Depletion of resident muscle stem cells negatively impacts running volume, physical function, and muscle fiber hypertrophy in response to lifelong physical activity

Abstract

To date, studies that have aimed to investigate the role of satellite cells during adult skeletal muscle adaptation and hypertrophy have utilized a nontranslational stimulus and/or have been performed over a relatively short time frame. Although it has been shown that satellite cell depletion throughout adulthood does not drive skeletal muscle loss in sedentary mice, it remains unknown how satellite cells participate in skeletal muscle adaptation to long-term physical activity. The current study was designed to determine whether reduced satellite cell content throughout adulthood would influence the transcriptome-wide response to physical activity and diminish the adaptive response of skeletal muscle. We administered vehicle or tamoxifen to adult Pax7-diphtheria toxin A (DTA) mice to deplete satellite cells and assigned them to sedentary or wheel-running conditions for 13 mo. Satellite cell depletion throughout adulthood reduced balance and coordination, overall running volume, and the size of muscle proprioceptors (spindle fibers). Furthermore, satellite cell participation was necessary for optimal muscle fiber hypertrophy but not adaptations in fiber type distribution in response to lifelong physical activity. Transcriptome-wide analysis of the plantaris and soleus revealed that satellite cell function is muscle type specific; satellite cell-dependent myonuclear accretion was apparent in oxidative muscles, whereas initiation of G protein-coupled receptor (GPCR) signaling in the glycolytic plantaris may require satellite cells to induce optimal adaptations to long-term physical activity. These findings suggest that satellite cells play a role in preserving physical function during aging and influence muscle adaptation during sustained periods of physical activity.

Keywords: aging; exercise; satellite cells; skeletal muscle; stem cells.

Fig. 1.

Higher satellite cell density in the plantaris and soleus muscles of running satellite cell-replete (SC+) compared with satellite cell-depleted (SC−) mice. A and B: representative images of satellite cell immunohistochemistry showing laminin (green), nuclei (blue), and Pax7 (red; white arrows). Scale bar, 20 µm. C and D: satellite cell density in the plantaris and soleus. Data represent means ± SE; n = 6–9 mice per group. Data were analyzed via a two-factor ANOVA. Post hoc comparisons were made with Šidák posttests. *P < 0.05, interaction effect between condition and treatment; †P < 0.05, running SC+ vs. sedentary SC+; ‡P < 0.05, running SC+ vs. running SC−.

Fig. 2.

Myonuclear accretion with lifelong physical activity was dependent on the presence of satellite cells. A: representative image of dystrophin (purple) and DAPI (blue) staining from muscle cross sections to identify myonuclei (white arrows). B: representative image of dystrophin (purple) and DAPI (blue) and pericentriolar material 1 (PCM1; green) staining from muscle cross sections to identify myonuclei (white arrows). Scale bar, 5 µm. C and D: myonuclear density of the plantaris and soleus. Data represent means ± SE; n = 6–9 mice per group. Data were analyzed via a two-factor ANOVA. #P < 0.05, running vs. sedentary mice; ¥P < 0.05, satellite cell-replete (SC+) vs. satellite cell-depleted (SC−) mice.

Fig. 3.

Lifelong physical activity led to a lower body weight and higher heart weight independent of satellite cells. A: body weight. B: heart weight normalized to body weight. Data represent means ± SE; n = 7–9 mice per group. Data were analyzed via a two-factor ANOVA. SC+, satellite cell replete; SC−, satellite cell depleted. #P < 0.05, running vs. sedentary mice.

Fig. 4.

Lifelong physical activity led to an oxidative shift in muscle fiber composition in the plantaris. A and B: representative images of the plantaris muscle stained for dystrophin (blue), myosin heavy chain 2a (green), and myosin heavy chain 2b (red). Scale bar, 50 µm. C and D: relative frequency of fiber type in the plantaris and soleus. Data represent means ± SE; n = 6–9 mice per group. Data were analyzed via a two-factor ANOVA. SC+, satellite cell replete; SC−, satellite cell depleted. #P < 0.05, running vs. sedentary mice.

Fig. 5.

Satellite cell depletion limited the hypertrophic response induced by lifelong physical activity. A–D: representative images of muscle fiber size in the soleus stained for dystrophin (red), type 1 myosin heavy chain (pink), and type 2a myosin heavy chain (green). Scale bar, 20 µm. E: whole muscle mean fiber cross-sectional area (CSA) in the plantaris. F: mean fiber CSA by fiber type in the plantaris. G: whole muscle mean fiber CSA in the soleus. H: mean fiber CSA by fiber type in the soleus. Data represent means ± SE; n = 6–9 mice per group. Data were analyzed via a two-factor ANOVA. Post hoc comparisons were made with Šidák posttests. *P < 0.05, interaction effect between condition and treatment; †P < 0.05, running satellite cell-replete (SC+) vs. sedentary SC+ mice; ‡P < 0.05, running SC+ vs. running satellite cell-depleted (SC−) mice; #P < 0.05, running vs. sedentary mice.

Fig. 6.

Lifelong exercise in the presence of satellite cells influenced global transcription and upregulated expression of genes involved in G protein-coupled receptor (GPCR) signaling pathways in the plantaris. A: Venn diagram of genes enriched in the soleus and plantaris of satellite cell-replete (SC+) runners. B and C: top 10 gene sets overrepresented in the plantaris (B) and soleus (C). D: gene set enrichment analysis highlighting the Gα_i2 signaling network in the plantaris. E: quantitative RT-PCR results of Gα_i2 genes verified the results seen in our gene set enrichment analysis. Data represent means ± SE; n = 6–9 mice per group. Data were analyzed via a two-factor ANOVA. Post hoc comparisons were made with Šidák posttests. *P < 0.05, interaction effect between condition and treatment; †P < 0.05, running SC+ vs. sedentary SC+ mice; ‡P < 0.05, running SC+ vs. running satellite cell-depleted (SC−) mice; #P < 0.05, running vs. sedentary mice; ¥P < 0.05, SC+ vs. SC− mice.

Fig. 7.

Satellite cell depletion negatively influenced physical function and reduced voluntary wheel running activity. A: time to fall in the rotarod test. B: time taken to transverse the balance beam. C: average distance run per month over the duration of the study. D: average distance run over the entire duration of the study. Data represent means ± SE; n = 7–9 mice per group. Data were analyzed via a two-factor ANOVA (A and B), a two-factor repeated measures ANOVA (C), or an unpaired Student’s t test (D). *P < 0.05, satellite cell-replete (SC+) vs. satellite cell-depleted (SC−) mice; ¥P < 0.05, SC+ vs. SC− mice.

Fig. 8.

Satellite cell depletion affected muscle spindle characteristics but not extracellular matrix (ECM) accumulation. A and B: representative images of muscle spindle fibers from the plantaris muscle. Scale bar, 5 µm. C–E: quantification of ECM index (C), spindle area (D), and spindle lobe size (E). Data represent means ± SE; n = 4 mice per group. Data were analyzed via a two-factor ANOVA. ¥P < 0.05, satellite cell-replete (SC+) vs. satellite cell-depleted (SC−) mice.

Fig. 9.

Satellite cell depletion did not cause fibrosis in response to lifelong physical activity. A–E: representative wheat germ agglutinin (WGA)-stained muscle cross sections and quantification. Scale bar, 50 µm. F–J: representative Picrosirius red (PSR)-stained muscle cross sections and quantification. Scale bar, 20 µm. Data represent means ± SE; n = 7–8 mice per group. SC+, satellite cell replete; SC−, satellite cell depleted.