Application of spontaneously immortalized odontoblast cells in tooth regeneration

Immortalized cell lines derived from dental/odontogenic tissue

Stem cells derived from dental/odontogenic tissue have the property of multiple differentiation and are prospective in tooth regenerative medicine and cellular and molecular studies. However, in the face of cellular senescence soon in vitro, the proliferation ability of the cells is limited, so studies are hindered to some extent. Fortunately, immortalization strategies are expected to solve the above issues. Cellular immortalization is that cells are immortalized by introducing oncogenes, human telomerase reverse transcriptase genes (hTERT), or miscellaneous immortalization genes to get unlimited proliferation. At present, a variety of immortalized stem cells from dental/odontogenic tissue has been successfully generated, such as dental pulp stem cells (DPSCs), periodontal ligament cells (PDLs), stem cells from human exfoliated deciduous teeth (SHEDs), dental papilla cells (DPCs), and tooth germ mesenchymal cells (TGMCs). This review summarized establishment and applications of immortalized stem cells from dental/odontogenic tissues and then discussed the advantages and challenges of immortalization.

Sinking Our Teeth in Getting Dental Stem Cells to Clinics for Bone Regeneration

Dental stem cells have been isolated from the medical waste of various dental tissues. They have been characterized by numerous markers, which are evaluated herein and differentiated into multiple cell types. They can also be used to generate cell lines and iPSCs for long-term in vitro research. Methods for utilizing these stem cells including cellular systems such as organoids or cell sheets, cell-free systems such as exosomes, and scaffold-based approaches with and without drug release concepts are reported in this review and presented with new pictures for clarification. These in vitro applications can be deployed in disease modeling and subsequent pharmaceutical research and also pave the way for tissue regeneration. The main focus herein is on the potential of dental stem cells for hard tissue regeneration, especially bone, by evaluating their potential for osteogenesis and angiogenesis, and the regulation of these two processes by growth factors and environmental stimulators. Current in vitro and in vivo publications show numerous benefits of using dental stem cells for research purposes and hard tissue regeneration. However, only a few clinical trials currently exist. The goal of this review is to pinpoint this imbalance and encourage scientists to pick up this research and proceed one step further to translation.

Intercellular signal communication among odontoblasts and trigeminal ganglion neurons via glutamate

Various stimuli to the exposed surface of dentin induce changes in the hydrodynamic force inside the dentinal tubules resulting in dentinal pain. Recent evidences indicate that mechano-sensor channels, such as the transient receptor potential channels, in odontoblasts receive these hydrodynamic forces and trigger the release of ATP to the pulpal neurons, to generate dentinal pain. A recent study, however, has shown that odontoblasts also express glutamate receptors (GluRs). This implies that cells in the dental pulp tissue have the ability to release glutamate, which acts as a functional intercellular mediator to establish inter-odontoblast and odontoblast-trigeminal ganglion (TG) neuron signal communication. To investigate the intercellular signal communication, we applied mechanical stimulation to odontoblasts and measured the intracellular free Ca²⁺ concentration ([Ca²⁺]_i). During mechanical stimulation in the presence of extracellular Ca²⁺, we observed a transient [Ca²⁺]_i increase not only in single stimulated odontoblasts, but also in adjacent odontoblasts. We could not observe these responses in the absence of extracellular Ca²⁺. [Ca²⁺]_i increases in the neighboring odontoblasts during mechanical stimulation of single odontoblasts were inhibited by antagonists of metabotropic glutamate receptors (mGluRs) as well as glutamate-permeable anion channels. In the odontoblast-TG neuron coculture, we observed an increase in [Ca²⁺]_i in the stimulated odontoblasts and TG neurons, in response to direct mechanical stimulation of single odontoblasts. These [Ca²⁺]_i increases in the neighboring TG neurons were inhibited by antagonists for mGluRs. The [Ca²⁺]_i increases in the stimulated odontoblasts were also inhibited by mGluRs antagonists. We further confirmed that the odontoblasts express group I, II, and III mGluRs. However, we could not record any currents evoked from odontoblasts near the mechanically stimulated odontoblast, with or without extracellular Mg²⁺, indicating that N-methyl-d-aspartic acid receptor does not contribute to inter-odontoblast signal communication. The results suggest that a mechanically stimulated odontoblast is capable of releasing glutamate into the extracellular space via glutamate-permeable anion channels. The released glutamate activates mGluRs on the odontoblasts in an autocrine/paracrine manner, forming an inter-odontoblasts communication, which drives dentin formation via odontoblast-odontoblast signal communication. Glutamate and mGluRs also mediate neurotransmission between the odontoblasts and neurons in the dental pulp to modulate sensory signal transmission for dentinal sensitivity.

Tooth Organ Bioengineering: Cell Sources and Innovative Approaches

Various treatment approaches for restoring missing teeth are being utilized nowadays by using artificial dental crowns/bridges or the use of dental implants. All aforementioned restorative modalities are considered to be the conventional way of treating such cases. Although these artificial therapies are commonly used for tooth loss rehabilitation, they are still less conservative, show less biocompatibility and fail to restore the natural biological and physiological function. Adding to that, they are considered to be costly due to the risk of failure and they also require regular maintenance. Regenerative dentistry is currently considered a novel therapeutic concept with high potential for a complete recovery of the natural function and esthetics of teeth. Biological-cell based dental therapies would involve replacement of teeth by using stem cells that will ultimately grow a bioengineered tooth, thereby restoring both the biological and physiological functions of the natural tooth, and are considered to be the ultimate goal in regenerative dentistry. In this review, various stem cell-based therapeutic approaches for tooth organ bioengineering will be discussed.

Three-dimensional Micro-culture System for Tooth Tissue Engineering

The arrangement of cells within a tissue plays an essential role in organogenesis, including tooth development. Progress is being made to regenerate teeth by reassociating dissociated embryonic dental cells and implanting them in vivo. In the present study, we tested the hanging drop method to study mixed epithelial-mesenchymal cell reorganization in a liquid instead of semisolid medium to see whether it could lead to tooth histogenesis and organogenesis. This method allowed the control of the proportion and number of cells to be used, and the forming microtissues showed homogeneous size. The liquid environment favored cell migrations as compared with collagen gels. Three protocols were compared. The one that sequentially combined the hanging drop and semisolid medium cultures prior to in vivo implantation gave the best results. Indeed, after implantation, teeth developed, showing a well-formed crown, mineralization of dentin and enamel, and the initiation of root formation. Vascularization and the cellular heterogeneity in the mesenchyme were similar to what was observed in developing molars. Finally, after coimplantation with a trigeminal ganglion, the dental mesenchyme, including the odontoblast layer, became innervated. The real advantage of this technique is the small number of cells required to make a tooth. This experimental model can be employed to study the development, physiology, metabolism, or toxicology in forming teeth and test other cell sources.

Immortalized mouse dental papilla mesenchymal cells preserve odontoblastic phenotype and respond to bone morphogenetic protein 2

Odontogenesis is the result of the reciprocal interactions between epithelial–mesenchymal cells leading to terminally differentiated odontoblasts. This process from dental papilla mesenchymal cells to odontoblasts is regulated by a complex signaling pathway. When isolated from the developing tooth germs, odontoblasts quickly lose their potential to maintain the odontoblast-specific phenotype. Therefore, generation of an odontoblast-like cell line would be a good surrogate model for studying the dental mesenchymal cell differentiation into odontoblasts and the molecular events of dentin formation. In this study, immortalized dental papilla mesenchymal cell lines were generated from the first mouse mandibular molars at postnatal day 3 using pSV40. These transformed cells were characterized by RT-PCR, immunohistochemistry, Western blot, and analyzed for alkaline phosphatase activity and mineralization nodule formation. One of these immortalized cell lines, iMDP-3, displayed a high proliferation rate, but retained the genotypic and phenotypic characteristics similar to primary cells as determined by expression of tooth-specific markers and demonstrated the ability to differentiate and form mineralized nodules. Furthermore, iMDP-3 cells had high transfection efficiency as well as were inducible and responded to BMP2 stimulation. We conclude that the establishment of the stable murine dental papilla mesenchymal cell line might be used for studying the mechanisms of dental cell differentiation and dentin formation.

Successful reconstruction of tooth germ with cell lines requires coordinated gene expressions from the initiation stage

Tooth morphogenesis is carried out by a series of reciprocal interactions between the epithelium and mesenchyme in embryonic germs. Previously clonal dental epithelial cell (epithelium of molar tooth germ (emtg)) lines were established from an embryonic germ. They were odontogenic when combined with a dental mesenchymal tissue, although the odontogenesis was quantitatively imperfect. To improve the microenvironment in the germs, freshly isolated dental epithelial cells were mixed with cells of lines, and germs were reconstructed in various combinations. The results demonstrated that successful tooth construction depends on the mixing ratio, the age of dental epithelial cells and the combination with cell lines. Analyses of gene expression in these germs suggest that some signal(s) from dental epithelial cells makes emtg cells competent to communicate with mesenchymal cells and the epithelial and mesenchymal compartments are able to progress odontogenesis from the initiation stage.

Tooth engineering: searching for dental mesenchymal cells sources

The implantation of cultured re-associations between embryonic dental mesenchymal cells and epithelial cells from mouse molars at embryonic day 14 (ED14) allowed making full teeth with crown, root, periodontal ligament fibers, and bone. Although representing valuable tools to set up methodologies embryonic cells are not easily available. This work thus aimed to replace the embryonic cells by dental mesenchymal cell lines or cultured expanded embryonic cells, and to test their ability to mediate tooth development in vitro when re-associated with a competent dental epithelium. Histology, immunostaining and RT-PCR allowed getting complementary sets of results. Two different immortalized cell lines from ED18 dental mesenchyme failed in mediating tooth formation. The potentialities of embryonic dental mesenchymal cells decreased from ED14 to ED16 and were lost at ED18. This is likely related to a change in the mesenchymal cell phenotype and/or populations during development. Attempts to cultivate ED14 or ED16 embryonic dental mesenchymal cells prior to re-association led to the loss of their ability to support tooth development. This was accompanied by a down-regulation of Fgf3 transcription. Supplementation of the culture medium with FGF2 allowed restoring Fgf3 expression, but not the ability of mesenchymal cells to engage in tooth formation. Altogether, these observations suggest that a competent cell population exists in the dental mesenchyme at ED14, progressively decreases during development, and cannot as such be maintained in vitro. This study evidenced the need for specific conditions to maintain the ability of dental mesenchymal cells to initiate whole tooth formation, when re-associated with an odontogenic epithelium. Efforts to improve the culture conditions will have to be combined with attempts to characterize the competent cells within the dental mesenchyme.

Cell differentiation and matrix organization in engineered teeth

Embryonic dental cells were used to check a series of criteria to be achieved for tooth engineering. Implantation of cultured cell-cell re-associations led to crown morphogenesis, epithelial histogenesis, organ vascularization, and root and periodontium development. The present work aimed to investigate the organization of predentin/dentin, enamel, and cementum which formed and mineralized after implantation. These implants were processed for histology, transmission electron microscopy, x-ray microanalysis, and electron diffraction. After two weeks of implantation, the re-associations showed gradients of differentiating odontoblasts. There were ciliated, polarized, and extended cell processes in predentin/dentin. Ameloblasts became functional. Enamel crystals showed a typical oriented arrangement in the inner and outer enamel. In the developing root, odontoblasts differentiated, cementogenesis occurred, and periodontal ligament fibroblasts interacted with the root surface and newly formed bone. The implantation of cultured dental cell re-associations allows for reproduction of complete functional differentiation at the cell, matrix, and mineral levels.

Expression, localisation and synthesis of versican by the enamel organ of developing mouse molar tooth germ: an in vivo and in vitro study

OBJECTIVE

Versican is a large, aggregating chondroitin sulphate proteoglycan. In dental tissue, versican expression occurs primarily in mesenchymal tissue but rarely in epithelial tissue. We investigated the expression, localisation and synthesis of versican in the enamel organ of the developing tooth germ.

DESIGN

To elucidate versican localisation in vivo, in situ hybridisation and immunohistochemistry were conducted in foetal ICR mice at E11.5-E18.5. Epithelium and mesenchyme from the lower first molars at E16.0 were enzymatically separated and versican mRNA expression was investigated by semi-quantitative RT-PCR. Organ culture of the separated samples combined with metabolic labelling with [(35)S], followed by gel filtration, was performed to analyse secreted proteoglycans.

RESULTS

Versican mRNA was first expressed in the thickened dental epithelium at E12.0 and continued to be expressed in the enamel organ until the bell stage. Versican immunostaining was detected in the stellate reticulum areas from the bud stage to the apposition stage. The enamel organ at E16.0 expressed versican mRNA at a level comparable to that in dental mesenchyme. Furthermore, when compared to dental mesenchyme, about 1/2-3/4 of the [(35)S]-labelled versican-like large proteoglycan was synthesised and released into tissue explants by the enamel organ.

CONCLUSIONS

The dental epithelium of developing tooth germ is able to synthesise significant amounts of versican.