ALBERT

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.