From skeletal muscle damage and regeneration to the hypertrophy induced by exercise: what is the role of different macrophage subsets?

Macrophages are one of the top players when considering immune cells involved with tissue homeostasis. Recently, increasing evidence has demonstrated that macrophages could also present two major subsets during tissue healing: proliferative macrophages (M1-like), which are responsible for increasing myogenic cell proliferation, and restorative macrophages (M2-like), which are involved in the end of the mature muscle myogenesis. The participation and characterization of these macrophage subsets are critical during myogenesis to understand the inflammatory role of macrophages during muscle recovery and to create supportive strategies that can improve mass muscle maintenance. Indeed, most of our knowledge about macrophage subsets comes from skeletal muscle damage protocols, and we still do not know how these subsets can contribute to skeletal muscle adaptation. Thus, this narrative review aims to collect and discuss studies demonstrating the involvement of different macrophage subsets during the skeletal muscle damage/regeneration process, showcasing an essential role of these macrophage subsets during muscle adaptation induced by acute and chronic exercise programs.

Skeletal muscles induce recruitment of Ly6C+ macrophage subtypes and release inflammatory cytokines 3 days after downhill exercise.

Macrophages are one of the most versatile cells of the immune system that can express distinct subtypes and functions depending on the physiological challenge. After skeletal muscle damage, inflammatory macrophage subtypes aid muscles to regenerate and are implicated in physical training adaption. Based on this information, this study aimed to evaluate two classic mice macrophage subtypes and determine whether some inflammatory cytokines might be involved in the muscle adaption process after exercise. For this purpose, mice were exposed to an intermittent experimental protocol of downhill exercise (18 bouts of running, each bout 5 min with a 2-min rest interval, slope -16°) and were euthanized before [control (CTRL)] and 1, 2 (D2), and 3 (D3) days after exercise. After euthanasia, the triceps brachii was harvested and submitted to protein extraction, immunostaining, and mononuclear digestion procedures. The muscle size, macrophage accumulation, and cytokines were determined. We observed an increase in the Ly6C+ macrophage subtype (P ≤ 0.05) in D2 and D3 compared with CTRL, as well as a significant inverse correlation coefficient (-0.52; P ≤ 0.05) between Ly6C+ and Ly6C- macrophage subtypes. Moreover, we also observed elevation in several cytokines (IL-1ß, IFN-γ, TNF-α, IL-6, and IL-13) at D3, although not IL-4, which tended to decrease at this time point (P = 0.06). Downhill exercises preferentially recruited Ly6C+ macrophages with important proinflammatory cytokine elevation at D3. Moreover, despite the elevation of several cytokines involved with myogenesis, an increase in IL-6 and IL-13, which potentially involve muscle adaption training after acute exercise, was also observed.

The Secretory Leukocyte Protease Inhibitor mRNA expression is involved with inflammatory control after downhill exercise in the triceps brachii intermediary head in Wistar rats.

After severe skeletal muscle damage, communication between inflammatory macrophages, myogenic cells, and modulatory secretion factors is essential to induce re-establishment of skeletal muscle structure. To analyze when characteristic gene expression of macrophages, myogenic cells, and SLPI are modulated after an exercise-induced muscle damage (EIMD) downhill protocol. Twenty-six rats were exposed to an experimental protocol of exercise and euthanized before (CTRL), immediately after (G0), and 24 (G24) and 48 (G48) hours after the exercise. After euthanasia, the Triceps brachii were dissected and analyzed by enzyme-linked immunosorbent assay and real time polymerase chain reaction. The CD68 expression was higher in the G24 when compared with all groups (p value < 0.05), whereas the CD163 was inhibited compared with G0 (p value < 0.05). MyoD and Myogenin were higher in the G24 when compared with G0 and G48 (p value < 0.05). The mRNA Secretory Leukocyte Protease Inhibitor (SLPI) was higher in the G48 when compared with the CTRL and G0 (p value < 0.05). IL-6 and TNF-α cytokines did not significantly change, but IL-10 presented a trend to be lower in the G0 when compared with G24 (p value = 0.054). A significant negative correlation was observed between CD68/CD163 (C.C = -0.71) and positive correlations between CD68/Myogenin (C.C = 0.65); MyoD/Myogenin (C.C = 0.72); IL-10/MyoD (C.C = 0.46), IL-10/MYOGENIN (C.C = 0.59); and IL-6/IL-10 (C.C = 0.64). A higher expression of CD68, concomitant with an elevation in MyoD and Myogenin 24 h after exercise, along with some correlations, suggests macrophage communication with myogenic cells independent of CD163 elevation. Additionally, the reestablishment of IL-10 in 24 h with the SLPI increased until 48 h indicate that these molecules are involved with anti-inflammatory transition after downhill exercise in the TBIH of Wistar rats.

Macrophage Polarization: Implications on Metabolic Diseases and the Role of Exercise.

Macrophages are cells of the innate immune response that trigger inflammation resolution. The phenotype of "classically activated macrophages" (M1) has anti-tumoricidal and anti-bactericidal activities. On the other hand, "alternatively activated macrophages" (M2) are involved in tissue remodeling and immunomodulatory functions. The change in the polarization of macrophages varies according to the diversity of cytokines present in the microenvironment or by the stimuli of an antigen. It involves such factors as interferon-regulatory factors, peroxisome proliferator-activated receptors (PPARs), hypoxia-inducible factors (HIFs), and signal transducers and activators of transcription (STATs). Switching the phenotype of macrophages can help attenuate the development of an inflammatory disease. Exercise can promote alterations in the number of innate immune cells and stimulates phagocytic function. Chronic exercise seems to inhibit macrophage infiltration into adipose tissue by attenuating the expression of F4/80 mRNA. Furthermore, exercise may also increase the expression of M2 markers and reduce TNF-α and TLR4 mRNA expression, which activates the inflammatory pathway of NF-κB. Chronic exercise reduces ß2-adrenergic receptors in monocytes and macrophages by modulating TLR4 signaling as well as suppressing IL-12 production, a stimulator of interferon Y. In this review, we discuss macrophage polarization in metabolic diseases and how exercise can modulate macrophage plasticity.

Downhill exercise-induced changes in gene expression related with macrophage polarization and myogenic cells in the triceps long head of rats.

Macrophages are one of the most heterogenic immune cells involved in skeletal muscle regeneration. After skeletal muscle damage, M1 phenotypes exhibit pro-inflammatory reaction. In a later stage, they are converted to M2 phenotypes with anti-inflammatory properties. To study when gene expressions of macrophage polarization are changed after damage induced by downhill exercise to exhaustion is the objective of this paper. Before (CTRL) and 0 h (G0), 24 h (G24), 48 h (G48) and 72 h (G72) after 18 bouts of downhill exercise, the animals were euthanised, and the triceps were dissected. We measured gene expression of macrophages (CD68 and CD163), myogenic cells (MyoD and myogenin) and quantified cytokine secretion (interleukin (IL)-6, IL-10 and tumour necrosis factor alpha (TNF-α)). The CD68 expression was lower in G72 compared with G24 (P = 0.005) while CD163 was higher in G48 compared with G24 (P = 0.04). The MyoD expression was higher in G72 compared with G0 (P = 0.04). The myogenin expression was lower in G24 compared with CTRL (P = 0.01) and restored in G72 compared with G24 (P = 0.007). The TNF-α was significantly higher at all times after 24 h (all compared with CTRL, with P = 0.03). The CD68 and CD163 expressions behaved distinctly after exercise, which indicates macrophage polarization between 24 and 48 h. The distinct expression of myogenin, concomitantly with MyoD elevation in G72, indicates that myogenic cell differentiation and the significant change of TNF-α level show an important role of this cytokine in these processes.

Exercise preconditioning modulates genotoxicity induced by doxorubicin in multiple organs of rats.

The aim of this study was to investigate the effects of exercise in multiple organs of rats treated with doxorubicin. Male adult Wistar rats were distributed into the following groups: sedentary + NaCl; exercise + NaCl; sedentary + doxorubicin; and exercise + doxorubicin. Animals were sacrificed 2 days following injections. Central fragments from heart, liver, and kidney were collected and minced in 0.9% NaCl being cellular suspensions used for the single-cell gel (comet) assay. The results showed that exercise was able to prevent genotoxicity induced by doxorubicin in heart cells. By contrast, exercise was not able to prevent genotoxicity induced by doxorubicin in liver cells. The same occurred to kidney cells, i.e. no statistically significant differences (p > 0.05) were found when compared with groups not exposed to doxorubicin. Taken together, our results support the idea that exercise could contribute to the protective effect against genotoxicity induced by doxorubicin in heart cells.