MSX2 in ameloblast cell fate and activity

Roles of SP/KLF transcription factors in odontoblast differentiation: From development to diseases

OBJECTIVES

A zinc-finger transcription factor family comprising specificity proteins (SPs) and Krüppel-like factor proteins (KLFs) plays an important role in dentin development and regeneration. However, a systematic regulatory network involving SPs/KLFs in odontoblast differentiation has not yet been described. This review examined the expression patterns of SP/KLF gene family members and their current known functions and mechanisms in odontoblast differentiation, and discussed prospective research directions for further exploration of mechanisms involving the SP/KLF gene family in dentin development.

MATERIALS AND METHODS

Relevant literature on SP/KLF gene family members and dentin development was acquired from PubMed and Web of Science.

RESULTS

We discuss the expression patterns, functions, and related mechanisms of eight members of the SP/KLF gene family in dentin development and genetic disorders with dental problems. We also summarize current knowledge about their complementary or synergistic actions. Finally, we propose future research directions for investigating the mechanisms of dentin development.

CONCLUSIONS

The SP/KLF gene family plays a vital role in tooth development. Studying the complex complementary or synergistic interactions between SPs/KLFs is helpful for understanding the process of odontoblast differentiation. Applications of single-cell and spatial multi-omics may provide a more complete investigation of the mechanism involved in dentin development.

Intrahepatic homeobox protein MSX-1 is a novel host restriction factor of hepatitis B virus

Chronic hepatitis B virus (HBV) infection (CHB) is a risk factor for the development of liver fibrosis, cirrhosis, and hepatocellular carcinoma. Covalently closed circular DNA serves as the sole transcription template for all viral RNAs and viral transcription is driven and enhanced by viral promoter and enhancer elements, respectively. Interactions between transcription factors and these cis-elements regulate their activities and change the production levels of viral RNAs. Here, we report the identification of homeobox protein MSX-1 (MSX1) as a novel host restriction factor of HBV in liver. In both HBV-transfected and HBV-infected cells, MSX1 suppresses viral gene expression and genome replication. Mechanistically, MSX1 downregulates enhancer II/core promoter (EnII/Cp) activity via direct binding to an MSX1 responsive element within EnII/Cp, and such binding competes with hepatocyte nuclear factor 4α binding to EnII/Cp due to partial overlap between their respective binding sites. Furthermore, CHB patients in immune active phase express higher levels of intrahepatic MSX1 but relatively lower levels of serum and intrahepatic HBV markers compared to those in immune tolerant phase. Finally, MSX1 was demonstrated to induce viral clearance in two mouse models of HBV persistence, suggesting possible therapeutic potential for CHB.IMPORTANCECovalently closed circular DNA plays a key role for the persistence of hepatitis B virus (HBV) since it serves as the template for viral transcription. Identification of transcription factors that regulate HBV transcription not only provides insights into molecular mechanisms of viral life cycle regulation but may also provide potential antiviral targets. In this work, we identified host MSX1 as a novel restriction factor of HBV transcription. Meanwhile, we observed higher intrahepatic MSX1 expression in chronic hepatitis B virus (CHB) patients in immune active phase compared to those in immune tolerant phase, suggesting possible involvement of MSX1 in the regulation of HBV activity by the host. Lastly, intrahepatic overexpression of MSX1 delivered by recombinant adenoviruses into two mouse models of HBV persistence demonstrated MSX1-mediated repression of HBV in vivo, and MSX1-induced clearance of intrahepatic HBV DNA in treated mice suggested its potential as a therapeutic target for the treatment of CHB.

Enam expression is regulated by Msx2

BACKGROUND

The precise formation of mineralized dental tissues such as enamel and/or dentin require tight transcriptional control of the secretion of matrix proteins. Here, we have investigated the transcriptional regulation of the second most prominent enamel matrix protein, enamelin, and its regulation through the major odontogenic transcription factor, MSX2.

RESULTS

Using in vitro and in vivo approaches, we identified that (a) Enam expression is reduced in the Msx2 mouse mutant pre-secretory and secretory ameloblasts, (b) Enam is an early response gene whose expression is under the control of Msx2, (c) Msx2 binds to Enam promoter in vitro, suggesting that enam is a direct target for Msx2 and that (d) Msx2 alone represses Enam gene expression.

CONCLUSIONS

Collectively, these results illustrate that Enam gene expression is controlled by Msx2 in a spatio-temporal manner. They also suggest that Msx2 may interact with other transcription factors to control spatial and temporal expression of Enam and hence amelogenesis and enamel biomineralization.

Molecular-based phenotype variations in amelogenesis imperfecta

Amelogenesis imperfecta (AI) is one of the typical dental genetic diseases in human. It can occur isolatedly or as part of a syndrome. Previous reports have mainly clarified the types and mechanisms of nonsyndromic AI. This review aimed to compare the phenotypic differences among the hereditary enamel defects with or without syndromes and their underlying pathogenic genes. We searched the articles in PubMed with different strategies or keywords including but not limited to amelogenesis imperfecta, enamel defects, hypoplastic/hypomaturation/hypocalcified, syndrome, or specific syndrome name. The articles with detailed clinical information about the enamel and other phenotypes and clear genetic background were used for the analysis. We totally summarized and compared enamel phenotypes of 18 nonsyndromic AI with 17 causative genes and 19 syndromic AI with 26 causative genes. According to the clinical features, radiographic or ultrastructural changes in enamel, the enamel defects were basically divided into hypoplastic and hypomineralized (hypomaturated and hypocalcified) and presented a higher heterogeneity which were closely related to the involved pathogenic genes, types of mutation, hereditary pattern, X chromosome inactivation, incomplete penetrance, and other mechanisms.The gene-specific enamel phenotypes could be an important indicator for diagnosing nonsyndromic and syndromic AI.

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.

Effects of Early Life Adversity on Tooth Enamel Formation

In a systemic effort to survive environmental stress, organ systems fluctuate and adapt to overcome external pressures. The evolutionary drive back toward homeostasis makes it difficult to determine if an organism experienced a toxic exposure to stress, especially in early prenatal and neonatal periods of development. Previous studies indicate that primary human teeth may provide historical records of experiences related to stressors during that early time window. To assess the molecular effects of early life adversity on enamel formation, we used a limited bedding and nesting (LBN) mouse model of early life adversity (ELA) to assess changes in the enamel organ gene expression and enamel matrix mineralization. On average, postnatal day 12 (P12) ELA mice weighed significantly less than the controls. When adjusted for animal weight, ELA molar enamel volume was reduced as compared with the controls, and the relative mineral density of molar enamel was significantly increased. There were no obvious changes in enamel matrix crystal morphology or structure in ELA as compared with the control mouse enamel. RNAseq showed extracellular matrix organization to be the most significantly affected GO and reactome pathways, whereas butanote metabolism was the most significantly altered KEGG pathway. Transcripts expressing the enamel matrix proteins amelogenin (Amelx) and enamelin (Enam) were among the top 4 most differentially expressed genes. When evaluating molecular mechanisms for the changes in gene expression in ELA enamel organs, we found significantly increased expression of Dlx3, while transcripts for clock genes Per1 and Nrd1 were downregulated. These findings support the possibility that the developing enamel organ is sensitive to the pressures of early life adversity and produces molecular and structural biomarkers reflecting these challenges.

Evaluating the Expression of Candidate Homeobox Genes and Their Role in Local-Site Inflammation in Mucosal Tissue Obtained from Children with Non-Syndromic Cleft Lip and Palate

Craniofacial development including palatogenesis is a complex process which requires an orchestrated and spatiotemporal expression of various genes and factors for proper embryogenesis and organogenesis. One such group of genes essential for craniofacial development is the homeobox genes, transcriptional factors that are commonly associated with congenital abnormalities. Amongst these genes, DLX4, HOXB3, and MSX2 have been recently shown to be involved in the etiology of non-syndromic cleft lip and palate. Hence, we investigated the gene and protein expression of these genes in normal and cleft affected mucosal tissue obtained from 22 children, along with analyzing their role in promoting local-site inflammation using NF-κB. Additionally, we investigated the role of PTX3, which plays a critical role in tissue remodeling and wound repair. We found a residual gene and protein expression of DLX4 in cleft mucosa, although no differences in gene expression levels of HOXB3 and MSX2 were noted. However, a significant increase in protein expression for these genes was noted in the cleft mucosa (p < 0.05), indicating increased cellular proliferation. This was coupled with a significant increase in NF-κB protein expression in cleft mucosa (p < 0.05), highlighting the role of these genes in promotion of pro-inflammatory environment. Finally, no differences in gene expression of PTX3 were noted.

MSX2 safeguards syncytiotrophoblast fate of human trophoblast stem cells

Multiple placental pathologies are associated with failures in trophoblast differentiation, yet the underlying transcriptional regulation is poorly understood. Here, we discovered msh homeobox 2 (MSX2) as a key transcriptional regulator of trophoblast identity using the human trophoblast stem cell model. Depletion of MSX2 resulted in activation of the syncytiotrophoblast transcriptional program, while forced expression of MSX2 blocked it. We demonstrated that a large proportion of the affected genes were directly bound and regulated by MSX2 and identified components of the SWItch/Sucrose nonfermentable (SWI/SNF) complex as strong MSX2 interactors and target gene cobinders. MSX2 cooperated specifically with the SWI/SNF canonical BAF (cBAF) subcomplex and cooccupied, together with H3K27ac, a number of differentiation genes. Increased H3K27ac and cBAF occupancy upon MSX2 depletion imply that MSX2 prevents premature syncytiotrophoblast differentiation. Our findings established MSX2 as a repressor of the syncytiotrophoblast lineage and demonstrated its pivotal role in cell fate decisions that govern human placental development and disease.

Identification of Biomarkers Controlling Cell Fate In Blood Cell Development

A blood cell lineage consists of several consecutive developmental stages starting from the pluri- or multipotent stem cell to a state of terminal differentiation. Despite their importance for human biology, the regulatory pathways and gene networks that govern these differentiation processes are not yet fully understood. This is in part due to challenges associated with delineating the interactions between transcription factors (TFs) and their corresponding target genes. A possible step forward in this case is provided by the increasing amount of expression data, as a basis for linking differentiation stages and gene activities. Here, we present a novel hierarchical approach to identify characteristic expression peak patterns that global regulators excert along the differentiation path of cell lineages. Based on such simple patterns, we identified cell state-specific marker genes and extracted TFs that likely drive their differentiation. Integration of the mean expression values of stage-specific “key player” genes yielded a distinct peaking pattern for each lineage that was used to identify further genes in the dataset which behave similarly. Incorporating the set of TFs that regulate these genes led to a set of stage-specific regulators that control the biological process of cell fate. As proof of concept, we considered two expression datasets covering key differentiation events in blood cell formation of mice.

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.

Bcl11b/Ctip2 in Skin, Tooth, and Craniofacial System

Ctip2/Bcl11b is a zinc finger transcription factor with dual action (repression/activation) that couples epigenetic regulation to gene transcription during the development of various tissues. It is involved in a variety of physiological responses under healthy and pathological conditions. Its role and mechanisms of action are best characterized in the immune and nervous systems. Furthermore, its implication in the development and homeostasis of other various tissues has also been reported. In the present review, we describe its role in skin development, adipogenesis, tooth formation and cranial suture ossification. Experimental data from several studies demonstrate the involvement of Bcl11b in the control of the balance between cell proliferation and differentiation during organ formation and repair, and more specifically in the context of stem cell self-renewal and fate determination. The impact of mutations in the coding sequences of Bcl11b on the development of diseases such as craniosynostosis is also presented. Finally, we discuss genome-wide association studies that suggest a potential influence of single nucleotide polymorphisms found in the 3’ regulatory region of Bcl11b on the homeostasis of the cardiovascular system.

Transcriptional Regulation of Dental Epithelial Cell Fate

Dental enamel is hardest tissue in the body and is produced by dental epithelial cells residing in the tooth. Their cell fates are tightly controlled by transcriptional programs that are facilitated by fate determining transcription factors and chromatin regulators. Understanding the transcriptional program controlling dental cell fate is critical for our efforts to build and repair teeth. In this review, we describe the current understanding of these regulators essential for regeneration of dental epithelial stem cells and progeny, which are identified through transgenic mouse models. We first describe the development and morphogenesis of mouse dental epithelium in which different subpopulations of epithelia such as ameloblasts contribute to enamel formation. Then, we describe the function of critical factors in stem cells or progeny to drive enamel lineages. We also show that gene mutations of these factors are associated with dental anomalies in craniofacial diseases in humans. We also describe the function of the master regulators to govern dental lineages, in which the genetic removal of each factor switches dental cell fate to that generating hair. The distinct and related mechanisms responsible for the lineage plasticity are discussed. This knowledge will lead us to develop a potential tool for bioengineering new teeth.

An Msx2-Sp6-Follistatin Pathway Operates During Late Stages of Tooth Development to Control Amelogenesis

Background

Ameloblasts are epithelially derived cells responsible for enamel formation through a process known as amelogenesis. Amongst the several transcription factors that are expressed during amelogenesis, both Msx2 and Sp6 transcription factors play important role. Msx2 and Sp6 mouse mutants, exhibit similar amelogenesis defects, namely enamel hypoplasia, while humans with amelogenesis imperfecta (AI) carry mutations in the human homologues of MSX2 or SP6 genes. These across species similarities in function indicate that these two transcription factors may reside in the same developmental pathway. In this paper, we test whether they work in a coordinated manner to exert their effect during amelogenesis.

Methods

Two different dental epithelial cell lines, the mouse LS8 and the rat G5 were used for either overexpression or silencing of Msx2 or Sp6 or both. Msx2 mutant mouse embryos or pups were used for in vivo studies. In situ hybridization, semi-quantitative and quantitative real time PCR were employed to study gene expression pattern. MatInspector was used to identify several potential putative Msx2 binding sites upstream of the murine Sp6 promoter region. Chromatin Immunoprecipitation (chIP) was used to confirm the binding of Msx2 to Sp6 promoter at the putative sites.

Results

Using the above methods we identified that (i) Msx2 and Sp6 exhibit overlapping expression in secretory ameloblasts, (ii) Sp6 expression is reduced in the Msx2 mouse mutant secretoty ameloblasts, and (iii) that Msx2, like Sp6 inhibits follistatin expression. Specifically, our loss-of function studies by silencing Msx2 and/or Sp6 in mouse dental epithelial (LS8) cells showed significant downregulation of Sp6 but upregulation of Fst expression. Transient transfection of Msx2 overexpression plasmid, up-regulated Sp6 and downregulated Fst expression. Additionally, using MatInspector, we identified several potential putative Msx2 binding sites, 3.5 kb upstream of the murine Sp6 promoter region. By chIP, we confirmed the binding of Msx2 to Sp6 promoter at these sites, thus suggesting that Sp6 is a direct target of Msx2.

Conclusion

Collectively, these results show that Sp6 and Msx2 work in a concerted manner to form part of a network of transcription factors that operate during later stages of tooth development controlling ameloblast life cycle and amelogenesis.

Dental cell type atlas reveals stem and differentiated cell types in mouse and human teeth

Understanding cell types and mechanisms of dental growth is essential for reconstruction and engineering of teeth. Therefore, we investigated cellular composition of growing and non-growing mouse and human teeth. As a result, we report an unappreciated cellular complexity of the continuously-growing mouse incisor, which suggests a coherent model of cell dynamics enabling unarrested growth. This model relies on spatially-restricted stem, progenitor and differentiated populations in the epithelial and mesenchymal compartments underlying the coordinated expansion of two major branches of pulpal cells and diverse epithelial subtypes. Further comparisons of human and mouse teeth yield both parallelisms and differences in tissue heterogeneity and highlight the specifics behind growing and non-growing modes. Despite being similar at a coarse level, mouse and human teeth reveal molecular differences and species-specific cell subtypes suggesting possible evolutionary divergence. Overall, here we provide an atlas of human and mouse teeth with a focus on growth and differentiation.

Msx2 plays an important role in BMP6-induced osteogenic differentiation of two mesenchymal cell lines: C3H10T1/2 and C2C12

Bone morphogenetic proteins (BMPs), have been shown to enhance the osteogenic differentiation of mesenchymal cells (MCs) and to promote bone formation. BMP6 is known to play an important role in the process of MCs towards osteogenic differentiation by virtue of their osteoinductive and cell type specific proliferative activity. However, the molecular mechanism relate to BMP6 osteoinductive activity is still unclear and continues to warrant further investigation. Msx2 is a member of the homeobox gene family of transcription factors and promotes calcification. Hence, we wondered if it might also play a role in BMP6-induced osteogenesis. In this study, two mouse mesenchymal cell lines were treated with BMP6, adenovirus-Msx2 (Ad-Msx2) or adenovirus-siMsx2 (Ad-siMsx2). Based on the results of mRNA and protein expression, it was indicated that BMP6 could enhance the expression of Msx2 and activate the phosphorylation of Smad 1/5/8, p38 and ERK1/2. Being transfected by Ad-Msx2, the BMP6-induced activation of phosphorylation was significantly promoted. On the contrary, two cell lines transfected by Ad-siMsx2 presented an inhibited expression of three phosphorylated proteins even after being induced by BMP6. The evaluation of ALP, OPN, OC and calcium deposits revealed the osteogenic results those were corresponding to the results of mRNA and protein. Taken together, these findings can be a novel viewpoint for the understanding of the mechanisms of BMP6-induced osteogenesis and provide therapeutic targets of bone defect.

Msx2 Prevents Stratified Squamous Epithelium Formation in the Enamel Organ

Tooth enamel is manufactured by the inner enamel epithelium of the multilayered enamel organ. Msx2 loss-of-function mutation in a mouse model causes an abnormal accumulation of epithelial cells in the enamel organ, but the underlying mechanism by which Msx2 regulates amelogenesis is poorly understood. We therefore performed detailed histological and molecular analyses of Msx2 null mice. Msx2 null ameloblasts and stratum intermedium (SI) cells differentiated normally in the early stages of amelogenesis. However, during subsequent developmental stages, the outer enamel epithelium (OEE) became highly proliferative and transformed into a keratinized stratified squamous epithelium that ectopically expressed stratified squamous epithelium markers, including Heat shock protein 25, Loricrin, and Keratin 10. Moreover, expression of hair follicle-specific keratin genes such as Keratin 26 and Keratin 73 was upregulated in the enamel organ of Msx2 mutants. With the accumulation of keratin in the stellate reticulum (SR) region and subsequent odontogenic cyst formation, SI cells gradually lost the ability to differentiate, and the expression of Sox2 and Notch1 was downregulated, leading to ameloblast depolarization. As a consequence, the organization of the Msx2 mutant enamel organ became disturbed and enamel failed to form in the normal location. Instead, there was ectopic mineralization that likely occurred within the SR. In summary, we show that during amelogenesis, Msx2 executes a bipartite function, repressing the transformation of OEE into a keratinized stratified squamous epithelium while simultaneously promoting the development of a properly differentiated enamel organ competent for enamel formation.

Bone resorption: an actor of dental and periodontal development?

Dental and periodontal tissue development is a complex process involving various cell-types. A finely orchestrated network of communications between these cells is implicated. During early development, communications between cells from the oral epithelium and the underlying mesenchyme govern the dental morphogenesis with successive bud, cap and bell stages. Later, interactions between epithelial and mesenchymal cells occur during dental root elongation. Root elongation and tooth eruption require resorption of surrounding alveolar bone to occur. For years, it was postulated that signaling molecules secreted by dental and periodontal cells control bone resorbing osteoclast precursor recruitment and differentiation. Reverse signaling originating from bone cells (osteoclasts and osteoblasts) toward dental cells was not suspected. Dental defects reported in osteopetrosis were associated with mechanical stress secondary to defective bone resorption. In the last decade, consequences of bone resorption over-activation on dental and periodontal tissue formation have been analyzed with transgenic animals (RANK^Tg and Opg^−∕− mice). Results suggest the existence of signals originating from osteoclasts toward dental and periodontal cells. Meanwhile, experiments consisting in transitory inhibition of bone resorption during root elongation, achieved with bone resorption inhibitors having different mechanisms of action (bisphosphonates and RANKL blocking antibodies), have evidenced dental and periodontal defects that support the presence of signals originating bone cells toward dental cells. The aim of the present manuscript is to present the data we have collected in the last years that support the hypothesis of a role of bone resorption in dental and periodontal development.