Induction and Diagnosis of Tumors in Drosophila Imaginal Disc Epithelia

Summary

Mosaic clone analysis in Drosophila imaginal disc epithelia is a powerful model system to study the genetic and cellular mechanisms of tumorigenesis. Here we describe a protocol to induce tumors in Drosophila wing imaginal discs using the GAL4-UAS system, and introduce a diagnosis method to classify the tumor phenotypes.

Abstract

In the early stages of cancer, transformed mutant cells show cytological abnormalities, begin uncontrolled overgrowth, and progressively disrupt tissue organization. Drosophila melanogaster has emerged as a popular experimental model system in cancer biology to study the genetic and cellular mechanisms of tumorigenesis. In particular, genetic tools for Drosophila imaginal discs (developing epithelia in larvae) enable the creation of transformed pro-tumor cells within a normal epithelial tissue, a situation similar to the initial stages of human cancer. A recent study of tumorigenesis in Drosophila wing imaginal discs, however, showed that tumor initiation depends on the tissue-intrinsic cytoarchitecture and the local microenvironment, suggesting that it is important to consider the region-specific susceptibility to tumorigenic stimuli in evaluating tumor phenotypes in imaginal discs. To facilitate phenotypic analysis of tumor progression in imaginal discs, here we describe a protocol for genetic experiments using the GAL4-UAS system to induce neoplastic tumors in wing imaginal discs. We further introduce a diagnosis method to classify the phenotypes of clonal lesions induced in imaginal epithelia, as a clear classification method to discriminate various stages of tumor progression (such as hyperplasia, dysplasia, or neoplasia) had not been described before. These methods might be broadly applicable to the clonal analysis of tumor phenotypes in various organs in Drosophila.

Introduction

Epithelial tissues are highly organized systems that have the remarkable homeostatic ability to maintain their organization through development and cell turnover. This robust self-organizing system, however, is progressively disrupted during tumor development. At the beginning of tumor development, individual mutant cells arising from oncogene activation or tumor-suppressor gene inactivation emerge within an epithelial layer. When this transformed "pro-tumor cell" evades a suppressive environment, disrupts epithelial organization, and begins uncontrolled proliferation, tumorigenesis occurs ¹. During the past few decades, outstanding tec....

Protocol

1. Fly Crosses and Clone Induction

1. Remove all flies in the vial 12 h before collecting virgin flies.
2. Anesthetize the flies in the vial by injecting CO₂ gas and place flies onto a CO₂ fly pad.
3. Transfer 10 - 20 virgin females and 10 males from the CO₂ fly pad into a fresh vial and incubate for 1 day at 25 °C.
4. Transfer these flies into a fresh vial and incubate for 12 h at 25 °C.
   NOTE: Discard the first vial as virgin females do not lay enough eggs in the first day.
5. For enhancer-GAL4 lines, remove the adult flies and incubate the vial at 25 °C until dissection.
....

Results

To demonstrate neoplastic tumor formation experimentally induced by RNAi-mediated nTSG-knockdown in Drosophila wing imaginal discs, three different GAL4 drivers were used to express UAS-RNAi for lgl or scrib: (1) sd-GAL4, which drives strong UAS expression in the wing pouch and mild expression in the hinge regions (Figure 2 and Figure 3A); (2) upd-GAL4, which drives i.......

Discussion

The GAL4-UAS system is one of the most powerful genetic tools for targeted gene expression in Drosophila ²⁶ and greatly facilitates tumor cell induction and analysis in vivo ⁴. This system enables the generation of clones bearing knockdown of tumor-suppressor genes or overexpression of oncogenes within wild-type epithelial tissue, a situation highly similar to the initial stages of human cancer where transformed pro-tumor cells are surrounded by normal epi.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Reagents
Phosphate buffered saline (PBS)	Wako	162-19321
TritonX-100	Wako	168-11805
Formaldehyde	Wako	064-00406
bovine serum albumin	Sigma	A7906
normal goat serum	Sigma	G6767
mounting medium, Vectashield	Vector Laboratories	H-1000
DAPI	Sigma	D9542
mouse-anti-Dlg 4F3	Developmental Studies Hybridoma Bank	4F3 anti-discs large, RRID:AB_528203	dilute in PBTG, 1:40
mouse-anti-MMP1	Developmental Studies Hybridoma Bank	3A6B4, RRID:AB_579780	3 mixed 1:1:1 and dilute in PBTG, 1:40
mouse-anti-MMP1	Developmental Studies Hybridoma Bank	3B8D12, RRID:AB_579781	3 mixed 1:1:1 and dilute in PBTG, 1:40
mouse-anti-MMP1	Developmental Studies Hybridoma Bank	5H7B11, RRID:AB_579779	3 mixed 1:1:1 and dilute in PBTG, 1:40
mouse-anti-atubulin	Developmental Studies Hybridoma Bank	AA4.3, RRID:AB_579793	dilute in PBTG, 1:100
Alexa Fluor 546 Phalloidin	Molecular probes	A22283	dilute in PBS, 1:40
goat anti-mouse IgG antibody, Alexa Fluor 546	Molecular probes	A11030	dilute in PBTG, 1:400
Name	Company	Catalog Number	Comments
Fly strains
sd-Gal4	Bloomington Drosophila Stock Center	#8609	recombined with UAS-EGFP
upd-Gal4	Bloomington Drosophila Stock Center	#26796	recombined with UAS-EGFP
UAS-lgl-RNAi	Vienna Drosophila RNAi center	#51247
UAS-scrib-RNAi	Vienna Drosophila RNAi center	#105412
UAS-RasV12	Bloomington Drosophila Stock Center	#64196
UAS-Yki3SA	Bloomington Drosophila Stock Center	#28817
hsFLP	Bloomington Drosophila Stock Center	#6
Act>CD2>GAL4 (flip-out GAL4)	Bloomington Drosophila Stock Center	#4780	recombined with UAS-EGFP
UAS-EGFP	Bloomington Drosophila Stock Center	#5428	X chromosome
UAS-EGFP	Bloomington Drosophila Stock Center	#6658	third chromosome
UAS-Dicer2	Bloomington Drosophila Stock Center	#24650	second chromosome
UAS-Dicer2	Bloomington Drosophila Stock Center	#24651	third chromosome
vkg-GFP	Morin et al. 2001		GFP protein trap