Roles of mitochondrial fission inhibition in developmental fluoride neurotoxicity: mechanisms of action in vitro and associations with cognition in rats and children.

Fluoride neurotoxicity is associated with mitochondrial disruption. Mitochondrial fission/fusion dynamics is crucial to maintain functional mitochondria, yet little is known about how fluoride perturbs this dynamics and whether such perturbation contributes to impaired neurodevelopment. Here in human neuroblastoma SH-SY5Y cells treated with sodium fluoride (NaF, 20, 40 and 60 mg/L), mitochondrial fission suppression exerted a central role in NaF-induced mitochondrial abnormalities and the resulting autophagy deficiency, apoptosis augmentation, and compromised neuronal survival. Mechanically, pharmacological inhibition of mitochondrial fission exacerbated NaF-induced mitochondrial defects and cell death through promoting apoptosis despite partial autophagy restoration. Conversely, genetic enhancement of mitochondrial fission alleviated NaF-produced detrimental mitochondrial and cellular outcomes by elevating autophagy and inhibiting apoptosis. Further suppressing autophagy was harmful, while blocking apoptosis was beneficial for cellular survival in this context. Consistently, using Sprague-Dawley rats developmentally exposed to NaF (10, 50, and 100 mg/L) from pre-pregnancy until 2 months of delivery to mimic human exposure, we showed that perinatal exposure to environmentally relevant levels of fluoride caused learning and memory impairments, accompanied by hippocampal mitochondrial morphological alterations manifested as fission suppression and fusion acceleration, along with defective autophagy, excessive apoptosis and neuronal loss. Intriguingly, the disturbed circulating levels of identified mitochondrial fission/fusion molecules were closely associated with intellectual loss in children under long-term environmental drinking water fluoride exposure. Collectively, our results suggest that mitochondrial fission inhibition induces mitochondrial abnormalities, triggering abnormal autophagy and apoptosis, thus contributing to neuronal death, and that the mitochondrial dynamics molecules may act as promising indicators for developmental fluoride neurotoxicity.