Enamel defects and ameloblast-specific expression in Enam knock-out/lacz knock-in mice

Enamelin is critical for proper dental enamel formation, and defects in the human enamelin gene cause autosomal dominant amelogenesis imperfecta. We used gene targeting to generate a knock-in mouse carrying a null allele of enamelin (Enam) that has a lacZ reporter gene replacing the Enam translation initiation site and gene sequences through exon 7. Correct targeting of the transgene was confirmed by Southern blotting and PCR analyses. No enamelin protein could be detected by Western blotting in the Enam-null mice. Histochemical 5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside (X-gal) staining demonstrated ameloblast-specific expression of enamelin. The enamel of the Enam(+/-) mice was nearly normal in the maxillary incisors, but the mandibular incisors were discolored and tended to wear rapidly where they contacted the maxillary incisors. The Enam(-/-) mice showed no true enamel. Radiography, microcomputed tomography, and light and scanning electron microscopy were used to document changes in the enamel of Enam(-/-) mice but did not discern any perturbations of bone, dentin, or any other tissue besides the enamel layer. Although a thick layer of enamel proteins covered normal-appearing dentin of unerupted teeth, von Kossa staining revealed almost a complete absence of mineral formation in this protein layer. However, a thin, highly irregular, mineralized crust covered the dentin on erupted teeth, apparently arising from the formation and fusion of small mineralization foci (calcospherites) in the deeper part of the accumulated enamel protein layer. These results demonstrate ameloblast-specific expression of enamelin and reveal that enamelin is essential for proper enamel matrix organization and mineralization.

[Construction of the stable cell line expressing the mouse recombinant enamelin gene]

OBJECTIVE

To construct a mouse recombinant enamelin eukaryocyte expression system, and establish the stable cell line which can produce the protein continuously.

METHODS

The mRNA transcript from the 3-day mouse jaw was extracted. and the enamelin gene fragment amplified with RT-PCR techniques. Then the PCR product was cat with two restriction enzymes, and subcloned into the eukaryotic gene expression vector pcDNA3.1TM/mycj His(-)B.The recombinant plasmid was transformed into E.coli DH5alpha bacterial cells, and harvested with plasmid midi kit. The recombinant expression plasmid was transferred to the HEK 293A eukaryocyte cells, cultured selectively with 800 mg/L G418, and examined with SDS-PAGE and Western Blot at the protein level.

RESULTS

The mouse enamelin gene was cloned to the eukaryotic expression plasmid successfully by sequence measuring. After the recombinant plasmid was transferred into the HEK 293A cells, about 32,000 enamelin protein was checked out by SDS-PAGE and Western Blot.

CONCLUSION

The recombinant eukaryocyte expression plasmid and the stable cell line were established. This is a basic research to obtain high-yeild biologically active enamelin protein, which may facilitate further investigation of its function.

Enamel dysplasia with hamartomatous atypical follicular hyperplasia (EDHFH) syndrome: suggested pathogenic mechanisms

The syndrome of enamel dysplasia with hamartomatous atypical follicular hyperplasia (EDHFH) is an unusual syndrome and is unique to black South Africans. Major criteria for the syndrome are enamel dysplasia with generalized amelogenesis imperfecta-like features and atypical hyperplastic dental follicles with microscopic features of central odontogenic fibroma WHO-type (follicle analogue) attached to the crowns of multiple impacted teeth. Minor features of some cases are anterior open-bite malocclusion, supernumerary teeth, pulpal calcification, aberrant roots with hypercementosis, and hypodontia. The pathogenic mechanisms that lead to the development of EDHFH are unknown. We speculate that faulty synthesis of enamel matrix proteins may interfere with enamel formation and play a role in the generalized enamel hypoplasia described in this syndrome. Alterations in inductive signalling by the odontogenic epithelium mediated by enamel matrix proteins may explain the development of the follicle analogues, the root hypercementosis and the presence of dysplastic cementum deposition juxtaposed to odontogenic epithelium in the gingival overgrowth. Thus, alterations in the function of enamel matrix protein function, may be the common denominator responsible for the development of the EDHFH phenotype.

The potential use of enamel matrix derivative for in situ anterior cruciate ligament tissue engineering: a translational in vitro investigation

Polyester scaffolds have been used as an alternative to autogenous tissues for the reconstruction of the anterior cruciate ligament (ACL). They are biocompatible and encourage tissue infiltration, leading to neoligament formation. However, rupture can occur, caused by abrasion of the scaffold against the bone tunnels through which it is implanted. Good early tissue induction is therefore considered essential to protect the scaffold from this abrasion. Enamel matrix derivative (EMD) is used clinically in the treatment of periodontal disease. It is a complex mix of proteins with growth factor-like activity, which enhances periodontal ligament fibroblast attachment, proliferation, and differentiation, leading to the regeneration of periodontal bone and ligament tissues. We hypothesized that EMD might, in a similar manner, enhance tissue induction around scaffolds used in ACL reconstruction. This preliminary investigation adopted a translational approach, modelling in vitro 3 possible clinical modes of EMD administration, to ascertain the suitability of each protocol for application in an animal model or clinically. Preliminary investigations in monolayer culture indicated that EMD had a significant dose-dependent stimulatory effect (p < 0.05, n = 6) on the proliferation of bovine primary synovial cells. However, pre-treating culture plates with EMD significantly inhibited cell attachment (p < 0.01, n = 6). EMD's effects on synovial cells, seeded onto ligament scaffolds, were then investigated in several in vitro experiments modelling 3 possible modes for clinical EMD administration (pre-, intra-, and post-operative). In the pre-operative model, EMD was adsorbed onto scaffolds before the addition of cells. In the intra-operative model, EMD and cells were added simultaneously to scaffolds in the culture medium. In the post-operative model, cells were pre-seeded onto scaffolds before EMD was administered. EMD significantly inhibited cell adhesion in the pre-operative model (p < 0.05, n = 6) and had no significant benefit in the intra-operative model. In the post-operative model, the addition of EMD to previously cell-seeded scaffolds significantly increased their total deoxyribonucleic acid content (p < 0.01, n = 5). EMD's stimulative effect on cell proliferation in vitro suggests that it may accelerate scaffold colonization by cells (and in turn tissue induction) in situ. However, its inhibitory effect on synovial cell attachment in vitro implies that it may only be suited to post-operative administration.

Rat wct mutation prevents differentiation of maturation-stage ameloblasts resulting in hypo-mineralization in incisor teeth

A recent study provided genetic and morphological evidence that rat autosomal-recessive mutation, whitish chalk-like teeth (wct), induced tooth enamel defects resembling those of human amelogenesis imperfecta (AI). The wct locus maps to a specific interval of rat chromosome 14 corresponding to human chromosome 4q21 where the ameloblastin and enamelin genes exist, although these genes are not included in the wct locus. The effect of the wct gene mutation on the enamel matrix synthesis and calcification remains to be elucidated. This study clarifies how the wct gene mutation influences the synthesis of enamel matrix and its calcification by immunocytochemistry for amelogenin, ameloblastin and enamelin, and by electron probe micro-analysis (EPMA). The immunoreactivity for enamel proteins such as amelogenin, ameloblastin, and enamelin in the ameloblasts in the homozygous teeth was the same as that in the heterozygous teeth from secretory to transitional stages, although the homozygous ameloblasts became detached from the enamel matrix in the transitional stage. The flattened ameloblasts in the maturation stage of the homozygous samples contained enamel proteins in their cytoplasm. Thus, the wct mutation was found to prevent the morphological transition of ameloblasts from secretory to maturation stages without disturbing the synthesis of enamel matrix proteins, resulting in the hypo-mineralization of incisor enamel and cyst formation between the enamel organ and matrix. This mutation also prevents the transfer of iron into the enamel.

Human dental follicle cells acquire cementoblast features under stimulation by BMP-2/-7 and enamel matrix derivatives (EMD) in vitro

The dental follicle (DF) surrounding the developing tooth germ is an ectomesenchymal tissue composed of various cell populations derived from the cranial neural crest. Human dental follicle cells (HDFC) are believed to contain precursor cells for cementoblasts, periodontal ligament cells, and osteoblasts. Bone morphogenetic proteins (BMPs) produced by Hertwig's epithelial root sheath or present in enamel matrix derivatives (EMD) seem to be involved in the control of DF cell differentiation, but their precise function remains largely unknown. We report the immunolocalization of STRO-1 (a marker of multipotential mesenchymal progenitor cells) and BMP receptors (BMPR) in DF in vivo. In culture, HDFC co-express STRO-1/BMPR and exhibit multilineage properties. Incubation with rhBMP-2 and rhBMP-7 or EMD for 24 h increases the expression of BMP-2 and BMP-7 by HDFC. Long-term stimulation of these cells by rhBMP-2 and/or rhBMP-7 or EMD significantly increases alkaline phosphatase activity (AP) and mineralization. Expression of cementum attachment protein (CAP) and cementum protein-23 (CP-23), two putative cementoblast markers, has been detected in EMD-stimulated whole DF and in cultured HDFC stimulated with EMD or BMP-2 and BMP-7. RhNoggin, a BMP antagonist, abolishes AP activity, mineralization, and CAP/CP-23 expression in HDFC cultures and the expression of BMP-2 and BMP-7 induced by EMD. Phosphorylation of Smad-1 and MAPK is stimulated by EMD or rhBMP-2. However, rhNoggin blocks only Smad-1 phosphorylation under these conditions. Thus, EMD may activate HDFC toward the cementoblastic phenotype, an effect mainly (but not exclusively) involving both exogenous and endogenous BMP-dependent pathways.

[Immunohistochemical localization of enamelin in developing rat tooth germ]

OBJECTIVE

To observe the immunohistochemical localization of enamelin in enamel formationand mineralization.

METHODS

Tissue sections of the first mandibular molar tooth germ from 1, 3, 7, 10, 14 days rats after birth were prepared, expression of the enamelin protein was identified by immunohistochemical technique.

RESULTS

Enamelin was found in the cytoplasm of ameloblasts in 1-10 days old rat postnatal first mandibular molar tooth germs. Enamelin appeared weakly in the tooth germs of 1 day rats. From 3 to 10 days, enamelin localized both in the cytoplasm of ameloblasts and the uncalcified enamel from the dentino-enamel junction to surfaces of the tooth. Enamelin protein was negative in the tooth germs of 14 days rats postnatally.

CONCLUSION

Enamelin protein is synthesised and secreted by ameloblasts, specially localized in enamel from DEJ to surfaces of the tooth, suggesting that enamelin has important roles in enamel formation.

Ameloblastin expression during craniofacial bone formation in rats

Based on previous results showing the expression of ameloblastin (Ambn; amelin) in the formation of mesenchymal dental hard tissues, we investigated its presence during bone development. Immunohistochemistry (IHC), in situ hybridization (ISH), and reverse transcription-polymerase chain reaction (RT-PCR) were used to investigate the expression of ameloblastin protein and mRNA during craniofacial development in rats. Tissue samples were collected on embryonic day 18 and from days 2-28 postnatally. IHC revealed the expression of ameloblastin during bone formation at embryonic and early postnatal stages with different patterns of expression in intramembranous and endochondral ossification. In intramembranous ossification, ameloblastin expression was detected in the superficial layer of the condensed vascularized primitive connective tissue and in the cellular layer covering the surface of the newly formed woven bone. In endochondral ossification, ameloblastin was expressed within the extracellular matrix of the cartilage templates and in the perichondrium. Between days 2 and 28 the expression decreased markedly, concordant with the maturation of the bone, and disappeared after completion of bone remodeling. The results obtained by IHC were confirmed by ISH and RT-PCR, showing the expression of ameloblastin mRNA during craniofacial bone formation. This study indicates the expression of the putative dental protein ameloblastin during craniofacial bone development in rats.

Cloning, sequencing, and expression of the amelogenin gene in two scincid lizards

Our knowledge of the gene coding for amelogenin, the major enamel protein, is mainly based on mammalian sequences. Only two sequences are available in reptiles. To know whether the snake sequence is representative of the amelogenin condition in squamates, we have studied amelogenin in two scincid lizards. Lizard amelogenin possesses numerous conserved residues in the N- and C-terminal regions, but its central region is highly variable, even when compared with the snake sequence. This rapid evolution rate indicates that a single squamate sequence was not representative, and that comparative studies of reptilian amelogenins might be useful to detect the residues which are really important for amelogenin structure and function. Reptilian and mammalian enamel structure is roughly similar, but no data support amelogenin being similarly expressed during amelogenesis. By performing in situ hybridization using a specific probe, we showed that lizard ameloblasts express amelogenin as described during mammalian amelogenesis. However, we have not found amelogenin transcripts in odontoblasts. This indicates that full-length amelogenin is specific to enamel matrix, at least in this lizard.

Fibromodulin-deficient Mice Display Impaired Collagen Fibrillogenesis in Predentin as Well as Altered Dentin Mineralization and Enamel Formation

To determine the functions of fibromodulin (Fmod), a small leucine-rich keratan sulfate proteoglycan in tooth formation, we investigated the distribution of Fmod in dental tissues by immunohistochemistry and characterized the dental phenotype of 1-day-old Fmod-deficient mice using light and transmission electron microscopy. Immunohistochemistry was also used to compare the relative protein expression of dentin sialoprotein (DSP), dentin matrix protein-1 (DMP 1), bone sialoprotein (BSP), and osteopontin (OPN) between Fmod-deficient mice and wild-type mice. In normal mice and rats, Fmod immunostaining was mostly detected in the distal cell bodies of odontoblasts and in the stratum intermedium and was weaker in odontoblast processes and predentin. The absence of Fmod impaired dentin mineralization, increased the diameter of the collagen fibrils throughout the whole predentin, and delayed enamel formation. Immunohistochemistry provides evidence for compensatory mechanisms in Fmod-deficient mice. Staining for DSP and OPN was decreased in molars, whereas DMP 1 and BSP were enhanced. In the incisors, labeling for DSP, DMP 1, and BSP was strongly increased in the pulp and odontoblasts, whereas OPN staining was decreased. Positive staining was also seen for DMP 1 and BSP in secretory ameloblasts. Together these studies indicate that Fmod restricts collagen fibrillogenesis in predentin while promoting dentin mineralization and the early stages of enamel formation. (J Histochem Cytochem 54:525-537, 2006)

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.

Reuptake of extracellular amelogenin by dental epithelial cells results in increased levels of amelogenin mRNA through enhanced mRNA stabilization

Amelogenin is an extracellular matrix protein secreted by ameloblasts and is a major component of enamel matrix. Recently, in addition to their role in enamel formation, the biological activity of enamel proteins in the process of cell differentiation has recently become widely appreciated. In this study, we examined the biological activity of amelogenin on ameloblast differentiation. Recombinant mouse amelogenin (rm-amelogenin) enhanced the expression of endogenous amelogenin mRNA in a cultured dental epithelial cell line (HAT-7), despite a lack of increased amelogenin promoter activity. To solve this discrepancy, we analyzed the effects of rm-amelogenin on the stability of amelogenin mRNA. The half-life of amelogenin mRNA is extremely short, but in the presence of rm-amelogenin its half-life was extended three times longer than the control. Furthermore, we showed the entry of exogenous fluorescein isothiocyanate-conjugated rm-amelogenin into the cytoplasm of HAT-7 cells. It follows from our results that exogenous amelogenin increases amelogenin mRNA levels through stabilization of mRNA in the cytoplasm of HAT-7 cells. Here we speculated that during differentiation, dental epithelial cells utilize a unique mechanism for increasing the production of amelogenin, the reuptake of secreted amelogenin.

Targeted disruption of two small leucine-rich proteoglycans, biglycan and decorin, excerpts divergent effects on enamel and dentin formation

Small leucine-rich proteoglycans have been suggested to affect mineralization of dental hard tissues. To determine the functions of two of these small proteoglycans during the early stages of tooth formation, we characterized the dental phenotypes of biglycan (BGN KO) and decorin deficient (DCN KO) mice and compared them to that of wild type mice. Each targeted gene disruption resulted in specific effects on dentin and enamel formation. Dentin was hypomineralized in both knock out mice, although the effect was more prominent in the absence of decorin. Enamel formation was dramatically increased in newborn biglycan knockout mice but delayed in absence of decorin. Increased enamel formation in the former case resulted from an upregulation of amelogenin synthesis whereas delayed enamel formation in the later case was most probably an indirect consequence of the high porosity of the underlying dentin. Enamelin expression was unchanged in BGN KO, and reduced in DCN KO. Dentin sialoprotein (DSP), a member of the family of phosphorylated extracellular matrix proteins that play a role in dentinogenesis, was overexpressed in BGN-KO odontoblasts and in the sub-odontoblastic layer. In contrast, a decreased expression of DSP was detected in DCN KO. Dentin matrix protein-1 (DMP-1), bone sialoprotein (BSP) and osteopontin (OPN) were upregulated in BGN KO and downregulated in the DCN KO. Despite the strong effects induced by these deficiencies in newborn mice, no significant difference was detected between the three genotypes in adult mice, suggesting that the effects reported here in newborn mice are transient and subjected to self-repair.

Expression of enamel proteins and LEF1 in adamantinomatous craniopharyngioma: evidence for its odontogenic epithelial differentiation

AIMS

Adamantinomatous craniopharyngioma (ACP) resembles histologically some odontogenic tumours, such as ameloblastoma and calcifying odontogenic cyst. However, there has been no evidence that ACP differentiates also functionally as odontogenic epithelium. The aim of this study was to gain evidence of odontogenic epithelial differentiation in ACP by means of immunohistochemistry. Among normal human tissues, enamel proteins are expressed exclusively in teeth, and lymphoid enhancer factor 1 (LEF1), in co-operation with beta-catenin, play an important role in tooth development. The expression of these proteins is therefore indicative of odontogenic epithelial differentiation.

METHODS AND RESULTS

The expression of enamel proteins and LEF1 was examined in 10 adamantinomatous and six papillary craniopharyngiomas. All the ACPs showed a variable degree of enamel protein expression, including amelogenin, enamelin and enamelysin, mainly in ghost cells. LEF1 was also heterogeneously expressed in ACPs; remarkably, its expression pattern was identical to that of nuclear beta-catenin accumulation. In contrast, none of the papillary craniopharyngiomas expressed enamel proteins or LEF1.

CONCLUSIONS

These results suggest that ACP consistently shows odontogenic epithelial differentiation. Since ACPs harbour beta-catenin mutation, the inappropriate activation of beta-catenin/LEF1 complex-dependent transcription may play a critical role in ACP tumorigenesis.

Characterization of epithelial cells derived from periodontal ligament by gene expression patterns of bone-related and enamel proteins

The cells of epithelial rests of Malassez (ERM) are thought to play a number of roles, such as protection against root resorption and cementoblast differentiation. However, little is known about characteristics of these cells. In the present study, we compared the expression patterns of the bone-related proteins osteopontin (OPN), osteonectin/secreted protein, acidic and rich in cysteine (SPARC), osteoprotegerin (OPG), bone morphogenetic proteins (BMP-2 and BMP-4) and the enamel matrix proteins amelogenin and tuftelin in epithelial cells derived from human periodontal ligament (ECHPL) with those of human gingival epithelial cells (HGEC) and oral mesenchymal cells (human gingival fibroblasts, human periodontal ligament fibroblasts and human pulp cells). The mRNA expression patterns were determined by the reverse-transcription polymerase chain reaction (RT-PCR). Cytokeratin 8 mRNA was expressed by oral epithelial cells but not oral mesenchymal cells. The OPN mRNA levels in ECHPL were by far the highest in the five cell types investigated. ECHPL and oral mesenchymal cells expressed OPG mRNA, whereas HGEC did not. BMP-2, SPARC and tuftelin mRNAs were detected in ECHPL and the other cells examined. The oral mesenchymal cells expressed BMP-4 mRNA much more strongly than did the oral epithelial cells. Amelogenin mRNA expression could not be detected in any of the cells. These findings suggest that cultured ERM cells are characterized by expression of the cytokeratin 8, OPG and OPN genes.

Ameloblastin and amelogenin expression in posnatal developing mouse molars

Ameloblastin and amelogenin are structural proteins present in the enamel matrix of developing teeth. Here we report the results of in situ hybridization analyses with DNA probes of ameloblastin and amelogenin expression in the mandibular first molars of ICR/Jcl mice from postnatal day 1 to day 15. Ameloblastin mRNA expression was observed in ameloblasts at day 2 while amelogenin mRNA was detected in secretory ameloblasts at day 3. Significant expression of both molecules was observed at days 4, 5 and 6, after which the levels decreased. Amelogenin expression ended on day 10, while ameloblastin mRNA was only weakly detected on day 12. Neither amelogenin nor ameloblastin expression was observed in day 15 mouse molars. Amelogenin and ameloblastin mRNAs were restricted to ameloblasts. We conclude that amelogenin and ameloblastin expression is enamel-specific, and suggest that these genes might be involved in the mineralization of enamel. It is possible that ameloblastin could participate in the attachment of ameloblasts to the enamel surface. In this case, the downregulation of expression may indicate the beginning of the maturation stage in which the ameloblasts tend to detach from the enamel layer.

[Enamelin transcriptional expression in developing postnatal rat tooth germ]

OBJECTIVE

To observe the transcriptional expression of enamelin in developing postnatal rat first mandibular molar germs, for further studies of functions of enamelin in enamel development and mineralization.

METHODS

Tissue slices of first mandibular molar germ of rat 1, 3, 7, 10, 14 days after birth were prepared. The enamelin mRNA expression was identified by in situ hybridization.

RESULTS

Enamelin mRNA was observed in both ameloblast and odontoblast in 1-10 day old rat postnatal first mandibular molar germs. Enamelin mRNA appeared very weakly at 1st day, and increased through 3rd day, reached the maximum at 7th day, and reduced at 10th day and became negative at 14th day postnatally; while the expression of enamelin mRNA in odontoblast maintained lower from 1st to 10th day and negative at 14th day postnatally.

CONCLUSION

Enamelin gene transcriptional expression lasts from preameloblast to maturation ameloblast, which suggests that enamelin may participate in the development of enamel and mantle dentin.

Role of Hertwig's epithelial root sheath cells in tooth root development

During tooth development, after the completion of crown formation, the apical mesenchyme forms the developing periodontium while the inner and outer enamel epithelia fuse below the level of the crown cervical margin to produce a bilayered epithelial sheath termed Hertwig's epithelial root sheath (HERS). The role of HERS cells in root formation is widely accepted; however, the precise function of these cells remains controversial. Functions suggested have ranged from structural (subdivide the dental ectomesenchymal tissues into dental papilla and dental follicle), regulators of timing of root development, inducers of mesenchymal cell differentiation into odontoblasts and cementoblasts, to cementoblast cell precursors. The characterization of the HERS phenotype has been hindered by the small amount of tissue present at a given time during root formation. In this study, we report the establishment of an immortal HERS-derived cell line that can be maintained in culture and then induced to differentiate in vitro. Characterization of the HERS phenotype using reverse transcriptase-polymerase chain reaction and Western blot immunostaining suggests that HERS cells initially synthesize and secrete some enamel-related proteins such as ameloblastin, and then these cells appear to change their morphology and produce a mineralized extracellular matrix resembling acellular cementum. These studies suggest that the acellular and cellular cementum are synthesized by two different types of cells, the first one by HERS-derived cementoblasts and the later by neural crest-derived cementoblasts.

[Cloning of human amelogenin gene encoding mature peptide]

PURPOSE

The purpose of this study was to clone human amelogenin gene encoding mature protein, which provides a basis for expressing the recombinant human amelogenin in Escherichia coli. in the future.

METHODS

In this study, total RNA was extracted from the dental germ of a legally aborted embryo by Trizol. Using RT-PCR technique we obtained synthesis of cDNA from the total RNA, and the desired DNA products were conducted with PCR from cDNA. The segment was inserted into expression vector PQE30 and the interesting plasmid was transformed into Escherichia coli. host DH5alpha. The double-stranded DNA of positive clone was analyzed by PCR, restriction endonuclease mapping and DNA sequence analysis.

RESULTS

The sequence analysis of recombinant plasmid showed that the human amelogenin encoding mature protein was inserted into vector PQE30 accurately.

CONCLUSIONS

We conducted human amelogenin encoding mature protein from dental germ of a legally aborted embryo and got the recombinant plasmid which may express amelogenin gene for further research.

Amelogenin self-assembly and the role of the proline located within the carboxyl-teleopeptide

A hallmark of biological systems is a reliance on protein assemblies to perform complex functions. We have focused attention on mammalian enamel formation because it relies on a self-assembling protein complex to direct mineral habit. The principle protein of enamel is amelogenin that self-assembles to form nanospheres. In mice, the principal amelogenin product is a 180 amino acid hydrophobic protein. The yeast two-hybrid assay has been used to demonstrate the importance of amelogenin self-assembly domains. We have generated specific variants of amelogenin to analyze contributions of individual amino acids to the self-assembly process. These amelogenin variants have been produced either by deleting carboxyl-terminal amino acids (to generate proteins that relate to the documented proteolytic products of mouse amelogenin) or by a site-directed mutagenesis approach. Assessment of variant amelogenins truncated at the carboxyl-terminal imply that the proline at position 169 of mouse amelogenin (M180) plays a significant role in amelogenin self-assembly. Site-directed mutagenesis of this particular proline, however, failed to disrupt the amelogenin self-assembly property. These conflicting data add to the complexity of protein-protein assembly mechanisms as they relate to the enamel matrix. Available data suggest a robustness of this enamel protein (amelogenin) that ensures a functional, even though mechanically less than optimal, enamel results despite either minor or major genetic errors to the amelogenin gene locus.

[Construction of eukaryotic expression clone for human amelogenin]

OBJECTIVE

To construct-the eukaryotic expression clone for human amelogenin.

METHODS

Total RNA was isolated from human fetal tooth buds. RT-PCR was used to amplify the amelogenin encoding region, and the amplified fragment for human amelogenin was inserted into eukaryotic expression vector PcDNA 3.1. The positive clones were selected and analyzed by restriction endonuclease mapping and DNA sequencing.

RESULTS

570 bp fragment was produced by RT-PCR; it was of the same size as expected based on human ameloginin mRNA encoding area length. The sequence of the inserted fragment from the recombinant clone PcDNA 3.1-AMG was consistent with that of AMELX from GenBank with one mismatch on 485 from G to C, without affecting the amino acid sequence.

CONCLUSION

The eukaryotic expression clone PcDNA 3.1-AMG was successfully constructed with the properly inserted DNA sequence encoding mature human amelogenin.

The structure of the rat ameloblastin gene and its expression in amelogenesis

Ameloblastin (also designated amelin and sheathlin) is an enamel matrix protein expressed within the ameloblast lineage. In this study we analyzed the structure of the rat ameloblastin gene and characterized its subtypes. The promoter sequence contains several E-box-like elements, and consensus sequences for AP1 and SP1. The gene is about 6 kb in length and contains 12 exons. Exon 1 was mapped by primer extension and encodes 90 bp of 5' untranslated leader sequence, followed by the coding sequences of exon 2 (309 bp), alternatively spliced exon 3a (45 bp), exon 3b (198 bp), exon 4 (36 bp), exon 5 (60 bp), exon 6 (45 bp), exon 7 (150 bp), and exon 8 (448 bp) containing coding sequence (426 bp) and 3' untranslated sequence (22 bp), followed by exon 9 (39 bp); exon 10 (143 bp); exon 11 (342 bp); and exon 12. Exon 3a, encoding YEYSLPVHPPPLPSQ, has a potential SH3 binding domain. Analysis of ameloblastin subclones showed that exon 3a and 11 were potential alternative splicing sites, producing 4 types of ameloblastin mRNA, from which ameloblastin I and II could be translated. Using in situ hybridization, immunohistochemistry, Western blot and RT-PCR methods we found that ameloblastin II, containing exon 3a, was more strongly expressed at the late maturation stage of ameloblasts than at the secretory stage, while a common probe for both ameloblastin subtypes showed wide expression throughout the presecretory, secretory and postsecretory stages. From the above results we propose that ameloblastin II plays an important role in the mineralization of ameloblasts during the maturation stages.

[Construction of the eukaryotic expression vector PsecTaq2A-AMG for human amelogenin]

OBJECTIVE

The purpose of this study was to construct a eukaryotic expression vector for human amelogenin (AMG).

METHODS

PCR was performed to amplify the AMG encoding region. Amplified fragments for human AMG were recovered and inserted into eukaryotic expression vectors PsecTaq2A. The recombinant plasmid PsecTaq2A-AMG was constructed and their positive clones were identified.

RESULTS

1. Amplified products were checked by electrophoresis and the results were satisfactory. 2. The recombinant plasmid PsecTaq2A-AMG was analyzed by restriction endonuclease mapping and DNA sequencing. The results of sequencing were consistent with those from GenBank.

CONCLUSION

The recombinant plasmid PsecTaq2A-AMG was successfully constructed with properly inserted DNA sequence encoding mature amelogenin.

Enamelysin and kallikrein-4 mRNA expression in developing mouse molars

Proteolytic processing and degradation of enamel matrix proteins appears to be an essential feature of dental enamel formation. The source and character of proteolytic activity in the enamel matrix of developing teeth changes as enamel formation progresses. Two proteinases have been isolated from the extracellular enamel matrix of developing teeth: enamelysin (MMP-20), a matrix metalloproteinase. and kallikrein-4 (KLK4), a serine proteinase. Here, we ask if the expression of MMP-20 and KLK4 correlate with the stage-associated changes in the digestion of enamel proteins. Using in situ hybridization, we localized MMP-20 and KLK4 mRNA in mouse maxillary first molars on postnatal days 1, 2, 3, 5, 6, 7, 9, 11, and 14. Enamelysin signal was first detected in preameloblasts, ameloblasts, and odontoblasts on day 2, but not in ameloblasts covering the enamel-free zone. Enamelysin signal declined in ameloblasts on day 6 but persisted in the dental pulp. In contrast, KLK4 transcripts were first observed on day 3 in pulp and on day 6 in ameloblasts covering the enamel-free zone. the KLK4 signal was present in maturation-stage ameloblasts on days 9, 11, and 14. The expression patterns of MMP-20 and KLK4 by ameloblasts in mouse molars are stage-specific and complementary.

Effects and distribution of the enamel matrix derivative Emdogain in the periodontal tissues of rat molars transplanted to the abdominal wall

The enamel matrix derivative Emdogain (EMD) has been found to promote regeneration of lost periodontal tissues. We have studied the effects and distribution of EMD in the periodontal tissues of maxillary rat molars transplanted to a subcutaneous position in the abdominal wall. The molars were transplanted with or without EMD either immediately after extraction or after drying for 30 min. After 2 days, 1, 2 or 4 weeks the rats were killed and the teeth were examined by means of light microscopy and immunohistochemistry with anti-amelogenin antibodies. Teeth transplanted immediately after extraction showed formation of alveolar bone separated from the dental roots by a periodontal space, regardless of the use of EMD. Among the teeth that were transplanted with EMD after drying for 30 min, new alveolar bone was formed in five out of eight teeth after 2 and 4 weeks. None of the teeth that were dried for 30 min and transplanted without EMD showed alveolar bone formation. Only one tooth transplanted with EMD showed root resorption after drying, while resorption was noted in all teeth transplanted without EMD. All teeth that were transplanted with EMD and none of the teeth that were transplanted without EMD showed an immunohistochemical reaction for amelogenin. After 2 days, amelogenin was precipitated on all surfaces exposed at the transplantation procedure. Later, the immunoreactive material was redistributed to cells at the root surface, where it was still demonstrable after 4 weeks. In conclusion, EMD is accumulated in cells at the root surface and promotes regeneration of the periodontal tissues of the transplanted teeth. It also seems to promote healing of root resorption.

A comparison of enamelin and amelogenin expression in developing mouse molars

Amelogenin and enamelin are structural proteins in the enamel matrix of developing teeth. The temporal and spatial patterns of enamelin expression in developing mouse molars have not been characterized, while controversy remains with respect to amelogenin expression by odontoblasts and cementoblasts. Here we report the results of in situ hybridization analyses of amelogenin and enamelin expression in mouse molars from postnatal days 1, 2, 3, 7, 9, 14, and 21. Amelogenin and enamelin mRNA in maxillary first molars was first observed in pre-ameloblasts on the cusp slopes at day 2. The onsets of amelogenin and enamelin expression were approximately synchronous with the initial accumulation of predentin matrix. Both proteins were expressed by ameloblasts throughout the secretory, transition, and early maturation stages. Enamelin expression terminated in maturation stage ameloblasts on day 9, while amelogenin expression is still detected in maturation stage ameloblasts on day 14. No amelogenin expression was observed in day 21 mouse molars. Amelogenin and enamelin RNA messages were restricted to ameloblasts. No expression was observed in pulp, bone, or along the developing root. We conclude that amelogenin and enamelin are enamel-specific and do not directly participate in the formation of dentin or cementum in developing mouse molars.

Histopathological and immunohistochemical analysis of calcifying odontogenic cysts

METHOD AND RESULTS

Calcifying odontogenic cysts (COCs) were examined histopathologically and immunohistochemically to characterize the histological and cytological properties of these lesions. Histopathologically, COCs showed thin or thick lining epithelium with ghost cells. COCs were classified according to proliferative type or nonproliferative type lining epithelium, the presence or absence of ameloblastomatous appearance, and the presence or absence of odontoma in the cyst walls. Immunohistochemically, amelogenin protein was expressed chiefly in ghost cells, whereas cytokeratin 19 (CK19) and bcl-2 proteins were expressed chiefly in lining epithelial cells. The proportion of cases positive for bcl-2 protein was slightly higher in COCs with odontoma than in those without odontoma. Lining epithelial cells sporadically showed positive reactions for Ki-67 antigen. Mean Ki-67 labeling index was slightly greater in COCs with proliferative type lining epithelium, COCs with ameloblastomatous appearance of the cyst walls, and COCs with odontoma of the cyst walls than in COCs without these histological features. Our results suggest that ghost cells or lining epithelial cells show ameloblastic cytodifferentiation or odontogenic epithelial characteristics, that bcl-2 protein is associated with survival of lining epithelial cells in COCs, and that high proliferation potential is associated with ameloblastomatous proliferation or combined odontoma. COCs exhibited various histological features with several transitional forms, and immunohistochemical examinations revealed little or no difference in cytodifferentiation and cellular activity among COCs.

CONCLUSION

We conclude that COCs with various histological features have neoplastic potential and may not be separate entities within the same histological spectrum.

Immunohistochemical detection of amelogenin and cytokeratin 19 in epithelial odontogenic tumors

OBJECTIVE

Epithelial odontogenic tumors exhibit considerable histological variation and are classified into several benign and malignant entities. Expression of amelogenin and cytokeratin 19 (CK19), that are potentially useful polypeptides for identification of odontogenic epithelial components, was evaluated in various types of epithelial odontogenic tumors.

MATERIALS AND METHODS

Specimens of 33 ameloblastomas, three calcifying epithelial odontogenic tumors (CEOTs), two clear cell odontogenic tumors (CCOTs) and five malignant ameloblastomas were examined by immunohistochemistry using anti-amelogenin and anti-CK19 antibodies.

RESULTS

Immunohistochemical reactivity for amelogenin was detected in many peripheral columnar or cuboidal cells and some central polyhedral cells in ameloblastomas, and histological variants showed various degrees of amelogenin expression. Expression of CK19 was diffusely present in neoplastic cells in ameloblastomas, and decreased expression was found in keratinizing cells of acanthomatous variants and some neoplastic cells of desmoplastic variants. In CEOTs, immunohistochemical reactivity for amelogenin was detected in neoplastic cells and intercellular amyloid-like materials, whereas CK19 was expressed in neoplastic cells. CCOTs showed positive reactivity for amelogenin and CK19 in neoplastic cells. Malignant ameloblastomas exhibited various degrees of amelogenin expression with constant CK19 expression in neoplastic cells.

CONCLUSION

Diverse types of epithelial odontogenic tumors express amelogenin and CK19, suggesting that these tumors have ameloblastic differentiation or odontogenic epithelial properties.

Induction of amelogenin and ameloblastin by insulin and insulin-like growth factors (IGF-I and IGF-II) during embryonic mouse tooth development in vitro

Insulin and insulin-like growth factors (IGF-I and IGF-II) are considered pleiotropic, acting as both mitogen and differentiation factors. Several investigators have demonstrated the expression of insulin, IGFs, their cognate receptors and IGF binding proteins during tooth morphogenesis. Previous work done in our laboratory indicated that exogenous insulin and IGFs induce the accumulation of enamel extracellular matrix on mouse mandibular molars cultured in a serumless, chemically defined medium. In order to determine the level of control of these factors in the induction of enamel biomineralization, we designed experiments to quantitate mRNAs for enamel specific-gene products. Mandibular first molars (MI) obtained from E15 Swiss Webster mice were placed in organ culture in the presence of insulin (1,000 ng/ml), IGF-I (100 ng/ml) or IGF-II (100 ng/ml) for 6, 12 and 18-days. At termination date, the RNA was extracted and the concentration of mRNAs for amelogenin, tuftelin and ameloblastin were determined using a quantitative competitive reverse transcription-polymerase chain reaction (RT-PCR) technique (PCR mimic). Our results showed that after 6-days in culture; treatment with insulin, IGF-I and IGF-II increased the synthesis of amelogenin and ameloblastin. In contrast, the expression of tuftelin mRNA was not affected by either factor. In conclusion, our studies showed that the increase in enamel matrix formation by overexpression of IGFs is the result of transcriptional regulation of enamel specific proteins like amelogenin and ameloblastin but not tuftelin. These studies also suggest that the regulatory mechanisms controlling tuftelin gene expression are different than the mechanisms regulating ameloblastin and amelogenin transcription.

Abortive secretion of an enamel matrix in the inner enamel epithelial cells during an enameloid formation in the gar-pike, Lepisosteus oculatus (Holostei, Actinopterygii)

The tooth in the gar-pike, Lepisosteus oculatus, an actinopterygian fish, is characterized by the occurrence of both enamel and enameloid, the former covering the tooth shaft and the latter, the tooth cap. Our previous research demonstrated that the enamel in this species was, as in the lungfish, immunoreactive for amelogenin, indicating its homologous nature with the mammalian tooth enamel, whereas the enameloid was completely immunonegative. The present study demonstrates that, during the early maturation stage of the enameloid formation, the inner enamel epithelial cells (IEECs) synthesize through a well-developed Golgi apparatus a fine-granular substance which is intensely immunoreactive for amelogenin. This substance was accumulated in a large saccule extended in a suprabasal zone of the cell; we were unable to find any morphological sign indicating a connection of the substance with the enameloid matrix. The abortive secretion of the enamel matrix-like substance in the IEEC during an enameloid formation was considered to be an instance of rudimental enamel formation. In the gar-pike, the synthesis of amelogenin in the IEEC has been demonstrated to occur independently from that of the enameloid matrix. The present findings demonstrate a prominent difference between the tooth enamel and enameloid.

[Expression of rat amelogenin gene in Escherichia coli]

OBJECTIVE

To detect the expression of amelogenin (Am) gene in Escherichia coli (E. coli).

METHODS

The cDNA fragmain of Am gene was obtained with EcoRI and Xhol I from the plasmid PUC18/Am. The fragment was inserted into prokaryotic gene fusion vector PGEX-4T-2 and an expression plasmid PGEX-4T-2/Am was constructed. PGEX-4T-2/Am was induced by IPTG for 4 h.

RESULTS

15% SDS-PAGE revealed a new forein protein band near 27,000.

CONCLUSION

The constructed plasmid expresses Am gene in E. coli.

Expression of amelin and amelogenin in epithelial root sheath remnants of fully formed rat molars

OBJECTIVE

To study the expression patterns of 2 enamel proteins, amelin and amelogenin, in the epithelial cells of the root sheath of fully formed rat molars.

STUDY DESIGN

Twelve Sprague-Dawley rats, 50, 65, and 85 days of age, were used in this study. The maxillae of the rats were dissected free, and sagittal serial sections were made through the mesial root of the first molar. In situ hybridization of amelin and amelogenin mRNAs was performed, and immunohistochemical examinations of the corresponding proteins in the epithelial cells of the root sheaths of fully formed rat molars were performed. Antibodies against epidermal keratins were used to identify epithelial root sheath remnants.

RESULTS

A group of epithelial cells, enclosed at the border between cellular cementum and dentin, expressed and synthesized amelin, but not amelogenin. Another group of epithelial cells, forming islands or strands, which were partially or totally incorporated in the matrix of the cellular cementum, expressed and synthesized both amelogenin and amelin. A third group of epithelial cells at the periphery of the cellular cementum expressed neither of the 2 proteins. Epithelial cells at the surface of acellular cementum did not express either of the 2 proteins.

CONCLUSION

This study showed that the epithelial root sheath remnants in rat molars express and synthesize amelogenin, as well as amelin. However, there are marked regional differences. The roles of the enamel proteins in the formation of the radicular hard tissues of the rat molars and in the maintenance of the periodontal tissues remain to be clarified.

Type X collagen in human enamel development: a possible role in mineralization

Although type X collagen is one of the key molecules in endochondral ossification, no data are available on whether it is present in dental structures when mineralization is proceeding. We therefore monitored the appearance of type X collagen in tooth germs of human samples ranging in gestational age from 17-week-old fetuses to 9-week-old newborn. Using immunohistochemistry, ELISA techniques, and Western blotting, we show that type X collagen is present in human tooth germ during enamel maturation. Intense immunohistochemical staining for collagen type X was observed in the enamel and in the apical parts of secretory ameloblast at the bell stage when the dentine and enamel matrix were already under formation. The odontoblasts, the dentine, and the pulp were not stained. In the early (9-week) postnatal stage, the staining for collagen type X in the enamel matrix was diminished, and only a very weak signal could be detected in the secretory ameloblasts. A positive reaction for collagen type X was also observed in ELISA assay of extracts obtained from human embryonic enamel and hypertrophic cartilage samples. The Western blot analysis of the enamel demonstrated that size of the molecule detected by MoAb X53 is characteristic of the type X collagen. This correlates well with our immunohistochemical findings. Based on these data, we propose that type X collagen is one of the candidate molecules present in the enamel matrix that might be involved in mineralization of the enamel.

Autoradiographic investigation of the effect of 1-hydroxyethylidene-1, 1-bisphosphonate on matrix protein synthesis and secretion by secretory ameloblasts in rat incisors

Seven daily subcutaneous injections of 1-hydroxyethylidene-1, 1-bisphosphonate (HEBP) can induce enamel hypoplasia. Several enamel-free zones were observed along the crown-analogue side of rat incisors during the secretory stage of amelogenesis. Ameloblasts related to the enamel-free zones lay directly on the abnormally non-mineralized mantle dentine, whereas the adjacent ameloblasts, which were forming the enamel matrix layer, were associated with the region where mineralization of dentine was proceeding. The further purpose of this study was to investigate the synthetic and secretory activity of these two groups of ameloblasts and to trace the fate of the radioactively labelled proteins. [(3)H]-proline was administered to Wistar rats 12 h after the last injection of HEBP. Light-microscopic autoradiography was performed. Quantitative analysis indicated that the ameloblasts of the enamel-forming zones in the drug-treated group showed a distribution pattern of silver grains similar to that of the controls. The ameloblasts of the enamel-free zones also demonstrated incorporation of [(3)H]-proline at the same level. There was some labelling over the non-mineralized mantle dentine, which was supposed to indicate the penetration of ameloblast products. From these results, it is concluded that HEBP does not affect the ameloblast activity in protein synthesis. The complete failure of enamel-layer formation in some specific regions is probably due to the failure in protein secretion and protein deposition. This study provides additional evidence that the mineralization of dentine is an essential factor in successful enamel matrix secretion and deposition.

Ghost cells in calcifying odontogenic cyst express enamel-related proteins

The so-called ghost cell is a unique cell type occurring in a variety of odontogenic and non-odontogenic lesions. However, the true nature of ghost cells has not been determined. In the present study, we examined the immunoreactivity of ghost cells in calcifying odontogenic cysts and dermal calcifying epitheliomas, with antibodies against amelogenin, enamelin, sheath protein (sheathlin) and enamelysin, in an attempt to clarify the nature of this unique cell. The cytoplasm of ghost cells in calcifying odontogenic cysts demonstrated distinct immunolocalization of the enamel-related proteins, while similar in the calcifying epitheliomas of the skin showed a negative reaction. The results indicate that the ghost cells in calcifying odontogenic cysts, as opposed to ghost cells in dermal calcifying epitheliomas, contain enamel-related proteins in their cytoplasm accumulated during the process of pathological transformation.

Gene expression and immunolocalization of amelogenin in enamel hypoplasia induced by successive injections of bisphosphonate in rat incisors

Successive injections of 1-hydroxyethylidene-1, 1-bisphosphonate (HEBP) in rats induce enamel hypoplasia. To elucidate the pathogenesis of this hypoplasia, male Wistar rats were daily injected with HEBP or physiological saline for 7 days. After the last injection, they were killed under anaesthesia and their maxillary incisors were examined using an in situ hybridization technique and immunohistochemical staining to detect the gene expression and localization of amelogenin protein, respectively. In the HEBP-injected rats, several islets of partially mineralized enamel were present along crown-analogous surface of the incisor in the secretory stage of amelogenesis and enamel-free zones existed between these islets. In situ hybridization demonstrated amelogenin gene expression over the ameloblasts facing the islets of the matrix enamel as well as over those of the enamel-free zones. Immunohistochemical studies using rabbit antiamelogenin antibody revealed positive reaction both in the enamel matrix of the control group and in the islets of enamel matrix of the HEBP-injected group. Some small granules immunoreactive to amelogenin antibody were found in the distal portions of the ameloblasts in the HEBP-injected rats. The results indicate that HEBP does not alter amelogenin gene expression over ameloblasts, or the protein's existence in enamel matrix. There appeared to be some accumulation of amelogenin in the HEBP-treated ameloblasts. It is therefore suggested that the enamel hypoplasia in this experiment may not be due to a disturbance in amelogenin synthesis but to a disturbance in a later process, presumably of protein secretion.

Expression of cytokeratins in human enamel organ

Cytokeratin (CK) is a filament which plays a central role in epithelial tissue and, like the polypeptides of intermediate filaments in general, shows a high degree of tissue specificity. The CK expression patterns of odontogenic epithelia are still poorly described. We studied the distribution of individual CK polypeptides in the human enamel organ at bell stage and in remnants of the dental lamina. Our immunohistochemical study showed that epithelial cells stained for CKs 7, 13, 14 and 19 with slight changes in their pattern during the differentiation phase of odontogenesis. There was negative staining for all other CK polypeptides tested (CKs 8, 10, 16, 17 and 18). Most of the CKs in the enamel organ epithelia did not show differences related to the stage-specific state of differentiation, except for CKs 14 and 19 at the inner enamel epithelium. A strong label for CK 14 was present at the inner dental epithelium at early bell stage, and this was substituted by CK 19 at the late bell stage when the ameloblasts were fully differentiated.

Identification of two additional exons at the 3' end of the amelogenin gene

Alternative splicing of the amelogenin gene generates a family of proteins secreted by ameloblasts that are primarily responsible for tooth enamel formation. Here the presence of two additional exons, downstream of exon 7, which are followed by an alternate polyadenylation site, is reported. Long polymerase chain reaction was used with a rat genomic amelogenin template to amplify sequences downstream from the primary polyadenylation site. Within the amplified sequences, two exons of 45 bp (exon 8) and 110 bp (exon 9) were identified. The presence of these additional exons in the human and mouse genome was indicated by Southern blot analysis. Antibodies raised against a synthetic peptide corresponding to a sequence encoded by exon 9 positively stained ameloblasts in sections of developing murine and porcine unerupted teeth. These results suggest that an amelogenin protein(s) that includes sequences derived from exons 8 and 9 is synthesized by the ameloblasts.

Expression and localization of ferritin mRNA in ameloblasts of rat incisor

At the maturation stage, the ameloblasts of the rat incisor incorporate iron, supplied through the bloodstream, and deposit it into the surface layer of the enamel. In this unique iron transport system, ferritin functions as a transient iron reservoir in the cells. Here the expression of ferritin mRNA and its localization in the rat enamel organ was examined. Among various tissues, the enamel organ showed the highest expression for both ferritin H- and L-chain mRNA, as quantified by reverse transcription-polymerase chain reaction. In situ hybridization using digoxigenin-labelled cRNA probes for each chain demonstrated that both ferritin H- and L-chain mRNA were abundantly expressed in presecretory and secretory ameloblasts. The intensity of the positive hybridization signal gradually decreased toward the incisal direction. Differing from the mRNA localization, ferritin protein was immunologically undetectable in presecretory or secretory ameloblasts but was found in ameloblasts at the maturation stage, into which iron is known to be incorporated from the bloodstream. Thus, the expression of ferritin mRNA precedes the protein expression in the developmental stages of rat incisor ameloblasts, and the translation of ferritin and its half-life are probably controlled by the iron entry, as has been reported for other cell types.

Protein-to-protein interactions: criteria defining the assembly of the enamel organic matrix

Enamel crystallites form in a protein matrix located proximal to the ameloblast cell layer. This unique organic extracellular matrix is constructed from structural protein components biosynthesized and secreted by ameloblasts. To date, three distinct classes of enamel matrix proteins have been cloned. These are the amelogenins, tuftelin, and ameloblastin, with recent data implicating ameloblastin gene expression during cementogenesis. The organic enamel extracellular matrix undergoes assembly to provide a three-dimensional array of protein domains that carry out the physiologic function of guiding enamel hydroxyapatite crystallite formation. Using the yeast two-hybrid system, we have surveyed these three known enamel gene products for their ability to direct self-assembly. We measured the capacity of the enamel gene products to direct protein-to-protein interactions, a characteristic of enamel proteins predicated to be required for self-assembly. We provide additional evidence for the self-assembly nature of amelogenin and tuftelin. Ameloblastin self-assembly could not be demonstrated, nor were protein-to-protein interactions observed between ameloblastin and either amelogenin or tuftelin. Within the limits of the yeast two-hybrid assay, these findings constrain the emerging model of enamel matrix assembly by helping to define the limits of enamel matrix protein-protein interactions that are believed to guide enamel mineral crystallite formation.

Sheath proteins: synthesis, secretion, degradation and fate in forming enamel

We investigated expression of ameloblastin and sheathlin, recently cloned enamel matrix proteins from the rat and pig, in forming enamel immunocytochemically and immunochemically, using region-specific antibodies. The results obtained from the rat and pig were essentially the same. Antibodies which recognize the N-terminal region stained the secretory machinery of the secretory ameloblast and the entire thickness of the enamel matrix, especially the peripheral region of the enamel rod. Immunostained protein bands were observed near 65 or 70 kDa and below 20 kDa. C-terminal-specific antibodies stained the secretory machinery of the ameloblast and the immature enamel adjacent to the secretion sites. Immunostained protein bands were found ranging from 25 to 70 kDa. Antibodies which recognize a region in the protein just prior to the C-terminal region stained the cis-side of the Golgi apparatus but not the enamel matrix. Immunostained protein bands were observed of about 55 kDa. These results suggest that post-translational and post-secretory modifications of ameloblastin and sheathlin are similar to each other, and further showed that their cleaved N-terminal polypeptides concentrate in the prism sheath. We propose that sheathlin and ameloblastin share the same role in amelogenesis and should be classified as sheath proteins.

Gene expression and immunolocalisation of amelogenins in developing embryonic and neonatal hamster teeth

Amelogenins are a group of related matrix proteins, synthesised and secreted by ameloblasts during the formation of dental enamel. We have examined expression patterns and the tissue distribution of amelogenins by in situ hybridisation and by immunohistochemistry of developing teeth of embryonic (E12-E15) and neonatal (1- to 4-day-old) golden hamsters. Amelogenin expression and (intracellular) immunostaining for amelogenins were first observed in late embryonic stages in E14 incisors and E15 first molars in partially polarised pre-ameloblasts located along a thin layer of predentine before any overt deposition of enamel. Expression of mRNA and protein staining for amelogenins increased with age and early pre-dentine became immunopositive. The highest mRNA levels and substantial immunostaining for amelogenins were noted in neonatal-stage secretory ameloblasts fully engaged in enamel matrix deposition. After completion of the secretory phase, amelogenin gene expression continued at a lower level in post-secretory stages and was seen in transition-phase and maturation-phase ameloblasts. No amelogenin transcripts were observed in odontoblasts at any stage of their development. However, young odontoblasts stained weakly with anti-amelogenin antibodies before they formed the first layer of dentine, although this staining disappeared in odontoblasts at later stages of development. We conclude that amelogenin gene transcription occurs as early as the polarisation stage of pre-ameloblasts and is closely followed by translation of mRNA into amelogenin proteins. Odontoblasts do not transcribe the amelogenin gene and probably endocytose and digest amelogenins from the pre-dentine. Amelogenins are also transcribed but at a low level in post-secretory stages of amelogenesis.

Protein interactions during assembly of the enamel organic extracellular matrix

Enamel is the outermost covering of teeth and contains the largest hydroxyapatite crystallites formed in the vertebrate body. Enamel forms extracellularly through the ordered assembly of a protein scaffolding that regulates crystallite dimensions. The two most studied proteins of the enamel extracellular matrix (ECM) are amelogenin and tuftelin. The underlying mechanism for assembly of the proteins within the enamel extracellular matrix and the regulatory role of crystallite-protein interactions have proven elusive. We used the two-hybrid system to identify and define minimal protein domains responsible for supra molecular assembly of the enamel ECM. We show that amelogenin proteins self-assemble, and this self-assembly depends on the amino-terminal 42 residues interacting either directly or indirectly with a 17-residue domain in the carboxyl region. Amelogenin and tuftelin fail to interact with each other. Based upon this data, and advances in the field, a model for amelogenin assemblies that direct enamel biomineralization is presented.

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.

The biomimetics of enamel: a paradigm for organized biomaterials synthesis

The formation of enamel takes place through a sequence of processes that can be mimicked in inorganic materials chemistry. This chapter describes the generic features of enamel biomineralization in terms of a house-building analogy. Four stages are identified: supramolecular preorganization and spatial patterning; interfacial molecular recognition in inorganic nucleation; vectorial crystallization; and pattern evolution and hierarchy. Each of these concepts can be translated into synthetic approaches to the formation of inorganic materials with organized architectures. An example of applying this biomimetic paradigm is described. Supersaturated water-in-oil microemulsions have been used to synthesize microskeletal calcium phosphates by controlled nucleation and vectorial growth in constrained reaction environments. The results of these preliminary studies suggest that biomimetic concepts could be useful in the fabrication of biomaterial implants with controlled porosity and microstructure.

Molecular strategies of tooth enamel formation are highly conserved during vertebrate evolution

The vertebrate body plan is determined by a variety of morphoregulatory genes that are highly conserved throughout evolution. This review presents a phylogenetic analysis of selected molecular and morphological features in vertebrates with particular emphasis upon the phylogeny of tooth morphogenesis and enamel formation. Three lines of evidence support our hypothesis that the agnathans (e.g. hagfishes) are the most primitive extant vertebrates and that enamel gene products are highly conserved during vertebrate evolution. First, an antibody raised against the polypeptide produced by exon 4 of the mouse amelogenin gene recognizes proteins in hagfish, sharks, reptiles and mammals. Second, electron photomicrographic evidence suggests heterochronic shifts in the relative time and rate of enamel formation during vertebrate tooth evolution. Third, mRNA phenotyping suggests significant homology between amelogenin transcripts expressed in species of various vertebrate phyla including agnathans and mammals. These three lines of evidence indicate that amelogenin gene products are expressed in agnathan, reptilian and mammalian teeth.

Amelogenin proteins of developing dental enamel

The amelogenins of developing dental enamel are tissue-specific proteins, rich in proline, leucine, histidine and glutamyl residues, and synthesized by the ameloblast cells of the inner enamel epithelium. These proteins comprise the bulk of the extracellular matrix that becomes mineralized with a hydroxyapatite phase to become the mature enamel. Examination of the amino acid sequences of amelogenins from a range of mammals shows a high degree of evolutionary sequence conservation, suggestive of specialized function. Recently it has been shown that multiple amelogenin components, observed in the matrix, arise both by a sequence of post-secretory proteolytic processing and by the expression of alternatively spliced mRNAs generated from the amelogenin gene(s) that are located on the sex chromosomes. Although the function of these amelogenins in enamel biomineralization is unknown, physico-chemical studies of recombinant amelogenins have shown that they undergo a self-assembly process in vitro generating supra-molecular 'nanosphere' structures, and recent observations in vivo point to a functional role for the nanospheres in the ultrastructural organization of the secretory enamel matrix, conducive to the organized development of the earliest mineral crystallites.

Expression patterns of RNAs for amelin and amelogenin in developing rat molars and incisors

We have recently identified a novel RNA sequence in ameloblasts, coding for amelin (Cerny et al., 1996). In the present paper, its expression has been compared with that of amelogenin in developing incisors and molars of rats, by means of in situ hybridization of paraffin sections. The RNAs for both amelin and amelogenin were highly expressed in secretory ameloblasts. The expression of RNA for amelogenin gradually decreased in the post-secretory ameloblasts. In contrast, the RNA expression for amelin remained high in post-secretory ameloblasts up to the stage of fusion between dental and oral epithelia at the time of tooth eruption. We suggest that amelin might be involved in the mineralization of enamel or in the attachment of ameloblasts to the enamel surface. The whole-mount in situ hybridization procedure is described for the first time in dental research. It proved to be a useful method and confirmed the results of the conventional in situ hybridization.

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.