Down-regulated genes in mouse dental papillae and pulp

Interference with the retinoic acid signalling pathway inhibits the initiation of teeth and caudal primary scales in the small-spotted catshark Scyliorhinus canicula

The retinoic acid (RA) pathway was shown to be important for tooth development in mammals, and suspected to play a key role in tooth evolution in teleosts. The general modalities of development of tooth and "tooth-like" structures (collectively named odontodes) seem to be conserved among all jawed vertebrates, both with regard to histogenesis and genetic regulation. We investigated the putative function of RA signalling in tooth and scale initiation in a cartilaginous fish, the small-spotted catshark Scyliorhinus canicula. To address this issue, we identified the expression pattern of genes from the RA pathway during both tooth and scale development and performed functional experiments by exposing small-spotted catshark embryos to exogenous RA or an inhibitor of RA synthesis. Our results showed that inhibiting RA synthesis affects tooth but not caudal primary scale development while exposure to exogenous RA inhibited both. We also showed that the reduced number of teeth observed with RA exposure is probably due to a specific inhibition of tooth bud initiation while the observed effects of the RA synthesis inhibitor is related to a general delay in embryonic development that interacts with tooth development. This study provides data complementary to previous studies of bony vertebrates and support an involvement of the RA signalling pathway toolkit in odontode initiation in all jawed vertebrates. However, the modalities of RA signalling may vary depending on the target location along the body, and depending on the species lineage.

Methylation of Cdkn1c may be involved in the regulation of tooth development through cell cycle inhibition

Cdkn1c, a member of the Cip/Kip cyclin-dependent kinase inhibitor family, is critically involved in regulating cell cycle and cellular differentiation during development in mammals. However, the functional role of Cdkn1c and the underlying mechanisms by which Cdkn1c affects odontogenesis remain largely unknown. In our study, we found that Cdkn1c expression dynamically changes from embryonic day 11.5 (E11.5) to postnatal day 3 (P3), and exhibits tissue-specific expression profiles. Evaluation of CDKN1C protein by immunohistochemistry and western blot, revealed that CDKN1C protein expression peaks at P3 and then is reduced at P5 and P7. Interestingly, we observed that CDKN1C expression is higher in immature odontoblasts than preodontoblasts, is lower in mature odontoblasts, and is practically absent from ameloblasts. We evaluated cell cycle progression to further investigate the mechanisms underlying CDKN1C-mediated regulation of odontogenesis, and found that pRB, cyclin D1 and CDK2 expression decreased from P1 to P3, and reduced at P5 and P7. In addition, we observed increased methylation of KvDMR1 at P1 and P3, and reduced KvDMR1 methylation at P5 and P7. However, the methylation levels of Cdkn1c-sDMR were relatively low from P1 to P7. In summary, we demonstrated that Cdkn1c expression and methylation status may be involved in early postnatal tooth development through regulating the cell cycle inhibition activity of Cdkn1c. Notably, Cdkn1c expression and methylation may associate with cell cycle exit and differentiation of odontoblasts.

Dental and periodontal phenotypes of Dlx2 overexpression in mice

Distal-less homeobox 2 (Dlx2) is a member of the homeodomain family of transcription factors and is important for the development of cranial neural crest cells (CNCCs)-derived craniofacial tissues. Previous studies revealed that Dlx2 was expressed in the cementum and a targeted null mutation disrupted tooth development in mice. However, whether Dlx2 overexpression may impair in vivo tooth morphogenesis remains to be elucidated. The present study used a transgenic mouse model to specifically overexpress Dlx2 in neural crest cells in order to identify the dental phenotypes in mice by observation, micro-computed tomography and histological examination. The Dlx2-overexpressed mice exhibited tooth abnormalities including incisor cross-bite, shortened tooth roots, increased cementum deposition, periodontal ligament disorganization and osteoporotic alveolar bone. Therefore, Dlx2 overexpression may alter the alveolar bone, cementum and periodontal ligament (PDL) phenotypes in mice.

Lhx8 mediated Wnt and TGFβ pathways in tooth development and regeneration

LIM homeobox 8 (Lhx8) is a highly conserved transcriptional factor with recently illustrated roles in cholinergic and GABAergic differentiation, and is expressed in neural crest derived craniofacial tissues during development. However, Lhx8 functions and signaling pathways are largely elusive. Here we showed that Lhx8 regulates dental mesenchyme differentiation and function via Wnt and TGFβ pathways. Lhx8 expression was restricted to dental mesenchyme from E11.5 to a peak at E14.5, and absent in dental epithelium. By reconstituting dental epithelium and mesenchyme in an E16.5 tooth organ, Lhx8 knockdown accelerated dental mesenchyme differentiation; conversely, Lhx8 overexpression attenuated dentin formation. Lhx8 overexpressed adult human dental pulp stem/progenitor cells in β-tricalcium phosphate cubes attenuated mineralized matrix production in vivo. Gene profiling revealed that postnatal dental pulp stem/progenitor cells upon Lhx8 overexpression modified matrix related gene expression including Dspp, Cola1 and osteocalcin. Lhx8 transcriptionally activated Wnt and TGFβ pathways, and its attenuation upregulated multiple dentinogenesis genes. Together, Lhx8 regulates dentin development and regeneration by fine-turning Wnt and TGFβ signaling.

Undifferentiated pulp cells and odontoblast-like cells share genes involved in the process of odontogenesis

OBJECTIVE

Expression of a large number of genes during differentiation of undifferentiated pulp cells into odontoblastic cells is still unknown, hence the aim of this investigation was to compare undifferentiated pulp cells (OD-21) and odontoblast-like cells (MDPC-23) through the assessment of cell stimulation and gene expression profiling.

DESIGN

The cells were cultured and after the experimental periods, there were evaluated cell proliferation and viability as well as alkaline phosphatase activity (ALP) and mineralization nodules. To evaluate gene expression it was used fluorescence cDNA microarray technology in addition to bioinformatics programmes such as SAM (significance analysis of microarrays). Gene expression was validated by Real Time PCR (qPCR).

RESULTS

The results showed that viability was above 80% in both cells, cell proliferation and ALP activity was higher in MDPC-23 cells and mineralization nodules were present only in the cultures of odontoblast-like cells. There were observed genes associated to odontogenesis with similar behaviour in both cell types, such as Il10, Traf6, Lef1 and Hspa8. Regions of the heatmap showed differences in induction and repression of genes such as Jak2 and Fas.

CONCLUSION

OD-21 cells share many genes with similar behaviour to MDPC-23 cells, suggesting their potential to differentiate into odontoblasts.

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.