Discovery and molecular cloning of piscidin-5-like gene from the large yellow croaker (Larimichthys crocea)

Identification and characterization of two novel antimicrobial peptides from Japanese sea bass (Lateolabrax japonicus) with antimicrobial activity and MO/MФ activation capability

Piscidins participate in the innate immune response of fish, which aims to eliminate recognized foreign microbes and restore the homeostasis of immune system. We characterized two piscidin-like antimicrobial peptides (LjPL-3 and LjPL-2) isolated from Japanese sea bass (Lateolabrax japonicus). LjPL-3 and LjPL-2 showed different expression patterns in tissues. After Vibrio harveyi infection, the mRNA expression of LjPL-3 and LjPL-2 was upregulated in the liver, spleen, head kidney, and trunk kidney. The synthetic mature peptides LjPL-3 and LjPL-2 exhibited different antimicrobial spectra. Furthermore, LjPL-3 and LjPL-2 treatments decreased inflammatory cytokine production while promoting chemotaxis and phagocytosis in monocytes/macrophages (MO/MФ). LjPL-2, but not LjPL-3, displayed bacterial killing capability in MO/MФ. LjPL-3 and LjPL-2 administration increased Japanese sea bass survival after V. harveyi challenge, which was accompanied by a decline in bacterial burden. These data suggested that LjPL-3 and LjPL-2 participate in immune response through direct bacterial killing and MO/MФ activation.

Antibacterial activity study of a novel piscidin 5-like type 4 from Larimichthys crocea

As one of the key components of innate immune system, piscidins are likely to play pivotal role in the first defense line in fish. Piscidins own multiple resistance activity. A novel piscidin 5-like type 4 was excavated from Larimichthys crocea (termed Lc-P5L4) liver transcriptome immuned by Cryptocaryon irritans, and upregulated at 7 days post infection when secondary bacterial infection occurred. In the study, we characterized the antibacterial activity of Lc-P5L4. The liquid growth inhibition assay detected the recombinant Lc-P5L4 (rLc-P5L) had potent antibacterial activity to Photobacterium damselae. Scanning electron microscope (SEM) observed the cell surface of P. damselae collapsed to form pit, and membrane of some bacteria ruptured after co-incubation with rLc-P5L. Further, transmission electron microscope (TEM) was also employed to observe the intracellular microstructural damage, rLc-P5L4 caused cytoplasm contraction, pores formation and contents leakage. After knowing about its antibacterial effects, the preliminary antibacterial mechanism was also explored, western blot analysis showed rLc-P5L4 could bind to P. damselae through targeting to LPS. Agarose gel eletrophoresis analysis further showed rLc-P5L4 could also penetrate into cells and brought about genome DNA degradation. Therefore, rLc-P5L4 was of potential being a candidate to explore new antimicrobial drug or additive agent, especially to P. damselae.

Defensing role of novel piscidins from largemouth bass (Micropterus salmoides) with evidence of bactericidal activities and inducible expressional delineation

Micropterus salmoides is an economical important species of freshwater-cultured fish, the in-depth knowledge of its immune system is in urgent development to cope with serious infectious diseases. Piscidin is an important antimicrobial peptide (AMP) family existing in almost all teleosts. However, no piscidin has been reported in largemouth bass. In this study, three novel piscidins (MSPiscidin-1, -2, and -3) were firstly identified and characterized from the largemouth bass. The predicted mature peptides of MSPiscidin-1, -2, and -3 (consists of 24, 27, 25 amino acid residues, respectively) all adopted an amphipathic α-helical conformation representative of cationic AMPs that are important for membrane permeabilization and antibacterial activity. MSPiscidin-2 and -3 indeed displayed strong, broad-spectrum, and highly efficient antimicrobial activities in vitro against aquatic pathogens, but MSPiscidin-1 didn't show direct antimicrobial activity. MSPiscidin-2 and -3 killed bacteria mainly by inducing membrane permeabilization, in addition, they also can interact with bacterial genomic DNA, which might influence the DNA replication and transcription. Besides, MSPiscidin-2 and -3 could effectively inhibit the formation of the bacterial biofilm and eliminate the preformed biofilms. In vivo, MSPiscidin-1-3 genes showed an inducible expression pattern in the tested tissues upon Vibrio harveyi infection, which further indicated the key roles of piscidins in innate immunity in largemouth bass. Overall, this study will supplement the understanding of M. salmoides innate immune system and provide candidates for the design of novel peptide antibacterial agents used in aquaculture.

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.

Characteristics delineation of piscidin 5 like from Larimichthys crocea with evidence for the potent antiparasitic activity

Several researches reported that piscidin members of teleosts owned strong antiparasitic activity. Cryptocaryon irritans, a type of ectoparasite, could infect most of the marine teleosts. Larimichthys crocea could severely suffer from marine white spot disease caused by C. irritans, and their mortality rate was significantly high. Concentrating on this problem, we have done many related works. Piscidin 5 like (termed Lc-P5L) was another piscidin member isolated from a comparative transcriptome of C. irritans-immuned L. crocea. In the paper, quantitative Real-time PCR (qRT-PCR) showed Lc-P5L was upregulated in examined tissues, including gill, head kidney, muscle, liver, spleen and intestine after challenged by C. irritans, the significant upregulation time was in accordance to key developmental stages of C. irritans, which implied different infection stages could result in host immune response. Furthermore, using microscope techniques, we observed theronts or trophonts became weakly motile, cilia became detached, cells were out of shape, membranes eventually lysed in different cell positions and cytoplasmic contents leaked. Laser confocal scanning microscope (LCSM) observed theronts macronucleus grew swell and depolymerized after treated by recombinant Lc-P5L (rLc-P5L). Data suggested rLc-P5L was significantly lethal to C. irritans, and the death state of the parasite incubated with rLc-P5L was remarkably similar to other piscidin members or other antiparasitic peptides (APPs). Thus, these data provided new insights into L. crocea immunity against C. irritans and potential of rLc-P5L as a therapeutic agent against pathogen invasion.

Molecular characterization and antiparasitic activity analysis of a novel piscidin 5-like type 4 from Larimichthys crocea

Cryptocaryon irritans is an obligate parasitic ciliate protozoan that can infect various commercially important mariculture teleosts and cause high lethality and economic loss, especially Larimichthys crocea. Current methods of controlling or preventing this parasite with chemicals or antibiotics are widely considered to be environmentally harmful. The antiparasitic activity of some antimicrobial peptides (AMPs) attracted extensive attention of scholars. In the study, a novel piscidin 5-like type 4 (termed Lc-P5L4) excavated from comparative transcriptome of C. irritans - immuned L. crocea was identified and characterized. Sequence analysis shows the full-length cDNA of Lc-P5L4 is 539 bp containing an open reading frame (ORF) of 198 bp which encodes a peptide of 65 amino acid residues. The genome consists of three exons and two introns which exist in its ORF, and all the exon-intron boundaries are in accordance with classical GT-AG rule (GT/intron/AG). Multiple alignments indicate the signal peptides share highly conserved identity, while mature peptides are more diverse. Phylogenetic analysis displays Lc-P5L4 clusters together with other members of piscidin 5-like family. Next, quantitative Real-time PCR (qRT-PCR) detection found C. irritans infection could upregulate Lc-P5L4 expression level in all tested tissues significantly, it appeared earliest upregulation in the theronts infection stage in the head kidney; the expression contents reached to maximum level in the intestine, gill and muscle during trophonts falling off stage; while it was just upregulated during secondary bacterial infection stage in the liver and spleen. The data showed Lc-P5L4 upregulation time points were in accordance with different infection stages. With recombinant Lc-P5L4 (rLc-P5L4) obtained through Escherichia coli system, in vitro assay showed rLc-P5L4 could cause cilia deactivation, cell bodiesclumping and sticking to each other, then cell membrane rupture and contents leakage. The data illustrated Lc-P5L4 played critical roles in the immune defense against C. irritans infection, and provided another proof that piscidins exhibit multiple anti- C. irritans features.

The Diverse Piscidin Repertoire of the European Sea Bass (Dicentrarchus labrax): Molecular Characterization and Antimicrobial Activities

Fish rely on their innate immune responses to cope with the challenging aquatic environment, with antimicrobial peptides (AMPs) being one of the first line of defenses. Piscidins are a group of fish specific AMPs isolated in several species. However, in the European sea bass (Dicentrarchuslabrax), the piscidin family remains poorly understood. We identified six different piscidins in sea bass, performed an in-depth molecular characterization and evaluated their antimicrobial activities against several bacterial and parasitic pathogens. Sea bass piscidins present variable amino acid sequences and antimicrobial activities, and can be divided in different sub groups: group 1, formed by piscidins 1 and 4; group 2, constituted by piscidins 2 and 5, and group 3, formed by piscidins 6 and 7. Additionally, we demonstrate that piscidins 1 to 5 possess a broad effect on multiple microorganisms, including mammalian parasites, while piscidins 6 and 7 have poor antibacterial and antiparasitic activities. These results raise questions on the functions of these peptides, particularly piscidins 6 and 7. Considering their limited antimicrobial activity, these piscidins might have other functional roles, but further studies are necessary to better understand what roles might those be.

Constructing a High-Density Genetic Linkage Map for Large Yellow Croaker (Larimichthys crocea) and Mapping Resistance Trait Against Ciliate Parasite Cryptocaryon irritans

The large yellow croaker (Larimichthys crocea) is the most economically important marine cage-farming fish in China in the past decade. However, the sustainable development of large yellow croaker aquaculture has been severely hampered by several diseases, of which, the white spot disease caused by ciliate protozoan parasite Cryptocaryon irritans ranks the most damaging disease in large yellow croaker cage farms. To better understand the genetic basis of parasite infection and disease resistance to C. irritans, it is vital to map the traits and localize the underlying candidate genes in L. crocea genome. Here, we constructed a high-density genetic linkage map using double-digest restriction-site associated DNA (ddRAD)-based high-throughput SNP genotyping data of a F1 mapping family, which had been challenged with C. irritans for resistant trait measure. A total of 5261 SNPs was grouped and oriented into 24 linkage groups (LGs), representing 24 chromosomes of L. crocea. The total genetic map length was 1885.67 cM with an average inter-locus distance of 0.36 cM. Quantitative trait loci (QTL) mapping identified seven significant QTLs in four LGs linked to C. irritans disease resistance. Candidate genes underlying disease resistance were identified from the reference genome, including ifnar1, ifngr2, ikbke, and CD112. Comparative genomic analysis between large yellow croaker and the four closely related species revealed high evolutionary conservation of chromosomes, though inter-chromosomal rearrangements do exist. Especially, the croaker genome structure was closer to the medaka genome than stickleback, indicating that the croaker genome might retain the teleost ancestral genome structure. The high-density genetic linkage map provides an important tool and resource for fine mapping, comparative genome analysis, and molecular selective breeding of large yellow croaker.

The antibacterial activity and mechanism analysis of piscidin 5 like from Larimichthys crocea

Chemical drugs, such as antibiotics, were still important materials to prevent and cure diseases of aquatic organisms. However, antibiotics abuse do not only make the effects little, but also cause other bad problems, such as bacterial resistance and drug residues. Therefore, seeking the effective substitutes of antibiotics was an approach needed to be explored. Antibacterial peptides (AMPs) attracted more and more attention in the recent years. The parasitism and secondary bacterial invasion caused by ectroparasite Cryptocaryon irritans was a disaster to almost all host fish, including Larimichthys crocea. Reports indicated many AMPs played a key role in the whole parasitic infection cycle. Piscidin 5 like was a member of piscidin family. In the study, the antibacterial activity and mechanisms of piscidin 5 like from L.coreca (Lc-P5L) were detected. Liquid growth inhibition results showed recombinant Lc-P5L (rLc-P5L) had broad antibacterial spectrum and strong bactericidal activity. The bactericidal activity functioned in dose- and time-dependent manners. SEM (scanning electron microscope) observed the relatively detailed bactericidal process, rLc-P5L treatment resulted in a mass of bacteria piling together, appearing plenty of strange filaments and covering on the bacteria. Besides, S.aureus overgrowed plenty of granules, formed holes on the membrane of a few cells, and contents poured out from the holes. At the same time, antibacterial mechanisms were explored. After direct incubation with bacteria, western blot detected the apparently positive signal of rLc-P5L on bacteria; secondly, the incubation first with LPS (lipopolysaccharide) or LTA (lipoteichoic acid) significantly affect the binding of rLc-P5L to bacteria again, which indicated rLc-P5L could bind to bacteria through interaction with some PAMPs (pathogen-associated molecular patterns). In addition, rLc-P5L could interact with bacterial genome DNA by dose- and time-dependent means. In summary, rLc-P5L binded to bacteria surface through targeting to some PAMPs to damage membrane, and entered into cells to interact with genome DNA to disturb normal metabolism when it reached to some certain time and concentration thresholds, which were likely to be its pathway to exert antibacterial activity.

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.

Antimicrobial peptides within the Yellowtail Kingfish (Seriola lalandi)

A number of Seriola species are currently farmed or being investigated as future aquaculture species in countries around the world. However they face a number of issues and limitations which will need to be overcome to ensure future stability and growth, one of which are disease outbreaks. Despite this, very little has been done to understand the immune system of Seriola species and very few immune genes have been characterised. Antimicrobial peptides (AMP) are naturally occurring low molecular weight polypeptides that play a major role in an organism's immune system and act effectively as a first line of defence. This investigation isolates the full length cDNA sequences of two AMP's, piscidin and hepcidin from the yellowtail kingfish (Seriola lalandi). The full-length cDNA of the piscidin gene encodes a 65 amino acid prepropeptide, containing a 25-residue peptide, predicted to form an amphipathic helix-loop-helix structure. Phylogenetic analysis using fish piscidin sequences, showed that this AMP is only found in bony fish within the Acanthomorpha clade and that a possible three groups within the piscidin family exists, with S. lalandi belonging to a particular group. The full-length cDNA of the hepcidin gene encodes a 90 amino acid preprohepcidin, which contains a typical RX(R/K)R motif for cleavage of the mature peptide which comprises of eight conserved cysteine residues. Phylogenetic analysis of known vertebrate hepcidin antimicrobial peptide (HAMP) sequences, shows sequences from the Neoteleostei clade of bony fish form two very separate groups, HAMP1 and HAMP2, with the S. lalandi hepcidin gene grouped with the HAMP1 sequences. HAMP2 sequences are found to have multiple copies within fish and genome analysis showed very clearly that these two groups of genes are located on separate regions on the genome, with the multiple HAMP2 copies formed from tandem gene duplications. Lastly, using qPCR the expression of the S. lalandi piscidin gene within healthy fish was highest within, spleen and gills and lowest in liver, whereas hepcidin was highest in the liver with little or no expression in the spleen and gills.

A novel stimulator of interferon gene (STING) from Larimichthys crocea and their involvement in immune response to ectoparasite Cryptocaryon irritans infection

The marine white spot disease caused by protozoan ectoparasite Cryptocaryon irritans is a severe problem to the large yellow croaker farming industry. To understand the molecular immune mechanisms and improve its immune capacity are particular important. STING, one of the important second messengers in innate immune response process, plays pivotal roles in defensing against different pathogenic microorganisms. Many reports have pointed that STING could not only combine the uncovered dsDNA, ssDNA directly in the cytoplasm from the pathogens or biology itself, but it also could recognize cyclic diguanylate monophosphate (c-di-GMP), cyclic diadenylate monophosphate (c-di-AMP). Based on the STING sequence, a variant of the L. crocea STING (termed LcSTING2) was found by accident. RACE was used to clone the full-length cDNA of LcSTING2 which contained a 5'- UTR of 154 bp, a 3'-UTR of 592 bp and an ORF of 1227 bp encoding 408 amino acids. The predicted protein molecular weight (Mw) was 45.83 KDa and the estimated theoretical isoelectric point (pI) was 6.24. The deduced protein of LcSTING2 contains no signal peptide. One transmembrane motif (TM) in the N-terminal region, a TMEM173 domain and five putative motifs (RXR) found in resident endoplasmic reticulum proteins were also conserved. According to the partial genomic sequence, the unknown sequences were amplified, it contained 6 exons and 5 introns, and all the exon-intron boundaries were in accordance with classical GT-AG rule (GT/intron/AG). The similarity shared with fishes was higher than other high vertebrates. qRT-PCR results showed that LcSTING (two variants) distributed in all examined tissues, and it was the most abundant in gill. After challenged by C. irritans, LcSTING mRNA only appeared instantaneous up-regulation during the infective stage of theronts in the head kidney and was also up-regulated during the whole infectious cycle of C. irritans in the liver, which implied LcSTING gene was likely to be involved in the defensing against C. irritans infection, it is the first time to explore the STING taking part in the immune response to ectoparasite rather than bacterium or viruses.

Ontogeny and modulation after PAMPs stimulation of β-defensin, hepcidin, and piscidin antimicrobial peptides in meagre (Argyrosomus regius)

Antimicrobial peptides (AMPs), components of innate immunity, play an important role in protecting fish. In this study we report the molecular cloning of full open reading frames and characterization of expression of three AMP genes (β-defensin (defb), hepcidin (hep2), piscidin (pisc) in meagre (Argyrosomus regius). A phylogenetic analysis of the expressed sequences obtained shows the defensin isoform forms a clade with the other members of the beta class of this family, hepcidin corresponds to hepcidin 2, and piscidin corresponds to class I of its respective family. Gene expression profiles of AMPs was investigated, by means of quantification of mRNA in nine development stages, from 8 days post-hatching (dph) to accomplishment of juvenile form (120 dph). During development it was demonstrated defb, hep2, pisc were expressed in all stages of larval development and in juvenile tissues (kidney, spleen gut and gill). Moreover, expression patterns suggest the expression levels of theses AMPs are influenced by live prey (rotifer, Artemia) and first intake of commercial diet. Induction experiments in vivo (24 h) and in vitro (4, 12, 24 h) with PAMPs (LPS, poly (I:C), β-glucan) revealed significant changes in gene expression of the three AMP genes, in kidney, spleen, gut and gill. However, expression profiles differed in magnitude and time course response. defb expression shows a similar trend in vivo and in vitro in kidney at 24 h after LPS and β-glucan stimulation. The hep2 expression levels were up-regulated upon β-glucan challenge in vivo, more in gut and gills than kidney, while in vitro hep2 expression was up-regulated in kidney cells by LPS, poly (I:C), β-glucan (4 h). pisc expression was up-regulated in kidney cells, splenocytes by β-glucan, but in gill cells by poly (I:C) and β-glucan in vivo. However, pisc expression was upregulated in kidney cells by β-glucan and gill cells by LPS at 4 post-stimulation in vitro. These data suggest that AMPs play an important role in defense against pathogens, with each AMP having differing efficacies against specific types of microorganisms, although follow-up studies focusing on the biological activities in fish are needed.

Molecular characterization and functional analysis of a piscidin gene in large yellow croaker (Larimichthys crocea)

The piscidin family, which includes potent antimicrobial peptides with broad-spectrum activity, plays an important role in the innate immune system of fish. In this study, we cloned piscidin-5-like type 3 (Lcpis5lt3) in large yellow croaker (Larimichthys crocea). Multiple alignments with other known piscidins revealed amino acid conservation throughout the fish, especially at the signal peptide (22 amino acids). The phylogenetic tree confirmed that Lcpis5lt3 and large yellow croaker piscidin-5-like proteins were grouped together to form a branch. Quantitative real-time PCR revealed that Lcpis5lt3 was expressed in a wide range of tissues, including the brain, muscle, gill, head kidney, intestine, kidney, liver, and spleen. The highest mRNA expression level of Lcpis5lt3 was found in the spleen. After Vibrio alginolyticus infection, mRNA expression was rapidly upregulated in the liver, head kidney, gill, kidney, and intestine at 4, 8, 12, and 24 h post infection (hpi), whereas there were no significant changes in the spleen. The antimicrobial spectrum showed that the synthetic mature peptide of Lcpis5lt3 exhibited different activity in vitro against various bacteria, such as Aeromonas hydrophila, V. anguillarum, V. alginolyticus, V. parahaemolyticus, Staphylococcus aureus, and Listeria monocytogenes. In addition, survival rates from the in vivo assay indicated that the synthetic peptide of Lcpis5lt3 increased the survival rate of large yellow croaker after V. alginolyticus challenge, resulting in a decline in bacterial burden and mRNA expression levels of interleukin-1β, interleukin-10, and tumor necrosis factor-α. These data suggest that Lcpis5lt3 plays an important role in innate immunity in large yellow croaker and might represent a potential therapeutic agent against pathogen invasion.

Transcriptome analysis of the Larimichthys crocea liver in response to Cryptocaryon irritans

The large yellow croaker (Larimichthys crocea) is an economically important marine fish cultured in China and East Asian countries and is facing a serious threat from Cryptocaryon irritans, which is a protozoan ectoparasite that infects most reared marine fish species. To understand the molecular immune mechanisms underlying the response to C. irritans, we first performed a comparative gene transcription analysis using livers from C. irritans-immunized L. croceas and from a control group through RNA-Seq technology. After the removal of low-quality sequences and assembly, 51360 contigs were obtained, with an average length of 1066.93 bp. Further, a blast analysis indicates that 30747 contigs can be annotated based on homology with matches in the NT, NR, gene, and string databases. A gene ontology analysis was used to classify 21598 genes according to three major functional categories: molecular function, cellular component, and biological process. Moreover, 14470 genes were found in 303 KEGG pathways. We used RSEM and EdgeR to determine that 3841 genes were significantly differentially expressed (FDR < 0.001), including 2129 up-regulated genes and 1712 down-regulated genes. A significant enrichment analysis of these differentially expressed genes and isogenes revealed major immune-related pathways, including the toll-like receptor, complement and coagulation cascades, and chemokine signaling pathways. In addition, 28748 potential simple sequence repeats (SSRs) were detected from 12776 transcripts, and 62992 candidate single nucleotide polymorphisms (SNPs) were identified in the L. croceas liver transcriptome. This study characterized a gene expression pattern for normal and C. irritans-immunized L. croceas for the first time and not only sheds new light on the molecular mechanisms underlying the host-C. irritans interaction but also facilitates future studies on L. croceas gene expression and functional genomics.

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.