Role of bitter contributors and bitter taste receptors:a comprehensive review of their sources,functions and future development

Bitterness,one of the 5“basic tastes”,is usually undesired by humans.However,abundant literature reported that bitter fruits and vegetables have beneficial health effects due to their bitter contributors.This review provided an updated overview of the main bitter contributors of typical bitter fruits and vegetables and their health benefits.The main bitter contributors,including phenolics,terpenoids,alkaloids,amino acids,nucleosides and purines,were summarized.The bioactivities and wide range of beneficial effects of them on anti-cancers,anti-inflammations,anti-microbes,neuroprotection,inhibiting chronic and acute injury in organs,as well as regulating behavior performance and metabolism were reported.Furthermore,not only did the bitter taste receptors(taste receptor type 2 family,T2Rs)show taste effects,but extra-oral T2Rs could also be activated by binding with bitter components,regulating physiological activities via modulating hormone secretion,immunity,metabolism,and cell proliferation.This review provided a new perspective on exploring and explaining the nutrition of bitter foods,revealing the relationship between the functions of bitter contributors from food and T2Rs.Future trends may focus on revealing the possibility of T2Rs being targets for the treatment of diseases,exploring the mechanism of T2Rs mediating the bioactivities,and making bitter foods more acceptable without getting rid of bitter contributors.

Discovery of the relationship between bitter taste and bitter flavor (efficacy) based on hTAS2Rs

Objective:To explore the relationship between bitter taste and bitter flavor(efficacy),their character-istics were analyzed at the target and pharmacodynamic points.Methods:Compounds with bitter receptor activity were obtained by high-throughput screening models of the human bitter taste receptors(hTAS2Rs),which were used to analyze bitter taste.The efficacy of anti-asthma medications was used as an example to research bitter flavor(efficacy).The pharmacological effects of bitter taste and bitter flavor(efficacy)were classified according to the functional modules.The coverage for the same targets or pharmacological effects in the overall protein interaction network(PIN)of bitter taste and bitter flavor(efficacy)was analyzed to reveal their relationship.The effect of the compound polydatin with anti-asthma activation on hTAS2R14 was studied to verify the reliability of the aforementioned idea.Results:A total of 121 Chinese materia medica(CMM)compounds that activate hTAS2R10,hTAS2R14,and hTAS2R49 were obtained.The analysis results indicated that 108 same targets for bitter taste and bitter flavor(efficacy)were obtained,accounting for 13.9%in the PIN of bitter taste and 72.5%in the PIN of bitter flavor(efficacy).The pharmacological effects shared by bitter taste and bitter flavor(efficacy)accounted for 79%of the PIN of bitter taste and 81%of the PIN of bitter flavor(efficacy).The activating effect of the anti-asthma compound polydatin on hTAS2R14 was dose-dependent with EC 50 of 4.3 m M.Conclusion:In this study,the relationship between bitter taste and bitter flavor(efficacy)has been demonstrated from the target and pharmacodynamic points,which are based on hTAS2Rs and anti-asthma effect.Bitter taste and bitter flavor(efficacy)exhibited a high correlation.This study provides a theoretical and scientific basis for the hypothesis that“the property theory of CMM is the clinical pharmacodynamics of CMM.”

Smallest bitter taste receptor (T2Rs) gene repertoire in carnivores

Bitter taste reception is presumably associated with dietary selection, preventing animals from ingesting potentially harmful compounds. Accordingly, carnivores, who encounter these toxic substances less often, should have fewer genes associated with bitter taste reception compared with herbivores and omnivores. To investigate the genetic basis of bitter taste reception, we confirmed bitter taste receptor （T2R） genes previously found in the genome sequences of two herbivores （cow and horse）, two omnivores （mouse and rat） and one carnivore （dog）. We also identified, for the first time, the T2R repertoire from the genome of other four carnivore species （ferret, giant panda, polar bear and cat） and detected 17-20 bitter receptor genes from the five carnivore genomes, including 12-16 intact genes, 0-1 partial but putatively functional genes, and 3-8 pseudogenes. Both the intact T2R genes and the total T2R gene number among carnivores were the smallest among the tested species, supporting earlier speculations that carnivores have fewer T2R genes, herbivores an intermediate number, and onmivores the largest T2R gene repertoire. To further explain the genetic basis for this disparity, we constructed a phylogenetic tree, which showed most of the T2R genes from the five carnivores were one-to-one orthologs across the tree, suggesting that carnivore T2Rs were conserved among mammals. Similarly, the small carnivore T2R family size was likely due to rare duplication events. Collectively, these results strengthen arguments for the connection between T2R gene family size, diet and habit.

Smallest bitter taste receptor (T2Rs) gene repertoire in carnivores

Bitter taste reception is presumably associated with dietary selection,preventing animals from ingesting potentially harmful compounds.Accordingly,carnivores,who encounter these toxic substances less often,should have fewer genes associated with bitter taste reception compared with herbivores and omnivores.To investigate the genetic basis of bitter taste reception,we confirmed bitter taste receptor(T2R)genes previously found in the genome sequences of two herbivores(cow and horse),two omnivores(mouse and rat)and one carnivore(dog).We also identified,for the first time,the T2R repertoire from the genome of other four carnivore species(ferret,giant panda,polar bear and cat)and detected 17−20 bitter receptor genes from the five carnivore genomes,including 12−16 intact genes,0−1 partial but putatively functional genes,and 3−8 pseudogenes.Both the intact T2R genes and the total T2R gene number among carnivores were the smallest among the tested species,supporting earlier speculations that carnivores have fewer T2R genes,herbivores an intermediate number,and omnivores the largest T2R gene repertoire.To further explain the genetic basis for this disparity,we constructed a phylogenetic tree,which showed most of the T2R genes from the five carnivores were one-to-one orthologs across the tree,suggesting that carnivore T2Rs were conserved among mammals.Similarly,the small carnivore T2R family size was likely due to rare duplication events.Collectively,these results strengthen arguments for the connection between T2R gene family size,diet and habit.

Etiopathogenesis of reproductive tract infections and the emerging role of bitter taste receptors:A scoping review

Reproductive tract infections pose an immense public health concern worldwide as over 600 million new cases are recorded annually along with several complications,including infertility,ectopic pregnancy,preterm delivery,and neonatal death.Despite improved understanding of the mechanisms of pathogenic invasion,the etiopathogenesis of reproductive tract infections is yet to be completely understood.Recent data has suggested a critical role of bitter taste receptors of the reproductive tract in etiopathogenesis of reproductive tract infections.Here,we review the literature on current etiopathogenesis of reproductive tract infections and the role of bitter taste receptors of the reproductive tract in etiopathogenesis of reproductive tract infections.Emerging evidence suggests a critical role of microbiota disorder of the reproductive tract in reproductive tract infections.Several bacterial,protozoan parasitic,helminthic parasitic and viral pathogens have been identified as causative agents of reproductive tract infections.These pathogens subvert host defenses and activate specific architectural units of the uroepithelium such as Toll-like receptors,which recognize conserved motifs on the pathogens.The activated Toll-like receptors mediate downstream signaling,stimulating nuclear factor-κB,which in turn activates the production of proinflammatory cytokines.This pathway is also associated with recruitment of immunocytes to the site of aggression and release of proteinases,which drive tissue damage in the reproductive tract.Defects in detection of pathogenic components by the bitter taste receptors of the reproductive tract may play a critical role in the etiopathogenesis of reproductive tract infections.This review provides important information for identification of novel frontiers for the treatment of reproductive tract infections.

Emulsifying property,antioxidant activity,and bitterness of soybean protein isolate hydrolysate obtained by Corolase PP under high hydrostatic pressure

Enzymatic hydrolysis of proteins can enhance their emulsifying properties and antioxidant activities.However,the problem related to the hydrolysis of proteins was the generation of the bitter taste.Recently,high hydrostatic pressure(HHP)treatment has attracted much interest and has been used in several studies on protein modification.Hence,the study aimed to investigate the effects of enzymatic hydrolysis by Corolase PP under different pressure treatments(0.1,100,200,and 300 MPa for 1-5 h at 50℃)on the emulsifying property,antioxidant activity,and bitterness of soybean protein isolate hydrolysate(SPIH).As observed,the hydrolysate obtained at 200 MPa for 4 h had the highest emulsifying activity index(47.49 m^(2)/g)and emulsifying stability index(92.98%),and it had higher antioxidant activities(44.77%DPPH free radical scavenging activity,31.12%superoxide anion radical scavenging activity,and 61.50%copper ion chelating activity).At the same time,the enhancement of emulsion stability was related to the increase of zeta potential and the decrease of mean particle size.In addition,the hydrolysate obtained at 200 MPa for 4 h had a lower bitterness value and showed better palatability.This study has a broad application prospect in developing food ingredients and healthy foods.

Genetic mutation of Tas2r104/Tas2r105/Tas2r114 cluster leads to a loss of taste perception to denatonium benzoate and cucurbitacin B

Background:Bitter taste receptors(Tas2rs)are generally considered to sense various bitter compounds to escape the intake of toxic substances.Bitter taste receptors have been found to widely express in extraoral tissues and have important physiological functions outside the gustatory system in vivo.Methods:To investigate the physiological functions of the bitter taste receptor cluster Tas2r106/Tas2r104/Tas2r105/Tas2r114 in lingual and extraoral tissues,multiple Tas2rs mutant mice and Gnat3 were produced using CRISPR/Cas9 gene-editing technique.A mixture containing Cas9 and sgRNA mRNAs for Tas2rs and Gnat3 gene was microinjected into the cytoplasm of the zygotes.Then,T7EN1 assays and sequencing were used to screen genetic mutation at the target sites in founder mice.Quantitative real-time polymerase chain reaction(qRT-PCR)and immunostaining were used to study the expression level of taste signaling cascade and bitter taste receptor in taste buds.Perception to taste substance was also studied using twobottle preference tests.Results:We successfully produced several Tas2rs and Gnat3 mutant mice using the CRISPR/Cas9 technique.Immunostaining results showed that the expression of GNAT3 and PLCB2 was not altered in Tas2rs mutant mice.But qRT-PCR results revealed the changed expression profile of m Tas2rs gene in taste buds of these mutant mice.With two-bottle preference tests,these mutant mice eliminate responses to cycloheximide due to genetic mutation of Tas2r105.In addition,these mutant mice showed a loss of taste perception to quinine dihydrochloride,denatonium benzoate,and cucurbitacin B(CuB).Gnat3-mediated taste receptor and its signal pathway contribute to CuB perception.Conclusions:These findings implied that these mutant mice would be a valuable means to understand the biological functions of TAS2Rs in extraoral tissues and investigate bitter compound-induced responses mediated by these TAS2Rs in many extraoral tissues.

Lineage-specific evolution of bitter taste receptor genes in the giant and red pandas implies dietary adaptation

Taste 2 receptors(TAS2R)mediate bitterness perception in mammals,thus are called bitter taste receptors.It is believed that these genes evolved in response to species-specific diets.The giant panda(Ailuropoda melanoleuca)and red panda(Ailurus fulgens styani)in the order Carnivora are specialized herbivores with an almost exclusive bamboo diet(>90%bamboo).Because bamboo is full of bitter tasting compounds,we hypothesized that adaptive evolution has occurred at TAS2R genes in giant and red pandas throughout the course of their dietary shift.Here,we characterized 195 TAS2R genes in 9 Carnivora species and examined selective pressures on these genes.We found that both pandas harbor more putative functional TAS2R genes than other carnivores,and pseudogenized TAS2R genes in the giant panda are different from the red panda.The purifying selection on TAS2R1,TAS2R9 and TAS2R38 in the giant panda,and TAS2R62 in the red panda,has been strengthened throughout the course of adaptation to bamboo diet,while selective constraint on TAS2R4 and TAS2R38 in the red panda is relaxed.Remarkably,a few positively selected sites on TAS2R42 have been specifically detected in the giant panda.These results suggest an adaptive response in both pandas to a dietary shift from carnivory to herbivory,and TAS2R genes evolved independently in the 2 pandas.Our findings provide new insight into the molecular basis of mammalian sensory evolution and the process of adaptation to new ecological niches.

Molecular evolution and deorphanization of bitter taste receptors in a vampire bat

Bats represent the largest dietary radiation in a single mammalian order,and have become an emerging model group for studying dietary evolution.Taste receptor genes have proven to be molecular signatures of dietary diversification in bats.For example,all 3 extant species of vampire bats have lost many bitter taste receptor genes(Tas2rs)in association with their dietary shift from insectivory to sanguivory.Indeed,only 8 full-length Tas2rs were identified from the high-quality genome of the common vampire bat(Desmodus rotundus).However,it is presently unknown whether these bitter receptors are functional,since the sense of taste is less important in vampire bats,which have an extremely narrow diet and rely on other senses for acquiring food.Here,we applied a molecular evolutionary analysis of Tas2rs in the common vampire bat compared with non-vampire bats.Furthermore,we provided the first attempt to deorphanize all bitter receptors of the vampire bat using a cell-based assay.We found that all Tas2r genes in the vampire bat have a level of selective pressure similar to that in non-vampire bats,suggesting that this species must have retained some bitter taste functions.We demonstrated that 5 of the 8 bitter receptors in the vampire bat can be activated by some bitter compounds,and observed that the vampire bat generally can not detect naturally occurring bitter compounds examined in this study.Our study demonstrates functional retention of bitter taste in vampire bats as suggested by cell-based functional assays,calling for an in-depth study of extra-oral functions of bitter taste receptors.

Bitter components related to alleviating intestinal obstruction in traditional Mongolian medicine

Intestinal obstruction is a blockage that keeps digesta from passing through upper or lower intestine.Traditional Mongolian medicine(TMM)has been proven to be efficacious in the clinical treatment of intestinal obstruction.However,the mechanism of its treatment has not been studied.The bitter taste receptors(T2Rs)are highly expressed in the extra-oral digestive system,such as gastrointestinal tract,which can regulate gastrointestinal peristalsis and contraction of gastrointestinal smooth muscle.In the respiratory system,T2Rs can relax the airway smooth muscle and effectively alleviate asthma symptoms.In this review,the theory and clinical applications of bitter herbs in TMM were discussed and the functional expression of T2Rs and bitter taste signal transduction pathway were analyzed to investigate whether bitter Mongolian medicine may play an effective role in promoting gastrointestinal peristalsis.Therefore,the scientific connotation of the theory of bitter medicinal property of TMM was interpreted by combining T2Rs research and application of modern technology.This new research approach may enrich and improve the basic theory and accelerate the modernization of TMM.

taBOX: A water-soluble tetraanionic rectangular molecular container for conjugated molecules and taste masking for berberine and palmatine

A water-soluble macrocycle that bears four carboxylate anions has been designed and prepared,which forms a rectangular cavity that can efficiently encapsulate discrete electron-deficient aromatic compounds,including berberine and palmatine.This macrocycle is revealed to be highly biocompatible and able to inhibit the bitter taste of the two drugs.

Molecular mechanisms of bitterness and astringency in the oral cavity induced by soyasaponin

The interaction mechanism between soyasaponin(Ssa)and bitter receptors/mucin,as well as the saliva interface behavior of Ssa,were investigated to explore the presentation mechanism of Ssa bitterness and astringency(BA).Strong bitterness arising from high Ssa concentrations(0.5–1.5 mg/mL)had a masking effect on astringency.At Ssa concentrations of 1.0–1.5 mg/mL,Ssa micelles altered the structure of mucin,exposing its internal tryptophan to a more polar environment.At Ssa concentrations of 0.05–1.50 mg/mL,its reaction with mucin increased the aggregation of particles in artificial saliva,which reduced the frictional lubricating properties of oral saliva.Ssa-mucin interactions affected the salivary interfacial adsorption layer,and their complexes synergistically reduced the interfacial tension.Ssa monomers and soyasapogenols bind to bitter receptors/mucin via hydrogen bonding and hydrophobic interactions.Class A Ssa binds more strongly than class B Ssa,and thus likely presents a higher BA.In conclusion,Ssa interacts with bitter receptors/mucin causing conformational changes and aggregation of salivary mucin,resulting in diminished frictional lubricating properties of oral saliva.This,in turn,affects taste perception and gustatory transmission.

The role of bitter and sweet taste receptors in upper airway innate immunity: Recent advances and future directions

Bitter(T2R)and sweet(T1R)taste receptors have been implicated in sinonasal innate immunity and in the pathophysiology of chronic rhinosinusitis(CRS).Taste receptors are expressed on several sinonasal cell types including ciliated epithelial cells and solitary chemosensory cells.Bitter agonists released by pathogenic microbes elicit a T2R dependent signaling cascade which induces the release of bactericidal nitric oxide,increases mucociliary clearance,and promotes secretion of antimicrobial peptides.Genetic variation conferred by polymorphisms in T2R related genes is associated with differential CRS susceptibility,symptomatology and post-treatment outcomes.More recently,based on our understanding of T1R and T2R function,investigators have discovered novel potential therapeutics in T2R agonists and T1R antagonists.This review will discuss bitter and sweet taste receptor function in sinonasal immunity,explore the emerging diagnostic and therapeutic implications stemming from the most recent findings,and suggest directions for future research.