Sequential expression and differential function of multiple enamel proteins during fetal, neonatal, and early postnatal stages of mouse molar organogenesis

Immunohistochemical demonstration of enamel proteins in odontogenic tumors

Immunohistochemical localization of two enamel proteins, amelogenin and enamelin, in comparison with that of keratin, was determined in odontogenic tumors and the allied lesions in order to verify functional differentiation of the tumor cells as ameloblasts. Amelogenin and enamelin were demonstrated in small mineralized foci and in the tumor cells surrounding them in adenomatoid odontogenic tumor (AOT), calcifying epithelial odontogenic tumor (CEOT), and calcifying odontogenic cyst (COC). Hyaline droplets in AOT showed positive staining for both enamel proteins. These mineralized and hyaline materials were not positive for keratin, although tumor cells were positive. On the other hand, no immunoreaction for enamel proteins was obtained in ameloblastoima and odontogenic epithelial cell nests within myxoma and epulis. The results suggest that tumor cells of AOT and CEOT and lining epithelial cells of COC show ameloblastic differentiation in part, but that ameloblastoma cells do not attain functional matauration as secretory phase ameloblasts.

Immunochemical and immunohistochemical studies, using antisera against porcine 25 kDa amelogenin, 89 kDa enamelin and the 13-17 kDa nonamelogenins, on immature enamel of the pig and rat

Enamel proteins were extracted from the newly formed layer of immature porcine enamel, and the 25 kDa amelogenin, 89 kDa enamelin and 13-17 kDa nonamelogenins were purified. Specific antisera were raised against these proteins. Antibodies specific to the C-terminal region (residues 149-173) of the 25 kDa amelogenin were generated by absorption of the anti-25 kDa amelogenin serum with 20 kDa amelogenin, which contains residues 1-148 of the antigen. Immunoelectro-transfer blotting of the extracted porcine enamel proteins showed that the anti-25 kDa amelogenin serum recognized the 25 kDa and other low and high molecular weight amelogenins. The C-terminal specific anti-25 kDa amelogenin serum reacted only with amelogenins having molecular weights over 23 kDa. The anti-89 kDa enamelin serum recognized the 89 kDa enamelin and lower molecular weight proteins, but neither the amelogenins nor the 13-17 kDa nonamelogenins. The antiserum against the 13-17 kDa nonamelogenins showed no cross reactivity to the 89 kDa enamelin, but recognized higher molecular weight nonamelogenins. In immunohistochemical preparations of the porcine tooth germs, the 25 kDa amelogenin-like immunoreactivity over immature enamel decreased in a gradient from the enamel surface to the middle layer. In the inner layer immunoreactivity was concentrated over the prism sheaths. The C-terminal specific 25 kDa amelogenin-like immunoreactivity was intense at the outer layer of immature enamel and decreased sharply toward the middle layer. Prism sheaths were intensely stained by the antiserum to the 13-17 kDa nonamelogenins.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphology of the enamel organ in the miniature swine

In recent years, the dentition of the pig has been increasingly used as a model for the study of amelogenesis. Indeed, much of our current knowledge on enamel formation derives from biochemical and physicochemical analyses of the organic and inorganic components, respectively, of porcine enamel. As an extension of this previous work, and as the first step in our attempt to correlate known enamel matrix and mineral changes with adjacent enamel organ morphology, the present study was undertaken to provide a description of the morphological events occurring in the enamel organ during porcine amelogenesis. Two-week-old miniature swine (minipigs) were fixed by vascular perfusion with glutaraldehyde, the deciduous teeth present at this age were embedded in Epon resin and sectioned, and the cells of the enamel organ at each of the various developmental stages of amelogenesis were examined by light and transmission electron microscopy. In many respects, the morphology of the porcine enamel organ was similar to that previously described in other mammalian species. On the other hand, several particularities were noted and these are discussed in the context of available data correlating cell ultrastructure with putative function during enamel formation.

Linkage of amelogenin (Amel) to the distal portion of the mouse X chromosome

Amelogenins are hydrophobic, proline-rich proteins that are the primary biosynthetic products of ameloblasts. These cells are responsible for the formation of tooth enamel, and amelogenins play an important role in the process of biomineralization. A cDNA, corresponding to the mouse 26-kDa amelogenin, has been molecularly cloned and sequenced. Southern blot analysis of genomic DNA from the mouse using this cDNA as a probe indicates that there is only one mouse amelogenin (Amel) gene. This paper describes restriction site variation for the Amel gene that we have identified between C57BL/6 and M. spretus and the segregation of that variation as an X-chromosome gene. The position of the amelogenin locus (Amel) relative to the loci for alpha-galactosidase (Ags), proteolipoprotein (Plp), and the random genomic probe DXWas31 has been determined. Amel is established as: (1) the most distal locus in the genetic map of the mouse X chromosome, (2) lying proximal to the X:Y pairing region, and (3) being restricted to the mouse X chromosome.

Immunohistochemical demonstration of amelogenin penetration toward the dental pulp in the early stages of ameloblast development in rat molar tooth germs

In order to examine the synthesis and secretion of enamel protein by ameloblasts in their early stages of development, immunohistochemical localization was carried out at light and electron microscopic levels using a monoclonal antibody produced in a preliminary experiment. Materials used were tooth germs of mandibular first molars of rats at 0-5 days after birth. Immunoblot analysis after two-dimensional electrophoresis revealed that antigen molecules recognized by the monoclonal antibody were amelogenins of 26-28 kDa (pI, 6.6-7.0). An immunohistochemical examination using this monoclonal antibody demonstrated that the presecretory ameloblasts in their early stages of differentiation both synthesized amelogenin and secreted through a classical merocrine secretory pathway. In some presecretory ameloblasts as well as ameloblasts we observed the distended cisternae of rough endoplasmic reticulum (rER) which demonstrated heterogenous immunolabelling. The immunolabellings were also detected in the predentin as well as the intercellular spaces of odontoblasts and dental pulp cells which indicated penetration of amelogenin from the presecretory ameloblast layer to the dental pulp. The presence of coated pits at the plasma membrane of odontoblasts in close proximity to enamel protein along with the immunolabelling of lysosomes of the odontoblasts suggests the phagocytosis of the enamel protein into the odontoblasts. These observations suggest the possibility that the penetration of enamel protein toward the dental pulp and odontoblasts plays a role in the interaction between ameloblasts and odontoblasts.

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.

Immunoblotting and cytochemical characterization of human enamel proteins

Mature enamel proteins (tuft proteins) and fetal enamel proteins were extracted by an homogenizing buffer method, subjected to SDS-PAGE and immunoblotted with a polyclonal antibody raised against the mature enamel proteins. Both fetal and tuft proteins were recognized by this immunoblotting. With the same antibody, immunolocalization of the developing enamel proteins was done on semi-thin-sections of human fetal tissue at the secretory stage, using an immunoperoxidase technique. Specific labelling of the enamel protein matrix was observed. It is concluded that a polyclonal antibody against mature enamel proteins (anti-tuft) can recognize the developing protein matrix at the secretory stage. This suggests that a common antigenic determinant is maintained throughout the course of amelogenesis in human enamel.

Immunocytochemical localization of enamelin proteins in developing bovine teeth

Enamelins were localized at both the light and electron microscopic level using an antienamelin monoclonal antibody and indirect immunogold methods. Bovine fetal incisors (crown-rump length 17-30 cm) were preserved in Karnovsky's fixative and embedded in Epon. For light microscopy, 2 microM thick sections were immunostained by the indirect method using the monoclonal antibody and goat anti-mouse IgG linked to 5 nM gold particles, followed by silver enhancement to increase the sensitivity of the method. For electron microscopy, thin sections were immunostained (indirect) with the antienamelin monoclonal antibody and goat anti-mouse IgG linked to 5 or 15 nM gold. Control samples were treated with an unrelated monoclonal antibody. The localization of enamelins was confined in the light microscopic sections to the extracellular enamel matrix. No gold staining was observed in the ameloblasts or other enamel organ cells even though the gold-silver technique is extremely sensitive. Ultrastructurally, enamelin was localized in the enamel extracellular matrix and associated ameloblasts. Both the crystal-containing and granular matrix were positively stained, with most gold particles being closely associated with the crystals. Counting of gold particles indicated more than 4 times as many amelogeninas enamelin-reactive antigenic sites in similar regions. Decalcification did not increase immunostaining with the anti-enamelin antibody in the extracellular matrix. Within ameloblasts, the gold particles were associated with secretory granules and Golgi complexes. Thus it appears that enamelins are synthesized in ameloblasts and secreted into the extracellular matrix in a similar manner to amelogenins and are preferentially associated with matrix hydroxyapatite crystals. Transient levels of enamelins within the ameloblasts are apparently too low to be detected by light microscopy.

Mouse ameloblasts do not transcribe the albumin gene

The interpretation of recent experimental evidence has lead to several discussions at international meetings which have included the suggestion that enamelins are not tissue-specific gene products but rather are similar to, or are identical to, albumin. Further complicating the issue has been the proposal by several investigators that albumin participates in the organization of hard tissues. The experimental strategy described in this study was to hybridize the mouse albumin gene cDNA clone, palb-3, to a Northern blot of secretory phase mouse mandibular first molar RNAs. This approach would permit the evaluation of the potential transcription of the albumin gene by differentiated ameloblasts as well as other cells of the odontogenic organ. The results of this approach indicated that odontogenic cells did not transcribe the albumin gene. Albumin polypeptides, therefore, cannot be synthesized by odontogenic cells and cannot be identical to odontogenic tissue-specific gene products. If albumin is incorporated into developing enamel matrix wherein it participates in undefined roles during enamel matrix organization, it cannot arise as a biosynthetic product of the odontogenic organ.

Amelogenesis in vitro: a model for studies of epithelial postsecretory processing during tissue-specific extracellular matrix biomineralization

The extracellular matrix (ECM) of developing mammalian enamel comprises a complex of unusual epithelial-derived proteins, which appear to function in concert to initiate and propagate tissue-specific biomineralization. Following enamel protein synthesis by ameloblast cells within the enamel organ, the subsequent steps of posttranslational modification, secretion, postsecretory processing and eventual removal of these proteins from forming enamel are largely unknown. To address this issue we have designed studies to investigate the hypothesis that enamel proteins are removed from enamel and translocated into the vasculature as relatively high-molecular-weight components. We examined enamel proteins recovered from serumless medium during prolonged organ culture of mouse capstage mandibular first molars. By 21 days in vitro the tooth crown formed and dentine and enamel biomineralization were apparent. At 31 days, explants retained metabolic activity and the enamel matrix showed extensive transformation. Immunologically identified enamel proteins of 26-18 k Da were produced by cultured tooth organs, translocated from tooth explants to the culture medium, recovered from the medium and then compared to control enamel protein from in vivo preparations. Comparable postsecretory processing of the 26-k Da amelogenin protein was observed in vitro and in vivo. We speculate that the pathway reported in the present studies is comparable to the processing of the enamel protein polypeptides of the maturing enamel which occurs in vivo. The in vitro organ culture model described in this report provides an approach with which to investigate the molecular events associated with epithelial-derived postsecretory processing of ECM molecules associated with tissue-specific biomineralization.

Biosynthesis and secretion of enamel proteins in the rat incisor

The synthesis and secretion of enamel proteins (EPs) in rat incisors was examined using cytochemical and biochemical methods. Radioautography after injection of 3H-methionine showed that ameloblasts in the presecretory, secretory, and maturation stages of amelogenesis actively synthesized and secreted proteins. Immunocytochemistry with an antibody to mouse amelogenins revealed the presence of EPs in the protein synthetic and secretory organelles of these cells at all three stages. Labeling was also found in elements of the endosomal/lysosomal compartment. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and silver staining of proteins extracted from enamel and enamel organ showed several protein bands. However, transfer to nitrocellulose paper and immunoblotting revealed that most of the proteins recognized by the antibody were situated between approximately 14 and 32 kDa. EPs were further characterized by using lectins to examine their carbohydrate content. Lectin-gold cytochemistry on sections showed the binding of wheat germ agglutinin and Helix pomatia lectin to secretory stage enamel. Lectin blotting indicated that the amelogenins were heterogeneously glycosylated and contained the sugars N-acetyl-glucosamine/N-acetyl-neuraminic acid and N-acetyl-D-galactosamine. Fluorography at 6 and 10 min and 1 h after injection of 35S-methionine revealed four labeled bands in the main amelogenin group near 22, 28, 30, and 32 kDa. A short-lived protein of approximately 58 kDa was also observed primarily in cells. The appearance of labeled proteins in enamel was paralleled by their disappearance from cells and the intensity of the radiolabeled protein bands, both, in enamel and in cells, decreased towards the maturation stage. These data are consistent with the concept that ameloblasts produce multiple amelogenins throughout amelogenesis.

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.

Distribution of intermediate-filament proteins in the human enamel organ: unusually complex pattern of coexpression of cytokeratin polypeptides and vimentin

We applied immunohistochemical techniques and gel electrophoresis to examine the distribution of intermediate filaments in human fetal oral epithelium and the epithelia of the human enamel organ. Both methods demonstrated that human enamel epithelia contain cytokeratins 5, 14, and 17, which are typical of the basal cells of stratified epithelia, as well as smaller quantities of cytokeratins 7, 8, 19, and in trace amounts 18, which are characteristic components of simple epithelial cells. In the external enamel epithelium and stellate-reticulum cells, most of these components appeared to be simultaneously expressed. In contrast, the parental oral epithelium was negative for cytokeratin 7, thus indicating possible "neoexpression" during the course of tooth formation. Immunohistochemical procedures using various monoclonal antibodies against vimentin revealed the transient coexpression of vimentin and cytokeratins in the external enamel epithelium and in stellate-reticulum cells during enamel development. The significance of the coexpression of cytokeratins and vimentin is discussed in relation to previous findings obtained in other normal tissues and in the light of the functional processes characteristic of these epithelia.

Splice of life: toward understanding genetic determinants of oral diseases

The introduction of recombinant DNA technology has led to a rapid advancement of our knowledge of genes and genomic structure. Such technology, applied to the human genome, has provided valuable information concerning the nature and possible treatment of inherited disorders throughout the human life span. A number of oral diseases can be attacked by use of molecular biological techniques in conjunction with genetic linkage analysis. It will be possible to identify carriers of inherited diseases, to utilize prenatal and antenatal diagnosis when appropriate, and to pursue studies on the mode of inheritance of specific oral diseases. Present and projected advances in this dimension of dental research provide the knowledge and strategies for approaching and solving a number of major problems as we continue to pave the way for improvements in the diagnosis, treatment, and prevention of many oral and craniofacial diseases. This position paper examines a few of the promising areas and suggests future goals toward establishing a knowledge of the genetic determinants for oral diseases.

Human and mouse cementum proteins immunologically related to enamel proteins

SDS-polyacrylamide gel electrophoresis, immunoblot and amino acid composition analyses were applied to human and mouse acellular cementum proteins immunologically related to enamelins and amelogenins. In this analysis, anti-mouse amelogenin, anti-human enamelin and synthetic peptide (e.g., -LPPHPGHPGYIC-) antibodies were shown to cross-react with tooth crown-derived enamelin with a molecular mass of 72,000 Da (72 kDa), amelogenins (26 kDa), and also to four human cementum proteins (72, 58, 50 and 26 kDa) and two mouse cementum proteins (72 and 26 kDa). Each of the antibodies recognized tooth root-derived cementum polypeptides which share one or more epitopes with tooth crown-derived enamel proteins. The molecular mass and isoelectric points for crown-derived and root-derived enamel-related proteins were similar. Analysis of human and mouse cementum proteins revealed a characteristic amino acid composition enriched in glutamyl, serine, glycine, alanine, proline, valine and leucine residues; compared to the major enamel protein amelogenin, cementum proteins were low in proline, histidine and methionine. The human and mouse putative intermediate cementum proteins appear to represent a distinct class of enamel-related proteins. Moreover, these results support the hypothesis that epithelial root sheath epithelia express several cementum proteins immunologically related to canonical enamel proteins.

Positional signalling and patterning for amelogenesis in mouse molar tooth development

Oral mandibular epithelium determines the position and number of tooth organs. Cranial neural crest-derived ectomesenchyme provides inductive signals for (i) tooth shape, and (ii) sequential determination and expression of several different epithelial-derived enamel proteins. Inductive signalling follows a precise sequence, can be expressed in serumless, chemically-defined medium in vitro, and appears to be highly conserved during vertebrate evolution.

Hertwig's epithelial root sheath differentiation and initial cementum and bone formation during long-term organ culture of mouse mandibular first molars using serumless, chemically-defined medium

Studies were designed to test the hypothesis that Hertwig's epithelial root sheath (HERS) synthesizes and secretes enamel-related proteins that participate in the process of acellular cementum formation. Our experimental strategy was to examine sequential root development of the mouse mandibular first molar in vivo and in long-term organ culture in vitro using serumless, chemically-defined medium. Using anti-amelogenin, anti-enamelin and anti-peptide antibodies, enamel-related antigens were localized within intermediate cementum during HERS differentiation and root formation in vivo. Cap stage molars maintained for periods of up to 31 days in organ culture expressed morphogenesis and cytodifferentiation as identified by tooth crown and initial root, cementum and bone formation. Metabolically-labeled HERS products were analyzed by immunodetection using enamel-related antibodies and one- and two-dimensional SDS gel electrophoresis. A 72 kDa and 26 kDa polypeptide were identified in forming mouse cementum. Both of these root putative cementum proteins yield similar (identical) amino acid compositions; however, both proteins differed from the compositions of either mouse crown enamelin or amelogenin proteins. This approach provides a new and novel in vitro model towards understanding HERS differentiation and functions related to root and bone formation. The data support the hypothesis that HERS cells synthesize polypeptides related to but also different from canonical crown enamel proteins.

1987 Kreshover lecture. Gene regulation in the development of oral tissues

The regulatory processes associated with tooth formation are being investigated by the identification of when, where, and how cell adhesion molecules (CAMs), substrate adhesion molecules (SAMs), dentin phosphoprotein, enamel gene products, and intermediate cementum products are expressed during sequential developmental stages of morphogenesis, cytodifferentiation, dentin, enamel and cementum extracellular matrix (ECM) formation, and biomineralization. Instructive and permissive signaling is required for both morphogenesis and cytodifferentiation based upon in vitro organotypic culture studies in serumless, chemically-defined medium. Intrinsic developmental instructions, independent of exogenous growth factors, mediate tooth morphogenesis from the initiation of the dental lamina through crown and initial root development. Recent progress using recombinant DNA methods has advanced descriptions of several dental structural genes. The complete nucleic acid sequence for mouse amelogenin has been defined. This sequence is located on the mouse X chromosome and on the human X and Y chromosomes. This discussion summarizes recent results using experimental embryology, recombinant DNA technology, and immunocytology in the context of instructive epithelial-mesenchymal interactions associated with epithelial differentiation into ameloblasts, ectomesenchyme differentiation into odontoblasts, and dentin and enamel ECM biomineralization. The tooth organ provides opportunities at several levels of biological organization to investigate cellular, molecular, and developmental processes.