Chronic exposure to environmentally relevant concentration of fluoride impairs osteoblast's collagen synthesis and matrix mineralization: Involvement of epigenetic regulation in skeletal fluorosis

Fluoride Exposure Modulates Skeletal Development and Mineralization in Zebrafish Larvae

The presence of high levels of fluoride (F) in groundwater is a major issue worldwide. Although F is essential for healthy teeth and bones, excessive exposure can cause fluorosis or F toxicity. This condition primarily affects the hard tissues due to their high F retention capacity. F accumulation alters bone formation and resorption mechanisms interfering with mineral homeostasis and eventually manifests as skeletal fluorosis. Albeit the numerous studies on skeletal fluorosis, the effect of F on developmental osteogenesis is inconclusive. In light of this, we studied the effect of F on osteogenic differentiation, bone development, and mineralization in zebrafish. Zebrafish embryos were subjected to a low (25 ppm NaF), and a moderately high (50 ppm NaF) dose, along with a control (E3 medium alone) until 7 days postfertilization (dpf). The F content in the larvae was quantified to reveal a dose-dependent increase in the exposed groups. Alizarin Red and alkaline phosphatase (ALP) staining suggested enhanced mineralization in the F-treated groups. Quantitative analyses of the ALP activity and hydroxyproline (Hyp) content revealed similar results. Alcian blue staining of pharyngeal cartilages showed that F exposure alters the morphology of the major cartilages, indicating a possible craniofacial defect. Moreover, gene expression analyses of the bone markers associated with osteogenic differentiation, early mineralization, and remodeling (runx2a/b, bmp4, ocn, osx, col1a1, alp, rank, rankl, and opg) showed enhanced expression in the low F group. While the 50 ppm F group showed a decline in osteogenic activity, a considerable increase in the expression of mineralization markers was observed. The expression levels of cartilage markers sox9a and sox9b, remained insignificant, indicating the effect of F toxicity on osteogenesis and mineralization. Also, F exposure interferes with bone metabolism through altered osteogenic differentiation, development, and mineralization in zebrafish larvae.

Fluoride Alters Gene Expression via Histone H3K27 Acetylation in Ameloblast-like LS8 Cells

Excessive fluoride ingestion during tooth development can cause dental fluorosis. Previously, we reported that fluoride activates histone acetyltransferase (HAT) to acetylate p53, promoting fluoride toxicity in mouse ameloblast-like LS8 cells. However, the roles of HAT and histone acetylation status in fluoride-mediated gene expression remain unidentified. Here, we demonstrate that fluoride-mediated histone modification causes gene expression alterations in LS8 cells. LS8 cells were treated with or without fluoride followed by ChIP-Seq analysis of H3K27ac. Genes were identified by differential H3K27ac peaks within ±1 kb from transcription start sites. The levels of mRNA of identified genes were assessed using rea-time PCR (qPCR). Fluoride increased H3K27ac peaks associated with Bax, p21, and Mdm2 genes and upregulated their mRNA levels. Fluoride decreased H3K27ac peaks and p53, Bad, and Bcl2 had suppressed transcription. HAT inhibitors (Anacardic acid or MG149) suppressed fluoride-induced mRNA of p21 and Mdm2, while fluoride and the histone deacetylase (HDAC) inhibitor sodium butyrate increased Bad and Bcl2 expression above that of fluoride treatment alone. To our knowledge, this is the first study that demonstrates epigenetic regulation via fluoride treatment via H3 acetylation. Further investigation is required to elucidate epigenetic mechanisms of fluoride toxicity in enamel development.

Remote coupling of electrical and mechanical cues by diurnal photothermal irradiation synergistically promotes bone regeneration

Recapitulating the natural extracellular physical microenvironment has emerged as a promising method for tissue regeneration, as multiple physical interventions, including ultrasound, thermal and electrical therapy, have shown great potential. However, simultaneous coupling of multiple physical cues to highly bio-mimick natural characteristics for improved tissue regeneration still remains formidable. Coupling of intrinsic electrical and mechanical cues has been regarded as an effective way to modulate tissue repair. Nevertheless, precise and convenient manipulation on coupling of mechano-electrical signals within extracellular environment to facilitate tissue regeneration remains challengeable. Herein, a photothermal-sensitive piezoelectric membrane was designed for simultaneous integration of electrical and mechanical signals in response to NIR irradiation. The high-performance mechano-electrical coupling under NIR exposure synergistically triggered the promotion of osteogenic differentiation of stem cells and enhances bone defect regeneration by increasing cellular mechanical sensing, attachment, spreading and cytoskeleton remodeling. This study highlights the coupling of mechanical signals and electrical cues for modulation of osteogenesis, and sheds light on alternative bone tissue engineering therapies with multiple integrated physical cues for tissue repair.

Mediation of mitochondrial DNA copy number and oxidative stress in fluoride-related bone mineral density alteration in Chinese farmers

Excessive fluoride can adversely affect bone mineral density (BMD). Oxidative stress and mitochondrial dysfunction are crucial mechanisms of health damage induced by fluoride. Here, a cross-sectional survey involving 907 Chinese farmers (aged 18-60) was carried out in Tongxu County in 2017, aiming to investigate the significance of mitochondrial DNA copy number (mtDNAcn) and oxidative stress in fluoride-related BMD change. Concentrations of urinary fluoride (UF), serum oxidative stress biomarkers, including total antioxidant capacity (T-AOC), total superoxide dismutase (T-SOD), glutathione peroxidase (GSH-Px), and malondialdehyde (MDA), as well as relative mtDNAcn in peripheral blood were determined. The multivariable linear model and mediation analysis were performed to assess associations between UF, oxidative stress, and relative mtDNAcn with BMD. Results showed that GSH-Px levels increased by 6.98 U/mL [95% confidence interval (CI) 3.41-10.56)] with each 1.0 mg/L increment of UF. After stratification, the T-AOC, relative mtDNAcn, and BMD decreased by 0.04 mmol/L (-0.08 ~ -0.01), 0.29-unit (-0.55 ~ -0.04), and 0.18-unit (-0.33 ~ -0.03) with every 1.0 mg/L elevation of UF in the excessive fluoride group (EFG, adults with UF > 1.6 mg/L), respectively. Furthermore, T-AOC and relative mtDNAcn were favorably related to the BMD in the EFG (β = 0.82, 95%CI 0.16-1.48 for T-AOC; β = 0.11, 95%CI 0.02-0.19 for relative mtDNAcn). Mediation analysis showed that relative mtDNAcn and T-AOC mediated 15.4% and 17.1% of the connection between excessive fluoride and reduced BMD, respectively. Findings suggested that excessive fluoride was related to lower BMD in adults, and the decrement of T-AOC and relative mtDNAcn partially mediate this relationship.

Effects of chronic fluorosis on the expression of VEGF/PI3K/AKT/eNOS in the gingival tissue of rats with orthodontic tooth movement

It has been reported that the force of orthodontic correction triggers periodontal tissue remodeling by affecting angiogenesis. However, the manifestation of the vascular response to orthodontic tooth movement in the setting of chronic fluorosis is unclear. The aim of the present study was to preliminarily explore the effect of orthodontic treatment on the angiogenesis of gingival tissue in rats with chronic fluorosis by monitoring changes in the expression of vascular endothelial growth factor (VEGF), phosphatidylinositol-3 kinase (PI3K), AKT (or protein kinase B) and endothelial nitric oxide synthase (eNOS) in the gingival tissue. A total of 60 rats were randomly divided equally into the orthodontic group (O group; n=30) and fluorosis orthodontic group (FO group; n=30). Each of these groups was divided into 0-, 3-, 7-, 14- and 21-day groups (n=6/group). Fluorosis and orthodontic tooth movement models were established, and rats in each group were sacrificed for tissue sampling at the corresponding time points. Tissue morphology was observed via hematoxylin and eosin (H&E) staining. The protein and mRNA expression levels of VEGF, PI3K, AKT and eNOS in gingival tissue were detected by western blotting and reverse transcription-quantitative polymerase chain reaction, respectively. The H&E staining images showed that the FO group had smaller blood vessels and reduced vascular proliferation compared with the O group. Furthermore, the mRNA and protein expression levels of VEGF, PI3K, AKT and eNOS were reduced in the gingiva of rats in the FO group compared with the O group, and certain reductions were significant during the delayed tooth movement period. In addition, with the extension of the application of orthodontic stress, the mRNA and protein expression levels of VEGF, PI3K, AKT and eNOS in the gingiva of the O and FO groups showed a trend of increasing at first and subsequently decreasing, which corresponds with the tooth movement cycle. In conclusion, chronic fluorosis may inhibit the angiogenesis and the expression of the VEGF/PI3K/AKT/eNOS pathway in gingival tissue of orthodontic tooth movement.