From Pluripotent Stem Cells to Organoids and Bioprinting: Recent Advances in Dental Epithelium and Ameloblast Models to Study Tooth Biology and Regeneration

Ameloblasts are the specialized dental epithelial cell type responsible for enamel formation. Following completion of enamel development in humans, ameloblasts are lost and biological repair or regeneration of enamel is not possible. In the past, in vitro models to study dental epithelium and ameloblast biology were limited to freshly isolated primary cells or immortalized cell lines, both with limited translational potential. In recent years, large strides have been made with the development of induced pluripotent stem cell and organoid models of this essential dental lineage - both enabling modeling of human dental epithelium. Upon induction with several different signaling factors (such as transforming growth factor and bone morphogenetic proteins) these models display elevated expression of ameloblast markers and enamel matrix proteins. The advent of 3D bioprinting, and its potential combination with these advanced cellular tools, is poised to revolutionize the field - and its potential for tissue engineering, regenerative and personalized medicine. As the advancements in these technologies are rapidly evolving, we evaluate the current state-of-the-art regarding in vitro cell culture models of dental epithelium and ameloblast lineage with a particular focus toward their applicability for translational tissue engineering and regenerative/personalized medicine.

Developmental Defects of Enamel: A Bibliometric Analysis of the Top 100 Most-Cited Papers

BACKGROUND

Developmental defects of enamel (DDE) are a result of disturbances during formation and maturation of the enamel. Evaluating the most-cited DDE papers can provide important tools that point to the gaps and strengths of this important topic in dentistry.

SUMMARY

This bibliometric study analyzed the 100 most-cited papers on DDE. Using a combined keyword search strategy, the 100 most-cited papers were selected in the Web of Science Core Collection. Papers that addressed any type of DDE were included. The extracted data were title, number of citations, study theme, authorship, journal, type of DDE, type of dentition (primary or permanent), type of diagnosis, study design, year, and country of publication. The bibliometric networks were generated through VOSviewer software. The 100 papers had a range from 78 to 459 citations. The main themes of studies were etiopathogenesis (53%), prevalence and incidence (22%), and diagnosis (8%). The authors with the highest number of citations were Goodman AH and Rose JC (459 citations). Most articles were published in dental journals (47%). The most studied types of DDE were fluorosis and amelogenesis imperfecta in the permanent dentition (47%). Observational (24%) and non-systematic reviews (24%) were the most common study designs and ranged from 1977 to 2019. The country with the highest number of publications was the USA (41%).

KEY MESSAGES

Most of the top 100 DDE papers were about fluorosis and amelogenesis imperfecta, with top papers from three continents with English as the native language. This topic is of great importance in dentistry, and the need for further studies is highlighted, especially regarding the diagnosis and treatment of some DDEs.

Multiloop edgewise archwire treatment for a patient with a severe anterior open bite and amelogenesis imperfecta

Amelogenesis imperfecta is a rare hereditary disorder that affects dental enamel and is often associated with an anterior open bite. Orthodontic treatment of a 16-year-old female patient with hypocalcified amelogenesis imperfecta and a 9-mm anterior open bite was presented. Radiographic examination revealed a steep mandibular plane angle, an increased lower face height, a Class II skeletal pattern, and a convex profile. Additionally, the patient had stainless steel crowns on all upper and lower posterior teeth and composite veneers on the upper anterior teeth. The patient was treated nonsurgically using a multiloop edgewise archwire (MEAW). MEAW mechanics allowed for successful correction of the anterior open bite, with significant reduction in the mandibular plane angle and improvement in the patient's profile. No fixed retainers were used, and the results remained stable 78 months after removal of orthodontic appliances. MEAW mechanics should be considered for patients with large anterior open bites, although this technique requires excellent patient compliance.

A genetic model for the secretory stage of dental enamel formation

The revolution in genetics has rapidly increased our knowledge of human and mouse genes that are critical for the formation of dental enamel and helps us understand how enamel evolved. In this graphical review we focus on the roles of 41 genes that are essential for the secretory stage of amelogenesis when characteristic enamel mineral ribbons initiate on dentin and elongate to expand the enamel layer to the future surface of the tooth. Based upon ultrastructural analyses of genetically modified mice, we propose a molecular model explaining how a cell attachment apparatus including collagen 17, α6ß4 and αvß6 integrins, laminin 332, and secreted enamel proteins could attach to individual enamel mineral ribbons and mold their cross-sectional dimensions as they simultaneously elongate and orient them in the direction of the retrograde movement of the ameloblast membrane.

Functional Role of Inorganic Trace Elements on Enamel and Dentin Formation: A Review

Calcium and phosphate are the major components of hydroxyapatite crystals that form the inorganic portion of the teeth. Apart from these, certain elements are present in little amounts in enamel and dentin of the human teeth. Although they are required in minute quantities, their absence may alter healthy development of enamel and dentin and may result in developmental tooth defects as well as dental caries. Furthermore, excessive intake of some trace elements may inversely affect tooth development and health. The exact of effects that trace elements have on teeth and oral health is still an unexplored territory. The present paper reviews the presence of trace elements in teeth and their role in tooth health and development.

Dental Fluorosis: the Risk of Misdiagnosis-a Review

Fluoride has been considered as the single factor most frequently responsible for causing enamel mottling. However, in humans, either endogenous and/or exogenous factors not related to fluoride exposure may also cause enamel mottling. In this sense, various studies in the international literature have reported severe mottling of the teeth that could not be attributed to fluoride exposure. Thus, misdiagnosis of non-fluoride-induced enamel defects may occur frequently. Reports of unexpectedly high population prevalence and individual cases of fluorosis, where such diagnoses are irreconcilable with the identified fluoride history, highlight the necessity for a more precise definition and diagnosis of dental fluorosis. Also, a more discriminating diagnostic procedure is suggested. Particularly, positive identification of environmental fluoride levels to which the communities and individuals are exposed shall be developed before the confirmation of a diagnosis of fluorosis. It is considered that a more critical methodology for the diagnosis of fluorosis will be helpful in the rational use and control of fluorides for dental health, and in the identification of factors that may induce enamel defects.

JNK Signaling Pathway Mediates Fluoride-Induced Upregulation of CK1α during Enamel Formation

Fluorosis is a defect in the enamel mineral content caused by excessive fluoride intake during amelogenesis; the interaction of various factors in the development and progression of fluorosis has not been defined. Casein kinase 1α (CK1α) is constitutively active in cells and is involved in diverse cellular processes; however, its expression in fluorosis has not been measured. This study aimed to investigate the effects of fluoride on CK1α expression and to assess the regulation of molecular signaling involving fluoride and CK1α during enamel development. Kunming mice were randomly divided into the control and F groups with induced clinical features of fluorosis. The F group mice, including mothers and newborns, were treated with 50 ppm fluoridated water. Immunohistochemical staining of the sections of the embryonic mandible regions was performed at the bell stage. Protein expression and signaling pathways in a mouse-derived ameloblast-like cell line (LS8) exposed to fluoride or a Jun N-terminal kinase (JNK) inhibitor were compared to those in control cells without exposure. CK1α and proteins of the JNK signaling pathways were assayed by quantitative real-time PCR and Western blotting. Mice of the F group developed dental fluorosis. Scanning electron microscopy showed a significant reduction in the degree of mineralization in the F group mice, which manifested as thin, loosely arranged, and disorganized enamel rods. Additional analysis revealed that the expression of CK1α in the F group was significantly elevated compared with that in the control group; LS8 cells responded to fluoride by upregulation of CK1α expression through the JNK pathway. Our findings identified the potential effects of CK1α on fluorosis using a mouse model and revealed that a high fluoride level increases the expression of CK1α and that JNK can be a key regulatory factor in CK1α expression.

Hypomagnesemia with Hypercalciuria Leading to Nephrocalcinosis, Amelogenesis Imperfecta, and Short Stature in a Child Carrying a Homozygous Deletion in the CLDN16 Gene

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessive disease caused by mutations in the CLDN16 or CLDN19 gene; however, few cases develop classical amelogenesis imperfecta. Herein, we report the case of a boy with early clinical renal manifestations that started at 1 year of age and presenting with dental hypoplasia and growth delay. The patient presented with vomiting, polyuria, and polydipsia. Apart from recurrent sterile leukocyturia, erroneously treated as infectious, he was normal, except for short stature and amelogenesis imperfecta with gradually discolored teeth. Laboratory tests revealed hyperparathyroidism, hypomagnesemia, severe hypercalciuria, and hypermagnesuria on 24-h urine testing. Helical computed tomography confirmed nephrocalcinosis. We performed whole-exome sequencing (WES) to test the hypothesis of FHHNC and oligogenic inheritance of amelogenesis. Analysis of the WES binary sequence alignment/map file revealed the presence of exon 1 of the CLDN16 and absence of the other exons [c.325_c918*? (E2_E5del)]. We confirmed a CLDN16 E2_E5 homozygous deletion by multiplex ligation-dependent probe amplification and polymerase chain reaction assays. Although most mutations causing FHHNC are missense and nonsense mutations in the CLDN16 or CLDN19 gene, large deletions occur and may be misled by WES, which is generally used for genetic screening of oligogenic disorders. The patient received cholecalciferol, magnesium oxide and potassium citrate. Later, the combination with hydrochlorothiazide plus amiloride was prescribed, with a good response during follow-up. Our report broadens the phenotype of FHHNC, including severe early-onset amelogenesis and short stature, and reinforces the phenotype-genotype correlation of the large deletion found in CLDN16.

Structure-function relationships of K+-dependent Na+/Ca2+ exchangers (NCKX)

K⁺-dependent Na⁺/Ca²⁺ exchanger proteins (NCKX1-5) of the SLC24 gene family play important roles in a wide range of biological processes including but not limited to rod and cone photoreceptor vision, olfaction, enamel formation and skin pigmentation. NCKX proteins are also widely expressed throughout the brain and NCKX2 and NCKX4 knockouts in mice have specific phenotypes. Here we review our work on structure-function relationships of NCKX proteins. We discuss membrane topology, domains critical to transport function, and residues critical to cation binding and transport function, all in the context of crystal structures that were obtained for the archaeal Na⁺/Ca²⁺ exchanger NCX_Mj.

Porcine keratin 75 in developing enamel

OBJECTIVE

To provide in vivo biochemical evidence for the isolation, identification, and characterization of porcine keratin 75 (K75) in developing enamel.

METHODS

Immunolocalization of K75 was observed in mandibles from mice at postnatal days 5 and 11. K75 gene expression was analyzed by quantitative reverse transcription-polymerase chain reaction using enamel organ epithelium (EOE) of incisors from pigs at 5 months of age. Enamel protein was extracted and isolated from both immature and mature enamel of second molars from 5-month-old pigs, and the K75 antibody-positive fraction was analyzed by liquid chromatography-mass spectrometry (LC-MS/MS). In vitro protease digestion of K75-antibody-positive fraction was carried out using porcine kallikrein 4 (pKLK4) or recombinant human enamelysin (rhMMP20) and their degradation patterns were characterized by both SDS-PAGE and western blotting.

RESULTS

Specific immunostaining for K75 was restricted to the layers of stratum intermedium and the enamel side of ameloblasts in mice at postnatal day 5, and to the papillary layer at postnatal day 11. Porcine K75 was expressed throughout enamel formation, but its transcript levels were significantly higher in the transition EOE than in the secretory- and maturation-stage EOE. Porcine K75 was extracted from the neutral soluble fraction from both immature and mature enamel. It was identified by LC-MS/MS analysis, and was found not to be degraded by either pKLK4 or rhMMP20.

CONCLUSION

We propose that K75 is present in the developing enamel and undergoes different processing/degradation compared to other enamel proteins.

Rodent Dental Fluorosis Model: Extraction of Enamel Organ from Rat Incisors

Chronic fluoride overexposure can cause dental fluorosis. Dental fluorosis is characterized by porous and soft enamel that is vulnerable to erosion and decay. Animal models often contribute to clinical applications by addressing pathogenic questions of disease. To study dental fluorosis, rodent models have been employed because rodent incisors erupt continuously and every stage of enamel development is present along the length of the rodent incisor. Here we present a protocol to induce dental fluorosis in mouse and rat and describe the procedure for extraction of stage specific enamel organ from rat mandibular incisors.

Effect of canonical NF-κB signaling pathway on the differentiation of rat dental epithelial stem cells

BACKGROUND

Nuclear factor-κB (NF-κB), an important transcription factor, participates in many physiological and pathological processes such as growth, differentiation, organogenesis, apoptosis, inflammation, and immune response, including tooth development. However, it is still unknown whether NF-κB participates in the regulation of dental epithelial stem cells (DESCs) in postnatal rat incisors. Here, we investigated the specific differentiation regulatory mechanisms of the canonical NF-κB signaling pathway in DESCs and provided the mechanism of cross-talk involved in DESC differentiation.

METHODS

After adding the activator or inhibitor of the NF-κB signaling pathway, Western blot and quantitative real-time PCR were used to analyze the expressions of amelogenesis-related genes and proteins and canonical transforming growth factor-β (TGF-β) signaling. In addition, we used amelogenesis induction in vitro by adding the activator or inhibitor of the NF-κB signaling pathway to the amelogenesis-induction medium, respectively. Recombinant TGF-β was used to activate the TGF-β pathway, and SMAD7 siRNA was used to downregulate the expression of SMAD7 in DESCs.

RESULTS

We found that the expression of amelogenesis-related genes and proteins as well as TGF-β signaling were downregulated, while SMAD7 expression was increased in NF-κB-activated DESCs. In addition, NF-κB-inhibited DESCs exhibited opposite results compared with NF-κB-activated DESCs. Furthermore, the canonical NF-κB signaling pathway suppressed the canonical TGF-β-SMAD signaling by inducing SMAD7 expression involved in the regulation of DESC differentiation.

CONCLUSIONS

These results indicate that the canonical NF-κB signaling pathway participated in the regulation of DESC differentiation, which was through upregulating SMAD7 expression and further suppressing the canonical TGF-β-SMAD signaling pathway.

Genes Regulating Immune Response and Amelogenesis Interact in Increasing the Susceptibility to Molar-Incisor Hypomineralization

Ameloblasts are sensitive cells whose metabolism and function may be affected by inflammatory stimuli. The aim of this study was to evaluate the possible association between polymorphisms in immune response-related genes and molar-incisor hypomineralization (MIH), and their interaction with polymorphisms in amelogenesis-related genes. DNA samples were obtained from 101 nuclear families that had at least 1 MIH-affected child. Eleven single-nucleotide polymorphisms (SNPs) were investigated in immune response genes using TaqMan® technology allele-specific probes. A transmission disequilibrium test was performed to verify overtransmission of alleles in all MIH families, as well as in families only with mild or severe MIH-affected children. Gene-gene interactions between the immune-related and amelogenesis-related polymorphisms were analyzed by determining whether alleles of those genes were transmitted from heterozygous parents more often in association than individually with MIH-affected children. In severe cases of MIH, significant results were observed for rs10733708 (TGFBR1, OR = 3.5, 95% CI = 1.1-10.6). Statistical evidence for gene-gene interactions between rs6654939 (AMELX) and the SNPs rs2070874 (IL4), rs2275913 (IL17A), rs1800872 (IL10), rs1800587 (IL1A), and rs3771300 (STAT1) was observed. The rs2070874 SNP (IL4) was also significantly overtransmitted from heterozygous parents with the rs7526319 (TUFT1) and the rs2355767 (BMP2) SNPs, suggesting a synergistic effect of the transmission of these alleles with susceptibility to MIH. This family-based study demonstrated an association between variation in TGFBR1 and MIH. Moreover, the polymorphisms in immune response and amelogenesis genes may have an additive effect on the risk of developing MIH.

Identification of major matrix metalloproteinase-20 proteolytic processing products of murine amelogenin and tyrosine-rich amelogenin peptide using a nuclear magnetic resonance spectroscopy based method

OBJECTIVE

The aim of this study was to identify major matrix metalloproteinase-20 (MMP20) proteolytic processing products of amelogenin over time and determine if the tyrosine-rich amelogenin peptide (TRAP) was a substrate of MMP20.

DESIGN

Recombinant¹⁵N-labeled murine amelogenin and ¹³C,¹⁵N-labeled TRAP were incubated with MMP20 under conditions where amelogenin self-assembles into nanospheres. Digestion products were fractionated by reverse-phase high-performance liquid chromatography at various time points. Product identification took advantage of the intrinsic disorder property of amelogenin that results in little change to its fingerprint ¹H-¹⁵N heteronuclear single-quantum coherence nuclear magnetic resonance spectrum in 2% acetic acid upon removing parts of the protein, allowing cleavage site identification by observing which amide cross peaks disappear.

RESULTS

The primary product in five out of the six major reverse-phase high-performance liquid chromatography bands generated after a 24 h incubation of murine amelogenin with MMP20 were: S55-L163, P2-L147, P2-E162, P2-A167, and P2-R176. After 72 h these products were replaced with five major reverse-phase high-performance liquid chromatography bands containing: L46-A170, P2-S152, P2-F151, P2-W45, and short N-terminal peptides. TRAP was completely digested by MMP20 into multiple small peptides with the initial primary site of cleavage between S16 and Y17.

CONCLUSIONS

Identification of the major MMP20 proteolytic products of amelogenin confirm a dynamic process, with sites towards the C-terminus more rapidly attacked than sites near the N-terminus. This observation is consistent with nanosphere models where the C-terminus is exposed and the N-terminus more protected. One previously reported end-product of the MMP20 proteolytic processing of amelogenin, TRAP, is shown to be an in vitro substrate for MMP20.

The overview of channels, transporters, and calcium signaling molecules during amelogenesis

Enamel is a highly calcified tissue. Its formation requires a progressive and dynamic system for the regulation of electrolyte concentration by enamel epithelia. A critical function of enamel epithelial cells, ameloblasts, is the secretion and movement of electrolytes via various channels and transporters to develop the enamel tissue. Enamel formation generates protons, which need to be neutralised. Thus, ameloblasts possess a buffering system to sustain mineral accretion. Normal tooth formation involves stage-dependent net fluctuations in pH during amelogenesis. To date, all of our information about ion transporters in dental enamel tissue is based solely on immunostaining-expression techniques. This review critically evaluates the current understanding and recent discoveries and physiological role of ion channels and transporters, Mg²⁺ transporters, and Ca²⁺ regulatory proteins during amelogenesis in enamel formation. The ways in which ameloblasts modulate ions are discussed in the context of current research for developing a novel morphologic-functional model of enamel maturation.

The Role of Epithelial Stat3 in Amelogenesis during Mouse Incisor Renewal

The aim of this study was to evaluate the role of epithelial signal transducer and activator of transcription 3 (STAT3) in mouse incisor amelogenesis. Since Stat3 is expressed in the epithelial component of developing and adult mouse teeth, we generated and analyzed Krt14Cre/+;Stat3fl/fl mutant mice in which Stat3 was inactivated in epithelia including ameloblast progenitors and ameloblasts, the cells responsible for enamel formation. Histological analysis showed little enamel matrix in mutant incisors compared to controls. Delayed incisor enamel mineralization was demonstrated using micro-computed X-ray tomography analysis and was supported by an increase in the pre-expression distance of enamel-enriched proteins such as amelogenin, ameloblastin, and kallikrein-4. Lastly, scanning electron microscopy analysis showed little enamel mineralization in mutant incisors underneath the mesial root of the 1st molar; however, the micro-architecture of enamel mineralization was similar in the erupted portion of control and mutant incisors. Taken together, our findings demonstrate for the first time that the absence of epithelial Stat3 in mice leads to delayed incisor amelogenesis.

Proteomic Mapping of Dental Enamel Matrix from Inbred Mouse Strains: Unraveling Potential New Players in Enamel

Enamel formation is a complex 2-step process by which proteins are secreted to form an extracellular matrix, followed by massive protein degradation and subsequent mineralization. Excessive systemic exposure to fluoride can disrupt this process and lead to a condition known as dental fluorosis. The genetic background influences the responses of mineralized tissues to fluoride, such as dental fluorosis, observed in A/J and 129P3/J mice. The aim of the present study was to map the protein profile of enamel matrix from A/J and 129P3/J strains. Enamel matrix samples were obtained from A/J and 129P3/J mice and analyzed by 2-dimensional electrophoresis and liquid chromatography coupled with mass spectrometry. A total of 120 proteins were identified, and 7 of them were classified as putative uncharacterized proteins and analyzed in silico for structural and functional characterization. An interesting finding was the possibility of the uncharacterized sequence Q8BIS2 being an enzyme involved in the degradation of matrix proteins. Thus, the results provide a comprehensive view of the structure and function for putative uncharacterized proteins found in the enamel matrix that could help to elucidate the mechanisms involved in enamel biomineralization and genetic susceptibility to dental fluorosis.

Bicarbonate Transport During Enamel Maturation

Amelogenesis (tooth enamel formation) is a biomineralization process consisting primarily of two stages (secretory stage and maturation stage) with unique features. During the secretory stage, the inner epithelium of the enamel organ (i.e., the ameloblast cells) synthesizes and secretes enamel matrix proteins (EMPs) into the enamel space. The protein-rich enamel matrix forms a highly organized architecture in a pH-neutral microenvironment. As amelogenesis transitions to maturation stage, EMPs are degraded and internalized by ameloblasts through endosomal-lysosomal pathways. Enamel crystallite formation is initiated early in the secretory stage, however, during maturation stage the more rapid deposition of calcium and phosphate into the enamel space results in a rapid expansion of crystallite length and mineral volume. During maturation-stage amelogenesis, the pH value of enamel varies considerably from slightly above neutral to acidic. Extracellular acid-base balance during enamel maturation is tightly controlled by ameloblast-mediated regulatory networks, which include significant synthesis and movement of bicarbonate ions from both the enamel papillary layer cells and ameloblasts. In this review we summarize the carbonic anhydrases and the carbonate transporters/exchangers involved in pH regulation in maturation-stage amelogenesis. Proteins that have been shown to be instrumental in this process include CA2, CA6, CFTR, AE2, NBCe1, SLC26A1/SAT1, SLC26A3/DRA, SLC26A4/PDS, SLC26A6/PAT1, and SLC26A7/SUT2. In addition, we discuss the association of miRNA regulation with bicarbonate transport in tooth enamel formation.

Amelogenesis imperfecta in familial hypomagnesaemia and hypercalciuria with nephrocalcinosis caused by CLDN19 gene mutations

BACKGROUND

Amelogenesis imperfecta (AI) is a group of genetic diseases characterised by tooth enamel defects. AI was recently described in patients with familial hypercalciuria and hypomagnesaemia with nephrocalcinosis (FHHNC) caused by CLDN16 mutations. In the kidney, claudin-16 interacts with claudin-19 to control the paracellular passage of calcium and magnesium. FHHNC can be linked to mutations in both genes. Claudin-16 was shown to be expressed during amelogenesis; however, no data are available on claudin-19. Moreover, the enamel phenotype of patients with CLDN19 mutations has never been described. In this study, we describe the clinical and genetic features of nine patients with FHHNC carrying CLDN19 mutations and the claudin-19 expression profile in rat ameloblasts.

METHODS

Six FHHNC Brazilian patients were subjected to mutational analysis. Three additional French patients were recruited for orodental characterisation. The expression profile of claudin-19 was evaluated by RT-qPCR and immunofluorescence using enamel epithelium from rat incisors.

RESULTS

All patients presented AI at different degrees of severity. Two new likely pathogenic variations in CLDN19 were found: p.Arg200Gln and p.Leu90Arg. RT-qPCR revealed low Cldn19 expression in ameloblasts. Confocal analysis indicated that claudin-19 was immunolocalised at the distal poles of secretory and maturing ameloblasts.

CONCLUSIONS

For the first time, it was demonstrated that AI is associated with FHHNC in patients carrying CLDN19 mutations. The data suggest claudin-19 as an additional determinant in enamel formation. Indeed, the coexistence of hypoplastic and hypomineralised AI in the patients was consistent with claudin-19 expression in both secretory and maturation stages. Additional indirect systemic effects cannot be excluded.

Laminin γ2 knockout mice rescued with the human protein exhibit enamel maturation defects

The epithelial ameloblasts are separated from the maturing enamel by an atypical basement membrane (BM) that is enriched in laminin 332 (LM-332). This heterotrimeric protein (α3, ß3 and γ2 chains) provides structural integrity to BMs and influences various epithelial cell processes including cell adhesion and differentiation. Mouse models that lack expression of individual LM-332 chains die shortly after birth. The lethal phenotype of laminin γ2 knockout mice can be rescued by human laminin γ2 (LAMC2) expressed using a doxycycline-inducible (Tet-on) cytokeratin 14 promoter-rtTA. These otherwise normal-looking rescued mice exhibit white spot lesions on incisors. We therefore investigated the effect of rescue with human LAMC2 on enamel maturation and structuring of the atypical BM. The maturation stage enamel organ in transgenic mice was severely altered as compared to wild type controls, a structured BM was no longer discernible, dystrophic matrix appeared in the maturing enamel layer, and there was residual enamel matrix late into the maturation stage. Microtomographic scans revealed excessive wear of occlusal surfaces on molars, chipping of enamel on incisor tips, and hypomineralization of the enamel layer. No structural alterations were observed at other epithelial sites, such as skin, palate and tongue. These results indicate that while this humanized mouse model is capable of rescue in various epithelial tissues, it is unable to sustain structuring of a proper BM at the interface between ameloblasts and maturing enamel. This failure may be related to the atypical composition of the BM in the maturation stage and reaffirms that the atypical BM is essential for enamel maturation.

V-type ATPase proton pump expression during enamel formation

Several diseases such as proximal and distal renal tubular acidosis and osteoporosis are related to intracellular pH dysregulation resulting from mutations in genes coding for ion channels, including proteins comprising the proton-pumping V-type ATPase. V-type ATPase is a multi-subunit protein complex expressed in enamel forming cells. V-type ATPase plays a key role in enamel development, specifically lysosomal acidification, yet our understanding of the relationship between the endocytotic activities and dental health and disease is limited. The objective of this study is to better understand the ameloblast-associated pH regulatory networks essential for amelogenesis. Quantitative RT-PCR was performed on tissues from secretory-stage and maturation-stage enamel organs to determine which of the V-type ATPase subunits are most highly upregulated during maturation-stage amelogenesis: a time when ameloblast endocytotic activity is highest. Western blot analyses, using specific antibodies to four of the V-type ATPase subunits (Atp6v0d2, Atp6v1b2, Atp6v1c1 and Atp6v1e1), were then applied to validate much of the qPCR data. Immunohistochemistry using these same four antibodies was also performed to identify the spatiotemporal expression profiles of individual V-type ATPase subunits. Our data show that cytoplasmic V-type ATPase is significantly upregulated in enamel organ cells during maturation-stage when compared to secretory-stage. These data likely relate to the higher endocytotic activities, and the greater need for lysosomal acidification, during maturation-stage amelogenesis. It is also apparent from our immunolocalization data, using antibodies against two of the V-type ATPase subunits (Atp6v1c1 and Atp6v1e1), that significant expression is seen at the apical membrane of maturation-stage ameloblasts. Others have also identified this V-type ATPase expression profile at the apical membrane of maturation ameloblasts. Collectively, these data better define the expression and role of the V-type ATPase proton pump in the enamel organ during amelogenesis.

Ameloblast transcriptome changes from secretory to maturation stages

The purpose of this study was to identify the major molecular components in the secretory and maturation stages of amelogenesis through transcriptome analyses. Ameloblasts (40 sections per age group) were laser micro-dissected from Day 5 (secretory stage) and Days 11-12 (maturation stage) first molars. PolyA+ RNA was isolated from the lysed cells, converted to cDNA, and amplified to generate a cDNA library. DNA sequences were obtained using next generation sequencing and analyzed to identify genes whose expression had increased or decreased at least 1.5-fold in maturation stage relative to secretory stage ameloblasts. Among the 9198 genes that surpassed the quality threshold, 373 showed higher expression in secretory stage, while 614 genes increased in maturation stage ameloblasts. The results were cross-checked against a previously published transcriptome generated from tissues overlying secretory and maturation stage mouse incisor enamel and 34 increasing and 26 decreasing expressers common to the two studies were identified. Expression of F2r, which encodes protease activated receptor 1 (PAR1) that showed 10-fold higher expression during the secretory stage in our transcriptome analysis, was characterized in mouse incisors by immunohistochemistry. PAR1 was detected in secretory, but not maturation stage ameloblasts. We conclude that transcriptome analyses are a good starting point for identifying genes/proteins that are critical for proper dental enamel formation and that PAR1 is specifically expressed by secretory stage ameloblasts.

Enamel hypomineralization due to endocrine disruptors

There has been increasing concerns over last 20 years about the potential adverse effects of endocrine disruptors (EDs). Bisphenol A (BPA), genistein (G) and vinclozolin (V) are three widely used EDs having similar effects. Tooth enamel has recently been found to be an additional target of BPA that may be a causal agent of molar incisor hypomineralization (MIH). However, populations are exposed to many diverse EDs simultaneously. The purpose of this study was therefore to assess the effects of the combination of G, V and BPA on tooth enamel. Rats were exposed daily in utero and after birth to low doses of EDs mimicking human exposure during the critical fetal and suckling periods when amelogenesis takes place. The proportion of rats presenting opaque areas of enamel hypomineralization was higher when rats were treated with BPA alone than with a combination of EDs. The levels of mRNAs encoding the main enamel proteins varied with BPA treatment alone and did not differ significantly between controls and combined treatment groups. In vitro, rat ameloblastic HAT-7 cells were treated with the three EDs. BPA induced enamelin and reduced klk4 expression, G had no such effects and V reduced enamelin expression. These findings suggest that combinations of EDs may affect enamel less severely than BPA alone, and indicate that enamel hypomineralization may differ according to the characteristics of the ED exposure.

Genes expressed in dental enamel development are associated with molar-incisor hypomineralization

Genetic disturbances during dental development influence variation of number and shape of the dentition. In this study, we tested if genetic variation in enamel formation genes is associated with molar-incisor hypomineralization (MIH), also taking into consideration caries experience. DNA samples from 163 cases with MIH and 82 unaffected controls from Turkey, and 71 cases with MIH and 89 unaffected controls from Brazil were studied. Eleven markers in five genes [ameloblastin (AMBN), amelogenin (AMELX), enamelin (ENAM), tuftelin (TUFT1), and tuftelin-interacting protein 11 (TFIP11)] were genotyped by the TaqMan method. Chi-square was used to compare allele and genotype frequencies between cases with MIH and controls. In the Brazilian data, distinct caries experience within the MIH group was also tested for association with genetic variation in enamel formation genes. The ENAM rs3796704 marker was associated with MIH in both populations (Brazil: p=0.03; OR=0.28; 95% C.I.=0.06-1.0; Turkey: p=1.22e-012; OR=17.36; 95% C.I.=5.98-56.78). Associations between TFIP11 (p=0.02), ENAM (p=0.00001), and AMELX (p=0.01) could be seen with caries independent of having MIH or genomic DNA copies of Streptococcus mutans detected by real time PCR in the Brazilian sample. Several genes involved in enamel formation appear to contribute to MIH.

Prospects and Pits on the Path of Biomimetics: The case of tooth enamel

This review presents a discourse on challenges in understanding and imitating the process of amelogenesis in vitro on the molecular scale. In light of the analysis of imitation of the growth of dental enamel, it also impends on the prospects and potential drawbacks of the biomimetic approach in general. As the formation of enamel proceeds with the protein matrix guiding the crystal growth, while at the same time conducting its own degradation and removal, it is argued that three aspects of amelogenesis need to be induced in parallel: a) crystal growth; b) protein assembly; c) proteolytic degradation. A particular emphasis is therefore placed on ensuring conditions for proteolysis-coupled protein-guided crystallization to occur. Discussed are structural and functional properties of the protein species involved in amelogenesis, mainly amelogenin and enamelysin, the main protein and the protease of the developing enamel matrix, respectively. A model of enamel growth based on controlled delivery of constituent ions or crystalline or amorphous building blocks by means of amelogenin is proposed. The importance of high viscosity of the enamel matrix and a more intricate role that water may play in such a gelatinous medium are also touched upon. The tendency of amelogenin to self-assemble into fibrous and rod-shaped morphologies is considered as potentially important in explaining the formation of elongated apatite crystals. The idea that a preassembling protein matrix serves as a template for the uniaxial growth of apatite crystals in enamel is finally challenged with the one based on co-assembly of the protein and the mineral phases.