Differentiation of Human Pluripotent Stem Cells into Functional Endothelial Cells in Scalable Suspension Culture

Abstract

Endothelial cells (ECs) are involved in a variety of cellular responses. As multifunctional components of vascular structures, endothelial (progenitor) cells have been utilized in cellular therapies and are required as an important cellular component of engineered tissue constructs and in vitro disease models. Although primary ECs from different sources are readily isolated and expanded, cell quantity and quality in terms of functionality and karyotype stability is limited. ECs derived from human induced pluripotent stem cells (hiPSCs) represent an alternative and potentially superior cell source, but traditional culture approaches and 2D differentiation protocols hardly allow for production of large cell numbers. Aiming at the production of ECs, we have developed a robust approach for efficient endothelial differentiation of hiPSCs in scalable suspension culture. The established protocol results in relevant numbers of ECs for regenerative approaches and industrial applications that show in vitro proliferation capacity and a high degree of chromosomal stability.

Keywords: endothelial cells; hiPSC differentiation; scalable culture.

Figure 1

EC Differentiation in Small-Scale Static Suspension Cultures hiPSC differentiation toward endothelial cells was performed according to the scheme depicted in (A). Cells were cultured in six-well suspension plates either 4 days or 1 day prior to initiation of differentiation conditions. Cell-only aggregates develop on day 1 and increase in size during differentiation (B). Scale bars, 100 μm. Flow cytometric analysis on day 7 of differentiation showed 27.86% ± 4.1% (combined data from hCBiPS2, hHSC1285iPS2, and hHSCSeViPS2; n = 15 independent experiments; mean ± SEM) CD31^pos cells after 4 days of pre-culture and 46.9% ± 3.9% (hCBiPS2; n = 3 independent experiments; mean ± SEM) CD31^pos cells and 43.5% ± 4.5% (hHSC1285iPS2; n = 3 independent experiments; mean ± SEM), 47.4% ± 1.4% (hHSCSeViPS2; n = 3 independent experiments; mean ± SEM) CD31^pos cells, respectively, for three independent cell lines with one day of pre-culture (C). See also Figure S1A.

Figure 2

Scale-Up of EC Differentiation to Agitated Erlenmeyer Flasks hiPSC differentiation toward endothelial cells in agitated Erlenmeyer flasks was performed according to the scheme presented in (A). Flow cytometric analysis showed increase of KDR (B) and CD31 (C) expression from day 3 to 6 onward (n = 6–9 independent experiments, hCBiPS2, hHSC1285iPS2, and hHSCSeViPS2; mean ± SEM). Flow cytometric analysis on day 7 of differentiation revealed for hHSC1285iPS2 63.7% ± 4.9% (n = 7, independent experiments), hCBiPSC2 64.1% ± 2.1% (n = 7, independent experiments), hHSCSeViPS2 49.2% ± 3.3% (n = 8, independent experiments), and CD31^pos ECs (mean ± SEM) (D). Generated CD31^pos cells show co-expression of CD34 (E) and KDR (F) (representative plots for hHSCSeViPS2 staining, red; isotype control, gray). See also Figure S1B.

Figure 3

Purified hiPSC-ECs Show Elevated Expression of Typical EC Markers Flow cytometric analysis resulted in enrichment of CD31^pos hiPSC-derived ECs from 74.1% ± 8.6% to 98.6% ± 0.5% (hCBiPS2; n = 3), 54.7% ± 2.1% to 97.5% ± 0.7% (hHSCSeViPS2; n = 12), and 68.2% ± 4.1% to 97.8% ± 0.7% (hCBiPS2CAGeGFP; n = 9) CD31^pos cells (mean ± SEM) by MACS separation (A). Purified CD31^pos cells showed co-expression of CD144 (VEcadherin) (representative plot for hCBiPS2CAGeGFP; staining, red; isotype control, gray) (B). Comparative gene expression analysis showed significant higher CD31 expression and VEcadherin expression in CD31^pos-sorted hiPSC-ECs compared with the CD31^neg population (hCBiPS2; n = 3 independent experiments; mean ± SEM, ^∗p < 0.05) (C). Immunocytological stainings for endothelial markers showed homogeneous expression of VEcadherin, vWF, and endothelial nitric oxide synthase (eNOS) in purified hiPSC-EC cultures (in red; DAPI in blue) (D). Scale bars, 100 μm. Quantitative real-time PCR analysis showed expression of markers for arterial (NRP1 and NOTCH1) as well as venous (NRP2 and EPHB4) ECs (hiPSC2, HSCSeViPS2, and hCBiPS2CAGeGFP; n = 3 each, independent experiments; mean ± SEM) (E). Immunocytological stainings showed co-expression of EPHB4 (venous markers, green) and DLL4 (arterial marker, red) in hiPSC-EC cultures; DAPI in blue; representative pictures for HSCSeViPS2-ECs (F). Scale bars, 100 μm.

Figure 4

hiPSC-ECs Show a Typical Angiogenic Response in Matrigel and Uptake of Ac-Dil-LDL and Functionally Respond to an IAV Infection A tube-forming assay demonstrated the angiogenic potential of the hiPSC-ECs comparable with hUVECs (P4 after isolation). Scale bars 100 μm (A). hiPSC-ECs showed uptake of Ac-Dil-LDL after MACS separation (B). Scale bars, 100 μm. Expression analysis showed upregulation of viral hemagglutinin indicating successful infection (C) as well as ISG15, IFN-β, and CXCL10 as cellular response (D) after incubation of the cells with IAV (H1N1) for 2 hr with MOI of 0.5 or 1 (hCBiPS2-EC) (n = 3 independent experiments; mean ± SEM).

Figure 5

Culture-Expanded hiPSC-Derived ECs Maintain Stable Marker Expression, Proliferation Rates, Cellular Function, and Karyotype Flow cytometric analysis showed stable expression of CD31 during in vitro passaging for up to 10 passages (n = 3–4 independent experiments; mean ± SEM) (A). Immunocytological stainings for endothelial markers showed homogenous expression of VEcadherin and vWF in hiPSC-EC cultures after ten passages (in red, DAPI in blue; scale bars represent 100 μm) (B). hiPSC-derived ECs showed stable population doubling time during in vitro passaging (n = 3) (C). A tube-forming assay showed angiogenic potential of the generated hiPSC-ECs; scale bars, 100 μm (D). hiPSC-ECs showed unchanged uptake of Ac-Dil-LDL after ten passages of in vitro culture. Scale bars, 100 μm (E). Karyotype analyses at passages 11–13 showed frequent karyotype changes in isolated hUVECs as well as hCBECs with abnormalities detected in 68% and 81% of analyzed metaphases, respectively. hSVECs and hPBECs showed abnormalities in 10% of analyzed samples already after up to six passages. Lower frequencies were detected in hiPSC-EC cultures at passages 4–12 with abnormalities in only one cell sample at passage 11 (2% of the analyzed metaphases) (F). See also Figures S2 and S3.

Figure 6

Vascular Competence of Early and Late hiPSC-Derived EC Passages in a Zebrafish Xenograft Model Early (P1) passage hiPSC-ECs (green, hCBiPSCAGeGFP) within the embryonic zebrafish vasculature [marked by Tg(fli1a:mCherry-NLS)^ubs10, red)] 1 day after transplantation (A). Scale bar, 300 μm. Details of vessels with P1 hiPSC-EC (green) integration into the Tg(fli1a:mCherry-NLS)^ubs10 transgenic embryonic zebrafish vasculature (red) (B–D). Scale bar, 50 μm. Late (P11) passage hiPSC-ECs (green) within the Tg(fli1a:mCherry-NLS)^ubs10 transgenic embryonic zebrafish vasculature (red) 1 day after transplantation (E). Scale bar, 300 μm. Details of vessel with P11 hiPSC-EC (green) integration into the Tg(fli1a:mCherry-NLS)^ubs10 transgenic embryonic zebrafish vasculature (red) (F–H). Scale bar, 50 μm. Quantification of integration rates of early and late hiPSC-ECs compared with early (P1, P4, and P5) and late (P14) hUVECs into the zebrafish embryonic vasculature 1 day after transplantation. Integration rates into the zebrafish vasculature were 61.9% (n = 83/134 embryos) for early hiPSC-ECs and 78.3% (n = 112/143 embryos) for late hiPSC-ECs. ^∗∗p ≤ 0.01, ^∗∗∗p ≤ 0.001 (I).

Figure 7

Scale-Up of EC Differentiation to Stirred-Tank Bioreactors Single-cell-inoculated cultures (5 × 10⁵ cells/mL, hCBiPs2CAGeGFP) formed aggregates with increasing diameter until day 6 of differentiation (A). Flow cytometric analysis showed robust generation of CD31^pos cells on day 6 of differentiation (56.8% ± 10.5%; n = 3 independent bioreactor runs, mean ± SEM, hCBiPS2CAGeGFP) representative plot (staining, red; isotype control, gray) (B). Gene expression analysis by quantitative real-time PCR showed downregulation of pluripotency-associated markers OCT4 and NANOG, as well as upregulation of mesodermal marker (KDR) and endothelial cell markers (VEcadherin, CD31) (n = 3 independent bioreactor runs; mean ± SEM, hCBiPS2CAGeGFP) during differentiation (C). MACS-sorted CD31^pos hiPSC-ECs showed typical EC morphology in culture (D), uptake of Ac-Dil-LDL (E) after MACS separation and tube-forming assay demonstrated the angiogenic potential (F). Scale bars, 100 μm. See also Figure S4.