The Dynamics of the Unreplicated DNA Checkpoint in Xenopus laevis Embryos and Extracts
When unreplicated or damaged DNA is present, cell cycle checkpoint pathways cause cell cycle arrest by inhibiting cyclin-dependent kinases (Cdks). In Xenopus laevis, early embryonic development consists of twelve rapid cleavage cycles between DNA replication (S) and mitosis (M) without checkpoints or gap phases. However, checkpoints are engaged in Xenopus once the embryo reaches the midblastula transition (MBT). At this point, the embryo initiates transcription, acquires gap phases between S and M phases, and establishes a functional apoptotic program. During the cell cycle, there are two main checkpoints that regulate entrance into S and M phases. The focus of this study is the role of protein kinase Chk1 and the phosphatase Cdc25A in the DNA replication checkpoint. In the absence of active Chk1, Cdc25A activates cyclin dependent kinases (Cdks) allowing the cell to progress into S or M phase. Chk1 regulates cell cycle arrest in the presence of unreplicated DNA in somatic cells by phosphorylating Cdc25A and leading to its degradation. Chk1 is also transiently activated at the MBT in Xenopus laevis embryos, even when there is no block to DNA replication or damaged DNA. One goal of this work is to understand the developmental role and regulation of checkpoint signaling pathways due to its monitoring of DNA integrity within the cell.
Chk1 plays a critical but not fully understood role in cell cycle remodeling and early embryonic development. In order to understand the function and regulation of Chk1 in checkpoints, the features of the MBT that activate Chk1 must be identified. The activation of Chk1 by two time-dependent events in the cell cycle, the critical nuclear to cytoplasmic (N/C) ratio and the cyclin E/Cdk2 maternal timer are explored in this study. Embryos treated with aphidicolin, resulting in a halted replication fork and therefore a reduced DNA concentration, were tested for Chk1 activation and Cdc25A degradation. Chk1 and Cdc25A were observed to undergo activation and degradation, respectively, in embryos with a reduced DNA concentration. In addition, embryos were injected with Δ34Xic cyclin E/Cdk2 inhibitor, in order to disturb the maternal timer and tested for Chk1 activation and Cdc25A degradation. Both Chk1 and Cdc25A were unaffected by the disruption of the cyclin E/Cdk2 maternal time in the embryo. Therefore, the N/C ratio and the cyclin E/Cdk2 maternal timer do not affect Chk1 activation and therefore Cdc25A degradation.
Another means of characterizing the unreplicated DNA checkpoint is through the use of mathematical modeling of the checkpoint-signaling cascade of the cell cycle. Mathematical modeling is the translating of biological pathways into mathematical equations that can simulate interactions without performing laboratory experiments. The NovÃ¡k-Tyson checkpoint model made important predictions of hysteresis and bistability in the frog egg checkpoint model, predictions that were later confirmed experimentally. The model was updated with additional interactions, such as those including Myt1, a second inhibitor kinase, and lamin proteins, which become phosphorylated at the onset of nuclear envelope breakdown (NEB) at entry into mitosis. Also, experimental data was fit into the model while maintaining hysteresis and bistability. Therefore, the unreplicated DNA checkpoint model was updated with new interactions and experimental data while still preserving previously identified dynamic characteristics of the system.
As described, Cdc25A regulation is dynamic in the embryo. The checkpoint original model represents the activity of Cdc25 phosphatase on the mitosis promoting factor (MPF) that leads the cell into mitosis. In the checkpoint model, Cdc25C is the phosphatase activating MPF. However, the model does not include Cdc25A, which is an integral part of the checkpoint-signaling pathway due to its role in activating the cyclin/Cdk complex allowing entry into S and possibly M phase. Experimental studies were performed in which Cdc25A levels were reduced in embryos and extracts using Cdc25A morpholinos. Embryos and extracts showed delayed cell cycle and mitotic entry, demonstrating the importance of Cdc25A plays in the cell cycle. Based upon experimental data, the mathematical model of the DNA replication checkpoint was expanded to include Cdc25A. The expanded model should more accurately demonstrate how checkpoints affect the core cell cycle machinery. Cdc25A was incorporated into the model by gathering experimental data and designing a signaling cascade, which was translated into differential equations. The updated model was then used to simulate the effect of synthesis and degradation rates of Cdc25A on the entry into mitosis dynamics. Therefore, using mathematical modeling and experimental design, we can further understand the role that Cdc25A plays in cell cycle progression during development.
Understanding the regulation of Chk1 activity at the MBT and the role of Cdc25A in checkpoint signaling will help us further characterize the dynamics of early embryonic development. The use of mathematical modeling and experimental tools both contribute to further our understanding of controls of the checkpoint signaling pathway and therefore leading us one step closer to truly being able to model a pathway and make predictions as to the behavior of the cell during early embryonic development.