Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P58: Self Organization in the CytoskeletonInvited
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Sponsoring Units: DBIO Chair: M. Betterton, University of Colorado - Boulder Room: LACC Petree Hall C |
Wednesday, March 7, 2018 2:30PM - 3:06PM |
P58.00001: Geometry of antiparallel microtubule bundles regulates relative sliding and stalling by PRC1 and Kif4A Invited Speaker: Radhika Subramanian Motor and non-motor crosslinking proteins play critical roles in determining the size and stability of microtubule-based architectures. Currently, we have a limited understanding of how the geometrical properties of microtubule arrays, in turn, regulate the output of the crosslinking proteins. Here we investigate this problem in the context of microtubule organization by two interacting proteins: the non-motor crosslinker PRC1 and the kinesin Kif4A. We show that the collective activity of both proteins results in both the relative sliding of antiparallel microtubules and the accumulation of PRC1 and Kif4A molecules at the plus-end of each microtubule (‘end-tag’). Sliding stalls when the end-tags on the two microtubules collide, resulting in the organization of a stable antiparallel overlap. Interestingly, we find that structural properties of the initial array regulate two aspects of PRC1-Kif4A mediated microtubule organization. First, sliding velocity scales with initial microtubule-overlap length. Second, the width of the final stable overlap scales with microtubule lengths. Our analyses provide insights into how the micron-scale geometrical features of antiparallel microtubule bundles can be decoded by nanometer-sized proteins to define the structure and mechanics of microtubule-based architectures. |
Wednesday, March 7, 2018 3:06PM - 3:42PM |
P58.00002: Mechanisms of Dynamic Stability in the Actomyosin Cytoskeleton Invited Speaker: Michael Murrell The actomyosin cytoskeleton is an active semi-flexible polymer network whose non-equilibrium behaviors coordinate both cell elasticity and fluidity to maintain or change cell shape. Unlike the induction of contractile flows, the maintenance of dynamic stability in highly labile yet internally pre-stressed active materials remains unknown. To this end, we synthesize a biomimetic active nematic liquid crystal from long semi-flexible actin filaments driven out-of-equilibrium by myosin motor activity. We identify diverse actomyosin interactions that govern the dynamic architecture and mechanical response of the network to active stresses. These responses include dynamic steady states, in which myosin reversibly bends actin filaments, whose curvatures show anomalous and strongly coupled fluctuations across a broad spectrum of filament bending modes. These fluctuations break detailed balance, enhance network elasticity, while maintaining dynamic stability. Furthermore, the actomyosin interactions that maintain dynamic stability are fundamentally distinct from those that drive contractile flows of actomyosin networks. |
Wednesday, March 7, 2018 3:42PM - 4:18PM |
P58.00003: A biophysical model for the formation of mitotic spindle bipolarity Invited Speaker: Robert Blackwell Mitotic spindles use an elegant bipolar architecture to segregate duplicated chromosomes with high fidelity. Bipolar spindles form from a monopolar initial condition; this is the most fundamental construction problem that the spindle must solve. Microtubules, motors, and cross-linkers are important for bipolarity, but the mechanisms necessary and sufficient for spindle assembly remain unknown. |
Wednesday, March 7, 2018 4:18PM - 4:54PM |
P58.00004: Mapping k-fiber load-bearing in the mammalian spindle reveals local anchorage that provides mechanical isolation and redundancy Invited Speaker: Mary Elting During cell division, a self-organizing microtubule-based machine called the mitotic spindle aligns and segregates chromosomes into two new cells. Forces transmitted through k-fiber microtubule bundles push and pull chromosomes to align them in the center of the cell. To oppose these forces, the spindle must robustly anchor its k-fibers, but where or how that anchorage occurs is not well-understood, nor are the mechanical properties of the k-fibers themselves. In part, this is due to the challenge of probing mechanics in live cells. To map load-bearing in the mammalian spindle across space and time, we laser ablate single k-fibers and quantify the immediate relaxation of chromosomes and surrounding microtubules and find only local load redistribution within a few microns of kinetochores. We describe a phenomenological model consistent with this behavior: dense, transient crosslinks that attach k-fibers to the spindle and bear load without limiting the timescale of spindle reorganization. We find that the microtubule crosslinker NuMA is required for wild-type levels of local load-bearing, whereas PRC1 and Eg5 are not. Local load-bearing provided by NuMA is well-suited to robust k-fiber anchorage that provides mechanical redundancy while permitting dynamic spindle rearrangements. |
Wednesday, March 7, 2018 4:54PM - 5:30PM |
P58.00005: Arcs, flows and waves: how the cytoskeleton shapes forces in immune cells Invited Speaker: Arpita Upadhyaya The activation of lymphocytes is an essential step in the adaptive immune response. Lymphocyte activation involves the binding of specialized receptors with antigen on the surface of antigen presenting cells. This leads to changes in cell morphology, large-scale reorganization and dynamics of the cytoskeleton and the movement and assembly of receptors and enzymes into signaling microclusters, which are critical for immune cell activation and the formation of the immune synapse. During this process, cells of the immune system interact with structures that possess a diverse range of physical properties. Coordinated dynamics of the acto-myosin and microtubule cytoskeleton enable the cell to respond these physical stimuli. I will summarize our recent studies that examine how T and B lymphocytes respond to physical cues such as stiffness, topography and ligand mobility. Specifically, I will highlight the distinct roles of the actin and microtubule cytoskeleton in the exertion of mechanical stresses that support signaling activation, receptor movement, microcluster assembly in lymphocytes. We find that traction forces generated by T cells are largely driven by actin dynamics but are also regulated by dynamic microtubules through Rho activation and non-muscle myosin II bipolar filament assembly at the interface between the T cell and antigen presenting surface. Force fluctuation analysis reveals two distinct sources of force generation in T cells and possible interplay between actin and microtubule dynamics. Our studies indicate that cytoskeletal forces may be important for receptor activation in T cells and influence their signaling activity. These findings establish the immune synapse as a mechanochemical module, which enables force generation during signaling activation via exquisitely coordinated spatiotemporal dynamics of cytoskeletal assemblies. |
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