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.

Human developing enamel proteins exhibit a sex-linked dimorphism

The amelogenin protein of developing dental enamel is generally accepted to mediate the regulation of the form and size of the hydroxyapatite crystallites during enamel biomineralization (1). A genetic disorder of enamel development (amelogenesis imperfecta) has been linked to the amelogenin gene AMEL(2-3), and loci regulating enamel thickness and tooth size have been mapped to the human sex chromosomes (4). In the human genome there are two AMEL loci with one copy of the gene on each of the sex chromosomes (AMELX and AMELY), whereas in the mouse only an AMELX locus is present (5). It is presently unknown if human AMELY is transcriptionally active. These observations prompted us to examine specimens of human developing enamel for sexual dimorphism at the protein level. We report here, for the first time, a diagnosis of differences in human enamel proteins which permits the distinction of specimens according to the sex of the individual.

Regulated expression of a cell division control-related protein kinase during development

Protein kinases are important signaling molecules that are known constituents of cellular pathways critical for normal cellular growth and development. We have recently identified a new protein kinase, p58, which contains a large domain that is highly homologous to the cell division control p34cdc2 protein kinase. This new cell division control-related protein kinase was originally identified as a component of semipurified galactosyltransferase; thus, it has been denoted galactosyltransferase-associated protein kinase. In vitro, this protein kinase has been shown to phosphorylate a number of substrates, including histone H1, casein, and galactosyltransferase. In vivo, we have found that this protein kinase affects galactosyltransferase enzyme activity and that it is apparently involved in some aspect of normal cell cycle regulation. In this report, we find that the p58 gene is evolutionarily well conserved and expressed ubiquitously, but to varying extents, in adult tissues. In developmentally staged embryos, p58 expression was elevated early in embryogenesis and then decreased dramatically. In the murine submandibular gland, p58 expression was elevated between day 14 and day 16 post coitus. Expression in the submandibular gland appeared to parallel the proliferation and differentiation of specific cell types as judged by in situ hybridization. These studies indicate that the p58 protein kinase may have a critical function during normal embryonic development and that this protein kinase continues to be expressed in differentiated adult tissues.

Cells from Hertwig's epithelial root sheath do not transcribe amelogenin

Recent experimental evidence has led to the interpretation that "enamel-like" material is deposited along the forming mouse molar root surface by cells of Hertwig's epithelial root sheath (HERS cells) and that this material is integral to the developmental program for cementogenesis. The experimental strategy described in this study was to examine selected developmental stages of root formation for mouse first and second mandibular molars in order to localize the cellular sites of amelogenin gene transcripts using high resolution in situ hybridization. Amelogenin is the major structural protein of coronal enamel and is highly conserved among mammalian species at the DNA and amino acid sequence level. Within the limits of sensitivity for in situ hybridization and utilizing either cRNAs or oligodeoxynucleotide probes, we were unable to localize amelogenin transcripts within HERS cells from selected developmental stages associated with mouse molar root formation. In contrast, previous studies using antipeptide antibodies have provided immuno-histochemical localization of amelogenin domains in HERS cell-derived products. For these HERS cell-derived proteins to contain both amelogenin epitopes and yet fail to yield nucleic acid hybridization signals suggests that either gene rearrangement and/or alternative processing of messenger RNAs from the structural gene locus operate to produce immunologically related motifs sharing insufficient complementarity at the nucleotide level to permit efficient detection by hybridization. It is postulated that HERS cells synthesize proteins which contain amelogenin domains and that these proteins participate during cementogenesis. However, these enamel-related proteins are neither identical to, nor collinear with coronal canonical amelogenin transcripts.

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.

Co-localization of EGF transcripts and peptides by combined immunohistochemistry and in situ hybridization

There is increasing evidence that autocrine- and paracrine-acting growth factors participate in cell and tissue development, maintenance, and renewal. Recent advances in histochemical techniques have facilitated the localization of growth factor messenger RNAs or polypeptides in tissue sections. However, the spatial relationships between the sites of growth factor transcription, translation, and post-translational processing to functional bioactive peptides have been difficult to correlate because each method of detection requires separate tissue sections. We undertook the simultaneous detection of epidermal growth factor (EGF) transcripts and EGF epitopes by combining immunohistochemistry methods with in situ hybridization. Adult mouse submandibular gland was chosen as a representative model because it contains sites of EGF biosynthesis which may participate in mediating the development, maintenance, and renewal of the organ through autocrine or paracrine mechanism(s). Granular duct (GD) cells demonstrated the presence of both EGF transcripts and EGF peptides. In contrast, the interstitial cells lying adjacent to glandular epithelium also contained relatively high levels of EGF transcripts, although no mature EGF peptides were detected. The experimental approach of co-localization and the resulting data indicate previously unreported sites of EGF transcription in glandular interstitial cells, which may provide molecular information required for the morphogenesis and differentiation of adjacent glandular epithelium.

Isolation and partial characterization of a human amelogenin from a single fetal dentition using HPLC techniques

A strategy based on high pressure liquid chromatography (HPLC) techniques for the isolation or a principal amelogenin molecule from a single human dentition is described. A partial sequence (33 residues) for this 24 kDa amelogenin is presented and related to earlier studies of human 5 kDa tyrosine-rich amelogenin polypeptides (TRAPs). A failure to identify amino acid residue #25 (tryptophan in other amelogenins) suggests that this 24 kDa amelogenin is the progenitor of the human TRAP-2 molecule and provides further support for the possibility of several human amelogenin gene products, generated by splice-junction selection, from the single amelogenin gene in the same individual. Alternatively, multiple amelogenins may arise by expression of both the AMELX and AMELY loci.

Analysis of human enamel genes: insights into genetic disorders of enamel

A number of inherited craniofacial diseases are known to be associated with gene mutations. Inherited genetic disorders of enamel formation called amelogenesis imperfecta (AI) affect the human population with a prevalence of 1 in 14,000 in the United States. Amelogenins, the major proteins in developing enamel matrix of mammalian teeth, have been suggested to participate in normal enamel matrix biomineralization, as well as with abnormal biomineralization such as seen in AI. The complementary DNA for mouse amelogenin gene (AMEL) has been cloned, characterized, and used as a probe to establish the chromosomal locations of AMEL for mouse and man. The human AMEL gene sequences have been located to the distal short arm p22.1----p22.3 region of the X chromosome, and the pericentromeric region of the Y chromosome. An assignment of human AMEL gene to the X chromosome p22 region together with a recent assignment of the X-linked AI disease locus to the Xp22.2 region support the association of the AMEL-X gene with AI. This also leads us to propose that a mutated AMEL-X gene produces altered amelogenin polypeptide, which is defective in its ability to participate in mineralization of enamel matrix, thus giving rise to the X-linked phenotypes of AI.

Cartilage, bone and tooth induction during early embryonic mouse mandibular morphogenesis using serumless, chemically-defined medium

Studies were designed to test the hypothesis that plasma- and serum-deprived embryonic cells and tissues in vitro are capable of producing growth regulating factors which augment cartilage, bone and tooth induction during mouse mandibular process development. Embryonic mouse first branchial arch-derived mandibular processes (E11-E12, Theiler stages 18-19) or cap stage molar tooth (M1) organs (E15-E16, Theiler stage 23) expressed morphogenesis, histogenesis and cytodifferentiation (e.g., Meckel's cartilage and mandibular bone) when cultured as explants in permissive serumless and chemically-defined BGJB medium for periods up to 31 days in vitro. Organ cultures of early mandibular process explants in serumless conditions showed DNA synthesis comparable to the time- and position-restricted patterns characteristic for control in vivo development. As a paradigm for embryonic cell expression of putative growth factors, sense and antisense oligodeoxynucleotide probes corresponding to amino acids 1070-1081 for preproEGF, and antibodies directed against amino acids 348-691 of preproEGF, were used to identify and localize mRNA transcripts and translation products. Our preliminary evidence suggests that odontogenic epithelial and ectomesenchyme cells produce EGF-like products during instructive phases of tooth development. We suggest that plasma- and serum-deprived cells and tissues in vitro produce autocrine and/or paracrine growth factors which mediate embryonic mandibular morphogenesis, histogenesis and cytodifferentiation.

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.

Human amelogenins: sequences of "TRAP" molecules

The extracellular protein matrix of developing enamel includes a major class of proteins, the amelogenins, which are believed to be concerned in regulating enamel biomineralization. Previous studies have shown the amelogenins of the extracellular matrix to be a complex of proline-rich hydrophobic proteins which, it is suggested, arise through posttranslational and postsecretory processing of a primary ameloblast gene product. More recently, it has been shown that the human amelogenin gene is located on both the X and Y chromosomes raising the possibility that polymorphism at the level of the gene may also contribute to the observed complexity of these enamel matrix proteins. To investigate such possible amelogenin polymorphism in developing human dental enamel, individual fractionated by size-exclusion and reversed-phase high pressure liquid chromatography (HPLC). Two tyrosine-rich amelogenin polypeptides (TRAPs) of approximately 5 kDa in size were isolated from an individual human dentition and characterized by automated gas-phase sequencing. These polypeptides were found to be of 42 (TRAP-2) and 44 (TRAP-1) amino acid residues in length; TRAP-2 lacked a carboxy-terminal -Gly-Trp sequence as has previously been described for analogous bovine TRAP molecules. However, residue #25 of the human TRAP-2 sequence was refractory to sequencing, apparently differing from the Trp-25 identified in TRAP-1. These findings suggest (1) two forms of TRAP molecules, differing only by cleavage of a carboxy-terminal dipeptide, are a general feature of human and other mammalian enamel proteins, probably being derived by postsecretory cleavage from the primary extracellular amelogenin; and (2) in human developing enamel four forms of TRAPs may arise either from polymorphism at the level of the gene, or by posttranscriptional alternative splicing of amelogenin mRNAs, coupled with specific post-secretory proteolytic processing.

Localization of epidermal growth factor precursor in tooth and lung during embryonic mouse development

The murine epidermal growth factor (EGF) precursor is a 1217 amino acid protein which contains mature EGF (amino acid residues 977-1029) as well as eight EGF-like repeats. Although the highest levels of EGF are found in the adult male mouse submandibular gland, the results of in situ hybridization studies and mRNA analyses suggest that EGF precursor mRNA is synthesized in several adult mouse tissues including the lung and the incisor. To determine if EGF precursor gene expression is intrinsic to the developmental program for either embryonic tooth or lung organogenesis, sense and antisense oligodeoxyribonucleotide probes corresponding to amino acids 1070-1081 of the precursor were used to localize cellular sites of synthesis of EGF precursor mRNA by in situ hybridization. Antibodies directed against amino acid residues 348-691 of the precursor were used in immunodetection techniques to identify either EGF precursor protein or processed derivatives. In contrast to earlier reports indicating that embryonic mouse tissues do not synthesize EGF precursor mRNA, we found that EGF precursor mRNA is present in clusters of ectoderm-, mesoderm-, and ectomesenchyme-derived cells associated with embryonic teeth and lung organs. Moreover, epitopes common to the EGF precursor were immunolocalized in both the epithelial and mesenchymal tissues of embryonic mouse tooth and lung organs. These results suggest that the EGF precursor and/or motifs contained within the precursor molecule, including mature EGF, may play an instructive or permissive role in epithelial-mesenchymal interactions pursuant to organogenesis.

Amelogenin gene expression in mouse incisor heterotopic recombinations

Mouse incisor tooth organs express the genes responsible for enamel extracellular matrix formation exclusively on the labial surface of the organ. A previous investigation has suggested that lingual inner dental epithelium of mouse incisor did not contain potential ameloblasts. The present work extends our histological observations, by analyzing the presence of mouse amelogenin mRNA in heterotopic mouse incisor tissue recombinations using in situ hybridization to 35S-labelled asymmetric complementary RNA probes from a cDNA specific to the mouse Mr 26 x 10(3) amelogenin. Labial polarized ameloblasts located in front of the lingual predentin layer were associated with numberous hybridization signals indicating an increased density of amelogenin mRNA. In contrast, the lingual inner dental epithelium in contact with labial predentin never showed amelogenin hybridization signals, indicating the absence of amelogenin transcripts. These results confirm our observation that the lingual inner dental epithelium does not contain potential ameloblasts, since these cells do not transcribe nor accumulate mRNA amelogenin even when in contact with a favorable microenvironment provided by the labial predentin.

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.

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.

Human and mouse amelogenin gene loci are on the sex chromosomes

Enamel is the outermost covering of teeth and is the hardest tissue in the vertebrate body. The enamel matrix is composed of enamelin and amelogenin classes of protein. We have determined the chromosomal locations for the human and mouse amelogenin (AMEL) loci using Southern blot analyses of DNA from human, mouse, or somatic cell hybrids by hybridization to a characterized mouse amelogenin cDNA. We have determined that human AMEL sequences are located on the distal short arm of the X chromosome in the p22.1----p22.3 region and near the centromere on the Y chromosome, possibly at the proximal long arm (Yq11) region. These chromosomal assignments are consistent with the hypothesis that perturbation of the amelogenin gene is involved in X-linked types of amelogenesis imperfecta, as well as with the Y-chromosomal locations for genes that participate in regulating tooth size and shape. Unlike the locus in humans, the mouse AMEL locus appears to be assigned solely to the X chromosome. Finally, together with the data on other X and Y chromosome sequences, these data for AMEL mapping support the notion of a pericentric inversion occurring in the human Y chromosome during primate 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.

In situ hybridization analysis of keratin gene expression in human ameloblastomas

Complementary DNA (cDNA) clones corresponding to the 55 kDa (K 14) and 59 kDa (K 10) keratins were used as probes for in situ hybridization analysis for the expression of keratin genes in human ameloblastomas and in oral mucosa. Transcripts for either the K 14 keratin or the K 10 keratin were restricted in their spatial distribution within stratified epithelia consistent with the stage of differentiation of the keratinocyte: the K 14 keratin gene transcript was restricted to the basal cell layers of the mucosa, while the K 10 keratin transcript was expressed predominantly in suprabasal cells, within the granular and prickle layers. In contrast, only the K 14 keratin transcript could be identified within the epithelial cells of human ameloblastomas. The differentiation-specific keratin transcript (K 10) was not present at detectable levels in this type of odontogenic tumors. In an atypical, infiltrating ameloblastoma, neither the K 10 nor the K 14 transcript could be identified. Granular cells within one ameloblastoma expressed the K 14 transcript. A detailed examination of the pattern of gene expression in these unique tumors may lead to a better understanding of their pathogenesis.

Spatial- and temporal-restricted pattern for amelogenin gene expression during mouse molar tooth organogenesis

Position- and time-restricted amelogenin gene transcription was analysed in developing tooth organs using in situ hybridization with asymmetric complementary RNA probes produced from a cDNA specific to the mouse 26 × 10(3) Mr amelogenin. In situ analysis was performed on developmentally staged fetal and neonatal mouse mandibular first (M1) and maxillary first (M1) molar tooth organs using serial sections and three-dimensional reconstruction. Amelogenin mRNA was first detected in a cluster of ameloblasts along one cusp of the M1 molar at the newborn stage of development. In subsequent developmental stages, amelogenin transcripts were detected within foci of ameloblasts lining each of the five cusps comprising the molar crown form. The number of amelogenin transcripts appeared to be position-dependent, being more abundant on one cusp surface while reduced along the opposite surface. Amelogenin gene transcription was found to be bilaterally symmetric between the developing right and left M1 molars, and complementary between the M1 and M1 developing molars; indicating position-restricted gene expression resulting in organ stereoisomerism. The application of in situ hybridization to forming tooth organ geometry provides a novel strategy to define epithelial-mesenchymal signal(s) which are believed to be responsible for organ morphogenesis, as well as for temporal- and spatial-restricted tissue-specific expression of enamel extracellular matrix.

Biosynthesis and characterization of rabbit tooth enamel extracellular-matrix proteins

Tooth enamel biomineralization is mediated by enamel proteins synthesized by ameloblast cells. Two classes of proteins have been described: enamelins and amelogenins. In lower vertebrates the absence of amelogenins is believed to give rise to aprismatic enamel; however, rabbit teeth, which apparently do not synthesize amelogenins, form prismatic enamel. The present study was designed to characterize the enamel proteins present in rabbit tooth organs and to gain an insight into the process of biomineralization. Rabbit enamel extracellular-matrix proteins were isolated and characterized during sequential stages of rabbit tooth organogenesis. The biosynthesis of enamel proteins was analysed by metabolic 'pulse-chase' experiments as well as mRNA-translation studies in cell-free systems. Our results indicated that rabbit enamel extracellular matrix contains 'amelogenin-like' proteins. However, these proteins are not synthesized as typical amelogenins, as in other mammalian species, thus suggesting that they are the processing products of higher-molecular-mass precursors. An N-terminal amino acid sequence of 29 residues, considered characteristic of mammalian amelogenins, was present in the rabbit 'amelogenin-like' proteins. By using anti-peptide antibodies to this region, similar epitopes were detected in all nascent enamel proteins, including enamelins. These studies suggest that the N-terminal sequence might be characteristic of all enamel proteins, not only amelogenins.

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

We have established the time and position of expression for multiple enamel proteins during the development of the mouse molar tooth organ. Using high-resolution two-dimensional gel electrophoresis coupled with immunoblotting and immunocytochemistry, a 46-kDa enamel protein (pI, 5.5) was detected during late cap stage (18-days gestation, E18d) within differentiation-zone-II inner enamel epithelia associated with an intact basal lamina. At E19d a second enamel polypeptide of 72 kDa (pI, 5.8) was identified at the time and position of initial biomineralization in differentiation zone V. At 20 days, differentiation-zone-VI ameloblasts without basal lamina (late bell stage) expressed 46- and 72-kDa enamel proteins and, in addition, expressed a relatively more basic 26-kDa enamel protein (pI, 6.5-6.7); detected after initial formation of calcium hydroxyapatite crystals. Antibodies raised against chemically synthesized enamel peptides cross-reacted with both the 72-kDa and 26-kDa polypeptides, but did not cross-react with the 46-kDa enamel polypeptide. The sequential expression of multiple enamel proteins suggests several functions: (a) the anionic enamel proteins may provide an instructive template for calcium hydroxyapatite crystal formation; (b) the more neutral proteins possibly serve to regulate size, shape and rates of enamel crystal formation. We suggest that initial expression of enamel gene products during mouse tooth development possibly recapitulates ancestral features of amelogenesis documented in prereptilian vertebrates. These results imply that multiple instructive signals may be responsible for mammalian enamel protein induction and that the sequential expression of a family of enamel proteins reflects the evolutionary acquisition of a more complex genetic program for amelogenesis.

Molecular determinants of cranial neural crest-derived odontogenic ectomesenchyme during dentinogenesis

Positional information on tooth morphogenesis is investigated by the identification of when and where phenotypic markers are expressed during odontogenesis. This temporal and positional information is correlated with the instructive and permissive signaling required for both dentinogenesis and amelogenesis. Of particular interest is the establishment of a map for the cranial neural crest-derived dental papilla ectomesenchyme and the odontoblast cell lineages. The expression of ectomesenchyme-derived cytotactin, dentin phosphoprotein, and epithelial-derived enamel proteins was studied in mice using embryonic, fetal, and postnatal mandibular first molar tooth organ development. This review summarizes the observations in the context of instructive epithelial-mesenchymal interactions and suggests that amelogenesis imperfecta and dentinogenesis imperfecta may in part be explained by alterations in these differentiation markers. Recombinant DNA methods should facilitate future investigations of these inherited dental disorders.

Examining the possible molecular origins for enamel protein complexity

Our strategy was to examine each of the three loci capable of contributing to the observed complexity 0 of mouse amelogenin proteins recovered from forming enamel: the genome (gene); the transcription apparatus (messenger RNA); and the translation apparatus (proteins). Our approach was based on recombinant DNA technology and a complementary DNA (cDNA) clone, pMa5-5, specific to the predominant mouse amelogenin protein. An "artificial ameloblast" was engineered based on pMa5-5 and the resulting synthetic products compared to those from authentic ameloblasts. First, the genome probably is not responsible for amelogenin complexity: Southern analysis indicates that the amelogenin gene exists as a single copy in either differentiated dental tissue or germ line tissue. Thus, ectomesenchymal-derived instructive signals for ameloblast differentiation do not lead to re-arrangement or amplification of the amelogenin gene. Next, using nucleic acid hybridization techniques, we examined messenger RNA from mouse ameloblasts. Northern analysis of authentic mRNA from mouse ameloblasts, with either the intact or 3'-end of pMa 5-5 used as the reporter molecule, indicates that only one size class of mRNA was detectable. We conclude that at the sensitivity of this assay there is no evidence for multiple mRNAs. Last, "artificial ameloblasts" were engineered so that the translation apparatus could be examined as a source of amelogenin complexity. Capped, artificial mRNAs were constructed to the pMa 5-5 template and used to program the synthesis of amelogenin polypeptides by translation in a cell-free system. When the resulting total translation products were immunoprecipitated with the rabbit anti-mouse amelogenin antibody, we observed multiple polypeptides, suggesting that the utilization of alternative start sites may also contribute to the observed complexity of amelogenin proteins, at least for artificial mRNAs translated in vitro.

Amelogenin antigenic domain defined by clonal epitope selection

To experimentally examine the participation of amelogenins in controlled mineral-phase maturation of mammalian enamel, the identification of the individual proteins and their corresponding gene(s) is required. For this purpose, cDNAs were constructed from polyadenylated RNA from 2-day postnatal murine teeth, molecularly cloned into lambda-gt11 expression vectors and transfected into E. coli. The cDNA library was screened for amelogenin gene(s) by using either antibody or nucleic acid probes. An amelogenin cDNA clone encoding 79 carboxy-terminal amino acid residues and 100 nucleotides of the 3' noncoding sequence was demonstrated to contain a major antigenic site for amelogenin protein by immunostaining of specific amelogenin proteins from total extracted enamel protein blots using clonal epitope selected antibody. This is the first report linking amelogenin epitope(s) to a defined DNA sequence, and consequently a defined portion of the amino acid sequence for amelogenins. Secondary structure analysis, based on the relative average linear hydropathy of the amino acid sequence of amelogenin, predicted epitopes in the amino terminus of the molecule rather than the carboxy terminus. Our present data suggest that the carboxy terminus of the amelogenins is sufficiently externalized to be an antigenic domain. These data may be useful in subsequent structural analysis of amelogenin proteins and enhancing our understanding of their physicochemical participation in biomineralization.