167 research outputs found
Patterning of polar active filaments on a tense cylindrical membrane
We study the dynamics and patterning of polar contractile filaments on the
surface of a cylindrical cell using active hydrodynamic equations that
incorporate couplings between curvature and filament orientation. Cables and
rings spontaneously emerge as steady state configurations on the cylinder, and
can be stationary or moving, helical or segments moving along helical
trajectories. Contractility induces coalescence of proximal rings. We observe
phase transitions in the steady state patterns upon changing cell diameter and
make several testable predictions. Our results are relevant to the dynamics and
patterning of a variety of active biopolymers in cylindrical cells.Comment: 10 pages, 8 figures, (Includes Supplementary information
Propagating Cell-Membrane Waves Driven by Curved Activators of Actin Polymerization
Cells exhibit propagating membrane waves which involve the actin cytoskeleton. One type of such membranal waves are Circular Dorsal Ruffles (CDR) which are related to endocytosis and receptor internalization. Experimentally, CDRs have been associated with membrane bound activators of actin polymerization of concave shape. We present experimental evidence for the localization of convex membrane proteins in these structures, and their insensitivity to inhibition of myosin II contractility in immortalized mouse embryo fibroblasts cell cultures. These observations lead us to propose a theoretical model which explains the formation of these waves due to the interplay between complexes that contain activators of actin polymerization and membrane-bound curved proteins of both types of curvature (concave and convex). Our model predicts that the activity of both types of curved proteins is essential for sustaining propagating waves, which are abolished when one type of curved activator is removed. Within this model waves are initiated when the level of actin polymerization induced by the curved activators is higher than some threshold value, which allows the cell to control CDR formation. We demonstrate that the model can explain many features of CDRs, and give several testable predictions. This work demonstrates the importance of curved membrane proteins in organizing the actin cytoskeleton and cell shape
Theoretical Model for Cellular Shapes Driven by Protrusive and Adhesive Forces
The forces that arise from the actin cytoskeleton play a crucial role in determining the cell shape. These include protrusive forces due to actin polymerization and adhesion to the external matrix. We present here a theoretical model for the cellular shapes resulting from the feedback between the membrane shape and the forces acting on the membrane, mediated by curvature-sensitive membrane complexes of a convex shape. In previous theoretical studies we have investigated the regimes of linear instability where spontaneous formation of cellular protrusions is initiated. Here we calculate the evolution of a two dimensional cell contour beyond the linear regime and determine the final steady-state shapes arising within the model. We find that shapes driven by adhesion or by actin polymerization (lamellipodia) have very different morphologies, as observed in cells. Furthermore, we find that as the strength of the protrusive forces diminish, the system approaches a stabilization of a periodic pattern of protrusions. This result can provide an explanation for a number of puzzling experimental observations regarding cellular shape dependence on the properties of the extra-cellular matrix
Physical Model of Contractile Ring Initiation in Dividing Cells
We present a physical mechanism to describe initiation of the contractile ring during cell division. The model couples the membrane curvature with the contractile forces produced by protein clusters attached to the membrane. These protein clusters are mobile on the membrane and possess either an isotropic or an anisotropic spontaneous curvature. Our results show that under these conditions the contraction force gives rise to an instability that corresponds in a closed cellular system to the initiation of the contractile ring. We find a quantization of this process at distinct length-scales, which we compare to available data for different types of eukaryote cells
Curved inclusions surf membrane waves
In this letter we describe how membrane inclusions that have a spontaneous
curvature, will be convected on the membrane due to the propagation of membrane waves. We
calculate the Stokes drift of such particles and the effect on their overall density field.
We solve analytically for a uniform sinusoidal wave in the absence of diffusion, and using
simulations for the more realistic case of decaying waves with diffusion. In the latter case
we provide some good analytic approximations. A variety of such membrane waves that propagate
over a significant proportion of the cell surface exists in living cells, and we therefore show
that they can play a role in transporting membrane proteins
Membrane-mediated interactions and the dynamics of dynamin oligomers on membrane tubes
Dynamin is a protein that plays a key role in the transport and recycling of membrane tubes and vesicles within a living cell. This protein adsorbs from solution to PIP2-containing membranes, and on these tubes it forms curved oligomers that condense into tight helical domains of uniform radius. The dynamics of this process is treated here in terms of the linear stability of a continuum model, whereby membrane-mediated interactions are shown to drive the spontaneous nucleation of condensed dynamin domains. We furthermore show that the deformation of the membrane outside the dynamin domains induces an energy barrier that can hinder the full coalescence of neighboring growing domains. We compare these calculations to experimental observations on dynamin dynamics in vitro
Association of Recanalization of the Left Umbilical Vein with Umbilical Hernia in Patients with Liver Disease
Transmission of portal hypertension to the umbilical region via a recanalized left umbilical vein may explain the higher prevalence of umbilical hernia than inguinal hernia in men with advanced liver disease. Images from a computed tomography of a 49-year-old man with cirrhosis and hepatocellular carcinoma from hepatitis C virus were reconstructed in 3-dimensional color format. Rupture of the web between the left portal vein and the recanalized left umbilical vein is seen. Penetration of abdominal wall by the varices at the umbilicus is demonstrated. A dilated inferior epigastric vein is seen to drain the varices inferiorly to the right external iliac vein
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