We thank Dr. Maria Reichenbach and Dr. Christian Hiepen for their support on the flow-set up system and Eleanor Fox and Yunyun Xiao for critically reading the manuscript. P-L.M. was funded by the international Max Planck Research School IMPRS-Biology and Computation (IMPRS-BAC). PK received funding by the DFG-SFB1444. Figure 1 was created using BioRender.

1. Cell culture and fluid shear stress exposure
NOTE: Human umbilical vein ECs (HUVECs) were used as an example to study SS induced interaction of SMADs. The protocol described below can be applied to every SS responsive cell type.
<ol>
	<li>Coat 6-channel slide with 0.1% porcine skin gelatin in PBS for 30 min at 37 &#176;C.</li>
	<li>Seed HUVECs in pre-coated 6-channel slides at a density of 2.5 x 106 cells per mL in 30 &#181;L of M199 full medium. 
	NOTE: For further inform...

<ol>
	<li>Yadin, D., Knaus, P., Mueller, T. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Structural+insights+into+BMP+receptors:+Specificity,+activation+and+inhibition.">Structural insights into BMP receptors: Specificity, activation and inhibition.</a> Cytokine and Growth Factor Reviews. 27, 13-34 (2016).</li><li>Sieber, C., Kopf, J., Hiepen, C., Knaus, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+advances+in+BMP+receptor+signaling.">Recent advances in BMP receptor signaling.</a> Cytokine and Growth Factor Reviews. 20 (5-6), 343-355 (2009).</li><li>Hiepen, 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=BMPR2+acts+as+a+gatekeeper+to+protect+endothelial+cells+from+increased+TGF&#946;+responses+and+altered+cell+mechanics.">BMPR2 acts as a gatekeeper to protect endothelial cells from increased TGF&#946; responses and altered cell mechanics.</a> PLoS Biology. 17 (12), 3000557 (2019).</li><li>Hildebrandt, 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=ActivinA+induced+SMAD1/5+Signaling+in+an+iPSC+derived+EC+model+of+Fibrodysplasia+Ossificans+Progressiva+(FOP)+can+be+rescued+by+the+drug+candidate+saracatinib.">ActivinA induced SMAD1/5 Signaling in an iPSC derived EC model of Fibrodysplasia Ossificans Progressiva (FOP) can be rescued by the drug candidate saracatinib.</a> Stem Cell Reviews and Reports. , (2021).</li><li>Goumans, M. J., 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=Balancing+the+activation+state+of+the+endothelium+via+two+distinct+TGF-beta+type+I+receptors.">Balancing the activation state of the endothelium via two distinct TGF-beta type I receptors.</a> The EMBO Journal. 21 (7), 1743-1753 (2002).</li><li>Goumans, M. J., 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=Activin+receptor-like+kinase+(ALK)1+is+an+antagonistic+mediator+of+lateral+TGFbeta/ALK5+signaling.">Activin receptor-like kinase (ALK)1 is an antagonistic mediator of lateral TGFbeta/ALK5 signaling.</a> Molecular Cell. 12 (4), 817-828 (2003).</li><li>Daly, A. C., Randall, R. A., Hill, C. 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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=Brightfield+proximity+ligation+assay+reveals+both+canonical+and+mixed+transforming+growth+factor-&#946;/bone+morphogenetic+protein+Smad+signaling+complexes+in+tissue+sections.">Brightfield proximity ligation assay reveals both canonical and mixed transforming growth factor-&#946;/bone morphogenetic protein Smad signaling complexes in tissue sections.</a> The Journal of Histochemistry and Cytochemistry : The Official Journal of The Histochemistry Society. 62 (12), 846-863 (2014).</li><li>Miyazono, K., Maeda, S., Imamura, T., Dijke, P. T., Heldin, C. -. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Smad Signal Transduction: Smads in Proliferation, Differentiation and Disease. , 277-293 (2006).</li><li>Goumans, M. J., Zwijsen, A., Ten Dijke, P., Bailly, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bone+morphogenetic+proteins+in+vascular+homeostasis+and+disease.">Bone morphogenetic proteins in vascular homeostasis and disease.</a> Cold Spring Harbor Perspectives in Biology. 10 (2), 031989 (2018).</li><li>Cai, J., Pardali, E., S&#225;nchez-Duffhues, G., ten Dijke, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=BMP+signaling+in+vascular+diseases.">BMP signaling in vascular diseases.</a> FEBS Letters. 586 (14), 1993-2002 (2012).</li><li>Cunha, S. I., Magnusson, P. U., Dejana, E., Lampugnani, M. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Deregulated+TGF-&#946;/BMP+signaling+in+vascular+malformations.">Deregulated TGF-&#946;/BMP signaling in vascular malformations.</a> Circulation research. 121 (8), 981-999 (2017).</li><li>MacCarrick, G., 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=Loeys-Dietz+syndrome:+a+primer+for+diagnosis+and+management.">Loeys-Dietz syndrome: a primer for diagnosis and management.</a> Genetics in Medicine : An Official Journal of the American College of Medical Genetics. 16 (8), 576-587 (2014).</li><li>Baeyens, N., Bandyopadhyay, C., Coon, B. G., Yun, S., Schwartz, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endothelial+fluid+shear+stress+sensing+in+vascular+health+and+disease.">Endothelial fluid shear stress sensing in vascular health and disease.</a> The Journal of Clinical Investigation. 126 (3), 821-828 (2016).</li><li>Min, 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=Activation+of+Smad+2/3+signaling+by+low+shear+stress+mediates+artery+inward+remodeling.">Activation of Smad 2/3 signaling by low shear stress mediates artery inward remodeling.</a> bioRxiv. , 691980 (2019).</li><li>Zhou, J., 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=BMP+receptor-integrin+interaction+mediates+responses+of+vascular+endothelial+Smad1/5+and+proliferation+to+disturbed+flow.">BMP receptor-integrin interaction mediates responses of vascular endothelial Smad1/5 and proliferation to disturbed flow.</a> Journal of Thrombosis and Haemostasis. 11 (4), 741-755 (2013).</li><li>Zhou, J., 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=Force-specific+activation+of+Smad1/5+regulates+vascular+endothelial+cell+cycle+progression+in+response+to+disturbed+flow.">Force-specific activation of Smad1/5 regulates vascular endothelial cell cycle progression in response to disturbed flow.</a> Proceedings of the National Academy of Sciences of the United States of America. 109 (20), 7770-7775 (2012).</li><li>van Dijk, R. A., 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=Visualizing+TGF-&#946;+and+BMP+signaling+in+human+atherosclerosis:+A+histological+evaluation+based+on+Smad+activation.">Visualizing TGF-&#946; and BMP signaling in human atherosclerosis: A histological evaluation based on Smad activation.</a> Histology and Histopathology. 27 (3), 387-396 (2012).</li><li>Derwall, 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=Inhibition+of+bone+morphogenetic+protein+signaling+reduces+vascular+calcification+and+atherosclerosis.">Inhibition of bone morphogenetic protein signaling reduces vascular calcification and atherosclerosis.</a> Arteriosclerosis, Thrombosis, and Vascular Biology. 32 (3), 613-622 (2012).</li><li>Fredriksson, 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=Protein+detection+using+proximity-dependent+DNA+ligation+assays.">Protein detection using proximity-dependent DNA ligation assays.</a> Nature Biotechnology. 20 (5), 473-477 (2002).</li><li>S&#246;derberg, O., 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=Direct+observation+of+individual+endogenous+protein+complexes+in+situ+by+proximity+ligation.">Direct observation of individual endogenous protein complexes in situ by proximity ligation.</a> Nature Methods. 3 (12), 995-1000 (2006).</li><li>Alam, M. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Proximity+Ligation+Assay+(PLA).">Proximity Ligation Assay (PLA).</a> Current Protocols in Immunology. 123 (1), 58 (2018).</li><li>Application Note 03: Growing Cells in &#181;-Channels. ibidi Available from: <a target="_blank" href="https://ibidi.com/img/cms/support/AN/AN03_Growing_cells.pdf">https://ibidi.com/img/cms/support/AN/AN03_Growing_cells.pdf</a> (2012)</li><li>Application Note 13: HUVECs under perfusion. ibidi Available from: <a target="_blank" href="https://ibidi.com/img/cms/support/AN/AN13_HUVECs_under_perfusion.pdf">https://ibidi.com/img/cms/support/AN/AN13_HUVECs_under_perfusion.pdf</a> (2019)</li><li>ibidi. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Application+Note+31:+Instructions+&#181;-Slide+VI+0.4.">Application Note 31: Instructions &#181;-Slide VI 0.4.</a> ibidi. , (2013).</li><li>Schindelin, J., 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=Fiji:+an+open-source+platform+for+biological-image+analysis.">Fiji: an open-source platform for biological-image analysis.</a> Nature Methods. 9 (7), 676-682 (2012).</li><li>Reichenbach, 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=Differential+impact+of+fluid+shear+stress+and+YAP/TAZ+on+BMP/TGF-&#946;+induced+osteogenic+target+genes.">Differential impact of fluid shear stress and YAP/TAZ on BMP/TGF-&#946; induced osteogenic target genes.</a> Advanced Biology. 5 (2), 2000051 (2021).</li><li>Hiepen, C., Mendez, P. L., Knaus, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=It+takes+two+to+tango:+Endothelial+TGF&#946;/BMP+signaling+crosstalk+with+mechanobiology.">It takes two to tango: Endothelial TGF&#946;/BMP signaling crosstalk with mechanobiology.</a> Cells. 9 (9), 1965 (2020).</li></ol>

The authors declare no conflict of interest.

The PLA based protocol described here offers an efficient way to determine close proximity of two proteins (e.g., their direct interaction) in ECs exposed to shear stress with quantitative and spatial resolution. By using flow slides with multiple parallel channels, several different protein interactions can be examined at the same time in cells under identical mechanical conditions. In contrast, custom-build flow chamber systems often make use of a single channel that is built around a glass coverslip, which would allow...

Proteins of the transforming growth factors beta (TGF&#946;) superfamily are pleiotropic cytokines with a variety of members, including TGF&#946;s, bone morphogenetic proteins (BMPs), and Activins1,2. Ligand binding induces the formation of receptor oligomers leading to the phosphorylation and, thereby, activation of cytosolic regulatory SMAD (R-SMAD). Depending on the sub-family of ligands, different R-SMADs are activated1,2. While TGF&#946;s and Activins mainly induce phosphorylation of SMAD2/3, BMPs induce SMAD1/5/8 phosphorylation. However, there are accumulating evidences that BMPs and TGF&#946;s/Activins also activate R-SMADs of the respective other sub-family, in a process termed as &#39;lateral signaling&#39;3,4,5,6,7,8 and that there are mixed SMAD complexes consisting of both, SMAD1/5 and SMAD2/3, members3,9. Two activated R-SMADs subsequently form trimeric complexes with the common mediator SMAD4. These transcription factor complexes are then able to translocate into the nucleus and regulate the transcription of target genes. SMADs can interact with a variety of different transcriptional co-activators and co-repressors, leading to the diversification of the possibilities to regulate target genes10. Deregulation of SMAD signaling has severe implications in a variety of diseases. In line with this, unbalanced TGF&#946;/BMP signaling may lead to severe vascular pathologies, such as pulmonary artery hypertension, hereditary hemorrhagic telangiectasia, or atherosclerosis3,11,12,13,14.
Endothelial cells (ECs) form the innermost layer of blood vessels and are, therefore, exposed to shear stress (SS), a frictional force exerted by the viscous flow of the blood. Interestingly, ECs residing at the parts of the vasculature, which are exposed to high levels of uniform, laminar SS, are kept in a homeostatic and quiescent state. In contrast, ECs that experience low, non-uniform SS, e.g., at bifurcations or the lesser curvature of the aortic arch, are proliferative and activate inflammatory pathways15. In turn, sites of dysfunctional ECs are prone to develop atherosclerosis. Interestingly, ECs in these atheroprone areas display aberrantly high levels of activated SMAD2/3 and SMAD1/516,17,18. In this context, enhanced TGF&#946;/BMP signaling was found to be an early event in the development of atherosclerotic lesions19 and interference with BMP signaling was found to markedly reduce vascular inflammation, atheroma formation, and associated calcification20.
Proximity Ligation Assay (PLA) is a biochemical technique to study protein-protein interactions in situ21,22. It relies on the specificity of antibodies of different species that can bind target proteins of interest, allowing highly specific detection of endogenous protein interactions at a single-cell level. Here, primary antibodies have to bind to their target epitope at a distance of less than 40 nm to allow for the detection23. Therefore, PLA is greatly beneficial over traditional co-immunoprecipitation approaches, where several million cells are needed to detect endogenous protein interactions. In PLA, species-specific secondary antibodies, covalently linked to DNA fragments (termed Plus and Minus probes), bind the primary antibodies and if the proteins of interest interact, Plus and Minus probes come in close proximity. The DNA gets ligated in the following step and the rolling circle amplification of the circular DNA is made possible. During amplification, fluorescently labeled complementary oligonucleotides bind to the synthesized DNA, allowing these protein interactions to be visualized by conventional fluorescence microscopy.
The protocol described here enables scientists to quantitatively compare the number of active SMAD transcription complexes at atheroprotective and atheroprone SS conditions in vitro using PLA. SS is generated via a programmable pneumatic pump system that is able to generate laminar unidirectional flow of defined levels and allows stepwise increases of flow rates. This method allows for the detection of interactions between SMAD1/5 or SMAD2/3 with SMAD4, as well as mixed-R-SMAD complexes. It can easily be expanded to analyze interactions of SMADs with transcriptional co-regulators or to transcription factor complexes other than SMADs. Figure 1 shows the major steps of the protocol presented below.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/62608/62608fig01.jpg" /> 
Figure 1: Schematic representation of the protocol described. (A)&#160;Cells seeded in 6-channel slides are exposed to shear stress with a pneumatic pump system. (B)&#160;Fixed cells are used for PLA experiment or for control conditions. (C)&#160;Images of PLA experiments are acquired with a fluorescence microscope and are analyzed using ImageJ analysis software. <a href="https://www.jove.com/files/ftp_upload/62608/62608fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>&#181;-Slide VI 0.4</td>
			<td>ibidi</td>
			<td>80606</td>
			<td>6-channel slide</td>
		</tr>
		<tr>
			<td>Ammonium Chloride</td>
			<td>Carl Roth</td>
			<td>K298.1</td>
			<td>Quenching</td>
		</tr>
		<tr>
			<td>Bovine Serum Albumin</td>
			<td>Carl Roth</td>
			<td>8076.4</td>
			<td>Blocking</td>
		</tr>
		<tr>
			<td>DAPI</td>
			<td>Sigma Aldrich/ Merck</td>
			<td>D9542</td>
			<td>Stain DNA/Nuclei</td>
		</tr>
		<tr>
			<td>DPBS</td>
			<td>PAN Biotech</td>
			<td>P04-53500</td>
			<td>PBS</td>
		</tr>
		<tr>
			<td>Duolink In Situ Detection Reagents Green</td>
			<td>Sigma Aldrich/ Merck</td>
			<td>DUO92014</td>
			<td>PLA kit containing Ligase, ligation buffer, polymerase and amplification buffer (with green labeled oligonucleotides)</td>
		</tr>
		<tr>
			<td>Duolink In Situ PLA Probe Anti-Mouse MINUS</td>
			<td>Sigma Aldrich/ Merck</td>
			<td>DUO92004</td>
			<td>MINUS probe</td>
		</tr>
		<tr>
			<td>Duolink In Situ PLA Probe Anti-Rabbit PLUS</td>
			<td>Sigma Aldrich/ Merck</td>
			<td>DUO92002</td>
			<td>PLUS probe</td>
		</tr>
		<tr>
			<td>Duolink In Situ Wash Buffers, Fluorescence</td>
			<td>Sigma Aldrich/ Merck</td>
			<td>DUO82049</td>
			<td>PLA wash buffers A and B</td>
		</tr>
		<tr>
			<td>Endothelial Cell Growth Supplement</td>
			<td>Corning</td>
			<td></td>
			<td>supplement for medium (ECGS)</td>
		</tr>
		<tr>
			<td>Fetal calf Serum</td>
			<td></td>
			<td></td>
			<td>supplement for medium</td>
		</tr>
		<tr>
			<td>FIJI</td>
			<td></td>
			<td></td>
			<td>Image Analysis software</td>
		</tr>
		<tr>
			<td>Formaldehyde solution 4% buffered</td>
			<td>KLINIPATH/VWR</td>
			<td>VWRK4186.BO1</td>
			<td>PFA</td>
		</tr>
		<tr>
			<td>Full medium</td>
			<td></td>
			<td></td>
			<td>M199 basal medium +20 % FCS +1 % P/S + 2 nM L-Glu +&#160; 25 &#181;g/mL Hep +&#160;&#160; 50 &#181;g/mL ECGS</td>
		</tr>
		<tr>
			<td>Gelatin from porcine skin, Type A</td>
			<td>Sigma Aldrich</td>
			<td>G2500</td>
			<td>Use 0.1% in PBS for coating of flow channels</td>
		</tr>
		<tr>
			<td>GraphPad Prism v.7</td>
			<td>GarphPad</td>
			<td></td>
			<td>Statistical Program used for the Plots and statistical calculations</td>
		</tr>
		<tr>
			<td>Heparin sodium salt from porcine intestinal mucosa</td>
			<td>Sigma Aldrich</td>
			<td>H4784-250MG</td>
			<td>supplement for medium (Hep)</td>
		</tr>
		<tr>
			<td>HUVECs</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>ibidi Mounting Medium</td>
			<td>ibidi</td>
			<td>50001</td>
			<td>Liquid mounting medium</td>
		</tr>
		<tr>
			<td>ibidi Pump System</td>
			<td>ibidi</td>
			<td>10902</td>
			<td>pneumatic pump</td>
		</tr>
		<tr>
			<td>Leica TCS SP8</td>
			<td>Leica</td>
			<td></td>
			<td>confocal microscope</td>
		</tr>
		<tr>
			<td>L-Glutamin 200mM</td>
			<td>PAN Biotech</td>
			<td>P04-80100</td>
			<td>supplement for medium (L-Glu)</td>
		</tr>
		<tr>
			<td>Medium 199</td>
			<td>Sigma Aldrich</td>
			<td>M2154</td>
			<td>Base medium</td>
		</tr>
		<tr>
			<td>mouse anti- SMAD1 Antibody</td>
			<td>Abcam</td>
			<td>ab53745</td>
			<td>Suited for PLA</td>
		</tr>
		<tr>
			<td>mouse anti- SMAD2/3 Antibody</td>
			<td>BD Bioscience</td>
			<td>610843</td>
			<td>Not suited for PLA in combination with CST 9515</td>
		</tr>
		<tr>
			<td>mousee anti- SMAD4 Antibody</td>
			<td>Sanata Cruz Biotechnology</td>
			<td>sc-7966</td>
			<td>Suited for PLA</td>
		</tr>
		<tr>
			<td>Penicillin 10.000U/ml /Streptomycin 10mg/ml</td>
			<td>PAN Biotech</td>
			<td>P06-07100</td>
			<td>supplement for medium (P/S)</td>
		</tr>
		<tr>
			<td>Perfusion Set WHITE</td>
			<td>ibidi</td>
			<td>10963</td>
			<td>Tubings used for 1 dyn/cm2</td>
		</tr>
		<tr>
			<td>Perfusion Set YELLOW and GREEN</td>
			<td>ibidi</td>
			<td>10964</td>
			<td>Tubings used for 30 dyn/cm2</td>
		</tr>
		<tr>
			<td>rabbit anti- phospho SMAD1/5 Antibody</td>
			<td>Cell Signaling Technologies</td>
			<td>9516</td>
			<td>Suited for PLA</td>
		</tr>
		<tr>
			<td>rabbit anti- SMAD2/3 XP Antibody</td>
			<td>Cell Signaling Technologies</td>
			<td>8685</td>
			<td>Suited for PLA</td>
		</tr>
		<tr>
			<td>rabbit anti- SMAD4 Antibody</td>
			<td>Cell Signaling Technologies</td>
			<td>9515</td>
			<td>Not suited for PLA in combination with BD 610843</td>
		</tr>
		<tr>
			<td>Serial Connector for &#181;-Slides</td>
			<td>ibidi</td>
			<td>10830</td>
			<td>serial connection tubes</td>
		</tr>
		<tr>
			<td>Triton X-100</td>
			<td>Carl Roth</td>
			<td>6683.1</td>
			<td>Permeabilization</td>
		</tr>
	</tbody>
</table>

visualization and quantification of tgf&#946;/bmp/smad signaling under different fluid shear stress conditions using proximity-ligation-assay

Transforming Growth Factor &#946; (TGF&#946;)/Bone Morphogenetic Protein (BMP) signaling is tightly regulated and balanced during the development and homeostasis of the vasculature system Therefore, deregulation in this signaling pathway results in severe vascular pathologies, such as pulmonary artery hypertension, hereditary hemorrhagic telangiectasia, and atherosclerosis. Endothelial cells (ECs), as the innermost layer of blood vessels, are constantly exposed to fluid shear stress (SS). Abnormal patterns of fluid SS have been shown to enhance TGF&#946;/BMP signaling, which, together with other stimuli, induce atherogenesis. In relation to this, atheroprone, low laminar SS was found to enhance TGF&#946;/BMP signaling while atheroprotective, high laminar SS, diminishes this signaling. To efficiently analyze the activation of these pathways, we designed a workflow to investigate the formation of transcription factor complexes under low laminar SS and high laminar SS conditions using a commercially available pneumatic pump system and proximity ligation assay (PLA).
Active TGF&#946;/BMP-signaling requires the formation of trimeric SMAD complexes consisting of two regulatory SMADs (R-SMAD); SMAD2/3 and SMAD1/5/8 for TGF&#946; and BMP signaling, respectively) with a common mediator SMAD (co-SMAD; SMAD4). Using PLA targeting different subunits of the trimeric SMAD-complex, i.e., either R-SMAD/co-SMAD or R-SMAD/R-SMAD, the formation of active SMAD transcription factor complexes can be measured quantitatively and spatially using fluorescence microscopy.
The usage of flow slides with 6 small parallel channels, that can be connected in series, allows for the investigation of the transcription factor complex formation and inclusion of necessary controls. 
The workflow explained here can be easily adapted for studies targeting the proximity of SMADs to other transcription factors or to transcription factor complexes other than SMADs, in different fluid SS conditions. The workflow presented here shows a quick and effective way to study the fluid SS induced TGF&#946;/BMP signaling in ECs, both quantitatively and spatially.

We have previously used PLA to detect interactions of different SMAD proteins3 and analyzed shear stress induced changes in SMAD phosphorylation28.
Here, both methods were combined with the protocol described above. HUVECs were subjected to shear stress of 1 dyn/cm2 and 30 dyn/cm2 and analyzed for interactions of SMAD transcription factors. We show that, when compared to the high shear stress (30 dyn/cm2), the low...

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Visualization and Quantification of TGF&amp;#946;/BMP/SMAD Signaling under Different Fluid Shear Stress Conditions using Proximity-Ligation-Assay

Here, we establish a protocol to simultaneously visualize and analyze multiple SMAD complexes using proximity ligation assay (PLA) in endothelial cells exposed to pathological and physiological fluid shear stress conditions.

Visualization and Quantification of TGF&#946;/BMP/SMAD Signaling under Different Fluid Shear Stress Conditions using Proximity-Ligation-Assay

Transforming Growth Factor &#946; (TGF&#946;)/Bone Morphogenetic Protein (BMP) signaling is tightly regulated and balanced during the development and ...

This protocol enables scientists to quantitatively compare the number of transcription factor complexes under different shear stress conditions, and will be beneficial for research in vascular biology. The main advantage of this protocol is the usage of microfluidic flow channels that allow investigations of several antibody pairs, including the respective controls simultaneously. To begin, coat the six channel slide by adding 0.1%porcine skin gelatin prepared in PBS into the channels for 30 minutes at 37 degrees Celsius.Empty the channels from the gelatin solution and seed Huvecs in the pre-coded six channel slides at a density of 2.5 times 10 to the sixth cells per milliliter in 30 microliters of M199 complete medium. Allow the cells to adhere for one hour at 37 degrees Celsius in the humidified incubator. Add 60 microliters of pre-warned M199 complete medium to each of the reservoirs.Culture the cells at 37 degrees Celsius for two days in a humidified incubator with medium replacement after 24 hours. Mount the silicon tubing on the fluidic units, fill the reservoirs with a minimum of 10 milliliters of pre-warned M199 complete medium. Connect the fluidic units with tubing to the pump system and perform the pre-run without cells for equilibrating the medium and removing any remaining air.Connect the single channels on the six channels slide using the serial connection tubing. Connect the first and last channel on the slide to the tubes mounted on the fluidics unit. For exposure of the cells to high levels of sheer stress, increase the sheer stress step wise with adaptation phases, setting the steps in increments of five dynes per square centimeter per 30 minutes.Use the clumps on the tubing to detach the slides from pumps after fluid shear stress exposure, avoiding medium spillage in the incubator, and transfer flow slides on ice. While detaching the tubing from the reservoirs, use the finger to close the other side of the reservoir to avoid trapping the air bubbles in the channel. Keeping the flow slide on ice, aspirate the medium carefully from the reservoirs but not from the channel where the cells reside.Wash the samples with 90 microliters of cold sterile PBS by adding PBS to one reservoir and aspirate carefully from the other reservoir, then repeat for all channels. Fix the cells by adding 90 microliters of buffered 4%PFA solution to the same reservoir where the PBS was added and aspirate the liquid from the other reservoir as demonstrated. After fixing the cells in all six channels, transfer the samples from ice to room temperature, and incubate for 20 minutes.Wash the cells three times with PBS by adding PBS to one reservoir and aspirating it carefully from the other reservoir in all channels, taking care to not dry out the channels. Quench access PFA by adding 90 microliters of ambient 15 millimolar ammonium chloride prepared in PBS to one of the reservoirs. Aspirate from the other reservoir and incubate the cells for 10 minutes.Permeabilize the cells by adding 90 microliters of 0.3%Triton X 100 prepared in PBS in the empty reservoir. Aspirate from the other reservoir for each channel and incubate for 10 minutes, then washed three times with PBS as demonstrated before. At 90 microliters of sterile PLA blocking solution to one reservoir.Aspirate from the other side for each channel and incubate for one hour at 37 degrees Celsius in a humidified chamber. Add primary antibodies in PLA antibody diluent and vortex. Carefully aspirate the blocking solution from the reservoirs and channels.Add 30 microliters of the primary antibody solution immediately into the empty channel by tilting the slide while adding the solution. Repeat for all channels and incubate at four degrees Celsius in the humidified chamber overnight or at 37 degrees Celsius for one hour. Dilute minus rabbit and plus mouse PLA probes one to five in the PLA antibody diluent.Wash all the channels two times by adding 90 microliters of wash buffer A and aspirating as demonstrated. After aspirating the wash buffer, add 30 microliters of PLA probe solution and incubate in the humidified chamber. Dilute the ligation buffer one to five in D ionized water and dilute the lygus enzyme one to 40 in the diluted ligation buffer.After washing and completely aspirating the wash buffer A, add 30 microliters of ligation solution to the empty channels while tilting the slide, and incubate in the humidified chamber. Dilute the amplification buffer one to five in deionized water, then dilute the polymerase enzyme one to 80 in the diluted amplification buffer on ice. After washing and completely aspirating the wash buffer A, add 30 microliters of amplification solution to the empty channels while tilting the slide, and incubate it in the humidified chamber.Dilute the DAPI one to 500 in wash buffer B and wash all the channels once by adding 90 microliters of DAPI containing wash buffer B, and aspirating as demonstrated to steam the nuclei. Then wash all the channels one time with wash buffer B without DAPI. Dilute the wash buffer B one to 10 in deionized water and wash all the channels once with 90 microliters of diluted wash buffer B.After aspirating wash buffer B, immediately add two to three drops of the liquid mounting medium to one reservoir, and tilt the slide to distribute the mounting medium in all channels.Store the samples at four degrees Celsius in a humidified environment until imaging. Huvecs were exposed to high and low sheer stress. The low shear stress increased SMAD protein interactions in the cytosol and nuclei.Single antibody controls showed no to very few signals, proving the success of the experiment. Increased antibody concentration resulted in a higher number of PLA events per cell, but the difference in PLA signals between high and low sheer stress exposed cells was reduced. Self-made buffers for blocking and antibody dilution can be used as an alternative for commercial buffers.The quantification of PLA events for commercial and self-made diluent and blocking solutions showed the same pattern of PLA signals in the cytosolic and nuclear areas. However, the total number of PLA events per cell was higher with commercial solutions. A combination of SMAD Two/Three, SMAD Four antibodies was used where the SMAD Four antibody was not suited for immunofluorescence experiments.There was no difference in PLA events in the experimental versus control condition, indicating the importance of selecting the correct antibody combination to detect the PLA events. The described protocol can be adapted to detect the interactions of transcription factor complexes different from SMADs and therefore help understand gene regulation in athero prone and athero protective conditions.

visualization-quantification-tgfbmpsmad-signaling-under-different

Research

JoVE Journal

Biochemistry

3.2K Views.  Freie Universität Berlin. This protocol enables scientists to quantitatively compare the number of transcription factor complexes under different shear stress conditions, and will be beneficial for research in vascular biology. The main advantage of this protocol is the usage of microfluidic flow channels that allow investigations of several antibody pairs, including the respective controls simultaneously. To begin, coat the six channel slide by adding 0.1%porcine skin gelatin prepared in PBS into the channels for 30 ...