This Week in Science

Science  01 Oct 2010:
Vol. 330, Issue 6000, pp. 9
  1. Avoiding Loss in a Quantum System

      CREDIT: ©TREMANI/DELFT UNIVERSITY OF TECHNOLOGY

      Single electron spins in solid-state environments have been explored as candidates for quantum information storage and computation; however, they often interact strongly with their surroundings and lose the stored information on the time scale of pico- to milliseconds. Dynamical decoupling schemes have been introduced to “undo” the effects of this interaction by applying a sequence of control pulses that reverse the undesirable evolution of the system. De Lange et al. (p. 60, published online 9 September) tested several decoupling schemes on a nitrogen vacancy center in diamond and found that a scheme with evenly spaced pulses with double-axis decoupling could prolong the coherence time of an arbitrary spin state up to 25-fold.

    1. Extra Exoplanet?

        A planet is said to transit its star if it can be seen to pass in front of the star; 19% of the known extrasolar planets are transiting planets. A lone planet will transit in an exactly periodic manner; if other planets are present, however, variations in transit duration are expected because of gravitational interactions. Holman et al. (p. 51, published online 26 August; see the cover; see the Perspective by Laughlin) report timing variations in the transits of two exoplanets detected by the Kepler space telescope. The planets have masses similar to that of Saturn and transit the same Sun-like star. A third planet several times the mass of Earth may also transit the star in an interior orbit.

      1. Smoke Gets in Your Lungs

          Chronic obstructive pulmonary disease (COPD) is a leading cause of death in the United States, primarily caused by cigarette smoking. The chronic inflammation that leads to tissue damage and organ dysfunction in COPD is mediated in large part by neutrophils, a type of granulocytic immune cell. Snelgrove et al. (p. 90, published online 2 September; see the Perspective by Barnes) now provide an explanation for why neutrophils stick around in the lung during COPD. The neutrophil chemoattractant Pro-Gly-Pro (PGP) is a biomarker for COPD and promotes neutrophil accumulation. The enzyme leukotriene A4 hydrolase degrades PGP in mice, and its activity was reduced by cigarette smoke both in vivo and in vitro. In contrast, during acute influenza infection in mice, leukotriene A4 hydrolase functioned normally, allowing for PGP degradation and the resolution of inflammation. Thus, in COPD, cigarette smoking may lead to the accumulation PGP—which, in turn, could keep neutrophils in the lung to drive inflammation and subsequent lung damage and dysfunction.

        1. New Guinea's Ancient Colonies

            CREDIT: SUMMERHAYES ET AL.

            Isolated by water, Australia and New Guinea were some of the last major parts of the world colonized by modern humans. Summerhayes et al. (p. 78; see the Perspective by Gosden) describe an archaeological site in the highlands of New Guinea that sheds light on this migration. The record extends back to nearly 50,000 years ago and thus represents one of the earliest known records. Nuts and yams were widely consumed, and the variety of stone tools discovered implies that the early humans may have cleared forest patches to promote the growth of useful plants.

          1. Two for One

              Solar cells often contain materials that absorb a broad spectrum of light above a certain frequency threshold, or band gap. Unfortunately, much of the energy contained in this light is wasted, because any balance exceeding the band gap tends to be dissipated as heat, rather than harnessed into electric current. Recent spectroscopic studies have shown that incident photons with energy several multiples of the band gap can transiently generate more than one current carrier, but the excess carriers tend to collapse before they can be diverted into the circuit. Sambur et al. (p. 63) now show that, when light-absorbing lead sulfide nanoparticles are carefully coupled to smoothly polished titanium dioxide crystalline electrodes, such excess carriers can be transferred into the circuit before collapsing.

            1. Toward Microbial Taxol

                Taxol, a minor chemical constituent of yew tree bark, has provided a potent cancer treatment. Production methods presently rely on plant cell cultures. Ajikumar et al. (p. 70; see the Perspective by Liu and Khosla) engineered Escherichia coli cells to produce a key taxol precursor, in which the polycyclic carbon skeleton is intact. The approach relied on optimizing the relative activity of two pathways, the first of which synthesized isoprenoid building blocks that were then stitched together with the second pathway. Accumulation of indole as a by-product inhibited the isoprenoid pathway—an insight that should facilitate more efficient engineered biosynthesis of a wide range of commercially important isoprenoid derivatives.

              1. Indirect Oxidation by Oxygen

                  Partial oxidation of alcohols to aldehydes and ketones must avoid complete oxidation to carbon dioxide and water. Zope et al. (p. 74) examined the partial oxidation of ethanol and glycerol to acids in alkaline aqueous solvents over gold and platinum catalysts. Conversions were highest for gold supported on titania, but studies with isotopically labeled molecular oxygen showed that oxygen incorporated into the acid comes from hydroxide ions. Direct incorporation of oxygen did not occur even for the platinum catalysts, despite the fact that oxygen can dissociate on this metal. Instead, molecular oxygen appeared to regenerate hydroxide ions at the metal surface through the formation of peroxide intermediates.

                1. Auroral Chorus

                    Energetic particles that arrive from near-Earth space produce photon emissions—the aurora—as they bombard the atmosphere in the polar regions. The pulsating aurora, which is characterized by temporal intensity variations, is thought to be caused by modulations in electron precipitation possibly produced by resonance with electromagnetic waves in Earth's magnetosphere. Nishimura et al. (p. 81) present a detailed study of an event that showed a good correlation between the temporal changes in auroral luminosity and chorus emission—a type of electromagnetic wave occurring in Earth's magnetosphere. The results points to chorus waves as the driver of the pulsating aurora.

                  1. Channel STIMulation

                      CREDIT: WANG ET AL.

                      The STIM1 protein functions as a calcium sensor and regulates entry of calcium into cells across the plasma membrane. When cell surface receptors are stimulated and cause release of calcium from internal stores in the endoplasmic reticulum (ER), STIM proteins in the ER membrane interact with the Orai channel pore protein in the plasma membrane to allow calcium entry from the outside of the cell (see the Perspective by Cahalan). Park et al. (p. 101) and Wang et al. (p. 105) now show that STIM also acts to suppress conductance by another calcium channel—the voltage-operated CaV1.2 channel. STIM1 appeared to interact directly with CaV1.2 channels in multiple cell types, including vascular smooth muscle cells, neurons, and cultured cells derived from T lymphocytes. The interaction inhibited opening of the CaV1.2 channels and caused depletion of the channel from the cell surface.

                    1. Improving Yeast for Biofuel Production

                        The biofuels industry uses the yeast Saccharomyces cerevisiae to produce ethanol from sugars derived from cornstarch or sugar cane. Plant cell walls are an attractive sugar source; however, yeast does not grow efficiently on cellulose–derived sugars (cellodextrins). Galazka et al. (p. 84, published online 9 September) now show that a model cellolytic fungus Neurospora crassa relies on a cellodextrin transport system to facilitate growth on cellulose. Yeast reconstituted with this transport system grew efficiently on cellodextrins, which could potentially improve the efficiency of cellulosic biofuel production.

                      1. Gaining from a Loss

                          Mitotic recombination can cause a cell carrying heterozygous mutations in a tumor suppressor gene to lose the wild-type copy of the gene, setting the cell on the pathway to uncontrolled growth. But can mitotic recombination have beneficial effects in other settings—that is, lead to phenotypic correction of a diseased cell by facilitating loss of the disease-causing mutation? Choate et al. (p. 94, published online 26 August; see the Perspective by Davis and Candotti) now find evidence for this type of event in a rare skin disease called ichthyosis with confetti (IWC). Patients with IWC display severe scaling of the skin but have widespread patches of normal skin that reflect clonal expansion of revertant cells. The revertant cells showed loss of heterozygosity on chromosome 17q and, as a result of mitotic recombination, these cells selectively lost dominant disease-causing mutations in the keratin 10 gene (KRT10), but retained the wild-type copy of the gene.

                        1. One, Two, Three, Remember Me

                            When a stimulus (such as a word or a face) is presented for the second, third, or fourth time, do the neural representations differ? And, if they do, are multiply represented stimuli remembered better? These questions and related ones have fascinated psychologists for decades, but only recently has it become feasible to begin tackling them using neuroimaging. Xue et al. (p. 97, published online 9 September) provide evidence that the greater the similarity in the patterns of neural activity during encoding of the item, the greater the likelihood that the item will be remembered.

                          1. Mechanical Responders Identified

                              Although many cells appear to respond to mechanical stimulation through increased conductance of ion channels in the plasma membrane, the actual channels that mediate these effects—which are important in diverse processes from hearing and touch to control of blood pressure—have remained elusive. Coste et al. (p. 55, published online 2 September) used RNA interference to decrease expression of candidate genes systematically in a mouse neuroblastoma cell line and identified two genes that encode proteins, Piezo1 and Piezo2, which are required for mechanically stimulated cation conductance in these cells and in cultured dorsal root ganglion neurons. Similar proteins are expressed in a range of species from protozoa to vertebrates. The proteins are not similar to known pore-forming proteins and thus could be unusual channels or regulatory components of a channel complex.

                            1. Chloride Control of a Catalyst

                                Allosteric regulation of proteins and enzymes allows their conformation to be controlled by changes in the concentration of effector-binding molecules; reactivity can be controlled by changing access to the active catalytic site. The supramolecular control of conformation through multiple weaker interactions has been exploited in organometallic catalysts that can open or close access to reactive sites around a metal center. Yoon et al. (p. 66) now show how chloride ions can be used to control a rhodium polymerization catalyst in which two metal centers are bridged by a linker bearing aromatic groups. When Cl is present, reactive sites remain available on the metal centers. When Cl is removed by abstracting agents, aromatic groups on one of the other ligands on the Rh centers can bind to the linker to form a triple-layer sandwich structure. Formation of this supramolecular structure blocks access of reactants to the metal centers.

                              1. Closing the Vector Circle

                                  The genome sequence of Culex quinquefasciatus offers a representative of the third major genus of mosquito disease vectors for comparative analysis. In a major international effort, Arensburger et al. (p. 86) uncovered divergences in the C. quinquefasciatus genome compared with the representatives of the other two genera Aedes aegypti and Anopheles gambiae. The main difference noted is the expansion of numbers of genes, particularly for immunity, oxidoreductive functions, and digestive enzymes, which may reflect specific aspects of the Culex life cycle. Bartholomay et al. (p. 88) explored infection-response genes in Culex in more depth and uncovered 500 immune response-related genes, similar to the numbers seen in Aedes, but fewer than seen in Anopheles or the fruit fly Drosophila melanogaster. The higher numbers of genes were attributed partly to expansions in those encoding serpins, C-type lectins, and fibrinogen-related proteins, consistent with greater immune surveillance and associated signaling needed to monitor the dangers of breeding in polluted, urbanized environments. Transcriptome analysis confirmed that inoculation with unfamiliar bacteria prompted strong immune responses in Culex. The worm and virus pathogens that the mosquitoes transmit naturally provoked little immune activation, however, suggesting that tolerance has evolved to any damage caused by replication of the pathogens in the insects.