Visualization of junctional epithelial cell replacement by oral gingival epithelial cells over a life time and after gingivectomy

Matrix Metalloproteinases in the Periodontium-Vital in Tissue Turnover and Unfortunate in Periodontitis

The extracellular matrix (ECM) is a complex non-cellular three-dimensional macromolecular network present within all tissues and organs, forming the foundation on which cells sit, and composed of proteins (such as collagen), glycosaminoglycans, proteoglycans, minerals, and water. The ECM provides a fundamental framework for the cellular constituents of tissue and biochemical support to surrounding cells. The ECM is a highly dynamic structure that is constantly being remodeled. Matrix metalloproteinases (MMPs) are among the most important proteolytic enzymes of the ECM and are capable of degrading all ECM molecules. MMPs play a relevant role in physiological as well as pathological processes; MMPs participate in embryogenesis, morphogenesis, wound healing, and tissue remodeling, and therefore, their impaired activity may result in several problems. MMP activity is also associated with chronic inflammation, tissue breakdown, fibrosis, and cancer invasion and metastasis. The periodontium is a unique anatomical site, composed of a variety of connective tissues, created by the ECM. During periodontitis, a chronic inflammation affecting the periodontium, increased presence and activity of MMPs is observed, resulting in irreversible losses of periodontal tissues. MMP expression and activity may be controlled in various ways, one of which is the inhibition of their activity by an endogenous group of tissue inhibitors of metalloproteinases (TIMPs), as well as reversion-inducing cysteine-rich protein with Kazal motifs (RECK).

Organoids from mouse molar and incisor as new tools to study tooth-specific biology and development

Organoid models provide powerful tools to study tissue biology and development in a dish. Presently, organoids have not yet been developed from mouse tooth. Here, we established tooth organoids (TOs) from early-postnatal mouse molar and incisor, which are long-term expandable, express dental epithelium stem cell (DESC) markers, and recapitulate key properties of the dental epithelium in a tooth-type-specific manner. TOs display in vitro differentiation capacity toward ameloblast-resembling cells, even more pronounced in assembloids in which dental mesenchymal (pulp) stem cells are combined with the organoid DESCs. Single-cell transcriptomics supports this developmental potential and reveals co-differentiation into junctional epithelium- and odontoblast-/cementoblast-like cells in the assembloids. Finally, TOs survive and show ameloblast-resembling differentiation also in vivo. The developed organoid models provide new tools to study mouse tooth-type-specific biology and development and gain deeper molecular and functional insights that may eventually help to achieve future human biological tooth repair and replacement.

Establishment of inclusive single-cell transcriptome atlases from mouse and human tooth as powerful resource for dental research

Single-cell (sc) omics has become a powerful tool to unravel a tissue’s cell landscape across health and disease. In recent years, sc transcriptomic interrogation has been applied to a variety of tooth tissues of both human and mouse, which has considerably advanced our fundamental understanding of tooth biology. Now, an overarching and integrated bird’s-view of the human and mouse tooth sc transcriptomic landscape would be a powerful multi-faceted tool for dental research, enabling further decipherment of tooth biology and development through constantly progressing state-of-the-art bioinformatic methods as well as the exploration of novel hypothesis-driven research. To this aim, we re-assessed and integrated recently published scRNA-sequencing datasets of different dental tissue types (healthy and diseased) from human and mouse to establish inclusive tooth sc atlases, and applied the consolidated data map to explore its power. For mouse tooth, we identified novel candidate transcriptional regulators of the ameloblast lineage. Regarding human tooth, we provide support for a developmental connection, not advanced before, between specific epithelial compartments. Taken together, we established inclusive mouse and human tooth sc atlases as powerful tools to potentiate innovative research into tooth biology, development and disease. The maps are provided online in an accessible format for interactive exploration.

Structure of junctional epithelium is maintained by cell populations supplied from multiple stem cells

The junctional epithelium (JE) is an epithelial component that attaches directly to the tooth surface and performs the unique function of protecting against bacterial infections; its destruction causes inflammation of the periodontal tissue and loss of alveolar bone. A recent study that used the single-color lineage tracing method reported that JE is maintained by its stem cells. However, the process by which individual stem cells form the entire JE around a whole tooth remains unclear. Using a 4-color lineage tracing method, we performed a detailed examination of the dynamics of individual stem cells that constitute the entire JE. The multicolor lineage tracing method showed that single-color areas, which were derived from each cell color, replaced all the constituent JE cells 168 d after the administration of tamoxifen. The horizontal section of the first molar showed that the single-color areas in the JE expanded widely. We detected putative stem cells at the external basal layer farthest from the enamel. In this study, JE cells that were supplied from different stem cells were visualized as individual monochromatic regions, and the JE around the first molar was maintained by several JE-specific stem cells. These findings indicated that the JE consisted of several cell populations that were supplied from their multiple stem cells and could help to explore the mechanisms involved in periodontal tissue homeostasis.

Formation and regeneration of a Wnt-responsive junctional epithelium

AIM

To identify the molecular mechanisms mediating the persistent defensive functions of the self-renewing junctional epithelium (JE).

MATERIALS AND METHODS

Two strains of Wnt reporter mice, Axin2^{CreErt2 /+} ;R26R^{mTmG /+} and Axin2^{LacZ /+} , were employed, along with three clinically relevant experimental scenarios where the function of the JE is disrupted: after tooth extraction, after a partial gingivectomy, and after a complete circumferential gingivectomy.

RESULTS

Using transgenic Wnt reporter strains of mice, we established the JE is a Wnt-responsive epithelium beginning at the time of its formation and that it maintains this status into adulthood. After tooth extraction, progeny of the initial Wnt-responsive JE population directly contributed to healing and ultimately adopted an oral epithelium (OE) phenotype. In the traditional partial gingivectomy model, the JE completely regenerated and did so via progeny of the original Wnt-responsive population. However, following circumferential gingivectomy, the OE was incapable of re-establishing a functional JE.

CONCLUSIONS

A Wnt-responsive niche at the interface between tooth and oral epithelia is required for a functional JE.