Steady-state MreB helices inside bacteria: dynamics without motors

Within individual bacteria, we combine force-dependent polymerization dynamics of individual MreB protofilaments with an elastic model of protofilament bundles buckled into helical configurations. We use variational techniques and stochastic simulations to relate the pitch of the MreB helix, the total abundance of MreB, and the number of protofilaments. By comparing our simulations with mean-field calculations, we find that stochastic fluctuations are significant. We examine the quasi-static evolution of the helical pitch with cell growth, as well as timescales of helix turnover and denovo establishment. We find that while the body of a polarized MreB helix treadmills towards its slow-growing end, the fast-growing tips of laterally associated protofilaments move towards the opposite fast-growing end of the MreB helix. This offers a possible mechanism for targeted polar localization without cytoplasmic motor proteins.Comment: 7 figures, 1 tabl

Curvature condensation and bifurcation in an elastic shell

We study the formation and evolution of localized geometrical defects in an indented cylindrical elastic shell using a combination of experiment and numerical simulation. We find that as a symmetric localized indentation on a semi-cylindrical shell increases, there is a transition from a global mode of deformation to a localized one which leads to the condensation of curvature along a symmetric parabolic crease. This process introduces a soft mode in the system, converting a load-bearing structure into a hinged, kinematic mechanism. Further indentation leads to twinning wherein the parabolic crease bifurcates into two creases that move apart on either side of the line of symmetry. A qualitative theory captures the main features of the phenomena and leads to sharper questions about the nucleation of these defects.Comment: 4 pages, 5 figures, submitted to Physical Review Letter

Elastic cavitation, tube hollowing, and differential growth in plants and biological tissues

Elastic cavitation is a well-known physical process by which elastic materials under stress can open cavities. Usually, cavitation is induced by applied loads on the elastic body. However, growing materials may generate stresses in the absence of applied loads and could induce cavity opening. Here, we demonstrate the possibility of spontaneous growth-induced cavitation in elastic materials and consider the implications of this phenomenon to biological tissues and in particular to the problem of schizogenous aerenchyma formation

Rotating strings

Analytical expressions are provided for the configurations of an inextensible, flexible, twistable inertial string rotating rigidly about a fixed axis. Solutions with trivial radial dependence are helices of arbitrary radius and pitch. Non-helical solutions are governed by a cubic equation whose roots delimit permissible values of the squared radial coordinate. Only curves coplanar with the axis of rotation make contact with it.Comment: added to discussion and made small revisions to tex

On the General Analytical Solution of the Kinematic Cosserat Equations

Based on a Lie symmetry analysis, we construct a closed form solution to the kinematic part of the (partial differential) Cosserat equations describing the mechanical behavior of elastic rods. The solution depends on two arbitrary analytical vector functions and is analytical everywhere except a certain domain of the independent variables in which one of the arbitrary vector functions satisfies a simple explicitly given algebraic relation. As our main theoretical result, in addition to the construction of the solution, we proof its generality. Based on this observation, a hybrid semi-analytical solver for highly viscous two-way coupled fluid-rod problems is developed which allows for the interactive high-fidelity simulations of flagellated microswimmers as a result of a substantial reduction of the numerical stiffness.Comment: 14 pages, 3 figure

Integrability of a conducting elastic rod in a magnetic field

We consider the equilibrium equations for a conducting elastic rod placed in a uniform magnetic field, motivated by the problem of electrodynamic space tethers. When expressed in body coordinates the equations are found to sit in a hierarchy of non-canonical Hamiltonian systems involving an increasing number of vector fields. These systems, which include the classical Euler and Kirchhoff rods, are shown to be completely integrable in the case of a transversely isotropic rod; they are in fact generated by a Lax pair. For the magnetic rod this gives a physical interpretation to a previously proposed abstract nine-dimensional integrable system. We use the conserved quantities to reduce the equations to a four-dimensional canonical Hamiltonian system, allowing the geometry of the phase space to be investigated through Poincar\'e sections. In the special case where the force in the rod is aligned with the magnetic field the system turns out to be superintegrable, meaning that the phase space breaks down completely into periodic orbits, corresponding to straight twisted rods.Comment: 19 pages, 1 figur

Mechanical response of plectonemic DNA: an analytical solution

We consider an elastic rod model for twisted DNA in the plectonemic regime. The molecule is treated as an impenetrable tube with an effective, adjustable radius. The model is solved analytically and we derive formulas for the contact pressure, twisting moment and geometrical parameters of the supercoiled region. We apply our model to magnetic tweezer experiments of a DNA molecule subjected to a tensile force and a torque, and extract mechanical and geometrical quantities from the linear part of the experimental response curve. These reconstructed values are derived in a self-contained manner, and are found to be consistent with those available in the literature.Comment: 14 pages, 4 figure

Dynamics of filaments and membranes in a viscous fluid

Motivated by the motion of biopolymers and membranes in solution, this article presents a formulation of the equations of motion for curves and surfaces in a viscous fluid. We focus on geometrical aspects and simple variational methods for calculating internal stresses and forces, and we derive the full nonlinear equations of motion. In the case of membranes, we pay particular attention to the formulation of the equations of hydrodynamics on a curved, deforming surface. The formalism is illustrated by two simple case studies: (1) the twirling instability of straight elastic rod rotating in a viscous fluid, and (2) the pearling and buckling instabilities of a tubular liposome or polymersome.Comment: 26 pages, 12 figures, to be published in Reviews of Modern Physic

Thrombosis Is Reduced by Inhibition of COX-1, but Unaffected by Inhibition of COX-2, in an Acute Model of Platelet Activation in the Mouse

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Comparison of the efficacy and safety outcomes of edoxaban in 8040 women versus 13 065 men with atrial fibrillation in the ENGAGE AF-TIMI 48 trial.

BACKGROUND: Female sex is an independent risk factor for stroke and systemic embolic events in patients with atrial fibrillation. This study aimed to examine the efficacy and safety profile of edoxaban in women versus men. METHODS: The ENGAGE AF-TIMI 48 trial (Effective Anticoagulation with Factor Xa Next Generation in Atrial Fibrillation-Thrombolysis in Myocardial Infarction 48) randomly assigned 21 105 patients (8040 women) with atrial fibrillation and CHADS2 score ≥2 either to a higher-dose edoxaban regimen, a lower-dose edoxaban regimen, or warfarin. The primary end points of the trial were the composite of stroke or systemic embolic events (efficacy), and International Society on Thrombosis and Haemostasis-defined major bleeding (safety). RESULTS: In comparison with men, women were older, had lower body weight, were more likely to have hypertension and renal dysfunction, but less likely to smoke, drink alcohol, or have diabetes or coronary artery disease. Pretreatment endogenous factor Xa activity was significantly higher in women than in men (92.5% versus 86.1%, P<0.001). Treatment with edoxaban in women resulted in greater peak edoxaban concentration and inhibition of endogenous factor Xa in comparison with men, resulting in similar endogenous factor Xa activity between the sexes 2 to 4 hours after dose. Treatment with higher-dose edoxaban regimen (versus warfarin) resulted in similar reduction in the risk of stroke/systemic embolic events (women: hazard ratio [HR], 0.87 [0.69-1.11], men: HR, 0.87 [0.71-1.06]; P-interaction=0.97) and major bleeding (women: HR, 0.74 [0.59-0.92], men: HR, 0.84 [0.72-0.99]; P-interaction=0.34) in women and men. However, women assigned to higher-dose edoxaban regimen experienced greater reductions in hemorrhagic stroke (HR, 0.30 [95% CI, 0.15-0.59] versus HR, 0.70 [95% CI, 0.46-1.06]), intracranial bleeding (HR, 0.20 [95% CI, 0.10-0.39] versus HR, 0.63 [95% CI, 0.44-0.89]), and life-threatening or fatal bleeding (HR, 0.25 [95% CI, 0.15-0.42] versus HR, 0.72 [95% CI, 0.54-0.96]) than men (each P-interaction<0.05). CONCLUSIONS: Despite many differences in baseline characteristics between women and men and higher baseline endogenous factor Xa levels in women, the intensity of anticoagulation achieved with edoxaban between the sexes was similar. Treatment with higher-dose edoxaban regimen resulted in an even greater reduction in hemorrhagic stroke and several serious bleeding outcomes in women than in men, whereas the efficacy profile was similar between sexes