Spatiotemporal expression of transcription factors is crucial for genomic reprogramming. Pou5f1 (Oct4) is an essential transcription factor for reprogramming. A recent study reported that OCT4A, which is crucial for establishment and maintenance of pluripotent cells, is expressed in oocytes, but maternal OCT4A is dispensable for totipotency induction. Whereas another study reported that OCT4B, which is not related to pluripotency, is predominantly expressed instead of OCT4A during early preimplantation phases in mice. To determine the expression states of OCT4 in murine preimplantation embryos, we conducted in-depth expression and functional analyses. We found that pluripotency-related OCT4 mainly localizes to the cytoplasm in early preimplantation phases, with no major nuclear localization until the 8–16-cell stage despite high expression in both oocytes and early embryos. RNA-sequencing analysis using oocytes and early preimplantation embryos could not identify the splice variants creating alternative forms of OCT4 protein. Forced expression of OCT4 in zygotes by the injection of polyadenylated mRNA clearly showed nuclear localization of OCT4 protein around 3–5-fold greater than physiological levels and impaired developmental competency in a dose-dependent manner. Embryos with modest overexpression of OCT4 could develop to the 16-cell stage; however, more than 50% of the embryos were arrested at this stage, similar to the results for OCT4 depletion. In contrast, extensive overexpression of OCT4 resulted in complete arrest at the 2-cell stage accompanied by downregulation of zygotically activated genes and repetitive elements related to the totipotent state. These results demonstrated that OCT4 protein localization was spatiotemporally altered during preimplantation development, and strict control of Oct4 protein levels was essential for proper totipotential reprogramming.
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Atsushi Fukuda, Atsushi Mitani, Toshiyuki Miyashita, Hisato Kobayashi, Akihiro Umezawa, and Hidenori Akutsu
Toshio Hamatani, Mitsutoshi Yamada, Hidenori Akutsu, Naoaki Kuji, Yoshiyuki Mochimaru, Mitsuko Takano, Masashi Toyoda, Kenji Miyado, Akihiro Umezawa, and Yasunori Yoshimura
Mammalian ooplasm supports the preimplantation development and reprograms the introduced nucleus transferred from a somatic cell to confer pluripotency in a cloning experiment. However, the underlying molecular mechanisms of oocyte competence remain unknown. Recent advances in microarray technologies have allowed gene expression profiling of such tiny specimens as oocytes and preimplantation embryos, generating a flood of information about gene expressions. So, what can we learn from it? Here, we review the initiative global gene expression studies of mouse and/or human oocytes, focusing on the lists of maternal transcripts and their expression patterns during oogenesis and preimplantation development. Especially, the genes expressed exclusively in oocytes should contribute to the uniqueness of oocyte competence, driving mammalian development systems of oocytes and preimplantation embryos. Furthermore, we discuss future directions for oocyte gene expression profiling, including discovering biomarkers of oocyte quality and exploiting the microarray data for ‘making oocytes’.