Fluoride exposure and children's intelligence: Gene-environment interaction based on SNP-set, gene and pathway analysis, using a case-control design based on a cross-sectional study

Fluoride Induces Neurocytotoxicity by Disrupting Lysosomal Iron Metabolism and Membrane Permeability

The detrimental effects of fluoride on neurotoxicity have been widely recorded, yet the detailed mechanisms underlying these effects remain unclear. This study explores lysosomal iron metabolism in fluoride-related neurotoxicity, with a focus on the Steap3/TRPML1 axis. Utilizing sodium fluoride (NaF)-treated human neuroblastoma (SH-SY5Y) and mouse hippocampal neuron (HT22) cell lines, our research demonstrates that NaF enhances the accumulation of ferrous ions (Fe2+) in these cells, disrupting lysosomal iron metabolism through the Steap3/TRPML1 axis. Notably, NaF exposure upregulated ACSL4 and downregulated GPX4, accompanied by reduced glutathione (GSH) levels and superoxide dismutase (SOD) activity and increased malondialdehyde (MDA) levels. These changes indicate increased vulnerability to ferroptosis within neuronal cells. The iron chelator deferoxamine (DFO) mitigates this disruption. DFO binds to lysosomal Fe2+ and inhibits the Steap3/TRPML1 axis, restoring normal lysosomal iron metabolism, preventing lysosomal membrane permeabilization (LMP), and reducing neuronal cell ferroptosis. Our findings suggest that interference in lysosomal iron metabolism may mitigate fluoride-induced neurotoxicity, underscoring the critical role of the Steap3/TRPML1 axis in this pathological process.

High fluoride aggravates cadmium-mediated nephrotoxicity of renal tubular epithelial cells through ROS-PINK1/Parkin pathway

Fluoride (F) and cadmium (Cd) as well known environmental pollutants can cause nephrotoxicity to damage human health, while the joint toxicity of F and Cd to the renal tubular epithelial cells remains still elusive. The interactive influence between F and Cd in oxidative stress, apoptosis, and mitochondrial autophagy of renal tubular epithelial cells was explored. Cells were submitted to varying concentrations with of NaF (1, 5, 10, and 15 μg/mL) combined with CdCl2·2.5H2O (1 μg/mL) for 12 h. Following this, the combined cytotoxicity was assessed. Our results show that different doses of F had varying effects on Cd-mediated nephrotoxicity, with a synergistic effect observed in the high F (15 μg/mL) co-treated with Cd. In response to the Cd induction, the high F treatment resulted in the formation of multiple autophagosomes and notably increased the levels of LDH, ROS, and MMP. It also elevated the MDA contents while decreasing the activities of SOD, GSH-Px, and CAT. Additionally, it yielded a higher Bax/Bcl-2 ratio, which further promotes the apoptotic process. The treatment also disturbed energy metabolism, resulting in a reduction of both ATP and ADP. Furthermore, autophagy-related genes and proteins, including PINK1, Parkin, LC3A, LC3B, and SQSTM1, were significantly improved. In brief, high F of 15 μg/mL aggravated Cd-mediated nephrotoxicity of renal tubular epithelial cells via the ROS-PINK1/Parkin pathway.

Fluoride exposure-induced gut microbiota alteration mediates colonic ferroptosis through N6-methyladenosine (m6A) mediated silencing of SLC7A11

Fluoride exposure is widespread worldwide and poses a significant threat to organisms, particularly to their gastrointestinal tracts. However, due to limited knowledge of the mechanism of fluoride induced intestinal injury, it has been challenging to develop an effective treatment. To address this issue, we used a series of molecular biology in vitro and in vivo experiments. NaF triggered m6A mediated ferroptosis to cause intestinal damage. Mechanistically, NaF exposure increased the m6A level of SLC7A11 mRNA, promoted YTHDF2 binding to m6A-modified SLC7A11 mRNA, drove the degradation of SLC7A11 mRNA, and led to a decrease in its protein expression, which eventually triggers ferroptosis. Moreover, NaF aggravated ferroptosis of the colon after antibiotics destroyed the composition of gut microbiota. 16 S rRNA sequencing and SPEC-OCCU plots, Zi-Pi relationships, and Spearman correlation coefficients verified that Lactobacillus murinus (ASV54, ASV58, and ASV82) plays a key role in the response to NaF-induced ferroptosis. Collectively, NaF-induced gut microbiota alteration mediates severe intestinal cell injury by inducing m6A modification-mediated ferroptosis. Our results highlight a key mechanism of the gut in response to NaF exposure and suggest a valuable theoretical basis for its prevention and treatment.

Dose dependence of prenatal fluoride exposure associations with cognitive performance at school age in three prospective studies

BACKGROUND

Fluoride may be a developmental neurotoxicant at elevated exposures. We merged new data from a prospective Odense Child Cohort (OCC) with results from two previous birth cohort studies from Mexico and Canada to characterize the dose-effect relationship in greater detail.

METHODS

The OCC contributed 837 mother-child pairs to the total of >1500. We measured creatinine-adjusted urine-fluoride concentrations in maternal urine samples obtained during late pregnancy. Child IQ was determined at age 7 years using an abbreviated version of the Wechsler Intelligence Scales for Children. Findings from the three cohorts were used to calculate the joint benchmark concentration (BMC) and the lower confidence limit (BMCL) after adjustment for covariables.

RESULTS

In the OCC, urine-fluoride concentrations varied between 0.08 and 3.04 mg/l (median 0.52 mg/l) but were not significantly associated with full-scale IQ at age 7 years (β = 0.08; 95% confidence interval -1.14 to 1.30 for a doubling in exposure). No difference was apparent between boys and girls. In the OCC, the BMC was 0.92 mg/l, with a BMCL of 0.30 mg/l. The joint analysis of all three cohorts showed a statistically significant association between urine-fluoride and IQ, with a BMC of 0.45 mg/l (BMCL, 0.28 mg/l), slightly higher than the BMC previously reported for the two North American cohorts alone.

CONCLUSIONS

As the BMCL reflects an approximate threshold for developmental neurotoxicity, the results suggest that pregnant women and children may need protection against fluoride toxicity.

Systematic review of epidemiological and toxicological evidence on health effects of fluoride in drinking water

INTRODUCTION

Fluoride is a naturally occurring substance that is also added to drinking water, dental hygiene products, and food supplements for preventing dental caries. Concerns have been raised about several other potential health risks of fluoride.

OBJECTIVE

To conduct a robust synthesis of evidence regarding human health risks due to exposure to fluoride in drinking water, and to develop a point of departure (POD) for setting a health-based value (HBV) for fluoride in drinking water.

METHODS

A systematic review of evidence published since recent reviews of human, animal, and in vitro data was carried out. Bradford Hill considerations were used to weigh the evidence for causality. Several key studies were considered for deriving PODs.

RESULTS

The current review identified 89 human studies, 199 animal studies, and 10 major in vitro reviews. The weight of evidence on 39 health endpoints was presented. In addition to dental fluorosis, evidence was considered strong for reduction in IQ scores in children, moderate for thyroid dysfunction, weak for kidney dysfunction, and limited for sex hormone disruptions.

CONCLUSION

The current review identified moderate dental fluorosis and reduction in IQ scores in children as the most relevant endpoints for establishing an HBV for fluoride in drinking water. PODs were derived for these two endpoints, although there is still some uncertainty in the causal weight of evidence for causality for reducing IQ scores in children and considerable uncertainty in the derivation of its POD. Given our evaluation of the overall weight of evidence, moderate dental fluorosis is suggested as the key endpoint until more evidence is accumulated on possible reduction of IQ scores effects. A POD of 1.56 mg fluoride/L for moderate dental fluorosis may be preferred as a starting point for setting an HBV for fluoride in drinking water to protect against moderate and severe dental fluorosis. Although outside the scope of the current review, precautionary concerns for potential neurodevelopmental cognitive effects may warrant special consideration in the derivation of the HBV for fluoride in drinking water.

Fecal microbiota transplantation and short-chain fatty acids improve learning and memory in fluorosis mice by BDNF-PI3K/AKT pathway

Fluoride, an environmental toxicant, not only arouses intestinal microbiota dysbiosis, but also causes neuronal apoptosis and a decline in learning and memory ability. The purpose of this study was to explore whether fecal microbiota transplantation (FMT) from healthy mice and bacteria-derived metabolites short-chain fatty acids (SCFAs) supplement protect against fluoride-induced learning and memory impairment. Results showed that FMT reversed the elevated percentage of working memory errors (WME) and reference memory errors (RME) in fluorosis mice during the eight-arm maze test. Nissl and TUNEL staining presented that fluoride led to a decreased proportion of Nissl bodies area in the hippocampal CA3 region and an increased apoptotic ratio of nerve cells in CA1, CA3 and DG areas, whereas FMT alleviated those pathological damages. Moreover, the expressions of mRNA in hippocampal BDNF, PDK1, AKT, Bcl-2, and Bcl-xL were downregulated in mice exposed to fluoride, but the levels of PI3K, Bax, Bak, and Caspase-7 mRNA were upregulated. NaF treatment had an increase in PI3K and Caspase-3 protein levels and reduced the expressions of these four proteins, including BDNF, p-PI3K, AKT and p-AKT. By contrast, FMT enhanced the expression of BDNF and thus activated the PI3K/AKT pathway. Besides, the 16S rRNA sequencing revealed that fluoride caused a reduction in certain SCFA producers in the colon as evidenced by a decline in Erysipelatoclostridiaceae, and a downward trend in Akkermansia, Blautia and Alistipes. However, the disordered gut microbiome was restored via frequent FMT. Of note, SCFAs administration also increased BDNF levels and regulated its downstream pathways, which contributed to cell survival and learning and memory function recovery. In conclusion, FMT and SCFAs may activate the BDNF-PI3K/AKT pathway to play an anti-apoptotic role and ultimately improve learning and memory deficits in fluorosis mice.

Association of Dental Fluorosis and Urinary Fluoride with Intelligence among Schoolchildren

Fluoride is present naturally in water and has been used worldwide for the prevention of caries. Several studies conducted in high water fluoride or endemic fluorosis areas reported that fluoride adversely affected children's cognitive function, but some studies had negative findings. This study aimed to assess the relationship between urinary fluoride, dental fluorosis, and intelligence among schoolchildren living in communities with non-fluoridated drinking water. This cross-sectional study was conducted on 562 children aged 6-12 years in Taichung, Taiwan. Each child's urinary fluoride level was determined by a fluoride-ion-selective electrode, and the dental fluorosis condition was evaluated according to the criteria of Dean's Index. The Raven's Colored Progressive Matrices-Parallel and Standard Progressive Matrices-Parallel were used to assess children's intelligence. The results showed that the mean (±standard deviation) urinary fluoride concentrations were 0.40 ± 0.27 mg/L (0.43 ± 0.23 mg/g creatinine) among participants. The prevalence of dental fluorosis was 23.67%. After extensive evaluation of potential confounders, dental fluorosis and urinary fluoride were not associated with intelligence quotient (IQ) scores or grades in the regression models. In conclusion, dental fluorosis and urinary fluoride levels were not significantly related to the IQ of schoolchildren living in areas with low drinking water fluoride.

Prolonged fluoride exposure induces spatial-memory deficit and hippocampal dysfunction by inhibiting small heat shock protein 22 in mice

Millions of residents in areas with high-fluoride drinking water supply ingest excessive levels of fluoride for long periods. This study investigated the mechanisms and impacts of lifelong exposure to naturally occurring moderate-high-fluoride drinking water on spatial-memory function by studying mice in controlled experiments. Spatial-memory deficits and disorders of hippocampal neuronal electrical activity were observed in mice exposed to 25-ppm or 50-ppm-fluoride drinking water for 56 weeks, but not in adult or old mice exposed to 50 ppm fluoride for 12 weeks. Ultrastructural analysis showed severely damaged hippocampal mitochondria, evidenced by reduced mitochondrial membrane potential and ATP content. Mitochondrial biogenesis was impaired in fluoride-exposed mice, manifesting as a significantly reduced mtDNA content, mtDNA-encoded subunits mtND6 and mtCO1, and respiratory complex activities. Fluoride reduced expression of Hsp22, a beneficial mediator of mitochondrial homeostasis, and decreased levels of signaling for the PGC-1α/TFAM pathway-which regulates mitochondrial biogenesis-and the NF-κβ/STAT3 pathway-which regulates mitochondrial respiratory chain enzyme activity. Hippocampus-specific Hsp22-overexpression improved fluoride-induced spatial-memory deficits by activating the PGC-1α/TFAM and STAT3 signaling pathways, while Hsp22-silencing aggravated the deficits by inhibiting both pathways. Downregulation of Hsp22 plays a vital role in fluoride-induced spatial-memory deficits by impacting mtDNA-encoding subsets and mitochondrial respiratory chain enzyme activity.

Low dosage fluorine ameliorates the bioaccumulation, hepatorenal dysfunction and oxidative stress, and gut microbiota perturbation of cadmium in rats

Many "hot spot" geographic areas around the world with soils and crops co-polluted with cadmium (Cd) and fluorine (F), two of the most representative pollutants in the environment. However, it still exists argumentative on the dose-effect relationship between F and Cd so far. To explore this, a rat model was established to evaluate the effects of F on Cd-mediated bioaccumulation, hepatorenal dysfunction and oxidative stress, and the disorder of intestinal microbiota as well. 30 healthy rats were randomly assigned to Control group (C group), Cd 1 mg/kg (Cd group), Cd 1 mg/kg and F 15 mg/kg (L group), Cd 1 mg/kg and F 45 mg/kg (M group), and Cd 1 mg/kg and F 75 mg/kg (H group) for 12 weeks by gavage. Our results showed that Cd exposure could accumulate in organs, cause hepatorenal function damage and oxidative stress, and disorder of gut microflora. However, different dosages of F showed various effects on Cd-induced damages in liver, kidney, and intestine, and only the low supplement of F showed a consistent trend. After low supplement of F, Cd levels were declined by 31.29% for liver, 18.31% for kidney, and 2.89% for colon, respectively. The serum aspartate aminotransferase (AST), blood urea nitrogen (BUN), creatinine (Cr), and N-acetyl-β-glucosaminidase (NAG) were significantly reduced (p < 0.01); The activity of superoxide dismutase (SOD) was elevated and mRNA expression level of NAD(P)H quinone oxidoreductase 1 (NQO1) was decreased in the liver and kidney (p < 0.05). Moreover, low F dosage up-regulated the abundance of Lactobacillus from 15.56% to 28.73% and the 6.23% of F/B ratio was declined to 3.70%. Collectively, this highlights that low dosage of F might be a potential strategy to ameliorate the hazardous effects by Cd-exposed in the environment.

Influence of the chronic groundwater fluoride consumption on cholinergic enzymes, ACHE and BCHE gene SNPs and pro-inflammatory cytokines: A study with Pakistani population groups

Fluoride is one of the abundant elements found in the Earth's crust and is a global environmental issue. The present work aimed to find the impact of chronic consumption of fluoride contained groundwater on human subjects. Five hundred and twelve volunteers from different areas of Pakistan were recruited. Cholinergic status, acetylcholinesterase and butyrylcholinesterase gene SNPs and pro-inflammatory cytokines were examined. Association analysis, regression and other standard statistical analyses were performed. Physical examination of the fluoride endemic areas' participants revealed the symptoms of dental and skeletal fluorosis. Cholinergic enzymes (AChE and BChE) were significantly increased among different exposure groups. ACHE gene 3'-UTR variant and BCHE K-variant showed a significant association with risk of fluorosis. Pro-inflammatory cytokines (TNF-α, IL-1β and IL-6) were found to be increased and have a significant correlation in response to fluoride exposure and cholinergic enzymes. The study concludes that chronic consumption of high fluoride-contained water is a risk factor for developing low-grade systemic inflammation through the cholinergic pathway and the studied cholinergic gene SNPs were identified to be associated with the risk of flurosis.

Fluoride exposure and cognitive neurodevelopment: Systematic review and dose-response meta-analysis

Many uncertainties still surround the possible harmful effect of fluoride exposure on cognitive neurodevelopment in children. The aim of this systematic review and meta-analysis was to characterize this relation through a dose-response approach, by comparing the intelligence quotient (IQ) scores in the highest versus the lowest fluoride exposure category with a random-effects model, within a one-stage dose-response meta-analysis based on a cubic spline random-effects model. Out of 1996 potentially relevant literature records, 33 studies were eligible for this review, 30 of which were also suitable for meta-analysis. The summary mean difference of IQ score, comparing highest versus lowest fluoride categories and considering all types of exposure, was -4.68 (95% confidence interval-CI -6.45; -2.92), with a value of -5.60 (95% CI -7.76; -3.44) for drinking water fluoride and -3.84 (95% CI -7.93; 0.24) for urinary fluoride. Dose-response analysis showed a substantially linear IQ decrease for increasing water fluoride above 1 mg/L, with -3.05 (95% CI -4.06; -2.04) IQ points per 1 mg/L up to 2 mg/L, becoming steeper above such level. A weaker and substantially linear decrease of -2.15 (95% CI -4.48; 0.18) IQ points with increasing urinary fluoride emerged above 0.28 mg/L (approximately reflecting a water fluoride content of 0.7 mg/L). The inverse association between fluoride exposure and IQ was particularly strong in the studies at high risk of bias, while no adverse effect emerged in the only study judged at low risk of bias. Overall, most studies suggested an adverse effect of fluoride exposure on children's IQ, starting at low levels of exposure. However, a major role of residual confounding could not be ruled out, thus indicating the need of additional prospective studies at low risk of bias to conclusively assess the relation between fluoride exposure and cognitive neurodevelopment.

Quality assessment and prediction of municipal drinking water using water quality index and artificial neural network: A case study of Wuhan, central China, from 2013 to 2019

The sanitary security of drinking water is closely related to human health, but its quality assessment mainly focused on limited types of indicators and relatively restricted time span. The current study was aimed to evaluate the long-term spatial-temporal distribution of municipal drinking water quality and explore the origin of water contamination based on multiple water indicators of 137 finished water samples and 863 tap water samples from Wuhan city, China. Water quality indexes (WQIs) were calculated to integrate the measured indicators. WQIs of the finished water samples ranged from 0.24 to 0.92, with the qualification rate and excellent rate of 100 % and 96.4 %, respectively, while those of the tap water samples ranged from 0.09 to 3.20, with the qualification rate of 99.9 %, and excellent rate of 95.5 %. Artificial neural network model was constructed based on the time series of WQIs from 2013 to 2019 to predict the water quality thereafter. The predicted WQIs of finished and tap water in 2020 and 2021 qualified on the whole, with the excellent rate of 87.5 % and 92.9 %, respectively. Except for three samples exceeding the limits of free chlorine residual, chloroform and fluoride, respectively, the majority of indicators reached the threshold values for drinking. Our study suggested that municipal drinking water quality in Wuhan was generally stable and in line with the national hygiene standards. Moreover, principal component analysis illustrated that the main potential sources of drinking water contamination were inorganic salts and organic matters, followed by pollution from distribution systems, the use of aluminum-containing coagulants and turbidity involved in water treatment, which need more attention.

The Associations of Erythrocyte Fatty Acids with Whole Blood Mineral Elements in Children

Background. Minerals play important biological roles in lipid metabolism. The primary aim of this study was to examine the relationships between erythrocyte fatty acids (FAs) levels with whole blood mineral elements concentrations among Chinese children. Methods. A cross-sectional study was conducted. A total of 435 children aged 4−7 years were recruited. Whole blood mineral elements were determined by atomic absorption spectrometry and erythrocyte FAs composition by gas chromatography-mass spectrometer. Results. There were direct correlations between Zn and C18:2n-6 (FDR corrected p = 0.019), total n-6 PUFAs (FDR corrected p = 0.034), and total PUFAs (FDR corrected p = 0.034). Direct correlations were found between whole blood Zn and C18:1n-9 (FDR corrected p = 0.035), C24:1n-9 (FDR corrected p = 0.023), total MUFAs (FDR corrected p = 0.023), and C18:2n-6 (FDR corrected p = 0.048) in the Cu < P50 group. In the Cu ≥ P50 group, Mg was inversely related to most FAs (All FDR corrected p < 0.05). In the Zn < P50 group, Cu was directly related to C24:1n-9, total MUFAs, C20:5n-3, C22:6n-3, total n-3 PUFAs, C20:4n-6, total n-6 PUFAs, total PUFAs, and total FAs (All FDR corrected p < 0.05). Conclusions. Whole blood Cu and Zn levels were directly linked to several FAs levels in the erythrocytes of children. The interactions of Mg, Cu, and Zn with fatty acids may affect FA metabolism, in which Mg influences FA absorption.