Ghrelin Signaling: GOAT and GHS-R1a Take a LEAP in Complexity

Abstract

Ghrelin and the growth hormone secretagogue receptor 1a (GHS-R1a) are important targets for disorders related to energy balance and metabolic regulation. Pharmacological control of ghrelin signaling is a promising avenue to address health issues involving appetite, weight gain, obesity, and related metabolic disorders, and may be an option for patients suffering from wasting conditions like cachexia. In this review, we summarize recent developments in the biochemistry of ghrelin and GHS-R1a signaling. These include unravelling the enzymatic transformations that generate active ghrelin and the discovery of multiple proteins that interact with ghrelin and GHS-R1a to regulate signaling. Furthermore, we propose that harnessing these processes will lead to highly selective treatments to address obesity, diabetes, and other metabolism-linked disorders.

Keywords: GHS-R1a; GPCR dimers; LEAP2; diabetes; energy balance; ghrelin; ghrelin O-acyltransferase; obesity.

Figure 1.. Ghrelin is activated for biological signaling by ghrelin O-acyltransferase (GOAT).

Following expression in endocrine cells, ghrelin undergoes processing including serine octanoylation by GOAT (orange tail) prior to secretion into the bloodstream. In circulation, esterases can remove the octanoyl group to produce unacylated ghrelin (UAG). When reaching a cell expressing the GHS-R1a receptor, acylated ghrelin can bind and activate downstream signaling. In some cell types, plasma membrane-exposed GOAT has been proposed to catalyze re-acylation of UAG to generate ghrelin which can then activate GHS-R1a signaling.

Figure 2.. GHS-R1a signaling.

Ghrelin and the liver-expressed antimicrobial peptide 2 (LEAP2) are endogenous ligands for GHS-R1a, with ghrelin acting as an agonist and LEAP2 acting as an antagonist. GHS-R1a is an αq G-coupled protein receptor that, upon activation, induces the activation of several intracellular signaling pathways including the activation and accumulation of phospholipase C γ (PLCγ), the activation of β-arrestin, and the accumulation of inositol phosphatase 3 kinase (IP3K). Activation of GHS-R1a also results in the production of diacylglycerol (DAG), an intracellular signal that in the presence of diacylglycerol lipase (DAGL) is converted into the endocannabinoid 2-arachidonoylglycerol (2-AG). Increased concentrations of PLCγ stimulate the release of Ca⁺ from the endoplasmic reticulum, with higher Ca⁺ concentrations then facilitating calcium/calmodulin kinase (CaCMK)-catalyzed phosphorylation of adenosine monophosphate-activated protein kinase (pAMPK). The presence of pAMPK is associated with many of the metabolic effects of ghrelin induced GHS-R1a activation. In the absence of ghrelin, GHS-R1a has relatively high constitutive activity that can be reduced by treatment with inverse agonists. Finally, GHS-R1a stimulation is also associated with increased phosphorylation of ERK via the β-arrestin pathway, and increase AKT phosphorylation via increased levels of PI3K.

Figure 3.. GHS-R1a agonists and antagonists.

The activation of GHS-R1a can be blocked by a number of compounds that act as antagonists in the presence of ghrelin. One such molecule is JMV2959, a commonly used compound that blocks ghrelin induced GHS-R1a signaling. This compound, however, acts as a partial agonist in the absence of ghrelin and can activate Gαq signaling through GHS-R1a. At the same time, JMV2959-induced activation prevents GHS-R1a internalization provoked by the activation of the β-arrestin pathway, potentially preventing GHS-R1a desensitization.

Figure 4.. GHS-R1a dimerizes with other GPCRs to bias receptor signaling.

GHS-R1a can form dimers with a number of other integral membrane proteins in the presence or absence of ghrelin. For example, GHS-R1a can dimerize with the melanocortin accessory protein 2 (MRAP-2), a protein that facilitates melanocortin receptor signaling. GHS-R1a also dimerizes with dopamine D1 and D2 receptors and can bias their downstream signaling in both ligand-dependent and ligand-independent manners.