Molecular programming of the hepatic lipid metabolism via a parental high carbohydrate and low protein diet in rainbow trout.

It is now recognised that parental diets could alter their offspring metabolism, concept known as nutritional programming. For agronomic purposes, it has been previously proposed that programming could be employed as a strategy to prepare individual for future nutritional challenges. Concerning cultured fish that belong to high trophic level, plant-derived carbohydrates are a possible substitute for the traditional protein-rich fishmeal in broodstock diet, lowering thus the dietary protein-to-carbohydrate ratio (HC/LP nutrition). However, in mammals, numerous studies have previously demonstrated that parental HC/LP nutrition negatively affects their offspring in the long term. Therefore, the question of possible adaptation to plant-based diets, via parental nutrition, should be explored. First, the maternal HC/LP nutrition induced a global DNA hypomethylation in the liver of their offspring. Interestingly at the gene expression level, the effects brought by the maternal and paternal HC/LP nutrition cumulated in the liver, as indicated by the altered transcriptome. The paternal HC/LP nutrition significantly enhanced cholesterol synthesis at the transcriptomic level. Furthermore, hepatic genes involved in long-chain polyunsaturated fatty acids were significantly increased by the parental HC/LP nutrition, affecting thus both hepatic and muscle fatty acid profiles. Overall, the present study demonstrated that lipid metabolism could be modulated via a parental nutrition in rainbow trout, and that such modulations have consequences on their progeny phenotypes.

Alteration of eggs biochemical composition and progeny survival by maternal high carbohydrate nutrition in a teleost fish.

Reproductive performances, and the factors affecting them, are of major importance especially for farmed fish in the context of the development of a sustainable aquaculture. Dietary maternal lipids have been identified as a major factor affecting reproductive performances. Nevertheless, the consequences of carbohydrates have been little studied while plant-derived carbohydrates could be increasingly used in broodstock diets. To explore this issue, 2-year-old female trout were fed either a control diet that contains no carbohydrate and a high protein content (65.7%) or a diet formulated with plant-derived carbohydrates containing 32.5% carbohydrate and 42.9% protein ('HC diet') for an entire reproductive cycle. The reproductive performances, the quality of the unfertilized eggs and the development of the progeny were carefully monitored. Although the one year HC nutrition had not impaired female growth nor spawns quality, such nutrition had increased the variability of eggs size within spawns (+ 34.0%). Moreover, the eggs produced had a modified fatty acid profile, including a significant reduction in EPA content (- 22.9%) and a significant increase in the AA/EPA ratio (+ 33.3%). The progeny were impacted by such alterations as their survival rates were significantly reduced. A lower plant-derived carbohydrate inclusion (20%) should be considered in aquafeed for female broodstock in trout.

Why Do Some Rainbow Trout Genotypes Grow Better With a Complete Plant-Based Diet? Transcriptomic and Physiological Analyses on Three Isogenic Lines.

Within the context of a growing aquaculture production coupled with a plateau of the production in the main components of aquafeeds (fish oil and fishmeal), recent studies have typically focused on replacing these feedstuffs with terrestrial plant ingredients for cultured carnivorous aquatic species, such as rainbow trout (Oncorhynchus mykiss). Substitution rates without adverse effects have, however, reached their limit. One potential way forward would be to take advantage of the genetic variability that exists in the salmonid population. However, to date, little is known about the underlying molecular mechanisms responsible for this genetic variability. The aim of the present research was to understand why some genotypes are better able to utilize plant-based diets devoid of marine resources. In this regard, three isogenic lines of rainbow trout (R23h, AB1h, and A22h), with similar growth when fed marine resources-based diets and which differ greatly in their responses to a plant-based diet, were fed with either a complete plant-based diet (V diet) or a marine resources-based diet (M diet) since first-feeding. Fish traits and the hepatic transcriptome of these three genotypes were compared after 5 months of feeding. First, differences in the ability to grow with the V diet observed amongst genotypes was not due to higher feed intake, but instead due to differences in feed efficiency. The comparison of transcriptome profiles revealed 575 (R23h vs. AB1h), 1,770 (R23h vs. A22h), and 2,973 (AB1h vs. A22h) probes differentially expressed amongst the three genotypes when fed the V diet. Interestingly, R23h and AB1h fish, which were the least affected by the V diet, exhibited the highest growth. These results demonstrate that these fish were able to maintain a high level of energy production and protein synthesis. Moreover, these genotypes were also able to activate pathways linked to lipid and cholesterol metabolisms, such as the biosynthesis of long-chain polyunsaturated fatty acids. Finally, as previously, immunity seems to also play an important role in the ability of fish to use the V diet, and further studies are needed to understand the mechanisms by which immunity interacts with growth.

No adverse effect of a maternal high carbohydrate diet on their offspring, in rainbow trout (Oncorhynchus mykiss).

In order to develop a sustainable salmonid aquaculture, it is essential to continue to reduce the use of the protein-rich fishmeal. One promising solution to do so is the use of plant-derived carbohydrates in diet destined to broodstock. However, in mammals, the reduction of protein content (replaced by carbohydrates) in parental diet is known to have strong adverse effects on offspring phenotypes and metabolism. For the first time, the effect of a paternal and a maternal high carbohydrate-low protein diet was assessed on progeny at long term in the rainbow trout. A 30% protein diminution in both males and females broodstock diet during 10 month and 5 months, respectively, did not trigger adverse consequences on their offspring. At the molecular level, offspring transcriptomes were not significantly altered, emphasizing no effect on metabolism. Tenuous differences in the biochemical composition of the liver and the viscera were observed. The recorded effects remained in the normal range of value and accordingly offspring growth were not negatively affected over the long term. Overall, we demonstrated here that a 30% protein diminution during gametogenesis is feasible, confirming the possibility to increase the proportion of plant-derived carbohydrates in female broodstock diets to replace fishmeal proteins.

Modulation of Energy Metabolism and Epigenetic Landscape in Rainbow Trout Fry by a Parental Low Protein/High Carbohydrate Diet.

It is now recognized that parental diets could highly affect offspring metabolism and growth. Studies in fish are, however, lacking. In particular, the effect of a parental diet high in carbohydrate (HC) and low in protein (LP) on progeny has never been examined in higher trophic level teleost fish. Thus, two-year old male and female rainbow trout (Oncorhynchus mykiss) were fed either a control diet (0% carbohydrate and 63.89% protein) or a diet containing 35% carbohydrate and 42.96% protein (HC/LP) for a complete reproductive cycle for females and over a 5-month period for males. Cross-fertilizations were then carried out. To evaluate the effect of the parental diet on their offspring, different phenotypic and metabolic traits were recorded for offspring before their first feeding and again three weeks later. When considering the paternal and maternal HC/LP nutrition independently, fry phenotypes and transcriptomes were only slightly affected. The combination of the maternal and paternal HC/LP diets altered the energy metabolism and mitochondrial dynamics of their progeny, demonstrating the existence of a synergistic effect. The global DNA methylation of whole fry was also highly affected by the HC/LP parental diet, indicating that it could be one of the fundamental mechanisms responsible for the effects of nutritional programming.

Exploring the Impact of a Low-Protein High-Carbohydrate Diet in Mature Broodstock of a Glucose-Intolerant Teleost, the Rainbow Trout.

Sustainable aquaculture production requires a greater reduction in the use of marine-derived ingredients, and one of the most promising solutions today is the augmentation in the proportion of digestible carbohydrates in aquafeed. This challenge is particularly difficult for high trophic level teleost fish as they are considered to be glucose-intolerant (growth delay and persistent postprandial hyperglycemia observed in juveniles fed a diet containing more than 20% of carbohydrates). It was previously suggested that broodstock could potentially use carbohydrates more efficiently than juveniles, probably due to important metabolic changes that occur during gametogenesis. To investigate this hypothesis, 2-year old male and female rainbow trout (Oncorhynchus mykiss) were either fed a diet containing no carbohydrates (NC) or a 35%-carbohydrate diet (HC) for an entire reproductive cycle. Zootechnical parameters as well as the activities of enzymes involved in carbohydrate metabolism were measured in livers and gonads. Fish were then reproduced to investigate the effects of such a diet on reproductive performance. Broodstock consumed the HC diet, and in contrast to what is commonly observed in juveniles, they were able to grow normally and they did not display postprandial hyperglycemia. The modulation of their hepatic metabolism, with an augmentation of the glycogenesis, the pentose phosphate pathway and a possible better regulation of gluconeogenesis, may explain their improved ability to use dietary carbohydrates. Although the HC diet did induce precocious maturation, the reproductive performance of fish was not affected, confirming that broodstock are able to reproduce when fed a low-protein high-carbohydrate diet. In conclusion, this exploratory work has shown that broodstock are able to use a diet containing digestible carbohydrates as high as 35% and can then grow and reproduce normally over an entire reproductive cycle for females and at least at the beginning of the cycle for males. These results are highly promising and suggest that dietary carbohydrates can at least partially replace proteins in broodstock aquafeed.

Nutritional regulation of glucose metabolism-related genes in the emerging teleost model Mexican tetra surface fish: a first exploration.

Astyanax mexicanus has gained importance as a laboratory model organism for evolutionary biology. However, little is known about its intermediary metabolism, and feeding regimes remain variable between laboratories holding this species. We thus aimed to evaluate the intermediary metabolism response to nutritional status and to low (NC) or high (HC) carbohydrate diets in various organs of the surface-dwelling form of the species. As expected, glycaemia increased after feeding. Fish fed the HC diet had higher glycaemia than fish fed the NC diet, but without displaying hyperglycaemia, suggesting that carbohydrates are efficiently used as an energy source. At molecular level, only fasn (Fatty Acid Synthase) transcripts increased in tissues after refeeding, suggesting an activation of lipogenesis. On the other hand, we monitored only moderate changes in glucose-related transcripts. Most changes observed were related to the nutritional status, but not to the NC versus HC diet. Such a metabolic pattern is suggestive of an omnivorous-related metabolism, and this species, at least at adult stage, may adapt to a fish meal-substituted diet with high carbohydrate content and low protein supply. Investigation to identify molecular actors explaining the efficient use of such a diet should be pursued to deepen our knowledge on this species.

Detection of new pathways involved in the acceptance and the utilisation of a plant-based diet in isogenic lines of rainbow trout fry.

To meet the growing demand of fish feed for aquaculture, an increasing proportion of marine ingredients are being replaced by blends of plant products. However, the total replacement of marine ingredients in salmonid diets impairs fish performance. This is particularly true during the early fry stage and this stage is therefore considered of particular importance. In rainbow trout (RBT), the existence of a genetic variability to survive and grow with plant-based diets devoid of marine ingredients has now been proved, but the mechanisms behind are little studied especially at early stage. To investigate these, we analysed the whole transcriptome of three isogenic lines of RBT fry, which have similar growth when fed a marine resources-based diet (M diet) but which highly differ in their responses to a plant-based diet (V diet). Analysis of transcriptomes profiles revealed 1740, 1834 and 246 probes differentially expressed among the three genotypes when fed the V diet. The use of these lines led to the discovery of potential molecular markers linked to plant-based diet utilisation, some of them belonging to new pathways, never described before. An important number of genes was related to immunity, but further investigations are needed to better understand the difference between the genotypes in their immune status response to V diet exposure. Finally, differences in expression of genes related to feed intake and sensory perception among genotypes suggested that the mechanisms underlying the differences in growth on plant-based diet are closely linked to diet acceptance. Research on plants components affecting feed intake should be thus further explored.

Successful selection of rainbow trout (Oncorhynchus mykiss) on their ability to grow with a diet completely devoid of fishmeal and fish oil, and correlated changes in nutritional traits.

In the context of limited marine resources, the exponential growth of aquaculture requires the substitution of fish oil and fishmeal, the traditional components of fish feeds by terrestrial plant ingredients. High levels of such substitution are known to negatively impact fish performance such as growth and survival in rainbow trout (Oncorhynchus mykiss) as in other salmonids. In this respect, genetic selection is a key enabler for improving those performances and hence for the further sustainable development of aquaculture. We selected a rainbow trout line over three generations for its ability to survive and grow on a 100% plant-based diet devoid of both fish oil and fishmeal (V diet) from the very first meal. In the present study, we compared the control line and the selected line after 3 generations of selection, both fed either the V diet or a marine resources-based diet (M diet). The objective of the study was to assess the efficiency of selection and the consequences on various correlated nutritional traits: feed intake, feed efficiency, digestibility, composition of whole fish, nutrient retention and fatty acid (FA) profile. We demonstrated that the genetic variability present in our rainbow trout population can be selected to improve survival and growth. The major result of the study is that after only three generations of selection, selected fish fed the V diet grew at the same rate as the control line fed the M diet, whilst the relative reduction of body weight was 36.8% before the selection. This enhanced performance on the V diet seems to be mostly linked to a higher feed intake for the selected fish.