This Week in Science

Science  03 Nov 2006:
Vol. 314, Issue 5800, pp. 721
  1. Alzheimer's Disease After 100 Years

    CREDIT: UNIVERSITY CLINIC OF TÜBINGEN

    In Tübingen, Germany, on 3 November 1906, Alois Alzheimer described the first documented case of Alzheimer's disease, a neurodegenerative disorder that impairs memory, cognition, and behavior (see the cover). Goedert and Spillantini (p. 777) review what is known about the molecular pathology of the disease, which is defined by the presence within the brain of amyloid-β-rich plaques and tau-containing neurofibrillary tangles. Roberson and Mucke (p. 781) review the prospects for therapy to help delay or prevent pathological processes within the brains of afflicted individuals, in order to prolong patients' cognitive abilities, and maintain for as long as possible their quality of life.

  2. A Need for a Sea Change

    The significance of the ocean's declining diversity on humanity has been difficult to assess. In a series of meta-analyses, Worm et al. (p. 787; see the news story by Stokstad) quantify how the loss of marine diversity on local, regional, and global scales has affected the functioning and stability of marine ecosystems, the flow of ecosystem services, and the rise of associated risks to humanity. Similar relationships occur between biodiversity change and ecosystem services at scales ranging from small square-meter plots to entire ocean basins; this finding implies that small-scale experiments can be used to predict large-scale ocean change. At current rates of diversity loss, this analysis indicates that there will be no more viable fish or invertebrate species available to fisheries by 2050. However, the results also show that the trends in loss of species are still reversible.

  3. Cosmic Shock Waves

    Clusters of galaxies grow by the infall of surrounding matter through gravitational effects, and peripheral shock waves are thought to be set up as material hits the cluster outskirts. Bagchi et al. (p. 791; see the Perspective by Enßlin) have used the Very Large Array to detect a ring of radio emission around a cluster that may signify such a shock wave. Giant twin radio arcs cup the cluster Abell 3376 and have size and brightness consistent with cosmological shock waves. The giant shock waves may provide sites for the acceleration of cosmic rays and particles associated with the structure-formation process.

  4. Sublimation in Two Acts

    Colloidal particles have been used as analogs for molecules for studying the formation of crystals or glassy jammed states. Savage et al. (p. 795; see the Perspective by Frenkel) use colloids to study the inverse problem, the sublimation of surface crystals. When the crystal is destabilized by a sudden temperature jump, large crystals first sublimate slowly. Once a critical size has been reached, the crystals suddenly melt into a metastable fluid before dissolving into the gas phase. In this regime of rapid change, the crystals were surrounded by a dense amorphous layer. The observations may correlate to the behavior of sublimating molecular systems and also to transitions in other systems like globular proteins.

  5. Quantum Wells Run Deep

    CREDIT: SPEER ET AL.

    Thin metal films grown on semiconductor substrates can display quantum-well states created by electron confinement. Photoemission studies by Speer et al. (p. 804; see the Perspective by Walldén) of atomically uniform sliver films grown on Si(111) surfaces reveal additional electronic-fringe features for substrates with high levels of n-type doping. Despite the lattice mismatch and incommensurate growth of the Ag film, these features, which resemble diffraction waves, result from the electronic states from the film extending deep into the substrate and interfering with propagating states below the band edge. For lightly n-doped or p-doped samples, the band bending is too shallow to allow sufficient overlap of the film's states with those of Si.

  6. From Heavy Fuels to Hydrogen

    Biomass can be converted in oils and heavy liquids, but for many applications, it is desirable to further process these fuels into hydrogen or synthesis gas (a mixture of CO and H2). However, the low volatility of these liquids often leads to long contact times with catalysts, and often some of the fuel is converted to carbon, which deactivates the catalyst. Salge et al. (p. 801) find that if heavy fuels such as soy oil or biodiesel are sprayed in the presence of O2 as fine droplets onto already-hot rhodium-cerium catalysts, the heat of reaction causes the droplet to further break up and fully convert to H2 (along with CO and CO2, the other main products) without any added heat. These reactions are very fast (total contact times of 50 milliseconds or less), and no deactivation was seen after 20 hours of operation.

  7. A Patchwork Solar Nebula

    The use of the long-lived neodymium (Nd)-samarium (Sm) radioactive decay system for understanding very early processes in our solar system and Earth's differentiation depends on knowing the initial solar isotopic ratios. However, there is wide variation among measured Nd isotope levels among meteorites and compared with terrestrial samples. Two reports indicate that the early solar nebula was not well mixed with respect to this dating system or barium isotopes (see the 6 October news story by Kerr). Variations in Nd isotopes seen between chondrites and earth samples led to the suggestion that some Nd isotopes were sequestered deep in the earth. Andreasen and Sharma (p. 806) have measured Nd and Sm isotopes in primitive carbonaceous chondrite meteorites and find that the variations among meteorites are real and are caused primarily by a p-process (photodissociation of nuclides) deficit in carbonaceous chondrites relative to ordinary chondrites, eucrites (from Vesta), and the terrestrial standard. Ranen and Jacobsen (p. 809) have measured barium isotopes in chondrites and found that they also exhibit variations among meteorite types, which they interpret as implying that the protosolar nebular was heterogeneous. Chondritic meteorites originated in a different place and were more enriched in supernova-derived material compared with Earth.

  8. Pores, Sieve, or Gel?

    Nuclear pore complexes behave like a sieve—they allow small molecules to pass freely but restrict passage of macromolecules (>30 kilodaltons) between the nucleus and cytosol. The so-called FG-rich nucleoporin repeats, which are intrinsically unfolded protein domains that contain short clusters of hydrophobic amino acids separated by hydrophilic spacer regions, are thought to form the barrier, but the functional organization of this barrier has remained a matter of speculation. Frey et al. (p. 815; see the Perspective by Burke and the Perspective by Elbaum) show that these FG-rich repeats occur in an extended conformation and form a noncovalent (and thus reversible) hydrogel. Hydrophobic bridges connecting the individual polypeptide chains and create a three-dimensional sieve-like structure that is crucial for nuclear pore complex function.

  9. Neurite Extension and Membrane Trafficking

    CREDIT: SHIRANE AND NAKAYAMA

    Neurons extend processes (neurites) that can reach more than 1 meter in length. Formation of neurites requires both cytoskeletal remodeling and membrane transport. Shirane and Nakayama (p. 818) have now identified a protein, protrudin, that is essential for neurite extension triggered by nerve growth factor. Protrudin binds to and inhibits the activity of the protein Rab11, which functions as a molecular switch in membrane transport to the cell surface.

  10. A Cool Way to a Long Life

    Caloric dietary restriction prolongs life span in a variety of organisms, and in mammals the resultant lowering of core body temperature has been offered as one potential explanation. Conti et al. (p. 825; see the Perspective by Saper) generated transgenic mice that overexpress mitochondrial uncoupling protein 2 in hypocretin-producing neurons within the hypothalamus, which lowers core body temperature by about 0.5°C. In the absence of caloric restriction, the median life span of these “cool mice” was about 15% greater than that of their wild-type littermates.

  11. Life Isn't Fair

    In the two-player ultimatum game, low offers made by the first player to divide the pot of money unequally are considered unfair. The second player can choose either to accept these low offers (in which case the first player walks away with more than half of the pot) or to reject them (in which case neither player receives anything), with the outcome reflecting the competition between selfishness and indignation. The dorsolateral prefrontal cortex (DLPFC) is thought to play a role in the decision-making process. Knoch et al. (p. 829, published online 5 October) used repetitive transcranial stimulation to test this directly by interfering with DLPFC function. Suppressing DLPFC activity tilted the competitive motivations toward the side of selfish behavior and a greater acceptance of unfair offers.

  12. Donning the Myelin Sheath

    The myelin sheath electrically insulates axons and makes the conductance of neuronal impulses much more efficient. Chan et al. (p. 832) examined how the Schwann cells begin myelination of an axon. In neuronal cultures, a cell polarity protein, Par-3, localized to where the Schwann cell meets the axon and promoted the recruitment of the brain-derived neurotrophic factor (BDNF)-receptor to the junction between Schwann cell and neuron. BDNF-dependent signaling between the axon and Schwann cell then ensures that myelination begins. This localized signaling arising from Par-3 polarity may help to ensure that the myelination begins at the right spot.

  13. Swell Gel Patterns

    The Belousov-Zhabotinsky reaction system involves chemical oscillations that show periodic behavior in space and time. The reaction is characterized by a periodic change in the oxidation state of a metal caused by local changes in the concentration of one or more reactants. If this reaction occurs within the confines of a polymer gel, the local concentration changes can also cause changes in the swelling of the gel. Previous models have captured these effects in one dimension, where the gel volume can contract or expand uniformly. Yashin and Balazs (p. 798) have extended this to two dimensions, which allows the gel to change shape as well. The richness of patterns and shape changes that arise depends on the dimensions of the gel.

  14. Predicting Changes in Biodiversity

    Predicting the effects of environmental change on biodiversity requires an understanding of whether those changes will affect which species occupy a given site, as well as their relative prevalence. Shipley et al. (p. 812, published online 5 October; see the Perspective by McGill) developed a quantitative method based on plant traits to answer these questions and test its effectiveness on a data set from a 42-year process of secondary succession after agricultural abandonment of sites in southern France. Their method successfully predicts community composition and relative abundance with a high level of accuracy.

  15. Questions and Answers in Photosystem II

    Oxidation of water to O2 is catalyzed by a Mn4Ca cluster within photosystem II (PSII). Recently described crystal structures of PSII have yielded important insights, but the structure of the Mn4Ca catalytic center has remained unclear. Yano et al. (p. 821) use polarized extended x-ray absorption fine structure (EXAFS) data to provide structural models of the Mn4Ca cluster and combined information from polarized EXAFS and x-ray diffraction data to place these models within the PSII protein environment. The model which provides the best fit differs from those proposed in the x-ray structure models.

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