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Skeletal muscle: Increasing the size of the locomotor cell
The International Journal of Biochemistry & Cell Biology 42 (2010) 1376–1379
Contents lists available at ScienceDirect
The International Journal of Biochemistry & Cell Biology journal homepage: www.elsevier.com/locate/biocel
Cells in focus
Skeletal muscle: Increasing the size of the locomotor cell Leonidas G. Karagounis∗ , John A. Hawley Health Innovations Research Institute, School of Medical Sciences, RMIT University, Bundoora 3083, Australia
a r t i c l e
i n f o
Article history: Received 15 December 2009 Received in revised form 29 April 2010 Accepted 30 May 2010 Available online 9 June 2010 Keywords: Ageing AKT Amino acids Atrophy Hypertrophy Skeletal muscle mTOR MyoD Protein synthesis
a b s t r a c t Skeletal muscle is the most abundant tissue in the body comprising 40–50% of body mass in humans and playing a central role in maintaining metabolic health. Skeletal muscle protein undergoes rapid turnover, a process that is intricately regulated by the balance between the rates of protein synthesis and degradation. The process of skeletal muscle hypertrophy and regeneration is an important adaptive response to both contractile activity (i.e., exercise) and nutrient availability (i.e., protein ingestion). Ageing and physical inactivity are two conditions associated with a loss of skeletal muscle protein (sarcopenia). Sarcopenia is characterised by a deterioration of muscle quantity and quality, although the precise mechanism(s) underlying this condition remain to be elucidated. This review will (1) summarise our current understanding of the origin and plasticity of skeletal muscle, (2) discuss the major effectors of muscle growth, and (3) highlight the importance of skeletal muscle health in the prevention of several common pathologies. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Cell facts • Approximately 50% of the body is comprised of skeletal muscle. • There are two main types of skeletal muscle fibres: red, oxidative (slow) and white glycolytic (fast). • Each skeletal muscle fibre contains several hundred to several thousand myofibrils which are comprised of large polymerized protein molecules (actin and myosin) responsible for contraction. • Skeletal muscle cells show enormous plasticity adapting to a variety of external stimuli such as mechanical loading/unloading to either increase (hypertrophy) or decrease in size (atrophy). • In healthy individuals ∼80% of insulin-stimulated glucose uptake is taken up by skeletal muscle, highlighting the importance of this tissue in metabolic health.
∗ Corresponding author at: Exercise Metabolism Group, School of Medical Sciences, RMIT University, PO Box 71, Bundoora 3083, Australia. Tel.: +61 3 992 56518; fax: +61 3 9467 8181. E-mail address: [email protected] (L.G. Karagounis). 1357-2725/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocel.2010.05.013
1.1. Morphology In humans, skeletal muscle is the most abundant tissue in the body comprising 40–50% of body mass and playing vital roles in locomotion, heat production during periods of cold stress, and overall metabolism. Skeletal muscle is composed of bundles of muscle fibres called fascicles. The cell membrane surrounding the muscle cell is the sarcolemma, and beneath the sarcolemma lies the sarcoplasm, which contains the cellular proteins, organelles, and myofibrils. The myofibrils are composed of two major types of protein filaments: the thin actin filament and the thicker myosin filament. The arrangement of these two protein filaments gives skeletal muscle its striated appearance. Human skeletal muscle fibres are classified in terms of contractile and metabolic properties (for review see Zierath and Hawley, 2004). Based upon histochemical staining, muscle fibres are commonly distinguished as slow-twitch (ST) oxidative (which stain dark or red), and fast-twitch (FT) glycolytic (which stain light or pale). In humans, a further subdivision of the FT fibres is made whereby the more aerobic (or oxidative) FT fibre is designated FTa , and the more anaerobic (glycolytic) fibre is termed FTb (Zierath and Hawley, 2004). There is a large degree of homogeneity within individual skeletal muscles in rodents, but this is not the case for humans. Indeed, the heterogeneity of fibre type composition between individuals helps explain, in part, the remarkable variation in metabolic potential and exercise capacity observed in
L.G. Karagounis, J.A. Hawley / The International Journal of Biochemistry & Cell Biology 42 (2010) 1376–1379
humans. This brief review will (1) summarise our current understanding of the origin and plasticity of skeletal muscle, (2) discuss the major effectors of muscle growth, and (3) highlight the importance of skeletal muscle health in the prevention of several common pathologies. 2. Cell origin and plasticity 2.1. Cell origin In 1961, two seminal studies demonstrated that the contractile unit of skeletal muscle, the myofibre, is formed by the fusion of large numbers of mononucleated myoblasts (Cooper and Konigsberg, 1961; Stockdale and Holtzer, 1961). Using electron microscopy techniques, it was observed that there was an apparently quiescent cell on the surface of the myofibre beneath its basement membrane. This cell was referred to as a ‘satellite cell’ in reference to its peripheral position (Mauro, 1961). Mammalian satellite cells originate from a group of transitory mesoderm-derived structures known as somites which differentiate into dermomyotome and sclerotome and the mesodermal cells become specified as skeletal muscle precursors within the nascent myotome (Zammit et al., 2006). Upon activation, satellite cells undergo a series of sequential events that lead ultimately to them fusing with existing myofibres and therefore increasing the size of the pre-existing muscle fibres. This process is carried out by a family of myogenic regulatory factors (MRF) including the myogenic differentiation factor (MyoD), myogenin, Myf5 and Myf6. Of these, MyoD is required to activate the satellite cell before being committed to differentiation by the second factor namely myogenin (see Fig. 1) (Arnold and Winter, 1998; Molkentin and Olson, 1996). The MyoD family of basic helix–loop–helix transcription factors function as heterodimers with members of the E-protein family to induce myogenic gene activation by binding to the E1 E-box of the myogenin promoter leading to the activation of the transcriptional machinery (Parker et al., 2006). In the embryonic stage, MyoD is involved in the regulation of transcription defining myogenesis, whereas in adult skeletal muscle it plays an important role in muscle plasticity (discussed subsequently). An interaction between the phosphoinositide-3 kinase (PI3K) insulin signalling pathway and MyoD muscle cell differentiation has previously been demonstrated in vitro. In a yeast two-hybrid study, it was shown that protein kinase B (PKB/AKT) specifically interacted with prohibitin 2 (PHB2), a ubiquitously expressed regulator of cell proliferation (Sun et al., 2004). It is believed that PHB2 is able to inhibit muscle differentiation by repressing the transcriptional activity of MyoD. Phosphorylation and activation of AKT causes its partial translo-
1377
cation into the nucleus where by it competitively binds to PHB2 releasing its inhibitory effect on MyoD and therefore allowing the initiation of muscle differentiation. 2.2. Cell plasticity Skeletal muscle is a highly heterogeneous tissue and demonstrates a remarkable plasticity, adapting to a variety of external stimuli including habitual level of contractile activity and loading state, substrate availability and the prevailing hormonal milieu. Within the basic functional unit of contraction, the sarcomere, there are a multitude of different structural, regulatory and contractile proteins, many of which exist as different isoforms, giving skeletal muscle a multiplicity of isoform expression (Schiaffino and Reggiani, 1996). Indeed, the inherent potential to increase the number of sarcomeres (i.e., hypertrophy), combined with an ability to alter protein isoform expression, gives muscle the unique ability to adapt to the many and varied challenges imposed upon it. The introduction of invasive surgical procedures to exercise physiology in the mid-1970s (Bergstrom, 1975), permitted small biopsy samples (∼100–150 mg) of human skeletal muscle to be excised, and by means of histological and biochemical analyses, the morphological, contractile, and metabolic properties were rapidly identified (Zierath and Hawley, 2004). Knowledge of the molecular and cellular events that regulate skeletal muscle plasticity is essential in order to define the capacity for adaptation in this tissue, as well as the potential for the discovery of novel genes and pathways in common clinical disease states (discussed subsequently). 3. Sending the signals for skeletal muscle growth 3.1. Effectors of muscle growth Rates of human skeletal muscle protein synthesis can be determined by employing stable isotope techniques, in which labelled amino acid tracers (e.g., [2 H5 ]phenylalanine) are infused intravenously for the duration of an intervention (i.e., exercise and/or nutrient challenge). Muscle protein synthesis is determined by measuring the incorporation of the tracer into small muscle samples obtained via single or multiple (time-course) biopsies (Rennie et al., 1982). These in vivo human techniques have the advantage of permitting the perfusion of the muscle by an intact circulatory system during a stable isotope infusion, coupled with the ability to combine direct measurements of protein turnover with simultaneous measures of protein and gene expression from the same muscle sample.
Fig. 1. Schematic of satellite cell activation in mature skeletal muscle. (A and B) Quiescent satellite cells are activated by external stimuli such as mechanical stimulation/contraction, (C) the activated satellite cell divides resulting in the formation of myoblasts that further proliferate and differentiate to form myotubes, (D1) at this stage, in a process of self renewal, some of the committed satellite cells withdraw from the cell cycle to replace the satellite cells that have been used up and therefore replenish the pool of quiescent satellite cells, and (D2 and E) the committed satellite cells which have fused to form myotubes then mature into myofibres. MyoD may act as a marker of satellite cell activation whereas myogenin is associated with the differentiation and formation of myofibres.
1378
L.G. Karagounis, J.A. Hawley / The International Journal of Biochemistry & Cell Biology 42 (2010) 1376–1379
When healthy individuals consume regular mixed meals and maintain activities of everyday living, skeletal muscle protein mass remains essentially unchanged for lengthy periods of time. This is because the rates of muscle protein synthesis (MPS) approximate the rates of muscle protein breakdown (MPB) over time. Mechanical stimulation of skeletal muscle in the form of resistance-like exercise results in an increase in muscle mass an effect that is more pronounced in FT compared to ST muscle fibres (Trappe et al., 2004). However, for an increase in the size of muscle protein mass, MPS must exceed MPB over a sustained period of time (i.e., 2–3 months). In addition to contractile activity, feeding increases the rate of protein synthesis compared to the unfed state, an effect that appears mainly due to the effects of increased amino acid availability (particularly essential amino acids) rather than the feeding-induced increase in insulin concentrations. Evidence to support this contention comes from investigations that have ‘clamped’ insulin levels (by the use of somatostatin and analogs) and still observed robust increases in protein synthesis (Greenhaff et al., 2008). Increased amino acid availability not only provides substrate for protein synthesis but also directly modulates intracellular signalling events regulating initiation and elongation phases of mRNA translation. Several recent investigations have found that the activation of proteins in the protein kinase B (PKB/AKT)—mammalian target of rapamycin (mTOR) and 70 kDa ribosomal protein kinase (p70S6K) pathway are enhanced following resistance exercise if amino acids or protein are ingested (Karlsson et al., 2004; Koopman et al., 2007; Dreyer et al., 2008; Drummond et al., 2009) compared to when resistance exercise is undertaken with no nutrient provision. These observations provide strong support for the notion that provision of amino acids/protein with concurrent resistance exercise synergistically enhances muscle protein synthesis (Biolo et al., 1997; Tipton et al., 1999; Rasmussen et al., 2000; Borsheim et al., 2002; Miller et al., 2003; Borsheim et al., 2004).
Fig. 2. Schematic depicting the activation of the AKT/mTOR signalling pathway in response to resistance-type exercise and increased insulin and nutrient availability (i.e., protein and essential amino acids) that result in increased translation and skeletal muscle hypertrophy. Arrowheads indicate activation by phosphorylation and rounded heads indicate inhibition by phosphorylation.
3.2. Sending the signal The results of many studies have demonstrated a central role of the AKT–mTOR cascade in anabolic processes following both acute chronic resistance exercise in humans and rodents (Deldicque et al., 2005; Karagounis et al., 2010) and also the proteins implicated in translational control, such as p70S6K and eIF4E-binding protein (4E-BP1) (Fig. 2). Early work by (Liu et al., 2002) and colleagues provides direct evidence for a fundamental role of p70S6K in skeletal muscle hypertrophy processes: increases in p70S6K phosphorylation after a bout of resistance exercise strongly correlate with the chronic increase in muscle mass and strength after a 14 wk resistance training programme in humans (Terzis et al., 2008). These results are highly relevant because they show that the acute response to exercise–nutrient protocols that have measured either dynamic changes in muscle protein turnover and the early activation of muscle signalling proteins (such as p70S6K) may act as qualitative predictors of long-term phenotypic changes. However, the extent to which the activation of the AKT–mTOR pathway results in net gains in skeletal muscle protein synthesis is not well understood. In this regard, we have previously shown a dissociation between intracellular signalling events and rates of muscle protein synthesis in both acute (Greenhaff et al., 2008) and chronic exercise/diet interventions (Karagounis et al., unpublished data). 4. Functions The prime functions of skeletal muscle are to maintain the integrity of our skeleton and generate force and power for locomotive activities of daily living. Approximately 2% of muscle proteins
are turned over daily accounting for 50% of basal metabolic rate in healthy adults. Skeletal muscle is also the largest reserve of protein in the body and acts as a dynamic metabolic store. This may be required in extreme situations, such as during times of starvation and prolonged illness. 5. Associated pathologies In healthy individuals ∼80% of insulin-stimulated glucose uptake is taken up by skeletal muscle, highlighting the importance of this tissue in metabolic health. Resistance to the normal action of insulin (i.e., insulin resistance) contributes to the pathogenesis of a number of common human disorders including obesity and type 2 diabetes. Indeed, impaired insulin action on whole body glucose uptake is largely attributable to impaired insulin-stimulated glucose transport in skeletal muscle. Insulin sensitivity also correlates with the proportion of ST oxidative fibres: insulin-stimulated glucose transport is greater in skeletal muscle with a greater proportion of ST than FT muscle fibres (Henriksen et al., 1990) Ageing and physical inactivity, two conditions accompanied by a diminished skeletal muscle oxidative capacity and insulin resistance are also associated with a loss of skeletal muscle protein (sarcopenia). Sarcopenia is characterised by a deterioration of muscle quantity beginning at age ∼30 years (Dorrens, 2003). Between the ages of 20 and 80 muscle mass of the Vastus lateralis is reduced by up to 40% (Lexell, 1995). The precise mechanism(s) to explain sarcopenia remain to be elucidated. However, muscle wasting in mid and later life is the result of a loss of muscle protein due to an imbalance between rates of MPS and MPB. The central approach to
L.G. Karagounis, J.A. Hawley / The International Journal of Biochemistry & Cell Biology 42 (2010) 1376–1379
the prevention and treatment of sarcopenia, and certainly the most potent natural stimulus to increase the rate of MPS, is mechanical loading. But while resistance-like exercise is widely acknowledged as an essential component of any programme aimed at increasing or maintaining muscle mass, exercise alone may not be an optimal intervention to prevent the age-associated degenerative changes in skeletal muscle. Indeed, it is now well accepted that increasing nutrient availability is critical for optimising many of the adaptations in skeletal muscle in response to resistance and endurance training (Hawley et al., 2006). Most prevalent chronic diseases and pathologies have an association with physical inactivity, and a number of risk factors for chronic diseases that are precipitated by physical inactivity have been discussed elsewhere (Booth et al., 2000; Karagounis and Hawley, 2009). Determining the basic underlying mechanisms of inactivity-related conditions and the associated loss of muscle mass and implementing effective treatment strategies to counter these conditions via primary interventions (i.e., exercise–diet manipulation to increase muscle mass and metabolic health) should be a top priority for basic scientists and health care professional alike in the forthcoming years. Acknowledgement The writing of this review was supported by an Australian Research Council Linkage Grant (ARCLP LP100100010 to JAH). References Arnold HH, Winter B. Muscle differentiation: more complexity to the network of myogenic regulators. Curr Opin Gen Dev 1998;8:539–44. Bergstrom J. Percutaneous needle biopsy of skeletal muscle in physiological and clinical research. Scand J Clin Lab Invest 1975;35:609–16. Biolo G, Tipton KD, Klein S, Wolfe RR. An abundant supply of amino acids enhances the metabolic effect of exercise on muscle protein. Am J Physiol Endocrinol Metab 1997;273:E122–9. Booth FW, Gordon SE, Carlson CJ, Hamilton MT. Waging war on modern chronic diseases: primary prevention through exercise biology. J Appl Physiol 2000;88:774–87. Borsheim E, Aarsland A, Wolfe RR. Effect of an amino acid, protein, and carbohydrate mixture on net muscle protein balance after resistance exercise. Int J Sport Nutr Exerc Metab 2004;14:255–71. Borsheim E, Tipton KD, Wolf SE, Wolfe RR. Essential amino acids and muscle protein recovery from resistance exercise. Am J Physiol Endocrinol Metab 2002;283:E648–57. Cooper WG, Konigsberg IR. Dynamics of myogenesis in vitro. Anat Rec 1961;140:195–205. Deldicque L, Theisen D, Francaux M. Regulation of mTOR by amino acids and resistance exercise in skeletal muscle. Eur J Appl Physiol 2005;94:1–10. Dorrens JMJR. Effects of ageing and human whole body and muscle protein turnover. Scand J Med Sci Sports 2003;13:26–33. Dreyer HC, Drummond MJ, Pennings B, Fujita S, Glynn EL, Chinkes DL, et al. Leucineenriched essential amino acid and carbohydrate ingestion following resistance exercise enhances mTOR signaling and protein synthesis in human muscle. Am J Physiol Endocrinol Metab 2008;294:E392–400. Drummond MJ, Miyazaki M, Dreyer HC, Pennings B, Dhanani S, Volpi E, et al. Expression of growth-related genes in young and older human skeletal
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muscle following an acute stimulation of protein synthesis. J Appl Physiol 2009;106:1403–11. Greenhaff PL, Karagounis LG, Peirce N, Simpson EJ, Hazell M, Layfield R, et al. Disassociation between the effects of amino acids and insulin on signaling, ubiquitin ligases, and protein turnover in human muscle. Am J Physiol Endocrinol Metab 2008;295:E595–604. Hawley JA, Tipton KD, Millard-Stafford ML. Promoting training adaptations through nutritional interventions. J Sports Sci 2006;24:709–21. Henriksen EJ, Bourey RE, Rodnick KJ, Koranyi L, Permutt MA, Holloszy JO. Glucose transporter protein content and glucose transport capacity in rat skeletal muscles. Am J Physiol Endocrinol Metab 1990;259:E593–8. Karagounis LG, Hawley JA. The 5 adenosine monophosphate-activated protein kinase: regulating the ebb and flow of cellular energetics. Int J Biochem Cell Biol 2009;41:2360–3. Karagounis LG, Yaspelkis 3rd BB, Reeder DW, Lancaster GI, Hawley JA, Coffey VG. Contraction-induced changes in TNFalpha and Akt-mediated signalling are associated with increased myofibrillar protein in rat skeletal muscle. Eur J Appl Physiol 2010, Epub ahead of print. Karlsson HK, Nilsson PA, Nilsson J, Chibalin AV, Zierath JR, Blomstrand E. Branchedchain amino acids increase p70S6k phosphorylation in human skeletal muscle after resistance exercise. Am J Physiol Endocrinol Metab 2004;287:E1–7. Koopman R, Beelen M, Stellingwerff T, Pennings B, Saris WHM, Kies AK, et al. Coingestion of carbohydrate with protein does not further augment postexercise muscle protein synthesis. Am J Physiol Endocrinol Metab 2007;293:E833–42. Lexell J. Human aging, muscle mass, and fiber type composition. J Gerontol A Biol Sci Med Sci 1995;50(Spec. No.):11–6. Liu Z, Jahn LA, Wei L, Long W, Barrett EJ. Amino acids stimulate translation initiation and protein synthesis through an Akt-independent pathway in human skeletal muscle. J Clin Endocrinol Metab 2002;87:5553–8. Mauro A. Satellite cell of skeletal muscle fibers. J Biophys Biochem Cytol 1961;9:493–5. Miller SL, Tipton KD, Chinkes DL, Wolf SE, Wolfe RR. Independent and combined effects of amino acids and glucose after resistance exercise. Med Sci Sports Exerc 2003;35:449–55. Molkentin JD, Olson EN. Defining the regulatory networks for muscle development. Curr Opin Gen Dev 1996;6:445–53. Parker MH, Perry RLS, Fauteux MC, Berkes CA, Rudnicki MA. MyoD synergizes with the E-protein HEB{beta} to induce myogenic differentiation. Mol Cell Biol 2006;26:5771–83. Rasmussen BB, Tipton KD, Miller SL, Wolf SE, Wolfe RR. An oral essential amino acid–carbohydrate supplement enhances muscle protein anabolism after resistance exercise. J Appl Physiol 2000;88:386–92. Rennie MJ, Edwards RH, Halliday D, Matthews DE, Wolman SL, Millward DJ. Muscle protein synthesis measured by stable isotope techniques in man: the effects of feeding and fasting. Clin Sci (Lond) 1982;63:519–23. Schiaffino S, Reggiani C. Molecular diversity of myofibrillar proteins: gene regulation and functional significance. Physiol Rev 1996;76:371–423. Stockdale FE, Holtzer H. DNA synthesis and myogenesis. Exp Cell Res 1961;24:508–20. Sun L, Liu L, Yang XJ, Wu Z. Akt binds prohibitin 2 and relieves its repression of MyoD and muscle differentiation. J Cell Sci 2004;117:3021–9. Terzis G, Georgiadis G, Stratakos G, Vogiatzis I, Kavouras S, Manta P, et al. Resistance exercise-induced increase in muscle mass correlates with p70S6 kinase phosphorylation in human subjects. Eur J Appl Physiol 2008;102:145–52. Tipton KD, Ferrando AA, Phillips SM, Doyle Jr D, Wolfe RR. Postexercise net protein synthesis in human muscle from orally administered amino acids. Am J Physiol Endocrinol Metab 1999;276:E628–34. Trappe TA, Raue U, Tesch PA. Human soleus muscle protein synthesis following resistance exercise. Acta Physiol Scand 2004;182:189–96. Zammit PS, Partridge TA, Yablonka-Reuveni Z. The skeletal muscle satellite cell: the stem cell that came in from the cold. J Histochem Cytochem 2006;54: 1177–91. Zierath JR, Hawley JA. Skeletal muscle fiber type: influence on contractile and metabolic properties. PLoS Biol 2004;2:e348.
Contents lists available at ScienceDirect
The International Journal of Biochemistry & Cell Biology journal homepage: www.elsevier.com/locate/biocel
Cells in focus
Skeletal muscle: Increasing the size of the locomotor cell Leonidas G. Karagounis∗ , John A. Hawley Health Innovations Research Institute, School of Medical Sciences, RMIT University, Bundoora 3083, Australia
a r t i c l e
i n f o
Article history: Received 15 December 2009 Received in revised form 29 April 2010 Accepted 30 May 2010 Available online 9 June 2010 Keywords: Ageing AKT Amino acids Atrophy Hypertrophy Skeletal muscle mTOR MyoD Protein synthesis
a b s t r a c t Skeletal muscle is the most abundant tissue in the body comprising 40–50% of body mass in humans and playing a central role in maintaining metabolic health. Skeletal muscle protein undergoes rapid turnover, a process that is intricately regulated by the balance between the rates of protein synthesis and degradation. The process of skeletal muscle hypertrophy and regeneration is an important adaptive response to both contractile activity (i.e., exercise) and nutrient availability (i.e., protein ingestion). Ageing and physical inactivity are two conditions associated with a loss of skeletal muscle protein (sarcopenia). Sarcopenia is characterised by a deterioration of muscle quantity and quality, although the precise mechanism(s) underlying this condition remain to be elucidated. This review will (1) summarise our current understanding of the origin and plasticity of skeletal muscle, (2) discuss the major effectors of muscle growth, and (3) highlight the importance of skeletal muscle health in the prevention of several common pathologies. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Cell facts • Approximately 50% of the body is comprised of skeletal muscle. • There are two main types of skeletal muscle fibres: red, oxidative (slow) and white glycolytic (fast). • Each skeletal muscle fibre contains several hundred to several thousand myofibrils which are comprised of large polymerized protein molecules (actin and myosin) responsible for contraction. • Skeletal muscle cells show enormous plasticity adapting to a variety of external stimuli such as mechanical loading/unloading to either increase (hypertrophy) or decrease in size (atrophy). • In healthy individuals ∼80% of insulin-stimulated glucose uptake is taken up by skeletal muscle, highlighting the importance of this tissue in metabolic health.
∗ Corresponding author at: Exercise Metabolism Group, School of Medical Sciences, RMIT University, PO Box 71, Bundoora 3083, Australia. Tel.: +61 3 992 56518; fax: +61 3 9467 8181. E-mail address: [email protected] (L.G. Karagounis). 1357-2725/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocel.2010.05.013
1.1. Morphology In humans, skeletal muscle is the most abundant tissue in the body comprising 40–50% of body mass and playing vital roles in locomotion, heat production during periods of cold stress, and overall metabolism. Skeletal muscle is composed of bundles of muscle fibres called fascicles. The cell membrane surrounding the muscle cell is the sarcolemma, and beneath the sarcolemma lies the sarcoplasm, which contains the cellular proteins, organelles, and myofibrils. The myofibrils are composed of two major types of protein filaments: the thin actin filament and the thicker myosin filament. The arrangement of these two protein filaments gives skeletal muscle its striated appearance. Human skeletal muscle fibres are classified in terms of contractile and metabolic properties (for review see Zierath and Hawley, 2004). Based upon histochemical staining, muscle fibres are commonly distinguished as slow-twitch (ST) oxidative (which stain dark or red), and fast-twitch (FT) glycolytic (which stain light or pale). In humans, a further subdivision of the FT fibres is made whereby the more aerobic (or oxidative) FT fibre is designated FTa , and the more anaerobic (glycolytic) fibre is termed FTb (Zierath and Hawley, 2004). There is a large degree of homogeneity within individual skeletal muscles in rodents, but this is not the case for humans. Indeed, the heterogeneity of fibre type composition between individuals helps explain, in part, the remarkable variation in metabolic potential and exercise capacity observed in
L.G. Karagounis, J.A. Hawley / The International Journal of Biochemistry & Cell Biology 42 (2010) 1376–1379
humans. This brief review will (1) summarise our current understanding of the origin and plasticity of skeletal muscle, (2) discuss the major effectors of muscle growth, and (3) highlight the importance of skeletal muscle health in the prevention of several common pathologies. 2. Cell origin and plasticity 2.1. Cell origin In 1961, two seminal studies demonstrated that the contractile unit of skeletal muscle, the myofibre, is formed by the fusion of large numbers of mononucleated myoblasts (Cooper and Konigsberg, 1961; Stockdale and Holtzer, 1961). Using electron microscopy techniques, it was observed that there was an apparently quiescent cell on the surface of the myofibre beneath its basement membrane. This cell was referred to as a ‘satellite cell’ in reference to its peripheral position (Mauro, 1961). Mammalian satellite cells originate from a group of transitory mesoderm-derived structures known as somites which differentiate into dermomyotome and sclerotome and the mesodermal cells become specified as skeletal muscle precursors within the nascent myotome (Zammit et al., 2006). Upon activation, satellite cells undergo a series of sequential events that lead ultimately to them fusing with existing myofibres and therefore increasing the size of the pre-existing muscle fibres. This process is carried out by a family of myogenic regulatory factors (MRF) including the myogenic differentiation factor (MyoD), myogenin, Myf5 and Myf6. Of these, MyoD is required to activate the satellite cell before being committed to differentiation by the second factor namely myogenin (see Fig. 1) (Arnold and Winter, 1998; Molkentin and Olson, 1996). The MyoD family of basic helix–loop–helix transcription factors function as heterodimers with members of the E-protein family to induce myogenic gene activation by binding to the E1 E-box of the myogenin promoter leading to the activation of the transcriptional machinery (Parker et al., 2006). In the embryonic stage, MyoD is involved in the regulation of transcription defining myogenesis, whereas in adult skeletal muscle it plays an important role in muscle plasticity (discussed subsequently). An interaction between the phosphoinositide-3 kinase (PI3K) insulin signalling pathway and MyoD muscle cell differentiation has previously been demonstrated in vitro. In a yeast two-hybrid study, it was shown that protein kinase B (PKB/AKT) specifically interacted with prohibitin 2 (PHB2), a ubiquitously expressed regulator of cell proliferation (Sun et al., 2004). It is believed that PHB2 is able to inhibit muscle differentiation by repressing the transcriptional activity of MyoD. Phosphorylation and activation of AKT causes its partial translo-
1377
cation into the nucleus where by it competitively binds to PHB2 releasing its inhibitory effect on MyoD and therefore allowing the initiation of muscle differentiation. 2.2. Cell plasticity Skeletal muscle is a highly heterogeneous tissue and demonstrates a remarkable plasticity, adapting to a variety of external stimuli including habitual level of contractile activity and loading state, substrate availability and the prevailing hormonal milieu. Within the basic functional unit of contraction, the sarcomere, there are a multitude of different structural, regulatory and contractile proteins, many of which exist as different isoforms, giving skeletal muscle a multiplicity of isoform expression (Schiaffino and Reggiani, 1996). Indeed, the inherent potential to increase the number of sarcomeres (i.e., hypertrophy), combined with an ability to alter protein isoform expression, gives muscle the unique ability to adapt to the many and varied challenges imposed upon it. The introduction of invasive surgical procedures to exercise physiology in the mid-1970s (Bergstrom, 1975), permitted small biopsy samples (∼100–150 mg) of human skeletal muscle to be excised, and by means of histological and biochemical analyses, the morphological, contractile, and metabolic properties were rapidly identified (Zierath and Hawley, 2004). Knowledge of the molecular and cellular events that regulate skeletal muscle plasticity is essential in order to define the capacity for adaptation in this tissue, as well as the potential for the discovery of novel genes and pathways in common clinical disease states (discussed subsequently). 3. Sending the signals for skeletal muscle growth 3.1. Effectors of muscle growth Rates of human skeletal muscle protein synthesis can be determined by employing stable isotope techniques, in which labelled amino acid tracers (e.g., [2 H5 ]phenylalanine) are infused intravenously for the duration of an intervention (i.e., exercise and/or nutrient challenge). Muscle protein synthesis is determined by measuring the incorporation of the tracer into small muscle samples obtained via single or multiple (time-course) biopsies (Rennie et al., 1982). These in vivo human techniques have the advantage of permitting the perfusion of the muscle by an intact circulatory system during a stable isotope infusion, coupled with the ability to combine direct measurements of protein turnover with simultaneous measures of protein and gene expression from the same muscle sample.
Fig. 1. Schematic of satellite cell activation in mature skeletal muscle. (A and B) Quiescent satellite cells are activated by external stimuli such as mechanical stimulation/contraction, (C) the activated satellite cell divides resulting in the formation of myoblasts that further proliferate and differentiate to form myotubes, (D1) at this stage, in a process of self renewal, some of the committed satellite cells withdraw from the cell cycle to replace the satellite cells that have been used up and therefore replenish the pool of quiescent satellite cells, and (D2 and E) the committed satellite cells which have fused to form myotubes then mature into myofibres. MyoD may act as a marker of satellite cell activation whereas myogenin is associated with the differentiation and formation of myofibres.
1378
L.G. Karagounis, J.A. Hawley / The International Journal of Biochemistry & Cell Biology 42 (2010) 1376–1379
When healthy individuals consume regular mixed meals and maintain activities of everyday living, skeletal muscle protein mass remains essentially unchanged for lengthy periods of time. This is because the rates of muscle protein synthesis (MPS) approximate the rates of muscle protein breakdown (MPB) over time. Mechanical stimulation of skeletal muscle in the form of resistance-like exercise results in an increase in muscle mass an effect that is more pronounced in FT compared to ST muscle fibres (Trappe et al., 2004). However, for an increase in the size of muscle protein mass, MPS must exceed MPB over a sustained period of time (i.e., 2–3 months). In addition to contractile activity, feeding increases the rate of protein synthesis compared to the unfed state, an effect that appears mainly due to the effects of increased amino acid availability (particularly essential amino acids) rather than the feeding-induced increase in insulin concentrations. Evidence to support this contention comes from investigations that have ‘clamped’ insulin levels (by the use of somatostatin and analogs) and still observed robust increases in protein synthesis (Greenhaff et al., 2008). Increased amino acid availability not only provides substrate for protein synthesis but also directly modulates intracellular signalling events regulating initiation and elongation phases of mRNA translation. Several recent investigations have found that the activation of proteins in the protein kinase B (PKB/AKT)—mammalian target of rapamycin (mTOR) and 70 kDa ribosomal protein kinase (p70S6K) pathway are enhanced following resistance exercise if amino acids or protein are ingested (Karlsson et al., 2004; Koopman et al., 2007; Dreyer et al., 2008; Drummond et al., 2009) compared to when resistance exercise is undertaken with no nutrient provision. These observations provide strong support for the notion that provision of amino acids/protein with concurrent resistance exercise synergistically enhances muscle protein synthesis (Biolo et al., 1997; Tipton et al., 1999; Rasmussen et al., 2000; Borsheim et al., 2002; Miller et al., 2003; Borsheim et al., 2004).
Fig. 2. Schematic depicting the activation of the AKT/mTOR signalling pathway in response to resistance-type exercise and increased insulin and nutrient availability (i.e., protein and essential amino acids) that result in increased translation and skeletal muscle hypertrophy. Arrowheads indicate activation by phosphorylation and rounded heads indicate inhibition by phosphorylation.
3.2. Sending the signal The results of many studies have demonstrated a central role of the AKT–mTOR cascade in anabolic processes following both acute chronic resistance exercise in humans and rodents (Deldicque et al., 2005; Karagounis et al., 2010) and also the proteins implicated in translational control, such as p70S6K and eIF4E-binding protein (4E-BP1) (Fig. 2). Early work by (Liu et al., 2002) and colleagues provides direct evidence for a fundamental role of p70S6K in skeletal muscle hypertrophy processes: increases in p70S6K phosphorylation after a bout of resistance exercise strongly correlate with the chronic increase in muscle mass and strength after a 14 wk resistance training programme in humans (Terzis et al., 2008). These results are highly relevant because they show that the acute response to exercise–nutrient protocols that have measured either dynamic changes in muscle protein turnover and the early activation of muscle signalling proteins (such as p70S6K) may act as qualitative predictors of long-term phenotypic changes. However, the extent to which the activation of the AKT–mTOR pathway results in net gains in skeletal muscle protein synthesis is not well understood. In this regard, we have previously shown a dissociation between intracellular signalling events and rates of muscle protein synthesis in both acute (Greenhaff et al., 2008) and chronic exercise/diet interventions (Karagounis et al., unpublished data). 4. Functions The prime functions of skeletal muscle are to maintain the integrity of our skeleton and generate force and power for locomotive activities of daily living. Approximately 2% of muscle proteins
are turned over daily accounting for 50% of basal metabolic rate in healthy adults. Skeletal muscle is also the largest reserve of protein in the body and acts as a dynamic metabolic store. This may be required in extreme situations, such as during times of starvation and prolonged illness. 5. Associated pathologies In healthy individuals ∼80% of insulin-stimulated glucose uptake is taken up by skeletal muscle, highlighting the importance of this tissue in metabolic health. Resistance to the normal action of insulin (i.e., insulin resistance) contributes to the pathogenesis of a number of common human disorders including obesity and type 2 diabetes. Indeed, impaired insulin action on whole body glucose uptake is largely attributable to impaired insulin-stimulated glucose transport in skeletal muscle. Insulin sensitivity also correlates with the proportion of ST oxidative fibres: insulin-stimulated glucose transport is greater in skeletal muscle with a greater proportion of ST than FT muscle fibres (Henriksen et al., 1990) Ageing and physical inactivity, two conditions accompanied by a diminished skeletal muscle oxidative capacity and insulin resistance are also associated with a loss of skeletal muscle protein (sarcopenia). Sarcopenia is characterised by a deterioration of muscle quantity beginning at age ∼30 years (Dorrens, 2003). Between the ages of 20 and 80 muscle mass of the Vastus lateralis is reduced by up to 40% (Lexell, 1995). The precise mechanism(s) to explain sarcopenia remain to be elucidated. However, muscle wasting in mid and later life is the result of a loss of muscle protein due to an imbalance between rates of MPS and MPB. The central approach to
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the prevention and treatment of sarcopenia, and certainly the most potent natural stimulus to increase the rate of MPS, is mechanical loading. But while resistance-like exercise is widely acknowledged as an essential component of any programme aimed at increasing or maintaining muscle mass, exercise alone may not be an optimal intervention to prevent the age-associated degenerative changes in skeletal muscle. Indeed, it is now well accepted that increasing nutrient availability is critical for optimising many of the adaptations in skeletal muscle in response to resistance and endurance training (Hawley et al., 2006). Most prevalent chronic diseases and pathologies have an association with physical inactivity, and a number of risk factors for chronic diseases that are precipitated by physical inactivity have been discussed elsewhere (Booth et al., 2000; Karagounis and Hawley, 2009). Determining the basic underlying mechanisms of inactivity-related conditions and the associated loss of muscle mass and implementing effective treatment strategies to counter these conditions via primary interventions (i.e., exercise–diet manipulation to increase muscle mass and metabolic health) should be a top priority for basic scientists and health care professional alike in the forthcoming years. Acknowledgement The writing of this review was supported by an Australian Research Council Linkage Grant (ARCLP LP100100010 to JAH). References Arnold HH, Winter B. Muscle differentiation: more complexity to the network of myogenic regulators. Curr Opin Gen Dev 1998;8:539–44. Bergstrom J. Percutaneous needle biopsy of skeletal muscle in physiological and clinical research. Scand J Clin Lab Invest 1975;35:609–16. Biolo G, Tipton KD, Klein S, Wolfe RR. An abundant supply of amino acids enhances the metabolic effect of exercise on muscle protein. Am J Physiol Endocrinol Metab 1997;273:E122–9. Booth FW, Gordon SE, Carlson CJ, Hamilton MT. Waging war on modern chronic diseases: primary prevention through exercise biology. J Appl Physiol 2000;88:774–87. Borsheim E, Aarsland A, Wolfe RR. Effect of an amino acid, protein, and carbohydrate mixture on net muscle protein balance after resistance exercise. Int J Sport Nutr Exerc Metab 2004;14:255–71. Borsheim E, Tipton KD, Wolf SE, Wolfe RR. Essential amino acids and muscle protein recovery from resistance exercise. Am J Physiol Endocrinol Metab 2002;283:E648–57. Cooper WG, Konigsberg IR. Dynamics of myogenesis in vitro. Anat Rec 1961;140:195–205. Deldicque L, Theisen D, Francaux M. Regulation of mTOR by amino acids and resistance exercise in skeletal muscle. Eur J Appl Physiol 2005;94:1–10. Dorrens JMJR. Effects of ageing and human whole body and muscle protein turnover. Scand J Med Sci Sports 2003;13:26–33. Dreyer HC, Drummond MJ, Pennings B, Fujita S, Glynn EL, Chinkes DL, et al. Leucineenriched essential amino acid and carbohydrate ingestion following resistance exercise enhances mTOR signaling and protein synthesis in human muscle. Am J Physiol Endocrinol Metab 2008;294:E392–400. Drummond MJ, Miyazaki M, Dreyer HC, Pennings B, Dhanani S, Volpi E, et al. Expression of growth-related genes in young and older human skeletal
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