Branching, Capping, and Severing in Dynamic Actin Structures

Branched actin networks at the leading edge of a crawling cell evolve via protein-regulated processes such as polymerization, depolymerization, capping, branching, and severing. A formulation of these processes is presented and analyzed to study steady-state network morphology. In bulk, we identify several scaling regimes in severing and branching protein concentrations and find that the coupling between severing and branching is optimally exploited for conditions {\it in vivo}. Near the leading edge, we find qualitative agreement with the {\it in vivo} morphology.Comment: 4 pages, 2 figure

Soft Listeria: actin-based propulsion of liquid drops

We study the motion of oil drops propelled by actin polymerization in cell extracts. Drops deform and acquire a pear-like shape under the action of the elastic stresses exerted by the actin comet. We solve this free boundary problem and calculate the drop shape taking into account the elasticity of the actin gel and the variation of the polymerization velocity with normal stress. The pressure balance on the liquid drop imposes a zero propulsive force if gradients in surface tension or internal pressure are not taken into account. Quantitative parameters of actin polymerization are obtained by fitting theory to experiment.Comment: 5 pages, 4 figure

Classical and Emerging Regulatory Mechanisms of Cytokinesis in Animal Cells

The primary goal of cytokinesis is to produce two daughter cells, each having a full set of chromosomes. To achieve this, cells assemble a dynamic structure between segregated sister chromatids called the contractile ring, which is made up of filamentous actin, myosin-II, and other regulatory proteins. Constriction of the actomyosin ring generates a cleavage furrow that divides the cytoplasm to produce two daughter cells. Decades of research have identified key regulators and underlying molecular mechanisms; however, many fundamental questions remain unanswered and are still being actively investigated. This review summarizes the key findings, computational modeling, and recent advances in understanding of the molecular mechanisms that control the formation of the cleavage furrow and cytokinesi

The Force-Velocity Relation for Growing Biopolymers

The process of force generation by the growth of biopolymers is simulated via a Langevin-dynamics approach. The interaction forces are taken to have simple forms that favor the growth of straight fibers from solution. The force-velocity relation is obtained from the simulations for two versions of the monomer-monomer force field. It is found that the growth rate drops off more rapidly with applied force than expected from the simplest theories based on thermal motion of the obstacle. The discrepancies amount to a factor of three or more when the applied force exceeds 2.5kT/a, where a is the step size for the polymer growth. These results are explained on the basis of restricted diffusion of monomers near the fiber tip. It is also found that the mobility of the obstacle has little effect on the growth rate, over a broad range.Comment: Latex source, 9 postscript figures, uses psfig.st

The Accumulation of Deficits with Age and Possible Invariants of Aging

This paper extends a method of apprising health status to a broad range of ages from adolescence to old age. The “frailty index” is based on the accumulation of deficits (symptoms, signs, disease classifications) as analyzed in the National Population Health Survey, a representative Canadian population sample (n = 81,859). The accumulation of deficits has both an age-independent (background) component and an age-dependent (exponential) component, akin to the well-known Gompertz-Makeham model for the risk of mortality. While women accumulate more deficits than men of the same age, on average, their rate of accumulation is lower, so the difference in the level of deficits between men and women decreases with age. Two possible invariants of the process of accumulation of deficits were found: (1) the age at which the average proportion of deficits coincides for men and women is 94 years, which closely matches the species-specific lifespan in humans (95 ± 2); (2) the value of the frailty index (proportion of deficits), which corresponds to that age (0.18). The similarity between mortality kinetics and the accumulation of deficits (frailty kinetics), and the coincidence of the time parameters in the frailty and mortality models make it possible to express mortality risk in terms of accumulated deficits. This provides a simple and accessible tool that might have potential in a number of biomedical applications

Self-organization in systems of self-propelled particles

We investigate a discrete model consisting of self-propelled particles that obey simple interaction rules. We show that this model can self-organize and exhibit coherent localized solutions in one- and in two-dimensions.In one-dimension, the self-organized solution is a localized flock of finite extent in which the density abruptly drops to zero at the edges.In two-dimensions, we focus on the vortex solution in which the particles rotate around a common center and show that this solution can be obtained from random initial conditions, even in the absence of a confining boundary. Furthermore, we develop a continuum version of our discrete model and demonstrate that the agreement between the discrete and the continuum model is excellent.Comment: 4 pages, 5 figure

New Proposed Mechanism of Actin-Polymerization-Driven Motility

We present the first numerical simulation of actin-driven propulsion by elastic filaments. Specifically, we use a Brownian dynamics formulation of the dendritic nucleation model of actin-driven propulsion. We show that the model leads to a self-assembled network that exerts forces on a disk and pushes it with an average speed. This simulation approach is the first to observe a speed that varies non-monotonically with the concentration of branching proteins (Arp2/3), capping protein and depolymerization rate (ADF), in accord with experimental observations. Our results suggest a new interpretation of the origin of motility that can be tested readily by experiment.Comment: 31 pages, 5 figure

Crecimiento in-vitro de raíces de Manihot esculenta en distintas condiciones de iluminación y temperatura

Se determinó el crecimiento in-vitro de raíces de Manihot esculenta en condiciones de iluminación y temperatura diferentes.Se utilizaron raíces obtenidas de ápices cultivados in-vitro en medio de Torrey (sólido), en cuartos, invernáculos y estufas, haciendo tres variantes: 1)- Luz contínua durante las 24 hs.; 2)- Día natural (de 11 hs. de luz y 13 hs. de oscuridad); 3)- Oscuridad durante las 24 hs. En cada variante se estudiaron dos subvariantes: a)- con temperaturas elevadas día y noche yb)- temperatura alta de día y baja de noche. La fuente de iluminación en la variante de luz contínua fue el sol durante el día y luz artificial de noche.Se encontró que la composición del medio nutritivo influye en el crecimiento de los ápices y en la rizogénesis, dando mejor resultado el medio de Torrey que el de White; que la temperatura Alta de día y Alta de noche con luz contínua o natural, coayudan al crecimiento de las raíces cultivadas invitro. Además se encontró una coloración verde en las raíces, posiblemente debido a la formación de clorofila por efecto de la iluminación