Density fields for branching, stiff networks in rigid confining regions.

We develop a formalism to describe the equilibrium distributions for segments of confined branched networks consisting of stiff filaments. This is applicable to certain situations of cytoskeleton in cells, such as for example actin filaments with branching due to the Arp2/3 complex. We develop a grand ensemble formalism that enables the computation of segment density and polarisation profiles within the confines of the cell. This is expressed in terms of the solution to nonlinear integral equations for auxiliary functions. We find three specific classes of behaviour depending on filament length, degree of branching and the ratio of persistence length to the dimensions of the geometry. Our method allows a numerical approach for semi-flexible filaments that are networked.

The Precision Control of Autophagic Flux and Vesicle Dynamics-A Micropattern Approach.

Autophagy failure is implicated in age-related human disease. A decrease in the rate of protein degradation through the entire autophagy pathway, i.e., autophagic flux, has been associated with the onset of cellular proteotoxity and cell death. Although the precision control of autophagy as a pharmacological intervention has received major attention, mammalian model systems that enable a dissection of the relationship between autophagic flux and pathway intermediate pool sizes remain largely underexplored. Here, we make use of a micropattern-based fluorescence life cell imaging approach, allowing a high degree of experimental control and cellular geometry constraints. By assessing two autophagy modulators in a system that achieves a similarly raised autophagic flux, we measure their impact on the pathway intermediate pool size, autophagosome velocity, and motion. Our results reveal a differential effect of autophagic flux enhancement on pathway intermediate pool sizes, velocities, and directionality of autophagosome motion, suggesting distinct control over autophagy function. These findings may be of importance for better understanding the fine-tuning autophagic activity and protein degradation proficiency in different cell and tissue types of age-associated pathologies.

Influence of weak reversible cross-linkers on entangled polymer melt dynamics.

In this paper, we study a system of entangled chains that bear reversible cross-links in a melt state. The cross-links are tethered uniformly on the backbone of each chain. A slip-link type model for the system is presented and solved for the relaxation modulus. The effects of entanglements and reversible cross-linkers are modelled as a discrete form of constraints that influence the motion of the primitive path. In contrast to a non-associating entangled system, the model calculations demonstrate that the elastic modulus has a much higher first plateau and a delayed terminal relaxation. These effects are attributed to the evolution of the entangled chains, as influenced by tethered reversible linkers. The model is solved for the case when the linker survival time τs is greater than the entanglement time τe, but less than the Rouse time τR.

Active force maintains the stability of a contractile ring.

We investigate a system of sufficiently dense polar actin filaments considered rigid and cross-linked by dimer myosin II protein within the contractile ring. The Langevin dynamics of this system is cast in a functional integral formalism and then transformed into density variables. Using the dynamical Random Phase Approximation (RPA) along with the a one-dimensional Langevin dynamics simulation (LDS), we investigate the structural integrity of the actin bundle network. The active force and the networking force reveal a non-trivial diffusive behaviour of the filaments within the ring. We conclude on when the active and networking forces lead to the contractile ring breaking down. The non-equilibrium active force is predominantly responsible for the prevention of the gaps in the ring.

In situ X-ray structural studies of a flexible host responding to incremental gas loading.

Crystallographic pressure-lapse snapshots of a porous material responding to gas loading were used to investigate the stepwise uptake of carbon dioxide and acetylene molecules into discrete confined spaces. Based on the data, a qualitative statistical mechanical model was devised that reproduces even subtle features in the experimental gas sorption isotherms.

Field-theoretical approach to a dense polymer with an ideal binary mixture of clustering centers.

We propose a field-theoretical approach to a polymer system immersed in an ideal mixture of clustering centers. The system contains several species of these clustering centers with different functionality, each of which connects a fixed number segments of the chain to each other. The field theory is solved using the saddle point approximation and evaluated for dense polymer melts using the random phase approximation. We find a short-ranged effective intersegment interaction with strength dependent on the average segment density and discuss the structure factor within this approximation. We also determine the fractions of linkers of the different functionalities.