Identification of CCAAT/enhancer-binding protein alpha as a transactivator of the mouse amelogenin gene

Enam expression is regulated by Msx2

BACKGROUND

The precise formation of mineralized dental tissues such as enamel and/or dentin require tight transcriptional control of the secretion of matrix proteins. Here, we have investigated the transcriptional regulation of the second most prominent enamel matrix protein, enamelin, and its regulation through the major odontogenic transcription factor, MSX2.

RESULTS

Using in vitro and in vivo approaches, we identified that (a) Enam expression is reduced in the Msx2 mouse mutant pre-secretory and secretory ameloblasts, (b) Enam is an early response gene whose expression is under the control of Msx2, (c) Msx2 binds to Enam promoter in vitro, suggesting that enam is a direct target for Msx2 and that (d) Msx2 alone represses Enam gene expression.

CONCLUSIONS

Collectively, these results illustrate that Enam gene expression is controlled by Msx2 in a spatio-temporal manner. They also suggest that Msx2 may interact with other transcription factors to control spatial and temporal expression of Enam and hence amelogenesis and enamel biomineralization.

Transcriptional Regulation of Dental Epithelial Cell Fate

Dental enamel is hardest tissue in the body and is produced by dental epithelial cells residing in the tooth. Their cell fates are tightly controlled by transcriptional programs that are facilitated by fate determining transcription factors and chromatin regulators. Understanding the transcriptional program controlling dental cell fate is critical for our efforts to build and repair teeth. In this review, we describe the current understanding of these regulators essential for regeneration of dental epithelial stem cells and progeny, which are identified through transgenic mouse models. We first describe the development and morphogenesis of mouse dental epithelium in which different subpopulations of epithelia such as ameloblasts contribute to enamel formation. Then, we describe the function of critical factors in stem cells or progeny to drive enamel lineages. We also show that gene mutations of these factors are associated with dental anomalies in craniofacial diseases in humans. We also describe the function of the master regulators to govern dental lineages, in which the genetic removal of each factor switches dental cell fate to that generating hair. The distinct and related mechanisms responsible for the lineage plasticity are discussed. This knowledge will lead us to develop a potential tool for bioengineering new teeth.

An Msx2-Sp6-Follistatin Pathway Operates During Late Stages of Tooth Development to Control Amelogenesis

Background

Ameloblasts are epithelially derived cells responsible for enamel formation through a process known as amelogenesis. Amongst the several transcription factors that are expressed during amelogenesis, both Msx2 and Sp6 transcription factors play important role. Msx2 and Sp6 mouse mutants, exhibit similar amelogenesis defects, namely enamel hypoplasia, while humans with amelogenesis imperfecta (AI) carry mutations in the human homologues of MSX2 or SP6 genes. These across species similarities in function indicate that these two transcription factors may reside in the same developmental pathway. In this paper, we test whether they work in a coordinated manner to exert their effect during amelogenesis.

Methods

Two different dental epithelial cell lines, the mouse LS8 and the rat G5 were used for either overexpression or silencing of Msx2 or Sp6 or both. Msx2 mutant mouse embryos or pups were used for in vivo studies. In situ hybridization, semi-quantitative and quantitative real time PCR were employed to study gene expression pattern. MatInspector was used to identify several potential putative Msx2 binding sites upstream of the murine Sp6 promoter region. Chromatin Immunoprecipitation (chIP) was used to confirm the binding of Msx2 to Sp6 promoter at the putative sites.

Results

Using the above methods we identified that (i) Msx2 and Sp6 exhibit overlapping expression in secretory ameloblasts, (ii) Sp6 expression is reduced in the Msx2 mouse mutant secretoty ameloblasts, and (iii) that Msx2, like Sp6 inhibits follistatin expression. Specifically, our loss-of function studies by silencing Msx2 and/or Sp6 in mouse dental epithelial (LS8) cells showed significant downregulation of Sp6 but upregulation of Fst expression. Transient transfection of Msx2 overexpression plasmid, up-regulated Sp6 and downregulated Fst expression. Additionally, using MatInspector, we identified several potential putative Msx2 binding sites, 3.5 kb upstream of the murine Sp6 promoter region. By chIP, we confirmed the binding of Msx2 to Sp6 promoter at these sites, thus suggesting that Sp6 is a direct target of Msx2.

Conclusion

Collectively, these results show that Sp6 and Msx2 work in a concerted manner to form part of a network of transcription factors that operate during later stages of tooth development controlling ameloblast life cycle and amelogenesis.

Transcriptomic analysis of MicroRNA expression in enamel-producing cells

There is little evidence for the involvement of microRNAs (miRNAs) in the regulation of circadian rhythms during enamel development. Few studies have used ameloblast-like cell line LS8 to study the circadian rhythm of gene activities related to enamel formation. However, the transcriptomic analysis of miRNA expression in LS8 cells has not been established yet. In this study, we analyze the oscillations of miRNAs in LS8 cells during one-day cycle of 24 h by next generation deep sequencing. After removal of low quality reads, contaminants, and ligation products, we obtained a high number of clean reads in all 12 samples from four different time points. The length distribution analysis indicated that 77.5% of clean reads were between 21 and 24 nucleotides (nt), of which 35.81% reads exhibited a length of 22 nt. In total, we identified 1471 miRNAs in LS8 cells throughout all four time-points. 1330 (90.41%) miRNAs were identified as known miRNA sequences, whereas 139 (9.59%) were unannotated and classified as novel miRNA sequences. The differential expression analysis showed that 191 known miRNAs exhibited significantly (P-value < 0.01) different levels of expression across three time-points investigated (T6, T12, and T18) compared to T0. Verification of sequencing data using qRT-PCR on six selected miRNAs suggested good correlation between the two methods. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed significant enrichment of predicted target genes of differentially expressed miRNAs. The present study shows that miRNAs are highly expressed in LS8 cells and that a significant number of them oscillate during one-day cycle of 24 h. This is the first transcriptomic analysis of miRNAs in ameloblast-like cell line LS8 that can be potentially used to further characterize the epigenetic regulation of miRNAs during enamel formation.

Transcriptional regulation of the matrix protein Shematrin-2 during shell formation in pearl oyster

The molluscan shell is a fascinating biomineral consisting of a highly organized calcium carbonate composite. Biomineralization is elaborately controlled and involves several macromolecules, especially matrix proteins, but little is known about the regulatory mechanisms. The matrix protein Shematrin-2, expression of which peaks in the mantle tissues and in the shell components of the pearl oyster Pinctada fucata, has been suggested to be a key participant in biomineralization. Here, we expressed and purified Shematrin-2 from P. fucata and explored its function and transcriptional regulation. An in vitro functional assay revealed that Shematrin-2 binds the calcite, aragonite, and chitin components of the shell, decreases the rate of calcium carbonate deposition, and changes the morphology of the deposited crystal in the calcite crystallization system. Furthermore, we cloned the Shematrin-2 gene promoter, and analysis of its sequence revealed putative binding sites for the transcription factors CCAAT enhancer-binding proteins (Pf-C/EBPs) and nuclear factor-Y (NF-Y). Using transient co-transfection and reporter gene assays, we found that cloned and recombinantly expressed Pf-C/EBP-A and Pf-C/EBP-B greatly and dose-dependently up-regulate the promoter activity of the Shematrin-2 gene. Importantly, Pf-C/EBP-A and Pf-C/EBP-B knockdowns decreased Shematrin-2 gene expression and induced changes in the inner-surface structures in prismatic layers that were similar to those of antibody-based Shematrin-2 inhibition. Altogether, our data reveal that the transcription factors Pf-C/EBP-A and Pf-C/EBP-B up-regulate the expression of the matrix protein Shematrin-2 during shell formation in P. fucata, improving our understanding of the transcriptional regulation of molluscan shell development at the molecular level.

Micro-dissection of Enamel Organ from Mandibular Incisor of Rats Exposed to Environmental Toxicants

Enamel defects resulting from environmental conditions and ways of life are public health concerns because of their high prevalence. These defects result from altered activity of cells responsible for enamel synthesis named ameloblasts, which present in enamel organ. During amelogenesis, ameloblasts follow a specific and precise sequence of events of proliferation, differentiation, and death. A rat continually growing incisors is a suitable experimental model to study ameloblast activity and differentiation stages in physiological and pathological conditions. Here, we describe a reliable and consistent method to micro-dissect enamel organ of rats exposed to environmental toxicants. The micro-dissected dental epithelia contain secretion- and maturation-stage ameloblasts that may be used for qualitative experiments, such as immunohistochemistry assays and in situ hybridization, as well as for quantitative analyses such as RT-qPCR, RNA-seq, and Western blotting.

The Fam50a positively regulates ameloblast differentiation via interacting with Runx2

Differentiated ameloblasts secret enamel matrix proteins such as amelogenin, ameloblastin, and enamelin. Expression levels of these proteins are regulated by various factors. To find a new regulatory factor for ameloblast differentiation, we performed 2D-PAGE analysis using mouse ameloblast lineage cell line (mALCs) cultured with mineralizing medium. Of identified proteins, family with sequence similarity 50 member A (Fam50a) was significantly increased during differentiation of mALCs. Fam50a protein was also highly expressed in secretory ameloblasts of mouse tooth germs. In mALCs cultures, forced expression of Fam50a up-regulated the expression of enamel matrix protein genes such as amelogenin, ameloblastin, and enamelin. In addition, up-regulation of Fam50a also increased ALP activity and mineralized nodule formation in a dose-dependent manner. In contrast, knockdown of Fam50a decreased expression levels of enamel matrix protein genes, ALP activity, and mineralized nodule formation. By fluorescence microscopy, endogenous Fam50a protein was found to be localized to the nucleus of ameloblasts. In addition, Fam50a synergistically increased Ambn transactivation by Runx2. Moreover, Fam50a increased binding affinity of Runx2 to Ambn promoter by physically interacting with Runx2. Taken together, these results suggest Fam50a might be a new positive regulator of ameloblast differentiation.

Role of MIZ-1 in AMELX gene expression

Amelogenin (AMELX) is the main component of the developing tooth enamel matrix and is essential for enamel thickness and structure. However, little is known about its transcriptional regulation. Using gene expression analysis, we found that MIZ-1, a potent transcriptional activator of CDKN1A, is expressed during odontoblastic differentiation of hDPSCs (human dental pulp stem cells), and is essential for odontoblast differentiation and mineralization. We also investigated how MIZ-1 regulates gene expression of AMELX. Oligonucleotide-pull down assays showed that MIZ-1 binds to an MRE (MIZ-1 binding element) of the AMELX proximal promoter region (bp, −170 to −25). Combined, our ChIP, transient transcription assays, and promoter mutagenesis revealed that MIZ-1 directly binds to the MRE of the Amelx promoter recruits p300 and induces Amelx gene transcription. Finally, we show that the zinc finger protein MIZ-1 is an essential transcriptional activator of Amelx, which is critical in odontogenesis and matrix mineralization in the developing tooth.

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MIZ-1is expressed during odontoblast differentiation.

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MIZ-1 activates transcription of theAmelxgene.

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MIZ-1 binds to the proximal promoter MRE (bp, −70 to −49).

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MIZ-1 is a critical factor of odontogenesis and matrix mineralization of hDPSCs.

Store-operated Ca2+ Entry Modulates the Expression of Enamel Genes

Dental enamel formation is an intricate process tightly regulated by ameloblast cells. The correct spatiotemporal patterning of enamel matrix protein (EMP) expression is fundamental to orchestrate the formation of enamel crystals, which depend on a robust supply of Ca2+. In the extracellular milieu, Ca2+ -EMP interactions occur at different levels. Despite its recognized role in enamel development, the molecular machinery involved in Ca2+ homeostasis in ameloblasts remains poorly understood. A common mechanism for Ca2+ influx is store-operated Ca2+ entry (SOCE). We evaluated the possibility that Ca2+ influx in enamel cells might be mediated by SOCE and the Ca2+ release-activated Ca2+ (CRAC) channel, the prototypical SOCE channel. Using ameloblast-like LS8 cells, we demonstrate that these cells express Ca2+ -handling molecules and mediate Ca2+ influx through SOCE. As a rise in the cytosolic Ca2+ concentration is a versatile signal that can modulate gene expression, we assessed whether SOCE in enamel cells had any effect on the expression of EMPs. Our results demonstrate that stimulating LS8 cells or murine primary enamel organ cells with thapsigargin to activate SOCE leads to increased expression of Amelx, Ambn, Enam, Mmp20. This effect is reversed when cells are treated with a CRAC channel inhibitor. These data indicate that Ca2+ influx in LS8 cells and enamel organ cells is mediated by CRAC channels and that Ca2+ signals enhance the expression of EMPs. Ca2+ plays an important role not only in mineralizing dental enamel but also in regulating the expression of EMPs.

Protein Interaction between Ameloblastin and Proteasome Subunit α Type 3 Can Facilitate Redistribution of Ameloblastin Domains within Forming Enamel

Enamel is a bioceramic tissue composed of thousands of hydroxyapatite crystallites aligned in parallel within boundaries fabricated by a single ameloblast cell. Enamel is the hardest tissue in the vertebrate body; however, it starts development as a self-organizing assembly of matrix proteins that control crystallite habit. Here, we examine ameloblastin, a protein that is initially distributed uniformly across the cell boundary but redistributes to the lateral margins of the extracellular matrix following secretion thus producing cell-defined boundaries within the matrix and the mineral phase. The yeast two-hybrid assay identified that proteasome subunit α type 3 (Psma3) interacts with ameloblastin. Confocal microscopy confirmed Psma3 co-distribution with ameloblastin at the ameloblast secretory end piece. Co-immunoprecipitation assay of mouse ameloblast cell lysates with either ameloblastin or Psma3 antibody identified each reciprocal protein partner. Protein engineering demonstrated that only the ameloblastin C terminus interacts with Psma3. We show that 20S proteasome digestion of ameloblastin in vitro generates an N-terminal cleavage fragment consistent with the in vivo pattern of ameloblastin distribution. These findings suggest a novel pathway participating in control of protein distribution within the extracellular space that serves to regulate the protein-mineral interactions essential to biomineralization.

Bioactive nanofibers enable the identification of thrombospondin 2 as a key player in enamel regeneration

Tissue regeneration and development involves highly synchronized signals both between cells and with the extracellular environment. Biomaterials can be tuned to mimic specific biological signals and control cell response(s). As a result, these materials can be used as tools to elucidate cell signaling pathways and candidate molecules involved with cellular processes. In this work, we explore enamel-forming cells, ameloblasts, which have a limited regenerative capacity. By exposing undifferentiated cells to a self-assembling matrix bearing RGDS epitopes, we elicited a regenerative signal at will that subsequently led to the identification of thrombospondin 2 (TSP2), an extracellular matrix protein that has not been previously recognized as a key player in enamel development and regeneration. Targeted disruption of the thrombospondin 2 gene (Thbs2) resulted in enamel formation with a disordered architecture that was highly susceptible to wear compared to their wild-type counterparts. To test the regenerative capacity, we injected the bioactive matrix into the enamel organ and discovered that the enamel organic epithelial cells in TSP-null mice failed to polarize on the surface of the artificial matrix, greatly reducing integrin β1 and Notch1 expression levels, which represent signaling pathways known to be associated with TSP2. These results suggest TSP2 plays an important role in regulating cell-matrix interactions during enamel formation. Exploiting the signaling pathways activated by biomaterials can provide insight into native signaling mechanisms crucial for tooth development and cell-based strategies for enamel regeneration.

MSX2 in ameloblast cell fate and activity

While many effectors have been identified in enamel matrix and cells via genetic studies, physiological networks underlying their expression levels and thus the natural spectrum of enamel thickness and degree of mineralization are now just emerging. Several transcription factors are candidates for enamel gene expression regulation and thus the control of enamel quality. Some of these factors, such as MSX2, are mainly confined to the dental epithelium. MSX2 homeoprotein controls several stages of the ameloblast life cycle. This chapter introduces MSX2 and its target genes in the ameloblast and provides an overview of knowledge regarding its effects in vivo in transgenic mouse models. Currently available in vitro data on the role of MSX2 as a transcription factor and its links to other players in ameloblast gene regulation are considered. MSX2 modulations are relevant to the interplay between developmental, hormonal and environmental pathways and in vivo investigations, notably in the rodent incisor, have provided insight into dental physiology. Indeed, in vivo models are particularly promising for investigating enamel formation and MSX2 function in ameloblast cell fate. MSX2 may be central to the temporal-spatial restriction of enamel protein production by the dental epithelium and thus regulation of enamel quality (thickness and mineralization level) under physiological and pathological conditions. Studies on MSX2 show that amelogenesis is not an isolated process but is part of the more general physiology of coordinated dental-bone complex growth.

Comparison of two mouse ameloblast-like cell lines for enamel-specific gene expression

Ameloblasts are ectoderm-derived cells that produce an extracellular enamel matrix that mineralizes to form enamel. The development and use of immortalized cell lines, with a stable phenotype, is an important contribution to biological studies as it allows for the investigation of molecular activities without the continuous need for animals. In this study we compare the expression profiles of enamel-specific genes in two mouse derived ameloblast-like cell lines: LS8 and ALC cells. Quantitative PCR analysis indicates that, relative to each other, LS8 cells express greater mRNA levels for genes that define secretory-stage activities (Amelx, Ambn, Enam, and Mmp20), while ALC express greater mRNA levels for genes that define maturation-stage activities (Odam and Klk4). Western blot analyses show that Amelx, Ambn, and Odam proteins are detectable in ALC, but not LS8 cells. Unstimulated ALC cells form calcified nodules, while LS8 cells do not. These data provide greater insight as to the suitability of both cell lines to contribute to biological studies on enamel formation and biomineralization, and highlight some of the strengths and weaknesses when relying on enamel epithelial organ-derived cell lines to study molecular activities of amelogenesis.

Tracking endogenous amelogenin and ameloblastin in vivo

Research on enamel matrix proteins (EMPs) is centered on understanding their role in enamel biomineralization and their bioactivity for tissue engineering. While therapeutic application of EMPs has been widely documented, their expression and biological function in non-enamel tissues is unclear. Our first aim was to screen for amelogenin (AMELX) and ameloblastin (AMBN) gene expression in mandibular bones and soft tissues isolated from adult mice (15 weeks old). Using RT-PCR, we showed mRNA expression of AMELX and AMBN in mandibular alveolar and basal bones and, at low levels, in several soft tissues; eyes and ovaries were RNA-positive for AMELX and eyes, tongues and testicles for AMBN. Moreover, in mandibular tissues AMELX and AMBN mRNA levels varied according to two parameters: 1) ontogenic stage (decreasing with age), and 2) tissue-type (e.g. higher level in dental epithelial cells and alveolar bone when compared to basal bone and dental mesenchymal cells in 1 week old mice). In situ hybridization and immunohistodetection were performed in mandibular tissues using AMELX KO mice as controls. We identified AMELX-producing (RNA-positive) cells lining the adjacent alveolar bone and AMBN and AMELX proteins in the microenvironment surrounding EMPs-producing cells. Western blotting of proteins extracted by non-dissociative means revealed that AMELX and AMBN are not exclusive to mineralized matrix; they are present to some degree in a solubilized state in mandibular bone and presumably have some capacity to diffuse. Our data support the notion that AMELX and AMBN may function as growth factor-like molecules solubilized in the aqueous microenvironment. In jaws, they might play some role in bone physiology through autocrine/paracrine pathways, particularly during development and stress-induced remodeling.

Adaptor protein complex 2-mediated, clathrin-dependent endocytosis, and related gene activities, are a prominent feature during maturation stage amelogenesis

Molecular events defining enamel matrix removal during amelogenesis are poorly understood. Early reports have suggested that adaptor proteins (AP) participate in ameloblast-mediated endocytosis. Enamel formation involves the secretory and maturation stages, with an increase in resorptive function during the latter. Here, using real-time PCR, we show that the expression of clathrin and adaptor protein subunits are upregulated in maturation stage rodent enamel organ cells. AP complex 2 (AP-2) is the most upregulated of the four distinct adaptor protein complexes. Immunolocalization confirms the presence of AP-2 and clathrin in ameloblasts, with strongest reactivity at the apical pole. These data suggest that the resorptive functions of enamel cells involve AP-2 mediated, clathrin-dependent endocytosis, thus implying the likelihood of specific membrane-bound receptor(s) of enamel matrix protein debris. The mRNA expression of other endocytosis-related gene products is also upregulated during maturation including: lysosomal-associated membrane protein 1 (Lamp1); cluster of differentiation 63 and 68 (Cd63 and Cd68); ATPase, H(+) transporting, lysosomal V0 subunit D2 (Atp6v0d2); ATPase, H(+) transporting, lysosomal V1 subunit B2 (Atp6v1b2); chloride channel, voltage-sensitive 7 (Clcn7); and cathepsin K (Ctsk). Immunohistologic data confirms the expression of a number of these proteins in maturation stage ameloblasts. The enamel of Cd63-null mice was also examined. Despite increased mRNA and protein expression in the enamel organ during maturation, the enamel of Cd63-null mice appeared normal. This may suggest inherent functional redundancies between Cd63 and related gene products, such as Lamp1 and Cd68. Ameloblast-like LS8 cells treated with the enamel matrix protein complex Emdogain showed upregulation of AP-2 and clathrin subunits, further supporting the existence of a membrane-bound receptor-regulated pathway for the endocytosis of enamel matrix proteins. These data together define an endocytotic pathway likely used by ameloblasts to remove the enamel matrix during enamel maturation.

The role of bioactive nanofibers in enamel regeneration mediated through integrin signals acting upon C/EBPα and c-Jun

Enamel formation involves highly orchestrated intracellular and extracellular events; following development, the tissue is unable to regenerate, making it a challenging target for tissue engineering. We previously demonstrated the ability to trigger enamel differentiation and regeneration in the embryonic mouse incisor using a self-assembling matrix that displayed the integrin-binding epitope RGDS (Arg-Gly-Asp-Ser). To further elucidate the intracellular signaling pathways responsible for this phenomenon, we explore here the coupling response of integrin receptors to the biomaterial and subsequent downstream gene expression profiles. We demonstrate that the artificial matrix activates focal adhesion kinase (FAK) to increase phosphorylation of both c-Jun N-terminal kinase (JNK) and its downstream transcription factor c-Jun (c-Jun). Inhibition of FAK blocked activation of the identified matrix-mediated pathways, while independent inhibition of JNK nearly abolished phosphorylated-c-Jun (p-c-Jun) and attenuated the pathways identified to promote enamel regeneration. Cognate binding sites in the amelogenin promoter were identified to be transcriptionally up-regulated in response to p-c-Jun. Furthermore, the artificial matrix induced gene expression as evidenced by an increased abundance of amelogenin, the main protein expressed during enamel formation, and the CCAAT enhancer binding protein alpha (C/EBPα), which is the known activator of amelogenin expression. Elucidating these cues not only provides guidelines for the design of synthetic regenerative strategies and opportunities to manipulate pathways to regulate enamel regeneration, but can provide insight into the molecular mechanisms involved in tissue formation.

Interaction of C/EBP-beta and NF-Y factors constrains activity levels of the nutritionally controlled promoter IA expressing the acetyl-CoA carboxylase-alpha gene in cattle

Background

The enzyme acetyl-CoA carboxylase-alpha (ACC-α) is rate limiting for de novo fatty acid synthesis. Among the four promoters expressing the bovine gene, promoter IA (PIA) is dominantly active in lipogenic tissues. This promoter is in principal repressed but activated under favorable nutritional conditions. Previous analyses already coarsely delineated the repressive elements on the distal promoter but did not resolve the molecular nature of the repressor. Knowledge about the molecular functioning of this repressor is fundamental to understanding the nutrition mediated regulation of PIA activity. We analyzed here the molecular mechanism calibrating PIA activity.

Results

We finely mapped the repressor binding sites in reporter gene assays and demonstrate together with Electrophoretic Mobility Shift Assays that nuclear factor-Y (NF-Y) and CCAAT/enhancer binding protein-β (C/EBPβ) each separately repress PIA activity by binding to their cognate low affinity sites, located on distal elements of the promoter. Simultaneous binding of both factors results in strongest repression. Paradoxically, over expression of NFY factors, but also - and even more so - of C/EBPβ significantly activated the promoter when bound to high affinity sites on the proximal promoter. However, co-transfection experiments revealed that NF-Y may eventually diminish the strong stimulatory effect of C/EBPβ at the proximal PIA in a dose dependent fashion. We validated by chromatin immunoprecipitation, that NF-Y and C/EBP factors may physically interact.

Conclusion

The proximal promoter segment of PIA appears to be principally in an active state, since even minute concentrations of both, NF-Y and C/EBPβ factors can saturate the high affinity activator sites. Higher factor concentrations will saturate the low affinity repressive sites on the distal promoter resulting in reduced and calibrated promoter activity. Based on measurements of the mRNA concentrations of those factors in different tissues we propose that the interplay of both factors may set tissue-specific limits for PIA activity.

Identification of a pH-responsive DNA region upstream of the transcription start site of human NBCe1-B

In rodent incisors two distinct stages of enamel formation can be identified visually based on cell morphology: the secretory stage and the maturation stage. The expression profiles of many genes characterize both stages, including the bicarbonate transport protein NBCe1. Bicarbonate is a requirement for the mineralizing enamel matrix to buffer excessive protons that form as a consequence of hydroxyapatite formation. NBCe1-B mRNA is up-regulated during the maturation stage of amelogenesis, where hydroxyapatite formation predominates. In this study, a presumed 572-bp NBCe1-B promoter region was subcloned into a reporter construct, and within this 572-bp region of DNA we characterized a 285-bp segment that shows an increase of ≈ 2.3-fold in gene-transcription activity when transfected into ameloblast-like cells and cultured in medium maintained at pH 6.8 (vs. pH 7.4). A presumed pH-responsive transcriptional factor-binding domain(s) thus resides in the 285-bp NBCe1-B promoter region where candidate domains include the nuclear factor of kappa light polypeptide gene enhancer in B-cells1(NFKB1), jun proto-oncogene (JUN), and tumor protein p53(TP53)-binding sites. Mutagenesis studies identify that both the NFKB1- and TP53-binding sites are responsive to changes in the extracellular pH. These data help to explain how ameloblasts respond to the altered extracellular milieu of protons by changing their gene-expression profile throughout the stages of amelogenesis.

Gene-expression analysis of early- and late-maturation-stage rat enamel organ

Enamel maturation is a dynamic process that involves high rates of mineral acquisition, associated fluctuations in extracellular pH, and resorption of extracellular enamel proteins. During maturation, ameloblasts change from having a tall, thin, and highly polarized organization, characteristic of the secretory stage, to having a low columnar and widened morphology in the maturation stage. To identify potential differences in gene expression throughout maturation, we obtained enamel organ epithelial cells derived from the early- and late-maturation stages of rat incisor and analyzed the global gene-expression profiles at each stage. Sixty-three candidate genes were identified as having potential roles in the maturation process. Quantitative PCR was used to confirm the results of this genome-wide analysis in a subset of genes. Transcripts enriched during late maturation (n = 38) included those associated with lysosomal activity, solute carrier transport, and calcium signaling. Also up-regulated were transcripts involved in cellular responses to oxidative stress, proton transport, cell death, and the immune system. Transcripts down-regulated during the late maturation stage (n =25) included those with functions related to cell adhesion, cell signaling, and T-cell activation. These results indicate that ameloblasts undergo widespread molecular changes during the maturation stage of amelogenesis and hence provide a basis for future functional investigations into the mechanistic basis of enamel mineralization.

An evolutionarily conserved enhancer regulates Bmp4 expression in developing incisor and limb bud

To elucidate the transcriptional regulation of Bmp4 expression during organogenesis, we used phylogenetic footprinting and transgenic reporter analyses to identify Bmp4 cis-regulatory modules (CRMs). These analyses identified a regulatory region located ∼46 kb upstream of the mouse Bmp4 transcription start site that had previously been shown to direct expression in lateral plate mesoderm. We refined this regulatory region to a 396-bp minimal enhancer, and show that it recapitulates features of endogenous Bmp4 expression in developing mandibular arch ectoderm and incisor epithelium during the initiation-stage of tooth development. In addition, this enhancer directs expression in the apical ectodermal ridge (AER) of the developing limb and in anterior and posterior limb mesenchyme. Transcript profiling of E11.5 mouse incisor dental lamina, together with protein binding microarray (PBM) analyses, allowed identification of a conserved DNA binding motif in the Bmp4 enhancer for Pitx homeoproteins, which are also expressed in the developing mandibular and incisor epithelium. In vitro electrophoretic mobility shift assays (EMSA) and in vivo transgenic reporter mutational analyses revealed that this site supports Pitx binding and that the site is necessary to recapitulate aspects of endogenous Bmp4 expression in developing craniofacial and limb tissues. Finally, Pitx2 chromatin immunoprecipitation (ChIP) demonstrated direct binding of Pitx2 to this Bmp4 enhancer site in a dental epithelial cell line. These results establish a direct molecular regulatory link between Pitx family members and Bmp4 gene expression in developing incisor epithelium.

The circadian clock modulates enamel development

Fully mature enamel is about 98% mineral by weight. While mineral crystals appear very early during its formative phase, the newly secreted enamel is a soft gel-like matrix containing several enamel matrix proteins of which the most abundant is amelogenin (Amelx). Histological analysis of mineralized dental enamel reveals markings called cross-striations associated with daily increments of enamel formation, as evidenced by injections of labeling dyes at known time intervals. The daily incremental growth of enamel has led to the hypothesis that the circadian clock might be involved in the regulation of enamel development. To identify daily rhythms of clock genes and Amelx, we subjected murine ameloblast cells to serum synchronization to analyze the expression of the circadian transcription factors Per2 and Bmal1 by real-time PCR. Results indicate that these key genetic regulators of the circadian clock are expressed in synchronized murine ameloblast cell cultures and that their expression profile follows a circadian pattern with acrophase and bathyphase for both gene transcripts in antiphase. Immunohistological analysis confirms the protein expression of Bmal and Cry in enamel cells. Amelx expression in 2-day postnatal mouse molars dissected every 4 hours for a duration of 48 hours oscillated with an approximately 24-hour period, with a significant approximately 2-fold decrease in expression during the dark period compared to the light period. The expression of genes involved in bicarbonate production (Car2) and transport (Slc4a4), as well as in enamel matrix endocytosis (Lamp1), was greater during the dark period, indicating that ameloblasts express these proteins when Amelx expression is at the nadir. The human and mouse Amelx genes each contain a single nonconserved E-box element within 10 kb upstream of their respective transcription start sites. We also found that within 2 kb of the transcription start site of the human NFYA gene, which encodes a positive regulator of amelogenin, there is an E-box element that is conserved in rodents and other mammals. Moreover, we found that Nfya expression in serum-synchronized murine ameloblasts oscillated with a strong 24-hour rhythm. Taken together, our data support the hypothesis that the circadian clock temporally regulates enamel development.

Targeted overexpression of amelotin disrupts the microstructure of dental enamel

We have previously identified amelotin (AMTN) as a novel protein expressed predominantly during the late stages of dental enamel formation, but its role during amelogenesis remains to be determined. In this study we generated transgenic mice that produce AMTN under the amelogenin (Amel) gene promoter to study the effect of AMTN overexpression on enamel formation in vivo. The specific overexpression of AMTN in secretory stage ameloblasts was confirmed by Western blot and immunohistochemistry. The gross histological appearance of ameloblasts or supporting cellular structures as well as the expression of the enamel proteins amelogenin (AMEL) and ameloblastin (AMBN) was not altered by AMTN overexpression, suggesting that protein production, processing and secretion occurred normally in transgenic mice. The expression of Odontogenic, Ameloblast-Associated (ODAM) was slightly increased in secretory stage ameloblasts of transgenic animals. The enamel in AMTN-overexpressing mice was much thinner and displayed a highly irregular surface structure compared to wild type littermates. Teeth of transgenic animals underwent rapid attrition due to the brittleness of the enamel layer. The microstructure of enamel, normally a highly ordered arrangement of hydroxyapatite crystals, was completely disorganized. Tomes' process, the hallmark of secretory stage ameloblasts, did not form in transgenic mice. Collectively our data demonstrate that the overexpression of amelotin has a profound effect on enamel structure by disrupting the formation of Tomes' process and the orderly growth of enamel prisms.

Transcription factor FoxO1 is essential for enamel biomineralization

The Transforming growth factor β (Tgf-β) pathway, by signaling via the activation of Smad transcription factors, induces the expression of many diverse downstream target genes thereby regulating a vast array of cellular events essential for proper development and homeostasis. In order for a specific cell type to properly interpret the Tgf-β signal and elicit a specific cellular response, cell-specific transcriptional co-factors often cooperate with the Smads to activate a discrete set of genes in the appropriate temporal and spatial manner. Here, via a conditional knockout approach, we show that mice mutant for Forkhead Box O transcription factor FoxO1 exhibit an enamel hypomaturation defect which phenocopies that of the Smad3 mutant mice. Furthermore, we determined that both the FoxO1 and Smad3 mutant teeth exhibit changes in the expression of similar cohort of genes encoding enamel matrix proteins required for proper enamel development. These data raise the possibility that FoxO1 and Smad3 act in concert to regulate a common repertoire of genes necessary for complete enamel maturation. This study is the first to define an essential role for the FoxO family of transcription factors in tooth development and provides a new molecular entry point which will allow researchers to delineate novel genetic pathways regulating the process of biomineralization which may also have significance for studies of human tooth diseases such as amelogenesis imperfecta.

Perturbed Amelogenin Protein Self-assembly Alters Nanosphere Properties Resulting in Defective Enamel Formation

Dental enamel is a unique composite bioceramic material that is the hardest tissue in the vertebrate body, containing long-, thin-crystallites of substituted hydroxyapatite. Enamel functions under immense loads in a bacterial-laden environment, and generally without catastrophic failure over a lifetime for the organism. Unlike all other biogenerated hard tissues of mesodermal origin, such as bone and dentin, enamel is produced by ectoderm-derived cells called ameloblasts. Recent investigations on the formation of enamel using cell and molecular approaches have been coupled to biomechanical investigations at the nanoscale and mesoscale levels. For amelogenin, the principle protein of forming enamel, two domains have been identified that are required for the proper assembly of multimeric units of amelogenin to form nanospheres. One domain is at the amino-terminus and the other domain in the carboxyl-terminal region. Amelogenin nanospheres are believed to influence the hydroxyapatite crystal habit. Both the yeast two-hybrid assay and surface plasmon resonance have been used to examine the assembly properties of engineered amelogenin proteins. Amelogenin protein was engineered using recombinant DNA techniques to contain deletions to either of the two self-assembly domains. Amelogenin protein was also engineered to contain single amino-acid mutations/substitutions in the amino-terminal self-assembly domain; and these amino-acid changes are based upon point mutations observed in humans affected with a hereditary disturbance of enamel formation. All of these alterations reveal significant defects in amelogenin self-assembly into nanospheres in vitro. Transgenic animals containing these same amelogenin deletions illustrate the importance of a physiologically correct bio-fabrication of the enamel protein extracellular matrix to allow for the organization of the enamel prismatic structure.

PERP regulates enamel formation via effects on cell-cell adhesion and gene expression

Little is known about the role of cell-cell adhesion in the development of mineralized tissues. Here we report that PERP, a tetraspan membrane protein essential for epithelial integrity, regulates enamel formation. PERP is necessary for proper cell attachment and gene expression during tooth development, and its expression is controlled by P63, a master regulator of stratified epithelial development. During enamel formation, PERP is localized to the interface between the enamel-producing ameloblasts and the stratum intermedium (SI), a layer of cells subjacent to the ameloblasts. Perp-null mice display dramatic enamel defects, which are caused, in part, by the detachment of ameloblasts from the SI. Microarray analysis comparing gene expression in teeth of wild-type and Perp-null mice identified several differentially expressed genes during enamel formation. Analysis of these genes in ameloblast-derived LS8 cells upon knockdown of PERP confirmed the role for PERP in the regulation of gene expression. Together, our data show that PERP is necessary for the integrity of the ameloblast-SI interface and that a lack of Perp causes downregulation of genes that are required for proper enamel formation.

Enamel protein regulation and dental and periodontal physiopathology in MSX2 mutant mice

Signaling pathways that underlie postnatal dental and periodontal physiopathology are less studied than those of early tooth development. Members of the muscle segment homeobox gene (Msx) family encode homeoproteins that show functional redundancy during development and are known to be involved in epithelial-mesenchymal interactions that lead to crown morphogenesis and ameloblast cell differentiation. This study analyzed the MSX2 protein during mouse postnatal growth as well as in the adult. The analysis focused on enamel and periodontal defects and enamel proteins in Msx2-null mutant mice. In the epithelial lifecycle, the levels of MSX2 expression and enamel protein secretion were inversely related. Msx2+/- mice showed increased amelogenin expression, enamel thickness, and rod size. Msx2-/- mice displayed compound phenotypic characteristics of enamel defects, related to both enamel-specific gene mutations (amelogenin and enamelin) in isolated amelogenesis imperfecta, and cell-cell junction elements (laminin 5 and cytokeratin 5) in other syndromes. These effects were also related to ameloblast disappearance, which differed between incisors and molars. In Msx2-/- roots, Malassez cells formed giant islands that overexpressed amelogenin and ameloblastin that grew over months. Aberrant expression of enamel proteins is proposed to underlie the regional osteopetrosis and hyperproduction of cellular cementum. These enamel and periodontal phenotypes of Msx2 mutants constitute the first case report of structural and signaling defects associated with enamel protein overexpression in a postnatal context.

Serotonin and fluoxetine receptors are expressed in enamel organs and LS8 cells and modulate gene expression in LS8 cells

The selective serotonin re-uptake inhibitor (SSRI) fluoxetine is widely used in the treatment of depression in children and fertile women, but its effect on developing tissues has been sparsely investigated. The aim of this study was to investigate if enamel organs and ameloblast-derived cells express serotonin receptors that are affected by peripherally circulating serotonin or fluoxetine. Using RT-PCR and western blot analysis we found that enamel organs from 3-d-old mice and ameloblast-like cells (LS8 cells) express functional serotonin receptors, the rate-limiting enzyme in serotonin synthesis (Thp1), as well as the serotonin transporter (5HTT), indicating that enamel organs and ameloblasts are able to respond to serotonin and regulate serotonin availability. Fluoxetine and serotonin enhanced the alkaline phosphatase activity in the cell culture medium from cultured LS8 cells, whereas the expression of enamelin (Enam), amelogenin (Amel), and matrix metalloproteinase-20 (MMP-20) were all significantly down-regulated. The secretion of vascular endothelial growth factor (VEGF), monocyte chemotactic protein 1 (MCP-1), and interferon-inducible protein 10 (IP-10) was also reduced compared with controls. In conclusion, enamel organs and ameloblast-like cells express functional serotonin receptors. Reduced transcription of enamel proteins and secretion of vascular factors may indicate possible adverse effects of fluoxetine on amelogenesis.

Resveratrol differentially modulates inflammatory responses of microglia and astrocytes

BACKGROUND

Inflammatory responses in the CNS mediated by activated glial cells play an important role in host-defense but are also involved in the development of neurodegenerative diseases. Resveratrol is a natural polyphenolic compound that has cardioprotective, anticancer and anti-inflammatory properties. We investigated the capacity of resveratrol to protect microglia and astrocyte from inflammatory insults and explored mechanisms underlying different inhibitory effects of resveratrol on microglia and astrocytes.

METHODS

A murine microglia cell line (N9), primary microglia, or astrocytes were stimulated by LPS with or without different concentrations of resveratrol. The expression and release of proinflammatory cytokines (TNF-alpha, IL-1beta, IL-6, MCP-1) and iNOS/NO by the cells were measured by PCR/real-time PCR and ELISA, respectively. The phosphorylation of the MAP kinase superfamily was analyzed by western blotting, and activation of NF-kappaB and AP-1 was measured by luciferase reporter assay and/or electrophoretic mobility shift assay.

RESULTS

We found that LPS stimulated the expression of TNF-alpha, IL-1beta, IL-6, MCP-1 and iNOS in murine microglia and astrocytes in which MAP kinases, NF-kappaB and AP-1 were differentially involved. Resveratrol inhibited LPS-induced expression and release of TNF-alpha, IL-6, MCP-1, and iNOS/NO in both cell types with more potency in microglia, and inhibited LPS-induced expression of IL-1beta in microglia but not astrocytes. Resveratrol had no effect on LPS-stimulated phosphorylation of ERK1/2 and p38 in microglia and astrocytes, but slightly inhibited LPS-stimulated phosphorylation of JNK in astrocytes. Resveratrol inhibited LPS-induced NF-kappaB activation in both cell types, but inhibited AP-1 activation only in microglia.

CONCLUSION

These results suggest that murine microglia and astrocytes produce proinflammatory cytokines and NO in response to LPS in a similar pattern with some differences in signaling molecules involved, and further suggest that resveratrol exerts anti-inflammatory effects in microglia and astrocytes by inhibiting different proinflammatory cytokines and key signaling molecules.

Regulation of dental enamel shape and hardness

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

Characterization of the mouse CP27 promoter and NF-Y mediated gene regulation

The cp27 gene is a highly conserved and unique gene with important roles related to craniofacial organogenesis. The present study is a first analysis of the CP27 promoter and its regulation. Here, we have cloned the promoter of the mouse cp27 gene, examined its transcriptional activity, and identified transcription factor binding sites in the proximal promoter region. Two major transcription start sites were mapped adjacent to exon 1. Promoter function analysis of the 5' flanking region by progressive 5' deletion mutations localized transcription repression elements between -1993bp and -969bp and several positive elements between -968bp and the preferred transcription start site. EMSA and functional studies indicated two function-cooperative CCAAT boxes and identified the NF-Y transcription factor as the CCAAT activator controlling transactivation of the CP27 promoter. In addition, this study demonstrated that for its effective binding and function, NF-Y required not only the minimal DNA segment length identified by deletion studies, but also a defined nucleotide sequence in the distal 3' flanking region of the CP27 proximal promoter CCAAT box. These results provide a basis for our understanding of the specific regulation of the cp27 gene in the NF-Y-mediated gene transcription network.

C/EBPalpha regulates SIRT1 expression during adipogenesis

SIRT1 plays an important role in adipogenesis, but how SIRT1 is regulated in adipogenesis is largely unknown. In this study, we show that both SIRT1 protein and mRNA levels were increased along with CCAAT/enhancer-binding protein alpha (C/EBPalpha) during adipocyte differentiation. C/EBPalpha, but not C/EBPalphap30, activated SIRT1 promoter in both HeLa cells and 3T3-L1 preadipocytes. Furthermore, C/EBPalpha upregulated SIRT1 mRNA and protein levels in HeLa cells and increased SIRT1 expression in a p53-independent manner in Soas2 cells. In preadipocytes, ectopic expression of C/EBPalpha upregulated SIRT1 protein level and knockdown of C/EBPalpha led to the decrease of SIRT1 protein level. Moreover, by promoter deletion analysis, gel shift assay and chromatin immunoprecipitation, we found that C/EBPalpha bound to the SIRT1 promoter at a consensus C/EBPalpha binding site. These data demonstrate that C/EBPalpha regulates SIRT1 expression during adipogenesis by directly binding to the SIRT1 promoter.

Sesamin inhibits bacterial formylpeptide-induced inflammatory responses in a murine air-pouch model and in THP-1 human monocytes

The reaction of human leukocytes to chemoattractants is an important component of the host immune response and also plays a crucial role in the development of inflammation. Sesamin has been shown to inhibit lipid peroxidation and regulate cytokine production. In this study, we examined the effect of sesamin on inflammatory responses elicited by the bacterial chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLF) in vitro and in vivo and explored the mechanisms involved. fMLF is recognized by a human G protein-coupled receptor formyl peptide receptor (FPR) on phagocytic leukocytes. Sesamin at concentrations between 12.5 and 50 micromol/L inhibited fMLF-induced chemotaxis of human monocyte cell line THP-1 differentiated with dibutyryl cyclic AMP (P < 0.01). Similarly, sesamin inhibited FPR-transfected rat basophilic leukemia cell [epitope-tagged human FPR (ETFR) cell] migration toward fMLF (P < 0.01). In fMLF-induced inflammation in a murine air-pouch model, intraperitoneal administration of sesamin (12 mgkg(-1)d(-1) for 2 d) suppressed leukocyte infiltration into the air pouch induced by fMLF [(62.89 +/- 7.93) x 10(4) vs. (19.67 +/- 4.43) x 10(4) cells/air pouch; n = 9; P < 0.001]. Ca(2+) mobilization and mitogen-activated protein kinase extracellular signal-regulated kinase (ERK1/2) activation are involved in fMLF-induced leukocyte migration. Pretreatment of ETFR cells with sesamin inhibited fMLF-induced ERK1/2 phosphorylation in a dose-dependent manner but did not affect fMLF-induced Ca(2+) flux. Electrophoretic mobility shift assay showed that pretreatment of THP-1 cells with sesamin dose dependently inhibited fMLF-induced nuclear factor-kappaB (NF-kappaB) activation. These results suggest that sesamin inhibits leukocyte activation by fMLF through ERK1/2- and NF-kappaB-related signaling pathways and thus is a potential compound for the management of inflammatory diseases.

Identification of a suppressor element in the amelogenin promoter

Amelogenin expression is regulated in a cell-type- specific manner. Investigators have previously identified an enhancer element by using the 5' flanking sequence of the amelogenin promoter. However, the cell-type-specific regulation of the amelogenin gene remains poorly understood. In some genes, the first intron regulates tissue-specific expression. We hypothesized that intron 1 is important for the cell-type-specific regulation of amelogenin expression. We identified a suppressor element between -74 and -464. We also found enhancer activity in intron 1. Additionally, we found that the suppressor element in the promoter region suppressed intron 1 enhancer activity. The suppressor and the enhancers acted in an ameloblast-like cell line, but not in HeLa cells. Mutation of the Oct-1 binding sites reversed the suppressor activity, suggesting that Oct-1 sites are essential for suppression. These results suggest that Oct-1 and intron 1 may contribute to cell-type-specific amelogenin expression.

Apc inhibition of Wnt signaling regulates supernumerary tooth formation during embryogenesis and throughout adulthood

The ablation of Apc function or the constitutive activation of beta-catenin in embryonic mouse oral epithelium results in supernumerary tooth formation, but the underlying mechanisms and whether adult tissues retain this potential are unknown. Here we show that supernumerary teeth can form from multiple regions of the jaw and that they are properly mineralized, vascularized, innervated and can start to form roots. Even adult dental tissues can form new teeth in response to either epithelial Apc loss-of-function or beta-catenin activation, and the effect of Apc deficiency is mediated by beta-catenin. The formation of supernumerary teeth via Apc loss-of-function is non-cell-autonomous. A small number of Apc-deficient cells is sufficient to induce surrounding wild-type epithelial and mesenchymal cells to participate in the formation of new teeth. Strikingly, Msx1, which is necessary for endogenous tooth development, is dispensable for supernumerary tooth formation. In addition, we identify Fgf8, a known tooth initiation marker, as a direct target of Wnt/beta-catenin signaling. These studies identify key mechanistic features responsible for supernumerary tooth formation.

Bioactive nanofibers instruct cells to proliferate and differentiate during enamel regeneration

During tooth development, ectoderm-derived ameloblast cells create enamel by synthesizing a complex protein mixture serving to control cell to matrix interactions and the habit of hydroxyapatite crystallites. Using an in vitro cell and organ culture system, we studied the effect of artificial bioactive nanostructures on ameloblasts with the long-term goal of developing cell-based strategies for tooth regeneration. We used branched peptide amphiphile molecules containing the peptide motif Arg-Gly-Asp, or "RGD" (abbreviated BRGD-PA), known to self-assemble in physiologic environments into nanofibers that display on their surfaces high densities of this biological signal. Ameloblast-like cells (line LS8) and primary enamel organ epithelial (EOE) cells were cultured within PA hydrogels, and the PA was injected into the enamel organ epithelia of mouse embryonic incisors. The expression of amelogenin, ameloblastin, integrin alpha 5, and integrin alpha 6 was detected by quantitative real-time PCR and immunodetection techniques. We performed cell proliferation assay using BrdU labeling and a biomineralization assay using Alizarin red S staining with quantitative Ca(2+) measurements. In the cell culture model, ameloblast-like cells (LS8) and primary EOE cells responded to the BRGD-PA nanostructures with enhanced proliferation and greater amelogenin, ameloblastin, and integrin expression levels. At the site of injection of the BRGD-PA in the organ culture model, we observed EOE cell proliferation with differentiation into ameloblasts as evidenced by their expression of enamel specific proteins. Ultrastructural analysis showed the nanofibers within the forming extracellular matrix, in contact with the EOE cells engaged in enamel formation and regeneration. This study shows that BRGD-PA nanofibers present with enamel proteins participate in integrin-mediated cell binding to the matrix with delivery of instructive signals for enamel formation.

The effects of LAMP1 and LAMP3 on M180 amelogenin uptake, localization and amelogenin mRNA induction by amelogenin protein

We previously demonstrated that the uptake of M180 amelogenin protein in dental epithelial cells (HAT-7) results in increased levels of amelogenin mRNA through enhanced mRNA stabilization. To determine the processes involved in the uptake of extracellular M180 amelogenin by cells and in amelogenin intracellular trafficking in the amelogenin protein-mediated amelogenin mRNA expression pathway, we investigated the effects of LAMP1 and LAMP3, which are candidate M180 amelogenin receptors, on M180 amelogenin uptake, localization and amelogenin mRNA induction by amelogenin protein, using anti-LAMP-1 and anti-LAMP-3 antibodies and siRNA analysis. The results indicate that LAMP3 blocking by anti-LAMP-3 decreases M180 amelogenin uptake, but does not affect amelogenin mRNA induction by amelogenin protein, suggesting that LAMP3 is related to amelogenin degradation. Down-regulation by siRNA of LAMP1, which is the receptor for small amelogenin protein (LRAP), does not affect M180 amelogenin uptake, localization or amelogenin mRNA induction by amelogenin protein. Thus, while LAMP1 is the specific receptor for LRAP, it is not a receptor for M180 amelogenin. These findings will aid further research into the understanding of M180 amelogenin function and expression.

Identifying promoter elements necessary for enamelin tissue-specific expression

Enamel development requires the strictly regulated spatiotemporal expression of genes encoding enamel matrix proteins. The mechanisms orchestrating the initiation and termination of gene transcription at each specific stage of amelogenesis are unknown. In this study, we identify cis- regulatory regions necessary for normal enamelin (Enam) expression. Sequence analysis of the Enam promoter 5'-noncoding region identified potentially important cis-regulatory elements located within 5.2 kb upstream of the Enam translation initiation site. DNA constructs containing 5.2 or 3.9 kb upstream of the Enam translation initiation site were linked to an LacZ reporter gene and used to generate transgenic mice. The 3.9-kb Enam-LacZ transgenic lines showed no expression in ameloblasts, but ectopic LacZ staining was detected in osteoblasts. In contrast, the 5.2-kb Enam-LacZ construct was sufficient to mimic the endogenous Enam ameloblast-specific expression pattern. Our study provides new insights into the molecular control of Enam cell- and stage-specific expression.

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.

Distal cis-regulatory elements are required for tissue-specific expression of enamelin (Enam)

Enamel formation is orchestrated by the sequential expression of genes encoding enamel matrix proteins; however, the mechanisms sustaining the spatio-temporal order of gene transcription during amelogenesis are poorly understood. The aim of this study was to characterize the cis-regulatory sequences necessary for normal expression of enamelin (Enam). Several enamelin transcription regulatory regions, showing high sequence homology among species, were identified. DNA constructs containing 5.2 or 3.9 kb regions upstream of the enamelin translation initiation site were linked to a LacZ reporter and used to generate transgenic mice. Only the 5.2-Enam-LacZ construct was sufficient to recapitulate the endogenous pattern of enamelin tooth-specific expression. The 3.9-Enam-LacZ transgenic lines showed no expression in dental cells, but ectopic beta-galactosidase activity was detected in osteoblasts. Potential transcription factor-binding sites were identified that may be important in controlling enamelin basal promoter activity and in conferring enamelin tissue-specific expression. Our study provides new insights into regulatory mechanisms governing enamelin expression.

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.

Establishment and characterization of rat dental epithelial derived ameloblast-lineage clones

Teeth are the hardest tissues covered with enamel produced by ameloblasts. The ameloblast differentiation is controlled by sequential epithelial-mesenchymal interactions during tooth morphogenesis. However, the molecular mechanism of ameloblast differentiation remains unclear. To address this question, we developed an in vitro assay system to evaluate the molecular mechanism of amelogenesis. First, we established dental epithelium-derived clones from 6-day-old rat incisors and established that cells of the clone SRE-G5 were the largest producers of amelogenin mRNA. Next, we analyzed the effects of several chemicals on the amelogenin expression in SRE-G5 cells. Only mitogen-activated protein kinase (MAPK) activators enhanced amelogenin mRNA expression. This finding corresponded to the immunohistochemical data showing the presence of phosphorylated forms of p38, c-Jun N-terminal kinase (JNK), and extracellular signal-regulated kinase (ERK) during ameloblast differentiation. To examine the roles of MAPK signals, we compared the effects of anisomycin and sodium salicylate on the expression of tooth-related differentiation markers. Both anisomycin and sodium salicylate induced amelogenin, Abcg2, and Bmp4 mRNA and down-regulated p75NGFR mRNA. On the other hand, ALP, ectodin, Bmp2 and Fgf8 mRNA were up-regulated only by anisomycin. These results indicate that MAPK signaling functions, at least in part, as the inducer of ameloblast differentiation.

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.

Establishment and characterization of an immortomouse-derived odontoblast-like cell line to evaluate the effect of insulin-like growth factors on odontoblast differentiation

Insulin-like growth factors (IGF-I and IGF-II) play important roles in regulating growth and differentiation of many different organs including teeth. The presence of these factors in the developing tooth has been demonstrated. In vitro studies using tooth explants grown in the presence of IGFs suggest that they promote differentiation of ameloblast and odontoblasts cells. This is achieved by inducing or repressing gene expression associated with these cells. Since some of the genes involved in tooth differentiation are expressed by both cells, to determine the effect of IGF on odontoblast cell differentiation we first need a cell line in which a controlled environment can be created. In this study, we report the establishment and characterization of an Immortomouse-derived odontoblast-like cell line. This conditional cell line can grow indefinitely under permissive conditions in the presence of INF-gamma at 33 degrees C, differentiate into odontoblast-like cells and produce a mineralized extracellular matrix when the INF-gamma is removed and cell maintained at 39 degrees C. Addition of exogenous IGFs to the media results in an accelerated production of a mineralized matrix. This is the result of increased transcription of genes associated with bone mineralization while down regulating genes associated with dentin formation like DSPP. This data suggest that IGFs induce dental papillae mesenchyme cells to produce a bone-like mineralized extracellular matrix.

Differentiation of an ameloblast-lineage cell line (ALC) is induced by Sonic hedgehog signaling

Sonic hedgehog (Shh), one of the essential molecules for embryogenesis and organogenesis, is strongly expressed in the enamel knot, which represents the signaling center for odontogenesis due to the presence of essential secretory molecules. Here, we investigated the effects of Shh on ameloblast differentiation using a mouse ameloblast-lineage cell line (ALC). In the presence of Shh, the expression of amelogenin and ameloblastin was up-regulated, and their promoter activities were also elevated. Subcutaneous implantation of Shh-treated ALC into the dorsa of C57/Bl6J mouse caused increased expression of amelogenin and ameloblastin. Forced expression of Gli1, a major transcription factor in Shh signaling, up-regulated both amelogenin and ameloblastin, while Gli1-specific RNAi blocked their expression. These results suggest that Shh directly promotes ALC differentiation, and that Gli1 may be responsible for the signal mediation of Shh.

The exon 6ABC region of amelogenin mRNA contribute to increased levels of amelogenin mRNA through amelogenin protein-enhanced mRNA stabilization

We recently demonstrated that the reuptake of full-length amelogenin protein results in increased levels of amelogenin mRNA through enhanced mRNA stabilization (Xu, L., Harada, H., Tamaki, T. Y., Matsumoto, S., Tanaka, J., and Taniguchi, A. (2006) J. Biol. Chem. 281, 2257-2262). Here, we examined the molecular mechanism of enhanced amelogenin mRNA stabilization. To identify the cis-regulatory region within amelogenin mRNA, we tested various reporter systems using a deletion series of reporter plasmids. A deletion at exon 6ABC of amelogenin mRNA resulted in a 2.5-fold increase in the amelogenin mRNA expression level when compared with that of full-length mRNA, indicating that a cis-element exists in exon 6ABC of amelogenin mRNA. Furthermore, Northwestern analysis demonstrated that amelogenin protein binds directly to its mRNA in vitro, suggesting that amelogenin protein acts as a trans-acting protein that specifically binds to this cis-element. Moreover, recombinant mouse amelogenin protein extended the half-life of full-length amelogenin mRNA but did not significantly alter the half-life of exon 6ABC-deletion mutant mRNA. The splice products produced by deletion of exon 6ABC are known as leucine-rich amelogenin peptides and have signaling effects on cells. Our findings also suggest that the regulation of full-length amelogenin protein expression differs from the regulation of leucine-rich amelogenin peptide expression.

Mandibular phenotype of p20C/EBPbeta transgenic mice: Reduced alveolar bone mass and site-specific dentin dysplasia

CCAAT enhancer binding proteins (C/EBP) comprise a family of basic-leucine zipper transcription factors that regulate cellular differentiation and function. To determine the role of C/EBP transcription factors in osteoblasts and odontoblasts, we generated a transgenic (TG) mouse model with Co1a1 (pOBCol3.6) promoter-targeted expression of a FLAG-tagged dominant negative C/EBP isoform, p20C/EBPbeta (previously LIP). Two of the four transgenic lines presented with abnormalities in the developing incisors, including breakage, overgrowth, and malocclusion. Histological examination revealed that the amount of alveolar bone was reduced in TG compared to wild-type (WT) mice. By microcomputed tomography (microCT), the bone volume fraction of the mandible was reduced at the level of the first and third molars, demonstrating a severe mandibular osteopenia. The lingual dentin morphology of TG incisors differed dramatically from WT. Labial dentin (enamel side) showed normal thickness and tubular dentin structure, whereas the lingual dentin was thinner (25-30% of WT at the alveolar crest) with an amorphous globular structure characteristic of dentin dysplasia. FLAG immunostaining was seen in both lingual and labial odontoblasts, indicating that the site-specific defect was not due to a lack of labial transgene expression. Northern blot analysis demonstrated reduced osteocalcin expression in TG mandibles, while bone sialoprotein was increased, consistent with prior results in calvariae and long bones. Dental sialophosphoprotein, a marker of the odontoblast lineage whose absence causes dentin dysplasia, was modestly reduced in TG mice by Northern blot and in situ hybridization analysis. By fluorescence microscopy, pOBCol2.3-GFP, a marker of the odontoblast lineage, was expressed in both labial and lingual odontoblasts, although GFP-marked lingual odontoblasts were more flattened than WT cells. Moreover, GFP-positive processes in the lingual dentin tubules were truncated and less organized than those in WT dentin. MicroCT analysis showed reduced tissue density in the lingual dentin. These data suggest that C/EBP transcription factors may be involved in the regulation of odontoblast polarization and dentin matrix production.

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.

Establishment of primary cultures for mouse ameloblasts as a model of their lifetime

To understand how the properties of ameloblasts are spatiotemporally regulated during amelogenesis, two primary cultures of ameloblasts in different stages of differentiation were established from mouse enamel epithelium. Mouse primary ameloblasts (MPAs) prepared from immature enamel epithelium (MPA-I) could proliferate, whereas those from mature enamel epithelium (MPA-M) could not. MPA-M but not MPA-I caused apoptosis during culture. The mRNA expression of amelogenin, a marker of immature ameloblasts, was down-regulated, and that of enamel matrix serine proteiase-1, a marker of mature ameloblasts, was induced in MPA-I during culture. Using green fluorescence protein as a reporter, a visualized reporter system was established to analyze the promoter activity of the amelogenin gene. The region between -1102bp and -261bp was required for the reporter expression in MPA-I. These results suggest that MPAs are valuable in vitro models for investigation of ameloblast biology, and that the visualized system is useful for promoter analysis in MPAs.