Substrate stiffness promotes dentinogenesis via LAMB1-FAK-MEK1/2 signaling axis

OBJECTIVES

In vivo, the principal function of mechanosensitive odontoblasts is to synthesize and secrete the matrix which then calcifies and forms reactive dentin after exposure to appropriate stimuli. This study aims to develop the influence of mechanical factors on dentinogenesis based on odontoblasts, which contribute to reparative dentin formation.

METHODS

We fabricated polydimethylsiloxane with different stiffnesses and seeded 17IIA11 odontoblast-like cells on the substrates in different stiffnesses. Cell morphology was detected by scanning electron microscope, and the mineralization phenotype was detected by alkaline phosphatase staining and alizarin red staining, while expression levels of dentinogenesis-related genes (including Runx2, Osx, and Alp) were assayed by qPCR. To explore mechanism, protein distribution and expression levels were detected by immunofluorescent staining, Western blotting, and immunoprecipitation.

RESULTS

In our results, during dentinogenesis, 17IIA11 odontoblast-like cells appeared better extension on stiffer substrates. The binding between LAMB1 and FAK contributed to converting mechanical stimuli into biochemical signaling, thereby controlling mitogen-activated protein kinase kinase 1/2 activity in stiffness-driven dentinogenesis.

CONCLUSION

The present study suggests odontoblast behaviors can be directly regulated by mechanical factors at cell-material interfaces, which offers fundamental mechanism in remodeling cell microenvironment, thereby contributing to physiological phenomena explanation and tissue engineering progress.

The Reparative Function of MMP13 in Tertiary Reactionary Dentinogenesis after Tooth Injury

MMP13 gene expression increases up to 2000-fold in mineralizing dental pulp cells (DPCs), with research previously demonstrating that global MMP13 deletion resulted in critical alterations in the dentine phenotype, affecting dentine-tubule regularity, the odontoblast palisade, and significantly reducing the dentine volume. Global MMP13-KO and wild-type mice of a range of ages had their molar teeth injured to stimulate reactionary tertiary dentinogenesis. The response was measured qualitatively and quantitatively using histology, immunohistochemistry, micro-CT, and qRT-PCR in order to assess changes in the nature and volume of dentine deposited as well as mechanistic links. MMP13 loss affected the reactionary tertiary dentine quality and volume after cuspal injury and reduced Nestin expression in a non-exposure injury model, as well as mechanistic links between MMP13 and the Wnt-responsive gene Axin2. Acute pulpal injury and pulp exposure to oral fluids in mice teeth showed upregulation of the MMP13 in vivo, with an increase in the gene expression of Mmp8, Mmp9, and Mmp13 evident. These results indicate that MMP13 is involved in tertiary reactionary dentine formation after tooth injury in vivo, potentially acting as a key molecule in the dental pulp during dentine-pulp repair processes.

Expression of BSP-GFPtpz Transgene during Osteogenesis and Reparative Dentinogenesis

Bone sialoprotein (BSP) is a member of the SIBLING family with essential roles in skeletogenesis. In the developing teeth, although the expression and function of BSP in the formation of acellular cementum and periodontal attachment are well documented, there are uncertainties regarding the expression and function of BSP by odontoblasts and dentin. Reporter mice are valuable animal models for biological research, providing a gene expression readout that can contribute to cellular characterization within the context of a developmental process. In the present study, we examined the expression of a BSP-GFPtpz reporter mouse line during odontoblast differentiation, reparative dentinogenesis, and bone. In the developing teeth, BSP-GFPtpz was expressed at high levels in cementoblasts but not in odontoblasts or dentin. In bones, the transgene was highly expressed in osteoblasts at an early stage of differentiation. Interestingly, despite its lack of expression in odontoblasts and dental pulp during tooth development, the BSP-GFPtpz transgene was detected during in vitro mineralization of primary pulp cultures and during reparative dentinogenesis following pulp exposures. Importantly, under these experimental contexts, the expression of BSP-GFPtpz was still exclusive to DSPP-Cerulean, an odontoblast-specific reporter gene. This suggests that the combinatorial use of BSP-GFPtpz and DSPP-Cerulean can be a valuable experimental tool to distinguish osteogenic from dentinogenic cells, thereby providing an avenue to investigate mechanisms that distinctly regulate the lineage progression of progenitors into odontoblasts versus osteoblasts.

Axin2-expressing cells differentiate into reparative odontoblasts via autocrine Wnt/β-catenin signaling in response to tooth damage

In non-growing teeth, such as mouse and human molars, primary odontoblasts are long-lived post-mitotic cells that secrete dentine throughout the life of the tooth. New odontoblast-like cells are only produced in response to a damage or trauma. Little is known about the molecular events that initiate mesenchymal stem cells to proliferate and differentiate into odontoblast-like cells in response to dentine damage. The reparative and regenerative capacity of multiple mammalian tissues depends on the activation of Wnt/β-catenin signaling pathway. In this study, we investigated the molecular role of Wnt/β-catenin signaling pathway in reparative dentinogenesis using an in vivo mouse tooth damage model. We found that Axin2 is rapidly upregulated in response to tooth damage and that these Axin2-expressing cells differentiate into new odontoblast-like cells that secrete reparative dentine. In addition, the Axin2-expressing cells produce a source of Wnt that acts in an autocrine manner to modulate reparative dentinogenesis.

Root Lengths in 47,XYY Males’ Permanent Teeth

Studies on individuals with sex chromosome anomalies have demonstrated the promoting effect of the Y chromosome on tooth crown enamel and dentin growth. The present research investigated permanent tooth root lengths in 47,XYY males. The measurements were made from panoramic radiographs. The results indicate longer tooth roots in 47,XYY males compared with those in control males and females. The promoting effect of the Y chromosome on dental growth thus continues in the form of root dentin after the completion of crown growth. The results, together with those on tooth crown sizes in 47,XYY males, suggest that growth excesses are evident and final, beginning a few months after birth and continuing up to the age of 14 years, at least. The excess root dentin growth in 47,XYY males, as well as sexual dimorphism in the growth of crown and root dentin, might be caused by the same factor on the Y chromosome.

Hyper-expression of Osteocalcin mRNA in Odontoblasts of Hyp Mice

The Hyp mouse is a murine homologue of human X-linked hypophosphatemia that displays hypo-mineralization in bone and dentin. In this study, we tested the hypothesis that the defect in Hyp mice leads to alterations in the expression of dentin matrix proteins that may be associated with the hypo-mineralization changes in the tissues. Quantitative RT-PCR analyses showed that expression of the osteocalcin gene in Hyp mice tooth germ samples was significantly higher than in wild-type mice, whereas the gene expressions of osteonectin, osteopontn, dentin matrix protein 1, and type I collagen in both types of mice were similar. Further, cultured Hyp mice tooth germ samples exhibited a higher expression of the osteocalcin gene than did those from wild-type mice, which was in accord with the results of our in vivo analysis. These findings suggest that osteocalcin mRNA is highly expressed in Hyp mice odontoblasts and may be associated with dentin hypo-mineralization.

Investigation of functional activity human dental pulp stem cells at acute and chronic pulpitis

It is already recognized that together with the other connective tissues organ-specific progenic stem cells are also found in postnatal dental pulp. This group of undifferentiated cells is only 1% of total cell population of the pulp. The aim of the study was the identification of stem cells in human dental pulp, detection of their localization and assessment of functional activity during inflammation process and/or at norm. The obtained results showed that at acute pulpitis the pulp stroma is hypocellular in comparison with the norm but cells proliferative activity is low. CD 133 and NCAM (CD 56) positive stem cells were found in perivascularl space of the pulp stroma and in Hohle layer. At process prolongation and transition to the chronic phase pulp stroma is hypercellular, the cells with large, rounded or oval-shaped nuclei with clear chromatin appear together with fibroblasts. They are distributed as about entire thickness of the stroma as especially Hohle layer. In such cells higher proliferative activity (Ki67 expression) was observed. The cells in the mentioned proliferation phase are intensively marked by CD133, the rate of which is high in Hohle layer and along it. A large number of NCAM (CD 56) positive cells appear in pulp stroma.

CONCLUSIONS

During pulpitis an involvement of stem cells into the process of reparative dentinogenesis should be conducted stepwise. In acute cases of the disease, stem cell perivascularl mobilization and proliferation and its migration to Hohle layer occur in response to irritation /stimulation. Chronification of the process leads not only to the migration of stem cells to the periphery of the pulp but also s their В«maturationВ» (increase of NCAM expression in the stem cells), which causes an increase the number of dentin producing active odontoblasts and initiation of reparative dentinogenesis.

Compounded PHOSPHO1/ALPL deficiencies reduce dentin mineralization

Phosphatases are involved in bone and tooth mineralization, but their mechanisms of action are not completely understood. Tissue-nonspecific alkaline phosphatase (TNAP, ALPL) regulates inhibitory extracellular pyrophosphate through its pyrophosphatase activity to control mineral propagation in the matrix; mice without TNAP lack acellular cementum, and have mineralization defects in dentin, enamel, and bone. PHOSPHO1 is a phosphatase found within membrane-bounded matrix vesicles in mineralized tissues, and double ablation of Alpl and Phospho1 in mice leads to a complete absence of skeletal mineralization. Here, we describe mineralization abnormalities in the teeth of Phospho1(-/-) mice, and in compound knockout mice lacking Phospho1 and one allele of Alpl (Phospho1(-/-);Alpl(+/-) ). In wild-type mice, PHOSPHO1 and TNAP co-localized to odontoblasts at early stages of dentinogenesis, coincident with the early mineralization of mantle dentin. In Phospho1 knockout mice, radiography, micro-computed tomography, histology, and transmission electron microscopy all demonstrated mineralization abnormalities of incisor dentin, with the most remarkable findings being reduced overall mineralization coincident with decreased matrix vesicle mineralization in the Phospho1(-/-) mice, and the almost complete absence of matrix vesicles in the Phospho1(-/-);Alpl(+/-) mice, whose incisors showed a further reduction in mineralization. Results from this study support prominent non-redundant roles for both PHOSPHO1 and TNAP in dentin mineralization.

Post-natal effect of overexpressed DKK1 on mandibular molar formation

Dickkopf-related protein 1 (DKK1) is a potent inhibitor of Wnt/β-catenin signaling. Dkk1-null mutant embryos display severe defects in head induction. Conversely, targeted expression of Dkk1 in dental epithelial cells leads to the formation of dysfunctional enamel knots and subsequent tooth defects during embryonic development. However, its role in post-natal dentinogenesis is largely unknown. To address this issue, we studied the role of DKK1 in post-natal dentin development using 2.3-kb Col1a1-Dkk1 transgenic mice, with the following key findings: (1) The Dkk1 transgene was highly expressed in pulp and odontoblast cells during post-natal developmental stages; (2) the 1(st) molar displayed short roots, an enlarged pulp/root canal region, and a decrease in the dentin formation rate; (3) a small malformed second molar and an absent third molar; (4) an increase of immature odontoblasts, few mature odontoblasts, and sharply reduced dentinal tubules; and (5) a dramatic change in Osx and nestin expression. We propose that DKK1 controls post-natal mandibular molar dentin formation either directly or indirectly via the inhibition of Wnt signaling at the following aspects: (i) post-natal dentin formation, (ii) formation and/or maintenance of the dentin tubular system, (iii) mineralization of the dentin, and (iv) regulation of molecules such as Osx and nestin.

Expression of clock proteins in developing tooth

Morphological and functional changes during ameloblast and odontoblast differentiation suggest that enamel and dentin formation is under circadian control. Circadian rhythms are endogenous self-sustained oscillations with periods of 24h that control diverse physiological and metabolic processes. Mammalian clock genes play a key role in synchronizing circadian functions in many organs. However, close to nothing is known on clock genes expression during tooth development. In this work, we investigated the expression of four clock genes during tooth development. Our results showed that circadian clock genes Bmal1, clock, per1, and per2 mRNAs were detected in teeth by RT-PCR. Immunohistochemistry showed that clock protein expression was first detected in teeth at the bell stage (E17), being expressed in EOE and dental papilla cells. At post-natal day four (PN4), all four clock proteins continued to be expressed in teeth but with different intensities, being strongly expressed within the nucleus of ameloblasts and odontoblasts and down-regulated in dental pulp cells. Interestingly, at PN21 incisor, expression of clock proteins was down-regulated in odontoblasts of the crown-analogue side but expression was persisting in root-analogue side odontoblasts. In contrast, both crown and root odontoblasts were strongly stained for all four clock proteins in first molars at PN21. Within the periodontal ligament (PDL) space, epithelial rests of Malassez (ERM) showed the strongest expression among other PDL cells. Our data suggests that clock genes might be involved in the regulation of ameloblast and odontoblast functions, such as enamel and dentin protein secretion and matrix mineralization.

Role of the transcription factor NFIC in odontoblast gene expression

The transcription factor NFI-C is essential for root development. Mice lacking NFI-C develop abnormal roots and lose their teeth, resembling radicular dentin dysplasia I in humans. The purpose of this study was to understand the role of NFI-C in dentinogenesis. The authors found statistically significant increases in the expression of several mRNAs in cells lacking NFI-C, suggesting that these molecules might interfere with odontoblast cell migration and differentiation, and consequently with root development.

MMP2-cleavage of DMP1 generates a bioactive peptide promoting differentiation of dental pulp stem/progenitor cell

Dentin Matrix Protein 1 (DMP1) plays a regulatory role in dentin mineralization and can also function as a signaling molecule. MMP-2 (matrix metalloproteinase-2) is a predominant protease in the dentin matrix that plays a prominent role in tooth formation and a potential role during the carious process. The possibility that MMP-2 can cleave DMP1 to release biologically active peptides was investigated in this study. DMP1, both in the recombinant form and in its native state within the dentin matrix, was shown to be a substrate for MMP-2. Proteolytic processing of DMP1 by MMP-2 produced two major peptides, one that contains the C-terminal region of the protein known to carry both the ASARM (aspartic acid and serine rich domain) domain involved in biomineralization and the DNA binding site of DMP1. In vitro experiments with recombinant N- and C-terminal polypeptides mimicking the MMP-2 cleavage products of DMP1 demonstrated an effect of the C-polypeptide on the differentiation of dental pulp stem/progenitor cells to a putative odontoblast phenotype. In vivo implantation of this peptide in a rat injured pulp model induced a rapid formation of a homogeneous dentin bridge covered by a palisade of orientated cells expressing dentin sialoprotein (DSP) and DMP1, attesting an efficient repair process. These data suggest that a peptide generated through the proteolytic processing of DMP1 by MMP-2 can regulate the differentiation of mesenchymal cells during dentinogenesis and thus sustain reparative dentin formation in pathological situations such as carious decay. In addition, these data open a new therapeutic possibility of using this peptide to regenerate dentin after an injury.

Expression and processing of small integrin-binding ligand N-linked glycoproteins in mouse odontoblastic cells

OBJECTIVE

Small integrin-binding ligand N-linked glycoproteins (SIBLINGs) are expressed in dentin and believed to control dentinogenesis. Five members of SIBLING family include bone sialoprotein (BSP), osteopontin (OPN), matrix extracellular phosphoglycoprotein (MEPE), dentin matrix protein 1 (DMP1) and dentin sialophosphoprotein (DSPP). These genes are clustered on chromosome 4q in humans and share similar biological features. DSPP and DMP1 are processed into given structural/functional fragments in rat and porcine. It still remains unclear whether these evidences occur in mouse and other SIBLING members are also processed into given fragments from their parent precursors. The aim of this study was to identify expression and processing of the five proteins in two mouse odontoblastic cell lines.

DESIGN

Two mouse odontoblastic cells were used to study expression and processing of the five SIBLING proteins by immunohistochemistry and Western blot analyses.

RESULTS

Immunohistochemistry study showed that all of the five SIBLING members were expressed within the cytoplasm and cellular processes in the mouse odontoblastic cell lines. Expression levels of DMP1 and DSPP were higher in differentiated mouse odontoblasts than undifferentiated mouse odontoblasts. Immunolabelling signal of DSP and MEPE was also detected within the nucleus in the two cell lines. Western blot assay indicated that all five members were processed into at least two fragments in these cells.

CONCLUSIONS

These results suggest that different processed products and expression levels of the SIBLING proteins may play distinct biological functions in tooth development and mineralisation.

NficGene Disruption Inhibits Differentiation of Odontoblasts Responsible for Root Formation and Results in Formation of Short and Abnormal Roots in Mice

BACKGROUND

Nuclear factor I genes play an important role in the development of the brain, lung, and roots of teeth. We had reported that Nfic-deficient mice form normal crowns, but abnormal roots of molar teeth. However, the mechanism by which the disruption of Nfic gene causes abnormal root formation remains unknown.

METHODS

To understand this mechanism, the root formation in Nfic-deficient mice was examined and compared to that of wild-type mice by morphological, immunohistochemical, and in situ hybridization analyses.

RESULTS

Nfic-deficient mice formed normal Hertwig's epithelial root sheath (HERS) but severely disrupted odontoblast differentiation, leading to the formation of aberrant odontoblasts in the early stage of root formation. They became dissociated and polygonal in shape, lost their orientation and polarity, and did not express dentin sialophosphoprotein. The abnormal roots contained trapped aberrant odontoblasts, thereby resembling osteodentin in overall morphology. No osteoclasts were associated with abnormal roots. Further, the abnormal roots exhibited a decreased number of cementoblasts and cementum formation on the root surface.

CONCLUSIONS

The loss of Nfic did not interfere with the formation of HERS, but it caused disrupted odontoblast differentiation, which resulted in the formation of short and abnormal roots, and decreased cementum. This finding suggests that root dentin is required for normal cementum formation. Therefore, Nfic may be a key regulator of root odontoblast differentiation and root formation.

Disorders of human dentin

Dentin, the most abundant tissue in teeth, is produced by odontoblasts, which differentiate from mesenchymal cells of the dental papilla. Dentinogenesis is a highly controlled process that results in the conversion of unmineralized predentin to mineralized dentin. By weight, 70% of the dentin matrix is mineralized, while the organic phase accounts for 20% and water constitutes the remaining 10%. Type I collagen is the primary component (>85%) of the organic portion of dentin. The non-collagenous part of the organic matrix is composed of various proteins, with dentin phosphoprotein predominating, accounting for about 50% of the non-collagenous part. Dentin defects are broadly classified into two major types: dentinogenesis imperfectas (DIs, types I-III) and dentin dysplasias (DDs, types I and II). To date, mutations in DSPP have been found to underlie the dentin disorders DI types II and III and DD type II. With the elucidation of the underlying genetic mechanisms has come the realization that the clinical characteristics associated with DSPP mutations appear to represent a continuum of phenotypes. Thus, these disorders should likely be called DSPP-associated dentin defects, with DD type II representing the mild end of the phenotypic spectrum and DI type III representing the severe end.

Formation of the dentino-enamel interface in enamelysin (MMP-20)-deficient mouse incisors

An anomalous dentino-enamel junction (DEJ), manifested by delamination of the enamel layer, was reported in enamelysin [matrix metalloproteinase-20 (MMP-20)] knockout (KO) mice. To better understand the possible role of MMP-20 in the formation of the DEJ, we performed transmission electron microscopy (TEM) studies of the DEJ at early stages of tooth morphogenesis in KO mice. Our TEM analysis revealed that in the incisors from KO mice the mantle dentin is hypomineralized at the onset of enamel mineralization. At this early stage, TEM revealed no apparent differences in nascent aprismatic enamel between the KO mice and the controls. Hypomineralized mantle dentin was also observed in the incisors from KO mice, as assessed by back-scattered SEM at the secretory and early maturation stages, but not in the late-maturation stage, suggesting that the mineralization of mantle dentin is not completely arrested, but rather postponed. Histological studies indicate that the organic content in the initial enamel layer remains very high throughout amelogenesis. These results imply that MMP-20 is involved in the regulation of mineralization in mantle dentin and demonstrate the complex nature of DEJ formation. They also suggest that the structural and functional properties of the DEJ are determined during the initial mineralization stages.

IGFs increase enamel formation by inducing expression of enamel mineralizing specific genes

Insulin-like growth factors (IGF-I and IGF-II) have been shown to play an important role in growth and differentiation in a number of tissues including mineralizing bone. Little is known about their role in tooth mineralization. Previous work in our laboratory has shown the presence of IGFs ligands, their receptors, and their binding proteins during mouse tooth morphogenesis. The expression of IGF I coincides with the expression of amelogenin, ameloblastin and enamelin at the late bell and secretory stage. The objective of this study is to determine the mechanisms by which IGFs modulate enamel and dentin formation. Mouse first mandibular molars were dissected from E16 and E17 mouse embryos and placed in organ culture in the presence of IGF-I or IGF-II. The molars were harvested after 12 days for histological examination or 1 day for mRNA expression analysis by real-time RT-PCR. Our results show an increase in enamel deposition, and an induction of enamelin, amelogenin and collagen type I mRNA expression, while expression of DSPP was down-regulated. These results suggest that IGFs increase enamel formation by the induction of gene expression of enamel related genes. Studies are underway to determine a possible mechanism for these factors.

Expression of bone morphogenetic proteins and Msx genes during root formation

Like crown development, root formation is also regulated by interactions between epithelial and mesenchymml tissues. Bone morphogenetic proteins (BMPs), together with the transcription factors Msx1 and Msx2, play important roles in these interactions during early tooth morphogenesis. To investigate the involvement of this signaling pathway in root development, we analyzed the expression patterns of Bmp2, Bmp3, Bmp4, and Bmp7 as well as Msx1 and Msx2 in the roots of mouse molars. Bmp4 was expressed in the apical mesenchyme and Msx2 in the root sheath. However, Bmps were not detected in the root sheath epithelium, and Msx transcripts were absent from the underlying mesenchyme. These findings indicate that this Bmp signaling pathway, required for tooth initiation, does not regulate root development, but we suggest that root shape may be regulated by a mechanism similar to that regulating crown shape in cap-stage tooth germs. Msx2 expression continued in the epithelial cell rests of Malassez, and the nearby cementoblasts intensely expressed Bmp3, which may regulate some functions of the fragmented epithelium.

Characterization of porcine dentin sialoprotein (DSP) and dentin sialophosphoprotein (DSPP) cDNA clones

Dentin sialophosphoprotein (DSPP) is a chimeric glycoprotein with dentin sialoprotein (DSP) on its N-terminus and dentin phosphoprotein (DPP) on its C-terminus. We have constructed and screened a unidirectional cDNA library derived from the pulp organ of developing pig teeth, and isolated cDNA clones encoding DSP-only, as well as two DSPP clones with alternative sequences in their 3' coding regions. The DSP-only transcript has an open reading frame of 386 codons, and is generated through the use of a polyadenylation signal within intron 4, immediately following the DSP coding region. the use of this polyadenylation signal deletes the DPP coding region and places a TGA translation termination signal as the fourth codon following the exon 4-encoded segment. The DSPP cDNAs contain open reading frames of 593 and 600 codons. Northern blots hybridized to radiolabeled DSP probes showed bands at 1.4, 2.5, 4.4, and 4.8 kb. Cloning and characterization of reverse transcriptase polymerase chain reaction products confirmed the existence of mRNA encoding pDSP386, pDSPP593, and pDSPP600in vivo, but also suggested that DNA sequence redundancies in the DSPP coding region make it prone to cloning artifacts.

Gene expression in a pure population of odontoblasts isolated by laser-capture microdissection

Studies of odontoblast differentiation and function have been limited due to difficulties in obtaining sufficient numbers of intact cells. We describe a novel approach of laser-capture microdissection to obtain homogenous populations of pre-odontoblasts and odontoblasts from tissue sections of mouse molar cusp tips. Fixation, processing, and staining conditions were assessed for the optimal retrieval of total RNA from microdissected odontoblasts. Fluorometric assays and RT-PCR analysis of alpha1(I) collagen, dentin sialophosphoprotein (Dspp), and osteocalcin (OC) confirmed that the total RNA from three-day-old captured odontoblasts was sufficient in quantity and quality. Odontoblast-specific gene expression was studied by RT-PCR analysis performed in a single streptavidin-coated tube. At E15.5, Days 0 and 3, gene expression in laser-captured odontoblasts resembled that seen in vivo by in situ hybridization. The use of LCM is thus a valuable means of retrieving quality RNA from discrete populations of odontoblasts at different stages of dentinogenesis.

A novel rat dentin mRNA coding only for dentin sialoprotein

Dentin sialoprotein (DSP) is a major glycoprotein present in the mineralized dentin matrix that is expressed mainly by young and mature odontoblasts. Mutations in the DSP coding regions are linked to Dentinogenesis imperfecta I and II. indicating the importance of DSP in tooth formation. Previous studies have identified multiple mRNA transcripts in dentin that code for both DSP and phosphophoryns (PPs). Using reverse transcriptase-polymerase chain reaction (RT-PCR) to characterize these mRNA transcripts, we have identified a cDNA that codes for DSP, but not PP. This cDNA codes for a protein with 324 amino acids, 303 amino acids being identical to the published rat DSP sequence. However, the subsequent 21 amino acids are unique to this cDNA. Based on the coding sequence, the core protein is predicted to have a pI=4.24, a net charge of -34, and to contain four potential N-glycosylation sites and six potential sites for phosphorylation by casein kinase. That the corresponding mRNA was present in day 5 molar tooth germs was confirmed using RNA protection assays. These data, therefore, identify a novel transcript in rat tooth germs that codes only for DSP (designated as DSPII).

New genes involved in odontoblast differentiation

The odontoblast phenotype has been mainly approached by the biochemical characterization of dentin matrix proteins and by extrapolation of the knowledge of bone cell biology, since dentin and bone share many similarities. In fact, direct investigations of the odontoblast phenotype have been hindered by the limited number of cells within the dental pulp and the difficulty in microdissection and isolation of a pure population of these cells. To overcome these obstacles, we previously developed a cell-culture system that promotes differentiation of human dental pulp cells into odontoblasts. This material now permits the study of odontoblasts through molecular biology techniques. Therefore, we constructed a cDNA library enriched for odontoblast-specific genes using the suppression subtractive hybridization technique (SSH). This library led us to identify new genes expressed by odontoblasts. In this paper, we will focus on some genes implied in various functions associated with odontoblast differentiation, such as cell polarization (MAP1B), dentin mineralization (PHEX, osteoadherin), and relationships between odontoblasts and nerve cells (reelin). Another important fact is that about 40% of the cDNA were unknown genes. Therefore, one can speculate that some of them will be odontoblast-specific, since, until now, only one gene (DSPP) presents this characteristic.

Highly regulated expression of subtilisin-like proprotein convertase PACE4 (SPC4) during dentinogenesis

Expressions of mRNAs for four subtilisin-like proprotein convertases (SPCs: furin, PACE4, PC6, and PC8) and bone morphogenetic protein 4 (BMP4) in the rat molar tooth during development were analyzed by Northern blotting, reverse transcriptase-polymerase chain reaction (RT-PCR), and in situ hybridization to explore the possible involvement of SPCs in the processing of proBMPs. We found a temporospacial expression of PACE4, but not one of the other SPCs, in this tissue; i.e., RT-PCR analysis revealed that the level of PACE4 mRNA, but not that of the other SPC mRNAs became high around the second postnatal day. This increase was in good accordance with the increase in BMP4 mRNA, indicating an apparent association of these molecules with the differentiation and establishment of functional ameloblasts and odontoblasts. During dentinogenesis, PACE4 mRNA was localized in the ameloblasts and odontoblasts. These observations suggest that PACE4 plays a crucial role in dentinogenesis, especially via the activation of BMPs.

Crown components of mandibular molar teeth in 45,X females (Turner syndrome)

This study was designed to determine the possible effect of one X-chromosome constitution on components of the human permanent and primary molar teeth. Enamel, dentine, pulp and crown dimensions were measured on radiographs of first and second permanent and second primary mandibular molars of 49 Finnish 45,X females (Turner syndrome), their 46 first-degree male and female relatives and 50 non-related males and females. In permanent first and second molars of the 45,X females, crown width and the dimensions of tooth components were less than those of normal females and males. Reduction in size affected first more than second molars, and in both teeth the enamel was relatively as well as absolutely thinner than in the controls. No differences were found in tooth components between normal relatives and unrelated controls. These data agree with previous studies which have demonstrated that the X chromosome promotes enamel apposition and that both X chromosomes in normal females are active in amelogenesis, while the Y chromosome influences both dentine and enamel growth. The relative reduction in "dentine" or the estimated mesiodistal width of the tooth germ in the 45,X females indicates that their tooth development is affected at an early stage of morphogenesis. Taken together with the results already reported for anterior teeth, the present results suggest that there is an inverse correlation between the duration of crown formation and the severity of size reduction.

Root development in mice lacking functional tissue non-specific alkaline phosphatase gene: inhibition of acellular cementum formation

Tissue non-specific alkaline phosphatase (TNAP) is richly present in developing teeth including the cells of the periodontal ligament. Here, we investigated tooth and root development in mice lacking the TNAP gene. Heterozygous mutants were obtained from The Jackson Laboratory, Animal Resources (Bar Harbor, ME, USA) and bred. TNAP-deficient mice and their littermates were killed from 6 to 25 days after birth and their molar blocks processed for light and electron microscopy. It was observed that the eruption of the incisors into the oral cavity was delayed for 2 to 3 days. Also, the onset of mineralization of the mantle dentin in the roots of the developing molars was delayed for 2 to 3 days. Yet, dentin and enamel formation in the homozygous mutants showed a more or less normal pattern, with the exception of localized enamel hypoplasias. The most conspicuous finding was the defective formation of acellular cementum along the molar roots. Instead of a continuous layer, the cementum was deposited as very thin and irregularly shaped patches around the bases of the periodontal ligament fibers. Sharpey's fibers were short and poorly developed. In contrast, the development of the alveolar bone, the periodontal ligament, and the cellular cementum was seemingly unaffected. It is concluded that TNAP represents an essential factor in mantle dentin mineralization and in the formation of acellular cementum.

Temporal and spatial expression of c-jun and jun-B proto-oncogenes in pulp cells involved with reparative dentinogenesis after cavity preparation of rat molars

c-jun and jun-B are nuclear proto-oncogenes induced by growth factors such as bone morphogenetic proteins (BMPs). These gene products enhance the expression of many genes, including osteocalcin and collagen types, indicating that c-jun and jun-B play important roles in the cell differentiation process. It is also known that BMPs affect the differentiation of pulp cells to odontoblast-like cells during reparative dentinogenesis, but little is known about the transcriptional regulation of genes in cells associated with reparative dentinogenesis. In this study, we examined the expression of c-jun and jun-B in pulp cells during reparative dentinogenesis after cavity preparation of rat molars by in situ hybridization. In rat tooth germs, c-jun and jun-B were co-expressed in the odontoblastic lineage. In rat adult molars, c-jun was expressed in the odontoblast layer, but the jun-B expression was absent in all pulp cells. After cavity preparation, we found that c-jun and jun-B were coexpressed in pulp cells underneath cavities. During the early phase of reparative dentinogenesis, levels of c-jun and jun-B greatly increased in pulp cells within and around the reparative dentin matrix formed adjacent to the cavity floor. Fourteen days after cavity preparation, c-jun and jun-B were expressed only in pulp cells lining the irregular surface of the thick reparative dentin. These results suggest that c-jun and jun-B may play important roles both in physiological and in reparative dentinogenesis; in particular, the limited distribution of the jun-B expression suggests a specific role of jun-B only in cells involved with the active formation of the dentin matrix during primary and reparative dentinogenesis.

Expression of collagen alpha1(I) mRNA variants during tooth and bone formation in the rat

Collagen alpha1(I) mRNA is composed of two variants of 5 and 6 kb, differing in the length of the 3' untranslated region. In this work, the nucleotide sequences of the two rat mRNA variants were compared, and their expression pattern in cells forming bone, dentin, and cementum were analyzed. The sequences were determined from cDNA inserts of tooth and bone libraries plus directly from PCR fragments, obtained from bone. A total of 5721 bases of the rat collagen alpha1(I) sequence from cDNA of tooth and bone was determined. All sequences of the short variant were represented in the long variant. Only the alternatively poly-A additions gave rise to the variants in hard tissue. Two oligonucleotides were chosen as probes, one of which recognized, on Northern blots, the two bands of 5 and 6 kb, and the other the 6-kb variant only. The oligonucleotides were used in in situ hybridization experiments, for study of the distribution of the variants in different extracellular matrix-forming cells. Osteoblasts, odontoblasts, and cementum-associated cells were closely examined in sections from rat maxillae from 2 to 25 days of age. A similar or identical pattern of mRNA expression was observed with both oligonucleotides, indicating that the two mRNA variants were co-expressed in all cases.

Dentin sialoprotein (DSP) transcripts: developmentally-sustained expression in odontoblasts and transient expression in pre-ameloblasts

Dentin sialoprotein (DSP), a 53 kDa glycoprotein, is believed to be present exclusively in dentin. Using rat and mouse digoxigenin labeled (DIG)-DSP and 35S-DSP riboprobes, and in situ hybridization techniques, we have studied the presence of DSP mRNA at specific developmental stages of dentinogenesis. In mouse and rat molars and incisors, DSP transcripts were localized in young odontoblasts associated with early stages of predentin formation, as well as in mature odontoblasts, cells with cytoplasmic extensions embedded in the forming dentin. No DSP transcripts were detected in dental pulp, enamel organ, ameloblasts, epithelial root sheath, Meckel's cartilage, alveolar bone or tibia. Furthermore, no DSP mRNA was observed in other soft tissues including heart, lung, kidney, intestine, eye, and muscle. In addition to the intense and prolonged expression by odontoblasts, DSP mRNA was transiently expressed by pre-ameloblasts in both developing molars and incisors. These observations are consistent with the results of previous immunohistochemical studies (1). The transient expression of DSP in pre-ameloblasts across from young odontoblasts suggests an involvement of DSP in epithelial-mesenchymal interactions that are crucial to later stages of tooth development.

Expressions of c-jun and jun-B proto-oncogenes in odontoblasts during development of bovine tooth germs

c-jun and jun-B genes are among the nuclear proto-oncogenes induced by growth factors such as the TGF-beta superfamily and play important roles in cell differentiation. These gene products enhance expressions of proteins including osteocalcin, alkaline phosphatase, and collagens. On the other hand, it is well-known that the TGF-beta superfamily affects odontoblast differentiation, and that differentiated odontoblasts express extracellular and membrane proteins as described above. However, there are few reports of factors that participate in the transcriptional regulation of odontoblasts. Especially, little is known about the expression of c-jun and jun-B genes. In this study, we focused on the examination of expressions of c-jun and jun-B genes in dental papillae of bovine tooth germs. Using in situ hybridization, we found that these genes were expressed only in the odontoblastic lineage, but not in other dental papilla cells. Levels of c-jun and jun-B mRNAs increased along the gradient of differentiation of odontoblasts. These levels of c-jun mRNAs were maintained in both young and mature odontoblasts. However, unlike the c-jun gene, expression of the jun-B gene became sparse in mature odontoblasts compared with young odontoblasts. For further analysis, Northern hybridization of total RNA extracted from differentiated odontoblasts was performed for the examination of levels of jun-B mRNAs, indicating that levels of jun-B mRNAs of mature odontoblasts were clearly less than those of young odontoblasts. These results suggest that c-jun and jun-B genes may participate in the transcriptional regulation of odontoblasts of bovine tooth germs, and may control the odontoblast phenotype. Furthermore, our results suggest that these genes can be markers of odontoblasts during dentinogenesis; especially, high expression of jun-B gene can be a marker of young odontoblasts that start to form the new dentin matrix.

Expression of genes for bone morphogenetic proteins and receptors in human dental pulp

Bone morphogenetic proteins (BMP) have been shown to induce reparative dentine formation experimentally but the cells responsible, which respond to BMPs, have not been identified. The BMP signal is probably mediated by interaction of type I and II BMP receptors (R). Here, the RNA of human adult dental pulp and pulp cells in culture was examined by reverse transcription (RT) polymerase chain reaction (PCR) for evidence of mRNA for BMPs. mRNAs for BMP-2, -4, osteogenic protein-1, ActR-1 (activin-like kinase receptor), BMPR-IA, -IB and -II were detected by RT-PCR. The 698-bp PCR fragment for BMPR-IB was used to probe pulp cells for expression of that receptor. Cell expression of BMPR-IB was detected by the hybridization probe. The findings suggest that resident pulp cells may be able to respond to BMPs to initiate tissue formation.

Gene expression of TGF-beta 1 and elaboration of extracellular matrix using in situ hybridization and EM radioautography during dentinogenesis

BACKGROUND AND METHODS

The expressions of TGF-beta 1 and Type I collagen mRNA were studied by in situ hybridization and immunohistochemistry then the secretory pathway of dentin phosphoprotein was investigated electron microscopic radioautography in rat incisors.

RESULTS AND CONCLUSIONS

Expression of TGF-beta 1 mRNA was observed in dental papilla cells before dentin formation. The signals were most intense in pre- and postodontoblasts and during dentinogenesis, but became weaker in the secretory region during the dentin formation. Type I collagen mRNA was expressed in essentially the same as that of TGF-beta 1. These results suggest that TGF-beta 1 plays an important role in the differentiation of, and collagen synthesis by odontoblasts. Radioautography showed radioactivity in the rough endoplasmic reticulum 5 min after injection of 3H-serine. Silver grains were observed over the cylindrical portions of the cis-face of the Golgi apparatus at 10 min and over the cylindrical portions of the transface at 20 min. The secretory granules showed the strongest reaction between 20 min and 1 h after injection. At 45 min, a significant labeled band appeared at the mineralization front. The pathway of 3H-proline was essentially the same as that of 3H-serine, but 3H-proline moved more slowly. Secretory granules were heavily labeled from 30 min; no labeling was found at the mineralization front at 45 min. The labeling pattern with 3H-serine appears to be closely related to the localization of phosphoproteins. Dentin phosphoproteins are related to secretory granules and are secreted by odontoblasts as the mineralization front, being involved in the process of dentin mineralization.

Partial cDNA sequencing of mouse dentine sialoprotein and detection of its specific expression by odontoblasts

Dentine sialoprotein (DSP), a 53-kDa acidic glycoprotein, is expressed by odontoblasts and secreted into the dentine extracellular matrix. Although little is known about its biological function, it might play a part in dentinogenesis. Because DSP has only been shown to occur in rat dentine, it is important to demonstrate its existence in another species. Here, the presence of DSP gene in the mouse genome, and the cloning of a mouse DSP cDNA coding for about one-fifth of the molecule with a nucleotide sequence similar to that for rat cDNA, are reported. Using in-situ hybridization, DSP mRNA was uniquely detected in mouse odontoblasts.

Nuclear DNA fragmentation during postnatal tooth development of mouse and hamster and during dentin repair in the rat

The TUNEL (transferase-mediated, dUTP-biotin nick end labeling) method for in situ labeling of DNA strands was utilized to localize DNA fragmentation in cells involved in tooth formation in the neonatal mouse and hamster. Positive reactions for the presence of DNA fragments were obtained in some epithelial cells of the cervical loop region of incisors, late secretory, transitional and early maturation stage ameloblasts, stratum intermedium cells and in shortened ameloblasts just before eruption. Also, cells of the periodontal ligament of the continuously erupting incisors stained positive shortly before eruption. Odontoblasts were negative but became strongly positive during the formation of physiological osteodentin at the tip of developing incisors. Osteodentin matrix and the surfaces of unerupted enamel and cementum just prior to eruption stained for DNA fragments as well. DNA fragmentation could be elicited in odontoblasts and underlying pulpal tissues of mature erupted molars after mechanical injury to the odontoblast processes during cavity preparation. We conclude that, in rodents, DNA fragmentation and cell death are biological processes which take place in a variety of cells involved in formation of teeth. The TUNEL staining technique is a simple but powerful tool to examine the fate of cells and tissues undergoing either programmed cell death (apoptosis) or fragmentation of nuclear DNA induced by external factors leading to pathological changes.

A new biological approach to vital pulp therapy

Molecular biology is providing opportunities to develop new strategies or agents for the treatment of a wide variety of diseases. The availability of large amounts of highly purified proteins produced by recombinant DNA techniques is an obvious example. Recent evidence has implicated proteins belonging to the bone morphogenetic protein (BMP) subgroup of the transforming growth factor beta supergene family in tooth formation and dentinogenesis. It has long been known that bone and dentin contain bone morphogenetic protein activity. Recently, recombinant human BMP-2, -4, and -7 (also known as OP-1), have been shown to induce reparative dentin formation in experimental models of large direct pulp exposures in permanent teeth. The manner in which these agents act appears unique. New reparative dentin replaces the stimulating agents applied directly to the partially amputated pulp. Hence, the new tissue forms contiguous with, largely superficial to, and not at the expense of the remaining vital pulp tissue. This suggests a therapeutic approach permitting the induction of a predetermined and controlled amount of reparative dentin. Additionally, OP-1 has been associated with the formation of reparative dentin after application to a freshly cut but intact layer of dentin. These findings may provide future clinicians with additional options for the treatment of substantially damaged or diseased vital teeth.

Temperature sensitive simian virus 40 large T antigen immortalization of murine odontoblast cell cultures: establishment of clonal odontoblast cell line

During tooth formation instructive epithelial-mesenchymal interactions result in the cytodifferentiation of ectomesenchymal cells into odontoblasts which produce the dentin extracellular matrix (DECM). The purpose of our study was to establish a stable murine odontoblast cell line by immortalization of odontoblasts using retrovirus transfection. In order to accomplish this goal, we utilized a previously characterized odontoblast monolayer cell culture system supportive of odontoblast cytodifferentiation from dental papilla mesenchyme (DPM), expression and secretion of a DECM and dentin biomineralization. First mandibular molars from E-18 Swiss Webster mice were dissected, the DPM isolated, and pulp cells dissociated. Pulp cells (5 x 10(5)/well) were plated as monolayers and grown in alpha-MEM supplemented with 10% FCS, 100 units/ml penicillin and streptomycin, 50 micrograms/ml ascorbic acid. Cultures were maintained for 6 days at 37 degrees C in a humidified atmosphere of 95% air and 5% CO2, with media changes every two days. Immortalization was performed using a recombinant defective retrovirus containing the temperature sensitive SV-40 large T antigen cDNA and the neomycin (G418) resistance gene recovered from CRE packaging cells. Cultures were infected for 24 h with CRE conditioned medium containing 8 micrograms/ml of polybrene, the media was replaced with selective media containing 300 micrograms/ml of G418, and the cultures incubated at 33 degrees C for one month with media changes every 3-5 days. Neomycin resistant cells were cloned by serial dilution to single cells in 96-well culture plates and grown in selection medium at 33 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular analysis of rat dentin sialoprotein

Dentin sialoprotein (DSP) is a noncollagenous protein originally isolated from rat dentin. Because it is made by odontoblasts that are actively synthesizing dentin. DSP may play an important role in dentinogenesis. We have isolated a full length DSP cDNA from a rat odontoblast/dental pulp cDNA library (Ritchie et al. [1994] J. Biol. Chem. 269:3698-3702) which codes for a 17 residue signal peptide and a 366 residue, 53 kDa mature protein. In situ hybridization revealed DSP mRNA expression by odontoblasts, but no other cells, in jaws from newborn rat. Northern analysis of various rat tissues demonstrated the presence of DSP transcripts in newborn tooth germs and 21 day old rat incisors. Moreover, multiple transcripts of 4.6 kb and 1.5 kb were found in these two tissues. To better understand the origin of these DSP mRNA multiple transcripts, we have isolated two rat genomic clones. Digestion of each clone with EcoRI followed by Southern analysis revealed that DSP cDNA hybridized to a 4 kb fragment in a lambda dash clone and to a 6 kb fragment in a cosmid clone. Since DSP cDNA hybridized to a 6 kb EcoRI fragment and a 4 kb EcoRI fragment obtained from a rat liver genomic cDNA digested with EcoRI, the multiple DSP mRNA transcripts are most likely derived from two related DSP genes which coexist in the rat genome.

Stage-specific expression of decapentaplegic-Vg-related genes 2, 4, and 6 (bone morphogenetic proteins 2, 4, and 6) during human tooth morphogenesis

Members of the decapentaplegic-Vg-related (DVR) gene family are diffusible signaling molecules regulating inductive tissue interactions during vertebrate development. Expression of DVR/bone morphogenetic protein (BMP) 2, 4, and 6 was studied in human fetal teeth. Sequential morphogenetic stage-specific studies of DVR/BMP 2 and 4 mRNA expression by in situ hybridization revealed transcripts for DVR/BMP 4 during compaction of the dental mesenchyme. In contrast, DVR/BMP 2 mRNA appeared later during tooth development and was located in differentiated cells (odontoblasts). These results were confirmed by reverse-transcription polymerase chain reaction (RT-PCR), which detected DVR/BMP 2 and 4 mRNA in human tooth-germ samples. DVR/BMP 6 protein was distributed in the early dental epithelium and, later, in pre-odontoblasts and odontoblasts, where it remained during dentin formation. These results suggest that DVR/BMP 4 is involved in the early tooth morphogenesis. DVR/BMP 6 may, in particular, be implicated in epithelial-mesenchymal interactions controlling cytodifferentiation. DVR/BMP 2 and 6 may also be involved in odontoblast secretory function. The results suggest that members of the DVR gene family may play regulatory roles during human tooth development.