Effects Of Folic Acid On Alzheimer’s Disease Models By DNA Methylation

ObjectiveAlzheimer’s disease(AD), the principal cause of senile dementia, is characterized by the presence of neurofibrillary tangles and senile plaques. AD is associated with malnutrition, folate acts through one-carbon metabolism to support the methylation of multiple substrates including DNA and protein, altered one-carbon metabolism and increased hippocampal amyloid-β peptide(Aβ) accumulation and tau phosphorylation. Aberrant DNA or protein methylation may be an epigenetic mechanism that underlies AD pathogenesis.MethodsThree kinds of cell models were used in this study. One was mouse Neuro-2a cells expressing human APP695, another was neuronal cell exposed oligomers Aβ,the third one was human neuroblastoma cells(SH-SY5Y) exposed to phosphoesterase inhibitor okadaic acid(OA).First, N2a-APP cells were incubated with folic acid(2.8-40 μmol/L), in the presence or absence of the DNMT inhibitor zebularine. The effects of folic acid on methylation potential, DNA methyltransferase(DNMT), APP, PS1 and Aβ production were determined. And then primary hippocampal neuronal cells and hippocampal HT-22 cells were incubated for 24 h with a combination of folic acid and either oligomers Aβ or vehicle and then were incubated for 72 h with various concentrations of folic acid. The effects of folic acid on DNMT, APP, PS1 and cell viability in AD cell models were determined. Last, to determine the function of folic acid on tau phosphorylation, an in vitro model of SH-SY5 Y were exposed to folic acid(0-40μmol/L) for 96 h, in the presence or absence of OA(10 nmol/L) for 9 h.In vivo, APP/PS1 mice were fed either folate-deficient or control diets and gavaged daily with 120 μg/kg folic acid or 13.3 mg/kg S-adenosylmethionine(SAM),or both. The effects of folic acid on APP, PS1 and Aβ production were determined.ResultsFirst, the results of N2a-APP cells showed that folic acid stimulated methylation potential, DNMT gene and protein expression, DNMT activity, and DNA methylationof APP and PS1 genes. Furthermore, folic acid decreased APP gene expression, PS1 gene and protein expression, and Aβ protein production, whereas inhibition of DNMT activity by zebularine increased Aβ production.And then it was observed that oligomers Aβ lowered DNMT activity, increased PS1 and APP expression, and decreased cell viability. Folic acid dose-dependently stimulated methylation potential and DNMT activity, altered PS1 and APP gene promoters’ methylation, decreased PS1 and APP expression, and partially preserved neuron cell viability.Last, the data of western blot showed tau phosphorylation at the Ser396 site in OA-incubated SH-SY5 Y cells was inhibited by folic acid in a concentration-dependent manner. Folic acid could downregulate tau protein phosphorylation by inhibiting the demethylation reactions of PP2 A. High folic acid concentrations(20 and 40 μmol/L) increased SAM: S-adenosylhomocysteine(SAH)ratios and cell viability.In vivo, the data showed that serum folate concentration increased with intake of folic acid but not SAM. Folate deficiency lowered endogenous SAM concentration,whereas neither intervention altered SAH concentration. DNMT activity increased with intake of folic acid raised DNMT activity. DNA methylation rate was stimulated by folic acid in APP and PS1 promoter. Folic acid deficiency elevated hippocampal APP, PS1 and Aβ protein levels and these rises were prevented by folic acid.ConclusionsFolic acid induces methylation potential-sensitive DNMT enzymes that methylate and silence APP and PS1 genes, thereby attenuating Aβ production. These results suggest a mechanism by which folic acid may prevent oligomers Aβ-induced neuronal toxicity. And then folate deficiency may be a cause of PP2 A deregulation,which can in turn lead to expression of the abnormal hyperphosphorylated form of tau.