Traction force microscopy for understanding cellular mechanotransduction

Abstract

Under physiological and pathological conditions, mechanical forces generated from cells themselves or transmitted from extracellular matrix (ECM) through focal adhesions (FAs) and adherens junctions (AJs) are known to play a significant role in regulating various cell behaviors. Substantial progresses have been made in the field of mechanobiology towards novel methods to understand how cells are able to sense and adapt to these mechanical forces over the years. To address these issues, this review will discuss recent advancements of traction force microscopy (TFM), intracellular force microscopy (IFM), and monolayer stress microscopy (MSM) to measure multiple aspects of cellular forces exerted by cells at cell-ECM and cell-cell junctional intracellular interfaces. We will also highlight how these methods can elucidate the roles of mechanical forces at interfaces of cell-cell/cell-ECM in regulating various cellular functions. [BMB Reports 2020; 53(2): 74-81].

Fig. 1

Traction force microscopy (TFM)-based cell-ECM force quantification. (A) Schematic diagram for typical TFM platform using deformable substrates, where fluorescence beads (orange dots) are embedded. Cells can adhere to the substrate through surface-conjugated ECMs or protein ligands (purple line). Traction forces (indicated by red arrows) exerted by cells can cause subtle deformation of a substrate, where traction forces can be measured by tracking the displacement of fluorescent beads within the substrate. (B) Traction force stress map showing human bone marrow-derived mesenchymal stem cells adhered onto hydrophobic-polydimethylsiloxane (PDMS) and hydrophilic-PDMS with polyethyleneoxide (PEO) (PEO-PDMS), with varying stiffness ranging from 0.2-0.3 kPa (soft, 70:1) to 5-6 kPa (intermediate, 60:1). (C, D) Schematic and scanning electron microscopy (SEM) image of 2D TFM by micropillars. Vertical arrays of PDMS microposts are fabricated by a photolithography technique. Cell spreads across multiple post beds on which ECMs are pre-coated. Adhered cells can exert traction forces. Traction forces are calculated from the deflection and material property (spring constant) of microposts. (E, F) Schematic representations of traditional 2D TFM method (E) and novel 3D TFM method (F). 3D TFM determines both horizontal (dh) and vertical (dv) components of the displacement vector (d), allowing the calculation of a 3D traction force vector. (G) A breast tumor cell (yellow, MDA-MB-231 cell line) is embedded in 3D type I collagen matrix, visualized by reflective confocal images (cyan). (H) 3D rendering images of bead displacements (blue) and cells (magenta) in 3D collagen matrix. *Figures adapted with permission from; Fig. 1A, B: ref. (, Fig. 1C, D: ref. (, Fig. 1E, F: ref. (, Fig. 1G, H: ref. (44).

Fig. 2

Intercellular junctional force quantification by IFM. (A) Schematic diagram 2D IFM by micropillars for a pair of endothelial cells at cell-cell junctional interfaces. For a doublet of contacting cells, the net force encompasses both traction force Ti (red arrows) and the intercellular force Fc, (blue arrows). Cell-cell junction or intracellular force Fc plotted over cell A is defined as the net tugging force that cell A is exerting on cell B at the cell-cell junctional interface. Cell B is expected to pull on cell A with an equal amount of opposite force. (B) Cells adhered onto microposts are constricted to have a shape of a bowtie pattern by micropatterned of fibronectin (Cyan) (top). Arrows present the force vectors with direction and magnitude (bottom). Red arrows show individual traction forces and white arrows exhibit tugging force between two cells. (C) 3D IFM by a deformable substrate. Schematic of two cells on a substrate with traction stress TS (blue arrows) and cell-cell tension JT (red arrows). Cell-cell and intracellular forces are determined in 3D by the force balance on the ground of Newton's first law. (D) A phase contrast image of a pair of endothelial cells in contact (left) and corresponding contour and vector map of displacement of two endothelial cells (right). *Figures adapted with permission from ref. (51) for Fig. 2A, B and from ref. (47) for Fig. 2C, 2D.