An increase in muscle mass requires a positive state of muscle net protein balance. Such a state can be achieved by changes, either separately or together, in muscle protein synthesis and/or breakdown rates. This mechanism of skeletal muscle growth, in our view, is firmly established given that there is no other physiological mechanism currently known through which muscle can gain or lose size independent of protein turnover. On the other hand, the role of skeletal muscle satellite cells in skeletal muscle hypertrophy remains unsettled as highlighted in a previous debate (O’Connor et al. 2007). A recent article in The Journal of Physiology by Wang and McPherron (2012) used well-designed experiments to contribute towards this current conversation that attempts to define the specific role of satellite cells play in skeletal muscle hypertrophy. Specifically, Wang & McPherron investigated the necessity of satellite cells in myostatin-mediated skeletal muscle hypertrophy. These authors induced muscle hypertrophy by blocking myostatin activity through the use of an established myostatin inhibitor while tracking satellite cell proliferation via bromodeoxyuridine (BrdU) incorporation into isolated muscle fibres. The authors modulated dose and timing of the myostatin inhibitor in an attempt to clarify the temporal relationship between satellite cell activation and muscle hypertrophy in response to myostatin inactivation. The impetus for the study was the discrepancy in the findings from previous studies using the myostatin knockout mouse that reported an increase, or no change, in the number of satellite cells. For example, recent studies report that myostatin null mice show satellite cell number is slightly decreased with no difference in satellite cell proliferation (Amthor et al. 2009), and are also resistant to sarcopenia (Siriett et al. 2006).
In the current study, Wang and McPherron (2012) reported that injection of low-dose myostatin inhibitor (5 and 10 mg (kg body weight)−1) resulted in 11% myofibre and whole muscle hypertrophy and was essentially absent of any BrdU incorporation (indicating there was no satellite cell proliferation). Although simultaneous staining was not clear, separate Pax7 (satellite cell marker) and BrdU+ staining is indicative of myonuclei that originated as a satellite cell that had proliferated and then fused into the myofibre. Interestingly, the degree of myonuclear addition was quite modest with BrdU-labelled nuclei, accounting for ∼0.4–3.4% of total nuclei when myofibre hypertrophy was 25–30%. These results suggest that if satellite cells are necessary for hypertrophy a small number of satellite cells induce a tremendous regulatory influence on the whole myofibre.
Until recently, as reviewed in the article of the Wang and McPherron work, the evidence supporting an obligatory role for satellite cells in muscle hypertrophy is based on indirect and controversial methodologies (O’Connor et al. 2007). For example, an earlier approach used gamma-irradiation to block satellite cell activity that clearly lacked cellular specificity. Moreover, in some of the gamma-irradiation studies young mice were used and the possibility exists of confounding results from incomplete post-natal development (i.e. muscle growth is, expectantly, dependent on satellite cell activation in a mature adult animal). Notable is the recent work of McCarthy et al. (2011) that demonstrated, through a genetic strategy, that satellite cell ablation can be inducible in skeletal muscle in adult mice that are past development (4 months old). Such an approach avoids the influence of satellite cells on maturation and instead specifically focuses on adult skeletal muscle hypertrophy. Also, this approach allowed the research group to assess the role of satellite cells in muscle hypertrophy, the same question Wang and McPherron addressed in their work. Rather than taking advantage of the myostatin pathway, McCarthy et al. (2011), chose a robust hypertrophic model in surgical synergist ablation to stimulate muscle growth after inducing satellite cell ablation. The workers demonstrated that muscle hypertrophy was not affected after eliminating the satellite cell response (McCarthy et al. 2011). Thus, these data also contribute to the body of literature suggesting that muscle hypertrophy induced through surgical mechanical overload, or myostatin inhibition, is not reliant on satellite cell activation.
Wang and McPherron (2012) highlight an important concept of ‘myonuclear domain’ that was introduced over 25 years ago (Cheek, 1985). Satellite cell researchers have argued both for, and against, the idea that the cytoplasmic volume/DNA ratio is a tightly regulated process and the driving force for a satellite cell to fuse into the myofibre. However, even now this process is understudied and not well understood. Since myonuclei are believed to be post-mitotic in nature, it is assumed the increase in myonuclei number is necessary to support the stability of myonuclear domain. To date, scientists remain relatively uncertain with regards to the exact nature of how the myofibre, existing myonuclei, and the ‘quiescent’ satellite cells communicate and ultimately signal incorporation and muscle growth.
This leads to further questions; although it appears satellite cells are not necessary for muscle hypertrophy, it would be narrow minded to suggest that satellite cells do not contribute towards overall skeletal muscle mass, or in other words a mistaken identity (i.e. no role for satellite cells in skeletal muscle hypertrophy). Certainly, independent of the satellite cell's role in development and regeneration, once muscle mass has increased the role of satellite cells may be necessary to sustain the mass and perhaps also be integral in maintaining muscle function/quality. This idea is merely speculation; however, it is especially intriguing with respect to sarcopenia, as highlighted by the Wang and McPherron study, demonstrating that myostatin null mice are not resistant to age-associated muscle loss or satellite cell loss. Also, the relationship between the acute muscle protein synthetic response (often used as a quantitative predictor of longer-term gains in muscle size) and the acute satellite cell response after anabolic stimuli remains relatively undefined. Are these two muscle ‘hypertrophic’ responses in complete discordance acutely (hours) and overtime (weeks) begin to share an intimately connected relationship that eventually allows for some ‘serious’ muscle size gains? Clearly, more work is necessary in this area to reach a more definitive answer to these questions. What is notable is that myostatin and synergist ablation represent powerful models to study muscle hypertrophy; however, other modalities that influence muscle mass (exercise, ageing, feeding, atrophy, etc.) may be dependent on satellite cells that have yet to be elucidated.
The findings from the current study, as well as others, are relevant to groups interested in athletic performance and physical therapy – a greater understanding of the intricacies of muscle size regulation will help in the development of more efficacious and proficient training programs and therapies. We congratulate Wang and McPherron for their well-designed study highlighting myostatin and its role in muscle hypertrophy and satellite cell biology. This research (and others) sparks discussion towards the idea that satellite cell biology may influence specific muscle growth pathways to varying degrees dependent on modality and muscle environment.