Isolation and preliminary characterization of epithelial-specific messenger ribonucleic acids and their products during embryonic tooth development

Alternative splicing of the mouse amelogenin primary RNA transcript contributes to amelogenin heterogeneity

A heterogeneous population of amelogenin proteins is derived from a single copy of the mouse amelogenin gene. To investigate the one gene--multiple protein enigma, we designed a study to distinguish between alternative splicing and proteolytic cleavage models. A pulse of [35S]methionine labeling demonstrated that multiple amelogenins are synthesized concurrently, a result consistent with an alternative splicing mechanism. Using reverse transcription and polymerase chain reaction we cloned a segment from the 5' end of a mouse amelogenin mRNA and connected it to a previously isolated abbreviated cDNA clone. Four additional cDNAs derived from alternatively spliced amelogenin mRNAs have been cloned and characterized. The five transcripts encode amelogenins 180, 156, 141, 74, and 59 amino acids in length.

Characteristics of phosphorylated and non-phosphorylated dentine phosphoprotein

Heterogeneity among the odontoblast-specific, highly phosphorylated acidic protein dentine phosphoprotein (DPP) obtained from different species has been reported by several investigators. In the present study, the apparent molecular-mass variations in rabbit and mouse DPP were investigated. Extracellular matrix (ECM) DPPs were isolated and characterized. Primary gene products, before post-translational phosphorylation, were analysed based upon translation products produced in a rabbit reticulocyte lysate cell-free system using a polyclonal mouse anti-DPP antibody. Nascent non-phosphorylated DPPs were also identified from intracellular protein extracts. Mouse and rabbit ECM phosphoproteins exhibited a 10 kDa difference in size. However, nascent intracellular or translation products from both species showed the same lower molecular mass (approx. 45 kDa). Furthermore, Northern-blot analysis showed a single mRNA of the same size in both species (approx. 1.6 kb) which contains information for a protein no larger than 50 kDa. Our results indicate that the difference in molecular mass (or electrophoretic behaviour) among DPPs from different species is due to post-translational modifications, in this case phosphorylation.

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.

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.

mRNA characterization of human fetal enamel matrix

Preliminary characterization of the human enamel matrix at 16-18 weeks in utero was performed. Using an homogenizing buffer, the proteins were extracted and analysed by gel electrophoresis. Total cellular RNA was isolated and the cell-free mRNA translated. The major component was a 68,000 protein with an enamelin-like amino-acid composition. Other translation products included a 55,000 polypeptide and lower mol. wt components of 26,000, 22,000 and 20,000 size of amelogenin size. It is suggested that high mol. wt component in the enamelin range is the most prevalent at the early stage of human tooth development.

Morphological studies on the distribution of enamel matrix proteins using routine electron microscopy and freeze-fracture replicas in the rat incisor

Enamel contains two categories of biochemically characterized proteins. Amelogenins are dissociated from enamel without physical disruption of the tissue whereas enamelins are obtained only when the crystallites are dissolved. Ultrastructural visualization of these proteins was attempted using routine electron microscopy and freeze-fracture replicas. Fresh, fixed, and 4.0 M guanidine-HCl-extracted samples of enamel from the secretory (young) and maturation (maturing) stages were compared. Decalcified and stained thin sections of fixed enamel revealed intercrystallite particulate material and "crystallite ghosts" which were identical to the crystallites themselves in young enamel and which corresponded to the periphery of the crystallites in maturing enamel. In contrast, 4.0 M guanidine-extracted enamel contained no intercrystallite particulate material but only "crystallite ghosts." Globular particles observed in freeze-fracture replicas of fresh and fixed enamel samples were also removed by 4.0 M guanidine extraction. Incubation of guanidine-extracted enamel with albumin and ovalbumin solutions restored the globular particles. It was concluded that amelogenins are the nonstructural, heterodispersed particulate material in the intercrystallite space. Enamelins constitute the integral template protein which initially provides for elongation of enamel crystallites. They then regulate the continuous growth in width and thickness during maturation and are progressively displaced to the periphery. The illusion that these "protein ghosts" are contained within the crystallite profile can be explained by the parallelepiped shape of the crystallite segment in thin sections.

Isolation of enamelinlike proteins from blue shark (Prionace glauca) enameloid

A sequential dissociative extraction scheme was used to extract proteins from developing Blue Shark enameloid. The first extraction solution (4 M guanidine HC1) solubilized the polypeptides, mainly collagenous, not closely associated with the hydroxyapatite. The next extraction solution (4 M guanidine HC1, 0.5 M ethylenediaminetetraacedic acid (EDTA] solubilized the proteins more closely associated with the tooth mineral component. After extraction, the proteins were separated and isolated with gel electrophoresis. Protein molecular weights were determined and selected proteins were isolated for amino acid composition analysis. The two proteins isolated were tested for mammalian enamel protein antigenic determinants by a "Dot" immunobinding assay. The isolated proteins were enamelinlike by extraction criteria and amino acid composition. Further, the two proteins share antigenic determinants with mammalian enamel proteins.

Enamelin gene expression during fetal and neonatal rabbit tooth organogenesis

Epithelial differentiation is a complex process which requires an integrated synthesis of DNA along with synthesis of a full complement of unique mRNAs and their respective proteins characteristic for each cell type. The time of initial transcription of enamel protein mRNAs and subsequent translation of proteins characteristic for secretory ameloblasts is not known. In order to determine when enamel protein mRNAs appear during New Zealand White rabbit molar tooth organogenesis, and when nascent enamel proteins are first translated, we analyzed early cap stages through late crown stages of molar tooth formation (i.e., 21-days gestation through 2-days postnatal). The biochemical phenotype which characterized rabbit ameloblasts were the acidic glycoproteins termed enamelins. Polyclonal antibodies were produced against the major fetal rabbit enamelin of approximately 70,000 daltons. Immunoprecipitation of enamelins from mRNA-directed translation products in a reticulocyte cell-free system, was used to characterize enamelin mRNAs. Enamelin mRNAs were first detected during bell stages (circa 23-days gestation), and persisted till crown stage (circa 28-days gestation). Indirect immunofluorescent localization of enamelin antigen showed staining over the extracellular enamel organ matrix by 23-days gestation. Neither enamelin mRNAs or polypeptides were detected during early or late cap stages of odontogenesis. Transcription of enamelin mRNAs coding for two enamelins of 65 an 58 kd (kilodaltons) appeared to be closely coupled with the translation of these enamel proteins. We assume that close-range ectomesenchyme- derived instructions mediate the biochemical differentiation of ameloblasts between 21-days and 23-days gestation during fetal rabbit development.

Protein biosynthesis during spiny dogfish (Squalus acanthias) enameloid formation

Little is known about protein biosynthesis during enameloid formation. The purpose of this study was to characterize protein biosynthesis during selachian amelogenesis. Experiments were conducted on intact shark tooth organs, surgically removed from the spiny dogfish, Squalus acanthias. These organs were cultured in medium labelled with [35S]-methionine during time periods ranging from 1 to 6 h in pulse/chase experiments. The proteins were extracted at the conclusion of the experiments for analysis by gel electrophoresis and fluorography. In addition, tooth organs were fixed and examined by light microscopic autoradiography. Through this outlined experimental strategy, protein biosynthesis during amelogenesis was examined. The findings suggest the ameloblasts, at early bell stage of tooth development, are biosynthetically more active than the incipient odontoblasts; and they are secreting two polypeptides, 55 and 43 kd, into the developing tooth extracellular matrix.

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.

Bovine high molecular weight amelogenin proteins

The existence of high molecular weight amelogenins in the matrix of developing fetal bovine dental enamel was investigated. After ion-exchange and gel filtration chromatography, a 40,000 dalton amelogenin protein has been isolated. This species was homogeneous on both guanidine hydrochloride gel filtration and sodium dodecyl sulfate (SDS) gel electrophoresis, but heterogeneous on conventional gel electrophoresis. This high molecular weight amelogenin may be a precursor to the complex of smaller sized amelogenins characteristic of the secretory phase of enamel formation.

Comparative protein biochemistry of developing dental enamel matrix from five mammalian species

The matrix proteins of the developing dental enamel of five mammalian species were isolated and subjected to chromatographic, electrophoretic, and amino acid analyses. It was found that the principal chromatographic fractions showed similarities of both size and amino acid composition among species. The major amelogenin protein of the cow, hamster, human, and sheep was of about 30,000 daltons and of the pig enamel matrix about 20,000 daltons. In each species a higher molecular weight fraction, greater than 40,000 daltons, was detected. In the lower molecular weight range an amelogenin polypeptide enriched in leucine, a fraction rich in tyrosine, and a fraction of intermediate size (Bovine matrix "Component-14") were identified in each case. It is suggested that these characteristic proteins arise during the degradation of the matrix which accompanied mineralization.

A histological and histochemical study of developing teeth in Polypterus (Pisces, Actinopterygii)

The tooth cap matrix contained proteins of ectodermal and mesodermal origin which extended the data already available on teleost teeth and showed that a combined origin for the tooth cap should be regarded as a primitive feature of actinopterygian fish. Furthermore, the dual nature of the Polypterus tooth cap matrix suggested that evolution of tetrapod enamel did not occur within the actinopterygians. The collar tissue was an unmodified dentine, in contrast to its enameloid nature in many other actinopterygian teeth. The presence of a range of developmental states from unmodified dentine to hypermineralized enameloid within Polypterus teeth themselves and also in other parts of the dermal skeleton, as well as the combined origin of the tooth cap, demonstrates that the enamel/enameloid/dentine system forms a continuum of tissues that have diverged from one another by changes in the relative timing of developmental events and matrix production.