Published data and the results of the authors’ own studies on the role of intracellular proteolytic enzymes and the metabolic and signaling processes regulated by these enzymes at certain stages of growth and development of salmonid fishes are analyzed in the present review. The major pathways of intracellular proteolysis relying on autophagy, proteasome activity, and calpain activity are considered, as well as the relative contribution of these pathways to proteolysis in skeletal muscle of the fish. Skeletal muscle accounts for more than half of the weight of the fish and undergoes the most significant changes due to the action of anabolic and catabolic signals. Special attention is paid to the intensity of protein degradation during the active growth period characterized by a high rate of protein synthesis and metabolism in fish, as well as to protein degradation during the reproductive period characterized by predomination of catabolic processes in contrast to the growth period. Skeletal muscle plays a unique role as a source of plastic and energy substrates in fish, and, therefore, the process of muscle protein degradation is regarded as a key mechanism for the regulation of growth intensity in juvenile salmon and for maintenance of viability and reproductive capacity of salmonid fish during the maturation of gametes, starvation, and migration related to spawning. The possibility of using a set of parameters of intracellular proteolysis to characterize the early development of salmonids is demonstrated in the review.

The review summarizes the literature and our own experimental data on the features of the structure, function, and protein metabolism in skeletal muscles of teleost fish (Teleostei). Despite the conserved basic mechanisms of muscle growth (myogenesis) and degradation in vertebrates, fish are characterized by unique features related to their poikilothermy, indeterminate growth, and a special role of skeletal muscle as a depot of plastic and energy substrates. Fish skeletal muscles show high plasticity in terms of their ability to undergo substantial anabolic or catabolic changes in response to environmental variables, such as temperature, photoperiod, and food availability. Under optimal (anabolic) conditions, fish muscle tissue grows by hypertrophy and hyperplasia at an extremely high rate, while during the periods of high energy demand, including migration, starvation, and gonad maturation, catabolism of skeletal muscle proteins temporarily dominates. However, degradation of fish muscle tissue can be profound enough to exceed its regenerative capacity: both genetic programs and responses to exogenous signals of excessive strength and duration can be implemented via this scenario. An extreme and demonstrative example of the mobilization of muscle protein reserves and utilization of resulting amino acids for energy production and synthesis of stage-specific gonadal proteins are Pacific salmonids whose exhaustion during spawning is so great that results in their death. Fish myopathies and the potential of fish as objects for human disease modeling are also considered.

Results of studies presented in recent papers and personal data related to investigation of structure, classification, phylogeny of calcium-dependent peptidases or calpains have been analyzed. The most extensively studied functions of calpains in cell activity have been examined. Some not yet resolved questions concerned with the biological role of a great number of proteins of the calpain family have been defined.