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

Ameloblastin binding to biomimetic models of cell membranes - A continuum of intrinsic disorder

OBJECTIVE

A 37-residue amino acid sequence corresponding to the segment encoded by exon-5 of murine ameloblastin (Ambn), AB2 (Y67-Q103), has been implicated with membrane association, ameloblastin self-assembly, and amelogenin-binding. Our aim was to characterize, at the residue level, the structural behavior of AB2 bound to chemical mimics of biological membranes using NMR spectroscopy.

DESIGN

To better define the structure of AB2 using NMR-based methods, recombinant 13C- and 15N-labelled AB2 (*AB2) was prepared and data collected free in solution and with deuterated dodecylphosphocholine (dPC) micelles, deuterated bicelles, and both small and large unilamellar vesicles.

RESULTS

Amide chemical shift and intensity perturbations observed in 1H-15N HSQC spectra of *AB2 in the presence of bicelles and dPC micelles suggest that a region of *AB2, S6-E36 (murine Ambn S68 - E98), associates with the membrane biomimetics. A CSI-3 analysis of the NMR chemical shift assignments for *AB2 free in solution and bound to dPC micelles indicated the peptide remains disordered except for the adoption of a short, 12-residue α-helix, F10-G21 (murine Ambn F72-G83). In dPC micelles, the NOE NMR data was void of patterns characteristic of long-lived helical structure indicating this helix was transient in nature.

CONCLUSIONS

A continuum of intrinsic disorder in the membrane-bound state may be responsible for ameloblastin's ability to dynamically interact with multiple partners at the same site during amelogenesis.

Loss of BMP2 and BMP4 Signaling in the Dental Epithelium Causes Defective Enamel Maturation and Aberrant Development of Ameloblasts

BMP signaling is crucial for differentiation of secretory ameloblasts, the cells that secrete enamel matrix. However, whether BMP signaling is required for differentiation of maturation-stage ameloblasts (MA), which are instrumental for enamel maturation into hard tissue, is hitherto unknown. To address this, we used an in vivo genetic approach which revealed that combined deactivation of the Bmp2 and Bmp4 genes in the murine dental epithelium causes development of dysmorphic and dysfunctional MA. These fail to exhibit a ruffled apical plasma membrane and to reabsorb enamel matrix proteins, leading to enamel defects mimicking hypomaturation amelogenesis imperfecta. Furthermore, subsets of mutant MA underwent pathological single or collective cell migration away from the ameloblast layer, forming cysts and/or exuberant tumor-like and gland-like structures. Massive apoptosis in the adjacent stratum intermedium and the abnormal cell-cell contacts and cell-matrix adhesion of MA may contribute to this aberrant behavior. The mutant MA also exhibited severely diminished tissue non-specific alkaline phosphatase activity, revealing that this enzyme's activity in MA crucially depends on BMP2 and BMP4 inputs. Our findings show that combined BMP2 and BMP4 signaling is crucial for survival of the stratum intermedium and for proper development and function of MA to ensure normal enamel maturation.

Ameloblastin Binds to Phospholipid Bilayers via a Helix-Forming Motif within the Sequence Encoded by Exon 5

Ameloblastin (Ambn), the most abundant non-amelogenin enamel protein, is intrinsically disordered and has the potential to interact with other enamel proteins and with cell membranes. Here, through multiple biophysical methods, we investigated the interactions between Ambn and large unilamellar vesicles (LUVs), whose lipid compositions mimicked cell membranes involved in epithelial cell-extracellular matrix adhesion. Using a series of Ambn Trp/Phe variants and Ambn mutants, we further showed that Ambn binds to LUVs through a highly conserved motif within the sequence encoded by exon 5. Synthetic peptides derived from different regions of Ambn confirmed that the sequence encoded by exon 5 is involved in LUV binding. Sequence analysis of Ambn across different species showed that the N-terminus of this sequence contains a highly conserved motif with a propensity to form an amphipathic helix. Mutations in the helix-forming sequence resulted in a loss of peptide binding to LUVs. Our in vitro data suggest that Ambn binds the lipid membrane directly through a conserved helical motif and have implications for biological events such as Ambn-cell interactions, Ambn signaling, and Ambn secretion via secretory vesicles.

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.

Intrinsically disordered enamel matrix protein ameloblastin forms ribbon-like supramolecular structures via an N-terminal segment encoded by exon 5

Tooth enamel, the hardest tissue in the body, is formed by the evolutionarily highly conserved biomineralization process that is controlled by extracellular matrix proteins. The intrinsically disordered matrix protein ameloblastin (AMBN) is the most abundant nonamelogenin protein of the developing enamel and a key element for correct enamel formation. AMBN was suggested to be a cell adhesion molecule that regulates proliferation and differentiation of ameloblasts. Nevertheless, detailed structural and functional studies on AMBN have been substantially limited by the paucity of the purified nondegraded protein. With this study, we have developed a procedure for production of a highly purified form of recombinant human AMBN in quantities that allowed its structural characterization. Using size exclusion chromatography, analytical ultracentrifugation, transmission electron, and atomic force microscopy techniques, we show that AMBN self-associates into ribbon-like supramolecular structures with average widths and thicknesses of 18 and 0.34 nm, respectively. The AMBN ribbons exhibited lengths ranging from tens to hundreds of nm. Deletion analysis and NMR spectroscopy revealed that an N-terminal segment encoded by exon 5 comprises two short independently structured regions and plays a key role in self-assembly of AMBN.

Binding of amelogenin to MMP-9 and their co-expression in developing mouse teeth

Amelogenin is the most abundant matrix protein in enamel. Proper amelogenin processing by proteinases is necessary for its biological functions during amelogenesis. Matrix metalloproteinase 9 (MMP-9) is responsible for the turnover of matrix components. The relationship between MMP-9 and amelogenin during tooth development remains unknown. We tested the hypothesis that MMP-9 binds to amelogenin and they are co-expressed in ameloblasts during amelogenesis. We evaluated the distribution of both proteins in the mouse teeth using immunohistochemistry and confocal microscopy. At postnatal day 2, the spatial distribution of amelogenin and MMP-9 was co-localized in preameloblasts, secretory ameloblasts, enamel matrix and odontoblasts. At the late stages of mouse tooth development, expression patterns of amelogenin and MMP-9 were similar to that seen in postnatal day 2. Their co-expression was further confirmed by RT-PCR, Western blot and enzymatic zymography analyses in enamel organ epithelial and odontoblast-like cells. Immunoprecipitation assay revealed that MMP-9 binds to amelogenin. The MMP-9 cleavage sites in amelogenin proteins across species were found using bio-informative software program. Analyses of these data suggest that MMP-9 may be involved in controlling amelogenin processing and enamel formation.

Adipose tissue-derived stem cell in vitro differentiation in a three-dimensional dental bud structure

Tooth morphogenesis requires sequential and reciprocal interactions between the cranial neural crest-derived mesenchymal cells and the stomadial epithelium, which regulate tooth morphogenesis and differentiation. We show how mesenchyme-derived single stem cell populations can be induced to transdifferentiate in vitro in a structure similar to a dental bud. The presence of stem cells in the adipose tissue has been previously reported. We incubated primary cultures of human adipose tissue-derived stem cells in a dental-inducing medium and cultured the aggregates in three-dimensional conditions. Four weeks later, cells formed a three-dimensional organized structure similar to a dental bud. Expression of dental tissue-related markers was tested assaying lineage-specific mRNA and proteins by RT-PCR, immunoblot, IHC, and physical-chemical analysis. In the induction medium, cells were positive for ameloblastic and odontoblastic markers as both mRNAs and proteins. Also, cells expressed epithelial, mesenchymal, and basement membrane markers with a positional relationship similar to the physiologic dental morphogenesis. Physical-chemical analysis revealed 200-nm and 50-nm oriented hydroxyapatite crystals as displayed in vivo by enamel and dentin, respectively. In conclusion, we show that adipose tissue-derived stem cells in vitro can transdifferentiate to produce a specific three-dimensional organization and phenotype resembling a dental bud even in the absence of structural matrix or scaffold to guide the developmental process.

Ameloblastin promotes bone growth by enhancing proliferation of progenitor cells and by stimulating immunoregulators

In this study, we examined the role of the enamel matrix protein, ameloblastin, in bone growth and remodelling, and attempted to identify some of the molecular mechanisms involved in these processes. The effects of recombinant ameloblastin (rAmbn) were tested in vivo in rats, and in vitro in primary human mesenchymal stem cells, osteoblasts, chondrocytes, and osteoclasts. We used a microarray technique to identify genes that were regulated in human osteoblasts and verified our findings using multiplex protein analysis and real-time RT-PCR. Recombinant ameloblastin was found to stimulate bone healing in vivo, and to enhance the proliferation of mesenchymal stem cells and osteoblasts, as well as the differentiation of osteoclast precursor cells in vitro. The most profound effect was on the regulation of genes related to immune responses as well as on the expression of cytokines and markers of bone cell differentiation, indicating that ameloblastin has an effect on mesenchymal cell differentiation. A receptor has not yet been identified, but we found rAmbn to induce, directly and indirectly, signal transducer and activator of transcription (STAT) 1 and 2 and downstream factors in the interferon pathway.

Fate of HERS during tooth root development

Tooth root development begins after the completion of crown formation in mammals. Previous studies have shown that Hertwig's epithelial root sheath (HERS) plays an important role in root development, but the fate of HERS has remained unknown. In order to investigate the morphological fate and analyze the dynamic movement of HERS cells in vivo, we generated K14-Cre;R26R mice. HERS cells are detectable on the surface of the root throughout root formation and do not disappear. Most of the HERS cells are attached to the surface of the cementum, and others separate to become the epithelial rest of Malassez. HERS cells secrete extracellular matrix components onto the surface of the dentin before dental follicle cells penetrate the HERS network to contact dentin. HERS cells also participate in the cementum development and may differentiate into cementocytes. During root development, the HERS is not interrupted, and instead the HERS cells continue to communicate with each other through the network structure. Furthermore, HERS cells interact with cranial neural crest derived mesenchyme to guide root development. Taken together, the network of HERS cells is crucial for tooth root development.

Spatiotemporal expression of ameloblastin isoforms during murine tooth development

Ameloblasts synthesize and secrete the enamel matrix proteins (amelogenin, ameloblastin, and enamelin). This investigation examined the profiles of ameloblastin in the ameloblasts and in the enamel matrix during different postnatal (PN) days (days 0-9) of development of mouse molar, using an antibody specific for C-terminal sequence of ameloblastin (Ct; GNKVHQPQVHNAWRF). Ameloblastin is found in three different molecular sizes (37, 55, and 66 kDa) in both ameloblasts and enamel matrix during PN development. In the ameloblasts, the sequence of expression of these fractions varied. The 37-kDa fraction was observed (even before the appearances of mRNA of the proteases, enamelysin and kallikrein-4) on days 0 and 1, persisted until day 3, and was not found thereafter. Other isoforms (55 and 66 kDa) distinctly appeared in ameloblasts after day 1, reached a peak on day 5, and remained thereafter. The Ct-positive granules appeared beaded in the ameloblasts on day 3. In the extracellular matrix, a 37-kDa (but not 66- or 55-kDa) fraction was detected on days 0 and 1 and remained in the matrix throughout the PN days. The larger isoforms (55 and 66 kDa) appeared in the enamel matrix from day 3 onward. On days 0-3, but not later, the 37-kDa isoform co-localizes with amelogenin in Tomes' process and formative enamel, as revealed by laser scan confocal microscopy. Autoradiography confirmed accumulation of 3H-labeled amelogenin trityrosyl motif peptide in the region of Tomes' process and formative enamel from day 0 to 3. These observations suggest that the 37-kDa isoform interacts with amelogenin during early tooth development.

A mutation in the enamelin gene in a mouse model

Amelogenesis imperfecta is an inherited disorder affecting tooth enamel formation. We previously isolated a mouse strain with an amelogenesis imperfecta phenotype (ATE1 mice) from a dominant ethylnitrosourea screen and mapped the disease-causing defect to a 9-cM region of mouse chromosome 5. In the current study, we tested the hypothesis that there is a mutation in enamelin (ENAM) or ameloblastin (AMBN), both of which are located within the linkage region, by sequencing these two candidate genes. Analysis of our data shows that the amelogenesis imperfecta phenotype is linked to a C > T transition in exon 8 of the enamelin gene. The mutation predicts a C826T transition, which is present in the enamelin transcript and changes the glutamine (Gln) codon at position 176 into a premature stop codon (Gln176X). Conversely, no mutation could be detected in the ameloblastin gene. These results define the ATE1 mice as a model for local hypoplastic autosomal-dominant amelogenesis imperfecta (AIH2), which is caused by enamelin truncation mutations in humans.

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.

Genes and related proteins involved in amelogenesis imperfecta

Dental enamel formation is a remarkable example of a biomineralization process. The exact mechanisms involved in this process remain partly obscure. Some of the genes encoding specific enamel proteins have been indicated as candidate genes for amelogenesis imperfecta. Mutational analyses within studied families have supported this hypothesis. Mutations in the amelogenin gene (AMELX) cause X-linked amelogenesis imperfecta, while mutations in the enamelin gene (ENAM) cause autosomal-inherited forms of amelogenesis imperfecta. Recent reports involve kallikrein-4 (KLK4), MMP-20, and DLX3 genes in the etiologies of some cases. This paper focuses mainly on the candidate genes involved in amelogenesis imperfecta and the proteins derived from them, and reviews current knowledge on their structure, localization within the tissue, and correlation with the various types of this disorder.

The genetics of amelogenesis imperfecta: a review of the literature

A melogenesis imperfecta (AI) is a group of inherited defects of dental enamel formation that show both clinical and genetic heterogeneity. Enamel findings in AI are highly variable, ranging from deficient enamel formation to defects in the mineral and protein content. Enamel formation requires the expression of multiple genes that transcribes matrix proteins and proteinases needed to control the complex process of crystal growth and mineralization. The AI phenotypes depend on the specific gene involved, the location and type of mutation, and the corresponding putative change at the protein level. Different inheritance patterns such as X-linked, autosomal dominant and autosomal recessive types have been reported. Mutations in the amelogenin, enamelin, and kallikrein-4 genes have been demonstrated to result in different types of AI and a number of other genes critical to enamel formation have been identified and proposed as candidates for AI. The aim of this article was to present an evaluation of the literature regarding role of proteins and proteinases important to enamel formation and mutation associated with AI.

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.

Laminin α2 Is Essential for Odontoblast Differentiation Regulating Dentin Sialoprotein Expression

Laminin alpha2 is subunit of laminin-2 (alpha2beta1gamma1), which is a major component of the muscle basement membrane. Although the laminin alpha2 chain is expressed in the early stage of dental mesenchyme development and localized in the tooth germ basement membrane, its expression pattern in the late stage of tooth germ development and molecular roles are not clearly understood. We analyzed the role of laminin alpha2 in tooth development by using targeted mice with a disrupted lama2 gene. Laminin alpha2 is expressed in dental mesenchymal cells, especially in odontoblasts and during the maturation stage of ameloblasts, but not in the pre-secretory or secretory stages of ameloblasts. Lama2 mutant mice have thin dentin and a widely opened dentinal tube, as compared with wild-type and heterozygote mice, which is similar to the phenotype of dentinogenesis imperfecta. During dentin formation, the expression of dentin sialoprotein, a marker of odontoblast differentiation, was found to be decreased in odontoblasts from mutant mice. Furthermore, in primary cultures of dental mesenchymal cells, dentin matrix protein, and dentin sialophosphoprotein, mRNA expression was increased in laminin-2 coated dishes but not in those coated with other matrices, fibronectin, or type I collagen. Our results suggest that laminin alpha2 is essential for odontoblast differentiation and regulates the expression of dentin matrix proteins.

Expression of alternatively spliced RNA transcripts of amelogenin gene exons 8 and 9 and its end products in the rat incisor

In addition to seven known exons of the amelogenin gene, recent studies have identified two exons downstream of amelogenin exon 7 in genomic DNA of mouse and rat. Here the spatial and temporal expression of mRNAs and of the translated proteins derived from alternative splicing of the amelogenin gene ending with exon 8 and exon 9 were examined by in situ hybridization (ISH) and immunohistochemistry (IHC). RNA signals for exons 8 and 9 were expressed in the ameloblast layer extending from early presecretory to postsecretory transitional stages of amelogenesis. IHC of amelogenin proteins that include sequences encoded by these exons demonstrated identical localization of these proteins in the ameloblast layer corresponding to RNA signals identified by ISH. There was intense immunostaining of the enamel matrix secreted by these cells. Western blotting analysis of rat enamel proteins revealed three distinct protein bands with sequences encoded by the new exons. These data confirmed the existence of the transcripts of alternatively spliced mRNAs coding for exons 8 and 9 of the amelogenin gene in rat tooth germs and suggest that the translated proteins contribute to the heterogeneity of amelogenins and have some significant roles in enamel formation and mineralization.

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.

Amelin extracellular processing and aggregation during rat incisor amelogenesis

Amelin (also known as ameloblastin and sheathlin) is a recently described protein that is secreted by ameloblasts during enamel formation. Here, the extracellular distribution and processing of amelin during rat incisor amelogenesis were investigated by Western blot probing using anti-recombinant rat amelin antibodies. In addition, the solubility behaviour and aggregative properties of rat amelin were investigated using a sequential extraction procedure involving (1) extraction with simulated enamel fluid to extract proteins most likely to be soluble in vivo; (2) extraction with phosphate buffer to desorb proteins bound to enamel crystal surfaces; (3) extraction with sodium dodecyl sulphate (SDS) to extract proteins present as insoluble aggregates; followed by (4) a final acid demineralization step to release any remaining proteins. Proteins immunoreactive to the anti-amelin antibodies were detectable in secretory- and transition-stage enamel. Maturation-stage enamel appeared devoid of amelin. The largest immunoreactive protein detected migrated at 68 kDa on SDS gels, corresponding to the M(r) of nascent amelin. Other immunoreactive bands at 52, 40, 37, 19, 17, 16, 15, 14 and 13 kDa were presumably amelin processing products. The sequential extraction procedure revealed that the 68-, 52-, 40-, 37- and 13-kDa amelins were completely extracted under solution conditions similar to those reported to exist in vivo. In contrast, the 19-, 17- and 16-kDa amelins were only partially extracted, whilst the 15- and 14-kDa amelins could not be extracted with simulated enamel fluid. A proportion of the remaining 17- and 16-kDa amelins was desorbed from the enamel crystals with phosphate buffer and appeared to have been mineral-bound. The 15- and 14-kDa amelins and the remainder of the 17- and 16-kDa amelins were extracted with SDS only, suggesting that these species were present in vivo as an insoluble aggregate. The results provide additional information on amelin processing and degradation, and on how such processing influences the solubility and aggregative properties of amelin-derived proteins.

The pH dependent amelogenin solubility and its biological significance

Amelogenins are a group of extracellular enamel matrix proteins which are believed to be involved in the regulation of the size and habit of enamel crystals. The aim of this study was to compare the solubility properties of several amelogenins in various pH (4.0-9.0) solutions with an ionic strength (IS) of 0.15 M using the Micro BCA protein assay at 25 degrees C or 37 degrees C. The solubility of the recombinant amelogenin rM179 was lowest (0.7 mg/ml) close to its isoelectric point and it increased below and above this point. The solubility of the recombinant amelogenin rM166 remained almost the same (1-2 mg/ml) as the pH rose from 6.0 to 9.0 and it increased as the solution became more acidic. Synthetic "tyrosine-rich amelogenin polypeptide" (TRAP) was extremely insoluble (<0.2 mg/ml) in the pH range studied while synthetic "leucine-rich amelogenin polypeptide" (LRAP) was readily soluble (>3.3 mg/ml). The native porcine amelogenin with apparent molecular weight 25 kDa shared similar solubility behavior to rM179. The porcine 23 kDa amelogenin was only sparingly soluble (0.3-0.8 mg/ml) over a wide range of pH. Interestingly, the porcine 20 kDa amelogenin was remarkably soluble in the pH range of 4.0 to 6.0 (approximately 12 mg/ml), but the solubility dropped strikingly to only approximately 0.2 mg/ml at pH larger than approximately 7.0. The strong dependence of amelogenin solubility on solution pH may be involved in the regulation of aggregation, enzymatic degradation and the binding properties of amelogenins, thus playing an important role in enamel biomineralization.

Primary culture and characterization of enamel organ epithelial cells

The cells of the enamel organ are programmed by signals such as growth factors and extracellular matrix components to differentiate and form dental enamel. To study how the enamel organ epithelial cells control enamel development, we have begun to characterize a primary porcine enamel organ epithelial cell culture system. The unerupted molars of 3 month old pigs were isolated, the cells were digested into a single cell suspension and grown in media either with or without serum. Expression of amelogenin and ameloblastin mRNA was monitored by RT PCR, and protein secretion was identified by immunohistochemistry. Cells grown in MEM formed a mixed cell population of epithelial- and fibroblast-like cells which grew past confluence, formed nodules, mineralized, and expressed low levels of amelogenin and ameloblastin protein. In LHC-9 media, which is selective for epithelial cells, the cells did not grow past confluence but secreted amelogenin and ameloblastin proteins more strongly. Cell viability was maintained in both serum-free and serum-containing media. However, in the serum-free media, cell proliferation proceeded slowly. Although cells grown in MEM mineralized, the mixed cell population may make studies of specific ameloblast-like cells more difficult. However, cells grown in a culture media selective for epithelial cells will require modifications such as cell immortalization to allow long term studies of cell regulation and interaction. In summary, we have established an enamel organ epithelial cell culture system which will enable us to study the role of ameloblasts in enamel matrix formation, ameloblast regulation, as well as cell-matrix interactions. Selection of specific culture conditions will depend on the questions being addressed in individual studies.

The structural biology of the developing dental enamel matrix

The biomineralization of the dental enamel matrix with a carbonated hydroxyapatite mineral generates one of the most remarkable examples of a vertebrate mineralized tissue. Recent advances in the molecular biology of ameloblast gene products have now revealed the primary structures of the principal proteins involved in this extracellular mineralizing system, amelogenins, tuftelins, ameloblastins, enamelins, and proteinases, but details of their secondary, tertiary, and quaternary structures, their interactions with other matrix and or cell surface proteins, and their functional role in dental enamel matrix mineralization are still largely unknown. This paper reviews our current knowledge of these molecules, the probable molecular structure of the enamel matrix, and the functional role of these extracellular matrix proteins. Recent studies on the major structural role played by the amelogenin proteins are discussed, and some new data on synthetic amelogenin matrices are reviewed.

Sequential expression of matrix protein genes in developing rat teeth

Tooth organogenesis is dependent on reciprocal and sequential epithelial-mesenchymal interactions and is marked by the appearance of phenotypic matrix macromolecules in both dentin and enamel. The organic matrix of enamel is composed of amelogenins, ameloblastin/amelin, enamelins and tuftelin. Dentin is mainly composed of type I collagen, but its specificity arises from the nature of the non-collagenous proteins (NCPs) involved in mineralization, phosphophoryn (DPP), dentin sialoprotein (DSP), osteocalcin, bone sialoprotein and dentin matrix protein-1 (Dmp1). In this paper, we studied the pattern of expression of four mineralizing protein genes (type I collagen, amelogenin, DSPP and osteocalcin) during the development of rat teeth by in situ hybridization on serial sections. For this purpose, we used an easy and rapid procedure to prepare highly-specific labeled single-stranded DNA probes using asymmetric polymerase chain reaction (PCR). Our results show that type I collagen is primarily expressed in polarizing odontoblasts, followed by the osteocalcin gene expression in the same polarized cells. Concomitantly, polarized ameloblasts start to accumulate amelogenin mRNAs and transiently express the DSPP gene. This latter expression switches over to odontoblasts whereas mineralization occurs. At the same time, osteocalcin gene expression decreases in secretory odontoblasts. Osteocalcin may thus act as an inhibitor of mineralization whereas DSP/DPP would be involved in more advanced steps of mineralization. Amelogenin and type I collagen gene expression increases during dentin mineralization. Their expression is spatially and temporally controlled, in relation with the biological role of their cognate proteins in epithelial-mesenchymal interactions and mineralization.

Comparative immunochemical analyses of the developmental expression and distribution of ameloblastin and amelogenin in rat incisors

Mineralized tissues are unique in using proteins to attract and organize calcium and phosphate ions into a structured mineral phase. A precise knowledge of the expression and extracellular distribution of matrix proteins is therefore very important in understanding their function. The purpose of this investigation was to obtain comparative information on the expression, intracellular and extracellular distribution, and dynamics of proteins representative of the two main classes of enamel matrix proteins. Amelogenins were visualized using an antibody and an mRNA probe prepared against the major alternatively spliced isoform in rodents, and nonamelogenins by antibodies and mRNA probes specific to one enamel protein referred to by three names: ameloblastin, amelin, and sheathlin. Qualitative and quantitative immunocytochemistry, in combination with immunoblotting and in situ hybridization, indicated a correlation between mRNA signal and sites of protein secretion for amelogenin, but not for ameloblastin, during the early presecretory and mid- to late maturation stages, during which mRNA signals were detected but no proteins appeared to be secreted. Extracellular amelogenin immunoreactivity was generally weak near secretory surfaces, increasing over a distance of about 1.25 microm to reach a level slightly above an amount expected if the protein were being deposited evenly across the enamel layer. Immunolabeling for ameloblastin showed an inverse pattern, with relatively more gold particles near secretory surfaces and much fewer deeper into the enamel layer. Administration of brefeldin A and cycloheximide to stop protein secretion revealed that the immunoblotting pattern of amelogenin was relatively stable, whereas ameloblastin broke down rapidly into lower molecular weight fragments. The distance from the cell surface at which immunolabeling for amelogenin stabilized generally corresponded to the point at which that for ameloblastin started to show a net reduction. These data suggest a correlation between the distribution of amelogenin and ameloblastin and that intact ameloblastin has a transient role in promoting/stabilizing crystal elongation. (J Histochem Cytochem 46:911-934, 1998)

Laminin gamma2 expression is developmentally regulated during murine tooth morphogenesis and is intense in ameloblasts

Mutations in the laminin gamma2 gene cause junctional epidermolysis bullosa, and enamel hypoplasias are frequently seen in these patients. Laminin gamma2 is one of the three polypeptide chains forming the basement membrane glycoprotein laminin-5. We have localized the expression of the laminin gamma2 gene by in situ hybridization during mouse tooth development from early morphogenesis to completion of crown development. The expression was restricted to epithelial cells. During the early morphogenesis of the tooth germ, laminin gamma2 was expressed by the outer dental epithelium and by the stellate reticulum cells. No expression was detected in the cells of the inner dental epithelium giving rise to ameloblasts. The pre-ameloblasts remained negative during the early bell stage, but, interestingly, expression was very prominently upregulated as the cells differentiated into ameloblasts. This upregulation appeared to coincide with the start of enamel matrix secretion. The ameloblasts expressed laminin gamma2 intensely throughout the period of active enamel deposition. The expression continued at a lower level in the maturation-stage ameloblasts covering the enamel surface. Immunolocalization of laminin-5 with polyclonal antibodies indicated that the protein formed a continuous lining at the basal surfaces of the cells expressing the laminin gamma2 transcripts. We suggest that the role of laminin-5 during enamel formation may be to strengthen the anchorage of the ameloblasts to the enamel matrix, and that the pathogenesis of enamel hypoplasias in cases of laminin-5 mutations could be associated with detachment of the ameloblast cell layer from the enamel surface.

Quantitative analysis of amelogenin solubility

Amelogenins are a group of extracellular enamel matrix proteins which are believed to be involved in the regulation of the size and habits of forming enamel crystals. The aim of this study was to compare the solubility properties of several amelogenins at various pH (from 4.0 to 9.0) at constant ionic strength (IS), and to examine the influence of buffer composition, IS, and divalent metal ions (including Ca2+, Mg2+, and Zn2+) on amelogenin solubility. The solubility of the recombinant murine amelogenin ("rM179") was minimum near its isoelectric point and increased rapidly below and above, regardless of buffer composition. A similar trend was observed for the native porcine ("25K") amelogenin. Porcine "23K" amelogenin was only sparingly soluble from pH of 4.0 to 9.0, in contrast to the analogous recombinant "rM166", which was more soluble in acidic solutions. The synthetic amelogenin polypeptide "TRAP" was extremely insoluble, while synthetic LRAP was readily soluble. Porcine "20K" amelogenin solubility increased strikingly as the solution pH was lowered from 7.0 to 6.0. Increasing IS decreased the solubility of rM179. While Zn2+ reduced rM179 solubility, Ca2+ and Mg2+ showed no significant effects. We conclude that the solubility of amelogenin was dependent on the primary structure, solution pH, and IS, and the low solubility of amelogenins under physiological conditions may result from their tendency to form quaternary (aggregate) structures in vivo.

Cellular and chemical events during enamel maturation

This review focuses on the process of enamel maturation, a series of events associated with slow, progressive growth in the width and thickness of apatitic crystals. This developmental step causes gradual physical hardening and transformation of soft, newly formed enamel into one of the most durable mineralized tissues produced biologically. Enamel is the secretory product of specialized epithelial cells, the ameloblasts, which make this covering on the crowns of teeth in two steps. First, they roughly "map out" the location and limits (overall thickness) of the entire extracellular layer as a protein-rich, acellular, and avascular matrix filled with thin, ribbon-like crystals of carbonated hydroxyapatite. These initial crystals are organized spatially into rod and interrod territories as they form, and rod crystals are lengthened by Tomes' processes in tandem with appositional movement of ameloblasts away from the dentin surface. Once the full thickness of enamel has been formed, ameloblasts initiate a series of repetitive morphological changes at the enamel surface in which tight junctions and deep membrane infoldings periodically appear (ruffle-ended), then disappear for short intervals (smooth-ended), from the apical ends of the cells. As this happens, the enamel covered by these cells changes rhythmically in net pH from mildly acidic (ruffle-ended) to near-physiologic (smooth-ended) as mineral crystals slowly expand into the "spaces" (volume) formerly occupied by matrix proteins and water. Matrix proteins are processed and degraded by proteinases throughout amelogenesis, but they undergo more rapid destruction once ameloblast modulation begins. Ruffle-ended ameloblasts appear to function primarily as a regulatory and transport epithelium for controlling the movement of calcium and other ions such as bicarbonate into enamel to maintain buffering capacity and driving forces optimized for surface crystal growth. The reason ruffle-ended ameloblasts become smooth-ended periodically is unknown, although this event seems to be crucial for sustaining long-term crystal growth.

Sheathlin: cloning, cDNA/polypeptide sequences, and immunolocalization of porcine enamel sheath proteins

Sheath proteins designate low-molecular-weight non-amelogenin enamel polypeptides and their parent protein, which concentrate in the sheath space separating rod and inter-rod enamel (Uchida et al., 1995). Two porcine sheath proteins, with apparent molecular weights of 13 and 15 kDa, are characterized by protein sequencing. The primary structures of these polypeptides match a portion of the derived amino acid sequences of clones isolated from a porcine enamel organ epithelia-specific cDNA library. Sheath protein RNA messages differ by the inclusion or deletion of a 45-nucleotide segment and by the use of three alternative polyadenylation/cleavage sites. The secreted proteins are 395 and 380 residues in length, with molecular masses of 42,358 and 40,279 Daltons and calculated isoelectric points of 6.3 and 6.7, respectively. Polyclonal antibodies were raised against a synthetic peptide having the sheathlin-specific sequence EHETQQYEYSGGC. Immunohistochemistry with this antibody demonstrates that the protein encoded by the sheathlin cDNA is preferentially localized in the sheath space. We propose that the porcine sheath proteins and their proteolytic cleavage products be designated "sheathlin".

Tuftelin: enamel mineralization and amelogenesis imperfecta

Tuftelin is a novel acidic enamel protein thought to play a major role in enamel mineralization. Its identity and localization has been confirmed by amino acid composition, enzyme-linked immunosorbant assay, Western blots, indirect immunohistochemistry and high resolution protein-A gold immunocytochemistry. The deduced tuftelin protein (pI 5.2) contains 389 amino acids and has a calculated peptide molecular mass of 43,814 Da. Immunological studies suggest conservation of tuftelin structure between species throughout vertebrate evolution. The cDNA sequence encodes for several putative post-translation sites including one N-glycosylation consensus site, seven O-glycosylation sites and seven phosphorylation sites, as well as an EF-hand calcium-binding domain (with mismatch), localized towards the N-terminal region. At the C-terminal region (residues 252-345) tuftelin contains structurally relevant determinants for self assembly. We recently cloned and partially sequenced the human tuftelin gene (four exons have now been sequenced). These sequences include exon 1 and over 1000 bases of the putative promoter region. Employing fluorescent in situ hybridization, we mapped the human tuftelin gene to chromosome 1q 21-31. Localization of the human tuftelin gene to a well-defined cytogenetic region may be important in understanding the aetiology of autosomally inherited amelogenesis imperfecta, the most common enamel hereditary disease.

Amelin: an enamel-related protein, transcribed in the cells of epithelial root sheath

Since 1974, when Slavkin and his collaborators proposed the epithelial origin of cementum, many experiments have been carried out to provide evidence for deposition of enamel-related proteins along the root surface. However, neither amelogenin nor other proteins have fully satisfied expectations. In previous studies, we have identified a novel mRNA coding for an extracellular-like protein which we called amelin. It was expressed at high levels in secretory and postsecretory ameloblasts in rat molars and incisors. In situ hybridization experiments described in the present study also localized the amelin message to epithelial cells adjacent to the peripheral surface of newly deposited dentin in the root end and to cells embedded in cellular cementum in molars. In incisors, the amelin RNA positive cells were detected in the area where cementum formation had been initiated. No amelogenin RNA signal was found in the cells at the root surface. We postulate that the epithelial cells of the root sheath as well as the ameloblasts are synthesizing amelin which might be one of the key proteins coupled to the process of cementogenesis.