Submitted to: Journal of Endocrinology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/17/2005
Publication Date: 2/20/2006
Citation: Leder, E.H., Silverstein, J. 2006. The Proopiomelanocortin Genes in Rainbow Trout (Oncorhynchus Mykiss): Duplications, Splice Variants, and Differential Expression. Journal of Endocrinology. 188:355-363. Interpretive Summary: In our efforts to improve feeding and growth in rainbow trout we have been studying the pro-opiomelanocortin (POMC) gene. Several proteins that are known to affect feed intake in mammals are encoded by POMC, such as melanocyte stimulating hormone and endorphin. This POMC gene is first expressed, then translated and then cleaved into several different functional peptides. Two copies of the POMC gene have been described in salmonids, POMC-A and POMC-B. While examining the POMC genes for their role in feed intake regulation, a third form of the POMC gene was identified. It is quite similar to one of the previously identified POMC genes (POMC-A), probably due to an ancestral duplication of the entire salmonid genome. While determining abundance of the different POMC’ (POMC-A1, POMC-A2, and POMC-B) expression levels, another variant of POMC-A2 (POMC-A2 short) was also observed. In this paper the distribution and abundance of the various POMC forms is described. The full length POMC-A2 appears to be the most widely distributed form present in the pituitary gland, all brain tissues, stomach, intestine, ovary, and muscle. Other than the pituitary and the hypothalamus, the shorter POMC-A2 was not significantly present in any tissue. POMC-A1 was abundant in the pituitary, moderately abundant in the hypothalamus, and detectable in the ovary. POMC-B was also highly expressed in the pituitary, but in the hypothalamus, it was not expressed as highly as the POMC-A genes. Regulation of various POMC actions is likely partly achieved by having different transcripts in different tissues.
Technical Abstract: The pro-opiomelanocortin gene (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 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 has added another degree of complexity when trying to determine 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 genome duplication. However, while investigating POMC involvement in the feeding response of rainbow trout, three distinct cDNA transcripts were identified for POMC-A. One of these transcripts is a splice-variant, but the other two appear to be separate genes. This novel POMC was more similar to POMC-A than POMC-B, but the deduced amino acid sequence of the coding sequence had 39 amino acid differences, 5 deleted amino acids, and 5 inserted amino acids. Real-time PCR assays were developed for the different POMC genes and transcripts, and relative expression was examined in a variety of tissues. Relative expression of POMC transcripts varied among tissues for the different POMC genes.