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ISSN : 1226-7155(Print)
ISSN : 2287-6618(Online)
International Journal of Oral Biology Vol.46 No.2 pp.67-73
DOI : https://doi.org/10.11620/IJOB.2021.46.2.67

New conceptual approaches toward dentin regeneration using the drug repositioning strategy with Wnt signaling pathways

Eui-Seon Lee1, Tae-Young Kim1, Yam Prasad Aryal1, Kihyun Kim1, Seongsoo Byun1, Dongju Song1, Yejin Shin1, Dany Lee1, Jooheon Lee1, Gilyoung Jung1, Seunghoon Chi1, Yoolim Choi1, Youngkyun Lee1, Chang-Hyeon An2*, Jae-Young Kim1*
1Department of Biochemistry, School of Dentistry, Institute for Hard Tissue and Bio-tooth Regeneration (IHBR), Kyungpook National University, Daegu 41940, Republic of Korea
2Department of Oral and Maxillofacial Radiology, School of Dentistry, IHBR, Kyungpook National University, Daegu 41940, Republic of Korea
*Correspondence to: Chang-Hyeon An, E-mail: chan@knu.ac.kr
*Correspondence to: Jae-Young Kim, E-mail: jykim91@knu.ac.kr
January 21, 2021 March 10, 2021 March 30, 2021

Abstract


This study summarizes the recent cutting-edge approaches for dentin regeneration that still do not offer adequate solutions. Tertiary dentin is formed when odontoblasts are directly affected by various stimuli. Recent preclinical studies have reported that stimulation of the Wnt/β-catenin signaling pathway could facilitate the formation of reparative dentin and thereby aid in the structural and functional development of the tertiary dentin. A range of signaling pathways, including the Wnt/β-catenin pathway, is activated when dental tissues are damaged and the pulp is exposed. The application of small molecules for dentin regeneration has been suggested as a drug repositioning approach. This study reviews the role of Wnt signaling in tooth formation, particularly dentin formation and dentin regeneration. In addition, the application of the drug repositioning strategy to facilitate the development of new drugs for dentin regeneration has been discussed in this study.



초록


    © The Korean Academy of Oral Biology. All rights reserved.

    This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/bync/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    Structure and Function of Dentin

    Dentin is the most spacious calcified tissues among the composition of tooth structure which is complex, porous and yellowish-hued [1]. There are three types of dentin - Primary, Secondary, and Tertiary. Primary dentin constitutes the most part of the dentin mass and lies between the enamel and pulp chamber, near dentinoenamel junction. It is produced relatively high rate. The outer most layer of dentin, close to enamel is known as mantle dentin. Mantle dentin is producedby newly differentiated odontoblasts and forms a layer consistently 15- 20 micrometers wide. Unlike primary dentin, mantle dentin lacks phosphorylation which makes loosely packed collagen fibrils and less mineralized composition. Below the mantle dentin, circumpulpal dentin lies. Circumpulpal dentin forms major part of the dentin layer and is produced before the root formation is completed [2,3]. Secondary dentin starts immediately after completion of root formation and continues throughout the life. The only difference between primary and secondary dentin is the tubule profile in which primary dentin is straight and secondary dentin is curved due to the gradual restriction in the space for inward migration of odontoblasts. It causes a decrease in the size of the pulp chamber with age as it deposits matrix continuously. Tertiary dentin is formed as a reaction to external stimuli such as chemical irritants, caries, attrition or other traumatic stress. There are two types of tertiary dentin - Reactionary and Reparative. Reactionary dentin is formed from a pre-existing odontoblasts, whereas thereparative dentin is formed from newly differentiated odontoblast-like cells which are formed due to death of original odontoblasts. Tertiary dentin is the only dentin formed by odontoblasts which are directly affected by a stimulus [2]. Recently, studies about differentiation and regeneration of dentin are continuously investigated using various methods such as modulation of signaling pathway, application of small molecules and drug repositioning method [4-11].

    Wnt Signalings in Tooth Formation

    Wnt/β-catenin signaling plays an important role in morphogenesis and cellular differentiation of tissues [12]. Several studies revealed that Wnt/β-catenin signaling involves in various stages of tooth morphogenesis and differentiation in both dental epithelium and mesenchyme which was organized and listed in Table 1 [13-17]. Many studies showed Wnt/β-catenin signaling acts as a factor in stages of tooth morphogenesis [18,19]. A study by Kim et al. [20] show that Wnt/β-catenin signaling acts as a key factor in the differentiation of odontoblast in root formation. In mice with the limited expression of β-catenin, incisors and molars showed distinct abnormalities in tooth morphology. In histological analysis, the differentiation of odontoblast in the inner layer remarkedly decreased and root was not formed properly despite the extension of the Hertwig’ s epithelial root sheath that determines the size and shape of the root [21]. These results suggest that Wnt/β-catenin signaling control the differentiation of odontoblast in root formation. Recent study by Bae et al. [11] showed that odontoblastspecific disruption of Wntless (Wls), a chaperone protein that regulates Wnt sorting and secretion, mediates severe defects in formation of dentin and root elongation, resulted in thin den-tin with enlarged pulp chambers and short roots. However, the study of Lim et al. [22], showed opposite phenotype, in which the deleted Wls in odontoblasts resulted in the increase of dentin thickness and the decrease of pulp volume in incisors. The differences are likely due to arrangement of expression of the osteocalcin-Cre recombinase. In summary, Wnt/β-catenin signaling in odontoblasts involves in regulating differentiation and matrix production. During tooth development, Wnt ligands secreted from odontoblasts are required for maturation of odontoblasts, dentin deposition and root elongation.

    Recent Studies in Regenerative Endodontics

    1. Cell transplantation and homing

    The goal of regenerative endodontics is to restore the functions of the dental pulp-dentin complex. There are two approaches of pulp-dentin regeneration - cell transplantation and cell homing [23]. Cell transplantation by delivering ex vivo cultivated cells in pulp-dentin regeneration is the most studied topic among pulp-dentin regeneration. When stem/progenitor cells are transplanted, not regarding their origins, they are thought to be in aid of regeneration or repair process not only by supplying cells but also by adding up growth factors or signaling molecule [24,25]. Stem and progenitor cells in dental pulp are anticipated to replenish odontoblasts upon infection or trauma in adult hood. Cell homing compared to cell transplantation has not been explored much yet. It is defined as migration or mobilization of cells involving stem/progenitor cells to the damaged tissue and this process is induced by biological signaling molecules [26,27]. The growth factors shown by Kim et al. [28] for cell homing approaches of pulp regeneration are vascular endothelial growth factor, basic fibroblast growth factor, platelet-derived growth factor, nerve growth factor and bone morphogenetic protein-7. The induction of stem/ progenitor cells from periapical tissue around the apical area of the root leads to the cell homing [29]. In cell homing strategy, scaffolds filled with growth factors are inserted into root canals. The placement of those scaffold initiates migration, proliferation, and differentiation of endogenous stem/progenitor cells located around the root apex like periodontal ligament stem cells [30]. The use of biological signaling molecules for cell transplantation is not yet applied in clinical approaches because of its immune rejection, and potential contamination during cell manipulation. For cell homing, the elaborated hurdles for cell transplantation are minimized. However, growth factors used in cell homing approaches for pulp regeneration require the U.S. Food and Drug Administration approval for the clinical application [31-33].

    2. Pulp revascularization

    Regenerative endodontics procedures designed to replace damaged structures, including dentin and root, as well as cells of the pulp-dentin complex [34]. Revascularization is one of candidates of regenerative endodontics. Revascularization mainly focuses on development of immature tooth root with promoting of dentinal wall by deposition of hard tissue of roots [35,36]. Another approach for regenerating dentin is recombinant human proteins combined with collagen-based matrixes. The mechanism of this procedure is relevant to stimulating agents which were placed in direct contact with dental pulp, however, the induction of reparative dentin was unsuccessful because of insufficient amount of active recombinant protein; protein has relatively short half-life and faster degradation rates when the pulp is inflamed. It is found that growth/differentiation factor11 and bone sialoprotein are considered as important factor to induce reparative dentin and to stimulate differentiation of dental pulp cells into cells that can secrete extracellular matrix respectively [37,38].

    3. Tertiary dentin formation using Wnt/β-catenin signaling

    Moving on to tertiary dentin problems and facilitation, the most obvious reparative response to pulp exposure is observed by reparative dentin because it offers odontoblasts and other pulp cells as well as protects pulp from harmful stimuli [39]. Furthermore, various factors may induce formation of tertiary dentin, also called reparative dentin: occlusal attrition, trauma, carious decay, dental restoration and any external harmful stimulus. A recent preclinical study found that stimulation of Wnt/β-catenin signaling promotes formation of reparative dentin [40]. When the tissue is damaged, Wnt/β-catenin signaling is activated immediately with the addition of small molecule, Wnt agonists generates reparative dentin formation and consequently restores the lost dentin structure by producing new dentin [11,20]. These results suggest that odontoblast differentiation and Wnt signaling pathways are significant factor in consideration of dentin regeneration.

    4. Reparative dentin formation using drug repositioning

    Recently, many studies employed the drug repositioning strategy, using approved drugs into other purposes from original one for saving time and cost [31-33]. In medical fields these applications were well understood and performed for overcoming various diseases, however, in dental field these approaches are limited and not well announced so far. In this study we prepared and summarized the recent reports which successfully showed the possibilities of drug repositioning in dentin regeneration.

    Bortezomib is introduced as the drug for dentin regeneration. Bortezomib is known to inhibit nuclear factor kappa B activation and interleukin-6 mediated cell growth [41]. The mechanism of Bortezomib involves various signaling pathways such as Wnt/β-catenin signaling pathway [42]. Bortezomib treatment up-regulated the Nestin and CD31 expression levels in odontoblasts and pulp tissue. Furthermore, stronger positive reaction against neutrophils marker has been found after Bortezomib application. Through these results, it is assumed that Bortezomib induces blood vessel formation in response to pulpal inflammation. Examination of the molecular reactions to Bortezomib using mesenchymal tissues showed that dental pulp and odontoblast-forming mesenchymal cells indicated different expression patterns of Bmp and Wnt genes [9]. Midazolam is considered as another potential drug for dentin regeneration. Midazolam regulates inhibitory neurotransmitters in the vertebrate nervous system. The Midazolam-only treatment increased the alkaline phosphatase activity and mRNA levels of odontoblast differentiation marker genes. In contrast, combination of midazolam and PPU-7 cells exhibited high potential of dentin regeneration. These results show that the repositioning of Midazolam stimulates dentin regeneration [10,40]. Moreover, glycogen synthase kinase 3 (GSK3) is a proline/ serine protein kinase ubiquitously expressed and associated with many cellular pathways such as controlling metabolism, differentiation and immunity, especially cell death and survival [43]. Interestingly, the delivery of small molecule inhibitors of GSK3 stimulates Wnt/β-catenin signaling when applied into exposed pulpal cavity and examined on dentin repair [44-46]. Other approaches are organized and listed at Table 2 [47,48]. This drug repositioning would be an effective way of providing practical solutions for dentin regeneration in near future.

    Conclusions

    Significant findings in regenerative endodontics (Table 2) contributes to development of treatment protocols and application of tissue engineering. As a clinical treatment strategy, regenerative endodontics has limited cases in permanent teeth and lack of robust diagnostic marker. In addition, infection control (for example, rubber dam, disinfection protocols, etc.), the extent of surgical excision of inflamed tissue and case selection in the context of preservation of tissue vitality are central features of regenerative endodontics [36]. Regenerative endodontics offers a number of exciting opportunities for preservation of pulp vitality. Treatment strategies also provides substantial clinical advantages in case of immature teeth. True pulp regeneration will arise as a practical clinical treatment. Such achievements will target recruitment of specific stem/ progenitor cell populations and develop endogenous signaling molecules in order to regenerate dentin-pulp tissue with physiological characteristics.

    Conflicts of Interest

    No potential conflict of interest relevant to this article was reported.

    Figure

    Table

    Wnt signaling molecules in developmental stage of tooth and their compartmental roles

    List of approaches for regeneration of tertiary dentin

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