G protein-coupled receptors in human fat taste perception

Association Between Arachidonic Acid and Chicken Meat and Egg Flavor, and Their Genetic Regulation

In Japan, the majority of chicken meat is obtained from fast-growing broiler chickens. Because most Japanese chicken breeds have a low meat yield and egg production, many of these breeds are endangered. Recently, the palatability of meat and eggs of native chickens has been reevaluated in the Japanese market. Jidori, which means chicken from the local, is an indigenous local chicken that is more delicious, firmer in texture, and more expensive than the broiler chickens. Most Japanese consumers recognize that the meat of Jidori chicken is richer in flavor than that of the broiler chicken. However, the reason for this rich flavor of the meat of Jidori chicken has not been elucidated. Recently, we found that arachidonic acid (AA) (C20:4n-6), a polyunsaturated fatty acid, is associated with the rich flavor of the meat and eggs of Jidori chicken. The present paper summarizes the discovery of the involvement of AA in the flavor characteristic of the meat and eggs of chicken, and also the genetic regulation of the AA content in the meat and eggs of Jidori chicken.

Nutrient-Sensing Biology in Mammals and Birds

Nutrient-sensing mechanisms have emerged as the fringe articulating nutritional needs with dietary choices. Carbohydrate, amino acid, fatty acid, mineral, and water-sensing receptors are highly conserved across mammals and birds, consisting of a repertoire of 22 genes known to date. In contrast, bitter receptors are highly divergent and have a high incidence of polymorphisms within and between mammals and birds and are involved in the adaptation of species to specific environments. In addition, the expression of nutrient-sensing genes outside the oral cavity seems to mediate the required decision-making dialogue between the gut and the brain by translating exogenous chemical stimuli into neuronal inputs, and vice versa, to translate the endogenous signals relevant to the nutritional status into specific appetites and the control of feed intake. The relevance of these sensors in nondigestive systems has uncovered fascinating potential as pharmacological targets relevant to respiratory and cardiovascular diseases.

The role of free-fatty acid receptor-4 (FFA4) in human cancers and cancer cell lines

A dietary influence on cancer progression has been evident for many decades, and dietary fatty acids, particularly long chain mono- and polyunsaturated fatty acids, have been shown to play significant roles in influencing growth of a variety of human cancers. The discovery of the family of cell-surface free-fatty acid receptors, which include the long-chain fatty acid receptors FFA1 and FFA4, suggest that many of the effects of dietary fats could be receptor-mediated. FFA4 is ubiquitously expressed and has recently been shown to modulate a variety of important anti-inflammatory and metabolic processes. Since FFA4 is currently an attractive drug target for treatment of metabolic disorders such as diabetes and obesity, understanding its role in cancer progression is critical towards the drug discovery process. In this research update, the current body of knowledge on the role of this receptor in regulating cancer cell proliferation, migration, and invasion, as well as in vivo tumorigenesis is reviewed.

Drosophila Gr64e mediates fatty acid sensing via the phospholipase C pathway

Animals use taste to sample and ingest essential nutrients for survival. Free fatty acids (FAs) are energy-rich nutrients that contribute to various cellular functions. Recent evidence suggests FAs are detected through the gustatory system to promote feeding. In Drosophila, phospholipase C (PLC) signaling in sweet-sensing cells is required for FA detection but other signaling molecules are unknown. Here, we show Gr64e is required for the behavioral and electrophysiological responses to FAs. GR64e and TRPA1 are interchangeable when they act downstream of PLC: TRPA1 can substitute for GR64e in FA but not glycerol sensing, and GR64e can substitute for TRPA1 in aristolochic acid but not N-methylmaleimide sensing. In contrast to its role in FA sensing, GR64e functions as a ligand-gated ion channel for glycerol detection. Our results identify a novel FA transduction molecule and reveal that Drosophila Grs can act via distinct molecular mechanisms depending on context.

Fatty acids (FAs) are energy-rich nutrients that are detected through the gustatory system to promote feeding. Here, we show FA detection requires a Drosophila gustatory receptor, Gr64e. Although GR64e functions as a ligand-gated ion channel for glycerol detection, in FA sensing, it acts downstream of phospholipase C signaling. We identified a novel signaling molecule for FA sensing in Drosophila. Furthermore, our findings suggest Drosophila GRs have multiple modes of action depending on their cellular and molecular context.

Salivary endocannabinoids andN-acylethanolamines upon mastication of a semisolid food: implications in fat taste, appetite and food liking

This study combined MSF and TDS methodologies to monitor salivary NAEs and ECs, fat taste, food liking and appetite in humans during pudding mastication.

Molecular basis of fatty acid taste in Drosophila

Behavioral studies have established that Drosophila appetitive taste responses towards fatty acids are mediated by sweet sensing Gustatory Receptor Neurons (GRNs). Here we show that sweet GRN activation requires the function of the Ionotropic Receptor genes IR25a, IR76b and IR56d. The former two IR genes are expressed in several neurons per sensillum, while IR56d expression is restricted to sweet GRNs. Importantly, loss of appetitive behavioral responses to fatty acids in IR25a and IR76b mutant flies can be completely rescued by expression of respective transgenes in sweet GRNs. Interestingly, appetitive behavioral responses of wild type flies to hexanoic acid reach a plateau at ~1%, but decrease with higher concentration, a property mediated through IR25a/IR76b independent activation of bitter GRNs. With our previous report on sour taste, our studies suggest that IR-based receptors mediate different taste qualities through cell-type specific IR subunits.

Omega-3 polyunsaturated fatty acids protect human hepatoma cells from developing steatosis through FFA4 (GPR120)

Protective effect of omega-3 polyunsaturated fatty acids (n-3 PUFA) on non-alcoholic fatty liver disease has been demonstrated. FFA4 (also known as GPR120; a G protein-coupled receptor) has been suggested to be a target of n-3 PUFA. FFA4 expression in hepatocytes has also been reported from liver biopsies in child fatty liver patients. In order to assess the functional role of FFA4 in hepatic steatosis, we used an in vitro model of liver X receptor (LXR)-mediated hepatocellular steatosis. FFA4 expression was confirmed in Hep3B and HepG2 human hepatoma cells. T0901317 (a specific LXR activator) induced lipid accumulation and docosahexaenoic acid (DHA; a representative n-3 PUFA) inhibited lipid accumulation. This DHA-induced inhibition was blunted by treatment of AH7614 (a FFA4 antagonist) and by transfection of FFA4 siRNA. SREBP-1c (a key transcription factor of lipogenesis) was induced by treatment with T0901317, and SREBP-1c induction was also inhibited by DHA at mRNA and protein levels. DHA-induced suppression of SREBP-1c expression was also blunted by FFA4-knockdown. Furthermore, DHA inhibited T0901317-induced lipid accumulation in primary hepatocytes from wild type mice, but not in those from FFA4 deficient mice. In addition, DHA-induced activations of G_q/11 proteins, CaMKK, and AMPK were found to be signaling components of the steatosis protective pathway. The results of this study suggest that n-3 PUFA protect hepatic steatosis by activating FFA4 in hepatocytes, and its signaling cascade sequentially involves FFA4, G_q/11 proteins, CaMKK, AMPK, and SREBP-1c suppression.

Salivary Proteome Patterns Affecting Human Salt Taste Sensitivity

To investigate the role of perireceptor events in inter-individual variability in salt taste sensitivity, 31 volunteers were monitored in their detection functions for sodium chloride (NaCl) and classified into sensitive (0.6-1.7 mmol/L), medium-sensitive (1.8-6.9 mmol/L), and nonsensitive (7.0-11.2 mmol/L) subjects. Chemosensory intervention of NaCl-sensitive (S⁺) and nonsensitive (S^-) panellists with potassium chloride, ammonium chloride, and sodium gluconate showed the salt taste sensitivity to be specific for NaCl. As no significant differences were found between S⁺ and S^- subjects in salivary sodium and protein content, salivary proteome differences and their stimulus-induced dynamic changes were analyzed by tryptic digestion, iTRAQ labeling, and liquid chromatography-tandem mass spectrometry analysis. Differences in the salivary proteome between S⁺ and S^- subjects were found primarily in resting saliva and were largely independent of the dynamic alterations observed upon salt stimulation. Gene ontology enrichment analysis of key proteins, i.e., immunoglobulin heavy constant y1, myeloblastin, cathepsin G, and kallikrein, revealed significantly increased serine-type endopeptidase activity for the S⁺ group, while the S^- group exhibited augmented cysteine-type endopeptidase inhibitor activity by increased abundances in lipocalin-1 and cystatin-D, -S, and -SN, respectively. As proteases have been suggested to facilitate transepithelial sodium transport by cleaving the y-subunit of the epithelial sodium channel (ENaC) and protease inhibitors have been shown to reduce ENaC-mediated sodium transport, the differentially modulated proteolytic activity patterns observed in vivo for S⁺ and S^- subjects show evidence of them playing a crucial role in affecting human NaCl sensitivity.

FFA4 (GPR120) as a fatty acid sensor involved in appetite control, insulin sensitivity and inflammation regulation

Unsaturated long-chain fatty acids have been suggested to be beneficial in the context of cardiovascular disorders based in epidemiologic studies conducted in Greenland and Mediterranean. DHA and EPA are omega-3 polyunsaturated fatty acids that are plentiful in fish oil, and oleic acid is an omega-9 monounsaturated fatty acid, rich in olive oil. Dietary intake of these unsaturated long-chain fatty acids have been associated with insulin sensitivity and weight loss, which contrasts with the impairment of insulin sensitivity and weight gain associated with high intakes of saturated long-chain fatty acids. The recent discovery that free fatty acid receptor 4 (FFA4, also known as GPR120) acts as a sensor for unsaturated long-chain fatty acids started to unveil the molecular mechanisms underlying the beneficial functions played by these unsaturated long-chain fatty acids in various physiological processes, which include the secretions of gastrointestinal peptide hormones and glucose homeostasis. In this review, the physiological roles and therapeutic significance of FFA4 in appetite control, insulin sensitization, and inflammation reduction are discussed in relation to obesity and type 2 diabetes from pharmacological viewpoints.

Oil Perception-Detection Thresholds for Varying Fatty Stimuli and Inter-individual Differences

Multiple lines of research have demonstrated that humans can perceive fat in the form of free fatty acids (FFAs). However, the dietary concentration of FFAs is generally very low and fat is mainly consumed as triacylglycerol (TAG). The aim of this study was to examine the perception of different fatty stimuli and possible associations between them. Therefore, detection thresholds for 4 fatty stimuli (oleic acid [FFA], paraffin oil [mixture of hydrocarbon molecules], canola oil [TAG-rich], and canola oil spiked with oleic acid [rich in TAGs and FFAs]) were determined in 30 healthy participants. Additionally, inter-individual differences in fat perception were examined. It was observed that oleic acid was perceivable at significantly lower concentrations than all other stimuli (P < 0.001). Similarly, canola oil with oleic acid was detectable at lower concentrations than canola oil alone (P < 0.001). Moreover, canola oil detection thresholds were significantly lower than paraffin oil detection thresholds (P = 0.017). Participants who were sensitive for low concentrations for oleic acid showed lower detection thresholds for canola oil with and without oleic acid, compared with participants that were less sensitive for oleic acid. The results of this study demonstrate that the higher the concentrations of FFAs in the stimuli, the lower the individual fat detection threshold. Moreover, participants being sensitive for lower concentrations of FFAs are also more likely to detect low concentrations of TAG-rich fats as it is found in the human diet.

Fat Sensation: Fatty Acid Taste and Olfaction Sensitivity and the Link with Disinhibited Eating Behaviour

Perception of fat taste, aroma, and texture are proposed to influence food preferences, thus shaping dietary intake and eating behaviour and consequently long-term health. In this study, we investigated associations between fatty acid taste, olfaction, mouthfeel of fat, dietary intake, eating behaviour, and body mass index (BMI). Fifty women attended three sessions to assess oleic acid taste and olfaction thresholds, the olfactory threshold for n-butanol and subjective mouthfeel ratings of custard samples. Dietary intake and eating behaviour were evaluated using a Food Frequency and Three-Factor Eating Questionnaire, respectively. Binomial regression analysis was used to model fat taste and olfaction data. Taste and olfactory detection for oleic acid were positively correlated (r = 0.325; p < 0.02). Oleic acid taste hypersensitive women had significantly increased n-butanol olfactory sensitivity (p < 0.03). The eating behaviour disinhibition and BMI were higher in women who were hyposensitive to oleic acid taste (p < 0.05). Dietary intake of nuts, nut spreads, and seeds were significantly correlated with high olfactory sensitivity to oleic acid (p < 0.01). These findings demonstrate a clear link between fatty acid taste sensitivity and olfaction and suggest that fat taste perception is associated with specific characteristics of eating behaviour and body composition.

GPR120: a critical role in adipogenesis, inflammation, and energy metabolism in adipose tissue

It is well known that adipose tissue has a critical role in the development of obesity and metabolic diseases and that adipose tissue acts as an endocrine organ to regulate lipid and glucose metabolism. Accumulating in the adipose tissue, fatty acids serve as a primary source of essential nutrients and act on intracellular and cell surface receptors to regulate biological events. G protein-coupled receptor 120 (GPR120) represents a promising target for the treatment of obesity-related metabolic disorders for its involvement in the regulation of adipogenesis, inflammation, glucose uptake, and insulin resistance. In this review, we summarize recent studies and advances regarding the systemic role of GPR120 in adipose tissue, including both white and brown adipocytes. We offer a new perspective by comparing the different roles in a variety of homeostatic processes from adipogenic development to adipocyte metabolism, and we also discuss the effects of natural and synthetic agonists that may be potential agents for the treatment of metabolic diseases.

The basal free fatty acid concentration in human saliva is related to salivary lipolytic activity

Fat perception during eating is a complex sensation that involves various sensory modalities, such as texture, aroma and taste. Taste is supported by the discovery of fatty acid receptors in the tongue papillae. Dietary fat is mainly composed of esterified fatty acids, whereas only free fatty acids can bind to taste receptors. Some authors have mentioned the necessity and efficiency of salivary lipolytic activity to hydrolyse the esterified fatty acids present in foods and enable fat perception. Our hypothesis is that salivary lipolytic activity is also involved in regulating the basal level of salivary fatty acids in humans. To test this hypothesis, total fatty acid (TFA) and free fatty acid (FFA) concentrations and selected salivary characteristics (such as lipolytic activity) were analysed in the resting saliva of 54 subjects. The results show differences in the TFA and FFA profiles, with TFA and FFA concentrations of 8.99 and 3.56 µg/mL of saliva, respectively. Interestingly, lipolytic activity had a significant positive correlation with FFA concentration (0.51, p < 0.01). This result highlights a possible physiological role of salivary lipolytic activity in the regulation of the basal FFA concentration. This regulation could be involved in fat taste sensitivity.

Taste buds: cells, signals and synapses

The past decade has witnessed a consolidation and refinement of the extraordinary progress made in taste research. This Review describes recent advances in our understanding of taste receptors, taste buds, and the connections between taste buds and sensory afferent fibres. The article discusses new findings regarding the cellular mechanisms for detecting tastes, new data on the transmitters involved in taste processing and new studies that address longstanding arguments about taste coding.

Extraoral Taste Receptor Discovery: New Light on Ayurvedic Pharmacology

More and more research studies are revealing unexpectedly important roles of taste for health and pathogenesis of various diseases. Only recently it has been shown that taste receptors have many extraoral locations (e.g., stomach, intestines, liver, pancreas, respiratory system, heart, brain, kidney, urinary bladder, pancreas, adipose tissue, testis, and ovary), being part of a large diffuse chemosensory system. The functional implications of these taste receptors widely dispersed in various organs or tissues shed a new light on several concepts used in ayurvedic pharmacology (dravyaguna vijnana), such as taste (rasa), postdigestive effect (vipaka), qualities (guna), and energetic nature (virya). This review summarizes the significance of extraoral taste receptors and transient receptor potential (TRP) channels for ayurvedic pharmacology, as well as the biological activities of various types of phytochemical tastants from an ayurvedic perspective. The relative importance of taste (rasa), postdigestive effect (vipaka), and energetic nature (virya) as ethnopharmacological descriptors within Ayurveda boundaries will also be discussed.

Genetics of fat intake in the determination of body mass

Body mass and fat intake are multifactorial traits that have genetic and environmental components. The gene with the greatest effect on body mass is FTO (fat mass and obesity-associated), but several studies have shown that the effect of FTO (and of other genes) on body mass can be modified by the intake of nutrients. The so-called gene–environment interactions may also be important for the effectiveness of weight-loss strategies. Food choices, and thus fat intake, depend to some extent on individual preferences. The most important biological component of food preference is taste, and the role of fat sensitivity in fat intake has recently been pointed out. Relatively few studies have analysed the genetic components of fat intake or fatty acid sensitivity in terms of their relation to obesity. It has been proposed that decreased oral fatty acid sensitivity leads to increased fat intake and thus increased body mass. One of the genes that affect fatty acid sensitivity is CD36 (cluster of differentiation 36). However, little is known so far about the genetic component of fat sensing. We performed a literature review to identify the state of knowledge regarding the genetics of fat intake and its relation to body-mass determination, and to identify the priorities for further investigations.

Polymorphic Variation in FFA Receptors: Functions and Consequences

Overfeeding of fat can cause various metabolic disorders including obesity and type 2 diabetes (T2D). Diet provided free fatty acids (FFAs) are not only essential nutrients, but they are also recognized as signaling molecules, which stimulate various important biological functions. Recently, several G protein-coupled receptors (GPCRs), including FFA1-4, have been identified as receptors of FFAs by various physiological and pharmacological studies. FFAs exert physiological functions through these FFA receptors (FFARs) depending on carbon chain length and degree of unsaturation. Functional analyses have revealed that several important metabolic processes, such as peptide hormone secretion, cell maturation and nerve activities, are regulated by FFARs and thereby FFARs contribute to the energy homeostasis through these physiological functions. Hence, FFARs are expected to be promising pharmacological targets for metabolic disorders since imbalances in energy homeostasis lead to metabolic disorders. In human, it is established that different responses of individuals to endogenous ligands and chemical drugs may be due to differences in the ability of such ligands to activate nucleotide polymorphic variants of receptors. However, the clear links between genetic variations that are involved in metabolic disorders and polymorphisms receptors have been relatively difficult to assess. In this review, I summarize current literature describing physiological functions of FFARs and genetic variations of those receptors to discuss the potential of FFARs as drug targets for metabolic disorders.

Consumer Acceptability of Intramuscular Fat

Fat in meat greatly improves eating quality, yet many consumers avoid visible fat, mainly because of health concerns. Generations of consumers, especially in the English-speaking world, have been convinced by health authorities that animal fat, particularly saturated or solid fat, should be reduced or avoided to maintain a healthy diet. Decades of negative messages regarding animal fats has resulted in general avoidance of fatty cuts of meat. Paradoxically, low fat or lean meat tends to have poor eating quality and flavor and low consumer acceptability. The failure of low-fat high-carbohydrate diets to curb "globesity" has prompted many experts to re-evaluate of the place of fat in human diets, including animal fat. Attitudes towards fat vary dramatically between and within cultures. Previous generations of humans sought out fatty cuts of meat for their superior sensory properties. Many consumers in East and Southeast Asia have traditionally valued more fatty meat cuts. As nutritional messages around dietary fat change, there is evidence that attitudes towards animal fat are changing and many consumers are rediscovering and embracing fattier cuts of meat, including marbled beef. The present work provides a short overview of the unique sensory characteristics of marbled beef and changing consumer preferences for fat in meat in general.

ω3-Polyunsaturated fatty acids for heart failure: Effects of dose on efficacy and novel signaling through free fatty acid receptor 4

Heart failure (HF) affects 5.7 million in the U.S., and despite well-established pharmacologic therapy, the 5-year mortality rate remains near 50%. Furthermore, the mortality rate for HF has not declined in years, highlighting the need for new therapeutic options. Omega-3 polyunsaturated fatty acids (ω3-PUFAs), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are important regulators of cardiovascular health. However, questions of efficacy and mechanism of action have made the use of ω3-PUFAs in all cardiovascular disease (CVD) controversial. Here, we review recent studies in animal models of HF indicating that ω3-PUFAs, particularly EPA, are cardioprotective, with the results indicating a threshold for efficacy. We also examine clinical studies suggesting that ω3-PUFAs improve outcomes in patients with HF. Due to the relatively small number of clinical studies of ω3-PUFAs in HF, we discuss EPA concentration-dependency on outcomes in clinical trials of CVD to gain insight into the perceived questionable efficacy of ω3-PUFAs clinically, with the results again indicating a threshold for efficacy. Ultimately, we suggest that the main failing of ω3-PUFAs in clinical trials might be a failure to reach a therapeutically effective concentration. We also examine mechanistic studies suggesting that ω3-PUFAs signal through free fatty acid receptor 4 (Ffar4), a G-protein coupled receptor (GPR) for long-chain fatty acids (FA), thereby identifying an entirely novel mechanism of action for ω3-PUFA mediated cardioprotection. Finally, based on mechanistic animal studies suggesting that EPA prevents interstitial fibrosis and diastolic dysfunction, we speculate about a potential benefit for EPA-Ffar4 signaling in heart failure preserved with ejection fraction.

Complex Pharmacology of Free Fatty Acid Receptors

G protein-coupled receptors (GPCRs) are historically the most successful family of drug targets. In recent times it has become clear that the pharmacology of these receptors is far more complex than previously imagined. Understanding of the pharmacological regulation of GPCRs now extends beyond simple competitive agonism or antagonism by ligands interacting with the orthosteric binding site of the receptor to incorporate concepts of allosteric agonism, allosteric modulation, signaling bias, constitutive activity, and inverse agonism. Herein, we consider how evolving concepts of GPCR pharmacology have shaped understanding of the complex pharmacology of receptors that recognize and are activated by nonesterified or "free" fatty acids (FFAs). The FFA family of receptors is a recently deorphanized set of GPCRs, the members of which are now receiving substantial interest as novel targets for the treatment of metabolic and inflammatory diseases. Further understanding of the complex pharmacology of these receptors will be critical to unlocking their ultimate therapeutic potential.

Lipids and obesity: Also a matter of taste?

Obesity is undoubtedly one of the major public health challenges worldwide because of its rapid progression and deleterious effects of associated diseases. The easier access to tasty and energy-dense foods is thought to greatly contribute to this epidemic. Studies also report that obese subjects and animals (rats and mice) preferentially consume foods rich in fat when they can choose. The origin of this eating behavior remains elusive. Over the last decade, the existence of a taste of fat, besides textural and olfactory cues, was supported by a growing number of studies. The existence of a sixth taste modality devoted to the detection/perception of dietary lipids might offer additive information on the quality of food. While the sense of taste is recognized to be a driving-force guiding food choice, interest in the putative relationships between lipids, gustation and obesity is only now emerging. This mini-review will attempt to summarize our current knowledge on this new field of research.

A Family of non-GPCR Chemosensors Defines an Alternative Logic for Mammalian Olfaction

Odor perception in mammals is mediated by parallel sensory pathways that convey distinct information about the olfactory world. Multiple olfactory subsystems express characteristic seven-transmembrane G-protein-coupled receptors (GPCRs) in a one-receptor-per-neuron pattern that facilitates odor discrimination. Sensory neurons of the "necklace" subsystem are nestled within the recesses of the olfactory epithelium and detect diverse odorants; however, they do not express known GPCR odor receptors. Here, we report that members of the four-pass transmembrane MS4A protein family are chemosensors expressed within necklace sensory neurons. These receptors localize to sensory endings and confer responses to ethologically relevant ligands, including pheromones and fatty acids, in vitro and in vivo. Individual necklace neurons co-express many MS4A proteins and are activated by multiple MS4A ligands; this pooling of information suggests that the necklace is organized more like subsystems for taste than for smell. The MS4As therefore define a distinct mechanism and functional logic for mammalian olfaction.

Dietary Fatty Acids and Their Potential for Controlling Metabolic Diseases Through Activation of FFA4/GPR120

It is well known that the amount and type of ingested fat impacts the development of obesity and metabolic diseases, but the potential for beneficial effects from fat has received less attention. It is becoming clear that the composition of the individual fatty acids in diet is important. Besides acting as precursors of potent signaling molecules, dietary fatty acids act directly on intracellular and cell surface receptors. The free fatty acid receptor 4 (FFA4, previously GPR120) is linked to the regulation of body weight, inflammation, and insulin resistance and represents a potential target for the treatment of metabolic disorders, including type 2 diabetes and obesity. In this review, we discuss the various types of dietary fatty acids, the link between FFA4 and metabolic diseases, the potential effects of the individual fatty acids on health, and the ability of fatty acids to activate FFA4. We also discuss the possibility of dietary schemes that implement activation of FFA4.

Cloning, Phylogenetic Analysis, and Distribution of Free Fatty Acid Receptor GPR120 Expression along the Gastrointestinal Tract of Housing versus Grazing Kid Goats

G-protein-coupled receptor 120 (GPR120) is reported as a long-chain fatty acid (LCFA) receptor that elicits free fatty acid (FFA) regulation on metabolism homeostasis. The study aimed to clone the gpr120 gene of goats (g-GPR120) and subsequently investigate phylogenetic analysis and tissue distribution throughout the digestive tracts of kid goats, as well as the effect of housing versus grazing (H vs G) feeding systems on GPR120 expression. Partial coding sequence (CDS) of g-GPR120 was cloned and submitted to NCBI (accession no. KU161270 ). Phylogenetic analysis revealed that g-GPR120 shared higher homology in both mRNA and amino acid sequences for ruminants than nonruminants. Immunochemistry, real-time PCR, and Western blot analysis showed that g-GPR120 was expressed throughout the digestive tracts of goats. The expression of g-GPR120 was affected by feeding system and age, with greater expression of g-GPR120 in the G group. It was concluded that the g-GPR120-mediated LCFA chemosensing mechanism is widely present in the tongue and gastrointestinal tract of goats and that its expression can be affected by feeding system and age.

Recent Advances in Molecular Mechanisms of Taste Signaling and Modifying

The sense of taste conveys crucial information about the quality and nutritional value of foods before it is ingested. Taste signaling begins with taste cells via taste receptors in oral cavity. Activation of these receptors drives the transduction systems in taste receptor cells. Then particular transmitters are released from the taste cells and activate corresponding afferent gustatory nerve fibers. Recent studies have revealed that taste sensitivities are defined by distinct taste receptors and modulated by endogenous humoral factors in a specific group of taste cells. Such peripheral taste generations and modifications would directly influence intake of nutritive substances. This review will highlight current understanding of molecular mechanisms for taste reception, signal transduction in taste bud cells, transmission between taste cells and nerves, regeneration from taste stem cells, and modification by humoral factors at peripheral taste organs.

Free-fatty acid receptor-4 (GPR120): Cellular and molecular function and its role in metabolic disorders

Over the last decade, a subfamily of G protein-coupled receptors that are agonized by endogenous and dietary free-fatty acids (FFA) has been discovered. These free-fatty acid receptors include FFA2 and FFA3, which are agonized by short-chained FFA, as well as FFA1 and FFA4, which are agonized by medium-to-long chained FFA. Ligands for FFA1 and FFA4 comprise the family of long chain polyunsaturated omega-3 fatty acids including α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), suggesting that many of the long-known beneficial effects of these fats may be receptor mediated. In this regard, FFA4 has gathered considerable interest due to its role in ameliorating inflammation, promoting insulin sensitization, and regulating energy metabolism in response to FFA ligands. The goal of this review is to summarize the body of evidence in regard to FFA4 signal transduction, its mechanisms of regulation, and its functional role in a variety of tissues. In addition, recent endeavors toward discovery of small molecules that modulate FFA4 activity are also presented.

Determinants of Obesity in Italian Adults: The Role of Taste Sensitivity, Food Liking, and Food Neophobia

Recent evidence has suggested that factors related to sensory perception may explain excess weight. The objective of this study was to consider multiple aspects while investigating the phenomenon of obesity. One goal was to compare taste acuity (taste threshold and density of fungiform papillae) in both normal weight and obese subjects. Thresholds for 4 basic tastes and the fat stimulus were investigated. A second research goal was to study the relationship between food neophobia and food liking according to the body mass index and taste sensitivity. The results showed that obese subjects seem to have higher threshold values and a reduced number of fungiform papillae than do normal weight subjects. Food neophobia did not vary with nutritional status, whereas differences were found for food liking, with obese subjects showing significantly higher liking ratings for high energy dense products compared with normal weight subjects.

Behavioral palatability of dietary fatty acids correlates with the intracellular calcium ion levels induced by the fatty acids in GPR120-expressing cells

We recently reported that G-protein-coupled receptor 120 (GPR120) is expressed on taste buds, and that rodents showed preference for long-chain fatty acids (LCFA) at a low concentration. We also showed that the LCFA (1% linoleic acid) increased the extracellular dopamine (DA) level in the nucleus accumbens (NAc), which participates in reward behavior. However, the mechanism underlying the involvement of the GPR120-agonistic activity of LCFA in the palatability of dietary fat remains elusive. Therefore, we examined the association between the GPR120-agonistic activity and palatability of LCFA. First, we measured Ca(2+) signaling in HEK293 cells stably expressing GPR120 under stimulation by various LCFAs. We then assessed the palatability of the various LCFAs by testing the licking behavior in mice and measured the changes in the NAc-DA level by in vivo microdialysis. Consequently, 14- to 22-carbon unsaturated LCFAs showed strong GPR120-agonistic activity. Additionally, mice displayed high licking response to unsaturated 16- and 18-carbon LCFAs, and unsaturated 18-carbon LCFA significantly increased the DA level. The licking rate and the LCFA-dependent increase in DA level also correlated well with the GPR120- agonistic activity. These findings demonstrate that chemoreception of LCFA by GPR120 is involved in the recognition and palatability of dietary fat.

Using Animal Models to Determine the Role of Gustatory Neural Input in the Control of Ingestive Behavior and the Maintenance of Body Weight

INTRODUCTION

Decades of research have suggested that nutritional intake contributes to the development of human disease, mainly by influencing the development of obesity and obesity-related conditions. A relatively large body of research indicates that functional variation in human taste perception can influence nutritional intake as well as body mass accumulation. However, there are a considerable number of studies that suggest that no link between these variables actually exists. These discrepancies in the literature likely result from the confounding influence of a variety of other, uncontrolled, factors that can influence ingestive behavior.

STRATEGY

In this review, the use of controlled animal experimentation to alleviate at least some of these issues related to the lack of control of experimental variables is discussed. Specific examples of the use of some of these techniques are examined.

DISCUSSION AND CONCLUSIONS

The review will close with some specific suggestions aimed at strengthening the link between gustatory neural input and its putative influence on ingestive behaviors and the maintenance of body weight.

Oleogustus: The Unique Taste of Fat

Considerable mechanistic data indicate there may be a sixth basic taste: fat. However, evidence demonstrating that the sensation of nonesterified fatty acids (NEFA, the proposed stimuli for "fat taste") differs qualitatively from other tastes is lacking. Using perceptual mapping, we demonstrate that medium and long-chain NEFA have a taste sensation that is distinct from other basic tastes (sweet, sour, salty, and bitter). Although some overlap was observed between these NEFA and umami taste, this overlap is likely due to unfamiliarity with umami sensations rather than true similarity. Shorter chain fatty acids stimulate a sensation similar to sour, but as chain length increases this sensation changes. Fat taste oral signaling, and the different signals caused by different alkyl chain lengths, may hold implications for food product development, clinical practice, and public health policy.

Omega-3 fatty acids induce Ca(2+) mobilization responses in human colon epithelial cell lines endogenously expressing FFA4

AIM

Free fatty acid receptor 4 (FFA4; formerly known as GPR120) is the G protein-coupled receptor (GPCR) for omega-3 polyunsaturated fatty acids. FFA4 has been found to express in the small intestines and colons of mice and humans. In this study we investigate the effects of omega-3 polyunsaturated fatty acids on FFA4 in human colon epithelial cells in vitro.

METHODS

HCT116 and HT-29 human colon epithelial cell lines endogenously expressing FFA4 were used. Intracellular Ca(2+) concentration ([Ca(2+)]i) was measured in fura 2-AM-loaded cells with fluorescence spectrophotometry. RT-PCR and immunohistochemistry were used to detect FFA4.

RESULTS

Ten to 100 μmol/L of omega-3 polyunsaturated fatty acids α-linolenic acid (αLA) or eicosapentaenoic acid (EPA) induced dose-dependent [Ca(2+)]i increase in HCT116 and HT-29 cells, whereas docosahexaenoic acid (DHA) had no effect. In addition, the omega-6 fatty acids linoleic acid and γ-linoleic acid also dose-dependently increase [Ca(2+)]i, but the mono-unsaturated fatty acid oleic acid and saturated fatty acids such as stearic acid and palmitic acid had no effect. In HCT116 and HT-29 cells, the αLA-induced [Ca(2+)]i increase was partially inhibited by pretreatment with EGTA, phospholipase C inhibitor edelfosine, cADPR inhibitors 8-bro-cADPR or DAB, and abolished by pretreatment with Ca(2+)ATPase inhibitor thapsigargin, but was not affected by Gi/o protein inhibitor PTX or IP3R inhibitor 2-APB.

CONCLUSION

Omega-3 and omega-6 long-chain polyunsaturated fatty acids (C18-20) induce Ca(2+) mobilization responses in human colonic epithelial cells in vitro through activation of FFA4 and PTX-insensitive Gi/o protein, followed by Ca(2+) release from thapsigargin-sensitive Ca(2+) stores and Ca(2+) influx across the plasma membrane.

The role of polyunsaturated fatty acids and GPR40 receptor in brain

Polyunsaturated fatty acids (PUFAs) are found in abundance in the nervous system. They perform significant functions for example boosting synaptogenesis, neurogenesis, inducing antinociception, stimulating gene expression and neuronal activity, preventing apoptosis and neuroinflammation. G-protein-coupled receptor 40 (GPR40), also called free fatty acid receptor 1 (FFA1), is ubiquitously expressed in various regions of the human brain including the olfactory bulb, midbrain, medulla oblongata, hippocampus, hypothalamus, cerebral cortex, cerebellum and in the spinal cord. GPR40, when binding with polyunsaturated fatty acids (PUFAs) has shown promising therapeutic potential. This review presents current knowledge regarding the pharmacological properties of GPR40 and addresses its functions in brain, with a focus on neurodevelopment & neurogenesis. Furthermore, the demonstration of GPR40 involvement in several neuropathological conditions such as apoptosis, inflammatory pain, Alzheimer's disease and Parkinson's disease. Although the results are encouraging, further research is needed to clarify their role in the treatment of inflammatory pain, Alzheimer's disease and Parkinson's disease. This article is part of the Special Issue entitled 'Lipid Sensing G Protein-Coupled Receptors in the CNS'.

Cardiac gene expression data and in silico analysis provide novel insights into human and mouse taste receptor gene regulation

G protein-coupled receptors are the principal mediators of the sweet, umami, bitter, and fat taste qualities in mammals. Intriguingly, the taste receptors are also expressed outside of the oral cavity, including in the gut, airways, brain, and heart, where they have additional functions and contribute to disease. However, there is little known about the mechanisms governing the transcriptional regulation of taste receptor genes. Following our recent delineation of taste receptors in the heart, we investigated the genomic loci encoding for taste receptors to gain insight into the regulatory mechanisms that drive their expression in the heart. Gene expression analyses of healthy and diseased human and mouse hearts showed coordinated expression for a subset of chromosomally clustered taste receptors. This chromosomal clustering mirrored the cardiac expression profile, suggesting that a common gene regulatory block may control the taste receptor locus. We identified unique domains with strong regulatory potential in the vicinity of taste receptor genes. We also performed de novo motif enrichment in the proximal promoter regions and found several overrepresented DNA motifs in cardiac taste receptor gene promoters corresponding to ubiquitous and cardiac-specific transcription factor binding sites. Thus, combining cardiac gene expression data with bioinformatic analyses, this study has provided insights into the noncoding regulatory landscape for taste GPCRs. These findings also have broader relevance for the study of taste GPCRs outside of the classical gustatory system, where understanding the mechanisms controlling the expression of these receptors may have implications for future therapeutic development.

Cloning and characterization of spliced variants of the porcine G protein coupled receptor 120

The polyunsaturated fatty acids (PUFAs) receptor GPR120 exerts a significant impact on systemic nutrient homeostasis in human and rodents. However, the porcine GPR120 (pGPR120) has not been well characterized. In the current study, we found that pGPR120 had 3 spliced variants. Transcript 1 encoded 362-amino acids (aa) wild type pGPR120-WT, which shared 88% homology with human short form GPR120. Transcript 1 was the mainly expressed transcript of pGPR120. It was expressed predominantly in ileum, jejunum, duodenum, spleen, and adipose. Transcript 3 (coding 320-aa isoform) was detected in spleen, while the transcript 2 (coding 310-aa isoform) was only slightly expressed in spleen. A selective agonist for human GPR120 (TUG-891) and PUFAs activated SRE-luc and NFAT-luc reporter in HEK293T cells transfected with construct for pGPR120-WT but not pGPR120-V2. However, 320-aa isoform was not a dominant negative isoform. The extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation levels in cells transfected with construct for pGPR120-WT were well activated by PUFAs, especially n-3 PUFA. These results showed that although pGPR120 had 3 transcripts, transcript 1 which encoded pGPR120-WT was the mainly expressed transcript. TUG-891 and PUFAs, especially n-3 PUFA, well activated pGPR120-WT. The current study contributed to dissecting the molecular regulation mechanisms of n-3 PUFA in pigs.

Functions of omega-3 fatty acids and FFA4 (GPR120) in macrophages

Omega-3 polyunsaturated fatty acids (n-3 PUFAs), which are plentiful in fish oil, have been known for decades to be beneficial functional nutrients in different disease states. GPR120 is a G protein-coupled receptor for long-chain unsaturated fatty acids, including n-3 PUFAs, and was recently renamed free fatty acid receptor 4 (FFA4). Studies on FFA4-deficient mice and the development of specific pharmacological tools have started to unravel the functions of FFA4 associated with the actions of n-3 PUFAs in obesity, type 2 diabetes, and inflammation-related diseases. Here, the state of the art regarding the roles and functions of FFA4 and n-3 PUFA in macrophages are reviewed from the pharmacological perspective. In particular, the functions of n-3 PUFA on the anti-inflammatory M2 phenotypes of macrophages in different organs, such as, adipose tissues and liver, are discussed along with future research directions.

Recent advances in fatty acid perception and genetics

This article summarizes new knowledge about the contribution of genetic variation to person-to-person differences underlying some sensory aspects of dietary fatty acids. Receptors on the taste cells of the human tongue arise from genes that have marked variation in DNA sequence, which, in some cases, is associated with differences in how these lipids in foods are perceived. These perceptual differences may affect food selection.

The endocrinology of taste receptors

Levels of obesity have reached epidemic proportions on a global scale, which has led to considerable increases in health problems and increased risk of several diseases, including cardiovascular and pulmonary diseases, cancer and diabetes mellitus. People with obesity consume more food than is needed to maintain an ideal body weight, despite the discrimination that accompanies being overweight and the wealth of available information that overconsumption is detrimental to health. The relationship between energy expenditure and energy intake throughout an individual's lifetime is far more complicated than previously thought. An improved comprehension of the relationships between taste, palatability, taste receptors and hedonic responses to food might lead to increased understanding of the biological underpinnings of energy acquisition, as well as why humans sometimes eat more than is needed and more than we know is healthy. This Review discusses the role of taste receptors in the tongue, gut, pancreas and brain and their hormonal involvement in taste perception, as well as the relationship between taste perception, overeating and the development of obesity.

The A allele of cluster of differentiation 36 (CD36) SNP 1761667 associates with decreased lipid taste perception in obese Tunisian women

Recent studies have suggested that excessive intake of dietary fat is associated with obesity. Some obese subjects have been reported to exhibit high thresholds for the gustatory detection of lipids via lipid receptors, such as cluster of differentiation 36 (CD36). We studied lingual detection thresholds for emulsions containing oleic acid in obese Tunisian women (n 203) using a three-alternative forced choice (3-AFC) method. Genotyping of the TNF-α (rs1800629), IL-6 (rs1800795) and CD36 (rs1761667) genes was performed to associate with lipid taste perception thresholds. The CD36 genotype distribution was as follows: GG (n 42), AG (n 102) and AA (n 59). Women with the CD36 GG genotype exhibited oral detection thresholds for oleic acid that were more than three times lower than those with the CD36 AA genotype. The present study confirms a high threshold of gustatory fat detection in obese women with the CD36 AA genotype, but there is no significant association with the IL-6 and TNF-α gene polymorphisms.

Humans are more sensitive to the taste of linoleic and α-linolenic than oleic acid

Health concerns have led to recommendations to replace saturated fats with unsaturated fats. However, addition of unsaturated fatty acids may lead to changes in the way foods are perceived in the oral cavity. This study tested the taste sensitivity to and emulsion characteristics of oleic, linoleic, and α-linolenic acids. The hypothesis tested was that oral sensitivity to nonesterified fatty acids would increase with degree of unsaturation but that in vitro viscosities and particle sizes of these emulsions would not differ. Oral taste thresholds were obtained using the three-alternative, forced-choice, ascending method. Each participant was tested on each fat 7 times, for a total of 21 study visits, to account for learning effects. Viscosities were obtained for the blank solutions and all three emulsions. Results indicate lower oral thresholds to linoleic and α-linolenic than oleic acid. At higher shear rates, 5% oleic and linoleic acid were more viscous than other samples. More-dilute emulsions showed no significant differences in viscosity. Particle sizes of the emulsions increased very slightly with increasing unsaturation. Together, the emulsion characteristics and oral sensitivity data support a taste mechanism for nonesterified fatty acid detection.