Picrotoxin inhibition mechanism of a gamma-aminobutyric acid A receptor investigated by a laser-pulse photolysis technique

The gamma-aminobutyric acid(A) (GABA(A)) receptor, a major inhibitory neurotransmitter receptor, belongs to a family of membrane-bound proteins that regulate signal transmission between approximately 10(12) cells of the nervous system. It plays a major role in many neurological disorders, including epilepsy. It is the target of many pharmacological agents, including the convulsant picrotoxin. Here, we present the mechanism of inhibition by picrotoxin of the rat alpha1beta2gamma2L GABA(A) receptor investigated using rapid kinetic techniques in combination with whole-cell current recordings. The following new results were obtained by using transient kinetic techniques, the cell-flow method and the laser-pulse photolysis (LaPP) technique with a microsecond to millisecond time resolution. (i) The apparent dissociation constant of picrotoxin for the open-channel form of the receptor was approximately 5 times higher than that of the closed-channel form. (ii) Picrotoxin increased the channel-closing rate constant (k(cl)) approximately 4-fold, while the rate constant for channel opening (k(op)) remained essentially unaffected. (iii) The mechanism indicates that picrotoxin binds to an allosteric site of the receptor with higher affinity for the closed-channel form than for the open-channel form and thereby inhibits the receptor by decreasing 4-fold its channel-opening equilibrium constant [Phi(I)(-)(1) = k(op(I))/k(cl(I))]. (iv) The mechanism further indicates that compounds that bind with equal affinity to the picrotoxin-binding site on the open-channel form of the receptor and the closed-channel form will not affect the channel-opening equilibrium and can, therefore, displace picrotoxin and prevent inhibition of the GABA(A) receptor by picrotoxin. Such compounds may be therapeutically useful in counteracting the effects of compounds and diseases that unfavorably affect the channel-opening equilibrium of the receptor channel.     

Mechanism-based approach to the successful prevention of cocaine inhibition of the neuronal (alpha 3 beta 4) nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (nAChR) belongs to a family of five channel-forming proteins that regulate communication between the approximately 10(12) cells of the nervous system. A minimum mechanism of inhibition of the muscle-type nAChR (1) by the noncompetitive inhibitors cocaine and MK-801 [(+)-dizocilpine, an anticonvulsant] indicated they bind to a regulatory site, with higher affinity for the closed-channel form than for the open-channel form, thus shifting the equilibrium toward the closed-channel form and inhibiting receptor function. The mechanism predicts that compounds that bind to this regulatory site with equal or higher affinity for the open-channel conformation than for the closed-channel conformation will prevent receptor inhibition (1). Does a neuronal form of the receptor behave similarly? The mechanism of inhibition of the neuronal nAChR by cocaine and MK-801 using rapid chemical kinetic techniques was investigated. The alpha3beta4 nAChR stably expressed in HEK 293 cells was used in these investigations. Whole-cell currents originated from a major and minor nAChR isoform. Only the major isoform has been characterized. For the dominant, rapidly desensitizing isoform, the carbamoylcholine dissociation constant for the site controlling receptor activation, Kd, is 2 mM; the channel-opening equilibrium constant, Phi(-1), is 4; and the dominant desensitization rate constant, k34, is 20 s(-1). Cocaine inhibits the receptor noncompetitively, with an apparent KI of 84 and 26 microM at high and low carbamoylcholine concentrations, at which concentrations the receptor is mainly in the open- or closed-channel form, respectively. Similar results were obtained with MK-801. A combinatorially synthesized RNA ligand and a cocaine analogue alleviated cocaine inhibition of this neuronal receptor.     

On the mechanism of alleviation by phenobarbital of the malfunction of an epilepsy-linked GABA(A) receptor

A mechanism for the alleviation of the malfunction of a mutated (gamma2(K289M)) epilepsy-linked gamma-aminobutyric acid (GABA) neurotransmitter receptor by phenobarbital is presented. Compared to the wild-type receptor, the GABA-induced current is considerably reduced in the mutated (alpha1beta2gamma2(K289M)) epilepsy-linked GABA(A) receptor [Baulac, S., Huberfeld, G., Gurfinkel-An, I., Mitropoulou, G., Beranger, A., Prud'homme, J. F., Baulac, M., Brice, A., Bruzzone, R., and LeGuer, E. (2001) Nat. Genet. 28, 46-48]. This is due to an impaired GABA-induced equilibrium between the closed- and open-channel forms of the receptor [Ramakrishnan, L., and Hess, G. P. (2004) Biochemistry 43, 7534-7540]. We report that a barbiturate anticonvulsant, phenobarbital, alleviates the effect of this mutation. Transient kinetic techniques with a millisecond-to-microsecond time resolution and the wild-type and mutated receptors recombinantly expressed in mammalian HEK293T cells were used. The efficacy of phenobarbital in potentiating currents elicited by a saturating concentration of GABA is about 3 times higher for the mutated receptor than for the wild type. The results indicate that phenobarbital alleviates the malfunction of the mutated receptor by increasing its channel-opening equilibrium constant (phi(-1) = k(op)/k(cl)) by about an order of magnitude. Phenobarbital changes the channel-opening rate constant (k(op)) by less than 2-fold but decreases the channel-closing rate constant (k(cl)) 8-fold. The dissociation constant of GABA is unaffected. The experiments also indicate that at saturating concentrations of GABA the mutated (gamma2(K289M)) form of the alpha1beta2gamma2 GABA(A) receptor is well suited for a rapid and simple screening of positive allosteric modulators of the receptor.     

On the mechanism of a mutated and abnormally functioning gamma-aminobutyric acid (A) receptor linked to epilepsy

A recent report indicates that a lysine-to-methionine mutation (K289M) in the gamma2 subunit of a human gamma-aminobutyric acid neurotransmitter receptor, the GABA(A) receptor, is linked to generalized epilepsy with febrile seizures [Baulac et al. (2001) Nat. Genet. 28, 46-48]. This mutation caused a decreased current response to GABA [Baulac et al. (2001) Nat. Genet. 28, 46-48]. Here we determine changes that occur in the mechanism of opening and closing of transmembrane channels formed by the GABA(A) receptor as a result of this mutation. The K289M mutation was introduced into the gamma2L subunit of the rat GABA(A) receptor, and the mutated subunit was coexpressed with the alpha1 and beta2 subunits in HEK293 cells. Transient kinetic techniques suitable for investigating reactions on cell surfaces with a microsecond-to-millisecond time resolution [Hess, G. P., and Grewer, C. (1998) Methods Enzymol. 291, 443-473] were used. They allow one to determine not only the channel-opening probability and rates of receptor desensitization but also the opening and closing rates of the mutated GABA(A) receptor channel. The channel-opening equilibrium constant of the mutated receptor was found to be 5-fold lower than that of the wild type. We calculated that this decrease in the channel-opening equilibrium accounts for the dysfunction of the mutated receptor. We discuss how a knowledge of the mechanism of the mutated receptor indicates an approach for alleviating this dysfunction.     

Channel-opening kinetics of GluR6 kainate receptor

GluR6 is an ionotropic glutamate receptor subunit of the kainate subtype. It plays an essential role in synaptic plasticity and epilepsy. We expressed this recombinant receptor in HEK-293 cells and characterized the glutamate-induced channel-opening reaction, using a laser-pulse photolysis technique with the caged glutamate (gamma-O-(alpha-carboxy-2-nitrobenzyl)glutamate). This technique permits glutamate to be liberated photolytically from the caged glutamate with a time constant of approximately 30 micros. Prior to laser photolysis, the caged glutamate did not activate the GluR6 channel, nor did it inhibit or potentiate the glutamate response. At the transmembrane voltage of -60 mV, pH 7.4 and 22 degrees C, the channel-opening and -closing rate constants were determined to be (1.1 +/- 0. 4) x 10(4) and (4.2 +/- 0.2) x 10(2) s(-1), respectively. The intrinsic dissociation constant of glutamate and the channel-opening probability were found to be 450 +/- 200 microM and 0.96, respectively. These constants are derived from a minimal kinetic mechanism of the channel activation involving the binding of two glutamate molecules. This mechanism describes the time course of the open-channel form of the receptor as a function of glutamate concentration. On the basis of the channel-opening rate constants obtained, the shortest rise time (20-80% of the receptor current response) or the fastest time by which the GluR6Q channel can open is predicted to be 120 micros. The open-channel form of the receptor determines the transmembrane voltage change, which in turn controls synaptic signal transmission between two neurons. The comparison of the channel-opening kinetic rate constants between GluR6Q and GluR2Q(flip), reported in the companion paper, suggests that at a glutamate concentration of 100 microM, for instance, the integrated neuronal signal will be dominated by a slower GluR6Q receptor response, as compared to the GluR2Q(flip) component.     

How fast does the gamma-aminobutyric acid receptor channel open? Kinetic investigations in the microsecond time region using a laser-pulse photolysis technique.

The gamma-aminobuytric acid(A) (GABA(A)) receptor is a membrane-bound protein that mediates signal transmission between neurons through formation of chloride ion channels. GABA is the activating ligand, which upon binding to the receptor triggers channel opening in the microsecond time domain and reversible desensitization of the receptor in the millisecond time region. We have investigated the channel-opening mechanism for this receptor in rat hippocampal neurons before the protein desensitizes by using a rapid flow method (cell-flow) with a 10 ms time resolution and a laser-pulse photolysis technique with a approximately 30 micros time resolution to determine the rate and equilibrium constants for channel opening and closing. Two different forms of the receptor, namely, a rapidly and a slowly desensitizing form, exist in the rat hippocampal cells and are characterized by their different rates for desensitization. At 250 microM GABA the rate constant for desensitization was 2.3 +/- 0.4 s(-)(1) for the rapidly desensitizing form and 0.4 +/- 0.1 s(-)(1) for the slowly desensitizing form. The dissociation constant of GABA from the site controlling channel opening was 100 +/- 40 microM for the rapidly desensitizing form and 120 +/- 60 microM for the slowly desensitizing form. The rate constants for channel closing did not differ significantly for the two forms, 85 +/- 20 s(-)(1) for the rapidly desensitizing and 100 +/- 60 s(-)(1) for the slowly desensitizing form. However, the channel-opening rate constant differed by a factor of 3, 1840 +/- 160 s(-)(1) for the rapidly desensitizing and 6700 +/- 330 s(-)(1) for the slowly desensitizing form. This difference in the rate constant for channel opening for the two forms, determined by the laser-pulse photolysis technique, is reflected as a shift in the channel-opening equilibrium constant, which is 7 +/- 5 and 20 +/- 15 for the rapidly and slowly desensitizing forms respectively, determined by the cell-flow method. These constants, together with the concentration of GABA and the concentration of receptor sites in the membrane, determine the number of channels that open as a function of GABA concentration, and the rate at which they open and close. These constants play an important role in determining the rate of the transmembrane ion flux and, therefore, the receptor-controlled changes in transmembrane voltage that trigger signal transmission.     

Inhibition of nicotinic acetylcholine receptor by philanthotoxin-343: kinetic investigations in the microsecond time region using a laser-pulse photolysis technique.

The mechanism of inhibition of a nicotinic acetylcholine receptor (nAChR) in BC(3)H1 muscle cells by philanthotoxin-343 (PhTX-343), a synthetic analogue of philanthotoxin-433, a wasp toxin, was investigated using a laser-pulse photolysis technique with microsecond time resolution and in a carbamoylcholine concentration range of 20-100 microM and PhTX-343 concentration range of 0-200 microM. The rate constant for nAChR channel opening determined by the chemical kinetic techniques decreased with increasing PhTX-343 concentration, whereas there was no significant effect on the rate constant for channel closing. The resulting decrease in the channel-opening equilibrium constant accounted quantitatively for the inhibition of the receptor by the toxin. Single-channel current measurements suggest an additional step in which the open channel:inhibitor complex isomerizes to a nonconducting receptor form. Cell-flow experiments with a time resolution of 10 ms indicate that this isomerization step is only important at very high inhibitor concentrations. The inhibitor binds to the open-channel receptor form, with an affinity that is at least 5 times smaller than that for the closed-channel form. This indicates that receptor inhibition mainly involves the binding of PhTX-343 to the closed-channel form of the receptor. PhTX-343, and an analogue of this polyamine, had no effect when applied to the inside of the cell membrane. However, there was significant inhibition of the nAChR when these compounds were applied to the outside of the cell membrane, indicating an extracellular site for inhibition. Furthermore, increasing the transmembrane potential results in a decrease in the ability of PhTX-343 to inhibit the receptor. This observation is related to the voltage dependence of the effect of PhTX-343 on the rate constant for nAChR channel opening with increasing transmembrane voltage (-60 to 50 mV). This suggests that the voltage dependence of inhibition mainly reflects the effect of transmembrane voltage on the rate constant of channel opening and, therefore, the channel-opening equilibrium constant. PhTX-343 competes with cocaine and procaine for its binding site. The finding that this toxin, which binds to a common inhibitory site with compounds such as cocaine, exerts its effect by decreasing the channel-opening equilibrium constant suggests an approach for the development of therapeutic agents. A compound that binds to this regulatory site but does not affect the channel-opening equilibrium constant may be developed. Such a compound can displace an abused drug such as cocaine and thereby alleviate the toxic effect of this compound on the organism.     

On the mechanism of inhibition of the nicotinic acetylcholine receptor by the anticonvulsant MK-801 investigated by laser-pulse photolysis in the microsecond-to-millisecond time region.

The mechanism of inhibition of the muscle nicotinic acetylcholine receptor is of interest because of the many drugs which are known to modify its function. The laser-pulse photolysis technique, using a photolabile, biologically inert ligand (caged carbamoylcholine) for the nicotinic acetylcholine receptor, and BC3H1 cells have been used to investigate the mechanism of inhibition of the receptor by MK-801 [(+)-dizocilpine] in the microsecond-to-millisecond time region. MK-801 is an anticonvulsant and a known inhibitor of the N-methyl-D-aspartate and nicotinic acetylcholine receptors. Both the chemical kinetic and the single-channel current-recording measurements reported here indicate the existence of two inhibition processes, one occurring within 50 ms and the other within about 1 s of equilibration of the receptor with the inhibitor. Unless stated otherwise, here we characterize the receptor inhibition observed when MK-801 is equilibrated with the receptor for only 50 ms. We determined the effect of MK-801 on the concentration of the open receptor-channels and the apparent dissociation constant of the inhibitor from the closed-channel (KI(obs) = 180 microM) and open-channel ( = 950 microM) forms. Within a few milliseconds after inhibitor binding, decreases to about 100 microM, due to an inhibitor-induced isomerization to an inactive receptor form. A mechanism that incorporates the new results is proposed. It includes the formation of an ion-conducting receptor:inhibitor complex with a channel-opening equilibrium constant that is unfavorable compared to the open-channel receptor form in the absence of inhibitor. In the MK-801 concentration range of 0-500 microM, this mechanism accounts for the observed MK-801-induced decrease in the concentration of open channels. At high concentrations of carbamoylcholine, when the receptor is mainly in the open-channel form, the conducting receptor:inhibitor complex isomerizes to a nonconducting state with a rate constant of about 2400 s-1 for the forward reaction and 230 s-1 for the back reaction. It is shown that the proposed new mechanism, based on transient kinetic measurements, also accounts for the results of previous investigations with other inhibitors (procaine, cocaine), which were carried out under both pre-steady-state and equilibrium conditions. A compound that binds to the same regulatory site on the receptor as MK-801 but does not affect the channel-opening equilibrium constant may have considerable use in protecting an organism from the effects of abused drugs.     

Generation of RNA aptamers to the G-protein-coupled receptor for neurotensin,NTS-1

G-protein-coupled receptors (GPCRs) are integral membrane proteins involved in signal transduction and constitute major drug targets for disease therapy. Aptamers, which are globular RNA or DNA molecules evolved to specifically bind a target, could represent a valuable tool with which to probe the role of such receptors in normal tissue and disease pathology and for cocrystallization with receptors for structure determination by X-ray crystallography. Using the bacterially expressed rat neurotensin receptor NTS-1 as an example, we describe a strategy for the generation of GPCR-specific RNA aptamers. Seven rounds of a "subtractive," paramagnetic bead-based selection protocol were used to enrich for neurotensin receptor-specific aptamers, while circumventing the evolution of aptamers reactive to minor protein contaminants. Representatives of each aptamer family were analyzed in Escherichia coli membrane nitrocellulose filter binding assays. Eight aptamers demonstrated specificity for the neurotensin receptor. One aptamer, P19, was characterized in detail and shown to bind to both the rat receptor and the human receptor with nanomolar affinity. P19 was also shown to interact with rat neurotensin receptor expressed in CHO cells, in both membrane preparations and intact cells. P19 represents the first example of a GPCR-specific RNA aptamer.     

Two novel residues in M2 of the gamma-aminobutyric acid type A receptor affecting gating by GABA and picrotoxin affinity

An amino acid residue was found in M2 of gamma-aminobutyric acid (GABA) type A receptors that has profound effects on the binding of picrotoxin to the receptor and therefore may form part of its binding pocket. In addition, it strongly affects channel gating. The residue is located N-terminally to residues suggested so far to be important for channel gating. Point mutated alpha1beta(3) receptors were expressed in Xenopus oocytes and analyzed using the electrophysiological techniques. Coexpression of the alpha(1) subunit with the mutated beta(3) subunit beta(3)L253F led to spontaneous picrotoxin-sensitive currents in the absence of GABA. Nanomolar concentrations of GABA further promoted channel opening. Upon washout of picrotoxin, a huge transient inward current was observed. The reversal potential of the inward current was indicative of a chloride ion selectivity. The amplitude of the inward current was strongly dependent on the picrotoxin concentration and on the duration of its application. There was more than a 100-fold decrease in picrotoxin affinity. A kinetic model is presented that mimics the gating behavior of the mutant receptor. The point mutation in the neighboring residue beta(3)A252V resulted in receptors that displayed an about 6-fold increased apparent affinity to GABA and an about 10-fold reduced sensitivity to picrotoxin.     

Minimal RNA Aptamer Sequences That Can Inhibit or Alleviate Noncompetitive Inhibition of the Muscle-Type Nicotinic Acetylcholine Receptor

Combinatorially synthesized nucleotide polymers have been used during the last decade to find ligands that bind to specific sites on biological molecules, including membrane-bound proteins such as the nicotinic acetylcholine receptors (nAChRs). The neurotransmitter receptors belong to a group of four structurally related proteins that regulate signal transmission between ~10¹¹ neurons of the mammalian nervous system. The nAChRs are inhibited by compounds such as the anticonvulsant MK-801 [(+)-dizocilpine] and abused drugs such as cocaine. Based on predictions arising from the mechanism of receptor inhibition by MK-801 and cocaine, we developed two classes of RNA aptamers: class I members, which inhibit the nAChR, and class II members, which alleviate inhibition of the receptor by MK-801 and cocaine. The systematic evolution of ligands by the exponential enrichment (SELEX) method was used to obtain these compounds. Here, we report that we have truncated RNA aptamers in each class to determine the minimal nucleic acid sequence that retains the characteristic function for which the aptamer was originally selected. We demonstrate that a truncated class I aptamer containing a sequence of seven nucleotides inhibits the nAChR and that a truncated class II aptamer containing a sequence of only four nucleotides can alleviate MK-801 inhibition.     

肝脏去唾液酸糖蛋白受体特异性RNA适配子的SELEX筛选与鉴定

目的获得能够特异性高亲和力结合肝脏特异性去唾液酸糖蛋白受体（asialoglycoprotein receptor,ASGPR）的RNA适配子,为开发诊断和治疗肝脏疾病的靶向性试剂和药物奠定基础。方法合成一个长度为115nt含有25个随机序列的单链DNA随机文库,通过体外转录构建出单链RNA适配子随机文库,以肝脏ASGPR大亚基为靶蛋白,采用SELEX（systematic evolution of ligands by exponential enrichment）技术筛选具有高亲和力的AsGPR特异性RNA适配子;通过膜结合测定实验、凝胶阻滞实验鉴定筛选适配子对靶蛋白的特异性和亲和力。结果经过12轮筛选获得了具有高亲和力的肝脏ASGPR特异性RNA适配子。结论成功地筛选出了具有离亲和力的肝脏ASGPR特异性RNA适配子库。     

Solution structure of an informationally complex high-affinity RNA aptamer to GTP

Higher-affinity RNA aptamers to GTP are more informationally complex than lower-affinity aptamers. Analog binding studies have shown that the additional information needed to improve affinity does not specify more interactions with the ligand. In light of those observations, we would like to understand the structural characteristics that enable complex aptamers to bind their ligands with higher affinity. Here we present the solution structure of the 41-nt Class I GTP aptamer (K(d) = 75 nM) as determined by NMR. The backbone of the aptamer forms a reverse-S that shapes the binding pocket. The ligand nucleobase stacks between purine platforms and makes hydrogen bonds with the edge of another base. Interestingly, the local modes of interaction for the Class I aptamer and an RNA aptamer that binds ATP with a K(d) of 6 microM are very much alike. The aptamers exhibit nearly identical levels of binding specificity and fraction of ligand sequestered from the solvent (81%-85%). However, the GTP aptamer is more informationally complex (approximately 45 vs. 35 bits) and has a larger recognition bulge (15 vs. 12 nucleotides) with many more stabilizing base-base interactions. Because the aptamers have similar modes of ligand binding, we conclude that the stabilizing structural elements in the Class I aptamer are responsible for much of the difference in K(d). These results are consistent with the hypothesis that increasing the number of intra-RNA interactions, rather than adding specific contacts to the ligand, is the simplest way to improve binding affinity.     

A sol-gel-based microfluidics system enhances the efficiency of RNA aptamer selection

RNA and DNA aptamers that bind to target molecules with high specificity and affinity have been a focus of diagnostics and therapeutic research. These aptamers are obtained by SELEX often requiring many rounds of selection and amplification. Recently, we have shown the efficient binding and elution of RNA aptamers against target proteins using a microfluidic chip that incorporates 5 sol-gel binding droplets within which specific target proteins are imbedded. Here, we demonstrate that our microfluidic chip in a SELEX experiment greatly improved selection efficiency of RNA aptamers to TATA-binding protein, reducing the number of selection cycles needed to produce high affinity aptamers by about 80%. Many aptamers were identical or homologous to those isolated previously by conventional filter-binding SELEX. The microfluidic chip SELEX is readily scalable using a sol-gel microarray-based target multiplexing. Additionally, we show that sol-gel embedded protein arrays can be used as a high-throughput assay for quantifying binding affinities of aptamers.     

An aptamer based competition assay for protein detection using CNT activated gold-interdigitated capacitor arrays

An aptamer can specifically bind to its target molecule, or hybridize with its complementary strand. A target bound aptamer complex has difficulty to hybridize with its complementary strand. It is possible to determine the concentration of target based on affinity separation system for the protein detection. Here, we exploited this property using C-reactive protein (CRP) specific RNA aptamers as probes that were immobilized by physical adsorption on carbon nanotubes (CNTs) activated gold interdigitated electrodes of capacitors. The selective binding ability of RNA aptamer with its target molecule was determined by change in capacitance after allowing competitive binding with CRP and complementary RNA (cRNA) strands in pure form and co-mixtures (CRP:cRNA=0:1, 1:0, 1:1, 1:2 and 2:1). The sensor showed significant capacitance change with pure forms of CRP/cRNA while responses reduced considerably in presence of CRP:cRNA in co-mixtures (1:1 and 1:2) because of the binding competition. At a critical CRP:cRNA ratio of 2:1, the capacitance response was dramatically lost because of the dissociation of adsorbed aptamers from the sensor surface to bind when excess CRP. Binding assays showed that the immobilized aptamers had strong affinity for cRNA (K(d)=1.98 μM) and CRP molecules (K(d)=2.4 μM) in pure forms, but low affinity for CRP:cRNA ratio of 2:1 (K(d)=8.58 μM). The dynamic detection range for CRP was determined to be 1-8 μM (0.58-4.6 μg/capacitor). The approach described in this study is a sensitive label-free method to detect proteins based on affinity separation of target molecules that can potentially be used for probing molecular interactions.     

RNA aptamers that functionally interact with green fluorescent protein and its derivatives

Green Fluorescent Protein (GFP) and related fluorescent proteins (FPs) have been widely used to tag proteins, allowing their expression and subcellular localization to be examined in real time in living cells and animals. Similar fluorescent methods are highly desirable to detect and track RNA and other biological molecules in living cells. For this purpose, we have developed a group of RNA aptamers that bind GFP and related proteins, which we term Fluorescent Protein-Binding Aptamers (FPBA). These aptamers bind GFP, YFP and CFP with low nanomolar affinity and binding decreases GFP fluorescence, whereas slightly augmenting YFP and CFP brightness. Aptamer binding results in an increase in the pKa of EGFP, decreasing the 475 nm excited green fluorescence at a given pH. We report the secondary structure of FPBA and the ability to synthesize functional multivalent dendrimers. FPBA expressed in live cells decreased GFP fluorescence in a valency-dependent manner, indicating that the RNA aptamers function within cells. The development of aptamers that bind fluorescent proteins with high affinity and alter their function, markedly expands their use in the study of biological pathways.     

Selection and stabilization of the RNA aptamers against the human immunodeficiency virus type-1 nucleocapsid protein

The nucleocapsid (NC) protein of the human immunodeficiency virus-1 (HIV-1) plays an important role in the encapsidation of viral RNA and assembly of viral particle. Since the NC protein is resistant for mutation, it might be an excellent target for the anti-viral therapy. RNA aptamers that bind to the mature form of the NC protein were isolated from a RNA library. Surface plasmon resonance measurement and gel shift assay showed that the RNA aptamers specifically bind to the NC protein with high affinity and compete for the psi RNA binding to the NC protein. Mapping of the RNA aptamer showed at least two sites for the protein binding, suggesting a multiple and cooperative binding by the NC to RNA. In addition, the circular form of RNA avidly binds to the NC protein as a linear counter does. Stabilized RNA aptamer is expected to act as an inhibitor for the viral packaging.     

GABA-mediated inhibition of primary olfactory receptor neurons

Applying GABA (1 microM-1 mM) to the soma of cultured lobster olfactory receptor neurons evokes an inward current (V(m) = -60 mV) accompanied by an increase in membrane conductance, with a half-effect of 487 microM GABA. The current-voltage relationship of this current is linear between -100 and 100 mV and reverses polarity at the equilibrium potential for Cl(-). The current is blocked by picrotoxin and bicuculline methiodide, and is evoked by trans-aminocrotonic acid, isoguvacine, muscimol, imidazole-4-acetic acid, and 3-amino-1-propanesulfonic acid, but not by the GABA(C)-receptor agonist cis-4-aminocrotonic acid and the GABA(B)-receptor agonist 3-aminopropylphosphonic. Applying GABA to the soma of the cells in situ reversibly suppresses the spontaneous discharge and substantially decreases the odor-evoked discharge. The effects of GABA on the cell soma in situ are antagonized by both picrotoxin and bicuculline methiodide. Taken together with evidence that GABA directly activates a chloride channel in outside-out patches excised from the soma of these neurons, we conclude that lobster olfactory receptor neurons express an ionotropic GABA receptor that can potentially regulate the output of these cells. Copyright Copyright 1999 S. Karger AG, Basel     

Bimodal loop-loop interactions increase the affinity of RNA aptamers for HIV-1 RNA structures

Multiple loop-loop interactions between adjacent RNA hairpins regulate gene expression in different organisms. To demonstrate that such natural interactions could be mimicked for generating RNA ligands that are able to recognize simultaneously at least two structured RNA targets, a double kissing complex model was designed. The target consisted of two HIV-1 transactivating responsive (TAR) RNA variants, BRU and MAL, connected by a non-nucleotidic linker. The double ligand was generated by combining the corresponding hairpin aptamers, R06BRU and R06MAL, identified previously by in vitro selection [Ducongé, F., and Toulmé, J. J (1999) RNA 5, 1605-1614]. The resulting interaction was analyzed by thermal denaturation monitored by UV spectroscopy, electrophoretic mobility shift assays (EMSAs), and surface plasmon resonance (SPR) experiments. The bimodal complex was characterized by a binding equilibrium constant increased by at least 1 order of magnitude compared to that of the complexes between the individual parent hairpins. This resulted from a slower dissociation rate. We then made use of such a strategy for targeting two structured functional motifs of the folded 5' untranslated region (5'UTR) of HIV-1. Two bivalent RNA ligands were designed that targeted simultaneously the TAR and dimerization initiation site (DIS) hairpins or the TAR and poly(A) ones. The results show that these ligands also displayed enhanced affinity for their target compared to the individual molecules. The work reported here suggests that bimodal structured RNA ligands might provide a way of increasing the affinity of aptamers for folded RNA targets.