Expression of Mash1 in basal cells of rat circumvallate taste buds is dependent upon gustatory innervation

Anterior and Posterior Tongue Regions and Taste Papillae: Distinct Roles and Regulatory Mechanisms with an Emphasis on Hedgehog Signaling and Antagonism

Sensory receptors across the entire tongue are engaged during eating. However, the tongue has distinctive regions with taste (fungiform and circumvallate) and non-taste (filiform) organs that are composed of specialized epithelia, connective tissues, and innervation. The tissue regions and papillae are adapted in form and function for taste and somatosensation associated with eating. It follows that homeostasis and regeneration of distinctive papillae and taste buds with particular functional roles require tailored molecular pathways. Nonetheless, in the chemosensory field, generalizations are often made between mechanisms that regulate anterior tongue fungiform and posterior circumvallate taste papillae, without a clear distinction that highlights the singular taste cell types and receptors in the papillae. We compare and contrast signaling regulation in the tongue and emphasize the Hedgehog pathway and antagonists as prime examples of signaling differences in anterior and posterior taste and non-taste papillae. Only with more attention to the roles and regulatory signals for different taste cells in distinct tongue regions can optimal treatments for taste dysfunctions be designed. In summary, if tissues are studied from one tongue region only, with associated specialized gustatory and non-gustatory organs, an incomplete and potentially misleading picture will emerge of how lingual sensory systems are involved in eating and altered in disease.

Expression of Oncofetal Antigen 5T4 in Murine Taste Papillae

Background: Multicellular taste buds located within taste papillae on the tongue mediate taste sensation. In taste papillae, taste bud cells (TBCs), such as taste receptor cells and taste precursor cells, and the surrounding lingual epithelium including epithelial progenitors (also called taste stem/progenitor cells) are maintained by continuous cell turnover throughout life. However, it remains unknown how the cells constituting taste buds proliferate and differentiate to maintain taste bud tissue. Based on in situ hybridization (ISH) screening, we demonstrated that the oncofetal antigen 5T4 (also known as trophoblast glycoprotein: TPBG) gene is expressed in the adult mouse tongue. Results: In immunohistochemistry of coronal tongue sections, 5T4 protein was detected at a low level exclusively in the basal part of the lingual epithelium in developing and adult mice, and at a high level particularly in foliate papillae and circumvallate papillae (CVPs). Furthermore, immunohistochemistry of the basal part of CVPs indicated that the proliferation marker PCNA (proliferating cell nuclear antigen) co-localized with 5T4. 5T4 was strongly expressed in Krt5+ epithelial progenitors and Shh+ taste precursor cells, but weakly in mature taste receptor cells. The number of proliferating cells in the CVP was higher in 5T4-knockout mice than in wild-type (WT) mice, while neither cell differentiation nor the size of taste buds differed between these two groups of mice. Notably, X-ray irradiation enhanced cell proliferation more in 5T4-knockout mice than in WT mice. Conclusion: Our results suggest that 5T4, expressed in epithelial progenitors (taste stem/progenitor cells), and taste precursor cells, may influence the maintenance of taste papillae under both normal and injury conditions.

Hedgehog Signaling Regulates Taste Organs and Oral Sensation: Distinctive Roles in the Epithelium, Stroma, and Innervation

The Hedgehog (Hh) pathway has regulatory roles in maintaining and restoring lingual taste organs, the papillae and taste buds, and taste sensation. Taste buds and taste nerve responses are eliminated if Hh signaling is genetically suppressed or pharmacologically inhibited, but regeneration can occur if signaling is reactivated within the lingual epithelium. Whereas Hh pathway disruption alters taste sensation, tactile and cold responses remain intact, indicating that Hh signaling is modality-specific in regulation of tongue sensation. However, although Hh regulation is essential in taste, the basic biology of pathway controls is not fully understood. With recent demonstrations that sonic hedgehog (Shh) is within both taste buds and the innervating ganglion neurons/nerve fibers, it is compelling to consider Hh signaling throughout the tongue and taste organ cell and tissue compartments. Distinctive signaling centers and niches are reviewed in taste papilla epithelium, taste buds, basal lamina, fibroblasts and lamellipodia, lingual nerves, and sensory ganglia. Several new roles for the innervation in lingual Hh signaling are proposed. Hh signaling within the lingual epithelium and an intact innervation each is necessary, but only together are sufficient to sustain and restore taste buds. Importantly, patients who use Hh pathway inhibiting drugs confront an altered chemosensory world with loss of taste buds and taste responses, intact lingual touch and cold sensation, and taste recovery after drug discontinuation.

Age-related decrease of IF5/BTG4 in oral and respiratory cavities in mice

An IF5 cDNA was isolated by expression cloning from a mouse oocyte cDNA library. It encoded a protein of 250 amino acids, and the region of it encoding amino acids 1-137 showed 86.8% alignment with the anti-proliferative domain of BTG/TOB family genes. This gene is also termed BTG4 or PC3B. Transiently expressed IF5/BTG4 induced alkaline phosphatase activity in human embryonic kidney (HEK293T) and 2T3 cells. IF5/BTG4 mRNA was detected by reverse transcription polymerase chain reaction in pharynx, larynx, trachea, oviduct, ovary, caput epididymis, and testis, but not in lung, intestine, or liver. Immunohistochemistry showed the IF5/BTG4 protein to be present in epithelial cells of the tongue, palate, pharynx, internal nose, and trachea. Both protein and mRNA levels of IF5/BTG4 were reduced by aging when comparing 4-week-old mice with 48-week-old mice. Our findings suggest that IF5/BTG4 may be an aging-related gene in epithelial cells.

Expression of GAD67 and Dlx5 in the taste buds of mice genetically lacking Mash1

It has been reported that a subset of type III taste cells express glutamate decarboxylase (GAD)67, which is a molecule that synthesizes gamma-aminobutyric acid (GABA), and that Mash1 could be a potential regulator of the development of GABAnergic neurons via Dlx transcription factors in the central nervous system. In this study, we investigated the expression of GAD67 and Dlx in the embryonic taste buds of the soft palate and circumvallate papilla using Mash1 knockout (KO)/GAD67-GFP knock-in mice. In the wild-type animal, a subset of type III taste cells contained GAD67 in the taste buds of the soft palate and the developing circumvallate papilla, whereas GAD67-expressing taste bud cells were missing from Mash1 KO mice. A subset of type III cells expressed mRNA for Dlx5 in the wild-type animals, whereas Dlx5-expressing cells were not evident in the apical part of the circumvallate papilla and taste buds in the soft palate of Mash1 KO mice. Our results suggest that Mash1 is required for the expression of GAD67 and Dlx5 in taste bud cells.

Sonic hedgehog-expressing basal cells are general post-mitotic precursors of functional taste receptor cells

BACKGROUND

Taste buds contain ∼60 elongate cells and several basal cells. Elongate cells comprise three functional taste cell types: I, glial cells; II, bitter/sweet/umami receptor cells; and III, sour detectors. Although taste cells are continuously renewed, lineage relationships among cell types are ill-defined. Basal cells have been proposed as taste bud stem cells, a subset of which express Sonic hedgehog (Shh). However, Shh+ basal cells turn over rapidly suggesting that Shh+ cells are post-mitotic precursors of some or all taste cell types.

RESULTS

To fate map Shh-expressing cells, mice carrying ShhCreER(T2) and a high (CAG-CAT-EGFP) or low (R26RLacZ) efficiency reporter allele were given tamoxifen to activate Cre in Shh+ cells. Using R26RLacZ, lineage-labeled cells occur singly within buds, supporting a post-mitotic state for Shh+ cells. Using either reporter, we show that Shh+ cells differentiate into all three taste cell types, in proportions reflecting cell type ratios in taste buds (I > II > III).

CONCLUSIONS

Shh+ cells are not stem cells, but are post-mitotic, immediate precursors of taste cells. Shh+ cells differentiate into each of the three taste cell types, and the choice of a specific taste cell fate is regulated to maintain the proper ratio within buds.

Mash1 is required for the differentiation of AADC-positive type III cells in mouse taste buds

Mash1 is expressed in subsets of neuronal precursors in both the central nervous system and the peripheral nervous system. However, involvement of Mash1 in taste cell differentiation has not previously been demonstrated. In this study, we investigated the role of Mash1 in regulating taste bud differentiation using Mash1 KO mice to begin to understand the mechanisms that regulate taste bud cell differentiation. We found that aromatic L-amino acid decarboxylase (AADC) cells were not evident in either the circumvallate papilla epithelia or in taste buds in the soft palates of Mash1 KO mice. However gustducin was expressed in taste buds in the soft palates of Mash1 KO mice. These results suggest that Mash1 plays an important role in regulating the expression of AADC in type III cells in taste buds, which supports the hypothesis that different taste bud cell types have progenitor cells that are specific to each cell type.

Lipopolysaccharide-induced inflammation attenuates taste progenitor cell proliferation and shortens the life span of taste bud cells

Background

The mammalian taste bud, a complex collection of taste sensory cells, supporting cells, and immature basal cells, is the structural unit for detecting taste stimuli in the oral cavity. Even though the cells of the taste bud undergo constant turnover, the structural homeostasis of the bud is maintained by balancing cell proliferation and cell death. Compared with nongustatory lingual epithelial cells, taste cells express higher levels of several inflammatory receptors and signalling proteins. Whether inflammation, an underlying condition in some diseases associated with taste disorders, interferes with taste cell renewal and turnover is unknown. Here we report the effects of lipopolysaccharide (LPS)-induced inflammation on taste progenitor cell proliferation and taste bud cell turnover in mouse taste tissues.

Results

Intraperitoneal injection of LPS rapidly induced expression of several inflammatory cytokines, including tumor necrosis factor (TNF)-α, interferon (IFN)-γ, and interleukin (IL)-6, in mouse circumvallate and foliate papillae. TNF-α and IFN-γ immunoreactivities were preferentially localized to subsets of cells in taste buds. LPS-induced inflammation significantly reduced the number of 5-bromo-2'-deoxyuridine (BrdU)-labeled newborn taste bud cells 1-3 days after LPS injection, suggesting an inhibition of taste bud cell renewal. BrdU pulse-chase experiments showed that BrdU-labeled taste cells had a shorter average life span in LPS-treated mice than in controls. To investigate whether LPS inhibits taste cell renewal by suppressing taste progenitor cell proliferation, we studied the expression of Ki67, a cell proliferation marker. Quantitative real-time RT-PCR revealed that LPS markedly reduced Ki67 mRNA levels in circumvallate and foliate epithelia. Immunofluorescent staining using anti-Ki67 antibodies showed that LPS decreased the number of Ki67-positive cells in the basal regions surrounding circumvallate taste buds, the niche for taste progenitor cells. PCR array experiments showed that the expression of cyclin B2 and E2F1, two key cell cycle regulators, was markedly downregulated by LPS in the circumvallate and foliate epithelia.

Conclusions

Our results show that LPS-induced inflammation inhibits taste progenitor cell proliferation and interferes with taste cell renewal. LPS accelerates cell turnover and modestly shortens the average life span of taste cells. These effects of inflammation may contribute to the development of taste disorders associated with infections.

Genome-wide analysis of gene expression in primate taste buds reveals links to diverse processes

Efforts to unravel the mechanisms underlying taste sensation (gustation) have largely focused on rodents. Here we present the first comprehensive characterization of gene expression in primate taste buds. Our findings reveal unique new insights into the biology of taste buds. We generated a taste bud gene expression database using laser capture microdissection (LCM) procured fungiform (FG) and circumvallate (CV) taste buds from primates. We also used LCM to collect the top and bottom portions of CV taste buds. Affymetrix genome wide arrays were used to analyze gene expression in all samples. Known taste receptors are preferentially expressed in the top portion of taste buds. Genes associated with the cell cycle and stem cells are preferentially expressed in the bottom portion of taste buds, suggesting that precursor cells are located there. Several chemokines including CXCL14 and CXCL8 are among the highest expressed genes in taste buds, indicating that immune system related processes are active in taste buds. Several genes expressed specifically in endocrine glands including growth hormone releasing hormone and its receptor are also strongly expressed in taste buds, suggesting a link between metabolism and taste. Cell type-specific expression of transcription factors and signaling molecules involved in cell fate, including KIT, reveals the taste bud as an active site of cell regeneration, differentiation, and development. IKBKAP, a gene mutated in familial dysautonomia, a disease that results in loss of taste buds, is expressed in taste cells that communicate with afferent nerve fibers via synaptic transmission. This database highlights the power of LCM coupled with transcriptional profiling to dissect the molecular composition of normal tissues, represents the most comprehensive molecular analysis of primate taste buds to date, and provides a foundation for further studies in diverse aspects of taste biology.

The bHLH transcription factors, Hes6 and Mash1, are expressed in distinct subsets of cells within adult mouse taste buds

Taste buds are multicellular receptor organs embedded in the lingual epithelium of vertebrates. Taste cells within these buds are modified epithelial cells as they lack axons and turnover rapidly throughout life, yet have neuronal properties enabling them to transduce taste stimuli and transmit this information to the nervous system. Taste cells are heterogeneous, comprising types I, II, III and basal cells, and are continually replaced during adult life, raising the question of how these different cells are generated. The molecular mechanisms governing taste cell differentiation are unknown, but the Notch signaling system has been implicated in this process based upon recent gene expression data. Here we investigate the expression in mature taste buds of Notch related transcription factors, Hes6 and Mash1, which are among the first genes expressed in embryonic taste buds. We further compare these patterns with those of immunocytochemical markers of discrete taste cell types. We find that Hes6 is expressed in a subset of basally located, possibly progenitor cells, yet is rarely coexpressed with taste cell markers. In contrast, Mash1 is detected in some basal cells and in the majority of differentiated type III taste cells, but never in type II cells. These data suggest a role for Notch signaling in taste cell differentiation in adult taste buds.

Taste bud contains both short-lived and long-lived cell populations

Taste bud cells undergo continual turnover even in adulthood, and their average lifespan has been estimated as approximately 10 days. However, it is not clear whether this figure can be applied to all the different cell types contained in a taste bud. Here, we describe the age and life cycle of taste bud cells in rat circumvallate papillae, and indicate that the lifespan is heterogeneous, ranging from 2 days to over 3 weeks. Taste bud cells were incorporated from the basal proliferative layer in 1-2 days after birth. After incorporation, approximately half of the cells were eliminated within 2-3 days, and the remaining half were maintained with gradual decrease, suggesting that there are at least two types of cells; short-lived cells and long-lived cells. Moreover, above 10% of the incorporated cells were maintained at 3 weeks. In order to gain information about the relationship between the cell functions and the cell age, we carried out double-labeling experiments using 5-bromo-2'-deoxyuridine and each of two markers for in situ hybridization: mammalian achaete-scute homolog 1 (Mash1) and phospholipase C beta 2 (PLCbeta2) as markers of early differentiation and functional taste signaling, respectively. Mash1 expression began immediately after the incorporation and reached a maximum at 5-6 days after birth. Fewer but distinct Mash1-positive cells were still observed after 3 weeks. PLCbeta2 expression was observed from day 5, reached a maximum at day 12, and continued over 3 weeks. Taken together, a taste bud contains both short-lived and long-lived cells: the short-lived cells are eliminated in a time course similar to the surrounding epithelial cells, and the long-lived cells including taste receptor cells have a lifespan longer than the previous estimation.

Building sensory receptors on the tongue

Neurotrophins, neurotrophin receptors and sensory neurons are required for the development of lingual sense organs. For example, neurotrophin 3 sustains lingual somatosensory neurons. In the traditional view, sensory axons will terminate where neurotrophin expression is most pronounced. Yet, lingual somatosensory axons characteristically terminate in each filiform papilla and in each somatosensory prominence within a cluster of cells expressing the p75 neurotrophin receptor (p75NTR), rather than terminating among the adjacent cells that secrete neurotrophin 3. The p75NTR on special specialized clusters of epithelial cells may promote axonal arborization in vivo since its over-expression by fibroblasts enhances neurite outgrowth from overlying somatosensory neurons in vitro. Two classical observations have implicated gustatory neurons in the development and maintenance of mammalian taste buds--the early arrival times of embryonic innervation and the loss of taste buds after their denervation in adults. In the modern era more than a dozen experimental studies have used early denervation or neurotrophin gene mutations to evaluate mammalian gustatory organ development. Necessary for taste organ development, brain-derived neurotrophic factor sustains developing gustatory neurons. The cardinal conclusion is readily summarized: taste buds in the palate and tongue are induced by innervation. Taste buds are unstable: the death and birth of taste receptor cells relentlessly remodels synaptic connections. As receptor cells turn over, the sensory code for taste quality is probably stabilized by selective synapse formation between each type of gustatory axon and its matching taste receptor cell. We anticipate important new discoveries of molecular interactions among the epithelium, the underlying mesenchyme and gustatory innervation that build the gustatory papillae, their specialized epithelial cells, and the resulting taste buds.

Specific induction of neuronal cells from bone marrow stromal cells and application for autologous transplantation

Bone marrow stromal cells (MSCs) have the capability under specific conditions of differentiating into various cell types such as osteocytes, chondrocytes, and adipocytes. Here we demonstrate a highly efficient and specific induction of cells with neuronal characteristics, without glial differentiation, from both rat and human MSCs using gene transfection with Notch intracellular domain (NICD) and subsequent treatment with bFGF, forskolin, and ciliary neurotrophic factor. MSCs expressed markers related to neural stem cells after transfection with NICD, and subsequent trophic factor administration induced neuronal cells. Some of them showed voltage-gated fast sodium and delayed rectifier potassium currents and action potentials compatible with characteristics of functional neurons. Further treatment of the induced neuronal cells with glial cell line-derived neurotrophic factor (GDNF) increased the proportion of tyrosine hydroxylase-positive and dopamine-producing cells. Transplantation of these GDNF-treated cells showed improvement in apomorphine-induced rotational behavior and adjusting step and paw-reaching tests following intrastriatal implantation in a 6-hydroxy dopamine rat model of Parkinson disease. This study shows that a population of neuronal cells can be specifically generated from MSCs and that induced cells may allow for a neuroreconstructive approach.

Specific induction of neuronal cells from bone marrow stromal cells and application for autologous transplantation

Bone marrow stromal cells (MSCs) have the capability under specific conditions of differentiating into various cell types such as osteocytes, chondrocytes, and adipocytes. Here we demonstrate a highly efficient and specific induction of cells with neuronal characteristics, without glial differentiation, from both rat and human MSCs using gene transfection with Notch intracellular domain (NICD) and subsequent treatment with bFGF, forskolin, and ciliary neurotrophic factor. MSCs expressed markers related to neural stem cells after transfection with NICD, and subsequent trophic factor administration induced neuronal cells. Some of them showed voltage-gated fast sodium and delayed rectifier potassium currents and action potentials compatible with characteristics of functional neurons. Further treatment of the induced neuronal cells with glial cell line-derived neurotrophic factor (GDNF) increased the proportion of tyrosine hydroxylase-positive and dopamine-producing cells. Transplantation of these GDNF-treated cells showed improvement in apomorphine-induced rotational behavior and adjusting step and paw-reaching tests following intrastriatal implantation in a 6-hydroxy dopamine rat model of Parkinson disease. This study shows that a population of neuronal cells can be specifically generated from MSCs and that induced cells may allow for a neuroreconstructive approach.

Notch-associated gene expression in embryonic and adult taste papillae and taste buds suggests a role in taste cell lineage decisions

The Notch signaling pathway is involved in cell fate decisions during development. To explore the role of this signaling cascade in the taste system, we investigated the expression patterns of Notch signaling genes in fetal and adult mouse tongues using in situ hybridization. Three of the four murine Notch receptors, their ligands, Delta-like 1 (Dll-1), Jagged1, and Jagged2, as well as three transcription factors, Hes1, Hes6, and Mash1, are expressed in the embryonic taste epithelium. Expression is first detected in the circumvallate papilla at embryonic day E14.5, when Notch1, Jagged1, and Jagged2 are expressed broadly in the papilla and general lingual epithelium. In contrast, Mash1 and Hes6 are restricted to only a few epithelial cells in the apical region of the developing papilla. By E18.5, many of the genes now exhibit a bimodal expression pattern in the papillary epithelium: apically and dorsally they are expressed in sparse clusters of cells, while more ventrally expression typically occurs throughout the lower regions of the trenches. The extent of papilla innervation was compared with Mash1 and Hes6 expression. At E14.5, when Hes6 and Mash1 are already expressed in small numbers of epithelial cells, PGP9.5 immunoreactive fibers have not yet invaded the epithelium, consistent with the specification of taste bud primordia prior to nerve contact. All of the genes examined (except Notch2) are also expressed in subsets of cells within circumvallate taste buds in adult mice, although Notch1 is restricted to basal cells adjacent to taste buds. The onset of embryonic Notch associated gene expression after the morphological differentiation of the circumvallate papilla argues that this signaling cascade may specify taste receptor cell lineages within an already specified taste papilla. Similarly, Notch gene expression in adult taste buds suggests continued roles in cell lineage determination and cell turnover.

The zinc finger transcription factor Gfi1, implicated in lymphomagenesis, is required for inner ear hair cell differentiation and survival

Gfi1 was first identified as causing interleukin 2-independent growth in T cells and lymphomagenesis in mice. Much work has shown that Gfi1 and Gfi1b, a second mouse homolog, play pivotal roles in blood cell lineage differentiation. However, neither Gfi1 nor Gfi1b has been implicated in nervous system development, even though their invertebrate homologues, senseless in Drosophila and pag-3 in C. elegans are expressed and required in the nervous system. We show that Gfi1 mRNA is expressed in many areas that give rise to neuronal cells during embryonic development in mouse, and that Gfi1 protein has a more restricted expression pattern. By E12.5 Gfi1 mRNA is expressed in both the CNS and PNS as well as in many sensory epithelia including the developing inner ear epithelia. At later developmental stages, Gfi1 expression in the ear is refined to the hair cells and neurons throughout the inner ear. Gfi1 protein is expressed in a more restricted pattern in specialized sensory cells of the PNS, including the eye, presumptive Merkel cells, the lung and hair cells of the inner ear. Gfi1 mutant mice display behavioral defects that are consistent with inner ear anomalies, as they are ataxic, circle, display head tilting behavior and do not respond to noise. They have a unique inner ear phenotype in that the vestibular and cochlear hair cells are differentially affected. Although Gfi1-deficient mice initially specify inner ear hair cells, these hair cells are disorganized in both the vestibule and cochlea. The outer hair cells of the cochlea are improperly innervated and express neuronal markers that are not normally expressed in these cells. Furthermore, Gfi1 mutant mice lose all cochlear hair cells just prior to and soon after birth through apoptosis. Finally, by five months of age there is also a dramatic reduction in the number of cochlear neurons. Hence, Gfi1 is expressed in the developing nervous system, is required for inner ear hair cell differentiation, and its loss causes programmed cell death.

Analysis of cell lineage relationships in taste buds

Taste buds are a heterogeneous population of cells exhibiting diverse morphological and biochemical characteristics. Because taste buds arise from multiple progenitors, the different types of taste cells may represent distinct lineages. The present study was undertaken to determine the following: (1) how many progenitors contribute to a taste bud, and (2) whether the specific subpopulation of serotonin-immunoreactive (IR) taste cells are related by lineage to a restricted set of progenitor cells. These questions were addressed using cell lineage analysis of taste buds from H253 X-inactivation mosaic mice. After random X-inactivation of the lacZ transgene, the tongue of hemizygous female mice displays discrete patches of epithelial cells, which are either beta-galactosidase (beta-gal) positive or beta-gal negative. By analyzing the proportion of the two differently stained cell populations in taste buds located at the boundary between positive and negative epithelial patches, we can determine the minimum number of progenitors that may contribute to the formation of a taste bud. The presence of taste buds containing only 6-12% labeled cells indicates that at least eight progenitors contribute to an average taste bud of 55 cells, assuming progenitors contribute equally to the cell population. Cell lineage analysis of serotonin-IR taste cells in such mixed taste buds suggests that this subpopulation likely arises from only one to two progenitors and often is related by lineage. Thus, at least some of the cell types in a taste bud represent distinct lineages of cells and are not merely phenotypic stages as a cell progresses from a young to a mature state.

Retinoic acid influences the expression of the neuronal regulatory genes Mash-1 and c-ret in the developing rat heart

We analyzed the expression of neuronal regulatory genes Mash-1 and c-ret by immunohistochemistry and reverse transcriptase-polymerase chain reaction in the developing heart of rat embryos following exogenous retinoic acid (RA) treatment of the pregnant dams. On E12, expression of Mash-1 and c-ret was confined to cells migrating via the common cardinal vein. On E16.5, Mash-1 and c-ret expression were restricted to cardiac ganglia around the great vessels and posterior atrial wall. While Mash-1 expression was down-regulated at birth, that of c-Ret was maintained. RA-treated hearts showed a down-regulation of both Mash-1 and c-Ret at the mRNA as well as at the protein level on E16.5. The present results show that differentiation of cardiac ganglionic cells is affected after RA treatment, by the down-regulation of Mash-1 and c-Ret.

Expression of cyclin-dependent kinase inhibitors in taste buds of mouse and hamster

Taste buds are specialized epithelial cell clusters in the oral squamous cell epithelium. Although taste buds have been reported to renew rapidly, the mechanism of cell cycle control in these specialized structures remains unresolved. To clarify the cell cycle status and role of cyclin-dependent kinase inhibitors (CDKI) for cell cycle control in the taste buds, we analyzed cell proliferation activity using bromodeoxyuridine (BrdU) and Ki-67 immunostainings and the expression of the Cip/Kip family of CDKI (p21Cip1, p27Kip1, and p57Kip2) in the circumvallate papillae of mouse and hamster. BrdU-positive cells were detected in the basal layer of the oral epithelium. In the taste buds, Ki-67-positive cells were seen in the basal area, with only a very few positive cells in the taste buds. Both p21Cip1 and p27Kip1 positive cells were seen in the suprabasal layer of the non-gustatory oral epithelium. In the taste buds, stronger p27Kip1 staining was detected than in the non-gustatory epithelium. Western blotting analysis revealed that p27Kip1 was abundant in the mucosal tissues from circumvallate papillae. Thus, our study suggests that the taste bud cells except for basal cells are post-mitotic cells and that the cell cycle arrest associated with taste bud cell differentiation could be regulated predominantly by p27Kip1.

Role of brain-derived neurotrophic factor in target invasion in the gustatory system

Brain-derived neurotrophic factor (BDNF) is a survival factor for different classes of neurons, including gustatory neurons. We have studied innervation and development of the gustatory system in transgenic mice overexpressing BDNF under the control of regulatory sequences from the nestin gene, an intermediate filament gene expressed in precursor cells of the developing nervous system and muscle. In transgenic mice, the number and size of gustatory papillae were decreased, circumvallate papillae had a deranged morphology, and there was also a severe loss of lingual taste buds. Paradoxically, similar deficits have been found in BDNF knock-out mice, which lack gustatory neurons. However, the number of neurons in gustatory ganglia was increased in BDNF-overproducing mice. Although gustatory fibers reached the tongue in normal numbers, the amount and density of nerve fibers in gustatory papillae were reduced in transgenic mice compared with wild-type littermates. Gustatory fibers appeared stalled at the base of the tongue, a site of ectopic BDNF expression, where they formed abnormal branches and sprouts. Interestingly, palatal taste buds, which are innervated by gustatory neurons whose afferents do not traverse sites of ectopic BDNF expression, appeared unaffected. We suggest that lingual gustatory deficits in BDNF overexpressing mice are a consequence of the failure of their BDNF-dependent afferents to reach their targets because of the effects of ectopically expressed BDNF on fiber growth. Our findings suggest that mammalian taste buds and gustatory papillae require proper BDNF-dependent gustatory innervation for development and that the correct spatial expression of BDNF in the tongue epithelium is crucial for appropriate target invasion and innervation.