Identification of methionine134 and alanine146 in the second transmembrane segment of the human tachykinin NK3 receptor as reduces involved in species-selective binding to SR 48968

Mapping the binding pocket of a novel, high-affinity, slow dissociating tachykinin NK3 receptor antagonist: biochemical and electrophysiological characterization

The NK3 receptor is a GPCR that is prominently expressed in limbic areas of the brain, many of which have been implicated in schizophrenia. Phase II clinical trials in schizophrenia with two selective NK3 antagonists (osanetant and talnetant) have demonstrated significant improvement in positive symptoms. The objective of this study was to characterize the properties of a novel dual NK2/NK3 antagonist, RO5328673. [(3)H]RO5328673 bound to a single saturable site on hNK2, hNK3 and gpNK3 with high-affinity. RO5328673 acted as an insurmountable antagonist at both human and guinea-pig NK3 receptors in the [(3)H]IP accumulation assay. In binding kinetic analyses, [(3)H]RO5328673 had fast association and dissociation rates at hNK2 while it had a fast association rate and a remarkably slow dissociation rate at gp and hNK3. In electrophysiological recordings of gp SNpc, RO5328673 inhibited the senktide-induced potentiation of spontaneous activity of dopaminergic neurons with an insurmountable mechanism of action. RO5328673 exhibited in-vivo activity in gerbils, robustly reversing the senktide-induced locomotor activity. The TM2 residue gpNK3-A114(2.58) (threonine in all other species) was identified as the critical residue for the RO5328673's slower dissociation kinetics and stronger insurmountable mode of antagonism in the guinea-pig as compared to hNK3-T139(2.58). Using site-directed mutagenesis, [(3)H]RO5328673 binding and rhodopsin-based modeling, the important molecular determinants of the RO5328673-binding pocket of hNK3 were determined. A comparison of the RO5328673-binding pocket with that of osanetant showed that two antagonists have similar contact sides on hNK3 binding crevice except for three mutations V95L(1.42), Y247W(5.38), V255I(5.46), which behaved differently between interacting modes of two antagonists in hNK3.

Progress in the development of neurokinin 3 modulators for the treatment of schizophrenia: molecule development and clinical progress

The neuropeptide NK3 receptor is expressed almost exclusively within the mammalian nervous system and its localization is commensurate with a role in modulating central monoaminergic neurotransmission. Following on from our previous work we review the rationale for NK3 receptor antagonists as wide spectrum antipsychotics and the recent scientific and patent literature that has highlighted new chemical strategies to identify selective NK3 and dual activity NK1/3 receptor ligands for the putative treatment of schizophrenia. We discuss the emerging structural biology and its use in the design of molecules with increased structural diversity and predictable receptor pharmacology. Particular attention is paid to the progress in improving ligand drug-like properties. The status of imaging and the development of translational technologies in the neurokinin field are also discussed. Finally, we summarize the available clinical information on the compounds that have progressed into psychiatric patient populations and evaluate the potential therapeutic utility of NK3 receptor targeted ligands.

Characterization of RO4583298 as a novel potent, dual antagonist with in vivo activity at tachykinin NK₁ and NK₃ receptors

BACKGROUND AND PURPOSE

Clinical results of osanetant and talnetant (selective-NK₃ antagonists) indicate that blocking the NK₃ receptor could be beneficial for the treatment of schizophrenia. The objective of this study was to characterize the in vitro and in vivo properties of a novel dual NK₁/NK₃ antagonist, RO4583298 (2-phenyl-N-(pyridin-3-yl)-N-methylisobutyramide derivative).

EXPERIMENTAL APPROACH

RO4583298 in vitro pharmacology was investigated using radioligand binding ([³H]-SP, [³H]-osanetant, [³H]-senktide), [³H]-inositol-phosphate accumulation Schild analysis (SP- or [MePhe⁷]-NKB-induced) and electrophysiological studies in guinea-pig substantia nigra pars compacta (SNpc). The in vivo activity of RO4583298 was assessed using reversal of GR73632-induced foot tapping in gerbils (GFT; NK₁) and senktide-induced tail whips in mice (MTW; NK₃).

KEY RESULTS

RO4583298 has a high-affinity for NK₁ (human and gerbil) and NK₃ (human, cynomolgus monkey, gerbil and guinea-pig) receptors and behaves as a pseudo-irreversible antagonist. Unusually it binds with high-affinity to mouse and rat NK₃, yet with a partial non-competitive mode of antagonism. In guinea-pig SNpc, RO4583298 inhibited the senktide-induced potentiation of spontaneous activity of dopaminergic neurones with an apparent non-competitive mechanism of action. RO4583298 (p.o.) robustly blocked the GFT response, and inhibited the MTW.

CONCLUSIONS AND IMPLICATIONS

RO4583298 is a high-affinity, non-competitive, long-acting in vivo NK₁/NK₃ antagonist; hence providing a useful in vitro and in vivo pharmacological tool to investigate the roles of NK₁ and NK₃ receptors in psychiatric disorders.

Characterization of species-related differences in the pharmacology of tachykinin NK receptors 1, 2 and 3

Tachykinin NK receptors (NKRs) differ to a large degree among species with respect to their affinities for small molecule antagonists. The aims of the present study were to clone NKRs from gerbil (NK2R and NK3R) and dog (NK1R, NK2R and NK3R) in which the sequence was previously unknown and to investigate the potency of several NKR antagonists at all known human, dog, gerbil and rat NKRs. The NKR protein coding sequences were cloned and expressed in CHO cells. The inhibitory concentrations of selective and non-selective NKR antagonists were determined by inhibition of agonist-induced mobilization of intracellular Ca2+. Receptor homology models were constructed based on the rhodopsin crystal structure to investigate and identify the antagonist binding sites and interaction points in the transmembrane (TM) regions of the NKRs. Data collected using the cloned dog NK1R confirmed that the dog NK1R displays similar pharmacology as the human and the gerbil NK1R, but differs greatly from the mouse and the rat NK1R. Despite species-related amino acid (AA) differences located close to the antagonist binding pocket of the NK2R, they did not affect the potency of the antagonists ZD6021 and saredutant. Two AA differences located close to the antagonist binding site of NK3R likely influence the NK3R antagonist potency, explaining the 3-10-fold decrease in potency observed for the rat NK3R. For the first time, detailed pharmacological experiments in vitro with cloned NKRs demonstrate that not only human, but also dog and gerbil NKR displays similar antagonist pharmacology while rat diverges significantly with respect to NK1R and NK3R.

Opinion: NK3 receptor antagonists: the next generation of antipsychotics?

Although current antipsychotic drugs are effective at treating the psychotic (positive) symptoms of schizophrenia, they have one or more serious side effects, including extrapyramidal symptoms, weight gain, cardiovascular liabilities and type II diabetes. However, recent data from clinical trials of selective neurokinin 3 (NK(3)) receptor antagonists in schizophrenia - osanetant and talnetant - have shown significant improvement in positive symptoms, with no major side-effects reported as yet. Here we discuss the preclinical and clinical evidence that indicates that NK(3) receptor antagonists might represent a new approach to the treatment of schizophrenia and possibly other neuropsychiatric disorders.

Molecular and pharmacological characterization of the murine tachykinin NK(3) receptor

Starting with a partial sequence from Genbank, polymerase chain reaction (PCR) was utilized to isolate the full-length cDNA for NK(3) receptor from mouse brain. The murine NK(3) receptor has a predicted sequence of 452 amino acids, sharing 96% and 86% identity to the rat and human NK(3) receptors, respectively. Binding affinities and functional potencies of tachykinin receptor agonists were similar in HEK (human embryonic kidney) 293 cells expressing murine NK(3) receptor and human NK(3) receptor, although substance P and neurokinin A were more potent stimulators of Ca(2+) mobilization in murine NK(3) receptor cells. NK(3) receptor-selective antagonists from two structural classes, had 10- to 100-fold lower binding affinities for murine NK(3) receptor compared to human NK(3) receptor, and about 5- to 10-fold reduced potency in the murine NK(3) receptor functional assay. The results demonstrate species differences in the potencies of tachykinin receptor antagonists in murine and human NK(3) receptors, and the lower potencies in the former should be taken into consideration when using murine disease models.

Molecular and pharmacological characterization of a functional tachykinin NK3 receptor cloned from the rabbit iris sphincter muscle

1. A functional tachykinin NK3 receptor was cloned from the rabbit iris sphincter muscle and its distribution investigated in ocular tissues. 2. Standard polymerase chain reaction (PCR) techniques were used to clone a full length rabbit NK3 receptor cDNA consisting of 1404 nucleotides. This cDNA encoded a protein of 467 amino acids with 91 and 87% homology to the human and rat NK3 receptors respectively. 3. In CHO-K1 cells transiently expressing the recombinant rabbit NK3 receptor, the relative order of potency of NKB>>NKA>/=SP to displace [125I]-[MePhe7]-NKB binding and to increase intracellular calcium, together with the high affinity of NK3 selective agonists (e.g. senktide, [MePhe7]-NKB) and antagonists (e.g. SR 142801, SB 223412) in both assays was consistent with NK3 receptor pharmacology. In binding and functional experiments, agonist concentration response curves were shallow (0.7 - 0.8), suggesting the possibility of multiple affinity states of the receptor. 4. Quantitative real time PCR analysis revealed highest expression of rabbit NK3 receptor mRNA in iris sphincter muscle, lower expression in retina and iris dilator muscle, and no expression in lens and cornea. In situ hybridization histochemistry revealed discrete specific localization of NK3 receptor mRNA in the iris muscle and associated ciliary processes. Discrete specific labelling of NK3 receptors with the selective NK3 receptor agonist [125I]-[MePhe7]-NKB was also observed in the ciliary processes using autoradiography. 5. Our study reveals a high molecular similarity between rabbit and human NK3 receptor mRNAs, as predicted from previous pharmacological studies, and provide the first evidence that NK3 receptors are precisely located on ciliary processes in the rabbit eye. In addition, there could be two affinity states of the receptor which may correspond to the typical and 'atypical' NK3 receptor subtypes previously reported.

Bacterial expression and characterization of human recombinant apolipoprotein(a) kringle IV type 9

Elevated plasma lipoprotein(a) [Lp(a)] is an independent risk factor for several vascular diseases. Lp(a) particles are generated through the formation of a disulfide bond between Cys4057 of kringle IV type 9, (KIVt9), of the multikringle apolipoprotein(a) [apo(a)] and a cysteine in apoB-100 low-density lipoprotein (LDL). To better understand this interaction, we have expressed and purified KIVt9 from Escherichia coli as a His-Tag fusionprotein. Dithiothreitol (DTT)-treated purified KIVt9 migrated as a single approximately 17. 3-kDa band on SDS-PAGE gels. Without DTT, an additional band twice the molecular weight of KIVt9 was observed. The double-size band presumably resulted from dimerization of individual kringles, through their unpaired cysteine residues, since a mutation Cys4057 --> Ser ([Ser4057]KIVt9) abolished dimer formation. Using a gel-shift assay, we showed that KIVt9 could couple to 14-amino-acid apoB-100 synthetic peptides (apoB3732-3745 and apoB4319-4332) containing Cys3734 or Cys4326. Both of these apoB-100 cysteines have been reported to associate with apo(a) to generate Lp(a). In the presence of either apoB-100 peptide, KIVt9 was shifted to a higher molecular weight that was consistent with the covalent addition of a 1.2-kDa apoB-100 peptide. Identical apoB-100 peptides in which the cysteine residues were replaced by alanine ([Ala3734]apoB3732-3745 and [Ala4326]apoB4319-4332) had no effect in the gel-shift assay. Furthermore, [Ser4057]KIVt9 did not covalently interact with apoB3732-3745 or apoB4319-4332. These results indicated that KIVt9 couples to the Cys-apoB-100 peptides through a disulfide linkage. This system may be suitable for further investigating the apo(a)/apoB-100 coupling reaction and the structure of KIVt9 through X-ray crystallographic studies.

The unpredicted high affinities of a large number of naturally occurring tachykinins for chimeric NK1/NK3 receptors suggest a role for an inhibitory domain in determining receptor specificity

Three chimeric receptors were constructed by exchanging exon sequences between human NK1 and NK3 receptor genes. The resulting chimeric receptors not only retained high affinities for their natural ligands substance P and neurokinin B but also exhibited surprisingly high affinities for other naturally occurring tachykinins including neurokinin A, neuropeptide K, neuropeptide gamma, eledoisin, kassinin, physalaemin, and phyllomedusin. In contrast, these chimeric receptors displayed a wide range of variability in their affinities for non-naturally occurring ligands including selective agonists and antagonists of NK1, NK2, and NK3 receptors. Since the only common feature among these naturally occurring neurokinin peptides is the conserved C-terminal sequences, our data suggest that these conserved sequences must play the major role in conferring high affinity binding to the chimeric receptors. To explain the apparently "improved" affinities of these naturally occurring ligands for the chimeric receptors as compared with their affinities for the parent NK1 and NK3 receptors, we are proposing that certain inhibitory domains that are present in the NK1 and/or NK3 receptors are compromised in these chimeric receptors. Upon disruption of these inhibitory domains during the formation of chimeras, the naturally occurring ligands can interact more favorably with chimeric receptors through their conserved C-terminal sequences. Based on this hypothesis, the binding affinities of natural tachykinin ligands may be largely determined by their conserved C-terminal sequences, whereas receptor selectivities of these ligands are influenced more by the presence or absence of inhibitory domains rather than specific binding domains on their target receptors.

Tachykinin receptors in gastrointestinal motility

For a long time research on the action of TKs on gastrointestinal tissue has been demonstrating the importance of the TKs as non-cholinergic stimulators of motility in most parts of the mammalian gastrointestinal tract. The past years witnessed the development of TK agonists and antagonists selective for the various receptor types, which prompted a wealth of new insight into the pharmacology and molecular biology of the TK receptors. This knowledge now allows a more specific elucidation of the role of TKs and their receptors in the various aspects of gastrointestinal motility, not only in normal tissue but also under pathological conditions.

Locating ligand-binding sites in 7TM receptors by protein engineering

Over the past year, mutational analysis of peptide receptors has started to change our understanding of the interaction between G protein coupled receptors and their ligands, an area previously almost totally dominated by results from studies of monoamine receptors. A picture is currently emerging, in which small ligands appear to bind in three (more or less) overlapping ligand-binding pockets in between the transmembrane segments. In contrast, contact residues for peptide and protein ligands have mainly been found in exterior regions of peptide and protein receptors. It is also becoming increasingly clear that agonists and antagonists may interact in vastly different manners, even though they are competitive ligands for a common receptor.