## 2 Histone Modifications in Fast- and Slow-Twitch Muscles  
Slow-twitch skeletal muscles are characterized by higher-energy metabolism. It is also known that slow-twitch muscle-specific characteristics are acquired through exercise training. The enhanced capability of mitochondrial oxidative phosphorylation in skeletal muscles by exercise training has been demonstrated in numerous rodent models. However, the limit of adaptive changes to exercise training in the skeletal muscles has also been shown in several studies [14; 15]. Running training attenuated the characteristics of fast-twitch muscles and produced muscle fibers that were more oxidative and insulin sensitive without showing any phenotypic transformations [14; 15]. Other previous studies have tested the transgene models. For example, [PERSON] et al. [16], using a transgenic mouse model, demonstrated that skeletal muscle-specific overexpression of peroxisome proliferatorator-activated receptor \(\gamma\) coactivator-1\(\alpha\) isoform b (PGC-1\(\alpha\)-b), which is known to be upregulated in response to exercise-related stimuli, exhibited the enhanced oxidative capacity of the mitochondria in skeletal muscle and longer endurance capability, whereas the gene expression of type I myosin heavy chain was not stimulated in the gastrocnemius muscle of the transgenic mice. Myozenin 1, a muscle-specific modulator of calcineurin activity, is exclusively expressed in fast-twitch fibers of the skeletal muscle [17; 18]. [PERSON] et al. [19] have reported that the phenotypic shift of muscle fibers from type II to type I did not occur in the fast-twitch gastrocnemius muscle of myozenin 1-deficient mice, although the distribution of type IIa fibers increased in these mice. Furthermore, lineage tracing performed using mice with _Myh/7_-dependent reporter gene expression by [PERSON] et al. [20], revealed that all fibers in the soleus muscle, but only a subset of fast-twitch fibers in the extensor digitorium longs muscle, activated type I myosin heavy chain gene transcription. These observations indicated that the limit for the fiber-type transformation capability lies between fast- and slow-twitch skeletal muscle fibers. We have previously determined the genome-wide differences in histone modifications between fast-twitch plantaris and slow-twitch soleus muscles in adult rats using chromatin immunoprecipitation (ChIP) followed by next-generation sequencing analysis [21]. Active histone modifications, such as H3K4 me3 and H3 acetylation, were prevalent at the transcriptionally activated loci in the plantaris muscle (Figure 1A,C). However, no correlation was observed between the distribution of active histone modifications and gene expression in the soleus muscle (Figure 1B,D). It was also noted that the distribution of total H3, which might reflect the level of nucleosome deposition, was slightly less at both upstream and downstream locations from the transcription start site of muscle-related loci in the soleus muscle than that observed in the plantaris muscle (Figure 1E,F). Enhanced muscle activity in the plantaris muscle by synergistically tendon transection resulted in significant upregulation of slow-twitch muscle-specific genes, while acetylated H3 distribution at these loci was decreased [21]. However, the expression of fast-twitch muscle-specific genes was not lowered by enhanced muscle activity due to H3K4 me3 level maintained at these loci. Interestingly, we also found that fast-twitch muscle-specific genes, in which active histone modifications were prevalent, were already upregulated at birth, independent of the muscle activity [21]. Conversely, activation of other slow genes started after the beginning of voluntary locomotion, although the magnitude of the upregulation was less pronounced when compared to that of the fast-twitch muscles. These results suggestedthat slow-twitch skeletal muscles have a unique epigenetic system that regulates the gene expression, which might be closely associated with muscle activity.