Catfish hepcidin gene is expressed in a wide range of tissues and exhibits tissue-specific upregulation after bacterial infection

Characterization, Expression, and Functional Analysis of the Northern Snakehead (Channa argus) Hepcidin

Hepcidin, an antimicrobial peptide (AMP), is a well-conserved molecule present in various species such as fish, amphibians, birds, reptiles, and mammals. It exhibits broad-spectrum antimicrobial activity and holds a significant role in the innate immune system of host organisms. The northern snakehead (Channa argus) has become a valuable freshwater fish in China and Asia. In this investigation, the cDNA encoding the hepcidin gene of northern snakehead was cloned and named caHep. The amino acid sequences and protein structure of caHep are similar to those of hepcidins from other fish. The eukaryotic expression product of the caHep gene showed broad-spectrum antibacterial activity. Scanning electron microscope analysis indicated that the caHep peptide inhibited bacterial growth by damaging their cell membranes. Lipopolysaccharide (LPS) injection induced significant expression of caHep, implying the involvement of caHep in the innate immune response of northern snakeheads. This investigation showed that the caHep peptide is potentially a robust antibacterial drug against bacterial diseases in aquaculture animals.

Novel hepcidin genes in gilthead seabream: Implications for immune response and iron metabolism

Antimicrobial peptides (AMPs) are highly conserved small molecules present in various organisms, including fish. In gilthead seabream (Sparus aurata), one hamp1 and 15 hamp2 genes have been identified. This study aimed to characterize two novel hamp2 genes, hamp2.0 and hamp2.15, located on chromosome 17 of the gilthead seabream genome. Evolutionary analysis revealed that orthologs of both genes first appeared in the Clupeocephala clade 229 million years ago. In silico analysis predicted that the mature peptides, Hamp2α and Hamp2Ω, possess antimicrobial properties. Both peptides exhibited bactericidal activity against Vibrio harveyi, with Hamp2α showing concentration-dependent inhibition and Hamp2Ω demonstrating time-dependent inhibition. Neither peptide displayed cytotoxicity against SAF-1 cells; instead, they promoted cell proliferation. Basal expression of both genes was observed in all tissues analyzed, with the highest levels in liver and gonad. In head kidney leucocytes (HKLs), expression of both genes increased upon stimulation with lypopolysaccharide, poly I:C, nodavirus, or V. anguillarum. In vivo, hamp2.0 expression significantly increased in various tissues of V. harveyi-infected fish, while hamp2.15 expression increased in liver, spleen, head kidney, skin, and brain. In nodavirus-infected fish, hamp2.15 expression decreased in head kidney and brain. Finally, both genes showed significantly increased expression in head kidney and liver 72 h post-iron dextran injection. These findings suggest that the two novel hamp2 genes in gilthead seabream play a role in the immune response to bacterial and viral infections and may be involved in iron metabolism regulation.

Expression and Immune Response Profiles in Nile Tilapia (Oreochromis niloticus) and European Sea Bass (Dicentrarchus labrax) During Pathogen Challenge and Infection

Nile tilapia (Oreochromis niloticus) and European sea bass (Dicentrarchus labrax) are economically significant species in Mediterranean countries, serving essential roles in the aquaculture industry due to high market demand and nutritional value. They experience substantial losses from bacterial pathogens Vibrio anguillarum and Streptococcus iniae, particularly at the onset of the summer season. The immune mechanisms involved in fish infections by V. anguillarum and S. iniae remain poorly understood. This study investigated their impact through experiments with control and V. anguillarum- and S. iniae-infected groups for each species. Blood samples were collected at 1, 3, and 7 days post bacterial injection to assess biochemical and immunological parameters, including enzyme activities (AST and ALT), oxidative markers (SOD, GPX, CAT, and MDA), and leukocyte counts. Further analyses included phagocyte activity, lysozyme activity, IgM levels, and complement C3 and C4 levels. Muscle tissues were sampled at 1, 3, and 7 days post injection to assess mRNA expression levels of 18 immune-relevant genes. The focus was on cytokines and immune-related genes, including pro-inflammatory cytokines (TNF-α, TNF-β, IL-2, IL-6, IL-8, IL-12, and IFN-γ), major histocompatibility complex components (MHC-IIα and MHC-IIβ), cytokine receptors (CXCL-10 and CD4-L2), antimicrobial peptides (Pleurocidin and β-defensin), immune regulatory peptides (Thymosin β12, Leap 2, and Lysozyme g), and Galectins (Galectin-8 and Galectin-9). β-actin was used as the housekeeping gene for normalization. Significant species-specific responses were observed in N. Tilapia and E. Sea Bass when infected with V. anguillarum and S. iniae, highlighting differences in biochemical, immune, and gene expression profiles. Notably, in N. Tilapia, AST levels significantly increased by day 7 during S. iniae infection, reaching 45.00 ± 3.00 (p < 0.05), indicating late-stage acute stress or tissue damage. Conversely, E. Sea Bass exhibited a significant rise in ALT levels by day 7 in the S. iniae group, peaking at 33.5 ± 3.20 (p < 0.05), suggesting liver distress or a systemic inflammatory response. On the immunological front, N. Tilapia showed significant increases in respiratory burst activity on day 1 for both pathogens, with values of 0.28 ± 0.03 for V. anguillarum and 0.25 ± 0.02 for S. iniae (p < 0.05), indicating robust initial immune activation. Finally, the gene expression analysis revealed a pronounced peak of TNF-α in E. Sea Bass by day 7 post V. anguillarum infection with a fold change of 6.120, suggesting a strong species-specific pro-inflammatory response strategy. Understanding these responses provides critical insights for enhancing disease management and productivity in aquaculture operations.

Effects of Berberine on Lipid Metabolism, Antioxidant Status, and Immune Response in Liver of Tilapia (Oreochromis niloticus) under a High-Fat Diet Feeding

Berberine, a natural alkaloid found abundantly in various medicinal plants, exhibits antioxidative, anti-inflammatory, and lipid metabolism-regulatory properties. Nonetheless, its protective effects and the molecular mechanisms underlying liver injury in fish have not been fully elucidated. The aims of this study were to investigate the antioxidative, anti-inflammatory, and lipid metabolism-regulating effects of berberine against high-fat diet (HFD)-induced liver damage and to clarify the underlying molecular mechanisms. Tilapia were fed diets containing two doses of berberine (50 and 100 mg/kg diet) alongside high fat for 60 days. The results showed that berberine treatments (50 and/or 100 mg/kg) significantly reduced elevated aminotransferases, triglycerides (TG), total cholesterol (TC), and low-density lipoprotein cholesterol (LDL-c) in the plasma. In the liver, berberine treatments significantly increased the expression of peroxisome proliferator-activated receptor α (pparα) and carnitine palmitoyltransferase 1 (cpt-1) genes, leading to a reduction in lipid accumulation. Meanwhile, berberine treatment suppressed lipid peroxidation formation and enhanced antioxidant capacity. Berberine upregulated the mRNA levels of erythroid 2-related factor 2 (nrf2) and its downstream genes including heme oxygenase 1 (ho-1) and glutathione-S-transferase (gstα). Additionally, berberine attenuated the inflammation by inhibiting the expression of toll-like receptor 2 (tlr2), myeloid differential protein-88 (myd88), relb, and inflammatory cytokines such as interleukin-1β (il-1β), tumor necrosis factor-α (tnf-α), and il-8. In summary, this study suggested that berberine offers protection against HFD-induced liver damage in tilapia via regulating lipid metabolism, antioxidant status, and immune response. This protective effect may be attributed to the modulation of the Nrf2, TLR2/MyD88/NF-κB, and PPARα signaling pathways.

Identification of antimicrobial peptide genes from transcriptomes in Mandarin fish (Siniperca chuatsi) and their response to infection with Aeromonas hydrophila

Mandarin fish (Siniperca chuatsi) is a valuable freshwater fish species widely cultured in China. Its aquaculture production is challenged by bacterial septicaemia, which is one of the most common bacterial diseases. Antimicrobial peptides (AMPs) play a critical role in the innate immune system of fish, exhibiting defensive and inhibitory effects against a wide range of pathogens. This study aimed to identify the antimicrobial peptide genes in mandarin fish using transcriptomes data obtained from 17 tissue in our laboratory. Through nucleotide sequence alignment and protein structural domain analysis, 15 antimicrobial peptide genes (moronecidin, pleurocidin, lysozyme g, thymosin β12, hepcidin, leap 2, β-defensin, galectin 8, galectin 9, apoB, apoD, apoE, apoF, apoM, and nk-lysin) were identified, of which 9 antimicrobial peptide genes were identified for the first time. In addition, 15 AMPs were subjected to sequence characterization and protein structure analysis. After injection with Aeromonas hydrophila, the number of red blood cells, hemoglobin concentration, and platelet counts in mandarin fish showed a decreasing trend, indicating partial hemolysis. The expression change patterns of 15 AMP genes in the intestine after A. hydrophila infection were examined by using qRT-PCR. The results revealed, marked up-regulation (approximately 116.04) of the hepcidin gene, down-regulation of the piscidin family genes expression. Moreover, most AMP genes were responded in the early stages after A. hydrophila challenge. This study provides fundamental information for investigating the role of the different antimicrobial peptide genes in mandarin fish in defense against A. hydrophila infection.

In silico molecular and functional characterization of a dual function antimicrobial peptide, hepcidin (GIFT-Hep), isolated from genetically improved farmed tilapia (GIFT, Oreochromis niloticus)

BACKGROUND

Antimicrobial peptides (AMPs), innate immune response molecules in organisms, are also known for their dual functionality, exemplified by hepcidin-an immunomodulator and iron regulator. Identifying and studying various AMPs from fish species can provide valuable insights into the immune profiles of aquaculturally significant fish, which can be made use of in its culture.

RESULTS

Hepcidin, a dual-function antimicrobial peptide, was isolated from the gill tissue of Genetically Improved Farmed Tilapia (GIFT-Hep). GIFT-Hep consists of a 90 amino acid pre-propeptide with a 24-mer signal, a 40-mer propeptide, and a 26-mer mature peptide region. The mature peptide had a molecular weight of 3015.61 Da, a theoretical pI of 8.78, a net charge of +4.25, and a protein-binding potential of 2.06 kcal/mol. Four disulfide bonds were formed by eight cysteine residues in the mature region. The presence of positively charged arginine residues renders the peptide 50% hydrophobic. Molecular analysis of GIFT-Hep revealed the presence of a furin propeptide convertase motif, RX(K/R)R, which facilitates trimming of the peptide to yield the mature GIFT-Hep. The hypothetical iron regulatory sequence, QSHLSL, was also identified in the mature peptide. In silico predictions about the characteristics of GIFT-Hep, such as charge, hydrophobicity, high surface accessibility, transmembrane helical regions, hydrophobic faces, hot spots, and cell-penetrating properties, suggest that the peptide functions as an iron regulatory antimicrobial agent.

CONCLUSIONS

This study reports a hepcidin antimicrobial peptide with both HAMP1 and HAMP2 properties isolated from genetically improved farmed tilapia, and further evaluation of the properties will prove the feasibility of GIFT-Hep being used as a therapeutant in aquaculture.

Genome-Wide Identification of Trachinotus ovatus Antimicrobial Peptides and Their Immune Response against Two Pathogen Challenges

Golden pompano, Trachinotus ovatus, as a highly nutritious commercially valuable marine fish, has become one of the preferred species for many fish farmers due to its rapid growth, wide adaptability, and ease of feeding and management. However, with the expansion of aquaculture scale, bacterial and parasitic diseases have also become major threats to the golden pompano industry. This study, based on comparative genomics, shows the possibility of preferential evolution of freshwater fish over marine fish by analyzing the phylogenetic relationships and divergence times of 14 marine fish and freshwater fish. Furthermore, we identified antimicrobial peptide genes from 14 species at the genomic level and found that the number of putative antimicrobial peptides may be related to species evolution. Subsequently, we classified the 341 identified AMPs from golden pompano into 38 categories based on the classification provided by the APD3. Among them, TCP represented the highest proportion, accounting for 23.2% of the total, followed by scolopendin, lectin, chemokine, BPTI, and histone-derived peptides. At the same time, the distribution of AMPs in chromosomes varied with type, and covariance analysis showed the frequency of its repeat events. Enrichment analysis and PPI indicated that AMP was mainly concentrated in pathways associated with disease immunity. In addition, our transcriptomic data measured the expression of putative AMPs of golden pompano in 12 normal tissues, as well as in the liver, spleen, and kidney infected with Streptococcus agalactiae and skin infected with Cryptocaryon irritans. As the infection with S. agalactiae and C. irritans progressed, we observed tissue specificity in the number and types of responsive AMPs. Positive selection of AMP genes may participate in the immune response through the MAPK signaling pathway. The genome-wide identification of antimicrobial peptides in the golden pompano provided a complete database of potential AMPs that can contribute to further understanding the immune mechanisms in pathogens. AMPs were expected to replace traditional antibiotics and be developed into targeted drugs against specific bacterial and parasitic pathogens for more precise and effective treatment to improve aquaculture production.

Early innate immune responses in European sea bass (Dicentrarchus labrax L.) following Tenacibaculum maritimum infection

Introduction

The marine aquaculture industry has been witnessing a worldwide emergence of tenacibaculosis, a poorly understood bacterial disease caused by Tenacibaculum maritimum that affects commercially important fish. So far, knowledge on the T. maritimum virulence mechanisms is scarce and the pathogen-host interaction operating in tenacibaculosis remain to be disclosed. This study aimed at contributing to a better understanding of this disease, by evaluating the early innate immune response triggered in European sea bass (Dicentrarchus labrax) by a bath-challenge with T. maritimum.

Methods

Groups of sea bass were bath-challenged with T. maritimum (challenged fish) or mock-challenged. Undisturbed fish were used as controls (time 0). Samples of blood, liver and mucosal organs (skin, gills and posterior-intestine) were collected at 0 h (control) and at 6, 24, 48 and 72 h post-challenge (n=12). Mucosal organs were used for analyzing the expression of immune-related genes by RT-qPCR, as well as blood samples for assessing haematological and innate humoral parameters and liver for oxidative stress assessment.

Results

An increased expression of il-1β, il8, mmp9 and hamp1 was detected in all mucosal organs of infected fish when compared with control and mock-challenged fish, suggesting a pro-inflammatory response against T. maritimum transversal to all organs. The faster induction of these pro-inflammatory genes was observed in the gills. Regarding the systemic response, challenged fish presented neutrophilia, monocytosis, signs of anemia, and a decrease of bactericidal and lysozyme activities in plasma. Almost no variations were observed regarding hepatic oxidative stress.

Discussion/Conclusions

The present study suggests that T. maritimum induces a local innate immune response upon bath infection not only in the skin of European sea bass, but also in the gills and posterior-intestine, likely triggered by the T. maritimum's capacity to adhere, colonize and damage these organs that can function as entry ways to bacteria, leading ultimately to the seen host's systemic response.

Efficacy and transcriptome analysis of golden pompano (Trachinotus ovatus) immunized with a formalin-inactived vaccine against Streptococcus iniae

Streptococcus iniae is a worldwide fish pathogen that cause tremendous economic losses to the global aquaculture industry. Vaccination is regarded as the most effective and safe way to control fish diseases. In our study, we developed a formalin-inactivated vaccine against S. iniae and evaluated its effect in golden pompano (Trachinotus ovatus). In addition, in order to clarify the molecular mechanisms underlying the vaccine protection, we compared the spleen transcriptomes of vaccinated and unvaccinated golden pompano at 1, 2 and 7 d post vaccination using the RNA-seq technology. The relative percentage survival (RPS) reached 71.1% at 28 days post-vaccination which suggested that the vaccine provided highly protection against S. iniae. KEGG pathway analysis revealed that phagosome, cytokine-cytokine receptor interaction, MAPK signaling pathway, and CAMs were activated by the vaccine. The most of strongly up-regulated genes in golden pompano spleen are involving in innate immunity. For adaptive immunity, the vaccine evoked a CD8⁺ CTL-mediated response by MHC Ⅰ pathway to achieve immune protection.

Antimicrobial peptide hepcidin contributes to restoration of the intestinal flora after Aeromonas hydrophila infection in Acrossocheilus fasciatus

Hepcidin is a cysteine-rich antimicrobial peptide that serves an important role in the immunity system of fishes. It exhibits antibacterial, antifungal, antiviral, and antitumor activities. However, the exact role of fish hepcidin in the regulation of the intestinal flora still remains a mystery. In our study, we sequenced and characterized hepcidin from the liver of Acrossocheilus fasciatus. Phylogenetic tree analysis showed that A. fasciatus hepcidin and Gobiocypris rarus hepcidin were the most closely related, and both belonged to the fish HAMP1 cluster. Studies conducted on in vivo tissue distribution showed that the expression of hepcidin was highest in healthy A. fasciatus liver. Aeromonas hydrophila infection was confirmed by the increased expression of pro-inflammatory cytokine genes and bacterial loads in A. fasciatus tissues. After A. hydrophila infection, hepcidin expression significantly increased in the liver, spleen, and head kidney. In vitro antibacterial assays showed that hepcidin exhibits strong broad spectrum antibacterial activity. Furthermore, we examined the regulatory effect of hepcidin on the intestinal flora and found that A. fasciatus hepcidin restored the reduced diversity and compositional changes in intestinal flora caused by A. hydrophila infection. Our results suggest that hepcidin could regulate the intestinal flora in fishes; however, the underlying mechanisms need to be explored in greater detail.

Molecular characterization of HEPCIDIN-1 (HAMP1) gene in red-bellied pacu (Piaractus brachypomus)

Hepcidins are cysteine-rich peptides, which participate in iron metabolism regulation, the inflammatory and antimicrobial response. This study characterizes the hepcidin-1 (HAMP1) gene, its transcript expression in different tissues, as well as its regulation in a model of brain injury in Piaractus brachypomus. Bioinformatic analysis was carried out to determine conserved domains, glycosylation sites and protein structure of HAMP1, and probability that HAMP1 corresponds to an antimicrobial peptide (AMP). Relative gene expression of the P. brachypomus HAMP1 gene was determined by qPCR from cDNA of several tissues, a brain injury model, an organophosphate sublethal toxicity model and anesthetic experiment using the 2^-ΔΔCt method. HAMP1 ORF encodes for a 91 aa pre-prohepcidin conformed for a prodomain with 42 aa and mature peptide of 25 aa. Mature domain was determined as an AMP. HAMP1 transcript is expressed in all the tissues, being higher in the spleen and liver. HAMP1 mRNA level was upregulated in the brain injury group, as well as in the olfactory bulb, optic chiasm and telencephalon of red-bellied pacu brain exposed to an organophosphate. In anesthetic experiment, HAMP1 mRNA level was upregulated in the liver and gills. HAMP1 gene of P. brachypomus may be involved in the inflammatory, antimicrobial, hypoxia and stress oxidative response.

Acute ammonia exposure combined with heat stress impaired the histological features of gills and liver tissues and the expression responses of immune and antioxidative related genes in Nile tilapia

Ammonia exposure can be considered more stressful for aquatic animals when it coincides with high temperature. This study was conducted to detect the effects of ammonia exposure and heat stress and their interactions on the histological features of gills and liver tissues and the expression responses of immune and antioxidative related genes in Nile tilapia. Thus, 180 fish were divided into four groups (triplicates), where the first and third groups were kept in clean water without total ammonium nitrogen (TAN) exposure. At the same time, the second and fourth groups were exposed to 5 mg TAN/L. After seven days, the water temperature was raised in the third (without ammonia toxicity) and fourth (exposed with 5 mg TAN/L) groups up to 32 °C and kept under these conditions for 24 h. While the first (without ammonia toxicity) and second (exposed with 5 mg TAN/L) groups were kept under optimum water temperature (27.28 °C) then gills and liver tissues were dissected. Marked upregulation of keap1 was seen in the gills of fish exposed to ammonia/heat stress. The expression of mRNA levels for nrf2, nqo-1, cat, and gpx genes were downregulated in all stressed groups, with the lowest was recorded in the ammonia/heat stress group. The transcription of ho-1 was upregulated in the ammonia and heat stress groups while downregulated in the ammonia/heat stress group. The transcription of the complement C3 gene was downregulated in the livers of heat stress and ammonia/heat stress groups, while the lysozyme gene was downregulated in the ammonia/heat stress group. The mRNA expression levels of nf-κB, il-1β, and tnf-α genes were higher in the ammonia group than in the heat stress group. The highest transcription level of nf-κB, il-1β, tnf-α, il-8, and hsp70 genes and the lowest C3 and lysozyme genes were observed in fish exposed to ammonia/heat stress. The co-exposure to ammonia with heat stress triggered degeneration of primary and secondary gill filaments with telangiectasia and vascular congestion of secondary epithelium while, the liver showed hepatic vascular congestion and visible necrotic changes with nuclear pyknosis. In conclusion, the combined exposure of ammonia and heat stress induced oxidative stress, immunosuppression, and inflammation in Nile tilapia.

Pharmacological importance of TG12 from tachykinin and its toxicological behavior against multidrug-resistant bacteria Klebsiella pneumonia

Development of antimicrobial drugs against multidrug-resistant (MDR) bacteria is a great focus in recent years. TG12, a short peptide molecule used in this study was screened from tachykinin (Tac) protein of an established teleost Channa striatus (Cs) transcriptome. Tachykinin cDNA has 345 coding sequence, that denotes a protein contained 115 amino acids; in which a short peptide (TG12) was identified at 83-94. Tachykinin mRNA upregulated in C. striatus treated with Aeromonas hydrophila and Escherichia coli lipopolysaccharide (LPS). The mRNA up-regulation was studied using real-time PCR. The up-regulation tachykinin mRNA pattern confirmed the immune involvement of tachykinin in C. striatus during infection. Further, the identified peptide, TG12 was synthesized and its toxicity was demonstrated in hemolytic and cytotoxic assays using human erythrocytes and human dermal fibroblast cells, respectively. The toxicity study exhibited that the toxicity of TG12 was similar to negative control, phosphate buffer saline (PBS). Moreover, the antibiogram of TG12 was active against Klebsiella pneumonia ATCC 27736, a major MDR bacterial pathogen. Further, the antimicrobial activity of TG12 against pathogenic bacteria was screened using minimum inhibitory concentration (MIC) and anti-biofilm assays, altogether TG12 showed potential activity against K. pneumonia. Fluorescence assisted cell sorter flow cytometer analysis (FACS) and field emission scanning electron microscopy (FESEM) was carried on TG12 with K. pneumonia; the results showed that TG12 significantly reduced K. pneumonia viability as well as TG12 disrupt its membrane. In conclusion, TG12 of CsTac is potentially involved in the antibacterial immune mechanisms, which has a prospectus efficiency in pharma industry against MDR strains, especially K. pneumonia.

Antimicrobial Peptides and Copper(II) Ions: Novel Therapeutic Opportunities

The emergence of new pathogens and multidrug resistant bacteria is an important public health issue that requires the development of novel classes of antibiotics. Antimicrobial peptides (AMPs) are a promising platform with great potential for the identification of new lead compounds that can combat the aforementioned pathogens due to their broad-spectrum antimicrobial activity and relatively low rate of resistance emergence. AMPs of multicellular organisms made their debut four decades ago thanks to ingenious researchers who asked simple questions about the resistance to bacterial infections of insects. Questions such as "Do fruit flies ever get sick?", combined with pioneering studies, have led to an understanding of AMPs as universal weapons of the immune system. This review focuses on a subclass of AMPs that feature a metal binding motif known as the amino terminal copper and nickel (ATCUN) motif. One of the metal-based strategies of hosts facing a pathogen, it includes wielding the inherent toxicity of copper and deliberately trafficking this metal ion into sites of infection. The sudden increase in the concentration of copper ions in the presence of ATCUN-containing AMPs (ATCUN-AMPs) likely results in a synergistic interaction. Herein, we examine common structural features in ATCUN-AMPs that exist across species, and we highlight unique features that deserve additional attention. We also present the current state of knowledge about the molecular mechanisms behind their antimicrobial activity and the methods available to study this promising class of AMPs.

Effects of chronic glyphosate exposure on antioxdative status, metabolism and immune response in tilapia (GIFT, Oreochromis niloticus)

Glyphosate (Gly) is an active ingredient of herbicide, its underlying toxicity on fish is still unclear. The aim of this study was to evaluate chronic toxicity of Gly on tilapia via determining antioxidative status, metabolism, inflammation and immune response. The fish were exposed to different concentrations of Gly (0, 0.2, 0.8, 4 and 16 mg/L) for 80 days. The blood, liver, gills and spleen were collected to assay biochemical parameters and genes expression after 80 days of exposure. The results showed that treatments with higher Gly (4 and/16 mg/L) significantly increased the levels of TC, TG, AST, ALT, LDL-C and MDA, and apparently decreased the levels of SOD, GSH, CAT, HDL-C, HK, G3PDH, FBPase and G6PD in serum, liver and/or gills. The gene expression data showed that the treatments with Gly adversely affected Nrf2 pathway in liver, gills and spleen, as shown by significant changes of nrf2, keap1, ho-1, nqo1 and gsta mRNA levels. Meanwhile, inflammatory response was activated via enhancing the mRNA levels of nf-κb2, rel, rela tnf-α, and il-1β, and immunotoxicity was caused through downregulating the genes expression of c-lzm, hep, igm, hsp70 and c3 in liver, gills and/or spleen of tilapia after Gly exposure. Moreover, the mRNA levels of cyp1a and cyp3a were upregulated in 16 or 0.2 mg/kg Gly group in liver. Overall results suggested chronic Gly exposure reduced antioxidative ability, disturbed liver metabolism, promoted inflammation and suppressed immunity. Interestingly, the Nrf2 and NF-κB signaling pathways played key roles in Gly chronic toxicity.

Sequence analysis of hepcidin in barbel steed (Hemibarbus labeo): QSHLS motif confers hepcidin iron-regulatory activity but limits its antibacterial activity

Fish hepcidin genes are generally classified into two groups: hamp1-and hamp2-type isoforms. Hamp1-type hepcidin exhibits iron regulatory and antimicrobial activity, while hamp2-type shows a unique role in the immune response against various pathogens. An iron-regulatory motif exists at the N-terminus of hamp1-type hepcidin; however, the functional effect of this motif in fish is not well understood. Here, cDNA of the barbel steed (Hemibarbus labeo) hepcidin gene was cloned and sequenced. The predicted amino acid sequence comprised a signal peptide, a prodomain, and a mature peptide. Phylogenetic tree analysis revealed that barbel steed hepcidin belongs to the fish HAMP1 cluster and is closely related to Chinese rare minnow (Gobiocypris rarus) hepcidin. Barbel steed hepcidin is constitutively expressed in healthy fish tissues, predominantly in the liver. Following iron dextran treatment or Aeromonas hydrophila infection, expression of barbel steed hepcidin increased significantly in tested tissues. In vivo administration of intact hepcidin mature peptide (hep25) significantly and dose-dependently reduced ferroportin 1 expression, while truncated hepcidin mature peptide (hep20) lacking a QSHLS motif had no such effect. In vitro treatment of barbel steed monocytes/macrophages with hep25, but not hep20, increased the labile iron pool levels. Hep25 and hep20 conferred antibacterial activity only against A. hydrophila and Vibrio vulnificus, with greater activity of the latter at low concentrations. Neither hep25 nor hep20 impaired the cell membrane integrity of A. hydrophila, but could hydrolyze its genomic DNA; lack of a QSHLS motif enables hep20 to have a better hydrolytic effect. In summary, we identified an iron-regulatory motif in a fish species and demonstrated that this motif confers hamp1-type hepcidin iron-regulatory activity, but attenuates its antibacterial activity.

Piscidin 4: Genetic expression and comparative immunolocalization in Nile tilapia (Oreochromis niloticus) following challenge using different local bacterial strains

The antimicrobial activity of tilapia piscidin 4 (TP4) was determined in vitro against four bacterial strains, Aeromonas hydrophilla, Pseudomonas fluorescens, Streptococcus iniae and Vibrio anguillarum. Nile tilapia were infected with low and high doses of the tested pathogens; after 3, 6, 24 h and 7 days of the specific TP4 gene expression, tissue immunolocalization was also performed. Histopathological examination revealed septicaemia and necrosis of hemopoietic tissue for all of the tested bacteria. Immunolocalization showed abundance in S. iniae-infected fish tissues. Quantitative RT-PCR analysis revealed that high doses raised mRNA expression levels compared to low doses and expression levels increased in the infected fish, particularly after 24 h, indicating that TP4 exerts potent bactericidal activity against some fish pathogens and plays an essential role in fish immunity.

Antibacterial and antioxidant activity of clove oil against Streptococcus iniae infection in Nile tilapia (Oreochromis niloticus) and its effect on hepatic hepcidin expression

This study was designed to evaluate the modulating effect dietary clove essential oil (CL) has on the antioxidant and immunological status of Nile tilapia following Streptococcus iniae (Si) infection. Fish were placed on either control or (1.5 and 3%) CL-supplemented diets for 4 weeks. After sampling, the remaining fish in the control group were divided into 2 groups: an unchallenged (negative control) and an Si-challenged positive control. On the other hand, the remaining fish in CL-supplemented groups were challenged with Si, and mortality was checked for two weeks before the final sampling. Serum immunological parameters, tissue antioxidants, and oxidative stress markers were determined. Moreover, hepatic hepcidin expression was also measured in different groups. The obtained results showed improvements in blood phagocytic, bactericidal, lysozyme, and respiratory burst activities in CL-supplemented fish before and after the Si challenge. Si-challenge caused a remarkable increase in tissue malondialdehyde (MDA) levels that was inhibited by CL supplementation. The activities of glutathione peroxidase (GPx) and superoxide dismutase (SOD) in tissues were significantly elevated in a dose-dependent manner in CL-supplemented groups in both pre- and post-challenge experiments; renal SOD did not show any differences. Hepatic nitric oxide (NO) level was significantly decreased in CL-supplemented fish in a dose-dependent manner. In the post-challenge experiment, nitrosative stress was apparent in the liver and kidney; however, CL supplementation was sufficient to reverse it. Interestingly, a remarkable induction of the hepatic hepcidin expression was observed in all CL-supplemented groups in the pre-challenge experiment and Si-challenged fish, underscoring the role of CL as an antibacterial through inducing hepatic hepcidin expression to combat S. iniae infection. CL-supplementation was associated with lower mortality rates after Si-challenge, which was more pronounced in CL-3% supplemented fish. In conclusion, our results demonstrate that CL has a potent antioxidant role via increasing antioxidant enzymes' activities and antagonizing lipid peroxidation. Moreover, CL has an immune-stimulant effect by inducing the hepatic hepcidin expression and immunological markers in response to S. iniae infection.

Immune Regulation, but Not Antibacterial Activity, Is a Crucial Function of Hepcidins in Resistance against Pathogenic Bacteria in Nile Tilapia (Oreochromis niloticus Linn.)

In this study, the functions of a recombinant propeptide (rProOn-Hep1) and the synthetic FITC-labelled mature peptides sMatOn-Hep1 and sMatOn-Hep2 were analyzed. Moreover, sMatOn-Hep1 and sMatOn-Hep2 were mildly detected in the lymphocytes of peripheral blood mononuclear cells (PBMCs) and strongly detected in head kidney macrophages. The in vitro binding and antibacterial activities of these peptides were slightly effective against several pathogenic bacteria. Immune regulation by sMatOn-Hep1 was also analyzed, and only sMatOn-Hep1 significantly enhanced the phagocytic index in vitro (p < 0.05). Interestingly, intraperitoneal injection of sMatOn-Hep1 (10 or 100 µg) significantly elevated the phagocytic activity, phagocytic index, and lysozyme activity and clearly decreased the iron ion levels in the livers of the treated fish (p < 0.05). Additionally, sMatOn-Hep1 enhanced the expression levels of CC and CXC chemokines, transferrin and both On-Hep genes in the liver, spleen and head kidney, for 1–96 h after injection, but did not properly protect the experimental fish from S. agalactiae infection after 7 days of treatment. However, the injection of S. agalactiae and On-Heps indicated that 100 μg of sMatOn-Hep1 was very effective, while 100 μg of rProOn-Hep1 and sMatOn-Hep2 demonstrated moderate protection. Therefore, On-Hep is a crucial iron-regulating molecule and a key immune regulator of disease resistance in Nile tilapia.

Administration of dietary recombinant hepcidin on grass carp (Ctenopharyngodon idella) against Flavobacterium columnare infection under cage aquaculture conditions

Hepcidin links iron metabolism with innate immunity during the inhibition of bacterial infection. Our previous studies had shown that recombinant hepcidin can significantly reduce the mortality rate of Ctenopharyngodon idella infected with Flavobacterium columnare under laboratory conditions. Here, we studied the preventive and therapeutic effects of feed supplemented with different doses of recombinant hepcidin on F. columnare-challenged C. idella reared in a cage culture environment. The results showed that in the prevention groups, 30 and 90 mg/kg of added purified and unpurified hepcidin respectively resulted in a higher survival rate in the early post-infection period, while 60 mg/kg of purified hepcidin significantly improved the survival rate in the therapy group (all compared to the control group). In the hepatopancreas, the expression of hepcidin and ferritin was significantly up-regulated, and the levels of ferroportin and serum iron were significantly decreased, especially in the therapy group. In addition, the expression of iron-related genes in spleen and intestine exhibited a similar trend to that in hepatopancreas. Meanwhile, immune genes were up-regulated to varying degrees, and the therapy group exhibited a significantly improved expression of pro-inflammatory cytokines and specific immunity. In summary, our study shows that different doses of recombinant hepcidin had protective effects against bacterial infection by regulating the iron distribution and immune gene expression, which provides a strong foundation for the application of recombinant hepcidin in aquaculture.

Bacillus velezensis LF01: in vitro antimicrobial activity against fish pathogens, growth performance enhancement, and disease resistance against streptococcosis in Nile tilapia (Oreochromis niloticus)

Streptococcus agalactiae is a major pathogen causing streptococcosis. To prevent and control this bacterial disease, antagonistic bacteria have become a new research hotspot. This study evaluated the probiotic potential of Bacillus velezensis LF01 strain, which is antagonistic to S. agalactiae. The active compounds produced by LF01 showed antimicrobial activity against a broad spectrum of fish pathogens, including S. agalactiae, Streptococcus iniae, Aeromonas hydrophila, Edwardsiella tarda, Edwardsiella ictaluri, Aeromonas schubertii, Aeromonas veronii, Aeromonas jandaei, and Vibrio harveyi. The antimicrobial compounds were heat stable, pH stable, UV stable, resistant to proteases, and could be stored for a long time. To evaluate the probiotic function of LF01 in Nile tilapia, juveniles were divided into three treatment groups: a control group, an interval feeding group, and a continuous feeding group. Tilapia fed with LF01-supplemented diets (1.0 × 10⁹ CFU/g) showed significantly better growth performances than those of the control group (P < 0.05). Tilapia fed with LF01-supplemented diets significantly increased lysozyme (LZY) and superoxide dismutase (SOD) activities. The expression of three immune-related genes (C3, lyzc, and MHC-IIβ) was higher in the intestine, head kidney, and gill of tilapia from the continuous feeding group than in those from the control group (P < 0.05). Tilapia fed with LF01-supplemented diets showed remarkably improved survival rates after S. agalactiae infection, and analysis of their intestinal tract pathogens revealed that the abundance of Edwardsiella and Plesiomonas had significantly decreased compared with the control group. Our findings demonstrate that LF01 is an effective antagonist against various fish pathogens and has potential for controlling infections by Streptococcus spp. and other pathogens in tilapia.

Antioxidative, anti-inflammatory and hepatoprotective effects of resveratrol on oxidative stress-induced liver damage in tilapia (Oreochromis niloticus)

Resveratrol, a dietary polyphenol, has been shown to exert antioxidation, hepatoprotection, anti-inflammation and immunostimulation. However, the effects and underlying mechanism of resveratrol on liver injury in fish are still unclear. In the present study, we investigated the potential protective effects and mechanism of resveratrol on oxidative stress-induced liver damage in tilapia. Fish were fed diet containing four doses of resveratrol (0, 0.1, 0.3, and 0.6 g/kg diet) for 60 days, and then given an intraperitoneal injection of H₂O₂ or saline. The results showed that administration of resveratrol significantly ameliorated H₂O₂-induced liver injury. In serum and liver, resveratrol treatment suppressed the oxidative stress, as evidenced by the decline of lipid peroxidation level and increase of antioxidant activity. Resveratrol also activated erythroid 2-related factor 2 (Nrf2) signaling pathway and enhanced the heme oxygenase 1 (HO-1), NAD(P) H:quinone oxidoreductase 1 (NQO-1), glutathione S-transferase (GST) mRNA levels. Meanwhile, resveratrol treatment repressed TLR2-Myd88-NF-κB signaling pathway to decrease the inflammatory response in H₂O₂-induced liver injury as evidenced by the lower interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) and IL-8 mRNA levels and higher IL-10 mRNA level. Moreover, resveratrol treatment attenuated immunotoxicity in liver of H₂O₂-treated fish, accompanied by upregulation of hepcidin (HEP), complement 3 (C3) and lysozyme (LZM) mRNA levels. Overall results suggested that the protection of resveratrol on H₂O₂-induced liver injury, inflammation and immunotoxicity was due to its antioxidant property and its ability to modulate the Nrf2 and TLR2-Myd88-NF-κB signaling pathways.

Antioxidative, inflammatory and immune responses in hydrogen peroxide-induced liver injury of tilapia (GIFT, Oreochromis niloticus)

Oxidative stress has been implicated in the pathogenesis of many liver diseases in fish, but the molecular mechanism is still obscure. Here, we used hydrogen peroxide (H₂O₂) as a reactive oxygen species (ROS) to induce liver injury and assess underlying molecular mechanism linking oxidative stress and liver injury in fish. Tilapia were injected with various concentrations of H₂O₂ (0, 40, 120, 200, 300 and 400 mM) for 72 h. The blood and liver were collected to assay biochemical parameters and genes expression after 24, 48 and 72 h of injection. The results showed that treatments with higher H₂O₂ levels (300 and/or 400 mM) significantly increased the levels of GPT, GOT, AKP and MDA, and apparently decreased the levels of TP, ALB, SOD, GSH, CAT, GST and T-AOC throughout of the 72 h. The gene expression data showed that treatments with 200, 300 and/or 400 H₂O₂ suppressed Nrf2/keap1 pathway and its downstream genes including ho-1, nqo1 and gsta, activated inflammatory response via enhancing the mRNA levels of nf-κb, tnf-α, il-1β and il-8, and attenuating il-10 mRNA level, and caused immunotoxicity through downregulating the genes expression of c3, hep, lzm and Igm for 24, 48 and/or 72 h. Additionally, there was a mild or strong increase in levels of nrf2 and its subsequent antioxidant genes or enzymes such as ho-1, nqo1, gst, CAT and SOD in treatments with lower concentrations of H₂O₂ (40 or 120 mM) for 24 and/or 48 h. Overall results suggested that H₂O₂ hepatotoxicity was mainly concerned with lipid peroxidation, impairment antioxidant defense systems, inflammatory response and immunotoxicity, and Nrf2/Keap1 and NF-κB signaling pathways played important roles in oxidative stress-induced liver injury in fish.

Molecular characterization of a hepcidin homologue in starry flounder (Platichthys stellatus) and its synergistic interaction with antibiotics

Hepcidins are small cysteine-rich antimicrobial peptides that play an important role in host immunity against pathogenic organisms. Most fish hepcidins exert bactericidal activities against a wide range of pathogens. In this study, we identified a cDNA sequence encoding a hepcidin homologue (PsHepcidin) in the starry flounder Platichthys stellatus. The predicted amino acid sequence of PsHepcidin comprises a signal peptide and a prodomain, which are followed by the mature peptide. Sequence analysis revealed that PsHepcidin belongs to the fish HAMP2 cluster and that it is closely related to mudskipper hepcidin-2. Expression of PsHepcidin mRNA was detected in all examined immune-related tissues, with the highest transcript levels being found in the liver. In response to lipopolysaccharide treatment, PsHepcidin was significantly up-regulated in the liver, kidney, and spleen in a time-dependent manner. Chemically synthesized mature peptides of PsHepcidin were found to exhibit broad antimicrobial activity in vitro. We also investigated the combined effect of PsHepcidin and conventional antibiotics and found that these combinations showed synergistic effects against most of the examined bacterial strains. Collectively, the results of this study indicate that PsHepcidin exhibits potent antibacterial activity both independently and when used in combination with conventional antibiotics.

Excavating differentially expressed antimicrobial peptides from transcriptome of Larimichthys crocea liver in response to Cryptocaryon irritans

Larimichthys crocea, the special marine economy fish, owns the largest annual yield for a single species in China. One of the most significant factors affecting large yellow croaker culture is the diseases, especially the threat of marine white spot disease which caused by a protozoan Cryptocaryon irritans. Antimicrobial peptides (AMPs) have been demonstrated to be active against bacterium, fungi and parasites, showing their potential usefulness in aquaculture as substitutes for antibiotics. Many researches have been carried out about the AMPs concentrating on the activity resist on C. irritans, and piscidin-like of L. crocea owning widely antibacterial spectrum and strong activity against C. irritans was screened in our team. In the paper, taking advantage of the large yellow croaker hepatic comparison transcriptome in response to C. irritans at 3d post infection, seven kinds of AMPs have been excavated from the differently expressed genes, including LEAP2 like, LEAP-2A, hepcidin, hepcidin-like, piscidin-5-like, piscidin-5-like type 4 and bactericidal permeability increasing protein (BPI). Hepcidin, hepcidin-like, piscidin-5-like, piscidin-5-like type4 and BPI were up-regulated to protect large yellow croaker from being damaged by C. irritans infection; while LEAP2 like and LEAP-2A were down-regulated, they might be as a negative-feedback regulation factor or some other regulatory mechanisms to adjust the immune response in the process of C. irritans infection. The differential expression changes were verified with quantitative real-time PCR (qRT-PCR) to illustrate the reliability of the sequenced data. Hearteningly, piscidin-5-like type 4 was a novel type which was high similar to other piscidin-5-like types. Interestingly, the infection may well cause alternative splicing of LEAP-2A mRNA, which was a surprised phenomenon and finding after C. irritans infection, but more further study was needed to be conducted. Therefore, the data showed that these AMPs were involved in the immune response to the C. irritans infection. In all, these results implied that the immune response of AMPs to C. irritans infection was a complex and sophisticated regulatory process.

Fish antimicrobial peptides (AMP's) as essential and promising molecular therapeutic agents: A review

Antimicrobial peptides (AMPs) are generally considered as an essential component of innate immunity, thereby providing the first line of defense against wide range of pathogens. In addition, they can also kill the pathogens which are generally resistant to number of antibiotics, thereby providing the avenues for the development of future therapeutic agents. Fishes are constantly challenged by variety of pathogens which not only shows detrimental effect on their health but also increases risk of becoming resistant to conventional antibiotics. As fishes rely more on innate immunity, AMPs can serve as a potential defensive weapons in fishes for combating emerging devastating diseases. Generally, AMPs show multidimensional properties like rapid diffusion to the site of infection, recruitment of other immune cells to infected tissues and vigorous potential to rapidly neutralize broad range of pathogens (bacterial, fungal and viral). AMPs also exhibit diverse biological effect like endotoxin neutralization, immunomodulation and induction of angiogenesis in mammals. Due to these properties AMPs have become one of the most promising therapeutic agents to be studied. Till date, many AMPs have been isolated from the fishes but not fully characterized at molecular level. This review provides an overview of the structures, functions, and putative mechanisms of major families of fish AMPs. Further, we also highlighted how fish AMPs can be used as a novel therapeutic tool which is the theme of future research in drug development.

Nutritional Immunity Triggers the Modulation of Iron Metabolism Genes in the Sub-Antarctic Notothenioid Eleginops maclovinus in Response to Piscirickettsia salmonis

Iron deprivation is a nutritional immunity mechanism through which fish can limit the amount of iron available to invading bacteria. The aim of this study was to evaluate the modulation of iron metabolism genes in the liver and brain of sub-Antarctic notothenioid Eleginops maclovinus challenged with Piscirickettsia salmonis. The specimens were inoculated with two P. salmonis strains: LF-89 (ATCC^® VR-1361™) and Austral-005 (antibiotic resistant). Hepatic and brain samples were collected at intervals over a period of 35 days. Gene expression (by RT-qPCR) of proteins involved in iron storage, transport, and binding were statistically modulated in infected fish when compared with control counterparts. Specifically, the expression profiles of the transferrin and hemopexin genes in the liver, as well as the expression profiles of ferritin-M, ferritin-L, and transferrin in the brain, were similar for both experimental groups. Nevertheless, the remaining genes such as ferritin-H, ceruloplasmin, hepcidin, and haptoglobin presented tissue-specific expression profiles that varied in relation to the injected bacterial strain and sampling time-point. These results suggest that nutritional immunity could be an important immune defense mechanism for E. maclovinus against P. salmonis injection. This study provides relevant information for understanding iron metabolism of a sub-Antarctic notothenioid fish.

Expression of ayu antimicrobial peptide genes after LPS stimulation

Plecoglossus altivelis (ayu) is one of the most important fish species in the Japanese islands and in internal fish hatcheries. Living in open aquatic environments exposes fish to many pathogens. Therefore, they require rapid and strong immune defenses. We investigated in vivo the direct association between the ayu innate immune response, represented by the relative transcription of genes encoding the cathelicidin and hepcidin antimicrobial peptides, and lipopolysaccharide (LPS), a conventional pathogen-associated molecular patterns (PAMPs) of Gram-negative bacteria. Different concentrations of LPS (1, 10 and 100 µg/fish) were injected intraperitoneally into young (sexually immature) and adult (fully sexually mature) ayu. The relative expression of the antimicrobial peptide genes was measured 6 hr, 24 hr and 1 week after stimulation with LPS. We found a direct association between the expression of the antimicrobial peptide genes investigated and LPS stimulation. This relationship was time-, dose- and age-dependent. Further research is required to determine the cell-specific transcriptional regulation and posttranscriptional regulation of these antimicrobial peptides.

Molecular Characterisation of a Novel Isoform of Hepatic Antimicrobial Peptide, Hepcidin (Le-Hepc), from Leiognathus equulus and Analysis of Its Functional Properties In Silico

Hepcidin represents a family of cysteine-rich antimicrobial peptides that are mainly expressed in the liver of living organisms. In this study, we have identified and characterised a novel isoform of hepcidin from the common pony fish, Leiognathus equulus (Le-Hepc). A 261-bp fragment cDNA coding for 86 amino acids was obtained. Homologous analysis showed that Le-Hepc belongs to the hepcidin super family and shares sequence identity with other known fish pre-propeptide hepcidin sequences. The ORF encodes for a 24-amino acid (aa) signal peptide coupled to a 36-aa prodomain followed by a 26-aa mature peptide. The mature peptide region has a calculated molecular weight of 2.73 kDa, a net positive charge of +2 and a theoretical pI of 8.23. Phylogenetic analysis of Le-Hepc showed a strong relationship with other fish hepcidin sequences and clustered into HAMP2 group hepcidins. Secondary structural analysis indicated that Le-Hepc mature peptide contains two antiparallel β-sheets strengthened by four disulphide bonds formed by eight conserved cysteine residues. The physicochemical properties of the peptide and its structural parameters are in agreement with characteristic features of an antimicrobial peptide. This is the first report of an antimicrobial peptide from the common pony fish, L. equulus.

H2A and Ca-L-hipposin gene: Characteristic analysis and expression responses to Aeromonas hydrophila infection in Carassius aurutus

Antimicrobial peptide is an important component of the host innate immune system and thus serves a crucial function in host defense against microbial invasion. In this study, H2A and derived antimicrobial peptide Ca-L-hipposin were cloned and characterized in Carassius aurutus. The gene H2A full-length cDNA is 908 bp and includes a 5'-terminal untranslated region (UTR) of 55 bp and a 3'-terminal UTR of 466 bp with a canonical polyadenylation signal sequence AATAA, as well as an open reading frame (ORF) of 387 bp encoding a polypeptide of 128 amino acids, with a molecular weight of 13.7 kDa, an isoelectric point of 10.7, and 94% homology with Danio rerio H2A. The secondary structure of H2A includes the α-spiral with 51 amino acids with a composition ratio of 39.8%, as well as a β-corner with 15 amino acids in a composition ratio of 11.7%. The online software ExPaSy predicted that a peptide sequence with 51 amino acids from the 2nd to 52nd amino acids in histone H2A can be produced through hydrolization by protease chymotrypsin, which indicates a difference of only three amino acids, compared with the antimicrobial peptide hipposin in Hippoglossus hippoglossus with a homology of 94%. Ca-L-hipposin includes 51 amino acids with a molecular weight of 5.4 kDa and an isoelectric point of 12.0, the secondary structure of which contains an α-helix of 17 amino acids accounting for 33.3% and a β-corner of 8 amino acids accounting for 15.7%. H2A was extensively expressed in the mRNA levels of various tissues, with higher expression levels in kidney and spleen. After C. aurutus was challenged with Aeromonas hydrophila, the mRNA expression levels of H2A were upregulated in the kidney, spleen, and liver. H2A serves an important function in the defense against the invasion of A. hydrophila. In addition, sequence characteristics reveal that Ca-L-hipposin could be a potential antimicrobial peptide for use in killing pathogenic bacteria in aquaculture.

Characterization of Two Antimicrobial Peptides from Antarctic Fishes (Notothenia coriiceps and Parachaenichthys charcoti)

We identified two antimicrobial peptides (AMPs) with similarity to moronecidin in Antarctic fishes. The characteristics of both AMPs were determined using moronecidin as a control. Moronecidin, which was first isolated from hybrid striped bass, is highly salt-resistant, and possesses broad-spectrum activity against various microbes. The moronecidin-like peptide from Notothenia coriiceps exhibited a narrower spectrum of activity and a higher salt sensitivity than moronecidin. The AMP from Parachaenichthys charcoti exhibited similar antimicrobial activity to moronecidin, and similar salt sensitivity. In an experiment to identify toxic effects, both of the moronecidin-like peptides from the Antarctic fishes exhibited lower hemolytic activity than moronecidin. In spite of its low toxicity, the AMP from N. coriiceps is unlikely to be considered as a candidate for antibiotic development, owing to its narrow spectrum of activity and high salt sensitivity. In contrast, the high salt resistance and broad-spectrum activity of the AMP from P. charcoti could be more advantageous for clinical use than moronecidin, since it could kill bacteria under physiological conditions with low toxicity. A further comparison of these two AMPs from Antarctic fishes with other AMPs could help to reduce the toxicity of AMPs for clinical use.

The chemokinome superfamily in channel catfish: I. CXC subfamily and their involvement in disease defense and hypoxia responses

Chemokines are a superfamily of structurally related chemotactic cytokines exerting significant roles in regulating cell migration and activation. They are defined by the presence of four conserved cysteine residues and are divided into four subfamilies depending on the arrangement of the first two conserved cysteines residues: CXC, CC, C and CX3C. In this study, a complete set of 17 CXC chemokine ligand (CXCL) genes was systematically identified and characterized from channel catfish genome through data mining of existing genomic resources. Phylogenetic analysis allowed annotation of the 17 CXC chemokines. Extensive comparative genomic analyses supported their annotations and orthologies, revealing the existence of fish-specific CXC chemokines and the expansion of CXC chemokines in the teleost genomes. The analysis of gene expression after bacterial infection indicated the CXC chemokines were expressed in a gene-specific manner. CXCL11.3 and CXCL20.3 were expressed significantly higher in resistant fish than in susceptible fish after ESC infection, while CXCL20.2 were expressed significantly higher in resistant fish than in susceptible fish after columnaris infection. The expression of those CXC chemokines, therefore can be a useful indicator of disease resistance. A similar pattern of expression was observed between resistant and susceptible fish with biotic and abiotic stresses, ESC, columnaris and hypoxia, suggesting that high levels of expression of the majority of CXC chemokines, with exception of CXC11 and CXC20, are detrimental to the host.

cDNA cloning and expression analysis of a hepcidin gene from yellow catfish Pelteobagrus fulvidraco (Siluriformes: Bagridae)

Hepcidin is a small, cysteine-rich antimicrobial peptide with a highly conserved β-sheet structure that plays a vital role in innate host immunity against pathogenic organisms. In this study, a hepcidin gene was identified in Pelteobagrus fulvidraco, an economically important freshwater fish in China. The gene is named PfHep. The complete PfHep cDNA was 723 bp, including a 5'-untranslated region (UTR) of 102 bp, a 3'-UTR of 339 bp and an open reading frame of 282 bp encoding a polypeptide of 93 amino acids, which includes a predicted signal peptide and the Hepcidin domain. The predicted mature, cationic PfHep protein has a typical hepcidin RX (K/R)R motif and eight conserved cysteine residues. The deduced PfHep protein sequence has 70%, 54% and 39% percent identity with hepcidins from Ictalurus punctatus, Danio rerio, and Homo sapiens, respectively. The predicted tertiary structure of PfHep is very similar to that of hepcidin in other animals. Phylogenetic analysis revealed that PfHep is closely related to the hepcidins of I. punctatus and I. furcatus. Real-time quantitative reverse transcription-PCR showed that the PfHep gene was expressed most in liver of healthy P. fulvidraco, and expressed to some extent in all the tissues tested. After challenge with lipopolysaccharide and polyriboinosinic:polyribocytidylic acid (poly I:C), respectively, the expression levels of PfHep were markedly upregulated in liver, spleen, head kidney and blood at different time points. Together these results imply that PfHep may be an important component of the innate immune system and be involved in immune defense against invading pathogens.

Genomic organization, expression and antimicrobial activity of a hepcidin from taimen (Hucho taimen, Pallas)

Hepcidin, an antimicrobial peptide, plays a crucial role in innate immune system of teleost fish. As a cysteine-rich peptide, hepcidin possesses a dual function including iron regulation and innate immunity. In the present study, a full-length hepcidin cDNA (HtHep) was cloned and characterized by RT-PCR and RACE techniques from taimen (Hucho taimen, Pallas), which is a type of rare, precious and cold-water fish species in China. The cDNA contains an open reading frame (ORF) of 267 bp encoding 88 amino acid (aa), with 170 bp located in the 5(') untranslated region (UTR) and 151 bp in the 3' UTR. The genomic sequences analysis showed that the HtHep gene consisted of three exons and two introns (with the length 94 and 251 bp, respectively). With a predicted molecular mass of 2881.4 Da and a theoretical pI of 8.53, the deduced amino acid encodes a signal peptide of 24 aa, prodomain of 39 aa and mature peptide of 25 aa. The signal peptidase (SA-VP) and the motif RX (K/R)R of propeptide convertase suggested the cleavage site of signal and mature peptide. Eight conserved cysteine residues were also identified and formed four disulfide bonds. Pair-wise alignments showed that HtHep clustered together with two fish species of Salmonidae family (Salmo salar and Oncorhynchus mykiss) in HAMP1 branch. Quantitative RT-PCR analysis indicated that the mRNA levels of HtHep were detected in a wide range of tissues and the highest level was detected in the liver. Its expression was also detected early during embryonic stage and could be up-regulated in the liver when challenged with pathogenic bacteria (Yersinia ruckeri). The recombinant HtHep (rHtHep) had antimicrobial activity against both gram-positive (Micrococcus lysodeikticus and Staphylococcus aureus) and gram-negative bacteria (Escherichia coli). Our results suggested that HtHep might be involved in the innate immune defense against bacterial pathogens in taimen.

Septin genes in channel catfish (Ictalurus punctatus) and their involvement in disease defense responses

Septins are an evolutionarily conserved family of GTP-binding proteins. They are involved in diverse processes including cytokinesis, apoptosis, infection, neurodegeneration and neoplasia. In this study, through thorough data mining of existed channel catfish genomic resources, we identified a complete set of 15 septin genes. Septins were classified into four subgroups according to phylogenetic analysis. Extensive comparative genomic analysis, including domain and syntenic analysis, supported their annotation and orthologies. The expression patterns of septins in channel catfish were examined in healthy tissues and after infection with two major bacterial pathogens, Edwardsiella ictaluri and Flavobacterium columnare. In healthy channel catfish, most septin genes were ubiquitously expressed and presented diversity patterns in various tissues, especially mucosal tissues, proposing the significant roles septin genes may play in maintaining homeostasis and host immune response activities. After bacterial infections, most septin genes were regulated, but opposite direction in expression profiles were found with the two bacterial pathogens: the differentially expressed septin genes were down-regulated in the intestine after E. ictaluri infection while generally up-regulated in the gill after F. columnare infection, suggesting a pathogen-specific and tissue-specific pattern of regulation. Taken together, these results suggested that septin genes may play complex and important roles in the host immune responses to bacterial pathogens in channel catfish.

Complement regulatory protein genes in channel catfish and their involvement in disease defense response

Complement system is one of the most important defense systems of innate immunity, which plays a crucial role in disease defense responses in channel catfish. However, inappropriate and excessive complement activation could lead to potential damage to the host cells. Therefore the complement system is controlled by a set of complement regulatory proteins to allow normal defensive functions, but prevent hazardous complement activation to host tissues. In this study, we identified nine complement regulatory protein genes from the channel catfish genome. Phylogenetic and syntenic analyses were conducted to determine their orthology relationships, supporting their correct annotation and potential functional inferences. The expression profiles of the complement regulatory protein genes were determined in channel catfish healthy tissues and after infection with the two main bacterial pathogens, Edwardsiella ictaluri and Flavobacterium columnare. The vast majority of complement regulatory protein genes were significantly regulated after bacterial infections, but interestingly were generally up-regulated after E. ictaluri infection while mostly down-regulated after F. columnare infection, suggesting a pathogen-specific pattern of regulation. Collectively, these findings suggested that complement regulatory protein genes may play complex roles in the host immune responses to bacterial pathogens in channel catfish.

The effects of feeding β-glucan to Pangasianodon hypophthalmus on immune gene expression and resistance to Edwardsiella ictaluri

Pangasianodon hypophthalmus (striped catfish) is an important aquaculture species and intensification of farming has increased disease problems, particularly Edwardsiella ictaluri. The effects of feeding β-glucans on immune gene expression and resistance to E. ictaluri in P. hypophthalmus were explored. Fish were fed 0.1% fungal-derived β-glucan or 0.1% commercial yeast-derived β-glucan or a basal control diet without glucan. After 14 days of feeding, the mRNA expression of immune genes (transferrin, C-reactive protein, precerebellin-like protein, Complement C3 and factor B, 2a MHC class II and interleukin-1 beta) in liver, kidney and spleen were determined. Following this fish from each of the three diet treatment groups were infected with E. ictaluri and further gene expression measured 24 h post-infection (h.p.i.), while the remaining fish were monitored over 2 weeks for mortalities. Cumulative percentage mortality at 14 days post-infection (d.p.i.) was less in β-glucan fed fish compared to controls. There was no difference in gene expression between dietary groups after feeding for 14 days, but there was a clear difference between infected and uninfected fish at 24 h.p.i., and based on principal component analysis β-glucans stimulated the overall expression of immune genes in the liver, kidney and spleen at 24 h.p.i.

Antimicrobial Peptides as Mediators of Innate Immunity in Teleosts

Antimicrobial peptides (AMPs) have been identified throughout the metazoa suggesting their evolutionarily conserved nature and their presence in teleosts is no exception. AMPs are short (18–46 amino acids), usually cationic, amphipathic peptides. While AMPs are diverse in amino acid sequence, with no two AMPs being identical, they collectively appear to have conserved functions in the innate immunity of animals towards the pathogens they encounter in their environment. Fish AMPs are upregulated in response to pathogens and appear to have direct broad-spectrum antimicrobial activity towards both human and fish pathogens. However, an emerging role for AMPs as immunomodulatory molecules has become apparent—the ability of AMPs to activate the innate immune system sheds light onto the multifaceted capacity of these small peptides to combat pathogens through direct and indirect means. Herein, this review focuses on the role of teleost AMPs as modulators of the innate immune system and their regulation in response to pathogens or other exogenous molecules. The capacity to regulate AMP expression by exogenous factors may prove useful in modulating AMP expression in fish to prevent disease, particularly in aquaculture settings where crowded conditions and environmental stress pre-dispose these fish to infection.

The new insights into the oyster antimicrobial defense: Cellular, molecular and genetic view

Oysters are sessile filter feeders that live in close association with abundant and diverse communities of microorganisms that form the oyster microbiota. In such an association, cellular and molecular mechanisms have evolved to maintain oyster homeostasis upon stressful conditions including infection and changing environments. We give here cellular and molecular insights into the Crassostrea gigas antimicrobial defense system with focus on antimicrobial peptides and proteins (AMPs). This review highlights the central role of the hemocytes in the modulation and control of oyster antimicrobial response. As vehicles for AMPs and other antimicrobial effectors, including reactive oxygen species (ROS), and together with epithelia, hemocytes provide the oyster with local defense reactions instead of systemic humoral ones. These reactions are largely based on phagocytosis but also, as recently described, on the extracellular release of antimicrobial histones (ETosis) which is triggered by ROS. Thus, ROS can signal danger and activate cellular responses in the oyster. From the current literature, AMP production/release could serve similar functions. We provide also new lights on the oyster genetic background that underlies a great diversity of AMP sequences but also an extraordinary individual polymorphism of AMP gene expression. We discuss here how this polymorphism could generate new immune functions, new pathogen resistances or support individual adaptation to environmental stresses.

Expression and cellular localization of hepcidin mRNA and protein in normal rat brain

Background

Hepcidin is a peptide hormone belonging to the defensin family of cationic antimicrobial molecules that has an essential role in systemic iron homeostasis. The peptide is synthesised by hepatocytes and transported in the circulation to target tissues where it regulates the iron export function of the ferrous iron permease, ferroportin. In the brain hepcidin protein has been identified using immuno-histochemistry and mRNA by real-time PCR but not by in situ hybridisation raising the question of whether there is measurable transcription of the hepcidin gene in the central nervous system. Alternatively hepcidin could be transported as a hormone to the brain via the circulation.

Results

By RT-PCR hepcidin mRNA was present at low level throughout normal rat brain while in situ hybridisation to detect low-abundant mRNA revealed that transcripts were restricted to endothelium of blood vessels and choroid plexus. In contrast, hepcidin protein analysed by immuno-histochemistry was highly expressed in blood vessels, in endothelium and in pericytes. Hepcidin was also present in glial cells and in the olfactory bulb, sub-ventricular zone and dentate gyrus, areas where neurogenesis and synaptic plasticity are maintained throughout adult life. The hepcidin species identified by Western blotting in sub-ventricular zone, cortex and hippocampus migrated as a ~2.8 kDa band, identical in size to hepcidin present in normal rat serum suggesting that hepcidin in brain was the full-length biologically active 25 amino acid peptide. Hepcidin co-localised with ferroportin in ependymal cells of the sub-ventricular zone and in the corpus callosum consistent with a regulatory role in iron metabolism at these sites.

Conclusions

Hepcidin protein was widely expressed in brain parenchyma while levels of hepcidin gene transcription appeared to be below the limits of detection of the in situ hybridisation probes. This disparity suggests that not all hepcidin in the brain is transcribed in situ and may originate in part outside the brain. The properties of hepcidin as a cationic peptide hormone are reflected in the finding of hepcidin in the walls of blood vessels and in pericytes and glia, cells that may be involved in transporting the peptide into brain interstitium.

Molecular cloning and functional characterization of the hepcidin gene from the convict cichlid (Amatitlania nigrofasciata) and its expression pattern in response to lipopolysaccharide challenge

The hepcidin gene is widely expressed in many fish species and functions as an antimicrobial peptide, suggesting that it plays an important role in the innate immune system of fish. In the present study, the Amatitlania nigrofasciata hepcidin gene (AN-hepc) was cloned from the liver and its expression during an immune response was characterized. The results of quantitative PCR and RT-PCR showed that the AN-hepc transcript was most abundant in the liver. The expression of AN-hepc mRNA was significantly increased in the liver, stomach, heart, intestine, gill and muscle but was not significantly altered in the spleen, kidney, brain or skin after lipopolysaccharide challenge. The synthetic AN-hepc peptide showed a wide spectrum of antimicrobial activity in vitro toward gram-positive and gram-negative bacteria. In particular, this peptide demonstrated potent antimicrobial activity against the aquatic pathogens Vibrio alginolyticus, V. parahaemolyticus, V. vulnificus, Aeromonas hydrophila and Streptococcus agalactiae. The in vivo bacterial challenge results demonstrated that the synthetic AN-hepc peptide significantly improved the survival rate of S. agalactiae- and V. vulnificus-infected zebrafish. Taken together, these data indicate an important role for AN-hepc in the innate immunity of A. nigrofasciata and suggest its potential application in aquaculture for increasing resistance to disease.

Pathogen recognition receptors in channel catfish: IV. Identification, phylogeny and expression analysis of peptidoglycan recognition proteins

Peptidoglycan recognition proteins (PGRPs) can recognize bacterial cell wall (peptidoglycan) and activate innate immune system. In addition to its function as pathogen recognition receptors (PRRs), PGRPs are also involved in directly killing bacteria, and regulating multiple signaling pathways. Recently, we have reported catfish PRRs including nucleotide-binding domain, leucine-rich repeat containing receptors (NLRs), retinoic acid inducible gene I (RIG-I) like receptors (RLRs), and Toll-like receptors (TLRs). In this study, we identified and characterized the PGRP gene family in channel catfish which included two members, PGLYRP-5 and PGLYRP-6. Phylogenetic analysis, syntenic analysis and protein structural analysis were conducted to determine their identities and evolutionary relationships. In order to gain insight into the roles of PGRPs in catfish innate immune responses, quantitative real-time PCR was used to investigate the expression profiles in catfish healthy tissues and after bacterial infection. Both PGLYRP-5 and PGLYRP-6 were ubiquitously expressed in all 12 healthy tissues, and most highly expressed in gill and spleen, respectively. Distinct expression patterns were observed for PGRPs after infection with Edwardsiella ictaluri and Flavobacterium columnare, both Gram-negative bacteria. After infection with E. ictaluri, both PGLYRP-5 and PGLYRP-6 were significantly down-regulated at a certain time-point, while both genes were generally up-regulated in the gill after infection with F. columnare. Collectively, these findings suggested that PGRPs may play complex roles in the host immune response to bacterial pathogens in catfish.

Characterization and structural analysis of hepcidin like antimicrobial peptide from Schizothorax richardsonii (Gray)

Innate immune system is a primary line of defense in fish that protects it from the invading pathogens. Antimicrobial peptides (AMPs) are widely distributed in nature and are essential components of innate immunity. These molecules enable the host's innate immune system to fight against a variety of infectious agents. One such AMP, hepcidin, is a cysteine rich amphipathic peptide. We have amplified, cloned and characterized hepcidin like AMP from Schizothorax richardsonii that inhabits one of the most difficult aquatic ecosystems in the Indian Himalayas. The cDNA encoding hepcidin like peptide was amplified as a 371 bp fragment with an open reading frame (ORF) of 279 nucleotides flanked by 5' and 3' UTRs of 70 and 22 bases respectively. This ORF encodes a peptide of 93 amino acids with a signal peptide of 24 amino acids and a mature peptide of 25 amino acids. The mature hepcidin like peptide of S. richardsonii has eight cystine residues that participate in the formation of four disulfide bonds, a unique feature of hepcidin like AMPs. A 3D model of hepcidin like mature peptide was generated using Modeller 9.10 which was validated using PROCHECK and ERRAT. Phylogenetic analysis of hepcidin like AMP from S. richardsonii revealed that it was closely related to hepcidin from olive barb (Puntius sarana).

Characterization and bioactivity of hepcidin-2 in zebrafish: dependence of antibacterial activity upon disulfide bridges

Hepcidin is an antimicrobial peptide and iron-regulatory molecule with highly conserved disulfide bridges among vertebrates, but structural insights into the function in fish remains largely missing. We demonstrate here that recombinant hepcidin-2 from zebrafish is capable of inhibiting the growth of the Gram-negative bacteria Escherichia coli and Vibrio anguillarum, and the Gram-positive bacteria Staphylococcus aureus and Bacillus subtilis with minimum inhibitory concentrations (MICs) of 18, 15, 13 and 9μM, respectively. We also show by TEM examination that recombinant hepcidin-2 is directly cidal to the cells of E. coli and S. aureus. Moreover, we find that hepcidin-2 displays affinity to LPS, LTA and PGN. All these data indicate that hepcidin-2 is both a pattern recognition molecule, capable of identifying LPS, LTA and PGN, and an antibacterial effector, capable of inhibiting the growth of bacteria. The data also show that the antibacterial activity of hepcidin-2 depends upon the disulfide bridges.

Turbot (Scophthalmus maximus) hepcidin-1 and hepcidin-2 possess antimicrobial activity and promote resistance against bacterial and viral infection

Hepcidin is an antimicrobial peptide and a regulator of iron homeostasis. In turbot (Scophthalmus maximus), two types of hepcidins have been identified, which share approximately 50% sequence identity. In this study, we examined the antimicrobial potentials of the two hepcidins in the form of synthesized peptides, SmHep1P and SmHep2P. We found that SmHep1P and SmHep2P exhibited apparent bactericidal activities against both Gram-positive and Gram-negative bacteria in a dose-dependent manner. The bactericidal effect of SmHep1P was stronger against Gram-positive bacteria, while the bactericidal effect of SmHep2P was stronger against Gram-negative bacteria. Fluorescence and electron microscopy showed that both peptides were able to bind to the target bacterial cells and alter the surface structure of the cells. In vitro studies showed that SmHep1P and SmHep2P reduced bacterial invasion into cultured fish cells. In vivo studies showed that turbot administered with SmHep1P and SmHep2P exhibited significantly enhanced resistance against bacterial and viral infection. In both in vivo and in vitro studies, the antimicrobial activities of SmHep2P were in most cases significantly stronger than those of SmHep1P. Together these results indicate that the two hepcidins of turbot most likely possess antimicrobial properties and play a role in the innate immune defense against bacterial and viral pathogens.

Molecular characterization, phylogeny and expression of a hepcidin gene in the blotched snakehead Channa maculata

A hepcidin-like gene (cmHep) was cloned and characterized from the liver of the blotched snakehead Channa maculata. The complete cmHep cDNA was 756 bp in length, containing an open reading frame of 270 bp (encoding 89 amino acids), flanked by 210 bp and 276 bp of 5' and 3' untranslated regions, respectively. The deduced peptide of 89 amino acids consisted of 24 aa, 40 aa and 25 aa for signal peptide, prodomain and mature peptide, respectively. The mature peptide had eight cysteines at the identical conserved positions in common with most of other known hepcidins in vertebrates. cmHepc gene displayed a tripartite structure (three exons interrupted by two introns), which organisation was conserved between the blotched snakehead and other fish species. Phylogenetic analysis of hepcidins from C. maculata and other vertebrates showed that major phylogenetic grouping of fish hepcidin coincided with the current euteleosts classification, indicating the multiphyletic evolution of hepcidin in the teleosts. In the Acanthopterygii subclade, there were two distinct additional subclades named as HAMP-Ac1 and HAMP-Ac2. The blotched snakehead hepcidin was in the group HAMP-Ac1, which has the hypothetical iron regulatory sequence [Q-S/I-H-L/I-S/A] motif in N-terminal of mature peptide. The RT-PCR showed cmHep mRNA transcripts were widely distributed in all tissues tested in the blotched snakehead including the liver, gill, intestine, spleen, head kidney and peripheral white blood cell. The most abundant of cmHep mRNA was detected in liver. A significant up-regulation of cmHep expression was detected only in head kidney at 24h post-challenge with Vibrio parahaemolyticus in blotched snakehead adults, no significant differences found in liver, gill, intestine and spleen. The cmHep expression was up-regulated in spleen, head kidney and intestine at 24h post-injection with LPS in blotched snakehead juveniles, liver cmHep expression was not altered. Iron overloading and poly I:C stimulation down-regulated cmHep expression in liver, but did not significantly change cmHep expression in spleen, head kidney and intestine in blotched snakehead juveniles.

Antimicrobial peptides from fish

Antimicrobial peptides (AMPs) are found widely distributed through Nature, and participate in the innate host defense of each species. Fish are a great source of these peptides, as they express all of the major classes of AMPs, including defensins, cathelicidins, hepcidins, histone-derived peptides, and a fish-specific class of the cecropin family, called piscidins. As with other species, the fish peptides exhibit broad-spectrum antimicrobial activity, killing both fish and human pathogens. They are also immunomodulatory, and their genes are highly responsive to microbes and innate immuno-stimulatory molecules. Recent research has demonstrated that some of the unique properties of fish peptides, including their ability to act even in very high salt concentrations, make them good potential targets for development as therapeutic antimicrobials. Further, the stimulation of their gene expression by exogenous factors could be useful in preventing pathogenic microbes in aquaculture.