Ligand-Specific Signaling Profiles and Resensitization Mechanisms of the Neuromedin U2 Receptor

The structurally related, but distinct neuropeptides, neuromedin U (NmU) and neuromedin S (NmS) are ligands of two G protein-coupled NmU receptors (NMU1, NMU2). Hypothalamic NMU2 regulates feeding behavior and energy expenditure and has therapeutic potential as an anti-obesity target, making an understanding of its signaling and regulation of particular interest. NMU2 binds both NmU and NmS with high affinity, resulting in receptor-ligand co-internalization. We have investigated whether receptor trafficking events post-internalization are 'biased' by the ligand bound and can therefore influence signaling function. Using recombinant cell-lines expressing human NMU2, we demonstrate that acute Ca2+ signaling responses to NmU or NmS are indistinguishable and that restoration of responsiveness (resensitization) requires receptor internalization and endosomal acidification. The rate of NMU2 resensitization is faster following NmU compared to NmS exposure, but is similar if endothelin-converting enzyme-1 activity is inhibited or knocked-down. Although acute activation of extracellular signal-regulated kinase (ERK) is also similar, activation by NMU2 is longer-lasting if NmS is the ligand. Furthermore, when cells were briefly challenged before removal of free, but not receptor-bound ligand, activation of ERK and p38 mitogen-activated protein kinase by NmS is more sustained, but only NmU responses are potentiated and extended by ECE-1 inhibition. These data indicate that differential intracellular ligand processing produces different signaling and receptor resensitization profiles and add to the findings of other studies demonstrating that intracellular ligand processing can shape receptor behavior and signal transduction.