TET3 downregulation induces circadian disruption of BMAL1 through its DNA methylation-mediated repression in Alzheimer’s disease

Circadian disruptions appear at the presymptomatic stage of Alzheimer's disease (AD) and may exacerbate mental dysfunction in AD. The downregulation of brain and muscle ARNT-like protein 1 (BMAL1), a key clock element for the maintenance of circadian rhythms, has been linked to epigenetic mechanisms. Our previous study revealed that the mRNA level of DNA demethylase ten-eleven translocation ( Tet) 3 was reduced in the hippocampi of APPswe/PS1dE9 (APP/PS1) mice. However, the effects of TET3 on BMAL1 downregulation and circadian dysregulation in AD are still unclear. Our investigation first confirms that Tet3 mRNA and protein levels are decreased in both APP/PS1 mice and APPswe cells. In addition, decreased levels of 5hmC are observed in HT22 cells after TET3 knockdown, whereas TET3 overexpression reverses the reduction in 5hmC. Critically, we report that TET3 knockdown remethylates the Bmal1 promoter, thus downregulating BMAL1 expression in HT22 cells. In contrast, TET3 overexpression could upregulate BMAL1 by decreasing its methylation level. These results indicate that reduced TET3 is responsible for BMAL1 downregulation through decreased TET3 demethylation. Additionally, TET3 knockdown could lead to circadian disruption of BMAL1 in U2OS cells, whereas overexpression of TET3 alleviates the dysregulated biological rhythm in Aβ-treated U2OS cells. Our data suggest that TET3 plays a vital role in modulating the circadian rhythm at the epigenetic level through DNA demethylation.