Ras Signaling Regulates Stem Cells and Amelogenesis in the Mouse Incisor

The Potential Reversible Transition between Stem Cells and Transient-Amplifying Cells: The Limbal Epithelial Stem Cell Perspective

Stem cells (SCs) undergo asymmetric division, producing transit-amplifying cells (TACs) with increased proliferative potential that move into tissues and ultimately differentiate into a specialized cell type. Thus, TACs represent an intermediary state between stem cells and differentiated cells. In the cornea, a population of stem cells resides in the limbal region, named the limbal epithelial stem cells (LESCs). As LESCs proliferate, they generate TACs that move centripetally into the cornea and differentiate into corneal epithelial cells. Upon limbal injury, research suggests a population of progenitor-like cells that exists within the cornea can move centrifugally into the limbus, where they dedifferentiate into LESCs. Herein, we summarize recent advances made in understanding the mechanism that governs the differentiation of LESCs into TACs, and thereafter, into corneal epithelial cells. We also outline the evidence in support of the existence of progenitor-like cells in the cornea and whether TACs could represent a population of cells with progenitor-like capabilities within the cornea. Furthermore, to gain further insights into the dynamics of TACs in the cornea, we outline the most recent findings in other organ systems that support the hypothesis that TACs can dedifferentiate into SCs.

AKT from dental epithelium to papilla promotes odontoblast differentiation

Epithelial-mesenchymal interactions occur during tooth development. The dental epithelium (DE) is regarded as the signal center that regulates tooth morphology. However, the mechanism by which DE regulates the differentiation of mesenchyme-derived dental papilla (DP) into odontoblasts remains unclear. Using miniature pigs as a model, we analyzed the expression profiles of the DE and DP during odontoblast differentiation using high-throughput RNA sequencing. The phosphatidylinositol-3-kinase (PI3K)/AKT pathway is one of the most enriched pathways in both DE and DP. The PI3K/AKT pathway was first activated in the inner enamel epithelium but not in the DP on embryonic day 50. This pathway was then activated in the odontoblast layer on embryonic day 60. We showed that AKT activation promoted odontoblast differentiation of DP cells. We further demonstrated that activation of PI3K/AKT signaling in the DE effectively increased the expression levels of AKT and dentin sialophosphoprotein in DP cells. Additionally, we found that DE cells secreted collagen type IV alpha 6 chain (COL4A6) downstream of epithelial AKT signaling to positively regulate mesenchymal AKT levels. Therefore, our data suggest that PI3K/AKT signaling from the DE to the DP promotes odontoblast differentiation via COL4A6 secretion.

Mediator1 involved in functional integration of Smad3 and Notch1 promoting enamel mineralization

Enamel hypoplasia is a tooth development defection due to the disruption of enamel matrix mineralization, manifesting as chalky white phenotype. Multiple genes may be involved in this tooth agenesis. It has been proved that ablation of coactivator Mediator1 (Med1) switches the cell fate of dental epithelia, resulting in abnormal tooth development via Notch1 signaling. Smad3 (-/-) mice displays the similar chalky white incisors. However, the expression of Smad3 in Med1 ablation mice and the impact of Med1 on functional integration between Smad3 and Notch1 remains unclear. Cre-loxP-based C57/BL6 mice with epithelial-specific Med1 knockout (Med1 KO) backgrounds were generated. Mandibles and dental epithelial stem cells (DE-SCs) from incisors cervical loop (CL) were isolated from wild-type (CON) mice and Med1 KO mice. Transcriptome sequencing was used to analyze the differences of CL tissue between KO and CON mice. The results revealed the enrichment of TGF-β signaling pathway. qRT-PCR and western blot were performed to show the gene and protein expression of Smad3, pSmad3, Notch1 and NICD, the key regulators of TGF-β and Notch1 signaling pathway. Expression of Notch1 and Smad3 was confirmed to be down-regulated in Med1 KO cells. Using activators of Smad3 and Notch1 on Med1 KO cells, both pSmad3 and NICD were rescued. Moreover, adding inhibitors and activators of Smad3 and Notch1 to cells of CON groups respectively, the protein expressions of Smad3, pSmad3, Notch1 and NICD were synergistically affected. In summary, Med1 participates in the functional integration of Smad3 and Notch1, thus promoting enamel mineralization.

The synergistic effects of TGF-β1 and RUNX2 on enamel mineralization through regulating ODAPH expression during the maturation stage

Transforming growth factor β1 (TGF-β1) and Runt-related transcription factor 2 (RUNX2) are critical factors promoting enamel development and maturation. Our previous studies reported that absence of TGF-β1 or RUNX2 resulted in abnormal secretion and absorption of enamel matrix proteins. However, the mechanism remained enigmatic. In this study, TGF-β1^-/-Runx2^-/- and TGF-β1^+/-Runx2^+/- mice were successfully generated to clarify the relationship between TGF-β1 and RUNX2 during amelogenesis. Lower mineralization was observed in TGF-β1^-/-Runx2^-/- and TGF-β1^+/-Runx2^+/- mice than single gene deficient mice. Micro-computed tomography (μCT) revealed a lower ratio of enamel to dentin density in TGF-β1^-/-Runx2^-/- mice. Although μCT elucidated a relatively constant enamel thickness, variation was identified by scanning electron microscopy, which revealed that TGF-β1^-/-Runx2^-/- mice were more vulnerable to acid etching with lower degree of enamel mineralization. Furthermore, the double gene knock-out mice exhibited more serious enamel dysplasia than the single gene deficient mice. Hematoxylin-eosin staining revealed abnormalities in ameloblast morphology and arrangement in TGF-β1^-/-Runx2^-/- mice, which was accompanied by the absence of atypical basal lamina (BL) and the ectopic of enamel matrix. Odontogenesis-associated phosphoprotein (ODAPH) has been identified as a component of an atypical BL. The protein and mRNA expression of ODAPH were down-regulated. In summary, TGF-β1 and RUNX2 might synergistically regulate enamel mineralization through the downstream target gene Odaph. However, the specific mechanism by which TGF-β1 and RUNX2 promote mineralization remains to be further studied.

Activated Epithelial FGF8 Signaling Induces Fused Supernumerary Incisors

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

Multiple versus solitary giant cell lesions of the jaw: Similar or distinct entities?

The majority of giant cell lesions of the jaw present as a solitary focus of disease in bones of the maxillofacial skeleton. Less frequently they occur as multifocal lesions. This raises the clinical dilemma if these should be considered distinct entities and therefore each need a specific therapeutic approach. Solitary giant cell lesions of the jaw present with a great diversity of symptoms. Recent molecular analysis revealed that these are associated with somatic gain-of-function mutations in KRAS, FGFR1 or TRPV4 in a large component of the mononuclear stromal cells which all act on the RAS/MAPK pathway. For multifocal lesions, a small group of neoplastic multifocal giant cell lesions of the jaw remain after ruling out hyperparathyroidism. Strikingly, most of these patients are diagnosed with jaw lesions before the age of 20 years, thus before the completion of dental and jaw development. These multifocal lesions are often accompanied by a diagnosis or strong clinical suspicion of a syndrome. Many of the frequently reported syndromes belong to the so-called RASopathies, with germline or mosaic mutations leading to downstream upregulation of the RAS/MAPK pathway. The other frequently reported syndrome is cherubism, with gain-of-function mutations in the SH3BP2 gene leading through assumed and unknown signaling to an autoinflammatory bone disorder with hyperactive osteoclasts and defective osteoblastogenesis. Based on this extensive literature review, a RAS/MAPK pathway activation is hypothesized in all giant cell lesions of the jaw. The different interaction between and contribution of deregulated signaling in individual cell lineages and crosstalk with other pathways among the different germline- and non-germline-based alterations causing giant cell lesions of the jaw can be explanatory for the characteristic clinical features. As such, this might also aid in the understanding of the age-dependent symptomatology of syndrome associated giant cell lesions of the jaw; hopefully guiding ideal timing when installing treatment strategies in the future.

Altered miRNA expression profiling in enamel organ of fluoride affected rat embryos

Evidence has shown that miRNAs could play a role in dental fluorosis, but there is no study has investigated the global expression miRNA profiles of fluoride-exposed enamel organ. In this study, we analysed the differentially expressed (DE) miRNAs between fluoride-treated and control enamel organ for the first time and found several candidate miRNAs and signaling pathways worthy of further research. Thirty Wistar rats were randomly distributed into three groups and exposed to drinking water with different fluoride contents for 10 weeks and during the gestation. The three groups were a control group (distilled water), medium fluoride group (75 mg/L NaF), and high fluoride group (150 mg/L NaF). On the embryonic day 19.5, the mandible was dissected for histological analysis, and the enamel organ of the mandibular first molar tooth germ was collected for miRNA sequencing (miRNA-seq) and quantitative real-time PCR analysis (qRT-PCR). Typical dental fluorosis was observed in the incisors of the prepregnant rats. In addition to the disorganized structure of enamel organ cells, 39 DE miRNAs were identified in the fluoride groups compared with the control group, and good agreement between the miRNA-seq data and qRT-PCR data was found. The functional annotation of the target genes of 39 DE miRNAs showed significant enrichment in metabolic process, cell differentiation, calcium signaling pathway, and mitogen-activated protein kinase(MAPK) signaling pathway terms. This study provides a theoretical reference for an extensive understanding of the mechanism of fluorosis and potential valuable miRNAs as therapeutic targets in fluorosis.

Role and application of stem cells in dental regeneration: A comprehensive overview

Recently, a growing attention has been observed toward potential advantages of stem cell (SC)-based therapies in regenerative treatments. Mesenchymal stem/stromal cells (MSCs) are now considered excellent candidates for tissue replacement therapies and tissue engineering. Autologous MSCs importantly contribute to the state-of-the-art clinical strategies for SC-based alveolar bone regeneration. The donor cells and immune cells play a prominent role in determining the clinical success of MSCs therapy. In line with the promising future that stem cell therapy has shown for tissue engineering applications, dental stem cells have also attracted the attention of the relevant researchers in recent years. The current literature review aims to survey the variety and extension of SC-application in tissue-regenerative dentistry. In this regard, the relevant English written literature was searched using keywords: "tissue engineering", "stem cells", "dental stem cells", and "dentistry strategies". According to the available database, SCs application has become increasingly widespread because of its accessibility, plasticity, and high proliferative ability. Among the growing recognized niches and tissues containing higher SCs, dental tissues are evidenced to be rich sources of MSCs. According to the literature, dental SCs are mostly present in the dental pulp, periodontal ligament, and dental follicle tissues. In this regard, the present review has described the recent findings on the potential of dental stem cells to be used in tissue regeneration.

Long noncoding RNA expression profiles in intermittent parathyroid hormone induced cementogenesis

The aim of this study was to explore the involvement of long noncoding RNAs (lncRNAs) during intermittent parathyroid hormone (PTH) induced cementogenesis. Expression profiles of lncRNAs and mRNAs were obtained using high-throughput microarray. Gene Ontology enrichment analysis, Kyoto Encyclopedia of Genes and Genomes pathway analysis, and coding-noncoding gene coexpression networks construction were performed. We identified 190 lncRNAs and 135 mRNAs that were differentially expressed during intermittent PTH-induced cementogenesis. In this process, the Wnt signaling pathway was negatively regulated, and eight lncRNAs were identified as possible core regulators of Wnt signaling. Based on the results of microarrray analysis, we further verified the repressed expression of Wnt signaling crucial components β-catenin, APC and Axin2. Above all, we speculated that lncRNAs may play important roles in PTH-induced cementogenesis via the negative regulation of Wnt pathway.

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

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

Molecular and cellular mechanisms of tooth development, homeostasis and repair

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

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

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