Effect of single point mutations of the human tachykinin NK1 receptor on antagonist affinity

Understanding Peptide Binding in Class A G Protein-Coupled Receptors

Many physiologic processes are controlled through the activation of G protein-coupled receptors (GPCRs) by regulatory peptides, making peptide GPCRs particularly useful targets for major human diseases such as diabetes and cancer. Peptide GPCRs are also being evaluated as next-generation targets for the development of novel antiparasite agents and insecticides in veterinary medicine and agriculture. Resolution of crystal structures for several peptide GPCRs has advanced our understanding of peptide-receptor interactions and fueled interest in correlating peptide heterogeneity with receptor-binding properties. In this review, the knowledge of recently crystalized peptide-GPCR complexes, previously accumulated peptide structure-activity relationship studies, receptor mutagenesis, and sequence alignment are integrated to better understand peptide binding to the transmembrane cavity of class A GPCRs. Using SAR data, we show that peptide class A GPCRs can be divided into groups with distinct hydrophilic residues. These characteristic residues help explain the preference of a receptor to bind the C-terminal free carboxyl group, the C-terminal amidated group, or the N-terminal ammonium group of peptides.

Crystal structure of the human NK1 tachykinin receptor

The NK₁ tachykinin G-protein-coupled receptor (GPCR) binds substance P, the first neuropeptide to be discovered in mammals. Through activation of NK₁R, substance P modulates a wide variety of physiological and disease processes including nociception, inflammation, and depression. Human NK₁R (hNK₁R) modulators have shown promise in clinical trials for migraine, depression, and emesis. However, the only currently approved drugs targeting hNK₁R are inhibitors for chemotherapy-induced nausea and vomiting (CINV). To better understand the molecular basis of ligand recognition and selectivity, we solved the crystal structure of hNK₁R bound to the inhibitor L760735, a close analog of the drug aprepitant. Our crystal structure reveals the basis for antagonist interaction in the deep and narrow orthosteric pocket of the receptor. We used our structure as a template for computational docking and molecular-dynamics simulations to dissect the energetic importance of binding pocket interactions and model the binding of aprepitant. The structure of hNK₁R is a valuable tool in the further development of tachykinin receptor modulators for multiple clinical applications.

An overview on GPCRs and drug discovery: structure-based drug design and structural biology on GPCRs

G protein-coupled receptors (GPCRs) represent 50-60% of the current drug targets. There is no doubt that this family of membrane proteins plays a crucial role in drug discovery today. Classically, a number of drugs based on GPCRs have been developed for such different indications as cardiovascular, metabolic, neurodegenerative, psychiatric, and oncologic diseases. Owing to the restricted structural information on GPCRs, only limited exploration of structure-based drug design has been possible. Much effort has been dedicated to structural biology on GPCRs and very recently an X-ray structure of the beta2-adrenergic receptor was obtained. This breakthrough will certainly increase the efforts in structural biology on GPCRs and furthermore speed up and facilitate the drug discovery process.

Molecular cloning, mutations and effects of NK1 receptor antagonists reveal the human-like pharmacology of gerbil NK1 receptors

The present study investigates the pharmacology of the cloned neurokinin 1 receptor from the gerbil (gNK(1)R), a species claimed to have human-like NK(1)R (hNK(1)R) pharmacology. The amino acid sequence of NK(1)R was cloned. The hNK(1)R, rat NK(1)R (rNK(1)R), gNK(1)R and mutants of the gNK(1)R were expressed in CHO cells. The affinity and potency of NKR agonists and the NK(1)R antagonists CP99994 and RP67580 (NK(1)R-selective) and ZD6021 (NK1/2R) were assessed in vitro by monitoring [(3)H]-SarMet SP binding and substance P-evoked mobilization of intracellular Ca(2+). The gerbil foot tap (GFT) method was used to assess the potency of the antagonists in vivo. The gNK(1)R coding sequence displayed an overall 95% and 97% homology with hNK(1)R and rNK(1)R, respectively. The affinity of the NK(1)R-selective agonist (3)H-SarMet SP for human and gerbil NK(1)R was similar (2.0 and 3.1 nM) but lower for rNK(1)R (12.4 nM). The rank order potency of the agonists for NK(1)R was SP > or = ASMSP > or = NKA >>> pro7NKB in all species. The NK(1)R antagonists, ZD6021 and CP99994, had comparable affinity and potency for gerbil and human NK(1)R, but were 1000-fold less potent for rNK(1)R. In contrast, RP67580 had comparable affinity and potency for all three species. Mutations in positions 116 and 290 did not affect agonist potency at the gNK(1)R while the potency of the antagonists ZD6021 and CP99994 were markedly decreased (10-20-fold). It is concluded that gNK(1)R has similar antagonist pharmacology as the human-like orthologue and that species differences in antagonist function depend on key residues in the coding sequence and antagonist structure.

Enhancing functional production of G protein-coupled receptors in Pichia pastoris to levels required for structural studies via a single expression screen

We have optimized the expression level of 20 mammalian G protein-coupled receptors (GPCRs) in the methylotrophic yeast Pichia pastoris. We found that altering expression parameters, including growth temperature, and supplementation of the culture medium with specific GPCR ligands, histidine, and DMSO increased the amount of functional receptor, as assessed by ligand binding, by more than eightfold over standard expression conditions. Unexpectedly, we found that the overall amount of GPCR proteins expressed, in most cases, varied only marginally between standard and optimized expression conditions. Accordingly, the optimized expression conditions resulted in a marked fractional increase in the ratio of ligand binding-competent receptor to total expressed receptor. The results of this study suggest a general approach for increasing yields of functional mammalian GPCRs severalfold over standard expression conditions by using a set of optimized expression condition parameters that we have characterized for the Pichia expression system. Overall, we have more than doubled the number of GPCR targets that can be produced in our laboratories in sufficient amounts for structural studies.

Structural biology of G protein-coupled receptors

More than 60% of the current drugs are based on G protein-coupled receptors. Paradoxically, high-resolution structures are not available to facilitate rational drug design. Difficulties in expression, purification, and crystallization of these transmembrane receptors are the reasons for the low success rate. Recent individual and network-based technology development has significantly improved our knowledge of structural biology and might soon bring a major breakthrough in this area.

Mutagenesis at the human tachykinin NK2 receptor to define the binding site of a novel class of antagonists

The pharmacological profile of novel antagonists endowed with high affinity for the human tachykinin NK(2) receptor is presented. MEN13918 (Ngamma[Nalpha[Nalpha(benzo[b]thiophen-2-yl)carbonyl]-1-aminocyclohexan-1-carboxy]-d-phenylalanyl]-3-cis-aminocyclohexan-1-carboxylic-acid-N-(1S,2R)-2-aminocyclohexyl)amide trifluoroacetate salt) and MEN14268 (Nalpha[Nalpha(benzo[b]thiophen-2-yl)carbonyl)-1-aminocyclopentane-1-carboxyl]-d-phenylalanine-N-[3(morpholin-4-yl)propyl]amide trifluoroacetate salt) were more potent in blocking neurokinin A (NKA, His-Lys-Thr-Asp-Ser-Phe-Val-Gly-Leu-Met-NH(2)) induced contraction in human, which induced greater contraction in human (pK(B) 9.1 and 8.3) than rat (pK(B) 6.8 and <6) urinary bladder smooth muscle preparation in vitro. In agreement with functional data, in membrane preparations of CHO cells stably expressing the human NK(2) receptors, both MEN13918 and MEN14268 potently inhibited the binding of agonist ([(125)I]NKA, K(i) 0.2 and 2.8 nM) and antagonist ([(3)H]nepadutant, K(i) 0.1 and 2.2 nM, [(3)H]SR48968 K(i) 0.4 and 6.9 nM) radioligands. Using site-directed mutagenesis and radioligands binding we identified six residues in the transmembrane (TM) helices that are critical determinants for the studied antagonists affinity. To visualize these experimental findings, we constructed a homology model based on the X-ray crystal structure of bovine rhodopsin and suggested a possible binding mode of these newly discovered antagonist ligands to the human tackykinin NK(2) receptor. Both MEN13918 and MEN14268 bind amongst TM4 (Cys167Gly), TM5 (Tyr206Ala), TM6 (Tyr266Ala, Phe270Ala), and TM7 (Tyr289Phe, Tyr289Thr). MEN13918 and MEN14268 diverging binding profile at Y289 mutations in TM7 (Tyr289Phe, Tyr289Thr) suggests a relation of their different chemical moieties with this residue. Moreover, the different influence on binding of these two ligands by mutations located deep along the inner side of TM6 (Phe270Ala, Tyr266Ala, Trp263Ala) indicates a nonequivalent positioning, although occupying the same binding crevice. Furthermore, binding data indicate the Ile202Phe mutation, which mimics the wild-type rat NK(2) receptor sequence, as a species selectivity determinant. In summary, data with mutant receptors describe, for these new tachykinin NK(2) receptor antagonists, a binding site which is partially overlapping either with that of the cyclized peptide antagonist nepadutant (cyclo-[[Asn(beta-d-GlcNAc)-Asp-Trp-Phe-Dpr-Leu]cyclo(2beta-5beta)] or the nonpeptide antagonist SR48968 ((S)-N-methyl-N[4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophenyl)butyl]benzamide).

Synthesis and biological evaluation of halogenated naphthyridone carboxamides as potential ligands for in vivo imaging studies of substance P receptors

With the aim of developing new radioligands for in vivo studies of substance P receptors using positron emission tomography or single photon emission computed tomography, 2- and 3-halo naphthyridone-6-carboxamide derivatives were synthesized. Their affinities toward the target receptors were evaluated on CHO cells and compared to the unsubstituted analogue EP 00652218 (IC(50) = 100 nM +/- 20). The IC(50) value was not altered in the case of 2-chloro compound 1 (IC(50) = 100 nM +/- 15) and only slightly reduced for the 2-fluoro and -iodo analogues 6 and 8 (IC(50) = 500 nM +/- 80). A drastic reduction in binding (IC(50) > 1000 nM) was observed for the halogenated compounds 2-5, 7, and 9.

Semliki Forest virus vectors for rapid and high-level expression of integral membrane proteins

Semliki Forest virus (SFV) vectors have been applied for the expression of recombinant integral membrane proteins in a wide range of mammalian host cells. More than 50 G protein-coupled receptors (GPCRs), several ion channels and other types of transmembrane or membrane-associated proteins have been expressed at high levels. The establishment of large-scale SFV technology has facilitated the production of large quantities of recombinant receptors, which have then been subjected to drug screening programs and structure-function studies on purified receptors. The recent Membrane Protein Network (MePNet) structural genomics initiative, where 100 GPCRs are overexpressed from SFV vectors, will further provide new methods and technologies for expression, solubilization, purification and crystallization of GPCRs.

The dynamics of bone marrow stromal cells in the proliferation of multipotent hematopoietic progenitors by substance P: an understanding of the effects of a neurotransmitter on the differentiating hematopoietic stem cell

Communication within the hematopoietic-neuroendocrine-immune axis is partly mediated by neurotransmitters (e.g. substance P, SP) and cytokines. SP mediates neuromodulation partly through the stimulation of bone marrow (BM) progenitors. This study shows that SP, through the neurokinin-1 receptor, stimulates the proliferation of primitive hematopoietic progenitors: cobblestone-forming cells (CAFC, CD34+). This effect is optimal when macrophage is included within the fibroblast support. Indirect induction of IL-1 could be important in the proliferation of CAFC colonies by SP. Phenotypic and functional studies suggest that SP might directly interact with the CD34+/CD45(dim) population. These studies indicate that SP can initiate a cascade of biological responses in the BM stroma and stem cells to stimulate hematopoiesis.

The neurokinin-1 and neurokinin-2 receptor binding sites of MDL103,392 differ

Several small molecule non-peptide antagonists of the NK-1 and NK-2 receptors have been developed. Mutational analysis of the receptor protein sequence has led to the conclusion that the binding site for these non-peptide antagonists lies within the bundle created by transmembrane domains IV-VII of the receptor and differs from the binding sites of peptide agonists and antagonists. The current investigation uses site-directed mutagenesis of the NK-1 and NK-2 receptors to elucidate the amino acids that are important for binding and functional activity of the first potent dual NK-1/NK-2 antagonist MDL103,392. The amino acids found to be important for MDL103,392 binding to the NK-1 receptor are Gln-165, His-197, Leu-203, Ile-204, Phe-264, His-265 and Tyr-272. The amino acids found to be important for MDL103,392 binding to the NK-2 receptor are Gln-166, His-198, Tyr-266 and Tyr-289. While residues in transmembrane (TM) domains IV and V are important in both receptors (Gln-165/166 and His-197/198), residues in TM V and VI are more important for the NK-1 receptor and residues in TM VII play a more important role in the NK-2 receptor. These data are the first report of the analysis of the binding site of a dual tachykinin receptor antagonist and indicate that a single compound (MDL103,392) binds to each receptor in a different manner despite there being a high degree of homology in the transmembrane bundles. In addition, this is the first report in which a model for the binding of a non-peptide antagonist to the NK-2 receptor is proposed.

Alphaviruses as tools in neurobiology and gene therapy

The broad host range and superior infectivity of alphaviruses have encouraged the development of efficient expression vectors for Semliki Forest virus (SFV) and Sindbis virus (SIN). The generation of high-titer recombinant alphavirus stocks has allowed high-level expression of a multitude of nuclear, cytoplasmic, membrane-associated and secreted proteins in a variety of different cell lines and primary cell cultures. Despite the viral cytopathogenic effects, functional assays on recombinant proteins are possible for a time-period of at least 24 hours post-infection. The high percentage (80-95%) of primary neurons infected with SFV has allowed localization and functional studies of recombinant proteins in these primary cell cultures. Through multiple infection studies the interaction of receptor and G protein subunits has become feasible. Establishment of efficient scale-up procedures has allowed production of large quantities of recombinant protein. Potential gene therapy applications of alphaviruses could be demonstrated by injection of recombinant SIN particles expressing beta-galactosidase into mouse brain. Tissue/cell specific infection has been achieved by introduction of an IgG-binding domain of protein A domain into one of the spike proteins of SIN. This enabled efficient targeting of infection to human lymphoblastoid cells.

Extracts and constituents of Hypericum perforatum inhibit the binding of various ligands to recombinant receptors expressed with the Semliki Forest virus system

Extracts, fractions and constituents of Hypericum perforatum were studied for in vitro receptor binding with various ligands to recombinant CNS receptors expressed with the Semliki Forest virus expression system. For this purpose we have prepared membranes of CHO cells with high density of several opioid, serotonin, estrogen, histamine, GABAA, neurokinin and metabotropic glutamate receptors, respectively. A lipophilic Hypericum fraction revealed relatively potent inhibition to the binding of the mu-, delta- and kappa-opioid and the 5-HT6 and 5-HT7 receptors. Moreover, Hypericum constituents such as the naphthodianthrones, hypericin and pseudohypericin, and the phloroglucinole hyperforin inhibited both binding to the opioid and serotonin receptors in the lower micromolar range. Estrogen binding was 50% inhibited by the biflavonoid I3,II8-biapigenin at micromolar concentration. The lipophilic Hypericum fraction provided a less potent inhibition of the neurokinin-1 receptor binding compared to the opioid and serotonin receptors. A total ethanolic Hypericum extract potently inhibited GABAA binding at approximately 3 micrograms/ml. This inhibition is however not specific to Hypericum, since extracts of plants like Valeriana officinalis and Passiflora incarnata showed similar inhibitions. Binding to neither histamine nor metabotropic glutamate receptors was affected by Hypericum extracts. These results support the hypothesis that several active constituents of Hypericum might in a synergistic way contribute to its antidepressant effect in the central nervous system.