FGF2-Induced Angiogenesis Model: Standard Operating Procedure (Protocol)

1. Overview

1.1 Purpose and scope

This protocol describes the establishment of angiogenesis models stimulated by fibroblast growth factor 2 (FGF2, also termed bFGF). The models are intended for assessing pro-angiogenic activity, screening inhibitors, dissecting FGFR downstream signaling, and interrogating matrix-dependent regulation. Two commonly used in vitro models are included:

(1) Endothelial tube formation on Matrigel (Matrigel tube formation).

(2) Endothelial spheroid sprouting in a 3D matrix (endothelial spheroid sprouting).

Either model may be used alone or in combination according to study objectives. For in vivo confirmation, a Matrigel plug assay can be incorporated into the same study design (key extension notes are provided at the end).

 

1.2 Principle

(1) FGF2 binds FGFR and forms a stabilized signaling complex in a heparan sulfate/heparin-dependent manner, activating pathways such as MAPK/ERK and PI3K/AKT to promote endothelial proliferation, migration, and cytoskeletal/shape remodeling.

(2) In basement-membrane-rich semi-3D/3D matrices, endothelial cells rapidly assemble into tube-like networks; when spheroids are embedded in collagen or matrix gels, they generate 3D sprouts that more closely resemble the “sprouting–extension” process.

(3) Quantification of network length, junctions, total segments, or sprout number/length enables reproducible and comparable assessment of FGF2-driven angiogenic responses.

 

2. Materials and reagents

2.1 Cells and culture conditions

(1) Endothelial cells

① HUVEC (human umbilical vein endothelial cells) or human microvascular endothelial cells (HMVEC).

② Low passage cells are recommended (typically ≤ P6) with stable morphology and growth.

(2) Media

① Endothelial basal medium (e.g., EBM-type media, or DMEM/F12 depending on cell source).

② Standard supplements (endothelial growth supplement kits, fetal bovine serum) for routine expansion.

③ Low-serum stimulation medium for assays (commonly 0.5%–2% FBS, or optimized serum-free basal medium).

 

2.2 Key stimulants and controls

(1) Recombinant human FGF2 (bFGF).

(2) Heparin sodium or an equivalent heparan sulfate mimic, used to improve FGF2 stability and presentation.

(3) Positive control (optional)

① Recombinant human VEGF-A165.

(4) Inhibitory controls (optional)

① FGFR inhibitor (e.g., SU5402 or a comparable FGFR inhibitor).

② MEK inhibitor (e.g., U0126) to verify ERK dependence (as dictated by study design).

 

2.3 Matrices and 3D materials

(1) Matrigel or equivalent basement membrane extract (for tube formation).

(2) Type I collagen (recommended for spheroid sprouting embedding); Matrigel–collagen mixtures may be used as an extension.

(3) Spheroid formation support reagent

① Methylcellulose (commonly used to support spheroid formation and stable suspension).

 

2.4 Assay reagents (optional)

(1) Viability/cytotoxicity assays (ATP-based or colorimetric) to exclude confounding cytotoxicity effects of inhibitors.

(2) Immunofluorescence staining

① CD31 (PECAM-1) and VE-cadherin for endothelial identity.

② Ki67 for proliferation readouts (if needed).

(3) Nuclear stains

① DAPI or equivalents.

 

2.5 Consumables and equipment

(1) Pre-chilled pipette tips and pre-chilled tubes (required for Matrigel handling).

(2) 96-well plates (tube formation); 24- or 48-well plates (spheroid sprouting).

(3) Cell culture incubator (37°C, 5% CO2), inverted microscope (phase contrast or brightfield), and imaging system.

(4) Ice bucket and pre-chilled metal cooling block to prevent premature Matrigel gelation.

 

3. Preparation of solutions and working reagents

3.1 FGF2 working solution

(1) Prepare an FGF2 stock according to the product instructions and aliquot to avoid repeated freeze–thaw cycles.

(2) A dose range is recommended to establish a dose–response relationship; typical final concentrations are 10–50 ng/mL (expandable to 1–100 ng/mL depending on cell sensitivity).

(3) Heparin is commonly used at a final concentration of 1–10 µg/mL and may be co-administered with FGF2 to improve stability and consistency.

 

3.2 Matrigel handling

(1) Thaw and aliquot Matrigel entirely at 4°C; avoid prolonged room-temperature exposure to prevent premature gelation.

(2) After coating, incubate at 37°C to ensure complete polymerization before cell seeding.

 

3.3 Low-serum stimulation medium

(1) Pre-equilibrate cells in low-serum medium (e.g., 4–16 h) to reduce basal growth factor background and increase the dynamic range for FGF2 induction.

(2) Because sensitivity to serum starvation differs across endothelial sources, monitor morphology and viability and avoid excessive starvation that increases baseline death.

 

4. Workflow A: Matrigel tube formation assay

4.1 Experimental design and grouping

(1) Minimal grouping

① Vehicle/blank control: low-serum medium.

② FGF2 groups: low-serum medium + FGF2 (at least two concentrations recommended).

(2) Optional extensions

① FGF2 + heparin (or include heparin uniformly across all groups as a standardized condition).

② VEGF positive control.

③ FGF2 + FGFR inhibitor (typical inhibitor pre-treatment 30–60 min).

(3) Replication

① Technical replicates: ≥ 3 wells per group.

② Biological repeats: ≥ 3 independent experiments.

 

4.2 Procedure

(1) Matrigel coating

① Place the 96-well plate on ice or a pre-chilled cooling block.

② Add pre-chilled Matrigel to each well (commonly 50 µL/well; adjust to ensure uniform bottom coverage).

③ Incubate at 37°C for 20–45 min until fully polymerized.

(2) Cell preparation

① Detach and collect endothelial cells; centrifuge gently and resuspend in low-serum medium.

② Adjust cell density; a typical seeding range is 1.5×10^4–3.0×10^4 cells/well (96-well plate), to be optimized empirically by cell type.

(3) Seeding and stimulation

① Seed cells gently onto the polymerized Matrigel surface.

② Immediately add stimulants and/or inhibitors according to group assignment. If pre-treatment is required, apply the inhibitor before seeding or immediately after seeding with strict consistency across groups.

(4) Incubation and imaging

① Incubate at 37°C. Stable networks typically develop within 4–8 h; optimization may range from 2–12 h by cell type.

② Acquire images at defined time points (e.g., 4 h and 6 h) to avoid late-stage network collapse that can bias comparisons.

 

4.3 Quantification recommendations

(1) Suggested metrics

① Total tube length.

② Number of junctions.

③ Mesh area/total covered area.

(2) Analytical principles

① Fix magnification, exposure, and field selection strategy within each experiment; collect at least three random fields per well.

② Avoid subjective “better-looking” comparisons; rely on pre-defined quantitative endpoints.

③ When inhibitor groups show pronounced decreases, provide concurrent viability/cytotoxicity data to exclude non-specific toxicity.

 

5. Workflow B: Endothelial spheroid sprouting assay

5.1 Experimental design and grouping

(1) Grouping principles follow Section 4.1; include at minimum a blank control and an FGF2 dose series.

(2) Because the spheroid assay is more sensitive to matrix and cell-state variability, heparin conditions are recommended to be uniform across groups to reduce inter-batch variance.

 

5.2 Spheroid preparation

(1) Cell preparation

① Harvest and count endothelial cells, ensuring high viability and minimal debris.

② Prepare a methylcellulose-containing medium to support spheroid formation (optimize concentration based on commonly used formulations).

(2) Spheroid formation

① Distribute cells under non-adherent conditions (e.g., U-bottom low-attachment plates or hanging-drop format).

② Incubate 12–24 h to generate spheroids with relatively uniform diameters.

③ Exclude spheroids with obvious size heterogeneity or loose morphology to maintain group comparability.

 

5.3 3D embedding and stimulation

(1) Matrix preparation

① Prepare collagen I to the target final concentration (commonly 1–2 mg/mL), on ice, adjusting pH to a gelation-permissive range.

② Mix spheroids evenly into the collagen solution and dispense into wells.

③ Incubate at 37°C until gelation is complete.

(2) Stimulation and incubation

① Overlay the gel with low-serum medium.

② Add FGF2 (dose series) and heparin; handle inhibitor groups according to the pre-treatment logic.

③ Incubate 16–24 h (or optimize based on sprouting kinetics) and image at fixed time points.

 

5.4 Quantification recommendations

(1) Primary endpoints

① Sprout number per spheroid.

② Mean sprout length and total sprout length.

③ Max sprout length (reflecting extension capacity under strong stimulation).

(2) Statistical unit

① Treat each spheroid as the unit of analysis to avoid pseudo-replication (do not treat multiple sprouts from one spheroid as independent samples).

② Analyze ≥ 10–20 spheroids per group and perform ≥ 3 independent repeats.

 

6. Quality control (QC) considerations

6.1 Cell QC

(1) Maintain stable endothelial phenotype; avoid high passage or chronic stress that blunts responsiveness.

(2) Inspect morphology and adherence before and after assays. If pronounced granularity, vacuolization, or extensive floating cells are observed, stop the assay and investigate root causes.

 

6.2 Matrix lot and temperature control

(1) Matrigel exhibits substantial lot-to-lot variability; use the same lot within a study phase whenever possible and perform lot qualification.

(2) Enforce strict temperature control for Matrigel and collagen, particularly avoiding room-temperature exposure for Matrigel that leads to uneven coating.

 

6.3 Factor and inhibitor handling

(1) FGF2 is susceptible to adsorption and freeze–thaw loss; aliquot into small volumes and use single-use portions.

(2) For inhibitors dissolved in solvents (e.g., DMSO), include a solvent control and keep final solvent concentrations identical across groups.

 

7. Troubleshooting and cause analysis

7.1 Weak tube formation/sprouting or non-significant differences

(1) Inappropriate cell density

① Too low: insufficient connectivity for network formation.

② Too high: aggregation or non-specific spreading.

(2) Inadequate low-serum conditioning

① Insufficient starvation: high baseline background and limited dynamic range.

② Excessive starvation: elevated baseline death suppressing network formation.

(3) Reduced FGF2 activity

① Repeated freeze–thaw, improper storage, or prolonged holding of working solutions.

② Absence of heparin reduces stability and presentation efficiency.

(4) Matrix-related issues

① Premature Matrigel gelation or uneven coating.

② Collagen gelation failure due to incorrect pH or ionic strength.

 

7.2 Large decreases in inhibitor groups that are difficult to interpret

(1) Inhibitor cytotoxicity or non-specific inhibition; confirm with parallel viability assays.

(2) Inconsistent final solvent concentrations causing baseline shifts.

(3) Imaging too late, when control networks have collapsed, obscuring true differences.

 

7.3 Poor within-group reproducibility

(1) Matrigel lot differences or temperature-control fluctuations during handling.

(2) Coating volume variability, edge effects, and incubator humidity/temperature gradients.

(3) Non-uniform imaging parameters; fix magnification and exposure and use randomized field selection.

 

8. Safety and compliance

8.1 Biosafety

(1) Endothelial cells are biological materials; handle under institutional biosafety requirements.

(2) Dispose of cell-containing waste as biohazard waste according to institutional procedures.

 

8.2 Chemical safety

(1) Manage inhibitors and solvents (e.g., DMSO) under chemical safety regulations.

(2) Matrigel is derived from murine basement membrane extracts; store and dispose of biologically derived materials according to applicable institutional requirements.

 

For more related articles, please see below:

[1] Fibroblast growth factor-induced angiogenesis model

[2] Comprehensive Overview of Vascular Endothelial Gth Factors (VEGF)

[3] The Fibroblast Development Factor (FGF) Family

[4] Regulation of TGF-beta activity by BMP-1

 

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