Muscle atrophy through thick bu...( During desperate times such as fasti...)

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 (www.jcb.org), 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 muscle 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.

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