Human skeletal muscle M2 macrophages are increased following aerobic exercise and may modulate Fn14/TWEAK signaling
Authors: Walton Grace , Kosmac Kate , Mula Jyothi , Fry Christopher S , Michaelis Samantha , Finlin Brian , Zhu Beibei , Kern Philip A. , Peterson Charlotte A. 
Published: November 2023

Skeletal muscle tissue resident macrophages are plastic and heterogeneous, and participate in tissue regeneration, repair, neovascularization, and homeostatic functions. We sought to determine whether skeletal muscle macrophages would change in response to aerobic exercise training. We also sought to determine whether changes in macrophage characteristics are associated with other physiological responses to aerobic training. Twenty six human subjects underwent muscle (vastus lateralis) biopsies before and after 12 weeks of cycle ergometer training. Subjects were 75% female and had a broad range of age (mean age 48.4, range 26–68), BMI (mean BMI 30.9, range 22.5–41.8), and insulin sensitivity (mean SI 3.1, range 0.65–7.1). In a sub‐set of 20 subjects, macrophages were quantified via immunohistochemistry for total macrophages (CD11b+), pro‐inflammatory M1 macrophages (CD11b+/CD206−), anti‐inflammatory M2 macrophages (CD11b+/CD206+), and the anti‐inflammatory and tissue remodeling M2c macrophages (CD206+/CD163+). Overall, M1 were less abundant than M2 macrophages (0.13 ± 0.02 SEM M1 macrophages per fiber versus 0.26 ± 0.02 SEM M2 macrophages per fiber); there was a trend toward increased M1 abundance following exercise (17% increase, P=0.06). However, total muscle macrophages were significantly increased following exercise by 30% (P<0.01), primarily due to an increase in M2 macrophages (32%, P<0.01), and M2c macrophages increased by 45% (P<0.05). In an overlapping subset of 20 subjects, gene expression of macrophage and inflammation‐related genes was quantified using the NanoString nCounter analysis system. In keeping with increased M2 and M2c macrophages following aerobic exercise training, we observed increased expression of IL‐4 (55% increase, P<0.05). We also observed increased gene expression of the TWEAK receptor Fn14 (78% increase, P<0.05), which is expressed in muscle fibers. Fn14 has recently been associated with muscle mass gains and we found that the change in Fn14 gene expression following exercise was significantly inversely associated with the change in MuRF (TRIM63, E3 ubiquitin ligase) gene expression (R=−0.46, P<0.05). We next hypothesized that M2c‐derived soluble CD163 increases following aerobic training, and functions to bind soluble TWEAK, promoting pro‐growth signaling through the Fn14 receptor. We therefore inhibited Fn14 expression using siRNA in vitro in mature human myotubes (77% decrease in Fn14 expression, P<0.001), and measured pro‐ and anti‐hypertrophy gene expression. Following Fn14 knock‐down, there were no changes in myogenin or myostatin, but there was a 3.3‐fold increase in MuRF expression (P<0.05), indicating that the CD163/TWEAK/Fn14 axis participates in regulation of protein degradation, that may be regulated by resident M2 macrophages in muscle.Support or Funding InformationThis work was supported by the following National Institutes of Health grants: R01 DK71349 and AG46920 (C.A.P. and P.A.K.). This project was supported by the institutional CTSA grant, NIH UL1 TR0001998