Longer collagen fibers trigger multicellular streaming on soft substrates via enhanced forces and cell-cell cooperation

Grouped cells often leave large cell colonies in the form of narrow multi-cellular streams. However, it remains unknown how collective cell streaming exploits specific matrix properties, like stiffness and fiber length. It is also unclear how cellular forces, cell-cell adhesion, and velocities are coordinated within streams. To independently tune stiffness and collagen fiber length, we developed new hydrogels and discovered invasion-like streaming of normal epithelial cells on soft substrates coated with long collagen fibers. Here, streams arise from a surge in cell velocities, forces, YAP activity, and mesenchymal markers in regions of high stress anisotropy. Coordinated velocities and symmetric distribution of tensile and compressive stresses support persistent stream growth. Stiff matrices diminish cell-cell adhesions, disrupt front-rear velocity coordination, and do not promote sustained fiber-dependent streaming. Rac inhibition reduces cell elongation and cell-cell cooperation, resulting in a complete loss of streaming in all matrix conditions. Our results reveal a stiffness-modulated effect of collagen fiber length on collective cell streaming and unveil a biophysical mechanism of streaming governed by a delicate balance of enhanced forces, monolayer cohesion, and cell-cell cooperation.