A novel tachykinin-related peptide receptor. Sequence, genomic organization, and functional analysis.

Structurally tachykinin-related peptides have been isolated from various invertebrate species and shown to exhibit their biological activities through a G-protein-coupled receptor (GPCR) for a tachykinin-related peptide. In this paper, we report the identification of a novel tachykinin-related peptide receptor, the urechistachykinin receptor (UTKR) from the echiuroid worm, Urechis unitinctus. The deduced UTKR precursor includes seven transmembrane domains and typical sites for mammalian tachykinin receptors and invertebrate tachykinin-related peptide receptors. A functional analysis of the UTKR expressed in Xenopus oocytes demonstrated that UTKR, like tachykinin receptors and tachykinin-related peptide receptors, activates calcium-dependent signal transduction upon binding to its endogenous ligands, urechistachykinins (Uru-TKs) I-V and VII, which were isolated as Urechis tachykinin-related peptides from the nervous tissue of the Urechis unitinctus in our previous study. UTKR responded to all Uru-TKs equivalently, showing that UTKR possesses no selective affinity with Uru-TKs. In contrast, UTKR was not activated by substance P or an Uru-TK analog containing a C-terminal Met-NH2 instead of Arg-NH2. Furthermore, the genomic analysis revealed that the UTKR gene, like mammalian tachykinin receptor genes, consists of five exons interrupted by four introns, and all the intron-inserted positions are completely compatible with those of mammalian tachykinin receptor genes. These results suggest that mammalian tachykinin receptors and invertebrate tachykinin-related peptide receptors were evolved from a common ancestral GPCR gene. This is the first identification of an invertebrate tachykinin-related peptide receptor from other species than insects and also of the genomic structure of a tachykinin-related peptide receptor gene.     

Direct linkage of three tachykinin receptors to stimulation of both phosphatidylinositol hydrolysis and cyclic AMP cascades in transfected Chinese hamster ovary cells.

The mammalian tachykinin system consists of three distinct peptides, substance P, substance K, and neuromedin K, and possesses three corresponding receptors. In this investigation we examined intracellular signal transduction of the individual tachykinin receptors by transfection and stable expression of these receptor cDNAs in Chinese hamster ovary cells. The three receptors commonly showed a rapid and marked stimulation in both phosphatidylinositol (PI) hydrolysis and cyclic AMP formation in response to tachykinin interaction. Direct linkage of the three receptors to both phospholipase C and adenylate cyclase was evidenced by the finding that tachykinin, added together with GTP, activated these enzyme activities in membrane preparations derived from tachykinin receptor-expressing cells. The stimulation of cyclic AMP formation was less efficient than that of PI hydrolysis in receptor-expressing cells as well as their membrane preparations (about 1 order of magnitude difference in the effective peptide concentrations). However, the stimulatory responses of the PI hydrolysis and cyclic AMP formation in both receptor-expressing cells and their membrane preparations were induced in complete agreement with the tachykinin binding selectivity of each subtype of the receptors. This investigation demonstrated unequivocally that the tachykinin receptors have the potential to couple directly to both phospholipase C and adenylate cyclase and to stimulate PI hydrolysis and cyclic AMP formation.     

Diversity in mammalian tachykinin peptidergic neurons: multiple peptides, receptors, and regulatory mechanisms

The tachykinins comprise a family of closely related peptides that participate in the regulation of diverse biological processes. The tachykinin peptides substance P, neurokinin A, neurokinin A(3-10), neuropeptide K, and neuropeptide gamma are produced from a single preprotachykinin gene as a result of differential RNA splicing and differential posttranslational processing. Another tachykinin, neurokinin B, is produced from a separate preprotachykinin gene. These preprotachykinin mRNAs and peptide products are differentially distributed throughout the nervous system. Three distinct G protein-coupled tachykinin receptors exist for these tachykinin peptides. The three receptors interact differentially with the tachykinin peptides and are uniquely distributed throughout the nervous system. The NK-1 receptor preferentially interacts with substance P, the NK-2 receptor prefers neurokinin A, neuropeptide K, and neuropeptide gamma, and the NK-3 receptor interacts best with neurokinin B. Examples of the roles of tachykinin peptidergic neuronal systems are taken from the spinal cord sensory system and the nigrostriatal extrapyramidal motor system. Analysis of the functional significance of multiple tachykinin peptide systems, receptor-second messenger coupling mechanisms, and developmental and regulatory mechanisms underlying peptide mRNA and receptor expression represent areas of current and future investigation.     

Comparison of the structures of beta amyloid peptide (25-35) and substance P in trifluoroethanol/water solution

The three-dimensional structure of beta amyloid peptide (25-35) in aqueous solution with 50% (vol/vol) 2,2,2-trifluoroethanol was determined by NMR spectroscopy. Beta amyloid peptide(Abeta) is the major component of senile plaques found in the brain of patient of Alzheimer's disease. Abeta25-35 is biologically active fragment of Abeta and exhibits some sequence homology with the tachykinin family. In this study, we present the structural similarity between Abeta25-35 and substance P which is a member of tachykinin family in order to examine the possibility of sharing pathways mediated by tachykinin receptors. Both peptides have alpha-helical structures in their C-terminal regions and aromatic rings or hydrophobic side chains in the center of the helix protrude outside. These conformational features are expected to be the key for the interaction with the receptors.     

The primary structures and myotropic activities of two tachykinins isolated from the African clawed frog,Xenopus laevis

Two peptides with limited structural similarity to mammalian substance P (SP) and neurokinin A (NKA) have been isolated from extracts of the intestine of the African clawed frog (Xenopus laevis). The primary structure of an SP-like peptide was established as: Lys-Pro-Arg-Pro-Asp-Gln-Phe-Tyr-Gly-Leu-Met.NH(2), which is identical to the previously characterized peptide, bufokinin isolated from the toad Bufo marinus. The primary structure of an NKA-related peptide was established as Thr-Leu-Thr-Thr-Gly-Lys-Asp-Phe-Val-Gly-Leu-Met.NH(2). Only the five amino acids at the C-terminal region of the peptide are identical to mammalian NKA whereas the N-terminal region shows no structural similarity to previously characterized tachykinins. Immunohistochemical investigations of the gut wall revealed a dense network of nerve fibres and nerve cell bodies containing SP/NKA-like substances. The myotropic effects of the Xenopus tachykinins were compared with the contractile effect of mammalian SP and NKA on isolated strips of circular smooth muscle from Xenopus stomach. No significant differences in potencies (-log EC(50)) or in intrinsic activities were observed between the Xenopus and mammalian peptides. The potencies for the Xenopus SP-like (8.49+/-0.15) and the NKA-like peptide (8.12+/-0.06) were similar suggesting that the amino acid sequence at the N-terminal region of the tachykinins is not important in activating the tachykinin receptors in Xenopus gastric smooth muscle. The maximum response to Xenopus SP (alpha=0.59+/-0.06) was significantly lower than to the NKA-like peptide (alpha=1.0) suggesting a more effective interaction of the NKA-like peptide with the tachykinin receptor(s) in Xenopus stomach.     

Expression and functional characterization of a Drosophila neuropeptide precursor with homology to mammalian preprotachykinin A

Peptides structurally related to mammalian tachykinins have recently been isolated from the brain and intestine of several insect species, where they are believed to function as both neuromodulators and hormones. Further evidence for the signaling role of insect tachykinin-related peptides was provided by the cloning and characterization of cDNAs for two tachykinin receptors from Drosophila melanogaster. However, no endogenous ligand has been isolated for the Drosophila tachykinin receptors to date. Analysis of the Drosophila genome allowed us to identify a putative tachykinin-related peptide prohormone (prepro-DTK) gene. A 1.5-kilobase pair cDNA amplified from a Drosophila head cDNA library contained an 870-base pair open reading frame, which encodes five novel Drosophila tachykinin-related peptides (called DTK peptides) with conserved C-terminal FXGXR-amide motifs common to other insect tachykinin-related peptides. The tachykinin-related peptide prohormone gene (Dtk) is both expressed and post-translationally processed in larval and adult midgut endocrine cells and in the central nervous system, with midgut expression starting at stage 17 of embryogenesis. The predicted Drosophila tachykinin peptides have potent stimulatory effects on the contractions of insect gut. These data provide additional evidence for the conservation of both the structure and function of the tachykinin peptides in the brain and gut during the course of evolution.     

Tachykinins and conformational aspects of their interactions with receptors]

Several peptides of the tachykinin family are reviewed. Special attention is spared to mammalian tachykinins as peptide neurotransmitters. Conformational possibilities of the tachykinins are considered in connection with their ability to interact with specific receptors. The results of theoretical and experimental studies suggest a significant identity of spatial structures of the tachykinins and explain the absence of the strict specificity in tachykinin-receptor interactions.     

Highly selective agonists for substance P receptor subtypes.

The existence of a third tachykinin receptor (SP-N) in the mammalian nervous system was demonstrated by development of highly selective agonists. Systematic N-methylation of individual peptide bonds in the C-terminal hexapeptide of substance P gave rise to agonists which specifically act on different receptor subtypes. The most selective analog of this series, succinyl-[Asp6,Me-Phe8]SP6-11, elicits half-maximal contraction of the guinea pig ileum through the neuronal SP-N receptor at a concentration of 0.5 nM. At least 60,000-fold higher concentrations of this peptide are required to stimulate the other two tachykinin receptors (SP-P and SP-E). The action of selective SP-N agonists in the guinea pig ileum is antagonized by opioid peptides, suggesting a functional counteraction between opiate and SP-N receptors. These results indicate that the tachykinin receptors are distinct entities which may mediate different physiological functions.     

The polyprotein nature of substance P precursors

Substance P and related tachykinin peptides probably act as neurotransmitters or modulators of neurotransmission, and regulate biological processes as diverse as salivary secretion and transmission of pain signals. Substance P peptide sequences are expressed in three distinct mRNAs that are generated from one gene by differential RNA splicing. In addition to substance P, as many as three other tachykinin peptides can be generated from the polyprotein precursors by differential posttranslational processing. Three tachykinin receptor subtypes have been extensively characterized which differentially interact with the naturally occurring tachykinin peptides. Therefore, the generation of diversity of tachykinin peptides results from differential precursor RNA splicing and differential posttranslational processing. The specificity of peptide responses is the result of selective receptor subtype expression.     

Bufokinin: immunoreactivity, receptor localization and actions in toad intestine and mesenteric circulation

In this study, we have mapped the immunoreactivity and the binding sites for bufokinin, a tachykinin peptide from the toad intestine. Dense bufokinin-immunoreactive fibers were present at the myenteric plexus, but no cell bodies were stained, suggesting an extrinsic origin. Bufokinin nerve fibers were also associated with submucosal blood vessels and mesenteric arteries. Autoradiographic binding sites for [(125)I]Bolton-Hunter-bufokinin were densely localized over the intestinal circular and longitudinal muscle, submucosal blood vessels and the endothelium of mesenteric arteries. Mesenteric veins had minimal immunoreactivity and binding sites. In the anesthetized toad, topical application of bufokinin onto the mesentery caused a 2.7-fold increase in arterial blood flow, observed using intravital microscopy. This study supports a role for bufokinin as an endogenous spasmogen and hemodynamic regulator in the toad intestine.     

Tachykinins and tachykinin receptors: a growing family

The peptides of the tachykinin family are widely distributed within the mammalian peripheral and central nervous systems and play a well-recognized role as excitatory neurotransmitters. Currently, the concept that tachykinins act exclusively as neuropeptides is being challenged, since the best known members of the family, substance P, neurokinin A and neurokinin B, are also present in non-neuronal cells and in non-innervated tissues. Moreover, the recently cloned mammalian tachykinins hemokinin-1 and endokinins are primarily expressed in non-neuronal cells, suggesting a widespread distribution and important role for these peptides as intercellular signaling molecules. The biological actions of tachykinins are mediated through three types of receptors denoted NK(1), NK(2) and NK(3) that belong to the family of G protein-coupled receptors. The identification of additional tachykinins has reopened the debate of whether more tachykinin receptors exist. In this review, we summarize the current knowledge of tachykinins and their receptors.     

Amphibian tachykinin precursor

The precursor of amphibian tachykinin has not been found although more than 30 tachykinins have been isolated from amphibians since 1964. In this report, two tachykinin-like peptides are identified from the skin secretions of the frog, Odorrana grahami. Their amino acid sequences are DDTEDLANKFIGLM-NH(2) (named tachykinin OG1) and DDASDRAKKFYGLM-NH(2) that is the same with ranamargarin found in Rana margaretae, respectively, with a conserved FXGLM-NH(2) C-terminal consensus motif. By cDNA cloning, their precursors were screened from the skin cDNA library of O. grahami. The precursors are composed of 61 amino acid (aa) residues including a signal peptide followed by an acidic spacer peptide and one copy of mature tachykinin-like peptide. Their overall structure is different from structures of other tachykinin precursors such as human protachykinin 1 precursor containing 143 aa including one copy of substance P (SP) and neurokinin A (NKA), and ascidian tachykinin 1 precursor containing 164 aa including two copies of tachykinin-like peptides. The current results demonstrate that the biosynthesis mode of tachykinins in amphibians is different from other animals.     

The smooth muscle pharmacology of maximakinin, a receptor-selective, bradykinin-related nonadecapeptide from the venom of the Chinese toad, Bombina maxima

Structural homologues of vertebrate regulatory peptides found in defensive skin secretions of anuran amphibians often display enhanced bioactivity and receptor binding when compared with endogenous mammalian peptide ligands. Maximakinin, a novel N-terminally extended bradykinin (DLPKINRKGPRPPGFSPFR) from the skin venom of a Chinese toad (Bombina maxima), displays such activity enhancement when compared with bradykinin but is additionally highly selective for mammalian arterial smooth muscle bradykinin receptors displaying a 50-fold increase in molar potency in this smooth muscle type. In contrast, a 100-fold decrease in molar potency was observed at bradykinin receptors in intestinal and uterine smooth muscle preparations. Maximakinin has thus evolved as a "smart" defensive weapon in the toad with receptor/tissue selective targeting. Natural selection of amphibian skin venom peptides for antipredator defence, through inter-species delivery by an exogenous secretory mode, produces subtle structural stabilisation modifications that can potentially provide new insights for the design of selectively targeted peptide therapeutics.     

Identification of multiple urechistachykinin peptides, gene expression, pharmacological activity, and detection using mass spectrometric analyses

Urechistachykinin I and II (Uru-TK I and II) are invertebrate tachykinin-related peptides (TRPs), which have been isolated from echiuroid worms. The cDNA sequence encoding the Uru-TK I and II revealed that the precursor also encoded five TRP-like peptides. Here, we report the characterization of these Uru-TK-like peptides named as Uru-TK III-VII. Northern and Southern blot analyses demonstrated that Uru-TK mRNA is localized in nerve tissue. In addition, the presence of the Uru-TK-like peptides as matured forms in the nerve tissue was detected by mass spectrometric analysis, and identified these peptides were shown to exhibit a contractile activity on cockroach hindgut that was as potent as that of Uru-TK II. Furthermore, synthetic Uru-TK-like peptide analogs which contained Met-NH₂ instead of Arg-NH₂ at their C-termini were shown to possess a potential to bind to a mammalian tachykinin receptor, indicating that Uru-TK-like peptides are likely to correspond to vertebrate tachykinins, except for the difference at the C-terminal residue. These findings show that Uru-TK-like peptides are essentially equivalent to Uru-TK I and II, leading to the proposal that Uru-TK-like peptides play an essential role as invertebrate tachykinin neuropeptides.     

Structure–activity relationship study of tachykinin peptides for the development of novel neurokinin-3 receptor selective agonists

Neurokinin B (NKB) is a potential regulator of pulsatile gonadotropin-releasing hormone (GnRH) secretion via activation of the neurokinin-3 receptor (NK3R). NKB with the consensus sequence of the tachykinin peptide family also binds to other tachykinin receptors [neurokinin-1 receptor (NK1R) and neurokinin-2 receptor (NK2R)] with low selectivity. In order to identify the structural requirements for the development of novel potent and selective NK3R agonists, a structure–activity relationship (SAR) study of [MePhe⁷]-NKB and other naturally occurring tachykinin peptides was performed. The substitutions to naturally occurring tachykinins with Asp and MePhe improved the receptor binding and agonistic activity for NK3R. The corresponding substitutions to NKB provided an NK3R selective analog.     

Functional characterization on invertebrate and vertebrate tissues of tachykinin peptides from octopus venoms

It has been previously shown that octopus venoms contain novel tachykinin peptides that despite being isolated from an invertebrate, contain the motifs characteristic of vertebrate tachykinin peptides rather than being more like conventional invertebrate tachykinin peptides. Therefore, in this study we examined the effect of three variants of octopus venom tachykinin peptides on invertebrate and vertebrate tissues. While there were differential potencies between the three peptides, their relative effects were uniquely consistent between invertebrate and vertebrae tissue assays. The most potent form (OCT-TK-III) was not only the most anionically charged but also was the most structurally stable. These results not only reveal that the interaction of tachykinin peptides is more complex than previous structure–function theories envisioned, but also reinforce the fundamental premise that animal venoms are rich resources of novel bioactive molecules, which are useful investigational ligands and some of which may be useful as lead compounds for drug design and development.     

Tissue distribution and quantitation of the mRNAs for three rat tachykinin receptors.

The family of mammalian tachykinin receptors consists of substance P receptor (SPR), neuromedin K receptor (NKR) and substance K receptor (SKR). In this investigation, tissue and regional distributions of the mRNAs for the three rat tachykinin receptors were investigated by blot-hybridization and RNase-protection analyses using the previously cloned receptor cDNAs. SPR mRNA is widely distributed in both the nervous system and peripheral tissues and is expressed abundantly in the hypothalamus and olfactory bulb, as well as in the urinary bladder, salivary glands and small and large intestines. In contrast, NKR mRNA is predominantly expressed in the nervous system, particularly in the cortex, hypothalamus and cerebellum, whereas SKR mRNA expression is restricted to the peripheral tissues, being abundant in the urinary bladder, large intestine, stomach and adrenal gland. Thus, the mRNAs for the three tachykinin receptors show distinct patterns of expression between the nervous system and peripheral tissues. Blot-hybridization analysis in combination with S1 nuclease protection and primer-extension analyses revealed that there are two large forms of SKR mRNA expressed commonly in the peripheral tissues, and two additional small forms of the mRNA expressed specifically in the adrenal gland and eye. These analyses also showed that the multiple forms of SKR mRNA differ in the lengths of the 5' mRNA portions, and that the two small forms of the mRNA, if translated, encode a truncated SKR polypeptide lacking the first two transmembrane domains. This investigation thus provides the comprehensive analysis of the distribution and mode of expression of the mRNAs for the multiple peptide receptors and offers a new basis on which to interpret the diverse functions of multiple tachykinin peptides in the CNS and peripheral tissues.     

Ligands for kappa-opioid and ORL1 receptors identified from a conformationally constrained peptide combinatorial library.

We have screened a synthetic peptide combinatorial library composed of 2 x 10(7) beta-turn-constrained peptides in binding assays on four structurally related receptors, the human opioid receptors mu, delta, and kappa and the opioid receptor-like ORL1. Sixty-six individual peptides were synthesized from the primary screening and tested in the four receptor binding assays. Three peptides composed essentially of unnatural amino acids were found to show high affinity for human kappa-opioid receptor. Investigation of their activity in agonist-promoted stimulation of [(35)S]guanosine 5'-3-O-(thio)triphosphate binding assay revealed that we have identified the first inverse agonist as well as peptidic antagonists for kappa-receptors. To fine-tune the potency and selectivity of these kappa-peptides we replaced their turn-forming template by other turn mimetic molecules. This "turn-scan" process allowed the discovery of compounds with modified selectivity and activity profiles. One peptide displayed comparable affinity and partial agonist activity toward all four receptors. Interestingly, another peptide showed selectivity for the ORL1 receptor and displayed antagonist activity at ORL1 and agonist activity at opioid receptors. In conclusion, we have identified peptides that represent an entirely new class of ligands for opioid and ORL1 receptors and exhibit novel pharmacological activity. This study demonstrates that conformationally constrained peptide combinatorial libraries are a rich source of ligands that are more suitable for the design of nonpeptidal drugs.