SUMMARY In decapod crustaceans, claw muscle undergoes atrophy in response to elevated ecdysteroids while thoracic muscle undergoes atrophy in response to unweighting. The signaling pathways that regulate muscle atrophy in crustaceans are largely unknown. Myostatin is a negative regulator of muscle growth in mammals, and a myostatin-like cDNA is preferentially expressed in muscle of the land crab, Gecarcinus lateralis (Gl-Mstn) . Contrary to prediction, levels of Gl-Mstn mRNA decreased dramatically in both the claw closer and weighted thoracic muscles when molting was induced by either eyestalk ablation (ESA) or multiple limb autotomy (MLA). However, the effect of molt induction was greater in the claw muscle. By late premolt, Gl-Mstn mRNA in the claw muscle decreased 81% and 94% in ESA and MLA animals, respectively, and was negatively correlated with ecdysteroids. Gl-Mstn mRNA in thoracic muscle decreased 68% and 82% in ESA and MLA animals, respectively, but was only weakly correlated with ecdysteroid. Claw and thoracic muscles also differed to varying extents in the expression of ecdysteroid receptor ( Gl-EcR and Gl-RXR ), elongation factor-2 ( Gl-EF-2 ), and calpain T ( Gl-CalpT ) in response to molt induction, but levels of the four transcripts were not correlated with ecdysteroid. The downregulation of Gl-Mstn expression in premolt claw muscle coincided with 11- and 13-fold increases in protein synthesis in the myofibrillar and soluble protein fractions, respectively. Furthermore, the rate of the increase in the synthesis of soluble proteins was greater than that of myofibrillar proteins during early premolt (1.4:1, soluble:myofibrillar), but the two were equivalent during late premolt. By contrast, Gl-Mstn mRNA increased 3-fold and Gl-CalpT mRNA decreased 40% in unweighted thoracic muscle; there was little or no effect on Gl-EF-2, Gl-EcR , and Gl-RXR mRNA levels. These data indicate that Gl-Mstn expression is negatively regulated by both ecdysteroids and load-bearing contractile activity. The downregulation of Gl-Mstn in claw muscle may induce the elevated protein turnover associated with remodeling of the contractile apparatus during molt-induced atrophy. The upregulation of Gl-Mstn in unweighted thoracic muscle suggests that this factor is also involved in disuse atrophy when hemolymph ecdysteroid levels are low.

SUMMARY In the crayfish and lobster opener neuromuscular preparations of the walking legs and claws, there are regional differences in synaptic transmission even though the entire muscle is innervated by a single excitatory tonic motor neuron. The innervation of the proximal fibres produced larger excitatory postsynaptic potentials (EPSPs) than those of the central fibres. The amplitudes of the EPSPs in the distal fibres were intermediate between those of the proximal and central regions. These differences in EPSP amplitudes were correlated with differences in short-term facilitation between the three regions. When given a 10- or 20-pulse train of stimuli, the proximal fibres showed greater short-term facilitation initially, often followed by a maximization of short-term facilitation towards the end of a train. In contrast, the central fibres showed a linear increase in short-term facilitation throughout a stimulus train. The distal fibres showed intermediate short-term facilitation compared with the other two regions. Analysis of myofibrillar isoforms showed that levels of troponin-T 1 (TnT 1 ), a 55 kDa isoform expressed in slow-tonic (S 2 ) fibres, were correlated with synaptic properties. Proximal fibres had the highest levels of TnT 1 , with lower levels in distal fibres; central fibres lacked TnT 1 , which is characteristic of slow-twitch (S 1 ) fibres. In addition, differences in troponin-I isoforms correlated with TnT 1 levels between the proximal, central and distal regions. The correlation between slow fibre phenotype and strength of innervation suggests a relationship between synaptic structure and expression of troponin isoforms.