Detection and Visualization of DNA Damage-induced Protein Complexes in Suspension Cell Cultures Using the Proximity Ligation Assay

Summary

Here, it is demonstrated how the in situ Proximity Ligation Assay (PLA) can be used to detect and visualize the direct protein-protein interactions between ATM and p53 in suspension cell cultures exposed to genotoxic stress.

Abstract

The DNA damage response orchestrates the repair of DNA lesions that occur spontaneously, are caused by genotoxic stress, or appear in the context of programmed DNA breaks in lymphocytes. The Ataxia-Telangiectasia Mutated kinase (ATM), ATM- and Rad3-Related kinase (ATR) and the catalytic subunit of DNA-dependent Protein Kinase (DNA-PKcs) are among the first that are activated upon induction of DNA damage, and are central regulators of a network that controls DNA repair, apoptosis and cell survival. As part of a tumor-suppressive pathway, ATM and ATR activate p53 through phosphorylation, thereby regulating the transcriptional activity of p53. DNA damage also results in the formation of so-called ionizing radiation-induced foci (IRIF) that represent complexes of DNA damage sensor and repair proteins that accumulate at the sites of DNA damage, which are visualized by fluorescence microscopy. Co-localization of proteins in IRIFs, however, does not necessarily imply direct protein-protein interactions, as the resolution of fluorescence microscopy is limited.

In situ Proximity Ligation Assay (PLA) is a novel technique that allows the direct visualization of protein-protein interactions in cells and tissues with unprecedented specificity and sensitivity. This technique is based on the spatial proximity of specific antibodies binding to the proteins of interest. When the interrogated proteins are within ~40 nm an amplification reaction is triggered by oligonucleotides that are conjugated to the antibodies, and the amplification product is visualized by fluorescent labeling, yielding a signal that corresponds to the subcellular location of the interacting proteins. Using the established functional interaction between ATM and p53 as an example, it is demonstrated here how PLA can be used in suspension cell cultures to study the direct interactions between proteins that are integral parts of the DNA damage response.

Introduction

DNA damage triggers a highly regulated series of events involving protein-protein interactions and post-translational modifications that ensures the efficient and rapid repair of DNA, thereby safeguarding genomic integrity¹. Typically, DNA repair is studied in cells exposed to ionizing radiation by monitoring the formation of so-called ionizing radiation-induced foci (IRIF) by (confocal) fluorescence microscopy. Many DNA repair and DNA damage-sensing proteins form IRIFs, which represent protein complexes that nucleate at chromatin sites sustaining DNA damage^2,3. The location and resolut....

Protocol

1. Treatment of Cells and DNA Damage Induction

1. Culture the human BCR-ABL+ B-cell acute lymphoblastic cell lines BV173 or SUP-B15 in IMDM supplemented with 20% FCS, 50 μM β-mercaptoethanol, 2 mM L-glutamine, 100 U/mL penicillin and 100 μg/mL streptomycin at 37 °C in an atmosphere of 5% CO₂. Count cells and plate at 2 x 10⁶ cell/mL in 6-wells plates, at 5 mL/well.
   NOTE: Optionally, suspension cell lines of various origin can used.
2. To arrest the BCR-ABL+ B-ALL cells in the G1-phase of the cell-cycle, add 5 μM of imatinib methanesulfonate (STI571), and incubate overnight.
   ....

Results

Phosphorylation of p53 at residue Ser15 was shown to be dependent on ATM kinase activity¹⁶. To demonstrate and confirm the specificity of the PLA technique on cytospin preparations of suspension cell cultures, it is shown that induction of DNA damage by 2 h NCS treatment of BCR-ABL+ B-ALL cells arrested in the G1-phase of the cell-cycle resulted in the specific interaction between ATM and phospho-Ser15-p53, as expected. Punctate PLA signals were observed in the nuc.......

Discussion

In this report, it is demonstrated that PLA can be used to determine and visualize the specific interaction between proteins in suspension cell cultures. Of note, the protocol described here is not restricted to the study of DNA repair complexes but also applies to visualize and quantify other protein-protein interactions in suspension cell cultures. It is shown that the ATM kinase interacts with phosphorylated p53 in G1-arrested BCR-ABL+ B-ALL cells when exposed to a DNA damage-inducing agent. Previously, the PLA techni.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
BV173 cell line	DSMZ	AC-20	BCR-ABL+ B-ALL cell line
SUP-B15 cell line	DSMZ	ACC-389	BCR-ABL+ B-ALL cell line
Iscove's Modified Dulbecco's Medium (IMDM)	Gibco (Life Technologies)	21980-032
Fetal Calf Serum	Sigma Aldrich	F7524	lot #: 064M3396
L-glutamine	Gibco (Life Technologies)	25030-024
penicillin/streptomycin	Gibco (Life Technologies)	15140-122
imatinib methanesulfonate	LC Laboratories	I-5508	Dissolve in DMSO, prepare 10 mM stock solution
neocarzinostatin	Sigma Aldrich	N9162	Mutagenic/teratogenic, handle with care
KU55933	Selleckchem	S1092	Dissolve in DMSO, prepare 5 mM stock solution
Starfrost Microscopy Slides	Waldemar Knittel	VA11200 003FKB
PAP pen liquid blocker	Sigma Aldrich	Z377821-1EA
Cytospin funnel	Q Path Labonord SAS	003411324
Duolink In Situ Red Starter Kit Goat/Rabbit	Sigma Aldrich	DUO92105	Available for different species/combinations, also available in FarRED, Orange and Green
goat-anti-ATM	Bethyl Laboratories	A300-136A	PLA-grade; we succesfully used lot#A300-136A-1 in our studies
rabbit-anti-phospho-Ser15-p53	Cell Signaling Technology	9284	We succesfully used lot #9284-4 in our studies
Vectashield antifading mounting medium with DAPI	Vector Labs	H-1200
Vectashield antifading mounting medium	Vector Labs	H-1000
4% paraformaldehyde in PBS	Santa Cruz Biotechnology	sc-281692	Also available from various other vendors