This Week in Science

Science  22 Feb 2019:
Vol. 363, Issue 6429, pp. 831
  1. Mass Extinction

    Two timelines for extinction

    1. Brent Grocholski

    The Deccan Traps in India were a source of large-scale volcanic activity that affected the climate 66 million years ago.

    IMAGE: GERTA KELLER

    The Cretaceous-Paleogene extinction that wiped out the nonavian dinosaurs 66 million years ago was correlated with two extreme events: The Chicxulub impact occurred at roughly the same time that massive amounts of lava were erupting from the Deccan Traps (see the Perspective by Burgess). Sprain et al. used argon-argon dating of the volcanic ash from the Deccan Traps to argue that a steady eruption of the flood basalts mostly occurred after the Chicxulub impact. Schoene et al. used uranium-lead dating of zircons from ash beds and concluded that four large magmatic pulses occurred during the flood basalt eruption, the first of which preceded the Chicxulub impact. Whatever the correct ordering of events, better constraints on the timing and rates of the eruption will help elucidate how volcanic gas influenced climate.

    Science, this issue p. 866, p. 862; see also p. 815

  2. Mesoscopic Physics

    Probing the dynamics of anyons

    1. Jelena Stajic

    A two-dimensional electron gas in the fractional quantum Hall regime has unusual excitations called anyons that carry only a fraction of the electron's charge. This fractional charge can be observed through a dynamical response to irradiation by microwaves, but such experiments require a combination of high magnetic fields with sensitive noise measurements and very low temperatures. Kapfer et al. observed this dynamical response in a GaAs/AlGaAs heterostructure hosting a high-mobility two-dimensional electron gas with fractional excitations of one-third and one-fifth of the electron's charge. The method may be of interest for use in topological quantum computing.

    Science, this issue p. 846

  3. Electrochemistry

    Scaled-up sodium-free Birch reductions

    1. Jake Yeston

    The so-called Birch reduction is frequently used by chemists despite its daunting conditions: Pyrophoric sodium is dissolved in pure liquified ammonia to achieve partial reduction of aromatics. Peters et al. surveyed and then optimized small-scale electrochemical alternatives to devise a safer protocol that can work on a larger scale with a broad range of functionally complex substrates.

    Science, this issue p. 838

  4. News and Politics

    Concentrated news precedes legislation

    1. Aaron Clauset

    News coverage can play a crucial role in shaping or reflecting public opinion and legislation. Sheshadri and Singh used machine-learning techniques to analyze 25 years of news articles from the New York Times and the Guardian, along with Google Trends data and U.S. federal legislation outcomes. Prolonged bursts of similarly framed articles were found to precede changes in public perception and new legislation. As an example, the framing around “surveillance” changed in conjunction with an uptick in the word “Snowden,” reflecting a shift in focus from national security to individual liberty. These findings illustrate the critical interplay between news, public attitudes, and new laws.

    Sci. Adv. 10.1126/sciadv.aat8296 (2019).

  5. Ion Channels

    A key to potassium channel activation

    1. Valda Vinson

    Using drugs to activate potassium channels has the potential to treat conditions like epilepsy, heart arrhythmias, and pain. Schewe et al. report a class of negatively charged activators (NCAs) with a defined pharmacore that use a similar mechanism to activate many types of potassium channels. X-ray crystallography, functional analysis, and molecular dynamics simulations showed that the NCAs bind below the selectivity filter to open the filter gate and activate the channels. Targeting this NCA site might be exploited in rational drug design.

    Science, this issue p. 875

  6. 3D Printing

    The key to keyhole formation

    1. Brent Grocholski

    The formation of keyholes, or vapor-filled depressions, during laser welding presents a large problem for additive manufacturing. Cunningham et al. used high-speed x-ray imaging to take a detailed look at keyhole formation in a titanium alloy. They found a simplified relationship between operational parameters and keyhole shape, which may allow for the prevention of pore formation going forward.

    Science, this issue p. 849

  7. Synthetic Biology

    Expanding the genetic code

    1. Laura M. Zahn

    DNA and RNA are naturally composed of four nucleotide bases that form hydrogen bonds in order to pair. Hoshika et al. added an additional four synthetic nucleotides to produce an eight-letter genetic code and generate so-called hachimoji DNA. Coupled with an engineered T7 RNA polymerase, this expanded DNA alphabet could be transcribed into RNA. Thus, new forms of DNA that add information density to genetic biopolymers can be generated that may be useful for future synthetic biological applications.

    Science, this issue p. 884

  8. Neurodegeneration

    Sleep may protect the brain from AD

    1. Stella M. Hurtley

    Two main proteins accumulate in the brain in Alzheimer's disease (AD), β-amyloid (Aβ) and tau. Aβ appears to instigate AD, but tau appears to drive brain damage and cognitive decline. Sleep deprivation is known to increase Aβ acutely and chronically. Now, Holth et al. show that chronic sleep deprivation strongly increases tau acutely over hours and also drives tau pathology spreading in the brains of mice and humans (see the Perspective by Noble and Spires-Jones). Thus, sleep appears to have a direct protective effect on a key protein that drives AD pathology.

    Science, this issue p. 880; see also p. 813

  9. Allergy

    Digging skin deep

    1. Lindsey Pujanandez

    Disrupted epithelial barriers are thought to be central to the development of allergic disorders such as atopic dermatitis, which is commonly associated with food allergy. Leung et al. performed repeated skin tape stripping on lesional and nonlesional skin of pediatric atopic dermatitis subjects, some of whom also had food allergies. Among other parameters, the authors measured lipids, barrier integrity, and the microbiome. Nonlesional skin from atopic dermatitis patients had different characteristics depending on whether the patient also had a food allergy. The findings could help in the development of food allergy biomarkers.

    Sci. Transl. Med. 11, aav2685 (2019).

  10. Topological Matter

    A topological paradigm shift

    1. Jelena Stajic

    The discovery of topological phases of matter forced condensed matter physicists to question and reexamine some of the basic notions of their discipline. Wen reviews the progress of the field that took a sharp turn from Landau's broken symmetry paradigm to arrive at the modern notions of topological order and quantum entanglement in many-body systems. This development was made possible by using increasingly sophisticated mathematical formalisms.

    Science, this issue p. eaal3099

  11. Plant Biology

    Chloroplast-associated protein degradation

    1. Stella M. Hurtley

    Protein degradation is vital for cellular functions, and it operates selectively with distinct mechanisms in different organelles. Some organellar proteins are targeted by the ubiquitin-proteasome system (UPS)—a major proteolytic network in the eukaryotic cytosol. In such cases, the organelle membrane presents a substantial barrier to protein degradation. Working in the model plant Arabidopsis, Ling et al. identified mechanisms underlying the UPS-mediated degradation of proteins in the outer membrane of chloroplasts (the organelles responsible for photosynthesis). They identified an Omp85-type β-barrel outer membrane channel and a cytosolic AAA+ chaperone that fulfill conductance and motor functions in the retrotranslocation of target proteins from chloroplasts. This process thus enabled outer membrane protein processing by the cytosolic proteasome. Such chloroplast-associated protein degradation was initiated by ubiquitination of the targets by the chloroplast-localized E3 ubiquitin ligase SP1.

    Science, this issue p. eaav4467

  12. Structural Biology

    Structures of the simplest replisome

    1. Steve Mao

    The DNA replisome performs concerted parental-strand separation and DNA synthesis on both strands. Gao et al. report the cryo–electron microscopy structures of the minimum set of bacteriophage T7 proteins that can carry out leading- and lagging-strand synthesis at the replication fork (see the Perspective by Li and O'Donnell). Three key enzymes involved in DNA replication—DNA polymerase, helicase, and primase—were visualized in complex with substrate DNA, demonstrating their highly dynamic organizations on both strands. Comparison of prokaryotic and eukaryotic replisomes reveals evolutionarily conserved operating principles and provides a structural basis for understanding coordination among DNA replication, recombination, and repair.

    Science, this issue p. eaav7003; see also p. 814

  13. Neuroscience

    Coincidence detection in synaptogenesis

    1. Stella M. Hurtley

    In the brain, synaptic connections are formed with exquisite specificity, but the underlying molecular mechanisms remain largely unexplored. Synapse formation is thought to involve bidirectional signaling by proteins that bind to each other across the synaptic cleft. Sando et al. used conditional genetic tools and in vitro assays to investigate the mechanisms of synapse formation. They found that synapse formation in the mouse hippocampus requires latrophilins. Latrophilins are G protein–coupled receptors that bind to cell-surface proteins called teneurins and fibronectin leucine-rich repeat transmembrane proteins (FLRTs). Two different latrophilins mediated formation of distinct synapses on the same hippocampal neuron. This function required binding of both teneurins and FLRTs. Thus, latrophilins may guide synapse formation by coincidence signaling, which could help to explain the specificity of synaptic connections.

    Science, this issue p. eaav7969

  14. Cold Molecules

    Coaxing quantumness in a molecular gas

    1. Jelena Stajic

    A dilute atomic gas cooled down to very cold temperatures can enter the so-called quantum degenerate regime, where quantum properties of the gas come to the fore. This regime has been achieved for both bosonic and fermionic atoms, but molecules, with their many internal states, present a special challenge. De Marco et al. cooled a bulk gas of fermionic potassium-rubidium molecules to quantum degeneracy (see the Perspective by Zelevinsky). The authors first cooled atomic potassium and rubidium gases separately, then bound them together into potassium-rubidium molecules, and finally brought the molecules down to their ground state. The density profile of the molecular gas revealed the system's quantum nature, which in turn kept the gas stable by suppressing chemical reactions.

    Science, this issue p. 853; see also p. 820

  15. Electrochemistry

    Harnessing self-tuned strain

    1. Phil Szuromi

    Strain can modify the electronic properties of a metal and has provided a method for enhancing electrocatalytic activity. For practical catalysts, nanomaterials with high surface areas are needed. However, for nanoparticles, strain is often induced with overlayers (adsorbates or heteroatoms) that can undergo reconstruction during operation that releases the induced strain. Wang et al. show that freestanding palladium nanosheets (three to five monolayers thick) form with an internal compressive strain of 1 to 2% and can be much more active for both the oxygen and hydrogen evolution reactions under alkaline conditions compared with nanoparticles.

    Science, this issue p. 870

  16. Microbiology

    Tiny modulators of parasite infection

    1. Gemma Alderton

    Extracellular vesicles are produced by diverse cell types, but what are they? Do they have a function, or are they cellular rubbish? In a Perspective, Ofir-Birin and Regev-Rudzki discuss the increasing evidence that extracellular vesicles are secreted by parasites to improve their chances of survival in host cells by modulating host immune responses. It is possible that parasite-derived extracellular vesicles could be manipulated to produce vaccines and/or be used in diagnostics.

    Science, this issue p. 817

  17. Thymic Selection

    Instructional signals for nascent T cells

    1. Ifor Williams

    Signals relayed through the T cell receptor (TCR)–CD3 complex are critical to determining whether immature T cells in the thymus undergo positive or negative selection. Positive selection enables T cell survival and subsequent export to the periphery, whereas negative selection leads to T cell death. Neier et al. developed a proteomics-based approach to profile proteins bound to the TCR–CD3 complex and compared differences between signaling associated with thymocyte survival and signaling associated with death. The full range of cellular outcomes after thymic selection could be attributed to quantitative differences in a shared core set of biochemical signals. Thus, differences in the affinity of TCRs for peptide–major histocompatibility complex complexes are transduced into downstream signals that are quantitatively variable, but qualitatively similar.

    Sci. Immunol. 4, eaal2201 (2019).

  18. Biochemistry

    Tumorigenic trio

    1. Leslie K. Ferrarelli

    Rho guanine nucleotide exchange factors (RhoGEFs) activate the Rho family of small guanosine triphosphatases that regulate cell migration and gene expression in normal development and disease, including some cancers. In many uveal (eye) melanomas, the RhoGEF module of Trio mediates signaling from oncogenic Gαq/11 proteins, which drives tumor progression. Bandekar et al. investigated the structural regulation of Trio and show that mutations found in patients relieve its autoinhibitory conformation, thereby enabling sustained Gαq/11-Trio-Rho signaling in cells.

    Sci. Signal. 12, eaav2449 (2019).

  19. Organic Chemistry

    Five-membered rings for two nickels

    1. Jake Yeston

    The Diels-Alder reaction is widely used to make six-membered rings by adding four-carbon dienes to two-carbon alkenes. It would seem straightforward to likewise access five-membered rings from dienes and one-carbon sources, or carbenes, but that does not tend to work. Instead, the carbene adds to just half of the diene to form a cyclopropane. Zhou and Uyeda now show that a catalyst with two nickel centers can steer this reaction toward the cyclopentyl products (see the Perspective by Johnson and Weix). A chiral version of the catalyst rendered the reaction enantioselective in intramolecular cases.

    Science, this issue p. 857; see also p. 819