Tissue Interactions Regulating Tooth Development and Renewal

Six1 Regulates Mouse Incisor Development by Promoting Dlx1/2/5 Expression

Tooth development is a complex process orchestrated by intricate gene regulatory networks, involving both odontogenic epithelium and ectomesenchyme. Six1, a pivotal transcription factor (TF), is involved in the development of the lower incisor. However, its precise role during incisor development and the molecular mechanisms underpinning its regulatory functions remain poorly understood. This study employs Six1 deletion mouse models to elucidate the critical regulatory role of Six1 in governing dental mesenchyme development. By performing single-cell RNA sequencing, we constructed a comprehensive transcriptome atlas of tooth germ development from the bud to bell stage. Our analyses suggest that the dental follicle and the dental papilla (DP) are differentiated from dental ectomesenchyme (DEM) and identify the key TFs underlying these distinct states. Notably, we show that Dlx1, Dlx2, and Dlx5 (Dlx1/2/5) may function as the key TFs that promote the formation of DP. We further show that the deletion of Six1 perturbs dental mesenchyme development by impeding the transitions from DEM to DP states. Importantly, SIX1 directly binds to the promoters of Dlx1/2/5 to promote their co-expression, which subsequently leads to widespread epigenetic and transcriptional remodeling. In summary, our findings unveil Six1's indispensable role in incisor development, offering key insights into TF-driven regulatory networks that govern dental mesenchyme cell fate transitions during tooth development.

Resilience of the replacing dentition in adult reptiles

The dentition is critical to animal survival and teeth are present in modern vertebrates including teleost fish, sharks, amphibians, mammals and reptiles. The developmental processes that give rise to teeth are not just preserved through evolution but also share high level of similarity with the embryogenesis of other ectodermal organs. In this review we go beyond the embryonic phase of tooth development to life-long tooth replacement. We will address the origins of successional teeth, the location of putative tissue-resident stem cells, how de novo tooth formation continues throughout life and how teeth are shed in a spatially and temporally controlled manner. We review the evidence that the dental epithelium, which is the earliest recognizable dental structure in the reptilian dentition, serves as a putative niche for tissue-resident epithelial stem cells and recent molecular findings from transcriptomics carried out in reptilian dentitions. We discuss how odontoclasts clear the eruption pathway and allow shedding of functional teeth. The reptiles, particularly lizards, are emerging as some of the most accessible animals to study tooth replacement which has relevance to evolution of the dentition and human dental disorders.

Evaluating predictions of the patterning cascade model of crown morphogenesis in the human lower mixed and permanent dentition

OBJECTIVE

The patterning cascade model of crown morphogenesis has been studied extensively in a variety of organisms to elucidate the evolutionary history surrounding postcanine tooth form. The current research is the first to use a large modern human sample to examine whether the crown configuration of lower deciduous and permanent molars aligns with expectations derived from the model. This study has two main goals: 1) to determine if metameric and antimeric pairs significantly differ in size, accessory trait expression, and relative intercusp spacing, and 2) assess whether the relative distance among early-forming cusps accounts for observed variation in accessory cusp expression.

METHODS

Tooth size, intercusp distance, and morphological trait expression data were collected from 3D scans of mandibular dental casts representing participants of the Harvard Solomon Islands Project. Paired tests were utilized to compare tooth size, accessory trait expression, and relative intercusp distance between diphyodont metameres and permanent antimeres. Proportional odds logistic regression was implemented to investigate how the odds of greater accessory cusp expression vary as a function of the distance between early-developing cusps.

RESULTS/SIGNIFICANCE

Comparing paired molars, significant differences were identified for tooth size and cusp 5 expression. Several relative intercusp distances emerged as important predictors of cusp 6 expression, however, results for cusp 5 and cusp 7 did not match expected patterns. These findings support previous quantitative genetic results and suggest the development of neighboring crown structures represents a zero-sum partitioning of cellular territory and resources. As such, this study contributes to a better understanding of the foundations of deciduous and permanent molar crown variation in humans.

Fibroblast growth factor signalling regulates the development of tooth root

Fibroblast growth factor (FGF) signalling plays a crucial role in the morphogenesis of multiple tissues including teeth. While the role of the signal has been studied in tooth crown development, little is known about root development. Of several FGF ligands involved in hard tissue formation, we suggest that FGF18 regulates the development of murine tooth roots. We implanted FGF18-soaked heparin beads into the lower first molar tooth buds at postnatal day 6 (P6), followed by transplantation under the kidney capsule. After 3 weeks, FGF18 significantly facilitated root elongation and periodontal tissue formation compared to the control. In situ hybridisation showed that Fgf18 transcripts were initially localised in the dental pulp along Hertwig's epithelial root sheath at P6 and P10 and subsequently in the dental follicle cells at P14. Fgf receptors were expressed in various dental tissues during these stages. In vitro analysis using the dental pulp stem cells revealed that FGF18 inhibited cell proliferation and decreased expression levels of osteogenic markers, Runx2, Alpl and Sp7. Consistently, after 1 week of kidney capsule transplantation, FGF18 application did not induce the expression of Sp7 and Bsp, but upregulated Periostin in the apical region of dental mesenchyme in the grafted molar. These findings suggest that FGF18 facilitates molar root development by regulating the calcification of periodontal tissues.

Orodental malformations associated with human MSX1 sequence variants

BACKGROUND

MSX1 sequence variants have been known to cause human tooth agenesis (TA) with or without orofacial clefts. However, their roles during the whole processes of tooth development are not fully understood. This study aimed to characterize a 4-membered family with TA carrying a novel MSX1 pathogenic variant and investigate the disease mechanism.

METHODS

The authors conducted whole exome analysis to define the disease-causing sequence variant. They performed microcomputed tomography, morphometric analyses, transcriptome profiling, and molecular characterization to study the affected teeth and the gene variant.

RESULTS

The authors identified an MSX1 pathogenic variant, p.Glu232∗, in affected family members with TA and concomitant orodental anomalies, namely, prominent maxillary labial frenum, central incisor diastema, median maxillary anterior alveolar cleft, tooth fusion, mandibular molar dysmorphology, thin dentin layer, and slender dental roots. MSX1-defective teeth were not apparently microdontic but had thin dentin layers. The mandibular molars showed a homeotic transformation to maxillary counterparts. Genes involved in extracellular matrix organization and dentinogenesis, such as DMP1 and MMP20, were downregulated in dental pulp tissues of MSX1-defective teeth. The p.Glu232∗-truncated MSX1 properly localized to the nucleus but partially lost its transactivation ability. Analyzing reported cases indicated that truncation sequence variants within the homeobox domain of MSX1 caused a more severe TA phenotype than those outside of the homeobox domain, probably due to dominant negativity compared with haploinsufficiency.

CONCLUSIONS

This study provides in vivo evidence that MSX1 contributes to developmental processes of various orodental tissues in humans.

PRACTICAL IMPLICATIONS

Clinically, hypertrophic labial frenum, incisor diastema, and median maxillary anterior alveolar cleft might be considered diagnostic for MSX1-associated TA.

The role of autophagy in odontogenesis, dental implant surgery, periapical and periodontal diseases

Autophagy is a cellular process that is evolutionarily conserved, involving the sequestration of damaged organelles and proteins into autophagic vesicles, which subsequently fuse with lysosomes for degradation. Autophagy controls the development of many diseases by influencing apoptosis, inflammation, the immune response and different cellular processes. Autophagy plays a significant role in the aetiology of disorders associated with dentistry. Autophagy controls odontogenesis. Furthermore, it is implicated in the pathophysiology of pulpitis and periapical disorders. It enhances the survival, penetration and colonization of periodontal pathogenic bacteria into the host periodontal tissues and facilitates their escape from host defences. Autophagy plays a crucial role in mitigating exaggerated inflammatory reactions within the host's system during instances of infection and inflammation. Autophagy also plays a role in the relationship between periodontal disease and systemic diseases. Autophagy promotes wound healing and may enhance implant osseointegration. This study reviews autophagy's dento-alveolar effects, focusing on its role in odontogenesis, periapical diseases, periodontal diseases and dental implant surgery, providing valuable insights for dentists on tooth development and dental applications. A thorough examination of autophagy has the potential to discover novel and efficacious treatment targets within the field of dentistry.

A Wnt10a-Notch signaling axis controls Hertwig's epithelial root sheath cell behaviors during root furcation patterning

Human with bi-allelic WNT10A mutations and epithelial Wnt10a knockout mice present enlarged pulp chamber and apical displacement of the root furcation of multi-rooted teeth, known as taurodontism; thus, indicating the critical role of Wnt10a in tooth root morphogenesis. However, the endogenous mechanism by which epithelial Wnt10a regulates Hertwig's epithelial root sheath (HERS) cellular behaviors and contributes to root furcation patterning remains unclear. In this study, we found that HERS in the presumptive root furcating region failed to elongate at an appropriate horizontal level in K14-Cre;Wnt10afl/fl mice from post-natal day 0.5 (PN0.5) to PN4.5. EdU assays and immunofluorescent staining of cyclin D1 revealed significantly decreased proliferation activity of inner enamel epithelial (IEE) cells of HERS in K14-Cre;Wnt10afl/fl mice at PN2.5 and PN3.5. Immunofluorescent staining of E-Cadherin and acetyl-α-Tubulin demonstrated that the IEE cells of HERS tended to divide perpendicularly to the horizontal plane, which impaired the horizontal extension of HERS in the presumptive root furcating region of K14-Cre;Wnt10afl/fl mice. RNA-seq and immunofluorescence showed that the expressions of Jag1 and Notch2 were downregulated in IEE cells of HERS in K14-Cre;Wnt10afl/fl mice. Furthermore, after activation of Notch signaling in K14-Cre;Wnt10afl/fl molars by Notch2 adenovirus and kidney capsule grafts, the root furcation defect was partially rescued. Taken together, our study demonstrates that an epithelial Wnt10a-Notch signaling axis is crucial for modulating HERS cell proper proliferation and horizontal-oriented division during tooth root furcation morphogenesis.

Global bibliometric mapping of dental development research for the period 2012-2021

BACKGROUND

Dental development is a complex long-term biological process, significant attention should be paid to the dental development and health of this critical time window in childhood for the oral health of the whole life cycle.

AIM

This study aimed to conduct bibliometric studies on the scientific outputs of global dental development research by the CiteSpace software.

DESIGN

The global scientific outputs about dental development between January 1, 2012, and December 31, 2021, retrieved from Web of Science Core Collection, CiteSpace, and Microsoft Excel were applied for this bibliometric study.

RESULTS

A total of 3746 reviews and articles were obtained from the Web of Science core database for exploring the basic publication characteristics, hotspots, and frontiers of this research field. The results showed that dental development is gaining more researcher's attention over time. In terms of countries, the USA and China were the major contributors to this research area. At the institutional level, Sichuan University ranked first. Meanwhile, international cooperation across regions was quite active. The Journal of Dental Research has exerted a broad and far-reaching influence on dental development research in both publications and citations. James P Simmer, Jungwook Kim, Charles E Smith, and Jan CC Hu are among the most influential scholars in this field. Finally, the future hotspots were proposed, covering three directions: dental analysis, tooth development, and post-translational phosphorylation of histones.

CONCLUSION

In the past decade, the field of dental development has developed rapidly, and the cooperation between scholars, institutions, and researchers has become increasingly close.

Developmental roles of glomerular epithelial protein-1 in mice molar morphogenesis

Glomerular epithelial protein-1 (Glepp1), a R3 subtype family of receptor-type protein tyrosine phosphatases, plays important role in the activation of Src family kinases and regulates cellular processes such as cell proliferation, differentiation, and apoptosis. In this study, we firstly examined the functional evaluation of Glepp1 in tooth development and morphogenesis. The precise expression level and developmental function of Glepp1 were examined by RT-qPCR, in situ hybridization, and loss and gain of functional study using a range of in vitro organ cultivation methods. Expression of Glepp1 was detected in the developing tooth germs in cap and bell stage of tooth development. Knocking down Glepp1 at E13 for 2 days showed the altered expression levels of tooth development-related signaling molecules, including Bmps, Dspp, Fgf4, Lef1, and Shh. Moreover, transient knock down of Glepp1 revealed alterations in cellular physiology, examined by the localization patterns of Ki67 and E-cadherin. Similarly, knocking down of Glepp1 showed disrupted enamel rod and interrod formation in 3-week renal transplanted teeth. In addition, due to attrition of odontoblastic layers, the expression signals of Dspp and the localization of NESTIN were almost not detected after knock down of Glepp1; however, their expressions were increased after Glepp1 overexpression. Thus, our results suggested that Glepp1 plays modulating roles during odontogenesis by regulating the expression levels of signaling molecules and cellular events to achieve the proper structural formation of hard tissue matrices in mice molar development.

Fundamentals and Translational Applications of Stem Cells and Biomaterials in Dental, Oral and Craniofacial Regenerative Medicine

Regenerative dental medicine continuously expands to improve treatments for prevalent clinical problems in dental and oral medicine. Stem cell based translational opportunities include regenerative therapies for tooth restoration, root canal therapy, and inflammatory processes (e.g., periodontitis). The potential of regenerative approaches relies on the biological properties of dental stem cells. These and other multipotent somatic mesenchymal stem cell (MSC) types can in principle be applied as either autologous or allogeneic sources in dental procedures. Dental stem cells have distinct developmental origins and biological markers that determine their translational utility. Dental regenerative medicine is supported by mechanistic knowledge of the molecular pathways that regulate dental stem cell growth and differentiation. Cell fate determination and lineage progression of dental stem cells is regulated by multiple cell signaling pathways (e.g., WNTs, BMPs) and epigenetic mechanisms, including DNA modifications, histone modifications, and non-coding RNAs (e.g., miRNAs and lncRNAs). This review also considers a broad range of novel approaches in which stem cells are applied in combination with biopolymers, ceramics, and composite materials, as well as small molecules (agonistic or anti-agonistic ligands) and natural compounds. Materials that mimic the microenvironment of the stem cell niche are also presented. Promising concepts in bone and dental tissue engineering continue to drive innovation in dental and non-dental restorative procedures.

ASH2L, Core Subunit of H3K4 Methylation Complex, Regulates Amelogenesis

Histone methylation assumes a crucial role in the intricate process of enamel development. Our study has illuminated the substantial prevalence of H3K4me3 distribution, spanning from the cap stage to the late bell stage of dental germs. In order to delve into the role of H3K4me3 modification in amelogenesis and unravel the underlying mechanisms, we performed a conditional knockout of Ash2l, a core subunit essential for the establishment of H3K4me3 within the dental epithelium of mice. The absence of Ash2l resulted in reduced H3K4me3 modification, subsequently leading to abnormal morphology of dental germ at the late bell stage. Notably, knockout of Ash2l resulted in a loss of polarity in ameloblasts and odontoblasts. The proliferation and apoptosis of the inner enamel epithelium cells underwent dysregulation. Moreover, there was a notable reduction in the expression of matrix-related genes, Amelx and Dspp, accompanied with impaired enamel and dentin formation. Cut&Tag-seq (cleavage under targets and tagmentation sequencing) analysis substantiated a reduction of H3K4me3 modification on Shh, Trp63, Sp6, and others in the dental epithelium of Ash2l knockout mice. Validation through real-time polymerase chain reaction, immunohistochemistry, and immunofluorescence consistently affirmed the observed downregulation of Shh and Sp6 in the dental epithelium following Ash2l knockout. Intriguingly, the expression of Trp63 isomers, DNp63 and TAp63, was perturbed in Ash2l defect dental epithelium. Furthermore, the downstream target of TAp63, P21, exhibited aberrant expression within the cervical loop of mandibular first molars and incisors. Collectively, our findings suggest that ASH2L orchestrates the regulation of crucial amelogenesis-associated genes, such as Shh, Trp63, and others, by modulating H3K4me3 modification. Loss of ASH2L and H3K4me3 can lead to aberrant differentiation, proliferation, and apoptosis of the dental epithelium by affecting the expression of Shh, Trp63, and others genes, thereby contributing to the defects of amelogenesis.

Adult dental epithelial stem cell-derived organoids deposit hydroxylapatite biomineral

Ameloblasts are specialized cells derived from the dental epithelium that produce enamel, a hierarchically structured tissue comprised of highly elongated hydroxylapatite (OHAp) crystallites. The unique function of the epithelial cells synthesizing crystallites and assembling them in a mechanically robust structure is not fully elucidated yet, partly due to limitations with in vitro experimental models. Herein, we demonstrate the ability to generate mineralizing dental epithelial organoids (DEOs) from adult dental epithelial stem cells (aDESCs) isolated from mouse incisor tissues. DEOs expressed ameloblast markers, could be maintained for more than five months (11 passages) in vitro in media containing modulators of Wnt, Egf, Bmp, Fgf and Notch signaling pathways, and were amenable to cryostorage. When transplanted underneath murine kidney capsules, organoids produced OHAp crystallites similar in composition, size, and shape to mineralized dental tissues, including some enamel-like elongated crystals. DEOs are thus a powerful in vitro model to study mineralization process by dental epithelium, which can pave the way to understanding amelogenesis and developing regenerative therapy of enamel.

Effects of antiresorptive medications on tooth root formation and tooth eruption in paediatric patients

Tooth eruption is a pivotal milestone for children's growth and development. This process involves with the formation of the tooth root, the periodontal ligament (PDL) and the alveolar bone, as the tooth crown penetrates the bone and gingiva to enter the oral cavity. This review aims to outline current knowledge of the adverse dental effects of antiresorptive medications. Recently, paediatric indications for antiresorptive medications, such as bisphosphonates (BPs), have emerged, and these agents are increasingly used in children and adolescents to cure pathological bone resorption associated with bone diseases and cancers. Since tooth eruption is accompanied by osteoclastic bone resorption, it is expected that the administration of antiresorptive medications during this period affects tooth development. Indeed, several articles studying human patient cohorts and animal models report the dental defects associated with the use of these antiresorptive medications. This review shows the summary of the possible factors related to tooth eruption and introduces the future research direction to understand the mechanisms underlying the dental defects caused by antiresorptive medications.

Splicing mutations in AMELX and ENAM cause amelogenesis imperfecta

BACKGROUND

Amelogenesis imperfecta (AI) is a developmental enamel defect affecting the structure of enamel, esthetic appearance, and the tooth masticatory function. Gene mutations are reported to be relevant to AI. However, the mechanism underlying AI caused by different mutations is still unclear. This study aimed to reveal the molecular pathogenesis in AI families with 2 novel pre-mRNA splicing mutations.

METHODS

Two Chinese families with AI were recruited. Whole-exome sequencing and Sanger sequencing were performed to identify mutations in candidate genes. Minigene splicing assays were performed to analyze the mutation effects on mRNA splicing alteration. Furthermore, three-dimensional structures of mutant proteins were predicted by AlphaFold2 to evaluate the detrimental effect.

RESULTS

The affected enamel in family 1 was thin, rough, and stained, which was diagnosed as hypoplastic-hypomature AI. Genomic analysis revealed a novel splicing mutation (NM_001142.2: c.570 + 1G > A) in the intron 6 of amelogenin (AMELX) gene in family 1, resulting in a partial intron 6 retention effect. The proband in family 2 exhibited a typical hypoplastic AI, and the splicing mutation (NM_031889.2: c.123 + 4 A > G) in the intron 4 of enamelin (ENAM) gene was observed in the proband and her father. This mutation led to exon 4 skipping. The predicted structures showed that there were obvious differences in the mutation proteins compared with wild type, leading to impaired function of mutant proteins.

CONCLUSIONS

In this study, we identified two new splicing mutations in AMELX and ENAM genes, which cause hypoplastic-hypomature and hypoplastic AI, respectively. These results expand the spectrum of genes causing AI and broaden our understanding of molecular genetic pathology of enamel formation.

AKT from dental epithelium to papilla promotes odontoblast differentiation

Epithelial-mesenchymal interactions occur during tooth development. The dental epithelium (DE) is regarded as the signal center that regulates tooth morphology. However, the mechanism by which DE regulates the differentiation of mesenchyme-derived dental papilla (DP) into odontoblasts remains unclear. Using miniature pigs as a model, we analyzed the expression profiles of the DE and DP during odontoblast differentiation using high-throughput RNA sequencing. The phosphatidylinositol-3-kinase (PI3K)/AKT pathway is one of the most enriched pathways in both DE and DP. The PI3K/AKT pathway was first activated in the inner enamel epithelium but not in the DP on embryonic day 50. This pathway was then activated in the odontoblast layer on embryonic day 60. We showed that AKT activation promoted odontoblast differentiation of DP cells. We further demonstrated that activation of PI3K/AKT signaling in the DE effectively increased the expression levels of AKT and dentin sialophosphoprotein in DP cells. Additionally, we found that DE cells secreted collagen type IV alpha 6 chain (COL4A6) downstream of epithelial AKT signaling to positively regulate mesenchymal AKT levels. Therefore, our data suggest that PI3K/AKT signaling from the DE to the DP promotes odontoblast differentiation via COL4A6 secretion.

Spatiotemporal expression of fibroblast growth factor 4 and 10 during the morphogenesis of deciduous molars in miniature pigs

OBJECTIVE

Fibroblast growth factors (FGFs) play pivotal roles in mediating interactions between dental epithelium and mesenchyme throughout tooth initiation and morphogenesis. This study aimed to elucidate the roles of FGF4 and FGF10 in the regulation of tooth development.

DESIGN

In this study, we investigated spatiotemporal expression patterns of FGF4 and FGF10 in the third deciduous molars (DM3) of miniature pigs at the cap, early bell, and late bell stages. Pregnant miniature pigs were obtained, and the samples were processed for histological staining. Non-radioactive in situ hybridization, immunohistochemistry, and real-time PCR were used to detect mRNA and protein expression levels of FGF4 and FGF10.

RESULTS

FGF4 was expressed in the dental epithelium and mesenchyme at the cap stage. At the early bell stage, epithelial expression of FGF4 was reduced while mesenchymal expression got stronger. At the late bell stage, the FGF4 expression was restricted to the inner enamel epithelium (IEE) and differentiating odontoblasts. FGF10 was expressed intensely in both epithelium and mesenchyme at the cap stage. The expression of FGF10 was concentrated in the secondary enamel knots and surrounding mesenchyme at the early bell stage. FGF10 was weakly detected in the IEE by the late bell stage.

CONCLUSIONS

Our results indicated that FGF4 and FGF10 might have partially redundant functions in regulating epithelium morphogenesis. FGF4 may be involved in regulatory signaling cascades mediating interactions between the epithelium and mesenchyme. In addition, the downregulation of FGF10 expression may be associated with the cessation of mesenchymal cell proliferation and initiation of preodontoblast polarization.

Single-cell census of human tooth development enables generation of human enamel

Tooth enamel secreted by ameloblasts (AMs) is the hardest material in the human body, acting as a shield to protect the teeth. However, the enamel is gradually damaged or partially lost in over 90% of adults and cannot be regenerated due to a lack of ameloblasts in erupted teeth. Here, we use single-cell combinatorial indexing RNA sequencing (sci-RNA-seq) to establish a spatiotemporal single-cell census for the developing human tooth and identify regulatory mechanisms controlling the differentiation process of human ameloblasts. We identify key signaling pathways involved between the support cells and ameloblasts during fetal development and recapitulate those findings in human ameloblast in vitro differentiation from induced pluripotent stem cells (iPSCs). We furthermore develop a disease model of amelogenesis imperfecta in a three-dimensional (3D) organoid system and show AM maturation to mineralized structure in vivo. These studies pave the way for future regenerative dentistry.

FGFR1 variants contributed to families with tooth agenesis

BACKGROUND

Tooth agenesis is a common dental anomaly that can substantially affect both the ability to chew and the esthetic appearance of patients. This study aims to identify possible genetic factors that underlie various forms of tooth agenesis and to investigate the possible molecular mechanisms through which human dental pulp stem cells may play a role in this condition.

RESULTS

Using whole-exome sequencing of a Han Chinese family with non-syndromic tooth agenesis, a rare mutation in FGFR1 (NM_001174063.2: c.103G > A, p.Gly35Arg) was identified as causative and confirmed by Sanger sequencing. Via GeneMatcher, another family with a known variant (NM_001174063.2: c.1859G > A, p.Arg620Gln) was identified and diagnosed with tooth agenesis and a rare genetic disorder with considerable intrafamilial variability. Fgfr1 is enriched in the ectoderm during early embryonic development of mice and showed sustained low expression during normal embryonic development of Xenopus laevis frogs. Functional studies of the highly conserved missense variant c.103G > A showed deleterious effects. FGFR1 (c.103G > A) was overexpressed compared to wildtype and promoted proliferation while inhibiting apoptosis in HEK293 and human dental pulp stem cells. Moreover, the c.103G > A variant was found to suppress the epithelial-mesenchymal transition. The variant could downregulate ID4 expression and deactivate the TGF-beta signaling pathway by promoting the expression of SMAD6 and SMAD7.

CONCLUSION

Our research broadens the mutation spectrum associated with tooth agenesis and enhances understanding of the underlying disease mechanisms of this condition.

Craniofacial developmental biology in the single-cell era

The evolution of a unique craniofacial complex in vertebrates made possible new ways of breathing, eating, communicating and sensing the environment. The head and face develop through interactions of all three germ layers, the endoderm, ectoderm and mesoderm, as well as the so-called fourth germ layer, the cranial neural crest. Over a century of experimental embryology and genetics have revealed an incredible diversity of cell types derived from each germ layer, signaling pathways and genes that coordinate craniofacial development, and how changes to these underlie human disease and vertebrate evolution. Yet for many diseases and congenital anomalies, we have an incomplete picture of the causative genomic changes, in particular how alterations to the non-coding genome might affect craniofacial gene expression. Emerging genomics and single-cell technologies provide an opportunity to obtain a more holistic view of the genes and gene regulatory elements orchestrating craniofacial development across vertebrates. These single-cell studies generate novel hypotheses that can be experimentally validated in vivo. In this Review, we highlight recent advances in single-cell studies of diverse craniofacial structures, as well as potential pitfalls and the need for extensive in vivo validation. We discuss how these studies inform the developmental sources and regulation of head structures, bringing new insights into the etiology of structural birth anomalies that affect the vertebrate head.

MEMO1 Is Required for Ameloblast Maturation and Functional Enamel Formation

Coordinated mineralization of soft tissue is central to organismal form and function, while dysregulated mineralization underlies several human pathologies. Oral epithelial-derived ameloblasts are polarized, secretory cells responsible for generating enamel, the most mineralized substance in the human body. Defects in ameloblast development result in enamel anomalies, including amelogenesis imperfecta. Identifying proteins critical in ameloblast development can provide insight into specific pathologies associated with enamel-related disorders or, more broadly, mechanisms of mineralization. Previous studies identified a role for MEMO1 in bone mineralization; however, whether MEMO1 functions in the generation of additional mineralized structures remains unknown. Here, we identify a critical role for MEMO1 in enamel mineralization. First, we show that Memo1 is expressed in ameloblasts and, second, that its conditional deletion from ameloblasts results in enamel defects, characterized by a decline in mineral density and tooth integrity. Histology revealed that the mineralization defects in Memo1 mutant ameloblasts correlated with a disruption in ameloblast morphology. Finally, molecular profiling of ameloblasts and their progenitors in Memo1 oral epithelial mutants revealed a disruption to cytoskeletal-associated genes and a reduction in late-stage ameloblast markers, relative to controls. Collectively, our findings integrate MEMO1 into an emerging network of molecules important for ameloblast development and provide a system to further interrogate the relationship of cytoskeletal and amelogenesis-related defects.

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.

The Notch-mediated circuitry in the evolution and generation of new cell lineages: the tooth model

The Notch pathway is an ancient, evolutionary conserved intercellular signaling mechanism that is involved in cell fate specification and proper embryonic development. The Jagged2 gene, which encodes a ligand for the Notch family of receptors, is expressed from the earliest stages of odontogenesis in epithelial cells that will later generate the enamel-producing ameloblasts. Homozygous Jagged2 mutant mice exhibit abnormal tooth morphology and impaired enamel deposition. Enamel composition and structure in mammals are tightly linked to the enamel organ that represents an evolutionary unit formed by distinct dental epithelial cell types. The physical cooperativity between Notch ligands and receptors suggests that Jagged2 deletion could alter the expression profile of Notch receptors, thus modifying the whole Notch signaling cascade in cells within the enamel organ. Indeed, both Notch1 and Notch2 expression are severely disturbed in the enamel organ of Jagged2 mutant teeth. It appears that the deregulation of the Notch signaling cascade reverts the evolutionary path generating dental structures more reminiscent of the enameloid of fishes rather than of mammalian enamel. Loss of interactions between Notch and Jagged proteins may initiate the suppression of complementary dental epithelial cell fates acquired during evolution. We propose that the increased number of Notch homologues in metazoa enabled incipient sister cell types to form and maintain distinctive cell fates within organs and tissues along evolution.

An essential overview of orofacial clefting

An appreciation of the embryologic development of the face is essential to understanding the anatomic variation observed in this phenotypically broad condition. Embryologically, the nose, lip and palate are separated into the primary and secondary palate, which are divided anatomically by the incisive foramen. The epidemiology of orofacial clefting is reviewed together with the contemporaneous cleft classification systems that enable comparisons to be made between international centres for audit and research purposes. A detailed examination of the clinical anatomy of the lip and palate informs the surgical priorities for the primary reconstruction of both form and function. The pathophysiology of the submucous cleft palate is also explored. The seismic impact of the 1998 Clinical Standards Advisory Group report on the organisation of UK cleft care provision is outlined. The importance of the Cleft Registry and Audit Network database in auditing UK cleft outcomes is highlighted. The potential for the Cleft Collective study to establish the causes of clefting, the optimal treatment protocols, and the impact of cleft on patients is tremendously exciting for all health care professionals involved in the management of this challenging congenital deformity.

Establishing and characterizing human stem cells from the apical papilla immortalized by hTERT gene transfer

Stem cells from the apical papilla (SCAPs) are promising candidates for regenerative endodontic treatment and tissue regeneration in general. However, harvesting enough cells from the limited apical papilla tissue is difficult, and the cells lose their primary phenotype over many passages. To get over these challenges, we immortalized human SCAPs with lentiviruses overexpressing human telomerase reverse transcriptase (hTERT). Human immortalized SCAPs (hiSCAPs) exhibited long-term proliferative activity without tumorigenic potential. Cells also expressed mesenchymal and progenitor biomarkers and exhibited multiple differentiation potentials. Interestingly, hiSCAPs gained a stronger potential for osteogenic differentiation than the primary cells. To further investigate whether hiSCAPs could become prospective seed cells in bone tissue engineering, in vitro and in vivo studies were performed, and the results indicated that hiSCAPs exhibited strong osteogenic differentiation ability after infection with recombinant adenoviruses expressing BMP9 (AdBMP9). In addition, we revealed that BMP9 could upregulate ALK1 and BMPRII, leading to an increase in phosphorylated Smad1 to induce the osteogenic differentiation of hiSCAPs. These results support the application of hiSCAPs in tissue engineering/regeneration schemes as a stable stem cell source for osteogenic differentiation and biomineralization, which could be further used in stem cell-based clinical therapies.

Loss of Stat3 in Osterix+ cells impairs dental hard tissues development

BACKGROUND

Mutations in the signal transducers and activators of transcription 3 (STAT3) gene result in hyper-IgE syndrome(HIES), a rare immunodeficiency that causes abnormalities in immune system, bones and teeth. However, the role of Stat3 in development of dental hard tissues was yet to investigate.

METHODS

In this study, a transgenic mouse of conditional knockout of Stat3 in dental mesenchymal cells (Osx-Cre; Stat3^fl/fl, Stat3 CKO) was made. The differences of postnatal tooth development between control and Stat3 CKO mice were compared by histology, µCT and scanning electron microscopy.

RESULT

Compared with the control, Stat3 CKO mice were presented with remarkable abnormal tooth phenotypes characterized by short root and thin dentin in molars and incisors. The enamel defects were also found on mandibular incisors. showed that Ki67-positive cells significantly decreased in dental mesenchymal of Stat3 CKO mice. In addition, β-catenin signaling was reduced in Hertwig's epithelial root sheath (HERS) and odontoblasts of Stat3 CKO mice.

CONCLUSIONS

Our results suggested that Stat3 played an important role in dental hard tissues development, and Stat3 may regulate dentin and tooth root development through the β-catenin signaling pathway.

Dentin defects caused by a Dspp-1 frameshift mutation are associated with the activation of autophagy

Dentin sialophosphoprotein (DSPP) is primarily expressed by differentiated odontoblasts (dentin-forming cells), and transiently expressed by presecretory ameloblasts (enamel-forming cells). Disease-causing DSPP mutations predominantly fall into two categories: 5' mutations affecting targeting and trafficking, and 3' - 1 frameshift mutations converting the repetitive, hydrophilic, acidic C-terminal domain into a hydrophobic one. We characterized the dental phenotypes and investigated the pathological mechanisms of DsppP19L and Dspp-1fs mice that replicate the two categories of human DSPP mutations. In DsppP19L mice, dentin is less mineralized but contains dentinal tubules. Enamel mineral density is reduced. Intracellular accumulation and ER retention of DSPP is observed in odontoblasts and ameloblasts. In Dspp-1fs mice, a thin layer of reparative dentin lacking dentinal tubules is deposited. Odontoblasts show severe pathosis, including intracellular accumulation and ER retention of DSPP, strong ubiquitin and autophagy activity, ER-phagy, and sporadic apoptosis. Ultrastructurally, odontoblasts show extensive autophagic vacuoles, some of which contain fragmented ER. Enamel formation is comparable to wild type. These findings distinguish molecular mechanisms underlying the dental phenotypes of DsppP19L and Dspp-1fs mice and support the recently revised Shields classification of dentinogenesis imperfecta caused by DSPP mutations in humans. The Dspp-1fs mice may be valuable for the study of autophagy and ER-phagy.

Heat Shock Proteins in Tooth Development and Injury Repair

Heat shock proteins (HSPs) are a class of molecular chaperones with expression increased in response to heat or other stresses. HSPs regulate cell homeostasis by modulating the folding and maturation of intracellular proteins. Tooth development is a complex process that involves many cell activities. During tooth preparation or trauma, teeth can be damaged. The damaged teeth start their repair process by remineralizing and regenerating tissue. During tooth development and injury repair, different HSPs have different expression patterns and play a special role in odontoblast differentiation and ameloblast secretion by mediating signaling pathways or participating in protein transport. This review explores the expression patterns and potential mechanisms of HSPs, particularly HSP25, HSP60 and HSP70, in tooth development and injury repair.

A shark-inspired general model of tooth morphogenesis unveils developmental asymmetries in phenotype transitions

Developmental complexity stemming from the dynamic interplay between genetic and biomechanic factors canalizes the ways genotypes and phenotypes can change in evolution. As a paradigmatic system, we explore how changes in developmental factors generate typical tooth shape transitions. Since tooth development has mainly been researched in mammals, we contribute to a more general understanding by studying the development of tooth diversity in sharks. To this end, we build a general, but realistic, mathematical model of odontogenesis. We show that it reproduces key shark-specific features of tooth development as well as real tooth shape variation in small-spotted catsharks Scyliorhinus canicula. We validate our model by comparison with experiments in vivo. Strikingly, we observe that developmental transitions between tooth shapes tend to be highly degenerate, even for complex phenotypes. We also discover that the sets of developmental parameters involved in tooth shape transitions tend to depend asymmetrically on the direction of that transition. Together, our findings provide a valuable base for furthering our understanding of how developmental changes can lead to both adaptive phenotypic change and trait convergence in complex, phenotypically highly diverse, structures.

Spontaneous spheroids from alveolar bone-derived mesenchymal stromal cells maintain pluripotency of stem cells by regulating hypoxia-inducible factors

BACKGROUND

Spontaneous spheroid culture is a novel three-dimensional (3D) culture strategy for the rapid and efficient selection of progenitor cells. The objectives of this study are to investigate the pluripotency and differentiation capability of spontaneous spheroids from alveolar bone-derived mesenchymal stromal cells (AB-MSCs); compare the advantages of spontaneous spheroids to those of mechanical spheroids; and explore the mechanisms of stemness enhancement during spheroid formation from two-dimensional (2D) cultured cells.

METHODS

AB-MSCs were isolated from the alveolar bones of C57BL/6 J mice. Spontaneous spheroids formed in low-adherence specific culture plates. The stemness, proliferation, and multi-differentiation capacities of spheroids and monolayer cultures were investigated by reverse transcription quantitative polymerase chain reaction (RT-qPCR), immunofluorescence, alkaline phosphatase (ALP) activity, and oil-red O staining. The pluripotency difference between the spontaneous and mechanical spheroids was analyzed using RT-qPCR. Hypoxia-inducible factor (HIFs) inhibition experiments were performed to explore the mechanisms of stemness maintenance in AB-MSC spheroids.

RESULTS

AB-MSCs successfully formed spontaneous spheroids after 24 h. AB-MSC spheroids were positive for MSC markers and pluripotency markers (Oct4, KLF4, Sox2, and cMyc). Spheroids showed higher Ki67 expression and lower Caspase3 expression at 24 h. Under the corresponding conditions, the spheroids were successfully differentiated into osteogenic and adipogenic lineages. AB-MSC spheroids can induce neural-like cells after neurogenic differentiation. Higher expression of osteogenic markers, adipogenic markers, and neurogenic markers (NF-M, NeuN, and GFAP) was found in spheroids than in the monolayer. Spontaneous spheroids exhibited higher stemness than mechanical spheroids did. HIF-1α and HIF-2α were remarkably upregulated in spheroids. After HIF-1/2α-specific inhibition, spheroid formation was significantly reduced. Moreover, the expression of the pluripotency genes was suppressed.

CONCLUSIONS

Spontaneous spheroids from AB-MSCs enhance stemness and pluripotency. HIF-1/2α plays an important role in the stemness regulation of spheroids. AB-MSC spheroids exhibit excellent multi-differentiation capability, which may be a potent therapy for craniomaxillofacial tissue regeneration.

Genome-wide identification of potential odontogenic genes involved in the dental epithelium-mesenchymal interaction during early odontogenesis

BACKGROUND

Epithelium-mesenchymal interactions are involved in odontogenic processes. Previous studies have focused on the intracellular signalling regulatory network in tooth development, but the functions of extracellular regulatory molecules have remained unclear. This study aims to explore the gene profile of extracellular proteoglycans and their glycosaminoglycan chains potentially involved in dental epithelium-mesenchymal interactions using high-throughput sequencing to provide new understanding of early odontogenesis.

RESULTS

Whole transcriptome profiles of the mouse dental epithelium and mesenchyme were investigated by RNA sequencing (RNA-seq). A total of 1,281 and 1,582 differentially expressed genes were identified between the dental epithelium and mesenchyme at E11.5 and E13.5, respectively. Enrichment analysis showed that extracellular regions and ECM-receptor interactions were significantly enriched at both E11.5 and E13.5. Polymerase chain reaction analysis confirmed that the extracellular proteoglycan family exhibited distinct changes during epithelium-mesenchymal interactions. Most proteoglycans showed higher transcript levels in the dental mesenchyme, whereas only a few were upregulated in the epithelium at both stages. In addition, 9 proteoglycans showed dynamic expression changes between these two tissue compartments. Gpc4, Sdc2, Spock2, Dcn and Lum were expressed at higher levels in the dental epithelium at E11.5, whereas their expression was significantly higher in the dental mesenchyme at E13.5, which coincides with the odontogenic potential shift. Moreover, the glycosaminoglycan biosynthetic enzymes Ext1, Hs3st1/5, Hs6st2/3, Ndst3 and Sulf1 also exhibited early upregulation in the epithelium but showed markedly higher expression in the mesenchyme after the odontogenic potential shift.

CONCLUSION

This study reveals the dynamic expression profile of extracellular proteoglycans and their biosynthetic enzymes during the dental epithelium-mesenchymal interaction. This study offers new insight into the roles of extracellular proteoglycans and their distinct sulfation underlying early odontogenesis.

Genotype-phenotype pattern analysis of pathogenic PAX9 variants in Chinese Han families with non-syndromic oligodontia

Background: Non-syndromic oligodontia is characterized by the absence of six or more permanent teeth, excluding third molars, and can have aesthetic, masticatory, and psychological consequences. Previous studies have shown that PAX9 is associated with autosomal dominant forms of oligodontia but the precise molecular mechanisms are still unknown.

Methods: Whole-exome and Sanger sequencing were performed on a cohort of approximately 28 probands with NSO, for mutation analysis. Bioinformatic analysis was performed on the potential variants. Immunofluorescence assay, western blotting, and qPCR were used to explore the preliminary functional impact of the variant PAX9 proteins. We reviewed PAX9-related NSO articles in PubMed to analyze the genotype-phenotype correlations.

Results: We identified three novel PAX9 variants in Chinese Han families: c.152G>T (p.Gly51Val), c.239delC (p.Thr82Profs*3), and c.409C>T (q.Gln137Ter). In addition, two previously reported missense variants were identified: c.140G>C (p.Arg47Pro) and c.146C>T (p.Ser49Leu) (reference sequence NM_006194.4). Structural modeling revealed that all missense variants were located in the highly conserved paired domain. The other variants led to premature termination of the protein, causing structural impairment of the PAX9 protein. Immunofluorescence assay showed abnormal subcellular localizations of the missense variants (R47P, S49L, and G51V). In human dental pulp stem cells, western blotting and qPCR showed decreased expression of PAX9 variants (c.140G>C, p.R47P, and c.152G>T, p.G51V) compared with the wild-type group at both the transcription and translation levels. A review of published papers identified 64 PAX9 variants related to NSO and found that the most dominant feature was the high incidence of missing upper second molars, first molars, second premolars, and lower second molars.

Conclusion: Three novel PAX9 variants were identified in Chinese Han families with NSO. These results extend the variant spectrum of PAX9 and provide a foundation for genetic diagnosis and counseling.

Genetic/Protein Association of Atopic Dermatitis and Tooth Agenesis

Atopic dermatitis and abnormalities in tooth development (including hypomineralization, hypodontia and microdontia) have been observed to co-occur in some patients. A common pathogenesis pathway that involves genes and protein interactions has been hypothesized. This review aims to first provide a description of the key gene mutations and signaling pathways associated with atopic dermatitis and tooth agenesis (i.e., the absence of teeth due to developmental failure) and identify the possible association between the two diseases. Second, utilizing a list of genes most commonly associated with the two diseases, we conducted a protein–protein network interaction analysis using the STRING database and identified a novel association between the Wnt/β-catenin signaling pathway (major pathway responsible for TA) and desmosomal proteins (component of skin barrier that affect the pathogenesis of AD). Further investigation into the mechanisms that may drive their co-occurrence and underlie the development of the two diseases is warranted.

Plasticity of Dental Cell Types in Development, Regeneration, and Evolution

Recent years have improved our understanding of the plasticity of cell types behind inducing, building, and maintaining different types of teeth. The latest efforts were aided by progress in single-cell transcriptomics, which helped to define not only cell states with mathematical precision but also transitions between them. This includes new aspects of dental epithelial and mesenchymal stem cell niches and beyond. These recent efforts revealed continuous and fluid trajectories connecting cell states during dental development and exposed the natural plasticity of tooth-building progenitors. Such "developmental" plasticity seems to be employed for organizing stem cell niches in adult continuously growing teeth. Furthermore, transitions between mature cell types elicited by trauma might represent a replay of embryonic continuous cell states. Alternatively, they could constitute transitions that evolved de novo, not known from the developmental paradigm. In this review, we discuss and exemplify how dental cell types exhibit plasticity during dynamic processes such as development, self-renewal, repair, and dental replacement. Hypothetically, minor plasticity of cell phenotypes and greater plasticity of transitions between cell subtypes might provide a better response to lifetime challenges, such as damage or dental loss. This plasticity might be additionally harnessed by the evolutionary process during the elaboration of dental cell subtypes in different animal lineages. In turn, the diversification of cell subtypes building teeth brings a diversity of their shape, structural properties, and functions.

Identification and Characterization of a Previously Undiscovered Anatomical Structure in Maxillary Second Molars: The Palato-Mesiobuccal Canal

INTRODUCTION

This report aims to communicate the discovery of a novel anatomy in a three-rooted maxillary second molar, named as palato-mesiobuccal canal METHODS: The tooth selected for this report was found incidentally as a result of a study on extracted maxillary molars that was being conducted for an unrelated purpose and involved hundreds of teeth. This three-rooted maxillary second molar was scanned using a micro-CT device set at a pixel size of 13.68 μm. The images were reconstructed with previously tested parameters resulting in the acquisition of 1655 axial cross-sections. Three-dimensional models of the internal and external anatomies were generated in STL format and were texturized to simulate the pulp tissue. The inner structure of the tooth was analyzed through the axial cross sections and the 3D volume was qualitatively evaluated.

RESULTS

The analysis of the 3D models revealed that the maxillary second molar under examination has 3 independent roots and 4 root canals. Each root contains 3 canals (mesiobuccal, distobuccal and palatal), while the fourth canal follows a unique path, originating in the coronal third of the palatal canal and proceeding in a buccal direction ultimately exiting through an independent foramen at the apex of the mesiobuccal root, near the mesiobuccal canal.

CONCLUSIONS

This brief communication reports the discovery of a novel anatomy in a three-rooted maxillary second molar, named as palato-mesiobuccal canal, and provides important insights into the complexity of the root canal system in this group of teeth.

Getting out of a mammalian egg: the egg tooth and caruncle of the echidna

In the echidna, after development in utero, the egg is laid in the pouch and incubated for 10 days. During this time, the fetuses develop an egg tooth and caruncle to help them hatch. Using rare and unprecedented access to limited echidna pre- and post-hatching tissues, development of the egg tooth and caruncle were assessed by micro-CT, histology and immunofluorescence. Unlike therian tooth germs that develop by placode invagination, the echidna egg tooth developed by evagination, similar to the first teeth in some reptiles and fish. The egg tooth ankylosed to the premaxilla, rather than forming a tooth root with ligamentous attachment found in other mammals, with loss of the egg tooth associated with high levels of activity odontoclasts and apoptosis. The caruncle formed as a separate mineralisation from the adjacent nasal capsule, and as observed in birds and turtles, the nasal region epithelium on top of the nose expressed markers of cornification. Together, this highlights that the monotreme egg tooth shares many similarities with typical reptilian teeth, suggesting that this tooth has been conserved from a common ancestor of mammals and reptiles.

Homeobox Genes in Odontogenic Lesions: A Scoping Review

BACKGROUND

Homeobox genes play crucial roles in tooth morphogenesis and development and thus mutations in homeobox genes cause developmental disorders such as odontogenic lesions. The aim of this scoping review is to identify and compile available data from the literatures on the topic of homeobox gene expression in odontogenic lesions.

METHOD

An electronic search to collate all the information on studies on homeobox gene expression in odontogenic lesions was carried out in four databases (PubMed, EBSCO host, Web of Science and Cochrane Library) with selected keywords. All papers which reported expression of homeobox genes in odontogenic lesions were considered.

RESULTS

A total of eleven (11) papers describing expression of homeobox genes in odontogenic lesions were identified. Methods of studies included next generation sequencing, microarray analysis, RT-PCR, Western blotting, in situ hybridization, and immunohistochemistry. The homeobox reported in odontogenic lesions includes LHX8 and DLX3 in odontoma; PITX2, MSX1, MSX2, DLX, DLX2, DLX3, DLX4, DLX5, DLX6, ISL1, OCT4 and HOX C in ameloblastoma; OCT4 in adenomatoid odontogenic tumour; PITX2 and MSX2 in primordial odontogenic tumour; PAX9 and BARX1 in odontogenic keratocyst; PITX2, ZEB1 and MEIS2 in ameloblastic carcinoma while there is absence of DLX2, DLX3 and MSX2 in clear cell odontogenic carcinoma.

CONCLUSIONS

This paper summarized and reviews the possible link between homeobox gene expression in odontogenic lesions. Based on the current available data, there are insufficient evidence to support any definite role of homeobox gene in odontogenic lesions.

Enamel Phenotypes: Genetic and Environmental Determinants

Dental enamel is a specialized tissue that has adapted over millions of years of evolution to enhance the survival of a variety of species. In humans, enamel evolved to form the exterior protective layer for the crown of the exposed tooth crown. Its unique composition, structure, physical properties and attachment to the underlying dentin tissue allow it to be a resilient, although not self-repairing, tissue. The process of enamel formation, known as amelogenesis, involves epithelial-derived cells called ameloblasts that secrete a unique extracellular matrix that influences the structure of the mineralizing enamel crystallites. There are over 115 known genetic conditions affecting amelogenesis that are associated with enamel phenotypes characterized by either a reduction of enamel amount and or mineralization. Amelogenesis involves many processes that are sensitive to perturbation and can be altered by numerous environmental stressors. Genetics, epigenetics, and environment factors can influence enamel formation and play a role in resistance/risk for developmental defects and the complex disease, dental caries. Understanding why and how enamel is affected and the enamel phenotypes seen clinically support diagnostics, prognosis prediction, and the selection of treatment approaches that are appropriate for the specific tissue defects (e.g., deficient amount, decreased mineral, reduced insulation and hypersensitivity). The current level of knowledge regarding the heritable enamel defects is sufficient to develop a new classification system and consensus nosology that effectively communicate the mode of inheritance, molecular defect/pathway, and the functional aberration and resulting enamel phenotype.

Tooth number abnormality: from bench to bedside

Tooth number abnormality is one of the most common dental developmental diseases, which includes both tooth agenesis and supernumerary teeth. Tooth development is regulated by numerous developmental signals, such as the well-known Wnt, BMP, FGF, Shh and Eda pathways, which mediate the ongoing complex interactions between epithelium and mesenchyme. Abnormal expression of these crutial signalling during this process may eventually lead to the development of anomalies in tooth number; however, the underlying mechanisms remain elusive. In this review, we summarized the major process of tooth development, the latest progress of mechanism studies and newly reported clinical investigations of tooth number abnormality. In addition, potential treatment approaches for tooth number abnormality based on developmental biology are also discussed. This review not only provides a reference for the diagnosis and treatment of tooth number abnormality in clinical practice but also facilitates the translation of basic research to the clinical application.

Creating a Microenvironment to Give Wings to Dental Pulp Regeneration-Bioactive Scaffolds

Dental pulp and periapical diseases make patients suffer from acute pain and economic loss. Although root canal therapies, as demonstrated through evidence-based medicine, can relieve symptoms and are commonly employed by dentists, it is still difficult to fully restore a dental pulp's nutrition, sensory, and immune-regulation functions. In recent years, researchers have made significant progress in tissue engineering to regenerate dental pulp in a desired microenvironment. With breakthroughs in regenerative medicine and material science, bioactive scaffolds play a pivotal role in creating a suitable microenvironment for cell survival, proliferation, and differentiation, following dental restoration and regeneration. This article focuses on current challenges and novel perspectives about bioactive scaffolds in creating a microenvironment to promote dental pulp regeneration. We hope our readers will gain a deeper understanding and new inspiration of dental pulp regeneration through our summary.

Novel PAX9 compound heterozygous variants in a Chinese family with non-syndromic oligodontia and genotype-phenotype analysis of PAX9 variants

OBJECTIVE

Studies have reported that >91.9% of non-syndromic tooth agenesis cases are caused by seven pathogenic genes. To report novel heterozygous PAX9 variants in a Chinese family with non-syndromic oligodontia and summarize the reported genotype-phenotype relationship of PAX9 variants.

METHODOLOGY

We recruited 28 patients with non-syndromic oligodontia who were admitted to the Hospital of Stomatology Hebei Medical University (China) from 2018 to 2021. Peripheral blood was collected from the probands and their core family members for whole-exome sequencing (WES) and variants were verified by Sanger sequencing. Bioinformatics tools were used to predict the pathogenicity of the variants. SWISS-MODEL homology modeling was used to analyze the three-dimensional structural changes of variant proteins. We also analyzed the genotype-phenotype relationships of PAX9 variants.

RESULTS

We identified novel compound heterozygous PAX9 variants (reference sequence NM_001372076.1) in a Chinese family with non-syndromic oligodontia: a new missense variant c.1010C>A (p.T337K) in exon 4 and a new frameshift variant c.330_331insGT (p.D113Afs*9) in exon 2, which was identified as the pathogenic variant in this family. This discovery expands the known variant spectrum of PAX9; then, we summarized the phenotypes of non-syndromic oligodontia with PAX9 variants.

CONCLUSION

We found that PAX9 variants commonly lead to loss of the second molars.

Dental niche cells directly contribute to tooth reconstitution and morphogenesis

Mammalian teeth develop from the inductive epithelial-mesenchymal interaction, an important mechanism shared by many organs. The cellular basis for such interaction remains elusive. Here, we generate a dual-fluorescence model to track and analyze dental cells from embryonic to postnatal stages, in which Pitx2⁺ epithelium and Msx1⁺ mesenchyme are sufficient for tooth reconstitution. Single-cell RNA sequencing and spatial mapping further revealed critical cellular dynamics during molar development, where tooth germs are organized by Msx1⁺Sdc1⁺ dental papilla and surrounding dental niche. Surprisingly, niche cells are more efficient in tooth reconstitution and can directly regenerate papilla cells through interaction with dental epithelium. Finally, from the dental niche, we identify a group of previously unappreciated migratory Msx1⁺ Sox9⁺ cells as the potential cell origin for dental papilla. Our results indicate that the dental niche cells directly contribute to tooth organogenesis and provide critical insights into the essential cell composition for tooth engineering.

Factors Influencing the Aggressive Behavior of Odontogenic Keratocyst: A Narrative Review

During odontogenesis, the dental lamina disintegrates, leaving behind the remnants. Odontogenic pathologies such as cysts and tumors can arise from these remnants. The odontogenic keratocyte (OKC) arises from dental lamina remnants. Among the cysts, the odontogenic keratocyst is the most controversial. There is convincing evidence that inflammation plays a significant role in the pathogenesis and expansion of OKCs. Several factors mediate the proliferative capacity of the epithelial lining. The presence of mast cells close to the epithelial lining, cystic pressure build-up by vascular endothelial growth factors (VEGFs), and other cytokines contribute to the cystic expansion. Fibroblast activation by inflammation in the connective tissue stroma and changes in the epithelial lining are responsible for the aggressive nature of OKC. The use of molecular methodologies gives more profound insights into the factors influencing the progression of the lesion and helps develop newer treatment modalities for OKC. This review describes the characteristics that determine the aggressive behavior of this unique cyst.

KDF1 Novel Variant Causes Unique Dental and Oral Epithelial Defects

Keratinocyte differentiation factor 1 (KDF1) is a recently identified and rare candidate gene for human tooth agenesis; however, KDF1-related morphological characteristics and pathological changes in dental tissue and the oral epithelium remain largely unknown. Here, we employed whole-exome sequencing (WES) and Sanger sequencing to screen for the suspected variants in a cohort of 151 tooth agenesis patients, and we segregated a novel KDF1 heterozygous missense variation, c.920G>C (p.R307P), in a non-syndromic tooth agenesis family. Essential bioinformatics analyses and tertiary structural predictions were performed to analyze the structural changes and functional impacts of the novel KDF1 variant. The subsequent functional assessment using a TOP-flash/FOP-flash luciferase reporter system demonstrated that KDF1 variants suppressed the activation of canonical Wnt signaling in 293T cells. To comprehensively investigate the KDF1-related oral morphological anomalies, we performed scanning electron microscopy and ground section of the lower right lateral deciduous incisor extracted from #285 proband, and histopathological assessment of the gingiva. The phenotypic analyses revealed a series of tooth morphological anomalies related to the KDF1 variant R307P, including a shovel-shaped lingual surface of incisors and cornicione-shaped marginal ridges with anomalous morphological occlusal grooves of premolars and molars. Notably, keratinized gingival epithelium abnormalities were revealed in the proband and characterized by epithelial dyskeratosis with residual nuclei, indistinct stratum granulosum, epithelial hyperproliferation, and impaired epithelial differentiation. Our findings revealed new developmental anomalies in the tooth and gingival epithelium of a non-syndromic tooth agenesis individual with a novel pathogenic KDF1 variant, broadening the phenotypic spectrum of KDF1-related disorders and providing new evidence for the crucial role of KDF1 in regulating human dental and oral epithelial development.

Novel Candidate Genes for Non-Syndromic Tooth Agenesis Identified Using Targeted Next-Generation Sequencing

Non-syndromic tooth agenesis (ns-TA) is one of the most common dental anomalies characterized by the congenital absence of at least one permanent tooth (excluding third molars). Regarding the essential role of genetic factors in ns-TA aetiology, the present study aimed to identify novel pathogenic variants underlying hypodontia and oligodontia. In a group of 65 ns-TA patients and 127 healthy individuals from the genetically homogenous Polish population, the coding sequences of 423 candidate genes were screened using targeted next-generation sequencing. Pathogenic and likely pathogenic variants were identified in 37 (56.92%) patients, including eight nucleotide alternations of genes not previously implicated in ns-TA (CHD7, CREBBP, EVC, LEF1, ROR2, TBX22 and TP63). However, since only single variants were detected, future research is required to confirm and fully understand their role in the aetiology of ns-TA. Additionally, our results support the importance of already known ns-TA candidate genes (AXIN2, EDA, EDAR, IRF6, LAMA3, LRP6, MSX1, PAX9 and WNT10A) and provide additional evidence that ns-TA might be an oligogenic condition involving the cumulative effect of rare variants in two or more distinct genes.

Atlas of human dental pulp cells at multiple spatial and temporal levels based on single-cell sequencing analysis

The dental pulp plays a crucial role in the long-term maintenance of tooth function. The progress of endodontic treatment and pulp tissue regeneration engineering has made pulp-regeneration therapy promising in clinical practice. However, the mechanisms of pulp regeneration and the role of dental stem cells in development and regeneration have not been fully elucidated. Bridging the gaps between clinical operation and basic research is urgently needed. With the application of single-cell sequencing technology in dental research, the landscapes of human dental pulp cells have begun being outlined. However, the specific cellular heterogeneity of dental pulp cells, especially that of dental stem cells, at different spatial and temporal levels, is still unclear. In this study, we used single-cell RNA sequencing analysis of pulp samples at four different developmental stages and combined the findings with immunohistochemical staining to explore the development of dental pulp and stem cells. The results revealed temporal changes in the proportion of pulp cells during development. For example, mononuclear phagocytes accounted for a higher proportion in early samples. Odontoblasts identified by DMP1 had a higher expression of ion channel-related and neurodevelopment-related genes. Subpopulations were identified in fibroblasts, odontoblasts, and mesenchymal stem cells. We identified a subclass of odontoblasts that expresses DGKI and RRBP1 present in early developmental samples. A population of earlier mesenchymal stem cells expressed the SEPTIN gene, which may have greater proliferative and differentiation potential. Furthermore, dental pulp stem cells can differentiate into two directions: mineralization and myogenesis. In summary, the specific cellular heterogeneity of dental pulp cells was revealed at different spatial and temporal levels. These findings may shed light on the mechanism of tooth development. The gene expression profile of developing pulp cells may help to select cells for regenerative engineering and improve the success of dental pulp regeneration.

Adam10-dependent Notch signaling establishes dental epithelial cell boundaries required for enamel formation

The disintegrin and metalloproteinase Adam10 is a membrane-bound sheddase that regulates Notch signaling and ensures epidermal integrity. To address the function of Adam10 in the continuously growing incisors, we used Keratin14^Cre/+;Adam10^fl/fl transgenic mice, in which Adam10 is conditionally deleted in the dental epithelium. Keratin14^Cre/+;Adam10^fl/fl mice exhibited severe abnormalities, including defective enamel formation reminiscent of human enamel pathologies. Histological analyses of mutant incisors revealed absence of stratum intermedium, and severe disorganization of enamel-secreting ameloblasts. In situ hybridization and immunostaining analyses in the Keratin14^Cre/+;Adam10^fl/fl incisors showed strong Notch1 downregulation in dental epithelium and ectopic distribution of enamel-specific molecules, including ameloblastin and amelogenin. Lineage tracing studies using Notch1^CreERT2;R26^mT/mG mice demonstrated that loss of the stratum intermedium cells was due to their fate switch toward the ameloblast lineage. Overall, our data reveal that in the continuously growing incisors the Adam10/Notch axis controls dental epithelial cell boundaries, cell fate switch and proper enamel formation.

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ADAM10 deletion in the dental epithelium causes the formation of defective enamel

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ADAM10 deletion leads to loss of stratum intermedium and Notch1 expression

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ADAM10 deletion leads to stratum intermedium-to-ameloblast cell fate switch

Impact of claudin-10 deficiency on amelogenesis: Lesson from a HELIX tooth

In epithelia, claudin proteins are important components of the tight junctions as they determine the permeability and specificity to ions of the paracellular pathway. Mutations in CLDN10 cause the rare autosomal recessive HELIX syndrome (Hypohidrosis, Electrolyte imbalance, Lacrimal gland dysfunction, Ichthyosis, and Xerostomia), in which patients display severe enamel wear. Here, we assess whether this enamel wear is caused by an innate fragility directly related to claudin-10 deficiency in addition to xerostomia. A third molar collected from a female HELIX patient was analyzed by a combination of microanatomical and physicochemical approaches (i.e., electron microscopy, elemental mapping, Raman microspectroscopy, and synchrotron-based X-ray fluorescence). The enamel morphology, formation time, organization, and microstructure appeared to be within the natural variability. However, we identified accentuated strontium variations within the HELIX enamel, with alternating enrichments and depletions following the direction of the periodical striae of Retzius. These markings were also present in dentin. These data suggest that the enamel wear associated with HELIX may not be related to a disruption of enamel microstructure but rather to xerostomia. However, the occurrence of events of strontium variations within dental tissues might indicate repeated episodes of worsening of the renal dysfunction that may require further investigations.

Epiprofin Transcriptional Activation Promotes Ameloblast Induction From Mouse Induced Pluripotent Stem Cells via the BMP-Smad Signaling Axis

The transcriptional regulation of induced pluripotent stem cells (iPSCs) holds promise for their directed differentiation into ameloblasts, which are usually lost after tooth eruption. Ameloblast differentiation is regulated by multiple signaling molecules, including bone morphogenetic proteins (BMPs). Epiprofin (Epfn), a transcription factor, is expressed in the dental epithelium, and epithelial Epfn overexpression results in ectopic ameloblast differentiation and enamel formation in mouse incisor, a striking phenotype resembling that of mice with deletion of follistatin (a BMP inhibitor). However, it remains unknown whether and how Epfn transcriptional activation promotes ameloblast induction from mouse iPSCs. Here, we generated doxycycline-inducible Epfn-expressing mouse iPSCs (Epfn-iPSCs). Ameloblasts, which are characterized by positive staining for keratin 14 and amelogenin and alizarin red S staining, were successfully derived from Epfn-iPSCs based on a stage-specific induction protocol, which involved the induction of the surface ectoderm, dental epithelial cells, and ameloblasts at stages 1, 2, and 3, respectively. Epfn activation by doxycycline at stages 2 and/or 3 decreased cell proliferation and promoted ameloblast differentiation, along with the upregulation of p-Smad1/5/8, a key regulator of the BMP-Smad signaling pathway. Gene analysis of the BMP-Smad signaling pathway-associated molecules revealed that Epfn activation decreased follistatin expression at stage 2, but increased BMP2/4/7 expression at stage 3. Perturbations in the ameloblast differentiation process were observed when the BMP-Smad signaling pathway was inhibited by a BMP receptor inhibitor (LDN-193189). Simultaneous LDN-193189 treatment and Epfn activation largely reversed the perturbations in ameloblast induction, with partial recovery of p-Smad1/5/8 expression, suggesting that Epfn activation promotes ameloblast induction from mouse iPSCs partially by upregulating BMP-Smad activity. These results reveal the potential regulatory networks between Epfn and the BMP-Smad pathway and suggest that Epfn is a promising target for inducing the differentiation of ameloblasts, which can be used in enamel and tooth regeneration.