Application of GPCR Structures for Modelling of Free Fatty Acid Receptors

Structural basis for the ligand recognition and signaling of free fatty acid receptors

Free fatty acid receptors 1 to 4 (FFA1 to FFA4) are class A G protein-coupled receptors (GPCRs). FFA1 to FFA3 share substantial sequence similarity, whereas FFA4 is unrelated. However, FFA1 and FFA4 are activated by long-chain fatty acids, while FFA2 and FFA3 respond to short-chain fatty acids generated by intestinal microbiota. FFA1, FFA2, and FFA4 are potential drug targets for metabolic and inflammatory conditions. Here, we determined the active structures of FFA1 and FFA4 bound to docosahexaenoic acid, FFA4 bound to the synthetic agonist TUG-891, and butyrate-bound FFA2, each complexed with an engineered heterotrimeric Gq protein (miniGq), by cryo-electron microscopy. Together with computational simulations and mutagenesis studies, we elucidated the similarities and differences in the binding modes of fatty acid ligands to their respective GPCRs. Our findings unveiled distinct mechanisms of receptor activation and G protein coupling. We anticipate that these outcomes will facilitate structure-based drug development and underpin future research on this group of GPCRs.

Structural basis for the ligand recognition and signaling of free fatty acid receptors

Free fatty acid receptors 1-4 (FFA1-4) are class A G protein-coupled receptors (GPCRs). FFA1-3 share substantial sequence similarity whereas FFA4 is unrelated. Despite this FFA1 and FFA4 are activated by the same range of long chain fatty acids (LCFAs) whilst FFA2 and FFA3 are instead activated by short chain fatty acids (SCFAs) generated by the intestinal microbiota. Each of FFA1, 2 and 4 are promising targets for novel drug development in metabolic and inflammatory conditions. To gain insights into the basis of ligand interactions with, and molecular mechanisms underlying activation of, FFAs by LCFAs and SCFAs, we determined the active structures of FFA1 and FFA4 bound to the polyunsaturated LCFA docosahexaenoic acid (DHA), FFA4 bound to the synthetic agonist TUG-891, as well as SCFA butyrate-bound FFA2, each complexed with an engineered heterotrimeric Gq protein (miniGq), by cryo-electron microscopy. Together with computational simulations and mutagenesis studies, we elucidated the similarities and differences in the binding modes of fatty acid ligands with varying chain lengths to their respective GPCRs. Our findings unveil distinct mechanisms of receptor activation and G protein coupling. We anticipate that these outcomes will facilitate structure-based drug development and underpin future research to understand allosteric modulation and biased signaling of this group of GPCRs.

Analysis of microbiota-host communication mediated by butyrate in Atlantic Salmon

Butyrate is a microbiota-produced metabolite, sensed by host short-chain fatty acid receptors FFAR2 (Gpr43), FFAR3 (Gpr41), HCAR2 (Gpr109A), and Histone deacetylase (HDAC) that promotes microbiota-host crosstalk. Butyrate influences energy uptake, developmental and immune response in mammals. This microbial metabolite is produced by around 79 anaerobic genera present in the mammalian gut, yet little is known about the role of butyrate in the host-microbiota interaction in salmonid fish. To further our knowledge of this interaction, we analyzed the intestinal microbiota and genome of Atlantic salmon (Salmo salar), searching for butyrate-producing genera and host butyrate receptors. We identified Firmicutes, Proteobacteria, and Actinobacteria as the main butyrate-producing bacteria in the salmon gut microbiota. In the Atlantic salmon genome, we identified an expansion of genes orthologous to FFAR2 and HCAR2 receptors, and class I and IIa HDACs that are sensitive to butyrate. In addition, we determined the expression levels of orthologous of HCAR2 in the gut, spleen, and head-kidney, and FFAR2 in RTgutGC cells. The effect of butyrate on the Atlantic salmon immune response was evaluated by analyzing the pro and anti-inflammatory cytokines response in vitro in SHK-1 cells by RT-qPCR. Butyrate decreased the expression of the pro-inflammatory cytokine IL-1β and increased anti-inflammatory IL-10 and TGF-β cytokines. Butyrate also reduced the expression of interferon-alpha, Mx, and PKR, and decreased the viral load at a higher concentration (4 mM) in cells treated with this molecule before the infection with Infectious Pancreatic Necrosis Virus (IPNV) by mechanisms independent of FFAR2, FFAR3 and HCAR2 expression that probably inhibit HDAC. Moreover, butyrate modified phosphorylation of cytoplasmic proteins in RTgutGC cells. Our data allow us to infer that Atlantic salmon have the ability to sense butyrate produced by their gut microbiota via different specific targets, through which butyrate modulates the immune response of pro and anti-inflammatory cytokines and the antiviral response.

Selective phosphorylation of threonine residues defines GPR84-arrestin interactions of biased ligands

GPR84 is an immune cell-expressed, proinflammatory receptor currently being assessed as a therapeutic target in conditions including fibrosis and inflammatory bowel disease. Although it was previously shown that the orthosteric GPR84 activators 2-HTP and 6-OAU promoted its interactions with arrestin-3, a G protein-biased agonist DL-175 did not. Here, we show that replacement of all 21 serine and threonine residues within i-loop 3 of GPR84, but not the two serines in the C-terminal tail, eliminated the incorporation of [32P] and greatly reduced receptor-arrestin-3 interactions promoted by 2-HTP. GPR84 was phosphorylated constitutively on residues Ser221 and Ser224, while various other amino acids are phosphorylated in response to 2-HTP. Consistent with this, an antiserum able to identify pSer221/pSer224 recognized GPR84 from cells treated with and without activators, whereas an antiserum able to identify pThr263/pThr264 only recognized GPR84 after exposure to 2-HTP and not DL-175. Two distinct GPR84 antagonists as well as inhibition of G protein-coupled receptor kinase 2/3 prevented phosphorylation of pThr263/pThr264, but neither strategy affected constitutive phosphorylation of Ser221/Ser224. Furthermore, mutation of residues Thr263 and Thr264 to alanine generated a variant of GPR84 also limited in 2-HTP-induced interactions with arrestin-2 and -3. By contrast, this mutant was unaffected in its capacity to reduce cAMP levels. Taken together, these results define a key pair of threonine residues, regulated only by subsets of GPR84 small molecule activators and by GRK2/3 that define effective interactions with arrestins and provide novel tools to monitor the phosphorylation and functional status of GPR84.

Dihydromyricetin Protects Against Ethanol-Induced Toxicity in SH-SY5Y Cell Line: Role of GABAA Receptor

Toxicity induced by binge alcohol drinking, particularly in adolescent and young adults, is of major medical and social consequence. Recently, we reported that butyrate, a short chain fatty acid, can protect against ethanol (ETOH)-induced toxicity in an in vitro model. In this study, we sought to evaluate the potential effectiveness of dihydromyricetin (DHM), a natural bioactive flavonoid, alone or in combination with butyrate in the same model. Exposure of SH-SY5Y cells for 24 h to 500 mM ETOH resulted in approximately 40% reduction in cell viability, which was completely prevented by 0.1 μM DHM. Combinations of DHM and butyrate provided synergistic protection against alcohol toxicity. Whereas butyrate effect was shown to be mediated primarily through fatty acid receptor 3 activation, DHM protection appears to be mediated primarily via benzodiazepine receptor site of GABA_A receptor. This is based on the finding that DHM's effect could be completely prevented by pretreatment with flumazenil, a selective antagonist at this site, but not by bicuculline, a selective antagonist at the actual GABA_A receptor binding site. These findings suggest potential utility of DHM alone or in combination with butyrate against ETOH-induced toxicity.

Genetic Inactivation of Free Fatty Acid Receptor 3 Impedes Behavioral Deficits and Pathological Hallmarks in the APPswe Alzheimer's Disease Mouse Model

The free fatty acid FFA3 receptor (FFA3R) belongs to the superfamily of G-protein-coupled receptors (GPCRs). In the intestine and adipose tissue, it is involved in the regulation of energy metabolism, but its function in the brain is unknown. We aimed, first, to investigate the expression of the receptor in the hippocampus of Alzheimer disease (AD) patients at different stages of the disease and, second, to assess whether genetic inactivation of the Ffar3 gene could affect the phenotypic features of the APP_swe mouse model. The expression of transcripts for FFA receptors in postmortem human hippocampal samples and in the hippocampus of wild-type and transgenic mice was analyzed by RT-qPCR. We generated a double transgenic mouse, FFA3R^−/−/APP_swe, to perform cognition studies and to assess, by immunoblotting Aβ and tau pathologies and the differential expression of synaptic plasticity-related proteins. For the first time, the occurrence of the FFA3R in the human hippocampus and its overexpression, even in the first stages of AD, was demonstrated. Remarkably, FFA3R^−/−/APP_swe mice do not have the characteristic memory impairment of 12-month-old APP_swe mice. Additionally, this newly generated transgenic line does not develop the most important Alzheimer’s disease (AD)-related features, such as amyloid beta (Aβ) brain accumulations and tau hyperphosphorylation. These findings are accompanied by increased levels of the insulin-degrading enzyme (IDE) and lower activity of the tau kinases GSK3β and Cdk5. We conclude that the brain FFA3R is involved in cognitive processes and that its inactivation prevents AD-like cognitive decline and pathological hallmarks.

Short-Chain Fatty Acid Receptors and Cardiovascular Function

Increasing experimental and clinical evidence points toward a very important role for the gut microbiome and its associated metabolism in human health and disease, including in cardiovascular disorders. Free fatty acids (FFAs) are metabolically produced and utilized as energy substrates during almost every biological process in the human body. Contrary to long- and medium-chain FFAs, which are mainly synthesized from dietary triglycerides, short-chain FFAs (SCFAs) derive from the gut microbiota-mediated fermentation of indigestible dietary fiber. Originally thought to serve only as energy sources, FFAs are now known to act as ligands for a specific group of cell surface receptors called FFA receptors (FFARs), thereby inducing intracellular signaling to exert a variety of cellular and tissue effects. All FFARs are G protein-coupled receptors (GPCRs) that play integral roles in the regulation of metabolism, immunity, inflammation, hormone/neurotransmitter secretion, etc. Four different FFAR types are known to date, with FFAR1 (formerly known as GPR40) and FFAR4 (formerly known as GPR120) mediating long- and medium-chain FFA actions, while FFAR3 (formerly GPR41) and FFAR2 (formerly GPR43) are essentially the SCFA receptors (SCFARs), responding to all SCFAs, including acetic acid, propionic acid, and butyric acid. As with various other organ systems/tissues, the important roles the SCFARs (FFAR2 and FFAR3) play in physiology and in various disorders of the cardiovascular system have been revealed over the last fifteen years. In this review, we discuss the cardiovascular implications of some key (patho)physiological functions of SCFAR signaling pathways, particularly those regulating the neurohormonal control of circulation and adipose tissue homeostasis. Wherever appropriate, we also highlight the potential of these receptors as therapeutic targets for cardiovascular disorders.

Replacing arginine 99 with leucine to study the kinetics of interconnected allosteric interactions between FFAR4 and naturally occurring fatty acids

The long-chain fatty acid receptor FFAR4 is the main G-protein-coupled receptor in the body for detecting long-chain fatty acids. It has been shown that Arg99 may be an important residue for fatty acid recognition and for the activation of hFFAR4, though direct evidence is still lacking. In this study, Arg99 on hFFAR4 was substituted with leucine by genetic manipulation, and a double-layer gold nanoparticle biosensor based on hFFAR4 (Arg99 → Leu) was constructed. The interconnected allosteric interaction between 11 naturally occurring fatty acid ligands and the receptor was determined. The results showed that Arg99 is the key residue on hFFAR4 for the recognition of the carboxyl group on fatty acids. This study offered direct quantitative evidence for the role played by different residues in receptor-ligand recognition and interconnected allosterism, providing a new approach for investigating the mechanisms and kinetics of interconnected receptor-ligand allosterism.

Sodium Butyrate Protects Against Ethanol-Induced Toxicity in SH-SY5Y Cell Line

Alcohol use disorder (AUD), brought about by excessive alcohol use, is associated with damages to several organs including the brain. Chronic excessive use of alcohol can compromise intestinal integrity, leading to changes in gut microbiota (GM) composition known as dysbiosis. Dysbiosis, by disruption of the gut-brain axis (GBA), further exacerbates the deleterious effects of alcohol. One of the fermentation by-products of GM is butyrate (BUT), a short-chain fatty acid (SCFA) that plays an important role in maintaining homeostasis of the GBA. Alcohol metabolism results in formation of acetaldehyde, a highly reactive compound that reacts with dopamine in the brain to form toxic adducts such as salsolinol. Recent studies indicate potential neuro-protective effects of BUT against various toxicants including salsolinol. Here, we sought to investigate whether BUT can also protect against alcohol toxicity. Pretreatment of neuroblastoma-derived SH-SY5Y cells with 500 mM ethanol (ETOH) for 24 h resulted in approximately 40% reduction in cell viability, which was totally blocked by 10 µM of either BUT or AR 420,626 (AR), a selective fatty acid 3 receptor (FA3R) agonist. The neuro-protective effects of both BUT and AR were significantly (80%) attenuated by beta-hydroxy butyrate (BHB), a selective FA3R antagonist. Interestingly, combination of BUT and AR resulted in synergistic protection against ETOH, which was totally blocked by BHB. These findings suggest potential utility of butyrate and/or FA3R agonists against ETOH-induced toxicity.

Discovery and Characterization of Novel Antagonists of the Proinflammatory Orphan Receptor GPR84

GPR84 is a poorly characterized, nominally orphan, proinflammatory G protein-coupled receptor that can be activated by medium chain length fatty acids. It is attracting considerable interest as a potential therapeutic target for antagonist ligands in both inflammatory bowel diseases and idiopathic pulmonary fibrosis. Successful screening of more than 300 000 compounds from a small molecule library followed by detailed analysis of some 50 drug-like hits identified 3-((5,6-bis(4-methoxyphenyl)-1,2,4-triazin-3-yl)methyl)-1H-indole as a high affinity and highly selective competitive antagonist of human GPR84. Tritiation of a di-iodinated form of the core structure produced [³H]3-((5,6-diphenyl-1,2,4-triazin-3-yl)methyl)-1H-indole, which allowed effective measurement of receptor levels in both transfected cell lines and lipopolysaccharide-treated THP-1 monocyte/macrophage cells. Although this compound series lacks significant affinity at mouse GPR84, homology modeling and molecular dynamics simulations provided a potential rationale for this difference, and alteration of two residues in mouse GPR84 to the equivalent amino acids in the human orthologue, predicted to open the antagonist binding pocket, validated this model. Sequence alignment of other species orthologues further predicted binding of the compounds as high affinity antagonists at macaque, pig, and dog GPR84 but not at the rat orthologue, and pharmacological experiments confirmed these predictions. These studies provide a new class of GPR84 antagonists that display species selectivity defined via receptor modeling and mutagenesis.

Novel Pharmacotherapies in Parkinson's Disease

Parkinson’s disease (PD), an age-related progressive neurodegenerative condition, is associated with loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc), which results in motor deficits characterized by the following: akinesia, rigidity, resting tremor, and postural instability, as well as nonmotor symptoms such as emotional changes, particularly depression, cognitive impairment, gastrointestinal, and autonomic dysfunction. The most common treatment for PD is focused on dopamine (DA) replacement (e.g., levodopa = L-Dopa), which unfortunately losses its efficacy over months or years and can induce severe dyskinesia. Hence, more efficacious interventions without such adverse effects are urgently needed. In this review, following a general description of PD, potential novel therapeutic interventions for this devastating disease are examined. Specifically, the focus is on nicotine and nicotinic cholinergic system, as well as butyrate, a short chain fatty acid (SCFA), and fatty acid receptors.

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.

Omega-3 Fatty Acids as Druggable Therapeutics for Neurodegenerative Disorders

Neurodegenerative disorders are commonly associated with a complex pattern of pathophysiological hallmarks, including increased oxidative stress and neuroinflammation, which makes their treatment challenging. Omega-3 Fatty Acids (O3FA) are natural products with reported neuroprotective, anti-inflammatory, and antioxidant effects. These effects have been attributed to their incorporation into neuronal membranes or through the activation of intracellular or recently discovered cell-surface receptors (i.e., Free-Fatty Acid Receptors; FFAR). Molecular docking studies have investigated the roles of O3FA as agonists of FFAR and have led to the development of receptor-specific targeted agonists for therapeutic purposes. Moreover, novel formulation strategies for targeted delivery of O3FA to the brain have supported their development as therapeutics for neurodegenerative disorders. Despite the compelling evidence of the beneficial effects of O3FA for several neuroprotective functions, they are currently only available as unregulated dietary supplements, with only a single FDA-approved prescription product, indicated for triglyceride reduction. This review highlights the relative safety and efficacy of O3FA, their drug-like properties, and their capacity to be formulated in clinically viable drug delivery systems. Interestingly, the presence of cardiac conditions such as hypertriglyceridemia is associated with brain pathophysiological hallmarks of neurodegeneration, such as neuroinflammation, thereby further suggesting potential therapeutic roles of O3FA for neurodegenerative disorders. Taken together, this review article summarizes and integrates the compelling evidence regarding the feasibility of developing O3FA and their synthetic derivatives as potential drugs for neurodegenerative disorders.

Weak Microbial Metabolites: a Treasure Trove for Using Biomimicry to Discover and Optimize Drugs

For decades, traditional drug discovery has used natural product and synthetic chemistry approaches to generate libraries of compounds, with some ending as promising drug candidates. A complementary approach has been to adopt the concept of biomimicry of natural products and metabolites so as to improve multiple drug-like features of the parent molecule. In this effort, promiscuous and weak interactions between ligands and receptors are often ignored in a drug discovery process. In this Emerging Concepts article, we highlight microbial metabolite mimicry, whereby parent metabolites have weak interactions with their receptors that then have led to discrete examples of more potent and effective drug-like molecules. We show specific examples of parent-metabolite mimics with potent effects in vitro and in vivo. Furthermore, we show examples of emerging microbial ligand-receptor interactions and provide a context in which these ligands could be improved as potential drugs. A balanced conceptual advance is provided in which we also acknowledge potential pitfalls-hyperstimulation of finely balanced receptor-ligand interactions could also be detrimental. However, with balance, we provide examples of where this emerging concept needs to be tested. SIGNIFICANCE STATEMENT: Microbial metabolite mimicry is a novel way to expand on the chemical repertoire of future drugs. The emerging concept is now explained using specific examples of the discovery of therapeutic leads from microbial metabolites.

Therapeutic validation of an orphan G protein-coupled receptor: The case of GPR84

Despite the importance of members of the GPCR superfamily as targets of a broad range of effective medicines many GPCRs remain poorly characterised. GPR84 is an example. Expression of GPR84 is strongly up regulated in immune cells in a range of pro-inflammatory settings and clinical trials to treat idiopathic pulmonary fibrosis are currently ongoing using ligands with differing levels of selectivity and affinity as GPR84 antagonists. Although blockade of GPR84 may potentially prove effective also in diseases associated with inflammation of the lower gut there is emerging interest in defining if agonists of GPR84 might find utility in conditions in which regulation of metabolism or energy sensing is compromised. Here, we consider the physiological and pathological expression profile of GPR84 and, in the absence of direct structural information, recent developments and use of GPR84 pharmacological tool compounds to study its broader role and biology.

GPR120 agonists for the treatment of diabetes: a patent review (2014 present)

INTRODUCTION

G protein-coupled receptor 120 (GPR120) is a G_αq coupled GPCR specifically activated by long-chain fatty acids (LCFAs). Functionally, it has been identified as a member of a family of lipid-binding free fatty acid receptors including GPR40, GPR41, and GPR43. Upon stimulation by LCFAs, GPR120 can directly or indirectly modulate hormone secretion from the gastrointestinal tract and pancreas, and regulate lipid and/or glucose metabolism in adipose, liver, and muscle tissues. Additionally, GPR120 is postulated to mediate anti-inflammatory and insulin-sensitizing effects in adipose and macrophages. These benefits suggest that GPR120 agonists have the potential to be an effective treatment for obesity, type 2 diabetes mellitus (T2DM), and other metabolic syndromes.

AREA COVERED

This article highlights and reviews research advances in this field that have been published in patent literature and peer-reviewed journals since 2014.

EXPERT OPINION

Current development has been hindered by species differences in GPR120 distribution, differences in GPR120-mediated signaling in distinct tissue types, and lack of available ligands with suitable selectivity for GPR120 over GPR40 in both human and rodents. The discovery of β-arrestin biased GPR120 agonists will help elucidate the potential of selective therapeutics that may discriminate between desirable and undesirable pharmacological effects.

ABBREVIATIONS

ALA: α-linolenic acid; AUC: area under the curve; BRET: bioluminescence resonance energy transfer; CCK: cholecystokinin; CHO-K1 cell: Chinese hamster ovary-K1 cell; db/db mouse: diabetic mouse; DHA: docosahexaenoic acid; DIO: diet-induced obesity; DMSO: dimethyl sulfoxide; DPP-4: dipeptidyl peptidase 4; EPA: eicosapentaenoic acid; FA(s): fatty acid(s); FFA(s): free fatty acid(s); FFAR: free fatty acid receptor; FLIPR: fluorescent imaging plate reader; GIR: glucose infusion rate; GLP-1: glucagon-like peptide 1; GP(C)R: G protein-coupled receptor; GSIS: glucose-stimulated insulin secretion; HEK293 cell: human embryonic kidney 293 cell; HOMA-IR: homeostatic measurement assessment of insulin resistance; IP1: inositol phosphate turnover; IPGTT: intraperitoneal glucose tolerance test; LCFA(s): long-chain fatty acid(s); MED_max: maximal efficacy; MIN6 cell: mouse insulin-secreting cell; NPY: neuropeptide Y; OGTT: oral glucose tolerance test; pERK: phosphorylated ERK; PPAR: peroxisome proliferator-activated receptor; QD: once daily; SAR: structure-activity relationship; siRNA: small interfering ribonucleic acid; STC-1: intestinal secretin tumor cell; T2DM: type 2 diabetes mellitus; U2OS cell: human bone osteosarcoma epithelial cell; uHTS: ultrahigh-throughput screening; ZDF: zucker diabetic fatty.

Addressing free fatty acid receptor 1 (FFAR1) activation using supervised molecular dynamics

The free fatty acid receptor 1 (FFAR1, formerly GPR40), is a potential G protein-coupled receptor (GPCR) target for the treatment of type 2 diabetes mellitus (T2DM), as it enhances glucose-dependent insulin secretion upon activation by endogenous long-chain free fatty acids. The presence of two allosterically communicating binding sites and the lack of the conserved GPCR structural motifs challenge the general knowledge of its activation mechanism. To date, four X-ray crystal structures are available for computer-aided drug design. In this study, we employed molecular dynamics (MD) and supervised molecular dynamics (SuMD) to deliver insights into the (un)binding mechanism of the agonist MK-8666, and the allosteric communications between the two experimentally determined FFAR1 binding sites. We found that FFAR1 extracellular loop 2 (ECL2) mediates the binding of the partial agonist MK-8666. Moreover, simulations showed that the agonists MK-8666 and AP8 are reciprocally stabilized and that AP8 influences MK-8666 unbinding from FFAR1.

Butyrate Protects Against Salsolinol-Induced Toxicity in SH-SY5Y Cells: Implication for Parkinson's Disease

Parkinson's disease (PD), a progressive neurodegenerative disorder, is associated with the destruction of dopamine neurons in the substantia nigra (SN) and the formation of Lewy bodies in basal ganglia. Risk factors for PD include aging, as well as environmental and genetic factors. Recent converging reports suggest a role for the gut microbiome and epigenetic factors in the onset and/or progression of PD. Of particular relevance and potential therapeutic targets in this regard are histone deacetylases (HDACs), enzymes that are involved in chromatin remodeling. Butyrate, a short-chain fatty acid (FA) produced in the gut and presumably acting via several G protein-coupled receptors (GPCRs) including FA3 receptors (FA3Rs), is a well-known HDAC inhibitor that plays an important role in maintaining homeostasis of the gut-brain axis. Recently, its significance in regulation of some critical brain functions and usefulness in neurodegenerative diseases such as PD has been suggested. In this study we sought to determine whether butyrate may have protective effects against salsolionl (SALS)-induced toxicity in SH-SY5Y cells. SALS, an endogenous product of aldehyde and dopamine condensation, may be selectively toxic to dopaminergic neurons. SH-SY5Y cells, derived from human neuroblastoma cells, are used as a model of these neurons. Exposure of SH-SY5Y cells for 24 h to 400 μM SALS resulted in approximately 60% cell death, which was concentration-dependently prevented by butyrate. The effects of butyrate in turn were significantly attenuated by beta-hydroxy butyrate (BHB), a selective FA3R antagonist. Moreover, a selective FA3R agonist (AR 420626) also provided protective effects against SALS, which was totally blocked by BHB. These findings provide further support that butyrate or an agonist of FA3R may be of therapeutic potential in PD.

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.

An Agonist Radioligand for the Proinflammatory Lipid-Activated G Protein-Coupled Receptor GPR84 Providing Structural Insights

The orphan G protein-coupled receptor (GPCR) GPR84 is expressed on immune cells mediating proinflammatory and immunostimulatory effects. In this study, we prepared the fully efficacious, nonbiased GPR84 agonist 6-hexylamino-2,4(1H,3H)-pyrimidinedione (6) in tritium-labeled form ([³H]PSB-1584) by hydrogenation of a hexenyl-substituted precursor with tritium gas. The radioligand was characterized by kinetic, saturation, and competition assays using membranes of Chinese hamster ovary cells recombinantly expressing the human GPR84. [³H]6 reversibly labeled the receptor with high affinity (K_D 2.08 nM). Structurally diverse orthosteric and allosteric ligands, including newly designed and synthesized compounds, were studied in competition binding assays. A homology model of GPR84 was generated to perform docking studies rationalizing the experimental data. The radioligand was additionally used for labeling GPR84 in native cells and tissues. [³H]6 constitutes the first GPR84 agonist radioligand representing a powerful tool for this poorly investigated GPCR, which has potential as a future drug target.

A Selectivity Study of FFAR4/FFAR1 Agonists by Molecular Modeling

FFAR4 has been considered as a potential target for metabolic diseases, including diabetes. Some compounds with biphenyl scaffold, represented by compound SR13 reported by our group, showed significant FFAR4 selectivity. However, the molecular basis for their selectivity has not been definitely disclosed. This study provided insights into the protein-ligand interactions between agonists and FFAR4/FFAR1 by molecular modeling. The important residues identified were consistent with those found in experimental studies. Moreover, the results proposed that the selectivity of SR13 between FFAR4 and FFAR1 depended on whether it can enter the ligand-binding site through the entrance region by adopting its preferential conformation. The big difference between the preferential conformation of SR13 and the narrow entrance region determined its poor agonist activity against FFAR1. These findings will facilitate the further development of selective FFAR4 agonists.

Three classes of ligands each bind to distinct sites on the orphan G protein-coupled receptor GPR84

Medium chain fatty acids can activate the pro-inflammatory receptor GPR84 but so also can molecules related to 3,3′-diindolylmethane. 3,3′-Diindolylmethane and decanoic acid acted as strong positive allosteric modulators of the function of each other and analysis showed the affinity of 3,3′-diindolylmethane to be at least 100 fold higher. Methyl decanoate was not an agonist at GPR84. This implies a key role in binding for the carboxylic acid of the fatty acid. Via homology modelling we predicted and confirmed an integral role of arginine¹⁷², located in the 2nd extracellular loop, in the action of decanoic acid but not of 3,3′-diindolylmethane. Exemplars from a patented series of GPR84 antagonists were able to block agonist actions of both decanoic acid and 3,3′-diindolylmethane at GPR84. However, although a radiolabelled form of a related antagonist, [³H]G9543, was able to bind with high affinity to GPR84, this was not competed for by increasing concentrations of either decanoic acid or 3,3′-diindolylmethane and was not affected adversely by mutation of arginine¹⁷². These studies identify three separable ligand binding sites within GPR84 and suggest that if medium chain fatty acids are true endogenous regulators then co-binding with a positive allosteric modulator would greatly enhance their function in physiological settings.

Discovery of an Isothiazole-Based Phenylpropanoic Acid GPR120 Agonist as a Development Candidate for Type 2 Diabetes

We have discovered a novel series of isothiazole-based phenylpropanoic acids as GPR120 agonists. Extensive structure-activity relationship studies led to the discovery of a potent GPR120 agonist 4x, which displayed good EC₅₀ values in both calcium and β-arrestin assays. It also presented good pharmaceutical properties and a favorable PK profile. Moreover, it demonstrated in vivo antidiabetic activity in C57BL/6 DIO mice. Studies in WT and knockout DIO mice showed that it improved glucose handling during an OGTT via GPR120. Overall, 4x possessed promising antidiabetic effect and good safety profile to be a development candidate.