Interdependent allosteric free fatty acid receptor 2 modulators synergistically induce functional selective activation and desensitization in neutrophils

Function and regulation of GPR84 in human neutrophils

Human neutrophils are components of the innate immune system and are the most abundant white blood cells in the circulation. They are professional phagocytes and express several G protein-coupled receptors (GPCRs), which are essential for proper neutrophil functions. So far, the two formyl peptide receptors, FPR1 and FPR2, have been the most extensively studied group of neutrophil GPCRs, but recently, a new group, the free fatty acid (FFA) receptors, has attracted growing attention. Neutrophils express two FFA receptors, GPR84 and FFA2, which sense medium- and short-chain fatty acids respectively, and display similar activation profiles. The exact pathophysiological role of GPR84 is not yet fully understood, but it is generally regarded as a pro-inflammatory receptor that mediates neutrophil activation. In this review, we summarize current knowledge of how GPR84 affects human neutrophil functions and discuss the regulatory mechanisms that control these responses, focusing on the similarities and differences in comparison to the two FPRs and FFA2. LINKED ARTICLES: This article is part of a themed issue GPR84 Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.10/issuetoc.

An increase in the cytosolic concentration of free calcium ions activates the neutrophil NADPH-oxidase provided that the free fatty acid receptor 2 has been allosterically modulated

Signals generated by free fatty acid receptor 2 (FFA2R) can activate the neutrophil NADPH-oxidase without involvement of any orthosteric FFA2R agonist. The initiating signals may be generated by P2Y₂R, the receptor for ATP. An FFA2R specific allosteric modulator (PAM; Cmp58) was required for this response and used to investigate the mechanism by which signals generated by ATP/P2Y₂R activate an FFA2R dependent process. The P2Y₂R induced signal that together with the modulated FFA2R activates neutrophils, was generated downstream of the Gα_q containing G protein coupled to P2Y₂R. A rise in the cytosolic concentration of ionized calcium ([Ca²⁺]_i) was hypothesized to be the important signal. The hypothesis gained support from the finding that the modulator transferred the neutrophils to a Ca²⁺sensitive state. The rise in [Ca²⁺]_i induced by the Ca²⁺ specific ionophore ionomycin, activated the neutrophils provided that an allosteric modulator was bound to FFA2R. The activity of the superoxide generating NADPH-oxidase induced by ionomycin was rapidly terminated and the FFA2Rs could then no longer be activated by the FFA2R agonist propionate or by the signal generated by ATP/P2Y₂R. The non-responding state of FFA2R was, however, revoked by a cross-activating allosteric FFA2R modulator. The [Ca²⁺]_i mediated activation of neutrophils with their FFA2Rs allosterically modulated, represent a unique regulatory receptor crosstalk mechanism by which the activation potency of a G protein coupled receptor is controlled by a receptor-crosstalk signaling system operating from the cytosolic side of the plasma membrane.

The Role of Short Chain Fatty Acids in Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder that is characterized by abdominal pain and disordered bowel habits. The etiology of IBS is multifactorial, including abnormal gut-brain interactions, visceral hypersensitivity, altered colon motility, and psychological factors. Recent studies have shown that the intestinal microbiota and its metabolites short chain fatty acids (SCFAs) may be involved in the pathogenesis of IBS. SCFAs play an important role in the pathophysiology of IBS. We discuss the underlying mechanisms of action of SCFAs in intestinal inflammation and immunity, intestinal barrier integrity, motility, and the microbiota-gut-brain axis. Limited to previous studies, further studies are required to investigate the mechanisms of action of SCFAs in IBS and provide more precise therapeutic strategies for IBS.

G protein coupled pattern recognition receptors expressed in neutrophils : Recognition, activation/modulation, signaling and receptor regulated functions

Neutrophils, the most abundant white blood cell in human blood, express receptors that recognize damage/microbial associated pattern molecules of importance for cell recruitment to sites of inflammation. Many of these receptors belong to the family of G protein coupled receptors (GPCRs). These receptor-proteins span the plasma membrane in expressing cells seven times and the down-stream signaling rely in most cases on an activation of heterotrimeric G proteins. The GPCRs expressed in neutrophils recognize a number of structurally diverse ligands (activating agonists, allosteric modulators, and inhibiting antagonists) and share significant sequence homologies. Studies of receptor structure and function have during the last 40 years generated important information on GPCR biology in general; this knowledge aids in the overall understanding of general pharmacological principles, governing regulation of neutrophil function and inflammatory processes, including novel leukocyte receptor activities related to ligand recognition, biased/functional selective signaling, allosteric modulation, desensitization, and reactivation mechanisms as well as communication (receptor transactivation/cross-talk) between GPCRs. This review summarizes the recent discoveries and pharmacological hallmarks with focus on some of the neutrophil expressed pattern recognition GPCRs. In addition, unmet challenges, including recognition by the receptors of diverse ligands and how biased signaling mediate different biological effects are described/discussed.

Allosteric receptor modulation uncovers an FFA2R antagonist as a positive orthosteric modulator/agonist in disguise

A novel receptor crosstalk activation mechanism, through which signals generated by the agonist-occupied P2Y₂R (the neutrophil receptor for ATP) activate allosterically modulated free fatty acid 2 receptor (FFA2R) without the involvement of any FFA2R agonist, was used to determine the inhibitor profiles of two earlier-described, FFA2R-specific antagonists, CATPB and GLPG0974. These antagonists have been shown to have somewhat different receptor-interaction characteristics at the molecular/functional level, although both are recognized by the orthosteric site in FFA2R. The antagonists inhibited neutrophil activation induced by ATP, an activation occurred only in the presence of either of the two positive allosteric FFA2R modulators (PAMs) AZ1729 and Cmp58. No neutrophil activation was induced by either AZ1729 or Cmp58 alone, whereas together they acted as co-agonistic PAMs and activated the superoxide-generating NADPH-oxidase in neutrophils. This response was inhibited by CATPB but not by GLPG0974. In contrast, GLPG0974 acted as a positive modulator, increasing the potency, albeit not the efficacy, of the co-agonistic PAMs. GLPG0974 also altered signaling downstream of FFA2R when activated by the co-agonistic PAMs. In the presence of GLPG0974, the response of neutrophils induced by the co-agonistic PAMs included an increase in the cytosolic concentration of free calcium ions (Ca²⁺), and this effect was reciprocal in that GLPG0974 triggered an increase in intracellular Ca²⁺, demonstrating that GLPG0974 acted as an FFA2R agonist. In summary, by studying the effects of the FFA2R ligand GLPG0974 on neutrophil activation induced by the co-agonists AZ1729 + Cmp58, we show that GLPG0974 is not only an FFA2R antagonist, but also displays agonistic and positive FFA2R-modulating functions that affect NADPH-oxidase activity and alter the receptor-downstream signaling induced by the co-agonistic PAMs.

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.

Participation of Short-Chain Fatty Acids and Their Receptors in Gut Inflammation and Colon Cancer

Short-chain fatty acids (SCFAs) are the main metabolites produced by the bacterial fermentation of dietary fiber, and they play a critical role in the maintenance of intestinal health. SCFAs are also essential for modulating different processes, and they have anti-inflammatory properties and immunomodulatory effects. As the inflammatory process predisposes the development of cancer and promotes all stages of tumorigenesis, an antitumor effect has also been associated with SCFAs. This is strongly supported by epidemiological studies showing that a diet rich in fiber is linked to a reduced risk of colon cancer and has significant clinical benefits in patients with inflammatory bowel disease (IBD). SCFAs may signal through the metabolite-sensing G protein-coupled receptors free fatty acid receptor 3 [FFAR3 or G protein-coupled receptor 41 (GPR41)], FFAR2 (GPR43), and GPR109A (also known as hydroxycarboxylic acid receptor 2 or HCAR2) expressed in the gut epithelium and immune cells. This review summarizes the existing knowledge regarding the SCFA-mediated suppression of inflammation and carcinogenesis in IBD and colon cancer.

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.

Functional selective FPR1 signaling in favor of an activation of the neutrophil superoxide generating NOX2 complex

The formyl peptide receptors FPR1 and FPR2 are abundantly expressed by neutrophils, in which they regulate proinflammatory tissue recruitment of inflammatory cells, the production of reactive oxygen species (ROS), and resolution of inflammatory reactions. The unique dual functionality of the FPRs makes them attractive targets to develop FPR‐based therapeutics as novel anti‐inflammatory treatments. The small compound RE‐04‐001 has earlier been identified as an inducer of ROS in differentiated HL60 cells but the precise target and the mechanism of action of the compound was has until now not been elucidated. In this study, we reveal that RE‐04‐001 specifically targets and activates FPR1, and the concentrations needed to activate the neutrophil NADPH‐oxidase was very low (EC₅₀ ∼1 nM). RE‐04‐001 was also found to be a neutrophil chemoattractant, but when compared to the prototype FPR1 agonist N‐formyl‐Met‐Leu‐Phe (fMLF), the concentrations required were comparably high, suggesting that signaling downstream of the RE‐04‐001‐activated‐FPR1 is functionally selective. In addition, the RE‐04‐001‐induced response was strongly biased toward the PLC‐PIP₂‐Ca²⁺ pathway and ERK1/2 activation but away from β‐arrestin recruitment. Compared to the peptide agonist fMLF, RE‐04‐001 is more resistant to inactivation by the MPO‐H₂O₂‐halide system. In summary, this study describes RE‐04‐001 as a novel small molecule agonist specific for FPR1, which displays a biased signaling profile that leads to a functional selective activating of human neutrophils. RE‐04‐001 is, therefore, a useful tool, not only for further mechanistic studies of the regulatory role of FPR1 in inflammation in vitro and in vivo, but also for developing FPR1‐specific drug therapeutics.

The PAR4-derived pepducin P4Pal10 lacks effect on neutrophil GPCRs that couple to Gαq for signaling but distinctly modulates function of the Gαi-coupled FPR2 and FFAR2

A novel mechanism of action was described for the protease-activated receptor 4 (PAR4)-derived pepducin (P4Pal₁₀), when it was shown to exhibit inhibitory efficacy towards G protein coupling to multiple Gαq-coupled receptors (Carr, R., 3rd et al., Mol. Pharmacol. 2016(89) 94). We could confirm that P4Pal₁₀, similar to an earlier-characterized Gαq inhibitor (YM-254890), inhibited platelet aggregation induced by agonists for the Gαq-coupled receptors PAR1 and PAR4. Next, we applied P4Pal₁₀ as a tool compound and investigated its modulatory effect on several Gαq- and Gαi-coupled GPCRs expressed by human neutrophils. P4Pal₁₀ had, however, no inhibitory effects on signaling downstream of the Gαq-coupled receptors for ATP (P2Y₂R) and PAF (PAFR). Instead, P4Pal₁₀ inhibited signaling downstream the Gαi-coupled FPR2. The inhibition was not due to a direct effect on Gαi as the closely related FPR1 was unaffected. In addition, we found that the pepducin activated allosterically modulated short chain fatty acid receptor (FFAR2), a Gαi/Gαq coupled GPCR that is functionally expressed in neutrophils. Taken together, we show that pepducins are unique tool-compounds for mechanistic studies of GPCR signaling and modulation in neutrophils. The data presented add also lipopeptides into the known ligand recognition lists for the two pattern recognition receptors FPR2 and FFAR2, receptors that primarily sense formylated peptides and short free fatty acids, respectively, inflammatory mediators of microbial origin.

Metabolic and inflammatory functions of short-chain fatty acid receptors

FFA2 and FFA3 are receptors for short-chain fatty acids which are produced in prodigious amounts by fermentation of poorly digested carbohydrates by gut bacteria. Understanding the roles of these receptors in regulating enteroendocrine, metabolic and immune functions has developed with the production and use of novel pharmacological tools and animal models. A complex (patho)physiological scenario is now emerging in which strategic expression of FFA2 and FFA3 in key cell types and selective modulation of their signalling might regulate body weight management, energy homoeostasis and inflammatory disorders.

Neutrophil Signaling That Challenges Dogmata of G Protein-Coupled Receptor Regulated Functions

Activation as well as recruitment of neutrophils, the most abundant leukocyte in human blood, to sites of infection/inflammation largely rely on surface-exposed chemoattractant receptors. These receptors belong to the family of 7-transmembrane domain receptors also known as G protein-coupled receptors (GPCRs) due to the fact that part of the downstream signaling relies on an activation of heterotrimeric G proteins. The neutrophil GPCRs share significant sequence homologies but bind many structurally diverse activating (agonistic) and inhibiting (antagonistic) ligands, ranging from fatty acids to purines, peptides, and lipopeptides. Recent structural and functional studies of neutrophil receptors have generated important information on GPCR biology in general; this knowledge aids in the overall understanding of general pharmacological principles, governing regulation of neutrophil function and inflammatory processes, including novel leukocyte receptor activities related to ligand recognition, biased/functional selective signaling, allosteric modulation, desensitization mechanisms and reactivation, and communication (cross-talk) between GPCRs. This review summarizes the recent discoveries and pharmacological hallmarks with focus on neutrophil GPCRs. In addition, unmet challenges are dealt with, including recognition by the receptors of diverse ligands and how biased signaling mediates different biological effects.