Identification of a novel isoform of mouse dentin matrix protein 1: spatial expression in mineralized tissues

Dual functional roles of dentin matrix protein 1. Implications in biomineralization and gene transcription by activation of intracellular Ca2+ store

Dentin matrix protein 1 (DMP1) is a bone- and teeth-specific protein initially identified from mineralized dentin. Here we report that DMP1 is primarily localized in the nuclear compartment of undifferentiated osteoblasts. In the nucleus, DMP1 acts as a transcriptional component for activation of osteoblast-specific genes like osteocalcin. During the early phase of osteoblast maturation, Ca(2+) surges into the nucleus from the cytoplasm, triggering the phosphorylation of DMP1 by a nuclear isoform of casein kinase II. This phosphorylated DMP1 is then exported out into the extracellular matrix, where it regulates nucleation of hydroxyapatite. Thus, DMP1 is a unique molecule that initiates osteoblast differentiation by transcription in the nucleus and orchestrates mineralized matrix formation extracellularly, at later stages of osteoblast maturation. The data presented here represent a paradigm shift in the understanding of DMP1 function. This information is crucial in understanding normal bone formation, remodeling, fracture healing, and skeletal tissue repair.

Mechanical loading stimulates dentin matrix protein 1 (DMP1) expression in osteocytes in vivo

Dentin matrix protein 1 (DMP1) was originally postulated to be dentin specific. Further analysis showed that DMP1 is also expressed in mature cartilage and bone. In bone tissue, DMP1 is expressed predominantly in late osteoblasts and osteocytes. DMP1 belongs to the SIBLING (Small Integrin Binding Ligand N-linked Glycoprotein) family of cellular matrix proteins that also includes osteopontin, bone sialoprotein, dentin sialophosphoprotein, and others. In this study, we examined the effect of mechanical loading on expression of DMP1 mRNA and DMP1 protein in alveolar bone in the mouse tooth movement model by in situ hybridization and immunocytochemistry. The expression of DMP1 mRNA was determined quantitatively in mechanically loaded and control sites of dento-alveolar tissue at several time points from 6 h to 7 days after loading. The tooth movement model allows simultaneous evaluation of bone resorption and bone formation sites. Expression of DMP1 mRNA in osteocytes increased 2-fold as early as 6 h after treatment in both the bone formation and bone resorption sites. After 4 days, DMP1 expression in osteocytes increased to a maximum of 3.7-fold in the bone formation sites and 3.5-fold in the resorption sites. Osteoblasts responded in the opposite manner and showed a transient 45% decrease of DMP1 mRNA in bone formation sites and a constant decrease of DMP1 mRNA during the entire course of treatment in the bone resorption sites, with a peak inhibition of 67% at day 2. By immunocytochemistry using a C-terminal region peptide antibody to DMP1, we found that there was a transient decrease in immunoreactivity at 3 days after treatment on both the formation side and the resorption side compared with the matched contralateral control tissue. However by 7 days of loading, there was a dramatic increase in DMP1 protein immunoreactivity on both the formation side and the resorption side. These results represent changes in epitope availability using this antibody or true changes in protein levels. The observations imply that the DMP1 protein is undergoing dynamic changes in either synthesis or other protein/matrix interaction after mechanical loading of alveolar bone. The findings indicate that DMP1 is involved in the responses of osteocytes and osteoblasts to mechanical loading of bone. These results support the hypothesis that osteocytes alter their matrix microenvironment in response to mechanical loading.

Type I collagen regulated dentin matrix protein-1 (Dmp-1) and osteocalcin (OCN) gene expression of rat dental pulp cells

In this study, we investigated the effect of type I collagen on dentin matrix protein-1 (Dmp-1) and osteocalcin (OCN) gene expression of dental pulp cells. The mRNA level of Dmp-1 gene was down-regulated; however, OCN gene expression was up-regulated by the culture of dental pulp cells with type I collagen. These findings imply that type I collagen regulates mRNA level of Dmp-1 and OCN gene that are predominantly expressed in active odontoblasts. The change of gene expression by type I collagen was suppressed by the blocking of collagen-integrin interaction. We could conclude that the effect of type I collagen was mediated via binding of collagen to integrin receptors.

Transcriptional regulation of dentin matrix protein 1 (DMP1) by AP-1 (c-fos/c-jun) factors

Dentin matrix protein 1 (DMP1) is an extracellular matrix phosphoprotein whose expression is precisely controlled temporally and spatially during the differentiation of the odontoblasts. Odontoblasts are postmitotic cells that differentiate to mature polarized cells and are responsible for the synthesis of a calcified dentin matrix. Although the DMP1 promoter has been isolated from the mouse and the rat, little is actually known about their control by specific transcription factors. Analysis of the rat DMP1 promoter has identified several regulatory transcription factors. These factors include AP-1 family members, SP1 and ETS. Therefore the transcription of DMP1 may be controlled positively or negatively by a variety of stimuli resulting in a developmentally regulated gene expression. This study demonstrates the role of c-fos and c-jun on the transcriptional regulation of DMP1 gene.

Phosphorylation of serine residues is fundamental for the calcium-binding ability of Orchestin, a soluble matrix protein from crustacean calcium storage structures

Orchestia cavimana is a terrestrial crustacean, which cyclically stores calcium in diverticula of the midgut, in the form of calcified amorphous concretions. These concretions are associated with a proteinaceous matrix, the main constituent of the soluble matrix is Orchestin, an acidic calcium-binding protein [Testenière et al., Biochem. J. 361 (2002) 327-335]. In the present paper, we clearly demonstrate that Orchestin is phosphorylated on serine and tyrosine residues, but that calcium binding only occurs via the phosphoserine residues. To our knowledge, this is the first example of an invertebrate mineralization for which a post-translational modification is clearly related to an important function of a calcifying protein.

Dentin matrix protein 1, a target molecule for Cbfa1 in bone, is a unique bone marker gene

Dentin matrix protein 1 (Dmp1), a phosphoprotein highly linked to dentin formation, has also been reported to be expressed in the skeleton. However, the role of Dmp1 in skeletal tissues remains unclear. To clarify the role of Dmp1 in bone formation, we characterized the expression profile of Dmp1 in bone and cartilage and examined whether Dmp1 expression was regulated by core-binding factor a1 (Cbfa1). Studies of fetal rat calvarial (FRC) cell cultures showed that the expression of Dmp1 was associated closely with "bone nodule" formation and mineralization in vitro. In situ hybridization studies were performed to examine the spatial and temporal expression patterns of Dmp1 during development in mouse embryos from 12.5 day postcoitus (dpc) to 8 weeks postnatal; these studies showed that Dmp1 first appeared in hypertrophic cartilage cells, followed by osteoblasts, and later was expressed strongly in osteocytes. The expression profiles of Cbfa1 and Dmp1 overlapped in both cartilage and bone during development, with Cbfa1 preceding Dmp1. Examination of Dmp1 expression in Cbfa1-/- mice revealed that Dmp1 was absent in the developing bones of Cbfa1-null mice, whereas there was essentially no change in Dmp1 expression in the arrested tooth bud. Transient transfection studies showed forced expression of Dmp1 under the control of Cbfa1 and gel shift data indicated the presence of a functional osteocalcin-specific element (OSE)-2 response element in the Dmp1 proximal promoter region. However, in vitro promoter studies suggested that regulation of Dmp1 by Cbfa1 was not mediated by direct binding of Cbfa1 to this site and may be through indirect mechanisms. These studies highlight Dmp1 as a unique marker gene for osteoblastic differentiation. The close association of Dmp1 and Cbfa1 in the developing skeleton suggests that Dmp1 may play an important role in bone formation.

Tumors associated with oncogenic osteomalacia express genes important in bone and mineral metabolism

Oncogenic osteomalacia (OOM) is associated with primitive mesenchymal tumors that secrete phosphaturic factors resulting in low serum concentrations of phosphate and calcitriol, phosphaturia, and defective bone mineralization. To identify overexpressed genes in these tumors, we compared gene expression profiles of tumors resected from patients with OOM and histologically similar control tumors using serial analysis of gene expression (SAGE). Three hundred and sixty-four genes were expressed at least twofold greater in OOM tumors compared with control tumors. A subset of 67 highly expressed genes underwent validation with an extended set of OOM and control tumors using array analysis or reverse-transcription polymerase chain reaction (RT-PCR). Ten of these validated genes were consistently overexpressed in all OOM tumors relative to control tumors. Strikingly, genes with roles in bone matrix formation, mineral ion transport, and bone mineralization were highly expressed in the OOM tumors.

A comparative study of sialic acid-rich proteins in rat bone and dentin

Four sialic acid-rich (SA-rich) proteins found in bone and dentin, osteopontin (OPN), bone sialoprotein (BSP), bone acidic glycoprotein-75 (BAG-75), and dentin matrix protein 1 (DMP1), share some common features. We used SDS-PAGE and Western immunoblots to analyze and compare SA-rich proteins in bone and dentin extracts from rats with a single chromatographic procedure. OPN was detected in dentin extracts, with a relative level less than one-seventieth of that in bone. Both bone and dentin BSP demonstrated an extremely broad distribution pattern, probably due to a high degree of heterogeneity in post-translational modifications. BAG-75 in both bone and dentin was detected as an 83 kDa band, dramatically distinct from that of DMPI. Using a polyclonal antibody raised against a purified bone 57 kDa protein (a portion of DMPI), we detected 150 kDa protein bands in bone fraction; the same bands were recognized by antirecombinant rat DMPI antibody. Bands from dentin migrating at about 150 kDa in earlier fractions and progressing to 200 kDa in later fractions showed a clear immunoreactivity to the anti-57 kDa antibody. We conclude that the majority of DMPI in rat bone is processed into fragments, whereas that in dentin remains intact.

Dentin matrix protein 1 is predominantly expressed in chicken and rat osteocytes but not in osteoblasts

Although osteocytes are the most abundant cells in bone, little is known about their function, and no specific marker protein for osteocytes has been described. Dentin matrix protein 1 (DMP1) is an acidic phosphoprotein expressed in tooth organ and bone. Our previous work showed that in the chicken, which is not capable of forming tooth, DMPI messenger RNA (mRNA) is highly expressed in bone by Northern blot analysis. To clarify the significance of DMP1 expression in bone, the expression of DMP1 mRNA and its protein was examined in the chicken and rat. In the chicken, DMPI mRNA was detected only in bone tissues and was localized in osteocytes and preosteocytes but not in osteoblasts. Similarly, in the rat, DMPI mRNA was predominantly expressed in osteocytes and preosteocytes in bone matrix but not in osteoblasts located at the bone surface. Antiserum was raised against the peptide from rat DMP1, and the localization of DMP1 was examined by immunohistochemistry. In the development of bone, DMP1 was first detected in newly formed bone matrix after osteoblastic cells had been embedded within it. After the appearance of typical osteocytes, DMP1 was localized in the pericellular bone matrix of osteocytes, including their processes. These data show that DMP1 is a bone matrix protein specifically expressed in osteocytes and preosteocytes and suggest that DMP1 plays a role in bone homeostasis because of its high calcium ion-binding capacity.

Differential expression of dentin matrix protein 1, type I collagen and osteocalcin genes in rat developing mandibular bone

The expression of dentin matrix protein 1 (Dmp1) mRNA has been compared with that of type I collagen and osteocalcin mRNAs during bone formation in the rat mandible, using in situ hybridization. At embryonic day 15 (E15), type I collagen and osteocalcin mRNAs were expressed by the majority of newly-differentiated osteoblasts attached to unmineralized bone matrices, whereas Dmp1 mRNA expression was confined to only a few osteoblasts. Expression of these genes increased as the number of osteoblasts increased in specimens from E16 to E18. At E20, expression of Dmp1, type I collagen and osteocalcin was also observed in osteocytes. Dmp1 expression continued in osteocytes as they matured up to the 90-day-old specimens, whereas type I collagen and osteocalcin expression in osteocytes almost disappeared at 30 days of postnatal life. In contrast, osteoblasts continued to express type I collagen and osteocalcin in 90-day-old rats, but transiently expressed Dmp1 mRNA, which was seen in the minority of osteoblasts at 14 days of postnatal life. These data show that the developmental expression patterns of Dmp1 in osteogenic differentiation differ from those of type I collagen and osteocalcin, and Dmp1 appears to be expressed by osteocytes throughout ossification in the skeleton. These observations indicate that Dmp1 may serve unique biological functions in osteocyte and bone metabolism.

Inositol hexasulphate, a casein kinase inhibitor, alters the distribution of dentin matrix protein 1 in cultured embryonic mouse tooth germs

Immunohistochemical studies using a polyclonal antibody, raised against the recombinant form of dentin matrix protein 1 (DMP1), show that DMP1 was detected mainly in odontoblasts in cultured mouse embryonic tooth germs. However, in restricted areas, DMP1 staining was also observed in secretory ameloblasts, in the stratum intermedium and stellate reticulum, but only when the odontoblasts located in front of them were unstained. When the embryonic tooth germs were cultured in the presence of inositol hexasulfate, a casein kinase I and II inhibitor, staining of odontoblasts was weak or nil, whereas, in contrast, ameloblasts and enamel organ were strongly immunolabelled, suggesting an enhanced translocation of DMP1 after secretion to the secretory ameloblasts and/or stratum intermedium and stellate reticulum. Moreover, DMP1--was shown to be a good substrate for gelatinase A (MMP-2), but not to gelatinase B (MMP- 9). We hypothesized that DMP1--or the sub-fractions cleaved by the MMP--could behave as diffusible signaling molecule (s) rather than as a true dentin extracellular matrix component.

Identification of osteoblast/osteocyte factor 45 (OF45), a bone-specific cDNA encoding an RGD-containing protein that is highly expressed in osteoblasts and osteocytes

We describe the cloning and characterization of a novel bone-specific cDNA predicted to encode an extracellular matrix protein. This cDNA was identified by subtractive hybridization based upon its high expression in bone marrow-derived osteoblasts. By Northern blot analysis, we detected a single 2-kilobase mRNA transcript in bone, whereas no expression was detected in other tissues. Immunohistochemistry revealed that the protein was expressed highly in osteocytes within trabecular and cortical bone. RNA and protein expression analysis using in vivo marrow ablation as a model of bone remodeling demonstrated that this gene was expressed only in cells that were embedded within bone matrix in contrast to the earlier expression of known osteoblast markers. The cDNA was predicted to encode a serine/glycine-rich secreted peptide containing numerous potential phosphorylation sites and one RGD sequence motif. The interaction of RGD domain-containing peptides with integrins has been shown previously to regulate bone remodeling by promoting recruitment, attachment, and differentiation of osteoblasts and osteoclasts. Secretion of this RGD-containing protein from osteocytes has the potential to regulate cellular activities within the bone environment and thereby may impact bone homeostasis. We propose the name OF45 (osteoblast/osteocyte factor of 45 kDa) for this novel cDNA.

Promotion of selective cell attachment by the RGD sequence in dentine matrix protein 1

Dentine matrix protein 1 (DMP1) is an important component of the non-collagenous extracellular matrix of developing teeth and bones. Functions of DMP1 other than a putative role in the initiation of mineralization are largely unknown. A first report on the DNA and deduced amino acid sequence showed that DMP1 has a single Arg-Gly-Asp (RGD) sequence. Here, whether the RGD sequence functions as a cell-attachment domain was tested. Using site-directed mutagenesis, two mutant recombinant DMP1 proteins with specific alterations at the RGD site were created. In the first mutant protein the RGD sequence was altered to a RGE (RGE) sequence; in the second the RGD domain was deleted (DEL). Mutated proteins were confirmed to be DMP1 by partial protein sequencing and dot-blot analysis with an anti-DMP1 antibody. Attachment of RPC-C2A (dental pulp cells), MC3T3-E1 (calvarial cells) or CHO (Chinese hamster ovary cells) to non-tissue-culture plastic coated with either DMP1, RGE or DEL proteins was compared. Bovine serum albumin and fibronectin served as negative and positive controls, respectively. The RGD-containing native DMP1 protein effectively allowed cell attachment and spreading. The RGE and DEL proteins with the altered and deleted RGD sites were significantly less effective in promoting cell attachment than the recombinant DMP1. Both RPC-C2A pulp cells and MC3T3-E1 cells showed similar reductions in attachment to mutated proteins. Treatment of RPC-C2A cells with a RGD-containing peptide prior to plating on DMP1-coated chambers abolished DMP1-mediated cell attachment. In contrast to RPC-C2A and MC3T3-E1cells, CHO cells, which normally do not express DMP1, failed to attach to DMP1. These data demonstrate that DMP1 promotes cell attachment through the RGD domain and that the attachment is cell- and tissue-specific. A basis for these observations is proposed using computer-generated models of the polypeptides within the DMP1 protein containing the RGD, RGE or DEL sequences.

Cloning and characterization of rat dentin matrix protein 1 (DMP1) gene and its 5'-upstream region

Rat dentin matrix protein 1 (DMP1) is a highly acidic 58-kDa phosphoprotein, and DMP1 was the first gene to be cloned from the mineralized dentin matrix. It exists as a highly phosphorylated protein with a pI of 3 in the dentin matrix and, in that state, might have an important role in the mineralization process. The spatio-temporal distribution during development indicates that the expression of this gene is tightly regulated in the odontoblasts. It is now known that DMP1 is not unique to dentin but is present in other mineralized tissues like long bone, calvaria, and ameloblasts. To study the transcriptional regulation and the function of DMP1 in these tissues, a genomic clone with a functional promoter, introns, and exons was isolated. Sequence analysis showed that the rat DMP1 gene is comprised of six exons and five introns and spans approximately 13 kilobases (kb). Exon 1 contains the 5'-untranslated sequences. Exon 2 encodes a total of 18 amino acids including the 16 amino acids of the signal sequence. Exons 3-5 encode 16, 11, and 15 amino acids, respectively. Exon 6 contains 1.3 kb of the coding sequence with the RGD domain, stop codon, and the 3'-untranslated region (1.1 kb). We have mapped two transcription start sites within the DMP1 promoter that are 280 and 321 base pairs, respectively, from the ATG start codon. The location of functional elements within the 5'-upstream DMP1 DNA fragment was determined by cloning it into a luciferase reporter gene. Transient transfection and luciferase assays revealed that the 3 kb fragment has the ability to drive the luciferase gene. However, this promoter activity was restricted to MC3T3-E1 cells (an osteoblast cell lineage). The promoter was silent in Chinese hamster ovary cells (an epithelial cell lineage), indicating the necessity of tissue-specific factors to drive the transcription.

Transcription factor Sp3 is essential for post-natal survival and late tooth development

Sp3 is a ubiquitously expressed transcription factor closely related to Sp1 (specificity protein 1). We have disrupted the mouse Sp3 gene by homologous recombination. Sp3-deficient embryos are growth retarded and invariably die at birth of respiratory failure. The cause for the observed breathing defect remains obscure since only minor morphological alterations were observed in the lung, and surfactant protein expression is indistinguishable from that in wild-type mice. Histological examinations of individual organs in Sp3(-/-) mice show a pronounced defect in late tooth formation. In Sp3 null mice, the dentin/enamel layer of the developing teeth is impaired due to the lack of ameloblast-specific gene products. Comparison of the Sp1 and Sp3 knockout phenotype shows that Sp1 and Sp3 have distinct functions in vivo, but also suggests a degree of functional redundancy.

Epithelial Dlx-2 homeogene expression and cementogenesis

The Dlx-2 (distal-less gene) homeoprotein transcription factor controls early tooth development but has not been studied during the late stages of biomineralization. Transgenic mice containing a Dlx-2/LacZ reporter construct were used to map the Dlx-2 expression pattern in cementoblasts, the dental cells most closely related to bone cells and therefore suggested to be uniquely positioned osteoblasts. During initial root formation, marked expression of Dlx-2 was evident in molar and incisor root epithelium, whereas dental papilla and follicle were negative. Dlx-2 was expressed in this epithelium from the apical loop to the area of its disruption. During acellular cementum formation in both incisors and molars, Dlx-2 expression was observed in the majority of differentiated cementoblasts from the apical region to the erupting zones. During cellular cementum formation, the presence of which characterizes growth-limited molars, Dlx-2 expression was restricted to the innermost cementoblasts and entrapped cementocytes. These data further support the hypothesis of a complex origin and fate of cementum-forming cells, as previously suggested by the expression patterns of a set of mesenchymal and epithelial markers, notably ameloblastin as shown here. Dlx-2 expression might constitute a landmark of cementoblast subpopulations of epithelial origin. (J Histochem Cytochem 48:277-283, 2000)

Identification of the chondrogenic-inducing activity from bovine dentin (bCIA) as a low-molecular-mass amelogenin polypeptide

Dentin extracellular matrix has been shown to contain components capable of inducing chondrogenesis and osteogenesis at ectopic sites when implanted in vivo, and chondrogenesis in cultures of embryonic muscle-derived fibroblasts (EMF) in vitro. The polypeptide responsible, called the chondrogenic-inducing agent (CIA), has been isolated from a 4.0-M guanidinium hydrochloride extract of demineralized bovine dentin matrix. Following Sephacryl S-100 chromatography, CIA activity was identified in fractions by assay for uptake of [35S]-SO4 into proteoglycan by the EMF after 24 hrs in culture. The active fraction induced the EMF to produce type II collagen mRNA and decrease production of type I collagen mRNA after 5 days in culture. The EMF + CIA, cultured for 4 to 7 wks, formed toluidine-blue- and alizarin-red-stainable nodules, indicative of chondrogenic induction. In vivo implants in rat muscle with collagen carrier produced ectopic bone after 7 wks. The CIA was brought to near-homogeneity by reverse-phase high-performance liquid chromatography, tested at each step by EMF [35S]-SO4-incorporation assays. The CIA components had masses in the ranges of 6000 to 10,000 Da by both mass spectroscopy and gel electrophoresis. The CIA amino acid composition, NH2-terminal, and internal amino acid sequences were determined. These data showed unequivocally that the CIA peptides were derived from bovine amelogenin. The peptides contain the amino-terminal portion of the bovine amelogenin. The presence of these chondrogenic/osteogenic amelogenin-polypeptides in dentin matrix leads us to hypothesize that they may be involved in epithelial-mesenchymal signaling during tooth development interactions-the first time a function has been indicated for these molecules.

Characterization of dentin matrix protein 1 gene in crocodilia

Several clones containing DMP1 cDNA were isolated from a caiman tooth library by screening with a platypus DMP1 probe. The caiman DMP1 shows little amino acid sequence similarity to mammalian DMP1s for much of its length. A few highly conserved regions can, however, be identified that correspond to the slowly evolving parts of the corresponding mammalian genes. Southern blot analysis using probes comprising either conserved regions or longer segments of the gene indicates that only a single DMP1 locus exists. In coding regions, exon-intron boundaries and reading frames are shared by caiman and mammalian genes with the exception of exons 1 and 5, which are longer in the caiman. The repetitive sequence of the last exon is shared by mammals and caiman as are the high Ser content and acidity due to a high proportion of Asp and Glu residues. The conserved mammalian cell-attachment signal Arg-Gly-Asp is absent in the caiman DMP1. In contrast to the amelogenin gene, the DMP1 gene appears to evolve rapidly in vertebrates.