The age-related progressive loss of skeletal muscle mass and function, termed sarcope-nia, is associated with physical disabilities, the loss of independence, and an increased risk of developing chronic metabolic disease. Skeletal muscle satellite cells play a key role in the maintenance, regeneration and growth of muscle tissue. Therefore, age-related changes in satellite cell content and/or function have been suggested to play an important role in the etiology of sarcopenia. In this thesis, we examined the potential regulatory role of satellite cell pool size and function in relation to both acute and more prolonged muscle atrophy and hypertrophy signals in humans.
In Chapter 2 of this thesis we determined muscle fiber characteristics in skeletal muscle tissue in a large group of people with ages ranging over the entire life span. We ob-served a decline in type II muscle fiber size with an increasing age. The type II muscle fiber atrophy with aging is accompanied by a muscle fiber type-specific decline in satel-lite cell content. However, the age-related reduction in type II muscle fiber size and satellite cell content can be completely reversed following prolonged resistance type exercise training in healthy elderly men. Obviously, the reduced level of physical activity may play an important role in the development of type II muscle fiber atrophy and asso-ciated changes in satellite cell content with aging. In Chapter 3 we studied the impact of reducing the level of physical activity on muscle fiber size and satellite cell content. Young adults were subjected to 2 weeks of one-legged knee immobilization by means of a full leg cast. Two weeks of single legged knee immobilization resulted in a considerable decline in both type I and type II muscle fiber size but without any changes in satellite and/or myonuclear content. Subsequently, the work in this chapter suggests that a de-cline in satellite cell content is not a mechanistic prerequisite for disuse induced muscle atrophy.
Apart from satellite cell content, impairments in the satellite cell activation response to anabolic stimuli may also contribute to the development of sarcopenia. Chapter 4 and 5 introduce novel immunohistological techniques that were applied to investigate changes in satellite cell activation status in human skeletal muscle biopsy samples. Skeletal mus-cle satellite cell activation status is shown to change within 9 hours of recovery after exercise. In addition, we report that changes in satellite cell content and/or activation status occur in a muscle fiber type-specific manner during the first 24 h after a single bout of exercise. These two studies emphasize the importance of analyzing changes in satellite cell activation, proliferation and/or differentiation in a muscle fiber type-specific manner in response to various anabolic stimuli in both health and disease. Sub-sequently, in Chapter 6 we determined the time-dependent changes in type I and type II muscle fiber satellite cell content and activation status in both healthy young and elderly men in response to a single bout of resistance type exercise. In this study we demon-strate that during post-exercise recovery the increase in satellite cell content is delayed with aging and is accompanied by a blunted satellite cell activation response. This atten-uated response may be instrumental in the reduced capacity of senescent muscle to respond to prolonged resistance type exercise training.
In a further attempt to unravel the different factors involved in the satellite cell re-sponse to an exercise stimulus, we also looked at the potential role of nutrition. It has been well-documented that during post-exercise recovery, dietary protein intake is essential to support the increase in myofibrillar muscle protein synthesis, thereby allow-ing net muscle protein accretion. On the other hand, for more long-term muscle adapta-tion, satellite cells are required to provide additional myonuclei to allow muscle fiber hypertrophy. In Chapter 7, we investigated whether dietary protein intake is also pre-requisite to allow a proper satellite response during recovery from a single bout of re-sistance type exercise. Here we show that an acute reduction in protein intake does not affect the increase in satellite cell content but does change the timeline of myostatin expression during 72 h of post-exercise recovery. We speculate that the altered myo-statin response may represent a compensatory response to allow muscle reconditioning to occur when dietary protein becomes available.
Moving forward in the line of a combined exercise and nutritional approach, in Chapter 8 we assessed whether dietary protein ingestion prior to sleep would have a surplus benefit on the increase in skeletal muscle mass and strength following 12 weeks of re-sistance type exercise training. In a previous study from our lab we have shown that protein ingestion prior to sleep increases muscle protein synthesis rates during post-exercise overnight recovery. However, whether these acute changes in muscle protein synthesis also translate to skeletal muscle adaptation to more prolonged resistance type exercise training remained to be established. In agreement with earlier work, progres-sive resistance type exercise training resulted in an increase in skeletal muscle mass and strength in both the placebo as well as the protein supplemented group of healthy young men. However, the increase in skeletal muscle mass and strength was significantly greater in the protein supplemented group. We concluded that protein ingestion prior to sleep represents an effective dietary strategy to augment skeletal muscle mass and strength gains during prolonged resistance type exercise training in healthy young males.
On the muscle fiber level, the gains in muscle mass and strength in response to the 12 weeks training program were accompanied by both type I and type II muscle fiber hy-pertrophy. It is generally believed that muscle fiber hypertrophy is initially supported by an increase in myonuclear domain size. However, the existing myonuclei can only sup-port the underlying increase in transcriptional activity to a certain extent. Subsequently, the incorporation of new, satellite cell derived myonuclei may be required to allow more extensive long-term muscle fiber growth, as was also shown in Chapter 2. In Chap-ter 9, we specifically examined whether an initial (temporary) increase in myonuclear domain size represents a crucial driving force for subsequent myonuclear accretion in response to prolonged resistance type exercise training. Therefore, the change in mus-cle fiber size, myonuclear domain size, myonuclear and satellite cell content were as-sessed at different time-points throughout 12 weeks of resistance type exercise train-ing. We show that muscle fiber hypertrophy is accompanied by a time-dependent in-crease in myonuclear and satellite cell content in response to 12 weeks of resistance type exercise training in young men. However, the exercise training induced muscle fiber hypertrophy is not accompanied by any temporary or permanent increase in myo-nuclear domain size. As such, changes in myonuclear domain size do not seem to be required to elicit myonuclear accretion and support subsequent muscle hypertrophy in healthy young males.
The final chapter addresses the implications of the findings presented in this thesis, and identifies a number of key topics that need to be addressed in future research. This thesis shows that skeletal muscle satellite cells represent an important factor in exercise induced muscle fiber hypertrophy. As such, an impairment in satellite cell function dur-ing post-exercise recovery, as observed in healthy elderly men, may be a crucial factor in the development of sarcopenia, and forms a primary target for intervention strategies aimed to counteract sarcopenia and improve muscle mass and function in the elderly.