Energy metabolic shift contributes to the phenotype modulation of maturation stage ameloblasts

Role of glucose metabolism in amelogenesis

BACKGROUND

Cell energy metabolism plays a pivotal role in organ development and function by regulating cell behavior in pathophysiological conditions. Glucose metabolism is the central cascade for obtaining energy in mammalian cells, and cells alter the glucose metabolic pathway depending on intra- and extracellular environments. Therefore, glucose metabolism is closely associated with cell differentiation stages, and cell energy metabolism plays a vital role not only in energy production but also in cell fate regulation in organogenesis.

HIGHLIGHT

During enamel formation, the timing of the expression of passive and active glucose transporters, glycogen synthesis, and glycogen degradation is strictly regulated according to the energy demand of ameloblast-lineage cells. These glucose metabolic reactions are particularly active in the maturation stage of ameloblasts. Furthermore, autophagy, a key regulator of cellular energy homeostasis that modulates glucose metabolism, occurs during both the secretory and maturation stages of ameloblasts. Disruption of glucose metabolism cascade and autophagy induces enamel hypoplasia, as demonstrated in both in vitro and in vivo models.

CONCLUSION

Adequate energy supply via glucose metabolism is essential for enamel matrix secretion and maturation. A thorough understanding of the precise regulation of energy metabolism in amelogenesis facilitates comprehension of the normal enamel formation process and pathological conditions affecting it. This review summarizes glucose metabolic processes during amelogenesis, focusing on glucose uptake, glycogenesis, and glycogenolysis.

Roles of calcium in ameloblasts during tooth development: A scoping review

Objectives

Calcium ions (Ca2+) play crucial role in tooth development, particularly in maintaining enamel density during amelogenesis. Ameloblasts require specific proteins such as amelogenin, ameloblastin, enamelin, kallikrein, and collagen for enamel growth. Recent research has highlighted the importance of calcium and fluoride ions, as well as the TRPM7, STIM, and SOCE pathways, in regulating various stages of enamel formation. This review synthesizes current knowledge, focusing on preclinical data elucidating the molecular mechanisms of calcium transport in ameloblasts, during normal tooth development and in response to external stimuli.

Methods

This scoping review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The literature search, conducted in December 2023, spanned multiple databases including PubMed (8.363 records), Google Scholar (5.630 records), and Science Direct (21.810 records). The primary aim was to examine the influence of calcium ion regulation on ameloblast development, with a focus on preclinical studies.

Results

After an initial screening of 396 titles and abstracts, 11 full-text articles (four in vitro studies and seven animal studies) met the inclusion and exclusion criteria. The studies, assessed for quality using the CAMRADES tool, ranged from low to moderate. Calcium deficiency, nutritional supplements, fluoride exposure, TRPM7, STIM proteins, and the SOCE pathway were found to influence amelogenesis.

Conclusion

Calcium transport mechanisms play a critical role in enamel formation, with factors such as TRPM7, Kir 4.2, CRAC channels, and the SOCE pathway supporting enamel mineralization, while disruptions like hypoxia, fluoride exposure, and circadian imbalances negatively impact amelogenesis. Understanding the interplay between calcium, environmental, and nutritional factors provides valuable insights into ameloblast function and offers potential avenues for improving enamel quality and addressing defects.

The circadian clock in enamel development

Circadian rhythms are self-sustaining oscillations within biological systems that play key roles in a diverse multitude of physiological processes. The circadian clock mechanisms in brain and peripheral tissues can oscillate independently or be synchronized/disrupted by external stimuli. Dental enamel is a type of mineralized tissue that forms the exterior surface of the tooth crown. Incremental Retzius lines are readily observable microstructures of mature tooth enamel that indicate the regulation of amelogenesis by circadian rhythms. Teeth enamel is formed by enamel-forming cells known as ameloblasts, which are regulated and orchestrated by the circadian clock during amelogenesis. This review will first examine the key roles of the circadian clock in regulating ameloblasts and amelogenesis. Several physiological processes are involved, including gene expression, cell morphology, metabolic changes, matrix deposition, ion transportation, and mineralization. Next, the potential detrimental effects of circadian rhythm disruption on enamel formation are discussed. Circadian rhythm disruption can directly lead to Enamel Hypoplasia, which might also be a potential causative mechanism of amelogenesis imperfecta. Finally, future research trajectory in this field is extrapolated. It is hoped that this review will inspire more intensive research efforts and provide relevant cues in formulating novel therapeutic strategies for preventing tooth enamel developmental abnormalities.

Loss of Autophagy Disrupts Stemness of Ameloblast-Lineage Cells in Aging

Autophagy is one of the intracellular degradation pathways and maintains cellular homeostasis, regulating the stress response, cell proliferation, and signal transduction. To elucidate the role of autophagy in the maintenance of dental epithelial stem cells and the subsequent enamel formation, we analyzed autophagy-deficient mice in epithelial cells (Atg7f/f;KRT14-Cre mice), focusing on the influence of aging and stress environments. We also performed in vitro cell and organ culture experiments with an autophagy inhibitor. In young Atg7f/f;KRT14-Cre mice, morphological change was not obvious in maxillary incisors, except for the remarkable cell death in the stratum intermedium of the transitional stage. However, under stress conditions of hyperglycemia, the incisor color changed to white in diabetes Atg7f/f;KRT14-Cre mice. Regarding dental epithelial stem cells, the shape of the apical bud region of the incisor became irregular with age, and odontoma was formed in aged Atg7f/f;KRT14-Cre mice. In addition, the shape of apical bud culture cells of Atg7f/f;KRT14-Cre mice became irregular and enlarged atypically, with epigenetic changes during culture, suggesting that autophagy deficiency may induce tumorigenesis in dental epithelial cells. The epigenetic change and upregulation of p21 expression were induced by autophagy inhibition in vivo and in vitro. These findings suggest that autophagy is important for the regulation of stem cell maintenance, proliferation, and differentiation of ameloblast-lineage cells, and an autophagy disorder may induce tumorigenesis in odontogenic epithelial cells.