TNF-alpha upregulates expression of BMP-2 and BMP-3 genes in the rat dental follicle--implications for tooth eruption

TNF reduces osteogenic cell fate in PDL cells at transcriptional and functional levels without alteration of periodontal proliferative capacity

AIMS

To investigate the effect of tumor necrosis factor (TNF) on the growth of human periodontal ligament (PDL) cells, their osteogenic differentiation and modulation of their matrix secretion in vitro.

METHODS

The influence of 10 ng/ml TNF on proliferation and metabolic activity of PDL cells was analyzed by cell counting (DAPI [4',6-diamidino-2-phenylindole] staining) and the MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) assay. In addition, cells were cultured under control conditions and osteogenic conditions (media containing 10 mM β-glycerophosphate). Quantitative expression analysis of genes encoding the osteogenic markers alkaline phosphatase (ALP), collagen type I alpha 1 chain (COL1A1), osteoprotegerin (OPG), and osteopontin (OPN) was performed after 7 and 14 days of cultivation. Calcium deposits were stained with alizarin red.

RESULTS

Our studies showed that 10 ng/ml TNF did not affect the survival and metabolic activity of PDL cells. Quantitative expression analysis revealed that long-term cultures with TNF impaired osteogenic cell fate at early and late developmental stages. Furthermore, TNF significantly reduced matrix secretion in PDL cells.

CONCLUSION

The present data confirm TNF as a regulatory factor of proinflammatory remodeling that influences the differentiation behavior but not the metabolism and cell proliferation of the periodontium. Therefore, TNF represents an interesting target for the regulation of orthodontic remodeling processes in the periodontium.

Metabolic Bone Diseases Affecting Tooth Eruption: A Narrative Review

Tooth eruption is an essential process for the development of the oral and maxillofacial system. Several inherited and acquired diseases might affect this tightly regulated process, resulting in premature, delayed, or even failed tooth eruption. The purpose of this article is to review the literature and the clinical parameters of metabolic bone diseases that affect tooth eruption. It examines the physiological aspects of tooth eruption and the pathophysiological changes induced by metabolic bone diseases, including changes in bone metabolism, density, and structure. The search strategy for this review included an electronic search in PubMed, Google Scholar, Medline, Scopus, and the Cochrane Library using the following keywords: "metabolic bone diseases", "tooth eruption", "delayed tooth eruption", and each reported disease in combination with "tooth eruption disorders", covering publications up to March 2024 and limited to English-language sources. Understanding the influence of metabolic bone diseases on tooth eruption is crucial for managing both dental and skeletal manifestations associated with these disorders. This review suggests that a multidisciplinary approach to treatment may significantly improve oral outcomes for patients suffering from such conditions. Clinicians should be aware of the specific dental abnormalities that may arise and consider comprehensive evaluations and individualized treatment plans. These findings underscore the need for further research into targeted therapies that address these abnormalities.

Epithelial-specific deletion of FAM20A leads to short root defects

Short Root Defects defined by a reduced ratio of root to crown, may culminate in root resorption and subsequent tooth loss, in spite of the absence of apparent symptoms. Such defects present considerable impediments to orthodontic treatment and restoration. Recent identification of Fam20a, an emergent pseudokinase, has been associated with enamel development and tooth eruption, yet its definitive role in root formation and eruption remains ambiguous. In this research, we initially ascertained that the targeted knockout of Fam20a within the epithelium led to truncated tooth roots, irregular breaks in the epithelial root sheath initiation of the WNT signaling pathway, and decreased expression of the cell polarity-related transcription factor Cdc42 in murine models. This was concomitant with the participation of the associated epithelial root sheath developmental pathways BMP2, Gli1, and Nfic. Furthermore, we observed that Fam20a predominantly affects the intraosseous eruption phase of tooth emergence. During this phase, the osteoclast peak around the mandibular first molar in cKO mice is delayed, leading to a slower formation of the eruption pathway, ultimately resulting in delayed tooth eruption in mice. The findings of this study enrich the extant knowledge regarding the role of Fam20a, suggesting its potential regulatory function in tooth root development through the WNT/β-catenin/Cdc42 pathway.

Tumor Necrosis Factor Alpha Signaling and Organogenesis

Tumor necrosis factor alpha (TNF-α) plays important roles in processes such as immunomodulation, fever, inflammatory response, inhibition of tumor formation, and inhibition of viral replication. TNF-α and its receptors are ubiquitously expressed in developing organs and they regulate the survival, proliferation, and apoptosis of embryonic stem cells (ESCs) and progenitor cells. TNF-α is an important inflammatory factor that also regulates the inflammatory response during organogenesis, and its cytotoxic effects can interfere with normal developmental processes, even leading to the onset of diseases. This review summarizes the various roles of TNF-α in organogenesis in terms of its secreting pattern, concentration-dependent activities, and interactions with other signaling pathways. We also explored new potential functions of TNF-α.

Rodent incisor and molar dental follicles show distinct characteristics in tooth eruption

OBJECTIVE

Rodent incisors and molars show different eruption patterns. Dental follicles and their interaction with dental epithelia play key roles in tooth eruption. However, little is known about the differences between incisor dental follicle (IF) and molar dental follicle (MF) during tooth eruption of rodents. This study aimed to investigate the differences between IF and MF during tooth eruption under induction with cervical-loop cells (CLC) and Hertwig's epithelial root sheath (HERS) cells of rats.

MATERIALS AND METHODS

CLC, HERS, IF, MF cells were isolated from 10 postnatal day 7 rats and identified by immunofluorescence staining. CLC or HERS cells-derived conditioned medium (CM) was obtained to induce IF and MF cells. Cell proliferation, mineralization, gene and protein expression related to tooth eruption were detected, and histological analysis was also performed.

RESULTS

The osteogenic differentiation and mineralization abilities of IF cells were stronger than those of MF cells. Both CLC and HERS cells-derived CM enhanced these abilities of IF cells, whereas they showed the opposite effect on MF cells. At 7, 10, and 15 d after birth, IF cells expressed more OPG and less RANKL than MF cells.

CONCLUSIONS

IF and MF cells present distinct characteristics in tooth eruption, CLC and HERS cells have significant inductive effects on them.

Diabetes and calcification: The potential role of anti-diabetic drugs on vascular calcification regression

Vascular calcification (VC) has been well-established as an independent and strong predictor of cardiovascular diseases (CVD) as well as major cardiac adverse events (MACE). VC is associated with increased mortality in patients with CVD. Pathologically, VC is now believed to be a multi-directional active process ultimately resulting in ectopic calcium deposition in vascular beds. On the other hand, prevalence of diabetes mellitus (DM) is gradually increasing thus making the current population more prone to future CVD. Although the mechanisms involved in development and progression of VC in DM patients are not fully understood, a series of evidences demonstrated positive association between DM and VC. It has been highlighted that different cellular pathways are involved in this process. These intermediates such as tumor necrosis factor alpha (TNF-α), various interleukins (ILs) and different cell-signaling pathways are over-expressed in DM patients leading to development of VC. Thus, considering the burden and significance of VC it is of great importance to find a therapeutic approach to prevent or minimize the development of VC in DM patients. Over the past few years various anti diabetic drugs (ADDs) have been introduced and many of them showed desired glucose control. But no study demonstrated the effects of these medications on regression of VC. In this review, we will briefly discuss the current understanding on DM and VC and how commonly used ADDs modulate the development or progression of VC.

[Expression patterns of ectodysplasin and ectodysplasin receptor during early dental development in zebrafish]

OBJECTIVE

This study aims to study the expression patterns of ectodysplasin (EDA) and ectodysplasin receptor (EDAR) during the early development of zebrafish and provide a foundation for further research of the Eda signaling pathway in tooth development.

METHODS

Total RNA was extracted from zebrafish embryos at 48 hours postfertilization (hpf) and then reverse transcribed for cDNA library generation. The corresponding RNA polymerase was selected for the synthesis of the digoxin-labeled antisense mRNA probe of zebrafish pharyngeal tooth specific marker dlx2b and Eda signaling-associated genes eda and edar in vitro. The three sequences were ligated into a pGEMT vector with a TA cloning kit, and polymerase chain reaction (PCR) was applied to linearize the plasmid. The resultant PCR sequences were used as templates for synthesizing Dig-labeled mRNA probe dlx2b, eda, and edar. Zebrafish embryos were collected at 36, 48, 56, 60, 72, and 84 hpf, then whole mount in situ hybridization was performed for the detection of eda and edar expression patterns. Then, their expression patterns at 72 hpf were compared with the expression pattern of dlx2b.

RESULTS

The mRNA antisense probes of dlx2b, eda, and edar were successfully obtained. The positive signals of eda and edar were observed in zebrafish pharyngeal tooth region at 48-72 hpf and thus conform to the signals of dlx2b in the positive regions.

CONCLUSIONS

The ligand eda and edar, which are associated with the Eda signaling pathway, are strongly expressed only at the pharyngeal tooth region in zebrafish from tooth initiation to the morphogenesis stage. Thus, the Eda signaling pathway may be involved in the regulation of the early development of zebrafish pharyngeal teeth.

RANKL is associated with persistent primary teeth and delayed permanent tooth emergence

BACKGROUND

Tooth eruption is a process that is not fully understood.

AIM

To evaluate whether genetic polymorphisms for RANK/RANKL/OPG are associated with delayed tooth emergence. To evaluate whether the relative expression of this genes is associated with persistent primary teeth.

DESIGN

To evaluate whether genetic polymorphisms for RANK/RANKL/OPG could be involved in delayed tooth emergence, saliva samples from 160 children, aged 6-13 years old, were analysed. To test if there is correlation between gene expression of RANK/RANKL/OPG in children with delayed tooth emergence and persistent primary teeth, periapical tissue from 15 children with persistent primary teeth and from 15 control subjects were collected for qPCR analysis.

RESULTS

Fifty-six children with delayed tooth emergence (35%) had at least one permanent tooth with delayed emergence. The T allele in RANKL (rs9594738) increased the risk of delayed tooth emergence (P = 0.02; OR = 1.71, 95%CI 1.09-2.75). The relative gene expression for RANKL and the ratio RANKL/OPG in children with delayed tooth emergence and persistent primary teeth were lower compared to controls (P = 0.02 and P = 0.005, respectively).

CONCLUSIONS

Data suggest that the polymorphism rs9594738 in RANKL is associated with delayed permanent tooth emergence. Moreover, reduced relative gene expression of RANKL in periapical tissue is associated with persistent primary teeth.

Matrix Metalloproteinase-1 and Acid Phosphatase in the Degradation of the Lamina Propria of Eruptive Pathway of Rat Molars

The comprehension of dental pathogenesis and disorders derived from eruption failure requires a deep understanding of the molecular mechanisms underlying normal tooth eruption. As intense remodelling is needed during tooth eruption, we hypothesize that matrix metalloproteinase-1 (MMP-1) and acid phosphatase (ACP) play a role in the eruptive pathway degradation. We evaluated MMP-1-immunoexpression and the collagen content in the lamina propria at different eruptive phases. Immunohistochemistry and ultrastructural cytochemistry for detection of ACP were also performed. In the maxillary sections containing first molars of 9-, 11-, 13-, and 16-day-old rats, the birefringent collagen of eruptive pathway was quantified. MMP-1 and ACP-2 immunohistochemical reactions were performed and the number of MMP-1-immunolabelled cells was computed. Data were analyzed by one-way ANOVA and Tukey post-test (p ≤ 0.05). ACP cytochemistry was evaluated in specimens incubated in sodium β-glycerophosphate. In the eruptive pathway of 13- and 16-day-old rats, the number of MMP-1-immunolabelled cells increased concomitantly to reduction of collagen in the lamina propria. Enhanced ACP-2-immunolabelling was observed in the lamina propria of 13- and 16-day-old rats. Fibroblasts and macrophages showed lysosomes and vacuoles containing fragmented material reactive to ACP. MMP-1 degrades extracellular matrix, including collagen fibers, being responsible for the reduction in the collagen content during tooth eruption. The enhanced ACP activity at the mucosal penetration stage indicates that this enzyme plays a role in the degradation of remnant material, which is engulfed by macrophages and fibroblasts of the eruptive pathway. Therefore, enzymatic failure in the eruptive pathway may disturbs tooth eruption.

Study on site-specific expression of bone formation and resorption factors in human dental follicles

We sought to investigate site‐specific expression of bone‐regulatory factors expressed by human dental follicles and to compare the stimulated expression of tumour necrosis factor (ligand) superfamily, member 11/tumour necrosis factor receptor superfamily, member 11b (RANKL/OPG) in human dental follicle cells (HDFCs) from different patients. Analysis of bone‐regulatory markers in follicles from 12 different study participants was performed using RT‐qPCR and immunofluorescence; apical and coronal segments from each dental follicle were processed independently. Four additional dental follicles were used for cell cultures; HDFCs were precultured in osteogenic medium to initiate differentiation and thereafter cultured with 10⁻⁶ M forskolin (FSK) to activate the protein kinase cAMP (PKA/cAMP) signalling pathway and induce RANKL/OPG expression. We demonstrate that RANKL expression is significantly higher in the coronal part of follicles than in the apical part. High levels of collagen type 1 (COL1), alkaline phosphatase (ALP) and Gap‐junction protein, alpha 1, 43 kDa (CX43) were expressed, whereas expression of Sp7 transcription factor (OSX), bone morphogenetic protein 2 (BMP2), colony‐stimulating factor 1 (CSF‐1), chemokine (C‐C motif) ligand 2 (MCP1), and OPG was low in all samples. The immunofluorescence localization of CSF‐1, MCP1, osteocalcin (OCN), RANKL, and BMP2 was not specific for either part of the follicles. In conclusion, a consistently high expression of CX43 suggests that gap‐junction communication in HDFCs is essential for the eruption process. Furthermore, the induced expression of RANKL in HDFCs varies significantly between individuals and may relate to clinical variations in tooth eruption.

Gene expression profiles in dental follicles from patients with impacted canines

Animal studies suggest that the dental follicle (DF) plays a major role in tooth eruption. However, the role of the DF during tooth impaction and related root resorptions in adjacent teeth is not clear. The hypothesis for the present study is that expression of regulatory factors involved in the bone remodelling process necessary for tooth eruption may differ between dental follicles from teeth with different clinical situations. We have analysed the gene expression profiles in the DF obtained from impacted canines, with (N = 3) or without (N = 5) signs of root resorption, and from control teeth (normal erupting teeth, mesiodens) (N = 3). DF from 11 patients (mean age: 13 years) obtains at the time of surgical exposure of the tooth. Due to the surgical time point, all teeth were in a late developmental stage. Gene expression related to osteoblast activation/bone formation, osteoclast recruitment and activation was analysed by RTqPCR. Genes related to bone formation (RUNX2, OSX, ALP, OCN, CX43) were highly expressed in all the samples, but osteoclast recruitment/activation markers (OPG, RANKL, MCP-1, CSF-1) were negligible. No apparent patterns or significant differences in gene expression were found between impacted canines, with or without signs of root resorption, or when compared to control teeth. Our results suggest the DF regulation of osteoclastic activity is limited in the late pre-emergent stage of tooth development, irrespective if the tooth is normally erupting or impacted. We suggest that the follicle may have an important regulatory function for alveolar bone formation in the final eruption process and CX43-gap junction communication could be an important signalling pathway.

Postmenopausal osteoporosis in rheumatoid arthritis: The estrogen deficiency-immune mechanisms link

Rheumatoid arthritis (RA) is characterized, among other factors, by systemic bone loss, reaching ~50% prevalence of osteoporosis in postmenopausal women. This is roughly a doubled prevalence in comparison with age-matched non-RA women. Postmenopausal RA women are more likely to be sero-positive for the anti-citrullinated peptide antibody (ACPA). Our extensive review of recent scientific literature enabled us to propose several mechanisms as responsible for the accelerated bone loss in ACPA(+) RA postmenopausal women. Menopause-associated estrogen deficiency plays a major role in these pathological mechanisms, as follows.

Epithelial - Mesenchymal Interactions in Tooth Development and the Significant Role of Growth Factors and Genes with Emphasis on Mesenchyme - A Review

The recent advancements in medical research field mainly highlights the genetic and molecular aspects of various disease processes and related treatment options, in a specialized "custom-made" approach. The medical and dental field has made tremendous progress in providing even with the smallest insight into pathological entities, thus, making patient management more fruitful. But, short comings have occurred in dental treatments involving odontogenic lesions mainly due to poor understanding of the developmental cycle involved during early stages of developmental process. Multiple numbers of interactions take place during embryo formation and further proliferation of tissue. One such important step is the interaction between epithelium and mesenchyme which tantamount to functional requirements of an individual tooth. The role of extra cellular molecules and genes has to be studied in depth to assess the impact and significance attached to it as the synergistic function of various elements underlines the complex process of development.

[Research progress on the cellular and molecular mechanisms of tooth eruption]

Tooth eruption is a series of complicated physiological processes occurring once the crown is formed completely, as well as when the tooth moves toward the occasion plane. As such, the tooth moves through the alveolar bone and the oral mucosa until it finally reaches its functional position. Most studies indicate that the process of tooth eruption involves the alveolar bone, dental follicles, osteoclasts, osteoblasts, and multiple cytokines. Dental follicles regulate both resorption and formation of the alveolar bone, which is required for tooth eruption. Furthermore, root formation with periodontal ligament facilitates continuous tooth eruption. However, the exact mechanism underlying tooth eruption remains unclear. Hence, this review describes the recent research progress on the cellular and molecular mechanisms of tooth eruption.

Inflammation in arthritis induces expression of BMP3, an inhibitor of bone formation

OBJECTIVES

Inflammation in diseases such as rheumatoid arthritis (RA) stimulates osteoclast-mediated articular bone erosion and inhibits osteoblast-mediated bone formation, leading to a net loss of bone. Pro-inflammatory cytokines and antagonists of the Wnt signalling pathway have been implicated in the inhibition of osteoblast differentiation and activity in RA, contributing to the erosive process and impairing erosion healing. Importantly, osteoblast differentiation and function are also regulated by the osteogenic bone morphogenetic protein (BMP) signalling pathway, which is antagonized by BMP3. We therefore examined the potential role of BMP3 in inflammatory arthritis.

METHOD

Two murine models of RA, K/BxN serum transfer arthritis (STA) and antigen-induced arthritis (AIA), were used to establish the temporal expression of BMP3 and the cellular sources of BMP3 mRNA and protein in inflammatory arthritis. To determine the effects of inflammation on the expression of BMP3 in osteoblasts, murine calvarial osteoblasts were treated with pro-inflammatory cytokines and BMP3 expression was assessed.

RESULTS

In both murine models of RA, BMP3 mRNA and protein are highly expressed by osteoblasts lining inflammation-bone interfaces late in the course of arthritis. Synovial tissues are not a significant source of BMP3. BMP3 expression is induced in osteocalcin-expressing osteoblasts in vitro following stimulation by tumour necrosis factor (TNF).

CONCLUSIONS

These data implicate BMP3 as a novel factor that may act locally to contribute to the erosive process and inhibit the repair of articular bone in RA through inhibition of osteoblast differentiation and function.

Evaluation of alveolar bone dimensions in unilateral palatally impacted canine: a cone-beam computed tomographic analyses

INTRODUCTION

The purpose of this study was to evaluate the alveolar bone dimensions and arch perimeter discrepancy in unilateral palatally impacted canines.

METHODS

This retrospective split-mouth study reviewed 207 cone-beam computed tomography (CBCT) scans of patients with unilateral or bilateral impacted canines referred for orthodontic treatment. Out of 207 CBCT scans, only 39 scans in which canines were unilaterally palatally impacted were approved for the study based on our inclusion and exclusion criteria. Two fiducial lines were used to orient the examiners and standardize the measurements. Arch perimeter, buccopalatal (BP) width, and alveolar height were measured both on the impacted and non-impacted side. Two different examiners measured all the parameters. A one-sample Kolmogorov-Smirnov test was used to examine normality of distribution of outcomes. Wilcoxon Signed Rank tests were used for comparing arch width and alveolar bone height between the impacted and non-impacted sides. For arch perimeter, the paired-sample t-test was used.

RESULTS

The arch perimeter, BP width, and alveolar bone height was significantly decreased on the impacted side (P < 0.05). The mean arch perimeter on the impacted side was 41.7 ± 2.5mm compared to 43.5 ± 2.37 on the non-impacted side. Similarly, BP width and alveolar bone height on the impacted side was 6.87 ± 1.08 mm and 18.12 ± 2.28 mm, respectively, whereas on the non-impacted side was 8.70 ± 1.13 mm and 19.49 ± 2.09 mm, respectively.

CONCLUSION

There was a significant decrease in the arch perimeter and alveolar bone dimensions (BP width and alveolar bone height) on the impacted side.

JNK MAPK is involved in BMP-2-induced odontoblastic differentiation of human dental pulp cells

Bone morphogenetic protein-2 (BMP-2) is a multi-functional growth factor belonging to the transforming growth factor β superfamily that has a broad range of activities that affect many different cell types. BMP-2 induces odontoblastic differentiation of human dental pulp cells (DPCs), but the underlying mechanism remains unclear. In this study, we investigated the potential role of the JNK mitogen-activated protein kinases (MAPK) pathway in BMP-2-induced odontoblastic differentiation of DPCs. The levels of phosphorylated and unphosphorylated JNK MAPK were quantified by Western blot analysis following treatment with BMP-2 and the JNK inhibitor SP600125. The role of JNK MAPK in the BMP-2-induced odontoblastic differentiation of DPCs was determined by measuring alkaline phosphatase (ALP) activity and by examining the expression of odontoblastic markers using quantitative real-time polymerase chain reaction analysis. The effect of JNK MAPK silencing on odontoblastic differentiation was also investigated. BMP-2 upregulated the phosphorylation of JNK in DPCs in a dose- and time-dependent manner. Early markers of odontoblastic differentiation, including ALP activity, osteopontin and dentin matrix protein-1, were not inhibited by the JNK inhibitor. However, the JNK inhibitor, SP600125, significantly inhibited late-stage differentiation of odontoblasts, including the gene expression of osteocalcin, dentin sialophosphoprotein and bone sialoprotein, and also reduced the formation of mineralized nodules in BMP-2-treated DPCs. Consistent with this observation, silencing of JNK MAPK also decreased late-stage odontoblastic differentiation. Taken together, these findings suggest that JNK activity is required for late-stage odontoblastic differentiation induced by BMP-2.

Expression of bone morphogenetic protein-6 in dental follicle stem cells and its effect on osteogenic differentiation

The dental follicle (DF) plays an essential role in tooth eruption via regulation of bone resorption and bone formation. Bone morphogenetic protein-6 (BMP6) expression in the DF is coincident with bone growth in the tooth crypt. DF stem cells (DFSCs) have been shown to possess strong osteogenic capability. This study aims to determine the expression of BMP6 in DFSCs and to elucidate the role of BMP6 in the osteogenesis of DFSCs. DFSCs and their non-stem cell counterpart, DF cells (DFCs), were obtained from the DFs of rat pups. We showed that expression of BMP6 was significantly higher in the DFSCs than in the DFCs. DFSCs lost osteogenic capability during in vitro expansion, and DFSCs in late passages had reduced BMP6 expression as compared to early passages of DFSCs when they were subjected to osteogenic induction. Addition of exogenous human recombinant BMP6 (hrBMP6) to the osteogenic medium dramatically enhanced the osteogenesis of the late-passage DFSCs. Knockdown of BMP6 by short interfering RNA in the DFSCs in early passages resulted in a decrease in osteogenesis, which could be restored by addition of hrBMP6. We concluded that DFSCs need to express high levels of BMP6 to maintain their osteogenesis capability. Increased BMP6 expression seen in vivo in the DF may reflect the activation of DFSCs for osteogenic differentiation for bone growth during tooth eruption.

Augmented BMPRIA-mediated BMP signaling in cranial neural crest lineage leads to cleft palate formation and delayed tooth differentiation

The importance of BMP receptor Ia (BMPRIa) mediated signaling in the development of craniofacial organs, including the tooth and palate, has been well illuminated in several mouse models of loss of function, and by its mutations associated with juvenile polyposis syndrome and facial defects in humans. In this study, we took a gain-of-function approach to further address the role of BMPR-IA-mediated signaling in the mesenchymal compartment during tooth and palate development. We generated transgenic mice expressing a constitutively active form of BmprIa (caBmprIa) in cranial neural crest (CNC) cells that contributes to the dental and palatal mesenchyme. Mice bearing enhanced BMPRIa-mediated signaling in CNC cells exhibit complete cleft palate and delayed odontogenic differentiation. We showed that the cleft palate defect in the transgenic animals is attributed to an altered cell proliferation rate in the anterior palatal mesenchyme and to the delayed palatal elevation in the posterior portion associated with ectopic cartilage formation. Despite enhanced activity of BMP signaling in the dental mesenchyme, tooth development and patterning in transgenic mice appeared normal except delayed odontogenic differentiation. These data support the hypothesis that a finely tuned level of BMPRIa-mediated signaling is essential for normal palate and tooth development.

Comparative genomics identifies the mouse Bmp3 promoter and an upstream evolutionary conserved region (ECR) in mammals

The Bone Morphogenetic Protein (BMP) pathway is a multi-member signaling cascade whose basic components are found in all animals. One member, BMP3, which arose more recently in evolution and is found only in deuterostomes, serves a unique role as an antagonist to both the canonical BMP and Activin pathways. However, the mechanisms that control BMP3 expression, and the cis-regulatory regions mediating this regulation, remain poorly defined. With this in mind, we sought to identify the Bmp3 promoter in mouse (M. musculus) through functional and comparative genomic analyses. We found that the minimal promoter required for expression in resides within 0.8 kb upstream of Bmp3 in a region that is highly conserved with rat (R. norvegicus). We also found that an upstream region abutting the minimal promoter acts as a repressor of the minimal promoter in HEK293T cells and osteoblasts. Strikingly, a portion of this region is conserved among all available eutherian mammal genomes (47/47), but not in any non-eutherian animal (0/136). We also identified multiple conserved transcription factor binding sites in the Bmp3 upstream ECR, suggesting that this region may preserve common cis-regulatory elements that govern Bmp3 expression across eutherian mammals. Since dysregulation of BMP signaling appears to play a role in human health and disease, our findings may have application in the development of novel therapeutics aimed at modulating BMP signaling in humans.

BMP activity is required for tooth development from the lamina to bud stage

Several Bmp genes are expressed in the developing mouse tooth germ from the initiation to the late-differentiation stages, and play pivotal roles in multiple steps of tooth development. In this study, we investigated the requirement of BMP activity in early tooth development by transgenic overexpression of the extracellular BMP antagonist Noggin. We show that overexpression of Noggin in the dental epithelium at the tooth initiation stage arrests tooth development at the lamina/early-bud stage. This phenotype is coupled with a significantly reduced level of cell proliferation rate and a down-regulation of Cyclin-D1 expression, specifically in the dental epithelium. Despite unaltered expression of genes known to be implicated in early tooth development in the dental mesenchyme and dental epithelium of transgenic embryos, the expression of Pitx2, a molecular marker for the dental epithelium, became down-regulated, suggesting the loss of odontogenic fate in the transgenic dental epithelium. Our results reveal a novel role for BMP signaling in the progression of tooth development from the lamina stage to the bud stage by regulating cell proliferation and by maintaining odontogenic fate of the dental epithelium.

Noggin is required for early development of murine upper incisors

BMP signaling plays crucial roles in the development of many organs, including the tooth. Equally important is BMP signaling homeostasis, as demonstrated by multiple organ defects in mice lacking the extracellular BMP antagonist Noggin. Here, we show that Noggin is initially expressed in the maxillary mesenchyme adjunct to the upper incisor at the initiation stage, and then in the developing teeth, including incisors and molars, from the bud stage. Noggin mutants develop normal molars and mandibular incisors, but form a single, medially located upper incisor that is arrested at the late bud stage. Histological and molecular marker analyses demonstrated that two distinct upper incisor placodes initiate independently at E11.5, but begin to fuse at E12.5, coupling with elevated cell proliferation rates in the developing tooth germs. We further found that Chordin and Gremlin, two other BMP antagonists, are co-expressed with Noggin in the developing lower incisor and molar teeth. These observations indicate the importance of BMP signaling homeostasis, and suggest a functional redundancy between BMP antagonists during tooth development.

BmprIa is required in mesenchymal tissue and has limited redundant function with BmprIb in tooth and palate development

The BMP signaling plays a pivotal role in the development of craniofacial organs, including the tooth and palate. BmprIa and BmprIb encode two type I BMP receptors that are primarily responsible for BMP signaling transduction. We investigated mesenchymal tissue-specific requirement of BmprIa and its functional redundancy with BmprIb during the development of mouse tooth and palate. BmprIa and BmprIb exhibit partially overlapping and distinct expression patterns in the developing tooth and palatal shelf. Neural crest-specific inactivation of BmprIa leads to formation of an unusual type of anterior clefting of the secondary palate, an arrest of tooth development at the bud/early cap stages, and severe hypoplasia of the mandible. Defective tooth and palate development is accompanied by the down-regulation of BMP-responsive genes and reduced cell proliferation levels in the palatal and dental mesenchyme. To determine if BmprIb could substitute for BmprIa during tooth and palate development, we expressed a constitutively active form of BmprIb (caBmprIb) in the neural crest cells in which BmprIa was simultaneously inactivated. We found that substitution of BmprIa by caBmprIb in neural rest cells rescues the development of molars and maxillary incisor, but the rescued teeth exhibit a delayed odontoblast and ameloblast differentiation. In contrast, caBmprIb fails to rescue the palatal and mandibular defects including the lack of lower incisors. Our results demonstrate an essential role for BmprIa in the mesenchymal component and a limited functional redundancy between BmprIa and BmprIb in a tissue-specific manner during tooth and palate development.