Wilding, Martin Graham; (1995) Calcium and mitosis entry in the early sea urchin embryo. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The cell cycle is characterised by a series of checkpoints that control passage through the individual phases of the cycle. The most well characterised checkpoint at the present is the mitosis entry checkpoint. Although the cell cycle proteins that control passage through this checkpoint are well defined, the precise mechanism that triggers mitosis entry is unclear at the present time. One hypothesis is that a calcium/calmodulin signal forms part of this mechanism. There is evidence for the role of calcium and calmodulin in this process, however this hypothesis has not gained universal acceptance. This is mainly because it has proved difficult to reliably measure calcium transients associated with mitosis entry. I study the role of calcium and calmodulin in mitosis entry, using the early sea urchin embryo as a model system. My results show that the mitosis entry transient is small and often localised, and this may obscure reliable detection of this transient under certain conditions. I also show that the mitosis entry calcium transient is generated by a mechanism involving inositol 1,4,5-trisphosphate, and is independent of the requirement for protein synthesis during the cell cycle. I use a pseudosubstrate inhibitor of calcium-dependent calmodulin signalling to test whether calmodulin has a function in the mitosis entry mechanism. Blocking the action of calmodulin also blocks mitosis entry. I complement these data by using a fluorescent calmodulin probe to measure calmodulin signals during mitosis. Calmodulin signals can be detected during mitosis entry and also during exit and throughout cleavage. The results also show that calmodulin signals are localised throughout mitosis. When taken as a whole, the data presented in this thesis shows that mitosis entry in the first cell cycle of the sea urchin embryo is triggered by localised calcium/calmodulin signals. The data also suggests that the specificity of action of calmodulin during the cell cycle may be controlled by localisation of the calcium/calmodulin signal to specific areas of the cell.

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