Catfish express two forms of insulin-like growth factor-I (IGF-I) in the brain. Ubiquitous IGF-I and brain-specific IGF-I

Insulin-like growth factor-I (IGF-I) is expressed not only in liver, but also in brain and other tissues. This ubiquitous IGF-I has a complex pattern of expression due to multiple transcription start sites, polyadenylation sites and exon skipping. We have isolated a cDNA encoding a brain-specific IGF-I from a catfish brain cDNA library. Also, a fragment encoding ubiquitous IGF-I was amplified from brain and liver mRNA and the deduced protein shown to be distinct (66% sequence identity) from brain-specific IGF-I. Consistent with other IGF-I prepropeptides, the brain-specific IGF-I has a 43-residue signal peptide followed by B, C, A, D, and E domains. Retained in the catfish brain-specific IGF-I peptide are residues predicted to be involved with the correct tertiary folding, disulfide linkages, and receptor binding. Northern blot analysis of poly(A+)-rich mRNA from brain indicated a single 1600-base pair transcript; a band was not detected from mRNA of liver, stomach, pancreas, pituitary, blood, herring brain, or brain poly(A-) RNA. A sensitive reverse transcriptase/polymerase chain reaction assay also showed that brain-specific IGF-I mRNA was expressed solely in the Thai catfish brain but not liver, stomach, pancreas, pituitary, ovary, and African catfish brain.