Noncollagenous proteins of dentin. A re-examination of proteins from rat incisor dentin utilizing techniques to avoid artifacts

Dentin defects caused by a Dspp-1 frameshift mutation are associated with the activation of autophagy

Dentin sialophosphoprotein (DSPP) is primarily expressed by differentiated odontoblasts (dentin-forming cells), and transiently expressed by presecretory ameloblasts (enamel-forming cells). Disease-causing DSPP mutations predominantly fall into two categories: 5' mutations affecting targeting and trafficking, and 3' - 1 frameshift mutations converting the repetitive, hydrophilic, acidic C-terminal domain into a hydrophobic one. We characterized the dental phenotypes and investigated the pathological mechanisms of DsppP19L and Dspp-1fs mice that replicate the two categories of human DSPP mutations. In DsppP19L mice, dentin is less mineralized but contains dentinal tubules. Enamel mineral density is reduced. Intracellular accumulation and ER retention of DSPP is observed in odontoblasts and ameloblasts. In Dspp-1fs mice, a thin layer of reparative dentin lacking dentinal tubules is deposited. Odontoblasts show severe pathosis, including intracellular accumulation and ER retention of DSPP, strong ubiquitin and autophagy activity, ER-phagy, and sporadic apoptosis. Ultrastructurally, odontoblasts show extensive autophagic vacuoles, some of which contain fragmented ER. Enamel formation is comparable to wild type. These findings distinguish molecular mechanisms underlying the dental phenotypes of DsppP19L and Dspp-1fs mice and support the recently revised Shields classification of dentinogenesis imperfecta caused by DSPP mutations in humans. The Dspp-1fs mice may be valuable for the study of autophagy and ER-phagy.

The Modified Shields Classification and 12 Families with Defined DSPP Mutations

Mutations in Dentin Sialophosphoprotein (DSPP) are known to cause, in order of increasing severity, dentin dysplasia type-II (DD-II), dentinogenesis imperfecta type-II (DGI-II), and dentinogenesis imperfecta type-III (DGI-III). DSPP mutations fall into two groups: a 5′-group that affects protein targeting and a 3′-group that shifts translation into the −1 reading frame. Using whole-exome sequence (WES) analyses and Single Molecule Real-Time (SMRT) sequencing, we identified disease-causing DSPP mutations in 12 families. Three of the mutations are novel: c.53T>C/p.(Val18Ala); c.3461delG/p.(Ser1154Metfs*160); and c.3700delA/p.(Ser1234Alafs*80). We propose genetic analysis start with WES analysis of proband DNA to identify mutations in COL1A1 and COL1A2 causing dominant forms of osteogenesis imperfecta, 5′-DSPP mutations, and 3′-DSPP frameshifts near the margins of the DSPP repeat region, and SMRT sequencing when the disease-causing mutation is not identified. After reviewing the literature and incorporating new information showing distinct differences in the cell pathology observed between knockin mice with 5′-Dspp or 3′-Dspp mutations, we propose a modified Shields Classification based upon the causative mutation rather than phenotypic severity such that patients identified with 5′-DSPP defects be diagnosed as DGI-III, while those with 3′-DSPP defects be diagnosed as DGI-II.

Mouse Dspp frameshift model of human dentinogenesis imperfecta

Non-syndromic inherited defects of tooth dentin are caused by two classes of dominant negative/gain-of-function mutations in dentin sialophosphoprotein (DSPP): 5' mutations affecting an N-terminal targeting sequence and 3' mutations that shift translation into the - 1 reading frame. DSPP defects cause an overlapping spectrum of phenotypes classified as dentin dysplasia type II and dentinogenesis imperfecta types II and III. Using CRISPR/Cas9, we generated a Dspp^-1fs mouse model by introducing a FLAG-tag followed by a single nucleotide deletion that translated 493 extraneous amino acids before termination. Developing incisors and/or molars from this mouse and a Dspp^P19L mouse were characterized by morphological assessment, bSEM, nanohardness testing, histological analysis, in situ hybridization and immunohistochemistry. Dspp^P19L dentin contained dentinal tubules but grew slowly and was softer and less mineralized than the wild-type. Dspp^P19L incisor enamel was softer than normal, while molar enamel showed reduced rod/interrod definition. Dspp^-1fs dentin formation was analogous to reparative dentin: it lacked dentinal tubules, contained cellular debris, and was significantly softer and thinner than Dspp^+/+ and Dspp^P19L dentin. The Dspp^-1fs incisor enamel appeared normal and was comparable to the wild-type in hardness. We conclude that 5' and 3' Dspp mutations cause dental malformations through different pathological mechanisms and can be regarded as distinct disorders.

A new method for dentine matrix metalloproteinase extraction

In addition to their involvement in tissue remodelling, matrix metalloproteinases (MMPs), in the oral environment, are linked to leakage in the resin-dentine interface through their involvement in the proteolytic degradation of the resin-dentine hybrid layer. Numerous studies have evaluated dentine MMP activity and the vast majority of those studies have used an MMP extraction protocol to semi-purify the MMPs from dentine, first described around 20 years ago. This is a protocol that requires 32 days for completion. The technique is based on the three-step sequential use of NaCl solution for pulverised dentine washing, then guanidine HCl and EDTA to demineralise the pulverised dentine to extract the MMPs. In this study, a new one-step dentine MMP extraction protocol was adapted to extract dentine MMPs in only four days. This was achieved by eliminating the NaCl washing step and combining the guanidine HCl and EDTA into one extraction solution that also contained proteinase inhibitors. Fifty-two dentine MMP extracts were obtained utilising the two different extraction methods. The amount of total and endogenously active MMP-2 in the specimens was assayed utilising a human MMP-2 activity enzyme-linked immunosorbent assay (ELISA). The study results showed that the new extraction method is as effective as the traditional three-step extraction method in semi-purifying dentine MMP-2.

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.

Structural features, processing mechanism and gene splice variants of dentin sialophosphoprotein

Dentin sialophosphoprotein (DSPP) plays an important role in the formation of dentin. Understanding its structure and function would provide important insights into the regulation of dentin mineralization. For the past 15 years, we have been studying DSPP-derived proteins isolated from pig dentin. Porcine DSPP is synthesized and secreted by odontoblasts and processed into three proteins, i.e., dentin sialoprotein (DSP), dentin glycoprotein (DGP), and dentin phosphoprotein (DPP), by bone morphogenetic protein 1 and matrix metalloproteinase-20 and -2. DSP is a proteoglycan that forms covalent dimers, DGP is a phosphorylated glycoprotein, and DPP is a highly phosphorylated intrinsically disordered protein with genetic polymorphisms. Furthermore, DPP is not detected in dental pulp. This is possibly due to the existence of two mRNA variants of the DSPP gene: one that encodes the DSP region alone and another that encodes full-length DSPP. The mRNA variant encoding DSP alone is expressed in dental pulp and odontoblasts, but the variant encoding full-length DSPP is predominantly expressed in odontoblasts and barely in dental pulp.

Dentin sialophosphoprotein-derived proteins in porcine pulp and dentin - Gene expression and function

BACKGROUND

Dentin sialophosphoprotein (DSPP) is the most abundant non-collagenous protein in dentin and is critical for the proper mineralization of tooth dentin. DSPP is processed by proteases into three major domains: dentin sialoprotein (DSP), dentin glycoprotein (DGP) and dentin phosphoprotein (DPP). Two mRNA variants are expressed from the Dspp gene. The larger transcript encodes full-length DSPP (DSP+DGP+DPP). The shorter transcript encodes only DSP.

HIGHLIGHT

We fractionated DSPP-derived proteins from the dental pulp of developing porcine incisors using heparin chromatography. DSP was identified, but little DPP could be detected in any fraction. Expression of full-length Dspp mRNA, determined by qPCR analysis, was significantly higher in odontoblasts than in pulp. Expression of DSP-only mRNA was almost equal in odontoblasts and in the body of pulp. Expression of full-length Dspp mRNA was also significantly higher than expression of DSP-only mRNA in odontoblasts. Both the full-length and DSP-only Dspp mRNA showed only trace expression in the pulp tip. We purified TGF-β1-unbound or -bound to DPP and DSP using high performance liquid chromatography (HPLC) and measured its alkaline phosphatase stimulating activity in human periodontal cells with or without TGF-β receptor inhibitor. We also incubated carrier-free human recombinant TGF-β1 (CF-hTGF-β1) protein with TGF-β1-unbound DPP or DSP and characterized binding ability.

CONCLUSION

DSP-only is expressed throughout odontoblast differentiation, while full-length DSPP is predominantly expressed by odontoblasts only after they have differentiated from mesenchymal cells. DPP and DSP rescued the loss of TGF-β1 activity. Type I collagen was infrequently bound to CF-hTGF-β1.

EDTA soluble chemical components and the conditioned medium from mobilized dental pulp stem cells contain an inductive microenvironment, promoting cell proliferation, migration, and odontoblastic differentiation

Background

The critical challenge in tissue engineering is to establish an optimal combination of stem cells, signaling morphogenetic molecules, and extracellular matrix scaffold/microenvironment. The extracellular matrix components of teeth may be reconstituted as an inductive microenvironment in an ectopic tooth transplantation bioassay. Thus, the isolation and identification of the chemical components of the inductive microenvironment in pulp/dentin regeneration will accelerate progress towards the goal of tissue engineering of the tooth.

Methods

The teeth demineralized in 0.6 M hydrochloric acid were sequentially extracted by 4.0 M guanidine hydrochloride (GdnHCl), pH 7.4, and 0.5 M ethylenediaminetetraacetic acid (EDTA), pH 7.4. The extracted teeth were transplanted into an ectopic site in severe combined immunodeficiency (SCID) mice with mobilized dental pulp stem cells (MDPSCs). The unextracted tooth served as a positive control. Furthermore, the soluble components for the inductive microenvironment, the GdnHCl extracts, or the EDTA extracts together with or without MDPSC conditioned medium (CM) were reconstituted systematically with autoclaved teeth in which the chemical components were completely inactivated and only the physical microenvironment was preserved. Their pulp/dentin regenerative potential and angiogenic potential were compared 28 days after ectopic tooth transplantation by histomorphometry and real-time RT-PCR analysis.

Results

Expression of an odontoblastic marker, enamelysin, and a pulp marker, thyrotropin-releasing hormone degrading enzyme (TRH-DE), was lower, and expression of a periodontal cell marker, anti-asporin/periodontal ligament-associated protein 1 (PLAP-1), was higher in the transplant of the EDTA-extracted teeth compared with the GdnHCl-extracted teeth. The autoclaved teeth reconstituted with the GdnHCl extracts or the EDTA extracts have weak regenerative potential and minimal angiogenic potential, and the CM significantly increased this potential. Combinatorial effects of the EDTA extracts and the CM on pulp/dentin regeneration were demonstrated in vivo, consistent with their in-vitro effects on enhanced proliferation, migration, and odontoblastic differentiation.

Conclusions

The EDTA-extracted teeth demonstrated significantly lower pulp/dentin regenerative potential compared with the GdnHCl-extracted teeth. The EDTA soluble chemical components when reconstituted with the physical structure of autoclaved teeth serve as an inductive microenvironment for pulp/dentin regeneration, promoting cell proliferation, migration, and odontoblastic differentiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0334-z) contains supplementary material, which is available to authorized users.

Family with sequence similarity member 20C is the primary but not the only kinase for the small-integrin-binding ligand N-linked glycoproteins in bone

Recent studies have identified family with sequence similarity member 20C (FAM20C) as a kinase that phosphorylates the Ser in Ser-X-Glu/phospho-Ser (pSer) motifs in the small-integrin-binding ligand N-linked glycoproteins (SIBLINGs). There is no in vivo evidence that validates this finding, and it is unclear whether FAM20C is the only kinase for SIBLINGs. We extracted bone noncollagenous proteins (NCPs) from Fam20C-knockout (KO) mice and analyzed the phosphorylation levels. The total NCPs were separated into osteopontin-, bone sialoprotein-, and dentin matrix protein-1-enriched fractions by anion-exchange chromatography and analyzed by SDS-PAGE, native PAGE, and Western immunoblot analysis. The NCP phosphorylation level in the KO mice was lower than that in the wild-type (WT). On the native gel, the SIBLINGs from KO mice showed a lower migration rate (Mr) than those from the WT. Calf intestine phosphatase treatment shifted SIBLINGs from the WT mice to the level adjacent to the KO, but failed to shift the latter, suggesting a phosphorylation loss of SIBLINGs in the KO mice. Mass spectrometry identified less pSers in the SIBLINGs from the KO mice [including the region of the acidic Ser- and aspartate-rich motif (ASARM) peptides]. In an intriguing finding, several pSers in the Ser-X-Glu motifs in the KO mice maintained their phosphorylation, whereas several others in non-Ser-X-Glu motifs did not. Phospho-Tyrs and phospho-Thrs in the SIBLINGs did not appear to be associated with FAM20C. Our results indicate that FAM20C is the primary, but not the only, kinase for the SIBLINGs.-Yang, X., Yan, W., Tian, Y., Ma, P., Opperman, L. A., Wang, X. Family with sequence similarity member 20C is the primary but not the only kinase for the small-integrin-binding ligand N-linked glycoproteins in bone.

Dentin Sialophosphoprotein-derived Proteins in the Dental Pulp

Porcine dentin sialophosphoprotein (DSPP), the most abundant noncollagenous protein in dentin, is critical for proper mineralization of tooth dentin. DSPP is processed by proteases into 3 major domains: dentin sialoprotein (DSP), dentin glycoprotein (DGP), and dentin phosphoprotein (DPP). There are at least 2 mRNA variants expressed from the Dspp gene: one encodes the full-length DSPP protein (DSP+DGP+DPP); the other encodes only DSP. The shorter transcript is generated through the use of a polyadenylation signal within intron 4, immediately following the DSP coding region (DGP and DPP are encoded by exon 5). We fractionated DSPP-derived proteins from the dental pulp of developing porcine incisors using heparin chromatography. DSP was identified, but little DPP could be detected in any fractions. BMP-1 digestion of DSPP-derived proteins extracted from dental pulp did not generate new DPP bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (indicating an absence of intact DSPP), although the results suggested another BMP-1 cleavage site within DSP. We further purified DSPP-derived protein by reversed-phase high-performance liquid chromatography. Its amino acid composition was similar to DSP. Expression of the full-length Dspp mRNA by quantitative real-time polymerase chain reaction analysis was significantly higher in odontoblasts than in pulp, while expression of the DSP-only mRNA was almost equal in odontoblasts and in the body of the pulp. Expression of the full-length Dspp mRNA was also significantly higher than the expression of DSP-only mRNA in odontoblasts. Both the full-length and the DSP-only Dspp mRNA showed only trace expression in the pulp tip. We conclude that use of the 3' polyadenylation signal in exon 5 predominates in fully differentiated odontoblasts, while both polyadenylation signals are used throughout odontoblast differentiation.

DPP and DSP are Necessary for Maintaining TGF-β1 Activity in Dentin

Porcine dentin sialophosphoprotein (DSPP) is the most abundant non-collagenous protein in dentin. It is processed by proteases into 3 independent proteins: dentin sialoprotein (DSP), dentin glycoprotein (DGP), and dentin phosphoprotein (DPP). We fractionated DPP and DSP along with TGF-β activity by ion exchange (IE) chromatography from developing pig molars and measured their alkaline phosphatase (ALP)-stimulating activity in human periodontal (HPDL) cells with or without TGF-β receptor inhibitor. We then purified TGF-β-unbound or -bound DPP and DSP by reverse-phase high-performance liquid chromatography (RP-HPLC) using the ALP-HPDL system. The TGF-β isoform bound to DPP and DSP was identified as being TGF-β1 by both ELISA and LC-MS/MS analysis. We incubated carrier-free human recombinant TGF-β1 (CF-hTGF-β1) with TGF-β-unbound DPP or DSP and characterized the binding on IE-HPLC using the ALP-HPDL system. When only CF-hTGF-β1 was incubated, approximately 3.6% of the ALP-stimulating activity remained. DPP and DSP rescued the loss of TGF-β1 activity. Approximately 19% and 10% of the ALP stimulating activities were retained by the binding of TGF-β to DPP and DSP, respectively. The type I collagen infrequently bound to CF-hTGF-β1. We conclude that both DPP and DSP help retain TGF-β1 activity in porcine dentin.

Dentin phosphophoryn activates Smad protein signaling through Ca2+-calmodulin-dependent protein kinase II in undifferentiated mesenchymal cells

Dentin phosphophoryn (DPP) is a major noncollagenous protein in the dentin matrix. In this study, we demonstrate that pluripotent stem cells such as C3H10T1/2 and human bone marrow cells can be committed to the osteogenic lineage by DPP. Treatment with DPP can stimulate the release of intracellular Ca(2+). This calcium flux triggered the activation of Ca(2+)-calmodulin-dependent protein kinase II (CaMKII). Activated CaMKII induced the phosphorylation of Smad1 and promoted nuclear translocation of p-Smad1. Inhibition of store Ca(2+) depletion by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) or down-regulation of CaMKII by KN-62, a selective cell-permeable pharmacological inhibitor or a dominant negative plasmid of CaMKII, blocked DPP-mediated Smad1 phosphorylation. Activation of Smad1 resulted in the expression of osteogenic markers such as Runx2, Osterix, DMP1, Bone sialoprotein, Osteocalcin, NFATc1, and Schnurri-2, which have been implicated in osteoblast differentiation. These findings suggest that DPP is capable of triggering commitment of pluripotent stem cells to the osteogenic lineage.

The chicken or the egg: PHEX, FGF23 and SIBLINGs unscrambled

The eggshell is an ancient innovation that helped the vertebrates' transition from the oceans and gain dominion over the land. Coincident with this conquest, several new eggshell and noncollagenous bone-matrix proteins (NCPs) emerged. The protein ovocleidin-116 is one of these proteins with an ancestry stretching back to the Triassic. Ovocleidin-116 is an avian homolog of Matrix Extracellular Phosphoglycoprotein (MEPE) and belongs to a group of proteins called Small Integrin-Binding Ligand Interacting Glycoproteins (SIBLINGs). The genes for these NCPs are all clustered on chromosome 5q in mice and chromosome 4q in humans. A unifying feature of the SIBLING proteins is an Acidic Serine Aspartate-Rich MEPE (ASARM)-associated motif. The ASARM motif and the released ASARM peptide play roles in mineralization, bone turnover, mechanotransduction, phosphate regulation and energy metabolism. ASARM peptides and motifs are physiological substrates for phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), a Zn metalloendopeptidase. Defects in PHEX are responsible for X-linked hypophosphatemic rickets. PHEX interacts with another ASARM motif containing SIBLING protein, Dentin Matrix Protein-1 (DMP1). DMP1 mutations cause bone-renal defects that are identical with the defects caused by loss of PHEX function. This results in autosomal recessive hypophosphatemic rickets (ARHR). In both X-linked hypophosphatemic rickets and ARHR, increased fibroblast growth factor 23 (FGF23) expression occurs, and activating mutations in FGF23 cause autosomal dominant hypophosphatemic rickets (ADHR). ASARM peptide administration in vitro and in vivo also induces increased FGF23 expression. This review will discuss the evidence for a new integrative pathway involved in bone formation, bone-renal mineralization, renal phosphate homeostasis and energy metabolism in disease and health.

Regulation of reactionary dentin formation by odontoblasts in response to polymicrobial invasion of dentin matrix

Odontoblast synthesis of dentin proceeds through discrete but overlapping phases characterized by formation of a patterned organic matrix followed by remodelling and active mineralization. Microbial invasion of dentin in caries triggers an adaptive response by odontoblasts, culminating in formation of a structurally altered reactionary dentin, marked by biochemical and architectonic modifications including diminished tubularity. Scanning electron microscopy of the collagen framework in reactionary dentin revealed a radically modified yet highly organized meshwork as indicated by fractal and lacunarity analyses. Immuno-gold labelling demonstrated increased density and regular spatial distribution of dentin sialoprotein (DSP) in reactionary dentin. DSP contributes putative hydroxyapatite nucleation sites on the collagen scaffold. To further dissect the formation of this altered dentin matrix, the associated enzymatic machinery was investigated. Analysis of extracted dentin matrix indicated increased activity of matrix metalloproteinase-2 (MMP-2) in the reactionary zone referenced to physiologic dentin. Likewise, gene expression analysis of micro-dissected odontoblast layer revealed up-regulation of MMP-2. Parallel up-regulation of tissue inhibitor of metalloproteinase-2 (TIMP-2) and membrane type 1- matrix metalloproteinase (MT1-MMP) was observed in response to caries. Next, modulation of odontoblastic dentinogenic enzyme repertoire was addressed. In the odontoblast layer expression of Toll-like receptors was markedly altered in response to bacterial invasion. In carious teeth TLR-2 and the gene encoding the corresponding adaptor protein MyD88 were down-regulated whereas genes encoding TLR-4 and adaptor proteins TRAM and Mal/TIRAP were up-regulated. TLR-4 signalling mediated by binding of bacterial products has been linked to up-regulation of MMP-2. Further, increased expression of genes encoding components of the TGF-β signalling pathway, namely SMAD-2 and SMAD-4, may explain the increased synthesis of collagen by odontoblasts in caries. These findings indicate a radical adaptive response of odontoblasts to microbial invasion of dentin with resultant synthesis of modified mineralized matrix.

Porcine dentin sialoprotein glycosylation and glycosaminoglycan attachments

Background

Dentin sialophosphoprotein (Dspp) is a multidomain, secreted protein that is critical for the formation of tooth dentin. Mutations in DSPP cause inherited dentin defects categorized as dentin dysplasia type II and dentinogenesis imperfecta type II and type III. Dentin sialoprotein (Dsp), the N-terminal domain of dentin sialophosphoprotein (Dspp), is a highly glycosylated proteoglycan, but little is known about the number, character, and attachment sites of its carbohydrate moieties.

Results

To identify its carbohydrate attachment sites we isolated Dsp from developing porcine molars and digested it with endoproteinase Glu-C or pronase, fractionated the digestion products, identified fractions containing glycosylated peptides using a phenol sulfuric acid assay, and characterized the glycopeptides by N-terminal sequencing, amino acid analyses, or LC/MSMS. To determine the average number of sialic acid attachments per N-glycosylation, we digested Dsp with glycopeptidase A, labeled the released N-glycosylations with 2-aminobenzoic acid, and quantified the moles of released glycosylations by comparison to labeled standards of known concentration. Sialic acid was released by sialidase digestion and quantified by measuring β-NADH reduction of pyruvic acid, which was generated stoichiometrically from sialic acid by aldolase. To determine its forms, sialic acid released by sialidase digestion was labeled with 1,2-diamino-4,5-methyleneoxybenzene (DMB) and compared to a DMB-labeled sialic acid reference panel by RP-HPLC. To determine the composition of Dsp glycosaminoglycan (GAG) attachments, we digested Dsp with chondroitinase ABC and compared the chromotagraphic profiles of the released disaccharides to commercial standards. N-glycosylations were identified at Asn³⁷, Asn⁷⁷, Asn¹³⁶, Asn¹⁵⁵, Asn¹⁶¹, and Asn¹⁷⁶. Dsp averages one sialic acid per N-glycosylation, which is always in the form of N-acetylneuraminic acid. O-glycosylations were tentatively assigned at Thr²⁰⁰, Thr²¹⁶ and Thr³¹⁶. Porcine Dsp GAG attachments were found at Ser²³⁸ and Ser²⁵⁰ and were comprised of chondroitin 6-sulfate and chondroitin 4-sulfate in a ratio of 7 to 3, respectively.

Conclusions

The distribution of porcine Dsp posttranslational modifications indicate that porcine Dsp has an N-terminal domain with at least six N-glycosylations and a C-terminal domain with two GAG attachments and at least two O-glycosylations.

Astacin proteases cleave dentin sialophosphoprotein (Dspp) to generate dentin phosphoprotein (Dpp)

Dentin sialophosphoprotein (Dspp) is critical for proper dentin biomineralization because genetic defects in DSPP cause dentin dysplasia type II and dentinogenesis imperfecta types II and III. Dspp is processed by proteases into smaller subunits; the initial cleavage releases dentin phosphoprotein (Dpp). We incubated fluorescence resonance energy transfer (FRET) peptides containing the amino acid context of the Dpp cleavage site (YEFDGKSMQGDDPN, designated Dspp-FRET) or a mutant version of that context (YEFDGKSIEGDDPN, designated mutDspp-FRET) with BMP-1, MEP1A, MEP1B, MMP-2, MMP-8, MMP-9, MT1-MMP, MT3-MMP, Klk4, MMP-20, plasmin, or porcine Dpp and characterized the peptide cleavage products. Only BMP-1, MEP1A, and MEP1B cleaved Dspp-FRET at the G-D peptide bond that releases Dpp from Dspp in vivo. We isolated Dspp proteoglycan from dentin power and incubated it with the three enzymes that cleaved Dspp-FRET at the G-D bond. In each case, the released Dpp domain was isolated, and its N-terminus was characterized by Edman degradation. BMP-1 and MEP1A both cleaved native Dspp at the correct site to generate Dpp, making both these enzymes prime candidates for the protease that cleaves Dspp in vivo. MEP1B was able to degrade Dpp when the Dpp was at sufficiently high concentration to deplete free calcium ion concentration. Immunohistochemistry of developing porcine molars demonstrated that astacins are expressed by odontoblasts, a result that is consistent with RT-PCR analyses. We conclude that during odontogenesis, astacins in the predentin matrix cleave Dspp before the DDPN sequence at the N-terminus of Dpp to release Dpp from the parent Dspp protein.

Application of polychromatic µCT for mineral density determination

Accurate assessment of mineral density (MD) provides information critical to the understanding of mineralization processes of calcified tissues, including bones and teeth. High-resolution three-dimensional assessment of the MD of teeth has been demonstrated by relatively inaccessible synchrotron radiation microcomputed tomography (SRµCT). While conventional desktop µCT (CµCT) technology is widely available, polychromatic source and cone-shaped beam geometry confound MD assessment. Recently, considerable attention has been given to optimizing quantitative data from CµCT systems with polychromatic x-ray sources. In this review, we focus on the approaches that minimize inaccuracies arising from beam hardening, in particular, beam filtration during the scan, beam-hardening correction during reconstruction, and mineral density calibration. Filtration along with lowest possible source voltage results in a narrow and near-single-peak spectrum, favoring high contrast and minimal beam-hardening artifacts. More effective beam monochromatization approaches are described. We also examine the significance of beam-hardening correction in determining the accuracy of mineral density estimation. In addition, standards for the calibration of reconstructed grey-scale attenuation values against MD, including K(2)PHO(4) liquid phantom, and polymer-hydroxyapatite (HA) and solid hydroxyapatite (HA) phantoms, are discussed.

Lysyl oxidase binds transforming growth factor-beta and regulates its signaling via amine oxidase activity

Lysyl oxidase (LOX), an amine oxidase critical for the initiation of collagen and elastin cross-linking, has recently been shown to regulate cellular activities possibly by modulating the functions of growth factors. In this study, we investigated the interaction between LOX and transforming growth factor-beta1 (TGF-beta1), a potent growth factor abundant in bone, the effect of LOX on TGF-beta1 signaling, and its potential mechanism. The specific binding between mature LOX and mature TGF-beta1 was demonstrated by immunoprecipitation and glutathione S-transferase pulldown assay in vitro. Both proteins were colocalized in the extracellular matrix in an osteoblastic cell culture system, and the binding complex was identified in the mineral-associated fraction of bone matrix. Furthermore, LOX suppressed TGF-beta1-induced Smad3 phosphorylation likely through its amine oxidase activity. The data indicate that LOX binds to mature TGF-beta1 and enzymatically regulates its signaling in bone and thus may play an important role in bone maintenance and remodeling.

The effect of stromelysin-1 (MMP-3) on non-collagenous extracellular matrix proteins of demineralized dentin and the adhesive properties of restorative resins

Dentin non-collagenous matrix components (NCPs) are structural proteins involved in the formation, the architecture and the mineralization of the extracellular matrix (ECM). We investigated here how recombinant metalloproteinase stromelysin-1, also termed MMP-3, initiates the release of ECM molecules from artificially demineralized human dentin. Analysis of the supernatants by Western blotting reveals that MMP-3 extracts PGs (decorin, biglycan), and also a series of phosphorylated proteins: dentin sialoprotein (DSP), osteopontin (OPN), bone sialoprotein (BSP) and MEPE, but neither dentin matrix protein-1 (DMP1), another member of the SIBLING family, nor osteocalcin (OC), a non-phosphorylated matrix molecule. After treatment of dentin surfaces by MMP-3, scanning electron microscope (SEM) examination of resin replica shows an increased penetration of the resin into the dentin tubules when compared to surfaces only treated by demineralizing solutions. This preclinical investigation suggests that MMP-3 may be used to improve the adhesive properties of restorative materials.

Distribution of SIBLING proteins in the organic and inorganic phases of rat dentin and bone

The SIBLING protein family is a group of non-collagenous proteins (NCPs) that includes dentin sialophosphoprotein (DSPP), dentin matrix protein 1 (DMP1), bone sialoprotein (BSP), and osteopontin (OPN). In the present study, we compared these four proteins in different phases of rat dentin and bone. First, we extracted NCPs in the unmineralized matrices and cellular compartments using guanidium-HCl (G1). Second, we extracted NCPs closely associated with hydroxyapatite using an EDTA solution (E). Last, we extracted the remaining NCPs again with guanidium-HCl (G2). Each fraction of Q-Sepharose ion-exchange chromatography was analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), Stains-All stain, and with western immunoblotting. In dentin, the NH(2)-terminal fragment of DSPP and its proteoglycan form were primarily present in the G1 extract, whereas the COOH-terminal fragment of DSPP was present exclusively in the E extract. The processed NH(2)-terminal fragment of DMP1 was present in G1 and E extracts, whereas the COOH-terminal fragment of DMP1 existed mainly in the E extract. Bone sialoprotein was present in all three extracts of dentin and bone, whereas OPN was present only in the G1 and E extracts of bone. The difference in the distribution of the SIBLING proteins between organic and inorganic phases supports the belief that these molecular species play different roles in dentinogenesis and osteogenesis.

Macromolecules of extracellular matrix: determination of selective structures and their functional significance

In this brief review, I recount events and scientific endeavors in which I have been privileged to participate. The descriptive information includes discovery and characterization of hydroxylysine glycosides from collagen, isolation of dentin sialoprotein (DSP), investigations on dentin phosphoprotein (DPP), and the discovery of a single gene for both DSP and DPP that requires posttranslational proteolytic cleavage of the parent DSPP molecule to generate the two fragments. Finally, I address our unexpected finding of fragments of DMP1 in bone extracts. These fragments are from the NH2-terminal (37 kDa) and COOH-terminal (57 kDa) regions of DMP1. Our studies showed that, similar to DSPP, DMP1 is proteolytically processed by cleavages at X-Asp bonds.

Dynamic processing of recombinant dentin sialoprotein-phosphophoryn protein

Dentin sialoprotein (DSP) and phosphophoryn (PP) are the two noncollagenous proteins classically linked to dentin but more recently found in bone, kidney, and salivary glands. These two proteins are derived from a single copy DSP-PP gene. Although this suggests that the DSP-PP gene is first transcribed into DSP-PP mRNAs, which later undergo processing to yield the DSP and PP proteins, this mechanism has not yet been demonstrated because of the inability to identify a DSP-PP precursor protein from any cell or tissue sample. To study this problem, we utilized a baculovirus expression system to produce recombinant DSP-PP precursor proteins from a DSP-PP(240) cDNA, which represents one of several endogenous DSP-PP transcripts that influence various tooth mineralization phases. Our in vitro results demonstrate that DSP-PP(240) precursor proteins are produced by this system and are capable of self-processing to yield both DSP and PP proteins. We further demonstrated that purified recombinant DSP-PP(240), purified recombinant PP(240), and the native highly phosphorylated protein (equivalent to the PP(523) isoform) have proteolytic activity. These newly identified tissue proteases may play key roles in tissue modeling during organogenesis.

Developmental biology and genetics of dental malformations

The synthesis of tooth development biology with human studies focusing on inherited conditions that specifically interfere with tooth development is improving our understanding of normal and pathological tooth formation. The type of inherited dental malformations observed in a given kindred relate to when, during odontogenesis, the defective gene is critically expressed. Information about the protein encoded by the defective gene and the resulting dental phenotype helps us understand the major processes underway at different stages during tooth development. Genes affecting early tooth development (PAX9, MSX1, and AXIN2) are associated with familial tooth agenesis or oligodontia. Genes expressed by odontoblasts (COL1A1, COL1A2, and DSPP), and ameloblasts (AMELX, ENAM, MMP20, and KLK4) during the crown formation stage, are associated with dentinogenesis imperfecta, dentin dysplasia, and amelogenesis imperfecta. Late genes expressed during root formation (ALPL and DLX3) are associated with cementum agenesis (hypophosphatasia) and taurodontism. Understanding the relationships between normal tooth development and the dental pathologies associated with inherited diseases improves our ability to diagnose and treat patients suffering the manifestations of inherited dental disorders.

Dentin sialophosphoprotein is processed by MMP-2 and MMP-20 in vitro and in vivo

Dentin sialophosphoprotein (DSPP) is a major secretory product of odontoblasts and is critical for proper tooth dentin formation. During dentinogenesis, DSPP is proteolytically cleaved into smaller subunits. These cleavages are proposed activation steps, and failure to make these cleavages is a potential cause of developmental tooth defects. We tested the hypothesis that dentin-resident matrix metalloproteinases catalyze the cleavages that process DSPP. We defined the exact DSPP cleavages that are catalyzed by proteases during crown formation by isolating DSPP-derived proteins from developing porcine molars and characterizing their N-terminal sequences and apparent size on SDS-PAGE and Western blots. The in vivo DSPP cleavage sites were on the N-terminal sides of Thr(200), Ser(330), Val(353), Leu(360), Ile(362), Ser(377), Ser(408), and Asp(458). The initial DSPP cleavage is between dentin glycoprotein (DGP) and dentin phosphoprotein (DPP), generating dentin sialoprotein (DSP)/DGP and DPP. Gelatin and casein zymograms identified MMP-2, MMP-20, and KLK4 in the dentin extracts. MMP-2 and MMP-20 were purified from over 150 g of porcine dentin powder and incubated with DSP-DGP and DPP. These enzymes show no activity in further cleaving DPP. MMP-20 cleaves DSP-DGP to generate DSP and DGP. MMP-20 also cleaves DSP at multiple sites, releasing N-terminal DSP cleavage products ranging in size from 25 to 38 kDa. MMP-2 makes multiple cleavages near the DSP C terminus, releasing larger forms of DGP, or "extended DGPs." Exact correspondence between DSPP cleavage sites that occur in vivo and those generated in vitro demonstrates that MMP-2 and MMP-20 process DSPP into smaller subunits in the dentin matrix during odontogenesis.

Adsorption and interactions of dentine phosphoprotein with hydroxyapatite and collagen

Dentine phosphoprotein (DPP) has been proposed to both promote and inhibit mineral deposition during dentinogenesis. The present study aimed to investigate the molecular interactions of DPP and dephosphorylated DPP (DPP-p) with hydroxyapatite (HAP). Bovine DPP was purified and dephosphorylated by alkaline phosphatase to obtain DPP-p. DPP and DPP-p adsorption to HAP was determined along with their ability, when free in solution or bound to collagen, to influence HAP-induced crystal growth. Absorption isotherms suggested that lower DPP concentrations (1.5-6.25 microg ml(-1)) demonstrated a reduced affinity for HAP compared with higher protein concentrations (12.5-50.0 microg ml(-1)). Dephosphorylated DPP had a much reduced affinity for HAP compared with DPP. Dentine phosphoprotein inhibited seeded HAP crystal growth, in a dose-dependent manner, whilst removal of the phosphate groups reduced this inhibition. When bound to collagen fibrils, DPP significantly promoted the rate of HAP crystal growth over 0-8 min. Conversely, DPP-p and collagen significantly decreased the rate of crystal growth over 0-18 min. These results indicate a major role for the phosphate groups present on DPP in HAP crystal growth. In addition, concentration-dependent conformational changes to DPP, and the interaction with other matrix components, such as collagen, are important in predicting its dual role in the mineralization of dentine.

Association of bovine dentine phosphophoryn with collagen fragments

Bovine dentine phosphophoryn (BDP), a protein rich in aspartyl (Asp) and O-phosphoseryl (Ser(P)) residues, is synthesized by odontoblasts and believed to be involved in matrix-mediated biomineralization of dentine. Phosphophoryn was purified from bovine dentine using EDTA extraction, Ca(2+) precipitation, anion exchange and size exclusion chromatography. The purified protein migrated on SDS-PAGGE as a single band. The protein was dephosphorylated using a chelex alkaline dialysis procedure, repurified using anion exchange and size exclusion chromatography and then subjected to cleavage with trypsin. The digest was subjected to reversed-phase HPLC and analysed by Q-TOF mass spectrometry. The only non-trypsin peptides that could be identified were two collagen Type I alpha2 peptides whose sequence was determined by fragmentation analysis. The association of collagen fragments with highly purified phosphophoryn suggests that the EDTA extraction method yields BDP that is strongly bound to collagen fragments. This association now helps explain discrepancies in molecular weight and amino acid composition data for various phosphophoryn preparations compared with the same data calculated from the C-terminal extension of mouse, rat and human dentine sialophosphoprotein (DSPP) gene products. Analysis of the mutation pattern of the clinical disorder Osteogenesis Imperfecta within the region enclosed by the identified collagen fragments reveals that phosphophoryn associates with a segment of collagen that is crucial for structure and/or function.

Extracellular matrix-mediated signaling by dentin phosphophoryn involves activation of the Smad pathway independent of bone morphogenetic protein

Cells have ingenious mechanisms for interpreting complex signals from their external microenvironment. Previously, we have shown that phosphophoryn (PP) regulates the expression of bone/dentin marker genes via the integrin/MAPK signaling pathway (Jadlowiec, J., Koch, H., Zhang, X., Campbell, P. G., Seyedain, M., and Sfeir, C. (2004) J. Biol. Chem. 279, 53323-53330). We hypothesize that other signaling pathways important for mineralized tissue morphogenesis such as the Smad pathway could be involved in PP signaling. We determined activation of the Smad pathway in human adult mesenchymal stem cells following treatment with recombinant PP (rPP). We observed that PP enhanced phosphorylation of Smad1 within 30 min and Smad1 translocation to the nucleus within 1 h. PP up-regulated the expression of Smad1 target genes, Smad6, Dlx5, and Runx2. The timing of PP activation of Smad1 implies this is a direct effect; however, we also investigated the possible involvement of bone morphogenetic proteins in PP stimulation of the Smad pathway. PP was shown to up-regulate Bmp-2 gene expression 12 h post-treatment with PP, which is much later than initial detection of Smad1 phosphorylation at 30 min. Furthermore, addition of Noggin did not block Smad1 phosphorylation by PP. We propose that PP could signal via the Smad pathway by either directly stimulating the phosphorylation of Smad1 via integrins or other mechanisms. These might include integrin/bone morphogenetic protein receptor interactions or involvement of PP with other growth factors leading to the modulation of intracellular signaling. It is noteworthy that a non-transforming growth factor-beta family member activates the Smad pathway. The role of PP in regulating the Smad pathway raises very interesting questions regarding the role of PP during bone and tooth development.

Targeted disruption of two small leucine-rich proteoglycans, biglycan and decorin, excerpts divergent effects on enamel and dentin formation

Small leucine-rich proteoglycans have been suggested to affect mineralization of dental hard tissues. To determine the functions of two of these small proteoglycans during the early stages of tooth formation, we characterized the dental phenotypes of biglycan (BGN KO) and decorin deficient (DCN KO) mice and compared them to that of wild type mice. Each targeted gene disruption resulted in specific effects on dentin and enamel formation. Dentin was hypomineralized in both knock out mice, although the effect was more prominent in the absence of decorin. Enamel formation was dramatically increased in newborn biglycan knockout mice but delayed in absence of decorin. Increased enamel formation in the former case resulted from an upregulation of amelogenin synthesis whereas delayed enamel formation in the later case was most probably an indirect consequence of the high porosity of the underlying dentin. Enamelin expression was unchanged in BGN KO, and reduced in DCN KO. Dentin sialoprotein (DSP), a member of the family of phosphorylated extracellular matrix proteins that play a role in dentinogenesis, was overexpressed in BGN-KO odontoblasts and in the sub-odontoblastic layer. In contrast, a decreased expression of DSP was detected in DCN KO. Dentin matrix protein-1 (DMP-1), bone sialoprotein (BSP) and osteopontin (OPN) were upregulated in BGN KO and downregulated in the DCN KO. Despite the strong effects induced by these deficiencies in newborn mice, no significant difference was detected between the three genotypes in adult mice, suggesting that the effects reported here in newborn mice are transient and subjected to self-repair.

Protein dynamics of bovine dentin phosphophoryn

Bovine dentin phosphophoryn (BDP), a protein rich in aspartyl (Asp) and o-phosphoseryl [Ser(P)] residues, is synthesized by odontoblasts and believed to be involved in matrix-mediated biomineralization of dentin. The elucidation of the structure-function relationship of phosphophoryn has been a challenge because of its high-molecular weight, high negative charge, repetitive sequence, and lability. We have used the dynamic behavior of the (1)H NMR signal at 600 MHz to provide insight into the molecular dynamics of phosphophoryn. Our results indicate that phosphophoryn is a molecule of uniformly high mobility, thus belonging to a recently identified class of intrinsically disordered proteins that are characterized by sequences of low complexity and rich in polar and charged residues. The significance of our results is that phosphophoryn, because of its uniform nature has the potential to be replaced by biomimetic synthetic peptide analogs that together with amorphous calcium phosphate may lead to the development of novel, nontoxic, apatite-based dental restorative materials.

Phosphophoryn regulates the gene expression and differentiation of NIH3T3, MC3T3-E1, and human mesenchymal stem cells via the integrin/MAPK signaling pathway

Extracellular matrix proteins (ECMs) serve as both a structural support for cells and a dynamic biochemical network that directs cellular activities. ECM proteins such as those of the SIBLING family (small integrin-binding ligand glycoprotein) could possess inherent growth factor activity. In this study, we demonstrate that exon 5 of dentin matrix protein 3 (phosphophoryn (PP)), a non-collagenous dentin ECM protein and SIBLING protein family member, up-regulates osteoblast marker genes in primary human adult mesenchymal stem cells (hMSCs), a mouse osteoblastic cell line (MC3T3-E1), and a mouse fibroblastic cell line (NIH3T3). Quantitative real-time PCR technology was used to quantify gene expression levels of bone markers such as Runx2, Osx (Osterix), bone/liver/kidney Alp (alkaline phosphatase), Ocn (osteocalcin), and Bsp (bone sialoprotein) in response to recombinant PP and stably transfected PP. PP up-regulated Runx2, Osx, and Ocn gene expression. PP increased OCN protein production in hMSCs and MC3T3-E1. ALP activity and calcium deposition was increased by PP in hMSC. Furthermore, an alpha(v)beta(3) integrin-blocking antibody significantly inhibited recombinant PP-induced expression of Runx2 in hMSCs, suggesting that signaling by PP is mediated through the integrin pathway. PP was also shown to activate p38, ERK1/2, and JNK, three components of the MAPK pathway. These data demonstrate a novel signaling function for PP in cell differentiation beyond the hypothesized role of PP in biomineralization.

Mechanical properties and microstructure of hypomineralised enamel of permanent teeth

Isolated enamel defects are commonly seen in first permanent molar teeth but there has been little work on the physical and morphological composition of affected molars. The aim of this study was to determine the mechanical and morphological properties of hypomineralised first permanent molar teeth, utilising the Ultra-Micro-Indentation System (UMIS) and scanning electron microscope, respectively. Further investigations using Energy Dispersive X-ray Spectrometery (EDS), Back Scatter Electron (BSE) Imaging, and X-ray diffraction were employed to attempt to determine the chemical composition, mineral content and crystalline structure of the hypomineralised tissue, respectively, of eight first permanent molars with severe enamel hypomineralisation. The hardness and modulus of elasticity were found to be statistically significantly lower (0.53+/-0.31 and 14.49+/-7.56 GPa, respectively) than normal enamel (3.66+/-0.75 and 75.57+/-9.98 GPa, respectively). Although the fractured surface of the hypomineralised enamel was significantly more disorganised and the relative mineral content was reduced by approximately 5% in comparison to sound enamel, the mineral phase and Ca/P ratio was similar in hypomineralised and sound enamel. The dramatic reduction in the mechanical properties of first permanent molar teeth has ramifications when clinicians are choosing restorative materials to restore the defects. The reason for the dramatic reduction in mechanical properties of hypomineralised first permanent molar teeth is at present unknown.

The conservation and regulation of rat DSP-PP gene

Two highly expressed noncollagenous proteins associated with dentin mineralization, dentin sialoprotein (DSP) and phosphophoryn (PP), are encoded by a single DSP-PP transcript. To better understand how DSP-PP transcripts are regulated, we have determined the DSP-PP transcription start site, sequenced its 5' flanking region, and analyzed the transcriptional activity of the gene promoter out to -1615 bp. Comparison of the rat cDNA sequence with the mouse, rat and human genes clearly indicates high sequence conservation within the DSP-PP 5' flanking region, implicating the possible presence of highly conserved gene regulatory cis elements. Among a number of conserved transcription sites identified in the 5' flanking region, we demonstrate that the conserved Y box sequence (ATTGG) can specifically bind nuclear extracts from mouse MDPC23 cells. This sequence (located within the -57 bp/-52 bp 5' flanking region) therefore likely represents one DSP-PP transcriptional regulatory sequence.

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

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

Phosphate and calcium uptake by rat odontoblast-like MRPC-1 cells concomitant with mineralization

It has been suggested that odontoblasts are instrumental in translocating Ca2+ and inorganic phosphate (Pi) ions during the mineralization of dentin. The aim of this study was to characterize cellular Pi and Ca2+ uptake in the novel rat odontoblast-like cell line mineralizing rat pulpal cell line (MRPC) 1 during mineralization to see if changes in the ion transport activity would occur as the cultures develop and begin forming a mineralized matrix. MRPC-1 cells were cultured in chemically defined medium containing ascorbate and Pi, and cultures were specifically analyzed for cellular P, and Ca2+ uptake activities and expression of type II high-capacity Na+-Pi cotransporters. The odontoblast-like phenotype of the cell line was ascertained by monitoring the expression of collagen type I and dentin phosphopoprotein (DPP). Mineralized nodule formation started at day 9 after confluency and then rapidly increased. Ca2+ uptake by the cells showed a maximum during the end of the proliferative phase (days 5-7). Pi uptake declined to a basal level during proliferation and then was up-regulated simultaneously with the onset of mineralization to a level fourfold of the basal uptake, suggesting an initiating and regulatory role for cellular Pi uptake in mineral formation. This up-regulation coincided with a conspicuously increased glycosylation of NaPi-2a, indicating an activation of this Na+-Pi cotransporter. The study showed that MRPC-1 cells express an odontoblast-like phenotype already at the onset of culture, but that to mineralize the collagenous extracellular matrix (ECM) that formed, a further differentiation involving their ion transporters is necessary.

A novel rat 523 amino acid phosphophoryn: nucleotide sequence and genomic organization

Phosphophoryns (PP), the major noncollagenous proteins (NCPs) in dentin, are believed to play a crucial role in mineral nucleation and hydroxyapatite growth during dentin mineralization. Previously we identified two mature rat PP transcripts, one coding for a 240 amino acid protein (designated as PP(240)) (H.H. Ritchie, L.-H. Wang, J. Biol. Chem. 271 (1996) 21695-21698), and another coding for a 171 amino acid protein (PP(171)) (H. Ritchie, L. Wang, Biochim. Biophys. Acta 1493 (2000) 27-32). We now have identified a third novel dentin sialoprotein (DSP)-PP cDNA transcript that encodes a 523 amino acid protein (PP(523)) with typical PP characteristics including DSS and DS motifs suitable as potential casein kinase I and II phosphorylation sites. Based on amino acid composition, the PP(523) protein product is identical to native rat HP2. We also show that the PP(523) sequence is identical to the corresponding genomic DNA sequence. Taken together, the existence of multiple DSP-PP transcripts, each significantly different from the other in net negative charge, suggests that dentin mineralization processes may be under fine-tune control by these PP protein isoforms.

The whereabouts of a morphogen: direct evidence for short- and graded long-range activity of hedgehog signaling peptides

Sonic Hedgehog (Shh) and Indian Hedgehog (Ihh) are members of the Hedgehog (Hh) family of signaling molecules known to be involved in embryonic patterning and morphogenesis. The Hh proteins undergo an autocatalytic cleavage to yield an N-terminal and a C-terminal peptide, with the signaling capacities confined to the N peptide. Drosophila Hh-N has been shown to act via both short- and long-range signaling. In vertebrates, however, attempts to directly demonstrate Shh (SHH) or Ihh (IHH) proteins at a distance from producing cells have been largely unsuccessful. Furthermore, the fact that the Hh N peptides occur in a cholesterol-modified, membrane-tethered form is not easily reconciled with long-range signaling. This study used optimized immunohistochemistry combined with tissue separation and biochemical analyses in vivo and in vitro to determine the range of action of SHH and IHH in the mouse embryo. In all embryonic structures studied, we detect signaling peptides in producing cells, but we also find that ligands move over considerable distances depending on the tissue. These data provide direct evidence for the presence of Hedgehog signaling peptides in target compartments, suggesting a direct long-range action without a need for secondary mediators. Visualization of Hedgehog proteins in target tissues was achieved only under conditions that allowed proteoglycan/glycosaminoglycan (PG/GAG) preservation. Furthermore, we show that induced changes of the composition of PG/GAG in the tooth alter SHH signaling. These data suggest a crucial role for PG/GAGs in Hedgehog movement.

The presence of multiple rat DSP-PP transcripts

Phosphoproteins or phosphophoryns (PPs) are the most abundant (>50%) non-collagenous proteins (NCPs) in dentin. PPs bind to calcium and hydroxyapatite and are believed to play a crucial role in dentin mineralization. Dentin sialoprotein (DSP), a highly glycosylated protein, comprised 5-8% of NCPs in dentin. The coding sequences for these two major NCPs are known to be contiguously located (i.e. DSP-PP) at the cDNA and genomic DNA levels in both rat and mouse. Previous studies have demonstrated the presence of multiple DSP-PP transcripts in the total RNA of adult rat incisors. To further understand the nature of these multiple transcripts, we performed reverse transcription-PCR and obtained a PP cDNA variant which encoded a 171 amino acid peptide (PP(171)) that shares many of the same characteristics as that of the published rat PP(240) sequence [Ritchie, H.H. and Wang, L.-H., J. Biol. Chem. 271 (1996) 21695-21698]. Due to its reduced size, as compared to PP(240), this cDNA encodes a phosphorylated protein with a reduced negative charge that may differentially affect mineralization processes. We provide evidence that there are multiple DSP-PP transcripts with various sizes of PP sequences in rat.

The matrix metalloproteinase gelatinase A in human dentine

A dentine protein extraction protocol was modified in order to identify matrix metalloproteinase gelatinolytic activities in the non-mineralized and mineralized phases of human dentine. Dentine proteins from 24 individual permanent molars from patients aged 15-73 years were sequentially extracted, first with guanidinium chloride (G1 extract), then EDTA (E extract), and after this demineralization step, again by guanidinium chloride (G2 extract) to dissociate collagen-associated proteins. Extracts were analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and the gels were processed by Western blotting and zymography to detect gelatinolytic activities. Active and latent forms of gelatinase A were identified in the non-mineralized dentine fraction (G1 extract) of 58% of the teeth. Other gelatinolytic species were also detected by zymography with apparent M(r) of 92, 54 and 30 kDa. Although gelatinase A was detected in the G1 extracts of teeth from all ages, indicating more recent synthesis and remodelling of the predentine, gelatinase A was never detected in any E extract or in the G2 extracts of patients older than 41 years. The presence of the active form of gelatinase A in mineralized human dentine implicates this enzyme in dentine mineralization.

A review of the methodological aspects of aspartic acid racemization analysis for use in forensic science

Accurate age determination of adult cadavers and human remains is a key requirement in forensic practice. The current morphological methods lack accuracy and precision, require specialist training and are costly. The use of aspartic acid racemization (AAR) in human dentine provides a simple, cost-effective solution and the method can achieve accuracies of +/- 3 years at best. Currently, there are differences in AAR methodology between laboratories which produce different results on the rate of racemization in teeth. These inconsistencies must be resolved if the technique is to be successfully applied to age determinations in forensic cases. This paper reviews the differences in protocol which have been used, discusses how each method will affect the results obtained from AAR analysis and gives recommendations for optimization of the methological protocol as a first step towards international standardization.

Molecular pathogenesis of root dentin caries

Human dentin has a higher content of organic matrix and more non-ideal hydroxyapatite than human enamel. Ultrastructural studies indicate that root caries involves both mineral dissolution and breakdown of the organic matrix. Factors involved in the root caries process seem more complicated than those in enamel caries. Moreover, the distinct roles of acids and enzymes and the sequence of events in the root caries process are not well-understood. Although Streptococcus mutans and Actinomyces viscosus are considered to be major pathogenic micro-organisms of root caries, their roles in degradation of the organic matrix components of root dentin need clarification. The purpose of this paper is to review the basic composition of root dentin and the roles of acids and both endogenous and bacterial enzymes in the root caries process.

Characterization of fibromodulin isolated from bovine periodontal ligament

Although several proteoglycans (PGs) have been reported in bovine periodontal ligament (PDL), the composition of PGs in PDL has been poorly characterized. In the present study, we isolated and characterized keratan sulfate-substituted PG (fibromodulin) in bovine PDL. Fibromodulin was purified from 4 M guanidine hydrochloride (GdmCl) extracts of bovine PDL tissues using DEAE Sephacel ion-exchange chromatography and preparative electrophoresis. Fibromodulin appeared as a single polydisperse band with an apparent molecular weight (MW) of 80,000 (80 kDa) on SDS-PAGE. Digestion of fibromodulin with keratanase or neuraminidase reduced the apparent molecular size, and N-glycanase treatment produced core protein bands of around 40 kDa. Fibromodulin reacted with keratan sulfate monoclonal antibody (5D4) and fibromodulin polyclonal antibodies (alpha-FM). The keratanase-digested fibromodulin reacted with alpha-FM, but not with 5D4. These data suggest that fibromodulin is one of the small PGs in the PDL-matrix and may fulfill construction and maintenance functions in this tissue.

A mammalian bicistronic transcript encoding two dentin-specific proteins

We previously reported that the cDNA for dentin phosphophoryn, a non-collagenous protein involved in dentin mineralization, was located immediately downstream from the 3' end of dentin sialoprotein, another functionally related non-collagenous dentin protein (Ritchie, H. H., and Wang, L-H. (1996) J. Biol. Chem. 269, 3689-3702). We now demonstrate unequivocally that at the genomic level in mammal, these two functionally related proteins are indeed transcribed as one transcriptional unit suggesting that their combined presence at specific mineralization sites in tooth germ is mandatory if dentin mineralization is to proceed.

Dentin phosphoprotein and dentin sialoprotein are cleavage products expressed from a single transcript coded by a gene on human chromosome 4. Dentin phosphoprotein DNA sequence determination

Dentin is the major mineralized extracellular matrix of the tooth. The organic components of dentin consist of type I collagen (90%) with 10% noncollagenous proteins, which are also components of bone. Two dentin proteins, dentin sialoprotein and dentin phosphoprotein, have been shown to be tooth-specific being expressed mostly by odontoblast cells. In this study, we screened a mouse molar tooth library for dentin sialoprotein and dentin phosphoprotein cDNA clones. Analysis of the clones resulted in characterization of a 4420-nucleotide cDNA that contained a 940-amino acid open reading frame. The signal peptide and NH2-terminal sequence was 75% homologous to the cDNA sequence of rat dentin sialoprotein. The continued open reading frame, however, contained a RGD sequence followed by a region of repeated aspartic acid and serine residues. This portion of the protein codes for amino acid sequence consistent with that of dentin phosphoprotein. The noncoding region contains three potential polyadenylation signals, two of which were shown to be utilized. Northern blot analysis indicated the presence of two major transcripts of 4.4 and 2.2 kilobases in odontoblasts. Chromosomal mapping localized the gene to human chromosome 4. These data suggest that the previously identified dentin extracellular matrix proteins, dentin sialoprotein and dentin phosphoprotein, are expressed as a single cDNA transcript coding for a protein that is specifically cleaved into two smaller polypeptides with unique physical-chemical characteristics. Therefore, we propose that the gene be named dentin sialophosphoprotein. The location of the human dentin sialophosphoprotein gene on chromosome 4 suggests that this gene may be a strong candidate gene for the genetic disease dentinogenesis imperfecta type II.

Characterization and identification of a human dentin phosphophoryn

The present study further characterizes an extract from immature, human tooth apicies from which an intact dentin phosphoprotein has been identified. Third molar apicies from developing roots were decalcified in 10% EDTA until Ca2+ was undetectable in the decalcifying solution. The crude extract was run on 7.5% SDS-PAGE and stained with "Stains-All." Four distinct bands were found and the molecular weights were 140, 60, 50, and 34 k. When run on a SDS-PAGE under nonreducing conditions the 60, 50, and 34 k bands were absent. These results suggest that the lower molecular weight bands may be subunits of the larger protein. The extract was then further purified by adding CaCl2 and MgCl2 to precipitate the phosphoprotein. The precipitate was subjected to a DEAE-Sepharose CL6B column and eluted by 0-0.7 M NaCl gradient solution. The amino acid composition of the purified phosphoprotein was determined and the extract was found to be rich in serine and aspartic acid residues. The N-terminal peptide Asp-Asp-Pro was identified. The sequence of the three amino acids is identical to rat incisor phosphoprotein.

Modification of the proteoglycans of rat incisor dentin-predentin during in vivo fluorosis

Proteoglycans (PGs) were isolated from the dentin-predentin of fluorotic and control rat incisor teeth using demineralization in EDTA, followed by extraction with 4 M guanidinium chloride in the presence of protease inhibitors. Differences in the behaviour of fluorotic and control PG were evident during purification by anion exchange chromatography on Q-Sepharose and MONO-Q interfaced with fast protein liquid chromatography. The PG from fluorotic teeth exhibited a more anionic profile, due to changes in glycosaminoglycan characteristics, since no apparent differences were evident between the respective core proteins, both of which were 45 kDa. The constituent glycosaminoglycan chains of fluorotic dentin were of lower molecular size and showed the additional presence of dermatan sulfate and heparan sulfate by comparison to nonfluorotic controls, where only chondroitin-4-sulfate was detected.