The product of the Drosophila polo gene is a serine/threonine protein kinase which is involved in mitosis. Drosophila
polo is related to the yeast CDC5 gene. The human homolog of the polo gene was cloned by Lake and
Jelinek (1993). The human polo-like kinase (PLK)
gene encodes a 603-amino acid polypeptide.
Smith et al. (1997) stated that Plk localizes to the mitotic spindle, suggesting that PLK is involved in regulating mitotic
spindle function. They showed that microinjection of PLK mRNA induced mitosis in quiescent NIH 3T3
cells.
Ovarian function
Oogenesis, Oocyte maturation
Comment
Tong C, et al reported that
Polo-like kinase-1 is a pivotal regulator of microtubule assembly during mouse oocyte meiotic maturation, fertilization, and early embryonic mitosis.
In this experiment, the changes in Plk1 expression were detected in mouse oocytes through Western blotting. The subcellular localization of Plk1 during oocyte meiotic maturation, fertilization, and early cleavage as well as after antibody microinjection or microtubule assembly disturbance was studied by confocal microscopy. The quantity of Plk1 protein remained stable during meiotic maturation and decreased gradually after fertilization. Plk1 was localized to the spindle poles of both meiotic and mitotic spindles at the early M phase and then translocated to the middle region. At anaphase and telophase, Plk1 was concentrated at the midbody of cytoplasmic cleavages. Plk1 was concentrated between the male and female pronuclei after fertilization. Plk1 disappeared at the spindle region when microtubule formation was inhibited by colchicine or staurosporine, while it was concentrated as several dots in the cytoplasm after taxol treatment. Plk1 antibody injection decreased the germinal vesicle breakdown rate and distorted MI spindle organization. Our results indicate that Plk1 is a pivotal regulator of microtubule organization during mouse oocyte meiosis, fertilization, and cleavage and that its functions may be regulated by other kinases, such as staurosporine-sensitive kinases.
Karaiskou A, et al reported that Polo-like kinase confers MPF autoamplification competence to growing Xenopus oocytes.
During oogenesis, the Xenopus oocyte is blocked in prophase of meiosis I. It becomes competent to resume meiosis in response to progesterone at the end of its growing period (stage VI of oogenesis). Stage IV oocytes contain a store of inactive pre-MPF (Tyr15-phosphorylated Cdc2 bound to cyclin B2); the Cdc25 phosphatase that catalyzes Tyr15 dephosphorylation of Cdc2 is also present. However, the positive feedback loop that allows MPF autoamplification is not functional at this stage of oocyte growth. We report that when cyclin B is overexpressed in stage IV oocytes, MPF autoamplification does not occur and the newly formed cyclin B-Cdc2 complexes are inactivated by Tyr15 phosphorylation, indicating that Myt1 kinase remains active and that Cdc25 is prevented to be activated. Plx1 kinase (or polo-like kinase), which is required for Cdc25 activation and MPF autoamplification in full grown oocytes is not expressed at the protein level in small stage IV oocytes. In order to determine if Plx1 could be the missing regulator that prevents MPF autoamplification, polo kinase was overexpressed in stage IV oocytes. Under these conditions, the MPF-positive feedback loop was restored. Moreover, we show that acquisition of autoamplification competence does not require the Mos/MAPK pathway.
Expression regulated by
Comment
Ovarian localization
Oocyte, Luteal cells, Stromal cells
Comment
Takai N et al examined the features
of PLK expression in the mouse placenta and ovary using immunohistochemical studies.
PLK was not detectable by
immunohistochemistry in ovarian follicles.
PLK expression seemed to be correlated with the expression of proliferation
cellular nuclear antigen (PCNA) in many placental and ovarian cells,
reflecting a role in cellular proliferation. Nevertheless, in ovarian stroma
and lutein bodies where PCNA was not expressed, PLK was strongly expressed.
Follicle stages
Comment
Plk1 expression, subcellular localization, and possible functions during rat oocyte meiotic maturation, fertilization, and embryonic cleavages were studied by Fan HY, et al using RT-PCR, Western blot, confocal microscopy, drug-treatments, and antibody microinjection. Both the mRNA and protein of this kinase were detected in rat maturing oocytes and developing embryos. Confocal microscopy revealed that Plk1 distributed abundantly in the nucleus at the germinal vesicle (GV) stage, was associated with spindle poles during the formation of M-phase spindle, and was translocated to the spindle mid-zone at anaphase. In fertilized eggs, Plk1 was strongly stained in the cytoplasm between the apposing male and female pronuclei, from where microtubules radiated. Throughout cytokinesis, Plk1 was localized to the division plane, both during oocyte meiosis and embryonic mitosis. The specific subcellular distribution of Plk1 was distorted after disrupting the M-phase spindle, while additional aggregation dots could be induced in the cytoplasm by taxol, suggesting its intimate association with active microtubule assembly. Plk1 antibody microinjection delayed the meiotic resumption and blocked the emission of polar bodies. In conclusion, Plk1 may be a multifunctional kinase that plays pivotal regulatory roles in microtubule assembly during rat oocyte meiotic maturation, fertilization, and early embryonic mitosis