Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A39: Cellular Biomechanics I |
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Sponsoring Units: DBP Chair: Jennifer Schwarz, Syracuse University Room: 411 |
Monday, March 16, 2009 8:00AM - 8:12AM |
A39.00001: Loss of an actin crosslinker uncouples cell spreading from cell stiffening on gels with a gradient of stiffness Qi Wen, Fitzroy J. Byfield, Kerstin Nordstrom, Paulo E. Arratia, R.Tyler Miller, Paul A. Janmey We use microfluidics techniques to produce gels with a gradient of stiffness to show the essential function of the actin crosslinker filamin A in cell responses to mechanical stimuli. M2 melanoma cells null for filamin A do not alter their adherent area in response to increased substrate stiffness when they link to the substrate only through collagen receptors, but change adherent area normally when bound through fibronectin receptors. In contrast, filamin A-replete A7 cells change adherent area on both substrates and respond more strongly to collagen 1-coated gels than to fibronectin-coated gels. A7 cells alter their stiffness, as measured by atomic force microscopy, to match the elastic modulus of the substrate immediately adjacent to them on the gradient. M2 cells, in contrast, maintain a constant stiffness on all substrates that is as low as that of A7 cells on the softest gels achievable (1000 Pa). By contrasting the responses of these cell types to different adhesive substrates, cell spreading can be dissociated from stiffening. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A39.00002: Substrate Induced Osteoblast-Like Differentiation of Stromal Stem Cells Jacqueline Belizar, Reena Glaser, Matthew Hung, Marcia Simon, Vladimir Jurukovski, Miriam Rafailovich, Alice Shih We have demonstrated that Adipose-derived stem cells (ASCs) can be induced to biomineralize on a polybutadiene (PB) coated Si substrate. The cells began to generate calcium phosphate deposits after a five-day incubation period in the absence of dexamethasone. Control cells plated on tissue culture PS culture dish (TCP) did not biomineralize. In addition, the biomineralizing culture retained proliferative cells In order to determine whether the induction was transient, we transferred the cells exposed to polybutadiene after 14 and 28-day incubation periods to TCP dishes. These cells continued to biominerlize. Genetic testing is underway which will determine whether differentiation is maintained after transfer. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A39.00003: Mechanical anisotropy of viscoelasticity in biological cells Ming-Tzo Wei, H.D. Ou-Yang The mechanism that biological cells use to remodel their cytoskeletal structure in response to external stress is unclear. Experimental observations suggest that the cells remodel their skeleton in a manner that is directionally responsive to the external stress. In order to understand these directional responses, we developed a method to measure the rheological response of the cell in orthogonal directions simultaneously. To achieve controlled stimulation and detection, we used a dual-beam optical tweezer, which used a pump and probe scheme to measure the storage and loss modulus of the cellular cytoskeleton. The pump was used to manipulate extracelluar micro-particles which were attached to the actin cytoskeleton through trans-membrane integrin alpha receptors. By measuring two independent regions of the cell, we were able to generate a localized mechanical stress on the outer surface of the cell while observing the directionally specific inside response. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A39.00004: Traction forces associated with shape changes in migrating amoeboid cells Baldomero Alonso-Latorre, Juan C. del Alamo, Effie Bastounis, Rudolph Meili, Richard Firtel, Juan C. Lasheras Amoeboid motility results from the repetition of a repertoire of shape changes (motility cycle). We studied the dominant changes and their relation to the activity and localization of cytoskeletal proteins by applying Principal Component Analysis (PCA) to measurements of cell shape, traction forces and F-actin concentration in migrating\textit{ Dictyostelium} cells. Using wild-type cells (\textit{wt}) as reference, we investigated myosin II activity by studying myosin II-null (\textit{mhc-}) and essential light chain-null cells (\textit{mlc-}). Only three PCA modes are enough to represent 67{\%} of the variance of cell area: dilation/elongation, a half-moon shape and a bulging of the front/back. These modes are similar for \textit{wt, mlc-} and \textit{mhc-} but they are implemented more slowly in \textit{mhc-}. The second mode, which represents sideways protrusion/retraction and is associated to lateral asymmetry in the traction forces, is significantly less important in \textit{mhc-}. These results suggest that migration speed decreases in the absence of myosin II due to a reduced control on the spatial organization of the cell stresses. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A39.00005: Imaging spatiotemporal redistribution of cellular traction stresses during fibroblast migration on a physiologically relevant ECM mimic Zhi Pan, Yajie Liu, Kaustabh Ghosh, Dhruv Nandamudi, Danny Stemp, Toshio Nakamura, Richard Clark, Miriam Rafailovich To better understand the dynamics of cell migration, we measured the spatiotemporal redistribution of cellular traction stresses during fibroblast migration at a submicron level and correlated it with nuclear translocation on a physiologically relevant ECM mimic. We found that nuclear translocation occurred in pulses whose magnitude was larger on the low ligand density surfaces (LLDS) than on the high ligand density surfaces (HLDS). Large nuclear translocations only occurred on LLDS when the rear traction forces completely relocated to a posterior nuclear location, while such relocation took much longer time on HLDS, probably due to the greater magnitude of traction forces. Our results suggest that the reinforcement of the traction forces around the nucleus is a critical step during fibroblast migration, serving as a speed regulator, which must be considered in any dynamic molecular reconstruction model of tissue cell migration. A traction gradient foreshortening model was proposed to explain how the relocation of rear traction forces leads to pulsed fibroblast migration. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A39.00006: Achieving in-vitro axonal polarization~by using micro-patterns Sophie Roth, Jacques Brocard, Sylvie Gory-Faure, Catherine Villard Our project is based on the elaboration of in vitro neuron networks as simplified models to explore the relation between neuronal architecture and biological function. Beyond a control of soma and neurite position, our first goal was to achieve in-vitro axonal differentiation of embryonic E18 hippocampal mice neurons by the mean of geometrical growth constraints, i.e. by the use of adhesive micro-patterns on silanized glass substrates. Such a process thus excludes chemical guidance or specific adhesion mechanisms. This study explores two different types of geometrical constraints. The first one, based on the centrosome role and localization, is applied to the soma, and force a choosen neurite to differentiate into an axon with 39{\%} of efficacy (N= 160 cells, 3 different cultures). The second one derives from the suggested relationship between neurite mechanical tension and axonal differentiation, and is based on the design of wavy neurite's shape. Its efficacy reach 0.51{\%} (N= 300 cells, 3 different cultures). The combinaison of these two constraints into a final pattern yields an efficacy of 82{\%} (N= 83 cells, 2 different cultures). These results not only provide an important tool for creating neural model networks but also point out an important role of intrinsic neurite tension during axon differentiation. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A39.00007: Matrix rigidity optimizes the polarization of stem cells Assaf Zemel, Florian Rehfeldt, Andre Brown, Dennis Discher, Samuel Safran We present a theoretical model and experiments to explain the non-monotonic dependence of stress-fiber polarization in stem cells on matrix rigidity. The theory generalizes the treatment of elastic inclusions to ``living'' inclusions (cells) whose active polarizability, unlike non-living matter, depends on the feedback of cellular forces that develop in response to matrix stresses. We demonstrate experimentally that the stress fibers in adult mesenchymal stem cells, generally orient parallel to the long axis of the cells, with an anisotropy that depends non-monotonically on substrate stiffness. Consistent with these experiments, our theory predicts that the magnitude of the cellular force increases monotonically with the matrix rigidity while the polarization anisotropy shows a maximum that depends on the cell shape and the elastic modulus of the medium. These findings offer a mechanical correlate for the observation that stem cell differentiation optimizes in a range of matrix rigidities that depends on the tissue type. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A39.00008: Structure at the Leading Edge D. A. Quint, J. M. Schwarz, M. C. Marchetti The leading edge of a crawling cell is propelled forward by a polymerizing network of branched actin filaments. This emergent structural array seems to be rigid enough to support and push against the cell membrane within the appropriate time scales under which cell motility can be realized. We seek to understand how such a network can optimize its structure to generate the rigidity required, particularly focusing on the role of branching in the network. For isolated elastic beams, which model semiflexible polymers, the critical buckling load is enhanced when branched supports are included. Therefore, we conjecture that an optimal branching angle is found by looking at the competition between branching providing collective structural support, which results in polymerization with a component perpendicular to the direction of motion, and polymerization along the direction of motion. To partially test this conjecture, we simulate a directed, branched network in the absence of forces. Preliminary results indicate a lower bound on the optimal branching angle of approximately 40 degrees (to be compared with the observed 70 degree branching angle). Studies of a directed, branched network with forces will also be addressed. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A39.00009: Chemotactic strategy of Vibrio alginolyticus studied with an optical trap Suddhashil Chattopadhyay, Tuba Altindal, Xie Li, Xia-Lun Wu The canonical ``run'' and ``tumble'' mode of chemotaxis, employed by multiple flagellated cells such as \emph{Escherichia coli, }has been studied in great detail over the years. In this work we will demonstrate the usage of an optical trap for studying the chemotaxis of cells belonging to the single flagellated strain of \emph{Vibrio alginolyticus. }This method allows precise and direct observation of chemotactic response, while the cell is exposed to various chemical signals (positive/negative gradient or no chemicals). We have studied the response of the flagellar motor with a precise control on the input signal (chemical gradient), such that a cell can be forced to move up or down a chemical gradient, a control which is not attainable for free swimming cells. The optical trap does not restrict the rotational motion of the bacterium and allows the state of the motor (clockwise or counter clockwise rotation) to be monitored continuously. Our group has recently observed an active flagellar movement (called the ``flagellar flick'') that is used by cells of \emph{V. algninolyticus} for randomizing swimming direction during chemotaxis. We consider this mode in addition to ``back and forth'' swimming employed by these cells. A modified chemotactic strategy is proposed and tested using the optical trap. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A39.00010: Chemotaxis in Marine Bacterium Vibrio alginolyticus Li Xie, Suddhashil Chattopadhyay, Tuba Altindal, Xiao-Lun Wu We investigated swimming behavior of marine bacterium \emph{Vibrio alginolyticus} in an uniform chemical environment. The typical bacterial trajectory consists of consecutive run (forward swimming) and reverse (backward swimming) intervals with occasional sudden changes of swimming directions, which we call \textquotedblleft{}flagellar flicks\textquotedblright{}. This mode of chemotaxis is different from the canonical run-and-tumble strategy adopted by \emph{Escherichia coli} and may be selected for in \emph{V. alginolyticus} due to the ocean environment where nutrients are scarce and are subject to rapid turbulent dispersion. We measured the statistical distributions of run $T_{run}$ and revers $T_{rev}$ time intervals, $P(T_{run})$ and $P(T_{rev})$, and found that while the back-swimming time appears to have a well-defined time scale of $0.5\, s$, the forward swimming time is more broadly distributed, suggestive of a Poisson process. Measurements of the time interval $T_{flick}$ between two consecutive directional changes show that $P(T_{flick})$ is also peaked at a finite time, $T_{flick}\sim1\, s$, and the mean directional change is $\Delta\theta\sim70\,^{0}$. Interestingly, this $\Delta\theta$ observed is nearly optimal for efficient randomization of swimming directions. Altogether, our experiments suggest that \emph{V. alginolyticus }employs both run-and-reverse and flicking activities for chemotaxis, and this behavior presumably optimizes their foraging efficiency in a turbulent environment. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A39.00011: The flagellar ``flick'': direction randomization in single falgellated cells of V. alginolyticus Tuba Altindal, Suddhashil Chattopadhyay, Li Xie, Xiao-Lun Wu Single flagellated bacteria such as \emph{Vibrio alginolyticus}, which possess a single flagellum are believed to be unable to use an active mechanism for direction randomization, and thus follow a {}``back'' and {}``forth'' swimming pattern. Inspired by the observation that \emph{V. alginolyticus }cells were able to rapidly accumulate around a point source of a chemoattractant, we have identified a previously unknown phenomenon in which an active movement of the flagellum is used to randomize the swimming direction. Fluorescently labelled cells clearly demostrated that bending of the flagellum is responsible for imparting direction changes to the cell body. Clues obtained from high speed video, bright-field microscopy and fluorescent imaging suggests a series of steps involved in the flagellar {}``flick''. An investigation of the energetics of the proposed mechanism leads to the conclusion that the directional change may be connected to the flagellar motor, which normally propels the cell body. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A39.00012: Probing the Dynamics of Cellular Traction Forces with Magnetic Micropost Arrays Corinne Kramer, Christopher Chen, Daniel Reich Mechanical forces on living cells are associated with changes in cellular function. For example, vascular smooth muscle cells are known to undergo a mechanical feedback response to increased stress, which can result in atherosclerosis. We have recently developed a magnetic micropost array, a novel device for measuring cellular traction forces that simultaneously enables the application of localized external forces to cells. The device consists of an array of micrometer scale elastomeric posts that act as force sensors for cells cultured on their tips. An external force is applied to the adherent surface of a cell via a magnetic torque on a cobalt nanowire embedded in a single post. Initial results showed an active and non-local cellular response to applied forces in mouse fibroblast cells.\footnote{N. Sniadecki, et. al, \emph{Proc Natl Acad Sci}, {\bf 104}, 14553 (2007).} We will present results on the spatially resolved dynamics of traction forces exerted by smooth muscle cells over time in response to constant and time-varying stimulation. The observation of biochemical and mechanical regulation of the subcellular redistribution of force may allow insights into cellular mechanotransduction and control of cell function. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A39.00013: Differential cellular response to linear and strain-stiffening hydrogel substrates Jessamine P. Winer, Shaina A. Oake, Bethany C. Baumann, Paul A. Janmey Many cell types act as tensiometers, modulating their spread area, motility, and protein expression in response to the substrate stiffness. Studies of stiffness sensing typically employ linear elastic materials whose stiffness is independent of the applied strain. Biological gels, however, often stiffen in response to increasing strain. Fibroblasts and mesenchymal stem cells adherent to linearly elastic gels typically display a small, round phenotype on soft substrates and increase spread area as the elastic modulus of their substrate increases. On the strain-stiffening biopolymer gel fibrin, the same cell types are maximally spread even when the gel's low strain elastic modulus would predict a round morphology. Traction microscopy reveals that cells apply active displacements of several microns up to five cell lengths away, and atomic force microscopy shows that these displacements locally stiffen the gel by deforming it beyond its linear range. The magnitude of cell-applied strains is inversely related to the gel's low strain elastic modulus and results in long distance cell-cell communication and alignment. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A39.00014: Elasticity of the eye's crystalline lens: A Brillouin light scattering study. S. Bailey, J. Gump, R. Sooryakumar, C. Jayaprakash, M.S. Venkiteshwar, M. Bullimore, M. Twa Focusing the eye on a near object results in an increase in its optical power brought about by contraction of the ciliary muscles and an increase in the lens surface curvature. Distant vision occurs when the muscular force flattens the lens. Central to the ability of the lens to alter shape are its mechanical properties. Thus, given that hardening of the lens would impede deformation and reduce its ability to undergo the changes required for accommodation, a noninvasive approach to measure the elastic properties of the lens is valuable. We present results of Brillouin scattering from bovine and human lenses (from the organ donor program at The Ohio State University) that measure their high frequency acoustic response. These measurements are conducted with a few milli-watts of laser power and, in the case of bovine lenses, from entire intact eye globes, allow the stiffness of the lens to be mapped across its cross-section. The results will be compared to values of the shear- and bulk-moduli determined from other techniques and the implications of differences in these moduli discussed. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A39.00015: Physical Description of Mitotic Spindle Orientation During Cell Division Andrea Jim\'enez-Dalmaroni, Manuel Th\'ery, Victor Racine, Michel Bornens, Frank J\"ulicher During cell division, the duplicated chromosomes are physically separated by the action of the mitotic spindle. The spindle is a dynamic structure of the cytoskeleton, which consists of two microtubule asters. Its orientation defines the axis along which the cell divides. Recent experiments show that the spindle orientation depends on the spatial distribution of cell adhesion sites. Here we show that the experimentally observed spindle orientation can be understood as the result of the action of cortical force generators acting on the spindle. We assume that the local activity of force generators is controlled by the spatial distribution of cell adhesion sites determined by the particular geometry of the adhesive substrate. We develop a simple physical description of the spindle mechanics, which allows us to calculate the torque acting on the spindle, as well as the energy profile and the angular distribution of spindle orientation. Our model accounts for the preferred spindle orientation, as well as the full shape of the angular distributions of spindle orientation observed in a large variety of pattern geometries. M. Th\'{e}ry, A. Jim\'{e}nez-Dalmaroni, et al., Nature 447, 493 (2007). [Preview Abstract] |
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