Normal human cementum-derived cells: isolation, clonal expansion, and in vitro and in vivo characterization

Differences in the stemness characteristics and molecular markers of distinct human oral tissue neural crest-derived multilineage cells

Objectives

Although multilineage cells derived from oral tissues, especially the dental pulp, apical papilla, periodontal ligament, and oral mucosa, have neural crest‐derived stem cell (NCSC)‐like properties, the differences in the characteristics of these progenitor cell compartments remain unknown. The current study aimed to elucidate these differences.

Material and methods

Sphere‐forming apical papilla‐derived cells (APDCs), periodontal ligament‐derived cells (PDLDCs), and oral mucosa stroma‐derived cells (OMSDCs) from the same individuals were isolated from impacted developing teeth. All sphere‐forming cells were characterized through biological analyses of stem cells.

Results

All sphere‐forming cells expressed neural crest‐related markers. The expression of certain tissue‐specific markers such as CD24 and CD56 (NCAM1) differed among tissue‐derived cells. Surprisingly, the expression of only CD24 and CD56 could be discriminated in human tissues. Although APDCs and PDLDCs exhibited greater mineralized cell differentiation than OMSDCs, they exhibited poorer differentiation into adipocytes in vitro. In immunocompromised mice, APDCs formed hard tissues better than PDLDCs and OMSDCs.

Conclusions

Although cells with NCSC‐like properties present the same phenotype, they differ in the expression of certain markers and differentiation abilities. This study is the first to demonstrate the differences in the differentiation ability and molecular markers among multilineage human APDCs, PDLDCs, and OMSDCs obtained from the same patients, and to identify tissue‐specific markers that distinguish tissues in the developing stage of the human tooth with immature apex.

Identification of stemness and differentially expressed genes in human cementum-derived cells

Purpose

Periodontal treatment aims at complete regeneration of the periodontium, and developing strategies for periodontal regeneration requires a deep understanding of the tissues composing the periodontium. In the present study, the stemness characteristics and gene expression profiles of cementum-derived cells (CDCs) were investigated and compared with previously established human stem cells. Candidate marker proteins for CDCs were also explored.

Methods

Periodontal ligament stem cells (PDLSCs), pulp stem cells (PULPSCs), and CDCs were isolated and cultured from extracted human mandibular third molars. Human bone marrow stem cells (BMSCs) were used as a positive control. To identify the stemness of CDCs, cell differentiation (osteogenic, adipogenic, and chondrogenic) and surface antigens were evaluated through flow cytometry. The expression of cementum protein 1 (CEMP1) and cementum attachment protein (CAP) was investigated to explore marker proteins for CDCs through reverse-transcription polymerase chain reaction. To compare the gene expression profiles of the 4 cell types, mRNA and miRNA microarray analysis of 10 samples of BMSCs (n=1), PDLSCs (n=3), PULPSCs (n=3), and CDCs (n=3) were performed.

Results

The expression of mesenchymal stem cell markers with a concomitant absence of hematopoietic markers was observed in PDLSCs, PULPSCs, CDCs and BMSCs. All 4 cell populations also showed differentiation into osteogenic, adipogenic, and chondrogenic lineages. CEMP1 was strongly expressed in CDCs, while it was weakly detected in the other 3 cell populations. Meanwhile, CAP was not found in any of the 4 cell populations. The mRNA and miRNA microarray analysis showed that 14 mRNA genes and 4 miRNA genes were differentially expressed in CDCs vs. PDLSCs and PULPSCs.

Conclusions

Within the limitations of the study, CDCs seem to have stemness and preferentially express CEMP1. Moreover, there were several up- or down-regulated genes in CDCs vs. PDLSCs, PULPSCs, and BMSCs and these genes could be candidate marker proteins of CDCs.

Establishment of in vitro three-dimensional cementocyte differentiation scaffolds to study orthodontic root resorption

Orthodontic-induced root resorption is a severe side effect that can lead to tooth root shortening and loss. Compressive force induces tissue stress in the cementum that covers the tooth root, which is associated with activation of bone metabolism and cementum resorption. To investigate the role of cementocytes in mechanotransduction and osteoclast differentiation, the present study established an in vitro three-dimensional (3D) model replicating cellular cementum and observed the effects of static compression on the cellular behavior of the cementocytes. Cell Counting Kit-8 assay, alkaline phosphatase staining and dentin matrix protein 1 quantification were used to evaluate the cementocyte differentiation in the 3D scaffolds. Cellular viability under static compression was evaluated using live/dead staining, and expression of mineral metabolism-related genes were analyzed via reverse transcription-quantitative PCR. The results suggested that the cementocytes maintained their phenotype and increased the expression of osteoprotegerin (OPG), receptor activator of NF-κB ligand (RANKL) and sclerostin (SOST) in the 3D model compared with cells cultured in two dimensions. Compression force increased cell death and induced osteoclastic differentiation via the upregulation of SOST and RANKL/OPG ratio, and the downregulation of osteocalcin. The effect of compression showed a force magnitude-dependent pattern. The present study established an in vitro model of cellular cementum to study the biology of cementocytes. The results indicated that cementocytes are sensitive to mechanical loading and may serve potential roles in the metabolic regulation of minerals during orthodontic root resorption. These findings provide a novel tool to study biological processes in the field of orthodontics and expand knowledge of the biological function of cementocytes.

Cellular therapy in periodontal regeneration

Periodontitis is a chronic inflammatory condition leading to destruction of the tooth supporting tissues, which if left untreated may cause tooth loss. The treatment of periodontitis mainly aims to arrest the inflammatory process by infection control measures, although in some specific lesions a limited periodontal regeneration can also be attained. Current regenerative approaches are aimed to guide the cells with regenerative capacity to repopulate the lesion and promote new cementum and new connective tissue attachment. The first phase in periodontal tissue regeneration involves the differentiation of mesenchymal cells into cementoblasts to promote new cementum, thus facilitating the attachment of new periodontal ligament fibers to the root and the alveolar bone. Current regenerative approaches limit themselves to the confines of the lesion by promoting the self-regenerative potential of periodontal tissues. With the advent of bioengineered therapies, several studies have investigated the potential use of cell therapies, mainly the use of undifferentiated mesenchymal cells combined with different scaffolds. The understanding of the origin and differentiation patterns of these cells is, therefore, important to elucidate their potential therapeutic use and their comparative efficacy with current technologies. This paper aims to review the in vitro and experimental studies using cell therapies based on application of cementoblasts and mesenchymal stem cells isolated from oral tissues when combined with different scaffolds.

Are Cementoblasts a Subpopulation of Osteoblasts or a Unique Phenotype?

Experimental studies have shown a great potential for periodontal regeneration. The limitations of periodontal regeneration largely depend on the regenerative potential at the root surface. Cellular intrinsic fiber cementum (CIFC), so-called bone-like tissue, may form instead of the desired acellular extrinsic fiber cementum (AEFC), and the interfacial tissue bonding may be weak. The periodontal ligament harbors progenitor cells that can differentiate into periodontal ligament fibroblasts, osteoblasts, and cementoblasts, but their precise location is unknown. It is also not known whether osteoblasts and cementoblasts arise from a common precursor cell line, or whether distinct precursor cell lines exist. Thus, there is limited knowledge about how cell diversity evolves in the space between the developing root and the alveolar bone. This review supports the hypothesis that AEFC is a unique tissue, while CIFC and bone share some similarities. Morphologically, functionally, and biochemically, however, CIFC is distinctly different from any bone type. There are several lines of evidence to propose that cementoblasts that produce both AEFC and CIFC are unique phenotypes that are unrelated to osteoblasts. Cementum attachment protein appears to be cementum-specific, and the expression of two proteoglycans, fibromodulin and lumican, appears to be stronger in CIFC than in bone. A theory is presented that may help explain how cell diversity evolves in the periodontal ligament. It proposes that Hertwig’s epithelial root sheath and cells derived from it play an essential role in the development and maintenance of the periodontium. The role of enamel matrix proteins in cementoblast and osteoblast differentiation and their potential use for tissue engineering are discussed.

Targeting and Immobilization of Bioactive Peptides on Dentin Matrix

The regeneration of structurally/functionally competent tooth root cementum is a critical step for the successful restoration of periodontal attachment. In this study, we tested whether a poly-glutamic acid-rich domain and glutamine-containing transglutaminase substrate can be used to target biologically active peptides to the mineralized root matrix and to bind such peptides covalently to the organic matrix. As a biologically active model molecule, the integrin-binding motif, RGD, was used. The effects of immobilization of such synthetic peptides to the dentin matrix on cementoblastic adhesion in vitro and cementogenesis in vivo were studied. In vitro, cementoblastic adhesion improved significantly when the dentin surface contained covalently bound peptides. In vivo, this bound peptide significantly increased cementum formation compared with that attained in control conditions. Transglutaminase-catalyzed covalent binding of bioactive peptides targeted to mineralized collagenous dentin matrix via the poly-glutamate domain can be readily achieved. This approach offers potential for clinical use in periodontal regeneration.

Redefining the induction of periodontal tissue regeneration in primates by the osteogenic proteins of the transforming growth factor-β supergene family

The molecular bases of periodontal tissue induction and regeneration are the osteogenic proteins of the transforming growth factor-β (TGF-β) supergene family. These morphogens act as soluble mediators for the induction of tissues morphogenesis sculpting the multicellular mineralized structures of the periodontal tissues with functionally oriented ligament fibers into newly formed cementum. Human TGF-β₃ (hTGF-β₃ ) in growth factor-reduced Matrigel^® matrix induces cementogenesis when implanted in class II mandibular furcation defects surgically prepared in the non-human primate Chacma baboon, Papio ursinus. The newly formed periodontal ligament space is characterized by running fibers tightly attached to the cementoid surface penetrating as mineralized constructs within the newly formed cementum assembling and initiating within the mineralized dentine. Angiogenesis heralds the newly formed periodontal ligament space, and newly sprouting capillaries are lined by cellular elements with condensed chromatin interpreted as angioblasts responsible for the rapid and sustained induction of angiogenesis. The inductive activity of hTGF-β₃ in Matrigel^® matrix is enhanced by the addition of autogenous morcellated fragments of the rectus abdominis muscle potentially providing myoblastic, pericytic/perivascular stem cells for continuous tissue induction and morphogenesis. The striated rectus abdominis muscle is endowed with stem cell niches in para/perivascular location, which can be dominant, thus imposing stem cell features or stemness to the surrounding cells. This capacity to impose stemness is morphologically shown by greater alveolar bone induction and cementogenesis when hTGF-β₃ in Matrigel^® matrix is combined with morcellated fragments of autogenous rectus abdominis muscle. The induction of periodontal tissue morphogenesis develops as a mosaic structure in which the osteogenic proteins of the TGF-β supergene family singly, synergistically and synchronously initiate and maintain tissue induction and morphogenesis. In primates, the presence of several homologous yet molecularly different isoforms with osteogenic activity highlights the biological significance of this apparent redundancy and indicates multiple interactions during embryonic development and bone regeneration in postnatal life. Molecular redundancy with associated different biological functionalities in primate tissues may simply represent the fine-tuning of speciation-related molecular evolution in anthropoid apes at the early Pliocene boundary, which resulted in finer tuning of the bone induction cascade.

IL-1β and compressive forces lead to a significant induction of RANKL-expression in primary human cementoblasts

AIM

The aim of this study was to investigate the response of primary human cementoblasts to conditions as they occur on the pressure side during orthodontic tooth movement.

METHODS

In our previous study, the cementoblasts were characterized using markers for osteoblastogenic differentiation and the cementoblast-specific marker CEMP-1. Initially, primary human cementoblasts were compressed for 1 h, 4 h, and 6 h (30 g/cm(2)). In the second experiment, the cementoblasts were stimulated with interleukin (IL)-1β for 24 h and for 96 h with 1 ng/ml and 10 ng/ml and subsequently compressed for 1 h and 6 h. Changes in mRNA expression for receptor activator of NF-κB (RANK), RANK ligand (RANKL), osteoprotegerin (OPG), and cyclooxygenase-2 (COX-2) were measured by quantitative real-time polymerase chain reaction (RT-PCR). RANK and RANKL were also examined by immunocytochemical staining at the protein level.

RESULTS

Compression (30 g/cm(2)) led to a significant increase in RANKL expression after 6 h. OPG expression in compressed cementoblasts was significantly reduced after 1 h. RANK remained unchanged during the course of the experiment. Stimulation with IL-1β induced RANKL and OPG expression. However, IL-1β-dependent induction of RANKL was more prominent than the induction of OPG, leading to a (significant) increase in the RANKL/OPG ratios. The expression of RANK remained unchanged after 24 h of stimulation with IL-1β and decreased significantly after 96 h. Compression of the prestimulated cells resulted in a further increase in RANKL expression significant after 6 h. OPG and RANK expression remained unchanged compared to the unstimulated sample. COX-2 increased significantly after both compression and stimulation with IL-1β. Combined stimulation and compression resulted in a significant further increase after 6 h compared to IL-1β stimulation alone.

CONCLUSION

Primary human cementoblasts in vitro express increased levels of RANKL, in particular during the combination of inflammation and compression. The increase in RANKL expression is not compensated by an increase in OPG expression. The induction of RANKL expression was associated with a significant increase in COX-2 expression. Since RANKL attracts osteoclasts, its increase might be associated with the progression of root resorption. The in vitro alterations in cementoblasts we observed may be indicators of cellular mechanisms that lead to the increased root resorption during orthodontic treatment.

Stem cell-delivery therapeutics for periodontal tissue regeneration

Periodontitis, an inflammatory disease, is the most common cause of tooth loss in adults. Attempts to regenerate the complex system of tooth-supporting apparatus (i.e., the periodontal ligament, alveolar bone and root cementum) after loss/damage due to periodontitis have made some progress recently and provide a useful experimental model for the evaluation of future regenerative therapies. Concentrated efforts have now moved from the use of guided tissue/bone regeneration technology, a variety of growth factors and various bone grafts/substitutes toward the design and practice of endogenous regenerative technology by recruitment of host cells (cell homing) or stem cell-based therapeutics by transplantation of outside cells to enhance periodontal tissue regeneration and its biomechanical integration. This shift is driven by the general inability of conventional therapies to deliver satisfactory outcomes, particularly in cases where the disease has caused large tissue defects in the periodontium. Cell homing and cell transplantation are both scientifically meritorious approaches that show promise to completely and reliably reconstitute all tissue and connections damaged through periodontal disease, and hence research into both directions should continue. In view of periodontal regeneration by paradigms that unlock the body's innate regenerative potential has been reviewed elsewhere, this paper specifically explores and analyses the stem cell types and cell delivery strategies that have been or have the potential to be used as therapeutics in periodontal regenerative medicine, with particular emphasis placed on the efficacy and safety concerns of current stem cell-based periodontal therapies that may eventually enter into the clinic.

Future dentistry: cell therapy meets tooth and periodontal repair and regeneration

Cell-based tissue repair of the tooth and – tooth-supporting – periodontal ligament (PDL) is a new attractive approach that complements traditional restorative or surgical techniques for replacement of injured or pathologically damaged tissues. In such therapeutic approaches, stem cells and/or progenitor cells are manipulated in vitro and administered to patients as living and dynamic biological agents. In this review, we discuss the clonogenic potential of human dental and periodontal tissues such as the dental pulp and the PDL and their potential for tooth and periodontal repair and/or regeneration. We propose novel therapeutic approaches using stem cells or progenitor cells, which are targeted to regenerate the lost dental or periodontal tissue.

Periodontal regeneration following implantation of cementum and periodontal ligament-derived cells

BACKGROUND AND OBJECTIVE

The periodontal regeneration of bone defects is often unsatisfactory and could be largely improved by cell therapy. Therefore, the purpose of this study was to evaluate the regenerative potential of implanting canine cementum-derived cells (CDCs) and canine periodontal ligament-derived cells (PDLDCs) in experimentally created periodontal intrabony defects in beagle dogs.

MATERIAL AND METHODS

Cells were obtained from premolars extracted from four beagle dogs. Three-wall intrabony periodontal defects, 3 mm wide and 4 mm deep, were surgically created in their second and fourth premolars and plaque was allowed to accumulate. Once the defects were surgically debrided, periodontal regeneration was attempted by random implantation of collagen sponges embedded with 750,000 CDCs, 750,000 PDLDCs or culture medium. After 3 mo of healing, specimens were obtained and periodontal regenerative outcomes were assessed histologically and histometrically.

RESULTS

The histological analysis showed that a minimal amount of new cementum was formed in the control group (1.56 ± 0.39 mm), whereas in both test groups, significantly higher amounts of new cementum were formed (3.98 ± 0.59 mm in the CDC group and 4.07 ± 0.97 mm in the PDLDC group). The test groups also demonstrated a larger dimension of new connective tissue, resulting in a significantly more coronal level of histological attachment.

CONCLUSION

This proof-of-principle study suggests that cellular therapy, in combination with a collagen sponge, promoted periodontal regeneration in experimental intrabony periodontal defects.

Differentiation of neural-crest-derived intermediate pluripotent progenitors into committed periodontal populations involves unique molecular signature changes, cohort shifts, and epigenetic modifications

Intermediate progenitor populations play a crucial role in the regional specification and differentiation of the cranial neural crest. On the basis of global gene expression profiles, gene cohort expression levels, and epigenetic modifications, we have defined key factors involved in the differentiation of dental follicle (DF) intermediate progenitors into periodontal lineages, including alveolar bone (AB) osteoblasts, cementoblasts, and periodontal ligament (PDL) cells. When comparing differentially expressed genes, PDL cells most closely resembled DF progenitors, followed by AB osteoblasts and cementoblasts as the most distant population. According to gene ontology analyses, extracellular matrix-adhesion proteins were substantially increased in PDL cells, osteogenesis factors were elevated in AB osteoblasts, and gene expression levels were lower in cementoblasts, especially in the cytokine group. Unique signature proteins included interleukin 6, paired-like homeodomain transcription factor 2, thrombospondin 2, and glial cell line-derived neurotrophic factor for DF progenitors; asporin and prostaglandin-H2 D-isomerase for AB osteoblasts; and keratin 18, Netrin 4, Jagged 1, and Dickkopf1 for cementoblasts, as verified by western blot analysis. Secreted frizzled-related protein 1 was preferentially expressed in PDL cells, whereas matrix Gla-protein, bone sialoprotein, and insulin-like growth factor binding protein 5 were higher in AB osteoblasts than in cementoblasts. On an epigenetic level, DF progenitors featured high levels of the euchromatin marker H3K4me3, whereas PDL cells, AB osteoblasts, and cementoblasts contained high levels of the transcriptional repressor H3K9me3. Together, our data indicate that in addition to changes in signature gene expression, unique shifts in gene cohort expression levels, epigenetic modifications, and changes in cell morphology contribute to the individuation of tissue populations from a common neural-crest-derived ancestor.

Mechanical tensile stress effects on the expression of bone sialoprotein in bovine cementoblasts

OBJECTIVE

To develop a new cementoblast culture method and to detect bone sialoprotein (BSP) expression in response to high and low mechanical tensile stress in cementoblast in vitro.

MATERIALS AND METHODS

Cementoblasts were collected from the roots of newborn bovine teeth and were identified with cementum-derived attachment protein (CAP) antibody 3G9. Cell proliferation was evaluated by MTT [3-(4,5-dimethylthazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay, and mineralization was confirmed by von Kossa staining. Mechanical tensile stress was applied in vitro to the cementoblast with the use of a uniaxial four-point bending system with 2000 or 4000 microstrains, at a frequency of 0.5 Hz for 3, 6, 12, 24, or 36 hours. BSP mRNA level was quantified by real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR).

RESULTS

A large amount of cementoblast was observed to be expressing CAP. Cementoblasts had a proliferation tendency similar to that of osteoblasts but different from that of periodontal ligament (PDL) cells. Cementoblasts had the ability to become mineralized between osteoblasts and PDL cells. The mechanical tensile stress significantly up-regulated BSP mRNA expression, which reached a peak at 24 hours in both 2000 and 4000 microstrain groups (P < .01) and was tenfold and sixfold higher than that of controls, respectively. BSP expression dropped toward baseline levels at 36 hours in both groups.

CONCLUSIONS

Mechanical tensile stress up-regulated the expression of BSP. Low mechanical tensile stress induced earlier and more intensive up-regulation of BSP mRNA; this might represent the optimal stimuli for cementoblast activity.

Formation of bone-like mineralized matrix by periodontal ligament cells in vivo: a morphological study in rats

Periodontal ligament (PDL) is a unique connective tissue that not only connects cementum and alveolar bone to support teeth, but also plays an important role in reconstructing periodontal tissues. Previous studies have suggested that PDL cells have osteogenic potential; however, they lack precise histological examinations. Here, we studied bone-like matrix formation by PDL cells in rats using morphological techniques. Rat and human PDL cells exhibited substantial alkaline phosphatase activity and induced mineralization in vitro. RT-PCR analyses showed that PDL cells expressed the osteoblast markers, Runx2, osterix, and osteocalcin. These results suggest that PDL cells share similar phenotypes with osteoblasts. To examine the bone-like matrix formation in vivo, PDL cells isolated from green fluorescent protein (GFP)-transgenic rats were inoculated with hydroxyapatite (HA) disks into wild-type rats. Five weeks after the implantation, the pores in HA disks were occupied by GFP-positive cells. Mineralized matrix formation was also found on the surface of HA pores. At 12 weeks, some of the pores were filled with bone-like mineralized matrices (BLMM), which were positive for the bone matrix proteins, osteopontin, bone sialoprotein, and osteocalcin. Immunohistochemical examination revealed that most of the osteoblast- and osteocyte-like cells on or in the BLMM were GFP-positive, suggesting that the BLMM were directly formed by the inoculated PDL cells. On the pore surfaces, Sharpey's fiber-like structures embedded in cementum-like mineralized layers were also observed. These results collectively suggest that PDL cells have the ability to form periodontal tissues and could be a useful source for regenerative therapies of periodontal diseases.

Differentiation and regenerative capacities of human odontoma-derived mesenchymal cells

Regenerating human tooth ex vivo and biological repair of dental caries are hampered by non-viable odontogenic stem cells that can regenerate different tooth components. Odontoma is a developmental dental anomaly that may contain putative post-natal stem cells with the ability to differentiate and regenerate in vivo new dental structures that may include enamel, dentin, cementum and pulp tissues. We evaluated odontoma tissues from 14 patients and further isolated and characterized human odontoma-derived mesenchymal cells (HODCs) with neural stem cell and hard tissue regenerative properties from a group of complex odontoma tissues from 1 of 14 patients. Complex odontoma was more common (9 of 14) than compound type and females (9 of 14) were more affected than males in our set of patients. HODCs were highly proliferative like dental pulp stem cells (DPSCs) but demonstrated stronger neural immunophenotype than both DPSCs and mandible bone marrow stromal cells (BMSCs) by expressing higher levels of nestin, Sox 2 and betaIII-tubulin. When transplanted with hydroxyapatite/tricalcium phosphate into immunocompromised mice, HODCs differentiated and regenerated calcified hard tissues in vivo that were morphologically and quantitatively comparable to those generated by DPSCs and BMSCs. When transplanted with polycaprolactone (biodegradable carrier), HODCs differentiated to form new predentin on the surface of a dentin platform. Newly formed predentin contained numerous distinct dentinal tubules and an apparent dentin-pulp arrangement. HODCs represent unique odontogenic progenitors that readily commit to formation of dental hard tissues.

Apical tooth germ cell-conditioned medium enhances the differentiation of periodontal ligament stem cells into cementum/periodontal ligament-like tissues

BACKGROUND AND OBJECTIVE

Limitations of current periodontal regeneration modalities in both predictability and extent of healing response, especially on new cementum and attachment formation, underscore the importance of restoring or providing a microenvironment that is capable of promoting the differentiation of periodontal ligament stem cells (PDLSCs) towards cementoblast-like cells and the formation of cementum/periodontal ligament-like tissues. The aim of this study was to investigate the biological effect of conditioned medium from developing apical tooth germ cells (APTG-CM) on the differentiation and cementogenesis of PDLSCs both in vitro and in vivo.

MATERIAL AND METHODS

Using the limiting dilution technique, single-colony-derived human PDLSCs were isolated and expanded to obtain homogeneous populations of PDLSCs. Morphological appearance, cell cycle analysis, bromodeoxyuridine incorporation, alkaline phosphatase (ALP) activity, mineralization behavior, gene expression of cementoblast phenotype and in vivo differentiation capacities of PDLSCs co-cultured with APTG-CM were evaluated.

RESULTS

The induced PDLSCs exhibited several characteristics of cementoblast lineages, as indicated by the morphological changes, increased proliferation, high ALP activity, and the expression of cementum-related genes and calcified nodule formation in vitro. When transplanted into immunocompromised mice, the induced PDLSCs showed tissue-regenerative capacity to produce cementum/periodontal ligament-like structures, characterized by a layer of cementum-like mineralized tissues and associated periodontal ligament-like collagen fibers connecting with the newly formed cementum-like deposits, whereas control, untreated PDLSCs transplants mainly formed connective tissues.

CONCLUSION

Our findings suggest that APTG-CM is able to provide a cementogenic microenvironment and induce differentiation of PDLSCs along the cementoblastic lineage. This has important implications for periodontal engineering.

A study of enamel matrix proteins on differentiation of porcine bone marrow stromal cells into cementoblasts

OBJECTIVE

To further explore the role of enamel matrix proteins (EMPs) in periodontal regeneration, we have used porcine bone marrow-derived stromal cells (BMSCs) to observe whether the EMPs could have an effect on their differentiation into cementoblasts.

MATERIALS AND METHODS

In this study, EMPs were extracted from porcine tooth germs by the use of acetic acid. BMSCs obtained from porcine iliac marrow aspiration were inoculated onto the surface of autologous root slices treated with or without EMPs. Following 7-day co-culture, all the BMSC-seeded root slices, with their respective non-cell-inoculated control specimens, were pocketed with expanded polytetrafluoroethylene membrane and were transplanted subcutaneously into 11 nude mice. The animals were sacrificed after 3 and 8 weeks, and the new specimens were processed for haematoxylin and eosin staining.

RESULTS

Histological analysis demonstrated new cellular cementum-like tissue formed along EMP-treated root slices.

CONCLUSION

Our work has indicated for the first time, differentiation of BMSCs into cementoblasts using an EMP-based protocol.

Human dental follicle cells acquire cementoblast features under stimulation by BMP-2/-7 and enamel matrix derivatives (EMD) in vitro

The dental follicle (DF) surrounding the developing tooth germ is an ectomesenchymal tissue composed of various cell populations derived from the cranial neural crest. Human dental follicle cells (HDFC) are believed to contain precursor cells for cementoblasts, periodontal ligament cells, and osteoblasts. Bone morphogenetic proteins (BMPs) produced by Hertwig's epithelial root sheath or present in enamel matrix derivatives (EMD) seem to be involved in the control of DF cell differentiation, but their precise function remains largely unknown. We report the immunolocalization of STRO-1 (a marker of multipotential mesenchymal progenitor cells) and BMP receptors (BMPR) in DF in vivo. In culture, HDFC co-express STRO-1/BMPR and exhibit multilineage properties. Incubation with rhBMP-2 and rhBMP-7 or EMD for 24 h increases the expression of BMP-2 and BMP-7 by HDFC. Long-term stimulation of these cells by rhBMP-2 and/or rhBMP-7 or EMD significantly increases alkaline phosphatase activity (AP) and mineralization. Expression of cementum attachment protein (CAP) and cementum protein-23 (CP-23), two putative cementoblast markers, has been detected in EMD-stimulated whole DF and in cultured HDFC stimulated with EMD or BMP-2 and BMP-7. RhNoggin, a BMP antagonist, abolishes AP activity, mineralization, and CAP/CP-23 expression in HDFC cultures and the expression of BMP-2 and BMP-7 induced by EMD. Phosphorylation of Smad-1 and MAPK is stimulated by EMD or rhBMP-2. However, rhNoggin blocks only Smad-1 phosphorylation under these conditions. Thus, EMD may activate HDFC toward the cementoblastic phenotype, an effect mainly (but not exclusively) involving both exogenous and endogenous BMP-dependent pathways.

Advances in defining regulators of cementum development and periodontal regeneration

Substantial advancements have been made in defining the cells and molecular signals that guide tooth crown morphogenesis and development. As a result, very encouraging progress has been made in regenerating crown tissues by using dental stem cells and recombining epithelial and mesenchymal tissues of specific developmental ages. To date, attempts to regenerate a complete tooth, including the critical periodontal tissues of the tooth root, have not been successful. This may be in part due to a lesser degree of understanding of the events leading to the initiation and development of root and periodontal tissues. Controversies still exist regarding the formation of periodontal tissues, including the origins and contributions of cells, the cues that direct root development, and the potential of these factors to direct regeneration of periodontal tissues when they are lost to disease. In recent years, great strides have been made in beginning to identify and characterize factors contributing to formation of the root and surrounding tissues, that is, cementum, periodontal ligament, and alveolar bone. This review focuses on the most exciting and important developments over the last 5 years toward defining the regulators of tooth root and periodontal tissue development, with special focus on cementogenesis and the potential for applying this knowledge toward developing regenerative therapies. Cells, genes, and proteins regulating root development are reviewed in a question-answer format in order to highlight areas of progress as well as areas of remaining uncertainty that warrant further study.

Characterization of established cementoblast-like cell lines from human cementum-lining cells in vitro and in vivo

To study cellular characteristics of human cementoblasts using a cellular model is important for understanding the mechanisms of homeostasis and regeneration of periodontal tissues. However, at present no immortalized human cementoblast cell line has been established due to limitation of the life span. In the present study, therefore, we attempted to establish human cementoblast-like cell lines by transfection with telomerase catalytic subunit hTERT gene. Two stable clones (HCEM-1 and -2) with high telomerase activity were obtained and they grew over 200 population doublings without significant growth retardation. The expression of mRNA for differentiation markers, type I collagen, alkaline phosphatase (ALP), runt-related transcription factor 2, osteocalcin, bone sialoprotein and cementum-derived protein was revealed in these clones by RT-PCR. Moreover, these cells showed high ALP activity and calcified nodule formation in vitro. Interestingly, HCEM-2 showed cementum like formation on the surface of hydroxyapatites granules by subcutaneous transplantation into immunodeficient mice with hydroxyapatite granules. Thus, we established human cementoblast-like cell lines. We suggest that HCEM cell lines can be useful cell models for investigating the characteristics of human cementoblasts.

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).

Primary culture of fibroblasts and cementoblasts of the equine periodontium

Fibroblasts and cementoblasts in the periodontal ligament (PDL) of equine cheek teeth were harvested, and monocultures were obtained by means of a "selective detachment" procedure. Cells were characterized by morphological criteria and by immunostaining for vimentin, FVIII, pan-cytokeratin, smooth muscle actin, and pro-collagen. Cementogenic potential of the cells was determined by immunostaining for osteopontin and by histochemical detection of alkaline phosphatase. Equine periodontal fibroblasts (EPF) were spindle-shaped and polygonal. Equine dental cementoblasts (EDC) grew in cobblestone-like clusters. Both EPF and EDC stained positive for vimentin. Only EPF contained smooth muscle actin, pro-collagen, and alkaline phosphatase. Few EDC stained positive for osteopontin. The phenotypes of EPF and EDC and their specific expression of proteins corresponded to PDL fibroblasts and dental cementoblasts of other species. These results indicate the potential use of EPF and EDC in an adequate in vitro model of equine cementogenesis and equine periodontal remodeling.

Regeneration of teeth using stem cell-based tissue engineering

Tooth autotransplantation, allotransplantation and dental implants have existed for many years, but have never been totally satisfactory. Thus, the development of new methods of tooth replacement has become desirable, and with the increasing knowledge of stem cell biology becomes a realistic possibility. Stem cell-based tissue engineering involving the recapitulation of the embryonic environment demonstrates that dental, non-dental, embryonic and adult stem cells can contribute to teeth formation in the appropriate setting. Evidence that stem cell populations may be present in human teeth provides the opportunity to consider biological tooth replacement 'new for old'.

Bisphosphonate modulates cementoblast behavior in vitro

BACKGROUND

Cementum formation is deemed to be instrumental for the successful regeneration of periodontal tissues, and thus events and modifiers of cementum formation and mineralization need to be determined. This study aimed to determine whether the bisphosphonate 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) altered the behavior of immortalized cementoblasts (osteocalcin-cementoblasts [OCCM]).

METHODS

OCCM from transgenic mice were exposed to HEBP at concentrations ranging from 0.01 to 10.0 microM. The assays performed included the count of cell number for proliferation, Northern blot analysis for gene expression (up to 10 days for core binding factor alpha-1 [Cbfa1], bone sialoprotein [BSP], osteocalcin [OCN], and osteopontin [OPN], markers for cementoblast/osteoblast maturation/mineralization), von Kossa stain and alizarin red S stain for mineralization, and enzyme assay (p-nitrophenol phosphate cleavage) for alkaline phosphatase (ALP) activity.

RESULTS

Mineral nodule formation was inhibited at the higher doses of HEBP (1.0 and 10.0 microM) only. At early stages (1, 3, and 6 days), gene expression assays revealed only subtle changes in treated cells versus untreated cells, but by day 10, groups treated with lower doses (0.01 and 0.1 microM) were markedly different at the gene expression level. OCN was significantly downregulated (70%) at the lowest dose, with less pronounced effects at higher doses. In concurrence, the master switch gene for osteoblasts, Cbfa1, was also downregulated at the lower doses. Inversely, OPN mRNA was enhanced at the lower doses. ALP activity was not altered by HEBP.

CONCLUSION

Bisphosphonate alters cementoblast function in vitro through the regulation of gene expression and mineral formation.

Biglycan modulates osteoblast differentiation and matrix mineralization

MC3T3-E1 cell-derived clones expressing higher (S) or lower (AS) levels of biglycan were generated and characterized. The processes of cell differentiation and matrix mineralization were accelerated in S but delayed in AS, indicating that BGN modulates osteoblastic cell differentiation.

INTRODUCTION

Biglycan (BGN), a member of the small leucine-rich proteoglycan family, is one of the major proteoglycans found in bone and has been implicated in bone formation. In this study, the effects of over- or underexpression of BGN on osteoblastic cell phenotypes and matrix mineralization were studied.

MATERIALS AND METHODS

MC3T3-E1 cells were transfected with vectors containing the BGN cDNA in a sense or antisense orientation to generate clones expressing higher (S clones) or lower (AS clones) levels of BGN. MC3T3-E1 cells and those transfected with an empty vector (EV) were used as controls. The levels of BGN synthesized by these clones were evaluated by Western blot analysis. Cell growth was analyzed by cell counting and cell differentiation by the gene expression patterns of several osteoblastic markers using quantitative real-time PCR. The abilities of these clones to form mineralized matrices were evaluated by in vitro and in vivo mineralization assays. Furthermore, the clones were treated with BMP-4 and their responsiveness was assessed.

RESULTS

The cell growth in these clones was unaffected; however, osteoblast differentiation was significantly accelerated in S clones and suppressed in AS clones. The in vitro matrix mineralization in S clones was significantly enhanced but severely impaired in AS clones. When transplanted into immunodeficient mice, S clone transplants exhibited larger areas of lamellar bonelike matrices, whereas only minute amounts of woven bone-type structure was found in AS transplants. The response to BMP-4 was higher in S clones but poorer in AS clones compared with that of controls.

CONCLUSIONS

BGN modulates osteoblast differentiation, possibly by regulating BMP signaling, and consequently matrix mineralization.

Differential effect of the cytolethal distending toxin of Actinobacillus actinomycetemcomitans on co-cultures of human oral cells

The periodontal pathogen Actinobacillus actinomycetemcomitans expresses a cytolethal distending toxin (CDT) that typically arrests the growth of eukaryotic cells at either the G0/G1 or G2/M phase of the cell cycle. It was previously found that CDT failed to arrest the growth of human periodontal ligament fibroblasts (HPLFs) when grown in pure culture. In contrast, proliferation of an oral epithelial cell line was rapidly inhibited by the toxin. In this study, the feasibility of using mixed-cell cultures and cell-specific markers to evaluate the response of oral cells, when in heterogeneous populations, to CDT was established. Proliferation of epithelial cells was rapidly inhibited and the cells were selectively eliminated in co-culture with HPLFs or cementoblasts by 24-48 h post-intoxication. Epithelial cells and HPLFs were detected and counted in co-cultures following cell-specific immunolabelling with antibodies against simian virus 40 large T antigen and the Ab-1 surface antigen, respectively. These results demonstrated that the activities of potential virulence factors, such as CDT, from periodontal pathogens can be successfully examined in mixed-cell cultures. This approach is especially relevant to infectious diseases that affect tissues with a diverse cellular composition, such as the periodontium.

The efficacy of mesenchymal stem cells to regenerate and repair dental structures

OBJECTIVES

Identification, characterization, and potential application of mesenchymal stem cells (MSC) derived from human dental tissues.

METHODS

Dental pulp and periodontal ligament were obtained from normal human impacted third molars. The tissues were digested in collagenase/dispase to generate single cell suspensions. Cells were cultured in alpha-MEM supplemented with 20% fetal bovine serum, 2 mM l-glutamine, 100 microM l-ascorbate-2-phosphate. Magnetic and fluorescence activated cell sorting were employed to characterize the phenotype of freshly isolated and ex vivo expanded cell populations. The developmental potential of cultured cells was assessed following co-transplantation with hydroxyapetite/tricalcium phosphate (HA/TCP) particles into immunocompromised mice for 8 weeks.

RESULTS

MSC were identified in adult human dental pulp (dental pulp stem cells, DPSC), human primary teeth (stem cells from human exfoliated deciduous teeth, SHED), and periodontal ligament (periodontal ligament stem cells, PDLSC) by their capacity to generate clongenic cell clusters in culture. Ex vivo expanded DPSC, SHED, and PDLSC populations expressed a heterogeneous assortment of makers associated with MSC, dentin, bone, smooth muscle, neural tissue, and endothelium. PDLSC were also found to express the tendon specific marker, Scleraxis. Xenogeneic transplants containing HA/TCP with either DPSC or SHED generated donor-derived dentin-pulp-like tissues with distinct odontoblast layers lining the mineralized dentin-matrix. In parallel studies, PDLSC generated cementum-like structures associated with PDL-like connective tissue when transplanted with HA/TCP into immunocompromised mice.

CONCLUSION

Collectively, these data revealed the presence of distinct MSC populations associated with dental structures with the potential of stem cells to regenerate living human dental tissues in vivo.

Stem cells in craniofacial and dental tissue engineering

Mesenchymal stem cells (MSC) have been identified in a variety of adult tissues as a population of pluripotential self-renewing cells. Based on their adherence and colony forming properties, a small number of MSC can be isolated from most mesenchymal tissues as well as bone marrow. In the presence of one or more growth factors, these cells commit to lineages that lead to the formation of bone, cartilage, muscle, tendon and adipose tissue; recent studies indicate that stem cells for cementum, dentine and the periodontal ligament also exist. All of these cells can be expanded in vitro, and, embedded in a scaffold, inserted into defects to promote healing and tissue replacement. Increased understanding of the molecular mechanism directing lineage specification and morphogenesis is providing a rational approach for the regeneration of craniofacial tissues and oral structures.

Alveolar bone marrow as a cell source for regenerative medicine: differences between alveolar and iliac bone marrow stromal cells

We isolated and expanded BMSCs from human alveolar/jaw bone at a high success rate (70%). These cells had potent osteogenic potential in vitro and in vivo, although their chondrogenic and adipogenic potential was less than that of iliac cells.

INTRODUCTION

Human bone marrow stromal cells (BMSCs) have osteogenic, chondrogenic, and adipogenic potential, but marrow aspiration from iliac crest is an invasive procedure. Alveolar BMSCs may be more useful for regenerative medicine, because the marrow can be aspirated from alveolar bone with minimal pain.

MATERIALS AND METHODS

In this study, alveolar bone marrow samples were obtained from 41 patients, 6-66 years of age, during the course of oral surgery. BMSCs were seeded and maintained in culture with 10% FBS and basic fibroblast growth factor. In addition, BMSCs were induced to differentiate into osteoblasts, chondrocytes, or adipocytes in appropriate medium.

RESULTS AND CONCLUSION

From a small volume (0.1-3 ml) of aspirates, alveolar BMSCs expanded at a success ratio of 29/41 (70%). The success rate decreased with increasing donor age, perhaps because of age-dependent decreases in the number and proliferative capacity of BMSCs. The expanded BMSCs differentiated into osteoblasts under osteogenic conditions in 21-28 days: the mRNA levels of osteocalcin, osteopontin, and bone sialoprotein, along with the calcium level, in alveolar BMSC cultures were similar to those in iliac cultures. However, unlike iliac BMSC, alveolar BMSC showed poor chondrogenic or adipogenic potential, and similar differences were observed between canine alveolar and iliac BMSCs. Subsequently, human alveolar BMSCs attached to beta-tricalcium phosphate were transplanted into immunodeficient mice. In transplants, new bone formed with osteoblasts and osteocytes that expressed human vimentin, human osteocalcin, and human GAPDH. These findings suggest that BMSCs have distinctive features depending on their in vivo location and that alveolar BMSCs will be useful in cell therapy for bone diseases.

Elevation of galectin-9 as an inflammatory response in the periodontal ligament cells exposed to Porphylomonas gingivalis lipopolysaccharide in vitro and in vivo

Considerable evidence suggests that periodontal disease not only is caused by bacterial infection but also is associated with host susceptibility. Using in-house cDNA microarray analysis, we attempted to identify gene expression changes in human periodontal ligament (PDL)-derived cells with and without treatment with lipopolysaccharide (LPS) extracted from Porphylomonas gingivalis (P. gingivalis LPS). Of the five up-regulated genes in the PDLs treated with P. gingivalis LPS, galectin-9, which was reported to have eosinophil chemoattraction, was selected for further analyses. By semiquantitative reverse transcriptase-polymerase chain reaction (sqRT-PCR), real-time quantitative RT-PCR, and Western blot analyses, elevated galectin-9 gene expression was detected in LPS-treated PDL-derived cells. Consequently, it was confirmed that the LPS enhances the expression level of galectin-9 mRNA and protein in a time-dependent manner together with interleukin-8. In addition, strong immunoreaction for galectin-9 was detected in the PDL consisting of the periodontal pocket of a patient with severe periodontal disease. Furthermore, significant up-regulation of galectin-9 mRNA expression was detected in the mRNA from PDLs of patients with periodontal disease when compared with healthy donors (P < 0.05). These results suggest that galectin-9 expression is associated with inflammatory reactions in the PDL.

GLUT1 is highly expressed in cementoblasts but not in osteoblasts

Cementum is a specialized mineralized tissue covering root surface of the tooth. Although the tissue's composition resembles bone, there are distinct structural and functional differences between the two mineralized tissues. In this study, the genes that are differentially expressed in putative cementoblasts (human cementum-derived cells [HCDCs]) compared with preosteoblastic cells (human bone marrow stromal cells [BMSCs]) were screened by two independent microarray systems, and some of the selected genes were further analyzed by quantitative real-time RT-PCR. The gene encoding glucose transporter 1 [GLUT1], which showed the greatest difference between the two groups by the latter analysis, was subjected to further analyses. High levels of the GLUT1 protein in HCDCs, but not in BMSCs, were detected by Western blotting and immunocytochemistry. Furthermore, intense immunoreactivities for GLUT1 were observed in cementoblasts and cementocytes but not in osteoblasts or osteocytes in human periodontal tissues. These results indicate that GLUT1 may play a role in cementogenesis and could serve as a biomarker to differentiate between cells of cementoblastic and osteoblastic lineage.

Investigation of multipotent postnatal stem cells from human periodontal ligament

BACKGROUND

Periodontal diseases that lead to the destruction of periodontal tissues--including periodontal ligament (PDL), cementum, and bone--are a major cause of tooth loss in adults and are a substantial public-health burden worldwide. PDL is a specialised connective tissue that connects cementum and alveolar bone to maintain and support teeth in situ and preserve tissue homoeostasis. We investigated the notion that human PDL contains stem cells that could be used to regenerate periodontal tissue.

METHODS

PDL tissue was obtained from 25 surgically extracted human third molars and used to isolate PDL stem cells (PDLSCs) by single-colony selection and magnetic activated cell sorting. Immunohistochemical staining, RT-PCR, and northern and western blot analyses were used to identify putative stem-cell markers. Human PDLSCs were transplanted into immunocompromised mice (n=12) and rats (n=6) to assess capacity for tissue regeneration and periodontal repair. Findings PDLSCs expressed the mesenchymal stem-cell markers STRO-1 and CD146/MUC18. Under defined culture conditions, PDLSCs differentiated into cementoblast-like cells, adipocytes, and collagen-forming cells. When transplanted into immunocompromised rodents, PDLSCs showed the capacity to generate a cementum/PDL-like structure and contribute to periodontal tissue repair.

INTERPRETATION

Our findings suggest that PDL contains stem cells that have the potential to generate cementum/PDL-like tissue in vivo. Transplantation of these cells, which can be obtained from an easily accessible tissue resource and expanded ex vivo, might hold promise as a therapeutic approach for reconstruction of tissues destroyed by periodontal diseases.

Cementum matrix formation in vivo by cultured dental follicle cells

Dental follicle is the fibrous tissue that surrounds the developing tooth germ, and it is believed to contain progenitors for cementoblasts, periodontal ligament cells, and osteoblasts. In this study, we report the presence of cementoblast progenitors in cultures of bovine dental follicle cells and demonstrate their differentiation capacity. Bovine dental follicle cells (BDFC) obtained from tooth germs by collagenase digestion were compared with bovine alveolar bone osteoblasts (BAOB) and bovine periodontal ligament cells (BPDL) in vitro and in vivo. In culture, BDFC exhibited low levels of alkaline phosphatase activity and expressed mRNA for osteopontin (OP) and type I collagen (COLI), as well as low levels of osteocalcin (OC) mRNA. In contrast, cultured BAOB exhibited high alkaline phosphatase activity levels and expressed mRNA for OC, OP, COLI, and bone sialoprotein (BSP). To elucidate the differentiation capacity of BDFC in vivo, cells were transplanted into severe combined immunodeficiency (SCID) mice and analyzed after 4 weeks. Transplanted BDFC formed fibrous tissue and cementum-like matrix, which stained positive for anti-cementum attachment protein (CAP) monoclonal antibody (3G9), and expressed mRNA for OC, OP, COLI, and BSP. On the other hand, transplanted BAOB formed bone-like matrix, but were negative for anti-CAP monoclonal antibody. The BPDL transplants formed fibrous tissue that contained a few cells expressing CAP. These results indicate that cementoblast progenitors are present in BDFC, which can provide a useful model for investigating the molecular mechanisms of cementogenesis.

A histopathological study of the role of periodontal ligament tissue in root resorption in the rat

Whether periodontal ligament (PDL) tissue is capable of inducing root resorption was examined. The distal root of the rat molar was sectioned at the furcation and the PDL tissue removed from the root (non-PDL group, n=40). The distal root with the PDL intact was also prepared (PDL-intact group, n=40). The roots were transplanted into the dorsal skin of the rat. On the 1st, 3rd, 5th, 7th, 10th, 14th, 21st or 28th day after transplantation, the roots were removed together with surrounding dorsal subcutaneous tissue and were fixed, demineralized and embedded in paraffin. Serial sections from each block were stained with haematoxylin and eosin or by the tartrate-resistant acid phosphatase (TRAP) method to observe root-resorbing cell formation. Cyclo-oxygenase-2 (COX2) was also detected immunohistologically to examine prostaglandin E(2) production. On the 7th day after transplantation, multinucleated root-resorbing cells with TRAP were observed in the PDL-intact group. The number of TRAP-positive cells peaked on the 10th day after transplantation. COX2-positive cells were observed in PDL during the early experimental stages. No root resorption was seen in the non-PDL group. These results suggest that PDL tissue is involved in the formation of root-resorbing cells and root resorption.

Enamel factors regulate expression of genes associated with cementoblasts

BACKGROUND

In order to design predictable periodontal regenerative therapies, it is important to understand the responsiveness of cells within the local environment to factors considered attractive candidates. The aim of this study was to determine the effect of an enamel matrix derivative (EMD) on cementoblast behavior in vitro and in vivo.

METHODS

Osteocalcin (OC) promoter SV40 transgenic mice were used to obtain cementoblasts. For comparison, preosteoblasts from these mice, as well as another murine pre-osteoblast cell line, MC3T3-E1 cells, were used. Cells exposed to EMD were evaluated for changes in: 1) proliferation over an 8-day period by cell counting; 2) gene expression using Northern blot analysis; and 3) biomineralization by von Kossa stain, in vitro and by preparing histological samples from implants retrieved from immunodeficient (SCID) mice, where cementoblasts were treated with EMD prior to implantation.

RESULTS

EMD promoted proliferation of all cell types. EMD down-regulated osteocalcin transcripts in cementoblasts and MC3T3-E1 cells and up-regulated osteopontin gene expression markedly in MC3T3-E1 cells and slightly in cementoblasts at day 8. In vitro, EMD decreased cementoblast-mediated biomineralization. In contrast, mineralization was noted in implants retrieved from SCID mice, where cells were pretreated with EMD.

CONCLUSION

These results indicate that EMD can influence activities of cementoblasts and osteoblasts, and thus may be able to regulate cell activities at a periodontal regenerative site.

Parathyroid hormone-related protein regulates extracellular matrix gene expression in cementoblasts and inhibits cementoblast-mediated mineralization in vitro

Parathyroid hormone-related protein (PTHrP) has been implicated in regulating tooth eruption and/or development. Formation of cementum, a mineralized tissue covering the tooth root surface, is a critical biological event for tooth root development. To test the hypothesis that PTHrP targets cementoblasts (CMs) and acts to regulate cementogenesis, CM cell lines were established and their responsiveness to PTHrP stimulation was determined, in vitro. First, subclones were derived from two immortalized murine cell populations that contained CMs; SV-CM/periodontal ligament (PDL) cells were obtained from the root surface of first mandibular molars of CD-1 mice and immortalized with SV40 T-antigen (TAg), and OC-CM cell population was established from OC-TAg transgenic mice in which their cells harbor an osteocalcin (OC and/or OCN) promoter-driving immortal gene SV40 TAg. Based on our previous in situ studies, CM subclones were identified as cells expressing bone sialoprotein (BSP) and OCN transcripts, while PDL cell lines were designated as cells lacking BSP and OCN messenger RNA (mRNA). CMs exhibited a cuboidal appearance and promoted biomineralization, both in vitro and in vivo. In contrast, PDL cells (PDL subclones) displayed a spindle-shaped morphology and lacked the ability to promote mineralized nodule formation, both in vitro and in vivo. Next, using these subclones, the effect of PTHrP on cementogenesis was studied. CMs, not PDL cells, expressed PTH/PTHrP receptor mRNA and exhibited PTHrP-mediated elevation in cyclic adenosine monophosphate (cAMP) levels and c-fos gene induction. PTHrP stimulation repressed mRNA expression of BSP and OCN in CMs and blocked CM-mediated mineralization, in vitro. Collectively, these data suggest that CMs possess PTH/PTHrP receptors and, thus, are direct targets for PTHrP action during cementogenesis and that PTHrP may serve as an important regulator of cementogenesis.

Growth factors regulate expression of mineral associated genes in cementoblasts

BACKGROUND

Knowledge of the responsiveness of cells within the periodontal region to specific bioactive agents is important for improving regenerative therapies. The aim of this study was to determine the effect of specific growth factors, insulin-like growth factor-I (IGF-I), platelet-derived growth factor-BB (PDGF-BB), and transforming growth factor-beta (TGF-beta) on cementoblasts in vitro and ex vivo.

METHODS

Osteocalcin (OC) promoter driven SV40 transgenic mice were used to obtain immortalized cementoblasts. Growth factor effects on DNA synthesis were assayed by [3H]-thymidine incorporation. Northern analysis was used to determine the effects of growth factors on gene expression profile. Effects of growth factors on cementoblast induced biomineralization were determined in vitro (von Kossa stain) and ex vivo (re-implantation of cells in immunodeficient (SCID) mice).

RESULTS

All growth factors stimulated DNA synthesis compared to control. Twenty-four hour exposure of cells to PDGF-BB or TGF-beta resulted in a decrease in bone sialoprotein (BSP) and osteocalcin (OCN) mRNAs while PDGF-BB also increased osteopontin (OPN) mRNA. Cells exposed to IGF-I for 24 hours exhibited decreased transcripts for OCN and OPN with an upregulation of BSP mRNA noted at 72 hours. In vitro mineralization was inhibited by continuous application of PDGF-BB or TGF-beta, while cells exposed to these factors prior to implantation into SCID mice still promoted biomineralization.

CONCLUSIONS

These data indicate IGF-I, PDGF-BB, and TGF-beta influence mitogenesis, phenotypic gene expression profile, and biomineralization potential of cementoblasts suggesting that such factors alone or in combination with other agents may provide trigger factors required for regenerating periodontal tissues.

Cementum-forming cells are phenotypically distinct from bone-forming cells

Normal human cementum-derived cells (HCDCs), expanded in vitro, formed mineralized matrix when attached to a ceramic carrier and transplanted subcutaneously into immunodeficient mice. The mineralized matrix elaborated by transplanted HCDC exhibited several features identical to cementum in situ and was significantly different from bone deposited by similarly transplanted human bone marrow stromal cells (BMSCs). No bone marrow formation and very few or no tartrate-resistant acid phosphatase (TRAP)-positive cells (osteoclasts and osteoclastic precursors) were found in HCDC transplants. In contrast, in BMSC transplants both hematopoiesis and TRAP-positive cells were routinely observed. Furthermore, compared with BMSC-derived matrix, HCDC-derived matrix was less cellular, numerous empty lacunae were present, and fewer cells were found on the cementum matrix/ceramic carrier interface. The organization of collagen fibers in HCDC-derived matrix, as visualized by using the Picrosirus red staining method, was similar to cementum, with typical unorganized bundles of collagen fibers. In contrast, bone matrix elaborated by transplanted BMSC had lamellar structure, identical to mature bone in situ. Finally, cementocytes embedded in the cementum-like matrix were immunopositive for fibromodulin and lumican, whereas osteocytes within the bonelike matrix were negative. This pattern is consistent with the cementum and bone in situ, respectively. These results indicate that human cementum cells are phenotypically distinct from bone cells and provide further validation of the combined in vitro/in vivo model of human cementogenesis recently developed in our laboratory.

Electron microscopy, micro-analysis, and X-ray diffraction characterization of the mineral-like tissue deposited by human cementum tumor-derived cells

The nature and characteristics of the mineralized-like tissue deposited by cementoblasts are not well-known due to the difficulties in obtaining and culturing cells representing the cementum phenotype. We hypothesized that a putative cementoblastic cell line derived from a human cementoblastoma could serve as a suitable model to study the physical, chemical, and morphological features of the cementum-like tissue deposited in vitro. The cementoblastoma cell line was studied by transmission electron, high resolution, scanning, and atomic force microscopy and compared with human cellular cementum, human osteoblasts, and human alveolar bone. The analyses of the crystals and mineral-like tissue in the cell line were performed by x-ray diffraction microscopy and energy-dispersive x-ray micro-analysis. TEM examination of cementoblastoma cells revealed the presence of electron-dense intracellular vesicles surrounded by a membrane that contained filaments and needle-like structures. The diffraction patterns obtained from the intracellular material and human cellular cementum were similar, with D-spacings of 3.36 and 2.8, consistent with those of hydroxyapatite (3.440 and 2.814). The composition of the mineral-like tissue had a Ca/P ratio of 1.60 for cementoblastoma cells and 1.97 for human cellular cementum. Na (5.29%) and Cl (1.47%) were present in the composition of cementoblastoma cells. Human cellular cementum additionally contained Mg (4.95%). Osteoblastic cells showed a Ca/P ratio of 1.6280. Na represented 4.52% and Cl 1.22% of its composition. Human alveolar bone had a Ca/P ratio value of 2.01. Na (6.63%), Mg (2.10%), and Cl (0.84%) were also present. All samples examined represented biological-type hydroxyapatite. Based on the compositional and morphological features, these findings indicate that cementoblastoma-derived cells express the human cellular cementum phenotype.