Ultrastructure of early amelogenesis in wild-type, Amelx-/-, and Enam-/- mice: enamel ribbon initiation on dentin mineral and ribbon orientation by ameloblasts

ENAM Mutations Can Cause Hypomaturation Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a diverse group of inherited diseases featured by various presentations of enamel malformations that are caused by disturbances at different stages of enamel formation. While hypoplastic AI suggests a thickness defect of enamel resulting from aberrations during the secretory stage of amelogenesis, hypomaturation AI indicates a deficiency of enamel mineralization and hardness established at the maturation stage. Mutations in ENAM, which encodes the largest enamel matrix protein, enamelin, have been demonstrated to cause generalized or local hypoplastic AI. Here, we characterized 2 AI families with disparate hypoplastic and hypomaturation enamel defects and identified 2 distinct indel mutations at the same location of ENAM, c588+1del and c.588+1dup. Minigene splicing assays demonstrated that they caused frameshifts and truncation of ENAM proteins, p.Asn197Ilefs*81 and p.Asn197Glufs*25, respectively. In situ hybridization of Enam on mouse mandibular incisors confirmed its restricted expression in secretory stage ameloblasts and suggested an indirect pathogenic mechanism underlying hypomaturation AI. In silico analyses indicated that these 2 truncated ENAMs might form amyloid structures and cause protein aggregation with themselves and with wild-type protein through the added aberrant region at their C-termini. Consistently, protein secretion assays demonstrated that the truncated proteins cannot be properly secreted and impede secretion of wild-type ENAM. Moreover, compared to the wild-type, overexpression of the mutant proteins significantly increased endoplasmic reticulum stress and upregulated the expression of unfolded protein response (UPR)-related genes and TNFRSF10B, a UPR-controlled proapoptotic gene. Caspase, terminal deoxynucleotidyl transferase UTP nick-end labeling (TUNEL), and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assays further revealed that both truncated proteins, especially p.Asn197Ilefs*81, induced cell apoptosis and decreased cell survival, suggesting that the 2 ENAM mutations cause AI through ameloblast cell pathology and death rather than through a simple loss of function. This study demonstrates that an ENAM mutation can lead to generalized hypomaturation enamel defects and suggests proteinopathy as a potential pathogenesis for ENAM-associated AI.

Ganoin and acrodin formation on scales and teeth in spotted gar: A vital role of enamelin in the unique process of enamel mineralization

Gars and bichirs develop scales and teeth with ancient actinopterygian characteristics. Their scale surface and tooth collar are covered with enamel, also known as ganoin, whereas the tooth cap is equipped with an enamel-like tissue, acrodin. Here, we investigated the formation and mineralization of the ganoin and acrodin matrices in spotted gar, and the evolution of the scpp5, ameloblastin (ambn), and enamelin (enam) genes, which encode matrix proteins of ganoin. Results suggest that, in bichirs and gars, all these genes retain structural characteristics of their orthologs in stem actinopterygians, presumably reflecting the presence of ganoin on scales and teeth. During scale formation, Scpp5 and Enam were initially found in the incipient ganoin matrix and the underlying collagen matrix, whereas Ambn was detected mostly in a surface region of the well-developed ganoin matrix. Although collagen is the principal acrodin matrix protein, Scpp5 was detected within the matrix. Similarities in timings of mineralization and the secretion of Scpp5 suggest that acrodin evolved by the loss of the matrix secretory stage of ganoin formation: dentin formation is immediately followed by the maturation stage. The late onset of Ambn secretion during ganoin formation implies that Ambn is not essential for mineral ribbon formation, the hallmark of the enamel matrix. Furthermore, Scpp5 resembles amelogenin that is not important for the initial formation of mineral ribbons in mammals. It is thus likely that the evolution of ENAM was vital to the origin of the unique mineralization process of the enamel matrix.

Dentin defects caused by a Dspp-1 frameshift mutation are associated with the activation of autophagy

Dentin sialophosphoprotein (DSPP) is primarily expressed by differentiated odontoblasts (dentin-forming cells), and transiently expressed by presecretory ameloblasts (enamel-forming cells). Disease-causing DSPP mutations predominantly fall into two categories: 5' mutations affecting targeting and trafficking, and 3' - 1 frameshift mutations converting the repetitive, hydrophilic, acidic C-terminal domain into a hydrophobic one. We characterized the dental phenotypes and investigated the pathological mechanisms of DsppP19L and Dspp-1fs mice that replicate the two categories of human DSPP mutations. In DsppP19L mice, dentin is less mineralized but contains dentinal tubules. Enamel mineral density is reduced. Intracellular accumulation and ER retention of DSPP is observed in odontoblasts and ameloblasts. In Dspp-1fs mice, a thin layer of reparative dentin lacking dentinal tubules is deposited. Odontoblasts show severe pathosis, including intracellular accumulation and ER retention of DSPP, strong ubiquitin and autophagy activity, ER-phagy, and sporadic apoptosis. Ultrastructurally, odontoblasts show extensive autophagic vacuoles, some of which contain fragmented ER. Enamel formation is comparable to wild type. These findings distinguish molecular mechanisms underlying the dental phenotypes of DsppP19L and Dspp-1fs mice and support the recently revised Shields classification of dentinogenesis imperfecta caused by DSPP mutations in humans. The Dspp-1fs mice may be valuable for the study of autophagy and ER-phagy.

A Suite of Mouse Reagents for Studying Amelogenesis

Amelogenesis, the formation of dental enamel, is driven by specialized epithelial cells called ameloblasts, which undergo successive stages of differentiation. Ameloblasts secrete enamel matrix proteins (EMPs), proteases, calcium, and phosphate ions in a stage-specific manner to form mature tooth enamel. Developmental defects in tooth enamel are common in humans, and they can greatly impact the well-being of affected individuals. Our understanding of amelogenesis and developmental pathologies is rooted in past studies using epithelial Cre driver and knockout alleles. However, the available mouse models are limited, as most do not allow targeting different ameloblast sub-populations, and constitutive loss of EMPs often results in severe phenotype in the mineral, making it difficult to interpret defect mechanisms. Herein, we report on the design and verification of a toolkit of twelve mouse alleles that include ameloblast-stage specific Cre recombinases, fluorescent reporter alleles, and conditional flox alleles for the major EMPs. We show how these models may be used for applications such as sorting of live stage specific ameloblasts, whole mount imaging, and experiments with incisor explants. The full list of new alleles is available at https://dev.facebase.org/enamelatlas/mouse-models/ .

Enamel defects in Acp4R110C/R110C mice and human ACP4 mutations

Human ACP4 (OMIM*606362) encodes a transmembrane protein that belongs to histidine acid phosphatase (ACP) family. Recessive mutations in ACP4 cause non-syndromic hypoplastic amelogenesis imperfecta (AI1J, OMIM#617297). While ACP activity has long been detected in developing teeth, its functions during tooth development and the pathogenesis of ACP4-associated AI remain largely unknown. Here, we characterized 2 AI1J families and identified a novel ACP4 disease-causing mutation: c.774_775del, p.Gly260Aspfs*29. To investigate the role of ACP4 during amelogenesis, we generated and characterized Acp4R110C mice that carry the p.(Arg110Cys) loss-of-function mutation. Mouse Acp4 expression was the strongest at secretory stage ameloblasts, and the protein localized primarily at Tomes' processes. While Acp4 heterozygous (Acp4+/R110C) mice showed no phenotypes, incisors and molars of homozygous (Acp4R110C/R110C) mice exhibited a thin layer of aplastic enamel with numerous ectopic mineralized nodules. Acp4R110C/R110C ameloblasts appeared normal initially but underwent pathology at mid-way of secretory stage. Ultrastructurally, sporadic enamel ribbons grew on mineralized dentin but failed to elongate, and aberrant needle-like crystals formed instead. Globs of organic matrix accumulated by the distal membranes of defective Tomes' processes. These results demonstrated a critical role for ACP4 in appositional growth of dental enamel probably by processing and regulating enamel matrix proteins around mineralization front apparatus.

The Modified Shields Classification and 12 Families with Defined DSPP Mutations

Mutations in Dentin Sialophosphoprotein (DSPP) are known to cause, in order of increasing severity, dentin dysplasia type-II (DD-II), dentinogenesis imperfecta type-II (DGI-II), and dentinogenesis imperfecta type-III (DGI-III). DSPP mutations fall into two groups: a 5′-group that affects protein targeting and a 3′-group that shifts translation into the −1 reading frame. Using whole-exome sequence (WES) analyses and Single Molecule Real-Time (SMRT) sequencing, we identified disease-causing DSPP mutations in 12 families. Three of the mutations are novel: c.53T>C/p.(Val18Ala); c.3461delG/p.(Ser1154Metfs*160); and c.3700delA/p.(Ser1234Alafs*80). We propose genetic analysis start with WES analysis of proband DNA to identify mutations in COL1A1 and COL1A2 causing dominant forms of osteogenesis imperfecta, 5′-DSPP mutations, and 3′-DSPP frameshifts near the margins of the DSPP repeat region, and SMRT sequencing when the disease-causing mutation is not identified. After reviewing the literature and incorporating new information showing distinct differences in the cell pathology observed between knockin mice with 5′-Dspp or 3′-Dspp mutations, we propose a modified Shields Classification based upon the causative mutation rather than phenotypic severity such that patients identified with 5′-DSPP defects be diagnosed as DGI-III, while those with 3′-DSPP defects be diagnosed as DGI-II.

Amyloid-like amelogenin nanoribbons template mineralization via a low-energy interface of ion binding sites

Protein scaffolds direct the organization of amorphous precursors that transform into mineralized tissues, but the templating mechanism remains elusive. Motivated by models for the biomineralization of tooth enamel, wherein amyloid-like amelogenin nanoribbons guide the mineralization of apatite filaments, we investigated the impact of nanoribbon structure, sequence, and chemistry on amorphous calcium phosphate (ACP) nucleation. Using full-length human amelogenin and peptide analogs with an amyloid-like domain, films of β-sheet nanoribbons were self-assembled on graphite and characterized by in situ atomic force microscopy and molecular dynamics simulations. All sequences substantially reduce nucleation barriers for ACP by creating low-energy interfaces, while phosphoserines along the length of the nanoribbons dramatically enhance kinetic factors associated with ion binding. Furthermore, the distribution of negatively charged residues along the nanoribbons presents a potential match to the Ca–Ca distances of the multi-ion complexes that constitute ACP. These findings show that amyloid-like amelogenin nanoribbons provide potent scaffolds for ACP mineralization by presenting energetically and stereochemically favorable templates of calcium phosphate ion binding and suggest enhanced surface wetting toward calcium phosphates in general.

Applications of scanning electron microscopy and focused ion beam milling in dental research

The abilities of scanning electron microscopy (SEM) and focused ion beam (FIB) milling for obtaining high-resolution images from top surfaces, cross-sectional surfaces, and even in three dimensions, are becoming increasingly important for imaging and analyzing tooth structures such as enamel and dentin. FIB was originally developed for material research in the semiconductor industry. However, use of SEM/FIB has been growing recently in dental research due to the versatility of dual platform instruments that can be used as a milling device to obtain low-artifact cross-sections of samples combined with high-resolution images. The advent of the SEM/FIB system and accessories may offer access to previously inaccessible length scales for characterizing tooth structures for dental research, opening exciting opportunities to address many central questions in dental research. New discoveries and fundamental breakthroughs in understanding are likely to follow. This review covers the applications, key findings, and future direction of SEM/FIB in dental research in morphology imaging, specimen preparation for transmission electron microscopy (TEM) analysis, and three-dimensional volume imaging using SEM/FIB tomography.

Pathophysiology of Demineralization, Part I: Attrition, Erosion, Abfraction, and Noncarious Cervical Lesions

PURPOSE OF THE REVIEW

Compare pathophysiology for infectious and noninfectious demineralization disease relative to mineral maintenance, physiologic fluoride levels, and mechanical degradation.

RECENT FINDINGS

Environmental acidity, biomechanics, and intercrystalline percolation of endemic fluoride regulate resistance to demineralization relative to osteopenia, noncarious cervical lesions, and dental caries. Demineralization is the most prevalent chronic disease in the world: osteoporosis (OP) >10%, dental caries ~100%. OP is severely debilitating while caries is potentially fatal. Mineralized tissues have a common physiology: cell-mediated apposition, protein matrix, fluid logistics (blood, saliva), intercrystalline ion percolation, cyclic demineralization/remineralization, and acid-based degradation (microbes, clastic cells). Etiology of demineralization involves fluid percolation, metabolism, homeostasis, biomechanics, mechanical wear (attrition or abrasion), and biofilm-related infections. Bone mineral density measurement assesses skeletal mass. Attrition, abrasion, erosion, and abfraction are diagnosed visually, but invisible subsurface caries <400μm cannot be detected. Controlling demineralization at all levels is an important horizon for cost-effective wellness worldwide.

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.

A N-Terminus Domain Determines Amelogenin's Stability to Guide the Development of Mouse Enamel Matrix

Amelogenins, the principal proteins in the developing enamel microenvironment, self-assemble into supramolecular structures to govern the remodeling of a proteinaceous organic matrix into longitudinally ordered hydroxyapatite nanocrystal arrays. Extensive in vitro studies using purified native or recombinant proteins have revealed the potential of N-terminal amelogenin on protein self-assembly and its ability to guide the mineral deposition. We have previously identified a 14-aa domain (P2) of N-terminal amelogenin that can self-assemble into amyloid-like fibrils in vitro. Here, we investigated how this domain affects the ability of amelogenin self-assembling and stability of enamel matrix protein scaffolding in an in vivo animal model. Mice harboring mutant amelogenin lacking P2 domain had a hypoplastic, hypomineralized, and aprismatic enamel. In vitro, the mutant recombinant amelogenin without P2 had a reduced tendency to self-assemble and was prone to accelerated hydrolysis by MMP20, the prevailing metalloproteinase in early developing enamel matrix. A reduced amount of amelogenins and a lack of elongated fibrous assemblies in the development enamel matrix of mutant mice were evident compared with that in the wild-type mouse enamel matrix. Our study is the first to demonstrate that a subdomain (P2) at the N-terminus of amelogenin controls amelogenin's assembly into a transient protein scaffold that resists rapid proteolysis during enamel development in an animal model. Understanding the building blocks of fibrous scaffold that guides the longitudinal growth of hydroxyapatites in enamel matrix sheds light on protein-mediated enamel bioengineering. © 2021 American Society for Bone and Mineral Research (ASBMR).

Biomimetic mineralisation systems for in situ enamel restoration inspired by amelogenesis

Caries and dental erosion are common oral diseases. Traditional treatments involve the mechanical removal of decay and filling but these methods are not suitable for cases involving large-scale enamel erosion, such as hypoplasia. To develop a noninvasive treatment, promoting remineralisation in the early stage of caries is of considerable clinical significance. Therefore, biomimetic mineralisation is an ideal approach for restoring enamel. Biomimetic mineralisation forms a new mineral layer that is tightly attached to the surface of the enamel. This review details the state-of-art achievements on the application of amelogenin and non-amelogenin, amorphous calcium phosphate, ions flow and other techniques in the biomimetic mineralisation of enamel. The ultimate goal of this review was to shed light on the requirements for enamel biomineralisation. Hence, herein, we summarise two strategies of biological minimisation systems for in situ enamel restoration inspired by amelogenesis that have been developed in recent years and compare their advantages and disadvantages.

Manifestation and Mechanisms of Abnormal Mineralization in Teeth

Tooth biomineralization is a dynamic and complicated process influenced by local and systemic factors. Abnormal mineralization in teeth occurs when factors related to physiologic mineralization are altered during tooth formation and after tooth maturation, resulting in microscopic and macroscopic manifestations. The present Review provides timely information on the mechanisms and structural alterations of different forms of pathological tooth mineralization. A comprehensive study of these alterations benefits diagnosis and biomimetic treatment of abnormal mineralization in patients.

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.

Mechanisms of Enamel Mineralization Guided by Amelogenin Nanoribbons

The nanofibrous nature and its intricate structural organization are the basis for the extraordinary ability of sound enamel to outlive masticatory forces at minimal failure rates. Apatite nanofibers of several hundreds of micrometers to possibly millimeters in length originate during the secretory stage of amelogenesis as 2-nm-thin and 15-nm-wide ribbons that develop and grow in length under the guidance of a dynamic mixture of specialized proteins, the developing enamel matrix (DEM). A critical role in the unidirectional and oriented growth of enamel mineral ribbons has been attributed to amelogenin, the major constituent of the DEM. This review elaborates on recent studies on the ability of ribbon-like assemblies of amelogenin to template the formation of an amorphous calcium phosphate precursor that transforms into apatite mineral ribbons similar to the ones observed in developing enamel. A mechanistic model of the biological processes that drive biomineralization in enamel is presented in the context of a comparative analysis of enamel mouse models and earlier structural data of the DEM emphasizing a regulatory role of the matrix metalloproteinase 20 in mineral deposition and the involvement of a process-directing agent for the templated mineral growth directed by amelogenin nanoribbons.

Coevolution of enamel, ganoin, enameloid, and their matrix SCPP genes in osteichthyans

We resolve debate over the evolution of vertebrate hypermineralized tissues through analyses of matrix protein-encoding secretory calcium-binding phosphoprotein (SCPP) genes and phylogenetic inference of hypermineralized tissues. Among these genes, AMBN and ENAM are found in both sarcopterygians and actinopterygians, whereas AMEL and SCPP5 are found only in sarcopterygians and actinopterygians, respectively. Actinopterygian AMBN, ENAM, and SCPP5 are expressed during the formation of hypermineralized tissues on scales and teeth: ganoin, acrodin, and collar enamel in gar, and acrodin and collar enameloid in zebrafish. Our phylogenetic analyses indicate the emergence of an ancestral enamel in stem-osteichthyans, whereas ganoin emerged in stem-actinopterygians and true enamel in stem-sarcopterygians. Thus, AMBN and ENAM originated in concert with ancestral enamel, SCPP5 evolved in association with ganoin, and AMEL evolved with true enamel. Shifts in gene expression domain and timing explain the evolution of different hypermineralized tissues. We propose that hypermineralized tissues in osteichthyans coevolved with matrix SCPP genes.

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Ganoin emerged in actinopterygians; true enamel arose in sarcopterygians

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Dental enamel, acrodin, and enameloid in actinopterygians are related to ganoin

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SCPP5 evolved in association with ganoin, whereas AMEL evolved with true enamel

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Shifts in SCPP gene expression explain the evolution of hypermineralized tissues

Odontogenesis-associated phosphoprotein truncation blocks ameloblast transition into maturation in OdaphC41*/C41* mice

Mutations of Odontogenesis-Associated Phosphoprotein (ODAPH, OMIM *614829) cause autosomal recessive amelogenesis imperfecta, however, the function of ODAPH during amelogenesis is unknown. Here we characterized normal Odaph expression by in situ hybridization, generated Odaph truncation mice using CRISPR/Cas9 to replace the TGC codon encoding Cys41 into a TGA translation termination codon, and characterized and compared molar and incisor tooth formation in Odaph^+/+, Odaph^+/C41*, and Odaph^C41*/C41* mice. We also searched genomes to determine when Odaph first appeared phylogenetically. We determined that tooth development in Odaph^+/+ and Odaph^+/C41* mice was indistinguishable in all respects, so the condition in mice is inherited in a recessive pattern, as it is in humans. Odaph is specifically expressed by ameloblasts starting with the onset of post-secretory transition and continues until mid-maturation. Based upon histological and ultrastructural analyses, we determined that the secretory stage of amelogenesis is not affected in Odaph^C41*/C41* mice. The enamel layer achieves a normal shape and contour, normal thickness, and normal rod decussation. The fundamental problem in Odaph^C41*/C41* mice starts during post-secretory transition, which fails to generate maturation stage ameloblasts. At the onset of what should be enamel maturation, a cyst forms that separates flattened ameloblasts from the enamel surface. The maturation stage fails completely.

Breaking new ground in mineralized tissue: Assessing tissue quality in clinical and laboratory studies

Mineralized tissues, such as bone and teeth, have extraordinary mechanical properties of both strength and toughness. This mechanical behavior originates from deformation and fracture resistance mechanisms in their multi-scale structure. The term quality describes the matrix composition, multi-scale structure, remodeling dynamics, water content, and micro-damage accumulation in the tissue. Aging and disease result in changes in the tissue quality that may reduce strength and toughness and lead to elevated fracture risk. Therefore, the capability to measure the quality of mineralized tissues provides critical information on disease progression and mechanical integrity. Here, we provide an overview of clinical and laboratory-based techniques to assess the quality of mineralized tissues in health and disease. Current techniques used in clinical settings include radiography-based (radiographs, dual energy x-ray absorptiometry, EOS) and x-ray tomography-based methods (high resolution peripheral quantitative computed tomography, cone beam computed tomography). In the laboratory, tissue quality can be investigated in ex vivo samples with x-ray imaging (micro and nano-computed tomography, x-ray microscopy), electron microscopy (scanning/transmission electron imaging (SEM/STEM), backscattered scanning electron microscopy, Focused Ion Beam-SEM), light microscopy, spectroscopy (Raman spectroscopy and Fourier transform infrared spectroscopy) and assessment of mechanical behavior (mechanical testing, fracture mechanics and reference point indentation). It is important for clinicians and basic science researchers to be aware of the techniques available in different types of research. While x-ray imaging techniques translated to the clinic have provided exceptional advancements in patient care, the future challenge will be to incorporate high-resolution laboratory-based bone quality measurements into clinical settings to broaden the depth of information available to clinicians during diagnostics, treatment and management of mineralized tissue pathologies.

Protein nanoribbons template enamel mineralization

As the hardest tissue formed by vertebrates, enamel represents nature's engineering masterpiece with complex organizations of fibrous apatite crystals at the nanometer scale. Supramolecular assemblies of enamel matrix proteins (EMPs) play a key role as the structural scaffolds for regulating mineral morphology during enamel development. However, to achieve maximum tissue hardness, most organic content in enamel is digested and removed at the maturation stage, and thus knowledge of a structural protein template that could guide enamel mineralization is limited at this date. Herein, by examining a gene-modified mouse that lacked enzymatic degradation of EMPs, we demonstrate the presence of protein nanoribbons as the structural scaffolds in developing enamel matrix. Using in vitro mineralization assays we showed that both recombinant and enamel-tissue-based amelogenin nanoribbons are capable of guiding fibrous apatite nanocrystal formation. In accordance with our understanding of the natural process of enamel formation, templated crystal growth was achieved by interaction of amelogenin scaffolds with acidic macromolecules that facilitate the formation of an amorphous calcium phosphate precursor which gradually transforms into oriented apatite fibers along the protein nanoribbons. Furthermore, this study elucidated that matrix metalloproteinase-20 is a critical regulator of the enamel mineralization as only a recombinant analog of a MMP20-cleavage product of amelogenin was capable of guiding apatite mineralization. This study highlights that supramolecular assembly of the scaffold protein, its enzymatic processing, and its ability to interact with acidic carrier proteins are critical steps for proper enamel development.

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.

The Evolution of Unusually Small Amelogenin Genes in Cetaceans; Pseudogenization, X-Y Gene Conversion, and Feeding Strategy

Among extant cetaceans, mysticetes are filter feeders that do not possess teeth and use their baleen for feeding, while most odontocetes are considered suction feeders, which capture prey by suction without biting or chewing with teeth. In the present study, we address the functionality of amelogenin (AMEL) genes in cetaceans. AMEL encodes a protein that is specifically involved in dental enamel formation and is located on the sex chromosomes in eutherians. The X-copy AMELX is functional in enamel-bearing eutherians, whereas the Y-copy AMELY appears to have undergone decay and was completely lost in some species. Consistent with these premises, we detected various deleterious mutations and/or non-canonical splice junctions in AMELX of mysticetes and four suction feeding odontocetes, Delphinapterus leucas, Monodon monoceros, Kogia breviceps, and Physeter macrocephalus, and in AMELY of mysticetes and odontocetes. Regardless of the functionality, both AMELX and AMELY are equally and unusually small in cetaceans, and even their functional AMELX genes presumably encode a degenerate core region, which is thought to be essential for enamel matrix assembly and enamel crystal growth. Furthermore, our results suggest that the most recent common ancestors of extant cetaceans had functional AMELX and AMELY, both of which are similar to AMELX of Platanista minor. Similar small AMELX and AMELY in archaic cetaceans can be explained by gene conversion between AMELX and AMELY. We speculate that common ancestors of modern cetaceans employed a degenerate AMELX, transferred from a decaying AMELY by gene conversion, at an early stage of their transition to suction feeders.

ENAM mutations and digenic inheritance

BACKGROUND

ENAM mutations cause autosomal dominant or recessive amelogenesis imperfecta (AI) and show a dose effect: enamel malformations are more severe or only penetrant when both ENAM alleles are defective.

METHODS

Whole exome sequences of recruited AI probands were initially screened for mutations in known AI candidate genes. Sanger sequencing was used to confirm sequence variations and their segregation with the disease phenotype. The co-occurrence of ENAM and LAMA3 mutations in one family raised the possibility of digenic inheritance. Enamel formed in Enam+/+ Ambn+/+ , Enam+/- , Ambn+/- , and Enam+/- Ambn+/- mice was characterized by dissection and backscattered scanning electron microscopy (bSEM).

RESULTS

ENAM mutations segregating with AI in five families were identified. Two novel ENAM frameshift mutations were identified. A single-nucleotide duplication (c.395dupA/p.Pro133Alafs*13) replaced amino acids 133-1142 with a 12 amino acid (ATTKAAFEAAIT*) sequence, and a single-nucleotide deletion (c.2763delT/p.Asp921Glufs*32) replaced amino acids 921-1142 with 31 amino acids (ESSPQQASYQAKETAQRRGKAKTLLEMMCPR*). Three families were heterozygous for a previously reported single-nucleotide ENAM deletion (c.588+1delG/p.Asn197Ilefs*81). One of these families also harbored a heterozygous LAMA3 mutation (c.1559G>A/p.Cys520Tyr) that cosegregated with both the AI phenotype and the ENAM mutation. In mice, Ambn+/- maxillary incisors were normal. Ambn+/- molars were also normal, except for minor surface roughness. Ambn+/- mandibular incisors were sometimes chalky and showed minor chipping. Enam+/- incisor enamel was thinner than normal with ectopic mineral deposited laterally. Enam+/- molars were sometimes chalky and rough surfaced. Enam+/- Ambn+/- enamel was thin and rough, in part due to ectopic mineralization, but also underwent accelerated attrition.

CONCLUSION

Novel ENAM mutations causing AI were identified, raising to 22 the number of ENAM variations known to cause AI. The severity of the enamel phenotype in Enam+/- Ambn+/- double heterozygous mice is caused by composite digenic effects. Digenic inheritance should be explored as a cause of AI in humans.

A large pool of actively cycling progenitors orchestrates self-renewal and injury repair of an ectodermal appendage

The classical model of tissue renewal posits that small numbers of quiescent stem cells (SCs) give rise to proliferating transit-amplifying cells before terminal differentiation. However, many organs house pools of SCs with proliferative and differentiation potentials that diverge from this template. Resolving SC identity and organization is therefore central to understanding tissue renewal. Here, using a combination of single-cell RNA sequencing (scRNA-seq), mouse genetics and tissue injury approaches, we uncover cellular hierarchies and mechanisms that underlie the maintenance and repair of the continuously growing mouse incisor. Our results reveal that, during homeostasis, a group of actively cycling epithelial progenitors generates enamel-producing ameloblasts and adjacent layers of non-ameloblast cells. After injury, tissue repair was achieved through transient increases in progenitor-cell proliferation and through direct conversion of Notch1-expressing cells to ameloblasts. We elucidate epithelial SC identity, position and function, providing a mechanistic basis for the homeostasis and repair of a fast-turnover ectodermal appendage.

AMBN mutations causing hypoplastic amelogenesis imperfecta and Ambn knockout-NLS-lacZ knockin mice exhibiting failed amelogenesis and Ambn tissue-specificity

BACKGROUND

Ameloblastin (AMBN) is a secreted matrix protein that is critical for the formation of dental enamel and is enamel-specific with respect to its essential functions. Biallelic AMBN defects cause non-syndromic autosomal recessive amelogenesis imperfecta. Homozygous Ambn mutant mice expressing an internally truncated AMBN protein deposit only a soft mineral crust on the surface of dentin.

METHODS

We characterized a family with hypoplastic amelogenesis imperfecta caused by AMBN compound heterozygous mutations (c.1061T>C; p.Leu354Pro/ c.1340C>T; p.Pro447Leu). We generated and characterized Ambn knockout/NLS-lacZ (AmbnlacZ/lacZ ) knockin mice.

RESULTS

No AMBN protein was detected using immunohistochemistry in null mice. ß-galactosidase activity was specific for ameloblasts in incisors and molars, and islands of cells along developing molar roots. AmbnlacZ/lacZ 7-week incisors and unerupted (D14) first molars showed extreme enamel surface roughness. No abnormalities were observed in dentin mineralization or in nondental tissues. Ameloblasts in the AmbnlacZ/lacZ mice were unable to initiate appositional growth and started to degenerate and deposit ectopic mineral. No layer of initial enamel ribbons formed in the AmbnlacZ/lacZ mice, but pockets of amelogenin accumulated on the dentin surface along the ameloblast distal membrane and within the enamel organ epithelia (EOE). NLS-lacZ signal was positive in the epididymis and nasal epithelium, but negative in ovary, oviduct, uterus, prostate, seminal vesicles, testis, submandibular salivary gland, kidney, liver, bladder, and bone, even after 15 hr of incubation with X-gal.

CONCLUSIONS

Ameloblastin is critical for the initiation of enamel ribbon formation, and its absence results in pathological mineralization within the enamel organ epithelia.

Mapping the Tooth Enamel Proteome and Amelogenin Phosphorylation Onto Mineralizing Porcine Tooth Crowns

Tooth enamel forms in an ephemeral protein matrix where changes in protein abundance, composition and posttranslational modifications are critical to achieve healthy enamel properties. Amelogenin (AMELX) with its splice variants is the most abundant enamel matrix protein, with only one known phosphorylation site at serine 16 shown in vitro to be critical for regulating mineralization. The phosphorylated form of AMELX stabilizes amorphous calcium phosphate, while crystalline hydroxyapatite forms in the presence of the unphosphorylated protein. While AMELX regulates mineral transitions over space and time, it is unknown whether and when un-phosphorylated amelogenin occurs during enamel mineralization. This study aims to reveal the spatiotemporal distribution of the cleavage products of the most abundant AMLEX splice variants including the full length P173, the shorter leucine-rich amelogenin protein (LRAP), and the exon 4-containing P190 in forming enamel, all within the context of the changing enamel matrix proteome during mineralization. We microsampled permanent pig molars, capturing known stages of enamel formation from both crown surface and inner enamel. Nano-LC-MS/MS proteomic analyses after tryptic digestion rendered more than 500 unique protein identifications in enamel, dentin, and bone. We mapped collagens, keratins, and proteolytic enzymes (CTSL, MMP2, MMP10) and determined distributions of P173, LRAP, and P190 products, the enamel proteins enamelin (ENAM) and ameloblastin (AMBN), and matrix-metalloprotease-20 (MMP20) and kallikrein-4 (KLK4). All enamel proteins and KLK4 were near-exclusive to enamel and in excellent agreement with published abundance levels. Phosphorylated P173 and LRAP products decreased in abundance from recently deposited matrix toward older enamel, mirrored by increasing abundances of testicular acid phosphatase (ACPT). Our results showed that hierarchical clustering analysis of secretory enamel links closely matching distributions of unphosphorylated P173 and LRAP products with ACPT and non-traditional amelogenesis proteins, many associated with enamel defects. We report higher protein diversity than previously published and Gene Ontology (GO)-defined protein functions related to the regulation of mineral formation in secretory enamel (e.g., casein α-S1, CSN1S1), immune response in erupted enamel (e.g., peptidoglycan recognition protein, PGRP), and phosphorylation. This study presents a novel approach to characterize and study functional relationships through spatiotemporal mapping of the ephemeral extracellular matrix proteome.

Relationships Between Enamel Prism Decussation and Organization of the Ameloblast Layer in Rodent Incisors

Rodent enamel microstructure has been extensively investigated, primarily on the basis of 2D electronic microscopy data. The nature and dynamics of the ameloblasts (the enamel-secreting cells) have also been well studied. However, critical issues still remain surrounding exactly how the ameloblasts produce the astonishing microstructural complexity of enamel, and how this subtle architecture evolved through time. In this article, we used a new methodology based on confocal laser microscopy to reconstruct the enamel microstructure of rodent incisors in three dimensions (3D) with the ameloblasts in situ. We proposed interpretations regarding the possible relationships between the workings of the ameloblasts and the resulting enamel prisms, especially how the phenomenon of decussation is generated. Finally, we were able to represent the two main types of modern rodent incisor microstructures (uniserial and multiserial decussations), as a set of parameters that have been entered into the 3D enamel simulation software Simulenam to generate 3D models that can be digitally manipulated. Associating 2D data of incisor enamel microstructure of fossil rodents and Simulenam, it was then possible to better understand how the various decussation parameters evolved through time and gave rise to the two modern microstructure types from the same ancestral type (pauciserial). This study also confirmed that rodent and artiodactyl enamel do not share the same mechanism of decussation formation. Anat Rec, 302:1195-1209, 2019. © 2018 Wiley Periodicals, Inc.

Enamel formation and growth in non-mammalian cynodonts

The early evolution of mammals is associated with the linked evolutionary origin of diphyodont tooth replacement, rapid juvenile growth and determinate adult growth. However, specific relationships among these characters during non-mammalian cynodont evolution require further exploration. Here, polarized light microscopy revealed incremental lines, resembling daily laminations of extant mammals, in histological sections of enamel in eight non-mammalian cynodont species. In the more basal non-probainognathian group, enamel extends extremely rapidly from cusp to cervix. By contrast, the enamel of mammaliamorphs is gradually accreted, with slow rates of crown extension, more typical of the majority of non-hypsodont crown mammals. These results are consistent with the reduction in dental replacement rate across the non-mammalian cynodont lineage, with greater rates of crown extension required in most non-probainognathians, and slower crown extension rates permitted in mammaliamorphs, which have reduced patterns of dental replacement in comparison with many non-probainognathians. The evolution of mammal-like growth patterns, with faster juvenile growth and more abruptly terminating adult growth, is linked with this reduction in dental replacement rates and may provide an additional explanation for the observed pattern in enamel growth rates. It is possible that the reduction in enamel extension rates in mammaliamorphs reflects an underlying reduction in skeletal growth rates at the time of postcanine formation, due to a more abruptly terminating pattern of adult growth in these more mammal-like, crownward species.

Peptide-Based Bioinspired Approach to Regrowing Multilayered Aprismatic Enamel

The gradual discovery of functional domains in native enamel matrix proteins has enabled the design of smart bioinspired peptides for tooth enamel mimetics and repair. In this study, we expanded upon the concept of biomineralization to design smaller amelogenin-inspired peptides with conserved functional domains for clinical translation. The synthetic peptides displayed a characteristic nanostructured scaffold reminiscent of 'nanospheres' seen in the enamel matrix and effectively controlled apatite nucleation in vitro resulting in the formation of smaller crystallites. Following application of the peptides to sectioned human molar teeth, a robust, oriented, synthetic aprismatic enamel was observed after 7 days of incubation in situ. There was a two-fold increase in the hardness and modulus of the regrown enamel-like apatite layers and an increase in the attachment of the tooth-regrown layer interface compared to control samples. Repeated peptide applications generated multiple enamel-like hydroxyapatite (HAP) layers of limited thickness produced by epitaxial growth in which c-axis oriented nanorods evolved on the surface of native enamel. We conclude that peptide analogues with active domains can effectively regulate the orientation of regenerated HAP layers to influence functional response. Moreover, this enamel biofabrication approach demonstrates the peptide-mediated growth of multiple microscale HAP arrays of organized microarchitecture with potential for enamel repair.

Meeting report: a hard look at the state of enamel research

The Encouraging Novel Amelogenesis Models and Ex vivo cell Lines (ENAMEL) Development workshop was held on 23 June 2017 at the Bethesda headquarters of the National Institute of Dental and Craniofacial Research (NIDCR). Discussion topics included model organisms, stem cells/cell lines, and tissues/3D cell culture/organoids. Scientists from a number of disciplines, representing institutions from across the United States, gathered to discuss advances in our understanding of enamel, as well as future directions for the field.

DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE

Dental enamel is the hardest and most mineralized tissue in extinct and extant vertebrate species and provides maximum durability that allows teeth to function as weapons and/or tools as well as for food processing. Enamel development and mineralization is an intricate process tightly regulated by cells of the enamel organ called ameloblasts. These heavily polarized cells form a monolayer around the developing enamel tissue and move as a single forming front in specified directions as they lay down a proteinaceous matrix that serves as a template for crystal growth. Ameloblasts maintain intercellular connections creating a semi-permeable barrier that at one end (basal/proximal) receives nutrients and ions from blood vessels, and at the opposite end (secretory/apical/distal) forms extracellular crystals within specified pH conditions. In this unique environment, ameloblasts orchestrate crystal growth via multiple cellular activities including modulating the transport of minerals and ions, pH regulation, proteolysis, and endocytosis. In many vertebrates, the bulk of the enamel tissue volume is first formed and subsequently mineralized by these same cells as they retransform their morphology and function. Cell death by apoptosis and regression are the fates of many ameloblasts following enamel maturation, and what cells remain of the enamel organ are shed during tooth eruption, or are incorporated into the tooth's epithelial attachment to the oral gingiva. In this review, we examine key aspects of dental enamel formation, from its developmental genesis to the ever-increasing wealth of data on the mechanisms mediating ionic transport, as well as the clinical outcomes resulting from abnormal ameloblast function.

Endocytosis and Enamel Formation

Enamel formation requires consecutive stages of development to achieve its characteristic extreme mineral hardness. Mineralization depends on the initial presence then removal of degraded enamel proteins from the matrix via endocytosis. The ameloblast membrane resides at the interface between matrix and cell. Enamel formation is controlled by ameloblasts that produce enamel in stages to build the enamel layer (secretory stage) and to reach final mineralization (maturation stage). Each stage has specific functional requirements for the ameloblasts. Ameloblasts adopt different cell morphologies during each stage. Protein trafficking including the secretion and endocytosis of enamel proteins is a fundamental task in ameloblasts. The sites of internalization of enamel proteins on the ameloblast membrane are specific for every stage. In this review, an overview of endocytosis and trafficking of vesicles in ameloblasts is presented. The pathways for internalization and routing of vesicles are described. Endocytosis is proposed as a mechanism to remove debris of degraded enamel protein and to obtain feedback from the matrix on the status of the maturing enamel.

Enamel: Molecular identity of its transepithelial ion transport system

Enamel is the most calcified tissue in vertebrates. It differs from bone in a number of characteristics including its origin from ectodermal epithelium, lack of remodeling capacity by the enamel forming cells, and absence of collagen. The enamel-forming cells known as ameloblasts, choreograph first the synthesis of a unique protein-rich matrix, followed by the mineralization of this matrix into a tissue that is ∼95% mineral. To do this, ameloblasts arrange the coordinated movement of ions across a cell barrier while removing matrix proteins and monitoring extracellular pH using a variety of buffering systems to enable the growth of carbonated apatite crystals. Although our knowledge of these processes and the molecular identity of the proteins involved in transepithelial ion transport has increased in the last decade, it remains limited compared to other cells. Here we present an overview of the evolution and development of enamel, its differences with bone, and describe the ion transport systems associated with ameloblasts.

SCPP Genes and Their Relatives in Gar: Rapid Expansion of Mineralization Genes in Osteichthyans

Gar is an actinopterygian that has bone, dentin, enameloid, and ganoin (enamel) in teeth and/or scales. Mineralization of these tissues involves genes encoding various secretory calcium-binding phosphoproteins (SCPPs) in osteichthyans, but no SCPP genes have been identified in chondrichthyans to date. In the gar genome, we identified 38 SCPP genes, seven of which encode "acidic-residue-rich" proteins and 31 encode "Pro/Gln (P/Q) rich" proteins. These gar SCPP genes constitute the largest known repertoire, including many newly identified P/Q-rich genes expressed in teeth and/or scales. Among gar SCPP genes, six acidic and three P/Q-rich genes were identified as orthologs of sarcopterygian genes. The sarcopterygian orthologs of most of these acidic genes are involved in bone and/or dentin formation, and sarcopterygian orthologs of all three P/Q-rich genes participate in enamel formation. The finding of these genes in gar suggests that an elaborate SCPP gene-based genetic system for tissue mineralization was already present in stem osteichthyans. While SCPP genes have been thought to originate from ancient SPARCL1, SPARCL1L1 appears to be more closely related to these genes, because it established a structure similar to acidic SCPP genes probably in stem gnathostomes, perhaps at about the same time with the origin of tissue mineralization. Assuming enamel evolved in stem osteichthyans, all P/Q-rich SCPP genes likely arose within the osteichthyan lineage. Furthermore, the absence of acidic SCPP genes in chondrichthyans might be explained by the secondary loss of earliest acidic genes. It appears that many SCPP genes expanded rapidly in stem osteichthyans and in basal actinopterygians.

Preparative SDS PAGE as an Alternative to His-Tag Purification of Recombinant Amelogenin

Recombinant protein technology provides an invaluable source of proteins for use in structure-function studies, as immunogens, and in the development of therapeutics. Recombinant proteins are typically engineered with “tags” that allow the protein to be purified from crude host cell extracts using affinity based chromatography techniques. Amelogenin is the principal component of the developing enamel matrix and a frequent focus for biomineralization researchers. Several groups have reported the successful production of recombinant amelogenins but the production of recombinant amelogenin free of any tags, and at single band purity on silver stained SDS PAGE is technically challenging. This is important, as rigorous structure-function research frequently demands a high degree of protein purity and fidelity of protein sequence. Our aim was to generate His-tagged recombinant amelogenin at single band purity on silver stained SDS PAGE for use in functionality studies after His-tag cleavage. An acetic acid extraction technique (previously reported to produce recombinant amelogenin at 95% purity directly from E. coli) followed by repeated rounds of nickel column affinity chromatography, failed to generate recombinant amelogenin at single band purity. This was because following an initial round of nickel column affinity chromatography, subsequent cleavage of the His-tag was not 100% efficient. A second round of nickel column affinity chromatography, used in attempts to separate the cleaved His-tag free recombinant from uncleaved His-tagged contaminants, was still unsatisfactory as cleaved recombinant amelogenin exhibited significant affinity for the nickel column. To solve this problem, we used preparative SDS PAGE to successfully purify cleaved recombinant amelogenins to single band purity on silver stained SDS PAGE. The resolving power of preparative SDS PAGE was such that His-tag based purification of recombinant amelogenin becomes redundant. We suggest that acetic acid extraction of recombinant amelogenin and subsequent purification using preparative SDS PAGE provides a simple route to highly purified His-tag free amelogenin for use in structure-function experiments and beyond.