Zbp1-positive cells are osteogenic progenitors in periodontal ligament

Osteoblast differentiation of Gli1⁺ cells via Wnt and BMP signaling pathways during orthodontic tooth movement

OBJECTIVES

Factors that induce bone formation during orthodontic tooth movement (OTM) remain unclear. Gli1 was recently identified as a stem cell marker in the periodontal ligament (PDL). Therefore, we evaluated the mechanism of differentiation of Cre/LoxP-mediated Gli1/Tomato+ cells into osteoblasts during OTM.

METHODS

After the final administration of tamoxifen to 8-week-old Gli1-CreERT2/ROSA26-loxP-stop-loxP-tdTomato mice for 2 days, nickel-titanium closed coil springs were attached between the upper anterior alveolar bone and the first molar. Immunohistochemical localizations of β-catenin, Smad4, and Runx2 were observed in the PDL on 2, 5, and 10 days after OTM initiation.

RESULTS

In the untreated tooth, few Gli1/Tomato+ cells were detected in the PDL. Two days after OTM initiation, the number of Gli1/Tomato+ cells increased in the PDL on the tension side. On this side, 49.3 ± 7.0% of β-catenin+ and 48.7 ± 5.7% of Smad4+ cells were found in the PDL, and Runx2 expression was detected in some Gli1/Tomato+ cells apart from the alveolar bone. The number of positive cells in the PDL reached a maximum on day 5. In contrast, on the compression side, β-catenin and Smad4 exhibited less immunoreactivity. On day 10, Gli1/Tomato+ cells were aligned on the alveolar bone on the tension side, with some expressing Runx2.

CONCLUSIONS

Gli1+ cells in the PDL differentiated into osteoblasts during OTM. Wnt and bone morphogenetic proteins signaling pathways may be involved in this differentiation.

Bone formation ability of Gli1+ cells in the periodontal ligament after tooth extraction

During the process of socket healing after tooth extraction, osteoblasts appear in the tooth socket and form alveolar bone; however, the source of these osteoblasts is still uncertain. Recently, it has been demonstrated that cells expressing Gli1, a downstream factor of sonic hedgehog signaling, exhibit stem cell properties in the periodontal ligament (PDL). Therefore, in the present study, the differentiation ability of Gli1⁺-PDL cells after tooth extraction was analyzed using Gli1-Cre^ERT2/ROSA26-loxP-stop-loxP-tdTomato (iGli1/Tomato) mice. After the final administration of tamoxifen to iGli1/Tomato mice, Gli1/Tomato⁺ cells were rarely detected in the PDL. One day after the tooth extraction, although inflammatory cells appeared in the tooth socket, Periostin⁺ PDL-like tissues having a few Gli1/Tomato⁺ cells remained near the alveolar bone. Three days after the extraction, the number of Gli1/Tomato⁺ cells increased as evidenced by numerous PCNA⁺ cells in the socket. Some of these Gli1/Tomato⁺ cells expressed BMP4 and Phosphorylated (P)-Smad1/5/8. After seven days, the Osteopontin⁺ bone matrix was formed in the tooth socket apart from the alveolar bone. Many Gli1/Tomato⁺ osteoblasts that were positive for Runx2⁺ were arranged on the surface of the newly formed bone matrix. In the absence of Gli1⁺-PDL cells in Gli1-Cre^ERT2/Rosa26-loxP-stop-loxP-tdDTA (iGli1/DTA) mice, the amount of newly formed bone matrix was significantly reduced in the tooth socket. Therefore, these results collectively suggest that Gli1⁺-PDL cells differentiate into osteoblasts to form the bone matrix in the tooth socket; thus, this differentiation might be regulated, at least in part, by bone morphogenetic protein (BMP) signaling.

Differentiation ability of Gli1+ cells during orthodontic tooth movement

Orthodontic tooth movement (OTM) induces bone formation on the alveolar bone of the tension side; however, the mechanism of osteoblast differentiation is not fully understood. Gli1 is an essential transcription factor for hedgehog signaling and functions in undifferentiated cells during embryogenesis. In this study, we examined the differentiation of Gli1⁺ cells in the periodontal ligament (PDL) during OTM using a lineage-tracing analysis. After the final administration of tamoxifen for 2 days to 8-week-old Gli1-Cre^ERT2/ROSA26-loxP-stop-loxP-tdTomato (iGli1/Tomato) mice, Gli1/Tomato⁺ cells were rarely observed near endomucin⁺ blood vessels in the PDL. Osteoblasts lining the alveolar bone did not exhibit Gli1/Tomato fluorescence. To move the first molar of iGli1/Tomato mice medially, nickel-titanium closed-coil springs were attached between the upper anterior alveolar bone and the first molar. Two days after OTM initiation, the number of Gli1/Tomato⁺ cells increased along with numerous PCNA⁺ cells in the PDL of the tension side. As some Gli1/Tomato⁺ cells exhibited positive expression of osterix, an osteoblast differentiation marker, Gli1⁺ cells probably differentiated into osteoblast progenitor cells. On day 10, the newly formed bone labeled by calcein administration during OTM was detected on the surface of the original alveolar bone of the tension side. Gli1/Tomato⁺ cells expressing osterix localized to the surface of the newly formed bone. In contrast, in the PDL of the compression side, Gli1/Tomato⁺ cells proliferated before day 10 and expressed type I collagen, suggesting that the Gli1⁺ cells also differentiated into fibroblasts. Collectively, these results demonstrate that Gli1⁺ cells in the PDL can differentiate into osteoblasts at the tension side and may function in bone remodeling as well as fibril formation in the PDL during OTM.

Structural analysis of melanosomes in living mammalian cells using scanning electron-assisted dielectric microscopy with deep neural network

Melanins are the main pigments found in mammals. Their synthesis and transfer to keratinocytes have been widely investigated for many years. However, analysis has been mainly carried out using fixed rather than live cells. In this study, we have analysed the melanosomes in living mammalian cells using newly developed scanning electron-assisted dielectric microscopy (SE-ADM). The melanosomes in human melanoma MNT-1 cells were observed as clear black particles in SE-ADM. The main structure of melanosomes was toroidal while that of normal melanocytes was ellipsoidal. In tyrosinase knockout MNT-1 cells, not only the black particles in the SE-ADM images but also the Raman shift of melanin peaks completely disappeared suggesting that the black particles were really melanosomes. We developed a deep neural network (DNN) system to automatically detect melanosomes in cells and analysed their diameter and roundness. In terms of melanosome morphology, the diameter of melanosomes in melanoma cells did not change while that in normal melanocytes increased during culture. The established DNN analysis system with SE-ADM can be used for other particles, e.g. exosomes, lysosomes, and other biological particles.

Periodontal tissue stem cells and mesenchymal stem cells in the periodontal ligament

Periodontal tissue stem cells, which play a crucial role in maintaining the homeostasis of periodontal tissues, are found in the periodontal ligament (PDL). These cells have long been referred to as mesenchymal stem/stromal cells (MSCs), and their clinical applications have been extensively studied. However, tissue stem cells in the PDL have not been thoroughly investigated, and they may be different from MSCs. Recent advances in stem cell biology, such as genetic lineage tracing, identification of label-retaining cells, and single-cell transcriptome analysis, have made it possible to analyze tissue stem cells in the PDL in vivo. In this review, we summarize recent findings on these stem cell populations in PDL and discuss future research directions toward developing periodontal regenerative therapy.

Plap-1 lineage tracing and single-cell transcriptomics reveal cellular dynamics in the periodontal ligament

Periodontal tissue supports teeth in the alveolar bone socket via fibrous attachment of the periodontal ligament (PDL). The PDL contains periodontal fibroblasts and stem/progenitor cells, collectively known as PDL cells (PDLCs), on top of osteoblasts and cementoblasts on the surface of alveolar bone and cementum, respectively. However, the characteristics and lineage hierarchy of each cell type remain poorly defined. This study identified periodontal ligament associated protein-1 (Plap-1) as a PDL-specific extracellular matrix protein. We generated knock-in mice expressing CreERT2 and GFP specifically in Plap-1-positive PDLCs. Genetic lineage tracing confirmed the long-standing hypothesis that PDLCs differentiate into osteoblasts and cementoblasts. A PDL single-cell atlas defined cementoblasts and osteoblasts as Plap-1−Ibsp+Sparcl1+ and Plap-1−Ibsp+Col11a2+, respectively. Other populations, such as Nes+ mural cells, S100B+ Schwann cells, and other non-stromal cells, were also identified. RNA velocity analysis suggested that a Plap-1highLy6a+ cell population was the source of PDLCs. Lineage tracing of Plap-1+ PDLCs during periodontal injury showed periodontal tissue regeneration by PDLCs. Our study defines diverse cell populations in PDL and clarifies the role of PDLCs in periodontal tissue homeostasis and repair.

Gli1+-PDL Cells Contribute to Alveolar Bone Homeostasis and Regeneration

The periodontal ligament (PDL) contains mesenchymal stem cells (MSCs) that can differentiate into osteoblasts, cementoblasts, and fibroblasts. Nevertheless, the distribution and characteristics of these cells remain uncertain. Gli1, an essential hedgehog signaling transcription factor, functions in undifferentiated cells during embryogenesis. Therefore, in the present study, the differentiation ability of Gli1⁺ cells was examined using Gli1-CreERT2/ROSA26-loxP-stop-loxP-tdTomato (iGli1/Tomato) mice. In 4-wk-old iGli1/Tomato mice, Gli1/Tomato⁺ cells were only slightly detected in the PDL, around endomucin-expressing blood vessels. These cells had proliferated over time, localizing in the PDL as well as on the bone and cementum surfaces at day 28. However, in 8-wk-old iGli1/Tomato mice, Gli1/Tomato⁺ cells were quiescent, as most cells were not immunoreactive for Ki-67. These cells in 8-wk-old mice exhibited high colony-forming unit fibroblast activity and were capable of osteogenic, chondrogenic, and adipogenic differentiation in vitro. In addition, after transplantation of teeth of iGli1/Tomato mice into the hypodermis of wild-type mice, Tomato fluorescence indicating the progeny of Gli1⁺ cells was detected in the osteoblasts and osteocytes of the regenerated bone. These results demonstrate that Gli1⁺ cells in the PDL were MSCs and could contribute to the alveolar bone regeneration.

Although Anatomically Micrometers Apart: Human Periodontal Ligament Cells Are Slightly More Active in Bone Remodeling Than Alveolar Bone Derived Cells

The periodontal ligament (PDL) and the alveolar bone are part of the periodontium, a complex structure that supports the teeth. The alveolar bone is continuously remodeled and is greatly affected by several complex oral events, like tooth extraction, orthodontic movement, and periodontitis. Until now, the role of PDL cells in terms of osteogenesis and osteoclastogenesis has been widely studied, whereas surprisingly little is known about the bone remodeling capacity of alveolar bone. Therefore, the purpose of this study was to compare the biological character of human alveolar bone cells and PDL cells in terms of osteogenesis and osteoclastogenesis in vitro. Paired samples of PDL cells and alveolar bone cells from seven patients with compromised general and oral health were collected and cultured. Bone A (early outgrowth) and bone B (late outgrowth) were included. PDL, bone A, bone B cell cultures all had a fibroblast appearance with similar expression pattern of six mesenchymal markers. These cultures were subjected to osteogenesis and osteoclastogenesis assays. For osteoclastogenesis assays, the cells were co-cultured with peripheral blood mononuclear cells, a source for osteoclast precursor cells. The total duration of the experiments was 21 days. Osteogenesis was slightly favored for PDL compared to bone A and B as shown by stronger Alizarin red staining and higher expression of RUNX2 and Collagen I at day 7 and for ALP at day 21. PDL induced approximately two times more osteoclasts than alveolar bone cells. In line with these findings was the higher expression of cell fusion marker DC-STAMP in PDL-PBMC co-cultures compared to bone B at day 21. In conclusion, alveolar bone contains remodeling activity, but to a different extent compared to PDL cells. We showed that human alveolar bone cells can be used as an in vitro model to study bone remodeling.