Phosphorylation-mimicking histone H3.3 rescues exercise-induced gene responses in an epigenetic aging model of mouse skeletal muscle.

Background: With aging, the canonical histone H3.1/3.2 in skeletal muscle is progressively replaced by the non-canonical variant H3.3. Although H3.3 is thought to be involved in age-related epigenetic regulation due to its role as a histone variant, its functional characteristics remain largely unknown. Serine 31 (S31) is a unique amino acid residue of H3.3 that undergoes phosphorylation. Therefore, the present study aimed to investigate the relationship between skeletal muscle aging and H3.3 phosphorylation at S31 (H3.3S31ph).

Results: We first demonstrated that H3.3S31ph levels were significantly reduced in the tibialis anterior muscle of 75-wk-old mice compared to 8-wk-old mice. We then examined the effects of viral vector-mediated expression of wild-type H3.3 or a phosphorylation-mimicking H3.3 mutant (H3.3S31E) on gene responsiveness to acute exercise in aging skeletal muscle. In muscles expressing wild-type H3.3, which simulates epigenetic alterations observed during skeletal muscle aging, the transcriptional response to acute exercise was lost by 30 weeks post-treatment (60 weeks of age). In contrast, expression of H3.3S31E successfully rescued the gene responses to acute exercise. This rescue was accompanied by increased enrichment of H3K4me3 and H3K27me3 following acute exercise in the H3.3S31E group, whereas no such histone modification changes were observed in the wild-type H3.3 group. Additionally, robust involvement of exogenous H3.3 in exercise-related histone turnover was observed in the wild-type H3.3 group, but not in the H3.3S31E group, suggesting that phosphorylation at S31 limits the dynamic behavior of H3.3.

Conclusions: Impaired transcriptional responsiveness to exercise in a simulated epigenetic aging model induced by exogenous H3.3 expression was rescued by the phosphorylation-mimicking H3.3S31E variant in middle-aged skeletal muscle. The findings of the present study demonstrate that H3.3S31ph plays a critical role in regulating the stability of H3.3 within chromatin.