A tough fishing game to separate mother and father chromosomes

It is known that the frequency of chromosome segregation errors is high during meiosis I in mammalian oocytes. By recording chromosome and kinetochore dynamics at high resolution in live mouse oocytes, we succeeded to track all kinetochores and chromosomes throughout meiosis I. Systematic and quantitative analysis of the datasets revealed that chromosome biorientation is highly error-prone, possibly explaining why chromosome segregation is so erroneous in mammalian oocytes.

HFSP Long-Term Fellow Tomoya Kitajima and colleagues
authored on Mon, 26 September 2011

To maintain the genetic information, cells must segregate the chromosomes equally into the daughter cells during cell division. However, it is known that the frequency of unequal segregation during meiosis I in mammalian oocytes is extremely high compared to other cell divisions. Such failure leads to generation of aneuploid eggs. Fertilization of aneuploid eggs in humans causes pregnancy loss and, if survived to term, results in severe genetic diseases such as Down's syndrome (trisomy 21).

 

Figure: The prometaphase belt. Chromosomes (purple) and kinetochores (green) during meiosis I in a mouse oocyte. Kinetochore positions (white spots) are tracked over time (color-coded lines).

To achieve equal chromosome segregation, kinetochore pairs must be pulled by microtubules from the opposite spindle poles by the end of metaphase, establishing stable chromosome biorientation. Unlike somatic cells, mammalian oocytes lack centrosomes, which would predefine two spindle poles. Instead, they have over 80 cytoplasmic microtubule organizing centres (MTOCs), which self-assemble  the bipolar spindle during meiosis I. The detailed process of chromosome biorientation and how it is coordinated with the acentrosomal spindle assembly were unclear.

In this study, we imaged kinetochore and chromosome dynamics throughout meiosis I in live mouse oocytes. To achieve high spatial and temporal resolution, we used an automated confocal microscope that can focus and track the region of interest within the oocyte. By processing the images with an in-house-developed computational pipeline, we tracked all kinetochores throughout the entire process of meiosis I, providing the first complete kinetochore tracking datasets throughout cell division.

Quantitative analysis of the datasets revealed that chromosomes congress to the spindle equator before biorientation. Chromosome congression formed a previously overlooked belt-like configuration, which we named 'the prometaphase belt'. The chromosomes start biorientation attempts on the prometaphase belt. Some of the bi-orienting chromosomes move to the central region of the belt, transforming the prometaphase belt into the well-known structure metaphase plate. The increase of bioriented chromosomes coincides with bipolar spindle formation, suggesting a functional link between the two processes.

Our complete kinetochore tracking datasets allowed us to analyze chromosome biorientation systematically for each individual chromosome. We found that almost 90% of chromosomes fail to establish stable biorientation by the initial attempt, because the kinetochores are incorrectly attached by microtubules. These incorrect kinetochore-microtubule attachments are corrected depending on Aurora kinase. On average two rounds of error correction per chromosome are required for establishing a stable biorientation. These results indicate that chromosome biorientation is highly error-prone during meiosis I in oocytes, which may explain the high incidence of aneuploid eggs in mammals including humans.

Reference

Complete Kinetochore Tracking Reveals Error-Prone Homologous Chromosome Biorientation in Mammalian Oocytes.  Kitajima, T. S., Ohsugi, M., and Ellenberg, J. (2011). Cell 146, 568–581.

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Commentary in Cell

Commentary in Biology of Reproduction