This Week in Science

Science  23 Mar 2012:
Vol. 335, Issue 6075, pp. 1405
  1. Trimprinting the Genome


    Reprogramming the parental genomes during the oocyte-to-embryo transition requires highly controlled epigenetic mechanisms. Although resetting the genome to a ground state is essential, conservation of inheritable marks is equally important. Now, Messerschmidt et al. (p. 1499) demonstrate that maternal deletion of the epigenetic modifier Trim28 in mice results in a strongly variable, yet ultimately embryonic, lethal phenotype. Aberrant loss of DNA methylation at imprinting control regions and thus partial loss of epigenetic memory was responsible for the phenotype. The stochastic time and mode of embryonic death reflect the exquisitely balanced interplay of maternal and zygotic factors in the early mammalian embryo.

  2. Interacting Topological Insulator

    Topological insulators (TIs) hold great promise as a setting for exotic fundamental phenomena, as well as for more practical applications such as quantum computing. This new state of matter has been discovered in materials such as Bi2Se3, where electrons' spins are correlated with their orbital motion, but interactions between the electrons themselves are negligible. For a TI to fulfill its potential, a material with interacting electrons is desirable. Zhang et al. (p. 1464) used density functional calculations to predict that strong interactions in actinide compounds such as AmN may drive a transition into a TI state, with a large insulating gap favorable for device applications.

  3. Tamer Triangulations

    Diazomethane is a broadly useful precursor to carbene (CH2), which, in turn, reacts with olefins to form triangular carbon cycles known as cyclopropanes. These small rings are of fundamental interest for their strained bonds and turn up periodically in natural products, as well as in pharmaceutical and agrochemical research. Unfortunately, the flip side of diazomethane's facile reactivity is its dangerous tendency to explode—a hazard exacerbated by the need to isolate the compound after its preparation in highly basic water before it can be reacted with hydrophobic olefins. Morandi and Carreira (p. 1471) now show that an iron cyclopropanation catalyst can circumvent the need for the isolation step, inducing reaction of the hazardous compound in a biphasic aqueous/organic medium immediately after its generation.

  4. Lignin to the Rescue?

    The increasing demand for rechargeable batteries is putting a strain on the availability of certain key raw materials. Lignin is the second most common biopolymer and typically makes up 25% of wood. Lignin derivatives are also readily available as by-products from the pulp and paper industry. Milczarek and Inganäs (p. 1468) combined lignin derivatives, which are electronic insulators, with polypyrole, a conductive polymer, into an interpenetrating composite suitable for use as a cathode.

  5. Waste Not

    The organic matter in wastewater is a potentially vast and sustainable energy source; however, most wastewater treatment plants consume energy. Cusick et al. (p. 1474, published online 1 March) combined a microbial fuel cell with a reverse-electrodialysis system to boost the voltage output and the power density over a simple microbial fuel cell. The use of ammonium bicarbonate as a fuel for reverse electrodialysis while microorganisms simultaneously turn organic matter into electricity not only allows for the capturing of waste heat, but could eventually produce enough energy to offset the energy used in conventional wastewater treatment systems.

  6. Building Blocks of Earth

    Earth formed from an explosive and energetic series of collisions that accreted material over millions of years. Comparisons between rocks from Earth's interior and more primitive extraterrestrial samples can help tease apart the composition of Earth's starting material; however, discrepancies between the abundance of certain elements or their isotope ratios often obscure their origin. Fitoussi and Bourdon (p. 1477, published online 1 March) analyzed the silicon isotopes of a suite of rocks from chondritic meteorites and the Moon to reconcile some of the previous models. By tuning Earth accretion models to account for these Si isotope signatures, enstatite chondrites could be ruled out as the sole end-member composition for bulk Earth. Instead, a heterogeneous mixture of several types of chondritic meteorites is more likely.

  7. Hidden from Magnetic View


    An electromagnetic cloak is a device within which electromagnetic fields cannot penetrate, but, more importantly, the device itself does not disturb the electromagnetic fields surrounding it. An article placed in the cloak therefore vanishes from view, creating no shadow or reflection. Such devices have been demonstrated, but only for a particular band of frequencies. Confirming theoretical work that predicts such cloaking should be possible down to zero frequency, Gömöry et al. (p. 1466) designed a cloak for a dc magnetic field. With a composite design of ferromagnetic and superconducting material, together with a relatively simple structure, the device could potentially find immediate application.

  8. Human Impact?

    Following the arrival of humans in Australia 40- to 50,000 years ago, many species of large vertebrates rapidly became extinct. By analyzing sediment cores from a site in northeastern Australia, Rule et al. (p. 1483; see the Perspective by McGlone) show that the extinction of the Australian megafauna caused important ecosystem shifts. Prominent among these were a shift from rainforest vegetation to sclerophyllous vegetation and a sustained increase in the incidence of fire. The cores also provide evidence of the cause of megafaunal extinction in Australia, ruling out climate and anthropogenic fire as possible causes while confirming that the extinctions closely followed human arrival. These findings show how landscapes sometimes have been fundamentally changed by the indirect effects of early humans—which underscores the impact that even prehistoric human societies had on natural systems.

  9. Untangling the Web

    Interspecific interactions link species within complex trophic and nontrophic webs (see the Perspective by Lewinsohn and Cagnolo). Theoretical work has suggested that certain characteristics of species, or even interactions, may predispose them to extinction from a network. Aizen et al. (p. 1486) provide empirical evidence that plant-pollinator interactions are lost nonrandomly following habitat reduction in isolated hills in the Argentine pampas. Some types of interaction were more vulnerable to disruption than others, particularly when the specialization of the interacting was high and when the interactions were infrequent. Stouffer et al. (p. 1489) applied network theory to predict the dynamical importance of species across different food webs. Characteristic three-node motifs were identified, and species were characterized according to the relative frequencies with which they occupied unique positions within the motifs. These relative frequencies and the dynamic importance of the motifs were then used to identify a species-level importance within a food web.

  10. Mistress of Meiosis


    Meiosis is essential for proper distribution of maternal chromosomes to eggs during oogenesis. Su et al. (p. 1496) identified a gene, meiosis arrest female 1 (Marf1), which is indispensable for meiosis and other oogenic processes. In mice, Marf1 mutations resulted in meiotic arrest and an increase in nuclear DNA double-strand breaks, phenotypes linked to up-regulated levels of specific messenger RNAs (mRNA). These findings place MARF1 as a key regulator of mammalian female fertility through its integration of oocyte mRNA homeostasis, meiosis, and maintenance of genomic integrity.

  11. Reversing Decline?

    Apolipoprotein E (apoE) normally helps in the clearance of β-amyloid from the brain, a process that is compromised in Alzheimer's disease. Cramer et al. (p. 1503, published online 9 February; see the Perspective by Strittmatter) now show that a drug that increases apoE expression rapidly promoted soluble β-amyloid clearance in a mouse model of Alzheimer's disease. The drug also improved cognitive, social, and olfactory performance and rapidly improved neural circuit function. Similar therapeutics may potentially help to ameliorate the symptoms of Alzheimer's disease and its prodromal states.

  12. Remembering Stressful Events

    Situations surrounding emotional events are better remembered than others that accompany neutral events. However, in severe pathological states such as posttraumatic stress disorder (PTSD), exposure to threatening situations can also result in memory impairment. In this case, a hypermnesia for a salient trauma-related cue is associated with loss of memory for important aspects of the traumatic event. The memory for the core traumatic event is enhanced, but the capacity to place it in the right place and in response to the right cues is reduced. Kaouane et al. (p. 1510, published online 23 February) associated a high-intensity threat with the infusion of corticosterone in the hippocampus to induce PTSD-like memory impairments in mice. The animals became unable to identify the threat context as the right predictor of the threat, and they showed a fear response for discrete salient cues normally identified as safe. The neural activation patterns in the amygdala and hippocampal regions of these mice were similar to those observed in human PTSD.

  13. Lights, Sound, Images

    Optical microscopy can readily image thin samples such as cells, but thicker samples, such as tissue, are more difficult to image directly, because of the multiple scattering of light. Wang and Hu (p. 1458) review methods for imaging biological samples on length scales ranging from organelles to whole organs that rely on the photoacoustic effect—the excitation of ultrasonic pressure waves when light is absorbed by molecules in solution. The incident light can be focused and scanned across a sample in a microscopy mode to create ultrasound images, or the entire region of interest can be illuminated and the ultrasound waves analyzed with a computer algorithm in a tomography mode. Imaging studies can reveal changes in oxygen metabolism and gene expression and in image biomarkers and vasculature.

  14. The Thin Melt Line

    Pronounced and persistent variations in the speed of seismic waves as they travel through Earth's interior imply the presence of a physical or chemical boundary layer. Near the lithosphere-asthenosphere boundary, these seismic discontinuities are often patchy and their origins are disputed. By analyzing a set of short-period seismic waves across the Pacific Ocean, Schmerr (p. 1480; see the Perspective by Kawakatsu) relates the intermittency of this seismic discontinuity to a layer of partial melt below the oceanic lithosphere. The melt may arise from a number of geodynamic processes, including upwellings from the mantle at hot spots like Hawaii, or small-scale thermal convection in the mantle.

  15. Donuts Dissociate

    In Arabidopsis, the UVR8 protein responds to ultraviolet-B (UV-B) light by dissociating into monomers, which are then available to interact with downstream factors that enact the plant's response to light. Christie et al. (p. 1492, published online 9 February; see the cover and see the Perspective by Gardner and Correa) have now determined the crystal structure of UVR8. Without ultraviolet-B light, UVR8 dimerizes, with two donut-shaped monomers joined by a network of salt bridges. Close-packing of a pyramid of tryptophan residues permits exciton coupling that is key to UV-B perception. Electron transfer after UV-B perception could dissociate the salt bridges that hold the dimer together and release monomeric UVR8 to initiate light-induced signaling.

  16. Inhibiting the Inhibitors

    Excitatory neurons in the hippocampus and entorhinal cortex are under control of local γ-aminobutyric acid–releasing (GABAergic) interneurons that are the major source of inhibition in the adult brain. In contrast to the wealth of knowledge regarding connectivity in the hippocampus, much less is known about the cross-talk between the hippocampus and other brain regions. Melzer et al. (p. 1506) investigated long-range GABAergic projections from the hippocampal formation to the medial entorhinal cortex and vice versa. These GABAergic neurons bidirectionally couple the hippocampus and medial entorhinal cortex and preferentially target local inhibitory neurons, forming disinhibitory loops that could help synchronize neuronal activity.

  17. Adding Artificial Associations

    In the mammalian cortex, there is significant spontaneous neural activity that is internally generated, rather than arising from sensory inputs, and this activity influences the processing of natural sensory stimuli. What role does this internally generated activity play in forming and accessing new memory representations? Using transgenic mice combined with controlled artificial activation of widely distributed neuronal ensembles associated with specific contexts, Garner et al. (p. 1513; see the Perspective by Morris and Takeuchi) investigated how spontaneous neuronal activity is integrated into a given context representation. Animals underwent fear conditioning in two distinct contexts. When a circuit in one context was artificially activated during fear conditioning in a separate context, a so-called “hybrid” memory was formed. This artificially stimulated network became a necessary component of the memory trace. These results are consistent with the emerging view that internally generated brain activity is not noise, but a coherent representation that can be incorporated into new associations and memories.

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