Effects of feeding level and sexual maturation on expression of genes regulating growth mechanisms in rainbow trout (Oncorhynchus mykiss)

The energy and nutrient demands of sexual maturation in salmonids requires a shift from somatic growth to gonadal development, although a loss of skeletal muscle can be avoided with adequate nutrition. How nutrient availability or ration affects the endocrine and growth factor systems that integrate nutrient partitioning, muscle metabolism, and growth during sexual maturation is poorly understood. Primary objectives of the current study were to determine how sexual maturation and ration affect expression of estrogen receptors, components of the GH/IGF axis and TGF8 superfamily system, and muscle specific regulatory factors during gonadal development in female rainbow trout (Oncorhynchus mykiss). Female diploid rainbow trout approaching ovulation were fed for 12 weeks at 0.25% and 0.50% tank biomass, and to apparent satiation. Controls for the effects of gonadal development included sterile triploid trout and two diploid females that failed to undergo gonadal development. Expression of era1 was increased in muscle of maturing diploids suggesting an increase in the catabolic effects of 178-estradiol (E2) on muscle. In the liver, expression of era2 increased with maturation, but expression of all estrogen receptors were increased with higher rations suggesting improved E2 sensitivity for vitellogenin pro-duction. Hepatic expression of ghrs and igfs were generally reduced with maturation suggesting a reduction in systemic growth promotion. Increased ration improved capacity for IGF signaling in muscle via increased igf1 and igfr1b expression, potentially lessening effects of reduced circulating IGF-I on muscle growth. The primary carrier of IGFs, hepatic igfbp2b1, increased with maturation likely reducing systemic IGF activity, and was also elevated with higher rations. All other hepatic IGF binding proteins, except igfbp1, showed some reduction with maturation and several increased with increased ration. Increased muscle igfbp6b1 suggests restricted myogenesis with maturation. Reductions in expression of muscle regulatory factors with maturation including pax7a, myog and myf5, support reduced myogenesis with maturation but were unaffected by ration. In addition, although mstn2a was the only myostatin gene affected by maturation or ration, altered expression of components of the TGF8 superfamily suggest increased ration may inhibit myostatin signaling. In summary, moderate feeding ap-pears to counter many changes in endocrine and growth factor system gene expression associated with growth reduction during maturation, perhaps altering endocrine and growth factor regulation to spare muscle atrophy and better prepare the fish for post-spawning recovery.