Softness, strength and self-repair in intermediate filament networks.
ABSTRACT One cellular function of intermediate filaments is to provide cells with compliance to small deformations while strengthening them when large stresses are applied. How IFs accomplish this mechanical role is revealed by recent studies of the elastic properties of single IF protein polymers and by viscoelastic characterization of the networks they form. IFs are unique among cytoskeletal filaments in withstanding large deformations. Single filaments can stretch to more than 3 times their initial length before breaking, and gels of IF withstand strains greater than 100% without damage. Even after mechanical disruption of gels formed by crossbridged neurofilaments, the elastic modulus of these gels rapidly recovers under conditions where gels formed by actin filaments are irreversibly ruptured. The polyelectrolyte properties of IFs may enable crossbridging by multivalent counterions, but identifying the mechanisms by which IFs link into bundles and networks in vivo remains a challenge.
Article: Properties of intermediate filament networks assembled from keratin 8 and 18 in the presence of Mg²⁺.[show abstract] [hide abstract]
ABSTRACT: The mechanical properties of epithelial cells are modulated by structural changes in keratin intermediate filament networks. To investigate the relationship between network architecture and viscoelasticity, we assembled keratin filaments from recombinant keratin proteins 8 (K8) and 18 (K18) in the presence of divalent ions (Mg(2+)). We probed the viscoelastic modulus of the network by tracking the movement of microspheres embedded in the network during assembly, and studied the network architecture using scanning electron microscopy. Addition of Mg(2+) at physiological concentrations (<1 mM) resulted in networks whose structure was similar to that of keratin networks in epithelial cells. Moreover, the elastic moduli of networks assembled in vitro were found to be within the same magnitude as those measured in keratin networks of detergent-extracted epithelial cells. These findings suggest that Mg(2+)-induced filament cross-linking represents a valid model for studying the cytoskeletal mechanics of keratin networks.Biophysical Journal 07/2012; 103(2):195-201. · 3.65 Impact Factor
Article: Divalent cations crosslink vimentin intermediate filament tail domains to regulate network mechanics.[show abstract] [hide abstract]
ABSTRACT: Intermediate filament networks in the cytoplasm and nucleus are critical for the mechanical integrity of metazoan cells. However, the mechanism of crosslinking in these networks and the origins of their mechanical properties are not understood. Here, we study the elastic behavior of in vitro networks of the intermediate filament protein vimentin. Rheological experiments reveal that vimentin networks stiffen with increasing concentrations of Ca(2+) and Mg(2+), showing that divalent cations act as crosslinkers. We quantitatively describe the elastic response of vimentin networks over five decades of applied stress using a theory that treats the divalent cations as crosslinkers: at low stress, the behavior is entropic in origin, and increasing stress pulls out thermal fluctuations from single filaments, giving rise to a nonlinear response; at high stress, enthalpic stretching of individual filaments significantly modifies the nonlinearity. We investigate the elastic properties of networks formed by a series of protein variants with stepwise tail truncations and find that the last 11 amino acids of the C-terminal tail domain mediate crosslinking by divalent ions. We determined the single-filament persistence length, l(P) approximately 0.5 mum, and Young's modulus, Y approximately 9 MPa; both are consistent with literature values. Our results provide insight into a crosslinking mechanism for vimentin networks and suggest that divalent ions may help regulate the cytoskeletal structure and mechanical properties of cells.Journal of Molecular Biology 05/2010; 399(4):637-44. · 4.00 Impact Factor
Conference Proceeding: Interactive cell injection simulation based on 3D biomechanical tensegrity model.[show abstract] [hide abstract]
ABSTRACT: To facilitate training of biological cell injection operations, we are developing an interactive virtual environment to simulate needle insertion into biological cells. A key challenge of deformable simulation is to satisfy the conflicting requirements of real-time interactivity and physical realism. This paper presents methodologies for dynamic modeling, visual/haptic display and model validation of cell injection. We first investigate the challenging issues in the modeling of the bio-mechanical properties of living cells. We propose a dynamic model to simulate cell deformation and puncture. The developed approach is based on the assumptions that the mechanical response of living cells is mainly determined by the cytoskeleton and that the cytoskeleton is organised as a tensegrity structure including microfilaments, microtubules and intermediate filaments. Equivalent microtubules struts are represented with a linear mass-tensor finite element model and equivalent microfilaments and intermediate filaments with viscoelastic Kelvin-Voigt elements. The virtual environment has been implemented with both graphic and haptic interfaces.2008 IEEE/RSJ International Conference on Intelligent Robots and Systems, September 22-26, 2008, Acropolis Convention Center, Nice, France; 01/2008