Regulation of muscle gene expression in Crustacea over the moult cycle

Muscle growth in Crustacea may occur during specific stages of the moult cycle, focused around ecdysis when the old cuticle is shed and the new cuticle expands. In order to determine the moult stages in which sarcomeric proteins are synthesized and the regulatory factors involved, actin mRNA levels have been measured in the muscles of two crustaceans. These levels have been followed throughout the moult cycle and in response to passive stretch of walking leg muscle in vivo and to exogenous ecydsteroids applied to muscle preparations in vitro. Actin mRNA levels in both claw and leg muscles were elevated during the pre and postmoult stages of the moult cycle. However, varying patterns of expression are found in claw and leg muscle at specific stages of pre and postmoult. There was no increase in actin mRNA expression in extensor leg muscles in vitro after 6 hours exposure to elevated premoult levels of ecdysteroids. Immobilization of intemoult walking legs to maintain the extensor muscle in continuous passive stretch did not result in increased levels of actin mRNA after 5 days. These results are discussed in relation to the regulation of muscle growth over the moult cycle and to the molecular processes which may be responsible for controlling muscle protein synthesis.