Date Published: July , 2010
Publisher: A.I. Gordeyev
Author(s): B.S. Shenkman, O.V. Turtikova, T.L. Nemirovskaya, A.I. Grigoriev.
Abstract Adult skeletal muscle fiber is a
symplast multinuclear structure developed in ontogenesis by the fusion of the
myoblasts (muscle progenitor cells). The nuclei of a muscle fiber (myonuclei)
are those located at the periphery of fiber in the space between myofibrils and
sarcolemma. In theory, a mass change in skeletal muscle during exercise or
unloading may be associated with the altered myonuclear number, ratio of the
transcription, and translation and proteolysis rates. Here we review the
literature data related to the phenomenology and hypothetical mechanisms of the
myonuclear number alterations during enhanced or reduced muscle contractile
activity. In many cases (during severe muscle and systemic diseases and
gravitational unloading), muscle atrophy is accompanied by a reduction in the
amount of myonuclei. Such reduction is usually explained by the development of
myonuclear apoptosis. A myonuclear number increase may be provided only by the
satellite cell nuclei incorporation via cell fusion with the adjacent myofiber.
It is believed that it is these cells which supply fiber with additional
nuclei, providing postnatal growth, work hypertrophy, and repair processes.
Here we discuss the possible mechanisms controlling satellite cell
proliferation during exercise, functional unloading, and passive stretch.
Skeletal muscle is the most flexible structure in mammalian organisms. High muscle
activity and load often lead to an increase in the transverse size (thickness) of the muscle,
myofibrils volume, and contractile properties (strength and power). The stable pattern of gene
expression underlies such transformation.
In a number of cases (during severe muscle and systemic diseases and under gravitational
unloading), muscle atrophy is accompanied by a decrease in the myonuclei number per myofiber,
along with a corresponding development of apoptotic processes in the myonuclei. Such a
reduction in the nuclei number was observed in cosmonauts’ quadriceps  and rat soleus after space flight [10, 12], under simulated unloading in
rats using the so–called hindlimb suspension technique [18, 19], and during soleus
immobilization. Myonuclei loss is most intensive in slow fibers . Studies of single fibers have demonstrated a decrease in the myonuclear
domain size under disuse in rat soleus, but not in plantaris . The myonuclear domain of rhesus monkeys also tends to decrease after 14
days in space flight . Wang et al
. showed a reduction of the cross sectional area, myonuclei number (25%), and nuclear domain
size of soleus fibers after 16 days of rat hindlimb suspension .
Satellite cells in a skeletal muscle are small mononucleate resting cells (remaining in
the Go phase of the cell cycle) which proliferate and fuse with muscle fibers when
activated, being an essential source of myonuclei during postembryonic development under tissue
hypertrophy and recovery . They can also fuse with
each other, forming new muscle fibers . Satellites
may be myoblasts resting in the muscle tissue. According to another opinion, satellites are
also believed to derive from some endothelial precursors associated with the embryonic vascular
system. They can rest in the skeletal muscle interstitial space and express CD 34 . However, skeletal muscle myogenic precursors are more
numerous than satellite cells, because of the migration or recruitment of the undifferentiated
stem cells from other sources. The precursor cell population from skeletal muscle was shown to
originate from virgin mesenchymal stem cells of bone marrow and differs from satellite cells.
Unlike satellites, precursor cells of the side population express Sca–1 (stem cell
antigen–1) and CD–45. Evidently, they take part in injured or transplanted muscle
regeneration and potentially form myocytes and myosatellite cells [36, 37]. Myosatellites can be
identified in a muscle by their location (between the sarcolemma and fiber basal lamina) and by
the immunohistochemical identification of different proteins expressed by these cells at
different stages of their cell cycles. Desmin, myf5, and MyoD were found in the activated
proliferating satellite cells, which normally express the regulatory muscle factors, such as
Pax–7 and c–Met. Myogenin and MRF4 synthesis is characteristic of
the final stage of differentiation [38;].
c–Met, the HGF receptor, is expressed in skeletal
muscle not only by satellites, but also by other myogenic precursor cells. Like resting cells,
active and proliferating satellite cells usually synthesize such cell adhesion molecules as
m–cadherin (Mcad) and NCAM (CD 56, Leu–19, neural cell adhesion molecule), which
are located in the narrow space between the satellite cell and muscle fiber. NCAM is expressed
in the activated satellite cells (myoblasts), in myotubes during muscle regeneration, and in
neuromuscular junctions. Recent data showed that NCAM is the earliest marker of committed
myoblasts; i.e., it determins their univocal transition from the proliferation to the
differentiation phase .
Three days of hindlimb unloading lead to an irreversible transformation in the muscles of
young rats. Therefore, the satellite cell number and their proliferative potential (according
to the BrdU incorporation data) declined in soleus in the same way as in
extensor digitorum longus. In this case, the program of muscle fiber development in growing
animals can change irreversibly, leading to a failure of the myonuclei number increase even
after reloading [42, 43]. Satellite cell mitotic activity decreased after 24 h of disuse and
completely deceased 3–5 days later. The most pronounced decline was observed in soleus.
Morphological atrophy features were revealed 48 h later . The increase in the proliferative processes in mice gastrocnemius
appeared after one week of hindlimb suspension . The
quantity of the resting and mitotically active satellites in muscle fiber fell by 57% when
compared to the control group . In the other work by
the same authors, 3 months of unloading caused no decrease in the satellite cell quantity or
muscle fiber length in young animals, but it led to an apoptosis–independent decrease in
the satellite cell and myonuclei contents and a decrease in the satellite mitotic activity
. However, Ferreira et al .
 observed unexpected proliferation reinforcement in
mice gastrocnemius after one week of suspension.
Myosatellites are supposed to rest when skeletal muscle is not active. Their activation
provides muscle mass maintenance, hypertrophy development, or the recovery of injured muscle.
Myosatellite activation can also be caused by strengthening exercise [46, 47].
Satellite cells possess high proliferative potential and are supposed to be important for
skeletal muscle regeneration and hypertrophy. Different stimuli, such as functional overload
during synergist removal, testosterone, clenbuterol, muscle stretch, and exercise can activate
the satellites, stimulate their entry to the cell cycle and their proliferation in both fast
and slow muscles. An increase in satellite proliferation was observed in the first days after
stimulus application. Cramery et al .  and Kadi et al . [13, 31] showed that a series of
intensive exercises stimulated an increase in the number of cells expressing NCAM. However,
this and other studies did not show proliferating cells fusing with muscle fibers. The question
of whether myosatellite nuclei incorporation into the fiber is necessary for muscle growth or
mass maintenance remains unanswered. Different points of view exist. Many authors deny the
necessity of incorporating myosatellite nuclei for muscle hypertrophy development , which has been proven by numerous studies with β
2–adrenoceptor agonists application, leading to muscle hypertropy without an increase in
DNA or the myonuclear number. According to Kadi et al . , muscle fiber size can change moderately without the
incorporation of new myonuclei. As was shown earlier, the myonuclear number is not normally the
determining factor for muscle fiber size; the myonuclear domain size varies during an
animal’s lifetime  and is unstable under
muscle atrophy . Despite the lack of dividing
myosatellites after ionizing radiation treatment, Lowe  observed hypertrophy of the stretched slow anterior latissimus dorsi of
the Japanese quail. Dupont–Versteegden et al .  showed that in spinalized animals, after myosatellite activation (during
resistive exercise), the latter did not fuse with the muscle fibers. Thus, training did not
promote the maintenance of the myonuclear number in the soleus of spinal animals. The number of
activated myosatellites was higher than that of divided ones. The physiological role of
activation of such a huge number of myosatellites without their incorporation into the growing
muscle fibers is unclear. Recently, Italian researchers showed that proteinkinase B activation
for 3 weeks caused muscle hypertrophy and a doubling of muscle weight, which was not
accompanied by satellite activation or the incorporation of new nuclei .
Gravitational unloading is a particular type of muscle contractile activity reduction. A
sharp decrease in the electric activity of soleus (to the zero level) is observed, as a rule,
immediately after support elimination and continues for 2–3 days of disuse. Then electric
activity begins its restoration slowly and reaches the control level by the 14th day of real or
simulated microgravity . However, gradually
increased muscle activity does not prevent muscle atrophy development. Evidently, the decreased
contractile activity has an affect alongside with the the significantly declined (to zero under
microgravity) resistance to muscle contraction ( weight bearing), which has a significant
influence on atrophy development . One approach to
studying this factor is chronic or repeated passive stretch of the muscle. This stretch
compensates for the lack of gravity loading and certainly prevents the development of muscle