This Week in Science

Science  11 Apr 2008:
Vol. 320, Issue 5873, pp. 151
  1. No Glue for Cuprate Superconductors?


    More than 20 years after their appearance, it is still unclear how the charge carriers in the high-temperature superconducting cuprates pair up and condense into the superconducting state. Some scenarios argue for a boson-mediated “glue” to bring them together, as in conventional superconductors, whereas in other scenarios, the glue is unnecessary and electronic correlation provides the necessary pairing energy. Pasupathy et al. (p. 196) now present temperature-dependent scanning tunneling spectroscopy data above and below the superconducting transition temperature that they believe to be strong evidence for the latter “no glue” picture.

  2. Magnetic Domain-Wall Memory

    Present memory storage is generally based on two architectures: random access memories, which are fast but expensive, and hard disk drives, which are cheap but slow. Other memory architectures are being developed, but they are also inherently two-dimensional and based on a one-bit, one-memory design. Thus, the expected improvement in memory density and speed is somewhat incremental. In an overview, Parkin et al. (p. 190) lay out the basic concept of “racetrack memory,” in which information is stored as a sequence of domain walls (interfaces between regions of different magnetization direction) on a magnetic strip. As many domain walls can be written on the wire and can be shifted along the strip by current-induced domain-wall motion, one-bit, many-memory designs are possible that could lead to orders of magnitude improvement in performance. A realization of the basic working principles of racetrack memory is presented by Hayashi et al. (p. 209), who show that it is possible to shift multiple domain walls to and fro along a magnetic nanowire repeatedly at high speed using nanosecond current pulses without any external magnetic fields.

  3. Reservoir Logs

    Although the contributions of melting glaciers and ice sheets to sea-level rise garner the most attention, global warming also increases sea level through the increased volume of warmer ocean water. When the thermal effects of ocean heating are calculated and the net observed rate of sea-level rise are subtracted, the remainder is commonly equated with ice-melt contributions. However, Chao et al. (p. 212; published online 13 March) have reconstructed the history of water impoundment in reservoirs filled during the last century and show that filling the reservoirs of the world depressed sea-level rise substantially. Thus, the amount of water contributed by ice melting was correspondingly higher than had been assumed.

  4. Hematite Currents

    A wide range of abundant minerals, such as hematite (Fe2O3), are semiconductors, but this electrical property has rarely been considered in analyses of these materials' natural geochemistry. Yanina and Rosso (p. 218, published online 6 March; see the Perspective by Eggleston) show through a series of carefully controlled experiments that when two crystallographically distinct faces of hematite are exposed to an acidic aqueous solution, a potential gradient is induced across the crystal by their differing ion adsorption properties. This potential drives redox chemistry at each surface by coupled current flow through the bulk mineral. Specifically, island growth is observed at one surface with compensatory etching at another. The authors argue that the finding should be broadly generalizable.

  5. Bacterial Speciation Uncovered


    The mating behavior, life history, and ecology of individual bacteria cannot be tracked, and our knowledge of the ecology of natural bacterial populations is rudimentary. By genotyping a large, broadly sampled collection of two important human zoonotic pathogens, Campylobacter jejuni and Campylobacter coli, Sheppard et al. (p. 237) have observed speciation going into reverse, probably as a consequence of intensive agricultural practice. These results offer a direct insight into mechanisms of bacterial speciation.

  6. Fast Receptors in Every Respect

    Lateral diffusion of receptors has been characterized as an important process for receptor trafficking in and out of synapses. The demonstration of this role has been limited to relatively slow events (in the range of minutes) that occur during development or turnover of receptors, during basal transmission, or during certain forms of neuronal plasticity. Heine et al. (p. 201; see the Perspective by Silver and Kanichay) now report that receptor lateral movements are also involved in fast (tens of milliseconds) regulation of synaptic transmission. Recovery from synaptic depression involves exchange of desensitized receptors with functional ones through lateral movements within or from nearby the postsynaptic density.

  7. Radioprotection: Taking the Toll Road

    Radiation therapy is a well-established and highly effective treatment for certain types of cancer, but it can destroy healthy cells in the body, especially bone-marrow cells and cells in the gastrointestinal tract. Burdelya et al. (p. 226; see news story by Bhattacharjee) have developed a drug that may prevent or reduce these side effects. The drug (a peptide called CBLB502) binds to Toll-like receptor 5 (TLR5) and activates the nuclear factor-κB signaling pathway, a pathway that cancer cells often activate to avoid cell death. When mice and rhesus monkeys were treated with CBLB502 shortly before exposure to lethal doses of total body irradiation, the animals exhibited less damage to healthy bone marrow and gastrointestinal cells and survived significantly longer than controls. In tumor-bearing mice, CBLB502 did not compromise the antitumor efficacy of radiation therapy.

  8. Mounting Mutations

    Of the numerous intracellular factors that help cells protect themselves from invading viruses, some of the most prominent belong to the APOBEC family of cytidine deaminases. These act against retroviruses such as HIV by peppering the single-stranded DNA generated during reverse transcription with deleterious C→U mutations. Now Vartanian et al. (p. 230) show that APOBEC editing also works on the human papillomavirus (HPV), a DNA virus. Equivalent mutations were observed in cells from plantar warts and precancerous cervical biopsies and in experiments in which plasmid DNA was cotransfected with different APOBEC family members. APOBEC enzymes seem to be able to mutate nuclear DNA, which raises the question whether elevated APOBEC activity, such as seen in HPV-infected cells, might have any causal link with the development of cancer.

  9. Keeping Axons on Track

    Like cars on a highway, neuronal signaling routes merge and diverge between origin and destination. By identifying developing motor and sensory neurons in the mouse embryo with distinguishable labels, Gallarda et al. (p. 233; see the Perspective by Murai and Pasquale) have visualized how the growing axons keep entanglement at bay. The axons grew into parallel but separated bundles. Signaling through the ephrin A receptors and ligands present on the growing axons served to keep like axons bundled with like.

  10. Leiomodin in Muscle Actin Assembly


    The dynamic assembly of the actin cytoskeleton plays a central role in many cellular functions, but actin polymerization does not occur spontaneously in cells. Filament nucleation factors mediate this process in a highly regulated and localized fashion. Chereau et al. (p. 239) describe an actin-filament nucleator from muscle cells, which they name leiomodin. Leiomodin nucleates actin filaments in muscle cell sarcomeres, with very high efficiency and without the need for other factors. The nucleation activity of leiomodin is enhanced by direct interaction with tropomyosin, which also mediates its localization to actin filament pointed ends in muscle sarcomeres. Thus, leiomodin appears to be adapted specifically to nucleate the de novo assembly of tropomyosin-decorated filaments in muscle cells.

  11. Promoting Polyketide Production

    Many useful natural products are produced in bacteria by modular polyketide synthases (PKS) in which each domain of a multidomain protein is used once in an “assembly-line” process. Eukaryotes also make polyketides, but reuse PKS domains in a poorly understood iterative process. Now Crawford et al. (p. 243; see the Perspective by Weissman) describe the functional dissection of a fungal PKS involved in aflatoxin biosynthesis that reveals the synthetic roles of each catalytic domain. The mechanistic features are likely to be general for the catalyzed production of aromatic polyketides and could guide rational engineering of these biosynthetic proteins.

  12. Synaptic Transmission in Living Color

    Actually observing microscopic events in real time can generate a richness of insight unattainable with static approaches. Westphal et al. (p. 246, published online 21 February; see the Perspective by Pinaud and Dahan) present a movie of synaptic vesicles in live neurons, using fluorescent markers to label the vesicles and then collecting video-rate optical images at subdiffraction resolution. As might have been anticipated, vesicles either moved short distances within a single synaptic bouton or made longer journeys into and out of multiple boutons. However, the ability to see nanoscale motion as it happens should promote ever deeper insight into synaptic transmission.

  13. Biased Reflectivity

    The optical properties of most materials generally respond rather weakly to applied electrical fields, but for graphene (individual layers of graphite), its combination of two-dimensional confinement and properties of its density of states that should cause the Fermi level to shift with applied voltages suggests that it could have a strong response. Wang et al. (p. 206, published online 13 March) applied voltages to graphene monolayers and bilayers in a simple gate geometry and observed changes in infrared reflectivity with applied voltages (up to 50 volts) of up to 2 to 6%, respectively. In addition to possible applications, these measurements allowed the band dispersion of the monolayer to be determined and revealed the presence of a van Hove singularity for the bilayer.

  14. Impactors Recorded in Osmium

    The terrestrial record of large impacts has been obscured by erosion and deformation of the continental crust and subduction of oceanic crust. Although some large impacts have been recognized, others may have been missed. One way to identify obscured impacts is to use records of elements that are concentrated in asteroid but rare in Earth's crust; such a search led to the discovery of the iridium anomaly at the Cretaceous-Tertiary boundary. Paquay et al. (p. 214) now show that the osmium isotope record is a sensitive recorder of impacts and can also be used to infer the size of the impactor, given the assumption of a chondritic source. This approach works because the osmium concentration of seawater is low and well-mixed, and a large impact, which predominantly vaporizes, adds osmium with a distinct isotopic composition to the oceans and sediments. Using this method, the authors constrain the size of the end Cretaceous (4 to 6 kilometers) and Eocene (3 kilometers) impactors.

  15. Biodiversity Maintenance in Madagascar

    Biodiversity is unevenly distributed across the globe, and areas with concentrations of high species diversity (“hotspots”) present tough challenges for conservation. Much conservation planning is based on low-resolution, expert-opinion driven inputs encompassing a small number of species, which lead to plans that may not adequately represent biodiversity. In an analysis of conservation priorities in Madagascar encompassing broad taxonomic and geographic breadth and spatial resolution, Kremen et al. (p. 222, see the cover) show that multitaxonomic, high-resolution approaches are critical for identifying areas likely to promote the persistence of the majority of species. This analysis has immediate relevance in Madagascar, where the government is tripling its protected-area network, and the approach is transferable to other global priority areas.