Distorted Patterns of Dentinogenesis and Eruption in Msx2 Null Mutants

"Biological responses of two calcium-silicate-based cements on a tissue-engineered 3D organotypic deciduous pulp analogue"

OBJECTIVES

The biological responses of MTA and Biodentine™ has been assessed on a three-dimensional, tissue-engineered organotypic deciduous pulp analogue.

METHODS

Human endothelial (HUVEC) and dental mesenchymal stem cells (SHED) at a ratio of 3:1, were incorporated into a collagen I/fibrin hydrogel; succeeding Biodentine™ and MTA cylindrical specimens were placed in direct contact with the pulp analogue 48 h later. Cell viability/proliferation and morphology were evaluated through live/dead staining, MTT assay and Scanning Electron Microscopy (SEM), and expression of angiogenic, odontogenic markers through real time PCR.

RESULTS

Viable cells dominated at day 3 after treatment presenting typical morphology, firmly attached within the hydrogel structures, as shown by live/dead staining and SEM images. MTT assay at day 1 presented a significant increase of cell proliferation in Biodentine™ group. Real-time PCR showed significant upregulation of odontogenic markers DSPP, BMP-2 (day 3,6), RUNX2, ALP (day 3) in contact with Biodentine™ compared to MTA and the control, whereas MTA promoted significant upregulation of DSPP, BMP-2, RUNX2, Osterix (day 3) and ALP (day 6) compared to the control. MSX1 presented downregulation in both experimental groups. Expression of angiogenic markers VEGFa and ANGPT-1 at day 3 was significantly upregulated in contact with Biodentine™ and MTA respectively, while the receptors VEGFR1, VEGFR2 and Tie-2, as well as PECAM-1 were downregulated.

SIGNIFICANCE

Both calcium silicate-based materials are biocompatible and exert positive angiogenic and odontogenic effects, although Biodentine™ during the first days of culture, seems to induce higher cell proliferation and provoke a more profound odontogenic and angiogenic response from SHED.

Effects of different signaling pathways on odontogenic differentiation of dental pulp stem cells: a review

Dental pulp stem cells (DPSCs) are a type of mesenchymal stem cells that can differentiate into odontoblast-like cells and protect the pulp. The differentiation of DPSCs can be influenced by biomaterials or growth factors that activate different signaling pathways in vitro or in vivo. In this review, we summarized six major pathways involved in the odontogenic differentiation of DPSCs, Wnt signaling pathways, Smad signaling pathways, MAPK signaling pathways, NF-kB signaling pathways, PI3K/AKT/mTOR signaling pathways, and Notch signaling pathways. Various factors can influence the odontogenic differentiation of DPSCs through one or more signaling pathways. By understanding the interactions between these signaling pathways, we can expand our knowledge of the mechanisms underlying the regeneration of the pulp-dentin complex.

Modulators of Wnt Signaling Pathway Implied in Dentin Pulp Complex Engineering: A Literature Review

The main goal of vital pulp therapy (VPT) is to preserve the vitality of the pulp tissue, even when it is exposed due to bacterial invasion, iatrogenic mechanical preparation, or trauma. The type of new dentin formed as a result of VPT can differ in its cellular origin, its microstructure, and its barrier function. It is generally agreed that the new dentin produced by odontoblasts (reactionary dentin) has a tubular structure, while the dentin produced by pulp cells (reparative dentin) does not or has less. Thus, even VPT aims to maintain the vitality of the pulp. It does not regenerate the dentin pulp complex integrity. Therefore, many studies have sought to identify new therapeutic strategies to successfully regenerate the dentin pulp complex. Among them is a Wnt protein-based strategy based on the fact that Wnt proteins seem to be powerful stem cell factors that allow control of the self-renewal and proliferation of multiple adult stem cell populations, suitable for homeostasis maintenance, tissue healing, and regeneration promotion. Thus, this review outlines the different agents targeting the Wnt signaling that could be applied in a tooth environment, and could be a potential therapy for dentin pulp complex and bone regeneration.

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.

Sclerostin is a promising therapeutic target for oral inflammation and regenerative dentistry

Sclerostin is the protein product of the SOST gene and is known for its inhibitory effects on bone formation. The monoclonal antibody against sclerostin has been approved as a novel treatment method for osteoporosis. Oral health is one of the essential aspects of general human health. Hereditary bone dysplasia syndrome caused by sclerostin deficiency is often accompanied by some dental malformations, inspiring the therapeutic exploration of sclerostin in the oral and dental fields. Recent studies have found that sclerostin is expressed in several functional cell types in oral tissues, and the expression level of sclerostin is altered in pathological conditions. Sclerostin not only exerts similar negative outcomes on the formation of alveolar bone and bone-like tissues, including dentin and cementum, but also participates in the development of oral inflammatory diseases such as periodontitis, pulpitis, and peri-implantitis. This review aims to highlight related research progress of sclerostin in oral cavity, propose necessary further research in this field, and discuss its potential as a therapeutic target for dental indications and regenerative dentistry.

The absence of muscle segment homeobox 2 leads to the pyroptosis of ameloblasts by inducing squamous epithelial hyperplasia in the enamel organ

Muscle segment homeobox 2 (MSX2) has been confirmed to be involved in the regulation of early tooth development. However, the role of MSX2 has not been fully elucidated in enamel development. To research the functions of MSX2 in enamel formation, we used a Msx2-/- (KO) mouse model with no full Msx2 gene. In the present study, the dental appearance and enamel microstructure were detected by scanning electron microscopy and micro-computed tomography. The results showed that the absence of Msx2 resulted in enamel defects, leading to severe tooth wear in KO mice. To further investigate the mechanism behind the phenotype, we performed detailed histological analyses of the enamel organ in KO mice. We discovered that ameloblasts without Msx2 could secrete a small amount of enamel matrix protein in the early stage. However, the enamel epithelium occurred squamous epithelial hyperplasia and partial keratinization in the enamel organ during subsequent developmental stages. Ameloblasts depolarized and underwent pyroptosis. Overall, during the development of enamel, MSX2 affects the formation of enamel by regulating the function of epithelial cells in the enamel organ.

Transcriptomic profiling of feline teeth highlights the role of matrix metalloproteinase 9 (MMP9) in tooth resorption

Tooth resorption (TR) in domestic cats is a common and painful disease characterised by the loss of mineralised tissues from the tooth. Due to its progressive nature and unclear aetiology the only treatment currently available is to extract affected teeth. To gain insight into TR pathogenesis, we characterised the transcriptomic changes involved in feline TR by sequencing RNA extracted from 14 teeth (7 with and 7 without signs of resorption) collected from 11 cats. A paired comparison of teeth from the same cat with and without signs of resorption identified 1,732 differentially expressed genes, many of which were characteristic of osteoclast activity and differentiation, in particular matrix metalloproteinase 9 (MMP9). MMP9 expression was confirmed by qPCR and immunocytochemistry of odontoclasts located in TR lesions. A hydroxamate-based MMP9 inhibitor reduced both osteoclast formation and resorption activity while siRNA targeting MMP9 also inhibited osteoclast differentiation although had little effect on resorption activity. Overall, these results suggest that increased MMP9 expression is involved in the progress of TR pathogenesis and that MMP9 may be a potential therapeutic target in feline TR.

Wnt/β-Catenin Signaling Promotes the Formation of Preodontoblasts In Vitro

Odontoblast differentiation is a complex and multistep process regulated by signaling pathways, including the Wnt/β-catenin signaling pathway. Both positive and negative effects of Wnt/β-catenin signaling on dentinogenesis have been reported, but the underlying mechanisms of these conflicting results are still unclear. To gain a better insight into the role of Wnt/β-catenin in dentinogenesis, we used dental pulp cells from a panel of transgenic mice, in which fluorescent protein expression identifies cells at different stages of odontoblast and osteoblast differentiation. Our results showed that exposure of pulp cells to WNT3a at various times and durations did not induce premature differentiation of odontoblasts. These treatments supported the survival of undifferentiated cells in dental pulp and promoted the formation of 2.3GFP⁺ preodontoblasts and their rapid transition into differentiated odontoblasts expressing DMP1-Cherry and DSPP-Cerulean transgenes. WNT3a also promoted osteogenesis in dental pulp cultures. These findings provide critical information for the development of improved treatments for vital pulp therapy and dentin regeneration.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Experimental periodontitis in Msx2 mutant mice induces alveolar bone necrosis

BACKGROUND

Msx2 homeoprotein is a key transcription factor of dental and periodontal tissue formation and is involved in many molecular pathways controlling mineralized tissue homeostasis such as Wnt/sclerostin pathway. This study evaluated the effect of Msx2-null mutation during experimental periodontitis in mice.

METHODS

Experimental periodontitis was induced for 30 days in wild-type and Msx2 knock-in Swiss mice using Porphyromonas gingivalis infected ligatures. In knock-in mice, Msx2 gene was replaced by n-LacZ gene encoding β-galactosidase. Periodontal tissue response was assessed by histomorphometry, tartrate-resistant acid phosphatase histoenzymology, β-galactosidase, sclerostin immunochemistry, and terminal deoxynucleotidyl transferase-mediated dUTP nickend labeling assay. Expression of Msx2 gene expression was also evaluated in human gingival biopsies using RT-qPCR.

RESULTS

During experimental periodontitis, osteonecrosis area and osteoclast number were significantly elevated in knock-in mice compared with wild-type mice. Epithelial downgrowth and bone loss was similar. Sclerostin expression in osteocytes appeared to be reduced during periodontitis in knock-in mice. Msx2 expression was detected in healthy and inflamed human gingival tissues.

CONCLUSION

These data indicated that Msx2 pathway influenced periodontal tissue response to experimental periodontitis and appeared to be a protective factor against alveolar bone osteonecrosis. As shown in other inflammatory processes such as atherothrombosis, genes initially characterized in early development could also play an important role in human periodontal pathogenesis.

Sostdc1 is expressed in all major compartments of developing and adult mammalian eyes

PURPOSE

This study was conducted in order to study Sostdc1 expression in rat and human developing and adult eyes.

METHODS

Using the yeast signal sequence trap screening method, we identified the Sostdc1 cDNA encoding a protein secreted by the adult rat retinal pigment epithelium. We determined by in situ hybridization, RT-PCR, immunohistochemistry, and western blot analysis Sostdc1 gene and protein expression in developing and postnatal rat ocular tissue sections. We also investigated Sostdc1 immunohistolocalization in developing and adult human ocular tissues.

RESULTS

We demonstrated a prominent Sostdc1 gene expression in the developing rat central nervous system (CNS) and eyes at early developmental stages from E10.5 days postconception (dpc) to E13 dpc. Specific Sostdc1 immunostaining was also detected in most adult cells of rat ocular tissue sections. We also identified the rat ocular embryonic compartments characterized by a specific Sostdc1 immunohistostaining and specific Pax6, Sox2, Otx2, and Vsx2 immunohistostaining from embryonic stages E10.5 to E13 dpc. Furthermore, we determined the localization of SOSTDC1 immunoreactivity in ocular tissue sections of developing and adult human eyes. Indeed, we detected SOSTDC1 immunostaining in developing and adult human retinal pigment epithelium (RPE) and neural retina (NR) as well as in several developing and adult human ocular compartments, including the walls of choroidal and scleral vessels. Of utmost importance, we observed a strong SOSTDC1 expression in a pathological ocular specimen of type 2 Peters' anomaly complicated by retinal neovascularization as well in the walls ofother pathological extra-ocular vessels.  CONCLUSION: As rat Sostdc1 and human SOSTDC1 are dual antagonists of the Wnt/β-catenin and BMP signaling pathways, these results underscore the potential crucial roles of these pathways and their antagonists, such as Sostdc1 and SOSTDC1, in developing and adult mammalian normal eyes as well as in syndromic and nonsyndromic congenital eye diseases.

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.

Biological interactions of a calcium silicate based cement (Biodentine™) with Stem Cells from Human Exfoliated Deciduous teeth

OBJECTIVE

To investigate the biological interactions of a calcium silicate based cement (Biodentine™) with Stem Cells from Human Exfoliated Deciduous teeth (SHED), focusing on viability/proliferation, odontogenic differentiation, biomineralization and elemental release/exchange.

METHODS

Biodentine™ specimens were used directly or for eluate preparation at serial dilutions (1:1-1:64). SHED cultures were established from deciduous teeth of healthy children. Viability/proliferation and morphological characteristics were evaluated by live/dead fluorescent staining, MTT assay and Scanning Electron Microscopy. Odontogenic differentiation by qRT-PCR, biomineralization by Alizarin red S staining, while ion elution by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES).

RESULTS

SHED effectively attached within the crystalline surface of Biodentine™ specimens acquiring a spindle-shaped phenotype. Statistically significant stimulation of cell proliferation was induced at day 3 by eluates in dilutions from 1:16 to 1:64. Differential, concentration- and time-dependent expression patterns of odontogenic genes were observed under non-inductive and inductive (osteogenic) conditions, with significant up-regulation of DSPP and Runx2 at higher dilutions and a peak in expression of BMP-2, BGLAP and MSX-2 at 1:8 dilution on day 7. Progressive increase in mineralized tissue formation was observed with increasing dilutions of Biodentine™ eluates. ICP-OES indicated that Biodentine™ absorbed Ca, Mg and P ions from culture medium, while releasing Si and Sr ions from its backbone.

SIGNIFICANCE

Biodentine™ interacts through elemental release/uptake with the cellular microenvironment, triggering odontogenic differentiation and biomineralization in a concentration-dependent manner. These results reveal a promising strategy for application of the calcium silicate based cement (Biodentine™) for vital pulp therapies of deciduous teeth in Paediatric Dentistry.

Bone Morphogenetic Protein 2 Coordinates Early Tooth Mineralization

Formation of highly organized dental hard tissues is a complex process involving sequential and ordered deposition of an extracellular scaffold, followed by its mineralization. Odontoblast and ameloblast differentiation involves reciprocal and sequential epithelial-mesenchymal interactions. Similar to early tooth development, various Bmps are expressed during this process, although their functions have not been explored in detail. Here, we investigated the role of odontoblast-derived Bmp2 for tooth mineralization using Bmp2 conditional knockout mice. In developing molars, Bmp2LacZ reporter mice revealed restricted expression of Bmp2 in early polarized and functional odontoblasts while it was not expressed in mature odontoblasts. Loss of Bmp2 in neural crest cells, which includes all dental mesenchyme, caused a delay in dentin and enamel deposition. Immunohistochemistry for nestin and dentin sialoprotein (Dsp) revealed polarization defects in odontoblasts, indicative of a role for Bmp2 in odontoblast organization. Surprisingly, pSmad1/5/8, an indicator of Bmp signaling, was predominantly reduced in ameloblasts, with reduced expression of amelogenin ( Amlx), ameloblastin ( Ambn), and matrix metalloproteinase ( Mmp20). Quantitative real-time polymerase chain reaction (RT-qPCR) analysis and immunohistochemistry showed that loss of Bmp2 resulted in increased expression of the Wnt antagonists dickkopf 1 ( Dkk1) in the epithelium and sclerostin ( Sost) in mesenchyme and epithelium. Odontoblasts showed reduced Wnt signaling, which is important for odontoblast differentiation, and a strong reduction in dentin sialophosphoprotein ( Dspp) but not collagen 1 a1 ( Col1a1) expression. Mature Bmp2-deficient teeth, which were obtained by transplanting tooth germs from Bmp2-deficient embryos under a kidney capsule, showed a dentinogenesis imperfecta type II-like appearance. Micro-computed tomography and scanning electron microscopy revealed reduced dentin and enamel thickness, indistinguishable primary and secondary dentin, and deposition of ectopic osteodentin. This establishes that Bmp2 provides an early temporal, nonredundant signal for directed and organized tooth mineralization.

Sclerostin Deficiency Promotes Reparative Dentinogenesis

In humans, the SOST gene encodes sclerostin, an inhibitor of bone growth and remodeling, which also negatively regulates the bone repair process. Sclerostin has also been implicated in tooth formation, but its potential role in pulp healing remains unknown. The aim of this study was to explore the role of sclerostin in reparative dentinogenesis using Sost knockout mice ( Sost^-/-). The pulps of the first maxillary molars were mechanically exposed in 3-mo-old Sost^-/- and wild-type (WT) mice ( n = 14 mice per group), capped with mineral trioxide aggregate cement, and the cavities were filled with a bonded composite resin. Reparative dentinogenesis was dynamically followed up by micro-computed tomography and characterized by histological analyses. Presurgical analysis revealed a significantly lower pulp volume in Sost^-/- mice compared with WT. At 30 and 49 d postsurgery, a large-forming reparative mineralized bridge, associated with osteopontin-positive mineralization foci, was observed in the Sost^-/- pulps, whereas a much smaller bridge was detected in WT. At the longer time points, the bridge, which was associated with dentin sialoprotein-positive cells, had expanded in both groups but remained significantly larger in Sost^-/- pulps. Sclerostin expression in the healing WT pulps was detected in the cells neighboring the forming dentin bridge. In vitro, mineralization induced by Sost^-/- dental pulp cells (DPCs) was also dramatically enhanced when compared with WT DPCs. These observations were associated with an increased Sost expression in WT cells. Taken together, our data show that sclerostin deficiency hastened reparative dentinogenesis after pulp injury, suggesting that the inhibition of sclerostin may constitute a promising therapeutic strategy for improving the healing of damaged pulps.