Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V6: Cell Motility |
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Sponsoring Units: DBP Chair: Henrik Flyvbjerg, Riso National Laboratory Room: Colorado Convention Center 207 |
Thursday, March 8, 2007 11:15AM - 11:51AM |
V6.00001: Extremes in motility: actin acrobatics, spasmin spasms and jellyfish jabs Invited Speaker: Fast movements in biology are functionally relevant in the context of avoidance and capture. I will talk about some of the adaptations in biology that lead to speed at the cellular level in a variety of organisms, and then discuss three in some detail: the explosive motility of jellyfish stings, the fast contraction of some pond weeds, and the extrusion of an actin spring. In each case, the morphology and mechano-chemistry come together in unusual ways that are adapted for functionality. This leads to questions of both a comparative and an evolutionary nature, and serve to perhaps move these questions from the realm of stamp collecting to physiology and physics. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V6.00002: Natural descriptions of motor behavior: examples from \textit{E. coli} and \textit{C. elegans}. Invited Speaker: \textit{E. coli} has a natural behavioral variable - the direction of rotation of its flagellar rotorary motor. Monitoring this one-dimensional behavioral response in reaction to chemical perturbation has been instrumental in the understanding of how \textit{E. coli} performs chemotaxis at the genetic, physiological, and computational level. Here we apply this experimental strategy to the study of bacterial thermotaxis - a sensory mode that is less well understood. We investigate bacterial thermosensation by studying the motor response of single cells subjected to impulses of heat produced by an IR laser. A simple temperature dependent modification to an existing chemotaxis model can explain the observed temperature response. Higher organisms may have a more complicated behavioral response due to the simple fact that their motions have more degrees of freedom. Here we provide a principled analysis of motor behavior of such an organism -- the roundworm \textit{C. elegans}. Using tracking video-microscopy we capture a worm's image and extract the skeleton of the shape as a head-to-tail ordered collection of tangent angles sampled along the curve. Applying principal components analysis we show that the space of shapes is remarkably low dimensional, with four dimensions accounting for $>$ 95{\%} of the shape variance. We also show that these dimensions align with behaviorally relevant states. As an application of this analysis we study the thermal response of worms stimulated by laser heating. Our quantitative description of \textit{C. elegans} movement should prove useful in a wide variety of contexts, from the linking of motor output with neural circuitry to the genetic basis of adaptive behavior. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V6.00003: How Molecular Motors Shape the Flagellar Beat Invited Speaker: Cilia and eukaryotic flagella are slender cellular appendages whose regular beating drives fluid flows across epithelia and propels cells and microorganisms through aqueous media. The beat is an oscillating pattern of propagating bends generated by dynein motor proteins that induce sliding between adjacent axonemal microtubules. A key open question is how the activity of the motors is coordinated in space and time to produce the observed regular oscillatory beat pattern. We have developed a physical description of Flagellar dynamics based on the interplay of collective action of dynein motors and relative sliding of microtubules in two and three dimensions. To elucidate the nature of motor coordination, we have inferred the mechanical properties of the motors by analyzing the shape of beating sperm. Steadily beating bull sperm were imaged at a high frame rate and their shapes were measured with high precision using a Fourier averaging technique. We compared our experimental data with theoretical waveforms and found that the observed flagellar beats were in accordance with a model based on sliding controlled motor activity, but not with curvature controlled motor activity. Furthermore, good agreement between observed and calculated waveforms was obtained only if significant sliding between microtubules occurred at the base. This highlights the role of basal sliding in shaping the flagellar waveform. Thus we conclude, that the flagellar beat patterns are determined by an interplay of the basal properties of the axoneme and the collective behavior of sliding controlled dynein motors that are coordinated mechanically via the sliding of adjacent microtubules. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V6.00004: Physical Aspects of Evolutionary Transitions to Multicellularity Invited Speaker: An important issue in evolutionary biology is the emergence of multicellular organisms from unicellular individuals. The accompanying differentiation from motile totipotent unicellular organisms to multicellular ones having cells specialized into reproductive (germ) and vegetative (soma) functions, such as motility, implies both costs and benefits, the analysis of which involves the physics of buoyancy, diffusion, and mixing. In this talk, I discuss recent results on this transition in a model lineage: the volvocine green algae. Particle Imaging Velocimetry of fluid flows generated by these organisms show that they exist in the regime of very large Peclet numbers, where the scaling of nutrient uptake rates with organism size is highly nontrivial. In concert with metabolic studies of deflagellated colonies, investigations of phenotypic plasticity under nutrient-deprived conditions, and theoretical studies of transport in the high-Peclet number regime, we find that flagella-generated fluid flows enhance the nutrient uptake rate per cell, and thereby provide a driving force for evolutionary transitions to multicellularity. Thus, there is a link between motility, mixing, and multicellularity. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 2:15PM |
V6.00005: Mechanics of actin-based motility Invited Speaker: The ability of cells to move is critical for organism evelopment, maintenance, and repair. Growth of actin filament networks drives a variety of cellular and intracellular motions and contributes to the mechanical rigidity of the cell's cytoskeleton. During motility, eukaryotic cells and intracellular pathogens are propelled by dendritic actin networks oriented in the direction of motion and characterized by a branched architecture. Nucleation-promoting factors activated near the cell membrane trigger the formation of nascent filaments from the side of existing filaments in the network. Here we use laser tracking and atomic force microscopy to test models of actin-based motility and actin network elasticity. A Brownian ratchet mechanism has been proposed to couple actin polymerization to cellular movements, whereby thermal motions are rectified by the addition of actin monomers at the end of elongating filaments. By following actin-propelled microspheres using three-dimensional laser tracking, we find that the movement of beads adhered to growing actin networks is consistent with an object-fluctuating Brownian ratchet. Elasticity of actin networks has been shown to arise in part from the resistance of filaments under extension. Using atomic force microscopy, we find that dendritic actin networks exhibit nonlinear stress softening behavior that points to an important role for filaments under compression. Together, these results raise new questions about how actin network architecture is involved in the propulsion and guidance of crawling cells. [Preview Abstract] |
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