This Week in Science

Science  27 Mar 2009:
Vol. 324, Issue 5923, pp. 11
  1. Warm in the Past


    During the Medieval Climate Anomaly (approximately 800 to 1300 A.D.) temperatures in Europe were generally warmer than immediately before or after. The source of that warmth is hotly debated. Trouet et al. (p. 78) present a reconstruction of the North Atlantic Oscillation—the dominant mode of atmospheric circulation variability in the North Atlantic region—that extends back to the middle of the Medieval Climate Anomaly, based on data from Moroccan tree rings and a Scottish stalagmite. The North Atlantic Oscillation, it seems, was in a persistently positive state during the Medieval Climate Anomaly. The authors suggest that prevailing La Niña-like conditions during medieval times was initiated by high levels of irradiance and amplified by enhanced Atlantic Meridional Overturning Circulation.

  2. Healing Heart

    Damage to the heart through age or disease can lead to loss of heart muscle cells. Although cell transplantation therapies are being investigated to replace these cells, there would be certain advantages if the heart tissue could regenerate its own cells. To find out how many, if any, cells the human heart normally replaces over time, Bergmann et al. (p. 98; see the Perspective by Murry and Lee) made use of 14C measurements in heart cells from adults of various ages. Atmospheric 14C levels were elevated during times of aboveground testing of nuclear bombs around the 1950s and have declined since such tests were banned. Cells formed in different years reflect these differing levels of 14C. Considerable amounts of DNA synthesis was observed in heart cells well after birth, such that a sturdy fraction of an adult's cardiac DNA seems not to be as old as the body it resides in. The extent to which this finding reflects new growth of cells during life, or represents tissue repair remains to be seen.

  3. Malleable Hydrogels


    Hydrogels consist of highly swollen polymer networks. Because their properties can be controlled through the specific constituent polymer chemistry, and because they can be made to degrade with time, hydrogels have been used, for example, as scaffolds in tissue engineering. Hydrogels, however, are difficult to pattern into complex shapes because of the high cross-link density, and it is difficult to modify or tune their properties after hydrogel formation. Kloxin et al. (p. 59) have developed a family of photodegradable hydrogels that can be patterned and shaped after gelation and show how dynamic tuning of the gel properties can be used to manipulate cell function and differentiation.

  4. Let Your Motor Do the Walking

    A challenge in designing a molecular motor is to ensure that its motion is biased in a particular direction to allow it to do work, potentially through harnessing otherwise random fluctuations. Omabegho et al. (p. 67; see the Perspective by Sherman) report the construction of a DNA-based bipedal motor system that can walk along a DNA track. The system operates using two fuel strands, where the by-products of the reaction from the first fuel strand are required for the second, driving a ratcheting motion along the track.

  5. Redox Redux in Alzheimer's Disease

    Neurodegenerative disorders involve a series of pathophysiological changes. Oxidative or nitrosative stress can induce a profound and abnormal degree of mitochondrial fission, leading to bioenergetic compromise, which may contribute to neurodegenerative disorders. Cho et al. (p. 102) describe a critical nitrosylation event induced by nitrosative stress in the pathogenesis of sporadic cases of Alzheimer's disease. Dynamin-related protein 1 (Drp1), which is known to be important for mitochondrial fission, is activated by S-nitrosylation, a redox reaction of NO with a critical cysteine thiol. The nitrosylation event is triggered by oligomerized amyloid-β peptide and appears to mediate the synaptic damage known to occur early in Alzheimer's disease. Thus, the pathogenesis of Alzheimer's disease involves a redox component, which may help to explain why redox metals can contribute to neuronal damage in Alzheimer's disease.

  6. Intelligent Machines?


    How can we make sense out of the enormous volume of scientific data that is now being generated, and would a robot be able to replace a research assistant, postdoc, or even the principal investigator in a biological laboratory? (See the Perspective by Waltz and Buchanan.) Schmidt and Lipson (p. 81) developed an algorithm that substitutes a combination of brute force and mathematical strategies to solve a problem that challenges human reasoning. Given raw data on the behavior of a physical system like a pendulum, the algorithm searched the gamut of possible equations of mathematical physics consistent with the data and converged on fundamental equations of motion originally derived by Hamilton and Lagrange. King et al. (p. 85) describe a robot programmed not only to conduct experiments on yeast metabolism with little to no human intervention, but also to reason about its results and plan appropriate next experiments. The robot, Adam, filled in the blanks of unknown enzymes required to account for a biochemical and bioinformatic description of metabolism and genomics, and identified orphan enzymes that were confirmed (by humans) to function in yeast metabolism.

  7. Modeling the Mantle

    In geology, the age-progression of the Hawaiian-Emperor chain, and its prominent bend, was thought to represent the motion of the Pacific plate over a fixed melting locus in Earth's mantle. Recent data has suggested that this melting locus has instead moved over millions of years, probably reflecting long-term motion in the Pacific mantle. Tarduno et al. (p. 50) review several possible explanations and constraints provided by numerical models of the mantle.

  8. The Phylogenetics of the Common Cold

    The common cold can be caused by one of several strains of rhinoviruses. Palmenberg et al. (p. 55, published online 12 February) present a phylogenetic analysis of over 100 whole-genome sequences of human rhinoviruses aligned using characteristics of the known viral capsid crystal structure. The resulting tree suggests that sequence evolution in rhinoviruses is more or less clocklike, identifies clades outside of the recognized two major groups, and also shows that recombination occurs between distantly related viruses. The findings clarify human rhinovirus serotype biology, evolution, diversity, and drug resistance and provide a framework for large-scale epidemiologic studies.

  9. Crystal Diode

    A diode restricts the flow of electricity, allowing current to flow in one direction but not in the reverse direction. The diode effect is usually observed at an asymmetric junction or interface. T. Choi et al. (p. 63, published online 19 February) observe a diode effect in a pure material consisting of monodomain crystals of bismuth iron oxide (BiFeO3). The unidirectional current flow is due to the bulk electric polarization, and the direction of current flow can be reversed by reversing the direction of electric polarization via an external applied voltage. The crystals also generate a current when exposed to light, even when there is no bias to drive the direction of the generated current.

  10. Improved Iron Availability in Fuel Cell Catalysts

    The scarcity and cost of platinum as the catalyst for fuel-cell reactions continues to drive a search for alternatives. Iron-based catalysts, in which iron cations coordinate at nitrogen-bearing sites in a graphitic support, catalyze the key oxygen-reduction reaction but at relatively low overall rates. Unlike platinum, in which the reactions can occur at numerous sites of metal catalyst particles, the density of accessible iron catalytic sites is low. Lefèvre et al. (p. 71; see the Perspective by Gasteiger and Marković describe a synthetic route for these materials that greatly increases the density of active iron sites. The resulting catalysts achieve comparable activity to platinum catalysts up to current densities of 0.1 amperes per square centimeter.

  11. Oxygen Elimination

    Plants are still well ahead of humans when it comes to harnessing sunlight to make oxygen from water. A particular sticking point in efforts to develop direct artificial catalysts for this purpose is the tendency of metal oxo or hydroxo intermediates not to let go of their coordinated oxygen groups. Kohl et al. (p. 74; see the Perspective by Eisenberg) present progress in this vein with a reaction sequence centered on an aqueous ruthenium complex. Initially, heating the compound in boiling water appends two OH groups to the metal center with accompanying liberation of hydrogen gas. Exposure to light then releases oxygen, in all likelihood through preliminary elimination of hydrogen peroxide, HOOH. Although currently the process takes several days, observation of clean oxygen elimination from a homogeneous metal complex bodes well for refinement of this and similar systems toward efficient catalysts.

  12. Secondary Messenger

    How do plants prime themselves to resist systemic pathogenic infections? Jung et al. (p. 89) report that a small mobile metabolite molecule, azelaic acid, found in Arabidopsis leaves, can contribute to defense priming during systemic acquired resistance to pathogens. Levels of azelaic acid increased in plants exposed to pathogens and triggered systemic resistance. When pure azelaic acid was sprayed onto leaves, it induced systemic resistance in wild-type, but not in systemic resistance-deficient mutants. Furthermore, azelaic acid stimulated the production of the systemic signaling molecule salicylic acid; consistent with a priming effect. The findings help to explain why salicylic acid is required both in locally infected tissue and in the distal tissue that will develop systemic acquired disease resistance, yet is not itself mobile.

  13. Eternal Optimist?

    During animal development, environmental conditions often vary and can be stressful. Baugh et al. (p. 92, published online 26 February) have analyzed growth and gene expression in nematode Caenorhabditis elegans larvae that have been starved or fed and then switched between feeding conditions. The nematode appears to behave like an optimist: When conditions are favorable, it responds rapidly, but when conditions become unfavorable, the worm is slower to respond. During starvation, larvae are developmentally arrested and RNA polymerase II accumulates on the promoters of growth and development genes. This accumulation anticipates recovery from developmental arrest when these genes are rapidly up-regulated in response to feeding.

  14. Regulating Gene Expression During Development

    Nuclear receptors convert a global hormone signal into cell-specific gene expression patterns. MicroRNAs repress target gene translation within cells. Bethke et al. (p. 95) now find that these two functions can be combined by showing that the Caenorhabditis elegans nuclear receptor DAF-12 directly activates transcription of let-7 microRNAs. The hormone-activated microRNAs then down-regulate their target gene, hbl-1, to allow developmental progression of epidermal stem cells. Thus, a hormone-coupled molecular switch can turn off earlier programs, which may also be relevant to developmental progression in other animals.

  15. Sleep and Memory

    Sleep is a biological process that is necessary for survival not only in vertebrates but also in Drosophila. Sleep is crucial for the consolidation of memories and is responsive to waking experience (see the news story by Miller). Donlea et al. (p. 105) found that core circadian clock neurons play an important role in mediating experience-dependent changes in sleep-need in fruit flies. Use-dependent changes in sleep-need were dependent upon the expression of the Drosophila homolog of Serum Response Factor in a specific subset of clock neurons. Within these neurons, Serum Response Factor altered experience-dependent changes in sleep by targeting epidermal growth factor receptor signaling. Experience-dependent changes in sleep were associated with an increase in the number of synaptic terminals in specific neuronal circuits. Moreover, the number of synaptic terminals was reduced during sleep and this decline was prevented by sleep deprivation. In an independent study, Gilestro et al. (p. 109) found that, in two different strains of Drosophila, levels of pre- and postsynaptic markers increased after sleep deprivation independent of time of day and gender. One of these synaptic markers increased after sleep deprivation in areas of the fly brain linked to memory and learning. Thus, in support of the synaptic homeostasis hypothesis of sleep, an important function of sleep, even in invertebrates, may be synaptic downscaling.