The ghrelin receptor is a 7 transmembrane (7TM) receptor involved in a variety of physiological functions including growth hormone secretion, increased food intake and fat accumulation as well as modulation of reward and cognitive functions. Because of its important role in metabolism and energy expenditure, the ghrelin receptor has become an important therapeutic target for drug design and the development of anti-obesity compounds. However, none of the compounds developed so far have been approved for commercial use. Interestingly, the ghrelin receptor is able to signal through several different signalling pathways including Gαq, Gαi/o, Gα12/13 and arrestin recruitment. These multiple signalling pathways allow for functionally biased signalling, where one signalling pathway may be favoured over another either by selective ligands or through mutations in the receptor. In the present review, we have described how ligands and mutations in the 7TM receptor may bias the receptors to favour either one G-protein over another or to promote G-protein independent signalling pathways rather than G-protein-dependent pathways. For the ghrelin receptor, both agonist and inverse agonists have been demonstrated to signal more strongly through the Gαq-coupled pathway than the Gα12/13-coupled pathway. Similarly a ligand that promotes Gαq coupling over Gαi coupling has been described and it has been suggested that several different active conformations of the receptor may exist dependent on the properties of the agonist. Importantly, ligands with such biased signalling properties may allow the development of drugs that selectively modulate only the therapeutically relevant physiological functions, thereby decreasing the risk of side effects. LINKED ARTICLESThis article is part of a themed section on Neuropeptides. To view the other articles in this section visit http://dx.doi. org/10.1111/bph.2013.170.issue-7 Abbreviations 7TM, 7 transmembrane segment; AgRP, agouti-related peptide; AMPK, AMP-activated PK; AP2, adaptor-related protein complex 2; AT1A receptor, angiotensin 1A receptor; AVP, arginine-vasopressin; CREB, cAMP response element binding protein; ECL, extracellular loop; FAS, fatty acid synthase; GH, growth hormone; GHS, growth hormone secretagogues; GRK, GPCR kinase; IP, inositol phosphate; IP3, inositol tris-1,4,5-phosphate; Isn, isonipecotic acid; MC4, melanocortin 4; NPY, neuropeptide Y; SRE, serum response element; UCP2, uncoupling protein 2; VTA, ventral tegmental area; WT, wild type Introduction to ghrelin receptor physiologyThe ghrelin receptor, previously known as the growth hormone secretagogue (GHS) receptor 1a (for receptor nomenclature see Alexander et al., 2011), is the receptor for the anabolic hormone ghrelin. This receptor is involved in GH secretion, appetite regulation, fat accumulation and energy expenditure. In addition it can also modulate behaviour and mood (van der Lely et al., 2004; Dickson et al., 2011; Figure 1). GHThe ghrelin receptor was initially cloned as a receptor important for GH secr...

]substance P, a series of novel, small, peptidemimetic agonists for the ghrelin receptor were generated. By using various simple, ring-constrained spacers connecting the D-Trp-Phe-D-Trp motif with the important C-terminal carboxyamide group, 40 nM agonism potency was obtained and also in one case (wFw-Isn-NH 2 , where Isn is isonipecotic acid) ϳ80% efficacy. However, in contrast to all previously reported ghrelin receptor agonists, the piperidine-constrained wFw-Isn-NH 2 was found to be a functionally biased agonist. Thus, wFw-Isn-NH 2 mediated potent and efficacious signaling through the G␣ q and ERK1/2 signaling pathways, but in contrast to all previous ghrelin receptor agonists it did not signal through the serum response element, conceivably the G␣ 12/13 pathway. The recognition pattern of wFw-Isn-NH 2 with the ghrelin receptor also differed significantly from that of all previously characterized unbiased agonists. Most importantly, wFw-Isn-NH 2 was not dependent on GluIII:09 (Glu3.33), which otherwise is an obligatory TM III anchor point residue for ghrelin agonists. Molecular modeling and docking experiments indicated that wFw-Isn-NH 2 binds in the classical agonist binding site between the extracellular segments of TMs III, VI, and VII, interacting closely with the aromatic cluster between TMs VI and VII, but that it does so in an opposite orientation as compared with, for example, the wFw peptide agonists. It is concluded that the novel peptide-mimetic ligand wFw-Isn-NH 2 is a biased ghrelin receptor agonist and that the selective signaling pattern presumably is due to its unique receptor recognition pattern lacking interaction with key residues especially in TM III.Ghrelin is a neuroendocrine hormone that differs from other peptide hormones by a fatty acid modification, which is crucial for both the binding and activation of its receptor (1). Ghrelin is synthesized mainly in the gastrointestinal tract, where the gene coding for the peptide sequence is expressed together with the enzyme responsible for the acylation of the fatty acid to the ghrelin peptide sequence (2, 3). Multiple functions have been described for ghrelin since it was discovered. Initially it was believed that growth hormone secretion induced by ghrelin receptors in the hypothalamus and the pituitary was the primary function of ghrelin (4). However, the function of ghrelin in the hypothalamus, and in particular in the arcuate nucleus, has become the focus of attention over the last decade. In the arcuate nucleus, ghrelin is responsible for increased activity in the NPY (neuropeptide Y) and AGRP (Agouti-related protein) neurones leading to increased appetite, decreased energy expenditure, and fat accumulation (5, 6). High receptor expression is also observed in the ventromedial nucleus of the hypothalamus, and the function in this area has been proposed to be orexigenic based on the regulation of fatty acid metabolism (7). More recently it has been demonstrated that ghrelin is also involved in reward-seeking behavior such as alcohol and ...

Ghrelin is a gastrointestinal polypeptide that acts through the ghrelin receptor (GHSR) to promote food intake and increase adiposity. Activation of GHSR requires the presence of a fatty-acid (FA) side chain on amino acid residue serine 3 of the ghrelin molecule. However, little is known about the role that the type of FA used for acylation plays in the biological action of ghrelin. We therefore evaluated a series of differentially acylated peptides to determine whether alterations in length or stability of the FA side chain have an impact on the ability of ghrelin to activate GHSR in vitro or to differentially alter food intake, body weight, and body composition in vivo. Fatty acids principally available in the diet (such as palmitate C16) and therefore representing potential substrates for the ghrelin-activating enzyme ghrelin O-acyltransferase (GOAT) were used for dose-, time-, and administration/route-dependent effects of ghrelin on food intake, body weight, and body composition in rats and mice. Our data demonstrate that altering the length of the FA side chain of ghrelin results in the differential activation of GHSR. Additionally, we found that acylation of ghrelin with a long-chain FA (C16) delays the acute central stimulation of food intake. Lastly, we found that, depending on acylation length, systemic and central chronic actions of ghrelin on adiposity can be enhanced or reduced. Together our data suggest that modification of the FA side-chain length can be a novel approach to modulate the efficacy of pharmacologically administered ghrelin.

The ghrelin receptor (nomenclature as agreed by the NC-IUPHAR Subcommittee for the Ghrelin receptor [19]) is activated by a 28 amino-acid peptide originally isolated from rat stomach, where it is cleaved from a 117 amino-acid precursor (GHRL, Q9UBU3). The human gene encoding the precursor peptide has 83% sequence homology to rat prepro-ghrelin, although the mature peptides from rat and human differ by only two amino acids [74]. Alternative splicing results in the formation of a second peptide, [des-Gln14]ghrelin with equipotent biological activity [49]. A unique post-translational modification (octanoylation of Ser3, catalysed by ghrelin Ο-acyltransferase (MBOAT4, Q96T53) [133] occurs in both peptides, essential for full activity in binding to ghrelin receptors in the hypothalamus and pituitary, and for the release of growth hormone from the pituitary [58]. Structure activity studies showed the first five N-terminal amino acids to be the minimum required for binding [4], and receptor mutagenesis has indicated overlap of the ghrelin binding site with those for small molecule agonists and allosteric modulators of ghrelin function [44]. An endogenous antagonist and inverse agonist called Liver enriched antimicrobial peptide 2 (Leap2), expressed primarily in hepatocytes and in enterocytes of the proximal intestine [35, 68] inhibits ghrelin receptor-induced GH secretion and food intake [35]. The secretion of Leap2 and ghrelin is inversely regulated under various metabolic conditions [71]. In cell systems, the ghrelin receptor is constitutively active [45], but this is abolished by a naturally occurring mutation (A204E) that results in decreased cell surface receptor expression and is associated with familial short stature [93].

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