The immune cell that heals your muscles can also turn them to bone

Imagine a healing process so precisely tuned that the same immune cell repairs your torn muscle after a fall - and so catastrophically misdirected that, in the wrong genetic context, it turns that muscle into bone. That is the story of Mrep, a macrophage population that researchers at Kanazawa University have now identified as the central player in both normal muscle regeneration and the devastating bone formation seen in fibrodysplasia ossificans progressiva (FOP).

Why muscle repair needs macrophages

Skeletal muscle accounts for roughly 40-50% of body weight and ranks among the most commonly injured tissues in daily life. When muscle fibers tear, an orchestrated sequence unfolds: inflammation sweeps in first, clearing debris, and then a repair phase restores the tissue. Muscle satellite cells - the resident stem cells - do the rebuilding, but they don't work alone. They need signals from immune cells to proliferate and differentiate. Which immune cells provide those signals, and through what mechanism, has remained surprisingly unclear.

The Kanazawa team, led by Hiroshi Takayanagi and Kazuo Okamoto, used a mouse model of hamstring muscle incision to investigate. They focused on macrophages, the dominant immune cells that accumulate in damaged muscle tissue, and performed RNA sequencing to catalog the soluble factors these cells produce.

Activin A and the cells that make it

The analysis revealed that muscle damage triggers substantial expression of activin A, a signaling molecule that promotes the proliferation of muscle satellite cells. But not all macrophages produce it equally. Using single-cell RNA sequencing, the team identified multiple macrophage subgroups within damaged muscle. One specific population - expressing the surface markers CD9, PDPN, and IL-7R - produced dramatically higher levels of activin A than its neighbors.

The researchers named this population Mrep (macrophage directing muscle tissue repair). When they depleted macrophages entirely in injured mice, repair slowed. When they transplanted Mrep cells back into those depleted mice, repair improved. Other macrophage populations had no such effect. And when they genetically blocked Mrep from producing activin A, muscle repair was significantly delayed - confirming that Mrep-derived activin A is the key driver.

The trigger for activin A production turned out to be DAMPs (damage-associated molecular patterns) - molecular alarm signals released by crushed or torn muscle fibers. These molecules activate the TLR4 receptor on Mrep cells, which in turn switches on activin A production. It is an elegant damage-response circuit: injured muscle signals danger, Mrep listens, and activin A tells satellite cells to start rebuilding.

When the repair signal builds bone

FOP is a rare hereditary disease in which trauma - sometimes as minor as a bruise or an injection - causes bone to form inside muscles, tendons, and ligaments. Over time, patients become progressively immobilized as soft tissue converts to a second skeleton. There is no cure. Surgery to remove the extra bone only triggers more bone formation. Patients must live with extraordinary caution to avoid any injury that might set off another ossification event.

The genetic cause is known: mutations in a receptor gene called ACVR1. Recent work had shown that activin A binds to the mutant form of ACVR1 and triggers aberrant bone-forming signals. But where the activin A came from after injury remained an open question.

The Kanazawa researchers hypothesized it was Mrep. They tested this in FOP mouse models and found exactly what they predicted. After muscle injury, Mrep cells accumulated at the damage site and produced activin A. That activin A bound to mutant ACVR1 on nearby mesenchymal progenitor cells, reprogramming them from their normal fate into osteoblasts - bone-forming cells. Heterotopic ossification followed.

Two interventions confirmed the mechanism. Blocking macrophages from producing activin A in FOP mice suppressed bone formation at injury sites. Administering TLR4 inhibitors alone - cutting off the upstream signal that tells Mrep to produce activin A in the first place - also prevented heterotopic ossification.

From mice to medicines - a long road

The dual role of Mrep creates both an opportunity and a challenge. Blocking Mrep activity could prevent pathological bone formation in FOP, but it might also impair normal muscle healing. Any therapeutic approach would need to be selective - targeting Mrep in FOP patients during or after trauma while preserving its beneficial function.

This work was conducted in mouse models, and FOP is extremely rare (estimated at roughly one in two million people worldwide), which makes clinical trials inherently difficult. The TLR4 inhibitor approach is intriguing because TLR4 inhibitors already exist in clinical development for other indications, but their effect on muscle repair would need careful evaluation.

The research was supported by the Japan Agency for Medical Research and Development (AMED), the Japan Science and Technology Agency, and the Japan Society for the Promotion of Science, among other funders.