Atrophy of Crustacean Somatic Muscle and the Proteinases that do the Job. A Review

As much as 60% of the muscle in the propodus of both chelae of some brachyurans atrophies during proecdysis and is restored during metecdysis. We describe here the structural changes that occur during atrophy of such chelae muscles as well as atrophy of somatic muscle from a number of species of Crustacea in response to one or more of several physiological stimuli. Atrophy is not random and whole fibers are not lost; the remaining cellular structures provide a framework into which newly synthesized myofibrillar proteins are packed during metecdysis. As seen in electron micrographs as well as in gel electropherograms, more thin filament proteins (actin, troponin, and tropomyosin) are degraded when compared to those from thick filaments (chiefly myosin). We also describe the properties of cytosolic proteinases isolated from muscle of land crabs and lobsters that degrade the myofibrillar proteins. Four of the five proteinases are activated by calcium and therefore are designated CDPs (calcium dependent proteinases). In addition to the CDPs, we summarize information about the first MCP (multicatalytic proteinase) to be isolated and characterized from Crustacea. Atrophy occurs only in chelae and not in walking legs during proecdysis; atrophy is twice as extensive in the major chela as in the minor chela of fiddler crabs. Clearly, the controls of this selective atrophy are fine-tuned. Some possible mechanisms that might provide the controls that determine which muscle fibers and which of their components are targeted are also discussed.