Smad3 is required for enamel biomineralization

The role of the TGF-β1 signaling pathway in the process of amelogenesis

Amelogenesis is a highly regulated process involving multiple signaling pathways, among which the transforming growth factor-β1 (TGF-β1) signaling pathway plays a pivotal role in enamel formation. This review firstly elucidates the critical functions of TGF-β1 in regulating ameloblast behavior and enamel development, encompassing ameloblast proliferation, differentiation, apoptosis, enamel matrix protein synthesis, and mineralization. Secondly, based on emerging evidence, we further discuss potential interactions between TGF-β signaling and circadian regulation in enamel formation, although this relationship requires further experimental validation. Finally, future research directions are proposed to further investigate the relationship between TGF-β1 and the circadian clock in the context of amelogenesis.

TGF-β1/Smad3 Signaling Is Required to Alleviate Fluoride-Induced Enamel Hypomineralization

Excessive fluoride intake during enamel development can affect enamel mineralization, leading to dental fluorosis. However, its potential mechanisms remain largely unexplored. In the present study, we aimed to investigate the impact of fluoride on the expressions of RUNX2 and ALPL during mineralization and the effect of TGF-β1 administration on fluoride treatment. A dental fluorosis model of newborn mice and an ameloblast cell line ALC were both used in the present study. The mice of the NaF group, including the mothers and newborns, were fed with water containing 150 ppm NaF after delivery to induce dental fluorosis. The mandibular incisors and molars showed significant abrasion in the NaF group. Immunostaining, qRT-PCR, and Western blotting analysis indicated that exposure to fluoride markedly down-regulated RUNX2 and ALPL in mouse ameloblasts and ALCs. Besides, fluoride treatment significantly decreased the mineralization level detected by ALP staining. Furthermore, exogenous TGF-β1 up-regulated RUNX2 and ALPL and promoted mineralization, while the addition of SIS3 could block such TGF-β1-induced up-regulation. In TGF-β1 conditional knockout mice, the immunostaining of RUNX2 and ALPL was weaker compared with wild-type mice. Exposure to fluoride inhibited the expressions of TGF-β1 and Smad3. Co-treatment of TGF-β1 and fluoride up-regulated RUNX2 and ALPL compared with the fluoride alone treatment, promoting mineralization. Collectively, our data indicated that TGF-β1/Smad3 signaling pathway was necessary for the regulatory effects of fluoride on RUNX2 and ALPL, and the fluoride-induced suppression of ameloblast mineralization was mitigated by activating TGF-β1/Smad3 signaling pathway.

Mediator1 involved in functional integration of Smad3 and Notch1 promoting enamel mineralization

Enamel hypoplasia is a tooth development defection due to the disruption of enamel matrix mineralization, manifesting as chalky white phenotype. Multiple genes may be involved in this tooth agenesis. It has been proved that ablation of coactivator Mediator1 (Med1) switches the cell fate of dental epithelia, resulting in abnormal tooth development via Notch1 signaling. Smad3 (-/-) mice displays the similar chalky white incisors. However, the expression of Smad3 in Med1 ablation mice and the impact of Med1 on functional integration between Smad3 and Notch1 remains unclear. Cre-loxP-based C57/BL6 mice with epithelial-specific Med1 knockout (Med1 KO) backgrounds were generated. Mandibles and dental epithelial stem cells (DE-SCs) from incisors cervical loop (CL) were isolated from wild-type (CON) mice and Med1 KO mice. Transcriptome sequencing was used to analyze the differences of CL tissue between KO and CON mice. The results revealed the enrichment of TGF-β signaling pathway. qRT-PCR and western blot were performed to show the gene and protein expression of Smad3, pSmad3, Notch1 and NICD, the key regulators of TGF-β and Notch1 signaling pathway. Expression of Notch1 and Smad3 was confirmed to be down-regulated in Med1 KO cells. Using activators of Smad3 and Notch1 on Med1 KO cells, both pSmad3 and NICD were rescued. Moreover, adding inhibitors and activators of Smad3 and Notch1 to cells of CON groups respectively, the protein expressions of Smad3, pSmad3, Notch1 and NICD were synergistically affected. In summary, Med1 participates in the functional integration of Smad3 and Notch1, thus promoting enamel mineralization.

Sodium Fluoride and Sulfur Dioxide Derivatives Induce TGF-β1-Mediated NBCe1 Downregulation Causing Acid-Base Disorder of LS8 Cells

The aim of the present work was to assess whether the combination of sodium fluoride (NaF) and sulfur dioxide derivatives (SO₂ derivatives) affects the expression of the electrogenic sodium bicarbonate cotransporter NBCe1 (SLC4A4), triggering an acid-base imbalance during enamel development, leading to enamel damage. LS8 cells was taken as the research objects and fluorescent probes, quantitative real-time polymerase chain reaction (qRT-PCR), western blot, and factorial analysis were used to clarify the nature of the fluoro-sulfur interaction and the potential signaling pathway involved in the regulation of NBCe1. The results showed that exposure to fluoride or SO₂ derivatives resulted in an acid-base imbalance, and these changes were accompanied by inhibited expression of NBCe1 and TGF-β1; these effects were more significant after fluoride exposure as compared to exposure to SO₂ derivatives. Interestingly, in most cases, the toxic effects during combined exposure were significantly reduced compared to the effects observed with fluoride or sulfur dioxide derivatives alone. The results also indicated that activation of TGF-β1 signaling significantly upregulated the expression of NBCe1, and this effect was suppressed after the Smad, ERK, and JNK signals were blocked. Furthermore, fluoride and SO₂ derivative-dependent NBCe1 regulation was found to require TGF-β1. In conclusion, this study indicates that the combined effect of fluorine and sulfur on LS8 cells is mainly antagonistic. TGF-β1 may regulate NBCe1 and may participate in the occurrence of dental fluorosis through the classic TGF-β1/Smad pathway and the unconventional ERK and JNK pathways.

Crucial Roles of microRNA-16-5p and microRNA-27b-3p in Ameloblast Differentiation Through Regulation of Genes Associated With Amelogenesis Imperfecta

Amelogenesis imperfecta is a congenital disorder within a heterogeneous group of conditions characterized by enamel hypoplasia. Patients suffer from early tooth loss, social embarrassment, eating difficulties, and pain due to an abnormally thin, soft, fragile, and discolored enamel with poor aesthetics and functionality. The etiology of amelogenesis imperfecta is complicated by genetic interactions. To identify mouse amelogenesis imperfecta-related genes (mAIGenes) and their respective phenotypes, we conducted a systematic literature review and database search and found and curated 70 mAIGenes across all of the databases. Our pathway enrichment analysis indicated that these genes were enriched in tooth development-associated pathways, forming four distinct groups. To explore how these genes are regulated and affect the phenotype, we predicted microRNA (miRNA)-gene interaction pairs using our bioinformatics pipeline. Our miRNA regulatory network analysis pinpointed that miR-16-5p, miR-27b-3p, and miR-23a/b-3p were hub miRNAs. The function of these hub miRNAs was evaluated through ameloblast differentiation assays with/without the candidate miRNA mimics using cultured mouse ameloblast cells. Our results revealed that overexpression of miR-16-5p and miR-27b-3p, but not miR-23a/b-3p, significantly inhibited ameloblast differentiation through regulation of mAIGenes. Thus, our study shows that miR-16-5p and miR-27b-3p are candidate pathogenic miRNAs for amelogenesis imperfecta.

Loss of transforming growth factor-β1 in epithelium cells affects enamel formation in mice

OBJECTIVES

In order to understand the specific in vivo function of transforming growth factor-beta1 (TGF-β1), we successfully established aTGF-β1 deficient mouse model using a conditional knockout method. In the present study, we aimed to further understand the potential role of TGF-β1 in enamel formation.

DESIGN

Transgenic mice withoutTGF-β1 in epithelial cells were generated. Scanning electron microscopy and micro-computed tomography analysis were used to detect the dental appearance, enamel microstructure and tooth density. Histological analysis was used to examine the residual organic matrix of enamel. Quantitative real-time polymerase chain reaction was used to analyze the expressions of enamel matrix proteins at the mRNA level.

RESULTS

The enamel of mandibular molars and incisors inTGF-β1 conditional knockout mice displayed severe attrition and lower density compared with the wild-type littermates. A slender microstructure of enamel rod was observed, and enamel matrix proteins were retained in the enamel space at the maturation stage in conditional knockout mice. Moreover, the expressions of enamel matrix protein-encoding genes, such as amelogenin (Amelx), ameloblastin (Ambn), Enamelin (Enam) and matrix metalloproteinase-20 (Mmp-20), were increased in enamel organs of conditional knockout mice. On the other hand, the expressions of Amelotin (Amtn), kallikrein-related peptidase-4 (Klk4), C4orf26 and WD repeat-containing protein 72 (Wdr72) were dramatically decreased at the transition and maturation stages.

CONCLUSIONS

TGF-β1 played an important role in enamel mineralization through decreasing synthesis ofAmelx, Ambn and Enam and increasing synthesis of Klk4, Amtn, Corf26 and Wdr72.

Enamel and dental anomalies in latent-transforming growth factor beta-binding protein 3 mutant mice

Latent‐transforming growth factor beta‐binding protein 3 (LTBP‐3) is important for craniofacial morphogenesis and hard tissue mineralization, as it is essential for activation of transforming growth factor‐β (TGF‐β). To investigate the role of LTBP‐3 in tooth formation we performed micro‐computed tomography (micro‐CT), histology, and scanning electron microscopy analyses of adult Ltbp3‐/‐ mice. The Ltbp3‐/‐ mutants presented with unique craniofacial malformations and reductions in enamel formation that began at the matrix formation stage. Organization of maturation‐stage ameloblasts was severely disrupted. The lateral side of the incisor was affected most. Reduced enamel mineralization, modification of the enamel prism pattern, and enamel nodules were observed throughout the incisors, as revealed by scanning electron microscopy. Molar roots had internal irregular bulbous‐like formations. The cementum thickness was reduced, and microscopic dentinal tubules showed minor nanostructural changes. Thus, LTBP‐3 is required for ameloblast differentiation and for the formation of decussating enamel prisms, to prevent enamel nodule formation, and for proper root morphogenesis. Also, and consistent with the role of TGF‐β signaling during mineralization, almost all craniofacial bone components were affected in Ltbp3‐/‐ mice, especially those involving the upper jaw and snout. This mouse model demonstrates phenotypic overlap with Verloes Bourguignon syndrome, also caused by mutation of LTBP3, which is hallmarked by craniofacial anomalies and amelogenesis imperfecta phenotypes.

Amelotin gene expression is temporarily being upregulated at the initiation of apoptosis induced by TGFβ1 in mouse gingival epithelial cells

Amelotin (AMTN) is expressed and secreted by ameloblasts in the maturation stage of amelogenesis and persist with low levels in the junctional epithelium (JE) of erupted teeth. The purpose of this study is to investigate the transcriptional regulation of the AMTN gene by transforming growth factor beta1 (TGFβ1) in gingival epithelial (GE1) cells in the apoptosis phase. Apoptosis was evaluated by the fragmentation of chromosomal DNA and TUNEL staining. A real-time PCR was carried out to examine the AMTN mRNA levels induced by TGFβ1 and Smad3 overexpression. Transient transfection analyses were completed using the various lengths of mouse AMTN gene promoter constructs with or without TGFβ1. Chromatin immunoprecipitation (ChIP) assays were performed to investigate the Smad3 bindings to the AMTN gene promoter by TGFβ1. TGFβ1-induced apoptosis in GE1 cells were detected at 24 and 48 h by DNA fragmentation and TUNEL staining. AMTN mRNA levels increased at 6 h and reached maximum at 24 h in GE1 cells. Luciferase activities of the mouse AMTN gene promoter constructs were induced by TGFβ1. The results of the ChIP assays showed that there was an increase in Smad3 binding to Smad-binding element (SBE)#1 and SBE#2 after stimulation by TGFβ1. Immunohistochemical localization of AMTN was detected in the JE, and the AMTN protein levels in Smad3-deficient mice were decreased compared with wild-type mice. AMTN mRNA levels were induced at the initiation of apoptosis by TGFβ1, which mediated through the Smad3 bindings to SBEs in the mouse AMTN gene promoter.

Immunolocalization of connective tissue growth factor, transforming growth factor-beta1 and phosphorylated-SMAD2/3 during the postnatal tooth development and formation of junctional epithelium

Connective tissue growth factor (CTGF) is a downstream mediator of transforming growth factor-beta 1 (TGF-β₁) and TGF-β₁-induced CTGF expression is regulated through SMAD pathway. However, there is no literature showing the expression of TGF-β₁-SMAD2/3-CTGF signaling pathway during postnatal tooth development and the formation of junctional epithelium (JE). Hence, we aimed to analyze the localization of TGF-β₁, CTGF and phosphorylated SMAD2/3 (p-SMAD2/3) in the developing postnatal rat molars. Wistar rats were killed at postnatal (PN) 0.5, 3.5, 7, 14 and 21days and the upper jaws were processed for immunohistochemistry. At PN0.5 and PN3.5, weak staining for TGF-β₁ and CTGF was evident in preameloblasts (PA), while moderate to strong staining was seen in odontoblasts (OD), dental papilla (DPL), secretary ameloblasts (SA), preodontoblasts (PO) and polarized odontoblasts (PoO). There was no staining for p-SMAD2/3 in PA, SA, PO and PoO, although strong staining was localized in DPL. OD was initially moderately positive and then negative for p-SMAD2/3. At PN7, intense staining for TGF-β₁ and CTGF was observed in SA, OD, dental pulp (DP) and predentin respectively. p-SMAD2/3 was strongly expressed in DP and moderately expressed in SA and OD. At PN14 and PN21, both reduced enamel epithelium (REE) and JE showed a strong reaction for TGF-β₁ and CTGF. p-SMAD2/3 was intensely and weakly expressed in REE and JE respectively. These data demonstrate that the expression of CTGF, TGF-β₁ and p-SNAD2/3 is tissue-specific and stage-specific, and indicate a regulatory role for a TGF-β₁-SMAD2/3-CTGF signaling pathway in amelogenesis, dentinogenesis and formation of JE.

DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE

Dental enamel is the hardest and most mineralized tissue in extinct and extant vertebrate species and provides maximum durability that allows teeth to function as weapons and/or tools as well as for food processing. Enamel development and mineralization is an intricate process tightly regulated by cells of the enamel organ called ameloblasts. These heavily polarized cells form a monolayer around the developing enamel tissue and move as a single forming front in specified directions as they lay down a proteinaceous matrix that serves as a template for crystal growth. Ameloblasts maintain intercellular connections creating a semi-permeable barrier that at one end (basal/proximal) receives nutrients and ions from blood vessels, and at the opposite end (secretory/apical/distal) forms extracellular crystals within specified pH conditions. In this unique environment, ameloblasts orchestrate crystal growth via multiple cellular activities including modulating the transport of minerals and ions, pH regulation, proteolysis, and endocytosis. In many vertebrates, the bulk of the enamel tissue volume is first formed and subsequently mineralized by these same cells as they retransform their morphology and function. Cell death by apoptosis and regression are the fates of many ameloblasts following enamel maturation, and what cells remain of the enamel organ are shed during tooth eruption, or are incorporated into the tooth's epithelial attachment to the oral gingiva. In this review, we examine key aspects of dental enamel formation, from its developmental genesis to the ever-increasing wealth of data on the mechanisms mediating ionic transport, as well as the clinical outcomes resulting from abnormal ameloblast function.

TGF-β1 autocrine signalling and enamel matrix components

Transforming growth factor-β1 (TGF-β1) is present in porcine enamel extracts and is critical for proper mineralization of tooth enamel. Here, we show that the mRNA of latent TGF-β1 is expressed throughout amelogenesis. Latent TGF-β1 is activated by matrix metalloproteinase 20 (MMP20), coinciding with amelogenin processing by the same proteinase. Activated TGF-β1 binds to the major amelogenin cleavage products, particularly the neutral-soluble P103 amelogenin, to maintain its activity. The P103 amelogenin-TGF-β1 complex binds to TGFBR1 to induce TGF-β1 signalling. The P103 amelogenin-TGF-β1 complex is slowly cleaved by kallikrein 4 (KLK4), which is secreted into the transition- and maturation-stage enamel matrix, thereby reducing TGF-β1 activity. To exert the multiple biological functions of TGF-β1 for amelogenesis, we propose that TGF-β1 is activated or inactivated by MMP20 or KLK4 and that the amelogenin cleavage product is necessary for the in-solution mobility of TGF-β1, which is necessary for binding to its receptor on ameloblasts and retention of its activity.

A systematic analysis for localization of predominant growth factors and their receptors involved in murine tooth germ differentiation using in situ hybridization technique

Tooth development is regulated by various growth factors and their receptors. However, the overall mechanism of growth factor-mediated odontogenesis remains to be elucidated. The present study examined expression sites and intensities of major growth factors and receptors in the tooth germ of murine fetuses and neonates. Signals of TGF-β and CTGF in fetuses were released from the enamel epithelium, while their neonatal signals arose in odontoblasts. Moreover, BMP/Smad signaling may affect the differentiation of ameloblasts, in contrast to PDGFα whose signals may cause odontoblast differentiation. Growth factors associated with the formation of the periodontium were IGF1, IGF2, IGFBP3, CTGF, and PDGFα. Concerning cusp formation, the enamel knot selectively expressed FGF4, BMP2, and BMP4 with an expression of PDGFα in the enamel-free area. It is concluded that many molecules play critical roles in the epithelium-mesenchyme interaction of tooth germ differentiation, and their expressions are precisely controlled.

Role of Smad3 in platelet-derived growth factor-C-induced liver fibrosis

Chronic liver injury leads to fibrosis and cirrhosis. Cirrhosis, the end stage of chronic liver disease, is a leading cause of death worldwide and increases the risk of developing hepatocellular carcinoma. Currently, there is a lack of effective antifibrotic therapies to treat fibrosis and cirrhosis. Development of antifibrotic therapies requires an in-depth understanding of the cellular and molecular mechanisms involved in inflammation and fibrosis after hepatic injury. Two growth factor signaling pathways that regulate liver fibrosis are transforming growth factor-β (TGFβ) and platelet-derived growth factor (PDGF). However, their specific contributions to fibrogenesis are not well understood. Using a genetic model of liver fibrosis, we investigated whether the canonical TGFβ signaling pathway was necessary for fibrogenesis. PDGF-C transgenic (PDGF-C Tg) mice were intercrossed with mice that lack Smad3, and molecular and histological fibrosis was analyzed. PDGF-C Tg mice that also lacked Smad3 had less fibrosis and improved liver lobule architecture. Loss of Smad3 also reduced expression of collagen genes, which were induced by PDGF-C, but not the expression of genes frequently associated with hepatic stellate cell (HSC) activation. In vitro HSCs isolated from Smad3-null mice proliferated more slowly than cells from wild-type mice. Taken together, these findings indicate that PDGF-C activates TGFβ/Smad3 signaling pathways to regulate HSC proliferation, collagen production and ultimately fibrosis. In summary, these results suggest that inhibition of both PDGF and TGFβ signaling pathways may be required to effectively attenuate fibrogenesis in patients with chronic liver disease.

Expression of cytokeratins in enamel organ, junctional epithelium and epithelial cell rests of Malassez

BACKGROUND AND OBJECTIVE

After tooth formation is complete, it is suggested that continuity exists between the epithelial cell rests of Malassez (ERM), reduced enamel epithelium (REE) and subsequently the junctional epithelium. However, the junctional epithelium was reported to differ from REE and ERM. The developmental relationships between and among them remain controversial. Therefore, in the present study we examined the expression of cytokeratins in the three types of epithelia to investigate the epithelial phenotypes.

MATERIAL AND METHODS

The maxillae of Wistar rats, 1, 2, 3 and 7 wk of age, were used, and the expression of CK14, CK17, CK19, CK10/CK13 and AE1/AE3 was detected using immunoperoxidase techniques.

RESULTS

There was negative staining for CK10/CK13 in all the epithelia. ERM stained strongly for AE1/AE3, CK14, CK17 and CK19. During the transformation of inner enamel epithelial (IEE) cells into reduced ameloblasts and subsequently into junctional epithelium, strong staining for CK14 was evident in IEE, REE and junctional epithelium, whereas the expression of AE1/AE3 and of CK19 were initially negative in IEE and then strong in REE and junctional epithelium, respectively. In particular, the expression of CK17 was strongly positive in ERM and REE, but was negative in IEE and junctional epithelium.

CONCLUSION

ERM are of odontogenic origin and junctional epithelium has an epithelial phenotype different from REE and ERM. This is the first report to demonstrate that CK17 can be used as a marker to distinguish junctional epithelium from ERM.

Fluoride affects enamel protein content via TGF-β1-mediated KLK4 inhibition

Dental fluorosis is caused by chronic high-level fluoride (F(-)) exposure during enamel development, and fluorosed enamel has a higher than normal protein content. Matrix metalloproteinase 20 cleaves enamel matrix proteins during the secretory stage, and KLK4 further cleaves these proteins during the maturation stage so that the proteins can be reabsorbed from the hardening enamel. We show that transforming growth factor β1 (TGF-β1) can induce Klk4 expression, and we examine the effect of F(-) on TGF-β1 and KLK4 expression. We found that in vivo F(-) inhibits Klk4 but not Mmp20 transcript levels. LacZ-C57BL/6-Klk4 (+/LacZ) mice have LacZ inserted in frame at the Klk4 translation initiation site so that the endogenous Klk4 promoter drives LacZ expression in the same temporal/spatial way as it does for Klk4. KLK4 protein levels in rat enamel and β-galactosidase staining in LacZ-C57BL/6-Klk4 (+/LacZ) mouse enamel were both significantly reduced by F(-) treatment. Since TGF-β1 induces KLK4 expression, we tested and found that F(-) significantly reduced Tgf-β1 transcript levels in rat enamel organ. These data suggest that F(-)-mediated downregulation of TGF-β1 expression contributes to reduced KLK4 protein levels in fluorosed enamel and provides an explanation for why fluorosed enamel has a higher than normal protein content.

TGF-ß regulates enamel mineralization and maturation through KLK4 expression

Transforming growth factor-ß (TGF-ß) signaling plays an important role in regulating crucial biological processes such as cell proliferation, differentiation, apoptosis, and extracellular matrix remodeling. Many of these processes are also an integral part of amelogenesis. In order to delineate a precise role of TGF-ß signaling during amelogenesis, we developed a transgenic mouse line that harbors bovine amelogenin promoter-driven Cre recombinase, and bred this line with TGF-ß receptor II floxed mice to generate ameloblast-specific TGF-ß receptor II conditional knockout (cKO) mice. Histological analysis of the teeth at postnatal day 7 (P7) showed altered enamel matrix composition in the cKO mice as compared to the floxed mice that had enamel similar to the wild-type mice. The µCT and SEM analyses revealed decreased mineral content in the cKO enamel concomitant with increased attrition and thinner enamel crystallites. Although the mRNA levels remained unaltered, immunostaining revealed increased amelogenin, ameloblastin, and enamelin localization in the cKO enamel at the maturation stage. Interestingly, KLK4 mRNA levels were significantly reduced in the cKO teeth along with a slight increase in MMP-20 levels, suggesting that normal enamel maturation is regulated by TGF-ß signaling through the expression of KLK4. Thus, our study indicates that TGF-ß signaling plays an important role in ameloblast functions and enamel maturation.

Iron deposition and ferritin heavy chain (Fth) localization in rodent teeth

Background

An iron rich layer on the labial surface is characteristic of the enamel of rodent incisors. In order to address a role for iron content in continuously growing incisors during odontogenesis, we studied iron deposition patterns in enamel and dentine using Perls’ blue staining and ferritin heavy chain (Fth) immunolocalization. Fth expression is regulated by iron level; therefore its localization can be used as a sensitive indicator for iron deposition.

Results

Sagittal sections of 4-week old rat incisors showed a gradual increase in iron level in the enamel organ from secretory to maturation stages. In addition, iron was detected in ameloblasts of erupting third molars of 4-week old rats, suggesting iron plays a role in both incisor and molar development. In odontoblasts, the presence of iron was demonstrated, and this is consistent with iron’s role in collagen synthesis. Using postnatal 3-, 6-, 9-day old mice, the spatial and temporal expression of Fth in tooth development again indicated the presence of iron in mature ameloblasts and odontoblasts.

Conclusions

While these data do not explain what functional role iron has in tooth formation, it does highlight a significant molecular activity associated with the formation of the rodent dentition.

Transcription factor FoxO1 is essential for enamel biomineralization

The Transforming growth factor β (Tgf-β) pathway, by signaling via the activation of Smad transcription factors, induces the expression of many diverse downstream target genes thereby regulating a vast array of cellular events essential for proper development and homeostasis. In order for a specific cell type to properly interpret the Tgf-β signal and elicit a specific cellular response, cell-specific transcriptional co-factors often cooperate with the Smads to activate a discrete set of genes in the appropriate temporal and spatial manner. Here, via a conditional knockout approach, we show that mice mutant for Forkhead Box O transcription factor FoxO1 exhibit an enamel hypomaturation defect which phenocopies that of the Smad3 mutant mice. Furthermore, we determined that both the FoxO1 and Smad3 mutant teeth exhibit changes in the expression of similar cohort of genes encoding enamel matrix proteins required for proper enamel development. These data raise the possibility that FoxO1 and Smad3 act in concert to regulate a common repertoire of genes necessary for complete enamel maturation. This study is the first to define an essential role for the FoxO family of transcription factors in tooth development and provides a new molecular entry point which will allow researchers to delineate novel genetic pathways regulating the process of biomineralization which may also have significance for studies of human tooth diseases such as amelogenesis imperfecta.

Critical role for αvβ6 integrin in enamel biomineralization

Tooth enamel has the highest degree of biomineralization of all vertebrate hard tissues. During the secretory stage of enamel formation, ameloblasts deposit an extracellular matrix that is in direct contact with ameloblast plasma membrane. Although it is known that integrins mediate cell-matrix adhesion and regulate cell signaling in most cell types, the receptors that regulate ameloblast adhesion and matrix production are not well characterized. Thus, we hypothesized that αvβ6 integrin is expressed in ameloblasts where it regulates biomineralization of enamel. Human and mouse ameloblasts were found to express both β6 integrin mRNA and protein. The maxillary incisors of Itgb6−/− mice lacked yellow pigment and their mandibular incisors appeared chalky and rounded. Molars of Itgb6−/− mice showed signs of reduced mineralization and severe attrition. The mineral-to-protein ratio in the incisors was significantly reduced in Itgb6−/− enamel, mimicking hypomineralized amelogenesis imperfecta. Interestingly, amelogenin-rich extracellular matrix abnormally accumulated between the ameloblast layer of Itgb6−/− mouse incisors and the forming enamel surface, and also between ameloblasts. This accumulation was related to increased synthesis of amelogenin, rather than to reduced removal of the matrix proteins. This was confirmed in cultured ameloblast-like cells, which did not use αvβ6 integrin as an endocytosis receptor for amelogenins, although it participated in cell adhesion on this matrix indirectly via endogenously produced matrix proteins. In summary, integrin αvβ6 is expressed by ameloblasts and it plays a crucial role in regulating amelogenin deposition/turnover and subsequent enamel biomineralization.

Acceleration of palatal wound healing in Smad3-deficient mice

Wound healing is a well-orchestrated complex process leading to the repair of injured tissues. It is suggested that transforming growth factor (TGF)-beta/Smad3 signaling is involved in wound healing. The purpose of this study was to investigate the role of TGF-beta/Smad3 signaling in palatal wound healing in Smad3-deficient (Smad3(-/-)) mice. Histological examination showed that wound closure was accelerated by the proliferation of epithelium and dermal cells in Smad3(-/-) mice compared with wild-type (WT) mice. Macrophage/monocyte infiltration at wounded regions in Smad3(-/-) mice was decreased in parallel with the diminished production of TGF-beta1, monocyte chemoattractant protein-1, and macrophage inflammatory protein-1alpha compared with WT mice. Fibrocytes, expressing hematopoietic surface marker and fibroblast products, were recruited and produced alpha-smooth-muscle actin in WT mice, but were not observed in Smad3(-/-) mice. These results suggest that TGF-beta/Smad3 signaling may play an important role in the regulation of palatal wound healing.

TGF-beta1 and TGFBR1 are expressed in ameloblasts and promote MMP20 expression

Transforming growth factor-beta (TGF-beta) signaling exerts a wide spectrum of biological functions. To investigate TGF-beta signaling in amelogenesis, we initially assessed the expression of TGF-beta1 and TGF-beta receptor 1 (TGFBR1) in developing teeth by immunohistochemistry. Both TGF-beta1 and TGFBR1 were strongly expressed in secreting ameloblasts. Next, we studied the effects of TGF-beta signaling on the expression of MMP20 and KLK4 mRNA using ameloblast-lineage cells (ALC) in vitro. Our RT-PCR study showed that TGF-beta1, TGFBR1, and enamel matrix proteases (MMP20 and KLK4) were expressed in ALC. Following TGF-beta1 treatment, the expression of MMP20 mRNA, but not KLK4 mRNA, was significantly upregulated. To further confirm the TGF-beta signaling involvement in the MMP20 expression, we constructed the activated TGFBR1 vector and transfected the construct into ALC. The activated TGFBR1 notably promoted MMP20 expression, but had no obvious effects on the KLK4 mRNA expression. Our studies strongly suggest that TGF-beta signaling involved in amelogenesis is partially mediated by regulating the expression of MMP20 mRNA.

Transforming growth factor-beta1 expression is up-regulated in maturation-stage enamel organ and may induce ameloblast apoptosis

Transforming growth factor-beta1 (TGF-beta1) regulates a variety of cellular responses that are dependent on the developmental stage and on the origins of the cell or the tissue. In mature tissues, and especially in tissues of epithelial origin, TGF-beta1 is generally considered to be a growth inhibitor that may also promote apoptosis. The ameloblast cells of the enamel organ epithelium are adjacent to and responsible for the developing enamel layer on unerupted teeth. Once the enamel layer reaches its full thickness, the tall columnar secretory-stage ameloblasts shorten, and a portion of these maturation-stage ameloblasts become apoptotic. Here we investigate whether TGF-beta1 plays a role in apoptosis of the maturation-stage ameloblasts. We demonstrate in vitro that ameloblast lineage cells are highly susceptible to TGF-beta1-mediated growth arrest and are prone to TGF-beta1-mediated cell death/apoptosis. We also demonstrate in vivo that TGF-beta1 is expressed in the maturation-stage enamel organ at significantly higher levels than in the earlier secretory-stage enamel organ. This increased expression of TGF-beta1 correlates with an increase in expression of the enamel organ immediate-early stress-response gene and with a decrease in the anti-apoptotic Bcl2 : Bax expression ratio. We conclude that TGF-beta1 may play an important role in ameloblast apoptosis during the maturation stage of enamel development.

A curriculum vitae of teeth: evolution, generation, regeneration

The ancestor of recent vertebrate teeth was a tooth-like structure on the outer body surface of jawless fishes. Over the course of 500,000,000 years of evolution, many of those structures migrated into the mouth cavity. In addition, the total number of teeth per dentition generally decreased and teeth morphological complexity increased. Teeth form mainly on the jaws within the mouth cavity through mutual, delicate interactions between dental epithelium and oral ectomesenchyme. These interactions involve spatially restricted expression of several, teeth-related genes and the secretion of various transcription and signaling factors. Congenital disturbances in tooth formation, acquired dental diseases and odontogenic tumors affect millions of people and rank human oral pathology as the second most frequent clinical problem. On the basis of substantial experimental evidence and advances in bioengineering, many scientists strongly believe that a deep knowledge of the evolutionary relationships and the cellular and molecular mechanisms regulating the morphogenesis of a given tooth in its natural position, in vivo, will be useful in the near future to prevent and treat teeth pathologies and malformations and for in vitro and in vivo teeth tissue regeneration.

Cloning and expression pattern of a Smad3 homolog from the pearl oyster, Pinctada fucata

Several transforming growth factor beta (TGFbeta) superfamily members have been identified from mollusks. The significant effects of TGFbeta signaling pathways on bone formation in vertebrates give clues to the signal transduction mechanism and how the shell of oysters is formed. However, what kinds of mediators are involved in the molluscan TGFbeta signaling pathways, and how they play their functions in mollusks has not been well explained due to a lack of genomic information and the failure to establish oyster cell lines. That is, if we knew the genome sequence we could search the TGFbeta superfamily members using the blast program (http://www.ncbi.nlm.nih.gov/), and if we established the molluscan cell line, we could transfect the gene of interest to the cell and detect its influence on expression levels of other genes, such as the matrix proteins. Therefore, to investigate whether there are similar TGFbeta pathways in mollusks, many important mediators should be identified. In this paper, we report a cDNA encoding a Smad3 homolog (designated Pf-Smad3) that was isolated from the pearl oyster, Pinctada fucata. Sequence alignment showed that Pf-Smad3 contains a DNA-binding MH1 domain and a Runx2/Cbfa1-binding MH2 domain, and shares an extremely high similarity with Smad3 proteins in vertebrates. However, Smad proteins in Drosophila and Caenorhabditis elegans are very different from other Smad3 proteins. Reverse transcription-polymerase chain reaction results indicated that Pf-Smad3 mRNA was expressed ubiquitously in adult Pin. fucata and was expressed at different levels at different developmental stages. In situ hybridization results showed that Pf-Smad3 mRNA was expressed mainly at the outer epithelial cells of the middle fold and the inner epithelial cells of the outer fold, especially around the gutter. These results suggested that Pf-Smad3 might take part in many physiological processes, including biomineralization, in oysters.

Characteristic distribution of immunoreaction for estrogen receptor alpha in rat ameloblasts

Estrogen has a diverse function, including cell proliferation and differentiation via estrogen receptors (ER), which have been reported to be the case in various tissues in addition to female reproductive organs. A recent immunocytochemical study has reported the expression of ERalpha, a subtype of ER, in rat odontoblasts, suggesting an involvement of estrogen in the differentiation of tooth-forming cells. However, there is no information on the ERalpha immunoexpression in ameloblasts. The present study was therefore undertaken to examine the localization of ERalpha immunoreaction in rat ameloblasts during amelogenesis. A computer-assisted quantitative analysis under a confocal laser scanning microscope was employed to demonstrate the stage-specific localization pattern of ERalpha immunoreaction. Immunohistochemistry of the rat enamel organ revealed ERalpha expression as nuclear localization in ameloblasts, stratum intermedium, stellate reticulum, and papillary layer, in addition to mature and immature odontoblasts. The ratio of immunopositive nuclei to total nuclei (immunopositive ratio) in ameloblasts was high at the apical loop region and gradually declined at the presecretory stage to zero at the secretory stage with statistically significant difference. The ERalpha immunolabeling pattern exhibited a periodic change at the maturation stage proper with constant higher labeling in ruffle-ended ameloblasts than in smooth-ended ameloblasts. The positive ratio was then followed by a statistically significant increase in immunolabeling thereafter. This stage-specific immunolabeling pattern during amelogenesis suggests a possible role of ERalpha in ameloblast proliferation and differentiation.