Generation of a Purified iPSC-Derived Smooth Muscle-like Population for Cell Sheet Engineering

Abstract

Induced pluripotent stem cells (iPSCs) provide a potential source for the derivation of smooth muscle cells (SMCs); however, current approaches are limited by the production of heterogeneous cell types and a paucity of tools or markers for tracking and purifying candidate SMCs. Here, we develop murine and human iPSC lines carrying fluorochrome reporters (Acta2^hrGFP and ACTA2^eGFP, respectively) that identify Acta2⁺/ACTA2⁺ cells as they emerge in vitro in real time during iPSC-directed differentiation. We find that Acta2^hrGFP+ and ACTA2^eGFP+ cells can be sorted to purity and are enriched in markers characteristic of an immature or synthetic SMC. We characterize the resulting GFP⁺ populations through global transcriptomic profiling and functional studies, including the capacity to form engineered cell sheets. We conclude that these reporter lines allow for generation of sortable, live iPSC-derived Acta2⁺/ACTA2⁺ cells highly enriched in smooth muscle lineages for basic developmental studies, tissue engineering, or future clinical regenerative applications.

Keywords: cell sheets; directed differentiation; fluorochrome reporters; induced pluripotent stem cells; smooth muscle cells.

Graphical abstract

Figure 1

Generation and Characterization of Mouse iPSC-Derived Acta2^hrGFP+ Cells (A) Schematic of the Acta2^hrGFP reporter construct and Acta2^hrGFP mouse fibroblast reprogramming to generate iPSCs, followed by differentiation into GFP-expressing cells. (B) Schematic of mouse iPSC-directed differentiation protocol. (C) Relative competence of day-5 KDR⁺ and KDR⁻ populations to differentiate to an Acta2^hrGFP+ population, scored by flow cytometry on day 13. Error bars represent average ± SD. ^∗∗p ≤ 0.01 and ^∗∗∗∗p ≤ 0.0001 by unpaired two-tailed Student's t test between KDR⁺ and KDR⁻ populations. n = 3 replicates for PT condition and n = 6 replicates for serum condition from independent experiments. (D) Flow-cytometry plot of Acta2^hrGFP and KDR expression on day 13, with accompanying dot plot showing Acta2^hrGFP+ efficiency on day 13. (E) Gene expression of characteristic SMC markers as well as markers of endothelial cells, podocytes, and epithelial cells. qRT-PCR measurements of fold change (FC) of mRNA expression (2^−ΔΔCt) is shown for presort, GFP⁺, and GFP⁻ populations. Undifferentiated miPSC mRNA expression is defined as FC = 1. Error bars represent ±SD. ^∗p ≤ 0.05, ^∗∗p ≤ 0.01, ^∗∗∗p ≤ 0.001, ^∗∗∗∗p ≤ 0.0001 by unpaired two-tailed Student's t test between GFP⁺ and GFP⁻ populations. n = 3 independent experiments. (F) Immunofluorescence images of hrGFP (green) and ACTA2 (red), with nuclear DAPI stain (blue). Scale bar, 50 μm. See also Figures S1–S3.

Figure 2

Global Transcriptomic Profiling of Primary Mouse Aortic SMCs and Key Stages of Acta2^hrGFP miPSC-Directed Differentiation toward Smooth Muscle-like Cells (A) Schematic of experiment. The following cell types were profiled using microarray expression analysis: undifferentiated miPSCs (D0), purified KDR⁺ mesodermal progenitors (D5 KDR⁺), purified Acta2^hrGFP+ and Acta2^hrGFP− cells (D13 GFP⁺ and D13 GFP⁻), and adult primary Acta2^hrGFP+ aortic SMCs. (B) Principal component (PC) analysis of all samples. (C) Hierarchical clustering of gene expression using all 2,386 differentially expressed genes with FDR < 1 × 10⁻⁷ and absolute FC > 5 revealed 10 different gene clusters. Key genes from four of these clusters of interest (mature/contractile SMC, immature/synthetic SMC, mesoderm, and iPSC) are shown to the right. (D) Dendrogram showing similarity between the samples through hierarchical clustering of all samples calculated across all genes. See also Figure S4 and Table S1.

Figure 3

Comparison of miPSC-Derived Populations with Primary Mouse Aortic SMCs (A and B) Differentially expressed genes and transcription factors, respectively, with FDR < 0.05 when comparing Acta2^hrGFP+ cells or aortic SMCs with KDR⁺ mesodermal progenitors. Numbers outside the circles represent number of genes differentially expressed between Acta2^hrGFP+ versus KDR⁺ and Aorta versus KDR⁺ comparisons. Number in the intersection of circles represents the number of overlapping genes between the comparisons. (C) Gene set enrichment analysis (GSEA) of Acta2^hrGFP+ cells or aortic SMCs compared with KDR⁺ mesodermal progenitors. (D) Heatmaps of top enriched genes in the “Myogenesis” gene set from the GSEA comparing Acta2^hrGFP+ and KDR⁺ populations (left) or aortic SMC and KDR⁺ populations (right). (E) GSEA showing enriched gene sets when comparing Acta2^hrGFP+ cells and aortic SMCs.

Figure 4

Human iPSC Reporter System for Generation of an ACTA2^eGFP+ Smooth Muscle-like Cells (A) Schematic showing targeting strategy to replace the endogenous stop codon of the ACTA2 locus with a 2A-eGFP cassette using the CRISPR/Cas9 system. (B) Schematic of hiPSC differentiation toward smooth muscle-like cells. (C and D) Flow-cytometry dot plots demonstrating the expression kinetics of KDR and eGFP over 4 days of directed differentiation into mesodermal progenitors using both monoallelically (BU3 AG8Cr1) and biallelically (BU3 AG18Cr1) targeted hiPSC lines. Line graphs in (D) represent n = 1 biological replicate for each hiPSC line. (E) Flow-cytometry dot plot of ACTA2^eGFP expression on day 30. (F) Relative gene expression (qRT-PCR) of markers of SMCs in ACTA2^eGFP+ and ACTA2^eGFP− cells compared with freshly isolated uncultured primary human airway SMC controls. Error bars represent average ± SD. ^∗p ≤ 0.05 and ^∗∗p ≤ 0.01 by unpaired two-tailed Student's t test between GFP⁺ and GFP⁻ populations. n = 3 independent experiments. See also Figure S5.

Figure 5

Global Transcriptomic Profiling of Human iPSC-Derived ACTA2^eGFP Populations (A) Schematic of microarray experiment. (B) Principal component (PC) analysis of ACTA2^eGFP+ and ACTA2^eGFP− populations. (C and D) Heatmap of top 25 upregulated and top 25 downregulated expressed genes (C) and transcription factors (D) with FDR < 0.05 between ACTA2^eGFP+ and ACTA2^eGFP− populations. (E) Heatmap of selected smooth muscle cell marker genes (FDR < 0.05). (F) GSEA showing enriched gene sets in ACTA2^eGFP+ cells compared with ACTA2^eGFP− cells. (G) Heatmap of top enriched genes in the “Myogenesis” gene set from the GSEA comparing ACTA2^eGFP+ and ACTA2^eGFP− populations.

Figure 6

Generation of iPSC-SMC Cell Sheets (A) Schematic of alginate-tyramine hydrogel and cell sheet fabrication. Soft lithography techniques were used to fabricate micropatterned alginate-tyramine hydrogels to generate aligned cell sheets, and a hydrogel-specific enzyme was used to degrade the hydrogel and leave cell-cell junctions and ECM proteins intact. (B) Images of Acta2^hrGFP+ and Acta2^hrGFP− cell sheets 96 h after seeding onto alginate-tyramine hydrogel substrates. Scale bar, 30 μm. (C) Representative stress-strain curves for Acta2^hrGFP+, Acta2^hrGFP−, and immortalized mouse aortic vascular SMC (MOVAS) cell sheets undergoing uniaxial tensile tests. (D–F) Elastic modulus (D), max stress (E), and max strain (F) of cell sheets derived from the following populations: Acta2^hrGFP+, Acta2^hrGFP−, MOVAS, ACTA2^eGFP+, and ACTA2^eGFP− cells. Error bars represent average ± SD. ^∗p ≤ 0.05 and ^∗∗p ≤ 0.05 by unpaired two-tailed Student's t test between GFP⁺ and GFP⁻ cell sheets and between GFP⁺ and MOVAS cell sheets. n = 11, 7, 6, 7, and 3 for the Acta2^hrGFP+, Acta2^hrGFP−, MOVAS, ACTA2^eGFP+, and ACTA2^eGFP− cell sheets from independent experiments, respectively.