Cognitive Decline of Rats with Chronic Fluorosis Is Associated with Alterations in Hippocampal Calpain Signaling

Optimal Reference Genes for RT-qPCR Experiments in Hippocampus and Cortex of Rats Chronically Exposed to Excessive Fluoride

Normalization of the quantitative real-time PCR (RT-qPCR) data to the stably expressed reference genes is critically important for obtaining reliable results. However, all previous studies focused on F- toxicity for brain tissues used a single, non-validated reference gene, what might be a cause of contradictory or false results. The present study was designed to analyze the expression of a series of reference genes to select optimal ones for RT-qPCR analysis in cortex and hippocampus of rats chronically exposed to excessive fluoride (F-) amounts. Six-week-old male Wistar rats randomly assigned to four groups consumed regular tap water with 0.4 (control), 5, 20, and 50 ppm F- (NaF) for 12 months. The expression of six genes (Gapdh, Pgk1, Eef1a1, Ppia, Tbp, Helz) was compared by RT-qPCR in brain tissues from control and F--exposed animals. The stability of candidate reference genes was evaluated by coefficient of variation (CV) analysis and RefFinder online program summarizing the results of four well-acknowledged statistical methods (Delta-Ct, BestKeeper, NormFinder, and GeNorm). In spite of some discrepancies in gene ranking between these algorisms, Pgk1, Eef1a1, and Ppia were found to be most valid in cortex, while Ppia, Eef1a1, and Helz showed the greatest expression stability in hippocampus. Tbp and Helz were identified as the least stable genes in cortex, whereas Gapdh and Tbp are unsuitable for hippocampus. These data indicate that reliable mRNA quantification in the cortex and hippocampus of F--poisoned rats is possible using normalization to geometric mean of Pgk1+Eef1a1 or Ppia+Eef1a1 expression, respectively.

SIRT1, a target of miR-708-3p, alleviates fluoride-induced neuronal damage via remodeling mitochondrial network dynamics

INTRODUCTION

Neurological dysfunction induced by fluoride contamination is still one of major concern worldwide. Recently, neuroprotective roles of silent information regulator 1 (SIRT1) focusing on mitochondrial function have been highlighted. However, what roles SIRT1 exerts and the underlying regulative mechanisms, remain largely uncharacterized in such neurotoxic process of fluoride.

OBJECTIVES

We aimed at evaluating the regulatory roles of SIRT1 in human neuroblastoma SH-SY5Y cells and Sprague-Dawley rats with fluoride treatment, and to further identify potential miRNA directly targeting SIRT1.

METHODS

Pharmacological suppression of SIRT1 by nicotinamide (NIC) and promotion of SIRT1 by adenovirus (Ad-SIRT1) or resveratrol (RSV) were employed to assess the effects of SIRT1 in mitochondrial dysfunction induced by fluoride. Also, miRNAs profiling and bioinformatic prediction were used to screen the miRNAs which can regulate SIRT1 directly. Further, chemical mimic or inhibitor of chosen miRNA was applied to validate the modulation of chosen miRNA.

RESULTS

NIC exacerbated defects in mitochondrial network dynamics and cytochrome c (Cyto C) release-driven apoptosis, contributing to fluoride-induced neuronal death. In contrast, the ameliorative effects were observed when overexpressing SIRT1 by Ad-SIRT1 in vitro or RSV in vivo. More importantly, miR-708-3p targeting SIRT1 directly was identified. And interestingly, moreover, treatment with chemically modified miR-708-3p mimic aggravated, while miR-708-3p inhibitor suppressed fluoride-caused neuronal death. Further confirmedly, overexpressing SIRT1 effectively neutralized miR-708-3p mimic-worsened fluoride neuronal death via correcting mitochondrial network dynamics. On contrary, inhibiting SIRT1 counteracted the promotive effects of miR-708-3p inhibitor against neurotoxic response by fluoride through aggravating abnormal mitochondrial network dynamics.

CONCLUSION

These data underscore the functional importance of SIRT1 to mitochondrial network dynamics in neurotoxic process of fluoride and further screen a novel unreported neuronal function of miR-708-3p as an upstream regulator of targeting SIRT1, which has important theoretical implications for a potential therapeutic and preventative target for treatment of neurotoxic progression by fluoride.

Sodium Butyrate Ameliorates Fluorosis-Induced Neurotoxicity by Regulating Hippocampal Glycolysis In Vivo

Fluorosis can induce neurotoxicity. Sodium butyrate (SB), a histone deacetylase inhibitor, has important research potential in correcting glucose metabolism disorders and is widely used in a variety of neurological diseases and metabolic diseases, but it is not yet known whether it plays a role in combating fluoride-induced neurotoxicity. This study aims to evaluate the effect of SB on fluoride neurotoxicity and the possible associated mechanisms. The results of HE staining and Morris water maze showed that, in mice exposed to 100 mg/L fluoride for 3 months, the hippocampal cells arranged in loosely with large cell gaps and diminished in number. One thousand milligram per kilogram per day SB treatment improved fluoride-induced neuronal cell damage and spatial learning memory impairment. Western blot results showed that the abundance of malate dehydrogenase 2 (MDH2) and pyruvate dehydrogenase (PDH) in the hippocampus of fluorosis mice was increased, the abundance of pyruvate kinase M (PKM), lactate dehydrogenase (LDH), hexokinase (HK), phosphatidylinositol 3-kinase (PI3K), phosphorylated Akt (P-AKT), and hypoxia-inducible factor 1α (HIF-1α) was inhibited, and the content of lactate and ATP was decreased. SB treatment reversed the decreased glycolysis in the hippocampus of fluorosis mice. These results suggested that SB could ameliorate fluorosis-induced neurotoxicity, which might be linked with its function in regulating glycolysis as well as inhibition of the PI3K/AKT/HIF-1α pathway. Sodium butyrate ameliorates fluorosis-induced neurotoxicity by regulating hippocampal glycolysis in vivo (created with MedPeer (www.medpeer.cn)).

Fluoride Induced Neurobehavioral Impairments in Experimental Animals: a Brief Review

Fluoride is one of the major toxicants in the environment and is often found in drinking water at higher concentrations. Living organisms including humans exposed to high fluoride levels are found to develop mild-to-severe detrimental pathological conditions called fluorosis. Fluoride can cross the hematoencephalic barrier and settle in various brain regions. This accumulation affects the structure and function of both the central and peripheral nervous systems. The neural ultrastructure damages are reflected in metabolic and cognitive activities. Hindrances in synaptic plasticity and signal transmission, early neuronal apoptosis, functional alterations of the intercellular signaling pathway components, improper protein synthesis, dyshomeostasis of the transcriptional and neurotrophic factors, oxidative stress, and inflammatory responses are accounted for the fluoride neurotoxicity. Fluoride causes a decline in brain functions that directly influence the overall quality of life in both humans and animals. Animal studies are widely used to explore the etiology of fluoride-induced neurotoxicity. A good number of these studies support a positive correlation between fluoride intake and toxicity phenotypes closely associated with neurotoxicity. However, the experimental dosages highly surpass the normal environmental concentrations and are difficult to compare with human exposures. The treatment procedures are highly dependent on the dosage, duration of exposure, sex, and age of specimens among other factors which make it difficult to arrive at general conclusions. Our review aims to explore fluoride-induced neuronal damage along with associated histopathological, behavioral, and cognitive effects in experimental models. Furthermore, the correlation of various molecular mechanisms upon fluoride intoxication and associated neurobehavioral deficits has been discussed. Since there is no well-established mechanism to prevent fluorosis, phytochemical-based alleviation of its characteristic indications has been proposed as a possible remedial measure.

Fluoride in the Central Nervous System and Its Potential Influence on the Development and Invasiveness of Brain Tumours-A Research Hypothesis

The purpose of this review is to attempt to outline the potential role of fluoride in the pathogenesis of brain tumours, including glioblastoma (GBM). In this paper, we show for the first time that fluoride can potentially affect the generally accepted signalling pathways implicated in the formation and clinical course of GBM. Fluorine compounds easily cross the blood-brain barrier. Enhanced oxidative stress, disruption of multiple cellular pathways, and microglial activation are just a few examples of recent reports on the role of fluoride in the central nervous system (CNS). We sought to present the key mechanisms underlying the development and invasiveness of GBM, as well as evidence on the current state of knowledge about the pleiotropic, direct, or indirect involvement of fluoride in the regulation of these mechanisms in various tissues, including neural and tumour tissue. The effects of fluoride on the human body are still a matter of controversy. However, given the growing incidence of brain tumours, especially in children, and numerous reports on the effects of fluoride on the CNS, it is worth taking a closer look at these mechanisms in the context of brain tumours, including gliomas.

Effects of neuron autophagy induced by arsenic and fluoride on spatial learning and memory in offspring rats

There are numerous studies showing that exposure to arsenic (As) or fluoride (F) damages the nervous system, but there is no literature investigating the effects of combined As and F exposure to induce autophagy on neurotoxicity in the offspring. In this study, we developed a rat model of As and/or F exposure through drinking water from before pregnancy to 90 days postnatal. The offspring rats were randomly divided into nine groups. Sodium arsenite (NaAsO₂) (0, 35, 70 mg/L) and Sodium fluoride (NaF) (0, 50, 100 mg/L) were designed according to 3 × 3 factorial design. Our results suggested that the presence of F might antagonize the excretion of total As in urine, and As-F co-exposure led to severe pathological damage in brain tissue and reduced spatial learning and memory ability. At the same time, the experiments showed that As and F increased Beclin1 expression and LC3B ratio to activate autophagy; both P62 and Lamp2 expression were increased, suggesting that autophagy lysosomal degradation was blocked; SYN and JIP1 expression were significantly decreased, disrupting synaptic structure and function. Axonal autophagosome reverse transport regulation might be affected by combined As-F exposure, exacerbating neuronal synaptic damage and inducing neurotoxicity. Further analysis showed that there was an interaction between As and F exposure-induced changes in autolysosome-related proteins in the hippocampus, which showed antagonism, and the antagonism of the high As combined exposure groups were stronger than that of the low As combined exposure groups. In conclusion, our study showed that combined As and F exposure might induce reverse transport impairment of autophagy on axons, leading to autophagy defects, which in turn led to disruption of synaptic morphology and function, induced neurotoxicity, and there was an interaction between As and F, the type of its combined effect was antagonism.

Fluoride-Induced Cortical Toxicity in Rats: the Role of Excessive Endoplasmic Reticulum Stress and Its Mediated Defective Autophagy

The cerebral cortex is closely associated with learning and memory, and fluoride is capable of inducing cortical toxicity, but its mechanism is unclear. This study aimed to investigate the role of endoplasmic reticulum stress and autophagy in fluoride-induced cortical toxicity. Rats exposed to sodium fluoride (NaF) were used as an in vivo model. The results showed that NaF exposure impaired the learning and memory capacities and increased urinary fluoride levels in rats. In addition, NaF exposure induced excessive endoplasmic reticulum stress and associated apoptosis, as evidenced by elevated IRE1α, GRP78, cleaved caspase-12, and cleaved caspase-3, as well as defective autophagy, as evidenced by increased expression of Beclin1, LC3-II, and p62 in cortical areas. Importantly, the endoplasmic reticulum stress inhibitor 4-phenylbutyric acid (4-PBA) alleviated endoplasmic reticulum stress as well as defective autophagy, thus confirming the critical role of endoplasmic reticulum stress and autophagy in fluoride-induced cortical toxicity. Taken together, these results suggest that excessive endoplasmic reticulum stress and its mediated defective autophagy lead to fluoride-induced cortical toxicity. This provides new insights into the mechanisms of fluoride-induced neurotoxicity and a new theoretical basis for the prevention and treatment of fluoride-induced neurotoxicity.

Study of Chitosan Ingestion Remitting the Bone Damage on Fluorosis Mice with Micro-CT

Chronic excessive fluoride exposure may lead to fluorosis, which causes health problems like a decrease in bone mechanical strength. It was speculated that chitosan may combine with fluorine to form in vivo organic fluorine, and may reduce the damage caused by fluorine. Hence, it is necessary to conduct a study to investigate the influence of chitosan on fluorosis mice. To investigate this problem, forty-four 4-week-old male Kunming mice were randomly divided into four groups, the control group, the fluoride group, the fluoride plus chitosan group, and the chitosan group. After 100 days of feeding, the femurs were collected to scan the Micro-CT image. The ultimate load of the femur in the fluoride group was significantly lower than control group. The trabecular separation was increased in the fluoride group compared with the fluoride plus chitosan group and the chitosan group. The level of trabecular thickness was increased in the fluoride plus chitosan group compared with the fluoride group. Our findings suggest that chitosan ingestion can improve the condition of cancellous bone and cortical bone affected by fluorine.

Weakened interaction of ATG14 and the SNARE complex blocks autophagosome-lysosome fusion contributes to fluoride-induced developmental neurotoxicity

Fluoride is capable of inducing developmental neurotoxicity, but the mechanisms involved remain unclear. We aimed to explore the role of autophagosome-lysosome fusion in developmental fluoride neurotoxicity, particularly focusing on the interaction between ATG14 and the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. We developed in vivo models of Sprague-Dawley rats exposed to sodium fluoride (NaF) from the pregnancy of parental rats until the offspring were two months old and in vitro models of NaF and/or Ad-ATG14-treated SH-SY5Y cells. We assessed neurobehavioral changes in offspring and further investigated the effects of NaF exposure on autophagic flux, apoptosis, autophagosome-lysosome fusion, and the interaction between ATG14 and the SNARE complex. NaF exposure impaired offspring learning and memory capabilities and induced the accumulation of autophagosomes and autophagic flux blockage and apoptosis, as indicated by increased LC3-II, p62, and cleaved-caspase-3 expression in vivo and in vitro. In addition, NaF treatment downregulated the protein expression of ATG14 and the SNARE complex and induced autophagosome-lysosome fusion blockage as evidenced by decreased ATG14, STX17, SNAP29, and VAMP8 expression and diminished colocalization of autophagosomes and lysosomes in vivo and in vitro. Furthermore, ATG14 upregulation enhanced the interaction of ATG14 and the SNARE complex to facilitate autophagosome-lysosome fusion, thereby restoring autophagic flux and alleviating NaF-induced apoptosis. In conclusion, NaF exhibited developmental neurotoxicity by restraining the interaction of ATG14 with the SNARE complex and hindering autophagosome-lysosome fusion, thereby participating in the occurrence and development of fluoride neurotoxicity. Notably, ATG14 upregulation protects against developmental fluoride neurotoxicity, and ATG14 may serve as a promising biomarker for further epidemiological investigation.

Differential protein expression of mitogen-activated protein kinases in erythrocytes and liver of lamprey Lampetra fluviatilis on the course of prespawning starvation

The study was designed to identify the types of mitogen-activated protein kinases (MAPKs) in erythrocytes and liver tissues of river lamprey Lampetra fluviatilis and monitor the changes in protein expression levels of found enzymes on the course of prespawning starvation (from November to the end of May). Immunoreactivity of the native and phosphorylated forms of ERK1/2, JNK and p38 was examined in the cytosolic and membrane cell fractions. Both lamprey erythrocytes and liver were found to highly express ERK1/2 and JNK, whereas only trace amounts of p38 were revealed in hepatic tissues. ERK1/2 was identified in cytosolic and membrane fractions, whereas JNK and p38 were predominantly cytosolic enzymes. Total cellular amounts of ERK1/2 and phospho-ERK1/2 in both erythrocytes and liver tissues appeared to be relatively stable on the course of prespawning starvation. However, before spawning ERK1/2 translocated from cytosol to membranes, with partial decline of its cytoplasmic expression being compensated by increases in membrane-bound pool. Immunoreactivity of cytoplasmic JNK, phospho-JNK and p38 were stable from November to March, but sharply decreased before spawning exhibiting almost negligible levels in May, which suggests the depletion of their cellular fractions. Most probably, ERK1/2 plays more important role in mediating adaptive responses of erythrocytes and liver tissues to conditions of natural starvation and maintenance of cell viability before spawning and death of animals in May.

Inorganic fluoride and functions of brain

Although actively disputed and questioned, it has been proposed that chronic exposure to inorganic fluoride (F^-) is toxic for brain. The major question for this review was whether an excessive F^- intake is causally related to adverse neurological and cognitive health conditions in human beings and animals. The paper systematically and critically summarizes the findings of the studies showing positive associations between F^- intoxication and various intellectual defects, as well as of those which attempted to clarify the nature of F^- neurotoxicity. Many works provide support for a link between pre- and postnatal F^- exposure and structural and functional changes in the central nervous system responsible for neurological and cognitive disorders. The mechanisms suggested to underlie F^- neurotoxicity include the disturbances in synaptic transmission and synaptic plasticity, premature death of neurons, altered activities of components of intracellular signaling cascades, impaired protein synthesis, deficit of neurotrophic and transcriptional factors, oxidative stress, metabolic changes, inflammatory processes. However, the majority of works have been performed on laboratory rodents using such F^- doses which are never exist in the nature even in the regions of endemic fluorosis. Thus, this kind of treatment is hardly comparable with human exposure even taking into account the higher rate of F^- clearance in animals. Of special importance are the data collected on humans chronically consuming excessive F^- doses in the regions of endemic fluorosis or contacting with toxic F^- compounds at industrial sites, but those works are scarce and often criticized due to low quality. New, expertly performed studies with repeated exposure assessment in independent populations are needed to prove an ability of F^- to impair neurological and intellectual development of human beings and to understand the molecular mechanisms implicated in F^--induced neurotoxicity.