Myo-inositol prevents the gill rot in fish caused by Flavobacterium columnare infection

Abstract

The gills are some of the most metabolically active organs in fish and are involved in gas exchange, osmotic regulation, acid-base balance, and ammonia nitrogen waste excretion. Here, we explored the actions of graded levels of dietary myo-inositol [27.0 (un-supplemented), 137.9, 286.8, 438.6, 587.7, and 737.3 mg/kg diet)] on the physical barrier functioning and signaling in the gills of grass carp (Ctenopharyngodon idella) for 10-weeks, and then infected by Flavobacterium columnare. It was found that optimal myo-inositol levels (approximately 137.9–438.6 mg myo-inositol /kg diet): (1) decreased the oxidative products such as malondialdehyde (MDA) and protein carbonyl (PC), while increased the activities and mRNA levels of SOD (CuZnSOD and MnSOD), CAT, GPx (GPx1a, GPx1b, GPx4a and GPx4b), GST (GSTO1, GSTO2, GSTP1 and GSTP2) and GR enzymes, possibly related to the Nrf2/Keap1 signaling pathway, suggesting that myo-inositol inhibited oxidative injury in fish gills; (2) inhibited cell apoptosis related to the down-regulation of caspases-2, −3, −7, −8, and-9 mRNA levels, possibly linked to the inhibition of the JNK/FasL/caspase 8 and p38MAPK/(Apaf-1, Bax, Bcl-2, Mcl-1, IAP)/caspase 9/caspase 3 signaling pathways; (3) promoted cell proliferation by up-regulating cyclins B, D, and E mRNA levels, possibly related to the E2F4 signaling; (4) maintained intercellular structural integrity by up-regulating tight junctional complexes (ZO-1, ZO-2, occludin, and claudins b, −c, f, −3c, −7a, −7b, and − 11), and down-regulating pore-forming TJs (claudins-12,-15a, and-15b) mRNA levels, possibly associated with the down-regulation of MLCK signaling. Based on the inhibition of gill rot and MDA content, the optimal myo-inositol intake was calculated as 285.8 and 305.4 mg/kg diet for grass carp (initial 221 g-final 792 g), respectively.