Muscle mass is determined by the difference between the rate of protein synthesis and degradation. If synthesis is greater than degradation, muscle mass will increase (hypertrophy) and when the reverse is true muscle mass will decrease (atrophy). Following resistance exercise/increased loading there is a transient increase in protein synthesis within muscle. This change in protein synthesis correlates with an increase in the activity of protein kinase B/Akt and mTOR (mammalian target of rapamycin). mTOR increases protein synthesis by increasing translation initiation and by inducing ribosomal biogenesis. By contrast, unloading or inactivity results in a decrease in protein synthesis and a significant increase in muscle protein breakdown. The decrease in synthesis is due in part to the inactivation of mTOR and therefore a decrease in translation initiation, but also to a decrease in the rate of translation elongation. The increase in degradation is the result of a co-ordinated response of the calpains, lysosomal proteases and the ATP-dependent ubiquitin-proteosome. Caspase 3 and the calpains act upstream of the ubiquitin–proteosome system to assist in the complete breakdown of the myofibrillar proteins. Two muscle specific E3 ubiquitin ligases, MuRF1 and MAFbx/atrogen-1, have been identified as key regulators of muscle atrophy. In this chapter, these pathways and how the balance between anabolism and catabolism is affected by loading and unloading will be discussed.
Skeletal muscle produces the forces that allow us to move, is the primary storehouse for both amino acids and glucose and, because of its high metabolic rate, is also the primary determinant of our basal metabolic rate . Our overall amount of muscle tissue is therefore not only essential for maintaining an active lifestyle, but is also an important determinant of our longevity , adiposity  and insulin sensitivity . Muscle mass is determined by the balance between protein synthesis and protein degradation. Following resistance exercise/increased loading in the fed state, the rate of protein synthesis exceeds breakdown, whilst unloading has the reverse effect. The result is that increasing loading results in skeletal muscle hypertrophy (increased muscle mass) and increased force production whereas unloading results in skeletal muscle atrophy (decreased muscle mass) and frailty. Over the last ten years a number of important discoveries have shed light on how loading determines muscle mass and much of this work will be briefly reviewed below. For a more extensive description, we suggest a number of very good reviews for both hypertrophy  and atrophy [5,6].
Skeletal muscle hypertrophy occurs following repeated bouts of high resistance exercise. Whilst each individual bout of high resistance exercise is necessary, it is not sufficient to produce hypertrophy. This indicates that, following acute exercise, there is a transient alteration within the muscle that, when repeated, produces skeletal muscle hypertrophy. Therefore, in order to understand what drives the increase in muscle mass we must understand what happens immediately following a single bout of resistance exercise. The most important acute response to resistance exercise is an increase in the rate of protein synthesis.
Control of protein synthesis by hypertrophic stimuli
In humans, a bout of high resistance exercise increases the fractional rate of protein synthesis 50% after 4 h, and 115% by 24 h . In some studies the increase in protein synthesis is maintained out to 48 h before returning to control levels . Theoretically, this increase could be due to an increase in RNA levels leading to an increase in protein synthesis through a mass action effect. However, needle biopsies taken from the subjects showed that there was no change in the RNA content of the muscle at either 4 h or 24 h following a single bout of resistance exercise , suggesting that the immediate changes in protein synthesis are the result of an increase in the amount of protein synthesized per molecule of RNA, not an increase in total RNA, thus it is the efficiency of translation or mRNA activity, that is increased following loading. An immediate increase in the rate of protein synthesis following an acute bout of resistance exercise has also been shown in a number of animal models. The most well-controlled studies addressing the initial response of skeletal muscle to a hypertrophic stimulus are those of Wong and Booth [9,10]. In these studies the initial response to an acute bout of shortening and lengthening contractions was a 25–50% increase in protein synthesis. This increase in protein synthesis was observed 12–17 h after the exercise bout at a time when the accumulation of RNA and DNA was unchanged. The authors concluded that increases in RNA do not have a primary role in increased protein synthesis after a single bout of resistance exercise . Hamosch et al.  conclusively demonstrated that an increase in protein synthesis occurs during hypertrophy using cell free extracts taken from hypertrophied muscle. This study demonstrated that following the addition of equal amounts of mRNA, the rate of amino acid incorporation was 120% higher in extracts from hypertrophied muscle than in extracts from normal muscle, indicating that the RNA activity of muscle is increased following high resistance exercise.
Control of protein synthesis by phosphorylation
Protein synthesis is the process by which the genetic code, carried by mRNA, is translated into the amino acids that form proteins. The polymerization of the amino acids is catalysed by a large molecule know as the ribosome that binds to the mRNA and scans to find the translation start site (initiation), translates the mRNA by sequentially inserting one amino acid for every codon as it travels the length of the mRNA (elongation), and is released from the mRNA when it completes the protein and reaches a stop codon (termination). To determine what step of protein synthesis was affected by resistance exercise, Baar and Esser used polysome profiles to show that there was an increase in the association of mRNA with ribosomes (Figure 1) suggesting that the rate of initiation of protein synthesis had increased more than the rate of elongation and termination .
Initiation can largely be separated into two regulated steps. The binding of the initiator tRNA to the 40S ribosomal subunit to make the 43S preinitiation complex which is regulated by eIF2 (eukaryotic initiation factor 2), and the cap-dependent binding of mRNA to the 43S preinitiation complex which is regulated by eIF4E (eukaryotic initiation factor 4E) and its repressor 4E-BP (eIF4E binding protein) (for a review see ). Both of these processes can be controlled following loading by the mTOR (mammalian target of rapamycin) and PDK1 (phosphoinositol-dependent protein kinase 1) pathway (Figure 2). mTOR exists in two important complexes within cells. The first is with its binding partners raptor and GβL in TORC1 (TOR complex 1) and the second is a complex with rictor and GβL termed TORC2 (TOR complex 2). TORC1 has been widely studied for its role in controlling protein synthesis through phosphorylation of its downstream targets the translational inhibitor 4E–BP and the 70 kDa S6K1 (ribosomal S6 kinase). TORC2 is the recently identified kinase upstream of PKB (protein kinase B)/Akt. Phosphorylation of S6K1 by TORC1 and PKB by TORC2 changes the conformation of the proteins making them substrates for the constitutively active PDK1. Following PDK1 phosphorylation, the proteins are fully active and function to increase the rate of protein synthesis. The importance of PDK1 in determining cell size has been demonstrated in mice with significantly decreased PDK1 protein levels. These mice are 40% smaller than their wild type littermates due to a decrease in cell size and no change in cell number .
Resistance exercise is known to induce a transient increase in the phosphorylation of mTOR , PKB , 4E-BP  and S6K1 , as well as the activity of eIF2 . The increase in S6K1 phosphorylation 6 h after a single bout of resistance exercise correlates with the increase in muscle mass following 6 weeks of training, suggesting that mTOR may play an important role in regulating muscle mass . In support of this hypothesis, blocking mTOR activity, with the bacterial macrolide antibiotic rapamycin, blocks the activation of S6K1, the increase in eIF2 activity and the increase in muscle mass following overload (Figure 3) [18,19] and muscle mass is decreased in S6K1 knockout mice .
Control of protein synthesis by increasing protein synthetic machinery
Along with increasing the rate of initiation, another way to promote increases in protein synthesis rates is to increase the number of ribosomes within muscle following increased loading. The number of ribosomes in a cell plays a fundamental role in growth regulation because it affects the amount of protein being synthesized per mRNA molecule. The cellular content of ribosomes is mainly regulated by an increase in their biosynthesis, which requires the co-ordinated synthesis of approx. 80 ribosomal proteins and four RNA species in addition to several hundred accessory enzymes .
Expression of the genes encoding ribosomal proteins can be regulated at both transcriptional and translational levels. Ribosomal protein gene transcription is regulated by the RNA polymerase II holoenzyme, and although transcriptional regulation is possible, ribosomal protein expression is controlled mainly by translational mechanisms. The typical ribosomal protein mRNA is characterized by a motif of 5–20 pyrimidines followed by a GC-rich sequence of approximately 40 nucleotides in its 5′ UTR (terminal oligopyrimidine tract or 5′ TOP). The 5? TOP confers selectivity for translational regulation of the ribosomal proteins during specific cellular demands, e.g. growth .
The expression of the ribosomal RNA subunits requires RNA polymerase I, UBF (upstream binding factor), and a series of associated factors. The ability of UBF to control rRNA expression is regulated by its own level of expression, phosphorylation on its C-terminus (that enhances UBF activity), as well as sequestration by retinoblastoma (that decreases UBF activity) . Retinoblastoma can in turn be regulated by phosphorylation by the cyclins and cdks (cyclin-dependent kinases) . Nader et al.  have recently reported that ribosome biogenesis during skeletal muscle hypertrophy is regulated, in part, by a cell cycle mechanism that is dependent on mTOR signalling. Serum stimulation of differentiated L6 rat myotubes resulted in an increase in ribosomal RNA content, cyclin D1-dependent phosphorylation of retinoblastoma, and release of the Rb binding partner UBF (Figure 4). All of these effects could be blocked by treatment with rapamycin, suggesting that mTOR controls ribosome synthesis via cyclin D1-dependent phosphorylation of retinoblastoma and an increase in UBF availability (Figure 2). Whether or not UBF phosphorylation plays a role in ribosome biosynthesis during muscle hypertrophy remains to be determined. However, in proliferating NIH 3T3 cells, mTOR regulates rRNA synthesis via release and phosphorylation of UBF  suggesting that increasing the amount of phosphorylated UBF within muscle may increase ribosome biogenesis in an mTOR dependent fashion.
Control of protein breakdown by increased loading
As well as increasing protein synthesis, resistance exercise increases the rate of protein degradation. The importance of the increase in degradation can be seen in the correlation between the fractional synthesis rate and the fractional breakdown rate in muscle following loading . As the rate of protein breakdown increases there is a concomitant rise in protein synthesis suggesting that there might be a molecular link between the two processes. However, the association is not always seen. Eating a meal rich in essential amino acids can decrease the effects of resistance exercise on protein degradation whilst at the same time increasing the rate of protein synthesis . Taking in a carbohydrate or mixed amino acids meal decreases the rate of degradation, possibly by decreasing circulating corticosteroids, without affecting the rate of synthesis. Therefore, it is only in the fed state that the net protein balance (protein synthesis minus degradation) becomes positive allowing the muscle to grow .
Muscle atrophy can result from a decrease in the recruitment of a muscle, a decrease in the loading of a muscle, ageing and metabolic diseases. Whilst all of these processes have the same global effect, a decrease in muscle mass, the underlying molecular mechanisms are different. What appears to be consistent across all forms of atrophy is that there is an overall shift in the balance between synthesis and degradation towards degradation leading to the loss of muscle mass. Under unloading conditions, the decrease in protein synthesis appears to acutely drive the decrease in muscle mass, whilst the rate of protein degradation remains fairly constant .
Control of protein synthesis by atrophic stimuli
As with muscle hypertrophy, the early changes in the rate of muscle protein synthesis correlate with alterations in the phosphorylation of translation factors. As opposed to resistance loading, unloading of muscle leads to a decrease in the phosphorylation of PKB and S6K1, an increase in the association of 4E-BP with eIF4E , and a likely decrease in the rate of translation initiation. However, initiation may not be the only site of translational regulation. Indeed, the wasted mouse shows a progressive muscle wasting as the result of a mutation in an elongation factor known as eEF1α2 . Further, muscle loss due to unloading has been associated with a decrease in translation elongation  and an increase in the phosphorylation of the elongation repressor eEF2 . These data suggest that multiple levels of translation are repressed under atrophic conditions resulting in a reduction of protein synthesis.
Control of protein breakdown by atrophic stimuli
Whilst protein synthesis is inhibited by atrophic stimuli, maintaining the rate of protein degradation is equally important in the development of muscle atrophy. Support for this hypothesis comes from transgenic mouse studies showing that inhibiting protein degradation can decrease muscle wasting in response to atrophic stimuli . A number of proteolytic systems are involved in muscle wasting and blocking one or more of these systems can attenuate muscle loss. The primary proteolytic systems: the calcium- dependent calpain system, the lysosomal proteases (i.e. cathepsins) and the ATP-dependent ubiquitin–proteosome system will be described below.
Calpains are a family of calcium-dependent cysteine proteases that localize to the Z-line of the sarcomere. These proteins are activated in response to elevated levels of calcium and phosphatidylinositides and are inactivated by the calpain-specific inhibitor calpistatin. The calpains can target structural proteins within the myofibrillar lattice such as titin, C-protein and nebulin  and may aid in the disassembly of the myofilaments and degradation by other proteases. Blocking calpain activity with either a dominant negative form of m-calpain or calpistatin overexpression reduced the rate of protein degradation, in muscle cells in vitro stimulated to atrophy by nutrient withdrawal, 30% and 60% respectively. Interestingly, blocking calpain activation in vivo blocks unloading induced muscle atrophy, but has no effect on inactivity-induced muscle loss . This suggests that calpains might play a specific role in controlling protein degradation in response to decreased loading.
Lysosomal proteases are primarily involved in the breakdown of proteins in the sarcolemma such as growth factor receptors, channels and transporters. Since growth factors like insulin and insulin-like growth factor 1 play an important role in the maintenance of muscle cell size, increasing the degradation of these receptors could lead to a progressive loss of muscle mass. Blocking lysosomal protease activity decreases the amount of muscle loss following denervation, but has no effect on atrophy due to unloading . Since denervation normally leads to greater atrophy than unloading, and inhibiting lysosomal proteases eliminates this difference, this suggests that denervation induces a lysosomal-dependent loss of growth factor signalling that results in greater muscle atrophy.
Another way that muscle protein can be degraded is through the caspase-3/ATP-dependent ubiquitination pathway (Figure 5). This system uses caspase-3 to breakdown intact sarcomeres and liberate fragments of actin and myosin that can later be degraded in the proteosome . In order for proteins to be degraded in the proteosome they have to be targeted by the addition of a polyubiquitin chain. Ubiquitin is added to proteins that are targeted for degradation in the proteosome by a process involving at least three classes of proteins called E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme) and E3 (ubiquitin ligase). In skeletal muscle, two novel E3 ubiquitin ligases have been clearly linked to muscle atrophy. The MAFbx (muscle atrophy F-box)/atrogin 1 and the MuRF1 (muscle ring finger protein -1) proteins are E3 ubiquitin ligases that are selectively expressed in skeletal, cardiac and smooth muscle. MAFbx is characteristic of F-box proteins that are components of Skp/Cullin/F-box ubiquitin–ligase complexes , and both MuRF1 and MAFbx exhibit E3 ligase activity in vitro when incubated with appropriate E1 and E2 ligases. Both MuRF1 and MAFbx are up-regulated in almost all atrophy models tested to date and this up-regulation appears to be the result of an increase in the amount of FOXO (forkhead transcription factor) in the nucleus [35,36]. One current theory is that a decrease in activity/loading in muscle decreases growth factor signalling, resulting in decreased phosphorylation of FOXO by its upstream kinase PKB. In the hypophosphorylated form, FOXO translocates to the nucleus and increases atrogene expression. Knocking out either MAFbx/atrogin-1 or MuRF1 spares muscle loss following denervation by 56% and 36% respectively (Figure 6). To date there is little information regarding the physiological function or target substrates of these genes; however, it is assumed that they function in some manner to regulate protein degradation since they are expressed early in the atrophy process and their peak expression occurs during the peak of protein degradation and muscle loss. MuRF1 is a member of a gene family that consists of MuRF1, MuRF2 and MuRF3, all of which are expressed in skeletal and cardiac muscle. Of the three family members, MuRF1 is the only one that is significantly up-regulated under atrophy inducing conditions. Using yeast two-hybrid methods, MuRF1 and MuRF2 have been shown to associate with each other, and both have been detected in the nucleus, indicating that they may play a role in the regulation of transcription via ubiquitination of specific transcription factors. MuRF proteins have been shown to interact with the kinase domain of titin, the 3 MDa muscle protein that extends from the Z-line to the M-line and provides passive tension within a muscle. When MuRF binding to titin is disrupted, either through expression of MuRF mutants or through inactivity, the different MuRF family members can move to the nucleus where they interact with the SRF (serum response factor) and GMEB-1 (glucocorticoid modulatory element binding protein 1) [37,38]. MuRF-1 appears to specifically bind and localize to the same nuclei as GMEB-1 whereas when MuRF-2 moves to the nucleus it specifically binds to SRF and induces the cytosolic translocation of SRF [37,38]. Through GMEB-1, nuclear MuRF-1 could increase the expression of degradation genes, whereas in excluding SRF from the nucleus, MuRF-2 would decrease the synthesis of muscle genes thus again increasing degradation and inhibiting synthesis. The transcriptional regulation of MuRF1 and MAFbx is intriguing since both genes are co-regulated under most atrophy-inducing conditions. Interestingly, the promoters of both MAFbx and MuRF1 contain FOXO binding sites, and the expression of MAFbx/atrogin 1 has been shown to be controlled by PKB–FOXO signalling, connecting growth factors to the regulation of protein degradation.
Muscle wasting is a major feature of ageing, and pathologies such as cancer, diabetes and neuromuscular diseases. Preventing the loss of muscle mass has been shown to increase lifespan . Further, having greater muscle mass decreases frailty and increases independence in the elderly, therefore maintaining muscle mass is essential to a long happy life. The primary determinant of muscle size is the interaction of molecular signalling pathways that affect the rates of protein synthesis and degradation. Increasing synthesis more than degradation leads to greater muscle mass whilst the opposite leads to decreased muscle mass. We are beginning to understand how a change in loading/recruitment is sensed by the muscle and converted into a chemical signal that increases or decreases muscle size. However, there are still a number of questions that remain unanswered. First, what is the sensor of the strain on a muscle? Titin seems to be involved in transducing a signal that controls degradation, but is this same signal important in activating synthesis? What other signals are important: integrin signals such as muselin, autophagy signals through Vps34, inactivation of phosphoinositide phosphatases such as SHIP (SH2-containing inositol phosphatase) and PTEN (phosphatase and tensin homologue) or could it be a new as yet undiscovered mechanism? Secondly, the activation of PKB following resistance exercise seems to be quite central to increasing muscle mass and yet its activation is quite transient, returning to baseline levels within 30 min whilst S6K1 activity remains high for 3–6 h. If PKB is required for the increase in S6K1 activity why are the kinetics of inactivation so different? Thirdly, what role does protein degradation play in the development of muscle hypertrophy? Is the increase in protein degradation after resistance exercise required for muscle hypertrophy? If so, why? Why doesn't decreasing degradation increase muscle cell size in either transgenic animals or tissue culture? What are the physiological targets and functions of the E3 ligases, MuRF1 and MAFbx? With the growing rise in muscle wasting conditions and the resulting metabolic syndrome, we must be able to answer these and other questions concerning the control of muscle size so that the increase in lifespan is associated with an increased quality of life.
• Muscle mass is determined by the sum of protein synthesis and protein degradation.
• Following resistance exercise there is an mTOR-dependent increase in translation initiation and ribosome biogenesis.
• The increase in protein degradation following resistance exercise correlates with the increase in the rate of protein synthesis suggesting that the two processes may be linked.
• Unloading results in attenuation of translation initiation and elongation leading to a decrease in protein synthesis.
• Calpains, lysosomal proteases and the ATP-dependent ubiquitin–proteosome system all play a role in muscle protein breakdown.
• MuRF1 and MAFbx contain FOXO binding sites and are co-regulated in response to atrophy-inducing conditions.
- © 2006 The Biochemical Society, London