Research

Chromosome Segregation during Mitosis and Meiosis

Our research aims to understand the control of chromosome segregation during both mitosis and meiosis. Accurate chromosome segregation relies on the cohesin complex that holds newly duplicated chromosomes together from the time of their replication to the time of their segregation. Each chromosome carries a specialized feature, known as the centromere, upon which the multi-subunit kinetochore assembles to enable attachment to microtubules. In preparation for segregation, pairs of duplicated chromosomes line up and their kinetochores attach to microtubules emanating from opposite poles of the cell. A surveillance mechanism known as the spindle checkpoint monitors the proper attachment of chromosomes to microtubules. Cohesin resists the pulling forces of microtubules enabling tension to be generated. Once all chromosomes have come under tension, signalling their correct attachment (biorientation) the checkpoint is silenced and cohesin is destroyed, triggering the segregation of chromosomes to opposite poles.

 

 

 

 

 

Cohesin is not present uniformly along chromosomes but is most highly enriched in the region surrounding the centromere, called the pericentromere. The pericentromere is also where the Shugoshin protein is localized, which is essential both for regulating the timing of cohesin loss during meiosis and as part of the surveillance mechanism monitoring tension between chromosomes. The pericentromere is therefore plays critical roles in ensuring the accuracy of chromosome segregation.

We aim to uncover the specialized roles of the pericentromere in chromosome segregation during mitosis and meiosis. We want to know what factors set up the specialized chromatin domain surrounding the centromere and what its roles are in mitosis and meiosis. To find this out, we use two genetically tractable and distantly related yeasts: budding yeast (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) as model systems to uncover conserved principles. To identify meiosis-specific proteins in vertebrates, we use frog eggs (oocytes). Our approach is broad and includes genetics, molecular biology, cell biology, live cell imaging, chromatin immunoprecipiation, biochemistry and proteomics. 

Our research can be broadly divided into three questions:

1. How does the centromere set up a cohesin-rich pericentromere?

The pericentromere, the region surrounding the centromere is essential for accurate chromosome segregation during mitosis and meiosis. A conserved feature of the pericentromere is that it attracts high levels of cohesin, the protein complex that holds newly duplicated chromosomes together until they separate during cell division. Pericentromeric cohesin plays unique roles in chromosome segregation and, accordingly, specialized mechanisms regulate its establishment and loss. 

We found that a conserved kinetochore subcomplex, known as the Ctf19 complex, is important for pericentromeric cohesion enrichment and accurate chromosome segregation during mitosis and meiosis in budding yeast. We are currently addressing the mechanism by which the Ctf19 complex enables cohesin recruitment to the surrounding chromatin.

(2) How does the pericentromeric protein, Shugoshin, ensure the accuracy of segregation?

Shugoshins are conserved pericentromeric proteins that play critical roles in chromosome segregation during mitosis and meiosis. In mitosis, Shugoshin halts the cell cycle in response to a lack of tension between sister chromatids. Futhermore, Shugoshin must be inactivated once this has occured to allow cell cycle progression. Our goals are both to determine the mechanism by which Shugoshin ensures the proper biorientation of chromosomes and uncover its mode of regulation.

We are also interested in understanding how shugoshin spatially regulated cohesin loss during meiosis. Cohesin (Rec8) is lost from chromosome arms during meiosis I but retained at centromeres until meiosis II. We are investigating the mechanism of this step-wise loss of cohesin, which also occurs during mammalian mitosis.

(3) How is chromosome segregation modified for meiosis?

During meiosis, haploid gametes are produced from a diploid progrenitor cell. To achieve this, following the duplication of the chromosomes, two consecutive chromosome segregation events occur, meiosis I and meiosis II. During meiosis I, paternal and maternal chromosomes, or homologs, are segregated. During meiosis II, sister chromatids are segregation. This requires many changes to the chromosome segregation machinery. We want to know how these changes are put in place. 

 

Our goal is to determine the modifications to the chromosome segregation machinery that occur during meiosis and uncover their molecular function. We are performing genomic and proteomic screens to identify new factors involved in these processes. We are also analyzing the role of known mitotic segregation proteins and cell cycle regulators in the specialized segregation that occurs during meiosis.

We purified meiotic kinetochores and used biophysical methods to determine how sister chromatids, co-migrate during meiosis I, the basis of Mendelian segregation.

 

We used a live cell assay to show that kinetochore proteins are important to prevent recombination near centromeres. This has provided insight into a long-known phenomenon, defects in which are known to be associated with birth defects.