Identification and characterization of a β-defensin gene involved in the immune defense response of channel catfish, Ictalurus punctatus

Antibacterial Efficacy and Mechanisms of Action of a Novel Beta-Defensin from Snakehead Murrel, Channa striata

Beta-defensins, identified from fishes, constitute a crucial category of antimicrobial peptides important in combating bacterial fish pathogens. The present investigation centers on the molecular and functional characterization of CsDef, a 63-amino acid beta-defensin antimicrobial peptide derived from snakehead murrel (Channa striata). The physicochemical attributes of CsDef align with the distinctive characteristics observed in AMPs. CsDef was recombinantly produced, and the recombinant peptide, rCsDef, exhibited notable antibacterial efficacy against bacterial fish pathogens with an MIC of 16 μM for V. proteolyticus. A. hydrophila exhibited 91% inhibition, E. tarda 92%, and V. harveyi 53% at 32 μM of rCsDef. The rCsDef exhibited a multifaceted mechanism of action against bacteria, i.e., through membrane depolarization, membrane permeabilization, and generation of ROS. The rCsDef was non-hemolytic to hRBCs and non-cytotoxic to normal mammalian cell line CHO-K1. However, it exhibited anticancer properties in MCF-7. rCsDef demonstrated notable stability with respect to pH, temperature, salt, metal ions, and proteases. These findings suggest it is a potential candidate molecule for prospective applications in aquaculture.

Effects of diets rich in Agaricus bisporus polysaccharides on the growth, antioxidant, immunity, and resistance to Yersinia ruckeri in channel catfish

To promote the application of Agaricus bisporus polysaccharides (ABPs) in channel catfish (Ictalurus punctatus) culture, we evaluated the effects of ABPs on the growth, immunity, antioxidant, and antibacterial activity of channel catfish. When the amount of ABPs was 250 mg/kg, channel catfish's weight gain and specific growth rates increased significantly while the feed coefficient decreased. We also found that adding ABPs in the feed effectively increased the activities of ACP, MDA, T-SOD, AKP, T-AOC, GSH, and CAT enzymes and immune-related genes such as IL-1β, Hsp70, and IgM in the head kidney of channel catfish. Besides, long-term addition will not cause pathological damage to the head kidney. When the amount of ABPs was over 125 mg/kg, the protection rate of channel catfish was more than 60%. According to the intestinal transcriptome analysis, the addition of ABPs promoted the expression of intestinal immunity genes and growth metabolism-related genes and enriched multiple related KEEG pathways. When challenged by Yersinia ruckeri infection, the immune response of channel catfish fed with ABPs was intenser and quicker. Additionally, the 16S rRNA gene sequencing analysis showed that the composition of the intestinal microbial community of channel catfish treated with ABPs significantly changed, and the abundance of microorganisms beneficial to channel catfish growth, such as Firmicutes and Bacteroidota increased. In conclusion, feeding channel catfish with ABPs promoted growth, enhanced immunity and antioxidant, and improved resistance to bacterial infections. Our current results might promote the use of ABPs in channel catfish and even other aquacultured fish species.

Transcriptome Revealed the Macrophages Inflammatory Response Mechanism and NOD-like Receptor Characterization in Siberian Sturgeon (Acipenser baerii)

Nucleotide-binding and oligomerization domain-like receptors (NOD-like receptors, NLRs) can regulate the inflammatory response to eliminate pathogens and maintain the host's homeostasis. In this study, the head kidney macrophages of Siberian sturgeon were treated with lipopolysaccharide (LPS) to induce inflammation by evaluating the expression of cytokines. The high-throughput sequencing for macrophages after 12 h treatment showed that 1224 differentially expressed genes (DEGs), including 779 upregulated and 445 downregulated, were identified. DEGs mainly focus on pattern recognition receptors (PRRs) and the adaptor proteins, cytokines, and cell adhesion molecules. In the NOD-like receptor signaling pathway, multiple NOD-like receptor family CARD domains containing 3-like (NLRC3-like) were significantly downregulated, and pro-inflammatory cytokines were upregulated. Based on the transcriptome database, 19 NLRs with NACHT structural domains were mined and named in Siberian sturgeon, including 5 NLR-A, 12 NLR-C, and 2 other NLRs. The NLR-C subfamily had the characteristics of expansion of the teleost NLRC3 family and lacked the B30.2 domain compared with other fish. This study revealed the inflammatory response mechanism and NLRs family characterization in Siberian sturgeon by transcriptome and provided basic data for further research on inflammation in teleost.

Expression and functional characterization of three β-defensins in grass carp (Ctenopharyngodon idella)

β-defensins (BDs) are a group of cysteine-rich cationic antimicrobial peptides and play important roles in the first line of defense against infection. In this study, the expression and antibacterial activities of three grass carp (Ctenopharyngodon idella) (Ci) β-defensin (BD) peptides were comparatively investigated. Expression analysis reveals that CiBD1-3 were constitutively expressed in tissues, with the highest expression detected in the skin. The CiBD-1 transcripts were more abundant than CiBD-2 and CiBD-3. In the primary head kidney leukocytes, CiBDs were induced by PHA, LPS, poly(I:C) and cytokines such as IL-1β and IFN-γ. In vivo challenge of fish with Aeromonas hydrophila resulted in the up-regulation of CiBDs in the head kidney and hindgut. To determine the biological activities, recombinant CiBD proteins were produced in the HEK293-F cells and purified for the minimum inhibitory concentration assay. It was found that all three recombinant CiBD proteins were effective to inhibit the growth of Gram-negative fish bacterial pathogens including Aeromonas hydrophila, Edwardsiella tarda, Flavobacterium columnare and Klebsiella pneumoniae and Gram-positive Staphylococcus aureus. CiBD-2 and CiBD-3 were more effective than CiBD-1. Our results demonstrate that all the three CiBDs have broad antibacterial activity against fish bacterial pathogens.

Expression profiling and inflammatory activation analysis of high-mobility group box 1 in Schizothorax prenanti

OBJECTIVE

High-mobility group box 1 (HMGB1) is a highly conserved nuclear protein and participates in the immune response to pathogens in bony fish. In this study, the structure and function of HMGB1 in the cyprinid fish Schizothorax prenanti (SpHMGB1) were investigated.

METHODS

The spatial structure of SpHMGB1 was predicted by CPHmodels. Quantitative reverse transcription PCR was used to detect the mRNA of SpHMGB1 in different tissues and Streptococcus agalactiae infection. The macrophage was treated with synthetic SpHMGB1-B box peptide to analyze the inflammatory activity.

RESULT

Structurally, SpHMGB1 had the conserved A box, B box, and acid tail compared with Zebrafish Danio rerio and mice Mus musculus. SpHMGB1 was universally expressed in various tissues, with the highest expression in the middle kidney. In vivo, SpHMGB1 was significantly induced in response to Streptococcus agalactiae infection in the blood and spleen. Synthetic SpHMGB1-B box peptide activated respiratory burst and up-regulated the messenger RNA expression of interleukin-1β, tumor necrosis factor α, interleukin-10, interferon regulatory factor 1, interferon regulatory factor 7, C-X-C motif chemokine ligand 11-1, C-X-C motif chemokine ligand 11-2, and toll-like receptor 4 in macrophages.

CONCLUSION

This study suggested that SpHMGB1 participated in the response to bacterial pathogens and that SpHMGB1-B box peptide played an important role in mediating the immune response of S. prenanti.

Efficacy of natural antimicrobial peptides versus peptidomimetic analogues: a systematic review

Aims: This systematic review was carried out to determine whether synthetic peptidomimetics exhibit significant advantages over antimicrobial peptides in terms of in vitro potency. Structural features - molecular weight, charge and length - were examined for correlations with activity. Methods: Original research articles reporting minimum inhibitory concentration  values against Escherichia coli, indexed until 31 December 2020, were searched in PubMed/ScienceDirect/Google Scholar and evaluated using mixed-effects models. Results: In vitro antimicrobial activity of peptidomimetics resembled that of antimicrobial peptides. Net charge significantly affected minimum inhibitory concentration values (p < 0.001) with a trend of 4.6% decrease for increments in charge by +1. Conclusion: AMPs and antibacterial peptidomimetics exhibit similar potencies, providing an opportunity to exploit the advantageous stability and bioavailability typically associated with peptidomimetics.

β-Defensin: An adroit saviour in teleosts

β-Defensin (BD) is an important first line innate defense molecule with potent antimicrobial and immunomodulatory activities in fish. The signatures of β-defensins are the presence of a net cationic charge and three intramolecular disulfide bonds mediated by six conserved cysteines. It consists of three exons and two introns. The signal peptide is usually conserved and sequence divergence is mostly seen in mature peptide region. The diverse amino acid sequences of matured peptide contribute to a strong positive selection and broad-spectrum antimicrobial activity. It is constitutively expressed in both mucosal as well as systemic sites. Increased expression of β-defensin was mostly reported in bacterial and viral infections in fish. Its role during parasitic and fungal infections is yet to be investigated. β-Defensin isoforms such as BD-1, BD-2, BD-3, BD-4 and BD-5 can be witnessed even in early developmental days to different pathogenic exposure in fish. β-Defensins possess adjuvant properties to enhance antigen-specific immunity promoting both cellular and humoral immune response. It significantly reduces/increases bacterial colonization or viral copy numbers when overexpressed/knockdown. Based on its chemotactic and activating potentials, it can contribute to both innate and adaptive immune responses. With mediated expression, it can also control inflammation. It is potent governing resistance in early developmental days as well. Its expression in pituitary and testis suggests its participation in reproduction and endocrine regulation in fish. Overall, β-defensins is an important member of antimicrobial peptides (AMPs) with multifunctional role in general homeostasis and to pathogen exposure possessing tremendous therapeutic approaches.

Interleukin-1β enhances the expression of two antimicrobial peptides in grass carp (Ctenopharyngodon idella) against Vibrio mimicus via activating NF-κB pathway

Vibrio mimicus (V. mimicus) is a pathogen causing serious vibriosis in aquatic animals. Hepcidin and β-Defensin1 are two important antibacterial peptides (AMPs) with broad-spectrum antibacterial activity in fish. In mammals, some evidences demonstrated that interleukin-1β (IL-1β) primarily promote AMPs expression via activating classical NF-κB pathway, but it still remains unclear in fish. Here, the temporal and spatial expression patterns of grass carp IL-1β (gcIL-1β) gene and two AMPs genes (gchepcidin and gcβ-defensin1) in tissues post-V. mimicus infection and anti-V. mimicus activity of these two AMPs in vitro were detected, showing that V. mimicus infection significantly elevated the mRNA levels of these three genes in the immune-related tissues although their expression patterns were not entirely consistent, and both gcHepcidin and gcβ-Defensin1 possessed anti-V. mimicus activity in vitro. Subsequently, the recombinant gcIL-1β (rgcIL-1β) was expressed prokaryotically in an inclusion body, which could promote proliferation of grass carp head kidney leukocytes (gcHKLs) and enhance respiratory burst activity and phagocytic activity of head kidney macrophages. Stimulation with rgcIL-1β was able to significantly regulate the mRNA expression of key regulatory genes (il-1RI, traf6, tak1, ikkβ, iκBα and p65) involved in the activation of classical NF-κB pathway, and then induce gcTAK1 phosphorylation, promote gcp65 nuclear translocation and enhance endogenous gcIL-1β expression at both mRNA and protein levels, implying NF-κB pathway was activated. More importantly, exogenous rgcIL-1β stimulation also significantly up-regulated both gcHepcidin and gcβ-Defensin1 mRNA levels against V. mimicus, and the regulatory effect was blocked or inhibited by NF-κB inhibitor PDTC. Taken together, our results demonstrated for the first time that grass carp IL-1β stimulation could significantly enhance the expression of these two anti-V.mimicus AMPs via activating classical NF-κB pathway.

TMT-based quantitative proteomic analysis of the effects of novel antimicrobial peptide AMP-17 against Candida albicans

Candida albicans is the most common human fungal pathogen in immunocompromised individuals. With the emergence of clinical fungal resistance, there is an urgent need to develop novel antifungal agents. AMP-17, a novel antimicrobial peptide from Musca domestica, has an antifungal effect against C. albicans, but its mechanism of antifungal action remains unclear. In the current study, we performed a proteomics analysis in C. albicans using TMT technique under the treatment of AMP-17. A total of 3931 proteins were identified, of which 3600 included quantitative information. With a 1.5-fold change threshold and a t-test p-value < 0.05 as standard, 423 differentially expressed proteins (DEPs) were up-regulated and 180 DEPs were down-regulated in the AMP-17/control. Notably, GO enrichment revealed that DEPs associated with the cell wall, RNA and oxidative stress were significantly up-regulated, while DEPs involved in ergosterol metabolism and membrane were significantly down-regulated in the AMP-17/control. KEGG pathway enrichment revealed that DEPs involved seven significant metabolic pathways, mainly involved oxidative phosphorylation, RNA degradation, propanoate metabolism and fatty acid metabolism. These results show that AMP-17 induces a complex organism response in C. albicans, indicating that AMP-17 may inhibit growth by affecting multiple targets in C. albicans cells. SIGNIFICANCE: Antimicrobial peptides (AMPs) are an important part of the innate immune system of organisms and having broad range of activity against fungi, bacteria and viruses. These AMPs are considered as probable candidate for forthcoming drugs, due to their broad range of activity, lesser toxicity and decreased resistance development by target cells. AMP-17, a novel antimicrobial peptide from M. domestica, has significant antifungal activity against C. albicans. It has been confirmed that AMP-17 can play an antifungal effect by destroying the cell wall and cell membrane of C. albicans in previous studies, but its mechanism of action at the protein level is currently unclear. In the current study, using the TMT-based quantitative proteomics method, 603 differentially expressed proteins were identified in the cells of C. albicans treated with AMP-17 for 12 h, and these DEPs were closely related to cell wall, cell membrane, RNA degradation and oxidative stress. The results provide new insights into the potential mechanism of action of AMP- 17 against C. albicans. Meanwhile, it provides certain technical support and theoretical basis for the research and development of novel peptide drugs.

Antimicrobial Peptides: Identification of two Beta-Defensins in a Teleost Fish, the European Sea Bass (Dicentrarchus labrax)

Beta-defensins consist in a group of cysteine-rich antimicrobial peptides (AMPs), widely found throughout vertebrate species, including teleost fish, with antimicrobial and immunomodulatory activities. However, although the European sea bass (Dicentrarchus labrax) is one of the most commercially important farmed fish species in the Mediterranean area, the characterization of its beta-defensins and its potential applications are still missing. In this study, we characterized two members of the beta-defensin family in this species. Phylogenetic and synteny analysis places sea bass peptides in the beta-defensin subfamilies 1 and 2, sharing similar features with the other members, including the six cysteines and the tertiary structure, that consists in three antiparallel beta-sheets, with beta-defensin 1 presenting an extra alpha-helix at the N-terminal. Further studies are necessary to uncover the functions of sea bass beta-defensins, particularly their antimicrobial and immunomodulatory properties, in order to develop novel prophylactic or therapeutic compounds to be used in aquaculture production.

β-Defensin from the Asian Sea Bass, Lates calcarifer: Molecular Prediction and Phylogenetic Analysis

Antimicrobial peptides (AMPs) are an important element of the innate immune system of all living organisms and serve as a barrier that safeguards the organisms against a wide range of pathogens. Fishes are proven to be a prospective source of AMPs, and β-defensins form an important family of AMPs with potent antimicrobial, chemotactic and immunomodulatory activities. The present study reports a β-defensin AMP sequence (Lc-BD) from the Asian sea bass, Lates calcarifer, a commercially important fish species in tropical and subtropical regions of Asia and the Pacific. A 202-bp cDNA fragment with an open reading frame encoding 63 amino acids (aa) was obtained from the mRNA of gill tissue by RT-PCR. The deduced aa sequence of Lc-BD possessed a signal and a mature peptide region with 20 and 43 aa residues, respectively. Lc-BD was characterized at the molecular level, and a molecular weight of 5.24 kDa and a net charge of +4.5 was predicted for the mature peptide. The molecular characterization of Lc-BD revealed the presence of three intramolecular disulphide bonds involving the six conserved cysteine residues in the sequence, and the phylogenetic analysis of Lc-BD showed a close relationship with β-defensins from fishes like Siniperca chuatsi, Argyrosomus regius, Trachinotus ovatus and Oplegnathus fasciatus.

Molecular and functional identification of a β-defensin homolog in large yellow croaker (Larimichthys crocea)

β-defensin (BD) is a cysteine-rich cationic antibacterial peptide that is active against a wide range of bacteria. Here, a β-defensin homolog (LcBD2) was identified in large yellow croaker (Larimichthys crocea). The open reading frame of LcBD2 contains 195 nucleotides, encoding a protein of 64 amino acids that possesses a typical arrangement of six conserved cysteine residues (C³¹ , C³⁷ , C⁴¹ , C⁵³ , C⁵⁹ and C⁶⁰ ). LcBD2 transcripts were constitutively expressed in all examined tissues and significantly increased in head kidney, spleen and gills by Vibrio alginolyticus. The synthetic LcBD2 peptide imparted antimicrobial effects on both Gram-negative bacteria (V. campbellii, V. parahaemolyticus, V. alginolyticus, V. harveyi and Pseudomonas plecoglossicida) and Gram-positive bacteria (Bacillus subtilis). We also observed that after treatment with synthetic LcBD2 peptide, numerous blisters appeared on the membrane of P. plecoglossicida, which in turn may result in cell membrane breakage and bacterial death. Moreover, the synthetic LcBD2 peptide significantly upregulated the expression levels of TNF-α2, IL-1β and CXCL8_L1 in monocytes/macrophages, while downregulated expression level of IL-10. The LcBD2 peptide also remarkedly enhanced the phagocytosis of monocytes/macrophages. These results indicate that LcBD2 not only protects large yellow croaker against multiple bacterial pathogens but also plays a role in activation of monocytes/macrophages.

MOSPD2 is a receptor mediating the LEAP-2 effect on monocytes/macrophages in a teleost, Boleophthalmus pectinirostris

Liver-expressed antimicrobial peptide 2 (LEAP-2) is a cationic peptide that plays an important role in a host's innate immune system. We previously demonstrated that mudskipper ( Boleophthalmus pectinirostris) LEAP-2 (BpLEAP-2) induces chemotaxis and activation of monocytes/ macrophages (MO/MФ). However, the molecular mechanism by which BpLEAP-2 regulates MO/MΦ remains unclear. In this study, we used yeast two-hybrid cDNA library screening to identify mudskipper protein(s) that interacted with BpLEAP-2, and characterized a sequence encoding motile sperm domain-containing protein 2 (BpMOSPD2). The interaction between BpLEAP-2 and BpMOSPD2 was subsequently confirmed by co-immunoprecipitation (Co-IP). Sequence analyses revealed that the predicted BpMOSPD2 contained an N-terminal extracellular portion composed of a CRAL-TRIO domain and a motile sperm domain, a C-terminal transmembrane domain, and a short cytoplasmic tail. Phylogenetic tree analysis indicated that BpMOSPD2 grouped tightly with fish MOSPD2 homologs and was most closely related to that of the Nile tilapia ( Oreochromis niloticus). The recombinant BpMOSPD2 was produced by prokaryotic expression and the corresponding antibody was prepared for protein concentration determination. RNA interference was used to knockdown BpMOSPD2 expression in the mudskipper MO/MФ, and the knockdown efficiency was confirmed by quantitative real-time polymerase chain reaction (qRT-PCR) and western blotting. Knockdown of BpMOSPD2 significantly inhibited BpLEAP-2-induced chemotaxis of mudskipper MO/MФ and BpLEAP-2-induced bacterial killing activity. Furthermore, knockdown of BpMOSPD2 inhibited the effect of BpLEAP-2 on mRNA expression levels of BpIL-10, BpTNFα, BpIL-1β, and BpTGFβ in MO/MФ. In general, BpMOSPD2 directly interacted with BpLEAP-2, and mediated the effects of BpLEAP-2 on chemotaxis and activation of mudskipper MO/MФ. This is the first identification of MOSPD2 as a receptor for LEAP-2.

Identification and characterization of SaRpAMP, a 60S ribosomal protein L27-derived antimicrobial peptide from amur catfish, Silurus asotus

Aquatic freshwater fish like catfish, Silurus asotus, lives in microbe-rich environments, which enable this fish to develop necessary defense mechanisms. Antimicrobial peptides, along with other innate immune factors, are regarded as an important group in this defense. An antimicrobial peptide, which was isolated from the skin of S. asotus, was identified as a C-terminal fragment of 60S ribosomal protein L27 from S. asotus. The peptide was, then, designated Silurus asotus 60S ribosomal protein L27-derived antimicrobial peptide, SaRpAMP. Primary structure analyses and cDNA cloning revealed that SaRpAMP was 4185.36 Da and composed of 33 amino acids (AAs). Its precursor had a total of 136 AAs containing a pro-sequence of 103 AAs encoded by the nucleotide sequence of 512 bp that comprises a 5' untranslated region (UTR) of 32 bp, an open reading frame (ORF) of 411 bp, and a 3' UTR of 69 bp. Secondary structure analyses showed that SaRpAMP had two α-helices with turns and coils and an amphiphilic structure, a finding consistent with the 3D model of the peptide. SaRpAMP exhibited potent antibacterial activity comparable to piscidin 1, a powerful positive control. Its antimicrobial activity against fungus C. albicans was relatively weak. The antimicrobial activity of SaRpAMP was not diminished by heat treatment and changes in pH but was abolished by proteolytic enzyme digestion. Membrane permeability assays suggested that SaRpAMP interacts with both the outer and inner bacterial membranes. This was consistent with the results of lipid titration and quenching of Trp fluorescence that demonstrated SaRpAMP's interaction with acidic liposomes. Collectively, these findings suggest that the identified peptide, SaRpAMP, was the first antimicrobial peptide reported to be derived from the C-terminal region of 60S ribosomal protein L27. The findings also suggest that the action mechanism of SaRpAMP involved the interaction of the peptide with the bacterial membranes.

Vaccination Route Determines the Kinetics and Magnitude of Nasal Innate Immune Responses in Rainbow Trout (Oncorhynchus mykiss)

Simple Summary

Many pathogens exploit the olfactory route to reach critical organs in the body such as the brain or lungs. Thus, effective onset of an early innate immune response in the nasal epithelium is key to stopping pathogen progression. The stimulation of nasal immunity by vaccines may depend on the type of vaccine and vaccination route. The goal of this study was to evaluate the ability of a live attenuated viral vaccine to stimulate innate immunity in the olfactory organ of rainbow trout, a teleost fish of commercial aquaculture value. The kinetics and magnitude of the innate immune response depended on the route of vaccination, with the strongest and fastest responses recorded in intranasally vaccinated fish. Injection vaccination had an intermediate effect, whereas immersion vaccination resulted in delayed and weak nasal innate immunity. Injection vaccination, even with the vehicle control, induced mortality in fingerlings, whereas nasal and immersion vaccines were safe. Challenge experiments with the live virus revealed that nasal and injected vaccines conferred very high and comparable levels of protection, but immersion vaccination only induced transient protection. In conclusion, the route of vaccination determines the type, magnitude and velocity of the innate immune response in the nasal epithelium of animals.

Abstract

Many pathogens infect animal hosts via the nasal route. Thus, understanding how vaccination stimulates early nasal immune responses is critical for animal and human health. Vaccination is the most effective method to prevent disease outbreaks in farmed fish. Nasal vaccination induces strong innate and adaptive immune responses in rainbow trout and was shown to be highly effective against infectious hematopoietic necrosis (IHN). However, direct comparisons between intranasal, injection and immersion vaccination routes have not been conducted in any fish species. Moreover, whether injection or immersion routes induce nasal innate immune responses is unknown. The goal of this study is to compare the effects of three different vaccine delivery routes, including intranasal (IN), intramuscular (i.m.) injection and immersion (imm) routes on the trout nasal innate immune response. Expression analyses of 13 immune-related genes in trout nasopharynx-associated lymphoid tissue (NALT), detected significant changes in immune expression in all genes analyzed in response to the three vaccination routes. However, nasal vaccination induced the strongest and fastest changes in innate immune gene expression compared to the other two routes. Challenge experiments 7 days post-vaccination (dpv) show the highest survival rates in the IN- and imm-vaccinated groups. However, survival rates in the imm group were significantly lower than the IN- and i.m.-vaccinated groups 28 dpv. Our results confirm that nasal vaccination of rainbow trout with live attenuated IHNV is highly effective and that the protection conferred by immersion vaccination is transient. These results also demonstrate for the first time that immersion vaccines stimulate NALT immune responses in salmonids.

Transcriptome analysis identifies immune-related genes and antimicrobial peptides in Siamese fighting fish (Betta splendens)

Siamese fighting fish (Betta splendens) is one of the most widely cultivated ornamental fish in global trade. However, transcriptomic data, which can reveal valuable genetic data for disease control and prevention, are extremely limited for this species. In this study, whole-body transcriptome sequencing of juvenile betta fish generated 4.457 GB of clean data and a total of 71,775 unigenes using the Illumina HiSeq4000 platform. These unigenes were functionally classified using 7 functional databases, yielding 45,316 NR (63.14%), 47,287 NT (65.88%), 39,105 Swiss-Prot (54.48%), 16,492 COG (22.98%), 37,694 KEGG (52.52%), 4,506 GO (6.28%), and 35,374 Interpro (49.28%) annotated unigenes. Furthermore, we also detected 13,834 SSRs distributed on 10,636 unigenes and 49,589 predicted CDSs. Based on KEGG analysis, five innate immune pathways (997 unigenes) were reported, including the NOD-like receptor signaling pathway, complement and coagulation cascades, toll-like receptor signaling pathway, RIG-I-like receptor signaling pathway and cytosolic DNA-sensing pathway. Moreover, four antimicrobial peptide (AMP) families (hepcidin, piscidin, LEAP-2, and defensins) from the betta fish transcriptome were also identified. Additionally, cDNA and genomic DNA of two β-defensins was successfully isolated from four betta fish species. RT-PCR analysis showed that BsBD1 transcripts were most abundant in the muscle and kidney and BsBD2 transcripts were most abundant in the gill. The genomic organization showed that the BD1 and BD2 genes consisted of three exons and two introns according to the GT-AG rule. Most importantly, this is the first report of the betta fish whole-body transcriptome obtained by high-throughput sequencing. Our transcriptomic data and the discovery of betta fish AMPs should promote a better understanding of molecular immunology for disease prevention for further ornamental fish aquaculture.

Five subfamilies of β-defensin genes are present in salmonids: Evolutionary insights and expression analysis in Atlantic salmon Salmo salar

β-defensins (BD) are the largest family of vertebrate defensins with potent antimicrobial, chemotactic and immune-regulatory activities. Four BD genes (BD1-4) have been cloned previously in rainbow trout but none have been reported in other salmonids. In this study seven BD genes (BD1a-b, 2-4, 5a-b) are characterised in Atlantic salmon and additional BD genes (BD1b and BD5) in rainbow trout. Bioinformatic analysis revealed up to seven BD genes in the genomes of other salmonids that belong to five subfamilies (BD1-5) due to whole genome duplications. BD1-2 and BD4-5 are also present in basal teleosts but only BD1 and/or BD5 are present in advanced teleosts due to loss of one chromosomal locus. BD3 is salmonid specific. Fish BD have a unique three-coding exon structure. Fish BD are highly divergent between subfamilies but conserved within each subfamily. Atlantic salmon BD genes are differentially expressed in tissues, often with low level expression in systemic immune organs (head kidney and spleen) yet with at least one BD gene highly expressed in mucosal tissues, heart, blood and liver. This suggests an important role of these BD genes in innate immunity in mucosa, liver and blood in Atlantic salmon.

Defensins: Transcriptional regulation and function beyond antimicrobial activity

Defensins are one the largest group of antimicrobial peptides and are part of the innate defence. Defensins are produced by animals, plants and fungi. In animals and plants, defensins can be constitutively or differentially expressed both locally or systemically which confer defence before and a stronger response after infection. Immune signalling pathways regulate the gene expression of defensins. These pathways include cellular receptors, which recognise pathogen-associated molecular patterns and are found both in plants and animals. After recognition, signalling pathways and, subsequently, transcriptional factors are activated. There is an increasing number of novel functions in defensins, such as immunomodulators and immune cell attractors. Identification of defensin triggers could help us to elucidate other new functions. The present article reviews the different elicitors of defensins with a main focus on human, fish and marine invertebrate defensins.

The host defense peptide β-defensin confers protection against Vibrio anguillarum in ayu, Plecoglossus altivelis

β-defensin is a cationic host defense peptide actively participating in host innate immune response against pathogens. In teleost fish, β-defensin exhibits a diversity in genotypes and functions. Herein, a β-defensin homolog (PaBD) was identified from ayu, Plecoglossus altivelis, showing multiple tissues' upregulation against Vibrio anguillarum challenge. In vivo experiments revealed that intraperitoneal injection of chemically synthesized mature PaBD (mPaBD) increased the survival rate of V. anguillarum-infected ayu, accompanied by reduced bacterial load and decreased tissue mRNA levels of tumor necrosis factor α (PaTNF-α) and interleukin 1β (PaIL-1β). However, in vitro, mPaBD showed weak bactericidal activity against V. anguillarum. Interestingly, mPaBD enhanced phagocytosis, intracellular bacterial killing, and respiratory burst of ayu monocytes/macrophages (MO/MΦ). Moreover, it inhibited mRNA levels of PaIL-1β and PaTNF-α in MO/MФ upon V. anguillarum infection. In conclusion, PaBD protects ayu against V. anguillarum challenge not only through its direct antibacterial ability, but also through its immunomodulation in MO/MΦ.

Identification and characterization of three novel antimicrobial peptides from Acipenser dabryanus

Antimicrobial peptides (AMPs) play essential roles in the innate immune system to protect against a wide variety of pathogens in aquatic environments. In this study, three very important AMPs, cathelicidin, hepcidin and defensin, were identified in the critically endangered Acipenser dabryanus. The full-length cDNA sequences of these three AMPs were identified from transcriptome sequencing and the rapid amplification of cDNA ends (RACE) technique. Phylogenetic analysis showed that cathelicidin formed a clade with the other members of the cathelicidin family, and similar results were obtained for hepcidin. The A. dabryanus β-defensin belonged to the fish class 2 β-defensins. A tissue distribution study showed that the three AMP transcripts could be detected constitutively in various tissues. The highest expression levels of cathelicidin and hepcidin were found in the liver, while defensin was primarily expressed in the skin. Bacterial challenge in vivo revealed significant changes in the gene expression of the three AMPs at both mucosal sites and systemic sites. Striking upregulation of cathelicidin and hepcidin was observed in the skin at 12 h post-challenge, with increases of more than 7000-fold and 1000-fold, respectively, compared to the control, and the expression of defensin mRNA was remarkably elevated in the hindgut (by 230-fold at 6 h post-challenge). Moreover, according to the expression profiles of the AMPs post-challenge, we found that the mucosal immune response occurred earlier than the systemic immune response following bacterial infection. Our results suggest that these three novel AMPs may play important roles in the innate immune system of A. dabryanus to protect against invading pathogens, especially during the mucosal immune response.

Characterization of grass carp (Ctenopharyngodon idella) beta-defensin 1: implications for its role in inflammation control

Considering that fish grows in a complex aquatic environment, there is an increasing interest in fish β-defensins, which is an important group of antimicrobial peptides (AMPs). In this study, grass carp (Ctenopharyngodon idella) β-defensin 1 (gcdefb1) was isolated using homology cloning technology. Tissue distribution assay showed that gcdefb1 transcripts were expressed with the highest levels in brain and liver, followed by some mucous tissues. To examine gcDefb1 bioactivities, the recombinant gcDefb1 proteins fused with thioredoxin tag protein (Trx) (Trx-Defb1) were induced for production in Escherichia coli Rosetta-gami2(DE3)pLysS under optimal expression conditions. The antibacterial activity of Trx-Defb1 against Aeromonas hydrophila was assessed and its minimum inhibitory concentration (MIC) was 36 μM. Interestingly, Trx-Defb1 significantly inhibited LPS-induced Tnf-α (gcTnf-α) secretion and nitric oxide production in grass carp head kidney monocytes/macrophages (HKM), although Trx-Defb1 alone had no effect. Our studies provide the first evidence of fish β-defensin 1 engaging in both antimicrobial and inflammation suppression process.