Exercise-induced histone modifications in human skeletal muscle

Abstract

Skeletal muscle adaptations to exercise confer many of the health benefits of physical activity and occur partly through alterations in skeletal muscle gene expression. The exact mechanisms mediating altered skeletal muscle gene expression in response to exercise are unknown. However, in recent years, chromatin remodelling through epigenetic histone modifications has emerged as a key regulatory mechanism controlling gene expression in general. The purpose of this study was to examine the effect of exercise on global histone modifications that mediate chromatin remodelling and transcriptional activation in human skeletal muscle in response to exercise. In addition, we sought to examine the signalling mechanisms regulating these processes. Following 60 min of cycling, global histone 3 acetylation at lysine 9 and 14, a modification associated with transcriptional initiation, was unchanged from basal levels, but was increased at lysine 36, a site associated with transcriptional elongation. We examined the regulation of the class IIa histone deacetylases (HDACs), which are enzymes that suppress histone acetylation and have been implicated in the adaptations to exercise. While we found no evidence of proteasomal degradation of the class IIa HDACs, we found that HDAC4 and 5 were exported from the nucleus during exercise, thereby removing their transcriptional repressive function. We also observed activation of the AMP-activated protein kinase (AMPK) and the calcium-calmodulin-dependent protein kinase II (CaMKII) in response to exercise, which are two kinases that induce phosphorylation-dependent class IIa HDAC nuclear export. These data delineate a signalling pathway that might mediate skeletal muscle adaptations in response to exercise.

Figure 1. Exercise induces chromatin remodelling

A, representative immunoblots of H3K9/14 and H3K36 acetylation and total H3 levels before and immediately after exercise. B, the H3K9/14 to total H3 ratio before and immediately after exercise. C, the H3K36 to total H3 ratio before and immediately after exercise. D, total HDAC activity before and immediately after exercise. All values are reported as means ±s.e.m. (n= 6–9). *Significantly different from rest (P= 0.01).

Figure 2. Class IIa HDAC expression in human skeletal muscle and following exercise

A, class IIa HDAC mRNA levels expressed relative to cyclophilin. B, representative immunoblots of total class IIa HDAC protein before and immediately after exercise. C, total class IIa HDAC protein immediately after exercise expressed relative to resting levels. D, ubiquitin-associated HDAC5 before and immediately after exercise. All values are reported as means ±s.e.m. (n= 6–9). *Significantly different from rest (P= 0.04).

Figure 3. Exercise induces the nuclear export of HDAC4 and 5

A, representative immunoblots confirming nuclear enrichment and the nuclear abundance of class IIa HDAC protein before and immediately after exercise. B, nuclear class IIa HDAC abundance immediately after exercise expressed relative to resting levels. All values are reported as means ±s.e.m. (n= 8–9). *Significantly different from rest (P < 0.01). #Significantly different from rest (P= 0.03).

Figure 4. Exercise activates kinases that mediate phosphorylation dependent nuclear export of HDAC4 and 5

A, representative immunoblots of pT172 AMPK α, pT287 CaMKII β_M and γ/δ and pS738/742 PKD before and immediately after exercise. B, pT172 AMPK α, pT287 CaMKII γ/δ and pS738/742 PKD phosphorylation immediately after exercise expressed relative to resting levels. All values are reported as means ±s.e.m. (n= 7–9). *Significantly different from rest (P < 0.01). #Significantly different from rest (P < 0.01).