Satellite cells are muscle-specific progenitor cells involved in the routine maintenance of skeletal muscle homeostasis, growth and regeneration. They are activated by various stimuli (myotrauma, growth factors etc), undergo rounds of proliferation as skeletal muscle myoblasts, to differentiate and fuse with each other to generate new myotubes or onto existing myofibres to augment growth or repair damaged fibres. Satellite cells contribute to hypertrophy by facilitating nuclear addition, which maintains contractile protein synthetic capacity. Conversely, during atrophy the dysregulation of satellite cells (e.g., via myogenic suppression), causes an opposing deficit in nuclear supplementation/contractile protein synthesis. The ‘activity status’ of satellite cells, an important determinant of muscle regenerative capacity is not routinely addressed in studies characterising mechanisms of muscle hypertrophy and atrophy. Therefore, the investigations described within this thesis examined the satellite cell specific signalling events that contribute to muscle loss or gain, in rodent models experiencing non-mechanically-induced muscle hypertrophy or atrophy. Chronic administration of an anabolic agent (BRL-47672, the pro-drug of clenbuterol) increased the expression of early components of satellite cell myogenesis (pax7, ki-67, myoD) but caused no alteration in myogenin expression, relative to control in rat soleus muscle. Pro-drug administration increased myostatin expression, with no concomitant change in follistation mRNA; this is likely a compensatory mechanism to check excessive muscle growth. These results provided evidence of increase satellite cell activity in hypertrophying muscle. In a lipopolysaccharide (LPS)-infusion model of muscle atrophy, satellite cells were inhibited in an inflammatory-dependent manner. LPS infusion caused early (<2hr) elevations inflammatory cytokines TNF-, IL-6 and NF-B. LPS-induced elevation in cytokine transcript levels paralleled increased myostatin and decreased pax7 and myoD mRNA and protein expression. The differential increase in cytokines also paralleled the reduction in the number of pax7+ and myoD+ satellite cells. These results suggest that alterations in satellite cell activity may contribute to the progression of muscle atrophy, due to the suppression of muscle compensatory mechanisms, which include satellite cell activation, differentiation and fusion for nuclear supplementation. Co-infusion with an anti-inflammatory agent, dexamethasone (Dex), blunted LPS-induced increase in inflammatory cytokines but had an additive effect on myogenic suppression. Dex+LPS infusion prevented LPS-induced increase in myogenin and resulted in an additional suppression of pax7 and myoD, greater than that elicited by either substance alone. Negative regulation of satellite cells by glucocorticoids could impede their efficacy in the treatment of inflammatory muscle disorders. The research within this thesis emphasise satellites are important for maintenance of muscle homeostasis and their activation/inhibition, may determine the magnitude of muscle loss or gain. This was demonstrated by the pattern of pax7 and myoD expression in hypertrophying muscle, where both markers were up-regulated and in atrophying muscle, where they were down-regulated. Down-regulation of these markers in atrophy could have implications for muscle regenerative capacity, especially myoD, whose expression was continuously inhibited across all time-points sampled in septic muscles. Satellite cells are a major source of compensatory action in skeletal muscle, their activation and subsequent myogenesis represents an auxiliary mechanism by which muscle responds to damaging stimuli; therefore their dysregulation (through the alteration of key myogenic markers) results in an alteration of normal function. Such dysregulation, as frequently reported in cases or progressive muscle degeneration and sarcopenia, limits the efficacy of muscle compensatory processes (i.e. satellite cell activation/proliferative or differentiation potential), thereby contributing to the progression of muscle atrophy and myopathy.
|Qualification||Doctor of Philosophy|
|Award date||1 May 2013|
|Publication status||Published - May 2013|