Association of Shh and Ptc with keratin localization in the initiation of the formation of circumvallate papilla and von Ebner's gland

The duct of von Ebner's glands is a source of Sox10 + taste bud progenitors and susceptible to pathogen infections

Introduction

We have recently demonstrated that Sox10-expressing (Sox10 +) cells give rise to mainly type-III neuronal taste bud cells that are responsible for sour and salt taste. The two tissue compartments containing Sox10 + cells in the surrounding of taste buds include the connective tissue core of taste papillae and von Ebner's glands (vEGs) that are connected to the trench of circumvallate and foliate papillae.

Methods

In this study, we performed single cell RNA-sequencing of the epithelium of Sox10-Cre/tdT mouse circumvallate/vEG complex and used inducible Cre mouse models to map the cell lineages of vEGs and/or connective tissue (including stromal and Schwann cells).

Results

Transcriptomic analysis indicated that Sox10 expression was enriched in the cell clusters of vEG ducts that contained abundant proliferating cells, while Sox10-Cre/tdT expression was enriched in type-III taste bud cells and vEG ductal cells. In vivo lineage mapping showed that the traced cells were distributed in circumvallate taste buds concurrently with those in the vEGs, but not in the connective tissue. Moreover, multiple genes encoding pathogen receptors were enriched in the vEG ducts hosting Sox10 + cells.

Discussion

Our data supports that it is the vEGs, not connective tissue core, that serve as the niche of Sox10 + taste bud progenitors. If this is also true in humans, our data indicates that vEG duct is a source of Sox10 + taste bud progenitors and susceptible to pathogen infections.

The role of Eya1 and Eya2 in the taste system of mice from embryonic stage to adulthood

Members of the Eya family, which are a class of transcription factors with phosphatase activity, are widely expressed in cranial sensory organs during development. However, it is unclear whether these genes are expressed in the taste system during development and whether they play any role in specifying taste cell fate. In this study, we report that Eya1 is not expressed during embryonic tongue development but that Eya1-expressing progenitors in somites or pharyngeal endoderm give rise to tongue musculature or taste organs, respectively. In the Eya1-deficient tongues, these progenitors do not proliferate properly, resulting in a smaller tongue at birth, impaired growth of taste papillae, and disrupted expression of Six1 in the papillary epithelium. On the other hand, Eya2 is specifically expressed in endoderm-derived circumvallate and foliate papillae located on the posterior tongue during development. In adult tongues, Eya1 is predominantly expressed in IP3R3-positive taste cells in the taste buds of the circumvallate and foliate papillae, while Eya2 is persistently expressed in these papillae at higher levels in some epithelial progenitors and at lower levels in some taste cells. We found that conditional knockout of Eya1 in the third week or Eya2 knockout reduced Pou2f3+, Six1+ and IP3R3+ taste cells. Our data define for the first time the expression patterns of Eya1 and Eya2 during the development and maintenance of the mouse taste system and suggest that Eya1 and Eya2 may act together to promote lineage commitment of taste cell subtypes.

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

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

Cellular mechanisms of reverse epithelial curvature in tissue morphogenesis

Epithelial bending plays an essential role during the multiple stages of organogenesis and can be classified into two types: invagination and evagination. The early stages of invaginating and evaginating organs are often depicted as simple concave and convex curves respectively, but in fact majority of the epithelial organs develop through a more complex pattern of curvature: concave flanked by convex and vice versa respectively. At the cellular level, this is far from a geometrical truism: locally cells must passively adapt to, or actively create such an epithelial structure that is typically composed of opposite and connected folds that form at least one s-shaped curve that we here, based on its appearance, term as "reverse curves." In recent years, invagination and evagination have been studied in increasing cellular detail. A diversity of mechanisms, including apical/basal constriction, vertical telescoping and extrinsic factors, all orchestrate epithelial bending to give different organs their final shape. However, how cells behave collectively to generate reverse curves remains less well-known. Here we review experimental models that characteristically form reverse curves during organogenesis. These include the circumvallate papillae in the tongue, crypt-villus structure in the intestine, and early tooth germ and describe how, in each case, reverse curves form to connect an invaginated or evaginated placode or opposite epithelial folds. Furthermore, by referring to the multicellular system that occur in the invagination and evagination, we attempt to provide a summary of mechanisms thought to be involved in reverse curvature consisting of apical/basal constriction, and extrinsic factors. Finally, we describe the emerging techniques in the current investigations, such as organoid culture, computational modelling and live imaging technologies that have been utilized to improve our understanding of the cellular mechanisms in early tissue morphogenesis.

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

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

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.

Gap junction-mediated cell-to-cell communication in oral development and oral diseases: a concise review of research progress

Homoeostasis depends on the close connection and intimate molecular exchange between extracellular, intracellular and intercellular networks. Intercellular communication is largely mediated by gap junctions (GJs), a type of specialized membrane contact composed of variable number of channels that enable direct communication between cells by allowing small molecules to pass directly into the cytoplasm of neighbouring cells. Although considerable evidence indicates that gap junctions contribute to the functions of many organs, such as the bone, intestine, kidney, heart, brain and nerve, less is known about their role in oral development and disease. In this review, the current progress in understanding the background of connexins and the functions of gap junctions in oral development and diseases is discussed. The homoeostasis of tooth and periodontal tissues, normal tooth and maxillofacial development, saliva secretion and the integrity of the oral mucosa depend on the proper function of gap junctions. Knowledge of this pattern of cell-cell communication is required for a better understanding of oral diseases. With the ever-increasing understanding of connexins in oral diseases, therapeutic strategies could be developed to target these membrane channels in various oral diseases and maxillofacial dysplasia.

FGF10 Is Required for Circumvallate Papilla Morphogenesis by Maintaining Lgr5 Activity

Taste buds develop in different regions of the mammal oral cavity. Adult stem cells in various organs including the tongue papillae are marked by leucine-rich repeat-containing G protein-coupled receptor 5 (Lgr5) and its homolog, Lgr6. Recent studies have reported that adult taste stem/progenitor cells in circumvallate papilla (CVP) on the posterior tongue are Lgr5-positive. In this study, we confirm the Lgr5 expression pattern during CVP development. A previous study reported that mesenchymal Fgf10 is necessary for maintaining epithelial Lgr5-positive stem/progenitor cells. To confirm the interaction between Lgr5-positive CVP epithelium and mesenchymal factor FGF10, reverse recombination (180-degree) was performed after tongue epithelium detachment. FGF10 protein-soaked bead implantation was performed after reverse recombination to rescue CVP development. Moreover, we reduced mesenchymal Fgf10 by BIO and SU5402 treatment which disrupted CVP morphogenesis. This study suggests that the crosstalk between epithelial Lgr5 and mesenchymal Fgf10 plays a pivotal role in CVP epithelium invagination during mouse tongue CVP development by maintaining Lgr5-positive stem/progenitor cells.

Immunolocalization patterns of cytokeratins during salivary acinar cell development in mice

Embryonic development of the mouse salivary glands begins with epithelial thickening and continues with sequential changes from the pre-bud to terminal bud stages. After birth, morphogenesis proceeds, and the glands develop into a highly branched epithelial structure that terminates with saliva-producing acinar cells at the adult stage. Acinar cells derived from the epithelium are differentiated into serous, mucous, and seromucous types. During differentiation, cytokeratins, intermediate filaments found in most epithelial cells, play vital roles. Although the localization patterns and developmental roles of cytokeratins in different epithelial organs, including the mammary glands, circumvallate papilla, and sweat glands, have been well studied, their stage-specific localization and morphogenetic roles during salivary gland development have yet to be elucidated. Therefore, the aim of this study was to determine the stage and acinar cell type-specific localization pattern of cytokeratins 4, 5, 7, 8, 13, 14, 18, and 19 in the major salivary glands (submandibular, sublingual, and parotid glands) of the mouse at the E15.5, PN0, PN10, and adult stages. In addition, cell physiology, including cell proliferation, was examined during development via immunostaining for Ki67 to understand the cellular mechanisms that govern acinar cell differentiation during salivary gland morphogenesis. The distinct localization patterns of cytokeratins in conjunction with cell physiology will reveal the roles of epithelial cells in salivary gland formation during the differentiation of serous, mucous or seromucous salivary glands.

Effects of perinatal undernutrition on the circumvallate papilla of developing Wistar rats

During the gestation and the lactating periods the gustatory papillae contain taste buds that respond to different flavors and aversive stimuli. The current study analyzed the effects of pre-and neonatal undernutrition on the circumvallate papillae of rats at 12, 20, and 30days of age. Early undernourishment occurred from gestational days G6 to G19 when dams received low percentages of food followed by a balanced diet from G20-21. After birth pups were underfed by rotating two lactating dams every 12h; in one of them, the nipples were tied. The pups were weaned at 25days of age, and then given an ad libitum diet. Under anesthesia the tongues were removed and stained with the hematoxylin-eosin (H-E) procedure. The results indicated that young underfed rats had significantly body weight reductions. The tongue measurements in underfed rats showed reduced total area and length of the anterior portion, but there were negligible effects on the posterior portion. The circumvallate papillae in underfed rats was significantly reduced in major length, major diameter, and total and upper areas, but unaffected in the lateral wall trench region. The taste bud areas and minor diameter were unaffected by undernutrition, but there were significant reductions in the total number of visible taste buds and the major diameter, delayed opening of taste bud pores, and an increased number of closed pores were also observed. These alterations by undernutrition reflect the vulnerability of structures in the gustatory oral cavity and suggest a possible interference with the receptorś activation, and transduction and perhaps with the taste encoding of signals to generate the gustatory sensory and hedonic responses.

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.

Morphological evidences in circumvallate papilla and von Ebners' gland development in mice

In rodents, the circumvallate papilla (CVP), with its underlying minor salivary gland, the von Ebners' gland (VEG), is located on the dorsal surface of the posterior tongue. Detailed morphological processes to form the proper structure of CVP and VEG have not been properly elucidated. In particular, the specific localization patterns of taste buds in CVP and the branching formation of VEG have not yet been elucidated. To understand the developmental mechanisms underlying CVP and VEG formation, detailed histological observations of CVP and VEG were examined using a three-dimensional computer-aided reconstruction method with serial histological sections and pan-Cytokeratins immunostainings. In addition, to define the developmental processes in CVP and VEG formation, we examined nerve innervations and cell proliferation using microinjections of AM1-43 and immunostainings with various markers, including phosphoinositide 3-kinase, Ki-67, PGP9.5, and Ulex europaeus agglutinin 1 (UEA1). Results revealed specific morphogenesis of CVP and VEG with nerve innervations patterns, evaluated by the coincided localization patterns of AM1-43 and UEA1. Based on these morphological and immunohistochemical results, we suggest that nerve innervations and cell proliferations play important roles in the positioning of taste buds in CVP and branching morphogenesis of VEG in tongue development.

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.

Importance of region-specific epithelial rearrangements in mouse rugae development

Epithelial appendages on palatal rugae develop during mouse palatogenesis through epithelial thickening and pattern formation. Recently, the patterned formation of nine rugae was observed together with the specific expression patterns of Shh in rodents. However, no crucial evidence was found for a direct association between Shh expression and the distinct structural formation of rugae. In order to reveal possible relationships, we investigated the morphological changes of rugae and expression patterns of Shh directly by in vitro organ culture at embryonic day 13 (E13) for 2 days. To compare and examine the diverse growing aspects of the palate and rugae, we carefully observed the detailed morphogenesis, with cell proliferation of the rugae occurring between E13 and E14.5. After 2 days of cultivation at E13, DiI micro-injections revealed that the middle part of the palate, adjacent to the upper molar-forming region, contributed to the formation of the subsequent structure of rugae by extensive cell rearrangement and proliferation within the epithelium in the preferred anteroposterior direction. The results also defined the intimate relationship between Shh expression and rugae formation.

Immunohistochemical localization of cytokeratins in the junctional region of ectoderm and endoderm

Although tridermic species have two junctional regions of ectoderm and endoderm between their epidermis and digestive tract, we actually know little about these particular boundaries. Cytokeratins are the major intermediate filaments of epithelial cells and show a high degree of tissue specificity. Therefore, to characterize the epithelial cells in the junctional region of ectoderm and endoderm, we immunohistochemically examined the localization of cytokeratins 5, 7/17, 14, 18, Sox17, and alpha-fetoprotein (AFP) in the oropharyngeal and anorectal regions during the mouse gastrulation process. At embryonic day (E) 9.5, cytokeratins 5, 7/17, 14, and 18 were detected in all epithelial cells of the oropharyngeal region. At E12.5, cytokeratin 5-positive cells were not observed in the middle area of the oral cavity; however, the immunoreactivity was strong in the anterior and posterior areas. The immunoreaction of cytokeratins 18 was seen only in the middle and posterior areas of the oral mucosa. Cytokeratins 7/17 and 14 were localized in all areas of the oropharyngeal region. Sox17 and AFP, which are endodermal markers, were detected in the middle and posterior areas of the oral mucosa, but not in the anterior area. Moreover, this same localization pattern of cytokeratins also existed in the anorectal region of the E12.5 embryo, suggesting that the localization of cytokeratins and endodermal markers might give an implication for the boundary between ectoderm and endoderm. These results also suggest that these cytokeratins are useful molecules for monitoring the epithelial cell differentiation in the junctional region of the germ layers.

Defects and rescue of the minor salivary glands in Eda pathway mutants

Despite their importance to oral health, the mechanisms of minor salivary gland (SG) development are largely unexplored. Here we present in vivo and in vitro analyses of developing minor SGs in wild type and mutant mice. Eda, Shh and Fgf signalling pathway genes are expressed in these glands from an early stage of development. Developing minor SGs are absent in Eda pathway mutant embryos, and these mice exhibit a dysplastic circumvallate papilla with disrupted Shh expression. Supplementation of Eda pathway mutant minor SG explants with recombinant EDA rescues minor SG induction. Supplementation with Fgf8 or Shh, previously reported targets of Eda signalling, leads to induction of gland like structures in a few cases, but these fail to develop into minor SGs.

Combined in silico and in vivo analyses reveal role of Hes1 in taste cell differentiation

The sense of taste is of critical importance to animal survival. Although studies of taste signal transduction mechanisms have provided detailed information regarding taste receptor calcium signaling molecules (TRCSMs, required for sweet/bitter/umami taste signal transduction), the ontogeny of taste cells is still largely unknown. We used a novel approach to investigate the molecular regulation of taste system development in mice by combining in silico and in vivo analyses. After discovering that TRCSMs colocalized within developing circumvallate papillae (CVP), we used computational analysis of the upstream regulatory regions of TRCSMs to investigate the possibility of a common regulatory network for TRCSM transcription. Based on this analysis, we identified Hes1 as a likely common regulatory factor, and examined its function in vivo. Expression profile analyses revealed that decreased expression of nuclear HES1 correlated with expression of type II taste cell markers. After stage E18, the CVP of Hes1(-/) (-) mutants displayed over 5-fold more TRCSM-immunoreactive cells than did the CVP of their wild-type littermates. Thus, according to our composite analyses, Hes1 is likely to play a role in orchestrating taste cell differentiation in developing taste buds.