Starvation beneficially influences the liver physiology and nutrient metabolism in Edwardsiella tarda infected red sea bream (Pagrus major)

Effects of Citrobacter freundii on sturgeon: Insights from skin mucosal immunology and microbiota

Skin mucus analysis has recently been used as a non-invasive method to evaluate for fish welfare. The present research study was conducted to examine the skin mucosal immunity and skin microbiota profiles of sturgeons infected with Citrobacter freundii. Our histology results showed that the thickness of the epidermal layer of skin remained thinner, and the number of mucous cells was significantly decreased in sturgeons after infection (p < 0.05). Total protein, alanine aminotransferase, aspartate aminotransferase, superoxide dismutase, and creatine kinase levels in the mucus showed biphasic pattern (decrease and then increase). Lactate dehydrogenase, lysozyme, and acid phosphatase activities in the mucus showed an increasing trend after infection. Furthermore, 16S rRNA sequencing also revealed that C. freundii infection also affected the diversity and community structure of the skin mucus microbiota. An increase in microbial diversity (p > 0.05) and a decrease in microbial abundance (p < 0.05) after infection were noted. The predominant bacterial phyla in the skin mucus were Proteobacteria, Fusobacteria, Bacteroidetes, Firmicutes, and Actinobacteria. Specifically, the relative abundance of Fusobacteria increased after infection. The predominant bacterial genera in the skin mucus were Cetobacterium, Pelomonas, Bradyrhizobium, Flavobacterium, and Pseudomonas. The relative abundance of Cetobacterium, Pseudomonas, and Flavobacterium increased after infection. Our current research findings will provide new insights into the theoretical basis for future research studies exploring the mechanism of sturgeon infection with C. freundii.

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.

The preferential utilization of hepatic glycogen as energy substrates in largemouth bass (Micropterus salmoides) under short-term starvation

To elucidate the underlying mechanism of the energy metabolism in largemouth bass (Micropterus salmoides), cultured fish (initial body weight: 77.57 ± 0.75 g) in the present study were starved for 0 h, 12 h, 24 h, 48 h, 96 h and 192 h, respectively. The proximate composition analysis showed that short-term starvation induced a significant up-regulation in crude protein proportion in hepatic of cultured fish (P < 0.05). However, short-term starvation significantly decreased the hepatosomatic index and the viscerosomatic index of cultured fish (P < 0.05). The exact hepatic glycogen content in the group starved for 92 h presented remarkable decrease (P < 0.05). Meanwhile, compared with the weight change of lipid and protein (mg) in hepatic (y = 0.0007x2 - 0.2827x + 49.402; y = 0.0013x2 - 0.5666x + 165.31), the decreasing trend of weight in glycogen (mg) was more pronounced (y = 0.0032x2 - 1.817x + 326.52), which suggested the preferential utilization of hepatic glycogen as energy substrates under short-term starvation. Gene expression analysis revealed that the starvation down-regulated the expression of insulin-like growth factor 1 and genes of TOR pathway, such as target of rapamycin (tor) and ribosomal protein S6 (s6) (P < 0.05). In addition, the starvation significantly enhanced expression of lipolysis-related genes, including hormone-sensitive lipase (hsl) and carnitine palmitoyl transferase I (cpt1), but down-regulated lipogenesis as indicated by the inhibited expression of fatty acids synthase (fas), acetyl-CoA carboxylase 1 (acc1) and acetyl-CoA carboxylase 2 (acc2) (P < 0.05). Starvation of 24 h up-regulated the expression of glycolysis genes, glucokinase (gk), phosphofructokinase liver type (pfkl) and pyruvate kinase (pk), and then their expression returned to the normal level. Meanwhile, the expression of gluconeogenesis genes, such as glucose-6-phosphatase catalytic subunit (g6pc), fructose-1,6-bisphosphatase-1 (fbp1) and phosphoenolpyruvate carboxy kinase (pepck), was significantly inhibited with the short-term starvation (P < 0.05). In conclusion, short-term starvation induced an overall decline in growth performance, but it could deplete the hepatic glycogen accumulation and mobilize glycogen for energy effectively.

Transcriptional regulation of the Japanese flounder Cu,Zn-SOD (Jfsod1) gene in RAW264.7 cells during oxidative stress caused by causative bacteria of edwardsiellosis

Edwardsiellosis is one of the most important bacterial diseases in fish, sometimes causing extensive economic losses in the aquaculture industry. Our previous studies demonstrated that the Cu,Zn-SOD (sod1) activity has significantly increased in Japanese flounder, Paralichthys olivaceus, hepatopancreas infected by causative bacteria of edwardsiellosis Edwardsiella tarda NUF251. In this study, NUF251 stimulated intracellular superoxide radical production in mouse macrophage RAW264.7 cells, which was reduced by N-acetylcysteine. This result suggests that NUF251 infection causes oxidative stress. To evaluate the regulatory mechanism of Jfsod1 at transcriptional levels under oxidative stress induced by NUF251 infection, we cloned and determined the nucleotide sequence (1124 bp) of the 5'-flanking region of the Jfsod1 gene. The sequence analysis demonstrated that the binding sites for the transcription factors C/EBPα and NF-IL6 involved in the transcriptional regulation of the mammalian sod1 gene existed. We constructed a luciferase reporter system with the 5'-flanking region (-1124/-1) of the Jfsod1 gene, and a highly increased transcriptional activity of the region was observed in NUF251-infected RAW264.7 cells. Further studies using several mutants indicated that deletion of the recognition region of NF-IL6 (-272/-132) resulted in a significant decrease in the transcriptional activity of the Jfsod1 gene in NUF251-infected RAW264.7 cells. In particular, the binding site (-202/-194) for NF-IL6 might play a major role in upregulating the transcriptional activity of the 5'-flanking region of the Jfsod1 gene in response to oxidative stress induced by NUF251 infection. These results could be provided a new insight to understand the pathogenic mechanism of causative bacteria of edwardsiellosis.

mitfa deficiency promotes immune vigor and potentiates antitumor effects in zebrafish

The mitfa gene is a well-known transcription factor associated with microphthalmia and is essential for early melanophore development. However, little is known about how mitfa affects the immune system. Here, we generated a novel mitfa knock-out zebrafish line using the CRISPR/Cas9 system. The mitfa-/- zebrafish exhibited reduced melanin levels compared to the nacre mutant. We investigated the impact on the immune system after exposure to Edwardsiella tarda and bifenazate in zebrafish larvae, and observed that the macrophage numbers were reduced in both treated groups. Remarkably, the expression levels of immune-related genes exhibited significant increases after bacterial challenge or bifenazate exposure in the mitfa-/- zebrafish, except for tlr4 and rela. Furthermore, we conducted xenograft experiments using mouse B16 melanoma cells. Notably, the cancer cells didn't show a high cell migration ratio, implying that the immune system was highly activated after the loss of mifta. Taken together, our findings suggest that mitfa-/- zebrafish serve as a valuable model for investigating the relationship between the immune system and melanocytes, providing new insights into the role of mitfa in immune responses.

Identification and characterization of a novel peptide from rainbow trout (Oncorhynchus mykiss) with antimicrobial activity against Streptococcus iniae

The overuse and misuse of antibiotics has led to the emergence of antibiotic-resistant bacterial species which remain a challenge to treat therapeutically. Novel and efficacious drugs are desperately needed to combat pathogens. One method to facilitate these discoveries is the use of in silico methods. Computational biology has the power to scan large data sets and screen for potential molecules with antibacterial function. In the current study, an in silico approach was used to identify an antimicrobial peptide (AMP) derived from rainbow trout von Willebrand Factor. The AMP was tested against a panel of aquatic bacterial pathogens and was found to possess antibacterial activity against Streptococcus iniae (S. iniae). Since S. iniae is a zoonotic pathogen, this may be useful in other species as well. The peptide was non-hemolytic and non-cytotoxic at the concentrations tested in rainbow trout cells. Pre-treatment of rainbow trout cells with the peptide did not result in an upregulation of immune genes but stimulating the rainbow trout macrophage/monocyte-like cell line, RTS11, with heat-killed S. iniae, did result in a significant upregulation of the tumor necrosis factor alpha (tnfa) gene. In this study, a new AMP has been identified but its expression, synthesis and role in vivo remains unknown. Nevertheless, the findings presented improve our understanding of fish gill and macrophage responses towards this important zoonotic pathogen.

Investigation of the Molecular Mechanisms of Antioxidant Damage and Immune Response Downregulation in Liver of Coilia nasus Under Starvation Stress

Commercial fishing of estuarine tapertail anchovy (Coilia nasus), an important anadromous fish species in the Yangtze River of China, has been prohibited due to the serious damage overfishing has caused to the wild population. Research regarding the energy metabolism is important for migratory fish to ensure the continuation of their existence. In this study, we performed, for the first time, a comparative transcriptome analysis of the liver of C. nasus subjected to long-term starvation stress. The results indicated that the damaging effects involved downregulation of the antioxidant capacity and immune response. The positive response to starvation involved upregulation of the anti-allergy and anticancer capacity, which supports the function of starvation in cancer inhibition, as has also been determined for human beings. This study revealed regulatory pathways, differentially expressed genes (DEGs), and mechanisms leading to damage of the liver in C. nasus affected by starvation. This research contributes information for the further study of the energy metabolism mechanism of C. nasus and provides a theoretical reference for starvation metabolism research of other fish species and even human beings.

Effects on hematological parameters, antioxidant and immune responses, AChE, and stress indicators of olive flounders, Paralichthys olivaceus, raised in bio-floc and seawater challenged by Edwardsiella tarda

Studies on the resistance of fish raised in bio-floc systems against bacterial infection are limited. We aimed to evaluate the changes in hematological parameters, antioxidant and immune responses, stress indicators, and acetylcholinesterase (AChE) in olive flounder, Paralichthys olivaceus, raised in bio-floc and seawater for 10 months and, then, infected with Edwardsiella tarda at concentrations of 0 (control), 6.61 × 10⁴, 6.61 × 10⁵, 6.61 × 10⁶, and 6.61 × 10⁷ CFU/g fish for 7 days. The lethal concentration 50% was 4.32 × 10⁷ in bio-floc and 3.11 × 10⁶ in seawater. Hematological parameters were significantly decreased by E. tarda challenge, and plasma components were significantly changed. The superoxide dismutase, catalase, and glutathione-S-transferase activities, as antioxidant responses, were significantly increased after infection, whereas the reduced glutathione level was significantly decreased. The lysozyme activity was significantly increased and the AChE level was significantly decreased after infection. Cortisol and HSP 70, as stress indicators, were also significantly increased. The results indicate that E. tarda infection affected various physiological factors in P. olivaceus. Additionally, P. olivaceus raised in seawater were more susceptible to E. tarda infection than those raised in bio-floc.

Effects of short-term fasting on the resistance of Nile tilapia (Oreochromis niloticus) to Streptococcus agalactiae infection

Short-term feed deprivation or fasting is commonly experienced by aquaculture fish species and may be caused by seasonal variations, production strategies, or diseases. To assess the effects of fasting on the resistance of Nile tilapia to Streptococcus agalactiae infection, vaccinated and unvaccinated fish were fasted for zero, one, three, and seven days prior to infection. The cortisol levels of both vaccinated and unvaccinated fish first decreased and then increased significantly as fasting time increased. Liver glycogen, triglycerides, and total cholesterol decreased significantly after seven days of fasting, but glucose content did not vary significantly between fish fasted for three and seven days. Hexokinase (HK) and pyruvate kinase (PK) activity levels were lowest after seven days of fasting, while phosphoenolpyruvate carboxykinase (PEPCK) activity levels varied in opposition to those of HK and PK. Serum superoxide dismutase (SOD) and catalase (CAT) activity levels first increased and then decreased as fasting time increased; SOD activity was highest after three days of fasting. Interleukin-1beta (IL-1β) and IL-6 mRNA expression levels first increased and then decreased significantly, peaking after three days of fasting. However, suppressor of cytokine signaling-1 (SOCS-1) mRNA expression levels were in opposition to those of IL-1β and IL-6. Specific antibody levels did not vary significantly among unvaccinated fish fasted for different periods. Although specific antibody level first increased and then decreased in the vaccinated fish as fasting duration increased, there were no significant differences in the survival rates of fasted vaccinated fish after challenge with S. agalactiae. The final survival rates of vaccinated fish fasted for zero, one, three, and seven days were 86.67 ± 5.44%, 80.00 ± 3.14%, 88.89 ± 6.28%, and 84.44 ± 8.32%, respectively. Among the unvaccinated fish, the survival rate was highest (35.56 ± 3.14%) in the fish fasted for three days and lowest (6.67 ± 3.14%) in the fish fasted for seven days. Therefore, our results indicated that short-term fasting (three days) prior to an infection might increase the resistance of unvaccinated Nile tilapia to S. agalactiae.

Effect of starvation and refeeding on oxidative stress and antioxidant defenses in Yangtze sturgeon (Acipenser dabryanus)

The present research aimed to evaluate the effects of long-term fasting and refeeding on the growth and antioxidant defenses in the liver and serum in Yangtze sturgeon (Acipenser dabryanus). The results showed that body mass and hepatosomatic index significantly decreased with long-term fasting, but they could be recovered after 4 weeks refeeding. Compared with controls, the antioxidant defense parameters of starvation indicated that the malondialdehyde (MDA) levels increased significantly in both tissues; the activities of superoxide dismutase (SOD) and glutathione peroxidase (GPx) increased obviously in serum and liver, respectively (p < 0.05). However, the activities of catalase (CAT) always decreased in two tissues including liver and serum during the whole starvation, as was the SOD in the liver (p < 0.05). Interestingly, the T-AOC levels of Yangtze sturgeon presented higher at early stage of starvation and dropped down at the end of starvation (p < 0.05). However, all of the antioxidant index above returned to origin level after 4 weeks refeeding. In conclusion, the present study indicated that long-time fasting induced oxidative stress in Yangtze sturgeon and it may easily adjust their physiological status under situations characterized by a long-term starvation and refeeding.

Molecular cloning, characterization and expression analysis of complement components in red sea bream (Pagrus major) after Edwardsiella tarda and red sea bream Iridovirus (RSIV) challenge

The complement system plays an important role in immune regulation and acts as the first line of defense against any pathogenic attack. To comprehend the red sea bream (Pagrus major) immune response, three complement genes, namely, pmC1r, pmMASP and pmC3, belonging to the classical, lectin and alternative complement cascade, respectively, were identified and characterized. pmC1r, pmMASP, and pmC3 were comprised of 2535, 3352, and 5735 base mRNA which encodes 732, 1029 and 1677 aa putative proteins, respectively. Phylogenetically, all the three studied genes clustered with their corresponding homologous clade. Tissue distribution and cellular localization data demonstrated a very high prevalence of all the three genes in the liver. Both bacterial and viral infection resulted in significant transcriptional alterations in all three genes in the liver with respect to their vehicle control counterparts. Specifically, bacterial challenge affected the pmMASP and pmC3 expression, while the viral infection resulted in pmC1r and pmC3 mRNA activation. Altogether, our data demonstrate the ability of pmC1r, pmMASP and pmC3 in bringing about an immune response against any pathogenic encroachment, and thus activating, not only one, but all the three complement pathways, in red sea bream.

Deletion of mstna and mstnb impairs the immune system and affects growth performance in zebrafish

Myostatin (Mstn) is a negative regulator of muscle development in vertebrates. Although its function in muscle growth has been well studied in mammals and fish, it remains unclear whether or how mstn functions in the immune system. In this study, mstna^-/- and mstnb^-/- homozygous zebrafish were firstly generated using CRISPR/Cas9 (Clustered regularly interspaced short palindromic repeats/CRISPR associated protein 9). Deletion of mstnb but not mstna enhanced growth performance. Although survival rates under normal conditions were slightly decreased in both strains, mortality after dexamethasone-induced stress was increased by ∼30%. Furthermore, transcriptional levels of several critical immune-related genes were decreased, and the ability to withstand exposure to pathogenic E. tarda was decreased, compared with that of controls. In mstnb^-/- but not mstna^-/- zebrafish, expression of NF-κB subunits and several pro-inflammatory cytokines failed to respond to E. tarda exposure except nfkb1, c-rel and tnfα. Taken together, these results indicate that mstnb but not mstna plays a key role in zebrafish muscle growth. While each paralogue contributes to the response to bacterial insult, mstnb affects the immune system through activation of the NF-κB pathway, and mstna is likely to act upstream of NF-κB at some as yet unidentified target.