ALBERT

Description: Tumor cell invasion is the most critical step of metastasis. Determination of the mode of invasion within the particular tumor is critical for effective cancer treatment. Protease-independent amoeboid mode of invasion has been described in carcinoma cells and more recently in sarcoma cells on treatment with protease inhibitors. To analyze invasive behavior, we compared highly metastatic sarcoma cells with parental nonmetastatic cells. The metastatic cells exhibited a functional up-regulation of Rho/ROCK signaling and, similarly to carcinoma cells, an amoeboid mode of invasion. Using confocal and traction force microscopy, we showed that an up-regulation of Rho/ROCK signaling leads to increased cytoskeletal dynamics, myosin light chain localization, and increased tractions at the leading edge of the cells and that all of these contributed to increased cell invasiveness in a three-dimensional collagen matrix. We conclude that cells of mesenchymal origin can use the amoeboid nonmesenchymal mode of invasion as their primary invading mechanism and show the dependence of ROCK-mediated amoeboid mode of invasion on the increased capacity of cells to generate force. (Mol Cancer Res 2008;6(9):141020)

Description: Cytoskeletal (CSK) dynamics such as remodeling and reorganization can be studied by tracking the spontaneous motion of CSK-bound particles. Particle motion is thought to be driven by local, ATP-dependent intracellular force fluctuations due to polymerization processes and motor proteins, and to be impeded by a viscoelastic, metastable cytoskeletal network. The mechanisms that link particle motion to force fluctuations and the CSK dynamics remain unclear. We report simultaneous measurements of the spontaneous motion of CSK-bound particles and of cellular force fluctuations. Cellular force fluctuations were measured by tracking fluorescent markers embedded in an elastic polyacrylamide hydrogel substrate that served as an extracellular matrix (ECM). The motion of CSK-bound particles and markers embedded in the ECM showed both persistence and superdiffusive behavior. Moreover, the movements of CSK-bound beads were temporally and spatially correlated with force fluctuations in the ECM. The findings suggest that the spontaneous motion of CSK-bound beads is driven not by random, local stress fluctuations within a viscoelastic continuum or cage, but rather by stress fluctuations within a tensed and constantly remodeling CSK network that transmits stresses over considerable distances to the ECM.

Description: Important processes of living cells, including intracellular transport, cell crawling, contraction, division, and mechanochemical signal transduction, are controlled by cytoskeletal (CSK) dynamics. CSK dynamics can be measured by tracking the motion of CSK-bound particles. Particle motion has been reported to follow a superdiffusive behavior that is believed to arise from ATP-driven intracellular stress fluctuations generated by polymerization processes and motor proteins. The power spectrum of intracellular stress fluctuations has been suggested to decay with 1/2 (Lau et al, Phys Rev Lett 91:198101). Here we report direct measurements of cellular force fluctuations that are transmitted to the extracellular matrix, and compared them with the spontaneous motion of CSK-bound beads. Fibronectin coated fluorescent beads (Ø 1 m) were bound to the CSK of confluent human vascular endothelial cells. Forces transmitted to the extracellular matrix (ECM) were quantified by plating these cells onto a collagen coated elastic polyacrylamide hydrogel, and measuring the gel deformation from the displacement of embedded fluorescent beads (Ø 0.5 m). Bead motion of both CSK-bound and ECM-bound beads were measured with nanometer-resolution and expressed as mean square displacement (MSD). The MSD of both CSK-bound and ECM-bound beads displayed a superdiffusive behavior that was well described by a power law: MSD = a*t^b. Surprisingly, we found an identical power law exponent for both CSK-bound and ECM-bound beads of b = 1.6. This finding suggests that the spontaneous motion of CSK-bound beads is driven by stress fluctuations with a 1/ b+1 power spectrum. This result is consistent with the notion that CSK dynamics and CSK stress fluctuations are closely coupled.

Description: Important processes of living cells, including intracellular transport, cell crawling, contraction, division and mechanochemical signal transduction, are controlled by cytoskeletal (CSK) dynamics. Some aspects of CSK dynamics have been studied by following the spontaneous motion of CSK-bound particles. Such particle exhibit a superdiffusive behavior that is believed to arise from random local ATP-driven intracellular force fluctuations generated by polymerization processes and motor proteins. (Lau et al, Phys Rev Lett 91:198101). Here we report simultaneous measurements of spontaneous particle motions and cellular force fluctuations. Human vascular endothelial cells were plated onto collagen coated elastic polyacrylamide hydrogels. Force fluctuations at the basal cell membrane(cell tractions) were computed from the displacements of gel-embedded fluorescent beads. Spontaneous particle motion was measured using fibronectin coated fluorescent beads that were bound to the apicell cell membrane via integrin receptors. Bead motion of both CSK-bound and ECM-bound beads were measured with nanometer-resolution and expressed as mean square displacement (MSD). The MSD of both CSK-bound and ECM-bound beads displayed a superdiffusive behavior that was well described by a power law: MSD = a*t^b. In contradiction to existing theories of stress dissipation within the CSK, we found an identical power law exponent for both CSK-bound and ECM-bound beads of b = 1.6. This finding suggests that the spontaneous motion of CSK-bound beads is driven not by random, local stress fluctuations within a viscoelastic continuum, but rather by large scale stress fluctuations within a CSK network that transmits these stresses with little or no dissipation to the ECM.

Description: For most cell types, adhesion, spreading and tension generation are crucial for cell survival. These processes are strongly influenced by the rigidity of the extracellular matrix: Cells spread more and faster, and generate higher tension on more rigid substrates. We report simulta- neous measurements of cell spreading and traction generation during adhesion of MDA-MB-231 breast carcinoma cells onto collagen coated polyacrylamid gels. The Youngs modulus of the gels was tuned between 1500 (’soft’) and 6000 (’hard’) Pa. The evolution of cell tractions was computed from the gel deformation measured every 30 sec by tracking the displacements of fluorescent beads (ø0.5µm) embedded at the gel surface. As a robust estimate of total force generation, we computed for each cell the elastic strain energy U stored within the gel. As ex- pected, cells generated a higher maximum strain energy U = 1.01pJ) and spread more (A = 6002 ± 961µm2) on harder gels compared to softer gels (U = 0.20pJ, A = 3012 ± 492µm2). When the strain energy vs. time data of individual cells were normalized by spreading area, they collapsed onto a single relationship, regardless of gel stiff- ness. These data extend earlier findings of a proportionality between cell spreading and tension generation (Reinhard-King, Biophys J 2005) and show that individual cells exhibit a constant rate of stress increase during early adhesion events regardless of the substrate rigidity.