Cathepsins are lysosomal cysteine proteases belonging to the papain family, whose members play important roles in normal metabolism for the maintenance of cellular homeostasis. Rock bream (Oplegnathus fasciatus) cathepsin H (RbCTSH) cDNAs were identified by expressed sequence tag analysis of a lipopolysaccharide-stimulated rock bream liver cDNA library. The full-length RbCTSH cDNA (1326 bp) contained an open reading frame of 978 bp encoding 325 amino acids. The presence of an ERFNIN-like motif was predicted in the propeptide region of RbCTSH. Furthermore, multiple alignments showed that the EPQNCSAT region was well conserved among other cathepsin H sequences. Phylogenetic analysis revealed that RbCTSH is most closely related to Nile tilapia cathepsin H. RbCTSH was expressed significantly in the intestine, spleen, head kidney and stomach. RbCTSH mRNA expression was also examined in several tissues under conditions of bacterial and viral challenge. All examined tissues of fish infected with Edwardsiella tarda, Streptococcus iniae and red sea bream iridovirus (RSIV) showed significant increases in RbCTSH expression compared to the control. In the kidney and spleen, RbCTSH mRNA expression was upregulated markedly following infection with bacterial pathogens. These findings indicate that RbCTSH plays an important role in the innate immune response of rock bream. Furthermore, these results provide important information for the identification of other cathepsin H genes in various fish species.

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The pH of water is one of the main environmental factors exerting selective pressure on marine and freshwater organisms. Here, we focus on the influence of pH on an organism's ability to maintain homeostasis and investigate the effects of acidification on immunity-related genes and osmotic pressure during early development of the olive flounder, Paralichthys olivaceus. The aim of our study was to determine the influence of various pH levels on the fertilized eggs and larvae of P. olivaceus. Gametes of P. olivaceus were artificially introduced and the resulting fertilized eggs were incubated at pH 4.0 (low), 6.0, and 8.0 (equivalent to natural sea water; control). We found that all eggs sank from the water column at pH 4.0. After 38 h, these eggs showed slow development. Hatching occurred more slowly at pH 4.0 and 6.0 and did not occur at all at pH 4.0. Result of gene expression, caspase and galectin-1 were expressed from the blastula to pre-hatch stages, with the exception of the two-cell stage. HSP 70 was also steadily expressed at all pH levels over the five days. The osmolality of fertilized eggs differed marginally at each stage and across pH levels. So, this results demonstrates that low pH level is detrimental to P. olivaceus fertilized eggs.

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