Mutational analysis of G-protein coupled receptor--FFA2

Multiple ligand recognition sites in free fatty acid receptor 2 (FFA2R) direct distinct neutrophil activation patterns

The allosteric modulating free fatty acid receptor 2 ligands Cmp58 and AZ1729, increased the activity induced by orthosteric receptor agonists mediating a rise in intracellular calcium ions and activation of the neutrophil NADPH-oxidase. Together, the two modulators triggered an orthosteric-agonist-independent activation of the oxidase without any rise in the concentration of intracellular calcium ions. In this study, structurally diverse compounds presumed to be ligands for free fatty acid receptor 2 were used to gain additional insights into receptor-modulation/signaling. We identified two molecules that activate neutrophils on their own and we classified one as allosteric agonist and the other as orthosteric agonist. Ten compounds were classified as allosteric FFA2R modulators. Of these, one activated neutrophils when combined with AZ1729; the nine remaining compounds activated neutrophils solely when combined with Cmp58. The activation signals were primarily biased when stimulated by two allosteric modulators interacting with different binding sites, such that two complementary modulators together triggered an activation of the NADPH-oxidase but no increase in the intracellular concentration of calcium ions. No neutrophil activation was induced when allosteric receptor modulators suggested to be recognized by the same binding site were combined, results in agreement with our proposed model for activation, in which the receptor has two different sites that selectively bind allosteric modulators. The down-stream signaling mediated by cross-sensitizing allosteric receptor modulators, occurring independent of any orthosteric agonist, represent a new mechanism for activation of the neutrophil NADPH oxidase.

Pharmacology of Free Fatty Acid Receptors and Their Allosteric Modulators

The physiological function of free fatty acids (FFAs) has long been regarded as indirect in terms of their activities as educts and products in metabolic pathways. The observation that FFAs can also act as signaling molecules at FFA receptors (FFARs), a family of G protein-coupled receptors (GPCRs), has changed the understanding of the interplay of metabolites and host responses. Free fatty acids of different chain lengths and saturation statuses activate FFARs as endogenous agonists via binding at the orthosteric receptor site. After FFAR deorphanization, researchers from the pharmaceutical industry as well as academia have identified several ligands targeting allosteric sites of FFARs with the aim of developing drugs to treat various diseases such as metabolic, (auto)inflammatory, infectious, endocrinological, cardiovascular, and renal disorders. GPCRs are the largest group of transmembrane proteins and constitute the most successful drug targets in medical history. To leverage the rich biology of this target class, the drug industry seeks alternative approaches to address GPCR signaling. Allosteric GPCR ligands are recognized as attractive modalities because of their auspicious pharmacological profiles compared to orthosteric ligands. While the majority of marketed GPCR drugs interact exclusively with the orthosteric binding site, allosteric mechanisms in GPCR biology stay medically underexploited, with only several allosteric ligands currently approved. This review summarizes the current knowledge on the biology of FFAR1 (GPR40), FFAR2 (GPR43), FFAR3 (GPR41), FFAR4 (GPR120), and GPR84, including structural aspects of FFAR1, and discusses the molecular pharmacology of FFAR allosteric ligands as well as the opportunities and challenges in research from the perspective of drug discovery.

Chemogenetic Approaches to Explore the Functions of Free Fatty Acid Receptor 2

Short-chain fatty acids are generated in large amounts by the intestinal microbiota. They activate both the closely related G protein-coupled receptors free fatty acid receptor 2 (FFA2) and free fatty acid receptor 3 (FFA3) that are considered therapeutic targets in diseases of immuno-metabolism. Limited and species-selective small-molecule pharmacology has restricted our understanding of the distinct roles of these receptors. Replacement of mouse FFA2 with a designer receptor exclusively activated by designer drug form of human FFA2 (hFFA2-DREADD) has allowed definition of specific roles of FFA2 in pharmacological and physiological studies conducted both ex vivo and in vivo, whilst overlay of murine disease models offers opportunities for therapeutic validation prior to human studies. Similar approaches can potentially be used to define roles of other poorly characterised receptors.

Free Fatty Acid Receptors 2 and 3 as Microbial Metabolite Sensors to Shape Host Health: Pharmacophysiological View

The role of the gut microbiome in human health is becoming apparent. The major functional impact of the gut microbiome is transmitted through the microbial metabolites that are produced in the gut and interact with host cells either in the local gut environment or are absorbed into circulation to impact distant cells/organs. Short-chain fatty acids (SCFAs) are the major microbial metabolites that are produced in the gut through the fermentation of non-digestible fibers. SCFAs are known to function through various mechanisms, however, their signaling through free fatty acid receptors 2 and 3 (FFAR2/3; type of G-coupled protein receptors) is a new therapeutic approach. FFAR2/3 are widely expressed in diverse cell types in human and mice, and function as sensors of SCFAs to change several physiological and cellular functions. FFAR2/3 modulate neurological signaling, energy metabolism, intestinal cellular homeostasis, immune response, and hormone synthesis. FFAR2/3 function through Gi and/or Gq signaling, that is mediated through specific structural features of SCFAs-FFAR2/3 bindings and modulating specific signaling pathway. In this review, we discuss the wide-spread expression and structural homologies between human and mice FFAR2/3, and their role in different human health conditions. This information can unlock opportunities to weigh the potential of FFAR2/3 as a drug target to prevent human diseases.

Free Fatty Acid Receptors in Health and Disease

Fatty acids are metabolized and synthesized as energy substrates during biological responses. Long- and medium-chain fatty acids derived mainly from dietary triglycerides, and short-chain fatty acids (SCFAs) produced by gut microbial fermentation of the otherwise indigestible dietary fiber, constitute the major sources of free fatty acids (FFAs) in the metabolic network. Recently, increasing evidence indicates that FFAs serve not only as energy sources but also as natural ligands for a group of orphan G protein-coupled receptors (GPCRs) termed free fatty acid receptors (FFARs), essentially intertwining metabolism and immunity in multiple ways, such as via inflammation regulation and secretion of peptide hormones. To date, several FFARs that are activated by the FFAs of various chain lengths have been identified and characterized. In particular, FFAR1 (GPR40) and FFAR4 (GPR120) are activated by long-chain saturated and unsaturated fatty acids, while FFAR3 (GPR41) and FFAR2 (GPR43) are activated by SCFAs, mainly acetate, butyrate, and propionate. In this review, we discuss the recent reports on the key physiological functions of the FFAR-mediated signaling transduction pathways in the regulation of metabolism and immune responses. We also attempt to reveal future research opportunities for developing therapeutics for metabolic and immune disorders.

Ligands at the Free Fatty Acid Receptors 2/3 (GPR43/GPR41)

A large number of reviews and commentaries have highlighted the potential role of the short-chain fatty acid receptors GPR41 (FFA3) and, particularly, GPR43 (FFA2) as an interface between the intestinal microbiota and metabolic and inflammatory disorders. However, short-chain fatty acids have very modest potency and display limited selectivity between these two receptors, and studies on receptor knockout mice have resulted in non-uniform conclusions; therefore, selective and high-potency/high-affinity synthetic ligands are required to further explore the contribution of these receptors to health and disease. Currently no useful orthosteric ligands of FFA3 have been reported and although a number of orthosteric FFA2 agonists and antagonists have been described, a lack of affinity of different chemotypes of FFA2 antagonists at the mouse and rat orthologs of this receptor has hindered progress. Selective allosteric regulators of both FFA2 and FFA3 have provided tools to address a number of basic questions in both in vitro and ex vivo preparations, but at least some of the positive modulators appear to be biased and able to regulate only a subset of the functional capabilities of the short-chain fatty acids. Significant further progress is required to provide improved tool compounds to better assess potential translational opportunities of these receptors for short-chain fatty acids.

SCFA Receptors in Pancreatic β Cells: Novel Diabetes Targets?

Nutrient sensing receptors are key metabolic mediators of responses to dietary and endogenously derived nutrients. These receptors are largely G-protein-coupled receptors (GPCRs) and many are gaining significant interest as drug targets with a potential therapeutic role in metabolic diseases. A distinct subclass of nutrient sensing GPCRs, two short chain fatty acid (SCFA) receptors (FFA2 and FFA3) are uniquely responsive to gut microbiota derived nutrients (such as acetate, propionate, and butyrate). Pharmacological, molecular, and genetic studies have investigated their role in organismal glucose metabolism and recently in pancreatic β cell biology. Here, we summarize the present knowledge on the role of these receptors as metabolic sensors in β cell function and physiology, revealing new therapeutic opportunities for type 2 diabetes.

A Molecular Mechanism for Sequential Activation of a G Protein-Coupled Receptor

Ligands targeting G protein-coupled receptors (GPCRs) are currently classified as either orthosteric, allosteric, or dualsteric/bitopic. Here, we introduce a new pharmacological concept for GPCR functional modulation: sequential receptor activation. A hallmark feature of this is a stepwise ligand binding mode with transient activation of a first receptor site followed by sustained activation of a second topographically distinct site. We identify 4-CMTB (2-(4-chlorophenyl)-3-methyl-N-(thiazol-2-yl)butanamide), previously classified as a pure allosteric agonist of the free fatty acid receptor 2, as the first sequential activator and corroborate its two-step activation in living cells by tracking integrated responses with innovative label-free biosensors that visualize multiple signaling inputs in real time. We validate this unique pharmacology with traditional cellular readouts, including mutational and pharmacological perturbations along with computational methods, and propose a kinetic model applicable to the analysis of sequential receptor activation. We envision this form of dynamic agonism as a common principle of nature to spatiotemporally encode cellular information.

Free fatty acid receptors: structural models and elucidation of ligand binding interactions

Background

The free fatty acid receptors (FFAs), including FFA1 (orphan name: GPR40), FFA2 (GPR43) and FFA3 (GPR41) are G protein-coupled receptors (GPCRs) involved in energy and metabolic homeostasis. Understanding the structural basis of ligand binding at FFAs is an essential step toward designing potent and selective small molecule modulators.

Results

We analyse earlier homology models of FFAs in light of the newly published FFA1 crystal structure co-crystallized with TAK-875, an ago-allosteric ligand, focusing on the architecture of the extracellular binding cavity and agonist-receptor interactions. The previous low-resolution homology models of FFAs were helpful in highlighting the location of the ligand binding site and the key residues for ligand anchoring. However, homology models were not accurate in establishing the nature of all ligand-receptor contacts and the precise ligand-binding mode. From analysis of structural models and mutagenesis, it appears that the position of helices 3, 4 and 5 is crucial in ligand docking. The FFA1-based homology models of FFA2 and FFA3 were constructed and used to compare the FFA subtypes. From docking studies we propose an alternative binding mode for orthosteric agonists at FFA1 and FFA2, involving the interhelical space between helices 4 and 5. This binding mode can explain mutagenesis results for residues at positions 4.56 and 5.42. The novel FFAs structural models highlight higher aromaticity of the FFA2 binding cavity and higher hydrophilicity of the FFA3 binding cavity. The role of the residues at the second extracellular loop used in mutagenesis is reanalysed. The third positively-charged residue in the binding cavity of FFAs, located in helix 2, is identified and predicted to coordinate allosteric modulators.

Conclusions

The novel structural models of FFAs provide information on specific modes of ligand binding at FFA subtypes and new suggestions for mutagenesis and ligand modification, guiding the development of novel orthosteric and allosteric chemical probes to validate the importance of FFAs in metabolic and inflammatory conditions. Using our FFA homology modelling experience, a strategy to model a GPCR, which is phylogenetically distant from GPCRs with the available crystal structures, is discussed.

Electronic supplementary material

The online version of this article (doi:10.1186/s12900-015-0044-2) contains supplementary material, which is available to authorized users.

Perspectives on the therapeutic potential of short-chain fatty acid receptors

There is rapidly growing interest in the human microbiome because of its implication in metabolic disorders and inflammatory diseases. Consequently, understanding the biology of short chain fatty acids and their receptors has become very important for identifying novel therapeutic avenues. GPR41 and GPR43 have been recognized as the cognate receptors for SCFAs and their roles in metabolism and inflammation have drawn much attention in recent years. GPR43 is highly expressed on immune cells and has been suggested to play a role in inflammatory diseases such as inflammatory bowel disease. Both GPR41 and GPR43 have been implicated in diabetes and obesity via the regulation of adipose tissue and gastrointestinal hormones. So far, many studies have provided contradictory results, and therefore further research is required to validate these receptors as drug targets. We will also discuss the synthetic modulators of GPR41 and GPR43 that are critical to understanding the functions of these receptors. [BMB Reports 2014; 47(3): 173-178]

Free fatty acid (FFA) and hydroxy carboxylic acid (HCA) receptors

Saturated and unsaturated free fatty acids (FFAs), as well as hydroxy carboxylic acids (HCAs) such as lactate and ketone bodies, are carriers of metabolic energy, precursors of biological mediators, and components of biological structures. However, they are also able to exert cellular effects through G protein-coupled receptors named FFA1-FFA4 and HCA1-HCA3. Work during the past decade has shown that these receptors are widely expressed in the human body and regulate the metabolic, endocrine, immune and other systems to maintain homeostasis under changing dietary conditions. The development of genetic mouse models and the generation of synthetic ligands of individual FFA and HCA receptors have been instrumental in identifying cellular and biological functions of these receptors. These studies have produced strong evidence that several FFA and HCA receptors can be targets for the prevention and treatment of various diseases, including type 2 diabetes mellitus, obesity, and inflammation.

A novel therapeutic target, GPR43; where it stands in drug discovery

With growing interest in human microbiome for its implication in metabolic disorders, inflammatory diseases, immune disorders and so forth, understanding the biology at the interface of the gut flora and the host becomes very important for identifying novel therapeutic avenues. GPR43 has been deorphanized and the metabolites of microbiome, such as short-chain fatty acids, serve as its natural ligands. There are numerous reports that GPR43 might be a crucial link to the novel therapies for the unmet medical needs and many drug discovery organizations are making their moves in response.

GPR43/FFA2: physiopathological relevance and therapeutic prospects

Research interest in free fatty acid-binding receptors has been growing during the past decade, with an aim to better understand the modulation of host physiology in response to nutrition. G-protein-coupled receptor 43 (GPR43), also called free fatty acid receptor 2 (FFA2/FFAR2), binds short-chain fatty acids (SCFAs) produced by the microbial fermentation of carbohydrates and has shown promising therapeutic potential. This review presents current knowledge regarding the pharmacological properties of GPR43 and addresses its functions in selected organs (adipose tissue, intestine and immune cells). Furthermore, the demonstration of GPR43 involvement in several pathological conditions such as obesity, inflammatory disease, and cancer suggests new fields of interest related to this receptor. Finally, GPR43 could be a key player in gut microbes-host crosstalk, although further research is needed to clearly evaluate its role in the management of host health by nutrients or treatments targeting the gut microbiota.

Low affinity GPCRs for metabolic intermediates: challenges for pharmacologists

The discovery that a number of metabolites and metabolic intermediates can act through G protein-coupled receptors has attracted great interest in the field and has led to new therapeutic targets for diseases such as hypertension, type 2 diabetes, inflammation, and metabolic syndrome. However, the low apparent affinity of these ligands for their cognate receptors poses a number of challenges for pharmacologists interested in investigating receptor structure, function or physiology. Furthermore, the endogenous ligands matched to their receptors have other, well established metabolic roles and thus selectivity is difficult to achieve. This review discusses some of the issues researchers face when working with these receptors and highlights the ways in which a number of these obstacles have been overcome.