinal cord neurones (6.8 fold) significantly more than responses of cortical neurones (4.8 fold). Responses of cortical neurones to KA were potentiated 3.5 fold by CTZ, while responses of spinal cord neurones were unaffected. Ultra-fast applications of AMPA to outside-out patches showed responses of spinal cord neurones desensitized by 97.5% and exhibit marked inward rectification, whereas cortical neurones desensitized by 91% and exhibited slight outward rectification. The time constants of deactivation and desensitization were about twice as fast in spinal cord than cortical neurones. In cortical neurones, single-cell RT - PCR showed GluR2 and GluR1 accounted for 91% of all subunits and were expressed together in 67% of neurones, predominantly as the flip variants (78%). GluR2 was detected alone in 24% of neurones. GluR3 and GluR4 were present in only 14 and 29% of neurones, respectively. For spinal cord neurones, GluR4(o) was detected in 81% of neurones, whereas predominantly flop versions of GluR1, 2 and 3 were detected in 38, 13 and 13% of neurones, respectively. These expression patterns are related to the respective pharmacological and mechanistic properties.

Coupling of AMPA receptors with the Na(+)/Ca(2+) exchanger in cultured rat astrocytes

Astrocytes exhibit three transmembrane Ca(2+) influx pathways: voltage-gated Ca(2+) channels (VGCCs), the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) class of glutamate receptors, and Na(+)/Ca(2+) exchangers. Each of these pathways is thought to be capable of mediating a significant increase in Ca(2+) concentration ([Ca(2+)](i)); however, the relative importance of each and their interdependence in the regulation astrocyte [Ca(2+)](i) is not known. We demonstrate here that 100 microM AMPA in the presence of 100 microM cyclothiazide (CTZ) causes an increase in [Ca(2+)](i) in cultured cerebral astrocytes that requires transmembrane Ca(2+) influx. This increase of [Ca(2+)](i) is blocked by 100 microM benzamil or 0.5 microM U-73122, which inhibit reverse-mode operation of the Na(+)/Ca(2+) exchanger by independent mechanisms. This response does not require Ca(2+) influx through VGCCs, nor does it depend upon a significant Ca(2+) influx through AMPA receptors (AMPARs). Additionally, AMPA in the presence of CTZ causes a depletion of thapsigargin-sensitive intracellular Ca(2+) stores, although depletion of these Ca(2+) stores does not decrease the peak [Ca(2+)](i) response to AMPA. We propose that activation of AMPARs in astrocytes can cause [Ca(2+)](i) to increase through the reverse mode operation of the Na(+)/Ca(2+) exchanger with an associated release of Ca(2+) from intracellular stores. This proposed mechanism requires neither Ca(2+)-permeant AMPARs nor the activation of VGCCs to be effective.

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.

Novel potent AMPA analogues differentially affect desensitisation of AMPA receptors in cultured hippocampal neurons

The agonist actions of two AMPA receptor analogues, (RS)-2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA) and (RS)-2-amino-3-(3-hydroxy-5-trfluoromethyl-4-isoxazolyl)prop ionic acid (Tri-F-AMPA) have been studied on cultured rat hippocampal neurons. Whole-cell recordings with semi-rapid application of the agonists were used to study steady-state (plateau) responses. ACPA was the most potent agonist (EC50, 1.2 microM), followed by AMPA (4.3 microM) and Tri-F-AMPA (4.6 microM), corresponding to a potency ratio of 4:1:1. Hill coefficients were close to 1 for AMPA and ACPA and close to 2 for Tri-F-AMPA, respectively. Plateau responses to maximal concentrations of the three agonists varied more than 2-fold. ACPA responses were 2.1 times greater and responses to Tri-F-AMPA were 1.6 times greater than responses to AMPA, respectively. Peak responses and desensitization were studied by using a fast piezoelectric device to apply agonists rapidly to outside-out patches. The time constants of desensitization were 8 ms for AMPA, 12 ms for Tri-F-AMPA and 17 ms for ACPA. There were no significant differences in the time-to-peak and 10-90% rise-time of the responses. The results indicate that of the three agonists tested, ACPA is the most potent at AMPA receptors expressed in cultured hippocampal neurons and that the maximum response to the agonists is inversely related to the rate of desensitization.

Comparison of the agonist binding site of homomeric, heteromeric, and chimeric GluR1(o) and GluR3(o) AMPA receptors

A series of AMPA [(R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid] analogues were evaluated for activity at homomeric, heteromeric, and chimeric rat GluR1(o) and GluR3(o) receptors expressed in Xenopus oocytes, using the two-electrode voltage clamp technique. The formation of heteromeric receptor complexes was demonstrated by cross-immunoprecipitation of both subunits from solubilized oocyte membranes. The AMPA analogue ACPA [(R,S)-2-amino-3(3-carboxy-5-methyl-4-isoxazolyl)propionic acid] was the most potent and selective agonist tested at GluR1(o) and GluR3(o), with a 10-fold selectivity for GluR3(o). ACPA showed an intermediate potency at both the GluR1(o) + 3(o) heteromeric complex as well as at the homomeric chimeric receptors. These experiments suggest that for receptor activation, agonist binding occurs between the interface of the GluR1 and GluR3 subunits in the heteromeric channel complex, perhaps between the S1 region of one subunit and the S2 region of another. Also, it seems that 1) electronegative group substitutions on the isoxazole ring of AMPA and 2) decreasing the pKa of the sub stituent at position 3 play a major role in determining the degree of receptor activation under steady-state conditions. Future studies will examine the effects of single amino acid mutations in these receptors, giving a more precise localization of the agonist binding site.