Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session X34: Shape Changes in Biological Membranes |
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Sponsoring Units: DBP Chair: Peter Jung, Ohio University Room: Colorado Convention Center 404 |
Friday, March 9, 2007 8:00AM - 8:12AM |
X34.00001: Polyunsaturated Fatty Acids in Lipid Bilayers and Tubules Linda S. Hirst, Jing Yuan, Yohannes Pramudya, Lam T. Nguyen Omega-3 polyunsaturated fatty acids (PUFAs) are found in a variety of biological membranes and have been implicated with lipid raft formation and possible function, typical molecules include DHA (Docosahexanoic Acid) and AA (Alphalinoleic Acid) which have been the focus of considerable attention in recent years. We are interested in the phase behavior of these molecules in the lipid bilayer. The addition of lipid molecules with polyunsaturated chains has a clear effect on the fluidity and curvature of the membrane and we investigate the effects the addition of polyunsaturated lipids on bilayer structure and tubule formation. Self-assembled cylindrical lipid tubules have attracted considerable attention because of their interesting structures and potential technological applications. Using x-ray diffraction techniques, Atomic Force Microscopy and confocal fluorescence imaging, both symmetric and mixed chain lipids were incorporated into model membranes and the effects on bilayer structure and tubule formation investigated. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X34.00002: Dynamics of Encapsulation and Budding in Lipid Membranes Kurt Smith The behavior of lipid membranes is important in cell biology, as well as in the development of synthetic vesicles for drug delivery and other applications. The fundamental role of the membrane is to control the passage of matter into and out of a cell or vesicle. We have examined two related processes - the encapsulation of a particle by an adhesive membrane (as in endocytosis) and the budding and vesiculation of a phase separated membrane domain. These processes require changes in membrane topology (i.e. pinch-off) which involve molecular-scale rearrangements. Thus they cannot be fully understood through a macroscopic free energy formulation. Using dissipative particle dynamics, we examine the pathway through which pinch-off occurs, and find that it depends upon the nucleation of a pore at the membrane neck. We use simulations to predict the range of conditions under which pinch-off is possible. [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X34.00003: Lipid tubules Formed by Flow-Controlled Hydration Jing Yuan, Linda S. Hirst Self-assembled cylindrical tubules from lipid molecules have attracted considerable attention because of their interesting supramolecular structures and technological applications. Schnur et al. [1] reported the formation of tubular microstructures from a series of diacetylenic phospholipids after liposomes were cooled through their chain melting transition. After that, several methods have been developed to fabricate such unique microstructures mainly by means of deforming preformed Giant unilamellar vesicles. Here we present a simple strategy to construct lipid microtubules through a flow-controlled lipid hydration. Fluorescent microscopy and Confocal Laser Microscopy were used to visualize the formation and the structure of the lipid tubules. Tubules were found to develop following the direction of the dynamic flow with highly parallel alignment. At high flow speeds, partial cross-linking of the lipid tubules was observed. To demonstrate the generality of this method, different types of phospholipids, such as Phosphatidic Acid (PA), Phosphatidylserine (PS), Phosphatidylethanolamine (PE), and Phosphatidylglycerol (PG) were investigated. \newline \newline [1] J.M. Schnur et al, Science, 264, 945 (1994). [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X34.00004: Spontaneous Formation of Lipid Nanotubes and Lipid Nanofibers from Giant Charged Dendrimer Lipids Alexandra Zidovska, Kai K. Ewert, Cyrus R. Safinya, Joel Quispe, Bridgett Carragher, Clinton S. Potter Liposomes have attracted much scientific interest due to their applications in model cells studies and in drug encapsulation. We report on the discovery of new vesicle phases formed in mixtures of MVLBG2, DOPC and water. MVLBG2 is a newly synthesized highly charged (16+) lipid (K. Ewert et al., \textit{JACS, }2006) with giant dendrimer headgroup thus leading to a high spontaneous curvature of the molecule. In combination with zero-curvature DOPC, MVLBG2 exhibits a rich phase diagram showing novel vesicle morphologies such as bones, lipid nanotubes and nanofibers as revealed by differential contrast microscopy (DIC) and cryo-TEM. At the micron scale DIC reveals a new phase consisting of bone-like vesicles. This novel morphology persists down to the nanometer scale as shown by cryo-TEM. The nanotubes are of diameter 10-50 nm, length $>$ 1$\mu $m and consist of a single lipid bilayer. A surprising new morphology arises resulting from a spontaneous topological transition from tubes to lipid nanorods. Funded by DOE DE-FG-02-06ER46314, NIH GM-59288, NSF DMR-0503347. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X34.00005: Hydrodynamic extrusion of membrane nanotubes: the role of the cytoskeleton Karine Guevorkian, Nicolas Borghi, S\'{e}bastien Kremer, Axel Buguin, Fran\c{c}oise Brochard We have investigated membrane-cytoskeleton adhesion properties by extrusion of tubes from tethered vesicles and cells using hydrodynamic flows. Our experimental results show that impermeable membranes (giant vesicles) act as entropic springs, i.e. the extruded tubes reach a stationary length, whereas porous membranes (vesicles decorated with pores) lead to tubes, which extrude at constant velocity without reaching a stationary length. On the other hand, experiments on red blood cells (RBC) suggest that the dynamics of extruded tubes is dominated by the detachment of the membrane from the cytoskeleton and the flow of lipids through the binding membrane proteins. We have estimated the membrane-cytoskeleton binding energy and the viscosity of the membrane for RBC-s. Tube extrusion from other cell types (S180, MDCK, BON) show phenomena such as healing time for the membrane-cytoskeleton rebinding, and cell aging (breakage of the tube after a few consecutive extrusions). We will discuss how these phenomena depend on the properties of the cytoskeleton and on the presence of cell adhesion molecules. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X34.00006: Shape transformations of active tubular membranes Elnaz Alipour-Assiabi, Thomas Powers Motivated by the action of enzymes that flip lipid molecules from one monolayer to another in a lipid bilayer membrane, we study shape instabilities of a tubular membrane driven by lipid-flipping. We begin with the instability of a tube with a fixed lipid number density distance, determining the relative importance of solvent viscosity, membrane viscosity, and bilayer friction. Then we consider the case of a uniform density of enzymes acting at a fixed rate. Implications for experiments will be discussed. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X34.00007: Effective surface tension of red blood cell membranes induced by cytoskeleton meshworks Rui Zhang, Frank Brown The membrane of red blood cell (RBC) consists of a lipid bilayer and a two dimensional cytoskeleton meshwork underneath. Its elastic properties are therefore different from a simple lipid bilayer. We introduced a simple entropic spring model to study the meshwork. In this model, adjacent nodes of the meshwork interact with each other through the link of an entropic spring. We run Monte Carlo and Brownian dynamics simulations, and developed some simple analytical theories to understand the simulation results. For a complete meshwork, we found that the cytoskeleton meshwork produced an effective surface tension to the RBC membrane, as far as the height fluctuation of the membrane is considered. This surface tension depends on the wave length of the fluctuation, and shows a crossover at the wave length of the average mesh size. We also studied the case when a fraction of randomly chosen links are disconnected from the nodes, possibly with the help of ATP. In this case, the surface tension changes with the fraction of connected links. Most interestingly, we found a percolation phase transition of the surface tension at long wave length limit. We discussed the experimental results related to our theory. Our model may improve the understanding of certain functions the RBC membrane related to its elastic properties. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X34.00008: Synthetic antimicrobial oligomers induce composition-dependent topological transition in membranes Lihua Yang, Vernita Gordon, Abhijit Mishra, Kirstin Purdy, John Cronan, Abhigyan Som, Gregory Tew, Gerard C.L. Wong Antimicrobial peptides comprise a key component of innate immunity for a wide range of multicellular organisms. Recently, their synthetic analogs have demonstrated broad-spectrum antimicrobial activity via permeating bacterial membranes selectively, although the precise molecular mechanism underlying the activity is still unknown. We systematically investigate interactions and self-assembled structures formed by model bacterial membranes and a prototypical family of phenylene ethynylene-based small molecule antimicrobials with controllable activity and selectivity. Synchrotron small angle x-ray scattering (SAXS) results correlate antibacterial activity and the induced formation of an inverted hexagonal phase, and indicate that the organization of negative curvature lipids such as DOPE are crucially important. Preliminary killing assays of DOPE-deficient mutant bacteria agree with the x-ray results. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X34.00009: Pore formation by antimicrobial peptides: structural tendencies in bulk and quasi-2D membrane systems Vernita Gordon, Lihua Yang, Matthew Davis, A. Som, G. Tew, Gerard Wong Antimicrobial peptides are cationic, amphiphilic structures that are key components of innate immunity. A prototypical family of synthetic analogs are the phenylene ethynylene antimicrobial oligomers (AMOs), which have hydrophobic alkyl chains connected to cationic hydrophilic regions. Synchrotron small-angle x-ray scattering (SAXS) shows that when AMO is mixed with concentrated model membranes, initially in the form of Small Unilamellar Vesicles, the sample forms the inverted hexagonal phase. This is a 3-dimensional phase characterized by a regular array of size-defined water channels. We demonstrate how this structural tendency is expressed when AMOs interact with dilute model membranes in the form of Giant Unilamellar Vesicles (GUVs). Using confocal microscopy, we see that applying AMO to the GUVs causes small encapsulated molecules to be released while large molecules are retained, indicating that size-defined pores have been created. Examining the partial release of polydisperse intermediately-sized molecules allows a closer measurement of the pore size, and there are indications that this single-vesicle microscopy will allow elucidation of the kinetics of the pore-forming process. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X34.00010: Role of membrane bending in ASAP1 protein activity Beatriz E. Burrola Gabilondo, Ruibai Luo, Wolfgang Losert, Paul A. Randazzo ASAP1 is part of the protein machinery that alters membranes and the actin cytoskeleton in cellular structures, called invadopodia, that mediate invasion of mammary cell carcinoma and uveal melanoma. The molecular mechanism by which ASAP1 contributes to these structures is not well defined. ASAP1 induces the hydrolysis of GTP that is bound to the protein Arf. Another activity is to deform lipid bilayers into tubules. We have set out to test the hypothesis that the enzymatic GAP activity is related to the mechanical activity. We contrast several reaction schemes for GAP activity, including steps that would be sensitive to physical changes in the membrane. We compare the numerical model predictions to data obtained from kinetics experiments. We are also developing assays such as FRET and tools like laser tweezer forcing of vesicle deformations to be used to determine the effect of ASAP1 and mutants with defects in enzymatic activity on the physical state of lipid vesicles. The ramifications of the results to the role of ASAP1 in invadopodia formation will be discussed. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X34.00011: Shape transformations of human red blood cells under osmotic deflation-inflation Gerald Lim, Michael Wortis We systematically study the mechanics of osmotically driven shape transformations of human red blood cells, based on a computational model we developed earlier that successfully describes the stomatocyte-discocyte-echinocyte shape transformations, which are driven by the bilayer couple mechanism. We obtain a surprisingly complex energy landscape, the prominent feature of which is a tricritical point that gives rise to self-intersection of the main minimum-energy surface in a line and shape transformations exhibiting hysteresis, metastability, and re-entry. These occur in physically accessible regions of parameter space and, thus, can be tested experimentally. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X34.00012: Dissipative Particle Dynamics Simulations of Deformable Red Blood Cells in Small Blood Vessels Igor Pivkin, Peter Richardson, George Karniadakis Explicit simulations of the blood cellular components require computational methods capable of tracking time-varying fluid-solid interface. The Dissipative Particle Dynamics (DPD) is an inherently adaptive method and potentially very effective in simulating complex fluid systems. In DPD, the fluid and solid objects are represented as a collection of interacting points, each representing a group of atoms or molecules. The red blood cell model takes into account bending and in-plane energies as well as constraints of constant surface and volume. We will present results of simulations of the deformable red blood cells in a small blood vessel using DPD. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X34.00013: Nucleation of holin domains and holes optimizes lysis timing of \textit{E. coli} by phage $\lambda$ Gillian Ryan, Andrew Rutenberg Holin proteins regulate the precise scheduling of \textit{Escherichia coli} lysis during infection by bacteriophage $\lambda$. Inserted into the host bacterium's inner membrane during infection, holins aggregate to form rafts and then holes within those rafts. We present a two-stage nucleation model of holin action, with the nucleation of condensed holin domains followed by the nucleation of holes within these domains. Late nucleation of holin rafts leads to a weak dependence of lysis timing on host cell size, though both nucleation events contribute equally to timing errors. Our simulations recover the accurate scheduling observed experimentally, and also suggest that phage-$\lambda$ lysis of \textit{E.coli} is optimized. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X34.00014: Physiological role of stochastic calcium signaling in subcellar microdomains Yohannes Shiferaw Calcium (Ca) plays an important role in regulating various cellular processes. In a variety of cell types, Ca signaling occurs within microdomains where Ca channels deliver localized pulses of Ca which activate a nearby collection of Ca sensitive receptors. The small number of channels in these microdomains ensures that the signaling process is stochastic. The aggregate response of several thousand of these micro-domains yields a whole cell response which dictates the observable cell behavior. Here, we study analytically the statistical properties of a population of these micro-domains in response to a trigger signal. We apply these results to understand the relationship between Ca influx and Ca release in cardiac myocytes. In particular, we explain why the global response is graded with respect to total Ca influx, even though Ca response at the micro-domain level is all-or-none. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X34.00015: A Systematic Study of Bilayer Failure on Engineered Surfaces Morgan Mager, Nicholas Melosh Ever since the invention of black lipid membranes, supported lipid bilayers have been an important tool for studying integral membrane proteins as well as fundamental bilayer behavior. In spite of this, these structures have a relatively short lifetime and little is known about their failure mechanisms. By systematically altering the geometry and surface chemistry of microfabricated pores, we are able to isolate the importance of several distinct failure mechanisms. These include pressure fluctuations, unsupported area, surface energy of the pore wall and surface roughness. We will also demonstrate that, even when not actively controlled, these parameters can inadvertently be altered depending in processing conditions. [Preview Abstract] |
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