Ubiquitin and actin expression in claw muscles of land crab, Gecarcinus lateralis, and american lobster, Homarus americanus: Differential expression of ubiquitin in two slow muscle fiber types during molt-induced atrophy
International Association of Astacology
Faculty of Computing, Health and Science
School of Natural Sciences
The closer muscle of large-clawed decapod crustaceans undergoes a proecdysial (premolt) atrophy to facilitate withdrawal of the appendage at ecdysis. This atrophy involves the activation of both calcium-dependent (calpains) and ubiquitin (Ub)/proteasome-dependent proteolytic systems that break down proteins to reduce muscle mass. Moreover, the large slow-twitch (S1) fibers undergo a greater atrophy than the small slow-tonic (S2) fibers. Both polyUb mRNA and Ub-protein conjugates increase during claw muscle atrophy. In this study in situ hybridization and RT-PCR were used to determine the temporal and spatial expression of polyUb and α-actin. A cDNA encoding the complete sequence of lobster muscle α-actin was characterized; a probe synthesized from the cDNA provided a positive control for optimizing RT-PCR and in situ hybridization. PolyUb was expressed at low levels in claw closer muscle from anecdysial (intermolt) land crab. By early proecdysis (premolt; stage D0), polyUb mRNA levels increased in medial fibers that insert along the midline of the apodeme, with greater expression in S1 than S2, while levels remained low in peripheral fibers. By late proecdysis, polyUb mRNA decreased in central fibers, while mRNA increased in peripheral S1 fibers. In contrast, α-actin was expressed in lobster claw muscles at relatively constant levels during the intermolt cycle. These results suggest that Ub/proteasome-dependent proteolysis contributes to enhanced turnover of myofibrillar proteins during claw closer muscle atrophy. Furthermore, atrophy is not synchronous within the muscle; it begins in medial fibers and then progresses peripherally.