Cloning, characterization, and heterologous expression of exon-4-containing amelogenin mRNAs

Leucine rich amelogenin peptide prevents ovariectomy-induced bone loss in mice

Amelogenins, major extra cellular matrix proteins of developing tooth enamel, are predominantly expressed by ameloblasts and play significant roles in the formation of enamel. Recently, amelogenin has been detected in various epithelial and mesenchymal tissues, implicating that it might have distinct functions in various tissues. We have previously reported that leucine rich amelogenin peptide (LRAP), one of the alternate splice forms of amelogenin, regulates receptor activator of NF-kappa B ligand (RANKL) expression in cementoblast/periodontal ligament cells, suggesting that the amelogenins, especially LRAP, might function as a signaling molecule in bone metabolism. The objective of this study was to identify and define LRAP functions in bone turnover. We engineered transgenic (TgLRAP) mice using a murine 2.3kb α1(I)-collagen promoter to drive expression of a transgene consisting of LRAP, an internal ribosome entry site (IRES) and enhanced green fluorescent protein (EGFP) to study functions of LRAP in bone formation and resorption. Calvarial cell cultures from the TgLRAP mice showed increased alkaline phosphatase (ALP) activity and increased formation of mineralized nodules compared to the cells derived from wild-type (WT) mice. The TgLRAP calvarial cells also showed an inhibitory effect on osteoclastogenesis in vitro. Gene expression comparison by quantitative polymerase chain reaction (Q-PCR) in calvarial cells indicated that bone formation makers such as Runx2, Alp, and osteocalcin were increased in TgLRAP compared to the WT cells. Meanwhile, Rankl expression was decreased in the TgLRAP cells in vitro. The ovariectomized (OVX) TgLRAP mice resisted bone loss induced by ovariectomy resulting in higher bone mineral density in comparison to OVX WT mice. The quantitative analysis of calcein intakes indicated that the ovariectomy resulted in increased bone formation in both WT and TgLRAP mice; OVX TgLRAP appeared to show the most remarkably increased bone formation. The parameters for bone resorption in tissue sections showed increased number of osteoclasts in OVX WT, but not in OVX TgLRAP over that of sham operated WT or TgLRAP mice, supporting the observed bone phenotypes in OVX mice. This is the first report identifying that LRAP, one of the amelogenin splice variants, affects bone turnover in vivo.

Circadian Rhythm Regulates Development of Enamel in Mouse Mandibular First Molar

Rhythmic incremental growth lines and the presence of melatonin receptors were discovered in tooth enamel, suggesting possible role of circadian rhythm. We therefore hypothesized that circadian rhythm may regulate enamel formation through melatonin receptors. To test this hypothesis, we examined expression of melatonin receptors (MTs) and amelogenin (AMELX), a maker of enamel formation, during tooth germ development in mouse. Using qRT-PCR and immunocytochemistry, we found that mRNA and protein levels of both MTs and AMELX in normal mandibular first molar tooth germs increased gradually after birth, peaked at 3 or 4 day postnatal, and then decreased. Expression of MTs and AMELX by immunocytochemistry was significantly delayed in neonatal mice raised in all-dark or all-light environment as well as the enamel development. Furthermore, development of tooth enamel was also delayed showing significant immature histology in those animals, especially for newborn mice raised in all daylight condition. Interestingly, disruption in circadian rhythm in pregnant mice also resulted in delayed enamel development in their babies. Treatment with melatonin receptor antagonist 4P-PDOT in pregnant mice caused underexpression of MTs and AMELX associated with long-lasting deficiency in baby enamel tissue. Electromicroscopic evidence demonstrated increased necrosis and poor enamel mineralization in ameloblasts. The above results suggest that circadian rhythm is important for normal enamel development at both pre- and postnatal stages. Melatonin receptors were partly responsible for the regulation.

Exon4 amelogenin transcripts in enamel biomineralization

Amelogenins are proteins formed by alternative splicing of the amelogenin gene, and are essential for tooth enamel formation. However, the unique functions of various alternatively spliced amelogenins in enamel formation are not well understood. In this study, we determined the spatiotemporal location of amelogenins derived from transcripts containing exon4 (AMG+4) in the enamel matrix, and the relative binding of recombinant AMG+4 to hydroxyapatite (HAP). Immunohistochemistry and mass spectrometry analyses showed that AMG+4 proteins were secreted into the enamel matrix at the early maturation stage. A stage-specific increase in the synthesis of AMG+4 was further supported by our observation that in mice overexpressing leucine-rich amelogenin peptide (TgLRAP), in which ameloblasts differentiate earlier, AMG+4 transcripts were also upregulated earlier. In vitro binding studies, supported by in silico modeling of protein binding to calcium and phosphate, showed that more recombinant AMG+4 bound to hydroxyapatite (HAP) as compared with recombinant AMG-4. The temporal and spatial localization of amelogenins containing exon4 peptide, and their functional differences in HAP binding, suggests that the unique properties of amelogenins containing exon4 cause a specific enhancement of biomineralization related to stabilization of early-formed HAP at the maturation stage.

Alternative Splicing of the Amelogenin Gene in a Caudate Amphibian, Plethodon cinereus

As the major enamel matrix protein contributing to tooth development, amelogenin has been demonstrated to play a crucial role in tooth enamel formation. Previous studies have revealed amelogenin alternative splicing as a mechanism for amelogenin heterogeneous expression in mammals. While amelogenin and its splicing forms in mammalian vertebrates have been characterized, splicing variants of amelogenin gene still remains largely unknown in non-mammalian species. Here, using PCR and sequence analysis we discovered two novel amelogenin transcript variants in tooth organ extracts from a caudate amphibian, the salamander Plethodoncinereus. The one was shorter -S- (416 nucleotides including untranslated regions, 5 exons) and the other larger -L- (851 nt, 7 exons) than the previously published “normal” gene in this species -M- (812 nucleotides, 6 exons). This is the first report demonstrating the amelogenin alternative splicing in amphibian, revealing a unique exon 2b and two novel amelogenin gene transcripts in Plethodoncinereus.

Leucine rich amelogenin peptide alters ameloblast differentiation in vivo

Highly mineralized tooth enamel develops from an extracellular matrix chiefly comprised of amelogenins formed by splicing of 7 (human) or 9 (rodent) exons secreted from specialized epithelial cells known as ameloblasts. Here we examined the role of the 59 amino acid alternatively spliced amelogenin known as leucine rich amelogenin peptide (LRAP) on enamel formation, using transgenic murine models in which LRAP overexpression is driven by an amelogenin promoter (TgLRAP). Beginning in the secretory stage of mouse amelogenesis, we found a reduced thickness of enamel matrix and a loss of Tomes' processes, followed by upregulated amelogenin mRNA expression, inhibited amelogenin secretion and loss of cell polarity. In the presecretory stage (P0) amelogenin m180 mRNA expression was increased 58 fold along with a 203 fold increase in MMP-20 expression and 3.5 and 3.2 fold increased in respectively enamelin and ameloblastin. When LRAP was overexpressed on an amelogenin knockout mouse model, the ameloblasts were not affected. Further, expression of the global chromatin organizer and transcription factor SATB1 was reduced in secretory stage TgLRAP ameloblasts. These findings identify a cellular role for LRAP in enamel formation that is not directly related to directing enamel crystal formation as is reported to be the primary function of full length amelogenins. The effect of LRAP overexpression in upregulating amelogenins, MMP-20 and SATB1, suggests a role in protein regulation critical to ameloblast secretion and matrix processing, to form a mineralized enamel matrix.

Identification of a novel splicing form of amelogenin gene in a reptile, Ctenosaura similis

Amelogenin, the major enamel matrix protein in tooth development, has been demonstrated to play a significant role in tooth enamel formation. Previous studies have identified the alternative splicing of amelogenin in many mammalian vertebrates as one mechanism for amelogenin heterogeneous expression in teeth. While amelogenin and its splicing forms in mammalian vertebrates have been cloned and sequenced, the amelogenin gene, especially its splicing forms in non-mammalian species, remains largely unknown. To better understand the mechanism underlying amelogenin evolution, we previously cloned and characterized an amelogenin gene sequence from a squamate, the green iguana. In this study, we employed RT-PCR to amplify the amelogenin gene from the black spiny-tailed iguana Ctenosaura similis teeth, and discovered a novel splicing form of the amelogenin gene. The transcript of the newly identified iguana amelogenin gene (named C. Similis-T2L) is 873 nucleotides long encoding an expected polypeptide of 206 amino acids. The C. Similis-T2L contains a unique exon denominated exon X, which is located between exon 5 and exon 6. The C. Similis-T2L contains 7 exons including exon 1, 2, 3, 5, X, 6, and 7. Analysis of the secondary and tertiary structures of T2L amelogenin protein demonstrated that exon X has a dramatic effect on the amelogenin structures. This is the first report to provide definitive evidence for the amelogenin alternative splicing in non-mammalian vertebrates, revealing a unique exon X and the splicing form of the amelogenin gene transcript in Ctenosaura similis.

Enamel proteins and proteases in Mmp20 and Klk4 null and double-null mice

Matrix metalloproteinase 20 (MMP20) and kallikrein-related peptidase 4 (KLK4) are thought to be necessary to clear proteins from the enamel matrix of developing teeth. We characterized Mmp20 and Klk4 null mice to better understand their roles in matrix degradation and removal. Histological examination showed retained organic matrix in Mmp20, Klk4, and Mmp20/Klk4 double-null mouse enamel matrix, but not in the wild-type. X-gal histostaining of Mmp20 null mice heterozygous for the Klk4 knockout/lacZ knockin showed that Klk4 is expressed normally in the Mmp20 null background. This finding was corroborated by zymogram and western blotting, which discovered a 40-kDa protease induced in the maturation stage of Mmp20 null mice. Proteins were extracted from secretory-stage or maturation-stage maxillary first molars from wild-type, Mmp20 null, Klk4 null, and Mmp20/Klk4 double-null mice and were analyzed by SDS-PAGE and western blotting. Only intact amelogenins and ameloblastin were observed in secretory-stage enamel of Mmp20 null mice, whereas the secretory-stage matrix from Klk4 null mice was identical to the matrix from wild-type mice. More residual matrix was observed in the double-null mice compared with either of the single-null mice. These results support the importance of MMP20 during the secretory stage and of KLK4 during the maturation stage and show there is only limited functional redundancy for these enzymes.

A mutation in the mouse Amelx tri-tyrosyl domain results in impaired secretion of amelogenin and phenocopies human X-linked amelogenesis imperfecta

Amelogenesis imperfecta (AI) describes a broad group of clinically and genetically heterogeneous inherited defects of dental enamel bio-mineralization. Despite identification of a number of genetic mutations underlying AI, the precise causal mechanisms have yet to be determined. Using a multi-disciplinary approach, we describe here a mis-sense mutation in the mouse Amelx gene resulting in a Y --> H substitution in the tri-tyrosyl domain of the enamel extracellular matrix protein amelogenin. The enamel in affected animals phenocopies human X-linked AI where similar mutations have been reported. Animals affected by the mutation have severe defects of enamel bio-mineralization associated with absence of full-length amelogenin protein in the developing enamel matrix, loss of ameloblast phenotype, increased ameloblast apoptosis and formation of multi-cellular masses. We present evidence to demonstrate that affected ameloblasts express but fail to secrete full-length amelogenin leading to engorgement of the endoplasmic reticulum/Golgi apparatus. Immunohistochemical analysis revealed accumulations of both amelogenin and ameloblastin in affected cells. Co-transfection of Ambn and mutant Amelx in a eukaryotic cell line also revealed intracellular abnormalities and increased cytotoxicity compared with cells singly transfected with wild-type Amelx, mutant Amelx or Ambn or co-transfected with both wild-type Amelx and Ambn. We hypothesize that intracellular protein-protein interactions mediated via the amelogenin tri-tyrosyl motif are a key mechanistic factor underpinning the molecular pathogenesis in this example of AI. This study therefore successfully links phenotype with underlying genetic lesion in a relevant murine model for human AI.

The amelogenin C-terminus is required for enamel development

The abundant amelogenin proteins are responsible for generating proper enamel thickness and structure, and most amelogenins include a conserved hydrophilic C-terminus. To evaluate the importance of the C-terminus, we generated transgenic mice that express an amelogenin lacking the C-terminal 13 amino acids (CTRNC). MicroCT analysis of TgCTRNC29 teeth (low transgene number) indicated that molar enamel density was similar to that of wild-type mice, but TgCTRNC18 molar enamel (high transgene number) was deficient, indicating that extra transgene copies were associated with a more severe phenotype. When amelogenin-null (KO) and TgCTRNC transgenic mice were mated, density and volume of molar enamel from TgCTRNCKO offspring were not different from those of KO mice, indicating that neither TgCTRNC18 nor TgCTRNC29 rescued enamel's physical characteristics. Because transgenic full-length amelogenin partially rescues both density and volume of KO molar enamel, it was concluded that the amelogenin C-terminus is essential for proper enamel density, volume, and organization.

The leucine-rich amelogenin peptide alters the amelogenin null enamel phenotype

INTRODUCTION

The amelogenin proteins secreted by ameloblasts during dental enamel development are required for normal enamel structure. Amelx null (KO) mice have hypoplastic, disorganized enamel similar to that of human patients with mutations in the AMELX gene, and provide a model system for studies of the enamel defect amelogenesis imperfecta. Because many amelogenin proteins are present in developing enamel due to RNA alternative splicing and proteolytic processing, understanding the function of individual amelogenins has been challenging.

PURPOSE

Our objective was to better understand the role of LRAP, a 59 amino acid leucine-rich amelogenin peptide, in the development of enamel.

APPROACH

Teeth from transgenic mice that express LRAP under control of the Amelx regulatory regions were analyzed for mechanical properties, and transgenic males were mated with female KO mice. Male offspring with a null background that were transgene positive or transgene negative were compared to determine phenotypic differences using microcomputed tomography (microCT) and scanning electron microscopy (SEM).

RESULTS

Nanoindentation revealed no differences between LRAP transgenic and wild-type murine enamel. Using microCT, LRAPKO enamel volume and density measurements were similar to those from KO mice. However, in etched samples examined by SEM, the organization of the enamel rod pattern was altered by the presence of the LRAP transgene.

CONCLUSIONS

The presence of LRAP leads to changes in enamel appearance compared to enamel from KO mice. Expression of a combination of amelogenin transgenes in KO mice may lead to rescue of the individual characteristics of normal enamel.

Identification of the functional activity of the [A-4] amelogenin gene splice product in newborn mouse ameloblasts

In the mouse tooth organ, shortly after birth, ameloblasts acquire their secretory phenotype, which is characterized by the prominent expression and subsequent secretion of two isoforms of amelogenin, M180 and M59 (LRAP, [A-4]). Amelogenin deposition into the ameloblast extracellular matrix promotes enamel biomineralization. A complex set of intercellular signaling events, reciprocal communications between the developing oral epithelium and its underlying dental mesenchyme, guide the expression of amelogenin mRNA, and limit it to a defined period of tooth development. In tooth germ organ culture, addition of the [A-4] isoform, lacking amelogenin exon 4 and exon 6 segments a, b, c, was shown to affect ameloblast development. To understand the basis for this regulatory activity, we have studied the effects of r[A-4] on ameloblast-like LS8 cells, and the role of the putative [A-4] cell surface receptor, LAMP1, as well as the related receptor LAMP3. In the LS8 cells, the expression of the spliced isoforms of amelogenin, LAMP1, and LAMP3 were identified by RT-PCR, and real-time PCR semi-quantitative analysis assessed the modulation of M180 message. M180 mRNA was up-regulated by exogenous [A-4], and this was further increased by blockade of LAMP1, suggesting additive effects between the intracellular signaling pathways activated by the discrete agonists. Immunofluorescence staining identified the patterns of [A-4] and LAMP1 localization in LS8 cells. Internalized r[A-4] was co-localized with LAMP1 in late endosomal/lysosomal compartments. Thus, the LAMP1 and [A-4] intracellular sorting pathways are interrelated. The nitric oxide (NO) signaling pathway was activated by exogenous [A-4]. [A-4] modulated inducible nitric oxide synthase (iNOS, NOS2) and endothelial nitric oxide synthase (eNOS, NOS3) expression, albeit, to different extents. NOS2 was significantly up-regulated after 4 h, while NOS3 increased slightly after 24 h. Co-treatment of LS8 cells with r[A-4] and anti-LAMP1 antibodies further enhanced NOS2 expression. Anti-LAMP1 antibodies did not abrogate NO production in LS8 cells treated for 4 h with r[A-4], but the iNOS inhibitor, l-Nil, down-regulated both NO production and the expression of M180 mRNA. These data suggest that [A-4] modulates M180 mRNA expression, partly, via the NO signaling pathway.

Partial rescue of the amelogenin null dental enamel phenotype

The amelogenins are the most abundant secreted proteins in developing dental enamel. Enamel from amelogenin (Amelx) null mice is hypoplastic and disorganized, similar to that observed in X-linked forms of the human enamel defect amelogenesis imperfecta resulting from amelogenin gene mutations. Both transgenic strains that express the most abundant amelogenin (TgM180) have relatively normal enamel, but strains of mice that express a mutated amelogenin (TgP70T), which leads to amelogenesis imperfecta in humans, have heterogeneous enamel structures. When Amelx null (KO) mice were mated with transgenic mice that produce M180 (TgM180), the resultant TgM180KO offspring showed evidence of rescue in enamel thickness, mineral density, and volume in molar teeth. Rescue was not observed in the molars from the TgP70TKO mice. It was concluded that a single amelogenin protein was able to significantly rescue the KO phenotype and that one amino acid change abrogated this function during development.

Transgenic mice that express normal and mutated amelogenins

Amelogenin proteins are secreted by ameloblasts within the enamel organ during tooth development. To better understand the function of the 180-amino-acid amelogenin (M180), and to test the hypothesis that a single proline-to-threonine (P70T) change would lead to an enamel defect similar to amelogenesis imperfecta (AI) in humans, we generated transgenic mice with expression of M180, or M180 with the proline-to-threonine (P70T) mutation, under control of the Amelx gene regulatory regions. M180 teeth had a relatively normal phenotype; however, P70T mineral was abnormally porous, with aprismatic regions similar to those in enamel of male amelogenesis imperfecta patients with an identical mutation. When Amelx null females were mated with P70T transgenic males, offspring developed structures similar to calcifying epithelial odontogenic tumors in humans. The phenotype argues for dominant-negative activity for the P70T amelogenin, and for the robust nature of the process of amelogenesis.

Origin, splicing, and expression of rodent amelogenin exon 8

Amelogenin RNA transcripts undergo extensive alternative splicing, and MMP-20 processes the isoforms following their secretion. Since amelogenins have been ascribed cell-signaling activities, we asked if a lack of proteolytic processing by MMP-20 affects amelogenin signaling and consequently alters amelogenin splice site selection. RT-PCR analyses of amelogenin mRNA between control and Mmp20(-/-)mice revealed no differences in the splicing pattern. We characterized 3 previously unidentified amelogenin alternatively spliced transcripts and demonstrated that exon-8-encoded amelogenin isoforms are processed by MMP-20. Transcripts with exon 8 were expressed approximately five-fold less than those with exon 7. Analyses of the mouse and rat amelogenin gene structures confirmed that exon 8 arose in a duplication of exons 4 through 5, with translocation of the copy downstream of exon 7. No downstream genomic sequences homologous to exons 4-5 were present in the bovine or human amelogenin genes, suggesting that this translocation occurred only in rodents.

Comparison of body weight and gene expression in amelogenin null and wild-type mice

Amelogenin (AmelX) null mice develop hypomineralized enamel lacking normal prism structure, but are healthy and fertile. Because these mice are smaller than wild-type mice prior to weaning, we undertook a detailed analysis of the weight of mice and analyzed AmelX expression in non-dental tissues. Wild-type mice had a greater average weight each day within the 3-wk period. Using reverse transcription-polymerase chain reaction (RT-PCR), products of approximately 200 bp in size were generated from wild-type teeth, brain, eye, and calvariae. DNA sequence analysis of RT-PCR products from calvariae indicated that the small amelogenin leucine-rich amelogenin peptide (LRAP), both with and without exon 4, was expressed. No products were obtained from any of the samples from the AmelX null mice. We also isolated mRNAs that included AmelX exons 8 and 9, and identified a duplication within the murine AmelX gene with 91% homology. Our results add additional support to the hypothesis that amelogenins are multifunctional proteins, with potential roles in non-ameloblasts and in non-mineralizing tissues during development. The smaller size of AmelX null mice could potentially be explained by the lack of LRAP expression in some of these tissues, leading to a delay in development.

The use of animal models to explore amelogenin variants in amelogenesis imperfecta

Amelogenin proteins are secreted by ameloblast cells during tooth development. Because of extensive alternative splicing of the amelogenin primary RNA transcript, and because systematic proteolysis results in many additional peptides during enamel maturation, it has been difficult to assign function to individual amelogenins. Targeted deletions and transgenic mice have been generated in order to better understand amelogenin protein function in vivo. From these murine models, we have determined that amelogenins are responsible for normal enamel thickness and structure, but not for initiation of enamel mineral formation at the dentin-enamel junction. Although it is now clear that the amelogenin (AmelX) gene exists in a nested orientation and that AmelX is expressed at a low level in various developing tissues, the significance of these findings is incompletely understood. Future studies are expected to answer remaining questions concerning structure/function relationships among these 'enamel proteins'.

Identification of temporal and spatial expression patterns of amelogenin isoforms during mouse molar development

Amelogenin synthesis is initiated in a restricted time frame during odontogenesis. Polypeptides translated from several alternatively spliced isoforms of amelogenin mRNA have been identified in ameloblasts and odontoblasts. Recent studies suggest that the isoforms deleting exons 6a, 6b, and 6c produce polypeptides that might exert regulatory functions governing the late stages of ameloblast and odontoblast differentiation. Herein, the spatial and temporal expression of mouse amelogenin mRNA isoforms M194, M180, M73, and M59 have been determined around the perinatal development period using splice form-specific probes. Expression levels and distribution patterns varied with developmental stage and cell location. Amelogenin mRNA expression was most prominent within the enamel organ at boundaries between cell layers, beginning at the newborn stage (PN0.5). Odontoblasts supported the expression of M73 and M59 mRNA from developmental stages PN0.5 to PN1.5 (1 d of age). In contrast, ameloblasts expressed predominantly the M180 mRNA isoform with full exon 6 but devoid of exon 4. In the enamel organ, the stratum intermediun cells supported expression of the full-length isoform, M194, including the full exon 6 and exon 4 sequences, and strikingly, expression of M180 message was inhibited. In conclusion, ameloblasts, odontoblasts, and stratum intermedium cells demonstrate selective alternative splicing patterns of the amelogenin pre-mRNA transcript.

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.

Nested genes: Biological implications and use of AFM for analysis

A "nested" gene is located within the boundaries of a larger gene, often within an intron and in the opposite orientation. Such structures are common in bacteria and viruses, but have also been described in higher species as diverse as Drosophila and humans. Expression of nested and host genes may be simultaneously up-regulated due to use of common enhancers, or down-regulated through steric hindrance or interference caused by annealing of the complementary RNAs, leading to degradation. Methods for RNA analysis such as RT-PCR and in situ hybridization reveal the presence of specific mRNAs, but do not address regulation of expression within a single cell at a single genetic locus. Atomic force microscopy is a relatively new technology, which allows visualization of the movement of an RNA polymerase along a DNA template. The potential of this technology includes a greater molecular understanding of cellular decision making processes, leading to enhanced opportunities to intervene in disease progression through use of novel treatment modalities.

Two related low molecular mass polypeptide isoforms of amelogenin have distinct activities in mouse tooth germ differentiation in vitro

Embryonic mouse tooth germs were cultured in vitro in the presence of two related amelogenin isoforms to determine their effects on tooth development. Our results show that these individual proteins have specific but quite different effects on epithelial-derived ameloblasts versus mesenchymal-derived odontoblasts.

INTRODUCTION

Amelogenins, the main protein components of enamel matrix, have been shown to have signaling activity. Amelogenin isoforms differing only by the presence or exclusion of exon 4, designated [A+4] (composed of exons 2, 3, 4, 5, 6d, and 7) and [A-4] (composed of exons 2, 3, 5, 6d, and 7), showed similar, but different, effects both in vitro and in vivo on postnatal teeth.

MATERIALS AND METHODS

Lower first molar tooth germs of E15/16 CD1 mice were microdissected and cultured in vitro in a semisolid media containing either 20% FBS, 2% FBS, or 2% FBS with either 1.5 nM [A+4], [A-4], or both for 6 days. Tooth germs were analyzed by H&E staining and immunohistochemistry for collagen I, dentin matrix protein 2, and DAPI nuclear staining.

RESULTS

Teeth cultured in media containing 20% FBS showed normal development with polarized ameloblasts, and odontoblasts producing dentin matrix, and DMP2 expression in odontoblasts and pre-ameloblasts. Culture in 2% FBS media resulted in no ameloblast polarization and modest odontoblast differentiation with scant dentin matrix. Tooth germs cultured with [A+4] in 2% FBS media had well-polarized odontoblasts with robust dentin production and concomitant ameloblast polarization. DMP2 expression was equal to or greater than seen in the 20% FBS culture condition. In cultures with [A-4] in 2% FBS media, odontoblast polarization and dentin production was reduced compared with [A+4]. However, the pre-ameloblast layer was disorganized, with no ameloblast polarization occurring along the dentin surface. DMP2 expression was reduced in the odontoblasts compared with the 20% FBS and [A+4] conditions and was almost completely abrogated in the pre-ameloblasts.

CONCLUSION

These data show different signaling activities of these closely related amelogenin isoforms on tooth development. Here we make the novel observation that [A-4] has an inhibitory effect on ameloblast development, whereas [A+4] strongly stimulates odontoblast development. We show for the first time that specific amelogenin isoforms have effects on embryonic tooth development in vitro and also hypothesize that DMP2 may play a role in the terminal differentiation of both ameloblasts and odontoblasts.

The small bovine amelogenin LRAP fails to rescue the amelogenin null phenotype

Amelogenins are the most abundant secreted proteins in developing dental enamel. These evolutionarily-conserved proteins have important roles in enamel mineral formation, as mutations within the amelogenin gene coding region lead to defects in enamel thickness or mineral structure. Because of extensive alternative splicing of the primary RNA transcript and proteolytic processing of the secreted proteins, it has been difficult to assign functions to individual amelogenins. To address the function of one of the amelogenins, we have created a transgenic mouse that expresses bovine leucine-rich amelogenin peptide (LRAP) in the enamel-secreting ameloblast cells of the dental organ. Our strategy was to breed this transgenic mouse with the recently generated amelogenin knockout mouse, which makes none of the amelogenin proteins and has a severe hypoplastic and disorganized enamel phenotype. It was found that LRAP does not rescue the enamel defect in amelogenin null mice, and enamel remains hypoplastic and disorganized in the presence of this small amelogenin. In addition, LRAP overexpression in the transgenic mouse (wildtype background) leads to pitting in the enamel surface, which may result from excess protein production or altered protein processing due to minor differences between the amino acid compositions of murine and bovine LRAP. Since introduction of bovine LRAP into the amelogenin null mouse does not restore normal enamel structure, it is concluded that other amelogenin proteins are essential for normal appearance and function.

Pig amelogenin gene expresses a unique exon 4

The pig amelogenin gene was isolated from a Lambda genomic library, and a 6.3 kb SalI/XbaI restriction fragment, inclusive of exons 3 through 7, was subcloned into a plasmid vector. DNA sequencing revealed two putative exon 4 sequences. The derived amino acid sequence of exon 4a, KSGRWGARLTAFVSSVQ, had previously been identified in a 190-amino-acid amelogenin isoform by protein sequencing. Exon 4b encoded the peptide DLYLEAIRIDRTAF, which is homologous to exon 4-encoded segments reported for human, mouse, and rat. Oligonucleotides from both of these exons were used to amplify cDNA generated from developing teeth. Amplification products were analyzed by agarose gel electrophoresis, cloned, and characterized by DNA sequencing. Exon 4a was found in transcripts encoding amelogenin isoforms having 190 and 73 amino acids. Exon 4b was found only in apparent splicing intermediates that retained intron 3, but was not detected in any final mRNA transcripts. Pig amelogenin having apparent molecular mass of 23 kD were isolated from the enamel matrix and characterized by mass spectrometry. Two mass values, 18,512.5, and 18,571.2 Da, were measured that match the values predicted for the 162-amino-acid cleavage product of the 173-amino-acid amelogenin, and the 165-amino-acid cleavage product of the 190-amino-acid amelogenin, which includes 17 amino acids encoded by exon 4a. We conclude that the pig amelogenin gene expresses a unique exon 4 that is not homologous to, or evolved from, the exon 4 segment expressed in humans and rodents.

Enamelysin (matrix metalloproteinase 20)-deficient mice display an amelogenesis imperfecta phenotype

Enamelysin is a tooth-specific matrix metalloproteinase that is expressed during the early through middle stages of enamel development. The enamel matrix proteins amelogenin, ameloblastin, and enamelin are also expressed during this same approximate developmental time period, suggesting that enamelysin may play a role in their hydrolysis. In support of this interpretation, recombinant enamelysin was previously demonstrated to cleave recombinant amelogenin at virtually all of the precise sites known to occur in vivo. Thus, enamelysin is likely an important amelogenin-processing enzyme. To characterize the in vivo biological role of enamelysin during tooth development, we generated an enamelysin-deficient mouse by gene targeting. Although mice heterozygous for the mutation have no apparent phenotype, the enamelysin null mouse has a severe and profound tooth phenotype. Specifically, the null mouse does not process amelogenin properly, possesses an altered enamel matrix and rod pattern, has hypoplastic enamel that delaminates from the dentin, and has a deteriorating enamel organ morphology as development progresses. Our findings demonstrate that enamelysin activity is essential for proper enamel development.

Model system for evaluation of alternative splicing: exon skipping

Alternative splicing of the primary RNA transcript is a common mechanism for generating protein diversity. A model system was developed to study this process in vitro that is useful for evaluation of splicing of transcripts expressed in cells that do not grow well in culture. The system was used to analyze skipping of exon 4 of the amelogenin message, normally expressed in ameloblast cells for a short interval during tooth enamel development. Amelogenins are highly conserved proteins resulting from extensive alternative splicing, with domains involved in a range of functions, including mineral formation and intercellular signaling. In the bovine gene, the very short intron 4 was predicted to inhibit inclusion of exon 4, because in murine ameloblasts, exon 4 is detectably included in mRNA, and intron 4 is longer than the bovine counterpart. Bovine intron 4 was lengthened, and this size increase enhanced exon 4 inclusion sixfold to eightfold, although splice site selection was inaccurate. Intron length, therefore, is not the sole determinant controlling amelogenin exon 4 inclusion, and cis-acting inhibitory elements may also be involved in exon skipping. This vector system allows evaluation of splicing of a tissue-specific RNA by focusing on exons of interest through transfection of heterologous cultured cells without complications attributable to background transcription of the gene being evaluated.

Amelogenin-deficient mice display an amelogenesis imperfecta phenotype

Dental enamel is the hardest tissue in the body and cannot be replaced or repaired, because the enamel secreting cells are lost at tooth eruption. X-linked amelogenesis imperfecta (MIM 301200), a phenotypically diverse hereditary disorder affecting enamel development, is caused by deletions or point mutations in the human X-chromosomal amelogenin gene. Although the precise functions of the amelogenin proteins in enamel formation are not well defined, these proteins constitute 90% of the enamel organic matrix. We have disrupted the amelogenin locus to generate amelogenin null mice, which display distinctly abnormal teeth as early as 2 weeks of age with chalky-white discoloration. Microradiography revealed broken tips of incisors and molars and scanning electron microscopy analysis indicated disorganized hypoplastic enamel. The amelogenin null phenotype reveals that the amelogenins are apparently not required for initiation of mineral crystal formation but rather for the organization of crystal pattern and regulation of enamel thickness. These null mice will be useful for understanding the functions of amelogenin proteins during enamel formation and for developing therapeutic approaches for treating this developmental defect that affects the enamel.

Biochemical characterization of recombinant mouse amelogenins: protein quantitation, proton absorption, and relative affinity for enamel crystals

Four recombinant mouse amelogenins, which varied by the presence or absence of the exon 4 encoded segment as well as the carboxyl-terminus were heterologously expressed and purified from bacteria. The rM193 and rM179 contain the carboxyl-terminus, whereas the rM180 and rM166 do not. The rM193 and rM180 contain the polypeptide segment encoded by exon 4 of the amelogenin gene. A precisely weighed sample of purified rM179 was quantified by Lowry, Bicinchoninic Acid and Bradford assays. It was determined that these protein quantification methods characteristically under or overestimate the amount of amelogenin. The calculated correction factors were: Lowry (x 1.35), BCA (x 1.96), and Bradford (x 0.78). Recombinant mouse amelogenin (rM179) was characterized with respect to its hydrogen ion binding properties. The protein absorbs 11.9 +/- 1.7 protons during a pH change from 8.0 to 5.0, suggesting that amelogenins buffer the enamel fluid in vivo. Crystal binding experiments were performed using rM193, rM180, rM179 and rM166. The carboxyl-terminus enhanced the binding of amelogenin to enamel crystals while the exon 4 encoded segment did not appreciably affect crystal binding.

Specific amelogenin gene splice products have signaling effects on cells in culture and in implants in vivo

Low molecular mass amelogenin-related polypeptides extracted from mineralized dentin have the ability to affect the differentiation pathway of embryonic muscle fibroblasts in culture and lead to the formation of mineralized matrix in in vivo implants. The objective of the present study was to determine whether the bioactive peptides could have been amelogenin protein degradation products or specific amelogenin gene splice products. Thus, the splice products were prepared, and their activities were determined in vitro and in vivo. A rat incisor tooth odontoblast pulp cDNA library was screened using probes based on the peptide amino acid sequencing data. Two specific cDNAs comprised from amelogenin gene exons 2,3,4,5,6d,7 and 2,3,5,6d, 7 were identified. The corresponding recombinant proteins, designated r[A+4] (8.1 kDa) and r[A-4] (6.9 kDa), were produced. Both peptides enhanced in vitro sulfate incorporation into proteoglycan, the induction of type II collagen, and Sox9 or Cbfa1 mRNA expression. In vivo implant assays demonstrated implant mineralization accompanied by vascularization and the presence of the bone matrix proteins, BSP and BAG-75. We postulate that during tooth development these specific amelogenin gene splice products, [A+4] and [A-4], may have a role in preodontoblast maturation. The [A+4] and [A-4] may thus be tissue-specific epithelial mesenchymal signaling molecules.

Mass spectrometry of native rat amelogenins: primary transcripts, secretory isoforms, and C-terminal degradation

Cloning technologies have established unambiguously that amelogenins always seem larger in molecular weight (Mr) by gel electrophoresis (SDS-PAGE) than by mass spectrometry (MS). This has caused many problems relating cloned versions of amelogenin to proteins actually secreted by ameloblasts in vivo. In this study, discrete protein fractions at 31-20 kDa (Mr(SDS)) were prepared from freeze-dried rat incisor enamel by techniques optimized for preserving protein integrity. N-terminal sequence and amino acid compositional analyses indicated that the major protein forming these fractions was amelogenin. As expected, the molecular weights estimated by matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) MS were significantly less than their apparent molecular weights estimated by SDS-PAGE. Plots of Mr(SDS) vs. Mr(MS) for all fractions showed high linear correlation (r = 0.992). Analysis of MS data further indicated that the major protein in the 27-kDa fraction corresponded to the R180 secretory isoform of rat amelogenin, whereas some minor proteins in the 23-kDa fraction likely corresponded to a R156 secretory isoform. This was in contrast to major proteins forming the 25-, 24-, and 23-kDa fractions (Mr(SDS)), which seemed to represent proteolytic fragments of R180 progressively altered at the P169-A170, P164-L165, and F151-S152 C-terminal cleavage sites, respectively. Proteins in the 20-kDa fraction (Mr(SDS)) most closely matched by ESI-MS fragments of the R156 secretory isoform that were C-terminally-modified at the equivalent P164-L165 site.

Proteinases in developing dental enamel

For almost three decades, proteinases have been known to reside within developing dental enamel. However, identification and characterization of these proteinases have been slow and difficult, because they are present in very small quantities and they are difficult to purify directly from the mineralizing enamel. Enamel matrix proteins such as amelogenin, ameloblastin, and enamelin are cleaved by proteinases soon after they are secreted, and their cleavage products accumulate in the deeper, more mature enamel layers, while the full-length proteins are observed only at the surface. These results suggest that proteinases are necessary for "activating" enamel proteins so the parent proteins and their cleavage products may perform different functions. A novel matrix metalloproteinase named enamelysin (MMP-20) was recently cloned from tooth tissues and was later shown to localize primarily within the most recently formed enamel. Furthermore, recombinant porcine enamelysin was demonstrated to cleave recombinant porcine amelogenin at virtually all of the sites that have previously been described in vivo. Therefore, enamelysin is at least one enzyme that may be important during early enamel development. As enamel development progresses to the later stages, a profound decrease in the enamel protein content is observed. Proteinases have traditionally been assumed to degrade the organic matrix prior to its removal from the enamel. Recently, a novel serine proteinase named enamel matrix serine proteinase-1 (EMSP1) was cloned from enamel organ epithelia. EMSP1 localizes primarily to the early maturation stage enamel and may, therefore, be involved in the degradation of proteins prior to their removal from the maturing enamel. Other, as yet unidentified, proteinases and proteinase inhibitors are almost certainly present within the forming enamel and await discovery.

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.

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.