Further delineation of the two binding sites (R*(n)) associated with tachykinin neurokinin-1 receptors using [3-Prolinomethionine(11)]SP analogues

Homotrimerization Approach in the Design of Thrombospondin-1 Mimetic Peptides with Improved Potency in Triggering Regulated Cell Death of Cancer Cells

In order to optimize the potency of the first serum-stable peptide agonist of CD47 (PKHB1) in triggering regulated cell death of cancer cells, we designed a maturation process aimed to mimic the trimeric structure of the thrombospondin-1/CD47 binding epitope. For that purpose, an N-methylation scan of the PKHB1 sequence was realized to prevent peptide aggregation. Structural and pharmacological analyses were conducted in order to assess the conformational impact of these chemical modifications on the backbone structure and the biological activity. This structure-activity relationship study led to the discovery of a highly soluble N-methylated peptide that we termed PKT16. Afterward, this monomer was used for the design of a homotrimeric peptide mimic that we termed [PKT16]₃, which proved to be 10-fold more potent than its monomeric counterpart. A pharmacological evaluation of [PKT16]₃ in inducing cell death of adherent (A549) and nonadherent (MEC-1) cancer cell lines was also performed.

Truncation of neurokinin-1 receptor-Negative regulation of substance P signaling

Substance P (SP) is a tachykinin peptide, which triggers intracellular signaling in the nervous and immune systems, as well as, other local and systemic events. The interaction between SP and its receptor, neurokinin-1 receptor (NK1R), results in major downstream cellular actions, which include changes in calcium fluxes, ERK, and p21-activated kinase phosphorylation and NFκB activation. Two naturally occurring variants of the NK1R, the full-length, 407 aa receptor (NK1R-F) and the truncated, 311 aa isoform (NK1R-T), mediate the actions of SP. Receptor truncation partially disrupts signaling motifs of the carboxyl tail, a critical site for mediating NK1R signaling, resulting in a "less-efficient" receptor. Although NK1R-F is the predominant isoform in the central and peripheral nervous systems, NK1R-T is expressed in several tissues and cells, which include monocytes, NK cells, and T-cells. The SP binding domain is not affected by truncation and this site is identical in both NK1R receptor isoforms. However, while cells expressing NK1R-F respond to nanomolar concentrations of SP, monocyte and macrophage activation, mediated through NK1R-T, requires micromolar concentrations of SP in order to elicit signaling responses. Elevated plasma levels of SP are associated with increased inflammatory responses and NK1R antagonists reduce inflammation and cytokine production in vivo. This mini review presents and discusses the novel hypothesis that the expression of NK1R-T on immune system cells prevents immune activation in a milieu, which usually contains low concentrations of SP and, thus, maintains immune homeostasis. In contrast, in the activated neuronal microenvironment, when SP levels reach the threshold at tissue sites, SP promotes immune activation and modulates monocyte/macrophage polarization.

Thrombospondin-1 Mimetic Agonist Peptides Induce Selective Death in Tumor Cells: Design, Synthesis, and Structure-Activity Relationship Studies

Thrombospondin-1 (TSP-1) is a glycoprotein considered as a key actor within the tumor microenvironment. Its binding to CD47, a cell surface receptor, triggers programmed cell death. Previous studies allowed the identification of 4N1K decapeptide derived from the TSP-1/CD47 binding epitope. Here, we demonstrate that this peptide is able to induce selective apoptosis of various cancer cell lines while sparing normal cells. A structure-activity relationship study led to the design of the first serum stable TSP-1 mimetic agonist peptide able to trigger selective programmed cell death (PCD) of at least lung, breast, and colorectal cancer cells. Altogether, these results will be of valuable interest for further investigation in the design of potent CD47 agonist peptides, opening new perspectives for the development of original anticancer therapies.

Seven transmembrane receptors as shapeshifting proteins: the impact of allosteric modulation and functional selectivity on new drug discovery

It is useful to consider seven transmembrane receptors (7TMRs) as disordered proteins able to allosterically respond to a number of binding partners. Considering 7TMRs as allosteric systems, affinity and efficacy can be thought of in terms of energy flow between a modulator, conduit (the receptor protein), and a number of guests. These guests can be other molecules, receptors, membrane-bound proteins, or signaling proteins in the cytosol. These vectorial flows of energy can yield standard canonical guest allostery (allosteric modification of drug effect), effects along the plane of the cell membrane (receptor oligomerization), or effects directed into the cytosol (differential signaling as functional selectivity). This review discusses these apparently diverse pharmacological effects in terms of molecular dynamics and protein ensemble theory, which tends to unify 7TMR behavior toward cells. Special consideration will be given to functional selectivity (biased agonism and biased antagonism) in terms of mechanism of action and potential therapeutic application. The explosion of technology that has enabled observation of diverse 7TMR behavior has also shown how drugs can have multiple (pluridimensional) efficacies and how this can cause paradoxical drug classification and nomenclatures.

Heptahelical terpsichory. Who calls the tune?

The discovery that arrestins can function as ligand-regulated signaling scaffolds has revealed a previously unappreciated level of complexity in G protein-coupled receptor (GPCR) signal transduction. Because arrestin-bound GPCRs are uncoupled from G proteins, arrestin binding can be viewed as switching receptors between two temporally and spatially distinct signaling modes. Recent work has established two factors that underscore this duality of GPCR signaling and suggest it may ultimately have therapeutic significance. The first is that signaling by receptor-arrestin "signalsomes" does not require heterotrimeric G protein activation. The second is that arrestin-dependent signals can be initiated by pathway-specific "biased agonists," creating the potential for drugs that selectively modulate different aspects of GPCR function. Currently, however, little is known about the physiological relevance of G protein-independent signals at the cellular or whole animal levels, and additional work is needed to determine whether arrestin pathway-selective drugs will find clinical application.

[3H]GR205171 displays similar NK1 receptor binding profile in gerbil and human brain

1 In this study, [(3)H]GR205171 (3(S)-(2-methoxy-5-(5-trifluoromethyltetrazol-1-yl)-phenylmethylamino)-2(S)-phenylpiperidine), a potent and selective NK1 receptor antagonist, was characterised in autoradiographic studies in gerbil brain and in binding experiments on homogenates from gerbil and human brain cortex and striatum. 2 In autoradiographic studies in gerbil brain, highest levels of [(3)H]GR205171 binding sites were observed in caudate putamen, nucleus accumbens, medial and cortical nuclei of the amygdala and intermediate levels were detected in the hypothalamus, basolateral amygdala, septum, and cortex. 3 Saturation experiments in homogenates of brain striatum from gerbil showed that [(3)H]GR205171 binds to a single receptor population with a pK(d) value of 10.8+/-0.2 and a B(max) value of 607+/-40 fmol mg(-1). A lower number of NK1 receptor sites was found in cortex, where a B(max) of 94+/-6 fmol mg(-1) protein was obtained. Saturation experiments performed on homogenates from brain striatum of two human subjects and brain cortex of three human subjects showed that [(3)H]GR205171 binds with pK(d) values not different from gerbil and B(max) values ranging from 318+/-51 to 432+/-27 fmol mg(-1) protein in striatum and from 59+/-1 to 74+/-21 fmol mg(-1) protein in cortex. The natural ligand [(3)H]Substance P (SP) bound with sub-nanomolar affinity to 15 and 6% sites compared to [(3)H]GR205171 in gerbil and human striatum, respectively. 4 In competition binding experiments, GR205171 and the NK1 receptor antagonists aprepitant (MK-869), L-733,060 and NKP-608 bound with similar pK(i) values in gerbil and human striatum, irrespective of the use of [(3)H]GR205171 or [(3)H]SP as radioligand. The following rank order was found in terms of pK(i) values: GR205171>aprepitant> or =L-733,060>NKP-608. In homologous displacement experiments in gerbil and human striatum, SP showed nanomolar affinity, whereas in [(3)H]GR205171 competition experiments SP bound with pIC(50) values in the micromolar range and Hill slopes significantly lower than one. 5 It is concluded that the similarities of [(3)H]GR205171 binding characteristics and pharmacology between gerbil and human in cortex and striatum support the use of gerbil in preclinical models to study the effects of NK1 receptor antagonists in the central nervous system.

Functional consequences of alteration of N-linked glycosylation sites on the neurokinin 1 receptor

The neurokinin 1 receptor (NK1R), a G protein-coupled receptor involved in diverse functions including pain and inflammation, has two putative N-linked glycosylation sites, Asn-14 and Asn-18. We studied the role of N-linked glycosylation in the functioning of the NK1R by constructing three receptor mutants: two single mutants (Asn --> Gln-14 and Asn --> Gln-18) and a double mutant, lacking both glycosylation sites. Using a lentiviral transfection system, the mutants were stably transfected into NCM 460 cells, a nontransformed human colonic epithelial cell line. We observed that the magnitude of glycosylation as estimated by changes in gel migration depends on the number of glycosylation sites available, with the wild-type receptor containing the greatest amount of glycosylation. All mutant receptors were able to bind to substance P and neurokinin A ligand with similar affinities; however, the double mutant, nonglycosylated NK1R showed only half the B(max) of the wild-type NK1R. In terms of receptor function, the ablation of both N-linked glycosylation sites did not have a profound effect on the receptors' abilities to activate the MAP kinase families (p42/p44, JNK, and p38), but did affect SP-induced IL-8 secretion. All mutants were able to internalize, but the kinetics of internalization of the double mutant receptor was more rapid, when compared with wild-type NK1R. Therefore, glycosylation of NK1R may stabilize the receptor in the plasma membrane. These results contribute to the ongoing elucidation of the role of glycosylation in G protein-coupled receptors and the study of the neurokinin receptors in particular.

Analysis of an Intact G-Protein Coupled Receptor by MALDI-TOF Mass Spectrometry: Molecular Heterogeneity of the Tachykinin NK-1 Receptor

Integral membrane proteins are among the most challenging targets for biomedical research as most important cellular functions are tied to these proteins. To analyze intrinsically their structure/function, their transduction mechanism, or both, these proteins are commonly expressed in cultured cells as recombinant proteins. However, it is not possible to check whether these recombinant proteins are homogeneously or heterogeneously expressed. Owing to difficulties in their purification, very few mass spectrometry studies have been performed with those proteins and even less with G-protein coupled receptors. Here we have set up a procedure that is highly compatible with MALDI-TOF mass spectrometry to analyze an intact histidine-tagged G-protein coupled, namely, the tachykinin NK-1 receptor expressed in CHO cells, solubilized and purified using cobalt or nickel chelating magnetic beads. The metal-chelating magnetic beads containing the receptor were directly spotted on the MALDI plate for analysis. SDS-PAGE, combined with in-gel digestion analyzed by mass spectrometry, Western blot ((His)6 and FLAG M2 tags), photoaffinity labeling with a radioactive agonist, and Edman sequencing, confirmed the identity of the purified protein as the human tachykinin NK-1 receptor. Mass spectrometry study of both the glycosylated and deglycosylated intact protein forms revealed the existence of several receptor species that is tempting to correlate with the unusual pharmacological behavior of the receptor.

The two NK-1 binding sites correspond to distinct, independent, and non-interconvertible receptor conformational states as confirmed by plasmon-waveguide resonance spectroscopy

Two nonstoichiometric ligand binding sites have been previously reported for the NK-1 receptor, with the use of classical methods (radioligand binding and second messenger assays). The most populated (major, NK-1M) binding site binds substance P (SP) and is related to the adenylyl cyclase pathway. The less populated (minor, NK-1m) binding site binds substance P, C-terminal hexa- and heptapeptide analogues of SP, and the NK-2 endogenous ligand, neurokinin A, and is coupled to the phospholipase C pathway. Here, we have examined these two binding sites with plasmon-waveguide resonance (PWR) spectroscopy that allows the thermodynamics and kinetics of ligand-receptor binding processes and the accompanying structural changes of the receptor to be monitored, through measurements of the anisotropic optical properties of lipid bilayers into which the receptor is incorporated. The binding of the three peptides, substance P, neurokinin A, and propionyl[Met(O(2))(11)]SP(7-11), to the partially purified NK-1 receptor has been analyzed by this method. Substance P and neurokinin A bind to the reconstituted receptor in a biphasic manner with two affinities (K(d1) = 0.14 +/- 0.02 nM and K(d2) = 1.4 +/- 0.18 nM, and K(d1) = 5.5 +/- 0.7 nM and K(d2) = 620 +/- 117 nM, respectively), whereas only one binding affinity (K(d) = 5.5 +/- 0.4 nM) could be observed for propionyl[Met(O(2))(11)]SP(7-11). Moreover, binding experiments in which one ligand was added after another one has been bound to the receptor have shown that the binding of these ligands to each binding site was unaffected by the fact that the other site was already occupied. These data strongly suggest that these two binding sites are independent and non-interconvertible on the time scale of these experiments (1-2 h).

Molecular mechanism of 7TM receptor activation--a global toggle switch model

The multitude of chemically highly different agonists for 7TM receptors apparently do not share a common binding mode or active site but nevertheless act through induction of a common molecular activation mechanism. A global toggle switch model is proposed for this activation mechanism to reconcile the accumulated biophysical data supporting an outward rigid-body movement of the intracellular segments, as well as the recent data derived from activating metal ion sites and tethered ligands, which suggests an opposite, inward movement of the extracellular segments of the transmembrane helices. According to this model, a vertical see-saw movement of TM-VI-and to some degree TM-VII-around a pivot corresponding to the highly conserved prolines will occur during receptor activation, which may involve the outer segment of TM-V in an as yet unclear fashion. Small-molecule agonists can stabilize such a proposed active conformation, where the extracellular segments of TM-VI and -VII are bent inward toward TM-III, by acting as molecular glue deep in the main ligand-binding pocket between the helices, whereas larger agonists, peptides, and proteins can stabilize a similar active conformation by acting as Velcro at the extracellular ends of the helices and the connecting loops.

Conformational analysis of the C-terminal Gly-Leu-Met-NH2 tripeptide of substance P bound to the NK-1 receptor

We examined the effect of simultaneously incorporating proline or proline-amino acid chimeras in positions 9, 10, and/or 11 of substance P, on the affinity for the two NK-1 binding sites and on second-messenger activation. Because these 3-substituted prolines constrain not only the (phi,psi) values of the peptide backbone, but also the chi space of the amino acid side chain, we were able to gather data on the structural requirements for high-affinity binding to the NK-1 receptor. We were able to confirm that this C-terminal component is crucial and that it should adopt an extended conformation close to a polyproline II structure when bound to the receptor. The partial additivity of these constraints, more specifically, for the NK-1M site, suggests that the peptide backbone flexibility around the hinge-point residue Gly9 is essential to subtly position crucial side chains.

Agonist-directed trafficking of response by endocannabinoids acting at CB2 receptors

This study examined the ability of the endocannabinoids 2-arachidonoyl glycerol (2-AG) and noladin ether as well as the synthetic cannabinoid CP-55,940 [(-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl) cyclohexanol] to regulate three intracellular effectors via CB2 receptors in transfected Chinese hamster ovary cells. Although the three agonists regulate all effectors with equivalent efficacy, the rank order of potencies differs depending on which effector is evaluated. Noladin ether and CP-55,940 most potently inhibit adenylyl cyclase, requiring higher concentrations to stimulate the extracellular signal-regulated kinase subgroup of the mitogen-activated protein kinases (extracellular signal-regulated kinase-mitogen-activated protein kinase; ERK-MAPK) and Ca(2+)-transients. In contrast, 2-AG most potently activates ERK-MAPK, necessitating greater concentrations to inhibit adenylyl cyclase and even higher amounts to stimulate Ca(2+)-transients. Endocannabinoids also seem to be more "efficient" agonists at CB2 receptors relative to synthetic agonists. 2-AG and noladin ether require occupancy of less than one-half the number of receptors to produce comparable regulation of adenylyl cyclase and ERK-MAPK, relative to the synthetic cannabinoid CP-55,940. The CB2 antagonist 6-iodo-2-methyl-1-[2-(4-morpholinyl)-ethyl]-1H-indol-3-yl](4-methoxyphenyl)-methanone (AM630) reverses the actions of all agonists except Ca(2+)-transient stimulation by 2-AG. However, the effect of 2-AG on Ca(2+)-transients is attenuated by a second CB2 antagonist N-[(1S)-endo-1,3,3-trimethylbicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-1-pyrazole-3-carboxamide (SR144528). This suggests that 2-AG stimulates Ca(2+)-transients by binding to sites on CB2 receptors distinct from those occupied by AM630 and the other cannabinoids examined. Agonists produce no effects in pertussis toxin-treated cells. In summary, cannabinoid agonists distinctly bind to CB2 receptors and display different rank order of potencies and fractional receptor occupancies for regulation of intracellular effectors. These data provide direct evidence for agonist-directed trafficking of response by endocannabinoids acting at CB2 receptors.

The origins of diversity and specificity in g protein-coupled receptor signaling

The modulation of transmembrane signaling by G protein-coupled receptors (GPCRs) constitutes the single most important therapeutic target in medicine. Drugs acting on GPCRs have traditionally been classified as agonists, partial agonists, or antagonists based on a two-state model of receptor function embodied in the ternary complex model. Over the past decade, however, many lines of investigation have shown that GPCR signaling exhibits greater diversity and "texture" than previously appreciated. Signal diversity arises from numerous factors, among which are the ability of receptors to adopt multiple "active" states with different effector-coupling profiles; the formation of receptor dimers that exhibit unique pharmacology, signaling, and trafficking; the dissociation of receptor "activation" from desensitization and internalization; and the discovery that non-G protein effectors mediate some aspects of GPCR signaling. At the same time, clustering of GPCRs with their downstream effectors in membrane microdomains and interactions between receptors and a plethora of multidomain scaffolding proteins and accessory/chaperone molecules confer signal preorganization, efficiency, and specificity. In this context, the concept of agonist-selective trafficking of receptor signaling, which recognizes that a bound ligand may select between a menu of active receptor conformations and induce only a subset of the possible response profile, presents the opportunity to develop drugs that change the quality as well as the quantity of efficacy. As a more comprehensive understanding of the complexity of GPCR signaling is developed, the rational design of ligands possessing increased specific efficacy and attenuated side effects may become the standard mode of drug development.

Incorporation of vinylogous scaffolds in the C-terminal tripeptide of substance P

Glycine-9 and leucine-10 of substance P (SP) are critical for (NK)-1 receptor recognition and agonist activity. Propsi(Z)-CH=CH(CH3)-CONH)Leu (or Met) and Propsi((E)-CH=CH(CH3)-CONH)Leu (or Met) have been introduced in the sequence of SP, in order to restrict the conformational flexibility of the C-terminal tripeptide, Gly-Leu-Met-NH2, of SP. Propsi((Z)-CH=C(CH2CH(CH3)2)-CONH)Met-NH2, with an isobutyl substituent to mimic the Leu side-chain, was also incorporated in place of the C-terminal tripeptide. The substituted-SP analogs were tested for their affinity to human NK-1 receptor specific binding sites (NK-1M and NK-1m) and their potency to stimulate adenylate cyclase and phospholipase C in Chinese Hamster ovary (CHO) cells transfected with the human NK-1 receptor. The most potent SP analogs [Pro9psi((Z)CH=C(CH3)CONH)Leu10]SP and [Pro9psi ((E)CH=C(CH3)CONH)Leu10]SP, are about 100-fold less potent than SP on both binding sites and second messenger pathways. These vinylogous (Z)- or (E)-CH=C(CH3)- or (Z)-CH=C(CH2CH(CH3)2) moieties hamper the correct positioning of the C-terminal tripeptide of SP within both the NK-1M- and NK-1m-specific binding sites. The origin of these lower potencies is related either to an incorrect peptidic backbone conformation and/or an unfavorable receptor interaction of the methyl or isobutyl group.

Coupling of the human A1 adenosine receptor to different heterotrimeric G proteins: evidence for agonist-specific G protein activation

The present study investigates the effect of varying ligand structure on the ability of agonists to activate guanine nucleotide-binding proteins of the Gi, Gs and Gq families via the A(1) adenosine receptor. In CHO cells expressing this receptor, inhibition or potentiation of forskolin-stimulated cAMP accumulation was used as an end point to measure the activation of Gi and, in Pertussis toxin (PTX)-treated cells, Gs, respectively. Stimulation of inositol phosphate accumulation in PTX-treated cells was used as an index of Gq activation. CPA (N(6)-cyclopentyladenosine), NECA (5'-N-ethyl-carboxyamidoadenosine) and eight analogues of these ligands presented a range of guanine nucleotide-binding protein (G-protein)-activating profiles. Some ligands could only activate Gi (e.g. 2'deoxyCPA), some primarily Gi and Gs (and only weakly Gq) (e.g. 3'deoxyCPA), highlighting the importance of the ribose hydroxyls in agonist activation of multiple G proteins. CHA (N(6)-cyclohexyladenosine) activated Gi, Gs and Gq, but was more efficacious than CPA in activating Gs. The NECA analogues 5'-N-cyclopropyl-carboxamidoadenosine, 5'-N-cyclobutyl-carboxamidoadenosine and 5'-N-cyclopentyl-carboxamidoadenosine (CPeCA) also activated all three G proteins, although their ability to activate Gs and Gq (relative to CPA) was reduced with increasing substituent size, such that CPeCA produced only a small stimulation (at 100 microM) at Gq, but was a full agonist, relative to CPA, at Gi and Gs. This study suggests that the A(1) adenosine receptor can adopt agonist-specific conformations, arising from small changes in ligand structure, which lead to the differential activation of Gi, Gs and Gq.

?-Adrenoceptor Blockers as Agonists: Coupling of ?2-Adrenoceptors to Multiple G-Proteins in the Failing Human Heart

Beta blockers have been shown in clinical trials to improve cardiac function and reduce mortality of heart failure patients. However, these agents require careful titration since they can produce an initial decrease in cardiac output. The authors have recently shown that beta blockers, including some used clinically, can directly depress contraction of myocardium from the failing (but not nonfailing) human heart. This occurs on single ventricular myocytes and is therefore completely independent of any inhibition of endogenous catecholamines. The effect appears to be mediated primarily by the beta2-adrenoceptor (AR) and is dependent on the inhibitory guanine nucleotide binding protein, Gi. Using a transgenic mouse model, as well as adenoviral vectors to overexpress Gi or the human beta2AR in adult myocytes of various species, the authors demonstrate that agents that are blockers for bAR/Gs coupling can be agonists at a beta2AR/Gi-coupled form of the receptor. The negative effect of beta blockers could contribute to the initial cardiodepression that is observed when introducing these agents in heart failure patients. However, in the long term, beta2AR/Gi coupling may enhance the ability of beta blockers to protect and improve the function of the failing heart.

Biological activity of silylated amino acid containing substance P analogues

The need to replace natural amino acids in peptides with nonproteinogenic counterparts to obtain new medicinal agents has stimulated a great deal of innovation on synthetic methods. Here, we report the incorporation of non-natural silylated amino acids in substance P (SP), the binding affinity for the two hNK-1 binding sites and, the potency to stimulate phospholipase C (PLC) and adenylate cyclase of the resulting peptide. We also assess the improvement of their stability towards enzyme degradation. Altogether, we found that replacing glycine with silaproline (Sip) in position 9 of SP leads to a potent analogue exhibiting an increased resistance to angiotensin-converting enzyme hydrolysis.

Probing the binding pocket and endocytosis of a G protein-coupled receptor in live cells reported by a spin-labeled substance P agonist

To probe the molecular nature of the binding pocket of a G protein-coupled receptor and the events immediately following the binding and activation, we have modified the substance P peptide, a potent agonist for the neurokinin-1 receptor, with a nitroxide spin probe specifically attached at Lys-3. The agonist properties and binding affinity of the spin-labeled substance P are similar to the native peptide. Using electron paramagnetic resonance (EPR) spectroscopy, the substance P analogue is capable of reporting the microenvironment found in the binding pocket of the receptor. The EPR spectrum of bound peptide indicates that the Lys-3 portion of the agonist is highly flexible. In addition, we detect a slight increase in the mobility of the bound peptide in the presence of a non-hydrolyzable analogue of GTP, indicative of the alternate conformational states described for this class of receptor. The down-regulation of neurokinin-tachykinin receptors is accomplished by a rapid internalization of the activated protein. Thus, it was also of interest to establish whether spin-labeled substance P could serve as a real time reporter for endocytosis. Our findings show the receptor agonist is efficiently endocytosed and the loss of EPR signal upon internalization provides a real time monitor of endocytosis. The rapid loss of signal suggests that endosomal trafficking vesicles maintain a reductive environment. Whereas the reductive capacity of the lysosome has been established, our findings indicate this capacity in early endosomes as well.

The new neurokinin 1-sensitive receptor mediates the facilitation by endogenous tachykinins of the NMDA-evoked release of acetylcholine after suppression of dopaminergic transmission in the matrix of the rat striatum

Using an in vitro microsuperfusion procedure, the NMDA-evoked release of [3H]ACh was studied after suppression of dopamine (DA) transmission (alpha-methyl-p-tyrosine) in striatal compartments of the rat. The effects of tachykinin neurokinin 1 (NK1) receptor antagonists and the ability of appropriate agonists to counteract the antagonist responses were investigated to determine whether tachykinin NK1 classic, septide-sensitive and/or new NK1-sensitive receptors mediate these regulations. The NK1 antagonists, SR140333, SSR240600, GR205171 but not GR82334 and RP67580 (0.1 and 1 microM) markedly reduced the NMDA (1 mm + D-serine 10 microM)-evoked release of [3H]ACh only in the matrix. These responses unchanged by coapplication with NMDA of NK2 or NK3 agonists, [Lys5,MeLeu9,Nle10]NKA(4-10) or senktide, respectively, were completely counteracted by the selective NK1 agonist, [Pro9]substance P but also by neurokinin A and neuropeptide K (1 nM each). According to the rank order of potency of agonists for counteracting the antagonist responses ([Pro9]substance P, 0.013 nM > neurokinin A, 0.15 nM >> substance P(6-11) 7.7 nM = septide 8.7 nM), the new NK1-sensitive receptors mediate the facilitation by endogenous tachykinins of the NMDA-evoked release of ACh in the matrix, after suppression of DA transmission. Solely the NK1 antagonists having a high affinity for these receptors could be used as indirect anti-cholinergic agents.

Differential involvement of intracellular domains of the rat NTS1 neurotensin receptor in coupling to G proteins: a molecular basis for agonist-directed trafficking of receptor stimulus

In this work, we evidenced characteristic features of agonist-induced trafficking of receptor stimulus for the rat neurotensin receptor 1 (NTS1). Thus, reverse potency orders between two agonists, EISAI-1 and neuromedin N, were observed in inositol 1,4,5-trisphosphate and cAMP assays in Chinese hamster ovary cells transfected with this receptor. Indeed, compared with other agonists, EISAI-1 presented lower relative potency toward inositol 1,4,5-trisphosphate production than toward cAMP accumulation, guanosine 5'-O -(3-[35 S]thio)triphosphate binding, and [3H]arachidonic acid production. These results indicated pathway-dependent differences in EISAI-1 intrinsic efficacies, favoring activations of Gs- and Gi/o-related pathways over the Gq/11-related pathway. Moreover, although coupling to Gq/11 and Gi/o involved the third intracellular loop and the C-terminal domain of the NTS1 receptor, respectively, we demonstrated that deletion of the latter domain suppressed agonist-induced cAMP accumulation, suggesting that this domain also mediated coupling to Gs. Together, these results indicated that, unlike other agonists, EISAI-1 discriminated between the pathways involving the receptor C-terminal domain and that involving the third intracellular loop. These properties of EISAI-1 were also observed in cortical neurons endogenously expressing the NTS1 receptor. They were further attributed to the functionalization of its COOH end by an ethyl group, because the unesterified analog EISAI-2 presented normal behavior on inositol 1,4,5-trisphosphate production. These findings support the hypothesis of agonist-selective receptor states with distinct conformations or accessibilities of intracellular domains. They also suggest that the differential involvement of these domains in coupling to G proteins might represent a molecular basis for agonist-selective responses through G protein-coupled receptors.

The two NK-1 binding sites are distinguished by one radiolabelled substance P analogue

Two non-stoichiometric binding sites had previously been characterized for the NK-1 receptor using two different types of radiolabelled analogues of substance P. However, the question remained on their eventual conformational interconversion induced or not by the ligand. In this study, kinetic, saturation, and competition studies using [3H]propionyl[Pro(9)]SP demonstrate the existence of two independent binding components in CHO cells transfected with the human NK-1 receptor, with K(d) values of 0.040 nM ( approximately 20% of total sites) and 5.9 nM ( approximately 80% of total sites) that correspond to those of the two previously described binding sites. These two binding sites do not seem to interconvert since the minor one can be selectively extinguished in saturation studies in the presence of a SP analogue specific of this binding site.

Characterization of the bioactive conformation of the C-terminal tripeptide Gly-Leu-Met-NH2 of substance P using [3-prolinoleucine10]SP analogues

Residue Leu10 of substance P (SP) is critical for NK-1 receptor recognition and agonist activity. In order to probe the bioactive conformation of this residue, cis- and trans-3-substituted prolinoleucines were introduced in position 10 of SP. The substituted SP analogues were tested for their affinity to human NK-1 receptor specific binding sites (NK-1M and NK-1m) and their potency to stimulate adenylate cyclase and phospholipase C in CHO cells transfected with the human NK-1 receptor. [trans-3-prolinoleucine10]SP retained affinity and potency similar to SP whereas [cis-3-prolinoleucine10]SP shows dramatic loss of affinity and potency. To analyze the structural implications of these biological results, the conformational preferences of the SP analogues were analyzed by NMR spectroscopy and minimum-energy conformers of Ac-cis-3-prolinoleucine-NHMe, Ac-trans-3-prolinoleucine-NHMe and model dipeptides were generated by molecular mechanics calculations. From NMR and modeling studies it can be proposed that residue Leu10 of SP adopts a gauche(+) conformation around the chi1 angle and a trans conformation around the chi2 angle in the bioactive conformation. Together with previously published results, our data indicate that the C-terminal SP tripeptide should preferentially adopt an extended conformation or a PPII helical structure when bound to the receptor.

Predicting therapeutic value in the lead optimization phase of drug discovery

Recombinant and natural cellular assays for human G-protein-coupled receptors are used to optimize initial lead molecules obtained from screening. Although the activity of these molecules can be assessed on human genotype receptors, there is increasing evidence that cells impose a phenotypic selectivity to molecules in various cellular backgrounds. This opens the possibility of dissimulations between activity seen in lead optimization assays and the intended therapeutic value in humans. This review discusses the mechanisms by which cells can impose phenotypic selectivity on molecules and approaches to reduce this practical problem for drug discovery.

Pharmacological properties of peptides derived from an antibody against the tachykinin NK1 receptor for the neuropeptide substance P

Two peptides were derived from the structural analysis of a previously described monoclonal antibody [Mol. Immunol. 37 (2000) 423] against the tachykinin NK(1) receptor for the neuropeptide substance P. Here we show that these two peptides were able to inhibit the inositol phosphate transduction pathway triggered both by substance P and neurokinin A, another high-affinity endogenous ligand for the tachykinin NK(1) receptor. They also reduced the cAMP production induced by substance P. By contrast, only one antagonist peptide was able to prevent substance P and neurokinin A from binding the receptor, as revealed both by biochemical and autoradiographic studies. First, these results illustrate the generality of the antibody-based strategy for developing new bioactive peptides. Second, they indicate that antagonists, even exhibiting very close amino acid composition, can interact with the tachykinin NK(1) receptor at different contact sites, some of them clearly distinct from the contact domains for endogenous agonists.

Amino-zinc-enolate carbometalation reactions: application to ring closure of terminally substituted olefin for the asymmetric synthesis of cis- and trans-3-prolinoleucine

The amino-zinc-enolate cyclization reaction is a straightforward route for the synthesis of 3-substituted prolines. As classical intramolecular carbometalation reactions, the applicability of the addition of zinc to a double bond was limited to a substrate in which the terminal alkene carbon was unsubstituted. Being interested in the synthesis of cis- and trans-3-prolinoleucine derivatives for our structure-activity relation (SAR) studies, we focused our effort on the preparation of these compounds by amino-zinc-enolate cyclization of terminally substituted double bonds. Herein we report that the attachment of an activating group such as cyclopropyl to the terminal olefin carbon allows the amino-zinc-enolate cyclization of a terminally substituted alkene. The reaction is stereospecific, leading to a trans-3-substituted proline derivative, whereas a cis stereochemistry was observed with the amino-zinc-enolate cyclization of terminally nonsubstituted olefins. Absolute configurations obtained for the 3-prolinoleucine were established by X-ray analysis, NMR, and optical activity comparison of the cis and trans derivatives obtained by an unambiguous pathway.

Cloning of a C-terminally truncated NK-1 receptor from guinea-pig nervous system

In order to examine the possibility that some actions of substance P may be mediated by a variant of the neurokinin-1 (NK-1) receptor, we isolated and sequenced the cDNA encoding a truncated NK-1 receptor from guinea-pig celiac ganglion and brain mRNA by two-step RT-PCR based on the 3'RACE method. The truncated NK-1 receptor sequence corresponded to a splice variant missing the final exon 5, and encoded a 311-amino acid protein that was truncated just after transmembrane domain 7, in an identical position to a truncated variant of the human NK-1 receptor. Thus, the truncated NK-1 receptor lacked the intracellular C-terminus sequence required for the phosphorylation and internalisation of the full-length NK-1 receptor. Using a sensitive one-step semi-quantitative RT-PCR assay, we detected mRNA for both the full length and truncated NK-1 receptors throughout the brain, spinal cord, sensory and autonomic ganglia, and viscera. Truncated NK-1 receptor mRNA was present in lower quantities than mRNA for the full-length NK-1R in all tissues. Highest levels of mRNA for the truncated NK-1 receptor were detected in coeliac ganglion, spinal cord, basal ganglia and hypothalamus. An antiserum to the N-terminus of the NK-1 receptor labelled dendrites of coeliac ganglion neurons that were not labelled with antisera to the C-terminus of the full length NK-1 receptor. These results show that a C-terminally truncated variant of the NK-1 receptor is likely to be widespread in central and peripheral nervous tissue. We predict that this receptor will mediate actions of substance P on neurons where immunohistochemical evidence for a full-length NK-1 receptor is lacking.

Structural and biological effects of a beta2- or beta3-amino acid insertion in a peptide

Molecular mechanics calculations on conformers of Ac-HGly-NHMe, Ac-beta2-HAla-NHMe and Ac-beta3-HAla-NHMe indicate that low-energy conformations of the beta-amino acids backbone, corresponding to gauche rotamers around the Calpha-Cbeta bond, may overlap canonical backbone conformers observed for alpha-amino acids. Therefore, Substance P (SP) was used as a model peptide to analyse the structural and biological consequences of the substitution of Phe7 and Phe8 by (R)-beta2-HPhe and of Gly9 by HGly (R)-beta2-HAla or (S)-beta3-HAla. [(R)-beta2-HAla9]SP has pharmacological potency similar to that of SP while [HGly9]SP and [(S)-beta3-HAla9]SP show a 30- to 50-fold decrease in biological activities. The three analogues modified at position 9 are more resistant to degradation by angiotensin converting enzyme than SP and [Ala9]SP. NMR analysis of these SP analogues suggest that a beta-amino acid insertion in position 9 does not affect the overall backbone conformation. Altogether these data suggest that [HGly9]SP, [(S)-beta3-HAla9]SP and [(R)-beta2-HAla9]SP could adopt backbone conformations similar to that of SP, [Ala9]SP and [Pro9]SP. In contrast, incorporation of beta2-HPhe in position 7 and 8 of SP led to peptides that are almost devoid of biological activity. Thus, a beta-amino acid could replace an alpha-amino acid within the sequence of a bioactive peptide provided that the additional methylene group does not cause steric hindrance and does not confine orientations of the side chain to regions of space different from those permitted in the alpha-amino acid.

Involvement of the second extracellular loop (E2) of the neurokinin-1 receptor in the binding of substance P. Photoaffinity labeling and modeling studies

Substance P (SP) interacts with the neurokinin-1 (NK-1) G-protein-coupled receptor, which has been cloned in several species. In the present study, the domains of the NK-1 receptor involved in the binding of SP and SP-(7-11) C-terminal fragment have been analyzed using two peptide analogs containing the photoreactive amino acid para-benzoylphenylalanine ((p-Bz)Phe) in position 8 of their sequence. This study was carried out with [BAPA-Lys(6),(p-Bz)Phe(8),Pro(9),Met(O(2))(11)]SP-(7-11) and [BAPA(0),(p-Bz)Phe(8)]SP on both rat and human NK-1 receptors expressed in CHO cells. Combined trypsin and endo-GluC enzymatic complete digestions and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis led to the identification of the same domain of covalent interaction, (173)TMPSR(177), for the two photoactivatable peptides. Further digestion of this fragment with carboxypeptidase Y led to the identification of (173)TMP(175) in the second extracellular loop (E2) of the NK-1 receptor as the site of covalent attachment. Models of the conformation of this E2 loop in the human NK-1 receptor were generated using two different strategies, one based on homology with bovine rhodopsin and the other based on the solution conformation preferences of a synthetic peptide corresponding to the E2 loop.

Analogs of Substance P modified at the C-terminus which are both agonist and antagonist of the NK-1 receptor depending on the second messenger pathway

The initial goal of this study was to analyze, using photolabeling, the interactions between Substance P and its tachykinin NK-1 receptor. Therefore, the photoreactive amino acid para-benzoyl-phenylalanine (pBzl)Phe was incorporated into the Substance P sequence from position 4 to 11 leading to Bapa0[(pBzl)Phex]SP analogs. Biotinyl sulfone-5-aminopentanoic acid (Bapa) was introduced in order to purify the covalent complex. These photoreactive SP analogs were first assayed for their affinity for the two binding sites associated with the NK-1 receptor, as well as for their potency in activating the phospholipase C and adenylate cyclase pathways. All analogs photoreactive from position 4 to 11 have moderate to high affinity for the two NK-1 receptor-binding sites, except for the analog modified at position 7. This affinity could be correlated to their potency to activate the phospholipase C and adenylate cyclase pathways, except for the analog photoreactive at position 11. Bapa0[(pBzl)Phe11]SP was found to be an agonist in the phospholipase C pathway and an antagonist in the adenylate cyclase pathway, other analogs modified at position 11 were therefore analyzed. Among these, Bapa0[Pro9, (pBzl)Hcy(O2)11]SP is a partial agonist, whereas Bapa0[Hcy(ethylaminodansyl)11]SP is a full agonist in the phospholipase C pathway, the two analogs being antagonist in the adenylate cyclase pathway. These results show that analogs of SP can be simultaneously agonist at one binding site and antagonist at the other binding site associated with the NK-1 receptor.

Drug efficacy at G protein-coupled receptors

Efficacy has been defined in receptor pharmacology as a proportionality factor denoting the amount of physiological response a given ligand imparts to a biological system for a given amount of receptor occupancy. While first defined in terms of response, the concept can be expanded to a wide variety of G protein-coupled receptor (GPCR) behaviors, which includes pleiotropic interaction with multiple G proteins, internalization, oligomerization, desensitization, and interaction with membrane auxilliary proteins. Thus, there can be numerous types of efficacy, and different ligands can have a range of efficacies for different receptor behaviors. This review discusses the use of the efficacy concept in GPCR models based on the thermodynamic linkage theory and also in terms of the protein ensemble theory, in which macroaffinity of ligands for an ensemble of receptor microstates produces a new ligand-bound ensemble. The pharmacological characteristics of the ligand emerge from the intersection of the ligand-bound ensemble with the various ensembles defining pharmacological receptor behaviors. Receptor behaviors discussed are activation of G proteins; ability to be phosphorylated, desensitized, and internalized; formation of dimers and oligomers; and the interaction with auxiliary membrane and cytosolic proteins. The concepts of ligand-specific receptor conformation and conditional efficacy are also discussed in the context of ligand control of physiological response.

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.

Calpha methylation in molecular recognition. Application to substance P and the two neurokinin-1 receptor binding sites

Two binding sites NK-1M (major, more abundant) and NK-1m (minor) are associated with the neurokinin-1 receptor. For the first time with a bioactive peptide, the Calpha methylation constraint, shown to be a helix stabiliser in model peptides, was systematically used to probe the molecular requirements of NK-1M and NK-1m binding sites and the previously postulated bioactive helical conformation of substance P (SP). Seven Calpha methylated analogues of the undecapeptide SP (from position 5-11) have been assayed for their affinities and their potencies to stimulate second messenger production. The consequences of Calpha methylation on the structure of SP have been analysed by circular dichroism and nuclear magnetic resonance combined with restrained molecular dynamics. The decreased potencies of six out of these seven Calpha methylated SP analogues do not allow the identification of any clear-cut differences in the structural requirements between the two binding sites. Strikingly, the most active analogue, [alphaMeMet5]SP, leads to variable subnanomolar affinity and potency when interacting with the NK-1m binding site. The conformational analyses show that the structural consequences associated with Calpha methylation of SP are sequence dependent. Moreover, a single Calpha methylation is not sufficient by itself to drastically stabilize a helical structure even pre-existing in solution, except when Gly9 is substituted by an alpha-aminoisobutyric acid. Furthermore, Calpha methylation of residues 5 and 6 of SP in the middle of the postulated helix does not stabilize, but decreases (to different extents) the stability of the helical structure previously observed in the 4-8 domain of other potent SP analogues.

Two active molecular phenotypes of the tachykinin NK1 receptor revealed by G-protein fusions and mutagenesis

The NK1 neurokinin receptor presents two non-ideal binding phenomena, two-component binding curves for all agonists and significant differences between agonist affinity determined by homologous versus heterologous competition binding. Whole cell binding with fusion proteins constructed between either Galpha(s) or Galpha(q) and the NK1 receptor with a truncated tail, which secured non-promiscuous G-protein interaction, demonstrated monocomponent agonist binding closely corresponding to either of the two affinity states found in the wild-type receptor. High affinity binding of both substance P and neurokinin A was observed in the tail-truncated Galpha(s) fusion construct, whereas the lower affinity component was displayed by the tail-truncated Galpha(q) fusion. The elusive difference between the affinity determined in heterologous versus homologous binding assays for substance P and especially for neurokinin A was eliminated in the G-protein fusions. An NK1 receptor mutant with a single substitution at the extracellular end of TM-III-(F111S), which totally uncoupled the receptor from Galpha(s) signaling, showed binding properties that were monocomponent and otherwise very similar to those observed in the tail-truncated Galpha(q) fusion construct. Thus, the heterogenous pharmacological phenotype displayed by the NK1 receptor is a reflection of the occurrence of two active conformations or molecular phenotypes representing complexes with the Galpha(s) and Galpha(q) species, respectively. We propose that these molecular forms do not interchange readily, conceivably because of the occurrence of microdomains or "signal-transductosomes" within the cell membrane.

Internalization of [3H]Substance P Analogues in NK-1 Receptor Transfected CHO Cells

The internalization of [3H]propionyl[Met(O2)11]SP(7-11) which binds one binding site and of [3H][Pro9]SP which binds the two binding sites associated with the NK-1 receptor has been examined in CHO cells. The quantity of [3H][Pro9]SP measured inside the cytoplasm in kinetic experiments is fully temperature-dependent. In contrast, [3H]propionyl[Met(O2)11]SP(7-11) internalization reaches the same extent whatever the temperature, although the rate slowed down with lower temperature. The extent of internalization of [3H][Pro(9)]SP relative to the total specific bound is biphasic, when the extent of internalization of [3H]propionyl[Met(O2)11]SP(7-11) remains constant. For [3H][Pro9]SP, a high-affinity high-yield component inhibited in the presence of propionyl[Met(O2)11]SP(7-11) and a low-affinity low-yield component in the internalization process could be determined. Saturation studies show that [3H][Pro9]SP-binding parameters are insensitive to both phenylarsine oxide and monensin treatment, whereas [3H]propionyl[Met(O2)11]SP(7-11) maximal binding is decreased in both cases. Altogether, these data suggest that the two radiolabeled peptides should not follow the same internalization pathway.

Inverse, protean, and ligand-selective agonism: matters of receptor conformation

Concepts regarding the mechanisms by which drugs activate receptors to produce physiological response have progressed beyond considering the receptor as a simple on-off switch. Current evidence suggests that the idea that agonists produce only varying degrees of receptor activation is obsolete and must be reconciled with data to show that agonist efficacy has texture as well as magnitude. Thus, agonists can block system constitutive response (inverse agonists), behave as positive and inverse agonists on the same receptor (protean agonists), and differ in the stimulus pattern they produce in physiological systems (ligand-selective agonists). The molecular mechanism for this seemingly diverse array of activities is the same, namely, the selective microaffinity of ligands for different conformational states of the receptor. This paper reviews evidence for the existence of the various types of agonism and the potential therapeutic utility of different agonist types.-Kenakin, T. Inverse, protean, and ligand-selective agonism: matters of receptor conformation.

Characterization of substance P-membrane interaction by transferred nuclear Overhauser effect

Substance P, one of the mammalian tachykinins, is known to interact strongly with lipid bilayers and this interaction may play a role in the receptor-peptide recognition process. The conformation of substance P bound to vesicles consisting of perdeuterated phosphatidylcholine has been investigated by means of two-dimensional transferred nuclear Overhauser (trNOE) spectroscopy. Nuclear magnetic resonance data analysis resulted in a unique conformational family characterized by a well-defined conformation of the last seven C-terminal amino acids, which consists of a sequence of nonstandard turns following each other in a helix-like manner. The absence of short- or medium-range trNOE in the N-terminal part indicates its structural flexibility.