Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E36: Physics of the Cytoskeleton Across Scales IInvited
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Sponsoring Units: DBIO Chair: Megan Valentine, University of California, Santa Barbara Room: BCEC 205C |
Tuesday, March 5, 2019 8:00AM - 8:36AM |
E36.00001: Controlling Epithelial Cell Shape Invited Speaker: Margaret Gardel I will discuss my lab’s recent work to study the biophysical mechanisms regulating control of cell shape in epithelial tissue. In particular, we have used optogenetics to locally regulate Rho activity at cell junctions to uncover how local Rho pulses drive junction contraction and stabilization via a ratcheting mechanism. We find that at short timescales, Rho activation drives junction contraction that reverses upon Rho reduction. Sustained RhoA activity shows a similar initial rapid contraction followed by a slow contractile phase. Upon removal of RhoA activation, the junction does not fully recover back to its original length, similar to junctional ratcheting observed in vivo. We find that this ratchet is dependent on both trans E-cadherin interactions and Formin activity. To understand these data, we model junction length in response to variable tension. In contrast to existing vertex models, our data argues that a junction has a rest length that is determined by the force-dependent junctional remodeling under acute periods of tension. This model then predicts that the slow contractile phase will eventually saturate to limit the amount of junction length changes and this is indeed what we see experimentally. We can overcome this saturation by inducing multiple activation periods, recapitulating junctional ratcheting seen in vivo. Altogether, these data provide insight into the underlying molecular and biophysical mechanisms of junction length changes seen in development. |
Tuesday, March 5, 2019 8:36AM - 9:12AM |
E36.00002: Posttranslational modification of microtubule mechanics Invited Speaker: Loren Hough Microtubules are built from tubulin dimers, which contain an intrinsically disordered C-terminal tail that makes up approximately 4% of the mass of a dimer. These tails coat the microtubule surface, and affect microtubule bending (the persistence length) and binding of other proteins. C-terminal tails are post-translationally modified by addition of chains of glycine or glutamate residues. We have developed methods for purifying tubulin with distinct levels of these modifications to study how tail modification affects microtubule behavior. We combine nuclear magnetic resonance spectroscopy with fluorescence measurements of the microtubule bending rigidity to determine the atomic and mechanical effects of varying post-translational modifications. Remarkably, we find that posttranslational modification that adds a small number of additional residues to the tubulin tail can lead to increase the microtubule persistence length by almost 50%. |
Tuesday, March 5, 2019 9:12AM - 9:48AM |
E36.00003: Active Stresses and Stress Relaxation in Cytoskeletal Networks Invited Speaker: Fred C. MacKintosh Living systems must operate out of thermodynamic equilibrium at the molecular scale. This lack of equilibrium results in directed fluxes through chemical states or phase space, corresponding to violations of the principle of detailed balance at the molecular scale. We explore consequences of such active, nonequilibrium dynamics at the meso and macro scales, with particular emphasis on cytoskeletal networks, which are driven by a variety of internal stresses. These stresses are often due to molecular motors, such as myosin that generates tension in the actin networks. These stresses can result in active remodeling of the networks and self organization of stress, particularly in networks near marginal stability. Active processes can also govern stress relaxation in such networks, through treadmilling and severing of filaments. We show how severing can lead to a surprising molecular weight independent relaxation in actin gels. We also show how transient crosslinking, together with prestress can lead to very slow, glassy relaxation in actin gels. Moreover, transient nonequilibrium (un)binding can also generate contractile stress in the absence of molecular motors. |
Tuesday, March 5, 2019 9:48AM - 10:24AM |
E36.00004: Dynamics and mechanics of actin cytoskeleton networks in vitro and in cellulo Invited Speaker: Olivia Du Roure Actin is a protein which self-assembles into highly dynamic filaments organized, within the cells, in different kinds of structures: bundled, crosslinked, contractile or branched networks. Actin interacts with a large number of partners either regulating the meshwork dynamics (polymerization, depolymerization and contractility) or the meshwork architecture (bundling, branching or crosslinking proteins). The architecture dictates the mechanics of the network and combines to its dynamic properties to enable the cell to achieve essential processes like migration, deformation or integration of external signals. I will show in this talk how we take advantage of the self-assembly of magnetic micro-particles to study simultaneously dynamic and mechanical properties of different actin networks. Results on actin cortex directly in living cells as well as on in vitro Arp2/3 actin networks resembling the ones at play in lamellipodia or in endocytosis will be discussed. |
Tuesday, March 5, 2019 10:24AM - 11:00AM |
E36.00005: What can we learn from self-organization for the understanding of the spatio-temporal dynamics of the cytoskeleton Invited Speaker: Eberhard Bodenschatz The response of the actin cytoskeleton to external chemical stimuli plays a fundamental role in numerous cellular functions. One of the key players that governs the dynamics of the actin network is the motor protein myosinII. Here we report on experiments and modeling on the interplay between myosinII and filamentous actin in Dictyostelium discoideum. In chemotactically compentent cells, upon uniform stimulation with the chemoattractant cAMP, myosinII is first released from the cortical region and then translocated back to the cortex with a time delay relative to the rapid increase of cortical actin. After the initial release of myosinII, the freshly formed filamentous actin pushed the membrane outward resulting in a change of cell morphology. By comparing with myosinII-null cells, we found that in the last stage of the cell response the coupling between myosinII and actin determines contraction dynamics. I shall also report on our approaches using phase field modeling. |
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