We have identified a macrophage population "Mrep" that plays an essential role in muscle repair. However, in Fibrodysplasia ossificans progressiva (FOP), Mrep functions as a pathogenic cell that triggers heterotopic ossification. These research findings would contribute not only to muscle regeneration therapy but also to the development of novel therapeutic approaches for FOP.
Musculoskeletal disorders are a primary cause of disability worldwide, especially in aging societies like Japan. As individuals age, reductions in muscle mass and physical activity weaken the body's structural support, increasing the likelihood of falls, bruises, fractures, and subsequent functional decline. These injuries not only cause pain but also impair mobility, diminishing quality of life and leading to further health complications. Addressing these pervasive issues necessitates an in-depth understanding of how muscles regenerate and repair after injury.
Skeletal muscle serves not only as the organ responsible for body movement but also constitutes the body's largest energy-consuming organ, accounting for approximately 40-50% of body weight. Muscle injury ranks among the most common daily injuries. When muscles are damaged, inflammation occurs first, followed by a repair process that restores tissue to its original state. This repair involves complex coordination between various immune cells and muscle satellite cells, the stem cells crucial for muscle regeneration. However, significant gaps remain in understanding which immune cells facilitate muscle repair and through what mechanisms.
When muscle regeneration fails, tissue fibrosis and heterotopic ossification can occur. A severe example is Fibrodysplasia Ossificans Progressiva (FOP), a rare hereditary disease where trauma triggers bone formation in muscles and tendons. No fundamental treatment currently exists, and surgical interventions are contraindicated. Patients must exercise extreme caution to avoid injuries, making new treatment development urgently needed.
Researchers utilized a mouse model involving incision of hamstring muscles to analyze muscle repair processes. Focusing on macrophages as the predominant immune cells accumulating in damaged tissue, they investigated soluble factors produced by macrophages using RNA sequencing. This analysis revealed that muscle damage induces substantial expression of activin A. They also found that activin A promotes the proliferation of muscle satellite cells, thus facilitating muscle regeneration.
Single-cell RNA sequencing of macrophages in damaged muscle identified multiple subgroups. One specific population expressing cell membrane proteins CD9, PDPN, and IL-7R produced particularly high levels of activin A and specialized in muscle repair. In macrophage-deficient mice with muscle injuries, repair was delayed compared to normal mice. However, transplanting this specific macrophage population improved muscle repair, while transplanting other populations showed no similar effect. Thus, researchers named this population "Mrep" (macrophage directing muscle tissue repair). Mice engineered to prevent Mrep from producing activin A showed significantly delayed muscle repair, confirming that Mrep-derived activin A is the primary factor for normal muscle repair. Furthermore, Mrep produces activin A upon receiving danger signal molecules called DAMPs released from damaged muscle through the TLR4 receptor.
The research team investigated the relationship between Mrep-mediated muscle repair and FOP pathology. Clinical observations had suggested immune system involvement in FOP, as intense inflammation occurs before heterotopic ossification and immunosuppressants can prevent new bone formation. The causative gene for FOP involves mutations in ACVR1. Recent research revealed that activin A binds to mutant ACVR1 and induces abnormal bone-forming signals, but how trauma-associated inflammation relates to activin A production remained unclear. Researchers hypothesized that Mrep accumulating after muscle injury serves as the cellular source for activin A that triggers heterotopic ossification in FOP. Experiments with FOP mouse models confirmed that muscle injury led to heterotopic ossification at injury sites, with Mrep producing activin A at these locations. Mrep-derived activin A acts on mutant ACVR1 in mesenchymal progenitor cells, inducing their differentiation into bone-forming osteoblasts. Importantly, preventing macrophages from producing activin A in FOP mouse models suppressed heterotopic bone formation. Additionally, administering TLR4 inhibitors alone also suppressed heterotopic ossification.
This research demonstrates that Mrep performs beneficial functions by promoting muscle repair under normal injury conditions. However, in FOP, Mrep accumulates following trauma and paradoxically triggers heterotopic ossification. These findings establish Mrep as an essential new macrophage population for muscle repair and elucidate the mechanism by which its normal function becomes pathological in FOP. This research suggests that therapeutic approaches targeting Mrep could lead to innovative treatments promoting muscle regeneration and breakthrough medications preventing heterotopic ossification in FOP patients.
Funding support
- Japan Agency for Medical Research and Development (AMED)–Core Research for Evolutional Science and Technology (JP23gm1210008 and JP26gm2110004 to HT; JP23gm1710003 to KO).
- AMED Practical Research Project for Rare/Intractable Diseases (JP18ek0109379 to AT).
- AMED Practical Research Project for Allergic Diseases and Immunology (JP23ek0410108 to HT).
- Japan Science and Technology Agency FOREST Program (JPMJFR205Z to KO).
- Japan Society for the Promotion of Science, Scientific Research S (21H05046 to HT) and Scientific Research B (25K02757 to KO).
- SECOM Science and Technology Foundation (to HT).
- Mitsui Sumitomo Insurance Welfare Foundation (to KO).
- The Hokkoku Cancer Foundation (to KO).
- Astellas Foundation for Research on Metabolic Disorders (to KO).
- The Japanese Society for Bone and Mineral Research Rising Stars Grant (to KO).
- Extramural Collaborative Research Grant of Cancer Research Institute, Kanazawa University (to KO).
- Daiichi Sankyo Foundation of Life Science (to KO).
- The Naito Foundation (to KO).
- Chugai Foundation for Innovative Drug Discovery Science (to KO).
- MEXT Promotion of Development of a Joint Usage/Research System Project: Coalition of Universities for Research Excellence (JPMXP1323015484 to KO).
- Japan Initiative for World-leading Vaccine Research and Development Centers (AMED) (JP223fa627001).
HT: Hiroshi Takayanagi
KO: Kazuo Okamoto
AT: Asuka Terashima
