Expression patterns of genes critical for BMP signaling pathway in developing human primary tooth germs

BMP Signaling Pathway in Dentin Development and Diseases

BMP signaling plays an important role in dentin development. BMPs and antagonists regulate odontoblast differentiation and downstream gene expression via canonical Smad and non-canonical Smad signaling pathways. The interaction of BMPs with their receptors leads to the formation of complexes and the transduction of signals to the canonical Smad signaling pathway (for example, BMP ligands, receptors, and Smads) and the non-canonical Smad signaling pathway (for example, MAPKs, p38, Erk, JNK, and PI3K/Akt) to regulate dental mesenchymal stem cell/progenitor proliferation and differentiation during dentin development and homeostasis. Both the canonical Smad and non-canonical Smad signaling pathways converge at transcription factors, such as Dlx3, Osx, Runx2, and others, to promote the differentiation of dental pulp mesenchymal cells into odontoblasts and downregulated gene expressions, such as those of DSPP and DMP1. Dysregulated BMP signaling causes a number of tooth disorders in humans. Mutation or knockout of BMP signaling-associated genes in mice results in dentin defects which enable a better understanding of the BMP signaling networks underlying odontoblast differentiation and dentin formation. This review summarizes the recent advances in our understanding of BMP signaling in odontoblast differentiation and dentin formation. It includes discussion of the expression of BMPs, their receptors, and the implicated downstream genes during dentinogenesis. In addition, the structures of BMPs, BMP receptors, antagonists, and dysregulation of BMP signaling pathways associated with dentin defects are described.

Operation of the Atypical Canonical Bone Morphogenetic Protein Signaling Pathway During Early Human Odontogenesis

Bone morphogenetic protein (BMP) signaling plays essential roles in the regulation of early tooth development. It is well acknowledged that extracellular BMP ligands bind to the type I and type II transmembrane serine/threonine kinase receptor complexes to trigger the BMP signaling pathway. Then, the receptor-activated Smad1/5/8 in cytoplasm binds to Smad4, the central mediator of the canonical BMP signaling pathway, to form transfer complexes for entering the nucleus and regulating target gene expression. However, a recent study revealed the functional operation of a novel BMP-mediated signaling pathway named the atypical BMP canonical signaling pathway in mouse developing tooth, which is Smad1/5/8 dependent but Smad4 independent. In this study, we investigated whether this atypical BMP canonical signaling is conserved in human odontogenesis. We showed that pSMAD1/5/8 is required for the expression of Msh homeobox 1 (MSX1), a well-defined BMP signaling target gene, in human dental mesenchyme, but the typical BMP canonical signaling is in fact not operating in the early human developing tooth, as evidenced by the absence of pSMAD1/5/8-SMAD4 complexes in the dental mesenchyme and translocation of pSMAD1/5/8, and the expression of MSX1 induced by BMP4 is mothers against decapentaplegic homolog 4 (SMAD4)-independent in human dental mesenchymal cells. Moreover, integrative analysis of RNA-Seq data sets comparing the transcriptome profiles of human dental mesenchymal cells with and without SMAD4 knockdown by siRNA displays unchanged expression profiles of pSMAD1/5/8 downstream target genes, further affirming the functional operation of the atypical canonical BMP signaling pathway in a SMAD1/5/8-dependent but SMAD4-independent manner in the dental mesenchyme during early odontogenesis in humans.

Differential lncRNA/mRNA Expression Profiling and Functional Network Analyses in Bmp2 Deletion of Mouse Dental Papilla Cells

Bmp2 is essential for dentin development and formation. Bmp2 conditional knock-out (KO) mice display a similar tooth phenotype of dentinogenesis imperfecta (DGI). To elucidate a foundation for subsequent functional studies of cross talk between mRNAs and lncRNAs in Bmp2-mediated dentinogenesis, we investigated the profiling of lncRNAs and mRNAs using immortalized mouse dental Bmp2 flox/flox (iBmp2^fx/fx) and Bmp2 knock-out (iBmp2^ko/ko) papilla cells. RNA sequencing was implemented to study the expression of the lncRNAs and mRNAs. Quantitative real-time PCR (RT-qPCR) was used to validate expressions of lncRNAs and mRNAs. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases were used to predict functions of differentially expressed genes (DEGs). Protein-protein interaction (PPI) and lncRNA-mRNA co-expression network were analyzed by using bioinformatics methods. As a result, a total of 22 differentially expressed lncRNAs (16 downregulated vs 6 upregulated) and 227 differentially expressed mRNAs (133 downregulated vs. 94 upregulated) were identified in the iBmp2^ko/ko cells compared with those of the iBmp2^fx/fx cells. RT-qPCR results showed significantly differential expressions of several lncRNAs and mRNAs which were consistent with the RNA-seq data. GO and KEGG analyses showed differentially expressed genes were closely related to cell differentiation, transcriptional regulation, and developmentally relevant signaling pathways. Moreover, network-based bioinformatics analysis depicted the co-expression network between lncRNAs and mRNAs regulated by Bmp2 in mouse dental papilla cells and symmetrically analyzed the effect of Bmp2 during dentinogenesis via coding and non-coding RNA signaling.

FGF8-mediated signaling regulates tooth developmental pace during odontogenesis

The developing human and mouse teeth constitute an ideal model system to study the regulatory mechanism underlying organ growth control since their teeth share highly conserved and well-characterized developmental processes and their developmental tempo varies notably. In the current study, we manipulated heterogenous recombination between human and mouse dental tissues and demonstrate that the dental mesenchyme dominates the tooth developmental tempo and FGF8 could be a critical player during this developmental process. Forced activation of FGF8 signaling in the dental mesenchyme of mice promoted cell proliferation, prevented cell apoptosis via p38 and perhaps PI3K-Akt intracellular signaling, and impelled the transition of the cell cycle from G1- to S-phase in the tooth germ, resulting in the slowdown of the tooth developmental pace. Our results provide compelling evidence that extrinsic signals can profoundly affect tooth developmental tempo and the dental mesenchymal FGF8 could be a pivotal factor in controlling the developmental pace in a non-cell-autonomous manner during mammalian odontogenesis.

Dentinogenesis and Tooth-Alveolar Bone Complex Defects in BMP9/GDF2 Knockout Mice

Tooth development is regulated by sequential and reciprocal epithelium-mesenchymal interactions and their related molecular signaling pathways, such as bone morphogenetic proteins (BMPs). Among the 14 types of BMPs, BMP9 (also known as growth differentiation factor 2) is one of the most potent BMPs to induce osteogenic differentiation of mesenchymal stem cells. The purpose of this study was to examine potential roles of BMP9 signaling in tooth development. First, we detected the expression pattern of BMP9 in tooth germ during postnatal tooth development, and we found that BMP9 was widely expressed in odontoblasts, ameloblasts, dental pulp cells, and osteoblasts in alveolar bones. Then, we established a BMP9-KO mouse model. Gross morphological examination revealed that the tooth cusps of BMP9-KO mice were significantly abraded with shorter roots. Micro-computed tomography and three-dimensional reconstruction analysis indicated that the first molars of the BMP9-KO mice exhibited a reduced thickness dentin, enlarged pulp canals, and shortened roots, resembling the phenotypes of the common hereditary dental disease dentinogenesis imperfecta. Further, the alveolar bone of the BMP9-KO mutants was found to be shorter and had a decreased mineral density and trabecular thickness and bone volume fraction compared with that of the wild-type control. Mechanistically, we demonstrated that both dentin sialophosphoprotein and dentin matrix protein 1 were induced in dental stem cells by BMP9, whereas their expression was reduced when BMP9 was silenced. Further studies are required to determine whether loss of or decreased BMP9 expression is clinically associated with dentinogenesis imperfecta. Collectively, our results strongly suggest that BMP9 may play an important role in regulating dentinogenesis and tooth development. Further research is recommended into the therapeutic uses of BMP9 to regenerate traumatized and diseased tissues and for the bioengineering of replacement teeth.

Distinctive role of ACVR1 in dentin formation: requirement for dentin thickness in molars and prevention of osteodentin formation in incisors of mice

Dentin is a major component of teeth that protects dental pulp and maintains tooth health. Bone morphogenetic protein (BMP) signaling is required for the formation of dentin. Mice lacking a BMP type I receptor, activin A receptor type 1 (ACVR1), in the neural crest display a deformed mandible. Acvr1 is known to be expressed in the dental mesenchyme. However, little is known about how BMP signaling mediated by ACVR1 regulates dentinogenesis. To explore the role of ACVR1 in dentin formation in molars and incisors in mice, Acvr1 was conditionally disrupted in Osterix-expressing cells (designated as cKO). We found that loss of Acvr1 in the dental mesenchyme led to dentin dysplasia in molars and osteodentin formation in incisors. Specifically, the cKO mice exhibited remarkable tooth phenotypes characterized by thinner dentin and thicker predentin, as well as compromised differentiation of odontoblasts in molars. We also found osteodentin formation in the coronal part of the cKO mandibular incisors, which was associated with a reduction in the expression of odontogenic gene Dsp and an increase in the expression of osteogenic gene Bsp, leading to an alteration of cell fate from odontoblasts to osteoblasts. In addition, the expressions of WNT antagonists, Dkk1 and Sost, were downregulated and B-catenin was up-regulated in the cKO incisors, while the expression levels were not changed in the cKO molars, compared with the corresponding controls. Our results indicate the distinct and critical roles of ACVR1 between incisors and molars, which is associated with alterations in the WNT signaling related molecules. This study demonstrates for the first time the physiological roles of ACVR1 during dentinogenesis.

Expression of BMP2/4/7 during the odontogenesis of deciduous molars in miniature pig embryos

Bone morphogenetic proteins (BMPs) play important roles in tooth development. However, their expression has not been studied in miniature pigs, which have many anatomical similarities in oral and maxillofacial region compared to human. This study investigated BMP2/4/7 expression patterns during deciduous molar development in miniature pigs on embryonic days (E) 40, 50, and 60. The mandibles were fixed, decalcified, and embedded before sectioning. H&E staining, immunohistochemistry, in situ hybridization using specific radionuclide-labeled cRNA probes, and real-time PCR were used to detect the BMP expression patterns during morphogenesis of the third deciduous molar. H&E staining showed that for the deciduous third molar, E40 represented the cap stage, E50 represented the early bell stage, and E60 represented the late bell stage or secretory stage. BMP2 was expressed in both the enamel organ and in the dental mesenchyme on E40 and E50 and was expressed mainly in pre-odontoblasts on E60. BMP7 expression was similar to BMP2 expression, but BMP7 was also expressed in the inner enamel epithelium on E60. On E40, BMP4 was expressed mainly in the epithelium, with some weak expression in the mesenchyme. On E50, BMP4 expression was stronger in the mesenchyme but weaker in the epithelium. On E60, BMP4 was expressed mainly in the mesenchyme. These data indicated that BMP2/4/7 showed differential spatial and temporal expression during the morphogenesis and odontogenesis of deciduous molars, suggesting that these molecules were associated with tooth morphogenesis and cell differentiation.

Mutant GDF5 enhances ameloblast differentiation via accelerated BMP2-induced Smad1/5/8 phosphorylation

Bone morphogenetic proteins (BMPs) regulate hard tissue formation, including bone and tooth. Growth differentiation factor 5 (GDF5), a known BMP, is expressed in cartilage and regulates chondrogenesis, and mutations have been shown to cause osteoarthritis. Notably, GDF5 is also expressed in periodontal ligament tissue; however, its role during tooth development is unclear. Here, we used cell culture and in vivo analyses to determine the role of GDF5 during tooth development. GDF5 and its associated BMP receptors are expressed at the protein and mRNA levels during postnatal tooth development, particularly at a stage associated with enamel formation. Furthermore, whereas BMP2 was observed to induce evidently the differentiation of enamel-forming ameloblasts, excess GDF5 induce mildly this differentiation. A mouse model harbouring a mutation in GDF5 (W408R) showed enhanced enamel formation in both the incisors and molars, but not in the tooth roots. Overexpression of the W408R GDF5 mutant protein was shown to induce BMP2-mediated mRNA expression of enamel matrix proteins and downstream phosphorylation of Smad1/5/8. These results suggest that mutant GDF5 enhances ameloblast differentiation via accelerated BMP2-signalling.

Effects of neurotrophin receptor-mediated MAGE homology on proliferation and odontoblastic differentiation of mouse dental pulp cells

OBJECTIVES

This study aimed to investigate effects of neurotrophin receptor-mediated melanoma antigen-encoding gene homology (NRAGE) on proliferation and odontoblastic differentiation of mouse dental pulp cells (mDPCs).

MATERIALS AND METHODS

Mouse dental pulp cells were infected with recombinant lentivirus to stably knockdown expression of NRAGE, and biological effects of NRAGE on the cells were detected. Proliferation and odontoblastic differentiation of mDPCs were observed. Simultaneously, mRNA and protein levels of NRAGE and nuclear factor-κB (NF-κB) protein expression were detected. Immunofluorescence assay was used to detect expression and location of NRAGE and NF-κB.

RESULTS

NRAGE mRNA and protein levels reduced significantly after mDPC odontoblastic induction. Knockdown of NRAGE inhibited the proliferation of mDPCs. However, knockdown of NRAGE enhanced their odontoblastic differentiation with up-regulated ALPase activity. It also promoted mineral nodule formation as well as mRNA and protein expressions of ALP, DSPP and DMP1. Protein levels of NF-κB/p50 significantly increased, whereas NF-κB/p105 protein expression decreased in the mDPC/shNRG group. Immunofluorescence revealed that relocation of NF-κB was similar to that of NRAGE during odontoblastic induction, in which NF-κB translocated from the cytoplasm to the nucleus.

CONCLUSION

NRAGE is a potent regulator of proliferation and odontoblastic differentiation of mDPCs, which might be via the NF-κB signalling pathway.

[Enamel: a unique self-assembling in mineral world]

Enamel is a unique tissue in vertebrates, acellular, formed on a labile scaffolding matrix and hypermineralized. The ameloblasts are epithelial cells in charge of amelogenesis. They secrete a number of matrix proteins degraded by enzymes during enamel mineralization. This ordered cellular and extracellular events imply that any genetic or environmental perturbation will produce indelible and recognizable defects. The specificity of defects will indicate the affected cellular process. Thus, depending on the specificity of alterations, the teratogenic event can be retrospectively established. Advances in the field allow to use enamel defects as diagnostic tools for molecular disorders. The multifunctionality of enamel peptides is presently identified from their chemical roles in mineralization to cell signaling, constituting a source of concrete innovations in regenerative medicine.

The Histochemistry and Cell Biology pandect: the year 2014 in review

This review encompasses a brief synopsis of the articles published in 2014 in Histochemistry and Cell Biology. Out of the total of 12 issues published in 2014, two special issues were devoted to "Single-Molecule Super-Resolution Microscopy." The present review is divided into 11 categories, providing an easy format for readers to quickly peruse topics of particular interest to them.