$240 Journal of Biomechanics 2006, Vol. 39 (Suppl 1) Oral Presentations

organization of filaments in a cylindrical geometry: a model for the plant pre- prophase band. Phys. Rev. Lett. 2005; 95: 258103.

5353 Th, 14:15-14:30 (P44) Elastic and dynamic properties of cytoskeletal biopolymer networks: probing microstructure and mechanical response F.C. MacKintosh. Department of Physics and Astronomy, Vrije Universiteit, Amsterdam, The Netherlands

The cyteskeleten consists in part of networks of filamentous proteins such as F-actin, together with associated proteins for crosslinking, bundling, and active force generation. We discuss recent experiments and theoretical models addressing the local structure and dynamics of in vitro models of crosslinked F-actin networks. We show how the elastic and dynamic properties of these model systems can be quantitatively understood in terms of equilibrium fluctu- ations of semiflexible polymers. We also report results on in vitro models with active force generation by molecular motors. These non-equilibrium networks exhibit clear evidence of non-thermal fluctuations and specifically contractile forces.

4201 Th, 14:30-14:45 (P44) The biomechanics of retraction in nematode sperm crawling C.W. Wolgemuth. Dept. of Cell Biology, University of Connecticut Health Center, Farmington, CT, USA

Cell crawling requires three main processes: polymerization at the leading edge, adhesion to the substrate, and retraction at the rear. Most cells dy- namically reorganize a cytoskeletal meshwork composed primarily of actin to accomplish polymerization and retraction. The function of molecular motors, such as myosin II, may also be responsible for retraction forces, but evidence regarding the role of myosin is ambiguous. In this talk, I will present research on physical models for the crawling of nematode sperm. These sperm dynamically reorganize a cytoskeleton composed of Major Sperm Protein (MSP). MSP is a-polar and no molecular motors have been identified, yet these cells show all three main processes of crawling, just like actin-based cells. Therefore, this system offers a more simple system to study cell crawling in which the dynamics of the cytoskeletal polymer alone is responsible for motility. New experiments suggest that depolymerization of the cytoskeletal network is responsible for producing the force for retraction, which hauls the back end of the cell forward. I will present a physical model that describes how depolymer- ization can produce force. This model provides quantitative agreement with in vitro experiments on nematode sperm cell extracts. In addition, applying this model to realistic cell geometries predicts that larger cells should crawl faster, which has been experimentally observed. This model also predicts that cell geometry affects crawling speed such that there is an optimum aspect ratio for crawling cells that favors cells that are longer perpendicular to their direction of motion. This result may have bearing on other crawling cells, such as keratocytes.

6668 Th, 14:45-15:00 (P44) Actin network organization at the leading edge of crawling cells S. Sun, D. Wirtz, E. Atilgan. Department ef Mechanical Engineering and Chemical Engineering, Johns Hopkins University, Baltimore, MD, USA

We examine the dynamical interaction between the growing actin network with the cell membrane using a 3-dimensional computational model. The roles of actin-associated proteins in generating the flat lamellipodium are explored. We show that the distinct pattern formed in the lamellipodium suggests a unique Arp2/3 branching mechanism. The mechanics of the plasma membrane and its effect on the growing lamellipodium and filopodium are discussed. Computations suggest that membrane fluctuations enhance the actin growth and protrusion speed.

4769 Th, 16:00-16:15 (P46) Pushing the limit: On building an artificial lamellipodium B. Stuhrmann, E Huber, J. K~s. Institute for Soft Matter Physics, University ef Leipzig, Leipzig, Germany

In cells displaying crawling motility, cell boundary advancement is governed by the assembly of actin protein, tightly regulated by a wealth of accessory proteins. The key molecular players involved in these processes have been identified [Loisel et al. Nature, 1999] and are used in this project to build a model system of the cell lamellipodium in vitro. A polymerizing actin gel is generated in a nanostructured, closed chamber. Buildup of actin meshwork is limited to the system periphery by localized functionalization of the chamber wall. Polymerization forces push the emerging actin gel backwards, where actin depolymerizing factors disassemble aged network. Monomeric actin diffuses to the functionalized zone for another round of assembly, closing the array treadmilling cycle typically observed in migrating cells [Pollard & Borisy, Cell,

2003]. In contrast to existing biomimetic assays, our system is constrained to lamellipodial volumes. This is a crucial step towards conditions in living cells, where confined geometry limits the total amount of protein available and restricts diffusional transport. By spatial confinement we create for the first time a self-sustaining, polymerizing machine mimicking the cell lamellipodium in vitro. Biochemical parameters, e.g., protein composition including actin crosslinkers and motor proteins, and physical parameters such as chamber geometry and system size, can be selectively changed. The system's response in terms of its structural, dynamical and rheological properties can be studied using fluorescence microscopy, electron microscopy, and dual-bead microrhe- ology. Thus, our assay presents a novel toolbox to explore biomechanical mechanisms underlying cell motility.

5131 Th, 16:15-16:30 (P46) Actin-binding proteins sensitively mediate actin bundle stiffness

M. Bathe 1 , M. Claessens 2, E. Frey 1 , A. Bausch 2. 1Ludwig Maximilian University, Munich, Germany, 2 Technical University of Munich, Munich, Germany

Animal cells express a multitude of actin-binding proteins (ABPs) that associate with filamentous actin (F-actin) to form stiff bundles in vivo. The physiological function of F-actin bundles varies from passive mechanical structures such as microvilli present on the surface of epithelial cells in the intestinal lining to active structures such as filopodia formed at the leading edge of cells during migration. A biomimetic emulsion technique, recently introduced by our lab, has been used to directly measure the bending stiffness of isolated F-actin bundles associated with biologically-relevant ABPs. Results demonstrate that bundle stiffness depends sensitively on the number of actin filaments constituting the bundle, bundle length, and ABP type and concentration. Here, we use a combination of molecular simulation and analytical theory to elucidate the origin of the observed bundle mechanical properties. We also determine for the first time the molecular stiffnesses of the various ABPs examined.

5347 Th, 16:30-16:45 (P46) Computational study of rheological properties and spontaneous fluctuations of particles within a prestressed cytoskeletal network C. Metzner, B. Fabry. Zentrum f~r Medizinische Physik und Technik, Universit~t Erlangen, Erlangen, Germany

Recent microrheological studies with beads tightly bound to the cytoskeleton of living cells revealed a power-law frequency response of the complex shear modulus [1]. In addition, beads coupled to the cytoskeletal network of cells show spontaneous superdiffusive motion that is clearly not of thermal origin. Based on these observations, the cell has been represented by a viscoelastic continuum with power-law rheology, interdispersed with point-like force centers that randomly fluctuate with a prescribed power spectral density [2]. Here we present an alternative model of cell mechanics based on a random network of force-generating stress fibers, with a part of the network nodes being attached to a semi-rigid substrate. The network structure is continuously changing due to fiber contractions, stochastic rupture, re-formation and adhe- sion events. Each fiber is described by two sliding filaments with dynamically attaching and detaching force-generating cross bridges, analogous to Huxley's 1957 model of muscle contraction. Due to the internal kinetics and linking of the fibers, the system exhibits a complex time-dependent distribution of pre- stress, resulting in random fluctuations of beads attached to the network. When an external force is applied to the bead, the fibers respond by length adaption and - for longer times and large spatial bead displacements - by topological network remodelling. In a series of computer simulations we show that the superdiffusive spon- taneous bead motion as well as the power-law creep response observed in living cells can be modelled with a pre-stressed fiber network using only a small number of model parameters.

References [1] B. Fabry et al. Phys. Rev. Lett. 2001; 87: 148102. [2] A.W.C. Lau et al. Phys. Rev. Lett. 2003; 91: 198101.

7485 Th, 16:45-17:00 (P46) Deformability and mechano-sensing in a cytoskeleton model with forced protein unfolding

J.C. Crocker 1,2, B.D. Hoffman 1 , G. Massiera 1 . 1Department ef Chemical and Biochemical Engineering, 2Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, PA, USA

Stem cell differentiation, tissue morphogenesis as well as cell growth and death are known to be affected by cell shape or the stiffness of the surrounding matrix. The molecular mechanisms by which such mechanical cues produce biochemical responses, however, remain essentially unknown. Similarly, the