Adapted physical exercise enhances activation and differentiation potential of satellite cells in the skeletal muscle of old mice

Abstract

During ageing, a progressive loss of skeletal muscle mass and a decrease in muscle strength and endurance take place, in the condition termed sarcopenia. The mechanisms of sarcopenia are complex and still unclear; however, it is known that muscle atrophy is associated with a decline in the number and/or efficiency of satellite cells, the main contributors to muscle regeneration. Physical exercise proved beneficial in sarcopenia; however, knowledge of the effect of adapted physical exercise on the myogenic properties of satellite cells in aged muscles is limited. In this study the amount and activation state of satellite cells as well as their proliferation and differentiation potential were assessed in situ by morphology, morphometry and immunocytochemistry at light and transmission electron microscopy on 28-month-old mice submitted to adapted aerobic physical exercise on a treadmill. Sedentary age-matched mice served as controls, and sedentary adult mice were used as a reference for an unperturbed control at an age when the capability of muscle regeneration is still high. The effect of physical exercise in aged muscles was further analysed by comparing the myogenic potential of satellite cells isolated from old running and old sedentary mice using an in vitro system that allows observation of the differentiation process under controlled experimental conditions. The results of this ex vivo and in vitro study demonstrated that adapted physical exercise increases the number and activation of satellite cells as well as their capability to differentiate into structurally and functionally correct myotubes (even though the age-related impairment in myotube formation is not fully reversed): this evidence further supports adapted physical exercise as a powerful, non-pharmacological approach to counteract sarcopenia and the age-related deterioration of satellite cell capabilities even at very advanced age.

Keywords: immunocytochemistry; sarcopenia; satellite cells; skeletal muscle; treadmill; ultrastructure.

Figure 1

SCs immunostained for Pax7. (a‐c) Immunohistochemical detection of Pax7 on quadriceps femoris sections from AS (a), OS (b), and OR (c) mice. Scale bar: 50 μm. (d) Mean percentage ± SD of myofibres with 0 to 6 associated Pax7⁺ SCs in the three experimental groups: it is worth noting that in OS mice, the fraction of myofibres without associated SCs is significantly higher than in AS (P = 0.04) or OR (P = 0.04) animals.

Figure 2

SCs immunostained for MyoD. (a) Representative micrograph showing the dual immunohistochemical detection of MyoD (blue signal) and laminin (brown) on a quadriceps femoris section from an OR mice: the black arrow points to an activated (MyoD⁺) SC, and the red and the green arrows respectively indicate a quiescent (MyoD⁻) SC surrounded by the basal lamina and a MyoD⁺ myonucleus. Scale bar: 50 μm. (b) Mean percentage ± SD of myofibres with 0–6 associated MyoD⁺ SCs in the three experimental groups: it is worth noting that in OS mice, the fraction of myofibres with 0 associated SCs is significantly higher than in AS (P = 0.03) or OR (P = 0.04) animals.

Figure 3

Light microscopy micrographs of cultured SC‐derived myoblasts and myotubes. Primary cultures of myoblasts from AS (a), OS (b), and OR (c) mice 12 h after seeding of freshly isolated SCs: in (a), nearly all myoblasts are flattened and firmly adhering (arrows), whereas adhering cells (arrows) are much less numerous in (b) and (c). Examples of multinucleated myoblast‐derived myotubes from AS (d), OS (e,e'), and OR (f) mice: myotubes from AS mice (d) exhibit regularly aligned nuclei (arrowheads), whereas those from OS mice (e) are often roundish in shape with clustered nuclei (arrowheads) and even at the beginning of myoblasts' fusion (e'), extensive vacuolization is observed in the cytoplasm (thin arrows). The myotubes from OR mice (f) are more similar to those from the adults, although the nuclei (arrowheads) are less regularly arranged. Scale bars: 30 μm (a‐c), 50 μm (d‐f).

Figure 4

Electron micrographs of SC‐derived myoblasts from muscles of AS (a‐c), OS (d,e), and OR (f,g) mice. In (a) the myoblast shows a roundish nucleus (N) with a large reticular nucleolus (Nu); the cytoplasm (b) is rich in RER (arrows), GAs (G), and mitochondria (M); some residual bodies (R) are also present. Moreover, bundles of cytoskeletal filaments show incipient sarcomere‐like arrangements (c). In (d,e), the myoblast shows an irregularly shaped nucleus (N) with a compact nucleolus (Nu), whereas the cytoplasm contains large amounts of heterogeneous residual bodies (R); RER (arrows), GAs (G), and mitochondria (M) are less abundant. In (g,f), the myoblast shows an irregularly shaped nucleus (N) with a compact nucleolus (Nu); RER (arrow), GAs (G), and mitochondria (M) are well developed similar to myoblasts from AS mice; moreover, these filament bundles show sarcomere‐like figures (inset in g). Scale bars: 2 μm (a,d,f), 1 μm (b,c,e,g, inset).

Figure 5

Electron micrographs of myotubes derived from myoblasts of AS mice. The myotubes show elongated shapes, with longitudinally arranged nuclei (a). Mitochondria (M), RER (arrows), and GAs (G) are numerous and well developed (b‐c). (d‐e) Several bundles of longitudinally arranged myofibrils (F) show sarcomere‐like arrangements (arrows), and elongated mitochondria (M) are often lined in the small cytoplasm cords (asterisks) between the bundles. Scale bars: 2 μm (a), 1 μm (b–e).

Figure 6

Electron micrographs of myotubes derived from myoblasts of OS (a) and OR (b) mice. OS myotubes (a) are roundish with irregularly shaped nuclei (N); residual bodies (asterisks) and vacuoles (v) are prominent. Mitochondria (M) and GAs (G) are scarce (a'). Myofibrils are rare and irregularly arranged (a''). OR myotubes (b) exhibit irregular shapes with centrally located nuclei (N); residual bodies (asterisks) and vacuoles (v) are numerous. Mitochondria (M) are abundant, and RER (arrow) and GAs (G) are well developed (b'). Myofibril bundles are thin and often show a sarcomere‐like pattern (arrows) (b''). Scale bars: 2 μm (a,b), 1 μm (a',a'',b',b'').

Figure 7

Immunoelectron microscopy of myoblast nuclei. Representative high magnification details of myoblast nuclei immunolabelled for (a) activated RNA polymerase II, (b) (Sm)snRNPs, and (c) Pax7 (12‐nm gold particles) and MyoD (6‐nm gold particles). The labelling distribution was similar in all animal groups: activated RNA polymerase II, Pax7, and MyoD were specifically associated to perichromatin fibrils (arrows), whereas (Sm)snRNPs occurred on perichromatin fibrils (arrows) and interchromatin granules (IG). Scale bars: 0.5 μm. (d) Labelling density (gold particles per μm²) of the four factors in the nucleoplasm is shown (means ± SD); all signals markedly decrease in OS mice and increase in OR animals. In each histogram, asterisks indicate values that are significantly different from each other (OR vs. OS P < 0.001 for all probes; OS vs. AS P < 0.001 for all probes; OR vs. AS P < 0.001 for MyoD and (Sm)snRNPs, P = 0.243 for activated polymerase II and P = 0.186 for Pax7).