This Week in Science

Science  12 Jun 2009:
Vol. 324, Issue 5933, pp. 1362
  1. It's All in the Delivery


    Pancreatic cancer is almost universally associated with a poor prognosis, in part because the tumors are resistant to chemotherapeutic drugs. Working with a mouse tumor model that displays many features of the human disease, Olive et al. (p. 1457, published online 21 May; see the Perspective by Olson and Hanahan) found that the tumors were poorly vascularized, a factor likely to impede drug delivery. Treatment of the mice with the chemotherapeutic drug gemcitabine in combination with a drug that depletes tumor-associated stromal tissue led to an increase in tumor vasculature, enhanced delivery of gemcitabine, and a delay in disease progression. Thus, drugs targeting the tumor stroma may merit investigation as a way to enhance the efficacy of conventional chemotherapy for pancreatic cancer.

  2. Stellar Hourglass Figure


    Star-forming clouds are thought to be supported against gravity by ordered interstellar magnetic fields, which are strong enough to slow gravitation collapse but too weak to prevent it. Girart et al. (p. 1408) measured polarized radio waves from dust particles around a forming massive star, which reveal an hourglass shape. The data imply that a magnetic field strength dominates over turbulence—the telltale signs of magnetically controlled star formation. These conditions mimic those found in low-mass star-forming regions, suggesting that the magnetic field plays an important role in star formation, irrespective of differences in mass.

  3. From X-ray Binary to Pulsar

    Pulsars with millisecond rotational periods are thought to originate from neutron stars in low-mass x-ray binaries that had their spin frequencies increased by long-lasting mass transfer from their companion stars. Using data from a radio pulsar survey, Archibald et al. (p. 1411, published online 21 May; see the Perspective by Kramer) found a neutron star in a low-mass X-ray binary that is in the process of turning into a radio millisecond pulsar. The system, which consists of a solar-like star and a 1.69-millisecond radio pulsar, has gone through a recent accretion phase, characteristic of low-mass X-ray binaries, but it shows no accretion disk anymore, confirming the evolutionary connection between millisecond radio pulsars and low-mass X-ray binaries.

  4. Confining Carriers in Diamond

    Charge carriers can be confined in thin layers created by quantum wells. These multilayer structures alternate nanoscale layers of materials with slightly different band-gap energies, such as AlGaAs and GaAs, and create a confining potential. Watanabe et al. (p. 1425) demonstrate quantum confinement of electrons by diamond multilayers that vary only in isotopic composition; the carbon-12 and carbon-13 layers differ in band-gap energy by ∼17 millielectron volts. Cathodoluminescence experiments performed at 80 kelvin showed that in alternating layers (as thin as 30 nanometers or as thick as 350 nanometers), emission comes from charge carriers recombining in the carbon-12 layer, which has a lower band gap. Carriers from the carbon-13 layer appear to transfer with little loss into carbon-12 layers.

  5. Cooling All Atoms

    Laser cooling can be used to chill atoms to ultralow temperatures. However, the characteristics of the atoms and of the lasers limit the technique to a select few elements of the periodic table. Techniques are thus being explored that can be applied more generally. Raizen (p. 1403) reviews recent work on the magnetic coilgun cooling technique—where a series of magnetic pulses are applied to a cloud of atoms or molecules and can cool them to millikelvin temperatures. The only requirement of the atoms is paramagnetism—a property of most of the elements in the periodic table.

  6. Spotting Charges

    Many nanoscale physical systems are sensitive to the position of isolated charges, such as single-electron transistors and molecular assemblies that separate charges with energy from photons. In order to probe the location of a charged atom, the most general methods would work on insulating surfaces. Gross et al. (p. 1428; see the Perspective by Meyer and Glatzel) show that a tuning-fork atomic force microscope (AFM) operating in a noncontact mode at cryogenic temperatures can resolve the charge state of gold and silver atoms absorbed on a sodium chloride film. Charged atoms set up image charges in the AFM tip, which creates an electrostatic force not present with a neutral atom.

  7. Planted Evidence

    To understand the spatial patterns and consequences of past climate change requires the identification of reliable proxies that reflect specific aspects of those changes, such as temperature or rainfall in a given location. Also of interest are proxies for broader categories of change, such as methane production or the sources of carbon dioxide. Severinghaus et al. (p. 1431) present a 100,000-year-long record of the oxygen isotopic composition of atmospheric O218Oatm) extracted from air from polar ice cores. δ18Oatm is a general measure of the strength of low-latitude terrestrial photosynthesis and thus of local rainfall because plant metabolism is controlled in large part by water availability. δ18Oatm changes were related to Heinrich and Dansgaard-Oeschger events, two modes of abrupt climate change common over that interval, and δ18Oatm was controlled mostly by the strength of the Asian and North African monsoons. The rapid changes observed should also help to synchronize ice core records.

  8. Helicopter Seed Lift

    The “helicopter” seeds of maple trees and other similar autorotating seeds detach from their parent tree under windy conditions and gyrate as they are dispersed by the wind. The reproductive success of the tree depends on the flight performance of its seeds. Autorotating seeds are known to generate high lift as they slowly descend through the air, but the means by which they do so is unclear. Lentink et al. (p. 1438, see the cover) have elucidated the aerodynamic mechanism for high lift in autorotating seeds using a robot model seed and a three-dimensional flow measurement technique. Maple seeds and a hornbeam seed create a prominent leading-edge vortex that is similar to the flow structures that are responsible for the high lift generated by the wings of hovering insects and bats. Thus, both animals and plants have converged on an identical aerodynamic solution for generating lift.

  9. Boom and Bust

    The Brazilian Amazon is renowned for its biodiversity and for its influence on climate regulation and geochemical cycles. It is also one of the country's poorest regions. For decades, much economic development has been pursued through conversion of forest for agriculture and cattle-ranching. Rodrigues et al. (p. 1435) investigated whether this pattern of land use brings lasting prosperity by analyzing data on the economic development of nearly 300 municipalities across the deforestation frontier. Relative development, in terms of life expectancy, literacy, and standard of living, increases as deforestation begins but then declines again as the frontier passes through. As a result, pre- and postfrontier levels of development are similarly low, indicating a pattern of boom and bust.

  10. Neurotransmission in Living Color

    Neurotransmission involves the release of small molecular neurotransmitters from one neuron to another across a synapse. Gubernator et al. (p. 1441, published online 7 May) introduce a means to observe neurotransmitter release optically, by the design of fluorescent false neurotransmitters, which act as substrates for the synaptic vesicle monoamine transporter. These endogenous monoamine optical tracers enabled visualization of neurotransmitter uptake and release from individual synaptic terminals and were used to evaluate dopamine neurotransmission in the striatum. The fraction of synaptic vesicles that release neurotransmitter per stimulus was frequency dependent, and a frequency-dependent selection of presynaptic terminals was observed.

  11. Rotavirus Rumbled


    Rotavirus infection is the primary cause of severe diarrhea in infants. For the virus to enter cells, a Ca2+-stabilized trimer of the outer layer protein VP7 must be dissociated. Aoki et al. (p. 1444) report the structure of the VP7 trimer in complex with the Fab fragment of a neutralizing monoclonal antibody. Based on the structure and an analysis of positions of neutralization escape mutations, the authors propose that many neutralizing antibodies inhibit cell entry by stabilizing the VP7 trimer even at low calcium concentrations. A disulfide-linked trimer was then produced that is a potential subunit immunogen.

  12. Dynamic Imprinting

    Gene imprinting—the silencing of either a maternally derived or paternally derived gene allele—is controlled in large part by DNA methylation. In plants, imprinting occurs in the endosperm, which nourishes the embryonic plant. Gehring et al. (p. 1447) and Hsieh et al. (p. 1451) analyzed the dynamics of DNA methylation in the endosperm and embryo of Arabidopsis and found extensive demethylation in the endosperm, suggesting that many imprinted genes are likely to exist. Gehring et al. characterized five imprinted genes in detail. Four of the 10 known imprinted genes are related homeodomain transcription factors. Furthermore, 5′ sequences demethylated in several of the genes were found to be derived from transposable elements, which supports the idea that imprinting arose as a by-product of silencing invading DNA.

  13. Evolving Entanglement

    Quantum mechanical entanglement is a powerful but fragile resource for quantum information processing. It lends itself to increased computational power over classical computers. However, when quantum systems interact with their environment, which they must do if you want to follow what they are doing, then the entanglement can be lost. Jiménez Farías et al. (p. 1414, published online 14 May) present an experimental and theoretical study on entangled photon pairs, showing that they can determine and understand how the entanglement evolves as the system interacts with its surroundings.

  14. Colloidal Nanocrystal Compounds

    Colloidal nanocrystals have properties that fall between those of the individual atoms but also differ from bulk due to confinement effects. They can thus be thought of as analogs of atoms, and, like atoms, there is a desire to bond together neighboring particles, which will also affect their properties. During synthesis, organic ligands are used to prevent the colloidal nanocrystals from growing too large or agglomerating, but these ligands result in poor interparticle coupling and communication. Kovalenko et al. (p. 1417) show that chalcogenide complexes (compounds based on S, Se, or Te) can effectively link together neighboring particles. Upon gentle heating, the ligands can be converted from insulating to semiconducting without altering the chemistry of the nanocrystals.

  15. Ferroelectric Patterning with High Fields

    Ferroelectric oxides have a net polarization that can switch direction upon application of a sufficiently high electric field. In principle, a ferroelectric thin film should be able to act as a polar switch—tunneling an electron through the film would effectively switch on or off depending on the direction of the polarization. In practice, the length scale needed for a sufficiently small tunneling barrier is nearly the same as the scale at which films no longer support ferroelectricity. Maksymovych et al. (p. 1421) now show that the tip of an atomic force microscope can be used to pattern polarization domains in a thin film of lead zirconate titanate in high electric fields similar to those for field emission tips.

  16. Recombining Resistance

    Homologous recombination is frequent in many bacteria, but few studies have addressed whether subpopulations within a species are more or less likely to undergo this process and whether it has consequences for their evolution. Taking a large data set from the pathogen Streptococcus pneumoniae, Hanage et al. (p. 1454) discovered a group of strains characterized by an anomalous sequence of housekeeping genes. This sequence appeared to have been horizontally acquired from other pneumococci and related species and was associated with resistance to all classes of antibiotics for which data are available. Thus, hyper-recombination (in contrast to hypermutation) is important in the evolution and spread of antibiotic resistance and may play a role in determining the emergence of species clusters and the phenotypes associated with them.