Phosphopeptides of enamel matrix

Role of carboxylic organic molecules in interfibrillar collagen mineralization

Bone is a composite material made up of inorganic and organic counterparts. Most of the inorganic counterpart accounts for calcium phosphate (CaP) whereas the major organic part is composed of collagen. The interfibrillar mineralization of collagen is an important step in the biomineralization of bone and tooth. Studies have shown that synthetic CaP undergoes auto-transformation to apatite nanocrystals before entering the gap zone of collagen. Also, the synthetic amorphous calcium phosphate/collagen combination alone is not capable of initiating apatite nucleation rapidly. Therefore, it was understood that there is the presence of a nucleation catalyst obstructing the auto-transformation of CaP before entering the collagen gap zone and initiating rapid nucleation after entering the collagen gap zone. Therefore, studies were focused on finding the nucleation catalyst responsible for the regulation of interfibrillar collagen mineralization. Organic macromolecules and low-molecular-weight carboxylic compounds are predominantly present in the bone and tooth. These organic compounds can interact with both apatite and collagen. Adsorption of the organic compounds on the apatite nanocrystal governs the nucleation, crystal growth, lattice orientation, particle size, and distribution. Additionally, they prevent the auto-transformation of CaP into apatite before entering the interfibrillar compartment of the collagen fibril. Therefore, many carboxylic organic compounds have been utilized in developing CaP. In this review, we have covered different carboxylate organic compounds governing collagen interfibrillar mineralization.

Alan Fincham and the era of enamel protein Biochemistry

Enamel research experienced an unprecedented period of growth during the latter part of the 20th century until today. This growth is in part due to the contributions of a number of iconic scientists such as Alan G. Fincham, the focus of the present review. Alan was involved in many of the seminal discoveries of this time, including the identification of the critical amelogenin peptides TRAP and LRAP, the determination of the amelogenin amino acid sequence, the identification of the sole serin-16 phosphorylation site, and the amelogenin nanosphere theory. Alan was also a superb mentor to graduate students and others. His experience and leadership related to problem-based learning greatly affected predoctoral dental education at the University of Southern California and in the United States.

Dentin: structure, composition and mineralization

We review firstly the specificities of the different types of dentin present in mammalian teeth. The outer layers include the mantle dentin, the Tomes' granular and the hyaline Hopewell-Smith's layers. Circumpulpal dentin forming the bulk of the tooth, comprises intertubular and peritubular dentin. In addition to physiological primary and secondary dentin formation, reactionary dentin is produced in response to pathological events. Secondly, we evaluate the role of odontoblasts in dentin formation, their implication in the synthesis and secretion of type I collagen fibrils and non-collagenous molecules. Thirdly, we study the composition and functions of dentin extracellular matrix (ECM) molecules implicated in dentinogenesis. As structural proteins they are mineralization promoters or inhibitors. They are also signaling molecules. Three different forms of dentinogenesis are identified: i) matrix vesicles are implicated in early dentin formation, ii) collagen and some proteoglycans are involved in the formation of predentin, further transformed into intertubular dentin, iii) the distal secretion of some non-collagenous ECM molecules and some serum proteins contribute to the formation of peritubular dentin.

Amelogenin Supra-Molecular Assembly in vitro Compared with the Architecture of the Forming Enamel Matrix

Tooth enamel is formed in the extracellular space within an organic matrix enriched in amelogenin proteins. Amelogenin nanosphere assembly is a key factor in controlling the oriented and organized growth of enamel apatite crystals. Recently, we have reported the formation of higher ordered structures resulting from organized association and self-orientation of amelogenin nanospheres in vitro. This remarkable hierarchical organization includes self-assembly of amelogenin molecules into subunits of 4-6 nm in diameter followed by their assembly to form nanospheres of 15-25 nm in radii. Chains of >100 nm length are then formed as the result of nanosphere association. These linear arrays of nanospheres assemble to form the microribbons that are hundreds of microns in length, tens of microns in width, and a few microns in thickness. Here, we review the step by step process of amelogenin self-assembly during the formation of microribbon structures in vitro. Assembly properties of selected amelogenins lacking the hydrophilic C terminus will then be reviewed. We will consider amelogenin as a template for the organized growth of crystals in vitro. Finally, we will compare the structures formed in vitro with globular and periodic structures observed earlier, in vivo, by different sample preparation conditions. We propose that the alignment of amelogenin nanospheres into long chains is evident in vivo, and is an important indication for the function of this protein in controlling the oriented and elongated growth of apatite crystals during enamel biomineralization.

Proteoglycans in dentinogenesis

The predominant proteoglycans present in predentin and dentin are the chondroitin-sulphate-rich decorin and biglycan and the keratan-sulphate-rich lumican and fibromodulin. These are small, interstitial, leucine-rich proteoglycans which have recently been shown to exist in gradients across the predentin. Antibodies recognizing chondroitin sulphate show a decreasing gradient from the pulpal aspect toward the mineralizing front, the converse being true for keratan sulphate. Antidecorin shows an increase toward the mineralization front. Evidence from biochemical, autoradiographic, and immunohistochemical studies implies that such changes may be brought about by gradients of metalloproteinases. This offers the possibility that the proteoglycans organize the collagen network for receipt of phosphoproteins and phospholipids, the former being evident only at the onset of dentin formation. The suggestion is raised that glycosaminoglycan-depleted leucine-rich protein cores act as sequester points for receipt of phosphoproteins in particular. The rigid, spatially oriented glycosaminoglycan chains on decorin and biglycan are known to bind calcium and may feature directly in mineral initiation.

Enzyme compartmentalization during biphasic enamel matrix processing

Processing of enamel matrix proteins is essentially biphasic. Secretory stage metalloprotease activity generates a discrete, presumably functional, spectrum of molecules which may also undergo dephosphorylation. Maturation stage serine proteases almost completely destroy the matrix. The present aim was to examine the tissue compartmentalization of these enzyme activities in relation to their possible function. A sequential extraction using synthetic enamel fluid, phosphate buffer and SDS was used to identify enzymes free in the enamel fluid, crystal bound or aggregated with the bulk matrix respectively. Results indicated that the metallo-proteases and alkaline phosphatase were free in the secretory stage enamel fluid while the serine proteases appeared to be largely bound to the maturation stage crystals. The mobility of the metallo-proteases and alkaline phosphatase would ensure efficient initial processing of secretory matrix, while the largely mineral bound serine proteases would ensure retention of protease activity despite massive destruction and protein removal.

Inositol hexasulphate, a casein kinase inhibitor, alters enamel formation in cultured embryonic mouse tooth germs

Post-translational modification of enamel proteins is regulated by casein kinases (CK) and results in binding sites for calcium ions that subsequently play a key role during the initial stages of mineralization. Phosphorylation may also influence the secretion and extracellular organization of enamel proteins. Previous studies indicated that inositol hexasulphate inhibited the activity of CK-I and/or CK-II in mouse tooth germs (Torres-Quintana et al., 1998). We hypothesized that inositol hexasulphate would also inhibit the activity of the specific casein kinase(s) identified in secretory ameloblasts, and would prove useful for determination of the extent to which phosphorylation might influence the organization of enamel proteins at early stages of enamel formation. To test this hypothesis, we dissected mandibular first molars from 18-day-old mouse embryos and cultured them for 11 days in the presence of 0-0.1 mM inositol hexasulphate. Ultastructural analysis revealed that the formation of enamel was largely impaired at an inhibitor concentration > or = 0.08 mM. Quantitative radioautographic analysis of [33P]phosphate incorporation indicated that radiolabeled phosphate normally secreted into forming enamel was retained within ameloblasts. In contrast, no significant difference was observed between control and inositol-hexasulphate-treated tooth germs when cultures were labeled with [3H]serine and [3H]proline. SDS-PAGE and Western blot analysis confirmed that while inositol hexasulphate inhibited CK-mediated phosphorylation, it did not significantly alter protein synthesis. We conclude that impairment of phosphorylation leads to intracellular accumulation of [3H]phosphate-containing material by ameloblasts. We also conclude that when non-phosphorylated enamel matrix proteins are secreted, they are either unable to form an enamel matrix that supports mineralization, or they diffuse throughout a poorly mineralized dentin.

Sequential expression of matrix protein genes in developing rat teeth

Tooth organogenesis is dependent on reciprocal and sequential epithelial-mesenchymal interactions and is marked by the appearance of phenotypic matrix macromolecules in both dentin and enamel. The organic matrix of enamel is composed of amelogenins, ameloblastin/amelin, enamelins and tuftelin. Dentin is mainly composed of type I collagen, but its specificity arises from the nature of the non-collagenous proteins (NCPs) involved in mineralization, phosphophoryn (DPP), dentin sialoprotein (DSP), osteocalcin, bone sialoprotein and dentin matrix protein-1 (Dmp1). In this paper, we studied the pattern of expression of four mineralizing protein genes (type I collagen, amelogenin, DSPP and osteocalcin) during the development of rat teeth by in situ hybridization on serial sections. For this purpose, we used an easy and rapid procedure to prepare highly-specific labeled single-stranded DNA probes using asymmetric polymerase chain reaction (PCR). Our results show that type I collagen is primarily expressed in polarizing odontoblasts, followed by the osteocalcin gene expression in the same polarized cells. Concomitantly, polarized ameloblasts start to accumulate amelogenin mRNAs and transiently express the DSPP gene. This latter expression switches over to odontoblasts whereas mineralization occurs. At the same time, osteocalcin gene expression decreases in secretory odontoblasts. Osteocalcin may thus act as an inhibitor of mineralization whereas DSP/DPP would be involved in more advanced steps of mineralization. Amelogenin and type I collagen gene expression increases during dentin mineralization. Their expression is spatially and temporally controlled, in relation with the biological role of their cognate proteins in epithelial-mesenchymal interactions and mineralization.

Immunohistochemical similarities and differences between amelogenin and tuftelin gene products during tooth development

Amelogenins and tuftelins are highly specialized proteins secreted into the developing enamel matrix during mammalian enamel formation. Both tuftelins and amelogenins have been associated with various functions during nucleation and maturation of the developing enamel matrix. In this study we conducted experiments to investigate whether tuftelins and portions of the amelogenin molecule were deposited and processed in spatially distinguished portions of the developing enamel matrix, using antibodies specific against tuftelin or amelogenins. The amelogenin antibodies were raised against recombinant and native amelogenins and also included an antibody against a polypeptide encoded by amelogenin exon 4. To compare spatial expression patterns of enamel protein epitopes, 3-day postnatal mouse molar tooth organs were processed for paraffin histology and cut into serial sections. Adjacent sections were exposed to antibodies against either tuftelin or various amelogenin epitopes. To investigate age-related changes of enamel protein expression, amelogenin and tuftelin antibodies were applied to tooth organs of developmental stages E19 and 1, 3, 5, 7, 9 and 11 postnatal days. Tuftelin was detected within the odontoblast processes during earlier stages of development (E19 and 1 day postnatal), whereas during later stages (3-11 days) it was recognized in a portion of the enamel layer adjacent to the dentine-enamel junction. In contrast, all four antibodies against amelogenins reacted with parts of the ameloblast cytoplasm and the entire enamel layer. Using immunohistochemistry, we were not able to detect any differences in the spatial distribution of the four amelogenin epitopes investigated. The spatial differences in the distribution of amelogenin and tuftelin as observed in this study may be interpreted as an indication of functional differences between both proteins during early enamel biomineralization.

Amelogenin proteins of developing dental enamel

The amelogenins of developing dental enamel are tissue-specific proteins, rich in proline, leucine, histidine and glutamyl residues, and synthesized by the ameloblast cells of the inner enamel epithelium. These proteins comprise the bulk of the extracellular matrix that becomes mineralized with a hydroxyapatite phase to become the mature enamel. Examination of the amino acid sequences of amelogenins from a range of mammals shows a high degree of evolutionary sequence conservation, suggestive of specialized function. Recently it has been shown that multiple amelogenin components, observed in the matrix, arise both by a sequence of post-secretory proteolytic processing and by the expression of alternatively spliced mRNAs generated from the amelogenin gene(s) that are located on the sex chromosomes. Although the function of these amelogenins in enamel biomineralization is unknown, physico-chemical studies of recombinant amelogenins have shown that they undergo a self-assembly process in vitro generating supra-molecular 'nanosphere' structures, and recent observations in vivo point to a functional role for the nanospheres in the ultrastructural organization of the secretory enamel matrix, conducive to the organized development of the earliest mineral crystallites.

Enamel matrix proteins and ameloblast biology

The paper reviews the changes in ameloblast ultrastructure, concomitant with the changes in its functions across the major stages of amelogenesis. It describes the mechanisms associated with the major events in biosynthesis and degradation of the major enamel proteins (amelogenins and tuftelin/enamelins) and with the presecretory and postsecretory mechanisms leading to the heterogeneity of these extracellular matrix proteins. The gene structure, chromosomal localization, protein, primary structure and possible function, and the involvement of the different proteins in X-linked (amelogenin) and possibly in autosomally linked (tuftelin) amelogenesis imperfecta, the most common hereditary disease of enamel, are also discussed.

Initial enamel crystals are not spatially associated with mineralized dentine

During epithelial-mesenchymal interactions associated with mammalian tooth development, epithelially-derived and mesenchymally-derived extracellular matrix molecules form a discrete dentine-enamel junction. The developmental and molecular processes required to form this junction are not known. To address this problem we designed studies to test the hypothesis that ectodermally-derived epithelial cells synthesize and secrete enamel proteins which function to nucleate and regulate the growth of enamel calcium phosphate crystals. Initial enamel crystals were detected separate from the adjacent dentine. Electron-microprobe analyses revealed that early enamel crystals were octacalciumphosphate or tricalciumphosphate rather than hydroxyapatite. Thereafter, enamel crystals became confluent with the adjacent, albeit significantly smaller hydroxyapatite crystals associated with mineralized dentine. Therefore, we interpret our data to indicate that de novo enamel crystal nucleation and growth are independent from the mineralization processes characterized for dentine. We further argue that gene expression of enamel protein appears to have a constitutive function during early enamel formation and that supramolecular aggregates of amelogenin and enamelin provide the microenvironment for the nucleation and crystal growth of the initial enamel matrix.

Recent advances in amelogenin biochemistry

This paper reviews advances in amelogenin biochemistry in three areas; (i) amelogenin expression; (ii) amelogenin post-translational and post-secretory processing, and (iii) amelogenin structure and function. Recent studies of amelogenin expression have demonstrated that alternative-splicing of mouse amelogenin RNA generates seven distinct mRNAs, coding for amelogenin proteins from 194 to 44 amino acid residues in length. A polyclonal antibody to a sequence of the 194-residue murine amelogenin identified this protein in vivo. While several studies have reported that amelogenins are post-translationally phosphorylated, it has proved difficult to confirm this view. Mass spectrometry studies of bovine and porcine TRAP and LRAP amelogenins have established a phosphoserine residue at position-16 as originally reported by Takagi et al. for a 180-residue bovine amelogenin. Also, we discovered that the detailed mechanism(s) of carboxy-terminal amelogenin proteolytic processing appear different than previously reported. In terms of amelogenin structure, it is well known that amelogenins form aggregated structures. Studies employing a recombinant amelogenin and dynamic light-scattering instrumentation demonstrated aggregate structures of 15-20 nm in radius, corresponding to a mass of 2-3 million daltons. Imaging these aggregates by transmission electron and atomic force microscopy suggested that these structures are equivalent to the "stippled" or "granular" material seen in electron photomicrographs of developing enamel. Collectively, these advances in amelogenin biochemistry lead to a new view of amelogenin structure, processing and functions in enamel biomineralization.

Mass-spectrographic analysis of a porcine amelogenin identifies a single phosphorylated locus

The amelogenins of the extracellular matrix of developing dental enamel, comprise a family of tissue-specific proteins which are postulated to play a central role in the biomineralization of dental enamel [1]. The primary structures of amelogenins derived from cow, pig, human, mouse and rat have now been elucidated by the interpretation of cDNA sequences or by direct amino acid sequence determinations [2-6] demonstrating a high degree of sequence homology between species [1]. However, the nature of post-translational modification of these proteins is less clear. In particular, early reports of amelogenin phosphorylation [7-8] have proved to be difficult to confirm by direct chemical analyses [1]. Using mass spectrographic analysis, we recently [9], reported that the lower molecular weight (5-7 kDa) bovine and porcine amelogenin polypeptides (TRAP and LRAP) contained a single phospho-serine residue at position 16Ser and, since these polypeptides are derived by proteolytic processing from the higher molecular weight "parent" amelogenins (18-25 kDa), we concluded that these precursor molecules must also be phosphorylated, as has previously been suggested [10]. In contrast to these observations, an extensive amino acid sequencing study of porcine amelogenins has recently reported no evidence for such phosphorylation [1]. We now report that a new analysis of the major porcine ("20K") amelogenin provides positive evidence for porcine amelogenin phosphorylation.

Alternative splicing of the mouse amelogenin primary RNA transcript

A heterogeneous mixture of amelogenins can be extracted from developing tooth enamel matrix. In an attempt to discover the extent to which alternative splicing of the amelogenin primary RNA transcript can generate unique isoforms, we have conducted a thorough search for cDNAs amplified by reverse transcription-polymerase chain reaction (RT-PCR). Over 2400 colonies were screened by colony hybridization. Seven different alternatively spliced amelogenin mRNAs were isolated. The predicted translation products of the messages are 194, 180, 156, 141, 74, 59, and 44 amino acids in length. RT-PCR amplification products not predicted by these seven amelogenin cDNAs were characterized. The intron separating exons 5 and 6 was cloned and sequenced. Using rapid amplification of cDNA ends (RACE) techniques, the 5' ends of the amelogenin mRNAs were cloned and characterized. The finding that the same exon 1 is common to all of the cloned mRNAs indicates that mouse amelogenin is transcribed from a single promoter. The mouse amelogenin transcription and translation initiation sites, the 5' untranslated leader, and the segment encoding the signal peptide were determined. The distinctly nonamelogenin-like exon 4, first observed in human amelogenin cDNAs, has also been found in mice. Antibodies were raised to synthetic exon 4-encoded polypeptides and used to immunostain Western transfers and histologic tooth sections.

Detection of monodisperse aggregates of a recombinant amelogenin by dynamic light scattering

Recombinant murine amelogenins M179 and M166 were expressed in Escherichia coli and purified. The aggregation properties of these amelogenins have been investigated in aqueous solutions as well as acetonitrile-containing solutions using dynamic light scattering. Dynamic light scattering provides direct measurement of the translational diffusion coefficient and hydrodynamic radius, and of an estimate of the molecular weight. Polydispersity and statistical parameters of how to interpret the analysis are also provided. Amelogenin aggregation was examined in solutions of a range of pH, ionic strengths, and protein concentrations. It was shown that at pH 7.8-8 and ionic strength of 0.02-0.05M the M179 molecules form monodispersed aggregates with hydrodynamic radii ranging from 15 to 19 nm. Analysis of hydrodynamic radii and size distribution of M179 aggregates in acetonitrile-containing solvents compared to that in aqueous solutions indicated a primary role for hydrophobic interactions in the association process of amelogenin molecules to form aggregates. Comparison between the aggregates formed by M179 and M166, which lacks the hydrophilic carboxy-terminal 13 residue sequence of M179, suggested that the self-assembly of amelogenin molecules to form stable and monodisperse aggregates requires the presence of the hydrophilic carboxy-terminal sequence of M179.

Seeded growth of hydroxyapatite in the presence of dissolved albumin

Hydroxyapatite (HAP) crystals were grown from a supersaturated solution by the addition of a suspension of seed crystals at a controlled pH value of 7.4 and a temperature of 37 degrees C. The degree of supersaturation was comparable to that in biological fluids and was such that all HAP precipitated would be expected to deposit on the seeds. Albumin was added to some of the solutions to give a concentration in the range 75-250 micrograms cm-3. Samples of solution were removed at known times after the addition of seed crystals and their calcium ion concentrations were determined by atomic absorption spectroscopy. The decrease in the dissolved calcium concentration was taken to be a measure of crystal growth. In the absence of seeds, no decrease in calcium concentration occurred. The initial rate of HAP growth decreased linearly with albumin concentration, i.e., albumin was found to inhibit crystal growth. Inhibition kinetics were consistent with a Langmuir model in which a single albumin molecule was capable of binding to more than one growth site on the crystal surface. Comparison with published results indicated that albumin was a less potent inhibitor of HAP growth than phosphoproteins but was a more potent inhibitor than magnesium or citrate ions.

Antisense inhibition of AMEL translation demonstrates supramolecular controls for enamel HAP crystal growth during embryonic mouse molar development

During tooth development, enamel organ epithelial cells express a tissue-specific gene product (amelogenin) which presumably functions to control calcium hydroxyapatite crystal growth patterns during enamel biomineralization. The present studies were designed to test the hypothesis that amelogenin as a supramolecular aggregate regulates crystal growth during enamel biomineralization. Antisense oligodeoxynucleotide strategy was used in a simple organ culture system to inhibit amelogenin translation. Under these experimental conditions, antisense treatment prior to and during amelogenin expression resulted in inhibition of amelogenin translation products within immunoprecipitated [35S]methionine metabolically labeled proteins. To determine the efficiency of antisense treatment in this model system, digoxigenin-labeled oligodeoxynucleotides were observed to diffuse throughout the tooth explants including the target ameloblast cells within 24 hours. Ultrastructural analyses of amelogenin supramolecular assembly as electron-dense stippled materials in antisense treated cultures demonstrated dysmorphology of the extracellular enamel matrix with a significant reduction in crystal length and width. We conclude that secreted extracellular proteins form a supramolecular aggregate, which controls both the orientation and dimensions of enamel crystal formation during tooth development.

Efficient solution-phase synthesis of multiple O-phosphoseryl-containing peptides related to casein and statherin

The multiple Ser(P)-containing peptides, H-Ser(P)-Ser(P)-Ser(P)-Glu-Glu-NHMe.TFA, H-Asp-Ser(P)-Ser(P)-Glu-Glu-NHMe.TFA and H-Glu-Ser(P)-Ser(P)-Glu-Glu-NHMe.TFA were prepared by the use of Boc-Ser(PO3Ph2)-OH in the Boc mode of solution phase peptide synthesis followed by platinum-mediated hydrogenolytic de-protection of the Ser(PO3Ph2)-containing peptides. The protected peptides were assembled using the mixed anhydride coupling methods with 40% TFA/CH2Cl2 used for removal of the Boc group from intermediate Boc-protected peptides.

Immunohistochemical localization of a approximately 66 kD glycosylated phosphoprotein during development of the embryonic chick tibia

Localization of a approximately 66 kD glycosylated phosphoprotein during morphogenesis of the embryonic chick tibia has been accomplished using immunohistochemistry. Although initial expression of the tibial osteoblast phenotype is detected as early as stage 28.5, with the deposition of osteoid matrix beginning at stage 30, little or no immunoreactivity against the approximately 66 kD glycosylated phosphoprotein is observed in pre-osteoblasts, osteoblasts, osteocytes, or in the uncalcified osteoid matrix during the early events of tibia development. Immunoreactivity was first observed at stage 32 when mineralization of the osteoid matrix is initiated. At this and all later stages, the phosphoprotein is located almost exclusively in the extracellular matrix at the mineralization front with essentially no detectable staining in the adjacent unmineralized osteoid matrix. Similarly, no cellular staining is observed when even the lightly mineralized extracellular matrix is strongly immunoreactive. Only scant immunostaining is present over the heavily mineralized regions, although demineralization of these areas with EDTA exposes a low intensity, punctate staining pattern. Additionally, cryosections of developing calvaria stained with this antiserum only display reactivity in regions of bone matrix undergoing mineralization. These localization studies support the hypothesis that this phosphoprotein is intimately associated with the process of bone matrix mineralization in the developing chick long bone.

Amelogenin post-secretory processing during biomineralization in the postnatal mouse molar tooth

The primary structures, molecular genetics and biosynthesis of the amelogenin protein of the developing tooth are established, but knowledge of their subsequent post-secretory processing and its relation to enamel biomineralization is fragmentary. Preparations of tooth matrix proteins were isolated from molars (M1) of mice from birth to 15 days and analysed by SDS-PAGE and immunochemical methods. Amelogenin proteins, isolated and partially purified by HPLC, were characterized by amino acid analysis and SDS-PAGE. At birth a 26 kDa amelogenin was present that during subsequent developmental stages generated a series of 20-25 kDa amelogenins differing in apparent size by approximately 1 kDa. Amino acid analyses showed that all these amelogenins have amino-terminal TRAP sequences; analyses for both glycosylation and phosphorylation were negative. It is suggested that these post-secretory amelogenins are generated by a sequence of specific carboxy-terminal cleavages, and that the observed post-secretory processing of amelogenin is functionally linked to the structure of the enamel matrix and the control of crystallite development.

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.

The dentino-enamel junction: a structural and microanalytical study of early mineralization

The spatial localization of enamel and dentin apatite crystals of the rat tooth has been studied by electron microscopic methods--bright field, selected-area dark field, and electron spectroscopic imaging. The sequential events of dentin calcification followed by the formation and growth of enamel crystals were determined and compared to previous studies. In dentin, initial sites of mineral deposition occur in areas subjacent to the dentino-enamel junction (DEJ). The subsequent expansion of these deposits progresses towards the DEJ to the terminal ends of dentin collagen fibrils. Concomitantly, an electron-dense enamel matrix is released by ameloblasts; with the presence of this matrix, the growth of enamel crystals occurs from the underlying calcified dentin. Enamel crystal growth continues to within close proximity of the plasma membrane of ameloblasts. A close spatial relationship between enamel and the crystals of calcified dentin collagen fibrils was observed by selected-area dark field imaging. Such areas of crystal intimacy show a co-localization of calcium and phosphorus extending from calcified collagen fibrils to enamel sheaths which encase enamel crystals. A working model of the spatial relationship between crystals of dentin and enamel is presented and discussed in light of mechanisms by which calcified dentin may promote the formation of enamel crystals.

X-ray microanalytical histochemistry of human circumpulpar and mantle dentine

An X-ray microanalytical histochemistry study was carried out, on thick sections observed under scanning electron microscopy, of five freshly extracted human premolars and five molars. In particular the circumpulpar and mantle dentine were examined to determine levels of calcium, phosphorus and sulphur, the basic elements involved in the process of biomineralization. Semiquantitative analysis was subsequently performed after the appropriate ZAF (Z, atomic number; A, absorption; F, secondary fluorescence) correction. Ca/P mass ratio was found to be similar for both regions in molars and slightly higher in circumpulpar dentine in premolars implying that this parameter is independent of the processes of biomineralization in the two dentinal regions. In contrast, statistically significant differences were recorded in sulphur content upon comparing mantle and circumpulpar dentine between premolars and molars (P less than 0.02). If sulphur, which has been associated with sulphated glycosaminoglycans (GAGs) decreases as mineralization progresses it would therefore seem reasonable to posit sulphur-rich areas in both topographical regions of the matrix--more significant in premolars than molars--able to undergo subsequent mineralization if required. The possible application of these findings in remineralization therapies is suggested.

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.

Structure and function of enamel gene products

The present paper reviews the main features of amelogenin and enamelin biochemistry, molecular biology, structural and ultrastructural localization, and immunology. It also examines recent studies concerning the origin, chemical characterization, suggested role, and participation of these two major classes of extracellular developing enamel matrix proteins in the complex process of "matrix-mediated" mineralization.

High resolution electron microscopy of nonstoichiometric apatite crystals

The application of high resolution electron microscopy, computer image processing, and image simulation techniques to the investigation of synthetic nonstoichiometric apatites has provided new details of apatite crystal growth mechanisms. Under certain precipitation conditions, calcium-deficient apatites with distinct octacalcium phosphate (OCP)-apatite intergrowths have been observed. Apatite crystals with unit-cell thick overgrowths of OCP on their surfaces confirmed the stepwise hydrolysis crystal growth mechanism initially proposed by Brown (Nature 196:1048-1050). However, many crystals also contained a central two-dimensional OCP inclusion one to two unit cells thick, embedded in an apatite matrix. Similar planar defects have been observed in dental enamel, dentin, and bone crystals. We have developed a modified version of Brown's stepwise OCP hydrolysis apatite crystal growth mechanism to explain the formation of crystals with OCP central planar defects. The mechanism involves the nucleation of an OCP seed that grows until it reaches a critical size, rh, before OCP hydrolysis occurs. Apatite subsequently grows epitaxially on the OCP seed, thereby embedding it in the center of an apatite crystal. Apatite growth is facilitated by partial screw dislocations emanating from the planar defect.

Tooth enamel protein, amelogenin, has a probable beta-spiral internal channel, Gln112-Leu138, within a single polypeptide chain: preliminary molecular mechanics and dynamics studies

Molecular dynamics simulation, with backbone constraints for 20 ps of equilibration and simulation, of a repeating polypeptide segment, Gln-Pro-His-Gln-Pro-Leu-Gln-Pro-His-Gln-Pro-Leu-Gln-Pro-Met-(Gln-Pro-Leu )4, constituting residues 112-138 of bovine amelolgenin, a 19.35 kD hydrophobic protein, are discussed. It is generally believed that the above polypeptide segment is important for the interaction of amelogenin with Ca++ ions, which occurs in the early phases of enamel mineralization. An energetically stable structure of the above polypeptide with recurrent beta-turns is observed and contains a pore of approximately 1 A radius along the helical that can accommodate an unhydrated Ca++ ion. The length of the polypeptide possesses correct dimensions to span a bilayer. The proposed structure is unique among known polypeptide and protein structures.

Production of monoclonal antibodies against enamelin and against amelogenin proteins of developing enamel matrix

The extracellular matrix of developing enamel contains two major classes of proteins, the hydrophobic proline-rich amelogenins and the acidic serine-, glycine-, and aspartic-rich enamelins. These proteins have been postulated as playing a major role in the mineralization and structural organization of developing enamel. To identify and further characterize these different proteins and their possible role in this complex process of biological mineralization, we have in recent years been concerned with the production of specific probes for these proteins. Previously, we have reported on the successful production of specific polyclonal antibodies against enamelin proteins, which did not cross-react with amelogenins, and against amelogenin proteins, which did not cross-react with enamelins (Deutsch et al., 1986, 1987). We now report the production of monoclonal antibodies against a major bovine amelogenin protein (28 kDa) and against a major bovine enamelin protein (66 kDa). One monoclonal antibody against amelogenin and one against enamelin are described. The results showed that the monoclonal antibody against the amelogenin protein reacted strongly with the 28-kDa amelogenin protein band but did not cross-react with enamelins, and the one against the enamelin protein reacted with the 66-kDa enamelin protein but did not cross-react with amelogenins. These monoclonal antibodies provide a specific and powerful tool to distinguish between and further characterize these different classes of proteins, and to improve our understanding of the process of enamel formation.

A characterization of amelogenin messenger RNA in the bovine tooth germ

In order to study the nature of amelogenin mRNA, we isolated ameloblast-rich tissue from the unerupted permanent incisor tooth germs of 18-month-old steers and subjected it to guanidine HC1 solubilization for extraction of mRNA. When poly A+ ameloblast RNA was incubated with radioactive deoxynucleotides and reverse transcriptase, four major transcripts were detected with sizes of 1.9, 1.4, 0.7, and 0.4 kb in length. One of the transcripts (0.7 kb) corresponded precisely in length to that predicted from the size of the major in vitro translated amelogenin proteins (27,000 daltons). To determine whether the transcripts did indeed encode amelogenin mRNA, we constructed a λgt11 cDNA library and isolated several amelogenin cDNA's by screening with amelogenin antibody. Four clones were amplified and insert sizes determined by acrylamide gel electrophoresis. Two of the clones had insert sizes of ~ 0.7 kb (λAm 16, XAm 7), and two had insert sizes of ~ 0.4 kb (λAm 11, λAm 4). When the amelogenin cDNA was radiolabeled and used for northern analysis, two species of amelogenin message (0.75 and 0.45 kb) were evident, both of which showed extensive hybridization to λAm 16 (large) and λAm 11 (small) cDNA. These data indicate that: (1) Amelogenin mRNA is heterogeneous in the bovine tooth germ, having two major species 800 and 400 bases long; and (2) the major species of amelogenin share extensive sequence homology. Based on these data, we suggest that at least part of the heterogeneity of amelogenin matrix protein may arise from the production of heterogeneous amelogenin mRNA's that share some common nucleotide sequences.

Selective adsorption of porcine-amelogenins onto hydroxyapatite and their inhibitory activity on hydroxyapatite growth in supersaturated solutions

The selective adsorption of amelogenins onto synthetic hydroxyapatite (HA) and their inhibitory activity on the seeded HA crystal growth were investigated using enamel proteins obtained from the outer layer of immature porcine-enamel (soft, cheeselike in consistency) of developing permanent incisors. Special interests were paid to the effect of a postsecretory degradation of the original amelogenin(s) on their adsorption and inhibitory activity. In the adsorption studies, it was apparent that the originally secreted amelogenin (25 K), proline, and histidine-rich protein (2a), as well as the higher molecular weight components (60-90 K), showed a strong adsorption affinity onto the HA. This adsorption of protein 2a was related to its inhibition of the crystal growth of seeded HA in a dilute supersaturated solution. On the other hand, the partially degraded product (20 K) of amelogenins, protein 2b, lost the high adsorption affinity onto the HA, and consequently showed no significant inhibitory activity. The observed selective adsorption of protein 2a onto HA was apparent at pH 6.0 and pH 7.4 even in the presence of dissociative solvents, such as 3 M urea or 2 and 4 M guanidine-HCl; however, this selective behavior was sensitive to changes in pH, and was not displayed at pH values of 7.8 or 10.8. The results suggest that the originally secreted amelogenin 2a may play an active role in amelogenesis, and that enamel mineralization could be regulated by the secretion of amelogenins and their inactivation through partial enzymic degradation, prior to their complete removal.

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.

Ultrastructural location of complex carbohydrates in developing rat incisor enamel

The ultrastructural distribution of complex carbohydrates in an early formation stage of rat incisor enamel was investigated by staining with the periodic acid-thiocarbohydrazide-silver proteinate reaction (PA-TCH-SP) for vicinal glycol-containing glycoconjugates, the phosphotungstic acid-chromic acid mixture (PTA) for glycoproteins, and the cationic dyes alcian blue or bismuth nitrate for sulfated glycoconjugates. In order to remove selectively sulfated complex carbohydrates, half of the serial sections obtained were digested with a bovine testicular hyaluronidase prior to staining. Far fewer electron-dense deposits were observed with the PA-TCH-SP method on hyaluronidase-treated sections, especially those subsequently treated for 48 hours with TCH. On the other hand, the minimal staining obtained with PTA was much more intense on sections treated with hyaluronidase where linear fiberlike structures were observed. With cationic dyes, staining of dotlike alignment structures and ground substance was obtained but was completely abolished by hyaluronidase treatment. Cuprolinic blue in a critical electrolyte concentration, ruthenium hexamine trichloride used with aldehyde during fixation, as well as rapid-freezing followed by freeze-substitution validate that this dotlike distribution is not an artefact of processing. The staining results demonstrated that the glycoproteins and sulfated complex carbohydrates in developing rat incisor enamel each display a specific distribution pattern. The glycoproteins were present as fiberlike structures and the sulfated carbohydrates appeared as dotlike formations located close to the surface of the fiberlike structures, and/or in the spaces between them.

Preliminary studies of the secondary structure in solution of two phosphoproteins of chicken bone matrix by circular dichroism and fourier transform-infrared spectroscopy

The secondary structures of two phosphoproteins from chicken bone matrix of Mr approximately 15kDa and approximately 28kDa, rich in Asx, Glx, and Ser, and containing Ser(P) and Thr(P) residues, have been investigated in solution by Circular Dichroism (CD) and Fourier Transform-Infrared Spectroscopy (FT-IR). CD spectroscopy, which yields useful information on the backbone conformation of polypeptides and proteins, suggests a predominantly beta-sheet structure for the two phosphoproteins. The FT-IR spectra of the approximately 15kDa protein, which is sensitive to secondary structure and hence provides complimentary information to CD spectroscopy, are consistent with the results obtained by CD studies.

Studies of the secondary structures of amelogenin from bovine tooth enamel

Circular dichroism and Fourier transform infrared spectroscopic studies of the major amelogenin protein of developing bovine tooth enamel in solution and in the solid state suggest a unique secondary structure containing beta-sheet and repetitive beta-turn structures. The repetitive beta-turn structure at the C-terminal end results from the unique primary structure of amelogenin.

Short-term effects of fluoride on biosynthesis of enamel-matrix proteins and dentine collagens and on mineralization during hamster tooth-germ development in organ culture

The effect of various concentrations of fluoride (F-) on cell proliferation, matrix formation and mineralization was examined in hamster molar tooth germs in premineralizing and mineralizing stages. The exposure lasted 16 h (mineralizing stages) and 24 h (premineralizing stages) and the F- levels ranged from 2.63 microM to 2.63 mM; [3H]-thymidine, [3H]-proline, 45Ca and 32PO4 were used as markers for cell proliferation, matrix formation and mineralization, respectively. The proline-labelled amelogenins were isolated by sequential extraction with water and formic acid and their nature examined by SDS-urea-polyacrylamide electrophoresis. Digestion by collagenase was used to assess the amount of proline incorporated into collagens. F- in concentrations up to 1.31 mM inhibited neither biosynthesis of DNA and amelogenins, nor synthesis of collagens and their hydroxylation. Amelogenins extracted from F- induced, non-mineralizing enamel matrix had the same electrophoretic mobility and the same degree of phosphorylation as amelogenins from normal, mineralizing enamel. However, F- increased the uptake of 45Ca and TCA-soluble 32P dose-dependently, starting with 52 microM. Thus, interference with secretion of enamel matrix by F- takes place at much lower concentrations than required to inhibit biosynthesis of enamel matrix.

Radioautographic visualization and biochemical identification of O-phosphoserine- and O-phosphothreonine-containing phosphoproteins in mineralizing embryonic chick bone

We injected NaH2(33)PO4 into normal 14-d-old embryonic chicks and examined the long bones by both radioautography and biochemical analyses from 10 to 240 min after the injection was completed. At 30 min, determination of the radiographic grain density revealed that 33P was concentrated principally in fibroblasts, preosteoblasts, and osteoblasts. With time, there was a progressive increase in the density of silver grains located over both the osteogenic cells and the regions of uncalcified (osteoid) and calcified extracellular organic matrices. Biochemical analyses identified 33P-O-phosphoserine as the major 33P component in glutaraldehyde-treated whole demineralized bone tissue and in EDTA-soluble, nondiffusible proteins extracted from the bones, both at the same time periods that 33P-induced silver grains were visualized by radioautography. 33P-O-phosphothreonine was also identified in experiments using a dosage of 10 mCi per embryo. The results provide the first combined direct biochemical and radioautographic identification that phosphoproteins are synthesized in bone and are located morphologically at the sites of mineralization. The data provide further evidence that phosphoproteins play a critical role in the biological calcification of vertebrate tissues.

Inhibition of seeded growth of enamel apatite crystals by amelogenin and enamelin proteins in vitro

The effect of enamel matrix proteins on the seeded growth of enamel apatite crystals was studied in stable supersaturated solutions at pH 7.4 and 37 degrees C. Of the two major protein classes in the enamel matrix, the enamelins were considerably more effective than the amelogenins in retarding seeded growth. However, the amelogenin species that did show significant inhibitory activity are those known to be lost first from the enamel matrix during the rapid mineralization stage of enamel maturation.

Multiple forms of rat dentin phosphoproteins

Previous studies have shown that the phosphoprotein from rat dentin is heterogenous and can be partially separated into two fractions by ion-exchange chromatography. These proteins were further characterized by polyacrylamide gel electrophoresis, gel chromatography, and amino acid and phosphate analysis, after chromatographic separations on ion-exchange columns. On 5-15% gradient gels, the phosphoproteins extracted from rat dentin and precipitated by CaCl2 gave three Alcian blue-staining bands with apparent molecular weights in the 90-95,000 range. The two slower-moving bands corresponded to highly phosphorylated proteins (HP) that had phosphoserine contents of greater than 400 residues per thousand and contained little or no valine, leucine, phenylalanine, or arginine. The faster-moving band corresponded to a moderately phosphorylated protein that contained about 250 residues per thousand of phosphoserine and greater quantities of glutamic acid, proline, and several other amino acids than HP. The nature of the phosphoproteins in HP was further studied after total removal of the phosphate with an insoluble form of bovine intestinal alkaline phosphatase. The dephosphorylated product (dP-HP) gave a single major band on gel electrophoresis but showed evidence for two closely related NH2-terminal sequences, Asp-Asp-Asp-Asn and Asp-Asp-Pro-Asn. The dephosphorylated material was separated into two components (dP-HP1 and dP-HP2) by chromatography on QAE-Sephadex A-25. The amino acid compositions of the two components showed that they differed in their primary structures. This conclusion was verified by the finding of the proline-containing sequence in dP-HP2. In addition to these two groups of phosphoproteins, a third class, LP, contains low levels of phosphoserine and high amounts of glutamic acid (W.T. Butler, M. Bhown, M.T. DiMuzio, and A. Linde, (1981) Coll. Res. 1, 187-199).

Biosynthesis of O-phosphoserine-containing phosphoproteins by isolated bone cells of mouse calvaria

Five groups of isolated bone cells from mouse calvaria were incubated with [3H]serine and the presence and amount of O-[3H]phosphoserine used as an indication of phosphoprotein synthesis. Cells in the osteoblastic fraction were the most active in synthesizing phosphoproteins, and unlike the other cell groups, released most of the phosphoproteins into the tissue culture medium. When subjected to molecular sieving and ion-exchange chromatography, the phosphoproteins synthesized by the bone cells of the osteoblastic group behaved like the phosphoproteins extracted from mouse calvaria by EDTA.

The proline-, glycine-, glutamic acid-rich pink-violet staining proteins in human parotid saliva are phosphoproteins

The major proteins in human parotid saliva, isolated in Fractions II-V following chromatography on Sephacryl S-200, DEAE-Sephadex A-50, or CM cellulose, contain 6 moles of phosphate per mole of protein, the phosphate probably bound to the protein via an ester linkage. This phosphate represents greater than 90% of the protein-bound phosphate in human parotid saliva. Neither purified gustin nor amylase, the two other major proteins in human parotid saliva, contain phosphate.

Structure of bovine milk calcium phosphate determined by X-ray absorption spectroscopy

Calcium in cow's milk is mainly in the form of calcium phosphate-phosphoprotein complexes known as casein micelles. These micelles, in contrast to other phosphoprotein complexes in bone and other tissues, can be readily isolated and studied, but conventional techniques have given ambiguous and conflicting evidence on the structure of milk calcium phosphate. Extended X-ray absorption fine structure and near-edge structure measurements at the newly commissioned Synchrotron Radiation Source at Daresbury indicate that it closely resembles brushite, CaHPO4 X 2H2O. This result, and chemical analysis, requires that phosphate groups from the matrix phosphoproteins be incorporated in the brushite lattice, probably in the surface, suggesting that these organic phosphate groups act as heterogeneous nucleation sites for phase separation of the calcium phosphate from solution.

Dental enamel matrix: sequences of two amelogenin polypeptides

The amino acid sequences of a leucine-rich amelogenin polypeptide (LRAP) and a tyrosine-rich amelogenin polypeptide (TRAP), isolated from foetal bovine enamel matrix, were determined. Both LRAP and TRAP occurred in two forms; in each case, one of the molecular species appeared to be shortened at the COOH terminus by 2 and 4 residues, respectively. A striking finding was that LRAP and TRAP had identical sequences for the first 33 residues but were almost completely different for the remaining 12 amino acids.