The cartography of cell motion

Cell motility plays an important role throughout biology, the polymerisation of actin being fundamental in producing protrusive force. However, it is increasingly apparent that intracellular pressure, arising from myosin-II contraction, is a co-driver of motility. In its extreme form, pressure manifests itself as hemispherical protrusions, referred to as blebs, where membrane is torn from the underlying cortex. Although many components and signalling pathways have been identified, we lack a complete model of motility, particularly of the regulation and mechanics of blebbing. Advances in microscopy are continually improving the quality of time series image data, but the absence of highthroughput tools for extracting quantitative numbers remains an analysis bottle-neck. We develop the next generation of the successful QuimP software designed for automated analysis of motile cells, producing quantitative spatio-temporal maps of protein distributions and changes in cell morphology. Key to QuimP's new functionality, we present the Electrostatic Contour Migration Method (ECMM) that provides high resolution tracking of local deformation with better uniformity and efficiency than rival methods. Photobleaching experiments are used to give insight into the accuracy and limitations of in silico membrane tracking algorithms. We employ ECMM to build an automated protrusion tracking method (ECMM-APT) sensitive not only to pseudopodia, but also the complex characteristics of high speed blebs. QuimP is applied to characterising the protrusive behaviour of Dictyostelium, induced to bleb by imaging under agar. We show blebs are characterised by distinct speed-displacement distributions, can reach speeds of 4.9μm/sec, and preferentially form at the anks during chemotaxis. Significantly, blebs emerge from at to concave membrane regions suggesting curvature is a major determinant of bleb location, size, and speed. We hypothesise that actin driven pseudopodia at the leading edge induce changes in curvature and therefore membrane tension, positive curvature inhibiting blebbing at the very front, and negative curvature enhancing blebbing at the sides. This possibly provides the necessary space for rear advancement. Furthermore, bleb kymographs reveal a retrograde shift of the cortex at the point of bleb expansion, suggesting inward contractive forces acting on the cortex even at concave regions. Strains defficient in phospholipid signalling show impaired chemotaxis and blebbing. Finally, we present further applications of QuimP, for example, we conclusively show that dishevelled is not polarised during Xenopus gastrulation, contrary to hypotheses in the literature

Pressure effects on charge, spin, and metal-insulator transitions in narrow bandwidth manganite Pr1−x_{1-x}Cax_{x}MnO3_{3}

Pressure effects on the charge and spin states and the relation between the ferromagnetic and metallic states were explored on the small bandwidth manganite Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ (x = 0.25, 0.3, 0.35). Under pressure, the charge ordering state is suppressed and a ferromagnetic metallic state is induced in all three samples. The metal-insulator transition temperature (T$_{MI}$) increases with pressure below a critical point P*, above which T$_{MI}$ decreases and the material becomes insulating as at the ambient pressure. The e$_{g}$ electron bandwidth and/or band-filling mediate the pressure effects on the metal-insulator transition and the magnetic transition. In the small bandwidth and low doping concentration compound (x = 0.25), the T$_{MI}$ and Curie temperature (T$_{C}$) change with pressure in a reverse way and do not couple under pressure. In the x = 0.3 compound, the relation of T$_{MI}$ and T$_{C}$ shows a critical behavior: They are coupled in the range of $\sim$0.8-5 GPa and decoupled outside of this range. In the x = 0.35 compound, T$_{MI}$ and T$_{C}$ are coupled in the measured pressure range where a ferromagnetic state is present

Spin-Coupled Local Distortions in Multiferroic Hexagonal HoMnO3

Local structural measurements have been performed on hexagonal HoMnO3 in order to ascertain the specific changes in bond distances which accompany magnetic ordering transitions. The transition from paramagnetic to the antiferromagetic (noncollinear) phase near ~70 K is dominated by changes in the a-b plane Mn-Mn bond distances. The spin rotation transition near ~40 K involves both Mn-Mn and nearest neighbor Ho-Mn interactions while the low temperature transition below 10 K involves all interactions, Mn-Mn, Ho-Mn (nearest and next nearest) and Ho-Ho correlations. These changes in bond distances reveal strong spin-lattice coupling. The similarity in magnitude of the change in J(Mn-Mn) and J(Ho-Mn) enhances the system frustration. The structural changes are interpreted in terms of a model of competing spin order and local structural distortions. Density functional calculations are used to estimate the energies associated with ionic displacements. The calculations also reveal asymmetric polarization of the charge density of Ho, O3 and O4 sites along the z-axis in the ferroelectric phase. This polarization facilitates coupling between Ho atoms on neighboring planes normal to the z-axis.Comment: 8 figure

X-ray absorption study of Ti-activated sodium aluminum hydride

Ti K-edge x-ray absorption near edge spectroscopy (XANES) was used to explore the Ti valence and coordination in Ti-activated sodium alanate. An empirical relationship was established between the Ti valence and the Ti K-edge onset based on a set of standards. This relationship was used to estimate oxidation states of the titanium catalyst in 2 mol% and 4 mol% Ti-doped NaAlH4. These results demonstrate that the formal titanium valence is zero in doped sodium alanate and nearly invariant during hydrogen cycling. A qualitative comparison of the edge fine structure suggests that the Ti is present on the surface in the form of amorphous TiAl3.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let

Classification of image distortions in terms of Petrov types

An observer surrounded by sufficiently small spherical light sources at a fixed distance will see a pattern of elliptical images distributed over the sky, owing to the distortion effect (shearing effect) of the spacetime geometry upon light bundles. In lowest non-trivial order with respect to the distance, this pattern is completely determined by the conformal curvature tensor (Weyl tensor) at the observation event. In this paper we derive formulas that allow to calculate these distortion patterns in terms of the Newman-Penrose formalism. Then we represent the distortion patterns graphically for all Petrov types, and we discuss their dependence on the velocity of the observer.Comment: 22 pages, 8 eps-figures; revised version, parts of Introduction and Conclusions rewritte

Methods for comparative evaluation of propulsion system designs for supersonic aircraft

The propulsion system comparative evaluation study was conducted to define a rapid, approximate method for evaluating the effects of propulsion system changes for an advanced supersonic cruise airplane, and to verify the approximate method by comparing its mission performance results with those from a more detailed analysis. A table look up computer program was developed to determine nacelle drag increments for a range of parametric nacelle shapes and sizes. Aircraft sensitivities to propulsion parameters were defined. Nacelle shapes, installed weights, and installed performance was determined for four study engines selected from the NASA supersonic cruise aircraft research (SCAR) engine studies program. Both rapid evaluation method (using sensitivities) and traditional preliminary design methods were then used to assess the four engines. The method was found to compare well with the more detailed analyses

The Size Distribution of Trans-Neptunian Bodies

[Condensed] We search 0.02 deg^2 for trans-Neptunian objects (TNOs) with m<=29.2 (diameter ~15 km) using the ACS on HST. Three new objects are discovered, roughly 25 times fewer than expected from extrapolation of the differential sky density Sigma(m) of brighter objects. The ACS and other recent TNO surveys show departures from a power law size distribution. Division of the TNO sample into ``classical Kuiper belt'' (CKB) and ``Excited'' samples reveals that Sigma(m) differs for the two populations at 96% confidence. A double power law adequately fits all data. Implications include: The total mass of the CKB is ~0.010 M_Earth, only a few times Pluto's mass, and is predominately in the form of ~100 km bodies. The mass of Excited objects is perhaps a few times larger. The Excited class has a shallower bright-end size distribution; the largest objects, including Pluto, comprise tens of percent of the total mass whereas the largest CKBOs are only ~2% of its mass. The predicted mass of the largest Excited body is close to the Pluto mass; the largest CKBO is ~60 times less massive. The deficit of small TNOs occurs for sizes subject to disruption by present-day collisions, suggesting extensive depletion by collisions. Both accretion and erosion appearing to have proceeded to more advanced stages in the Excited class than the CKB. The absence of distant TNOs implies that any distant (60 AU) population must have less than the CKB mass in the form of objects 40 km or larger. The CKB population is sparser than theoretical estimates of the required precursor population for short period comets, but the Excited population could be a viable precursor population.Comment: Revised version accepted to the Astronomical Journal. Numerical results are very slightly revised. Implications for the origins of short-period comets are substantially revised, and tedious material on statistical tests has been collected into a new Appendi

Guiding chemical pulses through geometry: Y-junctions

We study computationally and experimentally the propagation of chemical pulses in complex geometries.The reaction of interest, CO oxidation, takes place on single crystal Pt(110) surfaces that are microlithographically patterned; they are also addressable through a focused laser beam, manipulated through galvanometer mirrors, capable of locally altering the crystal temperature and thus affecting pulse propagation. We focus on sudden changes in the domain shape (corners in a Y-junction geometry) that can affect the pulse dynamics; we also show how brief, localized temperature perturbations can be used to control reactive pulse propagation.The computational results are corroborated through experimental studies in which the pulses are visualized using Reflection Anisotropy Microscopy.Comment: submitted to Phys. Rev.

Where is in a Name? A Survey of Mobility in Information-Centric Networks

Host mobility has been a long standing challenge in the current Internet architecture. Huge proportions of traffic are now attributed to mobile devices [1]; however, despite this promi-nence, mobility often remains a badly handled concept. Some have recently argued that the main reason for this lies in its choice of what to name [2]. The Internet Protocol (IP