Exploring the Origins of Cementoblasts and their Trigger Factors

Functional defects in cementoblasts with disrupted bone sialoprotein functional domains, in vitro

Bone sialoprotein (BSP) is a multifunctional extracellular matrix (ECM) protein present in bone and cementum. Global in vivo ablation of BSP leads to bone mineralization defects, lack of acellular cementum, and periodontal breakdown. BSP harbors three main functional domains: N-terminal collagen-binding domain, hydroxyapatite-nucleating domain, and C-terminal RGD integrin-binding signaling domain. How each of these domains contributes to BSP function(s) is not understood. We hypothesized that collagen-binding and RGD domains play distinct roles in cementoblast functions. Three CRISPR/Cas9 gene-edited cell lines were derived from control wild-type (WT) OCCM.30 murine immortalized cementoblasts: 1) deletion of the N-terminus of BSP after signal peptide, including entire collagen binding domain (Ibsp∆N-Term); 2) deletion of exon 4 (majority of collagen-binding domain; Ibsp∆Ex4); and 3) deletion of C-terminus of BSP including the integrin binding RGD domain (Ibsp∆C-Term). Compared to WT, Ibsp∆Ex4 and Ibsp∆C-Term cell lines showed reduced BSP secretion, in vitro. Abnormal cell morphology was observed in all mutant cell lines, with Ibsp∆C-Term showing highly disorganized cytoskeleton. All mutant cell lines showed significantly lower cell proliferation compared to WT at all timepoints. Ibsp∆N-Term cells showed reduced cell migration by 24 h. All mutants exhibited over 50 % significant reduced mineralization at days 6 and 10. While WT cells were largely unaffected by seeding density, mutant cells failed to mineralize at lower cell density. Mutant cell lines diverged from WT and from each other by dysregulated expression in 23 genes involved in mineralization, ECM, and cell signaling. In summary, disabling BSP functional domains led to profound and distinct changes in cementoblast cell functions, especially dysregulated gene expression and reduced mineralization, in a way did not align with a straightforward narrative where each functional domain caused specific, expected differences. Instead, the study uncovered a significant level of complexity in how different mutant forms of BSP affected cell functions, in vitro.

Overlapping functions of bone sialoprotein and pyrophosphate regulators in directing cementogenesis

Although acellular cementum is essential for tooth attachment, factors directing its development and regeneration remain poorly understood. Inorganic pyrophosphate (PP_i), a mineralization inhibitor, is a key regulator of cementum formation: tissue-nonspecific alkaline phosphatase (Alpl/TNAP) null mice (increased PP_i) feature deficient cementum, while progressive ankylosis protein (Ank/ANK) null mice (decreased PP_i) feature increased cementum. Bone sialoprotein (Bsp/BSP) and osteopontin (Spp1/OPN) are multifunctional extracellular matrix components of cementum proposed to have direct and indirect effects on cell activities and mineralization. Studies on dentoalveolar development of Bsp knockout (Bsp^-/-) mice revealed severely reduced acellular cementum, however underlying mechanisms remain unclear. The similarity in defective cementum phenotypes between Bsp^-/- mice and Alpl^-/- mice (the latter featuring elevated PP_i and OPN), prompted us to examine whether BSP is operating by modulating PP_i-associated genes. Genetic ablation of Bsp caused a 2-fold increase in circulating PP_i, altered mRNA expression of Alpl, Spp1, and Ank, and increased OPN protein in the periodontia. Generation of a Bsp knock-out (KO) cementoblast cell line revealed significantly decreased mineralization capacity, 50% increased PP_i in culture media, and increased Spp1 and Ank mRNA expression. While addition of 2μg/ml recombinant BSP altered Spp1, Ank, and Enpp1 expression in cementoblasts, changes resulting from this dose were not dependent on the integrin-binding RGD motif or MAPK/ERK signaling pathway. Decreasing PP_i by genetic ablation of Ank on the Bsp^-/- mouse background reestablished cementum formation, allowing >3-fold increased acellular cementum volume compared to wild-type (WT). However, deleting Ank did not fully compensate for the absence of BSP. Bsp^-/-; Ank^-/- double-deficient mice exhibited mean 20-27% reduced cementum thickness and volume compared to Ank^-/- mice. From these data, we conclude that the perturbations in PP_i metabolism are not solely driving the cementum pathology in Bsp^-/- mice, and that PP_i is more potent than BSP as a cementum regulator, as shown by the ability to override loss of BSP by lowering PP_i. We propose that BSP and PP_i work in concert to direct mineralization in cementum and likely other mineralized tissues.

Intermittent parathyroid hormone (PTH) promotes cementogenesis and alleviates the catabolic effects of mechanical strain in cementoblasts

Background

External root resorption, commonly starting from cementum, is a severe side effect of orthodontic treatment. In this pathological process and repairing course followed, cementoblasts play a significant role. Previous studies implicated that parathyroid hormone (PTH) could act on committed osteoblast precursors to promote differentiation, and inhibit apoptosis. But little was known about the role of PTH in cementoblasts. The purpose of this study was to investigate the effects of intermittent PTH on cementoblasts and its influence after mechanical strain treatment.

Results

Higher levels of cementogenesis- and differentiation-related biomarkers (bone sialoprotein (BSP), osteocalcin (OCN), Collagen type I (COL1) and Osterix (Osx)) were shown in 1–3 cycles of intermittent PTH treated groups than the control group. Additionally, intermittent PTH increased alkaline phosphatase (ALP) activity and mineralized nodules formation, as measured by ALP staining, quantitative ALP assay, Alizarin red S staining and quantitative calcium assay. The morphology of OCCM-30 cells changed after mechanical strain exertion. Expression of BSP, ALP, OCN, osteopontin (OPN) and Osx was restrained after 18 h mechanical strain. Furthermore, intermittent PTH significantly increased the expression of cementogenesis- and differentiation-related biomarkers in mechanical strain treated OCCM-30 cells.

Conclusions

Taken together, these data suggested that intermittent PTH promoted cementum formation through activating cementogenesis- and differentiation-related biomarkers, and attenuated the catabolic effects of mechanical strain in immortalized cementoblasts OCCM-30.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-017-0133-0) contains supplementary material, which is available to authorized users.

Early Healing Events after Periodontal Surgery: Observations on Soft Tissue Healing, Microcirculation, and Wound Fluid Cytokine Levels

Early wound healing after periodontal surgery with or without enamel matrix derivative/biphasic calcium phosphate (EMD/BCP) was characterized in terms of soft tissue closure, changes of microcirculation, and expression of pro- and anti-inflammatory cytokines in gingival crevicular fluid/wound fluid (GCF/WF). Periodontal surgery was carried out in 30 patients (18 patients: application of EMD/BCP for regeneration of bony defects; 12 patients: surgical crown lengthening (SCL)). Healthy sites were observed as untreated controls. GCF/WF samples were collected during two post-surgical weeks. Flap microcirculation was measured using laser Doppler flowmetry (LDF). Soft tissue healing was evaluated after two weeks. GCF/WF levels of interleukin 1β (IL-1β), tumour necrosis factor (TNF-α), IL-6, and IL-10 were determined using a multiplex immunoassay. Surgery caused similar reductions of flap microcirculation followed by recovery within two weeks in both EMD/BCP and SCL groups. GCF/WF and pro-inflammatory cytokine levels were immediately increased after surgery, and returned only partially to baseline levels within the two-week observation period. Levels of IL-10 were temporarily reduced in all surgical sites. Flap dehiscence caused prolonged elevated levels of GCF/WF, IL-1β, and TNF-α. These findings show that periodontal surgery triggers an immediate inflammatory reaction corresponding to the early inflammatory phase of wound healing, and these inflammation measures are temporary in case of maintained closure of the flap. However, flap dehiscence causes prolonged inflammatory exudation from the periodontal wound. If the biological pre-conditions for periodontal wound healing are considered important for the clinical outcome, care should be taken to maintain primary closure of the flap.

In vitro models for evaluation of periodontal wound healing/regeneration

Periodontal wound healing and regeneration are highly complex processes, involving cells, matrices, molecules and genes that must be properly choreographed and orchestrated. As we attempt to understand and influence these clinical entities, we need experimental models to mimic the various aspects of human wound healing and regeneration. In vivo animal models that simulate clinical situations of humans can be costly and cumbersome. In vitro models have been devised to dissect wound healing/regeneration processes into discrete, analyzable steps. For soft tissue (e.g. gingival) healing, in vitro models range from simple culture of cells grown in monolayers and exposed to biological modulators or physical effectors and materials, to models in which cells are 'injured' by scraping and subsequently the 'wound' is filled with new or migrating cells, to three-dimensional models of epithelial-mesenchymal recombination or tissue explants. The cells employed are gingival keratinocytes, fibroblasts or endothelial cells, and their proliferation, migration, attachment, differentiation, survival, gene expression, matrix production or capillary formation are measured. Studies of periodontal regeneration also include periodontal ligament fibroblasts or progenitors, osteoblasts or osteoprogenitors, and cementoblasts. Regeneration models measure cellular proliferation, attachment and migration, as well as gene expression, transfer and differentiation into a mineralizing phenotype and biomineralization. Only by integrating data from models on all levels (i.e. a single cell to the whole organism) can various critical aspects of periodontal wound healing/regeneration be fully evaluated.

Counter-regulatory phosphatases TNAP and NPP1 temporally regulate tooth root cementogenesis

Cementum is critical for anchoring the insertion of periodontal ligament fibers to the tooth root. Several aspects of cementogenesis remain unclear, including differences between acellular cementum and cellular cementum, and between cementum and bone. Biomineralization is regulated by the ratio of inorganic phosphate (P_i) to mineral inhibitor pyrophosphate (PP_i), where local P_i and PP_i concentrations are controlled by phosphatases including tissue-nonspecific alkaline phosphatase (TNAP) and ectonucleotide pyrophosphatase/phosphodiesterase 1 (NPP1). The focus of this study was to define the roles of these phosphatases in cementogenesis. TNAP was associated with earliest cementoblasts near forming acellular and cellular cementum. With loss of TNAP in the Alpl null mouse, acellular cementum was inhibited, while cellular cementum production increased, albeit as hypomineralized cementoid. In contrast, NPP1 was detected in cementoblasts after acellular cementum formation, and at low levels around cellular cementum. Loss of NPP1 in the Enpp1 null mouse increased acellular cementum, with little effect on cellular cementum. Developmental patterns were recapitulated in a mouse model for acellular cementum regeneration, with early TNAP expression and later NPP1 expression. In vitro, cementoblasts expressed Alpl gene/protein early, whereas Enpp1 gene/protein expression was significantly induced only under mineralization conditions. These patterns were confirmed in human teeth, including widespread TNAP, and NPP1 restricted to cementoblasts lining acellular cementum. These studies suggest that early TNAP expression creates a low PP_i environment promoting acellular cementum initiation, while later NPP1 expression increases PP_i, restricting acellular cementum apposition. Alterations in PP_i have little effect on cellular cementum formation, though matrix mineralization is affected.

Prospective potency of TGF-β1 on maintenance and regeneration of periodontal tissue

Periodontal ligament (PDL) tissue, central in the periodontium, plays crucial roles in sustaining tooth in the bone socket. Irreparable damages of this tissue provoke tooth loss, causing a decreased quality of life. The question arises as to how PDL tissue is maintained or how the lost PDL tissue can be regenerated. Stem cells included in PDL tissue (PDLSCs) are widely accepted to have the potential to maintain or regenerate the periodontium, but PDLSCs are very few in number. In recent studies, undifferentiated clonal human PDL cell lines were developed to elucidate the applicable potentials of PDLSCs for the periodontal regenerative medicine based on cell-based tissue engineering. In addition, it has been suggested that transforming growth factor-beta 1 is an eligible factor for the maintenance and regeneration of PDL tissue.

Central role of pyrophosphate in acellular cementum formation

BACKGROUND

Inorganic pyrophosphate (PP(i)) is a physiologic inhibitor of hydroxyapatite mineral precipitation involved in regulating mineralized tissue development and pathologic calcification. Local levels of PP(i) are controlled by antagonistic functions of factors that decrease PP(i) and promote mineralization (tissue-nonspecific alkaline phosphatase, Alpl/TNAP), and those that increase local PP(i) and restrict mineralization (progressive ankylosis protein, ANK; ectonucleotide pyrophosphatase phosphodiesterase-1, NPP1). The cementum enveloping the tooth root is essential for tooth function by providing attachment to the surrounding bone via the nonmineralized periodontal ligament. At present, the developmental regulation of cementum remains poorly understood, hampering efforts for regeneration. To elucidate the role of PP(i) in cementum formation, we analyzed root development in knock-out ((-/-)) mice featuring PP(i) dysregulation.

RESULTS

Excess PP(i) in the Alpl(-/-) mouse inhibited cementum formation, causing root detachment consistent with premature tooth loss in the human condition hypophosphatasia, though cementoblast phenotype was unperturbed. Deficient PP(i) in both Ank and Enpp1(-/-) mice significantly increased cementum apposition and overall thickness more than 12-fold vs. controls, while dentin and cellular cementum were unaltered. Though PP(i) regulators are widely expressed, cementoblasts selectively expressed greater ANK and NPP1 along the root surface, and dramatically increased ANK or NPP1 in models of reduced PP(i) output, in compensatory fashion. In vitro mechanistic studies confirmed that under low PP(i) mineralizing conditions, cementoblasts increased Ank (5-fold) and Enpp1 (20-fold), while increasing PP(i) inhibited mineralization and associated increases in Ank and Enpp1 mRNA.

CONCLUSIONS

Results from these studies demonstrate a novel developmental regulation of acellular cementum, wherein cementoblasts tune cementogenesis by modulating local levels of PP(i), directing and regulating mineral apposition. These findings underscore developmental differences in acellular versus cellular cementum, and suggest new approaches for cementum regeneration.

Enamel matrix derivative: a review of cellular effects in vitro and a model of molecular arrangement and functioning

BACKGROUND

Enamel matrix derivative (EMD), the active component of Emdogain®, is a viable option in the treatment of periodontal disease owing to its ability to regenerate lost tissue. It is believed to mimic odontogenesis, though the details of its functioning remain the focus of current research.

OBJECTIVE

The aim of this article is to review all relevant literature reporting on the composition/characterization of EMD as well as the effects of EMD, and its components amelogenin and ameloblastin, on the behavior of various cell types in vitro. In this way, insight into the underlying mechanism of regeneration will be garnered and utilized to propose a model for the molecular arrangement and functioning of EMD.

METHODS

A review of in vitro studies of EMD, or components of EMD, was performed using key words "enamel matrix proteins" OR "EMD" OR "Emdogain" OR "amelogenin" OR "ameloblastin" OR "sheath proteins" AND "cells." Results of this analysis, together with current knowledge on the molecular composition of EMD and the structure and regulation of its components, are then used to present a model of EMD functioning.

RESULTS

Characterization of the molecular composition of EMD confirmed that amelogenin proteins, including their enzymatically cleaved and alternatively spliced fragments, dominate the protein complex (>90%). A small presence of ameloblastin has also been reported. Analysis of the effects of EMD indicated that gene expression, protein production, proliferation, and differentiation of various cell types are affected and often enhanced by EMD, particularly for periodontal ligament and osteoblastic cell types. EMD also stimulated angiogenesis. In contrast, EMD had a cytostatic effect on epithelial cells. Full-length amelogenin elicited similar effects to EMD, though to a lesser extent. Both the leucine-rich amelogenin peptide and the ameloblastin peptides demonstrated osteogenic effects. A model for molecular structure and functioning of EMD involving nanosphere formation, aggregation, and dissolution is presented.

CONCLUSIONS

EMD elicits a regenerative response in periodontal tissues that is only partly replicated by amelogenin or ameloblastin components. A synergistic effect among the various proteins and with the cells, as well as a temporal effect, may prove important aspects of the EMD response in vivo.

Use of microarrays to find novel regulators of periodontal ligament fibroblast differentiation

Periodontal regeneration requires the coordinated movement and differentiation of several cell types in order to re-establish the cementum, periodontal ligament (PDL), and alveolar bone. Cells in culture are often used as model systems for mature tissues, although they may represent expanded progenitor cell populations. Comparison of transcript expression between fresh PDL tissue and PDL cell isolates by MicroArray analysis has revealed numerous molecular differences. Several transcripts (including alkaline phosphatase, bone sialoprotein, periostin, and fibromodulin) are expressed at higher levels in fresh PDL than in cultured PDL cells. In contrast, PDL cells in culture selectively express a variety of growth factors. Several of these growth factors alter PDL fibroblast behavior. Two members of the transforming growth factor beta family of growth factors, namely, bone morphogenic protein-7 (BMP7) and growth differentiation factor-5 (GDF5), reduce cell proliferation and Stro-1 expression (a bone marrow stromal stem cell marker), whereas only BMP7 induces alkaline phosphatase activity. In contrast, fibroblast growth factor-5 induces enhanced cell proliferation and Stro-1 expression, while repressing alkaline phosphatase activity. The stimulation of PDL cells to differentiate (either by BMP7 or GDF5) inhibits cell motility. Thus, PDL cells in culture are regulated by several factors that differentially stimulate a mineralized (cementoblast-like) fate, a non-mineralized fate (mature fibroblasts), or the propagation of a more naive phenotype (potential progenitors).

Defining the roots of cementum formation

Significant progress has been seen in research aimed at regeneration of the disease-damaged periodontium. Our own strategy has been to approach periodontal tissue development (i.e. root, cementum, periodontal ligament, and bone) as a source for the identification of key regulators of cellular processes that may be applicable to periodontal tissue repair. Specifically, enamel-like molecules, bone morphogenetic proteins (BMPs), and phosphates have been investigated for their role in altering gene expression and cell functions in follicle cells, periodontal ligament cells, and cementoblasts. Amelogenin, leucine-rich amelogenin peptide, and tyrosine-rich amelogenin peptide have been found to similarly affect cementoblast gene expression and cementoblast-mediated mineralization in vitro; however, these enamel-like factors do not increase cell proliferation as has been observed in cells treated with Emdogain (Biora AB, Malmö, Sweden), an enamel matrix derivative. BMP-2 has been found to promote differentiation of follicle cells into a cementoblast/osteoblast phenotype, and BMP-3 is being investigated as a negative regulator of mineralization. The increased ratio of phosphate to pyrophosphate in the local region during root development has been found to significantly enhance the extent of cementum formation in animal models. Furthermore, phosphate has been identified as a regulator of cementoblast SIBLING (small integrin-binding ligand N-linked glycoprotein) gene expression in vitro. These investigations of candidate factors for periodontal regeneration have uncovered mechanisms regulating gene expression and cell function in cells controlling the behavior of periodontal tissues (i.e. follicle cells, periodontal cells, and cementoblasts) and offer new directions to consider for clinical repair of periodontal defects.

Position Paper: Periodontal Regeneration

Untreated periodontal disease leads to tooth loss through destruction of the attachment apparatus and tooth-supporting structures. The goals of periodontal therapy include not only the arrest of periodontal disease progression,but also the regeneration of structures lost to disease where appropriate. Conventional surgical approaches (e.g., flap debridement) continue to offer time-tested and reliable methods to access root surfaces,reduce periodontal pockets, and attain improved periodontal form/architecture. However, these techniques offer only limited potential towards recovering tissues destroyed during earlier disease phases. Recently, surgical procedures aimed at greater and more predictable regeneration of periodontal tissues and functional attachment close to their original level have been developed, analyzed, and employed in clinical practice. This paper provides a review of the current understanding of the mechanisms, cells, and factors required for regeneration of the periodontium and of procedures used to restore periodontal tissues around natural teeth. Targeted audiences for this paper are periodontists and/or researchers with an interest in improving the predictability of regenerative procedures. This paper replaces the version published in 1993.

Establishment of cementoblast cell lines from rat cementum lining cells by transfection with temperature-sensitive simian virus-40 T-antigen gene

Defining the regulatory mechanisms promoting differentiation and proliferation of cementoblasts has not been well understood, because of the lack of cell models in vitro. To establish an in vitro cell model for the cementoblasts, extracted rat molars obtained from 8-week-old rats were used. Cells lining the root surface (cemetoblasts) were obtained by an enzymatic digestion method, and immediately immortalized by transfection of thermolabile SV40 T-antigen gene. The transfected cementum lining cell clones, RCM-C3 and -C4, were maintained for more than 200 population doublings (PD), while the original cells stopped their growth at 60 PD. Thus, immortalized cell lines decreased expression of SV40 T-antigen and subsequently cell proliferation at non-permissive temperature (39 degrees C). Reverse-transcribed-polymerase chain reaction indicated expression of gene for type I collagen, alkaline phosphatase (ALP), osteopontin, and osteocalcin mRNA at both permissive (33 degrees C) and non-permissive (39 degrees C) temperatures. RCM-C4 expressed higher bone siaploprotein (BSP) mRNA than RCM-C3, and further RCM-C4 showed higher BSP mRNA at 39 degrees C than 33 degrees C. High ALP activity and mineralized nodule formation were observed at 39 degrees C in both cell lines. These findings suggested that the cell lines, RCM-C3 and -C4, are useful model for studying the regulatory mechanisms of differentiation and proliferation of cementoblasts.