Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X51: Cell wall organization, growth and mechanicsFocus
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Sponsoring Units: DBIO Chair: David Quint, Univ of California - Merced Room: LACC 511C |
Friday, March 9, 2018 8:00AM - 8:36AM |
X51.00001: The role of microtubule organization in directional plant cell growth Invited Speaker: Jelmer Lindeboom This abstract not available. |
Friday, March 9, 2018 8:36AM - 8:48AM |
X51.00002: Coupled experimental and computational study of the interplay of mechanical properties and chemical signaling in stem cell patterns in plants. Mikahl Banwarth-Kuhn, Ali Nematbakhsh, Stephen Snipes, Venugopala Reddy, Mark Alber One of the central problems in developmental biology is the determination of how chemical and mechanical signals interact within a tissue to produce the final form, size and function of an organ. Cell walls in plants impose a unique constraint since cells are under turgor pressure and cannot move relative to one another. Cell wall extensibility and distribution of stress on the wall contribute to determining rates of cell expansion and orientation of cell division. How cell wall mechanical properties influence cell behavior is still largely unknown. First, a novel, multi-scale, computational model of the development of the shoot apical meristem (SAM) of Arabidopsis is described and calibrated using experimental data. Biologically relevant features of the model include separate representations of the cell wall and cytoplasm and a detailed description of extensibility and distribution of stress on the cell wall. Model predictive simulations reveal relative impacts of cell wall extensibility, distribution of stress, and chemical signals on growth rate and division plane orientation in the SAMs resulting in better understanding of the relationship between local morphogenetic processes and global tissue patterns in stem cell maintenance and differentiation. |
Friday, March 9, 2018 8:48AM - 9:00AM |
X51.00003: Computational models of the role of pectins in plant cell wall structure Peter Williams, Adam Saffer, Vivian Irish, Mark Shattuck, Corey O'Hern While it is established that pectins play a significant role in the structure of plant cell walls, little is known about the mechanisms pectins use to control cell architecture and growth. Recent experiments have shown that mutations in Rhamnose Biosynthesis 1 in A. thaliana give rise to a cellular phenotype characterized by a left-handed helical twist. These mutations suppress the synthesis of the pectic polysaccharide, rhamnogalacutonan I (RG-I). Mutations corresponding to other pectic polysaccharides fail to produce chiral phenotypes. It has also been widely observed that cellulose nanocrystals form left-handed chiral nematic phases. We hypothesize that the affinity of RG-I with cellulose microfibrils prevents the formation of a helical phase in wildtype cells. We have developed a multi-scale modeling approach to probe this system. Our all-atom models are able to recapitulate the backbone dihedral angles of cell wall polysaccharides and assess how efficiently pectins and cellulose intertwine and pack. To understand the formation of the left-handed helical twist of the mutants, we perform MD simulations of coarse-grained cellulose nanocrystals and explore the self-assembly of left-handed chiral nematic phases over a range of concentrations of pectic polysaccharides. |
Friday, March 9, 2018 9:00AM - 9:12AM |
X51.00004: Mechanical feedback coordinates cell wall expansion and assembly in yeast mating morphogenesis Samhita Banavar, Carlos Gomez, Michael Trogdon, Linda Petzold, Tau-Mu Yi, Otger Campas The shaping of individual cells requires a tight coordination of cell mechanics and growth. However, it is unclear how information about the mechanical state of the wall is relayed to the molecular processes building it, thereby enabling the coordination of cell wall expansion and assembly during morphogenesis. Combining theoretical and experimental approaches, we show that a mechanical feedback coordinating cell wall assembly and expansion is essential to sustain mating projection growth in budding yeast (Saccharomyces cerevisiae). Our theoretical results indicate that the mechanical feedback provided by the Cell Wall Integrity pathway, with cell wall stress sensors Wsc1 and Mid2 increasingly activating membrane-localized cell wall synthases Fks1/2 upon faster cell wall expansion, stabilizes mating projection growth without affecting cell shape. Experimental perturbation of the osmotic pressure and cell wall mechanics, as well as compromising the mechanical feedback through genetic deletion of the stress sensors, leads to cellular phenotypes in agreement with the theoretical predictions. Our results show that while the existence of mechanical feedback is essential to stabilize mating projection growth, the shape and size of the cell are insensitive to the feedback. |
Friday, March 9, 2018 9:12AM - 9:48AM |
X51.00005: For Whom the Cell Tolls: Regulation of Bacterial Growth and Division by Turgor Pressure Invited Speaker: Enrique Rojas A central goal of systems biology is to elucidate the pathways by which information is communicated within cells in order to control subcellular processes. While research often focuses on genetic and enzymatic pathways, I will highlight three novel paradigms by which microbes use physical factors, including mechanical forces and electric fields, as signals to control cell growth, division, and survival. First, I will describe how Gram-negative bacteria survive and grow robustly during mechanical perturbation by bearing mechanical forces in their outer membrane and by “storing” cell envelope synthesis during perturbation. Next, I will explain how the pathogenic bacterium Staphylococcus aureus harnesses internal hydrostatic pressure to drive sub-millisecond cell division. Finally, I will detail how Gram-positive bacteria use membrane tension and membrane depolarization as signals to ensure balanced synthesis of the plasma membrane and the cell wall. I will conclude by outlining an exciting roadmap for the future study of physical systems biology of microbes, including several questions that follow directly from my present research. |
Friday, March 9, 2018 9:48AM - 10:00AM |
X51.00006: How Bacteria Pop Felix Wong, Ariel Amir Membrane lysis, or rupture, is a cell death pathway in bacteria frequently caused by cell wall-targeting antibiotics. Although several studies have clarified some biochemical mechanisms of antibiotic action, a physical understanding of the processes leading to lysis remains lacking. Here, we model the dynamics of membrane bulging and lysis in Escherichia coli, where it has been observed that membrane bulging after cell wall digestion occurred on a characteristic timescale as fast as 100 ms. We show that bulging can be energetically favorable due to the relaxation of the combined stretching energies of the inner membrane, cell wall, and outer membrane and that experimentally observed bulge shapes, along with coarse-grained Monte Carlo simulations of pressurized fluid membranes, are consistent with model predictions. Our results elucidate the physics of cell envelope organization and may have implications for cellular morphogenesis and antibiotic discovery. |
Friday, March 9, 2018 10:00AM - 10:12AM |
X51.00007: Transient States with Long-Term Effects on Pattern Formation Jonas Denk, Simon Kretschmer, Jacob Halatek, Petra Schwille, Erwin Frey Protein patterning is vital for many cellular processes. A prototypical example is the bacterial Min system, where self-organized pole-to-pole oscillations of MinCDE proteins guide the cell division machinery to midcell. These oscillations are based on the cycling of the ATPase MinD and its activating protein MinE between the membrane and cytoplasm. Recent biochemical evidence suggests that MinE switches between a latent and reactive conformational state, dependent on MinD. Combining mathematical modeling and in vitro reconstitution of mutant proteins, we show that the MinD-dependent conformational switch of MinE is essential for patterns to emerge over a broad and physiological range of protein concentrations. Our results suggest that conformational switching of an ATPase activating protein can lead to the dynamic sequestration of its distinct functional states and thereby confer robustness to an intracellular protein network with vital roles in bacterial cell division. |
Friday, March 9, 2018 10:12AM - 10:24AM |
X51.00008: Dynamics of Escherichia coli’s passive response to a sudden decrease in external osmolarity Smitha Hegde, Renata Buda, Jin Yang, Yunxiao Liu, Fan Bai, Teuta Pilizota For most cells, a sudden decrease in external osmolarity results in fast water influx, which can burst the cell. To survive, cells rely on the passive response of mechanosensitive channels (Mscs), which open under increased membrane tension and allow the release of cytoplasmic solutes and water. Although the gating and the molecular structure of Mscs found in Escherichia coli have been extensively studied, the overall dynamics of the whole cell response remain poorly understood. |
Friday, March 9, 2018 10:24AM - 10:36AM |
X51.00009: Self-Consistent Field Theory For Virus Capsid Assembly Henri Orland, Siyu Li, Roya Zandi The Ground State Dominance Approximation(GSDA) has been extensively used to study the assembly of viral shells. In this work we employ the self-consistent field theory (SCFT) to investigate the adsorption of RNA onto the positively charged spherical viral shells and examine the validity of GSDA. We find that there are two regimes in which GSDA does work. First, when the genomic RNA length is long enough compared to the capsid radius, and second, when the interaction between the genome and capsid is so strong that the genome is localized next to the wall. In the case when RNA is more or less distributed uniformly in the shell, regardless of the length of RNA, GSDA is not a good approximation. We observe that as the polymer-shell interaction becomes stronger, the energy gap between the ground state and first excited state increases and thus GSDA becomes a better approximation. We also present our results corresponding to the genome persistent length obtained through the tangent-tangent correlation length in SCFT. |
Friday, March 9, 2018 10:36AM - 10:48AM |
X51.00010: A Self-Assembled Flagellated Bacterial Micropump Hiran Wijesinghe, Eric Mumper, Zachery Oestreicher, Zhixin Song, Christopher Pierce, Steven Lower, Brian Lower, Ratnasingham Sooryakumar Magnetotactic bacteria (MTB) are prokaryotic microorganisms that utilize innate lipid-bound chains of magnetic nanocrystals to orient their swimming direction in the weak (~0.5 Gauss) geomagnetic field. This magnetic guidance mechanism often reduces the search space in their quest for favorable environmental conditions. The present study is directed at developing a novel self-assembled flagellated bacterial micropump based on the response of the swarm dynamics to a sequence of designed magnetic fields. This microbial device is demonstrated on-chip through local constraints of oxygen due to the aerobic respiration of an MTB cluster. The fluid flow characteristics of the pump are modeled based on the cell response to ambient oxygen levels including the magneto-aerotactic polarity reversals and oxygen-dependent swimming speeds. The momentum transfer onto non-magnetic colloidal particles is described by considering the flagellar-driven hydrodynamic interactions, and experimentally verified using non-magnetic fluorescent nanoparticles. These findings, and the dynamics of the system including the role of oxygen diffusion, is numerically modeled. |
Friday, March 9, 2018 10:48AM - 11:00AM |
X51.00011: Memrane binding induced conformational changes in Marburg virus protein VP40 dimer Nisha Bhattarai, Bernard Gerstman, Prem Chapagain Marburg and Ebola viruses belong to the Filoviridae family and their infection causes hemorrhagic fever.The Marburg virus is a lipid enveloped virus and its viral matrix is formed by the matrix protein VP40. As in the Ebola VP40 (eVP40) dimer, the crystal structure of the Marburg VP40 (mVP40) dimer contains residues that form a basic patch at the membrane interface. However, compared to eVP40, the basic patch of mVP40 is significantly broader, suggesting differences between the dimers in plasma membrane (PM) localization. Using molecular dynamics simulations, we investigated the VP40 dimer interactions and the roles of various residues and lipid types in PM association as well as conformational changes. Despite the significant structural differences, mVP40 is found to adopt a configuration similar to eVP40 after associating with the membrane by reorienting the monomers in the dimer. We compared the structures of the mVP40 dimer in both lipid and lipid-free environments and found that the lipid interaction is needed for the observed conformational reorientation. Such conformational changes may be important for the mVP40 dimer to stabilize at the membrane surface and ensuring virus-like particle formation and budding from the plasma membrane. |
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