Cell regulation and the mitotic checkpoint

We use cultured human cells to study how they divide and multiply. To achieve this we use both molecular and biochemical approaches together with fluorescence imaging techniques to study protein function in both living and fixed cells. Our current work is focused on the study of the mitotic checkpoint (or spindle assembly checkpoint) and the relationship between the mitotic checkpoint and cell survival/cell death.

The Mitotic Checkpoint

We are interested in how a cell segregates its chromosomes into two identical sets at mitosis and the consequences for the cell if it fails to do so. The accurate segregation of chromosomes during anaphase is essential for the accurate transfer of genetic information during each cell division. Mistakes in this process result in the generation of cells with either extra or missing chromosomes (aneuploidy), which then have the potential to form cancer cells.

To avoid mistakes in chromosome segregation, cells have evolved a signalling mechanism called the mitotic checkpoint or spindle assembly checkpoint. Normally, cells only enter anaphase after a bipolar mitotic spindle has been assembled and all of the chromosomes have attached to the spindle through their kinetochores. However, the presence of a damaged spindle or a single unattached chromosome is sufficient to activate the spindle checkpoint which then signals metaphase arrest and so prevents premature chromosome segregation.

Our goal is to understand the biochemical and molecular basis of the spindle assembly checkpoint. Components of the spindle checkpoint such as the MAD (Mitotic Arrest Deficient) and BUB ( udding Unhibited by Benzimidazole) genes were initially isolated in the budding yeast. We have also isolated a homologue of Mad2 in humans (HsMad2A) and a related protein HsMad2B. While Mad2A plays a role in monitoring the attachment of kinetochores to the spindle microtubules and activating the spindle checkpoint by binding directly to unattached kinetochores, the function of Mad2B remains unknown. We are currently using a wide range of molecular, biochemical and cell biology techniques to analyse the role of Mad2B in the mammalian cell cycle.

The Mitotic Checkpoint and Apoptosis

A second aspect of the spindle checkpoint that we study is the cell's response to spindle checkpoint activation. In addition to causing cell cycle arrest, checkpoint-activated signalling pathways are also involved in the regulation of both cell survival and cell death.

Many anti-microtubule, anti-cancer drugs such as taxol (Paclitaxel), which is used clinically in the treatment of breast and ovarian cancer, activate the spindle checkpoint and cause the cell to arrest in mitosis. These mitotically-arrested cells eventually undergo programmed cell death or apoptosis.

However, our studies indicate that chemotherapeutic drugs also activate cell survival signals in addition to cell death signals. Therefore, our current studies are aimed at understanding the biochemical events that determine the fate of the cell in response to the activation of the spindle checkpoint. The analysis of these biochemical events will increase our understanding of the mechanism of action of anti-cancer drugs and more importantly, by identifying and suppressing the drug-induced survival signals, enable us to improve the effectiveness of anti-cancer agents.