Tachykinin peptide-induced activation and desensitization of neurokinin 1 receptors

Delineation of the motilin domain involved in desensitization and internalization of the motilin receptor by using full and partial antagonists

Studies with fragments of the gastrointestinal peptide, motilin, indicate that the C-terminal region of this peptide plays an important role in the desensitization of the motilin receptor (MTLR).

AIM

To verify this hypothesis we studied the desensitization, phosphorylation and internalization induced by motilin analogues of different chain length with agonistic and antagonistic properties in CHO-MTLR cells.

METHODS

We studied motilin [1-22], the [1-14] fragment, the analogues Phe(3)[1-22] and Phe(3)[1-14], and two putative antagonists, GM-109 and MA-2029 (modified 1-4 and 1-3 fragments). Activation and desensitization (2h preincubation with the motilin analogues 10muM) were studied in CHO-MTLR cells by an aequorin based luminescence assay. Phosphorylation was studied by immunoprecipitation and internalization was visualized in CHO-MTLR cells containing an enhanced green fluorescent protein (CHO-MTLR-EGFP).

RESULTS

Motilin [1-22] and [1-14] were more potent than Phe(3)[1-22] and Phe(3)[1-14] (pEC(50): 9.77, 8.78, 7.36 and 6.65, respectively) to induce Ca(2+) release. GM-109 and MA-2029 were without agonist activity. [1-22] and Phe(3)[1-22] decreased the second response to motilin from 78+/-2% to 11+/-3% and 34+/-3% (P<0.001), respectively, whereas [1-14], Phe(3)[1-14], GM-109 and MA-2029 had no desensitizing effect (68+/-5%, 78+/-3%, 78+/-6% and 78+/-5%, respectively, P>0.05). The rank order of MTLR-phosphorylation was: [1-22]>[1-14]>Phe(3)[1-22]=Phe(3)[1-14]>GM-109=MA-2029. Only motilin [1-22] and [1-14] induced receptor MTLR-EGFP internalization as shown by a decrease in membrane fluorescence: 20+/-3% and 7+/-3%, respectively.

CONCLUSION

The C-terminus of motilin enhances desensitization, phosphorylation and internalization of the MTLR while modifications of the N-terminus can favor a conformation of the receptor that is less susceptible to phosphorylation and internalization.

Functional Selectivity of NK1Receptor Signaling: Peptide Agonists Can Preferentially Produce Receptor Activation or Desensitization

A cascade of events follows binding of an agonist ligand to the tachykinin NK(1) receptor. These events include activation of multiple signal transduction pathways as well as cellular modulation of receptor function by the process of desensitization. This study examines the differences in the abilities of naturally occurring peptide agonist ligands of the tachykinin NK(1) receptor to preferentially direct signaling through the receptor to produce signal activation versus receptor desensitization. The differential effects of tachykinin peptides with respect to ligand competition binding, receptor-G protein coupling, intracellular Ca(2+) elevations, and receptor desensitization have been measured. In relation to its potency in competition binding studies, substance P produces desensitization at lower concentrations, whereas higher concentrations are required to elicit a Ca(2+) response. In contrast to this, a related peptide, ranatachykinin C, is more effective at activating a Ca(2+) response relative to its ability to produce desensitization. This difference in functional selectivity is conserved for an amphibian and two mammalian ortholog tachykinin receptors. The present study demonstrates that peptide agonist ligands of NK(1) receptors can preferentially produce signal activation or desensitization.

Effects of peptides, with emphasis on feeding, pain, and behavior A 5-year (1999-2003) review of publications in Peptides

Novel effects of naturally occurring peptides are continuing to be discovered, and their mechanisms of actions as well as interactions with other substances, organs, and systems have been elucidated. Synthetic analogs may have actions similar or antagonistic to the endogenous peptides, and both the native peptides and analogs have potential as drugs or drug targets. The journal Peptides publishes many leading articles on the structure-activity relationship of peptides as well as outstanding reviews on some families of peptides. Complementary to the reviews, here we extract information from the original papers published during the past five years in Peptides (1999-2003) to summarize the effects of different classes of peptides, their modulation by other chemicals and various pathophysiological states, and the mechanisms by which the effects are exerted. Special attention is given to peptides related to feeding, pain, and other behaviors. By presenting in condensed form the effects of peptides which are essential for systems biology, we hope that this summary of existing knowledge will encourage additional novel research to be presented in Peptides.