Effects of prolonged fasting on levels of metabolites, oxidative stress, immune-related gene expression, histopathology, and DNA damage in the liver and muscle tissues of rainbow trout (Oncorhynchus mykiss)

Gut physiology of rainbow trout (Oncorhynchus mykiss) is influenced more by short-term fasting followed by refeeding than by feeding fishmeal-free diets

Supplementing a fishmeal-free diet with yeast extract improves rainbow trout (Oncorhynchus mykiss) growth performance and modulates the hepatic and intestinal transcriptomic response. These effects are often observed in the long term but are not well documented after short periods of fasting. Fasting for a few days is a common practice in fish farming, especially before handling the fish, such as for short sorting, tank transfers, and vaccinations. In the present study, rainbow trout were subjected to a 4-day fast and then refed, for 8 days, a conventional diet containing fishmeal (control diet) or alternative diets composed of terrestrial animal by-products supplemented or not with a yeast extract. During the refeeding period alone, most of the parameters considered did not differ significantly in response to the different feeds. Only the expression of claudin-15 was upregulated in fish fed the yeast-supplemented diet compared to the control diet. Conversely, fasting followed by refeeding significantly influenced most of the parameters analyzed. In the proximal intestine, the surface area of villi significantly increased, and the density of goblet cell tended to decrease during refeeding. Although no distinct plasma immune response or major signs of gut inflammation were observed, some genes involved in the structure, complement pathway, antiviral functions, coagulation, and endoplasmic reticulum stress response of the liver and intestine were significantly regulated by refeeding after fasting. These results indicate that short-term fasting, as commonly practiced in fish farming, significantly alters the physiology of the liver and intestine regardless of the composition of the diet.

Assessment of deltamethrin-induced DNA damage, neurotoxic and neuroimmune effects in the brain tissue of brown trout (Salmo trutta fario)

This study investigated the impact of deltamethrin (DM) toxicity on brown trout (Salmo trutta fario), examining its effects on the immune system, including the white blood cell (WBC), lymphocyte (Lym), total immunoglobulin (T. Ig), and lysozyme levels, as well as its neurotoxic consequences on the brain tissue. The neurotoxic effects encompassed oxidative stress, the activity of the antioxidant enzymes, such as the superoxide dismutase (SOD) and catalase (CAT), acetylcholinesterase (AChE) activity, and DNA damage using 8-hydroxy-2-deoxyguanosine (8-OHdG). The DM exposure led to elevated levels of malondialdehyde (MDA), and 8-OHdG, while concurrently causing a reduction in the AChE activity, protein and lipid content, WBC count, Lym, lysozyme activity, T. Ig levels, as well as the SOD and CAT levels in the brain tissues of groups 2 and 3 when compared to those in group 1. In summary, the findings of this study strongly indicate that DM induces DNA damage, immunotoxicity, and neurotoxicity in the brain tissue of brown trout, primarily due to the excessive production of reactive oxygen species (ROS). Moreover, the observed dose-dependent responses of DM to the environmental concentrations on all the investigated parameters suggest its potential utility in aquaculture risk assessment.

Metabolomics and Proteomics Responses of Largemouth Bass (Micropterus salmoides) Muscle under Organic Selenium Temporary Rearing

Organic selenium has been widely studied as a nutritional supplement for animal feed. However, there are few studies on the effect of organic selenium on flesh quality. In this study, the effects of organic selenium (yeast selenium (YS), Se 0.002 mg/L) on the metabolism and protein expression in Micropterus salmoides muscle under temporary fasting condition (6 weeks) were investigated. The muscle structure was observed through a microscope, and regulatory pathways were analyzed using proteomics and metabolomics methods. Electron microscopy showed that YS made the muscle fibers of M. salmoides more closely aligned. Differential analysis identified 523 lipid molecules and 268 proteins. The numbers of upregulated and downregulated proteins were 178 and 90, respectively, including metabolism (46.15%), cytoskeleton (11.24%) and immune oxidative stress (9.47%), etc. Integrated analyses revealed that YS enhanced muscle glycolysis, the tricarboxylic acid cycle and oxidative phosphorylation metabolism. In the YS group, the content of eicosapentaenoic acid was increased, and that of docosahexaenoic acid was decreased. YS slowed down protein degradation by downregulating ubiquitin and ubiquitin ligase expression. These results suggest that organic selenium can improve M. salmoides muscle quality through the aforementioned pathways, which provides potential insights into the improvement of the quality of aquatic products, especially fish.

Starvation Affects the Muscular Morphology, Antioxidant Enzyme Activity, Expression of Lipid Metabolism-Related Genes, and Transcriptomic Profile of Javelin Goby (Synechogobius hasta)

Fish in natural and cultivated environments can be challenged by starvation. However, inducing starvation in a controlled manner cannot only reduce feed consumption but also reduces aquatic eutrophication and even improves farmed fish quality. This study investigated the effects of starvation on the muscular function, morphology, and regulatory signaling in javelin goby (Synechogobius hasta) by evaluating the biochemical, histological, antioxidant, and transcriptional changes in the musculature of S. hasta subjected to 3, 7, and 14 days fasting. The muscle glycogen and triglyceride levels in S. hasta were gradually reduced under starvation, reaching their lowest at the end of the trial (P < 0.05). The levels of glutathione and superoxide dismutase were significantly elevated after 3-7 days of starvation (P < 0.05), but later returned to the level of the control group. The muscle of starved S. hasta developed structural abnormalities in some areas after 7 days of food deprivation, and more vacuolation and more atrophic myofibers were observed in 14-day fasted fish. The transcript levels of stearoyl-CoA desaturase 1 (scd1), the key gene involved in the biosynthesis of monounsaturated fatty acids, were markedly lower in the groups starved for 7 or more days (P < 0.05). However, the relative expressions of genes associated with lipolysis were decreased in the fasting experiment (P < 0.05). Similar declines in the transcriptional response to starvation were found in muscle fatp1 and ppar γ abundance (P < 0.05). Furthermore, the de novo transcriptome of muscle tissue from the control, 3-day and 14-day starved S. hasta generated 79,255 unigenes. The numbers of differentially expressed genes (DEGs) identified by pairwise comparisons among three groups were 3276, 7354, and 542, respectively. The enrichment analysis revealed that the DEGs were primarily involved in metabolism-related pathways, including ribosome, TCA pathway, and pyruvate metabolism. Moreover, the qRT-PCR results of 12 DEGs validated the expression trends observed in the RNA-seq data. Taken together, these findings demonstrated the specific phenotypical and molecular responses of muscular function and morphology in starved S. hasta, which may offer preliminary reference data for optimizing operational strategies incorporating fasting/refeeding cycles in aquaculture.

Chemical and biochemical responses to sub-lethal doses of mercury and cadmium in gilthead seabream (Sparus aurata)

Specimens of Sparus aurata were exposed to sub-lethal concentrations of Hg and Cd for 25 days and the levels of both metals were investigated in organs and tissues. Bioaccumulation of Hg decreased as follow: gills > kidney > liver > skin > muscle, while the order of Cd bioaccumulation was: liver > kidney > gills > skin > muscle. Immediately after exposure, both metals showed the highest bioaccumulation in gills and skin indicating that these organs are reliable targets for biomonitoring studies after short term exposure. Metals introduction caused a significant time-dependent concentrations increase in kidney and liver, while in the muscle a significant increase of Hg was recorded only at the end of the experimentation. The effects of exposure were also investigated, at biochemical level, in the liver, which represents the main target of xenobiotics biotransformation and metabolism in fish. Exposed fishes exhibited a reduction of total lipid level, a decrease of polyunsaturated fatty acids (PUFA), together with a MDA increase. This suggests a direct effect of contaminants on oxidative stress induction that, through the MDA increase, altered the membrane fatty acids composition decreasing the PUFA content. As it regards molecular markers related to oxidative stress and lipid metanolism, a significant increase of Nrf2, Hif-1α and Ampk and a decrease of Fas were observed after exposure to both metals, while an Nf-kB increase was recorded in specimens exposed to Hg, docuemnting a correlation with oxidative stress and consequent metabolism adaptation. Finally, these results suggest the possibility to adopt these biomarkers to explore fish metabolic responses to environmental pollution.

Transcriptome Reveals the Effects of Early Weaning on Lipid Metabolism and Liver Health of Yangtze Sturgeon (Acipenser dabryanus)

The Yangtze sturgeon (Acipenser dabryanus) has recently been declared extinct in the wild, and artificial breeding is the only means to protect its germplasm resources, but it has difficulty in weaning (from live prey to artificial food). In this study, we first performed a histological observation, enzyme-activity determination, and transcriptome sequencing on the livers of juvenile Yangtze sturgeons, and we then cloned five critical genes of lipid metabolism according to the transcriptome-sequencing results. We designed a weaning experiment to analyze their expression levels during weaning. The results showed that the density of hepatocytes and the transaminase activity of the juveniles failed to wean. The differentially expressed genes were enriched significantly in the pathways involving steroid synthesis, amino acid metabolism, and pancreatic secretion. It was found that the mRNA level of the fatty acid-synthesis gene decreased, and the mRNA level of the lipolysis gene increased significantly during weaning. The results of this research indicated that weaning could affect the liver health of Yangtze sturgeon, and it could affect the liver lipid metabolism by inhibiting fatty acid synthesis and promoting lipolysis. This study enhances our understanding of the impact of weaning on the lipid metabolism in fish.

Exploring the impacts of different fasting and refeeding regimes on Nile tilapia (Oreochromis niloticus L.): growth performance, histopathological study, and expression levels of some muscle growth-related genes

The current study investigated how different fasting and refeeding regimes would impact Nile tilapia growth performance, histopathological examination, and gene expression of myostatin, myogenin, GH, IGF-1, and NPYa. Nile tilapia fish (n = 120) were randomly allocated into four groups, including the control group fed on a basal diet for 6 weeks (F6), group A starved for 1 week and then refed for 5 weeks (S1F5), group B starved for 2 weeks and then refed for 4 weeks (S2F4), while group C starved for 4 weeks and then refed for 2 weeks (S4F2). Fasting provoked a decrease in body weight coincided with more extended starvation periods. Also, it induced muscle and liver histological alterations; the severity was correlated with the length of fasting periods. Gene expression levels of GH, MSTN, MYOG, and NPYa were significantly increased, while IGF1 was markedly depressed in fasted fish compared to the control group. Interestingly, refeeding after well-planned short fasting period (S1F5) modulated the histopathological alterations. To some extent, these changes were restored after refeeding. Restored IGF-I and opposing fasting expression profiles of the genes mentioned above thus recovered weights almost like the control group and achieved satisfactory growth compensation. Conversely, refeeding following more extended fasting periods failed to restore body weight. In conclusion, refeeding after fasting can induce a compensatory response. Still, the restoration capacity is dependent on the length of fasting and refeeding periods through exhibiting differential morphological structure and expressions pattern for muscle and growth-related genes.