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

186 S.E. Alway and P.M. Siu 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 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 indepen-dent of p53, such as serum starvation, kinase inhibitors, glucocorticoids, endo-plasmic 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). Nuclear Apoptosis and Sarcopenia 187 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 exam-ple, 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 path-way (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 sig-nalling from the mitochondria (Grinberg et al. 2005). As has been previously shown, apoptotic signalling from the mitochondria stimulates cleavage of procas-pase 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 possibil-ity that cross talk between the extrinsic pathway and the intrinsic pathway may occur in aged skeletal muscles (Fig. 5). Nuclear Apoptosis and Sarcopenia 189 Fig. 5 The potential cross talk between the extrinsic and intrinsic apoptotic signalling pathways are shown 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 exer-cise improves mitochondria function (Daussin et al. 2008; Lanza et al. 2008) and reduces apoptotic signalling (Siu et al. 2004). 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 etal.2009) have shown that resistance exercise is an effective tool to reduce but not eliminate sarcopenia in aging humans. Although aging has gen-erally been shown to attenuate the absolute extent of muscle adaptations that are pos-sible 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 agedsubjectsasaresultofresistanceexercisetraining.Forexample,themitochondrial capacity for ATP synthesis increases after resistance training (Jubrias et al. 2001; Conley etal.2007b; Tarnopolsky 2009).Resistanceexercisealsoincreasesantioxidant 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 main-tain 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 buf-fer pro-oxidants in response to chronic muscle loading (Ryan etal.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 mito-chondrial biogenesis in aged individuals. If muscle loading improves not only antioxi-dant 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 ... - tailieumienphi.vn