Biomechanical adaptation to hindlimb suspension: it involves not only transcriptional mechanisms but also post‐translational modification of the molecular motor, myosin

Skeletal muscle compromises the largest tissue system in mammals including humans and consists of hundreds of individual muscles, which are responsible for postural control, locomotion, facial expression and speech in humans. Their large functional and metabolic heterogeneity renders them well adapted to meet these widely diverging motor tasks (Canepari et al. 2010, Schiaffino & Reggiani 2011). Muscles also play an important role in overall metabolism, and exercise is beneficial in the prevention of chronic diseases by virtue of myokines released from contracting muscles (Fiuza-Luces et al. 2013). Muscles exhibit a tremendous plasticity in response to a wide variety of environmental conditions (Pette & Staron 1990, Schiaffino & Reggiani 2011, Baldwin et al. 2013). In particular mechanical demand – increased or decreased – has a major impact on muscle structure, protein composition and biomechanical output. Its adaptation to increased mechanical demands has long been the topic of sports physiology. Space flight, on the other hand, spurred investigations into muscle0s adaptation to disuse (Fitts et al. 2001, di Prampero & Narici 2003). Terrestrial models of microgravity are hindlimb suspension in rodents (Thomason & Booth 1990) and bed rest in humans (di Prampero & Narici 2003). The latter is of obvious relevance to the bed ridden patient. Disuse affects all functional compartments of muscle fibres including excitation–contraction coupling, signalling to the nucleus, energy supply systems and the contractile machinery (Baldwin et al. 2013) and leads very rapidly to loss of muscle mass, that is atrophy, and deterioration of biomechanical function. Such changes also accompany old age (Canepari et al. 2010) and critical illness (Puthucheary et al. 2010). Therefore, understanding muscle physiology and its adaptation to various conditions is not only of interest to sport and space physiology, but also highly relevant for clinical medicine. The group of Bottinelli (Maffei et al. 2014) investigated the mechanical adaptations of a slow and a fast muscle in response to hindlimb suspension of mice with highly sophisticated technologies. In particular they show that biomechanical adaption of the fast gastrocnemius can occur independent of a shift in isoforms of myosin, that is transcriptional mechanisms (Baldwin et al. 2013) and involves phosphorylation of the regulatory light chains of myosin, that is covalent modification. Muscle fibres differ in their molecular composition, biomechanical performance and metabolic profile but those of a given motor unit are of the same fibre type. A motor unit compromises a motor neurone and all muscle fibres innervated by that motor neurone. The seminal investigations by Eccles and co-workers (Buller et al. 1960) demonstrated the prime importance of the pattern of neuronal activity of the motor unit for the fibre phenotype. Based on the contractile and metabolic profile, muscle fibres have been classified into oxidative, slow-twitch type 1 fibres, and in fast twitch type 2 fibres which are subclassified into oxidative-glycolytic, fast type 2A, and in glycolytic, fast 2B fibres (Pette & Staron 1990, Schiaffino 2010, Schiaffino & Reggiani 2011). Nearly, all muscles have a mixed fibre composition but the ratio of slow-to-fast fibres varies widely depending on the motor task. For example, in antigravity muscles like the soleus, fatigue resistant type 1 fibres predominate while the fast gastrocnemius contains mainly type 1B fibres. The precise fraction of slow-to-fast fibres in a given muscle is, however, species dependent, and in mice, also strain dependent (Stelzer & Widrick 2003). Force of contraction and shortening is due to the cyclic interaction of the molecular motor, myosin II, with actin in the so-called cross-bridge cycle, which is powered by the hydrolysis of ATP. Myosin II is a hexamer consisting of two heavy chains (MyHC), which contain the actin binding surface and the catalytic cleft for ATP hydrolysis, and two pairs of light chains (MyLC), the essential and regulatory light chains. Skeletal muscle expresses four isoforms of MyHC which differ in their ATPase activity and thus convey the specific functional and biochemical features of the fibre types (Reggiani & Bottinelli 2008). Slow, type I fibres express MyHC-1, fast fibres MyHC-2A (type2A fibre), MyHC-2X (type 2X fibre), and MyHC-2B (type 2B fibres), the latter was not found in humans. Fibres can also express more than one MyHC isoform, such fibres are called hybrid fibres giving rise to a huge functional diversity. From the pioneering work of B ar any (1967), we know that myosin ATPase activity positively correlates with the maximal speed of shortening, that is the mechanical output. The maximum shortening velocity of type 1 fibres is about 7 to 10-fold slower than that of MyHC-2B fibres, the fastest fibres (Reggiani & Bottinelli 2008). Thus, it is not