Iron-Regulatory Function of Hepcidin in the Channel Catfish and Western Clawed Frog
Type of DegreeDissertation
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Hepcidin, originally identified as a 25 amino acid (aa) antimicrobial peptide made in the liver, is a small peptide hormone that has been shown to be the long-sought regulator of iron metabolism in humans and mice. Although closely related hepcidin genes and peptides have been identified in other mammals, amphibians and various fish species, it is still largely unknown whether hepcidin can regulate iron metabolism in fish and amphibians. In the current study, it was found that in channel catfish hepcidin transcript levels in the liver were increased by 4, 19, and 22 fold at 4, 24, and 48 hours post Edwardsiella ictaluri challenge, respectively. However, augmented hepcidin expression in the gut and olfactory sac was detected only at 48 h post infection. In naturally occurring anemic fish, the concentration of serum iron, total iron binding capacity, and liver iron content were half of that in healthy controls. The levels of hepcidin transcript in the livers of catfish affected by anemia were only 14% of that of healthy catfish. Correlation analysis indicated that hepatic hepcidin transcript levels correlated significantly with serum iron concentrations (r = 0.54, P < 0.05) and with the percent saturation of transferrin (r = 0.63, P < 0.05). In Xenopus tropicalis study, we demonstrated for the first time that hepcidin may regulate iron metabolism in amphibians. Two hepcidin genes were identified in the X. tropicalis frog but only tHEP2 was responsive to iron loading, indicating a possible role of tHEP2 in the regulation of iron homeostasis. In contrast, tHEP1 was most likely involved in the host defense in response to corticosterone. The study on the promoter areas of the two hepcidin genes supports such notions. These preliminary data suggested that X. tropicalis could be a promising animal model which allows us to separate some features of the regulation of hepcidin expression at the transcriptional level. The phylogenetic study revealed the co-evolution of hepcidin and its receptor ferroportin. Multiple Sequence Analysis (MSA) demonstrated that charged residues (E, D, H, K, and R) within external segments (ESs) are most likely involved in the hepcidin-ferroportin interaction. In this study, highly conserved external segments (ES1-4, and ES6) were predicted to play a critical role during the binding between hepcidin and ferroportin. On the other hand, the most variable external segment ES5 was proposed to be responsible for the endurance of ferroportin to hepcidin variation.