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

Reawakening of Ancestral Dental Potential as a Mechanism to Explain Dental Pathologies

During evolution, there has been a trend to reduce both the number of teeth and the location where they are found within the oral cavity. In mammals, the formation of teeth is restricted to a horseshoe band of odontogenic tissue, creating a single dental arch on the top and bottom of the jaw. Additional teeth and structures containing dental tissue, such as odontogenic tumors or cysts, can appear as pathologies. These tooth-like structures can be associated with the normal dentition, appearing within the dental arch, or in nondental areas. The etiology of these pathologies is not well elucidated. Reawakening of the potential to form teeth in different parts of the oral cavity could explain the origin of dental pathologies outside the dental arch, thus such pathologies are a consequence of our evolutionary history. In this review, we look at the changing pattern of tooth formation within the oral cavity during vertebrate evolution, the potential to form additional tooth-like structures in mammals, and discuss how this knowledge shapes our understanding of dental pathologies in humans.

Cell signaling regulation in salivary gland development

The mammalian salivary gland develops as a highly branched structure designed to produce and secrete saliva. This review focuses on research conducted on mammalian salivary gland development, particularly on the differentiation of acinar, ductal, and myoepithelial cells. We discuss recent studies that provide conceptual advances in the understanding of the molecular mechanisms of salivary gland development. In addition, we describe the organogenesis of submandibular glands (SMGs), model systems used for the study of SMG development, and the key signaling pathways as well as cellular processes involved in salivary gland development. The findings from the recent studies elucidating the identity of stem/progenitor cells in the SMGs, and the process by which they are directed along a series of cell fate decisions to form functional glands, are also discussed. Advances in genetic tools and tissue engineering strategies will significantly increase our knowledge about the mechanisms by which signaling pathways and cells establish tissue architecture and function during salivary gland development, which may also be conserved in the growth and development of other organ systems. An increased knowledge of organ development mechanisms will have profound implications in the design of therapies for the regrowth or repair of injured tissues. In addition, understanding how the processes of cell survival, expansion, specification, movement, and communication with neighboring cells are regulated under physiological and pathological conditions is critical to the development of future treatments.

X-Linked Hypohidrotic Ectodermal Dysplasia-General Features and Dental Abnormalities in Affected Dogs Compared With Human Dental Abnormalities

X-linked hypohidrotic ectodermal dysplasia (XLHED) is a genetic disorder characterized by abnormalities in ectodermal derivatives such as sweat glands, hair, and teeth. In animals, the highest number of cases has been reported in dogs, which show characteristic congenital alopecia and develop abnormalities in the shape and number of teeth. Although the clinical phenotype of the affected individuals is typical, this disorder remains almost unknown in veterinary clinical practice. With the aim of making it better known, we gathered in this review the main clinical and genetic aspects of XLHED, placing emphasis on dental abnormalities.

MicroRNA-mediated regulation of BM-MSCs differentiation into sweat gland-like cells: targeting NF-κB

Sweat gland regeneration is important for patients with an extensive deep burn injury. In previous study, we reported that bone marrow-mesenchymal stem cells (BM-MSCs) could differentiate into sweat gland-like cells (SGLCs), but the underlying molecular mechanism remains unclear. Recently, microRNAs (miRNAs or miRs) are reported to manipulate many biological processes. However, whether the process of MSCs differentiation into sweat gland cells (SGCs) is regulated by miRNAs has not been reported. In this study, BM-MSCs were induced into SGLCs by co-culturing with SGCs. Differential expressions of miRNAs between BM-MSC and SGLCs were determined through miRNAs microarray and 68 miRNAs were found significantly changed in miRNA profile including hsa-miR-138-5p. Bioinformatics analysis showed that hsa-miR-138-5p targeted a group of nuclear factor-κB (NF-κB) related genes which play an important role in skin appendage development. As expected, hsa-miR-138-5p inhibitor transfected into BM-MSCs partly mimicked the effects of co-culture and increased the number of SGLCs by increasing the expression of NF-κB related genes. These results suggest that hsa-miR-138-5p and NF-κB are involved in the regulation of BM-MSCs differentiation into SGLCs. This study may also offer a new approach to yield SGCs for burn patients.

The Impact of the Eda Pathway on Tooth Root Development

The Eda pathway ( Eda, Edar, Edaradd) plays an important role in tooth development, determining tooth number, crown shape, and enamel formation. Here we show that the Eda pathway also plays a key role in root development. Edar (the receptor) is expressed in Hertwig's epithelial root sheath (HERS) during root development, with mutant mice showing a high incidence of taurodontism: large pulp chambers lacking or showing delayed bifurcation or trifurcation of the roots. The mouse upper second molars in the Eda pathway mutants show the highest incidence of taurodontism, this enhanced susceptibility being matched in human patients with mutations in EDA-A1. These taurodont teeth form due to defects in the direction of extension of the HERS from the crown, associated with a more extensive area of proliferation of the neighboring root mesenchyme. In those teeth where the angle at which the HERS extends from the crown is very wide and therefore more vertical, the mutant HERSs fail to reach toward the center of the tooth in the normal furcation region, and taurodont teeth are created. The phenotype is variable, however, with milder changes in angle and proliferation leading to normal or delayed furcation. This is the first analysis of the role of Eda in the root, showing a direct role for this pathway during postnatal mouse development, and it suggests that changes in proliferation and angle of HERS may underlie taurodontism in a range of syndromes.

Multiple Cranial Organ Defects after Conditionally Knocking Out Fgf10 in the Neural Crest

Fgf10 is necessary for the development of a number of organs that fail to develop or are reduced in size in the null mutant. Here we have knocked out Fgf10 specifically in the neural crest driven by Wnt1cre. The Wnt1creFgf10fl/fl mouse phenocopies many of the null mutant defects, including cleft palate, loss of salivary glands, and ocular glands, highlighting the neural crest origin of the Fgf10 expressing mesenchyme surrounding these organs. In contrast tissues such as the limbs and lungs, where Fgf10 is expressed by the surrounding mesoderm, were unaffected, as was the pituitary gland where Fgf10 is expressed by the neuroepithelium. The circumvallate papilla of the tongue formed but was hypoplastic in the conditional and Fgf10 null embryos, suggesting that other sources of FGF can compensate in development of this structure. The tracheal cartilage rings showed normal patterning in the conditional knockout, indicating that the source of Fgf10 for this tissue is mesodermal, which was confirmed using Wnt1cre-dtTom to lineage trace the boundary of the neural crest in this region. The thyroid, thymus, and parathyroid glands surrounding the trachea were present but hypoplastic in the conditional mutant, indicating that a neighboring source of mesodermal Fgf10 might be able to partially compensate for loss of neural crest derived Fgf10.

Pharmacological activation of the EDA/EDAR signaling pathway restores salivary gland function following radiation-induced damage

Radiotherapy of head and neck cancers often results in collateral damage to adjacent salivary glands associated with clinically significant hyposalivation and xerostomia. Due to the reduced capacity of salivary glands to regenerate, hyposalivation is treated by substitution with artificial saliva, rather than through functional restoration of the glands. During embryogenesis, the ectodysplasin/ectodysplasin receptor (EDA/EDAR) signaling pathway is a critical element in the development and growth of salivary glands. We have assessed the effects of pharmacological activation of this pathway in a mouse model of radiation-induced salivary gland dysfunction. We report that post-irradiation administration of an EDAR-agonist monoclonal antibody (mAbEDAR1) normalizes function of radiation damaged adult salivary glands as determined by stimulated salivary flow rates. In addition, salivary gland structure and homeostasis is restored to pre-irradiation levels. These results suggest that transient activation of pathways involved in salivary gland development could facilitate regeneration and restoration of function following damage.

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.

Molecular cues for development and regeneration of salivary glands

The hypofunction of salivary glands caused by Sjögren's Syndrome or radiotherapy for head and neck cancer significantly compromises the quality of life of millions patients. Currently no curative treatment is available for the irreversible hyposalivation, whereas regenerative strategies targeting salivary stem/progenitor cells are promising. However, the success of these strategies is constrained by the lack of insights on the molecular cues of salivary gland regeneration. Recent advances in the molecular controls of salivary gland morphogenesis provided valuable clues for identifying potential regenerative cues. A complicated network of signaling molecules between epithelia, mesenchyme, endothelia, extracellular matrix and innervating nerves orchestrate the salivary gland organogenesis. Here we discuss the roles of several cross-talking intercellular signaling pathways, i.e., FGF, Wnt, Hedgehog, Eda, Notch, Chrm1/HB-EGF and Laminin/Integrin pathways, in the development of salivary glands and their potentials to promote salivary regeneration.

A rat model of hypohidrotic ectodermal dysplasia carries a missense mutation in the Edaradd gene

Background

Hypohidrotic ectodermal dysplasia (HED) is a congenital disorder characterized by sparse hair, oligodontia, and inability to sweat. It is caused by mutations in any of three Eda pathway genes: ectodysplasin (Eda), Eda receptor (Edar), and Edar-associated death domain (Edaradd), which encode ligand, receptor, and intracellular adaptor molecule, respectively. The Eda signaling pathway activates NF-κB, which is central to ectodermal differentiation. Although the causative genes and the molecular pathway affecting HED have been identified, no curative treatment for HED has been established. Previously, we found a rat spontaneous mutation that caused defects in hair follicles and named it sparse-and-wavy (swh). Here, we have established the swh rat as the first rat model of HED and successfully identified the swh mutation.

Results

The swh/swh rat showed sparse hair, abnormal morphology of teeth, and absence of sweat glands. The ectoderm-derived glands, meibomian, preputial, and tongue glands, were absent. We mapped the swh mutation to the most telomeric part of rat Chr 7 and found a Pro153Ser missense mutation in the Edaradd gene. This mutation was located in the death domain of EDARADD, which is crucial for signal transduction and resulted in failure to activate NF-κB.

Conclusions

These findings suggest that swh is a loss-of-function mutation in the rat Edaradd and indicate that the swh/swh rat would be an excellent animal model of HED that could be used to investigate the pathological basis of the disease and the development of new therapies.

Shh is required for Tabby hair follicle development

In embryonic Eda mutant ("Tabby") mice, the development of one of the two major types of hair, "primary" hair fails, but other "secondary" hairs develop in normal numbers, though shorter and slightly aberrant. In Tabby mice, Shh is undetectable in skin early on, but is activated during secondary hair formation. We inferred that Shh may be involved in primary hair formation, activated normally by Eda, and also possibly in secondary hair formation, activated by an Eda-independent pathway. Varying the dosage of Shh now supports these inferences. In Shh knockout mice, mice were totally hairless: primary and secondary hair follicle germs were formed, but further progression failed. Consistent with these findings, when Shh loss was restricted to the skin, secondary hair follicle germs were initiated on time in Tabby mice, but their subsequent development (down-growth) failed. An Shh transgene expressed in Tabby skin could not restore induction of primary hair follicles, but restored normal length to the somewhat aberrant secondary hair that was formed and prolonged the anagen phase of hair cycling. Thus, Shh is required for primary and secondary hair down-growth and full secondary hair length, but is not itself sufficient to replace Eda or make fully normal secondary hair.

Ectodysplasin and Wnt pathways are required for salivary gland branching morphogenesis

The developing submandibular salivary gland (SMG) is a well-studied model for tissue interactions and branching morphogenesis. Its development shares similar features with other ectodermal appendages such as hair and tooth. The ectodysplasin (Eda) pathway is essential for the formation and function of several ectodermal organs. Mutations in the signaling components of the Eda pathway lead to a human syndrome known as hypohidrotic ectodermal dysplasia (HED), which is characterized by missing and malformed teeth, sparse hair and reduced sweating. Individuals with HED suffer also from dry mouth because of reduced saliva flow. In order to understand the underlying mechanism, we analyzed salivary gland development in mouse models with altered Eda pathway activities. We have found that Eda regulates growth and branching of the SMG via transcription factor NF-κB in the epithelium, and that the hedgehog pathway is an important mediator of Eda/NF-κB. We also sought to determine whether a similar reciprocal interplay between the Eda and Wnt/β-catenin pathways, which are known to operate in other skin appendages, functions in developing SMG. Surprisingly and unlike in developing hair follicles and teeth, canonical Wnt signaling activity did not colocalize with Edar/NF-κB in salivary gland epithelium. Instead, we observed high mesenchymal Wnt activity and show that ablation of mesenchymal Wnt signaling either in vitro or in vivo compromised branching morphogenesis. We also provide evidence suggesting that the effects of mesenchymal Wnt/β-catenin signaling are mediated, at least in part, through regulation of Eda expression.

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

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

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