Distribution and expression of salmon insulin-like growth factor I mRNA: Evidence for three IGF-I prohormones

Our studies of the hormonal regulation of growth in salmonids have focussed on expression of insulin-like growth factor I (IGF-I) mRNA in coho salmon, Oncorhynchus kisutch . RNA was prepared from tissues, and subjected to reverse transcriptase- PCR using coho salmon IGF-I (sIGF-I)-specific primers. Primers were designed to amplify a region from the signal peptide through the E-peptide. RT-PCR of coho parr liver RNA yielded three bands with sizes of 390, 469 (expected), and 510 bp. Partial DNA sequence data show that all three PCR products are authentic sIGF-I transcripts. Furthermore, we have sequenced through an alternative splicing site in the E-peptide, which results in removal of an 81 bp segment, converting the 469 bp band to the 390 bp band. Restriction enzyme data indicate that the 510 bp band differs from the 469 bp band by an insertion in the E-peptide. Thus, it appears that salmon have three pro-IGF-I molecules. RT-PCR analysis shows three sIGF-I mRNAs expressed in whole embryos, as well as liver from juveniles and adults. At least one pro-IGF-I mRNA is expressed in muscle, heart, spleen, kidney, adipose tissue, brain, gill, ovary, and testis.