Gene expression analysis of Atlantic salmon gills reveals mucin 5 and interleukin 4/13 as key molecules during amoebic gill disease

Mucosal barrier status in Atlantic salmon fed rapeseed oil and Schizochytrium oil partly or fully replacing fish oil through winter depression

The study was designed to investigate the effects of replacing fish oil by algal oil and rapeseed oil on histomorphology indices of the intestine, skin and gill, mucosal barrier status and immune-related genes of mucin and antimicrobial peptide (AMP) genes in Atlantic salmon (Salmo salar). For these purposes, Atlantic salmon smolts were fed three different diets. The first was a control diet containing fish oil but no Schizochytrium oil. In the second diet, almost 50 % of the fish oil was replaced with algal oil, and in the third diet, fish oil was replaced entirely with algal oil. The algal oil contained mostly docosahexaenoic acid (DHA) and some eicosapentaenoic acid (EPA). The study lasted for 49 days in freshwater (FW), after which some fish from each diet group were transferred to seawater (SW) for a 48-h challenge test at 33 ppt to test their ability to tolerate high salinity. Samples of skin, gills, and mid intestine [both distal (DI) and anterior (AI) portions of the mid intestine] were collected after the feeding trial in FW and after the SW-challenge test to assess the effects of the diets on the structure and immune functions of the mucosal surfaces. The results showed that the 50 % VMO (Veramaris® algal oil) dietary group had improved intestinal, skin, and gill structures. Principal component analysis (PCA) of the histomorphological parameters demonstrated a significant effect of the algal oil on the intestine, skin, and gills. In particular, the mucosal barrier function of the intestine, skin, and gills was enhanced in the VMO 50 % dietary group after the SW challenge, as evidenced by increased mucous cell density. Immunolabelling of heat shock protein 70 (HSP70) in the intestine (both DI and AI) revealed downregulation of the protein expression in the 50 % VMO group and a corresponding upregulation in the 100 % VMO group compared to 0 % VMO. The reactivity of HSP70 in the epithelial cells was higher after the SW challenge compared to the FW phase. Immune-related genes related to mucosal defense, such as mucin genes [muc2, muc5ac1 (DI), muc5ac1 (AI), muc5ac2, muc5b (skin), and muc5ac1 (gills)], and antimicrobial peptide genes [def3 (DI), def3 (AI), and cath1 (skin)] were significantly upregulated in the 50 % VMO group. PCA of gene expression demonstrated the positive influences on gene regulation in the 50 % VMO dietary group. In conclusion, this study demonstrated the positive effect of substituting 50 % of fish oil with algal oil in the diets of Atlantic salmon. The findings of histomorphometry, mucosal mapping, immunohistochemistry, and immune-related genes connected to mucosal responses all support this conclusion.

Mucosal affairs: glycosylation and expression changes of gill goblet cells and mucins in a fish-polyopisthocotylidan interaction

Introduction

Secreted mucins are highly O-glycosylated glycoproteins produced by goblet cells in mucosal epithelia. They constitute the protective viscous gel layer overlying the epithelia and are involved in pathogen recognition, adhesion and expulsion. The gill polyopisthocotylidan ectoparasite Sparicotyle chrysophrii, feeds on gilthead seabream (Sparus aurata) blood eliciting severe anemia.

Methods

Control unexposed and recipient (R) gill samples of gilthead seabream experimentally infected with S. chrysophrii were obtained at six consecutive times (0, 11, 20, 32, 41, and 61 days post-exposure (dpe)). In histological samples, goblet cell numbers and their intensity of lectin labelling was registered. Expression of nine mucin genes (muc2, muc2a, muc2b, muc5a/c, muc4, muc13, muc18, muc19, imuc) and three regulatory factors involved in goblet cell differentiation (hes1, elf3, agr2) was studied by qPCR. In addition, differential expression of glycosyltransferases and glycosidases was analyzed in silico from previously obtained RNAseq datasets of S. chrysophrii-infected gilthead seabream gills with two different infection intensities.

Results and Discussion

Increased goblet cell differentiation (up-regulated elf3 and agr2) leading to neutral goblet cell hyperplasia on gill lamellae of R fish gills was found from 32 dpe on, when adult parasite stages were first detected. At this time point, acute increased expression of both secreted (muc2a, muc2b, muc5a/c) and membrane-bound mucins (imuc, muc4, muc18) occurred in R gills. Mucins did not acidify during the course of infection, but their glycosylation pattern varied towards more complex glycoconjugates with sialylated, fucosylated and branched structures, according to lectin labelling and the shift of glycosyltransferase expression patterns. Gilthead seabream gill mucosal response against S. chrysophrii involved neutral mucus hypersecretion, which could contribute to worm expulsion and facilitate gas exchange to counterbalance parasite-induced hypoxia. Stress induced by the sparicotylosis condition seems to lead to changes in glycosylation characteristic of more structurally complex mucins.

In Situ Detection of Salmonid Alphavirus 3 (SAV3) in Tissues of Atlantic Salmon in a Cohabitation Challenge Model with a Special Focus on the Immune Response to the Virus in the Pseudobranch

Salmonid alphavirus strain 3 is responsible for outbreaks of pancreas disease in salmon and rainbow trout in Norway. Although the extensive amount of research on SAV3 focused mainly on the heart and pancreas (of clinical importance), tropism and pathogenesis studies of the virus in other salmon tissues are limited. Here, we used a combination of RT-qPCR (Q_nsp1 gene) and in situ hybridization (RNAscope®) to demonstrate the tropism of SAV3 in situ in tissues of Atlantic salmon, employing a challenge model (by cohabitation). In addition, as previous results suggested that the pseudobranch may harbor the virus, the change in the expression of different immune genes upon SAV3 infection (RT-qPCR) was focused on the pseudobranch in this study. In situ hybridization detected SAV3 in different tissues of Atlantic salmon during the acute phase of the infection, with the heart ventricle showing the most extensive infection. Furthermore, the detection of the virus in different adipose tissues associated with the internal organs of the salmon suggests a specific affinity of SAV3 to adipocyte components. The inconsistent immune response to SAV3 in the pseudobranch after infection did not mitigate the infection in that tissue and is probably responsible for the persistent low infection at 4 weeks post-challenge. The early detection of SAV3 in the pseudobranch after infection, along with the persistent low infection over the experimental infection course, suggests a pivotal role of the pseudobranch in SAV3 pathogenesis in Atlantic salmon.

The Immune Response to the Myxozoan Parasite Myxobolus cerebralis in Salmonids: A Review on Whirling Disease

Salmonids are affected by the economically significant whirling disease (WD) caused by the myxozoan parasite Myxobolus cerebralis. In the past, it was endemic to Eurasia, but it has now spread to different regions of North America, Europe, New Zealand, and South Africa. Among salmonids, rainbow trout is considered the most highly susceptible host. Upon entering to the host's body, the parasite invades the spine and cranium, resulting in whirling behaviour, a blackened tail, and destruction of cartilage. The disease is characterized by the infiltration of numerous inflammatory cells, primarily lymphocytes and macrophages, with the onset of fibrous tissue infiltration. Several efforts have been undertaken to investigate the role of various immune modulatory molecules and immune regulatory genes using advanced molecular methods including flow cytometry and transcriptional techniques. Investigation of the molecular and cellular responses, the role of STAT3 in Th17 cell differentiation, and the inhibitory actions of suppressors of cytokine signaling (SOCS) on interferons and interleukins, as well as the role of natural resistance-associated macrophage proteins (Nramp) in WD have significantly contributed to our understanding of the immune regulation mechanism in salmonids against M. cerebralis. This review thoroughly highlights previous research and discusses potential future directions for understanding the molecular immune response of salmonids and the possible development of prophylactic approaches against WD.

Identification of differentially expressed genes and SNPs linked to harvest body weight of genetically improved rohu carp, Labeo rohita

In most of the aquaculture selection programs, harvest body weight has been a preferred performance trait for improvement. Molecular interplay of genes linked to higher body weight is not elucidated in major carp species. The genetically improved rohu carp with 18% average genetic gain per generation with respect to harvest body weight is a promising candidate for studying genes' underlying performance traits. In the present study, muscle transcriptome sequencing of two groups of individuals, with significant difference in breeding value, belonging to the tenth generation of rohu carp was performed using the Illumina HiSeq 2000 platform. A total of 178 million paired-end raw reads were generated to give rise to 173 million reads after quality control and trimming. The genome-guided transcriptome assembly and differential gene expression produced 11,86,119 transcripts and 451 upregulated and 181 downregulated differentially expressed genes (DEGs) between high-breeding value and low-breeding value (HB & LB) groups, respectively. Similarly, 39,158 high-quality coding SNPs were identified with the Ts/Tv ratio of 1.23. Out of a total of 17 qPCR-validated transcripts, eight were associated with cellular growth and proliferation and harbored 13 SNPs. The gene expression pattern was observed to be positively correlated with RNA-seq data for genes such as myogenic factor 6, titin isoform X11, IGF-1 like, acetyl-CoA, and thyroid receptor hormone beta. A total of 26 miRNA target interactions were also identified to be associated with significant DETs (p-value < 0.05). Genes such as Myo6, IGF-1-like, and acetyl-CoA linked to higher harvest body weight may serve as candidate genes in marker-assisted breeding and SNP array construction for genome-wide association studies and genomic selection.

Transcriptome Signatures of Atlantic Salmon-Resistant Phenotypes against Sea Lice Infestation Are Associated with Tissue Repair

Salmon aquaculture is constantly threatened by pathogens that impact fish health, welfare, and productivity, including the sea louse Caligus rogercresseyi. This marine ectoparasite is mainly controlled through delousing drug treatments that have lost efficacy. Therein, strategies such as salmon breeding selection represent a sustainable alternative to produce fish with resistance to sea lice. This study explored the whole-transcriptome changes in Atlantic salmon families with contrasting resistance phenotypes against lice infestation. In total, 121 Atlantic salmon families were challenged with 35 copepodites per fish and ranked after 14 infestation days. Skin and head kidney tissue from the top two lowest (R) and highest (S) infested families were sequenced by the Illumina platform. Genome-scale transcriptome analysis showed different expression profiles between the phenotypes. Significant differences in chromosome modulation between the R and S families were observed in skin tissue. Notably, the upregulation of genes associated with tissue repairs, such as collagen and myosin, was found in R families. Furthermore, skin tissue of resistant families showed the highest number of genes associated with molecular functions such as ion binding, transferase, and cytokine activity, compared with the susceptible. Interestingly, lncRNAs differentially modulated in the R/S families are located near genes associated with immune response, which are upregulated in the R family. Finally, SNPs variations were identified in both salmon families, where the resistant ones showed the highest number of SNPs variations. Remarkably, among the genes with SPNs, genes associated with the tissue repair process were identified. This study reported Atlantic salmon chromosome regions exclusively expressed in R or S Atlantic salmon families' phenotypes. Furthermore, due to the presence of SNPs and high expression of tissue repair genes in the resistant families, it is possible to suggest mucosal immune activation associated with the Atlantic salmon resistance to sea louse infestation.

Regulation of the molecular repertoires of oxidative stress response in the gills and olfactory organ of Atlantic salmon following infection and treatment of the parasite Neoparameoba perurans

The present study investigated the involvement of key molecular regulators of oxidative stress in amoebic gill disease (AGD), a parasitic infestation in Atlantic salmon. In addition, the study evaluated how these molecular biomarkers responded when AGD-affected fish were exposed to a candidate chemotherapeutic peracetic acid (PAA). Atlantic salmon were experimentally infected with the parasite Neoparameoba perurans, the causative agent of AGD, by bath exposure and after 2 weeks, the fish were treated with three commercial PAA products (i.e., Perfectoxid, AquaDes and ADDIAqua) at a dose of 5 ppm. Two exposure durations were evaluated - 30 min and 60 min. Sampling was performed 24 h and 2 weeks after PAA treatment (equivalent to 2- and 4-weeks post infection). At each sampling point, the following parameters were evaluated: gross gill pathology, gill parasitic load, plasma reactive oxygen species (ROS) and total antioxidant capacity (TAC), histopathology and gene expression profiling of genes with key involvement in oxidative stress in the gills and olfactory organ. AGD did not result in systemic oxidative stress as ROS and TAC levels remained unchanged. There were no clear patterns of AGD-mediated regulation of the oxidative stress biomarkers in both the gills and olfactory organ; significant changes in the expression were mostly related to time rather than infection status. However, the expression profiles of the oxidative stress biomarkers in AGD-affected salmon, following treatment with PAA, revealed that gills and olfactory organ responded differently - upregulation was prominent in the gills while downregulation was more frequent in the olfactory organ. The expression of catalase, glutathione S-transferase and thioredoxin reductase 2 was significantly affected by the treatments, both in the gills and olfactory organ, and these alterations were influenced by the duration of exposure and PAA product type. Parasitic load in the gills did significantly increase after treatment regardless of the product and exposure duration; the parasite was undetectable in some fish treated with AquaDes for 30 mins. However, PAA treated groups for 30 min showed lower macroscopic gill scores than the infected-untreated fish. Histology disclosed the classic pathological findings such as multifocal hyperplasia and increased number of mucous cells in AGD-affected fish. Microscopic scoring of gill injuries showed that AGD-infected-PAA-treated fish had lower scores, however, an overall trend could not be established. The morphology and structural integrity of the olfactory organ were not significantly altered by parasitism or PAA treatment. Collectively, the results indicate that AGD did not affect the systemic and mucosal oxidative status of Atlantic salmon. However, such a striking profile was changed when AGD-affected fish were exposed to oxidative chemotherapeutics. Moreover, the gills and olfactory organ demonstrated distinct patterns of gene expression of oxidative stress biomarkers in AGD-infected-PAA-treated fish. Lastly, PAA treatment did not fully resolve the infection, but appeared not to worsen the mucosal health either.

Investigating the impacts of H2O2 treatment on gills of healthy Atlantic salmon reveals potential changes to mucus production with implications on immune activity

Current treatment strategies for relevant infectious diseases in Atlantic salmon (Salmo salar L.) include the use of low salinity or freshwater bathing. However, often availability is restricted, and hydrogen peroxide (H₂O₂) is used as an alternative. The potential impacts of H₂O₂ on fish mucosal tissues, especially the gills therefore need to be considered. In this study the mucosal and immunological effects of H₂O₂ treatment on the gills of healthy Atlantic salmon were examined by gene expression (qPCR) and immunohistochemistry (IHC) investigating T-cell, B-cell, and mucin activity. Healthy fish were treated with H₂O₂ and sampled at different times: 4 h, 24 h and 14 days post-H₂O₂ treatment (dpt) (total n = 18) to investigate the effect of holding time and H₂O₂ treatment. Treatment with H₂O₂ resulted in up-regulation of markers for T-cell activity and anti-inflammatory response and down-regulation of mucin expression in the gills at 14 dpt compared to fish sampled prior to treatment (0h; n = 5 fish). These findings were supported by IHC analysis, which despite being highly variable between samples, showed an increase in the number of CD3⁺ T cells at 14 dpt in 50% of treated fish compared to pre-treatment fish. The results from this study suggest that H₂O₂ treatment does not immune compromise healthy Atlantic salmon after 14 dpt (i.e., post-recovery) but modulates gill immune activity and disrupts the mucus covering of the gills. However, further studies are required to determine whether the effects observed are related to H₂O₂ treatment in isolation or other variables such as holding time or environmental factors.

Mucosal immune and stress responses of Neoparamoeba perurans-infected Atlantic salmon (Salmo salar) treated with peracetic acid shed light on the host-parasite-oxidant interactions

Treatment development for parasitic infestation is often limited to disease resolution as an endpoint response, and physiological and immunological consequences are not thoroughly considered. Here, we report the impact of exposing Atlantic salmon affected with amoebic gill disease (AGD) to peracetic acid (PAA), an oxidative chemotherapeutic. AGD-affected fish were treated with PAA either by exposing them to 5 ppm for 30 min or 10 ppm for 15 min. Unexposed fish from both infected and uninfected groups were also included. Samples for molecular, biochemical, and histological evaluations were collected at 24 h, 2 weeks, and 4 weeks post-treatment. Behavioral changes were observed during PAA exposure, and post-treatment mortality was higher in the infected and PAA treated groups, especially in 10 ppm for 15 min. Plasma indicators showed that liver health was affected by AGD, though PAA treatment did not exacerbate the infection-related changes. Transcriptome profiling in the gills showed significant changes, triggered by AGD and PAA treatments, and the effects of PAA were more notable 24 h after treatment. Genes related to immune pathways of B- and T- cells and protein synthesis and metabolism were downregulated, where the magnitude was more remarkable in 10 ppm for 15 min group. Even though treatment did not fully resolve the pathologies associated with AGD, 5 ppm for 30 min group showed lower parasite load at 4 weeks post-treatment. Mucous cell parameters (i.e., size and density) increased within 24 h post-treatment and were significantly higher at termination, especially in AGD-affected fish, with some treatment effects influenced by the dose of PAA. Infection and treatments resulted in oxidative stress—in the early phase in the gill mucosa, while systemic reactive oxygen species (ROS) dysregulation was evident at the later stage. Infected fish responded to elevated circulating ROS by increasing antioxidant production. Exposing the fish to a crowding stress revealed the interference in the post-stress responses. Lower cortisol response was displayed by AGD-affected groups. Collectively, the study established that PAA, within the evaluated treatment protocols, could not provide a convincing treatment resolution and, thus, requires further optimization. Nonetheless, PAA treatment altered the mucosal immune and stress responses of AGD-affected Atlantic salmon, shedding light on the host-parasite-treatment interactions.  

Environmentally relevant concentrations of microplastics modulated the immune response and swimming activity, and impaired the development of marine medaka Oryzias melastigma larvae

Microplastics (MPs), due to their impacts on the ecosystem and their integration into the food web either through trophic transfer or ingestion directly from the ambient environment, are an emerging class of environmental contaminants posing a great threat to marine organisms. Most reports on the toxic effects of MPs exclusively focus on bioaccumulation, oxidative stress, pathological damage, and metabolic disturbance in fish. However, the collected information on fish immunity in response to MPs is poorly defined. In particular, little is known regarding mucosal immunity and the role of mucins. In this study, marine medaka (Oryzias melastigma) larvae were exposed to 6.0 µm beads of polystyrene microplastics (PS-MPs) at three environmentally relevant concentrations (10² particles/L, 10⁴ particles/L, and 10⁶ particles/L) for 14 days. The experiment was carried out to explore the developmental and behavioural indices, the transcriptional profiles of mucins, pro-inflammatory, immune, metabolism and antioxidant responses related genes, as well as the accumulation of PS-MPs in larvae. The results revealed that PS-MPs were observed in the gastrointestinal tract, with a concentration- and exposure time-dependent manner. No significant difference in the larval mortality was found between the treatment groups and the control, whereas the body length of larvae demonstrated a significant reduction at 10⁶ particles/L on 14 days post-hatching. The swimming behaviour of the larvae became hyperactive under exposure to 10⁴ and 10⁶ particles/L PS-MPs. In addition, PS-MP exposure significantly up-regulated the mucin gene transcriptional levels of muc7-like and muc13-like, however down-regulated the mucin gene expression levels of heg1, muc2, muc5AC-like and muc13. The immune- and inflammation and metabolism-relevant genes (jak, stat-3, il-6, il-1β, tnf-а, ccl-11, nf-κb, and sod) were significantly induced by PS-MPs at 10⁴ and 10⁶ particles/L compared to the control. Taken together, this study suggests that PS-MPs induced inflammation response and might obstruct the immune functions and retarded the growth of the marine medaka larvae even at environmentally relevant concentrations.

Transcriptomic characterisation of a common skin lesion in farmed chinook salmon

Little is known about host responses of farmed Chinook salmon with skin lesions, despite the lesions being associated with increased water temperatures and elevated mortality rates. To address this shortfall, a transcriptomic approach was used to characterise the molecular landscape of spot lesions, the most commonly reported lesion type in New Zealand Chinook salmon, versus healthy appearing skin in fish with and without spot lesions. Many biological (gene ontology) pathways were enriched in lesion adjacent tissue, relative to control skin tissue, including proteolysis, fin regeneration, calcium ion binding, mitochondrial transport, actin cytoskeleton organisation, epithelium development, and tissue development. In terms of specific transcripts of interest, pro-inflammatory cytokines (interleukin 1β and tumour necrosis factor), annexin A1, mucin 2, and calreticulin were upregulated, while cathepsin H, mucin 5AC, and perforin 1 were downregulated in lesion tissue. In some instances, changes in gene expression were consistent between lesion and healthy appearing skin from the same fish relative to lesion free fish, suggesting that host responses weren't limited to the site of the lesion. Goblet cell density in skin histological sections was not different between skin sample types. Collectively, these results provide insights into the physiological changes associated with common spot lesions in farmed Chinook salmon.

Gill and Liver Transcript Expression Changes Associated With Gill Damage in Atlantic Salmon (Salmo salar)

Gill damage represents a significant challenge in the teleost fish aquaculture industry globally, due to the gill’s involvement in several vital functions and direct contact with the surrounding environment. To examine the local and systemic effects accompanying gill damage (which is likely to negatively affect gill function) of Atlantic salmon, we performed a field sampling to collect gill and liver tissue after several environmental insults (e.g., harmful algal blooms). Before sampling, gills were visually inspected and gill damage was scored; gill scores were assigned from pristine [gill score 0 (GS0)] to severely damaged gills (GS3). Using a 44K salmonid microarray platform, we aimed to compare the transcriptomes of pristine and moderately damaged (i.e., GS2) gill tissue. Rank Products analysis (5% percentage of false-positives) identified 254 and 34 upregulated and downregulated probes, respectively, in GS2 compared with GS0. Differentially expressed probes represented genes associated with functions including gill remodeling, wound healing, and stress and immune responses. We performed gill and liver qPCR for all four gill damage scores using microarray-identified and other damage-associated biomarker genes. Transcripts related to wound healing (e.g., neb and klhl41b) were significantly upregulated in GS2 compared with GS0 in the gills. Also, transcripts associated with immune and stress-relevant pathways were dysregulated (e.g., downregulation of snaclec 1-like and upregulation of igkv3) in GS2 compared with GS0 gills. The livers of salmon with moderate gill damage (i.e., GS2) showed significant upregulation of transcripts related to wound healing (i.e., chtop), apoptosis (e.g., bnip3l), blood coagulation (e.g., f2 and serpind1b), transcription regulation (i.e., pparg), and stress-responses (e.g., cyp3a27) compared with livers of GS0 fish. We performed principal component analysis (PCA) using transcript levels for gill and liver separately. The gill PCA showed that PC1 significantly separated GS2 from all other gill scores. The genes contributing most to this separation were pgam2, des, neb, tnnt2, and myom1. The liver PCA showed that PC1 significantly separated GS2 from GS0; levels of hsp70, cyp3a27, pparg, chtop, and serpind1b were the highest contributors to this separation. Also, hepatic acute phase biomarkers (e.g., serpind1b and f2) were positively correlated to each other and to gill damage. Gill damage-responsive biomarker genes and associated qPCR assays arising from this study will be valuable in future research aimed at developing therapeutic diets to improve farmed salmon welfare.

Size-dependent resistance to amoebic gill disease in naïve Atlantic salmon (Salmo salar)

Amoebic gill disease, caused by the protozoan ectoparasite Neoparamoeba perurans, remains a significant threat to commercial Atlantic salmon aquaculture operations worldwide, despite partial control afforded by selective breeding and therapeutic intervention. Anecdotal reports from commercial producers suggest that historically, smaller Atlantic salmon smolts are more susceptible to AGD than larger smolts. Here, large (>350 g) and small (<200 g) commercially sourced, AGD-naïve Atlantic salmon cohorts were experimentally exposed to 50 N. perurans trophozoites L^-1 without intervention. Progression and severity of AGD in challenged cohorts was evaluated through gill pathology, using gill score and histological examination, and quantification of gill-associated amoebae burden using qPCR. To determine the potential basis for differences in AGD susceptibility between cohorts, transcriptome analysis was conducted using RNA extracted from whole gill arches. Overall, the large Atlantic salmon cohort had significantly lower gill parasite burdens and reduced AGD-related gross pathology compared to the small cohort. Relative gill load of N. perurans appeared to be proportional to gill score in both size classes, with larger smolts typically observed to have comparatively reduced parasite burdens at a given gill score. Moreover, comparison between gene expression profiles of large and small smolts highlighted upregulation of genes consistent with elevated immune activity in large smolts. Combined, the results presented here provide strong evidence of size-dependent resistance to AGD in AGD-naïve Atlantic salmon.

Antimicrobial peptide gene expression in Atlantic salmon (Salmo salar) seven days post-challenge with Neoparamoeba perurans

Amoebic gill disease in teleost fish is caused by the marine parasite Neoparamoeba perurans. To date, the role of antimicrobial peptides β-defensins and cathelicidins in this infection have not been explored. Using a high-throughput microfluidics quantitative polymerase chain reaction system (Biomark HD™ by Fluidigm), this study aimed to: firstly, to investigate organ-specific expression of antimicrobial peptide genes β-defensin-1, -3 and -4 and cathelicidin 2 in healthy Atlantic salmon; secondly, to compare the expression of these antimicrobial peptide genes in healthy versus asymptomatic Atlantic salmon seven days post-challenge with Neoparamoeba perurans. Results from this study indicate expression of the β-defensin and cathelicidin genes in the selected organs from healthy Atlantic salmon. Furthermore, a statistically significant upregulation of β-defensins -3 and -4 and cathelicidin 2 was detected in gill of parasite-challenged salmon. The upregulated cathelicidin and β-defensin genes in gill could indicate novel potential roles in innate immune responses to Neoparamoeba perurans.

Investigation of the transcriptomic response in Atlantic salmon (Salmo salar) gill exposed to Paramoeba perurans during early onset of disease

Amoebic Gill Disease (AGD), caused by the protozoan extracellular parasite Paramoeba perurans (P. perurans) is a disease affecting Atlantic salmon (Salmo salar). This study investigated the gill transcriptomic profile of pre-clinical AGD using RNA-sequencing (RNA-seq) technology. RNA-seq libraries generated at 0, 4, 7, 14 and 16 days post infection (dpi) identified 19,251 differentially expressed genes (DEGs) of which 56.2% were up-regulated. DEGs mapped to 224 Gene Ontology (GO) terms including 140 biological processes (BP), 45 cellular components (CC), and 39 molecular functions (MF). A total of 27 reference pathways in the Kyoto Encyclopedia of Genes and Genomes (KEGG) and 15 Reactome gene sets were identified. The RNA-seq data was validated using real-time, quantitative PCR (qPCR). A host immune response though the activation of complement and the acute phase genes was evident at 7 dpi, with a concurrent immune suppression involving cytokine signalling, notably in interleukins, interferon regulatory factors and tumour necrosis factor-alpha (tnf-α) genes. Down-regulated gene expression with involvement in receptor signalling pathways (NOD-like, Toll-like and RIG-1) were also identified. The results of this study support the theory that P. perurans can evade immune surveillance during the initial stages of gill colonisation through interference of signal transduction pathways.

Teladorsagia Circumcincta Galectin-Mucosal Interactome in Sheep

Teladorsagia circumcincta is the most important gastrointestinal parasite in the livestock industry in temperate regions around the world, causing great economic losses. The infective third-stage larvae (L3) of Teladorsagia circumcincta secrete a large number of excretory-secretory (E/S) molecules, some of which are likely to play critical roles in modulating the host immune response. One of the most abundant E/S molecules is a protein termed Tci-gal-1, which has similarity to mammalian galectins. Galectins are a family of carbohydrate-binding molecules, with characteristic domain organisation and affinity for β-galactosids that mediate a variety of important cellular functions including inflammation and immune responses. To understand the role of Tci-gal-1 at the host–parasite interface, we used a proteomics pull-down approach to identify Tc-gal-1 interacting proteins from sheep abomasal scrapes and whole tissue. A total of 135 unique proteins were identified from whole abomasal tissue samples, while 89 proteins were isolated from abomasal scrape samples. Of these proteins, 63 were present in both samples. Many of the host proteins identified, such as trefoil factors and mucin-like proteins, play critical roles in the host response. The identification of Tci-gal-1 binding partners provides new insights on host–parasite interactions and could lead to the development of new control strategies.

Impact of dietary level and ratio of n-6 and n-3 fatty acids on disease progression and mRNA expression of immune and inflammatory markers in Atlantic salmon (Salmo salar) challenged with Paramoeba perurans

The aim of the study was to investigate the influence of dietary level and ratio of n-6/n-3 fatty acids (FA) on growth, disease progression and expression of immune and inflammatory markers in Atlantic salmon (Salmo salar) following challenge with Paramoeba perurans. Fish (80 g) were fed four different diets with different ratios of n-6/n-3 FA; at 1.3, 2.4 and 6.0 and one diet with ratio of 1.3 combined with a higher level of n-3 FA and n-6 FA. The diet with the n-6/n-3 FA ratio of 6.0 was included to ensure potential n-6 FA effects were revealed, while the three other diets were more commercially relevant n-6/n-3 FA ratios and levels. After a pre-feeding period of 3 months, fish from each diet regime were challenged with a standardized laboratory challenge using a clonal culture of P. perurans at the concentration of 1,000 cells L⁻¹. The subsequent development of the disease was monitored (by gross gill score), and sampling conducted before challenge and at weekly sampling points for 5 weeks post-challenge. Challenge with P. perurans did not have a significant impact on the growth of the fish during the challenge period, but fish given the feed with the highest n-6/n-3 FA ratio had reduced growth compared to the other groups. Total gill score for all surfaces showed a significant increase with time, reaching a maximum at 21 days post-challenge and declined thereafter, irrespective of diet groups. Challenge with P. perurans influenced the mRNA expression of examined genes involved in immune and inflammatory response (TNF-α, iNOS, IL4-13b, GATA-3, IL-1β, p53, COX2 and PGE₂-EP4), but diet did not influence the gene expression. In conclusion, an increase in dietary n-6/n-3 FA ratio influenced the growth of Atlantic salmon challenged with P. perurans; however, it did not alter the mRNA expression of immune genes or progression of the disease.

Experimental exposure to low concentrations of Neoparamoeba perurans induces amoebic gill disease in Atlantic salmon

Amoebic gill disease (AGD) is a significant issue in Atlantic salmon mariculture. Research on the development of treatments or vaccines uses experimental challenges where salmon is exposed to amoebae concentrations ranging from 500 to 5,000/L. However, the water concentrations of N. perurans on affected salmon farms are much lower. The lowest concentration of N. perurans previously reported to cause AGD was 10/L. Here, we report that concentrations as low as 0.1/L of N. perurans can cause AGD. We propose that concentrations of N. perurans that reflect those measured on salmon farms should be used for future experimental challenges.

Host-Parasite Interaction of Atlantic salmon (Salmo salar) and the Ectoparasite Neoparamoeba perurans in Amoebic Gill Disease

Marine farmed Atlantic salmon (Salmo salar) are susceptible to recurrent amoebic gill disease (AGD) caused by the ectoparasite Neoparamoeba perurans over the growout production cycle. The parasite elicits a highly localized response within the gill epithelium resulting in multifocal mucoid patches at the site of parasite attachment. This host-parasite response drives a complex immune reaction, which remains poorly understood. To generate a model for host-parasite interaction during pathogenesis of AGD in Atlantic salmon the local (gill) and systemic transcriptomic response in the host, and the parasite during AGD pathogenesis was explored. A dual RNA-seq approach together with differential gene expression and system-wide statistical analyses of gene and transcription factor networks was employed. A multi-tissue transcriptomic data set was generated from the gill (including both lesioned and non-lesioned tissue), head kidney and spleen tissues naïve and AGD-affected Atlantic salmon sourced from an in vivo AGD challenge trial. Differential gene expression of the salmon host indicates local and systemic upregulation of defense and immune responses. Two transcription factors, znfOZF-like and znf70-like, and their associated gene networks significantly altered with disease state. The majority of genes in these networks are candidates for mediators of the immune response, cellular proliferation and invasion. These include Aurora kinase B-like, rho guanine nucleotide exchange factor 25-like and protein NDNF-like inhibited. Analysis of the N. perurans transcriptome during AGD pathology compared to in vitro cultured N. perurans trophozoites, as a proxy for wild type trophozoites, identified multiple gene candidates for virulence and indicates a potential master regulatory gene system analogous to the two-component PhoP/Q system. Candidate genes identified are associated with invasion of host tissue, evasion of host defense mechanisms and formation of the mucoid lesion. We generated a novel model for host-parasite interaction during AGD pathogenesis through integration of host and parasite functional profiles. Collectively, this dual transcriptomic study provides novel molecular insights into the pathology of AGD and provides alternative theories for future research in a step towards improved management of AGD.

Host Response of Atlantic Salmon (Salmo salar) Re-Inoculated with Paramoeba perurans

In aquaculture, recurrence rates of amoebic gill disease (AGD) caused by the ectoparasite Paramoeba perurans are high and no prophylactic strategies exist for disease prevention. In this study, Atlantic salmon (Salmo salar) were initially inoculated with P. perurans and following the development of amoebic gill disease were treated with freshwater immersion on day 21 and day 35 post inoculation. Fish were re-inoculated following a negative qPCR analysis for the presence of P. perurans. The gill host immune response was investigated at 7, 14, and 18 days post re-inoculation. Differential proteome expression of immune related proteins was assessed by comparison of each time point against naïve controls. In the gill, some proteins of the innate immune system were expressed in response to gill re-colonization by P. perurans, while no features of adaptive immunity were found to be differentially expressed. Many of the proteins identified are novel in the context of AGD and their expression profiles suggest that their roles in the response to disease development and progression in single or multiple infections warrant further investigation.

Investigation of the Initial Host Response of Naïve Atlantic Salmon (Salmo salar) Inoculated with Paramoeba perurans

Amoebic Gill Disease (AGD), caused by the ectoparasite Paramoeba perurans is characterised by hyperplasia of the gill epithelium and lamellar fusion. In this study, the initial host response of naïve Atlantic salmon (Salmo salar) inoculated with P. perurans was investigated. Using gel-free proteomic techniques and mass spectrometry gill and serum samples were analysed at 7 timepoints (2, 3, 4, 7, 9, 11 and 14 days) post-inoculation with P. perurans. Differential expression of immune related proteins was assessed by comparison of protein expression from each time point against naïve controls. Few host immune molecules associated with innate immunity showed increased expression in response to gill colonisation by amoebae. Furthermore, many proteins with roles in immune signalling, phagocytosis and T-cell proliferation were found to be inhibited upon disease progression. Initially, various immune factors demonstrated the anticipated increase in expression in response to infection in the serum while some immune inhibition became apparent at the later stages of disease progression. Taken together, the pro-immune trend observed in serum, the lack of a robust early immune response in the gill and the diversity of those proteins in the gill whose altered expression negatively impact the immune response, support the concept of a pathogen-derived suppression of the host response.

Experimental Challenge Models and In Vitro Models to Investigate Efficacy of Treatments and Vaccines against Amoebic Gill Disease

Amoebic Gill Disease (AGD) severely affects salmonid mariculture due to fish losses and costs associated with management of the disease. Continued research into management solutions, including new treatments and vaccine development, is highly important for the future of salmonid production worldwide. This requires both in vitro (both pathogen only and host-pathogen models) and in vivo (disease challenge) testing. Challenge models are still widely varied, in particular with regard to: infection methods (cohabitation or immersion), source of the pathogen (isolated from infected fish or cultured), infectious dose, environmental conditions (in particular temperature) and the endpoints across experimental treatment and vaccine studies which makes comparisons between studies difficult. This review summarises in vitro assays, the challenge methods and endpoints used in studies of experimental treatments and vaccines for AGD.

Nutrient Digestibility, Growth, Mucosal Barrier Status, and Activity of Leucocytes From Head Kidney of Atlantic Salmon Fed Marine- or Plant-Derived Protein and Lipid Sources

Nutrient digestibility, growth, and mucosal barrier status of fish skin, gills, and distal intestine were studied in Atlantic salmon fed feeds based on marine or plant-derived ingredients. The barrier status was assessed by considering the expression of four mucin genes, five genes that encode antimicrobial proteins, distal intestine micromorphology, and design-based stereology of the midgut epithelium. In addition, the head kidney leukocytes were examined using flow cytometry; to understand the differences in their counts and function. Five experimental feeds containing the main components i) fishmeal and fish oil (BG1), ii) soybean meal (BG2; to induce enteritis), iii) fishmeal as the main protein source and rapeseed oil as the main lipid source (BG3), iv) a mix of plant protein concentrates as the protein sources and fish oil as the lipid source (BG4), and v) plant and marine ingredients in the ratio 70:30 (BG5) were produced for the study. Atlantic salmon with initial weight 72.7 ± 1.2 g was offered the experimental feeds for 65 days. The results revealed that the weights of all fish groups doubled, except for fish fed BG2. Fish fed the BG2 diet had lower blood cholesterol concentration, developed enteritis, had lower expression of muc2 in the distal intestine, and had a compromised barrier status in the intestine. Expression of both the mucin genes and genes that encode antimicrobial peptides were tissue-specific and some were significantly affected by diet. The fish fed BG1 and BG3 had more head kidney lymphocyte-like cells compared to BG5-fed fish, and the phagocytic activity of macrophage-like cells from the head kidney was the highest in fish fed BG1. The intestinal micromorphology and the mucosal mapping suggest two different ways by which plant-based diets can alter the gut barrier status; by either reducing the mucous cell sizes, volumetric densities and barrier status (as noted for BG2) or increasing volumetric density of mucous cells (as observed for BG4 and BG5). The results of the compromised intestinal barrier in fish fed plant ingredients should be further confirmed through transcriptomic and immunohistochemical studies to refine ingredient composition for sustainable and acceptable healthy diets.

Gill Mucus and Gill Mucin O-glycosylation in Healthy and Amebic Gill Disease-Affected Atlantic Salmon

Amoebic gill disease (AGD) causes poor performance and death in salmonids. Mucins are mainly comprised by carbohydrates and are main components of the mucus covering the gill. Since glycans regulate pathogen binding and growth, glycosylation changes may affect susceptibility to primary and secondary infections. We investigated gill mucin O-glycosylation from Atlantic salmon with and without AGD using liquid chromatography–mass spectrometry. Gill mucin glycans were larger and more complex, diverse and fucosylated than skin mucins. Confocal microscopy revealed that fucosylated mucus coated sialylated mucus strands in ex vivo gill mucus. Terminal HexNAcs were more abundant among O-glycans from AGD-affected Atlantic salmon, whereas core 1 structures and structures with acidic moieties such as N-acetylneuraminic acid (NeuAc) and sulfate groups were less abundant compared to non-infected fish. The fucosylated and NeuAc-containing O-glycans were inversely proportional, with infected fish on the lower scale of NeuAc abundance and high on fucosylated structures. The fucosylated epitopes were of three types: Fuc-HexNAc-R, Gal-[Fuc-]HexNAc-R and HexNAc-[Fuc-]HexNAc-R. These blood group-like structures could be an avenue to diversify the glycan repertoire to limit infection in the exposed gills. Furthermore, care must be taken when using skin mucus as proxy for gill mucus, as gill mucins are distinctly different from skin mucins.

The Atlantic Salmon Gill Transcriptome Response in a Natural Outbreak of Salmon Gill Pox Virus Infection Reveals New Biomarkers of Gill Pathology and Suppression of Mucosal Defense

The salmon gill poxvirus (SGPV) is a large DNA virus that infects gill epithelial cells in Atlantic salmon and is associated with acute high mortality disease outbreaks in aquaculture. The pathological effects of SGPV infection include gill epithelial apoptosis in the acute phase of the disease and hyperplasia of gill epithelial cells in surviving fish, causing damage to the gill respiratory surface. In this study, we sampled gills from Atlantic salmon presmolts during a natural outbreak of SGPV disease (SGPVD). Samples covered the early phase of infection, the acute mortality phase, the resolving phase of the disease and control fish from the same group and facility. Mortality, the presence and level of SGPV and gill epithelial apoptosis were clearly associated. The gene expression pattern in the acute phase of SGPVD was in tune with the pathological findings and revealed novel transcript-based disease biomarkers, including pro-apoptotic and proliferative genes, along with changes in expression of ion channels and mucins. The innate antiviral response was strongly upregulated in infected gills and chemokine expression was altered. The regenerating phase did not reveal adaptive immune activity within the study period, but several immune effector genes involved in mucosal protection were downregulated into the late phase, indicating that SGPV infection could compromise mucosal defense. These data provide novel insight into the infection mechanisms and host interaction of SGPV.

Zebrafish IL-4-like Cytokines and IL-10 Suppress Inflammation but Only IL-10 Is Essential for Gill Homeostasis

Mucosal surfaces such as fish gills interface between the organism and the external environment and as such are major sites of foreign Ag encounter. In the gills, the balance between inflammatory responses to waterborne pathogens and regulatory responses toward commensal microbes is critical for effective barrier function and overall fish health. In mammals, IL-4 and IL-13 in concert with IL-10 are essential for balancing immune responses to pathogens and suppressing inflammation. Although considerable progress has been made in the field of fish immunology in recent years, whether the fish counterparts of these key mammalian cytokines perform similar roles is still an open question. In this study, we have generated IL-4/13A and IL-4/13B mutant zebrafish (Danio rerio) and, together with an existing IL-10 mutant line, characterized the consequences of loss of function of these cytokines. We demonstrate that IL-4/13A and IL-4/13B are required for the maintenance of a Th2-like phenotype in the gills and the suppression of type 1 immune responses. As in mammals, IL-10 appears to have a more striking anti-inflammatory function than IL-4-like cytokines and is essential for gill homeostasis. Thus, both IL-4/13 and IL-10 paralogs in zebrafish exhibit aspects of conserved function with their mammalian counterparts.

Integration of Transcriptome, Gross Morphology and Histopathology in the Gill of Sea Farmed Atlantic Salmon (Salmo salar): Lessons From Multi-Site Sampling

The gill of teleost fish is a multifunctional organ involved in many physiological processes such as gas exchange, osmotic and ionic regulation, acid-base balance and excretion of nitrogenous waste. Due to its extensive interface with the environment, the gill plays a key role as a primary mucosal defense tissue against pathogens, as manifested by the presence of the gill-associated lymphoid tissue (GIALT). In recent years, the prevalence of multifactorial gill pathologies has increased significantly, causing substantial losses in Atlantic salmon aquaculture. The transition from healthy to unhealthy gill phenotypes and the progression of multifactorial gill pathologies, such as proliferative gill disease (PGD), proliferative gill inflammation (PGI) and complex gill disorder (CGD), are commonly characterized by epithelial hyperplasia, lamellar fusion and inflammation. Routine monitoring for PGD relies on visual inspection and non-invasive scoring of the gill tissue (gross morphology), coupled with histopathological examination of gill sections. To explore the underlying molecular events that are associated with the progression of PGD, we sampled Atlantic salmon from three different marine production sites in Scotland and examined the gill tissue at three different levels of organization: gross morphology with the use of PGD scores (macroscopic examination), whole transcriptome (gene expression by RNA-seq) and histopathology (microscopic examination). Our results strongly suggested that the changes in PGD scores of the gill tissue were not associated with the changes in gene expression or histopathology. In contrast, integration of the gill RNA-seq data with the gill histopathology enabled us to identify common gene expression patterns associated with multifactorial gill disease, independently from the origin of samples. We demonstrated that the gene expression patterns associated with multifactorial gill disease were dominated by two processes: a range of immune responses driven by pro-inflammatory cytokines and the events associated with tissue damage and repair, driven by caspases and angiogenin.

Characterising the mechanisms underlying genetic resistance to amoebic gill disease in Atlantic salmon using RNA sequencing

Background

Gill health is one of the main concerns for Atlantic salmon aquaculture, and Amoebic Gill Disease (AGD), attributable to infection by the amoeba Neoparamoeba perurans, is a frequent cause of morbidity. In the absence of preventive measures, increasing genetic resistance of salmon to AGD via selective breeding can reduce the incidence of the disease and mitigate gill damage. Understanding the mechanisms leading to AGD resistance and the underlying causative genomic features can aid in this effort, while also providing critical information for the development of other control strategies. AGD resistance is considered to be moderately heritable, and several putative QTL have been identified. The aim of the current study was to improve understanding of the mechanisms underlying AGD resistance, and to identify putative causative genomic factors underlying the QTL. To achieve this, RNA was extracted from the gill and head kidney of AGD resistant and susceptible animals following a challenge with N. perurans, and sequenced.

Results

Comparison between resistant and susceptible animals primarily highlighted differences mainly in the local immune response in the gill, involving red blood cell genes and genes related to immune function and cell adhesion. Differentially expressed immune genes pointed to a contrast in Th2 and Th17 responses, which is consistent with the increased heritability observed after successive challenges with the amoeba. Five QTL-region candidate genes showed differential expression, including a gene connected to interferon responses (GVINP1), a gene involved in systemic inflammation (MAP4K4), and a positive regulator of apoptosis (TRIM39). Analyses of allele-specific expression highlighted a gene in the QTL region on chromosome 17, cellular repressor of E1A-stimulated genes 1 (CREG1), showing allelic differential expression suggestive of a cis-acting regulatory variant.

Conclusions

In summary, this study provides new insights into the mechanisms of resistance to AGD in Atlantic salmon, and highlights candidate genes for further functional studies that can further elucidate the genomic mechanisms leading to resistance and contribute to enhancing salmon health via improved genomic selection.

Acting locally - affecting globally: RNA sequencing of gilthead sea bream with a mild Sparicotyle chrysophrii infection reveals effects on apoptosis, immune and hypoxia related genes

BACKGROUND

Monogenean flatworms are the main fish ectoparasites inflicting serious economic losses in aquaculture. The polyopisthocotylean Sparicotyle chrysophrii parasitizes the gills of gilthead sea bream (GSB, Sparus aurata) causing anaemia, lamellae fusion and sloughing of epithelial cells, with the consequent hypoxia, emaciation, lethargy and mortality. Currently no preventive or curative measures against this disease exist and therefore information on the host-parasite interaction is crucial to find mitigation solutions for sparicotylosis. The knowledge about gene regulation in monogenean-host models mostly comes from freshwater monopysthocotyleans and almost nothing is known about polyopisthocotyleans. The current study aims to decipher the host response at local (gills) and systemic (spleen, liver) levels in farmed GSB with a mild natural S. chrysophrii infection by transcriptomic analysis.

RESULTS

Using Illumina RNA sequencing and transcriptomic analysis, a total of 2581 differentially expressed transcripts were identified in infected fish when compared to uninfected controls. Gill tissues in contact with the parasite (P gills) displayed regulation of fewer genes (700) than gill portions not in contact with the parasite (NP gills) (1235), most likely due to a local silencing effect of the parasite. The systemic reaction in the spleen was much higher than that at the parasite attachment site (local) (1240), and higher than in liver (334). NP gills displayed a strong enrichment of genes mainly related to immune response and apoptosis. Processes such as apoptosis, inflammation and cell proliferation dominated gills, whereas inhibition of apoptosis, autophagy, platelet activation, signalling and aggregation, and inflammasome were observed in spleen. Proteasome markers were increased in all tissues, whereas hypoxia-related genes were down-regulated in gills and spleen.

CONCLUSIONS

Contrasting forces seem to be acting at local and systemic levels. The splenic down-regulation could be part of a hypometabolic response, to counteract the hypoxia induced by the parasite damage to the gills and to concentrate the energy on defence and repair responses. Alternatively, it can be also interpreted as the often observed action of helminths to modify host immunity in its own interest. These results provide the first toolkit for future studies towards understanding and management of this parasitosis.