Review Article

2011
Neumüller RA, Perrimon N. Where gene discovery turns into systems biology: genome-scale RNAi screens in Drosophila. Wiley Interdiscip Rev Syst Biol Med. 2011;3 (4) :471-8. Abstract

Systems biology aims to describe the complex interplays between cellular building blocks which, in their concurrence, give rise to the emergent properties observed in cellular behaviors and responses. This approach tries to determine the molecular players and the architectural principles of their interactions within the genetic networks that control certain biological processes. Large-scale loss-of-function screens, applicable in various different model systems, have begun to systematically interrogate entire genomes to identify the genes that contribute to a certain cellular response. In particular, RNA interference (RNAi)-based high-throughput screens have been instrumental in determining the composition of regulatory systems and paired with integrative data analyses have begun to delineate the genetic networks that control cell biological and developmental processes. Through the creation of tools for both, in vitro and in vivo genome-wide RNAi screens, Drosophila melanogaster has emerged as one of the key model organisms in systems biology research and over the last years has massively contributed to and hence shaped this discipline. WIREs Syst Biol Med 2011 3 471-478 DOI: 10.1002/wsbm.127

2011_Wiley_Neumuller.pdf
2010
Karpowicz P, Perrimon N. All for one, and one for all: the clonality of the intestinal stem cell niche. F1000 Biol Rep. 2010;2 :73. Abstract

Intestinal epithelia are maintained by intestinal stem cells (ISCs) that divide to replace dying absorptive and secretory cells that make up this tissue. Lineage labeling studies, both in vertebrates and Drosophila, have revealed the relationships between ISCs and their progeny. In addition, a number of signaling pathways involved in ISC proliferation and differentiation have been identified. Further studies will clarify the signals originating from the ISC niche and determine the processes that control the number and uniform distribution of niches throughout the epithelium.

2010_BioReports_Karpowicz.pdf
Rajan A, Perrimon N. Steroids make you bigger? Fat chance says Myc. Cell Metab. 2010;12 (1) :7-9. Abstract

In flies, ecdysone integrates growth with developmental transitions by antagonizing insulin signaling, which links growth with nutritional status. Work in Developmental Cell (Delanoue et. al, 2010) finds that ecdysone represses the transcription factor Myc in the larval fat body to inhibit systemic growth, revealing a mechanism for such coordination.

2010_Cell Metab_Rajan.pdf
Zirin J, Perrimon N. Drosophila as a model system to study autophagy. Semin Immunopathol. 2010;32 (4) :363-72. Abstract

Originally identified as a response to starvation in yeast, autophagy is now understood to fulfill a variety of roles in higher eukaryotes, from the maintenance of cellular homeostasis to the cellular response to stress, starvation, and infection. Although genetics and biochemical studies in yeast have identified many components involved in autophagy, the findings that some of the essential components of the yeast pathway are missing in higher organisms underscore the need to study autophagy in more complex systems. This review focuses on the use of the fruitfly, Drosophila melanogaster as a model system for analysis of autophagy. Drosophila is an organism well-suited for genetic analysis and represents an intermediate between yeast and mammals with respect to conservation of the autophagy machinery. Furthermore, the complex biology and physiology of Drosophila presents an opportunity to model human diseases in a tissue specific and analogous context.

2010_Semin Immunopathol_Zirin.pdf
Mohr S, Bakal C, Perrimon N. Genomic screening with RNAi: results and challenges. Annu Rev Biochem. 2010;79 :37-64. Abstract

RNA interference (RNAi) is an effective tool for genome-scale, high-throughput analysis of gene function. In the past five years, a number of genome-scale RNAi high-throughput screens (HTSs) have been done in both Drosophila and mammalian cultured cells to study diverse biological processes, including signal transduction, cancer biology, and host cell responses to infection. Results from these screens have led to the identification of new components of these processes and, importantly, have also provided insights into the complexity of biological systems, forcing new and innovative approaches to understanding functional networks in cells. Here, we review the main findings that have emerged from RNAi HTS and discuss technical issues that remain to be improved, in particular the verification of RNAi results and validation of their biological relevance. Furthermore, we discuss the importance of multiplexed and integrated experimental data analysis pipelines to RNAi HTS.

2010_An Rev Biochem_Mohr.pdf
Bakal C, Perrimon N. Realizing the promise of RNAi high throughput screening. Dev Cell. 2010;18 (4) :506-7. Abstract

Recently reporting in Nature, Collinet et al. describes the application of quantitative multiparametric methods to a genome-wide RNAi screen for regulators of endocytosis. The study illustrates the power of this approach beyond the identification of new endocytic components to providing insights into the design principles of the endocytic system.

2010_Dev Cell_Bakal.pdf
2009
Vinegoni C, Razansky D, Pitsouli C, Perrimon N, Ntziachristos V, Weissleder R. Mesoscopic fluorescence tomography for in-vivo imaging of developing Drosophila. J Vis Exp. 2009;(30). Abstract

Visualizing developing organ formation as well as progession and treatment of disease often heavily relies on the ability to optically interrogate molecular and functional changes in intact living organisms. Most existing optical imaging methods are inadequate for imaging at dimensions that lie between the penetration limits of modern optical microscopy (0.5-1mm) and the diffusion-imposed limits of optical macroscopy (>1cm) [1]. Thus, many important model organisms, e.g. insects, animal embryos or small animal extremities, remain inaccessible for in-vivo optical imaging. Although there is increasing interest towards the development of nanometer-resolution optical imaging methods, there have not been many successful efforts in improving the imaging penetration depth. The ability to perform in-vivo imaging beyond microscopy limits is in fact met with the difficulties associated with photon scattering present in tissues. Recent efforts to image entire embryos for example [2,3] require special chemical treatment of the specimen, to clear them from scattering, a procedure that makes them suitable only for post-mortem imaging. These methods however evidence the need for imaging larger specimens than the ones usually allowed by two-photon or confocal microscopy, especially in developmental biology and in drug discovery. We have developed a new optical imaging technique named Mesoscopic Fluorescence Tomography [4], which appropriate for non-invasive in-vivo imaging at dimensions of 1mm-5mm. The method exchanges resolution for penetration depth, but offers unprecedented tomographic imaging performance and it has been developed to add time as a new dimension in developmental biology observations (and possibly other areas of biological research) by imparting the ability to image the evolution of fluorescence-tagged responses over time. As such it can accelerate studies of morphological or functional dependencies on gene mutations or external stimuli, and can importantly, capture the complete picture of development or tissue function by allowing longitudinal time-lapse visualization of the same, developing organism. The technique utilizes a modified laboratory microscope and multi-projection illumination to collect data at 360-degree projections. It applies the Fermi simplification to Fokker-Plank solution of the photon transport equation, combined with geometrical optics principles in order to build a realistic inversion scheme suitable for mesoscopic range. This allows in-vivo whole-body visualization of non-transparent three-dimensional structures in samples up to several millimeters in size. We have demonstrated the in-vivo performance of the technique by imaging three-dimensional structures of developing Drosophila tissues in-vivo and by following the morphogenesis of the wings in the opaque Drosophila pupae in real time over six consecutive hours.

2009_JoVE_Vinegoni.pdf
Bai J, Sepp KJ, Perrimon N. Culture of Drosophila primary cells dissociated from gastrula embryos and their use in RNAi screening. Nat Protoc. 2009;4 (10) :1502-12. Abstract

We provide a detailed protocol for the mass culturing of primary cells dissociated from Drosophila embryos. The advantage of this protocol over others is that we have optimized it for a robust large-scale performance that is suitable for screening. More importantly, we further present conditions to treat these cells with double stranded (ds) RNAs for gene knockdown. Efficient RNAi in Drosophila primary cells is accomplished by simply bathing the cells in dsRNA-containing culture medium. This method provides the basis for functional genomic screens in differentiated cells, such as neurons and muscles, using RNAi or small molecules. The entire protocol takes approximately 14 d, whereas the preparation of primary cells from Drosophila embryos only requires 2-4 h.

2009_Nat Protocols_Bai.pdf
Pitsouli C, Apidianakis Y, Perrimon N. Homeostasis in infected epithelia: stem cells take the lead. Cell Host Microbe. 2009;6 (4) :301-7. Abstract

To maintain tissue homeostasis and avoid disease, epithelial cells damaged by pathogens need to be readily replenished, and this is mainly achieved by the activation of stem cells. In this Short Review, we discuss recent developments in the exciting field of host epithelia-pathogen interaction in Drosophila as well as in mammals.

2009_Cell Host_Pitsouli.pdf
2008
Pitsouli C, Perrimon N. Developmental biology: Our fly cousins' gut. Nature. 2008;454 (7204) :592-3. 2008_Nature_Pitsouli.pdf
2007
Perrimon N, Friedman A, Mathey-Prevot B, Eggert US. Drug-target identification in Drosophila cells: combining high-throughout RNAi and small-molecule screens. Drug Discov Today. 2007;12 (1-2) :28-33. Abstract

RNA interference (RNAi) and small-molecule approaches are synergistic on multiple levels, from technology and high-throughput screen development to target identification and functional studies. Here, we describe the RNAi screening platform that we have established and made available to the community through the Drosophila RNAi Screening Center at Harvard Medical School. We then illustrate how the combination of RNAi and small-molecule HTS can lead to effective identification of targets in drug discovery.

2007_Drug Discov Today_Perrimon.pdf
Perrimon N, Mathey-Prevot B. Matter arising: off-targets and genome-scale RNAi screens in Drosophila. Fly (Austin). 2007;1 (1) :1-5. Abstract

Recently, the issue of off-target effects (OTEs) associated with long double stranded RNAs (dsRNAs) used in RNAi screens, such as those performed at the Drosophila RNAi Screening Center and other laboratories, has become a focus of great interest and some concern. Although OTEs have been recognized as an important source of false positives in mammalian studies (where short siRNAs are used as triggers), they were generally thought to be inconsequential in Drosophila RNAi experiments because of the use of long dsRNAs. Two recent papers have disputed this contention and show that significant off-target effects can take place with the use of some long dsRNAs in Drosophila cells. Together, these studies provide evidence that OTEs mediated by short homology stretches of 19nt or greater within long dsRNAs can contribute to false positives in Drosophila RNAi screens. Here, we address how widespread the occurrence of OTE is in Drosophila screens, focusing on the DRSC dsRNA collections, and we discuss the implication for the interpretation of results reported in RNAi screens to-date. Lastly, we summarize steps taken by the DRSC to redress that situation and include a set of recommendations to observe in future RNAi screens.

2007_Fly_Perrimon.pdf
Ramadan N, Flockhart I, Booker M, Perrimon N, Mathey-Prevot B. Design and implementation of high-throughput RNAi screens in cultured Drosophila cells. Nat Protoc. 2007;2 (9) :2245-64. Abstract

This protocol describes the various steps and considerations involved in planning and carrying out RNA interference (RNAi) genome-wide screens in cultured Drosophila cells. We focus largely on the procedures that have been modified as a result of our experience over the past 3 years and of our better understanding of the underlying technology. Specifically, our protocol offers a set of suggestions and considerations for screen optimization and a step-by-step description of the procedures successfully used at the Drosophila RNAi Screening Center for screen implementation, data collection and analysis to identify potential hits. In addition, this protocol briefly covers postscreen analysis approaches that are often needed to finalize the hit list. Depending on the scope of the screen and subsequent analysis and validation involved, the full protocol can take anywhere from 3 months to 2 years to complete.

2007_Nat Prot_Ramadan.pdf
Mathey-Prevot B, Perrimon N. Do-it-yourself RNAi made easy?. Nat Methods. 2007;4 (4) :308-9. 2007_Nat Meth_Mathey-Prevot.pdf
Leopold P, Perrimon N. Drosophila and the genetics of the internal milieu. Nature. 2007;450 (7167) :186-8. Abstract

'Homeostasis', from the Greek words for 'same' and 'steady', refers to ways in which the body acts to maintain a stable internal environment despite perturbations. Recent studies in Drosophila exemplify the conservation of regulatory mechanisms involved in metabolic homeostasis. These new findings underscore the use of Drosophila as a model for the study of various human disorders.

2007_Nat_Leopold.pdf
Friedman A, Perrimon N. Genetic screening for signal transduction in the era of network biology. Cell. 2007;128 (2) :225-31. Abstract

In contrast to animal-based mutant phenotype assays, recent biochemical and quantitative genetic studies have identified hundreds of potential regulators of known signaling pathways. We discuss the discrepancy between previous models and new data, put forward a different signaling conceptual framework incorporating time-dependent quantitative contributions, and suggest how this new framework can impact our study of human disease.

2007_Cell_Friedman.pdf
Perrimon N, Mathey-Prevot B. Applications of high-throughput RNA interference screens to problems in cell and developmental biology. Genetics. 2007;175 (1) :7-16. Abstract

RNA interference (RNAi) in tissue culture cells has emerged as an excellent methodology for identifying gene functions systematically and in an unbiased manner. Here, we describe how RNAi high-throughput screening (HTS) in Drosophila cells are currently being performed and emphasize the strengths and weaknesses of the approach. Further, to demonstrate the versatility of the technology, we provide examples of the various applications of the method to problems in signal transduction and cell and developmental biology. Finally, we discuss emerging technological advances that will extend RNAi-based screening methods.

2007_Genetics_Perrimon.pdf
2006
Friedman A, Perrimon N. High-throughput approaches to dissecting MAPK signaling pathways. Methods. 2006;40 (3) :262-71. Abstract

With the development of genome-wide RNAi libraries, it is now possible to screen for novel components of mitogen-activated protein kinase (MAPK) pathways in cell culture. Although genetic dissection in model organisms and biochemical approaches in mammalian cells have been successful in identifying the core signaling cassettes of these pathways, high-throughput assays can yield unbiased, functional genomic insight into pathway regulation. We describe general high-throughput approaches to assaying MAPK signaling and the receptor tyrosine kinase (RTK)/extracellular signal-regulated kinase (ERK) pathway in particular using a phospho-specific antibody-based readout of pathway activity. We also provide examples of secondary validation screens and methods for managing large datasets for future in vivo functional characterization.

2006_Methods_Friedman.pdf
Echeverri CJ, Perrimon N. High-throughput RNAi screening in cultured cells: a user's guide. Nat Rev Genet. 2006;7 (5) :373-84. Abstract

RNA interference has re-energized the field of functional genomics by enabling genome-scale loss-of-function screens in cultured cells. Looking back on the lessons that have been learned from the first wave of technology developments and applications in this exciting field, we provide both a user's guide for newcomers to the field and a detailed examination of some more complex issues, particularly concerning optimization and quality control, for more advanced users. From a discussion of cell lines, screening paradigms, reagent types and read-out methodologies, we explore in particular the complexities of designing optimal controls and normalization strategies for these challenging but extremely powerful studies.

2006_Nat Rev Gene_Echeverri.pdf
Echeverri CJ, Beachy PA, Baum B, Boutros M, Buchholz F, Chanda SK, et al. Minimizing the risk of reporting false positives in large-scale RNAi screens. Nat Methods. 2006;3 (10) :777-9. Abstract

Large-scale RNA interference (RNAi)-based analyses, very much as other 'omic' approaches, have inherent rates of false positives and negatives. The variability in the standards of care applied to validate results from these studies, if left unchecked, could eventually begin to undermine the credibility of RNAi as a powerful functional approach. This Commentary is an invitation to an open discussion started among various users of RNAi to set forth accepted standards that would insure the quality and accuracy of information in the large datasets coming out of genome-scale screens.

2006_Nat Meth_Echeverri.pdf

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