A Single Amino Acid Mutation (R104P) in the E/DRY Motif of GPR40 Impairs Receptor Function

The orphan G protein-coupled receptor 141 expressed in myeloid cells functions as an inflammation suppressor

G protein-coupled receptors (GPCRs) regulate many cellular processes in response to various stimuli, including light, hormones, neurotransmitters, and odorants, some of which play critical roles in innate and adaptive immune responses. However, the physiological functions of many GPCRs and the involvement of them in autoimmune diseases of the central nervous system remain unclear. Here, we demonstrate that GPR141, an orphan GPCR belonging to the class A receptor family, suppresses immune responses. High GPR141 messenger RNA levels were expressed in myeloid-lineage cells, including neutrophils (CD11b + Gr1+), monocytes (CD11b + Gr1-Ly6C+ and CD11b + Gr1-Ly6C-), macrophages (F4/80+), and dendritic cells (CD11c+). Gpr141  -/- mice, which we independently generated, displayed almost no abnormalities in myeloid cell differentiation and compartmentalization in the spleen and bone marrow under steady-state conditions. However, Gpr141 deficiency exacerbated disease conditions of experimental autoimmune encephalomyelitis, an autoimmune disease model for multiple sclerosis, with increased inflammation in the spinal cord. Gpr141  -/- mice showed increased CD11b + Gr1+ neutrophils, CD11b + Gr1- monocytes, CD11c+ dendritic cells, and CD4+ T cell infiltration into the experimental autoimmune encephalomyelitis-induced spinal cord compared with littermate control mice. Lymphocytes enriched from Gpr141  -/- mice immunized with myelin oligodendrocyte glycoprotein 35-55 produced high amounts of interferon-γ, interleukin-17A, and interleukin-6 compared with those from wild-type mice. Moreover, CD11c+ dendritic cells (DCs) purified from Gpr141  -/- mice increased cytokine production of myelin oligodendrocyte glycoprotein 35-55-specific T cells. These findings suggest that GPR141 functions as a negative regulator of immune responses by controlling the functions of monocytes and dendritic cells and that targeting GPR141 may be a possible therapeutic intervention for modulating chronic inflammatory diseases.

Naturally occurring mutations in G protein-coupled receptors associated with obesity and type 2 diabetes mellitus

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors involved in the regulation of almost all known physiological processes. Dysfunctions of GPCR-mediated signaling have been shown to cause various diseases. The prevalence of obesity and type 2 diabetes mellitus (T2DM), two strongly associated disorders, is increasing worldwide, with tremendous economical and health burden. New safer and more efficacious drugs are required for successful weight reduction and T2DM treatment. Multiple GPCRs are involved in the regulation of energy and glucose homeostasis. Mutations in these GPCRs contribute to the development and progression of obesity and T2DM. Therefore, these receptors can be therapeutic targets for obesity and T2DM. Indeed some of these receptors, such as melanocortin-4 receptor and glucagon-like peptide 1 receptor, have provided important new drugs for treating obesity and T2DM. This review will focus on the naturally occurring mutations of several GPCRs associated with obesity and T2DM, especially incorporating recent large genomic data and insights from structure-function studies, providing leads for future investigations.

NPF activates a specific NPF receptor and regulates food intake in Pacific abalone Haliotis discus hannai

Neuropeptides function through G protein-coupled receptors (GPCRs) with high specificity, implying a significant degree of neuropeptide-GPCR coevolution. However, potential neuropeptide signaling systems in non-chordates are relatively elusive. We determined the specificity of the neuropeptide F (Hdh-NPF) signaling system with a cognate receptor (Hdh-NPFR) in the Pacific abalone, Haliotis discus hannai. Phylogenetic and exon–intron arrangement analyses of bilaterian NPF and the chordate ortholog NPY with their receptor sequences revealed a likely common ancestor, and Hdh-NPFR was similar to the NPYR2 subtype among the NPYR1, NPYR2, and NPYR5 subtypes. Among four Hdh-NPFR-related receptors, Hdh-NPFR specifically responded to Hdh-NPF peptide, supported by the dose–response luciferase reporter curve, intracellular Ca²⁺ mobilization, and phosphorylation of ERK1/2 and its inhibition with a protein kinase C inhibitor. Peptide fragmentations and shuffling of Hdh-NPF with human NPY could not activate the cellular response of Hdh-NPFR. Three-dimensional in silico modeling suggested that interaction of Hdh-NPF C-terminal amino acids with the extracellular loops of Hdh-NPFR is critical for Hdh-NPFR activation. In vivo injection of Hdh-NPF peptide increased food consumption, and knockdown of Hdh-NPF expression decreased food consumption in Pacific abalone. These findings provide evidence for co-evolution of the NPF/Y ligand-receptor system, enabling further research on mollusk orexigenic neuropeptides.

The LPA3 Receptor: Regulation and Activation of Signaling Pathways

The lysophosphatidic acid 3 receptor (LPA₃) participates in different physiological actions and in the pathogenesis of many diseases through the activation of different signal pathways. Knowledge of the regulation of the function of the LPA₃ receptor is a crucial element for defining its roles in health and disease. This review describes what is known about the signaling pathways activated in terms of its various actions. Next, we review knowledge on the structure of the LPA₃ receptor, the domains found, and the roles that the latter might play in ligand recognition, signaling, and cellular localization. Currently, there is some information on the action of LPA₃ in different cells and whole organisms, but very little is known about the regulation of its function. Areas in which there is a gap in our knowledge are indicated in order to further stimulate experimental work on this receptor and on other members of the LPA receptor family. We are convinced that knowledge on how this receptor is activated, the signaling pathways employed and how the receptor internalization and desensitization are controlled will help design new therapeutic interventions for treating diseases in which the LPA₃ receptor is implicated.

Targeting lipid GPCRs to treat type 2 diabetes mellitus - progress and challenges

Therapeutic approaches to the treatment of type 2 diabetes mellitus that are designed to increase insulin secretion either directly target β-cells or indirectly target gastrointestinal enteroendocrine cells (EECs), which release hormones that modulate insulin secretion (for example, incretins). Given that β-cells and EECs both express a large array of G protein-coupled receptors (GPCRs) that modulate insulin secretion, considerable research and development efforts have been undertaken to design therapeutic drugs targeting these GPCRs. Among them are GPCRs specific for free fatty acid ligands (lipid GPCRs), including free fatty acid receptor 1 (FFA1, otherwise known as GPR40), FFA2 (GPR43), FFA3 (GPR41) and FFA4 (GPR120), as well as the lipid metabolite binding glucose-dependent insulinotropic receptor (GPR119). These lipid GPCRs have demonstrated important roles in the control of islet and gut hormone secretion. Advances in lipid GPCR pharmacology have led to the identification of a number of synthetic agonists that exert beneficial effects on glucose homeostasis in preclinical studies. Yet, translation of these promising results to the clinic has so far been disappointing. In this Review, we present the physiological roles, pharmacology and clinical studies of these lipid receptors and discuss the challenges associated with their clinical development for the treatment of type 2 diabetes mellitus.

Point mutation of Ffar1 abrogates fatty acid-dependent insulin secretion, but protects against HFD-induced glucose intolerance

Objective

The fatty acid receptor 1 (FFAR1/GPR40) mediates fatty acid-dependent augmentation of glucose-induced insulin secretion (GIIS) in pancreatic β-cells. Genetically engineered Ffar1-knockout/congenic mice univocally displayed impaired fatty acid-mediated insulin secretion, but in vivo experiments delivered controversial results regarding the function of FFAR1 in glucose homeostasis and liver steatosis. This study presents a new coisogenic mouse model carrying a point mutation in Ffar1 with functional consequence. These mice reflect the situations in humans in which point mutations can lead to protein malfunction and disease development.

Methods

The Munich N-ethyl-N-nitrosourea (ENU) mutagenesis-derived F1 archive containing over 16,800 sperms and corresponding DNA samples was screened for mutations in the coding region of Ffar1. Two missense mutations (R258W and T146S) in the extracellular domain of the protein were chosen and homozygote mice were generated. The functional consequence of these mutations was examined in vitro in isolated islets and in vivo in chow diet and high fat diet fed mice.

Results

Palmitate, 50 μM, and the FFAR1 agonist TUG-469, 3 μM, stimulated insulin secretion in islets of Ffar1^T146S/T146S mutant mice and of wild-type littermates, while in islets of Ffar1^R258W/R258W mutant mice, these stimulatory effects were abolished. Insulin content and mRNA levels of Ffar1, Glp1r, Ins2, Slc2a2, Ppara, and Ppard were not significantly different between wild-type and Ffar1^R258W/R258W mouse islets. Palmitate exposure, 600 μM, significantly increased Ppara mRNA levels in wild-type but not in Ffar1^R258W/R258W mouse islets. On the contrary, Slc2a2 mRNA levels were significantly reduced in both wild-type and Ffar1^R258W/R258W mouse islets after palmitate treatment. HFD feeding induced glucose intolerance in wild-type mice. Ffar1^R258W/R258W mutant mice remained glucose tolerant although their body weight gain, liver steatosis, insulin resistance, and plasma insulin levels were not different from those of wild-type littermates. Worth mentioning, fasting plasma insulin levels were lower in Ffar1^R258W/R258W mice.

Conclusion

A point mutation in Ffar1 abrogates the stimulatory effect of palmitate on GIIS, an effect that does not necessarily translate to HFD-induced glucose intolerance.

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Generation of mice carrying point mutations in Ffar1 using ENU.

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FFAR1 point mutation R258W abrogates fatty acid-induced insulin secretion.

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Dysfunctional FFAR1 inhibits the development of diet-induced glucose intolerance.

The DRY motif and the four corners of the cubic ternary complex model

Recent structural data on GPCRs using a variety of spectroscopic approaches suggest that GPCRs adopt a dynamic conformational landscape, with ligands stabilizing subsets of these states to activate one or more downstream signaling effectors. A key outstanding question posed by this emerging dynamic structural model of GPCRs is what states, active, inactive, or intermediate are captured by the numerous crystal structures of GPCRs complexed with a variety of agonists, partial agonists, and antagonists. In the early nineties the discovery of inverse agonists and constitutive activity led to the idea that the active receptor state (R^⁎) is an intrinsic property of the receptor itself rather than of the RG complex, eventually leading to the formulation of the cubic ternary complex model (CTC). Here, by a careful analysis of a series of data obtained with a number of mutants of the highly conserved E/DRY motif, we show evidences for the existence of all the receptor states theorized by the CTC, four 'uncoupled (R, R^⁎ and HR and HR^⁎), and, consequently four 'coupled' (RG, R^⁎G, HRG and HR^⁎G). The E/DRY motif located at the cytosolic end of transmembrane helix III of Class A GPCRs has been widely studied and analyzed because it forms a network of interactions believed to lock receptors in the inactive conformation (R), and, thus, to play a key role in receptor activation. Our conclusions are supported by recent crystal and NMR spectra, as well as by results obtained with two prototypical GPCRs using a new FRET technology that de-couples G protein binding to the receptor from signal transduction. Thus, despite its complexity and limitations, we propose that the CTC is a useful framework to reconcile pharmacological, biochemical and structural data.

The Role and Future of FFA1 as a Therapeutic Target

Of the 415 million people suffering from diabetes worldwide, 90% have type 2 diabetes. Type 2 diabetes is characterized by hyperglycemia and occurs in obese individuals as a result of insulin resistance and inadequate insulin levels. Accordingly, diabetes drugs are tailored to enhance glucose disposal or target the pancreatic islet β cell to increase insulin secretion. The majority of the present-day insulin secretagogues, however, increase the risk of iatrogenic hypoglycemia, and hence alternatives are actively sought. The long-chain fatty acid, G protein-coupled receptor FFA1/Gpr40, is expressed in β cells, and its activation potentiates insulin secretion in a glucose-dependent manner. Preclinical data indicate that FFA1 agonism is an effective treatment to restore glucose homeostasis in rodent models of diabetes. This initial success prompted clinical trials in type 2 diabetes patients, the results of which were promising; however, the field suffered a significant setback when the lead compound TAK-875/fasiglifam was withdrawn from clinical development due to liver safety concerns. Nevertheless, recent developments have brought to light a surprising complexity of FFA1 agonist action, signaling diversity, and biological outcomes, raising hopes that with a greater understanding of the mechanisms at play the second round will be more successful.

Synthesis and Characterization of a Promising Novel FFAR1/GPR40 Targeting Fluorescent Probe for β-Cell Imaging

Diabetes affects an increasing number of patients worldwide and is responsible for a significant rise in healthcare expenses. Imaging of β-cells bears the potential to contribute to an improved understanding, diagnosis, and development of new treatment options for diabetes. Here, we describe the first small molecule fluorescent probe targeting the free fatty acid receptor 1 (FFAR1/GPR40). This receptor is highly expressed on β-cells, and was up to now unexplored for imaging purposes. We designed a novel probe by facile modification of the selective and potent FFAR1 agonist TAK-875. Effective and specific binding of the probe was demonstrated using FFAR1 overexpressing cells. We also successfully labeled FFAR1 on MIN6 and INS1E cells, two widely used β-cell models, by applying an effective amplification protocol. Finally, we showed that the probe is capable of inducing insulin secretion in a glucose-dependent manner, thus demonstrating that functional activity of the probe was maintained. These results suggest that our probe represents a first important step to successful β-cell imaging by targeting FFAR1. The developed probe may prove to be particularly useful for in vitro and ex vivo studies of diabetic cellular and animal models to gain new insights into disease pathogenesis.