We thank all the people in our laboratory who contributed to optimize and validate the protocol. This work was supported by INSERM and the national research agency (ANR-09-JCJC-0116 AND ANR-11-BSV1-033-02). E.T. was supported during her PhD by a research fellowship from the French ministry of higher education and research.

1. Preparation of Solutions
<ol>
	<li>Prepare 10% formaldehyde in PBS (low salt) by diluting 5.5 ml of 37% formaldehyde in 14.5 ml of PBS. Prepare 1 M glycine, pH 8.0, by dissolving 3.8 g of glycine in 50 ml of PBS; dilute this solution to obtain 100 mM glycine in 1x PBS.</li>
	<li>Prepare 0.1% Triton-X100 in 1x PBS. Prepare 20x Saline Sodium Citrate (SSC) by using 3.0 M sodium chloride and 0.30 M trisodium citrate prepared in a deionized water buffer with pH 7.0 at 25 &#176;C. Dilute this buffer to 1x and 0.01x using deionized water.</li>
</ol>
2. Fixation of Cells
NOTE: We used the HuH7 hepatocarcinoma cell line in this study, but this method is applicable to other adherent cell cultures.
<ol>
	<li>Plate HuH7 cells (cultured in DMEM 1 g/L glucose, supplemented with 10% fetal calf serum and 0.01% penicillin-streptomycin stock) at 150,000 cells per uncoated 35-mm glass-bottom dish. When working on primary cell cultures, use collagen-coated dishes.</li>
	<li>The next day, remove the culture medium. Wash the cells with 1 ml of PBS and aspirate. Fix the cells by adding 1 ml of formaldehyde 10% and incubate for 10 min at room temperature (RT) under agitation.</li>
	<li>Stop the reaction with 1 ml of 1 M glycine and mix by rotation. Remove the stop reaction solution and wash the cells by adding 1 ml of 1x PBS; agitate by rotation and aspirate. Add 1 ml of 100 mM glycine to the cells and incubate them for 15 min at RT under agitation, and then aspirate. 
	NOTE: The protocol can be stopped here and the following steps can be postponed to another day. In that case, add 1 ml of 100 mM glycine and keep at 4 &#176;C, if necessary.</li>
</ol>
3. Permeabilization of Cells
<ol>
	<li>Add 1 ml of 0.1% Triton-X100 in 1x PBS, incubate for 15 min at RT under agitation, and then aspirate. This incubation time could be increased up to 15 - 20 min when working on primary cell cultures (e.g., primary mice hepatocytes). Wash the cells by adding 1 ml of 1x PBS; aspirate.</li>
</ol>
4. Blocking
<ol>
	<li>Add 40 &#181;l of blocking solution to each sample (provided by the kit); this volume can be increased in order to cover the sample. Incubate the dishes for 30 min at 37 &#176;C in a humidity chamber.</li>
	<li>Tap the blocking solution off of the dishes. Try to obtain equal residual volumes for each slide, as this will affect reproducibility. Do not let the samples dry!</li>
</ol>
5. Primary Antibodies
<ol>
	<li>Dilute the primary antibodies in 1x PBS and add the solution to the dishes (VDAC1 mouse antibody: 1/100, IP3R1 rabbit antibody: 1/500). Alternatively, GRP75 or CypD antibodies (both mouse antibodies used at 1/500) can be used instead of VDAC1.</li>
	<li>Incubate overnight in a humidity chamber at 4 &#176;C. Wash the slides two times using Tris Buffered Saline with 0.01% Tween (TBS-T).</li>
</ol>
6. Proximity Ligation Assay Probes
<ol>
	<li>Proximity ligation assay probes are provided by the kit. Choose probes according to the species of primary antibodies.</li>
	<li>Prepare the two proximity ligation assay probes 1:5 in antibody diluent. Allow the mixture to sit for 20 min at RT. Add the diluted proximity ligation assay probe solution. Incubate the dishes in a pre-heated humidity chamber for 1 hr at 37 &#176;C. Wash the dishes two times with TBS-T.</li>
</ol>
7. Ligation
<ol>
	<li>Use the in situ detection reagent Texas red kit.</li>
	<li>Dilute the 5x ligation stock (provided by the kit) 1:5 in high-purity water and mix well. Dilute the ligase in the 1x ligation solution (provided by the kit) 1:40 and vortex. Wait to add the ligase until immediately before addition to the samples.</li>
	<li>Add this solution to each sample (40 &#181;l for 35-mm glass-bottom dishes) and incubate the slides in a pre-heated humidity chamber for 30 min at 37 &#176;C. Wash the slides two times with TBS-T.</li>
</ol>
8. Amplification
NOTE: Be careful, light sensitive reagents.
<ol>
	<li>Dilute the 5x amplification stock (provided by the kit) 1:5 in high-purity water. Remove the polymerase from the freezer using the freezing block (-20 &#176;C). Dilute the polymerase (provided by the kit) 1:80 in the 1x amplification solution and vortex. Add the polymerase immediately before using the mixture.</li>
	<li>Add this solution to each sample (40 &#181;l for 35-mm glass-bottom dishes). Incubate the slides in a pre-heated humidity chamber for 100 min at 37 &#176;C. Tap the Amplification-Polymerase solution off of the slides.</li>
</ol>
9. Final Washing
<ol>
	<li>Wash the slides in 1x SSC washing buffer for 2 min. Wash the slides in 0.01x SSC washing buffer for 2 min. Let the dishes dry at RT in the dark.</li>
</ol>
10. Preparation for Imaging
<ol>
	<li>Mount the slides using a minimal volume of mounting medium containing DAPI (aqueous and does not solidify), ensuring that no air bubbles get caught under the cover slip. Nail polish can be used to seal the edges. Wait for approximately 15 min before analyzing using a fluorescence or confocal microscope (excitation: 594 nm, emission: 624 nm, magnification: 63X).</li>
	<li>After imaging, store the slides at -20 &#176;C in the dark.</li>
</ol>

<ol><li>Bravo-Sagua, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Organelle+communication%3A+signaling+crossroads+between+homeostasis+and+disease%5BTitle%5D)+AND+%22The+international+journal+of+biochemistry+%26+cell+biology%22%5BJournal%5D)">Organelle communication: signaling crossroads between homeostasis and disease.</a> The international journal of biochemistry & cell biology. 50, 55-59 (2014).</li>
<li>Giorgi, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mitochondria-associated+membranes%3A+composition%2C+molecular+mechanisms%2C+and+physiopathological+implications%5BTitle%5D)+AND+%22Antioxidants+%26+redox+signaling%22%5BJournal%5D)">Mitochondria-associated membranes: composition, molecular mechanisms, and physiopathological implications.</a> Antioxidants & redox signaling. 22, 995-1019 (2015).</li>
<li>Phillips, M. J., Voeltz, G. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Structure+and+function+of+ER+membrane+contact+sites+with+other+organelles%5BTitle%5D)+AND+%22Nature+reviews.+Molecular+cell+biology%22%5BJournal%5D)">Structure and function of ER membrane contact sites with other organelles.</a> Nature reviews. Molecular cell biology. 17, 69-82 (2016).</li>
<li>Cosson, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+RTM+resistance+to+potyviruses+in+Arabidopsis+thaliana%3A+natural+variation+of+the+RTM+genes+and+evidence+for+the+implication+of+additional+genes%5BTitle%5D)+AND+%22PLoS+One%22%5BJournal%5D)">The RTM resistance to potyviruses in Arabidopsis thaliana: natural variation of the RTM genes and evidence for the implication of additional genes.</a> PLoS One. 7, 39169(2012).</li>
<li>Mannella, C. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Structure+and+dynamics+of+the+mitochondrial+inner+membrane+cristae%5BTitle%5D)+AND+%22Biochim+Biophys+Acta%22%5BJournal%5D)">Structure and dynamics of the mitochondrial inner membrane cristae.</a> Biochim Biophys Acta. 1763, 542-548 (2006).</li>
<li>Csordas, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Structural+and+functional+features+and+significance+of+the+physical+linkage+between+ER+and+mitochondria%5BTitle%5D)+AND+%22The+Journal+of+cell+biology%22%5BJournal%5D)">Structural and functional features and significance of the physical linkage between ER and mitochondria.</a> The Journal of cell biology. 174, 915-921 (2006).</li>
<li>Mannella, C. A., Buttle, K., Rath, B. K., Marko, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Electron+microscopic+tomography+of+rat-liver+mitochondria+and+their+interaction+with+the+endoplasmic+reticulum%5BTitle%5D)+AND+%22Biofactors%22%5BJournal%5D)">Electron microscopic tomography of rat-liver mitochondria and their interaction with the endoplasmic reticulum.</a> Biofactors. 8, 225-228 (1998).</li>
<li>Rizzuto, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Close+contacts+with+the+endoplasmic+reticulum+as+determinants+of+mitochondrial+Ca2%2B+responses%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Close contacts with the endoplasmic reticulum as determinants of mitochondrial Ca2+ responses.</a> Science. 280, 1763-1766 (1998).</li>
<li>Wieckowski, M. R., Giorgi, C., Lebiedzinska, M., Duszynski, J., Pinton, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Isolation+of+mitochondria-associated+membranes+and+mitochondria+from+animal+tissues+and+cells%5BTitle%5D)+AND+%22Nat+Protoc%22%5BJournal%5D)">Isolation of mitochondria-associated membranes and mitochondria from animal tissues and cells.</a> Nat Protoc. 4, 1582-1590 (2009).</li>
<li>Csordas, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Imaging+interorganelle+contacts+and+local+calcium+dynamics+at+the+ER-mitochondrial+interface%5BTitle%5D)+AND+%22Mol+Cell%22%5BJournal%5D)">Imaging interorganelle contacts and local calcium dynamics at the ER-mitochondrial interface.</a> Mol Cell. 39, 121-132 (2010).</li>
<li>Szabadkai, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Chaperone-mediated+coupling+of+endoplasmic+reticulum+and+mitochondrial+Ca2%2B+channels%5BTitle%5D)+AND+%22J+Cell+Biol%22%5BJournal%5D)">Chaperone-mediated coupling of endoplasmic reticulum and mitochondrial Ca2+ channels.</a> J Cell Biol. 175, 901-911 (2006).</li>
<li>Tubbs, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mitochondria-associated+endoplasmic+reticulum+membrane+%28MAM%29+integrity+is+required+for+insulin+signaling+and+is+implicated+in+hepatic+insulin+resistance%5BTitle%5D)+AND+%22Diabetes%22%5BJournal%5D)">Mitochondria-associated endoplasmic reticulum membrane (MAM) integrity is required for insulin signaling and is implicated in hepatic insulin resistance.</a> Diabetes. 63, 3279-3294 (2014).</li>
<li>Paillard, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Depressing+Mitochondria-Reticulum+Interactions+Protects+Cardiomyocytes+From+Lethal+Hypoxia-Reoxygenation+Injury%5BTitle%5D)+AND+%22Circulation%22%5BJournal%5D)">Depressing Mitochondria-Reticulum Interactions Protects Cardiomyocytes From Lethal Hypoxia-Reoxygenation Injury.</a> Circulation. 128, 1555-1565 (2013).</li>
<li>Rieusset, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Disruption+of+calcium+transfer+from+ER+to+mitochondria+links+alterations+of+mitochondria-associated+ER+membrane+integrity+to+hepatic+insulin+resistance%5BTitle%5D)+AND+%22Diabetologia%22%5BJournal%5D)">Disruption of calcium transfer from ER to mitochondria links alterations of mitochondria-associated ER membrane integrity to hepatic insulin resistance.</a> Diabetologia. 59, 614-623 (2016).</li>
<li>Allalou, A., Wahlby, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((BlobFinder%2C+a+tool+for+fluorescence+microscopy+image+cytometry%5BTitle%5D)+AND+%22Computer+methods+and+programs+in+biomedicine%22%5BJournal%5D)">BlobFinder, a tool for fluorescence microscopy image cytometry.</a> Computer methods and programs in biomedicine. 94, 58-65 (2009).</li>
<li>Theurey, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mitochondria-associated+endoplasmic+reticulum+membranes+allow+adaptation+of+mitochondrial+metabolism+to+glucose+availability+in+the+liver%5BTitle%5D)+AND+%22Journal+of+molecular+cell+biology%22%5BJournal%5D)">Mitochondria-associated endoplasmic reticulum membranes allow adaptation of mitochondrial metabolism to glucose availability in the liver.</a> Journal of molecular cell biology. , (2016).</li>
<li>de Brito, O. M., Scorrano, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mitofusin+2+tethers+endoplasmic+reticulum+to+mitochondria%5BTitle%5D)+AND+%22Nature%22%5BJournal%5D)">Mitofusin 2 tethers endoplasmic reticulum to mitochondria.</a> Nature. 456, 605-610 (2008).</li>
<li>Soderberg, O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Direct+observation+of+individual+endogenous+protein+complexes+in+situ+by+proximity+ligation%5BTitle%5D)+AND+%22Nature+methods%22%5BJournal%5D)">Direct observation of individual endogenous protein complexes in situ by proximity ligation.</a> Nature methods. 3, 995-1000 (2006).</li>
<li>De Pinto, V., Messina, A., Lane, D. J., Lawen, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Voltage-dependent+anion-selective+channel+%28VDAC%29+in+the+plasma+membrane%5BTitle%5D)+AND+%22FEBS+letters%22%5BJournal%5D)">Voltage-dependent anion-selective channel (VDAC) in the plasma membrane.</a> FEBS letters. 584, 1793-1799 (2010).</li>
<li>Kaul, S. C., Taira, K., Pereira-Smith, O. M., Wadhwa, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mortalin%3A+present+and+prospective%5BTitle%5D)+AND+%22Experimental+gerontology%22%5BJournal%5D)">Mortalin: present and prospective.</a> Experimental gerontology. 37, 1157-1164 (2002).</li>
</ol>

The authors declare that they have no competing financial interests.

Collectively, our studies indicate that the in situ proximity ligation assay is truly a relevant strategy to follow and quantify endogenous ER-mitochondria interactions in fixed cells, without the need for using organelle-specific fluorophores or fluorescent proteins. The specific use of VDAC1/IP3R1 antibodies has been adapted to study ER-mitochondria interactions in HuH7 cells. However, alternative isoforms of VDAC and IP3R may be used, depending on the cell type. In this case, antibodies need to be validated b...

Mitochondria and endoplasmic reticulum (ER) are not independent organelles in the cell, but they interact structurally and functionally at contact sites defined as mitochondria-associated endoplasmic reticulum membranes (MAM). In fact, MAMs correspond to regions where the membranes of the ER and mitochondria are closely apposed, allowing interactions between proteins from both sides. Nonetheless, the membranes of these organelles do not fuse within these regions, so they maintain their separate entities. The MAMs play a crucial role in calcium (Ca2+) and phospholipid transfer from ER to mitochondria, impacting energy metabolism and cell survival1-3.
The association between the ER and mitochondria was first visualized in the 1970s with electron microscopy. Since then, transmission electron microscopy4,5, electron tomography6,7 or immuno-localization of ER and mitochondria-specific fluorophores/fluorescent proteins8 were classically used to study ER-mitochondria interactions. Another useful tool for the analysis of MAM is based on the use of subcellular fractionation. It allows the isolation of MAM fractions by differential ultracentrifugation coupled to a Percoll gradient9. However, the final product contains enriched MAM fractions, rather than pure fractions. Altogether, these strategies are not particularly sensitive and/or quantitative, and they are not easily amenable to large screening. Alternatively, genetic approaches using drug-inducible fluorescent inter-organelle linkers have emerged, but they do not allow the analysis of organelle interactions at the endogenous expression levels of proteins10.
Based on Szabadkai&#39;s discovery of the IP3R/GRP75/VDAC complex at the MAM11, we developed a quantitative method to analyze ER-mitochondria interactions. We used the in situ proximity ligation assay to detect and quantify interactions between VDAC1 and IP3R1, two organelle-surface proteins involved in the Ca2+-channeling complex at the MAM interface in fixed cells12. Briefly, we probed VDAC1 at the outer mitochondrial membrane (mouse anti-VDAC1 primary antibody) and IP3R1 at the ER membrane (rabbit anti-IP3R1 primary antibody) (Figure 1, panel a). Then, according to the assay, we added both anti-mouse and anti-rabbit IgG (mouse and rabbit proximity ligation assay probes), which are conjugated to complementary oligonucleotide extensions. If the two targeted proteins are at a distance below 40 nm, the oligonucleotides can hybridize with the subsequently added connector oligos to allow the formation of a circular DNA template (Figure 1, panel b). This circular DNA molecule is ligated and amplified, creating a single-stranded DNA product covalently attached to one of the proximity probes (Figure 1, panel c). Since the distance between the ER and mitochondria at the MAM interface ranges from 10 nm to 25 nm6, proximity ligation and amplification can be done, leading to subsequent detection due to the hybridization of Texas red-labeled oligonucleotides probes (Figure 1, panel d). Each fluorescent dot represents interactions between VDAC1/IP3R1, thus allowing the quantification of in situ ER-mitochondria interactions in individual cells.
<img alt="Figure 1" src="/files/ftp_upload/54899/54899fig1.jpg" /> 
Figure 1: Schematic Illustration of the Detection of the Endoplasmic Reticulum-mitochondria Interactions by In Situ Proximity Ligation Assay. a) A mouse primary antibody directed against VDAC1 and a rabbit primary antibody directed against IP3R1 can bind to their epitopes in proximity at the MAM interface, b) The addition of a pair of proximity ligation probes directed against mouse and rabbit IgG. These probes have attached DNA strands that can form templates for the ligation of connector oligos. c) The circular DNA strand formed after ligation can be amplified and d) visualized by microscopy as a fluorescent dot by using Texas red-labeled oligonucleotides. <a href="http://cloudflare.jove.com/files/ftp_upload/54899/54899fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Similar in situ proximity ligation assay experiments can be performed with the GRP75/IP3R1 pair of antibodies, as well as cyclophilin D (CypD)/IP3R1 antibodies, considering that CypD was shown to interact with the IP3R/GRP75/VDAC complex at the MAM interface12-14.

<table><tbody><tr><td>Formaldehyde</td><td>Sigma</td><td>F-8775</td><td></td></tr><tr><td>Glycine</td><td>Sigma</td><td>G-8898</td><td></td></tr><tr><td>Triton</td><td>Sigma</td><td>T8532</td><td></td></tr><tr><td>35 mm Glass bottom culture dishes</td><td>MatTeK corporation</td><td>P35G-0-14-C</td><td></td></tr><tr><td>Blocking solution</td><td>Sigma</td><td>DUO-92004 or DUO-92002</td><td>provided in the Duolink PLA probes, Sigma</td></tr><tr><td>VDAC1 antibody</td><td>Abcam</td><td>ab14734</td><td></td></tr><tr><td>IP3R1-H80 antibody</td><td>Santa Cruz</td><td>sc28614</td><td></td></tr><tr><td>CypD antibody</td><td>Abcam</td><td>ab110324</td><td></td></tr><tr><td>Grp75 antibody</td><td>Santa Cruz</td><td>sc13967</td><td></td></tr><tr><td>TBS 10x</td><td>euromedex</td><td>ET220</td><td>Dilute to obtain 1x</td></tr><tr><td>Tween 100x</td><td>euromedex</td><td>2001-B</td><td>dilute in TBS to obtain 0,01%</td></tr><tr><td>PLA Probes Mouse MINUS</td><td>Sigma</td><td>DUO-92004</td><td>Duolink, Sigma</td></tr><tr><td>PLA Probes Rabbit PLUS</td><td>Sigma</td><td>DUO-92002</td><td>Duolink, Sigma</td></tr><tr><td>Duolink detection reagents red</td><td>Sigma</td><td>DUO-92008</td><td>Duolink, Sigma</td></tr><tr><td>Ligation solution</td><td>Sigma</td><td>DUO-92008</td><td>Part of the Duolink detection reagents red, Sigma</td></tr><tr><td>Ligase</td><td>Sigma</td><td>DUO-92008</td><td>Part of the Duolink detection reagents red, Sigma</td></tr><tr><td>Amplification solution</td><td>Sigma</td><td>DUO-92008</td><td>Part of the Duolink detection reagents red, Sigma</td></tr><tr><td>Polymerase</td><td>Sigma</td><td>DUO-92008</td><td>Part of the Duolink detection reagents red, Sigma</td></tr><tr><td>Duolink Mounting Medium</td><td>Sigma</td><td>DUO80102</td><td>Duolink, Sigma</td></tr><tr><td colspan="4">&lt;strong&gt;Softwares&lt;/strong&gt;</td></tr><tr><td>Blob-finder software</td><td></td><td></td><td>BlobFinder is a freely distributed software that can perform calculations on cells from fluorescence microscopy images. This software can be downloaded for free from The Centre for Image Analysis at Uppsala University who have developed the software and the work was supported by the EU FP6 Project ENLIGHT and Olink Bioscience. http://www.cb.uu.se/~amin/BlobFinder/index_files/Page430.htm</td></tr><tr><td>ImageJ software</td><td></td><td></td><td>Can be downloaded for free from: http://rsb.info.nih.gov/ij/download.html</td></tr></tbody></table>

study of endoplasmic reticulum and mitochondria interactions by in situ proximity ligation assay in fixed cells

Structural interactions between the endoplasmic reticular (ER) and mitochondrial membranes, in domains known as mitochondria-associated membranes (MAM), are crucial hubs for cellular signaling and cell fate. Particularly, these inter-organelle contact sites allow the transfer of calcium from the ER to mitochondria through the voltage-dependent anion channel (VDAC)/glucose-regulated protein 75 (GRP75)/inositol 1,4,5-triphosphate receptor (IP3R) calcium channeling complex. While this subcellular compartment is under intense investigation in both physiological and pathological conditions, no simple and sensitive method exists to quantify the endogenous amount of ER-mitochondria contact in cells. Similarly, MAMs are highly dynamic structures, and there is no suitable approach to follow modifications of ER-mitochondria interactions without protein overexpression. Here, we report an optimized protocol based on the use of an in situ proximity ligation assay to visualize and quantify endogenous ER-mitochondria interactions in fixed cells by using the close proximity between proteins of the outer mitochondrial membrane (VDAC1) and of the ER membrane (IP3R1) at the MAM interface. Similar in situ proximity ligation experiments can also be performed with the GRP75/IP3R1 and cyclophilin D/IP3R1 pairs of antibodies. This assay provides several advantages over other imaging procedures, as it is highly specific, sensitive, and suitable to multiple-condition testing. Therefore, the use of this in situ proximity ligation assay should be helpful to better understand the physiological regulations of ER-mitochondria interactions, as well as their role in pathological contexts.

Based on our experience using this protocol, we can safely recommend this method for the visualization and quantification of ER-mitochondria interactions in fixed cells. Representative images of in situ proximity ligation assay-visualized ER-mitochondria interactions in the HuH7 hepatocarcinoma cell line, using several pairs of antibodies, are shown. As shown in Figure 2 by fluorescent microscopy, each red dot represents an interaction between the two targeted pr...

Watch this Scientific Journal Video about Study of Endoplasmic Reticulum and Mitochondria Interactions by In Situ Proximity Ligation Assay in Fixed Cells at JoVE.com

Study of Endoplasmic Reticulum and Mitochondria Interactions by In Situ Proximity Ligation Assay in Fixed Cells

Here, we describe a procedure to visualize and quantify with high sensitivity the endogenous interactions between the endoplasmic reticulum and mitochondria in fixed cells. The protocol features an optimized in situ proximity ligation assay targeting the inositol 1,4,5-triphosphate receptor/glucose-regulated protein 75/voltage-dependent anion channel/cyclophilin D complex at the mitochondria-associated membrane interface.

Study of Endoplasmic Reticulum and Mitochondria Interactions by In Situ Proximity Ligation Assay in Fixed Cells

Structural interactions between the endoplasmic reticular (ER) and mitochondrial membranes, in domains known as mitochondria-associated membranes (MAM), ...

study-endoplasmic-reticulum-mitochondria-interactions-situ-proximity

Research

JoVE Journal

Biology

24.6K Views.  Lund University.  Hier beschrijven we een werkwijze voor het visualiseren en kwantificeren gevoeligheid een endogene interacties tussen het endoplasmatisch reticulum en mitochondria in gefixeerde cellen. Het protocol is voorzien van een geoptimaliseerde in situ nabijheid ligation assay gericht op de inositol 1,4,5-trifosfaat receptor / glucose-gereguleerd eiwit 75 / voltage-afhankelijke anion kanaal / cyclofiline D-complex aan de mitochondria bijbehorende membraan interface. 

Video: Study of Endoplasmic Reticulum and Mitochondria Interactions by In Situ Proximity Ligation Assay in Fixed Cells

Visualization of Protein-protein Interaction in Nuclear and Cytoplasmic Fractions by Co-immunoprecipitation and In Situ Proximity Ligation Assay

Protein-protein interactions are involved in thousands of cellular processes and occur in distinct spatial context. Traditionally, co-immunoprecipitation is a popular technique to detect protein-protein interactions. Subsequent Western blot analysis is the most common method to visualize co-immunoprecipitated proteins. Recently, the proximity ligation assay has become a powerful tool to visualize protein-protein interactions in situ and provides the possibility to quantify protein-protein interactions by this method. Similar to conventional immunocytochemistry, the proximity ligation assay technique is also based on the accessibility of primary antibodies to the antigens, but in contrast, proximity ligation assay detects protein-protein interactions with a unique technique involving rolling-circle PCR, while conventional immunocytochemistry only shows co-localization of proteins.
Nuclear factor 90 (NF90) and RNA-binding motif protein 3 (RBM3) have been previously demonstrated as interacting partners. They are predominantly localized in the nucleus, but also migrate into the cytoplasm and regulate signaling pathways in the cytoplasmic compartment. Here, we compared NF90-RBM3 interaction in both the nucleus and the cytoplasm by co-immunoprecipitation and proximity ligation assay. In addition, we discussed the advantages and limitations of these two techniques in visualizing protein-protein interactions in respect to spatial distribution and the properties of protein-protein interactions.

Visualization of Protein-protein Interaction in Nuclear and Cytoplasmic Fractions by Co-immunoprecipitation and In Situ Proximity Ligation Assay

Protein-protein interactions can occur in both the nucleus and the cytoplasm of a cell. To investigate these interactions, traditional co-immunoprecipitation and modern proximity ligation assay are applied. In this study, we compare these two methods to visualize the distribution of NF90-RBM3 interactions in the nucleus and the cytoplasm.

Protein-protein interactions are involved in thousands of cellular processes and occur in distinct spatial context. Traditionally, co-immunoprecipitation ...

Biochemistry

Detection of Heterodimerization of Protein Isoforms Using an in Situ Proximity Ligation Assay

Regulated protein-protein interactions are a guiding principle for many signaling events, and the detection of such events is an important element in understanding how such pathways are organized and how they function. There are many methods to detect protein-protein interactions in cells, but relatively few can be used to detect interactions between endogenous proteins. One such method, the proximity ligation assay (PLA), has several advantages to recommend its use. Compared to other common methods of protein-protein interaction analysis, PLA has relatively high sensitivity and specificity, can be performed with minimal cell manipulation, and, in the protocol described herein, requires only two target-specific antibodies derived from different species (e.g., from mouse and rabbit) and one specialized reagent: a set of secondary antibodies that are covalently linked to specific oligonucleotides that, when brought in close proximity of one another, create an amplifiable platform for in situ PCR or rolling circle amplification. In this presentation, we show how to apply the PLA technique to visualize changes in MST1 and MST2 proximity in fixed cells. The technique described in this manuscript is particularly applicable for the analysis of cell signaling studies.

Detection of Heterodimerization of Protein Isoforms Using an in Situ Proximity Ligation Assay

Here, we show how to use a Proximity Ligation Assay (PLA) to visualize MST1/MST2 heterodimerization in fixed cells with high sensitivity.

Regulated protein-protein interactions are a guiding principle for many signaling events, and the detection of such events is an important element in ...

Mitochondria and Endoplasmic Reticulum Imaging by Correlative Light and Volume Electron Microscopy

Cellular organelles, such as mitochondria and endoplasmic reticulum (ER), create a network to perform a variety of functions. These highly curved structures are folded into various shapes to form a dynamic network depending on the cellular conditions. Visualization of this network between mitochondria and ER has been attempted using super-resolution fluorescence imaging and light microscopy; however, the limited resolution is insufficient to observe the membranes between the mitochondria and ER in detail. Transmission electron microscopy provides good membrane contrast and nanometer-scale resolution for the observation of cellular organelles; however, it is exceptionally time-consuming when assessing the three-dimensional (3D) structure of highly curved organelles. Therefore, we observed the morphology of mitochondria and ER via correlative light-electron microscopy (CLEM) and volume electron microscopy techniques using enhanced ascorbate peroxidase 2 and horseradish peroxidase staining. An en bloc staining method, ultrathin serial sectioning (array tomography), and volume electron microscopy were applied to observe the 3D structure. In this protocol, we suggest a combination of CLEM and 3D electron microscopy to perform detailed structural studies of mitochondria and ER.

We present a protocol to study the distribution of mitochondria and endoplasmic reticulum in whole cells after genetic modification using correlative light and volume electron microscopy including ascorbate peroxidase 2 and horseradish peroxidase staining, serial sectioning of cells with and without the target gene in the same section, and serial imaging via electron microscopy.

Cellular organelles, such as mitochondria and endoplasmic reticulum (ER), create a network to perform a variety of functions. These highly curved ...

In Situ Detection of Ribonucleoprotein Complex Assembly in the C. elegans Germline using Proximity Ligation Assay

Understanding when and where protein-protein interactions (PPIs) occur is critical to understanding protein function in the cell and how broader processes such as development are affected. The Caenorhabditis elegans germline is a great model system for studying PPIs that are related to the regulation of stem cells, meiosis, and development. There are a variety of well-developed techniques that allow proteins of interest to be tagged for recognition by standard antibodies, making this system advantageous for proximity ligation assay (PLA) reactions. As a result, the PLA is able to show where PPIs occur in a spatial and temporal manner in germlines more effectively than alternative approaches. Described here is a protocol for the application and quantification of this technology to probe PPIs in the C. elegans germline.

In Situ Detection of Ribonucleoprotein Complex Assembly in the C. elegans Germline using Proximity Ligation Assay

This protocol demonstrates use of the proximity ligation assay to probe for protein-protein interactions in situ in the C. elegans germline.

Understanding when and where protein-protein interactions (PPIs) occur is critical to understanding protein function in the cell and how broader processes ...

Developmental Biology

Detection of In Situ Protein-protein Complexes at the Drosophila Larval Neuromuscular Junction Using Proximity Ligation Assay

Discs large (Dlg) is a conserved member of the membrane-associated guanylate kinase family, and serves as a major scaffolding protein at the larval neuromuscular junction (NMJ) in Drosophila. Previous studies have shown that the postsynaptic distribution of Dlg at the larval NMJ overlaps with that of Hu-li tai shao (Hts), a homologue to the mammalian adducins. In addition, Dlg and Hts are observed to form a complex with each other based on co-immunoprecipitation experiments involving whole adult fly lysates. Due to the nature of these experiments, however, it was unknown whether this complex exists specifically at the NMJ during larval development.
Proximity Ligation Assay (PLA) is a recently developed technique used mostly in cell and tissue culture that can detect protein-protein interactions in situ. In this assay, samples are incubated with primary antibodies against the two proteins of interest using standard immunohistochemical procedures. The primary antibodies are then detected with a specially designed pair of oligonucleotide-conjugated secondary antibodies, termed PLA probes, which can be used to generate a signal only when the two probes have bound in close proximity to each other. Thus, proteins that are in a complex can be visualized. Here, it is demonstrated how PLA can be used to detect in situ protein-protein interactions at the Drosophila larval NMJ. The technique is performed on larval body wall muscle preparations to show that a complex between Dlg and Hts does indeed exist at the postsynaptic region of NMJs.

Detection of In Situ Protein-protein Complexes at the Drosophila Larval Neuromuscular Junction Using Proximity Ligation Assay

This protocol demonstrates how Proximity Ligation Assay can be used to detect in situ protein-protein interactions at the Drosophila larval neuromuscular junction. With this technique, Discs large and Hu-li tai shao are shown to form a complex at the postsynaptic region, an association previously identified through co-immunoprecipitation.

Discs large (Dlg) is a conserved member of the membrane-associated guanylate kinase family, and serves as a major scaffolding protein at the larval ...

Neuroscience

A Simple Assay to Measure ER-Mitochondria Contacts for High-Throughput Screens

Split-Luciferase Reassembly Assay to Measure Endoplasmic Reticulum-Mitochondria Contacts in Live Cells

Endoplasmic reticulum (ER)-mitochondria contact sites play a critical role in cell health and homeostasis, such as the regulation of Ca2+ and lipid homeostasis, mitochondrial dynamics, autophagosome and mitophagosome biogenesis, and apoptosis. Failure to maintain normal ER-mitochondrial coupling is implicated in many neurodegenerative diseases such as Alzheimer&#39;s disease, Parkinson&#39;s disease, amyotrophic lateral sclerosis, and hereditary spastic paraplegia. It is of considerable significance to explore how the dysregulation of ER-mitochondrial contacts could lead to cell death and whether repairing these contacts to the normal level could ameliorate neurodegenerative conditions. Thus, improved assays that measure the level of these contacts could help to illuminate the pathogenic mechanisms of these diseases. Ultimately, establishing simple and reliable assays will facilitate the development of new therapeutic strategies. Here we describe&#160;a split-luciferase assay to quantitatively measure the level of ER-mitochondria contacts in live cells. This assay can be used to study the pathophysiological role of these contacts as well as to identify their modulators in high-throughput screening.

Author Spotlight: Regulation and Dysregulation of ER-Mitochondria Contacts &#8212; Implications for Neurodegenerative Disease Pathogenesis

We have established a split-luciferase reassembly assay to monitor the endoplasmic reticulum-mitochondria contacts in live cells. Using this assay, we describe a protocol to quantitatively measure the level of these inter-organelle couplings in HEK293T cells, under the condition of chemical treatment.

Endoplasmic reticulum (ER)-mitochondria contact sites play a critical role in cell health and homeostasis, such as the regulation of Ca2+ and lipid ...

Visualization of Endoplasmic Reticulum Subdomains in Cultured Cells

The lipids and proteins in eukaryotic cells are continuously exchanged between cell compartments, although these retain their distinctive composition and functions despite the intense interorganelle molecular traffic. The techniques described in this paper are powerful means of studying protein and lipid mobility and trafficking in vivo and in their physiological environment. Fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP) are widely used live-cell imaging techniques for studying intracellular trafficking through the exo-endocytic pathway, the continuity between organelles or subcompartments, the formation of protein complexes, and protein localization in lipid microdomains, all of which can be observed under physiological and pathological conditions. The limitations of these approaches are mainly due to the use of fluorescent fusion proteins, and their potential drawbacks include artifactual over-expression in cells and the possibility of differences in the folding and localization of tagged and native proteins. Finally, as the limit of resolution of optical microscopy (about 200 nm) does not allow investigation of the fine structure of the ER or the specific subcompartments that can originate in cells under stress (i.e. hypoxia, drug administration, the over-expression of transmembrane ER resident proteins) or under pathological conditions, we combine live-cell imaging of cultured transfected cells with ultrastructural analyses based on transmission electron microscopy.

We describe the imaging approaches we use to investigate the distribution and mobility of the transfected fluorescent proteins resident in the endoplasmic reticulum (ER) by means of the confocal imaging of living cells. We also ultrastructurally analyze the effect of their expression on the architecture of this subcellular compartment.

The lipids and proteins in eukaryotic cells are continuously exchanged between cell compartments, although these retain their distinctive composition and ...

Correlative Microscopy Reveals Stress-Induced Mitochondria-Lysosome Interactions

Dual-color Correlative Light and Electron Microscopy for the Visualization of Interactions between Mitochondria and Lysosomes

Cellular organelles, such as mitochondria and lysosomes, display dynamic structures. Despite the higher resolution of transmission electron microscopy for structural analysis, light microscopy is essential for the visualization of dynamic organelles by target-specific labeling. The following protocol describes a method that combines dual-color correlative light and electron microscopy (CLEM) to observe the interactions between mitochondria and lysosomes. In this study, mitochondria were labeled with mEosEM (Mito-mEosEM) and lysosomes with TMEM192-V5-APEX2. The results obtained from CLEM images enable us to observe the changes in the interactions between mitochondria and lysosomes under external stress conditions. Treatment with bafilomycin (BFA), which inhibits lysosomal function, resulted in an increase in contact between mitochondria and lysosomes, leading to the formation of fragmented mitochondria trapped inside lysosomes. Conversely, treatment with U18666A, which inhibits cholesterol export from lysosomes, caused lysosomes to be surrounded by mitochondria, indicating a distinct form of interaction. This study presents an effective method for observing the interactions between mitochondria and lysosomes in fixed cells. Furthermore, CLEM imaging with dual-color probes offers a powerful tool for future investigations of organelle dynamics and their implications for cell function and pathology.

This protocol describes a mechanism for using correlative light and electron microscopy to visualize the interaction of mitochondria and lysosomes labeled with mEosEM and APEX2, respectively.

Cellular organelles, such as mitochondria and lysosomes, display dynamic structures. Despite the higher resolution of transmission electron microscopy for ...

Visualization and Quantification of Endogenous Intra-Organelle Protein Interactions at ER-Mitochondria Contact Sites by Proximity Ligation Assays

Membrane contact sites (MCSs) are areas of close membrane proximity that allow and regulate the dynamic exchange of diverse biomolecules (i.e., calcium and lipids) between the juxtaposed organelles without involving membrane fusion. MCSs are essential for cellular homeostasis, and their functions are ensured by the resident components, which often exist as multimeric protein complexes. MCSs often involve the endoplasmic reticulum (ER), a major site of lipid synthesis and cellular calcium storage, and are particularly important for organelles, such as the mitochondria, which are excluded from the classical vesicular transport pathways. In the last years, MCSs between the ER and mitochondria have been extensively studied, as their functions strongly impact cellular metabolism/bioenergetics. Several proteins have started to be identified at these contact sites, including membrane tethers, calcium channels, and lipid transfer proteins, thus raising the need for new methodologies and technical approaches to study these MCS components. Here, we describe a protocol consisting of combined technical approaches, that include proximity ligation assay (PLA), mitochondria staining, and 3D imaging segmentation, that allows the detection of proteins that are physically close (&gt;40 nm) to each other and that reside on the same membrane at ER-mitochondria MCSs. For instance, we used two ER-anchored lipid transfer proteins, ORP5 and ORP8, which have previously been shown to interact and localize at ER-mitochondria and ER-plasma membrane MCSs. By associating the ORP5-ORP8 PLA with cell imaging software analysis, it was possible to estimate the distance of the ORP5-ORP8 complex from the mitochondrial surface and determine that about 50% of ORP5-ORP8 PLA interaction occurs at ER subdomains in close proximity to mitochondria.

The need for new approaches to study membrane contact sites (MCSs) has grown due to increasing interest in studying these cellular structures and their components. Here, we present a protocol that integrates previously available microscopy technologies to identify and quantify intra-organelle and inter-organelle protein complexes that reside at MCSs.

Membrane contact sites (MCSs) are areas of close membrane proximity that allow and regulate the dynamic exchange of diverse biomolecules (i.e., calcium ...

Proximity Ligation Assay to Study In Situ Protein-Protein Interactions

Source: Wang, X., et al., <a href="https://app.jove.com/v/60982">In Situ Detection of Ribonucleoprotein Complex Assembly in the C. elegans Germline using Proximity Ligation Assay.</a>&#160;J. Vis. Exp.&#160; (2020)
This video describes the proximity ligation assay to study in situ protein-protein interactions in nematode cells. The close proximity of the proteins of interest generates the circular DNA template with the help of a PLA probe attached to a secondary antibody. The probe is further amplified using the amplification solution, which produces a long single-stranded DNA followed by the hybridization of the detector probe, enabling the localization of protein-protein interactions in cells.

Source: Wang, X., et al., In Situ Detection of Ribonucleoprotein Complex Assembly in the C. elegans Germline using Proximity Ligation Assay.&#160;J. Vis. ...

Study of Endoplasmic Reticulum and Mitochondria Interactions by In Situ Proximity Ligation Assay in Fixed Cells

Samenvatting

Meer video's verkennen