1,409 research outputs found
Novel steady state of a microtubule assembly in a confined geometry
We study the steady state of an assembly of microtubules in a confined
volume, analogous to the situation inside a cell where the cell boundary forms
a natural barrier to growth. We show that the dynamical equations for growing
and shrinking microtubules predict the existence of two steady states, with
either exponentially decaying or exponentially increasing distribution of
microtubule lengths. We identify the regimes in parameter space corresponding
to these steady states. In the latter case, the apparent catastrophe frequency
near the boundary was found to be significantly larger than that in the
interior. Both the exponential distribution of lengths and the increase in the
catastrophe frequency near the cell margin is in excellent agreement with
recent experimental observations.Comment: 8 pages, submitted to Phys. Rev.
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ASSEMBLY OF PARTICLES ONTO RIGID CYLINDERS AND FLEXIBLE MEMBRANES: PROBING EFFECTS OF SURFACE CURVATURE AND DEFORMATION
In this thesis we explore two specific topics within the broad field of particle adhesion. First, we examine the effect of substrate shape and geometry on the self assembly of adsorbed particles, by performing molecular dynamics simulations of interacting particles constrained to the surface of cylinders of varying diameters. We find the diameter of the cylinder imposes a constraint on the shape and crystallographic orientation of the self-assembled lattice, essentially determining the optimal arrangement of particles a priori. We propose a simple one-dimensional model to explain the optimal arrangement of particles as a function of the particle interaction potential and the physical size of the constraining cylinder. We next investigate the stiffness of these cylindrical lattices, and find that thin cylindrical crystals are anomalously softer than large ones. We then propose this effect is a consequence of the geometric arrangement of particles in a tight cylindrical shape, and quantify how the stiffness depends on the circumference of the cylinder and on the strength of interaction between the particles.
Second, we explore how adhesion of particles can reshape the substrate, for the purpose of designing novel functional materials. We perform experiments exposing cationic nanoparticles to lipid bilayer vesicles, where we vary the adhesion energy between the two by adjusting the fraction of anionic lipid (DOPS) in the otherwise zwitterionic lipid (DOPC) bilayer membrane. We find two distinct types of behavior: when the DOPS content of the membrane is 5% or higher, the high adhesion energy causes the nanoparticles to disrupt the vesicles upon adsorption. When the DOPS content is 4% or less, the adhesion of nanoparticles caused the vesicles to adhere to one another and form a rigid liposome gel. We propose that these two behaviors are explained by a transition from a partial wrapping of the nanoparticles to their complete envelopment by the membrane when the DOPS content exceeds 4.5%. We also detail methods for producing large quantities of the vesicle gel using cationic polymers in place of the nanoparticles. These findings could be used to to engineer new solid, semi-permeable materials that can encapsulate cargo, or to create cargo-carrying liposomes with the ability to rupture on trigger
Intracellular transport driven by cytoskeletal motors: General mechanisms and defects
Cells are strongly out-of-equilibrium systems driven by continuous energy
supply. They carry out many vital functions requiring active transport of
various ingredients and organelles, some being small, others being large. The
cytoskeleton, composed of three types of filaments, determines the shape of the
cell and plays a role in cell motion. It also serves as a road network for the
so-called cytoskeletal motors. These molecules can attach to a cytoskeletal
filament, perform directed motion, possibly carrying along some cargo, and then
detach. It is a central issue to understand how intracellular transport driven
by molecular motors is regulated, in particular because its breakdown is one of
the signatures of some neuronal diseases like the Alzheimer.
We give a survey of the current knowledge on microtubule based intracellular
transport. We first review some biological facts obtained from experiments, and
present some modeling attempts based on cellular automata. We start with
background knowledge on the original and variants of the TASEP (Totally
Asymmetric Simple Exclusion Process), before turning to more application
oriented models. After addressing microtubule based transport in general, with
a focus on in vitro experiments, and on cooperative effects in the
transportation of large cargos by multiple motors, we concentrate on axonal
transport, because of its relevance for neuronal diseases. It is a challenge to
understand how this transport is organized, given that it takes place in a
confined environment and that several types of motors moving in opposite
directions are involved. We review several features that could contribute to
the efficiency of this transport, including the role of motor-motor
interactions and of the dynamics of the underlying microtubule network.
Finally, we discuss some still open questions.Comment: 74 pages, 43 figure
Efficiency of Organelle Capture by Microtubules as a Function of Centrosome Nucleation Capacity: General Theory and the Special Case of Polyspermia
Transport of organelles along microtubules is essential for the cell metabolism and morphogenesis. The presented analysis derives the probability that an organelle of a given size comes in contact with the microtubule aster. The question is asked how this measure of functionality of the microtubule aster is controlled by the centrosome. A quantitative model is developed to address this question. It is shown that for the given set of cellular parameters, such as size and total tubulin content, a centrosome nucleation capacity exists that maximizes the probability of the organelle capture. The developed general model is then applied to the capture of the female pronucleus by microtubules assembled on the sperm centrosome, following physiologically polyspermic fertilization. This application highlights an unintuitive reflection of nonlinearity of the nucleated polymerization of the cellular pool of tubulin. The prediction that the sperm centrosome should lower its nucleation capacity in the face of the competition from the other sperm is a stark illustration of the new optimality principle. Overall, the model calls attention to the capabilities of the centrosomal pathway of regulation of the transport-related functionality of the microtubule cytoskeleton. It establishes a quantitative and conceptual framework that can guide experiment design and interpretation
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