This Week in Science

Science  25 Nov 2005:
Vol. 310, Issue 5752, pp. 1241
  1. A Single Spiral Around Saturn


    The braided structure of Saturn's delicate F ring, with its wispy interweaving strands, has long puzzled astronomers. From sequences of detailed images taken by the Cassini spacecraft, Charnoz et al. (p. 1300; see the Perspective by Showalter) show that the F ring is not so complex and takes the form of a loose single-arm spiral that wraps around the planet three times. After using simulations to explore the spiral's origin, the authors propose that the passage of one of Saturn's tiny moonlets close to the main F-ring band may have expelled material which, after many orbits, has been strung out into a spiral pattern.

  2. Hard Nanowired

    In transistor fabrication, regions with different types of semiconductor doping can be created through ion implantation and lithographic patterning. Yang et al. (p. 1304) now report on the gold-nanocluster-catalyzed synthesis of silicon nanowires that are both highly uniform in diameter with lengths exceeding 10 micrometers and whose pattern of doping can be altered anywhere along the nanowire. Regions of light or heavy n-type doping were created by changing the amount of phosphine introduced during growth and were imaged by scanning gate microscopy. Nanowires with different patterns of doping regions were used to create an address decoder, and at low temperatures, the different doping regions defined quantum dots that exhibited Coulomb oscillations.

  3. Springing Back


    Many materials can recover their shape after compressive stress, but they can pass a limit after which they either fail completely or fail to reexpand. Cao et al. (p. 1307) have fabricated freestanding films consisting of aligned carbon nanotubes that behave as open-cell flexible foams. The films can be reversibly squeezed to only 15% of their original thickness without structural failure, despite the significant zigzag bucking of the nanotubes. The nanotubes act as elastic compression springs; they are highly compressible along their axis but regain most of their free length after a compressive load is released.

  4. Airing Out Older Glacial Cycles

    Air trapped in glacial ice contains the only reliable direct record of atmospheric composition before scientific sampling began in the 18th century. Since 1997, the oldest ice available for analysis was that from the Vostok, Antarctica, ice core, which extends back to 420,000 years ago and covers four complete glacial cycles. A new ice core from the EPICA Dome C site in Antarctica now extends back to an age of 740,000 years or more. Two reports present data on the composition of the atmosphere between 400,000 and 650,000 years ago, an interval soon after glacial cycles switched from a dominantly 41,000-year period to the dominantly 100,000-year period that occurs today (see the Perspective by Brook). Siegenthaler et al. (p. 1313) present measurements of the atmospheric concentration of CO2, the most important trace greenhouse gas, and show how its concentration varied during a much more narrow range than it did during the past 400,000 years. Spahni et al. (p. 1317; see the cover) present parallel measurements for two other important trace greenhouse gases, CH4 and N2O. As is the case for CO2, CH4 varied between much more narrow bounds during that time, although N2O varied just as much as it did in the nearly half-million years since then. These data will be keys to understanding how the carbon cycle has operated since the middle of the Pleistocene epoch.

  5. Hanging On to Introns

    Evolution has increased the complexity of organisms, especially bacteria and single-celled eukaryotes that are contrasted with vertebrates, but it does not necessarily follow that the genes and genomes of organisms that arose early in evolution should be less complex than those of newer species. Raible et al. (p. 1325) analyzed the genome of the marine ragworm, Platynereis dumerilii, a possible “living fossil,” and show that the structure of its genes is remarkably complex, and that its genome has an intron richness which resembles that of human genome. These two very different organisms have retained this genetic complexity, which has been lost in the other insects and nematodes whose genomes have been studied.

  6. Promoting Lipid Processing for Presentation

    A subpopulation of T cells recognizes antigens derived from lipids, rather than from proteins, and these lipid antigens are presented by members of the CD1 family of cell surface proteins. However, one CD1 family member, CD1e, does not seem to present lipids directly. De la Salle et al. (p. 1321) observed that a lipid antigen that depends on processing to stimulate T cells via another member of the CD1 family (CD1b) could not do so in the absence of CD1e. CD1e was required to assist in modifying a lipid precursor within the lysosome, which allowed intracellular association with CD1b and subsequent presentation to T cells. Thus, the role of this remaining CD1 family member appears to involve processing, rather than direct presentation of, antigenic lipids to T cells.

  7. Cell Fate Specification in the Worm

    Early in the development of the nematode worm Caenorhabditis elegans, the vulva is composed of six precursor cells that have the potential to develop into one of three vulval cell fates, termed 1°, 2°, and 3°. The 1° and 2° fates are patterned through the cross-talk between two signaling pathways, the EGFR-MAPK pathway and the LIN-12/Notch pathway. Yoo and Greenwald (p. 1330, published online 20 October; see the Perspective by Karp and Ambros) now show that a specific microRNA (miRNA), identified by a computational prediction analysis, is involved in specifying the 2° vulval cell fate. The miRNA mir-60 is a direct transcriptional target of LIN-12/Notch, and, in turn, an ortholog of the oncogene Vav is the target of mir-60. The regulatory circle is completed by the regulation of LIN-12 activity by Vav.

  8. Greenhouse Europe


    Assessing the likely affects of global climate change remains a high priority for all nations. Schröter et al. (p. 1333, published online 27 October) show how the pattern of Europe's vulnerability to global changes is likely to change in the 21st century caused by the decreased supply of ecosystem services such as plant growth, carbon sequestration, biodiversity, water, and soil fertility. They apply four climate models to Europe and combine them with socioeconomic scenarios to project the evolution of a range of ecosystem services for the coming century, ranging from carbon sequestration to freshwater provisioning and biodiversity. The loss of these services is likely to be accentuated particularly in the Mediterranean and in mountainous regions.

  9. Action, Choice, and Reward

    To attain specific goals, humans and animals choose actions based on current behavioral contexts and on past experiences. Samejima et al. (p. 1337) examined single unit activity within the basal ganglia in monkeys performing a simple motor decision task in which rewarded action and the relative reward value were independently manipulated. Cells were identified that showed activity associated with a preferred direction, amount of reward, or some combination of both. About one-third of neurons in the dorsal striatum coded for action value. A reinforcement learning algorithm, trained on the same sequence of trials presented to the animal, could predict trial-by-trial neural activity. The dorsal striatum may be the site of reinforcement learning of action values that are then used to select actions further downstream in the basal ganglia.

  10. Getting to the Bottom of Drug Cravings

    Behavioral sensitization, an animal model for drug craving, involves neural adaptations in the mesocorticolimbic regions of the brain, including the nucleus accumbens. Synaptic plasticity in the nucleus accumbens, especially long-term depression (LTD), plays an important role in behavioral sensitization. Using new synthetic peptide inhibitors, Brebner et al. (p. 1340) showed that LTD in nucleus accumbens is mediated by clathrin-dependent, regulated endocytosis of AMPA receptors. An AMPA-specific inhibitor delivered to neurons in the nucleus accumbens blocked behavioral sensitization. Thus, LTD in the nucleus accumbens is mediated by facilitated endocytosis of postsynaptic AMPA receptors and may be involved in the pathogenesis of drug craving.

  11. Signaling from Wingless

    Despite the extensive study of the Wingless (Wg) or Wnt signaling pathway in regulating development and cancer, a previously unrecognized mechanism has been uncovered for Wg signaling at developing synapses in the Drosophila nervous system. Mathew et al. (p. 1344; see the Perspective by Arias) found that the Wg receptor DFrizzled2 (DFz2) can be cleaved and translocated from the plasma membrane to the area of the cell just outside the nucleus. In response to Wg signals, the C terminal portion of the receptor then enters the nucleus, where it might act to regulate gene expression. Expression of a DFz2 mutant that could not be cleaved failed to rescue synapse formation in flies that expressed a mutant of DFz2 with defective signaling.

  12. The Ups and Downs of Sea Level

    Changes in sea level have exerted a powerful influence on climate, ocean chemistry, the evolution of terrestrial and marine organisms, and the geology of continents and the sea floor. These changes can be caused by the growth and decay of ice sheets, increases and decreases in the volume of water stored in lakes and groundwater, variations in ocean basin volume, and the thermal expansion and contraction of seawater. Miller et al. (p. 1293) review how sea level has changed during the past 543 million years, the Phanerozoic Eon, and concentrate on variations that occur on time scales of tens of thousands to hundreds of millions of years. They discuss the mechanisms that were responsible for producing those changes.

  13. Shuttling Wet Electrons

    Electron transfer (ET) between proteins is a key feature of processes such as photosynthesis and respiration. Lin et al. (p. 1311) have explored the role of the intervening water medium on ET between proteins by calculating rates for electron self-exchange reaction of trypsin-solubilized bovine liver cytochrome b5 at a variety of distances and relative protein orientations. Although experimental data are often fit to single exponential decays, the authors find three distinct distance regimes. At van der Waals contact, ET is mediated by the proteins. Upon initial separation, the structuring of water leads to an anomalously weak distance dependence that reverts to that of bulk water only at larger separations (>12 angstroms). These results may account for a number of unusual observations in biological ET kinetics.

  14. Control of Planarian Regeneration

    The conserved PIWI family of proteins has been implicated in germ cell regulation, but it is not clear whether they promote stem cell maintenance or differentiation. Reddien et al. (p. 1327) now focus on two PIWI-related genes, smedwi-1 and smedwi-2, in flatworms (planaria). SMEDWI-1 and -2 proteins are expressed in dividing neoblasts, which are pluripotent stem cells involved in planarian regeneration. SMEDWI-2 protein functions in the production of stem cell progeny that are competent to perform differentiated functions and in the long-term maintenance of those stem cells.

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