Interleukin-10 is produced by a specific subset of taste receptor cells and critical for maintaining structural integrity of mouse taste buds

Although inflammatory responses are a critical component in defense against pathogens, too much inflammation is harmful. Mechanisms have evolved to regulate inflammation, including modulation by the anti-inflammatory cytokine interleukin-10 (IL-10). Previously we have shown that taste buds express various molecules involved in innate immune responses, including the proinflammatory cytokine tumor necrosis factor (TNF). Here, using a reporter mouse strain, we show that taste cells also express the anti-inflammatory cytokine IL-10. Remarkably, IL-10 is produced by only a specific subset of taste cells, which are different from the TNF-producing cells in mouse circumvallate and foliate taste buds: IL-10 expression was found exclusively in the G-protein gustducin-expressing bitter receptor cells, while TNF was found in sweet and umami receptor cells as reported previously. In contrast, IL-10R1, the ligand-binding subunit of the IL-10 receptor, is predominantly expressed by TNF-producing cells, suggesting a novel cellular hierarchy for regulating TNF production and effects in taste buds. In response to inflammatory challenges, taste cells can increase IL-10 expression both in vivo and in vitro. These findings suggest that taste buds use separate populations of taste receptor cells that coincide with sweet/umami and bitter taste reception to modulate local inflammatory responses, a phenomenon that has not been previously reported. Furthermore, IL-10 deficiency in mice leads to significant reductions in the number and size of taste buds, as well as in the number of taste receptor cells per taste bud, suggesting that IL-10 plays critical roles in maintaining structural integrity of the peripheral gustatory system.

Apical CD36 immunolocalization in human and porcine taste buds from circumvallate and foliate papillae

CD36 is the receptor for long chain fatty acids (LCFA), and is expressed in lingual taste cells from rodents. In these animals, CD36 has been proposed to play an important role in oral detection of LCFA, and subsequently, determines their dietary fat preference. Humans also seem to detect LCFA in the oral cavity, however, information on the molecular mechanism of this human orosensory LCFA recognition is currently lacking. The aim of our study was to investigate whether CD36 is also expressed in lingual human and porcine taste buds cells. Using fluorescence immunohistochemistry, apical CD36 expression was revealed in human and porcine taste bud cells from circumvallate and foliate papillae. These data suggest CD36 as the putative orosensory receptor for dietary LCFA in human, and, therefore, may be involved in our preference for fatty foods.

Immune cells of the human peripheral taste system: dominant dendritic cells and CD4 T cells

Taste loss or alterations can seriously impact health and quality of life due to the resulting negative influence on eating habits and nutrition. Infection and inflammation are thought to be some of the most common causes of taste perception disorders. Supporting this view, neuro-immune interactions in the peripheral gustatory system have been identified, underlying the importance of this tissue in mucosal immunity, but we have little understanding of how these interactions influence taste perception directly or indirectly. This limited understanding is evident by the lack of even a basic knowledge of the resident immune cell populations in or near taste tissues. The present study characterized the distribution and population of the major immune cells and their subsets in healthy human anterior, lingual, fungiform papillae (FP) using immunohistochemistry. Dendritic cells (DCs) were the predominant innate immune cells in this tissue, including four subtypes: CD11c(+) DCs, DC-SIGN+ immature DCs, CD83(+) mature DCs, and CD1a(+) DCs (Langerhans cells). While most DCs were localized beneath the lamina propria and only moderately in the epithelium, CD1a(+) Langerhans cells were exclusively present within the epithelium and not in sub-strata. A small number of macrophages were observed. T lymphocytes were present throughout the FP with CD4(+) T cells more prevalent than CD8(+) T cells. Very few CD19(+) B lymphocytes were detected. The results show that DCs, macrophages, and T lymphocytes are the constitutive guardians of human FP taste tissue, with DCs and CD4 T cells being dominant, while B lymphocytes are rare under normal, healthy conditions. These observations provide a basic anatomical foundation for the immune response in the healthy human tongue as a basis for subsequent disease-related studies, but none of the present data indicate that the immune cell populations identified are, in fact, altered in individuals with abnormal taste perception.

Structure of bovine fungiform taste buds and their immunoreactivity for gustducin

The taste buds of bovine fungiform papillae were studied by light and electron microscopy using both histological and immunohistochemical methods. The taste buds existed in the epithelium of the apical region of the papillae. By electron microscopy, two types of taste cells, namely type I and type II cells, could be classified according to the presence of dense-cored vesicles, the cytoplasmic density and the cell shape. Type I cells were thin, had an electron-dense cytoplasm containing dense-cored vesicles, and possessed long thick apical processes in the taste pore. Type II cells were thick, had an electron-lucent cytoplasm containing many electron-lucent vesicles, rather than dense-cored vesicles, and possessed microvilli in the taste pore. Immunohistochemical staining with an antiserum against gustducin was investigated by both light and electron microscopy using the avidin-biotin complex (ABC) method. Some, but not all, of the type II cells exhibited gustducin immunoreactivity, whereas none of the type I cells showed any immunoreactivity.

Human blood group antigen H is not the specific marker for type I cells in the taste buds

We examined the localization of human blood antigen H (AbH) and its correlation with other cell type markers in the taste buds of circumvallate papillae of the adult rat. Immunoreactivity for AbH was localized in the membrane of two cell populations in the taste buds: in spindle-shaped cells extending from base to the apical portion of the taste buds as well as in round-shaped cells at the basal portion of the taste buds. Quantitative analysis revealed that approximately 47.8%, 24.4%, and 14.6% of cells within the taste buds displayed AbH-, alpha-gustducin- or protein gene product 9.5 (PGP 9.5)-immunoreactivity, respectively. Approximately 16.3% and 6.6% of AbH-immunoreactive taste bud cells displayed alpha-gustducin- or PGP 9.5-immunoreactivity, respectively. Although previous studies proposed that AbH immunoreactivity was specific for type I cells (dark cells or supporting cells), the present results indicate that AbH immunoreactivity is also present in some type II cells (alpha-gustducin immunoreactive cells) and type III cells (PGP 9.5-immunoreactive cells).

Up-regulation of activated macrophages in response to degeneration in the taste system: Effects of dietary sodium restriction

Dietary sodium restriction combined with unilateral chorda tympani nerve section leads to a rapid and specific decrease in neurophysiological taste responses to sodium in the contralateral, intact chorda tympani (Hill and Phillips [1994] J. Neurosci. 14:2904-2910). Previous work demonstrated that dietary sodium restriction may induce these early functional deficits by inhibiting immune activity after denervation (Phillips and Hill [1996] Am. J. Physiol. 271:R857-R862). However, little is known about the leukocyte response to denervation of taste buds in fungiform papillae. In the current study, it was hypothesized that T cells and macrophages are increased in the tongue after unilateral denervation in control-fed but not sodium-restricted animals. Adult, specified pathogen-free rats received unilateral chorda tympani nerve section or sham section followed by dietary sodium restriction or maintenance on control diet. At day 1, 2, 5, 7, or 50 postsectioning, immunostaining was used to detect the percentage of staining for activated macrophages, the number of alpha beta T cells, and the number of delta gamma epithelial T cells in the tongue. The number of lingual T cells did not significantly differ between treatment groups following denervation. However, there was a dramatic bilateral increase in ED1(+) staining for activated macrophages in control-fed rats that peaked at day 2 postsectioning. In contrast, sodium-restricted rats did not show an increase in activated macrophages above baseline at any time postsectioning. Further analysis of extralingual macrophages indicated that the deficit in immune activity in sodium-restricted rats is localized to the tongue and is not widespread. A model for immune modulation of taste receptor cell function is proposed based on these novel findings.

Expression of the metabotropic glutamate receptor, mGluR4a, in the taste hairs of taste buds in rat gustatory papillae

Taste-mGluR4, cloned from taste tissues, is a truncated variant of brain-expressed mGluR4a (brain-mGluR4), and is known to be a candidate for the receptor involved in the umami taste sense. Although the expression patterns of taste- and brain-mGluR4 mRNAs have been demonstrated, no mention has so far been made of the expression of these two mGluR4 proteins in taste tissues. The present study examined the expression of taste-mGluR4 and brain-mGluR4 proteins in rat taste tissues by using a specific antibody for mGluR4a which shared a C-terminus of both taste- and brain-mGluR4, for immunoblot analysis and immunohistochemistry. Immunoblot analysis showed that both brain-mGluR4 and taste-mGluR4 were expressed in the taste tissues. Taste-mGluR4 was not detected in the cerebellum. The immunoreactive band for brain-mGluR4 protein was much stronger than that for taste-mGluR4 protein. In the cryosections of fungiform, foliate and circumvallate papillae, the antibody against taste-mGluR4 exhibited intense labeling of the taste pores and taste hairs in all the taste buds of gustatory papillae examined; the immunoreaction to the antibody against brain-mGluR4 was more intense at the same sites of the taste buds. The portions of the taste bud cells below the taste pore and surrounding keratinocytes did not show any immunoreactivities. The results of the present study strongly suggest that, in addition to taste-mGluR4, brain-mGluR4 may function even more importantly than the former as a receptor for glutamate, i.e. the umami taste sensation.

Keratin 19-like immunoreactivity in receptor cells of mammalian taste buds

Three monoclonal antibodies, 4.62, LP2K and 170.2.14, were used to evaluate keratin 19-like immunoreactivity in gustatory epithelia. Keratin 19-like immunoreactivity was restricted to the intragemmal cells for all types of mammalian taste buds examined. These taste buds included fungiform, foliate and vallate taste buds in rat, gerbil and rabbit, and nasopalatine, epiglottal and palatine taste buds in rat. There was no keratin 19-like immunoreactivity in basal cells or in perigemmal cells lateral to the immunoreactive taste receptor cells. Denervation of the rat vallate papilla eliminated all taste buds, as well as all immunoreactive taste cells. That the immunoreactive material in the taste cells was keratin 19 was supported by the comparable staining of rat taste buds with each of three monoclonal antibodies specific for keratin 19. Furthermore, as predicted, these antibodies selectively stained luminal cells of rat bile ducts, bladder, salivary ducts, trachea, ureter and uterus. It was concluded that monoclonal antibodies against keratin 19 can usefully distinguish intragemmal taste receptor cells from keratinocytes, and from the perigemmal and basal cells of gustatory epithelia. Anti-keratin 19 antibodies may serve to identify differentiated taste cells in gustatory epithelia undergoing taste bud development, renewal, degeneration or regeneration.

Immunolocalization of different forms of neural cell adhesion molecule (NCAM) in rat taste buds

Taste buds consist of approximately 100 taste cells, including three morphological types of short receptor cells which synapse on the peripheral gustatory nerves. Some of the receptor cells produce neural cell adhesion molecule (NCAM), which may play a role in formation of specific connections in this system. Antibodies directed against different forms of NCAM were utilized in an attempt to define not only the distribution, but also the type of NCAM within taste buds. Within each taste bud approximately 10% of the taste cells exhibit abundant immunoreactivity for 180 kD (ld) or 140 kD (sd) forms of NCAM (i.e., those with an intracellular domain) along virtually the entire surface of the cell. Ultrastructural analysis reveals that these abundantly immunoreactive taste cells are of the intermediate morphological type, although not all of the intermediate taste cells within any bud are immunoreactive. In addition, the ultrastructural studies show that punctate (ld/sd) NCAM-immunoreactivity occurs on the membranes of taste cells and nerve fibers throughout each taste bud. The embryonic form of NCAM (E-NCAM), rich in polysialic acid residues, is present only in association with nerve fibers and other unidentified elongate, thin profiles of a few taste buds. The nerve plexus beneath the gustatory epithelium is also rich in NCAM-immunoreactivity. These fibers occasionally reveal immunoreactivity indicative of only the 120 kD (ssd) form of NCAM, typical of glial cells.

Immunohistochemical localization of neuron-specific enolase and calcitonin gene-related peptide in pig taste papillae

Immunoreactivity to neuron-specific enolase (NSE), a specific neuronal marker, and calcitonin gene-related peptide (CGRP) was localized in lingual taste papillae in the pigs. Sequential staining for NSE and CGRP by an elution technique allowed the identification of neuronal subpopulations. NSE-staining revealed a large neuronal network within the subepithelial layer of all taste papillae. NSE-positive fibers then penetrated the epithelium as isolated fibers, primarily in the foliate and circumvallate papillae, or as brush-shaped units formed by a multitude of fibers, especially in the fungiform papillae and in the apical epithelium of the circumvallate papilla. Taste buds of any type of taste papillae were found to express a dense subgemmal/intragemmal NSE-positive neuronal network. CGRP-positive nerve fibers were numerous in the subepithelial layer of all three types of taste papillae. In the foliate and circumvallate papillae, these fibers penetrated the epithelium to form extragemmal and intragemmal fibers, while in the fungiforms, they concentrated almost exclusively in the taste buds as intragemmal nerve fibers. Intragemmal NSE- and CGRP-positive fiber populations were not readily distinguishable by typical neural swellings as previously observed in the rat. The NSE-positive neuronal extragemmal brushes never expressed any CGRP-like immunoreactivity. Even more surprising, fungiform taste buds, whether richly innervated by or devoid of NSE-positive intragemmal fibers, always harboured numerous intragemmal CGRP-positive fibers. Consequently, NSE is not a general neuronal marker in porcine taste papillae. Our observations also suggest that subgemmal/intragemmal NSE-positive fibers are actively involved in synaptogenesis within taste buds. NSE-positive taste bud cells were found in all three types of taste papillae. CGRP-positive taste bud cells were never observed.

Immunohistochemical localization of neuron-specific enolase and calcitonin gene-related peptide in rat taste papillae

Calcitonin gene-related peptide-like and neuron-specific enolase-like immunoreactivity (CGRP-IR and NSE-IR) were surveyed immunohistochemically in the fungi-form, foliate and circumvallate papillae in rats. A dense CGRP-IR network (subgemmal and extragemmal) in the taste papillae is linked to the presence of taste buds, even though CGRP-IR fibers are rarely present in the taste buds. Three typical fiber populations were detected with these two markers. (a) A population of coarse NSE-IR intragemmal fibers characterized by thick neural swellings, never expressing CGRP-immunoreactivity. (b) A population of thin varicose intragemmal NSE/CGRP-IR fibers. (c) A population of subgemmal and extragemmal NSE-/CGRP-IR fibers that partly penetrated the epithelium. The common distribution of CGRP-IR and NSE-IR fibers at the base of taste buds, their differential distribution and morphology within taste buds, added to their restricted nature (gustatory or somatosensory) suggest that a population of CGRP-IR fibers undergoes a target-induced inhibition of its CGRP phenotype while entering the taste buds. The combined use of NSE and CGRP allowed a better characterization of nerve fibers within and between all three types of taste papillae. NSE was also a very good marker for a subtype of taste bud cells in the foliate and in the circumvallate papillae, but no such cells could be observed in the fungiform papillae.

A bitter substance induces a rise in intracellular calcium in a subpopulation of rat taste cells

The sense of taste permits animals to discriminate between foods that are safe and those that are toxic. Because most poisonous plant alkaloids are intensely bitter, bitter taste warns animals of potentially hazardous foods. To investigate the mechanism of bitter taste transduction, a preparation of dissociated rat taste cells was developed that can be studied with techniques designed for single-cell measurements. Denatonium, a very bitter substance, caused a rise in the intracellular calcium concentration due to release from internal stores in a small subpopulation of taste cells. Thus, the transduction of bitter taste may occur via a receptor-second messenger mechanism leading to neurotransmitter release and may not involve depolarization-mediated calcium entry.

Biochemical studies of taste sensation: monoclonal antibody against L-alanine binding activity of catfish taste epithelium

The L-alanine taste receptors of the channel catfish Ictalurus punctatus provide a useful biochemical model for studying taste receptor mechanisms. Mouse hybridomas that synthesize monoclonal antibodies have been produced. The antigen used to activate mouse spleen cells was the plasma membrane fraction obtained from the taste receptor-containing epithelium of the channel catfish. The spleen cells were fused with myeloma cells, Sp2/0-Ag14, to form hybridomas. To demonstrate inhibition of ligand binding by the product of these hybridomas, a catfish membrane fraction (fraction P2) was incubated with the antibody-containing preparation prior to assaying for L-[3H]alanine binding activity. We thereby demonstrated inhibition of binding of the taste ligand L-alanine to fraction P2. This approach should prove useful in further studies of receptor binding and transduction events in taste receptors.