Overview of our Research
Recently, we demonstrated that correct mitotic progression was dependent on maintaining a tightly regulated balance between the activities of the phosphatase PP2A, and kinase CDK1. Further, we identified the novel mitotic kinase Greatwall as the master regulator of this balance. These results dramatically altered our understanding of mitosis and opened up several new and exciting research pathways. The primary aim of the lab is to further explore and characterise these pathways, to identify new chemotherapeutic targets and improve the sensitivity and selectivity of existing cancer drugs.
The Cell Cycle
The cell cycle consists of five parts, two ‘active’ phases (S and M), two gap phases (G1 and G2), and a quiescent state (G0). G1 phase involves the synthesis of proteins required for DNA replication and increasing the cell size. This is followed by S phase where the cell makes a complete and identical copy of its genomic DNA. Once replication of the DNA is complete the second gap phase (G2) ensues and the cell prepares itself for mitosis (M), where the DNA condenses to form chromosomes, which are then separated into two identical daughter cells.
What is so Important about the Cell Cycle ?
De-regulation of the cell cycle is a common event in a wide variety of human diseases, such as arthritis, neurodegenerative, cardiovascular disease and of course cancer, which is often described as a “disease of the cell cycle”.
Many of the genes that are commonly deregulated in cancer are critical regulators of the cell cycle (e.g. Rb, p53, ATM, Cdk’s, p16).
What is Mitosis ?
Mitosis is the most active and arguable the most visually exciting phase of the cell cycle.
There are 5 main stages of Mitosis;
1) Prophase, 2) Pro-Metaphase, 3) Metaphse, 4) Anaphase, and 5) Teleophase.
Upon the completion of Mitosis cytokinesis occurs.
Why Study Mitosis ?
Just like the other phases of the cell cycle, many of the genes involved in regulating this critical phase are deregulated in human diseases, especially cancer. Mitosis is an incredibly rapid and complicated orchestra of events that requires absolute precision. Defects during mitosis tightly correlate with chromosomal/genomic instability, a hallmark of cancer, especially the more aggressive and metastatic forms. Normal cells have ‘checkpoints’ that prevent mitotic errors from being passed on by delaying division to allow time to either repair the damage or sacrifice themselves (via apoptosis). In contrast, cancer cells often have defective checkpoints and thus fail to delay. This provides cancer cells with a growth advantage, but it is also a weakness as they are no longer able to respond correctly to particular cellular insults. Thus identifying these defects and targeting them should provide a way to selectively kill cancer cells. In fact, a number of classical chemotherapeutic drugs (e.g. Taxol) work this method to selectively kill cancer cells.
While our knowledge of mitosis has and continues to increase dramatically, it is still incomplete. Furthermore, we still have a long way to go in translating this knowledge into improved therapies.
Greatwall, the Guardian of Mitosis
Greatwall in a stably expressed mitotic kinase, which is primarily localised in the nucleus of cells. However small amounts of the protein are present at the centrosome throughout the cell cycle. During Mitosis, greatwall becomes associated with spindle microtubules.
Knockdown of Greatwall in human cells results in a G2 arrest, as cells are unable to sufficiently inhibit PP2A-B55 activity to allow phosphorylations on Cdk1 substrates to be maintained. However, partial knockdown (around 30-50%) of Greatwall, leads to cells eventually entering mitosis with sub-optimal levels of Cdk1 substrate phosphorylation. This results in a highly aberrant mitosis with numerous defects.