Editors' Choice

Science  28 Apr 2006:
Vol. 312, Issue 5773, pp. 500

    Drying Out

    1. H. Jesse Smith

    The semiarid Sahel region, which bridges the Sahara desert and the savanna landscape in Africa, has endured multiple extreme droughts since the 1960s. Loss of vegetation has been attributed in part to periods of reduced rainfall, but the long-term contribution of livestock grazing to local desertification is still debated. Recent studies have interpreted satellite data to support a greening process, or recovery of vegetation, since rainfall began to increase in the mid-1980s, suggesting that grazing has had minimal lasting impact on the landscape.

    Hein and De Ridder argue that the satellite images have been systematically misinterpreted because of a flawed core assumption that rainfall variation would not alter rain-use efficiency (RUE): the ratio of annually generated plant material to rainfall. By analyzing data from six semiarid sites, they find that RUE instead appears to vary quadratically with rainfall. Correcting for this phenomenon suggests that anthropogenic degradation of the Sahel vegetation cover is a likely factor in the magnitude of the droughts over the past 40 years and suggests that future droughts may have a stronger impact than previously projected. — HJS

    Global Change Biol. 12, 10.1111/j.1365-2486.2006.01135x (2006).


    Fine Lines in Glass

    1. Phil D. Szuromi

    The feature resolution attainable using photolithography has generally been limited by the wavelength of the incident light. However, as light sources approach the extreme ultraviolet (EUV), the polymer resists become the limiting factor because etching leaves behind rough edges, probably due to polydispersity. A promising alternative is to fabricate resists from amorphous films composed of small organic molecules with high glass-transition temperatures. In this vein, Chang et al. prepared films with glass transitions at ∼120°C from derivatives of C-4-hydroxyphenyl-calix[4]resorcinarenes. A fluorinated photoacid was incorporated to solubilize local calixarenes on exposure to light, resulting in a positive-tone resist. The authors optimized the material by varying the extent of calixarene hydroxyl protection with bulky tert-butyloxycarbonyl (t-Boc) groups. At 70% t-Boc incorporation, EUV irradiation produced lines with 30 nm resolution. Moreover, a line-edge roughness below 5 nm was obtained for 50-nm lines. — PDS

    J. Mater. Chem. 16, 1470 (2006).


    Replenishing the Sheath

    1. Beverly A. Purnell

    After spinal cord injury, neuronal axons may survive; however, they often lose their myelin sheath, which is necessary for impulse conduction, and remyelination does not occur. Because of the ability of adult neural precursor cells (NPCs) to self-renew and to differentiate into multiple cell types, they serve as a potential source of cells to repair central nervous system injuries.

    Karimi-Abdolrezaee et al. have examined the ability of mouse NPCs to integrate with injured spinal cord tissue in rats that have been injured at the mid-thoracic level by aneurysm clip compression of the spinal cord. Adult NPCs from the mouse brain were transplanted, and growth factors, an anti-inflammatory drug, and an immunosuppressant were infused into the spinal cord of rats at 2 weeks after trauma, representing the subacute phase of spinal cord injury. This transplantation method promoted the survival and/or differentiation of adult neural progenitors with an oligodendrocyte lineage and resulted in remyelination of injured axons. Locomotion function and hindlimb movement improved after treatment with NPCs in the subacute model. These findings may lead to insights into spinal cord injury and therapeutic intervention. — BAP

    J. Neurosci. 26, 3377 (2006).


    Mass-Producing SET Sensors

    1. Phil D. Szuromi

    Weak electric fields at surfaces, whether in a solid-state device or a frozen cell section, can be mapped out noninvasively by mounting a single-electron transistor (SET) onto a scanning probe platform. However, the designs recently used to implement these scanning SETs have several drawbacks. Because the devices are easily damaged, elaborate methods for producing them one at a time are inefficient; moreover, the need for extremely low-temperature (<1 K) operating conditions, as well as laser-based feedback, limits the range of samples amenable to study.

    Brenning et al. have fabricated SETs on the ends of silicon nitride cantilevers, which in turn are mounted on rigid quartz crystal resonators. These noncontact atomic force microscopy tips use the change in resonant frequency as the feedback signal and scan at heights of a few nanometers. More than 200 tip assemblies can be fabricated at a time via electron-beam lithography, and they have large enough charging energies to operate at pumped liquid helium temperatures. The authors demonstrate the device by scanning a SiO2 surface at 4.2 K. — PDS

    Nano Lett. 6, 10.1021/nl052526t (2006).


    A Bad Outcome Implies Intent

    1. Gilbert J. Chin

    The last storyline on a once-popular television show described the prosecution of four defendants under the Good Samaritan law on the grounds that they had failed to act to prevent harm. The capacity to form judgments of morality (good/bad or helpful/harmful) and of intentionality (an outcome brought about deliberately/accidentally) has been one of the experimentally accessible aspects of investigations into how and when children develop a theory of mind and an understanding of causality.

    Leslie et al. have combined these two themes in a study of when children exhibit an adult-like asymmetry in making a distinction between a harmful side effect, which grown-ups commonly think of as being intentional and hence morally suspect, and a good side effect, which is usually regarded as an unintentional consequence of the action. They find evidence for this behavior, which they call the side-effect effect, in 4- and 5-year-olds but not in 3-year old children. In the specific scenario tested, that of Janine who disliked/liked a frog brought over by Andy, who did not care about her feelings about frogs, the older children were abler in correctly grasping his indifference, and then attributing purposefulness to the bad outcome but not the good one. — GJC

    Psychol. Sci. 17, 422 (2006).


    Sorting Sulfides

    1. Jake Yeston

    The abundant organosulfur compounds in crude oil are oxidized to acidic pollutants (such as sulfuric acid) during combustion. To minimize their environmental impact, gasoline and diesel are subjected to desulfurization processes before use. However, tighter regulations have spurred chemists to pursue more efficient desulfurization methods, which would treat heavy oil before the cracking process that yields transportation fuels.

    Toward this end, Choudhary et al. present a screening method to differentiate and quantify the organosulfur components of heavy oil. They first assay the aliphatic compounds by selective oxidation, followed by chromatographic/mass spectral analysis of the aromatics. Components are classified based on size and structure (mono- to hexacyclic, compact or extended geometry), and the relative reactivities of each class are then compared under varying desulfurization conditions. They find, for example, that phenanthrothiophenes are the least reactive toward hydrogenolysis (reductive removal of the sulfur as H2S) at 622 K but relatively more reactive at 655 K. They also determine which aromatics accept hydrogen more rapidly at carbon than at sulfur. These data offer useful projections for large-scale process optimizations. — JSY

    Angew. Chem. Int. Ed. 45, 10.1002/anie.200503660 (2006).

  7. STKE

    A Proton Gradient Signals Asymmetry

    1. Nancy Gough

    Adams et al. identified the H+-V-ATPase, which is a vacuolar and plasma membrane proton pump, in a pharmacological screen of Xenopus embryos in which defects in left-right asymmetry (heterotaxia) were scored. Inhibition of the H+-V-ATPase with drugs such as concanamycin or expression of a dominant-negative H+-V-ATPase subunit resulted in heterotaxia and the loss of asymmetric expression of one of the first genes with asymmetric expression, Nodal, suggesting that H+-V-ATPase provides a very early asymmetry signal indeed. Proton pump subunits were more abundant on the right side of the embryo as early as the two-cell stage, and proton efflux was greater on the right side of the embryo. In addition, the right side of the embryo was hyperpolarized relative to the left side, as expected from the electrogenic nature of the H+-V-ATPase. Elimination of asymmetric H+ flux by expression of a symmetrically localized plasma membrane H+ pump or exposure of the embryos to low pH, or elimination of the hyperpolarization of the membrane by incubating the embryos with palytoxin, both produced heterotaxia. This suggests that the activity of the H+-V-ATPase produces asymmetry through a combination of an effect on pH and the membrane potential. A role for H+-V-ATPase in asymmetry was also noted for chick and zebrafish embryos and appeared to serve as one of the earliest signals for asymmetry. Disruption of H+-V-ATPase activity randomized the expression of Nodal and Shh in chicks, and in zebrafish H+-V-ATPase activity was required for asymmetric expression of Southpaw and before formation of the Kupffer's vesicle, a ciliated organ involved in organ asymmetry. — NRG

    Development 133, 1657 (2006).