Dentin sialophosphoprotein (DSPP) is cleaved into its two natural dentin matrix products by three isoforms of bone morphogenetic protein-1 (BMP1)

Research progress of biomimetic materials in oral medicine

Biomimetic materials are able to mimic the structure and functional properties of native tissues especially natural oral tissues. They have attracted growing attention for their potential to achieve configurable and functional reconstruction in oral medicine. Though tremendous progress has been made regarding biomimetic materials, significant challenges still remain in terms of controversy on the mechanism of tooth tissue regeneration, lack of options for manufacturing such materials and insufficiency of in vivo experimental tests in related fields. In this review, the biomimetic materials used in oral medicine are summarized systematically, including tooth defect, tooth loss, periodontal diseases and maxillofacial bone defect. Various theoretical foundations of biomimetic materials research are reviewed, introducing the current and pertinent results. The benefits and limitations of these materials are summed up at the same time. Finally, challenges and potential of this field are discussed. This review provides the framework and support for further research in addition to giving a generally novel and fundamental basis for the utilization of biomimetic materials in the future.

Identification of DSPP novel variants and phenotype analysis in dentinogenesis dysplasia Shields type II patients

OBJECTIVES

To investigate the genetic causes and teeth characteristics of dentin dysplasia Shields type II(DD-II) in three Chinese families.

MATERIALS AND METHODS

Data from three Chinese families affected with DD-II were collected. Whole-exome sequencing (WES) and whole-genome sequencing (WGS) were conducted to screen for variations, and Sanger sequencing was used to verify mutation sites. The physical and chemical characteristics of the affected teeth including tooth structure, hardness, mineral content, and ultrastructure were investigated.

RESULTS

A novel frameshift deletion mutation c.1871_1874del(p.Ser624fs) in DSPP was found in families A and B, while no pathogenic mutation was found in family C. The affected teeth's pulp cavities were obliterated, and the root canals were smaller than normal teeth and irregularly distributed comprising a network. The patients' teeth also had reduced dentin hardness and highly irregular dentinal tubules. The Mg content of the teeth was significantly lower than that of the controls, but the Na content was obviously higher than that of the controls.

CONCLUSIONS

A novel frameshift deletion mutation, c.1871_1874del (p.Ser624fs), in the DPP region of the DSPP gene causes DD-II. The DD-II teeth demonstrated compromised mechanical properties and changed ultrastructure, suggesting an impaired function of DPP. Our findings expand the mutational spectrum of the DSPP gene and strengthen the understanding of clinical phenotypes related to the frameshift deletion in the DPP region of the DSPP gene.

CLINICAL RELEVANCE

A DSPP mutation can alter the characteristics of the affected teeth, including tooth structure, hardness, mineral content, and ultrastructure.

Dynamics of the secreted frizzled related protein Sizzled and potential implications for binding to bone morphogenetic protein-1 (BMP-1)

Sizzled (Szl) is both a secreted frizzled related protein (sFRP) and a naturally occurring inhibitor of the zinc metalloproteinase bone morphogenetic protein-1 (BMP-1), a key regulator of extracellular matrix assembly and growth factor activation. Here we present a new crystal structure for Szl which differs from that previously reported by a large scale (90°) hinge rotation between its cysteine-rich and netrin-like domains. We also present results of a molecular docking analysis showing interactions likely to be involved in the inhibition of BMP-1 activity by Szl. When compared with known structures of BMP-1 in complex with small molecule inhibitors, this reveals features that may be helpful in the design of new inhibitors to prevent the excessive accumulation of extracellular matrix that is the hallmark of fibrotic diseases.

Effects of inflammation in dental pulp cell differentiation and reparative response

The responsiveness of the dentin-pulp complex is possible due to the stimulation of dental pulp cells, which begin to synthesize and secrete dentin matrix. The inflammatory process generated by harmful stimuli should be understood as a natural event of the immune response, resulting in the recruitment of hematopoietic cells, which cross the endothelial barrier and reach the site affected by the injury in order to eliminate the damage and provide an appropriate environment for the restoration of homeostasis. The repair process occurs in the presence of adequate blood supply, absence of infection, and with the participation of pro-inflammatory cytokines, growth factors, extracellular matrix components, and other biologically active molecules. Prostaglandins and leukotrienes are bioactive molecules derived from the metabolism of arachidonic acid, as a result of a variable range of cellular stimuli. The aim of this review is to describe the process of formation and biomineralization of the dentin-pulp complex and how pro-inflammatory events can modify this response, with emphasis on the lipid mediators prostaglandins and leukotrienes derived from arachidonic acid metabolism.

Translated Mutant DSPP mRNA Expression Level Impacts the Severity of Dentin Defects

Hereditary dentin defects are conventionally classified into three types of dentinogenesis imperfecta (DGI) and two types of dentin dysplasia (DD). Mutations in the dentin sialophosphoprotein (DSPP) gene have been identified to cause DGI type II and III and DD type II; therefore, these are not three different conditions, but rather allelic disorders. In this study, we recruited three families with varying clinical phenotypes from DGI-III to DD-II and performed mutational analysis by candidate gene analysis or whole-exome sequencing. Three novel mutations including a silent mutation (NM_014208.3: c.52-2del, c.135+1G>C, and c.135G>A; p.(Gln45=)) were identified, all of which affected pre-mRNA splicing. Comparison of the splicing assay results revealed that the expression level of the DSPP exon 3 deletion transcript correlated with the severity of the dentin defects. This study did not only expand the mutational spectrum of DSPP gene, but also advanced our understanding of the molecular pathogenesis impacting the severity of hereditary dentin defects.

Enamel Defects Associated With Dentin Sialophosphoprotein Mutation in Mice

Dentin sialophosphoprotein (DSPP) is an extracellular matrix protein that is highly expressed in odontoblasts, but only transiently expressed in presecretory ameloblasts during tooth development. We previously generated a knockin mouse model expressing a mouse equivalent (DSPP, p.P19L) of human mutant DSPP (p.P17L; referred to as “Dspp^P19L/+”), and reported that Dspp^P19L/+ and Dspp^P19L/P19L mice manifested a dentin phenotype resembling human dentinogenesis imperfecta (DGI). In this study, we analyzed pathogenic effects of mutant P19L-DSPP on enamel development in Dspp^P19L/+ and Dspp^P19L/P19L mice. Micro-Computed Tomography (μCT) analyses of 7-week-old mouse mandibular incisors showed that Dspp^P19L/P19L mice had significantly decreased enamel volume and/or enamel density at different stages of amelogenesis examined. Acid-etched scanning electron microscopy (SEM) analyses of mouse incisors demonstrated that, at the mid-late maturation stage of amelogenesis, the enamel of wild-type mice already had apparent decussating pattern of enamel rods, whereas only minute particulates were found in Dspp^P19L/+ mice, and no discernible structures in Dspp^P19L/P19L mouse enamel. However, by the time that incisor enamel was about to erupt into oral cavity, distinct decussating enamel rods were evident in Dspp^P19L/+ mice, but only poorly-defined enamel rods were revealed in Dspp^P19L/P19L mice. Moreover, μCT analyses of the mandibular first molars showed that Dspp^P19L/+ and Dspp^P19L/P19L mice had a significant reduction in enamel volume and enamel density at the ages of 2, 3, and 24weeks after birth. Backscattered and acid-etched SEM analyses revealed that while 3-week-old Dspp^P19L/+ mice had similar pattern of enamel rods in the mandibular first molars as age-matched wild-type mice, no distinct enamel rods were observed in Dspp^P19L/P19L mice. Yet neither Dspp^P19L/+ nor Dspp^P19L/P19L mice showed well-defined enamel rods in the mandibular first molars by the age of 24weeks, as judged by backscattered and acid-etched SEM. In situ hybridization showed that DSPP mRNA level was markedly reduced in the presecretory ameloblasts, but immunohistochemistry revealed that DSP/DSPP immunostaining signals were much stronger within the presecretory ameloblasts in Dspp mutant mice than in wild-type mice. These results suggest that mutant P19L-DSPP protein caused developmental enamel defects in mice, which may be associated with intracellular retention of mutant DSPP in the presecretory ameloblasts.

Procollagen C-proteinase enhancer-1 (PCPE-1), a potential biomarker and therapeutic target for fibrosis

The correct balance between collagen synthesis and degradation is essential for almost every aspect of life, from development to healthy aging, reproduction and wound healing. When this balance is compromised by external or internal stress signals, it very often leads to disease as is the case in fibrotic conditions. Fibrosis occurs in the context of defective tissue repair and is characterized by the excessive, aberrant and debilitating deposition of fibril-forming collagens. Therefore, the numerous proteins involved in the biosynthesis of fibrillar collagens represent a potential and still underexploited source of therapeutic targets to prevent fibrosis. One such target is procollagen C-proteinase enhancer-1 (PCPE-1) which has the unique ability to accelerate procollagen maturation by BMP-1/tolloid-like proteinases (BTPs) and contributes to trigger collagen fibrillogenesis, without interfering with other BTP functions or the activities of other extracellular metalloproteinases. This role is achieved through a fine-tuned mechanism of action that is close to being elucidated and offers promising perspectives for drug design. Finally, the in vivo data accumulated in recent years also confirm that PCPE-1 overexpression is a general feature and early marker of fibrosis. In this review, we describe the results which presently support the driving role of PCPE-1 in fibrosis and discuss the questions that remain to be solved to validate its use as a biomarker or therapeutic target.

Dentine sialophosphoprotein signal in dentineogenesis and dentine regeneration

Dentineogenesis starts on odontoblasts, which synthesise and secrete non-collagenous proteins (NCPs) and collagen. When dentine is injured, dental pulp progenitors/mesenchymal stem cells (MSCs) can migrate to the injured area, differentiate into odontoblasts and facilitate formation of reactionary dentine. Dental pulp progenitor cell/MSC differentiation is controlled at given niches. Among dental NCPs, dentine sialophosphoprotein (DSPP) is a member of the small integrin-binding ligand N-linked glycoprotein (SIBLING) family, whose members share common biochemical characteristics such as an Arg-Gly-Asp (RGD) motif. DSPP expression is cell- and tissue-specific and highly seen in odontoblasts and dentine. DSPP mutations cause hereditary dentine diseases. DSPP is catalysed into dentine glycoprotein (DGP)/sialoprotein (DSP) and phosphoprotein (DPP) by proteolysis. DSP is further processed towards active molecules. DPP contains an RGD motif and abundant Ser-Asp/Asp-Ser repeat regions. DPP-RGD motif binds to integrin αVβ3 and activates intracellular signalling via mitogen-activated protein kinase (MAPK) and focal adhesion kinase (FAK)-ERK pathways. Unlike other SIBLING proteins, DPP lacks the RGD motif in some species. However, DPP Ser-Asp/Asp-Ser repeat regions bind to calcium-phosphate deposits and promote hydroxyapatite crystal growth and mineralisation via calmodulin-dependent protein kinase II (CaMKII) cascades. DSP lacks the RGD site but contains signal peptides. The tripeptides of the signal domains interact with cargo receptors within the endoplasmic reticulum that facilitate transport of DSPP from the endoplasmic reticulum to the extracellular matrix. Furthermore, the middle- and COOH-terminal regions of DSP bind to cellular membrane receptors, integrin β6 and occludin, inducing cell differentiation. The present review may shed light on DSPP roles during odontogenesis.

DSPP dosage affects tooth development and dentin mineralization

Dentin Sialoprotein (DSP) and phosphophoryn (PP) are two most dominant non-collagenous proteins in dentin, which are the cleavage products of the DSPP (dentin sialophosphoprotein) precursor protein. The absence of the DSPP gene in DSPP knock-out (KO) mice results in characteristics that are consistent with dentinogenesis imperfecta type III in humans. Symptoms include thin dentin, bigger pulp chamber with frequent pulp exposure as well as abnormal epithelial-mesenchymal interactions, and the appearance of chondrocyte-like cells in dental pulp. To better understand how DSPP influences tooth development and dentin formation, we used a bacterial artificial chromosome transgene construct (BAC-DSPP) that contained the complete DSPP gene and promoter to generate BAC-DSPP transgenic mice directly in a mouse DSPP KO background. Two BAC-DSPP transgenic mouse strains were generated and characterized. DSPP mRNA expression in BAC-DSPP Strain A incisors was similar to that from wild-type (wt) mice. DSPP mRNA expression in BAC-DSPP Strain B animals was only 10% that of wt mice. PP protein content in Strain A incisors was 25% of that found in wt mice, which was sufficient to completely rescue the DSPP KO defect in mineral density, since microCT dentin mineral density analysis in 21-day postnatal animal molars showed essentially identical mineral density in both strain A and wt mice. Strain B mouse incisors, with 5% PP expression, only partially rescued the DSPP KO defect in mineral density, as microCT scans of 21-day postnatal animal molars indicated a reduced dentin mineral density compared to wt mice, though the mineral density was still increased over that of DSPP KO. Furthermore, our findings showed that DSPP dosage in Strain A was sufficient to rescue the DSPP KO defect in terms of epithelial-mesenchymal interactions, odontoblast lineage maintenance, along with normal dentin thickness and normal mineral density while DSPP gene dosage in Strain B only partially rescued the aforementioned DSPP KO defect.

Cascade of interactions between candidate genes reveals convergent mechanisms in keratoconus disease pathogenesis

Keratoconus is a progressive thinning, steepening and distortion of the cornea which can lead to loss of vision if left untreated. Keratoconus has a complex multifactorial etiology, with genetic and environmental components contributing to the disease pathophysiology. Studies have observed high concordance between monozygotic twins, discordance between dizygotic twins, and high familial segregation indicating the presence of a very strong genetic component in the pathogenesis of keratoconus. The use of genome-wide linkage studies on families and twins, genome-wide association studies (GWAS) on case-controls, next-generation sequencing (NGS)-based genomic screens on both familial and non-familial cohorts have led to the identification of keratoconus candidate genes with much greater success and increased resproducibility of genetic findings. This review focuses on candidate genes identified till date and attempts to understand their role in biological processes underlying keratoconus pathogenesis. In addition, using these genes I propose molecular pathways that could contribute to keratoconus pathogenesis. The pathways identified the presence of direct cross-talk between known candidate genes of keratoconus and remarkably, 28 known candidate genes have a direct relationship among themselves that involves direct protein-protein binding, regulatory activities such as activation and inhibition, chaperone, transcriptional activation/co-activation, and enzyme catalysis. This review attempts to describe these relationships and cross-talks in the context of keratoconus pathogenesis.

Distinguished properties of cells isolated from the dentin-pulp interface

BACKGROUND

Dental odontoblasts produce dentin mineralized matrix, trigger immune responses and act as sensory cells. The understanding of the mechanisms of these functions has been particularly restricted due to the lack of odontoblasts being cultivable in vitro. Because of the lack of specific markers to identify cells of the odontoblastic lineage, properties of the cells isolated from the dentin-pulp interface were compared to dental pulp cells, periodontal ligament cells, osteoblasts, skin fibroblasts, epithelial cells (A549) and HeLa in the present study.

METHODS

After surgical procedures, the pulp tissue was removed from the tooth crown, and cells were scrapped off the dentin-pulp interface. Explants from teeth of three patients were routinely cultivated, and cells were harvested after several weeks. Cell morphology and ultrastructure was studied by light microscopy (LM), scanning (SEM) or transmission electron microscopy (TEM). Expression of dentin sialophosphoprotein (DSPP), dentin matrix protein 1 (DMP1), TRPV4, and S100 calcium binding protein A4 (S100A4) were analyzed at the protein level by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting using specific antibodies. The differential expression of S100A4 in the various cell lines was further investigated at the gene level by semiquantitative real-time PCR. Mineralization in the various cell types was observed after alizarin red staining after a 28 days incubation period. The immunophenotype of the cells was examined by flow cytometry using monoclonal anti-human antibodies CD90-FITC, CD73-PE, CD105-PE, CD29-PE, CD140a-FITC, CD144-PE, CD45-FITC or CD34-FITC. Differences between median values were statistically analyzed (Mann-Whitney U-test).

RESULTS

Cells from the dentin-pulp interface retain the polarity of odontoblast morphology in culture with an elongated, rounded cell body, and an extended cellular process. Ultrastructural analysis of the cells indicates high secretory activity including the extracellular deposition of fibrillar collagen. An extended rough endoplasmic reticulum is lined by a large number of ribosomes, and a vast number of secretory granules merges with the cell membrane. Protein expression of DSPP, DMP1, and TRPV4 as a transient receptor potential cation was detected in all cell lines. S100A4 was found differentially expressed in cultures of cells from tooth tissues. High expression of S100A4 was observed at the protein and gene level in two fractions of cells isolated from the dentin-pulp interface, but was absent or only weakly expressed in pulp cells. S100A4 expression in cells from the dentin-pulp interface and pulp cells is consistent with the intensity of the formation of mineralized nodules detected by alizarin red staining. Immunophenotyping revealed that a high percentage of CD73 (ecto-5-nucleotidase), an enzyme active on the surface of immune-competent cells, was expressed in cells of the dentin-pulp interface. While 72%-78% of positive cells were detected in dentin-pulp interface fractions, only 28-64% of the cells in pulp cell cultures were stained.

CONCLUSIONS

The present findings obtained with a variety of cells of different origin provide experimental evidence that cells isolated from the dentin-pulp interface express unique properties different from dental pulp cells in particular. The differential expression of S100A4 is a relevant marker candidate for differentiating between dental pulp cells and cells of the odontoblast lineage.

[Role of bone morphogenetic protein 1/tolloid proteinase family in the development of teeth and bone]

The bone morphogenetic protein (BMP) 1/tolloid (TLD) proteinase family is a group of important metalloproteinases, which play key roles in the growth and development of tissues and organs via regulating the biosynthetic processing of the extracellular matrix. Clinical reports have revealed that mutations in the genes encoding BMP1/TLD proteinases lead to dentinogenesis imperfecta type Ⅰ, accompanied with osteogenesis imperfecta. Therefore, this proteinase family is essential for the development of hard tissues. In this study, we review the research progress in the function and mechanism of the BMP1/TLD proteinase family in the development of teeth and bone.

The Effect of Mesoporous Bioactive Glass Nanoparticles/Graphene Oxide Composites on the Differentiation and Mineralization of Human Dental Pulp Stem Cells

The purpose of this study was to investigate the effects of mesoporous bioactive glass nanoparticle (MBN)/graphene oxide (GO) composites on the mineralization ability and differentiation potential of human dental pulp stem cells (hDPSCs). MBN/GO composites were synthesized using the sol-gel method and colloidal processing to enhance the bioactivity and mechanical properties of MBN. Characterization using FESEM, XRD, FTIR, and Raman spectrometry showed that the composites were successfully synthesized. hDPSCs were then cultured directly on the MBN/GO (40:1 and 20:1) composites in vitro. MBN/GO promoted the proliferation and alkaline phosphatase (ALP) activity of hDPSCs. In addition, qRT-PCR showed that MBN/GO regulated the mRNA levels of odontogenic markers (dentin sialophosphoprotein (DSPP), dentine matrix protein 1 (DMP-1), ALP, matrix extracellular phosphoglycoprotein (MEPE), bone morphogenetic protein 2 (BMP-2), and runt-related transcription factor 2 (RUNX-2)). The mRNA levels of DSPP and DMP-1, two odontogenesis-specific markers, were considerably upregulated in hDPSCs in response to growth on the MBN/GO composites. Western blot analysis revealed similar results. Alizarin red S staining was subsequently performed to further investigate MBN/GO-induced mineralization of hDPSCs. It was revealed that MBN/GO composites promote odontogenic differentiation via the Wnt/β-catenin signaling pathway. Collectively, the results of the present study suggest that MBN/GO composites may promote the differentiation of hDPSCs into odontoblast-like cells, and potentially induce dentin formation.

Dentin phosphoprotein inhibits lipopolysaccharide-induced macrophage activation independent of its serine/aspartic acid-rich repeats

OBJECTIVE

The objective of this study was to investigate the effects of dentin phosphoprotein (DPP) on lipopolysaccharide-induced inflammatory responses of macrophages in vitro.

DESIGN

Wildtype and mutant recombinant dentin phosphoprotein (rDPP) proteins were generated using a mammalian expression system. Macrophages, phorbol 12-myristate 13-acetate-differentiated THP-1 cells, were stimulated with lipopolysaccharide in the absence or presence of rDPP proteins. After the 24-hr incubation, the inflammatory gene expression levels were examined by quantitative reverse-transcription polymerase chain reaction and the amount of secreted TNF-α protein was evaluated by enzyme-linked immunosorbent assay. Furthermore, the subcellular localization of exogenously added rDPP was examined by immunocytochemistry, and the direct binding of rDPP to lipopolysaccharide was quantified by solid-phase binding assay.

RESULTS

rDPP dose-dependently reduced the expression of lipopolysaccharide-induced inflammatory genes, such as TNFα, IL-1β, and IL-8, and TNF-α protein secretion from the macrophages. Furthermore, mutant rDPP having a shortened serine/aspartic acid-rich repeats (SDrr) was also able to inhibit lipopolysaccharide-induced inflammatory responses of macrophages. rDPP was localized adjacent to the cellular membrane rather than in the cytoplasm, and rDPP was able to bind to lipopolysaccharide. These results suggested that rDPP inhibited lipopolysaccharide-induced inflammatory responses by binding to lipopolysaccharide.

CONCLUSIONS

In addition to the well-known functions of DPP for dentin mineralization that depend on the SDrr, we demonstrated that DPP possesses anti-inflammatory effects on lipopolysaccharide-stimulated macrophages that are independent of the SDrr.

Mutant Dentin Sialophosphoprotein Causes Dentinogenesis Imperfecta

Dentin sialophosphoprotein (DSPP) is an extracellular matrix protein highly expressed by odontoblasts in teeth. DSPP mutations in humans may cause dentinogenesis imperfecta (DGI), an autosomal dominant dentin disorder. We recently generated a mouse model (named "Dspp^P19L/+ mice") that expressed a mutant DSPP in which the proline residue at position 19 was replaced by a leucine residue. We found that the Dspp^P19L/+ and Dspp^P19L/P19L mice at a younger age displayed a tooth phenotype resembling human DGI type III characterized by enlarged dental pulp chambers, while the teeth of older Dspp^P19L/+ and Dspp^P19L/P19L mice had smaller dental pulp chambers mimicking DGI type II. The teeth of Dspp^P19L/+ and Dspp^P19L/P19L mice had a narrower pulp chamber roof predentin layer, thinner pulp chamber roof dentin, and thicker pulp chamber floor dentin. In addition, these mice also had increased enamel attrition, accompanied by excessive deposition of peritubular dentin. Immunohistochemistry, in situ hybridization, and real-time polymerase chain reaction analyses showed that the odontoblasts in both Dspp^P19L/+ and Dspp^P19L/P19L mice had reduced DSPP expression, compared to the wild-type mice. We also observed that the levels of DSPP expression were much higher in the roof-forming odontoblasts than in the floor-forming odontoblasts in the wild-type mice and mutant mice. Moreover, immunohistochemistry showed that while the immunostaining signals of dentin sialoprotein (N-terminal fragment of DSPP) were decreased in the dentin matrix, they were remarkably increased in the odontoblasts of the Dspp^P19L/+ and Dspp^P19L/P19L mice. Consistently, our in vitro studies showed that the secretion of the mutant DSPP was impaired and accumulated within endoplasmic reticulum. These findings suggest that the dental phenotypes of the mutant mice were associated with the intracellular retention of the mutant DSPP in the odontoblasts of the DSPP-mutant mice.

Conditional Knockout of Raptor/mTORC1 Results in Dentin Malformation

mTORC1 signaling plays an important role in extracellular and intracellular signals, including growth factors, nutrients, energy metabolism, and stress. However, the functional role of mTORC1 in dentinogenesis is unknown. To study the role of Raptor/mTORC1 in dentinogenesis, an Raptor^fl/fl; Osx-Cre (Rap-Osx) mouse, in which Raptor was conditionally deleted in odontoblasts and dental mesenchymal cells, was generated, and postnatal tooth development was compared between Rap-Osx mice and control littermates. Rap-Osx mice presented a phenotype known as dentinogenesis imperfecta and had smaller tooth volume, a thinner dentin layer and a larger pulp chamber. The proliferation and differentiation of odontoblasts/preodontoblasts were attenuated in mutant mice, which was likely responsible for the defects in dentinogenesis. Raptor/mTORC1-pS6K1 signaling was inactivated during tooth development in Rap-Osx mice, whereas it was activated in control mice. These results indicate that Raptor/mTORC1 plays a critical role in dentinogenesis via promoting odontoblasts/preodontoblasts proliferation and differentiation. Raptor/mTORC1 might regulate tooth development through the pS6K1 signaling pathway.

Procollagen C-Proteinase Enhancer 1 (PCPE-1) in Liver Fibrosis

Fibrosis is characterized by excessive deposition of collagen and additional extracellular matrix (ECM) components in response to chronic injuries. Liver fibrosis often results from chronic hepatitis C virus infection and alcohol abuse that can deteriorate to cirrhosis and liver failure. Current noninvasive diagnostic methods of liver fibrosis are limited in their ability to detect and differentiate between early and intermediate stages of fibrosis. New biomarkers of fibrosis that reflect ECM turnover are therefore badly needed. Procollagen C-proteinase enhancer 1 (PCPE-1), a connective tissue glycoprotein that functions as a positive regulator of C-terminal procollagen processing and subsequent collagen fibril assembly, is a promising candidate. Its tissue distribution and expression profile overlap those of collagen, and its expression in fibrosis is upregulated in parallel to the increase in collagen expression. The potential of PCPE-1 as a biomarker of liver fibrosis was recently established using a CCl₄ mouse model of liver fibrosis by showing that the increase in collagen and PCPE-1 content in the fibrotic mouse liver was reflected by elevated plasma levels of PCPE-1. This was achieved using a newly developed highly sensitive, specific, accurate, and reproducible ELISA for mouse PCPE-1, which is based on commercially available antibodies and is offered as a new research tool in the field. A similar ELISA test was developed for human PCPE-1, and preliminary results with plasma from liver fibrosis patients revealed increased plasma concentrations of PCPE-1 in some patients. The protocols of both ELISA tests are outlined herein in great detail to permit their application by any laboratory with similar interests.

Phosphophoryn and Dentin Sialoprotein Effects on Dental Pulp Cell Migration, Proliferation, and Differentiation

Phosphophoryn (PP) and dentin sialoprotein (DSP) are two of the most abundant dentin matrix non-collagenous proteins, and are derived from dentin sialoprotein-phosphophoryn (DSP-PP) mRNA. Mutations in the DSP-PP gene are linked to dentinogenesis imperfecta II and III. Previously, we reported transient DSP-PP expression in preameloblast cells first, followed by co-expression in preameloblasts and young odontoblasts, and finally sustained expression in odontoblasts. This phenomenon raised the possibility that DSP/PP proteins secreted by preameloblasts might promote dental pulp cell migration toward the dental pulp border and promote dental pulp cell differentiation. To examine the effects of DSP/PP proteins on dental pulp cell development, we investigated:(1) native PP effects on dental pulpcell migration and matrix protein expression; and (2) recombinant DSP/PP protein effects on cell proliferation and differentiation. We found that PP promoted cell migration and the expression of high levels of Col type I and PP in dental pulp cells. The addition of recombinant DSP/PP proteins affected cell proliferation and differentiation in a dental pulp cell line. These findings strongly suggest that DSP/PP may modulate cell migration, cell proliferation and differentiation, thus leading to dentin formation. DSP/PP protein may be useful clinically for pulp tissue regeneration.

Synergistic effect of PCPE1 and sFRP2 on the processing of procollagens via BMP1

Procollagen processing is essential for organ development and tissue functions. Both procollagen C-proteinase enhancer 1 (PCPE1) and secreted frizzled-related protein 2 (sFRP2) play vital roles in collagen formation via regulating the procollagen C-proteinase activity of bone morphogenetic protein 1 (BMP1). However, whether the two proteins exert a synergistic effect on BMP1 activity remains unclear. Here, simultaneous knockdown of sFRP2 and PCPE1 led to less collagen formation in mouse embryonic fibroblasts and dorsalized phenotypes in zebrafish embryos. Further studies revealed a direct interaction between the Frizzled domain of sFRP2 and the complement/Uegf/BMP-1 domain of PCPE1, which enhances the cleavage activity of BMP1 on procollagen. These results suggest that double silencing of sFRP2 and PCPE1 may provide a strategy for treating fibrosis diseases caused by collagen deposition.

The functional significance of dentin sialoprotein-phosphophoryn and dentin sialoprotein

Phosphophoryn (PP) and dentin sialoprotein (DSP) are the most dominant non-collagenous proteins in dentin. PP is an extremely acidic protein that can function as a mineral nucleator for dentin mineralization. DSP was first identified in 1981, yet its functional significance is still controversial. Historically, these two proteins were considered to be independently synthesized and secreted by dental pulp cells into the developing dentin matrix. However, with the identification of the DSP coding sequence in 1994, followed 2 years later by the finding that the PP coding sequence was located immediately downstream from the DSP sequence, it became immediately clear that DSP and PP proteins were derived from a single DSP-PP (i.e., dentin sialophosphoprotein, DSPP) transcript. Since DSPP cDNA became available, tremendous progress has been made in studying DSP-PP mRNA distribution and DSP generation from the DSP-PP precursor protein at specific cleavage sites by protease tolloid-related-1 (TLR1) or bone morphogenetic protein 1 (BMP1). The functions of DSP-PP and DSP were investigated via DSP-PP knockout (KO) and DSP knockin in DSP-PP KO mice. In addition, a number of in vitro studies aimed to elucidate DSPP and DSP function in dental pulp cells.

Along with phosphophoryn (PP), dental sialoprotein (DSP) is the dominant non-collagen protein in dentin, and in vitro studies have demonstrated that DSP is involved in inducing the differentiation of dental pulp cells into odontoblast-like cells, which form dentin. PP is known to be involved in the mineralization of dentin, but the functional significance of DSP had been controversial. Helena Ritchie of the University of Michigan School of Dentistry conducted a review of studies investigating the derivation, function and distribution of PP and DSP. It was originally thought that PP and DSP were synthesized independently; later, it became evident that they derive from a single DSP-PP gene. Wider DSP-PP distribution in various tissues, including kidney and salivary glands, and DSP or PP expression in non-mineralized tissues suggest that the proteins may have functions other than mineralization.

Surf4 (Erv29p) binds amino-terminal tripeptide motifs of soluble cargo proteins with different affinities, enabling prioritization of their exit from the endoplasmic reticulum

Some secreted proteins that assemble into large complexes, such as extracellular matrices or hormones and enzymes in storage granules, must be kept at subaggregation concentrations during intracellular trafficking. We show surfeit locus protein 4 (Surf4) is the cargo receptor that establishes different steady-state concentrations for a variety of soluble cargo proteins within the endoplasmic reticulum (ER) through interaction with the amino-terminal tripeptides exposed after removal of leader sequences. We call this motif the ER-Exit by Soluble Cargo using Amino-terminal Peptide-Encoding motif (ER-ESCAPE motif). Proteins that most readily aggregate in the ER lumen (e.g., dentin sialophosphoprotein [DSPP] and amelogenin, X-linked [AMELX]) have strong ER-ESCAPE motifs to inhibit aggregate formation, while less susceptible cargo exhibits weaker motifs. Specific changes in a single amino acid of the tripeptide result in aggregate formation and failure to efficiently traffic cargo out of the ER. A logical subset of 8,000 possible tripeptides starting a model soluble cargo protein (growth hormone) established a continuum of steady-state ER concentrations ranging from low (i.e., high affinity for receptor) to the highest concentrations associated with bulk flow-limited trafficking observed for nonbinding motifs. Human cells lacking Surf4 no longer preferentially trafficked cargo expressing strong ER-ESCAPE motifs. Reexpression of Surf4 or expression of yeast's ortholog, ER-derived vesicles protein 29 (Erv29p), rescued enhanced ER trafficking in Surf4-null cells. Hence our work describes a new way of preferentially exporting soluble cargo out of the ER that maintains proteins below the concentrations at which they form damaging aggregates.

SCPP Genes and Their Relatives in Gar: Rapid Expansion of Mineralization Genes in Osteichthyans

Gar is an actinopterygian that has bone, dentin, enameloid, and ganoin (enamel) in teeth and/or scales. Mineralization of these tissues involves genes encoding various secretory calcium-binding phosphoproteins (SCPPs) in osteichthyans, but no SCPP genes have been identified in chondrichthyans to date. In the gar genome, we identified 38 SCPP genes, seven of which encode "acidic-residue-rich" proteins and 31 encode "Pro/Gln (P/Q) rich" proteins. These gar SCPP genes constitute the largest known repertoire, including many newly identified P/Q-rich genes expressed in teeth and/or scales. Among gar SCPP genes, six acidic and three P/Q-rich genes were identified as orthologs of sarcopterygian genes. The sarcopterygian orthologs of most of these acidic genes are involved in bone and/or dentin formation, and sarcopterygian orthologs of all three P/Q-rich genes participate in enamel formation. The finding of these genes in gar suggests that an elaborate SCPP gene-based genetic system for tissue mineralization was already present in stem osteichthyans. While SCPP genes have been thought to originate from ancient SPARCL1, SPARCL1L1 appears to be more closely related to these genes, because it established a structure similar to acidic SCPP genes probably in stem gnathostomes, perhaps at about the same time with the origin of tissue mineralization. Assuming enamel evolved in stem osteichthyans, all P/Q-rich SCPP genes likely arose within the osteichthyan lineage. Furthermore, the absence of acidic SCPP genes in chondrichthyans might be explained by the secondary loss of earliest acidic genes. It appears that many SCPP genes expanded rapidly in stem osteichthyans and in basal actinopterygians.

Essential Roles of Bone Morphogenetic Protein-1 and Mammalian Tolloid-like 1 in Postnatal Root Dentin Formation

INTRODUCTION

Mutations in the proteinase bone morphogenetic protein-1 (BMP1) were recently identified in patients with osteogenesis imperfecta, which can be associated with type 1 dentinogenesis imperfecta. BMP1 is co-expressed in various tissues and has overlapping activities with the closely related proteinase mammalian tolloid-like 1 (TLL1). In this study we investigated whether removing the overlapping activities of BMP1 and TLL1 affects the mineralization of tooth root dentin.

METHODS

Floxed alleles of the BMP1 and TLL1 genes were excised via ubiquitously expressed Cre induced by tamoxifen treatment beginning at 3 days of age (harvested at 3 weeks of age) or beginning at 4 weeks of age (harvested at 8 weeks of age). Multiple techniques, including x-ray analysis, double-labeling with calcein and alizarin red stains for measurement of dentin formation rate, and histologic and immunostaining assays, were used to analyze the dentin phenotype.

RESULTS

BMP1/TLL1 double knockout mice displayed short and thin root dentin, defects in dentin mineralization, and delayed tooth eruption. Molecular mechanism studies revealed accumulation of collagens in dentin and a sharp reduction in non-collagenous proteins such as dentin matrix protein 1 and dentin sialophosphoprotein. Furthermore, we found a strong reduction in tartrate-resistant acid phosphatase, which is likely caused by defects in bone cells.

CONCLUSIONS

BMP1/TLL1 appear to play crucial roles in maintaining extracellular matrix homeostasis essential to root formation and dentin mineralization.

Inactivation of bone morphogenetic protein 1 (Bmp1) and tolloid-like 1 (Tll1) in cells expressing type I collagen leads to dental and periodontal defects in mice

Bone morphogenetic protein 1 (BMP1) and tolloid-like 1 (TLL1) belong to the BMP1/tolloid-like proteinase family, which cleaves secretory proteins. The constitutive deletion of the Bmp1 or Tll1 genes causes perinatal or embryonic lethality in mice. In this study, we first studied the β-galactosidase activity in mice in which an IRES-lacZ-Neo cassette was inserted in the intron of either the Bmp1 or the Tll1 gene; the β-galactosidase activities were used to reflect the expression of endogenous Bmp1 and Tll1, respectively. Our X-gal staining results showed that the odontoblasts in the tooth and cells in the periodontal ligament express both Bmp1 and Tll1. We then created Bmp1 ^flox/flox and Tll1 ^flox/flox mice by removing the IRES-lacZ-Neo cassette. By breeding 2.3 kb Col1a1-Cre mice with the Bmp1 ^flox/flox and Tll1 ^flox/flox mice, we further generated Col1a1-Cre;Bmp1 ^flox/flox ;Tll1 ^flox/flox mice in which both Bmp1 and Tll1 were inactivated in the Type I collagen-expressing cells. We employed X-ray radiography, histology and immunohistochemistry approaches to characterize the Col1a1-Cre;Bmp1 ^flox/flox ;Tll1 ^flox/flox mice. Our results showed that the molars of the Col1a1-Cre;Bmp1 ^flox/flox ;Tll1 ^flox/flox mice had wider predentin, thinner dentin and larger pulp chambers than those of the normal controls. The dentinal tubules of the molars in the Col1a1-Cre;Bmp1 ^flox/flox ;Tll1 ^flox/flox mice appeared disorganized. The level of dentin sialophosphoprotein in the molars of the 6-week-old Col1a1-Cre;Bmp1 ^flox/flox ;Tll1 ^flox/flox mice was lower than in the normal controls. The periodontal ligaments of the Col1a1-Cre;Bmp1 ^flox/flox ;Tll1 ^flox/flox mice were disorganized and had less fibrillin-1. Our findings indicate that the proteinases encoded by Bmp1 and Tll1 genes play essential roles in the development and maintenance of mouse dentin and periodontal ligaments.

sFRP2 activates Wnt/β-catenin signaling in cardiac fibroblasts: differential roles in cell growth, energy metabolism, and extracellular matrix remodeling

Secreted Frizzled-related protein 2 (sFRP2) plays a key role in chronic fibrosis after myocardial infarction and in heart failure. The aim of this study was to elucidate the mechanisms through which sFRP2 may regulate the growth and extracellular matrix (ECM) remodeling of adult mouse cardiac fibroblasts (CFs). We found that sFRP2 activates CFs in part through canonical Wnt/β-catenin signaling, as evidenced by increased expression of Axin2 and Wnt3a, but not Wnt5a, as well as accumulation of nuclear β-catenin. In response to sFRP2, CFs exhibited robust cell proliferation associated with increased glucose consumption and lactate production, a phenomenon termed "the Warburg effect" in oncology. The coupling between CF expansion and anaerobic glycolysis is marked by upregulation of glyceraldehyde-3-phosphate dehydrogenase and tissue-nonspecific alkaline phosphatase. In conjunction with these phenotypic changes, CFs accelerated ECM remodeling through upregulation of expression of the matrix metalloproteinase (MMP) 1 and MMP13 genes, two members of the collagenase subfamily, and enzyme activities of MMP2 and MMP9, two members of the gelatinase subfamily. Consistent with the induction of multiple MMPs possessing collagenolytic activities, the steady-state level of collagen type 1 in CF-spent medium was reduced by sFRP2. Analysis of non-CF cell types revealed that the multifaceted effects of sFRP2 on growth control, glucose metabolism, and ECM regulation are largely restricted to CFs and highly sensitive to Wnt signaling perturbation. The study provides a molecular framework on which the functional versatility and signaling complexity of sFRP2 in cardiac fibrosis may be better defined.

Procollagen C-Proteinase Enhancer 1 (PCPE-1) as a Plasma Marker of Muscle and Liver Fibrosis in Mice

Current non-invasive diagnostic methods of fibrosis are limited in their ability to identify early and intermediate stages of fibrosis and assess the efficacy of therapy. New biomarkers of fibrosis are therefore constantly sought for, leading us to evaluate procollagen C-proteinase enhancer 1 (PCPE-1), a fibrosis-related extracellular matrix glycoprotein, as a plasma marker of fibrosis. A sandwich ELISA that permitted accurate measurements of PCPE-1 concentrations in mouse plasma was established. Tissue fibrosis was assessed using histochemical, immunofluorescence, and immunoblotting analyses for type I collagen and PCPE-1. The normal plasma concentration of PCPE-1 in 6 weeks to 4 months old mice was ~200 ng/ml (189.5 ± 11.3 to 206.8 ± 13.8 ng/ml). PCPE-1 plasma concentrations in four and 8.5 months old mdx mice displaying fibrotic diaphragms increased 27 and 40% respectively relatively to age-matched control mice, an increase comparable to that of the N-propeptide of procollagen type III (PIIINP), a known blood marker of fibrosis. PCPE-1 plasma levels in mice with CCl4-induced liver fibrosis increased 34 to 50% relatively to respective controls and reflected the severity of the disease, namely increased gradually during the progression of fibrosis and went down to basal levels during recovery, in parallel to changes in the liver content of collagen I and PCPE-1. The results favor PCPE-1 as a potential new clinically valuable fibrosis biomarker.

Strontium (Sr) elicits odontogenic differentiation of human dental pulp stem cells (hDPSCs): A therapeutic role for Sr in dentine repair?

Strontium (Sr) forms a significant component of dental restorative materials and although it is widely used in toothpastes, the biological effects of Sr on the dentine-pulp complex have not been investigated. In this first study, we characterise the Sr elicited effects on human dental pulp stem cells (hDPSC) in vitro using exogenously Sr added to culture medium, and bioavailable Sr derived from a novel bioactive glass (BG). The related mechanisms were also investigated. Our results indicate that low dose Sr (between 0.1 and 2.5mM) induces proliferation and alkaline phosphatase (ALP) activity of hDPSCs, but has no effect on colony formation or cell migration. Sr at specific concentrations (1 and 2.5mM) stimulated collagen formation and mineralisation of the hDPSC generated matrix. In addition, qRT-PCR, Western blotting and immunocytochemistry revealed that Sr regulates gene expression and the protein secretion of the odontogenic markers: dentine sialophosphoprotein (DSPP) and dentine matrix protein 1 (DMP-1) and protein localisation (DSPP was localised to the Golgi, while no apparent changes occur in DMP-1 distribution which remains in both cytosol and the nucleus). Additionally, the calcium sensing receptor (CaSR) and downstream pathway MAPK/ERK signalling pathway in hDPSCs were activated by Sr. Bioavailable Sr from the BG revealed novel biological insights of regulating metabolic and ALP activities in hDPSCs. Taken together, these results suggest that Sr at specific doses significantly influences proliferation, odontogenic differentiation and mineralisation of hDPSCs in vitro via the CaSR using a pathway with similarities to osteoblast differentiation. These are the first such studies and indicate that Sr treatment of hDPSCs could be a promising therapeutic agent in dental applications. In conclusion, we propose that Sr from a substituted BG could be used more effectively in biomaterials designed for dental applications.

STATEMENT OF SIGNIFICANCE

Despite the fact that strontium (Sr) is used widely in dental practise, its potential effects on odontoblasts have been ignored. Our study provides the first evidence that Sr (exogenous and that derived from a bioglass (BG)) can stimulate dentinogenesis in human dental pulp stem cells (hDPSCs) by promoting their proliferation, differentiation and mineralisation in vitro. Therefore, while previously unrecognised, Sr BG is likely to be beneficial in atraumatic dentistry practise and maintenance of a competent tooth in conditions such as caries. Repair of defected dentine is still one of the main challenges in dental research and annually untreated caries results in the loss of productivity equivalent to US$ 27 billion. Advances in tissue engineering technology, alongside the use of dental pulp stem cells provide an approach to achieve dentine regeneration. Understanding the actions of Sr will permit a more controlled application of Sr in the clinic. These data are thus likely to be of great interest to the material scientists, biological researchers, clinicians and manufacturers of dental products.

Transgenic expression of dentin phosphoprotein inhibits skeletal development

Dentin sialophosphoprotein (DSPP) is proteolytically processed into an NH₂-terminal fragment called dentin sialoprotein (DSP) and a COOH-terminal fragment known as dentin phosphoprotein (DPP). These two fragments are believed to perform distinct roles in formation of bone and dentin. To investigate the functions of DPP in skeletal development, we generated transgenic mice to overexpress hemagglutinin (HA)-tagged DPP under the control of a 3.6 kb type I collagen (Col1a1) promoter (designated as Col1a1-HA-DPP). The Col1a1-HA-DPP transgenic mice were significantly smaller by weight, had smaller skeletons and shorter long bones than their wild type littermates, as demonstrated by X-ray radiography. They displayed reduced trabecular bone formation and narrower zones of proliferative and hypertrophic chondrocytes in the growth plates of the long bones. Histological analyses showed that the transgenic mice had reduced cell proliferation in the proliferating zone, but lacked obvious defects in the chondrocyte differentiation. In addition, the transgenic mice with a high level of transgene expression developed spontaneous long bone fractures. In conclusion, overexpressing DPP inhibited skeletal development, suggesting that the balanced actions between the NH₂- and COOH-terminal fragments of DSPP may be required for normal skeletal development.

The dentin phosphoprotein repeat region and inherited defects of dentin

Nonsyndromic dentin defects classified as type II dentin dysplasia and types II and III dentinogenesis imperfecta are caused by mutations in DSPP (dentin sialophosphoprotein). Most reported disease‐causing DSPP mutations occur within the repetitive DPP (dentin phosphoprotein) coding sequence. We characterized the DPP sequences of five probands with inherited dentin defects using single molecule real‐time (SMRT) DNA sequencing. Eight of the 10 sequences matched previously reported DPP length haplotypes and two were novel. Alignment with known DPP sequences showed 32 indels arranged in 36 different patterns. Sixteen of the 32 indels were not represented in more than one haplotype. The 25 haplotypes with confirmed indels were aligned to generate a tree that describes how the length variations might have evolved. Some indels were independently generated in multiple lines. A previously reported disease‐causing DSPP mutation in Family 1 was confirmed and its position clarified (c.3135delC; p.Ser1045Argfs*269). A novel frameshift mutation (c.3504_3508dup; p.Asp1170Alafs*146) caused the dentin defects in Family 2. A COL1A2 (c.2027G>A or p.Gly676Asp) missense mutation, discovered by whole‐exome sequencing, caused the dentin defects in Family 3. We conclude that SMRT sequencing characterizes the DPP repeat region without cloning and can improve our understanding of normal and pathological length variations in DSPP alleles.

Accelerated enamel mineralization in Dspp mutant mice

Dentin sialophosphoprotein (DSPP) is one of the major non-collagenous proteins present in dentin, cementum and alveolar bone; it is also transiently expressed by ameloblasts. In humans many mutations have been found in DSPP and are associated with two autosomal-dominant genetic diseases - dentinogenesis imperfecta II (DGI-II) and dentin dysplasia (DD). Both disorders result in the development of hypomineralized and mechanically compromised teeth. The erupted mature molars of Dspp(-/-) mice have a severe hypomineralized dentin phenotype. Since dentin and enamel formations are interdependent, we decided to investigate the process of enamel onset mineralization in young Dspp(-/-) animals. We focused our analysis on the constantly erupting mouse incisor, to capture all of the stages of odontogenesis in one tooth, and the unerupted first molars. Using high-resolution microCT, we revealed that the onset of enamel matrix deposition occurs closer to the cervical loop and both secretion and maturation of enamel are accelerated in Dspp(-/-) incisors compared to the Dspp(+/-) control. Importantly, these differences did not translate into major phenotypic differences in mature enamel in terms of the structural organization, mineral density or hardness. The only observable difference was the reduction in thickness of the outer enamel layer, while the total enamel thickness remained unchanged. We also observed a compromised dentin-enamel junction, leading to delamination between the dentin and enamel layers. The odontoblast processes were widened and lacked branching near the DEJ. Finally, for the first time we demonstrate expression of Dspp mRNA in secretory ameloblasts. In summary, our data show that DSPP is important for normal mineralization of both dentin and enamel.

The LPV Motif Is Essential for the Efficient Export of Secretory DMP1 From the Endoplasmic Reticulum

Dentin matrix protein 1 (DMP1) is found abundantly in the extracellular matrices of bone and dentin. Secretory DMP1 begins with a tripeptide of leucine-proline-valine (LPV) after the endoplasmic reticulum (ER)-entry signal peptide is cleaved. The goal of this study was to determine the role of the LPV motif in the secretion of DMP1. A series of DNA constructs was generated to express various forms of DMP1 with or without the LPV motif. These constructs were transfected into a preosteoblast cell line, the MC3T3-E1 cells, and the subcellular localization and secretion of various forms of DMP1 were examined by immunofluorescent staining and Western-blotting analyses. Immunofluorescent staining showed that the LPV-containing DMP1 variants were primarily localized in the Golgi complex, whereas the LPV-lacking DMP1 variants were found abundantly within the ER. Western-blotting analyses demonstrated that the LPV-containing DMP1 variants were rapidly secreted from the transfected cells, as they did not accumulate within the cells, and the amounts increased in the conditioned media over time. In contrast, the LPV-lacking DMP1 variants were predominantly retained within the cells, and only small amounts were secreted out of the cells over time. These results suggest that the LPV motif is essential for the efficient export of secretory DMP1 from the ER to the Golgi complex.

Dspp mutations disrupt mineralization homeostasis during odontoblast differentiation

The main pathological feature in isolated hereditary dentin disorders is the abnormality of dentin mineralization. Dentin sialophosphoprotein (DSPP) gene is the only identified causative gene for the disorders. The present study aims to explore the molecular association between Dspp mutations and the disrupted mineralization homeostasis during odontoblast differentiation. We generated lentivirus constructs with the mouse full-length wild type Dspp cDNA and 3 Dspp mutants and transfected them into mouse odontoblast-lineage cells (OLCs) which were then performed 21-day mineralization inducing differentiation. The formation of mineralized nodules was obviously fewer in mutants. Digital Gene Expression (DGE) showed that Dspp mutation affected the OLC differentiation in a degree. Further examination validated that Dspp (LV-Dspp) overexpressing OLCs possessed the ability to strictly orchestrate framework for mineralization inductors like Bmp2, Col1 and Runx2, and proliferative markers for mineralization like Alp and Ocn, as well as mineral homeostasis feedback regulators Mgp and Htra1. However, the missense mutation in Dspp signal peptide region (LV-M2) and the nonsense mutation (LV-M5) broke this orchestration. The results suggested that the mutant Dspp disrupt the dynamic homeostasis of mineralization during OLC differentiation. We are the first to use full-length mouse Dspp gene expression system to explore the mineralization mechanism by which inductors and inhibitors adjust each other during odontoblast differentiation. Our findings shed new light on association between Dspp and the dynamic homeostasis of mineralization inductors and inhibitors, and indicate the disruption of mineralization homeostasis might be a crucial reason for Dspp mutations resulting in dentin disorders.

DSPP Is Essential for Normal Development of the Dental-Craniofacial Complex

The craniofacial skeleton is derived from both neural crest cells and mesodermal cells; however, the majority of the bone, cartilage, and connective tissue is derived from the neural crest. Dentin sialophosphoprotein (DSPP) is a precursor protein that is expressed by the connective tissues of the craniofacial skeleton, namely, bone and dentin with high expression levels in the dentin matrix. Gene ablation studies have shown severe dental defects in DSPP-null mutant mice. Therefore, to elucidate the role of DSPP on the developing dental-craniofacial complex, we evaluated phenotypic changes in the structure of intramembranous bone and dentin mineralization using 3 different age groups of DSPP-null and wild-type mice. Results from micro-computed tomographic, radiographic, and optical microscopic analyses showed defective dentin, alveolar and calvarial bones, and sutures during development. The impaired mineralization of the cranial bone correlated well with low expression levels of Runx2, Col1, and OPN identified using calvarial cells from DSPP-null and wild-type mice in an in vitro culture system. However, the upregulation of MMP9, MMP2, FN, and BSP was observed. Interestingly, the null mice also displayed low serum phosphate levels, while calcium levels remained unchanged. Alizarin red and von Kossa staining confirmed the dysfunction in the terminal differentiation of osteoblasts obtained from the developing calvaria of DSPP-null mice. Immunohistochemical analysis of the developing molars showed changes in Runx2, Gli1, Numb, and Notch expression in the dental pulp cells and odontoblasts of DSPP-null mice when compared with wild-type mice. Overall, these observations provide insight into the role of DSPP in the normal development of the calvaria, alveolar bone, and dentin-pulp complex.

CCN2/CTGF expression via cellular uptake of BMP-1 is associated with reparative dentinogenesis

OBJECTIVE

CCN family member 2/connective tissue growth factor (CCN2/CTGF) is known as an osteogenesis-related molecule and is thought to be implicated in tooth growth. Bone morphogenetic protein-1 (BMP-1) contributes to tooth development by the degradation of dentin-specific substrates as a metalloprotease. In this study, we demonstrated the correlations between CCN2/CTGF and BMP-1 in human carious teeth and the subcellular dynamics of BMP-1 in human dental pulp cells.

MATERIALS AND METHODS

Expression of CCN2/CTGF and BMP-1 in human carious teeth was analyzed by immunohistochemistry. BMP-1-induced CCN2/CTGF protein expression in primary cultures of human dental pulp cells was observed by immunoblotting. Intracellular dynamics of exogenously administered fluorescence-labeled BMP-1 were observed using confocal microscope.

RESULTS

Immunoreactivities for CCN2/CTGF and BMP-1 were increased in odontoblast-like cells and reparative dentin-subjacent dental caries. BMP-1 induced the expression of CCN2/CTGF independently of protease activity in the cells but not that of dentin sialophosphoprotein (DSPP) or dentin matrix protein-1 (DMP-1). Exogenously added BMP-1 was internalized into the cytoplasm, and the potent dynamin inhibitor dynasore clearly suppressed the BMP-1-induced CCN2/CTGF expression in the cells.

CONCLUSION

CCN2/CTGF and BMP-1 coexist beneath caries lesion and CCN2/CTGF expression is regulated by dynamin-related cellular uptake of BMP-1, which suggests a novel property of metalloprotease in reparative dentinogenesis.

Relationship between length variations in Ser/Asp-rich repeats in phosphophoryn and in vitro precipitation of calcium phosphate

OBJECTIVE

Phosphophoryn (PP) is generated from the proteolytic cleavage of dentin sialophosphoprotein (DSPP). PP which contains tandem serine/asparatic acid rich repeats (SDrr) is known to enhance dentin mineralization. The nucleotide sequences coding SDrr are identified in the DSPP genes of toothed animals and the length variations of SDrr between intra- and inter-species have been reported. However, it remains unknown about the relationship between the length variations in SDrr and the functions of PP in matrix mineralization.

DESIGN

By utilizing a mammalian expression system, we generated several recombinant PP proteins (rPP) containing SDrr of different lengths and analyzed their effects on the precipitation of calcium phosphate with an in vitro gel diffusion system.

RESULTS

rPP-Δ37.6 SDrr and rPP-Δ63.5 SDrr, which possessed shortened SDrr that accounted for 62.4 and 36.5% the length of SDrr in full-length rPP (rPP-full), respectively, induced the precipitation of calcium phosphate similar to that of rPP-full at the same molar concentration, whereas rPP-ΔSDrr, in which SDrr were flipped, did not. Furthermore, rPP-Δ63.5 SDrr significantly increased the accumulation of calcium compared with rPP-full at adjusted concentrations so that the same amounts of SDrr were embedded. The results of an ELISA analysis indicated that the amounts of rPP-Δ37.6 SDrr and rPP-Δ63.5 SDrr secreted from transfected cells were 5.2- and 7.1-fold greater than that of rPP-full, respectively.

CONCLUSIONS

The generated rPP-Δ63.5 SDrr which can be substituted for rPP-full may be a candidate for a therapeutic molecule to facilitate hard tissue generation such as reparative dentin formation.

SCPP genes in the coelacanth: tissue mineralization genes shared by sarcopterygians

The coelacanth is the basal-most extant sarcopterygian that has teeth and tooth-like structures, comprising bone, dentin, and enamel or enameloid. Formation of these tissues involves many members of the secretory calcium-binding protein (SCPP) family. In tetrapods, acidic-residue-rich SCPPs are used in mineralization of bone and dentin, whereas Pro/Gln-rich SCPPs participate in enamel formation. Teleosts also employ many SCPPs for tissue mineralization. Nevertheless, the repertoire of SCPPs is largely different in teleosts and tetrapods; hence, filling this gap would be critical to elucidate early evolution of mineralized tissues in osteichthyans. In the present study, we searched for SCPP genes in the coelacanth genome and identified 11, of which two have clear orthologs in both tetrapods and teleosts, seven only in tetrapods, and two in neither of them. Given the divergence times of these vertebrate lineages, our discovery of this many SCPP genes shared between the coelacanth and tetrapods, but not with teleosts, suggests a complicated evolutionary scheme of SCPP genes in early osteichthyans. Our investigation also revealed both conserved and derived characteristics of SCPPs in the coelacanth and other vertebrates. Notably, acidic SCPPs independently evolved various acidic repeats in different lineages, while maintaining high acidity, presumably important for interactions with calcium. Furthermore, the three Pro/Gln-rich SCPP genes, required for mineralizing enamel matrix and confirmed only in tetrapods, were all identified in the coelacanth, strongly suggesting that enamel is equivalent in the coelacanth and tetrapods. This finding corroborates the previous proposition that true enamel evolved much earlier than the origin of tetrapods.

Adhesive and migratory effects of phosphophoryn are modulated by flanking peptides of the integrin binding motif

Phosphophoryn (PP) is generated from the proteolytic cleavage of dentin sialophosphoprotein (DSPP). Gene duplications in the ancestor dentin matrix protein-1 (DMP-1) genomic sequence created the DSPP gene in toothed animals. PP and DMP-1 are phosphorylated extracellular matrix proteins that belong to the family of small integrin-binding ligand N-linked glycoproteins (SIBLINGs). Many SIBLING members have been shown to evoke various cell responses through the integrin-binding Arg-Gly-Asp (RGD) domain; however, the RGD-dependent function of PP is not yet fully understood. We demonstrated that recombinant PP did not exhibit any obvious cell adhesion ability, whereas the simultaneously purified recombinant DMP-1 did. A cell adhesion inhibitory analysis was performed by pre-incubating human osteosarcoma MG63 cells with various PP peptides before seeding onto vitronectin. The results obtained revealed that the incorporation of more than one amino acid on both sides of the PP-RGD domain was unable to inhibit the adhesion of MG63 cells onto vitronectin. Furthermore, the inhibitory activity of a peptide containing the PP-RGD domain with an open carboxyl-terminal side (H-⁴⁶³SDESDTNSESANESGSRGDA⁴⁸²-OH) was more potent than that of a peptide containing the RGD domain with an open amino-terminal side (H-⁴⁷⁸SRGDASYTSDESSDDDNDSDSH⁴⁹⁹-OH). This phenomenon was supported by the potent cell adhesion and migration abilities of the recombinant truncated PP, which terminated with Ala⁴⁸². Furthermore, various point mutations in Ala⁴⁸² and/or Ser⁴⁸³ converted recombinant PP into cell-adhesive proteins. Therefore, we concluded that the Ala⁴⁸²-Ser⁴⁸³ flanking sequence, which was detected in primates and mice, was the key peptide bond that allowed the PP-RGD domain to be sequestered. The differential abilities of PP and DMP-1 to act on integrin imply that DSPP was duplicated from DMP-1 to serve as a crucial extracellular protein for tooth development rather than as an integrin-mediated signaling molecule.

The NTR domain of procollagen C-proteinase enhancer-1 (PCPE-1) mediates PCPE-1 binding to syndecans-1, -2 and -4 as well as fibronectin

Procollagen C-proteinase enhancer 1 (PCPE-1) is an extracellular matrix glycoprotein that can stimulate procollagen processing by procollagen C-proteinases (PCPs) such as bone morphogenetic protein-1 (BMP-1). PCPE-1 consists of two CUB domains that bind to the procollagen C-propeptide and are responsible for enhancing activity and a netrin-like (NTR) domain that binds to BMP-1 as well as heparin and heparan sulfate. The NTR domain also mediates binding of PCPE-1 to cells, an interaction inhibited by heparin, thus suggesting involvement of cell membrane heparan-sulfate proteoglycans (HSPGs). Using pull-down experiments and an ELISA type binding assay we show here that PCPE-1 binds to three cell membrane HSPGs, syndecans-1, -2 and -4. We also demonstrate that this binding is mediated by the NTR domain and depends on the glycosaminoglycan chains of the syndecans. Using co-immunoprecipitation and an ELISA type binding assay we show that PCPE-1 can also bind fibronectin (an established binding partner of BMP-1), another interaction involving the NTR domain. Consistently, fibronectin inhibits cell attachment to PCPE-1 although it does not affect PCPE-1 enhancing activity. PCPE-1 is not an adhesive protein since cell attachment to PCPE-1 is not associated with cell spreading and/or actin filaments formation. The results suggest that PCPE-1 binding to syndecans and/or fibronectin may control collagen fibril assembly on the cell surface. Further characterization of these interactions may pave the way for future design of new means to modulate collagen deposition in pathological conditions such as fibrosis.

Dentin sialophosphoprotein: a regulatory protein for dental pulp stem cell identity and fate

The dentin sialophosphoprotein (dspp) transcript is expressed during tooth development as a DSPP precursor protein, which then undergoes cleavage to form mature dentin sialoprotein (DSP) and phosphophoryn (PP) proteins. Previous studies using DSPP-knockout (KO) mice have reported that these animals have hypomineralized teeth, thin dentin, and a large dental pulp chamber, similar to those from patients with dentinogenesis imperfecta III. However, there is no information about factors that regulate dental pulp stem cell lineage fate, a critical early event in the odontoblast-dentin mineralization scheme. To reveal the role of DSPP in odontoblast lineage differentiation during tooth development, we systematically examined teeth from wild-type (wt) and DSPP-KO C57BL/6 mice between the ages of postnatal day 1 and 3 months. We found developmental abnormalities not previously reported, such as circular dentin formation within dental pulp cells and altered odontoblast differentiation in DSPP-KO mice, even as early as 1 day after birth. Surprisingly, we also identified chondrocyte-like cells in the dental pulp from KO-mice teeth. Thus, these studies that compare wt and DSPP-KO mice suggest that the expression of DSPP precursor protein is required for normal odontoblast lineage differentiation and that the absence of DSPP allows dental pulp cells to differentiate into chondrocyte-like cells, which could negatively impact pulpal wound healing and tissue regeneration.

Site specificity of DSP-PP cleavage by BMP1

Bone morphogenic protein 1 (BMP1), a metalloproteinase, is known to cleave a wide variety of extracellular matrix proteins, suggesting that a consensus substrate cleavage amino acid sequence might exist. However, while such a consensus sequence has been proposed based on P4 to P4' (i.e. the four amino acids flanking either side of the BMP1 cleavage site; P4P3P2P1|P1'P2'P3'P4') sequence homologies between two BMP1 substrates, dentin matrix protein 1 and dentin sialoprotein phosphophoryn (DSP-PP) (i.e. xMQx|DDP), no direct testing has so far been attempted. Using an Sf9 cell expression system, we have been able to produce large amounts of uncleaved DSP-PP. Point mutations introduced into this recombinant DSP-PP were then tested for their effects on DSP-PP cleavage by either Sf9 endogenous tolloid-related protein 1 (TLR-1) or by its human homolog, BMP1. Here, we have measured DSP-PP cleavage efficiencies after modifications based on P4-P4' sequence comparisons with dentin matrix protein 1, as well as for prolysyl oxidase and chordin, two other BMP1 substrates. Our results demonstrate that any mutations within or outside of the DSP-PP P4 to P4' cleavage site can block, impair or accelerate DSP-PP cleavage, and suggest that its BMP1 cleavage site is highly conserved in order to regulate its cleavage efficiency, possibly with additional assistance from its conserved exosites. Thus, BMP1 cleavage cannot be based on a consensus substrate cleavage site.

Multifunctional ECM proteins in bone and teeth

The extracellular matrix (ECM) of all tissues and organs is a highly organized and complex structure unique to the specific organ type. The ECM contains structural and functional proteins that define cellular function, organization, behavior and ultimately organ characteristics and function. The ECM was initially thought to contain only a specific set of secretory proteins. However, our group and several other groups have shown that the ECM contains functional proteins that have been previously defined as solely intracellular. In the present review, we have focused on the ECM of mineralized tissues namely bone and dentin. We provide here, a brief review of some non-classical ECM proteins that have been shown to possess both intra and extracellular roles in the formation of these mineralized matrices.

DSPP contains an IRES element responsible for the translation of dentin phosphophoryn

The major phosphoprotein in dentin is the aspartic acid and serine-rich protein called dentin phosphophoryn (DPP). DPP appears to be synthesized as a part of a larger compound protein, dentin sialophosphoprotein (DSPP). DSPP has never been isolated or detected in dentin extracts. It is now evident that DSPP is a chimeric protein composed of 3 parts: dentin sialoprotein (DSP), DPP, and dentin glycoprotein (DGP). Previous reports have suggested that the BMP1 protease is responsible for processing DSPP. However, unequal amounts of these products are present in the dentin matrix. Here, we provide evidence for an internal ribosome entry site in the DSPP gene that directs the synthesis of DPP. This mechanism would account for unequal amounts of intracellular DSP and DPP. The internal ribosomal entry site (IRES) activity varied in different cell types, suggesting the presence of additional regulatory elements during the translational regulation of DPP. Further, we provide evidence that DPP is transported to the extracellular matrix (ECM) through exosomes. Using tissue recombination and lentivirus-mediated gain-of-function approaches, we also demonstrate that DPP is essential for the formation of well-defined tooth structures with mineralized dentin matrix.

Domain of dentine sialoprotein mediates proliferation and differentiation of human periodontal ligament stem cells

Classic embryological studies have documented the inductive role of root dentin on adjacent periodontal ligament differentiation.  The biochemical composition of root dentin includes collagens and cleavage products of dentin sialophosphoprotein (DSPP), such as dentin sialoprotein (DSP).  The high abundance of DSP in root dentin prompted us to ask the question whether DSP or peptides derived thereof would serve as potent biological matrix components to induce periodontal progenitors to further differentiate into periodontal ligament cells. Here, we test the hypothesis that domain of DSP influences cell fate. In situ hybridization and immunohistochemical analyses showed that the COOH-terminal DSP domain is expressed in mouse periodontium at various stages of root development. The recombinant COOH-terminal DSP fragment (rC-DSP) enhanced attachment and migration of human periodontal ligament stem cells (PDLSC), human primary PDL cells without cell toxicity. rC-DSP induced PDLSC cell proliferation as well as differentiation and mineralization of PDLSC and PDL cells by formation of mineralized tissue and ALPase activity. Effect of rC-DSP on cell proliferation and differentiation was to promote gene expression of tooth/bone-relate markers, transcription factors and growth factors. The results for the first time showed that rC-DSP may be one of the components of cell niche for stimulating stem/progenitor cell proliferation and differentiation and a natural scaffold for periodontal regeneration application.