Effects of temperature and dietary n-3 and n-6 fatty acids on endocytic processes in isolated rainbow trout (Oncorhynchus mykiss, Walbaum) hepatocytes

Using a metabolomics approach to investigate the sensitivity of a potential Arctic-invader and its Arctic sister-species to marine heatwaves and traditional harvesting disturbances

Coastal species are threatened by fishing practices and changing environmental conditions, such as marine heatwaves (MHW). The mechanisms that confer tolerance to such stressors in marine invertebrates are poorly understood. However, differences in tolerance among different species may be attributed to their geographical distribution. To test the tolerance of species occupying different thermal ranges, we used two closely related bivalves the softshell clam Mya arenaria (Linnaeus, 1758), a cold-temperate invader with demonstrated potential for establishment in the Arctic, and the blunt gaper Mya truncata (Linnaeus, 1758), a native polar species. Clams were subjected to a thermal stress, mimicking a MHW, and harvesting stress in a controlled environment. Seven acute temperature changes (2, 7, 12, 17, 22, 27, and 32 °C) were tested at two harvesting disturbance intensities (with, without). Survival was measured after 12 days and three tissues (gills, mantle, and posterior adductor muscle) collected from surviving individuals for targeted metabolomic profiling. MHW tolerance differed significantly between species: 26.9 °C for M. arenaria and 17.8 °C for M. truncata, with a negligeable effect of harvesting. At the upper thermal limit, M. arenaria displayed a more profound metabolomic remodelling when compared to M. truncata, and this varied greatly between tissue types. Network analysis revealed differences in pathway utilization at the upper MHW limit, with M. arenaria displaying a greater reliance on multiple DNA repair and expression and cell signalling pathways, while M. truncata was limited to fewer pathways. This suggests that M. truncata is ill equipped to cope with warming environments. MHW patterning in the Northwest Atlantic may be a strong predictor of population survival and future range shifts in these two clam species. As polar environments undergo faster rates of warming compared to the global average, M. truncata may be outcompeted by M. arenaria expanding into its native range.

Fatty acid bioconversion in harpacticoid copepods in a changing environment: a transcriptomic approach

By 2100, global warming is predicted to significantly reduce the capacity of marine primary producers for long-chain polyunsaturated fatty acid (LC-PUFA) synthesis. Primary consumers such as harpacticoid copepods (Crustacea) might mitigate the resulting adverse effects on the food web by increased LC-PUFA bioconversion. Here, we present a high-quality de novo transcriptome assembly of the copepod Platychelipus littoralis, exposed to changes in both temperature (+3°C) and dietary LC-PUFA availability. Using this transcriptome, we detected multiple transcripts putatively coding for LC-PUFA-bioconverting front-end fatty acid (FA) desaturases and elongases, and performed phylogenetic analyses to identify their relationship with sequences of other (crustacean) taxa. While temperature affected the absolute FA concentrations in copepods, LC-PUFA levels remained unaltered even when copepods were fed an LC-PUFA-deficient diet. While this suggests plasticity of LC-PUFA bioconversion within P. littoralis, none of the putative front-end desaturase or elongase transcripts was differentially expressed under the applied treatments. Nevertheless, the transcriptome presented here provides a sound basis for future ecophysiological research on harpacticoid copepods. This article is part of the theme issue 'The next horizons for lipids as 'trophic biomarkers': evidence and significance of consumer modification of dietary fatty acids'.

Soya Saponins Induce Enteritis in Atlantic Salmon (Salmo salar L.)

Soybean meal-induced enteritis (SBMIE) is a well-described condition in the distal intestine of salmonids, and saponins have been implicated as the causal agent. However, the question remains whether saponins alone cause SBMIE. Moreover, the dose-response relationship has not been described. In a 10 week feeding trial with Atlantic salmon, a highly purified (95%) soya saponin preparation was supplemented (0, 2, 4, 6, or 10 g/kg) to two basal diets, one containing fishmeal as the major protein source (FM) and the other 25% lupin meal (LP). Saponins caused dose-dependent increases in the severity of inflammation independent of the basal diet, with concomitant alterations in digestive functions and immunological marker expression. Thus, saponins induced inflammation whether the diet contained other legume components or not. However, responses were often the same or stronger in fish fed the corresponding saponin-supplemented LP diets despite lower saponin exposure, suggesting potentiation by other legume component(s).

Ultrastructural features of hepatocytes in cultured Eurasian perch (Perca fluviatilisL.) as affected by nutritional and husbandry conditions

A wide range of factors can be attributed to the syndrome of fatty liver observed in some cultured fish species. The objective of the study was therefore to quantify different hepatocyte ultrastructural features as potentially influenced by twelve nutritional and husbandry factors, in order to discriminate the most influent factors in Eurasian perch (Perca fluviatilis), a typical carnivorous temperate fish species. Twenty-four groups of juveniles (initial weight 57·6 (sd14·4) g) were intensively reared for 116 d and fed sixteen different isoproteic diets. The distribution of the experimental treatments was based on a multivariate fractional factorial design (L24212) with either high (+1) or low ( − 1) level of each of the following factors: diet (lipid and protein sources, lipid content, astaxanthin enrichment), feeding level, daily and weekly distribution frequency, fish density, initial weight heterogeneity, temperature, photoperiod, and light spectrum. Liver lipid droplets, glycogen, mitochondria and rough endoplasmic reticulum (RER) were semi-quantified and analysed by a soft imaging system using transmission electronic microscopy photographs. Important variability of hepatocyte ultrastructural features was observed. The present study confirms that the rearing temperature, through its influences in the general metabolic activity, seems to be the main factor modifying mainly lipid droplet accumulation and RER development. However, factors that could be pooled under the designation of factors leading to food accessibility and lipid and protein quality intensify or compensate the effect of temperature.

Effects of dietary thia fatty acids on lipid composition, morphology and macrophage function of Atlantic salmon (Salmo salar L.) kidney

High lipid levels are being used in modern salmonid diets to promote rapid growth; however there is a limiting supply of the traditional fish oils as the fish farming industry expands. One way to utilize the lipid sources better, could be to find ways to stimulate fatty acid (FA) oxidation so that Atlantic salmon use more energy for muscle growth and less for storage in perivisceral adipose tissue. We have previously shown that dietary inclusion of the thia FA tetradecylthioacetic acid (TTA) promoted hepatic beta-oxidation and reduced total body lipid levels. However, dietary TTA also had some negative effects, leading to accumulation of sulfone and sulfoxide metabolites of TTA in the kidney and increasing mortality rates, particularly at low water temperatures. Therefore we also wish to investigate the effects of TTA on kidney function at high and low temperatures, including some immune system parameters. The production of leukotriene B4 (LTB4) and prostaglandin E2 (PGE2) immunoreactive material from exogenously added arachidonic acid in isolated head kidney macrophages was affected by both diet and temperature. The phagocytic activity in these cells was reduced by DTA in the 12 degrees C group and there was significantly higher protein degradation in head kidney macrophages at 12 degrees C compared to 5 degrees C in all dietary groups. Interestingly, the incorporation of thia FAs in the kidney was higher at 5 degrees C (0.3% TTA and 0.6% DTA) than at 12 degrees C (0.1% TTA and 0.5% DTA). Additionally, there were lower levels of saturated FAs, while higher levels of polyunsaturated FAs (PUFAs) in the kidney of TTA fed fish at 5 degrees C. We also observed temperature-independent tubular dilatation and a reduction in the density of melanomacrophages of the kidney in salmon fed TTA. Nevertheless, the mRNA expression of some immune-relevant genes in head kidney tissue was not affected by TTA-inclusion in salmon diets. In conclusion, it is clear that 0.6% TTA-inclusion in the feed leads to changes in the kidney function particularly at low water temperatures.

Effects of dietary supplementation of rapeseed oil on metabolism of [1-14C]18∶1n−9, [1-14C]20∶3n−6, and [1-14C]20∶4n−3 in atlantic salmon heaptocytes

Atlantic salmon were fed fish meal-based diets supplemented with either 100% fish oil (FO) or 100% rapeseed oil (RO) from an initial weight of 85 g to a final average weight of 280 g. The effects of these diets on the capacity of Atlantic salmon hepatocytes to elongate, desaturate, and esterify [1-14C] 18:1n-9 and the immediate substrates for the delta5 desaturase, [1-14C] 20:3 n-6 and [1-14C] 20:4n-3, were investigated. Radiolabeled 18:1n-9 was mainly esterified into cellular TAG, whereas the more polyunsaturated FA, [1-14C] 20:3n-6 and [1-14C] 20:4n-3, were primarily esterified into cellular PL. More of the elongation product, [1-14C] 20:1n-9, was produced from 18:1n-9 and more of the desaturation and elongation products, 22:5n-6 and 22:6n-3, were produced from [1-14C]20:3n-6 and [1-14C] 20:4n-3, respectively, in RO hepatocytes than in FO hepatocytes. Further, we studied whether increased addition of [1-14C]18:1n-9 to the hepatocyte culture media would affect the capacity of hepatocytes to oxidize 18:1n-9 to acid-soluble products and CO2. An increase in exogenous concentration of 18:1 n-9 from 7 to 100 microM resulted in a nearly twofold increase in the amount of 18:1n-9 that was oxidized. The conversion of 20:4n-3 and 20:3n-6 to the longer-chain 22:6n-3 and 22:5n-6 was enhanced by RO feeding in Atlantic salmon hepatocytes. The increased capacity of RO hepatocytes to produce 22:6n-3 was, however, not enough to achieve the levels found in FO hepatocytes. Our data further showed that there were no differences in the hepatocyte FA oxidation capacity and the lipid deposition of carcass and liver between the two groups.

Effects of 3-thia fatty acids on feed intake, growth, tissue fatty acid composition, beta-oxidation and Na+,K+-ATPase activity in Atlantic salmon

Atlantic salmon (Salmo salar) with an initial mass of 86 g were reared in 12 degrees C seawater for 8 weeks to a final average mass of 250 g. The fish were fed fish meal and fish oil-based diet supplemented with either 0%, 0.3% or 0.6% of tetradecylthioacetic acid (TTA), a 3-thia fatty acid. The specific growth rate (SGR) decreased with increasing dietary dose of TTA. The SGR of the group fed 0% of TTA (Control) was 1.8; that of the group fed 0.3% of TTA (TTA-L) was 1.7, and that of the group fed 0.6% of TTA (TTA-H) was 1.5. The mortality increased with increased dietary dose of TTA. The mitochondrial beta-oxidation capacity in the liver of fish fed the TTA diets was 1.5 to 2 times higher than that of the Control fish. TTA supplementation caused substantial changes in the fatty acid compositions of the phospholipids (PL), triacylglycerols (TAG) and free fatty acids (FFA) of gills, heart and liver. The percentages of n-3 fatty acids, particularly 22:6 n-3, increased in fish fed diets containing TTA, while the percentage of the saturated FAs 14:0 and 16:0 in the PL fractions of the gills and heart decreased. The sum of monounsaturated FAs in the PL and TAG fractions from liver was significantly higher in fish fed diets containing TTA. TTA itself was primarily incorporated into PL. Two catabolic products of TTA (sulphoxides of TTA) were identified, and these products were particularly abundant in the kidney. TTA supplementation had no significant effect on the activity of the membrane-bound enzyme Na(+),K(+)-ATPase.

Influence of temperature and high dietary linoleic acid content on esterification, elongation, and desaturation of PUFA in Atlantic salmon hepatocytes

The esterification, desaturation, and elongation of [1-14C]18:3n-3, [1-14C]18:2n-6, and [1-14C]20:5n-3 at 5 and at 12 degrees C were studied using cultivated hepatocytes from Atlantic salmon. The salmon were fed diets, in which 0, 50, or 100% of the supplementary fish oil had been replaced by soybean oil, for 950 day-degrees at 5 and 12 degrees C. The endogenous percentage of 18:2n-6 in hepatocyte lipids was 2% in cells from fish fed a diet with 100% of the supplemental lipid from fish oil, and it was slightly less than 25% in cells from fish fed the diet with 100% of the supplemental lipid from soybean oil. Furthermore, the percentages of 20:3n-6 and 20:4n-6 were significantly higher in hepatocytes from fish fed on soybean oil than they were in those of fish fed on fish oil. The percentages of 20:5n-3 and 22:6n-3, on the other hand, were lower. The endogenous levels of n-6 FA were not significantly correlated with the total amounts of radiolabeled FA esterified in hepatocyte lipids. The main radiolabeled products formed from 18:2n-6 were 20:2n-6 and 20:3n-6. The level of the important eicosanoid precursor 20:4n-6 was twice as high in hepatocyte phospholipids from fish fed the 100% soybean oil diet as it was in hepatocytes from fish fed the diet with 100% of supplemental lipid from fish oil. The main products formed from 18:3n-3 were 20:4n-3, 20:5n-3, and 22:6n-3. High levels of dietary 18:2n-6 do allow, or even seem to increase, the production of 22:6n-3 from 18:3n-3 in hepatocytes. The main products formed from 20:5n-3 were 22:5n-3 and 22:6n-3. The production of 22:6n-3 from 20:5n-3 was higher at 5 degrees C than at 12 degrees C. The percentage of 24:5n-3 was higher at 5 degrees C than it was at 12 degrees C, as was the ratio of 24:5 to 22:5. These results suggest that the elongation rate of 22:5n-3 to 24:5n-3 is higher at the lower temperature.

The effect of acclimation temperature on the fusion kinetics of lipid vesicles derived from endoplasmic reticulum membranes of rainbow trout (Oncorhynchus mykiss) liver

Membrane fusion is an obligatory step in many vital cellular processes. The well-established enrichment of bilayer-destabilizing lipids in membranes of poikilotherms subjected to growth at low temperatures leads to the prediction that such membranes will possess a greater propensity to undergo fusion. This hypothesis was explicitly tested in the present study by determining the kinetics of fusion between small unilamellar vesicles (SUVs) prepared from endoplasmic reticulum (ER) membranes of thermally-acclimated (to 5 and 20 degrees C) rainbow trout (Oncorhynchus mykiss) liver and bovine brain phosphatidylserine (BBPS). At temperatures above 10 degrees C, ER vesicles from 5 degrees C-acclimated trout, fused more rapidly and to a greater extent with BBPS vesicles (by average factors of 1.25- and 1.45-fold, respectively) than ER vesicles of 20 degrees C-acclimated trout. At temperatures >35 degrees C, apparent fusion rates declined while the extent of fusion increased in both acclimation groups. Fusion kinetics were found to be well correlated with and limited by the physical properties and phase state of the BBPS vesicles. These results indicate that dynamic attributes of biological membranes, such as the propensity to undergo fusion, are of potential regulatory significance and are partially conserved when growth or environmental temperature changes.

Effect of soybean oil and fish oil on individual molecular species of Atlantic salmon head kidney phospholipids determined by normal-phase liquid chromatography coupled to negative ion electrospray tandem mass spectrometry

The effect of soybean oil (SO) and fish oil (FO) on the relative molecular species distribution of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidylserine (PS) in Atlantic salmon head kidney was studied using normal-phase liquid chromatography coupled with negative ion electrospray tandem mass spectrometry. The conformation of identity of the phospholipid species was based on retention time, the mass of the [M-H]- ([M-15]- for PC) molecular ions and the carboxylate anion fragments in the product ion spectrum. The intensity ratio of sn-1/sn-2 fragment ions increased with increasing number of double bonds in the sn-2 acyl chain but was not affected by increasing number of double bonds in the sn-1 acyl chain of the species examined. The relative distribution of the molecular species was determined by multiple reaction monitoring of the carboxylate anion fragment from the sn-1 position. A total of 68 different phospholipid species were determined in the head kidney and the largest amount was found in PE (22 species). Depending on the diet, the main species identified in the different phospholipid classes were; PC 16:0/18:1, PE 16:0/22:6, PI 18:0/20:4 and PS 16:0/22:6. The SO diet significantly increased the 18:2, 20:3 and most 20:4 containing species and significantly reduced the 14:0 and most 20:5 and 22:6 fatty acid containing species. The increase of the 20:4 and the decrease of the 20:5 and 22:6 containing species were dependent on the fatty acid combination of the species.

Is fluid-phase endocytosis conserved in hepatocytes of species acclimated and adapted to different temperatures?

Our primary objective was to determine if rates of fluid-phase endocytosis (FPE) were conserved in hepatocytes from organisms acclimated and adapted to different temperatures. To this aim, the fluorescent dye Lucifer yellow was employed to measure FPE at different assay temperatures (AT) in hepatocytes from 5 degrees C- and 20 degrees C-acclimated trout, Oncorhynchus mykiss (at 5 and 20 degrees C AT), 22 degrees C- and 35 degrees C-acclimated tilapia, Oreochromis nilotica (at 22 and 35 degrees C AT), and the Sprague-Dawley rat (at 10, 20, and 37 degrees C AT). FPE was also studied in rats fed a long-chain polyunsaturated fatty acid (PUFA)-enriched diet (at 10 degrees C AT). Despite being temperature dependent, endocytic rates (values in pl. cell(-1). h(-1)) in both species of fish were compensated after a period of acclimation. For example, in 20 degrees C-acclimated trout, the rate of endocytosis declined from 1.84 to 1.07 when the AT was reduced from 20 to 5 degrees C; however, after a period of acclimation at 5 degrees C, the rate (at 5 degrees C AT) was largely restored (1.80) and almost perfectly compensated (95%). In tilapia, endocytic rates were also temperature compensated, although only partially (36%). Relatively similar rates obtained at 5 degrees C in 5 degrees C-acclimated trout (1.8), at 20 degrees C in 20 degrees C-acclimated trout (1.84), and at 22 degrees C in 22 degrees C-acclimated tilapia (2.2) suggest that endocytic rates are somewhat conserved in these two species of fish. In contrast, the rate in rat measured at 37 degrees C (16.83) was severalfold greater than in fish at their respective body temperatures. A role for lipids in determining rates of endocytosis was supported by data obtained at 10 degrees C in hepatocytes isolated from rats fed a long-chain PUFA-enriched diet: endocytic rates were higher (5.35 pl. cell(-1). h(-1)) than those of rats fed a standard chow diet (2.33 pl. cell(-1). h(-1)). The conservation of endocytic rates in fish may be related to their ability to conserve other membrane characteristics (i.e., order or phase behavior) by restructuring their membrane lipid composition or by modulating the activities of proteins that regulate endocytosis and membrane traffic, whereas the lack of conservation between fish and rat may be due to differences in metabolic rate.

Metabolism of n-3 and n-6 fatty acids in Atlantic salmon liver: stimulation by essential fatty acid deficiency

Oxidation, esterification, desaturation, and elongation of [1-14C]18:2n-6 and [1-14C]18:3n-3 were studied using hepatocytes from Atlantic salmon (Salmo salar L.) maintained on diets deficient in n-3 and n-6 polyunsaturated fatty acids (PUFA) or supplemented with n-3 PUFA. For both dietary groups, radioactivity from 18:3n-3 was incorporated into lipid fractions two to three times faster than from 18:2n-6, and essential fatty acids (EFA) deficiency doubled the incorporation. Oxidation to CO2 was very low and was independent of substrate or diet, whereas oxidation to acid-soluble products was stimulated by EFA deficiency. Products from 18:2n-6 were mainly 18:3n-6, 20:3n-6, and 20:4n-6, with minor amounts of 20:2n-6 and 22:5n-6. Products from 18:3n-3 were mainly 18:4n-3, 20:5n-3, and 22:6n-3, with small amounts of 20:3n-3. The percentage of 22:6n-3 in the polar lipid fraction of EFA-deficient hepatocytes was fourfold higher than in n-3 PUFA-supplemented cells. This correlated well with our other results obtained after abdominal injection of [1-14C]18:3n-3 and [1-14C]18:2n-6. In hepatocytes incubated with [4,5-3H]-22:6n-3, 20:5n-3 was the main product. This retroconversion was increased by EFA deficiency, as was peroxisomal betaoxidation activity. This study shows that 18:2n-6 and 18:3n-3 can be elongated and desaturated in Atlantic salmon liver, and that this conversion and the activity of retroconversion of very long chain PUFA is markedly enhanced by EFA deficiency.