p63 in tooth development

Synonymous alterations of cancer-associated Trp53 CpG mutational hotspots cause fatal developmental jaw malocclusions but no tumors in knock-in mice

Intragenic CpG dinucleotides are tightly conserved in evolution yet are also vulnerable to methylation-dependent mutation, raising the question as to why these functionally critical sites have not been deselected by more stable coding sequences. We previously showed in cell lines that altered exonic CpG methylation can modify promoter start sites, and hence protein isoform expression, for the human TP53 tumor suppressor gene. Here we extend this work to the in vivo setting by testing whether synonymous germline modifications of exonic CpG sites affect murine development, fertility, longevity, or cancer incidence. We substituted the DNA-binding exons 5-8 of Trp53, the mouse ortholog of human TP53, with variant-CpG (either CpG-depleted or -enriched) sequences predicted to encode the normal p53 amino acid sequence; a control construct was also created in which all non-CpG sites were synonymously substituted. Homozygous Trp53-null mice were the only genotype to develop tumors. Mice with variant-CpG Trp53 sequences remained tumor-free, but were uniquely prone to dental anomalies causing jaw malocclusion (p < .0001). Since the latter phenotype also characterises murine Rett syndrome due to dysfunction of the trans-repressive MeCP2 methyl-CpG-binding protein, we hypothesise that CpG sites may exert non-coding phenotypic effects via pre-translational cis-interactions of 5-methylcytosine with methyl-binding proteins which regulate mRNA transcript initiation, expression or splicing, although direct effects on mRNA structure or translation are also possible.

Stage-Specific Role of Amelx Activation in Stepwise Ameloblast Induction from Mouse Induced Pluripotent Stem Cells

Amelogenin comprises ~90% of enamel proteins; however, the involvement of Amelx transcriptional activation in regulating ameloblast differentiation from induced pluripotent stem cells (iPSCs) remains unknown. In this study, we generated doxycycline-inducible Amelx-expressing mouse iPSCs (Amelx-iPSCs). We then established a three-stage ameloblast induction strategy from Amelx-iPSCs, including induction of surface ectoderm (stage 1), dental epithelial cells (DECs; stage 2), and ameloblast lineage (stage 3) in sequence, by manipulating several signaling molecules. We found that adjunctive use of lithium chloride (LiCl) in addition to bone morphogenetic protein 4 and retinoic acid promoted concentration-dependent differentiation of DECs. The resulting cells had a cobblestone appearance and keratin14 positivity. Attenuation of LiCl at stage 3 together with transforming growth factor β1 and epidermal growth factor resulted in an ameloblast lineage with elongated cell morphology, positivity for ameloblast markers, and calcium deposition. Although stage-specific activation of Amelx did not produce noticeable phenotypic changes in ameloblast differentiation, Amelx activation at stage 3 significantly enhanced cell adhesion as well as decreased proliferation and migration. These results suggest that the combination of inducible Amelx transcription and stage-specific ameloblast induction for iPSCs represents a powerful tool to highlight underlying mechanisms in ameloblast differentiation and function in association with Amelx expression.

Optimization of culture conditions for the efficient differentiation of mouse-induced pluripotent stem cells into dental epithelial-like cells

The establishment of a method to derive dental epithelial cells seems to be an important challenge toward realizing the whole tooth regeneration. In order to obtain a source of dental epithelial-like cells, a new methodology has been previously developed by our research group. In the method, induced pluripotent stem cells are cultured in suspension in the presence of neurotrophin-4 to form embryoid bodies followed by further adherent culture of the embryoid bodies in DMEM basal nutrient medium. The present study was directed to improve the efficiency of dental epithelial-like cell production, by focusing on the optimization of initial cell number for the formation of embryoid bodies and the addition of epidermal growth factor as well as its timing. Our results demonstrated that an initial cell number of 1000 cells/drop gives the highest efficiency of dental epithelial-like cell production. It appears that, under this condition, medium deterioration is moderated, and that cell-cell interactions are optimized within embryoid bodies. On the other hand, epidermal growth factor serves to increase the abundance of dental epithelial-like cells when added to the medium together with neurotrophin-4 during embryoid body formation. The promotive effect of epidermal growth factor may involve the transactivation of TrkB, mediated by the effectors of epidermal growth factor receptor signaling.

Expression of new genes in vertebrate tooth development and p63 signaling

BACKGROUND

p63 is an evolutionarily ancient transcription factor essential to vertebrate tooth development. Our recent gene expression screen comparing wild-type and "toothless" p63^-/- mouse embryos implicated in tooth development several new genes that we hypothesized act downstream of p63 in dental epithelium, where p63 is also expressed.

RESULTS

Via in situ hybridization and immunohistochemistry, we probed mouse embryos (embryonic days 10.5-14.5) and spotted gar fish embryos (14 days postfertilization) for these newly linked genes, Cbln1, Cldn23, Fermt1, Krt15, Pltp and Prss8, which were expressed in mouse and gar dental epithelium. Loss of p63 altered expression levels but not domains. Expression was comparable between murine upper and lower tooth organs, implying conserved gene functions in maxillary and mandibular dentitions. Our meta-analysis of gene expression databases supported that these genes act within a p63-driven gene regulatory network important to tooth development in mammals and more evolutionary ancient vertebrates (fish, amphibians).

CONCLUSIONS

Cbln1, Cldn23, Fermt1, Krt15, Pltp, and Prss8 were expressed in mouse and fish dental epithelium at placode, bud, and/or cap stages. We theorize that these genes participate in cell-cell adhesion, cell polarity, and extracellular matrix signaling to support dental epithelium integrity, folding, and epithelial-mesenchymal cross talk during tooth development.

Differentiation of mouse-induced pluripotent stem cells into dental epithelial-like cells in the absence of added serum

Recent studies have successfully generated tooth-like structure by mimicking the reciprocal interaction between dental epithelial and mesenchymal cells in tooth organogenesis. However, clinical applications of these methods are limited primarily due to the lack of appropriate sources for dental epithelial cells. Induced pluripotent stem cells (iPSCs) are attractive as a source for dental epithelial cells due to their unique characteristics. In this study, we examined the effect of neurotrophin-4 (NT-4) on the differentiation of mouse iPSCs (miPSCs) into dental epithelial cells. Our results showed that the addition of NT-4 during the formation of embryoid body significantly triggered the upregulation of epithelial markers such as p63 and CK14, suggesting that NT-4 provides an inductive condition for the differentiation of miPSCs into epithelial cells. Expansion of the NT-4-treated cells under serum-free culture conditions improves the formation of cells with cobblestone-like morphology and significantly downregulated the expression of pluripotent and ectodermal markers. Phenotypic analysis revealed that a dental epithelial surface marker, CD49f, was highly expressed on these cells. Formation of miPSCs-derived dental epithelial-like cells was further confirmed by the expression of ameloblast-specific markers. These results suggest that the addition of NT-4 during the formation of embryoid body together with the serum-free culture condition promoted the differentiation of miPSCs into dental epithelial-like cells.

Upper jaw development in the absence of teeth: New insights for craniodental evo-devo integration

In p63-null mice (p63^-/- ), teeth fail to form but the mandible forms normally; conversely, the upper jaw skeleton is malformed. Here we explored whether lack of dental tissues contributed to midfacial dysmorphologies in p63^-/- mice by testing if facial prominence defects appeared before odontogenesis failed. We also investigated gene dose effects by testing if one wild type (WT) p63 allele (p63^+/- ) was sufficient for normal upper jaw skeleton formation. We micro-CT scanned PFA-fixed p63^-/- , p63^+/- , and WT (p63^+/+ ) adult and embryonic mice aged E10-E14. Next, we landmarked mandibular (MdP), maxillary (MxP) and nasal prominences (NPs), and facial bones. 3D landmark data were assessed using Principal Component, Canonical Variate, Partial Least Squares, and other statistical analyses. The p63^-/- embryos showed MxP and NP malformations by E12, despite the presence of dental tissues. MdP shape was comparable among p63^-/- , p63^+/- , and p63^+/+ embryos. Upper jaw shape was comparable between p63^+/+ and p63^+/- adults. The upper jaw and its dentition both require p63 signaling, but not each other's presence, to form properly. One WT p63 allele enables normal midfacial morphogenesis; gene dose may be a target for jaw macroevolution. Jaw-specific genetic mechanisms likely integrate the evo-devo of dentitions with upper versus lower jaws.

The p53 Family in Brain Disease

SIGNIFICANCE

The p53 family of transcription factors, including p53, p63, and p73, plays key roles in both biological and pathological processes, including cancer and neural development. Recent Advances: In recent years, a growing body of evidence has indicated that the entire p53 family is involved in the regulation of the central nervous system (CNS) functions as well as in the pathogenesis of several neurological disorders. Mechanistically, the p53 proteins control neuronal cell fate, terminal differentiation, and survival, via a complex interplay among the family members.

CRITICAL ISSUES

In this article, we discuss the involvement of the p53 family in neurobiology and in pathological conditions affecting the CNS, including neuroinflammation.

FUTURE DIRECTIONS

Understanding the molecular mechanism(s) underlying the function of the p53 family could improve our general knowledge of the pathogenesis of brain disorders and potentially pave the road for new therapeutic intervention. Antioxid. Redox Signal. 29, 1-14.

How Does p73 Cause Neuronal Defects?

The p53-family member, p73, plays a key role in the development of the central nervous system (CNS), in senescence, and in tumor formation. The role of p73 in neuronal differentiation is complex and involves several downstream pathways. Indeed, in the last few years, we have learnt that TAp73 directly or indirectly regulates several genes involved in neural biology. In particular, TAp73 is involved in the maintenance of neural stem/progenitor cell self-renewal and differentiation throughout the regulation of SOX-2, Hey-2, TRIM32 and Notch. In addition, TAp73 is also implicated in the regulation of the differentiation and function of postmitotic neurons by regulating the expression of p75NTR and GLS2 (glutamine metabolism). Further still, the regulation of miR-34a by TAp73 indicates that microRNAs can also participate in this multifunctional role of p73 in adult brain physiology. However, contradictory results still exist in the relationship between p73 and brain disorders, and this remains an important area for further investigation.

Towards unraveling the human tooth transcriptome: the dentome

The goal of the study was to characterize the transcriptome profiles of human ameloblasts and odontoblasts, evaluate molecular pathways and advance our knowledge of the human "dentome". Laser capture microdissection was used to isolate odontoblasts and ameloblasts from human tooth buds (15-20week gestational age) from 4 fetuses. RNA was examined using Agilent 41k whole genome arrays at 2 different stages of enamel formation, presecretory and secretory. Probe detection was considered against the array negative control to control for background noise. Differential expression was examined using Significance Analysis of Microarrays (SAM) 4.0 between different cell types and developmental stages with a false discovery rate of 20%. Pathway analysis was conducted using Ingenuity Pathway Analysis software. We found that during primary tooth formation, odontoblasts expressed 14,802 genes, presecretory ameloblasts 15,179 genes and secretory ameloblasts 14,526 genes. Genes known to be active during tooth development for each cell type (eg COL1A1, AMELX) were shown to be expressed by our approach. Exploring further into the list of differentially expressed genes between the motile odontoblasts and non-motile presecretory ameloblasts we found several genes of interest that could be involved in cell movement (FN1, LUM, ASTN1). Furthermore, our analysis indicated that the Phospholipase C and ERK5 pathways, that are important for cell movement, were activated in the motile odontoblasts. In addition our pathway analysis identified WNT3A and TGFB1 as important upstream contributors. Recent studies implicate these genes in the development of Schimke immuno-osseous dysplasia. The utility of laser capture microdissection can be a valuable tool in the examination of specific tissues or cell populations present in human tooth buds. Advancing our knowledge of the human dentome and related molecular pathways provides new insights into the complex mechanisms regulating odontogenesis and biomineralization. This knowledge could prove useful in future studies of odontogenic related pathologies.

Gene Expression Profiling during Murine Tooth Development

The aim of this study was to describe the expression of genes, including ameloblastin (Ambn), amelogenin X chromosome (Amelx), and enamelin (Enam) during early (pre-secretory) tooth development. The expression of these genes has predominantly been studied at post-secretory stages. Deoxyoligonucleotide microarrays were used to study gene expression during development of the murine first molar tooth germ at 24 h intervals, starting at the 11th embryonic day (E11.5), and up to the 7th day after birth (P7). The profile search function of Spotfire software was used to select genes with similar expression profile as the enamel genes (Ambn, Amelx, and Enam). Microarray results where validated using real-time reverse transcription-polymerase chain reaction (real-time RT-PCR), and translated proteins identified by Western-blotting. In situ localization of the Ambn, Amelx, and Enam mRNAs were monitored from E12.5 to E17.5 using deoxyoligonucleotide probes. Bioinformatics analysis was used to associate biological functions with differentially expressed (DE; p ≤ 0.05) genes. Microarray results showed a total of 4362 genes including Ambn, Amelx, and Enam to be significant DE throughout the time-course. The expression of the three enamel genes was low at pre-natal stages (E11.5–P0) increasing after birth (P1–P7). Profile search lead to isolation of 87 genes with significantly similar expression to the three enamel proteins. These mRNAs were expressed in dental epithelium and epithelium derived cells. Although expression of Ambn, Amelx, and Enam were lower during early tooth development compared to secretory stages enamel proteins were detectable by Western-blotting. Bioinformatic analysis associated the 87 genes with multiple biological functions. Around 35 genes were associated with 15 transcription factors.

N-methyl-N-nitrosourea-induced changes in epithelial rests of Malassez and the development of odontomas in rats

Morphological changes in the epithelial rests of Malassez (ERM) and the development of odontogenic tumors in the molars of female Lewis rats treated at 4 weeks of age with a single intraperitoneal injection of 50 mg/kg of N-methyl-N-nitrosourea (MNU) were examined at 12, 18 and 30 weeks of age. Following MNU exposure, the total number and average area of ERM in the cervical and furcational regions of the first, second and third molars of the mandible and maxilla were compared with age-matched control animals. The number of ERM at each time point was significantly greater in the MNU-treated group compared to the control group, but there was no time-dependent increase in the number of ERM in either group. The area of ERM was significantly larger in the MNU-treated group compared to the control group at each time point, and it increased in a time-dependent manner in the MNU-treated group. No increases in the number or area of ERM were observed in the control group. At 30 weeks of age, 23% of the MNU-treated rats had developed odontomas (complex type) in the molar region as well as in the incisor region. Immunohistochemically, the expression of tyrosine receptor kinase A (TrkA) and cytokeratin 14 (CK14) decreased, whereas p63 expression remained high during ERM enlargement. In tumors, ameloblast-like cells were positive for amelogenin, TrkA and CK14 but negative for p63, whereas odontoblast-like cells were negative for all antigens examined. In conclusion, a single intraperitoneal injection of MNU caused the development of odontomas in the molar region; these tumors were possibly derived from ERM.