Crystal growth in dental enamel: the role of amelogenins and albumin

Phenotypic Analysis, Molecular Characterization, and Antibiogram of Caries-Causing Bacteria Isolated from Dental Patients

Dental caries is a biofilm-mediated, sugar-driven, multifactorial, dynamic disease that results in the phasic demineralization and remineralization of dental hard tissues. Despite scientific advances in cariology, dental caries remains a severe global concern. The aim of this study was to determine the optimization of microbial and molecular techniques for the detection of cariogenic pathogens in dental caries patients, the prevalence of cariogenic bacteria on the basis of socioeconomic, climatological, and hygienic factors, and in vitro evaluation of the antimicrobial activity of selected synthetic antibiotics and herbal extracts. In this study, oral samples were collected from 900 patients for bacterial strain screening on a biochemical and molecular basis. Plant extracts, such as ginger, garlic, neem, tulsi, amla, and aloe vera, were used to check the antimicrobial activity against the isolated strains. Synthetic antimicrobial agents, such as penicillin, amoxicillin, erythromycin, clindamycin, metronidazole, doxycycline, ceftazidime, levofloxacin, and ciprofloxacin, were also used to access the antimicrobial activity. Among 900 patients, 63% were males and 37% were females, patients aged between 36 and 58 (45.7%) years were prone to disease, and the most common symptom was toothache (61%). For oral diseases, 21% used herbs, 36% used antibiotics, and 48% were self-medicated, owing to sweets consumption (60.66%) and fizzy drinks and fast food (51.56%). Staphylococcus mutans (29.11%) and Streptococcus sobrinus (28.11%) were found as the most abundant strains. Seven bacterial strains were successfully screened and predicted to be closely related to genera S. sobrinus, S. mutans, Actinomyces naeslundii, Lactobacillus acidophilus, Eubacterium nodatum, Propionibacterium acidifaciens, and Treponema Pallidum. Among plant extracts, the maximum zone of inhibition was recorded by ginger (22.36 mm) and amla (20.01 mm), while among synthetic antibiotics, ciprofloxacin and levofloxacin were most effective against all microbes. This study concluded that phyto extracts of ginger and amla were considered suitable alternatives to synthetic antibiotics to treat dental diseases.

Label-free quantitative proteomics reveals molecular correlates of altered biomechanical properties in molar incisor hypomineralization (MIH): an in vitro study

PURPOSE

Molar-incisor hypomineralization (MIH) is a qualitative developmental defect of enamel that affects first permanent molars with or without affecting permanent incisors. We aimed to carry out a quantitative proteomics-based study to compare and evaluate proteins in sound and MIH-affected enamel.

MATERIALS AND METHODS

Ten blocks each of the MIH-affected enamel and sound enamel were processed and prepared for LC-MS/MS analysis. Label-free quantitation was carried out to evaluate the differentially expressed proteins in the two groups of samples.

RESULTS

A significant increase in the number of proteins in MIH-affected enamel (50.3 ± 29.6) was observed compared to the sound enamel (21.4 ± 3.2). While proteins like collagens, α1-anti-trypsin, kallikrein-4 (KLK4), matrix metalloprotease-20 (MMP-20), alpha-2-macroglobulin, and alpha-2-HS-glycoprotein were upregulated in sound enamel, there was over-expression of albumin, calcium-binding proteins, anti-thrombin III, and dentin sialophosphoprotein (DSPP), along with proteins implicated in stress response and inflammatory processes in MIH.

CONCLUSION

We propose that altered biomechanical properties of the enamel in MIH samples arise due to (i) down-regulation of proteins contributing to collagen biosynthesis and fibril formation; (ii) an overall imbalance in required levels of proteases (KLK4 and MMP-20) and anti-proteases (anti-thrombin-III which inhibits KLK-4), essential for optimal mineralization; (iii) very low levels of alpha-2-macroglobulin with important consequences in enamel mineralization and amelogenesis; and (iv) increased albumin in MIH, preventing proper growth of hydroxyapatite crystals. Increased inflammatory component was also seen in MIH; however, whether inflammation is a cause or consequence of the poor mineralization process needs to be assessed.

A Breakthrough in Understanding the Pathogenesis of Molar Hypomineralisation: The Mineralisation-Poisoning Model

Popularly known as "chalky teeth", molar hypomineralisation (MH) affects over 1-in-5 children worldwide, triggering massive amounts of suffering from toothache and rapid decay. MH stems from childhood illness and so offers a medical-prevention avenue for improving oral and paediatric health. With a cross-sector translational research and education network (The D3 Group; thed3group.org) now highlighting this global health opportunity, aetiological understanding is urgently needed to enable better awareness, management and eventual prevention of MH. Causation and pathogenesis of "chalky enamel spots" (i.e., demarcated opacities, the defining pathology of MH) remain unclear despite 100 years of investigation. However, recent biochemical studies provided a pathomechanistic breakthrough by explaining several hallmarks of chalky opacities for the first time. This article outlines these findings in context of previous understanding and provides a working model for future investigations. The proposed pathomechanism, termed "mineralisation poisoning", involves localised exposure of immature enamel to serum albumin. Albumin binds to enamel-mineral crystals and blocks their growth, leading to chalky opacities with distinct borders. Being centred on extracellular fluid rather than enamel-forming cells as held by dogma, this localising pathomechanism invokes a new type of connection with childhood illness. These advances open a novel direction for research into pathogenesis and causation of MH, and offer prospects for better clinical management. Future research will require wide-ranging inputs that ideally should be coordinated through a worldwide translational network. We hope this breakthrough will ultimately lead to medical prevention of MH, prompting global health benefits including major reductions in childhood tooth decay.

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).

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.

On the Variable Clinical Presentation of Molar-Incisor Hypomineralization

Molar-incisor hypomineralization (MIH) is a condition that is defined based on its peculiar clinical presentation. Original reports on the etiology of the condition and possible risk factors were inconclusive, and we refuted the original suggestion that MIH is an idiopathic condition and suggested that MIH has complex inheritance and is due to the interaction of more than one gene and the environment. Our group was the first to suggest MIH has a genetic component that involves genetic variation in genes expressed during dental enamel formation. Later we expanded this work to include genes related to the immune response. In this report, we provide a rationale to explain the variation seen in the clinical presentation of MIH, which can affect just one molar out of the four or just a portion of a particular molar.

Bioinspired amphiphilic phosphate block copolymers as non-fluoride materials to prevent dental erosion

Inspired by the fact that certain natural proteins, e.g. casein phosphopeptide or amelogenin, are able to prevent tooth erosion (mineral loss) and to enhance tooth remineralization, a synthetic amphiphilic diblock copolymer, containing a hydrophilic methacryloyloxyethyl phosphate block (MOEP) and a hydrophobic methyl methacrylate block (MMA), was designed as a novel non-fluoride agent to prevent tooth erosion under acidic conditions. The structure of the polymer, synthesized by reversible addition-fragment transfer (RAFT) polymerization, was confirmed by gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR). While the hydrophilic PMOEP block within the amphiphilic block copolymer strongly binds to the enamel surface, the PMMA block forms a hydrophobic shell to prevent acid attack on tooth enamel, thus preventing/reducing acid erosion. The polymer treatment not only effectively decreased the mineral loss of hydroxyapatite (HAP) by 36-46% compared to the untreated control, but also protected the surface morphology of the enamel specimen following exposure to acid. Additionally, experimental results confirmed that low pH values and high polymer concentrations facilitate polymer binding. Thus, the preliminary data suggests that this new amphiphilic diblock copolymer has the potential to be used as a non-fluoride ingredient for mouth-rinse or toothpaste to prevent/reduce tooth erosion.

Evolutionary story of mammalian-specific amelogenin exons 4, "4b", 8, and 9

Amelogenin gene organization varies from 6 exons (1,2,3,5,6,7) in amphibians and sauropsids to 10 in rodents. The additional exons are exons 4, 8, 9, and "4b", the latter being as yet unidentified in AMELX transcripts. To learn more about the evolutionary origin of these exons, we used an in silico approach to find them in 39 tetrapod genomes. AMEL organization with 6 exons was the ancestral condition. Exon 4 was created in an ancestral therian (marsupials + placentals), then exon 9 in an ancestral placental, and finally exons "4b" and 8 in rodents, after divergence of the squirrel lineage. These exons were either inactivated in some lineages or remained functional: Exon 4 is functional from artiodactyls onward; exon 9 is known, to date, only in rodents, but could be coding in various mammals; and exon "4b" was probably coding in some rodents. We performed PCR of cDNA isolated from mouse and human tooth buds to identify the presence of these transcripts. A sequence analogous to exon "4b", and to exon 9, could not be amplified from the respective tooth cDNA, indicating that even though sequences similar to these exons are present, they are not transcribed in these species.

Protein content of molar-incisor hypomineralisation enamel

OBJECTIVES

The aim of the study was to compare the relative amounts and nature of the proteinous content of sound and molar-incisor hypomineralisation (MIH) enamel.

METHODS

TCA (20%) was used to dissolve the mineral phase and precipitate the proteins from enamel pieces sectioned from sound and MIH enamel. The protein content was estimated using a miniaturized version of the method of Lowry et al. Samples of the solubilised protein were separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), stained with Coomassie Blue R250 and tryptic fingerprint/mass spectrometry (MS/MS) of bands in excised gel pieces used for protein identification.

RESULTS

Compared to sound enamel, brown enamel showed a 15-21-fold higher protein content, and yellow and chalky enamel showed about 8-fold higher protein content. Tryptic fingerprint/MS performed on excised 50-70kDa areas demonstrated serum albumin, type I collagen and antitrypsin to be common to all types of enamel. Yellow and brown enamel showed more abundant serum albumin and antitrypsin, and the presence of serum antithrombin. Albumin is reported to be an inhibitor of crystal growth, and antitrypsin and antithrombin inhibit kallikrein 4 proteolytic activity.

CONCLUSIONS

The combination of the effects of serum proteins on developing enamel may result in elevated proteinous content and reduced mineral content as seen in MIH enamel.

Mineral acquisition rates in developing enamel on maxillary and mandibular incisors of rats and mice: implications to extracellular acid loading as apatite crystals mature

The formation rates of mineral in developing enamel were determined by microweighing of incisors of mice and rats. Computations indicated that a large excess of hydrogen ions would result from creating apatite at the calculated rates. Enamel organ cells (ameloblasts), therefore, likely excrete bicarbonate ions to prevent pH in fluid bathing enamel from becoming too acidic.

INTRODUCTION

Protons (H+) are generated whenever calcium and phosphate ions combine directly from aqueous solutions to form hydroxyapatite. Enamel is susceptible to potential acid loading during development because the amount of fluid bathing this tissue is small and its buffering capacity is low. The epithelial cells covering this tissue are also believed to form permeability barriers at times during the maturation stage when crystals grow at their fastest rates. The goal of this study was to measure the bulk weight of mineral present in rodent enamel at specific times in development and estimate the amount of acid potentially formed as the apatite crystals mature.

MATERIALS AND METHODS

Upper and lower jaws of mice and rats were freeze-dried, and the enamel layers on the incisors were partitioned into a series of 0.5 mm (mouse) or 1.0 mm (rat) strips along the length of each tooth. The strips were weighed on a microbalance, ashed at 575 degrees C for 18-24 h to remove organic material, and reweighed to determine the actual mineral weight for each strip.

RESULTS AND CONCLUSIONS

The data indicated that, despite differences in gross sizes and shapes of maxillary and mandibular incisors in rats and mice, the overall pattern and rates of mineral acquisition were remarkably similar. This included sharply increasing rates of mineral acquisition between the secretory and maturation stages, with peak levels approaching 40 microg/mm tooth length. Computer modeling indicated that quantities of H+ ions potentially generated as apatite crystals grew during the maturation stage greatly exceeded local buffering capacity of enamel fluid and matrix proteins. In other systems, bicarbonate ions are excreted to neutralize highly acidic materials generated extracellularly. Data from this study indicate that ameloblasts, and perhaps cells in other apatite-based hard tissues, use similar bicarbonate release mechanisms to control excess acid arising from mineral formation.

Crystallization at the polymer/calcium-phosphate interface in a sterilized injectable bone substitute IBS

Calcium phosphate (CaP) ceramics are the main raw materials used to elaborate blocks or granules for bone substitutes. In this study, injectable bone substitutes (IBS) were developed for applications in orthopedic or dental surgery. Sterile, ready-to-use composite containing CaP granules (biphasic calcium phosphate, BCP) and polymer (hydroxypropylmethylcellulose, HPMC) was prepared. Steam sterilization produced new phenomena at the CaP/polymer interface, resulting in crystal growth. These phenomena may constitute a model for the biomineralization study. Scanning electron microscopy showed that the formed crystallites organize themselves into a three-dimensional structure. Currently, the mechanisms of crystal growth are unknown and have been observed with only one combination of polymer/BCP ceramics after steam sterilization.

Degradative changes in whole enamel homogenates incubated in vitro in the presence of low calcium ion concentrations

The purpose of this study was to investigate overall degradative changes occurring to enamel matrix proteins in small, freeze-dried pieces of rat incisor enamel homogenized and incubated directly for 0-48 hours in a synthetic enamel fluid solution (165 mM total ionic strength with 0.153 mM calcium chloride) versus other samples homogenized and incubated for the same time intervals in distilled water. The results indicated that many alterations in the apparent molecular weights of enamel matrix proteins took place under both conditions although the rates for many degradative changes over a 48 hour period were often slower in distilled water than in synthetic enamel fluid. Freeze-dried enamel samples homogenized and incubated in 165 mM Tris-HCl buffer at pH 8.0 showed changes comparable to those seen with distilled water. This suggested that differences observed between samples incubated in enamel fluid versus distilled water were unrelated to pH or ionic strength of the solutions and may be the result of a requirement by some enamel proteinases for small amounts of free calcium ions in incubation media. Of interest were findings that some enamel matrix proteins, especially those in strips taken from the first half of the secretory stage of amelogenesis, were degraded much faster in distilled water than in synthetic enamel fluid. The reasons for this effect are unclear although, in this case, calcium ions could be inhibitory to hydrolysis of certain matrix proteins by the enamel proteinases.

Evidence for charge domains on developing enamel crystal surfaces

The control of hydroxyapatite crystal initiation and growth during enamel development is thought to be mediated via the proteins of the extracellular matrix. However, the precise nature of these matrix-mineral interactions remains obscure. The aim of the present study was to use a combination of atomic and chemical force microscopy to characterize developing enamel crystal surfaces and to determine their relationship with endogenous enamel matrix protein (amelogenin). The results show regular and discrete domains of various charges or charge densities on the surfaces of hydroxyapatite crystals derived from the maturation stage of enamel development. Binding of amelogenin to individual crystals at physiological pH was seen to be coincident with positively charged surface domains. These domains may therefore provide an instructional template for matrix-mineral interactions. Alternatively, the alternating array of charge on the crystal surfaces may reflect the original relationship with, and influence of, matrix interaction with the crystal surfaces during crystal growth.

Time-related changes of developing enamel crystals after exposure to the tissue fluid in vivo: analysis of a subcutaneously implanted rat incisor

To investigate the effects of tissue fluid on the growth of enamel crystals, upper and lower incisors extracted from 3-week-old Wistar rats were removed of the enamel organ, implanted subcutaneously in the dorsal portion of animals from the same litter, and harvested at 72 h or 1 week after implantation. The grafts were chemically fixed with surrounding tissues and prepared for light and electron microscopy, X-ray microanalysis, or for the immunohistochemistry of amelogenin. Mineralization of implanted enamel layers was examined by contact X-ray microradiography. The immunoreactivities for 25 kD amelogenin in immature enamel decreased sequentially, starting from the surface to the deeper layers; by 1 week after implantation, no positive reactivities remained in the entire enamel layers at the stages of matrix formation and early maturation. In accordance with the loss of enamel proteins, immature enamel gained mineral density until it attained higher radio opacity than that of the adjacent dentin by 1 week. In contrast, the radio opacity of the full thickness of the enamel at early maturation remained low except for a superficial thin layer. Electron microscopy revealed no sign of growth of original enamel crystals, but showed heavy precipitation of electron-dense fine granules of calcium phosphate in all layers of the secretory enamel and the superficial layer of enamel at early maturation, which showed high radio opacity. The Ca/P ratio and electron diffraction patterns of the granular materials precipitated between intrinsic enamel crystals indicated the property of hydroxy apatite or octacalcium phosphate though a characteristic ribbon-like profile of enamel crystals was lacking. These data indicate that the enamel organ blocks exogenous mineral precipitates in growing enamel during the stage of matrix formation and plays an essential regulatory role for fine enamel crystallites to grow into large hexagonal crystals.

Localization of EMSP1 expression during tooth formation and cloning of mouse cDNA

Enamel matrix serine proteinase 1 (EMSP1) is a proteolytic enzyme that has been isolated from the developing enamel of pig teeth. Its apparent function is to degrade the organic matrix in preparation for enamel maturation. The expression of EMSP1 has never been investigated in another organism besides the pig, and EMSP1 expression in the enamel organ has never been specifically demonstrated in ameloblasts. Here we report the expression of recombinant pig EMSP 1 (rpEMSP 1), the generation of rabbit polyclonal antibodies against rpEMSP1, the characterization of the antibodies and EMSP1 expression by Western blot and immunohistochemical analyses, the cloning and characterization of a full-length cDNA encoding mouse EMSP1, and the localization of EMSP1 expression in ameloblasts in mouse day 14 first and second molars by in situ hybridization. The full-length mouse EMSP1 cDNA clone has 1,237 nucleotides, excluding the poly(A+) tail, and encodes a preproprotein of 255 amino acids. Mouse EMSP1 shares 75% amino acid identity with pig EMSP1 and has three potential N-linked glycosylation sites, two of which are conserved in the pig homologue. Western blot analysis shows that the polyclonal antibodies are specific for EMSP1 and do not cross-react with trypsin. Immunohistochemistry of pig incisors shows discrete staining in the surface enamel at the earliest part of the maturation stage. In mouse molars, in situ hybridization gives a distinct and specific signal in maturation-stage ameloblasts, and in the junctional epithelium following tooth eruption. We conclude that EMSP1 is expressed by pig and mouse ameloblasts during the early maturation stage of amelogenesis.

The structural biology of the developing dental enamel matrix

The biomineralization of the dental enamel matrix with a carbonated hydroxyapatite mineral generates one of the most remarkable examples of a vertebrate mineralized tissue. Recent advances in the molecular biology of ameloblast gene products have now revealed the primary structures of the principal proteins involved in this extracellular mineralizing system, amelogenins, tuftelins, ameloblastins, enamelins, and proteinases, but details of their secondary, tertiary, and quaternary structures, their interactions with other matrix and or cell surface proteins, and their functional role in dental enamel matrix mineralization are still largely unknown. This paper reviews our current knowledge of these molecules, the probable molecular structure of the enamel matrix, and the functional role of these extracellular matrix proteins. Recent studies on the major structural role played by the amelogenin proteins are discussed, and some new data on synthetic amelogenin matrices are reviewed.

Cellular and chemical events during enamel maturation

This review focuses on the process of enamel maturation, a series of events associated with slow, progressive growth in the width and thickness of apatitic crystals. This developmental step causes gradual physical hardening and transformation of soft, newly formed enamel into one of the most durable mineralized tissues produced biologically. Enamel is the secretory product of specialized epithelial cells, the ameloblasts, which make this covering on the crowns of teeth in two steps. First, they roughly "map out" the location and limits (overall thickness) of the entire extracellular layer as a protein-rich, acellular, and avascular matrix filled with thin, ribbon-like crystals of carbonated hydroxyapatite. These initial crystals are organized spatially into rod and interrod territories as they form, and rod crystals are lengthened by Tomes' processes in tandem with appositional movement of ameloblasts away from the dentin surface. Once the full thickness of enamel has been formed, ameloblasts initiate a series of repetitive morphological changes at the enamel surface in which tight junctions and deep membrane infoldings periodically appear (ruffle-ended), then disappear for short intervals (smooth-ended), from the apical ends of the cells. As this happens, the enamel covered by these cells changes rhythmically in net pH from mildly acidic (ruffle-ended) to near-physiologic (smooth-ended) as mineral crystals slowly expand into the "spaces" (volume) formerly occupied by matrix proteins and water. Matrix proteins are processed and degraded by proteinases throughout amelogenesis, but they undergo more rapid destruction once ameloblast modulation begins. Ruffle-ended ameloblasts appear to function primarily as a regulatory and transport epithelium for controlling the movement of calcium and other ions such as bicarbonate into enamel to maintain buffering capacity and driving forces optimized for surface crystal growth. The reason ruffle-ended ameloblasts become smooth-ended periodically is unknown, although this event seems to be crucial for sustaining long-term crystal growth.