The pro-opiomelanocortin genes in rainbow trout (Oncorhynchus mykiss): duplications, splice variants, and differential expression

Pro-opiomelanocortin (POMC) is a precursor for several important peptide hormones involved in a variety of functions ranging from stress response to energy homeostasis. In mammals and fish, the POMC-derived peptide α-melanocyte stimulating hormone (MSH) is known to be involved in appetite suppression through its interaction with melanocortin-4 receptors. The details of energy homeostasis in fishes are beginning to be elucidated and many of the genes involved in mammalian neuroendocrine signaling pathways are being discovered in fish. In salmonid fishes such as the rainbow trout, genome duplication adds another degree of complexity when trying to compare gene function and homology with other vertebrates. This is true of the POMC gene. Two copies of the POMC gene were previously identified, A and B, presumably resulting from the salmonid duplication. However, while investigating POMC involvement in the feeding response of rainbow trout, a second copy of POMC-A was discovered which is more likely the result of the salmonid duplication and suggests that POMC-B is a duplicate resulting from the earlier teleost duplication prior to tetrapod divergence. The duplicated POMC-A had five deleted amino acids, five inserted amino acids, and 39 amino acid differences from the published POMC-A. In addition to the duplicate POMC-A, a splice variant of the published POMC-A sequence was also identified. Quantitative real-time PCR assays were developed for the different POMC transcripts, and expression was examined in a variety of tissues. Expression of POMC transcripts was highest in the pituitary for all POMC genes, but varied among other tissues for POMC-A1, POMC-A2, POMC-A2s, and POMC-B. POMC-A1 was the only transcript to respond significantly to food deprivation.