This Week in Science

Science  19 Jan 2007:
Vol. 315, Issue 5810, pp. 297
  1. Cellular Morphogenesis with a Twist


    Cellular morphogenesis is important throughout much of early development in multilcellular animals. During Drosophila gastrulation, epithelial cells on the ventral side of the embryo display apical constrictions that cause the invagination of the mesoderm and formation of a ventral furrow. Kölsch et al. (p. 384) now identify a target of the transcriptional activator Twist. This transmembrane protein, T48, coordinates with signaling factors Fog and Cta to localize the cytoskeletal modulator RhoGEF2 to the apical side of ventral cells in order to direct apical constriction and ventral furrow formation.

  2. Enabling Innovation

    In open innovation, different actors work together in a flexible manner so that they can develop products and services more efficiently than they could on their own. Dearing (p. 344) relates ways companies think about innovation within the policies that European governments are putting in place to foster productive innovation. Qualities that define effective ecosystems for innovation include the potential for market growth, straightforward and effective regulations, and the availability of skilled resources.

  3. Aluminum Pyramid

    The tendency of boron to compensate for its electron deficiency by forming elaborate polyhedral hydride clusters has long intrigued chemists and given rise to a detailed series of bonding rules that rationalize cage geometries based on electronic structure. In contrast, aluminum has not evinced a comparably rich hydride cluster chemistry, despite sharing boron's valence structure. However, photoelectron spectroscopy studies by Li et al. (p. 356) indicate that the neutral Al4H6 cluster is actually quite stable. Density functional theory supports a distorted tetrahedral aluminum arrangement. The stability of this molecule suggests the potential for synthesizing a broad range of (AlH)n structures.

  4. Random Stacking Beats the Heat

    Materials with low thermal conductivity are not only useful as thermal barriers but thermoelectric energy conversion. Chiritescu et al. (p. 351, published online 14 December; see the Perspective by Goodson) find that when tungsten selenide, a layered material similar to graphite in the weakness of attraction between its layers, is grown from alternating thin films of W and Se, the sheets stack in a random manner. This disordering, coupled with the high in-plane ordering of the layers, leads to an extremely low crossplane thermal conductivity for a fully solid film—as low as 0.05 watts per meter per kelvin at room temperature, or 30 times less than the c-axis thermal conductivity of single-crystal WSe2. Disruption of the in-plane ordering by ion bombardment actually increased the thermal conductivity of the material.

  5. Electric-Field-Modulated Magnetism


    The ability to modulate the magnetic properties of a material with an applied electric field offers the potential of low-power consumption and fast memory devices. Weisheit et al. (p. 349) report on the fabrication of a trilayer junction of metallic ferromagnetic thin films immersed in a dry propylene carbonate electrolyte. The ferromagnetic magnetization in the layer in contact with the electrolyte, a 2-nanometer-thick FePd film, could be modified considerably by applying a voltage across this barrier with a Pt counter electrode.

  6. The Ends Enable the Means

    During their synthesis, nanoparticles are often coated with a capping layer of rodlike molecules to prevent their further growth or agglomeration. Such a layer might be assumed to be isotropic, so that further derivatization or attempts to connect nanoparticles would be nonselective in terms of bonding directions. However, when rodlike molecules pack on a spherical object, at least two defect areas must form at opposite poles (much in the same way that hair on a person's head must adopt a whirl pattern). DeVries et al. (p. 358) exploit this phenomenon to selectively bond two different types of ligands to metal nanoparticles, such that they can be further reacted at the poles to give them directional bonding. The nanoparticles could then be formed into free-standing films.

  7. Water Marks the Asthenosphere

    Plate tectonics assumes that rigid plates float and move over the weaker asthenosphere, which extends from about 60 to 220 kilometers below the oceans and 150 kilometers below continents. The softness of the asthenosphere may be caused by pockets of hydrous melt. Mierdel et al. (p. 364; see the Perspective by Bolfan-Casanova) have performed experiments which show that the asthenosphere could coincide with a zone that marks a minimum in the solubility of water in mantle minerals. A sharp drop occurs in water solubility in aluminous orthopyroxene with pressure, or equivalently depth, whereas the water solubility in olivine continuously increases. The limits of the asthenosphere would be the regions where water comes out of solution and forms pockets of hydrous silicate melt.

  8. Modeling the Cytoskeleton

    The cytoskeleton, the mechanical framework of cells, represents a nonequilibrium active machinery that can adapt its mechanics to perform tasks such as cell locomotion. Mizuno et al. (p. 370) show that in a reconstituted model system comprising actin filaments, a cross-linker and a motor protein (myosin II), the motor activity controls the mechanical properties of the actin network. Adenosine triphosphate could promote near 100-fold increase in stiffness and change the viscoelastic response of the network. A quantitative model connects the large-scale properties of the gel to molecular force generation.

  9. Sideways with a Twist


    The family of ABC membrane transporters uses the energy from adenosine triphosphate, hydrolyzed by the nucleotide-binding domain (NBD), to power transport of substances into or out of the cell via the transmembrane domain (TMD). Pinkett et al. (p. 373, published online 7 December) report the NBD-TMD structure of an inward-facing conformation of a bacterial metal chelate importer and compare it to the outward-facing structure of another ABC family member, the vitamin B12 importer. Small shifts in the relative orientations of the membrane-spanning helices in the TMD suffice to switch the accessibility of the central cavity from periplasmic to cytoplasmic. Furthermore, these shifts are associated with, and perhaps driven by, twisting and translational conformational changes in the NBDs.

  10. Making (and Breaking) the Switch

    B cells produce different classes of antibodies by combining the highly variable antigen-binding front-end with one of a selection of functional rear-ends through class switch recombination, in which somatic gene rearrangement brings together intronic switch region sequences that flank the constant region segments. After generation of double-strand breaks by the cytidine deaminase AID and subsequent end-joining, intervening DNA is deleted and the upstream variable sequence meets its selected constant-region partner. To look more closely at the role AID and the switch regions themselves play in this process, Zarrin et al. (p. 377, published online 14 December; see the Perspective by Chaudhuri and Jasin) replaced the switch sequences with endonuclease sites from yeast, which allows the production of independent double-strand breaks. Surprisingly, class-switch recombination still took place with a measurable frequency and was independent of AID.

  11. Dance, Turtle, Dance

    The basic spinal network mechanisms underlying limb movements are still not fully understood. Investigating spinal cord preparations from adult turtles, Berg et al. (p. 390; see the cover and the Perspective by Kristan) describe how spinal networks operate during motor pattern generation. Balanced increases in synaptic excitation and inhibition operate in spinal motoneurons to produce rhythmic bursts of action potentials that are stochastic in nature. This activity contrasts strongly with the classical notion that antiphasic inhibition and excitation produces rhythmic activity in oscillatory spinal networks.

  12. Daydream Believer

    Despite the preponderance of daydreaming during everyday life, little is known about its neurocognitive underpinnings. How does the brain spontaneously produce the images, voices, thoughts, and feelings that constitute stimulus-independent thought? By analyzing functional magnetic resonance imaging signals associated with a cognitive task that was shown to induce a high frequency of mind-wandering, Mason et al. (p. 393) show that between periods of instrumental thought and goal-directed behavior, the mind exhibits tonic activity in a network of cortical regions. This so-called default network contributes to the production of stimulus-independent thought and the subjective experience of mind-wandering.

  13. Stabilizing Organic Glasses

    Unlike strong glass-formers such as silicates, most organic molecules will form glasses only under special conditions, such as by cooling the liquid phase rapidly or depositing the vapor on a cryogenically cooled substrate. Swallen et al. (p. 353, published online 7 December) show that controlled vapor-phase deposition onto substrate cooled to 50 kelvin below the conventional glass transition temperature, Tg, caused Tg of the resulting solid to increase; for [1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene], Tg increased from 347 to 363 kelvin. The authors attribute this increase in stability to the greater mobility of the molecules at higher substrate temperatures, which allowed more stable conformations to be accessed without actually forming crystallites.

  14. Predicting Sea-Level Rise

    Many different processes can contribute to sea-level rise, but too few of them are well enough understood for models to allow accurate process-based projections of sea-level rise to be made with much certainty. Rahmstorf (p. 368, published online 14 December) circumvents those critical gaps in understanding by using a semi-empirical technique, which uses the observed relation between rates of change of global mean surface air temperature and sea level for the 20th century to predict what sea level may be through 2100. In this way, using warming scenarios from the Intergovernmental Panel on Climate Change, he calculates a range of sea-level rise of 0.5 to 1.4 meters above the 1990 level by 2100. If correct, the uncertainty in current sea-level change predictions has been underestimated.

  15. Concentration-Controlled Release

    Litter decomposition is the primary source of fixed nitrogen for most terrestrial plants, but large-scale, long-term patterns of decomposition and nitrogen release are poorly understood. Using data from a 10-year global-scale experiment, Parton et al. (p. 361) present a simple model which shows that nitrogen release from leaf litter is mostly a function of initial concentration of nitrogen in the decomposing plant tissues and the mass of decaying matter remaining, regardless of climate, except in arid grasslands. This analysis suggests that fundamental physiological constraints on decomposers control the rate of nitrogen release regardless of climate or soil conditions, where decomposition is biologically mediated.

  16. Assessing Agriculture's Impact on Biodiversity

    Agriculture has been the major driver of biodiversity loss in many ecosystems, and the projected doubling of agricultural production by 2050 could have profound impacts on biodiversity and associated ecosystem services. Sustainable development requires that biodiversity conservation and increased agricultural production are reconciled. Butler et al. (p. 381; see the Perspective by Benton) present a riskassessment framework that predicts the impact of agricultural change on biodiversity and ecosystem services, using farmland birds in the United Kingdom as a model system. This generalizable framework accurately predicted both current conservation status and population growth rates.

  17. Finding Redox Active Cysteines

    Cysteine residues are found in a wide variety of proteins, in which they serve many functions. Identifying particular functions has required laborious individual characterization. Fomenko et al. (p. 387) present and experimentally verify an algorithm that can identify which cysteines act in redox reactions by searching for sporadic selenocysteine-Cys pairs in sequence databases. This methodology should assist in metagenomic analyses to identify oxidoreductase families.

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