Characterization of the target of ivermectin, the glutamate-gated chloride channel, from Anopheles gambiae

The use of insecticide-treated nets and indoor residual insecticides targeting adult mosquito vectors is a key element in malaria control programs. However, mosquito resistance to the insecticides used in these applications threatens malaria control efforts. Recently, the mass drug administration of ivermectin (IVM) has been shown to kill Anopheles gambiae mosquitoes and disrupt Plasmodium falciparum transmission in the field. We cloned the molecular target of IVM from A. gambiae, the glutamate-gated chloride channel (AgGluCl), and characterized its transcriptional patterns, protein expression and functional responses to glutamate and IVM. AgGluCl cloning revealed an unpredicted fourth splice isoform as well as a novel exon and splice site. The predicted gene products contained heterogeneity in the N-terminal extracellular domain and the intracellular loop region. Responses to glutamate and IVM were measured using two-electrode voltage clamp on Xenopus laevis oocytes expressing AgGluCl. IVM induced non-persistent currents in AgGluCl-a1 and did not potentiate glutamate responses. In contrast, AgGluCl-b was insensitive to IVM, suggesting that the AgGluCl gene could produce IVM-sensitive and -insensitive homomultimers from alternative splicing. AgGluCl isoform-specific transcripts were measured across tissues, ages, blood feeding status and sex, and were found to be differentially transcribed across these physiological variables. Lastly, we stained adult, female A. gambiae for GluCl expression. The channel was expressed in the antenna, Johnston's organ, supraesophageal ganglion and thoracic ganglia. In summary, we have characterized the first GluCl from a mosquito, A. gambiae, and described its unique activity and expression with respect to it as the target of the insecticide IVM.

Rational Design of a Novel AMPA Receptor Modulator through a Hybridization Approach

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are a family of glutamate ion channels of considerable interest in excitatory neurotransmission and associated disease processes. Here, we demonstrate how exploitation of the available X-ray crystal structure of the receptor ligand binding domain enabled the development of a new class of AMPA receptor positive allosteric modulators (7) through hybridization of known ligands (5 and 6), leading to a novel chemotype with promising pharmacological properties.

AMPA receptor potentiators: from drug design to cognitive enhancement

Positive allosteric modulators of ionotropic glutamate receptors have emerged as a target for treating cognitive impairment and neurodegeneration, but also mental illnesses such as major depressive disorder. The possibility of creating a new class of pharmaceutical agent to treat refractive mental health issues has compelled researchers to redouble their efforts to develop a safe, effective treatment for memory and cognition impairments. Coupled with the more robust research methodologies that have emerged, including more sophisticated high-throughput-screens, higher resolution structural biology techniques, and more focused assessment on pharmacokinetics, the development of positive modulators of AMPA receptors holds great promise. We describe recent approaches that improve our understanding of the basic physiology underlying memory and cognition, and their application toward promoting human health.

The challenges for scientists in avoiding plagiarism

Although it might seem to be a simple task for scientists to avoid plagiarism and thereby an allegation of research misconduct, assessment of trainees in the Responsible Conduct of Research and recent findings from the National Science Foundation Office of Inspector General regarding plagiarism suggests otherwise. Our experiences at a land-grant academic institution in assisting researchers in avoiding plagiarism are described. We provide evidence from a university-wide multi-disciplinary course that understanding how to avoid plagiarism in scientific writing is more difficult than it might appear, and that a failure to learn the rules of appropriate citation may cause dire consequences. We suggest that new strategies to provide training in avoiding plagiarism are required.

A charge-inverting mutation in the "linker" region of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors alters agonist binding and gating kinetics independently of allosteric modulators

AMPA receptors are gated through binding of glutamate to a solvent-accessible ligand-binding domain. Upon glutamate binding, these receptors undergo a series of conformational rearrangements regulating channel function. Allosteric modulators can bind within a pocket adjacent to the ligand-binding domain to stabilize specific conformations and prevent desensitization. Yelshansky et al. (Yelshansky, M. V., Sobolevsky, A. I., Jatzke, C., and Wollmuth, L. P. (2004) J. Neurosci. 24, 4728-4736) described a model of an electrostatic interaction between the ligand-binding domain and linker region to the pore that regulated channel desensitization. To test this hypothesis, we have conducted a series of experiments focusing on the R628E mutation. Using ultrafast perfusion with voltage clamp, we applied glutamate to outside-out patches pulled from transiently transfected HEK 293 cells expressing wild type or R628E mutant GluA2. In response to a brief pulse of glutamate (1 ms), mutant receptors deactivated with significantly slower kinetics than wild type receptors. In addition, R628E receptors showed significantly more steady-state current in response to a prolonged (500-ms) glutamate application. These changes in receptor kinetics occur through a pathway that is independent of that of allosteric modulators, which show an additive effect on R628E receptors. In addition, ligand binding assays revealed the R628E mutation to have increased affinity for agonist. Finally, we reconciled experimental data with computer simulations that explicitly model mutant and modulator interactions. Our data suggest that R628E stabilizes the receptor closed cleft conformation by reducing agonist dissociation and the transition to the desensitized state. These results suggest that the AMPA receptor external vestibule is a viable target for new positive allosteric modulators.

Structural and functional analysis of two new positive allosteric modulators of GluA2 desensitization and deactivation

At the dimer interface of the extracellular ligand-binding domain of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors a hydrophilic pocket is formed that is known to interact with two classes of positive allosteric modulators, represented by cyclothiazide and the ampakine 2H,3H,6aH-pyrrolidino(2,1-3',2')1,3-oxazino(6',5'-5,4)benzo(e)1,4-dioxan-10-one (CX614). Here, we present structural and functional data on two new positive allosteric modulators of AMPA receptors, phenyl-1,4-bis-alkylsulfonamide (CMPDA) and phenyl-1,4-bis-carboxythiophene (CMPDB). Crystallographic data show that these compounds bind within the modulator-binding pocket and that substituents of each compound overlap with distinct moieties of cyclothiazide and CX614. The goals of the present study were to determine 1) the degree of modulation by CMPDA and CMPDB of AMPA receptor deactivation and desensitization; 2) whether these compounds are splice isoform-selective; and 3) whether predictions of mechanism of action could be inferred by comparing molecular interactions between the ligand-binding domain and each compound with those of cyclothiazide and CX614. CMPDB was found to be more isoform-selective than would be predicted from initial binding assays. It is noteworthy that these new compounds are both more potent and more effective and may be more clinically relevant than the AMPA receptor modulators described previously.

Circadian difference in firing rate of isolated rat suprachiasmatic nucleus neurons

The suprachiasmatic nucleus (SCN) of the hypothalamus contains the primary circadian clock in mammals. Dissociated SCN neurons in long-term culture exhibit a circadian modulation of spontaneous electrical activity. To evaluate the presence of circadian differences in spontaneous activity of isolated SCN neurons without synaptic connections, dissociated rat SCN neurons were studied with on-cell recording 3-4 days after preparation, before the formation of dendrites, axons and synapses. A day-night difference in spontaneous electrical firing rate was found in acutely dissociated SCN neurons. During the first subjective day, the average firing rate (0.87+/-0.12 Hz) was significantly higher than during the first subjective night (0.24+/-0.06 Hz), while the firing rate on the next day (0.68+/-0.11 Hz) was significantly higher than during the preceding night. These data suggest that populations of isolated SCN neurons with no synaptic interactions contain a functioning circadian clock, and are particularly amenable to biophysical experiments.

The stargazin C terminus encodes an intrinsic and transferable membrane sorting signal

Activity-dependent plasticity of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors is regulated by their auxiliary subunit, stargazin. Association with stargazin enhances alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor surface expression and modifies the receptor's biophysical properties. Fusing the cytoplasmic C terminus of stargazin to the C-terminal domains of either GluR1 or the gonadotropin-releasing hormone receptor permits efficient trafficking from the endoplasmic reticulum and sorting to the basolateral membrane without altering other properties of either receptor.

Different Domains of the AMPA Receptor Direct Stargazin-mediated Trafficking and Stargazin-mediated Modulation of Kinetics

Stargazin is an accessory protein of AMPA receptors that enhances surface expression and also affects the biophysical properties of the receptor. AMPA receptor domains necessary for either of these two processes have not yet been identified. Here, we used confocal imaging and electrophysiology of heterologously expressed, fluorophore-tagged GluR1, GluR2, and stargazin to study surface expression and desensitization kinetics. Stargazin-mediated trafficking was sensitive to the nature of the AMPA receptor cytoplasmic domain. The insertion of YFP after residue 15 of the truncated cytoplasmic tail of GluR1i perturbed stargazin-mediated trafficking of the receptor but not its modulation of desensitization kinetics. This construct also failed to permit fluorescence resonance energy transfer (FRET) with stargazin in the endoplasmic reticulum (ER), whereas FRET between fluorophore-tagged stargazin and non-truncated AMPA receptors demonstrated a specific interaction between these proteins, both in the ER and the plasma membrane. Rather than encoding a specific binding site, the fluorophore-tagged C terminus may restrict access to one or more ER retention sites. Although perturbations of the C terminus impeded stargazin-mediated trafficking to the plasma membrane, the effects of stargazin on the biophysical properties of AMPA receptors (i.e. modulation of desensitization) remained intact. These data provide strong evidence that the AMPA receptor domains required for stargazin modulation of gating and trafficking are separable.

Kynurenic acid has a dual action on AMPA receptor responses

Glutamate is the predominant excitatory neurotransmitter in the central nervous system. The receptors that bind glutamate, including N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subtypes, are strongly implicated in higher cognitive processes, especially learning and memory. Loss of glutamate receptor function impairs the ability to acquire and retain information in some patients subsequent to stroke or brain injury, and positive allosteric modulators of glutamate receptors can restore learning and memory in some of these patients. Here we demonstrate that kynurenic acid (KYNA), an endogenous tryptophan metabolite, acts upon heterologous AMPA receptors via two distinct mechanisms. Low (nanomolar to micromolar) concentrations of KYNA facilitate AMPA receptor responses, whereas high (millimolar) concentrations of KYNA competitively antagonize glutamate receptors. Low concentrations of KYNA appear to increase current responses through allosteric modulation of desensitization of AMPA receptors. These findings suggest the possibility that low concentrations of endogenous KYNA acting at AMPA receptors may be a positive modulator of excitatory synaptic transmission.

Mechanism of positive allosteric modulators acting on AMPA receptors

Ligand-gated ion channels involved in the modulation of synaptic strength are the AMPA, kainate, and NMDA glutamate receptors. Small molecules that potentiate AMPA receptor currents relieve cognitive deficits caused by neurodegenerative diseases such as Alzheimer's disease and show promise in the treatment of depression. Previously, there has been limited understanding of the molecular mechanism of action for AMPA receptor potentiators. Here we present cocrystal structures of the glutamate receptor GluR2 S1S2 ligand-binding domain in complex with aniracetam [1-(4-methoxybenzoyl)-2-pyrrolidinone] or CX614 (pyrrolidino-1,3-oxazino benzo-1,4-dioxan-10-one), two AMPA receptor potentiators that preferentially slow AMPA receptor deactivation. Both potentiators bind within the dimer interface of the nondesensitized receptor at a common site located on the twofold axis of molecular symmetry. Importantly, the potentiator binding site is adjacent to the "hinge" in the ligand-binding core "clamshell" that undergoes conformational rearrangement after glutamate binding. Using rapid solution exchange, patch-clamp electrophysiology experiments, we show that point mutations of residues that interact with potentiators in the cocrystal disrupt potentiator function. We suggest that the potentiators slow deactivation by stabilizing the clamshell in its closed-cleft, glutamate-bound conformation.

Spike-dependent depolarizing afterpotentials contribute to endogenous bursting in gonadotropin releasing hormone neurons

Pulsatile secretion of gonadotropin releasing hormone in mammals is thought to depend on repetitive and prolonged bursts of action potentials in specific neuroendocrine cells. We have previously described episodes of electrical activity in isolated gonadotropin releasing hormone neurons, but the intrinsic mechanisms underlying the generation of spike bursts are unknown. In acutely isolated gonadotropin releasing hormone neurons, which had been genetically targeted to express enhanced green fluorescent protein, current pulses generated spike-mediated depolarizing afterpotentials in 69% of cells. Spike-dependent depolarizing afterpotentials could evoke bursts of action potentials that lasted for tens of seconds. Brief pulses of glutamate (as short as 1 ms), which simulated excitatory postsynaptic potentials, also triggered spike-mediated depolarizing afterpotentials and episodic activity. These data indicate that spike-dependent depolarizing afterpotentials, an endogenous mechanism in gonadotropin releasing hormone neurons, likely contribute to the episodic firing thought to underlie pulsatile secretion of gonadotropin releasing hormone. Furthermore, fast excitatory postsynaptic potentials mediated by glutamate can activate this intrinsic mechanism.

Acute dissociation for analyses of NMDA receptor function in cortical neurons during aging

The present study was designed to determine if functional age differences in the NMDA epsilon2 (NR2B) subunit were detectable at the level of individual cortical neurons. Neurons were acutely dissociated from the frontal and prefrontal cortices of young adult, middle-aged, or old mice, using a combination of proteinase K and trypsin followed by manual trituration. After overnight culture, patch-clamp electrophysiology and rapid perfusion were used to obtain whole-cell responses to 300 microM NMDA, with or without the potent NR2B antagonist ifenprodil. Healthy, phase-bright cortical neurons were isolated from animals of all ages. Cell diameter and capacitance was consistent between ages. We were able to perform kinetic analyses of the NMDA-evoked response, and demonstrated a significant increase in the rate of deactivation with increasing age. In addition, we observed a significant effect of high-concentration ifenprodil on the NMDA-evoked response in old animals. Thus, this method is ideal for the dissociation of neurons from the brain of both young and old animals, and offers a powerful tool for functional analysis at the level of the individual cell.

Identification of a site in GluR1 and GluR2 that is important for modulation of deactivation and desensitization

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype of ionotropic glutamate receptors consists of rapidly gating ion channels. Positive modulation of channel gating may slow gating kinetics through at least two distinct mechanisms, evidenced by the predominant slowing of either the rate of receptor desensitization or the rate of offset after agonist withdrawal (deactivation). This study compares the actions of two positive allosteric modulators [cyclothiazide, which modulates desensitization, and 1-(1,4-benzodioxan-6-ylcarbonyl)piperidine (CX546), which modulates deactivation] in a mutant shown previously to impede modulation by cyclothiazide. These experiments test the hypothesis that the point mutation, GluR1(S493T), would also cause a loss of modulation by CX546. Wild-type GluR1 through -4 receptors were modulated by CX546, as assayed by the potentiation of steady-state currents in the Xenopus laevis oocyte expression system. CX546 potentiated steady-state currents of both splice isoforms of GluR1. Modulation by CX546 was completely abolished in GluR1(S493T) and its homolog, GluR2(S497T), although this mutation did not affect apparent agonist affinity in the absence of CX546. Thus, the GluR1(S493T) mutation has a similar impairment of modulation by either cyclothiazide or CX546, indicating that some residues at the subunit interface of glutamate receptors play an important role in channel deactivation and desensitization.

Episodic bursting activity and response to excitatory amino acids in acutely dissociated gonadotropin-releasing hormone neurons genetically targeted with green fluorescent protein

The gonadotropin-releasing hormone (GnRH) system, considered to be the final common pathway for the control of reproduction, has been difficult to study because of a lack of distinguishing characteristics and the scattered distribution of neurons. The development of a transgenic mouse in which the GnRH promoter drives expression of enhanced green fluorescent protein (EGFP) has provided the opportunity to perform electrophysiological studies of GnRH neurons. In this study, neurons were dissociated from brain slices prepared from prepubertal female GnRH-EGFP mice. Both current- and voltage-clamp recordings were obtained from acutely dissociated GnRH neurons identified on the basis of EGFP expression. Most isolated GnRH-EGFP neurons fired spontaneous action potentials (recorded in cell-attached or whole-cell mode) that typically consisted of brief bursts (2-20 Hz) separated by 1-10 sec. At more negative resting potentials, GnRH-EGFP neurons exhibited oscillations in membrane potential, which could lead to bursting episodes lasting from seconds to minutes. These bursting episodes were often separated by minutes of inactivity. Rapid application of glutamate or NMDA increased firing activity in all neurons and usually generated small inward currents (<15 pA), although larger currents were evoked in the remaining neurons. Both AMPA and NMDA receptors mediated the glutamate-evoked inward currents. These results suggest that isolated GnRH-EGFP neurons from juvenile mice can generate episodes of repetitive burst discharges that may underlie the pulsatile secretion of GnRH, and glutamatergic inputs may contribute to the activation of endogenous bursts.

Domain interactions regulating ampa receptor desensitization

Desensitization is a common property of glutamate and other ligand-gated ion channels, yet its molecular mechanism is unknown. For glutamate receptors, agonist binding involves interactions with identified amino acids from two lobes and may result in stabilizing the lobes in a closed "clamshell" conformation. The present studies demonstrate that two structures, beta-strands 7 and 8 and alpha-helices J and K, functionally interact with each other and likely form hinges between the two lobes, influencing the coupling between agonist binding and desensitization. Two amino acids identified within these regions form a solvent-exposed interface with a third amino acid, a mutation of which was shown previously to block receptor desensitization (L(507) in glutamate receptor 3). This interface may regulate a concerted conformational shift of the AMPA subtype of glutamate receptor subunits to the desensitized state.

Electrophysiological analysis of NMDA receptor subunit changes in the aging mouse cortex

NMDA receptors play an important role in memory processes and plasticity in the brain. We have previously demonstrated a significant decrease in NMDARepsilon2 subunit mRNA and protein with increasing age in the C57Bl/6 mouse frontal cortex. In the present study, two-electrode voltage clamp electrophysiology on Xenopus oocytes injected with total RNA harvested from the frontal cortex of young and old C57Bl mice was used to detect changes in receptor composition during aging. Ifenprodil concentration-response curves, magnesium current-voltage curves, and single channel conductances were determined for native receptors. In addition, ifenprodil and magnesium curves were generated for recombinant NMDA receptors of varying subunit ratios. Ifenprodil dose-response curves for all receptors were biphasic. The low affinity component of the curve increased slightly with age, while the high affinity population decreased, mimicking recombinant receptors with decreasing levels of epsilon2. A decrease in maximal current was also observed in aged animals with decreased levels of epsilon2, although single channel conductances were identical between young and old mice. In addition, an increase in sensitivity to magnesium was observed for receptors from older animals. Results are consistent with the interpretation that the epsilon2 subunit is reduced in older mouse frontal cortex. A change in NMDA receptor subunit composition could influence memory processes during aging.

The contributions of GluR2 to allosteric modulation of AMPA receptors

Native AMPA receptor complexes in the CNS are composed of hetero-oligomers of the GluR1-4 subunits, and generally contain the GluR2 subunit. To determine the contributions of GluR2 to pharmacological properties of receptor complexes, the effect of hetero-oligomerization with GluR2 on allosteric modulation of recombinant AMPA receptors was studied. The study of homo-oligomeric GluR2 was facilitated with a site-directed mutant of the pore, GluR2(R607Q), which allowed robust currents from this normally low-conducting subunit. The efficacy of the allosteric modulators was tested on homo-oligomeric GluR1-4, and then compared with hetero-oligomeric GluR1/GluR2, GluR3/GluR2 and GluR4/GluR2. Two selective allosteric modulators were tested, a positive modulator, cyclothiazide, and a negative modulator, LY300164. The results show that the pharmacological properties of homo-oligomeric GluR2 are not significantly different from those of GluR1, GluR3 or GluR4. The apparent affinity of cyclothiazide is not significantly changed upon hetero-oligomerization. However, the extent of potentiation of kainate responses by cyclothiazide is significantly decreased upon hetero-oligomerization. Hetero-oligomerization increases the apparent affinity of LY300164, a (-) isomer of the 2,3-benzodiazepine LY293606. These data indicate that although GluR2 has a dominant effect on the permeation properties, this subunit does not have a similarly dominant effect on pharmacological properties of native receptors. However, the state of hetero-oligomerization can alter the pharmacological properties of AMPA receptors.

Amino acid substitutions in the pore of rat glutamate receptors at sites influencing block by polyamines

1. The effect on polyamine block of mutations at the Q/R site and the conserved negative charge +4 site in AMPA and kainate receptors was studied using the rat glutamate receptor GluR6 expressed in Xenopus oocytes and human embryonic kidney (HEK) cells. 2. Introduction of negative charge at the Q/R site increased the equilibrium dissociation constant at 0 mV (Kd(0)) for spermine from 1.3 to 4.0 microM (Q590E); the smaller side chains Q590D and Q590N had Kd(0) values of 47 and 20 microM. Reductions in spermine affinity were also obtained for the small hydrophobic residues Q590V and Q590A, with Kd(0) values of 3.6 and 8.8 microM. Positively charged side chains produced outward rectifying responses similar to those recorded for GluR6(Q) with polyamine-free conditions, suggesting a complete absence of voltage-dependent block by spermine. 3. Substitution of tryptophan at the Q/R site produced high-affinity block with a Kd(0) of 190 pM. In Xenopus oocytes no outward current was observed at potentials up to +200 mV. A much smaller increase in affinity was observed for Q590F and Q590Y, which had Kd(0) values of 0.28 and 0.83 microM respectively. 4. The Q590H mutant gave weakly birectifying responses strikingly different from those for other mutants. When ionization of the His group was increased by raising the external hydrogen ion concentration, responses became outward rectifying. The ratios of the conductance at 100 mV over that at -100 mV for Q590H were 0.52 at pH 8.3 and 2.5 at pH 5.3. 5. Neutralization of charge or aromatic residues at the +4 site produced a large reduction of spermine affinity, with Kd(0) values for E594N, E594Q and E594W of 109, 1020 and 2150 microM, respectively. In the absence of polyamines, E594K and E594R produced strongly inward rectifying responses while E594Q, E594A and E594W were birectifying. 6. A model for permeant block allowed quantitative comparisons between mutants. Despite large changes in well depth and barrier heights, there was little change in the voltage dependence of block for both Q/R and +4 site mutants. We propose a model with a distributed binding site for polyamines in which the +4 site is located near the entrance to the channel.

AMPA receptor flip/flop mutants affecting deactivation, desensitization, and modulation by cyclothiazide, aniracetam, and thiocyanate

AMPA receptor GluRA subunits with mutations at position 750, a residue shown previously to control allosteric regulation by cyclothiazide, were analyzed for modulation of deactivation and desensitization by cyclothiazide, aniracetam, and thiocyanate. Point mutations from Ser to Asn, Ala, Asp, Gly, Gln, Met, Cys, Thr, Leu, Val, and Tyr were constructed in GluRAflip. The last four of these mutants were not functional; S750D was active only in the presence of cyclothiazide, and the remaining mutants exhibited altered rates of deactivation and desensitization for control responses to glutamate, and showed differential modulation by cyclothiazide and aniracetam. Results from kinetic analysis are consistent with aniracetam and cyclothiazide acting via distinct mechanisms. Our experiments demonstrate for the first time the functional importance of residue 750 in regulating intrinsic channel-gating kinetics and emphasize the biological significance of alternative splicing in the M3-M4 extracellular loop.

Negative allosteric modulation of wild-type and mutant AMPA receptors by GYKI 53655

Benzothiadiazides such as cyclothiazide potentiate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor responses, whereas 2,3-benzodiazepines such as 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3,4 -dihydro- 5H-2,3-benzodiazepine (GYKI 53655) act as noncompetitive antagonists; both drugs act through allosteric modulation. Controversy exists as to whether cyclothiazide and GYKI 53655 act at a common site. Recent mutational analysis has led to the identification of a serine residue in flip splice variants that is critical for directing the interaction of cyclothiazide with AMPA receptors. We tested whether the mutation of this residue to glutamine, which abolishes potentiation by cyclothiazide, can in addition block antagonism by 2,3-benzodiazepines, as would be predicted for action at a common site. We found that the S to Q mutation does not alter antagonism by 2,3-benzodiazepines, suggesting that the molecular determinants directing the interaction between GYKI 53655 and AMPA receptors are not identical to those controlling sensitivity to cyclothiazide. Additional support for this was obtained from analysis of the responses of AMPA receptor flip/flop splice variants, which, despite differences in equilibrium desensitization and sensitivity to cyclothiazide, show only small differences in sensitivity to 2,3-benzodiazepines. Furthermore, introduction of the flip exon from GluRA into GluR6, conferred sensitivity to cyclothiazide but did not increase sensitivity to 2,3-benzodiazepines. Of interest, experiments with native AMPA receptors generated from hippocampal and forebrain poly(A)+ mRNA revealed greater sensitivity to 2,3-benzodiazepines than receptors generated by expression of recombinant AMPA receptors, possibly indicating the existence of an unidentified accessory protein or novel receptor subunit.

Structural determinants of allosteric regulation in alternatively spliced AMPA receptors

The flip and flop splice variants of AMPA receptors show strikingly different sensitivity to allosteric regulation by cyclothiazide; heteromers assembled from GluR-A and GluR-B also exhibit splice variant-dependent differences in efficacy for activation by glutamate and kainate. The sensitivity for attenuation of desensitization by cyclothiazide for homomeric GluR-A was solely dependent upon exchange of Ser-750 (flip) and Asn-750 (flop), and was unaffected by mutagenesis of other divergent residues. In contrast, substantial alteration of the relative efficacy of glutamate versus kainate required mutation of multiple residues in the flip/flop region. Modulation by cyclothiazide was abolished by mutation of Ser-750 to Gin, the residue found at the homologous site in kainate-preferring subunits, whereas introduction of Ser at this site in GluR6 imparted sensitivity to cyclothiazide.

Cyclothiazide differentially modulates desensitization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor splice variants

Agonist responses for flip splice variants of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits GluR-A, -C, and -D are more strongly potentiated by cyclothiazide than are those for the flop forms. Cyclothiazide shows both greater efficacy and higher apparent affinity for potentiation of GluR-Aflip versus GluR-Aflop. Consistent with higher affinity for the flip splice variant, recovery from potentiation by cyclothiazide proceeds 30 times more slowly for GluR-Aflip than for GluR-Aflop. In the presence of 300 microM cyclothiazide a 6-fold leftward shift in the kainate dose-response curve for GluR-Aflip but not GluR-Aflop additionally contributes to a difference in potentiation for these splice variants. Although control responses to glutamate show strong desensitization for both splice variants of GluR-A, in the presence of 100 microM cyclothiazide desensitization is strongly attenuated for GluR-Aflip, whereas for GluR-Aflop desensitization remains pronounced but with a rate of onset slowed 50-fold, compared with control. In heteromeric AMPA receptors formed from GluR-A and GluR-B, the flip splice variants are dominant for controlling both recovery from potentiation of responses to kainate and block of desensitization of responses to glutamate. Our results suggest that the flip/flop module could directly contribute to the binding site for cyclothiazide, raising the possibility that this site is located in an extracellular receptor domain.

Selective modulation of desensitization at AMPA versus kainate receptors by cyclothiazide and concanavalin A

Potentiation by cyclothiazide of recombinant glutamate receptor responses in Xenopus oocytes showed absolute selectivity for AMPA versus kainate receptors. In contrast, concanavalin A strongly potentiated responses at kainate but not AMPA receptors. Rapid desensitization in HEK 293 cells transfected with AMPA receptors was blocked by cyclothiazide, but only weakly attenuated by concanavalin A. Desensitization at kainate receptors was blocked by concanavalin A but unaffected by cyclothiazide. Selective effects of these modulators following coexpression of subunits from different families suggest independent assembly of functional AMPA and kainate receptors. Northern blot analysis of mRNA for dorsal root ganglia revealed a predominant expression of GluR5, indicating that modulation of desensitization by concanavalin A but not cyclothiazide in sensory neurons accurately predicts subunit expression for native glutamate receptors.

Identification of the activation domain of equine infectious anemia virus rev

Several members of the lentivirus family of complex retroviruses have been shown to encode proteins that are functionally equivalent to the Rev posttranscriptional regulatory protein of human immunodeficiency virus type 1 (HIV-1). Furthermore, the domain organization of HIV-1 Rev, featuring a highly basic N-terminal RNA binding domain and a leucin-rich C-terminal effector domain, has also been shown to be highly conserved among Rev proteins derived from not only the primate but also the ovine and caprine lentiviruses. Although it has therefore appeared highly probable that the lentivirus equine infectious anemia virus (EIAV) also encodes a Rev, the predicted amino acid sequence of this putative EIAV regulatory protein does not display any evident homology to the basic and leucine-rich motifs characteristic of other known Rev proteins. By fusion of different segments of the proposed EIAV Rev protein to the well-defined RNA binding domain of either HIV-1 or visna virus Rev, we have identified a segment of this EIAV protein that can efficiently substitute in cis for the otherwise essential activation motif. Interestingly, the minimal EIAV Rev activation motif identified in this study comprises approximately 18 amino acids located toward the protein N terminus that lack any evident similarity to the leucine-rich activation domains found in these other lentivirus Rev proteins. It therefore appears that the Rev protein of EIAV, while analogous in function to Rev proteins defined in lentiviruses of primate, ovine, and caprine origin, is nevertheless distinguished by an entirely novel domain organization.

Characterization of the transcriptional trans activator of human foamy retrovirus

The human foamy viruses, or spumaviruses, a distinct subfamily of complex human retroviruses, remain poorly understood both in terms of their pathogenic potential and in terms of the regulatory mechanisms that govern their replication. Here, we demonstrate that the human spumaretrovirus shares with other complex human retroviruses the property of encoding a transcriptional trans activator of the homologous viral long terminal repeat. This regulatory protein is encoded by the viral Bel-1 open reading frame and is localized to the nucleus of expressing cells. The Bel-1 trans activator is shown to function effectively in cell lines derived from human, simian, murine, and avian sources. The viral target sequence for Bel-1 has been mapped 5' to the start of viral transcription and is therefore likely to be recognized as a DNA sequence. Our results further suggest that the mechanism of action of the Bel-1 protein may be distinct from those reported for the transcriptional trans activators encoded by members of the other human retroviral subfamilies.