RINGO, a protein with no homology to cyclin B, has been reported to be involved in activation of CDC2 and regulation of meiotic maturation in Xenopus oocytes. Although the presence of homologues of RINGO families, which are known as SPDY families, has been reported in mammals, their roles in meiotic maturation of mammalian oocytes have never been examined. In the present study, the effects of SPDY on meiotic maturation of porcine oocytes were examined. At first, Xenopus RINGO (xRINGO) mRNA was injected into immature porcine oocytes and found to significantly accelerate CDC2 activation and meiotic resumption. The CCNB (also known as cyclin B) synthesis was prematurely started at 12 h of culture, whereas it started at 18 h in normal oocytes. We next cloned RINGO A2 homologue in pig (pigSPDYA2) from total RNA of immature porcine oocytes by RT-PCR and obtained full-length cDNA that was more than 85% and 40% homologous with mammalian SPDYA2 and xRINGO, respectively. Acceleration effects similar to those by xRINGO were observed in CDC2 activation, meiotic resumption, and the start of CCNB synthesis in pigSPDYA2 mRNA-injected porcine oocytes. In clear contrast with the effects of xRINGO, which was accumulated abnormally in porcine oocytes and arrested them in the first meiotic metaphase (M1), pigSPDYA2 accelerated the meiotic progression, with about half of pigSPDYA2 mRNA-injected oocytes completing meiotic maturation within 30 h. These results suggest that pigSPDYA2 has important roles on meiotic maturation of porcine oocytes and that the rapid degradation of SPDY was necessary for the normal maturation of oocytes.
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