Biphasic response of cell invasion to matrix stiffness in three-dimensional biopolymer networks

Abstract

When cells come in contact with an adhesive matrix, they begin to spread and migrate with a speed that depends on the stiffness of the extracellular matrix. On a flat surface, migration speed decreases with matrix stiffness mainly due to an increased stability of focal adhesions. In a 3-dimensional (3D) environment, cell migration is thought to be additionally impaired by the steric hindrance imposed by the surrounding matrix. For porous 3D biopolymer networks such as collagen gels, however, the effect of matrix stiffness on cell migration is difficult to separate from effects of matrix pore size and adhesive ligand density, and is therefore unknown. Here we used glutaraldehyde as a crosslinker to increase the stiffness of self-assembled collagen biopolymer networks independently of collagen concentration or pore size. Breast carcinoma cells were seeded onto the surface of 3D collagen gels, and the invasion depth was measured after 3 days of culture. Cell invasion in gels with pore sizes larger than 5 μm increased with higher gel stiffness, whereas invasion in gels with smaller pores decreased with higher gel stiffness. These data show that 3D cell invasion is enhanced by higher matrix stiffness, opposite to cell behavior in 2D, as long as the pore size does not fall below a critical value where it causes excessive steric hindrance. These findings may be important for optimizing the recellularization of soft tissue implants or for the design of 3D invasion models in cancer research.

Modeling the two-way feedback between contractility and matrix realignment reveals a nonlinear mode of cancer cell invasion

“…These key findings are also in agreement with the behavior observed in previous experimental studies (17). For example, Lang et al (17) used glutaraldehyde to enhance the stiffness of the gel, while maintaining the concentration of the ligands.…”

“…For example, Lang et al (17) used glutaraldehyde to enhance the stiffness of the gel, while maintaining the concentration of the ligands. They showed that adding glutaraldehyde to collagen matrices with dilute concentration of collagen, increases the cell invasion, suggesting that the stiffness of the matrix promotes cell invasion.…”

“…At the periphery of the spheroid, cells can sense and respond to the stiffness of the matrix by forming actin-rich focal adhesions with the matrix ligands. The cell-matrix interactions depend on the mechanical and microstructural properties of the matrix such as matrix rigidity (13,14), fiber alignment (5,6,15,16), interfibrillar pore size (17), and density of cell-adhesive ligands (18). Previous computational models of cell invasion have examined cellular interactions (19,20) and the interplay between protrusion forces (generated due to the polymerization of actin), traction forces (at the cell-matrix adhesions), and resisting forces (mostly the drag force caused by the viscosity of the matrix) (19,(21)(22)(23)(24).…”

The extracellular matrix in breast cancer predicts prognosis through composition, splicing, and crosslinking

“…It remains difficult to decouple the effects of fiber diameter, pore size and biomechanics, but it appears that all three act on cells (reviewed in [88]). For example, crosslinking and crosslinking density each affect cell invasion, with a significant interaction, such that crosslinking loose ECM environments increased invasiveness, whereas crosslinking dense ECM decreased invasiveness [89].…”

Heralding a new paradigm in 3D tumor modeling

“…A landmark study by the Weaver group showed that collagen crosslinking-induced ECM stiffening supports the invasive phenotype by enhancing integrin signaling [78]. By using glutaraldehyde as a crosslinker to increase the stiffness of collagen gels independently from pore size or collagen concentration, Lang et al showed that 3D invasion is dependent on pore size; while increased matrix stiffness promotes 3D invasion in gels with large pores (small steric hindrance), increased matrix stiffness hinders cell invasion in gels with small pores (large steric hindrance) [79]. In another interesting study, the Weaver group reported the development of a 3D tension bioreactor platform to facilitate studies on ECM stiffness; by mechanically loading collagen gels to induce gel stiffening (via collagen strain hardening) while maintaining composition and pore size, increasing matrix stiffness was also found to enhance tumor cell invasion [80].…”