Establishing protein expression profiles involved in tooth development using a proteomic approach

Various growth and transcription factors are involved in tooth development and developmental abnormalities; however, the protein dynamics do not always match the mRNA expression level. Using a proteomic approach, this study comprehensively analyzed protein expression in epithelial and mesenchymal tissues of the tooth germ during development. First molar tooth germs from embryonic day 14 and 16 Crlj:CD1 (ICR) mouse embryos were collected and separated into epithelial and mesenchymal tissues by laser microdissection. Mass spectrometry of the resulting proteins was carried out, and three types of highly expressed proteins [ATP synthase subunit beta (ATP5B), receptor of activated protein C kinase 1 (RACK1), and calreticulin (CALR)] were selected for immunohistochemical analysis. The expression profiles of these proteins were subsequently evaluated during all stages of amelogenesis using the continuously growing incisors of 3-week-old male ICR mice. Interestingly, these three proteins were specifically expressed depending on the stage of amelogenesis. RACK1 was highly expressed in dental epithelial and mesenchymal tissues during the proliferation and differentiation stages of odontogenesis, except for the pigmentation stage, whereas ATP5B and CALR immunoreactivity was weak in the enamel organ during the early stages, but became intense during the maturation and pigmentation stages, although the timing of the increased protein expression was different between the two. Overall, RACK1 plays an important role in maintaining the cell proliferation and differentiation in the apical end of incisors. In contrast, ATP5B and CALR are involved in the transport of minerals and the removal of organic materials as well as matrix deposition for CALR.

Altered miRNA expression profiling in enamel organ of fluoride affected rat embryos

Evidence has shown that miRNAs could play a role in dental fluorosis, but there is no study has investigated the global expression miRNA profiles of fluoride-exposed enamel organ. In this study, we analysed the differentially expressed (DE) miRNAs between fluoride-treated and control enamel organ for the first time and found several candidate miRNAs and signaling pathways worthy of further research. Thirty Wistar rats were randomly distributed into three groups and exposed to drinking water with different fluoride contents for 10 weeks and during the gestation. The three groups were a control group (distilled water), medium fluoride group (75 mg/L NaF), and high fluoride group (150 mg/L NaF). On the embryonic day 19.5, the mandible was dissected for histological analysis, and the enamel organ of the mandibular first molar tooth germ was collected for miRNA sequencing (miRNA-seq) and quantitative real-time PCR analysis (qRT-PCR). Typical dental fluorosis was observed in the incisors of the prepregnant rats. In addition to the disorganized structure of enamel organ cells, 39 DE miRNAs were identified in the fluoride groups compared with the control group, and good agreement between the miRNA-seq data and qRT-PCR data was found. The functional annotation of the target genes of 39 DE miRNAs showed significant enrichment in metabolic process, cell differentiation, calcium signaling pathway, and mitogen-activated protein kinase(MAPK) signaling pathway terms. This study provides a theoretical reference for an extensive understanding of the mechanism of fluorosis and potential valuable miRNAs as therapeutic targets in fluorosis.

Fluoride exposure alters Ca2+ signaling and mitochondrial function in enamel cells

Fluoride ions are highly reactive, and their incorporation in forming dental enamel at low concentrations promotes mineralization. In contrast, excessive fluoride intake causes dental fluorosis, visually recognizable enamel defects that can increase the risk of caries. To investigate the molecular bases of dental fluorosis, we analyzed the effects of fluoride exposure in enamel cells to assess its impact on Ca2+ signaling. Primary enamel cells and an enamel cell line (LS8) exposed to fluoride showed decreased internal Ca2+ stores and store-operated Ca2+ entry (SOCE). RNA-sequencing analysis revealed changes in gene expression suggestive of endoplasmic reticulum (ER) stress in fluoride-treated LS8 cells. Fluoride exposure did not alter Ca2+ homeostasis or increase the expression of ER stress-associated genes in HEK-293 cells. In enamel cells, fluoride exposure affected the functioning of the ER-localized Ca2+ channel IP3R and the activity of the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) pump during Ca2+ refilling of the ER. Fluoride negatively affected mitochondrial respiration, elicited mitochondrial membrane depolarization, and disrupted mitochondrial morphology. Together, these data provide a potential mechanism underlying dental fluorosis.

Porcine Amelogenin is Expressed from the X and Y Chromosomes

Amelogenin is the major enamel matrix component in developing teeth. In eutherian mammals, amelogenin is expressed from the X chromosome only, or from both the X and Y chromosomes. Two classes of porcine amelogenin cDNA clones have been characterized, but the chromosomal localization of the gene(s) encoding them is unknown. To determine if there are sex-based differences in the expression of porcine amelogenin, we paired PCR primers for exons 1a, 1b, 7a, and 7b, and amplified enamel organ-derived cDNA separately from porcine males and females. The results show that exons 1a/2a and 7a are always together and can be amplified from both males (XY) and females (XX). Exons 1b/2b and 7b are also always paired, but can be amplified only from females. We conclude that porcine amelogenin is expressed from separate genes on the X and Y chromosomes, and not, as previously proposed, from a single gene with two promoters.

Perlecan, a Basement Membrane-type Heparan Sulfate Proteoglycan, in the Enamel Organ: Its Intraepithelial Localization in the Stellate Reticulum

The localization and biosynthesis of perlecan, a basement membrane-type heparan sulfate proteoglycan, were studied in developing tooth germs by using murine molars in neonatal and postnatal stages and primary cultured cells of the enamel organ and dental papilla to demonstrate the role of perlecan in normal odontogenesis. Perlecan was immunolocalized mainly in the intercellular spaces of the enamel organ as well as in the dental papilla/pulp or in the dental follicle. By in situ hybridization, mRNA signals for perlecan core protein were intensely demonstrated in the cytoplasm of stellate reticulum cells and in dental papilla/pulp cells, including odontoblasts and fibroblastic cells in the dental follicle. Furthermore, the in vitro biosyntheses of perlecan core protein by the enamel organ and dental papilla/pulp cells were confirmed by immunofluorescence, immunoprecipitation, and reverse transcriptase-polymerase chain reaction. The results indicate that perlecan is synthesized by the dental epithelial cells and is accumulated in their intercellular spaces to form the characteristic stellate reticulum, whose function is still unknown.

TLR signalling can modify the mineralization of tooth germ

OBJECTIVE

The aim of this work is to investigate the possible role of Toll-like receptor 4 (TLR4) during the development of mouse tooth germ. TLR4 is well known to inhibit mineralization and cause inflammation in mature odontoblasts and dental pulp cells. However, unlike these pathological functions of TLR4, little is known about the developmental function(s) of TLR4 during tooth development.

MATERIALS AND METHODS

TLR4 expression was studied via Western blot in developing lower mouse incisors from E13.5 to E18.5. To generate functional data about the effects of TLR4, a specific agonist (LPS) was applied to the medium of in vitro tooth germ cultures, followed by Western blot, histochemical staining, ELISA assay, in situ hybridization and RT-qPCR.

RESULTS

Increased accumulation of biotin-labelled LPS was detected in the enamel organ and in preodontoblasts. LPS treatment induced degradation of the inhibitor molecule (IκB) of the NF-κB signalling pathway. However, no morphological alterations were detected in cultured tissue after LPS addition at the applied dosage. Activation of TLR4 inhibited the mineralization of enamel and dentin, as demonstrated by alizarin red staining and as decreased levels of collagen type X. mRNA expression of ameloblastin was elevated after LPS administration.

CONCLUSION

These results demonstrate that TLR4 may decrease the mineralization of hard tissues of the tooth germ and may trigger the maturation of ameloblasts; it can give valuable information to understand better congenital tooth abnormalities.

E-cadherin can replace N-cadherin during secretory-stage enamel development

BACKGROUND

N-cadherin is a cell-cell adhesion molecule and deletion of N-cadherin in mice is embryonic lethal. During the secretory stage of enamel development, E-cadherin is down-regulated and N-cadherin is specifically up-regulated in ameloblasts when groups of ameloblasts slide by one another to form the rodent decussating enamel rod pattern. Since N-cadherin promotes cell migration, we asked if N-cadherin is essential for ameloblast cell movement during enamel development.

METHODOLOGY/PRINCIPAL FINDINGS

The enamel organ, including its ameloblasts, is an epithelial tissue and for this study a mouse strain with N-cadherin ablated from epithelium was generated. Enamel from wild-type (WT) and N-cadherin conditional knockout (cKO) mice was analyzed. μCT and scanning electron microscopy showed that thickness, surface structure, and prism pattern of the cKO enamel looked identical to WT. No significant difference in hardness was observed between WT and cKO enamel. Interestingly, immunohistochemistry revealed the WT and N-cadherin cKO secretory stage ameloblasts expressed approximately equal amounts of total cadherins. Strikingly, E-cadherin was not normally down-regulated during the secretory stage in the cKO mice suggesting that E-cadherin can compensate for the loss of N-cadherin. Previously it was demonstrated that bone morphogenetic protein-2 (BMP2) induces E- and N-cadherin expression in human calvaria osteoblasts and we show that the N-cadherin cKO enamel organ expressed significantly more BMP2 and significantly less of the BMP antagonist Noggin than did WT enamel organ.

CONCLUSIONS/SIGNIFICANCE

The E- to N-cadherin switch at the secretory stage is not essential for enamel development or for forming the decussating enamel rod pattern. E-cadherin can substitute for N-cadherin during these developmental processes. Bmp2 expression may compensate for the loss of N-cadherin by inducing or maintaining E-cadherin expression when E-cadherin is normally down-regulated. Notably, this is the first demonstration of a natural endogenous increase in E-cadherin expression due to N-cadherin ablation in a healthy developing tissue.

Ameloblastin in Hertwig's epithelial root sheath regulates tooth root formation and development

Tooth root formation begins after the completion of crown morphogenesis. At the end edge of the tooth crown, inner and outer enamel epithelia form Hertwig’s epithelial root sheath (HERS). HERS extends along with dental follicular tissue for root formation. Ameloblastin (AMBN) is an enamel matrix protein secreted by ameloblasts and HERS derived cells. A number of enamel proteins are eliminated in root formation, except for AMBN. AMBN may be related to tooth root formation; however, its role in this process remains unclear. In this study, we found AMBN in the basal portion of HERS of lower first molar in mice, but not at the tip. We designed and synthesized small interfering RNA (siRNA) targeting AMBN based on the mouse sequence. When AMBN siRNA was injected into a prospective mandibular first molar of postnatal day 10 mice, the root became shorter 10 days later. Furthermore, HERS in these mice revealed a multilayered appearance and 5-bromo-2′-deoxyuridine (BrdU) positive cells increased in the outer layers. In vitro experiments, when cells were compared with and without transiently expressing AMBN mRNA, expression of growth suppressor genes such as p21^Cip1 and p27^Kip1 was enhanced without AMBN and BrdU incorporation increased. Thus, AMBN may regulate differentiation state of HERS derived cells. Moreover, our results suggest that the expression of AMBN in HERS functions as a trigger for normal root formation.

Morphogenetic roles of perlecan in the tooth enamel organ: an analysis of overexpression using transgenic mice

Perlecan, a heparan sulfate proteoglycan, is enriched in the intercellular space of the enamel organ. To understand the role of perlecan in tooth morphogenesis, we used a keratin 5 promoter to generate transgenic (Tg) mice that over-express perlecan in epithelial cells, and examined their tooth germs at tissue and cellular levels. Immunohistochemistry showed that perlecan was more strongly expressed in the enamel organ cells of Tg mice than in wild-type mice. Histopathology showed wider intercellular spaces in the stellate reticulum of the Tg molars and loss of cellular polarity in the enamel organ, especially in its cervical region. Hertwig's epithelial root sheath (HERS) cells in Tg mice were irregularly aligned due to excessive deposits of perlecan along the inner, as well as on the outer sides of the HERS. Tg molars had dull-ended crowns and outward-curved tooth roots and their enamel was poorly crystallized, resulting in pronounced attrition of molar cusp areas. In Tg mice, expression of integrin β1 mRNA was remarkably higher at E18, while expression of bFGF, TGF-β1, DSPP and Shh was more elevated at P1. The overexpression of perlecan in the enamel organ resulted in irregular morphology of teeth, suggesting that the expression of perlecan regulates growth factor signaling in a stage-dependent manner during each step of the interaction between ameloblast-lineage cells and mesenchymal cells.

Immunolocalization of hepatocyte growth factor receptor, c-Met, in human fetal tooth germ

To evaluate c-Met expression in human tooth germ development. An immunohistochemical study on c-Met expression in tooth germs of 8 human fetus between the 7th and 9th week. In the Bud stage C-Met immunopositivity was, moderately to strongly, detected both in the inner and the outer epithelium of the enamel organ. In particular, moderate staining was detected in a specific portion of tooth germs that corresponds to apical portion of the enamel organ. In the bell stage tooth germs were characterized by much stronger c-Met immunopositivity in cytoplasm, inner enamel epithelium, bilateral cusps and above all in the plasma apical membrane on the mesenchymal side. In conclusion because enamel organ cells can interact with mesenchymal cells directly, and c-Met is expressed in the stages at which mesenchymal induction is guided by the dental epithelium, it is conceivable that c-Met is related to tooth germ morphogenesis and cell differentiation.

Characterization of Apin, a secreted protein highly expressed in tooth-associated epithelia

We previously reported expression of a protein by enamel organ (EO) cells in rat incisors, originally isolated from the amyloid of Pindborg odontogenic tumors called Apin. The aim of the present study was to further characterize the Apin gene and its protein in various species, assess tissue specificity, and clarify its localization within the EO. Northern blotting and RT-PCR revealed that expression of Apin was highest in the EO and gingiva, moderate in nasal and salivary glands, and lowest in the epididymis. The protein sequences deduced from the cloned cDNA for rat, mouse, pig, and human were aligned together with those obtained from four other mammal genomes. Apin is highly conserved in mammals but is absent in fish, birds, and amphibians. Comparative SDS-PAGE analyses of the protein obtained from bacteria, transfected cells, and extracted from EOs all indicated that Apin is post-translationally modified, a finding consistent with the presence of predicted sites for phosphorylation and O-linked glycosylation. In rodent incisors, Apin was detected only in the ameloblast layer of the EO, starting at post-secretory transition and extending throughout the maturation stage. Intense labeling was visible over the Golgi region as well as on the apices of ameloblasts abutting the enamel matrix. Apin was also immunodetected in epithelial cells of the gingiva which bind it to the tooth surface (junctional epithelium). The presence of Apin at cell-tooth interfaces suggests involvement in adhesive mechanisms active at these sites, but its presence among other epithelial tissues indicates Apin likely possesses broader physiological roles.

[Hertwig's epithelial root sheath, bone matrix proteins, and initiation of cementogenesis in mouse teeth]

PURPOSE

The aim of this study was to analyze the association between Hertwig's epithelial root sheath (HERS), noncollagenous matrix proteins, and cementogenesis.

METHODS

Eighteen BALB/c postnatal mice were divided into five groups according to the developing stages. The mouse teeth were examined at the onset of root development by light and transmission electron microscopy. PV two-step immunohistochemical method was used to detect the expression of proliferating cell nuclear antigen(PCNA), OPN and BSP during the root formation.

RESULTS

Hertwig's epithelial root sheath, which derived from the apical extensions of inner and outer enamel epithelium, was formed at 5-day mouse teeth and expressed PCNA positively. During the fuse of inner and outer enamel epithelium, some cells, which displayed the cytologic features of protein synthesis and secretion, could be found between the two epithelium cells. Desmosomal cell junctions were observed. The positive expressions of OPN were observed at the beginning of HERS. An accumulation of OPN could also be observed at the dentin-cementum junction during cementogenesis, but no positive BSP signals were seen at the onset of HERS.

CONCLUSION

At onset of HERS, the inner enamel epithelium, outer enamel epithelium embrace several cells with developed cytoplasmic organelles, which may derive from stratum intermedium or stellate reticulum, and form a structure like enamel organ, which may be associated with initial cementogenesis. OPN, the non collagenous matrix proteins of cementum, may also take part in the formation of the dentin-cementum junction.

Evidence by signal peptide trap technology for the expression of carbonic anhydrase 6 in rat incisor enamel organs

During screening of a rat incisor enamel organ cDNA library by signal peptide trap technology, we identified a DNA fragment matching a predicted translation sequence for rat carbonic anhydrase 6 (CA6). This result was unexpected because CA6, to date, has been associated primarily with secretions from glandular tissues. To further characterize this observation, reverse transcription-polymerase chain reaction (RT-PCR) amplifications were carried out on total RNA extracted from freeze-dried secretory and maturation-stage rat incisor enamel organs. A cDNA fragment of the expected size was detected in control samples from rat salivary glands as well as within maturation-stage enamel organ samples. This CA6 RT-PCR fragment was further cloned and sequenced and found to match the nucleotide sequence 770-1079 from clone XM_216584 of GenBank. Northern blot analyses with the rat CA6 cDNA fragment confirmed its expression relative to maturation-stage enamel organ samples. It is at present unclear whether the CA6 expressed by enamel organ cells is secreted into the enamel layer or into the intercellular spaces of the enamel organ itself to assist in neutralizing excess protons arising from the growth of apatite crystals during the maturation stage of amelogenesis.

Expression of p63 transcription factor in ectoderm-derived oral tissues

The p63 gene encodes six splice variants expressed with transactivating or dominant-negative activities. Animal studies with p63 -/- mutants have suggested that p63 is important for proper development of several organs, including tooth and salivary gland. Moreover, mutations of p63 have been detected in patients affected by ectrodactyly, ectodermal dysplasia and facial clefts. To clarify the role of p63 in craniofacial development, we have studied the localization of p63 protein in human and rat oral tissues using immunohistochemistry. p63 immunostaining was identified in the enamel organ, oral epithelium and developing salivary glands. All compartments of the enamel organ were immunolabelled, whereas only basal and some suprabasal cells of the oral epithelium were stained. Ectomesenchyme-derived cells, including pulp cells, odontoblasts, bone cells and chondrocytes, were negative. The staining pattern was identical in human and rat tissues. These data lend support to the hypothesis that p63 is involved in growth and differentation of ectoderm-derived oral tissues and may be useful to clarify molecular and developmental aspects observed in animal knockout experiments and human syndromes related to p63 gene alteration.

Identification of secreted and membrane proteins in the rat incisor enamel organ using a signal-trap screening approach

The secretome represents the subset of proteins that are targeted by signal peptides to the endoplasmic reticulum. Among those, secreted proteins play a pivotal role because they regulate determinant cell activities such as differentiation and intercellular communication. In calcified tissues, they also represent key players in extracellular mineralization. This study was carried out to establish a secretome profile of rat enamel organ (EO) cells. A functional genomic technology, based on the signal trap methodology, was applied, starting with a library of 5'-enriched cDNA fragments prepared from rat incisor EOs. A total of 2,592 clones were analyzed by means of macroarray hybridizations and DNA sequencing. Ninety-four unique clones encoding a signal peptide were retrieved. Among those were 84 matched known genes, many not previously reported to be expressed by the EO. Most importantly, 10 clones were classified as being novel, with EO-009 identified as the rat homolog of human APin protein. These data indicate that many secreted and membrane-embedded EO proteins still remain to be identified, some of which may play crucial roles in regulating processes that create an optimal environment for the formation and organization of apatite crystals into a complex three-dimensional calcified matrix.

Intracellular nanosphere subunit assembly as revealed by amelogenin molecular cross-linking studies

Enamel matrix comprises nanospheres predominantly composed of amelogenin. Studies have shown that recombinant amelogenin forms nanospheres similar to those formed in vivo, but it is unclear exactly how nanospheres assemble in vivo. Are amelogenin monomers secreted into the enamel matrix where they then self-assemble to form nanospheres, or does nanosphere assembly actually occur intracellularly? The aim of this study was to attempt to answer this question. Rat enamel organs were treated with the bifunctional cross-linker, dithio bis (succinimidyl propionate) (DSP), which cross-links primary amines lying in close molecular proximity. The key to this technique is the fact that DSP cross-links are later sensitive to reductive cleavage. The cross-linked proteins were first subjected to non-reducing sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) in the first dimension and then to reducing SDS-PAGE in the second dimension (so-called diagonal electrophoresis) followed by western blot probing with anti-amelogenin. The results indicated that intracellular amelogenin monomers are in close neighbor contact, forming complexes comprising up to six individual amelogenin monomers. We suggest that these initial complexes are prefabricated intracellularly before secretion. Once secreted, these prefabricated subunits assemble further to form the mature full-size nanospheres containing hundreds of individual amelogenins characteristic of enamel matrix.

Expression of the Hedgehog antagonists Rab23 and Slimb/betaTrCP during mouse tooth development

The sonic hedgehog signalling peptide has been demonstrated to play an important role in the growth and patterning of several organs including the tooth. Inappropriate activation of Shh signalling in the embryo causes various patterning defects and complex regulation of this pathway is important during normal development. A growing list of diverse antagonists have been identified that restrict Shh signalling in the embryo, however, only Ptc1, Gas1 and Hip1 have been studied during tooth development. We have examined the expression pattern of the putative antagonists Rab23 and Slimb/betaTrCP during early murine odontogenesis and find that these molecules are expressed in the developing tooth. Interestingly, Rab23 demonstrates contrasting expression domains in the incisor and molar dentition during the cap stage, being restricted to the mesenchymal compartment of molar teeth and the epithelium of the enamel knot in incisor teeth. These findings provide the first evidence of distinct regulatory pathways for Shh in teeth of different classes.

Presence of anti-cystatin C-positive dendritic cells or macrophages and localization of cysteine proteases in the apical bud of the enamel organ in the rat incisor

Cystatin C, a cysteine protease inhibitor, was examined in the apical buds of rat incisors by immunohistochemistry, because in transition and maturation zones most of the dendritic cells in the papillary layer are anti-cystatin C-positive. Anti-cystatin C-labeled cells were sparse and localized to the proliferation and differentiation zones, constituting the apical bud of 5-week-old rat incisors. These cells were considered macrophages or dendritic cells, based on their reactivity with OX6 and ED1, as well as their ultrastructure. Basement membrane at the periphery of apical bud was also labeled by anti-cystatin C antibody. The apical buds included a few apoptotic fragments and weak reactivity with antibody to cathepsin L, a cysteine protease. Reactivity to anti-cystatin C and anti-cathepsin L antibodies was also detected in the apical bud of newborn rat incisors. These results suggest that the cystatin C-positive macrophages or dendritic cells are involved in normal incisor formation. They may be related to the clearance of apoptotic cells or protection from putative cysteine protease activity.

Apoptosis-related factors (Fas receptor, Fas ligand, FADD) in early tooth development of the field vole (Microtus agrestis)

Fas (CD95/APO-1) belongs to the TNF receptor (TNFR) family. Fas ligand binding followed by Fas-receptor oligomerisation leads to formation of a death-inducing signal complex starting with recruitment of the Fas-adapter protein (FADD). Components of this initiation complex (Fas, Fas-L, FADD) were correlated with apoptotic cells, detected by specific DNA fragmentation and morphological criteria. Apoptotic cells can be detected throughout the embryonic development of molar teeth. Restricted temporospatial distribution suggests several important roles for apoptosis in tooth morphogenesis. However, the mechanisms employed in dental apoptosis remain unclear. Frontal sections of the field vole at stage 13.5-15.5 of embryonic development were exploited to investigate and correlate location of Fas, Fas-ligand, FADD molecules and apoptosis in developing first molars by immunohistochemistry. During these stages the primary enamel knot appears and is gradually terminated by apoptosis. Initially, apoptotic cells were demonstrated in the most superficial layer of the dental lamina. The number of TUNEL-positive cells expanded from late bud to cap stages. Restricted areas of apoptotic cells were found in the stalk and primary enamel knot. Fas, Fas-L and FADD were co-localised, particularly in the primary enamel knot, and the stalk, correlating with the occurrence of apoptosis in these areas. Fas-L, however, was also found in proliferating parts of the developing tooth germ, such as in the cervical loops. Interestingly, FADD molecules were also observed in areas, where Fas protein was not detected. According to the immunohistochemical data, Fas-mediated signalling may have a triggering or enhancing role in dental apoptosis. This remains to be functionally confirmed.

Ameloblastin and amelogenin expression in posnatal developing mouse molars

Ameloblastin and amelogenin are structural proteins present in the enamel matrix of developing teeth. Here we report the results of in situ hybridization analyses with DNA probes of ameloblastin and amelogenin expression in the mandibular first molars of ICR/Jcl mice from postnatal day 1 to day 15. Ameloblastin mRNA expression was observed in ameloblasts at day 2 while amelogenin mRNA was detected in secretory ameloblasts at day 3. Significant expression of both molecules was observed at days 4, 5 and 6, after which the levels decreased. Amelogenin expression ended on day 10, while ameloblastin mRNA was only weakly detected on day 12. Neither amelogenin nor ameloblastin expression was observed in day 15 mouse molars. Amelogenin and ameloblastin mRNAs were restricted to ameloblasts. We conclude that amelogenin and ameloblastin expression is enamel-specific, and suggest that these genes might be involved in the mineralization of enamel. It is possible that ameloblastin could participate in the attachment of ameloblasts to the enamel surface. In this case, the downregulation of expression may indicate the beginning of the maturation stage in which the ameloblasts tend to detach from the enamel layer.

Relative levels of mRNA encoding enamel proteins in enamel organ epithelia and odontoblasts

Amelogenin, enamelin, sheathlin (ameloblastin/ amelin), enamelysin (MMP-20), and KLK4 (EMSP-1) are the major structural proteins and proteinases in developing tooth enamel. Recently, odontoblasts were reported to express amelogenin, the most abundant enamel protein. In this study, we hypothesized that odontoblasts express all enamel proteins and proteases, and we measured their relative mRNA levels in enamel organ epithelia and odontoblasts associated with porcine secretory- and maturation-stage enamel by RT-PCR, using a LightCycler instrument. The results showed that amelogenin mRNA in secretory-stage EOE is 320-fold higher than in odontoblasts beneath secretory-stage enamel, and over 20,000-fold higher than in odontoblasts under maturation-stage enamel. Similar results were obtained for enamelin and sheathlin. Enamelysin mRNA levels were equivalent in these two tissues, while KLK4 mRNA was higher in odontoblasts than in secretory-stage EOE. These results support the conclusion that odontoblasts are involved in the formation of the enamel layer adjacent to enamel-dentin junction.

Subcellular localization of �-catenin and cadherin expression in the cap-stage enamel organ of the mouse molar

We analyzed the subcellular distribution of beta-catenin in the cap-stage enamel organ and compared it with the expression of E- and P-cadherin by using confocal laser microscopy. The amounts of the molecules in the cytoplasm and the nucleus showed regional variations in the enamel organ, whereas cell surface-associated beta-catenin was ubiquitous. In both the enamel knot and the inner dental epithelium, beta-catenin was detected in the cytoplasm and in the nucleus. However, the amount of nuclear beta-catenin was apparently higher in the enamel knot than in the inner dental epithelium. P-cadherin also gave a stronger signal in the enamel knot than in other parts of the enamel organ. In the stellate reticulum, where E-cadherin was preferentially expressed, as well as in the cervical loop and outer dental epithelium, beta-catenin was localized in the cytoplasm but not in the nucleus. The nuclear localization of beta-catenin in the enamel knot suggests a specific activation of the canonical Wnt signaling pathway. A coincident upregulation of P-cadherin was observed in this area. Altogether, these observations suggest the possibility of a linkage between cell adhesion and Wnt signaling in the enamel knot.

Hertwig's epithelial root sheath, enamel matrix proteins, and initiation of cementogenesis in porcine teeth

OBJECTIVES

The aim of this study was to analyze the association between Hertwig's epithelial root sheath (HERS) cells, enamel matrix proteins (EMPs), and cementogenesis.

MATERIAL AND METHODS

Porcine teeth were examined at the beginning of root formation by light and transmission electron microscopy. Colloidal gold immunocytochemistry was used to analyze the protein expression of amelogenin and ameloblastin.

RESULTS

Before and during disintegration of HERS, its cells displayed the cytologic features of protein synthesis and secretion. While some cells assumed an ameloblast-like phenotype, others extended their territory away from the root surface. A collagenous matrix filled the widening intercellular spaces, and tonofilaments and desmosomes were still present in cells featuring the morphologic characteristics of cementoblasts. Labeling for amelogenin was observed but ameloblastin was not immunodetected. Labeling was associated with organic matrix deposits that were sporadically and randomly distributed both along the root surface and away from it among the dissipated epithelial cells.

CONCLUSIONS

These findings suggest that HERS' cells occasionally assume a lingering ameloblastic activity at the beginning of root formation in the pig. While the results do not support the hypothesis of a causal relationship between EMPs and cementogenesis, they lend support to the concept of an epithelial origin of cementoblasts.

Expression of heat-shock protein 25 immunoreactivity in the dental pulp and enamel organ during odontogenesis in the rat molar

The present immunocytochemical study reports on the expression of heat-shock protein (Hsp) 25 during odontogenesis in rat molars from postnatal 1 to 100 days. Hsp 25 immunoreactivity (IR) appeared in the immature dental mesenchymal cells and the differentiating and differentiated odontoblasts. At 30 days, the coronal odontoblasts retained intense Hsp25-IR, whereas the odontoblasts in the root and floor pulp were initially weak or negative but increased in IR in the later stages, indicating that the expression of Hsp 25 reflects the differentiation status of odontoblasts. During amelogenesis, the secretory ameloblasts were Hsp 25 immunopositive and the enamel free area (EFA) cells showed intense Hsp 25-IR when they developed a ruffled border. Ruffle-ended ameloblasts (RA) also consistently showed intense Hsp 25-IR, but smooth ended ameloblasts (SA) showed weak IR. These data suggest that Hsp 25 is related to the formation and maintenance of the ruffled border of RA and EFA cells.

Immunohistochemical localization of MMP-2, MMP-9, TIMP-1, and TIMP-2 in the forming rat incisor

Western blots analyses and gelatin zymography established the presence of matrix metalloproteinase (MMP)-2 and -9 in the forming zone of rat incisor. Light microscope immunohistochemistry carried out on undemineralized material provided evidence for strong MMP-2 staining in the secretory ameloblasts, odontoblasts, in the enamel organ, and in the pulp. A weaker staining was observed in predentin and in the outer part of the forming enamel. Using MMP-9 antibodies, the staining was generally weak, except for the secretory ameloblasts that were positively stained. Electron microscopic immunohistochemistry of undemineralized sections revealed a close association between gold-antibodies complexes and cytoskeletal microfilaments in the cytosol of secretory ameloblasts and odontoblasts, within the rough endoplasmic reticulum and along the plasma membrane. The striking feature of MMP-2 and -9 electron immunostaining was the particularly high labeling in the mantle dentin. By contrast, staining of tissue inhibitors of metalloproteinases (TIMP-1 and -2) was lowest in this region. We suggest that this uneven distribution may have some functional implications.

Shh signaling within the dental epithelium is necessary for cell proliferation, growth and polarization

Sonic hedgehog (Shh), a member of the mammalian Hedgehog (Hh) family, plays a key role during embryogenesis and organogenesis. Tooth development, odontogenesis, is governed by sequential and reciprocal epithelial-mesenchymal interactions. Genetic removal of Shh activity from the dental epithelium, the sole source of Shh during tooth development, alters tooth growth and cytological organization within both the dental epithelium and mesenchyme of the tooth. In this model it is not clear which aspects of the phenotype are the result of the direct action of Shh on a target tissue and which are indirect effects due to deficiencies in reciprocal signalings between the epithelial and mesenchymal components. To distinguish between these two alternatives and extend our understanding of Shh's actions in odontogenesis, we have used the Cre-loxP system to remove Smoothened (Smo) activity in the dental epithelium. Smo, a seven-pass membrane protein is essential for the transduction of all Hh signals. Hence, removal of Smo activity from the dental epithelium should block Shh signaling within dental epithelial derivatives while preserving normal mesenchymal signaling. Here we show that Shh-dependent interactions occur within the dental epithelium itself. The dental mesenchyme develops normally up until birth. In contrast, dental epithelial derivatives show altered proliferation, growth, differentiation and polarization. Our approach uncovers roles for Shh in controlling epithelial cell size, organelle development and polarization. Furthermore, we provide evidence that Shh signaling between ameloblasts and the overlying stratum intermedium may involve subcellular localization of Patched 2 and Gli1 mRNAs, both of which are targets of Shh signaling in these cells.

Expression of neural cell-adhesion molecule mRNA during mouse molar tooth development

This study employed in situ hybridisation using a probe recognising all isoforms of the molecule. Expression of the molecule in tooth germs started at embryonic day 13, when they were at the bud stage. Both inner cells of the epithelial bud and peripheral cells of the dental mesenchyme were positive. At the cap stage, positive cells were found in the inner part of the enamel organ but only in a limited area near the outer enamel epithelium. In the mesenchyme at the cap stage, expression was weak in the dental papilla and strong in the follicle. From the bell stage onward, epithelial cells in the enamel organ were negative except for the cells of the stratum intermedium, which were transiently positive at early and late bell stages. In the dental papilla, expression had mostly ceased during and after the bell stage, although transient expression was found in cuspal areas at the early bell stage. The dental follicle strongly expressed neural cell-adhesion molecule (NCAM) to the end of the experimental period, at post-natal day 4. In contrast to the first molar at its earliest stage of appearance, in which both the thickened epithelium and surrounding mesenchyme were negative for the expression of the molecule, the second molar appeared as a combination of extending epithelial thickenings and mesenchymal cells strongly positive for its expression. This study newly identifies the dental papilla and the stratum intermedium as NCAM-expressing sites.

The influx of serum albumin to enamel matrix in rat incisors after trauma

The most frequent results of trauma to tooth germs are enamel hypoplasia and enamel hypocalcification. These differing results may be due to the stage of amelogenesis at which trauma occurs. The cellular and biomolecullar events involved in the genesis of these defects are poorly understood. We hypothesized that one factor involved is the possibility that relatively high levels of serum albumin enter the enamel matrix through the damaged enamel organ, and impair mineralization of the matrix. The present study was undertaken to immunohistochemically and autoradiographically localize serum albumin in the enamel organs of rat incisors after trauma was inflicted to the mandibular incisor region of 4-day-old rats. Hemorrhage was seen surrounding the enamel organ and between the detached secretory-stage ameloblasts. One day after trauma, the most intense immunohistochemical (IHC) staining for albumin was localized in the outer layer of the enamel matrix adjacent to the detached secretory-stage ameloblasts. Albumin was also detected autoradiographically in the secretory-stage ameloblasts layer and enamel matrix. These findings indicate that serum albumin can leak between the detached ameloblasts and penetrate the enamel matrix after trauma. Leaked albumin was still present in the matrix during the maturation stage. Leaked albumin in the developing enamel could inhibit crystal growth and result in hypocalcification.

Spatiotemporal expression of NGFR during pre-natal human tooth development

OBJECTIVES

The relation between nerve growth factor receptor (NGFR) in the human pre-natal tooth buds and the dental follicle was investigated. In particular, we sought to determine if there is a specific pattern of p75NGFR expression in developing human tooth buds and their surrounding tissue.

SETTING AND SAMPLE POPULATION

The Department of Orthodontics at Copenhagen University, Denmark. Histological sections from 11 fetuses, aged 11-21 gestational weeks.

METHOD

The sections were studied by conventional immunohistochemistry.

RESULTS

Specific spatiotemporal patterns of p75NGFR reactions were observed in the tooth buds and dental follicle: Before matrix production by the ameloblasts, the entire inner enamel epithelium and the entire dental follicle display p75NGFR immunoreactivity; after matrix production is initiated, the immunoreactivity of the matrix producing cells is lost, as is that of the dental follicle adjacent to these matrix-producing cells.

CONCLUSION

A unique spatiotemporal distribution of NGFR in the pre-eruptive human tooth bud was demonstrated.

Cell-matrix interactions and cell-cell junctions during epithelial histo-morphogenesis in the developing mouse incisor

The continuously growing rodent incisor develops mainly along its antero-posterior axis. The labio-lingual asymmetry which characterizes this tooth is initiated at the cap stage and increases further during the cap to bell transition (ED14 to ED16) when histogenesis of the enamel organ proceeds. Histology, transmission electron microscopy (TEM), and immunostaining were used to document the changes in the basement membrane (BM) as well as the modifications of epithelial cell-matrix and cell-cell interactions during this period. The expression of plakoglobin, desmoglein and E-cadherin at ED14 suggested that the main cell-cell junctional complexes were adherens junctions. The expression of desmoglein and TEM observations suggested a progressive antero-posterior stabilization of the enamel organ by means of desmosomes from ED14 to ED18. alpha6 integrin, BP 230 and laminin gamma2 chain were all expressed in the developing incisor but were not always co-distributed. Immunostaining and TEM suggested that only primitive type II hemidesmosomes were present. At ED14, cells of the enamel knot (EK) did not show any specific expression for antigens involved in cell-cell interaction. However, strong staining for the laminin gamma2 chain characterized the BM in contact with EK cells. The BM in the labial part of the cervical loop demonstrated ultrastructural changes: the presence of loops of the lamina densa in this region preceeded the differential expression of the integrin alpha6 subunit and that of the laminin gamma2 chain in the labial/lingual parts of the cervical loop. Apoptosis was transiently observed in the contiguous mesenchyme. This affected osteoblasts and also nerve cells close to the labial part of the cervical loop.

Gelatinase A (MMP-2) in developing tooth tissues and amelogenin hydrolysis

Matrix metalloproteinases (MMPs) are thought to play important roles during enamel and dentin biomineralization. Previously, membrane type-1 matrix metalloproteinase (MT1-MMP) was localized to the plasma membranes of ameloblasts and odontoblasts of the developing tooth. The best-characterized function of MT1-MMP is to initiate the activation of gelatinase A (MMP-2). Thus, we hypothesized that gelatinase A may also be expressed by developing tooth tissues. A full-length porcine gelatinase A mRNA was isolated by RT-PCR homology cloning of an enamel-organ-specific cDNA library. Northern blot and in situ hybridization analyses demonstrated gelatinase A expression in developing tooth tissues. Immunohistochemical analysis localized gelatinase A close to the plasma membrane of these tissues. Furthermore, recombinant gelatinase A was demonstrated to cleave recombinant amelogenin into several fragments of differing molecular masses. Thus, gelatinase A is expressed by developing tooth tissues along with its activator MT1-MMP and may, therefore, play an important role during tooth development.

Ultrastructural immunolocalization of enamel matrix proteins during early stages of ameloblast differentiation

Enamel matrix proteins (EMP) represent specific molecular markers of ameloblast secretion. In order to study early differentiation stages of the cells of the inner enamel epithelium, we have investigated the ultrastructural localization of EMP-immunoreactivity in rat tooth germ. Pre-secretory stages of ameloblast differentiation were identified by the absence of EMP-immunoreactivity within epithelial cells as well as adjoining extra-cellular matrix. During subsequent secretory stages EMP-like immunoreactive material could be detected both within epithelial cells as well as within the adjoining extra-cellular matrix. The intensity of the immunoreactivity increased while advancing with the differentiation of epithelial cells. Intracellularly, EMP-immunoreactivity was detectable in cytoplasmic compartments involved in exocrine secretion pathway. During the early secretory stage, EMP-immunoreactive material was also detectable in the basement membrane of the epithelial-mesenchymal interface and within the pre-dentine, close to odontoblast plasma membranes and processes. It is thus suggested that EMP may cross the basement membrane between epithelial and mesenchymal cells. Our study suggests that this aspect might be important in molecular mechanisms that regulate epithelial-mesenchymal interactions during odontogenesis.

Localization of specific binding sites for 125I-TGF-beta1 to fenestrated endothelium in bone and anastomosing capillary networks in enamel organ suggests a role for TGF-beta1 in angiogenesis

Previous studies have shown endothelial cells to be a major target for endocrine TGF-beta in several soft tissues in the normal growing rat. The potent effect of TGF-beta1 on bone formation prompted us to analyze in detail the localization of specific binding sites for endocrine TGF-beta in hard tissues. At 2.5 minutes after injection of 125I-TGF-beta1, specific binding, as demonstrated by quantitative radioautography, was localized to fenestrated endothelium participating in angiogenesis in the vascular invasion region of the growth plate in bone as well as to anatomizing capillary networks in the maturation zone of the enamel organ. At 15 minutes after injection, the bound ligand was internalized into endocytic vesicles of endothelial cells. In bone, quantitation revealed significant differences in receptor density between endothelia undergoing proliferation vs those in a state of elongation and anastomosis with neighboring endothelial cells. In the rat incisor, specific binding of 125I-TGF-beta1 to endothelium correlated with increased formation of anastomotic capillary networks. These studies identify differential specific binding sites of 125I-TGF-beta1 in angiogenically active endothelium, providing an important link between TGF-beta1, the endothelium, and hard tissue development.

Cloning and characterization of the mouse and human enamelin genes

Enamelin is likely to be essential for proper dental enamel formation. It is secreted by ameloblasts throughout the secretory stage and can readily be isolated from the enamel matrix of developing teeth. The gene encoding human enamelin is located on the long arm of chromosome 4, in a region previously linked to an autosomal-dominant form of amelogenesis imperfecta (AI). To gain information on the structure of the enamelin gene and to facilitate the future assessment of the role of enamelin in normal and diseased enamel formation, we have cloned and characterized the mouse and human enamelin genes. Both genes are about 25 kilobases long. The enamelin gene has 10 exons interrupted by 9 introns. Translation initiates in exon 3 and terminates in exon 10. All of the intron/exon junctions within the mouse and human enamelin coding regions are between codons, so there are no partial codons in any exon, and deletion of one or more coding exons by alternative RNA splicing would not shift the downstream reading frame.

Isolation of amelogenin-positive ameloblasts from rat mandibular incisor enamel organs by flow cytometry and fluorescence activated cell sorting

The purpose of this study was to use amelogenin as a marker to examine the feasibility of isolating ameloblasts from enamel organ cell populations by fluorescence activated cell sorting. After treating dissected rat enamel organs with proteolytic enzymes to loosen cell attachments and labial connective tissues, dissociated cell suspensions were fixed, then immunostained with rabbit anti-rM179 recombinant amelogenin antibody and FITC-conjugated goat anti-rabbit Ig G antibody. Flow cytometry indicated that about 70% of the total cell sample and virtually all the larger cells therein were amelogenin-positive. Fluorescence activated cell sorting yielded a sample of amelogenin-positive cells at 97% purity. Immunofluorescence microscopy indicated that these isolated amelogenin-positive cells varied widely in size and morphology. This was attributed to loss of intercellular support for ameloblasts once they were dissociated from each other, and to some fragmentation caused when the cells were initially physically removed from the teeth. The results demonstrate that viable ameloblast cell fractions, especially representing cells at the secretory stage, can be purified from enzymic digests of rat enamel organ by sorting on the basis of cell size alone. From these fractions, subpopulations of ameloblasts may be identified when differentiation specific cell surface markers become available.

Primary culture and characterization of enamel organ epithelial cells

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

Identification of rat enamel organ RNA transcripts using differential-display

Enamel formation is a complex process which involves the expression of a number of genes, the most obvious being those related to the mineralized extracellular matrix. In this study the differential-display technique, first described by Liang and Pardee, has been used to identify genes specifically expressed in enamel organ cells. By comparing results obtained from RNA derived from rat enamel organ with RNA derived from other cellular sources, a number of differentially expressed transcripts have been identified. The nucleotide sequences of two of these have been analyzed and shown to have no homology with any previously published sequences. Further analysis will provide information on the type of protein that they may encode, their tissue distribution and their potential role in enamel formation.

Developmental regulation of dentin sialophosphoprotein during ameloblast differentiation: a potential enamel matrix nucleator

The two major dentin matrix proteins, dentin sialoprotein and dentin phosphoprotein have been shown to be expressed as a single large transcript termed dentin sialophosphoprotein (DSPP). These non-collagenous matrix proteins, identified biochemically by their unique physical-chemical properties, are specific cleavage products of a large parent acidic phosphorylated protein (pI 4.0). Previous studies have shown expression of dentin sialoprotein at the protein level by ameloblasts. The purpose of this study was to determine the temporal-spatial pattern of DSPP expression during amelogenesis. In situ hybridization and immunohistochemistry were performed on sections of developing mouse molars. These data were correlated with RT-PCR analysis of in vitro enamel organ epithelium monolayer cell cultures enriched for ameloblasts. Our data indicates initial expression of the DSPP transcripts and protein during early ameloblast differentiation prior to the secretory phase when the majority of the enamel matrix is formed. Ameloblasts appear to tightly down-regulate DSPP transcription as enamel matrix formation is up-regulated. These data demonstrate DSPP expression during amelogenesis is under highly controlled developmental regulation. Therefore, DSPP may have a primary role in the initial mineralization events of both enamel and dentin, acting as a potential nucleator of hydroxyapatite crystal formation.

Enamel cell biology. Towards a comprehensive biochemical understanding

Enamel cells ultimately determine the properties of dental enamel. Surprisingly little is known about enamel cell functions at the biochemical and molecular levels. Understanding of both normal and abnormal enamel formation should benefit from elucidation of this area. This paper reviews our recent efforts to establish microscale biochemical analyses of rat enamel cells, and the ensuing initial findings about their protein phenotype (i.e., proteome) and calcium-handling mechanisms. A perspective of the current status of enamel cell research, and where it might head, is also given.

Calbindin28kDa is specifically associated with extranuclear constituents of the dense particulate fraction

Recent attempts to understand the function of calbindin28kDa, a widely expressed calcium-binding protein, are confounded by uncertainties over its subcellular location. Using immunoblot analysis of rat brain subregions, we found that the proportion of particulate calbindin28kDa (24-43% of total) was independent of expression level and location. The association of calbindin28kDa with particulate structures appeared to be specific, since it persisted when soluble calbindin28kDa was sequestered by antibodies added before tissue disruption. Moreover, when exogenous calbindin28kDa was added during homogenisation of brain from calbindin28kDa-nullmutant mice, only 10% partitioned to the particulate fraction compared with 33% of endogenous calbindin28kDa in wildtype controls. Confocal microscopy showed that calbindin28kDa was predominantly extranuclear in all tissues analysed (i.e. various brain regions, isolated neurons, and dental enamel epithelium). Dual-label microscopy of neural dense particulate fractions confirmed the extranuclear location of calbindin28kDa and also showed that it partly colocalised with synaptosome and microtubule markers. Using sucrose step gradients, calbindin28kDa was separated from nuclei in parallel with synaptosome and endoplasmic reticulum markers. However, no association with the marker proteins (synaptophysin, ERp29, alpha/beta-tubulin) was detected by calbindin28kDa-immunoprecipitation analysis. Together these findings provide the first consistent picture that calbindin28kDa is located predominantly outside of the nucleus, irrespective of tissue type (neuronal vs. non-neuronal) and experimental approach (biochemical vs morphological). The evidence of a substantial, strong and specific association with insoluble cellular structures challenges the widely held view of calbindin28kDa as a mobile calcium buffer, and supports the existence of important alternative roles that involve target proteins.

Detection of immature dendritic cells in the enamel organ of rat incisors by using anti-cystatin C and anti-MHC class II immunocytochemistry

Dendritic cells in the enamel organ of rat incisors were examined with immunocytochemistry using an anti-cystatin C antibody for immature dendritic cells and macrophages, OX6 for MHC Class II, ED1 for macrophages and dendritic cells, and ED2 for macrophages. Single cells positive for anti-cystatin C appeared in the enamel organ in zones at which ameloblasts secrete enamel matrix proteins. They were also present in transition and enamel maturation zones. In addition, ameloblasts, osteocytes, and osteoclasts were labeled by anti-cystatin C. ED1 and ED2 immunocytochemistry revealed that there was no macrophage population in the enamel organ of secretion, transition, or enamel maturation zone. A double labeling study showed that most anti-cystatin C-positive cells in the enamel maturation zone were also positive for OX6, whereas anti-cystatin C-positive and OX6-negative cells were prevalent in the secretion zone. The results suggest that immature dendritic cells penetrate the enamel organ of the secretion zone and begin to mature in the zones of transition and enamel maturation. (J Histochem Cytochem 48:1243-1255, 2000)

Morphological and immunocytochemical analyses on the effects of diet-induced hypocalcemia on enamel maturation in the rat incisor

During the maturation stage of amelogenesis, the loss of matrix proteins combined with an accentuated but regulated influx of calcium and phosphate ions into the enamel layer results in the "hardest" tissue of the body. The aim of the present investigation was to examine the effects of chronic hypocalcemia on the maturation of enamel. Twenty-one-day old male Wistar rats were given a calcium-free diet and deionized water for 28 days, while control animals received a normal chow. The rats were perfused with aldehyde and the mandibular incisors were processed for histochemical and ultrastructural analyses and for postembedding colloidal gold immunolabeling with antibodies to amelogenin, ameloblastin, and albumin. The maturation stage enamel organ in hypocalcemic rats exhibited areas with an apparent increase in cell number and the presence of cyst-like structures. In both cases the cells expressed signals for ameloblastin and amelogenin. The content of the cysts was periodic acid-Schiff- and periodic acid-silver nitrate-methanamine-positive and immunolabeled for amelogenin, ameloblastin, and albumin. Masses of a similar material were also found at the enamel surface in depressions of the ameloblast layer. In addition, there were accumulations of glycoproteinaceous matrix at the interface between ameloblasts and enamel. In decalcified specimens, the superficial portion of the enamel matrix sometimes exhibited the presence of tubular crystal "ghosts." The basal lamina, normally separating ameloblasts and enamel during the maturation stage, was missing in some areas. Enamel crystals extended within membrane invaginations at the apical surface of ameloblasts in these areas. Immunolabeling for amelogenin, ameloblastin, and albumin over enamel was variable and showed a heterogeneous distribution. In contrast, enamel in control rats exhibited a homogeneous labeling for amelogenin, a concentration of ameloblastin at the surface, and weak reactivity for albumin. These results suggest that diet-induced chronic hypocalcemia interferes with both cellular and extracellular events during enamel maturation.

Quantitative image analysis of hyaluronan expression in human tooth germs

The expression of hyaluronan in human tooth germs was studied by using a biotinylated hyaluronan-binding complex and quantitative digital image analysis. At the cap stage, dental papilla exhibited a moderate staining, while intense reaction was observed in the apical portion of presecretory ameloblasts, stellate reticulum, and in dental basement membrane. When the enamel and dentine matrices started to develop, a strong hyaluronan reaction was evident in the young enamel and the apical portion of secretory ameloblasts. No hyaluronan could be detected in the secretory ameloblasts and enamel matrix of the early (9-wk-old) post-natal stage. It is concluded that hyaluronan may play a transitory role in the early phase of the development of the enamel matrix organization. A very weak signal was observed in the wall of dentin tubules, whereas the rest of the dentine matrix was not stained. The odontoblasts and the pulp were also moderately stained, and these reactions gradually decreased with age, suggesting that hyaluronan may also contribute to the development of dentine matrix and pulp.

The ameloblast movement in rat incisor. L.M., S.E.M. and C.L.S.M. study

The internal epithelium of enamel organ and the below enamel surface during growth of the lower incisor, were examinated in ten Wistar rat 12-27 weeks old and weighing between 150/200 gr, by means of immuno histochemical, light and scanning electron microscopy techniques. Our specimens indicate that during the outer enamel secretion the anti-actin positivity goes from distal terminal web to infra nuclear region of cell body. The results of the present study do not support the active movement hypothesis, conversely they support the Warshawsky (1992) hypothesis, i.e. the distal terminal web permits the maintenance and the assembling of ameloblasts during enamel growth. Hence we do agree with Osborn (1970) who reported that, during secretion, ameloblasts move passively in response to secretory forces.

Expression of amelogenins in developing embryonic and neonatal rat teeth

OBJECTIVE

Tooth enamel is formed by ameloblasts, which are derived from epitheliums and secrete an extracellular matrix containing a complex arrangement of protein components. The epithelial component, referred to as the enamel organ, contains a layer of cells that secrete an organic matrix that biomineralizes to become tooth enamel. Adjacent ectomesenchyme cells differentiate to become dentinproducing odontoblasts. These two mineralized matrices form the crown of the vertebrate tooth. Therefore, amelogenins play a critical role in tooth enamel formation. We have examined the expression patterns and tissue distribution of amelogenins in their developmental stages in order to build a foundation for further study.

METHODS

Amelogenin expression patterns and tissue distribution in developing teeth of embryonic (E17E19) and neonatal (1 to 9 days old) Wistar rats were examined by immunohistochemistry.

RESULTS

Positive immunostaining for amelogenin was first observed in the late embryonic stage, E18. The highest level of amelogenin was noted in neonatal secretary ameloblasts, fully engaged in enamel matrix deposition (3 to 5 days old). After that, amelogenin expression continued at a lower level (6, 7, 8 days old). There was no amelogenin staining observed in the maturation stage of development (9 days old).

CONCLUSIONS

Amelogenin expression occurs as early as the polarization stage of pre-ameloblasts. Amelogenin was also expressed, but at a low level, in post-secretary stages of amelogenesis. Odontoblasts did not contain detectable amelogenin.

Mechanism and timing of fluoride effects on developing enamel

Fluoride appears to specifically interact with mineralizing tissues, causing an alteration of the mineralization process. In enamel, fluorosis results in a subsurface hypomineralization. This hypomineralized enamel appears to be directly related to a delay in the removal of amelogenins at the early-maturation stage of enamel formation. The specific cause for this delay is not known, although existing evidence points to reduced proteolytic activity of proteinases that hydrolyze amelogenin. This delay in hydrolysis of amelogenins could be due to a direct effect of fluoride on proteinase secretion or proteolytic activity, or to a reduced effectiveness of the proteinase due to other changes in the protein or mineral of the fluorosed enamel matrix. The formation of dental fluorosis is highly dependent on the dose, duration, and timing of fluoride exposure. The early-maturation stage of enamel formation appears to be particularly sensitive to the effects of fluoride on enamel formation. Although the risk of enamel fluorosis is minimal with exposure only during the secretory stage, this risk is greatest when exposure occurs in both secretory and maturation stages of enamel formation. The risk of fluorosis appears to be best related to the total cumulative fluoride exposure to the developing dentition.

Expression of cytokeratins in human enamel organ

Cytokeratin (CK) is a filament which plays a central role in epithelial tissue and, like the polypeptides of intermediate filaments in general, shows a high degree of tissue specificity. The CK expression patterns of odontogenic epithelia are still poorly described. We studied the distribution of individual CK polypeptides in the human enamel organ at bell stage and in remnants of the dental lamina. Our immunohistochemical study showed that epithelial cells stained for CKs 7, 13, 14 and 19 with slight changes in their pattern during the differentiation phase of odontogenesis. There was negative staining for all other CK polypeptides tested (CKs 8, 10, 16, 17 and 18). Most of the CKs in the enamel organ epithelia did not show differences related to the stage-specific state of differentiation, except for CKs 14 and 19 at the inner enamel epithelium. A strong label for CK 14 was present at the inner dental epithelium at early bell stage, and this was substituted by CK 19 at the late bell stage when the ameloblasts were fully differentiated.

The immunohistochemical localization of phospholipase Cgamma and the epidermal growth-factor, platelet-derived growth-factor and fibroblast growth-factor receptors in the cells of the rat molar enamel organ during early amelogenesis

Findings on the localization and possible roles of the major growth factors, epidermal (EGF), platelet-derived (PDGF) and fibroblast (FGF) in early amelogenesis are contradictory and inconclusive. This study sought to localize immunohistochemically phospholipase (PLCgamma) and the EGF, PDGF and FGF receptors in the cells of the enamel organ during the events leading directly to early enamel formation in rat molars. PLCgamma is an immediate, downstream, signal-transduction pathway effector unique to the three receptors. A whole-head, freeze-dried sectioning method was used to reduce the possibilities of false-negative staining. A modification of the avidin/biotin complex method of immunohistochemical localization was used. Anti-PLCgamma and antibodies to each of EGF, PDGF and FGF receptors colocalized in the preameloblasts of the cervical loop, adjacent to the undifferentiated mesenchymal cells of the dental pulp. This staining disappeared shortly after the beginning of dentine mineralization. Staining for all four antibodies appeared on the proximal ends of the differentiating presecretory ameloblasts at the level of the beginning of predentine matrix deposition and continued in the secretory ameloblasts. It appears that EGF, PDGF and FGF have roles in the differentiation of ameloblasts and in control of cellular functions in presecretory and secretory ameloblasts. Their roles may represent redundancy of the kind seen in highly conserved tissues.

Inhibition of transforming growth factor-beta type II receptor signaling accelerates tooth formation in mouse first branchial arch explants

Members of the transforming growth factor-beta (TGF-beta) superfamily signal through their cognate receptors to determine cell phenotypes during embryogenesis. Our previous studies on the regulation of first branchial arch morphogenesis have identified critical components of a hierarchy of different TGF-beta isoforms and their possible functions in regulating tooth and cartilage formation during mandibular morphogenesis. Here we tested the hypothesis that TGF-beta type II receptor (TGF-beta IIR) is a critical component in the TGF-beta signaling pathway regulating tooth formation. To establish the precise location of TGF-beta ligand and its cognate receptor, we first performed detailed analyses of the localization of both TGF-beta2 and TGF-beta IIR during initiation and subsequent morphogenesis of developing embryonic mouse tooth organs. A possible autocrine functional role for TGF-beta and its cognate receptor (TGF-beta IIR) was inferred due to the temporal and spatial localization patterns during the early inductive stages of tooth morphogenesis. Second, loss of function of TGF-beta IIR in a mandibular explant culture model resulted in the acceleration of tooth formation to the cap stage while the mandibular explants in the control group only showed bud stage tooth formation. In addition, there was a significant increase in odontogenic epithelial cell proliferation following TGF-beta IIR abrogation. These results demonstrate, for the first time, that abrogation of the TGF-beta IIR stimulates embryonic tooth morphogenesis in culture and reverses the negative regulation of endogenous TGF-beta signaling upon enamel organ epithelial cell proliferation.