Recombinant bovine neurokinin-2 receptor stably expressed in Chinese hamster ovary cells couples to multiple signal transduction pathways

Allosteric ligands for G protein-coupled receptors: a novel strategy with attractive therapeutic opportunities

Allosteric receptor ligands bind to a recognition site that is distinct from the binding site of the endogenous messenger molecule. As a consequence, allosteric agents may attach to receptors that are already transmitter-bound. Ternary complex formation opens an avenue to qualitatively new drug actions at G protein-coupled receptors (GPCRs), in particular receptor subtype selective potentiation of endogenous transmitter action. Consequently, suitable exploitation of allosteric recognition sites as alternative molecular targets could pave the way to a drug discovery paradigm different from those aimed at mimicking or blocking the effects of endogenous (orthosteric) receptor activators. The number of allosteric ligands reported to modulate GPCR function is steadily increasing and some have already reached routine clinical use. This review aims at introducing into this fascinating field of drug discovery and at providing an overview about the achievements that have already been made. Various case examples will be discussed in the framework of GPCR classification (family A, B, and C receptors). In addition, the behavior at muscarinic receptors of hybrid derivatives incorporating both an allosteric and an orthosteric fragment in a common molecular skeleton will be illustrated.

A novel, conformation-specific allosteric inhibitor of the tachykinin NK2 receptor (NK2R) with functionally selective properties

The orthosteric agonist neurokinin A (NKA) interacts with the tachykinin NK2 receptors (NK2Rs) via an apparent sequential binding process, which stabilizes the receptor in at least two different active conformations (A1L and A2L). The A1L conformation exhibits fast NKA dissociation kinetics and triggers intracellular calcium elevation; the A2L conformation exhibits slow NKA dissociation kinetics and triggers cAMP production. The new compound LPI805 is a partial and noncompetitive inhibitor of NKA binding to NK2Rs. Analysis of NKA dissociation in the presence of LPI805 suggests that LPI805 decreases the number of NKA-NK2R complexes in A2L conformation while increasing those in the A1L conformation. Analysis of signaling pathways of NK2Rs shows that LPI805 dramatically inhibits the NKA-induced cAMP response while slightly enhancing the NKA-induced calcium response. Analysis of NKA association kinetics reveals that LPI805 promotes strong and specific destabilization of the NKA-NK2R complexes in the A2L conformation whereas access of NKA to the A1L conformations is unchanged. Thus, to our knowledge, LPI805 is the first example of a conformation-specific allosteric antagonist of a G-protein-coupled receptor. This work establishes the use of allosteric modulators in order to promote functional selectivity on certain agonist-receptor interactions.

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.

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.

Tachykinins mediate changes in ion transport in porcine jejunum through release of prostaglandins and neurotransmitters

In the present study, we investigated the mediators involved in substance P (SP) and neurokinin A (NKA) induced ion transport. Stripped preparations of porcine jejunal tissue were mounted in Ussing-chambers and short-circuited. The cyclo-oxygenase inhibitor, piroxicam (10 microM) and the neuronal conduction blocker, tetrodotoxin (TTX) (0.1 microM) both significantly decreased the SP (0.1 microM) (66% and 36%, respectively) and NKA (1 microM) (64% and 31%, respectively) induced increase in short-circuit current (SCC). Pretreatment with both piroxicam and TTX totally abolished the SP and NKA response. SP (0.1 microM) caused a significant release of prostaglandin E2 (PGE2), whereas the release of PGE2 induced by NKA was not significant. Experiments were performed to clarify if vasoactive intestinal polypeptide (VIP) was mediating SP or NKA responses. VIP caused a TTX-insensitive and a concentration-dependent increase in SCC. Two VIP antagonists did not change the response to VIP (10 nM and 0.1 microM). Thus, these antagonists could not be used to further elucidate the role of VIP. We were unable to measure a significant release of VIP after SP or NKA treatment. These results indicate, that SP and NKA regulate ion transport in porcine jejunum, entirely through the release of prostaglandins and enteric neurotransmitters.

Interaction between neurokinin A, VIP, prostanoids, and enteric nerves in regulation of duodenal function

Neurokinin A (NKA) induces duodenal motility and increases mucosal permeability and bicarbonate secretion in the in situ perfused duodenum in anesthetized rats. In the present study, the NKA-induced increase in mucosal permeability was potentiated by luminal perfusion with lidocaine and diminished by vasoactive intestinal peptide (VIP) but unaltered by elevated intraluminal pressure. Elevation of intraluminal pressure, however, potentiated the stimulatory effect of NKA on bicarbonate secretion. In contrast, the tachykinin decreased the rate of alkalinization in rats subjected to elevated intraluminal pressure and treated with indomethacin. Similarly, NKA partially inhibited the VIP-stimulated bicarbonate secretion. Luminal lidocaine did not affect the secretory response to NKA. The motility induced by NKA was unaffected by VIP or lidocaine but decreased by elevated intraluminal pressure. It is concluded that the NKA-induced increase in duodenal mucosal bicarbonate secretion is independent of neurons and possibly mediated by prostanoids. The increase in mucosal permeability in response to NKA may be suppressed by mucosal nerves, perhaps utilizing VIP as one of the transmitters.

Neurokinin A increases duodenal mucosal permeability, bicarbonate secretion, and fluid output in the rat

The aim of this study was to examine the integrative response to neurokinin A (NKA) on duodenal mucosal permeability, bicarbonate secretion, fluid flux, and motility in an in situ perfusion model in anesthetized rats. Intravenous infusion of NKA (100, 200, and 400 pmol.kg-1.min-1) induced duodenal motility. Furthermore, duodenal mucosal bicarbonate secretion, fluid output, and mucosal permeability increased in response to NKA. Pretreatment with the nicotinic antagonist hexamethonium did not change the response in any of the parameters investigated, whereas the NK2-receptor antagonist MEN 10,627 effectively inhibited all responses to NKA. Indomethacin induced duodenal motility and stimulated bicarbonate secretion. In indomethacin-treated rats, NKA further increased motility but decreased indomethacin-stimulated bicarbonate secretion by 70%. The NKA-induced increase in mucosal permeability was unaltered by indomethacin. It is concluded that NKA not only induces motility but also increases mucosal permeability and fluid output. Furthermore, the neuropeptide may have both stimulative and inhibitory effects on bicarbonate secretion. All responses to NKA are dependent on NK-2 receptor activation but are not mediated through nicotinic receptors.

Cloning and characterization of a specific receptor for the novel CC chemokine MIP-3alpha from lung dendritic cells

Dendritic cells are potent antigen-presenting cells involved in the initiation of immune responses. The trafficking of these cells to tissues and lymph nodes is mediated by members of the chemokine family. Recently, a novel CC chemokine known as MIP-3alpha or liver and activation-regulated chemokine has been identified from the EMBL/GenBank/DDBJ expressed sequence tag database. In the present study, we have shown that the messenger RNA for MIP-3alpha is expressed predominantly in inflamed and mucosal tissues. MIP-3alpha produced either synthetically or by human embryonic kidney 293 cells is chemotactic for CD34(+)-derived dendritic cells and T cells, but is inactive on monocytes and neutrophils. MIP-3alpha was unable to displace the binding of specific CC or CXC chemokines to stable cell lines expressing their respective high affinity receptors, namely CCR1-5 and CXCR1 and CXCR2, suggesting that MIP-3alpha acts through a novel CC chemokine receptor. Therefore, we used degenerate oligonucleotide-based reverse transcriptase PCR to identify candidate MIP-3alpha receptors in lung dendritic cells. Our results show that the orphan receptor known as GCY-4, CKRL-3, or STRL-22 is a specific receptor for MIP-3alpha, and that its activation leads to pertussis toxin-sensitive and phospholipase C-dependent intracellular Ca2+ mobilization when it is expressed in HEK 293 cells.

Mediation by prostaglandins of the stimulatory effect of substance P on cyclic AMP production in dog iris sphincter smooth muscle

The purpose of the present study was to examine the mechanism of the stimulatory effect of substance P (SP) on cyclic AMP (cAMP) accumulation in dog iris sphincter. We found that: (1) SP increased cAMP accumulation in a time- and concentration-dependent manner, the T1/2 and EC50 values being 1.2 min and 44 nM, respectively. SP has no effect on inositol trisphosphate and muscle contraction in this tissue. (2) SP-stimulated cAMP formation was inhibited by quinacrine, a non-specific phospholipase A2 inhibitor (IC50 = 9.5 microM), and by indomethacin (Indo), a cyclooxygenase inhibitor (IC50 = 3.5 nM), in a concentration-dependent manner, suggesting that SP induces cAMP accumulation via an Indo-sensitive pathway. (3) SP-induced arachidonic acid release and SP-induced prostaglandin E2 (PGE2) release were inhibited concentration dependently by quinacrine and Indo, with IC50 values of 11 microM and 0.8 nM, respectively. (4) PGE2 (1 microM) increased cAMP formation in the sphincter muscle by 94%, and, furthermore, the PG, but not SP, stimulated the activity of adenylyl cyclase in membrane fractions isolated from this tissue. (5) Indo (1 microM) blocked the relaxing effect of SP (1 microM) in iris sphincter precontracted with carbachol (1 microM). (6) The inhibitory effect of Indo on SP-induced cAMP accumulation was species specific. Increases in cAMP represent a mechanism by which extracellular SP can regulate smooth muscle function. Thus, we conclude from these studies that in dog iris sphincter SP-induced cAMP accumulation is mediated through PGs, and that in this cholinergically innervated muscle SP via cAMP could function, in part, to modulate the physiological responses to muscarinic receptor stimulation.

Palmitoylation of endothelin receptor A. Differential modulation of signal transduction activity by post-translational modification

Post-translational modifications such as phosphorylation and palmitoylation play important roles for the function and regulation of receptors coupled to heterotrimeric guanyl nucleotide-binding proteins. Here we demonstrate that the human endothelin receptor A (ETA) incorporates [3H]palmitate. Mutation of a cluster of five cysteine residues present in the cytoplasmic tail of ETA into serine or alanine residues completely prevented palmitoylation of the receptor. The ligand binding affinity of the non-palmitoylated ETA mutants was essentially unchanged as compared to the palmitoylated wild type ETA suggesting that the replacement of the cysteine residues did not alter the overall structure of the receptor. Furthermore, the ligand-induced stimulation of adenylyl cyclase by the mutant ETA was unaffected by the mutation. In contrast, the mutated non-palmitoylated receptors but not the wild type receptor failed to stimulate phosphatidylinositol hydrolysis by phospholipase C activation upon challenge by endothelin-1. Furthermore, the mutant receptors failed to stimulate the ligand-induced transient increase in the cytoplasmic calcium seen with the wild type ETA. Endothelin-1 induced mitogenic stimuli via the wild type receptors but not through the mutated receptors suggesting an important role for phospholipase C in this signal transduction pathway. The differential regulation of distinct signal transduction pathways by post-translational modification suggests that palmitoylation of the ETA provides a novel mechanism of modulating ETA receptor activity.

Effects of neuromedin-K on the rat hypothalamo-pituitary-adrenal axis

The effects of neuromedin-K (NMK) on the rat hypothalamo-pituitary-adrenal (HPA) axis were studied both in vivo and in vitro. A subcutaneous injection of 1 nmol/100 g NMK evoked a rise in plasma ACTH level at 30 min, increased plasma corticosterone (B) concentration (PBC) at 60 and 120 min, and did not alter plasma aldosterone (ALDO) concentration (PAC). The administration of 3 nmol/100 g NMK induced a rise in plasma ACTH level at 120 min and a drop of PBC at 30 min; it increased PBC and PAC at 60 and 120 min. NMK did not affect basal B secretion of dispersed zona fasciculata/reticularis (ZF/R) cells, but markedly enhanced basal ALDO production by dispersed zona glomerulosa (ZG) cells (minimal and maximal effective concentrations were 10(-9) M and 10(-7) M). Video-imaging analysis showed that NMK (10(-8) M) increased intracellular Ca2+ concentration in dispersed ZG cells, but not in ZF/R ones. These findings indicate that NMK exerts a complex modulatory action on the rat HPA axis: low doses of NMK appear to evoke a transient stimulation of ACTH release, while high doses seem to exert a short-term inhibition of glucocorticoid synthesis followed by the compensatory hypersecretion of ACTH; moreover, elevated doses of NMK also exert a strong ALDO secretagogue action by acting directly on the ZG cells.

Co-expression of the neurokinin NK2 receptor and G-protein components in the fission yeast Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe has proven useful for studying molecular interactions between a range of signal transduction components. We now report the first co-expression of a mammalian seven-transmembrane receptor and G-protein components in S. pombe. We selected the human neurokinin NK2 receptor together with its G-protein-signalling partner Gq for this study. Yeast membrane fractions showed high levels of NK2 receptor-binding activity (1159 +/- 534 (n = 3) fmol/mg protein) although initial experiments with intact cells revealed an absence of receptors at the cell surface. Using a construct comprising the NK2 coding sequence fused with the signal sequence from an endogenous phosphatase (phoI), we detected approximately 400 NK2 receptors/cell in unbroken yeast. Successful co-expression of the NK2 receptor with the G-protein subunits G alpha q, beta 1 or beta 2 and gamma 3 failed to modulate agonist binding, suggesting the absence of functional interaction between these components. As an alternative test of G alpha q function, we next expressed its downstream effector target phospholipase C-beta 1 (PLC beta 1) in S. pombe. Although PLC beta 1 undergoes powerful in vitro activation by G alpha q derived from baculovirus-infected Sf9 cells and mammalian cells, G alpha q expressed in S. pombe is totally ineffective. Similar results were also achieved with the G-protein subunit G alpha 16. Together, these data suggest that seven-transmembrane receptors can be expressed in S. pombe at high levels and directed to the cell surface although their interaction with co-expressed G-proteins in undetectable. Production of inactive G alpha-chains in S. pombe may account for these observations.

Effects of neurokinin-A on the rat hypothalamo-pituitary-adrenal axis

The effects of neurokinin-A (NKA) on the rat hypothalamo-pituitary-adrenal (HPA) axis were studied in vivo and in vitro. A subcutaneous injection of 1 or 3 nmol/100 g NKA did not alter plasma ACTH concentration. The lower dose of NKA evoked a transient rise in plasma corticosterone (B) concentration (PBC) at 30 min, and did not change plasma aldosterone (ALDO) concentration (PAC). The higher dose of NKA increased PBC at 30 and 60 min, and PAC at 30, 60 and 120 min. NKA did not affect basal ALDO secretion of dispersed zona glomerulosa (ZG) cells, but it markedly enhanced basal B production by dispersed zona fasciculata/reticularis (ZF/R) cells (minimal and maximal effective concentrations were 10(-9) M and 10(-6) M). Video-imaging analysis showed that 10(-6) M NKA increased intracellular Ca2+ concentration in dispersed ZF/R cells, but not in ZG ones. These findings indicate that NKA exerts a stimulatory action on the rat adrenal secretory activity, which is independent of any effect on the pituitary ACTH release: the B secretagogue action seems to be due to a direct effect of NKA on ZF/R cells, while the ALDO secretagogue action is not direct. but probably mediated by factor(s) other than ACTH.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

C-terminal truncation of the neurokinin-2 receptor causes enhanced and sustained agonist-induced signaling. Role of receptor phosphorylation in signal attenuation

The G protein-linked receptor for neurokinin A (NKA) couples to stimulation of phospholipase C and, in some cells, adenylyl cyclase. We have examined the function of the C-terminal cytoplasmic domain in receptor signaling and desensitization. We constructed C-terminal deletion mutants of the human NK-2 receptor (epitope tagged) to remove potential Ser/Thr phosphorylation sites, and expressed them in both mammalian and insect cells. When activated, truncated receptors mediate stronger and more prolonged phosphoinositide hydrolysis than wild-type receptor; however, the amplitude and kinetics of the NKA-induced rise in cytosolic Ca2+ remain unaltered. Protein kinase C (PKC)-activating phorbol ester abolishes wild-type receptor signaling but not mutant receptor signaling. Mutant receptors also mediate enhanced and prolonged cAMP generation, at least in part via PKC activation. When expressed in COS cells or Sf9 insect cells, the wild-type receptor is phosphorylated; receptor phosphorylation increases after addition of either NKA or phorbol ester. In contrast, mutant receptors are not phosphorylated by either treatment. Our results suggest that C-terminal Ser/Thr phosphorylation sites in the NK-2 receptor have a critical role in both homologous and heterologous desensitization. Removal of these phosphorylation sites results in a receptor that mediates sustained activation of signaling pathways and is insensitive to inhibition by PKC.

Pharmacological evidence for neurokinin receptors in murine neuroblastoma C1300 cells

We found that neurokinin A (NKA) and neurokinin B (NKB) induce an increase in the concentration of intracellular free Ca2+ ([Ca2+]i) in murine neuroblastoma C1300 cells (EC50: NKA 87 +/- 13 nM, NKB 97 +/- 15 nM). Substance P (SP) also caused a transient Ca2+ increase, although the potency of SP was much less than that of NKA and NKB. The increase in [Ca2+]i induced by NKA and NKB was inhibited by SR 48,968, a selective antagonist for NK2, and [beta Ala8]NKA(4-10), a selective agonist for NK2, did not stimulate the increase in [Ca2+]i. NKA- and NKB-induced Ca2+ mobilization was not inhibited by CP-96,345 and [Trp7, beta Ala8]NKA(4-10), selective antagonists for NK1 and NK3, respectively. These results suggested that C1300 cells express endogenous NK2 neurokinin receptors that have different features from known NK2 receptors.

Functional characterization of the human neurokinin receptors NK1, NK2, and NK3 based on a cellular assay system

The neurokinin receptor family is known to modulate phospholipase C activity. In order to find new compounds modulating the activity of these receptors we have developed a cellular screening system that measures the biological activity of receptors coupled to the IP3/DAG signal transduction pathway via the transcriptional activation of a reporter gene. For the establishment of neurokinin test cell lines the reporter cell line A20, stably transformed with the luciferase gene under the control of a promoter containing TPA response elements (TRE), which did not respond to neurokinin agonists, was used. Stable test cell lines were developed by transfecting the reporter cell line A20 with the genes for the human neurokinin receptors NK1, NK2 or NK3, respectively. In these cell lines, expression of luciferase was inducible by substance P, neurokinin A and neurokinin B, respectively. The order of potency of the three neurokinins substance P, neurokinin A and neurokinin B was consistent with published data and results from ligand binding studies performed with the NK1 and NK2 test cell lines. The agonistic effect of the neurokinins could be inhibited in a dose-dependent manner by simultaneous addition of neurokininspecific antagonists like the non-peptide antagonists CP-99,994 and SR 48968.

Palmitoylation but not the extreme amino-terminus of Gq alpha is required for coupling to the NK2 receptor

Gq alpha and G11 alpha differ from other G protein alpha subunits in that they have unique, conserved 6 residue amino-terminal extensions. Wild-type and amino-terminal mutants of Gq alpha expressed in COS cells were analyzed for their ability to functionally couple with co-expressed neurokinin NK2 receptor. Wild-type, T2A and delta 2-7 Gq alpha were able to stimulate agonist driven phospholipase C (PLC) activity in identical manners. Other activities of these two amino-terminal mutants including aluminum fluoride stimulated PLC activity, palmitoylation, interaction with G beta gamma subunits and GTP gamma S-induced trypsin resistance are also similar to the wild-type alpha subunit. This demonstrates that the NK2 receptor is able to functionally interact with the alpha subunit of Gq and that the first seven amino-acids of Gq alpha are not required for any of the alpha subunit functions tested. In contrast to the T2A and delta 2-7 mutants, a C9,10A Gq alpha mutant was not able to couple to either the NK2 receptor or PLC, as assessed by high-affinity agonist binding and activation of PLC either in intact cells or in vitro. The C9,10A protein was able to assume a GTP gamma S-induced trypsin-resistant conformation and partitioned primarily to the pelletable fraction in a manner similar to the wild-type protein. However, it was not labeled with [3H]palmitic acid. This suggests that blocking palmitoylation at the amino-terminus of Gq alpha results in a loss of functional activity which reflects an inability to interact with both the receptor and downstream signaling targets.

Role of prostaglandin-mediated cyclic AMP formation in protein kinase C-dependent secretion of atrial natriuretic peptide in rat cardiomyocytes

The role of endogenous prostaglandin production in phorbol diester-induced myocardial atrial natriuretic peptide (ANP) secretion was investigated in cultured spontaneously beating ventricular rat cardiomyocytes. Incubation of cells with 4 beta-phorbol 12-myristate 13-acetate (PMA; 0.1 microM) led to a rapid response in ANP release, a response accompanied by increases in cellular prostacyclin (PGI2) production, cyclic AMP (cAMP) formation and spontaneous contraction frequency. Although PMA-induced ANP secretion exhibited the pharmacological profile of a protein kinase C (PKC)-mediated event, the response was abolished in the presence of the cyclo-oxygenase inhibitors indomethacin (10 microM) and diclofenac (1 microM), indicating that endogenous prostaglandin production is responsible for PMA-induced ANP secretion in this system. Confirming this, PMA-induced ANP secretion was strongly correlated with endogenous formation of 6-oxo-prostaglandin F1 alpha (r = 0.93, P < 0.0005, n = 11), and exogenously applied PGI2, prostaglandin E2 (PGE2) or prostaglandin F2 alpha (PGF2 alpha) elicited simultaneous increases in cAMP formation, contraction frequency and ANP secretion in these cells. Furthermore, PMA-induced cAMP formation was abolished in the presence of either diclofenac or indomethacin, whereas the cAMP-elevating agent forskolin (0.1 microM) mimicked the secretory and chronotropic effect of PMA in these cells. A role for cAMP in PMA-induced ANP secretion was also apparent insofar as PMA-induced ANP release was substantially decreased in the presence of the Rp-diastereomer of 3',5'-cyclic adenosine monophosphorothioate (Rp-cAMPS; 10 microM), whereas the cAMP-mimetic agent dibutyryl cAMP (10 microM) provoked a rapid increase in ANP secretion in this system. Finally, the Ca(2+)-channel antagonist nifedipine (0.1 microM) severely decreased PGI2-, PGE2- and PMA-induced ANP secretion without affecting PGF2 alpha-induced peptide release, suggesting that PGI2 and/or PGE2, but not PGF2 alpha, are the prostanoids involved in PMA-induced ANP release. Taken together, these results suggest that PKC activation induces ANP secretion in spontaneously beating rat ventricular cardiomyocytes via an autocrine pathway involving increased PGI2 and/or PGE2 formation, a response leading to the activation of a myocardial adenylate cyclase and, subsequently, to that of a nifedipine-sensitive Ca2+ channel.

High-level expression of the human neurokinin-1 receptor in mammalian cell lines using the Semliki Forest virus expression system

Human neurokinin-1 receptor cDNA was introduced into the pSFV1 Semliki Forest virus (SFV) vector and the in vitro transcribed RNA was electroporated into BHK cells with pSFV-Helper RNA. This procedure resulted in the packaging of a high-titer SFV-NK-1 virus stock containing approximately 5 x 10(9) infective units/ml. Infection of baby hamster kidney, COS-7, Chinese hamster ovary and human osteosarcoma cells yielded high levels of human neurokinin-1 receptor expression as assessed by [3H]substance P binding. The maximal receptor expression level obtained was 4 x 10(6) receptors/cell and studies of the post-infection time indicated that a high level of receptor expression was observed 10-24 h post-infection. The human neurokinin-1 receptor expressed in infected baby hamster kidney, COS-7 and Chinese hamster ovary cells was able to stimulate Ca2+ mobilization indicating functional coupling to guanine-nucleotide-binding proteins. The application of the Semliki Forest virus expression system will permit the rapid and efficient production of large quantities of receptor protein for both pharmacological and structural studies.

Calcium influx in platelet activating factor-induced atrial natriuretic peptide release in rat cardiomyocytes

Atrial natriuretic peptide (ANP) is released from the myocardium after the activation of protein kinase C and/or ischemia, events that are associated with an increase in platelet activating factor (PAF) production in this tissue. In this study we demonstrate that PAF, but not lyso-PAF, induces a concentration-dependent increase in ANP secretion in spontaneously beating neonatal rat cardiomyocytes, a response associated with increases in cellular adenosine 3',5'-cyclic monophosphate (cAMP) formation, calcium influx, and the mobilization of calcium from intracellular stores. cAMP formation and calcium influx appear to play major roles in PAF-induced ANP secretion in this system, insofar as PAF-induced ANP release was substantially reduced in the presence of the (R)-p-diastereoisomer of adenosine 3',5'-cyclic monophosphorothioate (10 microM), whereas both PAF-induced calcium influx and ANP secretion were abolished in the presence of the calcium channel antagonist nifedipine (0.1 microM). Consistent with these results, N6-2'-O-dibutyryl cAMP (DBcAMP, 10 microM) and/or forskolin (0.1 microM) simultaneously increased cAMP production, calcium influx, and ANP release in these cells, with both DBcAMP- and forskolin-induced ANP secretion being fully abolished in the presence of 0.1 microM nifedipine. Taken together, these results suggest that PAF, DBcAMP, and forskolin promote ANP secretion in spontaneously beating cardiomyocytes via the activation of a cAMP-dependent, nifedipine-sensitive myocardial calcium channel and that calcium influx is a major requirement for cAMP-induced ANP secretion in this system.

Calcium influx and protein kinase C alpha activation mediate arachidonic acid mobilization by the human NK-2 receptor expressed in Chinese hamster ovary cells

We have cloned a cDNA encoding the human ileal neurokinin-2 (NK-2) receptor which mediates powerful neurokinin A-stimulated arachidonic acid (AA) and prostaglandin release when expressed in CHO cells. Two major signal transduction events appear to underlie this response. Firstly, AA liberation is critically dependent upon agonist-stimulated influx of extracellular Ca2+ although not release from intracellular stores. Secondly, NK-2 receptor-linked AA mobilization requires concomitant PKC activation and based upon limited subtype immunodetectability as well as toxin, identical pretreatment inhibits AA release partially and blocks PKC alpha translocation completely. These observations indicate that in this cell system AA liberation reflects NK-2 receptor-dependent activation of two distinct but converging signal transduction pathways regulated by different G-protein species and involving Ca2+ influx and PKC alpha activation.

Signal transduction mechanisms of recombinant bovine neurokinin-2 receptor stably expressed in baby hamster kidney cells

The bovine neurokinin-2 (NK-2) receptor gene was stably transfected into Baby hamster kidney (BHK-21) fibroblasts and one recombinant clone expressing 17,700 high-affinity [125I]neurokinin A (NKA) binding sites/cell characterized further. [125I]NKA binding was displaced by unlabeled NKA with an IC50 of 8.26 +/- 2 nM (n = 5) and with the rank order of potency NKA > neurokinin B (NKB) > Substance P (SP) confirming pharmacological characteristics of an NK-2 receptor subtype. Stimulation with NKA resulted in a rapid and dose-dependent increase in inositol 1,4,5-trisphosphate (IP3) levels (EC50 = 32 +/- 10 nM; n = 7) which was paralleled by a transient biphasic rise in intracellular free calcium concentration [Ca2+]i (EC50 = 35 +/- 20 nM; n = 3). In addition to phosphoinositide (PI) hydrolysis and Ca2+ mobilization, NKA was found to stimulate both cyclic AMP formation (EC50 = 1.02 +/- 0.26 microM; n = 7) and [3H]arachidonic acid mobilization (EC50 = 0.65 +/- 0.45 microM; n = 4). Interestingly, cyclic AMP levels also rose after addition of an exogenous arachidonic acid metabolite, prostaglandin E2 (PGE2) (EC50 = 11.5 +/- 2 microM). Similar observations of NKA-induced IP3 production, Ca2+ mobilization, arachidonic acid liberation, and cAMP formation have been made previously following expression of the bovine NK-2 receptor in Chinese hamster ovary (CHO) epithelial cells. The present results suggest that activation of NK-2 receptors leads to characteristic and reproducible intracellular second messenger responses in a subclass of cell types which includes fibroblasts and epithelial cells irrespective of their genetic and phenotypic background.

Ligand binding kinetics of substance P and neurokinin A receptors stably expressed in Chinese hamster ovary cells and evidence for differential stimulation of inositol 1,4,5-trisphosphate and cyclic AMP second messenger responses

Stably transfected Chinese hamster ovary cells expressing either the substance P receptor or neurokinin A receptor were constructed, isolated, and characterized. Equilibrium ligand binding studies performed on whole cells demonstrated that cell lines expressing either of these receptors contained a single class of high-affinity binding sites with an apparent KD of 0.16 nM for the substance P receptor and an apparent KD of 2.1 nM for the neurokinin A receptor. The higher affinity of substance P for its receptor was accounted for by both a greater association rate constant and a lesser dissociation rate constant. The time course and extent of ligand-stimulated inositol 1,4,5-trisphosphate mass increases in both cell lines were similar and displayed rapid and transient kinetics. Ligand-stimulated cyclic AMP accumulation was also apparent in the cell lines, although the time course and magnitude of the responses were substantially different, with the neurokinin A receptor mediating a greater and more prolonged response. These studies establish the presence of functional substance P receptors and neurokinin A receptors in the stably transfected cell lines and provide evidence for agonist-dependent differential stimulation of second messenger responses.

cDNA sequence and heterologous expression of the human neurokinin-3 receptor

Functional cDNA clones encoding the human neurokinin-3 receptor were isolated from human brain mRNA. The cloned human neurokinin-3 receptor was expressed in COS cells and Xenopus oocytes, where peptide binding affinity and intracellular effector activation were determined. Neurokinin B is the most potent agonist, followed by eledoisin, substance K and substance P. The binding affinities of these peptides at the human neurokinin-3 receptor differ quantitatively from the rat receptor, implying a functional consequence of the sequence divergence between the two species. Heterologous expression in oocytes revealed that, unlike the neurokinin-1 receptor, the efficacy of ion channel activation mediated by the neurokinin-3 receptor does not approximate the binding affinity. The heterologous expression of the human neurokinin-3 receptor will facilitate further investigation into its biochemical functions.

Molecular characterisation, expression and localisation of human neurokinin-3 receptor

The complete amino acid sequence of the human neurokinin-3 receptor was deduced by DNA sequence analysis of human genomic fragments. Comparison of the predicted primary structure with those for the human neurokinin receptors 1 and 2 shows a highly conserved pattern of seven hydrophobic regions with maximum divergence occurring at the amino- and carboxy-termini. The position of intron-exon junctions are identical to those in other reported neurokinin genes. Using a chimeric genomic-cDNA gene, the human NK-3 receptor was expressed in Xenopus laevis oocytes where it mediates membrane conductance changes in response to its agonist, neurokinin B. More significantly, expression of the gene in mammalian cells resulted in detection of receptor binding as well as neurokinin-stimulated calcium mobilization and arachidonic acid release, all displaying the pharmacological characteristics expected of a neurokinin-3 receptor. By using the polymerase chain reaction we have shown that mRNA for the human neurokinin-3 receptor is expressed predominantly in the central nervous system.