Interaction of tachykinins with their receptors studied with cyclic analogues of substance P and neurokinin B

Characterization of a conformationally sensitive TOAC spin-labeled substance P

To probe the binding of a peptide agonist to a G-protein coupled receptor in native membranes, the spin-labeled amino acid analogue 4-amino-4-carboxy-2,2,6,6-tetramethylpiperidino-1-oxyl (TOAC) was substituted at either position 4 or 9 within the substance P peptide (RPKPQQFFGLM-NH2), a potent agonist of the neurokinin-1 receptor. The affinity of the 4-TOAC analog is comparable to the native peptide while the affinity of the 9-TOAC derivative is approximately 250-fold lower. Both peptides activate receptor signaling, though the potency of the 9-TOAC peptide is substantially lower. The utility of these modified ligands for reporting conformational dynamics during the neurokinin-1 receptor activation was explored using EPR spectroscopy, which can determine the real-time dynamics of the TOAC nitroxides in solution. While the binding of both the 4-TOAC substance P and 9-TOAC substance P peptides to isolated cell membranes containing the neurokinin-1 receptor is detected, a bound signal for the 9-TOAC peptide is only obtained under conditions that maintain the receptor in its high-affinity binding state. In contrast, 4-TOAC substance P binding is observed by solution EPR under both low- and high-affinity receptor states, with evidence of a more strongly immobilized peptide in the presence of GDP. In addition, to better understand the conformational consequences of TOAC substitution into substance P as it relates to receptor binding and activation, atomistic models for both the 4- and 9-TOAC versions of the peptide were constructed, and the molecular dynamics calculated via simulated annealing to explore the influence of the TOAC substitutions on backbone structure.

Synthesis and chemical stability of a disulfide bond in a model cyclic pentapeptide: cyclo(1,4)-Cys-Gly-Phe-Cys-Gly-OH

Many cyclic peptides are formed using a disulfide bond to increase their conformational rigidity; this provides receptor selectivity and increased potency. However, degradation of the disulfide bond in formulation can lead to a loss of structural stability and biological activity of the peptide. Therefore, the objective of this study was to study the stability of peptide 1 (cyclo(1,4)-Cys-Gly-Phe-Cys-Gly-OH). This cyclic peptide was synthesized using Boc strategy via solution-phase peptide synthesis and purified using semi-preparative HPLC. The accelerated stability studies of the cyclic peptide were conducted in buffer solutions at pH 1.0-11.0 with controlled ionic strengths at 70 degrees C. The pH-rate profile shows that the peptide has an optimal stability around pH 3.0 with a V-shape between pH 1.0 and 5.0. Two small plateaus are observed at pH 5.0-7.0 and pH 8.0-10.0, indicating hydrolysis on different ionized forms of the cyclic peptide. One product was observed at acidic pH due to peptide bond hydrolysis at Gly2-Phe3. The number of degradation products increases as the pH increases from neutral to basic, and most of the degradation products at neutral and basic pH are derived from the degradation at the disulfide bond.

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.

A 25 year adventure in the field of tachykinins

Several aspects of our 25 year adventure in the field of tachykinins will be successively described. They concern: substance P (SP) synthesis and release in the basal ganglia, the identification and pharmacological characterization of central tachykinin NK(1), NK(2) and NK(3) binding sites and their topographical distribution, the description of some new biological tests for corresponding receptors, the identification of tachykinin NK(1) receptor subtypes or conformers sensitive to all endogenous tachykinins (substance P, neurokinin A (NKA), neurokinin B (NKB), neuropeptide gamma (NP gamma) and neuropeptide K (NPK)) and finally, the functional involvement of these receptors and their subtypes in tachykinin-induced regulations of dopamine and acetylcholine release in the striatum.

Substance P (free acid) adopts different conformation than native peptide in DMSO, water and DPPC bilayers

The conformation of substance P (free acid) (SPOH) has been investigated in dimethylsulfoxide (DMSO), water and dipalmitoylphosphotidylcholine (DPPC) bilayers by two-dimensional NMR and restraint molecular dynamics simulations. The observed NOE patterns for SPOH in these media are very much different from each other. Molecular modeling of the conformation of SPOH by incorporating NOEs as distance restraints shows wide differences in its conformation in three media. The main structural features for SPOH in DMSO are y-bends at Pro4 and Phe7 along with a non-specific bend around Lys3-Pro4-Gln5-Gln6, which are stabilized by Lys3CO-->Gln5NH, Gln6CO-->Phe8NH hydrogen bonding. The more flexible conformation of SPOH in water is transformed to an ordered structure after incorporation in DPPC bilayers. The conformation of SPOH in DPPC bilayers is characterized by gamma-bends at Pro4, Gln6 and Phe7, which are stabilized by hydrogen bonding between Lys3CO-->Gln5NH, Gln5CO-->Phe7NH and Gln6CO-->Phe8NH, respectively. The absence of biological activity in SPOH has been attributed to the absence of any helix like structure at the central residues and absence of any interresidue interaction with C-terminal OH group, in DPPC bilayers, a feature shown to be an important prerequisite for SP and SP agonists to bind to the NKI tachykinin receptor.

Lipid-induced conformation of substance P

Both the aqueous and the lipid-induced structure of a representative and widely studied tachykinin, substance P, has been investigated by two-dimensional proton nuclear magnetic resonance (2D 1H-NMR) spectroscopy and distance geometry calculations. Unambiguous NMR assignments of protons have been made with the aid of correlation spectroscopy (COSY and TOCSY) experiments and Overhauser enhancement spectroscopy (ROESY and NOESY; experiments. The NMR data obtained were utilized in a distance geometry algorithm to generate a family of structures which were further refined using restrained energy minimization. These data show that, while in water substance P appears to favour an extended chain conformation, in the presence of perdeuterated dodecylphosphocholine (DPC) micelles as membrane model system an amphiphilic helical conformation is induced in the mid-region (Q5-Q8) of substance P. The conformation adopted by substance P in the presence of DPC micelles yields a structural motif typical of neurokinin-1 selective ligands, as proposed by Convert and coworkers (O. Convert et al., Neuropeptides 19, 259-270 (1991)).

Computational molecular modeling in peptide drug design

The review concentrates on practical applications of computer molecular modeling in peptide drug design. The examples of the predictions (successful or not) made by computational modeling before synthesis of peptide analogs, not the explanations provided after synthesis and biological testing of peptides, are discussed. The review spans over 20 years of predictions made by computer molecular modeling for bradykinin, angiotensin, thyrotropin-releasing factor, tuftsin, substance P, CCK-related peptides, luliberin, alpha-melanotropin and opioid peptides. The described examples are discussed in terms of finding the optimal way to use computer modeling for peptide design. The step-by-step 'technology' of peptide design is outlined in detail.

Selective agonists of tachykinin binding sites

Three types of binding sites for the mammalian tachykinins, ie Substance P (SP) Neurokinin A (NKA) and Neurokinin B (NKB), have been found in both the central and peripheral nervous systems. Substance P binds to the NK-1 subclass of binding site while NKA and NKB are less selective endogenous ligands, which preferentially interact with the NK-2 and NK-3 subclasses of binding sites, respectively. Complementary strategies, including 3-dimensional structure analysis by NMR spectroscopy and structure-activity relationship led to the design of selective agonists of these binding sites. [Pro9] SP, [Pro10] SP and the cyclic analogues [Cys3,6, Tyr8, Pro9] SP and [Cys3,6, Tyr8, Pro10] SP are selective NK-1 agonists. [Lys5] NKA(4-10) is a water soluble NK-2 potent agonist. Finally, [Pro7] NKB, which completely discriminates NK-2 and NK-3 binding sites, is a water-soluble NK-3 selective agonist.

Tachykinin receptors of the NK1 type (substance P) coupled positively to phospholipase C on cortical astrocytes from the newborn mouse in primary culture

Specific 125I-Bolton-Hunter substance P (125I-BHSP) binding sites are present on intact cortical astrocytes of the newborn mouse in primary culture. Therefore, these cells were used to ascertain the existence of functional substance P (SP) receptors coupled positively to phospholipase C. SP stimulated phosphoinositide breakdown with an EC50 value (4.5 x 10(-10) M) similar to its IC50 value (3.8 x 10(-10) M) for inhibiting 125I-BHSP binding. The maximal response to (10(-6) M SP for 60 min) obtained was approximately 500% of control values. The rank order of potency of tachykinins was SP greater than neurokinin (NK) A greater than NKB. Long SP C-terminal fragments were more potent than shorter ones in stimulating the accumulation of 3H-inositol phosphates. SP free acid and SP N-terminal fragments were without effect. [L-Pro9]SP and SP methyl ester, two selective agonists of NK1 receptors, were almost as potent as SP. An excellent correlation was found when the abilities of tachykinins and their analogs for stimulating phosphoinositide breakdown and for inhibiting 125I-BHSP binding were compared. Finally, when used at a concentration of 3 x 10(-6) M, spantide [( D-Arg1, D-Trp7,9, Leu11]SP), an SP antagonist, competitively reduced the stimulatory effect of SP on accumulation of 3H-inositol phosphates. These results demonstrate the presence of functional SP receptors (NK1) on cortical astrocytes from the newborn mouse in primary culture.

Selective agonists for receptors of substance P and related neurokinins

Neurokinins and their receptors are a complex system consisting of at least three endogenous agents--substance P (SP), neurokinin A (NKA), and neurokinin B (NKB)--and their corresponding receptor types, respectively, NK-1, NK-2, and NK-3. Investigations on receptors have been made using sensitive and fairly selective pharmacological preparations (the dog carotid artery for the NK-1, the rabbit pulmonary artery devoid of endothelium for the NK-2, and the rat portal vein for the NK-3 receptor), and some natural peptides of mammalian and nonmammalian origin. Because of the nonselectivity of the natural peptides, analogues of the neurokinins have been found that act on one receptor only and show therefore high selectivity. The selective agonists [Sar9,Met(O2)11]SP, [Nle10]NKA (4-10), and [MePhe7]-NKB have been used successfully for (a) characterizing the three neurokinin receptors, (b) identifying isolated organs whose responses to neurokinins depend on the activation of a single (monoreceptor systems) or of more than one (multireceptor systems) receptor, and (c) elucidating some of the physiological function of the three receptor types. It is suggested that NK-1 mediate peripheral vasodilatation and exocrine secretions, NK-2 stimulate bronchial muscles and facilitate the release of catecholamines, and NK-3 promote the release of acetylcholine in peripheral organs.