Ecdysone signaling induces two phases of cell cycle exit in Drosophila cells

During development, cell proliferation and differentiation must be tightly coordinated to ensure proper tissue morphogenesis. Because steroid hormones are central regulators of developmental timing, understanding the links between steroid hormone signaling and cell proliferation is crucial to understanding the molecular basis of morphogenesis. Here we examined the mechanism by which the steroid hormone ecdysone regulates the cell cycle in Drosophila. We find that a cell cycle arrest induced by ecdysone in Drosophila cell culture is analogous to a G2 cell cycle arrest observed in the early pupa wing. We show that in the wing, ecdysone signaling at the larva-to-puparium transition induces Broad which in turn represses the cdc25c phosphatase String. The repression of String generates a temporary G2 arrest that synchronizes the cell cycle in the wing epithelium during early pupa wing elongation and flattening. As ecdysone levels decline after the larva-to-puparium pulse during early metamorphosis, Broad expression plummets, allowing String to become re-activated, which promotes rapid G2/M progression and a subsequent synchronized final cell cycle in the wing. In this manner, pulses of ecdysone can both synchronize the final cell cycle and promote the coordinated acquisition of terminal differentiation characteristics in the wing

NuA4 and Ecdysone Impact Cell Cycle Gene Expression to Regulate the Proper Timing of Cell Cycle Exit in Drosophila.

During organogenesis, many cells undergo a process called terminal differentiation, where cells acquire their final fates and begin to perform essential physiological functions. Terminal differentiation is often accompanied by a transition to a non-dividing state, called cell cycle exit. As development proceeds, many cells exit the cell cycle, and most cells in mature organisms remain in a non-dividing, post-mitotic state. Despite its prevalence, how the transition from a proliferative to a post-mitotic state is induced and maintained is not fully understood. In this thesis, I examine two aspects of cell cycle gene regulation that are essential for the proper transition of Drosophila epithelial cells from a proliferative to a post-mitotic state during development. In part 1 I describe a novel, critical role for the NuA4 chromatin remodeling and histone exchange complex in promoting the proper timing of cell cycle exit in vivo. Unlike other chromatin binding factors that repress cell cycle genes, NuA4 does not directly inhibit cell proliferation. NuA4 instead suppresses a previously unknown intrinsic DNA damage response that occurs during late S-phase. My work revealed that suppression of this endogenous DNA damage response is required in vivo to properly coordinate S and G2 cell cycle progression with differentiation and cell cycle gene expression during tissue development. In part 2, I examine how steroid hormone signaling plays a central role in coordinating the timing of cell cycle exit. Pulses of the steroid hormone ecdysone trigger a cascade of gene expression changes required to coordinate cell maturation with differentiation and cell cycle exit in insects. Here I reveal that ecdysone signaling in the Drosophila wing induces a transcription factor Broad that represses the expression of an essential mitosis promoting gene, the cdc25c phosphatase, to cause a G2-phase cell cycle arrest. After the ecdysone pulse, as Broad expression declines, the cdc25c phosphatase becomes re-expressed, which in turn promotes rapid mitotic entry into a synchronized final cell cycle and cell cycle exit. Altogether, my work has revealed two essential signaling pathways that impact cell cycle gene expression and cell cycle progression to properly coordinate cell cycle exit with terminal differentiation during development.PHDMolecular, Cellular, and Developmental BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135751/1/kerryann_1.pd

Ecdysone signaling induces two phases of cell cycle exit in Drosophila cells

During development, cell proliferation and differentiation must be tightly coordinated to ensure proper tissue morphogenesis. Because steroid hormones are central regulators of developmental timing, understanding the links between steroid hormone signaling and cell proliferation is crucial to understanding the molecular basis of morphogenesis. Here we examined the mechanism by which the steroid hormone ecdysone regulates the cell cycle in Drosophila. We find that a cell cycle arrest induced by ecdysone in Drosophila cell culture is analogous to a G2 cell cycle arrest observed in the early pupa wing. We show that in the wing, ecdysone signaling at the larva-to-puparium transition induces Broad which in turn represses the cdc25c phosphatase String. The repression of String generates a temporary G2 arrest that synchronizes the cell cycle in the wing epithelium during early pupa wing elongation and flattening. As ecdysone levels decline after the larva-to-puparium pulse during early metamorphosis, Broad expression plummets, allowing String to become re-activated, which promotes rapid G2/M progression and a subsequent synchronized final cell cycle in the wing. In this manner, pulses of ecdysone can both synchronize the final cell cycle and promote the coordinated acquisition of terminal differentiation characteristics in the wing