Muscles require motion to stay healthy, but that doesn’t mean pumping iron at the gym. The movements people make as they go about their daily lives give muscles the exercise they need, but when this baseline amount of movement becomes unattainable, bodies suffer the consequences.
While bed rest after injury, surgery, or illness is often restorative, this downtime leads to muscle atrophy. “It's really hard to recover, especially as an older adult, from this period of disuse,” said Marni Boppart, a professor in the Department of Kinesiology and Community Health at the University of Illinois at Urbana-Champaign. “We wanted to try to develop a novel therapy to help with the regrowth process after that period of disease.”
Boppart researches the factors that regulate skeletal muscle growth and remodeling during exercise and rehabilitation. After periods of extended rest where muscles are immobilized, resuming movement causes reactive oxygen species (ROS) accumulation. ROS triggers protein degradation, which hinders muscle recovery. Boppart’s team recently investigated this phenomenon and published their results in The Journal of Physiology.1
The researchers first took a close look at pericytes, which surround vessels within skeletal muscle tissue and secrete factors that help maintain muscle mass under normal conditions. Boppart didn’t know how the cells would respond to muscle disuse and rehabilitation, so the team set up a mouse model to study this situation. They immobilized mouse hindlimbs, and after a period of disuse, performed single cell RNA sequencing (scRNA-seq) on the muscle tissue.
The scRNA-seq data allowed the scientists to home in on the muscle pericytes and analyze the gene expression changes that occurred during immobilization. Boppart found that the pericytes from immobilized muscles were deficient in antioxidant gene expression, which might explain the ROS accumulation because antioxidants typically scavenge excess ROS.
Because pericytes produce important factors for recovery, the researchers previously tried injecting donor pericytes taken from healthy muscle directly into unused muscle. But only young mice benefitted from this cell-based therapy. “Cells typically are not viable in a collagen-enriched environment like we would observe in aged muscle,” Boppart said. “We needed to come up with a real creative approach to overcome this.”
Inspired by previous work,2 the researchers next stimulated healthy pericytes in cell culture with the ROS hydrogen peroxide, which caused them to release extracellular vesicles (EVs) filled with antioxidants. Boppart captured the EVs using a process that took three years to perfect and injected them into the unused muscles of aged mice.
Three days after treatment, she saw reduced stress caused by excess ROS and an increase in collagen turnover, which is important for muscle fiber regrowth. Additionally, the muscle fibers after EV treatment were large and fully functional. “These have the advantage of not being rejected like a cell might be within the muscle microenvironment,” Boppart said. “Our EV therapy was, in fact, effective.”
“Our EV therapy was, in fact, effective.”
– Marni Boppart, University of Illinois at Urbana-Champaign
While these initial results are promising, there are hurdles to overcome before EVs can be developed into a cell-free treatment strategy for rejuvenating aged skeletal muscle. “The use of these extracellular vesicles from the pericytes…is unique and hasn't been looked at before,” said Sue Bodine, a professor of internal medicine at the University of Iowa Carver College of Medicine, who was not involved in this study. “Scaling it up to a larger muscle would be the biggest limitation…If you're thinking about translation to humans, the muscles are much bigger, so how would you deliver these extracellular vesicles?”
Boppart next plans to derive EVs from human pericytes to see if they have the same effect as those from mice. Further investigation into the components of the hydrogen peroxide-stimulated EVs that drive muscle recovery will give researchers more information to drive future therapy development.
- Y.-F. Wu et al., “Development of a cell-free strategy to recover aged skeletal muscle after disuse,” J Physiol, 2022.
- R. Stavely, K. Nurgali, “The emerging antioxidant paradigm of mesenchymal stem cell therapy,” Stem Cells Transl Med, 9:985-1006, 2020.