This Week in Science

Science  15 Jun 2007:
Vol. 316, Issue 5831, pp. 1537
  1. Imperfect Inner Core


    The Earth's inner core is not as stiff as would be expected for a solidiron alloy, and one explanation for the deviation is that the inner core is partially molten. Belonoshko et al. (p. 1603) performed molecular dynamics simulations, which show that crystal defects could also reduce the inner core's rigidity and decrease its shear wave velocity. Their results, which are in closer agreement with seismic observations, suggest that the low rigidity of the Earth's inner core arises from viscous grain boundaries and high diffusion within iron crystals at high temperature.

  2. Signatures of the Sun's Center

    The Sun's vibrations can reveal details of its inner structure. Pressure-driven modes have long been observed, but they provide only limited information about the center of the Sun, which contains more than half the Sun's mass and is the region where fusion occurs. Gravity-driven modes can potentially reveal more about changes in buoyancy of the Sun's dense inner core but are very weak at the Sun's surface and have been hard to detect. García et al. (p. 1591, published online 3 May; see the cover, the 4 May news story by Kerr, and the Perspective by Hill) report the observation of a periodic structure in the solar power spectrum that is characteristic of gravity modes in 10 years of data taken with the Global Oscillation at Low Frequency (GOLF) instrument aboard the Solar and Heliospheric Observatory (SOHO). These signatures suggest that the Sun's core rotates faster than in the rest of the radiative zone.

  3. Controlling Quantum Stability

    When films undergo thinning, they often break up into droplets as this change in morphology decreases surface tension. However, recent work has shown that for metallic films there are thicknesses, counted in a sequential odd-even integer number of monolayers, where the films are remarkably stable. This quantum stability is thought to arise from the competing effects of surface tension and the energy of the electrons confined to the layer. Such quantum confinement effects can also affect properties such as superconducting transition temperatures. Özer et al. (p. 1594) show that quantum stability and the superconducting behavior can both be controlled by varying the electron density in the lead films by forming alloys with bismuth.

  4. Limiting Nanoparticle Dispersal


    Sulfate-reducing bacteria are known to produce metal-derived nanoparticles as part of the reduction process that helps reduce metal concentrations in anoxic waters, but the extent to which the resulting nanoparticles move about in the environment is uncertain. Moreau et al. (p. 1600) study the role of microbially derived proteins on the formation of zinc sulfide particles in a biofilm. The metal-binding polypeptides and proteins are important factors in extracellular metal-sulfide biomineralization, and they may play an important role in aggregation processes that limit the spread of nanoparticles in natural environments.

  5. Arctic Plant Dispersal

    Understanding of how the distributions of plant species might shift in response to climate change is hampered by the difficulty of obtaining good quantitative estimates of the frequency of long-distance dispersal. By analyzing the genetic variation of more than 4000 plant samples representing nine different flowering plant species from the arctic archipelago Svalbard and neighboring regions, Alsos et al. (p. 1606) show that long-distance dispersal has occurred from various source regions in the Arctic during plant colonization of the Svalbard archipelago since the Last Glacial Maximum. Long-distance dispersal appears to have been much more common than previously believed, which in turn suggests that northward range shifts could occur rapidly following global warming.

  6. From Local to Global Ecology

    Macroecology aims to build quantitative predictions about the distribution and abundance of organisms at the scale of regions and even continents, and is relevant to the understanding of biotic changes that may take place as a result of shifting climates. Kerr et al. (p. 1581) review recent progress in the macroecological research program and assess its potential practical benefits to the management of the consequences of global change.

  7. Focus on Fly Heterochromatin

    The DNA of eukaryotic genomes is packaged into chromatin. Euchromatin marks regions that are generally gene-rich and transcriptionally active, whereas heterochromatin (encompassing roughly 30% of the genome in flies and humans) includes regions that are generally transcriptionally repressed and often include densely repeated sequences. Such repetitive regions are intrinsically difficult to analyze with current DNA mapping and sequencing methods. Smith et al. (p. 1586) and Hoskins et al. (p. 1625) describe a tour de force analysis of roughly 15 to 25 megabases of heterochromatic DNA from pericentromeric regions in the fruit fly Drosophila melanogaster. Mapping, sequencing, and annotation of the regions reveals that the great majority of the sequences is repetitive, consisting of retrotransposons, DNA transposons, tandem repeats, and satellite repeats, with ~9% of a unique sequence housing several hundred protein-coding genes and a smaller number of non-protein-coding genes.

  8. Attention and Rhythm in Perception

    How does the nervous system enable communication between different brain areas or different neuronal groups? (See the Perspective by Knight.) Saalmann et al. (p. 1612) simultaneously recorded from two areas in the dorsal stream of the visual pathway, the lateral intraparietal area (LIP) and the medial temporal area (MT), while monkeys performed a delayed match-to-sample memory task. Activity in LIP predicted activity in MT when the receptive fields of the LIP and MT neurons were in the same place and when the monkey was attending to that place. LIP feedback can thus account for attention-enhanced MT responses. Womelsdorf et al. (p. 1609) combined multielectrode recording data from three different experimental preparations to test the hypothesis that the phase relation between the rhythmic activity of groups of neurons determines the strength of their mutual influence and found a heavy dependence of the correlation on phase. These results suggest a mechanism by which signals are matched and coupled during complex perceptual and cognitive operations.

  9. The Gift of Giving


    In Europe, taxation rates are high, and services are funded by government spending, whereas in the United States, low taxes and higher philanthropic donations are the norm. Harbaugh et al. (p. 1622) have carried out a neuroeconomic study to assess the degree of personal reward (as indexed by neural activation of reward-related brain areas) in response to mandatory (via taxation) and voluntary contributions to charity. Subjects experienced a hedonic reaction when tax revenues were transferred to a charity, and subjects who showed greater neural activation under this regime were more generous when charitable contributions were made voluntary. The sense of well-being in the voluntary giving condition surpassed that seen when subjects were taxed.

  10. Tuning Immune Stimulation

    During vaccination, an additional stimulus to the immune response is often needed and is provided by a material called an adjuvant. Lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria is a potent stimulant of the innate immune response, but the potential for toxic shock does not allow for its use in humans. A recently approved adjuvant, monophosphoryl lipid A (MPLA), has limited side effects compared to LPS from which it is derived (see the Perspective by Fitzgerald and Golenbock). Mata-Haro et al. (p. 1628) show that MPLA activates only a specific signaling component of the Toll-like receptor 4 (TLR4) pathway and avoids the myeloid differentiation factor 88 arm of TLR4 signaling, which can account for the much higher toxicity associated with LPS. Ohto et al. (p. 1632) determined crystal structures of the TLR4 co-receptor MD-2 alone and in complex with the antiendotoxic tetra-acylated lipid A core of LPS. MD-2 has a deep hydrophobic cavity that accommodates the four acyl chains of the lipid core.

  11. Signaling Central

    Wnt growth factors activate a signaling cascade that plays a key role in animal development, stem cell biology, and human cancer. Although many of the cascade components have been identified, how they come together to transmit the signal from the plasma membrane inside the cell is not clear. Bilic et al. (p. 1619) show that Wnt ligands induce formation of large, ribosome-sized complexes, termed the low-density lipoprotein receptor-related protein 6 (LRP6) signalosome at the plasma membrane. The large protein assemblies contain activated Wnt receptors and cytoplasmic adapter proteins. The scaffold protein Dishevelled and the Wnt co-receptor LRP6 play a central role in LRP6-signalosome formation, triggering subsequent molecular interactions that regulate the level of nuclear β-catenin.

  12. Sugar Shake-Up

    The effective use of biomass as an alternative to traditional petrochemical feedstocks will be enabled by generating versatile chemical intermediates using sugars. One such promising compound is hydroxymethylfurfural (HMF), which can be prepared from fructose via acid catalysis. Zhao et al. (p. 1597) find that efficient conversion of fructose to HMF is also possible under acid-free conditions using metal halide catalysts in ionic liquid solvent. Moreover, chromium dichloride catalyzes the analogous transformation of glucose in ~70% yield. Mechanistic studies suggest chromium promotes this unexpectedly selective transformation by promoting glucose to fructose isomerization.

  13. From Klotho to Calcium

    Klotho, a membrane protein with similarity to glycosidases, has been proposed to modify sugar residues on a TRP channel, to interact with the receptor for fibroblast growth factor 23, and to extend life span when overexpressed. Now Imura et al. (p. 1615) report that α-Klotho interacts with the α1 subunit of the Na+,K+-adenosine triphosphatase (Na+,K+-ATPase) in mouse cells. The amount of Na+,K+-ATPase at the plasma membrane appeared to decrease in cells exposed to high concentrations of Ca2+, a response that was blunted in cells lacking α-Klotho. The concentration of Ca2+ in cerebrospinal fluid of mice lacking α-Klotho was slightly lower than that in wild-type animals. Thus, α-Klotho may function in the control of Ca2+ concentrations by influencing the abundance of the Na+,K+-ATPase transporter in plasma membranes.