Parathyroid hormone receptor signalling in osterix-expressing mesenchymal progenitors is essential for tooth root formation

Tgfbr2 in Dental Pulp Cells Guides Neurite Outgrowth in Developing Teeth

Transforming growth factor β (TGFβ) plays an important role in tooth morphogenesis and mineralization. During postnatal development, the dental pulp (DP) mesenchyme secretes neurotrophic factors that guide trigeminal nerve fibers into and throughout the DP. This process is tightly linked with dentin formation and mineralization. Our laboratory established a mouse model in which Tgfbr2 was conditionally deleted in DP mesenchyme using an Osterix promoter-driven Cre recombinase (Tgfbr2 ^cko ). These mice survived postnatally with significant defects in bones and teeth, including reduced mineralization and short roots. Hematoxylin and eosin staining revealed reduced axon-like structures in the mutant mice. Reporter imaging demonstrated that Osterix-Cre activity within the tooth was active in the DP and derivatives, but not in neuronal afferents. Immunofluorescence staining for β3 tubulin (neuronal marker) was performed on serial cryosections from control and mutant molars on postnatal days 7 and 24 (P7, P24). Confocal imaging and pixel quantification demonstrated reduced innervation in Tgfbr2 ^cko first molars at both stages compared to controls, indicating that signals necessary to promote neurite outgrowth were disrupted by Tgfbr2 deletion. We performed mRNA-Sequence (RNA-Seq) and gene onotology analyses using RNA from the DP of P7 control and mutant mice to investigate the pathways involved in Tgfbr2-mediated tooth development. These analyses identified downregulation of several mineralization-related and neuronal genes in the Tgfbr2 ^cko DP compared to controls. Select gene expression patterns were confirmed by quantitative real-time PCR and immunofluorescence imaging. Lastly, trigeminal neurons were co-cultured atop Transwell filters overlying primary Tgfbr2 ^f/f DP cells. Tgfbr2 in the DP was deleted via Adenovirus-expressed Cre recombinase. Confocal imaging of axons through the filter pores showed increased axonal sprouting from neurons cultured with Tgfbr2-positive DP cells compared to neurons cultured alone. Axon sprouting was reduced when Tgfbr2 was knocked down in the DP cells. Immunofluorescence of dentin sialophosphoprotein in co-cultured DP cells confirmed reduced mineralization potential in cells with Tgfbr2 deletion. Both our proteomics and RNA-Seq analyses indicate that axonal guidance cues, particularly semaphorin signaling, were disrupted by Tgfbr2 deletion. Thus, Tgfbr2 in the DP mesenchyme appears to regulate differentiation and the cells' ability to guide neurite outgrowth during tooth mineralization and innervation.

Validating clinical characteristic of primary failure of eruption (PFE) associated with PTH1R variants

BACKGROUND

Primary failure of eruption (PFE) is a hereditary condition, and linkage with variants in the PTH1R gene has been demonstrated in many cases. The clinical severity and expression of PFE is variable, and the genotype-phenotype correlation remains elusive. Further, the similarity between some eruption disorders that are not associated with PTH1R alterations is striking. To better understand the genotype-phenotype correlation, we examined the relationship between the eruption phenotype and PTH1R genotype in 44 patients with suspected PFE and 27 unaffected relatives. Sanger sequencing was employed to analyze carefully selected PFE patients. Potential pathogenicity of variants was evaluated against multiple genetic databases for function prediction and frequency information.

RESULTS

Mutational analysis of the PTH1R coding sequence revealed 14 different variants in 38 individuals (30 patients and 8 first-degree relatives), 9 exonic and 5 intronic. Their pathogenicity has been reported and compared with the number and severity of clinical signs. In 72.7% of patients with pathogenic variants, five clinical and radiographic criteria have been found: involvement of posterior teeth, involvement of the distal teeth to the most mesial affected, supracrestal presentation, altered vertical growth of the alveolar process and posterior open-bite. In cases with mixed dentition (3), the deciduous molars of the affected quadrant were infraoccluded.

DISCUSSION

The probability of an affected patient having a PTH1R variant is greater when five specific clinical characteristics are present. The likelihood of an eruption defect in the absence of specific clinical characteristics is rarely associated with a PTH1R mutation.

CONCLUSIONS

We report here that systematic clinical and radiographic observation using a diagnostic rubric is highly valuable in confirming PFE and offers a reliable alternative for accurate diagnosis.

Parathyroid Hormone 1 Receptor Signaling in Dental Mesenchymal Stem Cells: Basic and Clinical Implications

Parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) are two peptides that regulate mineral ion homeostasis, skeletal development, and bone turnover by activating parathyroid hormone 1 receptor (PTH1R). PTH1R signaling is of profound clinical interest for its potential to stimulate bone formation and regeneration. Recent pre-clinical animal studies and clinical trials have investigated the effects of PTH and PTHrP analogs in the orofacial region. Dental mesenchymal stem cells (MSCs) are targets of PTH1R signaling and have long been known as major factors in tissue repair and regeneration. Previous studies have begun to reveal important roles for PTH1R signaling in modulating the proliferation and differentiation of MSCs in the orofacial region. A better understanding of the molecular networks and underlying mechanisms for modulating MSCs in dental diseases will pave the way for the therapeutic applications of PTH and PTHrP in the future. Here we review recent studies involving dental MSCs, focusing on relationships with PTH1R. We also summarize recent basic and clinical observations of PTH and PTHrP treatment to help understand their use in MSCs-based dental and bone regeneration.

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.

RNA-seq Analysis of Peri-Implant Tissue Shows Differences in Immune, Notch, Wnt, and Angiogenesis Pathways in Aged Versus Young Mice

The number of total joint replacements (TJRs) in the United States is increasing annually. Cementless implants are intended to improve upon traditional cemented implants by allowing bone growth directly on the surface to improve implant longevity. One major complication of TJR is implant loosening, which is related to deficient osseointegration in cementless TJRs. Although poor osseointegration in aged patients is typically attributed to decreased basal bone mass, little is known about the molecular pathways that compromise the growth of bone onto porous titanium implants. To identify the pathways important for osseointegration that are compromised by aging, we developed an approach for transcriptomic profiling of peri-implant tissue in young and aged mice using our murine model of osseointegration. Based on previous findings of changes of bone quality associated with aging, we hypothesized that aged mice have impaired activation of bone anabolic pathways at the bone-implant interface. We found that pathways most significantly downregulated in aged mice relative to young mice are related to angiogenic, Notch, and Wnt signaling. Downregulation of these pathways is associated with markedly increased expression of inflammatory and immune genes at the bone-implant interface in aged mice. These results identify osseointegration pathways affected by aging and suggest that an increased inflammatory response in aged mice may compromise peri-implant bone healing. Targeting the Notch and Wnt pathways, promoting angiogenesis, or modulating the immune response at the peri-implant site may enhance osseointegration and improve the outcome of joint replacement in older patients. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Combination of parathyroid hormone pretreatment and mechanical stretch promotes osteogenesis of periodontal ligament fibroblasts

INTRODUCTION

Parathyroid hormone (PTH) potentiates the mechanical loading induced bone formation in fracture healing and orthodontics. This study aimed to gain insight into the underlying mechanisms in periodontal ligament fibroblasts (PDLFs).

METHODS

Human PDLFs were cultured and subjected to uniaxial cyclic stretch at 0.5 Hz and 2000μ for 0, 6, 12, and 24 hours, respectively. 10 nM PTH was preadministered for 30 minutes before loading. The expression of PTH1R and osteogenic biomarkers Runx2, osteopontin, collagen type 1, alkaline phosphatase was assessed via immunofluorescence staining, quantitative polymerase chain reaction, or Western blot. Transfection of siPTH1R was applied, and alterations of osteogenic biomarkers were examined by Western blot. The expression of essential Wnt signal components Wnt3a, β-catenin, low-density lipoprotein receptor-related protein 5, Wnt5a, receptor tyrosine kinase-like orphan receptor 2 were examined, and the influence of dickkopf-related protein 1 on osteogenic biomarkers was evaluated.

RESULTS

The expression of PTH1R was instantaneously upregulated with PTH pretreatment and maintained a gradual increase until 24 hours. PTH synergistically enhanced the increase of Runx2, osteopontin, collagen type 1, and alkaline phosphatase under cyclic stretch, which was substantially attenuated by siPTH1R transfection. As for Wnt signal components, synergistic upregulation was detected on Wnt3a, β-catenin, and low-density lipoprotein receptor-related protein 5, whereas Wnt5a and receptor tyrosine kinase-like orphan receptor 2 increased relatively mildly. Blockage of the canonical Wnt/β-catenin pathway by dickkopf-related protein 1 impaired the boost of osteogenic biomarkers under the combined action of PTH and cyclic stretch.

CONCLUSIONS

The combination of PTH pretreatment and cyclic stretch promotes osteogenesis of PDLFs synergistically, and the canonical Wnt/β-catenin pathway is crucially involved in the underlying mechanism.

Epithelial Bone Morphogenic Protein 2 and 4 Are Indispensable for Tooth Development

The Bmp2 and Bmp4 expressed in root mesenchyme were essential for the patterning and cellular differentiation of tooth root. The role of the epithelium-derived Bmps in tooth root development, however, had not been reported. In this study, we found that the double abrogation of Bmp2 and Bmp4 from mouse epithelium caused short root anomaly (SRA). The K14-cre;Bmp2^f/f;Bmp4^f/f mice exhibited a persistent Hertwig’s Epithelial Root Sheath (HERS) with the reduced cell death, and the down-regulated BMP-Smad4 and Erk signaling pathways. Moreover, the Shh expression in the HERS, the Shh-Gli1 signaling, and Nfic expression in the root mesenchyme of the K14-cre;Bmp2^f/f;Bmp4^f/f mice were also decreased, indicating a disrupted epithelium- mesenchyme interaction between HERS and root mesenchyme. Such disruption suppressed the Osx and Dspp expression in the root mesenchyme, indicating an impairment on the differentiation and maturation of root odontoblasts. The impaired differentiation and maturation of root odontoblasts could be rescued partially by transgenic Dspp. Therefore, although required in a low dosage and with a functional redundancy, the epithelial Bmp2 and Bmp4 were indispensable for the HERS degeneration, as well as the differentiation and maturation of root mesenchyme.

Odontoblast death drives cell-rich zone-derived dental tissue regeneration

Severe dental tissue damage induces odontoblast death, after which dental pulp stem and progenitor cells (DPSCs) differentiate into odontoblast-like cells, contributing to reparative dentin. However, the damage-induced mechanism that triggers this regeneration process is still not clear. We aimed to understand the effect of odontoblast death without hard tissue damage on dental regeneration. Herein, using a Cre/LoxP-based strategy, we demonstrated that cell-rich zone (CZ)-localizing Nestin-GFP-positive and Nestin-GFP-negative cells proliferate and differentiate into odontoblast-like cells in response to odontoblast depletion. The regenerated odontoblast-like cells played a role in reparative dentin formation. RNA-sequencing analysis revealed that the expression of odontoblast differentiation- and activation-related genes was upregulated in the pulp in response to odontoblast depletion even without damage to dental tissue. In this regenerative process, the expression of type I parathyroid hormone receptor (PTH1R) increased in the odontoblast-depleted pulp, thereby boosting dentin formation. The levels of PTH1R and its downstream mediator, i.e., phosphorylated cyclic AMP response element-binding protein (Ser133) increased in the physically damaged pulp. Collectively, odontoblast death triggered the PTH1R cascade, which may represent a therapeutic target for inducing CZ-mediated dental regeneration.

Functions of the SNAI family in chondrocyte-to-osteocyte development

Different cellular mechanisms contribute to osteocyte development. And while critical roles for members of the zinc finger protein SNAI family (SNAIs) have been discussed in cancer-related models, there are few reviews summarizing their importance for chondrocyte-to-osteocyte development. To help fill this gap, we review the roles of SNAIs in the development of mature osteocytes from chondrocytes, including the regulation of chondro- and osteogenesis through different signaling pathways and in programmed cell death. We also discuss how epigenetic factors-including DNA methylation, histone methylation and acetylation, and noncoding RNAs-contribute differently to both chondrocyte and osteocyte development. To better grasp the important roles of SNAIs in bone development, we also review genotype-phenotype correlations in different animal models. We end with comments about the possible importance of the SNAI family in cartilage/bone development and the potential applications for therapeutic goals.

Physiological and Pharmacological Roles of PTH and PTHrP in Bone Using Their Shared Receptor, PTH1R

Parathyroid hormone (PTH) and the paracrine factor, PTH-related protein (PTHrP), have preserved in evolution sufficient identities in their amino-terminal domains to share equivalent actions upon a common G protein-coupled receptor, PTH1R, that predominantly uses the cyclic adenosine monophosphate-protein kinase A signaling pathway. Such a relationship between a hormone and local factor poses questions about how their common receptor mediates pharmacological and physiological actions of the two. Mouse genetic studies show that PTHrP is essential for endochondral bone lengthening in the fetus and is essential for bone remodeling. In contrast, the main postnatal function of PTH is hormonal control of calcium homeostasis, with no evidence that PTHrP contributes. Pharmacologically, amino-terminal PTH and PTHrP peptides (teriparatide and abaloparatide) promote bone formation when administered by intermittent (daily) injection. This anabolic effect is remodeling-based with a lesser contribution from modeling. The apparent lesser potency of PTHrP than PTH peptides as skeletal anabolic agents could be explained by lesser bioavailability to PTH1R. By contrast, prolongation of PTH1R stimulation by excessive dosing or infusion, converts the response to a predominantly resorptive one by stimulating osteoclast formation. Physiologically, locally generated PTHrP is better equipped than the circulating hormone to regulate bone remodeling, which occurs asynchronously at widely distributed sites throughout the skeleton where it is needed to replace old or damaged bone. While it remains possible that PTH, circulating within a narrow concentration range, could contribute in some way to remodeling and modeling, its main physiological role is in regulating calcium homeostasis.

Function of Dental Follicle Progenitor/Stem Cells and Their Potential in Regenerative Medicine: From Mechanisms to Applications

Dental follicle progenitor/stem cells (DFPCs) are a group of dental mesenchyme stem cells that lie in the dental follicle and play a critical role in tooth development and maintaining function. Originating from neural crest, DFPCs harbor a multipotential differentiation capacity. More importantly, they have superiorities, including the easy accessibility and abundant sources, active self-renewal ability and noncontroversial sources compared with other stem cells, making them an attractive candidate in the field of tissue engineering. Recent advances highlight the excellent properties of DFPCs in regeneration of orofacial tissues, including alveolar bone repair, periodontium regeneration and bio-root complex formation. Furthermore, they play a unique role in maintaining a favorable microenvironment for stem cells, immunomodulation and nervous related tissue regeneration. This review is intended to summarize the current knowledge of DFPCs, including their stem cell properties, physiological functions and clinical application potential. A deep understanding of DFPCs can thus inspire novel perspectives in regenerative medicine in the future.

The effect of parathyroid hormone on osteogenesis is mediated partly by osteolectin

We previously described a new osteogenic growth factor, osteolectin/Clec11a, which is required for the maintenance of skeletal bone mass during adulthood. Osteolectin binds to Integrin α11 (Itga11), promoting Wnt pathway activation and osteogenic differentiation by leptin receptor+ (LepR+) stromal cells in the bone marrow. Parathyroid hormone (PTH) and sclerostin inhibitor (SOSTi) are bone anabolic agents that are administered to patients with osteoporosis. Here we tested whether osteolectin mediates the effects of PTH or SOSTi on bone formation. We discovered that PTH promoted Osteolectin expression by bone marrow stromal cells within hours of administration and that PTH treatment increased serum osteolectin levels in mice and humans. Osteolectin deficiency in mice attenuated Wnt pathway activation by PTH in bone marrow stromal cells and reduced the osteogenic response to PTH in vitro and in vivo. In contrast, SOSTi did not affect serum osteolectin levels and osteolectin was not required for SOSTi-induced bone formation. Combined administration of osteolectin and PTH, but not osteolectin and SOSTi, additively increased bone volume. PTH thus promotes osteolectin expression and osteolectin mediates part of the effect of PTH on bone formation.

The role of biomineralization in disorders of skeletal development and tooth formation

The major mineralized tissues are bone and teeth, which share several mechanisms governing their development and mineralization. This crossover includes the hormones that regulate circulating calcium and phosphate concentrations, and the genes that regulate the differentiation and transdifferentiation of cells. In developing endochondral bone and in developing teeth, parathyroid hormone-related protein (PTHrP) acts in chondrocytes to delay terminal differentiation, thereby increasing the pool of precursor cells. Chondrocytes and (in specific circumstances) pre-odontoblasts can also transdifferentiate into osteoblasts. Moreover, bone and teeth share outcomes when affected by systemic disorders of mineral homeostasis or of the extracellular matrix, and by adverse effects of treatments such as bisphosphonates and fluoride. Unlike bone, teeth have more permanent effects from systemic disorders because they are not remodelled after they are formed. This Review discusses the normal processes of bone and tooth development, followed by disorders that have effects on both bone and teeth, versus disorders that have effects in one without affecting the other. The takeaway message is that bone specialists should know when to screen for dental disorders, just as dental specialists should recognize when a tooth disorder should raise suspicions about a possible underlying bone disorder.

PTH/PTHrP in controlled release hydrogel enhances orthodontic tooth movement by regulating periodontal bone remodaling

OBJECTIVE

This study aimed to evaluate the effects of local application of parathyroid hormone (PTH) or parathyroid hormone-related protein (PTHrP) on osteogenesis and osteoclastogenesis during orthodontic tooth movement (OTM).

BACKGROUND

Periodontal bone remodeling is the crucial biological process in the OTM that involves both bone resorption and formation, with the former more important as the initiator. PTH or PTHrP both play dual roles in bone remodeling regulation, and the balance may shift to the bone resorption side when they are given continuously, suggesting them as potential candidate medicine for OTM acceleration.

METHODS

A total of 40 rats underwent orthodontic mesialization of the maxillary first molars and received no micro-perforation (MOP), or MOP followed by injection of temperature-sensitive hydrogel containing PTH, PTHrP, or normal saline. The rats were sacrificed after 2-week OTM, except for the relapse groups, which had one more week of observation after removal of the force appliances. The amount of tooth movement, rate of relapse after OTM, and effects on the bone remodeling were assessed through micro-computed tomography (μCT) analysis, alkaline phosphatase (ALP) assay, alizarin red staining, tartrate-resistant acid phosphatase (TRAP) staining, immunohistochemistry (IHC) analysis, Western blot (WB), and quantitative real-time polymerase chain reaction (qRT-PCR). The effects of PTHrP on the osteogenic differentiation of human periodontal ligament cells (hPDLCs) were explored in vitro.

RESULTS

The cumulative release of PTH or PTHrP from PECE hydrogels was beyond 75% at 14 days in a sustained manner. After the intervention in vivo, the distance of OTM in the PTH (0.78 ± 0.06 mm) or PTHrP (0.81 ± 0.04 mm) group was significantly larger than that of the MOP only (0.51 ± 0.04 mm) or the no MOP (0.46 ± 0.05 mm) group. Moreover, PTH injection significantly reduced the rate of relapse after OTM (25.7 ± 4.3%) compared to the control (69.6 ± 6.1%). μCT analysis showed decreased BV/TV, BS/BV, and Tb.N, while increased Tb.Sp of alveolar bone in the PTH or PTHrP group. There were also more TRAP-positive osteoclasts in the PTH or PTHrP group with a significantly enhanced ratio of receptor activator of nuclear factor-κB ligand (RANKL)/osteoprotegerin (OPG). The protein expressions of PTH/PTHrP type 1 receptor (PTHR1), alkaline phosphatase (ALP), osteocalcin (OCN), runt-related transcription factor 2 (RUNX2), and β-catenin were significantly increased in the PTH or PTHrP group, as well as the gene expressions of Pth1r, Bglap, and Alpl. There was no significant difference between the effects of PTH and PTHrP. Nevertheless, inhibition of PTHrP on the osteogenic differentiation of hPDLCs was detected in vitro with decreased expression of OCN, RUNX2, COL-1, and ALP.

CONCLUSION

Local injection of either PTH or PTHrP carried by controlled release PECE hydrogel similarly enhances OTM in rats through regulating periodontal bone remodeling, which deserves further study for potential clinical application.

MicroRNA-1 affects the development of the neural crest and craniofacial skeleton via the mitochondrial apoptosis pathway

The neural crest is one of the key features of craniofacial development. MicroRNA-1 (miR-1) is a single-stranded noncoding RNA that serves an important role in embryonic development. However, the function of miR-1 in neural crest cells (NCCs) is unknown. Therefore, to evaluate the role of miR-1 in NCC development, a miR-1 mutant zebrafish was generated in the current study. Mouse NCCs were isolated from the first branchial arch of embryos at gestational day E9.5, and miR-1 was silenced using a miR-1 inhibitor. To the best of our knowledge, the present study was the first to report that homozygous zebrafish lacking miR-1 exhibited developmental defects in NCC-derived craniofacial bones, heart, melanocytes and iridophores. These defects may be caused by an increase in apoptosis of NCCs during their migration and differentiation in embryonic development. Moreover, the apoptosis analysis and western blotting results demonstrated that this effect was modulated via the mitochondrial apoptosis pathway, and miR-1 inhibited NCC apoptosis by modulating this pathway. These results collectively suggested that miR-1 in NCCs may be essential for craniofacial development.

Lhx6 regulates canonical Wnt signaling to control the fate of mesenchymal progenitor cells during mouse molar root patterning

Mammalian tooth crown formation has long served as a model for investigating how patterning and morphogenesis are orchestrated during development. However, the mechanism underlying root patterning and morphogenesis remains poorly understood. In this study, we find that Lhx6 labels a subpopulation of root progenitor cells in the apical dental mesenchyme, which is closely associated with furcation development. Loss of Lhx6 leads to furcation and root number defects, indicating that Lhx6 is a key root patterning regulator. Among the multiple cellular events regulated by Lhx6 is the odontoblast fate commitment of progenitor cells, which it controls in a cell-autonomous manner. Specifically, Lhx6 loss leads to elevated expression of the Wnt antagonist Sfrp2 and down-regulation of Wnt signaling in the furcation region, while overactivation of Wnt signaling in Lhx6+ progenitor cells partially restore the furcation defects in Lhx6-/- mice. Collectively, our findings have important implications for understanding organ morphogenesis and future strategies for tooth root regeneration.

METTL3-Mediated m6 A mRNA Methylation Modulates Tooth Root Formation by Affecting NFIC Translation

N6-methyladenosine (m⁶ A), as a eukaryotic mRNA modification catalyzed by methyltransferase METTL3, is involved in various processes of development or diseases via regulating RNA metabolism. However, the effect of METTL3-mediated m⁶ A modification in tooth development has remained elusive. Here we show that METTL3 is prevalently expressed in odontoblasts, dental pulp cells, dental follicle cells, and epithelial cells in Hertwig's epithelial root sheath during tooth root formation. Depletion of METTL3 in human dental pulp cells (hDPCs) impairs proliferation, migration, and odontogenic differentiation. Furthermore, conditional knockout of Mettl3 in Osterix-expressing cells leads to short molar roots and thinner root dentin featured by decreased secretion of pre-dentin matrix and formation of the odontoblast process. Mechanistically, loss of METTL3 cripples the translational efficiency of the key root-forming regulator nuclear factor I-C (NFIC). The odontogenic capacity of METTL3-silenced hDPCs is partially rescued via overexpressing NFIC. Our findings suggest that m⁶ A methyltransferase METTL3 is crucial for tooth root development, uncovering a novel epigenetic mechanism in tooth root formation. © 2020 American Society for Bone and Mineral Research (ASBMR).

Root dilaceration in maxillary impacted canines and adjacent teeth: A retrospective analysis of the difference between buccal and palatal impaction

INTRODUCTION

This research aimed to analyze the prevalence of root dilaceration in buccally impacted canines (BICs) and palatally impacted canines (PICs) with their adjacent teeth based on a retrospective cone-beam computed tomography (CBCT) investigation.

METHODS

Pretreatment CBCT images of 145 subjects with unilateral maxillary canine impaction and 145 age- and sex-matched subjects without impaction were used. Prevalence of dilaceration (subclassified to root curvature and apical hook based on severity) in canines and adjacent teeth was determined in CBCT records. The root length of maxillary impacted canines was measured for further morphologic evaluations.

RESULTS

Impacted canines had a significantly higher prevalence of root dilaceration than the control group and compared with the erupted contralateral canines in the experimental group (P < 0.001 for both). A significantly higher prevalence of root dilaceration was found in adjacent lateral incisors of the PICs subgroup than that of the control group (P < 0.001). Adjacent premolars had a higher prevalence of dilacerated roots in the PICs subgroup (P < 0.001) than the control group, but not for the BICs subgroup. Significantly higher prevalence of curvature (P < 0.001 for both) and hook (P = 0.008 and P < 0.001, respectively) were found in BICs and PICs roots compared with the control group. Both types of impacted canines had significantly shorter roots than the control group (P < 0.001 for both).

CONCLUSIONS

BICs and PICs have a higher tendency to present root dilaceration and shorter roots. Unlike BICs, adjacent teeth to PICs were more frequently observed to have root dilaceration.

Exploiting teeth as a model to study basic features of signaling pathways

Teeth constitute a classical model for the study of signaling pathways and their roles in mediating interactions between cells and tissues in organ development, homeostasis and regeneration. Rodent teeth are mostly used as experimental models. Rodent molars have proved fundamental in the study of epithelial–mesenchymal interactions and embryonic organ morphogenesis, as well as to faithfully model human diseases affecting dental tissues. The continuously growing rodent incisor is an excellent tool for the investigation of the mechanisms regulating stem cells dynamics in homeostasis and regeneration. In this review, we discuss the use of teeth as a model to investigate signaling pathways, providing an overview of the many unique experimental approaches offered by this organ. We discuss how complex networks of signaling pathways modulate the various aspects of tooth biology, and the models used to obtain this knowledge. Finally, we introduce new experimental approaches that allow the study of more complex interactions, such as the crosstalk between dental tissues, innervation and vascularization.

Runx2 Regulates Mouse Tooth Root Development Via Activation of WNT Inhibitor NOTUM

Progenitor cells are crucial in controlling organ morphogenesis. Tooth development is a well-established model for investigating the molecular and cellular mechanisms that regulate organogenesis. Despite advances in our understanding of how tooth crown formation is regulated, we have limited understanding of tooth root development. Runt-related transcription factor 2 (RUNX2) is a well-known transcription factor in osteogenic differentiation and early tooth development. However, the function of RUNX2 during tooth root formation remains unknown. We revealed in this study that RUNX2 is expressed in a subpopulation of GLI1+ root progenitor cells, and that loss of Runx2 in these GLI1+ progenitor cells and their progeny results in root developmental defects. Our results provide in vivo evidence that Runx2 plays a crucial role in tooth root development and in regulating the differentiation of root progenitor cells. Furthermore, we identified that Gli1, Pcp4, NOTUM, and Sfrp2 are downstream targets of Runx2 by integrating bulk and single-cell RNA sequencing analyses. Specifically, ablation of Runx2 results in downregulation of WNT inhibitor NOTUM and upregulation of canonical WNT signaling in the odontoblastic site, which disturbs normal odontoblastic differentiation. Significantly, exogenous NOTUM partially rescues the impaired root development in Runx2 mutant molars. Collectively, our studies elucidate how Runx2 achieves functional specificity in regulating the development of diverse organs and yields new insights into the network that regulates tooth root development. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Unveiling diversity of stem cells in dental pulp and apical papilla using mouse genetic models: a literature review

The dental pulp, a non-mineralized connective tissue uniquely encased within the cavity of the tooth, provides a niche for diverse arrays of dental mesenchymal stem cells. Stem cells in the dental pulp, including dental pulp stem cells (DPSCs), stem cells from human exfoliated deciduous teeth (SHEDs) and stem cells from apical papilla (SCAPs), have been isolated from human tissues with an emphasis on their potential application to regenerative therapies. Recent studies utilizing mouse genetic models shed light on the identities of these mesenchymal progenitor cells derived from neural crest cells (NCCs) in their native conditions, particularly regarding how they contribute to homeostasis and repair of the dental tissue. The current concept is that at least two distinct niches for stem cells exist in the dental pulp, e.g., the perivascular niche and the perineural niche. The precise identities of these stem cells and their niches are now beginning to be unraveled thanks to sophisticated mouse genetic models, which lead to better understanding of the fundamental properties of stem cells in the dental pulp and the apical papilla in humans. The new knowledge will be highly instrumental for developing more effective stem cell-based regenerative therapies to repair teeth in the future.

4-Hexylresorcinol Administration Increases Dental Hard Tissue Formation and Incisor Eruption Rate in Rats

Dental hard tissue formation and bone turnover are required for tooth eruption. 4-Hexylresorcinol (4HR) accelerates tooth movement by increasing bone turnover in orthodontic treatment. This study aimed to evaluate the following: (1) the effect of 4HR application on the expression of proteins associated with tooth formation, and (2) the effect of 4HR application on mandibular incisor eruption rate in a rat model. Primary cultured pulp cells received either 4HR (1 to 100 µM) or solvent only; western blotting was performed for transforming growth factor-beta 1 (TGF-β1), bone morphogenic protein-2/4 (BMP-2/4), runt-related transcription factor 2 (Runx2), osterix (OSX), dentin sialophosphoprotein (DSPP), and parathyroid hormone-related protein receptor (PTHrP-R). In in vivo study, rats (15 males and 15 females) received either solvent or 0.128 mg/kg or 12.8 mg/kg of 4HR via subcutaneous injection; mandibular incisor eruption rate was subsequently recorded. Immunohistochemical staining and western blotting for TGF-β1, BMP-2/4, Runx2, OSX, DSPP, and PTHrP-R were performed in the mandibular tissue samples. 4HR administration was found to increase TGF-β1, BMP-2/4, Runx2, OSX, DSPP, and PTHrP-R expression in both cell culture and tissue samples. Immunohistochemical staining of some markers showed site-specific expression, thereby indicating programmed differentiation of odontoblasts and ameloblasts. The eruption rate was significantly higher in the 12.8 mg/kg 4HR-administered group than in the untreated control (p = 0.001 and 0.010 for males and females, respectively). Collectively, 4HR administration increased the expression of markers related to dental hard tissue formation and accelerated the eruption rate of incisors in rats.

Gli1+ Periodontium Stem Cells Are Regulated by Osteocytes and Occlusal Force

Teeth are attached to alveolar bone by the periodontal ligament (PDL), which contains stem cells supporting tissue turnover. Here, we identified Gli1+ cells in adult mouse molar PDL as multi-potential stem cells (PDLSCs) giving rise to PDL, alveolar bone, and cementum. They support periodontium tissue turnover and injury repair. Gli1+ PDLSCs are surrounding the neurovascular bundle and more enriched in the apical region. Canonical Wnt signaling is essential for their activation. Alveolar bone osteocytes negatively regulate Gli1+ PDLSCs activity through sclerostin, a Wnt inhibitor. Blockage of sclerostin accelerates the PDLSCs lineage contribution rate in vivo. Sclerostin expression is modulated by physiological occlusal force. Removal of occlusal force upregulates sclerostin and inhibits PDLSCs activation. In summary, Gli1+ cells are the multipotential PDLSCs in vivo. Osteocytes provide negative feedback to PDLSCs and inhibit their activities through sclerostin. Physiological occlusal force indirectly regulates PDLSCs activities by fine-tuning this feedback loop.

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.

Primary Retention of Molars and RANKL Signaling Alteration during Craniofacial Growth

The primary retention of molars observed in clinic corresponds to a still-unexplained absence of molar eruption despite the presence of an eruption pathway, resembling the experimental transient inhibition of RANKL signaling in mice. The aim of the present study was to confront the hypothesis according to which the primary retention of molars is associated with transitory perturbations to RANKL signaling during growth as part of a wider craniofacial skeleton pattern. The experimental strategy was based on combining a clinical study and an animal study corresponding to the characterization of the craniofacial phenotypes of patients with primary retention of molars and analyses in mice of the consequences of transient inhibition of RANKL signaling on molar eruption and craniofacial growth. The clinical study validated the existence of a particular craniofacial phenotype in patients with primary retention of molars: a retromandibular skeletal class II typology with reduced mandibular dimensions which manifests itself at the dental level by a class II/2 with palatoversion of the upper incisors and anterior overbite. The animal study demonstrated that transient invalidation of RANKL signaling had an impact on the molar eruption process, the severity of which was dependent on the period of inhibition and was associated with a reduction in two craniofacial morphometric parameters: total skull length and craniofacial vault length. In conclusion, primary retention of molars may be proposed as part of the craniofacial skeleton phenotype associated with a transitory alteration in RANKL signaling during growth.

Immortalized Hertwig's epithelial root sheath cell line works as model for epithelial-mesenchymal interaction during tooth root formation

Hertwig's epithelial root sheath (HERS) is critical for epithelial-mesenchymal interaction (EMI) during tooth root formation. However, the exact roles of HERS in odontogenic differentiation by EMI have not been well characterized, because primary HERS cells are difficult to obtain. Immortalized cell lines constitute crucial scientific tools, while there are few HERS cell lines available. Our previous study has successfully established immortalized HERS cell lines. Here, we confirmed the phenotype of our HERS-H1 by verifying its characteristics and functions in odontogenic differentiation through EMI. The HERS-H1-conditioned medium (CM-H1) effectively enhanced odontogenic differentiation of dental papilla cells (DPCs) in vitro. Furthermore, Smad4 and p-Smad1/5/8 were significantly activated in DPCs treated with CM-H1, and this activation was attenuated by noggin. In vivo, our implanted recombinants of HERS-H1 and DPCs exhibited mineralized tissue formation and expression of Smad4, p-Smad1/5/8, and odontogenic differentiation markers. Our results indicated that HERS-H1 promoted DPCs odontoblastic differentiation via bone morphogenetic protein/Smad signaling. HERS-H1 exhibits relevant key molecular characteristics and constitutes a new biological model for basic research on HERS and the dental EMI during root development and regeneration.

The inflammatory response to bone infection - a review based on animal models and human patients

Bone infections are difficult to diagnose and treat, especially when a prosthetic joint replacement or implant is involved. Bone loss is a major complication of osteomyelitis, but the mechanism behind has mainly been investigated in cell cultures and has not been confirmed in human settings. Inflammation is important in initiating an appropriate immune response to invading pathogens. However, many of the signaling molecules used by the immune system can also modulate bone remodeling and contribute to bone resorption during osteomyelitis. Our current knowledge of the inflammatory response relies heavily on animal models as research based on human samples is scarce. Staphylococcus aureus is one of the most common causes of bone infections and is the pathogen of choice in animal models. The regulation of inflammatory genes during prosthetic joint infections and implant-associated osteomyelitis has only been studied in rodent models. It is important to consider the validity of an animal model when results are extrapolated to humans, and both bone composition and the immune system of pigs has been shown to be more similar to humans, than to rodents. Here in vivo studies on the inflammatory response to prosthetic joint infections and implant-associated osteomyelitis are reviewed.

Histone Acetylation as a Regenerative Target in the Dentine-Pulp Complex

If dental caries (or tooth decay) progresses without intervention, the infection will advance through the dentine leading to severe pulpal inflammation (irreversible pulpitis) and pulp death. The current management of irreversible pulpits is generally root-canal-treatment (RCT), a destructive, expensive, and often unnecessary procedure, as removal of the injurious stimulus alone creates an environment in which pulp regeneration may be possible. Current dental-restorative-materials stimulate repair non-specifically and have practical limitations; as a result, opportunities exist for the development of novel therapeutic strategies to regenerate the damaged dentine-pulp complex. Recently, epigenetic modification of DNA-associated histone ‘tails’ has been demonstrated to regulate the self-renewal and differentiation potential of dental-stem-cell (DSC) populations central to regenerative endodontic treatments. As a result, the activities of histone deacetylases (HDAC) are being recognised as important regulators of mineralisation in both tooth development and dental-pulp-repair processes, with HDAC-inhibition (HDACi) promoting pulp cell mineralisation in vitro and in vivo. Low concentration HDACi-application can promote de-differentiation of DSC populations and conversely, increase differentiation and accelerate mineralisation in DSC populations. Therapeutically, various HDACi solutions can release bioactive dentine-matrix-components (DMCs) from the tooth’s extracellular matrix; solubilised DMCs are rich in growth factors and can stimulate regenerative processes such as angiogenesis, neurogenesis, and mineralisation. The aim of this mini-review is to discuss the role of histone-acetylation in the regulation of DSC populations, while highlighting the importance of HDAC in tooth development and dental pulp regenerative-mineralisation processes, before considering the potential therapeutic application of HDACi in targeted biomaterials to the damaged pulp to stimulate regeneration.

BMP-Smad Signaling Regulates Postnatal Crown Dentinogenesis in Mouse Molar

Dentinogenesis, a formation of dentin by odontoblasts, is an essential process during tooth development. Bone morphogenetic proteins (BMPs) are one of the most crucial growth factors that contribute to dentin formation. However, it is still unclear how BMP signaling pathways regulate postnatal crown and root dentinogenesis. BMPs transduce signals through canonical Smad and non-Smad signaling pathways including p38 and ERK signaling pathways. To investigate the roles of Smad and non-Smad signaling pathways in dentinogenesis, we conditionally deleted Bmpr1a, which encodes the type 1A receptor for BMPs, to remove both Smad and non-Smad pathways in Osterix-expressing cells. We also expressed a constitutively activated form of Bmpr1a (caBmpr1a) to increase Smad1/5/9 signaling activity without altered non-Smad activity in odontoblasts. To understand the function of BMP signaling during postnatal dentin formation, Cre activity was induced at the day of birth. Our results showed that loss of BmpR1A in odontoblasts resulted in impaired dentin formation and short molar roots at postnatal day 21. Bmpr1a cKO mice displayed a reduction of dentin matrix production compared to controls associated with increased cell proliferation and reduced Osx and Dspp expression. In contrast, caBmpr1a mutant mice that show increased Smad1/5/9 signaling activity resulted in no overt tooth phenotype. To further dissect the functions of each signaling activity, we generated Bmpr1a cKO mice also expressing caBmpr1a to restore only Smad1/5/9 signaling activity. Restoring Smad activity in the compound mutant mice rescued impaired crown dentin formation in the Bmpr1a cKO mice; however, impaired root dentin formation and short roots were not changed. These results suggest that BMP-Smad signaling in odontoblasts is responsible for crown dentin formation, while non-Smad signaling may play a major role in root dentin formation and elongation. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Molecular mechanisms for short root anomaly

Short root anomaly (SRA) is a dental disorder that presents an abnormal root morphology with short and blunt dental roots. In this situation, many dental treatments face a difficult challenge, especially orthodontic and prosthodontic treatments. Therefore, an understanding of how SRA develops is urgently needed. Here we describe that the abnormal expression of nuclear factor I C-type (Nfic), osterix (Osx), hedgehog (Hh), bone morphogenetic proteins (BMPs), transforming growth factor-β (TGF-β), Smad, Wnt, β-catenin, and dickkopf-related protein 1 (DKK1) leads to SRA. These factors interact with each other and constitute complicated signaling network in tooth formation. Specifically, BMP signaling inhibits the activity of Wnt/β-catenin directly or by inducing Osx via Runx2-dependent and Runx2-independent pathways. And Osx is a main inhibitor of Wnt/β-catenin signaling. In return, Wnt/β-catenin signaling has an antagonistic action of BMP pathway and a stimulation of Runx2. We highlight the importance of Wnt/β-catenin signaling in the pathological mechanisms. Either suppression or overactivation of this signaling influences the normal odontogenesis. Finally, we list rescue experiments on animal models, which have been reported to restore the interrupted cell differentiation and impaired tooth formation. We hope to find potential treatments for SRA based on these evidences in the future.

A three-dimensional analysis of primary failure of eruption in humans and mice

OBJECTIVES

Primary failure of eruption (PFE) is a genetic disorder exhibiting the cessation of tooth eruption. Loss-of-function mutations in parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor (PTH/PTHrP receptor, PPR) were reported as the underlying cause of this disorder in humans. We showed in a PFE mouse model that PTHrP-PPR signaling is responsible for normal dental follicle cell differentiation and tooth eruption. However, the mechanism underlying the eruption defect in PFE remains undefined. In this descriptive study, we aim to chronologically observe tooth eruption and root formation of mouse PFE molars through 3D microCT analyses.

SETTING AND SAMPLE POPULATION

Two individuals with PFE were recruited at Showa University. A mouse PFE model was generated by deleting PPR specifically in PTHrP-expressing dental follicle and divided into three groups, PPR^fl/fl ;R26R^tdTomato/+ (Control), PTHrP-creER;PPR^fl/+ ;R26R^tdTomato/+ (cHet), and PTHrP-creER;PRR^fl/fl ;R26R^tdTomato/+ (cKO).

MATERIALS AND METHODS

Images from human PFE subjects were acquired by CBCT. All groups of mouse samples were studied at postnatal days 14, 25, 91, and 182 after a tamoxifen pulse at P3, and superimposition of 3D microCT images among three groups was rendered.

RESULTS

Mouse and human PFE molars exhibited a similar presentation in the 3D CT analyses. The quantitative analysis in mice demonstrated a statistically significant decrease in the eruption height of cKO first and second molars compared to other groups after postnatal day 25. Additionally, cKO molars demonstrated significantly shortened roots with dilacerations associated with the reduced interradicular bone height.

CONCLUSIONS

Mouse PFE molars erupt at a much slower rate compared to normal molars, associated with shortened and dilacerated roots and defective interradicular bones.

Salt-inducible kinases dictate parathyroid hormone 1 receptor action in bone development and remodeling

The parathyroid hormone 1 receptor (PTH1R) mediates the biologic actions of parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP). Here, we showed that salt-inducible kinases (SIKs) are key kinases that control the skeletal actions downstream of PTH1R and that this GPCR, when activated, inhibited cellular SIK activity. Sik gene deletion led to phenotypic changes that were remarkably similar to models of increased PTH1R signaling. In growth plate chondrocytes, PTHrP inhibited SIK3, and ablation of this kinase in proliferating chondrocytes rescued perinatal lethality of PTHrP-null mice. Combined deletion of Sik2 and Sik3 in osteoblasts and osteocytes led to a dramatic increase in bone mass that closely resembled the skeletal and molecular phenotypes observed when these bone cells express a constitutively active PTH1R that causes Jansen's metaphyseal chondrodysplasia. Finally, genetic evidence demonstrated that class IIa histone deacetylases were key PTH1R-regulated SIK substrates in both chondrocytes and osteocytes. Taken together, our findings establish that SIK inhibition is central to PTH1R action in bone development and remodeling. Furthermore, this work highlights the key role of cAMP-regulated SIKs downstream of GPCR action.

PTH/PTHrP Receptor Signaling, Allostery, and Structures

The parathyroid hormone (PTH) type 1 receptor (PTHR) is the canonical G protein-coupled receptor (GPCR) for PTH and PTH-related protein (PTHrP) and the key regulator of calcium homeostasis and bone turnover. PTHR function is critical for human health to maintain homeostatic control of ionized serum Ca²⁺ levels and has several unusual signaling features, such as endosomal cAMP signaling, that are well-studied but not structurally understood. In this review, we discuss how recently solved high resolution near-atomic structures of hormone-bound PTHR in its inactive and active signaling states and discovery of extracellular Ca²⁺ allosterism shed light on the structural basis for PTHR signaling and function.

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.

Dental Follicle Cells: Roles in Development and Beyond

Dental follicle cells (DFCs) are a group of mesenchymal progenitor cells surrounding the tooth germ, responsible for cementum, periodontal ligament, and alveolar bone formation in tooth development. Cascades of signaling pathways and transcriptional factors in DFCs are involved in directing tooth eruption and tooth root morphogenesis. Substantial researches have been made to decipher multiple aspects of DFCs, including multilineage differentiation, senescence, and immunomodulatory ability. DFCs were proved to be multipotent progenitors with decent amplification, immunosuppressed and acquisition ability. They are able to differentiate into osteoblasts/cementoblasts, adipocytes, neuron-like cells, and so forth. The excellent properties of DFCs facilitated clinical application, as exemplified by bone tissue engineering, tooth root regeneration, and periodontium regeneration. Except for the oral and maxillofacial regeneration, DFCs were also expected to be applied in other tissues such as spinal cord defects (SCD), cardiomyocyte destruction. This article reviewed roles of DFCs in tooth development, their properties, and clinical application potentials, thus providing a novel guidance for tissue engineering.

Notch Coordinates Periodontal Ligament Maturation through Regulating Lamin A

Tooth eruption is a continuous biological process with dynamic changes at cellular and tissue levels, particularly within the periodontal ligament (PDL). Occlusion completion is a significant physiological landmark of dentition establishment. However, the importance of the involvement of molecular networks engaging in occlusion establishment on the final PDL maturation is still largely unknown. In this study, using rat and mouse molar teeth and a human PDL cell line for RNAseq and proteomic analysis, we systematically screened the key molecular links in regulating PDL maturation before and after occlusion establishment. We discovered Notch, a key molecular pathway in regulating stem cell fate and differentiation, is a major player in the event. Intercepting the Notch pathway by deleting its key canonical transcriptional factor, RBP-Jkappa, using a conditional knockout strategy in the mice delayed PDL maturation. We also identified that Lamin A, a cell nuclear lamina member, is a unique marker of PDL maturation, and its expression is under the control of Notch signaling. Our study therefore provides a deep insight of how PDL maturation is regulated at the molecular level, and we expect the outcomes to be applied for a better understanding of the molecular regulation networks in physiological conditions such as tooth eruption and movement and also for periodontal diseases.

Regulation of Hedgehog signaling Offers A Novel Perspective for Bone Homeostasis Disorder Treatment

The hedgehog (HH) signaling pathway is central to the regulation of bone development and homeostasis. HH signaling is not only involved in osteoblast differentiation from bone marrow mesenchymal stem cells (BM-MSCs), but also acts upstream within osteoblasts via the OPG/RANK/RANKL axis to control the expression of RANKL. HH signaling has been found to up-regulate parathyroid hormone related protein (PTHrP) expression in osteoblasts, which in turn activates its downstream targets nuclear factor of activated T cells (NFAT) and cAMP responsive element binding protein (CREB), and as a result CREB and NFAT cooperatively increase RANKL expression and osteoclastogenesis. Osteoblasts must remain in balance with osteoclasts in order to avoid excessive bone formation or resorption, thereby maintaining bone homeostasis. This review systemically summarizes the mechanisms whereby HH signaling induces osteoblast development and controls RANKL expression through PTHrP in osteoblasts. Proper targeting of HH signaling may offer a therapeutic option for treating bone homeostasis disorders.

Antagonistic interaction between Ezh2 and Arid1a coordinates root patterning and development via Cdkn2a in mouse molars

Patterning is a critical step during organogenesis and is closely associated with the physiological function of organs. Tooth root shapes are finely tuned to provide precise occlusal support to facilitate the function of each tooth type. However, the mechanism regulating tooth root patterning and development is largely unknown. In this study, we provide the first in vivo evidence demonstrating that Ezh2 in the dental mesenchyme determines patterning and furcation formation during dental root development in mouse molars. Mechanistically, an antagonistic interaction between epigenetic regulators Ezh2 and Arid1a controls Cdkn2a expression in the dental mesenchyme to regulate dental root patterning and development. These findings indicate the importance of balanced epigenetic regulation in determining the tooth root pattern and the integration of roots with the jaw bones to achieve physiological function. Collectively, our study provides important clues about the regulation of organogenesis and has general implications for tooth regeneration in the future.

[Effects of post-natal inhibition of RANKL on molar eruption and root formation in C57BL/6 mice]

INTRODUCTION

Recent observations performed in the orthodontic department of La Pitié-Salpêtrière hospital in Paris reported an increase of non-familial eruption defects of permanent molars. Our recent data have evidenced the involvement of osteoclasts (OC) in both the eruption and the dental retention processes through the RANKL/RANK/OPG signaling pathway. These facts are at the origin of the hypothesis of the existence of an environmental etiology for those eruption defects that would correspond to the perturbation of cellular autocrine/paracrine signaling pathways as the RANKL/ RANK/OPG.

MATERIALS AND METHODS

C57BL/6 mice were submitted to repeated injections with anti-RANKL neutralizing antibody during the nine days following birth. A phenotypic comparison with transgenic mice overexpressing RANK was performed for the functional characterization of the RANKL/RANK/OPG pathway. The dento-alveolar complex was analyzed using micro-CT for bone density and Masson's trichrome staining for histological examination.

RESULTS

The RANKL transient invalidation of RANKL stopped the molar root development and tooth eruption contrary to transgenic mice overexpressing RANK. The recruitment and the OC activity were strongly impacted.

DISCUSSION

This research is of direct clinical interest in understanding the pathology of eruption as indirect in establishing orthodontic treatment protocols for particular cases.

PTH1R signalling regulates the mechanotransduction process of cementoblasts under cyclic tensile stress

Objective

To investigate the regulatory role of type I parathyroid hormone receptor (PTH1R) signalling in the mechanotransduction process of cementoblasts under cyclic tensile stress (CTS).

Materials and methods

Immortalized cementoblast cell line OCCM-30 were employed and subjected to cyclic tensile strain applied by a four-point bending system. The expression of PTHrP and PTH1R, as well as cementoblastic transcription factor Runx-2, Osterix, and extracellular matrix protein COL-1 and OPN were assessed by quantitative real-time polymerase chain reaction and western blot analysis. PTH1R expression was knocked down by siPTH1R transfection, and the alteration of cementoblastic biomarkers expression was examined to evaluate the function of PTH1R. Furthermore, to investigate possible downstream molecules, expression of signal molecule ERK1/2 with or without siPTH1R transfection, and the effect of ERK inhibitor PD98059 on the expression of cementoblastic biomarkers was also examined.

Results

Cyclic tensile strain elevated the expression of PTHrP and PTH1R, as well as cementoblastic biomarkers Runx-2, Osterix, COL-1, and OPN in a time-dependent manner, which was inhibited by siPTH1R transfection. The expression of phosphorylated ERK1/2 was upregulated time-dependently under cyclic stretch, which was also inhibited by siPTH1R transfection, and pretreatment of p-ERK1/2 inhibitor PD98059 undermined the increase of Runx-2, Osterix, COL-1, and OPN prominently.

Conclusion

The findings of the present study indicate that PTH1R signalling plays a regulatory role in the CTS induced cementoblastic differentiation in mature cementoblasts, and ERK1/2 is essentially involved as a downstream intracellular signal molecule in this mechanotransduction process.

Autocrine regulation of mesenchymal progenitor cell fates orchestrates tooth eruption

Mesenchymal progenitor cells play important roles in the formation of skeletal tissues; however, how cell fates of mesenchymal progenitor cells are regulated remains largely unclear. We made use of the dental follicle surrounding the developing tooth bud, which critically regulates tooth eruption and tooth root formation. Dental follicle mesenchymal progenitor cells express parathyroid hormone-related peptide (PTHrP), a locally acting autocrine/paracrine ligand, and become essential skeletal cell types establishing the root–bone interface. These PTHrP⁺ mesenchymal progenitors maintained their physiological cell fates through the PTH/PTHrP receptor, a deficiency of which resulted in failure of tooth eruption phenotypes closely resembling human genetic conditions. We conclude that proper cell fates of mesenchymal progenitor cells are maintained by autocrine signaling to achieve functional formation of skeletal tissues.

Formation of functional skeletal tissues requires highly organized steps of mesenchymal progenitor cell differentiation. The dental follicle (DF) surrounding the developing tooth harbors mesenchymal progenitor cells for various differentiated cells constituting the tooth root–bone interface and coordinates tooth eruption in a manner dependent on signaling by parathyroid hormone-related peptide (PTHrP) and the PTH/PTHrP receptor (PPR). However, the identity of mesenchymal progenitor cells in the DF and how they are regulated by PTHrP-PPR signaling remain unknown. Here, we show that the PTHrP-PPR autocrine signal maintains physiological cell fates of DF mesenchymal progenitor cells to establish the functional periodontal attachment apparatus and orchestrates tooth eruption. A single-cell RNA-seq analysis revealed cellular heterogeneity of PTHrP⁺ cells, wherein PTHrP⁺ DF subpopulations abundantly express PPR. Cell lineage analysis using tamoxifen-inducible PTHrP-creER mice revealed that PTHrP⁺ DF cells differentiate into cementoblasts on the acellular cementum, periodontal ligament cells, and alveolar cryptal bone osteoblasts during tooth root formation. PPR deficiency induced a cell fate shift of PTHrP⁺ DF mesenchymal progenitor cells to nonphysiological cementoblast-like cells precociously forming the cellular cementum on the root surface associated with up-regulation of Mef2c and matrix proteins, resulting in loss of the proper periodontal attachment apparatus and primary failure of tooth eruption, closely resembling human genetic conditions caused by PPR mutations. These findings reveal a unique mechanism whereby proper cell fates of mesenchymal progenitor cells are tightly maintained by an autocrine system mediated by PTHrP-PPR signaling to achieve functional formation of skeletal tissues.

Mettl3-mediated m6A RNA methylation regulates the fate of bone marrow mesenchymal stem cells and osteoporosis

N⁶-methyladenosine (m⁶A) is the most abundant epigenetic modification in eukaryotic mRNAs and is essential for multiple RNA processing events during mammalian development and disease control. Here we show that conditional knockout of the m⁶A methyltransferase Mettl3 in bone marrow mesenchymal stem cells (MSCs) induces pathological features of osteoporosis in mice. Mettl3 loss-of-function results in impaired bone formation, incompetent osteogenic differentiation potential and increased marrow adiposity. Moreover, Mettl3 overexpression in MSCs protects the mice from estrogen deficiency-induced osteoporosis. Mechanistically, we identify PTH (parathyroid hormone)/Pth1r (parathyroid hormone receptor-1) signaling axis as an important downstream pathway for m⁶A regulation in MSCs. Knockout of Mettl3 reduces the translation efficiency of MSCs lineage allocator Pth1r, and disrupts the PTH-induced osteogenic and adipogenic responses in vivo. Our results demonstrate the pathological outcomes of m⁶A mis-regulation in MSCs and unveil novel epitranscriptomic mechanism in skeletal health and diseases.

The Nfic-osterix pathway regulates ameloblast differentiation and enamel formation

Enamel makes up the outermost layer of the crown and its hardness protects other dental tissues from various stimuli. Enamel cannot be regenerated once damaged because ameloblasts are lost during the tooth eruption. Since the ameloblast differentiation mechanism is still unknown, further research is essential for developing treatments for defective or damaged enamel. Previously, we have reported that osteoblast differentiation and bone formation were regulated through the runt-related transcription factor 2 (Runx2)-nuclear factor 1-C (Nfic)-osterix (Osx) pathway where Nfic directly controls Osx expression. This pathway regulates odontoblast differentiation and dentin formation as well. The aim of this study was to investigate if the same pathway is applicable for ameloblast differentiation. Structural enamel defects with disorganized ameloblasts and decreased proliferation activity of the cervical loop were observed in Nfic^-/- mice incisors. Expression of the ameloblast differentiation markers was also downregulated significantly in Nfic^-/- mice. Real-time PCR analyses suggested that Runx2, Nfic, and Osx regulate the expression of ameloblast differentiation markers, where Runx2 is upstream of Nfic, and Nfic controls Osx expression. Therefore, we suggest the Runx2-Nfic-Osx pathway as one of the key factors that regulate ameloblast differentiation.

The fate of Osterix-expressing mesenchymal cells in dental root formation and maintenance

OBJECTIVES

Osterix (Osx)-expressing mesenchymal cells are progenitors for tooth root forming cells. The aim of this study was to reveal the fates of Osx-expressing cells during and after root formation using a lineage tracing experiment.

MATERIAL AND METHODS

To reveal the fates of Osx-expressing dental mesenchymal progenitors, we took advantage of tamoxifen-inducible Cre reporter system. Osx-creER; R26R-tdTomato mice received tamoxifen (0.1 mg/body) at postnatal day 3 (P3). In this system, Osx-expressing at P3 (Osx-P3) cells undergo recombination, and they and their descendants continue to express Tomato red fluorescence protein permanently. Mandibles were dissected at serial time points ranging from P4 to P116 to investigate how Osx-P3 cells participated in root formation. Tomato⁺ cells on frozen sections were imaged under fluorescence microscopy.

RESULTS

Osx-P3 cells and their descendants differentiated into all kinds of cells that contributed to the root and periodontal tissues, such as odontoblasts, cementoblasts, alveolar bone osteoblasts and periodontal ligament (PDL) cells during root formation. Even after root formation was completed, they persisted in dental pulp and PDL to provide progenitor cells for odontoblasts and cementoblasts.

CONCLUSION

Osx-expressing cells play important roles in the entire processes of tooth root formation; their progeny continue to contribute to maintenance of tooth root even after root formation is complete.

Specificity Protein 7 Is Required for Proliferation and Differentiation of Ameloblasts and Odontoblasts

The Sp7/Osterix transcription factor is essential for bone development. Mutations of the Sp7 gene in humans are associated with craniofacial anomalies and osteogenesis imperfecta. However, the role of Sp7 in embryonic tooth development remains unknown. Here we identified the functional requirement of Sp7 for dentin synthesis and tooth development. Sp7-null mice exhibit craniofacial dysmorphogenesis and are completely void of alveolar bone. Surprisingly, initial tooth morphogenesis progressed normally in Sp7-null mice. Thus the formation of alveolar bone is not a prerequisite for tooth morphogenesis. Sp7 is required for mineralization of palatal tissue but is not essential for palatal fusion. The reduced proliferative capacity of Sp7-deficient ectomesenchyme results in small and misshapen teeth with randomly arranged cuboidal preodontoblasts and preameloblasts. Sp7 promotes functional maturation and polarization of odontoblasts. Markers of mature odontoblast (Col1a, Oc, Dspp, Dmp1) and ameloblast (Enam, Amelx, Mmp20, Amtn, Klk4) are barely expressed in incisors and molar tissues of Sp7-null mice. Consequently, dentin and enamel matrix are absent in the Sp7-null littermates. Interestingly, the Sp7 expression is restricted to cells of the dental mesenchyme indicating the effect on oral epithelium-derived ameloblasts is cell-nonautonomous. Abundant expression of Fgf3 and Fgf8 ligand was noted in the developing tooth of wild-type mice. Both ligands were remarkably absent in the Sp7-null incisor and molar, suggesting cross-signaling between mesenchyme and epithelium is disrupted. Finally, promoter-reporter assays revealed that Sp7 directly controls the expression of Fgf-ligands. Together, our data demonstrate that Sp7 is obligatory for the differentiation of both ameloblasts and odontoblasts but not for the initial tooth morphogenesis. © 2018 American Society for Bone and Mineral Research.

GATA4 as a novel regulator involved in the development of the neural crest and craniofacial skeleton via Barx1

The role of GATA-binding protein 4 (GATA4) in neural crest cells (NCCs) is poorly defined. Here we showed that mouse NCCs lacking GATA4 exhibited developmental defects in craniofacial bone, teeth, and heart. The defects likely occurred due to decreased cell proliferation at the developmental stage. The in vitro results were consistent with the mouse model. The isobaric tags for relative and absolute quantitation assay revealed that BARX1 is one of the differentially expressed proteins after GATA4 knockdown in NCCs. On the basis of the results of dual-luciferase, electro-mobility shift, and chromatin immunoprecipitation assays, Barx1 expression is directly regulated by GATA4 in NCCs. In zebrafish, gata4 knockdown affects the development of NCCs derivatives. However, the phenotype in zebrafish could be partly rescued by co-injection of gata4 morpholino oligomers and barx1 mRNA. This study identified new downstream targets of GATA4 in NCCs and uncovered additional evidence of the complex regulatory functions of GATA4 in NCC development.

Expression of parathyroid hormone related protein (PTHRP) in ameloblastomas

BACKGROUND

Presence of parathyroid hormone related protein (PTHrP) might suggest that ameloblastomas recapitulate features of the enamel epithelium and induce bone resorption, which would facilitate their growth and local invasion. The aim of this study was to determine the expression of PTHrP in ameloblastomas.

MATERIAL AND METHODS

An observational research study was designed including 39 cases of histologically diagnosed ameloblastomas (39 out of 42 patients gave consent for the use of their medical records and all data required for this study). Gender, age, tumor location, histological type and subtype of the tumor were recorded and PTHrP expression was determined by indirect immunohistochemistry using monoclonal anti-human PTHrP (1D1 / Santa Cruz Biotechnology). Protein expression and intensity were evaluated under light microscope and finally data recorded and statistically analyzed. This research was approved by the Caracas West General Hospital review board.

RESULTS

39 cases of ameloblastomas were evenly distributed between genders (49% male and 51% female) with a mean age of 33 ± 3.53 years, mainly affecting the posterior mandible. 20 cases (51.28%) showed positive cytoplasmic immunoreactivity to PTHrP. 8 out of 15 cases of solid/multicystic ameloblastomas and 12 out of 23 cases of unicystic ameloblastomas were PTHrP positive. Intense expression of PTHrP was observed in 4 unicystic ameloblastomas (all luminal subtype) and in 5 cases of conventional ameloblastomas.

CONCLUSIONS

In the present study PTHrP expression in solid multicystic and unicystic ameloblastoma suggests its possible function in the biological behavior of the tumor. More studies are needed in order to determine the possible role of this protein related to bone invasion processes. Key words:Parathyroid hormone related protein, PTHrP, ameloblastoma, bone.

Essential Role of IFT140 in Promoting Dentinogenesis

Primary cilia, with highly regulated cellular sensory functions, play key roles in tissue development and function maintenance. Intraflagellar transport 140 (IFT140) is a subunit of IFT complex A, which is specialized for retrograde transportation in cilia. Mutations of Ift140 are usually associated with syndromic ciliopathy and may cause isolated diseases such as retinal dystrophy, short ribs, and polycystic kidney. However, the role of IFT140 in tooth development has not been well investigated. In this study, a close relationship between IFT140 and dentin formation is disclosed. During tooth development, IFT140 was highly expressed in odontoblasts. To further understand the role of IFT140 in dentinogenesis, Ift140^flox/flox/Osx-Cre mouse was generated. The dentin thickness of Ift140^flox/flox/Osx-Cre mouse is thinner and the dentin formation is slower than that in control. In vitro, deletion of IFT140 in odontoblasts led to poor odontogenic differentiation, abnormal primary cilia, and decreased Sonic hedgehog signaling molecules. More important, due to loss of primary cilia in odontoblasts by IFT140 deletion, reparative dentin formation was impaired in a tooth-drilling model. These results suggest that cilia gene IFT140 is essential in promoting dentin formation and reparation.