Subtype selective kainic acid receptor agonists: Discovery and approaches to rational design

Kainate Receptor Antagonists: Recent Advances and Therapeutic Perspective

Since the 1990s, ionotropic glutamate receptors have served as an outstanding target for drug discovery research aimed at the discovery of new neurotherapeutic agents. With the recent approval of perampanel, the first marketed non-competitive antagonist of AMPA receptors, particular interest has been directed toward 'non-NMDA' (AMPA and kainate) receptor inhibitors. Although the role of AMPA receptors in the development of neurological or psychiatric disorders has been well recognized and characterized, progress in understanding the function of kainate receptors (KARs) has been hampered, mainly due to the lack of specific and selective pharmacological tools. The latest findings in the biology of KA receptors indicate that they are involved in neurophysiological activity and play an important role in both health and disease, including conditions such as anxiety, schizophrenia, epilepsy, neuropathic pain, and migraine. Therefore, we reviewed recent advances in the field of competitive and non-competitive kainate receptor antagonists and their potential therapeutic applications. Due to the high level of structural divergence among the compounds described here, we decided to divide them into seven groups according to their overall structure, presenting a total of 72 active compounds.

Chemical Kindling as an Experimental Model to Assess the Conventional Drugs in the Treatment of Post-traumatic Epilepsy

BACKGROUND

Traumatic brain injury (TBI) is one of the leading causes of morbidity and mortality today, which will surpass many infectious diseases in the coming years/decades. Posttraumatic epilepsy (PTE) is one of the most common debilitating consequences of TBI. PTE is a secondary, acquired epilepsy that causes recurrent, spontaneous seizures more than a week after TBI. The extent of head injury in individuals who develop PTE is unknown; however, trauma is thought to account for 20% of symptomatic epilepsy worldwide. Understanding the mechanisms of epilepsy following TBI is crucial for the discovery of new anticonvulsant drugs for the treatment of PTE, as well as for improving the quality of life of patients with PTE.

OBJECTIVE

This review article explains the rationale for the usage of a chemical model to access new treatments for post-traumatic epilepsy.

RESULTS

There are multiple methods to control and manage PTE. The essential and available remedy for the management of epilepsy is the use of antiepileptic drugs. Antiepileptic drugs (AEDs) decrease the frequency of seizures without affecting the disease's causality. Antiepileptic drugs are administrated for the prevention and treatment of PTE; however, 30% of epilepsy patients are drug-resistant, and AED side effects are significant in PTE patients. There are different types of animal models, such as the liquid percussion model, intracortical ferric chloride injection, and cortical subincision model, to study PTE and neurophysiological mechanisms underlying the development of epilepsy after head injury. However, these animal models do not easily mimic the pathological events occurring in epilepsy. Therefore, animal models of PTE are an inappropriate tool for screening new and putatively effective AEDs. Chemical kindling is the most common animal model used to study epilepsy. There is a strong similarity between the kindling model and different types of human epilepsy.

CONCLUSION

Today, researchers use experimental animal models to evaluate new anticonvulsant drugs. The chemical kindling models, such as pentylenetetrazol, bicuculline, and picrotoxin-induced seizures, are important experimental models to analyze the impact of putative antiepileptic drugs.

Repeated exposure to microcystin-leucine-arginine potentiates excitotoxicity induced by a low dose of kainate

Microcystin-leucine-arginine (MLCR) is a cyanobacterial toxin, and has been demonstrated to cause neurotoxicity. In addition, MCLR has been identified as an inhibitor of protein phosphatase (PP)1 and PP2A, which are known to regulate the phosphorylation of various molecules related to synaptic excitability. Thus, in the present study, we examined whether MCLR exposure affects seizures induced by a low dose of kainic acid (KA; 0.05 μg, i.c.v.) administration. KA-induced seizure occurrence and seizure score significantly increased after repeated exposure to MCLR (2.5 or 5.0 μg/kg, i.p., once a day for 10 days), but not after acute MCLR exposure (2.5 or 5.0 μg/kg, i.p., 2 h and 30 min prior to KA administration), and hippocampal neuronal loss was consistently facilitated by repeated exposure to MCLR. In addition, repeated MCLR significantly elevated the membrane expression of kainate receptor GluK2 subunits, p-pan-protein kinase C (PKC), and p-extracellular signal-related kinase (ERK) at 1 h after KA. However, KA-induced membrane expression of Ca²⁺/calmodulin-dependent kinase II (CaMKII) was significantly reduced by repeated MCLR exposure. Consistent with the enhanced seizures and neurodegeneration, MCLR exposure significantly potentiated KA-induced oxidative stress and microglial activation, which was accompanied by increased expression of p-ERK and p-PKCδ in the hippocampus. The combined results suggest that repeated MCLR exposure potentiates KA-induced excitotoxicity in the hippocampus by increasing membrane GluK2 expression and enhancing oxidative stress and neuroinflammation through the modulation of p-CaMKII, p-PKC, and p-ERK.

Negative feedback control of neuronal activity by microglia

Microglia, the brain’s resident macrophages, help to regulate brain function by removing dying neurons, pruning non-functional synapses, and producing ligands that support neuronal survival¹. Here we show that microglia are also critical modulators of neuronal activity and associated behavioural responses in mice. Microglia respond to neuronal activation by suppressing neuronal activity, and ablation of microglia amplifies and synchronizes the activity of neurons, leading to seizures. Suppression of neuronal activation by microglia occurs in a highly region-specific fashion and depends on the ability of microglia to sense and catabolize extracellular ATP, which is released upon neuronal activation by neurons and astrocytes. ATP triggers the recruitment of microglial protrusions and is converted by the microglial ATP/ADP hydrolysing ectoenzyme CD39 into AMP; AMP is then converted into adenosine by CD73, which is expressed on microglia as well as other brain cells. Microglial sensing of ATP, the ensuing microglia-dependent production of adenosine, and the adenosine-mediated suppression of neuronal responses via the adenosine receptor A₁R are essential for the regulation of neuronal activity and animal behaviour. Our findings suggest that this microglia-driven negative feedback mechanism operates similarly to inhibitory neurons and is essential for protecting the brain from excessive activation in health and disease.

A Diversity Oriented Synthesis Approach to New 2,3-trans-Substituted l-Proline Analogs as Potential Ligands for the Ionotropic Glutamate Receptors

Discovery of chemical tools for the ionotropic glutamate receptors continues to be a challenging task. Herein we report a diversity-oriented approach to new 2,3-trans-l-proline analogs whereby we study how the spatial orientation of the distal carboxylate group influences the binding affinity and receptor class and subtype selectivity. In total, 10 new analogs were synthesized and 14 stereoisomers characterized in binding assays at native rat ionotropic glutamate receptors, and at cloned human homomeric kainic acid (KA) receptor subtypes GluK1-3. The study identified isoxazole analogs 3d,e, which displayed selectivity in binding at native N-methyl-d-aspartate (NMDA) receptors over native α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and KA receptors, in the high nanomolar to low micromolar range. Furthermore, analogs 3i-A/B showed a preference in binding affinity for GluK3 over GluK1,2. Finally, analog 3j displayed high nanomolar affinity for native NMDA receptors as well as for homomeric GluK3 receptors.

Kainic Acid-Based Agonists of Glutamate Receptors: SAR Analysis and Guidelines for Analog Design

A comprehensive survey of kainic acid analogs that have been tested for their biological activity is presented. Specifically, this review (1) gathers and compares over 100 kainoids according to a relative activity scale, (2) exposes structural features required to optimize affinity for kainate receptors, and (3) suggests design rules to create next-generation KA analogs. Literature SAR data are analyzed systematically and combined with the most recent crystallographic studies. In view of the renewed interest in neuroactive molecules, this review aims to help guide the efforts of organic synthesis laboratories, as well as to inform newcomers to KA/GluK research.

Design and Synthesis of 2,3- trans-Proline Analogues as Ligands for Ionotropic Glutamate Receptors and Excitatory Amino Acid Transporters

Development of pharmacological tools for the ionotropic glutamate receptors (iGluRs) is imperative for the study and understanding of the role and function of these receptors in the central nervous system. We report the synthesis of 18 analogues of (2 S,3 R)-2-carboxy-3-pyrrolidine acetic acid (3a), which explores the effect of introducing a substituent on the ε-carbon (3c-q). A new synthetic method was developed for the efficient synthesis of racemic 3a and applied to give expedited access to 13 racemic analogues of 3a. Pharmacological characterization was carried out at native iGluRs, cloned homomeric kainate receptors (GluK1-3), NMDA receptors (GluN1/GluN2A-D), and excitatory amino acid transporters (EAAT1-3). From the structure-activity relationship studies, several new ligands emerged, exemplified by triazole 3p-d1, GluK3-preferring (GluK1/GluK3 K_i ratio of 15), and the structurally closely related tetrazole 3q-s3-4 that displayed 4.4-100-fold preference as an antagonist for the GluN1/GluN2A receptor ( K_i = 0.61 μM) over GluN1/GluN2B-D ( K_i = 2.7-62 μM).

Total Synthesis of Neodysiherbaine A via 1,3-Dipolar Cycloaddition of a Chiral Nitrone Template

The total synthesis of neodysiherbaine A was achieved via 1,3-dipolar cycloaddition of a chiral nitrone template with a sugar-derived allyl alcohol in the presence of MgBr₂·OEt₂. This cycloaddition constructed the C2 and C4 asymmetric centers in a single step. Then reductive cleavage, intramolecular S_N2 reaction of the tertiary alcohol, and oxidation of the primary alcohol afforded neodysiherbaine A.

Revisiting the Quinoxalinedione Scaffold in the Construction of New Ligands for the Ionotropic Glutamate Receptors

More than two decades ago, the quinoxalinedione scaffold was shown to act as an α-amino acid bioisoster. Following extensive structure-activity relationship (SAR) studies, the antagonists DNQX, CNQX, and NBQX in the ionotropic glutamate receptor field were identified. In this work, we revisit the quinoxalinedione scaffold and explore the incorporation of an acid functionality in the 6-position. The SAR studies disclose that by this strategy it was possible to tune in iGluR selectivity among the AMPA, NMDA, and KA receptors, and to some extent also obtain full receptor subtype selectivity. Highlights of the study of 44 new analogues are compound 2m being a high affinity ligand for native AMPA receptors (IC₅₀= 0.48 μM), analogues 2e,f,h,k,v all displayed selectivity for native NMDA receptors, and compounds 2s,t,u are selective ligand for the GluK1 receptor. Most interestingly, compound 2w was shown to be a GluK3-preferring ligand with full selectivity over native AMPA, KA and NMDA receptors.

Evaluation of β-Aminocarboxylic Acid Derivatives in Hippocampal Excitatory Synaptic Transmission

β-Aminocarboxylic acid derivatives (LINS04 series) were screened with the aim to explore their potential functional role in excitatory synaptic transmission in the central nervous system. We used field recordings in rat hippocampal slices to investigate the effects of the LINS04 series on the synaptic transmission at hippocampal CA1 synapses. We found that LINS04008 and LINS04009 increase the size of the evoked field excitatory postsynaptic potential (EPSP) in a dose-dependent manner. The concentration-response curve shows that the efficacy of LINS04008 is highest in the series (EC₅₀  = 91.32 µM; maximum fEPSP 44.97%). The esters LINS04006 and LINS04005 did not affect the synaptic evoked activity. These data provide the first evidence of synaptic activity enhancement by these compounds and the importance of the acidic group to the activity. This set of data may provide direction for a strategic procedure to restore the glutamate synaptic transmission; however, further studies are needed to establish a more complete picture of how these molecules act on the glutamate transmission, which are in our mind for the next steps.

Ring Rearrangement Metathesis in 7-Oxabicyclo[2.2.1]heptene (7-Oxanorbornene) Derivatives. Some Applications in Natural Product Chemistry

Metathesis reactions is firmly established as a valuable synthetic tool in organic chemistry, clearly comparable with the venerable Diels-Alder and Wittig reactions and, more recently, with the metal-catalyzed cross-coupling reactions. Metathesis reactions can be considered as a fascinating synthetic methodology, allowing different variants regarding substrate (alkene and alkyne metathesis) and type of metathetical reactions. On the other hand, tandem metathesis reactions such Ring Rearrangement Metathesis (RRM) and the coupling of metathesis reaction with other reactions of alkenes such as Diels-Alder or Heck reactions, makes metathesis one of the most powerful and reliable synthetic procedure.

In particular, Ring-Rearrangement Metathesis (RRM) refers to the combination of several metathesis transformations into a domino process such as ring-opening metathesis (ROM)/ring-closing metathesis (RCM) and ROM-cross metathesis (CM) in a one-pot operation. RRM delivers complex frameworks that are difficult to assemble by conventional methods constitutingan atom economic process. RRM is applicable to mono- and polycyclic systems of varying ring sizes such as cyclopropene, cyclobutene, cyclopentene, cyclohexene, pyran systems, bicyclo[2.2.1]heptene derivatives, bicyclo[2.2.2]octene derivatives, bicyclo[3.2.1]octene derivatives and bicyclo[3.2.1]octene derivatives.

In this review our attention has focused on the RRM reactions in 7-oxabicyclo[2.2.1]heptene derivatives and on their application in the synthesis of natural products or significant subunits of them.

Synthesis and pharmacological characterization of the selective GluK1 radioligand (S)-2-amino-3-(6-[3H]-2,4-dioxo-3,4-dihydrothieno[3,2-d]pyrimidin-1(2H)-yl)propanoic acid ([3H]-NF608)

Expedite synthesis of [³H]NF608 – a new subtype selective GluK1 radioligand.

Beyond Membrane Protein Structure: Drug Discovery, Dynamics and Difficulties

Most of the previous content of this book has focused on obtaining the structures of membrane proteins. In this chapter we explore how those structures can be further used in two key ways. The first is their use in structure based drug design (SBDD) and the second is how they can be used to extend our understanding of their functional activity via the use of molecular dynamics. Both aspects now heavily rely on computations. This area is vast, and alas, too large to consider in depth in a single book chapter. Thus where appropriate we have referred the reader to recent reviews for deeper assessment of the field. We discuss progress via the use of examples from two main drug target areas; G-protein coupled receptors (GPCRs) and ion channels. We end with a discussion of some of the main challenges in the area.

Decursin attenuates kainic acid-induced seizures in mice

Epilepsy is a neurological disorder with recurrent unprovoked seizures as the main symptom. Of the coumarin derivatives in Angelica gigas, decursin, a major coumarin component, was reported to exhibit significant protective activity against glutamate-induced neurotoxicity when added to primary cultures of rat cortical cells. This study served to investigate the effects of decursin on a kainic acid (KA)-induced status epilepticus model. Thirty minutes after intraperitoneal injections of decursin (20 mg/kg) in male 7-week-old C57BL/6 mice, the animals were treated with KA (30 mg/kg, intraperitoneally) and then examined for behavioral seizure score, electroencephalogram, seizure-related expressed protein levels, neuronal cell loss, neurodegeneration, and astrogliosis. KA injections significantly enhanced neurodegenerative conditions but treatment with decursin 30 min before KA injection reduced the detrimental effects of KA in mice. The decursin-treated KA-injected group showed significantly decreased behavioral seizure activity and remarkably attenuated intense and high-frequency seizure discharges in the parietal cortex for 2 h compared with the group treated only with KA. Furthermore, in-vivo results indicated that decursin strongly inhibits selective neuronal death, astrogliosis, and oxidative stress induced by KA administration. Therefore decursin is able to attenuate KA-induced seizures and could have potential as an antiepileptic drug.

Functional and morphological characterization of glutamate transporters in the rat locus coeruleus

BACKGROUND AND PURPOSE

Excitatory amino acid transporters (EAATs) in the CNS contribute to the clearance of glutamate released during neurotransmission. The aim of this study was to explore the role of EAATs in the regulation of locus coeruleus (LC) neurons by glutamate.

EXPERIMENTAL APPROACH

We measured the effect of different EAAT subtype inhibitors/enhancers on glutamate- and KCl-induced activation of LC neurons in rat slices. EAAT2-3 expression in the LC was also characterized by immunohistochemistry.

KEY RESULTS

The EAAT2-5 inhibitor DL-threo-β-benzyloxaspartic acid (100 μM), but not the EAAT2, 4, 5 inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (100 μM) or the EAAT2 inhibitor dihydrokainic acid (DHK; 100 μM), enhanced the glutamate- and KCl-induced activation of the firing rate of LC neurons. These effects were blocked by ionotropic, but not metabotrobic, glutamate receptor antagonists. DHK (100 μM) was the only EAAT inhibitor that increased the spontaneous firing rate of LC cells, an effect that was due to inhibition of EAAT2 and subsequent AMPA receptor activation. Chronic treatment with ceftriaxone (200 mg·kg(-1) i.p., once daily, 7 days), an EAAT2 expression enhancer, increased the actions of glutamate and DHK, suggesting a functional impact of EAAT2 up-regulation on the glutamatergic system. Immuhistochemical data revealed the presence of EAAT2 and EAAT3 surrounding noradrenergic neurons and EAAT2 on glial cells in the LC.

CONCLUSIONS AND IMPLICATIONS

These results remark the importance of EAAT2 and EAAT3 in the regulation of rat LC by glutamate. Neuronal EAAT3 would be responsible for terminating the action of synaptically released glutamate, whereas glial EAAT2 would regulate tonic glutamate concentrations in this nucleus.

Chemoenzymatic synthesis of new 2,4-syn-functionalized (S)-glutamate analogues and structure-activity relationship studies at ionotropic glutamate receptors and excitatory amino acid transporters

In the mammalian central nervous system, (S)-glutamate (Glu) is released from the presynaptic neuron where it activates a plethora of pre- and postsynaptic Glu receptors. The fast acting ionotropic Glu receptors (iGluRs) are ligand gated ion channels and are believed to be involved in a vast number of neurological functions such as memory and learning, synaptic plasticity, and motor function. The synthesis of 14 enantiopure 2,4-syn-Glu analogues 2b-p is accessed by a short and efficient chemoenzymatic approach starting from readily available cyclohexanone 3. Pharmacological characterization at the iGluRs and EAAT1-3 subtypes revealed analogue 2i as a selective GluK1 ligand with low nanomolar affinity. Two X-ray crystal structures of the key analogue 2i in the ligand-binding domain (LBD) of GluA2 and GluK3 were determined. Partial domain closure was seen in the GluA2-LBD complex with 2i comparable to that induced by kainate. In contrast, full domain closure was observed in the GluK3-LBD complex with 2i, similar to that of GluK3-LBD with glutamate bound.

Neuroprotective and neurotrophic actions of glucagon-like peptide-1: an emerging opportunity to treat neurodegenerative and cerebrovascular disorders

Like type-2 diabetes mellitus (T2DM), neurodegenerative disorders and stroke are an ever increasing, health, social and economic burden for developed Westernized countries. Age is an important risk factor in all of these; due to the rapidly increasing rise in the elderly population T2DM and neurodegenerative disorders, both represent a looming threat to healthcare systems. Whereas several efficacious drugs are currently available to ameliorate T2DM, effective treatments to counteract pathogenic processes of neurodegenerative disorders are lacking and represent a major scientific and pharmaceutical challenge. Epidemiological data indicate an association between T2DM and most major neurodegenerative disorders, including Alzheimer's and Parkinson's diseases. Likewise, there is an association between T2DM and stroke incidence. Studies have revealed that common pathophysiological features, including oxidative stress, insulin resistance, abnormal protein processing and cognitive decline, occur across these. Based on the presence of shared mechanisms and signalling pathways in these seemingly distinct diseases, one could hypothesize that an effective treatment for one disorder could prove beneficial in the others. Glucagon-like peptide-1 (GLP-1)-based anti-diabetic drugs have drawn particular attention as an effective new strategy to not only regulate blood glucose but also to reduce apoptotic cell death of pancreatic beta cells in T2DM. Evidence supports a neurotrophic and neuroprotective role of GLP-1 receptor (R) stimulation in an increasing array of cellular and animal neurodegeneration models as well as in neurogenesis. Herein, we review the physiological role of GLP-1 in the nervous system, focused towards the potential benefit of GLP-1R stimulation as an immediately translatable treatment strategy for acute and chronic neurological disorders.

Pharmacological and structural characterization of conformationally restricted (S)-glutamate analogues at ionotropic glutamate receptors

Conformationally restricted glutamate analogues have been pharmacologically characterized at AMPA and kainate receptors and the crystal structures have been solved of the ligand (2S,1'R,2'S)-2-(2'-carboxycyclobutyl)glycine (CBG-IV) in complex with the ligand binding domains of the AMPA receptor GluA2 and the kainate receptor GluK3. These structures show that CBG-IV interacts with the binding pocket in the same way as (S)-glutamate. The binding affinities reveal that CBG-IV has high affinity at the AMPA and kainate receptor subtypes. Appreciable binding affinity of CBG-IV was not observed at NMDA receptors, where the introduction of the carbocyclic ring is expected to lead to a steric clash with binding site residues. CBG-IV was demonstrated to be an agonist at both GluA2 and the kainate receptor GluK1. CBG-IV showed high affinity binding to GluK1 compared to GluA2, GluK2 and GluK3, which exhibited lower affinity for CBG-IV. The structure of GluA2 LBD and GluK3 LBD in complex with CBG-IV revealed similar binding site interactions to those of (S)-glutamate. No major conformational rearrangements compared to the (S)-glutamate bound conformation were found in GluK3 in order to accommodate CBG-IV, in contrast with GluA2 where a shift in lobe D2 binding site residues occurs, leading to an increased binding cavity volume compared to the (S)-glutamate bound structure.

Exposure to prenatal stress enhances the development of seizures in young rats

A febrile seizure is a neurological disorder that occurs following an infection that results in a rapid rise in body temperature. It commonly affects 3-5% of children between the ages of 3 months and 5 years. Interleukin-1 beta IL-1β a pro-inflammatory cytokine has been suggested to play a role in the manifestation of febrile seizures. There is evidence suggesting that neurological disorders can be exacerbated in an offspring that was exposed to stress prenatally. The aim of our study was therefore to investigate whether febrile seizures are exacerbated in the offspring of rats that were prenatally stressed. The offspring of pregnant Sprague-Dawley dams were used in the study. Prenatal stress consisted of exposing the pregnant dams to 45 min of restraint, 3 times per day with 3 h intervals in-between, for 7 days starting on gestational day 14 (GND14). On postnatal day (PND) 14, the pups were injected with lipopolysaccharide (LPS, 200 μg/kg, i.p.) followed 2.5 h later by an i.p. injection of kainic acid (KA, 1.75 mg/kg). All the animals were decapitated on PND 21. Trunk blood was collected to detect plasma interleukin-1beta (IL-1β) levels in the various groups. Our data showed that i.p. injections of LPS followed by KA led to the development of seizure activity that was associated with increased plasma IL-1β levels. Prior exposure to prenatal stress resulted in the development of advanced stages of seizure development, leading to an exaggerated seizure response. Prenatal stress alone also led to elevated plasma IL-1β levels, while previously stressed animals receiving LPS and KA yielded the highest plasma levels of IL-1β levels. Our data therefore shows that IL-1β levels may play an important role in the development of febrile seizures.

Future therapeutics for osteoarthritis

Osteoarthritis (OA) is a disease of the joints that affects several million individuals worldwide. This disease, which involves mainly the diarthrodial joints, is chronic and develops slowly over decades, making it very difficult to precisely identify the different etiological and risk factors that influence its onset. At present, most therapies for OA are symptomatic. This review will focus on new OA therapeutics in development that are directed toward pain relief as well as others with the potential to reduce or stop the progression of the disease (DMOADs). This article is part of a Special Issue entitled "Osteoarthritis".

CaMKII antisense oligodeoxynucleotides protect against ischemia-induced neuronal death in the rat hippocampus

The present study was performed to investigate the effects of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) antisense oligodeoxynucleotides (ODNs) on the assembly of the CaMKII·GluR6·PSD-95 signaling module, GluR6 serine phosphorylation and c-Jun N-terminal kinase 3 (JNK3) activation. A further aim was to determine the neuroprotective mechanism of CaMKII antisense ODNs against ischemia-reperfusion (I/R)-induced neuronal death in the rat hippocampus. CaMKII antisense ODNs were intracerebroventricularly infused to inhibit CaMKII expression once daily for 3 days prior to the induction of ischemia. Transient cerebral ischemia (15 min) and reperfusion were induced by four-vessel occlusion in Sprague-Dawley rats as an animal model for transient cerebral I/R. The expression of related proteins was examined by immunoprecipitation and immunoblotting. Neuronal death in the rat hippocampus was detected by histology and histochemistry. The results indicate that CaMKII antisense ODNs inhibit several of the processes that are normally induced by cerebral I/R, including CaMKII expression, increased CaMKII·GluR6·PSD-95 signaling module assembly, GluR6 serine phosphorylation and JNK3 activation. Alternatively, CaMKII antisense ODNs also exhibit a significant neuroprotective role against cerebral I/R-induced cell death. These results provide the first evidence that CaMKII antisense ODNs can exert neuroprotective effects on cerebral I/R-induced cell death. The possible molecular mechanisms underlying this effect include 1) an inhibition of CaMKII expression and subsequent suppression of the assembly of the CaMKII·GluR6·PSD-95 signaling module, 2) GluR6 serine phosphorylation, and 3) reduced JNK3 activation.

First chemoenzymatic synthesis of (+)-2-carboxypyrrolidine-3-acetic acid, the nucleus of kainoid amino acids

The distinctive nucleus of kainoid amino acids, (2S,3R)-(+)-2-carboxypyrrolidine-3-acetic acid 6, was synthesized by a chemoenzymatic process, exploiting the diastereomeric cis/trans methyl pyroglutamate derivatives 10a-c/11a-c as key intermediates. These mixtures, when subjected to a kinetic resolution mediated by α-chymotrypsin, reacted diastereo-, regio-, and enantioselectively to give the trans derivatives (+)-10a-c possessing the correct (2S,3R) configuration. Subsequently, the desired product (2S,3R)-(+)-6 could be obtained after well-established transformations.

Rational design, synthesis and pharmacological evaluation of the (2R)- and (2S)-stereoisomers of 3-(2-carboxypyrrolidinyl)-2-methyl acetic acid as ligands for the ionotropic glutamate receptors

In this paper we describe the rational design, synthesis and pharmacological evaluation of two new stereoisomeric (S)-glutamate (Glu) analogues. The rational design was based on hybrid structures of the natural product kainic acid, a synthetic analogue CPAA and the high-affinity Glu analogue SYM2081. Pharmacological evaluation of the two stereoisomers revealed that one stereoisomer showed a subtype selectivity profile with low micromolar affinity for GluK1 and GluK3 and a 10- to 15-fold lower affinity for GluK2. The other stereoisomer displayed full selectivity for the KA over AMPA and NMDA receptors (GluK1-3: 0.39, 0.51 and 0.099 µM, respectively).

Medicinal chemistry of competitive kainate receptor antagonists

Kainic acid (KA) receptors belong to the group of ionotropic glutamate receptors and are expressed throughout in the central nervous system (CNS). The KA receptors have been shown to be involved in neurophysiological functions such as mossy fiber long-term potentiation (LTP) and synaptic plasticity and are thus potential therapeutic targets in CNS diseases such as schizophrenia, major depression, neuropathic pain and epilepsy. Extensive effort has been made to develop subtype-selective KA receptor antagonists in order to elucidate the physiological function of each of the five subunits known (GluK1-5). However, to date only selective antagonists for the GluK1 subunit have been discovered, which underlines the strong need for continued research in this area. The present review describes the structure-activity relationship and pharmacological profile for 10 chemically distinct classes of KA receptor antagonists comprising, in all, 45 compounds. To the medicinal chemist this information will serve as reference guidance as well as an inspiration for future effort in this field.

Discovery of a new class of ionotropic glutamate receptor antagonists by the rational design of (2S,3R)-3-(3-carboxyphenyl)-pyrrolidine-2-carboxylic acid

The kainic acid (KA) receptors belong to the class of glutamate (Glu) receptors in the brain and constitute a promising target for the treatment of neurological and/or psychiatric diseases such as schizophrenia, major depression, and epilepsy. Five KA subtypes have been identified and named GluK1-5. In this article, we present the discovery of (2S,3R)-3-(3-carboxyphenyl)-pyrrolidine-2-carboxylic acid (1) based on a rational design process. Target compound 1 was synthesized by a stereoselective strategy in 10 steps from commercially available starting materials. Binding affinities of 1 at native ionotropic Glu receptors were determined to be in the micromolar range (AMPA, 51 μM; KA, 22 μM; NMDA 6 μM), with the highest affinity for cloned homomeric KA receptor subtypes GluK1,3 (3.0 and 8.1 μM, respectively). Functional characterization of 1 by two electrode voltage clamp (TEVC) electrophysiology at a nondesensitizing mutant of GluK1 showed full competitive antagonistic behavior with a K(b) of 11.4 μM.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

4,4-Dimethyl- and diastereomeric 4-hydroxy-4-methyl- (2S)-glutamate analogues display distinct pharmacological profiles at ionotropic glutamate receptors and excitatory amino acid transporters

Subtype-selective ligands are of great interest to the scientific community, as they provide a tool for investigating the function of one receptor or transporter subtype when functioning in its native environment. Several 4-substituted (S)-glutamate (Glu) analogues were synthesized, and altogether this approach has provided important insight into the structure-activity relationships (SAR) for ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs), as well as the excitatory amino acid transporters (EAATs). In this work, three 4,4-disubstituted Glu analogues 1-3, which are hybrid structures of important 4-substituted Glu analogues 4-8, were investigated at iGluRs and EAATs. Collectively, their pharmacological profiles add new and valuable information to the SAR for the iGluRs and EAAT1-3.