Different muscles in the body heal at different rates after injury: the gene SIX2 activates a fast repair mode in a subset of muscle stem cells
Different muscles in the body heal at different rates after injury: the gene SIX2 activates a fast repair mode in a subset of muscle stem cells
French and Hong Kong biologists showed in mice that a subset of skeletal muscle stem cells retains the embryonic gene PAX3 in an active state, which makes them divide and repair tissue faster than the rest, while cells lacking PAX3 mostly stay quiescent and replenish the long-term reserve. The accelerated repair is driven by the gene SIX2, which is switched on only in PAX3-positive cells.
The human body contains roughly 640 distinct muscles, and each one repairs itself through its own resident satellite stem cells. After an injury these cells wake up, proliferate, and rebuild the damaged fiber. All of them share the marker PAX7, and for a long time they were considered essentially identical. Yet some of them retain an additional switch, PAX3, a gene that is normally active only in the embryo while future muscles are being laid down. The same group of biologists had previously shown that PAX3-positive cells survive toxic muscle injury better, and set out to test whether the same advantage holds in ordinary trauma.
The study, published on September 16 in Science Advances, shows that this embryonic gene remains active in the adult organism. The fraction of cells with active PAX3 varies from muscle to muscle: in the mouse gastrocnemius there are almost none, in the tibialis anterior about a fifth, and in the biceps brachii and diaphragm already half of all stem cells. Within 42 hours of injury, PAX3-positive cells enter the first division twice as often as the rest (35% versus 17%), and in culture their numbers grow threefold faster over three days. When transplanted into an injured muscle, it is these cells that more actively build new fibers, while PAX3-negative cells mainly replenish the stem cell reserve for the future.
To prove that PAX3 itself governs this behavior, the authors deleted the gene in adult mice, which reduced PAX3 expression in stem cells by 98%. In the biceps brachii, where PAX3-positive cells are abundant, this led to their massive death after injury and slower healing. In the tibialis anterior, where such cells are scarce to begin with, the effect was barely noticeable.
Single-cell RNA sequencing pointed to a downstream effector gene, SIX2, which is switched on almost exclusively in PAX3-positive cells. Knocking out SIX2 reproduces a substantial part of the effect of losing PAX3 itself, and forcing SIX2 on in cells that normally do not express it is enough to accelerate their division on its own. In PAX3-negative cells, artificially activated SIX2 simultaneously turns on cell cycle genes and DNA repair genes and rescues the cells from death during accelerated division.
The result is a stable division of labor within a single cell type: PAX3-positive cells serve as the rapid repair crew, PAX3-negative cells as the long-term reserve. The authors cite earlier work showing that the share of PAX3-positive cells declines with age alongside deteriorating maintenance of muscle fibers, and they frame an open hypothesis for future experiments: cells that operate at full capacity may become exhausted with aging faster than the rest.
The authors explicitly name a practical implication of the finding: the PAX3-SIX2 heterogeneity could potentially be used as a lever for targeted stimulation of regeneration in diseases that waste muscle, such as sarcopenia, muscular dystrophy, or cachexia. The vague idea of "waking up stem cells" now has a concrete molecular target with a name.