Strategies for improving the assessment of dental fluorosis: focus on chemical and biochemical aspects

Sulphur dioxide and fluoride co-exposure induce incisor hypomineralization and amelogenin upregulation via YAP/RUNX2 signaling pathway

Sulphur dioxide (SO₂) and fluoride are among the most common environmental pollutants affecting human health, and both co-exist in areas predominantly consuming coal. It is vital to analyse the combined toxicity of SO₂ and fluoride, and their effects on health and the underlying mechanisms of their co-exposure have not yet been adequately assessed. In the present study, we used ICR mice and LS8 cells to investigate the toxicity of SO₂ and fluoride exposure to the enamel, alone or in combination. Factorial design analysis was used to reveal the combined toxicity in vitro and in vivo. Co-exposure to SO₂ and fluoride exacerbated enamel injury, resulting in more severe hypomineralization of incisor, and enamel structure disorders in mice, and could induce the accumulation of protein residue in the matrix of the enamel. Amelogenin expression was increased upon exposure to SO₂ and fluoride, but enamel matrix proteases were not affected. Consistent with our in vivo results, co-exposure of SO₂ and fluoride aggravated amelogenin expression in LS8 cells, and increased the YAP and RUNX2 levels. Co-exposure to SO₂ and fluoride resulted in greater toxicity than individual exposure, both in vitro and in vivo, indicating that residents of areas exposed to SO₂ and fluoride may have an increased risk of developing enamel damage.

Development of a new approach to diagnosis of the early fluorosis forms by means of FTIR and Raman microspectroscopy

This study is aimed at investigating the features of mineralization of the enamel apatite at initial stages of fluorosis development. Samples of teeth with intact and fluorotic enamel in an early stage of the disease development (Thylstrup-Fejerskov Index = 1-3) were studied by Raman scattering and FTIR using Infrared Microspectroscopy beamline at Australian Synchrotron equipment. Based on the data obtained by optical microspectroscopy and calculation of the coefficient R [A-type/B-type], which represents the ratio of carbonation fraction of CO32-, replacing phosphate or hydroxyl radicals in the enamel apatite lattice, the features of mineralization of enamel apatite in the initial stages of development of the pathology caused by an increased content of fluorine in the oral cavity were established. Statistical analysis of the data showed significant differences in the mean values of R [A-type/B-type] ratio between the control and experimental groups for surface layers (p < 0.01). The data obtained are potentially significant as benchmarks in the development of a new approach to preventive diagnostics of the development of initial and clinically unregistered stages of human teeth fluorosis, as well as personalized control of the use of fluoride-containing caries-preventive agents.

Morphological and chemical characterization of tooth enamel exposed to alkaline agents

OBJECTIVES

In this study, morphological and chemical changes in teeth enamel exposed to alkaline agents, with or without surfactants, have been investigated. In addition, chemical effects of the organic surface layer, i.e. plaque and pellicle, were also investigated.

METHODS

The present study was conducted using several techniques: Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM).

RESULTS

From XPS-measurements, it was found that exposure to alkaline solutions resulted in a massive removal of carbon from the tooth surface, and that the addition of surfactants increased the rate under present conditions. Based on the results from the FTIR-analysis, no substitution reactions between phosphate, carbonate and hydroxide ions in the enamel apatite could be detected. From a minor SEM-analysis, degradation and loss of substance of the enamel surface was found for the exposed samples. From XRD-analysis, no changes in crystallinity of the enamel apatite could be found between the samples.

CONCLUSIONS

The findings in this study show that exposure to alkaline solutions results in a degradation of enamel surfaces very dissimilar from acidic erosion. No significant erosion or chemical substitution of the apatite crystals themselves could be discerned. However, significant loss of organic carbon at the enamel surface was found in all exposed samples. The degradation of the protective organic layer at the enamel surface may profoundly increase the risk for caries and dental erosion from acidic foods and beverages.

The dentin sialoprotein (DSP) expression in rat tooth germs following fluoride treatment: An immunohistochemical study

Fluoride is known to alter expression of dentin matrix proteins and affect their posttranslational modifications.

OBJECTIVE

The objective of our study was to examine dentin sialoprotein (DSP) expression in the early and late bell stages of development of the first molar tooth germs in rats treated with fluoride.

DESIGN AND METHODS

Pregnant dumps were divided into three groups. They were fed a standard diet and from the fifth day of pregnancy, each group received either tap water (with trace amounts of fluoride), tap water with a low concentration of fluoride, or tap water with a high concentration of fluoride. Changes in DSP expression and distribution were visualized by immunohistochemistry.

RESULTS

Immunoreactivity for DSP was detected in the cervical regions of the early bell stage in tooth germs of the 1-day-old animals. The earliest reaction was visible in the control group and the group supplemented with the low fluoride concentration (F(L)) but not in the group supplemented with the high fluoride concentration (F(H)). In early bell stages across all experimental groups, the immunoreactivity to DSP was observed in the cusp tip regions and was localized to preameloblasts, young and mature odontoblasts, dental pulp cells, predentin, and dentin. Generally, more intense positive staining for DSP was detected in animals supplemented with the high fluoride concentration. In the late bell stage found in the 4-day-old control group and the group supplemented with the low fluoride concentration, immunoreactivity for DSP was less intense compared with younger animals. However, immunoreactivity was greater in the group treated with the high dose of fluoride. In this group, the positive immunostaining for DSP, especially in young ameloblasts, was prolonged and relatively strong.

CONCLUSIONS

Fluoride supplementation causes changes in the developmental pattern of DSP expression and its distribution in rat tooth germs.

Dental fluorosis: chemistry and biology

This review aims at discussing the pathogenesis of enamel fluorosis in relation to a putative linkage among ameloblastic activities, secreted enamel matrix proteins and multiple proteases, growing enamel crystals, and fluid composition, including calcium and fluoride ions. Fluoride is the most important caries-preventive agent in dentistry. In the last two decades, increasing fluoride exposure in various forms and vehicles is most likely the explanation for an increase in the prevalence of mild-to-moderate forms of dental fluorosis in many communities, not the least in those in which controlled water fluoridation has been established. The effects of fluoride on enamel formation causing dental fluorosis in man are cumulative, rather than requiring a specific threshold dose, depending on the total fluoride intake from all sources and the duration of fluoride exposure. Enamel mineralization is highly sensitive to free fluoride ions, which uniquely promote the hydrolysis of acidic precursors such as octacalcium phosphate and precipitation of fluoridated apatite crystals. Once fluoride is incorporated into enamel crystals, the ion likely affects the subsequent mineralization process by reducing the solubility of the mineral and thereby modulating the ionic composition in the fluid surrounding the mineral. In the light of evidence obtained in human and animal studies, it is now most likely that enamel hypomineralization in fluorotic teeth is due predominantly to the aberrant effects of excess fluoride on the rates at which matrix proteins break down and/or the rates at which the by-products from this degradation are withdrawn from the maturing enamel. Any interference with enamel matrix removal could yield retarding effects on the accompanying crystal growth through the maturation stages, resulting in different magnitudes of enamel porosity at the time of tooth eruption. Currently, there is no direct proof that fluoride at micromolar levels affects proliferation and differentiation of enamel organ cells. Fluoride does not seem to affect the production and secretion of enamel matrix proteins and proteases within the dose range causing dental fluorosis in man. Most likely, the fluoride uptake interferes, indirectly, with the protease activities by decreasing free Ca(2+) concentration in the mineralizing milieu. The Ca(2+)-mediated regulation of protease activities is consistent with the in situ observations that (a) enzymatic cleavages of the amelogenins take place only at slow rates through the secretory phase with the limited calcium transport and that, (b) under normal amelogenesis, the amelogenin degradation appears to be accelerated during the transitional and early maturation stages with the increased calcium transport. Since the predominant cariostatic effect of fluoride is not due to its uptake by the enamel during tooth development, it is possible to obtain extensive caries reduction without a concomitant risk of dental fluorosis. Further efforts and research are needed to settle the currently uncertain issues, e.g., the incidence, prevalence, and causes of dental or skeletal fluorosis in relation to all sources of fluoride and the appropriate dose levels and timing of fluoride exposure for prevention and control of dental fluorosis and caries.

Enzyme compartmentalization during biphasic enamel matrix processing

Processing of enamel matrix proteins is essentially biphasic. Secretory stage metalloprotease activity generates a discrete, presumably functional, spectrum of molecules which may also undergo dephosphorylation. Maturation stage serine proteases almost completely destroy the matrix. The present aim was to examine the tissue compartmentalization of these enzyme activities in relation to their possible function. A sequential extraction using synthetic enamel fluid, phosphate buffer and SDS was used to identify enzymes free in the enamel fluid, crystal bound or aggregated with the bulk matrix respectively. Results indicated that the metallo-proteases and alkaline phosphatase were free in the secretory stage enamel fluid while the serine proteases appeared to be largely bound to the maturation stage crystals. The mobility of the metallo-proteases and alkaline phosphatase would ensure efficient initial processing of secretory matrix, while the largely mineral bound serine proteases would ensure retention of protease activity despite massive destruction and protein removal.

Effect of apatite crystals on the activity of amelogenin degrading enzymes in vitro

The objective of the present study was to determine the effect of apatite crystals on the activity of amelogenin degrading enzymes in vitro. Current experimental data, together with previous reports support the view that among the different proteinases present in the enamel extracellular matrix, serine proteinase(s) are responsible for the massive degradation of amelogenins during the maturation stage. For our in-vitro experiments we used the recombinant amelogenin M179 as substrate and a "65%-satd. (NH4)2SO4" fraction of enamel proteins as well as chymotrypsin as sources for serine-proteinase activity. We report preliminary experiments of amelogenin proteolysis in the presence of apatite crystals resulting in a different proteolysis pattern when compared to amelogenin proteolysis without apatite crystals. Quantitative analysis of the HPLC peaks corresponding to the proteolysis products indicates that the presence of apatite crystals in the proteolysis solution inhibits the ability of the serine-proteinases to degrade amelogenin. The present observations support the hypothesis that amelogenin degradation correlates with apatite crystal growth during enamel maturation.

The effect of fluoride on apatite structure and growth

Fluoride participates in many aspects of calcium phosphate formation in vivo and has enormous effects on the process and on the nature and properties of formed mineral. The most well-documented effect of fluoride is that this ion substitutes for a column hydroxyl in the apatite structure, giving rise to a reduction of crystal volume and a concomitant increase in structural stability. In the process of enamel mineralization during amelogenesis (a unique model for the cell-mediated formation of well-crystallized carbonatoapatite), free fluoride ions in the fluid phase are supposed to accelerate the hydrolysis of acidic precursor(s) and increase the driving force for the growth of apatitic mineral. Once fluoride is incorporated into the enamel mineral, the ion likely affects the subsequent mineralization process by reducing the solubility of the mineral and thereby modulating the ionic composition in the fluid surrounding the mineral, and enhancing the matrix protein-mineral interaction. But excess fluoride leads to anomalous enamel formation by retarding tissue maturation. It is worth noting that enameloid/enamel minerals found in vertebrate teeth have a wide range of CO3 and fluoride substitutions. In the evolutionary process from elasmobranch through enameloid to mammalian enamel, the biosystems appear to develop regulatory functions for limiting the fluoridation of the formed mineral, but this development is accompanied by an increase of carbonate substitution or defects in the mineral. In research on the cariostatic effect of fluoride, considerable emphasis is placed on the roles of free fluoride ions (i.e., preventing the dissolution and accelerating the kinetics of remineralization) in the oral fluid bathing tooth mineral. Fluoride also has been used for the treatment of osteoporosis, but much still remains to be learned about maximizing the benefit and minimizing the risk of fluoride when used as a public health measure.

Disturbed enamel mineralization in a rat incisor model

Possession of full-thickness hard enamel appears to be one of the indispensable life-saving characteristics of rats. Previous studies by Suga and his colleagues and by others demonstrated that various types of malformation are evoked in continuously erupting rat incisors. In the current report, we directed our effort to oversee various types of enamel malformation caused experimentally in rat incisors. We surveyed the specimens collected by Suga and his colleagues, as well as specimens we obtained. From the results, it is conceivable that perturbation of the programmed sequential events during enamel development is a major factor in the establishment of enamel malformation. Animal studies with either 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) or a multidentate phosphonic acid (EDTPO) confirmed that dentin mineralization provides a certain inductive effect on the secretion of enamel matrix and subsequent enamel crystallization. Our recent studies using anti-microtubular agents led to the conclusion that the acceleration of mineralization in outer enamel is a type of enamel malformation, most likely due to disruption of the cellular regulation of calcium transport under severe toxic regimens. In future work, experimental approaches combining measurements of kinetic factors with static observation of enamel lesions are required before we can gain a comprehensive understanding of the pathogenesis of disturbed enamel mineralization. The kinetic factors to be considered include the rates of tissue apposition and tooth eruption which determine the total volume of tooth substance formed, and the rate of mineral accretion. Furthermore, information as to the composition, crystallinity, solubility, and mechanical properties of enamel defects is needed before we can assess the susceptibility of teeth having those lesions to caries and other physico-chemical attacks in the oral environment.

Morphometry and autoradiography of altered rat enamel protein processing due to chronic exposure to fluoride

Female Sprague-Dawley rats had 6 weeks of 0 (control), 75 or 100 parts/10(6) sodium fluoride in their drinking water. Whole mandibular incisors were removed, fixed, demineralized and sections prepared for light-microscopic morphometric analysis of dose-related alterations in enamel protein retention. Other rats given 0 and 75 parts/10(6) only (control and experimental groups) were used for autoradiographic evaluation of alterations in enamel protein removal 35S-methionine was applied directly over secretory ameloblasts at the end of the fifth week of fluoride exposure. Incisors were removed either 5 or 7 days later and processed for autoradiographic analysis. The results indicated: (1) extended retention of enamel proteins in fluoride-exposed maturation enamel as well as reduced enamel protein synthesis and/or secretion in the secretory stage; (2) negative linear correlation between extended enamel protein retention and reduced enamel protein secretion among groups; and (3) repression of enamel protein removal. The data are also consistent with the concept that the fluoride effect is multifactorial.

Recent observations on enamel crystal formation during mammalian amelogenesis

BACKGROUND

Enamel mineralization taking place during amelogenesis is a unique model to investigate carbonatoapatite formation in vivo. The abundance of proteinaceous crystal growth inhibitors, in particular amelogenins, contributes significantly to the mineralization process. Their putative roles are to prevent random proliferation of crystal nuclei and to regulate the growth kinetics and orientation of the formed enamel crystals.

METHODS

The enamel fluid surrounding the forming enamel crystals contains high concentrations of carbonate and magnesium ions, both of which seem to modulate the mineralization process. Particularly, Mg ions can adsorb onto enamel crystal surfaces in a manner to compete with Ca ions. Enamel mineral formed during amelogenesis is featured as calcium-deficient, acid phosphate-rich carbonatoapatites. Currently the most putative stoichiometry model for enamel mineral is (Ca)5-x(HPO4)v(CO3)w(PO4)3-x (OH)1-x.

RESULTS

Very significant changes in the morphology, stoichiometry, and solubility of enamel crystals occur during the various stages of amelogenesis. The early enamel mineralization comprises two events: the initial precipitation of the well-documented thin ribbons and the subsequent overgrowth of apatite crystals on those templates. The thin ribbons precipitated in the vicinity of the secretory ameloblasts have the highest contents of acid phosphate, particularly in the form of exchangeable species, whereas both the exchangeable and unexchangeable acid phosphate decrease concomitantly with the progress of the apatite overgrowth and the appearance of elongated hexagonal crystals in the late secretory stages.

CONCLUSIONS

Those morphological and compositional features seem to be consistent with the formation of precursors, such as octacalcium phosphate.