Age-related differences in apoptosis with disuse atrophy in soleus muscle

Abstract

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 soleus muscle of young (6 mo) and old (32 mo) male Fischer 344 x Brown Norway rats subjected to acute disuse atrophy induced by 14 days of hindlimb suspension (HS). HS-induced atrophy (reduction in muscle weight and cross-sectional area) was associated with loss of myofiber nuclei in soleus muscle of young, but not old, rats. This resulted in a significant decrease in the myonuclear domain (cross-sectional area per nucleus) in young and old rats, with changes being more pronounced in old animals. Levels of apoptosis (TdT-mediated dUTP nick end labeling and DNA fragmentation) were higher in soleus muscles of old control rats than young animals. Levels were significantly increased with HS in young and old rats, with the greatest changes in old animals. Caspase-3 activity in soleus muscle tended to be increased with age, but changes were not statistically significant (P=0.052). However, with HS, caspase-3 activity significantly increased in young, but not old, rats. Immunohistochemistry showed that the proapoptotic 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 young, control animals. There was no difference between EndoG total protein content in young and old control rats, but EndoG increased almost fivefold in soleus muscle of old, but not young, rats after HS. These results show that deregulation of myonuclear number occurs in old skeletal muscle and that the pathways involved in apoptosis are distinct in young and old muscles. Apoptosis in skeletal muscle is partly mediated by the subsarcolemmal mitochondria through EndoG translocation to the nucleus in response to HS.

Multiple Pathways to the Same End: Mechanisms of Myonuclear Apoptosis in Sarcopenia of Aging

“…These observations have cast doubts about the involvement of mitochondrial caspase-dependent apoptosis in the pathogenesis of sarcopenia. In contrast, several studies have shown that caspase-independent apoptotic pathways are activated in old age [12,18,50,61]. It was found that translocation of mitochondrial EndoG to the nucleus was increased in the soleus muscle of old rats.…”

Death receptor-associated pro-apoptotic signaling in aged skeletal muscle

“…Although this phenomenon is well documented in animal and human studies, the underlying mechanisms leading to sarcopenia remain to be fully elucidated. The contributing mechanism(s) leading to sarcopenia are multi-factorial and may include: denervation and reinnervation of motor units, especially within skeletal muscles composed of a high degree of type II muscle fibers [1][2][3]; an alteration in the hormonal milieu in which anabolic hormone concentrations progressively decline [4,5]; elevated concentrations of inflammatory mediators that are not only associated with disease states, but also occur in healthy older adults [6][7][8][9][10][11][12][13]; and myonuclear loss through apoptotic mechanisms [14][15][16][17][18][19].…”

“…The process of nuclear apoptosis has been shown to contribute to skeletal muscle mass losses during disease states [20], periods of inactivity [15,17,19,21], and as a result of aging [14]. The presence of apoptotic nuclei within skeletal myofibers has been welldocumented [18,19,22,23], although the precise signaling mechanisms that result in these nuclear losses are just beginning to be fully understood.…”

“…The presence of apoptotic nuclei within skeletal myofibers has been welldocumented [18,19,22,23], although the precise signaling mechanisms that result in these nuclear losses are just beginning to be fully understood. Proapoptotic signaling has been shown to arise from mitochondria in aged skeletal muscles [14,15,17,19] and this has been termed the intrinsic apoptotic pathway. Pro-apoptotic members of the BCL-2 family of proteins, such as Bax and Bak, can promote cytochrome-c release from the mitochondrial intermembrane space initiating a caspase cascade resulting in apoptotic myonuclear loss [24].…”

“…Pro-apoptotic members of the BCL-2 family of proteins, such as Bax and Bak, can promote cytochrome-c release from the mitochondrial intermembrane space initiating a caspase cascade resulting in apoptotic myonuclear loss [24]. Although it appears that this intrinsic apoptotic program is well-conserved across muscles of differing fiber type and contractile activity, a pro-apoptotic environment appears to exist in aged skeletal muscles that can exacerbate myonuclear loss following apoptotic stimuli [15,17,19].…”