Biological synthesis of tooth enamel instructed by an artificial matrix

The regenerative capability of enamel, the hardest tissue in the vertebrate body, is fundamentally limited due to cell apoptosis following maturation of the tissue. Synthetic strategies to promote enamel formation have the potential to repair damage, increase the longevity of teeth and improve the understanding of the events leading to tissue formation. Using a self-assembling bioactive matrix, we demonstrate the ability to induce ectopic formation of enamel at chosen sites adjacent to a mouse incisor cultured in vivo under the kidney capsule. The resulting material reveals the highly organized, hierarchical structure of hydroxyapatite crystallites similar to native enamel. This artificially triggered formation of organized mineral demonstrates a pathway for developing cell fabricated materials for treatment of dental caries, the most ubiquitous disease in man. Additionally, the artificial matrix provides a unique tool to probe cellular mechanisms involved in tissue formation further enabling the development of tooth organ replacements.

Full length amelogenin binds to cell surface LAMP-1 on tooth root/periodontium associated cells

OBJECTIVES

Lysosome-associated membrane protein-1 (LAMP-1) has been suggested to be a cell surface receptor for a specific amelogenin isoform, leucine-rich amelogenin peptide or LRAP. However, it is unclear if LAMP-1 is an amelogenin receptor for dental mesenchymal cells. The goal of this study was to determine if LAMP-1 serves as a cell surface binding site for full length amelogenin on tooth root/periodontium associated mesenchymal cells.

DESIGN

Murine dental follicle cells and cementoblasts (OCCM-30) were cultured for 2 days followed by addition of full length recombinant mouse amelogenin, rp(H)M180. Dose-response (0-100 microg/ml) and time course (0-120 min) assays were performed to determine the optimal conditions for live cell surface binding using immunofluorescent microscopy. A competitive binding assay was performed to determine binding specificity by adding Emdogain (1 mg/ml) to the media. An antibody against LAMP-1 was used to detect the location of LAMP-1 on the cell surface and the pattern was compared to cell surface bound amelogenin. Both amelogenin and cell surface LAMP-1 were immuno-co-localized to compare the amount and distribution pattern.

RESULTS

Maximum surface binding was achieved with 50 microg/ml of rp(H)M180 for 120 min. This binding was specific as demonstrated by competitive inhibition (79% lower) with the addition of Emdogain. The binding pattern for rp(H)M180 was similar to the distribution of surface LAMP-1 on dental follicle cells and cementoblasts. The high co-localization coefficient (0.92) for rp(H)M180 and LAMP-1 supports rp(H)M180 binding to cell surface LAMP-1.

CONCLUSIONS

The data from this study suggest that LAMP-1 can serve as a cell surface binding site for amelogenin on dental follicle cells and cementoblasts.

A solution NMR investigation into the murine amelogenin splice-variant LRAP (Leucine-Rich Amelogenin Protein)

Amelogenins are the dominant proteins present in ameloblasts during the early stages of enamel biomineralization, making up >90% of the matrix protein. Along with the full-length protein there are several splice-variant isoforms of amelogenin present including LRAP (Leucine-Rich Amelogenin Protein), a protein that consists of the first 33 and the last 26 residues of full-length amelogenin. Using solution-state NMR spectroscopy we have assigned the (1)H-(15)N HSQC spectrum of murine LRAP (rp(H)LRAP) in 2% acetic acid at pH 3.0 by making extensive use of previous chemical shift assignments for full-length murine amelogenin (rp(H)M180). This correlation was possible because LRAP, like the full-length protein, is intrinsically disordered under these solution conditions. The major difference between the (1)H-(15)N HSQC spectra of rp(H)M180 and rp(H)LRAP was an additional set of amide resonances for each of the seven non-proline residues between S12 and Y12 near the N-terminus of rp(H)LRAP indicating that the N-terminal region of LRAP exists in two different conformations. Analysis of the proline carbon chemical shifts suggests that the molecular basis for the two states is not a cis-trans isomerization of one or more of the proline residues in the N-terminal region. Starting from 2% acetic acid, where rp(H)LRAP was monomeric in solution, NaCl addition effected residue specific changes in molecular dynamics manifested by the reduction in intensity and disappearance of (1)H-(15)N HSQC cross peaks. As observed for the full-length protein, these perturbations may signal early events governing supramolecular self-assembly of rp(H)LRAP into nanospheres. However, the different patterns of (1)H-(15)N HSQC cross peak perturbation between rp(H)LRAP and rp(H)M180 in high salt suggest that the termini may behave differently in their respective nanospheres, and perhaps, these differences contribute to the cell signaling properties attributable to LRAP but not to the full-length protein.

Dental enamel: genes define biomechanics

Regulated gene expression assembles an extracellular proteinaceous matrix to control biomineralization and the resultant biomechanical function of tooth enamel. The importance of the dominant enamel matrix protein, amelogenin (Amel); a minor transiently expressed protein, dentin sialoprotein (Dsp); an electrogenic sodium bicarbonate cotransporter (NBCe1); the timely removal of the proteinaceous matrix by a serine protease, Kallikrein-4 (Klk4); and the late-stage expression of Amelotin (Amtn) on enamel biomechanical function were demonstrated and measured using mouse models.

Enamel matrix proteins; old molecules for new applications

Emdogain (enamel matrix derivative, EMD) is well recognized in periodontology, where it is used as a local adjunct to periodontal surgery to stimulate regeneration of periodontal tissues lost to periodontal disease. The biological effect of EMD is through stimulation of local growth factor secretion and cytokine expression in the treated tissues, inducing a regenerative process that mimics odontogenesis. The major (>95%) component of EMD is Amelogenins (Amel). No other active components have so far been isolated from EMD, and several studies have shown that purified amelogenins can induce the same effect as the complete EMD. Amelogenins comprise a family of highly conserved extracellular matrix proteins derived from one gene. Amelogenin structure and function is evolutionary well conserved, suggesting a profound role in biomineralization and hard tissue formation. A special feature of amelogenins is that under physiological conditions the proteins self-assembles into nanospheres that constitute an extracellular matrix. In the body, this matrix is slowly digested by specific extracellular proteolytic enzymes (matrix metalloproteinase) in a controlled process, releasing bioactive peptides to the surrounding tissues for weeks after application. Based on clinical and experimental observations in periodontology indicating that amelogenins can have a significant positive influence on wound healing, bone formation and root resorption, several new applications for amelogenins have been suggested. New experiments now confirm that amelogenins have potential for being used also in the fields of endodontics, bone regeneration, implantology, traumatology, and wound care.

Science is the fuel for the engine of technology and clinical practice

BACKGROUND

The biological, chemical, behavioral and physical sciences provide the fuel for innovation, discovery and technology that continuously improves the quality of the human condition. Computer power derived from the dramatic breakthroughs of the digital revolution has made extraordinary computational capacity available for diagnostic imaging, bioinformatics (the science of information) and numerous aspects of how we practice dentistry in the 21st century.

OVERVIEW

The biological revolution was initiated by the identification of the structure for DNA in 1953, a discovery that continues to catalyze improvements in patient care through new and better diagnostics, treatments and biomaterials. Humanity's most basic and recognizable characteristics--including the face--are now better understood through the elucidation of our genome and proteome, the genes and proteins they encode. Health care providers are beginning to use personalized medicine that is based on a person's genetic makeup and predispositions to disease development.

CONCLUSIONS

Advances in the fields of genetics, developmental and stem cell biology, and many other disciplines continue to fuel innovative research findings that form the basis for new diagnostic tests, therapeutic interventions and procedures that improve the quality of life for patients. Scientists are on the threshold of applying knowledge in stem cell biology to regenerative medicine and dentistry, heralding an era when clinicians can consider using biological engineering to replace tissues and organs lost to disease or trauma.

A solution NMR investigation into the early events of amelogenin nanosphere self-assembly initiated with sodium chloride or calcium chloride

Using solution-state NMR spectroscopy, new insights into the early events governing amelogenin supramolecular self-assembly have been identified using sodium chloride and calcium chloride to trigger the association. Two-dimensional 1H-15N HSQC spectra were recorded for 15N- and 13C-labeled murine amelogenin as a function of increasing NaCl and CaCl2 concentration beginning with solution conditions of 2% acetic acid at pH 3.0, where amelogenin was monomeric. Residue specific changes in molecular dynamics, manifested by the reduction in intensity and disappearance of 1H-15N HSQC cross-peaks, were observed with the addition of either salt to the protein. With increasing NaCl concentrations, residues between T21 and R31 near the N-terminus were affected first, suggesting that these residues may initiate amelogenin dimerization, the first step in nanosphere assembly. At higher NaCl concentrations, more residues near the N-terminus (Y12-I51) were affected, and with further additions of NaCl, residues near the C-terminus (L141-T171) began to show a similar change in molecular dynamics. With increasing CaCl2 concentrations, a similar stepwise change in molecular dynamics involving essentially the same set of amelogenin residues was observed. As the concentration of either salt was increased, a concomitant increase in the estimated overall rotational correlation time (tau(c)) was observed, consistent with assembly. Self-assembly into a dimer or trimer was established with dynamic light scattering studies under similar conditions that showed an increase in diameter of the smallest species from 4.1 nm in the absence of salt to 10 nm in the presence of salt. These results suggest a possible stepwise interaction mechanism, starting with the N-terminus and followed by the C-terminus, leading to amelogenin nanosphere assembly.

Biglycan overexpression on tooth enamel formation in transgenic mice

Previously, it was shown that the volume of forming enamel of molar teeth in biglycan-null mice was greater than that in genetically matched wild-type mice. This phenotypic change appeared to result from an increase in amelogenin expression, implying that biglycan directly influences amelogenin synthesis. To determine whether biglycan overexpression resulted in decreased amelogenin expression, we engineered transgenic mice to overexpress biglycan in the enamel organ epithelium. Biglycan overexpression did not significantly affect the amelogenin expression in incisor and molar teeth in 3-day postnatal transgenic mice. In the transgenic animals, we observed that the immature and mature enamel appeared normal. These results suggested that increasing the biglycan expression, in the cells that synthesize the precursor protein matrix for enamel, has a negligible influence on amelogenesis.

Derivation of cranial neural crest-like cells from human embryonic stem cells

The neural crest is a transient population of multipotent progenitors contributing to a diverse array of tissues throughout the vertebrate embryo. Embryonic stem (ES) cells are able to form embryoid body and spontaneously differentiate to various lineages, following a reproducible temporal pattern of development that recapitulates early embryogenesis. Embryoid bodies were triturated and the dissociated cells were processed for fluorescence-activated cell sorting (FACS), and more than 1% of cells were identified as frizzled-3(+)/cadherin-11(+). Expression of marker genes associated with various terminal fates was detected for chondrocytes, glia, neurons, osteoblasts and smooth muscles, indicating that the FACS-sorted frizzled-3(+)/cadherin-11(+) cells were multipotent progenitor cells capable of differentiating to fates associated with cranial neural crest. Moreover, the sorted cells were able to self-renew and maintain multipotent differentiation potential. The derivation of cranial neural crest-like multipotent progenitor cells from ES cells provides a new tool for cell lineage analysis of neural crest in vitro.

Whole-tooth regeneration: it takes a village of scientists, clinicians, and patients

A team of senior scientists was formed in 2006 to create a blueprint for the regeneration of whole human teeth along with all of the supporting structure of the dentition. The team included experts from diverse fields, each with a reputation for stellar accomplishment. Participants attacked the scientific issues of tooth regeneration but, more importantly, each agreed to work collaboratively with experts from other disciplines to form a learning organization. A commitment to learn from one another produced a unique interdisciplinary and multidisciplinary team. Inspired by the Kennedy space program to send a man to the moon, with its myriad of problems and solutions that no one discipline could solve, this tooth regeneration team devised an ambitious plan that sought to use stem cell biology, engineering, and computational biology to replicate the developmental program for odontogenesis. In this manner, team members envisioned a solution that consisted of known or knowable fundamentals. They proposed a laboratory-grown tooth rudiment that would be capable of executing the complete program for odontogenesis when transplanted to a suitable host, recreating all of the dental tissues, periodontal ligament, cementum, and alveolar bone associated with the canonical tooth. This plan was designed to bring regenerative medicine fully into the dental surgery suite, although a lack of funding has so far prevented the plan from being carried out.

1H, 13C, and 15N resonance assignments of murine amelogenin, an enamel biomineralization protein

Amelogenin is the predominant matrix protein in developing dental enamel. Making extensive use of residue specific 15N-labeled amino acids samples, the majority of the main and side chain resonances for murine amelogenin were assigned in 2% aqueous acetic acid at pH 3.0.

Role of NBCe1 and AE2 in secretory ameloblasts

The H(+)/base transport processes that control the pH of the microenvironment adjacent to ameloblasts are not currently well-understood. Mice null for the AE2 anion exchanger have abnormal enamel. In addition, persons with mutations in the electrogenic sodium bicarbonate co-transporter NBCe1 and mice lacking NBCe1 have enamel abnormalities. These observations suggest that AE2 and NBCe1 play important roles in amelogenesis. In the present study, we aimed to understand the roles of AE2 and NBCe1 in ameloblasts. Analysis of the data showed that NBCe1 is expressed at the basolateral membrane of secretory ameloblasts, whereas AE2 is expressed at the apical membrane. Transcripts for AE2a and NBCe1-B were detected in RNA isolated from cultured ameloblast-like LS8 cells. Our data are the first evidence that AE2 and NBCe1 are expressed in ameloblasts in vivo in a polarized fashion, thereby providing a mechanism for ameloblast transcellular bicarbonate secretion in the process of enamel formation and maturation.

Leucine-rich amelogenin peptide induces osteogenesis in mouse embryonic stem cells

Leucine-rich amelogenin peptide (LRAP), an alternatively spliced amelogenin protein, possesses a signaling property shown to induce osteogenic differentiation. In the current study, we detected LRAP expression during osteogenesis of wild-type (WT) embryonic stem (ES) cells and observed the absence of LRAP expression in amelogenin-null (KO) ES cells. We explored the signaling effect of LRAP on wild-type ES cells, and the ability of LRAP to rescue the impaired osteogenesis phenotype observed in KO ES cells. Our data indicate that LRAP treatment of WT and KO ES cells induces a significant increase in mineral matrix formation, and significant increases in bone sialoprotein and osterix gene expression. In addition, the amelogenin KO phenotype is partially rescued by the addition of exogenous LRAP. These data suggest a unique function of LRAP during ES cell differentiation along osteogenic lineage.

Identification of novel genes expressed during mouse tooth development by microarray gene expression analysis

To identify genes heretofore undiscovered as critical players in the biogenesis of teeth, we have used microarray gene expression analysis of the developing mouse molar tooth (DMT) between postnatal day (P) 1 and P10 to identify genes differentially expressed when compared with 16 control tissues. Of the top 100 genes exhibiting increased expression in the DMT, 29 were found to have been previously associated with tooth development. Differential expression of the remaining 71 genes not previously associated with tooth development was confirmed by quantitative reverse transcription-polymerase chain reaction analysis. Further analysis of seven of the latter genes by mRNA in situ hybridization found that five were specific to the developing tooth in the craniofacial region (Rspo4, Papln, Amtn, Gja1, Maf). Of the remaining two, one was found to be more widely expressed (Sp7) and the other was found to be specific to the nasal serous gland, which is close to, but distinct from, the developing tooth (Vrm).

Factors influencing the expression of dental extracellular matrix biomineralization

The forming tooth organ provides a number of opportunities to investigate the cellular and molecular biology of cell-mediated extracellular matrix (ECM) biomineralization. Regulatory processes associated with tooth formation are being investigated by identifying when and where cell adhesion molecules (CAMs), substrate adhesion molecules (SAMs), dentine phosphoprotein and enamel gene products are expressed during sequential developmental stages. In vitro organotypic culture studies in serumless, chemically-defined medium, have shown that instructive and permissive signalling are required for both morphogenesis and cytodifferentiation. Intrinsic developmental instructions (autocrine and paracrine factors) act independently of long-range hormonal or exogenous growth factors and mediate morphogenesis from the initiation of the dental lamina to the crown stages of tooth development. This review summarizes the results of studies using experimental embryology, recombinant DNA technology and immunocytology to elucidate mechanisms responsive to instructive epithelial-mesenchymal interactions associated with ameloblast differentiation, odontoblast differentiation, and dentine and enamel ECM biomineralization.

CCAAT/enhancer-binding protein delta (C/EBPdelta) maintains amelogenin expression in the absence of C/EBPalpha in vivo

C/EBPalpha is implicated to regulate mouse amelogenin gene expression during tooth enamel formation in vitro. Because enamel formation occurs during postnatal development and C/EBPalpha-deficient mice die at birth, we used the Cre/loxP recombination system to characterize amelogenin expression in C/EBPalpha conditional knock-out mice. Mice carrying the Cre transgene under the control of the human keratin-14 promoter show robust Cre expression in the ameloblast cell lineage. Mating between mice bearing the floxed C/EBPalpha allele with keratin-14-Cre mice generate C/EBPalpha conditional knock-out mice. Real-time PCR analysis shows that removal of one C/EBPalpha allele from the molar enamel epithelial organ of 3-day postnatal mice results in dramatic decrease in endogenous C/EBPalpha mRNA levels and coordinately altered amelogenin mRNA abundance. Conditional deletion of both C/EBPalpha alleles further diminishes C/EBPalpha mRNA levels; however, rather than ablating amelogenin expression, we observe wild-type amelogenin mRNA abundance levels. We examined C/EBPbeta and nuclear factor YA expression, two transcription factors that had previously been shown to modestly participate in amelogenin expression, in vitro but found no significant changes in either of their mRNA abundance levels comparing conditional knock-out mice with wild-type counterparts. Although the abundance of C/EBPdelta is also unchanged in C/EBPalpha conditional knock-out mice, in vitro we find that C/EBPdelta activates the mouse amelogenin promoter and synergistically cooperates with nuclear factor Y, suggesting that C/EBPdelta can functionally substitute for C/EBPalpha to produce an enamel matrix competent to direct biomineralization.

The nucleation and growth of calcium phosphate by amelogenin

The nucleation processes involved in calcium phosphate formation in tooth enamel are not well understood but are believed to involve proteins in the extracellular matrix. The ability of one enamel protein, amelogenin, to promote the nucleation and growth of calcium phosphate was studied in an in vitro system involving metastable supersaturated solutions. It was found that recombinant amelogenin (rM179 and rp(H)M180) promoted the nucleation of calcium phosphate compared to solutions without protein. The amount of calcium phosphate increased with increasing supersaturation of the solutions and increasing protein concentrations up to 6.5 μg/mL. At higher protein concentrations, the amount of calcium phosphate decreased. The kinetics of nucleation was studied in situ and in real time using a quartz crystal microbalance (QCM) and showed that the protein reduced the induction time for nucleation compared to solutions without protein. This work shows a nucleation role for amelogenin in vitro which may be promoted by the association of amelogenin into nanosphere templates, exposing charged functionality at the surface.

Cellular uptake of amelogenin, and its localization to CD63, and Lamp1-positive vesicles

Proteins of the developing enamel matrix include amelogenin, ameloblastin and enamelin. Of these three proteins amelogenin predominates. Protein-protein interactions are likely to occur at the ameloblast Tomes' processes between membrane-bound proteins and secreted enamel matrix proteins. Such protein-protein interactions could be associated with cell signaling or endocytosis. CD63 and Lamp1 are ubiquitously expressed, are lysosomal integral membrane proteins, and localize to the plasma membrane. CD63 and Lamp1 interact with amelogenin in vitro. In this study our objective was to study the molecular events of intercellular trafficking of an exogenous source of amelogenin, and related this movement to the spatiotemporal expression of CD63 and Lamp1 using various cell lineages. Exogenously added amelogenin moves rapidly into the cell into established Lamp1-positive vesicles that subsequently localize to the perinuclear region. These data indicate a possible mechanism by which amelogenin, or degraded amelogenin peptides, are removed from the extracellular matrix during enamel formation and maturation.

Physical dissection of the CCAAT/enhancer-binding protein alpha in regulating the mouse amelogenin gene

The amelogenin gene is tightly regulated at the temporal and spatial level in accord with the developmental requirement for tooth formation. Previous studies have shown that CCAAT/enhancer-binding protein alpha (C/EBPalpha) is a transactivator of the mouse X-chromosomal amelogenin gene. C/EBPalpha contains four highly conserved regions (CR) named CR1, CR2, CR3, and CR4. Transient transfection assays showed that CR2 in isolation had an exceptional capacity to enhance transcription from the 2.3 kb mouse amelogenin promoter. The remaining conserved regions of C/EBPalpha, either in isolation or in selected combinations, were less effective in amelogenin transactivation than the full length C/EBPalpha. Msx2 has previously been shown to antagonize C/EBPalpha through protein-protein interactions with C/EBPalpha, and the carboxyl-terminus of Msx2 is required for protein-protein interactions. Co-immunoprecipitation analyses identified that the carboxyl-terminal domain (residues 218-359) of C/EBPalpha is required for the C/EBPalpha-Msx2 protein-protein interactions.

Ectopic expression of dentin sialoprotein during amelogenesis hardens bulk enamel

Dentin sialophosphpoprotein (Dspp) is transiently expressed in the early stage of secretory ameloblasts. The secretion of ameloblast-derived Dspp is short-lived, correlates to the establishment of the dentinoenamel junction (DEJ), and is consistent with Dspp having a role in producing the specialized first-formed harder enamel adjacent to the DEJ. Crack diffusion by branching and dissipation within this specialized first-formed enamel close to the DEJ prevents catastrophic interfacial damage and tooth failure. Once Dspp is secreted, it is subjected to proteolytic cleavage that results in two distinct proteins referred to as dentin sialoprotein (Dsp) and dentin phosphoprotein (Dpp). The purpose of this study was to investigate the biological and mechanical contribution of Dsp and Dpp to enamel formation. Transgenic mice were engineered to overexpress either Dsp or Dpp in their enamel organs. The mechanical properties (hardness and toughness) of the mature enamel of transgenic mice were compared with genetically matched and age-matched nontransgenic animals. Dsp and Dpp contributions to enamel formation greatly differed. The inclusion of Dsp in bulk enamel significantly and uniformly increased enamel hardness (20%), whereas the inclusion of Dpp weakened the bulk enamel. Thus, Dsp appears to make a unique contribution to the physical properties of the DEJ. Dsp transgenic animals have been engineered with superior enamel mechanical properties.

A new locus for autosomal dominant amelogenesis imperfecta on chromosome 8q24.3

Amelogenesis imperfecta (AI) is a collective term used to describe phenotypically diverse forms of defective tooth enamel development. AI has been reported to exhibit a variety of inheritance patterns, and several loci have been identified that are associated with AI. We have performed a genome-wide scan in a large Brazilian family segregating an autosomal dominant form of AI and mapped a novel locus to 8q24.3. A maximum multipoint LOD score of 7.5 was obtained at marker D8S2334 (146,101,309 bp). The disease locus lies in a 1.9 cM (2.1 Mb) region according to the Rutgers Combined Linkage-Physical map, between a VNTR marker (at 143,988,705 bp) and the telomere (146,274,826 bp). Ten candidate genes were identified based on gene ontology and microarray-facilitated gene selection using the expression of murine orthologues in dental tissue, and examined for the presence of a mutation. However, no causative mutation was identified.

Using the yeast two-hybrid assay to discover protein partners for the leucine-rich amelogenin peptide and for tuftelin-interacting protein 11

The established structural proteins of the enamel matrix are amelogenin, ameloblastin, and enamelin. Historically, tuftelin and tuftelin-interacting protein 11 (TFIP11) have also been discussed as possible enamel proteins. Protein complexes are achieved by protein-protein interactions, and it is protein complexes that control biomineralization. The purpose of our recent studies was to catalog protein partners for these proteins that are, or have been, implicated in tooth formation. We used the sensitive yeast two-hybrid assay to identify proteins that interact directly with amelogenin, ameloblastin, enamelin, the leucine-rich amelogenin peptide (LRAP) and TFIP11. In this manuscript we refer to, or document, potential protein partners for the proteins listed above. The yeast two-hybrid assay may ultimately prove to be a valuable proteomics methodology for using to decipher molecular events that ultimately result in enamel biomineralization.

Altering biomineralization by protein design

To create a bioceramic with unique materials properties, biomineralization exploits cells to create a tissue-specific protein matrix to control the crystal habit, timing, and position of the mineral phase. The biomineralized covering of vertebrate teeth is enamel, a distinctive tissue of ectodermal origin that is collagen-free. In forming enamel, amelogenin is the abundant protein that undergoes self-assembly to contribute to a matrix that guides its own replacement by mineral. Conserved domains in amelogenin suggest their importance to biomineralization. We used gene targeting in mice to replace native amelogenin with one of two engineered amelogenins. Replacement changed enamel organization by altering protein-to-crystallite interactions and crystallite stacking while diminishing the ability of the ameloblast to interact with the matrix. These data demonstrate that ameloblasts must continuously interact with the developing matrix to provide amelogenin-specific protein to protein, protein to mineral, and protein to membrane interactions critical to biomineralization and enamel architecture while suggesting that mutations within conserved amelogenin domains could account for enamel variations preserved in the fossil record.

Amelogenins regulate expression of genes associated with cementoblasts in vitro

Amelogenins are major proteins expressed by ameloblasts during development of the crown (enamel and dentin). These matrix proteins guide crystal habits of the mineral phase of developing enamel and are possible regulators of other genes/proteins during development and maturation of crown and root (dentin and cementum). This study focused on defining the effect that a specific proteolytic cleavage product of amelogenin, tyrosine-rich amelogenin peptide (TRAP), has on cementoblast behavior. Immortalized cementoblasts (OCCM-30) were exposed to TRAP in vitro. Cells treated with TRAP were evaluated for cell proliferation, gene expression for osteocalcin (OCN), osteopontin (OPN), and bone sialoprotein (BSP), and induction of mineral nodule formation. No significant difference in cell proliferation was found between vehicle-treated cells and those treated with TRAP for up to 9 d after treatment. Gene expression of OCN, OPN, and BSP in TRAP-treated cementoblasts showed down-regulation, up-regulation, and no significant change, respectively, relative to vehicle control. A marked decrease in mineral nodule formation was found in cells treated with TRAP compared with the vehicle control, in a dose-dependent manner. These data, along with our previous results demonstrating similar activity with full-length amelogenin and leucine-rich amelogenin peptide (LRAP), suggest that amelogenin-like molecules regulate mesenchymal cell behavior.

Transcription factor sumoylation and factor YY1 serve to modulate mouse amelogenin gene expression

Amelogenin proteins are essential in the control of enamel biomineralization and the amelogenin gene therefore is spatiotemporally regulated to ensure proper amelogenin protein expression. In this study, we examined the role of sumoylation to alter CCAAT/enhancer-binding protein alpha (C/EBPalpha) activity, and performed a search using a protein/DNA array system for other proteins that act co-operatively with C/EBPalpha to alter amelogenin expression. We observed that C/EBPalpha was modified by sumoylation, and that this modification played an indirect inhibitory role on the regulation of C/EBPalpha activity which appeared to act through other transcription factors. The protein/DNA array allowed us to single out the transcription factor, YY1, which acts in the absence of direct DNA binding to repress both the basal amelogenin promoter activity and C/EBPalpha-mediated transactivation. Taken together, these pathways may account for part of the physiological modulation of the amelogenin gene expression in accordance with tooth developmental and enamel biomineralization requirements.

NF-Y and CCAAT/enhancer-binding protein alpha synergistically activate the mouse amelogenin gene

Amelogenin is the major protein component of the forming enamel matrix. In situ hybridization revealed a periodicity for amelogenin mRNA hybridization signals ranging from low to high transcript abundance on serial sections of developing mouse teeth. This in vivo observation led us to examine the amelogenin promoter for the activity of transcription factor(s) that account for this expression aspect of the regulation for the amelogenin gene. We have previously shown that CCAAT/enhancer-binding protein alpha (C/EBPalpha) is a potent transactivator of the mouse X-chromosomal amelogenin gene acting at the C/EBPalpha cis-element located in the -70/+52 minimal promoter. The minimal promoter contains a reversed CCAAT box (-58/-54) that is four base pairs downstream from the C/EBPalpha binding site. Similar to the C/EBPalpha binding site, the integrity of the reversed CCAAT box is also required for maintaining the activity of the basal promoter. We therefore focused on transcription factors that interact with the reversed CCAAT box. Using electrophoretic mobility shift assays we demonstrated that NF-Y was directly bound to this reversed CCAAT site. Co-transfection of C/EBPalpha and NF-Y synergistically increased the promoter activity. In contrast, increased expression of NF-Y alone had only marginal effects on the promoter. A dominant-negative DNA binding-deficient NF-Y mutant (NF-YAm29) dramatically decreased the promoter activity both in the absence or presence of exogenous expression of C/EBPalpha. We identified protein-protein interactions between C/EBPalpha and NF-Y by a co-immunoprecipitation analysis. These results suggest that C/EBPalpha and NF-Y synergistically activate the mouse amelogenin gene and can contribute to its physiological regulation during amelogenesis.

Protein–Protein Interactions of the Developing Enamel Matrix

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

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

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

Morphoregulation of teeth: modulating the number, size, shape and differentiation by tuning Bmp activity

During development and evolution, the morphology of ectodermal organs can be modulated so that an organism can adapt to different environments. We have proposed that morphoregulation can be achieved by simply tilting the balance of molecular activity. We test the principles by analyzing the effects of partial downregulation of Bmp signaling in oral and dental epithelia of the keratin 14-Noggin transgenic mouse. We observed a wide spectrum of tooth phenotypes. The dental formula changed from 1.0.0.3/1.0.0.3 to 1.0.0.2(1)/1.0.0.0. All mandibular and M3 maxillary molars were selectively lost because of the developmental block at the early bud stage. First and second maxillary molars were reduced in size, exhibited altered crown patterns, and failed to form multiple roots. In these mice, incisors were not transformed into molars. Histogenesis and differentiation of ameloblasts and odontoblasts in molars and incisors were abnormal. Lack of enamel caused misocclusion of incisors, leading to deformation and enlargement in size. Therefore, subtle differences in the level, distribution, and timing of signaling molecules can have major morphoregulatory consequences. Modulation of Bmp signaling exemplifies morphoregulation hypothesis: simple alteration of key signaling pathways can be used to transform a prototypical conical-shaped tooth into one with complex morphology. The involvement of related pathways and the implication of morphoregulation in tooth evolution are discussed.

Controlled failure mechanisms toughen the dentino-enamel junction zone

STATEMENT OF PROBLEM

The dentino-enamel junction (DEJ) durably unites dissimilar hard brittle enamel and tough flexible dentin. In contrast to artificial bonds between restorations and dentin, the DEJ rarely fails except when it is affected by inherited disorders. Knowledge of DEJ toughening mechanisms is important in understanding inherited disorders, in biomimetic engineering of junctions between artificial restorations and teeth, and in tissue-engineering a DEJ.

PURPOSE

The purpose of this study was to identify specific DEJ-zone failure mechanisms and to survey the fracture toughness of the human DEJ zone.

MATERIAL AND METHODS

Fracture toughness indentations were made at 3 sites across the DEJ zone of 10 human incisor teeth. Failure modes identified using optical microscopy and fracture toughness (MPa.m(1/2)) were calculated following Vickers microindentation. Site mean values were then calculated and compared using 1-way analysis of variance (alpha=.05).

RESULTS

The DEJ did not undergo catastrophic interfacial delamination; instead, damage was distributed over a broad zone. The primary damage mode involved cracking and damage dispersion in the specialized first-formed enamel close to the DEJ. Multiple, somewhat convoluted and sometimes branching, cracks spread and diffused damage over a wide area of adjacent enamel rather than producing catastrophic interfacial failure. Other secondary mechanisms included short microcracks in the DEJ adjacent dentin with possible cracked bridging, as well as plastic deformation of the DEJ without delamination. A DEJ-zone fracture toughness of approximately 0.8 to 0.9 MPa.m(1/2) was calculated.

CONCLUSION

DEJ-zone damage occurred primarily within the adjacent layer of specialized first-formed enamel, and the optical DEJ interface resisted delamination.

Structural organization and cellular localization of tuftelin-interacting protein 11 (TFIP11)

Tuftelin-interacting protein (TFIP11) was first identified in a yeast two-hybrid screening as a protein interacting with tuftelin. The ubiquitous expression of TFIP11 suggested that it might have other functions in non-dental tissues. TFIP11 contains a G-patch, a protein domain believed to be involved in RNA binding. Using a green fluorescence protein tag, TFIP11 was found to locate in a novel subnuclear structure that we refer to as the TFIP body. An in vivo splicing assay demonstrated that TFIP11 is a novel splicing factor. TFIP11 diffuses from the TFIP body following RNase A treatment, suggesting that the retention of TFIP11 is RNA dependent. RNA polymerase II inhibitor (-amanitin and actinomycin D) treatment causes enlargement in size and decrease in number of TFIP bodies, suggesting that TFIP bodies perform a storage function rather than an active splicing function. The TFIP body may therefore represent a new subnuclear storage compartment for splicing components.

Dentin sialoprotein and dentin phosphoprotein overexpression during amelogenesis

The gene for dentin sialophosphoprotein produces a single protein that is post-translationally modified to generate two distinct extracellular proteins: dentin sialoprotein and dentin phosphoprotein. In teeth, dentin sialophosphoprotein is expressed primarily by odontoblast cells, but is also transiently expressed by presecretory ameloblasts. Because of this expression profile it appears that dentin sialophosphoprotein contributes to the early events of amelogenesis, and in particular to those events that result in the formation of the dentino-enamel junction and the adjacent "aprismatic" enamel. Using a transgenic animal approach we have extended dentin sialoprotein or dentin phosphoprotein expression throughout the developmental stages of amelogenesis. Overexpression of dentin sialoprotein results in an increased rate of enamel mineralization, however, the enamel morphology is not significantly altered. In wild-type animals, the inclusion of dentin sialoprotein in the forming aprismatic enamel may account for its increased hardness properties, when compared with bulk enamel. In contrast, the overexpression of dentin phosphoprotein creates "pitted" and "chalky" enamel of non-uniform thickness that is more prone to wear. Disruptions to the prismatic enamel structure are also a characteristic of the dentin phosphoprotein overexpressing animals. These data support the previous suggestion that dentin sialoprotein and dentin phosphoprotein have distinct functions related to tooth formation, and that the dentino-enamel junction should be viewed as a unique transition zone between enamel and the underlying dentin. These results support the notion that the dentin proteins expressed by presecretory ameloblasts contribute to the unique properties of the dentino-enamel junction.

Enamel matrix protein interactions

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

INTRODUCTION

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Fluoride induces endoplasmic reticulum stress in ameloblasts responsible for dental enamel formation

The mechanism of how fluoride causes fluorosis remains unknown. Exposure to fluoride can inhibit protein synthesis, and this may also occur by agents that cause endoplasmic reticulum (ER) stress. When translated proteins fail to fold properly or become misfolded, ER stress response genes are induced that together comprise the unfolded protein response. Because ameloblasts are responsible for dental enamel formation, we used an ameloblast-derived cell line (LS8) to characterize specific responses to fluoride treatment. LS8 cells were growth-inhibited by as little as 1.9-3.8 ppm fluoride, whereas higher doses induced ER stress and caspase-mediated DNA fragmentation. Growth arrest and DNA damage-inducible proteins (GADD153/CHOP, GADD45alpha), binding protein (BiP/glucose-responsive protein 78 (GRP78), the non-secreted form of carbonic anhydrase VI (CA-VI), and active X-box-binding protein-1 (Xbp-1) were all induced significantly after exposure to 38 ppm fluoride. Unexpectedly, DNA fragmentation increased when GADD153 expression was inhibited by short interfering RNA treatment but remained unaffected by transient GADD153 overexpression. Analysis of control and GADD153(-/-) embryonic fibroblasts demonstrated that caspase-3 mediated the increased DNA fragmentation observed in the GADD153 null cells. We also demonstrate that mouse incisor ameloblasts are sensitive to the toxic effects of high dose fluoride in drinking water. Activated Ire1 initiates an ER stress response pathway, and mouse ameloblasts were shown to express activated Ire1. Ire1 levels appeared induced by fluoride treatment, indicating that ER stress may play a role in dental fluorosis. Low dose fluoride, such as that present in fluoridated drinking water, did not induce ER stress.

Leucine-rich amelogenin peptide: a candidate signaling molecule during cementogenesis

BACKGROUND

Cementum is a critical mineralized tissue; however, control of its formation remains undefined. One hypothesis is that enamel matrix proteins/peptides secreted by ameloblasts and/or epithelial rest cells contribute to the control of cementum formation via epithelial-mesenchymal interactions. Here, we focused on determining whether or not leucine-rich amelogenin peptide (LRAP), translated from an alternatively spliced amelogenin RNA, altered cementoblast behavior.

METHODS

Immortalized murine cementoblasts (OCCM-30) were exposed to LRAP and evaluated for: 1) proliferative activity; 2) gene expression using Northern blot for Cbfal (core binding factor alpha-1); OCN (osteocalcin), OPN (osteopontin), and real-time reverse transcription-polymerase chain reaction (RT-PCR) for OPG (osteoprotegerin); and RANKL (receptor activator of NF-kappaB ligand); 3) signaling pathway using inhibitors of PKA (THFA), PKC (GF109203X), and MAPK (UO126); and 4) mineralization evaluated by von Kossa and Alizarin-red.

RESULTS

LRAP had no effect on cell proliferation up to 6 days, with a decrease in cell growth observed at the highest dose by 9 days versus untreated cells. LRAP down regulated OCN and up regulated OPN in a dose- and time-response fashion, and inhibited the capacity of mineral nodule formation. Transcripts for OPG were increased in LRAP-treated cells compared to control, but RANKL mRNA levels were not affected. Core binding factor alpha (Cbfa) mRNA, expressed constitutively, was not affected by LRAP. Signaling pathway assays suggested involvement of the MAPK pathway, since the addition of the MAPK inhibitor suppressed OPN expression in LRAP-treated cells.

CONCLUSION

Leucine-rich amelogenin peptide appears to have a direct effect on cementoblast activity that may prove significant during development as well as in regeneration of periodontal tissues.

Overexpression of TRAP in the enamel matrix does not alter the enamel structural hierarchy

The secreted, full-length amelogenin is the dominant protein of the forming enamel organ. As enamel mineralization progresses, amelogenin is quickly subjected to proteolytic activity, and eliminated from the enamel environment. Mature enamel contains only traces of structural proteins, including enamelin and the sheath protein ameloblastin. In addition, a proteolytic fragment of amelogenin, known as the tyrosine-rich amelogenin peptide or TRAP, is present in low but isolatable quantities. By overexpressing TRAP during enamel development we sought to determine if such overexpression would result in structural alterations to the mature enamel. We reasoned that overexpressing a protein associated with enamel maturation, at an inappropriate developmental stage, would result in alterations to the enamel protein assembly and hence, alterations in enamel structure and morphology. As judged by transmission and scanning electron microscopy, the enamel formed by overexpressing TRAP showed little morphological differences when compared to the enamel of normal nontransgenic animals. Based on scanning electron-microscopic images, there was modest hypomineralization evident in the interrod enamel of the TRAP-overexpressing animals. However, this finding was inconsistent and inconsequential from a structural and functional perspective. From these results it appears that additional amounts of TRAP protein in the immature enamel matrix are not sufficient to alter the properties of the enamel extracellular matrix to an extent that the hierarchical structure of mature enamel is altered.

The COOH terminus of the amelogenin, LRAP, is oriented next to the hydroxyapatite surface

The organic matrix in forming enamel consists largely of the amelogenin protein self-assembled into nanospheres that are necessary to guide the formation of the unusually long and highly ordered hydroxyapatite (HAP) crystallites that constitute enamel. Despite its ability to direct crystal growth, the interaction of the amelogenin protein with HAP is unknown. However, the demonstration of growth restricted to the c-axis suggests a specific protein-crystal interaction, and the charged COOH terminus is often implicated in this function. To elucidate whether the COOH terminus is important in the binding and orientation of amelogenin onto HAP, we have used solid state NMR to determine the orientation of the COOH terminus of an amelogenin splice variant, LRAP (leucine-rich amelogenin protein), which contains the charged COOH terminus of the full protein, on the HAP surface. These experiments demonstrate that the methyl 13C-labeled side chain of Ala46 is 8.0 A from the HAP surface under hydrated conditions, for the protein with and without phosphorylation. The experimental results provide direct evidence orienting the charged COOH-terminal region of the amelogenin protein on the HAP surface, optimized to exert control on developing enamel crystals.

Amelogenin protein exhibits a modular design: implications for form and function

The most abundant protein of forming enamel is amelogenin, a protein capable of self-assembly to form nanospheres. Naturally occurring mutations in the human amelogenin gene are responsible for at least some of the disease entities known collectively as amelogenesis imperfecta (AI), although it is clear that the AI phenotype may be caused by alteration to other genes responsible for the biogenesis of the enamel extracellular matrix. Mutations that create changes in the functional domains of the amelogenin protein do adversely affect enamel biomineralization. Protein engineering of amelogenin that phenocopies several of the known AI mutations exhibits defects in self-assembly. Amino acid alterations that occur within a domain of amelogenin appear to cause "mineral defects," that is to say hypocalcification of the enamel, whereas mutations that occur elsewhere in another domain of the amelogenin molecule result in "hypoplastic defects," a decrease in thickness of the enamel. However, not all patients with AI phenotypes segregate precisely into these arbitrary designations. Nonetheless, correlating the domain of the amelogenin protein that contains a specific mutation with the type of enamel structural alteration suggests a modular design for amelogenin that is corroborated by protein engineering using recombinant DNA techniques and transgenic animal studies.

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

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

In vivo Overexpression of Tuftelin in the Enamel Organic Matrix

The primary sequences of human and mouse tuftelin are 89% identical. Both proteins comprise 390 amino acids and produce an acidic protein with an isoelectric point of 5.7, and an unmodified molecular weight of 44 kD. Using fluorescent-tagged tuftelin and amelogenin plasmid constructs we saw little evidence that these two enamel proteins colocalize in ameloblast-like LS-8 cells. Tuftelin is primarily localized to distinct ‘speckled’ domains within the cell cytoplasm. In an attempt to better define a physiological function for tuftelin during amelogenesis, we have produced transgenic mice that overexpress tuftelin in ameloblasts and subsequently the enamel matrix. Tuftelin overexpression impacts dramatically upon the enamel crystallite habit and the enamel prismatic structure. Overexpressing tuftelin results in gross imperfections in enamel that is evident both at the nanoscale and the mesoscale. The most notable difference observed in the transgenic animals, when compared to wild-type animals, is an apparent loss of restricted growth of enamel crystallites along their a-axis and b-axis. This equates to a change in the crystallite aspect ratio. In the transgenic animals the crystallite structures appear more ‘plate’-like in contrast to the symmetric, ‘ribbon’-like crystallite morphology that is a characteristic feature of mammalian enamel.

Enamel structure properties controlled by engineered proteins in transgenic mice

Amelogenin protein has regulatory effects on enamel biofabrication in mammalian tooth. Using teeth obtained from transgenic mice that express two separate protein-engineered versions of amelogenins, we made structure-nanomechanical properties correlations and showed 21% hardness and 24% elastic modulus degradation compared with the age-matched wildtype littermates. We attribute the inferior properties to the disorganization of the protein matrix resulting in defective mineral formation.

INTRODUCTION

Enamel is a bioceramic initiated by the biosynthesis of a complex mixture of proteins that undergoes self-assembly to produce a super molecular ensemble that controls the nucleation and habit of the crystalline mineral phase. Ultimately, the inorganic crystals grow to almost fully replace the organic phase. This biofabrication process occurs at physiologic conditions of pH, temperature, pressure, and ion concentration and results in the hardest tissue in the vertebrate body, with the largest and longest substituted-hydroxyapatite crystals known to biomineralizing systems. The most abundant protein of forming mammalian enamel, amelogenin, has been shown to have a significant regulatory effect on this complex process.

MATERIALS AND METHODS

In this work, we present the effect of protein engineering of amelogenin on the mechanical properties of the resultant mouse enamel. We have produced two types of transgenic animals that express separate versions of amelogenin proteins that lack the required self-assembly domains. The resultant matured enamel was extensively characterized for its mechanical properties at the nanoscale by means of nanoindentation and atomic force microscopy (AFM). These techniques have enabled us to probe the mechanical properties that are representative of a single enamel rod.

RESULTS

Our nanoindentation measurements have revealed that the altered amelogenin with dysfunctional self-assembly properties resulted in a degradation by as much as 21% in hardness and 24% in elastic modulus compared with the age-matched wildtype littermates. Furthermore, the enamel formed by these defective proteins is found to display a decrease in indentation surface pile-up volume by up to 32%.

CONCLUSIONS

We attribute these inferior mechanical properties for the enamel grown by the engineered amelogenins to result from the disorganization of the nanospheres formed in the protein matrix starting at the mineral nucleation stage with a consequential alteration to the fully grown mineral component. By engineering the properties of proteins that contribute to the nanoscale level of hierarchy in enamel biomineralization, it is possible to regulate the properties of the resulting bioceramic at the mesoscale level of the tissue.

Amelogenin: A Potential Regulator of Cementum-Associated Genes

BACKGROUND

Studies suggest that enamel matrix proteins induce differentiation and mineralization of a variety of mesenchymal cells, including odontoblasts, osteoblasts, and cementoblasts. It has been postulated that this activity could be due to amelogenin-like proteins, known to be present in some mixtures of enamel matrix derivatives. Amelogenins have been reported to induce expression of a mineralized tissue-specific marker, bone sialoprotein (BSP), indicating that epithelial products can regulate the activity of mesenchyme-derived cells.

METHODS

To explore the molecular mechanisms involved in BSP regulation, a clonal population of immortalized murine cementoblasts (OCCM-30) was exposed to full-length murine amelogenin protein (rp(H)M180), 0.1 microg/ml to 10.0 microg/ml, for 8 days in vitro. To further investigate the potential epithelial-mesenchymal interaction, an amelogenin knockout mouse model was used to examine expression of BSP and other markers, including Type I collagen, in tissue samples.

RESULTS

The lowest dose of amelogenin slightly enhanced BSP expression, whereas at the highest dose, a dramatic decrease (three-fold) in BSP expression was observed. Parallel experiments showed a corresponding decrease in mineral nodule formation in vitro for cells treated with the higher dose of rp(H)M180. In situ hybridization and immunohistochemical analysis of sections from amelogenin null mice revealed a dramatic reduction in expression of BSP mRNA and protein in cementoblasts and surrounding osteoblasts in comparison to age-matched controls. In contrast, the expression of Type I collagen was not significantly different from controls.

CONCLUSION

These data suggest that amelogenin may be a critical signaling molecule required for appropriate development of the periodontium.

A transgenic animal model resembling amelogenesis imperfecta related to ameloblastin overexpression

Genetic diseases that affect tooth enamel are grouped under the classification amelogenesis imperfecta. Human pedigrees and experiments on transgenic and null mice have all demonstrated that mutations to the secreted proteins amelogenin, enamelin, and enamelysin result in visibly, structurally, or mechanically defective enamel. In an attempt to better define a physiologic function for ameloblastin during enamel formation, we have produced transgenic mice that misexpress the ameloblastin gene. These transgenic animals exhibit imperfections in their enamel that is evident at the nanoscale level. Specifically, ameloblastin overexpression influences enamel crystallite habit and enamel rod morphology. These findings suggest enamel crystallite habit and rod morphology are influenced by the temporal and spatial expression of ameloblastin and may implicate the role of the ameloblastin gene locus in the etiology of a number of undiagnosed autosomally dominant cases of amelogenesis imperfecta.

Functional domains for amelogenin revealed by compound genetic defects

We have previously used the yeast two-hybrid assay and multiple in vitro methodologies to show that amelogenin undergoes self-assembly involving two domains (A and B). Using transgenic animals, we show that unique enamel phenotypes result from disruptions to either the A- or B-domain, supporting the role of amelogenin in influencing enamel structural organization. By crossbreeding, animals bearing two defective amelogenin gene products have a more extreme enamel phenotype than the sum of the defects evident in the individual parental lines. At the nanoscale level, the forming matrix shows alteration in the size of the amelogenin nanospheres. At the mesoscale level of enamel structural hierarchy, 6-week-old enamel exhibits defects in enamel rod organization caused by perturbed organization of the precursor organic matrix. These studies reflect the critical dependency of amelogenin self-assembly to form a highly organized enamel organic matrix, and that amelogenins engineered to be defective in self-assembly produce compound defects in the structural organization of enamel.

Ultrastructure of forming enamel in mouse bearing a transgene that disrupts the amelogenin self-assembly domains

The mouse X-chromosomal amelogenin gene promoter was used to drive the expression of mutated amelogenin proteins in vivo. Two different transgenic mouse lines based on deletions to either the amino-terminal (A-domain deletions) or to the carboxyl-region (B-domain deletions) were bred. In the molars of newborn A-domain deleted transgenic mice the formation of the initial layer of aprismatic enamel was delayed. There were severe structural alterations in the enamel of incisors of newborn mice bearing the A-domain deletion which were not apparent in animals bearing the B-domain deletion. In the A-domain-deleted animals, stippled material accumulated throughout the entire thickness of the forming enamel apparently causing a disruption of the normal rod-to-inter-rod relationship. This stippled material was likened to and interpreted as being groupings of amelogenin nanospheres. In the B-domain-deleted animals the stippled material was detected only in minute defects of the forming enamel. These data suggest significant differences in nanosphere assembly properties for animals bearing either the A-domain or the B-domain-deleted transgene. The present in vivo experimental approach suggests that at early stages of enamel formation, the A-domain plays a greater role than does the B-domain in amelogenin self-assembly, and consequently in enamel architecture and structure.

Developmental patterning of the circumvallate papilla

Organogenesis is regulated by the sequential and reciprocal interactions between epithelial and mesenchymal tissues. Many molecules, including growth factors, transcription factors, extracellular matrices, cell surface receptors, and matrix degrading enzymes, have been found to be involved in this process. To investigate the molecular mechanism responsible for morphogenesis of the circumvallate papilla/von Ebners' gland complex, we examined the expression patterns of selected cell adhesion molecules, extracellular matrix molecules, innervation and cell division in the circumvallate papilla of mouse embryos from embryonic day 11.5 (E11.5) to E14. At E11.5-E13.5, the lingual epithelium, the site where the circumvallate papilla will develop, is negative for BrdU labeling. At E14-E15, we detected cell division in the papillary area, especially in the epithelial invagination where von Ebners' minor salivary gland will form. The basement membrane component, laminin, is expressed as a continuous thin line separating the epithelia from the underlying mesenchyme, but it is broadly and strongly expressed in the area wherethe nervefibers penetrate into the circumvallate papilla. At the E12-E12.5 stage of development, the epithelial thickening shows intense E-cadherin staining in the superficial and basal layers, but weak E-cadherin staining in the suprabasal layer. E-cadherin is strongly expressed, but appears dispersed among the basal layer of lingual epithelium, the site where nerve fibers will innervate. At E13, nerve fibers reach the circumvallate papilla. These nerve fibers penetrate into and split the epithelial cell mass into two stalks which will later differentiate to form the von Ebners' gland. These results suggest that 1) the formation of the circumvallate papilla does not initially depend on cell division, 2) cell migration likely plays a major role during circumvallate placode formation, 3) E-cadherin and laminin may play a role in nerve guidance and 4) innervation impacts the final morphogenesis of the circumvallate papilla.