Wnt5a plays a crucial role in determining tooth size during murine tooth development

DPP an extracellular matrix molecule induces Wnt5a mediated signaling to promote the differentiation of adult stem cells into odontogenic lineage

Dentin phosphophoryn (DPP) an extracellular matrix protein activates Wnt signaling in DPSCs (dental pulp stem cells). Wnt/β catenin signaling is essential for tooth development but the role of DPP-mediated Wnt5a signaling in odontogenesis is not well understood. Wnt5a is typically considered as a non-canonical Wnt ligand that elicits intracellular signals through association with a specific cohort of receptors and co-receptors in a cell and context-dependent manner. In this study, DPP facilitated the interaction of Wnt5a with Frizzled 5 and LRP6 to induce nuclear translocation of β-catenin. β-catenin has several nuclear binding partners that promote the activation of Wnt target genes responsible for odontogenic differentiation. Interestingly, steady increase in the expression of Vangl2 receptor suggest planar cell polarity signaling during odontogenic differentiation. In vitro observations were further strengthened by the low expression levels of Wnt5a and β-catenin in the teeth of DSPP KO mice which exhibit impaired odontoblast differentiation and defective dentin mineralization. Together, this study suggests that the DPP-mediated Wnt5a signaling could be exploited as a therapeutic approach for the differentiation of dental pulp stem cells into functional odontoblasts and dentin regeneration.

PRX1-positive mesenchymal stem cells drive molar morphogenesis

Mammalian teeth, developing inseparable from epithelial-mesenchymal interaction, come in many shapes and the key factors governing tooth morphology deserve to be answered. By merging single-cell RNA sequencing analysis with lineage tracing models, we have unearthed a captivating correlation between the contrasting morphology of mouse molars and the specific presence of PRX1+ cells within M1. These PRX1+ cells assume a profound responsibility in shaping tooth morphology through a remarkable divergence in dental mesenchymal cell proliferation. Deeper into the mechanisms, we have discovered that Wnt5a, bestowed by mesenchymal PRX1+ cells, stimulates mesenchymal cell proliferation while orchestrating molar morphogenesis through WNT signaling pathway. The loss of Wnt5a exhibits a defect phenotype similar to that of siPrx1. Exogenous addition of WNT5A can successfully reverse the inhibited cell proliferation and consequent deviant appearance exhibited in Prx1-deficient tooth germs. These findings bestow compelling evidence of PRX1-positive mesenchymal cells to be potential target in regulating tooth morphology.

Epigenetic Regulation of Methylation in Determining the Fate of Dental Mesenchymal Stem Cells

Dental mesenchymal stem cells (DMSCs) are crucial in tooth development and periodontal health, and their multipotential differentiation and self-renewal ability play a critical role in tissue engineering and regenerative medicine. Methylation modifications could promote the appropriate biological behavior by postsynthetic modification of DNA or protein and make the organism adapt to developmental and environmental prompts by regulating gene expression without changing the DNA sequence. Methylation modifications involved in DMSC fate include DNA methylation, RNA methylation, and histone modifications, which have been proven to exert a significant effect on the regulation of the fate of DMSCs, such as proliferation, self-renewal, and differentiation potential. Understanding the regulation of methylation modifications on the behavior and the immunoinflammatory responses involved in DMSCs contributes to further study of the mechanism of methylation on tissue regeneration and inflammation. In this review, we briefly summarize the key functions of histone methylation, RNA methylation, and DNA methylation in the differentiation potential and self-renewal of DMSCs as well as the opportunities and challenges for their application in tissue regeneration and disease therapy.

Role of Wnt5a in periodontal tissue development, maintenance, and periodontitis: Implications for periodontal regeneration (Review)

The periodontium is a highly dynamic microenvironment constantly adapting to changing external conditions. In the processes of periodontal tissue formation and remodeling, certain molecules may serve an essential role in maintaining periodontal homeostasis. Wnt family member 5a (Wnt5a), as a member of the Wnt family, has been identified to have extensive biological roles in development and disease, predominantly through the non‑canonical Wnt signaling pathway or through interplay with the canonical Wnt signaling pathway. An increasing number of studies has also demonstrated that it serves crucial roles in periodontal tissues. Wnt5a participates in the development of periodontal tissues, maintains a non‑mineralized state of periodontal ligament, and regulates bone homeostasis. In addition, Wnt5a is involved in the pathogenesis of periodontitis. Recently, it has been shown to serve a positive role in the regeneration of integrated periodontal complex. The present review article focuses on recent research studies of Wnt5a and its functions in development, maintenance, and pathological disorders of periodontal tissues, as well as its potential effect on periodontal regeneration.

WNT5A supports viability of senescent human dental follicle cells

The osteogenic differentiation of dental follicle cells (DFCs) is inhibited by the onset of cellular senescence, but the cause for this is largely unknown. Recently it was shown that WNT5a, which is an inductor of the non-canonical WNT pathway, stimulates both cellular senescence and osteogenic differentiation of different cell types. In this study, we investigated the role of WNT5a for viability and osteogenic differentiation in human DFCs after the induction of cellular senescence. DFCs were cultivated until the induction of cellular senescence. The induction of cellular senescence was confirmed by β-galactosidase staining, estimation of population doubling time, and slightly telomere length shortening. After induction of cellular senescence, the expression of WNT5A and the potential to induce the osteogenic differentiation decreased. Inhibition of WNT5A by specific siRNAs had significant effect on the viability of DFCs. Cell proliferation was reduced, whereas both cellular senescence and cell death were increased in DFCs. However, an inhibition of WNT5A did only slightly effect the osteogenic differentiation of DFCs. Our results suggest that WNT5A supports viability during both cell proliferation and osteogenic differentiation of DFCs.

SCAPs Regulate Differentiation of DFSCs During Tooth Root Development in Swine

The tooth root transmits and balances occlusal forces through the periodontium to the alveolar bone. The periodontium, including the gingiva, the periodontal ligament, the cementum and the partial alveolar bone, derives from the dental follicle (DF), except for the gingiva. In the early developmental stages, the DF surrounds the tooth germ as a sphere and functions to promote tooth eruption. However, the morphological dynamics and factors regulating the differentiation of the DF during root elongation remain largely unknown. Miniature pigs are regarded as a useful experimental animal for modeling in craniofacial research because they are similar to humans with respect to dentition and mandible anatomy. In the present study, we used the third deciduous incisor of miniature pig as the model to investigate the factors influencing DF differentiation during root development. We found that the DF was shaped like a crescent and was located between the root apical and the alveolar bone. The expression levels of WNT5a, β-Catenin, and COL-I gradually increased from the center of the DF (beneath the apical foramen) to the lateral coronal corner, where the DF differentiates into the periodontium. To determine the potential regulatory role of the apical papilla on DF cell differentiation, we co-cultured dental follicle stem cells (DFSCs) with stem cells of the apical papilla (SCAPs). The osteogenesis and fibrogenesis abilities of DFSCs were inhibited when being co-cultured with SCAPs, suggesting that the fate of the DF can be regulated by signals from the apical papilla. The apical papilla may sustain the undifferentiated status of DFSCs before root development finishes. These data yield insight into the interaction between the root apex and surrounding DF tissues in root and periodontium development and shed light on the future study of root regeneration in large mammals.

Bivalent Histone Codes on WNT5A during Odontogenic Differentiation

Lineage-committed differentiation is an essential biological program during odontogenesis, which is tightly regulated by lineage-specific genes. Some of these genes are modified by colocalization of H3K4me3 and H3K27me3 marks at promoter regions in progenitors. These modifications, named "bivalent domains," maintain genes in a poised state and then resolve for later activation or repression during differentiation. Wnt5a has been reported to promote odontogenic differentiation in dental mesenchyme. However, relatively little is known about the epigenetic modulations on Wnt5a activation during tooth development. Here, we investigated the spatiotemporal patterns of H3K4me3 and H3K27me3 marks in developing mouse molars. Associated H3K4me3 methylases (mixed-lineage leukemia [MLL] complex) and H3K27me3 demethylases (JMJD3 and UTX) were dynamically expressed between early and late bell stage of human tooth germs and in cultured human dental papilla cells (hDPCs) during odontogenic induction. Poised WNT5A gene was marked by bivalent domains containing repressive marks (H3K27me3) and active marks (H3K4me3) on promoters. The bivalent domains tended to resolve during inducted differentiation, with removal of the H3K27me3 mark in a JMJD3-dependent manner. When JMJD3 was knocked down in cultured hDPCs, odontogenic differentiation was suppressed. The depletion of JMJD3 epigenetically repressed WNT5A activation by increased H3K27me3 marks. In addition, JMJD3 could physically interact with ASH2L, a component of the MLL complex, to form a coactivator complex, cooperatively modulating H3K4me3 marks on WNT5A promoters. Overall, our study reveals that transcription activities of WNT5A were epigenetically regulated by the negotiated balance between H3K27me3 and H3K4me3 marks and tightly mediated by JMJD3 and MLL coactivator complex, ultimately modulating odontogenic commitment during dental mesenchymal cell differentiation.

Spatiotemporal Expression of Wnt/β-catenin Signaling during Morphogenesis and Odontogenesis of Deciduous Molar in Miniature Pig

The canonical Wnt/β-catenin signaling pathway has been shown to play essential roles in tooth initiation and early tooth development. However, the role of Wnt/β-catenin signaling in cusp patterning and crown calcification in large mammals are largely unknown. In our previous study, miniature pigs were used as the animal model due to the similarity of tooth anatomy and replacement pattern between miniature pig and human. Dynamic gene expression of third deciduous molar (DM3) in miniature pig at early stages was profiled using microarray method and expression of Wnt genes was significantly correlate with odontogenesis. In the present study, dynamic expression patterns of Wnt/β-catenin signaling genes of DM3 at cap, early bell and late bell (secretory) stage were identified. We found that Lef1 and Axin2 were expressed in the enamel knot and underlying mesenchyme regions. Meanwhile, Dkk1 was expressed in the peripheral and lower parts of dental papilla, thus forming the potential Wnt signaling gradient. We also found that β-Catenin, Axin2 and Lef1 were expressed strongly in undifferentiated cells of the inner enamel epithelium (IEE), but weakly in differentiated ameloblasts. Furthermore, we found that both Wnt signaling read-out gene Lef1 and the inhibitor Dkk1 were co-expressed in the pre-odontoblasts. In conclusion, the spatiotemporal distribution and potential gradient of Wnt signaling may contribute to cusp patterning and crown calcification. These data may yield insight into future study of precise control of crown morphogenesis and regeneration in large mammals.

Expression of planar cell polarity genes during mouse tooth development

OBJECTIVE

Planar cell polarity (PCP) refers to the cell polarity across the tissue plane and controls various cell behaviors and structures. Although the expression of several PCP signaling components has been detected in tooth germs, knowledge of the gene expression patterns of these PCP components during tooth development remains incomplete. The aim of this study is to characterize the temporal and spatial changes in PCP gene expression during tooth development.

DESIGN

Expression of Celsr1 and 2, Fzd3 and 6, Vangl1 and 2, and Dvl1-3 genes was analyzed in mouse molar germs from the bud to the bell stage using in situ hybridization.

RESULTS

At the bud stage, all target genes were expressed in all areas of the tooth bud. In the enamel organ at the cap stage, expression of Fzd3 was suppressed in the enamel knot, whereas Fzd6 was strongly expressed there. Expression of Vangl2 was strongly expressed in the inner dental epithelium from the cap stage onwards. In the inner dental epithelium, strong expression of Fzd3, Dvl2 and Vangl2 was noted at the early bell stage, and of Celsr1, Fzd3, Fzd6, Vangl2 and Dvl2 at the bell stage. Furthermore, differentiated odontoblasts strongly expressed Celsr1, Vangl2, and Dvl2.

CONCLUSION

The gene expression patterns delineated in this study improve our understanding of the role(s) of PCP components during tooth development.

Angled Growth of the Dental Lamina Is Accompanied by Asymmetrical Expression of the WNT Pathway Receptor Frizzled 6

Frizzled 6 (FZD6) belongs to a family of proteins that serve as receptors in the WNT signaling pathway. FZD6 plays an important role in the establishment of planar cell polarity in many embryonic processes such as convergent extension during gastrulation, neural tube closure, or hair patterning. Based on its role during hair development, we hypothesized that FZD6 may have similar expression pattern and function in the dental lamina, which is also a distinct epithelial protrusion growing characteristically angled into the mesenchyme. Diphyodont minipig was selected as a model species because its dentition closely resemble human ones with successional generation of teeth initiated from the dental lamina. We revealed asymmetrical expression of FZD6 in the dental lamina of early as well as late stages during its regression with stronger expression located on the labial side of the dental lamina. During lamina regression, FZD6-positive cells were found in its superficial part and the signal coincided with the upregulation of molecules involved in epithelial-mesenchymal transition and increased migratory potential of epithelial cells. FZD6-expression was also turned on during differentiation of cells producing hard tissues, in which mature odontoblasts, ameloblasts, or surrounding osteoblasts were FZD6-positive. On the other hand, the tip of successional lamina and its lingual part, in which progenitor cells are located, exhibited FZD6-negativity. In conclusion, asymmetrical expression of FZD6 correlates with the growth directionality and side-specific morphological differences in the dental lamina of diphyodont species. Based on observed expression pattern, we propose that the dental lamina is other epithelial tissue, where planar cell polarity signaling is involved during its asymmetrical growth.

Effects of Usag-1 and Bmp7 deficiencies on murine tooth morphogenesis

BACKGROUND

Wnt5a and Mrfzb1 genes are involved in the regulation of tooth size, and their expression levels are similar to that of Bmp7 during morphogenesis, including during the cap and early bell stages of tooth formation. We previously reported that Usag-1-deficient mice form supernumerary maxillary incisors. Thus, we hypothesized that BMP7 and USAG-1 signaling molecules may play important roles in tooth morphogenesis. In this study, we established double genetically modified mice to examine the in vivo inter-relationships between Bmp7 and Usag-1.

RESULTS

We measured the volume and cross-sectional areas of the mandibular incisors using micro-computed tomography (micro-CT) in adult Bmp7- and Usag-1-LacZ knock-in mice and their F2 generation upon interbreeding. The mandibular incisors of adult Bmp7+/- mice were significantly larger than those of wild-type (WT) mice. The mandibular incisors of adult Usag-1-/- mice were the largest of all genotypes examined. In the F2 generation, the effects of these genes were additive; Bmp7+/- was most strongly associated with the increase in tooth size using generalized linear models, and the total area of mandibular supernumerary incisors of Usag-1-/-Bmp7+/- mice was significantly larger than that of Usag-1-/-Bmp7 +/+ mice. At embryonic day 15 (E15), BrdU assays demonstrated that the labeling index of Bmp7+/- embryos was significantly higher than that of WT embryos in the cervical loop. Additionally, the labeling index of Usag-1-/- embryos was significantly the highest of all genotypes examined in dental papilla.

CONCLUSIONS

Bmp7 heterozygous mice exhibited significantly increased tooth sizes, suggesting that tooth size was controlled by specific gene expression. Our findings may be useful in applications of regenerative medicine and dentistry.

Role of the Wnt signaling molecules in the tooth

Wnt signaling plays a central role in many processes during embryonic development and adult homeostasis. At least 19 types of Wnt ligands, receptors, transducers, transcription factors, and antagonists have been identified in mammals. Two distinct Wnt signaling pathways, the canonical signaling pathway and the noncanonical signaling pathway, have been described. Some Wnt signaling pathway components are expressed in the dental epithelium and mesenchyme during tooth development in humans and mice. Functional studies and experimental analysis of relevant animal models confirm the effects of Wnt signaling pathway on the regulation of developing tooth formation and adult tooth homeostasis. Mutations in some Wnt signaling pathway components have been identified in syndromic and non-syndromic tooth agenesis. This review provides an overview of progress in elucidating the role of Wnt signaling pathway components in the tooth and the resulting possibilities for therapeutic development.

Genetic subdivision and candidate genes under selection in North American grey wolves

Previous genetic studies of the highly mobile grey wolf (Canis lupus) found population structure that coincides with habitat and phenotype differences. We hypothesized that these ecologically distinct populations (ecotypes) should exhibit signatures of selection in genes related to morphology, coat colour and metabolism. To test these predictions, we quantified population structure related to habitat using a genotyping array to assess variation in 42 036 single-nucleotide polymorphisms (SNPs) in 111 North American grey wolves. Using these SNP data and individual-level measurements of 12 environmental variables, we identified six ecotypes: West Forest, Boreal Forest, Arctic, High Arctic, British Columbia and Atlantic Forest. Next, we explored signals of selection across these wolf ecotypes through the use of three complementary methods to detect selection: FST /haplotype homozygosity bivariate percentilae, bayescan, and environmentally correlated directional selection with bayenv. Across all methods, we found consistent signals of selection on genes related to morphology, coat coloration, metabolism, as predicted, as well as vision and hearing. In several high-ranking candidate genes, including LEPR, TYR and SLC14A2, we found variation in allele frequencies that follow environmental changes in temperature and precipitation, a result that is consistent with local adaptation rather than genetic drift. Our findings show that local adaptation can occur despite gene flow in a highly mobile species and can be detected through a moderately dense genomic scan. These patterns of local adaptation revealed by SNP genotyping likely reflect high fidelity to natal habitats of dispersing wolves, strong ecological divergence among habitats, and moderate levels of linkage in the wolf genome.

Immunohistochemical expression of WNT5A and MMPs in odontogenic epithelial tumors and cysts

OBJECTIVE

The aim of this study was compare the expression of WNT5A and MMP2, 7 and 20, in frequent benign odontogenic tumors and odontogenic cysts, since these lesions have a different biological behavior.

MATERIALS AND METHODS

Eighty-one paraffin-embedded specimens of odontogenic tumors, including ameloblastoma and keratocystic odontogenic tumor, and thirty-two odontogenic cysts were used for immunohistochemical analysis.

RESULTS

The expression of WNT5A in odontogenic tumors and inflammatory cyst was higher than in developmental odontogenic cyst. There was no statistical difference (p<0.05) in the expression of WNT5A when comparing the analyzed tumors. The expression of MMP7 was lower in RC with a statistical difference when compared with all tumors and cysts. Statistical differences also occurred when comparing glandular odontogenic cyst (GOC) to keratocyst odontogenic tumor (KOT) and calcifying cystic odontogenic tumor (CCOT). MMP20 expression was higher in ameloblastoma when compared to adenomatoid odontogenic tumor (AOT), DC and GOC. The expression of MMP20 was lower in CCOT when compared to all tumors and cysts.

CONCLUSIONS

The expression of WNT5A in a group of odontogenic lesions suggests the participation of a non-canonical WNT signaling pathway in the progression and maintenance of these lesions. These molecules are possibly involved in the biological differences between odontogenic tumors and cysts. Considering previous studies, WNT5A may help promote the calcification seen in AOT, CCOT and CEOT by activating MMP7.

Wnt5a regulates dental follicle stem/progenitor cells of the periodontium

Introduction

Dental follicle gives rise to one or several tissues of the periodontium including the periodontal ligament, cementum and/or alveolar bone. Whether Wnt5a is expressed in the postnatal periodontium or regulates dental follicle stem/progenitor cells is unknown.

Methods

Dental follicle stem/progenitor cells were isolated from postnatal day 1 (p1) to p11 from rat mandibular first molars. Immunolocalization mapped Wnt5a expression in the alveolar bone, periodontal ligament, and the developing ameloblast and odontoblast layers. Mononucleated and adherent cells were isolated from p7 dental follicle. Wnt5a was overexpressed in dental follicle stem/progenitor cells to study their proliferation, osteogenic differentiation and migration behavior, with subpopulations of native dental follicle stem/progenitor cells as controls, using real-time PCR (Taqman), Lenti-viral transfection, Western blotting and immunofluorescence.

Results

Wnt5a was expressed consistently in p1 to p11 rat peridontium. Native, p7 dental follicle stem/progenitor cells had modest ability to mineralize in the tested 14 days. Even in chemically defined osteogenesis medium, dental follicle stem/progenitor cells only showed modest mineralization. Upon addition of 300 ng/mL Wnt5a protein in osteogenesis medium, dental follicle stem/progenitor cells displayed mineralization that was still unremarkable. Chemically induced or Wnt5a-induced mineralization of dental follicle cells only occurred sparsely. Combination of Wnt5a with 100 ng/mL BMP2 finally prompted dental follicle stem/progenitor cells to produce robust mineralization with elevated expression of Runx2, alkaline phosphatase, collagen 1α1 and osteocalcin. Thus, native dental follicle stem/progenitor cells or some of their fractions may be somewhat modest in mineralization. Strikingly, Wnt5a protein significantly augmented RANKL ligand, suggesting putative regulatory roles of dental follicle stem/progenitor cells for the monocyte/osteoclast lineage and potential involvement in alveolar bone remodeling and/or resorption. P-Jnk1/2 was activated in Wnt5a overexpressed dental follicle cells; conversely, exposure to SP600125, a c-Jun N-terminal kinase (JNK) inhibitor attenuated Runx2, collagen 1α1 and osteocalcin expression either in the presence or absence of Wnt5a. Wnt5a overexpression in dental follicle stem/progenitor cells significantly reduced their proliferation rates, but robustly augmented their migration capacity.

Conclusions

These findings provide a glimpse of Wnt5a’s putative roles in dental follicle stem/progenitor cells and the periodontium with implications in periodontal disease, tooth eruption, dental implant bone healing and orthodontic tooth movement.

Genome-wide analysis of miRNA and mRNA transcriptomes during amelogenesis

BACKGROUND

In the rodent incisor during amelogenesis, as ameloblast cells transition from secretory stage to maturation stage, their morphology and transcriptome profiles change dramatically. Prior whole genome transcriptome analysis has given a broad picture of the molecular activities dominating both stages of amelogenesis, but this type of analysis has not included miRNA transcript profiling. In this study, we set out to document which miRNAs and corresponding target genes change significantly as ameloblasts transition from secretory- to maturation-stage amelogenesis.

RESULTS

Total RNA samples from both secretory- and maturation-stage rat enamel organs were subjected to genome-wide miRNA and mRNA transcript profiling. We identified 59 miRNAs that were differentially expressed at the maturation stage relative to the secretory stage of enamel development (False Discovery Rate (FDR)<0.05, fold change (FC)≥1.8). In parallel, transcriptome profiling experiments identified 1,729 mRNA transcripts that were differentially expressed in the maturation stage compared to the secretory stage (FDR<0.05, FC≥1.8). Based on bioinformatics analyses, 5.8% (629 total) of these differentially expressed genes (DEGS) were highlighted as being the potential targets of 59 miRNAs that were differentially expressed in the opposite direction, in the same tissue samples. Although the number of predicted target DEGs was not higher than baseline expectations generated by examination of stably expressed miRNAs, Gene Ontology (GO) analysis showed that these 629 DEGS were enriched for ion transport, pH regulation, calcium handling, endocytotic, and apoptotic activities. Seven differentially expressed miRNAs (miR-21, miR-31, miR-488, miR-153, miR-135b, miR-135a and miR298) in secretory- and/or maturation-stage enamel organs were confirmed by in situ hybridization. Further, we used luciferase reporter assays to provide evidence that two of these differentially expressed miRNAs, miR-153 and miR-31, are potential regulators for their predicated target mRNAs, Lamp1 (miR-153) and Tfrc (miR-31).

CONCLUSIONS

In conclusion, these data indicate that miRNAs exhibit a dynamic expression pattern during the transition from secretory-stage to maturation-stage tooth enamel formation. Although they represent only one of numerous mechanisms influencing gene activities, miRNAs specific to the maturation stage could be involved in regulating several key processes of enamel maturation by influencing mRNA stability and translation.

Dental developmental abnormalities in a patient with subtelomeric 7q36 deletion syndrome may confirm a novel role for the SHH gene

Studies in mice demonstrated that the Shh gene is crucial for normal development of both incisors and molars, causing a severe retardation in tooth growth, which leads to abnormal placement of the tooth in the jaw and disrupted tooth morphogenesis. In humans the SHH gene is located on chromosome 7q36. Defects in its protein or signaling pathway may cause holoprosencephaly spectrum, a disorder in which the developing forebrain fails to correctly separate into right and left hemispheres and that can be manifested in microforms such as single maxillary central incisor. A novel role for this gene in the developing human primary dentition was recently demonstrated. We report a 12-year old boy with a de novo 7q36.1-qter deletion characterized by high-resolution karyotyping, oligonucleotide aCGH and FISH. His phenotype includes intellectual disability, non-verbal communication, hypospadia, partial sacral agenesis and absence of coccyx, which are distinctive features of the syndrome and mainly correlated with the MNX1, HTR5A and EN2 genes. No microforms of holoprosencephaly spectrum were observed; but the patient had diastema and dental developmental abnormalities, such as conical, asymmetric and tapered inferior central incisors. The dental anomalies are reported herein for the first time in subtelomeric 7q36 deletion syndrome and may confirm clinically a novel role for the SHH gene in dental development.

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We report a boy with 7q-, dental developmental abnormalities and sacral agenesis.

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We propose novel roles for SHH gene related to dental developmental abnormalities.

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The MNX1 gene may be associated with caudal deficiency sequence in 7q- patients.

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HTR5A and EN2 may be related to abnormal brain development in 7q- patients.

Inductive ability of human developing and differentiated dental mesenchyme

The development of cell-based therapeutic strategies to bioengineer tooth tissue is a promising approach for the treatment of lost or damaged tooth tissue. The lack of a readily available cell source for human dental epithelial cells (ECs) severely constrains the progress of tooth bioengineering. Previous studies in model organisms have demonstrated that developing dental mesenchyme can instruct nondental epithelium to differentiate into enamel-forming epithelium. In this study, we characterized the ability of fetal and adult human dental mesenchyme to promote differentiation of human embryonic stem cell (hESC)-derived ECs (ES-ECs) into ameloblast-lineage cells. ES-ECs were co-cultured either with human fetal dental mesenchymal cells (FDMCs) or with adult dental mesenchymal cells (ADMCs) in either a three-dimensional culture system, or in the renal capsules of SCID mice. When co-cultured with FDMCs in vitro, ES-ECs polarized and expressed amelogenin. Tooth organ-like structures assembled with epithelium and encased mesenchyme and developing enamel-like structures could be detected in the complexes resulting from in vitro and ex vivo co-culture of ES-ECs and FDMCs. In contrast, co-cultured ES-ECs and ADMCs formed amorphous spherical structures and occasionally formed hair. Transcription factors were significantly upregulated in FDMCs compared to ADMCs including MSX1, GLI1, LHX6, LHX8,LEF1 and TBX1. In summary, FDMCs but not ADMCs had the capacity to induce differentiation of ES-ECs into ameloblast lineage cells. Further characterization of the functional differences between these two types of dental mesenchyme could enable reprogramming of ADMCs to enhance their odontogenic inductive competence.

Common developmental pathways link tooth shape to regeneration

In many non-mammalian vertebrates, adult dentitions result from cyclical rounds of tooth regeneration wherein simple unicuspid teeth are replaced by more complex forms. Therefore and by contrast to mammalian models, the numerical majority of vertebrate teeth develop shape during the process of replacement. Here, we exploit the dental diversity of Lake Malawi cichlid fishes to ask how vertebrates generally replace their dentition and in turn how this process acts to influence resulting tooth morphologies. First, we used immunohistochemistry to chart organogenesis of continually replacing cichlid teeth and discovered an epithelial down-growth that initiates the replacement cycle via a labial proliferation bias. Next, we identified sets of co-expressed genes from common pathways active during de novo, lifelong tooth replacement and tooth morphogenesis. Of note, we found two distinct epithelial cell populations, expressing markers of dental competence and cell potency, which may be responsible for tooth regeneration. Related gene sets were simultaneously active in putative signaling centers associated with the differentiation of replacement teeth with complex shapes. Finally, we manipulated targeted pathways (BMP, FGF, Hh, Notch, Wnt/β-catenin) in vivo with small molecules and demonstrated dose-dependent effects on both tooth replacement and tooth shape. Our data suggest that the processes of tooth regeneration and tooth shape morphogenesis are integrated via a common set of molecular signals. This linkage has subsequently been lost or decoupled in mammalian dentitions where complex tooth shapes develop in first generation dentitions that lack the capacity for lifelong replacement. Our dissection of the molecular mechanics of vertebrate tooth replacement coupled to complex shape pinpoints aspects of odontogenesis that might be re-evolved in the lab to solve problems in regenerative dentistry.

Enhanced Wnt/β-catenin signalling during tooth morphogenesis impedes cell differentiation and leads to alterations in the structure and mineralisation of the adult tooth

BACKGROUND INFORMATION

Previous studies have indicated that over-activation of the wingless interaction site (Wnt)/β-catenin signalling pathway has important implications for tooth development, at the level of cell differentiation and morphology, as well as for the production of supernumerary teeth. Here, we provide evidence for a crucial role of this signalling pathway during the stage of tooth morphogenesis. We have developed an in vitro model consisting of 14.5-day-old mouse embryo first molars, in which the Wnt pathway is overactivated by the glycogen synthase kinase-3 inhibitor 6-bromoindirubin-3'-oxime (BIO; 20 µM).

RESULTS

We found that over-activation of the Wnt/β-catenin pathway delayed the differentiation and growth of the inner dental epithelium. In addition, in contrast to controls in which Nestin protein expression was restricted to differentiated odontoblasts, in BIO-treated molars, Nestin expression spread through sub-odontoblastic cellular layers. This alteration appears to be related to: (i) the over-expression of Bmp4 in the same region, (ii) the delay in odontoblast precursor cell differentiation and (iii) increased proliferation of mesenchymal cells. Furthermore, treatments longer than 6 days induced the malformation of typical dental structures and led to a total lack of cell differentiation. Finally, over-activation of the Wnt route during odontogenesis resulted in adult teeth which presented altered size, morphology and mineralisation.

CONCLUSIONS

Our results indicate that Wnt/β-catenin over-activation during tooth morphogenesis is sufficient to cause dramatic alterations in the adult tooth, by delaying cellular differentiation and stimulating proliferation of the dental mesenchyme of developing teeth.