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1 Biochemistry of Aging Laboratory, University of Florida, Gainesville, FL, USA
2 Geriatrics, University of Arkansas for Medical Sciences, Little Rock, AR, USA
3 Geriatrics, University of Arkansas for Medical Sciences, Little Rock, AR, USA; Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, AR, USA
* To whom correspondence should be addressed. E-mail: dupontesthere{at}uams.edu.
Muscle atrophy is associated with a loss of muscle fiber nuclei most likely through apoptosis. We investigated age-related differences in the extent of apoptosis in the soleus muscle of young (6 months) and old (32 months) male Fischer 344 X Brown Norway rats subjected to acute disuse atrophy, induced by 14 days of hind limb suspension (HS). HS induced-atrophy (reduction in muscle weight and cross-sectional area) was associated with a loss of myofiber nuclei in the soleus muscle of young, but not old rats. This resulted in a significant decrease in the myonuclear domain (cross-sectional area/nucleus) in young and old rats, with changes being more pronounced in the old animals. Moreover, levels of apoptosis (TUNEL and DNA fragmentation) were higher in the soleus muscles of old control rats than in young animals. Levels were significantly increased with HS in young and old rats, with the greatest changes seen in old animals. Caspase-3 activity in soleus tended to be increased with age, but changes were not statistically significant (p=0.052). However, upon HS caspase-3 activity significantly increased in young, but not old rats. Immunohistochemistry showed that the pro-apoptotic endonuclease G (EndoG; a mitochondrion-specific nuclease) was localized in the subsarcolemmal mitochondria in control muscles and translocation to the nucleus occurred in old, but not in young control animals. There was no difference between EndoG total protein content observed in young and old rats, but EndoG increased almost fivefold in the soleus muscle of old, but not young rats after HS. These results show that deregulation of myonuclear number occurs in skeletal muscle at old age and that the pathways involved in apoptosis are distinct in young and old muscles. Moreover, apoptosis in skeletal muscle is partly mediated by the subsarcolemmal mitochondria through EndoG translocation to the nucleus in response to hind limb suspension.
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