Age-related histone H3.3 accumulation associates with a repressive chromatin in mouse tibialis anterior muscle

Age-related alterations in epigenetic regulation are postulated to result in the disorganization of cellular functions of skeletal muscles. The accumulation of the non-canonical histone variant H3.3 increases with age in several organs and exhibits tissue-specific patterns of histone modifications. However, it is unclear how histone distribution and modifications in skeletal muscle are affected by aging. The present study aimed to investigate age-related changes in H3.3 and its role in the aging process of mouse skeletal muscles. We first analyzed age-related changes in the morphology of the tibialis anterior muscle and age-related changes in gene expression and histone distribution at target loci in the tibialis anterior muscles in mice of various ages. A significant decrease in muscle weight and the number of myonuclei was observed at 53-wk-old. H3.3 levels significantly increased with age and correlated with H3K27me3 levels. Chromatin immunoprecipitation analysis showed similar changes at both transcriptionally upregulated and downregulated loci. Next, we assessed the effect of acute exercise on gene expression and histone distribution between 8- and 53-wk-old mice. No upregulation in gene expression in response to acute exercise was noted in 53-wk-old mice. H3K27me3 levels were increased in certain loci in response to acute exercise in 8-wk-old mice. However, in 53-wk-old mice, H3.3 and H3K27me3 levels were increased at rest and were not affected by acute exercise. Furthermore, we assessed the effects of forced H3.3 expression in the skeletal muscles of 8-wk-old mice. The mice were given a viral vector expressing H3.3 under the control of a skeletal muscle-specific promoter. The latency to fall in the rotarod test significantly improved in mice with forced H3.3 expression. Downregulation of gene expression was noted in the tibialis anterior muscle of mice with forced H3.3 expression. H3.3 incorporation into the nucleosomes at these loci was promoted by forced H3.3 expression, although H3K27me3 distribution was reduced at these loci. Collectively, these results suggest that H3.3 accumulation increased with age in skeletal muscle and induced the formation of repressive chromatin in association with H3K27me3. Further, these results also suggest that H3.3 accumulation plays a positive role in muscle function if H3K27me3 is unmodified.