Abstract:  The control of the mechanical properties the cytoskeleton of cells appears to rely on the combination of the internal structure and passive equilibrium properties of crosslinked semiflexible filaments with the active nonequilibrium state of internal stress generated by molecular motors.  Recent experimental and theoretical studies have shown that changes in the equilibrium and nonequilibrium properties of these networks can have dramatic effects on the overall elasticity and architecture dynamics.  In this talk I will present results of computational modeling studies of both of these effects.  First, we consider the effects of network anisotropy and polydispersity, both of which are descriptive of real cytoskeletal networks in which filaments may have varied length and stiffness as well as locally preferred directions.  Specifically we show how anisotropy and polydispersity change the physics of crossover between affine and non-affine deformation behavior, which for uniform isotropic networks is determined by a single control parameter that is a function of the density of cross-links and the length and stiffness of constituent filaments.  Secondly, we consider the mechanisms by which molecular motors may control the structure and properties of cytoskeletal networks.  F-actin is strongly strain hardening under tension so that tensile stresses applied by myosin motors can stiffen the network dramatically. Experiments have shown that the action of these molecular motors can generate hundred-fold increases in the network's modulus.  Using a coarse-grained finite-element model incorporating wormlike chain behavior for individual filaments, we examine the effect of the internal force generation of myosin-like molecular motors on the material's elastic constants.  Lastly, we consider the viscoelastic behavior of semiflexible filaments networks as governed by the dynamics of labile crosslinkers.  The incorporation of a Bell model of crosslink binding and unbinding into our computational network models produces dissipation and partial elastic recovery consistent with experimental rheology of microtubule networks.  These models provide clues to the mechanisms used by cells to control their shape and mechanical properties.

Bio:  William Klug is an Associate Professor in the Mechanical and Aerospace Engineering Department at UCLA, where he has been since 2003.  He received a B.S. in Engineering Physics from Westmont College in 1997, a M.S. in Civil Engineering from UCLA in 1999, and a Ph.D. from Caltech in 2003. He is the recipient of a 2007 NSF CAREER award.  Professor Klug's primary scientific background is in continuum and computational modeling of the mechanics of solids and structures.  He has particular experience and interest in the development of numerical methods for modeling thin beam- and shell-like structures, and in the application of those methods to multi-scale and multi-physics problems in biology, including mechanics and assembly of viruses, mechanics of cell membranes, mechanics of DNA, mechanics of cytoskeletal networks, and electro-mechanics of the heart.

Host:  Prof. Megan Valentine