The role of spindle dynamics in segregating chromosomes in mammalian oocytes and embryos

One of the major causes of pregnancy miscarriage, and also the cause of downs syndrome, is a defect in the woman?s egg which means that the egg has too few or too many chromosomes. This defect arises just before the egg is fertilised, when chromosomes which are no longer required by the egg are separated and discarded in a special cell division called meiosis. Errors in this process are thought to occur in about 10-30% of eggs, and become much more likely as the woman gets older. However, it is not known why this is so common in eggs, or why the problem gets worse as women get older. Our approach is to try to understand how chromosomes are normally separated in eggs, to understand why errors are so common. We are beginning to understand that the forces which physically separate the chromosomes are generated differently in eggs than in other cells. We want to better understand this difference between eggs and other cells, to find out whether this might be part of the reason why errors are very common in eggs.

Errors in chromosome segregation during oocyte maturation are a major cause of pregnancy loss and mental retardation. However, little is known about how chromosomes are normally segregated in mammalian oocytes, or why the oocyte is so prone to errors. This project will translate some of the considerable advances that have recently been made in somatic cells and in-vitro systems in understanding the molecular mechanisms underpinning chromosome segregation into a clinically relevant system in the mammalian oocyte. In addition, we will use mouse oocytes and embryos to answer questions about anaphase mechanisms which have not been possible to answer with other model systems.

During anaphase chromosomes move towards opposite spindle poles virtue of two complementary features of microtubule dynamics. Poleward flux describes the continuous poleward movement of spindle microutbules, couples to ?ve end (pole end) depolymerisation. Pacman describes +ve end (kinetochore end) depolymerisation which causes chromosomes to ?chew? towards the pole, and is the major component of anaphase chromosome movement in somatic cells. We have established assays for measuring poleward flux and chromosome segregation in oocytes, and our preliminary data suggest that poleward flux is the main driver of chromosome segregation in the egg, and that pacman is not yet active.

The overall aim of this research is to examine the mechanisms which drive chromosome segregation in oocytes in order to be better placed to understand the reasons for chromosome missegragation.

We will address this aim by focussing on four specific questions:
1. What mechanisms drive poleward flux in mouse oocytes?
2. What is the role of poleward flux in oocytes?
3. When during development is pacman activated, and how is this switch regulated?
4. What is the contribution of poleward flux to chromosome segregation in human oocytes?