| Popis: |
Modulation of basic cell-cycle parameters during development is necessary to ensure spatio-temporal diversity of cell cycle behaviour and impose specific cell cycle constraints during developmental processes. We have used the zebrafish, Danio rerio, to examine the molecular-genetic control of cell cycle diversification in early vertebrate embryos. We have cloned genes encoding four crucial regulators of cell cycle progression including two members of the cdc25 gene family and two of the wee gene family. Together, these genes modify the activity of the Cdk1 protein, a central component of the cell cycle regulatory mechanism. We show that these genes are dynamically expressed during early development, and that the action of one, cdc25a, is rate limiting for cell cycle progression in the early embryo. To further understand how the developmental program imposes constraints on cell cycle behaviour, we have investigated the mechanisms underlying acquisition of cell-cycle diversity at the first cell cycle transition, the midblastula transition (MBT). In zebrafish, and many other organisms, development begins with a rapid and synchronous cell division program with no gap phases between successive rounds of DNA synthesis and mitosis. This early cell cycle program is terminated at the MBT (cycle 10) when zygotic transcription is initiated and gap phases are first observed, resulting in cell cycle lengthening and desynchronization. We demonstrate that overexpression of cdc25a, but not cdc25d is sufficient to largely abolish cell cycle lengthening at the MBT, and that this can be recapitulated by injection of constitutively active cdk1AF, but not cdk2AF. Overexpression of wee1 mRNA causes precocious lengthening of pre-MBT cell cycles. Given the complex expression pattern of cdk phosphoregulators in zebrafish embryos, we hypothesize that Cdk phosphoregulation this may be a general mechanism for ensuring cell cycle diversity in the developing embryo. |
