Age-related activation of mitochondrial caspase-independent apoptotic signaling in rat gastrocnemius muscle

Abstract

Mitochondria-mediated apoptosis represents a central process driving age-related muscle loss. However, the temporal relation between mitochondrial apoptotic signaling and sarcopenia as well as the regulation of release of pro-apoptotic factors from the mitochondria has not been elucidated. In this study, we investigated mitochondrial apoptotic signaling in skeletal muscle of rats across a wide age range. We also investigated whether mitochondrial-driven apoptosis was accompanied by changes in the expression of Bcl-2 proteins and components of the mitochondrial permeability transition pore (mPTP). Analyses were performed on gastrocnemius muscle of 8-, 18-, 29-and 37-month-old male Fischer 344 ×Brown Norway rats (9 per group). Muscle weight declined progressively with advancing age, concomitant with increased apoptotic DNA fragmentation. Cytosolic and nuclear levels of apoptosis inducing factor (AIF) and endonuclease G (EndoG) increased in old and senescent animals. In contrast, cytosolic levels of cytochrome c were unchanged with age. Mitochondrial Bcl-2, Bax and Bid increased dramatically in 37-month-old rats, with no changes in the Bax/Bcl-2 ratio in any of the age groups. Finally, expression of cyclophilin D was enhanced at very old age. Our findings indicate that the mitochondrial caspase-independent apoptotic pathway may play a more prominent role in skeletal muscle loss than caspase-mediated apoptosis.

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

“…Interestingly, elevated mitochondrial levels of CyPD relative to ANT and VDAC were detected in the gastrocnemius muscle of very old rats [18]. This finding underlines the major role postulated for CyPD in the formation of the mPTP, and suggests a higher susceptibility to mPTP opening at an older age.…”

“…Accordingly, levels of Apaf-1 [13,46,60], active caspase-9 content [52,54,60], and caspase-9 proteolytic activity [46] were also found to be higher in aged muscles compared to young controls. However, other studies did not observe increases in cytosolic cytochrome c and/or caspase-9 expression in skeletal muscles of old rats [12,13,18,49]. Surprisingly, it was observed that gene expression levels of Apaf-1 were even reduced in the plantaris of old and senescent rats relative to younger controls [50].…”

“…Alterations in the expression of Bcl-2 family proteins have been reported in skeletal muscles of aged experimental animals [18,46–49]. Specifically, a proapoptotic shift in the expression pattern of Bcl-2 proteins (i.e., increased Bax and decreased Bcl-2 levels) has been observed in muscles of aged rodents [47–49].…”

“…Specifically, a proapoptotic shift in the expression pattern of Bcl-2 proteins (i.e., increased Bax and decreased Bcl-2 levels) has been observed in muscles of aged rodents [47–49]. However, other studies did not detect such an adaptation [18,46]. Strikingly, lower levels of Bax and Bcl-2 mRNA were found in the plantaris muscle of old and senescent rats, resulting in a reduced, rather than increased, Bax-to-Bcl-2 ratio in very old animals compared to younger controls [50].…”

Mitochondria and aging: A role for the mitochondrial transition pore?

“…An increase in the ratio between CypD and ANT with aging was observed. This may contribute to higher susceptibility to mPTP opening (Marzetti et al., 2008) and to the reduction in affinity of ANT for CypD. An increased phospho‐GSK‐3β binding to ANT was suggested to be responsible for the inhibition of mPTP opening (Miura & Tanno, 2010; Nishihara et al., 2007).…”