Asymptotic dynamics of three-dimensional bipolar ultrashort electromagnetic pulses in an array of semiconductor carbon nanotubes.

We study the propagation of three-dimensional bipolar ultrashort electromagnetic pulses in an array of semiconductor carbon nanotubes at times much longer than the pulse duration, yet still shorter than the relaxation time in the system. The interaction of the electromagnetic field with the electronic subsystem of the medium is described by means of Maxwell's equations, taking into account the field inhomogeneity along the nanotube axis beyond the approximation of slowly varying amplitudes and phases. A model is proposed for the analysis of the dynamics of an electromagnetic pulse in the form of an effective equation for the vector potential of the field. Our numerical analysis demonstrates the possibility of a satisfactory description of the evolution of the pulse field at large times by means of a three-dimensional generalization of the sine-Gordon and double sine-Gordon equations.

Physical Model for Stabilization and Repair of Trans-endothelial Apertures.

Bacterial toxins that disrupt the stability of contractile structures in endothelial cells promote the opening of large-scale apertures, thereby breaching the endothelium barrier. These apertures are formed by fusion of the basal and apical membranes into a tunnel that spans the height of the cell. Subsequent to the aperture formation, an active repair process, driven by a stimulated polymerization of actin, results in asymmetrical membrane protrusions and, ultimately, the closure of the aperture. Here, we propose a physics-based model for the generation, stabilization and repair of trans-endothelial apertures. Our model is based on the mechanical interplay between tension in the plasma membrane and stresses that develop within different actin structures at the aperture's periphery. We suggest that accumulation of cytoskeletal fragments around the aperture's rim during the expansion phase results in parallel bundles of actin filaments and myosin motors, generating progressively greater contraction forces that resist further expansion of the aperture. Our results indicate that closure of the tunnel is driven by mechanical stresses that develop within a cross-linked actin gel that forms at localized regions of the aperture periphery. We show that stresses within the gel are due to continuous polymerization of actin filaments against the membrane surfaces of the aperture's edges. Based on our mechanical model, we construct a dynamic simulation of the aperture repair process. Our model fully accounts for the phenomenology of the trans-endothelial aperture formation and stabilization, and recaptures the experimentally observed asymmetry of the intermediate aperture shapes during closure. We make experimentally testable predictions for localization of myosin motors to the tunnel periphery and of adhesion complexes to the edges of apertures undergoing closure, and we estimate the minimal nucleation size of cross-linked actin gel that can lead to a successful repair of the aperture.

Large-scale curvature sensing by directional actin flow drives cellular migration mode switching.

Cell migration over heterogeneous substrates during wound healing or morphogenetic processes leads to shape changes driven by different organizations of the actin cytoskeleton and by functional changes including lamellipodial protrusions and contractile actin cables. Cells distinguish between cell-sized positive and negative curvatures in their physical environment by forming protrusions at positive ones and actin cables at negative ones; however, the cellular mechanisms remain unclear. Here, we report that concave edges promote polarized actin structures with actin flow directed towards the cell edge, in contrast to well-documented retrograde flow at convex edges. Anterograde flow and contractility induce a tension anisotropy gradient. A polarized actin network is formed, accompanied by a local polymerization-depolymerization gradient, together with leading-edge contractile actin cables in the front. These cables extend onto non-adherent regions while still maintaining contact with the substrate through focal adhesions. The contraction and dynamic reorganization of this actin structure allows forward movements enabling cell migration over non-adherent regions on the substrate. These versatile functional structures may help cells sense and navigate their environment by adapting to external geometric and mechanical cues.