Exercise builds the scaffold of life: muscle extracellular matrix biomarker responses to physical activity, inactivity, and aging

Skeletal muscle extracellular matrix (ECM) is critical for muscle force production and the regulation of important physiological processes during growth, regeneration, and remodelling. ECM remodelling is a tightly orchestrated process, sensitive to multi-directional tensile and compressive stresses and damaging stimuli, and its assessment can convey important information on rehabilitation effectiveness, injury, and disease. Despite its profound importance, ECM biomarkers are underused in studies examining the effects of exercise, disuse, or aging on muscle function, growth, and structure. This review examines patterns of short- and long-term changes in the synthesis and concentrations of ECM markers in biofluids and tissues, which may be useful for describing the time course of ECM remodelling following physical activity and disuse. Forces imposed on the ECM during physical activity critically affect cell signalling while disuse causes non-optimal adaptations, including connective tissue proliferation. The goal of this review is to inform researchers, and rehabilitation, medical, and exercise practitioners better about the role of ECM biomarkers in research and clinical environments to accelerate the development of targeted physical activity treatments, improve ECM status assessment, and enhance function in aging, injury, and disease

Eccentric Exercise Facilitates Mesenchymal Stem Cell Appearance in Skeletal Muscle

Eccentric, or lengthening, contractions result in injury and subsequently stimulate the activation and proliferation of satellite stem cells which are important for skeletal muscle regeneration. The discovery of alternative myogenic progenitors in skeletal muscle raises the question as to whether stem cells other than satellite cells accumulate in muscle in response to exercise and contribute to post-exercise repair and/or growth. In this study, stem cell antigen-1 (Sca-1) positive, non-hematopoetic (CD45-) cells were evaluated in wild type (WT) and α7 integrin transgenic (α7Tg) mouse muscle, which is resistant to injury yet liable to strain, 24 hr following a single bout of eccentric exercise. Sca-1+CD45− stem cells were increased 2-fold in WT muscle post-exercise. The α7 integrin regulated the presence of Sca-1+ cells, with expansion occurring in α7Tg muscle and minimal cells present in muscle lacking the α7 integrin. Sca-1+CD45− cells isolated from α7Tg muscle following exercise were characterized as mesenchymal-like stem cells (mMSCs), predominantly pericytes. In vitro multiaxial strain upregulated mMSC stem cells markers in the presence of laminin, but not gelatin, identifying a potential mechanistic basis for the accumulation of these cells in muscle following exercise. Transplantation of DiI-labeled mMSCs into WT muscle increased Pax7+ cells and facilitated formation of eMHC+DiI− fibers. This study provides the first demonstration that mMSCs rapidly appear in skeletal muscle in an α7 integrin dependent manner post-exercise, revealing an early event that may be necessary for effective repair and/or growth following exercise. The results from this study also support a role for the α7 integrin and/or mMSCs in molecular- and cellular-based therapeutic strategies that can effectively combat disuse muscle atrophy

Regulation of stress-activated protein kinases by exercise and contraction in skeletal muscle

Thesis (Sc.D.)--Boston UniversityPLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at [email protected]. Thank you.The c-Jun NH2-terminal kinase (JNK) and p38 intracellular signaling cascades are mitogen-activated protein kinase (MAPK) signaling pathways that are activated in mammalian cells by a variety of stressors, including proinflammatory cytokines, osmotic shock, and shear stress. The purpose of this dissertation research was to examine the effect of injury-producing exercise on JNK and p38 activities in human skeletal muscle and to determine whether mechanical stress is a primary stimulator of JNK and p38 activities with contraction. Twelve healthy subjects (7M/5F) completed maximal concentric or eccentric knee extensions on an isokinetic dynamometer (10 sets, 10 reps). Needle biopsies were obtained from the vastus lateralis muscle 24 h before exercise, immediately post-exercise, and 6 h post-exercise. While both forms of exercise increased JNK activity immediately post-exercise, eccentric contractions resulted in a much higher activation (15-fold vs. 4-fold increase above basal for eccentric and concentric, respectively). By 6 h post-exercise, JNK activity decreased back to baseline values. In a separate study, 14 male subjects completed a 42.2 km marathon. Biopsies were obtained from the vastus lateralis muscle 10 days prior to the marathon, immediately following the race, and 1, 3, and 5 days after the race. JNK activity increased 7-fold over basal immediately postexercise, but decreased back to basal 1, 3, and 5 days after the exercise. The activity of p38y also was increased and decreased in a similar pattern. However, no regulation was observed for p38α. In a third study, the effects of contraction and static stretch on JNK activity and p38 phosphorylation were determined in the rat soleus muscle in vitro. Static stretch dramatically increased JNK activity and p38 phosphorylation, whereas isometric contraction resulted in much smaller increases in JNK activity and p38 phosphorylation. The regulation of focal adhesion proteins also was examined following both exercise and contraction. The work presented in this thesis demonstrates that injury-producing exercise results in the marked activation of the JNK and p38 stress-activated protein kinases and provides evidence that mechanical stress may be a major contributor to increases in JNK and p38 activities observed following contraction in rat and human skeletal muscle.2031-01-0

Defining a role for non-satellite stem cells in the regulation of muscle repair following exercise

Skeletal muscle repair is essential for effective remodeling, tissue maintenance, and initiation of beneficial adaptations post-eccentric exercise. A series of well characterized events, such as recruitment of immune cells and activation of satellite cells, constitute the basis for muscle regeneration. However, details regarding the fine-tuned regulation of this process in response to different types of injury are open for investigation. Muscle-resident non-myogenic, non-satellite stem cells expressing conventional mesenchymal stem cell (MSC) markers, have the potential to significantly contribute to regeneration given the role for bone marrow-derived MSCs in whole body tissue repair in response to injury and disease. The purpose of this mini-review is to highlight a regulatory role for non-satellite stem cells in the process of skeletal muscle healing post-eccentric exercise. The non-myogenic, non-satellite stem cell fraction will be defined, its role in tissue repair will be briefly reviewed, and recent studies demonstrating a contribution to eccentric exercise-induced regeneration will be presented.

Increases in integrin-ILK-RICTOR-Akt proteins, muscle mass, and strength after eccentric cycling training

PURPOSE: Recently, it has been suggested that a cellular pathway composed of integrin, integrin-linked kinase (ILK), rapamycin-insensitive companion of mTOR (RICTOR), and Akt may facilitate long-term structural and functional adaptations associated with exercise, independent of the mTORC1 pathway. Therefore, we examined changes in integrin-ILK-RICTOR-Akt protein in vastus lateralis (VL) before and after 8 wk of eccentric cycling training (ECC), which was expected to increase muscle function and VL cross-sectional area (CSA). METHODS: Eleven men (23 ± 4 yr) completed 24 sessions of ECC with progressive increases in intensity and duration, resulting in a twofold increase in work from the first three (75.4 ± 14.1 kJ) to the last three sessions (150.7 ± 28.4 kJ). Outcome measures included lower limb lean mass, VL CSA, static strength, and peak and average cycling power output. These measures and VL samples were taken before and 4-5 d after the last training session. RESULTS: Significant (P \u3c 0.05) increases in integrin-β1 (1.64-fold) and RICTOR (2.99-fold) protein as well as the phosphorylated-to-total ILK ratio (1.70-fold) were found, but integrin-α7 and Akt did not change. Increases in lower limb, thigh, and trunk lean mass (2.8%-5.3%, P \u3c 0.05) and CSA (13.3% ± 9.0%, P \u3c 0.001) were observed. Static strength (18.1% ± 10.8%) and both peak (8.6% ± 10.5%) and average power output (7.4% ± 8.3%) also increased (P \u3c 0.05). However, no significant correlations were found between the magnitude of increases in protein and the magnitude of increases in CSA, static strength, or power output. CONCLUSIONS: In addition to increased muscle mass, strength, and power, we demonstrate that ECC increases integrin-β1 and RICTOR total protein and p-ILK/t-ILK, which may play a role in protection against muscle damage as well as anabolic signaling to induce muscle adaptations

Exercise promotes α7 integrin gene transcription and protection of skeletal muscle

The α7β1 integrin is increased in skeletal muscle in response to injury-producing exercise, and transgenic overexpression of this integrin in mice protects against exercise-induced muscle damage. The present study investigates whether the increase in the α7β1 integrin observed in wild-type mice in response to exercise is due to transcriptional regulation and examines whether mobilization of the integrin at the myotendinous junction (MTJ) is a key determinant in its protection against damage. A single bout of downhill running exercise selectively increased transcription of the α7 integrin gene in 5-wk-old wild-type mice 3 h postexercise, and an increased α7 chain was detected in muscle sarcolemma adjacent to tendinous tissue immediately following exercise. The α7B, but not α7A isoform, was found concentrated and colocalized with tenascin-C in muscle fibers lining the MTJ. To further validate the importance of the integrin in the protection against muscle damage following exercise, muscle injury was quantified in α7−/− mice. Muscle damage was extensive in α7−/− mice in response to both a single and repeated bouts of exercise and was largely restricted to areas of high MTJ concentration and high mechanical force near the Achilles tendon. These results suggest that exercise-induced muscle injury selectively increases transcription of the α7 integrin gene and promotes a rapid change in the α7β integrin at the MTJ. These combined molecular and cellular alterations are likely responsible for integrin-mediated attenuation of exercise-induced muscle damage

Sca-1⁺CD45⁻ cells extracted from α7Tg skeletal muscle post-exercise are multipotent mesenchymal-like stem cells, predominantly pericytes.

<p>(<i>A</i>) RT-PCR analysis of RNA transcripts from Sca-1⁺CD45⁻ cells extracted from α7Tg muscle post-exercise and primary muscle control cells. (<i>B</i>) Flow cytometry analysis of cell surface markers from Sca-1⁺CD45⁻ cells extracted from α7Tg muscle 24 hr post-exercise. (<i>C</i>) Localization of (a) Rgs5⁺ mononuclear cells (arrows, FITC-green) in the interstitium at 40×; (b) NG2⁺ mononuclear cells (arrow, FITC-green) in close proximity to vessels and nerves at 40×; (c) NG2⁺ mononuclear cells (arrows, FITC-green) in the interstitium; (d) CD90⁺ mononuclear cells (arrows, FITC-green) in close proximity to vessels at 40×. (<i>D</i>) NG2 (pericyte) protein expression in skeletal muscle. Scale bar = 5 µm. Data are means ± SEM, n = 4–5/group for <i>D</i>. § P<0.05 vs. all groups.</p