Transsynaptic Tracing from Taste Receptor Cells Reveals Local Taste Receptor Gene Expression in Gustatory Ganglia and Brain

The olfactory bulb: A neuroendocrine spotlight on feeding and metabolism

Olfaction is the most ancient sense and is needed for food-seeking, danger protection, mating and survival. It is often the first sensory modality to perceive changes in the external environment, before sight, taste or sound. Odour molecules activate olfactory sensory neurons that reside on the olfactory epithelium in the nasal cavity, which transmits this odour-specific information to the olfactory bulb (OB), where it is relayed to higher brain regions involved in olfactory perception and behaviour. Besides odour processing, recent studies suggest that the OB extends its function into the regulation of food intake and energy balance. Furthermore, numerous hormone receptors associated with appetite and metabolism are expressed within the OB, suggesting a neuroendocrine role outside the hypothalamus. Olfactory cues are important to promote food preparatory behaviours and consumption, such as enhancing appetite and salivation. In addition, altered metabolism or energy state (fasting, satiety and overnutrition) can change olfactory processing and perception. Similarly, various animal models and human pathologies indicate a strong link between olfactory impairment and metabolic dysfunction. Therefore, understanding the nature of this reciprocal relationship is critical to understand how olfactory or metabolic disorders arise. This present review elaborates on the connection between olfaction, feeding behaviour and metabolism and will shed light on the neuroendocrine role of the OB as an interface between the external and internal environments. Elucidating the specific mechanisms by which olfactory signals are integrated and translated into metabolic responses holds promise for the development of targeted therapeutic strategies and interventions aimed at modulating appetite and promoting metabolic health.

Investigating the Potential Influence of TAS2R16 Genetic Variants and Protein Levels on Multiple Sclerosis Development

The study aimed to investigate the association between the TAS2R16 gene (rs860170, rs978739, rs1357949), TAS2R16 serum levels, and multiple sclerosis (MS). A total of 265 healthy control subjects and 218 MS patients were included in the study. Single nucleotide polymorphisms (SNPs) were tested by real-time polymerase chain reaction (RT-PCR). The serum concentration of TAS2R16 was measured using the ELISA method. Analyses revealed that the TAS2R16 rs860170 TT genotype was statistically significantly less frequent in the MS group than in the control group (p = 0.041), and the CC genotype was statistically significantly more frequent in the MS group than in the control group (p < 0.001). In the most robust (codominant) model, the CC genotype was found to increase the odds of MS by ~27-fold (p = 0.002), and each C allele increased the odds of MS by 1.8-fold (p < 0.001). Haplotype analysis of the rs860170, rs978739, and rs1357949 polymorphisms showed that the C-C-A haplotype was associated with a ~12-fold increased odds of MS occurrence (p = 0.02). Serum TAS2R16 levels were elevated in the MS group compared to control subjects (p = 0.014). Conclusions: The rs860170, rs978739, and rs1357949 polymorphisms demonstrated that the C-C-A haplotype and elevated TAS2R16 serum levels can promote the development of MS. These preliminary findings underscore the importance of specific genetic variants, such as rs860170, rs978739, and rs1357949, in MS risk. Additionally, elevated TAS2R16 serum levels in MS patients suggest a potential role in MS pathogenesis. These findings provide insights into the genetic and molecular mechanisms underlying MS and pave the way for personalized diagnostic and therapeutic strategies. Integrating genetic and serum biomarker data in MS research offers promising avenues for improving clinical outcomes and advancing precision medicine approaches in the future.

The druggability of bitter taste receptors for the treatment of neurodegenerative disorders

The delivery of therapeutic drugs to the brain remains a major pharmacology challenge. A complex system of chemical surveillance to protect the brain from endogenous and exogenous toxicants at brain barriers hinders the uptake of many compounds with significant in vitro and ex vivo therapeutic properties. Despite the advances in the field in recent years, the components of this system are not completely understood. Recently, a large group of chemo-sensing receptors, have been identified in the blood-cerebrospinal fluid barrier. Among these chemo-sensing receptors, bitter taste receptors (TAS2R) hold promise as potential drug targets, as many TAS2R bind compounds with recognized neuroprotective activity (quercetin, resveratrol, among others). Whether activation of TAS2R by their ligands contributes to their diverse biological actions described in other cells and tissues is still debatable. In this review, we discuss the potential role of TAS2R gene family as the mediators of the biological activity of their ligands for the treatment of central nervous system disorders and discuss their potential to counteract drug resistance by improving drug delivery to the brain.

Integrating RNA-sequencing and untargeted LC-MS metabolomics to evaluate the effect of lysine deficiency on hepatic functions in Holstein calves

Lysine (Lys) is majorly metabolized in the liver. The liver functional consequences of a dietary Lys deficiency in young Holstein calves are unknown. This study aimed to investigate the effects of Lys deficiency in Holstein calf livers using RNA-sequencing and untargeted LC-MS metabolomics. Calves (n = 36; initial body weight 101.2 ± 10.8 kg; 90-day-old) were fed restricted diets, for 90 days, containing 19.2% crude protein that varied in Lys content (PC group 1.21%; PC-Lys group 0.85%; dry matter basis) for 90 days. Body weight, average daily gain, gain/feed, and Lys intake were significantly decreased in response to Lys deficiency (P < 0.05). Dry matter intake was not altered (P > 0.05). Network and pathway analyses revealed that noradrenaline, adenosine 5'-monophosphate, acetyl-CoA, and coenzyme A were significantly decreased. Regulating of lipolysis in adipocytes pathway and fatty acid degradation pathway were downregulated. We also identified eight significantly differentially expressed genes (SDEGs), among which adrenoceptor beta 2 (ADRB2), WAP four-disulfide core domain 2 (WFDC2), and claudin-4 (CLDN4) were associated with inhibition of lipolysis, and carbon catabolite repression 4-like (CCRN4L), FOS like 2 (FOSL2), and arginase 2 (ARG2) were associated with inhibiting lipid synthesis. Correlation tests showed that coenzyme A was strongly correlated with SDEGs (0.82 ≤|r|≤ 0.96). Acetyl-CoA and adenosine 5'-monophosphate were strongly correlated with CCRN4L (0.90 ≤|r|≤ 0.92), indicating a strong correlation between the changes in SDEGs and these metabolites. In conclusion, Lys deficiency caused dysplasia and affected lipid metabolism in the liver by inhibiting lipolysis and lipid synthesis in calves.

Extra-oral bitter taste receptors: New targets against obesity?

Taste perception on the tongue is essential to help us to identify nutritious or potential toxic food substances. Emerging evidence has demonstrated the expression and function of bitter taste receptors (TAS2Rs) in a wide range of extra-oral tissues. In particular, TAS2Rs in gastrointestinal enteroendocrine cells control the secretion of appetite regulating gut hormones and influence hunger and food intake. Furthermore, these effects may be reinforced by the presence of TAS2Rs on intestinal smooth muscle cells, adipocytes and the brain. This review summarises how activation of extra-oral TAS2Rs can influence appetite and body weight control and how obesity impacts the expression and function of TAS2Rs. Region-selective targeting of bitter taste receptors may be promising targets for the treatment of obesity.

Olfactory, Taste, and Photo Sensory Receptors in Non-sensory Organs: It Just Makes Sense

Sensory receptors that detect and respond to light, taste, and smell primarily belong to the G-protein-coupled receptor (GPCR) superfamily. In addition to their established roles in the nose, tongue, and eyes, these sensory GPCRs have been found in many ‘non-sensory' organs where they respond to different physicochemical stimuli, initiating signaling cascades in these extrasensory systems. For example, taste receptors in the airway, and photoreceptors in vascular smooth muscle cells, both cause smooth muscle relaxation when activated. In addition, olfactory receptors are present within the vascular system, where they play roles in angiogenesis as well as in modulating vascular tone. By better understanding the physiological and pathophysiological roles of sensory receptors in non-sensory organs, novel therapeutic agents can be developed targeting these receptors, ultimately leading to treatments for pathological conditions and potential cures for various disease states.

Tas2r125 functions as the main receptor for detecting bitterness of tea catechins in the oral cavity of mice

We attempted to identify mouse bitter taste receptors, Tas2rs, that respond to tea catechins. Among representative tea catechins, avoidance behavior of mice to (-)-epicatechin gallate (ECg) was the strongest, followed by (-)-epigallocatechin gallate (EGCg). Therefore, we measured ECg response using Tas2rs-expressing cells. Among the 35 members of Tas2r family, Tas2r108, 110, 113, 125, and 144 responded to ECg. Among these receptors, Tas2r113 and 125 also responded to EGCg. Because the response profiles of Tas2r125 were consistent with the results of the behavior assays, it was considered that Tas2r125 functions as the main receptor for detecting bitterness of tea catechins in the oral cavity. To determine the involvement of Tas2rs in the physiological action of catechins, mRNA expression of 5 Tas2rs was investigated in various tissues. Because mRNA expression of Tas2r108 was observed in some tissues including the gastrointestinal tract, it may be envisaged that Tas2r108 plays a part in exerting the physiological action of ECg. Tas2r125 expression was not observed in any of the tested tissues except the circumvallate papillae. Therefore, Tas2r125 was considered to mainly function in the events of catechin reception in the oral cavity.

Memory Function in Feeding Habit Transformation of Mandarin Fish (Siniperca chuatsi)

Mandarin fish refuse dead prey fish or artificial diets and can be trained to transform their inborn feeding habit. To investigate the effect of memory on feeding habit transformation, we compared the reaction time to dead prey fish and the success rate of feeding habit transformation to dead prey fish with training of mandarin fish in the 1st experimental group (trained once) and the 2nd experimental group (trained twice). The mandarin fish in the 2nd group had higher success rate of feeding habit transformation (100%) than those in the 1st group (67%), and shorter reaction time to dead prey fish (<1 s) than those in the 1st group (>1 s). Gene expression of cAMP responsive element binding protein I (Creb I), brain-derived neurotrophic factor (Bdnf), CCAAT enhancer binding protein delta (C/EBPD), fos-related antigen 2 (Fra2), and proto-oncogenes c-fos (c-fos) involved in long-term memory formation were significantly increased in the 2nd group after repeated training, and taste 1 receptor member 1 (T1R1), involved in feeding habit formation, was significantly increased in brains of the 2nd group after repeated training. DNA methylation levels at five candidate CpG (cytosine–guanine) sites contained in the predicted CpG island in the 5′-flanking region of T1R1 were significantly decreased in brains of the 2nd group compared with that of the 1st group. These results indicated that the repeated training can improve the feeding habit transformation through the memory formation of accepting dead prey fish. DNA methylation of the T1R1 might be a regulatory factor for feeding habit transformation from live prey fish to dead prey fish in mandarin fish.

Amino acid sensing in hypothalamic tanycytes via umami taste receptors

Objective

Hypothalamic tanycytes are glial cells that line the wall of the third ventricle and contact the cerebrospinal fluid (CSF). While they are known to detect glucose in the CSF we now show that tanycytes also detect amino acids, important nutrients that signal satiety.

Methods

Ca²⁺ imaging and ATP biosensing were used to detect tanycyte responses to l-amino acids. The downstream pathway of the responses was determined using ATP receptor antagonists and channel blockers. The receptors were characterized using mice lacking the Tas1r1 gene, as well as an mGluR4 receptor antagonist.

Results

Amino acids such as Arg, Lys, and Ala evoke Ca²⁺ signals in tanycytes and evoke the release of ATP via pannexin 1 and CalHM1, which amplifies the signal via a P2 receptor dependent mechanism. Tanycytes from mice lacking the Tas1r1 gene had diminished responses to lysine and arginine but not alanine. Antagonists of mGluR4 greatly reduced the responses to alanine and lysine.

Conclusion

Two receptors previously implicated in taste cells, the Tas1r1/Tas1r3 heterodimer and mGluR4, contribute to the detection of a range of amino acids by tanycytes in CSF.

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Hypothalamic tanycytes can detect amino acids in cerebrospinal fluid.

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The mechanism is taste receptor-dependent.

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Tas1r1/Tas1r3 mediates responses to l-arginine and l-lysine.

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mGluR4 mediates responses to l-alanine and partially those of l-lysine.

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ATP release from tanycytes evoked by amino acids reaches into the arcuate nucleus.

P2X receptors up-regulate the cell-surface expression of the neuronal glycine transporter GlyT2

Glycinergic inhibitory neurons of the spinal dorsal horn exert critical control over the conduction of nociceptive signals to higher brain areas. The neuronal glycine transporter 2 (GlyT2) is involved in the recycling of synaptic glycine from the inhibitory synaptic cleft and its activity modulates intra and extracellular glycine concentrations. In this report we show that the stimulation of P2X purinergic receptors with βγ-methylene adenosine 5'-triphosphate induces the up-regulation of GlyT2 transport activity by increasing total and plasma membrane expression and reducing transporter ubiquitination. We identified the receptor subtypes involved by combining pharmacological approaches, siRNA-mediated protein knockdown, and dorsal root ganglion cell enrichment in brainstem and spinal cord primary cultures. Up-regulation of GlyT2 required the combined stimulation of homomeric P2X₃ and P2X₂ receptors or heteromeric P2X_2/3 receptors. We measured the spontaneous glycinergic currents, glycine release and GlyT2 uptake concurrently in response to P2X receptor agonists, and showed that the impact of P2X3 receptor activation on glycinergic neurotransmission involves the modulation of GlyT2 expression or activity. The recognized pro-nociceptive action of P2X₃ receptors suggests that the fine-tuning of GlyT2 activity may have consequences in nociceptive signal conduction.

Nutrient Sensing: Another Chemosensitivity of the Olfactory System

Olfaction is a major sensory modality involved in real time perception of the chemical composition of the external environment. Olfaction favors anticipation and rapid adaptation of behavioral responses necessary for animal survival. Furthermore, recent studies have demonstrated that there is a direct action of metabolic peptides on the olfactory network. Orexigenic peptides such as ghrelin and orexin increase olfactory sensitivity, which in turn, is decreased by anorexigenic hormones such as insulin and leptin. In addition to peptides, nutrients can play a key role on neuronal activity. Very little is known about nutrient sensing in olfactory areas. Nutrients, such as carbohydrates, amino acids, and lipids, could play a key role in modulating olfactory sensitivity to adjust feeding behavior according to metabolic need. Here we summarize recent findings on nutrient-sensing neurons in olfactory areas and delineate the limits of our knowledge on this topic. The present review opens new lines of investigations on the relationship between olfaction and food intake, which could contribute to determining the etiology of metabolic disorders.

The Taste of Caffeine

Many people avidly consume foods and drinks containing caffeine, despite its bitter taste. Here, we review what is known about caffeine as a bitter taste stimulus. Topics include caffeine's action on the canonical bitter taste receptor pathway and caffeine's action on noncanonical receptor-dependent and -independent pathways in taste cells. Two conclusions are that (1) caffeine is a poor prototypical bitter taste stimulus because it acts on bitter taste receptor-independent pathways, and (2) caffeinated products most likely stimulate "taste" receptors in nongustatory cells. This review is relevant for taste researchers, manufacturers of caffeinated products, and caffeine consumers.

5-HT3A -driven green fluorescent protein delineates gustatory fibers innervating sour-responsive taste cells: A labeled line for sour taste?

Taste buds contain multiple cell types with each type expressing receptors and transduction components for a subset of taste qualities. The sour sensing cells, Type III cells, release serotonin (5-HT) in response to the presence of sour (acidic) tastants and this released 5-HT activates 5-HT₃ receptors on the gustatory nerves. We show here, using 5-HT_3A GFP mice, that 5-HT₃ -expressing nerve fibers preferentially contact and receive synaptic contact from Type III taste cells. Further, these 5-HT₃ -expressing nerve fibers terminate in a restricted central-lateral portion of the nucleus of the solitary tract (nTS)-the same area that shows increased c-Fos expression upon presentation of a sour tastant (30 mM citric acid). This acid stimulation also evokes c-Fos in the laterally adjacent mediodorsal spinal trigeminal nucleus (DMSp5), but this trigeminal activation is not associated with the presence of 5-HT₃ -expressing nerve fibers as it is in the nTS. Rather, the neuronal activation in the trigeminal complex likely is attributable to direct depolarization of acid-sensitive trigeminal nerve fibers, for example, polymodal nociceptors, rather than through taste buds. Taken together, these findings suggest that transmission of sour taste information involves communication between Type III taste cells and 5-HT₃ -expressing afferent nerve fibers that project to a restricted portion of the nTS consistent with a crude mapping of taste quality information in the primary gustatory nucleus.

A large increase of sour taste receptor cells in Skn-1-deficient mice does not alter the number of their sour taste signal-transmitting gustatory neurons

The connections between taste receptor cells (TRCs) and innervating gustatory neurons are formed in a mutually dependent manner during development. To investigate whether a change in the ratio of cell types that compose taste buds influences the number of innervating gustatory neurons, we analyzed the proportion of gustatory neurons that transmit sour taste signals in adult Skn-1a^-/- mice in which the number of sour TRCs is greatly increased. We generated polycystic kidney disease 1 like 3-wheat germ agglutinin (pkd1l3-WGA)/Skn-1a^+/+ and pkd1l3-WGA/Skn-1a^-/- mice by crossing Skn-1a^-/- mice and pkd1l3-WGA transgenic mice, in which neural pathways of sour taste signals can be visualized. The number of WGA-positive cells in the circumvallate papillae is 3-fold higher in taste buds of pkd1l3-WGA/Skn-1a^-/- mice relative to pkd1l3-WGA/Skn-1a^+/+ mice. Intriguingly, the ratio of WGA-positive neurons to P2X₂-expressing gustatory neurons in nodose/petrosal ganglia was similar between pkd1l3-WGA/Skn-1a^+/+ and pkd1l3-WGA/Skn-1a^-/- mice. In conclusion, an alteration in the ratio of cell types that compose taste buds does not influence the number of gustatory neurons that transmit sour taste signals.

Olfactory Receptors in Non-Chemosensory Organs: The Nervous System in Health and Disease

Olfactory receptors (ORs) and down-stream functional signaling molecules adenylyl cyclase 3 (AC3), olfactory G protein α subunit (Gαolf), OR transporters receptor transporter proteins 1 and 2 (RTP1 and RTP2), receptor expression enhancing protein 1 (REEP1), and UDP-glucuronosyltransferases (UGTs) are expressed in neurons of the human and murine central nervous system (CNS). In vitro studies have shown that these receptors react to external stimuli and therefore are equipped to be functional. However, ORs are not directly related to the detection of odors. Several molecules delivered from the blood, cerebrospinal fluid, neighboring local neurons and glial cells, distant cells through the extracellular space, and the cells’ own self-regulating internal homeostasis can be postulated as possible ligands. Moreover, a single neuron outside the olfactory epithelium expresses more than one receptor, and the mechanism of transcriptional regulation may be different in olfactory epithelia and brain neurons. OR gene expression is altered in several neurodegenerative diseases including Parkinson’s disease (PD), Alzheimer’s disease (AD), progressive supranuclear palsy (PSP) and sporadic Creutzfeldt-Jakob disease (sCJD) subtypes MM1 and VV2 with disease-, region- and subtype-specific patterns. Altered gene expression is also observed in the prefrontal cortex in schizophrenia with a major but not total influence of chlorpromazine treatment. Preliminary parallel observations have also shown the presence of taste receptors (TASRs), mainly of the bitter taste family, in the mammalian brain, whose function is not related to taste. TASRs in brain are also abnormally regulated in neurodegenerative diseases. These seminal observations point to the need for further studies on ORs and TASRs chemoreceptors in the mammalian brain.

The bitter truth about bitter taste receptors: beyond sensing bitter in the oral cavity

The bitter taste receptor (TAS2R)-family of G-protein-coupled receptors has been identified on the tongue as detectors of bitter taste over a decade ago. In the last few years, they have been discovered in an ever growing number of extra-oral tissues, including the airways, the gut, the brain and even the testis. In tissues that contact the exterior, protective functions for TAS2Rs have been proposed, in analogy to their function on the tongue as toxicity detector. However, TAS2Rs have also been found in internal organs, suggesting other roles for these receptors, perhaps involving as yet unidentified endogenous ligands. The current review gives an overview of the different proposed functions for TAS2Rs in tissues other than the oral cavity; from appetite regulation to the treatment of asthma, regulation of gastrointestinal motility and control of airway innate immunity.

Cre-Mediated Recombination in Tas2r131 Cells-A Unique Way to Explore Bitter Taste Receptor Function Inside and Outside of the Taste System

The type 2 taste receptors (Tas2rs) comprise a large family of G protein-coupled receptors that recognize compounds bitter to humans and aversive to vertebrates. Tas2rs are expressed in both gustatory and nongustatory tissues, however, identification and functional analyses of T2R-expressing cells have been difficult in most tissues. To overcome these limitations and to be able to manipulate Tas2r-expressing cells in vivo, we used gene-targeting to generate a Tas2r131-specific Cre knock-in mouse strain. We then employed a binary genetic approach to characterize Cre-mediated recombination in these animals and to investigate Tas2r131 expression during postnatal development. We demonstrate that a Cre-activated fluorescent reporter reliably visualizes Tas2r131-cells in gustatory tissue. We show that the onset of Tas2r131 as well as of α-Gustducin expression is initiated at different developmental stages depending on the type of taste bud. Furthermore, the number of Tas2r131- and α-Gustducin-expressing cells increased during postnatal development. Our results demonstrate that the Tas2r131-expressing cells constitute a subpopulation of α-Gustducin positive cells at all stages. We detected Tas2r131-expressing cells in several nongustatory tissues including lung, trachea, ovary, ganglia, and brain. Thus, the Tas2r131-Cre strain will help to dissect the functional role of Tas2r131 cells in both gustatory and nongustatory tissues in the future.