Progress and renewal in gustation: new insights into taste bud development

Leveraging Dental Stem Cells for Oral Health during Pregnancy: A Concise Review

Pregnancy induces significant changes in oral health because of hormonal fluctuations, making it a crucial period for preventive measures. Dental stem cells (DSCs), particularly those derived from the dental pulp and periodontal ligaments, offer promising avenues for regenerative therapies and, possibly, preventive interventions. While the use of DSCs already includes various applications in regenerative dentistry in the general population, their use during pregnancy requires careful consideration. This review explores recent advancements, challenges, and prospects in using DSCs to address oral health issues, possibly during pregnancy. Critical aspects of the responsible use of DSCs in pregnant women are discussed, including safety, ethical issues, regulatory frameworks, and the need for interdisciplinary collaborations. We aimed to provide a comprehensive understanding of leveraging DSCs to improve maternal oral health.

Regulatory role of m6A epitranscriptomic modifications in normal development and congenital malformations during embryogenesis

The discovery of N6-methyladenosine (m6A) methylation and its role in translation has led to the emergence of a new field of research. Despite accumulating evidence suggesting that m6A methylation is essential for the pathogenesis of cancers and aging diseases by influencing RNA stability, localization, transformation, and translation efficiency, its role in normal and abnormal embryonic development remains unclear. An increasing number of studies are addressing the development of the nervous and gonadal systems during embryonic development, but only few are assessing that of the immune, hematopoietic, urinary, and respiratory systems. Additionally, these studies are limited by the requirement for reliable embryonic animal models and the difficulty in collecting tissue samples of fetuses during development. Multiple studies on the function of m6A methylation have used suitable cell lines to mimic the complex biological processes of fetal development or the early postnatal phase; hence, the research is still in the primary stage. Herein, we discuss current advances in the extensive biological functions of m6A methylation in the development and maldevelopment of embryos/fetuses and conclude that m6A modification occurs extensively during fetal development. Aberrant expression of m6A regulators is probably correlated with single or multiple defects in organogenesis during the intrauterine life. This comprehensive review will enhance our understanding of the pivotal role of m6A modifications involved in fetal development and examine future research directions in embryogenesis.

A quantitative study of the development of taste pores in mice

OBJECTIVES

This study determined the early development of taste buds by observing the changes in the three-dimensional structures of taste pores and microvilli in the circumvallate papillae (CVP) of mice, from pre- and postnatal stages to the adult stages.

METHODS

Fragments of mouse CVP tissue were collected on embryonic day (E) 18 and postnatal days (P) 0, 3, 6, 7, 14, 21, 28, and 56. The surfaces of the tissue fragments located pore apertures via scanning electron microscopy, and the sizes of the CVP and maximum diameters of the pores were estimated from the recorded images. Likewise, changes in the structures of the epithelium around the pore aperture and microvilli protruding from the pores were examined.

RESULTS

The size of the CVP exhibited a linear increase with age from E18 to P56. The epithelium around the pore aperture demonstrated changes to form microridges, indicating a characteristic pattern during CVP development. The size of the pore aperture also increased with age from E18 to P56. Furthermore, an increase in the number of pores with protruding microvilli was observed at the base of the epithelial trench. A significant positive correlation was observed between the maximum diameter of the pore and the size of the CVP.

CONCLUSIONS

The expansion in the lateral view of the CVP was associated with the developmental stage from E18 to P56, suggesting that the growth of the CVP leads to the opening and enlargement of the taste pores with microvillus projections during these stages.

The effect of maternal period nutritional status on oro-sensorial fat perception and taste preference in rats

CD36 and GPR120 play an important role in the perception and preference for fat-rich food consumption. We aimed to investigate the relationship between oro-gustatory perception of lipids, fatty taste preference, and maternal (Gestation + Lactation)-maturation period nutrition status in offspring Sprague-Dawley rats. In our study, mother rats were fed with control (C) or high-fat diets (HFD) during gestation (21 days) and lactation (21 days) periods. After weaning, the offspring were fed with control (C) or high-fat diets (HFD) during the maturation (120 days) period. Daily calorie intake and weekly body weight measurements were monitored. Two-bottle preference (TBPT) and licking tests measured the fat perceptions and preferences. Plasma levels of insulin, leptin, glucose, and triglyceride were measured. The protein and mRNA expressions of CD36 and GPR120 in the circumvallate papillae (CVP) were determined. The 48 h TBPT results revealed that maternal HFD-exposed offspring rats significantly preferred 2% rapeseed oil solution regardless of the type of maturation diet. According to the licking test, C/C group (C diet exposed group in maternal and maturation periods) offspring licked 0.1% oleic acid-containing water more than C/HFD (C diet exposed in maternal period and HFD exposed group in maturation period) and HFD/HFD group. (HFD exposed group in maternal and maturation periods) groups. Plasma insulin and leptin concentrations significantly increased in HFD/HFD groups compared to C/C group. CD36 protein expressions were significantly lower in HFD/HFD than C/HFD and HFD/C groups. GPR120 and GNAT3 mRNA expressions in HFD/C group were significantly higher than in C/HFD group. Our results suggest that HFD exposure during maternal and maturation period may play a role in fat perception/preference through oral lipid sensors.

Vimentin Localization in the Zebrafish Oral Cavity: A Potential Role in Taste Buds Regeneration

The morphology of the oral cavity of fish is related to their feeding habits. In this context, taste buds are studied for their ability to catch chemical stimuli and their cell renewal capacity. Vimentin RV202 is a protein employed as a marker for mesenchymal cells that can differentiate along different lineages and to self-renew, while Calretinin N-18 is employed as a marker of sensory cells, and ubiquitin is a protein crucial for guiding the fate of stem cells throughout development. In this study, a surface morphology investigation and an immunohistochemical analysis have been conducted. The results of the present study reveal, for the first time, the presence of Vimentin RV202 in a taste bud cell population of zebrafish. Some taste bud cells are just Vimentin RV202-immunoreactive, while in other cells Vimentin RV202 and Calretinin N-18 colocalize. Some taste buds are just reactive to Calretinin N-18. Vimentin RV202-immunoreactive cells have been observed in the connective layer and in the basal portion of the taste buds. The immunoreactivity of ubiquitin was restricted to sensory cells. Further studies are needed to elucidate the role of Vimentin RV202 in the maturation of taste bud cells, its potential involvement in the regeneration of these chemosensory organs, and its eventual synergic work with ubiquitin.

Sweet Taste Receptors and Associated Sweet Peptides: Insights into Structure and Function

Long-term consumption of a high-sugar diet may contribute to the pathogenesis of several chronic diseases, such as obesity and type 2 diabetes. Sweet peptides derived from a wide range of food sources can enhance sweet taste without compromising the sensory properties. Therefore, the research and application of sweet peptides are promising strategies for reducing sugar consumption. This work first outlined the necessity for global sugar reduction, followed by the introduction of sweet taste receptors and their associated transduction mechanisms. Subsequently, recent research progress in sweet peptides from different protein sources was summarized. Furthermore, the main methods for the preparation and evaluation of sweet peptides were presented. In addition, the current challenges and potential applications are also discussed. Sweet peptides can stimulate sweetness perception by binding sweet taste receptors T1R2 and T1R3 in taste buds, which is an effective strategy for reducing sugar consumption. At present, sweet peptides are mainly prepared artificially by synthesis, hydrolysis, microbial fermentation, and bioengineering strategies. Furthermore, sensory evaluation, electronic tongues, and cell models have been used to assess the sweet taste intensity. The present review can provide a theoretical reference for reducing sugar consumption with the aid of sweet peptides in the food industry.

Taste papilla cell differentiation requires the regulation of secretory protein production by ALK3-BMP signaling in the tongue mesenchyme

Taste papillae are specialized organs, each of which comprises an epithelial wall hosting taste buds and a core of mesenchymal tissue. In the present study, we report that during early taste papilla development in mouse embryos, bone morphogenetic protein (BMP) signaling mediated by type 1 receptor ALK3 in the tongue mesenchyme is required for epithelial Wnt/β-catenin activity and taste papilla differentiation. Mesenchyme-specific knockout (cKO) of Alk3 using Wnt1-Cre and Sox10-Cre resulted in an absence of taste papillae at E12.0. Biochemical and cell differentiation analyses demonstrated that mesenchymal ALK3-BMP signaling governed the production of previously unappreciated secretory proteins, i.e. it suppressed those that inhibit and facilitated those that promote taste papilla differentiation. Bulk RNA-sequencing analysis revealed many more differentially expressed genes (DEGs) in the tongue epithelium than in the mesenchyme in Alk3 cKO versus control. Moreover, we detected downregulated epithelial Wnt/β-catenin signaling and found that taste papilla development in the Alk3 cKO was rescued by the GSK3β inhibitor LiCl, but not by Wnt3a. Our findings demonstrate for the first time the requirement of tongue mesenchyme in taste papilla cell differentiation.

Histological and molecular response of oral cavity tissues to Covid-19

The world is still dealing with Covid-19 waves, and maintaining good oral health has systemic effects on overall health. This review's objective is to identify the main oral manifestations of this illness, its effects on oral tissues at their histological bases, their molecular cell mechanisms, and the relationship issues between Covid-19 outcomes and oral health conditions. The main sources of the review are research articles published from 2000 to 2023. The main used terms in search were Covid-19 oral manifestations, Corona virus and Taste, or Olfaction, Covid and periodontitis, or Oral cavity. the angiotensin-converting enzyme II receptor (ACE2), which serves as a cellular entry point for viral entry into the cell to cause Covid-19 infection, is the target of corona virus attacks on human cells. The destruction of keratinocytes and oral fibroblasts, which is an indication of the virus's direct impact on oral tissues, results in inflammatory reactions in the salivary glands, tongue, and gingiva, which may explain both the loss of taste and the mouth ulceration. Additionally, there is a significant correlation between Covid-19 outcome and periodontitis. This results from the connection between hyperinflammation and poor oral hygiene.

Taste arbor structural variability analyzed across taste regions

Taste ganglion neurons are functionally and molecularly diverse, but until recently morphological diversity was completely unexplored. Specifically, taste arbors (the portion of the neuron within the taste bud) vary in structure, but the reason for this variability is unclear. Here, we analyzed structural variability in taste arbors to determine which factors determine their morphological diversity. To characterize arbor morphology and its relationship to taste bud cells capable of transducing taste stimuli (taste-transducing cell) number and type, we utilized sparse cell genetic labeling of taste ganglion neurons in combination with whole-mount immunohistochemistry. Reconstruction of 151 taste arbors revealed variation in arbor size, complexity, and symmetry. Overall, taste arbors exist on a continuum of complexity, cannot be categorized into discrete morphological groups, and do not have stereotyped endings. Arbor size/complexity was not related to the size of the taste bud in which it was located or the type of taste-transducing cell contacted (membranes within 180 nm). Instead, arbors could be broadly categorized into three groups: large asymmetrical arbors contacting many taste-transducing cells, small symmetrical arbors contacting one or two taste-transducing cells, and unbranched arbors. Neurons with multiple arbors had arbors in more than one of these categories, indicating that this variability is not an intrinsic feature of neuron type. Instead, we speculate that arbor structure is determined primarily by nerve fiber remodeling in response to cell turnover and that large asymmetrical arbors represent a particularly plastic state.

Early Steps towards Hearing: Placodes and Sensory Development

Sensorineural hearing loss is the most prevalent sensory deficit in humans. Most cases of hearing loss are due to the degeneration of key structures of the sensory pathway in the cochlea, such as the sensory hair cells, the primary auditory neurons, and their synaptic connection to the hair cells. Different cell-based strategies to replace damaged inner ear neurosensory tissue aiming at the restoration of regeneration or functional recovery are currently the subject of intensive research. Most of these cell-based treatment approaches require experimental in vitro models that rely on a fine understanding of the earliest morphogenetic steps that underlie the in vivo development of the inner ear since its initial induction from a common otic-epibranchial territory. This knowledge will be applied to various proposed experimental cell replacement strategies to either address the feasibility or identify novel therapeutic options for sensorineural hearing loss. In this review, we describe how ear and epibranchial placode development can be recapitulated by focusing on the cellular transformations that occur as the inner ear is converted from a thickening of the surface ectoderm next to the hindbrain known as the otic placode to an otocyst embedded in the head mesenchyme. Finally, we will highlight otic and epibranchial placode development and morphogenetic events towards progenitors of the inner ear and their neurosensory cell derivatives.

Two-generation exposure to a high-fat diet induces the change of salty taste preference in rats

High-fat diet (HFD) leads to multiple complications, including taste alteration. This study observed the effect of a two-generation exposure to an HFD on the peripheral taste system in offspring. Ten pregnant Wistar rats were assigned a standard diet (SD) (n = 5) or HFD (n = 5) from day 7 of pregnancy through the lactation. Thirty-six male and female 3-week-old offspring were measured for body weight and blood glucose level, and the circumvallate papillae were collected. The other twenty-four 3-week-old offspring were weaned on the same diet as their mothers and raised individually. The taste preference behaviors were studied using the two-bottle taste preference test and analyzed five basic tastes (sweet, bitter, umami, sour, and salty). The expressions of epithelial sodium channel alpha subunit (ENaCα) and angiotensin II receptor type 1 (AT1) in the circumvallate papilla were analyzed by immunohistochemical staining and reverse transcription-quantitative polymerase chain reaction (RT-qPCR). We found increased body weight and salty taste preference of offspring from the HFD group in both sexes. Correspondingly, the AT1 level of the taste bud cells significantly increased in 3-week-old female offspring from the HFD group. An increase in AT1 levels may be a risk factor for changes in salty taste preference.

Taste papilla cell differentiation requires tongue mesenchyme via ALK3-BMP signaling to regulate the production of secretory proteins

Taste papillae are specialized organs each of which is comprised of an epithelial wall hosting taste buds and a core of mesenchymal tissue. In the present study, we report that during the early stages of embryonic development, bone morphogenetic protein (BMP) signaling mediated by type 1 receptor ALK3 in the tongue mesenchyme is required for the epithelial Wnt/β-catenin activity and taste papilla cell differentiation. Mesenchyme-specific knockout ( cKO ) of Alk3 using Wnt1-Cre and Sox10-Cre resulted in an absence of taste papillae at E12.0. Biochemical and cell differentiation analyses demonstrated that mesenchymal ALK3-BMP signaling governs the production of previously unappreciated secretory proteins, i.e., suppresses those that inhibiting and facilitates those promoting taste cell differentiation. Bulk RNA-Sequencing analysis revealed many more differentially expressed genes (DEGs) in the tongue epithelium than in the mesenchyme in Alk3 cKO vs control. Moreover, we detected a down-regulated epithelial Wnt/β-catenin signaling, and taste papilla development in the Alk3 cKO was rescued by GSK3β inhibitor LiCl, but not Wnt3a. Our findings demonstrate for the first time the requirement of tongue mesenchyme in taste papilla cell differentiation.

Summary statement

This is the first set of data to implicate the requirement of tongue mesenchyme in taste papilla cell differentiation.

Actomyosin contractility in olfactory placode neurons opens the skin epithelium to form the zebrafish nostril

Despite their barrier function, epithelia can locally lose their integrity to create physiological openings during morphogenesis. The mechanisms driving the formation of these epithelial breaks are only starting to be investigated. Here, we study the formation of the zebrafish nostril (the olfactory orifice), which opens in the skin epithelium to expose the olfactory neurons to external odorant cues. Combining live imaging, drug treatments, laser ablation, and tissue-specific functional perturbations, we characterize a mechanical interplay between olfactory placode neurons and the skin, which plays a crucial role in the formation of the orifice: the neurons pull on the overlying skin cells in an actomyosin-dependent manner which, in combination with a local reorganization of the skin epithelium, triggers the opening of the orifice. This work identifies an original mechanism to break an epithelial sheet, in which an adjacent group of cells mechanically assists the epithelium to induce its local rupture.

Transcriptome analyses reveal differentially expressed genes associated with development of the palatal organ in bighead carp (Hypophthalmichthys nobilis)

The palatal organ is a filter-feeding related organ and occupies a considerable proportion of the head of bighead carp (Hypophthalmichthys nobilis), a large cyprinid fish intensive aquaculture in Asia. In this study, we performed RNA-seq of the palatal organ during growth periods of two (M2), six (M6) and 15 (M15) months of age after hatching. The numbers of differentially expressed genes (DEGs) were 1384, 481 and 1837 for M2 VS M6, M6 VS M15 and M2 VS M15 respectively. The following signaling pathways of energy metabolism and cytoskeleton function were enriched, including ECM-receptor interaction, Cardiac muscle contraction, Steroid biosynthesis and PPAR signaling pathway. Several members of collagen family (col1a1, col2a1, col6a2, col6a3, col9a2), Laminin gamma 1 (lamc1), integrin alpha 1 (itga1), Fatty acid binding protein 2 (fads2) and lipoprotein lipase (lpl), and Protein tyrosine kinase 7 (Ptk7) are candidate genes for growth and development of basic tissues of the palatal organ. Furthermore, taste-related genes such as fgfrl1, fgf8a, fsta and notch1a were also identified, which may be involved in the development of taste buds of the palatal organ. The transcriptome data obtained in this study provide insights into the understanding functions and development mechanisms of palatal organ, and potential candidate genes that may be related to the genetic modulation of head size of bighead carp.

Ascl1-expressing cell differentiation in initially developed taste buds and taste organoids

Mammalian taste bud cells are composed of several distinct cell types and differentiated from surrounding tongue epithelial cells. However, the detailed mechanisms underlying their differentiation have yet to be elucidated. In the present study, we examined an Ascl1-expressing cell lineage using circumvallate papillae (CVP) of newborn mice and taste organoids (three-dimensional self-organized tissue cultures), which allow studying the differentiation of taste bud cells in fine detail ex vivo. Using lineage-tracing analysis, we observed that Ascl1 lineage cells expressed type II and III taste cell markers both CVP of newborn mice and taste organoids. However, the coexpression rate in type II cells was lower than that in type III cells. Furthermore, we found that the generation of the cells which express type II and III cell markers was suppressed in taste organoids lacking Ascl1-expressing cells. These findings suggest that Ascl1-expressing precursor cells can differentiate into both type III and a subset of type II taste cells.

Role of Transient Receptor Potential Vanilloid 1 in Sonic Hedgehog-Dependent Taste Bud Differentiation

Taste bud cell differentiation is extremely important for taste sensation. Immature taste bud cells cannot function during taste perception transmission to the nerve. In this study, we investigated whether hedgehog signaling affected taste bud cell differentiation and whether transient receptor potential vanilloid 1 (TRPV1) played a key role in dry mouth. The induction of dry mouth due to salivary gland resection (SGR) was confirmed on the basis of reduced salivation and disrupted fungiform papillae. The expression of keratin 8 (K8) of taste bud cells, neurofilament (NF), sonic hedgehog (Shh), and glioma-associated oncogene homolog 1 (Gli1) around taste bud cells was downregulated; however, the expression of TRPV1, P2X purinoceptor 3 (P2X3), and hematopoietic stem cell factor (c-Kit) was upregulated at the NF ends in the dry mouth group. To investigate the effect of TRPV1 defect on dry mouth, we induced dry mouth in the TRPV-/- group. The K8, NF, and P2X3 expression patterns were the same in the TRPV1 wild-type and TRPV1-/- dry mouth groups. However, Shh and c-Kit expression decreased regardless of dry mouth in the case of TRPV1 deficiency. These results indicated that TRPV1 positively regulated proliferation during taste bud cell injury by blocking the Shh/Gli1 pathway. In addition, not only cell proliferation but also differentiation of taste bud cells could not be regulated under TRPV1-deficiency conditions. Thus, TRPV1 positively regulates taste bud cell innervation and differentiation; this finding could be valuable in the clinical treatment of dry mouth-related taste dysfunction.

Inter-individual characteristics on basic taste recognition thresholds in a college-aged cohort: potential predictive factors

Studying nutritional status from the perspective of taste sensitivity, rather than only dietary patterns, may provide new insights into the role of taste receptor signaling in the development of metabolic-associated diseases. In this cross-sectional study, we investigated the possible influence of sociodemographic (sex and smoking habit) and clinical variables (dental cavities, missing teeth, sinusitis, rhinitis, body mass index and metabolic high prevalence family antecedent diseases) on tastant (sucrose, monosodium glutamate, sodium chloride, citric acid, quinine, sinigrin, phenylthiocarbamide) recognition thresholds (RTs) in a college-aged cohort (n = 397). Predictive models for the tastant RTs were generated and a higher sucrose RT was found in females than in males, while sinusitis and rhinitis explained sucrose and sodium chloride RTs. Smoking habit was not an important predictive factor of taste sensitivity, although its long-term influence on RTs remains unclear. Additionally, a positive correlation was found between all the tastant RTs studied. Although results did not show a clear pattern, the statistical approach employed should prove useful in future studies of predictors of taste sensitivity.

Changes of the biophysical properties of voltage-gated Na+ currents during maturation of the sodium-taste cells in rat fungiform papillae

Taste cells are a heterogeneous population of sensory receptors that undergo continuous turnover. Different chemo-sensitive cell lines rely on action potentials to release the neurotransmitter onto nerve endings. The electrical excitability is due to the presence of a tetrodotoxin-sensitive, voltage-gated sodium current (I_Na ) similar to that found in neurons. Since the biophysical properties of neuronal I_Na change during development, we wondered whether the same also occurred in taste cells. Here, we used the patch-clamp recording technique to study I_Na in salt-sensing cells (sodium cells) of rat fungiform papillae. We identified these cells by exploiting the known blocking effect of amiloride on ENaC, the sodium (salt) receptor. Based on the amplitude of I_Na , which is known to increase during development, we subdivided sodium cells into two groups: cells with small sodium current (SSC cells; I_Na  < 1 nA) and cells with large sodium current (LSC cells; I_Na  > 1 nA). We found that: the voltage dependence of activation and inactivation significantly differed between these subsets; a slowly inactivating sodium current was more prominent in LSC cells; membrane capacitance in SSC cells was larger than in LSC cells. mRNA expression analysis of the α-subunits of voltage-gated sodium channels in fungiform taste buds supported the functional data. Lucifer Yellow labelling of recorded cells revealed that our electrophysiological criterion for distinguishing two broad groups of taste cells was in good agreement with morphological observations for cell maturity. Thus, all these findings are consistent with developmental changes in the voltage-dependent properties of sodium-taste cells. KEY POINTS: Taste cells are sensory receptors that undergo continuous turnover while they detect food chemicals and communicate with afferent nerve fibres. The voltage-gated sodium current (I_Na ) is a key ion current for generating action potentials in fully differentiated and chemo-sensitive taste cells, which use electrical signalling to release neurotransmitters. Here we show that, during the maturation of rat taste cells involved in salt detection (sodium cells), the biophysical properties of I_Na , such as voltage dependence of activation and inactivation, change significantly. Our results help reveal how taste cells gain electrical excitability during turnover, a property critical to their operation as chemical detectors that relay sensory information to nerve fibres.

Sweet Taste Signaling: The Core Pathways and Regulatory Mechanisms

Sweet taste, a proxy for sugar-derived calories, is an important driver of food intake, and animals have evolved robust molecular and cellular machinery for sweet taste signaling. The overconsumption of sugar-derived calories is a major driver of obesity and other metabolic diseases. A fine-grained appreciation of the dynamic regulation of sweet taste signaling mechanisms will be required for designing novel noncaloric sweeteners with better hedonic and metabolic profiles and improved consumer acceptance. Sweet taste receptor cells express at least two signaling pathways, one mediated by a heterodimeric G-protein coupled receptor encoded by taste 1 receptor members 2 and 3 (TAS1R2 + TAS1R3) genes and another by glucose transporters and the ATP-gated potassium (K_ATP) channel. Despite these important discoveries, we do not fully understand the mechanisms regulating sweet taste signaling. We will introduce the core components of the above sweet taste signaling pathways and the rationale for having multiple pathways for detecting sweet tastants. We will then highlight the roles of key regulators of the sweet taste signaling pathways, including downstream signal transduction pathway components expressed in sweet taste receptor cells and hormones and other signaling molecules such as leptin and endocannabinoids.

A Taste Bud Organoid-Based Microelectrode Array Biosensor for Taste Sensing

The biological taste system has the unique ability to detect taste substances. Biomaterials originating from a biological taste system have been recognized as ideal candidates to serve as sensitive elements in the development of taste-based biosensors. In this study, we developed a taste bud organoid-based biosensor for the research of taste sensation. Taste bud organoids prepared from newborn mice were cultured and loaded onto the surface of a 64-channel microelectrode array (MEA) chip to explore the electrophysiological changes upon taste; an MEA chip was used to simultaneously record multiple-neuron firing activities from taste bud organoids under different taste stimuli, which helped to reveal the role of taste buds in taste sensing. The obtained results show that taste cells separated from the taste epithelium grew well into spherical structures under 3D culture conditions. These structures were composed of multiple cells with obvious budding structures. Moreover, the multicellular spheres were seeded on a 64-channel microelectrode array and processed with different taste stimuli. It was indicated that the MEA chip could efficiently monitor the electrophysiological signals from taste bud organoids in response to various taste stimuli. This biosensor provides a new method for the study of taste sensations and taste bud functions.

METTL3-mediated m6A RNA methylation regulates dorsal lingual epithelium homeostasis

The dorsal lingual epithelium, which is composed of taste buds and keratinocytes differentiated from K14⁺ basal cells, discriminates taste compounds and maintains the epithelial barrier. N6-methyladenosine (m⁶A) is the most abundant mRNA modification in eukaryotic cells. How METTL3-mediated m⁶A modification regulates K14⁺ basal cell fate during dorsal lingual epithelium formation and regeneration remains unclear. Here we show knockout of Mettl3 in K14⁺ cells reduced the taste buds and enhanced keratinocytes. Deletion of Mettl3 led to increased basal cell proliferation and decreased cell division in taste buds. Conditional Mettl3 knock-in mice showed little impact on taste buds or keratinization, but displayed increased proliferation of cells around taste buds in a protective manner during post-irradiation recovery. Mechanically, we revealed that the most frequent m⁶A modifications were enriched in Hippo and Wnt signaling, and specific peaks were observed near the stop codons of Lats1 and FZD7. Our study elucidates that METTL3 is essential for taste bud formation and could promote the quantity recovery of taste bud after radiation.

The sense of taste: Development, regeneration, and dysfunction

Gustation or the sense of taste is a primary sense, which functions as a gatekeeper for substances that enter the body. Animals, including humans, ingest foods that contain appetitive taste stimuli, including those that have sweet, moderately salty and umami (glutamate) components, and tend to avoid bitter-tasting items, as many bitter compounds are toxic. Taste is mediated by clusters of heterogeneous taste receptors cells (TRCs) organized as taste buds on the tongue, and these convey taste information from the oral cavity to higher order brain centers via the gustatory sensory neurons of the seventh and ninth cranial ganglia. One remarkable aspect of taste is that taste perception is mostly uninterrupted throughout life yet TRCs within buds are constantly renewed; every 1-2 months all taste cells have been steadily replaced. In the past decades we have learned a substantial amount about the cellular and molecular regulation of taste bud cell renewal, and how taste buds are initially established during embryogenesis. Here I review more recent findings pertaining to taste development and regeneration, as well as discuss potential mechanisms underlying taste dysfunction that often occurs with disease or its treatment. This article is categorized under: Infectious Diseases > Stem Cells and Development Cancer > Stem Cells and Development Neurological Diseases > Stem Cells and Development.

Preparation and application of taste bud organoids in biomedicine towards chemical sensation mechanisms

Taste is one of the most basic and important sensations that is able to monitor the food quality and avoid intake of potential danger materials. Whether as an inevitable symptom of aging or a complication of cancer treatment, taste loss very seriously affects the patient's life quality. Taste bud organoids provide an alternative and convenient approach for the research of taste functions and the underlying mechanisms due to their characteristics of availability, strong maneuverability, and high similarity to the in-vivo taste buds. This review gives a systemic and comprehensive introduction to the preparation and application of taste bud organoids towards chemical sensing mechanisms. For the first, the basic structure and functions of taste buds will be briefly introduced. Then, the currently available approaches for the preparation of taste bud organoids are summarized and discussed, which are mainly divided into two categories, i.e. the stem/progenitor cell-derived approach and the tissue-derived approach. For the next, different applications of taste bud organoids in biomedicine are outlined based on their central roles such as disease modeling, biological sensing, gene regulation, and signal transduction. Finally, the current challenges, future development trends and prospects of research in taste bud organoids are proposed and discussed. This article is protected by copyright. All rights reserved.

Massively collaborative crowdsourced research on COVID19 and the chemical senses: Insights and outcomes

In March 2020, the Global Consortium of Chemosensory Research (GCCR) was founded by chemosensory researchers to address emerging reports of unusual smell and taste dysfunction arising from the SARS-CoV-2 pandemic. Over the next year, the GCCR used a highly collaborative model, along with contemporary Open Science practices, to produce multiple high impact publications on chemosensation and COVID19. This invited manuscript describes the founding of the GCCR, the tools and approaches it used, and a summary of findings to date. These findings are contextualized within a summary of some of the broader insights about chemosensation (smell, taste, and chemesthesis) and COVID19 gained over the last 18 months, including potential mechanisms of loss. Also, it includes a detailed discussion of some current Open Science approaches and practices used by the GCCR to increase transparency, rigor, and reproducibility.

Taste, smell and mouthfeel disturbances in patients with gastrointestinal stromal tumors treated with tyrosine-kinase inhibitors

CONTEXT

Taste, smell, and mouthfeel disturbances are underrated and underreported, but important side effects of anti-cancer medication. These symptoms are associated with a lower quality of life (QoL). The prevalence and the impact of taste, smell, and mouthfeel disturbances on daily life in patients with a gastrointestinal stromal tumor (GIST) are largely unknown.

OBJECTIVES

This exploratory study assessed the prevalence and type of taste, smell, and mouthfeel disturbances and their impact on daily life and QoL in patients with a GIST treated with a tyrosine-kinase inhibitor (TKI).

METHODS

Patients currently treated with TKIs for GIST completed a standardized questionnaire. The questionnaire addressed changes in taste, smell, and mouthfeel and, if changes occurred, impact on daily life and QoL. Statistics are descriptive.

RESULTS

A total of 65 GIST patients on TKI treatment completed the questionnaire. Of these patients, 79%, 12%, and 9% currently used imatinib, sunitinib, and regorafenib respectively. Taste, smell, and mouthfeel disturbances were reported by 25 (38%), 15 (23%), and 36 (55%) patients respectively. Salty and sweet tastes were mostly affected, respectively in 14 and 13 patients. A dry mouth was experienced by 29 (45%) patients. Taste disturbances were more often reported to have impact on daily life and QoL (80% and 60%) than smell (47% and 31%) and mouthfeel disturbances (47% and 30%).

CONCLUSION

Taste, smell, and mouthfeel disturbances are frequent side effects of TKIs in GIST patients. Daily life and QoL are affected in a considerable number of those patients.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NL7827 (2019-06-25).

Reciprocal signals between nerve and epithelium: how do neurons talk with epithelial cells?

Most epithelium tissues continuously undergo self-renewal through proliferation and differentiation of epithelial stem cells (known as homeostasis), within a specialized stem cell niche. In highly innervated epithelium, peripheral nerves compose perineural niche and support stem cell homeostasis by releasing a variety of neurotransmitters, hormones, and growth factors and supplying trophic factors to the stem cells. Emerging evidence has shown that both sensory and motor nerves can regulate the fate of epithelial stem cells, thus influencing epithelium homeostasis. Understanding the mechanism of crosstalk between epithelial stem cells and neurons will reveal the important role of the perineural niche in physiological and pathological conditions. Herein, we review recent discoveries of the perineural niche in epithelium mainly in tissue homeostasis, with a limited touch in wound repair and pathogenesis.

Possible Use of Phytochemicals for Recovery from COVID-19-Induced Anosmia and Ageusia

The year 2020 became the year of the outbreak of coronavirus, SARS-CoV-2, which escalated into a worldwide pandemic and continued into 2021. One of the unique symptoms of the SARS-CoV-2 disease, COVID-19, is the loss of chemical senses, i.e., smell and taste. Smell training is one of the methods used in facilitating recovery of the olfactory sense, and it uses essential oils of lemon, rose, clove, and eucalyptus. These essential oils were not selected based on their chemical constituents. Although scientific studies have shown that they improve recovery, there may be better combinations for facilitating recovery. Many phytochemicals have bioactive properties with anti-inflammatory and anti-viral effects. In this review, we describe the chemical compounds with anti- inflammatory and anti-viral effects, and we list the plants that contain these chemical compounds. We expand the review from terpenes to the less volatile flavonoids in order to propose a combination of essential oils and diets that can be used to develop a new taste training method, as there has been no taste training so far. Finally, we discuss the possible use of these in clinical settings.

Immortalization and Characterization of Rat Lingual Keratinocytes in a High-Calcium and Feeder-Free Culture System Using ROCK Inhibitor Y-27632

Cultured keratinocytes are desirable models for biological and medical studies. However, primary keratinocytes are difficult to maintain, and there has been little research on lingual keratinocyte culture. Here, we investigated the effect of Y-27632, a Rho kinase (ROCK) inhibitor, on the immortalization and characterization of cultured rat lingual keratinocyte (RLKs). Three Y-27632-supplemented media were screened for the cultivation of RLKs isolated from Sprague-Dawley rats. Phalloidin staining and TUNEL assay were applied to visualize cytoskeleton dynamics and cell apoptosis following Y-27632 removal. Label-free proteomics, RT-PCR, calcium imaging, and cytogenetic studies were conducted to characterize the cultured cells. Results showed that RLKs could be conditionally immortalized in a high-calcium medium in the absence of feeder cells, although they did not exhibit normal karyotypes. The removal of Y-27632 from the culture medium led to reversible cytoskeletal reorganization and nuclear enlargement without triggering apoptosis, and a total of 239 differentially expressed proteins were identified by proteomic analysis. Notably, RLKs derived from the non-taste epithelium expressed some molecular markers characteristic of taste bud cells, yet calcium imaging revealed that they rarely responded to tastants. Collectively, we established a high-calcium and feeder-free culture method for the long-term maintenance of RLKs. Our results shed some new light on the immortalization and differentiation of lingual keratinocytes.

Sweet Taste Is Complex: Signaling Cascades and Circuits Involved in Sweet Sensation

Sweetness is the preferred taste of humans and many animals, likely because sugars are a primary source of energy. In many mammals, sweet compounds are sensed in the tongue by the gustatory organ, the taste buds. Here, a group of taste bud cells expresses a canonical sweet taste receptor, whose activation induces Ca²⁺ rise, cell depolarization and ATP release to communicate with afferent gustatory nerves. The discovery of the sweet taste receptor, 20 years ago, was a milestone in the understanding of sweet signal transduction and is described here from a historical perspective. Our review briefly summarizes the major findings of the canonical sweet taste pathway, and then focuses on molecular details, about the related downstream signaling, that are still elusive or have been neglected. In this context, we discuss evidence supporting the existence of an alternative pathway, independent of the sweet taste receptor, to sense sugars and its proposed role in glucose homeostasis. Further, given that sweet taste receptor expression has been reported in many other organs, the physiological role of these extraoral receptors is addressed. Finally, and along these lines, we expand on the multiple direct and indirect effects of sugars on the brain. In summary, the review tries to stimulate a comprehensive understanding of how sweet compounds signal to the brain upon taste bud cells activation, and how this gustatory process is integrated with gastro-intestinal sugar sensing to create a hedonic and metabolic representation of sugars, which finally drives our behavior. Understanding of this is indeed a crucial step in developing new strategies to prevent obesity and associated diseases.

Nkx2-2 expressing taste cells in endoderm-derived taste papillae are committed to the type III lineage

In mammals, multiple cell-signaling pathways and transcription factors regulate development of the embryonic taste system and turnover of taste cells in the adult stage. Using single-cell RNA-Seq of mouse taste cells, we found that the homeobox-containing transcription factor Nkx2-2, a target of the Sonic Hedgehog pathway and a key regulator of the development and regeneration of multiple cell types in the body, is highly expressed in type III taste cells but not in type II or taste stem cells. Using in situ hybridization and immunostaining, we confirmed that Nkx2-2 is expressed specifically in type III taste cells in the endoderm-derived circumvallate and foliate taste papillae but not in the ectoderm-derived fungiform papillae. Lineage tracing revealed that Nkx2-2-expressing cells differentiate into type III, but not type II or type I cells in circumvallate and foliate papillae. Neonatal Nkx2-2-knockout mice did not express key type III taste cell marker genes, while the expression of type II and type I taste cell marker genes were unaffected in these mice. Our findings indicate that Nkx2-2-expressing cells are committed to the type III lineage and that Nkx2-2 may be critical for the development of type III taste cells in the posterior tongue, thus illustrating a key difference in the mechanism of type III cell lineage specification between ectoderm- and endoderm-derived taste fields.

Onset of taste bud cell renewal starts at birth and coincides with a shift in SHH function

Embryonic taste bud primordia are specified as taste placodes on the tongue surface and differentiate into the first taste receptor cells (TRCs) at birth. Throughout adult life, TRCs are continually regenerated from epithelial progenitors. Sonic hedgehog (SHH) signaling regulates TRC development and renewal, repressing taste fate embryonically, but promoting TRC differentiation in adults. Here, using mouse models, we show TRC renewal initiates at birth and coincides with onset of SHHs pro-taste function. Using transcriptional profiling to explore molecular regulators of renewal, we identified Foxa1 and Foxa2 as potential SHH target genes in lingual progenitors at birth and show that SHH overexpression in vivo alters FoxA1 and FoxA2 expression relevant to taste buds. We further bioinformatically identify genes relevant to cell adhesion and cell locomotion likely regulated by FOXA1;FOXA2 and show that expression of these candidates is also altered by forced SHH expression. We present a new model where SHH promotes TRC differentiation by regulating changes in epithelial cell adhesion and migration.

Taste Sensitivity of Elderly People Is Associated with Quality of Life and Inadequate Dietary Intake

Aging has been implicated in the alteration of taste acuity. Diet can affect taste sensitivity. We aimed to investigate the types of tastes altered in elderly Korean people and factors associated with taste alteration in relation to dietary intake and other factors. Elderly participants (≥65 years) and young adults were assessed to determine their recognition thresholds (RT) for sweet, salty, bitter, sour, and umami tastes. Elderly participants were further surveyed for dietary intake and non-nutritional factors. Five taste RTs were correlated with age, but only four taste RTs, except sweetness, differed between the elderly participants and young adults. Inadequate intake of iron, thiamin, folic acid, zinc, and phosphorus among the elderly participants was related to elevated taste RT levels, except for bitter taste. In both correlation and regression analyses, only salty and sour RTs were associated with energy, iron, thiamin, fiber, vitamin C, and riboflavin levels in the elderly participants. The elderly participants’ taste RTs exhibited strong associations with quality of life (QOL) but showed partial relationships with physical activity, number of medicine intakes, social gatherings, and education. Taste sensitivity may decrease with age, which is further influenced by insufficient dietary intake, especially iron and thiamin, and QOL.

Oral Mucosa, Saliva, and COVID-19 Infection in Oral Health Care

The SARS-CoV-2 virus has shaken the globe with an ongoing pandemic of COVID-19 and has set challenges to every corner of the modern health care setting. The oral mucosa and saliva are high risk sites for higher viral loads and dental health care professionals are considered a high risk group. COVID-19-induced oral lesions and loss of taste and smell are common clinical complaints in the dental health care setting. The SARS-CoV-2 virus has been found to cause a wide range of non-specific oral mucosal lesions, but the specific diagnosis of these mucocutaneous lesions as COVID-19 lesions will facilitate the prevention of SARS-CoV-2 in dental health care settings and aid in proper patient management. The reported loss of taste and smell needs further investigation at the receptor level as it will give new insights into SARS-CoV-2 pathogenicity. The high yield of virus in the salivary secretion is a common finding in this infection and ongoing research is focusing on developing saliva as a rapid diagnostic fluid in COVID-19. In this review, we discuss the significance of oral mucosa, saliva and the relevance of the COVID-19 pandemic in dentistry.

The Tryptophan/Kynurenine Pathway: A Novel Cross-Talk between Nutritional Obesity, Bariatric Surgery and Taste of Fat

Diet-induced obesity (DIO) reduces the orosensory perception of lipids in rodents and in some humans. Although bariatric surgery partially corrects this alteration, underlying mechanisms remain poorly understood. To explore whether metabolic changes might explain this fat taste disturbance, plasma metabolome analyses, two-bottle choice tests and fungiform papillae (Fun) counting were performed in vertical sleeve gastrectomized (VSG) mice and sham-operated controls. An exploratory clinic study was also carried out in adult patients undergone a VSG. In mice, we found that (i) the VSG reduces both the plasma neurotoxic signature due to the tryptophan/kynurenine (Trp/Kyn) pathway overactivation and the failure of fat preference found in sham-operated DIO mice, (ii) the activity of Trp/Kyn pathway is negatively correlated to the density of Fun, and (iii) the pharmacological inhibition of the Kyn synthesis mimics in non-operated DIO mice the positive effects of VSG (i.e., decrease of Kyn synthesis, increase of Fun number, improvement of the fat taste perception). In humans, a reduction of the plasma Kyn level is only found in patients displaying a post-surgery improvement of their fat taste sensitivity. Altogether these data provide a plausible metabolic explanation to the degradation of the orosensory lipid perception observed in obesity.

Generation and Culture of Lingual Organoids Derived from Adult Mouse Taste Stem Cells

The sense of taste is mediated by taste buds on the tongue, which are composed of rapidly renewing taste receptor cells (TRCs). This continual turnover is powered by local progenitor cells and renders taste function prone to disruption by a multitude of medical treatments, which in turn severely impacts the quality of life. Thus, studying this process in the context of drug treatment is vital to understanding if and how taste progenitor function and TRC production are affected. Given the ethical concerns and limited availability of human taste tissue, mouse models, which have a taste system similar to humans, are commonly used. Compared to in vivo methods, which are time-consuming, expensive, and not amenable to high throughput studies, murine lingual organoids can enable experiments to be run rapidly with many replicates and fewer mice. Here, previously published protocols have been adapted and a standardized method for generating taste organoids from taste progenitor cells isolated from the circumvallate papilla (CVP) of adult mice is presented. Taste progenitor cells in the CVP express LGR5 and can be isolated via EGFP fluorescence-activated cell sorting (FACS) from mice carrying an Lgr5^{EGFP-IRES-CreERT2} allele. Sorted cells are plated onto a matrix gel-based 3D culture system and cultured for 12 days. Organoids expand for the first 6 days of the culture period via proliferation and then enter a differentiation phase, during which they generate all three taste cell types along with non-taste epithelial cells. Organoids can be harvested upon maturation at day 12 or at any time during the growth process for RNA expression and immunohistochemical analysis. Standardizing culture methods for production of lingual organoids from adult stem cells will improve reproducibility and advance lingual organoids as a powerful drug screening tool in the fight to help patients experiencing taste dysfunction.

Cellular Diversity and Regeneration in Taste Buds

Taste buds are the sensory end organs for gustation, mediating sensations of salty, sour, bitter, sweet and umami as well as other possible modalities, e.g. fat and kokumi. Understanding of the structure and function of these sensory organs has increased greatly in the last decades with advances in ultrastructural methods, molecular genetics, and in vitro models. This review will focus on the cellular constituents of taste buds, and molecular regulation of taste bud cell renewal and differentiation.

The taste of neuroinflammation: Molecular mechanisms linking taste sensing to neuroinflammatory responses

Evidence indicates a critical role of neuroinflammatory response as an underlying pathophysiological process in several central nervous system disorders, including neurodegenerative diseases. However, the molecular mechanisms that trigger neuroinflammatory processes are not fully known. The discovery of bitter taste receptors in regions other than the oral cavity substantially increased research interests on their functional roles in extra-oral tissues. It is now widely accepted that bitter taste receptors, for instance, in the respiratory, intestinal, reproductive and urinary tracts, are crucial not only for sensing poisonous substances, but also, act as immune sentinels, mobilizing defense mechanisms against pathogenic aggression. The relatively recent discovery of bitter taste receptors in the brain has intensified research investigation on the functional implication of cerebral bitter taste receptor expression. Very recent data suggest that responses of bitter taste receptors to neurotoxins and microbial molecules, under normal condition, are necessary to prevent neuroinflammatory reactions. Furthermore, emerging data have revealed that downregulation of key components of the taste receptor signaling cascade leads to increased oxidative stress and inflammasome signaling in neurons that ultimately culminate in neuroinflammation. Nevertheless, the mechanisms that link taste receptor mediated surveillance of the extracellular milieu to neuroinflammatory responses are not completely understood. This review integrates new data on the molecular mechanisms that link bitter taste receptor sensing to neuroinflammatory responses. The role of bitter taste receptor-mediated sensing of toxigenic substances in brain disorders is also discussed. The therapeutic significance of targeting these receptors for potential treatment of neurodegenerative diseases is also highlighted.

LiCl Promotes Recovery of Radiation-Induced Oral Mucositis and Dysgeusia

Oral mucositis and taste dysfunction are frequently complained by patients with head and neck cancer receiving radiotherapy, challenging the clinical outcome of cancer treatment. Recent studies have indicated the protective role of Wnt/β-catenin signaling in radiation-induced oral mucositis (RIOM) and its pivotal role in the development and self-renewal of taste buds. The current study hypothesizes that lithium chloride (LiCl), a potent activator of the Wnt/β-catenin signaling pathway, can promote the postirradiation restoration of oral mucosa integrity and taste function. To validate this hypothesis, we established a RIOM mouse model and evaluated the treatment efficacy of LiCl on oral mucositis and taste dysfunction in comparison with keratinocyte growth factor (KGF), an agent approved by the US Food and Drug Administration for oral mucositis. The results showed that LiCl alleviated the weight loss and tongue ulceration of RIOM mice, promoted proliferation of basal epithelial cells, and inhibited epithelial-mesenchymal transition in tongue mucosa. More important, elevated taste bud renewal and dysgeusia recovery toward sweetness were observed in RIOM mice treated with LiCl as compared to those treated by KGF. Collectively, our data demonstrate that LiCl can mitigate oral mucositis and rescue taste alteration induced by irradiation, and activation of Wnt/β-catenin signaling may represent a promising therapy to improve the quality of life of patients receiving radiotherapy.

A Mechanistic Overview of Taste Bud Maintenance and Impairment in Cancer Therapies

Since the early 20th century, progress in cancer therapies has significantly improved disease prognosis. Nonetheless, cancer treatments are often associated with side effects that can negatively affect patient well-being and disrupt the course of treatment. Among the main side effects, taste impairment is associated with depression, malnutrition, and morbid weight loss. Although relatively common, taste disruption associated with cancer therapies remains poorly understood. Here, we review the current knowledge related to the molecular mechanisms underlying taste maintenance and disruption in the context of cancer therapies.

Whole-Mount Staining, Visualization, and Analysis of Fungiform, Circumvallate, and Palate Taste Buds

Taste buds are collections of taste-transducing cells specialized to detect subsets of chemical stimuli in the oral cavity. These transducing cells communicate with nerve fibers that carry this information to the brain. Because taste-transducing cells continuously die and are replaced throughout adulthood, the taste-bud environment is both complex and dynamic, requiring detailed analyses of its cell types, their locations, and any physical relationships between them. Detailed analyses have been limited by tongue-tissue heterogeneity and density that have significantly reduced antibody permeability. These obstacles require sectioning protocols that result in splitting taste buds across sections so that measurements are only approximated, and cell relationships are lost. To overcome these challenges, the methods described herein involve collecting, imaging, and analyzing whole taste buds and individual terminal arbors from three taste regions: fungiform papillae, circumvallate papillae, and the palate. Collecting whole taste buds reduces bias and technical variability and can be used to report absolute numbers for features including taste-bud volume, total taste-bud innervation, transducing-cell counts, and the morphology of individual terminal arbors. To demonstrate the advantages of this method, this paper provides comparisons of taste bud and innervation volumes between fungiform and circumvallate taste buds using a general taste-bud marker and a label for all taste fibers. A workflow for the use of sparse-cell genetic labeling of taste neurons (with labeled subsets of taste-transducing cells) is also provided. This workflow analyzes the structures of individual taste-nerve arbors, cell type numbers, and the physical relationships between cells using image analysis software. Together, these workflows provide a novel approach for tissue preparation and analysis of both whole taste buds and the complete morphology of their innervating arbors.

Implications of the specific localization of YAP signaling on the epithelial patterning of circumvallate papilla

Circumvallate papilla (CVP) is a distinctively structured with dome-shaped apex, and the surrounding trench which contains over two hundred taste buds on the lateral walls. Although CVP was extensively studied to determine the regulatory mechanisms during organogenesis, it still remains to be elucidated the principle mechanisms of signaling regulations on morphogenesis including taste buds formation. The key role of Yes-associated protein (YAP) in the regulation of organ size and cell proliferation in vertebrates is well understood, but little is known about the role of this signaling pathway in CVP development. We aimed to determine the putative roles of YAP signaling in the epithelial patterning during CVP morphogenesis. To evaluate the precise localization patterns of YAP and other related signaling molecules, including β-catenin, Ki67, cytokeratins, and PGP9.5, in CVP tissue, histology and immunohistochemistry were employed at E16 and adult mice. Our results suggested that there are specific localization patterns of YAP and Wnt signaling molecules in developing and adult CVP. These concrete localization patterns would provide putative involvements of YAP and Wnt signaling for proper epithelial cell differentiation including the formation and maintenance of taste buds.

GAD65Cre Drives Reporter Expression in Multiple Taste Cell Types

In taste buds, Type I cells represent the majority of cells (50-60%) and primarily have a glial-like function in taste buds. However, recent studies suggest that they have additional sensory and signaling functions including amiloride-sensitive salt transduction, oxytocin modulation of taste, and substance P mediated GABA release. Nonetheless, the overall function of Type I cells in transduction and signaling remains unclear, primarily because of the lack of a reliable reporter for this cell type. GAD65 expression is specific to Type I taste cells and GAD65 has been used as a Cre driver to study Type I cells in salt taste transduction. To test the specificity of transgene-driven expression, we crossed GAD65Cre mice with floxed tdTomato and Channelrhodopsin (ChR2) lines and examined the progeny with immunochemistry, chorda tympani recording, and calcium imaging. We report that while many tdTomato+ taste cells express NTPDase2, a specific marker of Type I cells, we see some expression of tdTomato in both Gustducin and SNAP25-positive taste cells. We also see ChR2 in cells just outside the fungiform taste buds. Chorda tympani recordings in the GAD65Cre/ChR2 mice show large responses to blue light. Furthermore, several isolated tdTomato-positive taste cells responded to KCl depolarization with increases in intracellular calcium, indicating the presence of voltage-gated calcium channels. Taken together, these data suggest that GAD65Cre mice drive expression in multiple taste cell types and thus cannot be considered a reliable reporter of Type I cell function.

The stability of tastant detection by mouse lingual chemosensory tissue requires Regulator of G protein Signaling-21 (RGS21)

The T1R and T2R families of G protein-coupled receptors (GPCRs) initiate tastant perception by signaling via guanine nucleotide exchange and hydrolysis performed by associated heterotrimeric G proteins (Gαβγ). Heterotrimeric G protein signal termination is sped up by Gα-directed GTPase-accelerating proteins (GAPs) known as the Regulators of G protein Signaling (RGS proteins). Of this family, RGS21 is highly expressed in lingual epithelial cells and we have shown it acting in vitro to decrease the potency of bitterants on cultured cells. However, constitutive RGS21 loss in mice reduces organismal response to GPCR-mediated tastants-opposite to expectations arising from observed in vitro activity of RGS21 as a GAP and inhibitor of T2R signaling. Here, we show reduced quinine aversion and reduced sucrose preference by mice lacking RGS21 does not result from post-ingestive effects, as taste-salient brief-access tests confirm the reduced bitterant aversion and reduced sweetener preference seen using two-bottle choice testing. Eliminating Rgs21 expression after chemosensory system development, via tamoxifen-induced Cre recombination in eight week-old mice, led to a reduction in quinine aversive behavior that advanced over time, suggesting that RGS21 functions as a negative regulator to sustain stable bitter tastant reception. Consistent with this notion, we observed downregulation of multiple T2R proteins in the lingual tissue of Rgs21-deficient mice. Reduced tastant-mediated responses exhibited by mice lacking Rgs21 expression either since birth or in adulthood has highlighted the potential requirement for a GPCR GAP to maintain the full character of tastant signaling, likely at the level of mitigating receptor downregulation.