Chemical character of proteins in rat incisors

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.

Hydrogen Peroxide Might Bleach Natural Dentin by Oxidizing Phosphoprotein

Recent studies suggested that bleaching agents may whiten teeth by oxidizing the fluorescent materials, which are the proteins located in the organic-inorganic interface. Therefore, we postulated that fluorescence of dentin came from dentin phosphoprotein (DPP) and that bleaching agents might bleach dentin by oxidizing DPP. Fifty-six specimens were randomly divided into 4 groups and exposed to distilled water, hydrogen peroxide (HP), ethylenediamine tetraacetic acid disodium salt (EDTA), and acetic acid for 24 h. After measuring the organic and inorganic components, fluorescence, and color characteristics of dentin before and after exposure, we found that when DPP was removed from dentin by EDTA, fluorescent intensity declined proportionally with the reduction in Raman relative intensity, and dentin was whitened considerably, with an Δ E value 6 times higher than that of the distilled water group. On the contrary, due to the incapability of acetic acid to dissolve DPP during decalcification, fluorescent intensity values and tooth color remained nearly unchanged after exposure to acetic acid. Dentin exposed to neutral HP showed no obvious morphologic and organic/inorganic component changes except for the destruction of DPP. Similarly, dramatically decreased fluorescent intensity and lightened color were found in the HP group. Moreover, DPP solution of the HP group exhibited decreased ultraviolet absorbance, especially between 250 and 300 nm, which arose from aromatic amino acids. The results indicated that DPP was responsible for the fluorescent properties of dentin and that HP might bleach dentin by the oxidization of aromatic amino acids in DPP. These findings are of great significance in promoting our further understanding of the mechanism of tooth bleaching and the fluorescent property of normal dentin.

Mineral Induction by Matrix from Mineralized Biological Tissues

The polymeric matrix of mineralized tissues controls the form and structure of the mineral that is deposited. This matrix has an insoluble fraction which provides a structural framework for the mineralized tissue, and a soluble fraction which is rich in polyanionic macromolecules. One hypothesis envisages mineral being nucleated by an atomic dimensional matching between crystal lattice and anionic spacing in the polyanionic macromolecules. An alternate hypothesis considers that fixed polyanions provide a surface for an adsorbed layer, enriched in lattice ions by ionotropy, to induce mineral formation from the metastable body fluids.

We found that soluble matrix polyanions, immobilized by attachment to insoluble substrates, would induce mineral from metastable solutions. The insoluble substrates included natural and synthetic hydrogels not derived from mineralized tissues. Whether the polyanions were prepared from apatitic or CaCO3 tissues, the mineral induced was independent of the source and was determined by the composition of the solution. Other immobilized, calcium-binding, polyanionic macromolecules, obtained from non-mineralizing tissues, also induced mineral.

These and other data indicate that mineral induction by biological matrices is less specific than implied in the atomic dimensional matching extension of the epitaxial hypothesis.

Ultrastructural observations and growth of occluding crystals in carious dentine

The aim of the present study was to investigate the ultrastructural mechanisms involved in the formation of caries-induced intratubular dentine. Conventional, high resolution and scanning transmission electron microscopy, electron diffraction and energy-dispersive X-ray spectroscopy techniques were used to study the ultrastructure of the inorganic phase in the transparent zone of carious dentine. The results demonstrated that the bulk of the inorganic phase in caries-induced intratubular dentine had an apatite crystal structure with the presence of additional Mg-substituted beta-TCP (beta-tricalcium phosphate) phase in the carious region. Highly oriented apatite crystallites observed in intratubular dentine demonstrated a regulated biomineralization process during the formation of inorganic phase in this region, whereas Mg beta-TCP crystals were presumably formed purely via "dissolution/precipitation" mechanism. The study demonstrated the importance of "dissolution/precipitation" process and the growth kinetics of Mg-substituted beta-TCP crystals in understanding the process of formation of calcium-phosphate crystallites in carious intratubular dentine.

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.

Study of the maturation of the organic (type I collagen) and mineral (nonstoichiometric apatite) constituents of a calcified tissue (dentin) as a function of location: a Fourier transform infrared microspectroscopic investigation

Fourier transform infrared microspectroscopy (FTIRM) was used to investigate the organic and mineral phases of a calcified tissue (dentin) as a function of its location from predentin toward enamel. Thin dentin slices (decalcified or not) were fixed in formaldehyde and embedded in glycolmethylmethacrylate (GMA). Fixation did not denature collagen, and GMA did not interact with organic or mineral constituents of dentin. The v1v3 PO4 domain was studied in particular in order to estimate mineral maturity and amide I, II, A, and B to obtain data on protein conformation. The results showed that dentin apatite became increasingly mature (stoichiometric) from the mineralization front toward the enamel, especially through loss of HPO4(2-) groups and vacancies. Moreover, collagen fibrils became less and less hydrated, suggesting that intrafibrillar mineralization partially dehydrated the collagen. Combined study of the organic and mineral fractions of calcified tissues may help clarify their relationships in physiological and pathological tissues.

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.

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

BACKGROUND AND METHODS

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

RESULTS AND CONCLUSIONS

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

Membrane-mediated precipitation of calcium phosphate in model liposomes with matrix vesicle-like lipid composition

The present study examined calcium phosphate precipitation in aqueous suspensions of artificial liposomes which closely resembled matrix vesicles (MV) in membrane lipid composition. At 22 degrees C, the liposomes per se did not initiate precipitation in the suspending medium for up to 120 h when the latter was made supersaturated with respect to hydroxyapatite (2.25 mM Ca2+, 1.5 mM PO4, 240 mosmol, pH 7.4). Likewise, the suspending medium remained stable for up to 72 h when precipitation was induced within the aqueous interiors of the liposomes by encapsulating pH 7.4-buffered 50 mM PO4 solutions in the interior spaces and making the enclosing membranes permeable to external solution Ca2+ ions with the ionophore X-537A. However, extraliposomal precipitation readily occurred under these latter conditions when phosphatidylserine (PS) and sphingomyelin (Sph) were deleted from the MV-like lipid formulation used to prepare the liposomes. These results suggest that lipidic membrane constituents such as PS and Sph may have a controlling influence on MV-mediated calcification in vivo by affecting the release of intravesicularly formed mineral crystals into the extracellular matrix space where they can subsequently grow and proliferate.

Concentration-dependent effects of dentin phosphophoryn in the regulation of in vitro hydroxyapatite formation and growth

The effect of dentin phosphophoryn on hydroxyapatite formation and growth was studied in an in vitro gelatin gel diffusion system. Phosphophoryn, in low concentrations (0.010-1 microgram/ml) promoted de novo hydroxyapatite formation; at a higher concentration (100 micrograms/ml) in the same system, the dentin matrix protein inhibited hydroxyapatite growth. Similar inhibition of hydroxyapatite growth was seen in solution. The intact phosphophoryn was not essential for either inhibition of seeded growth or promotion of mineralization, since the formic acid degraded protein was comparably effective. Transmission electron microscopy of the precipitates formed at 7 days showed no significant differences in crystallite size distribution in the presence and absence of phosphophoryn. However there was a dose-dependent decrease in the number of mineral clusters formed in the presence of increasing amounts of phosphophoryn, suggesting inhibition of secondary nucleation. These data provide support for the postulated 'multifunctional' role of the dentin phosphoprotein in the mineralization process.

Release of organic matrix components from bovine incisor roots during in vitro lesion formation

The solubilization of organic matrix components during demineralization of powdered and intact root sections from adult bovine incisors was investigated. Root powder was demineralized with 0.1 mol/L acetic acid, pH 4.0, at 4 degrees C and 37 degrees C. Surfaces of intact root sections were subjected to 0.1 mol/L acetic acid, pH 4.0 (for production of erosive lesions), or to 0.1 mol/L lactic acid, 0.2 mmol/L methane hydroxy diphosphonate, pH 5.0 (for production of subsurface lesions at 37 degrees C). The solubilized organic material was analyzed for collagen, total noncollagenous protein (NCP), organic phosphate (Po), and proteoglycans (PGs), which were measured as chondroitin 4-sulfate (C-4-S). For root powder, a maximal release of NCPs and PGs was found only after neutralization of the extraction mixture. For both temperatures tested, the average amounts of liberated noncollagenous components (NCCs) were the same, i.e., 0.68 micrograms [NCP - Po], 0.11 microns Po, and 0.10 micrograms C-4-S per mumols released calcium. The amino acid composition of the NCP fraction revealed relatively high amounts of aspartic acid and serine. These findings indicate that the NCCs were easily liberated from the tissue, and that the NCP fraction consisted mainly of phosphoprotein. Demineralization of intact root sections resulted in average amounts of solubilized NCCs of 0.21 micrograms [NCP - Po], less than 0.01 micrograms Po, and less than 0.01 micrograms C-4-S per mumols released calcium, independent of incubation time and lesion type. The amino acid composition of all NCP fractions was virtually the same, high in glutamic acid, but lower in aspartic acid and serine when compared with the neutralized powder extracts. For both demineralization solutions, the amounts of solubilized collagen were 0.07 and 0.16 micrograms per micromol released calcium after three and 28 days of incubation, respectively. Our experiments indicate that phosphoprotein and proteoglycans may be released from root surfaces during the periods of neutral pH that follow acid demineralization.

Phosphoprotein extraction from the dentine/cementum complex of human tooth roots

Root shards were placed in dialysis tubing and demineralized to completion in either 10% disodium EDTA, pH 7.4, 0.6 M HCl, 0.1 M HCl, 0.5 M acetic or 75 mM-25 mM lactic-acetic acids. The demineralized shards were then re-extracted with 0.05 M tris-HCl, 1.0 M NaCl. DEAE chromatography revealed that the major peak of the 0.6 M CHl and EDTA extracts contained organic phosphorus, whereas much less organic phosphorus was found in the major peak of the 0.1 M HCl extract. Analysis of the re-extracts gave a pattern opposite to that obtained from the initial extractions. Measurements of protein and organic phosphorus released during extraction and re-extraction confirmed these results. Staining of SDS-PAGE gels for phosphoprotein with Stains-All resulted in a blue smear in fractions containing organic phosphorus. Thus the extraction of phosphoproteins from human tooth roots differed depending upon the demineralizing conditions. This ability to remove phosphoprotein differentially will allow further investigation of the role of phosphoprotein in mineralization and remineralization.

Soluble glycosylated phosphoproteins of cementum

Ethylenediaminetetraacetate and hydrochloric acid (EDTA) (HCl) extracts of cementum were fractionated by molecular sieving, ion exchange chromotography, and reverse phase high precision liquid chromatography (HPLC). Nine fractions were isolated, all of which contained serine phosphate, threonine phosphate, and high concentrations of aspartic acid (asp) and glutamic acid (glu). Five of the fractions obtained by repeated HPLC consisted of a single band by SDS-PAGE; the others contained at least one other minor component. All of the protein bands stained with both Rhodamine B and alcian blue, the latter consistent with analytical determinations that demonstrated that the phosphoprotein component contained a significant amount of carbohydrate, including neuraminic acid.

Mechanism of calcification: role of collagen fibrils and collagen-phosphoprotein complexes in vitro and in vivo

Samples of decalcified chicken bone together with varying concentrations of phosphoproteins from bone or egg yolk (phosvitin) were used in vitro as heterogenous nucleators for the induction of Ca-P apatite crystals. The lag time between exposure of the collagen-phosphoprotein complexes and the time nucleation of crystals occurred decreased as the concentration of Ser(P) and Thr(P) increased. Enzymatic cleavage of the phosphate groups by wheat germ and phosphatase reversed this effort, indicating that the phosphate group per se principally facilitated the nucleation of Ca-P crystals by the phosphoprotein complex and collagen.

The effect of complexing phosphoproteins to decalcified collagen on in vitro calcification

Decalcified samples of chicken bone containing phosphoproteins of varying concentrations were used to assess the effect of phosphoproteins and of protein-bound Ser(P) and Thr(P) in the in vitro nucleation of a Ca-P solid phase from metastable solutions of Ca and P. Phosphoproteins of bone as well as the phosphoproteins from egg yolk (phosvitin) were used. Increasing concentrations of phosphoprotein [as measured by the amount of protein bound Ser(P) and Thr(P)] in the decalcified bone particles significantly reduced the time required for nucleation to occur after exposure to metastable solutions of Ca and P (decreased operational lag times). Treatment with wheat germ acid phosphatase markedly reduced the concentration of Ser(P) and Thr(P) in the decalcified bone samples and in the decalcified bone collagen samples complexed with phosphoproteins (almost to zero). The loss of the organic phosphate groups significantly increased the operational lag time, but did not abolish nucleation of apatite crystals by the bone collagen fibrils essentially devoid of Ser(P) and Thr(P). Bone phosphoproteins were not specific; substitution of phosvitin for bone phosphoproteins as complexes with bone collagen also proved to be effective facilitators of nucleation, which was interesting since both types of phosphoproteins have certain common chemical and structural characteristics. Noncollagenous components other than phosphoproteins were present in the decalcified bone samples. However, the marked dependence of the lag time on the Ser(P) and Thr(P) concentrations and the very marked diminution in the efficacy of the nucleation phenomenon as a result of treatment with wheat germ acid phosphatase, clearly suggests that the organic phosphate residues of the phosphoproteins play a direct and significant role in the process of in vitro nucleation of a solid phase of Ca and P (apatite) by bone collagen, and by implication, possibly in in vivo mineralization as well.

Phosphoprotein synthesis and secretion by odontoblasts in rat incisors as revealed by electron microscopic radioautography

The secretory pathway of dentin phosphoproteins in rat incisors was studied by electron microscopic radioautography after the injection of 3H-serine, and the results were compared with those using 3H-proline as a tracer. Five min after injection of 3H-serine, radioactivity was found in the rough endoplasmic reticulum. At 10 min, silver grains were observed over the spherical portions of the cisface of the Golgi apparatus. At 20 min after injection, silver grains were seen over the cylindrical portions of the transface of the Golgi apparatus. The secretory granules showed the strongest reaction from 20 min to 1 hr. At 45 min, a significant labeled band appeared at the mineralization front. At 1 hr, the labeling at the mineralization front began to appear in the mineralized dentin, and after 12 hr this labeled band was located within the mineralized dentin. The pathway of 3H-proline was essentially the same as that of 3H-serine, but 3H-proline moved more slowly than 3H-serine, especially in transit from the rough endoplasmic reticulum to the Golgi apparatus. Secretory granules were heavily labeled from 30 min to 1 hr after injection of 3H-proline; no labeling was found at the mineralization front at 45 min. The labeling seen initially over the predentin was over the mineralized dentin no earlier than 6 hr after injection. The labeling pattern with 3H-serine is closely related to the localization of phosphoproteins, whereas the pattern with 3H-proline reflects the production of collagen rather than of phosphoproteins. The present radioautographic results indicate that dentin phosphoproteins are related to secretory granules and are secreted by odontoblasts at the mineralization front and also that phosphoproteins are involved in the process of mineralization of the circumpulpal dentin.

In vitro study of the giant tubule collagen formation in bovine dentin by [3H]-proline incorporation

Unerupted permanent bovine incisors studied by routine autoradiography using radioactive proline incorporation revealed collagen formation within the giant tubules situated in the incisal dentin. A high [3H]-proline labeling was seen within the pulpal vascularized portion of the giant tubules, as well as in association with groups of cells immediately incisal to the blood vessel loops. The incisal portion of the giant tubules showed no or insignificant [3H]-proline labeling.

Difference in noncollagenous matrix composition between crown and root dentin of bovine incisor

A comparative study of the organic matrix in crown dentin and root dentin of the bovine incisor has been performed utilizing biochemical analyses and histochemical stainings. Root dentin contained only half the amount of dentin phosphophoryn present in crown dentin. Composition of noncollagenous matrix other than phosphophoryn was also suggested to be different between crown and root dentin. These data indicate that odontoblasts forming root dentin may secrete a noncollagenous matrix having a composition different from that of crown dentin, and that crown and root dentin may be separate in nature. The distinct compositions of noncollagenous matrix may reflect the difference in the mineralization process between crown and root dentin.

On the problem of covalent linkages between phosphoproteins and collagen in bovine dentin and bone

The majority of phosphoproteins in bovine bone and dentin are insoluble in EDTA and guanidine hydrochloride (Gu.HCl) at 2 degrees C. After removal of EDTA and Gu.HCl-soluble proteins at 2 degrees C, collagen alpha-chains and alpha-chain polymers were extracted from bovine bone and dentin in Gu.HCl at elevated temperatures and purified by several chromatographic techniques and SDS-PAGE. Small amounts of O-phosphoserine were found in all collagen components. In contrast, O-phosphoserine was not detected in the purified collagen components soluble in EDTA or Gu.HCl at 2 degrees C nor was hydroxyproline detected in the EDTA-soluble phosphoproteins. In contrast, although the vast majority of EDTA-insoluble collagen and phosphoprotein molecules can be readily dissociated by a variety of molecular sieving and ion-exchange chromatographic procedures, a small number are very strongly associated or covalently cross-linked. These results are consistent with the findings that both hydroxyproline and hydroxylysine are present in purified phosphoprotein components released from the EDTA-insoluble tissue by bacterial collagenase. The hydroxylysine/100 hydroxyproline ratios in the phosphoprotein-collagen complexes are much higher than those in dentin or bone collagens.

An ultrastructural study of dentinogenesis and amelogenesis in rat molar tooth germs cultured in vitro

Molar tooth germs from three-day-old rats were cultured successfully for fourteen days, permitting the study of the development in vitro of both extracellular matrix and cellular elements such as odontoblasts and ameloblasts. The ultrastructure of the cultured tooth germs was compared with the ultrastructure of tooth germs in vivo at a comparable developmental stage. Progenitor cells of odontoblasts and ameloblasts were found to differentiate in vitro. Odontoblasts seemed to contain more lysosome-like bodies and fewer secretory granules than in vivo. They formed normally mineralizing dentine or a thick layer of dense, unmineralized predentine with incidentally some amorphous, extracellular material. Enamel was exclusively present opposite well developed dentine. It was often hyper- or hypomineralized and enamel rods were not as regularly shaped as in vivo. In places where no enamel formation had taken place, large amounts of amorphous extracellular material were sometimes seen. From these observations it can be concluded that cellular development in cultured tooth germs appeared more or less normal, but extracellular matrix formation and mineralization were sometimes disturbed.

Histological distribution of phosphophoryn in normal and pathological human dentins

Dentin phosphophoryn is a highly phosphorylated protein which has a hydrophilic character but is not soluble in dilute acetic acid. A histochemical method was developed for staining this protein with Stains-all in situ utilizing those chemical properties. We have succeeded in detecting the presence of this protein in circumpulpal orthodentin of human permanent and deciduous teeth, but not in mantle dentin, secondary dentin and reparative dentin. Phosphophoryn staining was also absent in the dentin of dentinogenesis imperfecta (DI) Type II, a genetic disorder of dentin formation. From these results, it is suggested that phosphophoryn is synthesized and secreted only by physiologically-differentiated odontoblasts and that the mineralization processes of mantle, secondary, reparative and DI dentins may be different from that of circumpulpal orthodentin.

The biosynthesis of dentin phosphophoryns by rat incisor odontoblasts in organ culture

An in vitro system consisting of rat incisor fragments was used to study the process of dentinogenesis. In order to establish the usefulness of the organ culture, the biosynthesis and deposition of the major noncollagenous components of dentin, the phosphophoryns, were followed for specific lengths of time in culture. Three criteria were satisfied: (1) the synthesis of proteins which appeared to be chemically identical to the native proteins of dentin, (2) the accumulation of the phosphophoryns within the matrix or time, and (3) the association of the secreted proteins with the mineral phase of dentin. The synthesis of phosphophoryns was determined by using both (3H)-serine and (32P)-inorganic phosphate as precursors for synthesis of protein and post-translational modification of serine to phosphoserine. In vitro synthesized phosphophoryns were characterized by 1) their accumulation and EDTA extractability from within dentin, 2) calcium chloride precipitability, 3) elution on anion-exchange columns (DEAE cellulose and AGMP50), and 4) Mr's on SDS-PAGE and Sepharose CL-6B columns. This novel system of studying dentinogenesis provides a model with which to study the regulation of extracellular matrix protein synthesis and may be useful for revealing the effect of other agents which influence tooth development and mineralized tissue metabolism in general.

Odontoblast differentiation and the formation of the odontoblast layer

Origin, cell kinetics, and phenotypic aspects of odontoblast cell lineage are described. Epithelial-mesenchymal interactions regulate odontoblast differentiation. These interactions appear to be mediated by the extracellular matrix. Possible molecular mechanisms of cell-matrix interactions are discussed. Questions still unanswered are recommended for investigation.

Intercellular junctions in human tooth-pulp cells in culture in vitro revealed by freeze-fracture, lanthanum impregnation and filipin treatment

Three kinds of intercellular junctions were detected between human dental pulp cells in explant culture with electron microscopy included filipin detection for cholesterol; desmosome-like junctions observed on ultrathin sections probably contribute to the cohesiveness between cells in culture. Gap junctions, responsible for intercellular communication, exhibited two morphologies on freeze-fracture replicas: a conventional arrangement of their intramembranous particles and a crystalline array corresponding to the formation stage of junctions. Primitive tight junctions were detected on freeze-fracture replicas but not on ultrathin sections. It is likely that they contribute to the cell-to-cell adhesion under culture conditions.

Chemical nature of collagen in the placoid-scale dentine of the blue shark, Prionace glauca L

The dentine fraction was obtained from powdered placoid scales by differential density-flotation, and demineralized with 0.5 M EDTA. Monomeric alpha 1 and alpha 2 chains of collagen were extracted from the residual organic matrix, and the two alpha chains purified by chromatography. The two alpha chains were also isolated from shark-skin collagen. The corresponding alpha chains from shark dentine and skin collagens resembled each other closely in chromatographic and electrophoretic behaviour and in their CNBr-peptide maps but differed from calf-skin alpha chains. The differences in the chemical composition between shark dentine and skin were due only to post-translational modification (hydroxylation and phosphorylation), indicating that the collagens are of the same type. The hydroxylation of prolyl and lysyl residues occurred more in the dentine alpha chains than in the skin chains. Among the four alpha chains, the phosphate content was the highest in the alpha 2 chain of the dentine collagen. These differences in hydroxylation and phosphorylation have been observed among alpha chains of mammalian mineralized and unmineralized tissues. The preferential dimerization to form alpha 1-alpha 2, characteristic of shark-skin collagen, was not observed in the dentine collagen. Other dimers were hardly detectable in the latter.

Further evidence of an intravesicular Ca2+- pump in odontoblasts from rat incisors

Intracellular vesicles containing alkaline phosphatases were isolated from isolated odontoblasts using several centrifugation techniques, gradient media and filtering procedures. With a combined centrifugation technique using 0.32 M sucrose layered on 1.23 M sucrose, a fraction containing alkaline phosphatases with a 36-40-fold increased specific activity was obtained. This fraction also revealed a high Ca2+-accumulating ability. The vesicle fraction was totally free from mitochondria but to some extent contaminated by lysosomes. Characteristics of Ca2+-uptake were obtained. The Ca2+-uptake was maximal at 37-40 degrees C whereas no Ca2+-accumulated at 4 degrees C. Temperatures above 40 degrees C strongly inhibited Ca2+-uptake. ATP was the most potent stimulator of Ca2+-uptake whereas ITP, GTP, CTP, ADP, PPi and AMP also promoted Ca2+-uptake. Cysteine, EDTA and Triton X-100 were inhibitory to Ca2+-uptake. A correlation between alkaline phosphatases in intracellular vesicles as well as their relation to extracellular matrix vesicles and to the mineralization process is suggested.

Electron-microscopical studies of conformational changes in dentinal phosphophoryn

p67tinal phosphophoryn was isolated and purified from unerupted calves molars by the methods of Butler et al. (1981). The resulting proteins were concurrently analyzed by circular dichroism and electron microscopy after low-angle rotary shadowing. Electron microscopy of these proteins in aqueous solutions revealed extended bead-like chains that possessed intramolecular variations in morphology. The addition of calcium ion or methanol to solutions of these proteins produced circular dichroism spectra indicative of more ordered structures. Electron microscopy of these preparations revealed aggregates of 25-30 nm disc-like structures. Although correlations of domain sequences and structure were not possible, the resulting structures did possess molecular morphologies that are compatible with some of the functional roles advocated for these proteins as calcium hydroxyapatite nucleating sites in the mineralization of dentin (Lechner et al., 1981).

A histological and biochemical study of the effect of vitamin C-deficiency on induction of amelogenesis in hamster molars in vitro

Second maxillary molars of hamsters were cultured in the presence or absence of 250 micrograms/ml vitamin C for periods up to 12 days. At various days, cultured explants were studied with histological and biochemical methods to investigate effects of vitamin C-deficiency on matrix production and mineralization in vitro. As biochemical parameters for protein synthesis and mineralization, uptake and incorporation of [3H]-proline, 45Ca and 32PO4 were used. To discriminate between synthesis of collagenous and non-collagenous proteins digestion of the [3H]-proline-labelled material by purified collagenase and its degree of hydroxylation were measured. Histologically, in control explants cultured with vitamin C, normal dentinogenesis, amelogenesis and mineralization in vitro were observed. In the vitamin C-deficient explants, odontoblasts produced an abnormal predentine matrix and de-differentiated. Eventually, this matrix mineralized aspecifically. In the presence of this abnormal matrix, ameloblasts failed to differentiate, which suggests that a cell-matrix type of interaction is involved in the differentiation of the pre-ameloblasts. Biochemically, in the vitamin C-deficient explants protein synthesis and collagen synthesis were reduced by about the same extent; the in-vitro produced collagens appeared under-hydroxylated and could, in degraded form, easily be extracted in formic acid. An increase of [3H]-proline solubility in formic acid in the control explants, however, paralleled enamel matrix production and was attributed to the solubilization of proline-rich enamel matrix proteins. The production of acid insoluble phosphate was not affected by vitamin C-deficiency. The uptakes of 45Ca and 32PO4 were retarded and the molar Ca:PO4 uptake ratio was lower, reflecting the histologically observed aspecific mineralization.

Facts and hypotheses concerning the control of odontoblast differentiation

Numerous studies using amphibians have demonstrated that preodontoblasts emerging from the dental papilla are derived from cranial neural crest cells [4, 12, 46, 64]. However this has not been established for mammals. The history of odonotogenesis begins during the early stages of cranial-facial development when the maxillary and mandibular processes processes develop. Continuous epithelio-mesenchymal interactions condition the histogenesis and morphogenesis of the teeth [24-26, 43, 44, 49, 51, 58] as well as the terminal differentiation of odontoblasts and ameloblasts [23, 47, 52, 54, 59, 61, 67]. During recent years a considerable amount of experimental data relating to differentiation of odontoblasts has been published. We summarize these data and attempt to integrate them in deductive hypothesis concerning the control of odontoblast differentiation.

Neonatal hamster molar tooth development: extraction and characterization of amelogenins, enamelins, and soluble dentin proteins

Amelogenins, enamelins, and soluble dentin proteins were sequentially separated under dissociative conditions from morphologically characterized molar tooth germs of 4-, 6-, and 7-day-old hamsters. Polyacrylamide gel electrophoretic, gel filtration chromatographic, and amino acid compositional data of neonatal hamster amelogenin extracts were in general agreement with those obtained from fetal bovine enamel under similar extraction conditions. As development progressed (e.g., 4 vs. 7 days of life), changes in amelogenin proteins were manifested by altered values of all biochemical parameters measured. A high molecular weight (approximately 160,000-200,000 daltons) Stains-All-positive protein band was observed for all hamster enamelin extracts on SDS gels. Amino acid compositional data from this "enamel crystal protein," partially purified by dissociative gel-filtration chromatography, are presented. The hamster dentin phosphoprotein was partially purified by ion-exchange chromatography in 7M urea. The molecular weight (75,000-80,000 daltons) and amino acid composition of this protein were similar to those of rat incisor dentin phosphoprotein, but different from those of the fetal bovine phosphoprotein.

Purification and properties of bovine dental-pulp alkaline-phosphatase

Alkaline phosphatase (E.C.3.1.3.1.) from unerupted bovine pulp was extracted from the microsomal fraction with eta-butanol and purified 77-fold, using DEAE-cellulose chromatography, Sephadex G-200 gel-filtration and concanavalin-A affinity chromatography, to a final specific activity of 92.3 units/mg protein. Affinity chromatography confirmed the glycoprotein nature of the enzyme. The pH optimum for the purified enzyme was 10.0 with rho-nitrophenylphosphate, and 8.7 with phosphoserine. The apparent Km was estimated to be 0.7 mM, using rho-nitrophenylphosphate in glycine-NaOH buffer, pH 10.0. The enzyme was markedly inhibited by EDTA, bromotetramisole and homoarginine but was insensitive to phenylalanine, and therefore resembled the alkaline phosphatase of liver and bone, but not that of intestine and placenta. No protein phosphatase activity towards dentine phosphoprotein and phosvitin was observed.

Synthesis of collagen type I, type I trimer and type III by embryonic mouse dental epithelial and mesenchymal cells in vitro

Epithelial and mesenchymal dental cells were grown in primary monolayer culture and the ability of both cell types to synthesize interstitial collagens was investigated. Pepsin-solubilized collagens were analyzed by CM-cellulose chromatography and both cell types were found to synthesize collagen type I, type III and type I trimer. The collagen phenotype of mesenchymal cells (type I: 82.4%, type III: 8.5%, type I trimer: 9.1%) was different from that of epithelial cells (type I: 71.8%, type III: 9.5%, type I trimer: 18.7%). The radioactivity incorporated into collagen molecules by mesenchymal cells was 34-times greater than the radioactivity incorporated by epithelial cells. This result agreed with previous observations obtained from tissue culture experiments (Lesot, H. and Ruch, J.V. (1979) Biol. Cell. 34, 23--37) which indicated a low synthesis of interstitial collagens by isolated dental epithelia when compared to isolated dental mesenchymes.

Phosphoprotein phosphatase activity of bovine intestinal alkaline phosphatase

The phosphoprotein phosphatase activity of a commercial preparation of bovine intestinal alkaline phosphatase (EC 3.1.3.1) was examined using phosvitin and dentine phosphoprotein as substrates. Over 90% and 70% of the phosphorus from dentine phosphoprotein and phosvitin were hydrolyzed in 2 h. The optimum pH of the enzyme for the dephosphorylation of phosvitin and dentine phosphoprotein was nearly 6. No protein phosphatase activity was observed when the alkaline phosphatases from bovine liver and pulp were investigated.

Purification, composition, and 31P NMR spectroscopic properties of a noncollagenous phosphoprotein isolated from chicken bone matrix

Fractionation of the EDTA-soluble, noncollagenous proteins of the organic matrix of chicken bone by Sephadex G-100 molecular sieving has revealed that the majority of the organic phosphorus is present in two fractions, from one of which a homogeneous phosphoprotein has been isolated. The purified phosphoprotein has an apparent molecular weight of 12,000 and contains both O-phosphoserine and O-phosphothreonine. 31P-NMR spectroscopy demonstrates that all of the organic phosphorus exists in the form of phosphomonoesters which have an average pK2 of 6.8. The phosphoprotein is highly acidic due to its high content of dicarboxylic acids in addition to the presence of organic phosphorus. The characteristic amino acid composition of the phosphoprotein establishes its noncollagenous nature and highlights the differences among bone, dentin, and enamel phosphoproteins. The absence of gamma-carboxyglutamic acid distinguishes it from osteocalcin, the noncollagenous gamma-carboxyglutamic acid-containing peptide of bone matrix.

Ca2+-binding studies of the phosphoprotein from rat-incisor dentine

Rat incisor dentine was demineralized and extracted with 0.25 M EDTA containing protease inhibitors. The extract was purified by chromatography on DEAE-cellulose and sulfonated polystyrene. The Ca2+-finding properties of the phosphoprotein were studied by dynamic dialysis and by using a Ca2+-selective electrode. Two different binding sites were detected with Kd = 0.9 X 10(-7) M and 1.1 X 10(-5) M and displaying a Ca2+-binding capacity of 127 and 176 mol bound Ca2+/mol protein, respectively, assuming a molecular weight of 30 000. Upon enzymatic dephosphorylation of the phosphoprotein, the highest affinity sites disappeared and those with the lowest affinity were reduced. The optimum for Ca2+ binding by the phosphoprotein occurred at pH 8.2. The specificity of the Ca2+ ion interaction with the phosphoprotein was investigated by studying the competitive nature of other divalent and monovalent cations. It was found that Ca2+ ions were to a large extent displaced from the phosphoprotein by other cations in physiological concentrations.

Cation binding by the rat-incisor-dentine phosphoprotein. A spectroscopic investigation

Nuclear magnetic resonance and electron paramagnetic resonance spectroscopy have been used to quantify metal binding to the phosphoprotein extractable from demineralized rat incisor dentine. Paramagnetic cation probes enable identification of the metal binding sites. Cations are able to diffuse across the protein surface while forming a relatively long-lived metal-phosphoprotein complex. The ability of the protein to sequester surface-mobile Ca(II) is discussed in terms of its ability to act as a possible nucleation site for the initial localization of Ca(II) within the dentine matrix.

The effects of phosphoproteins on collagen self-assembly in tail tendon and incision dentin from rats

Phosphoproteins retard the rate at which collagen molecules undergo self-assembly into fibrils. The inhibition appears to be dependent on the amount of phosphoprotein present, with increasing phosphoprotein concentrations resulting in greater inhibition. Prior treatment of the phosphoprotein with calcium markedly increases the resultant inhibitory effect. Dentin phosphoproteins are considerably more effective than phosvitin in retarding collagen self-assembly, with retardation times for these hard tissue extracellular matrix proteins being 25-30 times greater than control values.

Comparative studies of phosphoprotein preparations from rat incisor dentin

Phosphoprotein was obtained from rat incisor dentin either by extraction at elevated ionic strength after acetic acid demineralization, or by extraction simultaneous with demineralization in neutral EDTA solution. Purification of solubilized proteins was achieved by Sepharose 4B and DEAE-cellulose chromatography. Polyacrylamide gel electrophoresis of the material from the two preparations resulted in one single band. Except for the amino acid analyses, no evidence for a difference between the two phosphoprotein preparations could be found. After additional purification by iso-electric focusing the amino acid analyses demonstrated a similar composition. It is concluded that the two methods for phosphoprotein extraction yield the same product when purified properly. The study did not give any unequivocal answer as to if any phosphoprotein component exists in rat incisor dentin which is covalently linked to the collagen matrix.

Phosphoprotein modulation of apatite crystallization

Several phosphoprotein preparations (phosvitin, rat incisor and fetal calf molar dentin phosphoproteins) all inhibit apatite growth/replication from pre-existing crystal seeds in metastable solutions. Two stages of the crystal growth process were inhibited by these phosphoproteins. First an initial lag period was induced, probably associated with seed surface phenomena. This period was prolonged indefinitely when a combination of phosphoprotein precoated seeds was used together with soluble phosphoproteins in the crystal growth reaction. Second, the phosphoproteins prolonged that stage of the reaction where octacalcium phosphate is the predominant mineral phase present prior to its conversion to the final apatite product. Pre-treatment of the phosphoproteins with calcium diminished their inhibitory activity to seeded crystal growth as well as towards de novo apatite formation in synthetic extracellular fluids. The presence of collagen diminished the inhibitory activity of the phosphoproteins towards de novo precipitation but had no effect on phosphoprotein-modulated apatite crystal growth in the seeded systems. These results suggest a potential regulatory role for phosphoproteins in dentin mineralization.

Identification of O-phosphoserine, O-phosphothreonine and gamma-carboxyglutamic acid in the non-collagenous proteins of bovine cementum; comparison with dentin, enamel and bone

O-phosphoserine [Ser(P)], O-phosphothreonine [Thr(P)], and gamma-carboxyglutamic acid (Gla) have been identified in native, calcified cementum and in non-collagenous proteins which can be extracted from the tissue in EDTA at neutral pH. The concentrations of Ser(P) and Thr(P) and the amino acid composition of the EDTA extractable proteins are more similar to those found in bone than in dentin or enamel. The concentration of Gla in cementum is lower than it is in bone and higher than it is in enamel, which contains essentially no Gla. Based on the contents of Gla in these mineralized tissues and the distribution of alkaline and acid phosphatases in these tissues, it is speculated that Gla may be part of these or other proenzymes rather than being involved directly and structurally with the deposition of the mineral phase.

The nature of covalent complexes of phosphoproteins with collagen in the bovine dentin matrix

The bovine dentin matrix still contains some noncollagenous proteins after thorough extraction and decalcification. These have been obtained following digestion of the matrix by cyanogen bromide. Peptides containing non-collagenous portions were isolated by chromatography on diethylaminoethyl cellulose columns and fractionated on hydroxyapatite columns. Several fractions were obtained. The principal component was a complex between a highly-phosphorylated serine-aspartic acid-rich protein and a collagen peptide. These collagenous and non-collagenous moieties could not be separated from each other even under highly dissociative solvent conditions. After digestion with collagenase, the resulting phosphoprotein fraction still contained a few residues of hydroxyproline and hydroxylysine, and an enhanced content of proline, compared to the equivalent directly extractable phosphophoryn of the matrix. These data were interpreted as indicating that the phosphophoryn which is not extractable in 0.5M ethylenediaminetetraacetic acid is in fact covalently bound to some specific section of the matrix collagen. The covalent modification of the collagen matrix with highly acidic phosphoproteins may have an important role in the mineralization process.

The association of amorphous mineral deposits with the plasma membrane of pre- and young odontoblasts and their relationship to the origin of dentinal matrix vesicles in rat incisor teeth

Young and preodontoblasts and matrix vesicles which occur in the presecretory region of incisor teeth of growing rats were examined in stained and unstained ultrathin sections in order to characterize sites involved in the initial mineralization of dentin. Common to pre- and young odontoblasts in the presecretory region were hemispherical membrane-associated amorphous densities, measuring 5-35 nm in diameter after fixation in glutaraldehyde-osmium tetroxide or glutaraldehyde only. Amorphous densities were associated also with the limiting membranes of some vesicles in the extracellular matrix. Other vesicles in the extracellular matrix contained needle-like crystalline deposits typical of dentinal matrix vesicles. Fully differentiated odontoblasts in more incisal regions of the tooth lacked plasma membrane-associated amorphous densities. Neither amorphous nor crystalline densities were associated with any other cellular or subcellular structures in cells of the presecretory region. Flotation of ultrathin sections on solutions of EDTA or EGTA removed the amorphous densities from the plasma membranes, suggesting that the amorphous densities are calcium-containing mineral deposits. Amorphous deposits were associated with the membrane of vesicular structures protruding from the surfaces of pre- and young odontoblasts, suggesting that vesicles found in the extracellular matrix arise by budding from the plasma membranes of pre- and young odontoblasts. The occurrence of amorphous mineral deposits in association with the limiting membrane of some vesicles in the extracellular matrix, and the occurrence of needle-like mineral crystals within other matrix vesicles, suggest that an amorphous-to-crystalline phase transformation of mineral takes place within the matrix vesicle. The results of this study suggest that calcium-binding sites associated with plasma membranes of pre- and young odontoblasts act as nucleating centers for primary mineral deposition in tooth dentin.