Expression of cyclin-dependent kinase inhibitors (CDKN1, CDKN5) in developmentally competent and incompetent porcine oocytes

Although several reports have been published regarding the cyclin-dependent kinases (Cdks) in developmentally competent mammalian oocytes, still little is known about their expression pattern in developmentally incompetent female gametes. Since Cdks are the main cell cycle division regulators, also described as "checkpoints" in the MI to MII transition, the aim of this study was to investigate the differential mRNA expression of genes encoding Cdkn1 and Cdkn5 in developmentally competent and incompetent porcine oocytes. Porcine cumulus-oocyte complexes (COCs) were collected from crossbred Landrace gilts after slaughter and partly subjected to brilliant cresyl blue (BCB) staining and in vitro maturation (IVM). Three groups of COCs were analysed: (i) oocytes soon after collection without BCB staining, (ii) oocytes which remained colourless after staining (BCB-) and (iii) COCs which stained blue (BCB+). BCB+ COCs were additionally cultured in standard porcine medium (TCM199) for 44 h. All oocytes were analysed using QT-PCR targeting CDKN1 and CDKN5 mRNA expression. The highest CDKN1 mRNA expression was found in oocytes without BCB staining and IVM (Group I) compared to BCB- and BCB+ oocytes, (P < 0.001, P < 0.01, respectively). An increased CDKN5 mRNA level was observed in BCB+ oocytes (Group III) compared to oocytes of Group I and Group II (P < 0.01), respectively. The mRNA expression of both genes was always higher in BCB+ compared to BCB- oocytes. Based on this study it can be supposed that CDKN1 and CDKN5 are differentially expressed in developmentally distinct porcine oocytes and in a maturation stage-dependent manner. Moreover, CDKN1 may play a role as a molecule which regulates cell cycle arrest and the specific "block of maturation" from MI to MII.