Showing papers by "Frank Jülicher published in 2007"
TL;DR: A vertex model is used for the epithelial junctional network in which cell packing geometries correspond to stable and stationary network configurations and accounts qualitatively and quantitatively for the observed packing geometry in the wing disc and its response to perturbation by laser ablation.
1,125 citations
TL;DR: It is shown that spindle orientation can be understood as the result of the action of cortical force generators, which interact with spindle microtubules and are activated by cortical cues.
Abstract: The architecture and adhesiveness of a cell microenvironment is a critical factor for the regulation of spindle orientation in vivo. Using a combination of theory and experiments, we have investigated spindle orientation in HeLa (human) cells. Here we show that spindle orientation can be understood as the result of the action of cortical force generators, which interact with spindle microtubules and are activated by cortical cues. We develop a simple physical description of this spindle mechanics, which allows us to calculate angular profiles of the torque acting on the spindle, as well as the angular distribution of spindle orientations. Our model accounts for the preferred spindle orientation and the shape of the full angular distribution of spindle orientations observed in a large variety of different cellular microenvironment geometries. It also correctly describes asymmetric spindle orientations, which are observed for certain distributions of cortical cues. We conclude that, on the basis of a few simple assumptions, we can provide a quantitative description of the spindle orientation of adherent cells.
409 citations
TL;DR: This review presents some of the recent results on active polar gels and uses the hydrodynamic approach to study the spontaneous generation of flow in an active polar film and the formation of vortex defects.
401 citations
TL;DR: The cellular mechanisms of Dpp and Wingless spreading are different: Dpp spreading requires endocytic, intracellular trafficking, and the range is controlled by the four key kinetic parameters governing morphogen spreading.
Abstract: In the developing fly wing, secreted morphogens such as Decapentaplegic (Dpp) and Wingless (Wg) form gradients of concentration providing positional information. Dpp forms a longer-range gradient than Wg. To understand how the range is controlled, we measured the four key kinetic parameters governing morphogen spreading: the production rate, the effective diffusion coefficient, the degradation rate, and the immobile fraction. The four parameters had different values for Dpp versus Wg. In addition, Dynamin-dependent endocytosis was required for spreading of Dpp, but not Wg. Thus, the cellular mechanisms of Dpp and Wingless spreading are different: Dpp spreading requires endocytic, intracellular trafficking.
386 citations
TL;DR: The flagellar beat patterns are determined by an interplay of the basal properties of the axoneme and the mechanical feedback of dynein motors, which suggests a novel mechanism by which changes in basal compliance could reverse the direction of beat propagation.
Abstract: Cilia and eukaryotic flagella are slender cellular appendages whose regular beating propels cells and microorganisms through aqueous media. The beat is an oscillating pattern of propagating bends generated by dynein motor proteins. A key open question is how the activity of the motors is coordinated in space and time. To elucidate the nature of this coordination we inferred the mechanical properties of the motors by analyzing the shape of beating sperm: Steadily beating bull sperm were imaged and their shapes were measured with high precision using a Fourier averaging technique. Comparing our experimental data with wave forms calculated for different scenarios of motor coordination we found that only the scenario of interdoublet sliding regulating motor activity gives rise to satisfactory fits. We propose that the microscopic origin of such "sliding control" is the load dependent detachment rate of motors. Agreement between observed and calculated wave forms was obtained only if significant sliding between microtubules occurred at the base. This suggests a novel mechanism by which changes in basal compliance could reverse the direction of beat propagation. We conclude that the flagellar beat patterns are determined by an interplay of the basal properties of the axoneme and the mechanical feedback of dynein motors.
287 citations
TL;DR: It is concluded that sampling a concentration field of chemoattractant along circular and helical swimming paths is a robust strategy for chemotaxis that works reliably for a vast range of parameters.
Abstract: We develop a theoretical description of sperm chemotaxis. Sperm cells of many species are guided to the egg by chemoattractants, a process called chemotaxis. Motor proteins in the flagellum of the sperm generate a regular beat of the flagellum, which propels the sperm in a fluid. In the absence of a chemoattractant, sperm swim in circles in two dimensions and along helical paths in three dimensions. Chemoattractants stimulate a signaling system in the flagellum, which regulates the motors to control sperm swimming. Our theoretical description of sperm chemotaxis in two and three dimensions is based on a generic signaling module that regulates the curvature and torsion of the swimming path. In the presence of a chemoattractant, swimming paths are drifting circles in two dimensions and deformed helices in three dimensions. The swimming paths can be described by a dynamical system that exhibits different dynamic regimes, which correspond to different chemotactic behaviours. We conclude that sampling a concentration field of chemoattractant along circular and helical swimming paths is a robust strategy for chemotaxis that works reliably for a vast range of parameters.
225 citations
TL;DR: A unified theoretical description, in which mechanical activity stems solely from myosin-based adaptation, could account for the fast and slow manifestations of active hair-bundle motility observed in frog, as well as in auditory organs of the turtle and the rat.
142 citations
TL;DR: A generic hydrodynamic theory of active fluids with several components that takes into account polar order and considers the case when one component is viscoelastic.
Abstract: We develop a generic hydrodynamic theory of active fluids with several components. We take into account polar order and consider the case when one component is viscoelastic. Our theory is motivated by the cytoskeleton which is a network of elastic filaments that are coupled to active processes such as the action of motor proteins which can generate relative forces between filaments as they hydrolyze a fuel (ATP). In addition to the filament gel, the system is embedded in a solvent component and free monomers constitute a third component. We derive constitutive material equations for the combined system which include reactive and dissipative couplings as well as the chemical driving by ATP hydrolysis and a possible chiral symmetry of the filaments. As an illustration of these equations, we discuss an active liquid in a simple shear gradient.
129 citations
TL;DR: It is shown that on a microscopic level contractile stresses can result from both the action of motor proteins, which cross-link filaments, and from the polymerization and depolymerization of filaments in the presence of end-tracking cross- linkers.
Abstract: We present a physical analysis of the dynamics and mechanics of contractile actin rings. In particular, we analyze the dynamics of ring contraction during cytokinesis in the Caenorhabditis elegans embryo. We present a general analysis of force balances and material exchange and estimate the relevant parameter values. We show that on a microscopic level contractile stresses can result from both the action of motor proteins, which cross-link filaments, and from the polymerization and depolymerization of filaments in the presence of end-tracking cross-linkers.
112 citations
TL;DR: It is shown that precise oscillations of high quality are possible even when the number of product molecules is low and the fluctuations of amplitude are large, and parameter values which can account for high quality oscillations as observed in single cells are discussed.
Abstract: We develop a stochastic description of feedback oscillators in which functional molecules are produced by an assembly line consisting of many identical steps. The initiation rate of this assembly is regulated by its products via a negative feedback. This model is motivated by genetic oscillators such as circadian clocks. We show that precise oscillations of high quality are possible even when the number of product molecules is low and the fluctuations of amplitude are large. We discuss parameter values which can account for high quality oscillations as observed in single cells. Furthermore, we discuss effects of stochastic amplification steps on precision to account for translational bursting.
67 citations
TL;DR: The effects of a directional bias on morphogen transport and those of the coupling of the morphogen and receptor kinetics are discussed and general properties of cellular transport processes such as the robustness of gradients are discussed.
Abstract: We present a general theoretical framework to discuss mechanisms of morphogen transport and gradient formation in a cell layer. Trafficking events on the cellular scale lead to transport on larger scales. We discuss in particular the case of transcytosis where morphogens undergo repeated rounds of internalization into cells and recycling. Based on a description on the cellular scale, we derive effective nonlinear transport equations in one and two dimensions which are valid on larger scales. We derive analytic expressions for the concentration dependence of the effective diffusion coefficient and the effective degradation rate. We discuss the effects of a directional bias on morphogen transport and those of the coupling of the morphogen and receptor kinetics. Furthermore, we discuss general properties of cellular transport processes such as the robustness of gradients and relate our results to recent experiments on the morphogen Decapentaplegic (Dpp) that acts in the wing disk of the fruit fly Drosophila.
TL;DR: A systematic framework is developed to calculate averages, autocorrelations, and response functions by treating the feedback as a weak perturbation of a signaling module consisting of a two-state stochastic point process with negative feedback.
Abstract: We study the stochastic kinetics of a signaling module consisting of a two-state stochastic point process with negative feedback. In the active state, a product is synthesized which increases the active-to-inactive transition rate of the process. We analyze this simple autoregulatory module using a path-integral technique based on the temporal statistics of state flips of the process. We develop a systematic framework to calculate averages, autocorrelations, and response functions by treating the feedback as a weak perturbation. Explicit analytical results are obtained to first order in the feedback strength. Monte Carlo simulations are performed to test the analytical results in the weak feedback limit and to investigate the strong feedback regime. We conclude by relating some of our results to experimental observations in the olfactory and visual sensory systems.
TL;DR: The physics of the cytoskeleton combines polymer physics, statistical mechanics and cell biophysics to address a class of soft matter with novel and highly dynamic properties.
Abstract: Living cells are organized by soft polymeric scaffolds which, from the perspective of condensed matter physics, are extraordinarily complex fluids Cells are active, highly dynamic and far from equilibrium states This dynamics is reflected in processes such as cell motility, mechanosensitivity, and cell division The complexity of biological cells involves activation patterns of many genes and a vast set of molecular interactions of their products By identifying cellular subunits which can act as independent functional modules, the complexity of biological cells becomes amenable to systematic study, both experimental and theoretical Furthermore, the study of cellular systems can lead to new ideas to create artificial devices and machines on scales from nanometres to micrometres An important example for such a module is the filament network known as the cytoskeleton together with sets of specific associated molecules The cytoskeleton consists mainly of long elastic filaments, namely actin filaments, intermediate filaments and microtubules These filaments are semiflexible polymers, ie polymers which behave as soft rods that bend under thermal and nonthermal forces in the cell The transiently gel-like network of filaments in the cell is driven in a state far from thermodynamic equilibrium and is therefore an inherently active material The cytoskeleton generates cellular motion sufficiently strong to push rigid AFM cantilevers out of the way The active forces are generated by molecular motor-based nano-structures and by the assembly of filament networks via polymerization and cross-linking Further, the nano-sized motors overcome the inherently slow, often glass-like Brownian polymer dynamics, resulting for cytoskeletal polymeric scaffolds in novel self-organization, rapid switching between fluid and solid states, and transitions between ordered and unordered states The physics of the cytoskeleton combines polymer physics, statistical mechanics and cell biophysics to address a class of soft matter with novel and highly dynamic properties We think that the present Focus Issue in New Journal of Physics can reflect the richness and excitement of this interdisciplinary field which is currently evolving rapidly It combines theoretical and experimental approaches as well as the investigation of physical model systems and biophysical studies of living cells Focus on Physics of the Cytoskeleton Contents Pattern formation in active cytoskeletal networks R Peter, V Schaller, F Ziebert and W Zimmermann Kinetics of stress fibers Matthew R Stachowiak and Ben O'Shaughnessy Actin-based propulsion of functionalized hard versus fluid spherical objects Vincent Delatour, Shashank Shekhar, Anne-Cecile Reymann, Dominique Didry, Kim Ho Diep Le, Guillaume Romet-Lemonne, Emmanuele Helfer and Marie-France Carlier Probing friction in actin-based motility Yann Marcy, Jean-Francois Joanny, Jacques Prost and Cecile Sykes Filament networks attached to membranes: cytoskeletal pressure and local bilayer deformation Thorsten Auth, S A Safran and Nir S Gov Less is more: removing membrane attachments stiffens the RBC cytoskeleton Nir S Gov Local and global deformations in a strain-stiffening fibrin gel Qi Wen, Anindita Basu, Jessamine P Winer, Arjun Yodh and Paul A Janmey Collective Langevin dynamics of flexible cytoskeletal fibers Francois Nedelec and Dietrich Foethke Statistical analysis of neuronal growth: edge dynamics and the effect of a focused laser on growth cone motility T Betz, D Koch, B Stuhrmann, A Ehrlicher and J Kas Coupling biochemistry and mechanics in cell adhesion: a model for inhomogeneous stress fiber contraction Achim Besser and Ulrich S Schwarz Programming protein self assembly with coiled coils Hendrik Dietz, Thomas Bornschlogl, Roland Heym, Frauke Konig and Matthias Rief Active-filament hydrodynamics: instabilities, boundary conditions and rheology Sriram Ramaswamy and Madan Rao Hydrodynamic theory for multi-component active polar gels J F Joanny, F Julicher, K Kruse and J Prost Effects of cross-links on motor-mediated filament organization Falko Ziebert, Igor S Aranson and Lev S Tsimring Actin network architecture and elasticity in lamellipodia of melanoma cells Frank Fleischer, Revathi Ananthakrishnan, Stefanie Eckel, Hendrik Schmidt, Josef Kas, Tatyana Svitkina, Volker Schmidt and Michael Beil Shear rheology of a cell monolayer Pablo Fernandez, Lutz Heymann, Albrecht Ott, Nuri Aksel and Pramod A Pullarkat Disassembly of actin networks by filament severing A E Carlsson Spontaneous waves in muscle fibres Stefan Gunther and Karsten Kruse The glassy wormlike chain Klaus Kroy and Jens Glaser Frank Julicher, Max Planck Institut fur Physik komplexer Systeme, Dresden, Germany Josef Kas, Abteilung fur die Physik weicher Materie, Universitat Leipzig, Germany