Transcriptome mining of apoptotic mechanisms in response to density and functional diets in Oncorhynchus mykiss and role in homeostatic regulation.

Husbandry conditions often expose fish to several stressors, compromising organism's global homeostasis, which has consequences on aquaculture production. In order to depict intestinal homeostatic responses to deleterious conditions and the potential beneficial effects of functional diets in aquaculture fish, the effects of chronic suboptimal density condition on the neuro-immune endocrine system of rainbow trout was investigated through a transcriptomic survey. After 30 days under high stocking density, 67% of genes involved in homeostasis were found down-regulated whereas when fed with a diet supplemented with the probiotic Saccharomyces cerevisiae, <1% of these genes were found down-regulated. Genes involved in apoptotic mechanisms were found to be particularly responsive to both density and diets. At high density, several genes involved in the intrinsic and extrinsic apoptotic pathway (Fadd, Fas, Bcl-2 and Bax) as well as caspases (Casp8 and Casp3) were found down-regulated (Fold change <-7). However, fish fed with a diet supplemented with the probiotic yeast had an opposite response, indicating mitigation of stress effects. Overall, these results indicate the activation of homeostatic machinery and particularly genes involved in the apoptotic pathway as a result of probiotic feeding, counteracting apoptotic molecular repression derived from chronic stress.