In older women, muscle recovers poorly after casting or bed rest because connective tissue cells turn against muscle stem cells, and their secretions alone are enough to cause the damage
In older women, muscle recovers poorly after casting or bed rest because connective tissue cells turn against muscle stem cells, and their secretions alone are enough to cause the damage
In a preprint posted on September 24, physiologists from the Universities of Utah and Kentucky compared the muscles of eight young and nine older women before and after two weeks of immobilizing one leg. In the older women, by day seven of recovery, the communication between muscle stem cells and connective tissue cells had broken down sharply. Their muscle cells fused into fibers less effectively and senesced more often, and conditioned medium from the connective tissue cells alone was sufficient to amplify both effects in fresh cells.
After injury or illness, older adults spend weeks without movement, and their muscles weaken. According to a review in JAMA, roughly one third of patients over 70 leave the hospital less able to care for themselves than they were before admission, even when the illness itself was treated successfully.
Older women have fewer muscle stem cells than men to begin with, so the authors compared eight young women (approximately 22 years old) and nine older women (approximately 66 years old): a knee brace unloaded one leg for two weeks, followed by one week of recovery through walking. In a subset, they performed single-nucleus RNA sequencing, a method that reveals gene activity in each individual cell rather than averaging across the tissue.
By recovery day seven, nearly 8,000 genes had changed their activity in the older women's muscle, compared with roughly 1,500 in the young. Most of the changes fell in two cell types: satellite cells, the muscle's stem cells that divide and fuse into fibers, and FAPs (fibro-adipogenic progenitors), connective tissue cells that normally support this process but, when overactivated, produce scar tissue.
The authors chose this pair of cell types based on two earlier findings. The same group had previously observed, in the same cohort of women, a rapid expansion of immune cells and markers of senescence during the first week of recovery. Separately, independent work in aged mice showed that the coordinated activity of satellite cells, immune cells, and FAPs is required for muscle remodeling under load, and that this coordination deteriorates with age.
In the older women, satellite cells activated the protein THBS1 and the TGF-β pathway instead of following a regenerative program. TGF-β is a well-established brake on muscle cell growth and renewal during aging. In the older women's FAPs, ADAMTS14 was upregulated, a marker of the transition toward a fibrogenic state. Intercellular signaling analysis showed that the predicted influence of FAPs on satellite cell gene expression increased sharply in the older group: the dialogue between these two cell types had gone wrong.
In culture, muscle progenitor cells from the older women on recovery day seven fused into muscle fibers less efficiently (fusion index 21% versus 44% in the young) and became senescent more often: they stopped dividing and secreted proinflammatory factors, with 19% of such cells compared to 7% in the young.
In a separate experiment, the authors tested whether secretions from connective tissue cells alone were sufficient to harm muscle cells. They applied conditioned medium to fresh cells of the same age. Medium from the older women reduced fiber fusion by 16 percentage points; medium from the young had almost no effect. The proportion of senescent cells rose in both groups, but more so with medium from the older women: by 27 percentage points versus 15.
The authors did not identify which specific molecule carries the damage. Candidate genes, including THBS1, showed only a trend toward increased expression without reaching full statistical significance, and the likely source is the secretome of senescent cells. The authors consider the disrupted FAP-to-satellite-cell dialogue a promising therapeutic target: a way to block it could eventually restore older women's ability to fully recover muscle after injury. A similar lever has already worked in mice: knocking out the gene P311, which is particularly active in FAPs, reduced scarring and increased muscle strength by 19%.