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

METTL14 regulates chondrogenesis through the GDF5-RUNX-extracellular matrix gene axis during limb development

m6A RNA methylation is essential for many aspects of mammalian development but its roles in chondrogenesis remain largely unknown. Here, we show that m6A is necessary for chondrogenesis and limb morphogenesis using limb progenitor-specific knockout mice of Mettl14, an essential subunit in the m6A methyltransferase complex. The knockout disrupts cartilage anlagen formation in limb buds with 11 downregulated proteins known to dysregulate chondrogenesis and shorten limb skeletons upon mutation in mice and humans. Further studies show a gene regulatory hierarchy among the 11 proteins. m6A stabilizes the transcript and increases the protein level of GDF5, a BMP family member. This activates the chondrogenic transcription factor genes Runx2 and Runx3, whose mRNAs are also stabilized by m6A. They promote the transcription of six collagen genes and two other chondrogenic genes, Ddrgk1 and Pbxip1. Thus, this study uncovers an m6A-based cascade essential for chondrogenesis during limb skeletal development.

Spatiotemporal Molecular Architecture of Lineage Allocation and Cellular Organization in Tooth Morphogenesis

The remarkable evolution of teeth morphological complexity represents a giant leap for vertebrate. Despite its importance in life history, the understanding of spatiotemporal organization of teeth remains rudimentary. Herein, a high-resolution genome-wide molecular patterning of lineage allocation and cellular organization in tooth morphogenesis is described, constructed by integrating spatial transcriptome and single-cell RNA sequencing. Twelve spatial compartments and seventeen heterogeneous cell clusters linked to tooth morphogenic milestones are identified. Eighty-eight percent of total lineage species has already appeared in the initial tooth bud rather than the generally considered sequential emergence. A previously unrecognized sprouting-like patterning mode of the dental papilla is discovered, that the inner compartment can break through the outer shell compartment to build up the final papilla cusp. Meanwhile, the continuum differentiation hierarchies of enamel knots in time and space are revealed. Furthermore, the regulatory network directing tooth morphogenesis is established, whereby a series of mechanotransduction signals are spatiotemporally involved beyond the well-established classical odontogenesis signals. Finally, genes underlying tooth dysplasia are successfully tracked to highly specific time points and cell types. The results raise the idea that tooth morphogenesis is orchestrated by mechanical niches combined with biochemical signaling.

Current Trends, Advances, and Challenges of Tissue Engineering-Based Approaches of Tooth Regeneration: A Review of the Literature

INTRODUCTION

Tooth loss is a significant health issue. Currently, this situation is often treated with the use of synthetic materials such as implants and prostheses. However, these treatment modalities do not fully meet patients' biological and mechanical needs and have limited longevity. Regenerative medicine focuses on the restoration of patients' natural tissues via tissue engineering techniques instead of rehabilitating with artificial appliances. Therefore, a tissue-engineered tooth regeneration strategy seems like a promising option to treat tooth loss.

OBJECTIVE

This review aims to demonstrate recent advances in tooth regeneration strategies and discoveries about underlying mechanisms and pathways of tooth formation.

RESULTS AND DISCUSSION

Whole tooth regeneration, tooth root formation, and dentin-pulp organoid generation have been achieved by using different seed cells and various materials for scaffold production. Bioactive agents are critical elements for the induction of cells into odontoblast or ameloblast lineage. Some substantial pathways enrolled in tooth development have been figured out, helping researchers design their experiments more effectively and aligned with the natural process of tooth formation.

CONCLUSION

According to current knowledge, tooth regeneration is possible in case of proper selection of stem cells, appropriate design and manufacturing of a biocompatible scaffold, and meticulous application of bioactive agents for odontogenic induction. Understanding innate odontogenesis pathways play a crucial role in accurately planning regenerative therapeutic interventions in order to reproduce teeth.

HIF-2α Inhibits Ameloblast Differentiation via Hey2 in Tooth Development

Enamel is the highly mineralized outer layer of teeth; the cells responsible for enamel formation are ameloblasts. Local hypoxia and hypoxia inducible factor (HIF) in embryonic tissues are important to promote normal organogenesis. However, hypoxic state in tooth germs and the roles of HIF in ameloblast differentiation have not been understood. The aim of this study is to clarify the role of HIF in ameloblast differentiation during tooth germ development. We found that tooth germs were under hypoxia and HIF-1α and HIF-2α were expressed in tooth germs in embryonic mice. Then, we used HIF inhibitors to evaluate the function of HIF during tooth germ development. The HIF-2α inhibitor significantly decreased the size of tooth germs in organ culture, while the HIF-1α inhibitor did not apparently affect the size of tooth germs. The HIF-2α inhibitor enhanced the expression of amelogenin, a marker of ameloblast differentiation, in the tooth germs in organ culture and rat dental epithelial SF2 cells. Moreover, we found that the HIF-2α inhibitor-stimulating amelogenin expression was regulated by hes-related family basic helix-loop-helix transcription factor with YRPW motif 2(Hey2) in SF2 cells. These findings suggest that the HIF-2α-Hey2 axis plays an important role in ameloblast differentiation during tooth germ development.

Transcriptional regulation of the basic helix-loop-helix factor AmeloD during tooth development

The epithelial-mesenchymal interactions are essential for the initiation and regulation of the development of teeth. Following the initiation of tooth development, numerous growth factors are secreted by the dental epithelium and mesenchyme that play critical roles in cellular differentiation. During tooth morphogenesis, the dental epithelial stem cells differentiate into several cell types, including inner enamel epithelial cells, which then differentiate into enamel matrix-secreting ameloblasts. Recently, we reported that the novel basic-helix-loop-helix transcription factor, AmeloD, is actively engaged in the development of teeth as a regulator of dental epithelial cell motility. However, the gene regulation mechanism of AmeloD is still unknown. In this study, we aimed to uncover the mechanisms regulating AmeloD expression during tooth development. By screening growth factors that are important in the early stages of tooth formation, we found that TGF-β1 induced AmeloD expression and ameloblast differentiation in the dental epithelial cell line, SF2. TGF-β1 phosphorylated ERK1/2 and Smad2/3 to induce AmeloD expression, whereas treatment with the MEK inhibitor, U0126, inhibited AmeloD induction. Promoter analysis of AmeloD revealed that the proximal promoter of AmeloD showed high activity in dental epithelial cell lines, which was enhanced following TGF-β1 stimulation. These results suggested that TGF-β1 activates AmeloD transcription via ERK1/2 phosphorylation. Our findings provide new insights into the mechanisms that govern tooth development.

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

OBJECTIVE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Expression Patterns of Claudin Family Members During Tooth Development and the Role of Claudin-10 (Cldn10) in Cytodifferentiation of Stratum Intermedium

There is growing evidence showing that tight junctions play an important role in developing enamel. Claudins are one of the main components of tight junctions and may have pivotal functions in modulating various cellular events, such as regulating cell differentiation and proliferation. Mutations in CLDN10 of humans are associated with HELIX syndrome and cause enamel defects. However, current knowledge regarding the expression patterns of claudins and the function of Cldn10 during tooth development remains fragmented. In this study, we aimed to analyze the expression patterns of claudin family members during tooth development and to investigate the role of Cldn10 in amelogenesis. Using cap analysis gene expression of developing mouse tooth germs compared with that of the whole body, we found that Cldn1 and Cldn10 were highly expressed in the tooth. Furthermore, single-cell RNA-sequence analysis using 7-day postnatal Krt14-RFP mouse incisors revealed Cldn1 and Cldn10 exhibited distinct expression patterns. Cldn10 has two isoforms, Cldn10a and Cldn10b, but only Cldn10b was expressed in the tooth. Immunostaining of developing tooth germs revealed claudin-10 was highly expressed in the inner enamel epithelium and stratum intermedium. We also found that overexpression of Cldn10 in the dental epithelial cell line, SF2, induced alkaline phosphatase (Alpl) expression, a marker of maturated stratum intermedium. Our findings suggest that Cldn10 may be a novel stratum intermedium marker and might play a role in cytodifferentiation of stratum intermedium.

STRAP and NME1 Mediate the Neurite Growth-Promoting Effects of the Neurotrophic Factor GDF5

Loss of midbrain dopaminergic (mDA) neurons and their axons is central to Parkinson's disease (PD). Growth differentiation factor (GDF)5 is a potential neurotrophic factor for PD therapy. However, the molecular mediators of its neurotrophic action are unknown. Our proteomics analysis shows that GDF5 increases the expression of serine threonine receptor-associated protein kinase (STRAP) and nucleoside diphosphate kinase (NME)1 in the SH-SY5Y neuronal cell line. GDF5 overexpression increased NME1 expression in adult rat brain in vivo. NME and STRAP mRNAs are expressed in developing and adult rodent midbrain. Expression of both STRAP and NME1 is necessary and sufficient for the promotion of neurite growth in SH-SY5Y cells by GDF5. NME1 treatment increased neurite growth in both SH-SY5Y cells and cultured mDA neurons. Expression patterns of NME and STRAP are altered in PD midbrain. NME1 and STRAP are thus key mediators of GDF5's neurotrophic effects, rationalizing their future study as therapeutic targets for PD.

Single-Cell RNA-Sequencing From Mouse Incisor Reveals Dental Epithelial Cell-Type Specific Genes

Dental epithelial stem cells give rise to four types of dental epithelial cells: inner enamel epithelium (IEE), outer enamel epithelium (OEE), stratum intermedium (SI), and stellate reticulum (SR). IEE cells further differentiate into enamel-forming ameloblasts, which play distinct roles, and are essential for enamel formation. These are conventionally classified by their shape, although their transcriptome and biological roles are yet to be fully understood. Here, we aimed to use single-cell RNA sequencing to clarify the heterogeneity of dental epithelial cell types. Unbiased clustering of 6,260 single cells from incisors of postnatal day 7 mice classified them into two clusters of ameloblast, IEE/OEE, SI/SR, and two mesenchymal populations. Secretory-stage ameloblasts expressed Amel and Enam were divided into Dspp + and Ambn + ameloblasts. Pseudo-time analysis indicated Dspp + ameloblasts differentiate into Ambn + ameloblasts. Further, Dspp and Ambn could be stage-specific markers of ameloblasts. Gene ontology analysis of each cluster indicated potent roles of cell types: OEE in the regulation of tooth size and SR in the transport of nutrients. Subsequently, we identified novel dental epithelial cell marker genes, namely Pttg1, Atf3, Cldn10, and Krt15. The results not only provided a resource of transcriptome data in dental cells but also contributed to the molecular analyses of enamel formation.

G protein-coupled receptor Gpr115 (Adgrf4) is required for enamel mineralization mediated by ameloblasts

Dental enamel, the hardest tissue in the human body, is derived from dental epithelial cell ameloblast-secreted enamel matrices. Enamel mineralization occurs in a strictly synchronized manner along with ameloblast maturation in association with ion transport and pH balance, and any disruption of these processes results in enamel hypomineralization. G protein-coupled receptors (GPCRs) function as transducers of external signals by activating associated G proteins and regulate cellular physiology. Tissue-specific GPCRs play important roles in organ development, although their activities in tooth development remain poorly understood. The present results show that the adhesion GPCR Gpr115 (Adgrf4) is highly and preferentially expressed in mature ameloblasts and plays a crucial role during enamel mineralization. To investigate the in vivo function of Gpr115, knockout (Gpr115-KO) mice were created and found to develop hypomineralized enamel, with a larger acidic area because of the dysregulation of ion composition. Transcriptomic analysis also revealed that deletion of Gpr115 disrupted pH homeostasis and ion transport processes in enamel formation. In addition, in vitro analyses using the dental epithelial cell line cervical loop-derived dental epithelial (CLDE) cell demonstrated that Gpr115 is indispensable for the expression of carbonic anhydrase 6 (Car6), which has a critical role in enamel mineralization. Furthermore, an acidic condition induced Car6 expression under the regulation of Gpr115 in CLDE cells. Thus, we concluded that Gpr115 plays an important role in enamel mineralization via regulation of Car6 expression in ameloblasts. The present findings indicate a novel function of Gpr115 in ectodermal organ development and clarify the molecular mechanism of enamel formation.

Sox21 Regulates Anapc10 Expression and Determines the Fate of Ectodermal Organ

The transcription factor Sox21 is expressed in the epithelium of developing teeth. The present study aimed to determine the role of Sox21 in tooth development. We found that disruption of Sox21 caused severe enamel hypoplasia, regional osteoporosis, and ectopic hair formation in the gingiva in Sox21 knockout incisors. Differentiation markers were lost in ameloblasts, which formed hair follicles expressing hair keratins. Molecular analysis and chromatin immunoprecipitation sequencing indicated that Sox21 regulated Anapc10, which recognizes substrates for ubiquitination-mediated degradation, and determined dental-epithelial versus hair follicle cell fate. Disruption of either Sox21 or Anapc10 induced Smad3 expression, accelerated TGF-β1-induced promotion of epithelial-to-mesenchymal transition (EMT), and resulted in E-cadherin degradation via Skp2. We conclude that Sox21 disruption in the dental epithelium leads to the formation of a unique microenvironment promoting hair formation and that Sox21 controls dental epithelial differentiation and enamel formation by inhibiting EMT via Anapc10.

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Sox21 was induced by Shh in dental epithelial cells

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Sox21 deficiency in dental epithelium caused differentiation into hair cells

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Sox21 deficiency did not cause differentiation into mature ameloblasts

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Anapc10 induced by Sox21 bound to Fzr1 and regulated EMT via Skp2

Decreased BMP-7 and p-Smad1/5/8 expression, and increased levels of gremlin in hepatocellular carcinoma

The aim of the present study was to investigate the expression of bone morphogenetic protein-7 (BMP-7), gremlin, and p-Smad/1/5/8 in carcinomatous and para-carcinoma tissue specimens from patients with hepatocellular carcinoma (HCC). The association of serum BMP-7 levels with clinicopathological parameters was examined to assess its relevance as a clinical biomarker for HCC. A total of 27 patients with HCC and 7 healthy subjects were included. Gene expression levels of BMP-7 and p-Smad1/5/8 were examined by reverse transcription-quantitative polymerase chain reaction. Immunohistochemical and western blot analysis were performed to determine the protein expression of target genes. The serum levels of BMP-7 were assessed by enzyme linked immunosorbent assay. The mRNA and protein expression of BMP-7 and gremlin were significantly down- and upregulated in HCC tumor tissues, respectively, compared with para-carcinoma tissues (P<0.05). The association of BMP-7 and gremlin expression with the differentiation status of HCC was also analyzed. There was a relatively higher level of BMP-7 and a lower level of gremlin expression in tumor tissues from patients with highly differentiated HCC when compared with poorly or moderately differentiated HCC (BMP-7, F=42.29, P<0.01; gremlin, F=37.93, P<0.01). Furthermore, the level of BMP-7 and p-Smad1/5/8 was decreased in patients with advanced stages of HCC, when compared with stage I HCC. The findings from the present study suggest that the BMP-7/p-Smad signaling pathway may be involved in the pathogenesis of HCC. The serum levels of BMP-7 may serve as a potential biomarker for HCC.

TGF-β Family Signaling in Epithelial Differentiation and Epithelial-Mesenchymal Transition

Epithelia exist in the animal body since the onset of embryonic development; they generate tissue barriers and specify organs and glands. Through epithelial-mesenchymal transitions (EMTs), epithelia generate mesenchymal cells that form new tissues and promote healing or disease manifestation when epithelial homeostasis is challenged physiologically or pathologically. Transforming growth factor-βs (TGF-βs), activins, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs) have been implicated in the regulation of epithelial differentiation. These TGF-β family ligands are expressed and secreted at sites where the epithelium interacts with the mesenchyme and provide paracrine queues from the mesenchyme to the neighboring epithelium, helping the specification of differentiated epithelial cell types within an organ. TGF-β ligands signal via Smads and cooperating kinase pathways and control the expression or activities of key transcription factors that promote either epithelial differentiation or mesenchymal transitions. In this review, we discuss evidence that illustrates how TGF-β family ligands contribute to epithelial differentiation and induce mesenchymal transitions, by focusing on the embryonic ectoderm and tissues that form the external mammalian body lining.

Combinatorial Signal Perception in the BMP Pathway

The bone morphogenetic protein (BMP) signaling pathway comprises multiple ligands and receptors that interact promiscuously with one another and typically appear in combinations. This feature is often explained in terms of redundancy and regulatory flexibility, but it has remained unclear what signal-processing capabilities it provides. Here, we show that the BMP pathway processes multi-ligand inputs using a specific repertoire of computations, including ratiometric sensing, balance detection, and imbalance detection. These computations operate on the relative levels of different ligands and can arise directly from competitive receptor-ligand interactions. Furthermore, cells can select different computations to perform on the same ligand combination through expression of alternative sets of receptor variants. These results provide a direct signal-processing role for promiscuous receptor-ligand interactions and establish operational principles for quantitatively controlling cells with BMP ligands. Similar principles could apply to other promiscuous signaling pathways.