Wnt activity is associated with cementum-type transition

Acellular Extrinsic Fiber Cementum Is Invariably Present in the Superficial Layer of Apical Cementum in Mouse Molar

The cementum is a highly mineralized tissue that covers the tooth root. The regional differences among the types of cementum, especially in the extrinsic fibers that contribute to tooth support, remain controversial. Therefore, this study used second harmonic generation imaging in conjunction with automated collagen extraction and image analysis algorithms to facilitate the quantitative examination of the fiber characteristics and the changes occurring in these fibers over time. Acellular extrinsic fiber cementum (AEFC) was invariably observed in the superficial layer of the apical cementum in mouse molars, indicating that this region of the cementum plays a crucial role in supporting the tooth. The apical AEFC exhibited continuity and fiber characteristics comparable with the cervical AEFC, suggesting a common cellular origin for their formation. The cellular intrinsic fiber cementum present in the inner layer of the apical cementum showed consistent growth in the apical direction without layering. This study highlights the dynamic nature of the cementum in mouse molars and underscores the requirement for re-examining its structure and roles. The findings of the present study elucidate the morphophysiological features of cementum and have broader implications for the maintenance of periodontal tissue health.

Activation of Wnt signaling in Axin2+ cells leads to osteodentin formation and cementum overgrowth

OBJECTIVES

In this study, we used the mouse incisor model to investigate the regulatory mechanisms of Wnt/β-catenin signaling on Axin2+ cells in tooth development.

MATERIALS AND METHODS

Axin2lacZ/+ reporter mice were used to define the expression pattern of Axin2 in mouse incisors. We traced the fate of Axin2+ cells from postnatal Day 21 (P21) to P56 using Axin2CreERT2/+ and R26RtdTomato/+ reporter mice. For constitutive activation of Wnt signaling, Axin2CreERT2/+ , β-cateninflox(Ex3)/+ , and R26RtdTomato/+ (CA-β-cat) mice were generated to investigate the gain of function (GOF) of β-catenin in mouse incisor growth.

RESULTS

The X-gal staining of Axin2lacZ/+ reporter mice and lineage tracing showed that Axin2 was widely expressed in dental mesenchyme of mouse incisors, and Axin2+ cells were essential cell sources for odontoblasts, pulp cells, and periodontal ligament cells. The constitutive activation of Wnt signaling in Axin2+ cells resulted in the formation of osteodentin featured with increased DMP1 and dispersed DSP expression and overgrowth of cementum.

CONCLUSION

Wnt signaling plays a key role in the differentiation and maturation of Axin2+ cells in mouse incisors.

LITTIP/Lgr6/HnRNPK complex regulates cementogenesis via Wnt signaling

Orthodontically induced tooth root resorption (OIRR) is a serious complication during orthodontic treatment. Stimulating cementum repair is the fundamental approach for the treatment of OIRR. Parathyroid hormone (PTH) might be a potential therapeutic agent for OIRR, but its effects still lack direct evidence, and the underlying mechanisms remain unclear. This study aims to explore the potential involvement of long noncoding RNAs (lncRNAs) in mediating the anabolic effects of intermittent PTH and contributing to cementum repair, as identifying lncRNA-disease associations can provide valuable insights for disease diagnosis and treatment. Here, we showed that intermittent PTH regulates cell proliferation and mineralization in immortalized murine cementoblast OCCM-30 via the regulation of the Wnt pathway. In vivo, daily administration of PTH is sufficient to accelerate root regeneration by locally inhibiting Wnt/β-catenin signaling. Through RNA microarray analysis, lncRNA LITTIP (LGR6 intergenic transcript under intermittent PTH) is identified as a key regulator of cementogenesis under intermittent PTH. Chromatin isolation by RNA purification (ChIRP) and RNA immunoprecipitation (RIP) assays revealed that LITTIP binds to mRNA of leucine-rich repeat-containing G-protein coupled receptor 6 (LGR6) and heterogeneous nuclear ribonucleoprotein K (HnRNPK) protein. Further co-transfection experiments confirmed that LITTIP plays a structural role in the formation of the LITTIP/Lgr6/HnRNPK complex. Moreover, LITTIP is able to promote the expression of LGR6 via the RNA-binding protein HnRNPK. Collectively, our results indicate that the intermittent PTH administration accelerates root regeneration via inhibiting Wnt pathway. The lncRNA LITTIP is identified to negatively regulate cementogenesis, which activates Wnt/β-catenin signaling via high expression of LGR6 promoted by HnRNPK.

The role of Evi/Wntless in exporting Wnt proteins

Intercellular communication by Wnt proteins governs many essential processes during development, tissue homeostasis and disease in all metazoans. Many context-dependent effects are initiated in the Wnt-producing cells and depend on the export of lipidated Wnt proteins. Although much focus has been on understanding intracellular Wnt signal transduction, the cellular machinery responsible for Wnt secretion became better understood only recently. After lipid modification by the acyl-transferase Porcupine, Wnt proteins bind their dedicated cargo protein Evi/Wntless for transport and secretion. Evi/Wntless and Porcupine are conserved transmembrane proteins, and their 3D structures were recently determined. In this Review, we summarise studies and structural data highlighting how Wnts are transported from the ER to the plasma membrane, and the role of SNX3-retromer during the recycling of its cargo receptor Evi/Wntless. We also describe the regulation of Wnt export through a post-translational mechanism and review the importance of Wnt secretion for organ development and cancer, and as a future biomarker.

Inactivation of Sufu in cementoblasts accelerates external tooth root resorption

Cementum has been empirically regarded as an antiresorptive barrier against tooth roots. However, little is known about the factors of homeostasis and resistant mechanisms of tooth roots against resorption. Here, we investigated cementum factors and their interaction against resorption using transgenic mice exhibiting external cervical root resorption (ECRR). Ectopically thickened cervical cementum caused by functional inactivation of ectonucleotide pyrophosphotase/phosphodiesterase 1 (Enpp1) was susceptible to ECRR with aging. In addition, the inactivation of the suppressor of fused (Sufu), a Hedgehog signaling inhibitor, in cementoblasts led to ECRR. Interestingly, concurrent inactivation of Sufu and Enpp1 in cementoblasts remarkably exacerbated ECRR with higher Rankl expression. Cellular and molecular analyses using cementoblasts and bone marrow-derived macrophages indicated that Dickkopf-related protein 1 (Dkk1) induced by the inactivation of Sufu in cementoblasts has roles in the acceleration of ECRR triggered by Enpp1 inactivation. Using compound mutant mice for concurrent Wntless and Enpp1 inactivation, this synergistic cooperation of Dkk1 and Npp1 for resorption found in double mutant Sufu and Enpp1 mice was confirmed by the reproduction of amplified ECRR. On the basis of these findings, we conclude that proper Npp1 function and sustained Wnt activity in the cervical cementum are essential for the homeostasis of tooth roots against resorption in a physiological state.

Npp1 prevents external tooth root resorption by regulation of cervical cementum integrity

Tooth roots embedded in the alveolar bone do not typically undergo resorption while the bone continues remodeling in its physiological state. In this study, we analyzed genetically modified mice with the functional inactivation of nucleotide pyrophosphatase 1 (Npp1), encoded by ectonucleotide pyrophosphatase/phosphodiesterase 1 (Enpp1). This mutation leads to the formation of ectopic cervical cementum vulnerable to external tooth root resorption. Cementoblasts with the inactivation of Enpp1 extensively expressed non-collagenous matrix proteins enriched with bone sialoprotein (Bsp), dentin matrix protein 1 (Dmp1), and osteopontin (Opn), which have roles in mineralization through nucleation and in cell adhesion through the Arg-Gly-Asp (RGD) motif. In cementoblasts with the inactivation of Enpp1, β-catenin was significantly activated and induced the expression of these non-collagenous matrix proteins. In addition, adenosine triphosphate (ATP), which is the most preferred substrate of Npp1, accumulated extracellularly and autocrinally induced the expression of the receptor activator of nuclear factor κB ligand (Rankl) in cementoblasts with inactivated Npp1. Consequently, these results strongly suggest that functional Npp1 preserves cervical cementum integrity and supports the anti-resorptive properties of tooth roots through ATP homeostasis in the physiological state of cervical cementum.

Connective tissue growth factor promotes cementogenesis and cementum repair via Cx43/β-catenin axis

Background

Orthodontic tooth movement inevitably induces cementum resorption, which is an urgent problem for orthodontists to confront. Human periodontal ligament stem cells (hPDLSCs) exert an important role in the orthodontic tooth movement and exhibit multidirectional differentiation ability in cementum regeneration. Connective tissue growth factor (CTGF) is an important extracellular matrix protein for bone homeostasis and cell differentiation. The purpose of our study was to explore the role of CTGF in cementum repair and cementogenesis and to elucidate its underlying mechanism.

Methods

A cementum defect model was established by tooth movement with heavy forces, and the cementum repair effect of CTGF was observed via micro-CT, HE staining and immunohistochemical staining. RT‒qPCR, western blotting (WB), alizarin red staining and ALP activity experiments verified the mineralization ability of hPDLSCs stimulated with CTGF. The expression of Cx43 in periodontal ligament cells was detected by WB and immunofluorescence (IF) experiments after CTGF stimulation in vivo and in vitro. Subsequently, the mineralization ability of hPDLSCs was observed after application of CTGF and the small interfering RNA Si-Cx43. Additionally, co-intervention via application of the small interfering RNA Si-CTGF and the Cx43 agonist ATRA in hPDLSCs was performed to deepen the mechanistic study. Next, WB, IF experiments and co-immunoprecipitation were conducted to confirm whether CTGF triggers the Cx43/β-catenin axis to regulate cementoblast differentiation of hPDLSCs.

Results

Local oral administration of CTGF to the cementum defects in vivo facilitated cementum repair. CTGF facilitated the cementogenesis of hPDLSCs in a concentration-dependent manner. Cx43 acted as a downstream effector of CTGF to regulate cementoblast differentiation. Si-Cx43 reduced CTGF-induced cementoblast differentiation. The Cx43 agonist ATRA restored the low differentiation capacity induced by Si-CTGF. Further mechanistic studies showed that CTGF triggered the activation of β-catenin in a dose-dependent manner. In addition, co-localization IF analysis and co-immunoprecipitation demonstrated that Cx43 interacted with β-catenin at cell‒cell connections. Si-Cx43 attenuated the substantial expression of β-catenin induced by CTGF. The Cx43 agonist reversed the inhibition of β-catenin induced by Si-CTGF. IF demonstrated that the nuclear importation of β-catenin was related to the immense expression of Cx43 at cell‒cell junctions.

Conclusions

Taken together, these data demonstrate that CTGF promotes cementum repair and cementogenesis through activation of the Cx43/β-catenin signalling axis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03149-8.

A Biphasic Feature of Gli1+-Mesenchymal Progenitors during Cementogenesis That Is Positively Controlled by Wnt/β-Catenin Signaling

Cementum, a specialized bony layer covering an entire molar root surface, anchors teeth into alveolar bone. Gli1, a key transcriptional activator in Hedgehog signaling, has been identified as a mesenchymal progenitor cell marker in various tissues, including the periodontal ligament (PDL). To address the mechanisms by which Gli1⁺ progenitor cells contribute to cementogenesis, we used the Gli1^lacZ/+ knock-in line to mark Gli1⁺ progenitors and the Gli1^CreERT2/+; R26R^tdTomato/+ line (named Gli1^Lin) to trace Gli1 progeny cells during cementogenesis. Our data unexpectedly displayed a biphasic feature of Gli1⁺ PDL progenitor cells and cementum growth: a negative relationship between Gli1⁺ progenitor cell number and cementogenesis but a positive correlation between Gli1-derived acellular and cellular cementoblast cell number and cementum growth. DTA-ablation of Gli1^Lin cells led to a cementum hypoplasia, including a significant reduction of both acellular and cellular cementoblast cells. Gain-of-function studies (by constitutive stabilization of β-catenin in Gli1^Lin cells) revealed a cementum hyperplasia. A loss of function (by conditional deletion of β-catenin in Gli1⁺ cells) resulted in a reduction of postnatal cementum growth. Together, our studies support a vital role of Gli1⁺ progenitor cells in contribution to both types of cementum, in which canonical Wnt/β-catenin signaling positively regulates the differentiation of Gli1⁺ progenitors to cementoblasts during cementogenesis.

Suppression of Hedgehog signaling is required for cementum apposition

Hedgehog (Hh) signaling plays a broad role in the development of many organs including bone and teeth. It is noted that sustained Hh activity in osteoblasts negatively regulates postnatal development in mice. However, it remains unknown whether Hh signaling contributes to cementum formation. In this study, to define the roles of Hh signaling in cementum formation, we analyzed two kinds of transgenic mouse models for Hh signaling activation designed by the inactivation of Suppressor of Fused (Sufu), a negative regulator of Hh signaling, (Sufu^OC) and a forced endogenous activation of Smo (SmoM2^OC) under the control of osteocalcin (OC) promoter-driven Cre recombinase. Interestingly, cellular cementum apposition was remarkably reduced in both mutants. Consistently, matrix formation and mineralization ability were down-regulated in OCCM-30, a cementoblast cell line, following treatment with a pharmaceutical Smo agonist. In addition, reductions in Osx expression and β-catenin activity, which are critical for cellular cementum formation, were also detected in vitro. Furthermore, the compound mutant mice designed for the stabilization of β-catenin with both Hh-Smo signaling activation in cementoblasts revealed a complete restoration of defective cellular cementum. In addition, Wnt antagonists such as Sostdc1 and Dkk1 were also induced by Smo activation and played a role in the reduction of Osx expression and β-catenin activity. Collectively, our data demonstrated that Hh signaling negatively regulates cementum apposition in a Wnt/β-catenin/Osx-dependent manner.

Axin2+-Mesenchymal PDL Cells, Instead of K14+ Epithelial Cells, Play a Key Role in Rapid Cementum Growth

To date, attempts to regenerate functional periodontal tissues (including cementum) are largely unsuccessful due to a lack of full understanding about the cellular origin (epithelial or mesenchymal cells) essential for root cementum growth. To address this issue, we first identified a rapid cementum growth window from the ages of postnatal day 28 (P28) to P56. Next, we showed that expression patterns of Axin2 and β-catenin within cementum-forming periodontal ligament (PDL) cells are negatively associated with rapid cementum growth. Furthermore, cell lineage tracing studies revealed that the Axin2+-mesenchymal PDL cells and their progeny rapidly expand and directly contribute to postnatal acellular and cellular cementum growth. In contrast, the number of K14+ epithelial cells, which were initially active at early stages of development, was reduced during rapid cementum formation from P28 to P56. The in vivo cell ablation of these Axin2+ cells using Axin2CreERT2/+; R26RDTA/+ mice led to severe cementum hypoplasia, whereas constitutive activation of β-catenin in the Axin2+ cells resulted in an acceleration in cellular cementogenesis plus a transition from acellular cementum to cellular cementum. Thus, we conclude that Axin2+-mesenchymal PDL cells, instead of K14+ epithelial cells, significantly contribute to rapid cementum growth.

Antagonistic interactions between osterix and pyrophosphate during cementum formation

During cementum formation, the key roles of osterix (Osx) and inorganic pyrophosphate (PPi), mainly controlled by nucleotide pyrophosphatase 1 (Npp1; encoded by the Enpp1 gene) and progressive ankylosis protein (Ank), have been demonstrated by animal models displaying altered cementum formation. In this study, we analyzed the relationship of Osx and local PPi during cementum formation using compound mutant mice with their wildtype and corresponding single gene mutants. Importantly, functional defects in PPi regulation led to the induction of Osx expression at the cervical cementum as demonstrated by Enpp1 mutant mice and cementoblasts with the retroviral transduction of small hairpin RNA for Enpp1 or Ank. Conversely, cementoblasts exposed to inorganic PPi or with the enforced expression of Enpp1 or Ank reduced Osx expression in a concentration-dependent manner. Furthermore, the loss of Osx induced the higher expression of Npp1 and Ank at the apical region of the developing tooth root as observed in Osx-deficient mice. The activity of PPi-generating ectoenzymes (nucleoside triphosphate pyrophosphohydrolase, NTPPPHase) and the level of extracellular PPi were significantly increased in Osx-knockdown cementoblasts. However, the formation of ectopic cervical cementum was not completely diminished by inactivation of Osx in Enpp1 mutant mice. In addition, fibroblast growth factor (FGF) receptor 1 (Fgfr1) was strongly localized in cementoblasts lining the acellular cementum and involved in the inhibitory regulation of matrix accumulation and further mineralization by supporting PPi production. Taken together, these results suggest that local PPi suppresses matrix accumulation and further mineralization through an antagonistic interaction with Osx under the synergistic influence of FGF signaling during cementum formation.

ENPP1 in the Regulation of Mineralization and Beyond

ENPP1 is well known for its role in regulating skeletal and soft tissue mineralization. It primarily exerts its function through the generation of pyrophosphate, a key inhibitor of hydroxyapatite formation. Several previous studies have suggested that ENPP1 also contributes to a range of human diseases including diabetes, cancer, cardiovascular disease, and osteoarthritis. In this review, we summarize the pathological roles of ENPP1 in mineralization and these soft tissue disorders. We also discuss the underlying mechanisms through which ENPP1 exerts its pathological effects. A fuller understanding of the pathways through which ENPP1 acts may help to develop novel therapeutic strategies for these commonly diagnosed morbidities.