Fish cathelicidins : novel antimicrobial peptides identified in rainbow trout Oncorhynchus mykiss and Atlantic salmon Salmo salar
This thesis reports a novel antimicrobial peptide gene, termed rtCATH_1, found in rainbow trout Oncorhynchus mykiss. The predicted 216-residue rtCATH_1 prepropeptide consists of three domains: a 22-residue signal peptide, a 128-residue cathelin-like region containing two identifiable cathelicidin family signatures, and a predicted 66-residue C-terminal cationic antimicrobial peptide. This predicted mature peptide was unique in possessing features of different known (mammalian) cathelicidin subgroups such as the cysteine bridged family and the specific amino-acid rich family. A 36-residue peptide corresponding to the core part of rtCATH_1 was chemically synthesised and shown to exhibit potent antimicrobial activity. Thus rtCATH_1 represents the first teleost cathelicidin. Further to the finding of rtCATH_1 gene, three more cathelicidin genes were found in salmonids; two in Atlantic salmon, named asCATH_1 and asCATH_2, and one in rainbow trout named rtCATH_2. All the four new salmonid cathelicidin genes share the common characteristics of mammalian cathelicidin genes, such as consisting of four exons, and possessing a highly conserved pre-proregion and four invariant cysteines clustered in the C-terminal region of the cathelin-like domain. Two 36-residue peptides corresponding to the core part of rtCATH_1 and rtCATH_2 were chemically synthesised and shown to exhibit potent antimicrobial activity. The differential expression between rtCATH_1 and rtCATH_2 were studied. rtCATH_2 was expressed constitutively in gill, head kidney, intestine, skin and spleen whilst the expression of rtCATH_1 was inducible in gill, head kidney and spleen after bacterial challenge.