Bone morphogenetic proteins and noggin: inhibiting and inducing fungiform taste papilla development

Morphogenesis of fungiform papillae in developing miniature pigs

Objective

Fungiform papillae contain taste buds and play a critical role in mastication and the gustatory system. In this study, we report a series of sequential observations of organogenesis of fungiform papillae in miniature pigs, as well as changes in the expression of BMP2, BMP4, Wnt5a, Sox2, and Notch1 signaling pathway components.

Design

In this study, we investigated the spatiotemporal expression patterns of BMP, Wnt, Sox2 and Notch in the fungiform papillae of miniature pigs at the bud stage (E40), cap stage (E50) and bell stage (E60). Pregnant miniature pigs were obtained, and the samples were processed for histological staining. Immunohistochemistry and real-time PCR were used to detect the mRNA and protein expression levels of BMP2, BMP4, Wnt5a, Sox2, and Notch1.

Results

At E40, fungiform papillae were present on the anterior two-thirds of the tongue in a specific array and pattern. The fungiform papillae were enlarged and basically developed at E50 and were largest at the earlier stage (E60). Most of the BMP2 was concentrated in the epithelial layer and the connective tissue core of the fungal papilloma and gradually accumulated from E40-E60. BMP-4 was weakly expressed in the fungiform papillae epithelia, but BMP-4-positive cells were also observed in the developing tongue muscle at E50 and E60. Wnt5a-positive cells were observed in the fungiform papillae epithelia and developing tongue muscle at all three time points. Sox2-positive cells were observed only in fungiform papillae epithelial cells, and Notch1-positive cells could not be detected.

Conclusions

This study provides primary data regarding the morphogenesis and expression of developmental signals in the fungiform papillae of miniature pigs, establishing a foundation for further research in both this model and humans.

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.

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.

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.

Quantitative gene expression dynamics of key placode signalling factors in the embryonic chicken scleral ossicle system

Development of the scleral ossicles, a ring of bony elements within the sclera, is directed by a series of papillae that arise from placodes in the conjunctival epithelium over a 1.5-day induction period in the chicken embryo. The regular spacing of the papillae around the corneal-scleral limbus suggests that their induction may be regulated by a reaction-diffusion mechanism, similar to other epithelial appendages. Some key placode signalling molecules, including β-catenin, are known to be expressed throughout the induction period. However, others have been studied only at certain stages or have not been successfully detected. Here we use qPCR to study the gene expression patterns of the wingless integration (WNT)/β-catenin, bone morphogenetic protein (BMP), ectodysplasin (EDA), fibroblast growth factor (FGF) and hedgehog (HH) signalling families in discrete regions of the eye throughout the complete conjunctival placode and papillae induction period. This comprehensive analysis revealed a variable level of gene expression within specific eye regions, with some genes exhibiting high, moderate or low changes. Most genes exhibited an initial increase in gene expression, followed by a decrease or plateau as development proceeded, suggesting that some genes are important for a brief initial period whilst the sustained elevated expression level of other genes is needed for developmental progression. The timing or magnitude of these changes, and/or the overall gene expression trend differed in the temporal, nasal and/or dorsal eye regions for some, but not all genes, demonstrating that gene expression may vary across different eye regions. Temporal and nasal EDA receptor (EDAR) had the greatest number of strong correlations (r > 0.700) with other genes and β-catenin had the greatest number of moderate correlations (r = 0.400-0.700), while EDA had the greatest range in correlation strengths. Among the strongly correlated genes, two distinct signalling modules were identified, connected by some intermediate genes. The dynamic gene expression patterns of the five signalling pathways studied here from conjunctival placode formation through to papillae development is consistent with other epithelial appendages and confirms the presence of a conserved induction and patterning signalling network. Two unique gene expression patterns and corresponding gene interaction modules suggest functionally distinct roles throughout placode development. Furthermore, spatial differences in gene expression patterns among the temporal, nasal and dorsal regions of the eye may indicate that the expression of certain genes is influenced by mechanical forces exerted throughout development. Therefore, this study identifies key placode signalling factors and their interactions, as well as some potential region-specific features of gene expression in the scleral ossicle system and provides a basis for further exploration of the spatial expression of these genes and the patterning mechanism(s) active throughout conjunctival placode and papillae formation.

Action of Actomyosin Contraction With Shh Modulation Drive Epithelial Folding in the Circumvallate Papilla

The mouse tongue possesses three types of gustatory papillae: large circumvallate papillae (CVP), foliate papillae (FOP) and fungiform papillae (FFP). Although CVP is the largest papilla and contain a high density of taste buds, little is known about CVP development. Their transition from placode to dome-shape is particularly ambiguous. Understanding this phase is crucial since dome-shaped morphology is essential for proper localization of the imminent nerve fibers and taste buds. Here, we report actomyosin-dependent apical and basal constriction of epithelial cells during dynamic epithelial folding. Furthermore, actomyosin-dependent basal constriction requires focal adhesion kinase to guide dome-shape formation. Sonic hedgehog (Shh) is closely associated with the differentiation or survival of the neurons in CVP ganglion and cytoskeletal alteration in trench epithelial cells which regulate CVP morphogenesis. Our results demonstrate the CVP morphogenesis mechanism from placode to dome-shape by actomyosin-dependent cell shape change and suggest roles that Shh may play in trench and stromal core formation during CVP development.

Increased activity of mesenchymal ALK2-BMP signaling causes posteriorly truncated microglossia and disorganization of lingual tissues

Proper development of taste organs including the tongue and taste papillae requires interactions with the underlying mesenchyme through multiple molecular signaling pathways. The effects of bone morphogenetic proteins (BMPs) and antagonists are profound, however, the tissue-specific roles of distinct receptors are largely unknown. Here, we report that constitutive activation (ca) of ALK2-BMP signaling in the tongue mesenchyme (marked by Wnt1-Cre) caused microglossia-a dramatically smaller and misshapen tongue with a progressively severe reduction in size along the anteroposterior axis and absence of a pharyngeal region. At E10.5, the tongue primordia (branchial arches 1-4) formed in Wnt1-Cre/caAlk2 mutants while each branchial arch responded to elevated BMP signaling distinctly in gene expression of BMP targets (Id1, Snai1, Snai2, and Runx2), proliferation (Cyclin-D1) and apoptosis (p53). Moreover, elevated ALK2-BMP signaling in the mesenchyme resulted in apparent defects of lingual epithelium, muscles, and nerves. In Wnt1-Cre/caAlk2 mutants, a circumvallate papilla was missing and further development of formed fungiform papillae was arrested in late embryos. Our data collectively demonstrate that ALK2-BMP signaling in the mesenchyme plays essential roles in orchestrating various tissues for proper development of the tongue and its appendages in a region-specific manner.

Effects of insulin signaling on mouse taste cell proliferation

Expression of insulin and its receptor (IR) in rodent taste cells has been proposed, but exactly which types of taste cells express IR and the function of insulin signaling in taste organ have yet to be determined. In this study, we analyzed expression of IR mRNA and protein in mouse taste bud cells in vivo and explored its function ex vivo in organoids, using RT-PCR, immunohistochemistry, and quantitative PCR. In mouse taste tissue, IR was expressed broadly in taste buds, including in type II and III taste cells. With using 3-D taste bud organoids, we found insulin in the culture medium significantly decreased the number of taste cell and mRNA expression levels of many taste cell genes, including nucleoside triphosphate diphosphohydrolase-2 (NTPDase2), Tas1R3 (T1R3), gustducin, carbonic anhydrase 4 (CA4), glucose transporter-8 (GLUT8), and sodium-glucose cotransporter-1 (SGLT1) in a concentration-dependent manner. Rapamycin, an inhibitor of mechanistic target of rapamycin (mTOR) signaling, diminished insulin's effects and increase taste cell generation. Altogether, circulating insulin might be an important regulator of taste cell growth and/or proliferation via activation of the mTOR pathway.

An overlooked placode: Recharacterizing the papillae in the embryonic eye of reptilia

The papillae in the chicken embryonic eye, described as scleral papillae in the well-known Hamburger and Hamilton (1951) staging table, are one of the key anatomical features used to stage reptilian (including bird) embryos from HH30-36. These papillae are epithelial thickenings of the conjunctiva and are situated above the mesenchymal sclera. Here, we present evidence that the conjunctival papillae, which are required for the induction and patterning of the underlying scleral ossicles, require epithelial pre-patterning and have a placodal stage similar to other placode systems. We also suggest modifications to the Hamburger Hamilton staging criteria that incorporate this change in terminology (from "scleral" to "conjunctival" papillae) and provide a more detailed description of this anatomical feature that includes its placode stage. This enables a more complete and accurate description of chick embryo staging. The acknowledgment of a placode phase, which shares molecular and morphological features with other cutaneous placodes, will direct future research into the early inductive events leading to scleral ossicle formation.

Early taste buds are from Shh+ epithelial cells of tongue primordium in distinction from mature taste bud cells which arise from surrounding tissue compartments

Mammalian taste buds emerge perinatally and most become mature 3-4 weeks after birth. Mature taste bud cells in rodents are known to be renewed by the surrounding K14+ basal epithelial cells and potentially other progenitor source(s), but the dynamics between initially developed taste buds and surrounding tissue compartments are unclear. Using the K14-Cre and Dermo1-Cre mouse lines to trace epithelial and mesenchymal cell lineages, we found that early taste buds in E18.5 and newborn mouse tongues are not derived from either lineage. At E11.5 when the tongue primordia (i.e., lingual swellings) emerge, the relatively homogeneous sonic hedgehog-expressing (Shh+) epithelial cells express Keratin (K) 8, a marker that is widely used to label taste buds. Mapping lineage of E11.0 Shh+ epithelium of the tongue rudiment with Shh-CreERT2/RFP mice demonstrated that both the early taste buds and the surrounding lingual epithelium are from the same population of progenitors - Shh+ epithelial cells of the tongue primordium. In combination with previous reports, we propose that Shh+K8+ cells in the homogeneous epithelium of tongue primordium at early embryonic stages are programmed to become taste papilla and taste bud cells. Switching off Shh and K8 expression in the Shh+ epithelial cells of the tongue primordium transforms the cells to non-gustatory cells surrounding papillae, including K14+ basal epithelial cells which will eventually contribute to the cell renewal of mature taste buds.

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.

Organoid Culture of Lingual Epithelial Cells in a Three-Dimensional Matrix

A novel lingual epithelial organoid culture system using a three-dimensional (3D) matrix and growth factors has recently been established. In the culture system, organoids with multilayered squamous keratinized epithelium and typical morphological features of filiform papillae are generated from single lingual epithelial cells at a high efficiency. The culture system is created in order to observe the differentiation and maturation process of each lingual epithelial stem cell and to observe abnormal organoid formation from malignant cells obtained from carcinogen-treated mice. Thus, our culture system will contribute to the advancement of research into the regulatory mechanism of lingual epithelium and the underlying mechanisms of carcinogenesis.

An Update on the Sense of Taste in Chickens: A Better Developed System than Previously Appreciated

Taste is important in guiding nutritive choices and motivating food intake. The sensory organs for taste are the taste buds, that transduce gustatory stimuli into neural signals. It has been reported that chickens have a low taste bud number and thus low taste acuity. However, more recent studies indicate that chickens have a well-developed taste system and the reported number and distribution of taste buds may have been significantly underestimated. Chickens, as a well-established animal model for research, are also the major species of animals in the poultry industry. Thus, a clear understanding of taste organ formation and the effects of taste sensation on nutrition and feeding practices is important for improving livestock production strategies. In this review, we provide an update on recent findings in chicken taste buds and taste sensation indicating that the chicken taste organ is better developed than previously thought and can serve as an ideal system for multidisciplinary studies including organogenesis, regenerative medicine, feeding and nutritional choices.

Epithelial cell behaviours during neurosensory organ formation

Perception of the environment in vertebrates relies on a variety of neurosensory mini-organs. These organs develop via a multi-step process that includes placode induction, cell differentiation, patterning and innervation. Ultimately, cells derived from one or more different tissues assemble to form a specific mini-organ that exhibits a particular structure and function. The initial building blocks of these organs are epithelial cells that undergo rearrangements and interact with neighbouring tissues, such as neural crest-derived mesenchymal cells and sensory neurons, to construct a functional sensory organ. In recent years, advances in in vivo imaging methods have allowed direct observation of these epithelial cells, showing that they can be displaced within the epithelium itself via several modes. This Review focuses on the diversity of epithelial cell behaviours that are involved in the formation of small neurosensory organs, using the examples of dental placodes, hair follicles, taste buds, lung neuroendocrine cells and zebrafish lateral line neuromasts to highlight both well-established and newly described modes of epithelial cell motility.

RNA-Seq analysis on chicken taste sensory organs: An ideal system to study organogenesis

RNA-Seq is a powerful tool in transcriptomic profiling of cells and tissues. We recently identified many more taste buds than previously appreciated in chickens using molecular markers to stain oral epithelial sheets of the palate, base of oral cavity, and posterior tongue. In this study, RNA-Seq was performed to understand the transcriptomic architecture of chicken gustatory tissues. Interestingly, taste sensation related genes and many more differentially expressed genes (DEGs) were found between the epithelium and mesenchyme in the base of oral cavity as compared to the palate and posterior tongue. Further RNA-Seq using specifically defined tissues of the base of oral cavity demonstrated that DEGs between gustatory (GE) and non-gustatory epithelium (NGE), and between GE and the underlying mesenchyme (GM) were enriched in multiple GO terms and KEGG pathways, including many biological processes. Well-known genes for taste sensation were highly expressed in the GE. Moreover, genes of signaling components important in organogenesis (Wnt, TGFβ/ BMP, FGF, Notch, SHH, Erbb) were differentially expressed between GE and GM. Combined with other features of chicken taste buds, e.g., uniquely patterned array and short turnover cycle, our data suggest that chicken gustatory tissue provides an ideal system for multidisciplinary studies, including organogenesis and regenerative medicine.

Transcriptome analyses of taste organoids reveal multiple pathways involved in taste cell generation

Taste cells undergo constant turnover throughout life; however, the molecular mechanisms governing taste cell generation are not well understood. Using RNA-Seq, we systematically surveyed the transcriptome landscape of taste organoids at different stages of growth. Our data show the staged expression of a variety of genes and identify multiple signaling pathways underlying taste cell differentiation and taste stem/progenitor cell proliferation. For example, transcripts of taste receptors appear only or predominantly in late-stage organoids. Prior to that, transcription factors and other signaling elements are upregulated. RNA-Seq identified a number of well-characterized signaling pathways in taste organoid cultures, such as those involving Wnt, bone morphogenetic proteins (BMPs), Notch, and Hedgehog (Hh). By pharmacological manipulation, we demonstrate that Wnt, BMPs, Notch, and Hh signaling pathways are necessary for taste cell proliferation, differentiation and cell fate determination. The temporal expression profiles displayed by taste organoids may also lead to the identification of currently unknown transducer elements underlying sour, salt, and other taste qualities, given the staged expression of taste receptor genes and taste transduction elements in cultured organoids.

FGF signaling refines Wnt gradients to regulate the patterning of taste papillae

The patterning of repeated structures is a major theme in developmental biology, and the inter-relationship between spacing and size of such structures is an unresolved issue. Fungiform papillae are repeated epithelial structures that house taste buds on the anterior tongue. Here, we report that FGF signaling is a crucial regulator of fungiform papillae development. We found that mesenchymal FGF10 controls the size of the papillary area, while overall patterning remains unchanged. Our results show that FGF signaling negatively affects the extent of canonical Wnt signaling, which is the main activation pathway during fungiform papillae development; however, this effect does not occur at the level of gene transcription. Rather, our experimental data, together with computational modeling, indicate that FGF10 modulates the range of Wnt effects, likely via induction of Sostdc1 expression. We suggest that modification of the reach of Wnt signaling could be due to local changes in morphogen diffusion, representing a novel mechanism in this tissue context, and we propose that this phenomenon might be involved in a broader array of mammalian developmental processes.

Tongue and Taste Organ Biology and Function: Homeostasis Maintained by Hedgehog Signaling

The tongue is an elaborate complex of heterogeneous tissues with taste organs of diverse embryonic origins. The lingual taste organs are papillae, composed of an epithelium that includes specialized taste buds, the basal lamina, and a lamina propria core with matrix molecules, fibroblasts, nerves, and vessels. Because taste organs are dynamic in cell biology and sensory function, homeostasis requires tight regulation in specific compartments or niches. Recently, the Hedgehog (Hh) pathway has emerged as an essential regulator that maintains lingual taste papillae, taste bud and progenitor cell proliferation and differentiation, and neurophysiological function. Activating or suppressing Hh signaling, with genetic models or pharmacological agents used in cancer treatments, disrupts taste papilla and taste bud integrity and can eliminate responses from taste nerves to chemical stimuli but not to touch or temperature. Understanding Hh regulation of taste organ homeostasis contributes knowledge about the basic biology underlying taste disruptions in patients treated with Hh pathway inhibitors.

Generation of intestinal surface: an absorbing tale

The vertebrate small intestine requires an enormous surface area to effectively absorb nutrients from food. Morphological adaptations required to establish this extensive surface include generation of an extremely long tube and convolution of the absorptive surface of the tube into villi and microvilli. In this Review, we discuss recent findings regarding the morphogenetic and molecular processes required for intestinal tube elongation and surface convolution, examine shared and unique aspects of these processes in different species, relate these processes to known human maladies that compromise absorptive function and highlight important questions for future research.

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

The sense of taste, or gustation, is mediated by taste buds, which are housed in specialized taste papillae found in a stereotyped pattern on the surface of the tongue. Each bud, regardless of its location, is a collection of ∼100 cells that belong to at least five different functional classes, which transduce sweet, bitter, salt, sour and umami (the taste of glutamate) signals. Taste receptor cells harbor functional similarities to neurons but, like epithelial cells, are rapidly and continuously renewed throughout adult life. Here, I review recent advances in our understanding of how the pattern of taste buds is established in embryos and discuss the cellular and molecular mechanisms governing taste cell turnover. I also highlight how these findings aid our understanding of how and why many cancer therapies result in taste dysfunction.

Villification in the mouse: Bmp signals control intestinal villus patterning

In the intestine, finger-like villi provide abundant surface area for nutrient absorption. During murine villus development, epithelial Hedgehog (Hh) signals promote aggregation of subepithelial mesenchymal clusters that drive villus emergence. Clusters arise first dorsally and proximally and spread over the entire intestine within 24 h, but the mechanism driving this pattern in the murine intestine is unknown. In chick, the driver of cluster pattern is tensile force from developing smooth muscle, which generates deep longitudinal epithelial folds that locally concentrate the Hh signal, promoting localized expression of cluster genes. By contrast, we show that in mouse, muscle-induced epithelial folding does not occur and artificial deformation of the epithelium does not determine the pattern of clusters or villi. In intestinal explants, modulation of Bmp signaling alters the spatial distribution of clusters and changes the pattern of emerging villi. Increasing Bmp signaling abolishes cluster formation, whereas inhibiting Bmp signaling leads to merged clusters. These dynamic changes in cluster pattern are faithfully simulated by a mathematical model of a Turing field in which an inhibitor of Bmp signaling acts as the Turing activator. In vivo, genetic interruption of Bmp signal reception in either epithelium or mesenchyme reveals that Bmp signaling in Hh-responsive mesenchymal cells controls cluster pattern. Thus, unlike in chick, the murine villus patterning system is independent of muscle-induced epithelial deformation. Rather, a complex cocktail of Bmps and Bmp signal modulators secreted from mesenchymal clusters determines the pattern of villi in a manner that mimics the spread of a self-organizing Turing field.

Lingual Epithelial Stem Cells and Organoid Culture of Them

As tongue cancer is one of the major malignant cancers in the world, understanding the mechanism of maintenance of lingual epithelial tissue, which is known to be the origin of tongue cancer, is unquestionably important. However, the actual stem cells that are responsible for the long-term maintenance of the lingual epithelium have not been identified. Moreover, a simple and convenient culture method for lingual epithelial stem cells has not yet been established. Recently, we have shown that Bmi1-positive cells, residing at the second or third layer of the epithelial cell layer at the base of the interpapillary pit (IPP), were slow-cycling and could supply keratinized epithelial cells for over one year, indicating that Bmi1-positive cells are long-term lingual epithelial stem cells. In addition, we have developed a novel lingual epithelium organoid culture system using a three-dimensional matrix and growth factors. Here, we discuss current progress in the identification of lingual stem cells and future applications of the lingual culture system for studying the regulatory mechanisms of the lingual epithelium and for regenerative medicine.

Diversity in cell motility reveals the dynamic nature of the formation of zebrafish taste sensory organs

Taste buds are sensory organs in jawed vertebrates, composed of distinct cell types that detect and transduce specific taste qualities. Taste bud cells differentiate from oropharyngeal epithelial progenitors localized mainly in proximity of the forming organs. Despite recent progress in elucidating the molecular interactions required for taste bud cell development and function, the cell behaviour underlying the organ assembly is poorly defined. Here, we used time-lapse imaging to observe the formation of taste buds in live zebrafish larvae. We found that tg(fgf8a.dr17) expressing cells form taste buds and get rearranged within the forming organs. In addition, differentiating cells move from the epithelium to the forming organs and can be displaced between developing organs. During organ formation, taste bud tg(fgf8a.dr17) and Type-II cells are displaced in random, directed or confined mode relative to the taste bud they join or are maintained. Finally, ascl1a activity in the 5-HT/Type-III cell is required to direct and maintain tg(fgf8a.dr17) expressing cells into the taste bud. We propose diversity in displacement modes of differentiating cells as a key mechanism for the highly dynamic process of taste bud assembly.

Coevolutionary patterning of teeth and taste buds

Teeth and taste buds are iteratively patterned structures that line the oro-pharynx of vertebrates. Biologists do not fully understand how teeth and taste buds develop from undifferentiated epithelium or how variation in organ density is regulated. These organs are typically studied independently because of their separate anatomical location in mammals: teeth on the jaw margin and taste buds on the tongue. However, in many aquatic animals like bony fishes, teeth and taste buds are colocalized one next to the other. Using genetic mapping in cichlid fishes, we identified shared loci controlling a positive correlation between tooth and taste bud densities. Genome intervals contained candidate genes expressed in tooth and taste bud fields. sfrp5 and bmper, notable for roles in Wingless (Wnt) and bone morphogenetic protein (BMP) signaling, were differentially expressed across cichlid species with divergent tooth and taste bud density, and were expressed in the development of both organs in mice. Synexpression analysis and chemical manipulation of Wnt, BMP, and Hedgehog (Hh) pathways suggest that a common cichlid oral lamina is competent to form teeth or taste buds. Wnt signaling couples tooth and taste bud density and BMP and Hh mediate distinct organ identity. Synthesizing data from fish and mouse, we suggest that the Wnt-BMP-Hh regulatory hierarchy that configures teeth and taste buds on mammalian jaws and tongues may be an evolutionary remnant inherited from ancestors wherein these organs were copatterned from common epithelium.

Migration of oligodendrocyte progenitor cells is controlled by transforming growth factor β family proteins during corticogenesis

During embryonic development oligodendrocyte precursor cells (OPCs) are generated first in the ventral forebrain and migrate dorsally to occupy the cortex. The molecular cues that guide this migratory route are currently completely unknown. Here, we show that bone morphogenetic protein-4 (Bmp4), Bmp7, and Tgfβ1 produced by the meninges and pericytes repelled ventral OPCs into the cortex at mouse embryonic stages. Ectopic activation of Bmp or Tgfβ1 signaling before the entrance of OPCs into the cortex hindered OPC migration into the cortical areas. OPCs without Smad4 signaling molecules also failed to migrate into the cortex efficiently and formed heterotopia in ventral areas. OPC migration into the cortex was also dramatically reduced by conditional inhibition of Tgfβ1 or Bmp expression from mesenchymal cells. The data suggest that mesenchymal Tgfβ family proteins promote migration of ventral OPCs into the cortex during corticogenesis.

Developing and regenerating a sense of taste

Taste is one of the fundamental senses, and it is essential for our ability to ingest nutritious substances and to detect and avoid potentially toxic ones. Taste buds, which are clusters of neuroepithelial receptor cells, are housed in highly organized structures called taste papillae in the oral cavity. Whereas the overall structure of the taste periphery is conserved in almost all vertebrates examined to date, the anatomical, histological, and cell biological, as well as potentially the molecular details of taste buds in the oral cavity are diverse across species and even among individuals. In mammals, several types of gustatory papillae reside on the tongue in highly ordered arrangements, and the patterning and distribution of the mature papillae depend on coordinated molecular events in embryogenesis. In this review, we highlight new findings in the field of taste development, including how taste buds are patterned and how taste cell fate is regulated. We discuss whether a specialized taste bud stem cell population exists and how extrinsic signals can define which cell lineages are generated. We also address the question of whether molecular regulation of taste cell renewal is analogous to that of taste bud development. Finally, we conclude with suggestions for future directions, including the potential influence of the maternal diet and maternal health on the sense of taste in utero.

The formation of endoderm-derived taste sensory organs requires a Pax9-dependent expansion of embryonic taste bud progenitor cells

In mammals, taste buds develop in different regions of the oral cavity. Small epithelial protrusions form fungiform papillae on the ectoderm-derived dorsum of the tongue and contain one or few taste buds, while taste buds in the soft palate develop without distinct papilla structures. In contrast, the endoderm-derived circumvallate and foliate papillae located at the back of the tongue contain a large number of taste buds. These taste buds cluster in deep epithelial trenches, which are generated by intercalating a period of epithelial growth between initial placode formation and conversion of epithelial cells into sensory cells. How epithelial trench formation is genetically regulated during development is largely unknown. Here we show that Pax9 acts upstream of Pax1 and Sox9 in the expanding taste progenitor field of the mouse circumvallate papilla. While a reduced number of taste buds develop in a growth-retarded circumvallate papilla of Pax1 mutant mice, its development arrests completely in Pax9-deficient mice. In addition, the Pax9 mutant circumvallate papilla trenches lack expression of K8 and Prox1 in the taste bud progenitor cells, and gradually differentiate into an epidermal-like epithelium. We also demonstrate that taste placodes of the soft palate develop through a Pax9-dependent induction. Unexpectedly, Pax9 is dispensable for patterning, morphogenesis and maintenance of taste buds that develop in ectoderm-derived fungiform papillae. Collectively, our data reveal an endoderm-specific developmental program for the formation of taste buds and their associated papilla structures. In this pathway, Pax9 is essential to generate a pool of taste bud progenitors and to maintain their competence towards prosensory cell fate induction.

The interactions of TGF-beta signalling pathway and Jagged2/Notch1 pathway induce acanthosis in lingual epithelia

The aims of this study were to distinguish between the primary and secondary effects of TGF-β signalling disruption by Dox treatment in NTPDase2+ cells; and to investigate the interactions between TGF-β signalling and Jagged2/Notch1 pathway in regulating the expansion of tongue epithelia stem cells.Transgenic mice expressing rtTA from the mouse NTPDase2 promoter or K14 promoter were used to generate an inducible dominant negative TGF-β receptor type II (Tgfbr2) mutant model.Disruption of TGF-β signalling in NTPDase2+ cells initially inhibited the formation of filiform papillae but led to their regeneration over time. In contrast, disruption of TGF-β signalling induced proliferation of lingual epithelia in the middle tongue. We also observed the proliferation of lingual epithelia in the posterior tongue near the circumvallate papillae. Interactions among the TGF-β signalling pathways, Jagged2/Notch1 signalling pathways and epigenetic modifications regulate the expansion of lingual epithelial stem cells. Different molecular mechanisms are involved in the developmental regulation of lingual epithelia and filiform papillae, dependent on the location along the whole tongue. The fluctuating phenotype of tongue epithelia, over time, may be the combined effects of signalling pathways and epigenetic modifications.

WT1 regulates the development of the posterior taste field

Despite the importance of taste in determining nutrient intake, our understanding of the processes that control the development of the peripheral taste system is lacking. Several early regulators of taste development have been identified, including sonic hedgehog, bone morphogenetic protein 4 and multiple members of the Wnt/β-catenin signaling pathway. However, the regulation of these factors, including their induction, remains poorly understood. Here, we identify a crucial role for the Wilms' tumor 1 protein (WT1) in circumvallate (CV) papillae development. WT1 is a transcription factor that is important in the normal development of multiple tissues, including both the olfactory and visual systems. In mice, WT1 expression is detectable by E12.5, when the CV taste placode begins to form. In mice lacking WT1, the CV fails to develop normally and markers of early taste development are dysregulated compared with wild type. We demonstrate that expression of the WT1 target genes Lef1, Ptch1 and Bmp4 is significantly reduced in developing tongue tissue derived from Wt1 knockout mice and that, in normal tongue, WT1 is bound to the promoter regions of these genes. Moreover, siRNA knockdown of WT1 in cultured taste cells leads to a reduction in the expression of Lef1 and Ptch1. Our data identify WT1 as a crucial transcription factor in the development of the CV through the regulation of multiple signaling pathways that have established roles in the formation and patterning of taste placodes.

Taste bud homeostasis in health, disease, and aging

The mammalian taste bud is an onion-shaped epithelial structure with 50-100 tightly packed cells, including taste receptor cells, supporting cells, and basal cells. Taste receptor cells detect nutrients and toxins in the oral cavity and transmit the sensory information to gustatory nerve endings in the buds. Supporting cells may play a role in the clearance of excess neurotransmitters after their release from taste receptor cells. Basal cells are precursor cells that differentiate into mature taste cells. Similar to other epithelial cells, taste cells turn over continuously, with an average life span of about 8-12 days. To maintain structural homeostasis in taste buds, new cells are generated to replace dying cells. Several recent studies using genetic lineage tracing methods have identified populations of progenitor/stem cells for taste buds, although contributions of these progenitor/stem cell populations to taste bud homeostasis have yet to be fully determined. Some regulatory factors of taste cell differentiation and degeneration have been identified, but our understanding of these aspects of taste bud homoeostasis remains limited. Many patients with various diseases develop taste disorders, including taste loss and taste distortion. Decline in taste function also occurs during aging. Recent studies suggest that disruption or alteration of taste bud homeostasis may contribute to taste dysfunction associated with disease and aging.

Establishment of a novel lingual organoid culture system: generation of organoids having mature keratinized epithelium from adult epithelial stem cells

Despite the strong need for the establishment of a lingual epithelial cell culture system, a simple and convenient culture method has not yet been established. Here, we report the establishment of a novel lingual epithelium organoid culture system using a three-dimensional matrix and growth factors. Histological analyses showed that the generated organoids had both a stratified squamous epithelial cell layer and a stratum corneum. Very recently, we showed via a multicolor lineage tracing method that Bmi1-positive stem cells exist at the base of the epithelial basal layer in the interpapillary pit. Using our new culture system, we found that organoids could be generated by single Bmi1-positive stem cells and that in the established organoids, multiple Bmi1-positive stem cells were generated at the outermost layer. Moreover, we observed that organoids harvested at an early point in culture could be engrafted and maturate in the tongue of recipient mice and that the organoids generated from carcinogen-treated mice had an abnormal morphology. Thus, this culture system presents valuable settings for studying not only the regulatory mechanisms of lingual epithelium but also lingual regeneration and carcinogenesis.

Multiple Shh signaling centers participate in fungiform papilla and taste bud formation and maintenance

The adult fungiform taste papilla is a complex of specialized cell types residing in the stratified squamous tongue epithelium. This unique sensory organ includes taste buds, papilla epithelium and lateral walls that extend into underlying connective tissue to surround a core of lamina propria cells. Fungiform papillae must contain long-lived, sustaining or stem cells and short-lived, maintaining or transit amplifying cells that support the papilla and specialized taste buds. Shh signaling has established roles in supporting fungiform induction, development and patterning. However, for a full understanding of how Shh transduced signals act in tongue, papilla and taste bud formation and maintenance, it is necessary to know where and when the Shh ligand and pathway components are positioned. We used immunostaining, in situ hybridization and mouse reporter strains for Shh, Ptch1, Gli1 and Gli2-expression and proliferation markers to identify cells that participate in hedgehog signaling. Whereas there is a progressive restriction in location of Shh ligand-expressing cells, from placode and apical papilla cells to taste bud cells only, a surrounding population of Ptch1 and Gli1 responding cells is maintained in signaling centers throughout papilla and taste bud development and differentiation. The Shh signaling targets are in regions of active cell proliferation. Using genetic-inducible lineage tracing for Gli1-expression, we found that Shh-responding cells contribute not only to maintenance of filiform and fungiform papillae, but also to taste buds. A requirement for normal Shh signaling in fungiform papilla, taste bud and filiform papilla maintenance was shown by Gli2 constitutive activation. We identified proliferation niches where Shh signaling is active and suggest that epithelial and mesenchymal compartments harbor potential stem and/or progenitor cell zones. In all, we report a set of hedgehog signaling centers that regulate development and maintenance of taste organs, the fungiform papilla and taste bud, and surrounding lingual cells. Shh signaling has roles in forming and maintaining fungiform papillae and taste buds, most likely via stage-specific autocrine and/or paracrine mechanisms, and by engaging epithelial/mesenchymal interactions.

Developing a sense of taste

Taste buds are found in a distributed array on the tongue surface, and are innervated by cranial nerves that convey taste information to the brain. For nearly a century, taste buds were thought to be induced by nerves late in embryonic development. However, this view has shifted dramatically. A host of studies now indicate that taste bud development is initiated and proceeds via processes that are nerve-independent, occur long before birth, and governed by cellular and molecular mechanisms intrinsic to the developing tongue. Here we review the state of our understanding of the molecular and cellular regulation of taste bud development, incorporating important new data obtained through the use of two powerful genetic systems, mouse and zebrafish.

Bmp signalling in filiform tongue papillae development

OBJECTIVE

Tongue papillae are critical organs in mastication. There are four different types of tongue papillae; fungiform, circumvallate, foliate, and filiform papillae. Unlike the other three taste papillae, non-gustatory papillae, filiform papillae cover the entire dorsal surface of the tongue and are important structures for the mechanical stress of sucking. Filiform papillae are further classified into two subtypes with different morphologies, depending on their location on the dorsum of the tongue. The filiform papillae at the intermolar eminence have pointed tips, whereas filiform papillae with rounded tips are found in other regions (anterior tongue). It remains unknown how the shape of each type of filiform papillae are determined during their development. Bmp signalling pathway has been known to regulate mechanisms that determine the shapes of many ectodermal organs. The aim of this study was to investigate the role of Bmp signalling in filiform papillae development.

DESIGN

Comparative in situ hybridization analysis of six Bmps (Bmp2-Bmp7) and two Bmpr genes (Bmpr1a and Bmpr1b) were carried out in filiform papillae development. We further examined tongue papillae in mice over-expressing Noggin under the keratin14 promoter (K14-Noggin).

RESULTS

We identified a dynamic temporo-spatial expression of Bmps in filiform papillae development. The K14-Noggin mice showed pointed filiform papillae in regions of the tongue normally occupied by the rounded type.

CONCLUSIONS

Bmp signalling thus regulates the shape of filiform papillae.

Separate and distinctive roles for Wnt5a in tongue, lingual tissue and taste papilla development

Although canonical Wnt signaling is known to regulate taste papilla induction and numbers, roles for noncanonical Wnt pathways in tongue and taste papilla development have not been explored. With mutant mice and whole tongue organ cultures we demonstrate that Wnt5a protein and message are within anterior tongue mesenchyme across embryo stages from the initiation of tongue formation, through papilla placode appearance and taste papilla development. The Wnt5a mutant tongue is severely shortened, with an ankyloglossia, and lingual mesenchyme is disorganized. However, fungiform papilla morphology, number and innervation are preserved, as is expression of the papilla marker, Shh. These data demonstrate that the genetic regulation for tongue size and shape can be separated from that directing lingual papilla development. Preserved number of papillae in a shortened tongue results in an increased density of fungiform papillae in the mutant tongues. In tongue organ cultures, exogenous Wnt5a profoundly suppresses papilla formation and simultaneously decreases canonical Wnt signaling as measured by the TOPGAL reporter. These findings suggest that Wnt5a antagonizes canonical Wnt signaling to dictate papilla number and spacing. In all, distinctive roles for Wnt5a in tongue size, fungiform papilla patterning and development are shown and a necessary balance between non-canonical and canonical Wnt paths in regulating tongue growth and fungiform papillae is proposed in a model, through the Ror2 receptor.

Fgf signaling controls pharyngeal taste bud formation through miR-200 and Delta-Notch activity

Taste buds, the taste sensory organs, are conserved in vertebrates and composed of distinct cell types, including taste receptor, basal/presynaptic and support cells. Here, we characterize zebrafish taste bud development and show that compromised Fgf signaling in the larva results in taste bud reduction and disorganization. We determine that Fgf activity is required within pharyngeal endoderm for formation of Calb2b(+) cells and reveal miR-200 and Delta-Notch signaling as key factors in this process. miR-200 knock down shows that miR-200 activity is required for taste bud formation and in particular for Calb2b(+) cell formation. Compromised delta activity in mib(-/-) dramatically reduces the number of Calb2b(+) cells and increases the number of 5HT(+) cells. Conversely, larvae with increased Notch activity and ascl1a(-/-) mutants are devoid of 5HT(+) cells, but have maintained and increased Calb2b(+) cells, respectively. These results show that Delta-Notch signaling is required for intact taste bud organ formation. Consistent with this, Notch activity restores Calb2b(+) cell formation in pharyngeal endoderm with compromised Fgf signaling, but fails to restore the formation of these cells after miR-200 knock down. Altogether, this study provides genetic evidence that supports a novel model where Fgf regulates Delta-Notch signaling, and subsequently miR-200 activity, in order to promote taste bud cell type differentiation.

Development of gustatory papillae in the absence of Six1 and Six4

Six family genes encode homeobox transcription factors, and a deficiency in them leads to abnormal structures of the sensory organs. In a previous paper, Six1 was reported to be expressed in the taste bud-bearing lingual papillae of mice, and loss of Six1 affected the development of these gustatory papillae. We show here that embryos lacking both Six1 and Six4 revealed more severe abnormalities than those lacking Six1 alone during morphogenesis of their gustatory papillae. By in situ hybridization, Six4 was shown to be broadly distributed in the epithelium of the lateral lingual swellings at embryonic day (E) 11.5, and in the tongue epithelium, mesenchyme, and muscles at E12.5. From E14, Six4 was similar in expression pattern to Six1, as previously reported. In the fungiform papillae, Six4 was expressed in the epithelium at E14-E16.5. In the circumvallate and foliate papillae, Six4 expression was observed in the trench wall of these papillae at E15.5-P0. Although Six4-deficient mice had no abnormalities, Six1/Six4-deficient mice showed distinct morphological changes: fusion of the lateral lingual swellings was delayed, and the tongue was poorly developed. The primordia of fungiform papillae appeared earlier than those in the wild-type or Six1-deficient mice, and the papillae rapidly increased in size; thus fusion of each papilla was evident. The circumvallate papillae showed severe defects; for example, invagination of the trenches started asymmetrically, which resulted in longer and shorter trenches. The foliate papillae elevated initially, and showed stunted trenches. Therefore, Six1 and Six4 function synergistically to form gustatory papillae during development of the tongue.

FGF signaling regulates the number of posterior taste papillae by controlling progenitor field size

The sense of taste is fundamental to our ability to ingest nutritious substances and to detect and avoid potentially toxic ones. Sensory taste buds are housed in papillae that develop from epithelial placodes. Three distinct types of gustatory papillae reside on the rodent tongue: small fungiform papillae are found in the anterior tongue, whereas the posterior tongue contains the larger foliate papillae and a single midline circumvallate papilla (CVP). Despite the great variation in the number of CVPs in mammals, its importance in taste function, and its status as the largest of the taste papillae, very little is known about the development of this structure. Here, we report that a balance between Sprouty (Spry) genes and Fgf10, which respectively antagonize and activate receptor tyrosine kinase (RTK) signaling, regulates the number of CVPs. Deletion of Spry2 alone resulted in duplication of the CVP as a result of an increase in the size of the placode progenitor field, and Spry1^−/−;Spry2^−/− embryos had multiple CVPs, demonstrating the redundancy of Sprouty genes in regulating the progenitor field size. By contrast, deletion of Fgf10 led to absence of the CVP, identifying FGF10 as the first inductive, mesenchyme-derived factor for taste papillae. Our results provide the first demonstration of the role of epithelial-mesenchymal FGF signaling in taste papilla development, indicate that regulation of the progenitor field size by FGF signaling is a critical determinant of papilla number, and suggest that the great variation in CVP number among mammalian species may be linked to levels of signaling by the FGF pathway.

The sense of taste is important for an animal's ability to survive and thrive, because it enables discrimination between nutritious substances and toxins. Taste buds are housed largely on the tongue in structures called papillae; of the three types of gustatory papillae, the circumvallate papilla (CVP) is the largest. In rodents, a single CVP is located in the posterior midline of the tongue housing hundreds of taste buds, whereas in other mammals up to dozens of CVPs can be found. However, despite the great variation in the number of CVPs in mammals, its status as the largest of the taste papillae, and its importance in taste function, very little is known about its development. We identified members of the FGF signaling pathway as determinants of CVP number. We propose that perturbations to the FGF signaling pathway may have been involved in the dramatic differences in CVP number that arose during mammalian evolution.

Taste bud cells of adult mice are responsive to Wnt/β-catenin signaling: implications for the renewal of mature taste cells

Wnt/β-catenin signaling initiates taste papilla development in mouse embryos, however, its involvement in taste cell turnover in adult mice has not been explored. Here we used the BATGAL reporter mouse model, which carries an engineered allele in which the LacZ gene is expressed in the presence of activated β-catenin, to determine the responsiveness of adult taste bud cells to canonical Wnt signaling. Double immunostaining with markers of differentiated taste cells revealed that a subset of Type I, II, and III taste cells express β-galactosidase. Using in situ hybridization, we showed that β-catenin activates the transcription of the LacZ gene mainly in intragemmal basal cells that are immature taste cells, identified by their expression of Sonic Hedgehog (Shh). Finally, we showed that β-catenin activity is significantly reduced in taste buds of 25-week-old mice compared with 10-week-old animals. Our data suggest that Wnt/β-catenin signaling may influence taste cell turnover by regulating cell differentiation. Reduced canonical Wnt signaling in older mice could explain in part the loss of taste sensitivity with aging, implicating a possible deficiency in the rate of taste cell renewal. More investigations are now necessary to understand if and how Wnt signaling regulates adult taste cell turnover.

Differential expression of a BMP4 reporter allele in anterior fungiform versus posterior circumvallate taste buds of mice

BACKGROUND

Bone Morphogenetic Protein 4 (BMP4) is a diffusible factor which regulates embryonic taste organ development. However, the role of BMP4 in taste buds of adult mice is unknown. We utilized transgenic mice with LacZ under the control of the BMP4 promoter to reveal the expression of BMP4 in the tongues of adult mice. Further we evaluate the pattern of BMP4 expression with that of markers of specific taste bud cell types and cell proliferation to define and compare the cell populations expressing BMP4 in anterior (fungiform papillae) and posterior (circumvallate papilla) tongue.

RESULTS

BMP4 is expressed in adult fungiform and circumvallate papillae, i.e., lingual structures composed of non-taste epithelium and taste buds. Unexpectedly, we find both differences and similarities with respect to expression of BMP4-driven ß-galactosidase. In circumvallate papillae, many fusiform cells within taste buds are BMP4-ß-gal positive. Further, a low percentage of BMP4-expressing cells within circumvallate taste buds is immunopositive for markers of each of the three differentiated taste cell types (I, II and III). BMP4-positive intragemmal cells also expressed a putative marker of immature taste cells, Sox2, and consistent with this finding, intragemmal cells expressed BMP4-ß-gal within 24 hours after their final mitosis, as determined by BrdU birthdating. By contrast, in fungiform papillae, BMP4-ß-gal positive cells are never encountered within taste buds. However, in both circumvallate and fungiform papillae, BMP4-ß-gal expressing cells are located in the perigemmal region, comprising basal and edge epithelial cells adjacent to taste buds proper. This region houses the proliferative cell population that gives rise to adult taste cells. However, perigemmal BMP4-ß-gal cells appear mitotically silent in both fungiform and circumvallate taste papillae, as we do not find evidence of their active proliferation using cell cycle immunomarkers and BrdU birthdating.

CONCLUSION

Our data suggest that intragemmal BMP4-ß-gal cells in circumvallate papillae are immature taste cells which eventually differentiate into each of the 3 taste cell types, whereas perigemmal BMP4-ß-gal cells in both circumvallate and fungiform papillae may be slow cycling stem cells, or belong to the stem cell niche to regulate taste cell renewal from the proliferative cell population.

Differential Shh, Bmp and Wnt gene expressions during craniofacial development in mice

In this study, Bmp-4, Wnt-5a and Shh gene expressions were compared during early craniofacial development in mice by comparative non-isotopic in situ hybridization. Wild-type C57BL/6J mice were studied at various stages of embryonic development (from 8.5- to 13.5-day-old embryos--E8.5-13.5). During early odontogenesis, transcripts for Bmp-4, Shh and Wnt-5a were co-localised at the tooth initiation stage. At E8.5, Shh mRNA expression was restricted to diencephalon and pharyngeal endoderm. Before maxillae and mandible ossification, Bmp-4 and Wnt-5a signals were detected in the mesenchymal cells and around Meckel's cartilage. During palatogenesis, Shh was expressed only in the epithelium and Wnt-5a only in the mesenchyme of the elevating palatal shelves. During tongue development, Shh expression was found in mesenchyme, probably contributing to tongue miogenesis, while Wnt-5a signal was in the epithelium, possibly during placode development and papillae formation. Taken together, these findings suggest that Bmp-4, Shh and Wnt-5a gene expressions may act together on the epithelial-mesenchymal interactions occurring in several aspects of the early mouse craniofacial development, such as odontogenesis, neuronal development, maxillae and mandible ossification, palatogenesis and tongue formation.

Role of bone morphogenetic proteins on cochlear hair cell formation: analyses of Noggin and Bmp2 mutant mice

The mammalian organ of Corti of the inner ear is a highly sophisticated sensory end organ responsible for detecting sound. Noggin is a secreted glycoprotein, which antagonizes bone morphogenetic proteins 2 and 4 (Bmp2 and Bmp4). The lack of this antagonist causes increased rows of inner and outer hair cells in the organ of Corti. In mice, Bmp2 is expressed transiently in nascent cochlear hair cells. To investigate whether Noggin normally modulates the levels of Bmp2 for hair cell formation, we deleted Bmp2 in the cochlear hair cells using two cre strains, Foxg1(cre/+) and Gfi1(cre/+). Bmp2 conditional knockout cochleae generated using these two cre strains show normal hair cells. Furthermore, Gfi1(cre/+);Bmp2(lox/-) mice are viable and have largely normal hearing. The combined results of Noggin and Bmp2 mutants suggest that Noggin is likely to regulate other Bmps in the cochlea such as Bmp4.

Wnt/beta-catenin signaling in oral tissue development and disease

The Wnt/beta-catenin signaling pathway is one of several key conserved intercellular signaling pathways in animals, and plays fundamental roles in the proliferation, regeneration, differentiation, and function of many cell and tissue types. This pathway is activated in a dynamic manner during the morphogenesis of oral organs, including teeth, taste papillae, and taste buds, and is essential for these processes to occur normally. Conversely, forced activation of Wnt/beta-catenin signaling promotes the formation of ectopic teeth and taste papillae. In this review, we discuss our current understanding of the roles of Wnt/beta-catenin signaling in oral tissue development and in related human diseases, and the potential of manipulating this pathway for therapeutic purposes.

WNT5a in tongue and fungiform Papilla development

Fungiform papillae are complex taste organs that develop in a pattern on anterior tongue in rodent embryos. Several intrinsic secreted molecules are important for papilla development and patterning, including sonic hedgehog, bone morphogenetic proteins, Noggin, epidermal growth factor, and WNTs. Recent data about roles of WNTs in regulation of tongue and fungiform papilla development lead to new insights about the importance of tissue and timing contexts when studying the effects of morphogenetic proteins. WNT/beta-catenin signaling is required for formation of fungiform papillae, but not for determining tongue size and shape. In contrast, WNT5a apparently is important for tongue outgrowth, but not papilla development. Preliminary data from WNT5a mutant mice separate genetic programs for papilla number from those for tongue shape and size.

In vivo fate tracing studies of mammalian taste cell progenitors

In mammals, the homogeneous lingual epithelium in the process of development forms specialized placodal cells that undergo a series of morphogenetic changes to form a papilla. Taste buds appear in the papillary epithelium around birth and thus papillae serve to house the taste buds in the adult. However, evidence for a precise lineage relationship between a putative embryonic taste progenitor population and functional adult taste buds has so far been elusive and is primarily indirect. Also, mammalian taste papillae are reminiscent of epithelial appendages suggesting that the mesenchymal tissue of the papillae could be involved in the formation of these lingual structures. These major questions in the field of mammalian taste development have remained unanswered due to lack of fate mapping studies that would label embryonic cell populations and remain indelibly marked in the adult. Taking advantage of a genetic fate mapping approach to label cell populations both in the lingual epithelium and mesenchyme and following their fate during development would be an ideal way to assess each of these tissues contribution in taste bud formation. Fate mapping studies using tissue specific cre strains crossed with reporter alleles would uncover unique features in the formation of these specialized sensory cells and also provide us with an in vivo model system for taste organ specific experimental manipulations during development.

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.

Fate mapping of mammalian embryonic taste bud progenitors

Mammalian taste buds have properties of both epithelial and neuronal cells, and are thus developmentally intriguing. Taste buds differentiate at birth within epithelial appendages, termed taste papillae, which arise at mid-gestation as epithelial thickenings or placodes. However, the embryonic relationship between placodes, papillae and adult taste buds has not been defined. Here, using an inducible Cre-lox fate mapping approach with the ShhcreER(T2) mouse line, we demonstrate that Shh-expressing embryonic taste placodes are taste bud progenitors, which give rise to at least two different adult taste cell types, but do not contribute to taste papillae. Strikingly, placodally descendant taste cells disappear early in adult life. As placodally derived taste cells are lost, we used Wnt1Cre mice to show that the neural crest does not supply cells to taste buds, either embryonically or postnatally, thus ruling out a mesenchymal contribution to taste buds. Finally, using Bdnf null mice, which lose neurons that innervate taste buds, we demonstrate that Shh-expressing taste bud progenitors are specified and produce differentiated taste cells normally, in the absence of gustatory nerve contact. This resolution of a direct relationship between embryonic taste placodes with adult taste buds, which is independent of mesenchymal contribution and nerve contact, allows us to better define the early development of this important sensory system. These studies further suggest that mammalian taste bud development is very distinct from that of other epithelial appendages.

Follistatin modulates a BMP autoregulatory loop to control the size and patterning of sensory domains in the developing tongue

The regenerative capacity of many placode-derived epithelial structures makes them of interest for understanding the molecular control of epithelial stem cells and their niches. Here, we investigate the interaction between the developing epithelium and its surrounding mesenchyme in one such system, the taste papillae and sensory taste buds of the mouse tongue. We identify follistatin (FST) as a mesenchymal factor that controls size, patterning and gustatory cell differentiation in developing taste papillae. FST limits expansion and differentiation of Sox2-expressing taste progenitor cells and negatively regulates the development of taste papillae in the lingual epithelium: in Fst(-/-) tongue, there is both ectopic development of Sox2-expressing taste progenitors and accelerated differentiation of gustatory cells. Loss of Fst leads to elevated activity and increased expression of epithelial Bmp7; the latter effect is consistent with BMP7 positive autoregulation, a phenomenon we demonstrate directly. We show that FST and BMP7 influence the activity and expression of other signaling systems that play important roles in the development of taste papillae and taste buds. In addition, using computational modeling, we show how aberrations in taste papillae patterning in Fst(-/-) mice could result from disruption of an FST-BMP7 regulatory circuit that normally suppresses noise in a process based on diffusion-driven instability. Because inactivation of Bmp7 rescues many of the defects observed in Fst(-/-) tongue, we conclude that interactions between mesenchyme-derived FST and epithelial BMP7 play a central role in the morphogenesis, innervation and maintenance of taste buds and their stem/progenitor cells.