QualificationsPh.D., University of Washington, Genetics, 1987. Expertise and Research InterestsThe development of any animal or plant requires a highly regulated program of cell growth and proliferation. Dr. Edgar's research group studies the genetic logic underlying Drosophila's cell proliferation program, aiming to characterize the different types of cell cycles that occur during development, identify critical regulators of cell growth and proliferation, and understand how these are in turn controlled by developmental programming and environmental factors such as nutrition and infection. We use classical and molecular genetics, cytological assays of cell growth and proliferation, gene expression profiling, and forward genetic screens. We seek to define new genes and mechanisms involved in growth control that will impact general paradigms in cell and developmental biology, and which have relevance to medical conditions involving cell and tissue growth including cancer diagnosis and therapy, stem cell biology and regeneration, wound healing, and metabolic diseases such as diabetes and obesity. Theme 1: Cell Growth and the Endocycle Objective: To understand how cell cycle progression is coordinated with cell growth, using endocycling cells. Overview: Endocycles are simplified cell cycles in which DNA Synthesis and Gap phases alternate without intervening mitoses. Endocycles are used by plants and animals to facilitate post-mitotic cell growth, and are so ubiquitous that they account for about half the earth's biomass. A notable characteristic of Drosophila's endocycles is that their progression is tightly coupled to rates of cell growth. Hence, we are studying these cycles as a model in which to understand the coordination of cell growth with cell cycle progression. We find that endocycles in Drosophila's salivary glands employ a novel biphasic oscillator in which E2F promotes cyclin E (cycE) transcription, CycE/Cdk2 triggers S-phase, and S-phase causes the destruction of E2F via a mechanism similar to that used to destroy the PreRC factor, Cdt1 (Fig 1). Periodic destruction of E2F is essential for endocycle progression, just as periodic destruction of G2-Cyclins is essential for progression of mitotic cell cycles. Furthermore, we find that parameters that alter protein synthesis, such as nutrition or TOR activity, regulate E2F protein levels, and thereby make endocycle progression growth-dependent. Our working hypothesis is that E2F is translationally regulated, and a current objective is to test this idea and determine the underlying mechanism. Many of the regulatory interactions essential to Drosophila's growth-dependent endocycle oscillator are broadly conserved, suggesting that elements of this mechanism may be used in many growth-dependent cell cycles. A long-term objective is to evaluate this view by characterizing plant endocycles, using Arabidopsis genetics. Theme 2: Developmental Control of Cell Cycle Exit Objective: To understand the genetic controls that terminate cell proliferation upon terminal differentiation, using the fly wing and eye. Overview: How are growth and form controlled in development? The regulation of cell proliferation is central to this problem, and the mechanisms controlling cell cycle exit at differentiation are particularly relevant. This project addresses the mechanism of cell cycle exit at differentiation in the Drosophila wing and eye. We have discovered that cell cycle exit involves a "double assurance" mechanism in which the activities of a critical transcription factor, E2F, and a G1 Cyclin/CDK complex, CycE/Cdk2, are independently and dominantly silenced. Bypassing this mechanism by co-expression of E2F and CycE promotes the indefinite proliferation of terminally differentiated cells in vivo. Interestingly, supra-physiological levels of E2F and CyclinE/Cdk2 are required to drive proliferation in differentiating cells. Since the critical targets of both E2F and Cdk2 reside on chromatin at transcriptional promoters and origins of replication, we like the idea that differentiating cells express factors that shield these targets from E2F binding and Cdk2 phosphorylation. To identify the factors that mediate cell cycle exit, we are performing F1 genetic screens. These employ an E2F-responsive pcna-white+ reporter gene to identify, using eye color, loss- and gain-of-function mutations that allow persistent E2F activity in the differentiating Drosophila eye (Fig 2). Secondary screens then identify those genes that can drive ectopic proliferation. To date we've screened ~180,000 mutant chromosomes covering ~60% of the genome, and ~4000 gain-of-function "EP" transgenics. We've identified 5 mutant loci and >10 overexpressed genes that deregulate E2F activity and drive unscheduled cell proliferation in differentiating wings and eyes. Three loci were expected (E2F, CycE, Ago/Fbw7), but most appear to be novel cell cycle regulators (e.g. Fig 2C-E). We will continue these genetic screens to saturation, and characterize in depth the genes isolated. In addition to the pcna-white+ screens we are using oligonucleotide microarrays to profile the gene expression changes that occur during wing differentiation. These experiments are tracking gene expression during wing differentiation in several genotypes that accomplish cell cycle exit (WT, dp mutants, E2F-overexpressing) and one genotype that bypasses cell cycle exit (co-overexpressed E2F and Cyclin E/Cdk2). The yield of the genetic screens combined with comparative analysis of the microarray data should identify new pathways that mediate cell cycle exit, and provide a paradigm for future studies of how differentiation signals interface with the cell cycle control apparatus to restrain cell proliferation. Theme 3: Targets of the TSC/Rheb/TOR signaling module Objective: To identify the molecular pathways via which the TSC1/2 tumor suppressor inhibits cell growth. Overview: Tuberous sclerosis (TS) is an autosomal dominant disease in which benign tumors arise throughout the body. Most cases are due to loss-of-heterozygosity in TSC1 or TSC2, which encode a protein complex. TSC1/2 restrains cell growth at least in part by inhibiting Rheb, a small GTPase that is an essential activator of the TOR kinase (Fig 3E). TOR regulates metabolic processes required for cell growth including protein synthesis, autophagy, and nutrient storage (Fig 3F, G, H). In vivo, TSC/Rheb/TOR signaling coordinates nutrient availability with cell growth rates. Hence cells mutant for TSC1/2, or overexpressing Rheb, have the striking ability to grow inappropriately without restraint, even in starvation-arrested animals (Fig 3C, D). Although TOR has two well-characterized targets (S6K, 4EBP), genetic analysis in flies and mice indicates that these cannot account for the striking cellular overgrowth phenotypes caused by TSC loss. Moreover, it is unclear that all the effects of TSC mutation are mediated by Rheb and/or TOR. The identification of TOR-independent effectors of the TSC1/2 complex would profoundly alter our understanding of the molecular basis of TS, and strategies for its treatment. We are taking two approaches to identify new targets of TSC1/2, Rheb, and TOR, and to test whether TSC1/2 executes all its functions via TOR. First, we are using heavy isotope labeling and quantitative mass spectrometry (the SILAC method) to comprehensively characterize the effects of loss-of-TSC function, and gain-of-Rheb function, on the expressed proteome in Drosophila and human cells. Second, we are using fly genetics to screen for suppressors of eye hypertrophy caused by loss of the TSC1/2 complex. Several strong suppressors of TSC1/2 and Rheb have been recovered and are being characterized. Theme 4: Homeostatic growth in the adult midgut Objective: To understand how cell-cell interactions, diet, and infection control the proliferation of adult Drosophila midgut stem cells, allowing homeostatic growth. Overview: Like mammals, adult insects have intestinal stem cells (ISCs) that allow dynamic self-renewal of the gut epithelium. The simplicity of the Drosophila midgut and the fine genetic tools available make this an attractive model for studies of stem cell biology and gut homeostasis, with potential relevance to diseases such as colorectal cancer and inflammatory bowel disease. Drosophila ISCs self-renew, and also generate short-lived differentiated enterocyctes (ECs; absorptive cells that make up the bulk of the intestinal epithelium) and enteroendocrine cells. We find that the induction of pro-apoptotic or stress signals in the ECs results in a burst of stem cell proliferation, allowing the intestine to rapidly regenerate. During regeneration the stem cell pool transiently expands, and then contracts to normal. These observations imply that signaling from spent ECs regulates stem cell proliferation, and that the number of stem cells is regulated according to need. To understand the nature of this regulation we have tested each of the major signaling pathways. We find that dietary stress induces JNK activity in the ECs, and that this triggers their production of a secreted cytokine (Upd3) that signals JAK/STAT activity in the ISCs, stimulating their proliferation. Accordingly, forced JAK/STAT signaling activates ISC proliferation and increases the stem cell pool, yielding grossly hyperplastic midguts. Current studies are investigating the effects of toxic enteric bacteria, and we find that these also stimulate cell turnover and renewal of the midgut epithelium, via JNK and JAK/STAT signaling. Critical regulatory components that mediate gut homeostasis remain unknown. We are taking two approaches to identify genes that regulate ISC proliferation and differentiation. First, we are performing an F1 genetic screen for genes and RNAi's that cause stem cell hyperplasia when overexpressed. Several novel genes have been identified (e.g. Fig 4). Second, we are using FACS and oligonucleotide microarrays to identify genes expressed specifically in ISCs. Potential ISC regulators identified by these screens will be functionally characterized using all the relevant tools of the Drosophila system. These studies will allow us to develop a working model of how signaling between the different midgut cell types mediates homeostatic growth in the intestine. Novel signaling components and stem cell markers should be discovered, which could be relevant to gut homeostasis and disease in humans. A long term objective, common to all of our developmental studies, is to understand how the signaling pathways that control ISC proliferation interface with the cell cycle control apparatus, and with the systems that regulate cell growth and metabolism. KeywordsCOS Keywords:Cell Cycle, Genetics.Languages(Reading, Writing, Speaking)German: (Basic, None, Basic) Honors and Awards1995-2000,
Rita Allen Scholar,
Rita Allen Foundation
1990-1999,
Lucille P. Markey Scholar,
Lucille P. Markey Charitable Trust
1988-1990,
American Cancer Society Postdoctoral Fellowship,
American Cancer Society
1988, Larry Sandler Memorial Award,
National Drosophila Research Conference
1983-1987,
Predoctoral Fellowship,
National Science Foundation (NSF)
Publications
Profile DetailsLast Verified: 9/10/2008 COS Expertise ID #450226 Reference this profile directly: http://myprofile.cos.com/bedgar Individual Expertise profile of Bruce A. Edgar, Copyright Bruce A. Edgar. © COS ExpertiseTM, 2009, ProQuest LLC All rights reserved. |