Regulation of small Hsp expression and phosphorylation in functionally overloaded rat muscles

Abstract

Functional overload (FO) is a powerful inducer of hypertrophy and both oxidative and mechanical stress in muscle fibers. The small Hsps may protect against these stressors and their expression can be regulated by muscle loading. This study tested the hypothesis that chronic FO increases Hsp20, Hsp25, and α β-crystallin (α β C) expression in hypertrophying rat muscle. Protein levels were quantified with Western analysis in the soleus and plantaris. Hsp25 phosphorylation (pHsp25) and shift to the insoluble fraction were measured to determine if FO increases translocation of Hsp25 and/or pHsp25 to the insoluble fraction. p38 protein and phosphorylation (p-p38) was measured to determine its association with pHsp25. Three or 7d of FO increased Hsp25 and pHsp25 in both muscles, with a greater response in the plantaris. Significant increases in Hsp20 and α β C were not observed until 7d of FO in either muscle. In the insoluble fraction, Hsp25 was increased after 3 or 7d in both muscles while pHsp25 was only increased after 7d in the plantaris. p38 and p-p38 increased in the plantaris at both time points, but only after 7d in the soleus. These data support the hypothesis that chronic FO is associated with significant time- and muscle- dependent increases in the small Hsps. Increases in Hsp25 in the insoluble fraction suggest it may help stabilize actin during muscle remodeling. This work was supported by NIAMS RO3 AR049855.