Effects of Long-Term Exposure to Cd2+ on the Common Carp (Cyprinus Carpio) HO Gene Family and Intestinal Immune Mechanisms

Abstract

\n Heavy metal Cadmium (Cd2+) is an environmental pollutant that poses serious health hazards. Due to the increasing contamination of aquatic systems with Cd2+, the increased accumulation of Cd2+ in fish has become a food safety and public health concern. Heme oxygenase(HO) is an important antioxidant enzyme that plays a key role in defending the body against oxidative damage, but less research has been done in common carp. In this study, 6 HO genes were identified in the common carp genome database. Comparative genomics analysis showed considerable expansion of the HO genes and verified the four-round whole genome duplication (WGD) event in common carp. Phylogenetic analysis revealed all HO genes of common carp were clustered into orthologous groups indicating the highly conservative during evolution. In addition, the results of tissue distribution showed that most HO genes had extensive tissue distribution and showed tissue-specific expression patterns. Exposure to 0.5 mg/L Cd2+ significantly reduced the expression of TGF-β and IL-10 in common carp, which may indicate that Cd2+ exposure can destroy the physical barrier function of the intestine, inhibit intestinal immune defense and induce intestinal inflammation. In order to find a suitable concentration of Bacillus coagulans that could activate HO-1 genes and the immunity of the organism, we investigated the changes of HO-1 gene expression levels in the intestinal tract of common carp under Cd2+ stress at 30d and 60d by adding different concentrations of B. coagulans to the feed. The results showed that L2 upregulated HO-1 gene expression at different infection time points compared to the control/stressed group, but L1 and L3 almost inhibited the expression of most HO-1 genes. This result suggests that the optimum concentration of 2.0´108 CFU/g for activation of HO-1 gene expression by B. coagulans when subjected to Cd2+ stress may have an important role in organism resistance to oxidative stress and immune response. Also compared to the Cd2+ stressed group, the addition of B. coagulans to the diet provoked an upregulation of IL10 and TGF-β gene expression in almost all periods and reduced the ability to suppress intestinal immune defenses in L1, L2 and L3, with the L2 group showing the best induction level. Therefore, the addition of appropriate concentrations of B. coagulans may be a good way to activate HO-1 genes and reduce oxidative damage.