Transcriptome analysis reveals the host immune response upon LMBV infection in largemouth bass (Micropterus salmoides)

Protective efficacy and immune responses of largemouth bass (Micropterus salmoides) immunized with an inactivated vaccine against the viral hemorrhagic septicemia virus genotype IVa

Viral hemorrhagic septicemia virus (VHSV) poses a significant threat to the aquaculture industry, prompting the need for effective preventive measures. Here, we developed an inactivated VHSV and revealed the molecular mechanisms underlying the host's protective response against VHSV. The vaccine was created by treating VHSV with 0.05% formalin at 16°C for 48 h, which was determined to be the most effective inactivation method. Compared with nonvaccinated fish, vaccinated fish exhibited a remarkable increase in survival rate (99%) and elevated levels of serum neutralizing antibodies, indicating strong immunization. To investigate the gene changes induced by vaccination, RNA sequencing was performed on spleen samples from control and vaccinated fish 14 days after vaccination. The analysis revealed 893 differentially expressed genes (DEGs), with notable upregulation of immune-related genes such as annexin A1a, coxsackievirus and adenovirus receptor homolog, V-set domain-containing T-cell activation inhibitor 1-like, and heat shock protein 90 alpha class A member 1 tandem duplicate 2, indicating a vigorous innate immune response. Furthermore, KEGG enrichment analysis highlighted significant enrichment of DEGs in processes related to antigen processing and presentation, necroptosis, and viral carcinogenesis. GO enrichment analysis further revealed enrichment of DEGs related to the regulation of type I interferon (IFN) production, type I IFN production, and negative regulation of viral processes. Moreover, protein-protein interaction network analysis identified central hub genes, including IRF3 and HSP90AA1.2, suggesting their crucial roles in coordinating the immune response elicited by the vaccine. These findings not only confirm the effectiveness of our vaccine formulation but also offer valuable insights into the underlying immunological mechanisms, which can be valuable for future vaccine development and disease management in the aquaculture industry.

The antiviral role of largemouth bass STING against iridovirus infection

Stimulator of interferon gene (STING) plays a crucial role in the innate immune response against viral and bacterial pathogens. However, its function in largemouth bass iridovirus (LMBV) infection remains uncertain. Here, a STING homolog (MsSTING) from largemouth bass (Micropterus salmoides) was cloned and characterized. MsSTING encoded a 407-amino-acid polypeptide, which shared 84.08% and 41.45% identity with golden perch (Perca flavescens) and human (Homo sapiens) homologs, respectively. MsSTING contained four transmembrane domains and a conserved C-terminal domain. The mRNA level of MsSTING was significantly increased in response to LMBV infection in vitro. Subcellular localization observation indicated that MsSTING encoded a cytoplasmic protein, which co-localized predominantly with endoplasmic reticulum (ER) and partially with mitochondria. Moreover, its accurate localization was dependent on the N-terminal transmembrane motif (TM) domains. MsSTING was able to activate interferon (IFN) response, evidenced by the activation of IFN1, IFN3 and ISRE promoters by its overexpression in vitro. Mutant analysis showed that both the N-terminal and C-terminal domain of MsSTING were essential for its activation on IFN response. In addition, overexpression of MsSTING inhibited the transcription and protein levels of viral core genes, indicating that MsSTING exerted antiviral action against LMBV. Consistently, the inhibitory effects were significantly attenuated when the N-terminal or C-terminal domains of MsSTING was deleted. Furthermore, MsSTING overexpression upregulated the transcriptions of interferon-related genes and pro-inflammatory factors, including TANK-binding kinase 1(TBK1), interferon regulatory factor 3 (IRF3), interferon regulatory factor 7 (IRF7), interferon stimulated exonuclease gene 20 (ISG20), interferon-induced transmembrane protein 1(IFITM1), interferon γ (IFN-γ), tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and interleukin 6 (IL-6). Together, MsSTING exerted antiviral action upon LMBV infection through positive regulation the innate immune response.

A nanobody-mediated drug system against largemouth bass virus delivered by bacterial nanocellulose in Micropterus salmoides

Diseases caused by pathogens severely hampered the development of aquaculture, especially largemouth bass virus (LMBV) has caused massive mortality and severe economic losses to the culture of largemouth bass (Micropterus salmoides). Considering the environmental hazards and human health, effective and environmentally friendly therapy strategy against LMBV is of vital importance and in pressing need. In the present study, a novel nanobody (NbE4) specific for LMBV was selected from a phage display nanobody library. Immunofluorescence and indirect ELISA showed that NbE4 could recognize LMBV virions and had strong binding capacity, but RT-qPCR evidenced that NBE4 did not render the virus uninfectious. Besides, antiviral drug ribavirin was used to construct a targeted drug system delivered by bacterial nanocellulose (BNC). RT-qPCR revealed that NbE4 could significantly enhance the antiviral activity of ribavirin in vitro and in vivo. The targeted drug delivery system (BNC-Ribavirin-NbE4, BRN) reduced the inflammatory response caused by LMBV infection and improved survival rate (BRN-L, 33.3 %; BRN-M, 46.7 %; BRN-H, 56.7 %)compared with control group (13.3 %), ribavirin group (RBV, 26.7 %) and BNC-ribavirin (BNC-R, 40.0 %), respectively. This research provided an effective antiviral strategy that improved the drug therapeutic effect and thus reduced the dosage.

Single nucleotide polymorphism SNP19140160 A > C is a potential breeding locus for fast-growth largemouth bass (Micropterus salmoides)

BACKGROUND

Largemouth bass (Micropterus salmoides) has significant economic value as a high-yielding fish species in China's freshwater aquaculture industry. Determining the major genes related to growth traits and identifying molecular markers associated with these traits serve as the foundation for breeding strategies involving gene pyramiding. In this study, we screened restriction-site associated DNA sequencing (RAD-seq) data to identify single nucleotide polymorphism (SNP) loci potentially associated with extreme growth differences between fast-growth and slow-growth groups in the F1 generation of a largemouth bass population.

RESULTS

We subsequently identified associations between these loci and specific candidate genes related to four key growth traits (body weight, body length, body height, and body thickness) based on SNP genotyping. In total, 4,196,486 high-quality SNPs were distributed across 23 chromosomes. Using a population-specific genotype frequency threshold of 0.7, we identified 30 potential SNPs associated with growth traits. Among the 30 SNPs, SNP19140160, SNP9639603, SNP9639605, and SNP23355498 showed significant associations; three of them (SNP9639603, SNP9639605, and SNP23355498) were significantly associated with one trait, body length, in the F1 generation, and one (SNP19140160) was significantly linked with four traits (body weight, height, length, and thickness) in the F1 generation. The markers SNP19140160 and SNP23355498 were located near two growth candidate genes, fam174b and ppip5k1b, respectively, and these candidate genes were closely linked with growth, development, and feeding. The average body weight of the group with four dominant genotypes at these SNP loci in the F1 generation population (703.86 g) was 19.63% higher than that of the group without dominant genotypes at these loci (588.36 g).

CONCLUSIONS

Thus, these four markers could be used to construct a population with dominant genotypes at loci related to fast growth. These findings demonstrate how markers can be used to identify genes related to fast growth, and will be useful for molecular marker-assisted selection in the breeding of high-quality largemouth bass.

Vitamin D3 can effectively and rapidly clear largemouth bass ranavirus by immunoregulation

Largemouth bass ranavirus (LMBV) is a highly destructive pathogen that causes significant mortality rates among largemouth bass populations. Unfortunately, there is a dearth of drug development efforts specifically aimed at treating LMBV. To address this, our study sought to investigate the potential effectiveness of incorporating varying doses of VD3 into the diet as a treatment for LMBV. Through qRT-PCR and semi-qPCR, we observed significant suppression and clearance of LMBV pathogens in largemouth bass fed with 15000 IU/Kg and 20000 IU/Kg of VD3 within 14 days. In addition, VD3 treatment significantly increased the expression levels of key immune-related genes such as IL-1β, IFN-γ, Mx, and IgM. Encouragingly, we observed that VD3 significantly increased antioxidant and immune activities such as TSOD, TAOC and C3 in serum and maintained total protein levels. Additionally, tissue pathology sections highlighted a dose-dependent relationship between VD3 supplementation and tissue damage, with the 15000 IU and 20000 IU groups exhibiting minimal damage. In conclusion, a reasonable concentration of VD3 effectively reduced LMBV replication and tissue damages, while improved immune-related genes expression and serum biochemical indices. These findings declare the considerable therapeutic potential of VD3 supplementation for combating LMBV disease and provide an alternative treatment option for fish farming.

Largemouth bass Rel exerts antiviral role against fish virus and regulates the expression of interleukin-10

Nuclear factor-κB (NF-κB)/Rel is a group of transcription factors that can be activated and regulates various aspects of innate and adaptive immune functions, which play a crucial role in mediating inflammatory responses. Interleukin-10 (IL-10) is a highly pleiotropic cytokine that has a central role in limiting the immune response to pathogens during infection and thereby alleviating damage to the host. This study aims to investigate the function of the Rel gene in virus infection and its regulatory effect on IL-10 in the largemouth bass (Micropterus salmoides). The ORF sequence of MsRel was 1941 bp, containing 646 amino acids with two conserved functional domains, including RHD and IPT domain. In healthy largemouth bass, the mRNA of MsRel was detected in all the tested tissues, including gill, liver, kidney, heart, spleen, intestine, stomach, skin, brain, fin and muscle. The expression of MsRel was induced by challenge with largemouth bass virus (LMBV) or red grouper nervous necrosis virus (RGNNV), as well as treatment with lipopolysaccharide (LPS) or poly (I:C) in vivo. As evidenced by the detection of viral gene mRNA levels, the infectivity of LMBV and morphological cytopathic effect (CPE), we found that overexpression of MsRel inhibited the infection and replication of LMBV, suggesting its antiviral roles in fish. Besides, the promoter analysis was carried out to determine whether MsRel was a regulator of MsIL-10. The results of the luciferase reporter assay indicated that MsRel has a positive regulatory role in MsIL-10 expression. Further analysis revealed that the potential binding sites of MsIL-10 may be located in the MsIL10-5-M (-42 to +8 bp) region of the MsIL-10 promoter. Furthermore, we observed that MsRel enhanced IFN-I and IFN-III promoter activities. Taken together, our findings demonstrated that MsRel affect LMBV infection by regulating the immune responses, and providing a new idea of the mechanisms how Rel regulate the expression of IL-10 in bony fish.

Adamantoyl chloride inhibited replication of the largemouth bass virus via enhanced immunity and inhibition of apoptosis

The largemouth bass virus (LMBV) is a commonly encountered pathogen in aquaculture and presents significant challenges to development of the largemouth bass industry due to the lack of effective treatment methods. Here, the inhibitory potential and underlying mechanisms of adamantoyl chloride (AdCl) against LMBV were assessed both in vitro and in vivo. The results showed that AdCl (IC50 = 72.35 μM) significantly inhibited replication of LMBV in epithelioma papulosum cyprini (EPC) cells. The results of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide and cytopathic effect (CPE) assays confirmed that AdCl inhibited replication of LMBV in EPC cells and significantly reduced the CPE effect, respectively. As a potential mechanism, AdCl inhibited apoptosis as determined by fluorescence and transmission electron microscopy. The results of flow cytometry showed that the apoptosis rate was decreased by 69 % in the AdCl-treated group as compared to the LMBV-infected group. Additionally, AdCl inhibited viral release. In vivo, the survival rate was 16.2 % higher in the AdCl-treated group as compared to the LMBV-infected group (26.9 % vs. 10.7 %, respectively). Additionally, the results of quantitative reverse transcription polymerase chain reaction (RT-qPCR) showed that AdCl significantly reduced the viral load of the fish liver, spleen, and kidneys at 3, 6, and 9 days postinfection. In addition, RT-qPCR analysis found that AdCl upregulated expression of immune-related genes to suppress replication of LMBV. Collectively, these results confirmed the anti-LMBV activities of AdCl for use in the aquaculture industry.