Immunochemical and biochemical studies of human enamel proteins during neonatal development

The present communication provides descriptions of the developmental, biochemical, and immunological properties of the human enamel extracellular matrix proteins. We report the isolation and partial characterization of the major human enamel proteins, the production of polyclonal antibodies directed against the human enamelins, and a comparison between the immunogenicity of enamelins and amelogenins from human and mouse enamel extracellular matrices. Our results indicate that although enamelins and amelogenins share some epitopes, each one of these proteins appears to invoke a different degree of immunogenicity.

DNA sequence for cloned cDNA for murine amelogenin reveal the amino acid sequence for enamel-specific protein

Enamel is the unique and highly mineralized extracellular matrix that covers vertebrate teeth. Amelogenin proteins represent the predominate subfamily of gene products found in developing mammalian enamel, and are implicated in the regulation of the formation of the largest hydroxyapatite crystals in the vertebrate body. Previous attempts to isolate, purify and characterize amelogenins extracted from developing matrix have proven difficult. We now have determined the DNA sequence for a cDNA for the 26-kDa class of murine amelogenin and deduced its corresponding amino acid sequence. The murine amino acid sequence is homologous to bovine or porcine amelogenins extracted from developing enamel matrices. However, an additional 10-residues were found at the carboxy terminus of the murine amelogenin. This is the most complete sequence database for amelogenin peptides and the only DNA sequence for enamel specific genes.

De novo gene expression detected by amelogenin gene transcript analysis

Reciprocal epithelial-mesenchymal interactions are responsible for mouse molar tooth organogenesis. Only dental ectomesenchymal cells are capable of instructing adjacent epithelial cells to become determined to synthesize and secrete enamel-specific proteins termed the amelogenins. To identify when inner enamel epithelial cells first express enamel specific gene products, cytoplasmic RNA has been analyzed from developing teeth by hybridization to a cloned cDNA probe to one of the amelogenins. It is reported that the de novo expression of amelogenin-encoding RNA as well as immunoprecipitated amelogenin polypeptides are first detected at Theiler stage 27. These data indicate that ectomesenchymal-mediated induction of inner enamel organ epithelia results in both the nascent transcription of amelogenin RNA and subsequent translation of amelogenin polypeptides, which are first detected at birth.

Concepts of epithelial-mesenchymal interactions during development: tooth and lung organogenesis

One of the major problems in developmental biology concerns how differential gene activity is regionally controlled. One approach to this problem is the use of mesenchyme specification of epithelial-specific gene expression, such as, during tooth morphogenesis or lung morphogenesis. In the example of tooth morphogenesis, dental papilla ectomesenchyme induces de novo gene expression as assayed by detection of amelogenin transcripts, or immunodetection of amelogenin polypeptides within ameloblast cells. This process does not require serum supplementation or exogenous factors during epithelial-mesenchymal interactions in vitro. In contrast, lung morphogenesis requires hormones to mediate mesenchyme-derived influences upon type II epithelial cell differentiation and the production of pulmonary surfactant (eg, neutral and phospholipids, surfactant proteins). Glucocorticoids are required to stimulate the release of fetal pneumonocyte factor (FPF) from fibroblasts which, in turn, enhance the production of pulmonary surfactant. Thyroxin appears to regulate the relative responsiveness of progenitor type II cells to steroid-stimulated release of FPF. This review will highlight key concepts associated with these developing organ systems and emphasize the problem of regional controls which regulate epithelial cell-specific gene activity.

Construction and identification of mouse amelogenin cDNA clones

The determination of the biochemical phenotype of tooth epithelium requires specification by the dental mesenchyme. This is a general feature of epithelial-mesenchymal interaction in a number of different epidermal organ systems (e.g., salivary gland, mammary gland, feather, skin, and hair morphogenesis). To investigate these developmental processes, we have identified a cDNA clone representing the major group of gene products associated with enamel extracellular matrix formation. The mRNAs for mouse amelogenins, representing approximately equal to 90% of the total enamel proteins, have been isolated and partially characterized by specific immunoprecipitation. The poly(A)-containing RNAs were used for the synthesis and cloning of the mouse amelogenin cDNA. Recombinant plasmids containing amelogenin cDNA sequences were identified by differential hybridization, hybrid-selected translation, and blot hybridization analyses. A cloned sequence was used to identify the expression of amelogenins during tooth development. The mouse cDNA sequence hybridized to genomic mouse and human DNAs. This amelogenin cDNA probe now enables molecular investigations of a number of classical problems in developmental biology.

Bromodeoxyuridine-DNA interactions associated with arrest of rat odontogenesis in vitro

To better characterize the molecular mechanism responsible for the bromodeoxyuridine (BrdU)-mediated arrest of mammalian odontogenesis in vitro, the nature of nuclear DNA-analogue interactions was determined. Bioactive doses of the radiolabelled analogue were added to tissue culture medium of 16-day old embryonic rat incisor primordia. Control rudiments were similarly exposed to equimolar, radiolabelled thymidine. After 16-18 h, DNA was isolated and purified from the labelled organ cultures. Following sedimentation to equilibrium through neutral CsCl density gradients, [3H]-BrdU-labelled DNA revealed a buoyant density indicative of a 12-15 per cent level of substitution in place of thymidine. Furthermore, similar centrifugation of DNA through alkaline density gradients suggested that the substitution was localized predominantly within a single-strand. DNA-DNA reassociation kinetics subsequently revealed that disproportionately more radiolabelled BrdU was concentrated within repetitive DNA nucleotide sequences in contrast to the more random distribution of [3H]-thymidine moieties. Thus it is likely that BrdU exerts its inhibitory effects on odontogenic differentiation through a relatively small proportion of rat embryo nuclear DNA.