A Strong Contractile Actin Fence and Large Adhesions Direct Human Pluripotent Colony Morphology and Adhesion

Abstract

Cell-type-specific functions and identity are tightly regulated by interactions between the cell cytoskeleton and the extracellular matrix (ECM). Human pluripotent stem cells (hPSCs) have ultimate differentiation capacity and exceptionally low-strength ECM contact, yet the organization and function of adhesion sites and associated actin cytoskeleton remain poorly defined. We imaged hPSCs at the cell-ECM interface with total internal reflection fluorescence microscopy and discovered that adhesions at the colony edge were exceptionally large and connected by thick ventral stress fibers. The actin fence encircling the colony was found to exert extensive Rho-ROCK-myosin-dependent mechanical stress to enforce colony morphology, compaction, and pluripotency and to define mitotic spindle orientation. Remarkably, differentiation altered adhesion organization and signaling characterized by a switch from ventral to dorsal stress fibers, reduced mechanical stress, and increased integrin activity and cell-ECM adhesion strength. Thus, pluripotency appears to be linked to unique colony organization and adhesion structure.

Keywords: Rho-ROCK-myosin signaling; actin cytoskeleton; focal adhesion; human pluripotent stem cell; integrin activity; pluripotency; total internal reflection fluorescence microscopy; traction force microscopy.

Graphical abstract

Figure 1

Prominent FAs and Actin Stress Fibers Define the hiPSC Colony Edge (A and C) TIRF images of parental fibroblasts (left) and hiPSC colony (right) stained as indicated, DAPI/NANOG (mid plane). (B) Confocal stacks of hiPSC colony (bottom and mid plane) stained as indicated, DAPI (blue). (D) A representative illustration of FAs detection areas and quantification of FA size, coverage, and density in parental fibroblasts (n = 6,752) and in the center (n = 7,418) and edge of the hiPSC colony (n = 4,767). Data are from more than three biologically independent experiments. Scale bars, 10 μm. See also Figure S1.

Figure 2

Mechanical Stress Depends on the Angle of Ventral Stress Fibers (A–C) TIRF images of hiPSC colonies stained as indicated. (D) TFM stress maps and representative confocal images of F-ACTIN (SiR-actin) in hiPSC colonies plated on ∼10-kPa VTN-functionalized polyacrylamide gels. White lines depict the colony edge. (E) Quantification of mechanical stress at FAs relative to inter-VSF angle (n = 3 biologically independent experiments; median IQR: 25^th–75^th percentile; whiskers: 1.5 × IQR; ANOVA, Turkey's HSD). (F) TIRF images of one (top) or two (bottom) hiPSCs stained as indicated. DAPI/NANOG (mid plane). Scale bars, 10 μm. See also Figure S2.

Figure 3

Rho Signaling Controls Large FAs with High SRC Activity, Reinforces Pluripotency, and Dictates Orientation of Mitosis (A–D) hiPSC colonies treated with blebbistatin (10 μM, 40 min [A] or 4 hr [B]), ROCKi (Y-27632, 10 μM, 4 hr [B]), or DMSO. (A) TIRF images of colonies stained as indicated. (B) Bright-field and DAPI-stained hiPSC colonies (white lines depict colony edge). (C and D) Relative colony area scored from live imaging in different time points (n = 3 biologically independent experiments, mean ± SD, t test). (E) Flow-cytometric analysis of SOX2:td-Tomato and SSEA-1 in the SOX2 reporter hiPSC line and SSEA5 and SSEA-1 in non-reporter hiPSC line after ROCKi exposure (10 μM, 3 days) relative to control (n > 3 biologically independent experiments, mean ± SD, t test). (F) Confocal images of DMSO-treated (left) or blebbistatin-treated (10 μM, 24 hr; right) hiPSC colonies and quantification of mitotic spindle orientation (angle between mitotic spindle and colony edge). Data are from three biologically independent experiments. (G and H) TIRF images of hiPSC colonies stained as indicated, DAPI (blue). (I) Western blot analysis of FA components and OCT4 (pluripotency marker), VIMENTIN (fibroblast marker), and GAPDH (loading control) in fibroblasts and in two different hiPSC lines HEL11.4 and HEL24.3, and quantification of pSRC and total SRC ratio (n = 3 biologically independent experiments, mean ± SD, t test). (J) TIRF images of parental fibroblasts and hiPSCs stained as indicated. Scale bars, 10 μm (A, G, H, J), 100 μm (B), and 20 μm (F). DAPI images are taken in the mid plane. See also Figure S3.

Figure 4

Differentiation Triggers Actin Reorientation, Altered FA Size, and Increased Integrin Activation hiPSC colonies were left untreated or were differentiated in the presence of retinoic acid for 3 days (RA 3d, 10 μM). (A) TIRF images stained as indicated, SSEA1 was used as marker of differentiation, DAPI (mid plane). (B) Comparison of FA size, coverage, and density in total hiPSC colony area (n = 12,185), post RA differentiation (n = 8,431) and in fibroblasts (n = 6,752). Data are from more than 3 biologically independent experiments. (C) Real-time adhesion of single cells on MG, VTN, and collagen (COL) over 1 hr (n = 3 biologically independent experiments, mean, t test). (D) TFM stress maps and representative confocal images of F-ACTIN (SiR-Actin) on 10 kPa VTN-functionalized gels. White lines depict the colony edge. (E) Western blot analysis of pSRC (Y416), SRC, OCT4, and GAPDH. (F) TIRF image of active β1-integrin (12G10); for PAXILLIN and F-ACTIN see Figure S4. (G) Quantification of active integrin levels in (F). Mean ± SD, ANOVA, Tukey's HSD. (H) Three-dimensional Imaris reconstruction. All scale bars represent 10 μm. See also Figure S4.