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Bacterial pathogens destroy the structural integrity of functional organs in fish, leading to severe challenges in the aquaculture industry. Vitamin D3(VD3) prevents bacterial infections and strengthens immune system function via vitamin D receptor (VDR). However, the correlation between VD3/VDR and the structural integrity of functional organs remains unclarified. This study aimed to investigate the influence of VD3supplementation on histological characteristics, apoptosis, and tight junction characteristics in fish intestine during pathogen infection. A total of 540 healthy grass carp (257.24 ± 0.63 g) were fed different levels of VD3(15.2, 364.3, 782.5, 1,167.9, 1,573.8, and 1,980.1 IU/kg) for 70 d. Subsequently, fish were challenged with Aeromonas hydrophila, a pathogen that causes intestinal inflammation. Our present study demonstrated that optimal supplementation with VD3(1) alleviated intestinal structural damage, and inhibited oxidative damage by reducing levels of oxidative stress biomarkers; (2) attenuated excessive apoptosis-related death receptor and mitochondrial pathway processes in relation to p38 mitogen-activated protein kinase signaling (P < 0.05); (3) enhanced tight junction protein expression by inhibiting myosin light chain kinase signaling (P < 0.05); and (4) elevated VDR isoform expression in fish intestine (P < 0.05). Overall, the results demonstrated that VD3alleviates oxidative injury, apoptosis, and the destruction of tight junction protein under pathogenic infection, thereby strengthening pathogen defenses in the intestine. This finding supports the rationale for VD3intervention as an essential practice in sustainable aquaculture.

The present study explored the impacts of Ile on muscle fatty acid and amino acid profiles, lipid metabolism, and autophagy in hybrid catfish. Seven isonitrogenous (387.8 g/kg protein) semi-purified diets were formulated to contain 5.0 (control), 7.5, 10.0, 12.5, 15.0, 17.5, and 20.0 g Ile/kg diet respectively. The fish (initial weight of 33.11 ± 0.09 g) were randomly assigned to 7 groups for a 56-day trial. Each group has 3 replicates with 30 fish per replicate, fed at 08:00 and 18:00 each day. Results showed that muscle protein and lipid, C14:0, C18:0, C22:0, C14:1, C18:1n-9, polyunsaturated fatty acid (PUFA), Arg, Ile, Ala, Cys, Gly, Tyr, essential amino acid (EAA), and total amino acid (TAA) contents and flavor amino acid (FAA)/TAA in muscle had positive linear and/or quadratic responses to dietary Ile levels (P < 0.05). Fatty acid synthase (FAS), stearoyl-CoA desaturase (SCD), acetyl-CoA carboxylase (ACC), and lipoprotein lipase (LPL) activities had positive linear and/or quadratic responses, but carnitine palmitoyl transferase 1 (CPT1) activity had a negative response with increasing dietary Ile levels (P < 0.05). The mRNA expressions of FAS, SCD, ACC, LPL, fatty acid binding protein 4 (FABP4), FATP1, sterol response element-binding protein 1c (SREBP-1c), sequestosome 1 (SQSTM1), and adenosine 5′-monophosphate-activated protein kinase (AMPK) had positive linear and/or quadratic responses to dietary Ile levels (P < 0.05). The mRNA expressions of hormone-sensitive lipase (HSL), CPT1, peroxisome proliferator-activated receptor α (PPARα), PPARγ, uncoordinated 51-like kinase 1 (ULK1), beclin1 (Becn1), autophagy-related protein 9α (Atg9α), Atg4b, Atg7, autophagy marker light chain 3 B (LC3B), and SQSTM1in muscle had negative linear and/or quadratic responses to dietary Ile levels (P < 0.05). The p-AMPK and ULK1 protein levels, and p-AMPK/AMPK were decreased by 12.5 g Ile/kg in the diet (P < 0.05). Finally, SQSTM1 protein level had the opposite effect (P < 0.05). The above results indicate that dietary Ile improves fish muscle fatty acid and amino acid profiles potentially via respectively regulating lipid metabolism and autophagy. The Ile requirement of hybrid catfish (33 to 72 g) were estimated to be 12.63, 13.77, 13.75, 11.45, 10.50, 12.53 and 12.21 g/kg diet based on the regression analysis of protein, lipid, SFA, PUFA, FAA, EAA, and TAA muscle contents, respectively.

Vitamin D3(VD3), an essential nutrient for animals, has been demonstrated to stimulate the uptake of certain amino acids. However, the role of VD3in the intestine, the primary site for digestion and absorption of nutrients, remains poorly characterized. Here, the grass carp (Ctenopharyngodon idella) was studied to assess the influence of different doses of VD3(15.2, 364.3, 782.5, 1,167.9, 1,573.8, and 1,980.1 IU/kg) on growth performance, intestinal morphology, digestive absorption, amino acid transport, and potential signaling molecule levels in a feeding experiment. As a result, dietary VD3improved growth performance, intestinal structure, and digestive and brush border enzyme activities. Additionally, most intestinal free amino acids and their transporters were upregulated after VD3intake, except for Ala, Lys, Asp, Leu, solute carrier (SLC) 7A7, SLC1A5, and SLC1A3mRNA in different segments, Leu and SLC6A14 mRNA in the proximal intestine, and SLC7A5mRNA in the mid and distal intestine. In the crucial target of rapamycin (TOR) signal pathway of amino acid transport, the gene and protein expression of TOR, S6 kinase 1, and activating transcription factor 4 were elevated, whereas 4E-binding protein 1 was decreased, further suggesting an advanced amino acid absorption capacity in the fish due to VD3supplementation. Based on percentage weight gain, feed efficiency, and trypsin activity, the VD3requirements of on-growing grass carp were estimated to be 968.33, 1,005.00, and 1,166.67 IU/kg, respectively. Our findings provide novel recommendations for VD3supplementation to promote digestion and absorption capacities of fish, contributing to the overall productivity of aquaculture.

Guanidine acetic acid (GAA) is a safe feed additive and the only precursor for the creatine synthesis of vertebrates. Aeromonas hydrophilais a major fish pathogen causing severe morbidity and mortality in the aquaculture sector. Under the current intensive farming system, A feed additive that can improve fish disease resistance is urgently needed. In teleost fish, the spleen (SP) and head kidney (HK) play a crucial role in peripheral immunity organs and fish skin (SK) serves as one of the most important mucosal barriers. Thus, the study focused on immune defense substances, inflammatory process, and mechanisms of molecular action in the HK, SP, and SK of on-growing grass carp (169.46–600.89 g). The fish were fed graded levels of GAA (0, 150, 300, 450 and 600 mg/kg) for 60 days, and then conducted a 14-day challenge experiment by intraperitoneal injection of Aeromonas hydrophila. In our results, we found that 300 or 450 mg/kg GAA could down-regulate the pro-inflammatory cytokines IL-(1β, 6, 12p40 and 17D), TNF-α and IFN-γ2 mRNA expression, and up-regulate the pro-inflammatory cytokines IL-(10, 11 and 4/13 A) and TGF-β1 mRNA expression in the HK, SP and SK. Furthermore, 300 or 450 mg/kg GAA could up-regulate the mRNA expression and promote the protein phosphorylation of STAT3 in the HK, SP and SK. Indicating that in the case of Aeromonas hydrophila.challenge, GAA could enhance the immune defense and mediated the inflammatory process, possibly via the JAKs-STATs signaling pathway in fish.