Foxo3 negatively regulates the activation of mouse primordial oocytes
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ORIGINAL ARTICLE
Foxo3 negatively regulates the activation of mouse primordial oocytes Yayoi Kashiwagi • Mohammad Moniruzzaman Takashi Miyano
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Received: 21 January 2012 / Accepted: 16 April 2012 / Published online: 29 April 2012 Japan Society for Reproductive Medicine 2012
Abstract Purpose The objective of this study is to know the role of Foxo3, a forkhead transcription factor, in the growth initiation of primordial oocytes in neonatal mice. Methods We studied the expression of Foxo3 in 0-, 1-, 2-, 7- and 21-day-old mouse ovaries by immunohistochemistry. Ovaries from 1-day-old mice were treated with Foxo3 siRNAs (small interfering RNAs) and subsequently organcultured for 6 days, and the oocyte growth was examined histologically. Results Expression of Foxo3 was low in newborn mouse ovaries. In 1-day-old ovaries, Foxo3 was expressed in the nuclei of 20 ± 7 % primordial oocytes. The percentage of Foxo3-positive primordial oocytes was increased to 48 ± 8, 37 ± 2 and 47 ± 4 in 2-, 7- and 21-day-old mice, respectively. After treatment of ovaries with Foxo3 siRNAs, higher proportion of oocytes entered the growth phase in cultured ovaries than that in control. Conclusions These results suggest that Foxo3 negatively regulates the growth initiation of primordial oocytes and knockdown of Foxo3 leads primordial oocytes to the growth phase in vitro.
Y. Kashiwagi M. Moniruzzaman T. Miyano (&) Graduate School of Agricultural Science, Kobe University, Nada-ku, Kobe 657-8501, Japan e-mail: [email protected] Present Address: M. Moniruzzaman Department of Animal Science, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh
Keywords Foxo3 Mouse Oocyte Organ culture Primordial follicle
Introduction After differentiation of primordial germ cells to oogonia, they undergo mitotic proliferation and enter meiosis to become oocytes. In mice, these oocytes are surrounded by a single layer of flat-shaped pre-granulosa cells around the time of birth. This unit, consisting of an oocyte and granulosa cells, is called the primordial follicle and the oocytes in primordial follicles are called primordial oocytes [1]. Activation of primordial oocytes causes their growth initiation and the transformation of their surrounding granulosa cells to cuboidal shape [2]. The follicles at this stage are called primary follicles that contain growing oocytes. The granulosa cells proliferate and become multilayered to form secondary follicles. As follicles develop through the primary, secondary and antral stages, they gain successive layers of granulosa cells and theca layers, and oocytes increase the size towards 70–75 lm in rodents and 120–125 lm in humans, cows and pigs. Finally, a large fluid-filled antral cavity is formed, and the follicles are called antral follicles. Primordial follicles start to develop at the time of birth in mice [3]. A small proportion of primordial oocytes enter the growth phase, while a large number are quiescent. The mechanism regulating this selection of primordial oocytes are not well understood.
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