CDK control of meiotic chromosome segregation
In meiosis, two consecutive rounds of nuclear division called meiosis I and meiosis II follow a single round of DNA replication to produce haploid gametes. During meiosis I, homologous chromosomes segregate. Meiosis II resembles mitosis in that sister chromatids split. The establishment of meiosis-specific chromosome segregation pattern requires three changes that modulate how chromosomes interact with each other and with the microtubule cytoskeleton: (1) reciprocal recombination between homologous chromosomes, (2) the way linkages between sister chromatids, known as sister-chromatid cohesion, are removed from chromosomes and (3) the manner in which chromosomes attach to the meiotic spindle.
We discovered a fourth key event essential for proper meiotic chromosome segregation: temporal restriction of microtubule-kinetochore interactions (Miller and Ünal et al., 2012). We found that microtubule-kinetochore interactions must be inhibited during prophase I to allow for two key aspects of MI chromosome morphogenesis: the assembly of a protected centromeric chromatin domain and the association of factors with sister kinetochores to enable attachment to microtubules emanating from the same pole (coorientation). When microtubules prematurely interact with kinetochores, sister kinetochores attach to microtubules emanating from opposite poles (biorientation). This geometry, in turn, interferes with meiosis I chromosome morphogenesis and segregation, eventually leading to the transformation of meiosis I into a mitosis-like division. We further defined the mechanism whereby premature microtubule-kinetochore interactions are inhibited prior to meiosis I. The inhibition occurs through regulation of cyclin-CDK activity and of outer kinetochore assembly.
Interestingly, a similar type of regulation also exists in mice, where during meiosis I, CDK1 activity gradually increases through prometaphase and metaphase I. Premature increase in CDK1 activity leads to precocious formation of stable microtubule-kinetochore attachments and eventually lagging chromosomes at anaphase I (Davydenko et al., 2013). This finding strongly indicates that the phenomenon we initially discovered in budding yeast appears to be an evolutionarily conserved mechanism, whereby the regulation of CDK1 activity in meiosis I acts as a timing mechanism to allow stable microtubule-kinetochore interactions only after bipolar spindle formation, thus preventing attachment errors.
Cyclin-CDKs differentially regulate microtubule-kinetochore interactions
Intriguingly, we found that misexpression of only a subset of cyclins causes stable microtubule-kinetochore interactions in prophase I. The different phenotypes do not correlate with total cyclin-CDK activity, strongly indicating substrate specificity among the meiotic cyclin-CDKs towards inducing stable microtubule-kinetochore attachments. Using a combination of biochemical, mass spectrometry and genetic approaches we aim to delineate the molecular mechanisms by which cyclin-CDKs regulate microtubule-kinetochore interactions during meiosis.
Protection of centromeric cohesin in meiosis I requires additional regulatory events
In meiosis I, cleavage of cohesin from the chromosome arms allows homologs to segregate. However, cohesin around the centromeres is protected from cleavage by Separase, as this cohesin pool is essential for accurate segregation of sister chromatids during meiosis II. Protection of centromeric cohesin is accomplished by preventing phosphorylation of Rec8, the meiosis-specific cohesin subunit. This occurs, at least in part, by Sgo1 (MEI-S332)-dependent recruitment of the protein phosphatase PP2A to centromeric regions where it antagonizes Rec8 phosphorylation.
We found that premature CLB3 expression causes increased Rec8 phosphorylation around centromeres and thus override the protection of centromeric cohesin during meiosis I. However, Sgo1/PP2A localization, which is thought to be the sole requirement for centromeric cohesin protection, is not affected by premature CLB3 expression. Our results reveal that the protection of centromeric cohesin requires additional events, which we aspire to delineate.