Independent coupling of the human tachykinin NK2 receptor to phospholipases C and A2 in transfected Chinese hamster ovary cells

NK2 receptor-mediated detrusor muscle contraction involves Gq/11-dependent activation of voltage-dependent Ca2+ channels and the RhoA-Rho kinase pathway

Tachykinins (TKs) are involved in both the physiological regulation of urinary bladder functions and development of overactive bladder syndrome. The aim of the present study was to investigate the signal transduction pathways of TKs in the detrusor muscle to provide potential pharmacological targets for the treatment of bladder dysfunctions related to enhanced TK production. Contraction force, intracellular Ca²⁺ concentration, and RhoA activity were measured in the mouse urinary bladder smooth muscle (UBSM). TKs and the NK2 receptor (NK2R)-specific agonist [β-Ala⁸]-NKA(4-10) evoked contraction, which was inhibited by the NKR2 antagonist MEN10376. In Gα_q/11-deficient mice, [β-Ala⁸]-NKA(4-10)-induced contraction and the intracellular Ca²⁺ concentration increase were abolished. Although G_q/11 proteins are linked principally to phospholipase Cβ and inositol trisphosphate-mediated Ca²⁺ release from intracellular stores, we found that phospholipase Cβ inhibition and sarcoplasmic reticulum Ca²⁺ depletion failed to have any effect on contraction induced by [β-Ala⁸]-NKA(4-10). In contrast, lack of extracellular Ca²⁺ or blockade of voltage-dependent Ca²⁺ channels (VDCCs) suppressed contraction. Furthermore, [β-Ala⁸]-NKA(4-10) increased RhoA activity in the UBSM in a G_q/11-dependent manner and inhibition of Rho kinase with Y-27632 decreased contraction force, whereas the combination of Y-27632 with either VDCC blockade or depletion of extracellular Ca²⁺ resulted in complete inhibition of [β-Ala⁸]-NKA(4-10)-induced contractions. In summary, our results indicate that NK2Rs are linked exclusively to G_q/11 proteins in the UBSM and that the intracellular signaling involves the simultaneous activation of VDCC and the RhoA-Rho kinase pathway. These findings may help to identify potential therapeutic targets of bladder dysfunctions related to upregulation of TKs.

Contractile effect of tachykinins on Suncus murinus (house musk shrew) isolated ileum

Recent studies used Suncus murinus to investigate the anti-emetic potential of NK(1) tachykinin receptor antagonists. However, the pharmacology of tachykinin receptors in this species has not been fully characterized. In the present studies, therefore, we examined a range of tachykinin receptor agonists for a capacity to induce contractions of the isolated ileum. The tachykinin NK1 receptor preferring agonists substance P, septide and [Sar9Met(O2)11] substance P, and the tachykinin NK2 preferring agonists neurokinin A and GR 64349 (Lys-Asp-Ser-Phe-Val-Gly-R-gamma-lactam-Leu-Met-NH2) caused concentration dependent contractions with EC50 values in the nanomolar range. However, the tachykinin NK3 preferring agonists neurokinin B and senktide (1nM-1microM) induced only weak contractions. The action of senktide, but not [Sar9Met(O2)11] substance P, septide, or GR 64349, was antagonized significantly by atropine (P<0.05); tetrodotoxin and hexamethonium were inactive. The tachykinin NK1 receptor antagonist CP-99,994 ((+)-[(2S,3S)-3-(2-methoxy-benzyl-amino)-2-phenylpiperidine]) (10-100nM) inhibited substance P- and septide-induced contractions non-competitively. The pA2 value estimated for CP-99,994 against septide was 7.3+/-0.1. It also non-competitively antagonized the contractile responses induced by [Sar9Met(O2)11] substance P with a pA2 of 7.4+/-0.1. CP-99,994 also had a slight inhibitory action on neurokinin A-induced contractions, but did not modify the action of GR 64349. Conversely, the tachykinin NK2 receptor antagonist, saredutant, competitively antagonized GR 64349-induced contractions with a pA2 of 7.34+/-0.02. On the other hand, the presence of both CP-99,994 and saredutant competitively antagonized substance P-induced contraction. The present studies indicate that tachykininNK1 and NK2 receptors exist in the ileum of S. murinus and are involved in mediating contractions directly on smooth muscle, whereas tachykinin NK3 receptors may play a minor role involving a release of acetylcholine.

Involvement of Ca2+ signaling in tachykinin-mediated contractile responses in swine trachea

Neuropeptide tachykinins, present within sensory nerves, have been implicated as neurotransmitters involved in nonadrenergic and noncholinergic airway muscle contraction. The signal transduction pathways of tachykinins on muscle contraction and Ca2+ mobilization were investigated in swine trachea. Tachykinins, substance P (SP) and neurokinin A (NKA), concentration (1 nM to 1 microM)-dependently induced contractile responses with removal of epithelium, whereas neurokinin B (NKB) did not alter the muscle tension. The SP- and NKA-evoked muscle contractions were inhibited by NK1-R antagonist L732138, but not by either NK2-R antagonist MDL29913 or NK3-R antagonist SB218795. Consistently, SP-elicited increase in [Ca2+]i was abolished by NK1-R antagonist, neither by NK2-R nor NK3-R antagonists. The SP-induced muscular responses were significantly inhibited by L-type Ca2+ channel blocker verapamil and withdrawal of external Ca2+. Caffeine (10 mM) or ryanodine (50 microM) also partly suppressed the SP-induced muscle responses. Inhibition of inositol 1,4,5-trisphosphate (InsP3) receptor with 2-APB (75 microM) potently attenuated SP-evoked Ca2+ mobilization and muscle contraction, which was further inhibited by 2-APB under Ca2+-free external solution, but not completely. Unexpectedly, simultaneous blockade of InsP3 receptor and ryanodine receptor (RyR) by 2-APB and ryanodine enhanced SP-evoked muscle contraction and Ca2+ mobilization. This potentiation was virtually abolished by removal of external Ca2+, suggesting native Ca2+ channels may contribute to this phenomenon. These results demonstrate that tachykinins produce a potent muscle contraction associated with Ca2+ mobilization via tachykinin NK1- R-dependent activation of multiple signal transduction pathways involving Ca2+ influx and release of Ca2+ from InsP3- and ryanodine-sensitive Ca2+ stores. Blockade of both InsP3 receptor and RyR enhances the Ca2+ influx through native Ca2+ channels in plasma membrane, which is crucial to Ca2+ signaling in response to NK1 receptor activation.

Protein kinase C is involved in neurokinin receptor modulation of N- and L-type Ca2+ channels in DRG neurons of the adult rat

Whole cell patch-clamp techniques were used to examine neurokinin receptor modulation of Ca2+ channels in small to medium size dorsal root ganglia neurons (<40 pF) that express mainly N- and L-type Ca2+ currents. Low concentrations of substance P enhanced Ca2+ currents (5-40%, <0.2 microM), while higher concentrations applied cumulatively reversed these enhancements (5-28% reductions, >0.5 microM). This apparent inhibition by high concentrations of substance P was blocked by the administration of the NK3 antagonist SB 235,375 (0.2 microM). The NK1 agonist, [Sar9,Met11]-substance P (0.05 to 1.0 microM) did not alter Ca2+ currents; whereas the NK2 agonist, [betaAla8]-neurokinin A (4-10), enhanced Ca2+ currents (5-36% increase, 0.05-0.5 microM). The enhancement was reversed by the NK2 antagonist MEN 10,376 (0.2 microM) but unaffected by the NK3 antagonist SB 235,375 (0.2 microM). The NK3 agonist [MePhe7]-neurokinin B (0.5-1.0 microM) inhibited Ca2+ currents (6-24% decrease). This inhibition was not prevented by the NK2 antagonist MEN 10,376 (0.2 microM) but was blocked by the NK3 antagonist SB 235,375 (0.2 microM). Both the enhancement and inhibition of Ca2+ currents by neurokinin agonists were reversed by the protein kinase C inhibitor bisindolylmaleimide I HCl (0.2-0.5 microM). Following inhibition of Ca2+ channels by [MePhe7]-neurokinin the facilitatory effect of BayK 8644 (5 microM) was increased and the inhibitory effect of the N-type Ca2+ channel blocker w -conotoxin GVIA (1 microM) was diminished, suggesting that the NK3 agonist inhibits N-type Ca2+ channels. Similarly, block of all but N-type Ca2+ channels, revealed that [betaAla8]-neurokinin A (4-10) enhanced the currents while [MePhe7]-neurokinin B inhibited the currents. Inhibition of all but L-type Ca2+ channels, revealed that [betaAla8]-neurokinin A (4-10) enhanced the currents while [MePhe7]-neurokinin B had no effect. Activation of protein kinase C with low concentrations of phorbol-12,13-dibutyrate enhanced Ca2+ currents, but high concentrations inhibited N- and L-type Ca2+ currents. In summary, these data suggest that in adult rat dorsal root ganglia neurons, NK2 receptors enhance both L- and N-type Ca2+ channels and NK3 receptors inhibit N-type Ca2+ channels and that these effects are mediated by protein kinase C phosphorylation of Ca2+ channels.

Mutations in the extracellular amino-terminal domain of the NK2 neurokinin receptor abolish cAMP signaling but preserve intracellular calcium responses

By combining real time measurements of agonist binding, by fluorescence resonance energy transfer, and of subsequent responses, we proposed previously that the neurokinin NK2 receptor preexists in equilibrium between three states: inactive, calcium-triggering, and cAMP-producing. Thr(24) and Phe(26) of the NK2 receptor extracellular domain are considered to interact with neuropeptide agonists based on the reduction of affinity when they are substituted by alanine. Using fluorescence resonance energy transfer, we now quantify the binding kinetics of two Texas Red-modified neurokinin A agonists to the fluorescent wild-type (Y-NK2wt) and the mutant (Y-NK2mut) receptor carrying Thr(24) --> Ala and Phe(26) --> Ala mutations. TR1-neurokinin A binds with a fast component and a slow component to the Y-NK2wt receptor and triggers both a calcium and a cAMP response. In contrast, on the mutant receptor, it binds in a single fast step with a lower apparent affinity and activates only the calcium response. Another agonist, TRC4-neurokinin A, binds to both wild-type and mutant receptors in a single fast step, with similar affinities and kinetics and promotes only calcium signaling. Kinetic modeling of ligand binding and receptor interconversions is carried out to analyze phenotypic changes in terms of binding alterations or changes in the transitions between conformational states. We show that the binding and response properties of the Y-NK2mut receptor are best described according to a phenotype where a reduction of the transition between the inactive and the active states occurs.

Peripheral tachykinin receptors as targets for new drugs

Tachykinins are widely distributed in the peripheral nervous system of the respiratory, urinary and gastrointestinal tract, stored in enteric neurons and in peripheral nerve endings of capsaicin-sensitive primary afferent neurons from which are released by stimuli having both pathological and physiological relevance. The most studied effects produced by tachykinins in these systems are smooth muscle contraction, plasma protein extravasation, mucus secretion and recruitment/activation of immune cells. The use of tachykinin receptor-selective antagonists and knockout animals has enabled to identify the involvement of tachykinin NK(1), NK(2) and NK(3) receptors as mediators of peripheral effects of tachykinins in different systems/species. The bulk of data obtained in experimental animal models suggests that tachykinins could contribute to the genesis of symptoms accompanying various human diseases including asthma/bronchial hyperreactivity, cystitis of various aetiology, inflammatory bowel diseases and irritable bowel syndrome. Tachykinin receptor antagonists are expected to afford therapeutically relevant effects.

The neurokinin A receptor activates calcium and cAMP responses through distinct conformational states

G protein-coupled receptors are thought to mediate agonist-evoked signal transduction by interconverting between discrete conformational states endowed with different pharmacological and functional properties. In order to address the question of multiple receptor states, we monitored rapid kinetics of fluorescent neurokinin A (NKA) binding to tachykinin NK2 receptors, in parallel with intracellular calcium, using rapid mixing equipment connected to real time fluorescence detection. Cyclic AMP accumulation responses were also monitored. The naturally truncated version of neurokinin A (NKA-(4-10)) binds to the receptor with a single rapid phase and evokes only calcium responses. In contrast, full-length NKA binding exhibits both a rapid phase that correlates with calcium responses and a slow phase that correlates with cAMP accumulation. Furthermore, activators (phorbol esters and forskolin) and inhibitors (Ro 31-8220 and H89) of protein kinase C or A, respectively, exhibit differential effects on NKA binding and associated responses; activated protein kinase C facilitates a switch between calcium and cAMP responses, whereas activation of protein kinase A diminishes cAMP responses. NK2 receptors thus adopt multiple activatable, active, and desensitized conformations with low, intermediate, or high affinities and with distinct signaling specificities.

A novel tachykinin NK2 receptor antagonist prevents motility-stimulating effects of neurokinin A in small intestine

1. MEN 11420 (nepadutant) is a potent, selective and competitive antagonist of tachykinin NK2 receptors. 2. The objective of the present study was to assess the capability of the drug to antagonize the stimulatory effects of neurokinin A (NKA) on gastrointestinal motility, as well as to change the fasting migrating motor complex (MMC). 3. Thirty-four male volunteers were randomized to treatment with either placebo or MEN 11420 in a double-blinded manner. Effects of MEN 11420 (8 mg intravenously) were evaluated as changes in phases I, II and III of MMC, as well as contraction frequency, amplitude and motility index during baseline conditions and during stimulation of motility using NKA (25 pmol kg(-1) min(-1) intravenously). 4. NKA preceded by placebo increased the fraction of time occupied by phase II, increased contraction frequency, amplitude and motility index. 5. MEN 11420 effectively antagonized the motility-stimulating effects of NKA. MEN 11420 reduced the phase II-stimulating effect of NKA. In addition, the stimulatory effect of NKA on contraction frequency and amplitude, as well as motility index were inhibited by MEN 11420. MEN 11420 did not affect the characteristics of MMC during saline infusion. 6. Plasma levels of MEN 11420 peaked during the first hour after infusion and decreased to less than half during the first 2 h. 7. In conclusion, intravenous MEN 11420 effectively inhibited NKA-stimulated, but not basal gastrointestinal motility, and was well tolerated by all subjects.

Tachykinin NK(2) receptor mediates contraction and ion transport in rat colon by different mechanisms

We have characterized the tachykinin NK(2) receptor-mediated contraction and vectorial ion transport responses in the muscularis mucosae and mucosa of the rat isolated distal colon, respectively. The tachykinin NK(2) receptor-selective antagonist nepadutant (c([(beta-D-GlcNAc)Asn-Asp-Trp-Phe-Dpr-Leu]c(2beta-5beta))) produced competitive antagonism of [betaAla(8)]neurokinin A-(4-10)-induced contraction (pK(B) = 9.3) in the muscularis mucosae, and insurmountable blockade of increases in short-circuit current (I(sc)) responses (pK(B) = 8.6) in the mucosa. However, this latter effect was completely reversed by washout of the antagonist. [betaAla(8)]Neurokinin A-(4-10)-induced contractions were unaffected by indomethacin (3 microM). In sharp contrast, I(sc) responses induced by [betaAla(8)]neurokinin A-(4-10) (100 nM) were inhibited (>70%) by indomethacin (3 microM), while I(sc) responses to substance P (3 microM) were unchanged. Our study provides the first evidence that in the same organ stimulation of tachykinin NK(2) receptors leads to two independent responses mediated by different effector mechanisms both of which are blocked (albeit with different kinetics) by the potent and selective tachykinin NK(2) receptor antagonist, nepadutant.

Defects of tyrosine289phenylalanine mutation on binding and functional properties of the human tachykinin NK2 receptor stably expressed in chinese hamster ovary cells

A point mutation was made at position 289 in the transmembrane segment 7 of the human tachykinin NK2 receptor to yield a tyrosine/phenylalanine (Tyr/Phe) substitution. Chinese hamster ovary cells stably transfected with the wild-type or Tyr289Phe mutant NK2 receptor both bound neurokinin A (NKA) and the synthetic NK2 receptor-selective agonists, GR 64349 and [betaAla8]NKA(4-10), with high and even affinities. Neurokinin B (NKB) and substance P (SP) also displayed sizeable binding affinities, albeit with lower affinity as compared to NKA. In a functional assay (production of inositol-1,4,5-trisphosphate, IP3), NKA, GR 64349, and [betaAla8]INKA(4-10) stimulated IP3 accumulation via the wild-type and mutant receptors with similar potencies. On the other hand, NKB and SP exhibited a dramatic reduction in their agonist efficacies at the mutant receptor, NKB acting as a partial agonist (maximum effect = 50% of the response to NKA) and SP being totally inactive. The results obtained with phenoxybenzamine inactivation experiments indicated that a large and similar receptor reserve existed for both the wild-type and the mutant receptor. SP, which displayed sizeable binding affinity for the mutant receptor but did not stimulate IP3 accumulation, antagonized the agonist effect of NKA. The antagonist action of SP at the mutant NK2 receptor cannot be ascribed to receptor internalization. The Tyr/Phe replacement at position 289 markedly reduced the binding affinity and antagonist potency of the non-peptide ligand, SR 48968, without affecting the binding affinity and antagonist potency of the bicyclic peptide antagonist MEN 11420. The results indicate that the hydroxyl radical function of Tyr289 in transmembrane segment 7 of the human NK2 receptor is, directly or indirectly, involved in stimulus transduction when the NK2 receptor is occupied by NKB or SP, but not when using NKA or NK2 receptor-selective agonists.