In mammals, feeding promotes protein accretion in skeletal muscle through a stimulation of the insulin- and amino acid- sensitive mammalian target of rapamycin (mTOR) signaling pathway, leading to the induction of mRNA translation. The purpose of the present study was to characterize both in vivo and in vitro the activation of several major kinases involved in the mTOR pathway in the muscle of the carnivorous rainbow trout. Our results showed that meal feeding enhanced the phosphorylation of the target of rapamycin (TOR), PKB, p70 S6 kinase, and eIF4E-binding protein-1, suggesting that the mechanisms involved in the regulation of mRNA translation are well conserved between lower and higher vertebrates. Our in vitro studies on primary culture of trout muscle cells indicate that insulin and amino acids regulate TOR signaling and thus may be involved in meal feeding effect in this species as in mammals. In conclusion, we report here for the first time in a fish species, the existence and the nutritional regulation of several major kinases involved in the TOR pathway, opening a new area of research on the molecular bases of amino acid utilization in teleosts.

Whole-genome duplication (WGD) was experienced twice by the vertebrate ancestor (2 rounds; 2R), again by the teleost fish ancestor (3R) and most recently in certain teleost lineages (4R). Consequently, vertebrate gene families are often expanded in 3R and 4R genomes. Arguably, many types of “functional divergence” present across 2R gene families will exceed that between 3R/4R paralogs of genes comprising 2R families. Accordingly, 4R offers a form of replication of 2R. Examining whether this concept has implications for molecular evolutionary research, we studied insulin-like growth factor (IGF) binding proteins (IGFBPs), whose six 2R family members carry IGF hormones and regulate interactions between IGFs and IGF1-receptors (IGF1Rs). Using phylogenomic approaches, we resolved the complete IGFBP repertoire of 4R-derived salmonid fishes (19 genes; 13 more than human) and established evolutionary relationships/nomenclature with respect to WGDs. Traits central to IGFBP action were determined for all genes, including atomic interactions in IGFBP–IGF1/IGF2 complexes regulating IGF–IGF1R binding. Using statistical methods, we demonstrate that attributes of these protein interfaces are overwhelming a product of 2R IGFBP family membership, explain 49–68% of variation in IGFBP mRNA concentration in several different tissues, and strongly predict the strength and direction of IGFBP transcriptional regulation under differing nutritional states. The results support a model where vertebrate IGFBP family members evolved divergent structural attributes to provide distinct competition for IGFs with IGF1Rs, predisposing different functions in the regulation of IGF signaling. Evolution of gene expression then acted to ensure the appropriate physiological production of IGFBPs according to their structural specializations, leading to optimal IGF-signaling according to nutritional-status and the endocrine/local mode of action. This study demonstrates that relatively recent gene family expansion can facilitate inference of functional evolution within ancient genetic systems.

The insulin-like growth factor (Igf) binding protein (Igfbp) family has a broad range of physiological functions and a fascinating evolutionary history. This review focuses on the Igfbps of teleost fishes, where genome duplication events have diversified gene repertoire, function, and physiological regulation—with six core Igfbps expanded into a family of over twenty genes in some lineages. In addition to briefly summarizing the current state of knowledge on teleost Igfbp evolution, function, and expression-level regulation, we highlight gaps in our understanding and promising areas for future work.

Calpains are non-lysosomal calcium-activated neutral proteases involved in a wide range of cellular processes including muscle proteolysis linked to post-mortem flesh softening. The aims of this study were (a) to characterise several members of the calpain system in gilthead sea bream and (b) to examine their expression in relation to nutritional status and muscle tenderisation. We identified the complete open reading frame of gilthead sea bream calpains1-3, sacapn1, sacapn2, sacapn3, and two paralogs of the calpain small subunit1, sacapns1a and sacapns1b. Proteins showed 63–90% sequence identity compared with sequences from mammals and other teleost fishes, and the characteristic domain structure of vertebrate calpains. Transcripts of sacapn1, sacapn2, sacapns1a and sacapns1b had a wide tissue distribution, whereas sacapn3 was almost exclusively detected in skeletal muscle. Next, we assessed transcript expression in skeletal muscle following alteration of nutritional status by (a) fasting and re-feeding or (b) feeding four experimental diets with different carbohydrate-to-protein ratios. Fasting significantly reduced plasma glucose and increased free fatty acids and triglycerides, together with a significant increase in sacapns1b expression. Following 7 days of re-feeding, plasma parameters returned to fed values and sacapn1, sacapn2, sacapns1a and sacapns1b expression was significantly reduced. Furthermore, an increase in dietary carbohydrate content (11 to 39%) diminished growth but increased muscle texture, which showed a significant correlation with decreased sacapn1 and sacapns1a expression, whilst the other calpains remained unaffected. This study has demonstrated that calpain expression is modulated by nutritional status and diet composition in gilthead sea bream, and that the expression of several calpain members is correlated with muscle texture, indicating their potential use as molecular markers for flesh quality in aquaculture production.