Identification of pluripotency genes in the fish medaka

Stem cell cultures can be derived directly from early developing embryos and indirectly from differentiated cells by forced expression of pluripotency transcription factors. Pluripotency genes are routinely used to characterize mammalian stem cell cultures at the molecular level. However, such genes have remained unknown in lower vertebrates. In this regard, the laboratory fish medaka is uniquely suited because it has embryonic stem (ES) cells and genome sequence data. We identified seven medaka pluripotency genes by homology search and expression in vivo and in vitro. By RT-PCR analysis, the seven genes fall into three groups of expression pattern. Group I includes nanog and oct4 showing gonad-specific expression; Group II contains sall4 and zfp281 displaying gonad-preferential expression; Group III has klf4, ronin and tcf3 exhibiting expression also in several somatic tissues apart from the gonads. The transcripts of the seven genes are maternally supplied and persist at a high level during early embryogenesis. We made use of early embryos and adult gonads to examine expression in stem cells and differentiated derivatives by in situ hybridization. Strikingly, nanog and oct4 are highly expressed in pluripotent blastomeres of 16-cell embryos. In the adult testis, nanog expression was specific to spermatogonia, the germ stem cells, whereas tcf3 expression occurred in spermatogonia and differentiated cells. Most importantly, all the seven genes are pluripotency markers in vitro, because they have high expression in undifferentiated ES cells but dramatic down-regulation upon differentiation. Therefore, these genes have conserved their pluripotency-specific expression in vitro from mammals to lower vertebrates.