Runx2 mediates FGF signaling from epithelium to mesenchyme during tooth morphogenesis

Derivation of dental epithelial-like cells from murine embryonic stem cells for tooth regeneration

Teeth are comprised of epithelial and mesenchymal cells, and regenerative teeth rely on the regeneration of both cell types. Transcription factors play a pivotal role in cell fate determination. In this study, we establish fluorescence models based on transcription factors to monitor and analyze dental epithelial cells. Using Pitx2-P2A-copGFP mice, we observe that Pitx2+ epithelial cells, when combined with E14.5 dental mesenchymal cells, are sufficient for the reconstitution of teeth. Induced-Pitx2+ cells, directly isolated from the embryoid body that employs the Pitx2-GFP embryonic stem cell line, exhibit the capacity to differentiate into ameloblasts and develop into teeth when combined with dental mesenchymal cells. The regenerated teeth exhibit a complete structure, including dental pulp, dentin, enamel, and periodontal ligaments. Subsequent exploration via RNA-seq reveals that induced-Pitx2+ cells exhibit enrichment in genes associated with FGF receptors and WNT ligands compared with induced-Pitx2- cells. Our results indicate that both primary Pitx2+ and induced Pitx2+ cells possess the capability to differentiate into enamel-secreting ameloblasts and grow into teeth when combined with dental mesenchymal cells.

"A Friend Among Strangers" or the Ambiguous Roles of Runx2

The transcription factor Runx2 plays a crucial role in regulating osteogenic differentiation and skeletal development. This factor not only controls the expression of genes involved in bone formation, but also interacts with signaling pathways such as the Notch pathway, which are essential for body development. However, studies have produced conflicting results regarding the relationship between Runx2 and the Notch pathway. Some studies suggest a synergistic interaction between these molecules, while others suggest an inhibitory one, for example, the interplay between Notch signaling, Runx2, and vitamin D3 in osteogenic differentiation and bone remodeling. The findings suggest a complex relationship between Notch signaling and osteogenic differentiation, with ongoing research needed to clarify the mechanisms involved and resolve existing contradictions regarding role of Notch in this process. Additionally, there is increasing evidence of contradictory roles for Runx2 in various tissues and organs, both under normal conditions and in pathological states. This diversity of roles makes Runx2 a potential therapeutic target, offering new directions for research. In this review, we have discussed the mechanisms of osteogenic differentiation and the important role of Runx2 in this process. We have also examined its relationship with different signaling pathways. However, there are still many uncertainties and inconsistencies in our current understanding of these interactions. Additionally, given that Runx2 is also involved in numerous other events in various tissues, we have tried to comprehensively examine its functions outside the skeletal system.

How do teeth erupt?

The development of normal occlusion requires eruptive migration of teeth from their developmental position in the jaw into a functional position within the oral cavity. This process involves significant and coordinated movement in an axial direction and appropriate eruption through the gingival tissues. The mechanisms regulating these developmental events are poorly understood, and teeth retain eruptive potential throughout their lifespan. In recent years, the use of mouse models has helped to elucidate some of the underlying molecular and biological mechanisms of mammalian tooth eruption. Here, we outline our current understanding of tooth eruption mechanisms and discuss their relevance in terms of known human disorders of tooth eruption.

Ectopic Activation of Fgf8 in Dental Mesenchyme Causes Incisor Agenesis and Molar Microdontia

Putatively, tooth agenesis was attributed to the initiation failure of tooth germs, though little is known about the histological and molecular alterations. To address if constitutively active FGF signaling is associated with tooth agenesis, we activated Fgf8 in dental mesenchyme with Osr-cre knock-in allele in mice (Osr2-creKI; Rosa26R-Fgf8) and found incisor agenesis and molar microdontia. The cell survival assay showed tremendous apoptosis in both the Osr2-creKI; Rosa26R-Fgf8 incisor epithelium and mesenchyme, which initiated incisor regression from cap stage. In situ hybridization displayed vanished Shh transcription, and immunostaining exhibited reduced Runx2 expression and enlarged mesenchymal Lef1 domain in Osr2-creKI; Rosa26R-Fgf8 incisors, both of which were suggested to enhance apoptosis. In contrast, Osr2-creKI; Rosa26R-Fgf8 molar germs displayed mildly suppressed Shh transcription, and the increased expression of Ectodin, Runx2 and Lef1. Although mildly smaller than WT controls prenatally, the Osr2-creKI; Rosa26R-Fgf8 molar germs produced a miniature tooth with impaired mineralization after a 6-week sub-renal culture. Intriguingly, the implanted Osr2-creKI; Rosa26R-Fgf8 molar germs exhibited delayed odontoblast differentiation and accelerated ameloblast maturation. Collectively, the ectopically activated Fgf8 in dental mesenchyme caused incisor agenesis by triggering incisor regression and postnatal molar microdontia. Our findings reported tooth agenesis resulting from the regression from the early bell stage and implicated a correlation between tooth agenesis and microdontia.

Spatiotemporal expression of fibroblast growth factor 4 and 10 during the morphogenesis of deciduous molars in miniature pigs

OBJECTIVE

Fibroblast growth factors (FGFs) play pivotal roles in mediating interactions between dental epithelium and mesenchyme throughout tooth initiation and morphogenesis. This study aimed to elucidate the roles of FGF4 and FGF10 in the regulation of tooth development.

DESIGN

In this study, we investigated spatiotemporal expression patterns of FGF4 and FGF10 in the third deciduous molars (DM3) of miniature pigs at the cap, early bell, and late bell stages. Pregnant miniature pigs were obtained, and the samples were processed for histological staining. Non-radioactive in situ hybridization, immunohistochemistry, and real-time PCR were used to detect mRNA and protein expression levels of FGF4 and FGF10.

RESULTS

FGF4 was expressed in the dental epithelium and mesenchyme at the cap stage. At the early bell stage, epithelial expression of FGF4 was reduced while mesenchymal expression got stronger. At the late bell stage, the FGF4 expression was restricted to the inner enamel epithelium (IEE) and differentiating odontoblasts. FGF10 was expressed intensely in both epithelium and mesenchyme at the cap stage. The expression of FGF10 was concentrated in the secondary enamel knots and surrounding mesenchyme at the early bell stage. FGF10 was weakly detected in the IEE by the late bell stage.

CONCLUSIONS

Our results indicated that FGF4 and FGF10 might have partially redundant functions in regulating epithelium morphogenesis. FGF4 may be involved in regulatory signaling cascades mediating interactions between the epithelium and mesenchyme. In addition, the downregulation of FGF10 expression may be associated with the cessation of mesenchymal cell proliferation and initiation of preodontoblast polarization.

Single Cell RNA Sequencing Reveals Human Tooth Type Identity and Guides In Vitro hiPSC Derived Odontoblast Differentiation (iOB)

Over 90% of the U.S. adult population suffers from tooth structure loss due to caries. Most of the mineralized tooth structure is composed of dentin, a material produced and mineralized by ectomesenchyme derived cells known as odontoblasts. Clinicians, scientists, and the general public share the desire to regenerate this missing tooth structure. To bioengineer missing dentin, increased understanding of human tooth development is required. Here we interrogate at the single cell level the signaling interactions that guide human odontoblast and ameloblast development and which determine incisor or molar tooth germ type identity. During human odontoblast development, computational analysis predicts that early FGF and BMP activation followed by later HH signaling is crucial. Application of this sci-RNA-seq analysis generates a differentiation protocol to produce mature hiPSC derived odontoblasts in vitro (iOB). Further, we elucidate the critical role of FGF signaling in odontoblast maturation and its biomineralization capacity using the de novo designed FGFR1/2c isoform specific minibinder scaffolded as a C6 oligomer that acts as a pathway agonist. We find that FGFR1c is upregulated in functional odontoblasts and specifically plays a crucial role in driving odontoblast maturity. Using computational tools, we show on a molecular level how human molar development is delayed compared to incisors. We reveal that enamel knot development is guided by FGF and WNT in incisors and BMP and ROBO in the molars, and that incisor and molar ameloblast development is guided by FGF, EGF and BMP signaling, with tooth type specific intensity of signaling interactions. Dental ectomesenchyme derived cells are the primary source of signaling ligands responsible for both enamel knot and ameloblast development.

Mesenchymal condensation in tooth development and regeneration: a focus on translational aspects of organogenesis

The teeth are vertebrate-specific, highly specialized organs performing fundamental functions of mastication and speech, the maintenance of which is crucial for orofacial homeostasis and is further linked to systemic health and human psychosocial well-being. However, with limited ability for self-repair, the teeth can often be impaired by traumatic, inflammatory, and progressive insults, leading to high prevalence of tooth loss and defects worldwide. Regenerative medicine holds the promise to achieve physiological restoration of lost or damaged organs, and in particular an evolving framework of developmental engineering has pioneered functional tooth regeneration by harnessing the odontogenic program. As a key event of tooth morphogenesis, mesenchymal condensation dictates dental tissue formation and patterning through cellular self-organization and signaling interaction with the epithelium, which provides a representative to decipher organogenetic mechanisms and can be leveraged for regenerative purposes. In this review, we summarize how mesenchymal condensation spatiotemporally assembles from dental stem cells (DSCs) and sequentially mediates tooth development. We highlight condensation-mimetic engineering efforts and mechanisms based on ex vivo aggregation of DSCs, which have achieved functionally robust and physiologically relevant tooth regeneration after implantation in animals and in humans. The discussion of this aspect will add to the knowledge of development-inspired tissue engineering strategies and will offer benefits to propel clinical organ regeneration.

Dental niche cells directly contribute to tooth reconstitution and morphogenesis

Mammalian teeth develop from the inductive epithelial-mesenchymal interaction, an important mechanism shared by many organs. The cellular basis for such interaction remains elusive. Here, we generate a dual-fluorescence model to track and analyze dental cells from embryonic to postnatal stages, in which Pitx2⁺ epithelium and Msx1⁺ mesenchyme are sufficient for tooth reconstitution. Single-cell RNA sequencing and spatial mapping further revealed critical cellular dynamics during molar development, where tooth germs are organized by Msx1⁺Sdc1⁺ dental papilla and surrounding dental niche. Surprisingly, niche cells are more efficient in tooth reconstitution and can directly regenerate papilla cells through interaction with dental epithelium. Finally, from the dental niche, we identify a group of previously unappreciated migratory Msx1⁺ Sox9⁺ cells as the potential cell origin for dental papilla. Our results indicate that the dental niche cells directly contribute to tooth organogenesis and provide critical insights into the essential cell composition for tooth engineering.

WNT10A and RUNX2 mutations associated with non-syndromic tooth agenesis

The goal of this study was to examine the prevalence of WNT10A and RUNX2 mutations and assess their potential impact on the phenotype of non-syndromic tooth agenesis. The study included 30 participants with non-syndromic tooth agenesis, divided into hypodontia (n = 24) and oligodontia forms (n = 6), and 42 unaffected family members. Genomic DNA from buccal epithelial cells was used for polymerase chain reaction amplification of functionally important exons of the WNT10A and RUNX2 genes. Direct sequencing reactions were performed to confirm the presence of mutations. The trend of increasing prevalence of WNT10A mutations and a slight increase in the prevalence of RUNX2 mutations were revealed in tooth agenesis cases compared to unaffected family members. There was a higher prevalence of hypodontia than oligodontia, increased frequency of females over males with missing teeth, and a wide phenotypic variability was observed in individuals and families analyzed. The common missense mutations (p.Phe228Ile, p.Arg113Cys, p.Asp217Asn, and p.Gly165Arg) and c.114-56T>C in the WNT10A gene and in-frame-deletion/insertions (11A, 24Q, 30Q), synonymous variant c.240G>A, and 424-33dupC in the RUNX2 gene were identified. These findings highlight an important role of WNT10A and RUNX2 mutations in the genetic etiology of non-syndromic tooth agenesis.

Integration of multimodal data in the developing tooth reveals candidate regulatory loci driving human odontogenic phenotypes

Human odontogenic aberrations such as abnormal tooth number and delayed tooth eruption can occur as a symptom of rare syndromes or, more commonly, as nonsyndromic phenotypes. These phenotypes can require extensive and expensive dental treatment, posing a significant burden. While many dental phenotypes are heritable, most nonsyndromic cases have not been linked to causal genes. We demonstrate the novel finding that common sequence variants associated with human odontogenic phenotypes are enriched in developmental craniofacial enhancers conserved between human and mouse. However, the bulk nature of these samples obscures if this finding is due to the tooth itself or the surrounding tissues. We therefore sought to identify enhancers specifically active in the tooth anlagen and quantify their contribution to the observed genetic enrichments. We systematically identified 22,001 conserved enhancers active in E13.5 mouse incisors using ChIP-seq and machine learning pipelines and demonstrated biologically relevant enrichments in putative target genes, transcription factor binding motifs, and in vivo activity. Multi-tissue comparisons of human and mouse enhancers revealed that these putative tooth enhancers had the strongest enrichment of odontogenic phenotype-associated variants, suggesting a role for dysregulation of tooth developmental enhancers in human dental phenotypes. The large number of these regions genome-wide necessitated prioritization of enhancer loci for future investigations. As enhancers modulate gene expression, we prioritized regions based on enhancers' putative target genes. We predicted these target genes and prioritized loci by integrating chromatin state, bulk gene expression and coexpression, GWAS variants, and cell type resolved gene expression to generate a prioritized list of putative odontogenic phenotype-driving loci active in the developing tooth. These genomic regions are of particular interest for downstream experiments determining the role of specific dental enhancer:gene pairs in odontogenesis.

Sulphur dioxide and fluoride co-exposure induce incisor hypomineralization and amelogenin upregulation via YAP/RUNX2 signaling pathway

Sulphur dioxide (SO₂) and fluoride are among the most common environmental pollutants affecting human health, and both co-exist in areas predominantly consuming coal. It is vital to analyse the combined toxicity of SO₂ and fluoride, and their effects on health and the underlying mechanisms of their co-exposure have not yet been adequately assessed. In the present study, we used ICR mice and LS8 cells to investigate the toxicity of SO₂ and fluoride exposure to the enamel, alone or in combination. Factorial design analysis was used to reveal the combined toxicity in vitro and in vivo. Co-exposure to SO₂ and fluoride exacerbated enamel injury, resulting in more severe hypomineralization of incisor, and enamel structure disorders in mice, and could induce the accumulation of protein residue in the matrix of the enamel. Amelogenin expression was increased upon exposure to SO₂ and fluoride, but enamel matrix proteases were not affected. Consistent with our in vivo results, co-exposure of SO₂ and fluoride aggravated amelogenin expression in LS8 cells, and increased the YAP and RUNX2 levels. Co-exposure to SO₂ and fluoride resulted in greater toxicity than individual exposure, both in vitro and in vivo, indicating that residents of areas exposed to SO₂ and fluoride may have an increased risk of developing enamel damage.

Multiple roles of Runt-related transcription factor-2 in tooth eruption: bone formation and resorption

OBJECTIVE

The aim was to provide a comprehensive review of the current knowledge of the multiple roles of Runt-related transcription factor-2 (RUNX2) in regulating tooth eruption, focusing on the molecular mechanisms regarding tooth eruption mediated by RUNX2.

DESIGN

Relevant literatures in PubMed, Medline, and Scopus database were searched, and a narrative review was performed. The multiple roles of RUNX2 in regulating tooth eruption was reviewed and discussed.

RESULTS

Aberrant RUNX2 expression leads to disturbed or failed tooth eruption. Tooth eruption involves both the process of bone formation and bone resorption. RUNX2 promotes osteogenesis around the radicular portion of the dental follicle that provides the biological force for tooth eruption through inducing the expression of osteogenesis-related genes in dental follicle cells/osteoblasts. On the other hand, through indirect and direct pathways, RUNX2 regulates osteoclastogenesis and the formation of the eruption pathway.

CONCLUSION

RUNX2 exerts a pivotal and complex influence in regulating tooth eruption. This review provides a better understanding of the function of RUNX2 in tooth eruption, which is beneficial to illuminate the precise molecular mechanism of osteogenesis and bone resorption, aiding the development of effective therapy for the failure of tooth eruption.

The Association Between Runx Signaling and Craniofacial Development and Disease

PURPOSE OF REVIEW

The Runx family genes (Runx1, Runx2, Runx3, and Cbfb) are important transcriptional regulators in the development of various tissues. We herein highlight the roles of the Runx family genes in morphogenesis in the craniofacial regions and in the pathogenesis of congenital morphological problems in these regions.

RECENT FINDINGS

A recent analysis using conditional Runx mutant animals and a human genetic study identified the novel roles of Runx genes in the development of the tooth, salivary glands, and the palate. In an animal study, Runx1/Cbfb signaling was found to regulate the Lgr5 expression and maintain the stem cells in the dental epithelium in the growing incisors. Aberrant Runx1/Cbfb signaling induced male-specific involution of the convoluted granular cell differentiation of the submandibular gland. In palatogenesis, Runx1/Cbfb signaling regulated the Tgfb3 expression in the fusing palatal epithelium through Stat3 activation. The combination of a human genetic study and a phenotype analysis of mutant animals revealed the various roles of Runx genes in the development of the tooth, palate, and salivary glands. Runx genes have functional redundancy in various tissues, which still hinder the roles of Runx genes in morphogenesis. Future studies may reveal the novel roles of Runx signaling.

Revitalizing mouse diphyodontic dentition formation by inhibiting the sonic hedgehog signaling pathway

BACKGROUND

Tooth regeneration depends on the longevity of the dental epithelial lamina. However, the exact mechanism of dental lamina regression has not yet been clarified. To explore the role of the Sonic hedgehog (Shh) signaling pathway in regression process of the rudimentary successional dental lamina (RSDL) in mice, we orally administered a single dose of a Shh signaling pathway inhibitor to pregnant mice between embryonic day 13.0 (E13.0) and E17.0.

RESULTS

We observed that the Shh signaling pathway inhibitor effectively inhibited the expression of Shh signaling pathway components and revitalized RSDL during E15.0-E17.0 by promoting cell proliferation. In addition, mRNA-seq, reverse transcription plus polymerase chain reaction (RT-qPCR), and immunohistochemical analyses indicated that diphyodontic dentition formation might be related to FGF signal up-regulation and the Sostdc1-Wnt negative feedback loop.

CONCLUSIONS

Overall, our results indicated that the Shh signaling pathway may play an initial role in preventing further development of mouse RSDL in a time-dependent manner.

Molecular Genetics of Cleidocranial Dysplasia

Cleidocranial dysplasia (CCD) is a genetic disorder with an autosomal dominant inheritance pattern. CCD characterized by abnormal clavicles, patent sutures and fontenelles, supernumerary teeth and short stature. Approximately 60-70% of CCD patients have mutations in the RUNX2 gene. The RUNX2 gene is an essential transcription factor for chondrocyte maturation, osteoblast differentiation and bone formation. Runx2 regulates mesenchymal cell proliferation in sutures and suture closure by inducing the signaling pathways of the genes of Fgf, Pthlh, hedgehog and Wnt. Material and Methods: We summarized molecular genetics aspects of CCD. Result: Approximately 94% of CCD patients have dental anomalies, the most common of which are supernumerary tooth. Dental anomalies are not determined solely by gene mutations of RUNX2, but are also affected by modifier genes, environmental factors, epigenetic factors and copy number variations. Conclusion: a definite diagnosis of CCD should include the patient's clinical history, symptoms and signs, as well as genetic analyses.

Sequencing analysis of exons 5 and 6 in RUNX2 in non-syndromic patients with supernumerary tooth in Kelantan, Malaysia

OBJECTIVE

The aim of this study is to do a sequencing analysis of RUNX2 in non-syndromic patients with supernumerary tooth.

MATERIALS AND METHODS

Fifty-three patients with supernumerary tooth were identified retrospectively from 1,275 radiographic reviews who attended the Hospital Universiti Sains Malaysia (USM) Dental Clinic. Informed consent was obtained from the patients prior to the study. Blood samples were collected from 41 patients and DNA extractions were performed out of which 10 samples were chosen randomly for PCR amplification using designated primers for RUNX2 followed by DNA sequencing analysis.

RESULTS

This study involved 28 male patients (68.3%) and 13 female patients (31.7%) with a gender ratio of 2.2:1 and mean age of 15.9 ± 6.2 years. DNA extraction yielded ~ 40 ng/μl of concentrated DNA, and each DNA sample had more than 1500 bp of DNA length. The purity ranged between 1.8 and 2.0. DNA sequencing analysis did not reveal any mutations in exons 5 and 6 of RUNX2.

CONCLUSION

This study did not reveal any mutations in exons 5 and 6 of RUNX2 in non-syndromic patients with supernumerary tooth.

CLINICAL RELEVANCE

Analysis of mutations in RUNX2 is important to enhance the understanding of tooth development in humans.

Skeletal and molecular findings in 51 Cleidocranial dysplasia patients from Turkey

Loss or decrease of function in runt-related transcription factor 2 encoded by RUNX2 is known to cause a rare autosomal-dominant skeletal disorder, cleidocranial dysplasia (CCD). Clinical spectrum and genetic findings in 51 CCD patients from 30 unrelated families are herein presented. In a majority of the patients, facial abnormalities, such as delayed fontanel closure (89%), parietal and frontal bossing (80%), metopic groove (77%), midface hypoplasia (94%), and abnormal mobility of shoulders (90%), were recorded following clinical examination. In approximately one-half of the subjects, wormian bone (51%), short stature (43%), bell-shaped thorax (42%), wide pubic symphysis (50%), hypoplastic iliac wing (59%), and chef's hat sign (44%) presented in available radiological examinations. Scoliosis was identified in 28% of the patients. Investigation of RUNX2 revealed small sequence alterations in 90% and gross deletions in 10% of the patients; collectively, 23 variants including 11 novel changes (c.29_30insT, c.203delAinsCG, c.423 + 2delT, c.443_454delTACCAGATGGGAinsG, c.505C > T, c.594_595delCTinsG, c.636_637insC, c.685 + 5G > A, c.1088G > T, c.1281delC, Exon 6-9 deletion) presented high allelic heterogeneity. Novel c.29_30insT is unique in affecting the P1-driven long isoform of RUNX2, which is expected to disrupt the N-terminal region of RUNX2; this was shown in two unrelated phenotypically discordant patients. The clinical findings highlighted mild intra-familial genotype-phenotype correlation in our CCD cohort.

Evolutionary developmental genetics of teeth and odontodes in jawed vertebrates: a perspective from the study of elasmobranchs

Most extant vertebrates display a high variety of tooth and tooth-like organs (odontodes) that vary in shape, position over the body and nature of composing tissues. The development of these structures is known to involve similar genetic cascades and teeth and odontodes are believed to share a common evolutionary history. Gene expression patterns have previously been compared between mammalian and teleost tooth development but we highlight how the comparative framework was not always properly defined to deal with different tooth types or tooth developmental stages. Larger-scale comparative analyses also included cartilaginous fishes: sharks display oral teeth and dermal scales for which the gene expression during development started to be investigated in the small-spotted catshark Scyliorhinus canicula during the past decade. We report several descriptive approaches to analyse the embryonic tooth and caudal scale gene expressions in S. canicula. We compare these expressions wih the ones reported in mouse molars and teleost oral and pharyngeal teeth and highlight contributions and biases that arise from these interspecific comparisons. We finally discuss the evolutionary processes that can explain the observed intra and interspecific similarities and divergences in the genetic cascades involved in tooth and odontode development in jawed vertebrates.

Odontogenesis-related candidate genes involved in variations of permanent teeth size

OBJECTIVE

The aim of the study was to evaluate the association between genetic polymorphisms in RUNX2, BMP4, BMP2, TGFβ1, EGF, and SMAD6 and variations in permanent tooth size (TS).

MATERIALS AND METHODS

This cross-sectional study evaluated 110 individuals' dental casts to determine the maximum tooth crown size of all fully erupted permanent teeth (third molars were excluded) in the mesiodistal (MD) and buccolingual (BL) dimensions. Genomic DNA was obtained from the epithelial cells of the oral mucosa to evaluate the genetic polymorphisms in RUNX2 (rs59983488 and rs1200425), BMP4 (rs17563), BMP2 (rs235768 and rs1005464), TGFβ1 (rs1800470), EGF (rs4444903), and SMAD6 (rs2119261 and rs3934908) through real-time PCR. The data were submitted to statistical analysis with a significance level of 0.05.

RESULTS

The genetic polymorphisms rs59983488, rs1200425, rs17563, rs235768, rs1005464, rs1800470, and rs4444903 were associated with MD and BL TS of the upper and lower arches (p < 0.05). The polymorphism rs2119261 was associated with variation in TS only in the upper arch (p < 0.05). The rs3934908 was not associated with any TS measurement (p > 0.05).

CONCLUSIONS

In summary, this study reports novel associations between variation in permanent TS and genetic polymorphisms in RUNX2, BMP4, BMP2, TGFβ1, EGF, and SMAD6 indicating a possible role of these genes in dental morphology.

CLINICAL RELEVANCE

Polymorphisms in odontogenesis-related genes may be involved in dental morphology enabling a prediction of permanent TS variability. The knowledge regarding genes involved in TS might impact the personalized dental treatment, considering that patients' genetic profile would soon be introduced into clinical practice to improve patient management.

Runx2 Regulates Mouse Tooth Root Development Via Activation of WNT Inhibitor NOTUM

Progenitor cells are crucial in controlling organ morphogenesis. Tooth development is a well-established model for investigating the molecular and cellular mechanisms that regulate organogenesis. Despite advances in our understanding of how tooth crown formation is regulated, we have limited understanding of tooth root development. Runt-related transcription factor 2 (RUNX2) is a well-known transcription factor in osteogenic differentiation and early tooth development. However, the function of RUNX2 during tooth root formation remains unknown. We revealed in this study that RUNX2 is expressed in a subpopulation of GLI1+ root progenitor cells, and that loss of Runx2 in these GLI1+ progenitor cells and their progeny results in root developmental defects. Our results provide in vivo evidence that Runx2 plays a crucial role in tooth root development and in regulating the differentiation of root progenitor cells. Furthermore, we identified that Gli1, Pcp4, NOTUM, and Sfrp2 are downstream targets of Runx2 by integrating bulk and single-cell RNA sequencing analyses. Specifically, ablation of Runx2 results in downregulation of WNT inhibitor NOTUM and upregulation of canonical WNT signaling in the odontoblastic site, which disturbs normal odontoblastic differentiation. Significantly, exogenous NOTUM partially rescues the impaired root development in Runx2 mutant molars. Collectively, our studies elucidate how Runx2 achieves functional specificity in regulating the development of diverse organs and yields new insights into the network that regulates tooth root development. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

In vitro tooth-shaped scaffold construction by mimicking late bell stage

Neogenesis of osseous and ligamentous interfacial structures is essential for the regeneration of large oral or craniofacial defects. However, current treatment strategies are inadequate in renewing supporting tissues of teeth after trauma, chronic infections or surgical resection. Combined use of 3D scaffolds with stem cells became a promising treatment option for these injuries. Matching different scaffolding materials with different tissues can induce the correct cytokines and the differentiation of cells corresponding to that particular tissue. In this study, a hydroxyapatite (HA) based scaffold was used together with human adipose stem cells (hASCs), human bone marrow stem cells (hBMSCs) and gingival epithelial cells to mimic human tooth dentin-pulp-enamel tissue complexes and model an immature tooth at the late bell stage in vitro. Characteristics of the scaffold were determined via SEM, FTIR, pore size and density measurements. Changes in gene expression, protein secretions and tissue histology resulting from cross-interactions of different dental tissues grown in the system were shown. Classical tooth tissues such as cementum, pulp and bone like tissues were formed within the scaffold. Our study suggests that a HA-based scaffold with different cell lineages can successfully mimic early stages of tooth development and can be a valuable tool for hard tissue engineering.

Three-dimensional evaluation of morphology and position of impacted supernumerary teeth in cases of cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is a congenital anomaly characterized by the presence of impacted supernumerary teeth and delayed eruption of permanent teeth. However, there has been no detailed investigation on supernumerary teeth in patients with CCD using three-dimensional (3D) imaging techniques. The purpose of this study was to elucidate the morphology and position of supernumerary teeth using 3D images reconstructed from cone-beam computed tomography (CBCT) data in a group of five Japanese subjects (male, 3; female, 2; age, 15.0-25.4 years) with CCD. All five subjects exhibited supernumerary teeth (39 in total; average, 7.8; range, 1-15). All supernumerary teeth were impacted and existed as pairs with adjacent permanent teeth. Comparison of the size (the crown and dental-root lengths, the crown mesiodistal and buccolingual diameters), the number of cusps and dental roots, the position, and direction of supernumerary teeth in relation to the adjacent permanent teeth was analyzed. The results of relationship analyses revealed that, at sites other than the molar region, supernumerary teeth were positioned on the lingual and distal sides and supernumerary teeth resembled the morphology of their adjacent permanent teeth in terms of the number of cusps but were smaller than the adjacent permanent teeth. In the molar region, supernumerary teeth were microdontia, which were apparently small and obscure morphologically. In addition, while all adjacent permanent teeth exhibited normal direction, five supernumerary teeth exhibited inverse direction. The findings of this study will improve our understanding of the characteristics of CCD and provide important information for the pathophysiology and clinical treatment.

Mesenchymal Sufu Regulates Development of Mandibular Molars via Shh Signaling

Sonic hedgehog (Shh) in dental epithelium regulates tooth morphogenesis by epithelial-mesenchymal signaling transduction. However, the action of Shh signaling regulation in this process is not well understood. Here we find that mesenchymal Suppressor of Fused (Sufu), a major negative regulator of Shh signaling, plays an important role in modulating the tooth germ morphogenesis during the bud-to-cap stage transition. Deletion of Sufu in dental mesenchyme by Dermo1-Cre mice leads to delayed development of mandibular molar into cap stage with defect of primary enamel knot (EK) formation. We show the disruption of cell proliferation and programmed cell death in dental epithelium and mesenchyme in Sufu mutants. Epithelial-specific adhesion molecule E-cadherin is evidently reduced in the bilateral basal cells of tooth germ at E14.5. The cells in the presumptive EK, predominantly expressing P-cadherin, appear stratified but fail to condense. Moreover, the transcripts of primary EK marker genes, including Shh, Fgf4, and p21, are significantly decreased compared to controls. In contrast, we find that deficiency of Sufu results in elevation of Shh signaling in mesenchyme, indicated by the significant upregulation of Gli1 and Ptch1. Meanwhile, the expression of Bmp4 and Fgf3, the critical factors of mesenchymal-epithelial induction, is significantly inhibited in dental mesenchyme. Furthermore, the expression of Runx2 experiences a transient decrease at the bud stage. Taken together, these data suggest that mesenchymal Sufu is necessary for tuning the Shh signaling, which may act as an upstream modulator of Bmp4 and Fgf3 to coordinate the interplay between the dental mesenchyme and epithelium of tooth germ.

Effects of Nel-like molecule-1 and bone morphogenetic protein 2 combination on rat pulp repair

Nel-like molecule-1 (NELL-1) is a novel highly specific growth factor that can induce osteoblast differentiation and bone formation as well as odontoblast differentiation. Recent studies have suggested that NELL-1 can synergistically increase bone formation and regeneration with bone morphogenetic protein 2 (BMP2) and inhibit adverse effects induced by BMP2. This study aimed to evaluate the combined effects of NELL-1 and BMP2 on rat pulp repair. The experiment used healthy non-carious maxillary first molars from 60 Wistar rats. Exposed pulps were capped with NELL-1 plus BMP2, NELL-1 alone, and BMP2 alone, and each was absorbed onto a sterile collagen sponge. In the control samples, the collagen sponge alone and Dycal were used as capping agents. After l, 2 and 4 weeks, the rats were sacrificed. The formation of reparative dentin, as well the situation of pulp repair, was detected by hematoxylin-eosin (HE) staining; moreover, the expression of dentin specific protein-dentin sialophosphoprotein (DSPP) and the pro-inflammatory cytokines interleukin-6 (IL6) and interleukin-8 (IL8) was detected by immunohistochemical staining. Quantitative real-time PCR experiment was used to investigate the mRNA levels of IL6 and IL8. The results showed that pulp capping with NELL-1 plus BMP2 in rats had superior ability in inducing reparative dentin formation with dentin tubules and in reducing the inflammatory cell response compared with the other groups. These findings suggested that combined use of NELL-1 and BMP2 could positively regulate pulp repair.

Baba T, Mori M, Komori T. Collapsin Response Mediator Protein 1, a Novel Marker Protein for Differentiated Odontoblasts

We previously reported that the terminal differentiation of odontoblasts was inhibited in Runx2 transgenic {Tg(Col1a1-Runx2)} mice under the control of the 2.3-kb Col1a1 promoter. Odontoblasts in Tg(Col1a1-Runx2) mice lose their characteristic long cellular processes, and show marked reductions in the protein levels of markers for odontoblasts, such as dentin sialophosphoprotein, nestin, and microtubule-associated protein tau (Mapt). We herein demonstrated that collapsin response mediator protein 1 (CRMP1), a neuronal phosphoprotein that participates in various aspects of neuronal development, was specifically expressed in the differentiated odontoblasts of wild-type, but not Tg(Col1a1-Runx2) tooth germs by comparing expression profiles in wild-type and Tg(Col1a1-Runx2) mouse molars using microarray and immunohistochemical analyses. CRMP1 expression was detected at a slightly later differentiation stage in odontoblasts than type 1 collagen, nestin, and Mapt expression, which was observed from the onset of dentinogenesis. Among these proteins, CRMP1 was the most specifically localized in odontoblasts in the tooth germ. In erupted molars, odontoblast-specific CRMP1 expression decreased with age. These results indicate that CRMP1 is a novel marker protein for differentiated odontoblasts in mouse tooth germs, and suggest that CRMP1 participates in the morphogenesis of functioning odontoblasts.

Persistent Wnt/β-catenin signaling in mouse epithelium induces the ectopic Dspp expression in cheek mesenchyme

Tooth development is accomplished by a series of epithelial-mesenchyme interactions. Epithelial Wnt/β-catenin signaling is sufficient to initiate tooth development by activating Shh, Bmps, Fgfs and Wnts in dental epithelium, which in turn, triggered the expression of odontogenic genes in the underlying mesenchyme. Although constitutive activation of Wnt/β-catenin signaling in oral ectoderm resulted in the continuous tooth formation throughout the life span, if the epithelial Wnt/β-catenin signaling could induce the mesenchyme other than oral mesenchyme still required to be elucidated. In this study, we found that in the K14-cre; Ctnnb1^ex3f mice, the markers of dental epithelium, such as Pitx2, Shh, Bmp2, Fgf4, and Fgf8, were not only activated in the oral ectoderm, but also in the cheek epithelium. Surprisingly, the underlying cheek mesenchymal cells were elongated and expressed Dspp. Further investigations detected that the expression of Msx1 and Runx2 extended from oral to cheek mesenchyme. These findings suggested that epithelial Wnt/β-catenin signaling was capable of inducing Dspp expression in non-dental mesenchyme. Moreover, Dspp expression in the K14-cre; Ctnnb1^ex3f oral mesenchyme was activated earlier than that in the wild type littermates. In contrast, although the elongated oral epithelial cells were detected in the K14-cre; Ctnnb1^ex3f mice, the Amelogenin expression was suppressed. The differential effects of the persistent epithelial Wnt/β-catenin signaling on ameloblast and odontoblast differentiation might result from the altered BMP signaling. In summary, our findings suggested that the epithelial Wnt/β-catenin signaling could induce craniofacial mesenchyme into odontogenic program and promote odontoblast differentiation.

An ancient Turing-like patterning mechanism regulates skin denticle development in sharks

Vertebrates have a vast array of epithelial appendages, including scales, feathers, and hair. The developmental patterning of these diverse structures can be theoretically explained by Alan Turing's reaction-diffusion system. However, the role of this system in epithelial appendage patterning of early diverging lineages (compared to tetrapods), such as the cartilaginous fishes, is poorly understood. We investigate patterning of the unique tooth-like skin denticles of sharks, which closely relates to their hydrodynamic and protective functions. We demonstrate through simulation models that a Turing-like mechanism can explain shark denticle patterning and verify this system using gene expression analysis and gene pathway inhibition experiments. This mechanism bears remarkable similarity to avian feather patterning, suggesting deep homology of the system. We propose that a diverse range of vertebrate appendages, from shark denticles to avian feathers and mammalian hair, use this ancient and conserved system, with slight genetic modulation accounting for broad variations in patterning.

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.

Specificity Protein 7 Is Required for Proliferation and Differentiation of Ameloblasts and Odontoblasts

The Sp7/Osterix transcription factor is essential for bone development. Mutations of the Sp7 gene in humans are associated with craniofacial anomalies and osteogenesis imperfecta. However, the role of Sp7 in embryonic tooth development remains unknown. Here we identified the functional requirement of Sp7 for dentin synthesis and tooth development. Sp7-null mice exhibit craniofacial dysmorphogenesis and are completely void of alveolar bone. Surprisingly, initial tooth morphogenesis progressed normally in Sp7-null mice. Thus the formation of alveolar bone is not a prerequisite for tooth morphogenesis. Sp7 is required for mineralization of palatal tissue but is not essential for palatal fusion. The reduced proliferative capacity of Sp7-deficient ectomesenchyme results in small and misshapen teeth with randomly arranged cuboidal preodontoblasts and preameloblasts. Sp7 promotes functional maturation and polarization of odontoblasts. Markers of mature odontoblast (Col1a, Oc, Dspp, Dmp1) and ameloblast (Enam, Amelx, Mmp20, Amtn, Klk4) are barely expressed in incisors and molar tissues of Sp7-null mice. Consequently, dentin and enamel matrix are absent in the Sp7-null littermates. Interestingly, the Sp7 expression is restricted to cells of the dental mesenchyme indicating the effect on oral epithelium-derived ameloblasts is cell-nonautonomous. Abundant expression of Fgf3 and Fgf8 ligand was noted in the developing tooth of wild-type mice. Both ligands were remarkably absent in the Sp7-null incisor and molar, suggesting cross-signaling between mesenchyme and epithelium is disrupted. Finally, promoter-reporter assays revealed that Sp7 directly controls the expression of Fgf-ligands. Together, our data demonstrate that Sp7 is obligatory for the differentiation of both ameloblasts and odontoblasts but not for the initial tooth morphogenesis. © 2018 American Society for Bone and Mineral Research.

The Role of Fibroblast Growth Factors in Tooth Development and Incisor Renewal

The mineralized tissue of the tooth is composed of enamel, dentin, cementum, and alveolar bone; enamel is a calcified tissue with no living cells that originates from oral ectoderm, while the three other tissues derive from the cranial neural crest. The fibroblast growth factors (FGFs) are critical during the tooth development. Accumulating evidence has shown that the formation of dental tissues, that is, enamel, dentin, and supporting alveolar bone, as well as the development and homeostasis of the stem cells in the continuously growing mouse incisor is mediated by multiple FGF family members. This review discusses the role of FGF signaling in these mineralized tissues, trying to separate its different functions and highlighting the crosstalk between FGFs and other signaling pathways.

Mesenchymal Wnt/β-catenin signaling limits tooth number

Tooth agenesis is one of the predominant developmental anomalies in humans, usually affecting the permanent dentition generated by sequential tooth formation and, in most cases, caused by mutations perturbing epithelial Wnt/β-catenin signaling. In addition, loss-of-function mutations in the Wnt feedback inhibitor AXIN2 lead to human tooth agenesis. We have investigated the functions of Wnt/β-catenin signaling during sequential formation of molar teeth using mouse models. Continuous initiation of new teeth, which is observed after genetic activation of Wnt/β-catenin signaling in the oral epithelium, was accompanied by enhanced expression of Wnt antagonists and a downregulation of Wnt/β-catenin signaling in the dental mesenchyme. Genetic and pharmacological activation of mesenchymal Wnt/β-catenin signaling negatively regulated sequential tooth formation, an effect partly mediated by Bmp4. Runx2, a gene whose loss-of-function mutations result in sequential formation of supernumerary teeth in the human cleidocranial dysplasia syndrome, suppressed the expression of Wnt inhibitors Axin2 and Drapc1 in dental mesenchyme. Our data indicate that increased mesenchymal Wnt signaling inhibits the sequential formation of teeth, and suggest that Axin2/Runx2 antagonistic interactions modulate the level of mesenchymal Wnt/β-catenin signaling, underlying the contrasting dental phenotypes caused by human AXIN2 and RUNX2 mutations.

Expression and localization of special AT-rich sequence binding protein 2 in murine molar development and the pulp-dentin complex of human healthy teeth and teeth with pulpitis

Special AT-rich sequence binding protein 2 (SATB2) is a member of the special family of AT-rich binding transcription factors and has a critical role in osteoblast differentiation and craniofacial patterning. However, the expression and distribution of SATB2 in tooth development is largely unknown. The aim of the present study was to detect the expression and distribution of SATB2 during murine molar development and, in human healthy teeth and teeth with pulpitis using immunohistochemistry. Molars were obtained from Kunming mice at embryonic day (E) 13.5, E14.5, E16.5 and E18.5, and postnatal day (P) 1, P5 and P7. In addition, 20 human teeth (10 healthy and 10 teeth with pulpitis) were obtained from young adult patients (age, 24.90±1.65 years) who were scheduled for routine extraction. Immunohistochemical analyses were performed to detect the expression and distribution of SATB2. The present results revealed that SATB2 exhibits a spatiotemporal expression pattern in murine molar development and was expressed in odontoblasts, predentin, dental pulp cells and the blood vessels in human teeth. These findings suggested that SATB2 may have an important role in odontoblast differentiation and dentin matrix mineralization during tooth development.

microRNA Regulation of Skeletal Development

PURPOSE OF REVIEW

Osteogenesis is a complex process involving the specification of multiple progenitor cells and their maturation and differentiation into matrix-secreting osteoblasts. Osteogenesis occurs not only during embryogenesis but also during growth, after an injury, and in normal homeostatic maintenance. While much is known about osteogenesis-associated regulatory genes, the role of microRNAs (miRNAs), which are epigenetic regulators of protein expression, is just beginning to be explored. While miRNAs do not abrogate all protein expression, their purpose is to finely tune it, allowing for a timely and temporary protein down-regulation.

RECENT FINDINGS

The last decade has unveiled a multitude of miRNAs that regulate key proteins within the osteogenic lineage, thus qualifying them as "ostemiRs." These miRNAs may endogenously target an activator or inhibitor of differentiation, and depending on the target, may either lead to the prolongation of a progenitor maintenance state or to early differentiation. Interestingly, cellular identity seems intimately coupled to the expression of miRNAs, which participate in the suppression of previous and subsequent differentiation steps. In such cases where key osteogenic proteins were identified as direct targets of miRNAs in non-bone cell types, or through bioinformatic prediction, future research illuminating the activity of these miRNAs during osteogenesis will be extremely valuable. Many bone-related diseases involve the dysregulation of transcription factors or other proteins found within osteoblasts and their progenitors, and the dysregulation of miRNAs, which target such factors, may play a pivotal role in disease etiology, or even as a possible therapy.

FGF2, FGF3 and FGF4 expression pattern during molars odontogenesis in Didelphis albiventris

Odontogenesis is guided by a complex signaling cascade in which several molecules, including FGF2-4, ensure all dental groups development and specificity. Most of the data on odontogenesis derives from rodents, which does not have all dental groups. Didelphis albiventris is an opossum with the closest dentition to humans, and the main odontogenesis stages occur when the newborns are in the pouch. In this study, D. albiventris postnatals were used to characterize the main stages of their molars development; and also to establish FGF2, FGF3 and FGF4 expression pattern. D. albiventris postnatals were processed for histological and indirect immunoperoxidase analysis of the tooth germs. Our results revealed similar dental structures between D. albiventris and mice. However, FGF2, FGF3 and FGF4 expression patterns were observed in a larger number of dental structures, suggesting broader functions for these molecules in this opossum species. The knowledge of the signaling that determinates odontogenesis in an animal model with complete dentition may contribute to the development of therapies for the replacement of lost teeth in humans. This study may also contribute to the implementation of D. albiventris as model for Developmental Biology studies.

Transcriptomic signatures shaped by cell proportions shed light on comparative developmental biology

Background

Comparative transcriptomics can answer many questions in developmental and evolutionary developmental biology. Most transcriptomic studies start by showing global patterns of variation in transcriptomes that differ between species or organs through developmental time. However, little is known about the kinds of expression differences that shape these patterns.

Results

We compared transcriptomes during the development of two morphologically distinct serial organs, the upper and lower first molars of the mouse. We found that these two types of teeth largely share the same gene expression dynamics but that three major transcriptomic signatures distinguish them, all of which are shaped by differences in the relative abundance of different cell types. First, lower/upper molar differences are maintained throughout morphogenesis and stem from differences in the relative abundance of mesenchyme and from constant differences in gene expression within tissues. Second, there are clear time-shift differences in the transcriptomes of the two molars related to cusp tissue abundance. Third, the transcriptomes differ most during early-mid crown morphogenesis, corresponding to exaggerated morphogenetic processes in the upper molar involving fewer mitotic cells but more migrating cells. From these findings, we formulate hypotheses about the mechanisms enabling the two molars to reach different phenotypes. We also successfully applied our approach to forelimb and hindlimb development.

Conclusions

Gene expression in a complex tissue reflects not only transcriptional regulation but also abundance of different cell types. This knowledge provides valuable insights into the cellular processes underpinning differences in organ development. Our approach should be applicable to most comparative developmental contexts.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1157-7) contains supplementary material, which is available to authorized users.

Dioxin Alters Gene Expression in Mouse Embryonic Tooth Explants

Dioxins are ubiquitous environmental poisons that cause disturbances in developing organs, including the teeth. Exposure to 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) at the cap stage leads to reduced tooth size and deformation of cuspal morphology. Our hypothesis was that TCDD affects the expression of genes specific for tooth development, which leads to these aberrations. Mouse embryonic E14 tooth germs were cultured for 24 hrs with/without 1 μM TCDD. Analysis of total RNA on Affymetrix arrays showed that TCDD altered the expression of 31 known genes by a fold factor of at least 2. Genes implied in tooth development expressed only slight changes. Genes active at the cap stage were selected for quantitative PCR analysis. Of these, the most highly up-regulated were Follistatin and Runx2, while TGFβ 1 and p21 were the most down-regulated genes. Incomplete tooth morphogenesis caused by TCDD may thus result from modified expression of developmentally regulated genes.

Making extra teeth: Lessons from a TRPS1 mutation

A Thai mother and her two daughters were affected with tricho-rhino-phalangeal syndrome type I. The daughters had 15 and 18 supernumerary teeth, respectively. The mother had normal dentition. Mutation analysis of TRPS1 showed a novel heterozygous c.3809_3811delACTinsCATGTTGTG mutation in all. This mutation is predicted to cause amino acid changes in the Ikaros-like zinc finger domain near the C-terminal end of TRPS1, which is important for repressive protein function. The results of our study and the comprehensive review of the literature show that pathways of forming supernumerary teeth appear to involve APC and RUNX2, the genes responsible for familial adenomatous polyposis syndrome and cleidocranial dysplasia, respectively. The final pathway resulting in supernumerary teeth seems to involve Wnt, a morphogen active during many stages of development. © 2016 Wiley Periodicals, Inc.

Comparison of Stemness and Gene Expression between Gingiva and Dental Follicles in Children

The aim of this study was to compare the differential gene expression and stemness in the human gingiva and dental follicles (DFs) according to their biological characteristics. Gingiva (n = 9) and DFs (n = 9) were collected from 18 children. Comparative gene expression profiles were collected using cDNA microarray. The expression of development, chemotaxis, mesenchymal stem cells (MSCs), and induced pluripotent stem cells (iPSs) related genes was assessed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Histological analysis was performed using hematoxylin-eosin and immunohistochemical staining. Gingiva had greater expression of genes related to keratinization, ectodermal development, and chemotaxis whereas DFs exhibited higher expression levels of genes related to tooth and embryo development. qRT-PCR analysis showed that the expression levels of iPSc factors including SOX2, KLF4, and C-MYC were 58.5 ± 26.3, 12.4 ± 3.5, and 12.2 ± 1.9 times higher in gingiva and VCAM1 (CD146) and ALCAM (CD166) were 33.5 ± 6.9 and 4.3 ± 0.8 times higher in DFs. Genes related to MSCs markers including CD13, CD34, CD73, CD90, and CD105 were expressed at higher levels in DFs. The results of qRT-PCR and IHC staining supported the microarray analysis results. Interestingly, this study demonstrated transcription factors of iPS cells were expressed at higher levels in the gingiva. Given the minimal surgical discomfort and simple accessibility, gingiva is a good candidate stem cell source in regenerative dentistry.

Wnt/β-Catenin Regulates the Activity of Epiprofin/Sp6, SHH, FGF, and BMP to Coordinate the Stages of Odontogenesis

BACKGROUND

We used an in vitro tooth development model to investigate the effects of overactivation of the Wnt/β-catenin pathway during odontogenesis by bromoindirubin oxime reagent (BIO), a specific inhibitor of GSK-3 activity.

RESULTS

Overactivating the Wnt/β-catenin pathway at tooth initiation upregulated and ectopically expressed the epithelial markers Sonic Hedgehog (Shh), Epiprofin (Epfn), and Fibroblast growth factor8 (Fgf8), which are involved in the delimitation of odontogenic fields in the oral ectoderm. This result indicated an ectopic extension of the odontogenic potential. During tooth morphogenesis, Fibroblast growth factor4 (Fgf4), Fibroblast growth factor10 (Fgf10), Muscle segment homeobox 1 (Msx-1), Bone Morphogenetic protein 4 (Bmp4), and Dickkopf WNT signaling pathway inhibitor 1 (Dkk-1) were overexpressed in first molars cultured with BIO. Conversely, the expression levels of Wingless integration site 10b (Wnt-10b) and Shh were reduced. Additionally, the odontoblast differentiation markers Nestin and Epfn showed ectopic overexpression in the dental mesenchyme of BIO-treated molars. Moreover, alkaline phosphatase activity increased in the dental mesenchyme, again suggesting aberrant, ectopic mesenchymal cell differentiation. Finally, Bmp4 downregulated Epfn expression during dental morphogenesis.

CONCLUSIONS

We suggest the presence of a positive feedback loop wherein Epfn and β-catenin activate each other. The balance of the expression of these two molecules is essential for proper tooth development. We propose a possible link between Wnt, Bmp, and Epfn that would critically determine the correct patterning of dental cusps and the differentiation of odontoblasts and ameloblasts.

Osteogenic Profile of Mesenchymal Cell Populations Contributing to Alveolar Bone Formation

Teeth develop within the surrounding periodontal tissues, involving the alveolar bone, periodontal ligament and cementum. The alveolar bone originates through the process of intramembranous ossification involving mesenchymal cells from the tooth germ. As most available data are related to endochondral ossification, we examined the molecular background of alveolar bone development. We investigated the osteogenic profile of mesenchymal cells dissected from mouse mandible slices at the stage of early alveolar bone formation. Relative monitoring of gene expression was undertaken using PCR Arrays; this included the profiles of 84 genes associated with osteogenesis. To examine the tooth-bone interface, stages with detectable changes in bone remodelling during development (E13.0, E14.0 and E15.0) were chosen and compared with each other. These results showed a statistically significant increase in the expression of the genes Fgf3, Ctsk, Icam-1, Mmp9, Itga3 and Tuft1, and of a wide range of collagens (Col1a2, Col3a1, Col7a1, Col12a1, Col14a1). Decreased expression was detected in the case of Col2a1, Sox9, Smad2 and Vegfb. To confirm these changes in gene expression, immunofluorescence analyses of Mmp9 and Sox9 proteins were performed in situ. Our research has identified several candidate genes that may be crucial for the initiation of alveolar bone formation and is the basis for further functional studies.

Klf10 regulates odontoblast differentiation and mineralization via promoting expression of dentin matrix protein 1 and dentin sialophosphoprotein genes

Klf10, a member of the Krüppel-like family of transcription factors, is critical for osteoblast differentiation, bone formation and mineralization. However, whether Klf10 is involved in odontoblastic differentiation and tooth development has not been determined. In this study, we investigate the expression patterns of Klf10 during murine tooth development in vivo and its role in odontoblastic differentiation in vitro. Klf10 protein was expressed in the enamel organ and the underlying mesenchyme, ameloblasts and odontoblasts at early and later stages of murine molar formation. Furthermore, the expression of Klf10, Dmp1, Dspp and Runx2 was significantly elevated during the process of mouse dental papilla mesenchymal differentiation and mineralization. The overexpression of Klf10 induced dental papilla mesenchymal cell differentiation and mineralization as detected by alkaline phosphatase staining and alizarin red S assay. Klf10 additionally up-regulated the expression of odontoblastic differentiation marker genes Dmp1, Dspp and Runx2 in mouse dental papilla mesenchymal cells. The molecular mechanism of Klf10 in controlling Dmp1 and Dspp expression is thus to activate their regulatory regions in a dosage-dependent manner. Our results suggest that Klf10 is involved in tooth development and promotes odontoblastic differentiation via the up-regulation of Dmp1 and Dspp transcription.

Phenotypic and evolutionary implications of modulating the ERK-MAPK cascade using the dentition as a model

The question of phenotypic convergence across a signalling pathway has important implications for both developmental and evolutionary biology. The ERK-MAPK cascade is known to play a central role in dental development, but the relative roles of its components remain unknown. Here we investigate the diversity of dental phenotypes in Spry2^−/−, Spry4^−/−, and Rsk2^−/Y mice, including the incidence of extra teeth, which were lost in the mouse lineage 45 million years ago (Ma). In addition, Sprouty-specific anomalies mimic a phenotype that is absent in extant mice but present in mouse ancestors prior to 9 Ma. Although the mutant lines studied display convergent phenotypes, each gene has a specific role in tooth number determination and crown patterning. The similarities found between teeth in fossils and mutants highlight the pivotal role of the ERK-MAPK cascade during the evolution of the dentition in rodents.

Control of Wnt5b secretion by Wntless modulates chondrogenic cell proliferation through fine-tuning fgf3 expression

Wnts and Fgfs regulate various tissues development in vertebrates. However, how regional Wnt or Fgf activities are established and how they interact in any given developmental event is elusive. Here, we investigated the Wnt-mediated craniofacial cartilage development in zebrafish and found that fgf3 expression in the pharyngeal pouches is differentially reduced along the anteroposterior axis in wnt5b mutants and wntless (wls) morphants, but its expression is normal in wnt9a and wnt11 morphants. Introducing fgf3 mRNAs rescued the cartilage defects in Wnt5b- and Wls-deficient larvae. In wls morphants, endogenous Wls expression is not detectable but maternally deposited Wls is present in eggs, which might account for the lack of axis defects in wls morphants. Secretion of endogenous Wnt5b but not Wnt11 was affected in the pharyngeal tissue of Wls morphants, indicating that Wls is not involved in every Wnt secretion event. Furthermore, cell proliferation but not apoptosis in the developing jaw was affected in Wnt5b- and Wls-deficient embryos. Therefore, Wnt5b requires Wls for its secretion and regulates the proliferation of chondrogenic cells through fine-tuning the expression of fgf3 during jaw cartilage development.

Deletion of Osr2 Partially Rescues Tooth Development in Runx2 Mutant Mice

Tooth organogenesis depends on genetically programmed sequential and reciprocal inductive interactions between the dental epithelium and neural crest-derived mesenchyme. Previous studies showed that the Msx1 and Runx2 transcription factors are required for activation of odontogenic signals, including Bmp4 and Fgf3, in the early tooth mesenchyme to drive tooth morphogenesis through the bud-to-cap transition and that Runx2 acts downstream of Msx1 to activate Fgf3 expression. Recent studies identified Osr2 as a repressor of tooth development and showed that inactivation of Osr2 rescued molar tooth morphogenesis in the Msx1(-/-) mutant mice as well as in mice with neural crest-specific inactivation of Bmp4. Here we show that Runx2 expression is expanded in the tooth bud mesenchyme in Osr2(-/-) mutant mouse embryos and is partially restored in the tooth mesenchyme in Msx1(-/-)Osr2(-/-) mutants in comparison with Msx1(-/-) and wild-type embryos. Whereas mandibular molar development arrested at the bud stage and maxillary molar development arrested at the bud-to-cap transition in Runx2(-/-) mutant mice, both mandibular and maxillary molar tooth germs progressed to the early bell stage, with rescued expression of Msx1 and Bmp4 in the dental papilla as well as expression of Bmp4, p21, and Shh in the primary enamel knot in the Osr2(-/-)Runx2(-/-) compound mutants. In contrast to the Msx1(-/-)Osr2(-/-) compound mutants, which exhibit nearly normal first molar morphogenesis, the Osr2(-/-)Runx2(-/-) compound mutant embryos failed to activate the expression of Fgf3 and Fgf10 in the dental papilla and exhibited significant deficit in cell proliferation in both the dental epithelium and mesenchyme in comparison with the control embryos. These data indicate that Runx2 synergizes with Msx1 to drive tooth morphogenesis through the bud-to-cap transition and that Runx2 controls continued tooth growth and morphogenesis beyond the cap stage through activation of Fgf3 and Fgf10 expression in the dental papilla.

Thymosin beta 4 is associated with RUNX2 expression through the Smad and Akt signaling pathways in mouse dental epithelial cells

In previous studies by our group, we reported that thymosin beta 4 (Tb4) is closely associated with the initiation and development of the tooth germ, and can induce the expression of runt-related transcription factor 2 (RUNX2) during the development of the tooth germ. RUNX2 regulates the expression of odontogenesis-related genes, such as amelogenin, X-linked (Amelx), ameloblastin (Ambn) and enamelin (Enam), as well as the differentiation of osteoblasts during bone formation. However, the mechanisms through which Tb4 induces the expression of RUNX2 remain unknown. In the present study, we employed a mouse dental epithelial cell line, mDE6, with the aim to elucidate these mechanisms. The mDE6 cells expressed odontogenesis-related genes, such as Runx2, Amelx, Ambn and Enam, and formed calcified matrices upon the induction of calcification, thus showing characteristics of odontogenic epithelial cells. The expression of odontogenesis-related genes, and the calcification of the mDE6 cells were reduced by the inhibition of phosphorylated Smad1/5 (p-Smad1/5) and phosphorylated Akt (p-Akt) proteins. Furthermore, we used siRNA against Tb4 to determine whether RUNX2 expression and calcification are associated with Tb4 expression in the mDE6 cells. The protein expression of p-Smad1/5 and p-Akt in the mDE6 cells was reduced by treatment with Tb4-siRNA. These results suggest that Tb4 is associated with RUNX2 expression through the Smad and PI3K-Akt signaling pathways, and with calcification through RUNX2 expression in the mDE6 cells. This study provides putative information concerning the signaling pathway through which Tb4 induces RUNX2 expression, which may help to understand the regulation of tooth development and tooth regeneration.

Microtubule-associated protein tau (Mapt) is expressed in terminally differentiated odontoblasts and severely down-regulated in morphologically disturbed odontoblasts of Runx2 transgenic mice

Runx2 is an essential transcription factor for osteoblast and odontoblast differentiation and the terminal differentiation of chondrocytes. We have previously shown that the terminal differentiation of odontoblasts is inhibited in Runx2 transgenic {Tg(Col1a1-Runx2)} mice under the control of the 2.3-kb Col1a1 promoter, which directs the transgene expression to osteoblasts and odontoblasts. Odontoblasts show severe reductions in Dspp and nestin expression and lose their characteristic polarized morphology, including a long process extending to dentin, in Tg(Col1a1-Runx2) mice. We study the molecular mechanism of odontoblast morphogenesis by comparing gene expression in the molars of wild-type and Tg(Col1a1-Runx2) mice, focusing on cytoskeleton-related genes. Using microarray, we found that the gene expression of microtubule-associated protein tau (Mapt), a neuronal phosphoprotein with important roles in neuronal biology and microtubule dynamics and assembly, was high in wild-type molars but severely reduced in Tg(Col1a1-Runx2) molars. Immunohistochemical analysis revealed that Mapt was specifically expressed in terminally differentiated odontoblasts including their processes in wild-type molars but its expression was barely detectable in Tg(Col1a1-Runx2) molars. Double-staining of Mapt and Runx2 showed their reciprocal expression in odontoblasts. Mapt and tubulin co-localized in odontoblasts in wild-type molars. Immunoelectron microscopic analysis demonstrated Mapt lying around α-tubulin-positive filamentous structures in odontoblast processes. Thus, Mapt is a useful marker for terminally differentiated odontoblasts and might play an important role in odontoblast morphogenesis.

Aberrant gene expression profiles, during in vitro osteoblast differentiation, of telomerase deficient mouse bone marrow stromal stem cells (mBMSCs)

Background

Telomerase deficiency has been associated with inadequate differentiation of mesenchymal stem cells. However, the effect of telomerase deficiency on differential regulation of osteoblast specific genes, based on functional gene grouping, during in vitro osteoblast differentiation has not been reported before.

Results

To examine these effects, Terc^-/- BMSCs (bone marrow stromal stem cells) were employed which exhibited reduced proliferation during in vitro osteogenesis along with increased population doubling time and level compared to wild type (WT) BMSCs during the normal culture. Osteogenic super array at day 10 of osteoblast differentiation revealed that telomerase deficiency strongly affected the osteoblast commitment by down-regulating Runx2, Twist and Vdr – known transcription regulators of osteogenesis. Similarly, in Terc^-/- BMSCs a marked reduction in other genes engaged in various phases of osteoblast differentiation were observed, such as Fgfr2 involved in bone mineralization, Phex and Dmp1 engaged in ossification, and Col11a1 and Col2a1 involved in cartilage condensation. A similar trend was observed for genes involved in osteoblast proliferation (Tgfb1, Fgfr2 and Pdgfa) and bone mineral metabolism (Col1a1, Col2a1, Col1a2 and Col11a1). More profound changes were observed in genes engaged in extracellular matrix production: Col1a1, Col1a2, Mmp10, Serpinh1 and Col4a1.

Conclusion

Taken together, these data suggest that telomerase deficiency causes impairment of BMSCs differentiation into osteoblasts affecting commitment, proliferation, matrix mineralization and maturation. Thus, modulating telomerase in BMSCs with advanced aging could improve BMSCs responsiveness towards osteoblast differentiation signals, optimal for osteoblast commitment, proliferation and maturation processes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-015-0116-4) contains supplementary material, which is available to authorized users.