### Session AW: Mini-Symposium on Intracellular Fluid Dynamics

Chair: Juan Carlos del Alamo, University of California, San Diego
Room: 208A-D

 Sunday, November 22, 2009 8:00AM - 8:26AM AW.00001: Cellular hydraulics: physics and physiology Invited Speaker: L. Mahadevan Hydraulics (from the Greek word $\delta \rho \alpha \upsilon \lambda \iota \kappa \varsigma )$ is the study of fluid movements and fluid power. In this brief talk, I will discuss the implications of water movements through soft porous structures for the dynamics of nuclear swelling and the cytoskeleton in animal cells. If time permits, I will conclude with a quantitative description of the growth of pollen tubes treated as hydraulic machines. Sunday, November 22, 2009 8:26AM - 8:52AM AW.00002: Active Gels and Cell Quakes: Exploring the non-equilibrium rheology and fluctuation spectrum of motor-driven polymer networks Invited Speaker: Alex Levine Recent experiments on molecular motor driven in vitro F-Actin networks have found anomalously large strain ?uctuations at low frequency. In addition, the shear modulus of these active networks becomes as much as one hundred times larger than that of the same system in equilibrium. In this talk we develop a theory of both these phenomena using a two-?uid model of a low-density isotropic semi?exible network driven by molecular motors. Relying on only simple assumptions regarding the motor activity in the system, we find that we can quantitatively understand both the low-frequency ?uctuation enhancement and the nonequilibrium stiffening of the network. We also show the results of new numerical studies of semiflexible networks driven by molecular motors that explore the effects of high motor density in isotropic networks and the effect of nematic order in the active filament network. These results have implications for the interpretation of microrheology in such active networks including the cytoskeleton of living cells. In addition, they may form the basis for theoretical studies of biomimetic nonequilibrium gels whose mechanical properties are tunable through the control of their nonequilibrium steady-state. Sunday, November 22, 2009 8:52AM - 9:18AM AW.00003: Resolving the Role of Actoymyosin Contractility in Cell Microrheology Invited Speaker: Denis Wirtz Einstein's original description of Brownian motion established a direct relationship between thermally-excited random forces and the transport properties of a submicron particle in a viscous liquid. Recent work based on reconstituted actin filament networks suggests that nonthermal forces driven by the motor protein myosin II can induce large non-equilibrium fluctuations that dominate the motion of particles in cytoskeletal networks. Here, using high-resolution particle tracking, we find that thermal forces, not myosin-induced fluctuating forces, drive the motion of submicron particles embedded in the cytoskeleton of living cells. These results resolve the roles of myosin II and contractile actomyosin structures in the motion of nanoparticles lodged in the cytoplasm, reveal the biphasic mechanical architecture of adherent cells-stiff contractile stress fibers interdigitating in a network at the cell cortex and a soft actin meshwork in the body of the cell, validate the method of particle tracking-microrheology, and reconcile seemingly disparate atomic force microscopy (AFM) and particle-tracking microrheology measurements of living cells. Sunday, November 22, 2009 9:18AM - 9:44AM AW.00004: Anisotropic viscoelastic properties and cytoskeletal structure of endothelial cells subject to shear flow Invited Speaker: Juan C. del Alamo Adherent cells remodel in response to mechanical stimuli leading to a redistribution of intracellular forces that depends on the viscoelastic properties of the cytoskeleton. We have analyzed the magnitude and anisotropy of these properties in confluent vascular endothelial cells subject to continuous flow. For this purpose we used Directional Particle Tracking Microrheology, which measures the second-order tensor of intracellular marker displacements, allowing us to determine the principal directions of highest and lowest shear modulus at each position. We studied the orientation of these principal directions relative to those of the actin stress fibers. After the application of flow shear the cells' stress fibers gradually orient parallel to the flow and the principal directions of the shear modulus become parallel and perpendicular to the flow. The role of ATP-driven myosin-II contractions in the observed anisotropy is analyzed by using cells treated with drugs inhibiting myosin-II function. Sunday, November 22, 2009 9:44AM - 10:10AM AW.00005: Structural response and remodeling of red blood cells - a multiscale modeling approach Invited Speaker: Qiang Zhu A red blood cell contains cytosol enclosed inside a composite membrane consisting of a fluidic lipid bilayer reinforced by a single layer of protein skeleton. It has been demonstrated that mechanical loads can trigger dissociation of inter-protein and protein-to-lipid linkages and cause structural remodeling and failure. To understand these effects, it is vital to quantitatively characterize the mechanical forces acting within the membrane. For this purpose we developed a multiscale model to study distributions of internal stress in response to external load. In this method, the cell is modeled at three length scales: in the complete-cell level it is depicted as two layers of continuum shells, one representing the lipid bilayer and the other the skeleton; a molecular-detailed model of the skeleton is developed to predict its constitutive properties; a nonlinear stain-stretch model of Sp (a major protein in the skeleton) is applied to study the mechanical properties of the cell in large deformations. With this model we investigated mechanical responses of the system under canonical experiments such as micropipette aspirations and optical tweezer stretching. Model validations were conducted through comparisons with benchmark experiments.