Structural characterization of nonactive site, TrkA-selective kinase inhibitors.

Current therapies for chronic pain can have insufficient efficacy and lead to side effects, necessitating research of novel targets against pain. Although originally identified as an oncogene, Tropomyosin-related kinase A (TrkA) is linked to pain and elevated levels of NGF (the ligand for TrkA) are associated with chronic pain. Antibodies that block TrkA interaction with its ligand, NGF, are in clinical trials for pain relief. Here, we describe the identification of TrkA-specific inhibitors and the structural basis for their selectivity over other Trk family kinases. The X-ray structures reveal a binding site outside the kinase active site that uses residues from the kinase domain and the juxtamembrane region. Three modes of binding with the juxtamembrane region are characterized through a series of ligand-bound complexes. The structures indicate a critical pharmacophore on the compounds that leads to the distinct binding modes. The mode of interaction can allow TrkA selectivity over TrkB and TrkC or promiscuous, pan-Trk inhibition. This finding highlights the difficulty in characterizing the structure-activity relationship of a chemical series in the absence of structural information because of substantial differences in the interacting residues. These structures illustrate the flexibility of binding to sequences outside of-but adjacent to-the kinase domain of TrkA. This knowledge allows development of compounds with specificity for TrkA or the family of Trk proteins.

[(3)H]chlorpromazine photolabeling of the torpedo nicotinic acetylcholine receptor identifies two state-dependent binding sites in the ion channel.

Chlorpromazine (CPZ), a potent nicotinic acetylcholine receptor (nAChR) noncompetitive antagonist, binds with higher affinity in the ion channel in the desensitized state than in the closed channel state and with low affinity to additional sites in nAChR-rich membranes. For nAChR equilibrated with agonist, we confirm previous reports that [(3)H]CPZ occupies a site near the cytoplasmic end of the M2 ion channel domain, photolabeling positions M2-2, M2-6, and/or M2-9 in each subunit. We find that [(3)H]CPZ also binds at the extracellular end of the channel, photolabeling amino acids at positions M2-16 (alpha,gamma), M2-17 (alpha,beta,delta), and M2-20 (alpha,beta,delta). The photolabeling at the cytoplasmic end of the channel is fully inhibitable by phencyclidine or proadifen, whereas neither drug inhibits [(3)H]CPZ photolabeling at the extracellular end, establishing that positively charged drugs can bind simultaneously at the cytoplasmic and extracellular ends of the ion channel. [(3)H]CPZ photolabeling is not detected in the transmembrane domain outside the ion channel, but it photolabels alphaMet-386 and alphaSer-393 in the cytoplasmic alphaMA helix. In the nAChR equilibrated with alpha-bungarotoxin to stabilize the nAChR in a closed state, [(3)H]CPZ photolabels amino acids at M2-5 (alpha), M2-6 (alpha,beta,delta), and M2-9 (beta,delta), with no labeling at M2-2. These results provide novel information about the modes of drug binding within the nAChR ion channel and indicate that within the nAChR transmembrane domain, the binding of cationic aromatic amine antagonists can be restricted to the ion channel domain, in contrast to the uncharged, allosteric potentiators and inhibitors that also bind within the delta subunit helix bundle and at subunit interfaces.

Affinity selection-mass spectrometry screening techniques for small molecule drug discovery.

Affinity selection-mass spectrometry (AS-MS) techniques assess the binding of candidate molecules to immobilized or soluble receptors, and these methods are gaining acceptance in high throughput screening laboratories as valuable complements to traditional drug discovery technologies. A diversity of receptor types have been evaluated by AS-MS, including those that are difficult to screen using traditional biochemical approaches. AS-MS techniques that couple liquid chromatography-MS with size-based separation methods, such as ultrafiltration, gel permeation, or size-exclusion chromatography, are particularly amenable to the demands of MS-based screening and have demonstrated the greatest success across a broad range of drug targets. MS measurements of receptor function have many of the same advantages as AS-MS screening and are increasingly used for drug discovery as well.

Discovery and characterization of orthosteric and allosteric muscarinic M2 acetylcholine receptor ligands by affinity selection-mass spectrometry.

Screening assays using target-based affinity selection coupled with high-sensitivity detection technologies to identify small-molecule hits from chemical libraries can provide a useful discovery approach that complements traditional assay systems. Affinity selection-mass spectrometry (AS-MS) is one such methodology that holds promise for providing selective and sensitive high-throughput screening platforms. Although AS-MS screening platforms have been used to discover small-molecule ligands of proteins from many target families, they have not yet been used routinely to screen integral membrane proteins. The authors present a proof-of-concept study using size exclusion chromatography coupled to AS-MS to perform a primary screen for small-molecule ligands of the purified muscarinic M2 acetylcholine receptor, a G-protein-coupled receptor. AS-MS is used to characterize the binding mechanisms of 2 newly discovered ligands. NGD-3350 is a novel M2-specific orthosteric antagonist of M2 function. NGD-3366 is an allosteric ligand with binding properties similar to the allosteric antagonist W-84, which decreases the dissociation rate of N-methyl-scopolamine from the M2 receptor. Binding properties of the ligands discerned from AS-MS assays agree with those from in vitro biochemical assays. The authors conclude that when used with appropriate small-molecule libraries, AS-MS may provide a useful high-throughput assay system for the discovery and characterization of all classes of integral membrane protein ligands, including allosteric modulators.

2-(3-Methyl-3H-diaziren-3-yl)ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate: a derivative of the stereoselective general anesthetic etomidate for photolabeling ligand-gated ion channels.

To locate general anesthetic binding sites on ligand-gated ion channels, a diazirine derivative of the potent intravenous anesthetic, R-(+)-etomidate (2-ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate), has been synthesized and characterized. R-(+)-Azietomidate [2-(3-methyl-3H-diaziren-3-yl)ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate] anesthetizes tadpoles with an EC(50) of 2.2 microM, identical to that of R-(+)-etomidate. At this concentration both agents equally enhanced GABA-induced currents and decreased binding of the caged-convulsant [(35)S]TBPS to GABA(A) receptors. In all of the above actions R-(+)-azietomidate is about an order of magnitude more potent than S-(-)-azietomidate, an enantioselectivity comparable to etomidate's. R-(+)-Azietomidate also inhibits acetylcholine-induced currents in nicotinic acetylcholine receptors, with about twice the potency of the parent compound. [(3)H]Azietomidate photoincorporated into Torpedo nicotinic acetylcholine receptor-rich membranes. Desensitization decreased photoincorporation into the delta-subunit and increased that into the alpha-subunit. The latter increase was confined to a proteolytic fragment containing the first three transmembrane segments. Thus, R-(+)-azietomidate is a potent stereoselective general anesthetic and an effective photolabel.

Photolabeling the Torpedo nicotinic acetylcholine receptor with 4-azido-2,3,5,6-tetrafluorobenzoylcholine, a partial agonist.

The interactions of a photoreactive analogue of benzoylcholine, 4-azido-2,3,5,6-tetrafluorobenzoylcholine (APFBzcholine), with nicotinic acetylcholine receptors (nAChRs) were studied using electrophysiology and photolabeling. APFBzcholine acted as a low-efficacy partial agonist, eliciting maximal responses that were 0.3 and 0.1% of that of acetylcholine for embryonic mouse and Torpedo nAChRs expressed in Xenopus oocytes, respectively. Equilibrium binding studies of [3H]APFBzcholine with nAChR-rich membranes from Torpedo electric organ revealed equal affinities (K(eq) = 12 microM) for the two agonist binding sites. Upon UV irradiation at 254 nm, [3H]APFBzcholine was photoincorporated into the nAChR alpha, gamma, and delta subunits in an agonist-inhibitable manner. Photolabeled amino acids in the agonist binding sites were identified by Edman degradation of isolated, labeled subunit fragments. [3H]APFBzcholine photolabeled gammaLeu-109/deltaLeu-111, gammaTyr-111, and gammaTyr-117 in binding site segment E as well as alphaTyr-198 in alpha subunit binding site segment C. The observed pattern of photolabeling is examined in relation to the predicted orientation of the azide when APFBzcholine is docked in the agonist binding site of a homology model of the nAChR extracellular domain based upon the structure of the snail acetylcholine binding protein.

Interactions of peptide mimics of hyaluronic acid with the receptor for hyaluronan mediated motility (RHAMM).

Using the hyaluronic acid (HA) binding region of the receptor for hyaluronan-mediated motility (RHAMM) as a model, a molecular perspective for peptide mimicry of the natural ligand was established by comparing the interaction sites of HA and unnatural peptide-ligands to RHAMM. This was accomplished by obtaining a series of octapeptide-ligands through screening experiments that bound to the HA binding domains of RHAMM (amino acids 517-576) and could be displaced by HA. These molecules were computationally docked onto a three-dimensional NMR based model of RHAMM. The NMR model showed that RHAMM(517-576) was a set of three helices, two of which contained the HA binding domains (HABDs) flanking a central groove. The structure was stabilized by hydrophobic interactions from four pairs of Val and Ile side chains extending into the groove. The presence of solvent exposed, positively charged side chains spaced 11 A apart matched the spacing of negative charges on HA. Docking experiments using flexible natural and artificial ligands demonstrated that HA and peptide-mimetics preferentially bound to the second helix that contains HABD-2. Three salt bridges between HA carboxylates and Lys548, Lys553 and Lys560 and two hydrophobic interactions involving Val538 and Val559 were predicted to stabilize the RHAMM-HA complex. The high affinity peptides and HA utilized the same charged residues, with additional contacts to other basic residues. However, hydrophobic contacts do not contribute to affinity for peptide ligand-RHAMM complexes. These results offer insight into how selectivity is achieved in the binding of HA to RHAMM, and how peptide competitors may compete for binding with HA on a single hyaladherin.

Identification of binding sites in the nicotinic acetylcholine receptor for [3H]azietomidate, a photoactivatable general anesthetic.

To identify binding domains in a ligand-gated ion channel for etomidate, an intravenous general anesthetic, we photolabeled nicotinic acetylcholine receptor (nAChR)-rich membranes from Torpedo electric organ with a photoactivatable analog, [(3)H]azietomidate. Based upon the inhibition of binding of the noncompetitive antagonist [(3)H]phencyclidine, azietomidate and etomidate bind with 10-fold higher affinity to nAChRs in the desensitized state (IC(50) = 70 microm) than in the closed channel state. In addition, both drugs between 0.1 and 1 mm produced a concentration-dependent enhancement of [(3)H]ACh equilibrium binding affinity, but they inhibited binding at higher concentrations. UV irradiation resulted in preferential [(3)H]azietomidate photoincorporation into the nAChR alpha and delta subunits. Photolabeled amino acids in both subunits were identified in the ion channel domain and in the ACh binding sites by Edman degradation. Within the nAChR ion channel in the desensitized state, there was labeling of alphaGlu-262 and deltaGln-276 at the extracellular end and deltaSer-258 and deltaSer-262 toward the cytoplasmic end. Within the acetylcholine binding sites, [(3)H]azietomidate photolabeled alphaTyr-93, alphaTyr-190, and alphaTyr-198 in the site at the alpha-gamma interface and deltaAsp-59 (but not the homologous position, gammaGlu-57). Increasing [(3)H]azietomidate concentration from 1.8 to 150 microm increased the efficiency of incorporation into amino acids within the ion channel by 10-fold and in the ACh sites by 100-fold, consistent with higher affinity binding within the ion channel. The state dependence and subunit selectivity of [(3)H]azietomidate photolabeling are discussed in terms of the structures of the nAChR transmembrane and extracellular domains.

Identification of amino acids in the nicotinic acetylcholine receptor agonist binding site and ion channel photolabeled by 4-[(3-trifluoromethyl)-3H-diazirin-3-yl]benzoylcholine, a novel photoaffinity antagonist.

[(3)H]4-[(3-trifluoromethyl)-3H-diazirin-3-yl]benzoylcholine (TDBzcholine) was synthesized and used as a photoaffinity probe to map the orientation of an aromatic choline ester within the agonist binding sites of the Torpedo nicotinic acetylcholine receptor (nAChR). TDBzcholine acts as a nAChR competitive antagonist that binds at equilibrium with equal affinity to both agonist sites (K(D) approximately 10 microM). Upon UV irradiation (350 nm), nAChR-rich membranes equilibrated with [(3)H]TDBzcholine incorporate (3)H into the alpha, gamma, and delta subunits in an agonist-inhibitable manner. The specific residues labeled by [(3)H]TDBzcholine were determined by N-terminal sequence analysis of subunit fragments produced by enzymatic cleavage and purified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and/or reversed-phase high-performance liquid chromatography. For the alpha subunit, [(3)H]TDBzcholine photoincorporated into alphaCys-192, alphaCys-193, and alphaPro-194. For the gamma and delta subunits, [(3)H]TDBzcholine incorporated into homologous leucine residues, gammaLeu-109 and deltaLeu-111. The photolabeling of these amino acids suggests that when the antagonist TDBzcholine occupies the agonist binding sites, the Cys-192-193 disulfide and Pro-194 from the alpha subunit Segment C are oriented toward the agonist site and are in proximity to gammaLeu-109/deltaLeu-111 in Segment E, a conclusion consistent with the structure of the binding site in the molluscan acetylcholine binding protein, a soluble protein that is homologous to the nAChR extracellular domain.