Abstract
The main objective of this study was to assess the effects of 4 g kg−1 dietary mannan oligosaccharides (MOS) inclusion in soybean oil (SBO)- and fish oil (FO)-based diets on the gut health and skin mucosa mucus production of European sea bass juveniles after 8 weeks of feeding. Dietary MOS, regardless of the oil source, promoted growth. The intestinal somatic index was not affected, however dietary SBO reduced the intestinal fold length, while dietary MOS increased it. The dietary oil source fed produced changes on the posterior intestine fatty acid profiles irrespective of MOS dietary supplementation. SBO down-regulated the gene expression of TCRβ, COX2, IL-1β, TNFα, IL-8, IL-6, IL-10, TGFβ, and Ig and up-regulated MHCII. MOS supplementation up-regulated the expression of MHCI, CD4, COX2, TNFα, and Ig when included in FO-based diets. However, there was a minor up-regulating effect on these genes when MOS was supplemented in the SBO-based diet. Both dietary oil sources and MOS affected mean mucous cell areas within the posterior gut, however the addition of MOS to a SBO diet increased the mucous cell size over the values shown in FO fed fish. Dietary SBO also trends to reduce mucous cell density in the anterior gut relative to FO, suggesting a lower overall mucosal secretion. There are no effects of dietary oil or MOS in the skin mucosal patterns. Complete replacement of FO by SBO, modified the gut fatty acid profile, altered posterior gut-associated immune system (GALT)-related gene expression and gut mucous cells patterns, induced shorter intestinal folds and tended to reduce European sea bass growth. However, when combined with MOS, the harmful effects of SBO appear to be partially balanced by moderating the down-regulation of certain GALT-related genes involved in the functioning of gut mucous barrier and increasing posterior gut mucous cell diffusion rates, thus helping to preserve immune homeostasis. This denotes the importance of a balanced dietary n–3/n–6 ratio for an appropriate GALT-immune response against MOS in European sea bass juveniles.
Highlights
The continuous expansion of the aquaculture sector requires securing access to sustainable feed ingredients for intensively cultured fish feed production [1]
Patterns of mucous cell distribution may be correlated with differential gene expression and be a vital clue to innate immune responses to a variety of pathogens and inflammatory agents
The results of this study have shown that complete replacement of fish oil (FO) by soybean oil (SBO), in addition to the FO provided by the dietary fish meal itself, markedly affects the morphology and physiology of the European sea bass gut, altering the fatty acid profile of gut, down-regulating genes related to both humoral and cellular GALT in the posterior gut, reducing mucous cell density in the anterior gut, and reducing fish growth
Summary
The continuous expansion of the aquaculture sector requires securing access to sustainable feed ingredients for intensively cultured fish feed production [1]. The negative effect of unbalanced dietary n–3/n–6 fatty acids on marine fish immune competence may depend on the VO fatty acid profile, the VO dietary inclusion levels and the specific ability of the fish species to synthesize LC-PUFA [6]. Dietary VOs have been associated with fluctuations on the immune cellular population proportions [10], altered intracellular and extracellular leukocytes killing capacity [9, 10, 12,13,14,15,16,17,18,19], variations in humoral response [9, 12, 15, 17, 18], variations in several physiological processes such as eicosanoids production [4, 9, 20,21,22,23,24,25,26,27], and modulation of signaling molecules and regulators of gene expression [6, 18, 26, 28]. Dietary alterations could stimulate the rate of production of mucous [41,42,43] and its biochemical composition [44], regulating the intestinal dynamic barrier formed by epithelium cells [45]
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