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.

Reciprocal role of PLAP-1 in HIF-1α-mediated responses to hypoxia.

OBJECTIVE: To investigate the mutual regulation of hypoxia-inducible factor (HIF)-1α activity and periodontal ligament-associated protein-1 (PLAP-1) expression in human periodontal ligament cells (HPDLs). BACKGROUND: Cellular responses to hypoxia regulate various biological events (e.g., inflammation and tissue regeneration) through activation of HIF-1α. PLAP-1, an extracellular matrix protein preferentially expressed in the periodontal ligament, plays important roles in the functions of HPDLs. Although PLAP-1 expression has been demonstrated in hypoxic regions, the involvement of PLAP-1 in responses to hypoxia has not been revealed. METHODS: HPDLs were cultured under normoxic (20% O2 ) or hypoxic (1% O2 ) conditions with or without deferoxamine mesylate (chemical hypoxia inducer) or chetomin (HIF signaling inhibitor). Expression levels of PLAP-1 and HIF-1α were examined by real-time reverse transcription-polymerase chain reaction and western blot analysis. Luciferase reporter assays of HIF-1α activity were performed using 293T cells stably transfected with a hypoxia response element (HRE)-containing luciferase vector in the presence or absence of recombinant PLAP-1 or PLAP-1 gene transfection. RESULTS: Cultivation under hypoxic conditions elevated the gene and protein expression levels of PLAP-1 in HPDLs. Deferoxamine mesylate treatment also enhanced PLAP-1 expression in HPDLs. Hypoxia-induced PLAP-1 expression was significantly suppressed in the presence of chetomin. PLAP-1-suppressed HPDLs showed increased HIF-1α accumulation in the nucleus during culture under hypoxic conditions, but not in the presence of recombinant PLAP-1. In the presence of recombinant PLAP-1, hypoxia-induced HRE activity of 293T cells was significantly suppressed in a dose-dependent manner. Transfection of the PLAP-1 gene resulted in a significant reduction of HRE activity during culture under hypoxic conditions. CONCLUSION: PLAP-1 expression is upregulated under hypoxic conditions through HIF-1α activation. Moreover, hypoxia-induced PLAP-1 expression regulates HIF-1α signaling.

Adjunctive use of fluoride rinsing and brush-on gel increased incipient caries-like lesion remineralization compared with fluoride toothpaste alone in situ.

Objective: The objective of this study was to compare the remineralizing effect of sodium fluoride (NaF) mouth rinse or NaF gel as an adjunct to NaF dentifrice on incipient caries-like lesions in an in situ cross-over design study, with three sessions of 30 days each. Materials and methods: Orthodontic brackets with artificial demineralized enamel slabs were attached to the upper first molars of 12 participants. A set of 3 test specimens from the same tooth was randomly assigned to each participant and allocated into three 30-day sessions: 1) brushing with 0.22% NaF dentifrice 2 times/day (F dentifrice), 2) brushing with 0.22% NaF dentifrice 2 times/day+ rinsing with 0.05% NaF before bedtime (F mouth rinse), 3) brushing with 0.22% NaF dentifrice 2 times/day + brushing with 1.1% NaF gel before bedtime (F brush-on gel). The mineral gain and lesion depth of the specimens were evaluated by micro-computed tomography. Results: The mean mineral gain from the NaF mouth rinse and the NaF brush-on gel was similar, but greater than that from the NaF dentifrice (p < .05). The NaF brush-on gel yielded the greatest mean depth of remineralization (168 µm), followed by the NaF mouth rinse (144 µm). Both depths were significantly greater than that of the NaF dentifrice (84 µm) (p < .05). Conclusions: Both 0.05% NaF mouth rinse and 1.1% NaF brush-on gel, used at bedtime, increased incipient caries-like lesion remineralization in situ in combination with brushing with NaF dentifrice twice a day.

Zbp1-positive cells are osteogenic progenitors in periodontal ligament.

Periodontal ligament (PDL) possesses a stem/progenitor population to maintain the homeostasis of periodontal tissue. However, transcription factors that regulate this population have not yet been identified. Thus, we aimed to identify a molecule related to the osteogenic differentiation of PDL progenitors using a single cell-based strategy in this study. We first devised a new protocol to isolate PDL cells from the surface of adult murine molars and established 35 new single cell-derived clones from the PDL explant. Among these clones, six clones with high (high clones, n = 3) and low (low clones, n = 3) osteogenic potential were selected. Despite a clear difference in the osteogenic potential of these clones, no significant differences in their cell morphology, progenitor cell marker expression, alkaline phosphatase activity, proliferation rate, and differentiation-related gene and protein expression were observed. RNA-seq analysis of these clones revealed that Z-DNA binding protein-1 (Zbp1) was significantly expressed in the high osteogenic clones, indicating that Zbp1 could be a possible marker and regulator of the osteogenic differentiation of PDL progenitor cells. Zbp1-positive cells were distributed sparsely throughout the PDL. In vitro Zbp1 expression in the PDL clones remained at a high level during osteogenic differentiation. The CRISPR/Cas9 mediated Zbp1 knockout in the high clones resulted in a delay in cell differentiation. On the other hand, Zbp1 overexpression in the low clones promoted cell differentiation. These findings suggested that Zbp1 marked the PDL progenitors with high osteogenic potential and promoted their osteogenic differentiation. Clarifying the mechanism of differentiation of PDL cells by Zbp1 and other factors in future studies will facilitate a better understanding of periodontal tissue homeostasis and repair, possibly leading to the development of novel therapeutic measures.