Cytoskeletal and Adhesion Dynamics are Coupled to Matrix Deformation in 3D Cell Migration

The generation and coordination of cellular traction forces play important roles in cell migration and extracellular matrix (ECM) remodeling, and hence in the development and repair of biological tissues. Historically, models for how cells generate and sense mechanical force have been derived from observations of cells adhering to two-dimensional (2D) surfaces. Here we sought to understand how the insights garnered from studies of cellular motility in 2D might apply to cell migration and traction force generation in more biologically realistic, 3D environments. We embedded primary human fibroblasts in a 3D fibrin matrix, and used multicolor, time-lapse confocal microscopy to simultaneously image matrix deformation induced by cellular traction and the dynamics of the actomyosin cytoskeleton and cell-matrix adhesions. We observed that traction forces were transduced to the matrix through contractile actomyosin fibers coupled to paxillin-rich adhesions. Spatial decomposition of the matrix deformation tensors, as quantified using digital volume correlation (DVC), revealed that cell-generated matrix deformations were largely tangential to the cell surface. Within protrusions, matrix deformations occurred in both the retrograde and anterograde directions relative to the protrusion tip. Automated tracking of paxillin-rich adhesions revealed persistent movements in both the retrograde and anterograde directions, with apparent slippage between adhesions and the underlying matrix. We tracked actin motion using both photoactivatable mCherry-actin and via automated particle tracking of alpha-actinin-EGFP puncta. These data revealed simultaneous anterograde and retrograde cytoskeletal motion within individual protrusions. In addition, the actin cytoskeleton moved at velocities higher than those of proximal paxillin plaques or proximal ECM. Together, these data suggest that a modified version of the molecular clutch model of cytoskeletal force transmission, which was originally developed to describe cell migration on flat surfaces, can be applied to understand cell migration in some 3D contexts.