Dietary phosphorus deficiency caused alteration of gill immune and physical barrier function in the grass carp (Ctenopharyngodon idella) after infection with Flavobacterium columnare

Abstract

Phosphorus (P) is an important and essential mineral for fish. To investigate the effects of dietary P on gill immune and physical barrier function in grass carp (Ctenopharyngodon idella), fish were fed six graded levels of dietary available P (0.95–8.75 g/kg diet) for 60 days, and then sampled 3 days after a challenge test (exposure to water containing Flavobacterium columnare; 1.0 × 108 colony-forming units (cfu)/ml for 3 h). P deficiency resulted in a decline in gill antimicrobial compound production, and liver-expressed antimicrobial peptide 2A (LEAP-2A), LEAP-2B, hepcidin and β-defensin-1 transcript abundance. P deficiency also decreased target of rapamycin transcript and protein expression and increased nuclear factor kappa B p65 mRNA expression, which because of their association with anti-inflammatory and pro-inflammatory cytokine gene expression, respectively, suggesting that P deficiency increased post-infection gill inflammation and could impair immune barrier function. Furthermore, P deficiency: (1) Enhanced Fas ligand /caspase-8 death receptor and Bcl-2associated X protein/apoptotic protease activating factor-1/caspase-9 mRNA expression (which is indicative of increased apoptosis). (2) Inhibited Kelch-like-ECH-associated protein 1b/NF-E2-related factor 2 mRNA expression, decreased the expression of antioxidant enzyme transcript levels and activities (with the exception of CuZnSOD mRNA expression), and increased ROS, malondialdehyde and protein carbonyl levels. This data suggests that protection against ROS-related damage was compromised with P deficiency. (3) Up-regulated myosin light chain kinase transcript levels, a response that was associated with the down-regulation zonula occludens 1 (but not ZO-2), occludin and claudin (except claudin-12) mRNA levels. This latter result suggesting that tight junction functional integrity (and thus, gill physical barrier function) was impaired. These data, combined with the fact that P deficiency led to opercular deformation and gill rot syndrome, suggest that insufficient dietary levels of this mineral (i.e. below ~5.75 mg/kg P) could increase the susceptibility of fishes to water borne pathogens.