Sox2 and Lef-1 interact with Pitx2 to regulate incisor development and stem cell renewal

Development of a new antibody drug to treat congenital tooth agenesis

BACKGROUND

This study aimed to develop a therapeutic agent promoting teeth regeneration from autologous tissues for congenital tooth agenesis, specifically for hypodontia (≤5 missing congenital teeth, 10% prevalence) and oligodontia (≥6 missing congenital teeth, 0.1% prevalence).

HIGHLIGHT

We studied mice genetically deficient in the USAG-1 protein, an antagonist of BMP/Wnt which forms excessive teeth. We identified USAG-1 as a target molecule for increasing the number of teeth. Crossing USAG-1-deficient mice with a congenital tooth agenesis model restored tooth formation. We produced anti-USAG-1 neutralizing antibodies as potential therapeutic agents for the treatment of congenital tooth agenesis. Mice anti-USAG-1 neutralizing antibodies can potentially rescue the developmentally arrested tooth germ programmed to a certain tooth type. A humanized anti-USAG-1 antibody was developed as the final candidate.

CONCLUSION

Targeting USAG-1 shows promise for treating missing congenital tooth. Anti-USAG-1 neutralizing antibodies have been developed and will progress towards clinical trials, which may regenerate missing congenital teeth in conditions, such as hypodontia and oligodontia. The protocol framework for a phase 1 study has been finalized, and preparation for future studies is underway.

Follistatin controls the number of murine teeth by limiting TGF-β signaling

Supernumerary teeth are common developmental anomalies of dentition. However, the factors and mechanisms driving their formation remain largely unknown. Here, we report that conditional knockout of Fst, encoding an antagonist for the transforming growth factor β (TGF-β) signaling pathway, in both oral epithelium and mesenchyme of mice (Fst CKO ) led to supernumerary upper incisor teeth, arising from the lingual dental epithelium of the native teeth and preceded by an enlarged and split lingual cervical loop. Fst-deficiency greatly activated TGF-β signaling in developing maxillary incisor teeth, associated with increased epithelium cell proliferation. Moreover, Fst CKO teeth exhibited increased expression of Tbx1, Sp6, and Sox2, which were identified as direct targets of TGF-β/SMAD2 signaling. Finally, we show that upregulation of Tbx1 in response to Fst-deficiency was largely responsible for the formation of extra teeth in Fst CKO mice. Taken together, our investigation indicates a novel role for Fst in controlling murine tooth number by restricting TGF-β signaling.

MAST4 regulates stem cell maintenance with DLX3 for epithelial development and amelogenesis

The asymmetric division of stem cells permits the maintenance of the cell population and differentiation for harmonious progress. Developing mouse incisors allows inspection of the role of the stem cell niche to provide specific insights into essential developmental phases. Microtubule-associated serine/threonine kinase family member 4 (Mast4) knockout (KO) mice showed abnormal incisor development with low hardness, as the size of the apical bud was decreased and preameloblasts were shifted to the apical side, resulting in amelogenesis imperfecta. In addition, Mast4 KO incisors showed abnormal enamel maturation, and stem cell maintenance was inhibited as amelogenesis was accelerated with Wnt signal downregulation. Distal-Less Homeobox 3 (DLX3), a critical factor in tooth amelogenesis, is considered to be responsible for the development of amelogenesis imperfecta in humans. MAST4 directly binds to DLX3 and induces phosphorylation at three residues within the nuclear localization site (NLS) that promotes the nuclear translocation of DLX3. MAST4-mediated phosphorylation of DLX3 ultimately controls the transcription of DLX3 target genes, which are carbonic anhydrase and ion transporter genes involved in the pH regulation process during ameloblast maturation. Taken together, our data reveal a novel role for MAST4 as a critical regulator of the entire amelogenesis process through its control of Wnt signaling and DLX3 transcriptional activity.

Transcriptional programs of Pitx2 and Tfap2a/Tfap2b controlling lineage specification of mandibular epithelium during tooth initiation

How the dorsal-ventral axis of the vertebrate jaw, particularly the position of tooth initiation site, is established remains a critical and unresolved question. Tooth development starts with the formation of the dental lamina, a localized thickened strip within the maxillary and mandibular epithelium. To identify transcriptional regulatory networks (TRN) controlling the specification of dental lamina from the naïve mandibular epithelium, we utilized Laser Microdissection coupled low-input RNA-seq (LMD-RNA-seq) to profile gene expression of different domains of the mandibular epithelium along the dorsal-ventral axis. We comprehensively identified transcription factors (TFs) and signaling pathways that are differentially expressed along mandibular epithelial domains (including the dental lamina). Specifically, we found that the TFs Sox2 and Tfap2 (Tfap2a/Tfap2b) formed complimentary expression domains along the dorsal-ventral axis of the mandibular epithelium. Interestingly, both classic and novel dental lamina specific TFs-such as Pitx2, Ascl5 and Zfp536-were found to localize near the Sox2:Tfap2a/Tfap2b interface. To explore the functional significance of these domain specific TFs, we next examined loss-of-function mouse models of these domain specific TFs, including the dental lamina specific TF, Pitx2, and the ventral surface ectoderm specific TFs Tfap2a and Tfap2b. We found that disruption of domain specific TFs leads to an upregulation and expansion of the alternative domain's TRN. The importance of this cross-repression is evident by the ectopic expansion of Pitx2 and Sox2 positive dental lamina structure in Tfap2a/Tfap2b ectodermal double knockouts and the emergence of an ectopic tooth in the ventral surface ectoderm. Finally, we uncovered an unappreciated interface of mesenchymal SHH and WNT signaling pathways, at the site of tooth initiation, that were established by the epithelial domain specific TFs including Pitx2 and Tfap2a/Tfap2b. These results uncover a previously unknown molecular mechanism involving cross-repression of domain specific TFs including Pitx2 and Tfap2a/Tfap2b in patterning the dorsal-ventral axis of the mouse mandible, specifically the regulation of tooth initiation site.

Computational identification of key transcription factors for embryonic and postnatal Sox2+ dental epithelial stem cell

While many reptiles can replace their tooth throughout life, human loss the tooth replacement capability after formation of the permanent teeth. It was thought that the difference in tooth regeneration capability depends on the persistence of a specialized dental epithelial structure, the dental lamina that contains dental epithelial stem cells (DESC). Currently, we know very little about DESC such as what genes are expressed or its chromatin accessibility profile. Multiple markers of DESC have been proposed such as Sox2 and Lgr5 . Few single cell RNA-seq experiments have been performed previously, but no obvious DESC cluster was identified in these scRNA-seq datasets, possible due to that the expression level of DESC markers such as Sox2 and Lgr5 is too low or the percentage of DESC is too low in whole tooth. We utilize a mouse line Sox2-GFP to enrich Sox2+ DESC and use Smart-Seq2 protocol and ATAC-seq protocol to generate transcriptome profile and chromatin accessibility profile of P2 Sox2+ DESC. Additionally, we generate transcriptome profile and chromatin accessibility profile of E11.5 Sox2+ dental lamina cells. With transcriptome profile and chromatin accessibility profile, we systematically identify potential key transcription factors for E11.5 Sox2+ cells and P2 Sox2+ cells. We identified transcription factors including Pitx2, Id3, Pitx1, Tbx1, Trp63, Nkx2-3, Grhl3, Dlx2, Runx1, Nfix, Zfp536 , etc potentially formed the core transcriptional regulatory networks of Sox2+ DESC in both embryonic and postnatal stages.

Wolf-Hirschhorn syndrome candidate 1 (Whsc1) methyltransferase signals via a Pitx2-miR-23/24 axis to effect tooth development

Wolf-Hirschhorn syndrome (WHS) is a developmental disorder attributed to a partial deletion on the short arm of chromosome 4. WHS patients suffer from oral manifestations including cleft lip and palate, hypodontia, and taurodontism. WHS candidate 1 (WHSC1) gene is a H3K36-specific methyltransferase that is deleted in every reported case of WHS. Mutation in this gene also results in tooth anomalies in patients. However, the correlation between genetic abnormalities and the tooth anomalies has remained controversial. In our study, we aimed to clarify the role of WHSC1 in tooth development. We profiled the Whsc1 expression pattern during mouse incisor and molar development by immunofluorescence staining and found Whsc1 expression is reduced as tooth development proceeds. Using real-time quantitative reverse transcription PCR, Western blot, chromatin immunoprecipitation, and luciferase assays, we determined that Whsc1 and Pitx2, the initial transcription factor involved in tooth development, positively and reciprocally regulate each other through their gene promoters. miRNAs are known to regulate gene expression posttranscriptionally during development. We previously reported miR-23a/b and miR-24-1/2 were highly expressed in the mature tooth germ. Interestingly, we demonstrate here that these two miRs directly target Whsc1 and repress its expression. Additionally, this miR cluster is also negatively regulated by Pitx2. We show the expression of these two miRs and Whsc1 are inversely correlated during mouse mandibular development. Taken together, our results provide new insights into the potential role of Whsc1 in regulating tooth development and a possible molecular mechanism underlying the dental defects in WHS.

Pitx2 suppression at meiotic stages associates with seasonal inhibition of testis development in Rattus norvegicus caraco

The bicoid-related transcription factor 2 (Pitx2) plays a crucial role in the development of many organs and tissues by affecting the mitotic cell cycle. Postnatal testis development is related to mitosis and meiosis in multiple cell types, but the role of Pitx2 gene in seasonal inhibition of testicular development remains unknown in rodents. We analyzed PITX2 protein and Pitx2 mRNA expression features using both laboratory and wild male Rattus norvegicus caraco. In postnatal testicle of laboratory colony, we found that PITX2 was expressed in Leydig cells, pachytene spermatocytes, round spermatids, and elongating spermatids rather than spermatogonia and leptotene/zygotene spermatocytes. Pitx2b expression significantly increased along with the occurrence of pachytene spermatocytes and round spermatids, while decreased along with the processes of elongated spermatids. In wild male rats with similar testes weight, a significantly suppressed Pitx2b expression occurred with an active meiotic stage in the inhibited testes in autumn and winter, compared with the normally developing testes in spring and summer. These results indicate that Pitx2b expression suppression plays a crucial role in the seasonal inhibition of testis development.

Development of a novel ex vivo organ culture system to improve preservation methods of regenerative tissues

Recent advances in regenerative technology have made the regeneration of various organs using pluripotent stem cells possible. However, a simpler screening method for evaluating regenerated organs is required to apply this technology to clinical regenerative medicine in the future. We have developed a simple evaluation method using a mouse tooth germ culture model of organs formed by epithelial-mesenchymal interactions. In this study, we successfully established a simple method that controls tissue development in a temperature-dependent manner using a mouse tooth germ ex vivo culture model. We observed that the development of the cultured tooth germ could be delayed by low-temperature culture and resumed by the subsequent culture at 37 °C. Furthermore, the optimal temperature for the long-term preservation of tooth germ was 25 °C, a subnormothermic temperature that maintains the expression of stem cell markers. We also found that subnormothermic temperature induces the expression of cold shock proteins, such as cold-inducible RNA-binding protein, RNA-binding motif protein 3, and serine and arginine rich splicing factor 5. This study provides a simple screening method to help establish the development of regenerative tissue technology using a tooth organ culture model. Our findings may be potentially useful for making advances in the field of regenerative medicine.

Nuclear Factor I-C Regulates Stemness Genes and Proliferation of Stem Cells in Various Mineralized Tissue through Epithelial-Mesenchymal Interactions in Dental Epithelial Stem Cells

Tooth development includes numerous cell divisions and cell-cell interactions generating the stem cell niche. After an indefinite number of divisions, pluripotent cells differentiate into various types of cells. Nuclear factor I (NFI) transcription factors are known as crucial regulators in various organ development and stem cell biology. Among its members, nuclear factor I-C (NFI-C) has been reported to play an essential role in odontogenesis. Nfic knockout mice show malformation in all mineralized tissues, but it remains unclear which stage of development Nfic is involved in. We previously reported that Nfic induces the differentiation of ameloblast, odontoblast, and osteoblast. However, the question remains whether Nfic participates in the late stage of development, perpetuating the proliferation of stem cells. This study aimed to elucidate the underlying mechanism of NFI-C function in stem cells capable of forming hard tissues. Here, we demonstrate that Nfic regulates Sox2 and cell proliferation in diverse mineralized tissue stem cells such as dental epithelial stem cells (DESCs), dental pulp stem cells, and bone marrow stem cells, but not in fibroblasts. It was also involved in the expression of pluripotency genes Lin28 and NANOG. Especially in DESCs, Nfic regulates the proliferation of epithelial cells via epithelial-mesenchymal interactions, which are the Fgf8-Nfic-Sox2 pathway in epithelium and Nfic-Fgf10 in the mesenchyme. Moreover, Nfic slightly increased reprogramming efficiency in induced pluripotent stem cells of mineralized tissues, but not in soft tissues. In conclusion, these results suggest that Nfic is a crucial factor for maintaining the stem cell niche of mineralized tissues and provides a possibility for Nfic as an additional factor in improving reprogramming efficiency.

Revisiting the miR-200 Family: A Clan of Five Siblings with Essential Roles in Development and Disease

Over two decades of studies on small noncoding RNA molecules illustrate the significance of microRNAs (miRNAs/miRs) in controlling multiple physiological and pathological functions through post-transcriptional and spatiotemporal gene expression. Among the plethora of miRs that are essential during animal embryonic development, in this review, we elaborate the indispensable role of the miR-200 family (comprising miR-200a, -200b, 200c, -141, and -429) in governing the cellular functions associated with epithelial homeostasis, such as epithelial differentiation and neurogenesis. Additionally, in pathological contexts, miR-200 family members are primarily involved in tumor-suppressive roles, including the reversal of the cancer-associated epithelial–mesenchymal transition dedifferentiation process, and are dysregulated during organ fibrosis. Moreover, recent eminent studies have elucidated the crucial roles of miR-200s in the pathophysiology of multiple neurodegenerative diseases and tissue fibrosis. Lastly, we summarize the key studies that have recognized the potential use of miR-200 members as biomarkers for the diagnosis and prognosis of cancers, elaborating the application of these small biomolecules in aiding early cancer detection and intervention.

Organoids from human tooth showing epithelial stemness phenotype and differentiation potential

Insight into human tooth epithelial stem cells and their biology is sparse. Tissue-derived organoid models typically replicate the tissue’s epithelial stem cell compartment. Here, we developed a first-in-time epithelial organoid model starting from human tooth. Dental follicle (DF) tissue, isolated from unerupted wisdom teeth, efficiently generated epithelial organoids that were long-term expandable. The organoids displayed a tooth epithelial stemness phenotype similar to the DF’s epithelial cell rests of Malassez (ERM), a compartment containing dental epithelial stem cells. Single-cell transcriptomics reinforced this organoid-ERM congruence, and uncovered novel, mouse-mirroring stem cell features. Exposure of the organoids to epidermal growth factor induced transient proliferation and eventual epithelial-mesenchymal transition, highly mimicking events taking place in the ERM in vivo. Moreover, the ERM stemness organoids were able to unfold an ameloblast differentiation process, further enhanced by transforming growth factor-β (TGFβ) and abrogated by TGFβ receptor inhibition, thereby reproducing TGFβ's known key position in amelogenesis. Interestingly, by creating a mesenchymal-epithelial composite organoid (assembloid) model, we demonstrated that the presence of dental mesenchymal cells (i.e. pulp stem cells) triggered ameloblast differentiation in the epithelial stem cells, thus replicating the known importance of mesenchyme-epithelium interaction in tooth development and amelogenesis. Also here, differentiation was abrogated by TGFβ receptor inhibition. Together, we developed novel organoid models empowering the exploration of human tooth epithelial stem cell biology and function as well as their interplay with dental mesenchyme, all at present only poorly defined in humans. Moreover, the new models may pave the way to future tooth-regenerative perspectives.

A novel mutation of SATB2 inhibits odontogenesis of human dental pulp stem cells through Wnt/β-catenin signaling pathway

BACKGROUND

SATB2-associated syndrome (SAS) is a multisystem disorder caused by mutation of human SATB2 gene. Tooth agenesis is one of the most common phenotypes observed in SAS. Our study aimed at identifying novel variant of SATB2 in a patient with SAS, and to investigate the cellular and molecular mechanism of tooth agenesis caused by SATB2 mutation.

METHODS

We applied whole exome sequencing (WES) to identify the novel mutation of SATB2 in a Chinese patient with SAS. Construction and overexpression of wild-type and the mutant vector was performed, followed by functional analysis including flow cytometry assay, fluorescent immunocytochemistry, western blot, quantitative real-time PCR and Alizarin Red S staining to investigate its impact on hDPSCs and the underlying mechanisms.

RESULTS

As a result, we identified a novel frameshift mutation of SATB2 (c. 376_378delinsTT) in a patient with SAS exhibiting tooth agenesis. Human DPSCs transfected with mutant SATB2 showed decreased cell proliferation and odontogenic differentiation capacity compared with hDPSCs transfected with wild-type SATB2 plasmid. Mechanistically, mutant SATB2 failed to translocate into nucleus and distributed in the cytoplasm, failing to activate Wnt/β-catenin signaling pathway, whereas the wild-type SATB2 translocated into the nucleus and upregulated the expression of active β-catenin. When we used Wnt inhibitor XAV939 to treat hDPSCs transfected with wild-type SATB2 plasmid, the increased odontogenic differentiation capacity was attenuated. Furthermore, we found that SATB2 mutation resulted in the upregulation of DKK1 and histone demethylase JHDM1D to inhibit Wnt/β-catenin signaling pathway.

CONCLUSION

We identified a novel frameshift mutation of SATB2 (c.376_378delinsTT, p.Leu126SerfsX6) in a Chinese patient with SATB2-associated syndrome (SAS) exhibiting tooth agenesis. Mechanistically, SATB2 regulated osteo/odontogenesis of human dental pulp stem cells through Wnt/β-catenin signaling pathway by regulating DKK1 and histone demethylase JHDM1D.

Intertwined Signaling Pathways Governing Tooth Development: A Give-and-Take Between Canonical Wnt and Shh

Teeth play essential roles in life. Their development relies on reciprocal interactions between the ectoderm-derived dental epithelium and the underlying neural crest-originated mesenchyme. This odontogenic process serves as a prototype model for the development of ectodermal appendages. In the mouse, developing teeth go through distinct morphological phases that are tightly controlled by epithelial signaling centers. Crucial molecular regulators of odontogenesis include the evolutionarily conserved Wnt, BMP, FGF and sonic hedgehog (Shh) pathways. These signaling modules do not act on their own, but are closely intertwined during tooth development, thereby outlining the path to be taken by specific cell populations including the resident dental stem cells. Recently, pivotal Wnt-Shh interaction and feedback loops have been uncovered during odontogenesis, showing conservation in other developing ectodermal appendages. This review provides an integrated overview of the interplay between canonical Wnt and Shh throughout mouse tooth formation stages, extending from the initiation of dental placode to the fully formed adult tooth.

Activated Epithelial FGF8 Signaling Induces Fused Supernumerary Incisors

FGF8, which is specifically expressed in the dental epithelium prior to the E12.5 bud stage, is a key player during odontogenesis, being responsible for the initiation of tooth development. Here, to investigate the impact of persistent FGF8 signaling on tooth development, we forcibly activated FGF8 signaling in the dental epithelium after the bud stage by generating K14-Cre;R26R-Fg8 mice. We found that a unique type of fused supernumerary incisors is formed, although morphologically resembling the features of type II dens invaginatus in humans. Further analysis revealed that ectopically activated epithelial FGF8 alters the cell fate of the incisor lingual outer enamel epithelium, endowing it with odontogenic potential by the activation of several key tooth genes, including Pitx2, Sox2, Lef-1, p38, and Erk1/2, and induces de novo formation of an extra incisor crown lingually in parallel to the original one, leading to the formation of an extra incisor crown and fused with the original incisor eventually. Meanwhile, the overdosed epithelial FGF8 signaling dramatically downregulates the expression of mesenchymal Bmp4, leading to severely impaired enamel mineralization. Based on the location of the extra incisors, we propose that they are likely to be rescued replacement teeth. Our results further demonstrate the essential role of FGF8 signaling for tooth initiation and the establishment of progenitor cells of dental epithelial stem cells during development.

Sox2 controls asymmetric patterning of ameloblast lineage commitment by regulation of FGF signaling in the mouse incisor

Mouse incisors are covered by enamel only on the labial side and the lingual side is covered by dentin without enamel. This asymmetric distribution of enamel makes it possible to be abrased on the lingual side, generating the sharp cutting edge of incisor on the labial side. The abrasion of mouse incisors is compensated by the continuous growth throughout life. Epithelium stem cells responsible for its continuous growth are reported to localize within the labial cervical loop. The transcription factor Sox2 plays important roles in the maintenance of stem cell pluripotency and organ formation. We previously found that Sox2 mainly expressed in the dental epithelium. Besides, Sox2 has been reported to be a dental epithelium stem cell marker in the incisor. However, the exact mechanism of Sox2 controlling amelogenesis is still not quite clearly elucidated. Here we report that conditional deletion of Sox2 in the dental epithelium using Shhcre caused impaired ameloblast differentiation in the labial side and induced ectopic ameloblast-like cell differentiation on the lingual side. Abnormal FGF gene expression was detected by RNAscope in situ hybridization in the mutant incisor. Collectively, we speculate that asymmetric ameloblast lineage commitment of mouse incisor might be regulated by Sox2 through FGF signaling.

Distinct tooth regeneration systems deploy a conserved battery of genes

BACKGROUND

Vertebrate teeth exhibit a wide range of regenerative systems. Many species, including most mammals, reptiles, and amphibians, form replacement teeth at a histologically distinct location called the successional dental lamina, while other species do not employ such a system. Notably, a 'lamina-less' tooth replacement condition is found in a paraphyletic array of ray-finned fishes, such as stickleback, trout, cod, medaka, and bichir. Furthermore, the position, renewal potential, and latency times appear to vary drastically across different vertebrate tooth regeneration systems. The progenitor cells underlying tooth regeneration thus present highly divergent arrangements and potentials. Given the spectrum of regeneration systems present in vertebrates, it is unclear if morphologically divergent tooth regeneration systems deploy an overlapping battery of genes in their naïve dental tissues.

RESULTS

In the present work, we aimed to determine whether or not tooth progenitor epithelia could be composed of a conserved cell type between vertebrate dentitions with divergent regeneration systems. To address this question, we compared the pharyngeal tooth regeneration processes in two ray-finned fishes: zebrafish (Danio rerio) and threespine stickleback (Gasterosteus aculeatus). These two teleost species diverged approximately 250 million years ago and demonstrate some stark differences in dental morphology and regeneration. Here, we find that the naïve successional dental lamina in zebrafish expresses a battery of nine genes (bmpr1aa, bmp6, cd34, gli1, igfbp5a, lgr4, lgr6, nfatc1, and pitx2), while active Wnt signaling and Lef1 expression occur during early morphogenesis stages of tooth development. We also find that, despite the absence of a histologically distinct successional dental lamina in stickleback tooth fields, the same battery of nine genes (Bmpr1a, Bmp6, CD34, Gli1, Igfbp5a, Lgr4, Lgr6, Nfatc1, and Pitx2) are expressed in the basalmost endodermal cell layer, which is the region most closely associated with replacement tooth germs. Like zebrafish, stickleback replacement tooth germs additionally express Lef1 and exhibit active Wnt signaling. Thus, two fish systems that either have an organized successional dental lamina (zebrafish) or lack a morphologically distinct successional dental lamina (sticklebacks) deploy similar genetic programs during tooth regeneration.

CONCLUSIONS

We propose that the expression domains described here delineate a highly conserved "successional dental epithelium" (SDE). Furthermore, a set of orthologous genes is known to mark hair follicle epithelial stem cells in mice, suggesting that regenerative systems in other epithelial appendages may utilize a related epithelial progenitor cell type, despite the highly derived nature of the resulting functional organs.

Gene expression patterns associated with dental replacement in the rabbit, a new model for the mammalian dental replacement mechanisms

BACKGROUND

Unlike many vertebrates with continuous dental replacement, mammals have a maximum of two dental generations. Due to the absence of dental replacement in the laboratory mouse, the mechanisms of the mammalian tooth replacement system are poorly known. In this study, we use the European rabbit as a model for mammalian tooth development and replacement.

RESULTS

We provide data on some key regulators of tooth development. We detected the presence of SOX2 in both the replacement dental lamina and the rudimentary successional dental lamina of unreplaced molars, indicating that SOX2 may not be sufficient to initiate and maintain tooth replacement. We showed that Shh does not seem to be directly involved in tooth replacement. The transient presence of the rudimentary successional dental lamina in the molar allowed us to identify genes that could be essential for the initiation or the maintenance of tooth replacement. Hence, the locations of Sostdc1, RUNX2, and LEF1 vary between the deciduous premolar, the replacement premolar, and the molar, indicating possible roles in tooth replacement.

CONCLUSION

According to our observations, initiation and the maintenance of tooth replacement correlate with the presence of LEF1⁺ cells and the absence of both mesenchymal RUNX2 and epithelial Sostdc1⁺ cells.

The miR-200 family is required for ectodermal organ development through the regulation of the epithelial stem cell niche

The murine lower incisor ectodermal organ contains a single epithelial stem cell (SC) niche that provides epithelial progenitor cells to the continuously growing rodent incisor. The dental stem cell niche gives rise to several cell types and we demonstrate that the miR‐200 family regulates these cell fates. The miR‐200 family is highly enriched in the differentiated dental epithelium and absent in the stem cell niche. In this study, we inhibited the miR‐200 family in developing murine embryos using new technology, resulting in an expanded epithelial stem cell niche and lack of cell differentiation. Inhibition of individual miRs within the miR‐200 cluster resulted in differential developmental and cell morphology defects. miR‐200 inhibition increased the expression of dental epithelial stem cell markers, expanded the stem cell niche and decreased progenitor cell differentiation. RNA‐seq. identified miR‐200 regulatory pathways involved in cell differentiation and compartmentalization of the stem cell niche. The miR‐200 family regulates signaling pathways required for cell differentiation and cell cycle progression. The inhibition of miR‐200 decreased the size of the lower incisor due to increased autophagy and cell death. New miR‐200 targets demonstrate gene networks and pathways controlling cell differentiation and maintenance of the stem cell niche. This is the first report demonstrating how the miR‐200 family is required for in vivo progenitor cell proliferation and differentiation.

Sorting Sox: Diverse Roles for Sox Transcription Factors During Neural Crest and Craniofacial Development

Sox transcription factors play many diverse roles during development, including regulating stem cell states, directing differentiation, and influencing the local chromatin landscape. Of the twenty vertebrate Sox factors, several play critical roles in the development the neural crest, a key vertebrate innovation, and the subsequent formation of neural crest-derived structures, including the craniofacial complex. Herein, we review the specific roles for individual Sox factors during neural crest cell formation and discuss how some factors may have been essential for the evolution of the neural crest. Additionally, we describe how Sox factors direct neural crest cell differentiation into diverse lineages such as melanocytes, glia, and cartilage and detail their involvement in the development of specific craniofacial structures. Finally, we highlight several SOXopathies associated with craniofacial phenotypes.

New insight into dental epithelial stem cells: Identification, regulation, and function in tooth homeostasis and repair

Tooth enamel, a highly mineralized tissue covering the outermost area of teeth, is always damaged by dental caries or trauma. Tooth enamel rarely repairs or renews itself, due to the loss of ameloblasts and dental epithelial stem cells (DESCs) once the tooth erupts. Unlike human teeth, mouse incisors grow continuously due to the presence of DESCs that generate enamel-producing ameloblasts and other supporting dental epithelial lineages. The ready accessibility of mouse DESCs and wide availability of related transgenic mouse lines make mouse incisors an excellent model to examine the identity and heterogeneity of dental epithelial stem/progenitor cells; explore the regulatory mechanisms underlying enamel formation; and help answer the open question regarding the therapeutic development of enamel engineering. In the present review, we update the current understanding about the identification of DESCs in mouse incisors and summarize the regulatory mechanisms of enamel formation driven by DESCs. The roles of DESCs during homeostasis and repair are also discussed, which should improve our knowledge regarding enamel tissue engineering.

Development and regeneration of the crushing dentition in skates (Rajidae)

Sharks and rays (elasmobranchs) have the remarkable capacity to continuously regenerate their teeth. The polyphyodont system is considered the ancestral condition of the gnathostome dentition. Despite this shared regenerative ability, sharks and rays exhibit dramatic interspecific variation in their tooth morphology. Ray (batoidea) teeth typically constitute crushing pads of flattened teeth, whereas shark teeth are pointed, multi-cuspid units. Although recent research has addressed the molecular development of the shark dentition, little is known about that of the ray. Furthermore, how dental diversity within the elasmobranch lineage is achieved remains unknown. Here, we examine dental development and regeneration in two Batoid species: the thornback skate (Raja clavata) and the little skate (Leucoraja erinacea). Using in situ hybridization and immunohistochemistry, we examine the expression of a core gnathostome dental gene set during early development of the skate dentition and compare it to development in the shark. Elasmobranch tooth development is highly conserved, with sox2 likely playing an important role in the initiation and regeneration of teeth. Alterations to conserved genes expressed in an enamel knot-like signalling centre may explain the morphological diversity of elasmobranch teeth, thereby enabling sharks and rays to occupy diverse dietary and ecological niches.

Sox2 Controls Periderm and Rugae Development to Inhibit Oral Adhesions

In humans, ankyloglossia and cleft palate are common congenital craniofacial anomalies, and these are regulated by a complex gene regulatory network. Understanding the genetic underpinnings of ankyloglossia and cleft palate will be an important step toward rational treatment of these complex anomalies. We inactivated the Sry (sex-determining region Y)-box 2 (Sox2) gene in the developing oral epithelium, including the periderm, a transient structure that prevents abnormal oral adhesions during development. This resulted in ankyloglossia and cleft palate with 100% penetrance in embryos examined after embryonic day 14.5. In Sox2 conditional knockout embryos, the oral epithelium failed to differentiate, as demonstrated by the lack of keratin 6, a marker of the periderm. Further examination revealed that the adhesion of the tongue and mandible expressed the epithelial markers E-Cad and P63. The expanded epithelia are Sox9-, Pitx2-, and Tbx1-positive cells, which are markers of the dental epithelium; thus, the dental epithelium contributes to the development of oral adhesions. Furthermore, we found that Sox2 is required for palatal shelf extension, as well as for the formation of palatal rugae, which are signaling centers that regulate palatogenesis. In conclusion, the deletion of Sox2 in oral epithelium disrupts palatal shelf extension, palatal rugae formation, tooth development, and periderm formation. The periderm is required to inhibit oral adhesions and ankyloglossia, which is regulated by Sox2. In addition, oral adhesions occur through an expanded dental epithelial layer that inhibits epithelial invagination and incisor development. This process may contribute to dental anomalies due to ankyloglossia.

Ectodermal Organ Development Is Regulated by a microRNA-26b-Lef-1-Wnt Signaling Axis

The developmental role of Lef-1 in ectodermal organs has been characterized using Lef-1 murine knockout models. We generated a Lef-1 conditional over-expression (COEL) mouse to determine the role of Lef-1 expression in epithelial structures at later stages of development after endogenous expression switches to the mesenchyme. Lef-1 over expression (OE) in the oral epithelium creates a new dental epithelial stem cell niche that significantly increases incisor growth. These data indicate that Lef-1 expression is switched off in the dental epithelial at early stages to maintain the stem cell niche and regulate incisor growth. Bioinformatics analyses indicated that miR-26b expression increased coinciding with decreased Lef-1 expression in the dental epithelium. We generated a murine model over-expressing miR-26b that targets endogenous Lef-1 expression and Lef-1-related developmental mechanisms. miR-26b OE mice have ectodermal organ defects including a lack of incisors, molars, and hair similar to the Lef-1 null mice. miR-26b OE rescues the Lef-1 OE phenotype demonstrating a critical genetic and developmental role for miR-26b in the temporal and spatial expression of Lef-1 in epithelial tissues. Lef-1 expression regulates Wnt signaling and Wnt target genes as well as cell proliferation mechanisms, while miR-26b OE reduced the levels of Wnt target gene expression. The extra stem cell compartment in the COEL mice expressed Lef-1 suggesting that Lef-1 is a stem cell factor, which was absent in the miR-26b OE/COEL rescue mice. This is the first demonstration of a microRNA OE mouse model that has ectodermal organ defects. These findings demonstrate that the levels of Lef-1 are critical for development and establish a role for miR-26b in the regulation of ectodermal organ development through the control of Lef-1 expression and an endogenous stem cell niche.

FAM20B-catalyzed glycosaminoglycans control murine tooth number by restricting FGFR2b signaling

BACKGROUND

The formation of supernumerary teeth is an excellent model for studying the molecular mechanisms that control stem/progenitor cell homeostasis needed to generate a renewable source of replacement cells and tissues. Although multiple growth factors and transcriptional factors have been associated with supernumerary tooth formation, the regulatory inputs of extracellular matrix in this regenerative process remains poorly understood.

RESULTS

In this study, we present evidence that disrupting glycosaminoglycans (GAGs) in the dental epithelium of mice by inactivating FAM20B, a xylose kinase essential for GAG assembly, leads to supernumerary tooth formation in a pattern reminiscent of replacement teeth. The dental epithelial GAGs confine murine tooth number by restricting the homeostasis of Sox2(+) dental epithelial stem/progenitor cells in a non-autonomous manner. FAM20B-catalyzed GAGs regulate the cell fate of dental lamina by restricting FGFR2b signaling at the initial stage of tooth development to maintain a subtle balance between the renewal and differentiation of Sox2(+) cells. At the later cap stage, WNT signaling functions as a relay cue to facilitate the supernumerary tooth formation.

CONCLUSIONS

The novel mechanism we have characterized through which GAGs control the tooth number in mice may also be more broadly relevant for potentiating signaling interactions in other tissues during development and tissue homeostasis.

Pitx2-Sox2-Lef1 interactions specify progenitor oral/dental epithelial cell signaling centers

Epithelial signaling centers control epithelial invagination and organ development, but how these centers are specified remains unclear. We report that Pitx2 (the first transcriptional marker for tooth development) controls the embryonic formation and patterning of epithelial signaling centers during incisor development. We demonstrate using Krt14Cre /Pitx2flox/flox (Pitx2cKO ) and Rosa26CreERT/Pitx2flox/flox mice that loss of Pitx2 delays epithelial invagination, and decreases progenitor cell proliferation and dental epithelium cell differentiation. Developmentally, Pitx2 regulates formation of the Sox2+ labial cervical loop (LaCL) stem cell niche in concert with two signaling centers: the initiation knot and enamel knot. The loss of Pitx2 disrupted the patterning of these two signaling centers, resulting in tooth arrest at E14.5. Mechanistically, Pitx2 transcriptional activity and DNA binding is inhibited by Sox2, and this interaction controls gene expression in specific Sox2 and Pitx2 co-expression progenitor cell domains. We demonstrate new transcriptional mechanisms regulating signaling centers by Pitx2, Sox2, Lef1 and Irx1.

Six1 is required for signaling center formation and labial-lingual asymmetry in developing lower incisors

BACKGROUND

The structure of the mouse incisor is characterized by its asymmetric accumulation of enamel matrix proteins on the labial side. The asymmetric structure originates from the patterning of the epithelial incisor placode through the interaction with dental mesenchymal cells. However, the molecular basis for the asymmetric patterning of the incisor germ is largely unknown.

RESULTS

A homeobox transcription factor SIX1 was shown to be produced in the mandibular mesenchyme, and its localization patterns changed dynamically during lower incisor development. Six1^-/- mice exhibited smaller lower incisor primordia than wild-type mice. Furthermore, Six1^-/- mice showed enamel matrix production on both the lingual and labial sides and disturbed odontoblast maturation. In the earlier stages of development, the formation of signaling centers, the initiation knot and the enamel knot, which are essential for the morphogenesis of tooth germs, were impaired in Six1^-/- embryos. Notably, Wnt signaling activity, which shows an anterior-posterior gradient, and the expression patterns of genes involved in incisor formation were altered in the mesenchyme in Six1^-/- embryos.

CONCLUSION

Our results indicate that Six1 is required for signaling center formation in lower incisor germs and the labial-lingual asymmetry of the lower incisors by regulating the anterior-posterior patterning of the mandibular mesenchyme.

Molecular and cellular mechanisms of tooth development, homeostasis and repair

The tooth provides an excellent system for deciphering the molecular mechanisms of organogenesis, and has thus been of longstanding interest to developmental and stem cell biologists studying embryonic morphogenesis and adult tissue renewal. In recent years, analyses of molecular signaling networks, together with new insights into cellular heterogeneity, have greatly improved our knowledge of the dynamic epithelial-mesenchymal interactions that take place during tooth development and homeostasis. Here, we review recent progress in the field of mammalian tooth morphogenesis and also discuss the mechanisms regulating stem cell-based dental tissue homeostasis, regeneration and repair. These exciting findings help to lay a foundation that will ultimately enable the application of fundamental research discoveries toward therapies to improve oral health.

Sox2 maintains epithelial cell proliferation in the successional dental lamina

OBJECTIVES

The successional dental lamina is the distinctive structure on the lingual side of the vertebrate tooth germ. The aim of this study was to investigate the relationship among Sox2, Claudin10 and laminin5 and the role of Sox2 in successional dental lamina proliferation during vertebrate tooth development.

MATERIALS AND METHODS

To understand the successional dental lamina, two types of successional tooth formation, that in geckos (with multiple rounds of tooth generation) and that in mice (with only one round of tooth generation), were analysed.

RESULTS

Unique coexpression patterns of Sox2 and Claudin10 expression were compared in the successional dental lamina from the cap stage to the late bell stage in the mouse tooth germ and in juvenile gecko teeth to support continuous tooth replacement. Furthermore, Laminin5 expression was shown in the cap stage and decreased after the bell stage. Upon comparing the epithelial cell cycles and cell proliferation in successional dental lamina regions between mouse and gecko molars using BrdU and IdU staining and pulse-chase methods, distinctive patterns of continuous expression were revealed. Moreover, Sox2 overexpression with a lentiviral system resulted in hyperplastic dental epithelium in mouse molars.

CONCLUSIONS

Our findings indicate that the regulation of Sox2 in dental lamina proliferation is fundamental to the successional dental lamina in both species.

High-fat diet increases labial groove formation in maxillary incisors and is related to aging in C57BL/6 mice

OBJECTIVES

The aim of this study was to explore the relationship between the consumption of a high-fat diet and aging-dependent formation of maxillary incisor grooves in C57BL/6 mice, and to identify putative maxillary incisor groove-related genes.

METHODS

We fed 2-month-old and 16-month-old C57BL/6 mice on either a chow diet or a high-fat diet for three months and observed changes in maxillary incisor grooves. We examined tissue sections of the maxillary incisors with grooves and carried out transcriptome analysis of the apical tissue fragments of maxillary incisors with/without grooves.

RESULTS

Consumption of a high-fat diet for three months resulted in significant increases in both body weight and the number of incisor grooves. Both the number and frequency of incisor grooves increased in an age-dependent manner from 26 to 28 months, during which time an additional groove appeared. There was abnormal differentiation and apoptosis of ameloblasts on the labial surface at the grooves of the maxillary incisors. Transcriptome analysis identified 23 genes as being specific to 24-month-old mice; these included several genes related to apoptosis and cell differentiation.

CONCLUSIONS

The study findings indicate that, in C57BL/6 mice, consumption of a high-fat diet increases labial groove formation in maxillary incisors, which is related to aging of the tissue stem cells in the apical root end of the teeth.

Six2 regulates Pax9 expression, palatogenesis and craniofacial bone formation

In this study, we investigated the role of the transcription factor Six2 in palate development. Six2 was selected using the SysFACE tool to predict genes from the 2p21 locus, a region associated with clefting in humans by GWAS, that are likely to be involved in palatogenesis. We functionally validated the predicted role of Six2 in palatogenesis by showing that 22% of Six2 null embryos develop cleft palate. Six2 contributes to palatogenesis by promoting mesenchymal cell proliferation and regulating bone formation. The clefting phenotype in Six2^-/- embryos is similar to Pax9 null embryos, so we examined the functional relationship of these two genes. Mechanistically, SIX2 binds to a PAX9 5' upstream regulatory element and activates PAX9 expression. In addition, we identified a human SIX2 coding variant (p.Gly264Glu) in a proband with cleft palate. We show this missense mutation affects the stability of the SIX2 protein and leads to decreased PAX9 expression. The low penetrance of clefting in the Six2 null mouse combined with the mutation in one patient with cleft palate underscores the potential combinatorial interactions of other genes in clefting. Our study demonstrates that Six2 interacts with the developmental gene regulatory network in the developing palate.

Missense Pathogenic variants in KIF4A Affect Dental Morphogenesis Resulting in X-linked Taurodontism, Microdontia and Dens-Invaginatus

The etiology of dental anomalies is multifactorial; and genetic and environmental factors that affect the dental lamina have been implicated. We investigated two families of European ancestry in which males were affected by taurodontism, microdontia and dens invaginatus. In both families, males were related to each other via unaffected females. A linkage analysis was conducted in a New Zealand family, followed by exome sequencing and focused analysis of the X-chromosome. In a US family, exome sequencing of the X-chromosome was followed by Sanger sequencing to conduct segregation analyses. We identified two independent missense variants in KIF4A that segregate in affected males and female carriers. The variant in a New Zealand family (p.Asp371His) predicts the substitution of a residue in the motor domain of the protein while the one in a US family (p.Arg771Lys) predicts the substitution of a residue in the domain that interacts with Protein Regulator of Cytokinesis 1 (PRC1). We demonstrated that the gene is expressed in the developing tooth bud during development, and that the p.Arg771Lys variant influences cell migration in an in vitro assay. These data implicate missense variations in KIF4A in a pathogenic mechanism that causes taurodontism, microdontia and dens invaginatus phenotypes.

Comparison of long non‑coding RNA expression profiles in human dental follicle cells and human periodontal ligament cells

The dental follicle develops into the periodontal ligament, cementum and alveolar bone. Human dental follicle cells (hDFCs) are the precursor cells of periodontal development. Long non-coding RNAs (lncRNAs) have been revealed to be crucial factors that regulate a variety of biological processes; however, whether lncRNAs serve a role in human periodontal development remains unknown. Therefore, the present study used microarrays to detect the differentially expressed lncRNAs and mRNAs between hDFCs and human periodontal ligament cells (hPDLCs). A total of 845 lncRNAs and 1,012 mRNAs were identified to be differentially expressed in hDFCs and hPDLCs (fold change >2.0 or <-2.0; P<0.05). Microarray data were validated by reverse transcription-quantitative polymerase chain reaction. Bioinformatics analyses, including gene ontology, pathway analysis and coding-non-coding gene co-expression network analysis, were performed to determine the functions of the differentially expressed lncRNAs and mRNAs. Bioinformatics analysis identified that a number of pathways may be associated with periodontal development, including the p53 and calcium signaling pathways. This analysis also revealed a number of lncRNAs, including NR_033932, T152410, ENST00000512129, ENST00000540293, uc021sxs.1 and ENST00000609146, which may serve important roles in the biological process of hDFCs. In addition, the lncRNA termed maternally expressed 3 (MEG3) was identified to be differentially expressed in hDFCs by reverse transcription-quantitative polymerase chain reaction. The knockdown of MEG3 was associated with a reduction of pluripotency makers in hDFCs. In conclusion, for the first time, to the best of our knowledge, the current study determined the different expression profiles of lncRNAs and mRNAs between hDFCs and hPDLCs. The observations made may provide a solid foundation for further research into the molecular mechanisms of lncRNAs in human periodontal development.

PITX2 Expression in Non-functional Pituitary Neuroendocrine Tumor with Cavernous Sinus Invasion

Although most pituitary neuroendocrine tumors (PitNETs) show benign behavior, a significant number of PitNETs exhibit an aggressive course including cavernous sinus (CS) invasion. To date, the cause of CS invasion has not been fully elucidated. In this study, we analyzed the relationship between CS invasion in PitNETs and the expression of PITX2 and SNAIL1, which are the transcription factors associated with the morphogenesis of pituitary gland. Sixty cases with non-functional PitNETs were classified into four types: type 1a, none of CS invasion and suprasellar expansion; type 1b, suprasellar expansion without CS invasion; type 2a, CS invasion without suprasellar expansion; and type 2b, CS invasion with suprasellar expansion. We analyzed the expression of PITX2 and SNAIL1 employing quantitative real-time polymerase chain reaction (qPCR) and immunohistochemistry. Other parameters such as mitotic count, Ki-67 index, and p53 expression were also analyzed, which were previously reported as potential tumor proliferative markers in PitNETs. PITX2 expression was significantly higher in PitNETs with CS invasion than PitNETs without CS invasion (P = 0.019). Expression of SNAIL1 was significantly elevated in PitNETs with suprasellar expansion compared with PitNETs without suprasellar expansion (P = 0.02). There was no apparent relationship between CS invasion and mitotic count, Ki-67 index, and p53 expression (mitotic count, P = 0.11; Ki-67 index, P = 0.61; p53, P = 0.66). High PITX2 expression was observed in non-functional PitNETs with CS invasion, suggesting that PITX2 may be involved in CS invasion of PitNETs.

Dental Epithelial Stem Cells Express the Developmental Regulator Meis1

MEIS1 is a key developmental regulator of several organs and participates in stem cell maintenance in different niches. However, despite the murine continuously growing incisor being a well described model for the study of adult stem cells, Meis1 has not been investigated in a dental context. Here, we uncover that Meis1 expression in the tooth is confined to the epithelial compartment. Its expression arises during morphogenesis and becomes restricted to the mouse incisor epithelial stem cell niche, the labial cervical loop. Meis1 is specifically expressed by Sox2⁺ stem cells, which give rise to all dental epithelial cell lineages. Also, we have found that Meis1 in the incisor is coexpressed with potential binding partner Pbx1 during both embryonic and adult stages. Interestingly, Meis2 is present in different areas of the forming tooth and it is not expressed by dental epithelial stem cells, suggesting different roles for these two largely homologous genes. Additionally, we have established the expression patterns of Meis1 and Meis2 during tongue, hair, salivary gland and palate formation. Finally, analysis of Meis1-null allele mice indicated that, similarly, to SOX2, MEIS1 is not essential for tooth initiation, but might have a role during adult incisor renewal.

Genomic analyses in African populations identify novel risk loci for cleft palate

Orofacial clefts are common developmental disorders that pose significant clinical, economical and psychological problems. We conducted genome-wide association analyses for cleft palate only (CPO) and cleft lip with or without palate (CL/P) with ~17 million markers in sub-Saharan Africans. After replication and combined analyses, we identified novel loci for CPO at or near genome-wide significance on chromosomes 2 (near CTNNA2) and 19 (near SULT2A1). In situ hybridization of Sult2a1 in mice showed expression of SULT2A1 in mesenchymal cells in palate, palatal rugae and palatal epithelium in the fused palate. The previously reported 8q24 was the most significant locus for CL/P in our study, and we replicated several previously reported loci including PAX7 and VAX1.

Revitalising the rudimentary replacement dentition in the mouse

Most mammals have two sets of teeth (diphyodont) - a deciduous dentition replaced by a permanent dentition; however, the mouse possesses only one tooth generation (monophyodont). In diphyodonts, the replacement tooth forms on the lingual side of the first tooth from the successional dental lamina. This lamina expresses the stem/progenitor marker Sox2 and has activated Wnt/β-catenin signalling at its tip. Although the mouse does not replace its teeth, a transient rudimentary successional dental lamina (RSDL) still forms during development. The mouse RSDL houses Sox2-positive cells, but no Wnt/β-catenin signalling. Here, we show that stabilising Wnt/β-catenin signalling in the RSDL in the mouse leads to proliferation of the RSDL and formation of lingually positioned teeth. Although Sox2 has been shown to repress Wnt activity, overexpression of Wnts leads to a downregulation of Sox2, suggesting a negative-feedback loop in the tooth. In the mouse, the first tooth represses the formation of the replacement, and isolation of the RSDL is sufficient to induce formation of a new tooth germ. Our data highlight key mechanisms that may have influenced the evolution of replacement teeth.This article has an associated 'The people behind the papers' interview.

Tissue Mechanical Forces and Evolutionary Developmental Changes Act Through Space and Time to Shape Tooth Morphology and Function

Efforts from diverse disciplines, including evolutionary studies and biomechanical experiments, have yielded new insights into the genetic, signaling, and mechanical control of tooth formation and functions. Evidence from fossils and non-model organisms has revealed that a common set of genes underlie tooth-forming potential of epithelia, and changes in signaling environments subsequently result in specialized dentitions, maintenance of dental stem cells, and other phenotypic adaptations. In addition to chemical signaling, tissue forces generated through epithelial contraction, differential growth, and skeletal constraints act in parallel to shape the tooth throughout development. Here recent advances in understanding dental development from these studies are reviewed and important gaps that can be filled through continued application of evolutionary and biomechanical approaches are discussed.

The transcription factor AmeloD stimulates epithelial cell motility essential for tooth morphology

The development of ectodermal organs, such as teeth, requires epithelial-mesenchymal interactions. Basic helix-loop-helix (bHLH) transcription factors regulate various aspects of tissue development, and we have previously identified a bHLH transcription factor, AmeloD, from a tooth germ cDNA library. Here, we provide both in vitro and in vivo evidence that AmeloD is important in tooth development. We created AmeloD-knockout (KO) mice to identify the in vivo functions of AmeloD that are critical for tooth morphogenesis. We found that AmeloD-KO mice developed enamel hypoplasia and small teeth because of increased expression of E-cadherin in inner enamel epithelial (IEE) cells, and it may cause inhibition of the cell migration. We used the CLDE dental epithelial cell line to conduct further mechanistic analyses to determine whether AmeloD overexpression in CLDE cells suppresses E-cadherin expression and promotes cell migration. Knockout of epiprofin (Epfn), another transcription factor required for tooth morphogenesis and development, and analysis of AmeloD expression and deletion revealed that AmeloD also contributed to multiple tooth formation in Epfn-KO mice by promoting the invasion of dental epithelial cells into the mesenchymal region. Thus, AmeloD appears to play an important role in tooth morphogenesis by modulating E-cadherin and dental epithelial-mesenchymal interactions. These findings provide detailed insights into the mechanism of ectodermal organ development.

FoxO6 regulates Hippo signaling and growth of the craniofacial complex

The mechanisms that regulate post-natal growth of the craniofacial complex and that ultimately determine the size and shape of our faces are not well understood. Hippo signaling is a general mechanism to control tissue growth and organ size, and although it is known that Hippo signaling functions in neural crest specification and patterning during embryogenesis and before birth, its specific role in postnatal craniofacial growth remains elusive. We have identified the transcription factor FoxO6 as an activator of Hippo signaling regulating neonatal growth of the face. During late stages of mouse development, FoxO6 is expressed specifically in craniofacial tissues and FoxO6-/- mice undergo expansion of the face, frontal cortex, olfactory component and skull. Enlargement of the mandible and maxilla and lengthening of the incisors in FoxO6-/- mice are associated with increases in cell proliferation. In vitro and in vivo studies demonstrated that FoxO6 activates Lats1 expression, thereby increasing Yap phosphorylation and activation of Hippo signaling. FoxO6-/- mice have significantly reduced Hippo Signaling caused by a decrease in Lats1 expression and decreases in Shh and Runx2 expression, suggesting that Shh and Runx2 are also linked to Hippo signaling. In vitro, FoxO6 activates Hippo reporter constructs and regulates cell proliferation. Furthermore PITX2, a regulator of Hippo signaling is associated with Axenfeld-Rieger Syndrome causing a flattened midface and we show that PITX2 activates FoxO6 expression. Craniofacial specific expression of FoxO6 postnatally regulates Hippo signaling and cell proliferation. Together, these results identify a FoxO6-Hippo regulatory pathway that controls skull growth, odontogenesis and face morphology.

A novel mutation of MSX1 in oligodontia inhibits odontogenesis of dental pulp stem cells via the ERK pathway

Background

Tooth agenesis, one of the most common developmental anomalies, can affect the function and esthetics of patients. The aim of the present study was to identify genetic clues for familial tooth agenesis and explore the underlying mechanisms, focusing on the role of human dental pulp stem cells (hDPSCs).

Methods

We applied Sanger sequencing to identify the cause of oligodontia in a Chinese family. DNA transfection and functional analysis in DPSCs was also performed to explore the impact of the identified mutation on this phenotype.

Results

In this study, a novel frameshift mutation, the twenty-nucleotide deletion (c.128_147del20, p.Met43Serfsx125), in exon1 of MSX1 was detected in a Chinese family causing autosomal dominant nonsyndromic oligodontia. The mutation cosegregated with the tooth agenesis phenotype in this family. DPSCs transfected with mutant MSX1 plasmid showed decreased capacity of osteo/odontogenic differentiation with a lower expression level of dentin sialophosphoprotein (DSPP) and bone sialoprotein (BSP) compared with those transfected with control MSX1 plasmid. Mechanically, control MSX1 showed nuclear localization while the mutant MSX1 inhibited its nuclear translocation and localized on the cytoplasm to inhibit ERK phosphorylation. Furthermore, we inhibited the ERK pathway using ERK inhibitor (U0126) treatment in control MSX1-transfected DPSCs which could downregulate mineralized nodule formation and the expression of odontogenic genes.

Conclusion

We demonstrated a novel MSX1 mutation causing familial nonsyndromic oligodontia and mechanically MSX1 regulates odontogenesis through the ERK signaling pathway in human dental pulp stem cells.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0965-3) contains supplementary material, which is available to authorized users.

Bioengineered Tooth Buds Exhibit Features of Natural Tooth Buds

Tooth loss is a significant health issue currently affecting millions of people worldwide. Artificial dental implants, the current gold standard tooth replacement therapy, do not exhibit many properties of natural teeth and can be associated with complications leading to implant failure. Here we propose bioengineered tooth buds as a superior alternative tooth replacement therapy. We describe improved methods to create highly cellularized bioengineered tooth bud constructs that formed hallmark features that resemble natural tooth buds such as the dental epithelial stem cell niche, enamel knot signaling centers, transient amplifying cells, and mineralized dental tissue formation. These constructs were composed of postnatal dental cells encapsulated within a hydrogel material that were implanted subcutaneously into immunocompromised rats. To our knowledge, this is the first report describing the use of postnatal dental cells to create bioengineered tooth buds that exhibit evidence of these features of natural tooth development. We propose future bioengineered tooth buds as a promising, clinically relevant tooth replacement therapy.

Submucosal Gland Myoepithelial Cells Are Reserve Stem Cells That Can Regenerate Mouse Tracheal Epithelium

The mouse trachea is thought to contain two distinct stem cell compartments that contribute to airway repair-basal cells in the surface airway epithelium (SAE) and an unknown submucosal gland (SMG) cell type. Whether a lineage relationship exists between these two stem cell compartments remains unclear. Using lineage tracing of glandular myoepithelial cells (MECs), we demonstrate that MECs can give rise to seven cell types of the SAE and SMGs following severe airway injury. MECs progressively adopted a basal cell phenotype on the SAE and established lasting progenitors capable of further regeneration following reinjury. MECs activate Wnt-regulated transcription factors (Lef-1/TCF7) following injury and Lef-1 induction in cultured MECs promoted transition to a basal cell phenotype. Surprisingly, dose-dependent MEC conditional activation of Lef-1 in vivo promoted self-limited airway regeneration in the absence of injury. Thus, modulating the Lef-1 transcriptional program in MEC-derived progenitors may have regenerative medicine applications for lung diseases.

Amelogenin X impacts age-dependent increase of frequency and number in labial incisor grooves in C57BL/6

Labial grooves in maxillary incisors have been reported in several wild-type rodent species. Previous studies have reported age-dependent labial grooves occur in moderate prevalence in C57BL/6 mice; however, very little is known about the occurrence of such grooves. In the present study, we observed age-dependent groove formation in C57BL/6 mice up to 26 months after birth and found that not only the frequency of the appearance of incisor grooves but also the number of grooves increased in an age-dependent manner. We examined the molecular mechanisms of age-dependent groove formation by performing DNA microarray analysis of the incisors of 12-month-old (12M) and 24-month-old (24M) mice. Amelx, encoding the major enamel matrix protein AMELOGENIN, was identified as a 12M-specific gene. Comparing with wild-type mice, the maxillary incisors of Amelx^-/- mutants indicated the increase of the frequency and number of labial grooves. These findings suggested that the Amelx gene impacts the age-dependent appearance of the labial incisor groove in C57BL/6 mice.

Plasticity within the niche ensures the maintenance of a Sox2+ stem cell population in the mouse incisor

In mice, the incisors grow throughout the animal's life, and this continuous renewal is driven by dental epithelial and mesenchymal stem cells. Sox2 is a principal marker of the epithelial stem cells that reside in the mouse incisor stem cell niche, called the labial cervical loop, but relatively little is known about the role of the Sox2⁺ stem cell population. In this study, we show that conditional deletion of Sox2 in the embryonic incisor epithelium leads to growth defects and impairment of ameloblast lineage commitment. Deletion of Sox2 specifically in Sox2⁺ cells during incisor renewal revealed cellular plasticity that leads to the relatively rapid restoration of a Sox2-expressing cell population. Furthermore, we show that Lgr5-expressing cells are a subpopulation of dental Sox2⁺ cells that also arise from Sox2⁺ cells during tooth formation. Finally, we show that the embryonic and adult Sox2⁺ populations are regulated by distinct signalling pathways, which is reflected in their distinct transcriptomic signatures. Together, our findings demonstrate that a Sox2⁺ stem cell population can be regenerated from Sox2^- cells, reinforcing its importance for incisor homeostasis.

Meeting report: a hard look at the state of enamel research

The Encouraging Novel Amelogenesis Models and Ex vivo cell Lines (ENAMEL) Development workshop was held on 23 June 2017 at the Bethesda headquarters of the National Institute of Dental and Craniofacial Research (NIDCR). Discussion topics included model organisms, stem cells/cell lines, and tissues/3D cell culture/organoids. Scientists from a number of disciplines, representing institutions from across the United States, gathered to discuss advances in our understanding of enamel, as well as future directions for the field.

Irx1 regulates dental outer enamel epithelial and lung alveolar type II epithelial differentiation

The Iroquois genes (Irx) appear to regulate fundamental processes that lead to cell proliferation, differentiation, and maturation during development. In this report, the Iroquois homeobox 1 (Irx1) transcription factor was functionally disrupted using a LacZ insert and LacZ expression demonstrated stage-specific expression during embryogenesis. Irx1 is highly expressed in the brain, lung, digits, kidney, testis and developing teeth. Irx1 null mice are neonatal lethal and this lethality it due to pulmonary immaturity. Irx1^-/- mice show delayed lung maturation characterized by defective surfactant protein secretion and Irx1 marks a population of SP-C expressing alveolar type II cells. Irx1 is specifically expressed in the outer enamel epithelium (OEE), stellate reticulum (SR) and stratum intermedium (SI) layers of the developing tooth. Irx1 mediates dental epithelial cell differentiation in the lower incisors resulting in delayed growth of the lower incisors. Irx1 is specifically and temporally expressed during developmental stages and we have focused on lung and dental development in this report. Irx1+ cells are unique to the development of the incisor outer enamel epithelium, patterning of Lef-1+ and Sox2+ cells as well as a new marker for lung alveolar type II cells. Mechanistically, Irx1 regulates Foxj1 and Sox9 to control cell differentiation during development.

Spatial and temporal expression of Sox9 during murine incisor development

Mouse incisors are capable of continuously growing due to the renewal of dental epithelium stem cells and mesenchymal stem cells residing at the proximal ends. The transcription factor Sox9 plays important roles in maintaining the stem cells of hair follicles, retinal progenitor cells and neural crest stem cells. Whether Sox9 is involved during mouse incisor development is not reported yet. In this study, we examined the expression pattern of Sox9 during mouse incisor development by in situ hybridization and immunohistochemistry. Sox9 mRNA and protein showed similar expression pattern from embryonic day (E) 13.5 to postnatal (PN) day 10. At E13.5 and E14.5, Sox9 was strongly expressed in the dental epithelium. At E16.5, Sox9 started to be detected in the mesenchymal cells within the dental pulp, especially the dental pulp cells that adjacent to the labial cervical loop. Similarly with E14.5, Sox9 was strongly detected in the labial cervical loop, including the basal epithelium, the stellate reticulum and the outer enamel epithelium from E16.5 to PN10. The mesenchyme adjacent to the labial cervical loop also showed strong signal of Sox9. The spatiotemporal expression of Sox9 suggested its possible involvement during mouse incisor development.