Notch pathway regulates mitotic checkpoints in the mitotic-to-endocycle transition in Drosophila follicle cells. H.R. Shcherbata, V. Schaeffer, C. Althauser, H. Ruohola-Baker. Biochemistry, University of Washington, Seattle, WA.

   The Notch pathway controls the follicle cell mitotic-to-endocycle transition in Drosophila oogenesis by stopping the mitotic cycle and promoting the endocycle. Loss of Notch function in the follicle cells or lack of its ligand Delta in the underlying germline disrupt the normal transition, mitotic cycling continues leading to an overproliferation phenotype. We have identified and carried out a functional analysis of genes whose transcription is responsive to the Notch pathway at the transition. These genes include the G2/M regulator Cdc25 phosphatase, String, a regulator of the APC ubiquitination complex Fzr and an inhibitor of the CyclinE/CDK complex, Dacapo. String is repressed in response to Notch activation just before the endocycle transition. However, the prolonged expression of stg alone is not sufficient to keep cells efficiently in mitotic cell cycle past stage 6, suggesting that Notch regulates other cell cycle components in the transition. Through an expression screen we found such a component; Fzr is expressed at mitotic-to-endocycle transition in Notch dependent manner. Mutant clones of fzr revealed that Fzr is dispensable for mitosis but essential for endocycles. We combined the action Stg and Fzr and discovered that ectopically expressed stg in the fzr mutant cells is efficient to keep the cells in mitotic cycle. Furthermore, Fzr is able to block mitosis and initiate precocious endocycling when expressed prematurely. However, the efficiency of this process is low suggesting that components in G1/S checkpoint might play a role. We show that CycE level is a critical factor in mitotic-to-endocycle transition and is controlled by post-translational regulators WD40-protein, Ago and Notch responsive CKI, Dacapo. Lack of ago or ectopic expression of dacapo lead to a halt in cell cycle progression in the transition. These data show that Notch activity executes the mitotic-to-endocycle transition by regulating independently all three mitotic checkpoints.