5.3.2Muscular Activity (i.e. Exercise) as a Regulator of Muscle Proteostasis

Repeated muscle use, in the form of manual labor, and lack of muscle use, in response to cessation of work or convalescence, have long been known to a ect muscle size; increasing and decreasing it respectively.29 We now appreciate that some form of muscular activity, even weight bearing per se, is essential for the maintenance of muscle mass. Indeed, it has become apparent that aside from nutrients the most potent regulators of proteostasis in postnatal skeletal muscle are Newton’s gravity and active muscle contraction (NB: in modern Western society this is largely only exercise). As examples, significant loss of soleus muscle cross-sectional area (CSA) of about 15–26%, depending on fiber type, is evident following removal of Newton’s force in the form of 17 days in Low Earth Orbit onboard a NASA Space Shuttle. Conversely, a 5% increase in muscle CSA is evident in leg muscles after fewer than ten individual bouts of heavy resistance exercise. These observations detail the importance of ambulatory activity in the maintenance of muscle mass and also demonstrate chronic high-force contractions stimulate muscle growth. Although muscle mass is ultimately regulated by the balance of protein synthesis and degradation, the prevailing view is that changes in protein synthesis are most critical to development of atrophy and hypertrophy since,whenexposedtospaceflightorexercise, the magnitudeofchangesinsynthesis are much larger than those of degradation. Consequently, the consensus is that changes in protein synthesis are facilitative, while changes in breakdown are adaptive.

Resistance exercise, in which each e ort is performed against a specific opposing force, is singly the most potent hypertrophic stimulus in adult human muscle. This is evidenced by the gross musculature achieved in bodybuilders engaged in routinely lifting heavy weights. The mechanism by which resistance training promotes muscle hypertrophy is chiefly through inducing transient increases in muscle protein synthesis after each exercise bout. The amplitude and duration of increases in muscle synthesis after exercise ranges B50–300% and lasts B4–48 h. Though largely unexplored, such a wide range in responsiveness is likely the function of exercise protocol (e.g. intensity/duration), nutritional status (e.g. post-absorptive/fed), subject characteristics (e.g. age, sex), training status and measurement duration. The crucial role for the stimulation of muscle synthesis by exercise in regulating adaptation is demonstrated by data that show that the amplitude of post-exercise increases in muscle synthesis are qualitatively predictive of long-term adaptation (i.e. muscle hypertrophy). After resistance exercise, increases in muscle protein breakdown are also observed and these can exceed the magnitude of increases in synthesis. Consequently, resistance exercise in post-absorptive conditions creates a net catabolic state despite increases in synthesis. Importantly this catabolic state is prevented when AA are ingested in close proximity to exercise. This incapacity to stimulate muscle growth without exogenous AA makes sense because one cannot achieve muscle growth without su cient building materials. This also highlights the important interaction between exogenous AA availability and muscular activity in the regulation of muscle proteostasis.

Interestingly, it seems that muscle protein synthesis is increased only in the ‘‘remodeling period’’ after resistance exercise (as described above) but is actually suppressed during muscular activity. It is likely this suppression is a direct result of cellular energy stress (i.e. ATP turnover) because the degree of suppression relates directly to the contraction duty-cycle. Consequently, the adaptive cellular response is to switch o ATP consuming processes such as mRNA translation (which requires 4 ATP per peptide bond) and synchronously switch on catabolic pathways (ATP creating pathways). This highlights the close links between cellular energy status and muscle proteostasis.

5.3.2.1Extracellular–Intracellular Signaling and Muscular Activity

The molecular mechanism(s) by which contractile activity modulates proteostasis is poorly defined.30 Most of the various intramuscular signaling molecules have been reported to be activated in response to exercise in some form or other. However, most recent work supports the notion that ostensibly anabolic hormones (i.e. testosterone, growth hormone and insulin-like growth factor 1) do not play a significant role, at least in alterations in short-term anabolic signaling and muscle proteostasis after muscular activity. There are several examples of this. Firstly, when the acute responses of muscle protein synthesis to resistance exercise are compared in arm muscles under conditions that either do generate increases in systemic hormones (i.e. previous intense exercise of large muscle groups) or do not (i.e. no prior exercise) the synthetic responses are identical. This is in spite of gross di erences in systemic concentrations of anabolic hormones such as testosterone and growth hormone. Furthermore, muscle hypertrophy after chronic exercise training under these conditions is identical and thus adaptation is independent of the systemic environment. Secondly, when both legs of the same person are trained, one using a resistance training protocol and one using an endurance protocol, both adapt distinctly. That is, the resistance-exercised leg hypertrophies while the endurance trained leg does not, but instead becomes fatigue resistant. Thus the molecular mechanism involved appears to be largely within a given muscle rather than from the systemic milieu (i.e. hormones). It also appears that known local factors (autocrine/paracrine) may not play a significant role in the acute responses in muscle proteostasis to contractile activity. For example, while changes in mTORc1 signaling (associated with a variety of peptide growth factors, such as IGFs; see Section 5.2.2.1) are observed in response to contraction, these changes are extremely fast, detectable within seconds, and are PI3K-independent (i.e. not going through receptor -4PI3K -4Akt -4mTORc1 pathways). This observation almost certainly excludes short-term receptormediated IGF-1/MGF inputs. Perhaps even more strikingly, a functional IGF-1 receptor is not necessary for load-induced skeletal muscle hypertrophy. Further evidence excludes other local factors as having a role because the medium in which stretched muscle is bathed (i.e. conditioned medium) does not induce the same anabolic signaling changes as stretch, for example

mTORc1 changes. Collectively, these data show that while signaling through these anabolic pathways is largely linear (i.e. ligand receptor - kinase-kinase- kinase - kinase-kinase - kinase - e ect) during development or specific disease states, complex non-linear signaling and cross-talk between what were previously thought of as distinct pathways occurs in adults subject to exercise.

Currently it seems that muscles control proteostasis in response to muscular activity largely from within. From the limited data available, it seems likely that physical deformation of the cell (i.e. mechanotransduction) and biochemical perturbations (i.e. chemotransduction) are key upstream regulators of the phosphoproteins regulating muscle proteostasis. Mechanotransduction is probably sensed though physical transmembrane links (i.e. the attachment complex) between the extracellular matrix and the actin cytoskeleton to which many of the anabolic signals may be physically tethered. However, titin, a major structural component of the contractile apparatus, has also been shown to be a stretch-activated kinase. Following stretch activation of the kinase substrates such as nbr1 and p62 are phosphorylated to create binding sites. Notably NFkB signaling can be activated via p62, although the relevance of this in human muscle remains to be established. The E3 ligase MuRF2 can also be activated by p62 and upon activation causes serum response factor (SRF) to translocate to the sarcoplasm from the nucleus. SRF acts as a transcription factor and participates in the expression of muscle genes, thus titin may modulate long-term muscle gene expression via modulation of SRF, which likely accounts for the presence of hereditary myopathy with early respiratory failure in individuals with mutations in titin kinase. Despite the promise of titin as a mechanosensor within muscle, this mechanism does not currently explain the activation of known anabolic signals in response to exercise.

On the basis of chemotransduction, muscular activity has been associated with many metabolic changes. As examples, muscular activity generates alterations in ion movements via opening of stretch-activated channels (SAC) in the plasma membrane and fluctuations in SR-derived [Ca21]i to facilitate contraction. Also increased ATP turnover during contraction produces metabolites, which are also able to modulate cellular signaling processes. Furthermore, as a by-product of increased ATP turnover, reactive oxygen species (ROS) are generated, which function to activate redox-sensitive pathways such as NFkB (see Section 5.2.2.4). Finally, production of lipid second messengers derived from the plasma membrane, which can be damaged during contraction, regulates cell signaling processes.

While the precise control of muscle proteostasis in human muscle subject to exercise remains elusive, there are several lines of evidence from in vitro studies that the anabolic signals observed are mechano-/chemosensitive (see Figure 5.2 for summary scheme). For example, inhibition of extracellular Ca21 influx through SAC inhibits mechanical stretch-dependent signaling to p70S6K1, suggesting that extracellular Ca21 is required for signaling to mTORc1. Likewise, disruption of the actin cytoskeleton using cytochalasin D suppresses stretch-induced signaling to p70S6K1 via altering the dynamics of the actin cytoskeleton. Indeed, a direct link from the cytoskeleton to signaling activity