Evidence for regulation of amelogenin gene expression by 1,25-dihydroxyvitamin D(3) in vivo

[Oral cavity as a target and a marker of environmental exposures: developmental dental defects]

The oral cavity is one of the main route for environmental contaminations associated to many chronic diseases (cancers, fertility and behavior disorders for example) via alimentation, medications and respiration. These environmental factors including, among others, endocrine disruptors and excessive fluoride can disrupt dental development and thus generate irreversible enamel defects. These defects are then treated with materials that may release molecules capable of generating these defects, leading to a vicious circle, particularly in pregnant women and young children. The present paper aims to review the state of knowledge, questions and controversies on common environmental factors in contact with the oral cavity. It also reviews their mechanisms of action and the mediators involved in enamel pathologies associated with environmental conditions. Dental tissues can not only be targeted by environmental factors but can also serve as early and easily accessible markers of exposure to these agents. Understanding and characterizing the environmental impact in the oral cavity will help to prevent multiple diseases, oral and distant, whose link with oral homeostasis is just being explored.

Toward Preventing Enamel Hypoplasia: Modeling Maternal and Neonatal Biomarkers of Human Calcium Homeostasis

AIM

The aim of this study was to assess biomarkers of calcium homeostasis and tooth development, in mothers during pregnancy and their children at birth, for enamel hypoplasia (EH) in the primary maxillary central incisor teeth.

METHODS

Bayesian methodology was used for secondary data analyses from a randomized, controlled trial of prenatal vitamin D3 supplementation in healthy mothers (N = 350) and a follow-up study of a subset of the children. The biomarkers were serum calcium (Ca), phosphorus (P), intact parathyroid hormone (iPTH), total circulating 25-dihydroxyvitamin D (25(OH)D), and 1,25-dihydroxyvitamin D (1,25(OH)2D). The maternal biomarkers were assayed monthly during pregnancy, and the child's biomarkers were derived from cord blood. Digital images of the child's 2 teeth were scored for EH using Enamel Defects Index criteria for each of the incisal, middle, and cervical regions for an EH extent score.

RESULTS

The child EH prevalence was 41% (60/145), with most defects present in the incisal and middle tooth regions. Cord blood iPTH and 1,25(OH)2D levels were significantly associated with EH extent after controlling for maternal factors. For every 1 pg/mL increase in cord blood iPTH, the EH extent decreased by approximately 6%. For every 10 pg/mL increase in cord blood 1,25(OH)2D, the EH extent increased by almost 30% (holding all other terms constant and adjusting for subject-level heterogeneity). The relationship between maternal 25(OH)D and maternal mean iPTH varied significantly by EH extent.

CONCLUSION

The results suggest possible modifiable relationships of maternal and neonatal factors of calcium homeostasis during pregnancy and at birth for EH, contributing to the frontier of knowledge regarding sound tooth development for dental caries prevention.

Expression of Steroid Receptors in Ameloblasts during Amelogenesis in Rat Incisors

Endocrine disrupting chemicals (EDCs) play a part in the modern burst of diseases and interfere with the steroid hormone axis. Bisphenol A (BPA), one of the most active and widely used EDCs, affects ameloblast functions, leading to an enamel hypomineralization pattern similar to that of Molar Incisor Hypomineralization (MIH). In order to explore the molecular pathways stimulated by BPA during amelogenesis, we thoroughly investigated the receptors known to directly or indirectly mediate the effects of BPA. The expression patterns of high affinity BPA receptors (ERRγ, GPR30), of ketosteroid receptors (ERs, AR, PGR, GR, MR), of the retinoid receptor RXRα, and PPARγ were established using RT-qPCR analysis of RNAs extracted from microdissected enamel organ of adult rats. Their expression was dependent on the stage of ameloblast differentiation, except that of ERβ and PPARγ which remained undetectable. An additional large scale microarray analysis revealed three main groups of receptors according to their level of expression in maturation-stage ameloblasts. The expression level of RXRα was the highest, similar to the vitamin D receptor (VDR), whereas the others were 13 to 612-fold lower, with AR and GR being intermediate. Immunofluorescent analysis of VDR, ERα and AR confirmed their presence mainly in maturation- stage ameloblasts. These data provide further evidence that ameloblasts express a specific combination of hormonal receptors depending on their developmental stage. This study represents the first step toward understanding dental endocrinology as well as some of the effects of EDCs on the pathophysiology of amelogenesis.

Distribution of the amelogenin protein in developing, injured and carious human teeth

Amelogenin is the major enamel matrix protein with key roles in amelogenesis. Although for many decades amelogenin was considered to be exclusively expressed by ameloblasts, more recent studies have shown that amelogenin is also expressed in other dental and no-dental cells. However, amelogenin expression in human tissues remains unclear. Here, we show that amelogenin protein is not only expressed during human embryonic development but also in pathological conditions such as carious lesions and injuries after dental cavity preparation. In developing embryonic teeth, amelogenin stage-specific expression is found in all dental epithelia cell populations but with different intensities. In the different layers of enamel matrix, waves of positive vs. negative immunostaining for amelogenin are detected suggesting that the secretion of amelogenin protein is orchestrated by a biological clock. Amelogenin is also expressed transiently in differentiating odontoblasts during predentin formation, but was absent in mature functional odontoblasts. In intact adult teeth, amelogenin was not present in dental pulp, odontoblasts, and dentin. However, in injured and carious adult human teeth amelogenin is strongly re-expressed in newly differentiated odontoblasts and is distributed in the dentinal tubuli under the lesion site. In an in vitro culture system, amelogenin is expressed preferentially in human dental pulp cells that start differentiating into odontoblast-like cells and form mineralization nodules. These data suggest that amelogenin plays important roles not only during cytodifferentiation, but also during tooth repair processes in humans.

Molecular and circadian controls of ameloblasts

Stage-specific expression of ameloblast-specific genes is controlled by differential expression of transcription factors. In addition, ameloblasts follow daily rhythms in their main activities (i.e. enamel protein secretion and enamel mineralization). This time-related control is orchestrated by oscillations of clock proteins involved in the regulation of circadian rhythms. Our aim was to identify the potential links between daily rhythms and developmental controls of ameloblast differentiation. The effects of the transcription factors distal-less homeobox 3 (Dlx3) and runt-related transcription factor 2 (Runx2), and the clock gene nuclear receptor subfamily 1, group D, member 1 (Nr1d1), on secretory and maturation ameloblasts [using stage-specific markers amelogenin (Amelx), enamelin (Enam), and kallikrein-related peptidase 4 (Klk4)] were evaluated in the HAT-7 ameloblast cell line. Amelx and Enam steady-state mRNA expression levels were down-regulated in Runx2 over-expressing cells and up-regulated in Dlx3 over-expressing cells. In contrast, Klk4 mRNA was up-regulated by both Dlx3 and Runx2. Furthermore, a temporal and spatial relationship between clock genes and ameloblast differentiation markers was detected. Of interest, clock genes not only affected rhythmic expression of ameloblast-specific genes but also influenced the expression of Runx2. Multiscale mathematical modeling is being explored to further understand the temporal and developmental controls of ameloblast differentiation. Our study provides novel insights into the regulatory mechanisms sustaining ameloblast differentiation.

Transcriptional activity analysis of promoter region of human PAX9 gene under dexamethasone, retinoic acid, and ergocalciferol treatment in MCF-7 and MDPC23

PAX9 gene is a member of the family homeobox of transcription factors and performs important function in development and organogenesis. Mutations in PAX9 coding sequences have been implicated in autosomal dominant oligodontia affecting predominantly permanent molars and second premolars. Previous studies have shown that PAX9 is required for secondary palate development and teratogens have been identified as inducers of a tooth and craniofacial malformations. This work focused on the analysis on the 5'-flanking region of the PAX9 gene studying the influence of retinoic acid, dexamethasone, and vitamin D on the expression of PAX9 by expression constructs that carry the reporter gene luciferase. As results, retinoic acid and dexamethasone showed progressive decrease of PAX9 expression. PAX9-pGL3B1 and PAX9-pGL3B2 promoter was inhibited under the treatment of dexamethasone and ergocalciferol. Retinoic acid and dexamethasone did not alter PAX9-pGL3B3 behavior indicating that sequences present between -1106 and +92 were important for the transcriptional activity of PAX9 promoter. In this study, we characterized the transcriptional activity of specific regions of the PAX9 promoter gene and we demonstrated that retinoic acid and ergocalciferol can modulate the transcriptional activity of PAX9 gene.

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.

Physiopathology of dental rickets in vitamin D receptor-ablated mice

1α25(OH)(2)vitaminD(3) and its nuclear receptor, VDR, are essential for normal tooth development. However, the relative contributions of the direct vs. indirect effects of vitamin D action on odontogenesis are unclear. The aim of this study was to discriminate among the specific roles of 1α25(OH)(2) vitaminD(3), calcemia/phosphatemia, and the maternal environment in mouse VDR null mutants. Microradiographic, histological, and molecular analyses were conducted on adult mice under hypocalcemic/hypophosphatemic vs. normocalcemic/normophosphatemic conditions, and pups of first- (VDR-/- born to VDR+/- dams) vs. second-generation (VDR-/- born to VDR-/- dams) mice. In VDR-/- mice, crown morphogenesis was affected exclusively in second-generation pups. In first-generation adult VDR-/- mice, both enamel and dentin were affected, and pathologic features of root resorption in both apical and cervical regions were observed. Nutritional calcium and phosphate normalization completely rescued the root resorption and partially rescued the dentin and enamel phenotypes (altered cell differentiation and matrix protein expression). Analysis of these data illustrates the co-existence of different pathways of vitamin D action in tooth differentiation and biomineralization. These targeted and cumulative effects would generate the diverse and wide spectrum of dental rickets phenotypes.

Immunohistochemical profile of odontogenic epithelium in developing dog teeth (Canis familiaris)

Tumors of the jaw bones and oral soft tissue are relatively common lesions in dogs. The aim of this study was to find cell markers to differentiate odontogenic epithelium from nonodontogenic epithelium for future research on the pathogenesis and pathology of odontogenic neoplasms in dogs. Keratin 14 and 19 staining was observed in odontogenic and nonodontogenic epithelium, whereas amelogenin and p75 neurotrophin receptor immunoreactivity was observed in certain odontogenic epithelial cells at various stages of development but not in other epithelial cells. Calretinin staining was observed in the alveolar epithelial cells directly overlying the developing tooth germ in 28 of 39 sections (71.8%), as well as the dental laminae in 30 of 35 sections (85.7%) and Serres rests in 24 of 28 sections (85.7%). Focal positivity was detected in the respiratory mucosa, some hair follicles, and fusion epithelium of the palate, but no calretinin staining was observed in other oral epithelial cells; therefore, calretinin has potential to be utilized as a marker to differentiate odontogenic form nonodontogenic epithelium.

Calcium-mediated differentiation of ameloblast lineage cells in vitro

Calcium is a key component of the mineralized enamel matrix, but may also have a role in ameloblast cell differentiation. In this study we used human ameloblast lineage cells to determine the effect of calcium on cell function. Primary human ameloblast lineage cells were isolated from human fetal tooth buds. Cells were treated with calcium ranging from 0.05 to -1.8 mM. Cell morphology was imaged by phase contrast microscopy, and amelogenin was immunolocalized. Proliferation of cells treated with calcium was measured by BrdU immunoassay. The effect of calcium on mRNA expression of amelogenin, Type 1 collagen, DSPP, amelotin, and KLK-4 was compared by PCR analysis. Von Kossa staining was used to detect mineral formation after cells were pretreated with calcium. Calcium induced cell organization and clustering at 0.1 and 0.3 mM concentrations. Increasing concentrations of calcium significantly reduced ameloblast lineage cell proliferation. The addition of 0.1 mM calcium to the cultures upregulated expression of amelogenin, Type I collagen, and amelotin. After pretreatment with 0.3 mM calcium, the cells could form a mineralized matrix. These studies, which utilized human ameloblast lineage cells grown in vitro, showed that the addition of calcium at 0.1 and 0.3 mM, induced cell differentiation and upregulation of amelogenin Type I collagen and amelotin.

Physiological implications of DLX homeoproteins in enamel formation

Tooth development is a complex process including successive stages of initiation, morphogenesis, and histogenesis. The role of the Dlx family of homeobox genes during the early stages of tooth development has been widely analyzed, while little data has been reported on their role in dental histogenesis. The expression pattern of Dlx2 has been described in the mouse incisor; an inverse linear relationship exists between the level of Dlx2 expression and enamel thickness, suggesting a role for Dlx2 in regulation of ameloblast differentiation and activity. In vitro data have revealed that DLX homeoproteins are able to regulate the expression of matrix proteins such as osteocalcin. The aim of the present study was to analyze the expression and function of Dlx genes during amelogenesis. Analysis of Dlx2/LacZ transgenic reporter mice, Dlx2 and Dlx1/Dlx2 null mutant mice, identified spatial variations in Dlx2 expression within molar tooth germs and suggests a role for Dlx2 in the organization of preameloblastic cells as a palisade in the labial region of molars. Later, during the secretory and maturation stages of amelogenesis, the expression pattern in molars was found to be similar to that described in incisors. The expression patterns of the other Dlx genes were examined in incisors and compared to Dlx2. Within the ameloblasts Dlx3 and Dlx6 are expressed constantly throughout presecretory, secretory, and maturation stages; during the secretory phase when Dlx2 is transitorily switched off, Dlx1 expression is upregulated. These data suggest a role for DLX homeoproteins in the morphological control of enamel. Sequence analysis of the amelogenin gene promoter revealed five potential responsive elements for DLX proteins that are shown to be functional for DLX2. Regulation of amelogenin in ameloblasts may be one method by which DLX homeoproteins may control enamel formation. To conclude, this study establishes supplementary functions of Dlx family members during tooth development: the participation in establishment of dental epithelial functional organization and the control of enamel morphogenesis via regulation of amelogenin 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.

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.

Vitamin D receptor deficiency affects dentin maturation in mice

Mutation of vitamin D receptors (vdr) results in resistance to the vitamin's normal effects which may compromise dentin formation. The objective of this study was to investigate the effect of vdr deficiency on post-natal dentin maturation in mice. The dentin in mandibular incisors of 70.5-day-old vdr wild-type and vdr knockout mice was compared at different levels along the long axis. Expression of biglycan and decorin was detected by immunolocalisation. Scanning electron microscopy was used to observe the ultrastructure of the dentin, and micro-computerised tomography was used to determine the degree of dentin mineralisation density. In the vdr knockout mice, the pulp chamber was larger and the dentin wall was thinner compared with the wild-type mice. In addition, the pre-dentin layer was thickened with an irregular front line and diffuse expression of biglycan and decorin. Fewer tubules, lower mineralisation density and pore-like defects were observed in the dentin at the eruptive region and level with the first molar. In conclusion, vdr deficiency compromises dentin maturation.

Vitamin D and tissue non-specific alkaline phosphatase in dental cells

Dental epithelium comprises different cell populations, including ameloblasts and stratum intermedium cells. Ameloblasts are vitamin D targets, and at least five proteins undergo specific modulation of their expression following the addition of 1alpha,25(OH)2 vitamin D3[1alpha,25(OH)2D3]. Stratum intermedium cells have not been studied in any great detail regarding vitamin D impact. Interestingly, in these cells, the tissue non-specific alkaline phosphatase (TNAP) is overexpressed. On the other hand, TNAP is a reliable bone marker of vitamin D action, similar to calbindins in kidney and intestine, previously used for studies of vitamin D activity in ameloblasts. Here, TNAP expression and activity were investigated in vivo in the microdissected epithelium and mesenchyme of mandible incisors. Physiological doses of 1alpha,25(OH)2D3 injected in control rats failed to modify TNAP activity in both dental epithelium and mesenchyme. No significant differences were observed in the steady-state levels of TNAP mRNAs of dental tissues from wild-type and vitamin D nuclear receptor (VDRnuc)-deficient mice of the same litters. These data suggest that, in contrast to ameloblasts, stratum intermedium cells are not sensitive to 1alpha,25(OH)2D3. An explanation for such a responsiveness of stratum intermedium cells to 1alpha,25(OH)2D3 is proposed based on the respective expressions of both vitamin D receptors (VDRnuc and 1,25D3-[MARRS]) and the Dlx2 homeobox gene.

Identification of differentially expressed cDNA transcripts from a rat odontoblast cell line

Odontoblasts and osteoblasts are two among the myriads of cell types present in the craniofacial complex. Both have a common ectomesenchymal origin and secrete macromolecules that are necessary for the formation of dentin and alveolar bone via matrix-mediated mechanisms. The mineralized matrices of bone and dentin differ in morphology and function but several mineral associated proteins, formerly thought to be tissue specific, have been found to be common in both tissues. To decipher the complex molecular mechanisms involved in mineralized dentin formation, the suppressive subtraction hybridization (SSH) approach has been used to identify the genes expressed by polarized odontoblasts. Employing SSH, 187 cDNA clones were identified from the subtracted cDNA library. Many of these genes have not been previously reported to be expressed by terminally differentiated odontoblasts. Genes were classified into seven groups based on the predicted function of the encoded proteins: extracellular matrix; cytoskeletal components, molecules involved in adhesion and cell-cell interaction; metabolic enzymes, transporters, ion channels; protein processing, protein transport and protein folding molecules; nuclear proteins (transcription factors, DNA processing enzymes); signaling molecules and genes of yet unknown function. Northern blot and in situ hybridization analysis performed for five putative novel genes and one new isoform of amelogenin revealed differential expression levels in the osteoblasts, ameloblasts and the odontoblasts of the developing rat molars. Some of the known genes isolated from this enriched pool were the cleavage products of dentin sialophosphoprotein (DSPP) namely, phosphophoryn (PP) and dentin sialoprotein (DSP). Interestingly amelogenin, ameloblastin and enamelin were also expressed in the odontoblasts during dentin formation.

Modulation of 1alpha,25-dihydroxyvitamin D3-membrane associated, rapid response steroid binding protein expression in mouse odontoblasts by 1alpha,25-(OH)2D3

The rapid, nongenomic effects of 1alpha,25-dihydroxyvitamin D3 (1alpha,25-(OH)2D3 have been related to a 1,25D3-membrane associated, rapid response steroid binding protein or 1,25D3-[MARRS]bp, with a molecular weight of 65 kDa, in several tissues and species. Currently, no information is available concerning the nongenomic responses to 1alpha,25-(OH)2D3 in dental tissues. In order to investigate the expression of 1,25D3-[MARRS]bp in dental cells, in the presence or absence of 1alpha,25-(OH)2D3, we have used rabbit polyclonal antibodies directed against the N-terminus of the 1,25D3-[MARRS]bp (Ab099) that recognizes the 1alpha,25-(OH)2D3 binding protein in chick intestinal basolateral membranes and a mouse odontoblast-like cell line (MO6-G3). Western blotting and flow cytometric analyses with Ab099 specifically detected 1,25D3-[MARRS]bp in MO6-G3 cells. Moreover, 1,25D3-[MARRS]bp was up-regulated, in vivo, in differentiated dental cells. Electron microscopic analysis confirmed the plasma membrane localization of this binding protein and also showed its intracellular presence. Incubation of MO6-G3 cells with different doses of 1alpha,25-(OH)2D3 for 36 h resulted in an inhibition of 1,25D3-[MARRS]bp expression with a maximal effect at 50 nM steroid. In addition, the culture media of MO6-G3 cells contains immunoreactive 1,25D3-[MARRS]bp. Immunogold positive membrane vesicle-like structures are present in the extracellular matrix of MO6-G3 cells. Altogether, these results indicate that the 1,25D3-[MARRS]bp expression in MO6-G3 cells is modulated by 1alpha,25-(OH)2D3. In conclusion, this 1alpha,25-(OH)2D3 binding protein could play an important role in the rapid, nongenomic responses to 1alpha,25-(OH)2D3 in dental cells.

Bipartite pattern discovery by entropy minimization-based multiple local alignment

Many multimeric transcription factors recognize DNA sequence patterns by cooperatively binding to bipartite elements composed of half sites separated by a flexible spacer. We developed a novel bipartite algorithm, bipartite pattern discovery (Bipad), which produces a mathematical model based on information maximization or Shannon's entropy minimization principle, for discovery of bipartite sequence patterns. Bipad is a C++ program that applies greedy methods to search the bipartite alignment space and examines the upstream or downstream regions of co-regulated genes, looking for cis-regulatory bipartite patterns. An input sequence file with zero or one site per locus is required, and the left and right motif widths and a range of possible gap lengths must be specified. Bipad can run in either single-block or bipartite pattern search modes, and it is capable of comprehensively searching all four orientations of half-site patterns. Simulation studies showed that the accuracy of this motif discovery algorithm depends on sample size and motif conservation level, but results were independent of background composition. Bipad performed equivalent with or better than other pattern search algorithms in correctly identifying Escherichia coli cyclic AMP receptor protein and Bacillus subtilis sigma factor binding site sequences based on experimentally defined benchmarks. Finally, a new bipartite information weight matrix for vitamin D3 receptor/retinoid X receptor alpha (VDR/RXRalpha) binding sites was derived that comprehensively models the natural variability inherent in these sequence elements.

Differential epithelial and mesenchymal regulation of tooth-specific matrix proteins expression by 1,25-dihydroxyvitamin D3 in vivo

Enamel defects have been reported in rickets and related to disturbed expression of amelogenin in ameloblasts. The present study is devoted to amelogenin, enamelin, ameloblastin, and dentin sialophosphoprotein (DSPP) expression in both the epithelium and mesenchyme of vitamin D-deficient rat incisors. Quantitative Northern blotting analysis (relatively to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA steady-state levels) was performed on microdissected cells of rachitic (-D) and control (+D) 56 day old rats. Steady-state levels of amelogenin and enamelin mRNA were significantly reduced in the -D epithelium versus the +D epithelium ones. In contrast, ameloblastin expression was slightly increased in -D epithelium. In the same samples, DSPP mRNA levels remained unchanged in -D dental mesenchyme. Comparative electron microscopy studies between +D and -D animals showed a dramatic decrease of intraprismatic enamel (amelogenin and enamelin immunoreactive) consistent with our molecular results. In conclusion, tooth formation results from the coordinated expression of several matrix proteins that may be controlled by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3].

Expression of amelogenin in odontoblasts

Amelogenin is the major enamel protein produced by ameloblasts. Its expression has been shown to be down-regulated in ameloblasts of vitamin-D-deficient (-D) rats. The potential expression and localization of amelogenin in odontoblasts and its regulation by vitamin D were investigated in this study. RT-PCR and semi-quantitative Northern blot analyses were performed using the odontoblast cell line MO6-G3 and microdissected dental pulp mesenchyme. Both in vitro and in vivo odontoblasts expressed various alternatively spliced amelogenin transcripts. In situ hybridization studies showed that amelogenin expression was restricted to young odontoblasts during mantle dentin deposition. Electron microscopy studies localized the amelogenin protein in the odontoblast cell process cytoplasm and mantle dentin. Amelogenin immunolabeling was stronger in -D rats, suggesting an inverse regulation by vitamin D in odontoblasts. Furthermore, amelogenin mRNA steady-state levels were significantly increased in -D dental pulp mesenchyme. In addition, a temporal-spatial lengthening of the mantle dentin stage was observed in -D animals, suggesting that developmental perturbations occur in relation to the vitamin D status and/or amelogenin expression. These data show that amelogenin is expressed by odontoblasts selectively during mantle dentin deposition. This developmental regulated expression pattern is enhanced under vitamin-D-deficiency status and in a broader context may play an important role during ameloblast and odontoblast differentiation and function.

Expression of a 1,25-dihydroxyvitamin D3 membrane-associated rapid-response steroid binding protein during human tooth and bone development and biomineralization

The calciotropic hormone 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] has been established to control skeletal tissue formation and biomineralization via the regulation of gene expression. This action involves the well-characterized nuclear 1,25(OH)2D3 receptor. However, it has been recognized that several cellular responses to 1,25(OH)2D3 may not to be related to the exclusive nuclear receptor. Indeed, this secosteroid is able to generate rapid responses that have been proposed to be mediated by interactions of the ligand, which is a putative cell membrane-associated rapid-response steroid (MARRS) binding protein for 1,25(OH)2D3 [1,25D3-MARRS]. The nongenomic pathway of 1,25(OH)2D3 was studied here in detail by immunolocalization of the 1,25D3-MARRS during the specific context of human prenatal development. Western blotting with proteins extracted from 4 week- to 27-week-old embryos was performed, evidencing a 65-kDa molecular species recognized by antibody Ab 099 generated against synthetic peptides corresponding to the N terminus of the 1,25D3-MARRS from chick intestinal basolateral membranes. Based on this biochemical conservation of protein in the human species, the temporospatial expression patterns were established in the craniofacial skeleton at the same ages. Comparative analysis was performed in teeth and bones from early morphogenesis to terminal cell differentiation and extracellular biomineralization. The data show the potential implication of 1,25D3-MARRS in the heterogeneous cell population including ameloblasts, odontoblasts, osteoblasts, and osteoclasts. The epithelial-mesenchymal cascade related to odontogenesis was coincident with a sequence of up- and down-regulation of immunoreactive 1,25D3-MARRS. Biomineralization was associated with a striking up-regulation in the adjoining secretory cells in all tissues. Finally, osteoclasts appeared also to express the 1,25D3-MARRS during these early phases of bone modeling. Previously obtained data of the nuclear vitamin D receptor (VDR) expression and this study on 1,25D3-MARRS suggest the existence of cross-talk between the genomic and nongenomic pathways during human development.

Identification of the calcium-sensing receptor in the developing tooth organ

Calcium (Ca2+) is a critical component of tooth enamel, dentin, and the surrounding extracellular matrix. Ca2+ also may regulate tooth formation, although the mechanisms for such action are poorly understood. The Ca2+-sensing receptor (CaR) that is expressed in the parathyroid gland, kidney, bone, and cartilage has provided a mechanism by which extracellular Ca2+ can regulate cell function. Because these tissues play an important role in maintaining mineral homeostasis and because Ca2+ is hypothesized to play a crucial role in tooth formation, we determined whether the CaR was present in teeth. In this study, using immunohistochemistry, CaR protein was detected in developing porcine molars localized in the predentin (pD), early secretory-stage ameloblasts, maturation-stage smooth-ended ameloblasts (SA), and certain cells in the stratum intermedium. CaR protein and messenger RNA (mRNA) were detected also in an immortalized ameloblast-like cell line (PABSo-E) using immunofluorescence, reverse-transcription polymerase chain reaction (RT-PCR), and Northern analysis. Based on the observation that the CaR is expressed in cultured ameloblasts, we determined whether increments in medium Ca2+ concentration could activate the intracellular Ca2+ signal transduction pathway. In PABSo-E cells, increasing extracellular Ca2+ in the medium from 0 (baseline) to 2.5mM or 5.0 mM resulted in an increase in intracellular Ca2+ above baseline to 534 +/- 69 nM and 838 +/- 86 nM, respectively. Taken together, these results suggest that the CaR is expressed in developing teeth and may provide a mechanism by which these cells can respond to alterations in extracellular Ca2+ to regulate cell function and, ultimately, tooth formation.

Vitamin D and genomic stability

1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] has been shown to act on novel target tissues not related to calcium homeostasis. There have been reports characterizing 1,25(OH)(2)D(3) receptors and activities in diverse tissues such as brain, pancreas, pituitary, skin, muscle, placenta, immune cells and parathyroid. The receptor hormone complex becomes localized in the nucleus, and undergoes phosphorylation by reacting with a kinase. This form of the receptor then interacts with the Vitamin D responsive element of target gene and modifies the transcription of those genes to develop the action. The modulation of gene transcription results in either the induction or repression of specific messenger RNAs (m-RNAs), ultimately resulting in changes in protein expression needed to produce biological responses. Genes for carbonic anhydrase that are expressed at high levels in osteoclast are known to be involved in bone resorption and Id genes role in osteoblast-osteoclast differentiation reflects the genomic effect of Vitamin D on bones. Genomic action of Vitamin D also explains the biosynthesis of oncogenes, polyamines, lymphokines and calcium binding proteins. However, there is a possibility that some of the actions of 1,25(OH)(2)D(3) may be mediated by non-genomic mechanisms and may not require the binding to Vitamin D receptor (VDR). Vitamin D offers a protection from genotoxic effects of Vitamin D deficiency by increasing the insulin receptor gene expression and BSP (bone sialoprotein), bone-remodeling by decreasing the osteopontin (OPN) m-RNAs, maintaining the normal epidermal structure and enamel matrix. Gonadal insufficiency in Vitamin D deficiency was corrected by vitamin mediated direct regulation of the expression of aramotase gene. The supportive role of Vitamin D in placental function is also evident by its influence on human placental lactogen (hpl) gene transcription accompanied by increase hpl m-RNA levels. Further role of Vitamin D is envisaged in identifying cyclin C as an important target for Vitamin D in cell-cycle regulation. Vitamin D at physiological concentration has been found to protect cell proteins and membranes against oxidative stress by inhibiting the peroxidative attack on membrane lipids. Vitamin D, at a concentration range of 2x10(-8)-5x10(-8)M, induces apoptosis in most cancer cells, stabilizes chromosomal structure and prevents DNA double-strand breaks induced either by endogenous or exogenous factors. Vitamin D is also effective in stimulating DNA synthesis in adult alveolar II cells and provides a novel mechanism of modulation of epithelial cell proliferation in the context of lung development and repair against injury. The regulation of various proto-oncogenes (c-myc, c-fos, c-jun), differentiation inducing properties, antiproliferative effects on keratinocytes and inhibitory effects in several human malignancy ranks Vitamin D as a novel hormone that may have physiological and clinical implication in the carcinogenic process.