Extracellular matrix rigidity governs smooth muscle cell motility in a biphasic fashion

Increasing evidence suggests that mechanical cues inherent to the extracellular matrix (ECM) may be equally as critical as its chemical identity in regulating cell behavior. We hypothesized that the mechanical properties of the ECM directly regulate the motility of vascular smooth muscle cells (SMCs) and tested this hypothesis using polyacrylamide substrates with tunable mechanical properties. Quantification of the migration speed on uniformly compliant hydrogels spanning a range of stiffnesses (Young's moduli values from 1.0 to 308 kPa for acrylamide/bisacrylamide ratios between 5/0.1% and 15/1.2%, respectively) revealed a biphasic dependence on substrate compliance, suggesting the existence of an optimal substrate stiffness capable of supporting maximal migration. The value of this optimal stiffness shifted depending on the concentration of ECM protein covalently attached to the substrate. Specifically, on substrates presenting a theoretical density of 0.8 microg/cm(2) fibronectin, the maximum speed of 0.74 +/- 0.09 microm/min was achieved on a 51.9 kPa gel; on substrates presenting a theoretical density of 8.0 microg/cm(2) fibronectin, the maximum speed of 0.72 +/- 0.06 microm/min occurred on a softer 21.6 kPa gel. Pre-treatment of cells with Y27632, an inhibitor of the Rho/Rho-kinase (ROCK) pathway, reduced these observed maxima to values comparable to those on non-optimal stiffnesses. In parallel, quantification of TritonX-insoluble vinculin via Western blotting, coupled with qualitative fluorescent microscopy, revealed that the formation of focal adhesions and actin stress fibers also depends on ECM stiffness. Combined, these data suggest that the mechanical properties of the underlying ECM regulate Rho-mediated contractility in SMCs by disrupting a presumptive cell-ECM force balance, which in turn regulates cytoskeletal assembly and ultimately, cell migration.