This Week in Science

Science  28 May 2010:
Vol. 328, Issue 5982, pp. 1071
  1. Thymocyte Egress

    A critical step during the development of the immune system is the egress of developing T lymphocytes, or thymocytes, from the thymus to peripheral organs, where they can defend against infectious microorganisms. Egress requires detection of the lipid mediator, sphingosine-1-phosphate (S1P), by thymocytes, but whether S1P detection is sufficient for egress, the source of S1P and the location where T lymphocytes exit the thymus are unknown. Zachariah and Cyster (p. 1129, published online 22 April) found that selective expression of a transgene for the S1P receptor-1 in mouse thymocytes was sufficient for thymocyte egress. Thymocyte egress occurred at corticomedullary junctions via blood vessels, rather than via lymphatics. Pericytes, neural crest-derived cells, which ensheath blood vessels, provided the S1P for exiting thymocytes.

  2. Taking the Rough with the Smooth

    Even with extensive annealing at high temperatures, most polycrystalline materials will not become a perfect single crystal, which would represent the thermodynamically preferred state. The stability of the polycrystalline state has been attributed to the presence of impurities that accumulate at the grain boundaries, but even very pure materials show grain growth stagnation. Using simulations, Holm and Foiles (p. 1138) show that grain boundaries can be classified as “rough” and “smooth.” Rough boundaries move continuously with well-defined activation energies, while the smooth boundaries have low mobility and move in a jerky, stepwise manner. With heating, a boundary can change from smooth to rough, but the transition temperature can vary by hundreds of degrees from one grain boundary to the next. These smooth, low-mobility boundaries thus pin the polycrystalline structure, even in the absence of impurities.

  3. Arsenic in Asia

    CREDIT: SCOTT FENDORF

    Millions of humans in South and Southeast Asia are exposed to potentially dangerous levels of the carcinogen arsenic via their drinking water every day. Although high arsenic levels are a known problem, a growing demand for drinking water drives the continued construction of new groundwater wells in these regions. Fendorf et al. (p. 1123) review chemical and hydrological factors controlling the release of arsenic in groundwater in South and Southeast Asia, which include the distribution of arsenic in groundwater aquifers used for drinking water and irrigation. Despite incomplete sampling and characterization of these factors across these regions, several key directions for improvements to water quality are presented.

  4. Catalysis at the Edge

    Many catalysts in solution, such as metalloenzymes and homogeneous metal complexes, create active sites where the metal ion is available to bind and activate reactants. Such coordinately unsaturated ferrous sites, or CUFs, have been created in a supported heterogeneous catalyst by Fu et al. (p. 1141). Ferrous oxide islands grown on platinum single-crystal surfaces were much more reactive for CO oxidation at low temperatures than more oxidized ferric islands. This difference arose from sites at the interface between the islands and the Pt surface that activated oxygen. Silica-supported Pt-Fe catalysts were active for CO removal from hydrogen streams, a reaction critical for maintaining the activity of fuel cells.

  5. Two Molecular Spheres

    Viruses form highly symmetrical coat structures, capsids, through the assembly of multiple lower-symmetry protein precursors. Recently, chemists have sought to emulate this process on a smaller scale, relying on the assembly of organic molecular struts and metal ions, rather than proteins. Sun et al. (p. 1144, published online 29 April; see the Perspective by Stefankiewicz and Sanders) now demonstrate that a mixture of palladium ions and V-shaped bridging ligands can self-assemble into a hollow, nearly spherical polyhedron with 24 vertices and a central diameter of 4 nanometers. The assembly process was highly sensitive to the ligand angle; a subtle average decrease generated instead a smaller 12-vertex product.

  6. Telling Up from Down

    It is generally believed that carbon dioxide accumulates in the deep ocean during cold periods and that it is released rapidly and in huge quantities during deglaciation, but evidence of deep ocean carbon dioxide storage has been elusive. Now Skinner et al. (p. 1147; see the Perspective by Anderson and Carr) present radiocarbon data from the Southern Ocean that indicate that the deep water circulating around Antarctica was about twice as old relative to the atmosphere as it is today, a condition considered indicative of carbon dioxide accumulation and storage.

  7. Collateral Damage

    Cotton crops that have been bioengineered to express the insecticidal toxin derived from Bacillus thuringiensis (Bt) carry their own insect control, particularly against the cotton bollworm, and are less dependent on externally applied pesticides. Lu et al. (p. 1151, published online 13 April) now show that reduction in general pesticide use in cotton-growing regions of northern China has shifted the balance of regional pest populations. Bt-expressing cotton now serves as a source of herbivorous insects of the Miridae family, rather than the sink that nonengineered cotton was when less specific pesticides were used. Because these insects will eat a variety of plants, they are emerging as a threat to other crops, including grape, apple, peach, and pear.

  8. Optical Nanoengineering

    CREDIT: NANFANG YU AND JON FAN

    Optics and electronics operate at very different length scales. Surface plasmons are light-induced electronic excitations that are being pursued as a route to bridge the length scales and bring the processing speed offered by optical communication down to the size scales of electronic chip circuitry. Now, Fan et al. (p. 1135) describe the self-assembly of nanoscale dielectric particles coated with gold. Functionalization of the gold surface with polymer ligands allowed controlled production of clusters of nanoparticles. The optical properties of the self-assembled nanostructures depended on the number of components within the cluster and each structure could be selected for its unique optical properties. Such a bottom-up approach should help in fabricating designed optical circuits on the nanoscale.

  9. CWD Strain Variation

    CREDIT: ANGERS ET AL.

    So-called prion diseases are fatal neurogenerative disorders that include chronic wasting disease (CWD) found in deer and other cervids. Prion diseases are thought to be caused by infectious proteins (prions) in the absence of associated infectious DNA. Nevertheless, prion strains have been isolated that can mutate in the absence of nucleic acids, and these strain properties control the ability of prions to cross species barriers. Angers et al. (p. 1154, published online 13 May; see the Perspective by Collinge) address the issue of strain variation in the context of CWD. Whereas the host range of this contagious disease continues to expand, the prevalence of CWD strains has not been determined. Understanding CWD strain variation may be important in predicting and preventing any future risks to human health.

  10. Fat-Burning Fat

    In mammals, fat exists in two forms—the well-known white adipose tissue (WAT), which stores energy and is associated with obesity, and the lesser-known brown adipose tissue (BAT), which burns energy to generate heat. BAT's role in human physiology was once thought to be restricted to newborns, but the recent discovery that adults also harbor functional BAT has re-ignited interest in the factors regulating BAT development and their potential as targets for anti-obesity therapies. Vegiopoulos et al. (p. 1158, published online 6 May; see the Perspective Ishibashi and Seale) now show that cyclooxygenase-2 (COX-2), an enzyme critical to prostaglandin synthesis, triggers fat progenitor cells in mice to differentiate into BAT rather than WAT. Mice overexpressing COX-2 displayed increased energy expenditure and were protected from diet-induced obesity.

  11. Everything Changes

    Nucleosomes package DNA, and their assembly and disassembly regulate access to the genome. The ability to follow these changes is limited to steady-state methods in higher eukaryotes, although direct kinetic analyses are available in yeast. To address this deficiency, Deal et al. (p. 1161) developed a general method for following the genome-wide dynamics of nucleosome assembly and disassembly. High levels of nucleosome turnover were observed across gene bodies and at the sites of epigenetic regulatory elements in fruit fly tissue culture cells. Nucleosomes were replaced multiple times during each 20-hour cell cycle, suggesting that histone modifications themselves are unlikely to transmit epigenetic information. Furthermore, analysis of replication origins indicates that they are determined by chromatin dynamics and not by sequence features.

  12. Fairness or Equality?

    Inequality in payments may be seen as inherently unfair, or as appropriate when it reflects differential achievement. Using an economic exchange game, Almås et al. (p. 1176) mapped how judgments changed from 5th-grade students to 13th graders: Fifth graders expressed a preference for equal division of rewards, whereas the 13th graders tolerated unequal outcomes, as long as they had been provided with evidence of unequal inputs. That is, the younger children were strict egalitarians, but the older ones—perhaps as a consequence of exposure to a variety of achievement-based social activities, such as sports—tended toward meritocracy.

  13. Global Biodiversity Target Missed

    In 2002, the Convention on Biological Diversity (CBD) committed to a significant reduction in the rate of biodiversity loss by 2010. There has been widespread conjecture that this target has not been met. Butchart et al. (p. 1164, published online 29 April) analyzed over 30 indicators developed within the CBD's framework. These indicators include the condition or state of biodiversity (e.g., species numbers, population sizes), the pressures on biodiversity (e.g., deforestation), and the responses to maintain biodiversity (e.g., protected areas) and were assessed between about 1970 and 2005. Taken together, the results confirm that we have indeed failed to meet the 2010 targets.

  14. Defining Defensins' Mode of Action

    Defensins are antimicrobial host defense peptides that play a role in innate immunity. Many such peptides act by disrupting the bacterial membrane; however, Schneider et al. (p. 1168) now show that the fungal defensin, plecstasin, targets cell wall biosynthesis. Biochemical studies identified Lipid II as the cellular target of plecstasin and the residues involved in complex formation were identified using NMR spectroscopy and computational modeling. Initial studies identified two defensins from invertebrates that also target Lipid II. Plecstasin is active against some drug-resistant Gram-positive bacteria, and its action against a validated target makes it a promising lead for further drug development.

  15. Proliferation Control

    The protein complex mTORC1, which contains the protein kinase known as mammalian target of rapamycin, is an important regulator of cell proliferation and cell size. Among many targets, mTORC1 phosphorylates the eukaryotic translation initiation factor eIF4E–binding proteins (4E-BPs), thus controlling translation of proteins that regulate proliferation. Dowling et al. (p. 1172) used mice lacking expression of the 4E-BPs to show that these proteins contribute to mTORC1's activation of cell proliferation, but are dispensable for the effects of mTORC1 on cell growth. The latter required another mTORC1 target—the ribosomal protein S6 kinase. mTORC1 inhibitors are being explored as potential anticancer agents, and the presence of 4E-BPs was necessary for mTORC1 inhibitors to reduce the number and size of colonies formed by transformed mouse cells.