Synthesis of fluoresceinyl-neurokinin-A, a biologically active probe for NK2 receptors

Probing the binding domain of the NK2 receptor with fluorescent ligands: evidence that heptapeptide agonists and antagonists bind differently

We have investigated the interaction of fluorescent peptide ligands with the G protein-coupled receptor NK2 using novel spectrofluorometric approaches. Several heptapeptide antagonists of structure PhCO-Xaa-Ala-D-Trp-Phe-D-Pro-Pro-Nle-NH2 were labelled on position 1 (Xaa) with the environment-sensitive nitrobenzoxadiazole (NBD) probe, differing only in the length of the spacer between the NBD group and the peptide. Upon binding of the labelled antagonist to NK2 receptors stably expressed in Chinese hamster ovary (CHO) cells, an increase in NBD fluorescence was observed when the spacer length was less than 10 A. Collisional quenching experiments using iodide and Co2+ ions were performed to define the accessibility of the NBD group on bound ligands to the solvent. By comparing ligands with spacer arms of varying lengths, we found that the binding pocket is buried at a depth of 5-10 A. In contrast, N-terminally NBD-labelled agonists, decapeptide neurokinin A (NKA) or heptapeptide Nle10-NKA[4-10], bound to the NK2 receptor were accessible to the solvent. Binding of fluorescent ligands to the NK2 receptor was accompanied by an enhancement in the fluorescence anisotropy. The changes in fluorescence properties were used to determine the kinetic parameters of antagonist binding and dissociation. These results indicate that the binding site on the NK2 receptor for the amino-terminal end of the heptapeptide antagonists is buried in the hydrophobic pocket of the receptor protein and clearly distinct from the binding site for the amino-terminal end of agonists, which is accessible to the solvent.(ABSTRACT TRUNCATED AT 250 WORDS)

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.