This Week in Science

Science  17 Sep 2010:
Vol. 329, Issue 5998, pp. 1437
  1. Location Matters

    CREDIT: MIWA SASAI

    Plasmacytoid dendritic cells (pDCs), an immune cell specialized to respond to viral infections, use Toll-like receptors (TLRs) 7 and 9 expressed in endosomes to sense viral nucleic acids. Triggering of TLR7 or 9 results in the induction of two distinct signaling pathways, one that leads to the production of proinflammatory cytokines and another that induces the expression of antiviral type I interferons. How one receptor can trigger two distinct signaling pathways, however, is not clear. Sasai et al. (p. 1530) now show that subcellular localization is key. Cells from mice deficient in the Adapter Protein 3 (AP-3) complex, which regulates protein sorting to intracellular vesicles, did not produce type I interferons in response to TLR9 ligand, but proinflammatory cytokine production remained intact. AP-3 was required for trafficking of TLR9 from early endosomes, where proinflammatory signaling can occur, to lysosome-related organelles, where the signaling machinery required for type I interferon induction is located. Such spatial segregation may represent a common mechanism whereby activation of one receptor can result in the induction of multiple independent signaling cascades.

  2. Tyrannosaurs Revisited

    Tyrannosaurs represent some of the most successful and largest carnivores in Earth's history. An expanding fossil record has allowed studies of their evolution and behavior that now allow broader comparisons with other groups, not just dinosaurs. Brusatte et al. (p. 1481) review the biology and evolutionary history of tyrannosaurs and update their phylogenetic relations to include several new fossils. The analysis suggests that tyrannosaurs remained relatively small (less than about 5 meters long) until the Late Cretaceous (about 80 million years ago).

  3. Clean or Dirty

    Aerosols strongly affect atmospheric properties and processes—including visibility, cloud formation, and radiative behavior. Knowing their effects in both clean and polluted air is necessary in order to understand their influence (see the Perspective by Baltensperger). Clarke and Kapustin (p. 1488) examine vertical atmospheric profiles collected above the Pacific Ocean, where air quality is affected by the transport of polluted air from the west, and find significant regional enhancements in light scattering, aerosol mass, and aerosol number associated with combustion. Aerosol particle concentrations in this region can exceed values in clean, unperturbed regions by over an order of magnitude. Thus combustion affects hemispheric aerosol optical depth and the distribution of cloud condensation nuclei. Pöschl et al. (p. 1513) discuss the composition of aerosols above the Amazon Basin, in the pristine conditions of the rainy season. The aerosols in this region are derived mostly from gaseous biogenic precursors, plants, and microorganisms, and particle concentration is orders of magnitude lower than in polluted continental regions.

  4. Glucose Metabolism Revisited

    Cancer cells are revved up to reproduce rapidly and typically consume glucose rapidly by glycolysis. Why then do cancer cells express an isoform of a rate-limiting enzyme in glycolysis, pyruvate kinase M2, which has decreased activity? Vander Heiden et al. (p. 1492) propose that consequent accumulation of phosphoenolpyruvate, with the help of an enzymatic activity that remains to be characterized, can lead to phosphate transfer to phosphoglycerate mutase, another glycolytic enzyme, providing the cell with a different way to make pyruvate. This may allow cancer cells to produce pyruvate without generating excess adenosine triphosphate, which can act through feedback to inhibit glycolyis.

  5. A Correlated Quantum Walk

    Random walks are powerful tools for modeling statistical events. The analogous quantum walk involves particles tunneling between available sites. Peruzzo et al. (p. 1500; see the Perspective by Hillery) now report on the quantum walk of a correlated pair of photons propagating through a coupled waveguide array. The output pattern resulting from the injection of two correlated photons possess quantum features, indicating that the photons retain their correlations as they walk randomly through the waveguide array, allowing scale-up and parallel searches over many possible paths.

  6. Under Pressure

    CREDIT: FIQUET ET AL.

    In order to understand the behavior of materials in the solid deep Earth, it is important to be able to estimate how a material melts at high pressure. To this end, Fiquet et al. (p. 1516) performed experiments using a laser-heated diamond anvil cell coupled to in situ synchrotron measurements of peridotite rock—a mixture of minerals thought to represent Earth's upper mantle—across a wide pressure range. The results suggest that liquid phases may exist at very high pressure values, such that seismically anomalous zones near the boundary between the core and the mantle may result from isolated pockets of melt. Along similar lines, the base of primitive Earth's mantle may have acquired its trace element signature from partial melting of certain mineral phases higher up in the mantle.

  7. Lunar Reconnaissance

    The Lunar Reconnaissance Orbiter reached lunar orbit on 23 June 2009. Global data acquired since then now tell us about the impact history of the Moon and the igneous processes that shaped it. Using the Lunar Orbiter Laser Altimeter, Head et al. (p. 1504; see the cover) provide a new catalog of large lunar craters. In the lunar highlands, large-impact craters have obliterated preexisting craters of similar size, implying that crater counts in this region cannot be used effectively to determine the age of the underlying terrain. Crater counts based on the global data set indicate that the nature of the Moon's impactor population has changed over time. Greenhagen et al. (p. 1507) and Glotch et al. (p. 1510) analyzed data from the Diviner Lunar Radiometer Experiment, which measures emitted thermal radiation and reflected solar radiation at infrared wavelengths. The silicate mineralogy revealed suggests the existence of more complex igneous processes on the Moon than previously assumed.

  8. Clever Crows

    CREDIT: RUTZ ET AL.

    Understanding the adaptive significance of animal tool use requires reliable information on the foraging behavior in the wild. New Caledonian crows consume a range of foods and use sticks as tools to extract wood-boring beetle larvae from their burrows. These larvae, with their unusual diet, have a distinct isotopic signature that can be traced after consumption by the crows in the crows' feathers and blood. By comparing the stable isotope profiles of crows' tissues with those of their food sources, Rutz et al. (p. 1523) estimated the proportion of larvae in crow diets, providing a proxy for tool-use dependence in individual crows. Just a few larvae can satisfy a crow's daily energy requirements, highlighting the substantial rewards available to competent tool users and their offspring. Thus, tool use provides New Caledonian crows with access to a very nutritional food source that is not easily exploited by beak alone.

  9. Making Modifications

    Arginylation of β-actin regulates cell motility and the actin cytoskeleton, but how differential arginylation of the two highly similar actin isoforms—β and γ—is achieved in vivo is unclear. Zhang et al. (p. 1534; see the Perspective by Weygand-Durasevic and Ibba) describe a cotranslational mechanism that selectively regulates the arginylation of proteins through degradation and is dependent on the nucleotide coding sequence coupled to the translation speed. The work provides an explanation for the different N-terminal arginylation states of β- and γ-actin in vivo and suggests translation rate affected by nucleotide coding sequence confers different posttranslational states to proteins and selectively regulates protein degradation.

  10. MicroRNA-16 and Depression

    Signal transmission between neurons is effected by neurotransmitters such as serotonin. Membrane-bound transporters remove excess neurotransmitters. Disruption of the delicate balance between neurotransmitter release and removal can lead to larger disruptions in neuronal circuitry. Depression and anxiety may be linked to dysfunction of some of these circuits. Uptake inhibitors can be used to treat depression, but the molecular pathways affected have been unclear. Baudry et al. (p. 1537) now show that microRNA-16 controls synthesis of the serotonin transporter and that the amount of microRNA-16 can be controlled by the same uptake inhibitors used to treat depression.

  11. Naval Gazing

    Simple perceptual tasks, such as detecting the contrast between light and dark bars in a grating, have been a mainstay of psychophysical research for decades. This kind of task makes it possible to obtain both an objective measure of how accurate subjects are and a subjective measure of how confident they are in their judgments. Fleming et al. (p. 1541; see the Perspective by Lau and Maniscalco) have taken this approach one step further by constructing a measure of how accurate subjects are in their confidence judgments. This capacity for introspection, which can be regarded as one facet of metacognition (thinking about thinking), is shown to vary across individuals and to correlate positively with the gray matter volume of the frontopolar cortex (the frontmost region of the brain) and also with white matter in the tracts of the corpus callosum that connect these regions in the left and right hemispheres.

  12. Rolling Snowballs

    The genetic incompatibilities that separate ongoing speciation events have been hypothesized by the Dobzhansky-Muller model of speciation to snowball—that is, accumulate mutations causing postzygotic isolation at a faster rate than the linear accumulation of mutations. This occurs because of potential deleterious epistatic interactions in hybrids involving two or more interacting genes. Testing QTLs (quantitative trait loci) in seed and pollen sterility between multiple species pairs in the plant group Solanum, Moyle and Nakazato (p. 1521) show that hybrid female (seed) sterility accumulates exponentially between increasingly distant species pairs, although not for hybrid male (pollen) sterility. In contrast, loci contributing to differences in other traits show no evidence for nonlinear accumulation over time. Matute et al. (p. 1518) come to similar conclusions through the use of deletion mapping in comparisons between two pairs of species of Drosophila. The number of genes causing postzygotic isolation grows as fast as the square of the number of substitutions between two species. Thus, a hybrid snowball effect is found in both plants and animals.

  13. Two Ways to Nucleotide Reduction

    Ribonucleotide reductases (RNRs) are essential for DNA synthesis and repair in all organisms, initiating nucleotide reduction through a free-radical mechanism. The class Ib RNRs are the primary aerobic RNRs for many human pathogens. NrdF, the class Ib RNR of Escherichia coli, can initiate nucleotide reduction through either a FeIII2-Y• or a MnIII2-Y• cofactor. Whereas the Fe-based cofactor can self-assemble, assembly of the Mn-based free radical requires a reduced flavoprotein, NrdI. Boal et al. (p. 1526, published online 5 August; see the Perspective by Sjöberg) have gained insight into the mechanism of cofactor activation by determining structures of MnII2-NrdF, FeII2-NrdF, and MnII2-NrdF in complex with reduced and oxidized NrdI. The structures show how a single protein, NrdF, can use two different oxidants to activate two different metallocofactors using distinct chemistries.