Muscle atrophy through thick but not thin

June 8, 2009

During desperate times, such as fasting, or muscle wasting that afflicts cancer or AIDS patients, the body cannibalizes itself, atrophying and breaking down skeletal muscle proteins to liberate amino acids. In a new study published online June 8 and in the June 15, 2009 print issue of the Journal of Cell Biology, Shenhav Cohen, Alfred Goldberg, and colleagues show that muscle atrophy is a more ordered process than was previously thought. These researchers find evidence that enzyme MuRF1 selectively degrades the thick filaments in muscle, while bypassing the thin filaments.

We depend on skeletal muscles because they can produce movement, but they serve another purpose too. "Skeletal is a protein reservoir that can be mobilized in times of need," says Goldberg. The structural core of a muscle cell is the myofibril, composed of myosin-containing thick filaments and actin-containing thin filaments. During atrophy, this structure is disassembled, but exactly how was not known. MuRF1, an atrophy-related gene, is a ubiquitin ligase that "ubiquitylates," or tags a protein, by attaching a ubiquitin molecule, marking it for degradation by the cell. It was unclear when and how ubiquitylation was involved in disassembling skeletal muscles. The researchers triggered atrophy in mice containing defective MuRF1 (lacking its RING-finger domain crucial for ubiquitylation). These mutant mice break down less muscle than wild-type mice, and less ubiquitylation takes place in the mutants.

Cohen and colleagues found that MuRF1 targets the thick filament, demolishing various components in a specific order. The researchers hypothesize that removal of certain thick filament components first permits subsequent MuRF1 access to the myosin heavy chain. However, MuRF1 doesn't exert the same power over the thin filament, which began to come apart even when MuRF1 was absent.

"Up to now, people thought the muscle just gets smaller" during atrophy, Goldberg says. Instead, these findings paint a picture of a well-regulated process of degradation and disassembly. This mechanism "allows the muscle to still be a muscle and function," Goldberg says. "Atrophy doesn't just destroy muscle cells, like apoptosis." The results indicate that MuRF1 doesn't have to wait for caspases or calpains to "pre-digest" the myofibril components. The work also bears on the practical question of whether can be halted or reversed with drugs. "It argues against MuRF1 inhibitors" for this purpose, Goldberg says, because the enzyme is responsible for degrading only some muscle components, whereas others fall victim to other ubiquitin ligases and autophagy. Inhibitors that work upstream to block signals that activate ubiquitin ligases and initiate autophagy are a better bet.

Source: Rockefeller University Press

Related Stories

Recommended for you

Researchers discover key signaling protein for muscle growth

November 20, 2017
Researchers at the University of Louisville have discovered the importance of a well-known protein, myeloid differentiation primary response gene 88 (MyD88), in the development and regeneration of muscles. Ashok Kumar, Ph.D., ...

New breast cell types discovered by multidisciplinary research team

November 20, 2017
A joint effort by breast cancer researchers and bioinformaticians has provided new insights into the molecular changes that drive breast development.

Brain cell advance brings hope for Creutzfeldt-Jakob disease

November 20, 2017
Scientists have developed a new system to study Creutzfeldt-Jakob disease in the laboratory, paving the way for research to find treatments for the fatal brain disorder.

Hibernating ground squirrels provide clues to new stroke treatments

November 17, 2017
In the fight against brain damage caused by stroke, researchers have turned to an unlikely source of inspiration: hibernating ground squirrels.

Age and gut bacteria contribute to multiple sclerosis disease progression

November 17, 2017
Researchers at Rutgers Robert Wood Johnson Medical School published a study suggesting that gut bacteria at young age can contribute to multiple sclerosis (MS) disease onset and progression.

Molecular guardian defends cells, organs against excess cholesterol

November 16, 2017
A team of researchers at the Harvard T. H. Chan School of Public Health has illuminated a critical player in cholesterol metabolism that acts as a molecular guardian in cells to help maintain cholesterol levels within a safe, ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

Nik_2213
not rated yet Jun 08, 2009
This may also offer a potential treatment for severe Anorexia--- Providing psychological issues are resolved, too...

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.