This work was supported by grants from the Nederlandse organisatie voor gezondheidsonderzoek en zorginnovatie (ZonMW 446002501), Health Holland (LSHM19057-H040), Leading Fellows Programme Marie Sk&#322;odowska-Curie COFUND, and by the Association Maladie de Rendu-Osler (AMRO).

1. Media preparation and culture of mESC
<ol>
	<li>Prepare conditioned medium +/- (CM+/-) using the supplement 1x Glasgow MEM (G-MEM BHK-21) medium with 10% (vol/vol) heat-inactivated Fetal Bovine Serum (FBS), 0.05 mM &#946;-mercaptoethanol, 1x non-essential amino acids (NEAA 1x), 2 mM L-glutamine, and 1 mM sodium pyruvate.</li>
	<li>Prepare conditioned medium +/+ (CM+/+) using the supplement CM+/- medium with Leukemia Inhibitory Factor (LIF) (1,500 U/mL) and 0.1 mM &#946;-mercaptoethanol.</li>
	<li>Prepare conditioned medium +/+ in the presence of two inhibitors (CM+/+ 2i) using the supplement CM+/+ medium with 1 &#181;M PD0325901 and 3 &#181;M CHIR99021.</li>
	<li>Prepare 0.1% gelatin solution by mixing 25 mL of pre-warmed 2% gelatin solution in 500 mL of phosphate-buffered saline without calcium and magnesium (DPBS).</li>
	<li>Prepare 10x Trypsin Versene Phosphate (TVP 10x) buffer by mixing Trypsin (2.5%) with TVP 1x (9.5% 10x DPBS, 1 mM EDTA, 0.01% chicken serum, 0.01% Trypsin (2.5%) in H2O) (1:10 ratio, vol/vol). 
	NOTE: Filter all the solutions through a 0.22 &#181;m pore filter. Store the culture medium at -20 &#176;C for long term and keep other reagents at 4 &#176;C for up to 3 weeks (see Table of Materials).</li>
	<li>Coat two 12-well cell culture plates with 0.1% gelatin solution (500 &#181;L per well) and incubate them for 30 min in a CO2 incubator (37 &#176;C, 5% CO2, humid atmosphere).</li>
	<li>Wash the gelatin-coated plates with PBS and add 500 &#181;L of CM+/- medium.</li>
	<li>Thaw two vials of 1 x 106 cryopreserved irradiated mouse embryonic fibroblasts (MEFs) at 37 &#176;C and transfer the cell suspension to a conical tube with 5 mL of CM+/- medium.</li>
	<li>Centrifuge the cells at 200 x g for 5 min at room temperature (RT). Aspirate the medium and gently resuspend the cell pellet in 12 mL of CM+/- medium at a concentration of 1.67 x 105 cells per mL.</li>
	<li>Seed 500 &#181;L of MEF suspension in a 12-well cell culture plate (2.4 x 105 cells per cm2) and incubate the plate overnight (o/n) in a CO2 incubator.</li>
	<li>Thaw one vial of cryopreserved mESCs (1 x 106) in CM+/+ 2i medium.</li>
	<li>Seed the mESC suspension on a pre-washed 12 well plate with MEFs in 1 mL of CM+/+ 2i medium and transfer the plate to a CO2 incubator. Refresh the medium daily.</li>
	<li>At 70% confluency, wash the mESC colonies with DPBS. Add 150 &#181;L of warm 10x TVP buffer and incubate at RT for 30 s to initiate enzymatic dissociation.</li>
	<li>Carefully remove the TVP buffer, resuspend the cells in 1 mL of CM+/+ 2i medium and dissociate the colonies into single cells by gentle pipetting.</li>
	<li>Passage the cells by transferring them to a new 12-well cell culture plate with MEFs (splitting ratio: 1:3-1:5). Mix gently to distribute the cells and incubate in a CO2 incubator.</li>
	<li>Refresh the culture medium (2-3 mL) and observe the cell growth/morphology daily. Repeat serial passaging at 70% confluency every 2 days. Switch to CM+/+ medium for two passages before initiating cell differentiation.</li>
</ol>
2. EB formation in hanging drop
<ol>
	<li>Prepare fresh mesoderm differentiation medium by supplementing the CM+/- medium with basic Fibroblast Growth Factor (bFGF) (50 ng&#183;mL-1) and with Bone Morphogenetic Protein 4 (BMP-4) (5 ng&#183;mL-1) and keep it at 4 &#176;C until use.</li>
	<li>Coat one well of the 6-well cell culture plate with 500 &#181;L of 0.1% gelatin solution and place it in a CO2 incubator for 30 min.</li>
	<li>Wash the gelatin-coated plates with PBS and add 500 &#181;L of CM+/- medium.</li>
	<li>To obtain a pure population of mESCs, trypsinize the cell culture plate with 10x TVP buffer for 30 s at RT, resuspend the cells in 1 mL mesoderm differentiation medium and then transfer them to the gelatin-coated 6-well plate for 30 min allowing the MEFs to attach while the mESCs stay in suspension.</li>
	<li>Collect the cell suspension in a 50 mL conical tube and count the cells using a Neubauer hemocytometer and Trypan blue dye for a live/dead cell exclusion.</li>
	<li>Centrifuge the cells at 200 x g for 5 min at RT. Remove the supernatant and resuspend the cell pellet in mesoderm differentiation medium to reach 4.55 x 104 cells per mL.</li>
	<li>Fill the bottom of 94 mm low attachment polystyrene dishes with 15 mL of sterile water. 
	&#8203;NOTE: Four dishes containing hanging drops (1.6 x 105 cells in 3.52 mL of medium) will be required for testing one particular condition using the 3D sprouting angiogenesis assay.</li>
	<li>Transfer the cell suspension into a sterile plastic reservoir and load four positions of a multichannel pipette with 22 &#181;L of cell suspension per channel (1 x 103 cells per 22 &#181;L drop).</li>
	<li>Lift and invert the lid of the 94 mm dish and place it on the clean surface of the flow cabinet with the inner side facing upwards.</li>
	<li>Deposit 40 drops of the cell suspension on the inner surface of each lid. Carefully invert the lid without disturbance and place it back on the dish, so that the drops face the water.</li>
	<li>Incubate the dishes in a CO2 incubator. Consider this as differentiation day 0. Maintain the plates for 4 days to form EBs.</li>
</ol>
3. Competition assay for the tip cell position
<ol>
	<li>Culture one fluorescent and one non-fluorescent mESC&#160;line as previously described13. As an example, 7ACS/EYFP mESCs labeled in yellow and R1 mESCs are used.</li>
	<li>Prepare mosaic EBs by mixing equal amounts of the two mESC lines (1:1 ratio) and incubate the hanging drop dishes in a CO2 incubator as described in step 2.</li>
</ol>
4. Floating EBs culture for vascular differentiation
<ol>
	<li>Before collecting EBs from the hanging drops, prepare the following.
	<ol>
		<li>Prepare 5% agar solution in H2O and sterilize it by autoclaving (20 min at 120 &#176;C).</li>
		<li>Use the warm 5% agar solution to prepare G-MEM BHK-21 medium containing 1% agar and quickly pour 3 mL in one of the 60 mm polystyrene dishes. Allow the agar to solidify for 1 h at RT. Store the dishes at 4 &#176;C until use.</li>
	</ol>
	</li>
	<li>Prepare fresh 2x vascular differentiation medium by supplementing the CM+/- medium with bFGF (100 ng&#183;mL-1) and VEGF-A (50 ng&#183;mL-1). Store the medium at 4 &#176;C until use.</li>
	<li>Collect the hanging drops in a 15 mL conical tube using a P1000 pipette and remove the supernatant after a few minutes of EB sedimentation.</li>
	<li>Resuspend the EBs in 3 mL of 2x vascular differentiation medium, transfer the EB suspension to one agar-coated dishand distribute the EBs homogenously to avoid aggregation.</li>
	<li>Incubate the dishes in CO2 incubator and refresh the medium every 2 days until day 9 using 1x vascular differentiation medium in the presence of bFGF (50 ng&#183;mL-1) and VEGF-A (25 ng&#183;mL-1).</li>
	<li>Alternatively, add Platelet Derived Growth Factor-BB (PDGF-BB) (10 ng&#183;mL-1 on day 4 and 5 ng&#183;mL-1 on day 6 and day 8) to the vascular differentiation medium to promote mural cell differentiation.</li>
</ol>
5. Flow cytometry analysis
<ol>
	<li>Collect the 9-day-old EBs in a 15 mL conical tube using a P1000 pipette and wash them once with warm PBS.</li>
	<li>Add 1mL of G-MEM BHK-21 medium containing 0.2 mg&#183;mL-1 of collagenase A and incubate the cells in a CO2 incubator for 5 min.</li>
	<li>Dissociate the EBs by gently pipetting up and down with a P1000 pipette.</li>
	<li>Stop the collagenase activity by adding 1 mL of cold G-MEM BHK-21 medium with 10% FBS.</li>
	<li>Centrifuge the cells at 200 x g for 5 min at RT.</li>
	<li>Resuspend the cells in 500 &#181;L of PBS with 2% FBS.</li>
	<li>Count the cells using a Neubauer hemocytometer.</li>
	<li>Resuspend 400,000 cells in 100 &#181;L of PBS with 2% FBS per staining condition.</li>
	<li>Incubate the cells for 45 min at 4 &#176;C with the following antibodies: APC conjugated rat anti-mouse PECAM-1 antibody (clone MEC13.3) and FITC conjugated rat anti-mouse CD45 (clone 30-F11) or isotype control antibodies.</li>
	<li>Wash the cells twice with 1 mL of PBS containing 2% FBS.</li>
	<li>Resuspend the cells to reach a final concentration of 5 x 106 cells per mL.</li>
	<li>Filter the cells using a round bottom polystyrene test tube, with a cell strainer snap cap.</li>
	<li>Analyze 20,000 PECAM-1 (+) events by flow cytometry.</li>
</ol>
6. 3D sprouting angiogenesis assay and immunofluorescence staining
<ol>
	<li>At day 9, prepare a sprouting medium by adding 10% FBS (vol/vol), bFGF (50 ng&#183;mL-1), VEGF-A (25 ng&#183;mL-1), human recombinant Erythropoietin (hEPO) (20 ng&#183;mL-1), human interleukin-6 (IL-6) (10 ng&#183;mL-1), 0.05 mM &#946;-mercaptoethanol, NEAA (1x), L-glutamine (1x), sodium pyruvate (1x), type I rat tail collagen (1.25 mg&#183;mL-1), and NaOH (3.1 mM) to GMEM BHK-1 medium.</li>
	<li>To avoid collagen gelation, maintain the sprouting medium on ice until use.</li>
	<li>To evaluate the effect of pro/anti-angiogenic molecules, add the selected drug or vehicle to the sprouting medium at the selected concentration.</li>
	<li>Collect 9-day-old EBs from one 60 mm agar dish in a 15 mL conical tube (equivalent to one condition) and remove the supernatant after a few minutes of sedimentation.</li>
	<li>Cover the bottom of a 35 mm culture dish with 1 mL of the sprouting medium and incubate at 37 &#176;C for 5 min to induce gelation.</li>
	<li>Resuspend the EBs in 2 mL of cold sprouting medium.</li>
	<li>Transfer the suspension to the 35 mm culture dish coated with the first layer of sprouting medium.</li>
	<li>Distribute the EBs all over the plate and ensure they are at an equal distance from each other. Incubate the dish in a CO2 incubator. The first sprout formation happens within 24-48 h.</li>
	<li>At day 12, the collagen gel containing EBs is carefully transferred to a glass slide (75 x 26 mm) using a spatula.</li>
	<li>Remove the excess liquid using a pipette (P1000) and dehydrate the gel by placing a gauze sheet of nylon linen and absorbent filter cards (gel blotting paper) on top of the gel. Apply pressure by placing a weight (250 g) for 2 min. Carefully remove the nylon/filter papers and allow the slides to air dry for 30 min at RT.</li>
	<li>Wash the slides three times with PBS for 5 min at RT.</li>
	<li>Fix the EBs using zinc solution (see Table of Materials) o/n at 4 &#176;C. Alternatively, fix the mosaic fluorescent EBs with Paraformaldehyde (PFA) (4%) o/n at 4 &#176;C in the dark.</li>
	<li>Remove the fixative. Wash the slides five times with PBS for 5 min at RT.</li>
	<li>Permeabilize the EBs in PBS containing 0.1% Triton-X100 for 15 min at RT.</li>
	<li>Remove the permeabilization solution. Wash the slides five times with PBS for 5 min.</li>
	<li>Incubate the EBs in the blocking buffer (PBS with 2% Bovine Serum Albumin, BSA) for 1 h at RT.</li>
	<li>To stain the endothelial sprouts, use a rat anti-mouse anti-PECAM-1 primary antibody (1:100 dilution) in blocking buffer o/n at 4 &#176;C.</li>
	<li>Wash the slides five times with PBS for 5 min.</li>
	<li>Incubate the slides with goat anti-rat Alexa 555 secondary antibody in blocking buffer (1:250 dilution) and when required, with FITC-conjugated anti-&#945;-SMA antibody in blocking buffer (1:250 dilution) to stain mural cells for 2 h at RT in the dark.</li>
	<li>Wash the slides three times with PBS for 5 min and one time with H20 before mounting them.</li>
</ol>
7. Confocal imaging, morphometric, and quantitative analysis of EB endothelial sprouts
<ol>
	<li>Acquire high-resolution images of the immunostained EBs by focal plane merging (z-stacking) using a confocal microscope. Use 10x magnification objective to image whole EBs.</li>
	<li>Analyze the images acquired using ImageJ to evaluate the morphology and quantify the characteristics of PECAM-1 positive endothelial sprouts according to established quantification methods13,14,21.
	<ol>
		<li>Calculate the mean number of endothelial sprouts per EBs by manually counting the total sprout number per individual EBs.</li>
		<li>Measure the individual sprout lengths using the ImageJ drawing tool. Define the base of the endothelial sprout starting at the EB core area and manually draw a line until the sprout tip end.</li>
		<li>Calculate the mean number of tip cells per sprout by manually counting the number of tip cells per individual sprout and then calculate the mean per EB.</li>
		<li>Calculate the filopodia orientation by manually determining the axis of the parent sprout and measure the acute angle between them using the ImageJ software angle tool. Calculate the number of sprouts with an orientation &#62;50&#176; and divide it by the total number of sprouts of the EB of interest. 
		NOTE: The angles always ranged from 0&#176; to 90&#176;.</li>
	</ol>
	</li>
	<li>Acquire high-resolution images of the immunostained EBs using a confocal microscope. Use 40x magnification objective to realize high-resolution images of single endothelial sprouts.</li>
	<li>Quantify the vessel coverage of PECAM-1 positive EC sprouts surrounded by &#945;-SMA positive MCs using the ImageJ software.
	<ol>
		<li>Split the merged images into separate red and green channels.</li>
		<li>Convert the images into its binary form.</li>
		<li>Measure the total cellular area of the sprout occupied separately by PECAM-1 (endothelial cells labeled in red) and by &#945;-SMA (mural cells labeled in green) positive cells.</li>
		<li>Generate the merged image using the image calculator function and the AND operator. Measure the total cellular area of the image. To calculate the coverage, divide the area of the colocalized image by the area of the PECAM-1 binary image.</li>
	</ol>
	</li>
	<li>To analyze the cell competition for endothelial tip/stalk cell position of sprouts developed by mosaic EBs, manually score the number of tip cells and mark their genotypic origin based on the fluorescence signal. Calculate the mean values per EB.</li>
</ol>

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(186), (2007).</li><li>Gau, D., 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=Pharmacological+intervention+of+MKL/SRF+signaling+by+CCG-1423+impedes+endothelial+cell+migration+and+angiogenesis.">Pharmacological intervention of MKL/SRF signaling by CCG-1423 impedes endothelial cell migration and angiogenesis.</a> Angiogenesis. 20 (4), 663-672 (2017).</li><li>Torres-Estay, V., 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=Androgens+modulate+male-derived+endothelial+cell+homeostasis+using+androgen+receptor-dependent+and+receptor-independent+mechanisms.">Androgens modulate male-derived endothelial cell homeostasis using androgen receptor-dependent and receptor-independent mechanisms.</a> Angiogenesis. 20 (1), 25-38 (2017).</li><li>Merjaneh, 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=Pro-angiogenic+capacities+of+microvesicles+produced+by+skin+wound+myofibroblasts.">Pro-angiogenic capacities of microvesicles produced by skin wound myofibroblasts.</a> Angiogenesis. 20 (3), 385-398 (2017).</li><li>Rezzola, 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=In+vitro+and+ex+vivo+retina+angiogenesis+assays.">In vitro and ex vivo retina angiogenesis assays.</a> Angiogenesis. 17 (3), 429-442 (2014).</li><li>Wang, X., Phan, D. T. T., George, S. C., Hughes, C. C. W., Lee, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+Anastomosed+microvascular+network+model+with+living+capillary+networks+and+endothelial+cell-lined+microfluidic+channels.">3D Anastomosed microvascular network model with living capillary networks and endothelial cell-lined microfluidic channels.</a> Methods in Molecular Biology. 1612, 325-344 (2017).</li><li>Nicosia, R. F., Ottinetti, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth+of+microvessels+in+serum-free+matrix+culture+of+rat+aorta.+A+quantitative+assay+of+angiogenesis+in+vitro.">Growth of microvessels in serum-free matrix culture of rat aorta. A quantitative assay of angiogenesis in vitro.</a> Laboratory Investigation; A Journal of Technical Methods and Pathology. 63 (1), 115-122 (1990).</li><li>Nicosia, R. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+aortic+ring+model+of+angiogenesis:+a+quarter+century+of+search+and+discovery.">The aortic ring model of angiogenesis: a quarter century of search and discovery.</a> Journal of Cellular and Molecular Medicine. 13 (10), 4113-4136 (2009).</li><li>Nowak-Sliwinska, 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=Consensus+guidelines+for+the+use+and+interpretation+of+angiogenesis+assays.">Consensus guidelines for the use and interpretation of angiogenesis assays.</a> Angiogenesis. 21 (3), 425 (2018).</li><li>Belair, D. G., Schwartz, M. P., Knudsen, T., Murphy, W. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Human+iPSC-derived+endothelial+cell+sprouting+assay+in+synthetic+hydrogel+arrays.">Human iPSC-derived endothelial cell sprouting assay in synthetic hydrogel arrays.</a> Acta Biomaterialia. 39, 12-24 (2016).</li><li>Bezenah, J. R., Kong, Y. P., Putnam, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evaluating+the+potential+of+endothelial+cells+derived+from+human+induced+pluripotent+stem+cells+to+form+microvascular+networks+in+3D+cultures.">Evaluating the potential of endothelial cells derived from human induced pluripotent stem cells to form microvascular networks in 3D cultures.</a> Scientific Reports. 8 (1), 2671 (2018).</li><li>Henderson, A. R., Choi, H., Lee, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Blood+and+lymphatic+vasculatures+on-chip+platforms+and+their+applications+for+organ-specific+in+vitro+modeling.">Blood and lymphatic vasculatures on-chip platforms and their applications for organ-specific in vitro modeling.</a> Micromachines (Basel). 11 (2), 147 (2020).</li><li>Lin, D. S. Y., Guo, F., Zhang, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modeling+organ-specific+vasculature+with+organ-on-a-chip+devices.">Modeling organ-specific vasculature with organ-on-a-chip devices.</a> Nanotechnology. 30 (2), 024002 (2019).</li><li>Pollet, A., den Toonder, J. M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recapitulating+the+vasculature+using+organ-on-chip+technology.">Recapitulating the vasculature using organ-on-chip technology.</a> Bioengineering. 7 (1), 17 (2020).</li><li>Cochrane, 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=Advanced+in+vitro+models+of+vascular+biology:+Human+induced+pluripotent+stem+cells+and+organ-on-chip+technology.">Advanced in vitro models of vascular biology: Human induced pluripotent stem cells and organ-on-chip technology.</a> Advanced Drug Delivery Reviews. 140, 68-77 (2019).</li></ol>

The authors have nothing to disclose.

This protocol describes an unbiased, robust, and reproducible 3D EB-based vascular sprouting assay that is amenable to screening for drugs and genes modulating angiogenesis. This method offers advantages over many widely used two dimensional (2D) assays using endothelial cell cultures such as Human Umbilical Vein Endothelial Cells (HUVECs) to monitor migration (lateral scratch assay or the Boyden chamber assay)22,23&#160;or proliferation (counting cell number, de...

In the past three decades, target-based drug discovery (TDD) has been widely employed in drug discovery by the pharmaceutical industry. TDD incorporates a defined molecular target playing an important role in a disease and relies on the development of relatively simple cell culture systems and readouts for drug screening1. Most typical disease models used in TDD programs include traditional cell culture methods such as cancer cells or immortalized cell lines grown within artificial environments and non-physiological substrates. Although many of these models have provided viable tools for identifying successful drug candidates, the use of such systems can be questionable owing to their poor disease relevance2.
For most diseases, the underlying mechanisms are indeed complex and various cell types, independent signaling pathways, and multiple sets of genes are often found to contribute to a specific disease phenotype. This is also true for inherited diseases where the primary cause is a mutation in one single gene. With the recent advent of human induced pluripotent stem cell (iPSC) technologies and gene editing tools, it is now possible to generate 3D organoids and organ-on-chip disease models that could better recapitulate the in vivo human complexity3,4. The development of such technologies is associated with a resurgence in interest in phenotypic drug discovery (PDD) programs1. PDD can be compared to empirical screening, as they do not rely on knowledge of the identity of a specific drug target or a hypothesis about its role in disease. The PDD approach is now increasingly recognized to strongly contribute to the discovery of first-in-class drugs5. Because the development of human organoid and organ-on-chip technologies is still in its infancy, it is expected that iPSC models (complemented with innovative imaging and machine-learning tools6,7) will provide, in the near future, multiple novel complex cell-based disease models for drug screening and associated PDD programs to overcome the poor productivity of the TDD approach8,9.
While human organoid and organ-on-chip models can provide important insights into disease complexity and to the identification of novel drugs, bringing drugs into new clinical practice also strongly relies on data from animal models to assess their efficacy and safety. Among them, genetically modified mice are certainly the most preferred mammalian models. They have many advantages as they have a relatively short generation time for mammals, have many similar phenotypes to human diseases, and can be easily genetically manipulated. They are therefore extensively used in drug discovery programs10. However, bridging the gap between mice and humans remains an important challenge11. The development of in vitro mouse models equivalent to human organoid and organ-on-chip models could at least partially fill this gap as it will allow direct drug efficacy and safety comparisons between in vivo mouse and in vitro human data.
Here, a vascular sprouting assay in mouse embryoid bodies (EBs) is described. Blood vessels are composed of endothelial cells (inner lining of vessel walls), mural cells (vascular smooth muscle cells&#160;and pericytes)12. This protocol is based on the differentiation of mouse embryonic stem cells (mESCs) into vascularized EBs using hanging droplets that recapitulate de novo endothelial cell and mural cell differentiation13,14. Mouse ESCs can be easily established in culture from isolated day 3.5 mouse blastocysts having different genetic backgrounds15. They also provide possibilities for clonal analysis, lineage tracing, and can be easily genetically manipulated to generate disease models13,16.
As blood vessels nourish all organs, it is not surprising that many diseases if not all, are associated with changes in the microvasculature. In pathological conditions, endothelial cells can adopt an activated state or can become dysfunctional resulting in mural cell death or migration away from blood vessels. These can result in excessive angiogenesis or in vessel rarefaction, can induce abnormal blood flow and defective blood vessel barrier leading to immune cell extravasation, and inflammation12,17,18,19. Research for the development of drugs modulating blood vessels is, therefore&#160;high, and multiple molecular players and concepts have already been identified for therapeutic targeting. In this context, the protocol described is particularly suitable for building disease models and for drug testing as it recapitulates key features of in vivo sprouting angiogenesis, including endothelial tip and stalk cell selection, endothelial cell migration and proliferation, endothelial cell guidance, tube formation, and mural cell recruitment. It also shows similarities with recently described 3D vascular assays based on human iPSC technologies20.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>2-mercaptoethanol</td>
			<td>Milipore, Merck</td>
			<td>805740</td>
			<td>Biohazard: adequate safety instructions should be taken when handling</td>
		</tr>
		<tr>
			<td>Agar Noble</td>
			<td>Difco, BD Pharmigen</td>
			<td>214220</td>
			<td></td>
		</tr>
		<tr>
			<td>Alexa Fluo 555 goat anti rat IgG</td>
			<td>Life technologies</td>
			<td>A21434</td>
			<td></td>
		</tr>
		<tr>
			<td>APC conjugated rat anti-mouse PECAM-1 antibody (clone MEC13.3)</td>
			<td>BD Biosciences</td>
			<td>551262</td>
			<td></td>
		</tr>
		<tr>
			<td>APC Rat IgG2a &#954; Isotype Control (Clone&#160; R35-95)</td>
			<td>BD Biosciences</td>
			<td>553932</td>
			<td></td>
		</tr>
		<tr>
			<td>Axiovert 25 inverted phase contrast tissue culture microscope</td>
			<td>ZEISS</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Basic Fibroblast Growth Factor-2 (bFGF)</td>
			<td>Peprotech</td>
			<td>450-33</td>
			<td></td>
		</tr>
		<tr>
			<td>Benchtop Centrifuge, Allegra X-15R</td>
			<td>Beckman Coulter</td>
			<td>392932</td>
			<td></td>
		</tr>
		<tr>
			<td>Biosafety cabinet BioVanguard (Green Line)</td>
			<td>Telstar</td>
			<td>133H401001</td>
			<td></td>
		</tr>
		<tr>
			<td>Bovine Serum Albumin (BSA)</td>
			<td>Sigma-Aldrich</td>
			<td>A9418</td>
			<td></td>
		</tr>
		<tr>
			<td>Cell counting chamber, Buerker, 0.100mm</td>
			<td>Marienfeld</td>
			<td>640211</td>
			<td></td>
		</tr>
		<tr>
			<td>Cell culture dishes 60 x 15mm</td>
			<td>Corning</td>
			<td>353802</td>
			<td></td>
		</tr>
		<tr>
			<td>Cell culture dishes, 35 x 10 mm</td>
			<td>Corning</td>
			<td>353801</td>
			<td></td>
		</tr>
		<tr>
			<td>Cell culture plates 12-well</td>
			<td>Corning</td>
			<td>3512</td>
			<td></td>
		</tr>
		<tr>
			<td>CFX96 Touch Real-Time PCR Detection System</td>
			<td>Biorad</td>
			<td>1855196</td>
			<td></td>
		</tr>
		<tr>
			<td>Chicken serum</td>
			<td>Sigma-Aldrich</td>
			<td>C5405</td>
			<td></td>
		</tr>
		<tr>
			<td>CHIR-99021 (CT99021) HCl</td>
			<td>Selleckchem</td>
			<td>S2924</td>
			<td></td>
		</tr>
		<tr>
			<td>Collagen I, High Concentration, Rat Tail, 100mg</td>
			<td>Corning</td>
			<td>354249</td>
			<td></td>
		</tr>
		<tr>
			<td>Collagenase A</td>
			<td>Roche</td>
			<td>10103586001</td>
			<td></td>
		</tr>
		<tr>
			<td>Confocal Laser Scanning Microscope, TCS SP5</td>
			<td>Leica</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cover glasses, 24 &#215; 50 mm</td>
			<td>Vwr</td>
			<td>631-0146</td>
			<td></td>
		</tr>
		<tr>
			<td>DAPT &#947;&#8209;secretase inhibitor</td>
			<td>Sigma Aldrich</td>
			<td>D5942</td>
			<td></td>
		</tr>
		<tr>
			<td>DC101 anti mouse VEGFR-2 Clone</td>
			<td>BioXcell</td>
			<td>BP0060</td>
			<td></td>
		</tr>
		<tr>
			<td>DC101 isotype rat IgG1</td>
			<td>BioXcell</td>
			<td>BP0290</td>
			<td></td>
		</tr>
		<tr>
			<td>Dimethyl sulfoxide (DMSO)</td>
			<td>Sigma-Aldrich</td>
			<td>D2438-5X</td>
			<td>Biohazard: adequate safety instructions should be taken when handling</td>
		</tr>
		<tr>
			<td>DPBS (10x), no calcium, no magnesium</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>14200067</td>
			<td></td>
		</tr>
		<tr>
			<td>EDTA 40 mM</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>15575-038</td>
			<td></td>
		</tr>
		<tr>
			<td>Embryonic stem-cell Fetal Bovine Serum</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>16141-079</td>
			<td>Should be lot-tested for maximum ES cell viability and growth. Heat inactivate at 60&#176;C and store at &#8722;20 &#176;C for up to 1 year</td>
		</tr>
		<tr>
			<td>Eppendorf Microcentrifuge 5415R</td>
			<td>Eppendorf AG</td>
			<td>&#160;Z605212</td>
			<td></td>
		</tr>
		<tr>
			<td>Erythropoietin, human (hEPO), 250 U (2.5 &#181;g) (1 mL)</td>
			<td>Roche</td>
			<td>11120166001</td>
			<td></td>
		</tr>
		<tr>
			<td>ESGRO Recombinant Mouse LIF Protein (10&#8311; units&#160;&#160;1 mL)</td>
			<td>Milipore, Merck</td>
			<td>ESG1107</td>
			<td></td>
		</tr>
		<tr>
			<td>Falcon tubes 15 mL</td>
			<td>Greiner Bio-One</td>
			<td>188271</td>
			<td></td>
		</tr>
		<tr>
			<td>Falcon tubes 50 mL</td>
			<td>Greiner Bio-0ne</td>
			<td>227270</td>
			<td></td>
		</tr>
		<tr>
			<td>Filter tip ,clear ,sterile F.Gilson, P-200</td>
			<td>Greiner Bio-One</td>
			<td>739288</td>
			<td></td>
		</tr>
		<tr>
			<td>Filter tip ,clear ,sterile F.Gilson, P10</td>
			<td>Greiner Bio-One</td>
			<td>771288</td>
			<td></td>
		</tr>
		<tr>
			<td>Filter tip ,clear ,sterile F.Gilson, P1000</td>
			<td>Greiner Bio-One</td>
			<td>740288</td>
			<td></td>
		</tr>
		<tr>
			<td>FITC conjugated anti-&#945; Smooth Muscle Actin (SMA) (clone 1A4)</td>
			<td>Sigma Aldrich</td>
			<td>F3777</td>
			<td></td>
		</tr>
		<tr>
			<td>FITC conjugated rat anti-mouse CD45 (clone 30-F11)</td>
			<td>Biolegend</td>
			<td>103107</td>
			<td></td>
		</tr>
		<tr>
			<td>FITC Rat IgG2b, &#954; Isotype Ctrl Antibody (clone RTK4530)</td>
			<td>Biolegend</td>
			<td>400605</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorscent mounting media</td>
			<td>DAKO</td>
			<td>S3023</td>
			<td></td>
		</tr>
		<tr>
			<td>Gascompress</td>
			<td>Cutisoft</td>
			<td>45846</td>
			<td></td>
		</tr>
		<tr>
			<td>Gauze Cutisoft 10 x 10 cm</td>
			<td>Bsn Medical</td>
			<td>45844_00</td>
			<td></td>
		</tr>
		<tr>
			<td>Gel blotting paper, Grade GB003</td>
			<td>Whatman</td>
			<td>WHA10547922</td>
			<td></td>
		</tr>
		<tr>
			<td>Gelatin solution, type B</td>
			<td>Sigma-Aldrich</td>
			<td>G1393-100 ml</td>
			<td></td>
		</tr>
		<tr>
			<td>Glasgow&#39;s MEM (GMEM)</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>21710082</td>
			<td></td>
		</tr>
		<tr>
			<td>IHC Zinc Fixative</td>
			<td>BD Pharmigen</td>
			<td>550523</td>
			<td></td>
		</tr>
		<tr>
			<td>IncuSafe CO2 Incubator</td>
			<td>PHCBi</td>
			<td>MCO-170AICUV-PE</td>
			<td></td>
		</tr>
		<tr>
			<td>Interleukin-6, human (hIL-6)</td>
			<td>Roche</td>
			<td>11138600001</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Glutamine 200 mM</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>25030-024</td>
			<td></td>
		</tr>
		<tr>
			<td>MEM Non-Essential Amino Acids Solution (100x)</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>11140035</td>
			<td></td>
		</tr>
		<tr>
			<td>Microscope slide box</td>
			<td>Kartell Labware</td>
			<td>278</td>
			<td></td>
		</tr>
		<tr>
			<td>Microscope slide, Starfrost</td>
			<td>Knittel glass</td>
			<td>VS113711FKB.0</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Cdh5_1_SG QuantiTect Primer Assay</td>
			<td>Qiagen</td>
			<td>QT00110467</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Eng_1_SG QuantiTect Primer Assay</td>
			<td>Qiagen</td>
			<td>QT00148981</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Epha4_1_SG QuantiTect Primer Assay</td>
			<td>Qiagen</td>
			<td>QT00093576</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Ephb2_1_SG QuantiTect Primer Assay</td>
			<td>Qiagen</td>
			<td>QT00154014</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Flt1_1_SG QuantiTect Primer Assay</td>
			<td>Qiagen</td>
			<td>QT00096292</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Flt4_1_SG QuantiTect Primer Assay</td>
			<td>Qiagen</td>
			<td>QT00099064</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Gapdh_3_SG QuantiTect Primer Assay</td>
			<td>Qiagen</td>
			<td>QT01658692</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Kdr_1_SG QuantiTect Primer Assay</td>
			<td>Qiagen</td>
			<td>QT00097020</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Notch1_1_SG QuantiTect Primer</td>
			<td>Qiagen</td>
			<td>QT00156982</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Nr2f2_1_SG QuantiTect Primer Assay</td>
			<td>Qiagen</td>
			<td>QT00153104</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Pecam1_1_SG QuantiTect Primer</td>
			<td>Qiagen</td>
			<td>QT01052044</td>
			<td></td>
		</tr>
		<tr>
			<td>Mm_Tek_1_SG QuantiTect Primer Assay</td>
			<td>Qiagen</td>
			<td>QT00114576</td>
			<td></td>
		</tr>
		<tr>
			<td>Mouse (ICR) Inactivated Embryonic Fibroblasts&#160; (2 M)</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>A24903</td>
			<td>Store vials in liquid nitrogen (195.79 &#176;C) indefinitely</td>
		</tr>
		<tr>
			<td>Mouse embryonic stem cell line 7AC5/EYFP (ATCC SCRC-1033)</td>
			<td>ATCC</td>
			<td>SCRC-1033</td>
			<td>Generated by Dr A Nagy, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Ave, Toronto, Ontario, M5G 1X5, Canada. [Hadjantonakis, A. K., et al. Mechanisms of Development. 76 (1&#8211;2), 79&#8211;90 (1998)].</td>
		</tr>
		<tr>
			<td>Mouse embryonic stem cell lines Acvrl1 +/- and Acvrl1 +/+</td>
			<td></td>
			<td></td>
			<td>Generated at Leiden University Medical Centre [Thalgott, J.H. et al. Circulation. 138 (23), 2698&#8211;2712 (2018)].</td>
		</tr>
		<tr>
			<td>Mouse embryonic stem cells line E14</td>
			<td></td>
			<td></td>
			<td>Provided by M Letarte laboratory and generated according to Cho, S. K., et al. Blood. 98 (13), 3635&#8211;3642 (2001).</td>
		</tr>
		<tr>
			<td>Mouse embryonic stem cells line R1 (ATCC SCRC-1011)</td>
			<td>ATCC</td>
			<td>SCRC-1011</td>
			<td>Generated by Dr A Nagy, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Ave, Toronto, Ontario, M5G 1X5, Canada. [Nagy, A., et al. Procedings of the National Academy of Sciences of the United States of America. 90 (18), 8424&#8211;8428 (1993)].</td>
		</tr>
		<tr>
			<td>Mouse embryonic stem cells line Z/Red (strain 129/Ola)</td>
			<td></td>
			<td></td>
			<td>Generated by Dr A Nagy, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Ave, Toronto, Ontario, M5G 1X5, Canada [Vintersten, K., et al. Genesis. 40 (4), 241&#8211;246 (2004)].</td>
		</tr>
		<tr>
			<td>NanoDrop 1000 UV/VIS Spectrophotometer</td>
			<td>Thermo Fischer Scientific</td>
			<td>ND-1000</td>
			<td></td>
		</tr>
		<tr>
			<td>PD0325901</td>
			<td>Selleckchem</td>
			<td>S1036</td>
			<td></td>
		</tr>
		<tr>
			<td>PDGF-BB, Recombinant Human</td>
			<td>Peprotech</td>
			<td>100-14B</td>
			<td></td>
		</tr>
		<tr>
			<td>Pecam-1 antibody, Rat Anti-Mouse</td>
			<td>BD Biosciences</td>
			<td>550274</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin-streptomycin (10,000 U/mL)</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>15140122</td>
			<td></td>
		</tr>
		<tr>
			<td>Petri dish, PS, 94/16 mm, standard ,with vents, sterile</td>
			<td>Greiner Bio-One</td>
			<td>633181</td>
			<td></td>
		</tr>
		<tr>
			<td>Pipetboy acu 2</td>
			<td>Integra-Biosciences</td>
			<td>155 019</td>
			<td></td>
		</tr>
		<tr>
			<td>Pipetman G Multichannel P8 x 200G</td>
			<td>Gilson</td>
			<td>F144072</td>
			<td></td>
		</tr>
		<tr>
			<td>Pipetman G Starter Kit, 4 Pipette Kit, P2G, P20G, P200G, P1000G</td>
			<td>Gilson</td>
			<td>F167360</td>
			<td></td>
		</tr>
		<tr>
			<td>Recombinant Human BMP-4 Protein</td>
			<td>R&#38;D Systems</td>
			<td>314-BP</td>
			<td></td>
		</tr>
		<tr>
			<td>RNeasy Plus mini Kit</td>
			<td>QIAGEN</td>
			<td>74134</td>
			<td></td>
		</tr>
		<tr>
			<td>Serological pipettes, 10 mL</td>
			<td>Greiner Bio-One</td>
			<td>607 180</td>
			<td></td>
		</tr>
		<tr>
			<td>Serological pipettes, 25 mL</td>
			<td>Greiner Bio-One</td>
			<td>760 180</td>
			<td></td>
		</tr>
		<tr>
			<td>Serological pipettes, 5 mL</td>
			<td>Greiner Bio-One</td>
			<td>606 180</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium hydroxide (NaOH)</td>
			<td>Merck</td>
			<td>106498</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium pyruvate 100 mM</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>11360039</td>
			<td></td>
		</tr>
		<tr>
			<td>Test tubes 5ml round-bottom with cell-strainer cap</td>
			<td>Corning</td>
			<td>352235</td>
			<td></td>
		</tr>
		<tr>
			<td>Thermal cycler, T100</td>
			<td>Biorad</td>
			<td>1861096</td>
			<td></td>
		</tr>
		<tr>
			<td>Triton X-100 (BioXtra)</td>
			<td>Sigma Aldrich</td>
			<td>T9284</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypan Blue Solution, 0.4%</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>15250061</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypsin (2.5%)</td>
			<td>Gibco, Thermofisher scientific</td>
			<td>15090046</td>
			<td></td>
		</tr>
		<tr>
			<td>Vacuum Filter/Storage Bottle System, 500 mL</td>
			<td>Corning</td>
			<td>430758</td>
			<td></td>
		</tr>
		<tr>
			<td>VEGFA165 , recombinant murine</td>
			<td>Peprotech</td>
			<td>450-32</td>
			<td></td>
		</tr>
		<tr>
			<td>Water, Sterile</td>
			<td>Fresenius-Kabi</td>
			<td>B230531</td>
			<td></td>
		</tr>
		<tr>
			<td>Waterbath, Lab-Line Digital</td>
			<td>Thermo Fischer Scientific</td>
			<td>18052A</td>
			<td></td>
		</tr>
	</tbody>
</table>

in vitro three-dimensional sprouting assay of angiogenesis using mouse embryonic stem cells for vascular disease modeling and drug testing

Recent advances in induced pluripotent stem cells (iPSC) and gene editing technologies enable the development of novel human cell-based disease models for phenotypic drug discovery (PDD) programs. Although these novel devices could predict the safety and efficacy of investigational drugs in humans more accurately, their development to the clinic still strongly rely on mammalian data, notably the use of mouse disease models. In parallel to human organoid or organ-on-chip disease models, the development of relevant in vitro mouse models is therefore an unmet need for evaluating direct drug efficacy and safety comparisons between species and in vivo and in vitro conditions. Here, a vascular sprouting assay that utilizes mouse embryonic stem cells differentiated into embryoid bodies (EBs) is described. Vascularized EBs cultured onto 3D-collagen gel develop new blood vessels that expand, a process called sprouting angiogenesis. This model recapitulates key features of in vivo sprouting angiogenesis-formation of blood vessels from a pre-existing vascular network-including endothelial tip cell selection, endothelial cell migration and proliferation, cell guidance, tube formation, and mural cell recruitment. It is amenable to screening for drugs and genes modulating angiogenesis and shows similarities with recently described three-dimensional (3D) vascular assays based on human iPSC technologies.

The protocol overview of the blood vessel sprouting assay is shown in Figure 1. Nine-day-old EBs derived from three independent 129/Ola mESC lines (Z/Red, R1, and E14) were enzymatically dissociated into single cells using collagenase A. Cells were stained for PECAM-1 and analyzed by Fluorescence-activated cell sorting (FACS) as described. All the cell lines exhibited robust endothelial differentiation, and no differences in their ability to differentiate into endothelial cells were observed...

Watch this Scientific Journal Video about In Vitro Three-Dimensional Sprouting Assay of Angiogenesis Using Mouse Embryonic Stem Cells for Vascular Disease Modeling and Drug Testing at JoVE.com

In Vitro Three-Dimensional Sprouting Assay of Angiogenesis Using Mouse Embryonic Stem Cells for Vascular Disease Modeling and Drug Testing

This assay utilizes mouse embryonic stem cells differentiated into embryoid bodies cultured in 3D-collagen gel to analyze the biological processes that control sprouting angiogenesis in vitro. The technique can be applied for testing drugs, modeling diseases, and for studying specific genes in the context of deletions that are embryonically lethal.

In Vitro Three-Dimensional Sprouting Assay of Angiogenesis Using Mouse Embryonic Stem Cells for Vascular Disease Modeling and Drug Testing

Recent advances in induced pluripotent stem cells (iPSC) and gene editing technologies enable the development of novel human cell-based disease models for ...

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2.7K Views.  Leiden University Medical Center. This assay utilizes mouse embryonic stem cells differentiated into embryoid bodies cultured in 3D-collagen gel to analyze the biological processes that control sprouting angiogenesis in vitro. The technique can be applied for testing drugs, modeling diseases, and for studying specific genes in the context of deletions that are embryonically lethal.

Video: In Vitro Three-Dimensional Sprouting Assay of Angiogenesis Using Mouse Embryonic Stem Cells for Vascular Disease Modeling and Drug Testing

In vitro Differentiation of Mouse Embryonic Stem (mES) Cells Using the Hanging Drop Method

Stem cells have the remarkable potential to develop into many different cell types. When a stem cell divides, each new cell has the potential to either remain a stem cell or become another type of cell with a more specialized function, This promising of science is leading scientists to investigate the possibility of cell-based therapies to treat disease.

When culture in suspension without antidifferentiation factors, embryonic stem cells spontaneously differentiate and form three-dimensional multicellular aggregates. These cell aggregates are called embryoid bodies(EB). Hanging drop culture is a widely used EB formation induction method. The rounded bottom of hanging drop allows the aggregation of ES cells which can provide mES cells a good environment for forming EBs. The number of ES cells aggregatied in a hanging drop can be controlled by varying the number of cells in the initial cell suspension to be hung as a drop from the lid of Petri dish. Using this method we can reproducibly form homogeneous EBs from a predetermined number of ES cells. 

In vitro Differentiation of Mouse Embryonic Stem mES Cells Using the Hanging Drop Method

This video demonstrates how to conduct in vitro differentiation of mouse embryonic stem cells to embryoid bodies using the hanging drop method.

Stem cells have the remarkable potential to develop into many different cell types. When a stem cell divides, each new cell has the potential to either ...

In vitro Labeling of Human Embryonic Stem Cells for Magnetic Resonance Imaging

Human embryonic stem cells (hESC) have demonstrated the ability to restore the injured myocardium. Magnetic resonance imaging (MRI) has emerged as one of the predominant imaging modalities to assess the restoration of the injured myocardium. Furthermore, ex-vivo labeling agents, such as iron-oxide nanoparticles, have been employed to track and localize the transplanted stem cells. However, this method does not monitor a fundamental cellular biology property regarding the viability of transplanted cells. It has been known that manganese chloride (MnCl2) enters the cells via voltage-gated calcium (Ca2+) channels when the cells are biologically active, and accumulates intracellularly to generate T1 shortening effect. Therefore, we suggest that manganese-guided MRI can be useful to monitor cell viability after the transplantation of hESC into the myocardium.
In this video, we will show how to label hESC with MnCl2 and how those cells can be clearly seen by using MRI in vitro. At the same time, biological activity of Ca2+-channels will be modulated utilizing both Ca2+-channel agonist and antagonist to evaluate concomitant signal changes.

In this video, we are showing how to label human embryonic stem cells (hESC) with manganese chloride (MnCl2) which can enter cells via voltage-gated calcium channels when the cells are biologically active. Additionally, we show the use of MnCl2 as cellular MRI contrast agent to determine the in vitro viability of hESC.

Human embryonic stem cells (hESC) have demonstrated the ability to restore the injured myocardium. Magnetic resonance imaging (MRI) has emerged as one of ...

Isolation and Derivation of Mouse Embryonic Germinal Cells

The ability of embryonic germinal cells (EG) to differentiate into primordial germinal cells (PGCs) and later into gametes during early developmental stages is a perfect model to address our hypothesis about cancer and infertility. This protocol shows how to isolate primordial germinal cells from developing gonads in 10.5-11.5 days post coitum (dpc) mouse embryos.  Developing gonadal ridges from mouse embryos (C57BL6J) were dissociated by mechanical disruption with collagenase, then plated in a mouse embryo fibroblast feeder layer (MEF-CF1) that was previously mitotically inactivated with mitomycin C in the presence of knockout media and supplemented with Leukemia Inhibitor Factor (LIF), basic Fibroblast Growth Factor (bFGF), and Stem Cell Factor (SCF).  Using these optimized methods for PCG identification, isolation, and establishment of culture conditions permits long term cultures of EG cells for more than 40 days.  The embryonic germinal cell lines showed embryonic phenotype and expression of common used markers of the pluripotent state.  Isolation and derivation of germinal cells in culture provide a tool to understand their development in vitro and offer the opportunity to monitor cumulative damage at genetic and epigenetic levels after exposure to oxidative stress.

The ability of embryonic germinal cells to differentiate into primordial germinal cells during early development stages is a perfect model to address our hypothesis about cancer and infertility. This protocol shows how to isolate primordial germinal cells from developing gonads in 10.5-11.5 days post coitum mouse embryos.

The ability of embryonic germinal cells (EG) to differentiate into primordial germinal cells (PGCs) and later into gametes during early developmental ...

Preparation of Mouse Embryonic Fibroblast Cells Suitable for Culturing Human Embryonic and Induced Pluripotent Stem Cells

In general, human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs)1 can be cultured under variable conditions. However, it is not easy to establish an effective system for culturing these cells. Since the culture conditions can influence gene expression that confers pluripotency in hESCs and hiPSCs, the optimization and standardization of the culture method is crucial.
The establishment of hESC lines was first described by using MEFs as feeder cells and fetal bovine serum (FBS)-containing culture medium2. Next, FBS was replaced with knockout serum replacement (KSR) and FGF2, which enhances proliferation of hESCs3. Finally, feeder-free culture systems enable culturing cells on Matrigel-coated plates in KSR-containing conditioned medium (medium conditioned by MEFs)4. Subsequently, hESCs culture conditions have moved towards feeder-free culture in chemically defined conditions5-7. Moreover, to avoid the potential contamination by pathogens and animal proteins culture methods using xeno-free components have been established8.
To obtain improved conditions mouse feeder cells have been replaced with human cell lines (e.g. fetal muscle and skin cells9, adult skin cells10, foreskin fibroblasts11-12, amniotic mesenchymal cells13). However, the efficiency of maintaining undifferentiated hESCs using human foreskin fibroblast-derived feeder layers is not as high as that from mouse feeder cells due to the lower level of secretion of Activin A14. Obviously, there is an evident difference in growth factor production by mouse and human feeder cells.
Analyses of the transcriptomes of mouse and human feeder cells revealed significant differences between supportive and non-supportive cells. Exogenous FGF2 is crucial for maintaining self-renewal of hESCs and hiPSCs, and has been identified as a key factor regulating the expression of Tgf&beta;1, Activin A and Gremlin (a BMP antagonist) in feeder cells. Activin A has been shown to induce the expression of OCT4, SOX2, and NANOG in hESCs15-16.
For long-term culture, hESCs and hiPSCs can be grown on mitotically inactivated MEFs or under feeder-free conditions in MEF-CM (MEF-Conditioned Medium) on Matrigel-coated plates to maintain their undifferentiated state. Success of both culture conditions fully depends on the quality of the feeder cells, since they directly affect the growth of hESCs.
Here, we present an optimized method for the isolation and culture of mouse embryonic fibroblasts (MEFs), preparation of conditioned medium (CM) and enzyme-linked immunosorbent assay (ELISA) to assess the levels of Activin A within the media.

The quality of mouse embryonic fibroblasts (MEFs) is dictated by the right strain of mouse such as CF-1. Pluripotency-supportive MEFs and conditioned media (CM) obtained from these should contain optimal concentrations of Activin A, Gremlin and Tgf&beta;1 needed for the Activin/Nodal and FGF pathways to co-operatively maintain self-renewal and pluripotency.

In general, human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs)1 can be cultured under variable conditions. However, it ...

Endothelial Cell Tube Formation Assay for the In Vitro Study of Angiogenesis

Angiogenesis is a vital process for normal tissue development and wound healing, but is also associated with a variety of pathological conditions. Using this protocol, angiogenesis may be measured in vitro in a fast, quantifiable manner. Primary or immortalized endothelial cells are mixed with conditioned media and plated on basement membrane matrix. The endothelial cells form capillary like structures in response to angiogenic signals found in conditioned media. The tube formation occurs quickly with endothelial cells beginning to align themselves within 1&#160;hr and lumen-containing tubules beginning to appear within 2 hr. Tubes can be visualized using a phase contrast inverted microscope, or the cells can be treated with calcein AM prior to the assay and tubes visualized through fluorescence or confocal microscopy. The number of branch sites/nodes, loops/meshes, or number or length of tubes formed can be easily quantified as a measure of in vitro angiogenesis. In summary, this assay can be used to identify genes and pathways that are involved in the promotion or inhibition of angiogenesis in a rapid, reproducible, and quantitative manner.

Endothelial Cell Tube Formation Assay for the In Vitro Study of Angiogenesis

The tube formation assay is a fast, quantifiable method for measuring in vitro angiogenesis. Endothelial cells are combined with conditioned media and plated on basement membrane extract. Tube formation occurs within hours and newly formed tubules easily quantified.

Angiogenesis is a vital process for normal tissue development and wound healing, but is also associated with a variety of pathological conditions. Using ...

Profiling Individual Human Embryonic Stem Cells by Quantitative RT-PCR

Heterogeneity of stem cell population hampers detailed understanding of stem cell biology, such as their differentiation propensity toward different lineages. A single cell transcriptome assay can be a new approach for dissecting individual variation. We have developed the single cell qRT-PCR method, and confirmed that this method works well in several gene expression profiles. In single cell level, each human embryonic stem cell, sorted by OCT4::EGFP positive cells, has high expression in OCT4, but a different level of NANOG expression. Our single cell gene expression assay should be useful to interrogate population heterogeneities.

Single cell gene expression assay is needed for understanding stem cell heterogeneities.

Heterogeneity of stem cell population hampers detailed understanding of stem cell biology, such as their differentiation propensity toward different ...

Zinc-finger Nuclease Enhanced Gene Targeting in Human Embryonic Stem Cells

One major limitation with current human embryonic stem cell (ESC) differentiation protocols is the generation of heterogeneous cell populations.&#160;These cultures contain the cells of interest, but are also contaminated with undifferentiated ESCs, non-neural derivatives and other neuronal subtypes.&#160; This limits their use in in vitro and in vivo applications, such as in vitro modeling for drug discovery or cell replacement therapy. To help overcome this, reporter cell lines, which offer a means to visualize, track and isolate cells of interest, can be engineered.&#160;However, to achieve this in human embryonic stem cells via conventional homologous recombination is extremely inefficient.&#160;This protocol describes targeting of the Pituitary homeobox 3 (PITX3) locus in human embryonic stem cells using custom designed zinc-finger nucleases, which introduce site-specific double-strand DNA breaks, together with a PITX3-EGFP-specific DNA donor vector. Following the generation of the PITX3 reporter cell line, it can then be differentiated using published protocols for use in studies such as in vitro Parkinson&#8217;s disease modeling or cell replacement therapy.

Reporter cell lines offer a means to visualize, track and isolate cells of interest from heterogeneous populations.&#160;However, gene-targeting using conventional homologous recombination in human embryonic stem cells is extremely inefficient. Herein, we describe targeting CNS midbrain specific transcription factor PITX3 locus with EGFP using zinc-finger nuclease enhanced homologous recombination.

One major limitation with current human embryonic stem cell (ESC) differentiation protocols is the generation of heterogeneous cell ...

Isolation and Primary Culture of Mouse Aortic Endothelial Cells

The vascular endothelium is essential to normal vascular homeostasis. Its dysfunction participates in various cardiovascular disorders. The mouse is an important model for cardiovascular disease research. This study demonstrates a simple method to isolate and culture endothelial cells from the mouse aorta without any special equipment. To isolate endothelial cells, the thoracic aorta is quickly removed from the mouse body, and the attached adipose tissue and connective tissue are removed from the aorta. The aorta is cut into 1 mm rings. Each aortic ring is opened and seeded onto a growth factor reduced matrix with the endothelium facing down. The segments are cultured in endothelial cell growth medium for about 4 days. The endothelial sprouting starts as early as day 2. The segments are then removed and the cells are cultured continually until they reach confluence. The endothelial cells are harvested using neutral proteinase and cultured in endothelial cell growth medium for another two passages before being used for experiments. Immunofluorescence staining indicated that after the second passage the majority of cells were double positive for Dil-ac-LDL uptake, Lectin binding, and CD31 staining, the typical characteristics of endothelial cells. It is suggested that cells at the second to third passages are suitable for in vitro and in vivo experiments to study the endothelial biology. Our protocol provides an effective means of identifying specific cellular and molecular mechanisms in endothelial cell physiopathology.

The vascular endothelial cells play a significant role in many important cardiovascular disorders. This article describes a simple method to isolate and expand endothelial cells from the mouse aorta without using any special equipment. Our protocol provides an effective means of identifying mechanisms in endothelial cell physiopathology.

The vascular endothelium is essential to normal vascular homeostasis. Its dysfunction participates in various cardiovascular disorders. The mouse is an ...

Three-dimensional Reconstruction of the Vascular Architecture of the Passive CLARITY-cleared Mouse Ovary

The ovary is the main organ of the female reproductive system and is essential for the production of female gametes and for controlling the endocrine system, but the complex structural relationships and three-dimensional (3D) vasculature architectures of the ovary are not well described. In order to visualize the 3D connections and architecture of blood vessels in the intact ovary, the first important step is to make the ovary optically clear. In order to avoid tissue shrinkage, we used the hydrogel fixation-based passive CLARITY (Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging/ Immunostaining/In situ-hybridization-compatible Tissue Hydrogel) protocol method to clear an intact ovary. Immunostaining, advanced multiphoton confocal microscopy, and 3D image-reconstructions were then used for the visualization of ovarian vessels and follicular capillaries. Using this approach, we showed a significant positive correlation (P &#60;0.01) between the length of the follicular capillaries and volume of the follicular wall.

Here we present an adaptation of the passive CLARITY and 3D reconstruction method for visualization of the ovarian vasculature and follicular capillaries in intact mouse ovaries.

The ovary is the main organ of the female reproductive system and is essential for the production of female gametes and for controlling the endocrine ...

Trans-inner Cell Mass Injection of Embryonic Stem Cells Leads to Higher Chimerism Rates

In an effort to increase efficiency in the creation of genetically modified mice via ES Cell methodologies, we present an adaptation to the current blastocyst injection protocol. Here we report that a simple rotation of the embryo, and injection through Trans-Inner cell mass (TICM) increased the percentage of chimeric mice from 31% to 50%, with no additional equipment or further specialized training. 26 different inbred clones, and 35 total clones were injected over a period of 9 months. There was no significant difference in either pregnancy rate or recovery rate of embryos between traditional injection techniques and TICM. Therefore, without any major alteration in the injection process and a simple positioning of the blastocyst and injecting through the ICM, releasing the ES cells into the blastocoel cavity can potentially improve the quantity of chimeric production and subsequent germline transmission.

Here we present a protocol to increase chimera production without the use of new equipment. A simple orientation change of the embryo for injection can increase the number of embryos produced, and potentially reduce the timeline to germline transmission.

In an effort to increase efficiency in the creation of genetically modified mice via ES Cell methodologies, we present an adaptation to the current ...