## 3 Histone Modifications Regulated by Exercise  
Acute and chronic resistance exercise alters histone modifications in human skeletal muscles. [PERSON] et al. [24] reported that acute resistance exercise significantly upregulated the expression of 153 genes with gene ontology terms including muscle development, stress response, metabolism, cell death, and transcription factors. Upregulation of the distribution of acetylated H3, H3K4 me1, and H3K27 me3 and downregulation of that of histone variant H3.3 were observed at these loci after acute resistance exercise compared to the pre-exercise levels. After 10 weeks of resistance exercise training, the expression of these genes was maintained at a higher level than the pretraining level. The distribution of acetylated H3 at these loci was found to be upregulated after resistance exercise training. These results indicated that histone modifications in the skeletal muscles robustly responded to acute exercise, although chronic exercise training transformed the skeletal muscle chromatin toward the transcriptionally active status. [PERSON] et al. [25] reported that disuse atrophy was not induced in the hindlimb muscles of adult rats with prior running training experience in early life. They also found in RNA sequencing analysis of the plantaris muscle that a subset of genes that were upregulated in sedentary rats after unloading were less responsive in the rats with prior running training experience. The distribution of total H3 was significantly decreased at these loci during the training period (Figure 3A), suggesting that nucleosome formation was diminished by running training. The nucleosomes were restored after the detraining period, although the incorporation of histone variant H3.3 was noted in the rats with prior running training experience (Figure 3A). This result also shows that histone H3 is accumulated with age, independent of the experience of exercise training. [PERSON] et al. [26] further tested the effects of total amount or duration of running training on histone modifications in rat plantaris muscle. An exercise training-associated decrease in total H3 distribution was observed to be dependent on the amount of daily exercise when the ChIP-qPCR analysis was performed at the same loci targeted by [PERSON] et al. [25] (Figure 3B). The results also showed that acetylated H3 accumulated during the running training if the total H3 distribution remained unchanged. These observations suggested that exercise training stimulated the dissociation of histones from nucleosomes as well as the exchange of histone components with newly synthesized histones such as H3.3, a phenomenon known as \"histone turnover.\"A nucleosome consists of DNA wrapped around an octameric protein complex composed of a histone H3-H4 tetramer flanked by two histone H2A-H2B dimers. It has been reported that H2A-H2B dimers and H3-H4 tetramers are independently exchanged in non-replicating yeast cells [27]. Histone turnover appears to correlate with gene transcription, as it regulates the accessibility of DNA to transcriptional machinery, such as transcription factors and Pol II [28; 29]. Our recent study [30] has revealed that chronic exercise training activated histone turnover in the skeletal muscle fibers (Figure 4). Using a tetracycline-inducible H2B-GFP expression model, it was demonstrated that 4 weeks of running training significantly promoted the incorporation of H2B-GFP into nucleosomes as well as the dissociation of total histones at both transcriptionally upregulated and nonresponsive loci in mouse tibialis anterior muscle fibers; such effects were not observed upon 2 weeks of training. Gene expression responses to a single bout of running were significantly enhanced in 4 week-trained mice when compared to those in 2 week-trained mice. These results indicated that activation of histone turnover by exercise training led to the loosening of nucleosomes, which enhanced the gene responsiveness to exercise.