Therapeutic Opportunities and Challenges in Targeting the Orphan G Protein-Coupled Receptor GPR35

The Complex World of Kynurenic Acid: Reflections on Biological Issues and Therapeutic Strategy

It has been unequivocally established that kynurenic acid has a number of actions in a variety of cells and tissues, raising, in principle, the possibility of targeting its generation, metabolism or sites of action to manipulate those effects to a beneficial therapeutic end. However, many basic aspects of the biology of kynurenic acid remain unclear, potentially leading to some confusion and misinterpretations of data. They include questions of the source, generation, targets, enzyme expression, endogenous concentrations and sites of action. This essay is intended to raise and discuss many of these aspects as a source of reference for more balanced discussion. Those issues are followed by examples of situations in which modulating and correcting kynurenic acid production or activity could bring significant therapeutic benefit, including neurological and psychiatric conditions, inflammatory diseases and cell protection. More information is required to obtain a clear overall view of the pharmacological environment relevant to kynurenic acid, especially with respect to the active concentrations of kynurenine metabolites in vivo and changed levels in disease. The data and ideas presented here should permit a greater confidence in appreciating the sites of action and interaction of kynurenic acid under different local conditions and pathologies, enhancing our understanding of kynurenic acid itself and the many clinical conditions in which manipulating its pharmacology could be of clinical value.

Application of FRET- and BRET-based live-cell biosensors in deorphanization and ligand discovery studies on orphan G protein-coupled receptors

Bioluminescence- and fluorescence-based resonance energy transfer assays have gained considerable attention in pharmacological research as high-throughput scalable tools applicable to drug discovery. To this end, G protein-coupled receptors represent the biggest target class for marketed drugs, and among them, orphan G protein-coupled receptors have the biggest untapped therapeutic potential. In this review, the cases where biophysical methods, BRET and FRET, were employed for deorphanization and ligand discovery studies on orphan G protein-coupled receptors are listed and discussed.

Mast cells help organize the Peyer's patch niche for induction of IgA responses

Peyer's patches (PPs) are lymphoid structures situated adjacent to the intestinal epithelium that support B cell responses that give rise to many intestinal IgA-secreting cells. Induction of isotype switching to IgA in PPs requires interactions between B cells and TGFβ-activating conventional dendritic cells type 2 (cDC2s) in the subepithelial dome (SED). However, the mechanisms promoting cDC2 positioning in the SED are unclear. Here, we found that PP cDC2s express GPR35, a receptor that promotes cell migration in response to various metabolites, including 5-hydroxyindoleacetic acid (5-HIAA). In mice lacking GPR35, fewer cDC2s were found in the SED, and frequencies of IgA+ germinal center (GC) B cells were reduced. IgA plasma cells were reduced in both the PPs and lamina propria. These phenotypes were also observed in chimeric mice that lacked GPR35 selectively in cDCs. GPR35 deficiency led to reduced coating of commensal bacteria with IgA and reduced IgA responses to cholera toxin. Mast cells were present in the SED, and mast cell-deficient mice had reduced PP cDC2s and IgA+ cells. Ablation of tryptophan hydroxylase 1 (Tph1) in mast cells to prevent their production of 5-HIAA similarly led to reduced PP cDC2s and IgA responses. Thus, mast cell-guided positioning of GPR35+ cDC2s in the PP SED supports induction of intestinal IgA responses.

Key aspects of modern GPCR drug discovery

G-protein-coupled receptors (GPCRs) are the largest and most versatile cell surface receptor family with a broad repertoire of ligands and functions. We've learned an enormous amount about discovering drugs of this receptor class since the first GPCR was cloned and expressed in 1986, such that it's now well-recognized that GPCRs are the most successful target class for approved drugs. Here we take the reader through a GPCR drug discovery journey from target to the clinic, highlighting the key learnings, best practices, challenges, trends and insights on discovering drugs that ultimately modulate GPCR function therapeutically in patients. The future of GPCR drug discovery is inspiring, with more desirable drug mechanisms and new technologies enabling the delivery of better and more successful drugs.

Pamoic acid-induced peripheral GPR35 activation improves pruritus and dermatitis

BACKGROUND AND PURPOSE

Pruritic dermatitis is a disease with a considerable unmet need for treatment and appears to present with not only epidermal but also peripheral neuronal complications. Here, we propose a novel pharmacological modulation targeting both peripheral dorsal root ganglion (DRG) sensory neurons and skin keratinocytes. GPR35 is an orphan G-protein-coupled receptor expressed in DRG neurons and has been predicted to downregulate neuronal excitability when activated. Modulator information is currently increasing for GPR35, and pamoic acid (PA), a salt-forming agent for drugs, has been shown to be an activator solely specific for GPR35. Here, we investigated its effects on dermatitic pathology.

EXPERIMENTAL APPROACH

We confirmed GPR35 expression in peripheral neurons and tissues. The effect of PA treatment was pharmacologically evaluated in cultured cells in vitro and in in vivo animal models for acute and chronic pruritus.

KEY RESULTS

Local PA application mitigated acute non-histaminergic itch and, consistently, obstructed DRG neuronal responses. Keratinocyte fragmentation under dermatitic simulation was also dampened following PA incubation. Chronic pruritus in 1-chloro-2,4-dinitrobenzene and psoriasis models were also moderately but significantly reversed by the repeated applications of PA. Dermatitic scores in the 1-chloro-2,4-dinitrobenzene and psoriatic models were also improved by its application, indicating that it is beneficial for mitigating disease pathology.

CONCLUSION AND IMPLICATIONS

Our findings suggest that pamoic acid activation of peripheral GPR35 can contribute to the improvement of pruritus and its associated diseases.

Versatile lactate signaling via HCAR1: a multifaceted GPCR involved in many biological processes

G-coupled protein receptors (GPCRs) are the ultimate refuge of pharmacology and medicine as more than 40% of all marketed drugs are directly targeting these receptors. Through cell surface expression, they are at the forefront of cellular communication with the outside world. Metabolites among the conveyors of this communication are becoming more prominent with the recognition of them as ligands for GPCRs. HCAR1 is a GPCR conveyor of lactate. It is a class A GPCR coupled to Gαi which reduces cellular cAMP along with the downstream Gβγ signaling. It was first found to inhibit lipolysis, and lately has been implicated in diverse cellular processes, including neural activities, angiogenesis, inflammation, vision, cardiovascular function, stem cell proliferation, and involved in promoting pathogenesis for different conditions, such as cancer. Other than signaling from the plasma membrane, HCAR1 shows nuclear localization with different location-biased activities therein. Although different functions for HCAR1 are being discovered, its cell and molecular mechanisms are yet ill understood. Here, we provide a comprehensive review on HCAR1, which covers the literature on the subject, and discusses its importance and relevance in various biological phenomena.

Comparative transcriptomic analysis of mammary gland tissues reveals the critical role of GPR110 in palmitic acid-stimulated milk protein and fat synthesis

The G protein-coupled receptors (GPCR) sensing nutritional signals (amino acids, fatty acids, glucose, etc.) are not fully understood. In this research, we used transcriptome sequencing to analyse differentially expressed genes (DEG) in mouse mammary gland tissues at puberty, lactation and involution stages, in which eight GPCR were selected out and verified by qRT-PCR assay. It was further identified the role of GPR110-mediating nutrients including palmitic acid (PA) and methionine (Met) to improve milk synthesis using mouse mammary epithelial cell line HC11. PA but not Met affected GPR110 expression in a dose-dependent manner. GPR110 knockdown decreased milk protein and fat synthesis and cell proliferation and blocked the stimulation of PA on mechanistic target of rapamycin (mTOR) phosphorylation and sterol-regulatory element binding protein 1c (SREBP-1c) expression. In summary, these experimental results disclose DEG related to lactation and reveal that GPR110 mediates PA to activate the mTOR and SREBP-1c pathways to promote milk protein and fat synthesis.

GPR35 acts a dual role and therapeutic target in inflammation

GPR35 is a G protein-coupled receptor with notable involvement in modulating inflammatory responses. Although the precise role of GPR35 in inflammation is not yet fully understood, studies have suggested that it may have both pro- and anti-inflammatory effects depending on the specific cellular environment. Some studies have shown that GPR35 activation can stimulate the production of pro-inflammatory cytokines and facilitate the movement of immune cells towards inflammatory tissues or infected areas. Conversely, other investigations have suggested that GPR35 may possess anti-inflammatory properties in the gastrointestinal tract, liver and certain other tissues by curbing the generation of inflammatory mediators and endorsing the differentiation of regulatory T cells. The intricate role of GPR35 in inflammation underscores the requirement for more in-depth research to thoroughly comprehend its functional mechanisms and its potential significance as a therapeutic target for inflammatory diseases. The purpose of this review is to concurrently investigate the pro-inflammatory and anti-inflammatory roles of GPR35, thus illuminating both facets of this complex issue.

G protein-receptor kinases 5/6 are the key regulators of G protein-coupled receptor 35-arrestin interactions

Human G protein-coupled receptor 35 is regulated by agonist-mediated phosphorylation of a set of five phospho-acceptor amino acids within its C-terminal tail. Alteration of both Ser300 and Ser303 to alanine in the GPR35a isoform greatly reduces the ability of receptor agonists to promote interactions with arrestin adapter proteins. Here, we have integrated the use of cell lines genome edited to lack expression of combinations of G protein receptor kinases (GRKs), selective small molecule inhibitors of subsets of these kinases, and antisera able to specifically identify either human GPR35a or mouse GPR35 only when Ser300 and Ser303 (orce; the equivalent residues in mouse GPR35) have become phosphorylated to demonstrate that GRK5 and GRK6 cause agonist-dependent phosphorylation of these residues. Extensions of these studies demonstrated the importance of the GRK5/6-mediated phosphorylation of these amino acids for agonist-induced internalization of the receptor. Homology and predictive modeling of the interaction of human GPR35 with GRKs showed that the N terminus of GRK5 is likely to dock in the same methionine pocket on the intracellular face of GPR35 as the C terminus of the α5 helix of Gα13 and, that while this is also the case for GRK6, GRK2 and GRK3 are unable to do so effectively. These studies provide unique and wide-ranging insights into modes of regulation of GPR35, a receptor that is currently attracting considerable interest as a novel therapeutic target in diseases including ulcerative colitis.

Pharmacohistory of Cannabis Use-A New Possibility in Future Drug Development for Gastrointestinal Diseases

Humans have employed cannabis for multiple uses including medicine, recreation, food, and fibre. The various components such as roots, flowers, seeds, and leaves have been utilized to alleviate pain, inflammation, anxiety, and gastrointestinal disorders like nausea, vomiting, diarrhoea, and inflammatory bowel diseases (IBDs). It has occupied a significant space in ethnomedicines across cultures and religions. Despite multi-dimensional uses, the global prohibition of cannabis by the USA through the introduction of the Marijuana Tax Act in 1937 led to prejudice about the perceived risks of cannabis, overshadowing its medicinal potential. Nevertheless, the discovery of tetrahydrocannabinol (THC), the primary psychoactive compound in cannabis, and the endocannabinoid system renewed scientific interest in understanding the role of cannabis in modulating different conditions, including gastrointestinal disorders. Preparations combining cannabidiol and THC have shown promise in mitigating gut symptoms through anti-inflammatory and motility-enhancing effects. This review revisits the ethnomedicinal use of cannabis in gastrointestinal diseases and emphasizes the need for further research to determine optimal dosages, formulations, and safety profiles of cannabis-based medicines. It also underscores the future potential of cannabinoid-based therapies by leveraging the role of the expanded endocannabinoid system, an endocannabinoidome, in the modulation of gastrointestinal ailments.

Effect of Yarrowia lipolytica yeast biomass with increased kynurenic acid content on selected metabolic indicators in mice

Background

The unconventional yeast species Yarrowia lipolytica is a valuable source of protein and many other nutrients. It can be used to produce hydrolytic enzymes and metabolites, including kynurenic acid (KYNA), an endogenous metabolite of tryptophan with a multidirectional effect on the body. The administration of Y. lipolytica with an increased content of KYNA in the diet may have a beneficial effect on metabolism, which was evaluated in a nutritional experiment on mice.

Methods

In the dry biomass of Y. lipolytica S12 enriched in KYNA (high-KYNA yeast) and low-KYNA (control) yeast, the content of KYNA was determined by high-performance liquid chromatography. Then, proximate and amino acid composition and selected indicators of antioxidant status were compared. The effect of 5% high-KYNA yeast content in the diet on the growth, hematological and biochemical indices of blood and the redox status of the liver was determined in a 7-week experiment on adult male mice from an outbred colony derived from A/St, BALB/c, BN/a and C57BL/6J inbred strains.

Results

High-KYNA yeast was characterized by a greater concentration of KYNA than low-KYNA yeast (0.80 ± 0.08 vs. 0.29 ± 0.01 g/kg dry matter), lower content of crude protein with a less favorable amino acid composition and minerals, higher level of crude fiber and fat and lower ferric-reducing antioxidant power, concentration of phenols and glutathione. Consumption of the high-KYNA yeast diet did not affect the cumulative body weight gain per cage, cumulative food intake per cage and protein efficiency ratio compared to the control diet. A trend towards lower mean corpuscular volume and hematocrit, higher mean corpuscular hemoglobin concentration and lower serum total protein and globulins was observed, increased serum total cholesterol and urea were noted. Its ingestion resulted in a trend towards greater ferric-reducing antioxidant power in the liver and did not affect the degree of liver lipid and protein oxidation.

Conclusions

The improvement of the quality of Y. lipolytica yeast biomass with increased content of KYNA, including its antioxidant potential, would be affected by the preserved level of protein and unchanged amino acid profile. It will be worth investigating the effect of such optimized yeast on model animals, including animals with metabolic diseases.

The cryptic role of CXCL17/CXCR8 axis in the pathogenesis of cancers: a review of the latest evidence

Chemokines are immune system mediators that mediate various activities and play a role in the pathogenesis of several cancers. Among these chemokines, C-X-C motif chemokine 17 (CXCL-17) is a relatively novel molecule produced along the airway epithelium in physiological and pathological conditions, and evidence shows that it plays a homeostatic role in most cases. CXCL17 has a protective role in some cancers and a pathological role in others, such as liver and lung cancer. This chemokine, along with its possible receptor termed G protein-coupled receptor 35 (GPR35) or CXCR8, are involved in recruiting myeloid cells, regulating angiogenesis, defending against pathogenic microorganisms, and numerous other mechanisms. Considering the few studies that have been performed on the dual role of CXCL17 in human malignancies, this review has investigated the possible pro-tumor and anti-tumor roles of this chemokine, as well as future treatment options in cancer therapy.

GPR35 and mediators from platelets and mast cells in neutrophil migration and inflammation

Neutrophil recruitment from circulation to sites of inflammation is guided by multiple chemoattractant cues emanating from tissue cells, immune cells, and platelets. Here, we focus on the function of one G-protein coupled receptor, GPR35, in neutrophil recruitment. GPR35 has been challenging to study due the description of multiple ligands and G-protein couplings. Recently, we found that GPR35-expressing hematopoietic cells respond to the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA). We discuss distinct response profiles of GPR35 to 5-HIAA compared to other ligands. To place the functions of 5-HIAA in context, we summarize the actions of serotonin in vascular biology and leukocyte recruitment. Important sources of serotonin and 5-HIAA are platelets and mast cells. We discuss the dynamics of cell migration into inflamed tissues and how multiple platelet and mast cell-derived mediators, including 5-HIAA, cooperate to promote neutrophil recruitment. Additional actions of GPR35 in tissue physiology are reviewed. Finally, we discuss how clinically approved drugs that modulate serotonin uptake and metabolism may influence 5-HIAA-GPR35 function, and we speculate about broader influences of the GPR35 ligand-receptor system in immunity and disease.

Recent advances in GPR35 pharmacology; 5-HIAA serotonin metabolite becomes a ligand

GPR35, an orphan receptor, has been waiting for its ligand since its cloning in 1998. Many endogenous and exogenous molecules have been suggested to act as agonists of GPR35 including kynurenic acid, zaprinast, lysophosphatidic acid, and CXCL17. However, complex and controversial responses to ligands among species have become a huge hurdle in the development of therapeutics in addition to the orphan state. Recently, a serotonin metabolite, 5-hydroxyindoleacetic acid (5-HIAA), is reported to be a high potency ligand for GPR35 by investigating the increased expression of GPR35 in neutrophils. In addition, a transgenic knock-in mouse line is developed, in which GPR35 was replaced with a human ortholog, making it possible not only to overcome the different selectivity of agonists among species but also to conduct therapeutic experiments on human GPR35 in mouse models. In the present article, I review the recent advances and prospective therapeutic directions in GPR35 research. Especially, I'd like to draw attention of readers to the finding of 5-HIAA as a ligand of GPR35 and lead to apply the 5-HIAA and human GPR35 knock-in mice to their research fields in a variety of pathophysiological conditions.

G Protein-Coupled Receptor 75 (GPR75) As a Novel Molecule for Targeted Therapy of Cancer and Metabolic Syndrome

In recent years, molecular targeted therapy has attracted more attention from researchers due to its high efficiency and fewer side effects. Researchers are attempting to find more specific ways to treat diseases. It has been found that there are different targets for the treatment of diseases such as cancer, obesity, and metabolic syndrome. It is important to find a potential target in order to lessen the side effects of current treatments. G Protein-coupled receptors (GPCRs) are a large family of transmembrane proteins that are expressed in many organs, leading to the activation of internal signal transduction cascades through the binding of different ligands, including neurotransmitters, peptides, and lipids. Due to the critical role of GPCRs in cells, it could be a potential target. G protein-coupled receptor 75 (GPR75) is a novel member of the GPCR family that has an important role in many diseases, such as obesity, cancer, and metabolic syndrome. Until now, three ligands have been detected for GPR75, including 20-HETE, CCL5, and RANTES. Recent studies suggest that 20-HETE, through GPR75, triggers signaling pathways including PI3K/Akt and RAS/MAPK, leading to a more aggressive phenotype in prostate cancer cells. Additionally, the PI3K/Akt and RAS/MAPK signaling pathways activate NF-κB, which is significant in various pathways of cancer development such as proliferation, migration, and apoptosis. The findings indicate that inhibiting GPR75 in humans leads to an increase in insulin sensitivity and glucose tolerance, as well as a reduction in body fat storage. According to these discoveries, GPR75 could be a potential target for drug treatment of diseases such as obesity, metabolic syndrome, and cancer. In this review, we aimed to discuss the therapeutic impact of GPR75 in cancer, metabolic syndrome, and obesity and underscore the possible pathways.

GPR35: from enigma to therapeutic target

The orphan G-protein-coupled receptor 35 (GPR35), although poorly characterised, is attracting considerable interest as a therapeutic target. Marked differences in pharmacology between human and rodent orthologues of the receptor and a dearth of antagonists with affinity for mouse and rat GPR35 have previously restricted the use of preclinical disease models. The development of improved ligands, novel transgenic knock-in mouse lines, and detailed analysis of the disease relevance of single-nucleotide polymorphisms (SNPs) have greatly enhanced understanding of the key roles of GPR35 and have stimulated efforts towards disease-targeted proof-of-concept studies. In this opinion article, new information on the biology of the receptor is considered, whilst insight into how GPR35 is currently being assessed for therapeutic utility - in areas ranging from inflammatory bowel diseases to nonalcoholic steatohepatitis and various cancers - is also provided.

Development and Characterization of Fluorescent Probes for the G Protein-Coupled Receptor 35

The orphan G protein-coupled receptor 35 (GPR35) is a potential target for the treatment of pain, inflammation, and metabolic diseases. Although many GPR35 agonists have been discovered, research on functional GPR35 ligands, such as fluorescent probes, is still limited. Herein, we developed a series of GPR35 fluorescent probes by conjugating a BODIPY fluorophore to DQDA, a known GPR35 agonist. All probes exhibited excellent GPR35 agonistic activity and desired spectroscopic properties, as determined by the DMR assay, bioluminescence resonance energy transfer (BRET)-based saturation, and kinetic binding experiments. Notably, compound 15 showed the highest binding potency and the weakest nonspecific BRET binding signal (K d = 3.9 nM). A BRET-based competition binding assay with 15 was also established and used to determine the binding constants and kinetics of unlabeled GPR35 ligands.

Aminosalicylates target GPR35, partly contributing to the prevention of DSS-induced colitis

GPR35, a class A G-protein-coupled receptor, is considered an orphan receptor; the endogenous ligand and precise physiological function of GPR35 remain obscure. GPR35 is expressed relatively highly in the gastrointestinal tract and immune cells. It plays a role in colorectal diseases like inflammatory bowel diseases (IBDs) and colon cancer. More recently, the development of GPR35 targeting anti-IBD drugs is in solid request. Nevertheless, the development process is in stagnation due to the lack of a highly potent GPR35 agonist that is also active comparably in both human and mouse orthologs. Therefore, we proposed to find compounds for GPR35 agonist development, especially for the human ortholog of GPR35. As an efficient way to pick up a safe and effective GPR35 targeting anti-IBD drug, we screened Food and Drug Administration (FDA)-approved 1850 drugs using a two-step DMR assay. Interestingly, we found aminosalicylates, first-line medicine for IBDs whose precise target remains unknown, exhibited activity on both human and mouse GPR35. Among these, pro-drug olsalazine showed the most potency on GPR35 agonism, inducing ERK phosphorylation and β-arrestin2 translocation. In dextran sodium sulfate (DSS)-induced colitis, the protective effect on disease progression and inhibitory effect on TNFα mRNA expression, NF-κB and JAK-STAT3 pathway of olsalazine are compromised in GPR35 knock-out mice. The present study identified a target for first-line medicine aminosalicylates, highlighted that uncleaved pro-drug olsalazine is effective, and provided a new concept for the design of aminosalicylic GPR35 targeting anti-IBD drug.

Receptor biology: Challenges and opportunities

Receptor biology provides a great opportunity to understand the ligand-receptor signaling involved in health and disease processes. Receptor endocytosis and signaling play a vital role in health conditions. Receptor-based signaling is the main form of communication between cells and cells with the environment. However, if any irregularities happen during these events, the consequences of pathophysiological conditions occur. Various methods are utilized to know structure, function, and regulation of receptor proteins. Further, live-cell imaging and genetic manipulations have aided in the understanding of receptor internalization, subcellular trafficking, signaling, metabolic degradation, etc. Understanding the genetics, biochemistry, and physiology of receptors and ligands is very helpful to explore various aspects such as prognosis, diagnosis, and treatment of disease. However, there are enormous challenges that exist to explore receptor biology further. This chapter briefly discusses the current challenges and emerging opportunities of receptor biology.

Insights into divalent cation regulation and G13-coupling of orphan receptor GPR35

Endogenous ions play important roles in the function and pharmacology of G protein-coupled receptors (GPCRs) with limited atomic evidence. In addition, compared with G protein subtypes G_s, G_i/o, and G_q/11, insufficient structural evidence is accessible to understand the coupling mechanism of G_12/13 protein by GPCRs. Orphan receptor GPR35, which is predominantly expressed in the gastrointestinal tract and is closely related to inflammatory bowel diseases (IBDs), stands out as a prototypical receptor for investigating ionic modulation and G₁₃ coupling. Here we report a cryo-electron microscopy structure of G₁₃-coupled GPR35 bound to an anti-allergic drug, lodoxamide. This structure reveals a novel divalent cation coordination site and a unique ionic regulatory mode of GPR35 and also presents a highly positively charged binding pocket and the complementary electrostatic ligand recognition mode, which explain the promiscuity of acidic ligand binding by GPR35. Structural comparison of the GPR35-G₁₃ complex with other G protein subtypes-coupled GPCRs reveals a notable movement of the C-terminus of α5 helix of the Gα₁₃ subunit towards the receptor core and the least outward displacement of the cytoplasmic end of GPR35 TM6. A featured 'methionine pocket' contributes to the G₁₃ coupling by GPR35. Together, our findings provide a structural basis for divalent cation modulation, ligand recognition, and subsequent G₁₃ protein coupling of GPR35 and offer a new opportunity for designing GPR35-targeted drugs for the treatment of IBDs.

Valorization of leftover green tea residues through conversion to bioactive peptides using probiotics-aided anaerobic digestion

Bioactive peptides (BPs) are protein fragments that benefit human health. To assess whether leftover green tea residues (GTRs) can serve as a resource for new BPs, we performed in silico proteolysis of GTRs using the BIOPEP database, revealing a wide range of BPs embedded in GTRs. Comparative genomics and the percentage of conserved protein analyses enabled us to select a few probiotic strains for GTR hydrolysis. The selected probiotics digested GTRs anaerobically to yield GTR-derived peptide fractions. To examine whether green tea (GT) peptide fractions could be potential mediators of host-microbe interactions, we comprehensively screened agonistic and antagonistic activities of 168 human G protein-coupled receptors (GPCRs). NanoLC-MS/MS analysis and thin-layer chromatography allowed the identification of peptide sequences and the composition of glycan moieties in the GTRs. Remarkably, GT peptide fractions produced by Lactiplantibacillus plantarum APsulloc 331261, a strain isolated from GT, showed a potent-binding activity for P2RY6, a GPCR involved in intestinal homeostasis. Therefore, this study suggests the potential use of probiotics-aided GTR hydrolysates as postbiotic BPs, providing a biological process for recycling GTRs from agro-waste into renewable resources as health-promoting BPs.

Kynurenic acid ameliorates NLRP3 inflammasome activation by blocking calcium mobilization via GPR35

The inflammasome has been linked to diverse inflammatory and metabolic diseases, and tight control of inflammasome activation is necessary to avoid excessive inflammation. Kynurenic acid (KA) is a tryptophan metabolite in the kynurenine pathway. However, the roles and mechanisms of the regulation of inflammasome activation by KA have not yet been fully elucidated. Here, we found that KA suppressed caspase-1 activation and IL-1β production in macrophages by specifically inhibiting canonical and noncanonical activation of the NLRP3 inflammasome. Mechanistically, KA reduced calcium mobilization through G-protein receptor 35 (GPR35), resulting in reduced mitochondrial damage and decreased mtROS production, thus blocking NLRP3 inflammasome assembly and activation. Importantly, KA prevented lipopolysaccharide-induced systemic inflammation, monosodium urate-induced peritoneal inflammation, and high-fat diet-induced metabolic disorder. Thus, KA ameliorated inflammation and metabolic disorders by blocking calcium mobilization-mediated NLRP3 inflammasome activation via GPR35. Our data reveal a novel mechanism for KA in the modulation of inflammasome activation and suggest that GPR35 might be a promising target for improving NLRP3 inflammasome-associated diseases by regulating calcium mobilization.

A Review of the Health Benefits of Food Enriched with Kynurenic Acid

Kynurenic acid (KYNA), a metabolite of tryptophan, is an endogenous substance produced intracellularly by various human cells. In addition, KYNA can be synthesized by the gut microbiome and delivered in food. However, its content in food is very low and the total alimentary supply with food accounts for only 1-3% of daily KYNA excretion. The only known exception is chestnut honey, which has a higher KYNA content than other foods by at least two orders of magnitude. KYNA is readily absorbed from the gastrointestinal tract; it is not metabolized and is excreted mainly in urine. It possesses well-defined molecular targets, which allows the study and elucidation of KYNA's role in various pathological conditions. Following a period of fascination with KYNA's importance for the central nervous system, research into its role in the peripheral system has been expanding rapidly in recent years, bringing some exciting discoveries. KYNA does not penetrate from the peripheral circulation into the brain; hence, the following review summarizes knowledge on the peripheral consequences of KYNA administration, presents data on KYNA content in food products, in the context of its daily supply in diets, and systematizes the available pharmacokinetic data. Finally, it provides an analysis of the rationale behind enriching foods with KYNA for health-promoting effects.

Isoforms of GPR35 have distinct extracellular N-termini that allosterically modify receptor-transducer coupling and mediate intracellular pathway bias

Within the intestine, the human G protein–coupled receptor (GPCR) GPR35 is involved in oncogenic signaling, bacterial infections, and inflammatory bowel disease. GPR35 is known to be expressed as two distinct isoforms that differ only in the length of their extracellular N-termini by 31 amino acids, but detailed insights into their functional differences are lacking. Through gene expression analysis in immune and gastrointestinal cells, we show that these isoforms emerge from distinct promoter usage and alternative splicing. Additionally, we employed optical assays in living cells to thoroughly profile both GPR35 isoforms for constitutive and ligand-induced activation and signaling of 10 different heterotrimeric G proteins, ligand-induced arrestin recruitment, and receptor internalization. Our results reveal that the extended N-terminus of the long isoform limits G protein activation yet elevates receptor–β-arrestin interaction. To better understand the structural basis for this bias, we examined structural models of GPR35 and conducted experiments with mutants of both isoforms. We found that a proposed disulfide bridge between the N-terminus and extracellular loop 3, present in both isoforms, is crucial for constitutive G₁₃ activation, while an additional cysteine contributed by the extended N-terminus of the long GPR35 isoform limits the extent of agonist-induced receptor–β-arrestin2 interaction. The pharmacological profiles and mechanistic insights of our study provide clues for the future design of isoform-specific GPR35 ligands that selectively modulate GPR35–transducer interactions and allow for mechanism-based therapies against, for example, inflammatory bowel disease or bacterial infections of the gastrointestinal system.

Recent Progress in Assays for GPCR Drug Discovery

G-protein coupled receptors (GPCRs), also known as 7 transmembrane receptors, are the largest family of cell surface receptors in eukaryotes. There are ~800 GPCRs in human, regulating diverse physiological processes. GPCRs are the most intensively studied drug targets. Drugs that target GPCRs account for about a quarter of the global market share of therapeutic drugs. Therefore, to develop physiologically relevant and robust assays to search new GPCR ligands or modulators remain the major focus of drug discovery research worldwide. Early functional GPCR assays are mainly depend on the measurement of G protein-mediated second messenger generation. Recent development in GPCR biology indicate the signaling of these receptors is much more complex than the oversimplified classical view. GPCRs have been found to activate multiple G proteins simultaneously and induce b-arrestin-mediated signaling. GPCRs have also been found to interacte with other cytosolic scaffolding proteins and form dimer or heteromer with GPCRs or other transmembrane proteins. Here we mainly discuss technologies focused on detecting protein-protein interactions, such as FRET/BRET, NanoBiT, Tango, etc, and their applications in measuring GPCRs interacting with various signaling partners. In the final part, we also discuss the species differences in GPCRs when using animal models to study the in vivofunctions of GPCR ligands, and possible ways to solve this problem with modern genetic tools.

An Emerging Cross-Species Marker for Organismal Health: Tryptophan-Kynurenine Pathway

Tryptophan (TRP) is an essential dietary amino acid that, unless otherwise committed to protein synthesis, undergoes metabolism via the Tryptophan-Kynurenine (TRP-KYN) pathway in vertebrate organisms. TRP and its metabolites have key roles in diverse physiological processes including cell growth and maintenance, immunity, disease states and the coordination of adaptive responses to environmental and dietary cues. Changes in TRP metabolism can alter the availability of TRP for protein and serotonin biosynthesis as well as alter levels of the immune-active KYN pathway metabolites. There is now considerable evidence which has shown that the TRP-KYN pathway can be influenced by various stressors including glucocorticoids (marker of chronic stress), infection, inflammation and oxidative stress, and environmental toxicants. While there is little known regarding the role of TRP metabolism following exposure to environmental contaminants, there is evidence of linkages between chemically induced metabolic perturbations and altered TRP enzymes and KYN metabolites. Moreover, the TRP-KYN pathway is conserved across vertebrate species and can be influenced by exposure to xenobiotics, therefore, understanding how this pathway is regulated may have broader implications for environmental and wildlife toxicology. The goal of this narrative review is to (1) identify key pathways affecting Trp-Kyn metabolism in vertebrates and (2) highlight consequences of altered tryptophan metabolism in mammals, birds, amphibians, and fish. We discuss current literature available across species, highlight gaps in the current state of knowledge, and further postulate that the kynurenine to tryptophan ratio can be used as a novel biomarker for assessing organismal and, more broadly, ecosystem health.

The GPR35 expression pattern is associated with overall survival in male patients with colorectal cancer

BACKGROUND

G protein-coupled receptor 35 (GPR35) is involved in the carcinogenesis; however, limited data exist on its relevance for overall survival (OS) and disease-specific survival (DSS) in patients with cancer.

METHODS

We have examined The Cancer Genome Atlas dataset to check the relations between GPR35 expression pattern and OS or DSS of patients with colorectal cancer (CRC).

RESULTS

The performed analysis showed a negative association between positive GPR35 expression Z score and OS in males, which remains statistically significant in advanced stages of colon (COAD) and rectal (READ) adenocarcinoma combined.

CONCLUSIONS

These findings suggest the prognostic value of early testing of GPR35 in male patients with an increased risk of CRC development and warrant further clinical confirmation.

Expression and clinical significance of CXCL17 and GPR35 in endometrial carcinoma

Endometrial carcinoma is one of the most common gynecologic malignancies. CXCL17-CXCR8 (GPR35) axis is reported to play an indispensability role in tumors. Our purpose is to screen possible prognostic and immune-related factors in endometrial carcinoma by detecting the mRNA and protein expression of CXCL17 and CXCR8. We use the qRT-PCR method to test the mRNA expression of CXCL17 and CXCR8 in 35 pairs of endometrial carcinoma and adjacent tissue. The protein expression of CXCL17 and CXCR8 in 30 cases of normal proliferative endometrium, 30 cases of endometrial atypical hyperplasia and 50 cases of endometrial carcinoma was detected by tissue microarray immunohistochemistry. There was no significant difference in the positive expression rate between endometrial adenocarcinoma tissue and endometrial atypical hyperplasia tissue (P > 0.05). But significantly better than normal proliferative tissue (P < 0.001). Correlation analysis of CXCR8 and CXCL17 in endometrial carcinoma showed a positive correlation (r = 0.9123, P < 0.0001). For patients with endometrial cancer, the overall survival (OS) of patients with high CXCL17 expression was significantly higher than that low CXCL17 expression (log-rank test, P < 0.0001), whereas CXCR8 had no statistical significance. But the expression of CXCR8 is an independent prognostic factor of OS in endometrial carcinoma patients. Our study showed that CXCL17 and CXCR8 may be involved in the occurrence and development of endometrial cancer. High expression of CXCL17 may be used as a biomarker for predicting survival. Because CXCL17 and CXCL18 are related to lymphocytes and immune regulation, they are expected to become potential targets for immunotherapy.

GPR35 promotes neutrophil recruitment in response to serotonin metabolite 5-HIAA

Rapid neutrophil recruitment to sites of inflammation is crucial for innate immune responses. Here, we reveal that the G-protein-coupled receptor GPR35 is upregulated in activated neutrophils, and it promotes their migration. GPR35-deficient neutrophils are less recruited from blood vessels into inflamed tissue, and the mice are less efficient in clearing peritoneal bacteria. Using a bioassay, we find that serum and activated platelet supernatant stimulate GPR35, and we identify the platelet-derived serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) as a GPR35 ligand. GPR35 function in neutrophil recruitment is strongly dependent on platelets, with the receptor promoting transmigration across platelet-coated endothelium. Mast cells also attract GPR35+ cells via 5-HIAA. Mice deficient in 5-HIAA show a loss of GPR35-mediated neutrophil recruitment to inflamed tissue. These findings identify 5-HIAA as a GPR35 ligand and neutrophil chemoattractant and establish a role for platelet- and mast cell-produced 5-HIAA in cell recruitment to the sites of inflammation and bacterial clearance.

Agonist-induced phosphorylation of orthologues of the orphan receptor GPR35 functions as an activation sensor

G protein-coupled receptor 35 (GPR35) is poorly characterized but nevertheless has been revealed to have diverse roles in areas including lower gut inflammation and pain. The development of novel reagents and tools will greatly enhance analysis of GPR35 functions in health and disease. Here, we used mass spectrometry, mutagenesis, and [³²P] orthophosphate labeling to identify that all five hydroxy-amino acids in the C-terminal tail of human GPR35a became phosphorylated in response to agonist occupancy of the receptor and that, apart from Ser²⁹⁴, each of these contributed to interactions with arretin-3, which inhibits further G protein-coupled receptor signaling. We found that Ser³⁰³ was key to such interactions; the serine corresponding to human GPR35a residue 303 also played a dominant role in arrestin-3 interactions for both mouse and rat GPR35. We also demonstrated that fully phospho-site–deficient mutants of human GPR35a and mouse GPR35 failed to interact effectively with arrestin-3, and the human phospho-deficient variant was not internalized from the surface of cells in response to agonist treatment. Even in cells stably expressing species orthologues of GPR35, a substantial proportion of the expressed protein(s) was determined to be immature. Finally, phospho-site–specific antisera targeting the region encompassing Ser³⁰³ in human (Ser³⁰¹ in mouse) GPR35a identified only the mature forms of GPR35 and provided effective sensors of the activation status of the receptors both in immunoblotting and immunocytochemical studies. Such antisera may be useful tools to evaluate target engagement in drug discovery and target validation programs.

Recent advances in clinical trials targeting the kynurenine pathway

The kynurenine pathway (KP) is the major catabolic pathway for the essential amino acid tryptophan leading to the production of nicotinamide adenine dinucleotide. In inflammatory conditions, the activation of the KP leads to the production of several bioactive metabolites including kynurenine, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, kynurenic acid and quinolinic acid. These metabolites can have redox and immune suppressive activity, be neurotoxic or neuroprotective. While the activity of the pathway is tightly regulated under normal physiological condition, it can be upregulated by immunological activation and inflammation. The dysregulation of the KP has been implicated in wide range of neurological diseases and psychiatric disorders. In this review, we discuss the mechanisms involved in KP-mediated neurotoxicity and immune suppression, and its role in diseases of our expertise including cancer, chronic pain and multiple sclerosis. We also provide updates on the clinical trials evaluating the efficacy of KP inhibitors and/or analogues in each respective disease.

G Protein-Coupled Receptor GPR35 Suppresses Lipid Accumulation in Hepatocytes

Although prevalent, nonalcoholic fatty liver disease is not currently treated effectively with medicines. Initially, using wild-type and genome-edited clones of the human hepatocyte cell line HepG2, we show that activation of the orphan G protein-coupled receptor GPR35 is both able and sufficient to block liver X-receptor-mediated lipid accumulation. Studies on hepatocytes isolated from both wild-type and GPR35 knock-out mice were consistent with a similar effect of GPR35 agonists in these cells, but because of marked differences in the pharmacology of GPR35 agonists and antagonists at the mouse and human orthologues, as well as elevated basal lipid levels in hepatocytes from the GPR35 knock-out mice, no definitive conclusion could be reached. To overcome this, we generated and characterized a transgenic knock-in mouse line in which the corresponding human GPR35 splice variant replaced the mouse orthologue. In hepatocytes from these humanized GPR35 mice, activation of this receptor was shown conclusively to prevent, and also reverse, lipid accumulation induced by liver X-receptor stimulation. These studies highlight the potential to target GPR35 in the context of fatty liver diseases.

CNS pharmacology of NKCC1 inhibitors

The Na-K-2Cl cotransporter NKCC1 and the neuron-specific K-Cl cotransporter KCC2 are considered attractive CNS drug targets because altered neuronal chloride regulation and consequent effects on GABAergic signaling have been implicated in numerous CNS disorders. While KCC2 modulators are not yet clinically available, the loop diuretic bumetanide has been used off-label in attempts to treat brain disorders and as a tool for NKCC1 inhibition in preclinical models. Bumetanide is known to have anticonvulsant and neuroprotective effects under some pathophysiological conditions. However, as shown in several species from neonates to adults (mice, rats, dogs, and by extrapolation in humans), at the low clinical doses of bumetanide approved for diuresis, this drug has negligible access into the CNS, reaching levels that are much lower than what is needed to inhibit NKCC1 in cells within the brain parenchyma. Several drug discovery strategies have been initiated over the last ∼15 years to develop brain-permeant compounds that, ideally, should be selective for NKCC1 to eliminate the diuresis mediated by inhibition of renal NKCC2. The strategies employed to improve the pharmacokinetic and pharmacodynamic properties of NKCC1 blockers include evaluation of other clinically approved loop diuretics; development of lipophilic prodrugs of bumetanide; development of side-chain derivatives of bumetanide; and unbiased high-throughput screening approaches of drug discovery based on large chemical compound libraries. The main outcomes are that (1), non-acidic loop diuretics such as azosemide and torasemide may have advantages as NKCC1 inhibitors vs. bumetanide; (2), bumetanide prodrugs lead to significantly higher brain levels than the parent drug and have lower diuretic activity; (3), the novel bumetanide side-chain derivatives do not exhibit any functionally relevant improvement of CNS accessibility or NKCC1 selectivity vs. bumetanide; (4) novel compounds discovered by high-throughput screening may resolve some of the inherent problems of bumetanide, but as yet this has not been achieved. Thus, further research is needed to optimize the design of brain-permeant NKCC1 inhibitors. In parallel, a major challenge is to identify the mechanisms whereby various NKCC1-expressing cellular targets of these drugs within (e.g., neurons, oligodendrocytes or astrocytes) and outside the brain parenchyma (e.g., the blood-brain barrier, the choroid plexus, and the endocrine system), as well as molecular off-target effects, might contribute to their reported therapeutic and adverse effects.

Metabolite G-Protein Coupled Receptors in Cardio-Metabolic Diseases

G protein-coupled receptors (GPCRs) have originally been described as a family of receptors activated by hormones, neurotransmitters, and other mediators. However, in recent years GPCRs have shown to bind endogenous metabolites, which serve functions other than as signaling mediators. These receptors respond to fatty acids, mono- and disaccharides, amino acids, or various intermediates and products of metabolism, including ketone bodies, lactate, succinate, or bile acids. Given that many of these metabolic processes are dysregulated under pathological conditions, including diabetes, dyslipidemia, and obesity, receptors of endogenous metabolites have also been recognized as potential drug targets to prevent and/or treat metabolic and cardiovascular diseases. This review describes G protein-coupled receptors activated by endogenous metabolites and summarizes their physiological, pathophysiological, and potential pharmacological roles.

Mechanism deconvolution of Qing Fei Pai Du decoction for treatment of Coronavirus Disease 2019 (COVID-19) by label-free integrative pharmacology assays

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional Chinese medicine (TCM) has a long history in the prevention and treatment of pandemics. The TCM formula Lung Cleansing and Detoxifying Decoction (LCDD), also known as Qing Fei Pai Du Decoction, has been demonstrated effective against Coronavirus Disease 2019 (COVID-19).

AIM OF THE STUDY

This work aimed to elucidate the active ingredients, targets and pathway mechanism of LCDD related to suppression of inflammatory, immunity regulation and relaxation of airway smooth muscle for the treatment of COVID-19.

MATERIALS AND METHODS

Mining chemical ingredients reported in LCDD, 144 compounds covering all herbs were selected and screened against inflammatory-, immunity- and respiratory-related GPCRs including GPR35, H1, CB2, B2, M3 and β2-adrenoceptor receptor using a label-free integrative pharmacology method. Further, all active compounds were detected using liquid chromatography-tandem mass spectrometry, and an herb-compound-target network based on potency and content of compounds was constructed to elucidate the multi-target and synergistic effect.

RESULTS

Thirteen compounds were identified as GPR35 agonists, including licochalcone B, isoliquiritigenin, etc. Licochalcone B, isoliquiritigenin and alisol A exhibited bradykinin receptor B2 antagonism activities. Atractyline and shogaol showed as a cannabinoid receptor CB2 agonist and a histamine receptor H1 antagonist, respectively. Tectorigenin and aristofone acted as muscarinic receptor M3 antagonists, while synephrine, ephedrine and pseudoephedrine were β2-adrenoceptor agonists. Pathway deconvolution assays suggested activation of GPR35 triggered PI3K, MEK, JNK pathways and EGFR transactivation, and the activation of β2-adrenoceptor mediated MEK and Ca²⁺. The herb-compound-target network analysis found that some compounds such as licochalcone B acted on multiple targets, and multiple components interacted with the same target such as GPR35, reflecting the synergistic mechanism of Chinese medicine. At the same time, some low-abundance compounds displayed high target activity, meaning its important role in LCDD for anti-COVID-19.

CONCLUSIONS

This study elucidates the active ingredients, targets and pathways of LCDD. This is useful for elucidating multitarget synergistic action for its clinical therapeutic efficacy.

GPR35 in Intestinal Diseases: From Risk Gene to Function

Diet and gut microbial metabolites mediate host immune responses and are central to the maintenance of intestinal health. The metabolite-sensing G-protein coupled receptors (GPCRs) bind metabolites and trigger signals that are important for the host cell function, survival, proliferation and expansion. On the contrary, inadequate signaling of these metabolite-sensing GPCRs most likely participate to the development of diseases including inflammatory bowel diseases (IBD). In the intestine, metabolite-sensing GPCRs are highly expressed by epithelial cells and by specific subsets of immune cells. Such receptors provide an important link between immune system, gut microbiota and metabolic system. Member of these receptors, GPR35, a class A rhodopsin-like GPCR, has been shown to be activated by the metabolites tryptophan-derived kynurenic acid (KYNA), the chemokine CXCL17 and phospholipid derivate lysophosphatidic acid (LPA) species. There have been studies on GPR35 in the context of intestinal diseases since its identification as a risk gene for IBD. In this review, we discuss the pharmacology of GPR35 including its proposed endogenous and synthetic ligands as well as its antagonists. We elaborate on the risk variants of GPR35 implicated in gut-related diseases and the mechanisms by which GPR35 contribute to intestinal homeostasis.

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.

Immunosuppressive metabolites in tumoral immune evasion: redundancies, clinical efforts, and pathways forward

Tumors accumulate metabolites that deactivate infiltrating immune cells and polarize them toward anti-inflammatory phenotypes. We provide a comprehensive review of the complex networks orchestrated by several of the most potent immunosuppressive metabolites, highlighting the impact of adenosine, kynurenines, prostaglandin E2, and norepinephrine and epinephrine, while discussing completed and ongoing clinical efforts to curtail their impact. Retrospective analyses of clinical data have elucidated that their activity is negatively associated with prognosis in diverse cancer indications, though there is a current paucity of approved therapies that disrupt their synthesis or downstream signaling axes. We hypothesize that prior lukewarm results may be attributed to redundancies in each metabolites' synthesis or signaling pathway and highlight routes for how therapeutic development and patient stratification might proceed in the future.

G-protein coupled receptor 35 (GPR35) regulates the colonic epithelial cell response to enterotoxigenic Bacteroides fragilis

G protein-coupled receptor (GPR)35 is highly expressed in the gastro-intestinal tract, predominantly in colon epithelial cells (CEC), and has been associated with inflammatory bowel diseases (IBD), suggesting a role in gastrointestinal inflammation. The enterotoxigenic Bacteroides fragilis (ETBF) toxin (BFT) is an important virulence factor causing gut inflammation in humans and animal models. We identified that BFT signals through GPR35. Blocking GPR35 function in CECs using the GPR35 antagonist ML145, in conjunction with shRNA knock-down and CRISPRcas-mediated knock-out, resulted in reduced CEC-response to BFT as measured by E-cadherin cleavage, beta-arrestin recruitment and IL-8 secretion. Importantly, GPR35 is required for the rapid onset of ETBF-induced colitis in mouse models. GPR35-deficient mice showed reduced death and disease severity compared to wild-type C57Bl6 mice. Our data support a role for GPR35 in the CEC and mucosal response to BFT and underscore the importance of this molecule for sensing ETBF in the colon.

Novel Approaches, Drug Candidates, and Targets in Pain Drug Discovery

Because of the problems associated with opioids, drug discovery efforts have been employed to develop opioids with reduced side effects using approaches such as biased opioid agonism, multifunctional opioids, and allosteric modulation of opioid receptors. Receptor targets such as adrenergic, cannabinoid, P2X3 and P2X7, NMDA, serotonin, and sigma, as well as ion channels like the voltage-gated sodium channels Nav1.7 and Nav1.8 have been targeted to develop novel analgesics. Several enzymes, such as soluble epoxide hydrolase, sepiapterin reductase, and MAGL/FAAH, have also been targeted to develop novel analgesics. In this review, old and recent targets involved in pain signaling and compounds acting at these targets are summarized. In addition, strategies employed to reduce side effects, increase potency, and efficacy of opioids are also elaborated. This review should aid in propelling drug discovery efforts to discover novel analgesics.

Beyond Alkaloids: Novel Bioactive Natural Products From Lobelia Species

In this work, we reviewed the progress in the phytochemical and biological investigations of bioactive components derived from medicinally valuable Lobelia species. In the last 60 years, Lobelia has garnered significant attention from the phytochemist from around the world, majorly due to the discovery of bioactive piperidine alkaloids (e.g., lobinaline and lobeline) in the early 1950s. Later, lobeline underwent clinical trials for several indications including the treatment of attention deficit hyperactivity disorder and a multicenter phase three trial for smoking cessation. Subsequently, several other alkaloids derived from different species of Lobelia were also investigated for their pharmacological characteristics. However, in the last few years, the research focus has started shifting to the characterization of the other novel chemical classes. The major shift has been noticed due to the structurally similar alkaloid components, which essentially share similar pharmacological, physicochemical, and toxicological profiles. In this review, we present an up-to-date overview of their progress with special attention to understanding the molecular mechanisms of the novel bioactive components.