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 present study aimed to elucidate how Atlantic salmon adipocytes pre-enriched with palmitic (16:0, PA), oleic (18:1n−9, OA), or eicosapentaenoic (20:5n−3, EPA) acid respond to a fasting condition mimicked by nutrient deprivation and glucagon. All experimental groups were supplemented with radiolabeled PA to trace secreted lipids and distribution of radioactivity in different lipid classes. There was a higher content of intracellular lipid droplets in adipocytes pre-enriched with OA than in adipocytes pre-enriched with PA or EPA. In the EPA group, the radiolabeled PA was mainly esterified in phospholipids and triacylglycerols, whereas in the OA and PA groups, the radioactivity was mainly recovered in phospholipids and cholesterol-ester. By subjecting the experimental groups to nutrient-deprived media supplemented with glucagon, lipolysis occurred in all groups, although to a lower extent in the OA group. The lipids were mainly secreted as esterified lipids in triacylglycerols and phospholipids, indicating mobilization in lipoproteins. A significant proportion was secreted as free fatty acids and glycerol. Leptin secretion was reduced in all experimental groups in response to fasting, while the mitochondria area responded to changes in the energy supply and demand by increasing after 3 h of fasting. Overall, different lipid classes in adipocytes influenced their mobilization during fasting.

It has been suggested that the high phospholipid (PL) requirement in Atlantic salmon (Salmo salar) fry is due to insufficient intestinal de-novo synthesis causing low lipoprotein (LP) production and reduced transport capacity of dietary lipids. However, in-depth ontogenetic analysis of intestinal PL and LP synthesis with the development of salmon has yet to be performed. Therefore, in this paper we used RNA-Seq technology to investigate the expression of genes involved in PL synthesis and LP formation throughout early developmental stages and associate insufficient expression of synthesis pathways in salmon fry with its higher dietary PL requirement. There was a special focus on the understanding homologous genes, especially those from salmonid-specific fourth vertebrate whole-genome duplication (Ss4R), and their contribution to salmonid specific features of regulation of PL metabolic pathways. Salmon fry were sampled at 0.16 g (1 day before first-feeding), 2.5 and 10 g stages of development and transcriptomic analysis was applied separately on stomach, pyloric caeca and hindgut of the fish. In general, we found up-regulated pathways involved in synthesis of phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), and LP in pyloric caeca of salmon between 0.16 and 10 g. Thirteen differentially expressed genes (q < 0.05) in these pathways were highly up-regulated in 2.5 g salmon compared to 0.16 g, while only five more differentially expressed (q < 0.05) genes were found when the fish grew up to 10 g. Different homologous genes were found dominating in stomach, pyloric caeca and hindgut. However, the expression of dominating genes in pathways of PL and LP synthesis were much higher in pyloric caeca than stomach and hindgut. Salmon-specific homologous genes (Ss4R) had similar expression during development, while other homologs had more diverged expression. The up-regulation of the de-novo PtdCho and PtdEtn pathways confirm that salmon have decreasing requirement for dietary PL as the fish develops. The similar expressions between Ss4R homologous genes suggest that the functional divergence of these genes was incomplete compared to homologs derived from other genome duplication. The results of the present study have provided new information on the molecular mechanisms of phospholipid synthesis and lipoprotein formation in fish.

The infectious pancreatic necrosis virus (IPNV) used to be one of the largest loss factors in Atlantic salmon farming. Since 2009, marker-assisted selection for resistance to IPN, targeting a single major quantitative trait locus (QTL), has led to a ten-fold decrease in the number of IPN outbreaks in Norway. However, some IPN-related problems remain, due to isolates of the virus which seem to bypass the resistance mechanism of the QTL. We comparatively characterized a classical isolate affected by the IPN-QTL (cIPNV) and an isolate that circumvents the QTL-based protection (rIPNV). Using both in vivo and in vitro challenges, the viral infection dynamics and host responses were evaluated by RT-qPCR and by gene ontology (GO) enrichment analysis from the RNA sequencing data of infected hepatocytes and the whole liver. Overall, cIPNV showed rapid replication with pronounced lytic cytopathology and enrichment for DNA damage, apoptosis and cell cycle disruption GO terms, while rIPNV exhibited slower accumulation of viral RNA and a transcriptional footprint consistent with pro-survival states in hepatocytes. While further research is needed to resolve the causality of QTL evasion, this work provides a first characterization of the pathogenicity of emerging QTL-insensitive IPNV isolates.

The infectious pancreatic necrosis virus (IPNV) used to be one of the largest loss factors in Atlantic salmon farming. Since 2009, marker-assisted selection for resistance to IPN, tar-geting a single major quantitative trait locus (QTL), has led to a ten-fold decrease in the number of IPN outbreaks in Norway. However, some IPN-related problems remain, due to isolates of the virus which seem to bypass the resistance mechanism of the QTL. We comparatively characterized a classical isolate affected by the IPN-QTL (cIPNV) and an isolate that circumvents the QTL-based protection (rIPNV). Using both in vivo and in vitro challenges, the viral infection dynamics and host responses were evaluated by RT-qPCR and by gene ontology (GO) enrichment analysis from RNA-sequencing data of infected hepatocytes and whole liver. Overall, cIPNV showed rapid replication with pronounced lytic cytopathology and enrichment for DNA damage, apoptosis and cell cycle disruption GO terms, while rIPNV exhibited slower accumulation of viral RNA and a transcriptional footprint consistent with pro-survival states in hepatocytes. While further research is needed to resolve causality of QTL evasion, this work provides a first characterization of the pathogenicity of emerging QTL-insensitive IPNV isolates.