Mechanotransduction of fluid stresses governs 3D cell migration.

Solid tumors are characterized by high interstitial fluid pressure, which drives fluid efflux from the tumor core. Tumor-associated interstitial flow (IF) at a rate of ∼3 µm/s has been shown to induce cell migration in the upstream direction (rheotaxis). However, the molecular biophysical mechanism that underlies upstream cell polarization and rheotaxis remains unclear. We developed a microfluidic platform to investigate the effects of IF fluid stresses imparted on cells embedded within a collagen type I hydrogel, and we demonstrate that IF stresses result in a transcellular gradient in ß1-integrin activation with vinculin, focal adhesion kinase (FAK), FAK(PY397), F actin, and paxillin-dependent protrusion formation localizing to the upstream side of the cell, where matrix adhesions are under maximum tension. This previously unknown mechanism is the result of a force balance between fluid drag on the cell and matrix adhesion tension and is therefore a fundamental, but previously unknown, stimulus for directing cell movement within porous extracellular matrix.

Paxillin controls endothelial cell migration and tumor angiogenesis by altering neuropilin 2 expression.

Although a number of growth factors and receptors are known to control tumor angiogenesis, relatively little is known about the mechanism by which these factors influence the directional endothelial cell migration required for cancer microvessel formation. Recently, it has been shown that the focal adhesion protein paxillin is required for directional migration of fibroblasts in vitro. Here, we show that paxillin knockdown enhances endothelial cell migration in vitro and stimulates angiogenesis during normal development and in response to tumor angiogenic factors in vivo. Paxillin produces these effects by decreasing expression of neuropilin 2 (NRP2). Moreover, soluble factors secreted by tumors that stimulate vascular ingrowth, including vascular endothelial growth factor (VEGF), also decrease endothelial cell expression of paxillin and NRP2, and overexpression of NRP2 reverses these effects. These results suggest that the VEGF-paxillin-NRP2 pathway could represent a new therapeutic target for cancer and other angiogenesis-related diseases.

Paxillin controls directional cell motility in response to physical cues.

Physical cues from the extracellular environment that influence cell shape and directional migration are transduced into changes in cytoskeletal organization and biochemistry through integrin-based cell adhesions to extracellular matrix (ECM). Paxillin is a focal adhesion (FA) scaffold protein that mediates integrin anchorage to the cytoskeleton, and has been implicated in regulation of FA assembly and cell migration. To determine whether paxillin is involved in coupling mechanical distortion with directional movement, cell shape was physically constrained by culturing cells on square-shaped fibronectin-coated adhesive islands surrounded by non-adhesive barrier regions that were created with a microcontact printing technique. Square-shaped cells preferentially formed FAs and extended lamellipodia from their corner regions when stimulated with PDGF, and loss of paxillin resulted in loss of this polarized response. Selective expression of the N- and C-terminal domains of paxillin produced opposite, but complementary, effects on suppressing or promoting lamellipodia formation in different regions of square cells, which corresponded to directional motility defects in vitro. Paxillin loss or mutation was also shown to affect the formation of circular dorsal ruffles, and this corresponded to changes in cell invasive behavior in 3D. This commentary addresses the implications of these findings in terms of how a multifunctional FA scaffold protein can link physical cues to cell adhesion, protrusion and membrane trafficking so as to control directional migration in 2D and 3D. We also discuss how microengineered ECM islands and in vivo model systems can be used to further elucidate the functions of paxillin in directional migration.