MEKK3 in hybrid snakehead (Channa maculate ♀ ×Channa argus ♂): Molecular characterization and immune response to infection with Nocardia seriolae and Aeromonas schubertii

Isolation and identification of hybrid snakehead rhabdovirus (HSHRV) and its immune response in the hybrid Snakehead((male Channa argus × female Channa maculata)

Hybrid snakehead is an emerging aquaculture species obtained from the mating of Channa argus (♂) and Channa maculate (♀). It has the advantages of fast growth and strong disease resistance. Viral diseases caused by hybrid snakehead rhabdovirus (HSHRV) critically affect the hybrid snakehead industry. We isolated and identified a highly virulent strain of HSHRV from a naturally occurring hybrid snakehead, namely HSHRV-GZ22. It showed clinical signs of sinking, superficial blackening, spinning, acute internal congestion, and hemorrhage, along with blackening and enlargement of the liver, spleen, and kidneys. Histopathological analysis showed multiple tissue lesions in the liver, spleen, and kidneys, characterized mainly by massive inflammatory cell infiltration, interstitial hemorrhage, and partial cell necrosis. Pathogen analysis identified the virus as HSHRV. Immunofluorescence analysis (IFA) with HSHRV-specific antibodies confirmed the virus and electron microscopic observation showed that the bullet-like virus particles had a size of approximately 150 nm. The replication efficiency of HSHRV was 107.33 TCID50/mL. The glycoproteins of the isolates were cloned and sequenced, and a phylogenetic tree was constructed. The HSHRV-GZ22 isolates clustered into a single branch with the reported HSHRV-C1207, and it had a high degree of homology with Siniperca chuatsi rhabdovirus (SCRV). HSHRV-GZ22 was regressively infected, clinical and pathological symptoms were similar to naturally occurring fish, with a fatality rate of about 85 %. qRT-PCR was performed to determine the viral replication in different tissues of hybrid snakehead, and the viral copies were found to be highly expressed in the liver, spleen, kidney, and intestine. HSHRV-GZ22 activated the antiviral immune pathway in hybrid snakeheads during infection, and the expressions of IgM, IRF7, ISG12, and IFNγ were significantly altered. In this study, we isolated a strong virulent strain of HSHRV and characterized it; in addition, it provided insights into the pathogenesis of HSHRV and immune response in hybrid snakehead, while also advancing the methods for diagnosing and preventing diseases caused by HSHRV.

Effect of kelp powder on the resistance of Aeromonas hydrophila in the gut of hybrid snakeheads (Channa maculata ♀ × Channa argus ♂)

To assess the level of oxidative stress, expression of immune-related genes, histomorphology, and changes in the intestinal tract of hybrid snakeheads(Channa maculata ♀ × Channa argus ♂) under stress from kelp powder in place of flour against Aeromonas hydrophila. We set up experimental diets: a control (C) diet of 20% flour, an experimental (MR) diet of 10% kelp powder and 10% flour, and an experimental (FR) diet of 0% starch and 15% kelp powder. The experimental fish in each group were infected with Aeromonas hydrophila after 60 days of feeding. For this experiment, some of the experimental fish in group C were injected with PBS as a negative control group (PBS). The results showed that the C group had significantly higher SOD, CAT, and T-AOC activity and expression of TAK1, IKKβ, IL-1β, and TNF-α genes in the MyD88 pathway than the PBS group. CAT activity and the expression of TAK1, IL-1β and TNF-α genes in the MyD88 pathway were significantly lower in the MR group than in the C group. Furthermore, the number of goblet cells in the MR group was significantly higher than in the C group. Furthermore, microorganisms such as Bacteroidota and Actinobacteriota were significantly lower in the C group than in the PBS and FR groups, as were beneficial bacteria such as Clostridium_sensu_stricto_1 and Sphingomonas. Replacing flour with kelp powder increases hybrid snakehead gut resistance to Aeromonas hydrophila.

Screening of genes encoding proteins that interact with ISG15: Probing a cDNA library from a snakehead fish cell line using a yeast two-hybrid system

Interferon-stimulated gene 15 (ISG15) regulates cellular life processes, including defense responses against infection by a variety of viral pathogens, by binding to target proteins. At present, various fish ISG15s have been identified, but the biological function of ISG15 in snakehead fish is still unclear. In this study, total RNA was extracted from snakehead fish cell line E11, ds cDNA was synthesized and purified using SMART technology, and the resulting cDNA library was screened by co-transforming yeast cells. The library titer was 4.28 × 10⁹ CFU/mL. Using snakehead ISG15 as the bait protein, the recombinant bait vector pGBKT7-ISG15 was constructed and transformed into the yeast strain Y2HGold. The toxicity and self-activation activity of the bait vector were detected on the deficient medium, and the prey proteins interacting with ISG15 were screened. In total, 19 interacting proteins of ISG15 were identified, including mitotic checkpoint protein BUB3, hypothetical protein SnRVgp6, elongation factor 1-beta, 60S ribosomal protein L9, dual specificity protein phosphatase 5-like, eukaryotic translation initiation factor 3 subunit I and ferritin. A yeast spotting assay further probed the interaction between ISG15 and DUSP5. These results increase our understanding of the interaction network of snakehead ISG15 and will aid in exploring the underlying mechanisms of snakehead ISG15 functions in the future.

Functional characterization of MEKK3 in the intestinal immune response to bacterial challenges in grass carp (Ctenopharyngodon idella)

Mitogen-activated protein kinase kinase kinase 3 (MEKK3) is an evolutionarily conserved Ser/Thr protein kinase of the MEKK family that is essential for the host immune response to pathogen challenges in mammals. However, the immune function of MEKK3s in lower vertebrate species, especially in bony fish, remains largely unknown. In this study, a fish MEKK3 (designated CiMEKK3) gene was cloned and identified from grass carp (Ctenopharyngodon idella). The present CiMEKK3 cDNA encoded a 620 amino acid polypeptide containing a conserved S-TKc domain and a typical PB1 domain. Several potential immune-related transcription factor-binding sites, including activating protein 1 (AP-1), nuclear factor kappa B (NF-κB) and signal transducer and activator of downstream transcription 3 (STAT3), were observed in the 5’ upstream DNA sequence of CiMEKK3. A phylogenetic tree showed that CiMEKK3 exhibits a close evolutionary relationship with MEKK3s from Cyprinus carpio and Carassius auratus. Quantitative real-time PCR analysis revealed that CiMEKK3 transcripts were widely distributed in all selected tissues of healthy grass carp, with a relatively high levels observed in the gill, head kidney and intestine. Upon in vitro challenge with bacterial pathogens (Aeromonas hydrophila and Aeromonas veronii) and pathogen-associated molecular patterns (PAMPs) (lipopolysaccharide (LPS), peptidoglycan (PGN), L-Ala-γ-D-Glu-mDAP (Tri-DAP) and muramyl dipeptide (MDP)), the expression levels of CiMEKK3 in the intestinal cells of grass carp were shown to be significantly upregulated in a time-dependent manner. In vivo injection experiments revealed that CiMEKK3 transcripts were significantly induced by MDP challenge in the intestine; however, these effects could be inhibited by the nutritional dipeptides carnosine and Ala-Gln. Moreover, subcellular localization analysis and luciferase reporter assays indicated that CiMEKK3 could act as a cytoplasmic signal-transducing activator involved in the regulation of NF-κB and MAPK/AP-1 signaling cascades in HEK293T cells. Taken together, these findings strongly suggest that CiMEKK3 plays vital roles in the intestinal immune response to bacterial challenges, which will aid in understanding the pathogenesis of inflammatory bowel disease in bony fish.

Characterization of snakehead (Channa argus) interleukin-21: Involvement in immune defense against two pathogenic bacteria, in leukocyte proliferation, and in activation of JAK-STAT signaling pathway

Interleukin-21 (IL-21), a crucial immune regulatory molecule, belongs to the common γ-chain family of type I cytokines, and exerts pleiotropic effects on multiple immune cell types in mammals. However, the characteristics and functions of fish IL-21 remain unclear. To further investigate the molecular mechanism of IL-21 in teleosts, we first cloned and identified the IL-21 gene (designated shIL-21) of the snakehead (Channa argus). The full-length open reading frame of shIL-21 is 438 bp in length, and encodes a predicted protein of 145 amino acid residues. A sequence analysis showed that shIL-21 has the typical structural characteristics of other IL-21 proteins, containing four α-helices and four conserved cysteine residues. In a phylogenetic analysis, shIL-21 clustered within a subgroup of IL-21 proteins from other teleost species and shared its closest evolutionary relationship with that of Lates calcarifer. The expression analysis showed that shIL-21 was ubiquitously expressed in all the healthy snakehead tissues tested, albeit at different levels. After infection with Nocardia seriolae or Aeromonas schubertii, the relative expression of shIL-21 was mainly upregulated in the head kidney and spleen in vivo. Similarly, after stimulation with the three pathogen analogues lipoteichoic acid, lipopolysaccharides, and polyinosinic-polycytidylic acid, the expression of shIL-21 was also induced in head kidney leukocytes in vitro. A recombinant shIL-21 protein was expressed and purified, and promoted the proliferation of head kidney leukocytes, induced the expression of genes encoding critical signaling molecules in the Janus kinase (JAK) and signal transducer and activator of transcription (STAT) pathway, including JAK1, JAK3, STAT1, and STAT3, and induced the expression of endogenous shIL-21 and genes encoding several key proinflammatory cytokines (tumor necrosis factor-α, interferon-γ, and IL-1β). Taken together, these preliminary findings suggest that shIL-21 is involved in the immune defense against bacterial infection, in leukocyte proliferation, and in the activation of the JAK-STAT pathway. They thus extend the functional studies of IL-21 in teleosts.