Fringe-dependent notch activation and tramtrack function are required for specification of the polar cells in Drosophila oogenesis.

During Drosophila oogenesis, each egg chamber is encapsulated through the coordinated signaling of multiple pathways, resulting in the formation of polar cells at the termini and a row of stalk cells in between each egg chamber. Notch signaling is required for specification of a precursor group containing both stalk and polar cells. Together, the Notch and JAK/STAT pathways specify the stalk cells as well as a group of prepolar cells, from within that group. The mechanism by which the polar cells differentiate from the prepolar group involves apoptosis, but the pathways which control that process are largely unknown. We now demonstrate that Notch signaling, activated by Delta and transduced by the transcription factor Tramtrack, is involved in the process of refining the prepolar cell group to two polar cells. The glycosyltransferase Fringe is expressed and required cell-autonomously in prepolar cells for this process. However, the transcription factor Mirror, which inhibits fringe expression in other tissues and stages of development, as well as Serrate, one of the two known ligands for Notch, are not required for maturation of prepolar cells. This finding suggests that Fringe is necessary for generating positional information in localizing a high-affinity interaction between Notch and its ligand Delta, even if a second ligand is not essential.

The mitotic-to-endocycle switch in Drosophila follicle cells is executed by Notch-dependent regulation of G1/S, G2/M and M/G1 cell-cycle transitions.

The Notch signaling pathway controls the follicle cell mitotic-to-endocycle transition in Drosophila oogenesis by stopping the mitotic cycle and promoting the endocycle. To understand how the Notch pathway coordinates this process, we have identified and performed a functional analysis of genes whose transcription is responsive to the Notch pathway at this transition. These genes include the G2/M regulator Cdc25 phosphatase, String; a regulator of the APC ubiquitination complex Hec/CdhFzr and an inhibitor of the CyclinE/CDK complex, Dacapo. Notch activity leads to downregulation of String and Dacapo, and activation of Fzr. All three genes are independently responsive to Notch. In addition, CdhFzr, an essential gene for endocycles, is sufficient to stop mitotic cycle and promote precocious endocycles when expressed prematurely during mitotic stages. In contrast, overexpression of the growth controller Myc does not induce premature endocycles but accelerates the kinetics of normal endocycles. We also show that Archipelago (Ago), a SCF-regulator is dispensable for mitosis, but crucial for endocycle progression in follicle epithelium. The results support a model in which Notch activity executes the mitotic-to-endocycle switch by regulating all three major cell cycle transitions. Repression of String blocks the M-phase, activation of Fzr allows G1 progression and repression of Dacapo assures entry into the S-phase. This study provides a comprehensive picture of the logic that external signaling pathways may use to control cell cycle transitions by the coordinated regulation of the cell cycle.

Notch-dependent Fizzy-related/Hec1/Cdh1 expression is required for the mitotic-to-endocycle transition in Drosophila follicle cells.

During Drosophila oogenesis, Notch function regulates the transition from mitotic cell cycle to endocycle in follicle cells at stage 6. Loss of either Notch function or its ligand Delta (Dl) disrupts the normal transition; this disruption causes mitotic cycling to continue and leads to an overproliferation phenotype. In this context, the only known cell cycle component that responds to the Notch pathway is String/Cdc25 (Stg), a G2/M cell cycle regulator. We found that prolonged expression of string is not sufficient to keep cells efficiently in mitotic cell cycle past stage 6, suggesting that Notch also regulates other cell cycle components in the transition. By using an expression screen, we found such a component: Fizzy-related/Hec1/Cdh1 (Fzr), a WD40 repeat protein. Fzr regulates the anaphase-promoting complex/cyclosome (APC/C) and is expressed at the mitotic-to-endocycle transition in a Notch-dependent manner. Mutant clones of Fzr revealed that Fzr is dispensable for mitosis but essential for endocycles. Unlike in Notch clones, in Fzr mutant cells mitotic markers are absent past stage 6. Only a combined reduction of Fzr and ectopic Stg expression prolongs mitotic cycles in follicle cells, suggesting that these two cell cycle regulators, Fzr and Stg, are important mediators of the Notch pathway in the mitotic-to-endocycle transition.