Regulation Of Asymmetric Division By CAMP In Meiosis I Of Mouse Oocytes

Asymmetric cell division refers to the process in which a mother cell divides intotwo daughter cells with unequal sized and different developmental potentials.Asymmetric cell division is an essential way to generate cell diversity in multicellularorganisms. Maturation of metazoal (especially mammalian) oocytes is a typicalexample of asymmetric division. Mammalian oocyte undergos first and secondmeiotic division asymmetrically, with separating chromosomes equally each time,finally produces a giant ovum and two tiny little polar bodies to preserve maternalresources for embryonic development. To divide asymmetrically, mammalian oocytesrelocate chromosomes from the center of the cell to the cortex, but little is knownabout the underlying mechanisms.In this study, by using in vitro culturing system, we changed the level ofintracellular cyclic adenosine monophsphate (cAMP) to investigate the role of cAMPduring mouse oocyte maturation. To examine the details of chromosome separationand cytokinesis, live cell imaging was employed to track the maturation process ofmouse oocyte. We combined immunostaining, immunoblotting and micromanipulation to further investigate the mechanism of oocyte asymmetric division. Wefound that: (1) Elevation of intracellular cAMP level induced oocytes dividedsymmetrically and produced two daughter cells with similar size. Inhibition of PKA, acAMP downstream protein kinase, rescued this symmetric division phenotype. (2)Live cell imaging revealed that the symmetrically positioned cleavage furrow causedsymmetric cell division. Detailed analyses demonstrated that symmetrically localizedcleavage furrows were caused by the inappropriate central positioning of chromosomeclusters at anaphase onset, indicating that chromosome cluster migration was impaired.Further investigations revealed that the cortex-oriented chromosome migration wasimpeded in treated oocytes, resulted the centrally localized chromosome at anaphaseinitiation. (3) Chromosome localization at anaphase initiation determined furrowpositioning at telophase start. (4) Microinjection of phosporylated myosin antibodydisrupted the cortex-oriented chromosome migration and asymmetric division,indicating that myosin activity is essential for chromosome migration. (5) Myosinactivity is significantly reduced in cAMP-elevating oocytes. Our results support thehypothesis that cAMP plays a role in regulating asymmetrical cell division by modulating myosin II activity during mouse oocyte meiosis I, providing a novelinsight into the regulation of female gamete formation in mammals.