Single cell atlas of developing mouse dental germs reveals populations of CD24+ and Plac8+ odontogenic cells

Unraveling the hidden complexity: Exploring dental tissues through single-cell transcriptional profiling

Understanding the composition and function of cells constituting tissues and organs is vital for unraveling biological processes. Single-cell analysis has allowed us to move beyond traditional methods of categorizing cell types. This innovative technology allows the transcriptional and epigenetic profiling of numerous individual cells, leading to significant insights into the development, homeostasis, and pathology of various organs and tissues in both animal models and human samples. In this review, we delve into the outcomes of major investigations using single-cell transcriptomics to decipher the cellular composition of mammalian teeth and periodontal tissues. The recent single-cell transcriptome-based studies have traced in detail the dental epithelium-ameloblast lineage and dental mesenchyme lineages in the mouse incisors and the tooth germ of both mice and humans; unraveled the microenvironment, the identity of niche cells, and cellular intricacies in the dental pulp; shed light on the molecular mechanisms orchestrating root formation; and characterized cellular dynamics of the periodontal ligament. Additionally, cellular components in dental pulps were compared between healthy and carious teeth at a single-cell level. Each section of this review contributes to a comprehensive understanding of tooth biology, offering valuable insights into developmental processes, niche cell identification, and the molecular secrets of the dental environment.

Single-cell RNA-seq of in vitro expanded cells from cranial neural crest reveals a rare odontogenic sub-population

Ecto-mesenchymal cells of mammalian tooth germ develops from cranial neural crest cells. These cells are recognised as a promising source for tooth development and regeneration. Despite the high heterogeneity of the neural crest, the cellular landscape of in vitro cultured cranial neural crest cells (CNCCs) for odontogenesis remains unclear. In this study, we used large-scale single-cell RNA sequencing to analyse the cellular landscape of in vitro cultured mouse CNCCs for odontogenesis. We revealed distinct cell trajectories from primary cells to passage 5 and identified a rare Alx3+/Barx1+ sub-population in primary CNCCs that differentiated into two odontogenic clusters characterised by the up-regulation of Pax9/Bmp3 and Lhx6/Dmp1. We successfully induced whole tooth-like structures containing enamel, dentin, and pulp under the mouse renal capsule using in vitro cultured cells from both cranial and trunk neural crests with induction rates of 26.7% and 22.1%, respectively. Importantly, we confirmed only cells sorted from odontogenic path can induce tooth-like structures. Cell cycle and DNA replication genes were concomitantly upregulated in the cultured NCCs of the tooth induction groups. Our data provide valuable insights into the cell heterogeneity of in vitro cultured CNCCs and their potential as a source for tooth regeneration.

Mx1 -labeled pulp progenitor cells are main contributors to postnatal odontoblasts and pulp cells in murine molars

Regeneration of dentin and odontoblasts from dental pulp stem cells (DPSCs) is essential for permanent tooth maintenance. However, the in vivo identity and role of endogenous DPSCs in reparative dentinogenesis are elusive. Here, using pulp single-cell analysis before and after molar eruption, we revealed that endogenous DPSCs are enriched in Cxcl12- GFP + coronal papilla-like cells with Mx1- Cre labeling. These Mx1 + Cxcl12- GFP + cells are long-term repopulating cells that contribute to the majority of pulp cells and new odontoblasts after eruption. Upon molar injury, Mx1 + DPSCs localize into the injury site and differentiate into new odontoblasts, forming scleraxis -GFP + and osteocalcin -GFP + dentinal tubules and reparative dentin. Single-cell and FACS analysis showed that Mx1 + Cxcl12- GFP + DPSCs are the most primitive cells with stem cell marker expression and odontoblast differentiation. Taken together, our findings demonstrate that Mx1 labels postnatal DSPCs, which are the main source of pulp cells and new odontoblasts with reparative dentinogenesis in vivo .

Ptip safeguards the epigenetic control of skeletal stem cell quiescence and potency in skeletogenesis

Stem cells remain in a quiescent state for long-term maintenance and preservation of potency; this process requires fine-tuning regulatory mechanisms. In this study, we identified the epigenetic landscape along the developmental trajectory of skeletal stem cells (SSCs) in skeletogenesis governed by a key regulator, Ptip (also known as Paxip1, Pax interaction with transcription-activation domain protein-1). Our results showed that Ptip is required for maintaining the quiescence and potency of SSCs, and loss of Ptip in type II collagen (Col2)+ progenitors causes abnormal activation and differentiation of SSCs, impaired growth plate morphogenesis, and long bone dysplasia. We also found that Ptip suppressed the glycolysis of SSCs through downregulation of phosphoglycerate kinase 1 (Pgk1) by repressing histone H3K27ac at the promoter region. Notably, inhibition of glycolysis improved the function of SSCs despite Ptip deficiency. To the best of our knowledge, this is the first study to establish an epigenetic framework based on Ptip, which safeguards skeletal stem cell quiescence and potency through metabolic control. This framework is expected to improve SSC-based treatments of bone developmental disorders.

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.

Effect of Bioactive Glasses and Basic Fibroblast Growth Factor on Dental Pulp Cells

Ideal regeneration of hard tissue and dental pulp has been reported with the use of a combination of bioactive glass and basic fibroblast growth factor (bFGF). However, no previous study has investigated the molecular mechanisms underlying the processes induced by this combination in dental pulp cells. This study aimed to examine the cellular phenotype and transcriptional changes induced by the combination of bioactive glass solution (BG) and bFGF in dental pulp cells using phase-contrast microscopy, a cell counting kit-8 assay, alkaline phosphatase staining, and RNA sequence analysis. bFGF induced elongation of the cell process and increased the number of cells. Whereas BG did not increase ALP activity, it induced extracellular matrix-related genes in the dental pulp. In addition, the combination of BG and bFGF induces gliogenesis-related genes in the nervous system. This is to say, bFGF increased the viability of dental pulp cells, bioactive glass induced odontogenesis, and a dual stimulation with bioactive glass and bFGF induced the wound healing of the nerve system in the dental pulp. Taken together, bioactive glass and bFGF may be useful for the regeneration of the dentin-pulp complex.

Single-cell dynamics of liver development in postnatal pigs

The postnatal development of the liver, an essential organ for metabolism and immunity, remains poorly characterized at the single-cell resolution. Here, we generated single-nucleus and single-cell transcriptomes of 84,824 pig liver cells at four postnatal time points: day 30, 42, 150, and 730. We uncovered 23 cell types, including three rare cell types: plasmacytoid dendritic cells, CAVIN3+IGF2+ endothelial cells, and EBF1+ fibroblasts. The latter two were verified by multiplex immunohistochemistry. Trajectory and gene regulatory analyses revealed 33 genes that encode transcription factors associated with hepatocyte development and function, including NFIL3 involved in regulating hepatic metabolism. We characterized the spatiotemporal heterogeneity of liver endothelial cells, identified and validated leucine zipper protein 2 (LUZP2) as a novel adult liver sinusoidal endothelial cell-specific transcription factor. Lymphoid cells (NK and T cells) governed the immune system of the pig liver since day 30. Furthermore, we identified a cluster of tissue-resident NK cells, which displayed virus defense functions, maintained proliferative features at day 730, and manifested a higher conservative transcription factor expression pattern in humans than in mouse liver. Our study presents the most comprehensive postnatal liver development single-cell atlas and demonstrates the metabolic and immune changes across the four age stages.

Mesenchymal condensation in tooth development and regeneration: a focus on translational aspects of organogenesis

The teeth are vertebrate-specific, highly specialized organs performing fundamental functions of mastication and speech, the maintenance of which is crucial for orofacial homeostasis and is further linked to systemic health and human psychosocial well-being. However, with limited ability for self-repair, the teeth can often be impaired by traumatic, inflammatory, and progressive insults, leading to high prevalence of tooth loss and defects worldwide. Regenerative medicine holds the promise to achieve physiological restoration of lost or damaged organs, and in particular an evolving framework of developmental engineering has pioneered functional tooth regeneration by harnessing the odontogenic program. As a key event of tooth morphogenesis, mesenchymal condensation dictates dental tissue formation and patterning through cellular self-organization and signaling interaction with the epithelium, which provides a representative to decipher organogenetic mechanisms and can be leveraged for regenerative purposes. In this review, we summarize how mesenchymal condensation spatiotemporally assembles from dental stem cells (DSCs) and sequentially mediates tooth development. We highlight condensation-mimetic engineering efforts and mechanisms based on ex vivo aggregation of DSCs, which have achieved functionally robust and physiologically relevant tooth regeneration after implantation in animals and in humans. The discussion of this aspect will add to the knowledge of development-inspired tissue engineering strategies and will offer benefits to propel clinical organ 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.

Establishment of inclusive single-cell transcriptome atlases from mouse and human tooth as powerful resource for dental research

Single-cell (sc) omics has become a powerful tool to unravel a tissue’s cell landscape across health and disease. In recent years, sc transcriptomic interrogation has been applied to a variety of tooth tissues of both human and mouse, which has considerably advanced our fundamental understanding of tooth biology. Now, an overarching and integrated bird’s-view of the human and mouse tooth sc transcriptomic landscape would be a powerful multi-faceted tool for dental research, enabling further decipherment of tooth biology and development through constantly progressing state-of-the-art bioinformatic methods as well as the exploration of novel hypothesis-driven research. To this aim, we re-assessed and integrated recently published scRNA-sequencing datasets of different dental tissue types (healthy and diseased) from human and mouse to establish inclusive tooth sc atlases, and applied the consolidated data map to explore its power. For mouse tooth, we identified novel candidate transcriptional regulators of the ameloblast lineage. Regarding human tooth, we provide support for a developmental connection, not advanced before, between specific epithelial compartments. Taken together, we established inclusive mouse and human tooth sc atlases as powerful tools to potentiate innovative research into tooth biology, development and disease. The maps are provided online in an accessible format for interactive exploration.