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During metastasis, cells can use proteolytic activity to form tube-like "microtracks" within the extracellular matrix (ECM). Using these microtracks, cells can migrate unimpeded through the stroma. To investigate the molecular mechanisms of microtrack migration, we developed an in vitro 3D micromolded collagen platform. When in microtracks, cells tend to migrate unidirectionally. Since focal adhesions are the primary mechanism by which cells interact with the ECM, we examined the roles of several focal adhesion molecules in driving unidirectional motion. Vinculin knockdown results in the repeated reversal of migration direction compared with control cells. Tracking the position of the Golgi centroid relative to the position of the nucleus centroid reveals that vinculin knockdown disrupts cell polarity in microtracks. Vinculin also directs migration on 2D substrates and in 3D uniform collagen matrices, indicated by reduced speed, shorter net displacement and decreased directionality in vinculin-deficient cells. In addition, vinculin is necessary for Focal Adhesion Kinase (FAK) activation in 3D as vinculin knockdown results in reduced FAK activation in both 3D uniform collagen matrices and microtracks, but not on 2D substrates, and accordingly, FAK inhibition halts cell migration in 3D microtracks. Together, these data indicate that vinculin plays a key role in polarization during migration., (© 2016 by The American Society for Cell Biology.)

Tumor cell invasion through the stromal extracellular matrix (ECM) is a key feature of cancer metastasis, and understanding the cellular mechanisms of invasive migration is critical to the development of effective diagnostic and therapeutic strategies. Since cancer cell migration is highly adaptable to physiochemical properties of the ECM, it is critical to define these migration mechanisms in a context-specific manner. Although extensive work has characterized cancer cell migration in two- and three-dimensional (3D) matrix environments, the migration program employed by cells to move through native and cell-derived microtracks within the stromal ECM remains unclear. We previously reported the development of an in vitro model of patterned type I collagen microtracks that enable matrix metalloproteinase-independent microtrack migration. Here we show that collagen microtracks closely resemble channel-like gaps in native mammary stroma ECM and examine the extracellular and intracellular mechanisms underlying microtrack migration. Cell-matrix mechanocoupling, while critical for migration through 3D matrix, is not necessary for microtrack migration. Instead, cytoskeletal dynamics, including actin polymerization, cortical tension, and microtubule turnover, enable persistent, polarized migration through physiological microtracks. These results indicate that tumor cells employ context-specific mechanisms to migrate and suggest that selective targeting of cytoskeletal dynamics, but not adhesion, proteolysis, or cell traction forces, may effectively inhibit cancer cell migration through preformed matrix microtracks within the tumor stroma., (Copyright © 2015 the American Physiological Society.)