Gly166 in the NK1 receptor regulates tachykinin selectivity and receptor conformation

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.

The tachykinin tale: molecular recognition in a historical perspective

Crystallography, mutational mapping and crosslinking are but a few of the experimental techniques that have helped to elucidate the underlying principles of molecular recognition between macromolecules and to improve our understanding of the evolution of the structure-activity relationship (SAR). While this development has been particularly successful for small and rigid ligands and substrates that bind to larger hydrophilic biomolecules, our understanding of membrane-embedded proteins is still rather limited. This review uses the example of the neuropeptide family of tachykinins and their G-protein coupled receptors (GPCR) to present how complementary experimental strategies over the past decades have nourished and modified conceptual models of the structural requisites of molecular recognition and function. Given the little we know, the pertinent question is how we proceed from here.

Evidence that L1AD3, an apoptosis-inducing cyclic peptide, binds a leukemic T-cell membrane protein receptor

Human leukemic T-lymphocytes undergo extensive and rapid apoptosis in the presence of L1AD3, a small cyclic peptide derivative of cobra cardiotoxin. The first step in this process involves its binding to membranes of susceptible cells. By the use of a biotin "handle" synthetically incorporated at the N-terminus of L1AD3, we show that binding is saturable and selective: normal human peripheral blood lymphocytes do not bind this peptide. Fluorescence resonance energy transfer experiments indicate that the binding sites are separated by at least 55 A. Loss of binding occurs if membrane proteins are enzymatically degraded, suggesting that L1AD3's target is a cell-membrane surface protein receptor. Finally, crosslinking of cyclic BTNL1AD3 peptide to a leukemic T-cell membrane surface receptor, as examined using a biotin-avidin blot, indicated a molecular weight of approximately 34,400.

Mutagenesis at the human tachykinin NK2 receptor to define the binding site of a novel class of antagonists

The pharmacological profile of novel antagonists endowed with high affinity for the human tachykinin NK(2) receptor is presented. MEN13918 (Ngamma[Nalpha[Nalpha(benzo[b]thiophen-2-yl)carbonyl]-1-aminocyclohexan-1-carboxy]-d-phenylalanyl]-3-cis-aminocyclohexan-1-carboxylic-acid-N-(1S,2R)-2-aminocyclohexyl)amide trifluoroacetate salt) and MEN14268 (Nalpha[Nalpha(benzo[b]thiophen-2-yl)carbonyl)-1-aminocyclopentane-1-carboxyl]-d-phenylalanine-N-[3(morpholin-4-yl)propyl]amide trifluoroacetate salt) were more potent in blocking neurokinin A (NKA, His-Lys-Thr-Asp-Ser-Phe-Val-Gly-Leu-Met-NH(2)) induced contraction in human, which induced greater contraction in human (pK(B) 9.1 and 8.3) than rat (pK(B) 6.8 and <6) urinary bladder smooth muscle preparation in vitro. In agreement with functional data, in membrane preparations of CHO cells stably expressing the human NK(2) receptors, both MEN13918 and MEN14268 potently inhibited the binding of agonist ([(125)I]NKA, K(i) 0.2 and 2.8 nM) and antagonist ([(3)H]nepadutant, K(i) 0.1 and 2.2 nM, [(3)H]SR48968 K(i) 0.4 and 6.9 nM) radioligands. Using site-directed mutagenesis and radioligands binding we identified six residues in the transmembrane (TM) helices that are critical determinants for the studied antagonists affinity. To visualize these experimental findings, we constructed a homology model based on the X-ray crystal structure of bovine rhodopsin and suggested a possible binding mode of these newly discovered antagonist ligands to the human tackykinin NK(2) receptor. Both MEN13918 and MEN14268 bind amongst TM4 (Cys167Gly), TM5 (Tyr206Ala), TM6 (Tyr266Ala, Phe270Ala), and TM7 (Tyr289Phe, Tyr289Thr). MEN13918 and MEN14268 diverging binding profile at Y289 mutations in TM7 (Tyr289Phe, Tyr289Thr) suggests a relation of their different chemical moieties with this residue. Moreover, the different influence on binding of these two ligands by mutations located deep along the inner side of TM6 (Phe270Ala, Tyr266Ala, Trp263Ala) indicates a nonequivalent positioning, although occupying the same binding crevice. Furthermore, binding data indicate the Ile202Phe mutation, which mimics the wild-type rat NK(2) receptor sequence, as a species selectivity determinant. In summary, data with mutant receptors describe, for these new tachykinin NK(2) receptor antagonists, a binding site which is partially overlapping either with that of the cyclized peptide antagonist nepadutant (cyclo-[[Asn(beta-d-GlcNAc)-Asp-Trp-Phe-Dpr-Leu]cyclo(2beta-5beta)] or the nonpeptide antagonist SR48968 ((S)-N-methyl-N[4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophenyl)butyl]benzamide).

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.

Mutational analysis and molecular modelling of the antagonist SR 144528 binding site on the human cannabinoid CB(2) receptor

We have investigated the binding site of the subtype specific antagonist SR 144528, (N-[(1S)-endo-1,3,3-trimethyl bicyclo [2.2. 1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methoxybenzyl)- pyrazo le-3-carboxamide) on the human cannabinoid CB(2) receptor based on functional studies with mutated receptors. Two serine residues in the fourth transmembrane region, Ser(161) and Ser(165), were singly mutated to the cognate cannabinoid CB(1) receptor residue, alanine, and each gave receptors with wild-type properties for the cannabinoid agonists CP 55,940 (1R,3R,4R)-3-[2-hydroxy-4-(1, 1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl)cyclohexan-1-ol) and WIN 55212-2 (R)-(+)[2, 3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1, 4-benzoxazin-6-yl](1-naphthalenyl) methanone, which SR 144528 completely failed to antagonise. Molecular modelling studies show that SR 144528 interacts with residues in transmembrane domains 3, 4, and 5 of the cannabinoid CB(2) receptor through a combination of hydrogen bonds and aromatic and hydrophobic interactions. In addition, the replacement by serine of a nearby cannabinoid CB(2) receptor-specific residue, Cys(175) resulted in wild-type receptor properties with CP 55,940, loss of SR 144528 binding and eight-fold reduced binding and activity of WIN 55212-2, a result compatible with a recently-proposed binding site model for WIN 55212-2.

Point mutation increases a form of the NK1 receptor with high affinity for neurokinin A and B and septide

1. The binding modalities of substance P and neurokinin A on the wild type and Gly166 to-Cys mutant NK1 receptors expressed on CHO cells were investigated in homologous and heterologous binding experiments using both radiolabelled substance P and neurokinin A. 2. On the wild type NK1 receptor NKA displaces radiolabelled substance P with very low apparent affinity, despite its high-affinity binding constant (determined in homologous binding experiments). The Gly166 to-Cys substitution in the NK1 tachykinin receptor greatly enhances the apparent affinity of neurokinin A in competition for radiolabelled substance P, but it does not change the binding constant of neurokinin A. The mutation, thereby, eliminates the discrepancy between the low apparent affinity and the high binding constant of neurokinin A. 3. On the wild type receptor the binding capacity of neurokinin A is significantly smaller than that of substance P. In contrast, the two tachykinins bind to approximately the same number of sites on the mutant receptor. 4. Simultaneous mass action law analysis of binding data in which multiple radioligands were employed in parallel demonstrated that a one-site model was unable to accommodate all the experimental data, whereas a two-site model provided a dramatically better description. 5. These two receptor-sites display equally high affinity for substance P, while neurokinin A strongly discriminates between a high and a low affinity component. The binding affinities of neurokinin A are not affected by the mutation, which instead specifically alters the distribution between receptor sites in favour of a high affinity neurokinin A binding form. 6. The low apparent affinity and binding capacity of neurokinin A on the wild type receptor results from neurokinin A binding with high affinity only to a fraction of the sites labelled by substance P. The mutation increases the proportion of this site, and consequently enhances the apparent affinity and binding capacity of neurokinin A. 7. The binding modalities of septide-like ligands (i.e. neurokinin B, SP(6-11), SP-methyl ester) are affected similarly to neurokinin A and are better resolved into two sites. The mutation leaves the affinity of these ligands for the two receptor forms unchanged, but increases the fraction of high-affinity sites. On the other hand, the binding of non-peptide and peptide antagonists (SR140.333 and FK888) behaved similarly to substance P with a single high affinity site that is unaffected by the mutation. 8. These findings may suggest that the NK1 receptor exists in two different forms with similar affinity for substance P and NK1 antagonists, but with a high and a low affinity for neurokinin A and septide-like ligands. Hence, the Gly166 in the NK1 receptor would seem to control the distribution between a pan-reactive form and a substance P-selective form of the receptor.