T1R1 expression in obscure puffer (Takifugu fasciatus) is associated with effect of dietary soybean antigenic protein on intestinal health

The effects of soybean antigenic protein on the gustatory receptor T1R1 mRNA expression and intestinal immune response were evaluated in the obscure puffer (Takifugu fasciatus; initial weight 39.84 ± 3.09 g). Fish were fed with eight different diets for two months; four of these were practical diets with either a high level of fish meal (FMH2), a low level of fish meal (FML3), a diet where FML was partially (18.6%) replaced by fermented soybean meal (FMLF4), and a diet where FML was partially (18.6%) replaced by soybean meal (FMLS5). Four other semi-purified diets were offered that contained different levels (0%, 5%, 8%, and 12.5%) of soybean antigenic proteins (AP0, AP5, AP8, and AP12.5, respectively). Feeding rate and intestine histological structure were investigated and the expression of IL-1mRNA, IL-10mRNA, and TNFmRNA in the intestine, HSP70mRNA in the liver, and T1R1mRNA in the tongue and intestine in four practical-diet groups and two semi-purified-diet groups (AP0% and AP8%) were also determined. The obtained results show feeding rate increased gradually as levels of soybean antigenic protein among the practical-diet groups, while the feeding rate of semi-purified-diet groups (AP5-AP12.5%) decreased and then increased with increasing soybean antigenic protein level. The expression level of T1R1mRNA in the tongue was higher than in the intestine; while they showed the same tendency, FMLS group was significantly higher than the control FML, AP8 group was significantly higher than AP0 group, and significantly lower than FMLS group (containing 8.8% soybean antigenic proteins similar with AP8 group). Soybean antigenic proteins did not affect the width of the intestine folds or the thickness of the muscular layer; however, they caused a difference in the height of intestine folds: higher soybean antigenic protein levels in the semi-purified-diet group led to lower height of the intestine folds with mild injuries in the intestine. The expression level of IL-1mRNA, IL-10mRNA and TNFmRNA in the intestine showed the same tendency as T1R1mRNA; and the expression level of HSP70 of FMLS group in the liver was significantly lower than in the control FML. No significant difference was found among most indexes between FMLF and FML. In conclusion, single or interacting antigenic proteins and other anti-nutritional factors may change the expression of gustatory receptor T1R1mRNA in both the intestine and tongue by affecting the intestinal health, and thus affected feeding. Excessive soybean antigenic proteins resulted in intestinal damage, therefore fermented soybean meal is better for partly replacement of fish meal than unfermented soybean meal.