Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J40: Biological Physics II |
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Sponsoring Units: DBP Chair: Joshua Socolar, Duke University Room: 412 |
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J40.00001: Cell Rheology and Embryogenesis Using the Subcellular Element Model Invited Speaker: I will present recent work on grid-free computational modeling of both cell biomechanics and multicellular collective dynamics, the latter in the context of gastrulation in the chick embryo. Mechanics at both subcellular and multicellular scales is modeled seamlessly with the Subcellular Element Model (SEM). The SEM is able to capture basic viscoelastic properties of cells at a semi-quantitative level, and is efficient enough to simulate thousands of cells in three dimensions allowing computational analysis of biological hypotheses regarding collective cell motion during gastrulation. Work done in collaboration with Sebastian Sandersius, Arizona State University. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J40.00002: The effects of viscoelastic polymer substrates on adult stem cell differentiation Chungchueh Chang, Adam Fields, Alex Ramek, Vladimir Jurukovski, Marcia Simon, Miriam Rafailovich Dental Pulp Stem Cells (DPSCs) are known to differentiate in either bone, dentine, or nerve tissue by different environment signals. In this study, we have determined whether differentiation could only through modification of the substrate mechanics. Atomic Force Microscopy (AFM) on Shear Modulation Force Microscopy (SMFM) mode indicated that the spun-cast polybutadiene (PB) thin films could be used to provide different stiffness substrates by changing the thicknesses of thin films. DPSCs were then plated on these substrates and cultured in standard media. After 28 days incubation, Lasar Scanning Confocal Microscopy (LSCM) with mercury lamp indicated that the crystals were observed only on hard surfaces. The Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX analysis) indicated that the crystals are calcium phosphates. The Glancing Incidence Diffraction (GID) was also used to determine the structure of crystals. These results indicate that DPSCs could be differentiated into osteoblasts by mechanical stimuli from substrate mechanics. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J40.00003: Translocation of double strand DNA into a biological nanopore Sunita Chatkaew, Lamia Mlayeh, Marc Leonetti, Fabrice Homble Translocation of double strand DNA across a unique mitochondrial biological nanopore (VDAC) is observed by an electrophysiological method. Characteristics of opened and sub-conductance states of VDAC are studied. When the applied electric potential is beyond $\pm$ 20 mV, VDAC transits to a sub-conductance state. Plasmids (circular double strand DNA) with a diameter greater than that of the channel shows the current reduction into the channel during the interaction but the state with zero-current is not observed. On the contrary, the interaction of linear double strand DNA with the channel shows the current reduction along with the zero-current state. These show the passages of linear double strand DNA across the channel and the electrostatic effect due to the surface charges of double strand DNA and channel for circular and linear double strand DNA. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J40.00004: On-chip growth of polymeric nanowires for electro-mechanical probing of live cells Bret Flanders, Prem Thapa This study characterizes the directed electrochemical nanowire assembly of amorphous polythiophene nanowires on micro-electrode arrays. In this approach, a long range component of an applied voltage signal defines a channel of maximum flux in the laboratory reference frame. Amorphous wires lack a natural growth axis. However, because polymerization is restricted to the channel-region, such materials may be grown with wire-like geometries, and the growth path of these wires may be controlled. The wire-laden electrode arrays are useful substrates for cell physiological studies. To this end, non-invasive methodology for inducing single Dictyostelium cells to approach and attach individual pseudopods to the tips of the polymeric wires will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J40.00005: What do cell rheology experiments really measure? Invited Speaker: It is now widely appreciated that normal tissue morphology and function rely upon cells? ability to sense and generate forces appropriate to their correct tissue context. While the effects of forces on cells have been studied for decades, our understanding of how those forces propagate through and act on different cell substructures remains at an early stage. The last decade has seen a resurgence of interest, with a variety of different micromechanical methods in current use that probe cells dynamic deformation in response to a time varying force. Recently, it has been shown that the seemingly disparate findings from different labs can be fit into a single, workable consensus description. The ability of researchers to carefully measure the mechanical properties of cells subjected to a variety of pharmacological and genetic interventions, however, currently outstrips our ability to quantitatively interpret the data in many cases. Despite these challenges, the stage is now set for the development of detailed models for cell deformability, motility and mechano-sensing that are rooted at the molecular level. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J40.00006: Cargo transport by several molecular motors Stefan Klumpp, Melanie M\"uller, Janina Beeg, Rumiana Dimova, Reinhard Lipowsky In cells, cargoes are often transported by small teams of molecular motors rather than by a single motor. Furthermore, many cargoes perform bidirectional movements, which are based on the presence of two motor species on the cargo. We study the transport by several motors theoretically using a model that describes the stochastic binding and unbinding of motors from filaments and that is based on the properties of individual motors as observed in single molecule experiments. We find that the cooperation of several motors leads to a strongly increased run length, which is confirmed experimentally for beads pulled by several kinesin motors. Furthermore, such cargoes exhibit a non-linear force-velocity relation. For the case of two motor teams pulling into opposite direction we find that a stochastic tug-of-war model, where the motors interact only by pulling their common cargo into opposite directions, leads to surprisingly complex motility. In particular, even for two motor teams with equal strength, we find that a tug-of-war leads to fast bidirectional motion similar to what is observed in cells and usually taken as evidence for some unknown coordination mechanism. This behavior is due to a dynamic instability, which arises from the strong force-dependence of the rate with which motors unbind from filaments. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J40.00007: Adverse Effects of TiO$_{2}$ Nanoparticles on Human Dermal Fibroblasts and How to Protect Cells Zhi Pan, Wilson Lee, Lenny Slutsky, Sowmya Sandaresh, Nicole Elstein, Richard Clark, Nadine Pernodet, Miriam Rafailovich We have studied the effects of exposure of human dermal fibroblasts to rutile and anatase TiO$_{2}$ nanoparticles. We found that these particles can impair cell functions, with the latter being more potent at producing damage. We showed that the exposure to nanoparticles decreases cell area, cell proliferation, mobility, and ability to contract collagen. Individual particles are shown to penetrate easily through the cell membrane, in the absence of endocytosis, while some endocytosis is observed for larger particle clusters. Once inside, the particles are sequestered in vesicles, which continue to fill up with increasing incubation time till they rupture. We also tested particles that were coated with a dense grafted polymer brush and, using flow cytometry, showed that the coating prevented the particles from adhering to the cell membrane and hence penetrating the cell, which effectively decreases reactive oxygen species (ROS) formation and protects cells, even in the absence of light exposure. Considering the broad applications of these nanoparticles in personal health care products, the functionalized polymer coating can potentially play an important role in protecting cells and tissue from damage. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J40.00008: Surface Morphological Studies on Nerve Cells by AFM Goksel Durkaya, Lei Zhong, Vincent Rehder, Nikolaus Dietz Surface morphological properties of fixed and living nerve cells removed from the buccal ganglion of \textit{Helisoma trivolvis} have been studied by using Atomic Force Microscopy (AFM). Identified, individual neurons were removed from the buccal ganglion of \textit{Helisoma trivolvis} and plated into poly-L-lysine coated glass cover-slips. The growth of the nerve cells was stopped and fixed with 0.1{\%} Glutaraldehyde and 4{\%} Formaldehyde solution after extension of growth cones at the tip of the axons. Topography and softness of growth cone filopodia and overlying lamellopodium (veil) were probed by AFM. Information obtained from AFM's amplitude and phase channels have been used for determination of softness of the region probed. The results of structural studies on the cells are linked to their mechanical properties and internal molecular density distribution. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J40.00009: Study of Charge and Spin Distribution Properties in Five-Liganded Helogen-Heme Systems Archana Dubey, Minakhi Pujari, K. Ramani Lata, Alyssa Garcia, A.F. Schulte, S.R. Badu, R.H. Pink, R.H. Scheicher, T.P. Das The current emphasis in biological physics is on the study of the functions of important systems, like for instance hemoglobin and cytochromes at a quantitative level. For these studies an accurate knowledge of the electronic structures of the entire molecules as well as parts of them are very important. In the heme proteins there is great current interest in both electron transport and in attachment and detachment of O$_{2}$, CO, and NO molecules to the iron. For this purpose an in depth understanding at the electronic level of the heme units, the protein chains, and the interactions between the two, is vital. With these aims in mind, we have studied quantitatively at a first principles level the electronic structures of all four halogen five liganded heme compounds, the natures of the charge and spin distributions over them, and the associated hyperfine interactions of the nuclei of the atoms . Results and trends of these properties over the four systems and comparisons with available data will be presented and discussed. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J40.00010: Kalman meets neuron - the intersection of control theory and neuroscience Steven Schiff Since the 1950s, we have developed mature theories of modern control theory and computational neuroscience with almost no interaction between these disciplines. With the advent of computationally efficient nonlinear Kalman filtering techniques, along with improved neuroscience models which provide increasingly accurate reconstruction of dynamics in a variety of important normal and disease states in the brain, the prospects for a synergistic interaction between these fields are now strong. I will show recent examples of the use of nonlinear control theory for the assimilation and control of single neuron dynamics, a novel framework for dynamic clamp, the modulation of oscillatory wave dynamics in brain cortex, a control framework for Parkinsonian dynamics and seizures, and the use of optimized parameter model networks to assimilate complex network data. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J40.00011: Protein Thermodynamics from Maxwell Constraint Counting Donald Jacobs, Dennis Livesay, Oleg Vorov Topological properties of network rigidity explain essential aspects of structural phase transitions and thermodynamic stability in proteins [1]. We present an exact transfer matrix method within a Distance Constraint Model (DCM) that maps interactions into distance constraints having energy and entropy contributions. Conformational entropy is reduced by interactions that rigidify structure, associated with independent constraints. Here, we solve the DCM using a mean-field treatment that assumes distance constraints are well distributed throughout the structure, meaning a distance constraint is independent until the structure is globally rigid. Experimental heat capacity curves are described markedly well with a few adjustable parameters. The universal character of this theory is analogous to the van der Waals model for a liquid-gas transition. This work is supported by NIH R01 GM073082. [Preview Abstract] |
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