Bora regulates meiotic spindle assembly and cell cycle during mouse oocyte meiosis.

Bora is the binding partner of Aurora A, which is required for its activation and phosphorylation of Polo like kinase 1 (Plk1), and is involved in the spindle assembly and progress of the cell cycle during mitosis. In this study, we examined the expression, localization, and function of Bora during mouse oocyte meiosis. The expression level of Bora was increased during oocyte meiotic maturation, with an elevated level at metaphase. Immunofluorescence analysis showed that Bora was concentrated as a dot shortly after germinal vesicle breakdown (GVBD), associating first with the surrounding chromosomes and then with the spindle throughout oocyte meiotic maturation. Further experiments confirmed that Bora co-localized with α-tubulin at prometaphase/metaphase, but dissociated from α-tubulin at anaphase/telophase. In metaphase-II-arrested oocytes, Bora was evenly distributed in the cytoplasm after treatment with a microtubule-depolymerizing agent, or recruited to the spindle after treatment with a microtubule-polymerizing agent, indicating that Bora was physically connected to the meiotic spindle and α-tubulin at metaphase. Furthermore, inhibition or depletion of Bora by either anti-Bora antibody or Bora siRNA microinjection significantly reduced the rates of GVBD and inhibited first polar body extrusion; caused morphologically defective spindles and misaligned chromosomes; arrested maturing oocytes at prometaphase/metaphase-I stage, or left oocytes and their first polar bodies with severely misaligned chromosomes and defective spindles; and/or caused the disappearance of Aurora A and Plk1 at the spindle. These results indicated that Bora acts as a critical regulator of Aurora A and Plk1, and is involved in microtubule organization during oocyte meiosis.