Degradation of enamelins by proteinases found in porcine secretory enamel in vitro

Mapping the Tooth Enamel Proteome and Amelogenin Phosphorylation Onto Mineralizing Porcine Tooth Crowns

Tooth enamel forms in an ephemeral protein matrix where changes in protein abundance, composition and posttranslational modifications are critical to achieve healthy enamel properties. Amelogenin (AMELX) with its splice variants is the most abundant enamel matrix protein, with only one known phosphorylation site at serine 16 shown in vitro to be critical for regulating mineralization. The phosphorylated form of AMELX stabilizes amorphous calcium phosphate, while crystalline hydroxyapatite forms in the presence of the unphosphorylated protein. While AMELX regulates mineral transitions over space and time, it is unknown whether and when un-phosphorylated amelogenin occurs during enamel mineralization. This study aims to reveal the spatiotemporal distribution of the cleavage products of the most abundant AMLEX splice variants including the full length P173, the shorter leucine-rich amelogenin protein (LRAP), and the exon 4-containing P190 in forming enamel, all within the context of the changing enamel matrix proteome during mineralization. We microsampled permanent pig molars, capturing known stages of enamel formation from both crown surface and inner enamel. Nano-LC-MS/MS proteomic analyses after tryptic digestion rendered more than 500 unique protein identifications in enamel, dentin, and bone. We mapped collagens, keratins, and proteolytic enzymes (CTSL, MMP2, MMP10) and determined distributions of P173, LRAP, and P190 products, the enamel proteins enamelin (ENAM) and ameloblastin (AMBN), and matrix-metalloprotease-20 (MMP20) and kallikrein-4 (KLK4). All enamel proteins and KLK4 were near-exclusive to enamel and in excellent agreement with published abundance levels. Phosphorylated P173 and LRAP products decreased in abundance from recently deposited matrix toward older enamel, mirrored by increasing abundances of testicular acid phosphatase (ACPT). Our results showed that hierarchical clustering analysis of secretory enamel links closely matching distributions of unphosphorylated P173 and LRAP products with ACPT and non-traditional amelogenesis proteins, many associated with enamel defects. We report higher protein diversity than previously published and Gene Ontology (GO)-defined protein functions related to the regulation of mineral formation in secretory enamel (e.g., casein α-S1, CSN1S1), immune response in erupted enamel (e.g., peptidoglycan recognition protein, PGRP), and phosphorylation. This study presents a novel approach to characterize and study functional relationships through spatiotemporal mapping of the ephemeral extracellular matrix proteome.

Dental enamel development: proteinases and their enamel matrix substrates

This review focuses on recent discoveries and delves in detail about what is known about each of the proteins (amelogenin, ameloblastin, and enamelin) and proteinases (matrix metalloproteinase-20 and kallikrein-related peptidase-4) that are secreted into the enamel matrix. After an overview of enamel development, this review focuses on these enamel proteins by describing their nomenclature, tissue expression, functions, proteinase activation, and proteinase substrate specificity. These proteins and their respective null mice and human mutations are also evaluated to shed light on the mechanisms that cause nonsyndromic enamel malformations termed amelogenesis imperfecta. Pertinent controversies are addressed. For example, do any of these proteins have a critical function in addition to their role in enamel development? Does amelogenin initiate crystallite growth, does it inhibit crystallite growth in width and thickness, or does it do neither? Detailed examination of the null mouse literature provides unmistakable clues and/or answers to these questions, and this data is thoroughly analyzed. Striking conclusions from this analysis reveal that widely held paradigms of enamel formation are inadequate. The final section of this review weaves the recent data into a plausible new mechanism by which these enamel matrix proteins support and promote enamel development.

Evolutionary analysis of mammalian enamelin, the largest enamel protein, supports a crucial role for the 32-kDa peptide and reveals selective adaptation in rodents and primates

Enamelin (ENAM) plays an important role in the mineralization of the forming enamel matrix. We have performed an evolutionary analysis of mammalian ENAM to identify highly conserved residues or regions that could have important function (selective pressure), to predict mutations that could be associated with amelogenesis imperfecta in humans, and to identify possible adaptive evolution of ENAM during 200 million years ago of mammalian evolution. In order to fulfil these objectives, we obtained 36-ENAM sequences that are representative of the mammalian lineages. Our results show a remarkably high conservation pattern in the region of the 32-kDa fragment of ENAM, especially its phosphorylation, glycosylation, and proteolytic sites. In primates and rodents we also identified several sites under positive selection, which could indicate recent evolutionary changes in ENAM function. Furthermore, the analysis of the unusual signal peptide provided new insights on the possible regulation of ENAM secretion, a hypothesis that should be tested in the near future. Taken together, these findings improve our understanding of ENAM evolution and provide new information that would be useful for further investigation of ENAM function as well as for the validation of mutations leading to amelogenesis imperfecta.

Degradative changes in whole enamel homogenates incubated in vitro in the presence of low calcium ion concentrations

The purpose of this study was to investigate overall degradative changes occurring to enamel matrix proteins in small, freeze-dried pieces of rat incisor enamel homogenized and incubated directly for 0-48 hours in a synthetic enamel fluid solution (165 mM total ionic strength with 0.153 mM calcium chloride) versus other samples homogenized and incubated for the same time intervals in distilled water. The results indicated that many alterations in the apparent molecular weights of enamel matrix proteins took place under both conditions although the rates for many degradative changes over a 48 hour period were often slower in distilled water than in synthetic enamel fluid. Freeze-dried enamel samples homogenized and incubated in 165 mM Tris-HCl buffer at pH 8.0 showed changes comparable to those seen with distilled water. This suggested that differences observed between samples incubated in enamel fluid versus distilled water were unrelated to pH or ionic strength of the solutions and may be the result of a requirement by some enamel proteinases for small amounts of free calcium ions in incubation media. Of interest were findings that some enamel matrix proteins, especially those in strips taken from the first half of the secretory stage of amelogenesis, were degraded much faster in distilled water than in synthetic enamel fluid. The reasons for this effect are unclear although, in this case, calcium ions could be inhibitory to hydrolysis of certain matrix proteins by the enamel proteinases.

Enamel matrix serine proteinase 1: stage-specific expression and molecular modeling

Enamel proteins are cleaved by proteinases soon after their secretion by ameloblasts. Intact proteins concentrate in the outer enamel at or near the growing tips of the enamel crystallites while cleavage products accumulate in the deeper enamel. In the transition and early maturation stages there is a dramatic increase in proteolytic activity. This activity, coupled with the diminished secretory and increased reabsorptive functions of ameloblasts, leads to a precipitous fall in the amount of enamel protein in the matrix. Recently we have cloned and characterized an mRNA encoding a tooth-specific serine proteinase designated enamel matrix serine proteinase 1 (EMSP1) [Simmer et al., JDR (1998) 77: 377]. EMSP1 can be detected in the inner enamel during the secretory stage and its activity increases sharply during the transition stage. Stage-specific Northern blot analysis demonstrates this increase is accompanied by a parallel increase in the amount EMSP1 mRNA. A 3-dimensional computer model of EMSP1, based upon the crystal structure of bovine trypsin, has been generated and analyzed. All six disulfide bridges as well as the active site are conserved. Changes in the peptide binding region and the specificity pocket suggest that interaction of the proteinase with protein substrates is altered, potentially causing a shift in substrate specificity. The calcium binding region of trypsin is thoroughly modified suggesting that the calcium independence of EMSP1 activity is due to an inability to bind calcium. The three potential N-linked glycosylation sites, N104, N139 and N184, are in surface accessible positions away from the active site.

Degradation of enamel matrix proteins in porcine secretory enamel

To elucidate the progressive disappearance of 25 kDa amelogenin occurring in a narrow space near the surface of enamel, the alkaline soluble fraction which contained 80% of the total proteins was extracted from a newly formed porcine enamel. When this fraction was incubated with the addition of Ca ions in an in vitro system, the degradation of the coexisting amelogenin and enamelin occurred without activation during the incubation period. Although the fraction contained mainly two kinds of metalloproteinases, 56 kDa and 61 kDa gelatinolytic, and 41 kDa and 46 kDa caseinolytic activities, it was demonstrated on amelogenin enzymography that the caseinolytic one was concerned with the conversion of the 25 kDa amelogenin into the 20 kDa amelogenin. The protein distribution of the newly formed enamel indicated that the metalloproteinases degraded the coexisting enamelin and amelogenin imperfectly. Nevertheless, during the next developing stage they demonstrated their full activities. It is suspected that these activities are regulated by Ca ions, which may be increased by a cascade system.

Immunohistochemical detection and distribution of enamelysin (MMP-20) in human odontogenic tumors

Enamelysin is a tooth-specific protease that was initially isolated from porcine enamel organ and subsequently from human odontoblasts. Since this protease is thought to play important roles in tooth development, the evaluation of enamelysin in odontogenic tumors may aid our understanding of the histogenesis and cell differentiation of such lesions. A monoclonal antibody (203-1C7) was generated against synthesized human enamelysin oligopeptide and was used to assess the immunolocalization of enamelysin in healthy developing tooth germs and various types of odontogenic lesions. In tooth germs, enamelysin expression was detected only in the secretory enamel. Thus, 203-1C7 may serve as an enamel-specific marker in the late stage of enamel matrix development and calcification. In odontogenic lesions, strong enamelysin staining was demonstrated in the immature enamel matrix of ameloblastic fibro-odontomas and odontomas. Furthermore, enamelysin was also detected in globular amyloid masses and calcified foci in calcifying epithelial odontogenic tumors, hyaline droplets, small and large mineralized areas in adenomatoid odontogenic tumors, and a portion of ghost cells in calcifying odontogenic cysts. Positive reactivity was also observed in selected tumor cells in some of these tumors. No intracellular staining for enamelysin was detected in ameloblastomas or the ameloblastic portion of ameloblastic fibro-odontomas. Also, enamelysin was not detected in dentin, dysplastic dentinoid hyaline matrices, and cementum that were present within the tumors examined. Thus, taken together, our results suggest that the enamelysin-specific monoclonal antibody (203-1C7) may be utilized as a marker of early enamel development and that enamelysin may be involved in the pathogenesis of specific odontogenic tumors.

Proteinases in developing dental enamel

For almost three decades, proteinases have been known to reside within developing dental enamel. However, identification and characterization of these proteinases have been slow and difficult, because they are present in very small quantities and they are difficult to purify directly from the mineralizing enamel. Enamel matrix proteins such as amelogenin, ameloblastin, and enamelin are cleaved by proteinases soon after they are secreted, and their cleavage products accumulate in the deeper, more mature enamel layers, while the full-length proteins are observed only at the surface. These results suggest that proteinases are necessary for "activating" enamel proteins so the parent proteins and their cleavage products may perform different functions. A novel matrix metalloproteinase named enamelysin (MMP-20) was recently cloned from tooth tissues and was later shown to localize primarily within the most recently formed enamel. Furthermore, recombinant porcine enamelysin was demonstrated to cleave recombinant porcine amelogenin at virtually all of the sites that have previously been described in vivo. Therefore, enamelysin is at least one enzyme that may be important during early enamel development. As enamel development progresses to the later stages, a profound decrease in the enamel protein content is observed. Proteinases have traditionally been assumed to degrade the organic matrix prior to its removal from the enamel. Recently, a novel serine proteinase named enamel matrix serine proteinase-1 (EMSP1) was cloned from enamel organ epithelia. EMSP1 localizes primarily to the early maturation stage enamel and may, therefore, be involved in the degradation of proteins prior to their removal from the maturing enamel. Other, as yet unidentified, proteinases and proteinase inhibitors are almost certainly present within the forming enamel and await discovery.

The structural biology of the developing dental enamel matrix

The biomineralization of the dental enamel matrix with a carbonated hydroxyapatite mineral generates one of the most remarkable examples of a vertebrate mineralized tissue. Recent advances in the molecular biology of ameloblast gene products have now revealed the primary structures of the principal proteins involved in this extracellular mineralizing system, amelogenins, tuftelins, ameloblastins, enamelins, and proteinases, but details of their secondary, tertiary, and quaternary structures, their interactions with other matrix and or cell surface proteins, and their functional role in dental enamel matrix mineralization are still largely unknown. This paper reviews our current knowledge of these molecules, the probable molecular structure of the enamel matrix, and the functional role of these extracellular matrix proteins. Recent studies on the major structural role played by the amelogenin proteins are discussed, and some new data on synthetic amelogenin matrices are reviewed.

Cellular and chemical events during enamel maturation

This review focuses on the process of enamel maturation, a series of events associated with slow, progressive growth in the width and thickness of apatitic crystals. This developmental step causes gradual physical hardening and transformation of soft, newly formed enamel into one of the most durable mineralized tissues produced biologically. Enamel is the secretory product of specialized epithelial cells, the ameloblasts, which make this covering on the crowns of teeth in two steps. First, they roughly "map out" the location and limits (overall thickness) of the entire extracellular layer as a protein-rich, acellular, and avascular matrix filled with thin, ribbon-like crystals of carbonated hydroxyapatite. These initial crystals are organized spatially into rod and interrod territories as they form, and rod crystals are lengthened by Tomes' processes in tandem with appositional movement of ameloblasts away from the dentin surface. Once the full thickness of enamel has been formed, ameloblasts initiate a series of repetitive morphological changes at the enamel surface in which tight junctions and deep membrane infoldings periodically appear (ruffle-ended), then disappear for short intervals (smooth-ended), from the apical ends of the cells. As this happens, the enamel covered by these cells changes rhythmically in net pH from mildly acidic (ruffle-ended) to near-physiologic (smooth-ended) as mineral crystals slowly expand into the "spaces" (volume) formerly occupied by matrix proteins and water. Matrix proteins are processed and degraded by proteinases throughout amelogenesis, but they undergo more rapid destruction once ameloblast modulation begins. Ruffle-ended ameloblasts appear to function primarily as a regulatory and transport epithelium for controlling the movement of calcium and other ions such as bicarbonate into enamel to maintain buffering capacity and driving forces optimized for surface crystal growth. The reason ruffle-ended ameloblasts become smooth-ended periodically is unknown, although this event seems to be crucial for sustaining long-term crystal growth.

Purification, characterization, and cloning of enamel matrix serine proteinase 1

The maturation of dental enamel succeeds the degradation of organic matrix. Inhibition studies have shown that this degradation is accomplished by a serine-type proteinase. To isolate and characterize cDNA clones encoding this proteinase, we used two degenerate primer approaches to amplify part of the coding region using polymerase chain-reaction (PCR). First, we purified the proteinase from porcine transition-stage enamel matrix and characterized it by partial protein sequencing. The enzyme was isolated from the neutral soluble enamel extract by successive ammonium sulfate precipitations, hydroxyapatite HPLC, reverse-phase HPLC, DEAE ion exchange, and affinity chromatography with a Benzamidine Sepharose 6B column. The intact protein and lysylendopeptidase-generated cleavage products were characterized by amino acid sequence analyses. Degenerate oligonucleotide primers encoding two of the polypeptide sequences were synthesized. In a complementary strategy, degenerate oligonucleotide primers were designed against highly conserved active-site regions of chymotrypsin-like proteinases. Both approaches yielded PCR amplification products that served as probes for screening a porcine enamel organ epithelia-specific cDNA library. The longest full-length clone is 1133 nucleotides and encodes a preproprotein of 254 amino acids. We designate this protein enamel matrix serine proteinase 1 or EMSP1. The active protein has 224 amino acids, an isotope-averaged molecular mass of 24.1 kDa, and an isoelectric point of 6.0. Multiple-tissue Northern analysis indicates that EMSP1 is a tooth-specific protein. Gelatin enzymography shows a dramatic increase in EMSP1 activity in the transition-stage enamel matrix. EMSP1 is most homologous to kallikriens and trypsins.

Cloning and characterization of porcine enamelin mRNAs

Dental enamel forms by matrix-mediated biomineralization. The components of the developing enamel matrix are generally specific for that matrix. The primary structures of three enamel proteins-amelogenin, tuftelin, and sheathlin (ameloblastin/amelin)-have been derived from cDNA sequences. Here we report the cloning and characterization of mRNA encoding a fourth enamel protein: enamelin. The longest porcine enamelin cDNA clone has 3907 nucleotides, exclusive of the poly(A) tail. The primary structure of the secreted protein is 1104 amino acids in length. Without post-translational modifications, the secreted protein has an isotope-averaged molecular mass of 124.3 kDa and an isoelectric point of 6.5. Polymerase chain-reaction phenotyping of enamelin cDNA suggests that porcine enamelin transcripts are not alternatively spliced and use a single polyadenylation/cleavage site. Immunohistochemical and Western blot analyses with an affinity-purified antipeptide antibody specific for the enamelin carboxyl terminus demonstrate that enamelin is synthesized and secreted by secretory-phase ameloblasts. The parent protein is a 186-kDa glycoprotein that concentrates along the secretory face of the ameloblast Tomes' process. Intact enamelin and proteolytic cleavage products containing its carboxyl terminus are limited to the most superficial layer of the developing enamel matrix, while other enamelin cleavage products are observed in deeper enamel.

The enamel proteins in human amelogenesis imperfecta

Amelogenesis imperfecta comprises a unique group of hereditary conditions that result in abnormal enamel development. The purpose of this study was to characterize the enamel proteins in different amelogenesis imperfecta types and to determine if amelogenin, the principal matrix protein in normal developing enamel, was retained. Primary and/or permanent amelogenesis imperfecta teeth were analysed from 11 individuals. Normal teeth served as controls. Thin sections were cut with a diamond blade and enamel was dissected for analysis. The enamel proteins were characterized by amino acid analysis, sodium dodecyl sulphate polyacrylamide gel electrophoresis, and Western blot analysis using antiamelogenin antibodies. An increased protein content was seen in all hypocalcified and hypomaturation amelogenesis imperfecta cases. A slightly increased protein content was seen in two of four hypoplastic amelogenesis imperfecta cases. The enamel protein amino acid composition varied between the different amelogenesis imperfecta types. All three cases of hypomaturation amelogenesis imperfecta enamel showed an increased proline content compared with normal enamel or other amelogenesis imperfecta types. Hypocalcified amelogenesis imperfecta enamel had an increased tyrosine content while the other amino acids were generally similar in amount to normal enamel. Fully developed hypomaturation and hypocalcified amelogenesis imperfecta enamel showed cross-reactivity to antiamelogenin antibodies while normal enamel did not. Although both amelogenesis imperfecta types showed cross-reactivity, the banding patterns on Western blot analyses were markedly different. This investigation provides additional evidence that abnormal post-secretory processing of amelogenin is involved in hypomaturation and hypocalcified amelogenesis imperfecta. Furthermore, these results indicate that amelogenin retention can occur in a variety of amelogenesis imperfecta types. The unique amino acid compositions and distinct enamel protein species seen by electrophoresis and Western blot analyses suggest that different developmental processes might be involved in hypomaturation and hypocalcified amelogenesis imperfecta.

Possible actions of metalloproteinases found in porcine enamel in an early secretory stage

In an outermost layer of porcine secretory enamel, metalloproteinases were detected by enzymography with gelatin used as a substrate. When the sample extracted from the outermost layer of the secretory enamel was incubated with calcium ions at 37 degrees C prior to electrophoresis, an increase of the 34-kDa proteinase activity and a decrease of the 76- and/or 78-kDa proteinase activities were observed. The results suggest that the metalloproteinases mediate the conversion from 76- and/or 78-kDa proteinases to the 34-kDa proteinase or the activation of a latent type of the 34-kDa proteinase, and that their activities are regulated by free Ca ions.

Developmental changes in the pH of enamel fluid and its effects on matrix-resident proteinases

The objectives of this study were to measure pH in developing enamel at progressively older (more mature) stages of amelogenesis in vivo, and then to formulate synthetic enamel fluid mixtures that approximated these pH values for in vitro studies. The ultimate goal was to characterize the molecular weights of proteinases visualized by enzymograms incubated in synthetic enamel fluid using gelatin and casein as substrates. For most experiments, the proteinases were extracted en masse from small freeze-dried enamel strips directly into a non-reducing sample preparation buffer. In some experiments, we pre-treated the enamel strips with acetic acid to determine if this common method for demineralization and protein extraction caused any changes in the activity levels of the enamel proteinases. In other experiments, we first soaked enamel strips in synthetic enamel fluid to determine solubility of the proteinases within an aqueous phase. The results indicated that the pH of developing enamel remained fairly constant near pH 7.23 across the secretory stage, but it was generally more acidic (6.93) and fluctuated in focal areas between mildly acidic (6.2-6.8) and near-neutral (7.2) conditions across the maturation stage. The pH then slowly rose to near 7.35 when the enamel was almost mature (hard). The acidic conditions were generally inhibitory to most enamel proteinases, but there were some caseinase activities in mid-maturation-stage enamel near 23-30 kDa which appeared to be activated by weakly acidic conditions (pH 6.28). Pre-treatment of enamel samples with 0.5 M acetic acid markedly altered the overall profile of enamel proteinases, causing activation of some latent proteinase activities and permanent inhibition of other activities. Most proteinases in whole homogenates were insoluble in synthetic enamel fluid. This suggests that they may be tightly bound, directly or indirectly, to matrix proteins or mineral components in situ.

Primary structure of the porcine 89-kDa enamelin

The primary structure of the 89-kDa enamelin found in porcine secretory enamel at an early stage of development was investigated. The fragments of the enamelin cDNA were amplified by polymerase chain-reaction from the first-strand enamelin cDNA, and were sequenced. The results indicated that the 89-kDa enamelin consisted of 627 amino acid residues and had a molecular mass of 70,448. A hydrophobic domain is located in the region of the 21st-62nd amino acid residues of the molecule. Acidic domains are located in two regions of the molecule-one in the region of the 135th-238th amino acid residues and the other in the C-terminal region. A basic domain is located in the region of the 239th-360th amino acid residues. The results also indicated that the low-molecular-weight enamelins were fragments derived from a prototype enamelin.

Identification of a novel proteinase (ameloprotease-I) responsible for the complete degradation of amelogenin during enamel maturation

During enamel formation the proteins of the extracellular matrix, particularly amelogenins, are removed prior to maturation. In order to investigate this process and to improve our understanding of the function of proteinases during enamel maturation, proteinase fractions were isolated from developing pig enamel and assayed for proteolytic activity in vitro. A recombinant murine amelogenin, M179, was used as a substrate. Two major groups of enamel proteinases were defined as high-molecular-mass ['high-molecular-weight' in Moradian-Oldak, Simmer, Sarte, Zeichner-David and Fincham (1994) Arch. Oral Biol.39, 647-656] and low-molecular-mass proteinases. Here we report the characterization of one of the proteinases present in the low-molecular-mass group. We demonstrate that this proteinase is a serine proteinase capable of degradation of M179 following cleavage of the tyrosine-rich amelogenin polypeptide from the N-terminal region. A partial N-terminal sequence of the proteinase was obtained (LPHVPHRIPPGYGRPXTXNEEGXNPYFXFFXXHG). An anti-peptide antibody directed against a synthetic peptide corresponding to the first 14 amino acids of the above sequence was produced. The presence of the proteinase in the acetic acid extract was confirmed by Western blotting. Searching using the amino acid sequence determined in this study showed it to be also present in the 32 kDa and 89 kDa enamelin proteins reported by Fukae, Tanabe, Murakami and Tohi [(1996) Adv. Dent. Res., in the press]. We therefore identify the 32 kDa enamelin as an enamel proteinase ('ameloprotease-I') which is responsible for amelogenin degradation in maturing enamel. We propose that the 89 kDa enamelin is a precursor of ameloprotease-I, the first enamel protein for which a function has been defined.

Protein dynamics of amelogenesis

BACKGROUND

The synthesis, secretion, and fate of matrix proteins released by ameloblasts during enamel formation was studied in continuously erupting rat incisors.

METHODS

Computerized image processing was used to quantify silver grain distribution in radioautographs of sections prepared from rats injected with 3H-methionine, and this was correlated with fluorographs defining radiolabeling patterns of proteins in enamel organ cell and enamel homogenates prepared from freeze-dried teeth of rats injected with 35S-methionine and other radioactive amino acids and precursors such as sugar, sulfate, and phosphate. Some rats were also treated with brefeldin A to characterize newly formed proteins blocked from being secreted from ameloblasts.

RESULTS

The results indicate that ameloblasts rapidly synthesize and secrete (minutes) at least five primary enamel matrix proteins, including a 65 kDa sugar-containing sulfated enamel protein and four nonsulfated proteins with molecular weights near 31, 29, 27, and 23 kDa as estimated by SDS-PAGE. The 27 kDa protein appears to correspond to the primary amelogenin described in many species. The cells also appear to release at least one phosphoprotein with molecular weight near 27 kDa, which may be an amelogenin, and up to five cysteine-containing proteins with molecular weights near 94, 90, 72, 55, and 27 kDa. The proteins collectively are released at interrod and rod growth sites where they appear to remain close to their point of release from ameloblasts. The 65 kDa sulfated protein and 31 kDa nonsulfated protein are rapidly converted into lower molecular weight forms (hours), whereas nonsulfated proteins near 29, 27, and 23 kDa are more slowly transformed into fragments near 20, 18, and 10 kDa in molecular weight (days). These fragments do not accumulate but appear to be removed from the enamel layer as they are created.

CONCLUSIONS

Enamel proteins seen by Coomassie blue (or silver) staining of one-dimensional polyacrylamide gels, therefore, represent a composite image of newly secreted and derived forms of sulfated and nonsulfated proteins that sometimes have similar molecular weights.

Enzymes in mineralizing systems: state of the art

The hallmark of biological mineralization is the precise regulation of mineral deposition in space and time. The cells which produce mineralized tissues are themselves controlled by developmental programs and hormonal signals which result in regulation of gene expression and modulation of protein function. These signals are transduced into changes in enzyme levels and/or activity. Upon activation, cellular enzymes then act to synthesize the organic matrix and process it extracellularly, utilize metabolic energy to transport ions from the blood to the matrix, and to initiate the mineralization cascade. The first enzyme activity described in mineralizing tissues was alkaline phosphatase and it is still the best characterized enzyme in the mineralization process. Yet, important questions about the role of this protein remain unanswered, and it continues to occupy a central focus in mineralized tissue investigation. Other phosphatases, including protein tyrosine phosphatases are important in regulating tyrosine kinase mediated signals. Investigators have now begun to look closely at several groups of kinases which are also important for proper mineralization. As peptide hormones are important modulators of mineralized tissues, protein kinase A has always been presumed to play a key role in phosphorylating intracellular proteins. There is also considerable interest in protein kinase C, as well as tyrosine kinases in mineralized tissue signal transduction. Another group of kinases important in mineralized tissues are the enzymes which phosphorylate the matrix phosphoproteins. Of these, casein kinase II appears to be involved in intracellular and extracellular protein phosphorylation. Several enzymes present in the premineralized matrix are thought to be significant in triggering mineralization. Alkaline phosphatase may act at this level, but new data also suggests that metalloproteases and gelatinases, by modifying or digesting matrix components, may be important in the initiation of calcification.

Enamel matrix protein turnover during amelogenesis: basic biochemical properties of short-lived sulfated enamel proteins

The formation and turnover of sulfated enamel proteins was investigated by SDS-PAGE, fluorography, and TCA-precipitations using freeze-dried incisors of rats injected intravenously with 35S-sulfate (35SO4) and processed at various intervals from 1.6 minutes to 4 hours thereafter. Some rats were injected first with 35SO4 followed 5 minutes later by 0.3 mg of cycloheximide. This was done to terminate protein translation and allow events related to extracellular processing and degradation of the sulfated enamel proteins to be visualized more distinctly. Other rats were injected with cycloheximide followed at 0 minutes (simultaneous injection) to 30 minutes later by 35SO4. This was done to characterize the time required for proteins to travel from endoplasmic reticulum to Golgi apparatus, where they became sulfated. The results indicated that enamel organ cells (ameloblasts) rapidly incorporated 35SO4 into a major approximately 65 kDa protein that was secreted into the enamel within 6-7.5 minutes. This parent protein appeared to be processed extracellularly within 15 minutes into major approximately 49 kDa and approximately 25 kDa fragments which themselves had apparent half-lives of about 1 and 2 hours, respectively. There were also many minor sulfated fragments varying in molecular weight (Mr) from approximately 13-42 kDa, which appeared to originate from extracellular processing and/or degradation of the parent approximately 65 kDa sulfated enamel protein or its major approximately 49 kDa and approximately 25 kDa fragments. Experiments with glycosidases further suggested that the majority of sulfate groups were attached to sugars N-linked by asparagine to the core of the approximately 65 kDa sulfated enamel protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular mechanisms of dental enamel formation

Tooth enamel is a unique mineralized tissue in that it is acellular, is more highly mineralized, and is comprised of individual crystallites that are larger and more oriented than other mineralized tissues. Dental enamel forms by matrix-mediated biomineralization. Enamel crystallites precipitate from a supersaturated solution within a well-delineated biological compartment. Mature enamel crystallites are comprised of non-stoichiometric carbonated calcium hydroxyapatite. The earliest crystallites appear suddenly at the dentino-enamel junction (DEJ) as rapidly growing thin ribbons. The shape and growth patterns of these crystallites can be interpreted as evidence for a precursor phase of octacalcium phosphate (OCP). An OCP crystal displays on its (100) face a surface that may act as a template for hydroxyapatite (OHAp) precipitation. Octacalcium phosphate is less stable than hydroxyapatite and can hydrolyze to OHAp. During this process, one unit cell of octacalcium phosphate is converted into two unit cells of hydroxyapatite. During the precipitation of the mineral phase, the degree of saturation of the enamel fluid is regulated. Proteins in the enamel matrix may buffer calcium and hydrogen ion concentrations as a strategy to preclude the precipitation of competing calcium phosphate solid phases. Tuftelin is an acidic enamel protein that concentrates at the DEJ and may participate in the nucleation of enamel crystals. Other enamel proteins may regulate crystal habit by binding to specific faces of the mineral and inhibiting growth. Structural analyses of recombinant amelogenin are consistent with a functional role in establishing and maintaining the spacing between enamel crystallites.