Role of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor regulation in stress-induced pain chronification

Persistent postsurgical pain is a serious issue in public health, which has received increased interest in recent years. Previous studies have reported that psychological factors promote the development of chronic postsurgical pain. However, it is unclear how chronification of postsurgical pain occurs. The α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor(AMPA) phosphorylation in the central nervous system plays a critical role in synaptic plasticity and contributes to central sensitization and chronic pain development. Here, we discuss the role of AMPA receptor regulation in stress-induced pain chronification after surgery.     

The N-terminal Domain Modulates α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) Receptor Desensitization

AMPA receptors are tetrameric glutamate-gated ion channels that mediate fast synaptic neurotransmission in mammalian brain. Their subunits contain a two-lobed N-terminal domain (NTD) that comprises over 40% of the mature polypeptide. The NTD is not obligatory for the assembly of tetrameric receptors, and its functional role is still unclear. By analyzing full-length and NTD-deleted GluA1–4 AMPA receptors expressed in HEK 293 cells, we found that the removal of the NTD leads to a significant reduction in receptor transport to the plasma membrane, a higher steady state-to-peak current ratio of glutamate responses, and strongly increased sensitivity to glutamate toxicity in cell culture. Further analyses showed that NTD-deleted receptors display both a slower onset of desensitization and a faster recovery from desensitization of agonist responses. Our results indicate that the NTD promotes the biosynthetic maturation of AMPA receptors and, for membrane-expressed channels, enhances the stability of the desensitized state. Moreover, these findings suggest that interactions of the NTD with extracellular/synaptic ligands may be able to fine-tune AMPA receptor-mediated responses, in analogy with the allosteric regulatory role demonstrated for the NTD of NMDA receptors.     

Contactin-associated protein 1 (Caspr1) regulates the traffic and synaptic content of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors

Glutamate receptors of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type mediate fast excitatory synaptic transmission in the CNS. Synaptic strength is modulated by AMPA receptor binding partners, which regulate receptor synaptic targeting and functional properties. We identify Contactin-associated protein 1 (Caspr1) as an AMPA receptor interactor. Caspr1 is present in synapses and interacts with AMPA receptors in brain synaptic fractions. Coexpression of Caspr1 with GluA1 increases the amplitude of glutamate-evoked currents. Caspr1 overexpression in hippocampal neurons increases the number and size of synaptic GluA1 clusters, whereas knockdown of Caspr1 decreases the intensity of synaptic GluA1 clusters. Hence, Caspr1 is a regulator of the trafficking of AMPA receptors to synapses.     

Interaction of the M4 segment with other transmembrane segments is required for surface expression of mammalian α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors

Ionotropic glutamate receptors (GluRs) are ligand-gated ion channels with a modular structure. The ion channel itself shares structural similarity, albeit an inverted membrane topology, with P-loop channels. Like P-loop channels, prokaryotic GluR subunits (e.g. GluR0) have two transmembrane segments. In contrast, eukaryotic GluRs have an additional transmembrane segment (M4), located C-terminal to the ion channel core. However, the structural/functional significance of this additional transmembrane segment is poorly defined. Although topologically similar to GluR0, mammalian AMPA receptor (GluA1) subunits lacking the M4 segment do not display surface expression. This lack of expression is not due to the M4 segment serving as an anchor to the ligand-binding domain because insertion of an artificial polyleucine transmembrane segment does not rescue surface expression. Specific interactions between M4 and the ligand-binding domain are also unlikely because insertion of polyglycines into the linker connecting them has no deleterious effects on function or surface expression. However, tryptophan and cysteine scanning mutagenesis of the M4 segment, as well as recovery of function in the polyleucine background, defined a unique face of the M4 helix that is required for GluR surface expression. In the AMPA receptor structure, this face forms intersubunit contacts with the transmembrane helices of the ion channel core (M1 and M3) from another subunit within the homotetramer. Thus, our experiments show that a highly specific interaction of the M4 segment with an adjacent subunit is required for surface expression of AMPA receptors. This interaction may represent a mechanism for regulating AMPA receptor biogenesis.     

α-Amino-3-hydroxy-5-methyl-4-isoxazole Propionic Acid (AMPA) and N-Methyl-d-aspartate (NMDA) Receptors Adopt Different Subunit Arrangements

Ionotropic glutamate receptors are widely distributed in the central nervous system and play a major role in excitatory synaptic transmission. All three ionotropic glutamate subfamilies (i.e. AMPA-type, kainate-type, and NMDA-type) assemble as tetramers of four homologous subunits. There is good evidence that both heteromeric AMPA and kainate receptors have a 2:2 subunit stoichiometry and an alternating subunit arrangement. Recent studies based on presumed structural homology have indicated that NMDA receptors adopt the same arrangement. Here, we use atomic force microscopy imaging of receptor-antibody complexes to show that whereas the GluA1/GluA2 AMPA receptor assembles with an alternating (i.e. 1/2/1/2) subunit arrangement, the GluN1/GluN2A NMDA receptor adopts an adjacent (i.e. 1/1/2/2) arrangement. We conclude that the two types of ionotropic glutamate receptor are built in different ways from their constituent subunits. This surprising finding necessitates a reassessment of the assembly of these important receptors.     

Synthese von O-(3-desoxy-α-d-manno-2-octulopyranosylonsäure)-(2→4)-O-(3-desoxy-α-d-manno-2-octulopyranosylonsäure)- (2→6)-O-(2-amino-2-desoxy-β-d-glucopyranosyl)-(1→6)-2-amino-2-desoxy-d-glucopyranose

Benzyl-3-O-benzyl-2-benzyloxycarbonylamino-6-O-[2-benzyloxycarbonyl-amino-2-deoxy-3,4-O-(tetraisopropyldisiloxane-1,3-diyl)- β-d-glucopyranosyl]-2-deoxy-α-d-glucopyranoside was coupled with methyl (4,5,7,8-tetra-O-acetyl-3-deoxy-α-d-manno-2-octulopyranosyl bromide)onate (13) to yield the α-glycosidically linked trisaccharide. After deacetylation and selective introduction of a second 7′,8′-O-tetraisopropyldisiloxane group, a further glycosidation reaction with 13 led regioselectively to the tetrasaccharide benzyl O-[methyl (4,5,7,8-tetra-O-acetyl-3-deoxy-α-d-manno-2-octulopyranosyl)onate]-(2→4)-O-{methyl [3-deoxy-7,8-O-(tetraisopropyldisiloxane-1,3-diyl)-α-d-manno-2-octulopyranosyl]-onate}-(2→6)-O- [2-benzyloxycarbonylamino-2-deoxy-3,4-O-(tetraisopropyldisiloxane-1,3-diyl)-β-d-glucopyranosyl]- (1→6)-3-O-benzyl-2-benzyloxycarbonyl-amino-2-deoxy-α-d-glucopyranoside. A series of deblocking steps gave O-(3-deoxy-α-d-manno-2-octulopyranosylonic acid)-(2→4)-O-(3-deoxy-α-d-manno-2-octulopyranosylonic acid)- (2→6)-O-(2-amino-2-deoxy-β-d-glucopyranosyl)-(1→6)-2-amino-2-deoxy-d-glucopyranose which was identical with a tetrasaccharide that had been isolated by hydrazinolysis of the lipopolysaccharide from Salmonella minnesota R 595. Hence, synthetic proof is provided for the linkages in this part of the inner core region of lipopolysaccharides.     

Synthesis of the disaccharides methyl 4-O-(2′/3′-O-sulfo-β-d-glucopyranosyluronic acid)-2-amino-2-deoxy-α-d-glucopyranoside disodium salts,related to heparin biosynthesis

The synthesis of the disaccharides methyl 4-O-(2/3-O-sulfo--d-glucopyranosyluronic acid)-2-amino-2-deoxy--d-glucopyranoside3 and4 as, disodium salts is described. Allyl 4,6-O-benzylidene--d-glucopyranoside6 was converted to trichloroacetimidate20 Glycosylation of20 with5 promoted by BF₃·OEt₂ gave disaccharide21. Deacetylation of21 followed by monoacetylation of the resultant diol22 afforded the two monoacetylated disaccharides23 and24. Sulfation and deprotection of each disaccharide gave the desired sulfated compounds3 and4.     

Effect of age on α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) binding sites in the rat brain studied by in vitro autoradiography

Receptors for excitatory amino acid,L-glutamate, have been classified into three subtypes named as N-methyl-D-aspartate (NMDA), quisqualate (QA) and kainate receptors. In the present study, an effect of age on binding sites of [³H]-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (³H-AMPA), a QA agonist, was studied in the rat brain through quantitative in vitro autoradiography.³H-AMPA binding sites were most concentrated in the hippocampus and cerebral cortex where glutamate receptors have been demonstrated to play a role in synaptic transmission. In aged rats,³H-AMPA binding sites in the hippocampus and cerebral cortex were not significantly changed. In our previous studies, it was noticed that strychnine-insensitive glycine receptors, which functionally coupled with NMDA receptors, showed marked age-dependent decreases in telencephalic regions. It has been shown that the glutamatergic neuronal system is involved in learning and memory. Nevertheless, it is considered that AMPA binding sites are not involved in the decline of neuronal functions, especially impairment of learning and memory, accompanying with aging process.     

Dynamic Regulation of N-Methyl-d-aspartate (NMDA) and α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptors by Posttranslational Modifications

Many molecular mechanisms underlie the changes in synaptic glutamate receptor content that are required by neuronal networks to generate cellular correlates of learning and memory. During the last decade, posttranslational modifications have emerged as critical regulators of synaptic transmission and plasticity. Notably, phosphorylation, ubiquitination, and palmitoylation control the stability, trafficking, and synaptic expression of glutamate receptors in the central nervous system. In the current review, we will summarize some of the progress made by the neuroscience community regarding our understanding of phosphorylation, ubiquitination, and palmitoylation of the NMDA and AMPA subtypes of glutamate receptors.     

Ca2+-permeable AMPA (α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid) Receptors and Dopamine D1 Receptors Regulate GluA1 Trafficking in Striatal Neurons

Regulation of striatal medium spiny neuron synapses underlies forms of motivated behavior and pathological drug seeking. A primary mechanism for increasing synaptic strength is the trafficking of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) into the postsynapse, a process mediated by GluA1 AMPAR subunit phosphorylation. We have examined the role of converging glutamate and dopamine inputs in regulating biochemical cascades upstream of GluA1 phosphorylation. We focused on the role of Ca²⁺-permeable AMPARs (CPARs), which lack the GluA2 AMPAR subunit. Under conditions that prevented depolarization, stimulation of CPARs activated neuronal nitric oxide synthase and production of cGMP. CPAR-dependent cGMP production was sufficient to induce synaptic insertion of GluA1, detected by confocal microscopy, through a mechanism dependent on GluA1 Ser-845 phosphorylation. Dopamine D1 receptors, in contrast, stimulate GluA1 extra synaptic insertion. Simultaneous activation of dopamine D1 receptors and CPARs induced additive increases in GluA1 membrane insertion, but only CPAR stimulation augmented CPAR-dependent GluA1 synaptic insertion. This incorporation into the synapse proceeded through a sequential two-step mechanism; that is, cGMP-dependent protein kinase II facilitated membrane insertion and/or retention, and protein kinase C activity was necessary for synaptic insertion. These data suggest a feed-forward mechanism for synaptic priming whereby an initial stimulus acting independently of voltage-gated conductance increases striatal neuron excitability, facilitating greater neuronal excitation by a subsequent stimulus.     

Characterization of quisqualate recognition sites in rat brain tissue usingDl-[3H]α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and a filtration assay

The binding of [³H]AMPA (Dl--amino-3-hydroxy-5-methylisoxazole-4-propionic acid), a ligand for the putative quisqualate excitatory amino acid receptor subtype, was evaluated using centrifugation and filtration receptor binding techniques in rat brain crude synaptosomal membrane preparations. Maximal specific binding of [³H]AMPA occurred in Triton X-100 treated membranes in the presence of the chaotropic agent potassium thiocyanate (KSCN). The effects of KSCN on binding were reversible and optimal at 100 mM. Supernatant obtained from detergent-treated membranes inhibited specific [³H]AMPA and [³H]kainic acid binding, suggesting the presence of an inhibitory agent which was tentatively identified as glutamate. Using centrifugation, saturation analysis revealed two distinct binding sites in both the absence and presence of KSCN. The chaotrope was most effective in increasing binding at the low affinity binding site, enhancing the affinity (K _d) without a concommitant change in the total number of binding sites. Using filtration, a single binding site was detected in Triton-treated membranes. Like the data obtained by centrifugation, KSCN enhanced the affinity of the receptor (K _d value=10 nM) without altering the number of binding sites (B _max=1.2 pmol/mg protein). The rank order of potency of various glutamate analogs in the [³H]AMPA binding assay was quisqualate > AMPA > l-glutamate > kainate > d-glutamate, consistent with the labeling of a quisqualate-type excitatory amino acid receptor subtype.l-glutamic acid diethylester, and 2-amino-7-phosphonoheptanoic acid (AP₇) were inactive. The present technique provides a rapid, reliable assay for the evaluation of quisqualate-type excitatory amino acid agonists and/or antagonists that may be used to discover more potent and selective agents.     

3-O-phosphate ester conjugates of 17-β-O-{1-[2-carboxy-(2-hydroxy-4-methoxy-3-carboxamido)anilido]ethyl}1,3,5(10)-estratriene as novel bone-targeting agents

A series of 3-O-phosphorylated analogs (4-10) of a novel bone-targeting estradiol analog (3) were synthesized after a thorough study of the reaction of 3 with a selection of phosphoryl chlorides under a variety of reaction conditions. Evaluation of these novel phosphate analogs for affinity for hydroxyapatite revealed that they bind with equal or higher affinity when compared to the bone tissue accumulator, tetracycline.     

Kinetics and mechanisms of Fe(III) complexation by lipophilic 3-hydroxy-2-methyl-1(γ-stearoamidopropyl)-4-pyridinone (HMSP)

 The kinetics of Fe(III) complexation by lipophilic 3-hydroxy-2-methyl-l(γ-stearoamidopropyl)-4-pyridinone (HMSP) were studied when [Fe(III)] > [HMSP] in MeOH/H₂O mixed solvent and [Fe(III)] < [HMSP] in MeOH, respectively. When Fe(III) was in excess, the observed rate constants depend on [Fe(III)]² _tot and on the reciprocal of [H⁺] and decrease with increasing pressure. ΔV ^‡ values are around +8.0 cm³ mol^–1. A mechanism consisting of the complexations of the hydrolyzed monomer Fe(H₂O)₅OH²⁺ and dimer species Fe₂(H₂O)₇ (μ-OH)₂OH³⁺ by HMSP is proposed. This mechanism is supported by the solvent effect and the work of other researchers. When HMSP is in excess, Fe(HMSP)₃ is formed and three kinetic steps on different time-scales are observed. An "intermolecular chelate ring-closure" mechanism is proposed, differing from the "intramolecular chelate ring-closure" complexation reported for the formation of ferrioxamine B. Received: 14 February 1997 / Accepted: 1 September 1997     

The Structure of (?)-Kaitocephalin Bound to the Ligand Binding Domain of the (S)-α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA)/Glutamate Receptor,GluA2

Glutamate receptors mediate the majority of excitatory synaptic transmission in the central nervous system, and excessive stimulation of these receptors is involved in a variety of neurological disorders and neuronal damage from stroke. The development of new subtype-specific antagonists would be of considerable therapeutic interest. Natural products can provide important new lead compounds for drug discovery. The only natural product known to inhibit glutamate receptors competitively is (−)-kaitocephalin, which was isolated from the fungus Eupenicillium shearii and found to protect CNS neurons from excitotoxicity. Previous work has shown that it is a potent antagonist of some subtypes of glutamate receptors (AMPA and NMDA, but not kainate). The structure of kaitocephalin bound to the ligand binding domain of the AMPA receptor subtype, GluA2, is reported here. The structure suggests how kaitocephalin can be used as a scaffold to develop more selective and high affinity antagonists for glutamate receptors.     

Discovery of (3S,4S)-3-methyl-3-(4-fluorophenyl)-4-(4-(1,1,1,3,3,3-hexafluoro-2-hydroxyprop-2-yl)phenyl)pyrrolidines as novel RORγt inverse agonists

A novel series of cis-3,4-diphenylpyrrolidines were designed as RORγt inverse agonists based on the binding conformation of previously reported bicyclic sulfonamide 1. Preliminary synthesis and structure–activity relationship (SAR) study established (3S,4S)-3-methyl-3-(4-fluorophenyl)-4-(4-(1,1,1,3,3,3-hexafluoro-2-hydroxyprop-2-yl)phenyl)pyrrolidine as the most effective scaffold. Subsequent SAR optimization led to identification of a piperidinyl carboxamide 31, which was potent against RORγt (EC₅₀ of 61 nM in an inverse agonist assay), selective relative to RORα, RORβ, LXRα and LXRβ, and stable in human and mouse liver microsomes. Furthermore, compound 31 exhibited considerably lower PXR Y_max (46%) and emerged as a promising lead. The binding mode of the diphenylpyrrolidine series was established with an X-ray co-crystal structure of 10A/RORγt.     

Optical resolution of 2-(3-indolyl)propionic acid with Mucor javanicus and α-chymotrypsin

Resolution of 2-(3-indolyl)propionic acid was achieved via biocatalytic hydrolysis of its chloroethyl ester. Of the enzymes tested, Mucor javanicus lipase (R selectivity) and -chymotrypsin (S selectivity) had high reactivity and enantioselectivity (E value > 50). Neither enzyme showed enantioselectivity (E value = 1) for 2-phenylpropionic acid.     

Structural and theoretical studies of [6-bromo-1-(4-fluorophenylmethyl)-4(1H)-quinolinon-3-yl)]-4-hydroxy-2-oxo-3-butenoïc acid as HIV-1 integrase inhibitor

Ethyl [6-bromo-1-(4-fluorophenylmethyl)-4(1H)-quinolinon-3-yl]-4-hydroxy-2-oxo-3-butenoate 1 and [6-bromo-1-(4-fluorophenylmethyl)-4(1H)-quinolinon-3-yl)]-4-hydroxy-2-oxo-3-butenoïc acid 2 were synthesized as potential HIV-1 integrase inhibitors and evaluated for their enzymatic and antiviral activity, acidic compound 2 being more potent than ester compound 1. X-ray diffraction analyses and theoretical calculations show that the diketoacid chain of compound 2 is preferentially coplanar with the quinolinone ring (dihedral angle of 0–30°). Docking studies suggest binding modes in agreement with structure–activity relationships.     

Identification and profiling of 3,5-dimethyl-isoxazole-4-carboxylic acid [2-methyl-4-((2S,3′S)-2-methyl-[1,3′]bipyrrolidinyl-1′-yl)phenyl] amide as histamine H3 receptor antagonist for the treatment of depression

Lead optimization guided by histamine H₃ receptor (H₃R) affinity and calculated physico-chemical properties enabled simultaneous improvement in potency and PK properties leading to the identification of a potent, selective, devoid of hERG issues, orally bioavailable, and CNS penetrable H₃R antagonist/inverse agonist 3h. The compound was active in forced-swimming tests suggesting its potential therapeutic utility as an anti-depressive agent. This Letter further includes its cardiovascular and neuropsychological/behavioral safety assessments.     

Inhibition of Calcineurin-mediated Endocytosis and ��-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptors Prevents Amyloid �� Oligomer-induced Synaptic Disruption

Synaptic degeneration, including impairment of synaptic plasticity and loss of synapses, is an important feature of Alzheimer disease pathogenesis. Increasing evidence suggests that these degenerative synaptic changes are associated with an accumulation of soluble oligomeric assemblies of amyloid β (Aβ) known as ADDLs. In primary hippocampal cultures ADDLs bind to a subpopulation of neurons. However the molecular basis of this cell type-selective interaction is not understood. Here, using siRNA screening technology, we identified α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits and calcineurin as candidate genes potentially involved in ADDL-neuron interactions. Immunocolocalization experiments confirmed that ADDL binding occurs in dendritic spines that express surface AMPA receptors, particularly the calcium-impermeable type II AMPA receptor subunit (GluR2). Pharmacological removal of the surface AMPA receptors or inhibition of AMPA receptors with antagonists reduces ADDL binding. Furthermore, using co-immunoprecipitation and photoreactive amino acid cross-linking, we found that ADDLs interact preferentially with GluR2-containing complexes. We demonstrate that calcineurin mediates an endocytotic process that is responsible for the rapid internalization of bound ADDLs along with surface AMPA receptor subunits, which then both colocalize with cpg2, a molecule localized specifically at the postsynaptic endocytic zone of excitatory synapses that plays an important role in activity-dependent glutamate receptor endocytosis. Both AMPA receptor and calcineurin inhibitors prevent oligomer-induced surface AMPAR and spine loss. These results support a model of disease pathogenesis in which Aβ oligomers interact selectively with neurotransmission pathways at excitatory synapses, resulting in synaptic loss via facilitated endocytosis. Validation of this model in human disease would identify therapeutic targets for Alzheimer disease.