Functional group and stereochemical requirements for substrate binding by ghrelin O-acyltransferase revealed by unnatural amino acid incorporation

Therapeutic potential of ghrelin/GOAT/GHSR system in gastrointestinal disorders

Ghrelin, a peptide primarily secreted in the stomach, acts via the growth hormone secretagogue receptor (GHSR). It regulates several physiological processes, such as feeding behavior, energy homeostasis, glucose and lipid metabolism, cardiovascular function, bone formation, stress response, and learning. GHSR exhibits significant expression within the central nervous system. However, numerous murine studies indicate that ghrelin is limited in its ability to enter the brain from the bloodstream and is primarily confined to specific regions, such as arcuate nucleus (ARC) and median eminence (ME). Nevertheless, the central ghrelin system plays an essential role in regulating feeding behavior. Furthermore, the role of vagal afferent fibers in regulating the functions of ghrelin remains a major topic of discussion among researchers. In recent times, numerous studies have elucidated the substantial therapeutic potential of ghrelin in most gastrointestinal (GI) diseases. This has led to the development of numerous pharmaceutical agents that target the ghrelin system, some of which are currently under examination in clinical trials. Furthermore, ghrelin is speculated to serve as a promising biomarker for GI tumors, which indicates its potential use in tumor grade and stage evaluation. This review presents a summary of recent findings in research conducted on both animals and humans, highlighting the therapeutic properties of ghrelin system in GI disorders.

Combined Computational-Biochemical Approach Offers an Accelerated Path to Membrane Protein Solubilization

Membrane proteins are difficult to isolate and purify due to their dependence on the surrounding lipid membrane for structural stability. Detergents are often used to solubilize these proteins, with this approach requiring a careful balance between protein solubilization and denaturation. Determining which detergent is most appropriate for a given protein has largely been done empirically through screening, which requires large amounts of membrane protein and associated resources. Here, we describe an alternative to conventional detergent screening using a computational modeling approach to identify the most likely candidate detergents for solubilizing a protein of interest. We demonstrate our approach using ghrelin O-acyltransferase (GOAT), a member of the membrane-bound O-acyltransferase family of integral membrane enzymes that has not been solubilized or purified in active form. A computationally derived GOAT structural model provides the only structural information required for this approach. Using computational analysis of detergent ability to penetrate phospholipid bilayers and stabilize the GOAT structure, a panel of common detergents were rank-ordered for their proposed ability to solubilize GOAT. The simulations were performed at all-atom resolution for a combined simulation time of 24 μs. Independently, we biologically screened these detergents for their solubilization of fluorescently tagged GOAT constructs. We found computational prediction of protein structural stabilization was the better predictor of detergent solubilization ability, but neither approach was effective for predicting detergents that would support GOAT enzymatic function. The current rapid expansion of membrane protein computational models lacking experimental structural information and our computational detergent screening approach can greatly improve the efficiency of membrane protein detergent solubilization, supporting downstream functional and structural studies.

Cellular Uptake of a Fluorescent Ligand Reveals Ghrelin O-Acyltransferase Interacts with Extracellular Peptides and Exhibits Unexpected Localization for a Secretory Pathway Enzyme

Ghrelin O-acyltransferase (GOAT) plays a central role in the maturation and activation of the peptide hormone ghrelin, which performs a wide range of endocrinological signaling roles. Using a tight-binding fluorescent ghrelin-derived peptide designed for high selectivity for GOAT over the ghrelin receptor GHSR, we demonstrate that GOAT interacts with extracellular ghrelin and facilitates ligand cell internalization in both transfected cells and prostate cancer cells endogenously expressing GOAT. Coupled with enzyme mutagenesis, ligand uptake studies support the interaction of the putative histidine general base within GOAT with the ghrelin peptide acylation site. Our work provides a new understanding of GOAT's catalytic mechanism, establishes that GOAT can interact with ghrelin and other peptides located outside the cell, and raises the possibility that other peptide hormones may exhibit similar complexity in their intercellular and organismal-level signaling pathways.

Ghrelin octanoylation by ghrelin O-acyltransferase: protein acylation impacting metabolic and neuroendocrine signalling

The acylated peptide hormone ghrelin impacts a wide range of physiological processes but is most well known for controlling hunger and metabolic regulation. Ghrelin requires a unique posttranslational modification, serine octanoylation, to bind and activate signalling through its cognate GHS-R1a receptor. Ghrelin acylation is catalysed by ghrelin O-acyltransferase (GOAT), a member of the membrane-bound O-acyltransferase (MBOAT) enzyme family. The ghrelin/GOAT/GHS-R1a system is defined by multiple unique aspects within both protein biochemistry and endocrinology. Ghrelin serves as the only substrate for GOAT within the human proteome and, among the multiple hormones involved in energy homeostasis and metabolism such as insulin and leptin, acts as the only known hormone in circulation that directly stimulates appetite and hunger signalling. Advances in GOAT enzymology, structural modelling and inhibitor development have revolutionized our understanding of this enzyme and offered new tools for investigating ghrelin signalling at the molecular and organismal levels. In this review, we briefly summarize the current state of knowledge regarding ghrelin signalling and ghrelin/GOAT enzymology, discuss the GOAT structural model in the context of recently reported MBOAT enzyme superfamily member structures, and highlight the growing complement of GOAT inhibitors that offer options for both ghrelin signalling studies and therapeutic applications.

Recent progress in the discovery of ghrelin O-acyltransferase (GOAT) inhibitors

Ghrelin is a stomach-derived peptide hormone which stimulates appetite. For ghrelin to exert its orexigenic effect, octanoylation on the serine-3 residue of this gut-brain peptide is essential. The octanoylation of ghrelin is mediated by a unique acyltransferase enzyme known as ghrelin O-acyltransferase (GOAT). Thus modulating this enzyme offers viable approaches to alter feeding behaviors. Over the past decade, several small-molecule based approaches have appeared dealing with the discovery of compounds able to modulate this enzyme for the treatment of obesity and type 2 diabetes. Drug discovery efforts from academic groups and several pharmaceutical companies have fielded compounds having efficacy in altering acylated ghrelin levels in animal models but to date, compounds modulating the activity of the GOAT enzyme do not yet represent clinical options. This mini-review covers the drug discovery approaches of the last decade since the discovery of the GOAT enzyme.

Current and emerging therapies for managing hyperphagia and obesity in Prader-Willi syndrome: A narrative review

In early childhood, individuals with Prader-Willi syndrome (PWS) experience excess weight gain and severe hyperphagia with food compulsivity, which often leads to early onset morbid obesity. Effective treatments for appetite suppression and weight control are currently unavailable for PWS. Our aim to further understand the pathogenesis of PWS led us to carry out a comprehensive search of the current and emerging therapies for managing hyperphagia and extreme weight gain in PWS. A literature search was performed using PubMed and the following keywords: "PWS" AND "therapy" OR "[drug name]"; reference lists, pharmaceutical websites, and the ClinicalTrials.gov registry were also reviewed. Articles presenting data from current standard treatments in PWS and also clinical trials of pharmacological agents in the pipeline were selected. Current standard treatments include dietary restriction/modifications, exercise, and growth hormone replacement, which appear to have limited efficacy for appetite and weight control in patients with PWS. The long-term safety and effectiveness of bariatric surgery in PWS remains unknown. However, many promising pharmacotherapies are in development and, if approved, will bring much needed choices into the PWS pharmacological armamentarium. With the progress that is currently being made in our understanding of PWS, an effective treatment may not be far off.

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

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.

Prader-Willi syndrome: A model for understanding the ghrelin system

Subsequent to the discovery of ghrelin as the endogenous ligand of growth hormone secretagogue receptor 1a, this unique gut peptide has been found to exert numerous physiological effects, such as appetite stimulation and lipid accumulation via the central regulating mechanisms in the hypothalamus, stimulation of gastric motility, regulation of glucose metabolism and brown fat thermogenesis, and modulation of stress, anxiety, taste sensation, reward-seeking behaviour and the sleep/wake cycle. Prader-Willi syndrome (PWS) has been described as a unique pathological state characterised by severe obesity and high circulating levels of ghrelin. It was hypothesised that hyperghrelinaemia would explain at least a part of the feeding behaviour and body composition of PWS patients, who are characterised by hyperphagia, an obsession with food and food-seeking, and increased adiposity. Initially, the link between hyperghrelinaemia and growth hormone deficiency, which is observed in 90% of the children with PWS, was not fully understood. Over the years, however, the increasing knowledge on ghrelin, PWS features and the natural history of the disease has led to a more comprehensive description of the abnormal ghrelin system and its role in the pathophysiology of this rare and complex neurodevelopmental genetic disease. In the present study, we (a) present the current view of PWS; (b) explain its natural history, including recent data on the ghrelin system in PWS patients; and (c) discuss the therapeutic approach of modulating the ghrelin system in these patients and the first promising results.

Ghrelin octanoylation by ghrelin O-acyltransferase: Unique protein biochemistry underlying metabolic signaling

Ghrelin is a small peptide hormone that requires a unique post-translational modification, serine octanoylation, to bind and activate the GHS-R1a receptor. Ghrelin signaling is implicated in a variety of neurological and physiological processes, but is most well known for its roles in controlling hunger and metabolic regulation. Ghrelin octanoylation is catalyzed by ghrelin O-acyltransferase (GOAT), a member of the membrane-bound O-acyltransferase (MBOAT) enzyme family. From the status of ghrelin as the only substrate for GOAT in the human genome to the source and requirement for the octanoyl acyl donor, the ghrelin-GOAT system is defined by multiple unique aspects within both protein biochemistry and endocrinology. In this review, we examine recent advances in our understanding of the interactions and mechanisms leading to ghrelin modification by GOAT, discuss the potential sources for the octanoyl acyl donor required for ghrelin's activation, and summarize the current landscape of molecules targeting ghrelin octanoylation through GOAT inhibition.