The general aim of our research
projects is to gain a better understanding of cell cycle regulation in meiosis.
We aim at elucidating the mechanisms of chromosome segregation during female
meiosis to understand why it goes wrong so frequently. Chromosomes are held
together through a protein complex named Cohesin, which has to be removed in a
two-step manner during the two meiotic divisions. We study how the centromere
associated pool of Cohesin is removed only in meiosis II. Chromosome
segregation in meiosis also requires a specific orientation of chromosomes in
meiosis I, and sister chromatids in meiosis II. The second aim of our projects
is to elucidate how the so-named spindle assembly checkpoint, which prevents
precocious Cohesin removal, recognizes and promotes correct attachments in
meiosis. A third aim is to gain a better understanding of cell cycle regulation in oocyte meiosis, by studying oocyte-specific roles of A- and B-type cyclins. The model system we are mainly using are mouse
oocytes, using genetically modified mouse strains and sophisticated
imaging approaches on live and fixed oocytes. Additionally we use
synchronized Xenopus laevis oocytes for
biochemical approaches not feasible in the mouse.
- 1) Deprotection of Centromeric Cohesin
in Meiosis II
Chromatid pairs are segregated in
meiosis I, and single chromatids in meiosis II. To maintain chromatid pairs
together throughout meiosis I, centromeric Cohesin has to be protected from proteolytic
cleavage by Separase through Shugoshin 2 (Sgo)- dependent recruitment of PP2A.
PP2A is a phosphatase that dephosphorylates a meiosis-specific subunit of the
Cohesin complex and thereby prevents Cohesin removal by Separase. PP2A and Sgo2
are localised to the centromeres in meiosis I , but strikingly, they also
co-localize in meiosis II. Therefore, localisation of Sgo2 and PP2A alone
cannot explain why centromeric Cohesin is protected in meiosis I, and why this
protection is lost in meiosis II ("deprotected"). We have found that the kinase Mps1 is required for Sgo2 dependent protection f centromeric cohesin. Furthermore we have identified I2PP2A and Cyclin A2 as being essential for deprotection of
centromeric Cohesin in oocytes. Our current projects aim
at identifying the signaling pathways required for protection and deprotection in female meiosis
- 2) Spindle Assembly Checkpoint Control
Chromosome segregation in mitosis and
meiosis depends on the activity of the APC/C (anaphase promoting
complex/cyclosome), an E3 ubiquitin ligase that targets key cell cycle
regulators for degradation. Degradation of Cyclin B and Securin is required for
full activation of the protease Separase. Separase cleaves one subunit of the
Cohesin complex (Scc1 in mitosis, Rec8 in meiosis) to allow the separation of
paired sister chromatids in mitosis, and paired chromosomes in meiosis I.
Activation of Separase depends on the inactivation of the Spindle Assembly
Checkpoint or SAC. The SAC verifies that correct tension-generating attachments
to the bipolar spindle are present on kinetochores. In the presence of
erroneous attachments, metaphase to anaphase transition is prevented through
the inhibition of the APC/C.
Chromosome attachment in meiosis I is
fundamentally different from mitosis: two sister chromatids are oriented
side-by-side towards the same pole (monopolar attachment), and not towards
opposite poles (bipolar attachment). Nevertheless, we and others have been
able to show that the SAC is active in meiosis I and components of the mitotic
SAC are essential for generating oocytes of the correct ploidy. Our
projects aim at elucidating the role of the SAC as oocytes progress from meiosis I into meiosis II. We address the role of SAC and the
molecular link between error correction and SAC control in
meiosis I and II.
- 3) Cell Cycle Control in Oocyte Meiosis
In meiosis, two cell divisions without intervening S-phase take place. Using mouse and X. laevis oocytes we are interested in understanding how cell cycle progression is regulated during the two divisions in oocytes, and how Separase is inactivated between meiosis I and II. We use both mouse and frog oocytes to determine the role of A-and B-type cyclins for meiotic progression. Furthermore we study how the meiotic cohesin Rec8 is recognized and cleaved by Separase in mouse oocyte meiosis I and II.