Effects of exportin 1 on nuclear transport and meiotic resumption in porcine full-grown and growing oocytes

Exportin 1 (XPO1) is a nuclear transport receptor involved in the nuclear export of majority proteins in somatic cells. In mammalian oocytes, however, only the presence of XPO1 has been reported at mRNA and protein levels, and the definitive functions of XPO1 and its effects on the meiotic maturation of oocytes have never been directly examined. In the present study, the expression state and the nuclear-export function of porcine XPO1 were analyzed in porcine oocytes. In addition, we investigated the effects of the overexpression and inhibition of XPO1 on meiotic regulation in full-grown and growing oocytes by mRNA injection and inhibitor treatment. Endogenous XPO1 was stably expressed in porcine oocytes during the germinal vesicle (GV) stage, and the expression of exogenous XPO1 significantly decreased the nuclear localization of XPO1 cargos, snurportin 1, and WEE1B. Inhibition of XPO1 by a specific inhibitor, leptomycin B, delayed the GV breakdown (GVBD), whereas the overexpression of XPO1 by mRNA injection accelerated the GVBD. XPO1 overexpression overcame the meiotic arrest induced by WEE1B expression in full-grown oocytes. Surprisingly, the GVBD of porcine growing oocytes, which could not resume meiosis by the maturation culture in vitro, was induced by the expression of exogenous XPO1. These results showed the presence of XPO1 and its function as a nuclear export receptor in mammalian oocytes, including growing oocytes, and they suggest that the regulation of nuclear transport has a large influence on the GV maintenance and meiotic resumption of oocytes.

Contributions of UBE2C and UBE2S to meiotic progression of porcine oocytes

Vertebrate oocytes arrested at the first meiotic prophase must proceed to the second meiotic metaphase (MII) before fertilization. This meiotic process requires the precise control of protein degradation. Part of the protein degradation in oocytes is controlled by members of the ubiquitin-conjugating enzyme family, UBE2C and UBE2S, which are known to participate in mono-ubiquitination and poly-ubiquitination, respectively. Although UBE2 enzymes have been well studied in mitosis, their contribution to mammalian oocyte meiosis is relatively unknown and has been studied only in mice. Here, we investigated the contribution of UBE2C and UBE2S to porcine oocyte maturation using an RNA injection method. Overexpression of UBE2S prevented MII arrest of oocytes and led to the formation of a pronucleus (PN) at 48 h of culture. This effect was also observed for prolonged cultures of UBE2C-overexpressing oocytes, suggesting the effectiveness of poly-ubiquitination in the rapid escape from M-phase in porcine oocytes. Although the inhibition of either UBE2C or UBE2S by antisense RNA (asRNA) injection had no effect on oocyte maturation, asRNA-injected oocytes showed inhibited PN formation after parthenogenetic activation. These results indicated that ubiquitination of certain factors by UBE2S and UBE2C plays a role in the escape from MII arrest in porcine oocytes. Further investigations to identify the factors and how mono- and/or poly-ubiquitination contributes to protein degradation could provide a better understanding of UBE2 roles in oocyte maturation.

Analyses of the involvement of PKA regulation mechanism in meiotic incompetence of porcine growing oocytes

Mammalian growing oocytes (GOs) lack the ability to resume meiosis, although the molecular mechanism of this limitation is not fully understood. In the present study, we cloned cDNAs of cAMP-dependent protein-kinase (PKA) subunits from porcine oocytes and analyzed the involvement of the PKA regulation mechanism in the meiotic incompetence of GOs at the molecular level. We found a cAMP-independent high PKA activity in GOs throughout the in vitro culture using a porcine PKA assay system we established, and inhibition of the activity by injection of the antisense RNA of the PKA catalytic subunit (PKA-C) induced meiotic resumption in GOs. Then we examined the possibility that the amount of the PKA regulatory subunit (PKA-R), which can bind and inhibit PKA-C, was insufficient to suppress PKA activity in GOs because of the overexpression of two PKA-Rs, PRKAR1A and PRKAR2A. We found that neither of them affected PKA activity and induced meiotic resumption in GO although PRKAR2A could inhibit PKA activity and induce meiosis in cAMP-treated full-grown oocytes (FGOs). Finally, we analyzed the subcellular localization of PKA subunits and found that all the subunits were localized in the cytoplasm during meiotic arrest and that PKA-C and PRKAR2A, but not PRKAR1A, entered into the nucleus just before meiotic resumption in FGOs, whereas all of them remained in the cytoplasm in GOs throughout the culture period. Our findings suggest that the continuous high PKA activity is a primary cause of the meiotic incompetence of porcine GOs and that this PKA activity is not simply caused by an insufficient expression level of PKA-R, but can be attributed to more complex spatial-temporal regulation mechanisms.