Apoptosis of the reduced enamel epithelium and its implications for bone resorption during tooth eruption

TMEM119 (c.G143A, p.S48L) Mutation Is Involved in Primary Failure of Eruption by Attenuating Glycolysis-Mediated Osteogenesis

Primary failure of eruption (PFE) is a rare oral disease with an incidence rate of 0.06%. It is characterized by abnormal eruption mechanisms that disrupt tooth eruption. The underlying pathogenic genetic variant and mechanism of PFE remain largely unknown. The purpose of this study was to explore the role of a novel transmembrane protein 119 (TMEM119) mutation in two PFE patients in a Chinese family. Information collection was performed on the family with a diagnosis of PFE, and blood samples from patients and healthy family members were extracted. Whole-exome sequencing was performed. Bioinformatics analysis revealed that a heterozygous variant in the TMEM119 gene (c.G143A, p.S48L) was a disease-associated mutation in this family. Recombinant pcDNA3.1 plasmid-containing wild-type and mutant TMEM119 expression cassettes were successfully constructed and transfected into MC3T3-E1 cells, respectively. The results of in vitro analysis suggested that the subcellular distribution of the TMEM119 protein was transferred from the cell cytoplasm to the nucleus, and the ability of cells to proliferate and migrate as well as glycolytic and mineralized capacities were reduced after mutation. Furthermore, rescue assays showed that activating transcription factor 4 (ATF4) overexpression rescued the attenuated glycolysis and mineralization ability of cells. Results of in vivo analysis demonstrated that TMEM119 was mainly expressed in the alveolar bone around the mouse molar germs, and the expression level increased with tooth eruption, demonstrated using immunohistochemistry and immunofluorescence. Collectively, the novel TMEM119 mutation is potentially pathogenic in the PFE family by affecting the glucose metabolism and mineralized function of osteoblasts, including interaction with ATF4. Our findings broaden the gene mutation spectrum of PFE and further elucidate the pathogenic mechanism of PFE.

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.

Runx2 and Nell-1 in dental follicle progenitor cells regulate bone remodeling and tooth eruption

Dental follicles are necessary for tooth eruption, surround the enamel organ and dental papilla, and regulate both the formation and resorption of alveolar bone. Dental follicle progenitor cells (DFPCs), which are stem cells found in dental follicles, differentiate into different kinds of cells that are necessary for tooth formation and eruption. Runt‐related transcription factor 2 (Runx2) is a transcription factor that is essential for osteoblasts and osteoclasts differentiation, as well as bone remodeling. Mutation of Runx2 causing cleidocranial dysplasia negatively affects osteogenesis and the osteoclastic ability of dental follicles, resulting in tooth eruption difficulties. Among a variety of cells and molecules, Nel-like molecule type 1 (Nell-1) plays an important role in neural crest-derived tissues and is strongly expressed in dental follicles. Nell-1 was originally identified in pathologically fused and fusing sutures of patients with unilateral coronal synostosis, and it plays indispensable roles in bone remodeling, including roles in osteoblast differentiation, bone formation and regeneration, craniofacial skeleton development, and the differentiation of many kinds of stem cells. Runx2 was proven to directly target the Nell-1 gene and regulate its expression. These studies suggested that Runx2/Nell-1 axis may play an important role in the process of tooth eruption by affecting DFPCs. Studies on short and long regulatory noncoding RNAs have revealed the complexity of RNA-mediated regulation of gene expression at the posttranscriptional level. This ceRNA network participates in the regulation of Runx2 and Nell-1 gene expression in a complex way. However, non-study indicated the potential connection between Runx2 and Nell-1, and further researches are still needed.

The Nfic-osterix pathway regulates ameloblast differentiation and enamel formation

Enamel makes up the outermost layer of the crown and its hardness protects other dental tissues from various stimuli. Enamel cannot be regenerated once damaged because ameloblasts are lost during the tooth eruption. Since the ameloblast differentiation mechanism is still unknown, further research is essential for developing treatments for defective or damaged enamel. Previously, we have reported that osteoblast differentiation and bone formation were regulated through the runt-related transcription factor 2 (Runx2)-nuclear factor 1-C (Nfic)-osterix (Osx) pathway where Nfic directly controls Osx expression. This pathway regulates odontoblast differentiation and dentin formation as well. The aim of this study was to investigate if the same pathway is applicable for ameloblast differentiation. Structural enamel defects with disorganized ameloblasts and decreased proliferation activity of the cervical loop were observed in Nfic^-/- mice incisors. Expression of the ameloblast differentiation markers was also downregulated significantly in Nfic^-/- mice. Real-time PCR analyses suggested that Runx2, Nfic, and Osx regulate the expression of ameloblast differentiation markers, where Runx2 is upstream of Nfic, and Nfic controls Osx expression. Therefore, we suggest the Runx2-Nfic-Osx pathway as one of the key factors that regulate ameloblast differentiation.

Primary intraosseous squamous cell carcinoma of the mandible: a case with atypical imaging features

Primary intraosseous squamous cell carcinoma is a rare malignant tumour that exclusively arises within the jaws. Its diagnosis requires an appropriate clinical, imaging and histological correlation. The exclusion of primary oral mucosa lesions and metastatic disease is mandatory. We report an atypical imaging appearance of this uncommon entity, characterized by new bone formation and periosteal reaction that resemble sarcomatous or malignant odontogenic tumours. A comprehensive discussion on the embryological principles of primary intraosseous squamous cell carcinoma is also provided.

Production and Functional Characterization of Murine Osteoclasts Differentiated from ER-Hoxb8-Immortalized Myeloid Progenitor Cells

In vitro differentiation into functional osteoclasts is routinely achieved by incubation of embryonic stem cells, induced pluripotent stem cells, or primary as well as cryopreserved spleen and bone marrow-derived cells with soluble receptor activator of nuclear factor kappa-B ligand and macrophage colony-stimulating factor. Additionally, osteoclasts can be derived from co-cultures with osteoblasts or by direct administration of soluble receptor activator of nuclear factor kappa-B ligand to RAW 264.7 macrophage lineage cells. However, despite their benefits for osteoclast-associated research, these different methods have several drawbacks with respect to differentiation yields, time and animal consumption, storage life of progenitor cells or the limited potential for genetic manipulation of osteoclast precursors. In the present study, we therefore established a novel protocol for the differentiation of osteoclasts from murine ER-Hoxb8-immortalized myeloid stem cells. We isolated and immortalized bone marrow cells from wild type and genetically manipulated mouse lines, optimized protocols for osteoclast differentiation and compared these cells to osteoclasts derived from conventional sources. In vitro generated ER-Hoxb8 osteoclasts displayed typical osteoclast characteristics such as multi-nucleation, tartrate-resistant acid phosphatase staining of supernatants and cells, F-actin ring formation and bone resorption activity. Furthermore, the osteoclast differentiation time course was traced on a gene expression level. Increased expression of osteoclast-specific genes and decreased expression of stem cell marker genes during differentiation of osteoclasts from ER-Hoxb8-immortalized myeloid progenitor cells were detected by gene array and confirmed by semi-quantitative and quantitative RT-PCR approaches. In summary, we established a novel method for the quantitative production of murine bona fide osteoclasts from ER-Hoxb8 stem cells generated from wild type or genetically manipulated mouse lines. These cells represent a standardized and theoretically unlimited source for osteoclast-associated research projects.

The protocol for the isolation and cryopreservation of osteoclast precursors from mouse bone marrow and spleen

Osteoclasts are responsible for physiological bone remodeling as well as pathological bone destruction in osteoporosis, periodontitis and rheumatoid arthritis, and thus represent a pharmacological target for drug development. We aimed to characterize and compare the cytokine-induced osteoclastogenesis of bone marrow and spleen precursors. Established protocols used to generate osteoclasts from bone marrow were modified to examine osteoclastogenesis of the spleen cells of healthy mice. Osteoclast formation was successfully induced from spleen precursors using receptor activator of nuclear factor κB ligand (50 ng/ml) and macrophage colony stimulating factor (50 ng/ml). Compared to bone marrow cultures, differentiation from spleen required a longer cultivation time (9 days for spleen, as compared to 5 days for marrow cultures) and a higher plating density of non-adherent cells (75,000/cm(2) for spleen, as compared to 50,000/cm(2) for bone marrow). Osteoclasts generated from spleen precursors expressed osteoclast marker genes calcitonin receptor, cathepsin K and matrix metalloproteinase 9 and were capable of resorbing hydroxyapatite. The differentiation capacity of spleen and bone marrow precursors was comparable for BALB/c, C57BL/6 and FVB mice. We also developed and tested a cryopreservation protocol for the osteoclast precursors. While 70-80 % of cells were lost during the first week of freezing, during the subsequent 5 weeks the losses were within 2-5 % per week. Osteoclastogenesis from the recovered bone marrow precursors was successful up to 5 weeks after freezing. Spleen precursors retained their osteoclastogenic capacity for 1 week after freezing, but not thereafter. The described protocol is useful for the studies of genetically modified animals as well as for screening new osteoclast-targeting therapeutics.