Molecular characterization of muscle atrophy and proteolysis associated with spawning in rainbow trout

Severe muscle deterioration is a physiological response to the energetic demands of fish spawning. This response represents a suitable model to study mechanisms of muscle degradation in fish where typical tetrapod methods, such as muscle unloading, are not applicable. Enzyme activities and mRNA accumulations of genes in major proteolytic pathways, including cathepsins, calpains and the multi-catalytic proteasome, were measured in white muscles of rainbow trout during spawning and post-spawning seasons of gravid fish for comparisons to sterile fish. Fertile fish at spawning had less muscle tissue and less muscle protein compared to sterile fish and post-spawning fertile fish. Muscle deterioration of the fertile fish during spawning was associated with greater mRNA accumulation and elevated activity of cathepsin-L. Concurrently, muscle of spawning fish showed increased mRNA accumulations of cathepsin-D, the calpain regulatory subunit and the proteasome catalytic subunit alpha without corresponding increases in enzyme activities. In addition, elevated activity and increased mRNA accumulation of caspase-9, but not caspase-3, were observed in fertile fish during spawning. This study indicates that cathepsins mediate protein catabolism during spawning in rainbow trout and the catabolic process may involve activation of the apoptosis mediator, caspase-9, but not the apoptosis executioner, caspase-3.