Identification of Oog1 Promoter Regions in Mouse Growing Oocytes.

Tissue specific promoter is useful tool for various fields from research on the gene function to genetic therapies. Analyses of male germ cell specific promoter have often been conducted by using transgenic technology; on the other hand, a few oocyte-specific promoters have been reported in mammals. Some oocyte-specific promoters have been described to work in growing oocytes such as Zp3 and Gdf9 promoter. However, no report has been seen on the promoter which works specifically in female germ cell at earlier stages of oogenesis. We previously reported the characteristics of Oog1 in female germ cells and embryos. Oog1 is an oocyte-specific gene and its expression starts in oocytes at E15.5, the time of the entry into meiotic arrest, continuing subsequently to the 2-cell stage. Identification of the promoter that functions at early stages of oocytes would help to analyse the mechanism of oogenesis and early embryogenesis, because the maternal mRNAs and proteins accumulated in the oocytes during oogenesis control oocyte quality and early embryo development. In the present study, we tried to analyze the promoter regions of Oog1. We selected 3 upstream regions (0.7 kb, 2.7 kb and 3.9 kb) of Oog1 coding sequence on the basis of the information of 5 copies of the gene on chromosome 4 and 12, and produced 3 kinds (6 lines) of transgenic mice having different Oog1 upstream region (0.7 kb, 2.7 kb and 3.9 kb) followed by GFP gene. The observation of GFP signal revealed that the longer 2 of 3 upstream regions functioned specifically in oocytes, but the shortest one did not function at all. GFP signals start to be observed in the oocytes of primary follicles in the transgenic lines with 3.9 kb promoter, while the fluorescence starts to be observed in the oocytes of secondary follicles of 2.7 kb promoter transgenic line. 3.9 kb promoter has 2 NOBOX binding element (NBE) which previously reported to promote oocyte-specific gene expression. The result of our experiment suggests that NBE strongly enhances the gene expression in growing oocytes. However, even in the transgenic mice with the longest promoter, GFP signal was not observed in oocytes of primordial follicles or germline cysts. Furthermore, comparison analysis of the upstream regions of 4 copies of Oog1 genes except that of Nm_178657 revealed that the homologous regions expand longer than 3.9 kb. Considering these results, there might be some regulatory elements in distal promoter regions in addition to the oocyte-specific core promoter.