This work was supported by the Grant Agency of the Czech Republic, project P302-12-G157. Experiments were also supported by the Czech-Norwegian Research Programme CZ09, which is supervised by Norwegian funds, and by the Ministry of Education, Youth and Sport of the Czech Republic (grant number: 7F14369).

<ol>
	<li>Cadet, J., Mouret, S., Ravanat, J. L., Douki, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photoinduced+damage+to+cellular+DNA:+direct+and+photosensitized+reactions.">Photoinduced damage to cellular DNA: direct and photosensitized reactions.</a> Photochem Photobiol. 88 (5), 1048-1065 (2012).</li><li>Cooke, M. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Immunochemical+detection+of+UV-induced+DNA+damage+and+repair.">Immunochemical detection of UV-induced DNA damage and repair.</a> J Immunol Methods. 280 (1-2), 125-133 (2003).</li><li>Lahtz, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gamma+irradiation+does+not+induce+detectable+changes+in+DNA+methylation+directly+following+exposure+of+human+cells.">Gamma irradiation does not induce detectable changes in DNA methylation directly following exposure of human cells.</a> PLoS One. 7 (9), e44858 (2012).</li><li>Cremer, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Detection+of+chromosome+aberrations+in+the+human+interphase+nucleus+by+visualization+of+specific+target+DNAs+with+radioactive+and+non-radioactive+in+situ+hybridization+techniques:+diagnosis+of+trisomy+18+with+probe+L1.84.">Detection of chromosome aberrations in the human interphase nucleus by visualization of specific target DNAs with radioactive and non-radioactive in situ hybridization techniques: diagnosis of trisomy 18 with probe L1.84.</a> Hum Genet. 74 (4), 346-352 (1986).</li><li>Zorn, C., Cremer, T., Cremer, C., Zimmer, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Laser+UV+microirradiation+of+interphase+nuclei+and+post-treatment+with+caffeine.+A+new+approach+to+establish+the+arrangement+of+interphase+chromosomes.">Laser UV microirradiation of interphase nuclei and post-treatment with caffeine. A new approach to establish the arrangement of interphase chromosomes.</a> Hum Genet. 35 (1), 83-89 (1976).</li><li>Bartova, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recruitment+of+Oct4+protein+to+UV-damaged+chromatin+in+embryonic+stem+cells.">Recruitment of Oct4 protein to UV-damaged chromatin in embryonic stem cells.</a> PLoS One. 6 (12), e27281 (2011).</li><li>Stixova, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=HP1beta-dependent+recruitment+of+UBF1+to+irradiated+chromatin+occurs+simultaneously+with+CPDs.">HP1beta-dependent recruitment of UBF1 to irradiated chromatin occurs simultaneously with CPDs.</a> Epigenetics Chromatin. 7 (1), 39 (2014).</li><li>Stixova, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advanced+microscopy+techniques+used+for+comparison+of+UVA-+and+gamma-irradiation-induced+DNA+damage+in+the+cell+nucleus+and+nucleolus.">Advanced microscopy techniques used for comparison of UVA- and gamma-irradiation-induced DNA damage in the cell nucleus and nucleolus.</a> Folia Biol (Praha). 60, 76-84 (2014).</li><li>Luijsterburg, M. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Heterochromatin+protein+1+is+recruited+to+various+types+of+DNA+damage.">Heterochromatin protein 1 is recruited to various types of DNA damage.</a> J Cell Biol. 185 (4), 577-586 (2009).</li><li>Bartova, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coilin+is+rapidly+recruited+to+UVA-induced+DNA+lesions+and+gamma-radiation+affects+localized+movement+of+Cajal+bodies.">Coilin is rapidly recruited to UVA-induced DNA lesions and gamma-radiation affects localized movement of Cajal bodies.</a> Nucleus. 5 (3), 460-468 (2014).</li><li>Franek, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advanced+Image+Acquisition+and+Analytical+Techniques+for+Studies+of+Living+Cells+and+Tissue+Sections.">Advanced Image Acquisition and Analytical Techniques for Studies of Living Cells and Tissue Sections.</a> Microsc Microanal. 22 (2), 326-341 (2016).</li><li>Lemmer, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Using+conventional+fluorescent+markers+for+far-field+fluorescence+localization+nanoscopy+allows+resolution+in+the+10-nm+range.">Using conventional fluorescent markers for far-field fluorescence localization nanoscopy allows resolution in the 10-nm range.</a> J Microsc. 235 (2), 163-171 (2009).</li><li>Reits, E. 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The authors declare that there are no conflicts of interest.

Microscopy techniques represent basic tools in research laboratories. Here, a brief description of the methods used for the study of protein recruitment and kinetics at DNA lesions is presented. We especially noted our experimental experience in the field of local microirradiation of living cells, and we discuss the study of protein kinetics by FRAP and protein-protein interaction at DNA lesions by acceptor-bleaching FRET28 and its advanced modification FRET-FLIM (Figure 4A</s...

DNA damage leads to the appearance of DNA lesions consisting of cyclobutane pyrimidine dimers (CPDs), 8-oxo-7,8-dihydro-2&#39;-deoxyguanosine, and single-strand or double-strand breaks1,2. &#947;-rays are the form of ionizing radiation with the highest energy and high penetrance, thus this source of radiation is widely used in radiotherapy3. On the other hand, experimentally induced DNA damage caused by UV lasers mimics natural exposure to UV light. UVA microirradiation, as a microscopic method, represents an experimental tool for studying DNA damage in individual living cells. Microirradiation was used for the first time 40 years ago in order to reveal the organization of chromosome regions4,5. This technique is highly dependent on either the functional properties of confocal microscopes or the technical limits of modern nanoscopy. To induce DNA lesions, cells can be presensitized by 5&#39; bromodeoxyuridine (BrdU) or Hoechst 33342 prior to UV irradiation. B&#225;rtov&#225; et al.6 previously described the presensitization step, and recently we optimized this microirradiation technique in order to avoid cell death, or apoptosis. For example, the use of a 405-nm UV laser (without Hoechst 33342 presensitization) leads to the induction of 53BP1-positive double-strand breaks (DSBs) at the expense of cyclobutane pyrimidine dimers (CPDs). On the other hand, presensitization steps combined with UV microirradiation induce very high levels of CPDs and DSBs simultaneously7,8. This methodology is difficult to apply to the study of a single DNA repair pathway.
With microirradiation, it is possible to analyze protein recruitment, kinetics, and interaction at DNA lesions in living cells. An example of this method was published by Luijsterburg et al.9 for heterochromatin protein 1&#946;, and we recently showed for the first time that the pluripotency factor Oct4 and a protein associated with Cajal bodies, coilin, are recruited to UV-induced DNA lesions6,10. Protein kinetics at these DNA lesions can also be studied using the FRAP (Fluorescence Recovery After Photobleaching)11,12,13 or FRET (Fluorescence Resonance Energy Transfer) techniques14,15. These methods have the potential to reveal simple diffusion of proteins at DNA lesions or protein-protein interactions. A useful tool for additional characterization of proteins is FLIM (Fluorescence Lifetime Imaging Microscopy) or its combination with FRET technology (FRET-FLIM)16. These methods enable the study of processes in living cells that are stably or transiently expressing the protein of interest tagged by a fluorescent molecule17. Here, an example of exponential decay time (&#964;) for GFP-tagged p53 protein and its interaction partner, mCherry-tagged 53BP1, playing an important role in DNA damage response18,19 is shown. The parameter &#964;, the lifetime of the fluorochrome provided by FLIM calculations, is specific for a given fluorescence dye, its binding abilities, and its cellular environment. Therefore, this method can show us distinctions between protein subpopulations, their binding abilities, and their functional properties after, for example, DNA damage.
Here, an outline of the methodological approaches of the advanced microscopy techniques that is used in our laboratory to study time-specific protein recruitment, kinetics, diffusion, and protein-protein interactions at the site of microirradiated chromatin is presented. The step-by-step methodology for the induction of local DNA lesions in living cells, and a description of methodologies useful for studies of DNA-damage-related events at locally induced DNA lesions caused by UV lasers are provided.

<table><tbody><tr><td>&lt;strong&gt;Cell cultivation&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>HeLa</td><td>ATCC</td><td>CCL-2TM</td><td></td></tr><tr><td>ES-D3 [D3]</td><td>ATCC</td><td>CRL-11632TM</td><td></td></tr><tr><td>HeLa -Fucci cells</td><td></td><td></td><td>http://ruo.mbl.co.jp/bio/e/product/flprotein/fucci.html</td></tr><tr><td>DMEM</td><td>PAN-Biotech</td><td>P03-0710</td><td></td></tr><tr><td>DMEM high glucose</td><td>Sigma-Aldrich</td><td>D6429-500ML</td><td></td></tr><tr><td>Fetal bovine serum (FBS)</td><td>HyClone</td><td>SV30180.03</td><td></td></tr><tr><td>ES Cell FBS</td><td>Gibco</td><td>16141-079</td><td></td></tr><tr><td>Non-Essential Amino Acids (NEAA)</td><td>Gibco</td><td>11140-035</td><td></td></tr><tr><td>mLIF (mouse Leukemia Inhibitor Factor)</td><td>Merck Millipore</td><td>ESG1107</td><td></td></tr><tr><td>MTG (1-Thioglycerol)</td><td>Sigma-Aldrich</td><td>M6145-25ML</td><td></td></tr><tr><td>Penicillin-Streptomycin Solution</td><td>Biosera</td><td>XC-A4122/100</td><td></td></tr><tr><td>Trypsin - EDTA</td><td>Biosera</td><td>XC-T1717/100</td><td>dilute with 1 &amp;times; PBS in ratio 1:6</td></tr><tr><td>Nunclon cell culture dishes</td><td>Sigma-Aldrich</td><td>P7866</td><td>cultivation of mESCs D3 cells</td></tr><tr><td>&amp;micro;-Dish 35m+A15:I35m Grid-500</td><td>Ibidi GmbH</td><td>81166</td><td>microscopic dish</td></tr><tr><td>0.2% Gelatine</td><td>Sigma-Aldrich</td><td>G1890-100G</td><td>dilute in destille water and autoclaved</td></tr><tr><td>&lt;strong&gt;Name&lt;/strong&gt;</td><td>&lt;strong&gt;Company&lt;/strong&gt;</td><td>&lt;strong&gt;Catalog Number&lt;/strong&gt;</td><td>&lt;strong&gt;Comments&lt;/strong&gt;</td></tr><tr><td>&lt;strong&gt;Cell transfection&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>GFP-p53 plasmid</td><td>Addgene</td><td>12091</td><td></td></tr><tr><td>mCherry-PCNA plasmid</td><td></td><td></td><td>generous gift from Cristina Cardoso, Technische Universit&amp;auml;t Darmstadt</td></tr><tr><td>mCherry-53BP1 plasmid</td><td>Addgene</td><td>19835</td><td></td></tr><tr><td>Metafecetene</td><td>Biontex Laboratories GmbH</td><td>T020&amp;ndash;2.0</td><td></td></tr><tr><td>10 &amp;times; PBS</td><td>Thermo Fisher Scientific</td><td>AM9625</td><td>for transfection use 1 &amp;times; PBS diluted in nuclease-free water</td></tr><tr><td>5-bromo-2&amp;rsquo;-deoxy-uridine</td><td>Sigma-Aldrich</td><td>11296736001</td><td></td></tr><tr><td>&lt;strong&gt;Name&lt;/strong&gt;</td><td>&lt;strong&gt;Company&lt;/strong&gt;</td><td>&lt;strong&gt;Catalog Number&lt;/strong&gt;</td><td>&lt;strong&gt;Comments&lt;/strong&gt;</td></tr><tr><td>&lt;strong&gt;Confocal microscopy&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Microscope Leica TCS SP5</td><td>Leica Microsystems</td><td></td><td></td></tr><tr><td>Microscope Leica TCS SP8</td><td>Leica Microsystems</td><td></td><td></td></tr><tr><td>White-light laser</td><td>Leica Microsystems</td><td></td><td></td></tr><tr><td>355-nm laser</td><td>Coherent Inc.</td><td></td><td>laser power 80 mW</td></tr><tr><td>405-nm laser</td><td>Leica Microsystems</td><td></td><td>laser power 50 mW</td></tr><tr><td>&lt;strong&gt;Name&lt;/strong&gt;</td><td>&lt;strong&gt;Company&lt;/strong&gt;</td><td>&lt;strong&gt;Catalog Number&lt;/strong&gt;</td><td>&lt;strong&gt;Comments&lt;/strong&gt;</td></tr><tr><td>&lt;strong&gt;Immunofluorescence staining&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Coverslip</td><td>VWR International Ltd</td><td>631-1580</td><td></td></tr><tr><td>4% paraformaldehyde</td><td>Affymetrix</td><td>19943 1 LT</td><td></td></tr><tr><td>Triton X100</td><td>MP Biomedicals</td><td>2194854</td><td></td></tr><tr><td>Saponin from quillaja bark</td><td>Sigma-Aldrich</td><td>S4521</td><td></td></tr><tr><td>BSA</td><td>Sigma-Aldrich</td><td>A2153</td><td></td></tr><tr><td>53BP1</td><td>Abcam</td><td>ab21083</td><td>primary antibody</td></tr><tr><td>AlexaFluore 647</td><td>Thermo Fisher Scientific</td><td>A27040</td><td>secondary antibody</td></tr><tr><td>Vectashield</td><td>Vector Laboratories Ltd</td><td>H-1000</td><td>mounting medium</td></tr></tbody></table>

advanced confocal microscopy techniques to study protein-protein interactions and kinetics at dna lesions

Local microirradiation with lasers represents a useful tool for studies of DNA-repair-related processes in live cells. Here, we describe a methodological approach to analyzing protein kinetics at DNA lesions over time or protein-protein interactions on locally microirradiated chromatin. We also show how to recognize individual phases of the cell cycle using the Fucci cellular system to study cell-cycle-dependent protein kinetics at DNA lesions. A methodological description of the use of two UV lasers (355 nm and 405 nm) to induce different types of DNA damage is also presented. Only the cells microirradiated by the 405-nm diode laser proceeded through mitosis normally and were devoid of cyclobutane pyrimidine dimers (CPDs). We also show how microirradiated cells can be fixed at a given time point to perform immunodetection of the endogenous proteins of interest. For the DNA repair studies, we additionally describe the use of biophysical methods including FRAP (Fluorescence Recovery After Photobleaching) and FLIM (Fluorescence Lifetime Imaging Microscopy) in cells with spontaneously occurring DNA damage foci. We also show an application of FLIM-FRET (Fluorescence Resonance Energy Transfer) in experimental studies of protein-protein interactions.

Using advanced confocal microscopy, we observed an accumulation of mCherry-tagged 53BP1 and mCherry-PCNA proteins at DNA lesions. Analyses were performed by local microirradiation of living cells. To recognize the nuclear distribution patterns of DNA-repair-related proteins in individual cell cycle phases, we used the Fucci cellular system, by which it is possible to determine the G1, early S, and G2 phases of the cell cycle (Figure 1). The biological applica...

Watch this Scientific Journal Video about Advanced Confocal Microscopy Techniques to Study Protein-protein Interactions and Kinetics at DNA Lesions at JoVE.com

Advanced Confocal Microscopy Techniques to Study Protein-protein Interactions and Kinetics at DNA Lesions

Laser microirradiation is a useful tool for studies of DNA repair in living cells. A methodological approach for the use of UVA lasers to induce various DNA lesions is shown. We have optimized a method for local microirradiation that maintains the normal cell cycle; thus, irradiated cells proceed through mitosis.

Local microirradiation with lasers represents a useful tool for studies of DNA-repair-related processes in live cells. Here, we describe a methodological ...

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Research

JoVE Journal

Biology

10.9K Views.  Institute of Biophysics of the Czech Academy of Sciences. Laser microirradiation is een nuttig instrument voor onderzoek naar DNA-herstel in levende cellen. Een methodologische aanpak voor het gebruik van UVA lasers voor het opwekken van verschillende DNA laesies wordt weergegeven. We hebben een methode voor het lokale microirradiation die de normale celcyclus; onderhoudt geoptimaliseerd zo doorloop bestraalde cellen mitose.

Video: Advanced Confocal Microscopy Techniques to Study Protein-protein Interactions and Kinetics at DNA Lesions

Atomic Force Microscopy Investigations of DNA Lesion Recognition in Nucleotide Excision Repair

AFM imaging is a powerful technique for the study of protein-DNA interactions. This single molecule method allows the simultaneous resolution of different molecules and molecular assemblies in a heterogeneous sample. In the particular context of DNA interacting protein systems, different protein complex forms and their corresponding binding positions on target sites containing DNA fragments can thus be distinguished. Here, an application of AFM to the study of DNA lesion recognition in the prokaryotic and eukaryotic nucleotide excision DNA repair (NER) systems is presented. The procedures of DNA and protein sample preparations are described and experimental as well as analytical details of the experiments are provided. The data allow important conclusions on the strategies by which target site verification may be achieved by the NER proteins. Interestingly, they indicate different approaches of lesion recognition and identification for the eukaryotic NER system, depending on the type of lesion. Furthermore, distinct structural properties of the two different helicases involved in prokaryotic and eukaryotic NER result in and explain the different strategies observed for these two systems. Importantly, these experimental and analytical approaches can be applied not only to the study of DNA repair but also very similarly to other DNA interacting protein systems such as those involved in replication or transcription processes.

Here, the study of different DNA lesion recognition approaches via single molecule AFM imaging is demonstrated with the nucleotide excision repair system as an example. The procedures of DNA and protein sample preparations and experimental as well as analytical details for the AFM experiments are described.

AFM imaging is a powerful technique for the study of protein-DNA interactions. This single molecule method allows the simultaneous resolution of different ...

Genetics

Single Molecule Analysis of Laser Localized Psoralen Adducts

The DNA Damage Response (DDR) has been extensively characterized in studies of double strand breaks (DSBs) induced by laser micro beam irradiation in live cells. The DDR to helix distorting covalent DNA modifications, including interstrand DNA crosslinks (ICLs), is not as well defined. We have studied the DDR stimulated by ICLs, localized by laser photoactivation of immunotagged psoralens, in the nuclei of live cells. In order to address fundamental questions about adduct distribution and replication fork encounters, we combined laser localization with two other technologies. DNA fibers are often used to display the progress of replication forks by immunofluorescence of nucleoside analogues incorporated during short pulses. Immunoquantum dots have been widely employed for single molecule imaging. In the new approach, DNA fibers from cells carrying laser localized ICLs are spread onto microscope slides. The tagged ICLs are displayed with immunoquantum dots and the inter-lesion distances determined. Replication fork collisions with ICLs can be visualized and different encounter patterns identified and quantitated.

Lasers are frequently used in studies of the cellular response to DNA damage. However, they generate lesions whose spacing, frequency, and collisions with replication forks are rarely characterized. Here, we describe an approach that enables the determination of these parameters with laser localized interstrand crosslinks.

The DNA Damage Response (DDR) has been extensively characterized in studies of double strand breaks (DSBs) induced by laser micro beam irradiation in live ...

Application of Laser Micro-irradiation for Examination of Single and Double Strand Break Repair in Mammalian Cells

Highly coordinated DNA repair pathways exist to detect, excise and replace damaged DNA bases, and coordinate repair of DNA strand breaks. While molecular biology techniques have clarified structure, enzymatic functions, and kinetics of repair proteins, there is still a need to understand how repair is coordinated within the nucleus. Laser micro-irradiation offers a powerful tool for inducing DNA damage and monitoring the recruitment of repair proteins. Induction of DNA damage by laser micro-irradiation can occur with a range of wavelengths, and users can reliably induce single strand breaks, base lesions and double strand breaks with a range of doses. Here, laser micro-irradiation is used to examine repair of single and double strand breaks induced by two common confocal laser wavelengths, 355 nm and 405 nm. Further, proper characterization of the applied laser dose for inducing specific damage mixtures is described, so users can reproducibly perform laser micro-irradiation data acquisition and analysis.

Confocal fluorescence microscopy and laser micro-irradiation offer tools for inducing DNA damage and monitoring the response of DNA repair proteins in selected sub-nuclear areas. This technique has significantly advanced our knowledge of damage detection, signaling, and recruitment. This manuscript demonstrates these technologies to examine single and double strand break repair.

Highly coordinated DNA repair pathways exist to detect, excise and replace damaged DNA bases, and coordinate repair of DNA strand breaks. While molecular ...

Cancer Research

Immunofluorescence Microscopy of &#947;H2AX and 53BP1 for Analyzing the Formation and Repair of DNA Double-strand Breaks

DNA double-strand breaks (DSB) are serious DNA lesions. Analysis of the formation and repair of DSB is relevant in a broad spectrum of research areas including genome integrity, genotoxicity, radiation biology, aging, cancer, and drug development. In response to DSB, the histone H2AX is phosphorylated at Serine 139 in a region of several megabase pairs forming discrete nuclear foci detectable by immunofluorescence microscopy. In addition, 53BP1 (p53 binding protein 1) is another important DSB-responsive protein promoting repair of DSB by nonhomologous end-joining while preventing homologous recombination. According to the specific functions of &#947;H2AX and 53BP1, the combined analysis of &#947;H2AX and 53BP1 by immunofluorescence microscopy may be a reasonable approach for a detailed analysis of DSB. This manuscript provides a step-by-step protocol supplemented with methodical notes for performing the technique. Specifically, the influence of the cell cycle on &#947;H2AX foci patterns is demonstrated in normal fibroblasts of the cell line NHDF. Further, the value of the &#947;H2AX foci as a biomarker is depicted in x-ray irradiated lymphocytes of a healthy individual. Finally, genetic instability is investigated in CD34+ cells of a patient with acute myeloid leukemia by immunofluorescence microscopy of &#947;H2AX and 53BP1.

This manuscript provides a protocol for the analysis of DNA double-strand breaks by immunofluorescence microscopy of &#947;H2AX and 53BP1.

DNA double-strand breaks (DSB) are serious DNA lesions. Analysis of the formation and repair of DSB is relevant in a broad spectrum of research areas ...

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

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.

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.

The DNA damage response orchestrates the repair of DNA lesions that occur spontaneously, are caused by genotoxic stress, or appear in the context of ...

Biochemistry

Investigation of Protein Recruitment to DNA Lesions Using 405 Nm Laser Micro-irradiation

The DNA Damage Response (DDR) uses a plethora of proteins to detect, signal, and repair DNA lesions. Delineating this response is critical to understand genome maintenance mechanisms. Since recruitment and exchange of proteins at lesions are highly dynamic, their study requires the ability to generate DNA damage in a rapid and spatially-delimited manner. Here, we describe procedures to locally induce DNA damage in human cells using a commonly available laser-scanning confocal microscope equipped with a 405 nm laser line. Accumulation of genome maintenance factors at laser stripes can be assessed by immunofluorescence (IF) or in real-time using proteins tagged with fluorescent reporters. Using phosphorylated histone H2A.X (&#947;-H2A.X) and Replication Protein A (RPA) as markers, the method provides sufficient resolution to discriminate locally-recruited factors from those that spread on adjacent chromatin. We further provide ImageJ-based scripts to efficiently monitor the kinetics of protein relocalization at DNA damage sites. These refinements greatly simplify the study of the DDR dynamics.

Studying DNA damage repair kinetics requires a system to induce lesions at defined sub-nuclear regions. We describe a method to create localized double-stranded breaks using a laser-scanning confocal microscope equipped with a 405 nm laser and provide automated procedures to quantify the dynamics of repair factors at these lesions.

The DNA Damage Response (DDR) uses a plethora of proteins to detect, signal, and repair DNA lesions. Delineating this response is critical to understand ...

Characterizing DNA Repair Processes at Transient and Long-lasting Double-strand DNA Breaks by Immunofluorescence Microscopy

The repair of double-stranded breaks (DSBs) in DNA is a highly coordinated process, necessitating the formation and resolution of multi-protein repair complexes. This process is regulated by a myriad of proteins that promote the association and disassociation of proteins to these lesions. Thanks in large part to the ability to perform functional screens of a vast library of proteins, there is a greater appreciation of the genes necessary for the double-strand DNA break repair. Often knockout or chemical inhibitor screens identify proteins involved in repair processes by using increased toxicity as a marker for a protein that is required for DSB repair. Although useful for identifying novel cellular proteins involved in maintaining genome fidelity, functional analysis requires the determination of whether the protein of interest promotes localization, formation, or resolution of repair complexes. 
The accumulation of repair proteins can be readily detected as distinct nuclear foci by immunofluorescence microscopy. Thus, association and disassociation of these proteins at sites of DNA damage can be accessed by observing these nuclear foci at representative intervals after the induction of double-strand DNA breaks. This approach can also identify mis-localized repair factor proteins, if repair defects do not simultaneously occur with incomplete delays in repair. In this scenario, long-lasting double-strand DNA breaks can be engineered by expressing a rare cutting endonuclease (e.g., I-SceI) in cells where the recognition site for the said enzyme has been integrated into the cellular genome. The resulting lesion is particularly hard to resolve as faithful repair will reintroduce the enzyme's recognition site, prompting another round of cleavage. As a result, differences in the kinetics of repair are eliminated. If repair complexes are not formed, localization has been impeded. This protocol describes the methodology necessary to identify changes in repair kinetics as well as repair protein localization.

Repair of double-strand DNA breaks is a dynamic process, requiring not only formation of repair complexes at the breaks, but also their resolution after the lesion is addressed. Here, we use immunofluorescence microscopy for transient and long-lasting double-stranded breaks as a tool to dissect this genome maintenance mechanism.

The repair of double-stranded breaks (DSBs) in DNA is a highly coordinated process, necessitating the formation and resolution of multi-protein repair ...

Visualization of DNA Repair Proteins Interaction by Immunofluorescence

Mammalian cells are constantly exposed to chemicals, radiations, and naturally occurring metabolic by-products, which create specific types of DNA insults. Genotoxic agents can damage the DNA backbone, break it, or modify the chemical nature of individual bases. Following DNA insult, DNA damage response (DDR) pathways are activated and proteins involved in the repair are recruited. A plethora of factors are involved in sensing the type of damage and activating the appropriate repair response. Failure to correctly activate and recruit DDR factors can lead to genomic instability, which underlies many human pathologies including cancer. Studies of DDR proteins can provide insights into genotoxic drug response and cellular mechanisms of drug resistance.
There are two major ways of visualizing&#160;proteins in vivo: direct observation, by tagging the protein of interest with a fluorescent protein and following it by live imaging, or indirect immunofluorescence on fixed samples. While visualization of fluorescently tagged proteins allows precise monitoring over time, direct tagging in N- or C-terminus can interfere with the protein localization or function. Observation of proteins in their unmodified, endogenous version is preferred. When DNA repair proteins are recruited to the DNA insult, their concentration increases locally and they form groups, or &#8220;foci&#8221;, that can be visualized by indirect immunofluorescence using specific antibodies.
Although detection of protein foci does not provide a definitive proof of direct interaction, co-localization of proteins in cells indicates that they regroup to the site of damage and can inform of the sequence of events required for complex formation. Careful analysis of foci spatial overlap in cells expressing wild type or mutant versions of a protein can provide precious clues on functional domains important for DNA repair function. Last, co-localization of proteins indicates possible direct interactions that can be verified by co-immunoprecipitation in cells, or direct pulldown using purified proteins.

Following DNA damage, human cells activate essential repair pathways to restore the integrity of their genome. Here, we describe the method of indirect immunofluorescence as a means to detect DNA repair proteins, analyze their spatial and temporal recruitment, and help interrogate protein-protein interaction at the sites of DNA damage.

Mammalian cells are constantly exposed to chemicals, radiations, and naturally occurring metabolic by-products, which create specific types of DNA ...

Laser Micro-Irradiation to Study DNA Recruitment During S Phase

DNA damage repair maintains the genetic integrity of cells in a highly reactive environment. Cells may accumulate various types of DNA damage due to both endogenous and exogenous sources such as metabolic activities or UV radiation. Without DNA repair, the cell's genetic code becomes compromised, undermining the structures and functions of proteins and potentially causing disease. 
Understanding the spatiotemporal dynamics of the different DNA repair pathways in various cell cycle phases is crucial in the field of DNA damage repair. Current fluorescent microscopy techniques provide great tools to measure the recruitment kinetics of different repair proteins after DNA damage induction. DNA synthesis during the S phase of the cell cycle is a peculiar point in cell fate regarding DNA repair. It provides a unique window to screen the entire genome for mistakes. At the same time, DNA synthesis errors also pose a threat to DNA integrity that is not encountered in non-dividing cells. Therefore, DNA repair processes differ significantly in S phase as compared to other phases of the cell cycle, and those differences are poorly understood. 
The following protocol describes the preparation of cell lines and the measurement of dynamics of DNA repair proteins in S phase at locally induced DNA damage sites, using a laser-scanning confocal microscope equipped with a 405 nm laser line. Tagged PCNA (with mPlum) is used as a cell cycle marker combined with an AcGFP-labeled repair protein of interest (i.e., EXO1b) to measure the DNA damage recruitment in S phase.

This protocol describes a non-invasive method to efficiently identify S-phase cells for downstream microscopy studies, such as measuring DNA repair protein recruitment by laser micro-irradiation.

DNA damage repair maintains the genetic integrity of cells in a highly reactive environment. Cells may accumulate various types of DNA damage due to both ...

Immunofluorescence Microscopy for the Analysis of DNA Double-Strand Breaks

Source: Popp, H. D., et al. <a href="https://app.jove.com/v/56617">Immunofluorescence Microscopy of &#947;H2AX and 53BP1 for Analyzing the Formation and Repair of DNA Double-strand Breaks.</a> J. Vis. Exp. (2017)
In this video, we describe the immunofluorescence microscopy technique to detect specific DNA damage response proteins localized at the sites of DNA double-strand breaks in human mononuclear cells. The proteins are recognized by specific primary antibodies, which in turn bind to fluorophore-tagged secondary antibodies that fluoresce during microscopic visualization, enabling visualization of the proteins as distinct fluorescent foci within the cell nuclei.

Source: Popp, H. D., et al. Immunofluorescence Microscopy of &#947;H2AX and 53BP1 for Analyzing the Formation and Repair of DNA Double-strand Breaks. J. ...

Advanced Confocal Microscopy Techniques to Study Protein-protein Interactions and Kinetics at DNA Lesions

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