This Week in Science

Science  02 May 2008:
Vol. 320, Issue 5876, pp. 581
  1. Journey to the Center of the Earth


    Our view of Earth's lowermost mantle has changed recently because of the likely presence of a newly identified mineral phase, post-perovskite. Furthermore, recent advances have improved seismic imaging of the structure and composition of this region. Garnero and McNamara (p. 626) review these developments, and, in combination with recent modeling efforts, provide an integrated view of the region and how it may influence mantle dynamics overall.

  2. Improving Thermoelectrics by Powdering and Pressing

    Thermoelectric materials are semiconductors that combine high electrical and low thermal conductivity to allow the recovery of electrical power from waste heat or direct cooling with applied current. One route to improving the figure of merit that describes these materials, ZT, is to form them as nanocrystalline materials, because the added grain boundaries should help to scatter phonons and lower their thermal conductivity. Poudel et al. (p. 634, published online 20 March) take one of the most widely used thermoelectric materials, p-type BixSbxTe3, which has ZT of 1 at room temperature, and convert it to a nanocrystalline form. They ball-milled the material into nanoparticles under an inert atmosphere, and then hot-pressed the powder back into bulk ingots. This material has a peak ZT of 1.4 near 100°C, and could produce temperature differentials of 100°C in a cooling mode.

  3. Shake Unrattled by Roll

    When molecules are excited by absorption of light, there is a brief period during which quantum mechanical oscillations retain a specific phase relationship and so can be manipulated productively. However, perturbations due to the onset of various vibrations and rotations, as well as the influence of neighboring molecules, soon randomize this coherent state and inhibit further active control. Branderhorst et al. (p. 638) explored the tendency of a vibrational mode in an ensemble of potassium dimers to lose coherence due to mixing with molecular rotation. As a marker of coherence, they detected the persistence of quantum beats, a result of wave interference in the fluorescence signal. By iteratively shaping their laser excitation pulse using this marker as feedback, they succeeded in prolonging coherence by a factor of 2 and accounting for the efficacy of the pulse shape with a model.

  4. From the Mouths of Baby Birds


    The youthful noises of young zebra finches sound different from adult zebra finches, somewhat like babbling in human infants. Using surgical and pharmacological lesions, Aronov et al. (p. 630) eliminate some of the brain regions and neural connections that support adult song. The lesions cause the adults to sound again like juveniles, but leave juvenile vocalizations intact. Thus, the brain connections upon which bird song depends differ between adults and juveniles, and the process of song maturation is not simply a refinement of an existing neural network, but involves switching from a youthful network to one required for adult song.

  5. Quantum Optical Chips

    While the quantum mechanical aspects of optics have been illustrated in a number of applications in communication, metrology, and lithography, these have generally been carried out on a room-sized optical bench. Quantum technologies-based photonics will require scaling down and operating on a platform that is robust and easy to implement. Patterning optical waveguides and circuits using silica-on-silicon, Politi et al. (p. 646, published online 27 March; cover) demonstrate that arbitrary photonic quantum circuits can be realized on silicon chips. Single photons launched into the devices showed high visibility interference and basic quantum logic operations with high fidelity.

  6. Prostratin Preparation

    Despite progress in drug development to target HIV, the virus remains very hard to root out of an infected system entirely, because of latent reservoirs beyond the reach of current treatments. Certain compounds, chief among them prostratin, have recently shown promise for accelerating emergence from these reservoirs, which may enhance the long-term effectiveness of other drugs. However, the scarcity of botanical sources for prostratin has hampered progress. Wender et al. (p. 649) present an efficient four-step chemical synthesis to produce prostratin from a much more abundant natural precursor. Moreover, the route can easily be modified to afford structural analogs that may enhance therapeutic efficacy.

  7. Mitochondria as Drivers of Metastasis

    Most cancer deaths occur when cells in a primary tumor metastasize, yet the mechanisms by which tumor cells acquire metastatic properties remain poorly understood. Ishikawa et al. (p. 661, published online 3 April) explored the role of mitochondria in this process by taking mouse tumor cell lines with either a high or low propensity to metastasize and swapping their mitochondrial DNA (mtDNA). Interestingly, the recipient cells acquired the metastatic potential of the cells donating the mtDNA. In one tumor cell line examined in detail, the mtDNA conferring high metastatic potential was found to harbor mutations that led to overproduction of reactive oxygen species (ROS), and up-regulation of nuclear genes involved in metastasis. Pretreatment of tumor cells with ROS scavengers reduced their ability to metastasize in mouse models, suggesting a possible avenue for the development of therapies to suppress metastasis.

  8. Sugars in Living Color

    The glycan structures that adorn the cell surface are a rich source of information about biochemical activities occurring inside the cell. Levels, distribution patterns, and structural changes of sugars making up the glycans reflect modifications in flux through metabolic pathways, alterations in gene expression, and changes in secretory pathway dynamics. The direct and noninvasive visualization of glycans in vivo has proved difficult. Now Laughlin et al. (p. 664) report the in vivo multicolor, time-resolved imaging of cell surface glycans in live, developing zebrafish. Dramatic bursts in glycan production were observed in the jaw, olfactory organ, and pectoral fin during the period of 60 to 72 hours post-fertilization, with major tissue-specific differences in the levels and trafficking patterns of glycans during embryogenesis.

  9. Expanding Low O2 Zones


    One of the consequences of a warming climate, warmer oceans, is expected to cause a decrease in the oxygen concentration of the oceans. This prediction is based on the fact that the solubility of oxygen decreases as water temperature increases, as well as the modeled result of a slower rate of advection of water to the deep ocean while sinking organic matter continues to decay in a process that consumes oxygen. Stramma et al. (p. 655) report measurements of dissolved oxygen concentrations in the tropical Atlantic and equatorial Pacific oceans that show a clear vertical expansion of the oxygen minimum zones over the past 50 years. A reduction of the concentration of dissolved oxygen could have serious effects on marine life, especially in regions that are already at the limits of oxygen concentration required to support many organisms.

  10. Testing Tethers

    The Golgi complex is composed of a set of flattened membrane cisternae interconnected by trafficking membrane vesicles. Inside the cell, during transport through the Golgi complex, membrane vesicles seem to rely on membrane tethers to avoid their escape from the Golgi region. Such membrane tethering has seldom been reconstituted using pure proteins and artificial membranes. Drin et al. (p. 670) present and test a simple model for how a long coiled-coil protein of the Golgin family, GMAP-210, forms a bridge between a highly curved membrane (a vesicle) and a flat one (a cisterna). This asymmetric tethering relies on motifs that sense membrane curvature. The tethering mechanism presented is asymmetric and reversible, which may explain how the Golgi keeps a constant morphology despite being constantly remodeled by membrane transport.

  11. A Sense of Danger in the Air

    Particulate airborne pollutants, such as asbestos and silica, are notorious for their negative effects on health, including lung inflammation and cancer, yet information on how such substances exert their effects is lacking. Dostert et al. (p. 674, published online 10 April; see the Perspective by O'Neill) reveal that a multiprotein complex known as the Nalp3 inflammasome can signal exposure of cells to internalized particles of asbestos and silica, which leads to the activation of a potent inflammatory response. In the absence of Nalp3, mice responded less vigorously to asbestos, supporting the idea that this inflammatory sensing complex plays a key role in the response to respiratory pollutants.

  12. Unintentional Uptake

    In many respects, our understanding of innate immune responses to protozoan parasites still lags behind that for other infectious organisms. However, recent work has shown that an important part of the armory against African trypanosomes is serum apolipoprotein L-I (apoL1), which can kill the parasite by causing lysis—why then would the parasites take it in? Vanhollebeke et al. (p. 677) show that apoL1 is taken up by the parasite via a specific glycoprotein receptor, which the parasite normally uses to supply heme for its growth and resistance to oxidative stress within the host. In human serum, however, the receptor also inadvertently recognizes a component of certain high-density lipoprotein complexes, of which apoL1 is a part, explaining how the uptake of this detrimental host protein is triggered.

  13. Understanding Random Lasers

    Conventional lasers are well understood—combining a local oscillator with a gain medium and suitable light-trapping cavity leads to build-up of a coherent excited state and subsequent emission into a single optical mode. Random lasers, in which lasing occurs in strongly disordered media, however, can emit into a number of modes across a wide range of wavelength and are not well understood. Türeci et al. (p. 643; see the Perspective by Bravo-Abad and Soljačić) describe a theoretical approach to understanding the nature of lasing in these complex structures, explaining how there is competition between different excitation modes, describing how and when the various modes switch on, and thereby providing a level of understanding that may carry over into improving their performance.

  14. Mysterious Polyphosphate

    Phosphorus is a vital macronutrient with a central influence on global oceanic primary production. The residence time of phosphorus in the ocean has been repeatedly revised downwards over the past several decades, as new sedimentary sinks have been discovered. One of these sinks is authigenic apatite, a mineral whose origin still is not understood. Diaz et al. (p. 652) show that polyphosphate, derived in abundance from diatoms, plays a key role in the formation of apatite in marine sediments, by circumventing the kinetic barrier faced by organic phosphates. Changes in diatom abundance due to paleoclimatic forcing in the geologic past may have affected phosphorus burial efficiency and the distribution of phosphorous minerals in marine sediments.

  15. Modeling Food Webs

    Food webs describe the interactions between species in ecological communities; they also consider questions of stability and robustness in ecosystems. Modeling has played a central role in understanding food web structure. However, evaluations of models with empirical data have so far been limited. Allesina et al. (p. 658) propose a model that can replicate the full empirical food web with a higher likelihood than alternative existing models.

  16. Signaling Survival Strategies

    Along with its role in control of metabolism, the enzyme glycogen synthase kinase 3β (GSK3β) functions in a signaling mechanism that controls transcription. When GSK3β is active, it phosphorylates β-catenin and promotes degradation of the β-catenin protein. Inhibition of GSK3 permits accumulation of β-catenin, which works in the nucleus with transcription factors to promote expression of target genes. Thornton et al. (p. 667) show that activation of the p38 mitogen-activated protein kinase (MAPK) appears to directly phosphorylate and thereby inhibit GSK3β. Mouse embryo fibroblasts lacking enzymes that lead to activation of p38 were deficient in phosphorylation of GSK3β. The authors propose that such a signaling pathway may allow cellular stresses and cytokines that activate p38 MAPK to influence cell survival.