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Objectives: Junctional epithelium (JE) connects the tooth surface and gingival epithelium and adheres directly to the tooth enamel. JE plays an important role as a barrier preventing the invasion of exogenous bacteria and substances. However, the cellular characteristics of this epithelium have not been adequately described, because no useful in vitro experimental model exists for JE., Methods: We generated a novel JE cell line, mHAT-JE01, using naturally immortalized dental epithelium derived from incisor labial cervical cells and by selecting cells that adhered to apatite. mHAT-JE01 was characterized by immunohistochemistry and quantitative reverse transcription-polymerase chain reaction and compared with the gingival epithelial cell line, mOE-PE01., Results: The mHAT-JE01 cells had a higher capacity for producing JE-specific markers than oral mucous epithelial cells. In addition, the presence of lipopolysaccharides from Porphyromonas gingivalis downregulated the expression of JE protein markers in mHAT-JE01 cells., Conclusions: This cell line is stable and presents the opportunity to characterize JE efficiently, which is essential for the prevention and treatment of periodontal disease., (Copyright © 2023 Japanese Association for Oral Biology. Published by Elsevier B.V. All rights reserved.)

Background and Objectives: Hertwig's epithelial root sheath (HERS) plays a role in root dentin formation. It produces the epithelial rests of Malassez (ERM) for the induction of periodontal tissue development during root formation. Although ERM is thought to be caused by epithelial-mesenchymal transition (EMT), the mechanism by which HERS is maintained as epithelium is unknown. Here, we aimed to elucidate the molecular mechanisms regulating the relationship between HERS maintenance and ERM development., Methods: To understand the relationship between HERS and ERM development during root formation, we observed the developing molar root using cytokeratin14 (CK14) Cre/tdTomato mice via stereomicroscopy. The relationship between semaphorin and transforming growth factor (TGF) signaling in the maintenance of HERS and ERM development was examined using CK14cre/R26-tdTomato mice and a HERS cell line., Results: tdTomato-positive cells were observed on HERS and the migrating cells from HERS. The migrating cells showed reduced E-cadherin expression. In contrast, HERS cells expressed semaphorin receptors and active RhoA. Semaphorin signaling was associated with RhoA activation and cell-cell adhesion, while TGF-β induced decreased E-cadherin and active RhoA expression, and consequently enhanced cell migration., Conclusion: HERS induces root formation by controlling epithelial maintenance and EMT through the opposing effects of semaphorin and TGF-β signaling., (© 2022 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Maturation stage ameloblasts (M-ABs) are responsible for terminal enamel mineralization in teeth and undergo characteristic cyclic changes in both morphology and function between ruffle-ended ameloblasts (RA) and smooth-ended ameloblasts (SA). Energy metabolism has recently emerged as a potential regulator of cell differentiation and fate decisions; however, its implication in M-ABs remains unclear. To elucidate the relationship between M-ABs and energy metabolism, we examined the expression pattern of energy metabolic enzymes in M-ABs of mouse incisors. Further, using the HAT7 cell line with M-AB characteristics, we designed experiments to induce an energy metabolic shift by changes in oxygen concentration. We revealed that RA preferentially utilizes oxidative phosphorylation, whereas SA depends on glycolysis-dominant energy metabolism in mouse incisors. In HAT7 cells, hypoxia induced an energy metabolic shift toward a more glycolytic-dominant state, and the energy metabolic shift reduced alkaline phosphatase (ALP) activity and calcium transport and deposition with a change in calcium-related gene expression, implying a phenotype shift from RA to SA. Taken together, these results indicate that the energy metabolic state is an important determinant of the RA/SA phenotype in M-ABs. This study sheds light on the biological significance of energy metabolism in governing M-ABs, providing a novel molecular basis for understanding enamel mineralization and elucidating the pathogenesis of enamel hypomineralization., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Arai, Inaba, Ikezaki, Kumakami-Sakano, Azumane, Ohshima, Morikawa, Harada and Otsu.)

Objectives: Lysophosphatidic acid (LPA) is a potent bioactive phospholipid that exerts various functions upon binding to six known G protein-coupled receptors (LPA 1-6 ); however; its role in a tooth remains unclear. This study aimed to explore the impact of the LPA/LPA receptor 6 (LPA 6 )/RhoA signaling axis on maturation stage ameloblasts (M-ABs), which are responsible for enamel mineralization., Methods: The expression of LPA 6 and LPA-producing synthetic enzymes during ameloblast differentiation was explored through immunobiological analysis of mouse incisors and molars. To elucidate the role of LPA 6 in ameloblasts, incisors of LPA 6 KO mice were analyzed. In vitro experiments using ameloblast cell lines were performed to validate the function of LPA-LPA 6 -RhoA signaling in ameloblasts., Results: LPA 6 and LPA-producing enzymes were strongly expressed in M-ABs. In LPA 6 knockout mice, M-ABs exhibited abnormal morphology with the loss of cell polarity, and an abnormal enamel epithelium containing cyst-like structures was formed. Moreover, the expression of E-cadherin and zonula occludens-1 (ZO-1) significantly decreased in M-ABs. In vitro experiments demonstrated that LPA upregulated the expression of E-cadherin, ZO-1, and filamentous actin (F-actin) at the cellular membrane, whereas LPA 6 knockdown decreased their expression and changed cell morphology. Furthermore, we showed that RhoA signaling mediates LPA-LPA 6 -induced junctional complexes., Conclusions: This study demonstrated that LPA-LPA 6 -RhoA signaling is essential for establishing proper cell morphology and polarity, via cell-cell junction and actin cytoskeleton expression and stability, of M-ABs. These results highlight the biological significance of bioactive lipids in a tooth, providing a novel molecular regulatory mechanism of ameloblasts., Competing Interests: Conflicts of interest The authors declare no competing financial interests., (Copyright © 2022 Japanese Association for Oral Biology. Published by Elsevier B.V. All rights reserved.)

Tooth and bone are major tissues involved in physiological calcification in the body, and they use similar molecular pathways for development, homeostasis, and regeneration. Harmine (HMN) is a natural small compound that stimulates osteoblast differentiation in vitro and in vivo. Here we examined the biological effect of HMN on the postnatal development of molar tooth roots and periodontal tissues. HMN supported the formation of tooth roots and periodontal tissues in developing tooth germs. In tooth germ organ culture, HMN promoted the elongation of Hertwig's epithelial root sheath (HERS) and stimulated cell proliferation in HERS and dental follicle-derived tissues, including dental papillae and dental follicles. HMN stimulated cell proliferation and cell movement of HERS-derived cells without mesenchymal cells in vitro and directly induced the phosphorylation of SMAD1/5/8 protein in HERS-derived cells. Our results indicated that HMN was the first natural small compound to stimulate postnatal development of tooth germs., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Teeth are composed of two domains, the enamel-covered crown and cementum-covered root. The mechanism for determining the transition from crown to root is important for understanding root anomaly diseases. Hertwig׳s epithelial root sheath (HERS) is derived from the dental epithelium and is known to drive the growth of root dentin and periodontal tissue. Some clinical cases of hypoplastic tooth root are caused by the cessation of HERS development. Understanding the mechanisms of HERS development will contribute to the study of the disease and dental regenerative medicine. However, the developmental biology of tooth root formation has not been fully studied, particularly regarding HERS formation. Here, we describe the mechanisms of HERS formation on the basis of analysis of cell dynamics using imaging and summarize how the growth factor and its receptor regulate cell behavior of the dental epithelium., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Stem cells are capable of renewing themselves through cell division and have the remarkable ability to differentiate into many different types of cells. They therefore have the potential to become a central tool in regenerative medicine. During the last decade, advances in tissue engineering and stem cell-based tooth regeneration have provided realistic and attractive means of replacing lost or damaged teeth. Investigation of embryonic and adult (tissue) stem cells as potential cell sources for tooth regeneration has led to many promising results. However, technical and ethical issues have hindered the availability of these cells for clinical application. The recent discovery of induced pluripotent stem (iPS) cells has provided the possibility to revolutionize the field of regenerative medicine (dentistry) by offering the option of autologous transplantation. In this article, we review the current progress in the field of stem cell-based tooth regeneration and discuss the possibility of using iPS cells for this purpose.