Disruption of sonic hedgehog signaling alters growth and patterning of lingual taste papillae

Development of Ectodermal and Endodermal Taste Buds

The sense of taste is mediated primarily by taste buds on the tongue. These multicellular sensory organs are induced, patterned and become innervated during embryogenesis such that a functional taste system is present at birth when animals begin to feed. While taste buds have been considered ectodermal appendages, this is only partly accurate as only fungiform taste buds in the anterior tongue arise from the ectoderm. Taste buds found in the posterior tongue actually derive from endoderm. Nonetheless, both anterior and posterior buds are functionally similar, despite their disparate embryonic origins. In this review, I compare the development of ectodermal vs endodermal taste buds, highlighting the many differences in the cellular and molecular genetic mechanisms governing their formation.

Invasive papillary carcinoma of the breast

Invasive papillary carcinoma is a rare form of breast cancer that is more likely to occur in postmenopausal women. Previous studies have been limited to case reports and small retrospective studies, leading to low awareness of this type of tumor and difficult clinical management. According to the available literature, invasive papillary carcinoma exhibits unique pathological features and biological behaviors. Invasive papillary carcinoma is mostly luminal type, with a low rate of lymph node metastasis, which underlies its favorable prognosis. The effectiveness of adjuvant therapy in reducing tumor burden and improving prognosis in patients with invasive papillary carcinoma remains uncertain. Due to the rarity of the lesion, conducting prospective clinical trials is impractical. The use of biological models, such as organoids, can help alleviate the impact of the scarcity of this condition on research. In addition, invasive papillary carcinoma is affected by specific genomic events, and more extensive studies of gene expression profiling may provide molecular-level insights to make optimal therapeutic decisions.

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.

Sonic hedgehog signaling in craniofacial development

Mutations in SHH and several other genes encoding components of the Hedgehog signaling pathway have been associated with holoprosencephaly syndromes, with craniofacial anomalies ranging in severity from cyclopia to facial cleft to midfacial and mandibular hypoplasia. Studies in animal models have revealed that SHH signaling plays crucial roles at multiple stages of craniofacial morphogenesis, from cranial neural crest cell survival to growth and patterning of the facial primordia to organogenesis of the palate, mandible, tongue, tooth, and taste bud formation and homeostasis. This article provides a summary of the major findings in studies of the roles of SHH signaling in craniofacial development, with emphasis on recent advances in the understanding of the molecular and cellular mechanisms regulating the SHH signaling pathway activity and those involving SHH signaling in the formation and patterning of craniofacial structures.

Development of a Patient-Reported Outcome Measure (PROM) for Dysgeusia During Treatment With Smoothened (SMO) Inhibitors for Basal Cell Carcinomas: The SMO-iD Questionnaire

INTRODUCTION

Dysgeusia may occur during conventional or target-therapies and affect patients adherence-to-treatment. Therefore, it should be monitored to improve clinical outcome. To date, available questionnaires on dysgeusia relate to traditional antineoplastics and do not apply to target-therapies as the pathogenetic mechanism and clinical expression differ.

OBJECTIVES

To develop a patient-reported outcome measure (PROM) to screen for and monitor the occurrence and severity of dysgeusia in patients under Smoothened (SMO) inhibitors: the SMO-iD questionnaire.

METHODS

Patients with locally advanced basal cell carcinomas referring dysgeusia under SMO inhibitors at the University Hospital of Naples Federico II, were enrolled between January-December 2020. The PROM was elaborated based on chemotherapy-induced dysgeusia (CiTas) scale (development phase) and then validated by measuring internal consistency and reliability (validation phase).

RESULTS

Thirty-nine patients were enrolled and interviewed every 8 weeks. In the first phase, 160 CiTas questionnaires were collected, and the SMO-iD questionnaire was developed. In the second phase, 195 SMO-iD questionnaires were recorded, and reliability and validity assessed. Cronbach alpha was 0.89.

CONCLUSIONS

The SMO-iD questionnaire is a validated questionnaire that shows high face and content validity as well as high internal consistency and reliability. Hence, it may be introduced in daily clinical setting to monitor dysgeusia in patients under SMO-inhibitors.

Early Steps towards Hearing: Placodes and Sensory Development

Sensorineural hearing loss is the most prevalent sensory deficit in humans. Most cases of hearing loss are due to the degeneration of key structures of the sensory pathway in the cochlea, such as the sensory hair cells, the primary auditory neurons, and their synaptic connection to the hair cells. Different cell-based strategies to replace damaged inner ear neurosensory tissue aiming at the restoration of regeneration or functional recovery are currently the subject of intensive research. Most of these cell-based treatment approaches require experimental in vitro models that rely on a fine understanding of the earliest morphogenetic steps that underlie the in vivo development of the inner ear since its initial induction from a common otic-epibranchial territory. This knowledge will be applied to various proposed experimental cell replacement strategies to either address the feasibility or identify novel therapeutic options for sensorineural hearing loss. In this review, we describe how ear and epibranchial placode development can be recapitulated by focusing on the cellular transformations that occur as the inner ear is converted from a thickening of the surface ectoderm next to the hindbrain known as the otic placode to an otocyst embedded in the head mesenchyme. Finally, we will highlight otic and epibranchial placode development and morphogenetic events towards progenitors of the inner ear and their neurosensory cell derivatives.

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.

Neurosensory development of the four brainstem-projecting sensory systems and their integration in the telencephalon

Somatosensory, taste, vestibular, and auditory information is first processed in the brainstem. From the brainstem, the respective information is relayed to specific regions within the cortex, where these inputs are further processed and integrated with other sensory systems to provide a comprehensive sensory experience. We provide the organization, genetics, and various neuronal connections of four sensory systems: trigeminal, taste, vestibular, and auditory systems. The development of trigeminal fibers is comparable to many sensory systems, for they project mostly contralaterally from the brainstem or spinal cord to the telencephalon. Taste bud information is primarily projected ipsilaterally through the thalamus to reach the insula. The vestibular fibers develop bilateral connections that eventually reach multiple areas of the cortex to provide a complex map. The auditory fibers project in a tonotopic contour to the auditory cortex. The spatial and tonotopic organization of trigeminal and auditory neuron projections are distinct from the taste and vestibular systems. The individual sensory projections within the cortex provide multi-sensory integration in the telencephalon that depends on context-dependent tertiary connections to integrate other cortical sensory systems across the four modalities.

The sense of taste: Development, regeneration, and dysfunction

Gustation or the sense of taste is a primary sense, which functions as a gatekeeper for substances that enter the body. Animals, including humans, ingest foods that contain appetitive taste stimuli, including those that have sweet, moderately salty and umami (glutamate) components, and tend to avoid bitter-tasting items, as many bitter compounds are toxic. Taste is mediated by clusters of heterogeneous taste receptors cells (TRCs) organized as taste buds on the tongue, and these convey taste information from the oral cavity to higher order brain centers via the gustatory sensory neurons of the seventh and ninth cranial ganglia. One remarkable aspect of taste is that taste perception is mostly uninterrupted throughout life yet TRCs within buds are constantly renewed; every 1-2 months all taste cells have been steadily replaced. In the past decades we have learned a substantial amount about the cellular and molecular regulation of taste bud cell renewal, and how taste buds are initially established during embryogenesis. Here I review more recent findings pertaining to taste development and regeneration, as well as discuss potential mechanisms underlying taste dysfunction that often occurs with disease or its treatment. This article is categorized under: Infectious Diseases > Stem Cells and Development Cancer > Stem Cells and Development Neurological Diseases > Stem Cells and Development.

Obesity-induced taste dysfunction, and its implications for dietary intake

The incidence of obesity has dramatically increased in recent years, and poses a public health challenge for which an effective and scalable intervention strategy is yet to be found. Our food choices are one of the primary drivers of obesity, where the overconsumption of energy from foods high in fat and sugar can be particularly problematic. Unfortunately, these same foods also tend to be highly palatable. We select foods more on their sensory properties than on any other factor, such as price, convenience, or healthfulness. Previous evidence from human sensory studies has suggested a depressed sense of taste in panelists with obesity. Evidence from animal models also demonstrates a clear deficiency in taste buds occurring with obesity, suggesting that damage to the taste system may result from an obese state. In this review only taste, as opposed to smell, will be examined. Here we seek to bring together evidence from a diverse array of human and animal studies into taste response, dietary intake, and physiology, to better understand changes in taste with obesity, with the goal of understanding whether taste may provide a novel target for intervention in the treatment of obesity.

Onset of taste bud cell renewal starts at birth and coincides with a shift in SHH function

Embryonic taste bud primordia are specified as taste placodes on the tongue surface and differentiate into the first taste receptor cells (TRCs) at birth. Throughout adult life, TRCs are continually regenerated from epithelial progenitors. Sonic hedgehog (SHH) signaling regulates TRC development and renewal, repressing taste fate embryonically, but promoting TRC differentiation in adults. Here, using mouse models, we show TRC renewal initiates at birth and coincides with onset of SHHs pro-taste function. Using transcriptional profiling to explore molecular regulators of renewal, we identified Foxa1 and Foxa2 as potential SHH target genes in lingual progenitors at birth and show that SHH overexpression in vivo alters FoxA1 and FoxA2 expression relevant to taste buds. We further bioinformatically identify genes relevant to cell adhesion and cell locomotion likely regulated by FOXA1;FOXA2 and show that expression of these candidates is also altered by forced SHH expression. We present a new model where SHH promotes TRC differentiation by regulating changes in epithelial cell adhesion and migration.

R-spondin substitutes for neuronal input for taste cell regeneration in adult mice

Taste bud cells regenerate throughout life. Taste bud maintenance depends on continuous replacement of senescent taste cells with new ones generated by adult taste stem cells. More than a century ago it was shown that taste buds degenerate after their innervating nerves are transected and that they are not restored until after reinnervation by distant gustatory ganglion neurons. Thus, neuronal input, likely via neuron-supplied factors, is required for generation of differentiated taste cells and taste bud maintenance. However, the identity of such a neuron-supplied niche factor(s) remains unclear. Here, by mining a published RNA-sequencing dataset of geniculate ganglion neurons and by in situ hybridization, we demonstrate that R-spondin-2, the ligand of Lgr5 and its homologs Lgr4/6 and stem-cell-expressed E3 ligases Rnf43/Znrf3, is expressed in nodose-petrosal and geniculate ganglion neurons. Using the glossopharyngeal nerve transection model, we show that systemic delivery of R-spondin via adenovirus can promote generation of differentiated taste cells despite denervation. Thus, exogenous R-spondin can substitute for neuronal input for taste bud cell replenishment and taste bud maintenance. Using taste organoid cultures, we show that R-spondin is required for generation of differentiated taste cells and that, in the absence of R-spondin in culture medium, taste bud cells are not generated ex vivo. Thus, we propose that R-spondin-2 may be the long-sought neuronal factor that acts on taste stem cells for maintaining taste tissue homeostasis.

Drinking Ice-Cold Water Reduces the Severity of Anticancer Drug-Induced Taste Dysfunction in Mice

Taste disorders are common adverse effects of cancer chemotherapy that can reduce quality of life and impair nutritional status. However, the molecular mechanisms underlying chemotherapy-induced taste disorders remain largely unknown. Furthermore, there are no effective preventive measures for chemotherapy-induced taste disorders. We investigated the effects of a combination of three anticancer drugs (TPF: docetaxel, cisplatin and 5-fluorouracil) on the structure and function of mouse taste tissues and examined whether the drinking of ice-cold water after TPF administration would attenuate these effects. TPF administration significantly increased the number of cells expressing apoptotic and proliferative markers. Furthermore, TPF administration significantly reduced the number of cells expressing taste cell markers and the magnitudes of the responses of taste nerves to tastants. The above results suggest that anticancer drug-induced taste dysfunction may be due to a reduction in the number of taste cells expressing taste-related molecules. The suppressive effects of TPF on taste cell marker expression and taste perception were reduced by the drinking of ice-cold water. We speculate that oral cryotherapy with an ice cube might be useful for prophylaxis against anticancer drug-induced taste disorders in humans.

Polycomb Repressive Complex 1 Controls Maintenance of Fungiform Papillae by Repressing Sonic Hedgehog Expression

How tissue patterns are formed and maintained are fundamental questions. The murine tongue epithelium, a paradigm for tissue patterning, consists of an array of specialized fungiform papillae structures that harbor taste cells. The formation of fungiform papillae is preceded by pronounced spatial changes in gene expression, in which taste cell genes such as Shh, initially diffused in lingual epithelial progenitors, become restricted to taste cells when their specification progresses. However, the requirement of spatial restriction of taste cell gene expression for patterning and formation of fungiform papillae is unknown. Here, we show that a chromatin regulator, Polycomb repressive complex (PRC) 1, is required for proper maintenance of fungiform papillae by repressing Shh and preventing ectopic SHH signaling in non-taste cells. Ablation of SHH signaling in PRC1-null non-taste cells rescues the maintenance of taste cells. Altogether, our studies exemplify how epigenetic regulation establishes spatial gene expression patterns necessary for specialized niche structures.

Formation and maintenance of patterns are critical for tissue development. Bar et al. show that PRC1, an epigenetic regulator, is critical for lingual papillae development. Specifically, PRC1 regulates maintenance of the developing fungiform papillae, harboring taste cells, by repressing Shh expression in the non-gustatory epithelium surrounding taste cells.

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.

Transcriptome analysis of axolotl oropharyngeal explants during taste bud differentiation stages

The Mexican salamander, Ambystoma mexicanum (Axolotl), is an excellent vertebrate model system to understand development and regeneration. Studies in axolotl embryos have provided important insights into taste bud development. Taste bud specification and determination occur in the oropharyngeal endoderm of axolotl embryos during gastrulation and neurulation, respectively, whereas taste bud innervation and taste cell differentiation occur later in development. Axolotl embryos are amenable to microsurgery, and tissue explants develop readily in vitro. We performed RNA-seq analysis to investigate the differential expression of genes in oropharyngeal explants at several stages of taste cell differentiation. Since the axolotl genome has only recently been sequenced, we used a Trinity pipeline to perform de novo assembly of sequencing reads. Linear models for RNA-seq data were used to identify differentially expressed genes. We found 1234 unique genes differentially expressed during taste cell differentiation stages. We validated four of these genes using RTqPCR and performed GO functional analysis. The differential expression of these genes suggests that they may play a role in taste cell differentiation in axolotls.

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.

Taste bud formation depends on taste nerves

It has been known for more than a century that, in adult vertebrates, the maintenance of taste buds depends on their afferent nerves. However, the initial formation of taste buds is proposed to be nerve-independent in amphibians, and evidence to the contrary in mammals has been endlessly debated, mostly due to indirect and incomplete means to impede innervation during the protracted perinatal period of taste bud differentiation. Here, by genetically ablating, in mice, all somatic (i.e. touch) or visceral (i.e. taste) neurons for the oral cavity, we show that the latter but not the former are absolutely required for the proper formation of their target organs, the taste buds.

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.

FGF- and SHH-based molecular signals regulate barbel and craniofacial development in catfish

BACKGROUND

Catfish (Siluriformes) are characterized by unique morphologies, including enlarged jaws with movable barbels and taste buds covering the entire body surface. Evolution of these characteristics was a crucial step in their adaptive radiation to freshwater environments. However, the developmental processes of the catfish craniofacial region and taste buds remain to be elucidated; moreover, little is known about the molecular mechanisms underlying the morphogenesis of these structures.

RESULTS

In Amur catfish (Silurus asotus), three pairs of barbel primordia are formed by 2 days post-fertilization (dpf). Innervation of the peripheral nerves and formation of muscle precursors are also established during early development. Taste buds from the oral region to the body trunk are formed by 4 dpf. We then isolated catfish cognates Shh (SaShh) and Fgf8 (SaFgf8), which are expressed in maxillary barbel primordium at 1-2 dpf. Further, SHH signal inhibition induces reduction of mandibular barbels with abnormal morphology of skeletal elements, whereas it causes no apparent abnormality in the trigeminal and facial nerve morphology. We also found that mandibular barbel lengths and number of taste buds are reduced by FGF inhibition, as seen in SHH signal inhibition. However, unlike with SHH inhibition, the abnormal morphology of the trigeminal and facial nerves was observed in FGF signal-inhibited embryos.

CONCLUSION

The developmental processes of Amur catfish are consistent with those reported for other catfish species. Thus, developmental aspects of craniofacial structures and taste buds may be conserved in Siluriformes. Our findings also suggest that SHH signaling plays a crucial role in the formation of barbels and taste buds, without affecting nerve projection, while FGF signaling is required for the development of barbels, taste buds, and branchial nerves. Thus, SHH and FGF signaling plays key roles in the ontogenesis and evolution of some catfish-specific characteristics.

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.

[Development and homeostasis of taste buds in mammals]

Taste is mediated by multicellular taste buds distributed throughout the oral and pharyngeal cavities. The taste buds can detect five basic tastes: sour, sweet, bitter, salty and umami, allowing mammals to select nutritious foods and avoid the ingestion of toxic and rotten foods. Once developed, the taste buds undergo continuous renewal throughout the adult life. In the past decade, significant progress has been achived in delineating the cellular and molecular mechanisms governing taste buds development and homeostasis. With this knowledges and in-depth investigations in the future, we can achieve the precise management of taste dysfunctions such as dysgeusia and ageusia.

Inflammation arising from obesity reduces taste bud abundance and inhibits renewal

Despite evidence that the ability to taste is weakened by obesity and can be rescued with weight loss intervention, few studies have investigated the molecular effects of obesity on the taste system. Taste bud cells undergo continual turnover even in adulthood, exhibiting an average life span of only a few weeks, tightly controlled by a balance of proliferation and cell death. Recent data reveal that an acute inflammation event can alter this balance. We demonstrate that chronic low-grade inflammation brought on by obesity reduces the number of taste buds in gustatory tissues of mice-and is likely the cause of taste dysfunction seen in obese populations-by upsetting this balance of renewal and cell death.

Gli3 is a negative regulator of Tas1r3-expressing taste cells

Mouse taste receptor cells survive from 3-24 days, necessitating their regeneration throughout adulthood. In anterior tongue, sonic hedgehog (SHH), released by a subpopulation of basal taste cells, regulates transcription factors Gli2 and Gli3 in stem cells to control taste cell regeneration. Using single-cell RNA-Seq we found that Gli3 is highly expressed in Tas1r3-expressing taste receptor cells and Lgr5+ taste stem cells in posterior tongue. By PCR and immunohistochemistry we found that Gli3 was expressed in taste buds in all taste fields. Conditional knockout mice lacking Gli3 in the posterior tongue (Gli3CKO) had larger taste buds containing more taste cells than did control wild-type (Gli3WT) mice. In comparison to wild-type mice, Gli3CKO mice had more Lgr5+ and Tas1r3+ cells, but fewer type III cells. Similar changes were observed ex vivo in Gli3CKO taste organoids cultured from Lgr5+ taste stem cells. Further, the expression of several taste marker and Gli3 target genes was altered in Gli3CKO mice and/or organoids. Mirroring these changes, Gli3CKO mice had increased lick responses to sweet and umami stimuli, decreased lick responses to bitter and sour taste stimuli, and increased glossopharyngeal taste nerve responses to sweet and bitter compounds. Our results indicate that Gli3 is a suppressor of stem cell proliferation that affects the number and function of mature taste cells, especially Tas1r3+ cells, in adult posterior tongue. Our findings shed light on the role of the Shh pathway in adult taste cell regeneration and may help devise strategies for treating taste distortions from chemotherapy and aging.

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.

Cell fate specification in the lingual epithelium is controlled by antagonistic activities of Sonic hedgehog and retinoic acid

The interaction between signaling pathways is a central question in the study of organogenesis. Using the developing murine tongue as a model, we uncovered unknown relationships between Sonic hedgehog (SHH) and retinoic acid (RA) signaling. Genetic loss of SHH signaling leads to enhanced RA activity subsequent to loss of SHH-dependent expression of Cyp26a1 and Cyp26c1. This causes a cell identity switch, prompting the epithelium of the tongue to form heterotopic minor salivary glands and to overproduce oversized taste buds. At developmental stages during which Wnt10b expression normally ceases and Shh becomes confined to taste bud cells, loss of SHH inputs causes the lingual epithelium to undergo an ectopic and anachronic expression of Shh and Wnt10b in the basal layer, specifying de novo taste placode induction. Surprisingly, in the absence of SHH signaling, lingual epithelial cells adopted a Merkel cell fate, but this was not caused by enhanced RA signaling. We show that RA promotes, whereas SHH, acting strictly within the lingual epithelium, inhibits taste placode and lingual gland formation by thwarting RA activity. These findings reveal key functions for SHH and RA in cell fate specification in the lingual epithelium and aid in deciphering the molecular mechanisms that assign cell identity.

Knowledge of the biological mechanisms controlling cell fate specification is of paramount importance for cell-based therapies. Sonic hedgehog (SHH) and retinoic acid (RA) pathways play key roles in development and disease. The role of SHH during in vivo tongue development is a subject of great interest, and whether RA signaling has any function in the developing tongue is unknown. The tongue is covered by a mucosa made of lingual epithelium and lingual mesenchyme. Various structures, including mechanosensory filiform papillae, gustatory papillae harboring taste buds, and minor salivary glands, arise from the epithelium, but how these entities are specified remains unclear. Here we show that in the mesenchyme SHH signaling drives growth and morphogenesis, whereas in the epithelium, SHH controls patterning and cell fate specification. We demonstrate that SHH inhibits taste placode and lingual gland formation by antagonizing RA inputs. We also show that loss of SHH signaling elicits Merkel cell formation in the lingual epithelium, a tissue normally bereft of Merkel cells. This is at odds with the hairy epidermis where Merkel cell specification has been shown to be SHH-dependent. Our study establishes SHH and RA as key players in the control of cell identity within the lingual epithelium.

Systems biology of facial development: contributions of ectoderm and mesenchyme

The rapid increase in gene-centric biological knowledge coupled with analytic approaches for genomewide data integration provides an opportunity to develop systems-level understanding of facial development. Experimental analyses have demonstrated the importance of signaling between the surface ectoderm and the underlying mesenchyme are coordinating facial patterning. However, current transcriptome data from the developing vertebrate face is dominated by the mesenchymal component, and the contributions of the ectoderm are not easily identified. We have generated transcriptome datasets from critical periods of mouse face formation that enable gene expression to be analyzed with respect to time, prominence, and tissue layer. Notably, by separating the ectoderm and mesenchyme we considerably improved the sensitivity compared to data obtained from whole prominences, with more genes detected over a wider dynamic range. From these data we generated a detailed description of ectoderm-specific developmental programs, including pan-ectodermal programs, prominence- specific programs and their temporal dynamics. The genes and pathways represented in these programs provide mechanistic insights into several aspects of ectodermal development. We also used these data to identify co-expression modules specific to facial development. We then used 14 co-expression modules enriched for genes involved in orofacial clefts to make specific mechanistic predictions about genes involved in tongue specification, in nasal process patterning and in jaw development. Our multidimensional gene expression dataset is a unique resource for systems analysis of the developing face; our co-expression modules are a resource for predicting functions of poorly annotated genes, or for predicting roles for genes that have yet to be studied in the context of facial development; and our analytic approaches provide a paradigm for analysis of other complex developmental programs.

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.

On the mechanism of smell loss in patients with Type II congenital hyposmia

BACKGROUND

Smell function has been initiated with theophylline treatment in 63% of patients with Type II congenital smell loss. Based upon a systematic evaluation of the protein components of nasal mucus we have demonstrated that interactions among four chemical moieties in nasal mucus may play significant roles in this initiation. Prior to treatment three of these moieties, cAMP, cGMP and sonic hedgehog (Shh), were significantly decreased in concentration whereas one of these moieties, TNFalpha, was increased in concentration. The mechanism(s) responsible for initiation of smell function in these patients, not immediately apparent, may depend upon understanding interactions among these moieties.

METHODS

Measurements of cAMP, cGMP, Shh and TNFalpha in nasal mucus by specific spectrophotometric immunoassays before and after treatment with theophylline.

RESULTS

Before theophylline treatment cAMP, cGMP and Shh in nasal mucus, which act as growth factors to support olfactory receptor function, were significantly decreased below normal levels whereas TNFalpha which acts as a "death factor" to inhibit olfactory receptor function was significantly increased above normal. After theophylline treatment cAMP, cGMP and Shh increased significantly whereas TNFalpha decreased significantly.

CONCLUSIONS

These results indicate that there are specific biochemical changes associated with smell loss in patients with Type II congenital smell loss and that correction of these biochemical changes are associated with initiation of smell function in these patients. Understanding these relationships play an important role in understanding receptor action in smell function.

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.

Sonic hedgehog is a regulator of extracellular glutamate levels and epilepsy

Sonic hedgehog (Shh), both as a mitogen and as a morphogen, plays an important role in cell proliferation and differentiation during early development. Here, we show that Shh inhibits glutamate transporter activities in neurons, rapidly enhances extracellular glutamate levels, and affects the development of epilepsy. Shh is quickly released in response to epileptic, but not physiological, stimuli. Inhibition of neuronal glutamate transporters by Shh depends on heterotrimeric G protein subunit Gαi and enhances extracellular glutamate levels. Inhibiting Shh signaling greatly reduces epileptiform activities in both cell cultures and hippocampal slices. Moreover, pharmacological or genetic inhibition of Shh signaling markedly suppresses epileptic phenotypes in kindling or pilocarpine models. Our results suggest that Shh contributes to the development of epilepsy and suppression of its signaling prevents the development of the disease. Thus, Shh can act as a modulator of neuronal activity, rapidly regulating glutamate levels and promoting epilepsy.

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.

Recent Advances in Molecular Mechanisms of Taste Signaling and Modifying

The sense of taste conveys crucial information about the quality and nutritional value of foods before it is ingested. Taste signaling begins with taste cells via taste receptors in oral cavity. Activation of these receptors drives the transduction systems in taste receptor cells. Then particular transmitters are released from the taste cells and activate corresponding afferent gustatory nerve fibers. Recent studies have revealed that taste sensitivities are defined by distinct taste receptors and modulated by endogenous humoral factors in a specific group of taste cells. Such peripheral taste generations and modifications would directly influence intake of nutritive substances. This review will highlight current understanding of molecular mechanisms for taste reception, signal transduction in taste bud cells, transmission between taste cells and nerves, regeneration from taste stem cells, and modification by humoral factors at peripheral taste organs.

β-Catenin signaling regulates temporally discrete phases of anterior taste bud development

The sense of taste is mediated by multicellular taste buds located within taste papillae on the tongue. In mice, individual taste buds reside in fungiform papillae, which develop at mid-gestation as epithelial placodes in the anterior tongue. Taste placodes comprise taste bud precursor cells, which express the secreted factor sonic hedgehog (Shh) and give rise to taste bud cells that differentiate around birth. We showed previously that epithelial activation of β-catenin is the primary inductive signal for taste placode formation, followed by taste papilla morphogenesis and taste bud differentiation, but the degree to which these later elements were direct or indirect consequences of β-catenin signaling was not explored. Here, we define discrete spatiotemporal functions of β-catenin in fungiform taste bud development. Specifically, we show that early epithelial activation of β-catenin, before taste placodes form, diverts lingual epithelial cells from a taste bud fate. By contrast, β-catenin activation a day later within Shh(+) placodes, expands taste bud precursors directly, but enlarges papillae indirectly. Further, placodal activation of β-catenin drives precocious differentiation of Type I glial-like taste cells, but not other taste cell types. Later activation of β-catenin within Shh(+) precursors during papilla morphogenesis also expands taste bud precursors and accelerates Type I cell differentiation, but papilla size is no longer enhanced. Finally, although Shh regulates taste placode patterning, we find that it is dispensable for the accelerated Type I cell differentiation induced by β-catenin.

Pharyngeal arch deficiencies affect taste bud development in the circumvallate papilla with aberrant glossopharyngeal nerve formation

BACKGROUND

The pharyngeal arches (PAs) generate cranial organs including the tongue. The taste placodes, formed in particular locations on the embryonic tongue surface, differentiate into taste buds harbored in distinct gustatory papillae. The developing tongue also has a complex supply of cranial nerves through each PA. However, the relationship between the PAs and taste bud development is not fully understood.

RESULTS

Ripply3 homozygous mutant mice, which have impaired third/fourth PAs, display a hypoplastic circumvallate papilla and lack taste buds, although the taste placode is normally formed. Formation of the glossopharyngeal ganglia is defective and innervation toward the posterior tongue is completely missing in Ripply3 mutant embryos at E12.5. Moreover, the distribution of neuroblasts derived from the epibranchial placode is severely, but not completely, atenuated, and the neural crest cells are diminished in the third PA region of Ripply3 mutant embryos at E9.5-E10.5. In Tbx1 homozygous mutant embryos, which exhibit another type of deficiency in PA development, the hypoplastic circumvallate papilla is observed along with abnormal formation of the glossopharyngeal ganglia and severely impaired innervation.

CONCLUSIONS

PA deficiencies affect multiple aspects of taste bud development, including formation of the cranial ganglia and innervation to the posterior tongue.

Cooperative integration between HEDGEHOG-GLI signalling and other oncogenic pathways: implications for cancer therapy

The HEDGEHOG-GLI (HH-GLI) signalling is a key pathway critical in embryonic development, stem cell biology and tissue homeostasis. In recent years, aberrant activation of HH-GLI signalling has been linked to several types of cancer, including those of the skin, brain, lungs, prostate, gastrointestinal tract and blood. HH-GLI signalling is initiated by binding of HH ligands to the transmembrane receptor PATCHED and is mediated by transcriptional effectors that belong to the GLI family, whose activity is finely tuned by a number of molecular interactions and post-translation modifications. Several reports suggest that the activity of the GLI proteins is regulated by several proliferative and oncogenic inputs, in addition or independent of upstream HH signalling. The identification of this complex crosstalk and the understanding of how the major oncogenic signalling pathways interact in cancer is a crucial step towards the establishment of efficient targeted combinatorial treatments. Here we review recent findings on the cooperative integration of HH-GLI signalling with the major oncogenic inputs and we discuss how these cues modulate the activity of the GLI proteins in cancer. We then summarise the latest advances on SMO and GLI inhibitors and alternative approaches to attenuate HH signalling through rational combinatorial therapies.

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.

During development intense Sox2 expression marks not only Prox1-expressing taste bud cell but also perigemmal cell lineages

Sox2 is proposed to regulate the differentiation of bipotential progenitor cells into taste bud cells. However, detailed expression of Sox2 remains unclear. In this report, Sox2 expression during taste bud development in the fungiform (FF), circumvallate (CV) and soft palate (SP) areas is examined together with Prox1. First, we immunohistochemically checked Prox1 expression in adults and found that almost all taste bud cells are Prox1-positive. During FF development, intense Sox2 expression was restricted to taste bud primordia expressing Prox1 at E12.5. However, at E14.5, Sox2 was intensely expressed outside the developing taste buds resolving to perigemmal Sox2 expression in adults. In the SP, at E14.5, taste bud primordia emerged as Prox1-expressing cell clusters. However, intense Sox2 expression was not restricted to taste bud primordia but was detected widely in the epithelium. During development, Sox2 expression outside developing taste buds was generally down-regulated but was retained in the perigemmal region similarly to that in the FF. In the CV, the initial stage of taste bud development remained unclear because of the lack of taste bud primordia comparable to that in the FF and SP. Here, we show that Prox1-expressing cells appear in the apical epithelium at E12.5, in the inner trench wall at E17.5 and in the outer trench wall at E18.5. Sox2 was again not restricted to developing taste bud cells expressing Prox1 during CV development. The expression patterns support that Sox2 does not serve as a cell fate selector between taste bud cells and surrounding keratinocytes but rather may contribute to them both.

Vismodegib, an antagonist of hedgehog signaling, directly alters taste molecular signaling in taste buds

Vismodegib, a highly selective inhibitor of hedgehog (Hh) pathway, is an approved treatment for basal-cell carcinoma. Patients on treatment with vismodegib often report profound alterations in taste sensation. The cellular mechanisms underlying the alterations have not been studied. Sonic Hh (Shh) signaling is required for cell growth and differentiation. In taste buds, Shh is exclusively expressed in type IV taste cells, which are undifferentiated basal cells and the precursors of the three types of taste sensing cells. Thus, we investigated if vismodegib has an inhibitory effect on taste cell turnover because of its known effects on Hh signaling. We gavaged C57BL/6J male mice daily with either vehicle or 30 mg/kg vismodegib for 15 weeks. The gustatory behavior and immunohistochemical profile of taste cells were examined. Vismodegib-treated mice showed decreased growth rate and behavioral responsivity to sweet and bitter stimuli, compared to vehicle-treated mice. We found that vismodegib-treated mice had significant reductions in taste bud size and numbers of taste cells per taste bud. Additionally, vismodegib treatment resulted in decreased numbers of Ki67- and Shh-expressing cells in taste buds. The numbers of phospholipase Cβ2- and α-gustducin-expressing cells, which contain biochemical machinery for sweet and bitter sensing, were reduced in vismodegib-treated mice. Furthermore, vismodegib treatment resulted in reduction in numbers of T1R3, glucagon-like peptide-1, and glucagon-expressing cells, which are known to modulate sweet taste sensitivity. These results suggest that inhibition of Shh signaling by vismodegib treatment directly results in alteration of taste due to local effects in taste buds.

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.

Induction of ectopic taste buds by SHH reveals the competency and plasticity of adult lingual epithelium

Taste buds are assemblies of elongated epithelial cells, which are innervated by gustatory nerves that transmit taste information to the brain stem. Taste cells are continuously renewed throughout life via proliferation of epithelial progenitors, but the molecular regulation of this process remains unknown. During embryogenesis, sonic hedgehog (SHH) negatively regulates taste bud patterning, such that inhibition of SHH causes the formation of more and larger taste bud primordia, including in regions of the tongue normally devoid of taste buds. Here, using a Cre-lox system to drive constitutive expression of SHH, we identify the effects of SHH on the lingual epithelium of adult mice. We show that misexpression of SHH transforms lingual epithelial cell fate, such that daughter cells of lingual epithelial progenitors form cell type-replete, onion-shaped taste buds, rather than non-taste, pseudostratified epithelium. These SHH-induced ectopic taste buds are found in regions of the adult tongue previously thought incapable of generating taste organs. The ectopic buds are composed of all taste cell types, including support cells and detectors of sweet, bitter, umami, salt and sour, and recapitulate the molecular differentiation process of endogenous taste buds. In contrast to the well-established nerve dependence of endogenous taste buds, however, ectopic taste buds form independently of both gustatory and somatosensory innervation. As innervation is required for SHH expression by endogenous taste buds, our data suggest that SHH can replace the need for innervation to drive the entire program of taste bud differentiation.

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.

Taste and smell function in chronic disease: a review of clinical and biochemical evaluations of taste and smell dysfunction in over 5000 patients at The Taste and Smell Clinic in Washington, DC

PURPOSE

To describe systematic methods developed over 40 years among over 5000 patients at The Taste and Smell Clinic in Washington, DC to evaluate taste and smell dysfunction.

MATERIALS AND METHODS

A tripartite methodology was developed. First, methods to determine clinical pathology underlying the multiple disease processes responsible for taste and smell dysfunction were developed. Second, methods to determine biochemical parameters responsible for these pathologies were developed. Third, methods to implement these techniques were developed to form a unified basis upon which treatment strategies can be developed to treat these patients.

RESULTS

Studies were performed in 5183 patients. Taste loss was present in 62% of patients, smell loss in 87%. Most patients with taste loss (52%) exhibited Type II hypogeusia; most patients with smell loss (56%) exhibited Type II hyposmia. Sensory distortions were present in 60%. Four common diagnostic entities were found: post influenza-type hyposmia and hypogeusia (27% of patients), idiopathic causes (16%), allergic rhinitis (15%) and post head injury (14%). Regardless of clinical diagnosis the major biochemical abnormality found in most patients (~70%) was diminished parotid salivary and nasal mucus secretion of cAMP and cGMP.

CONCLUSIONS

Taste and smell dysfunctions are common clinical problems associated with chronic disease processes. These symptoms require a systematic, integrated approach to understand their multiple and complex components. The approach presented here can and has led to effective treatment.

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.