Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J48: Focus Session: Physics of Cellular Organization I |
Hide Abstracts |
Sponsoring Units: DBIO Chair: Michael Gramlich, Unviersity of Massachusetts, Amherst Room: 217C |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J48.00001: Collective Dynamics of Dividing Chemotactic Cells Anatolij Gelimson, Ramin Golestanian The large scale behaviour of a population of cells that grow and interact through the concentration field of the chemicals they secrete is studied using dynamical renormalization group methods. The combination of the effective long-range chemotactic interaction and lack of number conservation leads to a rich variety of phase behaviour in the system, which includes a sharp transition from a phase that is controlled by a weakly coupled perturbatively accessible fixed point to a phase controlled by a nonaccessible strong coupling fixed point. For a range of parameters, the perturbatively accessible fixed point has nontrivial critical exponents. [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J48.00002: Nature's rheologists: Lymphatic endothelial cells control migration in response to shear stress Gerald Fuller, Alex Dunn, Vinay Surya Endothelial cells (ECs) line the inner surface of blood and lymphatic vessels and are sensitive to fluid flow as part of their physiological function. EC organization, migration and vessel development are profoundly influenced by shear stresses, with important implications in cardiovascular disease and tumor metastasis. How ECs sense fluid flow is a central and unanswered question in cardiovascular biology. We developed a high-throughput live-cell flow chamber that models the gradients in wall shear stress experienced by ECs in vivo. Live-cell imaging allows us to probe cellular responses to flow, most notably EC migration, which has a key role in vessel remodeling. We find that most EC subtypes, including ECs from the venous, arterial, and microvascular systems, migrate in the flow direction. In contrast, human lymphatic microvascular ECs (hLMVECs) migrate against flow and up spatial gradients in wall shear stress. Further experiments reveal that hLMVECs are sensitive to the magnitude, direction, and the local spatial gradients in wall shear stress. Lastly, recent efforts have aimed to link this directional migration to spatial gradients in cell-mediated small molecule emission that may be linked to the gradient in wall shear stress. [Preview Abstract] |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J48.00003: Obstructions Inhibit Long-range Motor Motility in Microtubule Bundles M.W. Gramlich Efficient cellular transport along the cytoskeletal network is essential for cell growth and maintenance. Everything from microtubules to plasma membrane components are transported along the cytoskeletal network. Long-range transport is accomplished by molecular motors carrying cargo along a microtubule network. Recently, the role of the microtubule bundle geometry has begun to be explored. Microtubules bundle together in order to efficiently direct transport. Consequently, bundled microtubules introduce a new set of parameters which affect cellular transport, such as bundle spacing or microtubule polarity. Even previously tested parameters need to be re-considered, such as the role of obstructions. In this talk I will focus on the relationship between obstructions and microtubule polarity, and their affects on long-range transport. Microtubule polarity varies from completely uniform, with all plus-ends pointing in the same direction, to completely random. I will quantitatively show how obstructions inhibit long-range motor motility in any bundle, regardless of the distribution of microtubule polarity within the bundle. However, inhibition of long-range transport is greater in mixed polarity bundles. This result has implications for how cells use microtubule polarity to accommodate obstructions in order to efficiently direct transport. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J48.00004: Tackling the single molecule counting problem Steve Press\'e Protein-protein interactions -- that give rise to spatiotemporal organization in the cell -- are the basis for most biological information processing and cellular control. Quantitatively understanding these interactions is an essential prerequisite for developing mechanistic models of cell biology. However, there is currently no routine answer to ``how many proteins of type X are in this complex?'' in living cells. Here we discuss methods developed in our group (Geoff Rollins, Kostas Tsekouras) for tackling this ``single molecule counting problem'' starting from photobleaching data and data from a superresolution microscopy technique called PALM (PhotoActivated Localization Microscopy). [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J48.00005: C. elegans uses Liquid-Liquid Demixing for the Assembly of Non-Membrane-Bound Compartments Christoph A. Weber, Frank Juelicher, Andres Felipe Diaz Delgadillo, Louise Jawerth, Anthony A. Hyman P granules are liquid cytoplasmic RNA/Protein condensates known to determine the germ lineage in Caenorhabditis elegans. They resemble striking similarities with liquid droplets, such as dripping, shearing and wetting. Assuming that P granules are liquid-like we consider how they form in the crowded cytoplasm. Using confocal and light-sheet microscopy, P granule formation in-vivo and in-vitro is shown to share all hallmarks with a liquid-liquid phase-separation. Specifically, demixing is determined by temperature and concentration, the droplet formation is reversible with respect to temperature quenches and there is evidence for droplet growth due to coalescence and Ostwald-ripening. Liquid-liquid demixing in-vivo breaks the paradigmatic view that a molecular machinery is necessary to build up organelles through complex biological pathways. Instead we propose that P granules form following a Flory-Huggins model. Liquid-liquid demixing could also serve as a mechanism for the assembly of non-membrane-bound compartments in other living organisms. [Preview Abstract] |
(Author Not Attending)
|
J48.00006: Testing Turing's Theory of Morphogenesis in Chemical Cells Nathan Tompkins, Ning Li, Camille Girabawe, Michael Heymann, G. Bard Ermentrout, Irving Epstein, Seth Fraden Alan Turing's 1952 paper ``The Chemical Basis of Morphogenesis'' described how reaction-diffusion dynamics could create six spatiotemporal patterns including a stationary pattern that could lead to physical morphogenesis (which now bears his name). This stationary ``Turing pattern'' has been observed in continuous media of various chemical systems but never in diffusively coupled discrete reactors as Turing theorized. We have created a system of microfluidically produced chemical compartments containing the Belousov-Zhabotinsky reaction that are designed to fulfill the assumptions of Turing's theoretical system. This system demonstrates all six spatiotemporal patterns that Turing predicted. In particular, we observe the stationary case that bears Turing's name where the cells create a pattern of oxidized and reduced states. As Turing predicted, this chemical heterogeneity gives rise to physical heterogeneity by driving an osmotic flow, swelling the reduced cells and shrinking the oxidized cells. In addition to the six patterns and physical morphogenesis predicted by Turing we observe a seventh pattern of mixed stationary/oscillatory states that is not predicted by Turing. This seventh pattern requires modifying Turing's theory to include slight heterogeneity to match experiments. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J48.00007: Specific Adhesion of Lipid Membranes Can Simultaneously Produce Two Types of Lipid and Protein Heterogeneities Orrin Shindell, Natalie Micah, Max Ritzer, Vernita Gordon Living cells adhere to one another and their environment. Adhesion is associated with re-organization of the lipid and protein components of the cell membrane. The resulting heterogeneities are functional structures involved in biological processes. We use artificial lipid membranes that contain a single type of binding protein. Before adhesion, the lipid, protein, and dye components in the membrane are well-mixed and constitute a single disordered-liquid phase (L$_{\mathrm{d}})$. After adhesion, two distinct types of heterogeneities coexist in the adhesion zone: a central domain of ordered lipid phase that excludes both binding proteins and membrane dye, and a peripheral domain of disordered lipid phase that is densely packed with adhesion proteins and enriched in membrane dye relative to the non-adhered portion of the vesicle. Thus, we show that adhesion that is mediated by only one type of protein can organize the lipid and protein components of the membranes into heterogeneities that resemble those found in biology, for example the immune synapse. [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J48.00008: Viscoelastic properties of actin networks influence material transport Samantha Stam, Kimberly Weirich, Margaret Gardel Directed flows of cytoplasmic material are important in a variety of biological processes including assembly of a mitotic spindle, retraction of the cell rear during migration, and asymmetric cell division. Networks of cytoskeletal polymers and molecular motors are known to be involved in these events, but how the network mechanical properties are tuned to perform such functions is not understood. Here, we construct networks of either semiflexible actin filaments or rigid bundles with varying connectivity. We find that solutions of rigid rods, where unimpeded sliding of filaments may enhance transport in comparison to unmoving tracks, are the fastest at transporting network components. Entangled solutions of semiflexible actin filaments also transport material, but the entanglements provide resistance. Increasing the elasticity of the actin networks with crosslinking proteins slows network deformation further. However, the length scale of correlated transport in these networks is increased. Our results reveal how the rigidity and connectivity of biopolymers allows material transport to occur over time and length scales required for physiological processes. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J48.00009: Regulation of kinesin-transport by microtubule age and polymerization conditions Jing Xu, Winnie Liang, Stephen King, K. Faysal Microtubules are fundamental biopolymers in cells, formed via self-assembly (``polymerization'') of tubulin dimers. Microtubule polymerization conditions have been shown to alter the presence of defects in microtubule lattices, including point defects (missing tubulin dimers) and line defects (protofilament disruption). Potential impact of these lattice defects on molecular motor-based transport is not yet understood. Here we investigate the impact of microtubule polymerization conditions on multiple-kinesin transport, using single-molecule-type optical trapping experiments. We find that kinesin-based cargoes pause preferentially at specific locations along individual microtubules, and that the pause frequency and duration is strongly dependent on microtubule age and polymerization condition. Within each polymerization condition and for fresh microtubules, we also observe significant variations in multiple-kinesin travel distances, depending on which microtubules the motors travel along. Taken together, our study suggests an important role of microtubule lattice defect in regulating intracellular transport. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:54PM |
J48.00010: Heterogeneity in motor driven transport Invited Speaker: Ali Tabei I will discuss quantitative analysis of particle tracking data for motor driven vesicles inside an insulin secreting cell. We use this method to study the dynamical and structural heterogeneity inside the cell. I will discuss our effort to explain the origin of observed heterogeneity in intracellular transport. Finally, I will explain how analyzing directional correlations in transport trajectories reveals self-similarity in the diffusion media. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J48.00011: Exploration of locomotion in the ParA/ParB system Lavisha Jindal, Eldon Emberly In many bacteria the ParA/ParB system is responsible for actively segregating DNA during replication. ParB precessively moves by hydrolyzing DNA bound ParA-ATP forming a depleted ParA region in its wake. Recent in-vitro experiments have shown that a ParB covered bead can traverse a ParA bound DNA substrate. It has been suggested that the formation of a gradient in ParA leads to diffusion-ratchet like motion of the ParB bead but its origin and potential consequences requires investigation. We have developed a deterministic model for the in-vitro ParA/ParB system and show that any amount of spatial noise in ParA can lead to the spontaneous formation of its gradient. The velocity of the bead is independent of this noise but depends on the scale over which ParA exerts a force on the bead and the scale over which ParB hydrolyzes ParA from the substrate. There is a particular ratio of these scales at which the velocity is a maximum. We also explore the effects of cooperative vs independent rebinding of ParA to the substrate. Our model shows how the driving force for ParB originates and highlights necessary conditions for directed motion in the in-vitro system that may provide insight into the in-vivo behaviour of the ParA/ParB system. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J48.00012: Pulse Dynamics in Endocytic Protein Patches Anders Carlsson, Xinxin Wang During the process of endocytosis in yeast, submicron-sized protein patches assemble, exert forces on the membrane to bend it, and finally disassemble. The patches contain an initial coat that establishes the endocytic site and binds cargo, polymers of the protein actin, ``nucleation-promoting factors'' (NPFs) that catalyze actin polymerization, and curvature-generating proteins. We model the dynamics of protein patches in yeast using a variant of the activator-inhibitor ``Fitzhugh-Nagumo'' model. We treat NPFs as the activator, and polymerized actin as the inhibitor, on the basis of findings that the lifetime of NPF patches is extended when actin polymerization is inhibited. Using this model, we find that as the polymerization rate is reduced, there is a discontinuous transition from protein pulses to persistent patches. We also find, surprisingly, that in some parameter regimes reducing the polymerization rate can increase the polymerized-actin content of the patch. We present data for NPF dynamics budding yeast, which confirm some of the predictions of the model. [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J48.00013: Analysis of protein dynamics in the pericellular matrix Jan Scrimgeour, Dylan Young The pericellular matrix (PCM) is a low density, hydrated polymer coating that extends into the extracellular space from the surface of many living cells. The PCM controls access to cell and tissue surfaces, regulating a diverse set of processes from cell adhesion to protein transport and storage. The cell coat consists of a malleable backbone - the large polysaccharide hyaluronan (HA) - with its structure, its material properties, and its bio-functionality tuned by a diverse set of HA binding proteins. These proteins add charge, cross-links and growth factor-like ligands into the brush. Dynamic interactions between the HA and its binding proteins can be observed using single particle tracking in a fluorescence microscope. The resulting single molecule trajectories can contain evidence of site hoping, with the proteins dynamically moving between different states of motion as they bind and unbind from the HA. Here, we present an evaluation of hidden Markov models for the analysis of such multi-mobility trajectories. Simulated trajectories are used to probe the limits of this approach for molecular trajectories of limited length and the results are used to inform the design of particle tracking experiments. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700