The melano-macrophage: The black leukocyte of fish immunity

Melanin and the process of melanin synthesis or melanogenesis have central roles in the immune system of insects, and production of melanin-synthesizing enzymes from their haemocytes may be induced following activation through danger signals. Melanin-containing macrophage-like cells have been extensively studied in amphibians and they are also present in reptiles. In fish, melano-macrophages are especially recognized with respect to melano-macrophage centres (MMCs), hypothesized to be analogues of germinal centres in secondary lymphoid organs of mammals and some birds. Melano-macrophages are in addition present in several inflammatory conditions, in particular melanised focal changes, or black spots, in the musculature of farmed Atlantic salmon, Salmo salar. Melanins are complex compounds that may be divided into different forms which all have the ability to absorb and scatter light. Other functions include the quenching of free radicals and a direct effect on the immune system. According to the common view held in the pigment cell community, vertebrate melanin synthesis with melanosome formation may only occur in cells of ectodermal origin. However, abundant information suggests that also myeloid cells of ectothermic vertebrates may be classified as melanocytes. Here, we discuss these opposing views and review relevant literature. Finally, we review the current status on the research concerning melanised focal muscle changes that represent the most severe quality problem in Norwegian salmon production, but also other diseases where melano-macrophages play important roles.

Design and functional characterization of Salmo salar TLR5 agonist peptides derived from high mobility group B1 acidic tail

Toll-like receptor 5 (TLR5) responds to the monomeric form of flagellin and induces the MyD88-depending signaling pathway, activating proinflammatory transcription factors such as NF-κB and the consequent induction of cytokines. On the other hand, HMGB1 is a highly conserved non-histone chromosomal protein shown to interact with and activate TLR5. The present work aimed to design and characterize TLR5 agonist peptides derived from the acidic tail of Salmo salar HMGB1 based on the structural knowledge of the TLR5 surface using global molecular docking platforms. Peptide binding poses complexed on TLR5 ectodomain model from each algorithm were filtrated based on docking scoring functions and predicted theoretical binding affinity of the complex. Circular dichroism spectra were recorded for each peptide selected for synthesis. Only intrinsically disordered peptides (6W, 11W, and SsOri) were selected for experimental functional assay. The functional characterization of the peptides was performed by NF-κB activation assays, RT-qPCR gene expression assays, and Piscirickettsia salmonis challenge in SHK-1 cells. The 6W and 11W peptides increased the nuclear translation of p65 and phosphorylation. In addition, the peptides induced the expression of genes related to the TLR5 pathway activation, pro- and anti-inflammatory response, and differentiation and activation of T lymphocytes towards phenotypes such as TH1, TH17, and TH2. Finally, it was shown that the 11W peptide protects immune cells against infection with P. salmonis bacteria. Overall, the results indicate the usefulness of novel peptides as potential immunostimulants in salmonids.

Thiamin dynamics during the adult life cycle of Atlantic salmon (Salmo salar)

Thiamin is an essential water-soluble B vitamin known for its wide range of metabolic functions and antioxidant properties. Over the past decades, reproductive failures induced by thiamin deficiency have been observed in several salmonid species worldwide, but it is unclear why this micronutrient deficiency arises. Few studies have compared thiamin concentrations in systems of salmonid populations with or without documented thiamin deficiency. Moreover, it is not well known whether and how thiamin concentration changes during the marine feeding phase and the spawning migration. Therefore, samples of Atlantic salmon (Salmo salar) were collected when actively feeding in the open Baltic Sea, after the sea migration to natal rivers, after river migration, and during the spawning period. To compare populations of Baltic salmon with systems without documented thiamin deficiency, a population of landlocked salmon located in Lake Vänern (Sweden) was sampled as well as salmon from Norwegian rivers draining into the North Atlantic Ocean. Results showed the highest mean thiamin concentrations in Lake Vänern salmon, followed by North Atlantic, and the lowest in Baltic populations. Therefore, salmon in the Baltic Sea seem to be consistently more constrained by thiamin than those in other systems. Condition factor and body length had little to no effect on thiamin concentrations in all systems, suggesting that there is no relation between the body condition of salmon and thiamin deficiency. In our large spatiotemporal comparison of salmon populations, thiamin concentrations declined toward spawning in all studied systems, suggesting that the reduction in thiamin concentration arises as a natural consequence of starvation rather than to be related to thiamin deficiency in the system. These results suggest that factors affecting accumulation during the marine feeding phase are key for understanding the thiamin deficiency in salmonids.

Loss of Fshr Prevents Testicular Maturation in Atlantic Salmon (Salmo salar L.)

Early puberty poses a significant challenge for male Atlantic salmon in aquaculture due to its negative impact on growth and welfare. The regulation of puberty in vertebrates involves 2 key reproductive hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and their gonadal receptors. In male mice lacking FSH receptor, testes size is reduced, but fertility is maintained, while medaka and zebrafish with a disrupted fshr gene exhibit near normal testis size and fertility. In these fishes both Fsh and Lh are present during puberty and Lh may rescue fertility, while in salmonid fish only Fsh is present in the circulation during puberty. Using CRISPR-Cas9, we produced crispants with a high prevalence of fshr mutations at the target site, which remained fertile, although more than half showed a testis development deviating from wild-type (wt) males. Crossing out these F0 crispants to each other produced a viable F1 generation showing frameshift (fshr-/-) or in-frame mutations (fshrif/if). Nearly all wt males matured while all fshr-/- males remained immature with small testes containing A spermatogonia as the furthest developed germ cell type and prepubertal plasma androgen levels. Also, the pituitary transcript levels of gnrhr2bba and lhb, but not for fshb, were reduced in the fshr-/- males compared with maturing males. More than half of the fshrif/if mutant males showed no or a delayed maturation. In conclusion, Atlantic salmon show the unique characteristic that loss of Fshr function alone results in male infertility, offering new opportunities to control precocious puberty or fertility in salmon.

Agonistic effect of peptides derived from a truncated HMGB1 acidic tail sequence in TLR5 from Salmo salar

Based on the structural knowledge of TLR5 surface and using blind docking platforms, peptides derived from a truncated HMGB1 acidic tail from Salmo salar was designed as TLR5 agonistic. Additionally, a template peptide with the native N-terminal of the acidic tail sequence as a reference was included (SsOri). Peptide binding poses complexed on TLR5 ectodomain model from each algorithm were filtrated based on docking scoring functions and predicted theoretical binding affinity of the complex. The best peptides, termed 6WK and 5LWK, were selected for chemical synthesis and experimental functional assay. The agonist activity by immunoblotting and immunocytochemistry was determined following the NF-κBp65 phosphorylation (p-NF-κBp65) and the nuclear translocation of the NF-κBp65 subunit from the cytosol, respectively. HeLa cells stably expressing a S. salar TLR5 chimeric form (TLR5c7) showed increased p-NF-κBp65 levels regarding extracts from flagellin-treated cells. No statistically significant differences (p > 0.05) were found in the detected p-NF-κBp65 levels between cellular extracts treated with peptides or flagellin by one-way ANOVA. The image analysis of NF-κBp65 immunolabeled cells obtained by confocal microscopy showed increased nuclear NF-κBp65 co-localization in cells both 5LWK and flagellin stimulated, while 6WK and SsOri showed less effect on p65 nuclear translocation (p < 0.05). Also, an increased transcript expression profile of proinflammatory cytokines such as TNFα, IL-1β, and IL-8 in HKL cells isolated from Salmo salar was evidenced in 5LWK - stimulated by RT-PCR analysis. Overall, the result indicates the usefulness of novel peptides as a potential immunostimulant in S. salar.

The gastric mucosa of Atlantic salmon (Salmo salar) is abundant in highly active chitinases

Atlantic salmon (Salmo salar) possesses a genome containing 10 genes encoding chitinases, yet their functional roles remain poorly understood. In other fish species, chitinases have been primarily linked to digestion, but also to other functions, as chitinase-encoding genes are transcribed in a variety of non-digestive organs. In this study, we investigated the properties of two chitinases belonging to the family 18 glycoside hydrolase group, namely Chia.3 and Chia.4, both isolated from the stomach mucosa. Chia.3 and Chia.4, exhibiting 95% sequence identity, proved inseparable using conventional chromatographic methods, necessitating their purification as a chitinase pair. Biochemical analysis revealed sustained chitinolytic activity against β-chitin for up to 24 h, spanning a pH range of 2 to 6. Moreover, subsequent in vitro investigations established that this chitinase pair efficiently degrades diverse chitin-containing substrates into chitobiose, highlighting the potential of Atlantic salmon to utilize novel chitin-containing feed sources. Analysis of the gastric matrix proteome demonstrates that the chitinases are secreted and rank among the most abundant proteins in the gastric matrix. This finding correlates well with the previously observed high transcription of the corresponding chitinase genes in Atlantic salmon stomach tissue. By shedding light on the secreted chitinases in the Atlantic salmon's stomach mucosa and elucidating their functional characteristics, this study enhances our understanding of chitinase biology in this species. Moreover, the observed capacity to effectively degrade chitin-containing materials implies the potential utilization of alternative feed sources rich in chitin, offering promising prospects for sustainable aquaculture practices.

Eumelanin Detection in Melanized Focal Changes but Not in Red Focal Changes on Atlantic Salmon (Salmo salar) Fillets

Superficial discolored spots on Atlantic salmon (Salmo salar) fillets are a serious quality problem for commercial seafood farming. Previous reports have proposed that the black spots (called melanized focal changes (MFCs)) may be melanin, but no convincing evidence has been reported. In this study, we performed chemical characterization of MFCs and of red pigment (called red focal changes (RFCs)) from salmon fillets using alkaline hydrogen peroxide oxidation and hydroiodic acid hydrolysis. This revealed that the MFCs contain 3,4-dihydroxyphenylalanine (DOPA)-derived eumelanin, whereas the RFCs contain only trace amounts of eumelanin. Therefore, it is probable that the black color of the MFCs can be explained by the presence of eumelanin from accumulated melanomacrophages. For the red pigment, we could not find a significant signature of either eumelanin or pheomelanin; the red color is probably predominantly hemorrhagic in nature. However, we found that the level of pigmentation in RFCs increased together with some melanogenic metabolites. Comparison with a "mimicking experiment", in which a mixture of a salmon homogenate + DOPA was oxidized with tyrosinase, suggested that the RFCs include conjugations of DOPAquinone and/or DOPAchrome with salmon muscle tissue proteins. In short, the results suggest that melanogenic metabolites in MFCs and RFCs derive from different chemical pathways, which would agree with the two different colorations deriving from distinct cellular origins, namely melanomacrophages and red blood cells, respectively.

Long-term stress induced cortisol downregulation, growth reduction and cardiac remodeling in Atlantic salmon

Stress and elevated plasma cortisol in salmonids have been linked with pathological remodeling of the heart and deterioration of fitness and welfare. However, these associations were based on biomarkers that fail to provide a retrospective view of stress. This study is the first whereby the association of long-term stress, using scale cortisol as a chronic stress biomarker, with cardiac morphology and growth performance of wild Atlantic salmon (Salmo salar) is made. Growth, heart morphology, plasma and scale cortisol levels, and expression of genes involved in cortisol regulation of the hypothalamic-pituitary-interrenal axis of undisturbed fish (control) were compared with those of fish exposed daily to stress for 8 weeks. Though scale cortisol levels showed a time-dependent accumulation in both groups, plasma and scale cortisol levels of stress group fish were 29.1% and 25.0% lower than those of control fish, respectively. These results correlated with the overall upregulation of stress-axis genes involved in the systemic negative feedback of cortisol, and local feedback via 11β-hydroxysteroid dehydrogenases, glucocorticoid and mineralocorticoid receptors in the stress treatment at the hypothalamus and pituitary level. These lower cortisol levels were, however, counterintuitive in terms of the growth performance as stress group fish grew 33.7% slower than control fish, which probably influenced the 8.4% increase in relative ventricle mass in the stress group. Though compact myocardium area between the treatments was comparable, these parameters showed significant linear correlations with scale cortisol levels, indicating the involvement of chronic stress in cardiac remodeling. These findings underscore the importance of scale cortisol as biomarker when associating chronic stress with long-term processes including cardiac remodeling.

Welfare and performance of ballan wrasse (Labrus bergylta) reared at two different temperatures after a preparatory feeding trial with enhanced dietary eicosapentaenoic acid

Concerns have long been raised about the welfare of ballan wrasse (Labrus bergylta) used for the biological control of sea lice in Atlantic salmon (Salmo salar) aquaculture. This study assessed the effect of increased dietary eicosapentaenoic acid (EPA) levels and initial condition factor (CF) on the subsequent performance and welfare of ballan wrasse farmed in high and low water temperatures. Fish were fed a diet with either commercial or high EPA levels for 3 months at 15°C. Subsequently, fish were tagged with a passive integrated transponder, measured for their CF and divided into two groups consisting of fish from both treatments and reared for 4.5 months at either 15 or 6°C fed a commercial diet. Each fish was categorized as high (≥2.7) or low CF (<2.7) fish based on the calculated average CF of the population. Dietary composition influenced the fatty acid (FA) profile of the stored lipids without affecting the growth or welfare of ballan wrasse. Fish reared at 15°C showed higher growth, more fat and energy reserves and less ash content. Fish reared at 6°C lost weight, using up their body lipids at the end of the temperature trial. Gene expression analyses showed upregulation of the positive growth marker (GHrα) and two genes involved in the synthesis and oxidation of FAs (elovl5, cpt1) and downregulation of the negative growth marker (mstn) in fish reared at 15°C compared to those reared at 6°C. Fish reared at 6°C showed upregulated levels of il-6 compared to those reared at 15°C, suggesting an enhanced immune reaction in response to low temperature. Fish with high CF showed better survival, growth and performance compared to those with low CF. External welfare scoring showed higher prevalence and severity in emaciation, scale loss and the sum index score (of all measured welfare parameters) in fish reared at 6°C compared to those reared at 15°C and better welfare in fish with high CF compared to those with low CF. Histological examination of the skin showed that fish reared at 6°C had decreased epidermal thickness, a lower overall number of mucous cells in the inner and outer epidermis and a different organization of mucous cells compared to fish reared at 15°C, indicating stress in fish reared at 6°C. Overall, low water temperatures had profound effects on the performance and external and internal welfare parameters of ballan wrasse and can be considered a stressor likely affecting the delousing efficacy. These findings support the seasonal use of different cleaner fish species. High CF, but not increased dietary EPA levels, appeared to help fish cope better with low water temperatures and should thus be assessed and considered before deploying them in salmon cages.

Identification of cortisol metabolites with LC-MS/MS in plasma, skin mucus, bile and faeces for stress evaluation of farmed Atlantic salmon

As a stress hormone, cortisol and more recently its metabolites are analysed when assessing fish stress and welfare status, although the exact identity of these metabolites is not clearly defined for the Atlantic salmon. LC-MS/MS techniques, owing to their specificity, sensitivity and ability to simultaneously identify and measure several relevant compounds, can be useful tools for this purpose. Using the guidelines provided by the European Decision no. 657/2002/EC for validation, the LC-MS/MS method presented here, can reliably identify and quantify cortisol and five of its metabolites (5β-THF, cortisone, 5β-DHE, 5β-THE and β-cortolone) in bile and faeces, and cortisol and cortisone in skin mucus and blood plasma of farmed Atlantic salmon within 15 min. Identified as the most predominant compound in faeces and bile, 5β-THE is proposed as a candidate stress biomarker when using these matrices. A decision limit (CCα) below 5 ng/mL, a detection capability (CCβ) and a limit of detection (LOD) below 10 ng/mL and a limit of quantitation (LOQ) below 30 ng/mL were typically obtained for most of the compounds. The concentrations of these compounds measured in either non-stressed or stressed fish were all above the CCα, CCβ, LOD and the LOQ of the method. The latter consequently demonstrated significant difference in cortisol metabolites concentrations between the two groups of fish. The present study further demonstrates that pooling of samples from several individuals could provide reliable results for farmed fish stress evaluation, when sample materials are insufficient in terms of quantity.

Eubiotic Effect of a Dietary Bio-Aqua® and Sodium Diformate (NaDF) on Salmo trutta caspius: Innate Immune System, Biochemical Indices, Antioxidant Defense, and Expression of Immunological and Growth-Related Genes

The present study investigated the effects of combined and singular oral administration of Bio-Aqua® with different dosages of sodium diformate (NaDF) on biochemical indices, innate immune responses, antioxidant effects, and expressions of immunological related genes of Caspian brown trout (Salmo trutta caspius). Fingerlings Salmo trutta caspius (n = 1800; initial weight 15 ± 3 g) were randomly allocated into five groups (120 fish group-1 in triplicates). Control diet: without any addition, G1, G2, G3, and G4 received diets containing 0.2 g kg-1 commercial probiotic Bio-Aqua® combined with 0, 0.5, 1.0, and 1.5% NaDF to the basal diet for 60 days according to recommended dosages reported in previous studies. Results indicated that serum bactericidal activity (G3 on day 60 and G1 on day 30) and classic complement in all groups (on day 60) (G1 and G2 on day 30) were significantly elevated (P < 0.05). The serum lysozyme, glucose, globulin, and albumin levels showed no significant differences between all groups compared to the control group (P > 0.05). On days 30 and 60 of the sampling, no significant difference was observed in the amount of superoxide disotase (SOD) and catalase (CAT) between the treatments (P > 0.05) but activity of malondialdehyde (MDA) was lower in G1 than the control (P < 0.05). The expression of the immune-regulating genes IL-10, IL-1β, GTP, FATP, and IGF was significantly improved in all probiotic + acidifier-treated groups (P < 0.05). The current findings showed that mixture of Bio-Aqua® and NaDF (1.5% + pro) is beneficial, as it effectively improves some immune parameters and expression of immunological and growth-related genes in Caspian brown trout.

Revealing the Salmo salar NLRP3 Inflammasome: Insights from Structural Modeling and Transcriptome Analysis

The NLRP3, one of the most heavily studied inflammasome-related proteins in mammals, remains inadequately characterized in Atlantic salmon (Salmo salar), despite the significant commercial importance of this salmonid. The NLRP3 inflammasome is composed of the NLRP3 protein, which is associated with procaspase-1 via an adapter molecule known as ASC. This work aims to characterize the Salmo salar NLRP3 inflammasome through in silico structural modeling, functional transcript expression determination in the SHK-1 cell line in vitro, and a transcriptome analysis on Atlantic salmon. The molecular docking results suggested a similar arrangement of the ternary complex between NLRP3, ASC, and caspase-1 in both the Atlantic salmon and the mammalian NLRP3 inflammasomes. Moreover, the expression results confirmed the functionality of the SsNLRP3 inflammasome in the SHK-1 cells, as evidenced by the lipopolysaccharide-induced increase in the transcription of genes involved in inflammasome activation, including ASC and NLRP3. Additionally, the transcriptome results revealed that most of the inflammasome-related genes, including ASC, NLRP3, and caspase-1, were down-regulated in the Atlantic salmon following its adaptation to seawater (also known as parr-smolt transformation). This is correlated with a temporary detrimental effected on the immune system. Collectively, these findings offer novel insights into the evolutionarily conserved role of NLRP3.

Atlantic salmon adapt to low dietary n-3 PUFA and warmer water temperatures by increasing feed intake and expression of n-3 biosynthesis-related transcripts

Climate change can have cascading impacts on biochemical reactions in aquatic ecosystems. Aquatic ectotherms can adapt to surrounding temperatures by using long-chain polyunsaturated fatty acids (LC-PUFAs) to maintain cell membrane fluidity. In a warming scenario, less LC-PUFA is needed to maintain fluidity. Our objective was to determine the impact of low dietary LC-PUFA and warm water temperature on growth, fatty acid (FA) storage, and expression of lipid metabolism-related transcripts in Atlantic salmon. Salmon (141 g) were fed two diets (high or low LC-PUFA) at either 12 °C or 16 °C for 16 weeks. Salmon weighed more and consumed more food at 16 °C and when fed the low-LC-PUFA diet. Liver and muscle FA mostly depended on diet rather than temperature. DHA in muscle was higher at 16 °C and in salmon fed the high-LC-PUFA diet. Levels of FA desaturation transcripts were more highly expressed at 16 °C and in salmon fed the low-LC-PUFA diet, which suggests synthesis of LC-PUFA. Overall, with slow, chronic temperature increases, salmon may adapt to low dietary LC-PUFA by synthesizing more when required.

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.

Salmonid alphavirus non-structural protein 2 is a key protein that activates the NF-κB signaling pathway to mediate inflammatory responses

Salmonid alphavirus (SAV) infection of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) causes pancreas disease (PD) with typical inflammatory responses, such as necrosis of the exocrine pancreas, cardiomyopathy and skeletal myopathy. However, the pathogenic mechanism underlying SAV infection is still unclear. Inflammation may cause damage to the body, but it is a defense response against infection by pathogenic microorganisms, of which nuclear factor-kappa B (NF-κB) is the main regulator. This study revealed that SAV can activate NF-κB, of which the viral nonstructural protein Nsp2 is the major activating protein. SAV activates the NF-κB signaling pathway by simultaneously up-regulating TLR3, 7, 8 and then the expression of the signaling molecule myeloid differentiation factor 88 (Myd88) and tumor necrosis factor receptor-associated factor 6 (TRAF6). We found that Nsp2 can induce IκB degradation and p65 phosphorylation and transnucleation, and activate NF-κB downstream inflammatory cytokines. Nsp2 may simultaneously activate NF-κB through TLR3,7,8-dependent signaling pathways. Overexpression of Nsp2 can up-regulate mitochondrial antiviral signaling protein (MAVS) and then promote the expression of IFNa1 and antiviral protein Mx, which inhibits viral replication. This study shows that Nsp2 acts as a key activator protein for the NF-κB signaling pathway, which induces inflammation post-SAV infection. This study systematically analyzes the molecular mechanism of SAV activation of the NF-κB signaling pathway, and provides a theoretical basis for revealing the mechanism of innate immune response and inflammatory injury caused by SAV.

Expression Analysis of Moritella viscosa-Challenged Atlantic Salmon Identifies Disease-Responding Genes, MicroRNAs and Their Predicted Target Genes and Pathways

Moritella viscosa is a bacterial pathogen causing winter-ulcer disease in Atlantic salmon. The lesions on affected fish lead to increased mortality, decreased fish welfare, and inferior meat quality in farmed salmon. MicroRNAs (miRNAs) are small non-coding RNAs involved in post-transcriptional regulation by guiding the miRNA-induced silencing complex to specific mRNA transcripts (target genes). The goal of this study was to identify miRNAs responding to Moritella viscosa in salmon by investigating miRNA expression in the head-kidney and the muscle/skin from lesion sites caused by the pathogen. Protein coding gene expression was investigated by microarray analysis in the same materials. Seventeen differentially expressed guide-miRNAs (gDE-miRNAs) were identified in the head-kidney, and thirty-nine in lesion sites, while the microarray analysis reproduced the differential expression signature of several thousand genes known as infection-responsive. In silico target prediction and enrichment analysis suggested that the gDE-miRNAs were predicted to target genes involved in immune responses, hemostasis, angiogenesis, stress responses, metabolism, cell growth, and apoptosis. The majority of the conserved gDE-miRNAs (e.g., miR-125, miR-132, miR-146, miR-152, miR-155, miR-223 and miR-2188) are known as infection-responsive in other vertebrates. Collectively, the findings indicate that gDE-miRNAs are important post-transcriptional gene regulators of the host response to bacterial infection.

Strong regulatory effects of vgll3 genotype on reproductive axis gene expression in juvenile male Atlantic salmon

Age at maturity is a major contributor to the diversity of life history strategies in organisms. The process of maturation is influenced by both genetics and the environment, and includes changes in levels of sex hormones and behavior, but the specific factors leading to variation in maturation timing are poorly understood. gnrh1 regulates the transcription of gonadotropin genes at pubertal onset in many species, but this gene is lacking in certain teleost species including Atlantic salmon (Salmo salar), which raises the possibility of the involvement of other important regulatory factors during this process. Earlier research has reported a strong association of alternative alleles of the vgll3 gene with maturation timing in Atlantic salmon, suggesting it as a potential candidate regulating reproductive axis genes. Here, we investigated the expression of reproductive axis genes in one-year-old Atlantic salmon males with immature gonads and different vgll3 genotypes during the spawning period. We detected strong vgll3 genotype-dependent differential expression of reproductive axis genes (such as fshb, lhb, amh and igf3) tested in the pituitary, and testis. In addition, we observed differential expression of jun (ap1) and nr5a1b (sf1), potential upstream regulators of gonadotropins in the pituitary, as well as axin2, id3, insl3, itch, ptgs2a and ptger4b, the downstream targets of amh and igf3 in the testis. Hereby, we provide evidence of strong vgll3 genotype-dependent transcriptional regulation of reproductive axis genes prior to sexual maturation and suggest alternative models for distinct actions of vgll3 genotypes on the related molecular processes.

Differential Expression of miRNAs and Their Predicted Target Genes Indicates That Gene Expression in Atlantic Salmon Gill Is Post-Transcriptionally Regulated by miRNAs in the Parr-Smolt Transformation and Adaptation to Sea Water

Smoltification (parr-smolt transformation) is a complex developmental process consisting of developmental changes that lead to remodeling of the Atlantic salmon gill. Here, the expression changes of miRNAs and mRNAs were studied by small-RNA sequencing and microarray analysis, respectively, to identify miRNAs and their predicted targets associated with smoltification and subsequent sea water adaptation (SWA). In total, 18 guide miRNAs were identified as differentially expressed (gDE miRNAs). Hierarchical clustering analysis of expression changes divided these into one cluster of 13 gDE miRNAs with decreasing expression during smoltification and SWA that included the miRNA-146, miRNA-30 and miRNA-7132 families. Another smaller cluster that showed increasing expression consisted of miR-101a-3p, miR-193b-5p, miR-499a-5p, miR-727a-3p and miR-8159-5p. The gDE miRNAs were predicted to target 747 of the genes (DE mRNAs), showing expression changes in the microarray analysis. The predicted targets included genes encoding NKA-subunits, aquaporin-subunits, cystic fibrosis transmembrane conductance regulator and the solute carrier family. Furthermore, the predicted target genes were enriched in biological processes associated with smoltification and SWA (e.g., immune system, reactive oxygen species, stress response and extracellular matrix organization). Collectively, the results indicate that remodeling of the gill involves the post-transcriptional regulation of gene expression by the characterized gDE miRNAs.

Molecular Regulation of Biosynthesis of Long Chain Polyunsaturated Fatty Acids in Atlantic Salmon

Salmon is a rich source of health-promoting omega-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA), such as eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3). The LC-PUFA biosynthetic pathway in Atlantic salmon is one of the most studied compared to other teleosts. This has largely been due to the massive replacement of LC-PUFA-rich ingredients in aquafeeds with terrestrial plant oils devoid of these essential fatty acids (EFA) which ultimately pushed dietary content towards the minimal requirement of EFA. The practice would also reduce tissue content of n-3 LC-PUFA compromising the nutritional value of salmon to the human consumer. These necessitated detailed studies of endogenous biosynthetic capability as a contributor to these EFA. This review seeks to provide a comprehensive and concise overview of the current knowledge about the molecular genetics of PUFA biosynthesis in Atlantic salmon, highlighting the enzymology and nutritional regulation as well as transcriptional control networks. Furthermore, we discuss the impact of genome duplication on the complexity of salmon LC-PUFA pathway and highlight probable implications on endogenous biosynthetic capabilities. Finally, we have also compiled and made available a large RNAseq dataset from 316 salmon liver samples together with an R-script visualization resource to aid in explorative and hypothesis-driven research into salmon lipid metabolism.

Mechanically Enhanced Salmo salar Gelatin by Enzymatic Cross-linking: Premise of a Bioinspired Material for Food Packaging, Cosmetics, and Biomedical Applications

Marine animal by-products of the food industry are a great source of valuable biomolecules. Skins and bones are rich in collagen, a protein with various applications in food, cosmetic, healthcare, and medical industries in its native form or partially hydrolyzed (gelatin). Salmon gelatin is a candidate of interest due to its high biomass production available through salmon consumption, its biodegradability, and its high biocompatibility. However, its low mechanical and thermal properties can be an obstacle for various applications requiring cohesive material. Thus, gelatin modification by cross-linking is necessary. Enzymatic cross-linking by microbial transglutaminase (MTG) is preferred to chemical cross-linking to avoid the formation of potentially cytotoxic residues. In this work, the potential of salmon skin gelatin was investigated, in a comparative study with porcine gelatin, and an enzymatic versus chemical cross-linking analysis. For this purpose, the two cross-linking methods were applied to produce three-dimensional, porous, and mechanically reinforced hydrogels and sponges with different MTG ratios (2%, 5%, and 10% w/w gelatin). Their biochemical, rheological, and structural properties were characterized, as well as the stability of the material, including the degree of syneresis and the water-binding capacity. The results showed that gelatin enzymatically cross-linked produced material with high cross-linking densities over 70% of free amines. The MTG addition seemed to play a crucial role, as shown by the increase in mechanical and thermal resistances with the production of a cohesive material stable above 40 °C for at least 7 days and comparable to porcine and chemically cross-linked gelatins. Two prototypes were obtained with similar thermal resistances but different microstructures and viscoelastic properties, due to different formation dynamics of the covalent network. Considering these results, the enzymatically cross-linked salmon gelatin is a relevant candidate as a biopolymer for the production of matrix for a wide range of biotechnological applications such as food packaging, cosmetic patch, wound healing dressing, or tissue substitute.

Competing endogenous RNA-networks reveal key regulatory microRNAs involved in the response of Atlantic salmon to a novel orthomyxovirus

One of the most intriguing discoveries of the genomic era is that only a small fraction of the genome is dedicated to protein coding. The remaining fraction of the genome contains, amongst other elements, a number of non-coding transcripts that regulate the transcription of protein coding genes. Here we used transcriptome sequencing data to explore these gene regulatory networks using RNA derived from gill tissue of Atlantic salmon (Salmo salar) infected with Pilchard orthomyxovirus (POMV), but showing no clinical signs of disease. We examined fish sampled early during the challenge trial (8-12 days after infection) to uncover potential biomarkers of early infection and innate immunity, and fish sampled late during the challenge trial (19 dpi) to elucidate potential markers of resistance to POMV. We analysed total RNA-sequencing data to find differentially expressed messenger RNAs (mRNA) and identify new long-noncoding RNAs (lncRNAs). We also evaluated small RNA sequencing data to find differentially transcribed microRNAs (miRNAs) and explore their role in gene regulatory networks. Whole-genome expression data (both coding and non-coding transcripts) were used to explore the crosstalk between RNA molecules by constructing competing endogenous RNA networks (ceRNA). The teleost specific miR-462/miR-731 cluster was strongly induced in POMV infected fish and deemed a potential biomarker of early infection. Gene networks also identified a selenoprotein (selja), downregulated in fish sampled late during the challenge, which may be associated to viral clearance and the return to homeostasis after infection. This study provides the basis for further investigations using molecular tools to overexpress or inhibit miRNAs to confirm the functional impact of the interactions presented here on gene expression and their potential application at commercial level.

Atlantic Salmon Mucins Inhibit LuxS-Dependent A. Salmonicida AI-2 Quorum Sensing in an N-Acetylneuraminic Acid-Dependent Manner

One of the most important bacterial diseases in salmonid aquaculture is furunculosis, caused by Aeromonas salmonicida. Bacterial communication through secreted autoinducer signals, quorum sensing, takes part in the regulation of gene expression in bacteria, influencing growth and virulence. The skin and mucosal surfaces, covered by a mucus layer, are the first point of contact between fish and bacteria. Mucins are highly glycosylated and are the main components of mucus. Here, we validate the Vibrio harveyi BB170 bioreporter assay for quantifying A. salmonicida quorum sensing and study the effects of Atlantic salmon mucins as well as mono- and disaccharides on the AI-2 levels of A. salmonicida. Atlantic salmon mucins from skin, pyloric ceca, proximal and distal intestine reduced A. salmonicida AI-2 levels. Among the saccharides abundant on mucins, fucose, N-acetylneuraminic acid and GlcNAcβ1-3Gal inhibited AI-2 A. salmonicida secretion. Removal of N-acetylneuraminic acid, which is the most abundant terminal residue on mucin glycans on Atlantic salmon mucins, attenuated the inhibitory effects on AI-2 levels of A. salmonicida. Deletion of A. salmonicida luxS abolished AI-2 production. In conclusion, Atlantic salmon mucins regulate A. salmonicida quorum sensing in a luxS and N-acetylneuraminic acid-dependent manner.

Dietary plant oil supplemented with arachidonic acid and eicosapentaenoic acid affects the fatty acid composition and eicosanoid metabolism of Atlantic salmon (Salmo salar L.) during smoltification

This study sought to investigate whether a "natural diet" (mimicking the fatty acid composition of freshwater aquatic insects eaten by salmon parr) during the freshwater (FW) life stage of pre-smolt Atlantic salmon (Salmo salar L.) affected red blood cells and gill fatty acid composition as well as eicosanoid metabolism in gill during smolting at different temperatures. Before being transferred to seawater (SW), salmon parr were fed with a modified (MO) diet containing vegetable oils (rapeseed, palm, and linseed oils) supplemented with eicosapentaenoic acid (EPA) and arachidonic acid (ARA) to completely replace the fish oil (FO). Fatty acid composition in red blood cells and gill tissues was determined before SW transfer and six weeks after. Additionally, the expression of genes associated with eicosanoid metabolism and Na⁺/K⁺-ATPase (NKA) activity in salmon gill was examined at different temperatures before SW transfer and 24 h after. The results showed the changes in fatty acid composition, including sum monounsaturated fatty acids (MUFAs), docosahexaenoic acid (DHA), ARA, EPA, and sum n-6 polyunsaturated fatty acids (n-6 PUFA) in both red blood cells and gill tissues at the FW stage were consistent with the fatty acid profiles of the supplied MO and FO fish diets; however sum EPA and DHA composition exhibited opposite trends to those of the FO diet. The proportion of ARA, EPA, and n-6 PUFA increased, whereas sum MUFAs and DHA decreased in the red blood cells and gill tissues of MO-fed fish compared to those fed with the FO diet at FW stage. Additionally, 5-lipoxygenase-activating protein (Flap) expression was downregulated in MO-fed fish prior to SW transfer. During the process of SW transfer at different temperatures, the MO diet remarkably suppressed NKAα1a expression in MO-fed fish both at 12 and 16 °C. The MO diet also upregulated phospholipase A2 group IV (PLA2g4) expression in gills at 8, 12, and 16 °C, but suppressed phospholipase A2 group VI (PLA2g6) expression in gills at 12 °C compared to FO-fed fish at 12 °C and MO-fed fish at 8 °C. The MO diet also upregulated Cyclooxygenase 2 (Cox-2) expression at 8 °C compared to FO-fed fish and increased Arachidonate 5-lipoxygenase (5-Lox) expression in MO-fed fish at 16 °C compared to both FO-fed fish at 16 °C and MO-fed fish at 8 °C. Our study also determined that both SW transfer water temperatures and diets during the FW period jointly influenced the mRNA expression of PLA2g4, PLA2g6, and Lpl, whereas 5-Lox was more sensitive to dietary changes. In conclusion, the MO diet affected the fatty acid composition in gill and in red blood cells. When transferred to SW, dietary ARA supplementation could promote the bioavailability for eicosanoid synthesis in gill mainly via PLA2g4 activation, and potentially inhibit the stress and inflammatory response caused by different water temperatures through dietary EPA supplementation.

Health benefits and risks associated with element uptake from grilled fish and fish products

BACKGROUND

The content of essential and toxic elements in grilled fish (Atlantic salmon, trout) and in 20 assortments of fish products was examined. The aim of this study was to assess the fulfilment of the demand for Zn, Fe, Mn and Cu based on recommended dietary allowances (RDAs) or adequate intakes (AIs) and to assess the health risk associated with the consumption of Al, Pb and Cd. The risk assessment was based on estimated weekly intake (EWI), hazard index (HI), target risk ratio (THQ) and percentage: provisional tolerable weekly intake (PTWI) for Al, tolerable weekly intake (TWI) for Cd and reference dose lower bound (BMDL) for Pb.

RESULTS

Taking into account the health benefits, the best source of Fe, Cu and Mn was found to be fish products in cans and jars, 150 g of which covered the daily requirement: Fe 9.39%, Cu 2.91% and Mn 1.21%. Smoked fish covered the RDA for Zn to 5.69%. Moreover, the uptake of toxic elements was as follows: Al 0.45% PTWI: Pb 0.74% BMDL10 and Cd 2.20% TWI. The THQ values for Pb and Cd were significantly lower than 1, whereas for Al it reached as high as 4.0.

CONCLUSION

The obtained results indicate that there is no risk related to the intake of Pb and Cd with the consumption of the tested fish products. Low consumption of fish by Poles, however, contributes to their small share in covering the demand for microelements, i.e. Zn, Fe, Mn and Cu. © 2021 Society of Chemical Industry.

Arsenolipids in salmon are partly converted to thioxo analogs during cooking

BACKGROUND

Arsenic hydrocarbons, major arsenolipids occurring naturally in marine fish, have substantial cytotoxicity leading to human health-related studies of their distribution and abundance in foods. These studies have all investigated fresh foods; because most fish are cooked before being consumed, it is both food- and health-relevant to determine the arsenolipids present in cooked fish.

METHODS

We used HPLC/mass spectrometry to investigate the arsenolipids present in salmon (Salmo salar) before and after cooking by either baking or steaming.

RESULTS

In raw salmon (total As 2.74 mg kg^-1 dry mass, of which 6% was lipid-soluble), major arsenolipids were three arsenic hydrocarbons (oxo-AsHC 332, oxo-AsHC 360, and oxo-AsHC 404, ca 55% of total arsenolipids) and a band of unidentified less-polar arsenolipids (ca 40%), trace amounts of another four arsenic hydrocarbons and two thioxo analogs were also detected. During the cooking process, 28% of the oxo-AsHCs were converted to their thioxo analogs.

CONCLUSION

Our study shows that arsenic hydrocarbons naturally present in fresh fish are partly converted to their thioxo analogs during cooking by either baking or steaming. The greater lipophilicity of the thioxo analogs could alter the mode of toxicity of arsenic hydrocarbons, and hence future food regulations for arsenic should consider the influence of cooking on the precise type of arsenolipid in fish.

Pituitary Gonadotropin Gene Expression During Induced Onset of Postsmolt Maturation in Male Atlantic Salmon: In Vivo and Tissue Culture Studies

Precocious male maturation causes reduced welfare and increased production costs in Atlantic salmon (Salmo salar) aquaculture. The pituitary produces and releases follicle-stimulating hormone (Fsh), the gonadotropin triggering puberty in male salmonids. However, little is known about how Fsh production is regulated in Atlantic salmon. We examined, in vivo and ex vivo, transcriptional changes of gonadotropin-related genes accompanying the initial steps of testis maturation, in pituitaries of males exposed to photoperiod and temperature conditions promoting maturation (constant light and 16°C). Pituitary fshb, lhb and gnrhr2bba transcripts increased in vivo in maturing males (gonado-somatic index > 0.1%). RNA sequencing (RNAseq) analysis using pituitaries from genetically similar males carrying the same genetic predisposition to mature, but differing by responding or not responding to stimulatory environmental conditions, revealed 144 differentially expressed genes, ~2/3rds being up-regulated in responders, including fshb and other pituitary hormones, steroid-related and other puberty-associated transcripts. Functional enrichment analyses confirmed gene involvement in hormone/steroid production and gonad development. In ex vivo studies, whole pituitaries were exposed to a selection of hormones and growth factors. Gonadotropin-releasing hormone (Gnrh), 17β-estradiol (E₂) and 11-ketotestosterone (11-KT) up-regulated gnrhr2bba and lhb, while fshb was up-regulated by Gnrh but down-regulated by 11-KT in pituitaries from immature males. Also pituitaries from maturing males responded to Gnrh and sex steroids by increased gnrhr2bba and lhb transcript levels, but fshb expression remained unchanged. Growth factors (inhibin A, activin A and insulin-like growth factor 1) did not change gnrhr2bba, lhb or fshb transcript levels in pituitaries either from immature or maturing males. Additional pituitary ex vivo studies on candidates identified by RNAseq showed that these transcripts were preferentially regulated by Gnrh and sex steroids, but not by growth factors, and that Gnrh/sex steroids were less effective when incubating pituitaries from maturing males. Our results suggest that a yet to be characterized mechanism up-regulating fshb expression in the salmon pituitary is activated in response to stimulatory environmental conditions prior to morphological signs of testis maturation, and that the transcriptional program associated with this mechanism becomes unresponsive or less responsive to most stimulators ex vivo once males had entered pubertal developmental in vivo.

The stress response in Atlantic salmon (Salmo salar L.): identification and functional characterization of the corticotropin-releasing factor (crf) paralogs

Corticotropin-Releasing Factor (CRF) is one of the main mediators of the Hypothalamic-Pituitary-Interrenal (HPI) axis to stress response. In Atlantic salmon, a comparative understanding of the crf1 paralogs role in the stress response is still incomplete. Our database searches have identified four crf1 genes in Atlantic salmon, named crf1a1, crf1a2, crf1b1 and crf1b2. Brain distribution analysis revealed that the four crf1 paralogs were widely distributed, and particularly abundant in the telencephalon, midbrain, and hypothalamus of Atlantic salmon post-smolts. To increase the knowledge on crf1-mediated response to stress, Atlantic salmon post-smolts were exposed to either repeated chasing, hypoxia or a combination of chasing and hypoxia for eight days, followed by a novel-acute stressor, confinement. Cortisol, glucose, lactate, and creatinine levels were used as markers for the stress response. The crf1 paralogs mRNA abundance showed to be dependent on the stress exposure regime. Both crf1 mRNA levels in the telencephalon and crf1a1 mRNA levels in the hypothalamus showed similar response profiles to the serum cortisol levels, i.e., increasing levels during the first 24 h after stress exposure followed by a decline during the eight-day exposure. The similar trend between crf1 and cortisol disappeared once exposed to the novel-acute stressor. There was a minor response to stress for both crf1b1 and crf1b2 in the hypothalamus, while no changes at mRNA level were observed in the hypothalamic crf1a2 under the different stress conditions. No or weak relationship was found between the crf1 paralogs mRNA expression and the other serum stress-indicators analysed. In summary, our data provide novel insights on the dynamic of the HPI axis activation in Atlantic salmon, and thus underline the involvement of the crf1 paralogs as additional factors in the regulation of the stress response in this species. Likewise, the data highlight the importance of analysing all crf1 paralogues response to a stress-condition, in particular in this premature knowledge stage of their functionality. Further analysis and a more detailed time-point series will help to elucidate the response of the HPI axis and the link of crf1 paralogs in the stress response mechanism.

Modulations of lipid metabolism and development of the Atlantic salmon (Salmo salar) fry in response to egg-to-fry rearing conditions

In stocking program, the use of artificial incubation conditions in hatcheries from the fertilisation of eggs to the release of unfed fry could reduce their ability to adapt to the natural environment. This study evaluates the effects of three factors on the fitness and physiology of salmon fry at their emergence, the origin of water (river vs drilling), the type of support in the incubator (support matrix vs plastic sheets) and the type of incubators (Californian vs vertical trays), and compares them to a semi-natural incubation method in river. Key biological functions including nutritional and immune status were compared among experimental conditions using biometric parameters, lipid composition and gene expression analyses. Our findings demonstrated that fry incubated in vertical trays supplied with river water had no significant difference in growth and lipid composition compared to those in semi-natural incubators. Besides, fry incubated on a substrate matrix in Californian trays exhibited phenotypic characteristics closest to those incubated in river. This support matrix improved fish growth, lipid consumption and distribution compared to fry on plastic sheets. Moreover, the large amounts of several PUFAs in these fry could allow a better membrane fluidity ensuring a better adaptation to temperature variation under cold conditions. In addition, drilling water improved the survival rate compared to river water due to lower numbers of fine particles, known to be responsible for the clogging of eggs. To conclude, using a substrate combined with drilling water in artificial incubators could increase fry fitness and its adaption to wild life.

Molecular Mechanism Involved in Carotenoid Metabolism in Post-Smolt Atlantic Salmon: Astaxanthin Metabolism During Flesh Pigmentation and Its Antioxidant Properties

A better understanding of carotenoid dynamics (transport, absorption, metabolism, and deposition) is essential to develop a better strategy to improve astaxanthin (Ax) retention in muscle of Atlantic salmon. To achieve that, a comparison of post-smolt salmon with (+ Ax) or without (- Ax) dietary Ax supplementation was established based on a transcriptomic approach targeting pyloric, hepatic, and muscular tissues. Results in post-smolts showed that the pyloric caeca transcriptome is more sensitive to dietary Ax supplementation compared to the other tissues. Key genes sensitive to Ax supplementation could be identified, such as cd36 in pylorus, agr2 in liver, or fbp1 in muscle. The most modulated genes in pylorus were related to absorption but also metabolism of Ax. Additionally, genes linked to upstream regulation of the ferroptosis pathway were significantly modulated in liver, evoking the involvement of Ax as an antioxidant in this process. Finally, the muscle seemed to be less impacted by dietary Ax supplementation, except for genes related to actin remodelling and glucose homeostasis. In conclusion, the transcriptome data generated from this study showed that Ax dynamics in Atlantic salmon is characterized by a high metabolism during absorption at pyloric caeca level. In liver, a link with a potential of ferroptosis process appears likely via cellular lipid peroxidation. Our data provide insights into a better understanding of molecular mechanisms involved in dietary Ax supplementation, as well as its beneficial effects in preventing oxidative stress and related inflammation in muscle.

Full recombinant flagellin B from Vibrio anguillarum (rFLA) and its recombinant D1 domain (rND1) promote a pro-inflammatory state and improve vaccination against P. salmonis in Atlantic salmon (S. salar)

Flagellin is the major component of the flagellum, and a ligand for Toll-like receptor 5. As reported, recombinant flagellin (rFLA) from Vibrio anguillarum and its D1 domain (rND1) are able to promote in vitro an upregulation of pro-inflammatory genes in gilthead seabream (Sparus aurata) and rainbow trout (Oncorhynchus mykiss) macrophages. This study evaluated the in vitro and in vivo stimulatory/adjuvant effect for rFLA and rND1 during P. salmonis vaccination in Atlantic salmon (Salmo salar). We demonstrated that rFLA and rND1 are molecules able to generate an acute upregulation of pro-inflammatory cytokines (IL-1β, IL-8, IL-12β), allowing the expression of genes associated with T-cell activation (IL-2, CD4, CD8β), and differentiation (IFNγ, IL-4/13, T-bet, Eomes, GATA3), in a differential manner, tissue/time dependent way. Altogether, our results suggest that rFLA and rND1 are valid candidates to be used as an immuno-stimulant or adjuvants with existing vaccines in farmed salmon.

Transcriptome Profiling of Long Non-coding RNAs During the Atlantic Salmon Smoltification Process

For salmon aquaculture, one of the most critical phase is the parr-smolt transformation. Studies around this process have mainly focused on physiological changes and the Na⁺/K⁺-ATPase activity during the osmoregulatory activity. However, understanding how the salmon genome regulates the parr-smolt transformation, specifically the molecular mechanisms involved, remains uncovered. This study aimed to explore the transcriptional modulation of long non-coding RNAs (lncRNAs), as key molecular regulators, during the freshwater (FW) to seawater (SW) transfer in Atlantic salmon. Transcriptome sequencing was performed from gill samples of Atlantic salmon adapted from FW to SW through gradual salinity changes from 0 to 30 PSU. The results showed that most transcripts differently modulated were downregulated in all salinity conditions. Relevant biological processes were associated with growth, collagen formation, immune response, metabolism, and heme transport. Notably, 2864 putative lncRNAs were identified in Atlantic salmon gills differently expressed during fish smoltification. The highest number of lncRNAs differently modulated was observed at 30 PSU. Correlation expression analysis suggests putative regulatory roles of lncRNAs with smoltification-related genes. Herein, co-localization of Na⁺/K⁺-ATPase, growth hormone receptor, and thyroid hormone receptor genes with lncRNAs differentially expressed suggest putative regulatory mechanisms in the Atlantic salmon genome. The lncRNAs can be used as novel biomarkers for the fish smoltification process. Here, the lncRNA_145326 and lncRNA_18762 are putatively related to the parr-smolt transfer in Atlantic salmon. This study is the first description of lncRNAs with putative regulatory roles in Atlantic salmon during the SW adaptation.

Modulation of hepatic miRNA expression in Atlantic salmon (Salmo salar) by family background and dietary fatty acid composition

This study finds significant differences in hepatic fatty acid composition between four groups of Atlantic salmon (Salmo salar) consisting of offspring from families selected for high and low capacities to express the delta 6 desaturase isomer b and fed diets with 10% or 75% fish oil. The results demonstrated that hepatic lipid metabolism was affected by experimental conditions (diet/family). The fatty acid composition in the four groups mirrored the differences in dietary composition, but it was also associated with the family groups. Small RNA sequencing followed by RT-qPCR identified 12 differentially expressed microRNAs (DE miRNAs), with expression associated with family groups (miR-146 family members, miR-200b, miR-214, miR-221, miR-125, miR-135, miR-137, miR_nov_1), diets (miR-203, miR-462) or both conditions. All the conserved DE miRNAs have been reported as associated with lipid metabolism in other vertebrates. In silico predictions revealed 37 lipid metabolism pathway genes, including desaturases, transcription factors and key enzymes in the synthesis pathways as putative targets (e.g., srebp-1 and 2, Δ6fad_b and c, hmdh, elovl4 and 5b, cdc42). RT-qPCR analysis of selected target genes showed expression changes that were associated with diet and with family groups (d5fad, d6fad_a, srebp-1). There was a reciprocal difference in the abundance of ssa-miR-203a-3p and srebp-1 in one group comparison, whereas other predicted targets did not reveal any evidence of being negatively regulated by degradation. More experimental studies are needed to validate and fully understand the predicted interactions and how the DE miRNAs may participate in the regulation of hepatic lipid metabolism.

Piscirickettsia salmonis shedding and tissue burden, and hematological responses during cohabitation infections in chum Oncorhynchus keta, pink O. gorbuscha and Atlantic salmon Salmo salar

BACKGROUND

Salmonid rickettsial septicemia is an emergent and geographically widespread disease of marine-farmed salmonids caused by infection with the water-borne bacterium Piscirickettsia salmonis. Very little is known about the route, timing, or magnitude of bacterial shedding from infected fish.

METHODOLOGY/PRINCIPAL FINDINGS

A cohabitation challenge model was used to assess shedding from chum Oncorhynchus keta, pink O. gorbuscha and Atlantic salmon Salmo salar. Infections in donor fish were established by intraperitoneal injection of P. salmonis. Naïve recipients were cohabitated with donor fish after which cumulative percent morbidity and mortality (CMM) was monitored, and bacterial burdens in kidney and in tank water were measured by qPCR. All donor fish died with mean days-to-death (MDD) among species ranging from 17.5 to 23.9. Among recipients, CMM ranged from 42.7% to 77.8% and MDD ranged from 49.7 to 56.4. In each trial, two peaks of bacterial DNA concentrations in tank water closely aligned with the MDD values of donor and recipient fish. Bacterial tissue burden and shedding rate, and plasma physiological parameters were obtained from individual donors and recipients. Statistically significant positive correlations between the shedding rate and P. salmonis kidney burden were measured in donor pink and in donor and recipient chum salmon, but not in donor or recipient Atlantic salmon. In Atlantic salmon, there was a negative correlation between kidney bacterial burden and hematocrit, plasma Ca++ and Mg++ values, whereas in infected chum salmon the correlation was positive for Na+ and Cl- and negative for glucose.

CONCLUSIONS

A dependency of bacterial shedding on species-specific patterns of pathogenesis was suggested. The coincidence of bacterial shedding with mortality will inform pathogen transmission models.

Shotgun Lipidomics for the Determination of Phospholipid and Eicosanoid Profiles in Atlantic Salmon (Salmo salar L.) Muscle Tissue Using Electrospray Ionization (ESI)-MS/MS Spectrometric Analysis

Shotgun lipidomics was applied to identify and quantify phospholipids (PLs) in salmon muscle tissue by focusing on the distribution of ω-3 fatty acids (e.g., docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA)) in the form of phospholipids, as well as to identify and quantify eicosanoids, which has not yet been attempted in Atlantic salmon muscle. Shotgun lipidomics enabled the identification of 43 PL species belonging to four different classes: phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), phosphatidylserines (PSs), and phosphatidylinositols (PIs). Among others, 16:0-22:6 PtdCho m/z [M + Na]⁺ at 828.4 was the predominant PL species in salmon muscle tissue. The present study provided the quantification of individual phospholipid species, which has not been performed for salmon muscle tissue so far. In addition, two eicosanoids-prostaglandin E2 (PGE2) and prostaglandin F3α (PGF3α)-were identified for the first time in salmon muscle. Thus, the rapid and high-throughput shotgun lipidomics approach should shed new light on phospholipids and eicosanoids in salmon muscle tissue.

Stress Impairs Skin Barrier Function and Induces α2-3 Linked N-Acetylneuraminic Acid and Core 1 O-Glycans on Skin Mucins in Atlantic Salmon, Salmo salar

The skin barrier consists of mucus, primarily comprising highly glycosylated mucins, and the epithelium. Host mucin glycosylation governs interactions with pathogens and stress is associated with impaired epithelial barrier function. We characterized Atlantic salmon skin barrier function during chronic stress (high density) and mucin O-glycosylation changes in response to acute and chronic stress. Fish held at low (LD: 14–30 kg/m³) and high densities (HD: 50-80 kg/m³) were subjected to acute stress 24 h before sampling at 17 and 21 weeks after start of the experiment. Blood parameters indicated primary and secondary stress responses at both sampling points. At the second sampling, skin barrier function towards molecules was reduced in the HD compared to the LD group (P_app mannitol; p < 0.01). Liquid chromatography–mass spectrometry revealed 81 O-glycan structures from the skin. Fish subjected to both chronic and acute stress had an increased proportion of large O-glycan structures. Overall, four of the O-glycan changes have potential as indicators of stress, especially for the combined chronic and acute stress. Stress thus impairs skin barrier function and induces glycosylation changes, which have potential to both affect interactions with pathogens and serve as stress indicators.

Growth hormone (GH) and growth hormone release factor (GRF) modulate the immune response in the SHK-1 cell line and leukocyte cultures of head kidney in Atlantic salmon

To clarify the role of growth hormone (GH) in the immune system of fish, we examine the comparative effect of GH and Growth Hormone Release Factor (GRF) on leukocytes culture of the head kidney of Atlantic salmon and the SHK-1 cell line. There are studies that associate the growth hormone (GH) / insulin-like growth factor (IGF) axis with the immune regulation of fish. However, there is no evidence that GH and GRF stimulate Atlantic salmon leukocyte cell lines, where there areńt reports on expression changes in different immune response markers. Thus, we investigated the effect of GH and GRF in Atlantic salmon leukocytes extracted from head kidney and the SHK-1 cell line on the different immune response markers such as: NLRC5, NLRC3, IL-1β, TNF-α, and IL-8 through qPCR. Our data suggest that GH increases the expression of NLRC5, NLRC3, and IL-1β mainly at 16 h post-stimulation in Atlantic salmon leukocytes. This indicates differential regulation between the two models used, helping us to better understand the independent action of GH on the immune system and the GH / IGF axis for future research.

Mycotoxin Identification and In Silico Toxicity Assessment Prediction in Atlantic Salmon

The present study aimed to identify mycotoxins in edible tissues of Atlantic salmon (Salmo salar) using liquid chromatography coupled to hybrid quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS). After using a non-targeted screening approach and a home-made spectral library, 233 mycotoxins were analyzed. Moreover, the occurrence of mycotoxins in fish filets was evaluated, and their potential toxicity was predicted by in silico methods. According to the obtained results, forty mycotoxins were identified in analyzed salmon samples, the predominant mycotoxins being enniatins (also rugulosin and 17 ophiobolins), commonly found in cereals and their by-products. Thus, mycotoxin carry-over can occur from feed to organs and edible tissues of cultivated fish. Moreover, the toxicity of detected mycotoxins was predicted by the in silico webserver ProTox-II, highlighting that special attention must be paid to some less reported mycotoxins due to their toxic predicted properties.

Minimizing marine ingredients in diets of farmed Atlantic salmon (Salmo salar): effects on liver and head kidney lipid class and fatty acid composition

Limited fish meal and fish oil supplies have necessitated research on alternatives for aquafeeds. Seven dietary treatments with different protein and lipid sources were formulated for farmed Atlantic salmon, and their effects on liver and head kidney lipid class, fatty acid, and elemental composition were studied. Fish meal, fish oil, and EPA + DHA content ranged from 5-35%, 0-12%, and 0.1-3%, respectively. Elemental analysis showed that the C to N ratio was higher in the head kidney than in the liver, which is consistent with higher content of total lipids in the head kidney compared with the liver. There was a greater susceptibility to dietary lipid alterations in the liver compared with the head kidney despite liver having a greater proportion of phospholipid and a much lower proportion of triacylglycerol. So long as fish oil levels were 5% or more of the diet, arachidonic acid (ARA) and docosahexaenoic acid (DHA) proportions were the same for each tissue as with feeding the marine diet with 12% fish oil; however, livers and head kidneys from fish fed the lowest amount of fish meal and fish oil had the lowest levels of eicosapentaenoic (EPA) and DHA and the highest ARA levels. Removal of fish oil and reduction of fish meal to 5% in diets of farmed Atlantic salmon affected elemental and lipid compositions of the liver and head kidney tissues potentially increasing susceptibility to inflammation. However, with 10% of the diet comprising fish meal and fish oil, lipid contents were comparable with fish fed marine-based diets.

Differential Regulation of the Expression of the Two Thyrotropin Beta Subunit Paralogs by Salmon Pituitary Cells In Vitro

We recently characterized two paralogs of the thyrotropin (TSH) beta subunit in Atlantic salmon, tshβa and tshβb, issued from teleost-specific whole genome duplication. The transcript expression of tshβb, but not of tshβa, peaks at the time of smoltification, which revealed a specific involvement of tshβb paralog in this metamorphic event. Tshβa and tshβb are expressed by distinct pituitary cells in salmon, likely related to TSH cells from the pars distalis and pars tuberalis, respectively, in mammals and birds. The present study aimed at investigating the neuroendocrine and endocrine factors potentially involved in the differential regulation of tshβa and tshβb paralogs, using primary cultures of Atlantic salmon pituitary cells. The effects of various neurohormones and endocrine factors potentially involved in the control of development, growth, and metabolism were tested. Transcript levels of tshβa and tshβb were measured by qPCR, as well as those of growth hormone (gh), for comparison and validation. Corticotropin-releasing hormone (CRH) stimulated tshβa transcript levels in agreement with its potential role in the thyrotropic axis in teleosts, but had no effect on tshβb paralog, while it also stimulated gh transcript levels. Thyrotropin-releasing hormone (TRH) had no effect on neither tshβ paralogs nor gh. Somatostatin (SRIH) had no effects on both tshβ paralogs, while it exerted a canonical inhibitory effect on gh transcript levels. Thyroid hormones [triiodothyronine (T3) and thyroxine (T4)] inhibited transcript levels of both tshβ paralogs, as well as gh, but with a much stronger effect on tshβa than on tshβb and gh. Conversely, cortisol had a stronger inhibitory effect on tshβb than tshβa, while no effect on gh. Remarkably, insulin-like growth factor 1 (IGF1) dose-dependently stimulated tshβb transcript levels, while it had no effect on tshβa, and a classical inhibitory effect on gh. This study provides the first data on the neuroendocrine factors involved in the differential regulation of the expression of the two tshβ paralogs. It suggests that IGF1 may be involved in triggering the expression peak of the tshβb paralog at smoltification, thus representing a potential internal signal in the link between body growth and smoltification metamorphosis.

Association of gut microbiota with metabolism in juvenile Atlantic salmon

The gut microbiome plays a key role in animal health and metabolism through the intricate functional interconnection between the feed, gut microbes, and the host. Unfortunately, in aquaculture, the links between gut microbes and fish genetics and production phenotypes are not well understood.In this study, we investigate the associations between gut microbial communities, fish feed conversion, and fish genetics in the domestic Atlantic salmon. Microbial community composition was determined for 230 juvenile fish from 23 full-sib families and was then regressed on growth, carbon and nitrogen metabolism, and feed efficiency. We only found weak associations between host genetics and microbial composition. However, we did identify significant (p < 0.05) associations between the abundance of three microbial operational taxonomical units (OTUs) and fish metabolism phenotypes. Two OTUs were associated with both carbon metabolism in adipose tissue and feed efficiency, while a third OTU was associated with weight gain.In conclusion, this study demonstrates an intriguing association between host lipid metabolism and the gut microbiota composition in Atlantic salmon. Video Abstract.

Nutritional Enhancement of Farmed Salmon Meat via Non-GMO Nannochloropsis Gaditana: Eicosapentaenoic Acid (EPA, 20:5 n-3), Docosapentaenoic Acid (DPA, 22:5 n-3) and Vitamin D3 for Human Health

Omega-3 long-chain polyunsaturated fatty acids (n-3 LC PUFAs) and vitamin D3 are essential components of human nutrition. A regular human diet is highly deficient in n-3 LC PUFAs. Fish like salmon are highly recommended in the human diet as they are a major source of high-value n-3 LC PUFAs and vitamin D3. The levels of these nutrients have been decreasing over the last few years in farmed salmon, whose production urgently needs sustainable sources of these nutrients. The microalga Nannochloropsis gaditana (NG) is known for its naturally high potential for the production of eicosapentaenoic (EPA, 20:5 n-3) fatty acid. A commercial diet for Atlantic salmon was supplemented with 1% and 10% of spray-dried NG grown under controlled conditions for a high EPA content. Salmon were harvested on day 49, following which, boneless and skinless salmon meat was recovered from fish and analyzed for the fatty acid profile, total fat, and vitamin D3. Vitamin D3, EPA, and docosapentaenoic fatty acid (DPA, 22:5 n-3) levels were significantly increased (p < 0.05) by supplementing the basal diet with 10% NG, thus, NG represents a novel, functional, natural ingredient and a sustainable source of n-3 LC-PUFAs that can raise the levels of healthy fats and vitamin D3 in farmed salmon meat.

The voltage-gated T-type Ca2+ channel is key to the sperm motility of Atlantic salmon (Salmo salar)

Ca²⁺ is a key element in the sperm activation process of Salmo salar. However, the molecular mechanisms by which this ion enters the sperm cell have been poorly studied. In this study, we examined, for the first time, the role of the voltage-gated T-type Ca²⁺ channel in the activation of sperm motility of Salmo salar. Using an in vitro inhibition assay, a significant decrease in total and progressive motility (P < 0.0001) was observed in Salmo salar sperm when they were treated with NNC-55-0396, a highly selective blocker. The in silico analysis showed that this blocker is docked with a strong affinity for the pore of the voltage-gated T-type calcium channel suggesting the blocking of Ca²⁺ ions. The results show that the T-type voltage-gated Ca²⁺ channel is key to sperm motility in Salmo salar.

Oxidant-induced modifications in the mucosal transcriptome and circulating metabolome of Atlantic salmon

Here we report the molecular networks associated with the mucosal and systemic responses to peracetic acid (PAA), a candidate oxidative chemotherapeutic in Atlantic salmon (Salmo salar). Smolts were exposed to different therapeutic doses (0, 0.6 and 2.4 mg/L) of PAA for 5 min, followed by a re-exposure to the same concentrations for 30 min 2 weeks later. PAA-exposed groups have higher external welfare score alterations, especially 2 weeks after the re-exposure. Cases of fin damage and scale loss were prevalent in the PAA-exposed groups. Transcriptomic profiling of mucosal tissues revealed that the skin had 12.5 % more differentially regulated genes (DEGs) than the gills following PAA exposure. The largest cluster of DEGs, both in the skin and gills, were involved in tissue extracellular matrix and metabolism. There were 22 DEGs common to both mucosal tissues, which were represented primarily by genes involved in the biophysical integrity of the mucosal barrier, including cadherin, collagen I α 2 chain, mucin-2 and spondin 1a. The absence of significant clustering in the plasma metabolomes amongst the three treatment groups indicates that PAA treatment did not induce any global metabolomic disturbances. Nonetheless, five metabolites with known functions during oxidative stress were remarkably affected by PAA treatments such as citrulline, histidine, tryptophan, methionine and trans-4-hydroxyproline. Collectively, these results indicate that salmon were able to mount mucosal and systemic adaptive responses to therapeutic doses of PAA and that the molecules identified are potential markers for assessing the health and welfare consequences of oxidant exposure.

Biotransformation of phytoestrogens from soy in enzymatically characterized liver microsomes and primary hepatocytes of Atlantic salmon

Efficient aquaculture is depending on sustainable protein sources. The shortage in marine raw materials has initiated a shift to "green aquafeeds" based on staple ingredients such as soy and wheat. Plant-based diets entail new challenges regarding fish health, product quality and consumer risks due to the possible presence of chemical contaminants, natural toxins and bioactive compounds like phytoestrogens. Daidzein (DAI), genistein (GEN) and glycitein (GLY) are major soy isoflavones with considerable estrogenic activities, potentially interfering with the piscine endocrine system and affecting consumers after carry-over. In this context, information on isoflavone biotransformation in fish is crucial for risk evaluation. We have therefore isolated hepatic fractions of Atlantic salmon (Salmo salar), the most important species in Norwegian aquaculture, and used them to study isoflavone elimination and metabolite formation. The salmon liver microsomes and primary hepatocytes were characterized with respect to phase I cytochrome P450 (CYP) and phase II uridine-diphosphate-glucuronosyltransferase (UGT) enzyme activities using specific probe substrates, which allowed comparison to results in other species. DAI, GEN and GLY were effectively cleared by UGT. Based on the measurement of exact masses, fragmentation patterns, and retention times in liquid chromatography high-resolution mass spectrometry, we preliminarily identified the 7-O-glucuronides as the main metabolites in salmon, possibly produced by UGT1A1 and UGT1A9-like activities. In contrast, the production of oxidative metabolites by CYP was insignificant. Under optimized assay conditions, only small amounts of mono-hydroxylated DAI were detectable. These findings suggested that bioaccumulation of phytoestrogens in farmed salmon and consumer risks from soy-containing aquafeeds are unlikely.

Negative impacts of the sea lice prophylactic emamectin benzoate on the survival of hatchery released salmon smolts in rivers

Emamectin benzoate (EB) is a prophylactic pharmaceutical used to protect Atlantic salmon (Salmo salar) smolts migrating out of rivers and into the ocean against sea lice parasites. Randomized control trials comparing the marine survival of smolts treated with EB to a control group is used to calculate the fraction of marine mortality attributable to sea lice parasitism. However, it is assumed that there is no baseline difference in survival induced by the application of EB treatment. We used a combined laboratory and field study approach to investigate the potential impacts of EB treatment on behaviour and survival of hatchery-reared Atlantic salmon in western Norway. In aquaria experiments, EB-treated salmon smolts did not differ significantly in exploratory behaviour. Fish from treated groups responded similarly to simulated predator attack with spontaneous escape and elevated gill beat rate. Three rivers in the Osterfjord system of western Norway were selected for field experiments, Dale, Vosso, and Modalen. Dale River smolts were treated with intraperitoneal EB injections and had lower probability of detection in a wolf trap downstream of the release site than control smolts. Salmon smolts raised in the Vosso River hatchery were treated with EB delivered in their food and were detected on PIT antennas at the rivermouth of Vosso and Modalen at lower rates than control fish, but only when released at downstream sites. Calculation of risk ratios suggested that the bias in mortality caused by treatment with EB decreased the estimated survival of treated fish from an expected 18%to 46%, reducing the observable negative impact of sea lice on Atlantic salmon smolts in randomized control trials. The results suggest that estimates of the fraction of mortality attributable to sea lice may be underestimated due to lower baseline survival of treated fish caused by treatment and bring urgent attention towards a potential systematic underestimation of the impacts of sea lice on wild salmon.

Triploid Atlantic salmon Salmo salar have a higher dietary phosphorus requirement for bone mineralization during early development

The effect of a dietary phosphorus regime in freshwater on vertebra bone mineralization was assessed in diploid and triploid Atlantic salmon, Salmo salar. Fish were fed either a low phosphorus (LP) diet containing 10.5 g kg^-1 total phosphorus or a normal phosphorus (NP) diet containing 17.4 g kg^-1 total phosphorus from ∼3 to ∼65 g (day 126) in body weight. Two further groups were fed the NP diet from ∼3 g in body weight, but were then switched to the LP diet after 38 (∼10 g in body weight) or 77 (∼30 g in body weight) days. Growth, vertebral ash content (% ash) and radiologically detectable vertebra pathologies were assessed. Triploids were initially smaller than diploids, and again on day 77, but there was no ploidy effect on days 38 or 126. Vertebral ash content increased with increasing body size and those fish fed the NP diet had higher vertebral ash content than those groups fed the LP diet during the intervening time period, but this diet effect became less apparent as fish grew, with all groups having relatively equal vertebral ash content at termination. In general, triploids had lower vertebral ash content than diploids on day 38 and this was most evident in the group fed the LP diet. On day 77, those triploids fed the LP diet during the intervening time period had lower vertebral ash content than diploids. At termination on day 126, the triploids had the same vertebral ash content as diploids, irrespective of diet. There was a ploidy × diet interaction on vertebral deformities, with triploids having higher prevalences of fish with ≥1 deformed vertebra in all dietary groups except continuous NP. In conclusion, between days 0 and 77 (3-30 g body size), triploids required more dietary phosphorus than diploids in order to maintain similar vertebral ash content. A possible link between phosphorus feeding history and phosphorus demand is also discussed.

DHA Modulates Immune Response and Mitochondrial Function of Atlantic Salmon Adipocytes after LPS Treatment

Adipocytes play a central role in overall energy homeostasis and are important contributors to the immune system. Fatty acids (FAs) act as signaling molecules capable to modulate adipocyte metabolism and functions. To identify the effects of two commonly used FAs in Atlantic salmon diets, primary adipocytes were cultured in the presence of oleic (OA) or docosahexaenoic (DHA) acid. DHA decreased adipocyte lipid droplet number and area compared to OA. The increase in lipid load in OA treated adipocytes was paralleled by an increase in iNOS activity and mitochondrial SOD2-GFP activity, which was probably directed to counteract increase in oxidative stress. Under lipopolysaccharide (LPS)-induced inflammation, DHA had a greater anti-inflammatory effect than OA, as evidenced by the higher SOD2 activity and the transcriptional regulation of antioxidant enzymes and pro- and anti-inflammatory markers. In addition, DHA maintained a healthy mitochondrial structure under induced inflammation while OA led to elongated mitochondria with a thin thread like structures in adipocytes exposed to LPS. Overall, DHA possess anti-inflammatory properties and protects Atlantic salmon against oxidative stress and limits lipid deposition. Furthermore, DHA plays a key role in protecting mitochondria shape and function.

The Effect of the Photoperiod on the Fatty Acid Profile and Weight in Hatchery-Reared Underyearlings and Yearlings of Atlantic Salmon Salmo salar L

The influence of two light regimes, 16:8 h light/dark (LD 16:8) and 24:0 h light/dark (LD 24:0), in comparison to a usual hatchery light regime (HL), on the fatty acids content and weight gain in hatchery-reared underyearlings (at 0+ age) and yearlings (at 1+ age) of Atlantic salmon in the summer–autumn period was studied. The total lipids were analyzed by Folch method, the lipid classes using HPTLC, and the fatty acids of total lipids using GC. The increase in EPA and DHA observed in October in underyearlings and yearlings salmon (especially under LD 24:0) suggests they were physiologically preparing for overwintering. The changes in fatty acids and their ratios in juvenile Atlantic salmon can be used as biochemical indicators of the degree to which hatchery-reared fish are ready to smoltify. These associated with an increase in marine-type specific DHA and EPA, an increase in the 16:0/18:1(n-9) ratio, in correlation with a reduction in MUFAs (mainly 18:1(n-9)). These biochemical modifications, accompanied by fish weight gain, were more pronounced in October in yearlings exposed to continuous light (LD 24:0). The mortality rate was lower in experimental groups of underyearliings with additional lighting. Exposure to prolonged and continuous light did not affect yearlings mortality rate.

Phospholipid and LC-PUFA metabolism in Atlantic salmon (Salmo salar) testes during sexual maturation

The importance of dietary lipids in male reproduction are not as well understood as in females, in which dietary lipids, such as phospholipids (PL) and associated fatty acids (FA), are important structural components of the eggs and provide energy for their offspring. In mammals, lipids are suggested to be important for spermatogenesis and to structural components of the spermatozoa that could improve fertilization rates. New knowledge of how lipids affect sexual maturation in male Atlantic salmon (Salmo salar), an important global aquaculture species, could provide tools to delay maturation and/or improve reproductive success. Therefore, changes in testicular composition of lipids and gene transcripts associated with spermatogenesis and lipid metabolism were studied in sexually maturing male salmon compared to immature males and females. An increase in total testis content of FA and PL, and a shift to higher PL composition was observed in maturing males, concomitant with increases in mRNA levels for genes involved in spermatogenesis, FA uptake and synthesis, and production of long chain-polyunsaturated fatty acids (LC-PUFA) and PL. A particularly interesting finding was elevated testis expression of acyl-CoA synthetase 4 (acsl4), and acyl-CoA thioesterase 2 (acot2), critical enzymes that regulate intra-mitochondrial levels of 20:4n-6 FA (arachidonic acid), which have been associated with improved cholesterol transport during steroidogenesis. This suggested that FA may have direct effects on sex steroid production in salmon. Furthermore, we observed increased testis expression of genes for endogenous synthesis of 16:0 and elongation/desaturation to 22:6n-3 (docosahexaenoic acid) in sexually maturing males relative to immature fish. Both of these FA are important structural components of the PL, phosphatidylcholine (PC), and were elevated concomitant with increases in the content of phosphatidic acid, an important precursor for PC, in maturing males compared to immature fish. Overall, this study suggests that, similar to mammals, lipids are important to spermatogenesis and serve as structural components during testicular growth and maturation in Atlantic salmon.

Heterogenic Origin of Micro RNAs in Atlantic Salmon (Salmo salar) Seminal Plasma

The origin and contribution of seminal plasma RNAs into the whole semen RNA repertoire are poorly known, frequently being overlooked or neglected. In this study, we used high-throughput sequencing and RT-qPCR to profile microRNA (miRNA) constituents in the whole semen, as well as in fractionated spermatozoa and seminal plasma of Atlantic salmon (Salmo salar). We found 85 differentially accumulated miRNAs between spermatozoa and the seminal plasma. We identified a number of seminal plasma-enriched and spermatozoa-enriched miRNAs. We localized the expression of some miRNAs in juvenile and mature testes. Two abundant miRNAs, miR-92a-3p and miR-202-5p, localized to both spermatogonia and somatic supporting cells in immature testis, and they were also highly abundant in somatic cells in mature testis. miR-15c-5p, miR-30d-5p, miR-93a-5p, and miR-730-5p were detected only in mature testis. miRs 92a-3p, 202-5p, 15c-5p, and 30d-5p were also detected in a juvenile ovary. The RT-qPCR experiment demonstrated lack of correlation in miRNA transcript levels in seminal plasma versus blood plasma. Our results indicate that salmon semen is rich in miRNAs, which are present in both spermatozoa and seminal plasma. Testicular-supporting somatic cells are likely the source of seminal plasma enrichment, whereas blood plasma is unlikely to contribute to the seminal plasma miRNA repertoire.