Rapamycin mitigates neurotoxicity of fluoride and aluminum by activating autophagy through the AMPK/mTOR/ULK1 pathway in hippocampal neurons and NG108-15 cells

Our previous studies have confirmed that fluoride combined with aluminum (FA) can induce hippocampal neuron damage in the second-generation offspring (F2) of rats; however, the underlying mechanisms remain unclear. In this study, we established an F2 rat model and an NG108-15 cell model to investigate the potential modes of action. The autophagy of F2 rat hippocampal neurons and NG108-15 cells was assessed using transmission electron microscopy and immunofluorescence/immunocytochemistry kit, respectively. Hippocampal morphology was evaluated via hematoxylin-eosin (HE) staining. We measured mRNA levels of AMPK, mTOR, ULK1, and LC3 using quantitative reverse transcription PCR, and protein expressions were analyzed by Western blotting. Following treatment with rapamycin (Rap) in FA-exposed F2 rats and NG108-15 cells, a small number of primary lysosomes and autophagosomes appeared within hippocampal cells, with HE staining indicating a near-normal restoration of pyramidal cell morphology. The quantity, intensity, and volume of green fluorescent spots in the cytoplasm of NG108-15 cells increased as observed through fluorescence microscopy. The mRNA expressions of AMPK, ULK1, and LC3 were upregulated while mTOR expressions were downregulated in NG108-15 cells. Correspondingly, protein levels for AMPK, p-AMPK, ULK1, p-ULK1 along with the LC3-II/LC3-I ratio increased whereas those for mTOR, p-mTOR and p62 decreased significantly. Similar trends regarding both mRNA and protein expression were noted within the hippocampus of F2 rats as well. Activation of the AMPK/mTOR/ULK1 signaling pathway by Rap enhances FA-induced autophagy thereby mitigating neuronal damage.