In Vivo Characterization of High Basal Signaling from the Ghrelin Receptor

Abstract

The receptor for the orexigenic peptide, ghrelin, is one of the most constitutively active 7TM receptors known, as demonstrated under in vitro conditions. Change in expression of a constitutively active receptor is associated with change in signaling independent of the endogenous ligand. In the following study, we found that the expression of the ghrelin receptor in the hypothalamus was up-regulated approximately 2-fold in rats both during 48-h fasting and by streptozotocin-induced hyperphagia. In a separate experiment, to probe for the effect of the high basal signaling of the ghrelin receptor in vivo, we used intracerebroventricular administration by osmotic pumps of a peptide [D-Arg(1), D-Phe(5), D-Trp(7,9), Leu(11)]-substance P. This peptide selectively displays inverse agonism at the ghrelin receptor as compared with an inactive control peptide with just a single amino acid substitution. Food intake and body weight were significantly decreased in the group of rats treated with the inverse agonist, as compared with the groups treated with the control peptide or the vehicle. In the hypothalamus, the expression of neuropeptide Y and uncoupling protein 2 was decreased by the inverse agonist. In a hypothalamic cell line that endogenously expresses the ghrelin receptor, we observed high basal activity of the cAMP response element binding protein, an important signaling transduction pathway for appetite regulation. The activation was further increased by ghrelin administration and decreased by administration of the inverse agonist. It is suggested that the high constitutive signaling activity is important for the in vivo function of the ghrelin receptor in the control of food intake and body weight.

Mutually opposite signal modulation by hypothalamic heterodimerization of ghrelin- and melanocortin-3 receptors

“…We further demonstrate that ligand-independent GHSR signaling activity plays a key role in this mutual and opposite receptor interrelation and is modulated by interaction with MC3R. This series of observations supports the importance of basal GHSR activity for body weight regulation (19) and for functional impact of GPCR dimerization and likely translates into the principal biology of other GPCRs characterized by a permanent basally active conformation.…”

Agonism, Antagonism, and Inverse Agonism Bias at the Ghrelin Receptor Signaling

“…These results highlighted the fact that ligands targeting GHS-R1a might be classified as neutral or partial agonists depending on the level of constitutive activity of the receptor. This point is of importance because constitutive activity of the GHS-R1a was demonstrated in vivo in rat brain (48) and in human somatotroph adenomas (49). However, the exact level of constitutive activity of the receptor was not ascertained due to the difficulty of its in vivo quantification.…”

Ligands and signaling proteins govern the conformational landscape explored by a G protein-coupled receptor

“…One of the main characteristics of the ghrelin receptor is its high constitutive activity in vitro (14), in heterologous expression systems (22), and in vivo (23). It is particularly striking that the receptor in the absence of an effector is characterized by a single, major conformation R. Indeed in the presence of Gq only does the receptor adopt a more complex conformational landscape with at least two different substates, R and R* 2 (Fig.…”