Sarcopenia Age-Related Muscle Wasting and Weakness: Mechanisms and Treatments P21 docx

186 S.E. Alway and P.M. Siu Karin 2009). However, NF-kB can also promote apoptosis when activated by pro-apoptotic proteins including p53, Fas and Fas ligand (Burstein and Duckett 2003; Dutta et al. 2006; Fan et al. 2008). p53 upregulated modulator of apoptosis (PUMA) is a downstream target of p53 and a BH3-only Bcl-2 family member(Lee et al. 2009; Chipuk and Green 2009; Ghosh et al. 2009b). It is induced by p53 following exposure to DNA- damaging agents, such as gamma-irradiation and commonly used chemothera- peutic drugs or oxidative stress (Lee et al. 2009; Chipuk and Green 2009; Ghosh et al. 2009a). It is also activated by a variety of nongenotoxic stimuli independent of p53, such as serum starvation, kinase inhibitors, glucocorticoids, endoplasmic reticulum stress, and ischemia/reperfusion (Nickson et al. 2007; Yu and Zhang 2008). The pro-apoptotic function of PUMA is mediated by its interac- tions with anti-apoptotic Bcl-2 family members such as Bcl-2 and Bcl-XL which lead to Bax/Bak-dependent mitochondrial dysfunction mitochondria permeabil- ity and caspase activation (Chipuk and Green 2009). In addition, PUMA is directly activated by NF-kB and contributes to TNF-a-induced apoptosis (Wang et al. 2009). Fig. 4 The extrinsic (death receptor) pathway is activated in aging and contributes to sarcopenia. A ligand (e.g., TNF-a) binds to the death receptor and TNFR1, activates procaspase 8 and caspase 8 which in turn activates caspase 3 and DNA fragmentation 187Nuclear Apoptosis and Sarcopenia Based on the well-documented increase in circulating TNF-a levels with aging (Bruunsgaard et al. 1999, 2001, 2003a, b; Bruunsgaard 2002; Visser et al. 2002; Pedersen et al. 2003; Sandmand et al. 2003; Schaap et al. 2006, 2009) and increases in apoptosis of myonuclei in aged skeletal muscles (Allen et al. 1997; Siu et al. 2005c; Pistilli et al. 2006b), we examined whether apoptotic signalling via the extrinsic pathway contributed to sarcopenia. Our data show that pro- and anti- apoptotic proteins in the extrinsic apoptotic pathway are affected by aging in fast (plantaris) and slow (soleus) skeletal muscles of rats (Pistilli et al. 2006b). Similarly, Marzetti et al. (2009a, b) report elevated TNF-a and TNF-receptor 1 in muscles of old rodents. Together, these data suggest that TNF-a mediated signalling may be an important element triggering the extrinsic apoptotic pathway in and leading to sarcopenia in aging muscles. Muscles from aged rats are significantly smaller and exhibit a larger incidence in fragmented DNA. This suggests that there is a higher level of nuclear apoptosis in muscles from aged animals. In addition, muscles from aged rodents have higher TNFR and FADD mRNA content (measured by semi-quantitative RT-PCR) and protein contents for FADD, Bid, and FLIP, and enzymatic activities of caspase 8 and caspase 3, when compared to muscles from young adult rodents. Although there is an increase in mRNA expression for the TNFR as measured by the semi- quantitative approach, the protein content for the TNFR remains unchanged (Pistilli et al. 2006a, b). This may be explained by the fact that the TNFR antibody utilized in western immunoblots recognizes the soluble form of the receptor. Thus, the changes in the membrane bound form of the receptor, measured by PCR, and the amount of the soluble TNFR may not be equivalent. While fast contracting muscles are generally more susceptible to apoptosis and sarcopenic muscle loss, the pro- apoptotic changes have been reported to be expressed in a similar fashion in both plantaris and soleus muscles; however strong relationships were observed between markers of apoptosis and muscle loss in the fast plantaris muscle that were not observed in the soleus (Pistilli et al. 2006a). These data extend the previous dem- onstration that type II fibres are preferentially affected by aging and suggest that type II fibre containing skeletal muscles may be more susceptible to muscle mass loses via the extrinsic apoptotic pathway (Pistilli et al. 2006b). We have found activation of the extrinsic apoptotic signalling pathway in muscles of old rats (Pistilli et al. 2006a, 2007; Siu et al. 2008), and therefore we speculate that circulating TNF-a may be the initiator of this pathway in skeletal muscle. Nevertheless, we cannot rule out the possibility that other pathways that we did not examine may have been activated by circulating TNF-a in aging muscle. For example, TNF-a has been shown to directly promote protein degradation (Garcia- Martinez, et al. 1993a, b; Llovera et al. 1997, 1998) and apoptosis within skeletal muscle (Carbo et al. 2002; Figueras et al. 2005). Furthermore, intravenous injection of recombinant TNF-a increases protein degradation in rat skeletal muscles and this is associated with the increased activity of the ubiquitin-dependent proteolytic pathway (Garcia-Martinez et al. 1993a, 1995; Llovera et al. 1997, 1998). In addition, elevated TNF-a concentrations in cell culture for 24–48 h increases apoptosis in skeletal myoblasts as determined by DNA fragmentation (Meadows et al. 2000; 188 S.E. Alway and P.M. Siu Foulstone et al. 2001). A reduction of procaspase 8 occurs within 6h of incubating myoblasts in vitro with recombinant TNF-a, suggesting a TNF-a mediated cleavage and activation of this initiator caspase in myoblast cultures (Stewart et al. 2004). Lees and co-workers (Lees et al. 2009) have recently shown that satellite cells (i.e., MPCs) isolated from hindlimb muscles of old rats have increased TNF-a- induced nuclear factor-kappa B (NF-kB) activation and expression of mRNA levels for TRAF2 and the cell death-inducing receptor, Fas (CD95), in response to pro- longed (24 h) TNF-a treatment compared to in MPCs isolated from muscles of young animals. These findings suggest that age-related differences may exist in the regulatory mechanisms responsible for NF-kB inactivation, which may in turn have an effect on TNF-a-induced apoptotic signalling. Systemic and muscle levels of TNF-a increase with aging, and this should have an even more profound increase in activation of apoptotic gene targets through the extrinsic pathway, as compared to MPCs in muscles of young adult rats (Krajnak et al. 2006; Lees et al. 2009). The effects of TNF-a on apoptosis are not limited to in vitro conditions, because a systemic elevation of TNF-a in vivo increases DNA fragmentation within rodent skeletal muscle (Carbo et al. 2002). Based on the observation that TNF-a mRNA was not different between muscles from young adult and aged rats, it is reasonable to assume that muscle-derived TNF-a does not act in an autocrine manner to stimulate the pro-apoptotic signalling observed in this study. Data from Pistilli and co-workers (Pistilli et al. 2006b) are consistent with the hypothesis that the well- documented systemic elevation of TNF-a with age, may increase the likelihood of ligand binding to the TNFR and stimulate apoptotic signalling of the extrinsic pathway downstream of the TNFR and contribute to sarcopenia in skeletal muscle of old rats. 5.2 Cross-talk Between Extrinsic and Intrinsic Apoptotic Signalling Cross-talk between extrinsic and intrinsic apoptotic pathways was recently reviewed (Sprick and Walczak 2004). Cross-talk between these pathways is the result of the cleavage of the pro-apoptotic BCL-2 family member Bid. Cleaved and activated caspase 8 cannot only serve to activate caspase 3, which is the execu- tioner caspase, but also cleave full-length Bid into a truncated version (tBid) (Tang et al. 2000). tBid then interacts with pro-apoptotic Bax, to stimulate apoptotic signalling from the mitochondria (Grinberg et al. 2005). As has been previously shown, apoptotic signalling from the mitochondria stimulates cleavage of procaspase 9, which then serves to activate caspase 3 (Johnson and Jarvis 2004). Thus, both the extrinsic and intrinsic apoptotic pathways converge on caspase 3, which then fully engages pro-apoptotic signalling. Skeletal muscles from aged rodents contained a greater protein expression of full-length Bid, which raises the possibility that cross talk between the extrinsic pathway and the intrinsic pathway may occur in aged skeletal muscles (Fig. 5). 189Nuclear Apoptosis and Sarcopenia 6 Exercise Modulation of Apoptosis in Sarcopenia Various perturbations have been used to determine if aging increases the sensitivity of skeletal muscle to apoptosis and apoptosis signalling cascades. These include increases in muscle loading, loading followed by a period of unloading, disuse, denervation or muscle unloading, and aerobic exercise. 6.1 Interventions by Muscle Loading The evidence presented above indicates that mitochondrial dysfunction is a major contributing factor to the path physiology of aging including sarcopenia. While muscle disuse decreases mitochondria function leading to apoptosis (Adhihetty et al. 2003; Siu and Alway 2005a; Bourdel-Marchasson et al. 2007), chronic exercise improves mitochondria function (Daussin et al. 2008; Lanza et al. 2008) and reduces apoptotic signalling (Siu et al. 2004). Fig. 5 The potential cross talk between the extrinsic and intrinsic apoptotic signalling pathways are shown 190 S.E. Alway and P.M. Siu Adaptation to chronic loading has been shown to improve anti-apoptotic proteins in skeletal muscle including XIAP (Siu et al. 2005d), Bcl2 (Song et al. 2006), and reduce DNA fragmentation (Siu and Alway 2006a) (Song et al. 2006) and lower pro-apoptotic proteins including Bax (Song et al. 2006), ARC (Siu and Alway 2006a), AIF (Siu and Alway 2006a). In contrast, models of muscle unloading show most of the appositive apoptotic signalling such as elevations in Bax, Apaf1, AIF (Pistilli et al. 2006b), cyto- solic levels of Id2 and p53 (Siu et al. 2006) and the Bax/Bcl2 ratio (Song et al. 2006). Reduced levels of pro-apoptotic proteins may provide one mechanism to explain the improvements in muscle mass and force that are observed in humans after a period of resistance exercise. Our lab (Roman et al. 1993; Ferketich et al. 1998) and others (Charette et al. 1991; Welle et al. 1995; Parise and Yarasheski 2000; Deschenes and Kraemer 2002; Mayhew et al. 2009) have shown that resistance exercise is an effective tool to reduce but not eliminate sarcopenia in aging humans. Although aging has generally been shown to attenuate the absolute extent of muscle adaptations that are possible with increased loading (Alway et al. 2002a; Degens and Alway 2003; Degens 2007; Degens et al. 2007), it is not known how much of this might be the result of increased nuclear apoptosis in skeletal muscle. Interestingly, several studies have reported unexpected improvements in mitochondrial function in both young adult and aged subjects as a result of resistance exercise training. For example, the mitochondrial capacity for ATP synthesis increases after resistance training (Jubrias et al. 2001; Conley et al. 2007b; Tarnopolsky 2009). Resistance exercise also increases antioxidant enzymes and decreases oxidative stress (Parise et al. 2005; Johnston et al. 2008). Furthermore, 26 weeks of whole body resistance exercise was shown to reverse the gene expression of mitochondrial proteins that were associated with normal aging, to that observed in young subjects (Melov et al. 2007). Although we have found that resistance training did not increase the relative volume of mitochondria in muscle fibres of young adults, resistance exercise stimulated mitochondria biogenesis to maintain the myofibrillar to mitochondria volume (Alway et al. 1989; Alway 1991). In addition, aging attenuates the adaptive response to improve the muscle’s ability to buffer pro-oxidants in response to chronic muscle loading (Ryan et al. 2008). Nevertheless, there is some improvement in antioxidant enzymes and the ability to buffer oxidative stress in response to loading conditions (Ryan et al. 2008). Therefore, it is possible that, resistance training could also improve mitochondria function and stimulate mitochondrial biogenesis in aged individuals. If muscle loading improves not only antioxidant enzymes levels but it also reduces Bax accumulation in mitochondria, we would expect that apoptosis signalling should be decreased. This would lead to improved muscle recovery following disuse and reduce sarcopenia. 6.2 Apoptotic Elimination of MPCs Reduces Muscle Hypertrophic Adaptation to Loading It is thought that myonuclei maintain a constant cytoplasm to nuclei ratio, (i.e. “nuclear domain”, see Fig. 1), and that hypertrophy requires that fibres add new 191Nuclear Apoptosis and Sarcopenia nuclei (Schultz 1989, 1996; Schultz and McCormick 1994). Because myonuclei are post mitotic (Schultz 1989, 1996; Schultz and McCormick 1994), satellite cells/ MPCs provide the only important source for adding new nuclei to initiate muscle regeneration, muscle hypertrophy, and postnatal muscle growth in muscles of both young and aged animals (Rosenblatt et al. 1994; Phelan and Gonyea 1997; McCall et al. 1998; Allen et al. 1999; Hawke and Garry 2001; Adams et al. 2002). MPCs are critical for muscle growth because muscle hypertrophy is markedly reduced or eliminated completely after irradiation to prevent MPC activation (Rosenblatt et al. 1994; Hawke and Garry 2001). Growth of adult skeletal muscle requires activation and differentiation of satellite cells/MPCs and increased protein synthesis and accumulation of proteins, and this necessitates increased transcription of muscle genes (Dirks and Leeuwenburgh 2002; Pollack et al. 2002; Alway et al. 2002b; Leeuwenburgh 2003; Dirks and Leeuwenburgh 2004; Siu et al. 2005c). Thus, there is little doubt that MPC activation and differentiation are critical components in determining muscle adaptation and growth. If MPCs are activated normally, but they either do not differentiate or do not survive to participate in increased protein synthesis, then muscle adaptation would be compromised. Elevation of apoptosis (lower MPC survival) in muscles from aged animals (Renault et al. 2002; Siu et al. 2005c) could explain the poorer adaptation to repetitive loading in aging. We have shown that the most recently activated satellite cells/MPCs during loading are also the most susceptible to apoptosis (Pollack et al. 2002; Alway et al. 2002a, b; Leeuwenburgh 2003; Dirks and Leeuwenburgh 2004). Based on these data, we hypothesize that MPC contribution to chronic loading-induced adaptation (hypertrophy) is lower in muscles of old animals because apoptosis is higher (Degens and Alway 2003), and fewer MPCs survive to contribute to muscle adaptation (Chakravarthy et al. 2001). 6.3 Regulation of Apoptotic Signalling by Aerobic Exercise Although acute endurance exercise has been shown to increase apoptotic signalling under some conditions including dystrophies and other pathologies (Sandri et al. 1997; Podhorska-Okolow et al. 1998, 1999) long-term adaptation to endurance exercise has been shown to lower mitochondria-associated apoptosis in heart and skeletal muscle of rodents (Siu et al. 2004; Kwak et al. 2006; Song et al. 2006; Peterson et al. 2008); however, it does not improve muscle mass or act as a coun- termeasure to sarcopenia (Alway et al. 1996; Marzetti et al. 2008a). This might be in part due to aerobically-induced pathways that are generally inhibitory to muscle growth (e.g., AMPK). Apoptosis has been shown to occur in cardiac (Dalla et al. 2001; Hu et al. 2008; Molina et al. 2009) and skeletal muscles (Dalla et al. 2001; Vescovo and Dalla 2006; Libera et al. 2009) of experimental models of chronic heart failure. Apoptosis in skeletal muscle has been linked to elevated circulating levels of TNF-a (Adams et al. 1999; Vescovo et al. 2000). Although nuclear apoptosis has been detected in 192 S.E. Alway and P.M. Siu muscles of humans with severe chronic heart failure (Conraads et al. 2009), it does not appear to be a large component of muscle loss associated when the disease is less severe (Dirks and Jones 2006; Yu et al. 2009a). Complicating the treatment of heart failure and related cardiovascular diseases is the likelihood that drugs including statins which are routinely prescribed to reduce hypercholesterolemia, may themselves have a pro-apoptotic role in skeletal muscle (Adams et al. 2008). Such increases in apoptosis are likely to have devastating effects when statins are combined with sarcopenia, where muscle loss is already high. Although aerobic exercise appears to reduce several skeletal muscle problems of persons suffering from severe chronic heart failure (Linke et al. 2005) and an exercise-induced improvement in antioxidant enzymes is correlated to reduced apoptosis in muscles of patients with chronic heart failure (Siu et al. 2004, 2005a; Song et al. 2006), currently there are no data to definitively address if aerobic exercise reduces apoptosis in heart failure patients. The role or aerobic exercise on nuclear apoptosis of skeletal muscle has not been well-studied but limited data suggest that apoptosis signalling is reduced by aerobic exercise in cardiac and skeletal muscle of young, diseased and aged animals (Siu et al. 2004; Kwak et al. 2006; Song et al. 2006; Peterson et al. 2008; Marzetti et al. 2008a, b). 7 Summary and Conclusions Sarcopenia involves complex of several cellular mechanisms which together contribute to muscle loss during aging. Among them, nuclear apoptosis has recently emerged as an important factor involved in the pathophysiology of sarcopenia. Several lines of evidence support the hypothesis that mitochondrial (intrinsic), extrinsic (death receptor) and endoplasmic reticulum-calcium stress activated apoptotic signalling, occurs in skeletal muscles of old mammals. Nevertheless, it has not been determined to what extent sarcopenia would be reduced, if apoptotic signalling could be fully blocked. Although there is evidence that reducing Bax markedly reduces apoptosis associated muscle loss with denervation (Siu and Alway 2006b), it is not known if this is also the case with aging. We cannot rule the possibility that the apoptotic signalling events may occur to simply eliminate dysfunctional nuclei and/or damaged muscle fibres, whose perseverance would be detrimental for organ function. Even though a cause and effect relationship between apoptosis and sarcopenia has not been unequivocally determined, evidence that muscle loss is reduced in Bax null mice (Siu and Alway 2006b), and experimental interventions to accelerate muscle loss in aged animals also elevates apoptosis (Siu and Alway 2005a; Siu et al. 2005b, c, d, 2006, 2008; Pistilli et al. 2007) strongly suggests that a causal relationship likely exists between nuclear apoptosis and muscle loss, and this may also extend to aging associated muscle loss. Furthermore, activation of mitochondrial apoptotic signalling during the early phases of disuse muscle atrophy (Siu and Alway 2005b; Siu and Alway 2009) suggests that this may exist to balance muscle 193Nuclear Apoptosis and Sarcopenia size and the metabolic or functional needs of the animal. If this is true, nuclear apoptosis may be a fundamentally important mechanism that regulates myonuclei number and, therefore controls the extent of muscle growth (or atrophy) in aging. Apoptotic signalling may be modified by loading and aerobic forms of exercise, but it remains to be seen how effective exercise might be in slowing or preventing apoptosis in sarcopenia. Clearly further research is required to better understand the complex cellular mechanisms underlying muscle atrophy that occurs in sarcopenia, and the importance of apoptosis in this process. 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Schultz and McCormick 1994), satellite cells/ MPCs provide the only important source for adding new nuclei to initiate muscle regeneration, muscle hypertrophy, and postnatal muscle growth in muscles. activation and differentiation of satellite cells/MPCs and increased protein synthesis and accumulation of proteins, and this necessitates increased transcription of muscle genes (Dirks and Leeuwenburgh. phases of disuse muscle atrophy (Siu and Alway 2005b; Siu and Alway 2009) suggests that this may exist to balance muscle 193Nuclear Apoptosis and Sarcopenia size and the metabolic or functional