Erucic acid inhibits growth performance and disrupts intestinal structural integrity of on-growing grass carp (Ctenopharyngodon idella)
Introduction
In recent years, plant protein was considered as an ideal protein source largely incorporated into aquafeeds (Gatlin III et al., 2007). However, most plant proteins contain various kinds of antinutritional factors (ANFs) (Francis et al., 2001). The large incorporation of plant protein into aquafeeds may expose fish intestine to a wide variety of ANFs derived from plant proteins. In addition, earlier study reported that fish intestine contains a higher level of polyunsaturated fatty acids (PUFAs) (Liu, 1991). Considering the above, it was obvious that fish intestinal structure may be highly susceptible to different kinds of ANFs. Earlier studies from our laboratory confirmed the detrimental effects of ANFs on intestinal structural integrity, but the mechanism was not always in line with each other. For example, in fish intestine, glycinin decreased the content of GSH (one indicator of intestinal structural integrity), whereas β-conglycinin had no effect on it (Jiang et al., 2015a, Jiang et al., 2015b; Zhang et al., 2013). EA is one of major antinutritional factors naturally present in rapeseed meal (Do et al., 2017). An earlier study showed that EA increased ROS content in human polymorphonuclear leukocytes (Heiskanen and Savolainen, 1997), which could cause oxidative damage in Jian carp (Jiang et al., 2015a, Jiang et al., 2015b) and result in histopathological lesion of intestine in rat (Ramachandran et al., 2002). However, no information regarding the impacts of EA on fish intestinal structural integrity and related mechanisms was provided, which needs systematic and in-depth exploration.
In fish, the disrupted structural integrity of intestine is closely correlated with oxidative damage, cell apoptosis and dysfunction of cell-cell tight junction (Jiang et al., 2015a, Jiang et al., 2015b), which was associated with the inhibition of NF-E2-related factor 2 (Nrf2) signaling pathway, and the activations of p38 mitogen-activated protein kinase (p38MAPK)/c-Jun N-terminal kinase (JNK) and myosin light-chain kinase (MLCK) signaling pathways, respectively (Huang et al., 2018; Zeng et al., 2019). However, to date, no data about the effect of EA on these three aspects in animals was published. It was reported that EA could cause lipid accumulation in rat heart (Kramer et al., 1992). Studies carried out with grass carp by Feng et al. (2017) concluded that a high lipid level decreased intestinal antioxidant enzyme CuZnSOD activity. In rat, it was reported that EA could be converted into stearic acid (Murphy et al., 2008) that stimulated the insulin secretion (Stein et al., 1997). Studies conducted with diabetic mice kidney by Ghosh et al. (2017) found that insulin down-regulated Nrf2 gene expression. In rat heart, it was reported that EA elevated rat heart arachidonic acid content (Sato et al., 1983). Previous study performed on human indicated that the high arachidonic acid content caused polymorphonuclear neutrophil granulocytes apoptosis (Köller et al., 1997). In rat, it was reported that EA could be metabolized to be oleic acid (Thomassen et al., 1985), which could activate p38MAPK and JNK (Gu et al., 2015; Lager et al., 2013). And earlier studies conducted with weaning piglets found that EA decreased the content of carnitine (Sauer et al., 1989), which could down-regulate tight junction protein ZO-1 expression (Wei et al., 2016). Meanwhile, in rat serum, Rahman et al. (2014) concluded that EA increased cholesterol content. A study carried out by Zhu et al. (2011) reported that a high cholesterol level increased MLCK mRNA level in grass carp. These results mentioned above indicated that EA might disrupt structural integrity of animal intestine accompanied by oxidative damage, cell apoptosis and dysfunction of cell-cell tight junction associated with Nrf2, p38MAPK (or JNK) and MLCK signaling pathway, respectively, which is also essential to be investigated.
Grass carp is the largest farmed fish species in the world (FAO, 2018). The large aquaculture scale of grass carp requires more formulated feed for this species. Although the formulated feed of this species could be allowed to use a large amount of rapeseed meal, excessive rapeseed meal supplementation also bring some negative effects (Tan et al., 2013). The negative effect was partly ascribed to anti-nutritional factors (such as EA) existed in rapeseed meal. Thus, our study is the first to focus on assessing the maximum allowable level of EA for grass carp, which could give some references for the proper use of rapeseed meal. And our study further assessed the impacts of EA on the intestinal structural integrity referred to oxidative damage, cell apoptosis and dysfunction of cell-cell tight junction and relevant signaling pathways (Nrf2, p38MAPK and MLCK signaling) in grass carp, which could supply a theoretical basis for exploring the high level of rapeseed meal-inhibited growth of fish.
Section snippets
Trial diet
The basal diet formula is given in Table 1. Fishmeal, casein and gelatin were selected as protein source. Fish oil and soybean oil were selected as lipid source. The planned levels of erucic acid (EA) in six different diets were 0.00%, 0.30%, 0.60%, 0.90%, 1.20% and 1.50% diet, respectively, and the amount of lauric acid was added to guarantee the equal lipid level in each experimental treatment according to Yang and Dick (1994). The actual EA contents of six different diets were 0.00%, 0.29%,
EA inhibited fish growth
Incremental EA level in the diet linearly (P < .001) and quadratically (P < .05) decreased FBW, PWG, SGR, FI, FE, IW and ISI, and linearly (P < .001) decreased IL and ILI (Table 3). In addition, we also measured the residues of EA in three intestinal segments of fish. The residue of EA in the PI of fish increased in a linear and quadratic fashion, and in the MI and DI increased in a linear fashion with incremental EA level in the diet (Table 3).
EA caused the intestinal hyperemia and hyperplasia of intestine villi of fish
We were the first to find intestinal hyperemia of
EA inhibits fish growth
Data from our study showed that EA caused an adverse impact on fish growth, which was in line with Renner et al. (1979), who working with chick found that EA had a negative effect on its weight gain. The growth performance of fish was closely associated with immune function of intestine, which was correlated with its structural integrity (Guo et al., 2017). In addition, previous study performed on rat by Carroll (1962) observed that EA residue was mainly detected in intestine. Similarly,
Conclusion
In summary (Fig. 9), our study found that EA deteriorated the growth performance of on-growing grass carp closely related to the EA residue that disrupted intestinal structural integrity. Furthermore, we were the first to explore the related mechanisms about this detrimental effect of EA on the structural integrity of three intestinal segments of on-growing grass carp, which were as follows: (1) depressing antioxidant ability partly by increasing Keap1a (not Keap1b) expression to suppress Nrf2
Acknowledgments
This research was financially supported by National Key R&D Program of China (2018YFD0900400), the Earmarked Fund for China Agriculture Research System (CARS-45), Outstanding Talents and Innovative Team of Agricultural Scientific Research (Ministry of Agriculture), Foundation of Sichuan Youth Science and Technology Innovation Research Team (2017TD0002), Supported by Sichuan Science and Technology Program (2019YFN0036). The authors would like to thank the personnel of these teams for their kind
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Erucic acid impairs intestinal immune function of on-growing grass carp (Ctenopharyngodon idella)
2020, AquacultureCitation Excerpt :EA is a major ANF in rapeseed meal (Do et al., 2017). Our previous study found that EA could inhibit the growth performance of grass carp (Gan et al., 2019). The inhibited growth of fish is closely related to the occurrence of enteritis (Gu et al., 2018).
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These two authors contributed to this work equally.