Sarcopenia Age-Related Muscle Wasting and Weakness: Mechanisms and Treatments P17 docx

146 R.T. Hepple burdens of mtDNA damage at the whole muscle level and very much higher fractions of muscle fibers exhibiting complex IV enzyme activity deficiency, and yet in these patients neither individual muscle fibers lacking complex IV activity (Fig. 6) nor their muscles as a whole are grossly atrophied relative to healthy individuals of the same age (Jacobs 2003). As such, the degree to which this phenomenon might contribute to sarcopenia remains an important area of investigation. As suggested above, one specific manner in which mitochondria are proposed to be involved in sarcopenia involves apoptosis (Pollack and Leeuwenburgh 2001; Chabi et al. 2008; Seo et al. 2008). Mitochondria play a key role in regulating apoptosis, via the mitochondrial permeability transition pore (mPTP) which regu- lates the release of cytochrome c into the cytoplasm. A variety of stimuli, such as high Ca 2+ and high ROS exposure, can lead to opening of the mPTP, allowing cyto- chrome c to leak out of the mitochondria and into the cytoplasm. Once released into the cytoplasm, cytochrome c binds with Apaf-1 and caspase 9, leading to the formation of an apoptosome, activation of caspase 9 and subsequent commitment of the apoptotic pathway via activation of caspase 3. In support of a role for apoptosis in age-related muscle atrophy, many studies have reported an increase in pro-apoptotic signaling in aged muscles (Alway et al. 2002; Dirks and Leeuwenburgh 2002; Giresi et al. 2005; Baker and Hepple 2006; Rice and Blough 2006; Chabi et al. 2008). On the other hand, differences in the degree of muscle atrophy between Fig. 6 Succinate dehydrogenase and complex IV doubly-stained cross-section of muscle from a patient with heteroplasmic mtDNA mutation. Note that the complex IV deficient fibers (blue fibers) are no different in size than fibers with normal complex IV activity (brown-orange fibers) (Reproduced from Taivassalo and Haller [2005], with permission from The American College of Sports Medicine) 147Alterations in Mitochondria and Their Impact in Aging Skeletal Muscle individuals in senescent animals do not track well with differences in expression of pro-apoptotic transcripts (Baker and Hepple 2006). In addition, although the pro- gression of muscle atrophy with aging correlates generally with an increase in number of apoptotic nuclei in both fast-twitch and slow-twitch muscles, it is strik- ing that there are markedly more apoptotic nuclei in the slow-twitch soleus muscle than the fast-twitch extensor digitorum longus muscle, despite very similar amounts of atrophy (Fig. 7; data taken from (Rice and Blough 2006)). This difference may relate to the fact that muscle fibers are multi-nucleated and, therefore, apoptotic loss of a nucleus within a given myocyte does not need to result in loss of the myo- cyte entirely. As such, a difference in the incidence of apoptotic nuclei between muscles having the same amount of atrophy could reflect differences in the ability of these muscles to regenerate and repair, e.g., via recruitment of satellite cells. Whether this or another explanation applies awaits further investigation. Notwithstanding some uncertainty about the degree to which apoptosis directly explains the degree of muscle atrophy with aging, recent data suggests that accu- mulation of non-heme iron in skeletal muscle mitochondria may be one mechanism leading to an increased incidence of mitochondrial-mediated apoptosis in aged skeletal muscle. Specifically, accumulation of non-heme iron with aging is hypoth- esized to exacerbate mitochondrial ROS generation (and thus oxidative damage) via the Fenton reaction, wherein the increased mitochondrial damage leads to an increased probability of mPTP opening (Seo et al. 2008). This notion is consistent with the aforementioned increase in mitochondrial ROS generation in aged skeletal muscles (Section 2.2), and observations indicating greater accumulation of non- heme iron in mitochondria isolated from aged skeletal muscle (Seo et al. 2008). In addition, mitochondria from aged muscles exhibit a greater release of cytochrome Fig. 7 Muscle mass in the fast-twitch extensor digitorum longus (EDL) muscle and slow-twitch soleus muscle (Sol) versus the density of TUNEL-positive nuclei (a marker of apoptotic nuclei) as sarcopenia progresses with aging (Data reproduced from Rice et al. [2006]) 148 R.T. Hepple c in response to ROS-induced stress (Chabi et al. 2008), which may in part explain the increased susceptibility to mitochondrial-driven apopotosis in aging muscle. Thus, collectively, there is substantial evidence that apoptosis increases in aged muscles and that age-related changes in mitochondria are likely to be involved. 4.2 Involvement of Mitochondria in Age-Related Muscle Dysfunction In addition to the potential involvement of mitochondria in the age-related loss of muscle mass, there is considerable support for the involvement of mitochondria in impaired muscle function with aging. For example, there is a progressive decline in skeletal muscle aerobic function with aging that is not due to loss of capillaries (Hepple and Vogell 2004; Mathieu-Costello et al. 2005), but rather correlates with a progressive loss of mitochondrial oxidative capacity in aging muscles (Hagen et al. 2004) (Fig. 8). As noted in Section 3, a decline in muscle mitochondrial oxi- dative capacity may be caused by a reduction in the expression of PGC-1a in aged muscles (Baker et al. 2006; Chabi et al. 2008). In this context, it is important to note that aged muscles, particularly in senescence, are characterized by an accumulation of very small muscle fibers. Although this area requires further study, it seems likely that a large proportion of these small fibers are denervated (Hepple et al. 2004b) and that a sub-fraction of these may be attempting to regenerate. The reason this is relevant here is that these small fibers have lower levels of markers of Fig. 8 Muscle maximal oxygen uptake (VO 2max ) in pump-perfused rat hindlimb versus the flux capacity of complex I–III in homogenates of gastrocnemius muscle. The figure shows that the age-related decline in VO 2max parallels the decline in flux capacity through a key part of the mitochondrial electron transport chain (Reproduced from Hagen et al. [2004], with permission from The Gerontological Society of America) 149Alterations in Mitochondria and Their Impact in Aging Skeletal Muscle mitochondrial content (e.g., complex IV activity) (Fig. 9), and because of this they contribute significantly to the lower muscle oxidative capacity. Furthermore, den- ervation, or perhaps failure to reinnervate, may be constraining the mitochondrial content of these fibers, secondary to the aforementioned reduction in drive on mito- chondrial biogenesis that occurs in denervated muscle (Adhihetty et al. 2007) (Section 3.1). Thus, the reduction of muscle mitochondrial oxidative capacity with aging may have an important neurological involvement. This point needs further consideration in the experimental literature. As noted in Section 3.2, aged muscles are also characterized by mitochondria that emit higher levels of ROS. This increase in mitochondrial ROS generation in aging skeletal muscles can exacerbate oxidative damage to proteins, which has been shown to inhibit the biological activity of enzymes, particularly those contain- ing iron-sulfur centers (Bota et al. 2002; Ma et al. 2009). In addition, several pro- teins involved in muscle contraction are known to be specifically targeted by oxidative stress, and thus, likely contribute to the impairment in muscle contractile function with aging. Prochniewicz et al. (2005) previously showed using in vitro motility assays that although actin function was unaltered with aging, the catalyti- cally active portion of myosin (heavy meromyosin) was impaired in muscles of aged versus young adult rats. In addition, this difference in actin versus myosin function with aging corresponded to differences in the susceptibility of actin versus Fig. 9 Senescent rat gastrocnemius muscle cross-section stained for complex IV activity. Note that the very small fibers have a lower complex IV activity than the larger fibers, showing that the accumulation of these very small fibers in aged muscle, particularly in senescence, contributes to the overall decline in muscle oxidative capacity with aging (R.T. Hepple [unpublished]) 150 R.T. Hepple myosin to accumulate oxidative damage to cysteine molecules (Prochniewicz et al. 2005). Similarly, there is an increase in oxidative damage, particularly nitrotyrosine damage, to the sarcoplasmic reticulum ATPase in aged muscles (Fugere et al. 2006; Thomas et al. 2009), and this is thought to contribute to decreases in maximal SERCA activity in aged muscle (Thomas et al. 2009). As such, the collective evi- dence suggests that oxidative damage to various proteins within skeletal muscle, and the mitochondria therein, can lead to functional deterioration in aging skeletal muscle. 5 Plasticity of Mitochondria in Aging Muscles Given the above evidence of reduced mitochondrial oxidative capacity and increased ROS generation with aging, both of which have been attributed in part to accumulation of damaged mitochondria secondary to reduced mitochondrial renewal, an obvious question is whether aged muscle simply loses the capacity to increase its mitochondrial content. The majority of what we know about this ques- tion has been obtained from experiments examining changes in muscle mitochon- drial oxidative capacity in response to exercise training or chronic electrical stimulation. Significantly, an emerging concept is that the capacity for mitochon- drial biogenesis in response to muscle activation, while relatively preserved in the younger of the old, becomes severely impaired in the oldest old. There are many studies showing that aged muscles can respond favorably by increasing markers of mitochondrial content in response to endurance exercise training in both the human (Orlander and Aniansson 1980; Hagberg et al. 1989; Meredith et al. 1989; Short et al. 2003) and animal model (Cartee and Farrar 1987; Rossiter et al. 2005; Betik et al. 2008) literature. However, it is important to realize that these prior studies have not considered potential differences in the endurance training responses between late middle age versus the senescent period (i.e., when survival rates drop below 50%), and it is the senescent period when the consequences of aging for skeletal muscle become most severe. To address this issue, we recently examined the effect of aging on the responses of the skeletal muscle aerobic machinery to endurance training in rat skeletal muscles. Interestingly, whereas skeletal muscle aerobic function (in situ maximal oxygen consumption) and mitochondrial enzyme activities increased significantly when endurance exercise training was imposed in late middle age and continued for 7 weeks (Betik et al. 2008) (Fig. 10), the skeletal muscles completely lost this positive adaptation when the training was continued for 7 months into the senescent period (Betik et al. 2009) (Fig. 11). Further to this, the normally robust response of PGC-1a expression to endurance exercise training seen in studies of rodents (Baar et al. 2002; Terada et al. 2002) and young adult humans (Norrbom et al. 2004) was abolished in senescent rat skeletal muscles following 7 months of endurance exercise training in both the slow-twitch soleus muscle and the fast-twitch plantaris muscle (Fig. 12) (Betik et al. 2009). On the basis of these results, therefore, it appears that senescent 151Alterations in Mitochondria and Their Impact in Aging Skeletal Muscle muscle in particular has a markedly diminished capacity to increase mitochondrial biogenesis in response to an endurance training stimulus, and that this is due in part to an impaired ability to up-regulate PGC-1a. This finding of reduced adaptability with endurance training in senescence is consistent with studies demonstrating that skeletal muscle from the oldest old also has a diminished plasticity in response to resistance exercise training (Slivka et al. 2008; Raue et al. 2009) and functional overload (Blough and Linderman 2000). The aforementioned results indicate that senescent skeletal muscle loses its abil- ity to generate new mitochondria in advanced age, suggesting that the reduced Fig. 10 Muscle oxygen uptake during incremental muscle contractions in distal rat hindlimb muscles pump-perfused in situ (top) and the activity of complex IV in homogenates of plantaris (Plan) and gastrocnemius (Gas) muscle (bottom) in sedentary late middle aged rats and late middle aged rats exercise-trained for 7 weeks (Reproduced from Betik et al. [2008], with permission from The Physiological Society [London]) 152 R.T. Hepple mitochondrial turnover rate with aging is secondary to this diminished capacity to make new mitochondria. However, an important question remains: is it that senes- cent muscle loses its adaptive plasticity per se, or is the limitation the result of the much lower exercise stimulus that can be sustained in very old age. To help address this issue, a recent study examined the response of young adult versus senescent skeletal muscle to an acute bout of low frequency electrical stimulation. Interestingly, these experiments revealed that whereas the cell signaling pathway, including molecules involved in driving mitochondrial biogenesis (e.g., adenosine monophosphate protein kinase [AMPK] activation), was relatively intact in the highly oxidative region of the tibialis anterior muscle, there was a blunted response Fig. 11 Muscle maximal oxygen uptake (VO 2 max) during incremental muscle contractions in distal rat hindlimb muscles pump-perfused in situ (top) and the activity of complex IV in homo- genates of plantaris (Plan) and Soleus (Sol) muscle (bottom) in sedentary senescent rats and senescent rats trained for 7 months beginning in late middle age (Data reproduced from Betik et al. [2009]) 153Alterations in Mitochondria and Their Impact in Aging Skeletal Muscle in the highly glycolytic region of this muscle in senescence (Ljubicic and Hood 2009). These data are generally consistent with another study showing that AMPK activation is markedly blunted in aged muscles following either pharamacological stimuli or an acute exercise bout (Reznick et al. 2007). What is not yet clear, how- ever, is the degree to which an attenuated mitochondrial biogenesis response is a general property of all muscle fibers in an aged muscle, versus there being an increasing proportion of muscle fibers which cannot contribute to the whole muscle mitochondrial biogenesis response (e.g., those that have become denervated and/or which are undergoing regeneration). Irrespective of this point, the growing consen- sus is that aging muscle, particularly in senescence, displays an impaired ability to up-regulate mitochondrial biogenesis and this in turn plays an important role in the attenuated benefits of endurance exercise training for skeletal muscle aerobic capacity in senescence. Future studies need to address whether this loss of adaptive plasticity is an immutable consequence of aging, or if other interventions yet to be identified can help restore the adaptive response to increased muscle use. 6 Conclusions Mitochondrial changes in aging skeletal muscles, and the implications these have for the decline in both muscle mass and its function with aging, have constituted an intensive area of study. The aforementioned chapter provides some context for the current knowledge in this area and areas that will be refined through further study. Given the central importance of mitochondrial biology to so many facets of normal Fig. 12 PGC-1 protein expression in plantaris (Plan) and soleus (Sol) muscles of sedentary senes- cent rats and senescent rats trained for 7 months beginning in late middle age. *P < 0.05 versus Sedentary group (Data reproduced from Betik et al. [2009]) 154 R.T. Hepple cell function, particularly in tissues with a wide metabolic scope like skeletal muscle, new discoveries about the significance of changes in mitochondria for aging skeletal muscles, and their potential remedy through lifestyle modification (e.g., exercise training, diet) and/or medical intervention (e.g., pharmaceuticals, gene therapy), will remain at the forefront of our quest to promote healthy aging. References Adhihetty, P. J., O’Leary, M. F., Chabi, B., Wicks, K. L., Hood, D. A. (2007). Effect of denervation on mitochondrially mediated apoptosis in skeletal muscle. Journal of Applied Physiology, 102, 1143–1151. Altmann, R. (1890). Die Elementarorganismen und ihre Beziehungen zu den Zellen. Verlag von Veit & Comp Leipzig. Alway, S. E., Degens, H., Krishnamurthy, G., Smith, C. A. (2002). Potential role for Id myogenic repressors in apoptosis and attenuation of hypertrophy in muscles of aged rats. AJP – Cell Physiology, 283, C66–C76. Baar, K., Wende, A. R., Jones, T. E., Marison, M., Nolte, L. A., Chen, M., Kelly, D. P., Holloszy, J. O. (2002). Adaptations of skeletal muscle to exercise: rapid increase in the transcriptional coactivator PGC-1. The FASEB Journal, 16, 1879–1886. Bakeeva, L. E., Chentsov, Y. S., Skulachev, V. P. (1978). Mitochondrial framework (reticulum mitochondriale) in rat diaphragm muscle. Biochim Biophys Acta, 501, 349–369. Baker, D. J. & Hepple, R. T. (2006). Elevated caspase and AIF signaling correlates with the pro- gression of sarcopenia during aging in male F344BN rats. Experimental Gerontology, 41, 1149–1156. Baker, D. J., Betik, A. C., Krause, D. J., Hepple, R. T. (2006). No decline in skeletal muscle oxida- tive capacity with aging in long-term caloric restricted rats: effects are independent of mtDNA integrity. The Journals of Gerontology. Series A: Biological Sciences and Medical Sciences, 61A, 675–684. Bejma, J. & Ji, L. L. (1999). Aging and acute exercise enhance free radical generation in rat skel- etal muscle. Journal of Applied Physiology, 87, 465–470. Betik, A. C., Baker, D. J., Krause, D. J., McConkey, M. J., Hepple, R. T. (2008). Exercise training in late middle-aged male Fischer 344 x Brown Norway F1-hybrid rats improves skeletal muscle aerobic function. Experimental Physiology, 93, 863–871. Betik, A. C., Thomas, M. M., Wright, K. J., Riel, C. D., Hepple, R. T. (2009). Exercise training from late middle age until senescence does not attenuate the declines in skeletal muscle aero- bic function. American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, 297, R744–R755. Bhattacharya, A., Muller, F. L., Liu, Y., Sabia, M., Liang, H., Song, W., Jang, Y. C., Ran, Q., Van Remmen, H. (2009). Denervation induces cytosolic phospholipase A2-mediated fatty acid hydroperoxide generation by muscle mitochondria. The Journal of Biological Chemistry, 284, 46–55. Blough, E. R. & Linderman, J. K. (2000). Lack of skeletal muscle hypertrophy in very aged male Fischer 344 X Brown Norway rats. Journal of Applied Physiology, 88, 1265–1270. Boncompagni, S., d’Amelio, L., Fulle, S., Fano, G., Protasi, F. (2006). Progressive disorgani- zation of the excitation-contraction coupling apparatus in aging human skeletal muscle as revealed by electron microscopy: A possible role in the decline of muscle performance. The Journals of Gerontology. Series A: Biological Sciences and Medical Sciences, 61, 995–1008. Booth, F. W. & Holloszy, J. O. (1977). Cytochrome c turnover in rat skeletal muscles. The Journal of Biological Chemistry, 252, 416–419. 155Alterations in Mitochondria and Their Impact in Aging Skeletal Muscle Bota, D. A., Van Remmen, H., Davies, K. J. (2002). Modulation of Lon protease activity and aconitase turnover during aging and oxidative stress. FEBS Letters, 532, 103–106. Bua, E., Johnson, J., Herbst, A., Delong, B., McKenzie, D., Salamat, S., Aiken, J. M. (2006). Mitochondrial DNA-deletion mutations accumulate intracellularly to detrimental levels in aged human skeletal muscle fibers. American Journal of Human Genetics, 79, 469–480. Bua, E. A., McKiernan, S. H., Wanagat, J., McKenzie, D., Aiken, J. M. (2002). Mitochondrial abnormalities are more frequent in muscles undergoing sarcopenia. Journal of Applied Physiology, 92, 2617–2624. Capel, F., Buffiere, C., Patureau, M. P., Mosoni, L. (2004). Differential variation of mitochondrial H2O2 release during aging in oxidative and glycolytic muscles in rats. Mechanisms of Ageing and Development, 125, 367–373. Capel, F., Rimbert, V., Lioger, D., Diot, A., Rousset, P., Mirand, P. P., Boirie, Y., Morio, B., Mosoni, L. (2005). Due to reverse electron transfer, mitochondrial H2O2 release increases with age in human vastus lateralis muscle although oxidative capacity is preserved. Mechanisms of Ageing and Development, 126, 505–511. Cartee, G. D. & Farrar, R. P. (1987). Muscle respiratory capacity and VO 2max in identically trained young and old rats. Journal of Applied Physiology, 63, 257–261. Chabi, B., Ljubicic, V., Menzies, K. J., Huang, J. H., Saleem, A., Hood, D. A. (2008). Mitochondrial function and apoptotic susceptibility in aging skeletal muscle. Aging Cell, 7, 2–12. Choksi, K. B., Nuss, J. E., Deford, J. H., Papaconstantinou, J. (2008). Age-related alterations in oxidatively damaged proteins of mouse skeletal muscle mitochondrial electron transport chain complexes. Free Radical Biology & Medicine, 45, 826–838. Coggan, A. R., Spina, R. J., King, D. S., Rogers, M. A., Brown, M., Nemeth, P. M., Holloszy, J. O. (1992). Histochemical and enzymatic comparison of the gastrocnemius muscle of young and elderly men and women. Journal of Gerontology, 47, B71–B76. Conley, K. E., Jubrias, S. A., Esselman, P. C. (2000). Oxidative capacity and aging in human muscle. The Journal of Physiology, 526.1, 203–210. Davies, K. J., Packer, L., Brooks, G. A. (1981). Biochemical adaptation of mitochondria, muscle, and whole-animal respiration to endurance training. Archives of Biochemistry and Biophysics, 209, 539–554. Desai, V. G., Weindruch, R., Hart, R. W., Feuers, R. J. (1996). Influences of age and dietary restriction on gastrocnemius electron transport system activities in mice. Archives of Biochemistry and Biophysics, 333, 145–151. Dirks, A. & Leeuwenburgh, C. (2002). Apoptosis in skeletal muscle with aging. AJP - Regulatory, Integrative and Comparative Physiology, 282, R519–R527. Drew, B., Phaneuf, S., Dirks, A., Selman, C., Gredilla, R., Lezza, A., Barja, G., Leeuwenburgh, C. (2003). Effects of aging and caloric restriction on mitochondrial energy production in gastroc- nemius muscle and heart. American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, 284, R474–R480. Droge, W. (2002). Free radicals in the physiological control of cell function. Physiological Reviews, 82, 47–95. Dupuis, L., Gonzalez de Aguilar, J. L., Echaniz-Laguna, A., Eschbach, J., Rene, F., Oudart, H., Halter, B., Huze, C., Schaeffer, L., Bouillaud, F., Loeffler, J. P. (2009). Muscle mitochondrial uncoupling dismantles neuromuscular junction and triggers distal degeneration of motor neurons. PLoS ONE, 4, e5390. Essen-Gustavsson, B. & Borges, O. (1986). Histochemical and metabolic characteristics of human skeletal muscle in relation to age. Acta Physiologica Scandinavica, 126, 107–114. Fano, G., Mecocci, P., Vecchiet, J., Belia, S., Fulle, S., Polidori, M. C., Felzani, G., Senin, U., Vecchiet, L., Beal, M. F. (2001). Age and sex influence on oxidative damage and functional status in human skeletal muscle. Journal of Muscle Research and Cell Motility, 22, 345–351. Fugere, N. A., Ferrington, D. A., Thompson, L. V. (2006). Protein nitration with aging in the rat semimembranosus and soleus muscles. The Journals of Gerontology. Series A: Biological Sciences and Medical Sciences, 61, 806–812. . damage at the whole muscle level and very much higher fractions of muscle fibers exhibiting complex IV enzyme activity deficiency, and yet in these patients neither individual muscle fibers lacking. in age-related muscle atrophy, many studies have reported an increase in pro-apoptotic signaling in aged muscles (Alway et al. 2002; Dirks and Leeuwenburgh 2002; Giresi et al. 2005; Baker and. Rice and Blough 2006; Chabi et al. 2008). On the other hand, differences in the degree of muscle atrophy between Fig. 6 Succinate dehydrogenase and complex IV doubly-stained cross-section of muscle