Ionotropic Glutamate Receptor Recognition and Activation

Frost tolerance in apricot (Prunus armeniaca L.) receptacle and pistil organs: how is the relationship among amino acids, minerals, and cell death points?

To the better management of spring frost problem in the apricot cultivars, evaluation of biochemical changes in flower and/or flower organs during bud break could be one of the key factors. In this study, the relationship between the biochemical metabolites such as amino acids and minerals in the receptacle and pistil organs of two different apricot cultivars (frost-sensitive and frost-tolerant) and their relative effects on the frost tolerance of the cultivars and their organs were investigated during full blooming stage. In both apricot cultivars, it was found that the cell death points (CDP) of flower receptacle (- 6.3 to - 8.4 °C) were at higher temperatures than the CDP of flower pistil organs (- 13.1 to - 14.5 °C). Receptacle organs in flower, therefore, had less tolerance to spring frost damage. In addition, significant differences in mineral and amino acid contents were detected both between apricot cultivars and between the receptacle and pistil organs of the cultivars. Amino acid and mineral contents were lower both in the freezing-sensitive apricot cultivar ("Mihralibey") and the freezing-sensitive organ (receptacle) in comparison with the freezing-tolerant apricot cultivar ("Iğdır Şalak") and the freezing-tolerant organ (pistil). A significant negative correlation was also observed between the mean CDP values and both amino acid and mineral contents in the receptacle and pistil organs of both apricot cultivars. A negative correlation was found between CDP values and glutamate from amino acids and N, K, and Mg from minerals, and also these were determined that they had positive effects on frost tolerance increase. An important finding from our work revealed that the amount of each mineral and amino acid allocated differently to the receptacle and pistil organs of the apricot cultivars. The understanding of the amino acids and the mineral dynamics may contribute to improving the tolerance of flowers of apricot or other deciduous species to frost damage during spring. In the future, we may conclude that protection strategies such as increasing amino acids and mineral content in the receptacle organ of flowers would be necessary to cope with the negative effects of spring frost.

Configurational Preference of the Glutamate Receptor Ligand Binding Domain Dimers

Ionotropic glutamate receptors are a family of tetrameric ion channels with functional states consisting of nonconducting, conducting, and desensitized states that are starting to become well characterized by electrophysiological and biophysical studies. However, the structure and relative energetics of these states beyond the general structure of the receptor are still not well understood. It is known that the interface between monomeric subunits of the tetramer plays a major role in distinguishing these functional states. We have used umbrella sampling and multimicrosecond molecular dynamics simulations of the GluA2 AMPA subtype glutamate receptor ligand-binding domain (LBD) dimers to characterize a natural propensity of the LBD dimers for various configurational states. Our results show a proposed desensitized conformation of the LBD dimer is a highly preferable conformation of the LBD dimer without the influence of other receptor domains or crystallographic conditions. This has been demonstrated by both free protein simulations of 5 μs duration, as well as by computed free energy difference between the active and desensitized states. At the same time, the simulations performed using the same protocols revealed that for the LBD mutant L483Y, known to lack desensitization, the postulated active state of the LBD dimer is indeed the preferred configurational state, which remained stable in the simulations. Our findings pave the path for developing more detailed hypotheses of the full receptor activation mechanism. Combined with the energetics of glutamate binding to the LBD and the energy required to open the transmembrane pore helices, our results strongly support a hypothesis that the low absolute free-energy state is the desensitized state of the intact AMPA receptor.

EGAR, A Food Protein-Derived Tetrapeptide, Reduces Seizure Activity in Pentylenetetrazole-Induced Epilepsy Models Through α-Amino-3-Hydroxy-5-Methyl-4-Isoxazole Propionate Receptors

A primary pathogeny of epilepsy is excessive activation of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPARs). To find potential molecules to inhibit AMPARs, high-throughput screening was performed in a library of tetrapeptides in silico. Computational results suggest that some tetrapeptides bind stably to the AMPAR. We aligned these sequences of tetrapeptide candidates with those from in vitro digestion of the trout skin protein. Among salmon-derived products, Glu-Gly-Ala-Arg (EGAR) showed a high biological affinity toward AMPAR when tested in silico. Accordingly, natural EGAR was hypothesized to have anticonvulsant activity, and in vitro experiments showed that EGAR selectively inhibited AMPAR-mediated synaptic transmission without affecting the electrophysiological properties of hippocampal pyramidal neurons. In addition, EGAR reduced neuronal spiking in an in vitro seizure model. Moreover, the ability of EGAR to reduce seizures was evaluated in a rodent epilepsy model. Briefer and less severe seizures versus controls were shown after mice were treated with EGAR. In conclusion, the promising experimental results suggest that EGAR inhibitor against AMPARs may be a target for antiepilepsy pharmaceuticals. Epilepsy is a common brain disorder characterized by the occurrence of recurring, unprovoked seizures. Twenty to 30 % of persons with epilepsy do not achieve adequate seizure control with any drug. Here we provide a possibility in which a natural and edible tetrapeptide, EGAR, can act as an antiepileptic agent. We have combined computation with in vitro experiments to show how EGAR modulates epilepsy. We also used an animal model of epilepsy to prove that EGAR can inhibit seizures in vivo. This study suggests EGAR as a potential pharmaceutical for the treatment of epilepsy.

Calcium-permeable channels in plant cells

Calcium signal transduction is a central mechanism by which plants sense and respond to endogenous and environmental stimuli. Cytosolic Ca(2+) elevation is achieved via two cellular pathways, Ca(2+) influx through Ca(2+) channels in the plasma membrane and Ca(2+) release from intracellular Ca(2+) stores. Because of the significance of Ca(2+) channels in cellular signaling, interaction with the environment and developmental processes in plants, a great deal of effort has been invested in recent years with regard to these important membrane proteins. Because of limited space, in this review we focus on recent findings giving insight into both the molecular identity and physiological function of channels that have been suggested to be responsible for the elevation in cytosolic Ca(2+) level, including cyclic nucleotide gated channels, glutamate receptor homologs, two-pore channels and mechanosensitive Ca(2+) -permeable channels. We provide an overview of the regulation of these Ca(2+) channels and their physiological roles and discuss remaining questions.

Comparative genetics of the central nervous system in epigean and hypogean Astyanax mexicanus

The extreme environment of subterranean caves presents an adaptive challenge to troglobitic organisms. The mechanisms by which natural selection modify an ancestral surface neural circuit to produce a novel subterranean behavior remain a mystery. To address this question, we performed cross species microarray experiments to compare differences in gene expression levels in the adult brain of the teleost Astyanax mexicanus. This species provides a unique opportunity for comparative genetic studies as it consists of extant epigean (surface) and hypogean (cave) conspecifics. Microarray experiments herein revealed significant changes in transcription levels of seventeen genes, several of which are important for behaviors involved in metabolic management. We focused on genes central to three neurotransmission and neuromodulation networks: the endocannabinoid system (Cannabinoid receptor CB1), the dopaminergic system (Tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein) and the glutamatergic system (glutamate receptor AMPA 2a). All three genes were upregulated in the hypogean form of A. mexicanus compared to the epigean form, indicating that behavioral differences in the hypogean form of the species could be due to alterations in expression levels of several key genes. This information provides insights into the complex relationships among environmental factors, genetics, nervous systems and adaptive behavior, and can subsequently help us understand how these interactions affect behavior in other biological systems.

Peripheral glutamate signaling in head and neck areas

The major excitatory neurotransmitter glutamate is also found in the periphery in an increasing number of nonexcitable cells. In line with this it became apparent that glutamate can regulate a broad array of peripheral biological responses, as well. Of particular interest is the discovery that glutamate receptor reactive reagents can influence tumor biology. However, the knowledge of glutamate signaling in peripheral tissues is still incomplete and, in the case of head and neck areas, is almost lacking. The roles of glutamate signaling pathways in these regions are manifold and include orofacial pain, periodontal bone production, skin and airway inflammation, as well as salivation. Furthermore, the interrelations between glutamate and cancers in the oral cavity, thyroid gland, and other regions are discussed. In summary, this review shall strengthen the view that glutamate receptor reagents may also be promising targets for novel therapeutic concepts suitable for a number of diseases in peripheral tissues. The contents of this review cover the following sections: Introduction; The "Glutamate System"; The Taste of Glutamate; Glutamate Signaling in Dental Regions; Glutamate Signaling in Head and Neck Areas; Glutamate Signaling in Head and Neck Cancer; A Brief Overview of Glutamate Signaling in Other Cancers; and Conclusion.

Piracetam defines a new binding site for allosteric modulators of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors

Glutamate receptors are the most prevalent excitatory neurotransmitter receptors in the vertebrate central nervous system and are important potential drug targets for cognitive enhancement and the treatment of schizophrenia. Allosteric modulators of AMPA receptors promote dimerization by binding to a dimer interface and reducing desensitization and deactivation. The pyrrolidine allosteric modulators, piracetam and aniracetam, were among the first of this class of drugs to be discovered. We have determined the structure of the ligand binding domain of the AMPA receptor subtypes GluA2 and GluA3 with piracetam and a corresponding structure of GluA3 with aniracetam. Both drugs bind to GluA2 and GluA3 in a very similar manner, suggesting little subunit specificity. However, the binding sites for piracetam and aniracetam differ considerably. Aniracetam binds to a symmetrical site at the center of the dimer interface. Piracetam binds to multiple sites along the dimer interface with low occupation, one of which is a unique binding site for potential allosteric modulators. This new site may be of importance in the design of new allosteric regulators.

On the mechanisms of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor binding to glutamate and kainate

The alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtype of ionotropic glutamate receptors mediates much of the fast excitatory neurotransmission in the central nervous system. The ability of these receptors to shape such responses appears to be due in part to dynamic processes induced by agonists in the ligand-binding domain. Previous studies employing fluorescence spectroscopy and whole cell recording suggest that agonist binding is followed by sequential transitions to one or more distinct conformational states. Here, we used hydrogen-deuterium exchange to determine the mechanisms of binding of glutamate and kainate (full and partial agonists, respectively) to a soluble ligand-binding domain of GluR2. Our results provide a structural basis for sequential state models of agonist binding and the free energy changes of the associated state-to-state transitions. For glutamate, a multi-equilibrium binding reaction was discerned involving distinct ligand docking, domain isomerization, and lobe-locking steps. In contrast, kainate binding involves a simpler dock-isomerization process in which the isomerization equilibrium is shifted dramatically toward open domain conformations. In light of increasing evidence that the stability, in addition to the extent, of domain closure is a critical component of the channel activation mechanism, the differences in domain opening and closing equilibria detected for glutamate and kainate should be useful structural measures for interpreting the markedly different current responses evoked by these agonists.

Probing the allosteric modulator binding site of GluR2 with thiazide derivatives

Ionotropic glutamate receptors mediate the majority of vertebrate excitatory synaptic transmission and are therapeutic targets for cognitive enhancement and treatment of schizophrenia. The binding domains of these tetrameric receptors consist of two dimers, and the dissociation of the dimer interface of the ligand-binding domain leads to desensitization in the continued presence of agonist. Positive allosteric modulators act by strengthening the dimer interface and reducing the level of desensitization, thereby increasing steady-state activation. Removing the desensitized state for simplified analysis of receptor activation is commonly achieved using cyclothiazide (CTZ), the most potent modulator of the benzothiadiazide class, with the flip form of the AMPA receptor subtype. IDRA-21, the first benzothiadiazide to have an effect in behavioral tests, is an important lead compound in clinical trials for cognitive enhancement as it can cross the blood-brain barrier. Intermediate structures between CTZ and IDRA-21 show reduced potency, suggesting that these two compounds have different contact points associated with binding. To understand how benzothiadiazides bind to the pocket bridging the dimer interface, we generated a series of crystal structures of the GluR2 ligand-binding domain complexed with benzothiadiazide derivatives (IDRA-21, hydroflumethiazide, hydrochlorothiazide, chlorothiazide, trichlormethiazide, and althiazide) for comparison with an existing structure for cyclothiazide. The structures detail how changes in the substituents at the 3- and 7-positions of the hydrobenzothiadiazide ring shift the orientation of the drug in the binding site and, in some cases, change the stoichiometry of binding. All derivatives maintain a hydrogen bond with the Ser754 hydroxyl, affirming the partial selectivity of the benzothiadiazides for the flip form of AMPA receptors.

Vibrational spectroscopic investigation of the ligand binding domain of kainate receptors

Fourier transform infrared spectroscopy has been used to probe the agonist-protein interactions in the ligand binding domain of the GluR6 subunit, one subunit of the kainate subtype of glutamate receptors. In order to study the changes in the interactions over a range of activations the investigations were performed using the wild type, N690S, and T661E mutations. These studies show that the strength of the interactions at the alpha-amine group of the agonist, as probed by studying the environment of the nondisulphide bonded Cys 432, acts as a switch with weaker interactions at lower activations and stronger interactions at higher activations. The alpha-carboxylate interactions of the agonist, however, are not significantly different over the wide range of activations, as measured by the maximum currents mediated by the receptors at saturating concentrations of agonists. Previous investigations of AMPA receptors show a similar dependence of the alpha-amine interactions on activation indicating that the roles of the alpha-amine interactions in mediating receptor activation are similar for both subtypes of receptors; however, in the case of the AMPA receptors a tug of war type of change was observed between the alpha-amine and alpha-carboxylate interactions and this is not observed in kainate receptors. This decoupling of the two interactions could arise due to the larger cleft observed in kainate receptors, which allows for a more flexible interaction for the alpha-amine and alpha-carboxylate groups of the agonists.

Structure of the S1S2 glutamate binding domain of GLuR3

Glutamate receptors are the most prevalent excitatory neurotransmitter receptors in the vertebrate central nervous system. Determining the structural differences between the binding sites of different subtypes is crucial to our understanding of neuronal circuits and to the development of subtype specific drugs. The structures of the binding domain (S1S2) of the GluR3 (flip) AMPA receptor subunit bound to glutamate and AMPA and the GluR2 (flop) subunit bound to glutamate were determined by X-ray crystallography to 1.9, 2.1, and 1.55 A, respectively. Overall, the structure of GluR3 (flip) S1S2 is very similar to GluR2 (flop) S1S2 (backbone RMSD of 0.30 +/- 0.05 for glutamate-bound and 0.26 +/- 0.01 for AMPA-bound). The differences in the flip and flop isoforms are subtle and largely arise from one hydrogen bond across the dimer interface and associated water molecules. Comparison of the binding affinity for various agonists and partial agonists suggest that the S1S2 domains of GluR2 and GluR3 show only small differences in affinity, unlike what is found for the intact receptors (with the exception of one ligand, Cl-HIBO, which has a 10-fold difference in affinity for GluR2 vs. GluR3).

Correlating AMPA receptor activation and cleft closure across subunits: crystal structures of the GluR4 ligand-binding domain in complex with full and partial agonists

AMPA receptors are glutamate-gated ion channels that are essential mediators of synaptic signals in the central nervous system. They form tetramers that are assembled as combinations of subunits GluR1-4, each of which contains a ligand-binding domain (LBD). Crystal structures of the GluR2 LBD have revealed an agonist-binding cleft, which is located between two lobes and which acts like a Venus flytrap. In general, agonist efficacy is correlated with the extent of cleft closure. However, recent observations show that cleft closure is not the sole determinant of the relative efficacy for glutamate receptors. In addition, these studies have focused on the GluR2 subunit, which is the specific target of a physiologically important RNA-editing modification in vivo. We therefore sought to test the generality of the cleft closure-efficacy correlation for other AMPA-R subunits. Here, we present crystal structures of the GluR4(flip) LBD in complex with both full and partial agonists. As for GluR2, both agonists stabilize a closed-cleft conformation, and the partial agonist induces a smaller cleft closure than the full agonist. However, a detailed analysis of LBD-kainate interactions reveals the importance of subtle backbone conformational changes in the ligand-binding pocket in determining the magnitude of agonist-associated conformational changes. Furthermore, the GluR4 subunit exhibits a different correlation between receptor activation and LBD cleft closure than does GluR2.

Luminescence resonance energy transfer investigation of conformational changes in the ligand binding domain of a kainate receptor

The apo state structure of the isolated ligand binding domain of the GluR6 subunit and the conformational changes induced by agonist binding to this protein have been investigated by luminescence resonance energy transfer (LRET) measurements. The LRET-based distances show that agonist binding induces cleft closure, and the extent of cleft closure is proportional to the extent of activation over a wide range of activations, thus establishing that the cleft closure conformational change is one of the mechanisms by which the agonist mediates receptor activation. The LRET distances also provide insight into the apo state structure, for which there is currently no crystal structure available. The distance change between the glutamate-bound state and the apo state is similar to that observed between the glutamate-bound and antagonist UBP-310-bound form of the GluR5 ligand binding domain, indicating that the cleft for the apo state of the GluR6 ligand binding domain should be similar to the UBP-310-bound form of GluR5. This observation implies that te apo state of GluR6 undergoes a cleft closure of 29-30 degrees upon binding full agonists, one of the largest observed in the glutamate receptor family.

Mechanism of partial agonism at the GluR2 AMPA receptor: Measurements of lobe orientation in solution

The mechanism by which the binding of a neurotransmitter to a receptor leads to channel opening is a central issue in molecular neurobiology. The structure of the agonist binding domain of ionotropic glutamate receptors has led to an improved understanding of the changes in structure that accompany agonist binding and have provided important clues about the link between these structural changes and channel activation and desensitization. However, because the binding domain has exhibited different structures under different crystallization conditions, understanding the structure in the absence of crystal packing is of considerable importance. The orientation of the two lobes of the binding domain in the presence of a full agonist, an antagonist, and several partial agonists was measured using NMR spectroscopy by employing residual dipolar couplings. For some partial agonists, the solution conformation differs from that observed in the crystal. A model of channel activation based on the results is discussed.

LRET investigations of conformational changes in the ligand binding domain of a functional AMPA receptor

The structural investigations using the soluble ligand binding domain of the AMPA subtype of the glutamate receptor have provided invaluable insight into the mechanistic pathway by which agonist binding to this extracellular domain mediates the formation of cation-selective channels in this protein. These structures, however, are in the absence of the transmembrane segments, the primary functional component of the protein. Here, we have used a modified luminescence resonance energy transfer based method to obtain distance changes due to agonist binding in the ligand binding domain in the presence of the transmembrane segments. These distance changes show that the cleft closure conformational change observed in the isolated ligand binding domain upon binding agonist is conserved in the receptor with the channel segments, thus establishing that the isolated ligand binding domain is a good model of the domain in the receptor containing the transmembrane segments.

Glutamate receptors as seen by light: spectroscopic studies of structure-function relationships

Ionotropic glutamate receptors are major excitatory receptors in the central nervous system and also have a far reaching influence in other areas of the body. Their modular nature has allowed for the isolation of the ligand-binding domain and for subsequent structural studies using a variety of spectroscopic techniques. This review will discuss the role of specific ligand:protein interactions in mediating activation in the a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype of glutamate receptors as established by various spectroscopic investigations of the GluR2 and GluR4 subunits of this receptor. Specifically, this review will provide an introduction to the insight gained from X-ray crystallography and nuclear magnetic resonance investigations and then go on to focus on studies utilizing vibrational spectroscopy and fluorescence resonance energy transfer to study the behavior of the isolated ligand-binding domain in solution and discuss the importance of specific ligand:protein interactions in the mechanism of receptor activation.

Purification and crystallization of a non-GluR2 AMPA-receptor ligand-binding domain: a case of cryo-incompatibility addressed by room-temperature data collection

Glutamate is the major excitatory neurotransmitter in the brain. Among the cognate ionotropic glutamate receptors, the subfamily selective for AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) is responsible for most fast excitatory synaptic signaling and plays key roles in synaptic plasticity. AMPA receptors (AMPA-Rs) have also been implicated in a number of neurological disorders. To investigate subunit-specific differences in the ligand binding and activation of AMPA-Rs, the GluR4 AMPA-R ligand-binding domain (LBD) was crystallized in complex with full and partial agonists. This is the first non-GluR2 AMPA-R LBD available for structural analysis. Standard cryoprotection protocols yielded high-resolution diffraction from flash-cooled crystals of the complex with the full agonist glutamate. However, for cocrystals with the partial agonist kainate, systematic screening and optimization of cryoprotection conditions yielded at best mosaic, weak diffraction at 100 K. In contrast, room-temperature data collection from capillary-mounted kainate cocrystals exhibited reproducible diffraction to better than 3 A resolution. Together, these crystals lay the foundation for a structural comparison of LBD-agonist interactions in distinct AMPA-R subunits.

Novel aspects of glutamatergic signalling in the neuroendocrine system

L-glutamate, the main excitatory neurotransmitter, influences virtually all neurones of the neuroendocrine hypothalamus via synaptic mechanisms. Vesicular glutamate transporters (VGLUT1-3), which selectively accumulate L-glutamate into synaptic vesicles, provide markers with which to visualise glutamatergic neurones in histological preparations; excitatory neurones in the endocrine hypothalamus synthesise the VGLUT2 isoform. Results of recent dual-label in situ hybridisation studies indicate that glutamatergic neurones in the preoptic area and the hypothalamic paraventricular, supraoptic and periventricular nuclei include parvocellular and magnocellular neurosecretory neurones which secrete peptide neurohormones into the bloodstream to regulate endocrine functions. Neurosecretory terminals of GnRH, TRH, CRF-, somatostatin-, oxytocin- and vasopressin-secreting neurones contain VGLUT2 immunoreactivity, suggesting the co-release of glutamate with hypophysiotrophic peptides. The presence of VGLUT2 also indicates glutamate secretion from non-neuronal endocrine cells, including gonadotrophs and thyrotrophs of the anterior pituitary. Results of in vitro studies show that ionotropic glutamate receptor analogues can elicit hormone secretion at neuroendocrine/endocrine release sites. Structural constituents of the median eminence, adenohypophysis and neurohypophysis contain elements of glutamatergic transmission, including glutamate receptors and enzymes of the glutamate/glutamine cycle. The synthesis of VGLUT2 exhibits robust up-regulation in response to certain endocrine challenges, indicating that altered glutamatergic signalling may represent an important adaptive mechanism. This review article discusses the newly emerged non-synaptic role of glutamate in neuroendocrine and endocrine communication.

Chemical interplay in the mechanism of partial agonist activation in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, one subtype in the family of ionotropic glutamate receptors, are the main receptors responsible for excitatory signaling in the mammalian central nervous system. Previous studies utilitizing the isolated ligand binding domain of these receptors have provided insight into the role of specific ligand-protein interactions in mediating receptor activation. However, these studies relied heavily on the partial agonist kainate, in which the alpha-amine group is constrained in a pyrrolidine ring. Here we have studied a series of substituted and unsubstituted willardiines with primary alpha-amine groups similar to that of the full agonist glutamate whose activation can be varied depending on the size of the substituent. The specific ligand-protein interactions in the mechanism of partial agonism in this subtype were investigated using vibrational spectroscopy, and the large-scale conformational changes in the ligand binding domain were studied with fluorescence resonance energy transfer (FRET). These investigations show that the strength of the interaction at the alpha-amine group correlates with the extent of cleft closure and extent of activation, with the agonist of higher efficacy showing larger cleft closure and stronger interactions at this group, suggesting that this is one of the mechanisms by which the agonist controls receptor activation.

Dynamics of the S1S2 Glutamate Binding Domain of GluR2 Measured Using 19F NMR Spectroscopy

Ionotropic glutamate receptors mediate the majority of vertebrate excitatory synaptic transmission. Although the structure of the GluR2 binding domain (S1S2) is well known (agonist binding site between two lobes), little is known about the time scales of conformational transitions or the relationship between dynamics and function. (19)F NMR ((19)F-labeled tryptophan) spectroscopy was used to monitor motions in the S1S2 domain bound to ligands with varying efficacy and in the apo state. One tryptophan (Trp-671) undergoes chemical exchange in some but not all agonists, consistent with mus-ms motion. The dynamics can be correlated to ligand affinity, and a likely source of the motion is a peptide bond capable of transiently forming hydrogen bonds across the lobe interface. Another tryptophan (Trp-767) appears to monitor motions of the relative positions of the lobes and suggests that the relative orientation in the apo- and antagonist-bound forms can exchange between at least two conformations on the ms time scale.

Role of the chemical interactions of the agonist in controlling alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor activation

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are the main excitatory neurotransmitter receptors in the mammalian central nervous system. Structures of the isolated ligand binding domain of this receptor have provided significant insight into the large-scale conformational changes, which when propagated to the channel segments leads to receptor activation. However, to establish the role of specific molecular interactions in controlling fine details such as the magnitude of the functional response, we have used a multiscale approach, where changes at specific moieties of the agonists have been studied by vibrational spectroscopy, while large-scale conformational changes have been studied using fluorescence resonance energy transfer (FRET) investigations. By exploiting the wide range of activations by the agonists, glutamate, kainate, and AMPA, for the wild type and Y450F and L650T mutants of the GluR2 subtype, and by using the multiscale investigation, we show that the strength of the interactions at the alpha-amine group of the agonist with the protein in all but one case tracks the extent of activation. Since the alpha-amine group forms bridging interactions at the cusp of the ligand binding cleft, this appears to be a critical interaction through which the agonist controls the extent of activation of the receptor.

Measurement of conformational changes accompanying desensitization in an ionotropic glutamate receptor

The canonical conformational states occupied by most ligand-gated ion channels, and many cell-surface receptors, are the resting, activated, and desensitized states. While the resting and activated states of multiple receptors are well characterized, elaboration of the structural properties of the desensitized state, a state that is by definition inactive, has proven difficult. Here we use electrical, chemical, and crystallographic experiments on the AMPA-sensitive GluR2 receptor, defining the conformational rearrangements of the agonist binding cores that occur upon desensitization of this ligand-gated ion channel. These studies demonstrate that desensitization involves the rupture of an extensive interface between domain 1 of 2-fold related glutamate-binding core subunits, compensating for the ca. 21 degrees of domain closure induced by glutamate binding. The rupture of the domain 1 interface allows the ion channel to close and thereby provides a simple explanation to the long-standing question of how agonist binding is decoupled from ion channel gating upon receptor desensitization.

Allosteric mechanism in AMPA receptors: a FRET-based investigation of conformational changes

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are the primary mediators of fast excitatory synaptic transmission in the mammalian CNS. Structures of the extracellular ligand-binding domain suggest that the extent of cleft closure in the ligand-binding domain controls the extent of activation of the receptor. Here we have developed a fluorescence resonance energy transfer-based probe that allows us to study the extent of cleft closure in the isolated ligand-binding domain in solution. These investigations show that the wild-type protein exhibits a graded cleft closure that correlates to the extent of activation, which is in qualitative agreement with the crystal structures. However, the changes in extent of cleft closure between the apo and agonist-bound states are smaller than that observed in the crystal structures. We have also used this method to study the L650T mutant and show that in solution the alpha-amino-5-methyl-3-hydroxy-4-isoxazole propionate-bound form of this mutant exists primarily in a conformation that is more closed than predicted based on the activity, indicating that the degree of cleft closure alone cannot be used as a measure of extent of activation of the receptor, and there are possibly other mechanisms in addition to cleft closure that mediate the subtleties in extent of activation by a given agonist.

N-methyl-D-aspartate (NMDA) and the regulation of mitogen-activated protein kinase (MAPK) signaling pathways: a revolving neurochemical axis for therapeutic intervention?

Excitatory synaptic transmission in the central nervous system (CNS) is mediated by the release of glutamate from presynaptic terminals onto postsynaptic channels gated by N-methyl-D-aspartate (NMDA) and non-NMDA (AMPA and KA) receptors. Extracellular signals control diverse neuronal functions and are responsible for mediating activity-dependent changes in synaptic strength and neuronal survival. Influx of extracellular calcium ([Ca(2+)](e)) through the NMDA receptor (NMDAR) is required for neuronal activity to change the strength of many synapses. At the molecular level, the NMDAR interacts with signaling modules, which, like the mitogen-activated protein kinase (MAPK) superfamily, transduce excitatory signals across neurons. Recent burgeoning evidence points to the fact that MAPKs play a crucial role in regulating the neurochemistry of NMDARs, their physiologic and biochemical/biophysical properties, and their potential role in pathophysiology. It is the purpose of this review to discuss: (i) the MAPKs and their role in a plethora of cellular functions; (ii) the role of MAPKs in regulating the biochemistry and physiology of NMDA receptors; (iii) the kinetics of MAPK-NMDA interactions and their biologic and neurochemical properties; (iv) how cellular signaling pathways, related cofactors and intracellular conditions affect NMDA-MAPK interactions and (v) the role of NMDA-MAPK pathways in pathophysiology and the evolution of disease conditions. Given the versatility of the NMDA-MAPK interactions, the NMDA-MAPK axis will likely form a neurochemical target for therapeutic interventions.

Substrate specificity of human kallikrein 6: salt and glycosaminoglycan activation effects

Human kallikrein 6 (hK6) is abundantly expressed in the central nervous system and is implicated in demyelinating disease. This study provided biochemical data about the substrate specificity and activation of hK6 by glycosaminoglycans and by kosmotropic salts, which followed the Hofmeister series. The screening of fluorescence resonance energy transfer (FRET) peptide families derived from Abz-KLRSSKQ-EDDnp resulted in the finding that Abz-AFRFSQ-EDDnp (where Abz is ortho-aminobenzoic acid and EDDnp is N-[2,4-dinitrophenyl]ethylenediamine)) is the best synthetic substrate described so far for hK6 (kcat/Km 38,667 s(-1) mm(-1)). It is noteworthy that the AFRFS sequence was found as a motif in the amino-terminal domain of seven human ionotropic glutamate receptor subunits. We also examined the hK6 hydrolytic activity on FRET peptides derived from human myelin basic protein, precursor of the Abeta amyloid peptide, reactive center loop of alpha1-antichymotrypsin, plasminogen, and maturation and inactivation cleavage sites of hK6, which were described earlier as natural substrates for hK6. The best substrates were derived from myelin basic protein. The hK6 maturation cleavage site was poorly hydrolyzed, and no evidence was found to support a two-step self-activation process reported previously. Finally, we assayed FRET peptides derived from sequences that span the cleavage sites for activation of protease-activated receptors (PAR) 1-4, and only the substrate with the PAR 2 sequence was hydrolyzed. These results further supported the hypothesis that hK6 expressed in the central nervous system is involved in normal myelin turnover/demyelination processes, but it is unlikely to self-activate. This report also suggested the possible modulation of ionotropic glutamate receptors and activation of PAR 2 by hK6.

Structural determinants of agonist-specific kinetics at the ionotropic glutamate receptor 2

Glutamate receptors (GluRs) are the most abundant mediators of the fast excitatory neurotransmission in the human brain. Agonists will, after activation of the receptors, induce different degrees of desensitization. The efficacy of agonists strongly correlates with the agonist-induced closure of the ligand-binding domain. However, the differences in desensitization properties are less well understood. By using high-resolution x-ray structure of the GluR2 flop (GluR2o) ligand-binding core protein in complex with the partial glutamate receptor agonist (S)-2-amino-3-(3-hydroxy-5-tert-butyl-4-isothiazolyl)propionic acid [(S)-thio-ATPA], we show that (S)-thio-ATPA induces an 18 degrees closure of the binding core similar to another partial agonist, (S)-2-amino-3-(4-bromo-3-hydroxy-5-isoxazolyl)propionic acid [(S)-Br-HIBO]. Despite the similar closure of the ligand-binding domain, we find in electrophysiological studies that (S)-thio-ATPA induced a 6.4-fold larger steady-state current than (RS)-Br-HIBO, and rapid agonist applications show that (S)-thio-ATPA induces a 3.6-fold higher steady-state/peak ratio and a 2.2-fold slower desensitization time constant than (RS)-Br-HIBO. Structural comparisons reveal that (S)-Br-HIBO, but not (S)-thio-ATPA, induces a twist of the ligand-binding core compared with the apostructure, and the agonist-specific conformation of Leu-650 correlates with the different kinetic profiles pointing at a key role in defining the desensitization kinetics. We conclude that, especially for intermediate efficacious agonists, the desensitization properties are influenced by additional ligand-induced factors beyond domain closure.