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

Influence of bone morphogenetic protein (BMP) signaling and masticatory load on morphological alterations of the mouse mandible during postnatal development

OBJECTIVE

Bone homeostasis relies on several contributing factors, encompassing growth factors and mechanical stimuli. While bone morphogenetic protein (BMP) signaling is acknowledged for its essential role in skeletal development, its specific impact on mandibular morphogenesis remains unexplored. Here, we investigated the involvement of BMP signaling and mechanical loading through mastication in postnatal mandibular morphogenesis.

DESIGN

We employed conditional deletion of Bmpr1a in osteoblasts and chondrocytes via Osterix-Cre. Cre activity was induced at birth for the 3-week group and at three weeks for the 9-week and 12-week groups, respectively. The conditional knockout (cKO) and control mice were given either a regular diet (hard diet, HD) or a powdered diet (soft diet, SD) from 3 weeks until sample collection, followed by micro-CT and histological analysis.

RESULTS

The cKO mice exhibited shorter anterior lengths and a posteriorly inclined ramus across all age groups compared to the control mice. The cKO mice displayed an enlarged hypertrophic cartilage area along with fewer osteoclast numbers in the subchondral bone of the condyle compared to the control group at three weeks, followed by a reduction in the cartilage area in the posterior region at twelve weeks. Superimposed imaging and histomorphometrical analysis of the condyle revealed that BMP signaling primarily affects the posterior part of the condyle, while mastication affects the anterior part.

CONCLUSIONS

Using 3D landmark-based geometric morphometrics and histological assessments of the mandible, we demonstrated that BMP signaling and mechanical loading reciprocally contribute to the morphological alterations of the mandible and condyle during postnatal development.

Animal models and related techniques for dentin study

The intricate and protracted process of dentin formation has been extensively explored, thanks to the significant advancements facilitated by the use of animal models and related techniques. Despite variations in their effectiveness, taking into account factors such as sensitivity, visibility, and reliability, these models or techniques are indispensable tools for investigating the complexities of dentin formation. This article focuses on the latest advances in animal models and related technologies, shedding light on the key molecular mechanisms that are essential in dentin formation. A deeper understanding of this phenomenon enables the careful selection of appropriate animal models, considering their suitability in unraveling the underlying molecular intricacies. These insights are crucial for the advancement of clinical drugs targeting dentin-related ailments and the development of comprehensive treatment strategies throughout the duration of the disease.

Delta like Non-Canonical Notch Ligand 2 inhibits chondrogenic differentiation and cell proliferation of bone marrow mesenchymal stem cells through the Notch1 signaling pathway

Bone marrow mesenchymal stem cells (BMSCs) is the candidate for the treatment of cartilage defects because of their directional induction potential and natural anti-inflammatory properties. As one of the non-canonical receptors of Notch1, Delta Like Non-Canonical Notch Ligand 2 (DLK2) involves in stem cells' adipogenesis and chondrogenic differentiation. However, the specific regulatory mechanism of DLK2 in the chondrogenic differentiation of BMSCs is still unclear. In this study, we found that the expression of DLK2 was reduced and the expression of Col2a1, Col10a1, Acan, Sox9, and Notch1 was raised in the process of BMSCs chondrogenic differentiation. However, the expression of Col2a1, Col10a1, Acan, and Sox9 reduced significantly, and the signal factor Notch1 and the chondrogenic differentiation capacity of BMSCs turned down in the DLK2 overexpression group. Furthermore, the expression of Col2a1, Col10a1, Acan, and Sox9 significantly enhanced, Notch1 expression was also increased, and the chondrogenic differentiation capacity of BMSCs turned up in the DLK2 suppression group. Concurrently, the proliferation of BMSCs was weakened after overexpression of DLK2, and there was no significant change in cell migration. However, the proliferation and migration of BMSCs were significantly enhanced after the inhibition of DLK2 expression. Therefore, these results suggest that DLK2 negatively regulates chondrogenic differentiation and cell proliferation in BMSCs by inhibiting the Notch1 signaling pathway.

BMP signaling in the development and regeneration of tooth roots: from mechanisms to applications

Short root anomaly (SRA), along with caries, periodontitis, and trauma, can cause tooth loss, affecting the physical and mental health of patients. Dental implants have become widely utilized for tooth restoration; however, they exhibit certain limitations compared to natural tooth roots. Tissue engineering-mediated root regeneration offers a strategy to sustain a tooth with a physiologically more natural function by regenerating the bioengineered tooth root (bio-root) based on the bionic principle. While the process of tooth root development has been reported in previous studies, the specific molecular mechanisms remain unclear. The Bone Morphogenetic Proteins (BMPs) family is an essential factor regulating cellular activities and is involved in almost all tissue development. Recent studies have focused on exploring the mechanism of BMP signaling in tooth root development by using transgenic animal models and developing better tissue engineering strategies for bio-root regeneration. This article reviews the unique roles of BMP signaling in tooth root development and 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.

IGFs in Dentin Formation and Regeneration: Progress and Remaining Challenges

Tertiary dentin results from the interplay between the host defense and dental injury or infection. Modern endodontics aiming vital pulp treatment take the tertiary dentin formation as the interim step, with the final goal of a physiological pulp-dentin like tissue regeneration. Dental pulp stem cells have been nominated for contributing to differentiating into odontoblast-like cells who are responsible for reparative dentin formation. Understanding the original dentin formation mechanism provides us a blueprint while exploring the reparative dentin formation mechanism builds bridge to bonafide pulp-dentin tissue regeneration. Among all the regulators, growth factors have long been revealed under the spotlight. The insulin-like growth factor (IGF) family has been implicated in critical events of inducing dentin formation, which is essential for pulp treatment. The expression of IGF family members including IGF1, IGF1R, IGF2, and IGF2R has been well characterized in dental papilla cells, dental pulp stem cells, and periodontal ligament cells. Recent studies indicated IGF binding to the receptors activated pathways, including MAPK pathway, and AKT pathway, orchestrated proliferation, and differentiation, and finally, contributed to dentin formation. This review summarizes the role of IGF family in dentin formation during tooth development and tertiary dentin formation during dentin-pulp repair and sheds light on key parts of research for future treatment improvements.

Gene regulatory network from cranial neural crest cells to osteoblast differentiation and calvarial bone development

Calvarial bone is one of the most complex sequences of developmental events in embryology, featuring a uniquely transient, pluripotent stem cell-like population known as the cranial neural crest (CNC). The skull is formed through intramembranous ossification with distinct tissue lineages (e.g. neural crest derived frontal bone and mesoderm derived parietal bone). Due to CNC's vast cell fate potential, in response to a series of inductive secreted cues including BMP/TGF-β, Wnt, FGF, Notch, Hedgehog, Hippo and PDGF signaling, CNC enables generations of a diverse spectrum of differentiated cell types in vivo such as osteoblasts and chondrocytes at the craniofacial level. In recent years, since the studies from a genetic mouse model and single-cell sequencing, new discoveries are uncovered upon CNC patterning, differentiation, and the contribution to the development of cranial bones. In this review, we summarized the differences upon the potential gene regulatory network to regulate CNC derived osteogenic potential in mouse and human, and highlighted specific functions of genetic molecules from multiple signaling pathways and the crosstalk, transcription factors and epigenetic factors in orchestrating CNC commitment and differentiation into osteogenic mesenchyme and bone formation. Disorders in gene regulatory network in CNC patterning indicate highly close relevance to clinical birth defects and diseases, providing valuable transgenic mouse models for subsequent discoveries in delineating the underlying molecular mechanisms. We also emphasized the potential regenerative alternative through scientific discoveries from CNC patterning and genetic molecules in interfering with or alleviating clinical disorders or diseases, which will be beneficial for the molecular targets to be integrated for novel therapeutic strategies in the clinic.

Acvr1 deletion in osteoblasts impaired mandibular bone mass through compromised osteoblast differentiation and enhanced sRANKL-induced osteoclastogenesis

Bone morphogenetic protein (BMP) signaling is well known in bone homeostasis. However, the physiological effects of BMP signaling on mandibles are largely unknown, as the mandible has distinct functions and characteristics from other bones. In this study, we investigated the roles of BMP signaling in bone homeostasis of the mandibles by deleting BMP type I receptor Acvr1 in osteoblast lineage cells with Osterix-Cre. We found mandibular bone loss in conditional knockout mice at the ages of postnatal day 21 and 42 in an age-dependent manner. The decreased bone mass was related to compromised osteoblast differentiation together with enhanced osteoclastogenesis, which was secondary to the changes in osteoblasts in vivo. In vitro study revealed that deletion of Acvr1 in the mandibular bone marrow stromal cells (BMSCs) significantly compromised osteoblast differentiation. When wild type bone marrow macrophages were cocultured with BMSCs lacking Acvr1 both directly and indirectly, both proliferation and differentiation of osteoclasts were induced as evidenced by an increase of multinucleated cells, compared with cocultured with control BMSCs. Furthermore, we demonstrated that the increased osteoclastogenesis in vitro was at least partially due to the secretion of soluble receptor activator of nuclear factor-κB ligand (sRANKL), which is probably the reason for the mandibular bone loss in vivo. Overall, our results proposed that ACVR1 played essential roles in maintaining mandibular bone homeostasis through osteoblast differentiation and osteoblast-osteoclast communication via sRANKL.

Constitutive activation of β-catenin in odontoblasts induces aberrant pulp calcification in mouse incisors

During dentin formation, odontoblast polarization ensures that odontoblasts directionally secrete dentin matrix protein, leading to tubular dentin formation; however, little is known about the major features and regulatory mechanisms of odontoblast polarization. In a study of epithelial cell polarization, β-catenin was shown to serve as a structural component of cadherin-based adherens junctions to initiate cell polarity. However, the role of β-catenin in odontoblast polarization has not been well investigated. In this study, we explored whether β-catenin participated in odontoblast polarization to regulate the secretion of mineralization proteins. We established Col1-CreErt2; β-catenin exon3fl/fl (CA-β-catenin) mice, which constitutively activate β-catenin in odontoblasts. CA-β-catenin mice exhibited disorganization and depolarization of incisor odontoblasts. Moreover, the incisor dentin was hypomineralized, and ectopic calcification was found in mouse incisor pulp. In addition, by constitutive activation of β-catenin, the expression levels of the core polarity molecule Cdc42 and its downstream polarity protein complex Par3-Par6-aPKC were decreased in the incisors of CA-β-catenin mice. These findings suggest that β-catenin plays an essential role in dentin formation by regulating odontoblast polarization.

Influence on proliferation and apoptosis of intestinal epithelial cells and expression of ACVR1 by Helicobacter pylori

To discuss the influence on proliferation and apoptosis of human intestinal epithelial cells by Helicobacter pylori (Hp). CCK-8 method and flow cytometry to test the influence on proliferation and apoptosis of intestinal epithelial cells by Hp and cell cycle distribution. Immunocytochemistry, qRT-PCR, and WB to analyze the expression and activation of ACVR1. Functional studies of TNBS-induced IBD on mice and DSS-induced IBD on C57BL/6J mice with HP used for intervention were also performed. Hp facilitated the proliferation of intestinal epithelial cells with the breeding ratios on the first and third day are 5.24% ± 3.26% and 34.18% ± 6.68% respectively. ACVR1 is activated and the expression of anti-apoptosis protein Bcl-2 and Bcl-XL is up-regulated. In vivo, TNBS-induced and Hp-induced exhibited inflammatory lesions of intestine, ACVR1 and Bcl-2 higher expressions as compared to wild-type mice. Hp is likely to play a very important role in the proliferation, apoptosis, and malignant transformation of intestinal epithelial cells through ACVR1 pathway.

Comparison of osteogenic and dentinogenic potentials of mice incisor and molar pulps in vitro

OBJECTIVE

In the present study, we compared the in vitro osteogenic and dentinogenic potential of pulp cells from incisors and molars.

DESIGN

Primary pulp cultures were established from DSPP-Cerulean/DMP1-Cherry and BSP-GFPtpz reporter mouse lines and processed for various assays.

RESULTS

Our results showed marked differences in dentinogenic and osteogenic potentials of primary cultures from unerupted molars and incisors isolated from 5 to 7 days old pups. While primary cultures from both incisors and molars differentiated into odontoblasts and osteoblasts, cultures from molars differentiated into more DSPP-Cerulean + cells (∼5.5 %) compared to incisor cultures (∼0.7 %) at Day 14 and appear to be more committed to the odontogenic lineage. On the other hand, cultures from incisors show more differentiation into BSP-GFPtpz + cells (∼25 %) compared to molar cultures (∼16 %) and were more committed to the osteogenic lineage. Data were analyzed by Student's t-test and statistical significance was set at P ≤ 0.05.

CONCLUSION

Since in the dental pulp, MSCs are the primary source of progenitors capable of giving rise to osteoblasts and odontoblasts, our results provide evidence for differences in the commitment of MSCs in molars and incisors to the odontogenic and osteogenic lineages, respectively.

BMP-Smad Signaling Regulates Postnatal Crown Dentinogenesis in Mouse Molar

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

ACVR1 Function in Health and Disease

Activin A receptor type I (ACVR1) encodes for a bone morphogenetic protein type I receptor of the TGFβ receptor superfamily. It is involved in a wide variety of biological processes, including bone, heart, cartilage, nervous, and reproductive system development and regulation. Moreover, ACVR1 has been extensively studied for its causal role in fibrodysplasia ossificans progressiva (FOP), a rare genetic disorder characterised by progressive heterotopic ossification. ACVR1 is linked to different pathologies, including cardiac malformations and alterations in the reproductive system. More recently, ACVR1 has been experimentally validated as a cancer driver gene in diffuse intrinsic pontine glioma (DIPG), a malignant childhood brainstem glioma, and its function is being studied in other cancer types. Here, we review ACVR1 receptor function and signalling in physiological and pathological processes and its regulation according to cell type and mutational status. Learning from different functions and alterations linked to ACVR1 is a key step in the development of interdisciplinary research towards the identification of novel treatments for these pathologies.

Effects of Nel-like molecule-1 and bone morphogenetic protein 2 combination on rat pulp repair

Nel-like molecule-1 (NELL-1) is a novel highly specific growth factor that can induce osteoblast differentiation and bone formation as well as odontoblast differentiation. Recent studies have suggested that NELL-1 can synergistically increase bone formation and regeneration with bone morphogenetic protein 2 (BMP2) and inhibit adverse effects induced by BMP2. This study aimed to evaluate the combined effects of NELL-1 and BMP2 on rat pulp repair. The experiment used healthy non-carious maxillary first molars from 60 Wistar rats. Exposed pulps were capped with NELL-1 plus BMP2, NELL-1 alone, and BMP2 alone, and each was absorbed onto a sterile collagen sponge. In the control samples, the collagen sponge alone and Dycal were used as capping agents. After l, 2 and 4 weeks, the rats were sacrificed. The formation of reparative dentin, as well the situation of pulp repair, was detected by hematoxylin-eosin (HE) staining; moreover, the expression of dentin specific protein-dentin sialophosphoprotein (DSPP) and the pro-inflammatory cytokines interleukin-6 (IL6) and interleukin-8 (IL8) was detected by immunohistochemical staining. Quantitative real-time PCR experiment was used to investigate the mRNA levels of IL6 and IL8. The results showed that pulp capping with NELL-1 plus BMP2 in rats had superior ability in inducing reparative dentin formation with dentin tubules and in reducing the inflammatory cell response compared with the other groups. These findings suggested that combined use of NELL-1 and BMP2 could positively regulate pulp repair.