Immune relevant molecules identified in the skin mucus of fish using -omics technologies

Evaluation of the antibacterial activity and protein profiling of Nile tilapia (Oreochromis niloticus) epidermal mucus under different feeds and culture systems (biofloc technology and earthen pond)

The mucus layers of fish serve as the main interface between the organism and the environment. They play an important biological and ecological role. The current study focuses on Nile tilapia epidermal mucus reared under different commercial feeds (coded A and B) and environments (biofloc technology and earthen pond systems). Crude protein levels in feed A and B were 30% and 28%, respectively. Water parameters in all culturing systems were suitable for tilapia throughout the study period. The antimicrobial potency of tilapia (n = 5 from each) epidermal mucus was tested in vitro against human and fish pathogenic strains viz. Staphylococcus epidermidis, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Francisella noatunensis, and Aeromonas hydrophila. To determine the antimicrobial activity, zones of inhibition (ZOI) were measured in millimetres and compared with two antibiotics (chloramphenicol and ciprofloxacin). SDS-PAGE analysis was performed on skin mucus samples of tilapia to determine protein quantity and size (molecular weight). Results of tilapia skin mucus (crude and aqueous) revealed a strong antibacterial effect against all the selected pathogenic strains. However, variation has been observed in the mucus potency and ZOI values between the biofloc and pond tilapia mucus. The crude mucus of tilapia fed on feed A and cultured in the pond exhibited strong antibacterial effects and high ZOI values compared to the mucus of biofloc tilapia, aqueous mucus extracts and positive control chloramphenicol (antibiotic). The SDS-PAGE results showed that the high molecular weight proteins were found in the collected epidermal mucus of BFT-B (240 kDa) and EP-B (230 kDa). Several peptides in fish skin mucus may play a crucial role in the protection of fish against disease-causing pathogens. Thus, it can be utilized in the human and veterinary sectors as an 'antimicrobial' for treating various bacterial infections.

Nasal responses to elevated temperature and Francisella noatunensis infection in Atlantic cod (Gadus morhua)

We report the histological and transcriptomic changes in the olfactory organ of Atlantic cod exposed to Francisella noatunensis. Experimental infection was performed at either 12 °C or 17 °C. Infected fish presented the classic gross pathologies of francisellosis. Nasal morpho-phenotypic parameters were not significantly affected by elevated temperature and infection, except for the number of mucus cells in the 12 °C group seven weeks after the challenge. A higher number of genes were altered through time in the group reared at 17 °C. At termination, the nasal transcriptome of infected fish in both groups was similar to the control. When both infected groups were compared, 754 DEGs were identified, many of which were involved in signalling, defence, transmembrane and enzymatic processes. In conclusion, the study reveals that elevated temperature could trigger responses in the olfactory organ of Atlantic cod and shape the nasal response to F. noatunensis infection.

Single-nuclei RNA-seq reveals skin cell responses to Aeromonas hydrophila infection in Chinese longsnout catfish Leiocassis longirostris

As the primary natural barrier that protects against adverse environmental conditions, the skin plays a crucial role in the innate immune response of fish, particularly in relation to bacterial infections. However, due to the diverse functionality and intricate anatomical and cellular composition of the skin, deciphering the immune response of the host is a challenging task. In this study, single nuclei RNA-sequencing (snRNA-seq) was performed on skin biopsies obtained from Chinese longsnout catfish (Leiocassis longirostris), comparing Aeromonas hydrophila-infected subjects to healthy control subjects. A total of 19,581 single nuclei cells were sequenced using 10x Genomics (10,400 in the control group and 9,181 in the treated group). Based on expressed unique transcriptional profiles, 33 cell clusters were identified and classified into 12 cell types including keratinocyte (KC), fibroblast (FB), endothelial cells (EC), secretory cells (SC), immune cells, smooth muscle cells (SMC), and other cells such as pericyte (PC), brush cell (BC), red blood cell (RBC), neuroendocrine cell (NDC), neuron cells (NC), and melanocyte (MC). Among these, three clusters of KCs, namely, KC1, KC2, and KC5 exhibited significant expansion after A. hydrophila infection. Analysis of pathway enrichment revealed that KC1 was primarily involved in environmental signal transduction, KC2 was primarily involved in endocrine function, and KC5 was primarily involved in metabolism. Finally, our findings suggest that neutrophils may play a crucial role in combating A. hydrophila infections. In summary, this study not only provides the first detailed comprehensive map of all cell types present in the skin of teleost fish but also sheds light on the immune response mechanism of the skin following A. hydrophila infection in Chinese longsnout catfish.

Teleost skin microbiome: An intimate interplay between the environment and the host immunity

The mucosal microbiome plays a role in regulating host health. The research conducted in humans and mice has governed and detailed the information on microbiome-host immunity interactions. Teleost fish, different from humans and mice, lives in and relies on the aquatic environment and is subjected to environmental variation. The growth of teleost mucosal microbiome studies, the majority in the gastrointestinal tract, has emphasized the essential role of the teleost microbiome in growth and health. However, research in the teleost external surface microbiome, as the skin microbiome, has just started. In this review, we examine the general findings in the colonization of the skin microbiome, how the skin microbiome is subjected to environmental change and the reciprocal regulation with the host immune system, and the current challenges that potential study models can address. The information collected from teleost skin microbiome-host immunity research would help future teleost culturing from the potential parasitic infestation and bacterial infection as foreseeing growing threats.

Fish Skin Mucus Extracts: An Underexplored Source of Antimicrobial Agents

The slow discovery of new antibiotics combined with the alarming emergence of antibiotic-resistant bacteria underscores the need for alternative treatments. In this regard, fish skin mucus has been demonstrated to contain a diverse array of bioactive molecules with antimicrobial properties, including peptides, proteins, and other metabolites. This review aims to provide an overview of the antimicrobial molecules found in fish skin mucus and its reported in vitro antimicrobial capacity against bacteria, fungi, and viruses. Additionally, the different methods of mucus extraction, which can be grouped as aqueous, organic, and acidic extractions, are presented. Finally, omic techniques (genomics, transcriptomics, proteomics, metabolomics, and multiomics) are described as key tools for the identification and isolation of new antimicrobial compounds. Overall, this study provides valuable insight into the potential of fish skin mucus as a promising source for the discovery of new antimicrobial agents.

Skin mucus metabolomics provides insights into the interplay between diet and wound in gilthead seabream (Sparus aurata)

The molecular processes underlying skin wound healing in several fish species have been elucidated in the last years, however, metabolomic insights are scarce. Here we report the skin mucus metabolome of wounded and non-wounded gilthead seabream (Sparus aurata) fed with silk fibroin microparticles, a functional additive considered to accelerate the wound healing process. The three experimental diets (commercial diet enriched with 0 mg (control), 50 mg or 100 mg of silk fibroin microparticles Kg^-1) were administered for 30 days and thereafter, a skin wound was inflicted. Skin mucus was collected on day 30 of feeding and 7 days post-wounding and subjected to metabolomic analysis by Ultra Performance Liquid Chromatography coupled with a high-resolution quadrupole-orbitrap mass spectrometry. The most enriched metabolite class was amino acids and derivatives, followed by nucleotides, nucleosides and analogues and carbohydrates and their derivatives. Metabolomic profiles revealed that the diet had a more profound effect than wounding in skin mucus. Metabolic pathway analysis of significantly affected metabolites revealed perturbations in the aminoacyl t-RNA biosynthesis in the skin. In particular, skin wound resulted in a decreased methionine level in mucus. Further, silk fibroin supplementation increased methionine level in skin mucus, which correlated with several wound morphometric parameters that characterized the epithelial healing capacity in seabream. The results provided new insight into the physiological consequences of skin wounds and how these processes could be influenced by dietary manipulation.

A novel gnotobiotic experimental system for Atlantic salmon (Salmo salar L.) reveals a microbial influence on mucosal barrier function and adipose tissue accumulation during the yolk sac stage

Gnotobiotic models have had a crucial role in studying the effect that commensal microbiota has on the health of their animal hosts. Despite their physiological and ecological diversity, teleost fishes are still underrepresented in gnotobiotic research. Moreover, a better understanding of host-microbe interactions in farmed fish has the potential to contribute to sustainable global food supply. We have developed a novel gnotobiotic experimental system that includes the derivation of fertilized eggs of farmed and wild Atlantic salmon, and gnotobiotic husbandry of fry during the yolk sac stage. We used a microscopy-based approach to estimate the barrier function of the skin mucus layer and used this measurement to select the derivation procedure that minimized adverse effects on the skin mucosa. We also used this method to demonstrate that the mucus barrier was reduced in germ-free fry when compared to fry colonized with two different bacterial communities. This alteration in the mucus barrier was preceded by an increase in the number of cells containing neutral mucosubstances in the anterior segment of the body, but without changes in the number of cells containing acidic substances in any of the other segments studied along the body axis. In addition, we showed how the microbial status of the fry temporarily affected body size and the utilization of internal yolk stores during the yolk sac stage. Finally, we showed that the presence of bacterial communities associated with the fry, as well as their composition, affected the size of adipose tissue. Fry colonized with water from a lake had a larger visceral adipose tissue depot than both conventionally raised and germ-free fry. Together, our results show that this novel gnotobiotic experimental system is a useful tool for the study of host-microbe interactions in this species of aquacultural importance.

The feasibility of skin mucus replacing exosome as a pool for bacteria-infected markers development via comparative proteomic screening in teleost

In fish, skin mucus forms a protective barrier between the body surface and the external water environment, thus providing the most direct and intuitive clues to monitor the subject's health condition. To explore the impact of the Vibrio harveyi pathogen on teleost, the proteome of epidermal mucus from control and sick Cynoglossus semilaevis were screened through iTRAQ followed with LC-MS/MS. 1531 credible proteins were obtained relating to structural, metabolic and immunological functions. 335 different expressed proteins (DEPs) were identified, with 166 up-regulated and 169 down-regulated in MS. 62 proteins were characterized, including 22 up-regulated proteins and 40 down-regulated proteins. Integrated analysis of DE-miRNAs and DEPs from miRomics and proteomics were conducted to show the indirect regulatory relationship. Comparative analysis of DEPs between mucus and exosomes demonstrated that exosomes contributed the most DEPs of all mucus DEPs. 125 proteins are DEPs only in exosomes, which presented minor difference in total mucus. Expression of Aminopeptidase (anpep), Calcium-transporting ATPase, Histone H2B and H2A confirmed implied fine discriminative power with infected C. semilaevis, among which Calcium-transporting ATPase and H2B also appeared in list of exosomal markers. This study might shed the light on effective biomarker digging at other extended screening scenarios.

Sex and reproductive development impact skin mucosal epithelium immunity, antimicrobial capacity, and up-regulation of immune-related gene of goldfish (Carassius auratus)

This study evaluated the epidermis mucosal capacity of goldfish (Carassius auratus) during different stages of reproductive development in both females and males. In this regard, the activity of mucolytic immune enzymes, i.e., lysozyme, complement and peroxidase, as well as the activity of alkaline phosphatase (ALP) were evaluated. There were five stages for females i.e., immature (f1), cortical alveoli (f2), early and late-vitellogenesis (vtg) (f3 and f4) and ripe (f5); as well as two stages for males spermatogenesis (m1) and spermiation (m2). Some stages were also examined for the mucosal antimicrobial activity against specific pathogens. The results showed that the mucosal lysozyme activity increased significantly during vitellogenesis (P < 0.05), but no lysozyme activity was detected in plasma. On the contrary, the complement activity was only observed in female plasma, and it was significantly higher at f3 compared to the other developmental stages. Both the plasma and mucosal ALP and peroxidase activities showed a significant increase by female reproductive development with the highest amounts at f4. Contrary to the female, no significant changes were observed in plasma and mucosal immune agents and biochemistry of the male. The f5-staged goldfish showed the highest antimicrobial activities against Gram-positive bacteria, i.e., Streptococcus faecium, Staphylococcus aureus and Micrococcus luteus (P < 0.05). Our results also represented the up-regulation of lysozyme (c-lys) gene expression by effects of female maturational development in ovary, liver and skin, while male goldfish showed no significant changes in c-lys expression. Moreover, there were positive correlations between c-lys expression, mucosal lysozyme activity and calcium levels in females (P < 0.01). Overall, our findings revealed that vtg process improves mucosal innate immunity that leads to activate antimicrobial components at spawning season.

Triphenyltin exposure induced abnormal morphological colouration in adult male guppies (Poecilia reticulata)

Fish morphological colouration is essential for their survival and reproduction success; however, it is vulnerable to environmental factors, such as pollutants. Triphenyltin (TPT) is widespread in aquatic ecosystems, and its impacts on fish have been problematic. Therefore, the purpose of this study was to investigate the effects of TPT at environment-related concentrations (0, 1, 10 and 100 ng Sn/L) on morphological colouration in male guppies (Poecilia reticulata). The results showed that TPT exposure affected both orange/red and dark morphological colouration in guppies. The faded orange/red colouration might be related to the decrease of coloured pteridine and Pts (6-Pyruvoyltetrahydropterin Synthase) expression. In addition, TPT exposure induced melanogenesis, however, much melanin was distributed diffusely in the skin and did not seem to form a spot pattern, giving the fish a dull appearance. According to the skin transcriptional profiles, the changes of dark morphological colouration might be related to the changes in genes related to the functions of melanosome components (Gpnmb, Slc45a2 and Tyr), construction (Ap3d1, Fig4, Hps3, Hps5, Lyst, Rabggta, Txndc5 and Vps33a), and transport (Rab27a). Additionally, genes related to the regulation of melanogenesis (Atrn and Pomc) and system effects (Atox1, Atp6ap2, Atp6v1f, Atp6v1h, Rpl24, Rps19 and Rps20) might also be involved in the molecular mechanisms of abnormal morphological colouration induced by TPT. The present study provides crucial data on the molecular basis of abnormal morphological colouration in fish exposed to TPT and underscores the importance of toxicological studies of the effects of pollutants in aquatic environments on fish morphological colouration.

Perturbation by Antimicrobial Bacteria of the Epidermal Bacterial Flora of Rainbow Trout in Flow-Through Aquaculture

The bacterial flora of the epidermal mucus of fish is closely associated with the host’s health and susceptibility to pathogenic infections. In this study, we analyzed the epidermal mucus bacteria of rainbow trout (Oncorhynchus mykiss) reared in flow-through aquaculture under environmental perturbations. Over ~2 years, the bacteria present in the skin mucus and water were analyzed based on the 16S rDNA sequences. The composition of the mucus bacterial community showed significant monthly fluctuations, with frequent changes in the dominant bacterial species. Analysis of the beta- and alpha-diversity of the mucus bacterial flora showed the fluctuations of the composition of the flora were caused by the genera Pseudomonas, Yersinia, and Flavobacterium, and some species of Pseudomonas and Yersinia in the mucus were identified as antimicrobial bacteria. Examination of the antimicrobial bacteria in the lab aquarium showed that the natural presence of antimicrobial bacteria in the mucus and water, or the purposeful addition of them to the rearing water, caused a transition in the mucus bacteria community composition. These results demonstrate that specific antimicrobial bacteria in the water or in epidermal mucus comprise one of the causes of changes in fish epidermal mucus microflora.

Mucoadhesive cationic lipid-based Flavobacterium oreochromis nanoencapsulation enhanced the efficacy of mucoadhesive immersion vaccination against columnaris disease and strengthened immunity in Asian sea bass (Lates calcarifer)

Columnaris is a bacterial disease, found in freshwater fish, caused by Flavobacterium oreochromis. The disease has a devastating impact on a range of cultured and wild freshwater fish species e.g. Lates calcarifer (Asian sea bass), which is a serious economic losses to the freshwater aquaculture in Thailand. The disease can be prevented by an efficacious vaccine, however, no licensed effective vaccine is available to date. Current study was based on the development of a novel mucoadhesive nano-encapsulated vaccine (EncapFlavoNP⁺⁺), where, cationic lipid-based nanoparticles were combined with an antigen obtained from F. oreochromis. Various parameters including transmission electron microscopy (TEM), physiochemical properties; zeta potential, and polydispersity index were determined. The TEM results depicted well-formed circular-shaped nano-encapsulates complexed with cationic lipid surfactants. The average diameter of the molecules was 200 nm, having a zeta potential of 31.82 mV, while, the polydispersity index (PDI) was 0.31. The in vivo study lasted for 8 weeks, the immunologic and protective potentials of the prepared molecules were determined by challenging the fish for 8 weeks. The most effective dilutions of EncapFlavoNP⁺⁺ solution were 1:100 and 1:200, which significantly improved the efficacy of the immunity by increasing the level of antibody specific to F. oreochromis. A trend of upregulation was found in the immune-related genes including immunoglobulin M heavy chain (IgM), major histocompatibility complex class IIα molecules (MHC-IIα), and dendritic cell specific transcript (DCs) in gills, skin, liver, peripheral blood lymphocytes (PBLs), head kidneys, and spleen as compared to the control group (P < 0.05 and P < 0.01). Upon immunization with EncapFlavoNP⁺⁺ solution at the dilution of 1:100 and 1:200, the significant increase in survival rate (SR) and relative percent survival (RPS) were found in fish challenged with virulent F. oreochromis bacterium (SR 72.50% and RPS 62.07) and (SR 65.83% and RPS 52.87), respectively as compared to the control group (P < 0.05). It can be concluded that immunization with EncapFlavoNP⁺⁺ solution has significant immunologic and protective effects against Columnaris disease. Furthermore, the prepared vaccine candidate has more potential as compared to whole-cell immersion vaccination (FK-WC). It can be used on a large scale in the freshwater aquaculture industry to boost immunity against Columnaris disease.

Skin immune response to Aeromonas hydrophila infection in crucian carp Carassius auratus revealed by multi-omics analysis

Fish skin is an essential protective barrier and functions as the first line of immune defense against pathogens. However, the molecular mechanism at the proteome-level remains unclear in the skin of fish. In this study, the comparative proteomics of skin immune responses of crucian carp Carassius auratus infected with Aeromonas hydrophila was investigated by isobaric tags for relative and absolute quantification (iTRAQ), two-dimensional gel electrophoresis combined with mass spectrometry (2-DE/MS) as well as high-throughput transcriptome (RNA-seq) techniques. A total of 241 and 178 differentially expressed proteins (DEPs) at 6 and 12 h post-infection (hpi) were respectively identified by iTRAQ, and key-DEPs were furtherly verified with 2-DE/MS analysis. GO and KEGG analysis showed that these DEPs were mostly related to metabolism, regulation of the cytoskeleton, stress and immune responses. Co-association results of proteome and transcriptome revealed the lysozyme (LYZ), complement C3, DnaJ (Hsp40) homolog subfamily C member 8 (DNAJC8) and allograft inflammatory factor 1-like (AIF1L) play important roles in skin immune responses of crucian carp. The significantly up-regulated expression of detected immune-related genes (c3, mapk3, f5, nlr, hsp90, itgb2, fnl, flnca, p47, mhc and pros1) were validated by qRT-PCR analysis. To our knowledge, this is first report on multi-omics analysis of the differential proteomics for the skin immune response of C. auratus against A.hydrophila infection, which contribute to the understanding the mechanisms of skin mucosal immunity in cyprinid fish.

Immune responses of Cyprinus carpio induced by protein extracts of Lernaea cyprinacea Linnaeus, 1758

Lernaea cyprinacea Linnaeus, 1758 is an ectoparasite showing widespread infections in tropical aquaculture, and the present study aimed to determine the specific immune responses against this parasite. For the experiment, whole parasite extracts were injected intraperitoneally into Cyprinus carpio Linnaeus, 1758, and samples of epidermal mucus and blood were drawn at 0, 1, 7 and 14-days post-injection (DPI). The results revealed high levels of protein, protease and lysozyme activities in the experimental fish which were injected with L. cyprinacea protein extract. In the epidermal mucus, the total protein concentration of the control fish was 460 μg/mL, and the level raised significantly to 800 μg/mL in the experimental fish. The lysozyme activity increased from 741.5 u/mL to a peak level of 1448.5 u/mL at 7DPI. The protease activity was also found elated gradually from 2.91 u/μL to 4.49 u/μL at 1 to 14 DPI. In the serum samples, the protein concentration remained steady throughout the experiment period. However, all the experimental fish displayed statistically high levels of lysozyme and protease activity, from 890 u/mL to 1220 u/mL, and 6.10 u/μL to 11.88 u/μL, respectively. In the whole blood samples, the haemoglobin content and the red blood cells (RBC) count did not show any significant change in any of the experimental groups. But, the percentage of lymphocytes showed a marginal increase from 0.47 to 0.6 in the experimental groups. Overall, the immune responses induced by L. cyprinacea protein extracts depicts a pattern of specific responses, in which the local humoral responses dominate the systemic humoral/cellular response. The results further revealed the possibility of futuristic approaches to control freshwater ectoparasites.

Integrated Analysis of lncRNA and circRNA Mediated ceRNA Regulatory Networks in Skin Reveals Innate Immunity Differences Between Wild-Type and Yellow Mutant Rainbow Trout (Oncorhynchus mykiss)

Fish skin is a vital immune organ that forms the first protective barrier preventing entry of external pathogens. Rainbow trout is an important aquaculture fish species that is farmed worldwide. However, our knowledge of innate immunity differences between wild-type (WR_S) and yellow mutant rainbow trout (YR_S) remains limited. In this study, we performed whole transcriptome analysis of skin from WR_S and YR_S cultured in a natural flowing water pond. A total of 2448 mRNAs, 1630 lncRNAs, 22 circRNAs and 50 miRNAs were found to be differentially expressed (DE). Among these DEmRNAs, numerous key immune-related genes, including ifih1, dhx58, trim25, atp6v1e1, tap1, tap2, cd209, hsp90a.1, nlrp3, nlrc3, and several other genes associated with metabolism (gstp1, nampt, naprt and cd38) were identified. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses of DEmRNAs revealed that many were significantly enriched in innate immune-related GO terms and pathways, including NAD+ADP-ribosyltransferase activity, complement binding, immune response and response to bacterium GO terms, and RIG-I-like receptor signaling, NOD-like receptor signaling and phagosome KEGG pathways. Furthermore, the immune-related competing endogenous RNA networks were constructed, from which we found that lncRNAs MSTRG.11484.2, MSTRG.32014.1 and MSTRG.29012.1 regulated at least three immune-related genes (ifih1, dhx58 and irf3) through PC-5p-43254_34, PC-3p-28352_70 and bta-miR-11987_L-1R-1_1ss8TA, and tap2 was regulated by two circRNAs (circRNA5279 and circRNA5277) by oni-mir-124a-2-p5_1ss13GA. The findings expand our understanding of the innate immune system of rainbow trout, and lay the foundation for further study of immune mechanisms and disease resistance breeding.

A Transcriptomic Study Reveals That Fish Vibriosis Due to the Zoonotic Pathogen Vibrio vulnificus Is an Acute Inflammatory Disease in Which Erythrocytes May Play an Important Role

Vibrio vulnificus is a marine zoonotic pathogen associated with fish farms that is considered a biomarker of climate change. Zoonotic strains trigger a rapid death of their susceptible hosts (fish or humans) by septicemia that has been linked to a cytokine storm in mice. Therefore, we hypothesize that V. vulnificus also causes fish death by triggering a cytokine storm in which red blood cells (RBCs), as nucleated cells in fish, could play an active role. To do it, we used the eel immersion infection model and then analyzed the transcriptome in RBCs, white BCs, and whole blood using an eel-specific microarray platform. Our results demonstrate that V. vulnificus triggers an acute but atypical inflammatory response that occurs in two main phases. The early phase (3 h post-infection [hpi]) is characterized by the upregulation of several genes for proinflammatory cytokines related to the mucosal immune response (il17a/f1 and il20) along with genes for antiviral cytokines (il12β) and antiviral factors (ifna and ifnc). In contrast, the late phase (12 hpi) is based on the upregulation of genes for typical inflammatory cytokines (il1β), endothelial destruction (mmp9 and hyal2), and, interestingly, genes related to an RNA-based immune response (sidt1). Functional assays revealed significant proteolytic and hemolytic activity in serum at 12 hpi that would explain the hemorrhages characteristic of this septicemia in fish. As expected, we found evidence that RBCs are transcriptionally active and contribute to this atypical immune response, especially in the short term. Based on a selected set of marker genes, we propose here an in vivo RT-qPCR assay that allows detection of early sepsis caused by V. vulnificus. Finally, we develop a model of sepsis that could serve as a basis for understanding sepsis caused by V. vulnificus not only in fish but also in humans.

Molecular cloning, expression analysis of the IgT gene and detection of IgT+ B cells in the half-smooth tongue sole (Cynoglossus semilaevis)

IgT is a specific Ig isotype in teleosts, which plays extremely important roles in the mucosal immunity of fish. In the present study, the membrane-bound and secretory IgT of the half-smooth tongue sole (Cynoglossus semilaevis) were identified for the first time. The V-D-J-C structure of two forms of csIgT are translated by the same Cτ gene, and the secretory tail and transmembrane domain were encoded through alternative splicing at the 3' end of the Cτ4. The CH regions of csIgT had high similarity with that of other flatfish (P. olivaceus and S. maximus). In healthy C. semilaevis, sIgT and mIgT were mainly expressed in mucus related tissues such as skin, intestine and gill. The transcript levels of sIgT and mIgT mRNA showed a significant induction in the immune-related tissues upon Vibrio Harveyi infection. A polyclonal rabbit anti-csIgT was successfully prepared using the csIgT heavy chain recombinant protein. Using this antibody, we detected the native IgT with the molecular mass at 220 kDa in skin total protein under non-reducing SDS-PAGE condition. Immunofluorescence analysis indicated that IgT⁺ B lymphocytes were intensively located in the skin, gill, intestine, and head kidney of C. semilaevis. These results suggest that IgT may participate in the immune response of C. semilaevis, which will facilitate the investigations of the immunoglobulins of marine fish.

Interacting Effects of Sea Louse (Lepeophtheirus salmonis) Infection and Formalin-Killed Aeromonas salmonicida on Atlantic Salmon Skin Transcriptome

Lepeophtheirus salmonis (sea lice) and bacterial co-infection threatens wild and farmed Atlantic salmon performance and welfare. In the present study, pre-adult L. salmonis-infected and non-infected salmon were intraperitoneally injected with either formalin-killed Aeromonas salmonicida bacterin (ASAL) or phosphate-buffered saline (PBS). Dorsal skin samples from each injection/infection group (PBS/no lice, PBS/lice, ASAL/no lice, and ASAL/lice) were collected at 24 h post-injection and used for transcriptome profiling using a 44K salmonid microarray platform. Microarray results showed no clear inflammation gene expression signatures and revealed extensive gene repression effects by pre-adult lice (2,189 down and 345 up-regulated probes) in the PBS-injected salmon (PBS/lice vs. PBS/no lice), which involved basic cellular (e.g., RNA and protein metabolism) processes. Lice repressive effects were not observed within the group of ASAL-injected salmon (ASAL/lice vs. ASAL/no lice); on the contrary, the observed skin transcriptome changes –albeit of lesser magnitude (82 up and 1 down-regulated probes)– suggested the activation in key immune and wound healing processes (e.g., neutrophil degranulation, keratinocyte differentiation). The molecular skin response to ASAL was more intense in the lice-infected (ASAL/lice vs. PBS/lice; 272 up and 11 down-regulated probes) than in the non-infected fish (ASAL/no lice vs. PBS/no lice; 27 up-regulated probes). Regardless of lice infection, the skin’s response to ASAL was characterized by the putative activation of both antibacterial and wound healing pathways. The transcriptomic changes prompted by ASAL+lice co-stimulation (ASAL/lice vs. PBS/no lice; 1878 up and 3120 down-regulated probes) confirmed partial mitigation of lice repressive effects on fundamental cellular processes and the activation of pathways involved in innate (e.g., neutrophil degranulation) and adaptive immunity (e.g., antibody formation), as well as endothelial cell migration. The qPCR analyses evidenced immune-relevant genes co-stimulated by ASAL and lice in an additive (e.g., mbl2b, bcl6) and synergistic (e.g., hampa, il4r) manner. These results provided insight on the physiological response of the skin of L. salmonis-infected salmon 24 h after ASAL stimulation, which revealed immunostimulatory properties by the bacterin with potential applications in anti-lice treatments for aquaculture. As a simulated co-infection model, the present study also serves as a source of candidate gene biomarkers for sea lice and bacterial co-infection.

Characterization of TRAF2 in Nile tilapia: Expression profiles and the role in decreasing NF-κB pathway

Mammals TRAF2 played a dual role in several immune signaling transduction processes. In this study, TRAF2 was cloned from Nile tilapia, Oreochromis niloticus, which named OnTRAF2. The open reading frame was 1797 bp, encoding 598 amino acids. Amino acid alignment and phylogenetic analysis indicated that OnTRAF2 showed relatively low identify with other teleost TRAF2 proteins, with the exception of TRAF2s from Epinephelus coioides. In healthy tilapia, OnTRAF2 was expressed widely in all the examined tissues, which had highest expression level in the brain. After Streptococcus agalactiae infection, the expression level of OnTRAF2 was increased significantly at different times in several organs, implying that OnTRAF2 may be involved in host defense against S. agalactiae infection. The result of subcellular localization showed that OnTRAF2 presented in cytoplasm and nucleus of HEK293T cells. Additionally, overexpression of OnTRAF2 significantly decreased the transcriptional activity of the NF-κB reporter in HEK293T cells, yeast two-hybrid results revealed that OnTRAF2 had no interaction with E3 ubiquitin ligase OnNEDD4. These results indicated that OnTRAF2 played important function during bacterial infection, and negatively mediated the immune signaling transduction in Nile tilapia, while the mechanism need further study.

Immunotoxic effects of metal-based nanoparticles in fish and bivalves

There is a global research interest in metal nanoparticles (MNPs) due to their diverse applications, rapidly increasing use, and increased presence in the aquatic environment. Currently, most MNPs in the environment are at levels unlikely to cause overt toxicity. Sub-lethal effects that MNPs may induce, notable immunotoxicity, could however have significant health implications. Thus, deciphering the immunological interactions of MNPs with aquatic organisms constitutes a much-needed area of research. In this article, we critically assess the evidence for immunotoxic effects of MNPs in bivalves and fish, as key wildlife sentinels with widely differing ecological niches that are used as models in ecotoxicology. The first part of this review details the properties, fate, and fundamental physicochemical behavior of MNPs in the aquatic ecosystem. We then consider the toxicokinetics of MNP uptake, accumulation, and deposition in fish and bivalves. The main body of the review then focuses on immune reactions in response to MNPs exposure in bivalves and fish illustrating their immunotoxic potential. Finally, we identify major knowledge gaps in our current understanding of the implications of MNPs exposure for immunological functions and the associated health consequences for bivalves and fish, as well as the general lessons learned on the immunotoxic properties of the emerging class of nanoparticulate contaminants in fish and bivalves.

Comparative Mucous miRomics in Cynoglossus semilaevis Related to Vibrio harveyi Caused Infection

Epidermal mucus is an important barrier and regulating mediator in fish. MicroRNAs (miRNAs) are proved to be involved in various biological processes, also as promising biomarkers for disease diagnosis. Vibrio harveyi has long been a noticeable bacterial pathogen in Cynoglossus semilaevis aquaculture. To find the evidence whether there are indicating miRNAs in mucus and whether the miRNAs are related to infections caused by V. harveyi, miRNA profiles of mucus from V. harveyi infected fish and healthy controls were screened by small RNA sequencing and verified by quantitative real-time PCR. This is the first report about miRNA profiling of flatfish mucus, aiming at illustrating the pathogenesis of V. harveyi caused infection and developing disease-related biomarkers. The results revealed significant differences in expression levels of some miRNAs between infected fish and healthy ones. Three hundred differentially expressed miRNAs were obtained after filtering through FC > 2 or FC < 0.5 and most of the differential miRNAs were downregulated. After verification through qRT-PCR, four unique miRNAs, dre-miR-451, dre-miR-184, dre-miR-205-5p > ssa-miR-205b-5p, and dre-miR-181a-5p > ssa-miR-181a-5p, were identified as V. harveyi infection-related signatures, consistent with sequencing trend. The expression levels of these four miRNAs in the infected fish were all significantly lower than controls. These miRNAs in mucus could be used to differentiate diseased and healthy fish in a non-invasive way with practical value for large-scale disease screening. They also provided new insights into the mechanism underlying the bacterial infections in fish.

Impact of cinnamaldehyde on innate immunity and immune gene expression in Channa striatus against Aphanomyces invadans

The effect of cinnamaldehyde (CM) enriched diet on immunity and cytokine gene expression in Channa striatus against Aphanomyces invadans is reported. C. striatus was uniformly divided into eight groups (n = 25 fish each) and fed with formulated diets with 0, 5, 10, and 15 mg kg^-1 CM enriched diet. In healthy and infected groups fed with 5 mg kg^-1 diet the leukocytes count increased significantly after 4th week; with 10 mg kg^-1 CM diet the increase manifested after 6th week, but with 15 mg kg^-1 not even after 8th week. In both groups, 5 mg kg^-1 CM diet resulted in a significant increase in the serum total protein, albumin, and globulin levels after 4th week, whereas with other diets this effect was observed only after 6th week. Similarly, with any enriched diet the lysozyme activity increased significantly, but with 15 mg kg^-1 CM diet only after 6th week. In both groups the complement activity and lymphocyte production increased significantly when fed with 5 mg kg^-1 CM diet after 4th week while with other enriched diets only after 6th week. The phagocytic activity increased significantly in both groups fed with 5 mg kg^-1 CM diet after 6th week, whereas the SOD activity increased after 4th week. The IgM production increased significantly in both groups fed with 5 mg kg^-1 CM diet after 2nd week, while with 5 and 10 mg kg^-1 CM diet after 4th week. In both groups, the expression of CXCR3α was significant on 4th week when fed with 10 mg kg^-1 CM diet, while in the healthy group fed with 15 mg kg^-1 CM diet the expression manifested earlier than 4th week. However, when fed with 10 and 15 mg kg^-1 CM diets the increase was observed on 6th week; whereas, the expression of MHC-I reached the maximum on 6th week with any enriched diet. The results indicate that in C. striatus the innate immunity and expression of cytokine and immune related genes were significantly modulated when fed with 5 mg kg^-1 CM diet on 4th week against A. invadans.

Dietary inclusion of watermelon rind powder and Lactobacillus plantarum: Effects on Nile tilapia's growth, skin mucus and serum immunities, and disease resistance

An eight-week investigation was conducted to access the potential impact of dietary watermelon rind powder (WMRP) and L. plantarum CR1T5 (LP) administered individually or in combination on immunity, disease resistance, and growth rate of Nile tilapia fingerlings cultured in a biofloc system. Three hundred twenty fish (average weight 16.57 ± 0.14 g) were distributed into 16 tanks at a rate of 20 fish per tank. The fish were fed different diets: Diet 1 (0 g kg^-1 WMRP and 0 CFU g^-1 L. plantarum) (control), Diet 2 (40 g kg^-1 WMRP), Diet 3 (10⁸ CFU g^-1 LP), and Diet 4 (40 g kg^-1 WMRP + 10⁸ CFU g^-1 LP) for eight weeks. A completely randomized design (CRD) with four replications was applied. Skin mucus, serum immunity, and growth parameters were analyzed every 4 weeks, and a challenge study against S. agalactiae was conducted at the end of the experiment. The findings showed that the inclusion of WMRP + LP, administrated individually or in a mixture, significantly (P<0.05) stimulated growth, skin mucus, and serum immune parameters of Nile tilapia fingerlings compared with the control. The highest values were detected in fish fed the combination of WMRP and LP, as opposed to individual administration of either WMRP or LP, in which no significant differences were detected. Within the challenge study, the relative percent survival (RPS) in Diet 2, Diet 3, and Diet 4 was 48.0%, 52.0%, and 68.0%, respectively. Fish fed 40 g kg^-1 WMRP + LP produced significantly higher RPS and protection against S. agalactiae than the other treated groups. Current results suggest that the dual administration of WMRP and LP maybe an effective feed additive for Nile tilapia grown in an indoor biofloc system, capable of improving growth parameters and increasing resistance to S. agalactiae infection.

Effect of Dietary Sugarcane Bagasse Supplementation on Growth Performance, Immune Response, and Immune and Antioxidant-Related Gene Expressions of Nile Tilapia (Oreochromis niloticus) Cultured under Biofloc System

Simple Summary

Supplementation of agriculture by-product as functional feed additives in combination with biofloc technology (a sustainable and environmentally friendly technology) has recently gained much attention in aquaculture. In the present study, sugarcane bagasse powder can possibly be applied as a feed additive to improve growth performance, immune response, and immune and antioxidant-related gene expression.

Abstract

We investigated, herein, the effects of dietary inclusion of sugarcane bagasse powder (SB) on Nile tilapia development, mucosal and serum immunities, and relative immune and antioxidant genes. Fish (15.12 ± 0.04 g) were provided a basal diet (SB0) or basal diet incorporated with SB at 10 (SB10), 20 (SB20), 40 (SB40), or 80 (SB80) g kg⁻¹ for 8 weeks. Our results demonstrated that the dietary incorporation of sugarcane bagasse powder (SB) at 20 and 40 g kg⁻¹ significantly ameliorated FW, WG, and SGR as opposed to fish fed basal, SB10, and SB80 diets. However, no significant changes in FCR and survivability were observed between the SB supplemented diets and the control (basal diet). The mucosal immunity exhibited significantly higher SMLA and SMPA activities (p < 0.005) in fish treated with SB diets after eight weeks. The highest SMLA and SMPA levels were recorded in fish fed SB80 followed by SB20, SB40, and SB10, respectively. For serum immunity, fish fed SB incorporated diets significantly ameliorated SL and RB levels (p < 0.05) compared with the control. However, SP was not affected by the inclusion of SB in any diet throughout the experiment. The expression of IL1, IL8, LBP, GSTa, GPX, and GSR genes in the fish liver was significantly increased in fish fed the SB20 and SB10 diets relative to the basal diet fed fish (p < 0.05); whereas only the IL8, LBP, and GPX genes in the intestines were substantially augmented via the SB20 and SB80 diets (p < 0.05). IL1 and GSR were not influenced by the SB incorporated diets (p > 0.05). In summary, sugarcane bagasse powder (SB) may be applied as a feed additive to improve growth performance, immune response, and immune and antioxidant-related gene expression in Nile tilapia.

Identification and quantification of dicarboxylic fatty acids in head tissue of farmed Nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) was grown in Bangladesh with four different feeding treatments as part of a project that aims to produce fish in a cost-effective way for low-income consumers in developing countries. Fillet and head tissue was analysed because both tissues were destined for human consumption. Gas chromatography with mass spectrometry (GC/MS) analyses of transesterified fatty acid methyl ester extracts indicated the presence of ~ 50 fatty acids. Major fatty acids in fillet and head tissue were palmitic acid and oleic acid. Both linoleic acid and polyunsaturated fatty acids with three or more double bonds were presented in quantities > 10% of total fatty acids in fillet, but lower in head tissue. Erucic acid levels were below the newly proposed tolerable daily intake in the European Union, based on the consumption of 200 g fillet per day. Moreover, further analysis produced evidence for the presence of the dicarboxylic fatty acid azelaic acid (nonanedioic acid, Di9:0) in head tissue. To verify this uncommon finding, countercurrent chromatography was used to isolate Di9:0 and other dicarboxylic acids from a technical standard followed by its quantification. Di9:0 contributed to 0.4–1.3% of the fatty acid profile in head tissue, but was not detected in fillet. Fish fed with increasing quantities of flaxseed indicated that linoleic acid was the likely precursor of Di9:0 in the head tissue samples.

Proteomics of mucosal exosomes of Cynoglossus semilaevis altered when infected by Vibrio harveyi

The cargo of exosomes contains proteins with various functions, which might be promising biomarkers for disease diagnosis and prognosis. To explore the impact of the Vibrio harveyi pathogen on Cynoglossus semilaevis from a different perspective and develop promising biomarkers for infection, the exosomes from epidermal mucus of healthy controls（EC）and sick fish（ES）were extracted and identified, coupled with proteomic screening through iTRAQ followed with LC-MS/MS. 1531 credible proteins were obtained relating to structural, metabolic and immunological functions. 359 different expressed proteins (DEPs) (FC > 2 or FC < 0.5) were found, with 161 up-regulated and 198 down-regulated in ES. Based on the database of C. semilaevis on Uniprot, 71 proteins were characterized as concrete names, including 19 up-regulated proteins and 52 down-regulated proteins, and were selected as subjects for further studies. Ferritin, Toll-like receptor 5S protein and Calcium-transporting ATPase were upregulated, while Histone H2B and Eukaryotic translation initiation factor 5A were downregulated, consistent with the expression levels of related mRNAs in skin tissue verified by qRT-PCR. The integrated analysis between miRomics and proteomics also provided possible regulatory relationships mediated by mucous exosomes during infection. The signature proteins in mucosal exosomes could make sense in the explanation of the infection defending mechanism and the development of biomarkers which can differentiate diseased and healthy C. semilaevis individuals.

Extracellular vesicles piwi-interacting RNAs from skin mucus for identification of infected Cynoglossus semilaevis with Vibrio harveyi

Extracellular vesicles play a regulatory role in intracellular and intercellular transmission through a variety of biological information molecules, including mRNA, small RNAs and proteins. piRNAs are one kind of regulatory small RNAs in the vesicles at the post transcriptional level. Hereby, we isolated the extracellular vesicles from skin mucus and screened the piRNA profiles of these vesicles, aiming at developing biomarkers related to bacterial infections in Cynoglossus semilaevis. The different profilings of piRNAs in mucous extracellular vesicles of C. semilaevis were compared through small RNA sequencing, between fish infected with Vibrio harveyi and healthy ones. The number of clean reads on the alignment of exosome sick (ES) group was 105, 345 and that of exosome control (EC) group was 455, 144. GO and KEGG pathway enrichment analysis showed that most of the target genes were involved in cellular process, response to stimulus, biological regulation, immune system process and signal transduction, signal molecular and interaction, transport and catabolism. The 45 final candidate piRNAs related to immunity or infectious diseases included 20 piRNAs with high expression in the ES group and 25 piRNAs with a low expression in the ES group. After verification by qRT-PCR, there was significant difference of five piRNAs expression level between infected fish and healthy fish, in line with the sequencing. The expression level of piR-mmu-16401212, piR-mmu-26829319 and piR-gga-244092 in infected fish were significantly lower than that of control group, while piR-gga-71717 and piR-gga-99034 were higher, which implying that these piRNAs in mucous extracellular vesicles can be used to identify diseased fish from normal ones. This work supplied a novel class of biomarker for infection diagnosis in fish, and it will be benefit for screening disease resistant breeding of C. semilaevis.

Insights into the Gut and Skin Microbiome of Freshwater Fish, Smelt (Hypomesus nipponensis)

Freshwater smelt (Hypomesus nipponensis) is a planktivorous fish found in the river of South Korea, Japan, China, and Russia. Because of its specific characteristics living in the cold temperature, this species is economically valuable in the various countries that held winter festival. The body size of the smelt is too small, so people consumed raw smelt during the winter festival sometimes. However, the microbial studies of smelt are nonexistent. Here, we characterized and compared the bacterial communities in the gut and skin of freshwater smelts. We amplified, sequenced, and analyzed the V4 regions of bacterial 16S rRNA genes from freshwater smelts. The microbial diversity in the skin (375 OTUs) was much greater than that in the gut (250 OTUs). At the phylum level, Proteobacteria (gut: 51.5%; skin: 52.9%), Firmicutes (gut: 30.6%; skin: 25.4%), Bacteroidetes (gut: 7.7%; skin: 14.7%), and Actinobacteria (gut: 5.2%; skin: 3.8%) were predominant in both organs. At the genus level, Sphingomonas (gut: 24.9%; skin: 4.4%, P < 0.01) was more abundant in the gut, whereas Acinetobacter (gut: 0.8%; skin: 11.8%, P = 0.02) and Pseudomonas (gut: 0.3%; skin: 2.1%, P = 0.01) were more abundant in the skin. Both beneficial (Lactobacillus) and harmful (Staphylococcus and Streptococcus) bacteria were detected in both organs, even under freshwater conditions. These results revealed that smelts have their own unique microbial communities in the gut and skin. Our findings broaden the understanding of planktivorous freshwater fish microbiomes and provide new insights into fish microbiomes for ensuring food safety.

Proteomic profile and protease activity in the skin mucus of greater amberjack (Seriola dumerili) infected with the ectoparasite Neobenedenia girellae - An immunological approach

Skin mucus is considered the first barrier against diseases in fish. The skin mucus protein profile of the greater amberjack (Seriola dumerili) and its changes due to experimental infection with Neobenedenia girellae were studied by combining 2-DE-MS/MS and gel-free LC-MS/MS proteomic approaches. The 2-DE results led to the identification of 69 and 55 proteins in noninfected and infected fish, respectively, and revealed that keratins were specifically cleaved in parasitized fish. Therefore, the skin mucus of the infected fish showed a higher protease activity due to, at least in part, an increase of metal-dependent protease and serine-type protease activities. Additionally, through a gel-free LC-MS/MS analysis, 1377 and 1251 different proteins were identified in the skin mucus of healthy and parasitized fish, respectively. The functional analysis of these proteins demonstrated a statistical overrepresentation of ribosomal proteins (a well-known source of antimicrobial peptides) in N. girellae-infected fish. In contrast, the components of membranes and protein transport GO categories were underrepresented after infection. Immune system process-related proteins constituted 2.5% of the total skin mucosal proteins. Among these skin mucosal proteins, 14 and 15 proteins exclusive to non-parasitized and parasitized fish were found, respectively, including specific serine-type proteases and metalloproteases in the parasitized fish. Moreover, the finding of tryptic peptides exclusive to some bacterial genera, obtained by gel-free LC-MS/MS, allowed us to construct a preliminary map of the microbiota living in the mucus of S. dumerili, with Pseudomonas and Paracoccus the most represented genera in both noninfected and infected fish.

Establishing a Percutaneous Infection Model Using Zebrafish and a Salmon Pathogen

Simple Summary

The epidermis and mucus layer of fish act as barriers that protect them against waterborne pathogens, and provide niches for symbiotic microorganisms that benefit the host’s health. However, our understanding of the relationship between fish skin bacterial flora and fish pathogen infection is limited. In order to elucidate this relationship, an experimental model for infection through fish skin is necessary. Such a model must also pose a low biohazard risk in a laboratory setting. We established a percutaneous infection model using zebrafish (Danio rerio), a typical fish experimental model, and Yersinia ruckeri, a salmon pathogen. Our experimental data indicate that Y. ruckeri colonizes niches on the skin surface generated by transient changes in the skin microflora caused by stress, dominates the skin bacterial flora, occupies the surface of the fish skin, invades the fish body through injury, and finally, causes fatal enteric redmouth disease. This percutaneous infection model can be used to study the interaction between fish skin bacterial flora and fish pathogens in water, or the relationship between pathogens and the host’s skin immune system.

Abstract

To uncover the relationship between skin bacterial flora and pathogen infection, we developed a percutaneous infection model using zebrafish and Yersinia ruckeri, a pathogen causing enteric redmouth disease in salmon and in trout. Pathogen challenge, either alone or together with pricking by a small needle, did not cause infection of the fish. However, cold stress given by water temperature shift from the optimum 28 °C for zebrafish to 20 °C caused fatal infection of injured fish following pathogen challenge. We investigated the effects of cold stress, injury, and pathogen challenge, alone and in combination, on fish skin bacterial flora using 16S rDNA metagenomics. We found that cold stress drastically altered the skin bacterial flora, which was dominated by Y. ruckeri on infected fish. In addition, fish whose intrinsic skin bacterial flora was disrupted by antibiotics had their skin occupied by Y. ruckeri following a challenge with this pathogen, although the fish survived without injury to create a route for invasion into the fish body. Our results suggest that the intrinsic skin bacterial flora of fish protects them from pathogen colonization, and that its disruption by stress allows pathogens to colonize and dominate their skin.

Assessing the impacts of skin mucus from Salmo salar and Oncorhynchus mykiss on the growth and in vitro infectivity of the fish pathogen Piscirickettsia salmonis

Piscirickettsiosis is a fish disease caused by the facultative intracellular bacterium, Piscirickettsia salmonis. Even though entry routes of P. salmonis in fish are not fully clear yet, the skin seems to be the main portal in some salmonid species. Despite the importance of fish mucous skin barrier in fighting waterborne pathogens, the interaction between salmonid skin mucus and the bacterium is unknown. This study seeks to determine the in vitro changes in the growth of two Chilean P. salmonis strains (LF-89-like and EM-90-like genotypes) and the type strain LF-89^T under exposures to skin mucus from Salmo salar and Oncorhynchus mykiss, as well as changes in the cytotoxic effect of P. salmonis on the SHK-1 cells following exposures. The results suggest that the growth of three P. salmonis strains was not significantly negatively affected under exposures to skin mucus (adjusted at 100 μg total protein ml^-1 ) of O. mykiss (69 ± 18 U lysozyme ml^-1 ) and S. salar (48 ± 33 U lysozyme ml^-1 ) over time. However, the cytotoxic effect of P. salmonis, pre-exposed to salmonid skin mucus, on the SHK-1 cell line was reliably identified only towards the end of the incubation period, suggesting that the mucus had a delaying effect on the cytotoxic response of the cell line to the bacterium. These results represent a baseline knowledge to open new avenues of research intended to understand how P. salmonis faces the fish mucous skin barrier.

Development of a Mucus Gland Bioreactor in Loach Paramisgurnus dabryanus

Most currently available bioreactors have some defects in the expression, activity, or purification of target protein and peptide molecules, whereas the mucus gland of fish can overcome these defects to become a novel bioreactor for the biopharmaceutical industry. In this study, we have evaluated the practicability of developing a mucus gland bioreactor in loach (Paramisgurnus dabryanus). A transgenic construct pT2-krt8-IFN1 was obtained by subcloning the promoter of zebrafish keratin 8 gene and the type I interferon (IFN1) cDNA of grass carp into the SB transposon. The IFN1 expressed in CIK cells exhibited an antiviral activity against the replication of GCRV873 and activated two genes downstream of JAK-STAT signaling pathway. A transgenic loach line was then generated by microinjection of the pT2-krt8-IFN1 plasmids and in vitro synthesized capped SB11 mRNA. Southern blots indicated that a single copy of IFN1 gene was stably integrated into the genome of transgenic loach. The expression of grass carp IFN1 in transgenic loaches was detected with RT-PCR and Western blots. About 0.0825 µg of grass carp IFN1 was detected in 20 µL mucus from transgenic loaches. At a viral titer of 1 × 10³ PFU/mL, plaque numbers on plates containing mucus from transgenic loaches reduced by 18% in comparison with those of the control, indicating that mucus of IFN1-transgenic loaches exhibited an antiviral activity. Thus, we have successfully created a mucus gland bioreactor that has great potential for the production of various proteins and peptides.

(2-Carboxyethyl) dimethylsulfonium bromide supplementation in non-fish meal diets for on-growing grass carp (Ctenopharyngodon idella): Beneficial effects on immune function of the immune organs via modulation of NF-κB and TOR signalling pathway

The immune function of immune organs is extremely crucial for maintaining organism health status, which ultimately affects fish growth. Our previous study has found that dietary supplementation of (2-carboxyethyl)dimethylsulfonium Bromide (Br-DMPT) in non-fish meal (NFM) diet could promote the growth of grass carp (Ctenopharyngodon idella), whereas the underlying reason or mechanism for this results is largely unclear. Herein, we further explored whether dietary supplementation of Br-DMPT promoted fish growth is connected with the enhanced immune function in the immune organs (the head kidney, spleen and skin). In this study, 540 fish (216.49 ± 0.29 g) were irregularly distributed to six groups with three replicates (30 fish replicate^-1) and fed corresponding diets group containing a fish meal (FM) diet group and five different NFM diets supplemented with gradational Br-DMPT (0-520.0 mg/kg level) group for 60 days. After the 60-days feeding trial, 8 fish from each replicate were selected and then conducted a challenge test with A. hydrophila for 14 days. Our results indicated that in the NFM diets, appropriate Br-DMPT: (1) significantly decreased the morbidity of skin haemorrhage and lesion after A. hydrophila infection (P < 0.05). (2) significantly improved the innate and adaptive immune components (lysozyme, complement 3, immunoglobulin M and antibacterial peptides et al.) (P < 0.05). (3) increased the gene expressions of main anti-inflammatory cytokines partially by referring to TOR signalling pathway, and decreased the gene expressions of main pro-inflammatory cytokines partially by referring to NF-kB signalling pathway (P < 0.05). Strikingly, no statistical difference could be found in the most of above immune parameters between 260.0 mg/kg Br-DMPT diet group and FM diet group (P > 0.05). Taken together, in non-fish meal diet, appropriate supplementation of Br-DMPT could improve the disease resistance capacity, non-specific immunity and ameliorate inflammation, and simultaneously could mitigate these adverse effects induced by the non-fish meal diet in fish immune organs. Moreover, this study may be helpful to decipher the underlying mechanisms of how Br-DMPT promote fish growth by immune organs and also provide scientific theoretical evidence for the future application of Br-DMPT as a new immunopotentiator in aquaculture industry.

Detecting Cynoglossus semilaevis infected with Vibrio harveyi using micro RNAs from mucous exosomes

Exosomes are important mediators of vesicle transportation and contain microRNAs (miRNAs) that mediate transcriptional gene knockout and silencing in biological processes. Moreover, exosomic miRNAs are promising biomarkers for disease diagnosis and physiological status indication in many species, including fish. The impact of the Vibrio harveyi pathogen on Cynoglossus semilaevis aquaculture is becoming more and more serious as the industry expands. To overcome this challenge, miRNAs in mucous exosomes were screened by small RNA sequencing and verified by quantitative real-time PCR to develop biomarkers. This is the first capture of exosomes from flatfish mucus coupled with miRNA profiling. The results revealed significant differences in expression levels of some miRNAs between infected and healthy fish. Three unique miRNAs were identified for V. harveyi infection diagnosis; expression levels of dre-miR-205-5p and dre-miR-205-5p in infected fish were significantly lower than controls, while dre-miR-100-5p expression was higher. These miRNAs in mucous exosomes could be used to differentiate diseased and healthy fish in an early screening method with practical value for breeding disease-resistant C. semilaevis.

Skin transcriptome, tissue distribution of mucin genes and discovery of simple sequence repeats in crucian carp (Carassius auratus)

Crucian carp (Carassius auratus) is one of the major freshwater species and important food fish in China. Fish skin acts as the first line of defence against pathogens, yet its molecular and immune mechanism remains unclear. In this study, a de novo transcriptome assembly of C. auratus skin was performed with the Illumina Hiseq 2000 platform. A total of 49,154,776 clean reads were assembled, among which 60,824 (46.86%), 37,103 (28.59%), 43,269 (33.33%) unigenes were annotated against National Center for Biotechnology Information, Gene Onotology and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases, respectively. KEGG Orthology categories were significantly involved in immune system (20.50%), signal transduction (18.04%) and mucosal mucin genes (e.g., muc2, muc5AC, muc5B, muc17, muc18). The high expression of muc18 gene was observed in brain; that of muc2 in intestine; and that of muc5AC in skin, liver, spleen, intestine and muscle. Moreover, the potential 28,928 simple sequence repeats with the three most abundant dinucleotide repeat motifs (AC/GT, AG/CT, AT/AT) were detected in C. auratus. To authors' knowledge, this is the first report to describe the transcriptome analysis of C. auratus skin, and the outcome of this study contributed to the understanding of mucosal immune response of the skin and molecular markers in cyprinid species.

Skin proteome profiling of tongue sole (Cynoglossus semilaevis) challenged with Vibrio vulnificus

Vibrio vulnificus is a major pathogen of cultured Cynoglossus semilaevis and results in skin ulceration and haemorrhage, but the proteomic mechanism of skin immunity against V. vulnificus remains unclear. In this study, we investigated the histopathology and skin immune response in C. semilaevis with V. vulnificus infection at the protein levels, the differential proteomic profiling of its skin was examined by using iTRAQ and LC-MS/MS analyses. A total of 951 proteins were identified in skin, in which 134 and 102 DEPs were screened at 12 and 36 hpi, respectively. Selected eleven immune-related DEPs (pvβ, Hsp71, MLC1, F2, α2ML, HCII, C3, C5, C8β, C9 and CD59) were verified for their immune roles in the V. vulnificus infection via using qRT-PCR assay. KEGG enrichment analysis revealed that most of the identified immune proteins were significantly associated with complement and coagulation cascades, antigen processing and presentation, salivary secretion and phagosome pathways. To our knowledge, this study is the first to describe the proteome response of C. semilaevis skin against V. vulnificus infection. The outcome of this study contributed to provide a new perspective for understanding the molecular mechanism of local skin mucosal immunity, and facilitating the development of novel mucosal vaccination strategies in fish.

Effect of dietary isoleucine on skin mucus barrier and epithelial physical barrier functions of hybrid bagrid catfish Pelteobagrus vachelli × Leiocassis longirostris

The study investigated the effects of dietary isoleucine (Ile) on skin mucus barrier and epithelial physical barrier functions of hybrid bagrid catfish Pelteobagrus vachelli × Leiocassis longirostris. A total of 630 fish (33.11 ± 0.09 g) were fed semi-purified isonitrogenous diets containing 5.0 (control), 7.5, 10.0, 12.5, 15.0, 17.5, and 20.0 g Ile kg ^-1 diet for 8 weeks. The results indicated that dietary Ile increased (P < 0.05) in skin (1) mucus protein content and antimicrobial activity against three gram-negative bacteria (Aeromonas hydrophila, Escherichia coli, and Yersinia ruckeri) and two gram-positive bacteria (Streptococcus agalactiae and Staphylococcus aureus), (2) mucus lysofew information is available about the influencezyme (LZM), acid phosphatase (ACP), and alkaline phosphatase (AKP) activities, and complement 3 and 4 (C3 and C4) and immunoglobulin M (IgM) contents, (3) intelectin 1 (intl1), intelectin 2 (intl2), c-type-lysozyme (c-LZM), g-type-lysozyme (g-LZM), and β-defensin mRNA levels. Dietary Ile decreased (P < 0.05) reactive oxygen species (ROS), malondialdehyde (MDA), and protein carbonyl (PC) contents, and up-regulated (P < 0.05) CuZnSOD, GST, GPX1a, muc5ac, muc5b, zonula occludens-1 (ZO-1), zonula occludens-2 (ZO-2), occludin, and claudin 3 mRNA levels in skin. These results indicated that Ile improved skin mucus barrier function via increasing mucus protein, C3 and C4, and IgM contents and antibacterial factors activities, and promoted epithelial physical barrier function via decreasing skin antioxidant damage and improving tight junction structure in hybrid bagrid catfish.

Quantification of Defensive Proteins in Skin Mucus of Atlantic salmon Using Minimally Invasive Sampling and High-Sensitivity ELISA

Simple Summary

The external surfaces of fish, including the skin, are covered by mucus. Mucus is an important and multifunctional matrix of substantial complexity. The mucus is viscous and sticky, and adheres to the underlying epithelium, making the sampling of mucus challenging. To help define more standardized protocols for mucus sampling, we here compare three different sampling methods. The methods include scraping of mucus, wiping of mucus, and absorption of the liquid part of mucus. We compare the resulting damage to the fish skin and compare the content of two specific immune proteins in the three sample types. Using histological examination, we show that absorption leads to very limited damage to the skin epithelium while scraping causes substantial damage. Using Enzyme-Linked Immunosorbent Assay (ELISA) methods, we show that the mucus sample types contain similar amounts of antigen specific immunoglobulin M (IgM) and complement component 5 (C5), respectively. The levels of the IgM but not the C5 were moderately correlated between mucus and blood from the same fish, suggesting the importance of fish skin mucus for analyzing antigen-specific IgM after vaccination procedures. We conclude that absorption is an easily performed and minimally invasive sampling method that produces mucus samples with comparable contents of IgM and C5.

Abstract

Protocols used to collect fish skin mucus may inadvertently compromise the sampled fish or the resulting sample. Here, we used three methods (wiping, scraping, and absorption) to collect skin mucus from Atlantic salmon and compared their invasiveness on fish skin epithelium. We found that the absorption method was the least invasive. We also compared the abundance of antigen-specific immunoglobulin M subtype A antibodies (IgM-A Ab) and complement component 5 (C5) in mucus samples collected from vaccinated fish by the three methods. An enzyme-cascade-amplification strategy colorimetric immune assay was optimized and used to analyze IgM-A, and ELISA was used to analyze C5. The abundance of antigen-specific IgM-A in skin mucus was comparable between the three methods, but C5 was significantly lower in absorbed mucus in comparison to in the wiped or scraped mucus samples. Absorbed skin mucus samples collected from various body regions of salmon, levels of C5 were comparable, while specific IgM-A amounts varied between the regions. By comparing three mucus-absorbing materials (medical wipe, gauze, and cotton) for their ability to absorb and release IgM-A and C5, medical wipes proved to be ideal for IgM-A analysis, whereas gauze was the best for C5 analysis.

Improved understanding of biofilm development by Piscirickettsia salmonis reveals potential risks for the persistence and dissemination of piscirickettsiosis

Piscirickettsia salmonis is the causative agent of piscirickettsiosis, a disease with high socio-economic impacts for Chilean salmonid aquaculture. The identification of major environmental reservoirs for P. salmonis has long been ignored. Most microbial life occurs in biofilms, with possible implications in disease outbreaks as pathogen seed banks. Herein, we report on an in vitro analysis of biofilm formation by P. salmonis Psal-103 (LF-89-like genotype) and Psal-104 (EM-90-like genotype), the aim of which was to gain new insights into the ecological role of biofilms using multiple approaches. The cytotoxic response of the salmon head kidney cell line to P. salmonis showed interisolate differences, depending on the source of the bacterial inoculum (biofilm or planktonic). Biofilm formation showed a variable-length lag-phase, which was associated with wider fluctuations in biofilm viability. Interisolate differences in the lag phase emerged regardless of the nutritional content of the medium, but both isolates formed mature biofilms from 288 h onwards. Psal-103 biofilms were sensitive to Atlantic salmon skin mucus during early formation, whereas Psal-104 biofilms were more tolerant. The ability of P. salmonis to form viable and mucus-tolerant biofilms on plastic surfaces in seawater represents a potentially important environmental risk for the persistence and dissemination of piscirickettsiosis.

Proteomic profiling of yellow catfish (Pelteobagrus fulvidraco) skin mucus identifies differentially-expressed proteins in response to Edwardsiella ictaluri infection

Fish mucus acts as a physiological and immunological barrier for maintaining normal fish physiology and conferring defense against pathogens infection. Here we report proteomic profiling of skin mucus of yellow catfish before and after E. ictaluri infection by Label-free LC-MS/MS approach. A total of 918 non-redundant proteins were identified from 54443 spectra referring to yellow catfish genome database. Further annotation via GO and KEGG database revealed complex protein composition of yellow catfish mucus. Besides structural proteins in mucus, a lot of immune-related proteins were retrieved, such as lectins, complement components, antibacterial peptides and immunoglobins. 133 differentially-expressed proteins (DEPs), including 76 up-regulated and 57 down-regulated proteins, were identified, most of which were enriched into 17 pathways centering on "immune system" category with 33 proteins involved. Consistently, significant proliferation of mucus-secreting goblet cells and CYPA-expressing cells were observed along outside of yellow catfish skin after E. ictaluri infection, indicating an enhanced immune response to E. ictaluri infection in yellow catfish skin mucus. The proteomic data provide systematic protein information to comprehensively understand the biological function of yellow catfish skin mucus in response to bacterial infection.

A Truncated Galectin-3 Isolated from Skin Mucus of Atlantic Salmon Salmo salar Binds to and Modulates the Proteome of the Gram-Negative Bacteria Moritella viscosa

The mucus of fish skin plays a vital role in innate immune defense. Some mucus proteins have the potential to incapacitate pathogens and/or inhibit their passage through the skin. In this study the aim was to isolate and characterize galectin(s), β-galactosides binding proteins, present in skin mucus. A novel short form of galectin-3 was isolated from Atlantic salmon skin mucus by α-lactose agarose based affinity chromatography followed by Sephadex G-15 gel filtration. Mass spectrometric analysis showed that the isolated protein was the C-terminal half of galectin-3 (galectin-3C). Galectin-3C showed calcium independent and lactose inhabitable hemagglutination, and agglutinated the Gram-negative pathogenic bacteria Moritella viscosa. Galectin-3 mRNA was highly expressed in skin and gill, followed by muscle, hindgut, spleen, stomach, foregut, head kidney, and liver. Moritella viscosa incubated with galectin-3C had a modified proteome. Proteins with changed abundance included multidrug transporter and three ribosomal proteins L7/12, S2, and S13. Overall, this study shows the isolation and characterization of a novel galectin-3 short form involved in pathogen recognition and modulation, and hence in immune defense of Atlantic salmon.

Transcriptome analysis in the skin of Carassius auratus challenged with Aeromonas hydrophila

Skin plays an important role in the innate immune responses of fish, particularly towards bacterial infection. To understand the molecular mechanism of mucosal immunity of fish during bacterial challenge, a de novo transcriptome assembly of crucian carp Carassius auratus skin upon Aeromonas hydrophila infection was performed, the latter with Illumina Hiseq 2000 platform. A total of 118111 unigenes were generated and of these, 9693 and 8580 genes were differentially expressed at 6 and 12 h post-infection, respectively. The validity of the transcriptome results of eleven representative genes was verified by quantitative real-time PCR (qRT-PCR) analysis. A comparison with the transcriptome profiling of zebrafish skin to A. hydrophila with regards to the mucosal immune responses revealed similarities in the complement system, chemokines, heat shock proteins and the acute-phase response. GO and KEGG enrichment pathway analyses displayed the significant immune responses included TLR, MAPK, JAK-STAT, phagosome and three infection-related pathways (ie., Salmonella, Vibrio cholerae and pathogenic Escherichia coli) in skin. To our knowledge, this study is the first to describe the transcriptome analysis of C. auratus skin during A. hydrophila infection. The outcome of this study contributed to the understanding of the mucosal defense mechanisms in cyprinid species.

Biochemical and molecular identification of a novel hepcidin type 2-like antimicrobial peptide in the skin mucus of the pufferfish Takifugu pardalis

Fish skin mucus is considered to act as the first line of defense against waterborne pathogens and to be potential source of novel antimicrobial components. Here we report the purification and characterization of a novel hepcidin type 2-like antimicrobial peptide (TpHAMP2) from the skin mucus of the pufferfish Takifugu pardalis. The purified TpHAMP2 comprised of 23 amino acids (AAs) with eight Cys residues that form four intramolecular disulfide bonds. The TpHAMP2 gene shared overall structural characteristics with all known hepcidins, which have a tripartite exon-intron gene organization and three structural signatures in the precursor protein. Phylogenetically, TpHAMP2 was classified as HAMP2 class in acanthopterygian fish. Interestingly, the AA sequence of TpHAMP2 did not contain a proprotein cleavage site (RXXR motif) that conserved in most hepcidins and showed a highly positive charged (RKR-) short N-terminus and Val¹⁸ and Gly²² residues, which are distinctive structures compared to other known active hepcidins. Recombinant TpHAMP2 identical to the native form exhibited a broad spectrum and potent antimicrobial activity against tested gram-positive and -negative bacteria. Expression of TpHAMP2 mRNA was predominant in the liver and was upregulated in the liver, the spleen, the intestine, and the skin of T. pardalis post immune challenge. Thus, our findings suggests that TpHAMP2 might be of importance in the framework of discovering the fish hepcidins, especially type 2s, and provide noteworthy insight into its gene structure and expression and in the innate immunity as well as the mucosal immunity in regard to hepcidins' evolutionary history in fish species.

Biochemical and biological characterization of the Hypanus americanus mucus: A perspective on stingray immunity and toxins

Stingrays skin secretions are largely studied due to the human envenoming medical relevance of the sting puncture that evolves to inflammatory events, including necrosis. Such toxic effects can be correlated to the biochemical composition of the sting mucus, according to the literature. Fish skin plays important biological roles, such as the control of the osmotic pressure gradient, protection against mechanical forces and microorganism infections. The mucus, on the other hand, is a rich and complex fluid, acting on swimming, nutrition and the innate immune system. The elasmobranch's epidermis is a tissue composed mainly by mucus secretory cells, and marine stingrays have already been described to present secretory glands spread throughout the body. Little is known about the biochemical composition of the stingray mucus, but recent studies have corroborated the importance of mucus in the envenomation process. Aiming to assess the mucus composition, a new non-invasive mucus collection method was developed that focused on peptides and proteins, and biological assays were performed to analyze the toxic and immune activities of the Hypanus americanus mucus. Pathophysiological characterization showed the presence of peptidases on the mucus, as well as the induction of edema and leukocyte recruitment in mice. The fractionated mucus improved phagocytosis on macrophages and showed antimicrobial activity against T. rubrumç. neoformans and C. albicans in vitro. The proteomic analyses showed the presence of immune-related proteins like actin, histones, hemoglobin, and ribosomal proteins. This protein pattern is similar to those reported for other fish mucus and stingray venoms. This is the first report depicting the Hypanus stingray mucus composition, highlighting its biochemical composition and importance for the stingray immune system and the possible role on the envenomation process.

The Farmed Atlantic Salmon (Salmo salar) Skin-Mucus Proteome and Its Nutrient Potential for the Resident Bacterial Community

Norway is the largest producer and exporter of farmed Atlantic salmon (Salmo salar) worldwide. Skin disorders correlated with bacterial infections represent an important challenge for fish farmers due to the economic losses caused. Little is known about this topic, thus studying the skin-mucus of Salmo salar and its bacterial community depict a step forward in understanding fish welfare in aquaculture. In this study, we used label free quantitative mass spectrometry to investigate the skin-mucus proteins associated with both Atlantic salmon and bacteria. In particular, the microbial temporal proteome dynamics during nine days of mucus incubation with sterilized seawater was investigated, in order to evaluate their capacity to utilize mucus components for growth in this environment. At the start of the incubation period, the largest proportion of proteins (~99%) belonged to the salmon and many of these proteins were assigned to protecting functions, confirming the defensive role of mucus. On the contrary, after nine days of incubation, most of the proteins detected were assigned to bacteria, mainly to the genera Vibrio and Pseudoalteromonas. Most of the predicted secreted proteins were affiliated with transport and metabolic processes. In particular, a large abundance and variety of bacterial proteases were observed, highlighting the capacity of bacteria to degrade the skin-mucus proteins of Atlantic salmon.

Methacarn preserves mucus integrity and improves visualization of amoebae in gills of Atlantic salmon (Salmo salar L.)

Two aqueous fixation methods (modified Davidson's solution and modified Davidson's solution with 2% (w/v) Alcian blue) were compared against two non-aqueous fixation methods (methacarn solution and methacarn solution with 2% (w/v) Alcian blue) along with the standard buffered formalin fixation method to (a) improve preservation of the mucous coat on Atlantic salmon, Salmo salar L., gills and (b) to examine the interaction between the amoebae and mucus on the gill during an infection with amoebic gill disease. Aqueous fixatives demonstrated excellent cytological preservation but failed to deliver the preservation of the mucus when compared to the non-aqueous-based fixatives; qualitative and semi-quantitative analysis revealed a greater preservation of the gill mucus using the non-aqueous methacarn solution. A combination of this fixation method and an Alcian blue/Periodic acid-Schiff staining was tested in gills of Atlantic salmon infected with amoebic gill disease; lectin labelling was also used to confirm the mucus preservation in the methacarn-fixed tissue. Amoebae were observed closely associated with the mucus demonstrating that the techniques employed for preservation of the mucous coat can indeed avoid the loss of potential mucus-embedded parasites, thus providing a better understanding of the relationship between the mucus and parasite.

Proteomic and structural differences in lumpfish skin among the dorsal, caudal and ventral regions

Fish skin is a vital organ that serves a multitude of functions including mechanical protection, homeostasis, osmoregulation and protection against diseases. The expression of skin proteins changes under different physiological conditions. However, little is known about differences in protein expression among various body sites in naïve fish. The objectives of this work is to study potential differences in protein and gene expression among dorsal, caudal and ventral regions of lumpfish skin employing 2D gel based proteomics and real-time PCR and to assess structural differences between these regions by using Alcian blue and Periodic acid Schiff stained skin sections. The proteins collagen alfa-1, collagen alfa-2, heat shock cognate 71 kDa, histone H4, parvalbumin, natterin-2, 40S ribosomal protein S12, topoisomerase A and topoisomerase B were differentially expressed among the three regions. mRNA expression of apoa1, hspa8 and hist1h2b showed significant differences between regions. Skin photomicrographs showed differences in epidermal thickness and goblet cell counts. The ventral region showed relatively high protein expression, goblet cell count and epidermal thickness compared to dorsal and caudal regions. Overall, this study provides an important benchmark for comparative analysis of skin proteins and structure between different parts of the lumpfish body.