In vivo identification of periodontal progenitor cells

Biomaterials and therapeutic strategies designed for tooth extraction socket healing

Tooth extraction is the most commonly performed oral surgical procedure, with a wide range of clinical indications. The oral cavity is a complex microenvironment, influenced by oral movements, salivary flow, and bacterial biofilms. These factors can contribute to delayed socket healing and the onset of post-extraction complications, which can burden patients' esthetic and functional rehabilitation. Achieving effective extraction socket healing requires a multidisciplinary approach. Recent advancements in materials science and bioengineering have paved the way for developing novel strategies. This review outlines the fundamental healing processes and cellular-molecular interactions involved in the healing of extraction sockets. It then delves into the current landscape of biomaterials for socket healing, highlighting emerging strategies and potential targets that could transform the treatment paradigm. Building upon this foundation, this review also presents future directions and identifies challenges associated with the clinical application of biomaterials for extraction socket healing.

Investigation of Impact of Oxidative Stress on Human Periodontal Ligament Cells Exposed to Static Compression

Oxidative stress (OS) is a common feature of many inflammatory diseases, oral pathologies, and aging processes. The impact of OS on periodontal ligament cells (PDLCs) in relation to oral pathologies, including periodontal diseases, has been investigated in different studies. However, its impact on orthodontic tooth movement (OTM) remains poorly understood. This study used an in vitro model with human PDLCs previously exposed to H2O2 to investigate the effects of OS under a static compressive force which simulated the conditions of OTM. Human PDLCs were treated with varying concentrations of H2O2 to identify sub-lethal doses that affected viability minimally. To mimic compromised conditions resembling OTM under OS, the cells were pretreated with the selected H2O2 concentrations for 24 h. Using an in vitro loading model, a static compressive force (2 g/cm2) was applied for an additional 24 h. The cell viability, proliferation, and cytotoxicity were evaluated using live/dead and resazurin assays. Apoptosis induction was assessed based on caspase-3/7 activity. The gene expression related to bone remodeling (RUNX2 , TNFRSF11B /OPG , BGLAP ), inflammation (IL6 , CXCL8 /IL8 , PTGS2 /COX2 ), apoptosis (CASP3 , CASP8 ), and autophagy (MAP1LC3A /LC3 , BECN1 ) was analyzed using RT-qPCR. This study suggests an altering effect of previous OS exposure on static-compression-related mechanosensing. Further research is needed to fully elucidate these mechanisms.

In vivo dynamics of hard tissue-forming cell origins: Insights from Cre/loxP-based cell lineage tracing studies

Bone tissue provides structural support for our bodies, with the inner bone marrow (BM) acting as a hematopoietic organ. Within the BM tissue, two types of stem cells play crucial roles: mesenchymal stem cells (MSCs) (or skeletal stem cells) and hematopoietic stem cells (HSCs). These stem cells are intricately connected, where BM-MSCs give rise to bone-forming osteoblasts and serve as essential components in the BM microenvironment for sustaining HSCs. Despite the mid-20th century proposal of BM-MSCs, their in vivo identification remained elusive owing to a lack of tools for analyzing stemness, specifically self-renewal and multipotency. To address this challenge, Cre/loxP-based cell lineage tracing analyses are being employed. This technology facilitated the in vivo labeling of specific cells, enabling the tracking of their lineage, determining their stemness, and providing a deeper understanding of the in vivo dynamics governing stem cell populations responsible for maintaining hard tissues. This review delves into cell lineage tracing studies conducted using commonly employed genetically modified mice expressing Cre under the influence of LepR, Gli1, and Axin2 genes. These studies focus on research fields spanning long bones and oral/maxillofacial hard tissues, offering insights into the in vivo dynamics of stem cell populations crucial for hard tissue homeostasis.

Wnt/β-catenin Promotes Cementum Apposition in Periodontal Regeneration

Regeneration of periodontal tissue, particularly the cementum-periodontal ligament (PDL)-bone complex, has long been challenging because the differentiation kinetics of cells and the molecular pathways contributing to the regeneration process are largely unknown. We aimed to evaluate the cell behavior and molecular pathways that contribute to periodontal tissue regeneration in vivo. We analyzed the process of periodontal tissue regeneration through subrenal capsule transplantation of immediately extracted molars in mice. We showed that the regenerated periodontal tissue in the subrenal capsule was morphologically comparable to the intact periodontal tissue, with increased cellular cementum thickness in the apical region. Cell tracing analysis revealed that the cells comprising the regenerated periodontal tissue were derived from transplanted teeth and were indispensable for periodontal tissue regeneration, whereas recipient mouse-derived cells partly contributed to angiogenesis. Bioinformatics analysis based on the gene expression profile in the transplanted teeth indicated that Wnt/β-catenin signaling is involved in periodontal tissue regeneration, which was further confirmed through β-catenin immunohistochemistry. Moreover, the constitutive activation of β-catenin in the cells of transplanted teeth was found to promote accelerated cellular cementum apposition, while the conditional knockout of β-catenin in the cells of transplanted teeth suppressed cellular cementum apposition. Notably, the manipulation of Wnt/β-catenin signaling did not interfere with the bone-PDL-cementum complex, while endogenous osteoclast activity was affected in bone. Our results demonstrated the essential roles of endogenous PDL cells in periodontal tissue regeneration and that Wnt/β-catenin signaling is involved in this process, particularly cellular cementum apposition. Hence, controlling this pathway could promote cementum regeneration, which is a critical process for the regeneration of the cementum-PDL-bone complex. This study provides novel insights into cell behavior and signaling pathways that will advance practical periodontal tissue regeneration.

Localization of α-smooth muscle actin in osteoblast differentiation during periodontal development

α-Smooth muscle actin (α-SMA) is an actin isoform commonly found within vascular smooth muscle cells. Moreover, α-SMA-positive cells are localized in the dental follicle (DF). DF is derived from alveolar bone (AB), cementum, and periodontal ligament (PDL). Therefore, α-SMA-positive cells in the periodontal tissue are speculated to be a marker for mesenchymal stem cells during tooth development. In particular, the mechanism of osteoblast differentiation is not clear. This study demonstrated the fate of α-SMA-positive cells around the tooth germ immunohistochemically. First, α-SMA- and Runx2-positive localization at embryonic days (E) 13, E14, postnatal days (P) 9, and P15 was demonstrated. α-SMA- and Runx2-positive cells were detected in the upper part of the DF at P1. At P9 and P15, α-SMA-positive cells in the PDL were detected in the upper and lower parts. The positive reaction of Runx2 was also localized in the PDL. Then, the distribution of α-SMA-positive cell progeny at P9 and P15 were clarified using α-SMA-CreERT2/ROSA26-loxP-stop-loxP-tdTomato (α-SMA/tomato) mice. It has known that Runx2-positive cells differentiate into osteoblasts. In this study, some Runx2 and α-SMA-positive cells were localized in the DF and PDL. The lineage-tracing analysis demonstrated that the α-SMA/tomato-positive cells expressing Runx2 or Osterix were detected on the AB surface at P15. α-SMA/tomato-positive cells expressing type I collagen were found in the AB matrix. These results indicate that the progeny of the α-SMA-positive cells in the DF could differentiate into osteogenic cells. In conclusion, α-SMA could be a potential marker of progenitor cells that differentiate into osteoblasts.

Engineered 3D Periodontal Ligament Model with Magnetic Tensile Loading

In vitro models are invaluable tools for deconstructing the biological complexity of the periodontal ligament (PDL). Model systems that closely reproduce the 3-dimensional (3D) configuration of cell-cell and cell-matrix interactions in native tissue can deliver physiologically relevant insights. However, 3D models of the PDL that incorporate mechanical loading are currently lacking. Hence, we developed a model where periodontal tissue constructs (PTCs) are made by casting PDL cells in a collagen gel suspended between a pair of slender, silicone posts for magnetic tensile loading. Specifically, one of the posts was rigid and the other was flexible with a magnet embedded in its tip so that PTCs could be subjected to tensile loading with an external magnet. Additionally, the deflection of the flexible post could be used to measure the contractile force of PDL cells in the PTCs. Prior to tensile loading, second harmonics generation analysis of collagen fibers in PTCs revealed that incorporation of PDL cells resulted in collagen remodeling. Biomechanical testing of PTCs by tensile loading revealed an elastic response at 4 h, permanent deformation by 1 d, and creep elongation by 1 wk. Subsequently, contractile forces of PDL cells were substantially lower for PTCs under tensile loading. Immunofluorescence analysis revealed that tensile loading caused PDL cells to increase in number, express higher levels of F-actin and α-smooth muscle actin, and become aligned to the tensile axis. Second harmonics generation analysis indicated that collagen fibers in PTCs progressively remodeled over time with tensile loading. Gene expression analysis also confirmed tension-mediated upregulation of the F-actin/Rho pathway and osteogenic genes. Our model is novel in demonstrating the mechanobiological behavior that results in cell-mediated remodeling of the PDL tissue in a 3D context. Hence, it can be a valuable tool to develop therapeutics for periodontitis, periodontal regeneration, and orthodontics.

The role of fibroblast growth factor-2 in modulating the differentiation of periodontal ligament and alveolar bone-derived stem cells

OBJECTIVE

This study examined how range concentrations of Fibroblast Growth Factor-2 (FGF-2) influence the differentiation and activity of human-derived periodontal ligament (hPDLSCs) and alveolar bone-derived stem cells (haBMSCs).

DESIGN

hPDLSCs and haBMSCs were cultured with varying concentrations of FGF-2 (0, 1, 2.5, 5, 10, 20 ng/mL) and monitored for osteogenic differentiation through alkaline phosphatase (ALP) activity and quantification of gene expression (qRT-PCR) for osteogenesis markers. Additionally, alizarin red staining and a hydroxyproline colorimetric assay evaluated and quantified osteogenic matrix mineralization and collagen deposition. Statistical analyses were performed using one-way ANOVA or two-way ANOVA for multiple comparisons between groups.

RESULTS

At low FGF-2 concentrations, hPDLSCs differentiated toward an osteogenic lineage, whereas higher concentrations of FGF-2 inhibited osteogenesis and promoted fibroblastic differentiation. The effect of FGF-2 at the lowest concentration tested (1 ng/mL) led to significantly higher ALP activity than osteogenically induced positive controls at early time points and equivalent RUNX2 expression at early and later time points. FGF-2 supplementation of haBMSC cultures was sufficient, at all concentrations, to increase ALP activity at an earlier time point. Mineralization of haBMSC cultures increased significantly within 5-20 ng/mL FGF-2 concentrations under basal growth media conditions (α-minimal essential medium supplemented with 15 % fetal bovine serum and 1 % penicillin/streptomycin).

CONCLUSIONS

FGF-2 has a dual capacity in promoting osteogenic and fibroblastic differentiation within hPDLSCs contingent upon the dosage and timing of administration, alongside supporting osteogenic differentiation in haBMSCs. These findings underscore the need for precision growth factors dosing when considering the design of biomaterials for periodontal regeneration.

Impact of Inflammatory Markers and Senescence-Associated Secretory Phenotype in the Gingival Crevicular Fluid on the Outcomes of Periodontal Regeneration

The aim of this study was to test the molecular expression profile (senescence-associated secretory phenotype; SASP) in gingival crevicular fluid (GCF) prior to surgery in relation to the distribution of clinical success of periodontal regeneration. Forty consecutive patients presenting sites with residual probing pocket depth (PPD) ≥ 6 mm and intrabony defects ≥ 3 mm were treated through a minimally invasive surgical technique. Pre-operatively, GCF was sampled for inflammatory biomarker analysis related to SASP [interleukin (IL)-1β, IL-6, and IL-12; matrix-metalloproteinases (MMP)-8 and -9]. Better or worse responders were classified depending on the achievement of a composite outcome measure at 1-year [COM; PPD ≤ 4 mm and clinical attachment gain (CAL) gain ≥ 3 mm]. Correlation analyses and logistic regression models were performed. Periodontal regeneration led to significant improvements in mean clinical and radiographic parameters. Teeth achieving COM presented significantly lower amounts of SASP factors compared with non-successful teeth. Higher CAL gain, PPD reduction, and radiographic bone fill were negatively correlated with IL-1β and MMP-8 and -9 (p < 0.001), while IL-12 showed a direct relationship with CAL gain (p = 0.005) and PPD reduction (p = 0.038). Sites expressing higher SASP expression in the GCF before periodontal regeneration achieved worse clinical and radiographic outcomes.

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.

Orally Derived Stem Cell-Based Therapy in Periodontal Regeneration: A Systematic Review and Meta-Analysis of Randomized Clinical Studies

The present systematic review was performed to assess the application of orally derived stem cells in periodontal regenerative therapy, and because of this, the following PICO question was proposed: "In patients with periodontitis, can the adjunctive use of orally derived stem cells provide additional clinical and radiographic benefits for periodontal regeneration?". Randomized clinical studies were electronically and manually searched up until December 2023. Quantitative analyses were performed with the aim of evaluating the mean differences (MDs) between the treatment and control groups in terms of clinical attachment level (CAL) gain, probing pocket depth (PPD) reduction, gingival recession (GR), and radiographic bone gain (RBG) using random effect models. A total of seven studies were selected for the systematic review. Meta-analyses excluding studies with a high risk of bias highlighted a non-statistically significant result for the use of stem cells when compared to the control groups in terms of CAL gain [MD = 1.05; 95% CI (-0.88, 2.97) p = 0.29] and PPD reduction [MD = 1.32; 95% CI (-0.25, 2.88) p = 0.10]. The same also applied to GR [MD = -0.08; 95% CI (-0.79, 0.63) p = 0.83] and RBG [MD = 0.50; 95% CI (-0.88, 1.88) p = 0.48]. Based on the high heterogeneity, there is not enough evidence to consider the adjunctive application of orally derived mesenchymal stem cells as a preferential approach for periodontal regenerative treatment, as compared to standard procedures.

Osteogenic mesenchymal stem cells/progenitors in the periodontium

Periodontitis is the major cause of tooth loss in adults and is mainly characterized by alveolar bone destruction. Elucidating the mesenchymal stem cell (MSC)/progenitor populations of alveolar bone formation will provide valuable insights into regenerative approaches to clinical practice, such as endogenous regeneration and stem-cell-based tissue engineering therapies. Classically, MSCs residing in the bone marrow, periosteum, periodontal ligament (PDL), and even the gingiva are considered to be osteogenic progenitors. Furthermore, the contributions of MSCs expressing specific markers, including Gli1, Axin2, PTHrP, LepR, and α-SMA, to alveolar bone formation have been studied using cell lineage tracing and gene knockout models. In this review, we describe the MSCs/progenitors of alveolar bone and the biological properties of different subpopulations of MSCs involved in alveolar bone development, remodeling, injury repair, and regeneration.

Gli1+ Periodontal Mesenchymal Stem Cells in Periodontitis

Periodontal mesenchymal stem cells (MSCs) play a crucial role in maintaining periodontium homeostasis and in tissue repair. However, little is known about how periodontal MSCs in vivo respond under periodontal disease conditions, posing a challenge for periodontium tissue regeneration. In this study, Gli1 was used as a periodontal MSC marker and combined with a Gli1-cre ERT2 mouse model for lineage tracing to investigate periodontal MSC fate in an induced periodontitis model. Our findings show significant changes in the number and contribution of Gli1+ MSCs within the inflamed periodontium. The number of Gli1+ MSCs that contributed to periodontal ligament homeostasis decreased in the periodontitis-induced teeth. While the proliferation of Gli1+ MSCs had no significant difference between the periodontitis and the control groups, more Gli1+ MSCs underwent apoptosis in diseased teeth. In addition, the number of Gli1+ MSCs for osteogenic differentiation decreased during the progression of periodontitis. Following tooth extraction, the contribution of Gli1+ MSCs to the tooth socket repair was significantly reduced in the periodontitis-induced teeth. Collectively, these findings indicate that the function of Gli1+ MSCs in periodontitis was compromised, including reduced contribution to periodontium homeostasis and impaired injury response.

A novel macrolide-Del-1 axis to regenerate bone in old age

Aging is associated with increased susceptibility to chronic inflammatory bone loss disorders, such as periodontitis, in large part due to the impaired regenerative potential of aging tissues. DEL-1 exerts osteogenic activity and promotes bone regeneration. However, DEL-1 expression declines with age. Here we show that systemically administered macrolide antibiotics and a non-antibiotic erythromycin derivative, EM-523, restore DEL-1 expression in 18-month-old ("aged") mice while promoting regeneration of bone lost due to naturally occurring age-related periodontitis. These compounds failed to induce bone regeneration in age-matched DEL-1-deficient mice. Consequently, these drugs promoted DEL-1-dependent functions, including alkaline phosphatase activity and osteogenic gene expression in the periodontal tissue while inhibiting osteoclastogenesis, leading to net bone growth. Macrolide-treated aged mice exhibited increased skeletal bone mass, suggesting that this treatment may be pertinent to systemic bone loss disorders. In conclusion, we identified a macrolide-DEL-1 axis that can regenerate bone lost due to aging-related disease.

LRP1-deficient leptin receptor-positive cells in periodontal ligament tissue reduce alveolar bone mass by inhibiting bone formation

OBJECTIVE

Leptin receptor-positive (LepR+) periodontal ligament (PDL) cells play a crucial role in osteogenesis during tooth socket healing and orthodontic tooth movement; however, the factors regulating osteoblast differentiation remain unclear. This study aimed to demonstrate the function of low-density lipoprotein receptor-related protein 1 (LRP1) in alveolar bone formation by examining conditional knockout (cKO) mice lacking LRP1 in LepR+ cells.

DESIGN

Bone mass and formation were examined via bone morphometric analysis. Bone formation and resorption activities were determined via histochemical staining. Additionally, PDL cells collected from molars were induced to differentiate into osteoblasts with the addition of BMP2 and to mineralize with the addition of osteogenic medium. Osteoblast differentiation of PDL cells was examined by measuring the expression of osteoblast markers.

RESULTS

Bone morphometry analysis revealed decreased mineral apposition rate and alveolar bone mass in cKO mice. Additionally, cKO mice showed a decreased number of osterix-positive cells in the PDL. cKO mice had a large number of osteoclasts around the alveolar bone near the root apex and mesial surface of the tooth. In the PDL cells from cKO mice, inhibition of mineralized matrix formation and decreased expression of alkaline phosphatase, osterix, bone sialoprotein, and osteocalcin were observed even when BMP2 was added to the medium. BMP2, BMP4, and osteoprotegerin expression also decreased, but RANKL expression increased dominantly.

CONCLUSION

LRP1 in LepR+ cells promotes bone formation by stimulating osteoblast differentiation. Our findings can contribute to clinical research on bone diseases and help elucidate bone metabolism in the periodontal tissue.

Multiomics analysis of cultured mouse periodontal ligament cell-derived extracellular matrix

A comprehensive understanding of the extracellular matrix (ECM) is essential for developing biomimetic ECM scaffolds for tissue regeneration. As the periodontal ligament cell (PDLC)-derived ECM has shown potential for periodontal tissue regeneration, it is vital to gain a deeper understanding of its comprehensive profile. Although the PDLC-derived ECM exhibits extracellular environment similar to that of periodontal ligament (PDL) tissue, details of its molecular composition are lacking. Thus, using a multiomics approach, we systematically analyzed cultured mouse PDLC-derived ECM and compared it to mouse PDL tissue as a reference. Proteomic analysis revealed that, compared to PDL tissue, the cultured PDLC-derived ECM had a lower proportion of fibrillar collagens with increased levels of glycoprotein, corresponding to an immature ECM status. The gene expression signature was maintained in cultured PDLCs and was similar to that in cells from PDL tissues, with additional characteristics representative of naturally occurring progenitor cells. A combination of proteomic and transcriptomic analyses revealed that the cultured mouse PDLC-derived ECM has multiple advantages in tissue regeneration, providing an extracellular environment that closely mimics the environment in the native PDL tissue. These findings provide valuable insights for understanding PDLC-derived ECM and should contribute to the development of biomimetic ECM scaffolds for reliable periodontal tissue regeneration.

Lepr-Expressing PDLSCs Contribute to Periodontal Homeostasis and Respond to Mechanical Force by Piezo1

Periodontium supports teeth in a mechanically stimulated tissue environment, where heterogenous stem/progenitor populations contribute to periodontal homeostasis. In this study, Leptin receptor+ (Lepr+) cells are identified as a distinct periodontal ligament stem cell (PDLSC) population by single-cell RNA sequencing and lineage tracing. These Lepr+ PDLSCs are located in the peri-vascular niche, possessing multilineage potential and contributing to tissue repair in response to injury. Ablation of Lepr+ PDLSCs disrupts periodontal homeostasis. Hyper-loading and unloading of occlusal forces modulate Lepr+ PDLSCs activation. Piezo1 is demonstrated that mediates the mechanosensing of Lepr+ PDLSCs by conditional Piezo1-deficient mice. Meanwhile, Yoda1, a selective activator of Piezo1, significantly accelerates periodontal tissue growth via the induction of Lepr+ cells. In summary, Lepr marks a unique multipotent PDLSC population in vivo, to contribute toward periodontal homeostasis via Piezo1-mediated mechanosensing.

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.

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.

Subset of the periodontal ligament expressed leptin receptor contributes to part of hard tissue-forming cells

The lineage of periodontal ligament (PDL) stem cells contributes to alveolar bone (AB) and cementum formation, which are essential for tooth-jawbone attachment. Leptin receptor (LepR), a skeletal stem cell marker, is expressed in PDL; however, the stem cell capacity of LepR⁺ PDL cells remains unclear. We used a Cre/LoxP-based approach and detected LepR-cre-labeled cells in the perivascular around the root apex; their number increased with age. In the juvenile stage, LepR⁺ PDL cells differentiated into AB-embedded osteocytes rather than cementocytes, but their contribution to both increased with age. The frequency of LepR⁺ PDL cell-derived lineages in hard tissue was < 20% per total cells at 1-year-old. Similarly, LepR⁺ PDL cells differentiated into osteocytes following tooth extraction, but their frequency was < 9%. Additionally, both LepR⁺ and LepR^- PDL cells demonstrated spheroid-forming capacity, which is an indicator of self-renewal. These results indicate that both LepR⁺ and LepR^- PDL populations contributed to hard tissue formation. LepR^- PDL cells increased the expression of LepR during spheroid formation, suggesting that the LepR^- PDL cells may hierarchically sit upstream of LepR⁺ PDL cells. Collectively, the origin of hard tissue-forming cells in the PDL is heterogeneous, some of which express LepR.

The role of PRX1-expressing cells in periodontal regeneration and wound healing

The ideal outcome of wound healing is the complete restoration of the structure and function of the original tissue. Stem cells are one of the key factors in this process. Currently, the strategy of periodontal regeneration based on mesenchymal stem cells (MSCs) is generally used to expand stem cells in vitro and then transplant them in vivo. However, their clinical application is limited. In fact, the human body has the capacity to regenerate through stem cells residing in different tissues, even without external therapeutic intervention. Stem cell niches are present in many adult tissues, such as the periodontal ligament and gingiva, and stem cells might remain in a quiescent state in their niches until they are activated in response to a regenerative need. Activated stem cells can exit the niche and proliferate, self-renew, and differentiate to regenerate original structures. Thus, harnessing the regenerative potential of endogenous stem cells in situ has gained increasing attention as a simpler, safer, and more applicable alternative to stem cell transplantation. Nevertheless, there are several key problems to be solved in the application of periodontal mesenchymal stem cells. Thus, animal studies will be especially important to deepen our knowledge of the in vivo mechanisms of mesenchymal stem cells. Studies with conditional knockout mice, in which the expression of different proteins can be eliminated in a tissue-specific manner, are especially important. Post-natal cells expressing the paired-related homeobox protein 1 (PRX1 or PRRX1), a transcription factor expressed in the mesenchyme during craniofacial and limb development, have been shown to have characteristics of skeletal stem cells. Additionally, following wounding, dermal Prx1+ cells are found out of their dermal niches and contribute to subcutaneous tissue repair. Postnatal Prx1+ cells are uniquely injury-responsive. Meanwhile, current evidence shows that Prx1+ cells contribute to promote dentin formation, wound healing of alveolar bone and formation of mouse molar and periodontal ligament. Initial result of our research group also indicates Prx1-expressing cells in bone tissue around the punch wound area of gingiva increased gradually. Collectively, this review supports the future use of PRX1 cells to stimulate their potential to play an important role in endogenous regeneration during periodontal therapy.

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.

Prolyl-hydroxylase inhibitor-induced regeneration of alveolar bone and soft tissue in a mouse model of periodontitis through metabolic reprogramming

Bone injuries and fractures reliably heal through a process of regeneration with restoration to original structure and function when the gap between adjacent sides of a fracture site is small. However, when there is significant volumetric loss of bone, bone regeneration usually does not occur. In the present studies, we explore a particular case of volumetric bone loss in a mouse model of human periodontal disease (PD) in which alveolar bone surrounding teeth is permanently lost and not replaced. This model employs the placement a ligature around the upper second molar for 10 days leading to inflammation and bone breakdown and faithfully replicates the bacterially-induced inflammatory etiology of human PD to induce bone degeneration. After ligature removal, mice are treated with a timed-release formulation of a small molecule inhibitor of prolylhydroxylases (PHDi; 1,4-DPCA) previously shown to induce epimorphic regeneration of soft tissue in non-regenerating mice. This PHDi induces high expression of HIF-1α and is able to shift the metabolic state from OXPHOS to aerobic glycolysis, an energetic state used by stem cells and embryonic tissue. This regenerative response was completely blocked by siHIF1a. In these studies, we show that timed-release 1,4-DPCA rapidly and completely restores PD-affected bone and soft tissue with normal anatomic fidelity and with increased stem cell markers due to site-specific stem cell migration and/or de-differentiation of local tissue, periodontal ligament (PDL) cell proliferation, and increased vascularization. In-vitro studies using gingival tissue show that 1,4-DPCA indeed induces de-differentiation and the expression of stem cell markers but does not exclude the role of migrating stem cells. Evidence of metabolic reprogramming is seen by the expression of not only HIF-1a, its gene targets, and resultant de-differentiation markers, but also the metabolic genes Glut-1, Gapdh, Pdk1, Pgk1 and Ldh-a in jaw periodontal tissue.

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.

Dual peptide-functionalized hydrogels differentially control periodontal cell function and promote tissue regeneration

Restoring the tooth-supporting tissues lost during periodontitis is a significant clinical challenge, despite advances in both biomaterial and cell-based approaches. This study investigated poly(ethylene glycol) (PEG) hydrogels functionalized with integrin-binding peptides RGD and GFOGER for controlling periodontal ligament cell (PDLC) activity and promoting periodontal tissue regeneration. Dual presentation of RGD and GFOGER within PEG hydrogels potentiated two key PDLC functions, alkaline phosphatase (ALP) activity and matrix mineralization, over either peptide alone and could be tuned to differentially promote each function. Hydrogel matrix mineralization, fostered by high concentrations of GFOGER together with RGD, identified a PDLC phenotype with accelerated matrix adhesion formation and expression of cementoblast and osteoblast genes. In contrast, maximizing ALP activity through high RGD and low GFOGER levels resulted in minimal hydrogel mineralization, in part, through altered PDLC pyrophosphate regulation. Transplantation of PDLCs in hydrogels optimized for either outcome promoted cementum formation in rat periodontal defects; however, only hydrogels optimized for in vitro mineralization improved new bone formation. Overall, these results highlight the utility of engineered hydrogel systems for controlling PDLC functions and their promise for promoting periodontal tissue regeneration.

TNFAIP6 defines the MSC subpopulation with enhanced immune suppression activities

Background

Mesenchymal stromal/stem cells (MSCs) have been intensively investigated in both pre-clinical and clinical studies. However, the therapeutic efficacy varies resulting from the heterogenicity of MSCs. Therefore, purifying the specific MSC subpopulation with specialized function is necessary for their therapeutic applications.

Methods

The large-scale RNA sequencing analysis was performed to identify potential cell markers for the mouse MSCs. Then, the immune suppression activities of the purified MSC subpopulation were assessed in vitro and in vivo.

Results

The TNFAIP6 (tumor necrosis factor alpha-induced protein 6) has been identified as a potential cell marker for mouse MSCs, irrespective of tissue origin and laboratory origin. The TNFAIP6⁺ mouse MSCs showed enhanced immune suppression activities and improved therapeutic effects on the mouse model of acute inflammation, resulting from faster response to immune stimulation.

Conclusions

Therefore, we have demonstrated that the TNFAIP6⁺ MSC subpopulation has enhanced immune suppression capabilities.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03176-5.

Diverse stem cells for periodontal tissue formation and regeneration

The periodontium is comprised of multiple units of mineralized and nonmineralized tissues including the cementum on the root surface, the alveolar bone, periodontal ligament (PDL), and the gingiva. PDL contains a variety of cell populations including mesenchymal stem/progenitor cells (MSCs) termed PDLSCs, which contribute to periodontal regeneration. Recent studies utilizing mouse genetic models shed light on the identities of these mesenchymal progenitors in their native environment, particularly regarding how they contribute to homeostasis and repair of the periodontium. The current concept is that mesenchymal progenitors in the PDL are localized to the perivascular niche. Single-cell RNA sequencing (scRNA-seq) analyses reveal heterogeneity and cell-type specific markers of cells in the periodontium, as well as their developmental relationship with precursor cells in the dental follicle. The characteristics of PDLSCs and their diversity in vivo are now beginning to be unraveled thanks to insights from mouse genetic models and scRNA-seq analyses, which aid to uncover the fundamental properties of stem cells in the human PDL. The new knowledge will be highly important for developing more effective stem cell-based regenerative therapies to repair periodontal tissues in the future.

Tooth transplantation and replantation: Biological insights towards therapeutic improvements

Transplantation and replantation of teeth are effective therapeutic approaches for tooth repositioning and avulsion, respectively. Transplantation involves transplanting an extracted tooth from the original site into another site, regenerating tissue including the periodontal ligament (PDL) and alveolar bone, around the transplanted tooth. Replantation places the avulsed tooth back to its original site, regenerating functional periodontal tissue. In clinical settings, transplantation and replantation result in favorable outcomes with regenerated PDL tissue in many cases. However, they often result in poor outcomes with two major complications: tooth ankylosis and root resorption. In tooth ankylosis, the root surface and alveolar bone are fused, reducing the PDL tissue between them. The root is subjected to remodeling processes and is partially replaced by bone. In severe cases, the resorbed root is completely replaced by bone tissue, which is called as "replacement resorption." Resorption is sometimes accompanied by infection-mediated inflammation. The molecular mechanisms of ankylosis and root resorption remain unclear, although some signaling mechanisms have been proposed. In this mini-review, we summarized the biological basis of repair mechanisms of tissues in transplantation and replantation and the pathogenesis of their healing failure. We also discussed possible therapeutic interventions to improve treatment success rates.

Advances in periodontal stem cells and the regulating niche: From in vitro to in vivo

Periodontium possesses stem cell populations for its self-maintenance and regeneration, and has been proved to be an optimal stem cell source for tissue engineering. In vitro studies have shown that stem cells can be isolated from periodontal ligament, alveolar bone marrow and gingiva. In recent years, more studies have focused on identification of periodontal stem cells in vivo. Multiple genetic markers, including Gli1, Prx1, Axin2, αSMA, and LepR, were identified with the lineage tracing approaches. Characteristics, functions, and regulatory mechanisms of specific populations expressing one of these markers have been investigated. In vivo studies also revealed that periodontal stem cells can be regulafrted by different niche and mechanisms including intercellular interactions, ECM and multiple secreted factors. In this review, we summarized the current knowledge of in vitro characteristics and in vivo markers of periodontal stem cells, and discussed the specific regulating niche.

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.

SM22α-lineage niche cells regulate intramembranous bone regeneration via PDGFRβ-triggered hydrogen sulfide production

Bone stromal cells are critical for bone homeostasis and regeneration. Growing evidence suggests that non-stem bone niche cells support bone homeostasis and regeneration via paracrine mechanisms, which remain to be elucidated. Here, we show that physiologically quiescent SM22α-lineage stromal cells expand after bone injury to regulate diverse processes of intramembranous bone regeneration. The majority of SM22α-lineage cells neither act as stem cells in vivo nor show their expression patterns. Dysfunction of SM22α-lineage niche cells induced by loss of platelet-derived growth factor receptor β (PDGFRβ) impairs bone repair. We further show that PDGFRβ-triggered hydrogen sulfide (H₂S) generation in SM22α-lineage niche cells facilitates osteogenesis and angiogenesis and suppresses overactive osteoclastogenesis. Collectively, these data demonstrate that non-stem SM22α-lineage niche cells support the niche for bone regeneration with a PDGFRβ/H₂S-dependent regulatory mechanism. Our findings provide further insight into non-stem bone stromal niche cell populations and niche-regulation strategy for bone repair.

Identification of Dental Stem Cells Similar to Skeletal Stem Cells

Stem and progenitor cells play important roles in the development and maintenance of teeth and bone. Surface markers expressed in bone marrow–derived mesenchymal stem cells are also expressed in dental tissue–derived stem cells. Mouse skeletal stem cells (mSSCs, CD45−Ter119−Tie2−CD51+Thy−6C3−CD105−CD200+) and human skeletal stem cells (hSSCs, CD45−CD235a−TIE2−CD31−CD146−PDPN+CD73+CD164+) have been identified in bone and shown to play important roles in skeletal development and regeneration. However, it is unclear whether dental tissues also harbor mSSC or hSSC populations. Here, we employed rainbow tracers and found that clonal expansion occurred in mouse dental tissues similar to that in bone. We sorted the mSSC population from mouse periodontal ligament (mPDL) tissue and mouse dental pulp (mDP) tissue in the lower incisors by fluorescence-activated cell sorting (FACS). In addition, we demonstrated that mPDL-derived skeletal stem cells (mPDL-SSCs) and mDP-derived skeletal stem cells (mDP-SSCs) have similar clonogenic capacity, as well as cementogenic and odontogenic potential, but not adipogenic potential, similar to the characteristics of mSSCs. Moreover, we found that the dental tissue–derived mSSC population plays an important role in repairing clipped incisors. Importantly, we sorted the hSSC population from human periodontal ligament (hPDL) and human dental pulp (hDP) tissue in molars and identified its stem cell characteristics. Finally, hPDL-like and hDP-like structures were generated after transplanting hPDL-SSCs and hDP-SSCs beneath the renal capsules. In conclusion, we demonstrated that mouse and human PDL and DP tissues harbor dental stem cells similar to mSSCs and hSSCs, respectively, providing a precise stem cell population for the exploration of dental diseases.

Periodontal Wound Healing and Regeneration: Insights for Engineering New Therapeutic Approaches

Periodontitis is a widespread inflammatory disease that leads to loss of the tooth supporting periodontal tissues. The few therapies available to regenerate periodontal tissues have high costs and inherent limitations, inspiring the development of new approaches. Studies have shown that periodontal tissues have an inherent capacity for regeneration, driven by multipotent cells residing in the periodontal ligament (PDL). The purpose of this review is to describe the current understanding of the mechanisms driving periodontal wound healing and regeneration that can inform the development of new treatment approaches. The biologic basis underlying established therapies such as guided tissue regeneration (GTR) and growth factor delivery are reviewed, along with examples of biomaterials that have been engineered to improve the effectiveness of these approaches. Emerging therapies such as those targeting Wnt signaling, periodontal cell delivery or recruitment, and tissue engineered scaffolds are described in the context of periodontal wound healing, using key in vivo studies to illustrate the impact these approaches can have on the formation of new cementum, alveolar bone, and PDL. Finally, design principles for engineering new therapies are suggested which build on current knowledge of periodontal wound healing and regeneration.

Gli1+ Mesenchymal Stem Cells in Bone and Teeth

Mesenchymal stem cells (MSCs) are remarkable and noteworthy. Identification of markers for MSCs enables the study of their niche in vivo. It has been identified that glioma-associated oncogene 1 positive (Gli1+) cells are mesenchymal stem cells supporting homeostasis and injury repair, especially in the skeletal system and teeth. This review outlines the role of Gli1+ cells as an MSC subpopulation in both bones and teeth, suggesting the prospects of Gli1+ cells in stem cell-based tissue engineering.

Alveolar Bone Marrow Gli1+ Stem Cells Support Implant Osseointegration

Osseointegration is the key issue for implant success. The in vivo properties of cell populations driving the osseointegration process have remained largely unknown. In the current study, using tissue clearing-based 3-dimensional imaging and transgenic mouse model-based lineage tracing methods, we identified Gli1+ cells within alveolar bone marrow and their progeny as the cell population participating in extraction socket healing and implant osseointegration. These Gli1⁺ cells are surrounding blood vessels and do not express lineage differentiation markers. After tooth extraction and delayed placement of a dental implant, Gli1⁺ cells were activated into proliferation, and their descendants contributed significantly to new bone formation. Ablation of Gli1⁺ cells severely compromised the healing and osseointegration processes. Blockage of canonical Wnt signaling resulted in impaired recruitment of Gli1⁺ cells and compromised bone healing surrounding implants. Collectively, these findings demonstrate that Gli1⁺ cells surrounding alveolar bone marrow vasculature are stem cells supporting dental implant osseointegration. Canonical Wnt signal plays critical roles in regulating Gli1⁺ stem cells.

Specialized Proresolving Mediators Facilitate the Immunomodulation of the Periodontal Ligament Stem Cells

Recent investigations into the regulation of the inflammation in the periodontitis have revealed that chronic inflammatory diseases such as periodontitis are characterized by an imbalance in the proinflammatory and proresolution mediators and can be characterized by a failure of the resolution pathways in the late stages of the acute inflammatory response. The proresolution mediators, termed as specialized proresolving mediators (SPMs), comprise the lipoxins, resolvins, protectins, and maresins that are derived from the arachidonic acid or omega-3 polyunsaturated fatty acids. In the animal studies, treatment of the periodontitis with the topical SPMs return the inflammatory lesion to the homeostasis with the regeneration of all the components of the periodontal organ lost to the disease. In this article, the study investigates the immunomodulatory role of SPMs in the periodontal ligament stem cells (PDLSCs). Primary porcine PDLSCs (pPDLSCs) were stimulated with interleukin-1β (IL-1β) and interleukin-17 (IL-17) in vitro to simulate the periodontal inflammation in the presence or absence of SPMs. This study found that IL-1β and IL-17 synergistically activated the proinflammatory genes of pPDLSCs and altered the immune phenotype of pPDLSCs including the key signaling pathways. Addition of SPMs rescued the pPDLSCs phenotype and induced further production of the additional SPMs, which was reflected by upregulation of the requisite enzymes 12- and 15-lipoxygenase by pPDLSCs. This study interrogated the immunomodulatory actions of pPDLSCs on the monocytes/macrophages, focusing on the porcine CD14/CD16/CD163 markers by using flow cytometry. This study utilized the CD14+CD16+/CD14+CD16− ratio and CD163 on the monocytes/macrophages to differentiate between a proinflammation phenotype (lower ratio) and a resolution of the inflammation phenotype (higher ratio). This study also found that the conditioned medium from pPDLSCs treated with the cytokines and Maresin1 increased the CD14+CD16+/CD14+CD16− ratio and had the highest CD163 expression. This study concludes that in an inflammatory environment, pPDLSCs become proinflammatory and exert immunomodulatory functions. Maresin 1 resolves the inflammation by acting on pPDLSCs directly and by shifting the monocytes/macrophages phenotype to the proresolution dominance.

Enhancement of Osteoblast Differentiation Using No-Ozone Cold Plasma on Human Periodontal Ligament Cells

Periodontitis is an inflammatory disease that leads to periodontal tissue destruction and bone resorption. Proliferation and differentiation of cells capable of differentiating into osteoblasts is important for reconstructing periodontal tissues destroyed by periodontitis. In this study, the effects of the nozone (no-ozone) cold plasma (NCP) treatment on osteoblastic differentiation in periodontal ligament (PDL) cells were investigated. To test the toxicity of NCP on PDL cells, various NCP treatment methods and durations were tested, and time-dependent cell proliferation was analyzed using a water-soluble tetrazolium salts-1 assay. To determine the effect of NCP on PDL cell differentiation, the cells were provided with osteogenic media immediately after an NCP treatment to induce differentiation; the cells were then analyzed using alkaline phosphatase (ALP) staining, an ALP activity assay, real time PCR, and Alizarin Red S staining. The NCP treatment without toxicity on PDL cells was the condition of 1-min NCP treatment immediately followed by the replacement with fresh media. NCP increased ALP, osteocalcin, osteonectin, and osteopontin expression, as well as mineralization nodule formation. NCP treatment promotes osteoblastic differentiation of PDL cells; therefore, it may be beneficial for treating periodontitis.

Diabetic wound healing in soft and hard oral tissues

There is significant interest in understanding the cellular mechanisms responsible for expedited healing response in various oral tissues and how they are impacted by systemic diseases. Depending upon the types of oral tissue, wound healing may occur by predominantly re-eptihelialization, by re-epithelialization with substantial new connective tissue formation, or by a a combination of both plus new bone formation. As a result, the cells involved differ and are impacted by systemic diaseses in various ways. Diabetes mellitus is a prevalent metabolic disorder that impairs barrier function and healing responses throughout the human body. In the oral cavity, diabetes is a known risk factor for exacerbated periodontal disease and delayed wound healing, which includes both soft and hard tissue components. Here, we review the mechanisms of diabetic oral wound healing, particularly on impaired keratinocyte proliferation and migration, altered level of inflammation, and reduced formation of new connective tissue and bone. In particular, diabetes inhibits the expression of mitogenic growth factors whereas that of pro-inflammatory cytokines is elevated through epigenetic mechanisms. Moreover, hyperglycemia and oxidative stress induced by diabetes prevents the expansion of mesengenic cells that are involved in both soft and hard tissue oral wounds. A better understanding of how diabetes influences the healing processes is crucial for the prevention and treatment of diabetes-associated oral complications.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

Notch signaling in the dynamics of perivascular stem cells and their niches

The Notch signaling pathway is a fundamental regulator of cell fate determination in homeostasis and regeneration. In this work, we aimed to determine how Notch signaling mediates the interactions between perivascular stem cells and their niches in human dental mesenchymal tissues, both in homeostatic and regenerative conditions. By single cell RNA sequencing analysis, we showed that perivascular cells across the dental pulp and periodontal human tissues all express NOTCH3, and that these cells are important for the response to traumatic injuries in vivo in a transgenic mouse model. We further showed that the behavior of perivascular NOTCH3‐expressing stem cells could be modulated by cellular and molecular cues deriving from their microenvironments. Taken together, the present studies, reinforced by single‐cell analysis, reveal the pivotal importance of Notch signaling in the crosstalk between perivascular stem cells and their niches in tissue homeostasis and regeneration.

MicroRNAs: Harbingers and shapers of periodontal inflammation

Periodontal disease is an inflammatory reaction of the periodontal tissues to oral pathogens. In the present review we discuss the intricate effects of a regulatory network of gene expression modulators, microRNAs (miRNAs), as they affect periodontal morphology, function and gene expression during periodontal disease. These miRNAs are small RNAs involved in RNA silencing and post-transcriptional regulation and affect all stages of periodontal disease, from the earliest signs of gingivitis to the regulation of periodontal homeostasis and immunity and to the involvement in periodontal tissue destruction. MiRNAs coordinate periodontal disease progression not only directly but also through long non-coding RNAs (lncRNAs), which have been demonstrated to act as endogenous sponges or decoys that regulate the expression and function of miRNAs, and which in turn suppress the targeting of mRNAs involved in the inflammatory response, cell proliferation, migration and differentiation. While the integrity of miRNA function is essential for periodontal health and immunity, miRNA sequence variations (genetic polymorphisms) contribute toward an enhanced risk for periodontal disease progression and severity. Several polymorphisms in miRNA genes have been linked to an increased risk of periodontitis, and among those, miR-146a, miR-196, and miR-499 polymorphisms have been identified as risk factors for periodontal disease. The role of miRNAs in periodontal disease progression is not limited to the host tissues but also extends to the viruses that reside in periodontal lesions, such as herpesviruses (human herpesvirus, HHV). In advanced periodontal lesions, HHV infections result in the release of cytokines from periodontal tissues and impair antibacterial immune mechanisms that promote bacterial overgrowth. In turn, controlling the exacerbation of periodontal disease by minimizing the effect of periodontal HHV in periodontal lesions may provide novel avenues for therapeutic intervention. In summary, this review highlights multiple levels of miRNA-mediated control of periodontal disease progression, (i) through their role in periodontal inflammation and the dysregulation of homeostasis, (ii) as a regulatory target of lncRNAs, (iii) by contributing toward periodontal disease susceptibility through miRNA polymorphism, and (iv) as periodontal microflora modulators via viral miRNAs.

In vivo cell proliferation analysis and cell-tracing reveal the global cellular dynamics of periodontal ligament cells under mechanical-loading

Periodontal ligament (PDL) is a uniquely differentiated tissue that anchors the tooth to the alveolar bone socket and plays key roles in oral function. PDL cells can respond rapidly to mechanical stimuli, resulting in accelerated tissue remodeling. Cell proliferation is an initial event in tissue remodeling and participates in maintaining the cell supply; therefore, analyzing cell-proliferative activity might provide a comprehensive view of cellular dynamics at the tissue level. In this study, we investigated proliferating cells in mouse molar PDL during orthodontic tooth movement (OTM)-induced tissue remodeling. Our results demonstrated that the mechanical stimuli evoked a dynamic change in the proliferative-cell profile at the entire PDL. Additionally, cell-tracing analysis revealed that the proliferated cells underwent further division and subsequently contributed to tissue remodeling. Moreover, OTM-induced proliferating cells expressed various molecular markers that most likely arise from a wide range of cell types, indicating the lineage plasticity of PDL cells in vivo. Although further studies are required, these findings partially elucidated the global views of the cell trajectory in mouse molar PDL under mechanical-loading conditions, which is vital for understanding the cellular dynamics of the PDL and beneficial for dental treatment in humans.

A single-cell atlas of human teeth

Teeth exert fundamental functions related to mastication and speech. Despite their great biomedical importance, an overall picture of their cellular and molecular composition is still missing. In this study, we have mapped the transcriptional landscape of the various cell populations that compose human teeth at single-cell resolution, and we analyzed in deeper detail their stem cell populations and their microenvironment. Our study identified great cellular heterogeneity in the dental pulp and the periodontium. Unexpectedly, we found that the molecular signatures of the stem cell populations were very similar, while their respective microenvironments strongly diverged. Our findings suggest that the microenvironmental specificity is a potential source for functional differences between highly similar stem cells located in the various tooth compartments and open new perspectives toward cell-based dental therapeutic approaches.

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Dental atlas of the pulp and periodontal tissues of human teeth

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Identification of three common MSC subclusters between dental pulp and periodontium

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Dental pulp and periodontal MSCs are similar, and their niches diverge

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.

Heterogeneity of murine periosteum progenitors involved in fracture healing

The periosteum is the major source of cells involved in fracture healing. We sought to characterize progenitor cells and their contribution to bone fracture healing. The periosteum is highly enriched with progenitor cells, including Sca1+ cells, fibroblast colony-forming units, and label-retaining cells compared to the endosteum and bone marrow. Using lineage tracing, we demonstrate that alpha smooth muscle actin (αSMA) identifies long-term, slow-cycling, self-renewing osteochondroprogenitors in the adult periosteum that are functionally important for bone formation during fracture healing. In addition, Col2.3CreER-labeled osteoblast cells contribute around 10% of osteoblasts but no chondrocytes in fracture calluses. Most periosteal osteochondroprogenitors following fracture can be targeted by αSMACreER. Previously identified skeletal stem cell populations were common in periosteum but contained high proportions of mature osteoblasts. We have demonstrated that the periosteum is highly enriched with skeletal progenitor cells, and there is heterogeneity in the populations of cells that contribute to mature lineages during periosteal fracture healing.

The vital role of Gli1+ mesenchymal stem cells in tissue development and homeostasis

The hedgehog (Hh) signaling pathway plays an essential role in both tissue development and homeostasis. Glioma-associated oncogene homolog 1 (Gli1) is one of the vital transcriptional factors as well as the direct target gene in the Hh signaling pathway. The cells expressing the Gli1 gene (Gli1⁺ cells) have been identified as mesenchymal stem cells (MSCs) that are responsible for various tissue developments, homeostasis, and injury repair. This review outlines some recent discoveries on the crucial roles of Gli1⁺ MSCs in the development and homeostasis of varieties of hard and soft tissues.

M2-like macrophage infiltration and transforming growth factor-β secretion during socket healing process in mice

OBJECTIVE

Macrophages are involved in tissue inflammation and repair through cytokine secretion. However, the contribution of macrophages to healing and osteogenesis after tooth extraction remains unclear. Therefore, we investigated the distribution of osteoblastic cells and macrophages in the early healing process after tooth extraction.

METHODS

The maxillary first molars of 6-week-old male mice were extracted. The maxilla was collected 1, 3, and 7 days after extraction. The states of socket healing, localization of osteoblastic markers, and macrophage infiltration were sequentially observed by micro-CT imaging and immunohistochemistry.

RESULTS

On day 3 after tooth extraction, α-smooth muscle actin (SMA)-positive cells, osteoprogenitor cells at fracture healing, were observed in the socket. Several α-SMA-positive cells also expressed Runx2, the early osteoblast differentiation marker. The infiltration of F4/80-positive, mature macrophages and CD206-positive, M2-like macrophages was noted in the socket. However, CD169-positive macrophages (Osteomac), which are involved in fracture healing, were not detected in the socket. F4/80-positive and CD206-positive macrophages also showed the localization of transforming growth factor-β (TGF-β), which promotes osteoprogenitor cell proliferation and early differentiation. Phosphorylated Smad3, a downstream mediator of the signal activity of TGF-β, was detected in α-SMA-positive cells. On day 7, the extracted socket contained a large amount of new bone. Tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts were detected on bone surfaces.

CONCLUSION

Our data indicate that M2-like macrophages regulate the proliferation and differentiation of α-SMA-positive cells by secreting TGF-β at the early stage of socket healing, and also suggest the importance of macrophages in healing and bone formation after tooth extraction.

Matrix Control of Periodontal Ligament Cell Activity Via Synthetic Hydrogel Scaffolds

Rebuilding the tooth-supporting tissues (periodontium) destroyed by periodontitis remains a clinical challenge. Periodontal ligament cells (PDLCs), multipotent cells within the periodontal ligament (PDL), differentiate and form new PDL and mineralized tissues (cementum and bone) during native tissue repair in response to specific extracellular matrix (ECM) cues. Thus, harnessing ECM cues to control PDLC activity ex vivo, and ultimately, to design a PDLC delivery vehicle for tissue regeneration is an important goal. In this study, poly(ethylene glycol) hydrogels were used as a synthetic PDL ECM to interrogate the roles of cell-matrix interactions and cell-mediated matrix remodeling in controlling PDLC activity. Results showed that PDLCs within matrix metalloproteinase (MMP)-degradable hydrogels expressed key PDL matrix genes and showed a six to eightfold increase in alkaline phosphatase (ALP) activity compared with PDLCs in nondegradable hydrogel controls. The increase in ALP activity, commonly considered an early marker of cementogenic/osteogenic differentiation, occurred independent of the presentation of the cell-binding ligand RGD or soluble media cues and remained elevated when inhibiting PDLC-matrix binding and intracellular tension. ALP activity was further increased in softer hydrogels regardless of degradability and was accompanied by an increase in PDLC volume. However, scaffolds that fostered PDLC ALP activity did not necessarily promote hydrogel ECM mineralization. Rather, matrix mineralization was greatest in stiffer, MMP-degradable hydrogels and required the presence of soluble media cues. These divergent outcomes illustrate the complexity of the PDLC response to ECM cues and the limitations of current scaffold materials. Nevertheless, key biomaterial design principles for controlling PDLC activity were identified for incorporation into scaffolds for periodontal tissue regeneration. Impact statement Engineered scaffolds are an attractive approach for delivering periodontal ligament cells (PDLCs) to rebuild the tooth-supporting tissues. Replicating key extracellular matrix (ECM) cues within tissue engineered scaffolds may maximize PDLC potential. However, the identity of important ECM cues and how they can be harnessed to control PDLC activity is still unknown. In this study, matrix degradability, cell-matrix binding, and stiffness were varied using synthetic poly(ethylene glycol) hydrogels for three-dimensional PDLC culture. PDLCs exhibited dramatic and divergent responses to these cues, supporting further investigation of ECM-replicating scaffolds for control of PDLC behavior and periodontal tissue regeneration.

New insights on the reparative cells in bone regeneration and repair

Bone possesses a remarkable repair capacity to regenerate completely without scar tissue formation. This unique characteristic, expressed during bone development, maintenance and injury (fracture) healing, is performed by the reparative cells including skeletal stem cells (SSCs) and their descendants. However, the identity and functional roles of SSCs remain controversial due to technological difficulties and the heterogeneity and plasticity of SSCs. Moreover, for many years, there has been a biased view that bone marrow is the main cell source for bone repair. Together, these limitations have greatly hampered our understanding of these important cell populations and their potential applications in the treatment of fractures and skeletal diseases. Here, we reanalyse and summarize current understanding of the reparative cells in bone regeneration and repair and outline recent progress in this area, with a particular emphasis on the temporal and spatial process of fracture healing, the sources of reparative cells, an updated definition of SSCs, and markers of skeletal stem/progenitor cells contributing to the repair of craniofacial and long bones, as well as the debate between SSCs and pericytes. Finally, we also discuss the existing problems, emerging novel technologies and future research directions in this field.