Proteolysis by MMP20 Prevents Aberrant Mineralization in Secretory Enamel

Role of amelogenin phosphorylation in regulating dental enamel formation

Amelogenin (AMELX), the predominant matrix protein in enamel formation, contains a singular phosphorylation site at Serine 16 (S16) that greatly enhances AMELX's capacity to stabilize amorphous calcium phosphate (ACP) and inhibit its transformation to apatitic enamel crystals. To explore the potential role of AMELX phosphorylation in vivo, we developed a knock-in (KI) mouse model in which AMELX phosphorylation is prevented by substituting S16 with Ala (A). As anticipated, AMELXS16A KI mice displayed a severe phenotype characterized by weak hypoplastic enamel, absence of enamel rods, extensive ectopic calcifications, a greater rate of ACP transformation to apatitic crystals, and progressive cell pathology in enamel-forming cells (ameloblasts). In the present investigation, our focus was on understanding the mechanisms of action of phosphorylated AMELX in amelogenesis. We have hypothesized that the absence of AMELX phosphorylation would result in a loss of controlled mineralization during the secretory stage of amelogenesis, leading to an enhanced rate of enamel mineralization that causes enamel acidification due to excessive proton release. To test these hypotheses, we employed microcomputed tomography (µCT), colorimetric pH assessment, and Fourier Transform Infrared (FTIR) microspectroscopy of apical portions of mandibular incisors from 8-week old wildtype (WT) and KI mice. As hypothesized, µCT analyses demonstrated significantly higher rates of enamel mineral densification in KI mice during the secretory stage compared to the WT. Despite a greater rate of enamel densification, maximal KI enamel thickness increased at a significantly lower rate than that of the WT during the secretory stage of amelogenesis, reaching a thickness in mid-maturation that is approximately half that of the WT. pH assessments revealed a lower pH in secretory enamel in KI compared to WT mice, as hypothesized. FTIR findings further demonstrated that KI enamel is comprised of significantly greater amounts of acid phosphate compared to the WT, consistent with our pH assessments. Furthermore, FTIR microspectroscopy indicated a significantly higher mineral-to-organic ratio in KI enamel, as supported by µCT findings. Collectively, our current findings demonstrate that phosphorylated AMELX plays crucial mechanistic roles in regulating the rate of enamel mineral formation, and in maintaining physico-chemical homeostasis and the enamel growth pattern during early stages of amelogenesis.

Biomimetic Self-Maturation Mineralization System for Enamel Repair

Enamel repair is crucial for restoring tooth function and halting dental caries. However, contemporary research often overlooks the retention of organic residues within the repair layer, which hinders the growth of dense crystals and compromises the properties of the repaired enamel. During the maturation of natural enamel, the organic matrix undergoes enzymatic processing to facilitate further crystal growth, resulting in a highly mineralized tissue. Inspired by this process, a biomimetic self-maturation mineralization system is developed, comprising ribonucleic acid-stabilized amorphous calcium phosphate (RNA-ACP) and ribonuclease (RNase). The RNA-ACP induces initial mineralization in the form of epitaxial crystal growth, while the RNase present in saliva automatically triggers a biomimetic self-maturation process. The mechanistic study further indicates that RNA degradation prompts conformational rearrangement of the RNA-ACP, effectively excluding the organic matter introduced earlier. This exclusion process promotes lateral crystal growth, resulting in the generation of denser enamel-like apatite crystals that are devoid of organic residues. This strategy of eliminating organic residues from enamel crystals enhances the mechanical and physiochemical properties of the repaired enamel. The present study introduces a conceptual biomimetic mineralization strategy for effective enamel repair in clinical practice and offers potential insights into the mechanisms of biomineral formation.

Crucial Roles of microRNA-16-5p and microRNA-27b-3p in Ameloblast Differentiation Through Regulation of Genes Associated With Amelogenesis Imperfecta

Amelogenesis imperfecta is a congenital disorder within a heterogeneous group of conditions characterized by enamel hypoplasia. Patients suffer from early tooth loss, social embarrassment, eating difficulties, and pain due to an abnormally thin, soft, fragile, and discolored enamel with poor aesthetics and functionality. The etiology of amelogenesis imperfecta is complicated by genetic interactions. To identify mouse amelogenesis imperfecta-related genes (mAIGenes) and their respective phenotypes, we conducted a systematic literature review and database search and found and curated 70 mAIGenes across all of the databases. Our pathway enrichment analysis indicated that these genes were enriched in tooth development-associated pathways, forming four distinct groups. To explore how these genes are regulated and affect the phenotype, we predicted microRNA (miRNA)-gene interaction pairs using our bioinformatics pipeline. Our miRNA regulatory network analysis pinpointed that miR-16-5p, miR-27b-3p, and miR-23a/b-3p were hub miRNAs. The function of these hub miRNAs was evaluated through ameloblast differentiation assays with/without the candidate miRNA mimics using cultured mouse ameloblast cells. Our results revealed that overexpression of miR-16-5p and miR-27b-3p, but not miR-23a/b-3p, significantly inhibited ameloblast differentiation through regulation of mAIGenes. Thus, our study shows that miR-16-5p and miR-27b-3p are candidate pathogenic miRNAs for amelogenesis imperfecta.

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.

Quantitative label-free optical technique to analyze the ultrastructure changes and spatiotemporal relationship of enamel induced by Msx2 deletion

New advances in the molecular mechanism of enamel mineralization reveal the practical significance of regenerative medicine in clinical transformation. Muscle segment homeobox 2 (MSX2), a transcription factor, is recently reported to be closely associated with the amelogenesis imperfecta (AI). To elucidate the biomineralization framework of AI enamel, herein, Msx2 gene mutant mice are investigated by dual-mode noninvasive spectroscopic analytical techniques for the first time. Optical coherence tomography (OCT) records the depth-resolved structural information of mice teeth, where a dramatic decrease in enamel thickness and quality occurred in Msx2 deficient (Msx2^-/- ) enamel. And it has the advantages of fast, noninvasive and low cost. Raman spectroscopy, a powerful molecular fingerprint tool, further witnesses an imbalance of inorganic and organic contents in Msx2^-/- enamel. In addition, abnormal expression of MSX2 also influences the spatial distribution of phosphate in enamel according to the Raman spectral imaging. Therefore, OCT integrated with Raman spectroscopy provides the quantitative label-free optical parameters of both the physical structure and chemical component in mice enamel, which strengthens the understanding of the biomineralization process underlying the Msx2-related amelogenesis imperfect.

MMP20-generated amelogenin cleavage products prevent formation of fan-shaped enamel malformations

Dental enamel forms extracellularly as thin ribbons of amorphous calcium phosphate (ACP) that initiate on dentin mineral in close proximity to the ameloblast distal membrane. Secreted proteins are critical for this process. Enam-/- and Ambn-/- mice fail to form enamel. We characterize enamel ribbon formation in wild-type (WT), Amelx-/- and Mmp20-/- mouse mandibular incisors using focused ion beam scanning electron microscopy (FIB-SEM) in inverted backscatter mode. In Amelx-/- mice, initial enamel mineral ribbons extending from dentin are similar in form to those of WT mice. As early enamel development progresses, the Amelx-/- mineral ribbons develop multiple branches, resembling the staves of a Japanese fan. These striking fan-shaped structures cease growing after attaining ~ 20 µm of enamel thickness (WT is ~ 120 µm). The initial enamel mineral ribbons in Mmp20-/- mice, like those of the Amelx-/- and WT, extend from the dentin surface to the ameloblast membrane, but appear to be fewer in number and coated on their sides with organic material. Remarkably, Mmp20-/- mineral ribbons also form fan-like structures that extend to ~ 20 µm from the dentin surface. However, these fans are subsequently capped with a hard, disorganized outer mineral layer. Amelogenin cleavage products are the only matrix components absent in both Amelx-/- and Mmp20-/- mice. We conclude that MMP20 and amelogenin are not critical for enamel mineral ribbon initiation, orientation, or initial shape. The pathological fan-like plates in these mice may form from the lack of amelogenin cleavage products, which appear necessary to form ordered hydroxyapatite.

Tooth Enamel and its Dynamic Protein Matrix

Tooth enamel is the outer covering of tooth crowns, the hardest material in the mammalian body, yet fracture resistant. The extremely high content of 95 wt% calcium phosphate in healthy adult teeth is achieved through mineralization of a proteinaceous matrix that changes in abundance and composition. Enamel-specific proteins and proteases are known to be critical for proper enamel formation. Recent proteomics analyses revealed many other proteins with their roles in enamel formation yet to be unraveled. Although the exact protein composition of healthy tooth enamel is still unknown, it is apparent that compromised enamel deviates in amount and composition of its organic material. Why these differences affect both the mineralization process before tooth eruption and the properties of erupted teeth will become apparent as proteomics protocols are adjusted to the variability between species, tooth size, sample size and ephemeral organic content of forming teeth. This review summarizes the current knowledge and published proteomics data of healthy and diseased tooth enamel, including advancements in forensic applications and disease models in animals. A summary and discussion of the status quo highlights how recent proteomics findings advance our understating of the complexity and temporal changes of extracellular matrix composition during tooth enamel formation.

How Fluoride Protects Dental Enamel from Demineralization

Introduction:

How fluoride (F^–) protects dental enamel from caries is here conveyed to dental health-care providers by making simplifying approximations that accurately convey the essential principles, without obscuring them in a myriad of qualifications.

Materials and Methods:

We approximate that dental enamel is composed of calcium hydroxyapatite (HAP), a sparingly soluble ionic solid with the chemical formula Ca₁₀(PO₄)₆(OH)₂.

Results:

The electrostatic forces binding ionic solids together are described by Coulomb’s law, which shows that attractions between opposite charges increase greatly as their separation decreases. Relatively large phosphate ions (PO₄^3–) dominate the structure of HAP, which approximates a hexagonal close-packed structure. The smaller Ca²⁺ and OH^– ions fit into the small spaces (interstices) between phosphates, slightly expanding the close-packed structure. F^– ions are smaller than OH^– ions, so substituting F^– for OH^– allows packing the same number of ions into a smaller volume, increasing their forces of attraction. Dental decay results from tipping the solubility equilibrium Ca₁₀(PO₄)₆(OH)₂ (s) ⇔ 10Ca²⁺ (aq) + 6PO₄^2– (aq) + 2OH^– (aq) toward dissolution. HAP dissolves when the product of its ion concentrations, [Ca²⁺]¹⁰×[PO₄^3–]⁶×[OH^–]², falls below the solubility product constant (Ksp) for HAP.

Conclusion:

Because of its more compact crystal structure, the Ksp for fluorapatite (FAP) is lower than the Ksp for HAP, so its ion product, [Ca²⁺]¹⁰×[PO₄^3–]⁶×[F^–]², must fall further before demineralization can occur. Lowering the pH of the fluid surrounding enamel greatly reduces [PO₄^3–] (lowering the ion products of HAP and FAP equally), but [OH^–] falls much more rapidly than [F^–], so FAP better resists acid attack.

Amelogenin phosphorylation regulates tooth enamel formation by stabilizing a transient amorphous mineral precursor

Dental enamel comprises interwoven arrays of extremely long and narrow crystals of carbonated hydroxyapatite called enamel rods. Amelogenin (AMELX) is the predominant extracellular enamel matrix protein and plays an essential role in enamel formation (amelogenesis). Previously, we have demonstrated that full-length AMELX forms higher-order supramolecular assemblies that regulate ordered mineralization in vitro, as observed in enamel rods. Phosphorylation of the sole AMELX phosphorylation site (Ser-16) in vitro greatly enhances its capacity to stabilize amorphous calcium phosphate (ACP), the first mineral phase formed in developing enamel, and prevents apatitic crystal formation. To test our hypothesis that AMELX phosphorylation is critical for amelogenesis, we generated and characterized a hemizygous knockin (KI) mouse model with a phosphorylation-defective Ser-16 to Ala-16 substitution in AMELX. Using EM analysis, we demonstrate that in the absence of phosphorylated AMELX, KI enamel lacks enamel rods, the hallmark component of mammalian enamel, and, unlike WT enamel, appears to be composed of less organized arrays of shorter crystals oriented normal to the dentinoenamel junction. KI enamel also exhibited hypoplasia and numerous surface defects, whereas heterozygous enamel displayed highly variable mosaic structures with both KI and WT features. Importantly, ACP-to-apatitic crystal transformation occurred significantly faster in KI enamel. Secretory KI ameloblasts also lacked Tomes' processes, consistent with the absence of enamel rods, and underwent progressive cell pathology throughout enamel development. In conclusion, AMELX phosphorylation plays critical mechanistic roles in regulating ACP-phase transformation and enamel crystal growth, and in maintaining ameloblast integrity and function during amelogenesis.

Trafficking and secretion of keratin 75 by ameloblasts in vivo

A highly specialized cytoskeletal protein, keratin 75 (K75), expressed primarily in hair follicles, nail beds, and lingual papillae, was recently discovered in dental enamel, the most highly mineralized hard tissue in the human body. Among many questions this discovery poses, the fundamental question regarding the trafficking and secretion of this protein, which lacks a signal peptide, is of an utmost importance. Here, we present evidence that K75 is expressed during the secretory stage of enamel formation and is present in the forming enamel matrix. We further show that K75 is secreted together with major enamel matrix proteins amelogenin and ameloblastin, and it was detected in Golgi and the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) but not in rough ER (rER). Inhibition of ER-Golgi transport by brefeldin A did not affect the association of K75 with Golgi, whereas ameloblastin accumulated in rER, and its transport from rER into Golgi was disrupted. Together, these results indicate that K75, a cytosolic protein lacking a signal sequence, is secreted into the forming enamel matrix utilizing portions of the conventional ER-Golgi secretory pathway. To the best of our knowledge, this is the first study providing insights into mechanisms of keratin secretion.