Aberrantly elevated Wnt signaling is responsible for cementum overgrowth and dental ankylosis

Effects of ECM Components on Periodontal Ligament Stem Cell Differentiation Under Conditions of Disruption of Wnt and TGF-β Signaling Pathways

Periodontitis is accompanied by inflammation that causes dysregulation of the Wnt/β-catenin and TGF-β signaling pathways. This leads to a violation of the homeostasis of periodontal tissues. Components of the extracellular matrix (ECM) are an important part of biomaterials used for the repair of periodontal tissue. The purpose of this study was to evaluate the components of the effect of ECM (hyaluronic acid (HA), fibronectin (Fn), and laminin (Lam)) on the osteogenic and odontogenic differentiation of periodontal ligament stem cells (PDLSCs) in the collagen I hydrogel under conditions of disruption of the Wnt/β-catenin and TGF-β signaling pathways. The study showed that the addition of components of the ECM restored the expression of odontogenic markers in PDLSCs, which was absent during inhibition of the canonical Wnt signaling pathway, and their multidirectional effect on the secretion of transforming growth factor-β1 (TGF-β1) and bone morphogenetic protein 2 (BMP-2). Fn and Lam suppressed the expression of odontogenic markers in PDLSCs against the background of inhibition of the TGF-β signaling pathway. The addition of HA under the conditions of the TGF-β signaling pathway improved BMP-2 secretion, preserving odontogenic differentiation. Thus, our results demonstrated that disruption of the Wnt/β-catenin and TGF-β signaling pathways causes disorders in the differentiation of PDLSCs, preventing the regeneration of periodontal tissues. This should be taken into account when developing multicomponent scaffolds that recapitulate the ECM microenvironment at endogenic regeneration of the periodontium. Inclusion of hyaluronic acid as one of these components may enhance the therapeutic effect of such biomaterials.

Wnt/β-catenin Promotes Cementum Apposition in Periodontal Regeneration

Regeneration of periodontal tissue, particularly the cementum-periodontal ligament (PDL)-bone complex, has long been challenging because the differentiation kinetics of cells and the molecular pathways contributing to the regeneration process are largely unknown. We aimed to evaluate the cell behavior and molecular pathways that contribute to periodontal tissue regeneration in vivo. We analyzed the process of periodontal tissue regeneration through subrenal capsule transplantation of immediately extracted molars in mice. We showed that the regenerated periodontal tissue in the subrenal capsule was morphologically comparable to the intact periodontal tissue, with increased cellular cementum thickness in the apical region. Cell tracing analysis revealed that the cells comprising the regenerated periodontal tissue were derived from transplanted teeth and were indispensable for periodontal tissue regeneration, whereas recipient mouse-derived cells partly contributed to angiogenesis. Bioinformatics analysis based on the gene expression profile in the transplanted teeth indicated that Wnt/β-catenin signaling is involved in periodontal tissue regeneration, which was further confirmed through β-catenin immunohistochemistry. Moreover, the constitutive activation of β-catenin in the cells of transplanted teeth was found to promote accelerated cellular cementum apposition, while the conditional knockout of β-catenin in the cells of transplanted teeth suppressed cellular cementum apposition. Notably, the manipulation of Wnt/β-catenin signaling did not interfere with the bone-PDL-cementum complex, while endogenous osteoclast activity was affected in bone. Our results demonstrated the essential roles of endogenous PDL cells in periodontal tissue regeneration and that Wnt/β-catenin signaling is involved in this process, particularly cellular cementum apposition. Hence, controlling this pathway could promote cementum regeneration, which is a critical process for the regeneration of the cementum-PDL-bone complex. This study provides novel insights into cell behavior and signaling pathways that will advance practical periodontal tissue regeneration.

Abnormal dental follicle cells: A crucial determinant in tooth eruption disorders (Review)

The dental follicle (DF) plays an indispensable role in tooth eruption by regulating bone remodeling through their influence on osteoblast and osteoclast activity. The process of tooth eruption involves a series of intricate regulatory mechanisms and signaling pathways. Disruption of the parathyroid hormone‑related protein (PTHrP) in the PTHrP‑PTHrP receptor signaling pathway inhibits osteoclast differentiation by DF cells (DFCs), thus resulting in obstructed tooth eruption. Furthermore, parathyroid hormone receptor‑1 mutations are linked to primary tooth eruption failure. Additionally, the Wnt/β‑catenin, TGF‑β, bone morphogenetic protein and Hedgehog signaling pathways have crucial roles in DFC involvement in tooth eruption. DFC signal loss or alteration inhibits osteoclast differentiation, affects osteoblast and cementoblast differentiation, and suppresses DFC proliferation, thus resulting in failed tooth eruptions. Abnormal tooth eruption is also associated with a range of systemic syndromes and genetic diseases, predominantly resulting from pathogenic gene mutations. Among these conditions, the following disorders arise due to genetic mutations that disrupt DFCs and impede proper tooth eruption: Cleidocranial dysplasia associated with Runt‑related gene 2 gene mutations; osteosclerosis caused by CLCN7 gene mutations; mucopolysaccharidosis type VI resulting from arylsulfatase B gene mutations; enamel renal syndrome due to FAM20A gene mutations; and dentin dysplasia caused by mutations in the VPS4B gene. In addition, regional odontodysplasia and multiple calcific hyperplastic DFs are involved in tooth eruption failure; however, they are not related to gene mutations. The specific mechanism for this effect requires further investigation. To the best of our knowledge, previous reviews have not comprehensively summarized the syndromes associated with DF abnormalities manifesting as abnormal tooth eruption. Therefore, the present review aims to consolidate the current knowledge on DFC signaling pathways implicated in abnormal tooth eruption, and their association with disorders of tooth eruption in genetic diseases and syndromes, thereby providing a valuable reference for future related research.

FGF signaling modulates mechanotransduction/WNT signaling in progenitors during tooth root development

Stem/progenitor cells differentiate into different cell lineages during organ development and morphogenesis. Signaling pathway networks and mechanotransduction are important factors to guide the lineage commitment of stem/progenitor cells during craniofacial tissue morphogenesis. Here, we used tooth root development as a model to explore the roles of FGF signaling and mechanotransduction as well as their interaction in regulating the progenitor cell fate decision. We show that Fgfr1 is expressed in the mesenchymal progenitor cells and their progeny during tooth root development. Loss of Fgfr1 in Gli1+ progenitors leads to hyperproliferation and differentiation, which causes narrowed periodontal ligament (PDL) space with abnormal cementum/bone formation leading to ankylosis. We further show that aberrant activation of WNT signaling and mechanosensitive channel Piezo2 occurs after loss of FGF signaling in Gli1-CreER;Fgfr1fl/fl mice. Overexpression of Piezo2 leads to increased osteoblastic differentiation and decreased Piezo2 leads to downregulation of WNT signaling. Mechanistically, an FGF/PIEZO2/WNT signaling cascade plays a crucial role in modulating the fate of progenitors during root morphogenesis. Downregulation of WNT signaling rescues tooth ankylosis in Fgfr1 mutant mice. Collectively, our findings uncover the mechanism by which FGF signaling regulates the fate decisions of stem/progenitor cells, and the interactions among signaling pathways and mechanotransduction during tooth root development, providing insights for future tooth root regeneration.

Functional defects in cementoblasts with disrupted bone sialoprotein functional domains, in vitro

Bone sialoprotein (BSP) is a multifunctional extracellular matrix (ECM) protein present in bone and cementum. Global in vivo ablation of BSP leads to bone mineralization defects, lack of acellular cementum, and periodontal breakdown. BSP harbors three main functional domains: N-terminal collagen-binding domain, hydroxyapatite-nucleating domain, and C-terminal RGD integrin-binding signaling domain. How each of these domains contributes to BSP function(s) is not understood. We hypothesized that collagen-binding and RGD domains play distinct roles in cementoblast functions. Three CRISPR/Cas9 gene-edited cell lines were derived from control wild-type (WT) OCCM.30 murine immortalized cementoblasts: 1) deletion of the N-terminus of BSP after signal peptide, including entire collagen binding domain (Ibsp∆N-Term); 2) deletion of exon 4 (majority of collagen-binding domain; Ibsp∆Ex4); and 3) deletion of C-terminus of BSP including the integrin binding RGD domain (Ibsp∆C-Term). Compared to WT, Ibsp∆Ex4 and Ibsp∆C-Term cell lines showed reduced BSP secretion, in vitro. Abnormal cell morphology was observed in all mutant cell lines, with Ibsp∆C-Term showing highly disorganized cytoskeleton. All mutant cell lines showed significantly lower cell proliferation compared to WT at all timepoints. Ibsp∆N-Term cells showed reduced cell migration by 24 h. All mutants exhibited over 50 % significant reduced mineralization at days 6 and 10. While WT cells were largely unaffected by seeding density, mutant cells failed to mineralize at lower cell density. Mutant cell lines diverged from WT and from each other by dysregulated expression in 23 genes involved in mineralization, ECM, and cell signaling. In summary, disabling BSP functional domains led to profound and distinct changes in cementoblast cell functions, especially dysregulated gene expression and reduced mineralization, in a way did not align with a straightforward narrative where each functional domain caused specific, expected differences. Instead, the study uncovered a significant level of complexity in how different mutant forms of BSP affected cell functions, in vitro.

Glycogen synthase kinase 3 inhibition enhances mineral nodule formation by cementoblasts in vitro

This study aimed to investigate whether GSK-3 inhibition (CHIR99021) effectively promoted mineralization by cementoblasts (OCCM-30). OCCM-30 cells were used and treated with different concentrations of CHIR99021 (2.5, 5, and 10 mM). Experiments included proliferation and viability, cellular metabolic activity, gene expression, and mineral nodule formation by Xylene Orange at the experimental time points. In general, CHIR99021 did not significantly affect OCCM-30 viability and cell metabolism (MTT assay) (p > 0.05), but increased OCCM-30 proliferation at 2.5 mM on days 2 and 4 (p < 0.05). Data analysis further showed that inhibition of GSK-3 resulted in increased transcript levels of Axin2 in OCCM-30 cells starting as early as 4 h, and regulated the expression of key bone markers including alkaline phosphatase (Alp), runt-related transcription factor 2 (Runx-2), osteocalcin (Ocn), and osterix (Osx). In addition, CHIR99021 led to an enhanced mineral nodule formation in vitro under both osteogenic and non-osteogenic conditions as early as 5 days after treatment. Altogether, the results of the current study suggest that inhibition of GSK-3 has the potential to promote cementoblast differentiation leading to increased mineral deposition in vitro.

Activation of Wnt signaling in Axin2+ cells leads to osteodentin formation and cementum overgrowth

OBJECTIVES

In this study, we used the mouse incisor model to investigate the regulatory mechanisms of Wnt/β-catenin signaling on Axin2+ cells in tooth development.

MATERIALS AND METHODS

Axin2lacZ/+ reporter mice were used to define the expression pattern of Axin2 in mouse incisors. We traced the fate of Axin2+ cells from postnatal Day 21 (P21) to P56 using Axin2CreERT2/+ and R26RtdTomato/+ reporter mice. For constitutive activation of Wnt signaling, Axin2CreERT2/+ , β-cateninflox(Ex3)/+ , and R26RtdTomato/+ (CA-β-cat) mice were generated to investigate the gain of function (GOF) of β-catenin in mouse incisor growth.

RESULTS

The X-gal staining of Axin2lacZ/+ reporter mice and lineage tracing showed that Axin2 was widely expressed in dental mesenchyme of mouse incisors, and Axin2+ cells were essential cell sources for odontoblasts, pulp cells, and periodontal ligament cells. The constitutive activation of Wnt signaling in Axin2+ cells resulted in the formation of osteodentin featured with increased DMP1 and dispersed DSP expression and overgrowth of cementum.

CONCLUSION

Wnt signaling plays a key role in the differentiation and maturation of Axin2+ cells in mouse incisors.

LITTIP/Lgr6/HnRNPK complex regulates cementogenesis via Wnt signaling

Orthodontically induced tooth root resorption (OIRR) is a serious complication during orthodontic treatment. Stimulating cementum repair is the fundamental approach for the treatment of OIRR. Parathyroid hormone (PTH) might be a potential therapeutic agent for OIRR, but its effects still lack direct evidence, and the underlying mechanisms remain unclear. This study aims to explore the potential involvement of long noncoding RNAs (lncRNAs) in mediating the anabolic effects of intermittent PTH and contributing to cementum repair, as identifying lncRNA-disease associations can provide valuable insights for disease diagnosis and treatment. Here, we showed that intermittent PTH regulates cell proliferation and mineralization in immortalized murine cementoblast OCCM-30 via the regulation of the Wnt pathway. In vivo, daily administration of PTH is sufficient to accelerate root regeneration by locally inhibiting Wnt/β-catenin signaling. Through RNA microarray analysis, lncRNA LITTIP (LGR6 intergenic transcript under intermittent PTH) is identified as a key regulator of cementogenesis under intermittent PTH. Chromatin isolation by RNA purification (ChIRP) and RNA immunoprecipitation (RIP) assays revealed that LITTIP binds to mRNA of leucine-rich repeat-containing G-protein coupled receptor 6 (LGR6) and heterogeneous nuclear ribonucleoprotein K (HnRNPK) protein. Further co-transfection experiments confirmed that LITTIP plays a structural role in the formation of the LITTIP/Lgr6/HnRNPK complex. Moreover, LITTIP is able to promote the expression of LGR6 via the RNA-binding protein HnRNPK. Collectively, our results indicate that the intermittent PTH administration accelerates root regeneration via inhibiting Wnt pathway. The lncRNA LITTIP is identified to negatively regulate cementogenesis, which activates Wnt/β-catenin signaling via high expression of LGR6 promoted by HnRNPK.

The LRP5 high-bone-mass mutation causes alveolar bone accrual with minor craniofacial alteration

BACKGROUND AND OBJECTIVE

Mutations in low-density lipoprotein receptor-related protein 5 (LRP5) cause various bone diseases. Several mouse models were generated to study the role of LRP5 in bone development. But most of the studies were confined to the appendicular skeleton. The role of LRP5 in the axial skeleton, especially in the craniofacial skeleton, is largely unknown. The aim of this study was to investigate the craniofacial phenotype with the LRP5G171V mutation.

METHODS

To understand how LRP5 affects craniofacial bone properties, we analyzed LRP5 high-bone-mass mutant mice carrying the G171V missense mutation (LRP5HBM ). Quantitative microcomputed tomographic imaging and histomorphometric analyses were used to study craniofacial phenotypes and bone density. Histology, immunohistochemistry, and in vivo fluorochrome labeling were used to study molecular mechanisms.

RESULTS

LRP5HBM mice showed overall minor changes in the craniofacial bone development but with increased bone mass in the interradicular alveolar bone, edentulous ridge, palatine bone, and premaxillary suture. Elevated osteocyte density was observed in LRP5HBM mice, along with increased Runx2 expression and unmineralized bone surrounding osteocytes. Meanwhile, LRP5HBM mice exhibited increased osteoprogenitors, but no significant changes were observed in osteoclasts. This led to a high-bone-mass phenotype, and an increased osteocyte density in the alveolar bone and edentulous ridge.

CONCLUSION

LRP5HBM mice display increased bone mass in the alveolar bone with minor changes in the craniofacial morphology. Collectively, these data elucidated the important role of LRP5 in axial bone development and homeostasis and provided clues into the therapeutical potential of LRP5 signaling in treating alveolar bone loss.

Loss of β-catenin causes cementum hypoplasia by hampering cementogenic differentiation of Axin2-expressing cells

BACKGROUND AND OBJECTIVE

Although cementum plays an essential role in tooth attachment and adaptation to occlusal force, the regulatory mechanisms of cementogenesis remain largely unknown. We have previously reported that Axin2-expressing (Axin2⁺ ) mesenchymal cells in periodontal ligament (PDL) are the main cell source for cementum growth, and constitutive activation of Wnt/β-catenin signaling in Axin2⁺ cells results in hypercementosis. Therefore, the aim of the present study was to further evaluate the effects of β-catenin deletion in Axin2⁺ cells on cementogenesis.

MATERIALS AND METHODS

We generated triple transgenic mice to conditionally delete β-catenin in Axin2-lineage cells by crossing Axin2^CreERT2/+ ; R26R^tdTomato/+ mice with β-catenin^flox/flox mice. Multiple approaches, including X-ray analysis, micro-CT, histological stainings, and immunostaining assays, were used to analyze cementum phenotypes and molecular mechanisms.

RESULTS

Our data revealed that loss of β-catenin in Axin2⁺ cells led to a cementum hypoplasia phenotype characterized by a sharp reduction in the formation of both acellular and cellular cementum. Mechanistically, we found that conditional removal of β-catenin in Axin2⁺ cells severely impaired the secretion of cementum matrix proteins, for example, bone sialoprotein (BSP), dentin matrix protein 1 (DMP1) and osteopontin (OPN), and markedly inhibited the differentiation of Axin2⁺ mesenchymal cells into osterix⁺ cementoblasts.

CONCLUSIONS

Our findings confirm the vital role of Axin2⁺ mesenchymal PDL cells in cementum growth and demonstrate that Wnt/β-catenin signaling shows a positive correlation with cementogenic differentiation of Axin2⁺ cells.

Wnt/β-Catenin Signaling in Craniomaxillofacial Osteocytes

PURPOSE OF REVIEW

There is a growing appreciation within the scientific community that cells exhibit regional variation. Whether the variation is attributable to differences in embryonic origin or anatomical location and mechanical loading has not been elucidated; what is clear, however, is that adult cells carry positional information that ultimately affects their functions. The purpose of this review is to highlight the functions of osteocytes in the craniomaxillofacial (CMF) skeleton as opposed to elsewhere in the body, and in doing so gain mechanistic insights into genetic conditions and chemically-induced diseases that particularly affect this region of our anatomy.

RECENT FINDINGS

In the CMF skeleton, elevated Wnt/β-catenin signaling affects not only bone mass and volume, but also mineralization of the canalicular network and osteocyte lacunae. Aberrant elevation in the Wnt/β-catenin pathway can also produce micropetrosis and osteonecrosis of CMF bone, presumably due to a disruption in the signaling network that connects osteocytes to one another, and to osteoblasts on the bone surface.

Between a rock and a hard place: Regulation of mineralization in the periodontium

The periodontium supports and attaches teeth via mineralized and nonmineralized tissues. It consists of two, unique mineralized tissues, cementum and alveolar bone. In between these tissues, lies an unmineralized, fibrous periodontal ligament (PDL), which distributes occlusal forces, nourishes and invests teeth, and harbors progenitor cells for dentoalveolar repair. Many unanswered questions remain regarding periodontal biology. This review will focus on recent research providing insights into one enduring mystery: the precise regulation of the hard-soft tissue borders in the periodontium which define the interfaces of the cementum-PDL-alveolar bone structure. We will focus on advances in understanding the molecular mechanisms that maintain the unmineralized PDL "between a rock and a hard place" by regulating the mineralization of cementum and alveolar bone.

Tooth transplantation and replantation: Biological insights towards therapeutic improvements

Transplantation and replantation of teeth are effective therapeutic approaches for tooth repositioning and avulsion, respectively. Transplantation involves transplanting an extracted tooth from the original site into another site, regenerating tissue including the periodontal ligament (PDL) and alveolar bone, around the transplanted tooth. Replantation places the avulsed tooth back to its original site, regenerating functional periodontal tissue. In clinical settings, transplantation and replantation result in favorable outcomes with regenerated PDL tissue in many cases. However, they often result in poor outcomes with two major complications: tooth ankylosis and root resorption. In tooth ankylosis, the root surface and alveolar bone are fused, reducing the PDL tissue between them. The root is subjected to remodeling processes and is partially replaced by bone. In severe cases, the resorbed root is completely replaced by bone tissue, which is called as "replacement resorption." Resorption is sometimes accompanied by infection-mediated inflammation. The molecular mechanisms of ankylosis and root resorption remain unclear, although some signaling mechanisms have been proposed. In this mini-review, we summarized the biological basis of repair mechanisms of tissues in transplantation and replantation and the pathogenesis of their healing failure. We also discussed possible therapeutic interventions to improve treatment success rates.

Platelet-derived growth factor-BB regenerates functional periodontal ligament in the tooth replantation

Tooth ankylosis is a pathological condition of periodontal ligament (PDL) restoration after tooth replantation. Platelet-derived growth factor-BB (PDGF-BB) has been proposed as a promising factor for preventing tooth ankylosis. Using rat tooth replantation model, we investigated whether PDGF-BB accelerates the repair of PDL after tooth replantation without ankylosis, and its molecular mechanisms. In PDGF-BB pretreated replanted teeth (PDGF-BB group), ankylosis was markedly reduced and functionally organized PDL collagen fibers were restored; the mechanical strength of the healing PDL was restored to an average of 76% of that in non-replanted normal teeth at 21 days. The numbers of PDGF-Rβ- and BrdU-positive cells in the periodontal tissues of the PDGF-BB group were greater than those of atelocollagen pretreated replanted teeth (AC group). Moreover, in the PDGF-BB group, the periodontal tissues had fewer osteocalcin-positive cells and decreased number of nuclear β-catenin-positive cells compared to those in the AC group. In vitro analyses showed that PDGF-BB increased the proliferation and migration of human periodontal fibroblasts. PDGF-BB downregulated mRNA expressions of RUNX2 and ALP, and inhibited upregulatory effects of Wnt3a on β-catenin, AXIN2, RUNX2, COL1A1, and ALP mRNA expressions. These findings indicate that in tooth replantation, topical PDGF-BB treatment enhances cell proliferation and migration, and inhibits canonical Wnt signaling activation in bone-tooth ankylosis, leading to occlusal loading of the PDL tissues and subsequent functional restoration of the healing PDL. This suggests a possible clinical application of PDGF-BB to reduce ankylosis after tooth replantation and promote proper regeneration of PDL.

The osteocyte as a signaling cell

Osteocytes, former osteoblasts encapsulated by mineralized bone matrix, are far from being passive and metabolically inactive bone cells. Instead, osteocytes are multifunctional and dynamic cells capable of integrating hormonal and mechanical signals and transmitting them to effector cells in bone and in distant tissues. Osteocytes are a major source of molecules that regulate bone homeostasis by integrating both mechanical cues and hormonal signals that coordinate the differentiation and function of osteoclasts and osteoblasts. Osteocyte function is altered in both rare and common bone diseases, suggesting that osteocyte dysfunction is directly involved in the pathophysiology of several disorders affecting the skeleton. Advances in osteocyte biology initiated the development of novel therapeutics interfering with osteocyte-secreted molecules. Moreover, osteocytes are targets and key distributors of biological signals mediating the beneficial effects of several bone therapeutics used in the clinic. Here we review the most recent discoveries in osteocyte biology demonstrating that osteocytes regulate bone homeostasis and bone marrow fat via paracrine signaling, influence body composition and energy metabolism via endocrine signaling, and contribute to the damaging effects of diabetes mellitus and hematologic and metastatic cancers in the skeleton.

Wnt Signaling in Periodontal Disease

Periodontitis is a multifactorial and chronic condition associated with the formation of a dysbiotic biofilm, leading to a pro-inflammatory environment that can modulate cell signaling. The Wnt pathway plays fundamental roles during homeostasis and disease, and emerging evidence suggests its involvement in the maintenance of the periodontium and the development of periodontitis. Here, we summarize the role of the Wnt/β-catenin and non-canonical Wnt signaling pathways in periodontitis. The accumulated data suggests specific roles for each branch of the Wnt pathway. Wnt5a emerges as a critical player promoting periodontal ligament remodeling and impairing regenerative responses modulated by the Wnt/β-catenin pathway, such as alveolar bone formation. Collectively, the evidence suggests that achieving a proper balance between the Wnt/β-catenin and non-canonical pathways, rather than their independent modulation, might contribute to controlling the progression and severity of the periodontal disease.

The Role of Wnt Signaling in Postnatal Tooth Root Development

Appropriate tooth root formation and tooth eruption are critical for achieving and maintaining good oral health and quality of life. Tooth eruption is the process through which teeth emerge from their intraosseous position to their functional position in the oral cavity. This temporospatial process occurs simultaneously with tooth root formation through a cascade of interactions between the epithelial and adjoining mesenchymal cells. Here, we will review the role of the Wnt system in postnatal tooth root development. This signaling pathway orchestrates the process of tooth root formation and tooth eruption in conjunction with several other major signaling pathways. The Wnt signaling pathway is comprised of the canonical, or Wnt/β-catenin, and the non-Canonical signaling pathway. The expression of multiple Wnt ligands and their downstream transcription factors including β-catenin is found in the cells in the epithelia and mesenchyme starting from the initiation stage of tooth development. The inhibition of canonical Wnt signaling in an early stage arrests odontogenesis. Wnt transcription factors continue to be present in dental follicle cells, the progenitor cells responsible for differentiation into cells constituting the tooth root and the periodontal tissue apparatus. This expression occurs concurrently with osteogenesis and cementogenesis. The conditional ablation of β-catenin in osteoblast and odontoblast causes the malformation of the root dentin and cementum. On the contrary, the overexpression of β-catenin led to shorter molar roots with thin and hypo-mineralized dentin, along with the failure of tooth eruption. Therefore, the proper expression of Wnt signaling during dental development is crucial for regulating the proliferation, differentiation, as well as epithelial-mesenchymal interaction essential for tooth root formation and tooth eruption.

Wnt signalling in oral and maxillofacial diseases

Wnts include more than 19 types of secreted glycoproteins that are involved in a wide range of pathological processes in oral and maxillofacial diseases. The transmission of Wnt signalling from the extracellular matrix into the nucleus includes canonical pathways and noncanonical pathways, which play an important role in tooth development, alveolar bone regeneration, and related diseases. In recent years, with the in-depth study of Wnt signalling in oral and maxillofacial-related diseases, many new conclusions and perspectives have been reached, and there are also some controversies. This article aims to summarise the roles of Wnt signalling in various oral diseases, including periodontitis, dental pulp disease, jaw disease, cleft palate, and abnormal tooth development, to provide researchers with a better and more comprehensive understanding of Wnts in oral and maxillofacial diseases.

Although Anatomically Micrometers Apart: Human Periodontal Ligament Cells Are Slightly More Active in Bone Remodeling Than Alveolar Bone Derived Cells

The periodontal ligament (PDL) and the alveolar bone are part of the periodontium, a complex structure that supports the teeth. The alveolar bone is continuously remodeled and is greatly affected by several complex oral events, like tooth extraction, orthodontic movement, and periodontitis. Until now, the role of PDL cells in terms of osteogenesis and osteoclastogenesis has been widely studied, whereas surprisingly little is known about the bone remodeling capacity of alveolar bone. Therefore, the purpose of this study was to compare the biological character of human alveolar bone cells and PDL cells in terms of osteogenesis and osteoclastogenesis in vitro. Paired samples of PDL cells and alveolar bone cells from seven patients with compromised general and oral health were collected and cultured. Bone A (early outgrowth) and bone B (late outgrowth) were included. PDL, bone A, bone B cell cultures all had a fibroblast appearance with similar expression pattern of six mesenchymal markers. These cultures were subjected to osteogenesis and osteoclastogenesis assays. For osteoclastogenesis assays, the cells were co-cultured with peripheral blood mononuclear cells, a source for osteoclast precursor cells. The total duration of the experiments was 21 days. Osteogenesis was slightly favored for PDL compared to bone A and B as shown by stronger Alizarin red staining and higher expression of RUNX2 and Collagen I at day 7 and for ALP at day 21. PDL induced approximately two times more osteoclasts than alveolar bone cells. In line with these findings was the higher expression of cell fusion marker DC-STAMP in PDL-PBMC co-cultures compared to bone B at day 21. In conclusion, alveolar bone contains remodeling activity, but to a different extent compared to PDL cells. We showed that human alveolar bone cells can be used as an in vitro model to study bone remodeling.

Wnt signaling: An attractive target for periodontitis treatment

Periodontitis is the most common chronic inflammatory disease, and a leading cause of tooth loss. Characterized by resorption of alveolar process and destruction of periodontal ligaments, periodontitis can impact not only periodontal tissues but also systemic diseases, such as diabetes, cardiovascular diseases, and respiratory infections. Currently, it is a hotspot to manage destruction and gain regeneration of periodontal tissues. Increasing evidence indicates that the Wnt signaling plays an important role in homeostasis of periodontal tissues, functions of periodontal derived cells, and progression of periodontitis. Its molecule expressions were abnormal in periodontitis. As such, modulators targeting the Wnt signaling may be an adjuvant therapy for periodontitis treatment. This review elucidates the role of Wnt signaling and its molecules, with a view to develop a potential application of drugs targeting the Wnt signaling for periodontitis treatment.

Periodontal plastic surgery for the management of an ankylosed permanent maxillary lateral incisor: A clinical report with 5-year follow-up

Anterior maxillary tooth ankylosis disturbs the development of the alveolar bone process, leading to discrepancies between the cervical gingival margin and incisal edge position of the affected tooth, and therefore, the esthetics is compromised. Proposed treatments in adults and growing patients have been used successfully, but they have disadvantages and are contraindicated in some circumstances. This article proposes an alternative treatment for an ankylosed permanent maxillary anterior tooth with a slow replacement resorption rate in an adult patient, for whom a combination of a periodontal plastic surgery procedure and a fixed dental prosthesis was used to correct the esthetics. This treatment has less risk of complications, preserves the ankylosed tooth as long as possible, creates an optimal gingival contour, and maintains the alveolar bone for further treatment should the tooth be lost.

Molecular Basis for Craniofacial Phenotypes Caused by Sclerostin Deletion

Some genetic disorders are associated with distinctive facial features, which can aid in diagnosis. While considerable advances have been made in identifying causal genes, relatively little progress has been made toward understanding how a particular genotype results in a characteristic craniofacial phenotype. An example is sclerosteosis/van Buchem disease, which is caused by mutations in the Wnt inhibitor sclerostin (SOST). Affected patients have a high bone mass coupled with a distinctive appearance where the mandible is enlarged and the maxilla is foreshortened. Here, mice carrying a null mutation in Sost were analyzed using quantitative micro-computed tomographic (µCT) imaging and histomorphometric analyses to determine the extent to which the size and shape of craniofacial skeleton were altered. Sost^-/- mice exhibited a significant increase in appositional bone growth, which increased the height and width of the mandible and reduced the diameters of foramina. In vivo fluorochrome labeling, histology, and immunohistochemical analyses indicated that excessive bone deposition in the premaxillary suture mesenchyme curtailed overall growth, leading to midfacial hypoplasia. The amount of bone extracellular matrix produced by Sost^-/- cells was significantly increased; as a consequence, osteoid seams were evident throughout the facial skeleton. Collectively, these analyses revealed a remarkable fidelity between human characteristics of sclerosteosis/van Buchem disease and the Sost^-/- phenotype and provide clues into the conserved role for sclerostin signaling in modulating craniofacial morphology.

Histological, Histochemical and Immunohistochemical Evaluation of the Role of Bone Marrow-Derived Mesenchymal Stem Cells on the Structure of Periodontal Tissues in Carbimazole-Treated Albino Rats

OBJECTIVE

To elucidate the role of bone marrow-mesenchymal stem cells (BM-MSCs) on the structure of periodontal tissues in carbimazole (antithyroid drug) treated rats at different durations.

DESIGN

28 albino rats were divided into: Group I: received distilled water. Group II: received therapeutic dose of carbimazole. Group III: received carbimazole then single injection of BM-MSCs by the end of 3^rd week. Group IV: received carbimazole and single injection of BM-MSCs at the beginning of the experiment. Specimens were examined by light microscope. New collagen and β-catenin-immunoreactivity area% were assessed histomorphometrically, and statistically using ANOVA test.

RESULTS

Histological examination revealed normal periodontal tissues structure in Groups I & IV. Group II showed disorganized periodontal ligament fibers and different stainability of cementum and alveolar bone. Group III illustrated dense periodontal ligament fibers, normal stainability of cementum and most of alveolar bone. Masson's trichrome results of Groups I & IV illustrated large areas of new collagen in periodontal ligament, old collagen in cementum and intermingled old and new collagen in alveolar bone. Group II showed old collagen. Group III revealed only new collagen. β-catenin-immunoreactivity was strong in Groups I & IV, negative in Group II and moderate in Group III. Statistically, Group III showed highest mean of new collagen area% followed by Groups I, IV and II respectively. Highest mean of β-catenin-immunoreactivity area% was for Group I followed by Groups IV, III and II respectively.

CONCLUSIONS

Carbimazole has damaging effects and BM-MSCs are capable to mend these destructive outcomes in time dependent manner.

Long non-coding RNA TUG1 participates in LPS-induced periodontitis by regulating miR-498/RORA pathway

AIM

This study was aimed to investigate the role of TUG1 in LPS-stimulated hPDLCs and to evaluate the potential functions of TUG1 in the pathogenesis of periodontitis.

METHODS

LPS-stimulated hPDLCs were established as the cell model. CCK-8 assay was performed to assess cell proliferation ability. Flow cytometry was performed to detect cell cycle distribution, and quantitative RT-PCR and Western blotting were conducted to measure gene expressions. ELISA kits were used to evaluate the production of inflammatory cytokines. The putative binding site between TUG1 and miR-498 was verified using luciferase reporter and RNA immunoprecipitation assays.

RESULTS

TUG1 was downregulated upon LPS stimulation in hPDLCs. TUG1 overexpression promoted cell proliferation through regulating the cell cycle distribution, along with the decreased expression of p21 and increased expression of CDK2 and cyclin D1. Besides, TUG1 overexpression decreased the production of inflammatory cytokines. The effects were opposite upon TUG1 knockdown. TUG1 negatively regulated its target miR-498, and influenced the expression of RORA, the direct target of miR-498. Simultaneous TUG1 overexpression and miR-498 reversed the effect of TUG1 overexpression alone on alleviating LPS-induced cell injury and inhibition of Wnt/β-catenin signaling, which was further changeover after co-overexpression with RORA.

CONCLUSION

Therefore, TUG1 could protect against periodontitis via regulating miR-498/RORA mediated Wnt/β-catenin signaling.

Bioactivating a bone substitute accelerates graft incorporation in a murine model of vertical ridge augmentation

Objective.

Compared to autologous bone grafts, allogeneic bone grafts integrate slowly, which can adversely affect clinical outcomes. Here, our goal was to understand the molecular mechanisms underlying graft incorporation, and then test clinically feasible methods to accelerate this process.

Methods.

Wild-type and transgenic Wnt “reporter” mice were used in a vertical ridge augmentation procedure. The surgery consisted of tunneling procedure to elevate the maxillary edentulous ridge periosteum, followed by the insertion of bone graft. Micro-computed tomographic imaging, and molecular/cellular analyses were used to follow the bone graft over time. Sclerostin null mice, and mice carrying an activated form of β-catenin were evaluated to understand how elevated Wnt signaling impacted edentulous ridge height and based on these data, a biomimetic strategy was employed to combine bone graft particles with a formulation of recombinant WNT protein. Thereafter, the rate of graft incorporation was evaluated.

Results.

Tunneling activated osteoprogenitor cell proliferation from the periosteum. If graft particles were present, then osteoprogenitor cells attached to the matrix and gave rise to new bone that augmented edentulous ridge height. Graft particles alone did not stimulate osteoprogenitor cell proliferation. Based on the thicker edentulous ridges in mice with amplified Wnt signaling, a strategy was undertaken to load bone graft particles with WNT; this combination was sufficient to accelerate the initial step of graft incorporation.

Significance.

Local delivery of a WNT protein therapeutic has the potential to accelerate graft incorporation, and thus shorten the time to when the graft can support a dental implant.

Role of PTH1R Signaling in Prx1+ Mesenchymal Progenitors during Eruption

Tooth eruption is a complex process requiring precise interaction between teeth and adjacent tissues. Molecular analysis demonstrates that bone remodeling plays an essential role during eruption, suggesting that a parathyroid hormone 1 receptor (PTH1R) gene mutation is associated with disturbances in bone remodeling and results in primary failure of eruption (PFE). Recent research reveals the function of PTH1R signaling in mesenchymal progenitors, whereas the function of PTH1R in mesenchymal stem cells during tooth eruption remains incompletely understood. We investigated the specific role of PTH1R in Prx1⁺ progenitor expression during eruption. We found that Prx1⁺-progenitors occur in mesenchymal stem cells residing in alveolar bone marrow surrounding incisors, at the base of molars and in the dental follicle and pulp of incisors. Mice with conditional deletion of PTH1R using the Prx1 promoter exhibited arrested mandibular incisor eruption and delayed molar eruption. Micro-computed tomography, histomorphometry, and molecular analyses revealed that mutant mice had significantly reduced alveolar bone formation concomitant with downregulated gene expression of key regulators of osteogenesis in PTH1R-deficient cells. Moreover, culturing orofacial bone-marrow-derived mesenchymal stem cells (OMSCs) from Prx1Cre;PTH1R^fl/fl mice or from transfecting Cre recombinase adenovirus in OMSCs from PTH1R^fl/fl mice suggested that lack of Pth1r expression inhibited osteogenic differentiation in vitro. However, bone resorption was not affected by PTH1R ablation, indicating the observed reduced alveolar bone volume was mainly due to impaired bone formation. Furthermore, we found irregular periodontal ligaments and reduced Periostin expression in mutant incisors, implying loss of PTH1R results in aberrant differentiation of periodontal ligament cells. Collectively, these data suggest that PTH1R signaling in Prx1⁺ progenitors plays a critical role in alveolar bone formation and periodontal ligament development during eruption. These findings have implications for our understanding of the physiologic and pathologic function of PTH1R signaling in tooth eruption and the progression of PFE.

Transcriptome analysis of ankylosed primary molars with infraocclusion

Primary molar ankylosis with infraocclusion can retard dental arch development and cause dental asymmetry. Despite its widespread prevalence, little is known about its molecular etiology and pathogenesis. To address this, RNA sequencing was used to generate transcriptomes of furcal bone from infraoccluded (n = 7) and non-infraoccluded (n = 9) primary second molars, all without succeeding biscuspids. Of the 18 529 expressed genes, 432 (2.3%) genes were differentially expressed between the two groups (false discovery rate < 0.05). Hierarchical clustering and principal component analysis showed clear separation in gene expression between infraoccluded and non-infraoccluded samples. Pathway analyses indicated that molar ankylosis is associated with the expression of genes consistent with the cellular inflammatory response and epithelial cell turnover. Independent validation using six expressed genes by immunohistochemical analysis demonstrated that the corresponding proteins are strongly expressed in the developing molar tooth germ, in particular the dental follicle and inner enamel epithelium. The descendants of these structures include the periodontal ligament, cementum, bone and epithelial rests of Malassez; tissues that are central to the ankylotic process. We therefore propose that ankylosis involves an increased inflammatory response associated with disruptions to the developmental remnants of the dental follicle and epithelial rests of Malassez.

A Correlation between Wnt/Beta-catenin Signaling and the Rate of Dentin Secretion

INTRODUCTION

Odontoblasts produce dentin throughout life and in response to trauma. The purpose of this study was to identify the roles of endogenous Wnt signaling in regulating the rate of dentin accumulation.

METHODS

Histology, immunohistochemistry, vital dye labeling, and histomorphometric assays were used to quantify the rate of dentin accumulation as a function of age. Two strains of Wnt reporter mice were used to identify and follow the distribution and number of Wnt-responsive odontoblasts as a function of age. To show a causal relationship between dentin secretion and Wnt signaling, dentin accumulation was monitored in a strain of mice in which Wnt signaling was aberrantly elevated.

RESULTS

Dentin deposition occurs throughout life, but the rate of accumulation slows with age. This decline in dentin secretion correlates with a decrease in endogenous Wnt signaling. In a genetically modified strain of mice, instead of tubular dentin, aberrantly elevated Wnt signaling resulted in accumulation of reparative dentin or osteodentin secreted from predontoblasts.

CONCLUSIONS

Wnt signaling regulates dentin secretion by odontoblasts, and the formation of reparative or osteodentin is the direct consequence of elevated Wnt signaling. These preclinical data have therapeutic implications for the development of a biologically based pulp capping medicant.

Mesenchymal Progenitor Regulation of Tooth Eruption: A View from PTHrP

Tooth eruption is a unique biological process by which highly mineralized tissues emerge into the outer world, and it occurs concomitantly with tooth root formation. These 2 processes have been considered independent phenomena; however, recent studies support the theory that they are indeed intertwined. Dental mesenchymal progenitor cells in the dental follicle lie at the heart of the coupling of these 2 processes, providing a source for diverse mesenchymal cells that support formation of the highly functional tooth root and the periodontal attachment apparatus, while facilitating formation of osteoclasts. These cells are regulated by autocrine signaling by parathyroid hormone-related protein (PTHrP) and its parathyroid hormone/PTHrP receptor PPR. This PTHrP-PPR signaling appears to crosstalk with other signaling pathways and regulates proper cell fates of mesenchymal progenitor cell populations. Disruption of this autocrine PTHrP-PPR signaling in these cells leads to defective formation of the periodontal attachment apparatus, tooth root malformation, and failure of tooth eruption in molars, which essentially recapitulate primary failure of eruption in humans, a rare genetic disorder exclusively affecting tooth eruption. Diversity and distinct functionality of these mesenchymal progenitor cell populations that regulate tooth eruption and tooth root formation are beginning to be unraveled.

Mechanoadaptive Responses in the Periodontium Are Coordinated by Wnt

Despite an extensive literature documenting the adaptive changes of bones and ligaments to mechanical forces, our understanding of how tissues actually mount a coordinated response to physical loading is astonishingly inadequate. Here, using finite element (FE) modeling and an in vivo murine model, we demonstrate the stress distributions within the periodontal ligament (PDL) caused by occlusal hyperloading. In direct response, a spatially restricted pattern of apoptosis is triggered in the stressed PDL, the temporal peak of which is coordinated with a spatially restricted burst in PDL cell proliferation. This culminates in increased collagen deposition and a thicker, stiffer PDL that is adapted to its new hyperloading status. Meanwhile, in the adjacent alveolar bone, hyperloading activates bone resorption, the peak of which is followed by a bone formation phase, leading ultimately to an accelerated rate of mineral apposition and an increase in alveolar bone density. All of these adaptive responses are orchestrated by a population of Wnt-responsive stem/progenitor cells residing in the PDL and bone, whose death and revival are ultimately responsible for directly giving rise to new PDL fibers and new bone.