This Week in Science

Science  30 Nov 2012:
Vol. 338, Issue 6111, pp. 1124
  1. Warming and Melting


    Mass loss from the ice sheets of Greenland and Antarctica account for a large fraction of global sea-level rise. Part of this loss is because of the effects of warmer air temperatures, and another because of the rising ocean temperatures to which they are being exposed. Joughin et al. (p. 1172) review how ocean-ice interactions are impacting ice sheets and discuss the possible ways that exposure of floating ice shelves and grounded ice margins are subject to the influences of warming ocean currents. Estimates of the mass balance of the ice sheets of Greenland and Antarctica have differed greatly—in some cases, not even agreeing about whether there is a net loss or a net gain—making it more difficult to project accurately future sea-level change. Shepherd et al. (p. 1183) combined data sets produced by satellite altimetry, interferometry, and gravimetry to construct a more robust ice-sheet mass balance for the period between 1992 and 2011. All major regions of the two ice sheets appear to be losing mass, except for East Antarctica. All told, mass loss from the polar ice sheets is contributing about 0.6 millimeters per year (roughly 20% of the total) to the current rate of global sea-level rise.

  2. Building with DNA

    One route for assembling three-dimensional (3D) DNA nanostructures is to start with a long natural DNA single strand and attach short strands, or “staples,” that cause the entire “origami” structure to fold into a desired shape. Ke et al. (p. 1177, see the cover; see the Perspective by Gothelf) present an alternative approach to 3D assembly that builds upon modular assembly of 2D DNA tiles. One hundred and two distinct shapes were created from four-domain, 32-nucleotide single-stranded DNAs that assembled like children's blocks; each block could bind to four neighboring blocks through specific pairing interactions. Computer design and stepwise assembly allowed assembly of hollow shapes with a variety of internal cavities.

  3. Glow from the Past

    Extragalactic background light (EBL) is the integrated radiation from all extragalactic sources in the universe. Foreground emission from our solar system and galaxy makes direct detection of the EBL very difficult. However, it is possible to measure EBL from gamma-ray spectra of distant sources, because gamma-ray photons from these sources interact with the EBL. Ackermann et al. (p. 1190, published online 1 November; see the Perspective by Bromm) report a measurement of the EBL based on an attenuation feature seen in the combined spectra of distant active galaxies detected by the Fermi Gamma-Ray Space Telescope. The result puts constraints on the cosmic history of star formation.

  4. What Do You Know? A Dome


    The superconducting dome—the appearance of a maximum in the transition temperature as a function of a tuning parameter—has been observed in compounds such as cuprates, pnictides, and heavy fermion materials and is thought of as a signature of unconventional superconductivity. Ye et al. (p. 1193) used a liquid gating technique combined with back gating to finely tune the carrier density in the band insulator MoS2, which allowed them to observe the formation of a dome. The unexpected finding awaits theoretical explanation but may suggest that the appearance of an optimal carrier density may be a more common occurrence than was previously thought.

  5. Forming the Regular Moons

    There is a good understanding of how Jupiter's Galilean moons (Io, Europa, Ganymede, and Callisto) and Saturn's largest moon, Titan, formed, but the same is not true for the other regular moons of the giant planets. Crida and Charnoz (p. 1196) describe a model whereby most of the regular moons (those with little orbital inclination or eccentricity) in the solar system were born in ancient massive rings that surrounded their planets. This model does not apply to Jupiter, but it can explain the regular moons of the other giant planets, including Titan, and even those of Earth and Pluto. The model further predicts that Uranus and Neptune once had massive, Saturn-like rings, which gave birth to moons and then disappeared.

  6. Swimming in Iron Pools

    Because iron is essential for marine phytoplankton growth, its availability limits the primary productivity of the oceans. Iron is typically bioavailable only when present in a dissolved state; however, a large fraction of the total iron in the oceans exists as tiny solid-phase particles ranging in size from a few nanometers to a few micrometers. von der Heyden et al. (p. 1199) used high-resolution x-ray microscopy and spectroscopy to characterize the distribution of iron particles along two transects of the Southern Ocean. Analysis of a number of individual particles reveals strong variation in iron oxidation state, particle mineralogy, and substitution of aluminum for iron—all of which control the solubility, and hence bioavailability, of iron.

  7. Modeling the Brain

    Neurons are pretty complicated cells. They display an endless variety of shapes that sprout highly variable numbers of axons and dendrites; they sport time- and voltage-dependent ion channels along with an impressive array of neurotransmitter receptors; and they connect intimately with near neighbors as well as former neighbors who have since moved away. Simulating a sizeable chunk of brain tissue has recently become achievable, thanks to advances in computer hardware and software. Eliasmith et al. (p. 1202; see the Perspective by Machens) present their million-neuron model of the brain and show that it can recognize numerals, remember lists of digits, and write down those lists—tasks that seem effortless for a human but that encompass the triad of perception, cognition, and behavior.

  8. More Genes, Tougher Plant

    Soybean crops, which supply valuable protein, oil, and renewable fuel, are under attack by a nematode for which there is no effective pesticide. Instead, agriculture relies on resistance derived from a genetic locus, which is now represented in most of the soybean crops cultivated in the United States. Cook et al. (p. 1206, published online 11 October) elucidated the mechanisms by which this rhg1-b allele protects against disease. The region carries several genes, none of which resemble other known immune receptor genes. Experiments silencing one or another of the genes showed that the genes work as a cluster. However, one set of the genes is not enough: Plants need multiple repeats of the locus to acquire resistance.

  9. Discerning a Difference


    Neighboring cells communicate via the Notch signaling pathway to make numerous decisions. Notch receptors are known to distinguish between two distinct ligand families, Delta and Serrate/Jagged, in different contexts. Posttranslational sugar modifications have been shown to play a role in this process, but it is not clear if other features of Notch are involved. Using a forward genetic approach in fruit flies, Yamamoto et al. (p. 1229) identified an evolutionarily conserved amino acid in the extracellular domain of Notch necessary for Serrate/Jagged signaling but dispensable for Delta signaling.

  10. DNA Repair in Vitro

    Accurate replication of the genome is critical to an organism's continued survival. Damaged DNA not repaired before the commencement of replication can cause the DNA replication fork to stall or collapse, which can result in mutation or recombination, with potentially serious consequences for cell and organism. Fork regression involving a so-called “chicken foot” structure (Holliday junction) is thought to provide one mechanism for dealing with unrepaired DNA damage during replication. Manosas et al. (p. 1217) analyzed the action of the T4 bacteriophage replisome and helicase UvsW on a stalled-fork mimic in vitro, using a magnetic trap. UvsW was able to switch migration directions, which was essential for remodeling the stalled fork. Together, UvsW and T4 holoenzyme were able to drive template switching and lesion bypass in vitro.

  11. Cooperation Is the Key to Control

    Chronic infections like hepatitis C virus (HCV) or HIV are hard on the immune system. In the face of a constant threat, some immune cells like T lymphocytes become “exhausted”; although present, they can no longer mount responses that are effective enough to eliminate the virus. These responses, however, are still important because in many cases they do keep the virus relatively controlled. The mechanisms underlying the population dynamics of T cell responses during chronic viral infection, however, are not well understood. Paley et al. (p. 1220) now demonstrate that the T-box transcription factors T-bet and Eomesodermin differentially regulate two phenotypically and functionally distinct subsets of antiviral CD8+ T cells in mice. The cooperation of these subsets may be important for antiviral immunity during chronic viral infections in humans.

  12. Joy or Pain?

    Face recognition and processing are so completely central to human social interactions that these functions are supported by specialized regions in the brain. One of the fundamental aspects being processed is emotion, particularly whether the emotion being expressed is positive or negative. Nevertheless, neuroimaging studies have documented that perceiving opposite emotions often activates the same or overlapping regions. Aviezer et al. (p. 1225) report that the recognition of positive versus negative emotions actually relies on information communicated by the body—the extent to which perceivers identified joy versus grief in composite figures was driven by whether the body came from a joyous (versus grievous) image rather than the face.

  13. Imprinting Evolution

    Many mammals have imprinted alleles, where the paternal or maternal version is solely expressed during reproduction. In humans, one such imprinted gene set is the growth promoter insulin-like growth factor 2 (IGF2) and its binding inhibitor, IGF2R mannose 6-phosphate/IGF2 receptor. To avoid parental conflict in fetal growth, imprinting regulates expression of these genes so that expression of IGF2R in the fetus quenches IGF2 and prevents fetus overgrowth through high-affinity binding of IGF2 to IGF2R. Williams et al. (p. 1209) demonstrate that high-affinity binding of IGF2 to IGF2R is present in placental and marsupial mammals; absent in birds and fish; and present, with a 10-fold lower affinity, in monotreme (egg-laying) mammals. The appearance of exonic splicing enhancers in exon 34 of the IGF2R of monotremes appears to have been a key mutational event leading to the establishment of higher affinity, which may have been driven by selection to minimize parental conflict.

  14. Powering LPS Export

    The surface of Gram-negative bacteria such as Escherichia coli is covered in lipopolysaccharide (LPS), which must be synthesized within the cytoplasm and then exported across the periplasm to the outer leaflet of the outer membrane. Okuda et al. (p. 1214, published online 8 November) describe the detection of LPS molecules bound to specific components of the E. coli lipopolysaccharide exporter during transport in vivo. In vitro reconstitution of LPS transport revealed the energetic requirements for the first two steps of transport, release from the membrane and transit along the periplasmic bridge of the exporter that connects the inner and outer membrane components. The findings suggest that adenosine triphosphate hydrolysis by a cytoplasmic component of the machine provides energy to push a continuous stream of LPS against its concentration gradient to the cell surface.