Editors' Choice

Science  09 May 2008:
Vol. 320, Issue 5877, pp. 718

    Ringing in the Old Years

    The annual rings of the bristlecone pine trees in eastern California, extending back in live trees about 8000 years, are a lynchpin for calibrating radiocarbon dates. They have also provided important local climate records, as ring width is thought to reflect temperature and moisture; the length of the record is particularly useful in applying the data toward long reconstructions of global climate. Berkelhammer and Stott have now obtained an annual resolved record, extending back to the year 1700, of the oxygen isotope composition of cellulose from the rings of two California bristlecone pines. Interpreting the record is somewhat complicated, as isotopes are fractionated within the tree and by transpiration from the leaves as relative humidity varies, but large changes in values probably reflect variation in the main sources of storms in the region. The record shows small oscillations every 20 years or so that probably reflect Pacific climate variations. More dramatic is a steep change in the mid-1800s that indicates a change from southern-derived moisture during the time of the Little Ice Age to more northern winter storms since. The effect of such changes on the overall growth of the trees, and thus the ring width used in longer reconstructions, need further study. — BH

    Geochem. Geophys. Geosys. 9, 10.1029/2007GC001803 (2008).


    Inflammation Activates Neurons

    Cell fusion is common in some tissues such as skeletal muscle. Studies using transplanted bone marrow-derived cells (BMDCs) have been complicated by observations that cell fusions between BMDCs and resident cell types can occur. Injury may increase the number of fusion events; however, it is not known whether fusogenic cells are functional.

    Johansson et al. transplanted single hematopoietic stem cells (HSCs) into lethally irradiated mice and found that Purkinje neurons fused with the HSCs. An alternate method for detecting such fusions entailed joining mice surgically, so that the parabiotic mice (one of which expressed green fluorescent protein) shared a circulatory system; 5 to 6 months after parabiosis, the cerebellums of both mice contained fluorescent Purkinje neurons. When chronic inflammation was present, cerebellar heterokaryon formation was enhanced, and differential gene expression analysis showed that the heterokaryons repressed hematopoietic genes and activated Purkinje neuron genes. — BAP

    Nat. Cell Biol. 10, 10.1038/ncb1720 (2008).


    Polar Preferences

    In rod-shaped bacteria, proteins that mediate chemotaxis, cell division, and development are observed to localize to the poles. Recent evidence of heterogeneity in the composition of the bacterial cytoplasmic membrane and of the preferential binding of proteins to particular lipids suggests that membrane architecture may contribute to protein localization. For example, cardiolipin is involved in the polar and septal targeting of the Escherichia coli osmosensory transporter ProP. Cardiolipin is a phospholipid with high intrinsic curvature, and one proposal is that clusters of cardiolipin localize spontaneously to the poles, where the membrane is constrained by the curved cell wall. Mukhopadhyay et al. have extended their quantitative biophysical model to include the elastic energy of the membrane (which penalizes a deformation of the membrane away from the cell wall-imposed geometry), the interaction energy between lipids, and entropy. They show that variations in osmotic pressure, the force that pins the membrane to the cell wall, can regulate localization. The model predicts the minimum membrane fraction of cardiolipin required for domain formation and a polar localization that are consistent with the experimentally observed critical concentration of cardiolipin required for polar localization of ProP. — VV

    Biophys. J. 94, 10.1529/biophysj.107.126920 (2008).


    Guiding Gamma Rays

    Early exposure in high-school science labs to geometrical optics with lenses and mirrors fosters wide familiarity with how to focus and manipulate light in the visible spectrum. A very broad range of convenient focal parameters is available at these wavelengths. As the energy of the photons increases and their wavelengths shrink into the x-ray and gamma-ray regimes, however, conventional lenses and mirrors no longer work. Instead, such high-energy photons must be guided along their path by shallow-angle reflection from specially designed mirrors. Because of the small angles required to avoid absorption losses, the typical focal distances involved are on the order of 10 m or so. Though manageable for the likes of large, Earth-based observation stations, these focal parameters are rather unwieldy for medical imaging, space-based observatories, or mobile radioactive material detectors. Tournear et al. show that multilayered stacks of gold-palladium and polymer films deposited on a curved silicon substrate can be used to effectively guide gamma rays with a much smaller focal length. Demonstrating the technique for gamma rays of 122 keV, they claim that the focal lengths can be shrunk down to 1 m or less for radiation in the ∼100-keV to low-MeV range. — ISO

    Appl. Phys. Lett. 92, 153502 (2008).


    Can SWNTs Swim Apart?

    Single-walled carbon nanotubes (SWNTs) form as bundles, and to facilitate their processing, they are usually solubilized by some sort of surface treatment, such as coating with surfactants or protonation with superacids. However, unadorned SWNT bundles, when sonicated in dilute N-methyl-pyrrolidone (NMP) suspensions, have been observed to disperse and even exfoliate. Bergin et al. now show that this process represents true thermodynamic solubility; in other words, the free energy of mixing is negative. This situation is unusual for SWNTs in that the entropy term of mixing for such large molecules is very small, so the enthalpy of mixing must be near zero or even negative. The authors found that as the degree of dilution of SWNT samples in NMP increased, an analysis of bundle sizes (via atomic force microscopy of droplets evaporated on substrates) showed a decrease in bundle diameter, indicating that the samples were approaching equilibrium in solution through spontaneous desorption of SWNTs from the bundles. Light-scattering studies afforded a measurement of the Flory-Huggins parameter, whose sign indicated that the enthalpy of mixing was negative. The authors note that the surface energy of NMP is similar to that of the graphitic surface of the nanotubes; scanning tunneling microscopy (STM) images of the SWNTs after sonication in NMP showed evidence of NMP molecules adsorbed on the SWNTs migrating from inside the bundles to the silicon deposition surface. The authors argue that strong physisorption of NMP onto the SWNT surfaces drives solu- bilization and that this observation will be key in developing better solvents. — PDS

    Adv. Mater. 20, 10.1002/adma.200702451 (2008).


    Are You a Morning Person?

    Some neurotransmitters, such as dopamine, have been implicated in adjusting a person's mood. The circadian clock mechanisms, meanwhile, keep the organism's physiology tuned for appropriate responses to day or night. Hampp et al. have demonstrated how the molecular signaling pathways for circadian rhythms might intersect with the brain's establishment of general mood. They found that the promoter of the gene encoding monoamine oxidase A, which stabilizes some aspects of mood and breaks down dopamine and serotonin, contains binding sites for several clock proteins and showed that circadian oscillation was driven by the Maoa promoter in neuroblastoma cells. Mice lacking Per2, a gene that stabilizes circadian rhythms, showed damped expression from the Maoa promoter. Observations of the Per2 mutant mice in response to an unavoidable problematic situation—taken as a proxy for despair in humans—showed correlations with disorders of mood. — PJH

    Curr. Biol. 18, 10.1016/j.cub.2008.04.012 (2008).


    Seen in a Positive Light

    A frequently observed and seemingly antisocial behavior is that a morally dubious action taken by oneself will be regarded as less sinful than the same act committed by someone else. Within the currently popular framework of a dual-process model for rendering moral judgments, this asymmetry might arise either at the automatic/intuitive stage of assessment or at the conscious/deliberative stage. Valdesolo and DeSteno describe a study that reveals that the bias in favor of the self grows out of cognitive processes. Diverting some of these cognitive resources by imposing an onerous numerical task while actions were being rated for fairness eliminated the moral bonus awarded to one's own acts, suggesting that the rapid intuitive ranking of behaviors according to norms of fairness operates in an evenhanded and disinterested fashion. In contrast, the higher-level thinking so characteristic of us may, in fact, overrule baser animal instincts for all-too-human motivations, such as the drive toward a positive self-image. — GJC

    J. Exp. Soc. Psychol. 44, 10.1016/j.jesp.2008.03.010 (2008).

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