Muscle loss from space travel, prolonged inactivity linked to two genes

Findings point way to identifying therapeutics to stem muscle atrophy

In research that could benefit astronauts posted to the International Space Station as well as individuals whose universe is defined by their sick bed, Boston University Sargent College researchers Susan Kandarian and R. Bridge Hunter have found that disrupting either one of two genes, nfêb1 and bcl3, can block the biological process of muscle wasting known as atrophy.

Their findings will inform efforts to identify therapeutics that could inhibit muscle atrophy caused by a chronic reduction in muscle use. Such treatments could end the muscle loss, weakness, and fatigue that can plague space travelers, sedentary or bed-ridden individuals, or anyone whose muscles undergo long periods of disuse. “By understanding the genes necessary for muscle atrophy,” says Kandarian, lead researcher and professor of health sciences at BU Sargent College of Health and Rehabilitation Sciences, “we can begin to study the protein products required for atrophy. Our ongoing research shows that the process of atrophy might be partially moderated by something as straightforward as aspirin or other non-steroidal anti-inflammatory drugs [NSAIDs].”

It is well known that prolonged muscle disuse causes muscles to lose protein in two ways: by decreasing the amount of protein synthesized and by increasing the rate at which muscle protein is degraded. The intracellular signals that drive muscle protein to these extremes, however, are not well known. When designing her study, Kandarian chose to investigate the role of nfêb1 and bcl3, two gene members from a family of transcription factors — genes that produce proteins that regulate the activities of other genes. Her previous research had shown these transcriptional regulators were implicated in the process of muscle atrophy.

By studying strains of mice bred to be without the nfêb1 gene or without the bcl3 gene, so-called knockout mice, and then comparing knockout results with those from control groups as well as those from genetically unaltered (“wild-type”) mice, Kandarian was able to isolate the effects either gene had on triggering atrophy in each of two weight-bearing muscles in the mouse hind limb: the soleus, a so-called slow-fast muscle, and the plantaris, a fast muscle. The differing muscle fiber make-up allowed Kandarian to assess what changes in the muscle’s phenotype were linked to the atrophy process. A slow-to-fast change in phenotype is typical in muscle atrophy.

After 10 days of reduced weight-bearing use, all groups of mice were assessed for changes in muscle fiber size, fiber phenotype, and activation of an injected NF-êB reporter gene. This reporter gene reflects the muscles’ transcriptional activity of the two genes the scientists were studying. Kandarian found that when muscle fiber size of the soleus in knockouts and wild types were compared, knockout mice showed virtually no changes after the long period of disuse. Plantaris muscle fiber size in knockout mice also showed very little atrophy compared to wild type; fiber atrophy was inhibited by 67 percent.

Kandarian also found an absence of a slow-to-fast shift in phenotype in the knockout groups, a shift that remained in the wild type mice, and an absence of NF-êB reporter activity in the muscles of the knockout mice. In comparison, wild type muscles subjected to disuse show a sevenfold increase in NF-êB reporter activity.

The researchers conclude that atrophy associated with prolonged muscular inactivity requires the involvement of nfêb1 and bcl3. Their findings are reported in the latest issue of The Journal of Clinical Investigation. The work was supported by grants from the National Space Biomedical Research Institute and the National Institute of Arthritis and Musculoskeletal and Skin Diseases.

To begin to assess directly just what types of chemical interventions might stall the activity of these genes, Kandarian’s laboratory currently is testing whether NSAIDs such as aspirin can reduce skeletal muscle weakness and loss under the condition of weightlessness. A two-year study led by her research associate, Andrew Judge, seeks to determine whether high-dose aspirin treatment can inhibit muscle fiber atrophy and activation of the NF-êB reporter gene.