Thyroid Hormone-mediated Histone Modification Protects Cortical Neurons From the Toxic Effects of Hypoxic Injury

Thyroid hormone has been shown to have a protective role in neuronal injury, although the mechanisms have not been established. The cellular response to stress that promotes adaptation and survival has been shown to involve epigenetic modifications.

We hypothesized that the neuroprotective role of thyroid hormone was associated with epigenetic modifications of histone proteins. We used hypoxic neurons as a model system for hypoxia-induced brain injury.

Mouse primary cortical neurons were exposed to 0.2% oxygen for 7 hours, with or without, treatment with triiodothyronine (T3). We analyzed the expression of histone-modifying enzymes by RNA-seq and the post-translationally modified histone 3 proteins by enzyme-linked immunosorbent assay (ELISA) and Western blot.

We found that methylation of H3K27, associated with inactive promoters, was highly induced in hypoxic neurons, and this histone methylation was reduced by T3 treatment. H3K4 methylation is the hallmark of active promoters. The expression of 3 (Set1db, Kmta2c, and Kmt2e) out of 6 H3K4 methyltransferases was downregulated by hypoxia and expression was restored by T3 treatment. H3K4me3 protein, measured by ELISA, was increased 76% in T3-treated hypoxic neurons compared with the levels without T3 treatment. H3K56ac plays a critical role in transcription initiation and was markedly increased in T3-treated hypoxic neurons compared with those without T3 treatment, indicating stimulation of gene transcription. Additionally, T3 treatment restored hypoxia-induced downregulation of histone acetyltransferase, Kat6a, Kat6b, and Crebbp, which function as transcription factors.

These findings indicate that T3 treatment mitigates hypoxia-induced histone modifications and protects neurons from hypoxia-induced injury.