The goal of our laboratory is to define post-transcriptional mechanisms of cell cycle regulation in the early embryonic cell cycle of Xenopus laevis and in the cancer cell cycle. We study RNA binding proteins and microRNAs, post-transcriptional regulators responsible for regulating the early embryonic cell cycle, and whose expression is disrupted in cancer. Specifically, we are assessing the role of Human antigen R (HuR), cold-inducible RNA binding protein (CIRP) and microRNAs in co-reulating targets implicated in breast cancer etiology.

The long term goal of the Principal Investigator's laboratory is to understand how the cell cycle is regulated during the development of a multicellular organism. Ultimately, we wish to apply this knowledge to developing novel therapies for cancer and genetic diseases. Towards this goal, we are analyzing gap phase and checkpoint addition to the mitotic cell cycles of Xenopus laevis. Gap phases (G1 and G2) are important periods in the adult somatic cell cycle during which correct replication and division of the chromosomes are ensured. The first gap phase (G1) is also when cells grow, and either commit to another round of cell division or exit the cell cycle, to become quiescent or differentiate. Xenopus laevis embryonic cell cycles lack G phases. The first 12 rapid and synchronous divisions alternate between DNA synthesis and mitosis, and are driven by maternal mRNAs and proteins stored in the egg. The cell cycle remodels after the twelfth divison, lengthening as G phases are added during the switch from maternal to zygotic control of development, the maternal to zygotic transition (MZT). After the MZT in Xenopus, an increase in the length of S-phase extends cycles 13 and 14. Cycle 15 resembles a typical adult somatic cell cycle, with G1 and G2 phases. Cell cycle times now range from 3 to 6 hours, differing for subpopulations of cells in specific regions of the embryo. This natural process of G phase and checkpoint addition to the cell cycle is the converse of the pathway normal cells traverse in becoming cancer cells. Thus, cell cycle remodeling (lengthening due to G phase addition) provides a unique opportunity to study the establishment of controls in a normal and homogeneous cell population. Insight into this process may lead to novel strategies not only to combat uncontrolled cell division, but to prevent mutations leading to neoplastic and malignant states. In addition, identification of novel regulatory mechanisms will result in a better understanding of how the acquisition of a regulated cell cycle is integrated with development.

The mechanism of G-phase addition is unknown, but cell cycle remodeling is associated with dramatic changes in cell cycle regulators. We are studying the mechanisms of these changes. Maternal cyclins and cyclin dependent kinases (cdks), like their somatic counterparts, drive progression through the cell cycle. In somatic cell cycles, cyclin E is synthesized during G1, and cyclin E/Cdk2 kinase activity is rate limiting for progression through S phase, after which cyclin E is degraded. Unlike somatic cycles, in embryonic cycles cyclin E1 is present at constant levels, until it is degraded after the twelfth cell cycle, at the MZT. We believe that constant levels of cyclin E1 during the first 12 cycles eliminate G1 phase, and its degradation is necessary for the addition of G1 after the MBT. We are currently determining the mechanism of cyclin E1 stability during the early cycles, as well as its degradation at the MZT using both in vivo expression and in vitro degradation assays. Degradation of maternal cyclins A1 and B2 follows that of cyclin E1, as does the upregulation of zygotic cyclin A2 and Xic1. Cyclin A2 is the Xenopus homolog of somatic cyclin A, and Xic1 is a member of the p27 family of cyclin dependent kinase inhibitors that specifically inhibits cyclin E/Cdk2. Injection of Xic1 before the MZT lengthens the mitotic cell cycles, and delays the degradation of maternal cyclins and the MZT. These results suggest that cyclin E1/Cdk2 regulates cell cycle length and remodeling, as well as the timing of the MZT. We are testing whether the changes in gene expression that occur during cell cycle remodeling are dependent on repression of cyclin E1/Cdk2.

In addition, we are determining the role of translational control in modulating expression of cyclins and cdk inhibitors during cell cycle remodeling. Current studies focus on identifying and isolating novel regulatory elements in the untranslated regions of mRNAs encoding cell cycle regulators and their interacting RNA-binding proteins. Using these approaches, translation of specific cell cycle proteins can be altered in vivo to determine the role of translational control in cell cycle regulation and remodeling. Linking translational control to cell cycle regulation will suggest pathways that may be altered during tumorigenesis as well as therapeutic targets.