The effect of prolonged fasting on total lipid synthesis and enzyme activities in the liver of the European eel (Anguilla anguilla)

Unravelling the changes during induced vitellogenesis in female European eel through RNA-Seq: What happens to the liver?

The life cycle of European eel (Anguilla anguilla), a catadromous species, is complex and enigmatic. In nature, during the silvering process prior to their long spawning migration, reproductive development is arrested, and they cease feeding. In studies of reproduction using hormonal induction, eels are equivalently not feed. Therefore, in female eels that undergo vitellogenesis, the liver plays different, essential roles being involved both in vitellogenins synthesis and in reallocating resources for the maintenance of vital functions, performing the transoceanic reproductive migration and completing reproductive development. The present work aimed at unravelling the major transcriptomic changes that occur in the liver during induced vitellogenesis in female eels. mRNA-Seq data from 16 animals (eight before induced vitellogenesis and eight after nine weeks of hormonal treatment) were generated and differential expression analysis was performed comparing the two groups. This analysis detected 1,328 upregulated and 1,490 downregulated transcripts. Overrepresentation analysis of the upregulated genes included biological processes related to biosynthesis, response to estrogens, mitochondrial activity and localization, while downregulated genes were enriched in processes related to morphogenesis and development of several organs and tissues, including liver and immune system. Among key genes, the upregulated ones included vitellogenin genes (VTG1 and VTG2) that are central in vitellogenesis, together with ESR1 and two novel genes not previously investigated in European eel (LMAN1 and NUPR1), which have been linked with reproduction in other species. Moreover, several upregulated genes, such as CYC1, ELOVL5, KARS and ACSS1, are involved in the management of the effect of fasting and NOTCH, VEGFA and NCOR are linked with development, autophagy and liver maintenance in other species. These results increase the understanding of the molecular changes that occur in the liver during vitellogenesis in this complex and distinctive fish species, bringing new insights on European eel reproduction and broodstock management.

How the European eel (Anguilla anguilla) loses its skeletal framework across lifetime

European eels (Anguilla anguilla) undertake an impressive 5 000 km long migration from European fresh waters through the North Atlantic Ocean to the Sargasso Sea. Along with sexual maturation, the eel skeleton undergoes a remarkable morphological transformation during migration, where a hitherto completely obscure bone loss phenomenon occurs. To unravel mechanisms of the maturation-related decay of the skeleton, we performed a multiscale assessment of eels' bones at different life-cycle stages. Accordingly, the skeleton reflects extensive bone loss that is mediated via multinucleated bone-resorbing osteoclasts, while other resorption mechanisms such as osteocytic osteolysis or matrix demineralization were not observed. Preserving mechanical stability and releasing minerals for energy metabolism are two mutually exclusive functions of the skeleton that are orchestrated in eels through the presence of two spatially segregated hard tissues: cellular bone and acellular notochord. The cellular bone serves as a source of mineral release following osteoclastic resorption, whereas the mineralized notochord sheath, which is inaccessible for resorption processes due to an unmineralized cover layer, ensures sufficient mechanical stability as a part of the notochord sheath. Clearly, an eel's skeleton is structurally optimized to meet the metabolic challenge of fasting and simultaneous sexual development during an exhausting journey to spawning areas, while the function of the vertebral column is maintained to achieve this goal.

The influence of zinc chloride and zinc oxide nanoparticles on air-time survival in freshwater mussels

The purpose of this study was to determine the cumulative effects of exposure to either dissolved zinc or nanozinc oxide (nanoZnO) and air-time survival in freshwater mussels. Mussels were exposed to each forms of zinc for 96h then placed in air to determine survival time. A sub-group of mussels before and after 7days of exposure to air were kept aside for the determination of the following biomarkers: arachidonate-dependent cyclooxygenase (COX) and peroxidase (inflammation and oxidative stress), lipid metabolism (total lipids, esterases activity, HO-glycerol, acetyl CoA and phospholipase A2) and lipid damage (lipid peroxidation [LPO]). The results showed that air-time survival was decreased from a mean value of 18.5days to a mean value of 12days in mussels exposed to 2.5mg/L of nanoZnO although it was not lethal based on shell opening at concentrations below 50mg/L after 96h. In mussels exposed to zinc only, the median lethal concentration was estimated at 16mg/L (10-25 95% CI). The air-time survival did not significantly change in mussels exposed to the same concentration of dissolved Zn. Significant weight losses were observed at 0.5mg/L of nanoZnO and at 2.5mg/L for dissolved zinc chloride, and were also significantly correlated with air-time survival (r=0.53; p<0.01). Air exposure significantly increased COX activity in control mussels and in mussels exposed to 0.5mg/L of nanoZnO and zinc chloride. The data also suggested fatty acid breakdown and β-oxidation. Mussels exposed to contaminants are more susceptible to prolonged exposure to air during low water levels.

Starvation induced cholesterogenesis in hepatic and extra hepatic tissues of climbing Perch, Anabas testudineus (Bloch)

Cholesterol is a structural lipid, which may be differentially utilized or synthesized in response to stress or during insulin deficient states such as starvation. In the present investigation we estimated the levels of cholesterol in Anabas testudineus, which was subjected to brief (15 days) and prolonged fasting (60 days). Tissues such as liver, kidney, brain, accessory respiratory organ, pectoral and lateral line muscle were selected for the study. Cholesterol content was estimated by the Crawford method (1958). Both the starvation regimes showed a significant increase in cholesterol levels in almost all the tissues, but for liver, which strangely showed an insignificant decline during the short-term starvation. This overall upsurge in cholesterol levels observed in all extra hepatic tissues may be attributed to the synthesis of stress hormones such as glucocorticoids, which may promote gluconeogenesis and adrenocorticoids, which may help the animal to combat the stressful condition of starvation. Anabas adapted well to starvation stress and survived all throughout the experimental period.

Impact of starvation-refeeding on kinetics and protein expression of trout liver NADPH-production systems

Herein we report on the kinetic and protein expression of glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase, and malic enzyme (ME) in the liver of the trout (Oncorhynchus mykiss) during a long-term starvation-refeeding cycle. Starvation significantly depressed the activity of these enzymes by almost 60%, without changing the Michaelis constant. The time response to this nutritional stimulus increased with fish weight. The sharp decline in G6PDH and ME activities was due to a specific protein-repression phenomenon, as demonstrated by molecular and immunohistochemical analyses. Also, the dimeric banding pattern of liver G6PDH shifted from the fully reduced and partially oxidized forms, predominant in control, to a fully oxidized form, more sensitive to proteolytic inactivation. Refeeding caused opposite effects in both protein concentration and enzyme activities of about twice the control values in the first stages, later reaching the normal enzyme activity levels. Additionally, the partially oxidized form of G6PDH increased. The kinetics of these enzymes were examined in relation to the various metabolic roles of NADPH. These results clearly indicate that trout liver undergoes protein repression-induction processes under these two contrasting nutritional conditions.