Immunolocalization of SNARE proteins in both type II and type III cells of rat taste buds

The existence of cells exhibiting characteristics of both Type II and Type III cells in rat taste buds. An immunohistochemical and electron-microscopic study

Taste bud cells are classified into four types by their ultrastructural features. Immunohistochemical detection of taste-signaling molecules is used to distinguish cell types of taste bud cells; however, the characteristics of taste cell types such as the immunoreactivity for taste-signaling molecules have long remained unclear. We investigated the detailed characteristics of taste cells in rat vallate taste buds by electron microscopy and immunohistochemistry for gustducin, neural cell adhesion molecule (NCAM) and vesicle-associated membrane protein 2 (VAMP2), which are known as markers of Type II cells, Type III cells and both cell types, respectively. Triple immunostaining for these molecules discriminated seven kinds of cell, including the totally immunopositive cell. Electron microscopy revealed Type III cells with a typical synaptic structure and subsurface cisterna as a specialized contact between a nerve and a Type II cell. The present study clarified the existence of cells with features of both Type II and Type III cells as a subtype of taste bud cells in the rat taste bud.

N-cadherin localization in taste buds of mouse circumvallate papillae

Taste buds, the receptor organs for taste, contain 50-100 taste bud cells. Although these cells undergo continuous turnover, the structural and functional integrity of taste buds is maintained. The molecular mechanisms by which synaptic connectivity between taste buds and afferent fibers is formed and maintained remain ambiguous. In the present study, we examined the localization of N-cadherin in the taste buds of the mouse circumvallate papillae because N-cadherin, one of the classical cadherins, is important for the formation and maintenance of synapses. At the light microscopic level, N-cadherin was predominantly detected in type II cells and nerve fibers in the connective tissues in and around the vallate papillae. At the ultrastructural level, N-cadherin immunoreactivity appears along the cell membrane and in the intracellular vesicles of type II cells. N-cadherin immunoreactivity also is evident in the membranes of afferent terminals at the contact sites to N-cadherin-positive type II cells. At channel type synapses between type II cells and nerve fibers, N-cadherin is present surrounding, but not within, the presumed neurotransmitter release zone, identified by large mitochondria apposed to the taste cells. The present results suggest that N-cadherin is important for the formation or maintenance of type II cell afferent synapses in taste buds.

Whole transcriptome profiling of taste bud cells

Analysis of single-cell RNA-Seq data can provide insights into the specific functions of individual cell types that compose complex tissues. Here, we examined gene expression in two distinct subpopulations of mouse taste cells: Tas1r3-expressing type II cells and physiologically identified type III cells. Our RNA-Seq libraries met high quality control standards and accurately captured differential expression of marker genes for type II (e.g. the Tas1r genes, Plcb2, Trpm5) and type III (e.g. Pkd2l1, Ncam, Snap25) taste cells. Bioinformatics analysis showed that genes regulating responses to stimuli were up-regulated in type II cells, while pathways related to neuronal function were up-regulated in type III cells. We also identified highly expressed genes and pathways associated with chemotaxis and axon guidance, providing new insights into the mechanisms underlying integration of new taste cells into the taste bud. We validated our results by immunohistochemically confirming expression of selected genes encoding synaptic (Cplx2 and Pclo) and semaphorin signalling pathway (Crmp2, PlexinB1, Fes and Sema4a) components. The approach described here could provide a comprehensive map of gene expression for all taste cell subpopulations and will be particularly relevant for cell types in taste buds and other tissues that can be identified only by physiological methods.

Expression of the synaptic exocytosis-regulating molecule complexin 2 in taste buds and its participation in peripheral taste transduction

Taste information from type III taste cells to gustatory neurons is thought to be transmitted via synapses. However, the molecular mechanisms underlying taste transduction through this pathway have not been fully elucidated. In this study, to identify molecules that participate in synaptic taste transduction, we investigated whether complexins (Cplxs), which play roles in regulating membrane fusion in synaptic vesicle exocytosis, were expressed in taste bud cells. Among four Cplx isoforms, strong expression of Cplx2 mRNA was detected in type III taste cells. To investigate the function of CPLX2 in taste transduction, we observed taste responses in CPLX2‐knockout mice. When assessed with electrophysiological and behavioral assays, taste responses to some sour stimuli in CPLX2‐knockout mice were significantly lower than those in wild‐type mice. These results suggested that CPLX2 participated in synaptic taste transduction from type III taste cells to gustatory neurons. A part of taste information is thought to be transmitted via synapses. However, the molecular mechanisms have not been fully elucidated. To identify molecules that participate in synaptic taste transduction, we investigated complexins (Cplxs) expression in taste bud cells. Strong expression of Cplx2 mRNA was detected in taste bud cells. Furthermore, taste responses to some sour stimuli in CPLX2‐ knockout mice were significantly lower than those in wild‐type mice. These suggested that CPLX2 participated in synaptic taste transduction.

A physiologic role for serotonergic transmission in adult rat taste buds

Of the multiple neurotransmitters and neuropeptides expressed in the mammalian taste bud, serotonin remains both the most studied and least understood. Serotonin is expressed in a subset of taste receptor cells that form synapses with afferent nerve fibers (type III cells) and was once thought to be essential to neurotransmission (now understood as purinergic). However, the discovery of the 5-HT1A serotonin receptor in a subset of taste receptor cells paracrine to type III cell suggested a role in cell-to-cell communication during the processing of taste information. Functional data describing this role are lacking. Using anatomical and neurophysiological techniques, this study proposes a modulatory role for serotonin during the processing of taste information. Double labeling immunocytochemical and single cell RT-PCR technique experiments documented that 5-HT1A-expressing cells co-expressed markers for type II cells, cells which express T1R or T2R receptors and release ATP. These cells did not co-express type III cells markers. Neurophysiological recordings from the chorda tympani nerve, which innervates anterior taste buds, were performed prior to and during intravenous injection of a 5-HT1A receptor antagonist. These experiments revealed that serotonin facilitates processing of taste information for tastants representing sweet, sour, salty, and bitter taste qualities. On the other hand, injection of ondansetron, a 5-HT3 receptor antagonist, was without effect. Collectively, these data support the hypothesis that serotonin is a crucial element in a finely-tuned feedback loop involving the 5-HT1A receptor, ATP, and purinoceptors. It is hypothesized that serotonin facilitates gustatory signals by regulating the release of ATP through ATP-release channels possibly through phosphatidylinositol 4,5-bisphosphate resynthesis. By doing so, 5-HT1A activation prevents desensitization of post-synaptic purinergic receptors expressed on afferent nerve fibers and enhances the afferent signal. Serotonin may thus play a major modulatory role within peripheral taste in shaping the afferent taste signals prior to their transmission across gustatory nerves.

Patterns of immunoreactivity specific for gustducin and for NCAM differ in developing rat circumvallate papillae and their taste buds

α-Gustducin and neural cell adhesion molecule (NCAM) are molecules previously found to be expressed in different cell types of mammalian taste buds. We examined the expression of α-gustducin and NCAM during the morphogenesis of circumvallate papillae and the formation of their taste buds by immunofluorescence staining and laser-scanning microscopy of semi-ultrathin sections of fetal and juvenile rat tongues. Images obtained by confocal laser scanning microscopy in transmission mode were also examined to provide outlines of histology and cell morphology. Morphogenesis of circumvallate papillae had already started on embryonic day 13 (E13) and was evident as the formation of placode. By contrast, taste buds in the circumvallate papillae started to appear between postnatal day 0 (P0) and P7. Although no cells with immunoreactivity specific for α-gustducin were detected in fetuses from E13 to E19, cells with NCAM-specific immunoreactivity were clearly apparent in the entire epithelium of the circumvallate papillary placode, the rudiment of each circumvallate papilla and the developing circumvallate papilla itself from E13 to E19. However, postnatally, both α-gustducin and NCAM became concentrated within taste cells as the formation of taste buds advanced. After P14, neither NCAM nor α-gustducin was detectable in the epithelium around the taste buds. In conclusion, α-gustducin appeared in the cytoplasm of taste cells during their formation after birth, while NCAM appeared in the epithelium of the circumvallate papilla-forming area. However, these two markers of taste cells were similarly distributed within mature taste cells.

Characterization of the expression pattern of adrenergic receptors in rat taste buds

Taste buds signal the presence of chemical stimuli in the oral cavity to the central nervous system using both early transduction mechanisms, which allow single cells to be depolarized via receptor-mediated signaling pathways, and late transduction mechanisms, which involve extensive cell-to-cell communication among the cells in the bud. The latter mechanisms, which involve a large number of neurotransmitters and neuropeptides, are less well understood. Among neurotransmitters, multiple lines of evidence suggest that norepinephrine plays a yet unknown role in the taste bud. This study investigated the expression pattern of adrenergic receptors in the rat posterior taste bud. Expression of alpha1A, alpha1B, alpha1D, alpha2A, alpha2B, alpha2C, beta1, and the beta2 adrenoceptor subtypes was observed in taste buds using RT-PCR and immunocytochemical techniques. Taste buds also expressed the biosynthetic enzyme for norepinephrine, dopamine beta-hydroxylase (DbetaH), as well as the norepinephrine transporter. Further, expression of the epinephrine synthetic enzyme, phenylethanolamine N-methyltransferase (PNMT), was observed suggesting a possible role for this transmitter in the bud. Phenotyping adrenoceptor expression patterns with double labeling experiments to gustducin, synaptosomal-associated protein 25 (SNAP-25), and neural cell adhesion molecule (NCAM) suggests they are prominently expressed in subsets of cells known to express taste receptor molecules but segregated from cells known to have synapses with the afferent nerve fiber. Alpha and beta adrenoceptors co-express with one another in unique patterns as observed with immunocytochemistry and single cell reverse transcription polymerase chain reaction (RT-PCR). These data suggest that single cells express multiple adrenergic receptors and that adrenergic signaling may be particularly important in bitter, sweet, and umami taste qualities. In summary, adrenergic signaling in the taste bud occurs through complex pathways that include presynaptic and postsynaptic receptors and likely play modulatory roles in processing of gustatory information similar to other peripheral sensory systems such as the retina, cochlea, and olfactory bulb.

Histochemical changes and apoptosis in degenerating taste buds of the rat circumvallate papilla

The present study was designed to examine the histochemical changes and occurrence of apoptosis in taste buds of rat circumvallate papillae following bilateral transection of the glossopharyngeal nerve. Following transection of the glossopharyngeal nerve, the number of taste buds was not altered until post-operative day 3 (PO3), but decreased significantly thereafter. The number of cells within a taste bud, however, decreased significantly from PO2. In normal, uninjured animals, approximately 15.4%, 9.0%, and 7.7% of taste bud cells were labeled with antibodies for phospholipase C beta2 subunit (PLCbeta2), a marker for type II cells, neural cell adhesion molecule (NCAM), a marker for type III cells, and Jacalin, a marker for type IV cells, respectively. Following gustatory nerve injury, the ratio of cells expressing markers of type III and type IV decreased gradually from PO2, and Jacalin-labeled taste bud cells disappeared on PO3. Under normal conditions, immunoreactivity for single-strand DNA (ssDNA), a marker of apoptosis, was detected in the nuclei of PLC beta2-immunoreactive cells and cells showing no labeling for PLCbeta2, NCAM, or Jacalin. On PO1, the number of taste bud cells showing ssDNA immunoreactivity increased to double that of normal uninjured animals; these ssDNA-immunoreactive cells were also labeled with NCAM and Jacalin as well as PLCbeta2. The present results suggest that denervation of the gustatory nerve causes apoptosis in all types of taste bud cells, resulting in the rapid degeneration of taste buds.

Discrimination of taste qualities among mouse fungiform taste bud cells

Multiple lines of evidence from molecular studies indicate that individual taste qualities are encoded by distinct taste receptor cells. In contrast, many physiological studies have found that a significant proportion of taste cells respond to multiple taste qualities. To reconcile this apparent discrepancy and to identify taste cells that underlie each taste quality, we investigated taste responses of individual mouse fungiform taste cells that express gustducin or GAD67, markers for specific types of taste cells. Type II taste cells respond to sweet, bitter or umami tastants, express taste receptors, gustducin and other transduction components. Type III cells possess putative sour taste receptors, and have well elaborated conventional synapses. Consistent with these findings we found that gustducin-expressing Type II taste cells responded best to sweet (25/49), bitter (20/49) or umami (4/49) stimuli, while all GAD67 (Type III) taste cells examined (44/44) responded to sour stimuli and a portion of them showed multiple taste sensitivities, suggesting discrimination of each taste quality among taste bud cells. These results were largely consistent with those previously reported with circumvallate papillae taste cells. Bitter-best taste cells responded to multiple bitter compounds such as quinine, denatonium and cyclohexamide. Three sour compounds, HCl, acetic acid and citric acid, elicited responses in sour-best taste cells. These results suggest that taste cells may be capable of recognizing multiple taste compounds that elicit similar taste sensation. We did not find any NaCl-best cells among the gustducin and GAD67 taste cells, raising the possibility that salt sensitive taste cells comprise a different population.

Identification of the vesicular nucleotide transporter (VNUT) in taste cells

Taste cells are chemosensory epithelial cells that sense distinct taste qualities. It is the type II taste cell that express G-protein coupled receptors to sense either umami, sweet, or bitter compounds. Whereas several reports have suggested involvement of ATP in taste signal transduction, there is a paucity of molecular information about how ATP is stored and being released. The recent discovery of a novel vesicular nucleotide transporter (VNUT) led us to examine whether VNUT exist in the taste tissue where ATP is to be released for taste signal transmission. Here, we report that VNUT is selectively expressed in type II cell but not in type III taste cell. In addition, we show that during taste bud development VNUT expression is always accompanied by the expression of type II taste cell markers. Our results, together with previous studies, strongly suggest that VNUT plays a role in type II taste cell.

GABA expression in the mammalian taste bud functions as a route of inhibitory cell-to-cell communication

Recent advances have underscored cell-to-cell communication as an important component of the operation of taste buds with individual taste receptor cells (TRCs) communicating with one another by means of a number of neurotransmitters and neuropeptides, although functional roles are not yet understood. Here, we characterize the presence, distribution pattern, phenotype, and functional consequences of a previously undescribed inhibitory route within the taste bud mediated by the classic neurotransmitter GABA and its receptors. By using immunocytochemistry, subsets of TRCs within rat taste buds were identified as expressing GABA, and its synthetic enzyme glutamate decarboxylase (GAD). GAD expression was verified with Western blotting. Immunofluorescent studies revealed complex coexpression patterns of GAD with the TRC protein markers gustducin, neural cell adhesion molecule, protein gene product 9.5, and synaptosomal-associated protein of 25 kDa that collectively outline hardwired signaling pathways of GABAergic TRCs. RT-PCR and immunocytochemistry demonstrated that both GABA(A) and GABA(B) receptors are expressed in the taste bud. The later was observed in a subset TRCs paracrine to GAD-expressing TRCs. Physiological effects of GABA were examined by patch clamp recordings. GABA and the GABA(A) agonists muscimol and isoguvacine enhanced isolated chloride currents in a dose-dependent manner. Also, GABA and the GABA(B) agonist baclofen both elicited increases of the inwardly rectifying potassium currents that could be blocked by the GABA(B) receptor antagonist CGP 35348 and the G protein blocker GDP-betaS. Collectively, these data suggest that GABAergic TRCs are able to shape the final chemosensory output of the bud by means of processes of cell-to-cell modulation.

Cell-type specific occurrence of apoptosis in taste buds of the rat circumvallate papilla

The present study employed immunohistochemistry for single-stranded DNA (ssDNA) to detect apoptotic cells in taste buds of the rat circumvallate papilla. Double-labeling of ssDNA and markers for each cell type - phospholipase C beta2 (PLCbeta2) and alpha-gustducin for type II cells, neural cell adhesion molecule (NCAM) for type III cells, and Jacalin for type IV cells - was also performed to reveal which types of cells die by apoptosis. We detected approximately 16.8% and 14.0% of ssDNA-immunoreactive nuclei among PLCbeta2-immunoreactive and alpha-gustducinimmunoreactive cells, respectively, but rarely found ssDNA-immunoreactive cells among NCAM-immunoreactive or Jacalin-labeled cells, indicating that type II cells die by apoptosis. We also applied double labeling of ssDNA and human blood group antigen H (AbH) - which mostly labels type I cells as well as other cell types - and found that approximately 78% of ssDNA-immunoreactive cells were labeled with AbH, indicating that apoptosis also occurs in type I cells. The present results revealed that apoptosis occurs in both type I cells (dark cells) and type II cells (light cells), suggesting that there are two major cell lineages (dark cell and light cell lineages) for the differentiation of taste bud cells. In summury, type IV cells differentiate into dark and light cells and type III cells differentiate to type II cells within the light cell line.