In fish, chronic stress causes the release of the stress hormone cortisol into the bloodstream, which in turn triggers changes in gene transcription that lead to the breakdown of skeletal muscle. Ultimately, such stress-related transcriptional changes restrict growth, disrupt the immune system and hamper normal behaviors such as eating and swimming, which is problematic in aquaculture facilities, where fish are often stressed as a result of crowding. Although the effects of stress on fish muscle growth are well established, the specific molecular mechanisms at play remain unclear. Cristián Valenzuela of the Universidad Andres Bello and the Interdisciplinary Center for Aquaculture Research in Chile, along with colleagues in Chile and at the University of Gothenburg in Sweden, wanted to learn more about how chronic stress affects various levels of the stress response in the fine flounder.
To perform the multi-level stress assessments, the researchers first exposed juvenile flounders to chronic stress by crowding them at two different densities (low and high) for 4 or 7 weeks. After the crowding periods, the group analyzed overall growth (length, mass and condition factor) of the fish and the concentrations of cortisol, growth hormone and growth factors in their blood. The scientists also took muscle samples from the flounders to determine expression levels of genes involved in stress, growth and atrophy processes and to measure quantities of molecules critical to two separate protein-breakdown pathways: one that marks proteins for degradation using a special targeting molecule and another that breaks down and recycles cellular components. The researchers also measured molecules involved in processes that induce cell death.
After 4 weeks of crowding, the group found that flounders in the more-crowded group had higher levels of stress hormones in their blood, as well as greater expression of certain members of the stress pathway in their muscles, than their less-crowded counterparts. In addition, higher crowding levels also changed growth hormone levels in the blood and decreased expression of genes and molecules involved in fish growth, while simultaneously stimulating both targeted protein degradation and induced cell death mechanisms. After 7 weeks of crowding, plasma cortisol levels were no longer elevated in the more-crowded fish, but some components of the stress pathway were still upregulated in their muscles, and growth processes remained inhibited. In addition, the intracellular recycling pathway was activated in the more-crowded fish after 7 weeks, while the targeted protein degradation pathway and induced cell death processes were no longer stimulated. On a whole-animal level, 7 weeks of stress significantly limited the mass gain and body condition of the flounders that were severely overcrowded in contrast to those kept at low densities.
In all, this study reveals that the negative influence of chronic stress on fish growth is linked to specific molecular mechanisms, including the independent pathways involved in targeted protein breakdown and intracellular recycling. The results also suggest that the relative involvement of these mechanisms may change with the duration of stress. Such findings may ultimately help develop effective aquaculture methods by explaining why stressed flatfish won't be fat fish.
