Study pronuclear union in interphase using a photo-convertable fluorescence protein - Dendra2

I found this clean study is very interesting. There are so many biological questions left !!!!

PubMed Abstract:

Curr Biol. 2008 Sep 9;18(17):1308-13. Epub 2008 Aug 14.
Cyclin B-cdk1 controls pronuclear union in interphase.

Tachibana K, Hara M, Hattori Y, Kishimoto T.

Laboratory of Cell and Developmental Biology, Graduate School of Bioscience, Tokyo Institute of Technology, Nagatsuta, Midoriku, Yokohama 226-8501, Japan.

In sexual reproduction, the union of the male and female pronuclei occurs in fertilized eggs to mix genetic materials derived from both parents, thereby creating a new genome for the next generation [1-4]. The process leading to pronuclear union consists of pronuclear congression, which depends on astral microtubules derived from sperm centrosome [5-8], and the subsequent pronuclear fusion or karyogamy. The union process progresses in parallel with the first embryonic cell cycle, but the molecular mechanisms involved are poorly understood. Here, we devise a labeling method with Dendra2 to track both pronuclei individually in living starfish eggs. Although pronuclear union naturally proceeds while G1 arrest is released by fertilization and S phase progresses [9], we show that the cell-cycle resumption and progression are not prerequisites for pronuclear union. However, low levels of cyclin B- (but not cyclin A-) Cdk1 activity are detectable even in interphase, and are indispensable for pronuclear union, by contributing at least to pronuclear congression through formation of sperm aster. Pronuclear congression thus requires the activity of M-phase cell-cycle regulator in interphase, independently of the cell-cycle regulation. These findings not only provide a clue to the regulatory aspect of creation of new genome with fertilization, but also reveal a novel role for the M-phase Cdk1 during interphase.

PMID: 18701285

F1000 review:

Jonathon Pines
University of Cambridge, United Kingdom
Cell Biology

In this elegant study, the authors have found an unexpected role for cyclin B-Cdk1 kinase before germinal vesicle breakdown in meiosis. Using a photoconvertible form of GFP (green fluorescent protein) to follow the small male pronucleus at the same time as the female after fertilising starfish oocytes, the authors find that the pronuclei can only migrate towards each other when cyclin B-Cdk1 is active. They go on to show that a small amount of cyclin B-Cdk1 activity is needed for the the sperm centriole to form a microtubule aster and thereby find its way towards the female pronucleus. Thus, it seems that cyclin B1-Cdk1 is more subtly regulated than just the explosive auto-activation that drives nuclear envelope breakdown in meiosis and mitosis, but how it can be partially activated is still unclear.