Analysis of the regulatory region of the bovine X-chromosomal amelogenin gene

Amelogenin Gene Expression in Porcine Odontoblasts

Amelogenin is the major organic component in the enamel matrix of developing teeth and plays an important role in enamel biomineralization. Amelogenin has been reported to be a specific secretory product of ameloblasts. In this study, we examined amelogenin gene expression in various cell layers prepared from a porcine permanent tooth germ using reverse transcription-polymerase chain-reaction (RT-PCR). Amelogenin amplification products were detected only in the secretory ameloblast layer after 20 cycles of PCR. After 30 cycles of PCR, amelogenin amplification products were detected in secretory and maturation-stage ameloblasts and in odontoblasts. The relative levels of amelogenin gene expression in secretory and maturation-stage ameloblasts and odontoblasts were determined. Secretory ameloblasts expressed over 1000 times the level of amelogenin mRNA found in odontoblasts. Amelogenin gene expression in odontoblasts was confirmed in an erupted porcine permanent first molar, which has no ameloblasts. Amelogenin PCR amplification products were identified from 4 different alternatively spliced transcripts in the ameloblast samples, and the same spliced forms were detected in the odontoblast samples.

Distribution of the amelogenin protein in developing, injured and carious human teeth

Amelogenin is the major enamel matrix protein with key roles in amelogenesis. Although for many decades amelogenin was considered to be exclusively expressed by ameloblasts, more recent studies have shown that amelogenin is also expressed in other dental and no-dental cells. However, amelogenin expression in human tissues remains unclear. Here, we show that amelogenin protein is not only expressed during human embryonic development but also in pathological conditions such as carious lesions and injuries after dental cavity preparation. In developing embryonic teeth, amelogenin stage-specific expression is found in all dental epithelia cell populations but with different intensities. In the different layers of enamel matrix, waves of positive vs. negative immunostaining for amelogenin are detected suggesting that the secretion of amelogenin protein is orchestrated by a biological clock. Amelogenin is also expressed transiently in differentiating odontoblasts during predentin formation, but was absent in mature functional odontoblasts. In intact adult teeth, amelogenin was not present in dental pulp, odontoblasts, and dentin. However, in injured and carious adult human teeth amelogenin is strongly re-expressed in newly differentiated odontoblasts and is distributed in the dentinal tubuli under the lesion site. In an in vitro culture system, amelogenin is expressed preferentially in human dental pulp cells that start differentiating into odontoblast-like cells and form mineralization nodules. These data suggest that amelogenin plays important roles not only during cytodifferentiation, but also during tooth repair processes in humans.

Regulation of dental enamel shape and hardness

Epithelial-mesenchymal interactions guide tooth development through its early stages and establish the morphology of the dentin surface upon which enamel will be deposited. Starting with the onset of amelogenesis beneath the future cusp tips, the shape of the enamel layer covering the crown is determined by five growth parameters: the (1) appositional growth rate, (2) duration of appositional growth (at the cusp tip), (3) ameloblast extension rate, (4) duration of ameloblast extension, and (5) spreading rate of appositional termination. Appositional growth occurs at a mineralization front along the ameloblast distal membrane in which amorphous calcium phosphate (ACP) ribbons form and lengthen. The ACP ribbons convert into hydroxyapatite crystallites as the ribbons elongate. Appositional growth involves a secretory cycle that is reflected in a series of incremental lines. A potentially important function of enamel proteins is to ensure alignment of successive mineral increments on the tips of enamel ribbons deposited in the previous cycle, causing the crystallites to lengthen with each cycle. Enamel hardens in a maturation process that involves mineral deposition onto the sides of existing crystallites until they interlock with adjacent crystallites. Neutralization of acidity generated by hydroxyapatite formation is a key part of the mechanism. Here we review the growth parameters that determine the shape of the enamel crown as well as the mechanisms of enamel appositional growth and maturation.

Amelogenin, a major structural protein in mineralizing enamel, is also expressed in soft tissues: brain and cells of the hematopoietic system

The amelogenin protein is considered as the major molecular marker of developing and mineralizing ectodermal enamel. It regulates the shape, size, and direction of growth of the enamel mineral crystallite. Recent data suggest other roles for amelogenin beyond regulation of enamel mineral crystal growth. The present study describes our recent discovery of amelogenin expression in soft tissues: in brain and in cells of the hematopoietic system, such as macrophages, megakaryocytes and in some of the hematopoietic stem cells. Reverse transcription-polymerase chain reaction (RT-PCR) followed by cDNA sequencing revealed, in mouse brain, two amelogenin mRNA isoforms: the full-length amelogenin including exon 4, and the isoform lacking exon 4. Immunohistochemistry revealed amelogenin expression in brain glial cells. Mouse macrophages were found to express the full-length amelogenin sequence lacking exon 4. Confocal microscopy revealed colocalization of amelogenin and CD41 (a megakaryocyte marker), as well as amelogenin and CD34 (a hematopoietic stem cell marker) in some of the bone marrow cells. The expression of amelogenin, a major structural protein of the mineralizing extracellular enamel matrix, also in cells of non-mineralizing soft tissues, suggests that amelogenin is multifunctional. Several different potential functions of amelogenin are discussed.

Protein–Protein Interactions of the Developing Enamel Matrix

Extracellular matrix proteins control the formation of the inorganic component of hard tissues including bone, dentin, and enamel. The structural proteins expressed primarily in the enamel matrix are amelogenin, ameloblastin, enamelin, and amelotin. Other proteins, like biglycan, are also present in the enamel matrix as well as in other mineralizing and nonmineralizing tissues of mammals. In addition, the presence of sulfated enamel proteins, and "tuft" proteins has been examined and discussed in relation to enamel formation. The structural proteins of the enamel matrix must have specific protein-protein interactions to produce a matrix capable of directing the highly ordered structure of the enamel crystallites. Protein-protein interactions are also likely to occur between the secreted enamel proteins and the plasma membrane of the enamel producing cells, the ameloblasts. Such protein-protein interactions are hypothesized to influence the secretion of enamel proteins, establish short-term order of the forming matrix, and to mediate feedback signals to the transcriptional machinery of these cells. Membrane-bound proteins identified in ameloblasts, and which interact with the structural enamel proteins, include Cd63 (cluster of differentiation 63 antigen), annexin A2 (Anxa2), and lysosomal-associated glycoprotein 1 (Lamp1). These and related data help explain the molecular and cellular mechanisms responsible for the removal of the organic enamel matrix during the events of enamel mineralization, and how the enamel matrix influences its own fate through signaling initiated at the cell surface. The knowledge gained from enamel developmental studies may lead to better dental and nondental materials, or materials inspired by Nature. These data will be critical to scientists, engineers, and dentists in their pursuits to regenerate an entire tooth. For tooth regeneration to become a reality, the protein-protein interactions involving the key dental proteins must be identified and understood. The scope of this review is to discuss the current understanding of protein-protein interactions of the developing enamel matrix, and relate this knowledge to enamel biomineralization.

High yield of biologically active recombinant human amelogenin using the baculovirus expression system

The amelogenins are secreted by the ameloblast cells of developing teeth; they constitute about 90% of the enamel matrix proteins and play an important role in enamel biomineralization. Recent evidence suggests that amelogenin may also be involved in the regeneration of the periodontal tissues and that different isoforms may have cell-signalling effects. During enamel development and mineralization, the amelogenins are lost from the tissue due to sequential degradation by specific proteases, making isolation of substantial purified quantities of full-length amelogenin challenging. The aim of the present study was to express and characterize a recombinant human amelogenin protein in the eukaryotic baculovirus system in quantities sufficient for structural and functional studies. Human cDNA coding for a 175 amino acid amelogenin protein was subcloned into the pFastBac HTb vector (Invitrogen), this system adds a hexa-histidine tag and an rTEV protease cleavage site to the amino terminus of the expressed protein, enabling effective one-step purification by Ni2+-NTA affinity chromatography. The recombinant protein was expressed in Spodoptera frugiperda (Sf9) insect cells and the yield of purified his-tagged human amelogenin (rHAM+) was up to 10 mg/L culture. Recombinant human amelogenin (rHAM+) was characterized by SDS-PAGE, Western blot, ESI-TOF spectrometry, peptide mapping, and MS/MS sequencing. Production of significant amounts of pure, full-length amelogenin opened up the possibility to investigate novel functions of amelogenin. Our recent in vivo regeneration studies reveal that the rHAM+ alone could bring about regeneration of the periodontal tissues; cementum, periodontal ligament, and bone.

Tooth developmental biology: disruptions to enamel-matrix assembly and its impact on biomineralization

Dental enamel is a composite bioceramic material that is the hardest tissue in the vertebrate body, containing long, thin crystallites of substituted hydroxyapatite (HAP). Over a lifetime of an organism, enamel functions under repeated and immense loads, generally without catastrophic failure. Enamel is a product of ectoderm-derived cells called ameloblasts. Recent investigations on the formation of enamel using cell and molecular approaches are now being coupled to biomechanical investigations at the nanoscale and mesoscale levels. For amelogenin, the principal structural protein for forming enamel, we have identified two domains that are required for its proper self-assembly into supramolecular structures referred to as nanospheres. Nanospheres are believed to control HAP crystal habit. Other structural proteins of the enamel matrix include ameloblastin and enamelin, but little is known about their biological importance. Transgenic animals have been prepared to investigate the effect of overexpression of wild-type or mutated enamel proteins on the developing enamel matrix. Amelogenin transgenes were engineered to contain deletions to either of the two self-assembly domains and these alterations produced significant defects in the enamel. Additional transgenic animal lines have been prepared and studied and each gives additional insights into the mechanisms for enamel biofabrication. This study summarizes the observed enamel phenotypes of recently derived transgenic animals. These data are being used to help define the role of each of the enamel structural proteins in enamel and study how each of these proteins impact on enamel biomineralization.

Enamel matrix protein interactions

The recognized structural proteins of the enamel matrix are amelogenin, ameloblastin, and enamelin. While a large volume of data exists showing that amelogenin self-assembles into multimeric units referred to as nanospheres, other reports of enamel matrix protein-protein interactions are scant. We believe that each of these enamel matrix proteins must interact with other organic components of ameloblasts and the enamel matrix. Likely protein partners would include integral membrane proteins and additional secreted proteins.

INTRODUCTION

The purpose of this study was to identify and catalog additional proteins that play a significant role in enamel formation.

MATERIALS AND METHODS

We used the yeast two-hybrid assay to identify protein partners for amelogenin, ameloblastin, and enamelin. Once identified, RT-PCR was used to assess gene transcription of these newly identified and potential "enamel" proteins in ameloblast-like LS8 cells.

RESULTS

In the context of this yeast assay, we identified a number of secreted proteins and integral membrane proteins that interact with amelogenin, ameloblastin, and enamelin. Additionally, proteins whose functions range from the inhibition of soft tissue mineralization, calcium ion transport, and phosphorylation events have been identified as protein partners to these enamel matrix proteins. For each protein identified using this screening strategy, future studies are planned to confirm this physiological relationship to biomineralization in vivo.

CONCLUSION

Identifying integral membrane proteins of the secretory surface of ameloblast cells (Tomes' processes) and additional enamel matrix proteins, based on their abilities to interact with the most abundant enamel matrix proteins, will better define the molecular mechanisms of enamel formation at its most rudimentary level.

Expression of amelogenin in odontoblasts

Amelogenin is the major enamel protein produced by ameloblasts. Its expression has been shown to be down-regulated in ameloblasts of vitamin-D-deficient (-D) rats. The potential expression and localization of amelogenin in odontoblasts and its regulation by vitamin D were investigated in this study. RT-PCR and semi-quantitative Northern blot analyses were performed using the odontoblast cell line MO6-G3 and microdissected dental pulp mesenchyme. Both in vitro and in vivo odontoblasts expressed various alternatively spliced amelogenin transcripts. In situ hybridization studies showed that amelogenin expression was restricted to young odontoblasts during mantle dentin deposition. Electron microscopy studies localized the amelogenin protein in the odontoblast cell process cytoplasm and mantle dentin. Amelogenin immunolabeling was stronger in -D rats, suggesting an inverse regulation by vitamin D in odontoblasts. Furthermore, amelogenin mRNA steady-state levels were significantly increased in -D dental pulp mesenchyme. In addition, a temporal-spatial lengthening of the mantle dentin stage was observed in -D animals, suggesting that developmental perturbations occur in relation to the vitamin D status and/or amelogenin expression. These data show that amelogenin is expressed by odontoblasts selectively during mantle dentin deposition. This developmental regulated expression pattern is enhanced under vitamin-D-deficiency status and in a broader context may play an important role during ameloblast and odontoblast differentiation and function.

Comparison of upstream regions of X- and Y-chromosomal amelogenin genes

The amelogenin genes encode abundant enamel proteins that are required for the development of normal tooth enamel. These genes are active only in enamel-forming ameloblasts within the dental organ of the developing tooth, and are part of a small group of genes that are active on both sex chromosomes. The upstream regions of the bovine X- and Y-chromosomal and the sole murine X-chromosomal amelogenin genes have been cloned and sequenced, and conservation at nearly 60% is found in the 300 bp upstream of exon 1 for the 3 genes. A region of the bovine X-chromosomal gene that has inhibitory activity when assayed by gene transfer into heterologous cells includes motifs that have a silencing activity in other genes, and may be important to the mechanism that represses amelogenin expression in non-ameloblast cells in vivo. A comparison of sequences from three genes has led to the identification of several regions with conserved motifs that are strong candidates for having positive or negative regulatory functions, and these regions can now be tested further for interaction with nuclear proteins, and for their ability to regulate expression in vivo.

Regulation of amelogenin gene expression

The amelogenins are found uniquely in enamel, where they constitute the predominant class of secreted matrix proteins and where they play a fundamental role in normal enamel formation. To better understand the high level of tissue-specific expression, we cloned the bovine X and Y chromosomal amelogenin genes and the murine amelogenin gene and determined the DNA sequences for the regions upstream of the transcription start sites. We observed segments of strong homology among species, and identified consensus sequences for the binding of various transcription factors, including the glucocorticoid receptor, AP1, RXR and p53. Although specific sis-elements conferring enhanced transcription have not yet been identified, elements have been localized that have silencing effect in non-ameloblast cells. Conserved sequences are likely to be involved in tissue-specific expression. Transgenic mouse studies have shown that 3.5 kb of upstream region is sufficient but 900 bp is insufficient for specific expression in vivo. Alternative splicing of the primary transcript is an effective mechanism for generating molecular heterogeneity. Amelogenin genes contain seven exons, and exons 3, 4, 5 and most of 6 can be deleted by alternative splicing. However, the pattern of exon splicing varies according to the species, and skipping of bovine exon 3 appears to be developmentally regulated. It will be important to determine whether the relative amounts of translation products differ among species as do the mRNAs, and to correlate the various protein structures with function. These findings also suggest that the regulation of amelogenin gene expression is complex and takes place at several levels.

High expression of human amelogenin in E. coli

A human cDNA, encoding for the 175-amino-acid human amelogenin, was prepared by RT PCR from tooth bud mRNA and sub-cloned into pGEX-KG expression plasmid for over-expression in E. coli. The expressed protein was characterized by SDS-PAGE Western blotting, and N-terminal amino acid sequencing.