Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W7: Physics of Cell Elasticity, Interactions and Tissue Formation |
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Sponsoring Units: DBP DPOLY Chair: Philip Nelson, University of Pennsylvania Room: Baltimore Convention Center 307 |
Thursday, March 16, 2006 2:30PM - 3:06PM |
W7.00001: Nucleation and growth of cell contacts Invited Speaker: Living cells develop adhesive contacts with their environments. We present experiments which are dedicated to measure geometric and density evolutions of these contacts in living cells. We focus on two contacts: focal contacts, formed between a cell and the extracellular matrix, and adherens junctions, assembled between neighbouring cells. Both include: (i) a transmembraneous protein dictating the given adhesive junction assembly, (ii) a specific protein complex with tens of different components, (iii) an actin cytoskeletal structure. Our experiments involve the observations of fluorescently labelled proteins of these contacts in living cells, local force application, force measurements, and optical development such as evanescent wave excitation. These adhesive cellular entities are usually described as a result of activation of signalling events. However cell adhesive contacts can be seen as discrete \textit{particles }aggregates, which undergo nucleation and growth like in a first order phase transition. We will show that self-assembly processes are indeed imposing contacts shapes and dynamics. Far from being two antagonistic ways of describing cells dynamics, signalling pathways and cell self-assembly complement each other to dictate contacts shapes. In addition, eventhough focal contacts and adherens junctions involve different proteins, we will show that they share common features such as mechanosensitivity. Via these contacts, cells behave as climbers seeking to probe the resistance of their environment in order to reinforce appropriately specific adhesive areas. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:42PM |
W7.00002: Integrin activation and cell adhesion by mechanical forces Invited Speaker: |
Thursday, March 16, 2006 3:42PM - 4:18PM |
W7.00003: Cell morphologies depend on substrate rigidity. Invited Speaker: Extracellular matrices and intracellular cytoskeletons are composed mainly of open meshworks formed by semi-flexible polymers linked by accessory proteins in networks with specific geometries. The viscoelastic properties of such networks often differ strongly from those of flexible polymer gels and are characterized by relatively large elastic moduli for low volume fractions, increased rigidity at increasing strains, and in some cases, negative normal stresses when deformed in simple shear. Cell structure and function depend on the stiffness of the surfaces on which cells adhere as well as on the type of adhesion complex by which the cell binds its extracellular ligand. Most cell types, including fibroblasts and endothelial cells, switch from round to spread morphology as stiffness is increased between 1000 and 10,000 Pa. In contrast, other cells types such as neutrophils do not require rigid substrates in order to spread, and neurons extend processes better on soft (50 Pa) materials than on stiffer gels. Differences in rigidity sensing and response can be exploited to design soft matrices optimized for growth and differentiation of specific cell types. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:54PM |
W7.00004: Physics of adhesion and elasticity of biological cells Invited Speaker: Forces exerted by adherent cells are important for many physiological processes such as wound healing and tissue formation. By pulling on their environment, cells sense rigidity gradients, boundaries and strains induced by the presence of other cells. Many cell types respond to these signals by actively adjusting the magnitude and direction of the adhesions that connect cells to surfaces or to each other. These adhesions are formed from membrane-bound integrin proteins and other cytoplasmic proteins that form condensed domains that grow in the direction of externally applied or internal, cytoskeletal forces. We present a model for the adsorption of adhesion proteins from the cell interior to the adhesion site and the resulting, force-sensitive anisotropic growth. The theory couples the mechanical forces to the non- linear adsorption dynamics and predicts the growth velocities of the back and front of the adhesion in qualitative agreement with experiment. The adhesion forces generated by a collection of cells in a tissue significantly alter the overall elastic response of the system. We model an ensemble of cells by an extension of the treatment of dielectric response of polar molecules to elastic interactions. By introducing the elastic analogy of the dielectric constant of the medium, we are able to predict the average cell polarization, their orientational order, and the effective material constants. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:30PM |
W7.00005: Cell mechanics and human disease states Invited Speaker: This presentation will provide summary of our very recent studies exploring the effects of biochemical factors, influenced by foreign organisms or \textit{in vivo} processes, on intracellular structural reorganization, single-cell mechanical response and motility of a population of cells in the context of two human diseases: malaria induced by \textit{Plasmodium falciparum }merozoites that invade red blood cells, and gastrointestinal cancer metastasis involving epithelial cells. In both cases, particular attention will be devoted to systematic changes induced in specific molecular species in response to controlled alterations in disease state. The role of critical proteins in influencing the mechanical response of human red bloods during the intra-erythrocytic development of \textit{P. falciparum} merozoites has also been assessed quantitatively using specific protein knock-out experiments by recourse to gene inactivation methods. Single-cell mechanical response characterization entails such tools as optical tweezers and mechanical plate stretchers whereas cell motility assays and cell-population biorheology characterization involves microfluidic channels. The experimental studies are accompanied by three-dimensional computational simulations at the continuum and mesoscopic scales of cell deformation. An outcome of such combined experimental and computational biophysical studies is the realization of how chemical factors influence single-cell mechanical response, cytoadherence, the biorheology of a large population of cells through microchannels representative of \textit{in vivo} conditions, and the onset and progression of disease states. [Preview Abstract] |
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