Crustaceans must shed their shells periodically in order to grow, a process called molting. Many crabs and lobsters have large claws, which must be pulled through the small joints that connect the appendage to the body when the animal molts. A reduction, or atrophy, in the claw muscle facilitates the withdrawal of the claws at molt. The PI hypothesizes that myostatin (Mstn), a negative regulator of muscle growth in mammals, mediates claw muscle atrophy that is under the control of steroid molting hormones (ecdysteroids). The specific aims of the project are to (1) determine the regulation of Mstn/Smad signaling by ecdysteroid in the land crab and (2) determine the interaction of fiber type and ecdysteroid on claw muscle atrophy in lobster. The experiments on land crab take advantage of their ability to manipulate the molt cycle and to culture claw muscle. The experiments on lobster take advantage of their ability to quantify mRNAs and proteins in muscles that differ in fiber type composition in the dimorphic claws. An advantage of both species is that only the claw muscles are responsive to the atrophy-inducing signal. Muscles that do not respond to ecdysteroid (thoracic muscle in land crab and abdominal muscles in lobster) serve as internal controls, as they are exposed to the same hormonal environment. The discovery of Mstn in crustacean muscles suggests that the regulation of muscle mass is highly conserved between vertebrates and invertebrates. It constitutes an entirely novel and exciting area of investigation. As crustacean muscles have few stem cells, they are an ideal model to determine the direct effects of Mstn on differentiated fibers in adults and therefore can provide insights into Mstn action in humans. Understanding the signaling pathway(s) regulating skeletal muscle mass could have significant impacts on crustacean aquaculture. For example, blocking Mstn signaling could enhance growth and improve product quality. The project will provide training in modern biochemical and molecular technologies for a Native American postdoctoral fellow, 2 graduate students, and 4-6 undergraduates.
Crabs have a pair of claws used for capturing prey and defense. The large claws give the animal a competitive advantage, but make it more difficult to extricate the claws when the animal molts. The solution to this problem is a large reduction (as much as 78%) in the claw closer muscle as the animal prepares to molt, which allows the claw to pass through the small basal segments at molt. The process is reversed after the animal is free of the old shell. Muscle growth during the postmolt period fills the enlarged space created by expansion of the new shell. Thus, the claw muscle undergoes a sequential atrophy and growth each time the animal molts. The ability of skeletal muscle to change in size is not unique to crustaceans. In mammals, skeletal muscle is altered by a variety of physiological conditions. We found that crustaceans use the same mechanisms for controlling protein synthesis and degradation as mammals. Myostatin, an inhibitor of muscle growth, is expressed in crustacean muscle. Moreover, crustacean muscles express mTOR, which is a highly conserved protein that controls protein synthesis in all animal cells. Enzymes that degrade muscle proteins are up-regulated in atrophic muscle. This research has provided new insights into the function of myostatin and mTOR in skeletal muscles.
