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1 Geriatrics, University of Arkansas for Medical Sciences, Little Rock, Arkansas, United States; Physiology and Biophysics, United States
2 Little Rock, Arkansas, United States; Geriatrics, University of Arkansas for Medical Sciences, Little Rock, Arkansas, United States
3 Geriatrics, University of Arkansas for Medical Sciences, Little Rock, Arkansas, United States
4 Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, Arkansas, United States; Center for Orthopaedic Research, University of Arkansas for Medical Sciences, Little Rock, Arkansas, 72205, United States
5 Health and Kinesiology, Texas A&M University, College Station, Texas, United States
6 Geriatrics, University of Arkansas for Medical Sciences, Little Rock, Arkansas, United States; Physiology and Biophysics, United States; Central Arkansas Veterans Health Care System, Little Rock, Arkansas, United States
* To whom correspondence should be addressed. E-mail: dupontesthere{at}uams.edu.
Skeletal muscle atrophy is associated with an increase in apoptosis and we showed previously that endonuclease G (EndoG) is localized to nuclei following unloading. The goal of this study was to determine if the onset of apoptosis in response to disuse was consistent with the hypothesis that EndoG is involved in myofiber nuclear loss. Atrophy was induced by hindlimb suspension for 12 hours, 1, 2, 4 and 7 days in 6 month-old rats. Soleus myofiber cross sectional area decreased significantly by 2 days, while muscle mass and muscle-to-body mass ratio decreased by 4 and 7 days, respectively. By contrast, a significant increase in apoptosis, evidenced by TUNEL-positive nuclei, occurred as early as 12 hours after suspension, preceding the elevation in muscle atrophy F-box gene expression. The early increase in apoptosis appeared to be specific to myofiber nuclei, whereas TUNEL-positive interstitial cells did not become significantly elevated until 2 days after suspension. Further, TUNEL-positive myofiber nuclei co-localized with EndoG as early as 12 hours after suspension and no such localization was observed in interstitial cells. Although no significant change in total activated caspase-3, -7, or -12 protein abundance was apparent, activated caspase-3 was expressed in interstitial cells undergoing apoptosis, some of which were endothelial cells. These data indicate that apoptosis is an early, and therefore possibly causative, event in the process of muscle atrophy, and that EndoG nuclear translocation is specific for myofiber nuclear apoptosis, while interstitial cells may undergo apoptosis via a more classical, caspase-dependent pathway.
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