Prolonged exposure to high fluoride levels during adolescence to adulthood elicits molecular, morphological, and functional impairments in the hippocampus

Effects of low concentration of fluoride exposure during fetal on behavior and neurotransmitters in adult mice

Fluoride (F) naturally occurs in water in China and India, and in excess, can cause skeletal fluorosis and mottled teeth. Chronic exposure to F during gestation can affect the development of the brain, reducing intelligence quotient and inducing autism spectrum disorder-like behavior. In the present study, it was aimed to clarify the effects of chronic exposure to low concentrations of F in utero on brain function. The behavior was assessed, the levels of brain neurotransmitters were measured in mice and their relationships were analyzed. ICR mice consumed water containing sodium fluoride (F concentrations: 0, 15, or 30 ppm) from 3 weeks of age until the weaning of their pups (F1). The pups then consumed water containing the same concentration of F as their parents from weaning. At 8-weeks old, the F1 mice underwent behavioral testing using the Y-maze, elevated plus maze, Barnes maze (BM) and open-field test (OFT). At 10 weeks of age, they were euthanized, their brains were collected, and the levels of neurotransmitters were measured. Grooming events in the OFT were more frequent in F-exposed groups than in the control group, indicating that F exposure causes anxiety-like behavior. In the BM, the time taken to reach the escape box and the number of errors were higher during the training and test, suggesting spatial memory impairment. Cerebellar glutamate (Glu) concentrations were significantly lower in the F-exposed groups than in the control group. Low Glu concentration was associated with greater grooming frequency in the OFT, lower mean speed and more errors in the BM, and a delay in reaching the escape box. In the F-exposed groups, the midbrain noradrenaline concentrations were significantly lower and the number of errors in the BM was larger than in controls. Thus, F-exposed mice showed poorer spatial memory and differences in the levels of neurotransmitter, suggesting that F is an environmental contributor to disease.

Unraveling the role of abnormal AMPK and CRMP-2 phosphorylation in developmental fluoride neurotoxicity: Implications for synaptic damage and neurological disorders

Excessive fluoride exposure can be neurotoxic, although the exact mechanism remains unknown. This study aimed to investigate the neurotoxicity of continuous sodium fluoride exposure in offspring rats, focusing on the potential effects of fluoride exposure on hippocampal synaptic function and the role of AMPK and CRMP-2 in synaptic damage. We established an SD rat model of fluoride exposure (25, 50, and 100 mg/L NaF) and found that fluoride exposure damaged the learning and memory ability of F1 generation rats and caused ultrastructural changes in the hippocampus. Additionally, after the proteomic and phosphoproteomic analysis of rat hippocampal tissues, the Gene Ontology analysis revealed that sodium fluoride was involved in the enrichment of neuronal differentiation, synaptic signaling, and cytoskeleton-related biological processes. The Kyoto Encyclopedia of Genes and Genomes analysis showed that differential genes were enriched in synapse-related signaling pathways. Thus, we screened three differentially expressed proteins related to synaptic function for validation. The Western blotting analysis showed that AMPK and CRMP-2 were hyperphosphorylated in the hippocampus of fluoride-exposed rats. Our study found that abnormal AMPK and CRMP-2 phosphorylation leads to synaptic damage. This may be an important cause of memory impairment in fluorosis, offering new insights into the mechanism of fluoride-induced neurotoxicity.

Dose-dependent developmental fluoride exposure leads to neurotoxicity and impairs excitatory synapse development

Developmental fluoride exposure has been implicated in cognitive deficits and neurotoxicity, yet the mechanisms underlying these effects remain unclear. Here, we investigated the dose- and time-dependent impacts of sodium fluoride (NaF) on neuronal morphology, viability, oxidative stress, and synaptic function using both in vitro and in vivo mouse models. Cultured primary embryonic mouse cortical neurons were exposed to varying concentrations of NaF (0-200 μg/ml). Acute exposure led to neuronal swelling at higher concentrations (≥ 50 μg/ml), while prolonged exposure reduced neuronal viability. Notably, NaF dose-dependently elevated reactive oxygen species (ROS) production, implicating oxidative stress as a key mechanism of fluoride-induced neurotoxicity. Synaptic development was also impaired, as evidenced by reduced density and co-localization of excitatory synapse markers with prolonged 2 μg/ml NaF exposure. To extend these in vitro findings, pregnant mice were exposed to 50 mg/L NaF in drinking water, and offspring brain functions were evaluated postnatally. Whole-cell patch-clamp recordings in layer V pyramidal neurons in the prefrontal cortex revealed reduced frequency and amplitude of miniature excitatory post-synaptic currents (mEPSCs), indicating impaired synaptic function. Morphological analysis showed decreased dendritic spine density and head diameter. These findings suggest that fluoride exposure during critical period of brain development disrupts synaptic integrity and function through excitatory synapse impairments.

The role of GSK3β signaling mediated lysosomal biosynthesis dysregulation in fluoride-induced neurological impairment

Neurological dysfunction induced by fluoride is still one of major concern worldwide, yet the underlying mechanisms remain elusive. To explore whether fluoride disrupts lysosomal biosynthesis via the GSK3β signaling, leading to neurological damage, both in vivo rat models and in vitro PC12 cell models were conducted. Subsequent findings revealed reduced spatial learning and memory abilities, decreased hippocampal neurons, and disrupted neuronal arrangement in NaF-treated rats. In vitro, PC12 cells exhibited decreased cell viability and increased apoptosis rates after NaF treatment for 24 h. Moreover, immunofluorescence assays demonstrated that there is a reduction in the number of mature lysosomes and an increase in immature lysosomes in NaF-treated PC12 cells, evident by decreased co-localization of LAMP1 with Arl8b, and increased co-localization of LAMP1 with Rab7. Furthermore, both in vivo and in vitro, the protein expression of cleaved caspase-3 was upregulated, whereas the protein expressions of TFEB and CTSB were downregulated. The GSK3β signaling activation was detected, and this was confirmed by silencing GSK3β with siRNA in vitro. Collectively, these results indicate that NaF can impair lysosomal biosynthesis via GSK3β signaling, promoting neuronal apoptosis, and consequently impairing neurological function in rats.

Fluoride Exposure Increases the Activity of the Cystine/Glutamate Exchanger in Glia Cells

Fluoride exposure in drinking water has been widely related to impairment of cognitive function. Even though this ion has been described as neurotoxic for more than two decades, the molecular mechanisms of fluoride neurotoxicity are not fully understood, however, increasing evidence suggests that glial cells are the site of early injury in fluoride neurotoxicity. Nevertheless, a convergence point of many studies is the effect on glutamatergic neurotransmission and the generation of reactive oxygen species. In this context, we evaluated here the expression and regulation of the cystine/glutamate exchanger upon fluoride exposure since this transporter is in the interface between excitotoxicity and the antioxidant response. We demonstrate here the functional expression of the cystine /glutamate exchanger in both the U373 human glioblastoma cells and chick cerebellar Bergmann glia cells. Using a [3H]-L-Glutamate uptake assay, we demonstrate that fluoride increases the activity of the exchanger in a time and dose-dependent manner. This augmentation is mitigated by the antioxidant Trolox. To gain insight into fluoride neurotoxicity mechanisms, we evaluated its effect on human antigen R, a RNA binding protein, that binds to the 3'-UTR region of exchanger mRNA increasing its half time life. An increase in human antigen R protein was recorded after a 6 h fluoride exposure, suggesting that this ion regulates the exchanger through this RNA-binding protein. Furthermore, we show that fluoride exposure increases both the exchanger and human antigen R mRNAs half-life. These results provide insights into fluoride neurotoxicity mechanisms and support the notion of a central role of glial cells in neuronal glutamatergic transmission disruption that leads to neuronal cell death.

Sodium butyrate attenuates oxidative stress, apoptosis, and excessive mitophagy in sodium fluoride-induced hepatotoxicity in rats

AIM

Long-term exposure to excess sodium fluoride (NaF) can cause chronic fluorosis. Liver, the most important detoxification organ, is the most vulnerable to the effects of fluoride. Sodium butyrate (NaB), a short-chain fatty acid produced in the intestinal tract, maintains normal mitochondrial function in vivo and reduces liver inflammation and oxidative stress. This study aims to investigate the protective effect and potential mechanism of NaB on liver injury in fluoride poisoned rats, particularly through the mitophagy pathway.

METHODS

Rats were randomly divided into four groups of 12 male rats each: control, NaF (100 mg/mL), NaB (1000 mg/kg), and NaF (100 mg/mL)+NaB (1000 mg/kg) group.

KEY FINDINGS

Changes in the levels of liver enzymes (alanine aminotransferase [ALT] and aspartate aminotransferase [AST]) and antioxidant enzymes (superoxide dismutase [SOD], catalase [CAT], and malondialdehyde [MDA]) confirmed NaF-induced liver injury. NaF also changed the levels of autophagy markers (Beclin-1, LC3α/β, P62), and increased the level of apoptosis. The combined use of NaB and NaF significantly ameliorated these indices.

SIGNIFICANCE

These findings indicate that NaB may provide effective protection against NaF-induced liver injury through its attenuates oxidative stress, apoptosis, and excessive mitophagy mechanisms.

Quercetin, the new stress buster: Investigating the transcriptional and behavioral effects of this flavonoid on multiple stressors using Lymnaea stagnalis

Growing evidence suggests that a flavonoid-rich diet can prevent or reverse the effects of stressors, although the underlying mechanisms remain poorly understood. One common and abundant flavonoid found in numerous foods is quercetin. This study utilizes the pond snail Lymnaea stagnalis, a valid model organism for learning and memory, and a simple but robust learning paradigm-operant conditioning of aerial respiration-to explore the behavioral and transcriptional effects of different stressors on snails' cognitive functions and to investigate whether quercetin exposure can prevent stress effects on learning and memory formation. Our findings demonstrate that three different stressors-severe food deprivation, lipopolysaccharide injection (an inflammatory challenge), and fluoride exposure (a neurotoxic agent)-block memory formation for operant conditioning and affect the expression levels of key targets related to stress response, energy balance, and immune response in the snails' central ring ganglia. Remarkably, exposing snails to quercetin for 1 h before stress presentation prevents these effects at both the behavioral and transcriptional levels, demonstrating the potent stress-preventive properties of quercetin. Despite the evolutionary distance from humans, L. stagnalis has proven to be a valuable model for studying conserved mechanisms by which bioactive compounds like quercetin mitigate the adverse effects of various stressors on cognitive functions across species. Moreover, these findings offer insights into quercetin's potential for mitigating stress-induced physiological and cognitive impairments.

The role of mTORC1/TFEB axis mediated lysosomal biogenesis and autophagy impairment in fluoride neurotoxicity and the intervention effects of resveratrol

Elevated exposures to fluoride have been linked to neurological diseases. Identifying mechanisms of fluoride neurotoxicity and finding ways for prevention and treatment of epidemic fluorosis are important issues of public health. In this study, fluoride inhibited TFEB nuclear translocation by activating p-mTORC1/p-p70S6K, thus inhibiting lysosomal biogenesis, leading to dysfunctional lysosome accumulation, which further negatively affected autophagosome and lysosome fusion, thus impairing autophagy degradation, evidenced by the blocked conversion of LC3II to LC3I, and the increased p62 levels. Interestingly, RSV alleviated rats' cognition by improving fluoride-induced nerve damage and promoted lysosomal biogenesis demonstrated by the increased nucleus translocation of TFEB via inhibiting p-mTORC1 and p-p70S6K, the decreased expression of LC3II and p62. Collectively, we clarified the correlation between fluoride neurotoxicity and mTORC1/TFEB-mediated lysosomal biogenesis and autophagy. Meanwhile, RSV appeared to be a promising drug for the prevention and treatment of epidemic fluorosis.

A new insight into fluoride induces cardiotoxicity in chickens: Involving the regulation of PERK/IRE1/ATF6 pathway and heat shock proteins

Fluorosis poses a significant threat to human and animal health and is an urgent public safety concern in various countries. Subchronic exposure to fluoride has the potential to result in pathological damage to the heart, but its potential mechanism requires further investigation. This study investigated the effects of long-term exposure to sodium fluoride (0, 500, 1000, and 2000 mg/kg) on the hearts of chickens were investigated. The results showed that an elevated exposure dose of sodium fluoride led to congested cardiac tissue and disrupted myofiber organisation. Sodium fluoride exposure activated the ERS pathways of PERK, IRE1, and ATF6, increasing HSP60 and HSP70 and decreasing HSP90. The NF-κB pathway and the activation of TNF-α and iNOS elicited an inflammatory response. BAX, cytc, and cleaved-caspase3 were increased, triggering apoptosis and leading to cardiac injury. The abnormal expression of HSP90 and HSP70 affected the stability and function of RIPK1, RIPK3, and MLKL, which are crucial necroptosis markers. HSPs inhibited TNF-α-mediated necroptosis and apoptosis of the death receptor pathway. Sodium fluoride resulted in heart injury in chickens because of the ERS and variations in HSPs, inducing inflammation and apoptosis. Cardiac-adapted HSPs impeded the activation of necroptosis. This paper may provide a reference for examining the potential cardiotoxic effects of sodium fluoride.

Association between fluoride exposure in drinking water and cognitive deficits in children: A pilot study

Fluoride (F) exposure in drinking water may lead to reduced cognitive function among children; however, findings largely remain inconclusive. In this pilot study, we examined associations between a range of chronic F exposures (low to high: 0.4 to 15.5 mg/L) in drinking water and cognition in school-aged children (5-14 years, n = 74) in rural Ethiopia. Fluoride exposure was determined from samples of community-based drinking water wells and urine. Cognitive performance was measured using: 1) assessments of ability to draw familiar objects (donkey, house, and person), and 2) a validated Cambridge Neuropsychological Test Automated Battery's (CANTAB) Paired Associate Learning (PAL), which examines memory and new learning and is closely associated with hippocampus function of the brain. Associations between F and cognitive outcomes were evaluated using regression analysis, adjusting for demographic, health status, and other covariates. The median (range) of water and urine F levels was 7.6 (0.4-15.5 mg/L) and 6.3 (0.5-15.7 mg/L), respectively; these measures were strongly correlated (r = 0.74), indicating that water is the primary source of F exposure. Fluoride in drinking water was negatively associated with cognitive function, measured by both drawing and CANTAB test performance. Inverse relationships were also found between F and drawing objects task scores, after adjusting for covariates (p < 0.05). Further analysis using CANTAB PAL tasks in the children confirmed that F level in drinking water was positively associated with the number of errors made by children (p < 0.01), also after adjusting for covariates (p < 0.05). This association between water F and total errors made became markedly stronger as PAL task difficulty increased. Fluoride exposure was also inversely associated with other PAL tasksthe number of patterns reached, first attempt memory score and mean errors to success. These findings provide supportive evidence that high F exposures may be associated with cognitive deficits in children. Additional well-designed studies are critically needed to establish the neurotoxicity of F in children and adults exposed to both low levels known to protect dental caries, as well as excess F levels in drinking water.