Catfish hepcidin gene is expressed in a wide range of tissues and exhibits tissue-specific upregulation after bacterial infection

Abstract

Antimicrobial peptides (AMPs) are important components of the host innate immune response against microbial invasion. The cysteine-rich AMPs such as defensin and hepcidin have been extensively studied from various organisms, but their role in disease defense in catfish is unknown. As a first step, we sequenced a hepcidin cDNA from both channel catfish and blue catfish, and characterized the channel catfish hepcidin gene. The channel catfish hepcidin gene consists of two introns and three exons that encode a peptide of 96 amino acids. The amino acid sequences and gene organization were conserved between catfish and other organisms. In contrast to its almost exclusive expression in the liver in humans, the channel catfish hepcidin gene was expressed in a wide range of tissues except brain. Its expression was detected early during embryonic and larval development, and induced after bacterial infection with Edwardsiella ictaluri, the causative agent of enteric septicemia of catfish (ESC) in a tissue-specific manner. The upregulation was observed in the spleen and head kidney, but not in the liver. The expression of hepcidin was upregulated 1–3 days after challenge, but returned to normal levels at 7 days after challenge. The expression profile of the catfish hepcidin gene during the course of bacterial infection mirrors those of inflammatory proteins such as chemokines, suggesting an important role for hepcidin during inflammatory responses.

Introduction

Hepcidin was initially isolated from human plasma ultrafiltrate and urine and shown to possess antimicrobial activities [1], [2]. Subsequent research has demonstrated that it is an iron regulatory molecule involved in hereditary hemochromatosis and anemia of chronic disease [3], [4], [5], that it is part of the acute phase response to infection and inflammation [6], [7], [8], and that its expression changes during hypoxia [7]. Significant overlaps exist between these functions, which were examined together by Nicolas et al. [7]. Hepcidin expression is now understood to increase with the body's iron levels [6] and to be induced by inflammatory cytokine IL-6 during infection and inflammation [9]. The upregulation of hepcidin results in a decrease in circulating iron, increased iron sequestration to reticuloendothelial macrophages, and decreased intestinal iron absorption. Hypoxia and anemia down-regulate hepcidin gene expression and can override the inflammatory upregulation [7]. Several recent editorials and reviews contemplate the significant implications of these findings and provide insight into the role of iron in disease [10], [11], [12].

Hepcidin complementary deoxynucleic acids (cDNAs) have been cloned and characterized from various species including human, mouse, rat, striped bass, medaka, rainbow trout, Japanese flounder, winter flounder, long-jawed mudsucker, Atlantic salmon, and zebrafish [6], [13], [14], [15], [16]. However, hepcidin genes have been characterized only from human, white bass [14], Atlantic Salmon [15], mouse [17], and zebrafish [16]. The antimicrobial activity of hepcidin may lend itself to potential applications in fighting the bacterial and viral outbreaks that plague aquaculture species such as catfish. Toward this end, and as a part of our efforts to elucidate the innate immune components in catfish, we have characterized hepcidin cDNAs from both channel catfish and blue catfish, sequenced and characterized the channel catfish hepcidin gene, and analyzed its expression in various tissues, after bacterial challenge, and during development. The catfish hepcidin gene was also mapped to specific BACs for comparative genome analysis.