The authors thank Sophie Kr&#252;ger and Gabriele Prinz for their excellent technical support. We thank Sebastian Maier for developing the ImageJ plugin to quantify spheroid sprouts and the Lighthouse Core Facility, Zentrum f&#252;r Translationale Zellforschung (ZTZ), Department of Medicine I, University Hospital Freiburg for the use of the IncuCyte system. The graphics were created with biorender.com. This work was supported by the Deutsche Forschungsgemeinschaft [Bu3135/3-1 + Bu3135/3-2 to F.B], the Medizinische Fakult&#228;t der Albert-Ludwigs- Universit&#228;t Freiburg [Berta-Ottenstein-Program for Clinician Scientists and Advanced Clinician Scientists to F.B.], the Else Kr&#246;ner-Fresenius-Stiftung [2021_EKEA.80 to F.B.] the German Cancer Consortium [CORTEX fellowship for Clinician Scientists to J.R.] and the Volker Homann Stiftung [to J.N.+F.B.] and the &#34;Freunde der Universit&#228;ts-Augenklinik Freiburg e.V.&#34; [to P.L.]

1. HUVEC cell culture
NOTE: Perform all following steps under sterile working conditions (sterile working bench).
<ol>
	<li>Expanding HUVECs
	<ol>
		<li>For both angiogenesis assays, use pooled human umbilical vein endothelial cells (HUVECs) or human microvascular endothelial cells (HMVECs).</li>
		<li>Cultivate the cells until they reach 90% confluence in an uncoated T75 flask with endothelial cell growth medium (EGM) before performing a split. Perform medium change 3 times...

Study of Angiogenesis Using Scratch Wound Migration and Spheroid Sprouting Assays

<ol>
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A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Angiogenesis+in+wound+healing.">Angiogenesis in wound healing.</a> J Investig Dermatol Symp Proc. 5 (1), 40-46 (2000).</li><li>Folkman, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Role+of+angiogenesis+in+tumor+growth+and+metastasis.">Role of angiogenesis in tumor growth and metastasis.</a> Semin Oncol. 29, 15-18 (2002).</li><li>Crawford, T. N., Alfaro, D. V., Kerrison, J. B., Jablon, E. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diabetic+retinopathy+and+angiogenesis.">Diabetic retinopathy and angiogenesis.</a> Curr Diabetes Rev. 5 (1), 8-13 (2009).</li><li>Ng, E. W., Adamis, 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=Targeting+angiogenesis,+the+underlying+disorder+in+neovascular+age-related+macular+degeneration.">Targeting angiogenesis, the underlying disorder in neovascular age-related macular degeneration.</a> Can J Ophthalmol. 40 (3), 352-368 (2005).</li><li>Blanco, R., Gerhardt, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vegf+and+notch+in+tip+and+stalk+cell+selection.">Vegf and notch in tip and stalk cell selection.</a> Cold Spring Harb Perspect Med. 3 (1), 006569 (2013).</li><li>Hellstr&#246;m, M., Kal&#233;n, M., Lindahl, P., Abramsson, A., Betsholtz, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Role+of+PDGF-B+and+PDGFR-&#946;+in+recruitment+of+vascular+smooth+muscle+cells+and+pericytes+during+embryonic+blood+vessel+formation+in+the+mouse.">Role of PDGF-B and PDGFR-&#946; in recruitment of vascular smooth muscle cells and pericytes during embryonic blood vessel formation in the mouse.</a> Development. 126 (14), 3047-3055 (1999).</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), 425-532 (2018).</li><li>Korff, T., Augustin, H. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tensional+forces+in+fibrillar+extracellular+matrices+control+directional+capillary+sprouting.">Tensional forces in fibrillar extracellular matrices control directional capillary sprouting.</a> J Cell Sci. 112, 3249-3258 (1999).</li><li>Rapp, 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=2D+and+3D+in+vitro+angiogenesis+assays+highlight+different+aspects+of+angiogenesis.">2D and 3D in vitro angiogenesis assays highlight different aspects of angiogenesis.</a> Biochim Biophys Acta Mol Basis Dis. 1870 (3), 167028 (2024).</li><li>Rapp, 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=STAT3+signaling+induced+by+the+IL-6+family+of+cytokines+modulates+angiogenesis.">STAT3 signaling induced by the IL-6 family of cytokines modulates angiogenesis.</a> J Cell Sci. 136 (1), 260182 (2023).</li><li>Heiss, 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=Endothelial+cell+spheroids+as+a+versatile+tool+to+study+angiogenesis+in+vitro.">Endothelial cell spheroids as a versatile tool to study angiogenesis in vitro.</a> FASEB J. 29 (7), 3076-3084 (2015).</li><li>Rapp, 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=Oncostatin+m+reduces+pathological+neovascularization+in+the+retina+through+m&#252;ller+cell+activation.">Oncostatin m reduces pathological neovascularization in the retina through m&#252;ller cell activation.</a> Invest Ophthalmol Vis Sci. 65 (1), 22 (2024).</li><li>Fagotto, F., Gumbiner, B. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cell+contact-dependent+signaling.">Cell contact-dependent signaling.</a> Dev Biol. 180 (2), 445-454 (1996).</li><li>Rapp, 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=Addressing+bias+in+manual+segmentation+of+spheroid+sprouting+assays+with+U-Net.">Addressing bias in manual segmentation of spheroid sprouting assays with U-Net.</a> Mol Vis. 29, 197-205 (2023).</li><li>Boyden, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+chemotactic+effect+of+mixtures+of+antibody+and+antigen+on+polymorphonuclear+leucocytes.">The chemotactic effect of mixtures of antibody and antigen on polymorphonuclear leucocytes.</a> J Exp Med. 115 (3), 453-466 (1962).</li><li>Guy, J. B., 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=Evaluation+of+the+cell+invasion+and+migration+process:+A+comparison+of+the+video+microscope-based+scratch+wound+assay+and+the+Boyden+chamber+assay.">Evaluation of the cell invasion and migration process: A comparison of the video microscope-based scratch wound assay and the Boyden chamber assay.</a> J Vis Exp. (129), e56337 (2017).</li><li>Decicco-Skinner, K. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endothelial+cell+tube+formation+assay+for+the+in+vitro+study+of+angiogenesis.">Endothelial cell tube formation assay for the in vitro study of angiogenesis.</a> J Vis Exp. (91), e51312 (2014).</li><li>Vernon, R. B., Angello, J. C., Iruela-Arispe, M. L., Lane, T. F., Sage, E. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reorganization+of+basement+membrane+matrices+by+cellular+traction+promotes+the+formation+of+cellular+networks+in+vitro.">Reorganization of basement membrane matrices by cellular traction promotes the formation of cellular networks in vitro.</a> Lab Invest. 66 (5), 536-547 (1992).</li><li>Craig, L. E., Spelman, J. P., Strandberg, J. D., Zink, M. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endothelial+cells+from+diverse+tissues+exhibit+differences+in+growth+and+morphology.">Endothelial cells from diverse tissues exhibit differences in growth and morphology.</a> Microvasc Res. 55 (1), 65-76 (1998).</li><li>M&#252;ller, A. M., Hermanns, M. I., Cronen, C., Kirkpatrick, C. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparative+study+of+adhesion+molecule+expression+in+cultured+human+macro-and+microvascular+endothelial+cells.">Comparative study of adhesion molecule expression in cultured human macro-and microvascular endothelial cells.</a> Exp Mol Pathol. 73 (3), 171-180 (2002).</li><li>Chiew, G. G. Y., Wei, N., Sultania, S., Lim, S., Luo, K. Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bioengineered+three-dimensional+co-culture+of+cancer+cells+and+endothelial+cells:+A+model+system+for+dual+analysis+of+tumor+growth+and+angiogenesis.">Bioengineered three-dimensional co-culture of cancer cells and endothelial cells: A model system for dual analysis of tumor growth and angiogenesis.</a> Biotechnol Bioeng. 114 (8), 1865-1877 (2017).</li><li>Smith, L. 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In this report, we presented a spectrum of techniques with functional and molecular readouts to study angiogenesis in vitro. 
The migration assay represents a well-established technique used across all fields of wet laboratory work. We chose the commercially available live-cell imaging approach to take advantage of the 96-well format suitable for screening and dose-response experiments, the standardized and reproducible wound size created by the WoundMaker tool, the opportunity to ob...

Angiogenesis, which refers to the formation of new blood vessels from pre-existing ones1, is a crucial process during physiological development and pathologic conditions. It is indispensable for providing energy to highly metabolically active tissues such as the retina2 or the developing central nervous system3 and during the healing of damaged tissue4. Abnormal angiogenesis, on the other hand, is the basis for numerous diseases. Solid tumors, such as colorectal cancer or non-small cell lung cancer, facilitate their growth and the necessary energy supply by promoting angiogenesis5. Apart from cancer, neovascular diseases of the eye like diabetic retinopathy or age-related macular degeneration, which represent leading causes of blindness in the developed world, result from aberrant vessel growth6,7. A detailed understanding of the underlying pathomechanism is crucial to comprehend how physiological angiogenesis can be enhanced, for instance, in wound healing while better controlling pathological conditions such as vasoproliferative eye diseases.
On a cellular level, vascular endothelial cells are activated by various signaling molecules in angiogenesis, initiating cell proliferation and migration8. The cells subsequently organize into a hierarchy, with non-proliferating tip cells forming filopodia at the leading edge of the developing vessel branch8. Alongside, fast-proliferating stalk cells trail the tip cells, contributing to the formation of the emerging vessel. Subsequently, other cell types, such as pericytes or smooth muscle cells, are recruited to further stabilize the nascent branch9.
To explore molecular processes at the vascular endothelial cell level, numerous in vitro protocols have been developed and recently reviewed10. These assays typically fall into two categories: more simplistic but scalable 2D approaches and more elaborate 3D protocols. In a recent project, we conducted a comprehensive comparative analysis between the 2D scratch wound migration assay and the 3D spheroid sprouting assay11 to assess the extent of their differences and their ability to model various aspects of angiogenesis12. 
While both offer the advantages of being reliable and easily implementable, on a molecular level, the 3D spheroid sprouting assay was favorable in addressing key aspects of angiogenesis compared to in vivo data, such as metabolic switches or cell-matrix interactions. Since in vitro angiogenesis assays are used to evaluate the angiomodulatory potential of signaling pathways13 and to screen for therapeutic agents, transferability of in vitro results to in vivo settings is essential. Furthermore, the opportunity for omics-based analyses on the RNA and protein levels to characterize the molecular changes in response to targeted modulation of angiogenic processes under controlled conditions remains an important benefit compared to in vivo settings14,15.
In this publication, we present key assays for exploring angiogenesis-related questions through the utilization of a live-cell imaging migration assay and a spheroid sprouting assay, including subsequent molecular analyses like RNA sequencing for transcriptomic analysis and immunohistochemistry on the protein level.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>10x Medium 199</td>
			<td>Sigma-Aldrich</td>
			<td>M0650</td>
			<td></td>
		</tr>
		<tr>
			<td>2-(4-(2-Hydroxyethyl)1-piperazinyl)-ethan-sulfons&#228;ure</td>
			<td>PAN-Biotec</td>
			<td>P05-01100</td>
			<td>HEPES</td>
		</tr>
		<tr>
			<td>Alexa Fluor 647-conjugated AffiniPure F(ab)&#8216;2-Fragment</td>
			<td>Jackson IR&#160;</td>
			<td>115-606-072</td>
			<td></td>
		</tr>
		<tr>
			<td>Axio Vert. A1</td>
			<td>Zeiss</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>CapturePro 2.10.0.1</td>
			<td>JENOTIK Optical Systems</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Collagen Type 1 rat tail</td>
			<td>Corning</td>
			<td>354236</td>
			<td></td>
		</tr>
		<tr>
			<td>Collagenase D</td>
			<td>Roche&#160;</td>
			<td>11088858001</td>
			<td></td>
		</tr>
		<tr>
			<td>Endothelial Cell Basal Medium</td>
			<td>Lonza</td>
			<td>CC-3156</td>
			<td>EBM</td>
		</tr>
		<tr>
			<td>Endothelial Cell Growth Medium</td>
			<td>Lonza</td>
			<td>CC-3162</td>
			<td>EGM</td>
		</tr>
		<tr>
			<td>Ethylenediaminetetraacetic acid&#160;</td>
			<td>Serva</td>
			<td>11290.02</td>
			<td>EDTA</td>
		</tr>
		<tr>
			<td>Fetal bovine serum</td>
			<td>Bio&#38;SELL</td>
			<td>S 0615</td>
			<td>FBS</td>
		</tr>
		<tr>
			<td>Human Umbilical Vein Endothelial Cells, pooled</td>
			<td>Lonza</td>
			<td>C2519A</td>
			<td>HUVEC</td>
		</tr>
		<tr>
			<td>IncuCyte ImageLock 96-well plates&#160;</td>
			<td>Sartorius</td>
			<td>4379</td>
			<td></td>
		</tr>
		<tr>
			<td>Incucyte S3 Live-Cell Analysis System</td>
			<td>Sartorius</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Methocel</td>
			<td>Sigma</td>
			<td>m-0512</td>
			<td></td>
		</tr>
		<tr>
			<td>Microvascular Endothelial Cell Medium with 10% FBS</td>
			<td>PB-MH-100-4090-GFP</td>
			<td>PELOBiotech</td>
			<td></td>
		</tr>
		<tr>
			<td>NaOH</td>
			<td>Carl Roth</td>
			<td>P031.2</td>
			<td></td>
		</tr>
		<tr>
			<td>Phalloidin-Fluorescein Isothiocyanate Labeled (0.5 mg/mL Methanol)</td>
			<td>Sigma-Aldrich</td>
			<td>P5282-.1MG</td>
			<td>Phalloidin-FITC</td>
		</tr>
		<tr>
			<td>Phosphate-buffered saline</td>
			<td>Thermo Fisher Scientfic</td>
			<td>14190-094</td>
			<td>PBS</td>
		</tr>
		<tr>
			<td>Primary Human Retinal Microvascular Endothelial Cells</td>
			<td>Cell Systems</td>
			<td>ACBRI 181</td>
			<td></td>
		</tr>
		<tr>
			<td>ProLong Glass Antifade Mountant with NucBlue</td>
			<td>Invitrogen by ThermoFisher Scientific</td>
			<td>2260939</td>
			<td></td>
		</tr>
		<tr>
			<td>QIAzol Lysis Reagent</td>
			<td>QIAGEN</td>
			<td>79306</td>
			<td></td>
		</tr>
		<tr>
			<td>recombinant human Vascular Endothelial Growth Factor</td>
			<td>PeproTech</td>
			<td>100-20</td>
			<td>VEGF</td>
		</tr>
		<tr>
			<td>Squared petri dish</td>
			<td>Greiner</td>
			<td>688102</td>
			<td></td>
		</tr>
		<tr>
			<td>Trizol</td>
			<td>Qiagen</td>
			<td>79306</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypsin</td>
			<td>PAN-Biotec</td>
			<td>P10-024100</td>
			<td></td>
		</tr>
		<tr>
			<td>VEGF-R2 (monoclonal)</td>
			<td>ThermoFisher Scientific Inc.</td>
			<td>B.309.4</td>
			<td></td>
		</tr>
		<tr>
			<td>WoundMaker</td>
			<td>Sartorius</td>
			<td>4493</td>
			<td></td>
		</tr>
	</tbody>
</table>

investigating angiogenesis on a functional and molecular level by leveraging the scratch wound migration assay and the spheroid sprouting assay

Angiogenesis plays a crucial role in both physiological and pathological processes within the body including tumor growth or neovascular eye disease. A detailed understanding of the underlying molecular mechanisms and reliable screening models are essential for targeting diseases effectively and developing new therapeutic options. Several in vitro assays have been developed to model angiogenesis, capitalizing on the opportunities a controlled environment provides to elucidate angiogenic drivers at a molecular level and screen for therapeutic targets.
This study presents workflows for investigating angiogenesis in vitro using human umbilical vein endothelial cells (HUVECs). We detail a scratch wound migration assay utilizing a live cell imaging system measuring endothelial cell migration in a 2D setting and the spheroid sprouting assay assessing endothelial cell sprouting in a 3D setting provided by a collagen matrix. Additionally, we outline strategies for sample preparation to enable further molecular analyses such as transcriptomics, particularly in the 3D setting, including RNA extraction as well as immunocytochemistry. Altogether, this framework offers scientists a reliable and versatile toolset to pursue their scientific inquiries in in vitro angiogenesis assays.

Author Spotlight: Investigating Angiogenesis Through Challenges and Innovations in Assay Development

Investigating Angiogenesis on a Functional and Molecular Level by Leveraging the Scratch Wound Migration Assay and the Spheroid Sprouting Assay

Angiogenesis is a hallmark process involved in embryology and in disease development, including tumor growth and vasoproliferative eye disease. In this project, we aim to present a comprehensive framework of in vitro techniques that can be used to study angiogenesis on a molecular level in a very controlled setting, and to screen for potential therapeutic options. Translating in vitro to in vivo and ultimately to the clinical setting is challenging due to the risk of false positive or false negative results.Selecting the right angiogenesis assay is therefore crucial and knowing its limitations. This study aims to offer guidelines in how to interpret and how to establish two common angiogenesis assays. Recently, we compared a 2D scratch wound migration assay with a 3D spheroid sprouting assay.While the 2D assay is easily scalable, the 3D assay captures angiogenesis in greater detail, including tip stalk cell formation, matrix interaction, and the glycolytic switch. This data offer a scientists guidelines on which assay to choose next in their projects. Our laboratory will continue to characterize key aspects of the presented in vitro assays by directly comparing them to in vivo settings in order to facilitate translations of in vitro results to clinical applications.

For the migration assay, it is crucial to thoroughly examine the images captured at the t = 0 h time point to ensure the presence of a fully formed cell monolayer is accurately detected by the system (Figure 1B). Additionally, the clarity and straightness of the scratch border should be confirmed (Figure 1B). The cell-free area ought to be largely free of debris. At the end of the assay, a group stimulated with, for example, 25 ng/mL VEGF as a positive control s...

This study presents the two-dimensional (2D) scratch wound migration assay and the three-dimensional (3D) spheroid sprouting assay, along with their respective downstream analysis methods, including RNA extraction and immunocytochemistry, as suitable assays to study angiogenesis in vitro.

Angiogenesis plays a crucial role in both physiological and pathological processes within the body including tumor growth or neovascular eye disease. A ...