Discovery of chicken microRNAs associated with lipogenesis and cell proliferation.

The primary function of microRNA (miRNA, a class of small regulatory RNA) is to regulate gene expression. Studies of miRNA in mammals suggest that many liver-associated miRNAs are expressed, with a wide range of functions. To characterize miRNA expressed in the avian liver, we created two small RNA libraries from embryonic chick livers at embryonic day (E)15 and E20, a time at which the embryo begins to grow rapidly and so its energy demands increase. It is of interest to examine miRNAs expressed at these developmental stages because miRNAs involved in regulating metabolic pathways and cell proliferation are likely to be identified. The small RNA libraries were sequenced with 454 Life Sciences deep sequencing. Of the 49,937 sequences obtained, 29,390 represented known chicken miRNAs and 1,233 reads represented homologous miRNAs that have not been previously identified in chickens. Additionally, 1,032 reads represented 17 potential novel miRNAs not previously identified in any species. To further investigate the possible functions of avian liver miRNAs we identified the potential targets of two differentially expressed novel miRNAs, nc-miR-5 and nc-miR-33. These two miRNAs were predicted to target metabolic genes, including the lipid metabolism-associated gene fatty acid synthase (FAS), and genes involved in the control of cell proliferation, such as peroxisome proliferator-activated binding protein (Pparbp) and bone morphogenetic protein 4 (BMP4). Our findings demonstrate that a diverse group of miRNAs are expressed in developing avian livers. In addition, some of the identified miRNAs have been suggested to play a key role(s) in regulating metabolic pathways.