Male hormones help keep muscle stem cells in reserve for repair, a study in mice finds

Muscle has a remarkable ability to heal after injury. Behind that repair is a reserve of muscle stem cells that sit quietly until they are needed. A new study in male mice suggests that androgens, the group of hormones that includes testosterone, play a surprisingly specific role in keeping that reserve from running dry.

When the researchers removed the receptor for these hormones from muscle stem cells, the cells woke up too early and the reserve dwindled, in ways that resembled an aging muscle. Giving older mice extra hormone improved repair.

What are muscle stem cells?

Muscle stem cells, also known as satellite cells, sit alongside muscle fibers in a dormant state. The authors describe them as the guardians of muscle regeneration. When muscle is injured, some of them wake up, multiply and fuse to build new fibers.

The trick is balance. If every stem cell rushed to repair the damage, none would be left for the next injury. So some must return to rest, keeping a reserve in place. With age, both the number of these stem cells and their ability to repair muscle decline.

Why study androgens in muscle repair?

Androgens act on cells through the androgen receptor, a protein that switches particular genes on or off when the hormone binds to it. Androgen signaling is an essential hormonal pathway for male muscle, best known for helping build muscle mass.

Its role in the stem cells themselves had remained largely unexplored. Previous work had hinted at a link, and the researchers note that androgen supplementation increases the number of these stem cells in both animal models and humans. But the authors say the receptor’s control over whether these stem cells renew themselves or commit to repair had not previously been shown in living animals.

What happened when the androgen receptor was removed?

Writing in Science Advances, the team deleted the androgen receptor only in the muscle stem cells of male mice, starting at nine weeks old. They then injured a leg muscle and watched how it healed.

The receptor turned out to mark the stem cells that were most deeply at rest, and to act as a safeguard keeping them dormant. Without it, stem cells entered the cycle of cell division too early and divided in skewed ways, producing cells committed to repair at the expense of cells that renew the reserve. Over repeated injuries the reserve was depleted, and repair suffered.

Gene activity studies showed why. In resting cells, the receptor sat on genes that maintain dormancy. During repair it shifted toward genes that drive activation and changes in how the cells use energy.

How does this relate to aging muscle?

The researchers measured testosterone in mice of different ages. Levels fell from about 25 nmol/liter in young mice to a plateau of about 5 in older ones, and the receptor itself became less abundant in muscle.

The damage seen in young mice lacking the receptor resembled features of aging, including a weaker response to repeated injuries. The authors are careful not to call the two identical, since aging also involves broader loss of stem cells and changes in the tissue around them. Mice that were both older and lacking the receptor fared worst of all.

Did adding hormone help older mice heal?

To test the idea directly, the team gave one-year-old mice an implant releasing dihydrotestosterone, a potent androgen. A week after muscle injury, those mice showed noticeably better repair, with healthier-looking new fibers. The total number of stem cells did not change significantly, though slightly more of them were actively dividing.

Human muscle cells grown in the lab pointed the same way. When the researchers reduced the androgen receptor in those cells, they struggled to mature into muscle.

Should anyone take testosterone for muscle repair?

No, and this is the most important point to make clearly. The work was done in male mice and lab-grown cells. Nobody knows from this study whether giving hormones to people would help their muscle stem cells, at what dose, or for whom.

Testosterone is also far from harmless. MedlinePlus warns that testosterone may cause serious side effects if used at higher doses or in ways other than directed by a doctor, including heart attack, stroke, liver disease and changes in mental health. The study also says nothing about women.

What does the muscle stem cell finding change?

For now, it mainly sharpens the scientific picture. MedlinePlus notes that your muscles help you move and help your body work, and muscle disorders can cause weakness or pain. This study identifies the androgen receptor as one of the switches that decides whether a muscle stem cell stays in reserve or gets spent.

That could eventually help researchers understand why repair falters in later life, and whether targeting this pathway more precisely than simply adding hormone might help. Those are questions for future research, not for the medicine cabinet.